FOS TE R. N. 
a text-Book of 
E®. 
c A S' . . 1 AMERICAN PUBLISHER'S NOTICE 
TO THE SIXTH EDITION. 
In the preparation of the Sixth American Edition every page has been 
subjected to careful, scrutiny and considerable liberty taken with the text. 
Useless verbiage has been omitted, obscure sentences have been revised or 
entirely rewritten, a large number of typographical errors have been cor- 
rected, histological details (except of the nervous system) have been mate- 
rially abridged, much that was too theoretical has been omitted, and such 
other alterations and additions have been made as to bring the book up to 
date and render it in a more advantageous form for the student. The his- 
tology of the nervous system has been retained in full, as has also the 
valuable Chemical Appendix-features that will be appreciated by both 
student and teacher. The more important additions have been dis- 
tinguished by enclosure in brackets [ ]. 
Philadelphia, September, 1895. 
7 CONTENTS. 
PAGE 
Introduction 17 
BOOK I. 
BLOOD. THE TISSUES OF MOVEMENT. THE VASCULAR MECHANISM. 
CHAPTER I. 
BLOOD. 
The Clotting of Blood 24 
The Corpuscles of the Blood : The Red Corpuscles 37 
The White or Colorless Corpuscles 43 
Blood Platelets 49 
The Chemical Composition of Blood 50 
The Quantity of Blood, and its Distribution in the Body 53 
CHAPTER II. 
The Phenomena of Muscle and Nerve : Muscular and Nervous Irritability ... 55 
The Phenomena of a Simple Muscular Contraction 65 
Tetanic Contractions 73 
On the Changes which Take Place in a Muscle during a Contraction: The 
Change in Form 77 
The Chemistry of Muscle 83 
Thermal Changes 90 
Electrical Changes 92 
The Changes in a Nerve during the Passage of a Nervous Impulse 97 
The Nature of the Changes through which an Electric Current is Able to 
Generate a Nervous Impulse: Action of the Constant Current 100 
The Muscle-nerve Preparation as a Machine 107 
The Circumstances which Determine the Degree of Irritability of Muscles and 
Nerves Ill 
The Energy of Muscle and Nerve, and the Nature of Muscular and Nervous 
Action 115 
On Some Other Forms of Contractile Tissue: Plain, Smooth or Unstriated Mus- 
cular Tissue 116 
Ciliary Movement 119 
Amoeboid Movements 121 
THE CONTRACTILE TISSUES. 
9 10 
CONTENTS. 
CHAPTER III. 
PAGE 
GENERAL FEATURES OF NERVOUS TISSUES 122 
CHAPTER IV. 
THE VASCULAR MECHANISM. 
The Structure and Main Features of the Vascular Apparatus 133 
The Structure of Arteries, Capillaries, and Veins 134 
The Main Features of the Apparatus 134 
The Main Facts of the Circulation 136 
Hydraulic Principles of the Circulation 144 
Circumstances Determining the Rate of the Flow 150 
The Heart 157 
The Phenomena of the Normal Beat 157 
Endocardiac Pressure . . 164 
Summary 174 
The Work Done 175 
The Pulse ... 176 
The Regulation and Adaptation of the Vascular Mechanism: The Regulation of 
the Beat of the Heart 191 
The Development of the Normal Beat 192 
The Government of Heart beat by the Nervous System 199 
Other Influences Regulating or Modifying the Beat of the Heart 209 
Changes in the Calibre of the Minute Arteries. Vasomotor Actions 211 
The Course of Vaso-constrictor and Vaso-dilator Fibres 219 
The Effects of Vasomotor Actions 221 
Vasomotor Functions of the Central Nervous System 222 
The Capillary Circulation 231 
Changes in the Quantity of Blood 236 
A Review of Some of the Features of the Circulation 238 
BOOK II. 
THE TISSUES OF CHEMICAL ACTION WITH THEIR RESPECTIVE 
MECHANISMS. NUTRITION. 
CHAPTER I. 
THE TISSUES AND MECHANISMS OF DIGESTION. 
The Characters and Properties of Saliva and Gastric Juice : Saliva 246 
Gastric Juice 251 
The Act of Secretion of Saliva and Gastric Juice and the Nervous Mechanisms 
which Regulate It 260 
The Changes in a Gland Constituting the Act of Secretion 268 
The Properties and Characters of Bile. Pancreatic Juice, and Succus Entericus • . 278 
Bile 278 CONTENTS. 
11 
PAGE 
Pancreatic J nice 281 
Succus Entericus 285 
The Secretion of Pancreatic Juice and of Bile 286 
The Structure of the Intestines : The Small Intestine 292 
The Muscular Mechanisms of Digestion 293 
The Changes which the Food Undergoes in the Alimentary Canal 306 
The Changes in the Stomach 306 
In the Small Intestine 308 
In the Large Intestine 312 
The Feces .... 313 
The Lacteals and the Lymphatic System 313 
The Nature and Movements of Lymph (including Chyle) 314 
The Characters of Lymph . 315 
The Movements of Lymph 317 
Absorption from the Alimentary Canal 325 
The Course Taken by the Several Products of Digestion 325 
The Mechanism of Absorption 329 
CHAPTER II. 
RESPIRATION. 
The Mechanics of Pulmonary Respiration 333 
The Respiratory Movements 339 
Changes of the Air in Respiration 344 
The Respiratory Changes in the Blood 346 
The Relations of Oxygen in the Blood 350 
Products of the Decomposition of {hemoglobin 358 
The Relations of the Carbonic Acid in the Blood 360 
The Relations of the Nitrogen in the Blood 361 
The Respiratory Changes in the Lungs : The Entrance of Oxygen 361 
The Exit of Carbonic Acid 364 
The Respiratory Changes in the Tissues 365 
The Nervous Mechanism of Respiration . 369 
The Effects of Changes in the Composition and Pressure of the Air Breathed . . 386 
The Relations of the Respiratory System to the Vascular and Other Systems . . 390 
Modified Respiratory Movements 403 
CHAPTER III. 
THE ELIMINATION OF WASTE PRODUCTS. 
The Composition and Characters of Urine 404 
Amounts of the Several Urinary Constituents Passed in Twenty-four Hours. 
(After Parkes.) 409 
The Secretion of Urine 410 
Secretion of the Renal Epithelium 417 
The Discharge of Urine 426 
Micturition 427 
The Nature and Amount of Perspiration . 431 
Cutaneous Respiration 432 
The Mechanism of the Secretion of Sweat 434 12 
CONTENTS. 
CHAPTER IV. 
THE METABOLIC PROCESSES OF THE BODY. 
PAGE 
The History of Glycogen 437 
Diabetes 448 
The Formation of the Constituents of Bile 453 
On Urea and on Nitrogenous Metabolism in General 457 
On Some Structures and Processes of Obscure Nature 465 
The History of Fat. Adipose Tissue 470 
The Mammary Gland 477 
Average Composition of Milk in Different Animals 480 
CHAPTER V. 
NUTRITION. 
Statistics of Nutrition 482 
Comparison of Income and Output of Material 485 
The Energy of the Body: The Income of Energy 492 
The Expenditure 493 
Animal Heat 496 
On Nutrition in General 506 
On Diet 513 
BOOK in. 
THE CENTRAL NERVOUS SYSTEM AND ITS INSTRUMENTS. 
CHAPTER I. 
THE SPINAL CORD. 
On Some Features of the Spinal Nerves 525 
The Structure of the Spinal Cord $ . 529 
The Reflex Actions of the Spinal Cord 564 
The Automatic Actions of the Spinal Cord 578 
CHAPTER II. 
THE BRAIN. 
On Some General Features of the Structure of the Brain 584 
The Bulb 589 
The Disposition and Connections of the Gray and White Matter of the Brain : 
The Gray Matter 501 
The Central Gray Matter and the Nuclei of the Cranial Nerves 601 CONTENTS. 
13 
PAGE 
The Superficial Gray Matter 615 
The Intermediate Gray Matter of the Crural System 615 
Other Collections of Gray Matter 625 
The Arrangement of the Fibres of the Brain 627 
Longitudinal Fibres of the Pedal System • 628 
Longitudinal Fibres of the Tegmental System 631 
Transverse or so-called Commissural Fibres ... 634 
Summary 635 
On the Phenomena Exhibited by an Animal Deprived of its Cerebral Hemispheres. 637 
The Machinery of Coordinated Movements . 643 
On Some Histological Features of the Brain 652 
The Superficial Gray Matter of the Cerebellum 653 
The Cerebral Cortex 655 
On Voluntary Movements 661 
On the Development within the Central Nervous System of Visual and of some 
Other Sensations: Visual Sensations . . 689 
Sensations of Smell 700 
Sensations of Taste 701 
Sensations of Hearing . 701 
On the Development of Cutaneous and Some Other Sensations 703 
Some Other Aspects of the Functions of the Brain 716 
On the Time Taken up by Cerebral Operations 724 
The Lymphatic Arrangements of the Brain and Spinal Cord 728 
The Vascular Arrangements of the Brain and Spinal Cord 732 
CHAPTER III. 
The Eye 738 
Dioptric Mechanisms: The Formation of the Image 744 
Accommodation 745 
Imperfections of the Dioptric Apparatus 757 
Visual Sensations 759 
The Origin of Visual Impulses . . . . 760 
Simple Sensations 766 
Color Sensations 769 
Visual Perceptions 776 
Modified Perceptions 778 
Binocular Vision : Corresponding or Identical Points 781 
Movements of the Eyeballs 782 
The Horopter 786 
Visual Judgments 786 
The Protected Mechanisms of the Eye 789 
SIGHT. 
CHAPTER IV. 
HEARING, SMELL, AND TASTE. 
Hearing: The Ear 790 
The Acoustic Apparatus ... 798 14 
CONTENTS. 
PAGE 
Auditory Sensations 800 
Auditory Judgments 805 
Smell: The Nasal Fossae 806 
Taste: The Gustatory Mucous Membrane 808 
CHAPTER V. 
FEELING AND TOUCH. 
General Sensibility and Tactile Perceptions 812 
Tactile Sensations: Sensations of Pressure 814 
Sensations of Temperature 814 
Tactile Perceptions and Judgments 816 
The Muscular Sense 818 
CHAPTER VI. 
The Voice: The Larynx 819 
Speech: Vowels 826 
Consonants 827 
Locomotor Mechanisms 829 
SPECIAL MUSCULAR MECHANISMS. 
BOOK IV. 
THE TISSUES AND MECHANISMS OF REPRODUCTION. 
CHAPTER I. 
ORGANS OF REPRODUCTION 833 
CHAPTER II. 
MENSTRUATION 838 
CHAPTER III. 
IMPREGNATION 841 
CHAPTER IV. 
THE NUTRITION OF THE EMBRYO 847 
PARTURITION 853 
CHAPTER V. CONTENTS. 
15 
CHAPTER VI. 
PAGE 
THE PHASES OF LIFE 855 
CHAPTER VII. 
DEATH 863 
APPENDIX. 
THE CHEMICAL BASIS OF THE ANIMAL BODY 865 A TEXT-BOOK OF PHYSIOLOGY. 
INTRODUCTION. 
§ 1. Dissection, aided by microscopical examination, teaches us that 
the body of man is made up of certain kinds of material, so differing from 
each other in optical and other physical characters and so built up together 
as to give the body certain structural features. Chemical examination 
further teaches us that these kinds of material are composed of various 
chemical substances, a large number of which have this characteristic, that 
they possess a considerable amount of potential energy capable of being set 
free, rendered actual, by oxidation or some other chemical change. Thus 
the body, as a whole, may, from a chemical point of view, be considered as 
a mass of various chemical substances, representing altogether a considera- 
ble capital of potential energy. 
§ 2. This body may exist either as a living body or (for a certain time 
at least) as a dead body, and the living body may at any time become a 
dead body. At what is generally called the moment of death (but arti- 
ficially so, for as we shall see the processes of death are numerous and 
gradual) the dead body so far as structure and chemical composition are 
concerned is exceedingly like the living body ; indeed the differences be- 
tween the two are such as can be determined only by very careful examina- 
tion, and are still to a large extent estimated by drawing inferences rather 
than actually observed. At any rate the dead body at the moment of death 
resembles the living body in so far as it represents a capital of potential 
energy. From that moment onward, however, the capital is expended ; by 
processes which are largely those of oxidation, the energy is gradually dis- 
sipated, leaving the body chiefly in the form of heat. While these chemical 
processes are going on the structural features disappear, and the body, with 
the loss of nearly all its energy, is at last resolved into " dust and ashes." 
The characteristic of the dead body then is that, being a mass of sub- 
stances of considerable potential energy, it is always more or less slowly 
losing energy, never gaining energy ; the capital of energy present at the 
moment of death is more or less slowly diminished, is never increased or 
replaced. 
§ 3. When on the other hand we study a living body we are struck with 
the following salient facts: 
1. The living bodv moves of itself, either moving one part of the body 
on another or moving the whole body from place to place. These move- 
ments are active; the body is not simply pulled or pushed by external 
forces, but the motive power is in the body itself, the energy of each move- 
ment is supplied by the body itself. 
2. These movements are determined and influenced, indeed often seem to 
be started, by changes in the surroundings of the body. Sudden contact 
between the surface of the body and some foreign object will often call 
17 18 
INTRODUCTION. 
forth a movement. The body is sensitive to changes in its surroundings, 
and this sensitiveness is manifested not only by movements but by other 
changes in the body. 
3. It is continually generating heat and giving out beat to surrounding 
things, the production and loss of heat, in the case of man and certain 
other animals, being so adjusted that the whole body is warm, that is, of a 
temperature higher than that of surrounding things. 
4. From time to time it eats, that is to say takes into itself supplies of 
certain substances known as food, these substances being in the main similar 
to those which compose the body, and being like them chemical bodies of 
considerable potential energy, capable through oxidation or other chemical 
changes of setting free a considerable quantity of energy. 
5. It is continually breathing; that is, taking in from the surrounding 
air supplies of oxygen. 
6. It is continually, or from time to time, discharging from itself into its 
surroundings so-called waste matters, which waste matters may be broadly 
described as products of oxidation of the substances taken in as food, or of 
the substances composing the body. 
Hence the living body may be said to be distinguished from the dead 
body by three main features. 
The living body like the dead is continually losing energy (and losing it 
more rapidly than the dead body, the special breathing arrangements per- 
mitting a more rapid oxidation of its substance), but unlike the dead body 
is by means of food continually restoring its substance and replenishing its 
store of energy. 
The energy set free in the dead body by the oxidation and other chem- 
ical changes of its substance leaves the body almost exclusively in the form 
of heat, whereas a great deal of energy leaves the living body as mechanical 
work, the result of various movements of the body, and as we shall see a 
great deal of the energy which ultimately leaves the body as heat, exists for 
a while within the living body in other forms than heat, though eventually 
transformed into heat. 
The changes in the surroundings affect the dead body at a slow rate and 
in a general way only, simply lessening or increasing the amount or rate 
of chemical change and the quantity of heat thereby set free, but never 
diverting the energy into some other form such as that of movement; 
whereas changes in the surroundings may in the case of the living body 
rapidly, profoundly, and in special ways affect not only the amount but also 
the kind of energy set free. The dead body left to itself slowly falls to 
pieces, slowly dissipates its store of energy, and slowly gives out heat; a 
higher or lower temperature, more or less moisture, a free or scanty supply 
of oxygen, the advent of many or few putrefactive organisms, these may 
quicken or slacken the rate at which energy is being dissipated but do not 
divert that energy from heat into motion ; whereas in the living body so 
slight a change of surroundings as the mere touch by a hair of some par- 
ticular surface, may so affect the setting free of energy as to lead to such a 
discharge of energy in the form of movement that the previously appar- 
ently quiescent body may be suddenly thrown into the most violent con- 
vulsions. 
The differences, therefore, between living substance and dead substance 
though recondite are very great, and the ultimate object of physiology is 
to ascertain how it is that living substance can do what dead substance 
cannot-that is, can renew its substance and replenish the energy which it 
is continually losing, and ^an, according to the nature of its surroundings, 
vary not only the amount but also the kind of energy which it sets free. INTRODUCTION. 
19 
Thus there are two great divisions of physiology : one having to do with the 
renewal of substance and the replenishment of energy, the other having to 
do with the setting free of energy. 
§ 4. Now the body of man (or one of the higher animals) is a very com- 
plicated structure, consisting of different kinds of material, which we call 
tissues, such as muscular, nervous, connective, and the like, variously 
arranged in organs such as heart, lungs, muscles, skin, etc., all built up to 
form the body according to certain morphological laws. But all this com- 
plication, though advantageous and indeed necessary for the fuller life of 
man, is not essential to the existence of life. The amoeba [Fig. 1] is a 
[Fig. 1. 
Amoeba princeps, shown in different forms (A, B, C) assumed by the same animal.] 
living being; it renews its substance, replenishes its store of energy, and 
sets free energy now in one form, now in another; and yet the amoeba may 
be said to have no tissues and no organs; at all events this is true of closely 
allied but not so well known simple beings. Using the more familiar 
amoeba as a type, and, therefore, leaving on one side the nucleus, and any 
distinction between endosarc and ectosarc, we may say that its body is 
homogeneous in the sense that if we divided it into small pieces, each piece 
would be like all the others. In another sense it is not homogeneous. For 
we know that the amoeba receives into its substance material as food, and 
that this food or part of it remains lodged in the body, until it is made use 
of and built up into the living substance of the body, and each piece of the 
living substance of the body must have in or near it some of the material 
which it is about to build up into itself. Further, we know that the amoeba 
gives out waste matters such as carbonic acid and other substances, and each 
piece of the amoeba must contain some of these waste matters about to be, 
but not yet, discharged from the piece. Each piece of the amoeba will, 
therefore, contain these three things, the actual living substance, the food 
about to become living substance, and the waste matters which have ceased 
to be living substance. 
Moreover, we have reasons to think that the living substance does not 
break down into the waste matters which leave the body at a single bound, 
but there are stages in the downward progress between the one and the 
other. Similarly, though our knowledge on this point is less sure, we have 
reason to think that the food is not incorporated into the living substance at 
a single step, but that there are stages in the upper progress from the 
dead food to the living substance. Each piece of the body of the amoeba 20 
INTRODUCTION. 
will, therefore, contain substances representing various stages of becoming 
living, and of ceasing to be living, as well as the living substance itself. 
And we may safely make this statement though we are quite unable to 
draw the line where the dead food on its way up becomes living, or the 
living substance on its way down becomes dead. 
§ 5. Nor is it necessary for our present purpose to be able to point out 
under the microscope, or to describe from a histological point of view, the 
parts which are living and the parts which are dead food or dead waste. 
The body of the amoeba is frequently spoken of as consisting of " proto- 
plasm." The name was originally given to the matter forming the primor- 
dial utricle of the vegetable cell as distinguished from the cell wall on the 
one hand, and from the fluid contents of the cell or cell sap on the other, 
and also we may add from the nucleus. It has since been applied very 
generally to such parts of animal bodies as resemble, in their general fea- 
tures, the primordial utricle. Thus the body of a white blood-corpuscle, or 
of a gland cell, or of a nerve cell, is said to consist of protoplasm. Such 
parts of animal bodies as do not in their general features resemble the 
matter of the primordial utricle are not called protoplasm, or, if they at 
some earlier stage did bear such resemblance, but no longer do so, are some- 
times, as in the case of the substance of a muscular fibre, called " differen- 
tiated protoplasm." Protoplasm in this sense sometimes appears, as in the 
outer part of most amoebae, as a mass of glassy-looking material, either con- 
tinuous or interrupted by more or less spherical spaces or vacuoles filled 
with fluid, sometimes as in a gland cell as a more refractive, cloudy-looking, 
or finely granular material arranged in a more or less irregular network, or 
spongework, the interstices of which are occupied either by fluid or by some 
material different from itself. We shall return, however, to the features of 
this " protoplasm " when we come to treat of white blood-corpuscles and 
other " protoplasmic" structures. Meanwhile it is sufficient for our present 
purpose to note that lodged in the protoplasm, discontinuous with it, and 
forming no part of it, are in the first place collections of fluid, of watery 
solutions of various substances, occupying the more regular vacuoles or 
the more irregular spaces of the network, and in the second place discrete 
granules of one kind or another, also forming no part of the protoplasm 
itself, but lodged either in the bars or substance of the protoplasm or in the 
vacuoles or meshes. 
Now, there can be little doubt that the fluids and the discrete granules are 
dead food or dead waste, but the present state of our knowledge will not 
permit us to make any very definite statement about the protoplasm itself. 
We may probably conclude, indeed we may be almost sure, that protoplasm 
in the above sense is not all living substance, that it is made up partly of 
the real living substance, and partly of material which is becoming living or 
has ceased to be living; and in the case where protoplasm is described as 
forming a network, it is possible that some of the material occupying the 
meshes of the network may be, like part of the network itself, really alive. 
" Protoplasm " in fact, as in the sense in which we are now using it, and shall 
continue to use it, is a morphological term ; but it must be borne in mind that 
the same word " protoplasm " is also frequently used to denote what we have 
just now called " the real living substance." The word then embodies a.phy- 
siological idea ; so used it may be applied to the living substance of all living 
structures, whatever the microscopical features of those structures ; in this 
sense it cannot at present, and possibly never will be recognized by the 
microscope, and our knowledge of its nature must be based on inferences. 
Keeping then to the phrase " living substance" we may say that each 
piece of the body of the amoeba consists of living substance, in which are INTRODUCTION. 
21 
lodged, or with which are built up in some way or other, food and waste in 
various stages. 
Now, an amoeba may divide itself into two, each half exhibiting all the 
phenomena of the whole; and we can easily imagine the process to be 
repeated, until the amoeba was divided into a multitude of exceedingly 
minute amoebae, each having all the properties of the original. But it is 
obvious, as in the like division of a mass of a chemical substance, that the 
division could not be repeated indefinitely. Just as in division of the chem- 
ical mass we come to the chemical molecule, further division of which 
changes the properties of the substance, so in the continued division of the 
amoeba we should come to a stage in which further division interfered with 
the physiological actions, we should come to a physiological unit correspond- 
ing to but greatly more complex than the chemical molecule. This unit to 
remain a physiological unit and to continue to live must contain not only a 
portion of the living substance but also the food for that living substance, 
in several at least of the stages, from the initial raw food up to the final 
" living " stages, and must similarly contain various stages of waste. 
§ 6. Now, the great characteristic of the typical amoeba (leaving out the 
nucleus) is that, as far as we can ascertain, all the physiological units are 
alike; they all do the same things. Each and every part of the body 
receives food more or less raw and builds it up into its own living substance ; 
each and every part of the body may be at one time quiescent and at another 
in motion ; each and every part is sensitive and responds by movement or 
otherwise to various changes in its surroundings. 
The body of man, in its first stage, while it is yet an ovum, if we leave 
aside the nucleus and neglect differences caused by the unequal distribution 
of food material or yolk, may also be said to be composed of like parts or 
like physiological units. 
By the act of segmentation, however, the ovum is divided into parts or 
cells which early show differences from each other; and these differences 
rapidly increase as development proceeds. Some cells put on certain char- 
acters and others other characters-that is to say, the cells undergo histologi- 
cal differentiation. And this takes place in such a way that a number of 
cells lying together in a group become eventually converted into a tissue, 
and the whole body becomes a collection of such tissues arranged together 
according to morphological laws, each tissue having a definite structure, its 
cellular nature being sometimes preserved, sometimes obscured or even lost. 
This histological differentiation is accompanied by a. physiological division 
of labor. Each tissue may be supposed to be composed of physiological units, 
the units of the same tissue being alike but differing from the units of other 
tissues ; and corresponding to this difference of structure, the units of different 
tissues behave or act differently. Instead of all the units, as in the amoeba, 
doing the same things equally well, the units of one tissue are told off, as it 
were, to do one thing especially well, or especially fully, and thus the whole 
labor of the body is divided among the several tissues. 
§ 7. The several tissues may thus be classified according to the work 
which they have to do; and the first great distinction is into (1) the tissues 
which are concerned in the setting free of energy in special ways, and (2) 
the tissues which are concerned in replenishing the substance and so renew- 
ing the energy of the body. 
Each physiological unit of the amoeba while it is engaged in setting free 
energy so as to move itself, and by reason of its sensitiveness so directing 
that energy as to produce a movement suitable to the conditions of its sur- 
roundings, has at the same time to bear the labor of taking in raw food, of 
selecting that part of the raw food which is useful and rejecting that which 22 
INTRODUCTION. 
is useless, and of working up the accepted part through a variety of stages 
into its own living substance-that is to say, it has at the same time that it 
is feeling and moving to carry on the work of digesting and assimilating. 
It has, moreover, at the same time to throw out the waste matters arising 
from the changes taking place in its own substance, having first brought 
these waste matters into a condition suitable for being thrown out. 
§ 8. In the body of man, movements, as we shall see, are, broadly speak- 
ing, carried out by means of muscular tissue, and the changes in muscular 
tissue which lead to the setting free of energy in the form of movement are 
directed, governed, and adapted to the surroundings of man, by means of 
nervous tissue. Rays of light fall on the nervous substance of the eye called 
the retina, and set up in the retina changes which induce in the optic nerve 
other changes, which in turn are propagated to the brain as nervous impulses, 
both the excitation and the propagation involving an expenditure of energy. 
These nervous impulses reaching the brain may induce other nervous im- 
pulses which, travelling down certain nerves to certain muscles, may lead to 
changes in those muscles by which they suddenly grow short and pull upon 
the bones or other structures to which they are attached, in which case we 
say the man starts ; or the nervous impulses reaching the brain may produce 
some other effects. Similarly sound falling on the ear, or contact between 
the skin and some foreign body, or some change in the air or other surround- 
ings of the body, or some change within the body itself may so affect the 
nervous tissue of the body that nervous impulses are started and travel to 
this point or that, to the brain or elsewhere, and eventually may either reach 
some muscular tissue and so give rise to movements, or may reach other 
tissues and produce some other effect. 
The muscular tissue then may be considered as given up to the produc- 
tion of movement, and the nervous tissue as given up to the generation, 
transformation, and propagation of nervous impulses. In each case there 
is an expenditure of energy, which in the case of the muscle, as we shall 
see, leaves the body partly as heat and partly as work done, but in the case 
of nervous tissue is wholly or almost wholly transformed into heat before it 
leaves the body ; and this expenditure necessitates a replenishment of energy 
and a renewal of substance. 
§ 9. In order that these master tissues, the nervous and muscular tissues, 
may carry on their important works to the best advantage, they are relieved 
of much of the labor that falls upon each physiological unit of the amoeba. 
They are not presented with raw food, they are not required to carry out the 
necessary transformations of their immediate waste matters. The whole of 
the rest of the body is engaged (1) in so preparing the raw food, and so 
bringing it to the nervous and muscular tissues that these may build it up 
into their own substance with the least trouble, and (2) in receiving the 
waste matters which arise in muscular and nervous tissues, and preparing 
them for rapid and easy ejection from the body. 
Thus to certain tissues, which we may speak of broadly as " tissues of 
digestion," is allotted the duty of acting on the food and preparing it for the 
use of the muscular and nervous tissues ; and to other tissues, which we may 
speak of as "tissues of excretion," is allotted the duty of clearing the body 
from the waste matters generated by the muscular and nervous tissues. 
§ 10. These tissues are for the most part arranged in machines or mech- 
anisms called organs, and the workings of these organs involves movement. 
The movements of these organs are carried out, like the other movements of 
the body, chiefly by means of muscular tissue governed by nervous tissue. 
Hence we may make a distinction between the muscles which are concerned 
in producing an effect on the world outside man's body, the muscles by which INTRODUCTION. 
23 
mail does his work in the world, and the muscles which are concerned in 
carrying out the movements of the internal organs. And we may similarly 
make a distinction between the nervous tissue concerned in carrying out the 
external work of the body and that concerned in regulating the movements 
and, as we shall see, the general conduct of the internal organs. But these 
two classes of muscular and nervous tissue, though distinct in work and, as 
we shall see, often different in structure, are not separated or isolated. On 
the contrary, while it is the main duty of the nervous tissue as a whole, the 
nervous system as we may call it, to carry out, by means of nervous impulses 
passing hither and thither, what may be spoken of as the work of man, and 
in this sense it is the master tissue, it also serves as a bond of union between 
itself and the muscles doing external work on the one hand, and the organs 
of digestion or excretion on the other, so that the activity and conduct of 
the latter mav be adequately adapted to the needs of the former. 
§11- Lastly, the food prepared and elaborated by the digestive organs 
is carried and presented to the muscular and nervous tissues in the form of 
a complex fluid known as blood, which, driven by means of a complicated 
mechanism known as the vascular system, circulates all over the body, visit- 
ing in turn all the tissues of the body, and by a special arrangement known 
as the respiratory mechanism, carrying in itself to the several tissues a sup- 
ply of oxygen as well as of food more properly so called. 
The motive power of this vascular system is supplied, as in the case of 
the digestive system, by means of muscular tissue, the activity of which is 
similarly governed by the nervous system, and hence the flow of blood to 
this part or that part is regulated according to the needs of the part. 
§ 12. The above slight sketch will perhaps suflice to show not only how 
numerous but how varied are the problems with which physiology has to 
deal. 
In the first place, there are what may be called general problems, such as, 
How the food after its preparation and elaboration into blood is built up 
into the living substance of the several tissues? How the living substance 
breaks down into the dead waste ? How the building up and breaking 
down differ in the different tissues in such a way that energy is set free in 
different modes, the muscular tissue contracting, the nervous tissue thrilling 
with a nervous impulse, the secreting tissue doing chemical work, and the 
like? To these general questions the answers which we can at present give 
can hardly be called answers at all. 
In the second place, there are what maybe called special problems, such 
as, What are the various steps by which the blood is kept replenished with 
food and oxygen, and kept free from an accumulation of waste, and how is 
the activity of the digestive, respiratory, and excretory organs, which effect 
this, regulated and adapted to the stress of circumstances? What are the 
details of the working of the vascular mechanism by which each and every 
tissue is forever bathed with fresh blood, and how is that working delicately 
adapted to all the various changes of the body? And, compared with which 
all other special problems are insignificant and preparatory only, How do 
nervous impulses so flit to and fro within the nervous system as to issue in 
the movements which make up what we sometimes call the life of man? It 
is to these special problems that we must chiefly confine our attention, and 
we may fitly begin with a study of the blood. BOOK I. 
BLOOD. THE TISSUES OF MOVEMENT. THE VASCULAR MECHANISM. 
CHAPTER I. 
BLOOD. 
§ 13. The several tissues are traversed by minute tubes, the capillary 
bloodvessels, to which blood is brought by the arteries, and from which 
blood is carried away by the veins. These capillaries form networks the 
meshes of which, differing in form and size in the different tissues, are occu- 
pied by the elements of the tissue, which consequently lie outside the capil- 
laries. 
The blood flowing through the capillaries consists, under normal condi- 
tions, of an almost colorless fluid, the plasma, in which are carried a num- 
ber of bodies, the red and the white corpuscles. Outside the capillary walls, 
filling up such spaces as exist between the capillary walls and the cells or 
fibres of the tissue, or between the elements of the tissue themselves, is found 
a colorless fluid resembling in many respects the plasma of blood, and called 
lymph. Thus all the elements of the tissue and the outsides of all the capil- 
laries are bathed with lymph, which, as we shall see hereafter, is continually 
flowing away from the tissue along special channels to pass into lymphatic 
vessels and thence into the blood. 
As the blood flows through the capillaries certain constituents of the 
plasma (together with, at times, white corpuscles, and under exceptional 
circumstances red corpuscles') pass through the capillary wall into the 
lymph, and certain constituents of the lymph pass through the capillary 
wall into the blood within the capillary. There is thus an interchange of 
material between the blood within the capillary and the lymph outside. A 
similar interchange of material is at the same time going on between the 
lymph and the tissue itself. Hence, by means of the lymph acting as mid- 
dleman, a double interchange of material takes place between the blood 
within the capillary and the tissue outside the capillary. In every tissue, 
so long as life lasts and the blood flows through the bloodvessels, a double 
stream, now rapid, now slow, is passing from the blood to the tissue and 
from the tissue to the blood. The stream from the blood to the tissue car- 
ries to the tissue the material which the tissue needs for building itself up 
and for doing its work, including the all-important oxygen. The stream 
from the tissue to the blood carries into the blood certain of the products of 
the chemical changes which have been taking place in the tissue, products 
which may be simple waste, to be cast out of the body as soon as possible, 
or which may be bodies capable of being made use of by some other tissue. 
A third stream, that from the lymph lying in the chinks and crannies 
of the tissue along the lymph channels to the larger lymph vessels, carries 
away from the tissue such parts of the material coming from the blood as 
25 26 
BLOOD. 
are not taken up by the tissue itself and such parts of the material coming 
from the tissue as do not find their way into the bloodvessel. 
In most tissues, as in muscle for instance, the capillary network is so 
close set and the muscular fibre lies so near to the bloodvessel that the 
lymph between the two exists only as a very thin sheet; but in some tis- 
sues, as in cartilage, the bloodvessels lie on the outside of a large mass of 
tissue, the interchange between the central parts of which and the nearest 
capillary bloodvessel is carried on through a long stretch of lymph passages. 
But in each case the principle is the same ; the tissue, by the help of lymph, 
lives on the blood; and when in succeeding pages we speak of changes 
between the blood and the tissues, it will be understood, whether expressly 
stated so or not, that the changes are effected by means of the lymph. The 
blood may thus be regarded as an internal medium bearing the same rela- 
tions to the constituent tissues that the external medium, the world, does to 
the whole individual. Just as the whole organism lives on the things 
around it, its air and its food, so the several tissues live on the complex 
fluid by which they are all bathed, and which is to them their immediate 
air and food. 
All the tissues take up oxygen from the blood and give up carbonic acid 
to the blood, but not always at the same rate or at the same time. More- 
over, the several tissues take up from the blood and give up to the blood 
either different things or the same things at different rates or at different 
times. 
From this it follows, on the one hand, that the composition and charac- 
ters of the blood must be forever varying in different parts of the body and 
at different times; and on the other hand, that the united action of all the 
tissues must tend to establish and maintain an average uniform composition 
of the whole mass of blood. The special changes which blood is known to 
undergo while it passes through the several tissues will best be dealt with 
when the individual tissues and organs come under our consideration. At 
present it will be sufficient to study the main features which are presented 
by blood brought, so to speak, into a state of equilibrium by the common 
action of all the tissues. 
Of all these main features of blood the most striking, if not the most 
important, is the property it possesses of clotting when shed. 
The Clotting of Blood. 
§ 14. Blood, when shed from the bloodvessels of a living body, is per- 
fectly fluid. In a short time it becomes viscid, this viscidity increasing rap- 
idly until the whole mass of blood under observation becomes a complete 
jelly. The vessel into which it has been shed can at this stage be inverted 
without a drop of the blood being spilt. The jelly is of the same bulk as 
the previously fluid blood, and if carefully shaken out will present a com- 
plete mould of the interior of the vessel. [Fig. 2.] If the blood in this 
jelly stage be left untouched in a glass vessel, a few drops of an almost 
colorless fluid soon make their appearance on the surface of the jelly. 
Increasing in number, and running together, the drops after a while form 
a superficial layer of pale, straw-colored fluid. Later on, similar layers 
of the same fluid are seen at the sides and finally at the bottom of the jelly, 
which, shrunk to a smaller size and of firmer consistency, now forms a clot 
or crassamentum, floating in a perfectly fluid serum. [Fig. 3.] The shrinking 
and condensation of the clot, and the corresponding increase of the serum, 
continue for some time. The upper surface of the clot is generally slightly 
concave. A portion of the clot examined under the microscope is seen to THE CLOTTING OF BLOOD. 
27 
consist of a feltwork of fine granular fibrils, in the meshes of which are 
entangled the red and white corpuscles of the blood. In the serum nothing 
[Fig. 2. 
[Fig. 3. 
Bowl of recently coagulated blood, 
showing the whole mass uniformly 
solidified. After Dalton.] 
Bowl of coagulated blood, after twelve 
hours, showingthe clot contracted and float- 
ing in the fluid serum. After Dalton.] 
can be seen but a few stray corpuscles, chiefly white. The fibrils are com- 
posed of a substance called fibrin. [Fig. 4.] Hence we may speak of the 
clot as consisting of fibrin and cor- 
puscles ; and the act of clotting is 
obviously a substitution for the 
plasma of fibrin and serum, fol- 
lowed by a separation of the fibrin 
and corpuscles from the serum. 
In man, blood when shed becomes 
viscid in about two or three minutes, 
and enters the jelly stage in about 
five or ten minutes. After the lapse 
of another few minutes the first 
drops of serum are seen, and clotting 
is generally complete in from one to 
several hours. The time, however, 
will be found to vary according to 
circumstances. Among animals the 
rapidity of clotting varies exceed- 
ingly in different species. The 
blood of the horse clots with re- 
markable slowness; so slowly, in- 
deed, that many of the red and also ■ -www-- ■ • 
some of the white corpuscles (both these being specifically heavier than 
the plasma) have time to sink before viscidity sets in. In consequence 
there appears on the surface of the blood an upper layer of colorless plasma 
containing in its deeper portions many colorless corpuscles (which are lighter 
than the red). This layer clots like the other parts of the blood, forming 
the so-called "buffy coat." A similar buffy coat is sometimes seen in the 
blood of man in certain abnormal conditions of the body. 
If a portion of horse's blood be surrounded by a cooling mixture of ice 
and salt, and thus kept about 0° C., clotting may be almost indefinitely 
postponed. Under these circumstances a more complete descent of the cor- 
puscles takes place, and a considerable quantity of colorless transparent 
plasma free from blood corpuscles may be obtained. A portion of this 
plasma removed from the freezing mixture clots in the same manner as does 
the entire blood. It first becomes vascid and then forms a jelly, which sub- 
sequently separates into a colorless shrunken clot and serum. This shows 
that the corpuscles are not an essential part of the clot. 
[Fig. 4. 
Coagulated fibrin, showing its fibrillated con- 
dition. After Dalton.] 28 
BLOOD 
If a few cubic centimetres of this colorless plasma, or of a similar plasma 
which may be obtained from almost any blood by means which we will 
presently describe, be diluted with many times its bulk of a 0.6 per cent, 
solution of sodium chloride1 clotting is much retarded, and the various stages 
may be more easily watched. As the fluid is becoming viscid, fine fibrils 
of fibrin will be seen to be developed in it, especially at the sides of the con- 
taining vessel. As these fibrils multiply in number, the fluid becomes more 
and more of the consistence of a jelly and at the same time somewhat opaque. 
Stirred or pulled about with a needle, the fibrils shrink up into a small 
opaque stringy mass; and a very considerable bulk of the jelly may by 
agitation be resolved into a minute fragment of shrunken fibrin floating in a 
quantity of what is really diluted serum. If a specimen of such diluted 
plasma be stirred from time to time, as soon as clotting begins, with a needle 
or glass rod, the fibrin may be removed piecemeal as it forms, and the jelly 
stage may be altogether done away with. When fresh blood which has not 
yet had time to clot is stirred or whipped with a bundle of rods (or anything 
presenting a large amount of rough surface), no jelly-like clotting takes 
place, but the rods become covered with a mass of shrunken fibrin. Blood 
thus whipped until fibrin ceases to be deposited, is found to have entirely 
lost its power of clotting. 
Putting these facts together, it is very clear that the phenomena of the 
clotting of blood are caused by the appearance in the plasma of fine fibrils 
of fibrin. So long as these are scanty, the blood is simply viscid. When 
they become sufficiently numerous, they give the blood the firmness of a 
jelly. Soon after their formation they begin to shrink, and while shrinking 
enclose in their meshes the corpuscles, but squeeze out the fluid parts of 
the blood. Hence the appearance of the shrunken colored clot and the 
colorless serum. 
$ 15. Fibrin, whether obtained by whipping freshly shed blood, or by 
washing either a normal clot, or a clot obtained from colorless plasma, 
exhibits the same general characters. It belongs to that class of complex 
unstable nitrogenous bodies called proteids, which form a large portion of 
all living bodies and an essential part of all living structures. 
Our knowledge of proteids is at present too imperfect, and probably none 
of them have yet been prepared in adequate purity to justify us in attempt- 
ing to assign to them any definite formula ; but it is important to remember 
their general composition. 100 parts of a proteid contain rather more than 
50 parts of carbon, rather more than 15 of nitrogen, about 7 of hydrogen, 
and rather more than 20 of oxygen ; that is to say, they contain about half 
their weight of carbon, and only about I their weight of nitrogen ; and yet, 
as we shall see, they are eminently the nitrogenous substances of the body. 
They usually contain a small quantity (1 or 2 per cent.) of sulphur, and 
many also have some phosphorus attached to them in some way or other. 
When burnt they leave a variable quantity of ash, consisting of inorganic 
salts of which the bases are chiefly sodium and potassium, and the acids 
chiefly hydrochloric, sulphuric, phosphoric, and carbonic. 
They all give certain reactions, by which their presence may be recog- 
nized; of these the most characteristic are the following: Boiled with nitric 
acid they give a yellow color, which deepens into orange upon the addition of 
ammonia. This is called the xanthoproteic test; the color is due to a product 
of decomposition. Boiled with the mixture of mercuric and mercurous 
nitrates known as Millon's reagent, they give a pink color. Mixed with a 
strong solution of sodic hydrate they give, on the addition of a drop or two of 
1 A solution of sodium chloride of this strength will hereafter be spoken of as " normal 
saline solution." THE CLOTTING OF BLOOD. 
29 
a very weak solution of cupric sulphate, a violet or pink color which deepens 
on heating. These are artificial reactions, not throwing much if any light 
on the constitution of proteids ; but they are useful as practical tests enabling 
us to detect their presence. 
The several members of the proteid group are at present distinguished 
from each other chiefly by their respective solubilities, especially in various 
saline solutions. Fibrin is one of the least soluble; it is insoluble in water, 
almost insoluble in dilute neutral saline solutions, and very sparingly soluble 
in more concentrated neutral saline solutions and in dilute acids and alkalies. 
In strong acids and alkalies it dissolves, but in the process becomes com- 
pletely changed into something which is no longer fibrin. In dilute acids it 
swells up and becomes transparent, but when the acid is neutralized returns 
to its previous condition. When suspended in water and heated to 100° C., 
or even to 75° C., it becomes changed, and still less soluble than before ; it is 
said in this case to be coagulated by the heat, and, as we shall see, nearly all 
proteids have the property of being changed in nature, of undergoing 
coagulation and so becoming less soluble than before, by being exposed to a 
certain high temperature. 
Fibrin, then, is a proteid distinguished from other proteids by its smaller 
solubility; it is further distinguished by its peculiar filamentous structure, 
the other proteids when obtained in a solid form appearing either in amor- 
phous granules or, at most, in viscid masses. 
$ 16. We may now return to the serum. 
This is perfectly fluid, and remains fluid until it decomposes. It is of a 
faint straw-color, due to the presence of a special pigment substance, differing 
from the red matter which gives redness to the red corpuscles. 
Tested by the xanthoproteic and other tests it obviously contains a large 
quantity of proteid matter, and upon examination we find that at least two 
distinct proteid substances are present in it. 
If crystals of magnesium sulphate be added to serum and gently stirred 
until they dissolve, it will be seen that the serum as it approaches saturation 
with the salt becomes turbid instead of remaining clear, and eventually a 
white amorphous granular or flocculent precipitate makes its appearance. 
This precipitate may be separated by decantation or filtration, washed with 
saturated solutions of magnesium sulphate, in which it is insoluble, until it 
is freed from all other constituents of the serum, and thus obtained fairly 
pure. It is then found to be a proteid body, distinguished by the following 
characters among others: 
1. It is (when freed from any adherent magnesium sulphate) insoluble in 
distilled water; it is insoluble in concentrated solutions of neutral saline 
bodies, such as magnesium sulphate, sodium chloride, etc., but readily soluble 
in dilute (e. g., 1 per cent.) solutions of the same neutral saline bodies. 
Hence from its solutions in the latter it maybe precipitated either by adding 
more neutral saline substance or by removing by dialysis the small quantity 
of saline substance present. When obtained in a precipitated form, and 
suspended in distilled water, it readily dissolves into a clear solution upon 
the addition of a small quantity of some neutral saline body. By these 
various solutions and precipitations it is not really changed in nature. 
2. It readily dissolves in very dilute acids (e. g., in hydrochloric acid even 
when diluted to far less than 1 per cent.), and it is similarly soluble in dilute 
alkalies, but in being thus dissolved it is wholly changed in nature, and the 
solutions of it in dilute acid and dilute alkalies gives reactions quite different 
from those of the solution of the substance in dilute neutral saline solutions. 
By the acid it is converted into what is called acid-albumin, by the alkali 
into alkali-albumin, both of which bodies we shall have to study later on. 30 
BLOOD. 
3. When it is suspended in water and heated it becomes altered in char- 
acter, coagulated, and all its reactions are changed. It is no longer soluble 
in dilute neutral saline solutions, noteven in dilute acids and alkalies; it 
has become coagulated proteid, and is now even less soluble than fresh fibrin. 
When a solution of it in dilute neutral saline solution is similarly heated, a 
similar change takes place, a precipitate falls down which on examination 
is found to be coagulated proteid. The temperature at which this change 
takes place is somewhere about 75° C., though shifting slightly according to 
the quantity of saline substance present in the solution. 
One of the proteids present in blood-serum is paraglobulin, characterized 
by its solubility in dilute neutral saline solutions, its insolubility in distilled 
water and concentrated saline solutions, its ready solubility, and at the same 
time conversion into other bodies, in dilute acids and alkalies, and in its 
becoming converted into coagulated proteid, and so being precipitated from 
its solutions at 75° C. 
These reactions are given by a number of proteid bodies forming a group 
called globulins, the particular globulin present in blood-serum being para- 
globulin. 
The amount of it present in blood-serum varies in various animals, and 
apparently in the same animal at different times. In 100 parts by weight of 
serum there are generally present about 8 or 9 parts of proteids altogether, 
and of these some 3 or 4, more or less, may be taken as paraglobulin. 
§ 17. If the serum from which the paraglobulin has been precipitated by 
the addition of neutral salt, and removed by filtration, be subjected to di- 
alysis, the salt added may be removed, and a clear, somewhat diluted serum 
free from paraglobulin may be obtained. 
This still gives abundant proteid reactions, so that the serum still contains 
a proteid, or some proteids still more soluble than the globulins, since they 
will remain in solution, and are not precipitated, even when dialysis is con- 
tinued until the serum is practically freed from both the neutral salt added 
to it and the diffusible salts previously present in the natural serum. 
When this serum is heated to 75° C. a precipitate makes its appearance ; 
the proteids still present are coagulated at this temperature. 
We have some reasons for thinking that more than one proteid is present, 
but they are all closely allied to each other, and we may for the present 
speak of them as if they were one, and call the proteid left in serum, after 
removal of the paraglobulin, by the name of albumin, or, to distinguish it 
from other albumins found elsewhere, serum-albumin. Serum-albumin is 
distinguished by being more soluble than the globulins, since it is soluble in 
distilled water, even in the absence of all neutral salts. Like the glubulins, 
though with much less ease, it is converted by dilute acids and dilute alkalies 
into acid- or into alkali-albumin. The percentage amount of serum-albumin 
in serum may be put down as 4 or 5, more or less, but it varies and some- 
times is less abundant than paraglobulin. In some animals (snakes) it is 
said to disappear during starvation. 
The more important characters of the three proteids which we have just 
studied may be stated as follows: 
Soluble in distilled water and in saline solutions of all strengths . . serum-albumin. 
Insoluble in distilled water, readily soluble in dilute saline solutions, 
insoluble in concentrated saline solutions paraglobulin. 
Insoluble in distilled water, hardly soluble at all in dilute saline 
solutions, and very little soluble in more concentrated saline solu- 
tions fibrin. 
Besides paraglobulin and serum-albumin, serum contains a very large 
number of substances, generally in small quantity, which, since they have 31 
THE CLOTTING OF BLOOD. 
to be extracted by special methods, are called extractives; of these some are 
nitrogenous, some non-nitrogenous. Serum contains besides important inor- 
ganic saline substances; but to these we shall return. 
§ 18. With the knowledge which we have gained of the proteids of clotted 
blood we may go back to the question : Clotting being due to the appearance 
in blood plasma of a proteid substance, fibrin, which previously did not exist 
in it as such, what are the causes which led to the appearance of fibrin ? 
We learn something by studying circumstances which affect the rapidity 
with which the blood of the same individual clots when shed. These are as 
follows: 
A temperature of 40° C., which is about or slightly above the tempera- 
ture of the blood of warm-blooded animals, is perhaps the most favorable to 
clotting. A further rise of a few degrees is apparently also beneficial, or at 
least not injurious; but upon a still further rise the effect changes, and when 
blood is rapidly heated to 56° C. no clotting at all may take place. At this 
temperature certain proteids of the blood are coagulated and precipitated 
before clotting can take place, and with this change the power of the blood 
to clot is wholly lost. If, however, the heating be not very rapid, the blood 
may clot before this change has time to come on. When the temperature 
instead of being raised is lowered below 40° C. the clotting becomes delayed 
and prolonged ; and at the temperature of 0° or 1° C. the blood will remain 
fluid, and yet capable of clotting when withdrawn from the adverse circum- 
stances, for a very long, it might almost be said, for an indefinite time. 
A small quantity of blood shed into a small vessel clots sooner than a 
large quantity shed into a larger one; and in general the greater the amount 
of foreign surface with which the blood comes in contact the more rapid the 
clotting. When shed blood is stirred or " whipped " the fibrin makes its 
appearance sooner than when the blood is left to clot in the ordinary way; 
so that here, too, the accelerating influence of contact with foreign bodies 
makes itself felt. Similarly, movement of shed blood hastens clotting, since 
it increases the amount of contact with foreign bodies. So also the addition 
of spongy platinum or of powdered charcoal, or of other inert powders, to 
tardily clotting blood will, by influence of surface, hasten clotting. Con- 
versely, blood brought into contact with pure oil does not clot so rapidly as 
when in contact with glass or metal; and blood will continue to flow for a 
longer time without clotting through a tube smeared inside with oil than 
through a tube not so smeared. The influence of the. oil in such cases is a 
physical not a chemical one; any pure neutral inert oil will do. As far as 
we know these influences affect only the rapidity with which the clotting 
takes place-that is, the rapidity with which the fibrin makes its appear- 
ance, not the amount of clot, not the quantity of fibrin formed, though 
when clotting is very much retarded by cold changes may ensue whereby 
the amount of clotting which eventually takes place is indirectly affected. 
Mere exposure to air exerts apparently little influence on the process of 
clotting. Blood collected direct from a bloodvessel over mercury so as 
wholly to exclude the air, clots as readily as blood freely exposed to the air. 
It is only when blood is much laden with carbonic acid, the presence of 
which is antagonistic to clotting, that exclusion of air, by hindering the 
escape of the excess of carbonic acid, delays clotting. 
These facts teach us that fibrin does not, as was once thought, make its 
appearance in shed blood because the blood when shed ceases to share in the 
movement of the circulation, or because the blood is cooled on leaving the 
warm body, or because the blood is then more freely exposed to the air; they 
further suggest the view that the fibrin is the result of some chemical change, 
the conversion into fibrin of something which is not fibrin, the change like 32 
BLOOD. 
other chemical changes being most active at an optimum temperature, and 
like so many other chemical changes, being assisted by the influences exerted 
by the presence of inert bodies. 
And we have direct experimental evidence that plasma does contain an 
antecedent of fibrin which, by chemical change, is converted into fibrin. 
§ 19. If blood be received direct from the bloodvessels into one-third its 
bulk of a saturated solution of some neutral salt such as magnesium sulphate, 
and the two gently but thoroughly mixed, clotting, especially at a moder- 
ately low temperature, will be deferred for a very long time. If the mixture 
be allowed to stand, the corpuscles will sink, and a colorless plasma will be 
obtained similar to the plasma gained from horse's blood by cold, except that 
it contains an excess of the neutral salt. The presence of the neutral salt 
has acted in the same direction as cold; it has prevented the occurrence of 
clotting. It has not destroyed the fibrin ; for if some of the plasma be diluted 
with from five to ten times its bulk of water, it will clot speedily in quite a 
normal fashion, with the production of quite normal fibrin. 
The separation of the fluid plasma from the corpuscles and from other bodies 
heavier than the plasma is much facilitated by the use of the centrifugal machine. 
This consists essentially of a tireless wheel with several spokes, placed in a hori- 
zontal position and made to revolve with great velocity (1000 revolutions per min- 
ute for instance) around its axis. Tubes of metal or of very strong glass are sus- 
pended at the ends of the spokes by carefully adjusted joints. As the wheel 
rotates with increasing velocity, each tube gradually assumes a horizontal posi- 
tion, bottom outward, without spilling any of its contents. As the rapid rotation 
continues the corpuscles and heavier particles are driven to the bottom of the tube, 
and if a very rapid movement be continued for a long time will form a compact 
cake at the bottom of the tube. When the rotation is stopped the tubes gradually 
return to their upright position again without anything being spilt, and the clear 
plasma in each tube can then be decanted off. 
If some of the colorless transparent plasma, obtained either by the action 
of neutral salts from any blood, or by the help of cold from horse's blood, be 
treated with some solid neutral salt, such as sodium chloride, to saturation, 
a white flaky, somewhat sticky precipitate will make its appearance. If this 
precipitate be removed, the fluid no longer possesses the power of clotting 
(or very slightly so), even though the neutral salt present be removed by 
dialysis, or its influence lessened by dilution. With the removal of the sub- 
stance precipitated, the plasma has lost its power of clotting. 
If the precipitate itself, after being washed with a saturated solution of 
the neutral salt (in which it is insoluble) so as to get rid of all serum and 
other constituents of the plasma, be treated with a small quantity of water, 
it readily dissolves,1 and the solution rapidly filtered gives a clear, colorless 
filtrate, which is at first perfectly fluid. Soon, however, the fluidity gives 
way to viscidity, and this in turn to a jelly condition, and finally the jelly 
shrinks into a clot floating in a clear field ; in other words, the filtrate clots 
like plasma. Thus there is present in cooled plasma, and in plasma kept 
from clotting by the presence of neutral salts, a something precipitable by 
saturation with neutral salts-a something which, since it is soluble in very 
dilute saline solutions, cannot be fibrin itself, but which in solution speedily 
gives rise to the appearance of fibrin. To this substance its discoverer, 
Denis, gave the name of plasmine. 
The substance thus precipitated is not however a single body, but a mix- 
turo of at least two bodies. If sodium chloride be carefully added to plasma 
to an extent of about 13 per cent, a white flaky viscid precipitate is thrown 
1 The substance itself is not soluble in distilled water, but a quantity of the neutral 
salt always clings to the precipitate, and thus the addition of water virtually gives rise 
to dilute saline solution, in which the substance is readily soluble. THE CLOTTING OF THE BLOOD. 
33 
down very much like plasmine. If after the removal of the first precipitate 
more sodium chloride, and especially if magnesium sulphate be added, a 
second precipitate is thrown down, less viscid and more granular than the 
first. 
The second precipitate when examined is found to be identical with the 
paraglobulin, coagulating at 75° C., which we have already seen to be a 
constituent of serum. 
The first precipitate is also a proteid belonging to the globulin group, 
but differs from paraglobulin, not only in being more readily precipitated 
by sodium chloride, and in being when precipitated more viscid, but also in 
other respects, and especially in being coagulated at a far lower temperature 
than paraglobulin, viz., at 56° C. Now, while isolated paraglobulin cannot 
by any means known to us be converted into fibrin, and as its presence in 
the so-called plasmine does not seem to be essential to the formation of 
fibrin out of plasmine, the presence in plasmine of the body coagulating at 
56° C. does seem essential to the conversion of plasmine into fibrin, ami we 
have reason for thinking that it is itself converted, in part at least, into 
fibrin. Hence it has received the name of fibrinogen. 
$ 20. The reasons for this view areas follows : 
Besides blood, which clots naturally when shed, there are certain fluids 
in the body which do not clot naturally, either in the body or when shed, 
but which by certain artificial means may be made to clot, and in clotting 
to yield quite normal fibrin. 
Thus the so-called serous fluid taken some hours after death1 from the 
pericardial, pleural, or peritoneal cavities, the fluid found in the enlarged 
serous sac of the testis, known as hydrocele fluid, and other similar fluids, 
will in the majority of cases, when obtained free from blood or other admix- 
tures, remain fluid almost indefinitely, showing no disposition whatever to 
clot.2 Yet, in most cases at all events, these fluids, when a little blood, or a 
piece of blood clot, or a little serum is added to them, will clot rapidly and 
firmly,3 giving rise to an unmistakable clot of normal fibrin, differing only 
from the clot of blood in that, when serum is used, it is colorless, being free 
from red corpuscles. 
Now blood (or blood clot, or serum) contains many things, to any one of 
which the clotting power thus seen might be attributed. But it is found 
that in many cases clotting may be induced in the fluids of which we are 
speaking by the mere addition, and that even in exceedingly small quantity, 
of a substance which can be extracted from blood, or from serum, or from 
blood clot, or even from washed fibrin, or indeed from other sources, a sub- 
stance whose exact nature is uncertain, it being doubtful whether it is a 
proteid at all, and whose action is peculiar. 
If serum, or whipped blood or a broken-up clot be mixed with a large 
quantity of alcohol and allowed to stand some days, the proteids present are 
in time so changed by the alcohol as to become insoluble in water. Hence 
if the copious precipitate caused by the alcohol, after longstanding, be sepa- 
rated by filtration from the alcohol, dried at a low temperature, not exceed- 
ing 40° C., and extracted with distilled water, the aqueous extract contains 
very little proteid matter, indeed very little organic matter at all. Never- 
theless, even a small quantity of this aqueous extract added alone to certain 
specimens of hydrocele fluid or other of the fluids spoken of above, will bring 
about a speedy clotting. The same aqueous extract has also a remarkable 
1 If it be removed immediately after death it generally clots readily and firmly, giving 
a colorless clot consisting of fibrin and white corpuscles. 
2 In some specimens, however, a spontaneous coagulation, generally slight, but in ei- 
ceptional cases massive, may be observed. 
3 In a few cases no coagulation can thus be induced. 34 
BLOOD. 
effect in hastening the clotting of fluids which, though they will eventually 
clot, do so very slowly. Thus plasma may, by the careful addition of a cer- 
tain quantity of neutral salt and water, be reduced to such a condition that 
it clots very slowly indeed, taking perhaps days to complete the process. The 
addition of a small quantity of the aqueous extract we are describing will, 
however, bring about a clotting which is at once rapid and complete. 
The active substance, whatever it be, in this aqueous extract exists in 
small quantity only, and its clotting virtues are at once and forever lost 
when the solution is boiled. Further, there is no reason to think that the 
active substance actually enters into the formation of the fibrin to which it 
gives rise. It appears to belong to a class of bodies playing an important 
part in physiological processes, and called ferments, of which we shall have 
more to say hereafter. We may, therefore, speak of it as the fibrin ferment, 
the name given to it by its discoverer, Alexander Schmidt. 
This fibrin ferment is present in and may be extracted from clotted or 
whipped blood, and from both the clot1 and the serum of clotted blood ; and 
since in most, if not all, cases where blood or blood clot or serum produces 
clotting in hydrocele or pericardial fluid, an exactly similar clotting may be 
induced by the mere addition of fibrin ferment, we seem justified in con- 
cluding that the clotting virtues of the former are due to the ferment which 
they contain. 
Now, when fibrinogen is precipitated from plasma, as above described, by 
sodium chloride, redissolved, and reprecipitated, more than once, it may be 
obtained in solution, by help of a dilute neutral saline solution, in an ap- 
proximately pure condition, at all events free from other proteids. Such a 
solution will not clot spontaneously; it may remain fluid indefinitely ; and 
yet on the addition of a little fibrin ferment it will clot readily and firmly, 
yielding quite normal fibrin. 
This body fibrinogen is also present and may be separated out from the 
specimens of hydrocele, pericardial, and other fluids which clot on the 
addition of fibrin ferment, and when the fibrinogen has been wholly removed 
from these fluids they refuse to clot on the addition of fibrin ferment. 
Paraglobulin, on the other hand, whether prepared from plasmine by 
separation of the fibrinogen, or from serum, or from other fluids in which it 
is found, cannot be converted by fibrin ferment, or indeed by any other 
means, into fibrin. And fibrinogen isolated, as described above, or serous 
fluids which contain fibrinogen, can be made, by means of fibrin ferment, to 
yield quite normal fibrin in the complete absence of paraglobulin. A solu- 
tion of paraglobulin obtained from serum or blood clot will, it is true, clot 
pericardial or hydrocele fluids containing fibrinogen, or indeed a solution of 
fibrinogen, but this is apparently due to the fact that the paraglobulin has 
in these cases some fibrin ferment mixed with it; it is also possible that, 
under certain conditions, the presence of paraglobulin may be favorable to 
the action of the ferment. 
When the so-called plasmine is precipitated, as directed in § 19, fibrin 
ferment is carried down with the fibrinogen and paraglobulin, and when the 
plasmine is re-dissolved the ferment is present in the solution and ready to 
act on the fibrinogen. Hence the re-dissolved plasmine clots spontaneously. 
When fibrinogen is isolated from plasma by repeated precipitation and 
solution, the ferment is washed away from it, and the pure ferment-free 
fibrinogen, ultimately obtained, does not clot spontaneously. 
So far it seems clear that there does exist a proteid body, fibrinogen, 
which may by the action of fibrin ferment be directly, without the interven- 
1 A powerful solution of fibrin ferment may be readily prepared by simply extracting 
a washed blood clot with a 10 per cent, solution of sodium chloride. THE CLOTTING OF BLOOD. 
35 
tion of other proteids, converted into the less soluble fibrin. Our knowledge 
of the constitution of proteid bodies is too imperfect to enable us to make 
any very definite statement as to the exact nature of the change thus 
effected ; but we may say this much. Fibrinogen and fibrin have about the 
same elementary composition, fibrin containing a trifle more nitrogen. 
When fibrinogen is converted into fibrin by means of fibrin ferment, the 
weight of the fibrin produced is always less than that of the fibrinogen 
which is consumed, and there is always produced at the same time a certain 
quantity of another proteid, belonging to the globulin family. There are 
reasons, however, why we cannot speak of the ferment as splitting up 
fibrinogen into fibrin and a globulin ; it seems more probable that the fer- 
ment converts the fibrinogen first into a body which we might call soluble 
fibrin, and then turns this body into a veritable fibrin ; but further inquiries 
on the subject are needed. 
It may be added that among the conditions necessary for the due action 
of fibrin ferment on fibrinogen, the presence of a certain quantity of some 
neutral salt seems to be one. In the total absence of all neutral salts the 
ferment cannot convert the fibrinogen into fibrin. There are some reasons 
also for thinking that the presence of a lime salt, such as calcium sulphate, 
though it may be in minute quantity only, is essential. If the calcium 
salts are taken out of fibrinogen, the fibrinogen no longer coagulates upon 
the addition of fibrin ferment free from lime salts. Oxalate of potassium 
added to freshly drawn blood also prevents coagulation, apparently because 
of the precipitation of the lime salts; the addition now of calcium salts is 
promptly followed by coagulation. 
§ 21. We may conclude, then, that the plasma of blood when shed, or, 
at all events, soon after it has been shed, contains fibrinogen ; and it also 
seems probable that the clotting conies about because the fibrinogen is con- 
verted into fibrin by the action of fibrin ferment; but we are still far from 
a definite answer to the question, why blood remains fluid in the body and 
yet clots when shed ? 
We have already said that blood, or blood plasma, brought up to a tem- 
perature of 56° C. as soon as possible after its removal from the living 
bloodvessels, gives a proteid precipitate and loses its power of clotting. 
This may be taken to show that blood, as it circulates in the living blood- 
vessels, contains fibrinogen as such, and that when the blood is heated to 
56° C., which is the coagulating point of fibrinogen, the fibrinogen present 
is coagulated and precipitated, and consequently no fibrin can be formed. 
Further, while clotted blood undoubtedly contains an abundance of 
fibrin ferment, no ferment, or a minimal quantity only, is present in blood 
as it leaves the bloodvessels. If the blood be received directly from the 
bloodvessels into alcohol, the aqueous extract prepared as directed above 
contains no ferment, or merely a trace. Apparently the ferment makes its 
appearance in the blood as the result of changes taking place in the blood 
after it has been shed. 
We might from this be inclined to conclude that blood clots when shed, 
but not before, because, fibrinogen being always present, the shedding brings 
about changes which produce fibrin ferment, not previously existing, and 
this acting on the fibrinogen gives rise to fibrin. But we meet with the fol- 
lowing difficulty: A very considerable quantity of very active ferment may 
be injected into the blood-current of a living animal without necessarily 
producing any clotting at all. Obviously either blood within the blood- 
vessels does not contain fibrinogen as such, and the fibrinogen detected by 
heating the blood to 56° C. is the result of changes which have already 
ensued before the temperature is reached ; or in the living circulation there 36 
BLOOD. 
are agencies at work which prevent any ferment which may be introduced 
into the circulation from producing its usual effect on fibrinogen ; or there 
are agencies at work which destroy, or do away with the fibrin, little by 
little, as it is formed. 
§ 22. And indeed, when we reflect how complex blood is and the many 
and great changes of which it is susceptible, we shall not wonder that the 
question we are putting cannot be offered off* hand. 
The corpuscles with which blood is crowded are living structures, and 
consequently are continually acting upon and being acted upon by the 
plasma. The red corpuscles it is true are, as we shall see, peculiar bodies, 
with a restricted life and a very specialized work, and possibly their influ- 
ence on the plasma is not very great; but we have reason to think that the 
relations between the white corpuscles and the plasma are close and im- 
portant. 
Then again the blood is not only acting upon and being acted upon by 
the several tissues as it flows through the various capillaries, but along 
the whole of its course through the heart, arteries, capillaries, and veins, is 
acting upon and being acted upon by the vascular walls, which like the rest 
of the body are alive, and being alive are continually undergoing and pro- 
moting change. 
That relations of some kind, having a direct influence on the clotting of 
blood, do exist between the blood and the vascular walls is shown by the 
following facts: 
After death, when all motion of the blood has ceased, the blood remains 
for a long time fluid. It is not until some time afterward, at an epoch 
when post-mortem changes in the blood and in the bloodvessels have had 
time to develop themselves, that clotting begins. Thus some hours after 
death the blood in the great veins may be found still perfectly fluid. Yet 
such blood has not lost its power of clotting; it still clots when removed 
from the body, and chits too when received over mercury without exposure 
to air, showing that, though the blood, being highly venous, is rich in car- 
bonic acid and contains little or no oxygen, its fluidity is not due to any 
excess of carbonic acid or absence of oxygen. Eventually it does clot even 
within the vessels, but perhaps never so firmly and completely as when 
shed. It clots first in the larger vessels, but remains fluid in the smaller 
vessels for a very long time, for many hours in fact, since in these the same 
bulk of blood is exposed to the influence of, and reciprocally exerts an 
influence on, a larger surface of the vascular walls than in the larger ves- 
sels. And if it be urged that the result is here due to influences exerted 
by the body at large, by the tissues as well as by the vascular walls, this 
objection will not hold good against the following experiment. 
If the jugular vein of a large animal, such as an ox or horse, be carefully 
ligatured when full of blood, and the ligatured portion excised, the blood 
in many cases remains perfectly fluid, along the greater part of the length 
of the piece, for twenty-four or even forty-eight hours. The piece so liga- 
tured may be suspended in a framework and opened at the top so as to imi- 
tate a living test-tube, and yet the blood will often remain long fluid, though 
a portion removed at any time into a glass or other vessel will clot in a few 
minutes. If two such living test-tubes be prepared, the blood may be 
poured from one to the other without clotting taking place. 
A similar relation of the fluid to its containing living wall is seen in the 
case of those serous fluids which clot spontaneously. If, as soon after death 
as the body is cold and the fat is solidified, the pericardium be carefully re- 
moved from a sheep by an incision round the base of the heart, the peri- 
cardial fluid (which, as we have already seen, during life and some little THE CLOTTING OF BLOOD. 
37 
time after death, possesses the power of clotting) may be kept in the peri- 
cardial bag as in a living cup for many hours without clotting, and yet a 
small portion removed with a pipette clots at once. 
This relation between the blood and the vascular wall may be disturbed 
or overridden : clotting may take place or maybe induced within the living 
bloodvessel. When the living membrane is injured, as when an artery or 
vein is sharply ligatured, or when it is diseased, as for instance in aneurism, 
a clot is apt to be formed at the injured or diseased spot; and in certain 
morbid conditions of the body clots are formed in various vascular tracts. 
Absence of motion, which in shed blood as we have seen is unfavorable to 
clotting, is apt within the body to lead to clotting. Thus, when an artery 
is ligatured, the blood in the tract of the artery on the cardiac side of the 
ligature, between the ligature and the branch last given off* by the artery, 
ceasing to share in the circulation, remains motionless or nearly so, and 
along this tract a clot forms, firmest next to the ligature and ending near 
where the branch is given off; this perhaps may be explained by the fact 
that the walls of the tract suffer in their nutrition by the stagnation of the 
blood, and that consequently the normal relation between them and the 
contained blood is disturbed. 
That the blood within the living bloodvessels, though not actually clot- 
ting under normal circumstances, may easily be made to clot, that the blood 
is always on the point of clotting, is shown by the fact that a foreign body 
such as a needle thrust into the interior of a bloodvessel or a thread drawn 
through and left in a bloodvessel, is apt to become covered with fibrin. 
Some influence exerted by the needle or thread, whatever may be the cha- 
racter of that influence, is sufficient to determine a clotting, which other- 
wise would not have taken place. 
The same instability of the blood, as regards clotting, is strikingly shown, 
in the case of the rabbit, by the result of injecting into the bloodvessels a 
small quantity of a solution of a peculiar proteid, prepared from certain 
structures such as the thymus body. Massive clotting of the blood in 
almost all the bloodvessels, small and large, takes place with great rapid- 
ity, leading to the sudden death of the animal. In contrast to this effect 
may be mentioned the result of injecting into the bloodvessels of a dog a 
quantity of a solution of a body called albumose, of which we shall here- 
after have to treat as a product of the digestion of proteid substances, to 
the extent of 0.8 gramme per kilo of body weight. So far from producing 
clotting, the injected albumose has such an effect on the blood that for several 
hours after the injection shed blood will refuse to clot of itself and remain 
quite fluid, though it can be made to clot by special treatment. 
§ 23. All the foregoing facts tend to show that the blood as it is flow- 
ing through the healthy bloodvessels is, as far as clotting is concerned, in a 
state of unstable equilibrium, which may at any moment be upset, even 
within the bloodvessels, and which is upset directly the blood is shed, 
with clotting as a result. Our present knowledge does not permit us to 
make au authoritative statement as to the exact nature of this equilib- 
rium. There are reasons, however, for thinking that the white corpuscles 
play an important part in the matter. Wherever clotting occurs natu- 
rally, white corpuscles are present; and this is true not only of blood but 
also of such specimens of pericardial or other serous fluids as clot natu- 
rally. And many arguments which we cannot enter upon here, may be 
adduced all pointing to the same conclusion, that the white corpuscles play 
an important part in the process of clotting. But it would lead us too far 
into controversial matters to attempt to define what that part is, or to explain 
the exact nature of the equilibrium of which we have spoken. 38 
BLOOD. 
What we do know is that in blood soon after it has been shed the body 
which we have called fibrinogen is present, as also the body which we have 
called fibrin ferment, that the latter acting on the former will produce 
fibrin, and that the appearance of fibrin is undoubtedly the cause of what 
is called clotting. We seem justified in concluding that the clotting of shed 
blood is due to the conversion by ferment of fibrinogen into fibrin. The 
further inference that clotting within the body is the same thing as clotting 
outside the body, and similarly due to the transformation of fibrinogen by 
ferment into fibrin, though probable, is not proved. We do not yet know 
the exact nature and condition of the blood within the living bloodvessels, 
and until we know that we cannot satisfactorily explain why blood in the 
living bloodvessels is usually fluid but can at times clot. 
The Corpuscles of the Blood. 
§ 24. The redness of blood is due exclusively to the red corpuscles. 
The plasma as seen in thin layers within the living bloodvessels appears 
colorless, as does also a thin layer of serum ; but a thick layer of serum 
(and probably of plasma) has a faint yellowish tinge due, as we have 
said, to the presence of a small quantity of a special pigment. 
The corpuscles appear under the microscope as fairly homogeneous, im- 
perfectly translucent biconcave discs with a diameter of 7 to 8 p and a 
The Red Corpuscles. 
[Fig. 5. 
Fig. 5.-Human Blood as seen on the Wann Stage (magnified about 1200 diameters): r, r, 
single red corpuscles seen lying flat; r', r', red corpuscles on their edge and viewed in profile; r", 
red corpuscles arranged in rouleaux ; c, c, crenate red corpuscles; p, a finely granular pale cor- 
puscle; g, a coarsely granular pale corpuscle. Both have two or three distinct vacuoles, and 
were undergoing changes of shape at the moment of observation ; in g a nucleus also was visible 
Fig. 6.-Human Red Corpuscles Lying Singly and Collected into Rolls. (As seen under an 
ordinary high power of the microscope.)] 
thickness of 1 to 2 p. Being discs they are circular in outline when seen 
on the Hat, but rod-shaped when seen in profile. [Fig. 5.] Being bicon- 
cave, with a thicker rounded rim surrounding a thinner centre, the rays of THE CORPUSCLES OF THE BLOOD. 
39 
light in passing through them, when they are examined by transmitted light, 
are more refracted at the rim than in the centre. The effect of this is that, 
when viewed at what may be considered the proper focus, the centre of a 
corpuscle appears clear, while a slight opacity marks out indistinctly the 
inner margin of the thicker rim, whereas, when the focus is shifted either 
up or down, the centre becomes dark and the rest of the corpuscle clear. 
Any body of the same shape, and composed of substances of the same 
refractive power, would produce the same optical effects. Otherwise the 
corpuscle appears homogeneous, without distinction of parts and without 
a nucleus. A single corpuscle seen by itself has a very faint color, look- 
ing yellow rather than red, but when several corpuscles lie one upon the 
top of the other the mass is distinctly red. 
The red corpuscle is elastic, in the sense that it may be deformed by 
pressure or traction, but when the pressure or traction is removed regains its 
previous form. Its shape is also much influenced by the physical conditions 
of the plasma, serum, or fluid in which for the time being it is. If the 
plasma or serum be diluted with water, the disc, absorbing water, swells up 
into a sphere [Fig. 7], becoming a disc again on the removal of the dilution. 
If the serum be concentrated, the disc, giving 
out water, shrinks irregularly and assumes 
various forms; one of these forms is that of a 
number of blunted protuberances projectingall 
over the surface of the corpuscle, which is then 
said to be crenate ; in a drop of blood examined 
under the microscope, crenate corpuscles are 
often seen at the edge of the cover-slip where 
evaporation is leading to concentration of the 
plasma, or, as it should then perhaps rather be 
called, serum. In blood just shed the red cor- 
puscles are apt to adhere to each other by their 
flat surfaces, much more than to the glass or 
other surface with which the blood is in contact, and hence arrange them- 
selves in rolls. This tendency, however, to form rolls very soon diminishes 
after the blood is shed. 
Though a single corpuscle is somewhat translucent, a comparatively thin 
layer of blood is opaque; type, for instance, cannot be read through even a 
thin layer of blood. 
When a quantity of whipped blood (or blood otherwise deprived of fibrin) 
is frozen and thawed several times it changes color, becoming of a darker 
hue, and is then found to be much more transparent, so that type can now 
be easily read through a moderately thin layer. It is then spoken of as 
laky blood. The same change may be effected by shaking the blood with 
ether, or by adding a small quantity of bile salts, and in other ways. Upon 
examination of laky blood it is found that the red corpuscles are " broken 
up" or at least altered, and that the redness which previously was confined 
to them is now diffused through the serum. Normal blood is opaque because 
each corpuscle, while permitting some rays of light (chiefly red) to pass 
through, reflects many others, and the brightness of the hue of normal blood 
is due to this reflection of light from the surfaces of the several corpuscles. 
Laky blood is transparent because there are no longer intact corpuscles to 
present surfaces for the reflection of light, and the darker hue of laky blood 
is similarly due to the absence of reflection from the several corpuscles. 
When laky blood is allowed to stand a sediment is formed (and may be 
separated by the centrifugal machine) which on examination is found to 
consist of discs, or fragments of discs, of a colorless substance exhibiting 
! Fig. 7. 
a-e, successive effects of water 
upon a red corpuscle; f, effect of 
solution of salt, crenated; g, effect 
of tannic acid.] 40 
BLOOD. 
under high powers an obscurely spongy or reticular structure. These color- 
less, thin discs seen flatwise often appear as mere rings. The substance 
composing them stains with various reagents and may thus be made more 
evident. 
The red corpuscle, then, consists obviously of a colorless framework, with 
which in normal conditions a red coloring matter is associated ; but by 
various means the coloring matter may be driven from the framework and 
dissolved in the serum. 
The framework is spoken of as stroma; it is a modified or differentiated 
protoplasm, and upon chemical analysis yields proteid substances, some of 
them at least belonging to the globulin group, and other matters, among 
which is a peculiar complex fat called lecithin, of which we shall have to 
speak in treating of nervous tissue. 
The red coloring matter which in normal conditions is associated with 
this stroma is called haemoglobin, and may by proper methods be split up 
into a proteid belonging to the globulin group, and into a colored pigment, 
containing iron, called hamaiin. Haemoglobin is, therefore, a very complex 
body. It is found to have remarkable relations to oxygen, and indeed, as 
we shall see, the red corpuscles by virtue of their haemoglobin have a special 
work in respiration ; they carry oxygen from the lungs to the several tissues. 
We shall therefore defer the further study of haemoglobin until we have to 
deal with respiration. 
The red corpuscle, then, consists of a disc of colorless stroma with which 
is associated in a peculiar way the complex colored body haemoglobin. 
Though the haemoglobin, as is seen in laky blood, is readily soluble in serum 
(and it is also soluble in plasma), in the intact normal blood it remains con- 
fined to the corpuscle; obviously there is some special connection between 
the stroma and the haemoglobin ; it is not until the stroma is altered, we may 
perhaps say killed (as by repeated freezing and thawing), that it loses its hold 
on the haemoglobin, which thus set free passes into solution in the serum. 
The disc of stroma when separated from the haemoglobin has, as we have just 
said, an obscurely spongy texture ; but we do not know accurately the exact 
condition of the stroma in the intact corpuscle or how it holds the haemo- 
globin. There is certainly no definite membrane or envelope to the corpuscle, 
for by exposing blood to a high temperature, 60° C., the corpuscle will break 
up into more or less spherical pieces, each still consisting of stroma and 
haemoglobin. 
The quantity of stroma necessary to hold a quantity of haemoglobin is 
exceedingly small. Of the total solid matter of a corpuscle more than 90 
per cent, is haemoglobin. A red corpuscle in fact is a quantity of haemo- 
globin held together in the form of a disc by a minimal amount of stroma. 
Hence whatever effect the stroma per se may have upon the plasma, this, in 
the case of mammals at all events, must be insignificant; the red corpuscle 
is practically simply a carrier of haemoglobin. 
§ 25. The average number of red corpuscles in human blood may be 
probably put down at about 5 millions in a cubic millimetre (the range in 
different mammals is said to be from 3 to 18 millions), but the relation of 
corpuscle to plasma varies a great deal even in health, and very much in 
disease. Obviously the relation maybe affected (1) by an increase or de- 
crease of the plasma, (2) by an actual decrease or increase of red corpus- 
cles. Now, the former must frequently take place. The blood, as we have 
already urged, is always being acted upon by changes in the tissues and, 
indeed, is an index of those changes; hence the plasma must be continu- 
ally changing, though always striving to return to the normal condition. 
Thus when a large quantity of water is discharged by the kidney, the THE CORPUSCLES OF THE BLOOD. 
41 
skin or the bowels, that water comes really from the blood, and the drain 
of water must tend to diminish the bulk of the plasma, and so to increase 
the relative number of red corpuscles, though the effect is probably soon 
remedied by the passage of water from the tissues into the blood. So 
again when a large quantity of water is drunk, this passes into the blood 
and tends temporarily to dilute the plasma (and so to diminish the relative 
number of red corpuscles), though this condition is in turn soon remedied 
by the passage of the superfluous fluid to the tissues and excretory organs. 
The greater or less number of red corpuscles, then, in a given bulk of blood 
may be simply due to less or more plasma, but we have reason to think that 
the actual number of the corpuscles in the blood does vary from time to 
time. This is especially seen in certain forms of disease which may be 
spoken of under the general term of anaemia (there being several kinds of 
anaemia), in which the number of red corpuscles is distinctly diminished. 
The redness of blood may, however, be influenced not only by the number 
of red corpuscles in each cubic millimetre of blood, but also by the amount 
of haemoglobin in each corpuscle, and to a less degree by the size of the 
corpuscles. If we compare, with a common standard, the redness of two 
specimens of blood unequally red, and then determine the relative number 
of corpuscles in each, we may find that the less red specimen has as many 
corpuscles as the redder one, or at least the deficiency in redness is greater 
than can be accounted for by the paucity of red corpuscles. Obviously, in 
such a case, the red corpuscles have too little haemoglobin. In some cases 
of anaemia the deficiency of haemoglobin in each corpuscle is more striking 
than the scantiness of red corpuscles. 
The number of corpuscles in a specimen of blood is determined by mixing a 
small but carefully measured quantity of the blood with a large quantity of some 
[Fig. 8. 
Hsemacytometer of Gowers: A, pipette for measuring the diluting solution ; B, capillary tube 
for measuring blood ; C, cell with divisions on slide, cover-glass and springs; D, vessel to mix 
solutions; E, mixer; F, guarded spear-pointed needle for sticking finger.] 
indifferent fluid, e. c/., a 5 percent, solution of sodium sulphate, and then actually 
counting the corpuscles in a known minimal bulk of the mixture. 42 
BLOOD. 
This, perhaps, may be most conveniently done by the method generally known 
as that of Gowers (Haemacytometer) [Fig. 8], improved by Malassez. A glass 
slide, in a metal frame, is ruled into minute rectangles, e. g. | mm. by i mm., so 
as to give a convenient area of of a square mm. Three small screws in the frame 
permit a coverslip to be brought to a fixed distance, e. g. £ mm., from the surface 
of the slide. The blood having been diluted, e. g. to 100 times its volume, a small 
quantity of the diluted (and thoroughly mixed) blood, sufficient to occupy fully the 
space between the coverslip and the glass slide when the former is brought to its 
proper position, is placed on the slide, and the coverslip brought down. The 
volume of diluted blood now lying over each of the rectangles will be (^yX|) 
of a cubic mm.; and if, when the corpuscles have subsided, the number of cor- 
puscles lying within a rectangle be counted, the result will give the number of 
corpuscles previously distributed through of a cubic mm. of the diluted 
blood. This multiplied by 100 will give the number of corpuscles in 1 cubic 
mm. of the diluted blood, and again multiplied by 100 the number in 1 cubic 
mm. of the entire blood. Tt is advisable to count the number of corpuscles in 
several of the rectangles, and to take the average. For the convenience of count- 
ing, each rectangle is subdivided into a number of very small squares, e. g. into 20, 
each with a side of mm., and so an area of of a square mm. 
Since the actual number of red corpuscles in a specimen of blood (which 
may be taken as a sample of the whole blood) is sometimes more, sometimes 
less, it is obvious that either red corpuscles may be temporarily withdrawn 
from and returned to the general blood current, or that certain red corpuscles 
are, after a while, made away with, and that new ones take their place. We 
have no satisfactory evidence of the former being the case in normal condi- 
tions, whereas we have evidence that old corpuscles do die and that new 
ones are born. 
§ 26. The red corpuscles, we have already said, are continually engaged 
in carrying oxygen, by means of their haemoglobin, from the lungs to the 
tissues; they load themselves with oxygen at the lungs and unload at the 
tissues. It is extremely unlikely that this act should be repeated indefinitely 
without leading to changes which may be familiarly described as wear and 
tear, and which would ultimately lead to the death of the corpuscles. 
We shall have to state later on that the liver discharges into the alimen- 
tary canal, as a constituent of bile, a considerable quantity of a pigment 
known as bilirubin, and that this substance has remarkable relations with, 
and, indeed, may be regarded as a derivative of hoematin, which, as we have 
seen (§ 24), is a product of the decomposition of haemoglobin. It appears 
probable, in fact, that the bilirubin of bile (and this as we shall see is the 
chief biliary pigment, and the source of the other biliary pigments) is not 
formed wholly anew in the body, but is manufactured in some way or other 
out of haematin derived from haemoglobin. This must entail a daily con- 
sumption of a considerable quantity of haemoglobin, and since we know no 
other source of haemoglobin besides the red corpuscles, and have no evidence 
of red corpuscles continuing to exist after having lost their haemoglobin, 
must, therefore, entail a daily destruction of many red corpuscles. 
Even in health, then, a number of red corpuscles must be continually 
disappearing ; and in disease the rapid and great diminution which may 
take place in the number of red corpuscles shows that large destruction 
may occur. 
We cannot at present accurately trace out the steps of this disappearance 
of red corpuscles. In the spleen pulp, red corpuscles have been seen in 
various stages of disorganization, some of them lying within the substance 
of large colorless corpuscles, and as it were being eaten by them. There is 
also evidence that destruction takes place in the liver itself, and, indeed, 
elsewhere. But the subject has not yet been adequately worked out. 
§ 27. This destruction of red corpuscles necessitates the birth of new THE CORPUSCLES OF THE BLOOD. 
43 
corpuscles, to keep up the normal supply of haemoglobin ; and, indeed, the 
cases in which after even great loss of blood by hemorrhage a healthy 
ruddiness returns, and that often rapidly, showing that the lost corpuscles 
have been replaced, as well as the cases of recovery from the disease anaemia, 
prove that red corpuscles are, even in adult life, born somewhere in the 
body. 
In the adult, as in the embryo, the red corpuscles appear to be formed 
out of preceding colored nucleated cells. 
In the interior of bones is a peculiar tissue called marrow, which, in most 
parts being very full of bloodvessels, is called red marrow. In this red 
marrow the capillaries and minute veins form an intricate labyrinth of rel- 
atively wide passages with very thin walls, and through this labyrinth the 
flow of blood is comparatively slow. In the passages of this labyrinth are 
found colored nucleated cells, that is to say, cells the cell substance of which 
has undergone more or less differentiation into haemoglobin and stroma. 
And there seems to be going on in red marrow a multiplication of such 
colored nucleated cells, which become transformed into red non-nucleated 
discs, that is, into ordinary red corpuscles, and pass into the general blood 
current. In other words, a formation of red corpuscles, not wholly unlike 
that which takes place in the embryo, is in the adult continually going on 
in the red marrow of the bones. 
A similar formation of red corpuscles has also been described, though 
with less evidence, as taking place in the spleen, especially under particular 
circumstances, such as after great loss of blood. 
The formation of red corpuscles is, therefore, a special process, taking 
place in special regions; we have no satisfactory evidence that the ordinary 
white corpuscles of the blood are, as they travel in the current of the circu- 
lation, transformed into red corpuscles. 
The red corpuscles, then, to sum up, are useful to the body on account of 
the haemoglobin, which constitutes so nearly the whole of their solid matter. 
What functions the stroma may have besides the mere, so to speak, mechan- 
ical one of holding the haemoglobin in the form of a corpuscle we do not 
know. The primary use of the haemoglobin is to carry oxygen from the 
lungs to the tissues, and it would appear that it is advantageous to the econ- 
omy that the haemoglobin should be as it were bottled up in corpuscles rather 
than simply diffused through the plasma. How long a corpuscle may live 
carrying oxygen we do not exactly know ; the red corpuscles of one animal, 
e. g., a bird, injected into the vessels of another, e. g., a mammal, disappear 
within a fewr days; but this affords no measure of the life of a corpuscle in 
its own home. Eventually, however, the red corpuscle dies, its place being 
supplied by a new one. The haemoglobin set free from the dead corpuscles 
appears to have a secondary use in forming the pigment of the bile and 
possibly other pigments. 
The White or Colorless Corpuscles. 
§ 28. The white corpuscles are far less numerous than the red ; a speci- 
men of ordinary healthy blood will contain several hundred red corpuscles 
to each white corpuscle, though the proportion, even in health, varies consid- 
erably under different circumstances, ranging from 1 in 300 to 1 in 700. 
But though less numerous, the white corpuscles are probably of greater 
importance to the blood itself than are the red corpuscles; the latter are 
chiefly limited to the special work of carrying oxygen from the lungs to the 
tissues, while the former probably exert a considerable influence on the blood 
plasma itself, and help to maintain it in a proper condition. 44 
BLOOD. 
When seen in a normal condition, and " at rest," the white corpuscle is a 
small, spherical, colorless mass, varying in size, but with an average diameter 
of about 10 /z, and presenting generally a finely but sometimes a coarsely 
granular appearance. [Fig. 9.] The surface, even when the corpuscle is 
[Fig. 9. 
a, white corpuscles of human blood; d, red corpuscles (high power).] 
perfectly at rest, is not absolutely smooth and even, but somewhat irregular, 
thereby contributing to the granular appearance; and at times these irreg- 
ularities are exaggerated into protuberances or "pseudopodia" of varying 
size or form, the corpuscle in this way assuming various forms without 
changing its bulk, and by the assumption of a series of forms shifting its 
place. Of these " amoeboid movements," as they are called, we shall have 
to speak later on. 
In carrying on these amoeboid movements the corpuscle may transform 
itself from a spherical mass into a thin, flat, irregular plate; and when this 
occurs there may be seen at times in the midst of the extended finely gran- 
ular mass or cell body, a smaller body of different aspect and refractive power, 
the nucleus. The normal presence of a nucleus in the white corpuscles may 
also be shown by treating the corpuscle with dilute acetic acid, which swells 
up and renders more transparent the cell body but makes the nucleus more 
refractive and more sharply defined, and so more conspicuous, or by the use 
of staining reagents, the majority of which stain the nucleus more readily 
and more deeply than the cell body. In what perhaps may be considered 
a typical white corpuscle, the nucleus is a spherical mass about 2-3 /z in 
diameter, but it varies in size in different corpuscles, and not unfrequently 
is irregular in form, at least after the action of reagents. It occasionally 
appears as if about to divide into fragments, and sometimes a corpuscle may 
contain two or even more (then generally small) nuclei. Though staining 
readily with staining reagents, the nucleus of an ordinary white corpuscle 
does not show the nuclear network which is so characteristic, as we shall see, 
of the nuclei of many cells, and which in these is the part of the nucleus 
which especially stains; in the closely allied lymph corpuscles, to which we 
shall have immediately to refer, a nuclear network is present. 
The cell body of the white corpuscle may be taken as a good example of 
what we have called undifferentiated protoplasm. Optically, it consists of a 
uniformly transparent but somewhat refractive material or basis, in which, 
are imbedded minute particles, generally spherical in form, and in which 
sometimes occur minute vacuoles filled with fluid; it is rarely, if ever, that 
any distinct network, like that which is sometimes observed in other cells, 
can be seen in the cell body of a white corpuscle whether stained or not. The 
imbedded particles are generally very small, and for the most part distributed 
uniformly over the cell body, giving it a finely granular aspect; sometimes, 
however, the particles are relatively large, making the corpuscles coarsely 
granular, the coarse granules being frequently confined to one or another THE CORPUSCLES OF THE BLOOD. 
45 
part of the cell body. These particles or granules, whether coarse or fine, 
vary in nature; some of them, as shown by their greater refractive power, 
their staining with osmic acid, and their solution by solvents of fat, are fatty 
in nature; others may similarly be shown by their reactions to be proteid in 
nature. 
The material in which these granules are imbedded, and which forms the 
greater part of the cell body, has no special optical features; so far as can 
be ascertained, it appears under the microscope to be homogeneous; no 
definite structure can be detected in it. It must be borne in mind that the 
whole corpuscle consists largely of water, the total solid matter amounting 
to not much more than 10 per cent. The transparent material of the cell 
body must, therefore, be in a condition which we may call semifluid, or semi- 
solid, without being called upon to define what we exactly mean by these 
terms. This approach to fluidity appears to be connected with the great 
mobility of the cell body, as shown in its amoeboid movements. 
§ 29. When we submit to chemical examination a sufficient mass of 
white corpuscles, separated out from the blood by special means and obtained 
tolerably free from red corpuscles and plasma (or apply to the white blood- 
corpuscles the chemical results obtained from the more easily procured lymph- 
corpuscles, which, as we shall see, are very similar to, and, indeed, in many 
ways related to the white corpuscles of the blood), we find that this small 
solid matter of the corpuscle consists largely of certain proteids. 
One of these proteids is a body either identical with, or closely allied to, 
the proteid called myosin, which we shall have to study more fully in con- 
nection with muscular tissue. At present we may simply say that myosin is 
a body intermediate between fibrin and globulin, being less soluble than the 
latter and more soluble than the former ; thus while it is hardly at all soluble 
in a 1 per cent, solution of sodium chloride or other neutral salt, it is, unlike 
fibrin, speedily and wholly dissolved by a 10 per cent, solution. Myosin is 
further interesting because, as we shall see, just as fibrin is formed in the 
clotting of blood from fibrinogen, so myosin is formed out of a preceding 
myosinogen, during a kind of clotting which takes place in muscular fibre 
ami which is spoken of as rigor mortis. And we have reasons for thinking 
that in the living white blood-corpuscle there does exist a body identical 
with or allied to myosinogen, which we may speak of as being in a fluid con- 
dition; and which on the death of the corpuscle is converted, by a kind of 
clotting, into myosin, or into an allied body which, being solid, gives the 
body of the corpuscle a stiffness and rigidity which it did not possess during 
life. 
Besides this myosin or myosin-like proteid, the white corpuscles also 
contain either paraglobulin itself or some other member of the globulin 
group, as well as a body or bodies like or identical with serum-albumin. 
In addition, there is present, in somewhat considerable quantity, a sub- 
stance of a peculiar nature, which since it is confined to the nuclei of the 
corpuscles, and further seems to be present in all nuclei, has been called 
nuclein. This nuclein, which though a complex nitrogenous body is very 
different in composition and nature from proteids, is remarkable on the one 
hand for being a very stable inert body, and on the other for containing a 
large quantity (according to some observers nearly 10 per cent.) of phos- 
phorus, which appears to enter more closely into the structure of the molecule 
than it does in the case of proteids. 
Next in importance to the proteids, as constant constituents of the white 
corpuscles, come certain fats. Among these the most conspicuous is the 
complex fatty body lecithin. 
In the case of many corpuscles at all events we have evidence of the 46 
BLOOD. 
presence of a member of the large group of carbohydrates, comprising 
starches and sugar, viz., the starch-like body glycogen, which we shall have 
to study more fully hereafter. This glycogen may exist in the living cor- 
puscle as glycogen, but it is very apt, after the death of the corpuscles, to 
become changed by hydration into some form of sugar, such as maltose or 
dextrose. 
Lastly, the ash of the white corpuscles is characterized by containing a 
relatively large quantity of potassium and of phosphates and by being rela- 
tive poor in chlorides and in sodium. But in this respect the corpuscle is 
merely an example of what seems to be a general rule (to which, however, 
there may be exceptions) that while the elements of the tissues themselves 
are rich in potassium and phosphates, the blood plasma or lymph on which 
they live abounds in chlorides and sodium salts. 
§ 30. In the broad features above mentioned, the white blood-corpuscle 
may be taken as a picture and example of all living tissues. If we examine 
the histological elements of any tissue, whether we take an epithelium cell, 
or a nerve cell, or a cartilage cell, or a muscular fibre, we meet with very 
similar features. Studying the element morphologically, we find a nucleus1 
and a cell body, the nucleus having the general characters described above 
with frequently other characters introduced, and the cell body consisting of 
at least more than one kind of material, the materials being sometimes so 
disposed as to produce the optical effect simply of a transparent mass in 
which granules are imbedded, in which case we speak of the cell body as 
protoplasmic, but at other times so arranged that the cell body possesses dif- 
ferentiated structure. Studying the element from a chemical point of view, 
we find proteids always present, and among these bodies identical with or 
more or less closely allied to myosin, we generally have evidence of the 
presence also of fat of some kind and of some member or members of the 
carbohydrate group, and the ash always contains potassium and phosphates, 
with sulphates, chlorides, sodium, and calcium, to which may be added mag- 
nesium and iron. 
We stated in the Introduction that living matter is always undergoing 
chemical change; this continued chemical change we may denote by the 
term metabolism. We further urged that as long as living matter is alive, 
the chemical change or metabolism is of a double kind. On the one hand, 
the living substance is continually breaking down into simpler bodies, with 
a setting free of energy; this part of the metabolism we may speak of as 
made up of katabolic changes. On the other hand, the living substance is 
continually building itself up, embodying energy into itself and so replen- 
ishing its store of energy ; this part of the metabolism we may speak of as 
made up of anabolic changes. We also urged that in every piece of living 
tissue there might be (1) the actual living substance itself, (2) material which 
is present for the purpose of becoming, and is on the way to become living 
substance-that is to say, food undergoing or about to undergo anabolic 
changes, and (3) material which has resulted from, or is resulting from, the 
breaking down of the living substance-that is to say, material which has 
undergone or is undergoing katabolic changes, and which we speak of as 
waste. In using the word " living substance," however, we must remember 
that in reality it is not a substance in the chemical sense of the word, but 
material undergoing a series of changes. 
If, now, we ask the question, which part of the body of the white cor- 
puscle (or of a similar element of another tissue) is the real living substance, 
and which part is food or waste, we ask a question which we cannot as yet 
definitely answer. We have at present no adequate morphological criteria 
1 The existence of niultinuclear structures does not affect the present argument. THE CORPUSCLES OF THE BLOOD. 
47 
to enable us to judge, by optical characters, what is really living and what 
is not. 
The material which appears in the cell body in the form of distinct 
granules, merely lodged in the more transparent material, cannot be part 
of the real living substance; it must be either food or waste. Many of 
these granules are fat, and we have at times an opportunity of observing 
that they have been introduced into the corpuscle from the surrounding 
plasma. The white corpuscle, as we have said, has the power of executing 
amoeboid movements; it can creep around objects, envelope them with its 
own substance, and so put them inside itself. The granules of fat thus in- 
troduced may be subsequently extruded or may disappear within the cor- 
puscle ; in the latter case they are obviously changed, and apparently made 
use of by the corpuscle. In other words, these fatty granules are apparently 
food material on their way to be worked up in the living substance of the 
corpuscle. 
But we have also evidence that similar granules of fat may make their 
appearance wholly within the corpuscle ; they are products of the activity of 
the corpuscle. We have further reason to think that in some cases, at all 
events, they arise from the breaking down of the living substance of the 
corpuscle, that they are what we have called waste products. 
But all the granules visible in a corpuscle are not necessarily fatty in 
nature ; some of them may undoubtedly be proteid granules, and it is possi- 
ble that some of them may at times be of carbohydrate or other nature. In 
all cases, however, they are either food material or waste products. And 
what is true of the easily distinguished granules is also true of other sub- 
stances, in solution or in a solid form, but so disposed as not to be optically 
recognized. 
Hence a part, and it may be no inconsiderable part, of the white corpus- 
cle, may be not living substance at all, but either food or waste. Further,- it 
does not necessarily follow that the whole of any quantity of material, fatty 
or otherwise, introduced into the corpuscle from without, should actually 
be built up into and so become part of the living substance; the changes 
from raw food to living substance are, as we have already said, probably 
many, and it may be that after a certain number of changes, few or many, 
part only of the material is accepted as worthy of being made alive, and 
the rest, being rejected, becomes at once waste matter ; or the material may, 
even after it has undergone this or that change, never actually enter into 
the living substance, but all become waste matter. We say waste matter, 
but this does not mean useless matter. The matter so formed may without 
entering into the living substance be of some subsidiary use to the corpuscle, 
or, as probably more often happens, being discharged from the corpuscle, 
may be of use to some other part of the body. We do not know how the 
living substance builds itself up, but we seem compelled to admit that, 
in certain eases at all events, it is able in some way or other to produce 
changes on material while that material is still outside the living substance 
as it were, before it enters into and indeed without its ever actually entering 
into the composition of the living substance. On the other hand, we must 
equally admit that some of the waste substances are the direct products of 
the katabolic changes of the living substance itself-were actually once part 
of the living substance. Hence we ought, perhaps, to distinguish the products 
of the activity of living matter into waste products proper, the direct results 
of katabolic changes, and into by-products which are the results of changes 
effected by the living matter outside itself, and which cannot, therefore, be 
considered as necessarily either anabolic or katabolic. 
Concerning the chemical characters of the living matter itself we cannot 48 
BLOOD. 
at present make any very definite statement. We may say that the proteid 
myosin, or rather the proteid antecedent or antecedents of myosin, enter 
in some way into its structure, but we are not justified in saying that the 
living substance consists only of proteid matter in a peculiar condition. 
And, indeed, the persistency with which some representative of fatty bodies 
and some representative of carbohydrates always appear in living tissue 
would, perhaps, rather lead us to suppose that these equally with proteid 
material were essential to its structure. Again, though the behavior of the 
nucleus, as contrasted with that of the cell body, leads us to suppose that 
the living substance of the former is a different kind from that of the latter, 
we do not know exactly in what the difference consists. The nucleus, as we 
have seen, contains nuclein which, perhaps, we may regard as a largely 
modified proteid ; but being a body which is remarkable for its stability, for 
the difficulty with which it is changed by chemical reagents, cannot be 
regarded as an integral part of the essentially mobile living substance of 
the nucleus. 
In this connection it may be worth while to again call attention to the 
fact that the corpuscle contains a very large quantity of water, viz., about 
90 per cent. Part of this, we do not know how much, probably exists in a 
more or less definite combination with the protoplasm, somewhat after the 
manner of, to use what is a mere illustration, the water of crystallization of 
salts. If we imagine a whole group of different complex salts continually 
occupied in turn in being crystallized and being decrystallized, the water 
thus engaged by the salts will give us a rough image of the water which 
passes in and out of the substance of the corpuscle as the result of its meta- 
bolic activity. We might call this " water of metabolism." Another part 
of the water, carrying in this case substances in solution, probably exists in 
spaces or interstices too small to be seen with even the highest powers of the 
microscope. Still another part of the water similarly holding substances in 
solution exists at times in definite spaces visible under the microscope, more 
or less regularly spherical, and called vacuoles. 
We have dwelt thus at length on the white corpuscle in the first place, 
because, as we have already said, what takes place in it is in a sense a picture 
of what takes place in all living structures, and in the second place because 
the facts which we have mentioned help us to understand how the white 
corpuscle may carry on in the blood a work of no important kind ; for from 
what has been said it is obvious that the white corpuscle is continually 
acting upon and being acted upon by the plasma. 
§ 31. To understand, however, the work of these white corpuscles, we 
must learn what is known of their history. 
In successive drops of blood taken at different times from the same 
individual, the number of colorless corpuscles will be found to vary very 
much, not only relatively to the red corpuscles, but also absolutely. They 
must, therefore, " come and go." 
In treating of the lymphatic system we shall have to point out that a 
very large quantity of fluid called lymph, containing a very considerable 
number of bodies, very similar in their general characters to the white cor- 
puscles of the blood, is being continually poured into the vascular system at 
the point where the thoracic duct joins the great veins on the left side of 
the neck, and to a less extent where the other large lymphatics join the 
venous system on the right side of the neck. These corpuscles of lymph, 
which, as we have just said, closely resemble, and, indeed, are with difficulty 
distinguished from the white corpuscles of the blood, but of which, when 
they exist outside the vascular system, it will be convenient to speak of as 
leucocytes, are found along the whole length of the lymphatic system, but THE CORPUSCLES OF THE BLOOD. 
49 
are more numerous in the lymphatic vessels after these have passed through 
the lymphatic glands. These lymphatic glands are partly composed of 
what is known as adenoid tissue, a special kind of connective tissue arranged 
as a delicate network. The meshes of this are crowded with colorless nucle- 
ated cells, which, though varying in size, are, for the most part, small, the 
nucleus being surrounded by a relatively small quantity of cell substance. 
Many of these cells show signs that they are undergoing cell-division, and 
we have reason to think that cells so formed, acquiring a larger amount of 
cell substance, become veritable leucocytes. In other words, leucocytes 
multiply in the lymphatic glands, and leaving the glands by the lymphatic 
vessels, make their way to the blood. Patches and tracts of similar adenoid 
tissue, not arranged, however, as distinct glands, but similarly occupied by 
developing leucocytes and similarly connected with lymphatic vessels, are 
found in various parts of the body, especially in the mucous membranes. 
Hence we may conclude that from various parts of the body, the lymph- 
atics are continually bringing to the blood an abundant supply of leuco- 
cytes, and that these in the blood become ordinary white corpuscles. This 
is probably the chief source of the white corpuscles, for though the white 
corpuscles have been seen dividing in the blood itself, no large increase 
takes place in that way. 
§ 32. It follows that since white corpuscles are thus continually being 
added to the blood, white corpuscles must as continually either be destroyed, 
or be transformed, or escape from the interior of the bloodvessels; otherwise 
the blood would soon be blocked with white corpuscles. 
Some do leave the bloodvessels. In treating of the circulation we shall 
have to point out that white corpuscles are able to pierce the walls of the 
capillaries and minute veins, and thus to make their way from the interior 
of the bloodvessels into spaces filled with lymph, the " lymph spaces," as 
they are called, of the tissue lying outside the bloodvessels. This is spoken 
of as the " migration of the white corpuscles." In an "inflamed area" 
large numbers of white corpuscles are thus drained away from the blood 
into the lymph spaces of the tissue; and it is probable that a similar loss 
takes place, more or less, under normal conditions. These migrating cor- 
puscles may, by following the devious tracks of the lymph, find their way 
back into the blood ; some of them, however, may remain, and undergo 
various changes. Thus, in inflamed areas, when suppuration follows in- 
flammation, the white corpuscles which have migrated may become " pus 
corpuscles," or, where thickening and growth follow7 upon inflammation, 
may, according to many authorities, become transformed into temporary or 
permanent tissue, especially connective tissue; but this transformation into 
tissue is disputed. When an inflammation subsides without leaving any 
effect a few corpuscles only will be found in the tissue; those which had 
previously migrated must, therefore, have been disposed of in some way or 
another. 
In speaking of the formation of red corpuscles (§ 27) we saw that not 
only it is not proved that the nucleated corpuscles which give rise to red 
corpuscles are ordinary white corpuscles, but that in all probability the real 
hsematoblasts, the parents of red corpuscles, are special corpuscles developed 
in the situations where the manufacture of red corpuscles takes place. So 
far, therefore, from assuming, as is sometimes done, that the white corpuscles 
of the blood are all of them on their way to become red corpuscles, it may 
be doubted whether any of them are. In any case, however, even making 
allowance for those which migrate, a very considerable number of the white 
corpuscles must " disappear" in some way or other from the blood stream, 
and we may, perhaps, speak of their disappearance as being a " destruction " 50 
BLOOD. 
or "dissolution." We have, as yet, no exact knowledge to guide us in the 
matter, but we can readily imagine, that upon the death of the corpuscle, 
the substances composing it, after undergoing changes, are dissolved by and 
become part of the plasma. If so, the corpuscles, as they die, must repeat- 
edly influence the composition and nature of the plasma. 
But if they thus affect the plasma in their death, it is even more probable 
that they influence it during their life. Being alive, they must be continually 
taking in and giving out. As we have already said, they are known to ingest, 
after the fashion of an amoeba, solid particles of various kinds, such as fat 
or carmine, present in the plasma, and probably digest such of these particles 
as are nutritious. But if they ingest these solid matters they probably also 
carry out the easier task of ingesting dissolved matters. If, however, they 
thus take in, they must also give out; and thus by the removal on the one 
hand of various substances from the plasma, and by the addition on the other 
hand of others, they must be continually influencing the plasma. We have 
already said that the white corpuscles in shed blood as they die are supposed 
to play an important part in the clotting of blood ; similarly they may dur- 
ing their whole life be engaged in carrying out changes in the protends of 
the plasma which do not lead to clotting, but which prepare them for their 
various uses in the body. 
Pathological facts afford support to this view. The disease called leuco- 
cythsemia (or leukaemia) is characterized by an increase of the white cor- 
puscles, both absolute and relative to the red corpuscles, the increase, due 
to an augmented production or possibly to a retarded destruction, being at 
times so great as to give the blood a pinkish-gray appearance, like that of 
blood mixed with pus. We accordingly find that in this disease the plasma 
is in many ways profoundly affected and fails to nourish the tissues. As 
a further illustration of the possible action of the white corpuscles we may 
state that, according to some observers in certain diseases in which minute 
organisms, such as bacteria, make their appearance in the blood, the white 
corpuscles " take up " these bacteria into their substance, and thus probably, 
by exerting an influence on them, modify the course of the disease of which 
these organisms are the essential cause. 
If the white corpuscles are thus engaged during their life in carrying on 
important labors, we may expect them to differ in appearance according to 
their condition. Some of the corpuscles are spoken of as " faintly " or 
" finely " granular. Other corpuscles are spoken of as "coarsely " granular, 
their cell substance being loaded with conspicuously discrete granules. It 
may be, of course, that there are two distinct kinds of corpuscles, having 
different functions and possibly different origins and histories; but since 
intermediate forms are met with containing a few coarse granules only, it 
is more probable that the one form is a phase of the other ; that a faintly 
granular corpuscle, by taking in granules from without or by producing 
granules within itself as products of its metabolism, may become a coarsely 
granular corpuscle. 
Whether, however, the white corpuscles are really all of one kind, or 
whether they are different kinds performing different functions, must at 
present be left an open question. 
Blood Platelets. 
§ 33. In a drop of blood examined with care immediately after removal 
may be seen a number of exceedingly small bodies (2/?. to 3// in diameter), 
frequently disc-shaped, but sometimes of a rounded or irregular form, homo- 
geneous in appearance when quite fresh, but apt to assume a faintly gran- THE CHEMICAL COMPOSITION OF BLOOD. 
51 
ular aspect, They are called blood platelets, or blood plaques, they have 
been supposed by some to become developed into, and, indeed, to be early 
stages of, the red corpuscles, and hence have 
been called luematoblasts; but this view has 
not been confirmed ; indeed, as we have seen 
(§ 27), the real hsematoblasts, or developing red 
corpuscles, are of quite a different nature. 
They speedily undergo change after removal 
from the body, apparently dissolving in the 
plasma ; they break up, part of their substance 
disappearing, while the rest becomes granular. 
Their granular remains are apt to run together, 
forming in the plasma the shapeless masses 
which have long been known and described as 
"lumps of protoplasm." By appropriate re- 
agents, however, these platelets may be fixed 
and stained in the condition in which they 
appear after leaving the body. 
The substance composing them is peculiar, 
and, though we may perhaps speak of them as 
consisting of living material, their nature is at 
present obscure. They may be seen within the 
living bloodvessels [Fig. 10], and therefore 
must be regarded as real parts of the blood, 
and not as products of the changes taking 
place in blood after it has been shed. 
When a needle or thread or other foreign body is introduced into the 
interior of a bloodvessel, they are apt to collect upon, and, indeed, are the 
precursors of the clot which in most cases forms around the needle or thread. 
They are also found in the thrombi or plugs which sometimes form in the 
bloodvessels as the result'of disease or in jury. Indeed, it has been main- 
tained that what are called white thrombi (to distinguish them from red 
thrombi, which are plugs of corpuscles and fibrin) are in reality aggrega- 
tions of blood platelets; and for various reasons blood platelets have been 
supposed to play an important part in the clotting of blood, carrying out 
the work which, in this respect, is by others attributed to the white corpus- 
cles. But no very definite statement can at present be made about this; 
and, indeed, the origin and whole nature of these blood platelets is at pre- 
sent obscure. 
[Fig. 10. 
Fibrin Filaments and Blood 
Platelets: A, network of fibrin, 
shown after washing away the 
corpuscles from a preparation of 
blood that has been allowed to 
clot; many of the filaments radiate 
from small clumps of blood plate- 
lets. B (from Osler), blood cor- 
puscles and elementary particles 
or blood platelets within a small 
vein.] 
The Chemical Composition of Blood. 
§ 34. We may now pass briefly in review the chief chemical characters 
of blood, remembering always that, as we have already urged, the chief 
chemical interests of blood are attached to the changes which it undergoes 
in the several tissues; these will be considered in connection with each 
tissue at the appropriate place. 
The average specific gravity of human blood is 1055, varying from 1045 
to 1075 within the limits of health. 
The reaction of blood as it flows from the bloodvessels is found to be 
distinctly though feebly alkaline. If a drop be placed on a piece of faintly 
red, highly glazed litmus paper, and then wiped off, a blue stain will be left. 
The whole blood contains, in a certain quantity, gases-viz., oxygen, car- 
bonic acid, and nitrogen, which are held in the blood in a peculiar way- 
which vary in different kinds of blood, and so serve especially to distin- 52 
BLOOD. 
guish arterial from venous blood, and which may be given off from blood 
whan exposed to an atmosphere, according to the composition of that 
atmosphere. These gases of blood we shall study in connection with res- 
piration. 
The normal blood consists of corpuscles and plasma. 
If the corpuscles be supposed to retain the amount of water proper to 
them, blood may, in general terms, be considered as consisting by weight 
of from about one-third to somewhat less than one-half of corpuscles, the 
rest being plasma. As we have already seen, the number of corpuscles in 
a specimen of blood is found to vary considerably, not only in different ani- 
mals and in different individuals, but in the same individual at different 
times. 
The plasma is resolved by the clotting of the blood into serum and fibrin. 
§ 35. The serum contains in 100 parts : 
Proteid substances about 8 or 9 parts. 
Fats, various extractives, and saline matters " 2 or 1 part. 
Water " 90 parts. 
The proteids are paraglobulin and serum-albumin (there being probably 
more than one kind of serum-albumin) in varying proportion. We may, 
perhaps, roughly speaking, say that they occur in about equal quantities. 
Conspicuous and striking as are the results of clotting, massive as appears 
to be the clot which is formed, it must be remembered that by far the greater 
part of the clot consists of corpuscles. The amount by weight of fibrin re- 
quired to bind together a number of corpuscles, in order to form even a 
large, firm clot, is exceedingly small. Thus, the average quantity by weight 
of fibrin in human blood is said to be 0.2 per cent.; the amount, however, 
which can be obtained from a given quantity of plasma varies extremely, 
the variation being due not only to circumstances affecting the blood, but to 
the method employed. 
The fats, which are scanty, except after a meal or in certain pathological 
conditions, consist of the neutral fats-stearin, palmitin, and olein-with a 
certain quantity of their respective alkaline soaps. The peculiar complex 
fat lecithin occurs in very small quantities only; the amount present of the 
peculiar alcohol cholesterin, which had so fatty an appearance is also small. 
Among the extractives present in serum may be put down nearly all the 
nitrogenous and other substances which form the extractives of the body 
and of food, such as urea, kreatin, sugar, lactic acid, etc. A very large 
number of these have been tdiscovered in the blood under various circum- 
stances, the consideration of which must be left for the present. The pecu- 
liar odor of blood or of serum is probably due to the presence of volatile 
bodies of the fatty acid series. The faint yellow color of serum is due to a 
special yellow pigment. The most characteristic and important chemical 
feature of the saline constitution of the serum is the preponderance, at 
least in man and most animals, of sodium salts over those of potassium. 
In this respect the serum offers a marked contrast to the corpuscles. Less 
marked, but still striking, is the abundance of chlorides and the poverty 
of phosphates in the serum, as compared with the corpuscles. The salts 
may in fact briefly be described as consisting chiefly of sodium chloride, 
with some amount of sodium carbonate, or more correctly sodium bicar- 
bonate, and potassium chloride with small quantities of sodium sulphate, 
sodium phosphate, calcium phosphate, and magnesium phosphate. And 
of even the small quantity of phosphates found in the ash, part of the 
phosphorus exists in the serum itself, not as a phosphate, but as phos- 
phorus in some organic body. 53 
QUANTITY OF BLOOD, AND ITS DISTRIBUTION IN THE BODY. 
§ 36. The red corpuscles contain less water than the serum, the amount 
of solid matter being variously estimated at from 30 to 40 or more percent. 
The solids are almost entirely organic matter, the inorganic salts amounting 
to less than 1 per cent. Of the organic matter again by far the larger part 
consists of haemoglobin. In 100 parts of the dried organic matter of the 
corpuscles of human blood, about 90 parts are haemoglobin, about 8 parts 
are proteid substances, and about 2 parts are other substances. Of the 
last, one of the most important, forming about a quarter of them and 
apparently being always present, is lecithin. Cholesterin appears also to 
be normally present. The proteids which form the stroma of the red cor- 
puscles appear to belong chiefly to the globulin family. As regards the 
inorganic constituents, the corpuscles are distinguished by the relative 
abundance of the salts of potassium and of phosphates. This at least is 
the case in man ; the relative quantities of sodium and potassium in the 
corpuscles and serum respectively appear, however, to vary in different 
animals; in some the sodium salts are in excess even in the corpuscles. 
§ 37. The proteid matrix of the white corpuscles we have stated to be 
composed of myosin (or an allied body), paraglobulin and possibly other 
proteids. The nuclei contain nuclein. The white corpuscles are found to 
contain, in addition to proteid material, lecithin and other fats, glycogen, 
extractives and inorganic salts, there being in the ash, as in that of the red 
corpuscles, a preponderance of potassium salts and of phosphates. 
The main facts of interest then in the chemical composition of the blood 
are as follows : The red corpuscles consist chiefly of haemoglobin. The or- 
ganic solids of serum consist partly of serum-albumin, and partly of para- 
globulin. The serum or plasma contrasts, in man at least, with the corpus- 
cles, inasmuch as the former contains chiefly chlorides and sodium salts, 
while the latter are richer in phosphates and potassium salts. The extrac- 
tives of the blood are remarkable rather for their number and variability 
than for their abundance, the most constant and important being perhaps 
urea, kreatin, sugar, and lactic acid. 
The Quantity of Blood, and its Distribution in the Body. 
§ 38. The quantity of blood contained in the whole vascular system is a 
balance struck between the tissues which give to, and those which takeaway 
from, the blood. Thus the tissues of the alimentary canal largely add to the 
blood water and the material derived from food, while the excretory organs 
largely take away water and the other substances constituting the excretions. 
Other tissues both give and take, and the considerable drain from the blood 
to the lymph spaces which takes place in the capillaries is met by the flow 
of lymph into the great veins. 
From the result of a few observations on executed criminals it has been 
concluded that the total quantity of blood in the human body is about 73th 
of the body weight. But in various animals, the proportion of the weight 
of the blood to that of the body has been found to vary very considerably 
in different individuals; and probably this holds good for man also, at all 
events within certain limits. 
In the same individual the quantity probably does not vary largely. A 
sudden drain upon the water of the blood by great activity of the excretory 
organs, as by profuse sweating, or a sudden addition to the water of the blood, 
as by drinking large quantities of water or by injecting fluid into the blood- 
vessels, is rapidly compensated for by the passage of water from the tissues 
to the blood, or from the blood to the tissues. As we have already said, the 
tissues are continually striving to keep up an average composition of the 54 
THE CONTRACTILE TISSUES. 
blood, and in so doing keep up an average quantity. In starvation the 
quantity (and quality) of the blood is maintained for a long time at the 
expense of the tissues, so that after some days' privation of food and drink, 
while the fat, the muscles, and other tissues have been largely diminished, 
the quantity of blood remains nearly the same. 
The total quantity of blood present in an animal body is estimated in the fol- 
lowing way: As much blood as possible is allowed to escape from the vessels; 
this is measured directly. The vessels are then washed out with water or normal 
saline solution, and the washings carefully collected, mixed, and measured. A 
known quantity of blood is diluted with water or normal saline solution until it 
possesses the same tint as a measured specimen of the washings. This gives the 
amount of blood (or rather of haemoglobin) in the measured specimen, from which 
the total quantity in the whole washings is calculated. Lastly, the whole body is 
carefully minced and washed free from blood. The washings are collected and 
filtered, and the amount of blood in them is estimated, as before, by comparison 
with a specimen of diluted blood. The quantity of blood, as calculated from the 
two washings, together with the escaped and directly measured blood, gives the 
total quantity of blood in the body. 
The method is not free from objections, but other methods are even more 
imperfect. 
The blood is in round numbers distributed as follows: 
About one-fourth in the heart, lungs, large arteries and veins. 
About one-fourth in the liver. 
About one-fourth in the skeletal muscles. 
About one-fourth in the other organs. 
Since in the heart and great bloodvessels the blood is simply in transit, 
without undergoing any great changes (and in the lungs, as far as we know, 
being limited to respiratory changes), it follows that the alterations which 
take place in the blood passing through the liver and skeletal muscles far 
exceed those which occur in the rest of the body. 
CHAPTER II. 
THE CONTRACTILE TISSUES. 
§ 39. In order that the blood may nourish the several tissues it is carried 
to and from them by the vascular mechanism ; and this carriage entails 
active movements. In order that the blood may adequately nourish the tis- 
sues, it must be replenished by food from the alimentary canal, and purified 
from waste by the excretory organs; and both these processes entail move- 
ments. Hence before we proceed further we must study some of the general 
characters of the movements of the body. 
Most of the movements of the body are carried out by means of the mus- 
cles of the trunk and limbs, which being connected with the skeleton are 
frequently called skeletal muscles. A skeletal muscle when subjected to 
certain influences suddenly shortens, bringing its two ends nearer together; 
and it is the shortening, acting upon various bony levers or by help of other 
mechanical arrangements, which produces the movement. Such a temporarv 
shortening, called forth by certain influences, and due as we shall see to 
changes taking place in the muscular tissue forming the chief part of the THE PHENOMENA OF MUSCLE AND NERVE. 
55 
muscle, is technically called a contraction of the muscle; and the muscular 
tissue is spoken of as a contractile tissue. The heart is chiefly composed of 
muscular tissue, differing in certain minor features from the muscular tissue 
of the skeletal muscles, and the beat of the heart is essentially a contraction 
of the muscular tissue composing it, a shortening of the peculiar muscular 
fibres of which the heart is chiefly made up. The movements of the ali- 
mentary canal and of many other organs are similarly the results of the 
contraction of the muscular tissue entering into the composition of those 
organs, of the shortening of certain muscular fibres built up into those 
organs. In fact, almost all the movements of the body are the result of 
the contraction of muscular fibres, of various nature and variously disposed. 
Some few movements, however, are carried out by structures which can- 
not be called muscular. Thus, in the pulmonary passages and elsewhere, 
movement is effected by means of cilia attached to epithelium cells; and 
elsewhere, as in the case of the migrating white corpuscles of the blood, 
transference from place to place in the body is brought about by amoeboid 
movements. But as we shall see the changes in the epithelium cell or white 
corpuscle which are at the bottom of ciliary or amoeboid movements are, in 
all probability, fundamentally the same as those which take place in a mus- 
cular fibre when it contracts: they are of the nature of a contraction, and 
hence we may speak of all these as different forms of contractile tissue. 
Of all these various forms of contractile tissue, the skeletal muscles, on 
account of the more complete development of their functions, will be better 
studied first; the others, on account of their very simplicity, are in many 
respects less satisfactorily understood. 
All the ordinary skeletal muscles are connected with nerves. We have 
no reason for thinking that they are thrown into contraction, under normal 
conditions, otherwise than by the agency of nerves. 
Muscles and nerves being thus so closely allied, and having besides so 
many properties in common, it will conduce to clearness and brevity if we 
treat them together. 
The Phenomena of Muscle and Nerve. 
Muscular and Nervous Irritability. 
§ 40. The skeletal muscles of a frog, the brain and spinal cord of which 
have been destroyed, do not exhibit any spontaneous movements or contrac- 
tions, even though the nerves be otherwise quite intact. Left undisturbed, 
the whole body may decompose without any contraction of any of the skele- 
tal muscles having been witnessed. Neither the skeletal muscles nor the 
nerves distributed to them possess any power of automatic action. 
If, however, a muscle be laid bare and be more or less violently disturbed 
-if, for instance, it be pinched, or touched with a hot wire, or brought into 
contact with certain chemical substances, or subjected to the action of gal- 
vanic currents-it will move, that is, contract, whenever it is thus disturbed. 
Though not exhibiting any spontaneous activity, the muscle is (and con- 
tinues for some time after the general death of the animal to be) irritable. 
Though it remains quite quiescent when left untouched, its powers are then 
dormant only-not absent. These require to be roused or " stimulated " by 
some change or disturbance in order that they may manifest themselves. 
The substances or agents which are thus able to evoke the activity of an 
irritable muscle are spoken of as stimuli. 
But to produce a contraction in a muscle the stimulus need not be ap- 
plied directly to the muscle; it may be applied indirectly by means of the 56 
THE CONTRACTILE TISSUES. 
nerve. Thus, if the trunk of a nerve be pinched, or subjected to sudden 
heat, or dipped in certain chemical substances, or acted upon by various 
galvanic currents, contractions are seen in the muscles to which branches of 
the nerves are distributed. 
The nerve, like the muscle, is irritable; it is thrown into a state of 
activity by a stimulus: but, unlike the muscle, it does not itself contract. 
The stimulus does not give rise in the nerve to any visible change of form ; 
but that changes of some kind or other are set up and propagated along the 
nerve down to the muscle is shown by the fact that the muscle contracts 
when a part of the nerve at some distance from itself is stimulated. Both 
nerve and muscle are irritable, but only the muscle is contractile-i. e., mani- 
fests its irritability by contraction. The nerve manifests its irritability by 
transmitting along itself, without any visible alteration of form, certain 
molecular changes set up by the stimulus. We shall call these changes 
thus propagated along a nerve " nervous impulses." 
§41. We have stated above that the muscle may be thrown into con- 
traction by stimuli applied directly to itself. But it might fairly be urged 
that the contractions so produced are in reality due to the fact that the 
stimulus, although apparently applied directly to the muscle, is, after 
all, brought to bear on some of the many fine nerve-branches, which, as 
we shall see, are abundant in the muscle, passing along and between the 
muscular fibres, in which they finally end. The following facts, however, 
go far to prove that the muscular fibres themselves are capable of being 
directly stimulated without the intervention of any nerves: When a frog 
(or other animal) is poisoned with urari, the nerves may be subjected to the 
strongest stimuli without causing any contractions in the muscles to which 
they are distributed ; yet even ordinary stimuli, applied directly to the 
muscle, readily cause contractions. If, before introducing the urari into the 
system, a ligature be passed underneath the sciatic nerve in one leg-for 
instance, the right-and drawn tightly round the whole leg to the exclusion 
of the nerve, it is evident that the urari, when injected into the back of the 
animal, will gain access to the right sciatic nerve above the ligature, but 
not below, while it will have free access to the rest of the body, including 
the whole left sciatic. If, as soon as the urari has taken effect, the two 
sciatic nerves be stimulated, no movement of the left leg will be produced 
by stimulating the left sciatic, whereas strong contractions of the muscles 
of Jie right leg below the ligature will follow stimulation of the right 
sciatic, whether the nerve be stimulated above or below the ligature. Now, 
since the upper parts of both sciatics are equally exposed to the action of 
the poison, it is clear that the failure of the left nerve to cause contraction 
is not attributable to any change having taken place in the upper portion 
of the nerve, else why should not the right, which has in its upper portion 
been equally exposed to the action of the poison, also fail? Evidently the 
poison acts on some parts of the nerve lower down. If a single muscle be 
removed from the circulation (by ligaturing its bloodvessels), previous to 
the poisoning with urari, that muscle will contract when any part of the 
nerve going to it is stimulated, though no other muscle in the body will 
contract when its nerve is stimulated. Here the whole nerve right down to 
the muscle has been exposed to the action of the poison, and yet it has lost 
none of its power over the muscle. On the other hand, if the muscle be 
allowed to remain in the body, and so be exposed to the action of the poison, 
but the nerve be divided high up and the part connected with the muscle 
gently lifted up before the urari is introduced into the system, so that no 
blood flows to it. and so that it is protected from tl influence of the 
poison, stimulation of the nerve will be found to produce no contractions in THE PHENOMENA OF MUSCLE AND NERVE. 
57 
the muscle, though stimuli applied directly to the muscle at once causes it 
to contract. From these facts it is clear that urari poisons the ends of the 
nerve within the muscle long before it affects the trunk ; and it is exceedingly 
probable that it is the very extreme ends of the nerves (possibly the end- 
plates or peculiar structures in which the nerve fibres end in the' muscular 
fibres, for urari poisoning, at least when profound, causes a slight, but yet 
distinctly recognizable effect in the microscopic appearance of these struc- 
tures) which are affected. The phenomena of urari poisoning go far to 
prove that muscles are capable of being made to contract by stimuli ap- 
plied directly to the muscular fibres themselves; and there are other facts 
which support this view. 
§ 42. When, in a recently killed frog, we stimulate by various means 
and in various ways the muscles and nerves, it will be observed that the 
movements thus produced, though very various, may be distinguished to be 
of two kinds. On the one hand, the result may be a mere twitch, as it 
were, of this or that muscle ; on the other hand, one or more muscles may 
remain shortened or contracted for a considerable time-a limb, for instance, 
being raised up or stretched out, and kept raised up or stretched out for 
many seconds. And we find, upon examination, that a stimulus may be 
applied either in such a way as to produce a mere twitch, a passing rapid 
contraction which is over and gone in a fraction of a second, or in such a 
way as to keep the muscle shortened or contracted for as long a time as, up 
to certain limits, we may choose. The mere twitch is called a single or 
simple muscular contraction; the sustained contraction, which, as we shall 
see, is really the result of rapidly repeated simple contractions, is called a 
tetanic contraction. 
§ 43. In order to study these contractions adequately, we must have 
recourse to the "graphic method," as it is called, and obtains a tracing or 
other record of the change of form of the muscle. To do this conveniently, 
it is best to operate with a muscle isolated from the rest of the body of a 
recently killed animal, and carefully prepared in such a way as to remain 
irritable for some time. The muscles of cold-blooded animals remain irri- 
table after removal from the body far longer than those of warm-blooded 
animals, and hence those of the frog are generally made use of. We shall 
study presently the conditions which determine this maintenance of the irri- 
tability of muscles and nerves after removed from the body. 
A muscle thus isolated, with its nerve left attached to it, is called a 
muscle-nerve preparation. The most convenient muscle for this purpose in 
the frog is, perhaps, the gastrocnemius, which should be dissected out so as 
to leave carefully preserved the attachment to the femur above, some portion 
of the tendon (tendo Achillis) below, and a considerable length of the sciatic 
nerve with its branches going to the muscle. (Fig. 11.) 
§ 44. We may apply to such a muscle-nerve preparation the various 
kinds of stimuli (mechanical, such as pricking or pinching; thermal, such 
as sudden heating; chemical, such as acids or other active chemical sub- 
stances ; or electrical) and these we may apply either to the muscle directly 
or to the nerve, thus affecting the muscle indirectly. Of all these stimuli 
by far the most convenient for general purposes are electrical stimuli of 
various kinds ; and these, except for special purposes, are best applied to the 
nerve, and not directly to the muscle. 
Of electrical stimuli, again, the currents, as they are called, generated by 
a voltaic cell, are most convenient, though the electricity generated by a 
rotating magnet, or that produced by friction may be employed. Making 
use of a cell or battery of cells-Daniell's, Grove's, Leclanche, or any other 
-we must distinguish between the current produced by the cell itself, the 58 
THE CONTRACTILE TISSUES. 
constant current, as we shall call it, and the induced current obtained from 
the constant current by means of an induction coil, as it is called ; for 
Fig. 11. 
A Muscle-nerve Preparation: m, the muscle, gastrocnemius of frog; n, the sciatic nerve, all 
the branches being cut away except that supplying the muscle ; J, femur; cl., clamp ; t. a., tendo 
Achillis; sp. c., end of spinal canal. 
the physiological effects of the two kinds of current are in many ways 
different. 
It may, perhaps, be worth while to remind the reader of the following facts: 
In a galvanic battery, the substance (plate of zinc, for instance) which is acted 
upon and used by the liquid is called the positive element, and the substance 
which is not so acted upon and used up (plate, etc., of copper, platinum, or carbon, 
etc.) is called the negative element. A galvanic action is set up when the positive 
(zinc) and the negative (copper) elements are connected outside the battery by 
some conducting material, such as a wire, and the current is said to flow in a cir- 
cuit or circle from the zinc or positive element to the copper or negative element 
inside the battery, and then from the copper or negative element back to the zinc 
or positive element through the wire outside the battery. If the conducting wire 
be cut through, the current ceases to flow ; but if the cut ends be brought into 
contact, the current is re-established and continues to flow so long as the contact 
is good. The ends of the wires are called "poles," or when used for physiological 
purposes, in which case they may be fashioned in various ways, are spoken of as 
electrodes. When the poles are brought into contact or are connected by some 
conducting material, galvanic action is set up, and the current flows through the 
battery and wires; this is spoken of as "making the current" or "completing or 
closing the circuit." When the poles are drawn apart from each other, or when 
some non-conducting material is interposed between them, the galvanic action is 
arrested; this is spoken of as "breaking the current" or "opening the circuit." 
The current passes from the wire connected with the negative (copper) element in 
the battery to the wire connected with the positive (zinc) element in the battery; 
hence, the pole connected with the copper (negative) element is called the positive 
pole, and that connected with the zinc (positive) element is called the negative pole. 
When used for physiological purposes the positive pole becomes the positive elec- THE PHENOMENA OF MUSCLE AND NERVE. 
59 
trode, and the negative pole the negative electrode. The positive electrode is often 
spoken of as the anode (ana, up), and the negative electrode as the kathode (kata, 
down). 
A piece of nerve of ordinary length, though not a good conductor, is still a 
conductor, and when placed on the electrodes completes the circuit, permitting 
the current to pass through it; in order to remove the nerve from the influence 
of the current it must be lifted off from the electrodes. This is obviously incon- 
venient; and hence it is usual to arrange a means of opening or closing the cir- 
cuit at some point along one of the two wires. This may be done in various ways 
-by fastening one part of the wire into a cup of mercury, and so by dipping the 
other part of the wire into the cup to close the circuit and make the current, and 
by lifting it out of the mercury to open the circuit and break the current; or by 
arranging between the two parts of the wires a movable bridge of good con- 
ducting material, such as brass, which can be put down to close the circuit or 
raised up to open the circuit: or in other ways. Such a means ofclosing and 
opening a circuit, and so of making or breaking a current, is called a key. 
A key which is frequently used by physiologists goes by the name of Du Bois- 
Reymond's key. Though undesirable in many respects, it has the advantage that it 
can be used in two different ways; when arranged as in A, Fig. 12, the brass bridge 
Fig. 12. 
Diagram of Du Bois-Reymond Key used, A, for making and breaking ; B, for short-circuiting 
of K, the key, being down, and forming a means of good conduction between the 
brass plates to which the wires are screwed, the circuit is closed and the current 
passes from the positive pole (end of the negative (copper) element) to the posi- 
tive electrode, or anode, An., through the nerve, to the negative electrode, or 
kathode, Kat., and thence back to the negative pole (end of the positive (zinc) 
element) in the battery; on raising the brass bridge, the circuit is opened, the 
current is broken, and no current passes through the electrodes. When arranged as 
in B, if the brass bridge be " down," the resistance offered by it is so small, compared 
with the resistance offered by the nerve between the electrodes, that the whole 
current from the battery passes through the bridge back to the battery, and none, 
or only an infinitesimal portion, passes into the nerve. When, on the other hand, 
the bridge is raised, ami so the conduction between the two sides suspended, the 
current is not able to pass directly from one side to the other, but can and does 
pass along the wire through the nerve back to the battery. Hence, in arrange- 
ment A, ''putting down the key," as it is called, makes a current in the nerve, 
and ''raising" or "opening the key" breaks the current. In arrangement B, 
however, putting down the key diverts the current from the nerve by sending it 
through the bridge, and so back to the battery; the current, instead of making 
the longer circuit through the electrodes, makes the shorter circuit through the 60 
THE CONTRACTILE TISSUES. 
key ; lienee, this is called "short-circuiting.'' When the bridge is raised the cur- 
rent passes through the nerve on the electrodes. Thus, "putting down and 
"raising '' or " opening " the key have contrary effects in A and B. In B, it will 
be observed, the battery is always at work, the current is always flowing either 
through the electrodes (key up) or through the key (key down); in A, the battery 
is not at work until the circuit is made by putting down the key. And in many 
cases it is desirable to take, so to speak, a sample of the current while the battery 
is in full swing, rather than just as it begins to work. Moreover, in B the elec- 
trodes are, when the key is down, wholly shut off from the current; whereas, in 
A, when the key is up, one electrode is still in direct connection with the battery, 
and this connection leading to what is known as unipolar action, may give rise to 
stimulation of the nerve. Hence the use of the key in the form B. 
Other forms of key may be used. Thus, in the Morse key (F, Fig. 13) contact 
is made by pressing down a lever handle (ha) ; when the pressure is removed, the 
handle, driven up by a spring, breaks contact. In the arrangement shown in the 
figure, one wire from the battery being brought to the binding screw b, while the 
binding screw a is connected with the other wire, putting down the handle makes 
connection between a and b, and thus makes a current. By arranging the wires 
in the several binding screw's in a different way, the making contact by depressing 
the handle may be used to short circuit. 
In an " induction coil,'' Figs. 13 and 14, the wire connecting the two elements 
of a battery is twisted at some part of its course into a close spiral, called the 
primary coil. Thus, in Fig. 13. the wire x//z, connected with the copper or nega- 
tive plate c. p. of the battery, A. joins the primary coil, pr. c., and then passes on 
as y'^. through the "key'' /I to the positive (zinc) plate, z.p., of the battery. 
Over this primary coil, but quite unconnected with it, slides another coil, the 
secondary coil, s. c. ; the ends of the wire forming this coil, y" and x", are con- 
tinued on in the arrangement illustrated in the figure as y' and y, and as x' and 
x, and terminate in electrodes. If these electrodes are in contact or connected 
with conducting material, the circuit of the secondary coil is said to be closed; 
otherwise it is open. 
In such an arrangement it is found that at the moment when the primary cir- 
cuit is closed, i. e., when the primary current is " made,'' a secondary "induced " 
current is, for an exceedingly brief period of time, set up in the secondary coil. 
Thus, in Fig. 13, when, by moving the "key" A, y'" and x'", previously not in 
connection with each other, are put into connection, and the primary current thus 
made, at that instant a current appears in the wires, y", x", etc., but almost imme- 
diately disappears. A similar almost instantaneous current is also developed when 
the primary current is "broken," but not till then. So long as the primary cur- 
rent flows with uniform intensity, no current is induced in the secondary coil. It 
is only when the primary current is either made or broken, or suddenly varies 
in intensity, that a current appears in the secondary coil. In each case the cur- 
rent is of very brief duration, gone in an instant almost, and may therefore be 
spoken of as "a shock," an induction shock; being called a "making shock," 
when it is caused by the making, and a " breaking shock," when it is caused by 
the breaking of the primary circuit. The direction of the current in the making 
shock is opposed to that of the primary current; thus, in the figure, while, the 
primary current flows from a/,zto y'", the induced making shock flow's from y to 
x. The current of the breaking shock, on the other hand, flows in the same 
direction as the primary current from x to y, and is therefore in direction the 
reverse of the making shock. Compare Fig. 14, where arrangement is shown in 
a diagrammatic manner. 
The current from the battery, upon its first entrance into the primary coil, as it 
passes along each twist of that coil, gives rise in the neighboring twists of the 
same coil to a momentary induced current having a direction opposite to its own. 
and therefore tending to weaken itself. It is not until this "self-induction" has 
passed oft that the current in the primary coil is established in its full strength. 
Owing to this delay in the full establishment of the current in the primary coil, 
the induced current in the secondary coil is developed more slowly than it would 
be were no such " self-induction " present. On the other hand, when the current 
from the battery is "broken or "shut off'" from the primary coil, no such delay 
is offered to its disappearance, and consequently the induced current in the second- THE PHENOMENA OF MUSCLE AND NERVE. 
61 
Fig. 13. 
Diagram illustrating Apparatus arranged for Experiments with Muscle and Nerve: A. The 
moist chamber containing the muscle-nerve preparation. The muscle m, supported by the clamp 
cl., which firmly grasps the end of the femur f, is connected by means of the S-hooks and a thread 
with the lever I, placed below the moist chamber. The nerve n, with a portion of the spinal col- 
umn n' still attached to it, is placed on the electrode-holder el, in contact with the wires x, y. The 
whole of the interior of the glass case gl, is kept saturated witli moisture, and the electrode-holder 
is so constructed that a piece of moistened blotting-paper may be placed on it without coming 
into contact with the nerve. 
B. The revolving cylinder bearing the smoked paper on which the lever writes. 
C. Du Bois-Reymond's key arranged for short-circuiting. The wires x and y of the electrode- 62 
THE CONTRACTILE TISSUES. 
ary coil is developed with unimpeded rapidity. We shall see later on that a 
rapidly developed current is more effective as a stimulus than is a more slowly 
Fig. 14. 
Diagram of an Induction Coil: + positive pole, end of negative element; - negative pole, 
end of positive element of battery; K, Du Bois-Reymond's key; pr. c. primary coil, current 
shown by feathered arrow; sc. c. secondary coil, current shown by unfeathered arrow. 
developed current. Hence the making shock, where rapidity of production is 
interfered 'with by the self-induction of the primary coil, is less effective as a 
stimulus than the breaking shock whose development is not thus interfered with. 
The strength of the induced current depends, on the one hand, on the strength 
of the current passing through the primary coil-that is, on the strength of the 
battery. It also depends on the relative position of the two coils. Thus a second- 
ary coil is brought nearer and nearer to the primary coil and made to overlap it 
more and more; the induced current becomes stronger and stronger, though the 
current from the battery remains the same. With an ordinary battery, the sec- 
ondary coil may be pushed to some distance away from the primary coil, and yet 
shocks sufficient to stimulate a muscle will be obtained. For this purpose, how- 
ever, the two coils should be in the same line ; when the secondary coil is placed 
crosswise, at right angles to the primary, no induced current is developed, and at 
intermediate angles the induced current has intermediate strengths. 
When the primary current is repeatedly and rapidly made and broken, the 
secondary current being developed with each make and with each break, a rapidly 
recurring series of alternating currents is developed in the secondary coil and 
holder are connected through binding screws in the floor of the moist chamber with the wires 
x', y', and these are secured in the key, one on either side. To the same key are attached the 
wires x", y", coming from the secondary coils s. c. of the induction coil I). This secondary coil 
can be made to slide up and down over the primary coil pr. c., with which are connected the two 
wires x"' and y'"; z"'is connected directly with one pole-for instance, the copper pole c. p. of the 
battery E; y"' is carried to a binding screw, a, of the Morse key F, and is continued as yiv from 
another binding screw, b, of the key to the zinc pole z. p. of the battery. 
Supposing everything to be arranged, and the battery charged; on depressing the handle ha, 
of the Morse key F, a current will be made in the primary coil pr. c., passing from c. p. through 
x'" to pr. c., and thence through y" to a, thence to b, and so through y^ to z. p. On removing the 
linger from the handle of F, a spring thrusts up the handle, and the primary circuit is in conse- 
quence immediately broken. 
At the instant that the primary current is either made or broken, an induced current is for the 
instant developed in the secondary coil s. c. If the cross-bar h in the Du Bois-Reymond's key be 
raised (as shown in the thick line in the figure), the wires x", x', x, the nerve between the elec- 
trodes and the wires y, y', y", form the complete secondary circuit, and the nerve consequently 
experiences a making or breaking induction-shock whenever the primary current is made or 
broken. If the cross-bar of the Du Bois-Reymond key be shut down, as in the dotted line h' in 
the figure, the resistance of the cross-bar is so slight compared with that of the nerve and of the 
wires going from the key to the nerve, that the whole secondary (induced) current passes from 
x" to y" (or from y" to x") along the cross-bar, and practically none passes into the nerve. The 
nerve, being thus " short-circuited," is not affected by any changes in the current. 
The figure is intended merely to illustrate the general method of studying muscular contraction ; 
it is not to be supposed that the details here given are uni versail v adopted, or, indeed, the best 
for all purposes. 63 
THE PHENOMENA OF MUSCLE AND NERVE. 
passes through its electrodes. We shall frequently speak of this as the interrupted 
induction current, or more briefly the interrupted current; it is sometimes spoken 
of as the faradic current, and the application of it to any tissue is spoken of as 
faradization. 
Such a repeated breaking and making of the primary current may be effected 
in many various ways. In the instruments commonly used for the purpose, the 
primary current is made and broken by means of a vibrating steel slip working 
against a magnet : hence the instrument is called a magnetic interruptor. See 
Fig. 15. 
The two wires x and y from the battery are connected with the two brass pillars 
a and d by means of screws. Directly contact is thus made current, indicated 
Fig. 15. 
The Magnetic Interrupter. 
in the figure by the thick interrupted line, passes in the direction of the arrows, 
up the pillar a. along the steel spring b, as far as the screw c, the point of which, 
armed with platinum, is in contact with a small platinum plate on b. The cur- 
rent passes from b through c and a connecting wire into the primary coil p. Upon 
its entering into the primary coil, an induced (making) current is for the instant 
developed in the secondary coil (not shown in the figure). From the primary coil 
p the current passes, by a connecting wire, through the double spiral m, and. did 
nothing happen, would continue to pass from m by a connecting wire to the pillar 
d, and so by the wire y to the battery. The whole of this course is indicated by 
the thick interrupted line with its arrows. 
As the current, however, passes through the spirals m, the iron cores of these 
are made magnetic. They, in consequence, draw down the iron bar e, fixed at the 
end of the spring />, the flexibility of the spring allowing this. But when e is 
drawn down, the platinum plate on the upper surface of b is also drawn away from 
the screw c, and thus the current is "broken" at b. (Sometimes the screw / is 
so arranged that when e is drawn down a platinum plate on the under surface of b 
is brought into contact with the platinum armed point of the screw f. The cur- 
rent then passes from b, not to c. but to f and so down the pillar d, in the direction 
indicated by the thin interrupted line, and out of the battery by the wire y, and is 
thus cut off from the primary coil. But this arrangement is unnecessary.) At the 
instant that the current is thus broken and so cut off from the primary coil, an 
induced (breaking) current is for the moment developed in the secondary coil. But 
the current is cut off not only from the primary coil, but also from the spirals m ; 
in consequence, their cores cease to be magnetized, the bare ceases to be attracted 
by them, and the spring b, by virtue of its elasticity, resumes its former position 64 
THE CONTRACTILE TISSUES. 
in contact with the screw c. This return of the spring, however, re-establishes 
the current in the primary coil and in the spirals, and the spring is drawn down, 
to be released once more in the same manner as before. Thus, as long as the 
current is passing along x, the contact of b with c is alternately being made and 
broken, and the current is constantly passing into and being shut off from p, 
the periods of alternation being determined by the periods of vibration of the 
spring b. With each passage of the current into or withdrawal from the pri- 
mary coil, an induced (making and, respectively, breaking) current is developed 
in a secondary coil. 
As thus used, each " making shock," as explained above, is less powerful than 
the corresponding "breaking shock;" and, indeed, it sometimes happens that 
instead of each make, as well as each break, acting as a stimulus, giving rise 
to a contraction, the "breaks" only are effective, the several "makes" giving 
rise to no contraction. 
But what is known as Helmholtz's arrangement (Fig. 16), however, the making 
and breaking shocks may be equalized. For this purpose the screw c is raised out 
Fig. 16. 
The Magnetic Interrupter with Helmholtz's Arrangement for Equalizing 
the Make and Break Shock. 
of reach of the excursions of the spring 6, and a moderately thick wire w, offering 
a certain amount only of resistance, is interposed between the upper binding screw 
a' on the pillar a, and the binding screw c' leading to the primary coil. Under 
these arrangements the current from the battery passes through a'along the inter- 
posed wire to cz, through the primary coil, and thus, as before, to m. As before, 
by the magnetism of m, e is drawn down and b brought in contact with f. As the 
result of this contact, the current from the battery can now pass by a, f ami d 
(shown by the thin interrupted line), back to the battery; but not the whole of 
the current, some of it can still pass along the wire w to the primary coil, the rela- 
tive amount being determined by the relative resistance offered by tbe two courses. 
Hence at each successive magnetization of m, the current in the primary coil does 
not entirely disappear when b is brought in contact with /; it is only so far dimin- 
ished that m ceases to attract e, and hence by the release of b from f the whole 
current once more passes along w. Since, at what corresponds to the '"break " the 
current in the primary coil is diminished only, not absolutely done away with, 
self-induction makes its appearance at the "break" as well as at the "make;" 
thus the "breaking" and "making" induced currents or shocks in the secondary 
coil are equalized. They are both reduced to the lower efficiency of the " mak- 
ing " shock in the old arrangement; hence to produce the same strength of THE PHENOMENA OF MUSCLE AND NERVE. 
65 
stimulus with this arrangement a stronger current must be applied or the sec- 
ondary coil pushed over the primary coil to a greater extent than with the other 
arrangement. 
The Phenomena of a Simple Muscular Contraction. 
§ 45. If the far end of the nerve of a muscle-nerve preparation (Figs. 
11 and 13) be laid on electrodes connected with the secondary coil of an 
induction-machine, the passage of a single induction-shock, which may be 
taken as a convenient form of an almost momentary stimulus, will produce 
no visible change in the nerve, but the muscle will give a twitch, a short, 
sharp contraction, i. e., will for an instant shorten itself, becoming thicker 
the while, and then return to its previous condition. If one end of the mus- 
cle be attached to a lever, while the other is fixed, the lever will by its move- 
ments indicate the extent and duration of the shortening. If the point of 
the lever be brought to bear on some rapidly travelling surface, on which it 
leaves a mark (being for this purpose armed with a pen and ink if the sur- 
face be plain paper, or with a bristle or finely pointed piece of platinum foil 
if the surface be smoked glass or paper), so long as the muscle remains at 
rest the lever will describe an even line, which we may call the base line. 
If, however, the muscle shortens the lever will rise above the base line and 
thus describe some sort of curve above the base line. Now it is found that 
when a single induction-shock is sent through the nerve the twitch which the 
muscle gives causes the lever to describe some such curve as that shown in 
Fig. 17 ; the lever (after a brief interval immediately succeeding the open- 
ing or shutting the key, of which we shall speak presently) rises at first rap- 
idly but afterward more slowly, showing that the muscle is correspondingly 
shortening; then ceases to rise, showing that the muscle is ceasing to grow 
shorter ; then descends, showing that the muscle is lengthening again, and 
finally, sooner or later, reaches and joins the base line, showing that the 
muscle after the shortening has regained its previous natural growth. Such 
a curve described by a muscle during a twitch or simple muscular contrac- 
tion, caused by a single induction-shock or by any other stimulus producing 
the same effect, is called a curve of a simple muscular contraction, or more 
shortly, a " muscle-curve." It is obvious that the exact form of the curve 
described by identical contractions of a muscle will depend on the rapidity 
with which the recording surface is travelling. Thus if the surface be travel- 
ling slowly the up-stroke corresponding to the shortening will be very abrupt 
Fig. 17. 
Fig. 18. 
A Muscle-curve from the Gastrocnemius of the Frog: This curve, like all succeeding ones, 
unless otherwise indicated, is to be read from left to right-that is to say, while the lever and 
tuning-fork were stationary the recording surface was travelling from right to left. 
a indicates the moment at which the induetion-shock is sent into the nerve, b the com- 
mencement, c the maximum, and d the close of the contraction. 
Below the muscle-curve is the curve drawn by a tuning-fork making 100 double vibrations 
a second, each complete curve representing therefore one-hundredth of a second. 
and the down stroke also very steep, as in Fig. 18, which is a curve from a 
gastrocnemius muscle of a frog, taken with a slowly moving drum, the 66 
THE CONTRACTILE TISSUES. 
tuning-fork being the same as that used in Fig. 17 ; indeed, with a very 
slow movement, the two may be hardly separable from each other. On 
the other hand, if the surface travel very rapidly the 
curve may be immediately long drawn out, as in Fig. 
19, which is a curve from a gastrocnemius muscle of 
a frog, taken with a very rapidly moving pendulum 
myograph, the tuning-fork making about 500 vibra- 
tions a second. On examination, however, it will be 
found that both these extreme curves are fundamen- 
tally the same as the medium one, when account is 
taken of the different rapidities of the travelling sur- 
face in the several cases. 
In order to make the " muscle-curve " complete, 
it is necessary to mark on the recording surface the 
exact time at which the induction shock is sent into 
the nerve, and also to note the speed at which the 
recording surface is travelling. 
In the pendulum myograph the rate of movement 
can be calculated from the length of the pendulum ; 
but even in this it is convenient, and in the case of 
the spring myograph and revolving cylinder is neces- 
sary, to measure the rate of movement directly by 
means of a vibrating tuning-fork, or of some body 
vibrating regularly. Indeed it is best to make such 
a direct measurement with each curve that is taken. 
A tuning-fork, as is known, vibrates so many times 
a second according to its pitch. If a tuning-fork, 
armed with a light marker on one of its prongs and 
vibrating say 100 a second-i. e., executing a double 
vibration, moving forward and backward, 100 times 
a second-be brought while vibrating to make a trac- 
ing on the recording surface immediately below the 
lever belonging to the muscle, we can use the curve 
or rather curves described by the tuning-fork to 
measure the duration of any part or of the whole of 
the muscle-curve. It is essential that at starting the 
point of the marker of the tuning-fork should be ex- 
actly underneath the marker of the lever, or rather, 
since the point of the lever as it moves up and down 
describes not a straight line but an arc of a circle of 
which its fulcrum is the centre and itself (from the 
fulcrum to the tip of the marker) the radius, that 
the point of the marker of the tuning-fork should 
be exactly on the arc described by the marker of the 
lever, either above or below it, as may prove most 
convenient. If, then, at starting the tuning-fork 
marker be thus on the arc of the lever marker, and 
we note on the curve of the tuning fork the place 
where the arc of the lever cuts it at the beginning 
and at the end of the muscle-curve, as at Fig. 17, 
we can count the number of vibrations of the tuning- 
fork which have taken place between the two marks, 
and so ascertain the whole time of the muscle-curve; 
if, for instance, there have been 10 double vibrations, each occupying 
second, the whole curve has taken second to make. In the same way we 
Fig. 19. THE PHENOMENA OF MUSCLE AND NERVE. 
67 
can measure the duration of the rise of the curve or of the fall, or of any 
part of it. 
Though the tuning-fork may, by simply striking it, be set going long 
enough for the purposes of an observation, it is convenient to keep it going 
by means of an electric current and a magnet, very much as the spring in 
the magnetic interrupter (Fig. 15) is kept going. 
It is not necessary to use an actual tuning-fork ; any rod, armed with a 
marker, which can be made to vibrate regularly, and whose time of vibra- 
tion is known, may be used for the purpose: thus a reed, made to vibrate 
by a blast of air, is sometimes employed. 
The exact moment at which the induction shock is thrown into the 
nerve may be recorded on the muscle-curve by means of a " signal," which 
may be applied in various ways. 
A large steel lever armed with a marker is arranged over a small coil by means 
of a light spring in such a way that when the coil by the passage of a' current 
through it becomes a magnet it pulls the lever down to itself; on the current being 
broken, and the magnetization of the coil ceasing, the lever by help of the spring 
flies up. The marker of such a lever is placed immediately under-i. e., at some 
point on the arc described by-the marker of the muscle (or other) lever. Hence 
by making a current in the coil and putting the signal lever down, or by breaking 
an already existing current, and letting the signal lever fly up. we can make at 
pleasure a mark corresponding to any part we please of the muscle (or other) curve. 
If, in order to magnetize the coil of the signal, we use. as we may do, the pri- 
mary current which generates the induction-shock, the breaking or making of the 
primary current, whichever we use to produce the induction-shock, will make the 
signal lever fly up or come down. Hence we shall have on the recording surface, 
under the muscle, a mark indicating the exact moment at which the primary cur- 
rent was broken or made. Now the time taken up by the generation of the induced 
current and its passage into the nerve between the electrodes is so infinitesimally 
small, that we may, without appreciable error, take the moment of the breaking or 
making of the primary current as the moment of the entrance of the induction- 
shock into the nerve. Thus we can mark below the muscle-curve, or by describing 
the arc of the muscle lever, on the muscle-curve itself, the exact moment at which 
the induction-shock falls into the nerve between the electrodes, as is done at a in 
Figs. 17. 18, 19. 
In the pendulum myograph a separate signal is not needed. If, having placed 
the muscle lever in the position in which we intend to make it record, we allow 
the glass plate to descend until the tooth a' just touches the rod c (so that the rod 
is just about to be knocked down, and so break the primary circuit) and make on 
the base line, which is meanwhile being described by the lever marker, a mark to 
indicate where the point of the marker is under these circumstances, and then 
bring back the plate to its proper position, the mark which we have made will 
mark the moment of the breaking of the primary circuit, and so of the entrance 
of the induction-shock into the nerve. For it is just when, as the glass plate 
swings down, the marker of the lever comes to the mark which we have made 
that the rod c is knocked back and the primary current is broken. 
A ''signal" like the above, in an improved form known as Desprez's, maybe 
used also to record time, and thus the awkwardness of bringing a large tuning- 
fork up to the recording surface obviated. For this purpose the signal is intro- 
duced into a circuit the current of which is continually being made and broken by 
a tuning-fork (Fig. 21). The tuning-fork once set vibrating continues to make and 
break the current at each of its vibrations, and as stated above is kept vibrating by 
the current. But each make or break caused by the tuning-fork affects also the 
small coil of the signal, causing the lever of the signal to fall down or fly up. Thus 
the signal describes vibration-curves synchronous with those of the tuning-fork 
driving it. The signal may similarly be worked by means of vibrating agents other 
than a tuning-fork. 
Various recording surfaces may be used. The form most generally useful is a 
cylinder covered with smoked paper and made to revolve by clockwork or other- 
wise ; such a cylinder driven by clockwork is shown in Fig. 13, B. By using a 68 
THE CONTRACTILE TISSUES. 
Fig. 20. 
The Pendulum Myograph: The figure is diagrammatic, the essentials only of the instrument 
being shown. The smoked glass plate A swings with the pendulum B on carefully adjusted 
bearings at C. The contrivances by which the glass plate can be removed and replaced at 
pleasure are not shown. A second glass plate so arranged that the first glass plate may be 
moved up and down without altering the swing of the pendulum is also omitted. Before com- 
mencing an experiment the pendulum is raised up (in the figure to the right), and is kept in that 
position by the tooth a catching on the spring-catch b. On depressing the catch b the glass plate 
is set free, swings into the new position indicated by the dotted lines, and is held in that posi- 
tion by the tooth a' catching on the catch b'. In the course of its swing the tooth a' coming in 
contact with the projecting steel rod c, knocks it on one side into the position indicated by the THE PHENOMENA OF MUSCLE AND NERVE. 
cylinder of large radius with adequate gear, a high speed for instance, in a second, 
can be obtained, in the spring myograph a smoked glass plate is thrust rapidly 
69 
Fig. 21. 
Diagram of an Arrangement of a Vibrating Tnning-fork with a Desprez Signal: The current 
flows along the wire J connected with the positive (+) pole or end of the negative plate (IV) of 
the battery, through the tuning-fork, down the pin connected with the end of the lower prong, 
to the mercury in the cup Hg, and so by a wire (shown in figure) to the binding screw e. From 
this binding screw part of the current flows through the coil d between the prongs of the tuning- 
fork, and thence by the wire c to the binding screw a, while another part flows through the wire 
g, through the coil of the Desprez signal back by the wire b, to the binding screw a. From the 
binding screw a the current passes back to the negative (-) pole or end of the positive element 
(P) of the battery. As the current flows through the coil of the Desprez signal from g to b, the 
core of coil becoming magnetized draws down the marker of the signal. As the current flows 
through the coil d, the core of that coil, also becoming magnetized, draws up the lower prong of 
the fork. But the pin is so adjusted that the drawing up of the prong lifts the point of the pin 
out of the mercury. In consequence, the current, being thus broken at Hg, flows neither 
through d nor through the Desprez signal. The core of the Desprez thus ceasing to be magnetized, 
the marker flies back, being usually assisted by a spring (not shown in the figure). But since 
the current ceases to flow through d, the core of d ceases to lift up the prong, and the pin, in the 
descent of the prong, makes contact once more with the mercury. The re-establishment of the 
current, however, once more acting on the two coils, again pulls down the marker of the signal, 
and again by magnetizing the core of d pulls up the prong and once more breaks the current. 
Thus the current is continually made and broken, the rapidity of the interruptions being 
determined by the vibration periods of the tuning-fork, and the lever of the signal rising and 
falling synchronously with the movements of the tuning-fork. 
forward along a groove by means of a spring suddenly thrown into action. In the 
pendulum myograph, Fig. 20, a smoked glass plate attached to the lower end of a 
long frame swinging like a pendulum, is suddenly let go at a certain height, and 
so swings rapidly through an arc of a circle. The disadvantage of the last two 
methods is that the surface travels at a continually changing rate, whereas, in the 
revolving cylinder, careful construction and adjustment will secure a very uniform 
rate. 
§ 46. Having thus obtained a time record, and an indication of the exact 
moment at which the induction-shock falls into the nerve, we may for present 
purposes consider the muscle-curve complete. The study of such a curve, 
as, for instance, that shown in Fig. 17, taken from the gastrocnemius of a 
frog, teaches us the following facts: 
1. That although the passage of the induced current from electrode to 
electrode is practically instantaneous, its effect, measured from the entrance 
of the shock into the nerve to the return of the muscle to its natural length 
dotted line c'. The rod c is in electric continuity with the wire x of the primary coil of an 
induction-machine. The screw d is similarly in electric continuity with the wire y of the same 
primary coil. The sdrew d and the rod c are armed with platinum at the points at which they 
are in contact, and both are insulated by means of the ebonite block e. As long as c and d are 
in contact the circuit of the primary coil to which x and y belong is closed. When in its swing 
the tooth a' knocks c away from d, at that instant the circuit is broken, and a " breaking " shock 
is sent through the electrodes connected with the secondary coil of the machine, and so through 
the nerve. The lever I, the end only of which is shown in the figure, is brought to bear on the 
glass plate, and when at rest describes a straight line, or more exactly an arc of a circle of large 
radius. The tuning-fork f, the ends only of the two limbs of which are shown in the figure 
placed immediately below the lever, serves to mark the time. 70 
THE CONTRACTILE TISSUES. 
after the shortening, takes an appreciable time. In the figure, the whole 
curve from a to d takes up about the same time as eleven double vibrations 
of the tuning-fork. Since each double vibration here represents second, 
the duration of the whole curve is rather more than second. 
2. In the first portion of this period, from a to b, there is no visible 
change, no raising of the lever, no shortening of the muscle. 
3. It is not until b, that is to say, after the lapse of about second, 
that the shortening begins. The shortening, as shown by the curve is at 
first slow, but soon becomes more rapid, and then slackens again until it 
reaches a maximum at c; the whole shortening occupying rather more than 
second. 
4. Arrived at the maximum of shortening, the muscle at once begins to 
relax, the lever descending at first slowly, then more rapidly, and at last 
more slowly again, until at d the muscle has regained its natural length ; 
the whole return from the maximum of contraction to the natural length 
occupying rather more than T40 second. 
Thus a simple muscular contraction, a simple spasm or twitch, produced 
by a momentary stimulus, such as a single induction-shock, consists of three 
main phases: 
1. A phase antecedent to any visible alteration in the muscle. This 
phase, during which invisible preparatory changes are taking place in the 
nerve and muscle, is called the " latent period." 
2. A phase of shortening or, in the more strict meaning of the word, 
contraction. 
3. A phase of relaxation or return to the original length. 
In the case we are considering, the electrodes are supposed to be applied 
to the nerve at some distance from the muscle. Consequently the latent 
period of the curve comprises not only the preparatory actions going on in 
the muscle itself, but also the changes necessary to conduct the immediate 
effect of the induction-shock from the part of the nerve between the elec- 
trodes along a considerable length of nerve down to the muscle. It is obvi- 
ous that these latter changes might be eliminated by placing the electrodes 
on the muscle itself or on the nerve close to the muscle. If this were done, 
the muscle and lever being exactly as before, and care were taken that the 
induction-shock entered into the nerve at the new spot, at the moment when 
the point of the lever had reached exactly the same point of the travelling 
surface as before, two curves would be gained having the relations shown in 
Fig. 22. The two curves resemble each other in almost all points, except 
that in the curve taken with the shorter piece of nerve, the latent period, 
the distance a to b as compared with the distance a to b' is shortened ; the 
contraction begins rather earlier. A study of the two curves teaches us the 
following two facts: 
1. Shifting the electrodes from a point of the nerve at some distance 
from the muscle to a point of the nerve close to the muscle has only short- 
ened the latent period a very little. Even when a very long piece of nerve 
is taken the difference in the two curves is very small, and, indeed, in order 
that it may be clearly recognized or measured, the travelling surface must 
be made to travel very rapidly. It is obvious, therefore, that by far the 
greater part of the latent period is taken up by changes in the muscle itself, 
changes preparatory to the actual visible shortening. Of course, even when 
the electrodes are placed close to the muscle, the latent period includes the 
changes going on in the short piece of nerve still lying between the elec- 
trodes and the muscular fibres. To eliminate this with a view of determin- 
ing the latent period in the muscle itself, the electrodes might be placed 
directly on the muscle poisoned with urari. If this were done, it would be THE PHENOMENA OF MUSCLE AND NERVE. 
71 
found that the latent period remained about the same, that is to say, that in 
all cases the latent period is chiefly taken up by changes in the muscular as 
distinguished from the nervous elements. 
2. Such difference as does exist between the two curves in the fio-ure 
indicates the time taken up by the propagation, along the piece of nerve, of 
Fig. 22. 
Curves Illustrating the Measurement of the Velocity of a Nervous Impulse: The same muscle- 
nerve preparation is stimulated (1) as far as possible from the muscle, (2) as near as possible to 
the muscle; both contractions are registered in exactly the same way. 
In (1) the stimulus enters the nerve at the time indicated by the line a, the contraction begins 
at b'; the whole latent period, therefore, is indicated by the distance from a to b'. 
In (2) the stimulus enters the nerve at exactly the same time a; the contraction begins at b ; 
the latent period, therefore, is indicated by the distance between a and b. 
The time taken up by the nervous impulse in passing along the length of nerve between 1 and 
2 is, therefore, indicated by the distance between b and b', which may be measured by the tuning- 
fork curve below; each double vibration of the tuning-fork corresponds to 1-120 or 0.0083 second. 
the changes set up at the far end of the nerve by the induction-shock. 
These changes we have already spoken of as constituting a nervous impulse ; 
and the above experiment shows that it takes a small but yet distinctly 
appreciable time for a nervous impulse to travel along a nerve. In the 
figure the difference between the two latent periods, the distance between b 
and b', seems almost too small to measure accurately ; but if a long piece 
of nerve be used for the experiment, and the recording surface be made to 
travel very fast, the difference between the duration of the latent period 
when the induction-shock is sent in at a point close to the muscle, and that 
when it is sent in at a point as far away as possible from the muscle, may 
be satisfactorily measured in fractions of a second. If the length of nerve 
between the two points be accurately measured, the rate at which a nervous 
impulse travels along the nerve to a muscle can thus be easily calculated. 
This has been found to be in the frog about 28, and in man about 33 
metres per second, but varies considerably, especially in warm-blooded 
animals. 
Thus when a momentary stimulus, such as a single induction-shock, is sent 
into a nerve connected with a muscle, the following events take place: a 
nervous impulse is started in the nerve and this travelling down to the muscle 
produces in the muscle, first the invisible changes which constitute the latent 
period, secondly the changes which bring about the shortening or contraction 
proper, and thirdly the changes which bring about the relaxation and return 
to the original length. The changes taking place in each of these three phases 
are changes of living matter ; they vary with the condition of the living sub- 
stance of the muscle, and only take place so long as the muscle is alive. 
Though the relaxation which brings back the muscle to its original length 
is assisted by the muscle being loaded with a weight or otherwise stretched, 
this is not essential to the actual relaxation, and with the same load the 
return will vary according to the condition of the muscle; the relaxation 
must be considered as an essential part of the whole contraction no less than 
the shortening itself. 
§ 47. Not only, as we shall see later on, does the whole contraction vary 72 
THE CONTRACTILE TISSUES. 
in extent and character according to the condition of the muscle, the strength 
of the induction-shock, the load which the muscle is bearing, and various 
attendant circumstances, but the three phases may vary independently. The 
latent period may be longer or shorter, the shortening may take a longer or 
shorter time to reach the same height, and especially the relaxation may be 
slow or rapid, complete or imperfect. Even when the same strength of 
induction-shock is used the contraction may be short and sharp or very long 
drawn out, so that the curves described on a recording surface travelling at 
the same rate in the two cases appear very different; and under certain cir- 
cumstances, as when a muscle is fatigued, the relaxation, more particularly 
the last part of it, may be so slow, that it may be several seconds before the 
muscle really regains its original length. 
Hence, if we say that the duration of a simple muscular contraction of the 
gastrocnemius of a frog under ordinary circumstances is about second, of 
which is taken up by the latent period, by the contraction, and 
by the relaxation, these must be taken as " round numbers," stated so as to 
be easily remembered. The duration of each phase as well as of the whole 
contraction varies in different animals, in different muscles of the same 
animal, and in the same muscle under different conditions. 
The muscle-curve which we have been discussing is a curve of changes in 
the length only of the muscle ; but if the muscle, instead of being suspended, 
were laid flat on a glass plate and a lever laid over its belly, we should find, 
upon sending an induction-shock into the nerve, that the lever was raised, 
showing that the muscle during the contraction became thicker. And, if we 
took a graphic record of the movements of the lever, we should obtain a 
curve very similar to the one just discussed ; after a latent period the lever 
would rise, showing that the muscle was getting thicker, and afterward would 
fall, showing that the muscle was becoming thin again. In other words, in 
contraction the lessening of the muscle lengthwise is accompanied by an 
increase crosswise ; indeed, as we shall see later on, the muscle in contracting 
is not diminished in bulk at all (or only to an exceedingly small extent, 
about of its total bulk), but makes up for its diminution in length by 
increasing in its other diameters. 
§ 48. A single induction shock is, as we have said, the most convenient 
form of stimulus for producing a simple muscular contraction, but this may 
also be obtained by other stimuli, provided that these are sufficiently sudden 
and short in their action, as, for instance, by a prick of, or a sharp blow on, 
the nerve or muscle. For the production of a single simple muscular con- 
traction the changes in the nerve leading to the muscle must be of such a 
kind as to constitute what may be called a single nervous impulse, and any 
stimulus which will evoke a single nervous impulse only may be used to 
produce a simple muscular contraction. 
As a rule, however, most stimuli, other than single induction-shocks, tend 
to produce in a nerve several nervous impulses, and, as we shall see, the 
nervous impulses which issue from the central nervous system, and so pass 
along nerves to muscles, are, as a rule, not single and simple, but complex. 
Hence, as a matter of fact, a simple muscular contraction is within the living 
body a comparatively rare event (at least as far as the skeletal muscles are 
concerned), and cannot easily be produced outside the body otherwise than 
by a single induction-shock. The ordinary form of muscular contraction is 
not a simple muscular contraction, but the more complex form known as a 
tetanic contraction, to the study of which we must now turn. THE PHENOMENA OF MUSCLE AND NERVE. 
73 
§ 49. If a single induction-shock be followed at a certain interval by a 
second shock of the same strength, the first simple contraction will be fol- 
lowed by a second simple contraction, both contractions being separate and 
distinct; and if the shocks be repeated a series of rhythmically recurring 
separate simple contractions may be obtained. If, however, the interval 
between two shocks be made short, for instance, only just long enough to 
allow the first contraction to have passed its maximum before the latent 
period of the second is over, the curves of the two contractions will bear 
some such relation to each other as that shown in Fig. 23. It will be 
Tetanic Contraction. 
Fig. 23. 
Tracing of a Double Muscle-curve. While the muscle (gastrocnemius of frog) was engaged in 
the first contraction (whose complete course, had nothing intervened, is indicated by the dotted 
line), a second induction-shock was thrown in, at such a time that the second contraction began 
just as the first was beginning to decline. The second curve is seen to start from the first, as 
does the first from the base-line. 
observed that the second curve is almost in all respects like the first except 
that it starts, so to speak, from the first curve instead of from the base-line. 
The second nervous impulse has acted on the already contracted muscle, 
and made it contract again just as it would have done if there had been no 
first impulse and the muscle had been at rest. The two contractions are 
added together and the lever is raised nearly double the height it would 
have been by either alone. If in the same way a third shock follows the 
second at a sufficiently short interval, a third curve is piled on top of the 
second ; the same with a fourth, and so on. A more or less similar result 
would occur if the second contraction began at another phase of the first. 
The combined effect is, of course, greatest when the second contraction begins 
at the maximum of the first, being less both before and afterward. 
Hence, the result of a repetition of shocks will depend largely on the 
rate of repetition. If, as in Fig. 24, the shocks follow each other so slowly 
Fig. 24. 
Muscle-curve. Single Induction-shock Repealed Slowly. 
that one contraction is over, or almost over, before the next begins, each 
contraction will be distinct, or nearly distinct, and there will be little or no 
combined effect. 
If, however, the shocks be repeated more rapidly, as in Fig. 25, each sue- 74 
THE CONTRACTILE TISSUES. 
ceeding contraction will start from some part of the preceding one, and the 
lever will be raised to a greater height at each contraction. 
Fig. 25. 
Muscle-curve. Single Induction-shock Repeated More Rapidly. 
If the frequency of the shocks be still further increased, as in Fig. 26, 
the rise due to the combination of contraction will be still more rapid, and 
a smaller part of each contraction will be visible on the curve. 
In each of these three curves it will be noticed that the character of the 
curve changes somewhat during its development. The change is the result 
Fig. 26. 
Muscle-curve. Single Induction-shock Still More Rapidly. 
of commencing fatigue, caused by the repetition of the contractions, the 
fatigue manifesting itself by an increasing prolongation of each contrac- 
tion, shown especially in a delay of relaxation, and by an increasing dimi- 
nution in the height of the contraction. Thus, in Fig. 24, the contractions, 
quite distinct at first, become fused later; the fifth contraction, for instance, 
is prolonged so that the sixth begins before the lever has reached the base 
line; yet the summit of the sixth is hardly higher than the summit of the 
fifth, since the sixth, though starting at a higher level, is a somewhat 
weaker contraction. See also, in Fig. 25, the level' rises rapidly at first but 
more slowly afterward, owing to an increasing diminution in the height of 
the single contractions. In Fig. 26 the increment of rise of the curve due 
to each contraction diminishes very rapidly, and though the lever does con- 
tinue to rise during the whole series, the ascent after about the sixth con- 
traction is very gradual indeed, and the indications of the individual con- 
tractions are much less marked than at first. 
Hence, when shocks are repeated with sufficient rapidity, it results that 
after a certain number of shocks, the succeeding impulses do not cause any 
further shortening of the muscle, any further raising of the lever, but 
merely keep up the contraction already existing. The curve thus reaches 
a maximum, which it maintains, subject to the depressing effects of exhaus- 
tion, so long as the shocks are repeated. When these cease to be given, the 
muscle returns to its natural length. THE PHENOMENA OF MUSCLE AND NERVE. 
75 
When the shocks succeed each other still more rapidly than in Fig. 26, 
the individual contractions, visible at first, may become fused together and 
wholly lost to view in the latter part of the curve. When the shocks succeed 
each other still more rapidly (the second contraction beginning in the 
ascending portion of the first) it becomes difficult or impossible to trace 
out any of the single contractions.1 The curve then described by the lever 
is of the kind shown in Fig. 27, where the primary current of an induction- 
Fig. 27. 
Tetanus Produced with the Ordinary Magnetic Interrupter of an Induction-machine. (Recording 
surface travelling slowly.) The interrupted current is thrown in at a. 
machine was rapidly made and broken by the magnetic interrupter, Fig. 15. 
The lever, it will be observed, rises at a (the recording surface is travelling 
too slowly to allow the latent period to be distinguished), at first very 
rapidly, in fact in an unbroken and almost a vertical line, and so very 
speedily reaches the maximum, which is maintained so long as the shocks 
continue to be given ; when these cease to be given, the curve descends at 
first very rapidly and then more and more gradually toward the base-line, 
which it reaches just at the end of the figure. 
This condition of muscle, brought about by rapidly repeated shocks, this 
fusion of a number of simple twitches into an apparently smooth continuous 
effort, is known as tetanus or tetanic contraction. The above facts are most 
clearly shown w'hen induction-shocks, or at least galvanic currents in some 
form or other, are employed. They are seen, however, whatever be the form 
of stimulus employed. Thus, in the case of mechanical stimuli, while a 
single quick blow may cause a single twitch, a pronounced tetanus may be 
obtained by rapidly striking successively fresh portions of a nerve. With 
chemical stimulation, as when a nerve is dipped in acid, it is impossible to 
secure a momentary application ; hence tetanus, generally irregular in cha- 
racter, is the normal result of this mode of stimulation. In the living body, 
the contractions of the skeletal muscles, brought about either by the will or 
otherwise, are generally tetanic in character. Even very short sharp move- 
ments, such as a sudden jerk of a limb or a wink of the eyelid, are in reality 
examples of tetanus of short duration. 
If the lever, instead of being fastened to the tendon of a muscle hung 
vertically, be laid across the belly of a muscle placed in a horizontal position 
and the muscle be thrown into tetanus by a repetition of induction-shocks, it 
will be seen that each shortening of the muscle is accompanied by a corre- 
sponding thickening, and that the total shortening of the tetanus is accom- 
panied by a corresponding total thickening. And, indeed, in tetanus we can 
observe more easily than in a single contraction that the muscle in contract- 
1 The ease with which the individual contractions can be made out depends in part, it 
need hardly be said, on the rapidity with which the recording surface travels. 76 
THE CONTRACTILE TISSUES. 
ing changes in form only-not in bulk. If a living muscle or group of 
muscles be placed in a glass jar or chamber, the closed top of which is pro- 
longed into a narrow glass tube, and the chamber be filled with water (or 
preferably with a solution of sodium chloride, 0.6 per cent, in strength, 
usually called " normal saline solution," which is less injurious to the tissue 
than simple water) until the water rises into the narrow tube, it is obvious 
that any change in the bulk of the muscle will be easily shown by a rising 
or falling of the column of fluid in the narrow tube. It is found that when 
the muscle is made to contract, even in the most forcible manner, the change 
of level in the height of the column which can be observed is practically 
insignificant; there appears to be a fall indicating a diminution of bulk to 
the extent of about one ten-thousandth of the total bulk of the muscle. So that 
we may fairly say that in a tetanus, and hence in a simple contraction, the 
lessening of the length of the muscle causes a corresponding increase in the 
other directions; the substance of the muscle is displaced, not diminished. 
§ 50. So far we have spoken simply of an induction-shock or of induction- 
shocks without any reference to their strength, and of a living or irritable 
muscle without any reference to the degree or extent of its irritability. But 
induction-shocks may vary in strength, and the irritability of the muscle 
may vary. 
If we slide the secondary coil a long way from the primary coil, and thus 
make use of extremely feeble induction-shocks, we shall probably find that 
these shocks, applied even to a quite fresh muscle-nerve preparation, produce 
no contraction. If we then gradually slide the secondary coil nearer and 
nearer the primary coil, and keep on trying the effects of the shocks, we shall 
find that after a while, in a certain position of the coils, a very feeble con- 
traction makes its appearance. As the secondary coil comes still nearer to 
the primary coil, the contractions grow greater and greater. After a while, 
however-and that, indeed, in ordinary circumstances, very speedily increas- 
ing-the strength of the shock no longer increases the height of the contrac- 
tion ; the maximum contraction of which the muscle is capable with such 
shocks, however strong, has been reached. 
If we use a tetanizing or interrupted current, we shall obtain the same 
general results; we may, according to the strength of the current, get no 
contraction at all, or contractions of various extent up to a maximum, which 
cannot be exceeded. Under favorable conditions the maximum contraction 
may be very considerable ; the shortening in tetanus may amount to three- 
fifths of the total length of the muscle. 
The amount of contraction, then, depends on the strength of the stimulus, 
whatever be the stimulus; but this holds good within certain limits only; 
to this point, however, we shall return later on. 
§ 51. If, having ascertained in a perfectly fresh muscle-nerve prepara- 
tion the amount of contraction produced by this and that strength of stim- 
ulus, we leave the preparation by itself for some time-say for a few hours 
-and then repeat the observations, we shall find that stronger stimuli- 
stronger shocks, for instance-are required to produce the same amount of 
contraction as before ; that is to say, the irritability of the preparation, the 
power to respond to stimuli, has in the meanwhile diminished. After a 
further interval we should find the irritability still further diminished ; even 
very strong shocks would be unable to evoke contractions as large as those 
previously caused by weak shocks. At last we should find that no shocks, 
no stimuli, however strong, were able to produce any visible contraction what- 
ever. The amount of contraction, in fact, evoked by a stimulus depends not 
only on the strength of the stimulus, but also on the degree of irritability of 
the muscle-nerve preparation. CHANGES IN A MUSCLE DURING CONTRACTION. 
77 
Immediately upon removal from the body, the preparation possesses a 
certain amount of irritability, not differing very materially from that which 
the muscle and nerve possess while within and forming an integral part of 
the body ; but after removal from the body the preparation loses irritability, 
the rate of loss being dependent on a variety of circumstances; and this goes 
on until, since no stimulus which we can apply will give rise to a contrac- 
tion, we say the irritability has wholly disappeared. 
We might take this disappearance of irritability as marking the death of 
the preparation, but it is followed sooner or later by a curious change in the 
muscle, which is called rigor mortis, and which we shall study presently ; and 
it is convenient to regard this rigor mortis as marking the death of the muscle. 
The irritable muscle, then, when stimulated either directly, the stimulus 
being applied to itself, or indirectly, the stimulus being applied to its nerve, 
responds to the stimulus by a change of form which is essentially a shorten- 
ing and thickening. By the shortening (and thickening) the muscle in con- 
tracting is able to do work, to move the parts to which it is attached ; it thus 
sets free energy. We have now to study more in detail how this energy is 
set free, and the laws which regulate its expenditure. 
On the Changes which take place in a Muscle during a 
Contraction. 
The Change in Form. 
§ 52. An ordinary skeletal muscle consists of elementary muscle fibres, 
bound together in variously arranged bundles by connective tissue which 
carries bloodvessels, nerves, and lymphatics. 
The contraction of a muscle is the contraction of all or some of its ele- 
mentary fibres, the connective tissue being passive; hence while those fibres 
of muscle which end directly in the tendon, in contracting pull directly on 
the tendon, those which do not so end pull indirectly on the tendon by 
means of the connective tissue between the bundles, which connective 
tissue is continuous with the tendon. 
Each muscle is supplied by one or more branches of nerves composed of 
medu Hated fibres, with a certain proportion of non-medullated fibres. These 
branches running in the connective tissue divide into smaller branches and 
twigs between the bundles and fibres. Some of the nerve fibres are dis- 
tributed to the bloodvessels, and others end in a manner of which we 
shall speak later on in treating of muscular sensations; but by far the 
greater part of the medullated fibres and in the muscular fibres, the 
arrangement being such that every muscular fibre is supplied with at least 
one medullated nerve fibre, which joins the muscular fibre somewhere about 
the middle between its two ends or sometimes nearer one end, in a special 
nerve ending, of which we shall presently have to speak, called an end- 
plate. The nerve fibres thus destined to end in the muscular fibres divide 
as they enter the muscle, so that what, as it enters the muscle, is a single 
nerve fibre, may, by dividing, end as several nerve fibres in several muscu- 
lar fibres. Sometimes two nerve fibres join one muscular fibre, but in this 
case the end-plate of each nerve fibre is still at some distance from the end 
of the muscular fibre. It follows that when a muscular fibre is stimulated 
by means of a nerve fibre, the nervous impulse travelling down the nerve 
fibre falls into the muscular fibre not at one end but at about its middle ; it is 
the middle of the fibre which is affected first by the nervous impulse, and 
the changes in the muscular substance started in the middle of the muscu- 
lar fibre travel thence to the two ends of the fibre. In an ordinary skele- 78 
THE CONTRACTILE TISSUES. 
tai muscle, however, as we have said, the fibres and bundles of fibres begin 
and end at different distances from the ends of the muscle, and the nerve or 
nerves going to the muscle divide and spread out in the muscle in such a 
way that the end-plates, in which the individual fibres of the nerve end, are 
distributed widely over the muscle at very different distances from the ends 
of the muscle. Hence, if we suppose a single nervous impulse, such as that 
generated by a single induction-shock, or a series of such impulses to be 
started at the same time at some part of the trunk of the nerve in each of 
the fibres of the nerve going to the muscle, these impulses will reach very 
different parts of the muscle at about the same time and the contractions 
which they set going will begin, so to speak, nearly all over the whole mus- 
cle at the same time, and will not all start in any particular zone or area of 
the muscle. 
§ 53. The wave of contraction. We have seen, however, that under the 
influence of urari the nerve fibre is unable to excite contractions in a mus- 
cular fibre, although the irritability of the muscular fibre itself is retained. 
Hence in a muscle poisoned by urari the contraction begins at that part of 
the muscular substance which is first affected by the stimulus, and we may 
start a contraction in what part of the muscle we please by properly placing 
the electrodes. 
Some muscles, such, for instance, as the sartorius of the frog, though of 
some length, are composed of fibres which run parallel to each other from 
one end of the muscle to the other. If such a muscle be poisoned with 
urari so as to eliminate the action of the nerves and stimulated at one end 
(an induction-shock sent through a pair of electrodes placed at some little 
distance apart from each other at the end of the muscle may be employed, 
but better results are obtained if a mode of stimulation, of which we shall 
have to speak presently, viz. the application of the " constant current," be 
adopted), the contraction which ensues starts from the end stimulated, and 
travels thence along the muscle. If two levers be made to rest on, or be 
suspended from, two parts of such a muscle placed horizontally, the parts 
being at a known distance from each other and from the part stimulated, 
the progress of the contraction may be studied. 
The movements of the levers indicate in this case the thickening of the 
fibres which is taking place at the parts on which the levers rest or to which 
they are attached ; and if we take a graphic record of these movements, 
bringing the two levers to mark one immediately below the other, we shall 
find that the lever nearer the part stimulated begins to move earlier, reaches 
its maximum earlier, and returns to rest earlier than does the further lever. 
The contraction, started by the stimulus, in travelling along the muscle from 
the part stimulated reaches the nearer lever some little time before it reaches 
the further lever, and has passed by the nearer lever some little time before 
it has passed by the further lever; and the further apart the two levers are 
the greater will be the difference in time between their movements. In other 
words, the contraction travels along the muscle in the form of a wave, each 
part of the muscle in succession from the end stimulated swelling out and 
shortening as the contraction reaches it, and then returning to its original 
state. And what is true of the collection of parallel fibres which we call 
the muscle is also true of each fibre, for the swelling at any part of the 
muscle is only the sum of the swelling of the individual fibres ; and if 
we were able to take a single long fibre and stimulate it at one end, we 
should be able, under the microscope, to see a swelling or bulging accom- 
panied by a corresponding shortening, i. e., to see a contraction, sweep 
along the fibre from end to end. 
If, in the graphic record of the two levers just mentioned, we count the CHANGES IN A MUSCLE DURING CONTRACTION. 
79 
number of vibrations of the tuning-fork which intervene between the mark 
on the record which indicates the beginning of the rise of the near lever 
(that is, the arrival of the contraction wave at this lever) and the mark 
which indicates the beginning of the rise of the far lever, this will give us 
the time which it has taken the contraction wave to travel from the near to 
the far lever. Let us suppose this to be 0.005 second. Let us suppose the 
distance between the two levers to be 15 mm. The contraction wave, then, 
has taken 0.005 second to travel 15 mm., that is to say it has travelled at the 
rate of 3 metres per second. And, indeed, we find by this, or by other 
methods, that in the frog's muscles the contraction wave does travel at a 
rate which may be put down as from 3 to 4 metres a second, though it 
varies under different conditions. In the warm-blooded mammal the rate 
is somewhat greater, and may probably be put down at 5 metres a second 
in the excised muscle, rising possibly to 10 metres in a muscle within the 
living body. 
If, again, in the graphic record of the two levers we count, in the case of 
either lever, the number of vibrations of the tuning-fork which intervene 
between the mark where the lever begins to rise and the mark where it has 
finished its fall and returned to the base-line, we can measure the time inter- 
vening between the contraction wave reaching the lever and leaving the lever 
on its way onward, that is to say we can measure the time which it has taken 
the contraction wave to pass over the part of the muscle on which the lever 
is resting. Let us suppose this time to be, say, 0.1 second. But a wrave which 
is travelling at the rate of 3 metres a second, and takes 0.1 second to pass 
over any point must be 300 mm. long. And, indeed, we find that in the 
frog the length of the contraction wave may be put down as varying from 
200 to 400 mm., and in the mammal it is not very different. 
Now, as we have said, the very longest muscular fibre is stated to be at 
most only about 40 mm. in length ; hence, in an ordinary contraction, dur- 
ing the greater part of the duration of the contraction the whole length of 
the fibre will be occupied by the contraction wave. Just at the beginning 
of the contraction there will be a time when the front of the contraction 
wave has reached, for instance, only half way down the fibre (supposing the 
stimulus to be applied, as in the case we have been discussing, at one end 
only), and just at the end of contraction there will be a time, for instance, 
when the contraction has left the half of the fibre next to the stimulus, but 
has not yet cleared away from the other half. But nearly all the rest of the 
time every part of the fibre will be in some phase or other of contraction, 
though the parts nearer the stimulus will be in more advanced phases than 
the parts further from the stimulus. 
This is true when a muscle of parallel fibres is stimulated artificially at 
one end of the muscles, and when, therefore, each fibre is stimulated at one 
end. It is, of course, all the more true when a muscle of ordinary construc- 
tion is stimulated by means of its nerve. The stimulus of the nervous 
impulse impinges in this case on the muscle fibre at the end-plate which, 
as we have said, is placed toward the middle of the fibre, and the contrac- 
tion wave travels from the end-plate in opposite directions toward each end, 
and has accordingly only about half the length of the fibre to run in. All 
the more, therefore, must the whole fibre be in a state of contraction at the 
same time. 
It will be observed that the contraction wave includes not only the con- 
traction proper and the thickening and shortening, but the relaxation and 
return to the natural form ; the first part of the wave up to the summit of 
the crest corresponds to the shortening and thickening; the decline from 
the summit onward corresponds to the relaxation. But we have already 80 
THE CONTRACTILE TISSUES. 
insisted that the relaxation is an essential part of the whole act; indeed, 
in a certain sense, as essential as the shortening itself. 
§ 54. Optical changes in a muscular fibre during contraction. So far we 
have been dealing with the muscle as a whole and as observed with the 
naked eye, though we have incidentally spoken of fibres. We have now, 
confining our attention exclusively to skeletal muscles, to consider what mi- 
croscopic changes take place during a contraction, what are the relations of 
the histological features of the muscle fibre to the act of contraction. Un- 
fortunately, our knowledge of the minute structure of the fibre is as yet so 
limited that any statements must of necessity be but speculative. When 
muscle contracts there is a translocation of molecules whereby there occurs 
not only a change of form, but other optical (polariscopical and microscopi- 
cal) alterations which are due to the movement of refractive particles. 
The long cylindrical sheath of sarcolemma is occupied by muscle sub- 
stance. After death the muscle substance may separate from the sarco- 
lemma, leaving the latter as a distinct sheath, but during life the muscle 
substance is adherent to the sarcolemma, so that no line of separation between 
the two can be made out; the movements of the one follow exactly all the 
movements of the other. 
Scattered in the muscle substance, but, in the mammal, lying for the most 
part close under the sarcolemma, are a number of nuclei, oval in shape, with 
their long axes parallel to the length of the fibre. Around each nucleus is 
a thin layer of granular-looking substance very similar in appearance to 
that forming the body of a white blood-corpuscle, and like that often spoken 
of as undifferentiated protoplasm. A small quantity of the same granular 
substance is prolonged for some distance, as a narrow conical streak from 
each end of the nucleus, along the length of the fibre. 
With the exception of these nuclei with their granular-looking bed and 
the end-plate or end-plates, to be presently described, all the rest of the 
space enclosed by the sarcolemma from one end of the fibre to the other 
appears to be occupied by a peculiar material, striated muscle substance. 
It is called striated because it is marked out, and that along the whole 
length of the fibre, by transverse bands [Fig. 28], stretching right across 
the fibre, of substance which is very transparent, bright 
substance, alternating with similar bands of substance 
which has a dim cloudy appearance, dim substance; 
that is to say, the fibre is marked out along its whole 
length by alternate bright bands and dim bands. The 
bright bands are on an average about 1 ji or 1.5 /z and 
the dim bands about 2.5 /z or 3 /z thick. By careful 
focusing, both bright bands and dim bands may be 
traced through the whole thickness of the fibre, so that 
the whole fibre appears to be composed of bright discs 
and dim discs placed alternately one upon the other along the whole length 
of the fibre, the arrangement being broken by the end-plate and here and 
there by the nuclei. 
§ 55. We may now return to the question, What happens when a con- 
traction wave sweeps over the fibre? 
Muscular fibres may be examined even under high powers of the micro- 
scope while they are yet living and contractile ; the contraction itself may 
be seen, but the rate at which the wave travels is too rapid to permit satis- 
factory observations to be made as to the minute changes which accompany 
the contraction. It frequently happens however that when living muscle 
has been treated with certain reagents, as for instance with osmic acid vapor, 
and subsequently prepared for examination, fibres are found in which a 
TFro. 28. 
Diagrammatic Repre- 
sentation of a Muscle- 
case : an, dim bands; 
is, bright bands.] CHANGES IN A MUSCLE DURING CONTRACTION. 
81 
bulging, a thickening and shortening, over a greater or less part of the 
length of the fibre, has been fixed by the osmic acid or other reagent. Such 
a bulging obviously differs from a normal contraction in being confined to a 
part of the length of the fibre, whereas, as we have said, a normal wave of 
contraction, being very much longer than any fibre, occupies the whole 
length of the fibre at once. We may however regard this bulging as a very 
short, a very abbreviated wave of contraction, and assume that the changes 
visible in such a short bulging also take place in a normal contraction. 
Admitting this assumption, we learn from such preparations that in the 
contracting region of the fibre, while both dim and bright bands become 
broader across the fibre, and correspondingly thinner along the length of the 
fibre, a remarkable change takes place between the dim bands, bright bands, 
and granular lines. We have seen that in the fibre at rest the intermediate 
line in the bright band is in most cases inconspicuous; in the contracting 
fibre, on the contrary, a dark line in the middle of the bright band in the 
position of the intermediate line becomes very distinct. As we pass along 
the fibre from the beginning of the contraction wrave to the summit of the 
wave, where the thickening is greatest, this line becomes more and more 
striking, until at the height of the contraction it becomes a very marked 
dark line or thin dark band. Parc passu with this change, the distinction 
between the dim and the bright bands become less and less marked ; these 
appear to become confused together, until at the height of the contraction, 
the whole space between each two now conspicuous dark lines is occupied by 
a substance which can be called neither dim nor bright, but which in con- 
trast to the dark line appears more or less bright and transparent. So that 
in the contracting part there is, at the height of the contraction, a reversal 
of the state of things proper to the part at rest. The place occupied by the 
bright band, in the state of rest, is now largely filled by a conspicuous dark 
line which previously was represented by the inconspicuous intermediate 
line, and the place occupied by the conspicuous dim band of the fibre at rest 
now seems by comparison with the dark line the brighter part of the fibre. 
The contracting fibre is, like the fibre at rest, striated, but its striation is dif- 
ferent in its nature from the natural striation of the resting fibre; and it is 
held by some that in the earlier phases of the contraction, while the old nat- 
ural striation is being replaced by the new striation, there is a stage in which 
all striation is lost. 
We may add that the outline of the sarcolemma, which in the fibre at 
rest is quite even, becomes during the contraction indented opposite the 
intermediate line, and bulges out in the interval between each two interme- 
diate lines, the bulging and indentation becoming more marked the greater 
the contraction. 
§ 56. We can learn something further about this remarkable change by 
examining the fibre under polarized light. 
When ordinary light is sent through a Nicol prism (which is a rhomb of Fee- 
land spar divided into two in a certain direction, the halves being subsequently 
cemented together in a special way) it undergoes a change in passing through the 
prism and is said to be polarized. One effect of this polarization is that a ray 
of light which has passed through one Nicol prism will or will not pass through a 
second Nicol according to the relative position of the two prisms. Thus, if the 
second Nicol be so placed that what is called its "optic axis" be in a line with or 
parallel to the optic axis of the first Nicol the light passing through the first Nicol 
will also pass through the second. But if the second Nicol be rotated until its 
optic axis is at right angles with the optic axis of the first Nicol none of the light 
passing through the former will pass through the latter; the prisms in this position 
are said to be "crossed." In intermediate positions more or less light passes 
through the second Nicol according to the angle between the two optic axes. 82 
THE CONTRACTILE TISSUES. 
Hence when one Nicol is placed beneath the stage of a microscope so that the 
light from the mirror is sent through it, and another Nicol is placed in the eye- 
piece, the field of the microscope will appear dark when the eye-piece Nicol is 
rotated so that its optic axis is at right angles to the optic axis of the lower Nicol, 
and consequently the light passing through the lower Nicol is stopped by it. If, 
however, the optic axis of the eye-piece Nicol is parallel to that of the lower Nicol, 
the light from the latter will pass through the former and the field will be bright; 
and as the eye-piece is gradually rotated from one position to the other the bright- 
ness of the field will diminish or increase. 
Both the Nicols are composed of doubly refractive material. If now a third 
doubly refractive material be placed on the stage, and therefore between the two 
Nicols, the light passing through the lower Nicol will (in a certain position of the 
doubly refractive material on the stage, that is to say, when its optic axes have a 
certain position) pass through it. and also through the crossed Nicol in the eye- 
piece. Hence the doubly refractive material on the stage (or such parts of it as 
are in the proper position in respect to their optic axes) will, when the eye-piece 
Nicol is crossed, appear illuminated and bright on a dark field. In this way the 
existence of doubly refractive material in a preparation may be detected. 
When muscle prepared and mounted in Canada balsam is examined in 
the microscope between Nicol prisms, one on the stage below the object, and 
the other in the eye-piece, the fibres stand out as bright objects on the dark 
ground of the field when the axes of the prisms are crossed. On closer 
examination it is seen that the parts -which are bright are chiefly the dim 
bands. This indicates that it is the dim bands which are doubly refractive, 
anisotropic, or are chiefly made up of anisotropic substance; there seems, 
however, to be some slight amount of anisotropic substance in the bright 
bands, though these as a whole appear singly refractive or isotropic. The 
fibre accordingly appears banded or striated with alternate bands of aniso- 
tropic and isotropic material. According to most authors such an alterna- 
tion of anisotropic and (chiefly) isotropic bands which is obvious in a dead 
and prepared fibre.exists also in the living fibre ; but some maintain that the 
living fibre is uniformly anisotropic. 
Now, when a fibre contracts, in spite of the confusion previously men- 
tioned between dim and bright bands, there is no confusion between the 
anisotropic and isotropic material. The anisotropic, doubly refractive bands, 
bright under crossed Nicols, occupying the position of the dim bands in the 
resting fibre, remain doubly refractive, bright under crossed Nicols, even at 
the very height of the contraction. The isotropic, singly refractive bands, 
dark under crossed Nicols, occupying the position of the bright bands in the 
fibre at rest, remain isotropic and dark under crossed Nicols at the very 
height of the contraction. All that can be seen is that the singly refractive 
isotropic bands become very thin indeed during the contraction, while the 
anistropic bands, though of course becoming thinner and broader in the 
contraction, do not become so thin as do the isotropic bands ; in other words, 
while both bands become thinner and broader, the doubly refractive aniso- 
tropic band seems to increase at the expense of the singly refractive isotropic 
band. 
§ 57. The mere broadening and shortening of each section of the fibre 
is at bottom a translocation of the molecules of the muscle substance. If 
we imagine a company of 100 soldiers, ten ranks deep, with ten men in each 
rank, rapidly, but by a series of gradations, to extend out into a double line 
with 50 men in each line, we shall have a rough image of the movement of 
the molecules during a muscular contraction. But, from what has been said, 
it is obvious that the movement, in striated muscle at least, is a very com- 
plicated one ; in other forms of contractile tissue it may be, as we shall see, 
more simple. Why the movement is so complicated in striated muscle, what 
purposes it serves, why the skeletal muscles are striated, we do not at present CHANGES IN A MUSCLE DURING CONTRACTION. 
83 
know. Apparently where swift and rapid contraction is required the con- 
tractile tissue is striated muscle ; but how the striation helps, so to speak, the 
contraction we do not know. We cannot say what share in the act of con- 
traction is to be allotted to the several parts. Since, during a contraction, 
the fibre bulges out more opposite to each dim disc, and is indented opposite 
to each bright disc, since the dim disc is more largely composed of aniso- 
tropic material than the rest of the fibre, and since the anisotropic material 
in the position of the dim disc increases during a contraction, we might per- 
haps infer that the dim disc rather than the bright disc is the essentially 
active part. Assuming that the fibrillar substance is more abundant in the 
dim discs, while the interfibrillar substance is more abundant in the bright 
discs, and that the fibrillar substance is anisotropic (and hence the dim discs 
largely anisotropic), while the interfibrillar substance is isotropic, we might 
also be inclined to infer it is the fibrillar and not the interfibrillar substance 
which really carries out the contraction ; but even this much is not yet 
definitely proved. 
One thing must be remembered. The muscle substance, though it pos- 
sesses the complicated structure, and goes through the remarkable changes 
which we have described, is while it is living and intact in a condition which 
we are driven to speak of as semi-fluid. The whole of it is essentially mobile. 
The very act of contraction indeed shows this; but it is mobile in the sense 
that no part of it, except of course the nuclei and sarcolemma, neither dim 
nor bright substance, neither fibrillar nor interfibrillar substance, can be 
regarded as a hard and fast structure. A minute nematoid worm has been 
seen wandering in the midst of the substance of a living contractile fibre; 
as it moved along, the muscle substance gave way before it, and closed up 
again behind it, dim bands and bright bands all falling back into their proper 
places. We may suppose that in this case the worm threaded its way in a 
fluid interfibrillar substance between and among highly extensible and elastic 
fibrillte. But even on such a view, and still more on the view that the 
fibrillar substance also was broken and closed up again, the maintenance of 
such definite histological features as those which we have described in 
material so mobile can only be effected, even in the fibre at rest, at some con- 
siderable expenditure of energy, which energy it may be expected has a 
chemical source. During the contraction there is a still further expenditure 
of energy, some of which, as we have seen, may leave the muscle as " work 
done ; " this energy, likewise, may be expected to have a chemical source. 
We must, therefore, now turn to the chemistry of muscle. 
§ 58. We said in the Introduction that it was difficult to make out with 
certainty the exact chemical differences between dead and living substance. 
Muscle, however, in dying undergoes a remarkable chemical change, which 
may be studied with comparative ease. All muscles, within a certain time 
after "general " death of the body, lose their irritability, which is succeeded 
by an event somewhat more sudden, viz., the entrance into the condition 
known as rigor mortis. The occurrence of rigor mortis, or cadaveric rigidity, 
as it is sometimes called, which may be considered as a token of the death 
of the muscle, is marked by the following features: The living muscle 
possesses a certain translucency, the rigid muscle is distinctly more opaque. 
The living muscle is very extensible and elastic, it stretches readily and to a 
considerable extent when a weight is hung upon it, or when any traction is 
applied to it, but speedily and, under normal circumstances, completely 
returns to its original length when the weight of traction is removed ; 'as we 
The Chemistry of Muscle. 84 
THE CONTRACTILE TISSUES. 
shall see, however, the rapidity and completeness of the return depends on 
the condition of the muscle, a well-nourished active muscle regaining its 
normal length much more rapidly and completely than a tired and exhausted 
muscle. A dead rigid muscle is much less extensible, and at the same time 
much less elastic; the muscle now requires considerable force to stretch it, 
and when the force is removed, does not, as before, return to its former 
length. To the touch the rigid muscle has lost much of its former softness, 
and has become firmer and more resistant. The entrance into rigor mortis 
is, moreover, accompanied by a shortening or contraction, which may, under 
certain circumstances, be considerable. The energy of this contraction is 
not great, so that any actual shortening is easily prevented by the presence 
of even a slight opposing force. 
Now the chemical features of the dead rigid muscle are also strikingly 
different from those of the living muscle. 
§ 59. If a dead muscle, from which all fat, tendon, fascia, and connec- 
tive tissue have been as much as possible removed, and which has been 
freed from blood by the injection of " normal " saline solution, be minced 
and repeatedly washed with water, the washings will contain certain forms 
of albumin and certain extractive bodies of which we shall speak directly. 
When the washing has been continued until the wash-water gives no proteid 
reaction, a large portion of muscle will still remain undissolved. If this 
be treated with a 10 per cent, solution of a neutral salt, ammonium 
chloride being the best, a large portion of it will become dissolved ; the 
solution, however, is more or less imperfect and filters with difficulty. If 
the filtrate be allowed to fall drop by drop into a large quantity of dis- 
tilled water, a white flocculent matter will be precipitated. This flocculent 
precipitate is myosin. Myosin is a proteid, giving the ordinary proteid 
reactions, and having the same general elementary composition as other 
proteids. It is soluble in dilute saline solutions, especially those of ammo- 
nium chloride, and may be classed in the globulin family, though it is 
not so soluble as paraglobulin, requiring a stronger solution of a neutral 
salt to dissolve it; thus, while soluble in a 5 or 10 per cent, solution of such 
a salt, it is far less soluble in a 1 per cent, solution, which, as we have seen, 
readily dissolves paraglobulin. From its solutions in neutral saline solu- 
tion it is precipitated by saturation with a neutral salt, preferably 
sodium chloride, and may be purified by being washed with a saturated 
solution, dissolved again in a weaker solution, and reprecipitated by satu- 
ration. Dissolved in saline solutions it readily coagulates when heated-i. e., 
is converted into coagulated proteid-and it is worthy of notice that it 
coagulates at a comparatively low temperature, viz., about 56° C.; this, 
it will be remembered, is the temperature at which fibrinogen is coagulated, 
whereas paraglobulin, serum-albumin, and many other proteids, do not 
coagulate until a higher temperature (75° C.) is reached. Solutions of 
myosin are precipitated by alcohol, and the precipitate, as in the case of 
other proteids, becomes, by continued action of the alcohol, altered into 
coagulated insoluble proteid. 
We have seen that paraglobulin, and, indeed, any member of the 
globulin group, is very readily changed by the action of dilute acids into a 
body called acid-albumin, characterized by not being soluble either in water 
or in dilute saline solutions, but readily soluble in dilute acids and alkalies, 
from its solutions in either of which it is precipitated by neutralization, 
and by the fact that the solutions in dilute acids and alkalies are not coagu- 
lated by heat. When, therefore, a globulin is dissolved in dilute acid, 
what takes place is not a mere solution, but a chemical change ; the globulin 
cannot be got back from the solution, it has been changed into acid-albu- CHANGES IN A MUSCLE DURING CONTRACTION. 
85 
min. Similarly when globulin is dissolved in dilute alkalies it is changed 
into alkali-albumin; and, broadly speaking, alkali-albumin precipitated by 
neutralization can be changed by solution with dilute acids into acid-albu- 
min, and acid-albumin by dilute alkalies into alkali-albumin. 
Now myosin is similarly, and even more readily than is globulin, con- 
verted into acid-albumin, and by treating a muscle, either washed or not, 
directly with dilute hydrochloric acid, the myosin may be converted into 
acid-albumin and dissolved out. Acid-albumin obtained by dissolving 
muscle in dilute acid used to be called syntonin, and it used to be said that 
a muscle contained syntonin ; the muscle, however, contains myosin, not 
syntonin, but it may be useful to retain the word syntonin to denote acid- 
albumin obtained by the action of dilute acid on myosin. By the action of 
dilute alkalies, myosin may similarly be converted into alkali-albumin. 
From what has been above stated it is obvious that myosin has many 
analogies with fibrin, and we have yet to mention some striking analogies; 
it is, however, much more soluble than fibrin, and, speaking generally, it 
may be said to be intermediate in its character between fibrin and globulin. 
On keeping, and especially on drying, its solubility is much diminished. 
Of the substances which are left in washed muscle, from which all the 
myosin has been extracted by ammonium chloride solution, little is 
known. If washed muscle be treated directly with dilute hydrochloric 
acid, a large part of the material of the muscle passes, as we have said, 
at once into syntonin. The quantity of syntonin thus obtained may be 
taken as roughly representing the quantity of myosin previously existing 
in the muscle. A more prolonged action of the acid may dissolve out other 
proteids, besides myosin, left after the washing. The portion insoluble in 
dilute hydrochloric acid consists in part of the gelatin-yielding and other 
substances of the sarcolemma and of the connective and other tissues 
between the bundles, of the nuclei of these tissues and of the fibres them- 
selves, and in part, possibly, of some portions of the muscle substance itself. 
We are not, however, at present in a position to make any very definite 
statement as to the relation of the myosin to the structural features of 
muscle. Since the dim bands are rendered very indistinct by the action of 
a 10 per cent, sodium chloride solution, we may, perhaps, infer that myosin 
enters largely into the composition of the dim bands, and, therefore, of the 
fibrillne; but it would be hazardous to say much more than this. 
§ 60. Living muscle may be frozen, and yet, after certain precautions, 
will, on being thawed, regain its irritability, or, at all events, will for a time 
be found to be still living in the sense that it has not yet passed into rigor 
mortis. We may, therefore, take living muscle which has been frozen as 
still living. 
If living contractile muscle, freed as much as possible from blood, be frozen, 
and while frozen minced and rubbed up in a mortar with four times its 
weight of snow containing 1 per cent, of sodium chloride, a mixture is 
obtained which, at a temperature just below 0° C., is sufficiently fluid to be 
filtered, though with difficulty. The slightly opalescent filtrate, or muscle- 
plasma, as it is called, is at first quite fluid, but will, when exposed to the 
ordinary temperature, become a solid jelly, and afterward separate into a 
clot and serum. It will, in fact, coagulate like blood-plasma, with this dif- 
ference, that the clot is not firm and fibrillar, but loose, granular, and floc- 
culent. During the coagulation the fluid, which before was neutral or 
slightly alkaline, becomes distinctly acid. 
The clot is myosin. It gives ail the reactions of myosin obtained from 
dead muscle. 
The serum contains an albumin very similar to, if not identical with 86 
THE CONTRACTILE TISSUES. 
serum-albumin, a globulin differing somewhat from and coagulating at a 
lower temperature than paraglobulin, and which to distinguish it from the 
globulin of blood has been called myoglobulin, some other proteids which 
need not be described here, and various "extractives" of which we shall 
speak directly. Such muscles as are red also contain a small quantity of 
haemoglobin, and of another allied pigment called histohamatin, to which 
pigments, indeed, their redness is due. 
Thus, while dead muscle contains myosin, albumin, and other proteids, 
extractives, and certain insoluble matters, together with gelatinous and other 
substances not referable to the muscle substance itself, living muscle contains 
no myosin, but some substance or substances which bear somewhat the same 
relation to myosin that the antecedents of fibrin do to fibrin, and which give 
rise to myosin upon the death of the muscle. There are, indeed, reasons for 
thinking that the myosin arises from the conversion of a previously existing 
body which may be called myosinogen, and that the conversion takes place, 
or may take place, by the action of a special ferment, the conversion of 
myosinogen into myosin being very analogous to the conversion of fibrinogen 
into fibrin. 
We may, in fact, speak of rigor mortis as characterized by a coagulation 
of the muscle-plasma, comparable to the coagulation of blood-plasma, but 
differing from it inasmuch as the product is not fibrin, but myosin. The 
rigidity, the loss of suppleness, and the diminished translucency appear to 
be at all events largely, though probably not wholly, due to the change from 
the fluid plasma to the solid myosin. We might compare a living muscle 
to a number of fine transparent membranous tubes containing blood-plasma. 
When this blood-plasma entered into the " jelly" stage of coagulation, the 
system of tubes would present many of the phenomena of rigor mortis. 
They would lose much of their suppleness and translucency, and acquire 
a certain amount of rigidity. 
§ 61. There is, however, one very marked and important difference be- 
tween the rigor mortis of muscle and the coagulation of blood. Blood 
during its coagulation undergoes a slight change only in its reaction ; but 
muscle during the onset of rigor mortis becomes distinctly acid. 
A living muscle at rest is in reaction neutral, or, possibly from some 
remains of lymph adhering to it, faintly alkaline. If, on the other hand, 
the reaction of a thoroughly rigid muscle be tested, it will be found to be 
most distinctly acid. This development of an acid reaction is witnessed not 
only in the solid untouched fibre but also in expressed muscle-plasma; it 
seems to be associated in some way with the appearance of the myosin. 
The exact causation of this acid reaction has not at present been clearly 
worked out. Since the coloration of the litmus produced is permanent, 
carbonic acid, which, as we shall immediately state, is set free at the same 
time, cannot be regarded as the active acid, for the reddening of litmus 
produced by carbonic acid speedily disappears on exposure. On the other 
hand, it is possible to extract from rigid muscle a certain quantity of lactic 
acid, or rather of a variety of lactic acid known as sarcolactic acid;1 and 
it has been thought that the appearance of the acid reaction of rigid muscle 
is due to a new formation or to an increased formation of this sarcolactic 
acid. There is much to be said in favor of this view, but it cannot at 
present be regarded as established beyond dispute. 
Coincident with the appearance of this acid reaction, though, as we have 
said, not the direct cause of it, a large development of carbonic acid takes 
1 There are many varieties of lactic acid, which are isomeric, having the same compo- 
sition, CsHgOs, but differ in their reactions and especially in the solubility of their zinc 
salts. The variety present in muscle is distinguished as sarcolactic acid. CHANGES IN A MUSCLE DURING CONTRACTION. 
87 
place when muscle becomes rigid. Irritable living muscular substance, like 
all living substance, is continually respiring, that is to say, is continually 
consuming oxygen and giving out carbonic acid. In the body, the arterial 
blood going to the muscle gives up some of its oxygen, and gains a quantity 
of carbonic acid, thus becoming venous as it passes through the muscle 
capillaries. Even after removal from the body, the living muscle continues 
to take up from the surrounding atmosphere a certain quantity of oxygen 
and to give out a certain quantity of carbonic acid. 
At the onset of rigor mortis there is a very large and sudden increase in 
this production of carbonic acid, in fact an outburst, as it were, of that gas. 
This is a phenomena deserving special attention. Knowing that the car- 
bonic acid which is the outcome of the respiration of the whole body is the 
result of the oxidation of carbon-holding substances, we might very natu- 
rally suppose that the increased production of carbonic acid attendant on 
the development of rigor mortis is due to the fact that during that event a 
certain quantity of the carbon-holding constituents of the muscle are sud- 
denly oxidized. But such a view is negatived by the following facts : In 
the first place, the increased production of carbonic acid during rigor mortis 
is not accompanied by a corresponding increase in the consumption of 
oxygen. In the second place, a muscle (of a frog, for instance) contains in 
itself no free or loosely attached oxygen ; when subjected to the action of a 
mercurial air-pump it gives off no oxygen to a vacuum, offering in this 
respect a marked contrast to blood ; and yet, when placed in an atmosphere 
free from oxygen, it will not only continue to give off carbonic acid while it 
remains alive, but will also exhibit at the onset of rigor mortis the same 
increased production of carbonic acid that is shown by a muscle placed in 
an atmosphere containing oxygen. It is obvious that in such a case the 
carbonic acid does not arise from the direct oxidation of the muscle sub- 
stance, for there is no oxygen present at the time to carry on that oxidation. 
We are driven to suppose that during rigor mortis, some complex body, con- 
taining in itself ready-formed carbonic acid, so to speak, is split up, and 
thus carbonic acid is set free, the process of oxidation by which that car- 
bonic acid was formed out of the carbon-holding constituents of the muscle 
having taken place at some anterior date. 
Living resting muscle, then, is alkaline or neutral in reaction, and the 
substance of its fibres contains a coagulable plasma. Dead rigid muscle, on 
the other hand, is acid in reaction, and no longer contains a coagulable 
plasma, but is laden with the solid myosin. Further, the change from the 
living irritable condition to that of rigor mortis is accompanied by a large 
and sudden development of carbonic acid. 
It is found, moreover, that there is a certain amount or parallelism 
between the intensity of the rigor mortis, the degree of acid reaction and 
the quantity of carbonic acid given ont. If we suppose, as we fairly 
may do, that the intensity of the rigidity is dependent on the quantity of 
myosin deposited in the fibres, and the acid reaction to the development, if 
not of lactic acid, at least of some other substance, the parallelism between 
the three products, myosin, acid-producing substance, and carbonic acid, 
would suggest the idea that all three are the results of the splitting-up of th§ 
same highly complex substance. No one has at present, however, succeeded 
in isolating or in otherwise definitely proving the existence of such a body, and 
though the idea seems tempting, it may in the end prove totally erroneous. 
$ 62. As to the other proteids of muscle, such as the albumin and the 
globulin, we know as yet nothing concerning the parts which they play and 
the changes which they undergo in the living muscle or in rigor mortis. 
Besides the/a^ which is found, and that not infrequently in abundance, 88 
THE CONTRACTILE TISSUES. 
in the connective tissue between the fibres, there is also present in the 
muscular substance within the sarcolemma, always some and at times 
a great deal of fat, chiefly ordinary fat, viz., stearin, palmitin, and olein 
in variable proportion, but also the more complex fat lecithin. As to 
the function of these several fats in the life of the muscle we know little 
or nothing 
Carbohydrates, the third of the three great classes in which wre may group 
the energy-holding substances of which the animal body and its food are 
alike composed, viz., proteids, fat, and carbohydrates, are represented in 
muscle by a peculiar body, glycogen, which we shall have to study in detail 
later on. We must here merely say that glycogen is a body closely allied 
to starch, having a formula, which may be included under the general 
formula for starches, x (C6HI0O5), and may like it be converted by the action 
of acids, or by the action of particular ferments known as amylolytic fer- 
ments, into some form of sugar, dextrose (C6H12O6), or some allied sugar. 
Many, if not all, living muscles contain a certain amount, and some, under 
certain circumstances, a considerable amount, of glycogen. During or after 
rigor mortis this glycogen is very apt to be converted into dextrose, or an 
allied sugar. The muscles of the embryo at an early stage contain a rela- 
tively enormous quantity of glycogen, a fact which suggests that the glyco- 
gen of muscle is carbohydrate food of the muscle about to be wrought up 
into the living muscular substance. 
The bodies which we have called extractives are numerous and varied. 
They are especially interesting, since it seems probable that they are waste 
products of the metabolism of the muscular substance, and the study of 
them may be expected to throw light on the chemical change which mus- 
cular substance undergoes during life. Since, as we shall see, muscular 
substance forms by far the greater part of the nitrogenous, that is, proteid, 
portion of the body, the nitrogenous extractives of muscle demand peculiar 
attention. Now the body urea, which we shall have to study in detail later 
on, far exceeds in importance all the other nitrogenous extractives of the 
body as a whole, since it is practically the one form in which nitrogenous 
wastes leave the body ; if we include with urea the closely allied uric acid 
(which for present purposes may simply be regarded as a variety of urea), 
we may say that all the nitrogen taken in as food sooner or later leaves the 
body as urea; compared with this all other nitrogenous waste thrown out 
from the body is insignificant. Of the urea which thus leaves the body, a 
considerable portion must at some time or other have existed, or to speak 
more exactly, its nitrogen must have existed as the nitrogen of the proteids 
of muscular substance. Nevertheless, no urea at all is, in normal condi- 
tions, present in muscular substance either living and irritable or dead and 
rigid; urea does not arise in muscular substance itself as one of the imme- 
diate waste products of muscular substance. 
There is, however, always present in relatively considerable amount, on 
an average about 0.25 per cent, of wet muscle, a remarkable body, kreatin. 
This is, in one sense, a compound of urea; it may be split up into urea and 
sarcosin. This latter body is a methyl glycin, that is to say, a glycin in 
which methyl has been substituted for hydrogen, and glycin itself is amido- 
acetic acid, a compound of amidogen, that is a representative of ammonia 
and acetic acid Hence kreatin contains urea, which has close relations 
with ammonia, together with another representative of ammonia, and a sur- 
plus of carbon and hydrogen arranged as a body belonging to the fatty acid 
series. We shall have to return to this kreatin and consider its relation to 
urea and to muscle when we come to deal with urine. 
The other nitrogenous extractives, such as karnin, hypoxanthin (or CHANGES IN A MUSCLE DURING CONTRACTION. 
89 
sarkin), xanthin, taurin, etc., occur in small quantity, and need not be 
dwelt on here. 
Among non-nitrogenous extractives the most important is the sarcolactic 
acid, of which we have already spoken ; to this may be added sugar in some 
form or other, either coming from glycogen or from some other source. 
The ash of muscle, like the ash of the blood corpuscles, and, indeed the 
ash of the tissues in general, as distinguished from the blood, or plasma, or 
lymph on which the tissues live, is characterized by the preponderance of 
potassium salts and of phosphates ; these form, in fact, nearly 80 per cent, of 
the whole ash. 
§ 63. We may now pass on to the question, What are the chemical 
changes which take place when a living resting muscle enters into a con- 
traction ? These changes are most evident after the muscle has been sub- 
jected to a prolonged tetanus; but there can be no doubt that the chemical 
events of a tetanus are, like the physical events, simply the sum of the results 
of the constituent single contractions. 
In the first place the muscle becomes acid, not so acid as in rigor mortis, 
but still sufficiently so after a vigorous tetanus to turn blue litmus distinctly 
red. The cause of the acid reaction, like that of rigor mortis, is doubtful, 
but is in all probability the same in both cases. 
In the second place, a considerable quantity of carbonic acid is set free; 
and the production of carbonic acid in muscular contraction is altogether 
similar to the production of carbonic acid during rigor mortis; it is not 
accompanied by any corresponding increase in the consumption of oxygen. 
This is evident even in a muscle through which the circulation of blood is 
still going on ; for though the blood passing through a contracting muscle 
gives up more oxygen than the blood passing through a resting muscle, the 
increase in the amount of oxygen taken up falls below the increase in the 
carbonic acid given out. But it is still more markedly shown in a muscle 
removed from the body ; for in such a muscle both the contraction and the 
increase in the production of carbonic acid will goon in the absence of oxy- 
gen. A frog's muscle suspended in an atmosphere of nitrogen will remain 
irritable for some considerable time, and at each vigorous tetanus an increase 
in the production of carbonic acid may be readily ascertained. 
Moreover, there seems to be a correspondence between the energy of the 
contraction and the amount of carbonic acid and the degree of acid reaction 
produced, so that, though we are now treading on somewhat uncertain 
ground, we are naturally led to the view that the essential chemical process 
lying at the bottom of a muscular contraction as of rigor mortis is the split- 
ting up of some highly complex substance. But here the resemblance 
between rigor mortis and contraction ends. We have no satisfactory evi- 
dence of the formation during a contraction of any body like myosin. And 
this difference in chemical results tallies with an important difference between 
rigid muscle and contracting muscle. The rigid muscle, as we have seen, 
becomes less extensible, less elastic, less translucent; the contracting muscle 
remains no less translucent, elastic, and extensible than the resting muscle, 
indeed, there are reasons for thinking that the muscle in contracting becomes 
actually more extensible for the time being. 
But if during a contraction myosin is not formed, what changes of proteid 
or nitrogenous matter do take place? We do not know. We have no evi- 
dence that kreatin, or any other nitrogenous extractive, is increased by the 
contraction of muscle; we have no evidence of any nitrogen waste at all as 
the result of a contraction ; and, indeed, as we shall see later on, the study 
of the waste products of the body as a whole lead us to believe that the 
energy of the work done by the muscles of the body comes from the poten- 90 
THE CONTRACTILE TISSUES. 
tial energy of carbon compounds, and not of nitrogen compounds at all. 
But to this point we shall have to return. 
§ 64. We may sum up the chemistry of muscle somewhat as follows: 
During life the muscular substance is continually taking up from the 
blood, that is, from the lymph, proteid, fatty and carbohydrate material, 
saline matters and oxygen ; these it builds up into itself, how we do not 
know, and so forms the peculiar complex living muscular substance. The 
exact nature of this living substance is unknown to us. What we do know 
is that it is largely composed of proteid material, and that such bodies as 
myosinogen, myoglobulin, and albumin have something to do with the build- 
ing of it up. 
During rest this muscular substance, while taking in and building itself 
up out of or by means of the above-mentioned materials is continually giving 
off carbonic acid and continually forming nitrogenous waste such as kreatin. 
It also probably gives off some amount of sarcolactic acid, and possibly 
other non-nitrogenous waste matters. 
During a contraction there is a great increase in the quantity of 
carbonic acid given off, of lactic acid and some other substance formed 
giving an acid reaction, a greater consumption of oxygen, though the 
increase is not equal to the increase of carbonic acid, but as far as we 
can learn, no increase of nitrogenous waste. 
During rigor mortis there is a similar increased production of carbonic 
acid and of some other acid-producing substance, accompanied by remark- 
able conversion of myosinogen into myosin, by which the rigidity of the 
dead fibre is brought about. 
Thermal Changes. 
§ 65. The chemical changes during a contraction set free a quantity of 
energy, but only a portion of this energy appears in the "work done," a con- 
siderable portion takes on the form of heat. Though we shall have hereafter 
to treat this subject more fully, the leading facts may be given here. 
Whenever a muscle contracts its temperature rises, indicating that heat 
is given out. When a mercury thermometer is plunged into a mass of mus- 
cles, such as those of the thigh of the dog, a rise of the mercury is observed 
upon the muscles being thrown into a prolonged contraction. More exact 
results, however, are obtained by means of a thermopile, by the help of 
which the rise of temperature caused by a few repeated single contractions, 
or indeed by a single contraction, may be observed, and the amount of heat 
given out approximately measured. 
'fhe thermopile may consist either of a single junction in the form of a needle 
plunged into the substance of the muscle, or of several junctions, either in the 
shape of a flat surface carefully opposed to the surface of muscle (the pile being 
balanced so as to move with the contracting muscle, and thus to keep the contact 
exact), or in the shape of a thin wedge, the edge of which comprising the actual 
junctions, is thrust into a mass of muscles and held in position by them. In all 
cases the fellow-junction or junctions must be kept at a constant temperature. 
Another delicate method of determining the changes of temperature of a tissue 
is based upon the measurement of alterations in electric; resistance which a fine 
wire, in contact with or plunged into the tissue, undergoes as the temperature of 
the tissue changes. 
It has been calculated that the heat given out by the muscles of the 
thigh of a frog in a single contraction amounts to 3.1 micro-units of heat1 
for each gramme of muscle, the result being obtained by dividing by five 
the total amount of heat given out in five successive single contractions. It 
1 The micro-unit being a milligramme of water raised one degree Centigrade. CHANGES IN A MUSCLE DURING CONTRACTION. 
91 
will, however, be safer to regard these figures as illustrative of the fact that 
the heat given out is considerable, rather than as data for elaborate calcula- 
tions. Moreover, we have no satisfactory quantitative determinations of the 
heat given out by the muscles of warm-blooded animals, though there can 
be no doubt that it is much greater than that given out by the muscles of 
the frog. 
There can hardly be any doubt that the heat thus set free is the product 
of chemical changes within the muscle-changes which, though they cannot 
for the reasons given above (§ 63) be regarded as simple and direct oxida- 
tions, yet, since they are processes dependent on the antecedent entrance of 
oxygen into the muscle, may be spoken of in general terms as a combustion ; 
so that the muscle may be likened to a steam-engine, in which the combustion 
of a certain amount of material gives rise to the development of energy in 
two forms, as heat and as movement, there being certain quantitative rela- 
tions between the amount of energy set free as heat and that giving rise to 
movement. We must, however, carefully guard ourselves against pressing 
this analogy too closely. In the steam-engine we can distinguish clearly 
between the fuel which, through its combustion, is the sole source of energy, 
and the machinery, which is not consumed to provide energy, and only suffers 
wear and tear. In the muscle we cannot with certainty at present make such 
a distinction. It may be that the chemical changes at the bottom of a con- 
traction do not involve the real living material of the fibre, but only some 
substance manufactured by the living material and lodged in some way, we 
do not know how, in the living material. It may be that when a fibre con- 
tracts it is this substance within the fibre which explodes, and not the fibre 
itself. If we further suppose that this substance is some complex compound 
of carbon and hydrogen, into which no nitrogen enters, we shall have an 
explanation of the difficulty referred to above (§ 63), namely, that nitro- 
genous waste is not increased by a contraction. The special contractile 
carbon-hydrogen substance, may then be compared to the charge of a gun, 
the products of its explosion being carbonic and sarcolactic acids, while the 
real living material of the fibre may be compared to the gun itself, but to a 
gun which itself is continually undergoing change far beyond mere wear and 
tear, among the products of which change nitrogenous bodies like kreatin 
are conspicuous. This view will certainly explain why kreatin is not 
increased during the contraction, while the carbonic and lactic acids are. 
But it must be remembered that such a view is not yet proved ; it may be 
the living material of the fibre, as a whole, which is continually breaking 
down in an explosive decomposition, and as continually building itself up 
again out of the material supplied by the blood. 
In a steam-engine only a certain amount of the total potential energy of 
the fuel issues as work, the rest being lost as heat, the proportion varying, 
but the work rarely, if ever, exceeding one-tenth of the total energy, and 
generally being less. In the case of the muscle we are not at present in a 
position to draw up an exact equation between the latent energy on the one 
hand, and the two forms of actual energy on the other. We have reason to 
think that the proportion between heat and work varies considerably under 
different circumstances, the work sometimes rising as high as one-fifth, some- 
times possibly sinking as low as one-twenty-fourth of the total energy; and 
observations seem to show that the greater the resistance which the muscle 
has to overcome, the larger the proportion of the total energy expended 
which goes out as work done. The muscle, in fact, seems to be so far self- 
regulating that the more work it has to do the greater, within certain 
limits, is the economy with which it works. 
Lastly, it must be remembered that the giving out of heat by the muscle 92 
THE CONTRACTILE TISSUES. 
is not confined to the occasions when it is actually contracting. When, at a 
later period, we treat of the heat of the body generally, evidence will be 
brought forward that the muscles, even when at rest, are giving rise to 
heat, so that the heat given out at a contraction is not some wholly new 
phenomenon, but a temporary exaggeration of what is continually going 
on at a inore feeble rate. 
Electrical Changes. 
§ 66. Besides chemical and thermal changes, a remarkable electric 
change takes place whenever a muscle contracts. 
Muscle-currents.-If a muscle be removed in an ordinary manner from 
the body, and two non-polarizable electrodes,1 connected with a delicate 
galvanometer and many convolutions and high resistance, be placed on two 
Fig. 29. 
Non-polarizable Electrodes: a,the glass tube : z, the amalgamated zinc slips connected with 
their respective wires; z. s., the zinc sulphate solution ; ch. c., the plug of china-clay ; c', the por- 
tion of the china-clay plug projecting from the end of the tube ; this can be moulded into any 
required form. 
points of the surface of the muscle, a deflection of the galvanometer will 
take place, indicating the existence of a current passing through the gal- 
vanometer from the one point of the muscle to the other, the direction and 
amount of the deflection varying according to the position of the points. 
The "muscle-currents" thus revealed are seen to the best advantage when 
the muscle chosen is a cylindrical or prismatic one with parallel fibres, and 
when the two tendinous ends are cut off by clean incisions at right angles to 
the long axis of the muscle. The muscle then presents a transverse section 
(artificial) at each end and a longitudinal surface. We may speak of the 
latter as being divided into two equal parts by an imaginary transverse line 
on its surface called the " equator," containing all the points of the surface 
midway between the two ends. Fig. 30 is a diagrammatic representation of 
such a muscle, the line ab being the equator. In such a muscle the develop- 
ment of the muscle-currents is found to be as follows: 
The greatest deflection is observed when one electrode is placed at the 
1 These (Fig. 29) consist essentially of a slip of thoroughly amalgamated zinc, dipping 
into a saturated solution of zinc sulphate, which in turn is brought into connection with 
the nerve or muscle by means of a plug or bridge of china-clay moistened with normal 
sodium chloride solution; it is important that the zinc should be thoroughly amalga- 
mated. This form of electrode gives rise to less polarization than do simple platinum or 
copper electrodes. The clay affords a connection between the zinc and the tissue, which 
neither acts on the tissue nor is acted on by the tissue. Contact of any tissue with copper 
or platinum is in itself sufficient to develop a current. CHANGES IN A MUSCLE DURING CONTRACTION. 
93 
mid-point or equator of the muscle, and the other at either cut end ; and 
the deflection is of such a kind as to show that positive currents are con- 
tinually passing from the equator through the galvanometer to the cut end ; 
that is to say, the cut end is negative, relatively to the equator. The cur- 
rents outside the muscle may be considered as completed by currents in the 
muscle from the cut end to the equator. In the diagram, Fig. 30, the arrows 
Fig. 30. 
Diagram illustrating the Electric Currents of Nerve and Muscle: Being purely diagrammatic, 
it may serve for a piece either of nerve or of muscle, except that the currents at the transverse 
section cannot be shown in a nerve. The arrows show the direction of the current through the 
galvanometer. 
ab, the equator. The strongest currents are those shown by the dark lines, as from a, at equator, 
to x or to y at the cut ends. The current from a to c is weaker than from a to y, though both, as 
shown by the arrows, have the same direction. A current is shown from e, which is near the 
equator, to f, which is further from the equator. The current (in muscle) from a point in the cir- 
cumference to a point nearer the centre of the transverse section is shown at gh. From a to b, or 
from x to y, there is no current, as indicated by the dotted lines. 
indicate the direction of the currents. If one electrode be placed at the 
equator ab, the effect is the same at whichever of the two cut ends, x or y, 
the other is placed. If, one electrode remaining at the equator, the other be 
shifted from the cut end to a spot (c) nearer to the equator, the current con- 
tinues to have the same direction, but is of less intensity in proportion to the 
nearness of the electrodes to each other. If the two electrodes be placed at 
unequal distances (c and f), one on either side of the equator, there will be 
a feeble current from the one nearer the equator to the one further off, and 
the current will be the feebler the more nearly they are equidistant from the 
equator. If they are quite equidistant-as, for instance, when one is placed 
on one cut end Qr) and the other on the other cut end (y)-there will be no 
current at all. 
If one electrode be placed at the circumference of the transverse section 
and the other at the centre of the transverse section, there will be a current 
through the galvanometer from the former to the latter ; there will be a cur- 
rent of similar direction, but of less intensity, when one electrode is at the 
circumference (g) of the transverse section and the other at some point (h) 
nearer the centre of the transverse section. In fact, the points which are 
relatively most positive and most negative to each other are points on the 
equator and the two centres of the transverse sections ; and the intensity of 
the current between any two points will depend on the respective distances 
of those points from the equator and from the centre of the transverse 
section. 94 
THE CONTRACTILE TISSUES. 
Similar currents may be observed when the longitudinal surface is not 
the natural but an artificial one; indeed, they may be witnessed in even a 
piece of muscle, provided it be of cylindrical shape and composed of 
parallel fibres. 
These " muscle-currents" are not mere transitory currents, disappearing 
as soon as the circuit is closed ; on the contrary, they last a very consider- 
able time. They must, therefore, be maintained by some changes going on 
in the muscle-by continual chemical action, in fact. They disappear as 
the irritability of the muscle vanishes, and are connected with those nutri- 
tive, so-called vital, changes which maintain the irritability of the muscle. 
Muscle-currents, such as have just been described, may, we repeat, be 
observed in any cylindrical muscle suitably prepared, and similar currents, 
with variations which need not be discussed here, may be seen in muscles 
of irregular shape with obliquely or otherwise arranged fibres. And Du 
Bois-Reymond, to whom chiefly we are indebted for our knowledge of 
these currents, has been led to regard them as essential and important prop- 
erties of living muscle. He has, moreover, advanced the theory that muscle 
may be considered as composed of electro-motive particles or molecules, 
each of which, like the muscle at large, has a positive equator and neg- 
ative ends, the whole muscle being made up of these molecules in some- 
what the same way (to use an illustration, which must not, however, be 
strained or considered as an exact one) as a magnet may be supposed to 
be made up of magnetic particles, each with its north and south pole. 
There are reasons, however, for thinking that these muscle-currents have 
no such fundamental origin, that they are, in fact, of surface, and, indeed, 
of artificial origin. Without entering into the controversy on this question, 
the following important facts may be mentioned: 
1. When a muscle is examined while it still retains uninjured its natural 
tendinous terminations, the currents are much weaker than when artificial 
transverse sections have been made; the natural tendinous end is less nega- 
tive than the cut surface. But the tendinous end becomes at once negative 
when it is dipped in water or acid-indeed, when it is in any way injured. 
The less roughly, in fact, a muscle is treated the less evident are the 
muscle-currents; audit is maintained that if adequate care be taken to 
maintain a muscle in an absolutely natural condition, no such currents as 
those we have been describing exist at all-that natural living muscle is 
isoelectric, as it is called. 
2. The surface of the uninjured inactive1 ventricle of the frog's heart, 
which is practically a mass of muscle, is isoelectric; no current is obtained 
when the electrodes are placed on any two points of the surface. If, how- 
ever, any part of the surface be injured, or if the ventricle be cut across 
so as to expose a cut surface, the injured spot or the cut surface becomes 
at once more powerfully negative toward the uninjured surface, a strong 
current being developed which passes through the galvanometer from the 
uninjured surface to the cut surface or to the injured spot. The negativity 
thus developed in a cut surface passes off* in the course of some hours, but 
may be restored by making a fresh cut and exposing a fresh surface. 
The temporary duration of the negativity after injury, and its renewal 
upon fresh injury, in the case of the ventricle, in contrast to the more per- 
manent negativity of injured skeletal muscle, is explained by the different 
structure of the two kinds of muscle. The cardiac muscle, as we shall 
hereafter see, is composed of short fibre-cells; when a cut is made a cer- 
tain number of these fibre-cells are injured, giving rise to negativity, but 
the injury done to them stops with them, and is not propagated to the cells 
1 The necessity of its being inactive will be seen subsequently. CHANGES IN A MUSCLE DURING CONTRACTION. 
95 
with which they are in contact; hence, upon their death, the negativity 
and the current disappear. A fresh cut, involving new cells, produces 
fresh negativity and a new current. In the long fibres of the skeletal 
muscle, on the other hand, the effects of the injury arc slowly propagated 
along the fibre from the spot injured. 
Now, when a muscle is cut or injured, the substance of the fibres dies 
at the cut or injured surface. And many physiologists, among whom the 
most prominent is Hermann, have been led by the above and other facts 
to the conclusion that muscle-currents do not exist naturally in un- 
touched, uninjured muscles, that the muscular substance is naturally, 
when living, isoelectric, but that whenever a portion of the muscular sub- 
stance dies, it becomes, while dying, negative to the living substance, and 
thus gives rise to currents. They explain the typical currents (as they 
might be called) manifested by a muscle with a natural longitudinal surface 
and artificial transverse sections, by the fact that the dying cut ends are neg- 
ative relatively to the rest of the muscle. 
Du Bois-Reymond and those with him offer special explanations of 
the above facts and of other objections which have been urged against 
the theory of naturally existing electro-motive molecules. Into these we 
cannot enter here. We must rest content with the statement that in 
an ordinary muscle currents such as have been described may be witnessed, 
but that strong arguments may be adduced in favor of the view that these 
currents are not " natural " phenomena, but essentially of artificial origin. 
It will, therefore, be best to speak of them as currents of rest. 
§ 67. Current of action. Negative variation of the muscle-current.-The 
controversy whether the "currents of rest" observable in a muscle be of 
natural origin or not, does not affect the truth or the importance of the fact 
that an electrical change takes place and a current is developed in a muscle 
whenever it enters into a contraction. When currents of rest are observ- 
able in a muscle, these are found to undergo a diminution upon the occur- 
rence of a contraction, and this diminution is spoken of as " the negative 
variation " of the currents of rest. The negative variation may be seen 
when a muscle is thrown into a single contraction, but is most readily 
shown when the muscle is tetanized. Thus, if a pair of electrodes be 
placed on a muscle, one at the equator and the other at or near the trans- 
verse section, so that a considerable deflection of the galvanometer needle, 
indicating a considerable current of rest, be gained, the needle of the gal- 
vanometer will, when the muscle is tetanized by an interrupted current 
sent through its nerve (at a point too far from the muscle to allow of 
any escape of the current into the electrodes connected with the galvano- 
meter), swing back toward zero; it returns to its original deflection when 
the tetanizing current is shut off. 
Not only may this negative variation be shown by the galvanometer, but 
it, as well as the current of rest, may be used as a galvanic shock, and so 
employed to stimulate a muscle, as in the experiment known as " the rheo- 
scopic frog." For this purpose the muscles and nerves need to be very 
irritable and in thoroughly good condition. Two muscle-nerve preparations, 
A and B, having been made, and each placed on a glass plate for the sake 
of insulation, the nerve of the one, B, is allowed to fall on the muscle of the 
other, A, in such a way that one point of the nerve comes in contact with 
the equator of the muscle, and another point with one end of the muscle or 
with a point at some distance from the equator. At the moment the nerve 
is let fall and contact made, a current-viz., the "current of rest" of the 
muscle A-passes through the nerve; this acts as a stimulus to the nerve, 
and so causes a contraction in the muscle connected with the nerve. Thus, 96 
THE CONTRACTILE TISSUES. 
the muscle A acts as a battery, the completion of the circuit of which by 
means of the nerve of B serves as a stimulus, causing the muscle B to 
contract. 
If, while the nerve of B is still in contact with the muscle of A, the 
nerve of the latter is tetanized with an interrupted current, not only is the 
muscle of A thrown into tetanus, but also that of B, the reason being as 
follows: At each spasm of which the tetanus of A is made up, there is a 
negative variation of the muscle-current of A. Each negative variation of 
the muscle-current of A serves as a stimulus to the nerve of B, and is hence 
the cause of a spasm in the muscle of B; and the stimuli following each 
other rapidly, as being. produced by the tetanus of A they must do, the 
spasms in B to which they give rise are also fused into a tetanus in B. B, 
in fact, contracts in harmony with A. This experiment shows that the 
negative variation accompanying the tetanus of a muscle, though it causes 
only a single swing of the galvanometer, is really made up of a series of 
negative variations, each single negative variation corresponding to the 
single spasms of which the tetanus is made up. 
But an electrical change may be manifested even in cases when no cur- 
rents of rest exist. We have stated (§ 66) that the surface of the uninjured 
inactive ventricle of the frog's heart is isoelectric, no currents being observed 
when the electrodes of a galvanometer are placed on two points of the sur- 
face. Nevertheless, a most distinct current is developed whenever the 
ventricle contracts. This may be shown either by the galvanometer or by 
the rheoscopic frog. If the nerve of an irritable muscle-nerve preparation 
be laid over a pulsating ventricle, each beat is responded to by a twitch of 
the muscle of the preparation. In the case of ordinary muscles, too, 
instances occur in which it seems impossible to regard the electrical change 
manifested during the contraction as the mere diminution of a pre-existing 
current. 
Accordingly, those who deny the existence of " natural " muscle-currents 
speak of a muscle as developing during a contraction a " current of action," 
occasioned, as they believe, by the muscular substance as it is entering into 
the state of contraction becoming negative toward the muscular substance 
which is still at rest, or has returned to a 
state of rest. In fact, they regard the 
negativity of muscular substance as cha- 
racteristic alike of beginning death and of a 
beginning contraction. So that, in muscu- 
lar contraction a wave of negativity, start- 
ing from the end-plate when indirect, or 
from the point stimulated when direct 
stimulation is used, passes along the mus- 
cular substance to the ends or end of the 
fibre. 
For instance, we will suppose two elec- 
trodes placed on two points (Fig. 31), A 
and B, of a fibre about to be stimulated by 
a single induction-shock at one end. Before 
the stimulation the fibre is isoelectric, and 
the needle of the galvanometer stands at zero. At a certain time after the 
shock has been sent through the stimulating electrodes (a;), as the wave of 
contraction is travelling down the fibre, the section of the fibre beneath A 
will become negative toward the rest of the fibre, and so negative toward 
the portion of the fibre under B-i.e., A will be negative relatively to B, 
and this will be shown by a deflection of the needle. A little later B will 
Fig. 31. CHANGES IN A .MUSCLE DURING CONTRACTION. 
97 
be entering into contraction, and will be becoming negative toward the rest 
of the fibre, including the part under A, whose negativity by this time is 
passing off-that is to say, B will now be negative toward A, and this will 
be shown by a deflection of the needle in a direction opposite to that of the 
deflection which has just previously taken place. Hence, between two elec- 
trodes placed along a fibre a single wave of contraction will give rise to two 
currents of different phases, to a diphasic change ; and this, indeed, is found 
to be the case. 
This being so, it is obvious that the electrical result of tetanizing a 
muscle when wave after wave follows along each fibre is a complex matter; 
but it is maintained that the apparent negative variation of tetanus can 
be explained as the net result of a series of currents of action due to the 
individual contractions, the second phase of the current in each contrac- 
tion being less marked than the first phase. We cannot, however, enter 
more fully here into a discussion of this difficult subject. 
Whichever view be taken of the nature of these phenomena, it is im- 
portant to remember that the electrical changes are closely allied to the 
chemical events involved in the contraction of the muscle and to the change 
of form of the muscle. Their exact relations to each other await research. 
§ 68. The change in the form of a muscle during its contraction is a 
thing which can be seen and felt; but the changes in a nerve during its 
activity are invisible and impalpable. We stimulate one end of a nerve 
going to a muscle, and we see this followed by a contraction of the muscle 
attached to the other end ; or we stimulate a nerve still connected with the 
central nervous system, and we see this followed by certain movements, or 
by other tokens which show that disturbances have been set up in the cen- 
tral nervous system. We know, therefore, that some changes or other, con- 
stituting what we have called a nervous impulse, have been propagated 
along the nerve ; but the changes are such as we cannot see. It is possible, 
however, to learn something about them. 
§ 69. The chemistry of a nerve. We have spoken of the medulla as fatty, 
and yet it is in reality very largely composed of a substance which is not (in 
the strict sense of the word) a fat. When we examine chemically a quantity 
of nerve (or what is practically the same thing, a quantity of that part of the 
central nervous system which is called white matter, and which as we shall 
see is chiefly composed, like a nerve, of medullated nerves, and is to be pre- 
ferred for chemical examination because it contains a relatively small quantity 
of connective tissue), we find that a very large proportion, according to some 
observers about half, of the dried matter consists of a peculiar body, 
cholesterin. Now, cholesterin is not a flit but an alcohol; like glycerin, how- 
ever, which is also an alcohol, it forms compounds with fatty acids; and 
though we do not know definitely the chemical condition in which cholesterin 
exists during life in the medulla, it is more than probable that it exists in 
some combination with some of the really fatty bodies also present in the 
medulla, and not in a free isolated state. It is singular that besides being 
present in such large quantities in nervous tissue, and to a small extent in 
other tissues and in blood, cholesterin is a normal constituent of bile, and 
forms the greater part of gall-stones when these are present; in gall-stones it 
is undoubtedly present in a free state. Besides cholesterin, " white" nervous 
matter contains a less but still considerable quantity of a complex fat, whose 
nature is disputed. According to some authorities rather less than half this 
complex fat consists of the peculiar body lecithin, which we have already 
The Changes in a Nerve chiring the Passage of a Nervous Impulse. 98 
THE CONTRACTILE TISSUES. 
seen to be present also in blood corpuscles and in muscle. Lecithin contains 
the radicle of stearic acid (or of oleic, or of palmitic acid) associated not, as 
ip ordinary fats, with simple glycerin, but with the more complex glycerin- 
phosphoric acid, and further combined with a nitrogenous body, neurin, 
an ammonia compound of some considerable complexity ; it is therefore of 
remarkable nature, since, though a fat, it contains both nitrogen and phos- 
phorus. According to the same authorities the remainder of the complex 
fat consists of another fatty body, also apparently containing nitrogen but 
no phosphorus, called cerebrin. Other authorities regard both these bodies, 
lecithin and cerebrin, as products of decomposition of a still more complex 
fat, called protagon. Obviously the fat of the white matter of the central 
nervous system and of spinal nerves (of which fat by far the greater part 
must exist in the medulla, and form nearly the whole of the medulla) is a 
very complex body indeed, especially so if the cholesterin exists in combina- 
tion with the lecithin, or cerebrin (or protagon). Being so complex it is 
naturally very unstable, and indeed, in its stability resembles proteid matter. 
Hence, probably, the reason why the medulla changes so rapidly and so pro- 
foundly after the death of the nerve. 
The presence in such large quantity of this complex fatty medulla renders 
the chemical examination of the other constituents of a nerve very difficult, 
and our knowledge of the chemical nature of, and of the chemical changes 
going on in the axis-cylinder, is very limited. Examined under the micro- 
scope the axis-cylinder gives the xanthoproteic reaction and other indications 
that it is proteid in nature ; beyond this we are largely confined to inferences. 
After the fats of the medulla (and the much smaller quantity of fat 
present in the axis-cylinder), the proteids of the axis-cylinder, and the other 
soluble substances present in one or the other, or gathered round the nuclei 
of the neurilemma, have by various means been dissolved out of a nerve 
fibre, certain substances still remain. One of these in small quantity is the 
nuclein of the nuclei; another in larger quantity is the substance neuro- 
keratin which forms, as we have seen, a supporting framework for the 
medulla, and whose most marked characteristic is, perhaps, its resistance 
to solution. 
In the ash of nerves there is a preponderance of potassium salts and 
phosphates, but not so marked as in the case of muscle. 
§ 70. The nervous impulse.-The chemical analogy between the substance 
of the muscle and that of the axis-cylinder would naturally lead us to sup- 
pose that the progress of a nervous impulse along a nerve fibre was accom- 
panied by chemical changes similar to those taking place in a muscle fibre. 
Whatever changes, however, do or may take place are too slight to be 
recognized by the means at our disposal. We have no satisfactory evidence 
that in a nerve even repeated nervous impulses can give rise to an acid 
reaction, or that the death of a nerve fibre leads to such reaction. The gray 
matter of the central nervous system, it is true, is said to be slightly acid 
during life and to become more acid after death ; but in this gray matter, 
nerve cells are relatively abundant; the white matter, composed chiefly of 
nerve fibre, is and remains, during action as well as rest, and even after 
death, neutral or slightly alkaline. 
Nor have we satisfactory evidence that the progress of a nervous impulse 
is accompanied by any setting free of energy in the form of heat. 
In fact, beyond the terminal results, such as a muscular contraction in the 
case of a nerve going to a muscle, or some affection of the central nervous 
system in the case of a nerve still in connection with its nervous centre, 
there is one event and one event only which we are able to recognize as the 
objective token of a nervous impulse, and that is an electric change. For a CHANGES IN A MUSCLE DURING CONTRACTION. 
99 
piece of nerve removed from the body exhibits nearly the same electric 
phenomena as a piece of muscle. It has an equator which is electrically 
positive relatively to the two cut ends. In fact, the diagram Fig. 30, and 
the description which was given in § 66 of the electric changes in muscle 
may be applied almost as well to a nerve, except that the currents are in 
all cases much more feeble in the case of nerves than of muscles, and the 
special currents from the circumference to the centre of the transverse sec- 
tions cannot well be shown in a slender nerve ; indeed, it is doubtful if they 
exist at all. 
During the passage of a nervous impulse the " natural nerve current " 
undergoes a negative variation, just as the " natural muscle current" under- 
goes a negative variation during a contraction. There are, moreover, reasons 
in the case of the nerve, as in the case of the muscle, which lead us to doubt 
the pre-existence of any such " natural " currents.' A nerve in an absolutely 
natural condition appears to be, like a muscle, isoelectric; hence we may 
say that in a nerve during the passage of a nervous impulse, as in a mus- 
cle during a muscular contraction, a " current of action " is developed. 
This " current of action " or " negative variation " may be shown either 
by a galvanometer or by the rheoscopic frog. If the nerve of the " muscle 
nerve preparation," B (see § 67) be placed in an appropriate manner on a 
thoroughly irritable nerve, A (to which, of course, no muscle need be attached), 
touching for instance the equator and one end of the nerve, then single 
induction-shocks sent into the far end of A will cause single spasms in the 
muscle of B, while tetanization of A, i. e., rapidly repeated shocks sent into 
A, will cause tetanus of the muscle of B. 
That this current, whether it be regarded as an independent " current of 
action " or as a negative variation of a " pre-existing" current, is an essential 
feature of a nervous impulse is shown by the fact that the degree of intensity 
of the one varies with that of the other. They both travel, too, at the same 
rate. In describing the muscle-curve, and the method of measuring the 
muscular latent period, we have incidentally shown (§ 46) how at the same 
time the velocity of the nervous impulse may be measured, and stated that 
the rate in the nerves of a frog is about 28 metres per second. By means of 
a special and somewhat complicated apparatus it is ascertained that the cur- 
rent of action travels along an isolated piece of nerve at the same rate. It 
also, like the molecular change in a muscle preceding the contraction, and 
indeed like the contraction itself, travels in the form of a wave, rising rapidly 
to a maximum at each point of the nerve and then more gradually declin- 
ing again. The length of the wave may by special means be measured, and 
is found to be about 18 mm. 
When an isolated piece of nerve is stimulated in the middle, the current 
of action is propagated equally well in both directions, and that whether the 
nerve be a chiefly sensory or a chiefly motor nerve, or indeed if it be a nerve- 
root composed exclusively of motor or of sensory fibres. Taking the current 
of action as the token of a nervous impulse, we infer from this that when a 
nerve fibre is stimulated artificially at any part of its course, the nervous 
impulse set going travels in both directions. 
We used just now the phrase " tetanization of a nerve," meaning the 
application to a nerve of rapidly repeated shocks such as would produce 
tetanus in the muscle to which the nerve was attached, and we shall have 
frequent occasion to employ the phrase. It must, however, be understood 
that there is in the nerve, in an ordinary way, no summation of nervous 
impulses comparable to the summation of muscular contractions. Putting 
aside certain cases which we cannot discuss here, we may say that the series 
of shocks sent in at the far end of the nerve start a series of impulses ; these 100 
THE CONTRACTILE TISSUES. 
travel down the nerve and reach the muscle as a series of distinct impulses; 
and the first changes in the muscle, the molecular latent-period changes, 
also form a series the members of which are distinct. It is not until these 
molecular changes become transformed into visible changes of form that any 
fusion or summation takes place. 
§ 71. Putting together the facts contained in this and the preceding sec- 
tions, the following may be taken as a brief approximate history of what 
takes place in a muscle and nerve when the latter is subjected to a single 
induction-shock. At the instant that the induced current passes into the 
nerve, changes occur, of whose nature we know nothing certain, except that 
they cause a " current of action " or " negative variation " of the " natural " 
nerve-current. These changes propagate themselves along the nerve in both 
directions as a nervous impulse in the form of a wave, having a wave-length 
of about 18 mm., and a velocity (in frog's nerve) of about 28 m. per second. 
Passing down the nerve fibres to the muscle, flowing along the branching 
and narrowing tracts, the wave at last breaks on the end-plates of the fibres 
of the muscle. Here it is transmitted into what we may call a muscle 
impulse, with a shorter, steeper wave, and a greatly diminished velocity 
(about 3 m. per second). This muscle impulse, of which we know hardly 
more than that it is marked by a current of action, travels from each end- 
plate in both directions to the end of the fibre, where it appears to be lost; 
at all events, wTe do not know what becomes of it. As this impulse wave, 
whose development takes place entirely within the latent period, leaves the 
end-plate, it is followed by an explosive decomposition of material, leading 
to a discharge of carbonic acid, to the appearance of some substance or 
substances with an acid reaction, and probably of other unknown things, 
with a considerable development of heat. This explosive decomposition gives 
rise to the visible contraction wave, which travels behind the invisible mus- 
cle impulse at about the same rate, but with a vastly increased wave-length. 
The fibre, as the wave passes over it, swells and shortens, and thus brings its 
two ends nearer together. 
When repeated shocks are given, wave follows wave of nervous impulse, 
muscle impulse, and visible contraction ; but the last do not keep distinct; 
they are fused into the continued shortening which we call tetanus. 
The Nature of the Changes through which an Electric Cur- 
rent is Able to Generate a Nervous Impulse. 
Action of the Constant Current. 
§ 72. In the preceding account, the stimulus applied in order to give 
rise to a nervous impulse has always been supposed to be an induction-shock, 
single or repeated. This choice of stimulus has been made on account of 
the almost momentary duration of the induced current. Had we used a 
current lasting for some considerable time, the problems before us would 
have become more complex, in consequence of our having to distinguish 
between the events taking place while the current was passing through the 
nerve from those which occurred at the moment when the current was 
thrown into the nerve or at the moment when it was shut off from the nerve. 
These complications do arise when, instead of employing the induced current 
as a stimulus, we use a constant current, i. e., when we pass through the nerve 
(or muscle) a current direct from the battery without the intervention of 
any induction-coil. 
Before making the actual experiment, we might, perhaps, naturally sup- 
pose that the constant current would act as a stimulus throughout the whole 101 
STIMULUS BY ELECTRIC CURRENT. 
time during which it was applied ; that, so long as the current passed along 
the nerve, nervous impulses would be generated ; and that these would throw 
the muscle into something, at all events, like tetanus. And, under certain 
conditions, this does take place; occasionally it does happen that at the 
moment the current is thrown into the nerve the muscle of the muscle-nerve 
preparation falls into a tetanus, which is continued until the current is shut 
off; but such a result is exceptional. In the vast majority of cases what 
happens is as follows: At the moment that the circuit is made, the mo- 
ment that the current is thrown into the nerve, a single twitch, a simple 
contraction, the so-called making contraction, is witnessed ; but after this has 
passed away the muscle remains absolutely quiescent, in spite of the current 
continuing to pass through the nerve, and this quiescence is maintained until 
the circuit is broken, until the current is shut off from the nerve, when 
another simple contraction, the so-called breaking contraction, is observed. 
The mere passage of a constant current of uniform intensity through a nerve 
does not, under ordinary circumstances, act as a stimulus generating a nerv- 
ous impulse ; such an impulse is only set up when the current either falls into 
or is shut off from the nerve. It is the entrance or the exit of the current, 
and not the continuance of the current, which is the stimulus. The quies- 
cence of the nerve and muscle during the passage of the current is, however, 
dependent on the current remaining uniform in intensity, or, at least, not 
being suddenly increased or diminished. Any sufficiently sudden and large 
increase or diminution of the intensity of the current will act like the 
entrance or exit of a current, and by generating a nervous impulse give rise 
to a contraction. If the intensity of the current, however, be very slowly 
and gradually increased or diminished, a very wide range of intensity may 
be passed through without any contraction being seen. It is the sudden 
change from one condition to another, and not the condition itself, which 
causes the nervous impulse. 
In many cases, both a " making" and a " breaking " contraction, each a 
simple twitch, are observed, and this is, perhaps, the commonest event; but 
when the current is very weak, and again when the current is very strong, 
either the breaking or the making contraction may be absent ; i. e., there 
may be a contraction only when the current is thrown into the nerve, or 
only when it is shut off* from the nerve. 
Under ordinary circumstances the contractions witnessed with the con- 
stant current either at the make or at the break, are of the nature of a 
"simple" contraction ; but, as has already been said, the application of the 
current may give rise to very pronounced tetanus. Such a tetanus is seen 
sometimes when the current is made, lasting during the application of the 
current, sometimes when the current is broken, lasting some time after the 
current has been wholly removed from the nerve. The former is spoken of 
as a " making," the latter as a " breaking" tetanus. But these exceptional 
results of the application of the constant current need not detain us now. 
The great interest attached to the action of the constant current lies in 
the fact that, during the passage of the current, in spite of the absence of 
all nervous impulses, and therefore of all muscular contractions, the nerve 
is for the time both between and on each side of the electrodes profoundly 
modified in a most peculiar manner. This modification, important both for 
the light it throws on the generation of nervous impulses and for its practi- 
cal applications, is known under the name of electrotonus. 
§ 73. Electrotonus.-1The marked feature of the electrotonic condition is 
that the nerve, though apparently quiescent, is changed in respect to its irri- 
tability ; and that in a different way in the neighborhood of the two elec- 
trodes respectively. 102 
THE CONTRACTILE TISSUES. 
Suppose that on the nerve of a muscle-nerve preparation are placed two 
(non-polarizable) electrodes (Fig. 32, a, k), connected with a battery and 
arranged with a key, so that a constant current can at pleasure be thrown 
into or shut off from the nerve. This constant current, whose effects we are 
about to study, may be called the " polarizing current." Let a be the posi- 
tive electrode or anode, and k the negative electrode or kathode, both placed 
at some distance from the muscle, and also with a certain interval between 
each other. At the point x let there be applied a pair of electrodes con- 
nected with an induction-coil. Let the muscle further be connected with a 
lever, so that its contractions can be recorded and their amount measured. 
Before the polarizing current is thrown into the nerve, let a single induction- 
shock of known intensity (a weak one being chosen, or, at least, not one 
which would cause in the muscle a maximum contraction) be thrown in at 
x. A contraction of a certain amount will follow. The contraction may 
Fig. 32. 
Muscle-nerve Preparations: with the nerve exposed in A to a descending and in B to an ascend- 
ing constant current. In each, a is the anode, k the kathode of the constant current; x represents 
the spot where the induction-shocks, used to test the irritability of the nerve, are sent in. 
be taken as a measure of the irritability of the nerve at the point x. Now 
let the polarizing current be thrown in and let the kathode or negative pole 
be nearest the muscle, as in Fig. 32, A, so that the current passes along the 
nerve in a direction from the central nervous system toward the muscle; 
such a current is spoken of as a descending one. The entrance of the polar- 
izing current into the nerve will produce a " making" contraction ; this we 
may neglect. If while the current is passing the same induction-shock as 
before be sent through x, the contraction which results will be found to be 
greater than on the former occasion. If the polarizing current be now shut 
off, a "breaking" contraction will probably be produced; this we also may 
neglect. If, now, the point x, after a short interval, be again tested with the 
same induction-shock as before, the contraction will be no longer greater, 
but of the same amount, or perhaps not so great as at first. During the 
passage of the polarizing current, therefore, the irritability of the nerve at 
the point x has been temporarily increased, since the same shock applied to 
it causes a greater contraction during the presence than in the absence of the 
current. But this is only true so long as the polarizing current is a descend- 
ing one-so long as the point x lies on the side of the kathode. On the other 
hand, if the polarizing current had been an ascending one, with the anode or STIMULUS BY ELECTRIC CURRENT. 
103 
positive pole nearest the muscle, as in Fig. 32, B, the irritability of the nerve 
at x would have been found to be diminished, instead of increased, by the polar- 
izing current; the contraction obtained during the passage of the constant 
current would be less than before the passage of the current, or might be 
absent altogether, and the contraction after the current had been shut off 
would be as great, or perhaps greater, than before. That is to say, when a 
constant current is applied to a nerve, the irritability of the nerve between 
the polarizing electrodes and the muscle is, during the passage of the 
current, increased when the kathode is nearest the muscle (and the polariz- 
ing current descending) and diminished when the anode is nearest the 
muscle (and the polarizing current ascending). The same result, mutatis 
mutandis, and with some qualifications which we need not discuss, would be 
gained if x were placed, not between the muscle and the polarizing current, 
but on the far side of the latter. Hence, it may be stated generally that 
during the passage of a constant current through a nerve the irritability of 
the nerve is increased in the region of the kathode, and diminished in the 
region of the anode. The changes in the nerve which give rise to this 
increase of irritability in the region of the kathode are spoken of as kat- 
electrotonus, and the nerve is said to be in a katelectrotonic condition. Simi- 
larly the changes in the region of the anode are spoken of as anelectrotonus, 
and the nerve is said to be in an anelectrotonic condition. It is also often 
usual to speak of the katelectrotonic increase, and anelectrotonic decrease of 
irritability. 
This law remains true whatever be the mode adopted for determining 
the irritability. The result holds good not only with a single induction- 
shock, but also with a tetanizing interrupted current, with chemical and 
mechanical stimuli. It further appears to hold good not only in a dissected 
nerve-muscle preparation but also in the intact nerves of the living body. 
The increase and decrease of irritability are most marked in the immediate 
neighborhood of the electrodes, but spread for a considerable distance in 
each direction in the extrapolar regions. The same modification is not con- 
fined to the extrapolar region, but exists also in the intrapolar region. In 
the intrapolar region there must be, of course, a neutral or indifferent point, 
where the katelectrotonic increase merges into the anelectrotonic decrease, 
Fig. 33. 
Diagram illustrating the Variations of Irritability during Electrotonus, with Polarizing Cur- 
rents of Increasing Intensity (From Pfliiger) The anode is supposed to be placed at A, the 
kathode at B ; AB is consequently the intrapolar district. In each of the three curves, the por- 
tion of the curve below the base line represents diminished irritability, that above, increased ir- 
ritability. «/i represents the effect of a weak current; the indifferent point is near the anode A. 
In y2, a stronger current, the indifferent point ,r2 is nearer the kathode B, the diminution of irri- 
tability in anelecttotonus and the increase in katelectrotonus being greater than in ; the effect 
also spreads for a greater distance along the extrapolar regions in both directions. In ys the same 
events are seen to be still more marked. 
and where, therefore, the irritability is unchanged. When the polarizing 
current is a weak one, this indifferent point is nearer the anode than the 104 
THE CONTRACTILE TISSUES. 
kathode, but as the polarizing current increases in intensity, draws nearer 
and nearer the kathode (see Fig. 33). 
The amount of increase and decrease is dependent: (1) On the strength 
of the current, the stronger current up to a certain limit producing the 
greater effect. (2) On the irritability of the nerve, the more irritable, 
better conditioned nerve being the more affected by a current of the same 
intensity. 
In the experiments just described the increase or decrease of irritability 
is taken to mean that the same stimulus starts in the one case a larger or 
more powerful, and in the other case a smaller or less energetic impulse; 
but we have reason to think that the mere propagation or condition of im- 
pulses started elsewhere is also affected by the electrotonic condition. At all 
events anelectrotonus appears to offer an obstacle to the passage of a nervous 
impulse. 
? 74. Electrotonic currents. During the passage of a constant current through 
a nerve, variations in the electric currents belonging to the nerve itself may be 
Fig. 34. 
Diagram illustrating Elcctrotonic Currents: P the polarizing battery, with k a key, p the 
anode, and p' the kathode. At the left end of the piece of nerve the natural current flows through 
the galvanometer G from g to g', in the direction of the arrows; its direction, therefore, is the 
same as that of the polarizing current; consequently it appears increased, as indicated by the 
sign +. The current at the other end of the piece of nerve, from h to h' through the galvan- 
ometer H, flows in a contrary direction to the polarizing current; it consequently appears to be 
diminished, as indicated by the sign -. 
N. B.-For simplicity's sake, the polarizing current is here supposed to be thrown in at the 
middle of a piece of nerve, and the galvanometer placed at the two ends. Of course it will be 
understood that the former may be thrown in anywhere, and the latter connected with any two 
pairs of points which will give currents. 
observed ; and these variations have certain relations to the variations of the irri- 
tability of the nerve. Thus if a constant current supplied by the battery P 
(Fig. 34) be applied to a piece of nerve by means of two non-polarizable electrodes STIMULI'S BY ELECTRIC CURRENT. 
105 
n, p'. the "currents of rest" obtainable from the various points of the nerve will 
be different during the passage of the polarizing current from those which were 
manifest before or after the current was applied ; and, moreover, the changes in the 
nerve-currents produced by the polarizing current will not be the same in the 
neighborhood of the anode (p) as those in the neighborhood of the kathode (p'). 
Thus, let G and JI be two galvanometers so connected with the two ends of the 
nerve as to afford good and clear evidence of the "currents of rest." Before the 
polarizing current is thrown into the nerve, the needle of II will occupy a posi- 
tion indicating the passage of a current of a certain intensity from A to h' 
though the galvanometer (from the positive longitudinal surface to the negative 
cut end of the nerve), the circuit being completed by a current in the nerve from 
h' to h, i. e., the current will flow in the direction of the arrow. Similarly the 
needle of G will, by its deflection, indicate the existence of a current flowing from 
g to g' through the galvanometer, and from g' to g through the nerve, in the 
direction of the arrow. 
At the instant that the polarizing current is thrown into the nerve at p p', the 
currents at g g', hl/ will undergo a "negative variation." that is, the nerve at 
each point will exhibit a "current of action" corresponding to the nervous im- 
pulse, which, at the making of the polarizing current, passes in both directions 
along the nerve, and may cause a contraction in the attached muscle. The cur- 
rent of action is, as we have seen, of extremely short duration, it is over and 
gone in a small fraction of a second. It, therefore, must not be confounded with 
a permanent effect which, in the case we are dealing with, is observed in both 
galvanometers. This effect, which is dependent on the direction of the polariz- 
ing current, is as follows: Supposing that the polarizing current is flowing in the 
direction of the arrow in the figure, that is, passes in the nerve from the positive 
electrode or anode p to the negative electrode or kathode p', it is found that the 
current through the galvanometer G is increased, while that through H is dimin- 
ished. The polarizing current has caused the appearance, in the nerve outside 
the electrodes, of a current having the same direction as itself, called the "elec- 
trotonic" current; and this electrotonic current adds to, or takes away from, the 
natural nerve-current or "current of rest" according as it is flowing in the same 
direction as that or in an opposite direction. 
The strength of the electrotonic current is dependent on the strength of the 
polarizing current, and on the length of the intrapolar region which is exposed 
to the polarizing current. When a strong polarizing current is used, the electro- 
motive force of the electrotonic current may be much greater than that of the 
natural nerve-current. 
The strength of the electrotonic current varies with the irritability, or vital 
condition of the nerve, being greater with the more irritable nerve; and a dead 
nerve will not manifest electrotonic currents. Moreover, the propagation of the 
current is stopped by a ligature, or by crushing the nerve. 
We may speak of the conditions which give rise to this electrotonic current as 
a physical electrotonus analogous to that physiological electrotonus which is made 
known by variations in irritability. The physical electrotonic current is probably 
due to the escape of the polarizing current along the nerve under the peculiar 
conditions of the living nerve ; but we must not attempt to enter here into this 
difficult subject or into the allied question as to the exact connection between the 
physical and the physiological electrotonus, though there can be little doubt that 
the latter is dependent on the former. 
§75. These variations of irritability at the kathode and anode respect- 
ively, thus brought about by the action of the constant current, are inter- 
esting theoretically, because we may trace a connection between them and 
the nervous impulse which is the result of the making or breaking of a 
constant current. 
For we have evidence that a nervous impulse is generated when a portion 
of the nerve passes suddenly from a normal condition to a state of katelec- 
trotonus or from a state of anelectrotonus back to a normal condition, but 
that the passage from a normal condition to anelectrotonus or from katelec- 
trotonus back to a normal condition is unable to generate an impulse. 
Hence when a constant current is " made" the impulse is generated only at 106 
THE CONTRACTILE TISSUES. 
the kathode where the nerve passes suddenly into katelectrotonus; when 
the current on the other hand is " broken " the impulse is generated only at 
the anode where the nerve passes suddenly back from anelectrotonus into 
a normal condition. We have an indirect proof of this in the facts to 
which we drew attention a little while back, viz., that a contraction some- 
times occurs at the " breaking" only, sometimes at the " making" only of 
the constant current, sometimes at both. For it is found that this depends 
partly on the strength of the current in relation to the irritability of the 
nerve, partly on the direction of the current, whether ascending or descend- 
ing; and the results obtained with strong, medium and weak descending 
and ascending currents have been stated in the form of a "law of contrac- 
tion." We need not enter into the details of this " law," but will merely 
say that the results which it formulates are best explained by the hypothe- 
sis just stated. We may add that when the constant current is applied to 
certain structures composed of plain muscular fibres, whose rate of contrac- 
tion we have seen to be slow, the making contraction may be actually seen 
to begin at the kathode and travel toward the anode, and the breaking con- 
traction to begin at the anode and travel thence toward the kathode. 
Since in katelectrotonus the irritability is increased, and in anelectrotonus 
decreased, both the entrance from the normal condition into katelectrotonus 
and the return from anelectrotonus to the normal condition are instances of 
a passage from a lower stage of irritability to a higher stage of irritability. 
Hence, the phenomena of electrotonus would lead us to the conception that 
a stimulus in provoking a nervous impulse produces its effect by, in some 
way or other, suddenly raising the irritability to a higher pitch. But what 
we are exactly to understand by raising the irritability, what molecular 
change is the cause of the rise, and how either electric or other stimuli can 
produce this change, are matters we cannot discuss here. 
Besides their theoretical importance, the phenomena of electrotonus have 
also a practical interest. When an ascending current is passed along a nerve 
going to a muscle or group of muscles, the region between the electrodes and 
the muscle is thrown into anelectrotonus and its irritability is diminished. 
If the current be of adequate strength, the irritability may be so much 
lessened that nervous impulses cannot be generated in that part of the nerve 
or cannot pass along it. Hence, by this means the irregular contractions of 
muscles known as "cramp" may be abolished. Similarly, by bringing into 
a condition of anelectrotonus a portion of a sensory nerve in which violent 
impulses are being generated, giving rise in the central nervous system to 
sensations of pain, the impulses are toned down or wholly abolished, and the 
pain ceases. So, on the other hand, we may at pleasure heighten the irrita- 
bility of a part by throwing it into katelectrotonus. In this way the con- 
stant current, properly applied, becomes a powerful remedial means. 
Lastly, though we are dealing now with nerves going to muscles-that 
is to say, with motor nerves only-we may add that what we have said about 
electrotonus and the development of nervous impulses by it appears to apply 
equally well to sensory nerves. 
§ 76. In a general way muscular fibres behave toward an electric cur- 
rent very much as do nerve fibres; but there are certain important dif- 
ferences. 
In the first place, muscular fibres, devoid of nerve fibres, are much more 
readily thrown into contractions by the breaking and making of a constant 
current than by the more transient induction-shock; the muscular substance 
seems to be more sluggish than the nervous substance, and requires to be 
acted upon for a longer time. This fact may be made use of, and, indeed, 
is in medical practice made use of, to determine the condition of the nerves THE MUSCLE-NERVE PREPARATION AS A MACHINE. 
107 
supplying a muscle. If the intra-muscular nerves be still in good condition, 
the muscle, as a whole, responds readily to single induction-shocks, because 
these can act upon the intra-muscular nerves. If these nerves, on the other 
hand, have lost their irritability, the muscle does not respond readily to 
single induction-shocks, or to the interrupted current, but can still easily be 
thrown into contraction by the constant current. 
In the second place, while in a nerve no impulse is, as a rule, generated 
during the passage of a constant current, between the break and the make, 
provided that it is not too strong, and that it remains uniform in strength, 
in a urarized muscle, on the other hand, even with moderate and perfectly 
uniform currents, a kind of tetanus or apparently a series of rhythmically 
repeated contractions is very frequently witnessed during the passage of the 
current. The exact nature and cause of these phenomena in muscle, we 
must not, however, discuss here. 
The Muscle-nerve Preparation as a Machine. 
§ 77. The facts described in the foregoing sections show that a muscle 
with its nerve may be justly regarded as a machine which, when stimulated, 
will do a certain amount of work. But the actual amount of work which a 
muscle-nerve preparation will do is found to depend on a large number of 
circumstances, and consequently to vary within very wide limits. These 
variations will be largely determined by the condition of the muscle and 
nerve in respect to their nutrition ; in other words, by the degree of irrita- 
bility manifested by the muscle or by the nerve, or by both. But quite 
apart from the general influences affecting its nutrition and thus its irrita- 
bility, a muscle-nerve preparation is affected as regards the amount of its 
work by a variety of other circumstances, which we may briefly consider 
here, reserving to a succeeding section the study of variations in irritability. 
The influence of the nature and mode of application of the stimulus. When 
we apply a weak stimulus-a weak induction-shock-to a nerve we get a 
small contraction, a slight shortening of the muscle; when we apply a 
stronger stimulus-a stronger induction-shock-we get a larger contraction, 
a greater shortening of the muscle. We take, other things being equal, the 
amount of contraction of the muscle as a measure of the nervous impulse, 
and say that in the former case a weak or slight, in the latter case a stronger 
or larger, nervous impulse has been generated. Now, the muscle of the 
muscle-nerve preparation consists of many muscular fibres, and the nerve of 
many nerve fibres; and we may fairly suppose that in two experiments we 
may in the one experiment bring the induction-shock or other stimulus to 
bear on a few fibres only, and in the other experiment on many or even all 
the fibres of the nerve. In the former case, only those muscular fibres in 
which the few nerve fibres stimulated end will be thrown into contraction, 
the others remaining quiet, and the shortening of the muscle, as a whole, 
since only a few fibres take part in it, will necessarily be less than when all 
the fibres of the nerve are stimulated and all the fibres of the muscles con- 
tract. That is to say, the amount of contraction will depend on the number 
of fibres stimulated. For simplicity's sake, however, we will in what fol- 
lows, except when otherwise indicated, suppose that when a nerve is stimu- 
lated, all the fibres are stimulated and all the muscular fibres contract. 
This being premised, we may say that, other things being equal, the mag- 
nitude of a nervous impulse, and so the magnitude of the ensuing contraction, 
is directly dependent on what we may call the strength of the stimulus. 
Thus taking a single induction-shock as the most manageable stimulus, we 
find that if, before we begin, we place the secondary coil (Fig. 14, sec. c.) a 108 
THE CONTRACTILE TISSUES. 
long way off the primary coil pr. c., no visible effect at all follows upon the 
discharge of the induction-shock. The passage of the momentary weak cur- 
rent is either unable to produce any nervous impulse at all, or the weak 
nervous impulse to which it gives rise is unable to stir the sluggish muscular 
substance to a visible contraction. As we slide the secondary coil toward 
the primary, sending in an induction-shock at each new position, we find 
that at a certain distance between the secondary and primary coils, the mus- 
cle responds to each induction-shock1 with a contraction which makes itself 
visible by the slightest possible rise of the attached lever. This position of 
the coils, the battery remaining the same and other things being equal, 
marks the minimal stimulus giving rise to the minimal contraction. As the 
secondary coil is brought nearer to the primary, the contractions increase in 
height corresponding to the increase in the intensity of the stimulus. Very 
soon, however, an increase in the stimulus caused by further sliding the 
secondary coil over the primary fails to cause any increase in the contrac- 
tion. This indicates that the maximal stimulus giving rise to the maximal 
contraction has been reached ; though the shocks increase in intensity as 
the secondary coil is pushed further and further over the primary, the con- 
tractions remain of the same height, until fatigue lowers them. 
With single induction-shocks then the muscular contraction, and by 
inference the nervous impulse, increases with an increase in the intensity of 
the stimulus, between the limits of the minimal and maximal stimuli; and 
this dependence of the nervous impulse, and so of the contraction, on the 
strength of the stimulus may be observed not only in electric but in all 
kinds of stimuli. 
It may here be remarked that in order for a stimulus to be effective, a 
certain abruptness in its action is necessary. Thus as we have seen the con- 
stant current when it is passing through a nerve with uniform intensity does 
not give rise to a nervous impulse, and indeed it may be increased or 
diminished to almost any extent without generating nervous impulses, pro- 
vided that the change be made gradually enough ; it is only when there is 
a sudden change that the current becomes effective as a stimulus. And the 
reason why the breaking induction-shock is more potent as a stimulus than 
the making shock is because as we have seen (§ 44) the current which is 
induced in the secondary coil of an induction-machine at the breaking of the 
primary circuit is more rapidly developed, and has a sharper rise, than the 
current which appears when the primary circuit is made. Similarly a sharp 
tap on a nerve will produce a contraction, when a gradually increasing 
pressure will fail to do so ; and in general the efficiency of a stimulus of any 
kind will depend in part on the suddenness or abruptness of its action. 
A stimulus in order that it may be effective must have an action of a 
certain duration, the time necessary to produce an effect varying according 
to the strength of the stimulus and being different in the case of a nerve 
from what it is in the case of a muscle. It would appear that an electric cur- 
rent applied to a nerve must have a duration of at least about 0.0015 second 
to cause any contraction at all, and needs a longer time than this to produce 
its full effect. A muscle fibre apart from its nerve fibre requires a still 
longer duration of the stimulus, and hence, as we have already stated, a 
muscle poisoned by urari, or which has otherwise lost the action of its 
nerves, will not respond as readily to induction-shocks as to the more slowly 
acting, breaking and making of a constant current. 
In the case of electric stimuli, the same current will produce a stronger 
1 In these experiments either the breaking or making shock must be used, not some- 
times one and sometimes the other, for, as we have stated, the two kinds of shock differ 
in efficiency, the breaking being the most potent. THE MUSCLE-NERVE PREPARATION AS A MACHINE. 
109 
contraction when it is sent along the nerve than when it is sent across the 
nerve ; indeed it is maintained that a current which passes through a nerve 
in an absolutely transverse direction is powerless to generate impulses. 
It would also appear, at all events up to certain limits, that the longer 
the piece of nerve through which the current passes, the greater is the effect 
of the stimulus. 
When two pairs of electrodes are placed on the nerve of a long and per- 
fectly fresh and successful nerve-preparation, one near to the cut end, and 
the other nearer the muscle, it is found that the same stimulus produces a 
greater contraction when applied through the former pair of electrodes than 
through the latter. This has been interpreted as meaning that the impulse 
started at the further electrodes gathers strength, like an avalanche, in its 
progress to the muscle. It is more probable, however, that the larger con- 
traction produced by stimulation of the part of the nerve near the cut end 
is due to the stimulus setting free a larger impulse, i. e., to this part of the 
nerve being more irritable. The mere section, possibly by developing nerve 
currents, increases for a time the irritability at the cut end. A similar 
greater irritability may, however, also be observed in the part of the nerve 
nearer the spinal cord while it is still in connection with the spinal cord ; 
and it is possible that the irritability of a nerve may vary considerably at 
different points of its course. 
§ 78. We have seen that when single stimuli are repeated with sufficient 
frequency, the individual contractions are fused into tetanus; as the fre- 
quency of the repetition is increased, the individual contractions are less 
obvious on the curve, until at last we get a curve on which they seem to be 
entirely lost and which we may speak of as a complete tetanus. By such a 
tetanus a much greater contraction, a much greater shortening of the muscle, 
is of course obtained than by single contractions. 
The exact frequency of repetition required to produce complete tetanus 
will depend chiefly on the length of the individual contractions, and this 
varies in different animals, in different muscles of the same animal, and in 
the same muscle under different conditions. In a cold-blooded animal, a 
single contraction is as a rule more prolonged than in a warm-blooded ani- 
mal, and tetanus is consequently produced in the former by a less frequent 
repetition of the stimulus. A tired muscle has a longer contraction than a 
fresh muscle, and hence in many tetanus curves the individual contractions, 
easily recognized at first, disappear later on, owing to the individual contrac- 
tions being lengthened out by the exhaustion caused by the tetanus itself. 
In many animals, e. g., the rabbit, some muscles (such as the adductor mag- 
nus femoris) are pale, while others (such as the semitendinosus) are red. 
The red muscles are not only more richly supplied with bloodvessels, but the 
muscle substance of the fibres contains more haemoglobin than the pale, and 
there are other structural differences. Now the single contraction of one of 
these red muscles is more prolonged than a single contraction of one of the 
pale muscles produced by the same stimulus. Hence the red muscles are 
thrown into complete tetanus with a repetition of much less frequency than 
that required for the pale muscles. Thus, ten stimuli in a second are quite 
sufficient to throw the red muscles of the rabbit into complete tetanus, while 
the pale muscles require at least twenty stimuli in a second. 
So long as signs of the individual contractions are visible on the curve of 
tetanus it is easy to recognize that each stimulation produces one of the con- 
stituent single contractions, and that the number so to speak of the vibra- 
tions of the muscle making up the tetanus corresponds to the number of 
stimulations ; but the question whether, when we increase the number of 
stimulations beyond that necessary to produce a complete tetanus, we still 110 
THE CONTRACTILE TISSUES. 
increase the number of constituent single contractions is one not so easy to 
answer. And connected with this question is another difficult one. What is 
the rate of repetition of single contractions making up those tetanic contrac- 
tions which as we have said are the kind of contractions by which the vol- 
untary, and indeed other natural, movements of the body are carried out? 
What is the evidence that these are really tetanic in character? 
When a muscle is thrown into tetanus, a more or less musical sound is 
produced. This may be heard by applying a stethoscope directly over a 
contracting muscle, and a similar sound but of a more mixed origin and 
less trustworthy may be heard when the masseter muscles are forcibly con- 
tracted or when a finger is placed in the ear, and the muscles of the same 
arm are contracted. 
When the stethoscope is placed over a muscle, the nerve of which is 
stimulated by induction-shocks repeated with varying frequency, the note 
heard will vary with the frequency of the shocks, being of higher pitch with 
the more frequent shocks. Now it has been thought that the vibrations of 
the muscle giving rise to the " muscle sound " are identical with the single 
contractions making up the tetanus of the muscle. And since, in the 
human body, when a muscle is thrown into contraction in a voluntary 
effort, or indeed in any of the ordinary natural movements of the body, 
the fundamental tone of the sound corresponds to about 19 or 20 vibrations 
a second, it has been concluded that the contraction taking place in such 
cases is a tetanus of which the individual contractions follow each other 
about 19 or 20 times a second. But investigations seem to show that the 
vibrations giving rise to the muscle sound do not really correspond to the 
shortenings and relaxations of the individual contractions, and that the 
pitch of the note cannot therefore be taken as an indication of the number 
of single contractions making up the tetanus; indeed, as we shall see in 
speaking of the sounds of the heart, a single muscular contraction may 
produce a sound which though differing from the sound given out during 
tetanus has to a certain extent musical characters. Nevertheless the special 
characters of the muscle sound given out by muscles in the natural move- 
ments of the body may be taken as showing at least that the contractions 
of the muscle in these movements are tetanic in nature, and the similarity 
of the note in all the voluntary efforts of the body and indeed in all move- 
ments carried out by the central nervous system is at least consonant with 
the view that the repetition of single contractions is of about the same fre- 
quency in all these movements. What that frequency is, and whether it is 
exactly identical in all these movements, is not at present perhaps abso- 
lutely determined ; but certain markings on the myographic tracings of 
these movements and other facts seem to indicate that it is about 12 a 
second. 
§ 79. The influence of the load. It might be imagined that a muscle 
which, when loaded with a given weight and stimulated by a current of a 
given intensity, had contracted to a certain extent, would only contract to 
half that extent when loaded with twice the weight and stimulated with the 
same stimulus. Such, however, is not necessarily the case; the height to 
which the weight is raised may be in the second instance as great, or even 
greater than in the first. That is to say, the resistance offered to the con- 
traction actually augments the contraction ; the tension of the muscular 
fibre increases the facility with which the explosive changes resulting in a 
contraction take place. And we have other evidence that anything which 
tends to stretch the muscular fibres-that any tension of the muscular 
fibres, whether during rest or during contraction-increases the metabolism 
of the muscle. There is, of course, a limit to this favorable action of the DEGREE OF IRRITABILITY OF MUSCLES AND NERVES. 
111 
resistance. As the load continues to be increased, the height of the con- 
traction is diminished, and at last a point is reached at which the muscle is 
unable (even when the stimulus chosen is the strongest possible) to lift the 
load at all. 
In a muscle viewed as a machine we have to deal, not merely with the 
height of the contraction-that is, with the amount of shortening-but with 
the work done. And this is measured by multiplying the number of units 
of height to which the load is raised into the number of units of weight of 
the load. Hence, it is obvious from the foregoing observations that the 
work done must be largely dependent on the weight itself. Thus, there is a 
certain weight of load with which, in any given muscle stimulated by a 
given stimulus, the most work will be done, as may be seen from the follow- 
ing example: 
Load, in grammes .... 
0 
50 
100 
150 
200 
250 
Height of contractions, in millimetres 
14 
9 
7 
5 
2 
0 
Work done, in gram-millimetres 
0 
450 
700 
750 
400 
0 
§ 80. The influence of the size and form of the muscle. Since all known 
muscular fibres are much shorter than the wave-length of a contraction, it 
is obvious that the longer the fibre the greater will be the shortening caused 
by the same contraction wave; the greater will be the height of the con- 
traction with the same stimulus. Hence, in a muscle of parallel fibres, the 
height to which the load is raised as the result of a given stimulus applied 
to its nerve, will depend on the length of the fibres, while the maximum 
weight of load capable of being lifted will depend on the number of the 
fibres, since the load is distributed among them. Of two muscles, therefore, 
of equal length (and of the same quality) the most work will be done by 
that which has the larger number of fibres-that is to say, the fibres being 
of equal width, which has the greatest sectional area; and of two muscles 
with equal sectional areas, the most work will be done by that which is the 
longer. If the two muscles are unequal both in length and sectional area, 
the work done will be the greater in the one which has the larger bulk, 
which contains the greater number of cubic units. In speaking, therefore, 
of the work which can be done by a muscle, we may use as a standard a 
cubic unit of bulk; or, the specific gravity of the muscle being the same, a 
unit of weight. 
We learn, then, from the foregoing paragraphs that the work done by a 
muscle-nerve preparation will depend, not only on the activity of the nerve 
and muscle as determined by their own irritability, but also on the character 
and mode of application of the stimulus; on the kind of contraction 
(whether a single spasm, ora slowly repeated or a rapidly repeated tetanus) 
on the load itself, and on the size and form of the muscle. Taking the most 
favorable circumstances-viz., a well-nourished, lively preparation, a maxi- 
mum stimulus causing a rapid tetanus, and an appropriate load-we may 
determine the maximum work done by a given weight of muscle, say one 
gramme. This in the case of the muscles of the frog has been estimated at 
about four gram-metres for one gramme of muscle. 
The Circumstances which Determine the Degree of Irritability 
of Muscles and Nerves. 
§81. A muscle-nerve preparation at the time that it is removed from 
the body possesses a certain degree of irritability; it responds by a contrac- 
tion of a certain amount to a stimulus of a certain strength applied to the 
nerve or to the muscle. After a while, the exact period depending on a 112 
THE CONTRACTILE TISSUES. 
variety of circumstances, the same stimulus produces a smaller contraction, 
i. e., the irritability of the preparation has diminished. In other words, the 
muscle or nerve or both have become partially "exhausted," and the 
exhaustion subsequently increases, the same stimulus producing smaller con- 
tractions, until at last all irritability is lost, no stimulus, however strong, 
producing any contraction, whether applied to the nerve or directly to the 
muscle; and eventually the muscle, as we have seen, becomes rigid. The 
progress of this exhaustion is more rapid in the nerves than in the muscles; 
for some time after the nerve-trunk has ceased to respond to even the 
strongest stimulus, contractions may be obtained by applying the stimulus 
directly to the muscle. It is much more rapid in the warm-blooded than in 
the cold-blooded animals. The muscles and nerves of the former lose their 
irritability when removed from the body, after a period varying according 
to circumstances, from a few minutes to two or three hours; those of cold- 
blooded animals (or at least of an amphibian or a reptile) may, under 
favorable conditions, remain irritable for two, three, or even more days. 
The duration of irritability in warm-blooded animals may, however, be 
considerably prolonged by reducing the temperature of the body before 
death. 
if with some thin body a sharp blow be struck across a muscle which has 
entered into the later stages of exhaustion a wheal lasting for several seconds is 
developed. This wheal appears to be a contraction wave limited to the part struck, 
and disappearing very slowly without extending to the neighboring muscular sub- 
stance. It has been called an " idio-muscular" contraction, because it may be 
brought out even when ordinary stimuli have ceased to produce any effect. It 
may, however, be accompanied at its beginning by an ordinary contraction. It is 
readily produced in the living body on the pectoral and other muscles of persons 
suffering from phthisis and other exhausting diseases. 
This natural exhaustion and diminution of irritability in muscles and 
nerves removed from the bodv may be modified both in the case of the 
muscle and of the nerve by a variety of circumstances. Similarly, while 
the nerve and muscle still remain in the body, the irritability of the one or 
of the other may be modified either in the way of increase or of decrease 
by certain general influences, of which the most important are severance 
from the central nervous system and variations in temperature, in blood- 
supply, and in functional activity. 
The effects of severance from the central nervous system. When a nerve, 
such, for instance, as the sciatic, is divided in situ, in the living body, there 
is, first of all, observed a light increase of irritability, noticeable especially 
near the cut end, but after a while the irritability diminishes and gradually 
disappears. Both the slight initial increase and the subsequent decrease 
begin at the cut end and advance centrifugally toward the peripheral ter- 
minations. This centrifugal feature of the loss of irritability is often 
spoken of as the Ritter-Valli law. In a mammal it may be two or three 
days ; in a frog, as many, or even more weeks, before irritability has dis- 
appeared from the nerve-trunk. It is maintained in the small (and espe- 
cially in the intramuscular) branches for still longer periods. 
In the central portion of the divided nerve similar changes may be 
traced as far only as the next node of Ranvier. Beyond this the nerve 
usually remains in a normal condition. 
If the muscle thus deprived of its nervous elements be left to itself its 
irritability, however tested, sooner or later diminishes ; but if the muscle be 
periodically thrown into contractions by artificial stimulation with the con- 
stant current, the decline of irritability and attendant loss of nutritive 
power may be postponed for some considerable time. But as far as our ex- DEGREE OF IRRITABILITY OF MUSCLES AND NERVES. 
113 
perience goes at present the artificial stimulation cannot fully replace the 
natural one, and sooner or later the muscle like the nerve suffers degenera- 
tion, loses all irritability, and ultimately its place is taken by connective 
tissue. 
§82. The influence of temperature. We have already seen that sudden 
heat (and the same might be said of cold when sufficiently intense), applied 
to a limited part of a nerve or muscle, as when the nerve or muscle is 
touched with a hot wire, will act as a stimulus. It is, however, much 
more difficult to generate nervous or muscular impulses by exposing a whole 
nerve or muscle to a gradual rise of temperature. 
A muscle may be gradually cooled to 0° C. or below without any con- 
traction being caused ; but when it is heated to a limit, which in the case of 
frog's muscles is about 45° C., of mammalian muscles about 50° C., a sud- 
den change takes place : the muscle falls at the limiting temperature into a 
rigor mortis, which is initiated by a forcible contraction or at least short- 
ening. 
Moderate warmth, e. g., in the frog an increase of temperature up to 
somewhat below 45° C., favors both muscular and nervous irritability. All 
the molecular processes are hastened and facilitated: the contraction is for 
a given stimulus greater and more rapid, i. e., of shorter duration, and ner- 
vous impulses are generated more readily by slight stimuli. Owing to the 
quickening of the chemical changes, the supply of new material may prove 
insufficient; hence muscles and nerves removed from the body lose their 
irritability more rapidly at a high than at a low temperature. 
The gradual application of cold to a nerve, especially when the tempera- 
ture is thus brought near to 0° C., slackens all the molecular processes, so 
that the wave of nervous impulse is lessened and prolonged, the velocity of 
its passage being much diminished, e. g., from 28 metres to 1 metre per second. 
At about 0° C., the irritability of the nerve disappears altogether. 
When a muscle is exposed to similar cold, e. g., to a temperature very 
little above zero, the contractions are remarkably prolonged; they are 
diminished in height at the same time, but not in proportion to the increase 
of their duration. Exposed to a temperature of zero or below, muscles soon 
lose their irritability, without, however, undergoing rigor mortis. 
§ 83. The influence of blood-supply. When a muscle still within the 
body is deprived by any means of its proper blood-supply, as when the 
blood-vessels going to it are ligatured, the same gradual loss of irritability 
and final appearance of rigor mortis are observed as in muscles removed 
from the body. Thus, if the abdominal aorta be ligatured, the muscles of 
the lower limbs lose their irritability and finally become rigid. So also in 
systemic death, when the blood-supply to the muscles is cut off by the cessa- 
tion of the circulation, loss of irritability ensues, and rigor mortis eventually 
follows. In a human corpse the muscles of the body enter into rigor mortis 
in a fixed order; first, those of the jaw and neck, then those of the trunk, 
next those of the arms, and lastly those of the legs. The rapidity with 
which rigor mortis comes on after death varies considerably, being deter- 
mined both by external circumstances and by the internal conditions of the 
body. Thus, external warmth hastens and cold retards the onset. After 
great muscular exertion, as in hunted animals, and when death closes 
wasting diseases, rigor mortis in most cases comes on rapidly. As a general 
rule, it may be said that the later it is in making its appearance the more 
pronounced it is, and the longer it lasts; but there are many exceptions, 
and when the state is recognized as being fundamentally due to a clotting of 
myosin, it is easy to understand that the amount of rigidity, i. e.,the amount 
of the clot, and the rapidity of the onset, i. e., the quickness with which 114 
THE CONTRACTILE TISSUES. 
coagulation takes place, may vary independently. The rapidity of onset 
after muscular exercise and wasting disease may, perhaps, be in part depend- 
ent on an increase of acid reaction, which is produced under those circum- 
stances in the muscle, for this seems to be favorable to the coagulation of 
the muscle plasma. When rigor mortis has once become thoroughly estab- 
lished in a muscle through deprivation of blood, it cannot be removed by 
any subsequent supply of blood. Thus, when the abdominal aorta has 
remained ligatured until the lower limbs have become completely rigid, 
untying the ligature will not restore the muscles to an irritable condition ; 
it simply hastens the decomposition of the dead tissues by supplying them 
with oxygen and, in the case of the mammal, with warmth also. A muscle, 
however, may acquire as a whole a certain amount of rigidity on account of 
some of the fibres becoming rigid, while the remainder, though they have 
lost their irritability, have not yet advanced into rigor mortis. At such a 
juncture a renewal of the blood-stream may restore the irritability of those 
fibres which are not yet rigid, and thus appear to do away with rigor mortis ; 
yet, it appears that in such cases the fibres which have actually become 
rigid never regain their irritability, but undergo degeneration. 
Mere loss of irritability, even though complete, if stopping short of the 
actual coagulation of the muscle substance, may be with care removed. 
The influence of blood-supply cannot be so satisfactorily studied in the 
case of nerves as in the case of muscles; there can, however, be little doubt 
that the effects are analogous. 
§ 84. The influence of functional activity. This, too, is more easily studied 
in the case of muscles than of nerves. 
When a muscle within the body is unused, it wastes; when used it (within 
certain limits) grows. Both these facts show that the nutrition of a muscle 
is favorably affected by its functional activity. Part of this may be an 
indirect effect of the increased blood-supply which occurs when a muscle 
contracts. When a nerve going to a muscle is stimulated, the bloodvessels 
of the muscle dilate. Hence at the time of the contraction more blood 
flows through the muscle, and this increased flow continues for some little 
while after the contraction of the muscle has ceased. But, apart from the 
blood-supply, it is probable that the exhaustion caused by a contraction is 
immediately followed by a reaction favorable to the nutrition of the muscle; 
and this is a reason, possibly the chief reason, why a muscle is increased by 
use, that is to say, the loss of substance and energy caused by the contrac- 
tion is subsequently more than made up for by increased metabolism during 
the following period of rest. 
A muscle, even within the body, after prolonged action is fatigued, i. e., 
a stronger stimulus is required to produce the same contraction ; in other 
words, its irritability may be lessened by functional activity. Whether 
functional activity, therefore, is injurious or beneficial depends on its amount 
in relation to the condition of the muscle. The muscle is sooner fatigued 
and exhausted than the motor nerve. 
The sense of fatigue of which, after prolonged or unusual exertion, we are 
conscious in our own bodies, is probably of complex origin, and its nature, 
like that of the normal muscular sense of which we shall have to speak 
hereafter, is at present not thoroughly understood. It seems to be in the first 
place the result of changes in the muscles themselves, but it is possibly also 
caused by changes in the nervous apparatus concerned in muscular action, 
and especially in those parts of the central nervous system which are con- 
cerned in the production of voluntary impulses. In any case it cannot be 
taken as an adequate measure of the actual fatigue of the muscles; for a 
man who says he is absolutely exhausted may, under excitement, perform a THE ENERGY OF MUSCLE AND NERVE. 
115 
very large amount of work with his already weary muscles. The will, in 
fact, rarely if ever calls forth the greatest contractions of which the muscles 
are capable. 
Absolute (temporary) exhaustion of the muscles, so that the strongest 
stimuli produce no contraction, may be produced even within the body by 
artificial stimulation ; recovery takes place on rest. Out of the body absolute 
exhaustion takes place readily. Here, also, recovery may take place. 
Whether in any given case it does occur or not, is determined by the amount 
of contraction causing the exhaustion, and by the previous condition of the 
muscle. In all cases recovery is hastened by renewal (natural or artificial) 
of the blood-stream. 
The more rapidly the contractions follow each other, the less the interval 
between any two contractions, the more rapid the exhaustion. A certain 
number of single induction-shocks repeated rapidly, say every second or 
oftener, bring about exhaustive loss of irritability more rapidly than the 
same number of shocks repeated less rapidly, for instance every 5 or 10 
seconds. Hence tetanus is a ready means of producing exhaustion. 
In exhausted muscles the elasticity is much diminished ; the tired muscle 
returns less readily to its natural length than does the fresh one. 
The exhaustion due to contraction may be the result either of the con- 
sumption of the store of really contractile material present in the muscle; 
or of the accumulation in the tissue of the products of the act of contrac- 
tion ; or of both of these causes. 
The restorative influence of rest, in the case of a muscle removed from 
the circulation, may be explained by supposing that during the repose, either 
the internal changes of the tissue manufacture new explosive material out of 
the comparatively raw material already present in the fibres, or the directly 
hurtful products of the act of contraction undergo changes by which they 
are converted into comparatively inert bodies. A stream of fresh blood 
may exert its restorative influence not only by quickening the above two 
events, but also by carrying off the immediate waste products while at the 
same time it brings new raw material. It is not known to what extent each 
of these parts is played. That the products of contraction are exhausting in 
their effect, is shown by the facts that the injection of a solution of the 
muscle-extractives into the vessels of a muscle produces exhaustion, and that 
exhausted muscles are recovered by the simple injection of inert saline 
solutions into their bloodvessels. But the matter has not yet been fully 
worked out. 
One important element brought by fresh blood is oxygen. This, as we 
have seen, is not necessary for the carrying out of the actual contraction, and 
yet is essential to the maintenance of irritability. The oxygen absorbed by 
the muscle apparently enters in some peculiar way into the formation of that 
complex explosive material the decomposition of which in the act of contrac- 
tion, though it gives rise to carbonic acid and other products of oxidation, 
is not in itself a process of direct oxidation. 
The Energy of Muscle and Nerve, and the Nature of 
Muscular and Nervous Action. 
§ 85. We may briefly recapitulate some of the chief results arrived at 
in the preceding pages as follows: 
A muscular contraction itself is essentially a translocation of molecules, 
a change of form, not of bulk. We cannot say, however, anything definite 
as to the nature of this translocation or as to the way in which it is brought 
about. For instance we cannot satisfactorily explain the contraction between 116 
THE CONTRACTILE TISSUES. 
the striation of a muscular fibre and a muscular contraction. Nearly all 
rapidly contracting muscles are striated, and we must suppose that the 
striation is of some use ; but it is not essential to the carrying out of a con- 
traction, for, as we shall see, the contraction of a non-striated muscle is 
fundamentally the same as that of a striated muscle. But whatever be the 
exact way in which the translocation is effected, it is in some way or other 
the result of a chemical change, of an explosive decomposition of certain 
parts of the muscle substance. The energy which is expended in the 
mechanical work done by the muscle has its source in the energy latent in 
the muscle substance and set free by that explosion. Concerning the nature 
of that explosion we only know at present that it results in the production 
of carbonic acid and in an increase of the acid reaction, and that heat is set 
free as well as the specific muscular energy. There is a general parallelism 
between the extent of metabolism taking place and the amount of energy set 
free ; the greater the development of carbonic acid, the larger is the con- 
traction and the higher the temperature. 
It is important to remember that, as we have already urged, relaxation, 
the return to the original length, is an essential part of the whole contraction 
no less than shortening itself. It is true that the return to the original 
length is assisted by the stretching exerted by the load, and in the case of 
muscles within the living body is secured by the action of antagonistic 
muscles or by various anatomical relations; but the fact that the complete- 
ness and rapidity of the return are dependent on the condition of the muscle, 
that is, on the complex changes within the muscle making up what we call 
its nutrition, the tired muscle relaxing much more slowly than the untired 
muscle, shows that the relaxation is due in the main to intrinsic processes 
going on in the muscle itself, processes which we might characterize as the 
reverse of those of contraction. In fact, to put the matter forcibly, adopting 
the illustration used in § 57, and regarding relaxation as a change of molecules 
from a " formation " of one hundred in two lines of fifty each to a formation 
of ten columns each ten deep, it would be possible to support the theory that 
the really active forces in muscle are those striving to maintain the latter 
formation in columns, and that the falling into double lines, that is to say 
the contraction, is the result of these forces ceasing to act; in other words, 
that the contracted state of the muscular fibre is what may be called the 
natural state, that the relaxed condition is only brought about at the expense 
of changes counteracting the natural tendencies of the fibre. Without going 
so far as this, however, we may still recognize that both contraction and relax- 
ation are the result of changes which, since they seem to be of a chemical 
nature in the one case, are probably so in the other also. 
Ox Some Other Forms of Contractile Tissue. 
Plain, Smooth or Unstriated Muscular Tissue. 
§86. This, in vertebrates at all events, rarely occurs in isolated masses 
or muscles, as does striated muscular tissue, but is usually found taking part 
in the structure of complex organs,such for instance as the intestines; hence 
the investigation of its properties is beset with many difficulties. 
§ 87. As far as we know, plain muscular tissue in its chemical features 
resembles striated muscular tissue. It contains albumin, some forms of 
globulin, and antecedents of myosin which upon the death of the fibres 
become myosin ; for plain muscular tissue after death becomes rigid, losing 
its extensibility and probably becoming acid, though the acidity is not so 
marked as in striated muscle. Kreatin has also been found, as well as ON SOME OTHER FORMS OF CONTRACTILE TISSUE. 
117 
glycogen, and, indeed, it seems probable that the whole metabolism of 
plain muscular tissue is fundamentally the same as that of the striated 
muscles. 
§ 88. In their general physical features plain muscular fibres also resem- 
ble striated fibres, and like them they are irritable and contractile; when 
stimulated they contract. The fibres vary in natural length in different 
situations, those of the bloodvessels, for instance, being shorter and stouter 
than those of the intestine; but in the same situation the fibres may also be 
found in one of two different conditions. In the one case the fibres are 
long and thin, in the other case they are reduced in length, it may be to 
one-half or even to one-third, and are correspondingly thicker, broader, 
and less pointed at the ends, their total bulk remaining unaltered. In 
the former case they are relaxed or elongated, in the latter case they are 
contracted. 
The facts of the contraction of plain muscular tissue may be studied in 
the intestine, the muscular coat of which consists of an outer thin sheet 
composed of fibres and bundles of fibres disposed longitudinally, and of an 
inner much thicker sheet of fibres disposed circularly ; in the ureter a similar 
arrangement of two coats obtains. 
If a mechanical or electrical (or, indeed, any other) stimulus be brought 
to bear on a part of a fresh, living, still warm intestine (the small intestine 
is the best to work with) a circular contraction is seen to take place at the 
spot stimulated ; the intestine seems nipped in ringwise, as if tied round with 
an invisible cord, and the part so constricted, previously vascular and red, 
becomes pale and bloodless. The individual fibres of the circular coat in 
the region stimulated have each become shorter, and the total effect of the 
shortening of the multitude of fibres all having the same circular disposition 
is to constrict or narrow the lumen or tube of the intestine. The longitudi- 
nally disposed fibres of the outer longitudinal coat will at the same time 
similarly contract or shorten in a longitudinal direction, but this coat being 
relatively much thinner than the circular coat, the longitudinal contraction 
is altogether overshadowed by the circular contraction. A similar mode of 
contraction is also seen when the ureter is similarly stimulated. 
The contraction thus induced is preceded by a very long latent period and 
lasts a very considerable time, in fact, several seconds, after which relaxa- 
tion slowly takes place. We may say then that over the circularly disposed 
fibres of the intestine (or ureter) at the spot in question there has passed a 
contraction-wave remarkable for its long latent period and for the slowness 
of its development, the wave being propagated from fibre to fibre. From 
the spot so directly stimulated the contraction may pass also as a wave (with 
a length of 1 cm. and a velocity of from 20 to 30 millimetres a second in 
the ureter), along the circular coat both upwards and downwards. The 
longitudinal fibres at the spot stimulated are, as we have said, also thrown 
into contractions of altogether similar character, and a wave of contraction 
may thus also travel longitudinally along the longitudinal coat both upwards 
and downwards. It is evident, however, that the wave of contraction of 
which we are now speaking is, in one respect, different from the wave of 
contraction treated of in dealing with striated muscle. In the latter case 
the contraction-wave is a simple wave propagated along the individual fibre, 
and starting from the end-plate or, in the case of direct stimulation, from 
the part of the fibre first affected by the stimulus; we have no evidence that 
the contraction of one fibre can communicate contraction to neighboring 
fibres, or, indeed, in any way influence neighboring fibres. In the case of 
the intestine or ureter the wave is complex, being the sum of the contraction- 
waves of several fibres engaged in different phases, and is propagated from 118 
THE CONTRACTILE TISSUES. 
fibre to fibre, both in the direction of the fibres, as when the whole circum- 
ference of the intestine is engaged in the contraction, or when the wave 
travels longitudinally along the longitudinal coat, and also in a direction at 
right angles to the axes of the fibres, as when the contraction-wave travels 
lengthways along the circular coat of the intestine, or when it passes across 
a breadth of the longitudinal coat; that is to say, the changes leading to 
contraction are communicated not only in a direct manner across the cement 
substance uniting the fibres of a bundle, but also in an indirect manner, 
probably by means of nerve fibres from bundle to bundle across the connective 
tissue between them. Moreover, it is obvious that even the contraction-wave 
which passes along a single unstriated fibre differs from that passing along a 
striated fibre, in the very great length both of its latent period and of the 
duration of its contraction. Hence, much more even than in the case of a 
striated muscle, the whole of each fibre must be occupied by the contraction- 
wave, and, indeed, be in nearly the same phase of the contraction at the 
same time. 
Waves of contraction thus passing along the circular and longitudinal 
coats of the intestine constitute what is called peristaltic action. 
Like the contractions of striated muscle the contractions of plain mus- 
cles may be started by stimulation of nerves going to the part, the nerves 
supplying plain muscular tissue running for the most part in the so-called 
sympathetic system, but being, as wre shall see, ultimately connected with 
the spinal cord or brain. Here, however, we come upon an important dis- 
tinction between the striated skeletal muscles and the plain muscles of the 
viscera. As a general rule, the skeletal muscles are thrown into contrac- 
tion only by nervous impulses reaching them along their nerves; spontane- 
ous movements of the skeletal muscles, that is, contractions, arising out of 
changes in the muscles themselves, are extremely rare and when they occur 
are abnormal; so-called " cramps " for instance, which are prolonged tetanic 
contractions of skeletal muscles independent of the will, though their occur- 
rence is largely due to the condition of the muscle itself, generally the re- 
sult of overwork, are probably actually started by nervous impulses reaching 
them from without. On the other hand, the plain muscles of the viscera, 
of the intestine, uterus, and ureter, for instance, and of the bloodvessels very 
frequently fall into contractions and so carry out movements of the organs 
to which they belong quite independently of the central nervous system. 
These organs exhibit " spontaneous " movements quite apart from the will, 
quite apart from the central nervous system, and under favorable circum- 
stances continue to do this for some time after they have been entirely isolated 
and removed from the body. So slight, indeed, is the connection between 
the movements of organs and parts supplied with plain muscular fibres and 
the will, that these muscular fibres have sometimes been called involuntary 
muscles; but this name is undesirable, since some muscles consisting entirely 
of plain muscular fibres (e. g., the ciliary muscles by which the eye is accom- 
modated for viewing objects at different distances) are directly under the 
influence of the will, and some muscles composed of striated fibres (c. g., 
those of the heart) are wholly removed from the influence of the will. 
We shall best study, however, the facts relating to the movements of 
parts provided with plain muscular fibres when we come to consider the 
parts themselves. 
Like the skeletal muscles, whose nervous elements have been rendered 
functionally incapable 76), plain muscles are much more sensitive to the 
making and breaking of a constant current than to induction-shocks; a 
current, when very brief, like that of an induction-shock, produces little or 
no effect. ON SOME OTHER FORMS OF CONTRACTILE TISSUE. 
119 
The contraction of plain muscular fibres is, as we said, very slow in its 
development and very long in its duration, even when started by a momen- 
tary stimulus, such as a single induction-shock. The contraction after a 
stimulation often lasts so long as to raise the question, whether what has been 
produced is not a single contraction but a tetanus. Tetanus, however, that 
is, the fusion of a series of contractions, seems to be of rare occurrence, 
though probably it may be induced in plain muscular tissue; but the ends 
of tetanus are gained by a kind of contraction which, rare or at least not 
prominent in skeletal muscle, becomes of great importance in plain muscular 
tissue by a kind of contraction called a tonic contraction. The subject is 
one not without difficulties, but it would appear that a plain muscular fibre 
may remain for a very considerable time in a state of contraction, the 
amount of shortening thus maintained being either small or great; it is then 
said to be in a state of tonic contraction. This is especially seen in the case 
of the plain muscular tissue of the arteries, and we shall have to return to 
this matter in dealing with the circulation. 
The muscular tissue which enters into the construction of the heart is of 
a peculiar nature, being on the one hand striated and on the other in some 
respects similar to plain muscular tissue, but this we shall consider in deal- 
ing with the heart itself. 
Ciliary Movement. 
§ 89. Nearly all the movements of the body which are not due to physi- 
cal causes, such as gravity, the diffusion of liquids, etc., are carried out by 
muscles, either striated or plain ; but some small and important effects in the 
way of movement are produced by the action ofcilia, and by those changes 
of protoplasm which are called amoeboid. 
§ 90. Ciliary action, in the form in which it is most common, in mam- 
mals and, indeed, vertebrates, consists in the cilium being at one moment 
straight or vertical, at the next moment being bent down suddenly into 
a hook or sickle form, and then more slowly returning to the straight erect 
position. When the cilia are vigorous this double movement is repeated 
with very great rapidity-so rapidly that the individual movements cannot 
be seen ; it is only when, by reason of fatigue, the action becomes slow that 
the movement itself can be seen ; what is seen otherwise is simply the effect 
of the movement. The movements when slow have been counted at about 
eight (double movements) in a second; probably when vigorous they are 
repeated from twelve to twenty times a second. 
The flexion takes place in one direction only, and all the cilia of each 
cell and, indeed, of all the cells of the same epithelium move in the same 
direction. Moreover, the same direction is maintained during the whole 
life of the epithelium ; thus the cilia of the epithelium of the trachea and 
bronchial passages move during the whole of life in such a way as to drive 
the fluid lying upon them upward toward the mouth ; as far as we know, in 
vertebrates, or at least in mammals, the direction is not and cannot by any 
means be reversed. 
The flexion is very rapid, but the return to the erect position is much 
slower; hence the total effect of the blow, supposing the cilium and the cell 
to be fixed, is to drive the thin layer of fluid in which the cilium is 
working, and which always exists over the epithelium, and any particles 
which may be floating in that fluid, in the same direction as that in which 
the blow is given. If the cell be not attached, but floating free, the effect 
of the blow may be to drive the cell itself backward ; and when perfectly 
fresh ciliated epithelium is teased out and examined in an inert fluid, such 120 
THE CONTRACTILE TISSUES. 
as normal saline solution, isolated cells or small groups of cells may be seen 
rowing themselves about, as it were, by the action of their cilia. 
All the cilia of a cell move, as we have just said, in the same direction, 
but not quite at the same time. If we call the side of the cell toward which 
the cilia bend the front of the cell and the opposite side the back, the cilia 
at the back move a trifle before those at the front, so that the move- 
ment runs over the cell in the direction of the movement itself. Sim- 
ilarly, taking any one cell, the cilia of the cells behind it move slightly 
before and the cilia of the cells in front of it slightly after its own cilia 
move. Hence, in this way, along a whole stretch of epithelium, the 
movement or bending of the cilia sweeps over the surface in ripples or 
waves, very much as, when the wind blows, similar waves of bending sweep 
over a field of corn or tall grass. By this arrangement the efficacy of the 
movement is secured, and a steady stream of fluid carrying particles is 
driven over the surface in a uniform continued direction ; if the cilia of 
separate cells, and still more if the separate cilia of each cell, moved 
independently of the others, all that would be produced would be a series 
of minute " wabbles," of as little use for driving the fluid definitely on- 
ward as the efforts of a boat's crew all rowing out of time are for propelling 
the boat. 
Swift bending and slower straightening is the form of ciliary movement 
generally met with in the ciliated epithelium of mammals and, indeed, of 
vertebrates; but among the invertebrates we find other kinds of movement, 
such as a to-and-fro movement, equally rapid in both directions, a cork- 
screw movement, a simple undulatory movement, and many others. In 
each case the kind of movement seems adapted to secure a special end. 
Thus even in the mammal while the one-sided blow of the cilia of the epi- 
thelial cells secures a flow of fluid over the epithelium, the tail of the sper- 
matozoon, which is practically a single cilium, by moving to and fro in an 
undulatory fashion drives the head of the spermatozoon onward in a straight 
line, like a boat driven by a single oar worked at the stern. 
Why and exactly how the cilium of the epithelial cells bends swiftly and 
straightens slowly, always acting in the same direction, is a problem dif- 
ficult at present to answer fully. Some have thought that the body of 
the cell is contractile, or contains contractile mechanisms pulling upon the 
cilia, which are thus simple passive puppets in the hands of the cells. But 
there is no satisfactory evidence for such a view. On the whole the evi- 
dence is in favor of the view that the action is carried out by the cilium 
itself, that the bending is a contraction of the cilium, and that the straight- 
ening corresponds to the relaxation of a muscular fibre. But even then 
the exact manner in which the contraction bends and the relaxation 
straightens the filament is not fully explained. We have no positive evi- 
dence that a longitudinal half, the inside we might say, of the filament is 
contractile, and the other half, the outside, elastic, a supposition which has 
been made to explain the bending and straightening. In fact, no adequate 
explanation of the matter has as yet been given, and it is really only on 
general grounds we conclude that the action is an effect of contractility. 
In the vertebrate animal cilia are, as far as we know, wholly independent 
of the nervous system, and their movement is probably ceaseless. In such 
animals, however, as infusoria, hydrozoa, etc., the movements in a ciliary 
tract may often be seen to stop and to go on again, to be now fast, now slow, 
according to the needs of the economy, and, as it almost seems, according to 
the will of the creature ; indeed, in some of these animals the ciliary move- 
ments are clearly under the influence of the nervous system. 
Observations with galvanic currents, constant and interrupted, have not ON SOME OTHER FORMS OF CONTRACTILE TISSUE. 
121 
led to any satisfactory results, and, as far as we know at present, cilian 
action is most affected by changes of temperature and chemical media. 
Moderate heat quickens the movements, but a rise of temperature beyond a 
certain limit (about 40° C., in the case of the pharyngeal membrane of the 
frog) becomes injurious; cold retards. Very dilute alkalies are favorable, 
acids are injurious. An excess of carbonic acid or an absence of oxygen 
diminishes or arrests the movements, either temporarily or permanently, 
according to the length of the exposure. Chloroform or ether in slight 
doses diminishes or suspends the action temporarily, in excess kills and dis- 
organizes the cells. 
Amoeboid Movements. 
§ 91. The white blood-corpuscles, as we have said (§ 28), are able of 
themselves to change their form and by repeated changes of form to move 
from place to place. Such movements of the substance of the corpuscles are 
called amoeboid, since they closely resemble and appear to be identical in 
nature with the movements executed by the amoeba and similar organisms. 
The movements of the endoplasm of the vegetable cell seems also to be of 
the same kind. 
The amoeba changes its form (and shifts its place) by throwing out pro- 
jections of its substance, called pseudopodia which may be blunt and short, 
broad bulgings as it were, or may be so long and thin as to be mere filaments, 
or may be of an intermediate character. As we watch the outline of the 
hyaline ectosarc we may see a pseudopodium beginning by a slight bulging 
of the outline; the bulging increases by the neighboring portions of the 
ectosarc moving into it, the movement under the microscope reminding one 
of the flowing of melted glass. As the pseudopodium grows larger and 
engages the whole thickness of the ectosarc at the spot, the granules of the 
endosarc may be seen streaming into it, forming a core of endosarc in the 
middle of the bulging of ectosarc. The pseudopodium may continue to 
grow larger and larger at the expense of the rest of the body, and eventually 
the whole of the amoeba including the nucleus may, as it were, have 
passed into the pseudopodium; the body of the amoeba will now occupy 
the place of the pseudopodium instead of its old place; in other words, it 
will in changing its form have also changed its place. 
During all these movements, and during all similar amoeboid movements, 
the bulk of the organism will, as far as can be ascertained, have remained 
unchanged ; the throwing out a pseudopodium in one direction is accom- 
panied by a corresponding retraction of the body in other directions. If, as 
sometimes happens, the organism throws out pseudopodia in various direc- 
tions at the same time, the main body from which the pseudopodia project is 
reduced in thickness; from being a spherical lump, for instance, it becomes a 
branched film. The movement is brought about not by increase or decrease 
of substance but by mere translocation of particles ; a particle which at one 
moment was in one position moves into a new position, several particles thus 
moving toward the same point cause a bulging at that point, and several 
particles moving away from the same point cause a retraction at that point; 
but no two particles get nearer to each other so as to occupy together less 
space and thus lead to condensation of substance, or get further from each 
other so as to occupy more space and thus lead to increase of bulk. 
In this respect, in that there is no change of bulk but only a shifting of 
particles in their relative position to each other, the amoeboid movement 
resembles a muscular contraction ; but in other respects the two kinds of 
movement seem different, and the question arises, have we the right to speak 122 
GENERAL . FEATURES OF NERVOUS TISSUES. 
of the substance which can only execute amoeboid movements as being con- 
tractile ? 
We may, if we admit that contractility is at bottom simply the power of 
shifting the relative position of particles, admit that muscular contraction is 
a specialized form of contraction. In a plain muscular fibre (which we may 
take as simpler than the striated muscle) the shifting of particles is special- 
ized in the sense that it has always a definite relation to the long axis of the 
fibre; when the fibre contracts a certain number of particles assume a new 
position by moving at right angles to the long axis of the fibre, and the fibre 
in consequence becomes shorter and broader. In a white blood-corpuscle, 
amoeba, or other organism executing amoeboid movements, the shifting of the 
particles is not limited to any axis of the body of the organism ; at the same 
moment one particle or one set of particles may be moving in one direction, 
and another particle or another set of particles in another direction. A 
pseudopodium short and broad, or long, thin, and filamentous, may be thrust 
out from any part of the surface of the body and in any direction ; and a 
previously existing pseudopodium may be shortened, or be wholly drawn 
back into the substance of the body. 
CHAPTER III. 
ON THE MORE GENERAL FEATURES OF NERVOUS TISSUES. 
§ 92. In the preceding chapter we have dealt with the properties of nerves 
going to muscles, the nerves which we called motor, and have incidentally 
spoken of other nerves which we called sensory. Both these kinds of nerves 
are connected with the brain and spinal cord, and form part of the general 
nervous system. We shall have to study hereafter in detail the brain and 
spinal cord ; but the nervous system intervenes so repeatedly in the processes 
carried out by other tissues that it will be desirable, before proceeding 
further, to discuss some of its more general features. 
The nervous system consists (1) of the brain and spinal cord forming 
together the cerebrospinal axis or central nervous system, (2) of the nerves 
passing from that axis to nearly all parts of the body, those which are con- 
nected with the spinal cord being called spinal and those which are connected 
with the brain, within the cranium, being called cranial, and (3) of ganglia 
distributed along the nerves in various parts of the body. 
The spinal cord obviously consists of a number of segments or metameres, 
following in succession along its axis, each metamere giving off on each side 
a pair of spinal nerves ; and a similar division into metameres may be traced 
in the brain, though less distinctly, since the cranial nerves are arranged in 
manner somewhat different from that of the spinal nerves. We may take a 
single spinal metamere, represented diagrammatically in Fig. 35, as illus- 
trating the general features of the nervous system ; and since the half on 
one side of the median line resembles the half on the other side we may 
deal with one lateral half only. 
Each spinal nerve arises by two roots. The metamere of the central ner- 
vous system C consists, as we shall hereafter see, of gray matter Gr in the 
interior and white matter W on the outside. From the anterior part of 
gray matter is given off the anterior nerve root A and from the posterior 
part the posterior nerve root P. The latter passes into a swelling or gan- 
glion G, " the ganglion of the posterior root," or more shortly " the spinal GENERAL FEATURES OF NERVOUS TISSUES. 
123 
Fig. 35. 
Scheme of the Nerves of a Segment of the Spinal Cord : Gr gray, IF white matter of spinal 
cord. A anterior, P posterior root G ganglion on the posterior root A'whole nerve, A" spinal 
nerve proper,ending in M skeletal or somatic muscle, S somatic sensory cell or surface, Xin 
other ways. V visceral nerve (white ramus communicans) passing to a ganglion of the sympa- 
thetic chain S, and passing on as V to supply the more distant ganglion a, then as V" to the peri- 
pheral ganglion a' and ending in m splanchnic muscle, s splanchnic sensory cell or surface, x 
other possible splanchnic endings. From S is given off the revehent nerve r. v (gray ramus 
communicans) which partly passes backward toward the spinal cord, and partly runs as v. m, in 
connection with the spinal nerve, to supply vasomotor (constrictor) fibres to the muscles (m') of 
bloodvessels in certain parts, for example, in the limbs. Sy, the sympathetic chain uniting the 
ganglia of the series 2. The terminations of the other nerves arising from S, a, a' are not 
shown. 124 
GENERAL FEATURES OF NERVOUS TISSUES. 
ganglion the anterior root does not pass into this ganglion. Beyond the 
ganglion the roots join to form the nerve trunk N. We shall later on give 
the evidence that the nerve fibres composing the posterior root P are, as far 
as we know at present, exclusively occupied in carrying nervous impulses 
from the tissues of the body to the central nervous system, and that the fibres 
composing the anterior root A are similarly occupied in carrying impulses 
from the central nervous system to the several tissues: that is to say, the 
former is made up of sensory fibres, or (since the impulses passing along 
them to the central system may give rise to effects other than sensations) 
afferent fibres, while the latter is made up of motor, or (since the impulses 
passing along them from the central nervous system may produce effects 
other than movements), efferent fibres. The nerve trunk N is consequently 
a mixed nerve composed of afferent and efferent fibres. 
By far the greater part of this mixed nerve, dividing into various branches, 
is distributed (Ar) to the skin and the skeletal muscles, some of the fibres 
(motor) ending in muscular fibres (M), others (sensory) ending in epithelial 
cells (S) connected with the skin, which we shall consider hereafter under 
the name of sensory epithelial cells, while others, A, after dividing into 
minute branches and forming plexuses end, in ways not yet definitely deter- 
mined, in tissues associated with the skin and skeletal muscles. Morpholo- 
gists distinguish the parts which go to form the skin, skeletal muscles, etc., 
as somatic, from the splanchnic parts which go to form the viscera. We may, 
accordingly, call this main part of the spinal nerve the somatic division of 
the nerve. 
Soon after the mixed nerve N leaves the spinal canal, it gives off a small 
branch F, which under the name of (white) ramus communicans, joins one 
of a longitudinal series of ganglia (ff) conspicuous in the thorax as the main 
sympathetic chain. This branch is destined to supply the viscera, and might, 
therefore, be called the splanchnic division of the spinal nerve. We may say 
at once, without entering into details, that the whole of the sympathetic 
system, with its ganglia, plexuses, and nerves, is to be regarded as a develop- 
ment or expansion of the visceral or splanchnic divisions of certain spinal 
nerves. By means of this system splanchnic fibres from the central nervous 
system are distributed to the tissues of the viscera, some of them on their 
way passing through secondary ganglia a, and, it may be, tertiary ganglia. 
There are, however, as we shall see, certain nerves or fibres which do not run 
in the sympathetic system, and yet are distributed to the viscera and are 
"splanchnic" in nature. We cannot, therefore, use the word sympathetic 
to denote all the fibres which are splanchnic in nature. On the other hand, 
the " splanchnic nerves " of the anatomist form a part only of the splanchnic 
system in the above sense, the term thus used is limited to particular nerves 
of the splanchnic system distributed to the abdomen ; and the double use of 
the term splanchnic might lead to confusion. The difficulty, may, perhaps, 
be avoided by calling the splanchnic nerves of the anatomist " abdominal 
splanchnic." The majority of these splanchnic fibres seem to be efferent in 
nature, carrying impulses from the central nervous system to the tissues, 
some ending in plain muscular fibres (m), others in other ways (x) ; but some 
of the fibres are afferent and convey impulses from the viscera to the central 
nervous system, and it is probable that some of these begin or end in epithe- 
lial cells of the viscera (s). 
We shall have occasion in the next chapter to speak of nerves which 
govern the bloodvessels of the body, the so-called vasomotor nerves. A cer- 
tain class of these, namely, the vaso-constrictor nerves or fibres are branches 
of the splanchnic divisions of the cerebro-spinal nerves, and, as we shall see, 
the vaso-constrictor nerves of the skeletal muscles, skin, and other parts sup- 125 
GENERAL FEATURES OF NERVOUS TISSUES. 
plied by somatic nerves, after running for some distance in the splanchnic 
division (V), turn aside (r. v and v. m) and join the somatic division, the 
fibres of which they accompany on their way to the tissues whose bloodvessels 
(m') they supply. 
We have seen (§ 68) that a nerve going to a muscle is composed of nerve 
fibres, chiefly medullated, some, however, being non-medullated, bound 
together by connective tissues. The same description holds good for the 
whole somatic division of each of the spinal nerves. The splanchnic division 
also consists of medullated and non-medullated fibres bound together by con- 
nective tissue, but in it the non-medullated fibres preponderate, some branches 
appearing to contain hardly any medullated fibres at all. The non-medul- 
lated fibres which are found in the somatic division appear to be fibres which 
have joined that division from the splanchnic division. So prominent are 
non-medullated fibres in splanchnic nerves and hence in the sympathetic 
system that they are sometimes called sympathetic fibres. 
We have said that the axis-cylinder, whethex' of a medullated or non- 
medullated fibre, is to be considered as a long-drawn-out process of a nerve 
cell. Nerve cells are found in three main situations. 1. In the central ner- 
vous system, the brain, and spinal cord. 2. In the several ganglia placed 
along the course of the nerves, both the spinal ganglia, and the ganglia of 
the splanchnic or sympathetic system. 3. At the terminations of nerves in 
certain tissues. Some of these lattei' are to be regarded as small, more or 
less terminal, ganglia, and similar minute ganglia consisting of two oi' three 
cells only are found frequently along the course of splanchnic nerves and 
occasionally along the course of spinal nerves; such cells really, therefore, 
belong to the second group. But besides this, in certain situations, as, for 
instance, in certain organs of the skin, and in the organs of special sense, 
nerves, generally afferent or sensory in nature, either actually end in, or at 
their termination are connected with, cells which appear to be of a nervous 
nature; such cells form a distinct category by themselves. 
Hence along its whole course a nerve consists exclusively of nerve fibres 
(and the connective tissue supporting them), except in the central nervous 
system from which it springs in the ganglia, great and small, through which 
it passes, or which are attached to it at one part or another of its course, in 
both of which situations nerve cells are found, and at its termination where 
its fibres may end in nerve cells. 
The features of these nerve cells differ in these several situations. The 
characters of the terminal cells which, as we have said, are chiefly sensory, 
and the structure of the brain and spinal cord, we shall study in detail later 
on. We may here confine our attention to the nerve cells of the ganglia 
and to some of the broad features of the nerve cells of the spinal cord. 
$ 93. Spinal ganglia. When a longitudinal section of a spinal ganglion 
is examined under a low power, the fibres of the posterior root as they enter 
the ganglion are observed to spread out and pass between relatively large 
and conspicuously nucleated cells, which are to a large extent arranged in 
groups, somewhat after the fashion of a bunch of grapes. These are the 
nerve cells ; they have frequently a diameter of about 100/', but may be still 
larger or may be much smaller. In a transverse section it will be observed 
that a large compact mass of these cells lies on the outer side of the gang- 
lion, and that the racemose groups on the inner side are smaller. A quantity 
of connective tissue carrying bloodvessels and lymphatics runs between the 
groups, and passing into each group runs between the cells and fibres; and 
a thick wrapping of connective tissue continuous with the sheath of the 
nerves surrounds and forms a sheath for the whole ganglion. 
Each of the nerve cells-ganglionic cells, as they are called-examined 126 
GENERAL FEATURES OF NERVOUS TISSUES. 
under a higher power, either after having been isolated or in an adequately 
thin and prepared section, will present the following features: 
The cell consists of a cell body which is, normally, pear-shaped, having a 
broad end in which is placed the nucleus, and a narrow' end w'hich thins out 
into a stalk, and is eventually continued on as a nerve fibre. The substance 
of the cell body is of the kind which we call finely granular protoplasm ; 
sometimes there is an appearance of fibrillation, the fibrillae passing in 
various directions in the body of the cell and being gathered together in a 
longitudinal direction in the stalk. Sometimes the cell body immediately 
around the nucleus appears of a different grain from that nearer the stalk, 
and not unfrequently near the nucleus is an aggregation of discrete pigment 
granules imbedded in the protoplasm. 
The nucleus, like the nuclei of nearly all nerve cells, is large and con- 
spicuous, and when in a normal condition is remarkably clear and refractive, 
though it appears to consist like other nuclei of a nuclear membrane and 
network and nuclear interstitial material. Even more conspicuous, perhaps, 
is a very large spherical, highly refractive nucleolus; occasionally more than 
one nucleolus is present. 
Surrounding the cell body is a distinct sheath or capsule consisting of a 
transparent, hyaline, or faintly fibrillated membrane, lined on the inside by 
one layer or by two layers of flat, polygonal, nucleated epithelioid cells or 
plates; that is to say, cells which resemble epithelium cells, but differ not 
only in being extremely flattened, but also in the cell body being transformed 
from ordinary granular protoplasm into a more transparent differentiated 
material. In stained specimens the nuclei of these plates are very conspicu- 
ous. Under normal conditions this sheath is in close contact with the whole 
body of the cell, but in hardened and prepared specimens the cell body is 
sometimes seen shrunk away from the sheath, leaving a space between them. 
Occasionally the cell body while remaining attached to the sheath at three 
or four or more points is retracted elsewhere, and accordingly assumes a 
more or less stellate form ; but this artificial condition must not be con- 
founded with the natural branched form which, as we shall see, other kinds 
of nerve cells possess. 
When a section is made through a hardened ganglion, the plane of the 
section passes through the stalks of few only of the cells, and that rarely for 
any great distance along the stalk, since in the case of many of the cells the 
stalk is more or less curved and consequently runs out of the plane of sec- 
tion ; but in properly isolated cells we can see that in many cases, and we 
have reasons to believe that in all cases, the stalk of the cell is, as we have 
said, continued on into a nerve fibre. As the cell body narrows into the 
stalk several nuclei make their appearance, lodged on it; these are small 
granular nuclei, wholly unlike the nucleus of the cell body itself, and more 
like, though not quite like, the nuclei of the neurilemma of a nerve. They 
are probably of the same nature as the latter; and, indeed, as we trace the 
narrowing stalk downward a fine delicate sheath, which, if present, is at 
least not obvious over the cell body, makes its appearance, and a little further 
on, between this sheath, which is now clearly a neurilemma, and the stalk of 
the cell body, which has by this time become a cylinder of uniform width 
and is now obviously an axis-cylinder, a layer of medulla, very fine at first, 
but rapidly thickening, is established. The stalk of the nerve cell thus 
becomes an ordinary medullated nerve fibre. The sheath of the cell is con- 
tinued also on to the nerve fibre, not as was once thought as the neurilemma, 
but as that special sheath of connective tissue of which we have already 
spoken as Henle's sheath, and which ultimately becomes fused with the con- 
nective tissue of the nerve. GENERAL FEATURES OF NERVOUS TISSUES. 
127 
At some variable distance from the cell the nerve fibre bears the first 
node, and either at this or some early succeeding node the fibre divides into 
two; as we have seen, division of a medullated nerve fibre always takes 
place at a node. The two divisions thus arising run in opposite directions, 
forming in this way a T-piece; and while one division runs in one direction 
toward the posterior root, the other runs in an opposite direction toward the 
nerve trunk. The nerve cell is thus, as it were, a side piece attached to a 
fibre passing through the ganglion on its way from the posterior root to the 
nerve trunk. It cannot be said that in any one ganglion this connection has 
been traced in the case of every nerve cell of the ganglion ; but the more 
care is taken, and the more successful the preparation, the greater is the 
number of cells which may be isolated with their respective T-pieces; so 
that we may conclude that, normally, every cell of a ganglion is connected 
on the one hand with a fibre of the posterior root, and on the other hand 
with a fibre of the nerve trunk. We have reasons further to believe that 
every fibre of the posterior root in passing through the ganglion on its way 
to the mixed nerve trunk is thus connected with a nerve cell; but this has 
been called in question. In certain animals-for instance, certain fishes- 
the cells of the spinal ganglia are not pear-shaped, but oval or fusiform, and 
each narrow end is prolonged into a nerve fibre, one end thus being con- 
nected with the posterior root and the other with the nerve trunk. In such 
a case the nerve cell is simply a direct enlargement of the axis-cylinder, with 
a nucleus placed in the enlargement. The nerve cells above described are 
similar enlargements, also bearing nuclei, placed not directly in the course 
of the axis-cylinder, but on one side, and connected with the axis-cylinder 
by the cross-piece of the T-piece. Hence the ordinary ganglion cell is 
spoken of as being unipolar, those of fishes being called bipolar. 
In examining spinal ganglia cells are sometimes found which bear no 
trace of any process connecting them with a nerve fibre. Such cells are 
spoken of as apolar. It is possible that such a cell may be a young cell 
which has not yet developed its nerve process, or an old cell which has by 
degeneration lost its nerve process. 
§ 94. The ganglia of the splanchnic system, like the spinal ganglia, con- 
sist of nerve cells and fibres imbedded in connective tissue, which, however, 
is of a looser and less compact nature in them than in the spinal ganglia. 
As far as the characters of their nuclei, the nature of their cell substance, 
and the possession of a sheath, are concerned, what has been said concerning 
the nerve cells of spinal ganglia holds, in general, good for those of splanch- 
nic ganglia; and, indeed, in certain ganglia of the splanchnic system con- 
nected with the cranial nerves, the nerve cells appear to be wholly like those 
of spinal ganglia. In most splanchnic ganglia, however, in those which are 
generally called sympathetic ganglia, two important differences may be ob- 
served between what we may call the characteristic nerve cell of the splanch- 
nic ganglion and the cell of the spinal ganglion. 
In the first place, while the nerve cell of the spinal ganglia has one 
process only, the nerve cell of the splanchnic ganglia may have, and fre- 
quently has, two, three, or even four or five processes; it is a multipolar 
cell. 
In the second place, while these processes of the splanchnic ganglion 
cell are continued on as nerve fibres, as is the single process of the spinal 
ganglion cell, the nerve fibres so formed are, in the case of most of the pro- 
cesses of a cell, and sometimes in the case of all the processes, non-medul- 
lated fibres, and remain non-medullated ^s far as they can be traced. In 
some instances one process becomes at a little distance from the cell a 
medullated fibre, while the other processes become non-medullated fibres; 128 
GENERAL FEATURES OF NERVOUS TISSUES. 
and we are led to believe that in this case the medullated fibre is proceeding 
to the cell on its way from the central nervous system, and that the non- 
medullated fibres are proceeding from the cell on their way to more peri- 
pherally placed parts; the nerve cell seems to serve as a centre for the 
division of nerve fibres, and also for the change from medullated to non- 
medullated fibres. 
In consequence of its thus possessing several processes, the splanchnic 
ganglion cell is more or less irregular and often star-like in form, in con- 
trast to the pear shape of the spinal ganglion cell. But in certain situations 
in certain animals-for instance, in the frog-in many of the ganglia of the 
abdomen, and in the small ganglia in the heart, pear-shaped splanchnic 
ganglion cells are met with. In such cases the nucleated sheath is distinctly 
pear-shaped or balloon-shaped, and the large conspicuous nucleus is placed, 
as in the spinal ganglion cell, near the broad end, but the cell substance of 
the cell is gathered at the stalk, not into a single fibre, but into two fibres, 
one of which is straight and the other twisted spirally round the straight 
one. The two fibres run for some distance togther in the same funnel-shaped 
prolongation of the nucleated sheath of the cell, but eventually separate, 
each fibre acquiring a sheath (sheath of Henle) of its own. Generally, if not 
always, one fibre, usually the straight one, becomes a medullated fibre, while 
the other, usually the twisted or spiral one, is continued as a non-medullated 
fibre, while within the common nucleated sheath both fibres, especially the 
spiral one, bear nuclei of the same character as those seen in a corresponding 
situation in the spinal ganglion cell. It has been maintained that the straight 
and spiral fibres take origin from different parts of the nerve cell, but this 
has not been definitely proved. 
In the walls of the intestine, in connection with splanchnic nerves, are 
found peculiar nerve cells forming what are known as the plexuses of Meiss- 
ner and Auerbach, but we shall postpone for the present any description of 
these or of other peculiar splanchnic cells. 
§ 95. In the central nervous system nerve cells are found in the so-called 
gray matter only; they are absent from the white matter. In the gray matter 
of the spinal cord, in the parts spoken of as the anterior cornua, we meet 
with remarkable nerve cells of the following characters. The cells are large, 
varying in diameter from 50/z to 140m, and each consists of a cell body sur- 
rounding a large conspicuous refractive nucleus, in which is placed an even 
still more conspicuous nucleolus. The nucleus resembles the nuclei of the 
ganglion cells already described, and the cell body, like the cell body of the 
ganglion cells, is composed of finely granular protoplasm, often fibrillated, 
though generally obscurely so; frequently a yellowish-brown pigment is 
deposited in a part of the cell body not far from the nucleus. The cell body 
is prolonged sometimes into two or three only, but generally into several 
processes, which appear more distinctly fibrillated than the more central 
parts of the cell body. These processes are of two kinds. One process, and, 
apparently, one only, but in the case of the cells of the anterior cornu, 
always one, is prolonged as a thin unbranched band, which retains a fairly 
uniform diameter for a considerable distance from the cell, and when suc- 
cessfully traced is found sooner or later to acquire a medulla and to become 
the axis-cylinder of a nerve fibre; the processes which thus pass out from 
the gray matter of the anterior cornu through the white matter form the 
anterior roots of the spinal nerve. Such a process is accordingly called the 
axis-cylinder process. The other processes of the cell rapidly branch, and so 
divide into very delicate filaments which are soon lost to view in the sub- 
stance of the gray matter. Indeed, the gray matter is partly made up of a 
plexus of delicate filaments arising, on the one hand, from the divisions of GENERAL FEATURES OF NERVOUS TISSUES. 
129 
processes of the nerve cells, and on the other, from the division of the axis- 
cylinders of fibres running in the gray matter. 
The cell is not surrounded like the ganglion cell by a distinct sheath. As 
we shall see later on, while treating in detail of the central nervous system, 
all the nervous elements of the spinal cord are supported by a network or 
spongework of delicate peculiar tissue called neuroglia, analogous to and 
serving much the same function as, but different in origin and nature from, 
connective tissue. This neuroglia forms a sheath to the nerve cell and to its 
processes, as well as to the nerve fibres running both in the white and the 
gray matter; hence within the central nervous system the fibres, whether 
medullated or not possess no separate neurilemma; tubular sheaths of the 
neuroglia give the axis-cylinder and medulla all the support they need. 
All the nerve cells of the anterior cornu probably possess an axis-cylinder 
process, and other cells similarly provided with an axis-cylinder process are 
found in other parts of the gray matter. But in certain parts, as, for instance, 
in the posterior cornu, many of the cells appear to possess no axis-cylinder 
process ; in such cases all the processes appear to branch out rapidly into fine 
filaments. Except for this absence, apparent or real, of an axis-cylinder 
process, such cells resemble in their general features the cells of the anterior 
cornu, though they are generally somewhat smaller. Speaking generally, 
the great feature of the nerve cells of the central nervous system as distin- 
guished from the ganglion cells is the remarkable way in which their pro- 
cesses branch off into a number of delicate filaments, corresponding to the 
delicate filaments or fibrillae in which at its termination in the tissues the 
axis-cylinder of a nerve often ends. 
§ 96. From the above description it is obvious that in the spinal cord 
(to which as representing the central nervous system we may at present con- 
fine ourselves, leaving the brain for later study) afferent fibres (fibres of the 
posterior root) are in some way by means of the gray matter brought into 
connection with efferent fibres (fibres of the anterior root) ; in other words, 
the spinal cord is a centre uniting afferent and efferent fibres. The spinal 
ganglia are not centres in this sense ; the nerve cells composing the ganglia 
are simply relays on the afferent fibres of the posterior root, they have no 
connection whatever with efferent fibres, they are connected with fibres of 
one kind only. Concerning the ganglia of the splanchnic system we cannot 
in all cases make at present a positive statement, but the evidence so far at 
our disposal points to the conclusion that in them as in the spinal ganglia 
each nerve cell belongs to fibres of one function only, that where several 
processes of a cell are prolonged into nerve fibres these fibres have all the 
same function, the nerve cell being, as in the spinal ganglia, a mere relay. 
We have no satisfactory evidence that in a ganglion the fibres springing 
from or connected with one cell join another cell so as to convert the 
ganglion into a centre joining together cells whose nerve fibres have dif- 
ferent functions. 
We shall have later on to bring forward evidence that the nucleated cell 
body of a nerve cell in a ganglion or elsewhere is in some way or other con- 
nected with the nutrition, the growth and repair of the nerve fibres springing 
from it. Besides this nutritive function the multipolar cells of the splanchnic 
ganglia appear to serve the purpose of multiplying the tracts along which 
nervous impulses may pass. An impulse, for instance, reaching a multipolar 
cell in one of the proximal sympathetic ganglia along one fibre or process 
(the fibre in very many cases being a medullated fibre) can pass out of the 
cell in various directions along several processes or fibres, which, in the 
majority of cases, if not always, are non-medullated fibres. Thus these nerve 
cells are organs of distribution for impulses of the same kind. What further 130 
GENERAL FEATURES OF NERVOUS TISSUES. 
modifications of the impulses thus passing through them these ganglia may 
bring about we do not know. 
It is only in some few instances that we have any indications, and those 
of a very doubtful character, that the ganglia of the splanchnic system can 
carry out either of the two great functions belonging to what is physiologi- 
cally called a nerve centre, namely the function of starting nervous impulses 
anew from within itself, the function of an automatic centre so called, and the 
function of being so affected by the advent of afferent impulses as to send 
forth in response efferent impulses, of converting, as it were, afferent into 
efferent impulses, the function of a reflex centre so called. 
In the central nervous system, the brain with the spinal cord, which 
supplies the nervous centres for automatic actions and for reflex actions- 
indeed all the processes taking place in the central nervous system (at 
least all such as come within the province of physiology)-fall into or 
may be considered as forming part of one or the other of these two 
categories. 
§ 97. Reflex actions. In a reflex action afferent impulses reaching the 
nervous centre give rise to the discharge of efferent impulses, the discharge 
following so rapidly and in such away as to leave no doubt that it is caused 
by the advent at the centre of the afferent impulses. Thus a frog from 
which the brain has been removed while the rest of the body has been left 
intact will frequently remain quite motionless (as far at least as the skeletal 
muscles are concerned) for an almost indefinite time ; but if its skin be 
pricked, or if in other ways afferent impulses be generated in afferent 
fibres by adequate stimulation, movement of the limbs or body will imme- 
diately follow. Obviously in this instance the stimulation of afferent fibres 
has been the cause of the discharge of impulses along efferent fibres. 
The machinery involved in a such a reflex act consists of three parts : (1) 
the afferent fibres, (2) the nerve centre, in this case the spinal cord, and (3) 
the efferent fibres. [Fig. 36.] If any one of these three 
parts be missing the reflex act cannot take place; if, for 
instance, the afferent nerves or the efferent nerves be 
cut across in their course, or if the centre, the spinal 
cord, be destroyed, the reflex action cannot take place. 
Reflex actions can be carried out by means of the 
brain, as we shall see while studying that organ in de- 
tail, but the best and clearest examples of reflex action 
are manifested by the spinal cord ; in fact, reflex action 
is one of the most important functions of the spinal 
cord. We shall have to study the various reflex actions 
of the spinal cord in detail hereafter, but it will be 
desirable to point out here some of their general fea- 
tures. 
When we stimulate the nerve of a muscle-nerve preparation the 
result, though modified in part by the condition of the muscle and 
nerve, whether fresh and irritable or exhausted, for instance, is directly 
dependent on the nature and strength of the stimulus. If we use a single 
induction-shock we get a simple contraction, if the interrupted current 
we get a tetanus, if we use a weak shock we get a slight contraction, if 
a strong shock a large contraction, and so on ; and throughout our study 
of muscular contractions we assumed that the amount of contraction might 
be taken as a measure of the magnitude of the nervous impulses generated 
by the stimulus. And it need hardly be said that when we stimulate cer- 
tain fibres only of a motor nerve, it is only the muscular fibres in which 
those nerve fibres end which are thrown into contraction. 
[Fig. 36. 
Diagram, illustrating 
Simplest Form of Re- 
flex Apparatus.] 131 
GENERAL FEATURES OF NERVOUS TISSUES. 
Iii a reflex action, on the other hand, the movements called forth by the 
same stimulus may be in one case insignificant and in another violent and 
excessive, the result depending on the arrangements and condition of the 
reflex mechanism. Thus the mere contact of a hair with the mucous mem- 
brane lining the larynx, a contact which can originate only the very slightest 
afferent impulses, may call forth a convulsive fit of coughing, in which a very 
large number of muscles are thrown into violent contractions; whereas the 
same contact of the hair with other surfaces of the body may produce no 
obvious effect at all. Similarly, while in the brainless but otherwise normal 
frog, a slight touch on the skin of the flank will produce nothing but a faint 
flicker of the underlying muscles, the same touch on the same part of a frog 
poisoned with strychnine will produce violent lasting tetanic contractions of 
nearly all the muscles of the body. Motor impulses, as we have seen, travel 
along motor nerves without any great expenditure of energy, and probably 
without increasing that expenditure as they proceed ; and the same is appa- 
rently the case with afferent impulses passing along afferent nerves. When, 
however, in a reflex action afferent impulses reach the nerve centre, a change 
in the nature and magnitude of the impulses takes place. It is not that in 
the nerve centre the afferent impulses are simply turned aside or reflected 
into efferent impulses ; and hence the name " reflex " action is a bad one. 
It is rather that the afferent impulses act afresh, as it were, as a stimulus to 
the nerve centre, producing according to circumstances and conditions either 
a few weak efferent impulses or a multitude of strong ones. The nerve centre 
may be regarded as a collection of explosive charges ready to be discharged 
and so to start efferent impulses along certain efferent nerves, and these 
charges are so arranged and so related to certain afferent nerves, that afferent 
impulses reaching the centre along those nerves may in one case discharge a 
few only of the charges, and so give rise to feeble movements, and in another 
case discharge a very large number, and so give rise to large and violent 
movements. In a reflex action then the number, intensity, character, and 
distribution of the efferent impulses, and so the kind and amount of move- 
ment, will depend chiefly on what takes place in the centre, and this will, in 
turn, depend, on the one hand, on the condition of the centre, and, on the 
other, on the special relations of the centre of the afferent impulses. At 
the same time we are able to recognize in most reflex actions a certain rela- 
tion between the strength of the stimulus, or the magnitude of the afferent 
impulses and the extent of the movement or the magnitude of the efferent 
impulses. 
We may add, without going more fully into the subject here, that in 
most reflex actions a special relation may be observed between the part 
stimulated and the resulting movement. In the simplest cases of reflex 
action this relation is merely of such a kind that the muscles thrown into 
action are those governed by a motor nerve which is the fellow of the sen- 
sory nerve, the stimulation of which calls forth the movement. In the more 
complex reflex action of the brainless frog, and in other cases, the relation 
is of such a kind that the resulting movement bears an adaptation to the 
stimulus; the foot is withdrawn from the stimulus, or the movement is cal- 
culated to push or wipe away the stimulus. In other words, a certain 
purpose is evident in the reflex action. 
Thus in all eases, except perhaps the very simplest, the movements called 
forth by a reflex action are exceedingly complex compared with those which 
result from the direct stimulation of a motor trunk. 
§ 98. Automatic actions. Efferent impulses frequently issue from the 
brain and spinal cord and so give rise to movements without being obviously 
preceded by any stimulation. Such movements are spoken of as automatic 132 
GENERAL FEATURES OF NERVOUS TISSUES. 
or spontaneous. The efferent impulses in such cases are started by changes 
in the nerve centre which are not the immediate result of the arrival at the 
nerve centre of afferent impulses from without, but which appear to arise in 
the nerve centre itself. Changes of this kind may recur rhythmically ; thus, 
as we shall see, we have reason to think that in a certain part of the medulla 
oblongata changes of the nervous material, recurring rhythmically, lead to 
the rhythmic discharge along certain nerves of efferent impulses whereby 
muscles connected with the chest are rhythmically thrown into action and a 
rhythmically repeated breathing is brought about. And other similar rhyth- 
mic automatic movements may be carried out by other parts of the spinal 
cord. 
From the brain itself a much more varied and apparently irregular dis- 
charge of efferent impulses, not the obvious result of any immediately fore- 
going afferent impulses, and therefore not forming part of reflex actions, is 
very common, constituting what we speak of as volition, efferent impulses 
thus arising being called volitional or voluntary impulses. The spinal cord 
apart from the brain does not appear capable of executing these voluntary 
movements; but to this subject we shall return when we come to speak of 
the central nervous system in detail. 
We said just now that there is no satisfactory evidence that the ganglia 
of the splanchnic system ever act as centres of reflex action. The evidence, 
however, that these ganglia may serve as centres of rhythmic automatic 
action seems at first sight of some strength. Several organs of the body 
containing muscular tissue, the most notable being the heart, are during life 
engaged in rhythmic automatic movements, and in many cases continue these 
movements after removal from the body. In nearly all these cases ganglia 
are present in connection with the muscular tissue; and the presence and 
intact condition of these ganglia seem, at all events in many cases, in some 
way essential to the due performance of the rhythmic automatic movements. 
Indeed, it has been thought that the movements in question are really due 
to the rhythmic automatic generation in the cells of these ganglia of efferent 
impulses which passing down to the appropriate muscular fibres call forth 
the rhythmic movement. When we come to study these movements in detail, 
we shall find reasons for coming to the conclusion that this view is not sup- 
ported by adequate evidence; and, indeed, though it is perhaps immature to 
make a dogmatic statement, all the evidence goes, as we have already said, 
to show that the great use of the ganglia of the splanchnic system, like that 
of the spinal ganglia, is connected with the nutrition of the nerves, and that 
these structures do not, like the central nervous system, act as centres either 
automatic or reflex. 
§ 99. Inhibitory nerves. We have said that the fibres of the anterior 
root should be called efferent rather than motor, because though they all 
carry impulses outward from the central nervous system to the tissues, the 
impulses which they carry do not in all cases lead to the contraction of mus- 
cular fibres. Some of these efferent fibres are distributed to glandular struc- 
tures, for instance to the salivary glands, and impulses passing along these 
lead to changes in epithelial cells and their surroundings whereby, without 
any muscular contraction necessarily intervening, secretion is brought about; 
the action of these fibres of secretion we shall study in connection with 
digestion. 
Besides this there are efferent fibres going to muscular tissue or at all 
events to muscular organs, the impulses passing along which, so far from 
bringing about muscular contraction, diminish, hinder, or stop movements 
already in progress. Thus, if when the heart is beating regularly, that is 
to say, when the muscular fibres which make up the greater part of the FEATURES OF VASCULAR APPARATUS. 
133 
heart are rhythmically contracting, the branches of the pneumogastric nerve 
going to the heart be adequately stimulated, for instance with the interrupted 
current, the heart will stop beating; and that not because the muscles of 
the heart are thrown into a continued tetanus, the rhythmic alternation of 
contraction and relaxation being replaced by sustained contraction, but be- 
cause contraction disappears altogether, all the muscular fibres of the heart 
remaining for a considerable time in complete relaxation and the whole heart 
being quite flaccid. If a weaker stimulus be employed the beat may not be 
actually stopped, but slowed or weakened. And. as we shall see, there are 
many other cases where the stimulation of efferent fibres hinders, weakens, 
or altogether stops a movement already in progress. Such an effect is called 
an inhibition, and the fibres stimulation of which produces the effect are 
called " inhibitory " fibres. 
The phenomena of inhibition are not, however, confined to such cases as 
the heart, where the efferent nerves are connected with muscular tissues. 
Thus the activity of a secreting gland may be inhibited, as for instance when 
emotion stops the secretion of saliva, and the mouth becomes dry from fear. 
In this instance, however, it is probable that inhibition is brought about not 
by inhibitory impulses passing to the gland and arresting secretion in the 
gland itself, but rather by an arrest, in the central nervous system, of the 
nervous impulses which, normally, passing down to the gland, excite it 
to action. And, indeed, as we shall see later on, there are many illus- 
trations of the fact that afferent impulses reaching a nervous centre, 
instead of stimulating it to activity, may stop or inhibit an activity previ- 
ously going on. In fact it is probable, though not actually proved in every 
case, that wherever in any tissue, energy is being set free, nervous impulses 
brought to bear on the tissue may affect the rate or amount of the energy 
set free in two different ways ; on the one hand, they may increase or quicken 
the setting free of energy, and on the other hand, they may slacken, hinder, 
or inhibit the setting free of energy. And in at all events a large number 
of cases it is possible to produce the one effect by means of one set of nerve 
fibres and the other effect by another set of nerve fibres. We shall have 
occasion, however, to study the several instances of this double action in the 
appropriate places. 
CHAPTER IV. 
THE VASCULAR MECHANISM. 
The Structure and Main Features of the Vascular Apparatus. 
§ 100. The blood is the internal medium on which the tissues live; 
from it the tissues draw their food and oxygen, to it they give up the pro- 
ducts of waste matters which they form. The tissues, with some few excep- 
tions, are traversed by, and thus the elements of the tissues surrounded by, 
networks of minute thin-walled tubes, the capillary bloodvessels. The ele- 
mentary striated muscle fibre, for instance, is surrounded by capillaries, 
running in the connective tissue outside but close to the sarcolemma, arranged 
in a network with more or less rectangular meshes. These capillaries are 
closed tubes with continuous walls, and the blood, which, as we shall see, is 
continually streaming through them, is as a whole confined to their channels 
and does not escape from them. The elements of the tissues lie outside the 
capillaries and form extra-vascular islets, of different form and size in the 134 
THE VASCULAR MECHANISM. 
different tissues, surrounded by capillary networks. But the walls of the 
capillaries are so thin and of such a nature that certain of the constituents 
of the blood pass from the interior of the capillary, through the capillary 
wall to the elements of the tissue outside the capillary, and similarly certain 
of the constituents of the tissue, to wit, certain substances the result of the 
metabolism continually going on in the tissue, pass from the tissue outside 
the capillary through the capillary wall into the blood flowing through the 
capillary. Thus as we have already said, § 13, there is a continual inter- 
change of material between the blood in the capillary and the elements of 
the tissue outside the capillary, the lymph acting as middle-man. By this 
interchange the tissue lives on the blood, and the blood is affected by its 
passage through the tissue. In the small arteries which end in, and in the 
small veins which begin in the capillaries, a similar interchange takes place ; 
but the amount of interchange diminishes as, passing in each direction from 
the capillaries, the walls of the arteries and veins become thicker; and 
indeed, in all but the minute veins and arteries, the interchange is so small 
that it may practically be neglected. It is in the capillaries (and minute 
arteries and veins) that the business of the blood is done; it is in these that 
the interchange takes place ; and the object of the vascular mechanism is to 
cause the blood to flow through these in a manner best adapted for carrying 
on this interchange under varying circumstances. The use of the arteries is 
in the main simply to carry the blood in a suitable manner from the heart 
to the capillaries, the use of the veins is in the main simply to carry the 
blood from the capillaries back to the heart, and the use of the heart is in 
the main simply to drive the blood in a suitable manner through the 
arteries into the capillaries and from the capillaries back along the veins 
to itself again. The structure of these several parts is adapted to these 
several uses. 
Main Features of the Apparatus. 
§ 101. We may now pass briefly in review some of the main features of 
the several parts of the vascular apparatus-heart, arteries, veins, and 
capilliaries. 
The heart is a muscular pump-that is, a pump the force of whose strokes 
is supplied by the contraction of muscular fibres working intermittently, the 
strokes being repeated so many times (in man about 72 times) a minute. 
It is so constructed and furnished with valves in such a way that at each 
stroke it drives a certain quantity of blood with a certain force and a certain 
rapidity from the left ventricle into the aorta, and so into the arteries, receiv- 
ing during the stroke and the interval between that stroke and the next the 
same quantity of blood from the veins into the right auricle. We omit for 
simplicity's sake the pulmonary circulation by which the same quantity of 
blood is driven at each stroke from the right ventricle into the lungs and 
received into the left auricle. The rhythm of the beat, that is the frequency 
of repetition of the strokes, and the characters of each beat or stroke, are 
determined by changes taking place in the tissues of the heart itself, though 
they are also influenced by causes working from without. 
The arteries are tubes, with relatively stout walls, branching from the 
aorta all over the body. The constitution of their walls, especially of their 
middle coat, gives the arteries two salient properties. In the first place 
they are very elastic, in the sense that they will stretch readily, both length- 
wise and crosswise, when pulled, and return readily to their former size and 
shape when the pull is taken off*. If fluid be driven into one end of a piece 
of artery, the other end of which is tied, the artery will swell out to a very 
great extent, but return immediately to its former calibre when the fluid is FEATURES OF VASCULAR APPARATUS. 
135 
let out. This elasticity is chiefly due to the elastic elements in the coats, 
elastic membranes, and feltworks, but the muscular fibres, being themselves 
also elastic, contribute to the result. By reason of their possessing such 
stout, elastic walls, the arteries when empty do not collapse, but remain as 
open tubes. In the second place the arteries, by virtue of their muscular 
elements, are contractile; when stimulated either directly, as by applying an 
electric or mechanical stimulus to the arterial walls, or indirectly, by means 
of the so-called vasomotor nerves, which we shall have to study presently, 
the arteries shrink in calibre, the circularly disposed muscular fibres 
contracting, and so, in proportion to the amount of their contraction, 
narrowing the lumen or bore of the vessel. The contraction of these 
arterial muscular fibres, like that of all plain, non-striated muscular fibres, 
is slow and long-continued, with a long latent period, as compared with 
the contraction of skeletal striated muscular fibres. Owing to this mus- 
cular element in the arterial walls, the calibre of an artery may be very 
narrow or very wide, or in an intermediate condition between the two, 
neither very narrow nor very wide, accordingas the muscular fibres are very 
much contracted or not contracted at all, or only moderately contracted. 
We have further seen that, while the relative proportion of elastic and 
muscular elements differs in different arteries, as a general rule the elastic 
elements predominate in the larger arteries and the muscular elements in 
the smaller arteries, so that the larger arteries may be spoken of as eminently 
elastic, or as especially useful on account of their elastic properties, and the 
smaller arteries as eminently muscular, or as especially useful on account of 
their muscular properties. Thus, in the minute arteries which are just pass- 
ing into capillaries the muscular coat, though composed often of a single 
layer, and that sometimes an imperfect one, of muscular fibres, is a much 
more conspicuous and important part of the arterial wall than that furnished 
by the elastic elements. 
The arteries branching out from a single aorta down to multitudinous 
capillaries in nearly every part of the body diminish in bore as they divide. 
Where an artery divides into two or gives off a branch, though the bore of 
each division is less than that of the artery before the division or branching, 
the two together are greater; that is to say, the united sectional area of the 
branches is greater than the sectional area of the trunk. Hence, the sec- 
tional area of the arterial bed through which the blood flows goes on increas- 
ing from the aorta to the capillaries. If all the arterial branches were thrown 
together into one channel, this would form a hollow cone with its apex at 
the aorta and its base at the capillaries. The united sectional area of the 
capillaries may be taken as several hundred times that of the sectional area 
of the aorta, so greatly does the arterial bed widen out. 
The capillaries are channels of variable but exceedingly small size. 
The thin sheet of cemented epithelioid plates which forms the only wall of 
a capillary is elastic, permitting the channel offered by the same capillary 
to differ much in width at different times, to widen when blood and blood- 
corpuscles are being pressed through it and to narrow again when the 
pressure is lessened or cut off. The same thin sheet permits water and sub- 
stances, including gases, in solution to pass through itself from the blood to 
the tissue outside the capillary, and from the tissue to the blood, and thus 
carries on the interchange of material between the blood and the tissue. In 
certain circumstances, at all events, white or even red corpuscles may also 
pass through the wall to the tissue outside. 
The minute arteries and veins with which the capillaries are continuous 
allow of a similar interchange of material, the more so the smaller they are. 
The walls of the veins are thinner, weaker and less elastic than those of 136 
THE VASCULAR MECHANISM. 
the arteries, and possess a very variable amount of muscular tissue; they 
collapse when the veins are empty. Though all veins are more or less elastic 
and some veins are distinctly muscular, the veins as a whole cannot, like the 
arteries, be characterized as eminently elastic and contractile tubes; they 
are rather to be regarded as simple channels for conveying the blood from 
the capillaries to the heart, having just so much elasticity as will enable 
them to accommodate themselves to the quantity of blood passing through 
them, the same vein being at one time full and distended, and at another 
time empty and shrunk, and only gifted with any great amount of muscular 
contractility in special cases for special reasons. The united sectional area 
of the veins, like that of the arteries, diminishes from the capillaries to the 
heart; but the united sectional area of the venae cavae at their junction with 
the right auricle is greater than, nearly twice as great as, that of the aorta 
at its origin. The total capacity also of the veins is much greater than that 
of the arteries. The veins alone can hold the total mass of blood which in 
life is distributed over both arteries and veins. Indeed, nearly the whole 
blood is capable of being received by what is merely a part of the venous 
system, viz., the vena porta and its branches. 
The Main Facts of the Circulation. 
§ 102. Before we attempt to study in detail the working of these several 
parts of the mechanism, it will be well, even at the risk of some future repe- 
tition, to take a very brief survey of some of the salient points. 
At each beat of the heart, which in man is repeated about 72 times a 
minute, the contraction or systole of the ventricles drives a certain quantity 
of blood, probably amounting to about 180 c.c. (4 to 6 oz.), with very great 
force into the aorta (and the same quantity of blood with less force into the 
pulmonary artery). The discharge of blood from the ventricle into the 
aorta is very rapid, and the time taken up by it is, as we shall see, much 
less than the time which intervenes between it and the next discharge 
of the next beat. So that the flow from the heart into the arteries is 
most distinctly intermittent, sudden rapid discharges alternating with 
relatively long intervals during which the arteries receive no blood from 
the heart. 
At each beat of the heart just as much blood flows, as we shall see, from 
the veins into the right auricle as escapes from the left ventricle into the 
aorta ; but, as we shall also see, this inflow is much slower, takes a longer 
time, than the discharge from the ventricle. 
When the finger is placed on an artery in the living body a sense of resist- 
ance is felt, and this resistance seems to be increased at intervals, correspond- 
ing to the heart-beats, the artery at each heart-beat being felt to rise up or 
expand under the finger, constituting what we shall study hereafter as the 
pulse. In certain arteries this pulse may be seen by the eye. When the 
finger is similarly placed on a corresponding vein very little resistance is felt, 
and, under ordinary circumstances, no pulse can be perceived by the touch 
or by the eye. 
When an artery is severed, the flow of blood from the proximal cut end, 
that on the heart side, is not equable, but conies in jets, corresponding to the 
heart-beats, though the flow does not cease between the jets. The blood is 
ejected with considerable force, and may in a large artery of a large animal 
be spurted out to the distance of some feet. The larger the artery and the 
nearer to the heart, the greater the force with which the blood issues, and 
the more marked the intermittence of the flow. The flow from the distal cut 
end, that away from the heart, may be very slight or may take place with THE MAIN FACTS OF THE CIRCULATION. 
137 
considerable force and marked intermittence, according to the amount of 
collateral communication. 
When a corresponding vein is severed, the flow of blood, which is chiefly 
from the distal cut end, that in connection with the capillaries, is not jerked 
but continuous; the blood conies out with comparatively little force, and 
" wells up " rather than " spurts out." The flow from the proximal cut end, 
that on the heart side, may amount to nothing at all or may be slight or 
may be considerable, depending on the presence or absence of valves and the 
amount of collateral communication. 
When an artery is ligatured the vessel swells on the proximal side, toward 
the heart, and the throbbing of the pulse may be felt right up to the liga- 
ture. On the distal side the vessel is empty and shrunk, and no pulse can 
be felt in it unless there be free collateral communication. 
When a vein is ligatured the vessel swells on the distal side, away from 
the heart, bat no pulse is felt; while on the proximal side, toward the heart, 
it is empty and collapsed unless there be too free collateral communication. 
§ 103. When the interior of an artery-for instance, the carotid-is 
placed in communication with a long glass tube of not too great a bore, held 
vertically, the blood, immediately upon the communication being effected, 
may be seen to rush into and to fill the tube for a certain distance, forming 
in it a column of blood of a certain height. The column rises not steadily, 
but by leaps, each leap corresponding to a heart-beat, and each leap being 
less than its predecessor; and this goes on, the increase in the height of the 
column at each heart-beat each time diminishing, until at last the column 
ceases to rise and remains for a while at a mean level, above and below 
which it oscillates with slight excursions at each heart-beat. 
To introduce such a tube an artery-say the carotid of a rabbit-is laid bare, 
ligatured at a convenient spot, V Fig. 37, and further temporarily closed a little 
distance lower down nearer the heart by a small pair of "bull-dog" forceps, bd, 
or by a ligature which can be easily slipped. A longitudinal incision is now made 
in the artery between the forceps, bd, and the ligature I' (only the drop or two of 
blood which happens to remain inclosed between the two being lost); the end of 
the tube, represented by c in the figure, is introduced into the artery and secured 
by the ligature I. The interior of the tube is now in free communication with the 
interior of the artery, but the latter is by means of the forceps at present shut off 
from the heart. On removing the forceps a direct communication is at once estab- 
lished between the tube and the artery below; in consequence the blood from the 
heart flows through the artery into the tube. 
This experiment shows that the blood as it is flowing into the carotid is 
exerting a considerable pressure on the walls of the artery. At the moment 
when the forceps are removed there is nothing but the ordinary pressure of 
the atmosphere to counterbalance this pressure within the artery, and con- 
sequently a quantity of blood is pressed out into the tube; and this goes on 
until the column of blood in the tube reaches such a height that its weight is 
equal to the pressure within the artery, whereupon no more blood escapes. 
The whole column continues to be raised a little at each heart-beat, but sinks 
as much during the interval between each two beats, and thus oscillates, as 
we have said, above and below a mean level. In a rabbit this column of 
blood will generally have the height of about 90 cm. (3 feet) ; that is to say, 
the pressure which the blood exerts on the walls of the carotid of a rabbit is 
equal to the pressure exerted by a column of rabbit's blood 90 cm. high. 
This is equal to the pressure of a column of water about 95 cm. high, and to 
the pressure of a column of mercury about 70 mm. high. 
If a like tube be similarly introduced into a corresponding vein-say 
the jugular vein-it will be found that the column of blood, similarly formed 138 
THE VASCULAR MECHANISM. 
in the tube, will be a very low one, not more than a very few centimetres 
high, and that while the level of the column may vary a good deal, owing, 
as we shall see later, to the influence of the respiratory movement, there will 
not, as in the artery, be oscillations corresponding to the heart-beats. 
We learn, then, from this simple experiment, that in the carotid of the 
rabbit the blood while it flows through that vessel is exerting a considerable 
mean pressure on the arterial walls, equivalent to that of a column of mer- 
cury about 70 mm. high, but that in the jugular vein the blood exerts on 
the venous walls a very slight mean pressure, equivalent to that of a column 
of mercury 3 or 4 mm. high. We speak of this mean pressure exerted by 
the blood on the walls of the bloodvessels as blood-pressure, and we say that 
the blood-pressure in the carotid of the rabbit is very high (70 mm. Hg), 
while that in the jugular vein is very low (only 3 or 4 mm. Hg.). 
In the normal state of things the blood flows through the carotid to the 
arterial branches beyond, and through the jugular vein toward the heart; 
the pressure exerted by the blood on the artery or on the vein is a lateral 
pressure on the walls of the artery and vein, respectively. In the above ex- 
periment the pressure measured is not exactly this, but the pressure exerted 
at the end of the artery (or of the vein) where the tube is attached. We 
might directly measure the lateral pressure in the carotid by somewhat 
modifying the procedure described above. We might connect the carotid 
with a tube the end of which was not straight, but made in the form of a 
T-piece, and might introduce the T-piece in such a way that the blood 
should flow along one limb (the vertical limb) of the T-piece from the 
proximal to the distal part of the carotid, and at the same time by the other 
(horizontal) limb of the T-piece into the main upright part of the glass 
tube. The column of blood in the tube would then be a measure of the 
pressure which the blood as it is flowing along the carotid is exerting on a 
portion of its walls corresponding to the mouth of the horizontal limb of the 
T-piece. If we were to introduce into the aorta, at the place of origin of the 
carotid, a similar (larger) T-piece, and to connect the glass tube with the 
horizontal limb of the T-piece by a piece of elastic tubing of the same length 
and bore as the carotid, the column of blood rising up in the tube would be 
the measure of the lateral pressure exerted by the blood on the walls of the 
aorta at the origin of the carotid artery and transmitted to the rigid glass 
tube through a certain length of elastic tubing. And, indeed, what is 
measured in the experiment previously described is not the lateral pressure 
in the carotid itself at the spot where the glass tube is introduced, but the 
lateral pressure of the aorta at the origin of the carotid modified by the 
influences exerted by the length of the carotid between its origin and the 
spot where the tube is introduced. 
§ 104. Such an experiment as the one described has the disadvantages 
that the animal is weakened by the loss of the blood which goes to form the 
column in the tube, and that the blood in the tube soon clots, and so brings 
the experiment to an end. Blood-pressure may be more conveniently studied 
by connecting the interior of the artery (or vein) with a mercury gauge or 
manometer (Fig. 37) the proximal descending limb of which, in, is filled 
above the mercury with some innocuous fluid, as is also the tube connecting 
the manometer with the artery. Using such an instrument, we should 
observe very much the same facts as in the more simple experiment. 
Immediately that communication is established between the interior of 
the artery and the manometer, blood rushes from the former into the latter, 
driving some of the mercury from the descending limb, in, into the ascending 
limb, in', and thus causing the level of the mercury in the ascending limb to 
rise rapidly. This rise is marked by jerks corresponding with the heart- THE MAIN FACTS OF THE CIRCULATION. 
139 
Fig. 37. 
Apparatus for Investigating Blood-pressure, At the upper right-hand corner is seen, on an 
enlarged scale, the carotid artery, clamped by the forceps bd, with the vagus nerve v lying by its 
side. The artery has been ligatured at I' and the glass canula c has been introduced into the 
artery between the ligature I' and the forceps bd, and secured in position by the ligature I. The 
shrunken artery on the distal side of the canula is seen at ca'. 
p.b is a box containing a bottle holding a saturated solution of sodium carbonate or a solution 
of sodium bicarbonate of sp. gr. 1083, and capable of being raised or lowered at pleasure. The 
solution flows by the tube p.t regulated by the clamp c" into the tube t. A syringe, with a stop- 
cock, may be substituted for the bottle, and attached at c". This, indeed, is in many respects a 140 
THE VASCULAR MECHANISM. 
beats. Having reached a certain level, the mercury ceases to rise any more. 
It does not, however, remain absolutely at rest, but undergoes oscillations; 
it keeps rising and falling. Each rise, which is very slight compared with 
the total height to which the mercury has risen, has the same rhythm as the 
systole of the ventricle. Similarly, each fall corresponds with the diastole. 
If a float, swimming on the top of the mercury in the ascending limb of 
the manometer, and bearing a brush or other marker, be brought to bear on 
a travelling surface, some such tracing as that represented in Fig. 38 will be 
Fig. 38. 
Tracing of Arterial Pressure with a Mercury Manometer: The smaller curves p p are the pulse 
curves. The space from r to r embraces a respiratory undulation. The tracing is taken from a 
dog, and the irregularities visible in it are those frequently met with in this animal. 
described. Each of the smaller curves (p p) corresponds to a heart-beat, 
the rise corresponding to the systole and the fall to the diastole of the ven- 
tricle. The larger undulations (r r) in the tracing, which are respiratory 
in origin, will be discussed hereafter. 
In Fig. 39 are given twro tracings 
taken from the carotid of a rabbit; in 
the lower curve the recording surface 
is travelling more rapidly than in the 
upper curve; otherwise the curves are 
alike and repeat the general features 
of the curve from the dog. 
Description of experiment. In a carotid, 
or other bloodvessel, prepared as explained, 
a small glass tube, of suitable bore, called 
a canula is introduced by the method 
described above, and is subsequently con- 
nected, by means of a short piece of India- 
rubber tubing (Fig. 37 7,) and a leaden or 
other tube t which is at once flexible and 
yet not extensible, with the descending 
limb, m. of the manometer or mercury 
gauge. The canula, tube, and descending 
limb of the manometer are all filled with some fluid, which tends to prevent clot- 
ting of the blood, the one chosen being generally a strong solution (sp. gr. 1083) 
of sodium bicarbonate, but other fluids may be chosen. In order to avoid loss 
of blood, a quantity of fluid is injected into the flexible tube sufficient to raise the 
Fig. 39. 
Blood-pressure Curves from the Carotid 
of Rabbit, the time marker in each case 
marking seconds. 
more convenient plan. The tube t is connected with the leaden tube t, and the stopcock c with 
the manometer, of which m is the descending and m' the ascending limb, and s the support. The 
mercury in the ascending limb bears on its surface the float fl, a long rod attached to which is 
fitted with the pen p, writing on the recording surface r. The clamp cl. at the end of the tube t 
has an arrangement shown on a larger scale at the right hand upper corner. 
The descending tube m of the manometer and the tube t being completely filled along its whole 
length with fluid to the exclusion of all air, the canula c is filled with fluid, slipped into the 
open end of the thick-walled India-rubber tube i, until it meets the tube t (whose position within 
the India-rubber tube is shown by the dotted lines), and is then securely fixed in this position by 
the clamp cl. 
The stopcocks cand c" are now opened, and the pressure bottle raised or fluid driven in by the 
sj ringe until the mercury in the manometer is raised to the required height. The clamp crt is 
then closed and the forceps bd removed from the artery. The pressure of the blood in the carotid 
ca is in consequence brought to bear through t upon the mercury in the manometer. THE MAIN FACTS OF THE CIRCULATION. 
141 
mercury in the ascending limb of the manometer to a level a very little below 
what may be beforehand guessed at as the probable mean pressure. When the 
forceps bd are removed, the pressure of the blood in the carotid is transmit- 
ted through the flexible tube to the manometer, the level of tjie mercury in the 
ascending limb of which falls a little, or sinks a little at first, or may do neither, 
according to the success with which the probable mean pressure has been guessed, 
and continues to exhibit the characteristic oscillations until the experiment is 
brought to an end by the blood clotting or otherwise. 
Tracings of the movements of the column of mercury in the manometer may 
be taken either on a smoked surface of a revolving cylinder ( Fig. 11), or by means 
of a brush and ink on a continuous roll of paper, as in the more complex kymo- 
graph (Fig. 40). 
§ 105. By the help of the manometer applied to various arteries and 
veins we learn the following facts: 
1. The mean blood-pressure is high in all the arteries, but is greater in 
the larger arteries nearer the heart than in the smaller arteries further from 
Fig. 40. 
Ludwig's Kymograph for Recording on a Continuous Roll of Paper. 
the heart; it diminishes, in fact, along the arterial tract from the heart 
toward the capillaries. 
2. The mean blood-pressnre is low in the veins, but is greater in the 
smaller veins nearer the capillaries than in the larger veins nearer the heart, 
diminishing, in fact, from the capillaries toward the heart. In the large 
veins near the heart it may be negative, that is to say, the pressure of blood 
in the vein bearing on the proximal descending limb of the manometer may 
be less than the pressure of the atmosphere on the ascending distal limb, so 142 
THE VASCULAR MECHANISM. 
that when communication is made between the interior of the vein and the 
manometer, the mercury sinks in the distal and rises in the proximal limb, 
being sucked up toward the vein. 
The manometer cannot well be applied to the capillaries, but we may 
measure the blood-pressure in the capillaries in an indirect way. It is well 
known that when any portion of the skin is pressed upon, it becomes pale 
and bloodless ; this is due to the pressure driving the blood out of the capil- 
laries and minute vessels and preventing any fresh blood entering into them. 
By carefully investigating the amount of pressure necessary to prevent the 
blood entering the capillaries and minute arteries of the web of the frog's 
foot, or of the skin beneath the nail or elsewhere in man, the internal pres- 
sure which the blood is exercising on the walls of the capillaries and minute 
arteries and veins may be approximately determined. In the frog's web 
this has been found to be equal to about 7 to 11 mm. of mercury. In the 
mammal the capillary blood-pressure is naturally higher than this and may 
be put down at from 20 to 30 mm. It is, therefore, considerable, being 
greater than that in the veins, though less than that in the arteries. 
3. There is thus a continued decline of blood-pressure from the root of 
the aorta, through the arteries, capillaries, and veins to the right auricle. 
We find, however, on examination that the most marked fall of pressure 
takes place between the small arteries 
on the one side of the capillaries and 
the small veins on the other, the 
curve of pressure being somewhat 
of the form given in Fig. 41, which 
is simply intended to show this fact 
graphically and has not been con- 
structed by exact measurements. 
4. In the arteries this mean pres- 
sure is marked by oscillations corre- 
sponding to the heart beats, each 
oscillation consisting of a rise (in- 
crease of pressure above the mean) 
corresponding to the systole of the 
ventricle, followed by a fall (de- 
crease of pressure below the mean) 
corresponding to the diastole of the 
ventricle. 
5. These oscillations, which we may speak of as the pulse, are largest 
and most conspicuous in the large arteries near the heart, diminish from the 
heart toward the capillaries, and are, under ordinary circumstances, wholly 
absent from the veins along the whole extent from the capillaries to the 
heart. 
Obviously a great change takes place in that portion of the circulation 
which comprises the capillaries, the minute arteries leading to and the 
minute veins leading away from the capillaries, and which we may speak 
of as the " peripheral region." It is here that a great drop of pressure 
takes place; it is here also that the pulse disappears. 
§ 106. If the web of a frog's foot be examined with a microscope, the 
blood, as judged of by the movements of the corpuscles, is seen to be passing 
in a continuous stream from the small arteries through the capillaries to the 
veins. The velocity is greater in the arteries than in the veins, and greater 
in both than in the capillaries. In the arteries faint pulsations, synchronous 
with the heart's beat, are frequently visible; but these disappear in the 
capillaries, in which the flow is even, that is, not broken by pulsations, and 
Fig. 41. 
Diagram of Blood-pressure : A, arteries; P, pe- 
ripheral region (minute arteries, capillaries, and 
veins); V, veins. THE MAIN FACTS OF THE CIRCULATION. 
143 
this evenness of flow is continued on along the veins as far as we can trace 
them. Not infrequently variations in velocity and in the distribution of 
the blood, due to causes which will be hereafter discussed, are witnessed 
from time to time. 
The character of the flow through the smaller capillaries is very 
variable. Sometimes the corpuscles are seen passing through the chan- 
nel in single file with great regularity; at other times they may be 
few and far between. Some of the capillaries are wide enough to permit 
two or more corpuscles abreast. In all cases the blood as it passes through 
the capillary stretches and expands the walls. Sometimes a corpuscle may 
remain stationary at the entrance into a capillary, the channel itself being 
for some little distance entirely free from corpuscles. Sometimes many 
corpuscles will appear to remain stationary in one or more capillaries for a 
brief period and then to move on again. Any one of these conditions 
readily passes into another ; and, especially with a somewhat feeble circu- 
lation, instances of all of them may be seen in the same field of the micro- 
scope. It is only when the vessels of the web are unusually full of blood 
that all the capillaries can be seen equally filled with corpuscles. The 
long, oval red corpuscle moves with its long axis parallel to the stream, 
occasionally rotating on its long axis, and sometimes, in the larger chan- 
nels, on its short axis. The flexibility and elasticity of a corpuscle are 
well seen when it is being driven into a capillary narrower than itself, or 
when it becomes temporarily lodged at the angle between two diverging 
channels. 
These and other phenomena, on which we shall dwell later on, may be 
readily seen in the web of the frog's foot or in the stretched-out tongue or in 
the mesentery of the frog; and essentially similar phenomena may be 
observed in the mesentery or other transparent tissues of a mammal. All 
over the body, wherever capillaries are present, the corpuscles and the 
plasma are being driven in a continuous and though somewhat irregular 
yet on the whole steady flow through channels so minute that the passage 
is manifestly attended with considerable difficulties. 
It is obvious that the peculiar characters of the flow through the minute 
arteries, capillaries, and veins affords an explanation of the great change 
taking place in the peripheral region between the arterial flow and the 
venous flow. The united sectional area of the capillaries is, as we have seen, 
some hundreds of times greater than the sectional area of the aorta ; but 
this united sectional area is made up of thousands of minute passages, 
varying in man from 5 to 20 /a, some of them, therefore, being in an undis- 
tended condition, smaller than the diameter of a red corpuscle. Even were 
the blood a simple liquid free from all corpuscles, these extremely minute 
passages would occasion an enormous amount of friction, and thus present a 
considerable obstacle or resistance to the flow of blood through them. Still 
greater must be the friction and resistance occasioned by the actual blood 
with its red and white corpuscles. The blood in fact meets with great dif- 
ficulties in its passage through the peripheral region, and sometimes, as we 
shall see, the friction and resistance are so great in the peripheral vessels of 
this or that area that no blood passes through them at all, and an arrest of 
the flow takes place in the area. 
The resistance to the flow of blood thus caused by the friction generated 
in so many minute passages is one of the most important physical facts in the 
circulation. In the large arteries the friction is small; it increases gradually 
as they divide, but receives its chief and most important addition in the 
minute arteries and capillaries; it is relatively greater in the minute arteries 
than in the capillaries on account of the flow being more rapid in the former, 144 
THE VASCULAR MECHANISM. 
for friction diminishes rapidly with a diminution in the rate of flow. We 
may speak of it as the " peripheral friction," and the resistance which it 
offers as the " peripheral resistance." It need perhaps hardly be said that 
this peripheral resistance not only opposes the flow of blood through the 
capillaries and minute arteries themselves where it is generated, but, working 
backward along the whole arterial system, has to be overcome by the heart 
at each systole of the ventricle. 
Hydraulic Principles of the Circulation. 
§ 107. In the circulation, then, the following three facts of fundamental 
importance are met with : 
1. The systole of the ventricle, driving at intervals a certain quantity 
of blood, with a certain force, into the aorta. 
2. The peripheral resistance just described. 
3. A long stretch of elastic tubing (the arteries), reaching from the 
ventricle to the region of peripheral resistance. 
From these facts we may explain the main phenomena of the circula- 
tion, which we have previously sketched, on purely physical principles 
without any appeal to the special properties of living tissues, beyond the 
provision that the ventricle remains capable of good rhythmical contrac- 
tions, that the arterial walls retain their elasticity, and that the friction be- 
tween the blood and the lining of the peripheral vessels remains the same ; 
we may thus explain the high pressure and pulsatile flow in the arteries, 
the steady stream through the capillaries, the low pressure and the uniform 
pulseless flow in the veins, and finally the continued flow of the blood from 
the aorta to the mouths of the venae cavae. 
All the above phenomena in fact are the simple results of an intermit- 
tent force (like that of the systole of the ventricle) working in a closed 
circuit of branching tubes, so arranged that while the individual tubes first 
diminish in calibre (from the heart to the capillaries) and then increase 
(from the capillaries to the heart), the area of the bed first increases and 
then diminishes, the tubes together thus forming two cones placed base to 
base at the capillaries, with their apices converging at the heart, and pre- 
senting at their conjoined bases a conspicuous peripheral resistance, the 
tubing on one side, the arterial, being eminently elastic, and, on the other, 
the venous, affording a free and easy passage for the blood. It is the 
peripheral resistance (for the resistance offered by the friction in the larger 
vessels may, when compared with this, be practically neglected), reacting 
through the elastic walls of the arteries upon the intermittent force of the 
heart, which gives the circulation of the blood its peculiar features. 
§ 108. Circumstances determining the character of the flow. When fluid 
is driven by an intermittent force, as by a pump, through a perfectly rigid 
tube, such as a glass one (or a system of such tubes), there escapes at each 
stroke of the pump from the distal end of the tube (or system of tubes) just 
as much fluid as enters it at the proximal end. What happens is very like 
what would happen if, with a wide glass tube completely filled with billiard 
balls lying in a row, an additional ball were pushed in at one end: each 
ball would be pushed on in turn a stage further and the last ball at the 
further end would tumble out. The escape, moreover, takes place at the 
same time as the entrance. 
This result remains the same when any resistance to the flow is intro- 
duced into the tube, as for instance when the end of the tube is narrowed. 
The force of the pump remaining the same, the introduction of the resist- 
ance undoubtedly lessens the quantity of fluid issuing at the distal end at THE MAIN FACTS OF THE CIRCULATION. 
145 
each stroke, but it at the same time lessens the quantity entering at the 
proximal end ; the inflow and outflow remain equal to each other, and still 
occur at the same time. 
In an elastic tube, such as an India-rubber one (or in a system of such 
tubes), whose sectional area is sufficiently great to offer but little resistance 
to the progress of the fluid, the flow caused by an intermittent force is also 
intermittent. The outflow being nearly as easy as the inflow, the elasticity 
of the walls of the tube is scarcely at all called into play. The tube be- 
haves practically like a rigid tube. When, however, sufficient resistance is 
introduced into any part of the course, the fluid being unable to pass by 
the resistance as rapidly as it enters the tube from the pump, tends to accu- 
mulate on the proximal side of the resistance. This it is able to do by ex- 
panding the elastic walls of the tube. At each stroke of the pump a cer- 
tain quantity of fluid enters the tube at the proximal end. Of this only a 
fraction can pass through the resistance during the stroke. At the moment 
when the stroke ceases, the rest still remains on the proximal side of the 
resistance, the elastic tube having expanded to receive it. During the 
interval between this and the next stroke, the distended elastic tube, striving 
to return to its natural undistended condition, presses on this extra quantity 
of fluid which it contains and tends to drive it past the resistance. 
Thus, in the rigid tube (and in the elastic tube without the resistance) 
there issues, from the distal end of the tube at each stroke, just as much 
fluid as enters it at the proximal end, while between the strokes there is 
perfect quiet. In the elastic tube with resistance, on the contrary, the quan- 
tity which passes the resistance is only a fraction of that which enters the 
tube from the pump at any one stroke, the remainder or a portion of the 
remainder continuing to pass during the interval between the strokes. In 
the former case the tube is no fuller at the end of the stroke than at the 
beginning; in the latter case there is an accumulation of fluid between the 
pump and the resistance, and a corresponding distention of that part of the 
tube at the close of each stroke-an accumulation and distention, however, 
which go on diminishing during the interval between that stroke and the 
next. The amount of fluid thus remaining after the stroke will depend on 
the amount of resistance in relation to the force of the stroke and on the 
distensibility of the tube ; and the amount which passes the resistance before 
the next stroke will depend on the degree of elastic reaction of which the 
tube is capable. Thus, if the resistance be very considerable in relation to 
the force of the stroke, and the tube very distensible, only a small portion 
of the fluid will pass the resistance, the greater part remaining lodged be- 
tween the pump and the resistance. If the elastic reaction be great, a large 
portion of this will be passed on through the resistance before the next 
stroke comes. In other words, the greater the resistance (in relation to the 
force of the stroke), and the more the elastic force is brought into play, the 
less intermittent, the more nearly continuous, will be the flow on the far 
side of the resistance. 
If the first stroke be succeeded by a second stroke before its quantity of 
fluid has all passed by the resistance, there will be an additional accumula- 
tion of fluid on the near side of the resistance, an additional distention of 
the tube, an additional strain on its elastic powers, and, in consequence, the 
flow between this second stroke and the third will be even more marked than 
that between the first and second, though all three strokes were of the same 
force, the addition being due to the extra amount of elastic force called into 
play. In fact, it is evident that, if there be a sufficient store of elastic power 
to fall back upon, by continually repeating the stroke a state of things will 
be at last arrived at in which the elastic force, called into play by the con- 146 
THE VASCULAR MECHANISM. 
tinually increasing distention of the tube on the near side of the resistance, 
will be sufficient to drive through the resistance, between each two strokes, 
just as much fluid as enters the near end of the system as each stroke. In 
other words, the elastic reaction of the walls of the tube will have converted 
the intermittent into a continuous How. The How on the far side of the 
resistance is in this case not the direct result of the strokes of the pump. All 
the force of the pump is spent, first in getting up, and afterward in keeping 
up, the distention of the tube on the near side of the resistance; the imme- 
diate cause of the continuous flow lies in the distention of the tube which 
leads it to empty itself into the far side of the resistance at such a rate that 
it discharges through the resistance during a stroke and in the succeeding 
interval just as much as it receives from the pump by the stroke itself. 
This is exactly what takes place in the vascular system. The friction in 
the minute arteries and capillaries presents a considerable resistance to the 
flow of blood through them into the small veins. In consequence of this 
resistance the force of the heart's beat is spent in maintaining the whole of 
the arterial system in a state of great distention ; the arterial walls are put 
greatly on the stretch by the pressure of the blood thrust into them by the 
repeated strokes of the heart; this is the pressure which we spoke of above 
as blood-pressure. The greatly distended arterial system is, by the elastic 
reaction of its elastic walls, continually tending to empty itself by overflow- 
ing through the capillaries into the venous system ; and it overflows at such 
a rate that just as much blood passes from the arteries to the veins during 
each systole and its succeeding diastole as enters the aorta at each systole. 
§ 109. Indeed, the important facts of the circulation which we have not 
as yet studied may be roughly but successfully imitated on an artificial model, 
Fig. 42, in which an elastic syringe represents the heart, a long piece of 
elastic India-rubber tubing the arteries, another piece of tubing the veins, 
and a number of smaller connecting pieces the minute arteries and capil- 
laries. If these connecting pieces be made at first somewhat wide, so as to 
offer no great resistance to the flow from the artificial arteries to the artificial 
veins, but be so arranged that they may be made narrow by the screwing-up 
of clamps or otherwise, it is possible to illustrate the behavior of the vascular 
mechanism when the peripheral resistance is less than usual (and as we shall 
see later on it is possible in the living organism either to reduce or to increase 
what may be considered as the normal peripheral resistance), and to compare 
that behavior with the behavior of the mechanism when the peripheral 
resistance is increased. 
The whole apparatus being placed flat on a table, so as to avoid differences 
in level in different parts of it, and filled with water, but so as not to distend 
the tubing, the two manometers attached, one (A) to the arterial side of the 
tubing and the other ( F) to the venous side, ought to show the mercury 
standing at equal heights in both limbs of both instruments, since nothing 
but the pressure of the atmosphere is bearing on the fluid in the tubes, and 
that equally all over. 
If, now, the connecting pieces being freely open, that is to say, the perii>h- 
eral resistance being very little, we imitate a ventricular beat by the stroke of 
the pump, we shall observe the following : Almost immediately after the stroke 
the mercury in the arterial manometer will rise, but will at once fall again, 
and very shortly afterward the mercury in the venous tube will in a similar 
manner rise and fall. If we repeat the strokes with a not too rapid rhythm, 
each stroke having the same force, and make, as may by a simple contrivance 
be effected, the two manometers write on the same recording surface, we shall 
obtain curves like those of Fig. 43, A and F. At each stroke of the pump 
the mercury in the arterial manometers rises, but forthwith falls again to or THE MAIN FACTS OF THE CIRCULATION. 
147 
nearly to the base line ; no mean arterial pressure, or very little, is established. 
The contents of the ventricle (syringe) thrown into the arterial system dis- 
Fig. 42. 
Arterial Scheme: P, unshaded, is an elastic tube to represent the arterial system, branching 
at A" and Y, and ending in the region of peripheral resistance, including the capillaries, which 
are imitated by filling loosely with small pieces of sponge the parts shown as dilated in the figure. 
The capillaries are gathered up into the venous system, shaded, which terminates at 0. Water is 
driven into the arterial system at Pby means of an elastic bag syringe orany other form of pump. 
Clamps are placed on the undilated tubes, c, c', c". When these clamps are tightened, the only 
access for the water from the arterial to the venous side is through the dilated parts filled with 
sponge, which offers a considerable resistance to the flow of fluid through them. When the 
clamps are unloosed the fluid passes, with much less resistance, through the undilated tubes. 
Thus, by tightening or loosening the clamps the "peripheral" resistance may be increased or 
diminished at pleasure. 
At A, on the arterial side, and at V, on the venous side, manometers can be attached. At a 
and v (and also at x and y), by means of clamps, the flow of fluid from an artery and from a vein, 
under various conditions, may be observed. At Sa, S'a, and Sv, sphygmographs may be applied. 
tend it, but the passage through the peripheral region is so free that an equal 
quantity of fluid passes through to the veins immediately, and hence the 
Fig. 43. 
Tracings taken from an Artificial Scheme, with the Peripheral Resistance Slight: A, arterial; 
V, venous manometer. This figure, to save space, is on a smaller scale than the corresponding 
Fig. 44. 
mercury at once falls. But the fluid thus passing easily into the veins dis- 
tends these too, and the mercury in their manometer rises too, but only to 148 
THE VASCULAR MECHANISM. 
fall again, as a corresponding quantity issues from the ends of the veins into 
the basin, which serves as an artificial auricle. Now introduce " peripheral 
resistance" by screwing up the clamps on the connecting tubes, and set the 
pump to work again as before. With the first stroke the mercury in the 
arterial manometer (Fig. 44, A1) rises as before, but instead of falling rapidly 
it falls slowly, because it now takes a longer time for a quantity of fluid 
equal to that which has been thrust into the arterial system by the ventricu- 
lar stroke to passthrough the narrowed peripheral region. Before the curve 
has fallen to the base line, before the arterial system has had time to dis- 
charge through the narrowed peripheral region as much fluid as it received 
from the ventricle, a second stroke drives more fluid into the arteries, dis- 
tending them this time more than it did before, and raising the mercury to 
Fig. 44. 
Tracings taken from an Artificial Scheme, with the Peripheral Resistance Considerable: A', 
arterial; V1, venous manometer. 
a still higher level. A third, a fourth, and succeeding strokes produce the 
same effect, except that the additional height to which the mercury is raised 
at each stroke becomes at each stroke less and less, until a state of things is 
reached in which the mercury, being on the fall when the stroke takes place, 
is by the stroke raised just as high as it was before, and then beginning to 
fall again is again raised just as high, and so on. With each succeeding 
stroke the arterial system has become more and more distended ; but the 
more distended it is the greater is the elastic reaction brought into play ; this 
greater elastic reaction more and more overcomes the obstacle presented by 
the peripheral resistance and drives the fluid more and more rapidly through 
the peripheral region. At last the arterial system is so distended, and the 
force of the elastic reaction so great, that during the stroke and the succeed- 
ing interval just as much fluid passes through the peripheral region as enters 
the arteries at the stroke. In other words, the repeated strokes have estab- 
lished a mean arterial pressure which, at the point where the manometer THE MAIN FACTS OF THE CIRCULATION. 
149 
is affixed, is raised slightly at each ventricular stroke, and falls slightly 
between the strokes. 
Turning now to the venous manometer, Fig. 44, F1, we observe that each 
stroke of the pump produces on this much less effect than it did before the 
introduction of the increased peripheral resistance. The mercury, instead of 
distinctly rising and falling at each stroke, now shows nothing more than 
very gentle undulations; it feels to a very slight degree only the direct 
effect of the ventricular stroke; it is simply raised slightly above the base 
line, and remains fairly steady at this level. The slight rise marks the 
mean pressure exerted by the fluid at the place of attachment of the manom- 
eter. This mean " venous" pressure is a continuation of the mean arterial 
pressure so obvious in the arterial manometer, but is much less than that be- 
cause a large part of the arterial mean pressure has been expended in driv- 
ing the fluid past the peripheral resistance. What remains is, however, 
sufficient to drive the fluid along the wide venous tubing right to the open 
end. 
Thus this artificial model may be made to illustrate how it comes about 
that the blood flows in the arteries at a relatively high pressure, which at 
each ventricular systole is raised slightly above and at each diastole falls 
slightly below a certain mean level, and flows in the veins at a much lower 
pressure, which does not show the immediate effects of each heart-beat. 
If two manometers, instead of one, were attached to the arterial system, 
one near the pump and the other further off, close to the peripheral resist- 
ance, the pressure shown by the near manometer would be found to be greater 
than that shown by the far one. The pressure at the far point is less 
because some of the pressure exerted at the near point has been used to drive 
the fluid from the near point to the far one. Similarly on the venous side, a 
manometer placed close to the peripheral region would show a higher pres- 
sure than that shown by one further off, because it is the pressure still remain- 
ing in the veins near the capillaries which, assisted, as we shall see, by other 
events, drives the blood onward to the larger veins. The blood-pressure is 
at its highest at the root of the aorta and at its lowest at the mouths of the 
venae cavae, and is falling all the way from one point to the other, because 
all the way it is being used up to move the blood from one point to the 
other. The great drop of pressure is, as we have said, in the peripheral 
region, because more work has to be done in driving the blood through this 
region than in driving the blood from the heart to this region, or from this 
region to the heart. 
The manometer on the arterial side of the model shows, as we have seen, 
an oscillation of pressure, a pulse due to each heart-beat, and the same pulse 
may be felt by placing a finger, or rendered visible by placing a light lever, 
on the arterial tube. It may further be seen that this pulse is most marked 
nearest the pump, and becomes fainter as we pass to the periphery; but we 
must reserve the features of the pulse for a special study. On the venous 
side of the model no pulse can be detected by the manometer or by the 
finger, provided that the peripheral resistance be adequate. If the periph- 
eral resistance be diminished, as by unscrewing the clamps, then, as neces- 
sarily follows from what has gone before, the pulse passes over on to the 
venous side; and, as we shall have occasion to point out later on, in the 
living organism the peripheral resistance in particular areas may be at 
times so much lessened that a distinct pulsation appears in the veins. 
If in the model, when the pump is in full swing, and arterial pressure well 
established, the arterial tube be pricked or cut, or the small side tube a 
be opened, the water will gush out in jets, as does blood from a cut artery in 
the living body ; whereas, if the venous tube be similarly pricked or cut, or 150 
THE VASCULAR MECHANISM. 
the small tube v be opened, the water will simply ooze out or well up, as 
does blood from a vein in the living body. If the arterial tube be ligatured, 
it will swell on the pump side and shrink on the peripheral side; if the 
venous tube be ligatured, it will swell on the side nearest the capillaries and 
shrink on the other side. In short, the dead model will show all the main 
facts of the circulation which we have as yet described. 
§ 110. In the living body, however, there are certain helps to the circu- 
lation which cannot be imitated by such a model without introducing great 
and undesirable complications; but these chiefly affect the flow along the 
veins. 
The veins are in many places provided with valves so constructed as to 
offer little or no resistance to the flow from the capillaries to the heart, but 
effectually to block a return toward the capillaries. Hence any external 
pressure brought to bear upon a vein tends to help the blood to move for- 
ward toward the heart. In the various movements carried out by the 
skeletal muscles, such an external pressure is brought to bear on many of 
the veins, and hence these movements assist the circulation. Even passive 
movements of the limbs have a similar effect. So, also, the movements of 
the alimentary canal, carried out by means of plain muscular tissue, promote 
the flow along the veins coming from that canal, and when we come to deal 
with the spleen we shall see that the plain muscular fibres which are so 
abundant in that organ in some animals, serve by rhythmical contractions 
to pump the blood regularly away from the spleen along the splenic veins. 
When we come to deal with respiration, we shall see that each enlarge- 
ment of the chest constituting an inspiration tends to draw the blood toward 
the chest, and each return or retraction of the chest walls in expiration tends 
to drive the blood away from the chest. The arrangement of the valves of 
the heart causes this action of the respiratory pump to promote the flow of 
blood in the direction of the normal circulation ; and, indeed, were the heart 
perfectly motionless, the working of this respiratory pump alone would tend 
to drive the blood from the vente cavae through the heart into the aorta, and 
so to keep up the circulation ; the force so exerted, however, would, without 
the aid of the heart, be able to overcome a very small part only of the 
resistance in the capillaries and small vessels of the lungs, and so would 
prove actually ineffectual. 
There are, then, several helps to the flow along the veins, but it must be 
remembered that, however useful, they are helps only, and not the real cause 
of the circulation. The real cause of the flow is the ventricular stroke, and 
this is sufficient to drive the blood from the left ventricle to the right auricle, 
even when every muscle of the body is at rest and breathing is for a while 
stopped, when, therefore, all the helps we are speaking of are wanting. 
Circumstances Determining the Rate of the Flow. 
§ 111. We may now pass on to consider briefly the rate at which the 
blood flows through the vessels, and first the rate of flow in the arteries. 
When even a small artery is severed, a considerable quantity of blood 
escapes from the proximal cut end in a very short space of time. That is to 
say, the blood moves in the arteries from the heart to the capillaries with a 
very considerable velocity. By various methods, this velocity of the blood- 
current has been measured at different parts of the arterial system ; the 
results, owing to imperfections in the methods employed, cannot be regarded 
as satisfactorily exact, but may be accepted as approximately true. They 
show that the velocity of the arterial stream is greatest in the largest arteries 
near the heart, and diminishes from the heart toward the capillaries. Thus, THE MAIN FACTS OF THE CIRCULATION. 
151 
in a large artery of a large animal, such as the carotid of a dog or horse, and 
probably in the carotid of a man, the blood flows at the rate of 300 or 500 
mm. a second. In the very small arteries the rate is probably only a few 
mm. a second. 
Methods. The hmmadromometer of Volkmann. [Fig. 45.] An artery-e.g., a 
carotid-is clamped in two places, and divided between the clamps. Two canulte, 
[Fig. 45. 
Volkmann's Hsemadromometer: The conical portions of the instrument are inserted in the 
cut ends of a vein or artery. By a simple arrangement of a double stopcock the blood-current 
can be made to pass immediately through the transverse arm, as in A, or to pass through the 
graduated U-shaped tube, as in B.] 
of a bore as nearly equal as possible to that of the artery, or of a known bore, are 
inserted in the two ends. The two canulae are connected by means of two stop- 
cocks, which work together, with the two ends of a long glass tube, bent in the 
shape of a U, and filled with normal saline solution, or with a colored innocuous 
fluid. The clamps on the artery being released, a turn of the stopcocks permits 
the blood to enter the proximal end of the long U-tube. along which it courses, 
driving the fluid out into the artery through the distal end. Attached to the tube 
is a graduated scale, by means of which the velocity with which the blood flows 
along the tube may be read off. Even supposing the canulae to be of the same 
bore as the artery, it is evident that the conditions of the flow through the tube are 
such as will only admit of the result thus gained being considered as an approxi- 
mative estimation of the real velocity in the artery itself. 
The rheometer (Stromuhr) of Ludwig. This consists of two glass bulbs, A and 
ZA Fig. 46, communicating above with each other and with the common tube C, 
by which they can be filled. Their lower ends are fixed in the metal disc 1). 
which can be made to rotate, through two right angles, round the lower disc E. 
In the upper disc are two holes, a and 6, continuous with A and B respectively, 
and in the lower disc are two similar holes a' and I/, similarly continuous with 
the tubes G and H Hence, in the position of the discs shown in the figure, the 
tube G is continuous through the two discs with the bulb A, and the tube H 
with the bulb B. On turning the disc D through two right angles, the tube G 152 
THE VASCULAR MECHANISM. 
becomes continuous with B instead of A. and the tube If with A instead of B. 
There is a further arrangement, omitted from the figure for the sake of simplicity, 
by which when the disc D is turned through one instead of two right angles from 
either of the above positions, G becomes directly continuous with //, both being 
completely shut oil' from the bulbs. 
The ends of the tubes II and G are made to fit exactly into two canulae inserted 
into the two cut ends of the artery about to be experimented upon, and having a 
bore as nearly equal as possible to that of the artery. 
The method of experimenting is as follows: The disc D, being placed in the 
intermediate position, so that a and 6 are both cut off from az and //, the bulb A 
is filled with pure olive oil up to the mark x, and the bulb B. the rest of A, and 
the junction C, with defibrinated blood; and C is then clamped. The tubes II 
and G are also filled with defibrinated blood, and G is inserted into the canula 
of the central, Il into that of the peripheral, end of the artery. On removing 
Fig. 46. 
Ludwig's Stromuhr and a Diagrammatic Representation of the Same. 
the clamps from the artery the blood flows through G to II, and so back into the 
artery. The observation now begins by turning the disc D into the position 
shown in the figure; the blood then flows into A, driving the oil there contained 
out before it into the bulb B. in the direction of the arrow, the defibrinated blood 
previously present in B passing by H into the artery, and so into the system. At 
the moment that the blood is seen to rise to the mark x, the disc I) is with all 
possible rapidity turned through two right angles ; and thus the bulb B, now 
largely filled with oil, placed in communication with G. The blood-stream now 
drives the oil back into A, and the new blood in A through If into the artery. 
As soon as the oil has wholly returned to its original position, the disc is again 
turned round, and A once more placed in communication with G, and the oil once 
more driven from A to B. And this is repeated several times, indeed generally 
until the clotting of the blood or the admixture of the oil with the blood puts an 
end to the experiment. Thus the flow of blood is used to fill alternately with 
blood or oil the space of the bulb A, whose cavity as far as the mark x has been 
exactly measured; hence if the number of times in any given time the disc I) 
has to be turned round be known, the number of times A has been filled is also 
known, and thus the quantity of blood which has passed in that time through 
the canula connected with the tube G is directly measured. For instance, sup- THE MAIN FACTS OF THE CIRCULATION. 
153 
posing that the quantity held by the bulb A when filled up to the mark x is 5 c.c., 
and supposing that from the moment of allowing the first 5 c.c. of blood to begin 
to enter the tube to the moment when the escape of the last 5 c.c. from the artery 
into the tube was complete, 100 seconds had elapsed, during which time 5 c.c. 
has been received ten times into the tube from the artery (all but the last 5 c.c. 
being returned into the distal portion of the artery), obviously 0.5 c.c. of blood 
had flowed from the proximal section of the artery in one second. Hence, sup- 
posing that the diameter of the canula (and of the artery, they being the same) 
were 2 mm., with area therefore of 3.14 square mm., an outflow through the sec- 
tion of 0.5 c.c. or 500 mm. in a second would give ( ) a velocity of about 159 
mm. in a second. 
The haematachometer of Vierordt [Fig. 47] is constructed on the principle of 
measuring the velocity of the current by observing the amount of deviation under- 
gone by a pendulum, the free end of which hangs loosely in the stream. A square 
or rectangular chamber, one side of which is of glass 
and marked with a graduated scale in the form of an 
arc of a circle, is connected by means of two short 
tubes with the two cut ends of an artery; the blood 
consequently flows from the proximal (central) por- 
tion of the artery through the chamber into the distal 
portion of the artery. Within the chamber and sus- 
pended from its roof is a short pendulum, which when 
the blood-stream is cut oft from the chamber hangs 
motionless in a vertical position, but when the blood 
is allowed to flow through the chamber, is driven by 
the force of the current out of its position of rest. The 
pendulum is so placed that a marker attached to its free end travels close to the 
inner surface of the glass side along the arc of the graduated side. Hence the 
amount of deviation from a vertical position may easily be read off on the scale 
from the outside. The graduation of the scale having been carried out by experi- 
menting with streams of known velocity, the velocity can at once be calculated from 
the amount of deviation. 
An instrument based on the same principle has been invented by Chauveau and 
improved by Lortet, Fig. 48. In this the part which corresponds to the pendulum 
[Fig. 47. 
Hsematachometer of Vierordt • 
a, b, mouthpieces.] 
Fig. 48. 
Heematachometer of Chauveau and Lortet. 
in Vierordt's instrument is prolonged outside the chamber, and thus the portion 
within the chamber is made to form the short arm of a lever, the fulcrum of which 
is at the point where the wall of the chamber is traversed, and the long arm of 
which projects outside. A somewhat wide tube, the wall of which is at one point 154 
THE VASCULAR MECHANISM. 
composed of an India-rubber membrane, is introduced between the two cut ends 
of an artery. A long light lever pierces the India-rubber membrane. The short 
expanded arm of this lever projecting within the tube is moved on its fulcrum in 
the India-rubber ring by the current of blood passing through the tube, the 
greater the velocity of the current the larger being the excursion of the lever. 
The movements of the short arm give rise to corresponding movements in the oppo- 
site direction of the long arm outside the tube, and these, by means of a marker 
attached to the end of the long arm. may be directly inscribed on a recording sur- 
face. This instrument is very well adapted for observing changes in the velocity 
of the flow. In determining actual velocities, for which purpose it has to be 
experimentally graduated, it is not so useful. 
In the capillaries, the rate is slowest of all. In the web of the frog the 
flow as judged by the movement of the red corpuscles may be directly 
measured under the microscope by means of a micrometer, and is found to 
be about half a millimeter in a second ; but this is probably a low estimate, 
since it is only when the circulation is somewhat slow, slower perhaps than 
what ought to be considered the normal rate, that the red corpuscles can be 
distinctly seen. In the mammal the rate has been estimated at about 0.75 
millimetres a second, but is probably quicker than even this. 
As regards the veins, the flow is very slow in the small veins emerging 
from the capillaries, but increases as these join into larger trunks, until in a 
large vein, such as the jugular of the dog, the rate is about 200 mm. a 
second. 
§ 112. It will be seen, then, that the velocity of the flow is in inverse 
proportion to the width of the bed, to the united sectional areas of the 
vessels. It is greatest at the aorta, it diminishes along the arterial system to 
the capillaries, to the united bases of the cones spoken of in § 101, where it 
is least, and from thence increases again along the venous system. 
And, indeed, it is this width of the bed, and this alone, which determines 
the general velocity of the flow at various parts of the system. The slowness 
of the flow in the capillaries is not due to there being so much more friction 
in their narrow channels than in the wider canals of the larger arteries. 
For the peripheral resistance caused by the friction in the capillaries and 
small arteries is an obstacle not only to the flow of blood through these 
small vessels where the resistance is actually generated, but also to the 
escape of the blood from the large into the small arteries, and indeed from 
the heart into the large arteries. It exerts its influence along the whole 
arterial tract. And it is obvious that if it were this peripheral resistance 
which checked the flow in the capillaries, there could be no recovery of 
velocity along the venous tract. 
The blood is flowing through a closed system of tubes, the bloodvessels, 
under the influence of one propelling force, the systole of the ventricle, for 
this is the force which drives the blood from ventricle to auricle, though, as 
we have seen, its action is modified in the several parts of the system. In 
such a system the same quantity of fluid must pass each section of the sys- 
tem at the same time, otherwise there would be a block at one place and a 
deficiency at another. If, for instance, a fluid 
is made to flow by some one force, pressure or 
gravity, through a tube A (Fig. 49) with an 
enlargement B, it is obvious that the same 
quantity of fluid must pass through the sec- 
tion b as passes through the section a in the 
same time-for instance, a second. Other- 
wise, if less passes through b than a, the 
fluid would accumulate in B, or if more, 
B would be emptied. In the same way just as much must pass in the 
Fig. 49. THE MAIN FACTS OF THE CIRCULATION. 
155 
same time through the section c as passes through a or b. But if just as 
many particles of water have to get through the narrow section a in the 
same time as they have to get through the broader section c, they must 
move quicker through a than through c, or more slowly through c than 
through a. For the same reason water flowing along a river impelled by 
one force-viz., that of gravity-rushes rapidly through a " narrow " and 
flows sluggishly when the river widens out into a " broad." The flow 
through B will be similarly slackened if B, instead of being simply a single en- 
largement of the tube A, consists of a number of small tubes branching out 
from A, with a united sectional area greater than the sectional area of A. 
In each of such small tubes, at the line c, for instance, the flow will be 
slower than at a, where the small tubes branch out from A, or at b, where 
they join again to form a single tube. Hence it is that the blood rushes 
swiftly through the arteries, tarries slowly through the capillaries, but 
quickens its pace again in the veins. 
An apparent contradiction to this principle that the rate of flow is de- 
pendent on the width of the bed is seen in the case where, the fluid having 
alternative routes, one of the routes is temporarily widened. Suppose a tube 
A dividing into two branches of equal length x and y which unite again to 
form the tube V. Suppose, to start with, x and y are of equal diameter ; 
then the resistance offered by each being equal, the flow will be equally 
rapid through the two, being just so rapid that as much fluid passes in a 
given time through x and y together as passes through A or through F- 
But now suppose y to be widened ; the widening will diminish the resistance 
offered by y, and in consequence, supposing that no material change takes 
place in the pressure or force which is driving the fluid along, more fluid 
will now pass along y in a given time than did before; that is to say, the 
rapidity of the flow in y will be increased. It will be increased at the ex- 
pense of the flow through x, since it will still hold good that the flow through 
x and y together is equal to the flow through A and through V. We shall 
have occasion later on to point out that a small artery, or a set of small 
arteries, may be more or less suddenly widened without materially affecting 
the general blood-pressure which is driving the blood through the artery or 
set of arteries. In such cases the flow of blood through the widened artery 
or arteries is for the time being increased in rapidity, not only in spite of, 
but actually in consequence of, the artery being widened. 
It must be understood in fact that this dependence of the rapidity of the 
flow on the width of the bed applies to the general rate of flow of the whole 
circulation, and that, besides the above instance, other special and temporary 
variations occur due to particular circumstances. Thus changes of pressure 
may alter the rapidity of flow. The cause of the flow through the whole 
system is the pressure of the ventricular systole manifested as what we have 
called blood-pressure. At each point along the system nearer the left ven- 
tricle, and therefore further from the right auricle, the pressure is greater 
than at a point further from the left ventricle and so nearer the right auricle ; 
it is this difference of pressure which is the real cause of the flow from the 
one point to the other; and other things being equal the rapidity of the flow 
will depend on the amount of the difference of pressure. Hence, temporary 
or local variations in rapidity of flow may be caused by the establishment 
of temporary or local differences of pressure. For example, at any point 
along the arterial system the flow is increased in rapidity during the tem- 
porary increase of pressure due to the ventricular systole, i. e., the pulse, 
and diminished during the subsequent temporary decrease, the increase and 
decrease being the more marked the nearer the point to the heart. And we 
shall probably meet later on with other instances. 156 
THE VASCULAR MECHANISM. 
§113. Time of the entire circuit. It is obvious from the foregoing that a 
red corpuscle in performing the whole circuit, in travelling from the left 
ventricle back to the left ventricle, would spend a large portion of its time 
in the capillaries, minute arteries, and veins. The entire time taken up in 
the whole circuit has been approximately estimated by measuring the time 
it takes for an easily recognized chemical substance after injection into the 
jugular vein of one side to appear in the blood of the jugular vein of the 
other side. 
While small quantities of blood are being drawn at frequently repeated intervals 
from the jugular vein of one side, or while the blood from the vein is being allowed 
to fall in a minute stream on an absorbent paper covering some travelling surface, 
an iron salt such as potassium ferrocyanide (or preferably sodium ferrocyanide as 
being more innocuous) is injected into the jugular vein of the other side. If the 
time of the injection be noted, and the time after the injection into one side at 
which evidence of the presence of the iron salt can be detected in the sample of 
blood from the vein of the other side be noted, this gives the time it has taken the 
salt to perform the circuit: and on the supposition that mere diffusion does not 
materially affect the result, the time which it takes the blood to perform the same 
circuit is thereby given. 
In the horse this time has been experimentally determined at about 30 
seconds and in the dog at about 15 seconds. In man it is probably from 20 
to 25 seconds. 
Taking the rate of flow through the capillaries at about 1 mm. a second it 
would take a corpuscle as long a time to get through about 20 mm. of capil- 
laries as to perform the whole circuit. Hence, if any corpuscle had in its 
circuit to pass through 10 mm. of capillaries, half the whole time of its 
journey would be spent in the narrow channels of the capillaries. Inasmuch 
as the purposes served by the blood are chiefly carried out in the capillaries, 
it is obviously of advantage that its stay in them should be prolonged. 
Since, however, the average length of a capillary is about 0.5 mm., about 
half a second is spent in the capillaries of the tissues and another half sec- 
ond in the capillaries of the lungs. 
§ 114. We may now briefly summarize the broad features of the circu- 
lation, which we have seen may be explained on purely physical principles, 
it being assumed that the ventricle delivers a certain quantity of blood with 
a certain force into the aorta at regular intervals, and that the physical 
properties of the bloodvessels remain the same. 
We have seen that owing to the peripheral resistance offered by the 
capillaries and small vessels the direct effect of the ventricular stroke is to 
establish in the arteries a mean arterial pressure which is greatest at the root 
of the aorta and diminishes toward the small arteries, some of it being used 
up to drive the blood from the aorta to the small arteries, but which retains 
at the region of the small arteries sufficient power to drive through the 
small arteries, capillaries, and veins just as much blood as is being thrown 
into the aorta by the ventricular stroke. We have seen, further, that in the 
large arteries at each stroke the pressure rises and falls a little above and 
below the mean, thus constituting the pulse, but that this extra distention 
with its subsequent recoil diminishes along the arterial tract and finally 
vanishes; it diminishes and vanishes because it too, like the whole force of 
the ventricular stroke, of a fraction of which it is the expression, is used up 
in establishing the mean pressure; we shall, however, consider again later 
on the special features of this pulse. We have seen, further, that the task 
of driving the blood through the peripheral resistance of the small arteries 
and capillaries consumes much of this mean pressure, which consequently is 
much less in the small veins than in the corresponding small arteries, but THE HEART. 
157 
that sufficient remains to drive the blood, even without the help of the aux- 
iliary agents which are generally in action, from the small veins right back 
to the auricle. Lastly we have seen that while the above is the cause of the 
How from ventricle to auricle, the changing rate of the flow, the diminish- 
ing swiftness in the arteries, the sluggish crawl through the capillaries, the 
increasing quickness through the veins are determined by the changing width 
of the vascular " bed." 
Before we proceed to consider any further details as to the phenomena 
of the flow through the vessels, we must turn aside to study the heart. 
The Heart. 
§115. The heart is a valvular pump which works on mechanical princi- 
ples, but the motive power of which is supplied by the contraction of its 
muscular fibres. Its action consequently presents problems which are partly 
mechanical and partly vital. Regarded as a pump, its effects are determined 
by the frequency of the beats, by the force of each beat, by the character of 
each beat-whether, for instance, slow and lingering, or sudden and sharp- 
and by the quantity of fluid ejected at each beat. Hence, with a given 
frequency, force, and character of beat, and a given quantity ejected at each 
beat, the problems which have to be dealt with are for the most part 
mechanical. The vital problems are chiefly connected with the causes which 
determine the frequency, force, and character of the beat. The quantity 
ejected at each beat is governed more by the state of the rest of the body 
than by that of the heart itself. 
The Phenomena of the Normal Beat. 
The visible movements. When the chest of a mammal is opened and arti- 
ficial respiration kept up the heart may be watched beating. Owing to the 
removal of the chest-wall, what is seen is not absolutely identical with what 
takes place within the intact chest, but the main events are the same in both 
cases. A complete beat of the whole heart or cardiac cycle may be observed 
to take place as follows : 
The great veins, inferior and superior vente cavse, and pulmonary veins 
are seen, while full of blood, to contract in the neighborhood of the heart; 
the contraction runs in a peristaltic wave toward the auricles, increasing in 
intensity as it goes. Arrived at the auricles, which are then full of blood, 
the wave suddenly spreads, at a rate too rapid to be fairly judged by the eye, 
over the whole of those organs, which accordingly contract with a sudden 
sharp systole. In the systole, the walls of the auricles press toward the 
auriculo-ventricular orifices, and the auricular appendages are drawn inward, 
becoming smaller and paler. During the auricular systole, the ventricles 
may be seen to become turgid. Then follows, as it were immediately, the 
ventricular systole, during which the ventricles become more conical. Held 
between the fingers they are felt to become tense and hard. As the systole 
progresses, the aorta and pulmonary arteries expand and elongate, the apex 
is tilted slightly upward, and the heart twists somewhat on its long axis, 
moving from the left and behind toward the front and right so that more of 
the left ventricle becomes displayed. As the systole gives way to the suc- 
ceeding diastole, the ventricles resume their previous form and position, the 
aorta and pulmonary artery shrink and shorten, the heart turns back toward 
the left, and thus the cycle is completed. 
In the normal beat, the two ventricles are perfectly synchronous in ac- 
tion, they contract at the same time and relax at the same time, and the 158 
THE VASCULAR MECHANISM. 
two auricles are similarly synchronous in action. It has been maintained, 
however, that the synchronism may at times not be perfect. 
Before we attempt to study in detail the several parts of this compli- 
cated series of events, it will be convenient to take a rapid survey of what 
is taking place within the heart during such a cycle. 
§ 116. The cardiac cycle. We may take as the end of the cycle the 
moment at which the ventricles having emptied their contents have relaxed 
and returned to the diastolic or resting position and form. At this moment 
the blood is flowing freely with a fair rapidity, but as we have seen at a very 
low pressure, through the veme cavse into the right auricle (we may confine 
ourselves at first to the right side), and since there is now nothing to keep 
the tricuspid valve shut, some of this blood probably finds its way into the 
ventricle also. This goes on for some little time, and then comes the sharp, 
short systole of the auricle, which, since it begins, as we have seen, as a wave 
of contraction running forward along the ends of the veme cavse, drives the 
blood not backward into the veins but forward • into the ventricle; this end 
is further secured by the fact that the systole has behind it on the venous 
side the pressure of the blood in the veins, increasing, as we have seen, back- 
ward toward the capillaries, and before it the relatively empty cavity of the 
ventricle, in which the pressure is at first very low. By the complete con- 
traction of the auricular walls the complete or nearly complete emptying of 
the cavity is insured. No valves are present in the mouth of the superior 
vena cava, for they are not needed; and the imperfect Eustachian valve at 
the mouth of the inferior vena cava cannot be of any great use in the adult, 
though in its more developed state in the foetus it had an important function 
in directing the blood of the inferior vena cava through the foramen ovale 
into the left auricle. The valves in the coronary vein are, however, probably 
of some use in preventing a reflux into that vessel. 
As the blood is being driven by the auricular systole into the ventricle, a 
reflux current is probably set up, by which the blood, passing along the 
sides of the ventricle, gets between them and the flaps of the tricuspid valve 
and so tends to float these up. [Figs. 50, 51.] It is further probable that 
[Fig. 50. 
Fig. 51. 
Diagrams of Valves of the Heart. After Dalton.] 
the same reflux current, continuing somewhat later than the flow into the 
ventricle, is sufticient to bring the flaps into apposition, without any regurgi- 
tation into the auricle, at the close of the auricular systole, before the ven- THE HEART. 
159 
tricular systole has begun. According to some authors, however, the closure 
of the valve is effected, at the very beginning of the ventricular systole, by 
the contraction of the papillary muscles ; the chordae tendineae of a papillary 
muscle are attached to the adjacent edges of two flaps, so that the shorten- 
ing of the muscle tends to bring these edges into apposition. 
The auricular systole is, as we have said, immediately followed by that of 
the ventricle. Whether the contraction of the ventricular walls (which as 
we shall see is a simple though prolonged contraction and not a tetanus) 
begins at one point and swiftly travels over the rest of the fibres, or begins 
all over the ventricle at once, is a question not at present definitely settled ; 
but in any case the walls exert on the contents a pressure which is soon 
brought to bear on the whole contents and very rapidly rises to a maximum. 
The only effect upon the valve of this increasing intra-ventricular pressure 
is to render the valve more and more tense, and in consequence more secure, 
the chordse tendineae (the slackening of which through the change of form of 
the ventricle is probably obviated by a regulative contraction of the papillary 
muscles) at the same time preventing the valve from being inverted or even 
bulging largely into the auricle, and indeed, according to some observers, 
keeping the valvular sheet actually convex to the ventricular cavity, by 
which means the complete emptying of the ventricle is more fully effected. 
[Figs. 50, 51.] The connection, to which we have just referred, of the chordae 
of the same papillary muscle with the adjacent edges of two flaps, also assists 
in keeping the flaps in more complete apposition. Morever the extreme 
borders of the valves, outside the attachments of the chordae, are excessively 
thin, so that when the valve is closed, these thin portions are pressed flat 
together back to back ; hence, while the tougher central parts of the valves 
bear the force of the ventricular systole, the opposed thin membranous 
edges, pressed together by the blood, more completely secure the closure of 
the orifice. 
At the commencement of the ventricular systole the semilunar valves of 
the pulmonary artery are closed, and are kept closed by the high pressure of 
the blood in the artery. As, however, the ventricle continues to press with 
greater and greater force on its contents, making the ventricle hard and 
tense to the touch, the pressure within the ventricle becomes greater than 
that in the pulmonary artery, and this greater pressure forces open the semi- 
lunar valves and allows the escape of the contents into the artery. The 
ventricular systole may be seen and felt in the exposed heart to be of some 
duration ; it is strong enough and long enough to empty the ventricle com- 
pletely ; indeed, as we shall see, it probably lasts longer than the discharge 
of blood, so that there is a brief period during which the ventricle is empty 
but yet contracted. 
During the ventricular systole the semilunar valves are pressed outward 
toward but not close to the arterial walls, reflux currents probably keeping 
them in an intermediate position, so that their orifice forms an equilateral 
triangle with curved sides; thus they offer little obstacle to the escape of 
blood from the cavity of the ventricle. The ventricle, as we have seen, pro- 
pels the blood with great force and rapidity into the pulmonary artery, and 
the whole contents are speedily ejected. Now, when a force which is driving 
a fluid with great rapidity along a closed channel suddenly ceases to act, 
the fluid, by its momentum, continues to move onward after the force has 
ceased ; in consequence of this a negative pressure makes its appearance in 
the rear of the fluid, and, sucking the fluid back again, sets up a reflux cur- 
rent. So when the last portions of blood leave the ventricle a negative 
pressure makes its appearance behind them, and leads to a reflux current 
from the artery toward the ventricle. This alone would be sufficient to bring 160 
THE VASCULAR MECHANISM. 
the valves together; and, in the opinion of some, is the real cause of the 
closure of the valves; others, however, as we shall see later on, maintain 
that subsequent to this reflux due to mere negative pressure a somewhat later 
reflux, in which the elastic reaction of the arterial walls is concerned, more 
completely fills and renders tense the pockets, causing their free margins to 
come into close and firm contact, and thus entirely blocks the wav. The 
corpora Arantii meet in the centre, and the thin membranous festoons or 
lunula are brought into exact apposition. As in the tricuspid valves, so 
here, while the pressure of the blood is borne by the tougher bodies of the 
several valves, each two thin adjacent lunulae, pressed together by the blood 
acting on both sides of them, are kept in complete contact, without any 
strain being put upon them ; in this way the orifice is closed in a most 
efficient manner. 
The ventricular systole now passes off, the muscular walls relax, the 
ventricle returns to its previous form and position, and the cycle is once 
more ended. 
What thus takes place in the right side takes place in the left side also. 
There is the same sudden sharp auricular systole beginning at the roots of 
the pulmonary veins, the same systole of the ventricle, but, as we shall see, 
one much more powerful and exerting much more force; the mitral valve 
with its two flaps acts exactly like the tricuspid valve, and the action of the 
semilunar valves of the aorta simply repeats that of the valves of the 
pulmonary artery. 
We may now proceed to study some of the cardiac events in detail. 
§ 117. The change of form. The exact determination of the changes in 
form and position of the heart, especially of the ventricles, during a cardiac 
cycle is attended with difficulties. 
The ventricles, for instance, are continually changing their form : they 
change while their cavities are being filled from the auricles, they change 
while the contraction of their walls is getting up the pressure on their con- 
tents, they change while under the influence of that pressure, their contents 
being discharged into the arteries, and they change when, their cavities 
having been emptied, their muscular walls relax. 
We may take it for granted that the internal cavities are obliterated by 
the systole, for it is probable that practically the whole contents are driven 
out at each stroke, and probably also each cavity is emptied from its apex 
toward the mouth of the artery. 
With regard to changes in external form, there seems no doubt that the 
side-to-side diameter is much lessened. It seems also clear that the front-to 
back diameter is greater during the whole time of the systole than during 
the diastole, the increase taking place during the first part of the systole. If 
a light lever be placed on the surface of the heart of a mammal, the chest 
having been opened and artificial respiration being kept up, some such curve 
as that represented in Fig. 52 is obtained. The rise of the lever in describing 
such a curve is due to the elevation of the part of the front surface of the 
heart on which the lever is resting. Such an elevation might be caused, 
especially if the lever were placed near the apex, by the heart being " tilted " 
upward during the systole, but only a small portion at most of the rise can 
be attributed to this cause; the rise is perhaps best seen when the lever is 
placed in the middle portion of the ventricle, and must be chiefly due to an 
increase in the front-to-back diameter of the ventricle during the beat. We 
shall discuss this curve later on in connection with other curves and may 
here simply say that the part of the curve from b' to d probably corresponds 
to the actual systole of the ventricle, that is, to the time during which the 
fibres of the ventricle are undergoing contraction, the sudden fall from d THE HE A KT. 
161 
onward representing the relaxation which forms the first part of the diastole. 
If this interpretation of the curve be correct, it is obvious that the front-to- 
Fig. 52.' 
Tracing from Heart of Cat, obtained by placing a Light Lever on the Ventricle, the Chest 
having been Opened. The tuning-fork curve marks 50 vibrations per second. 
back diameter is greater during the whole of the systole than it is during 
diastole, since the lever is raised up all this time. 
This increase of the front-to-back diameter combined with a decrease of 
the side-to-side diameter has for a result a change in the form of the section 
of the base of the ventricles. During the diastole this has somewhat the 
form of an ellipse with the long axis from side to side, but with the front 
part of the ellipse much more convex than the back, since the back surface 
of the ventricle is somewhat flattened. During the systole this ellipse is by 
the shortening of the side-to-side diameter and the increase of the front-to- 
back diameter converted into a figure much more nearly resembling a circle. 
It is urged, moreover, that the whole of the base is constricted, and that the 
greater efficiency of the auriculo-ventricular valves is thereby secured. 
As to the behavior of the long diameter from base to apex observers are 
not agreed. Some maintain that it is shortened, and others that it is prac- 
tically unchanged. If any shortening does take place, it must be largely 
compensated by the elongation of the great vessels, which, as stated above, 
1 The vertical or rather curved lines (segments of circles) introduced into this and 
many other curves are of use for the purpose of measuring parts of the curve. A complete 
curve should exhibit an "abscissa" line. This may be drawn by allowing the lever, ar- 
ranged for the experiment but remaining at rest, to mark with its point on the recording 
surface set in motion; a straight line, the abscissa line, is thus described, and may be 
drawn before or after the curve itself is made, and may be placed above or preferably be- 
low the curve. When a tuning-fork or other time-marker is used, the line of the time- 
marker or a line drawn through the curves of the tuning-fork will serve as an abscissa 
line. After a tracing has been made, the recording surface should be brought back to 
such a position that the point of the lever coincides with some point of the curve which 
it is desired to mark ; if the lever be then gently moved up and down, the point of the 
lever will describe a segment of a circle (the centre of which lies at the axis of the lever), 
which segment should be made long enough to cut both the curve and the abscissa line 
(the tuning-fork curves or other time-marking line) where this is drawn. By moving 
the recording surface backward and forward similar segments of circles may be drawn 
through other points of the curve. The lines a, b, c in Fig. 52 were thus drawn. The 
distance between any two of these points may thus be measured on the tuning-fork curve or 
other time curve, or on the abscissa line. Similar lines may be drawn on the tracing 
after its removal from the recording instrument in the following way: Take a pair of 
compasses, the two points of which are fixed just as far apart as the length of the lever 
used in the experiment, measured from its axis to its writing point. By means of the 
compasses find the position on the tracing of the centre of the circle of which any one 
of the previously drawn curved lines forms a segment. Through this centre draw a line 
parallel to the abscissa. By keeping one point of the compass on this line but moving it 
along the line backward or forward a segment of a circle may be drawn so as to cut any 
point of the curve that may be desired, and also the abscissa line or the time line. Such 
a segment of a circle may be used for the same purposes as the original one, and any 
number of such segments may be drawn. 162 
THE VASCULAR MECHANISM. 
may be seen in an inspection of the beating heart. For there is evidence 
that the apex, though, as we have seen, it is during the systole somewhat 
twisted round and at the same time brought closer to the chest-wall, does 
not change its position up or down-i. e., in the long axis of the body. If 
in a rabbit or dog a needle be thrust through the chest-wall so that its point 
plunges into the apex of the heart, though the needle quivers its head 
moves neither up nor down, as it would do if its point in the apex moved 
down or up. 
§ 118. Cardiac impulse. If the hand be placed on the chest, a shock or 
impulse will be felt at each beat, and on examination this impulse, "cardiac 
impulse," will be found to be synchronous with the systole of the ventricle. 
In man, the cardiac impulse may be most distinctly felt in the fifth costal 
interspace, about an inch below and a little to the median side of the left 
nipple. In an animal the same impulse may also be felt in another way- 
viz., by making an incision through the diaphragm from the abdomen, and 
placing the finger between the chest-wall and the apex. It then can be dis- 
tinctly recognized as the result of the hardening of the ventricle during the 
systole. And the impulse which is felt on the outside of the chest is chiefly 
the effect of the same hardening of the stationary portion nf the ventricle in 
contact with the chest-wall, transmitted through the chest-wall to the finger. 
In its flaccid state, during diastole, the apex is (in a standing position, at 
least) at this point in contact with the chest-wall, lying between it and the 
tolerably resistant diaphragm. During the systole, while being brought 
even closer to the chest-wall by the tilting of the ventricle and by the move- 
ment to the front and to the right, of which we have already spoken, it 
suddenly grows tense and hard. The ventricles, in executing their systole, 
have to contract against resistance. They have to produce within their 
cavities pressures greater than those in the aorta and pulmonary arteries, 
respectively. This is, in fact, the object of the systole. Hence, during the 
swift systole, the ventricular portion of the heart becomes suddenly tense, 
somewhat in the same way as a bladder full of fluid would become tense and 
hard when forcibly squeezed. The sudden pressure exerted by the ventricle, 
thus become suddenly tense and hard, aided by the closer contact of the 
apex with the chest-wall (which, however, by itself without the hardening 
of contraction would be insufficient to produce the effect), gives an impulse 
of shock both to the chest-wall and to the diaphragm, which may be felt 
readily both on the chest-wall, and also through the diaphragm when the 
abdomen is opened and the finger inserted. If the modification of the 
sphygmograph (of which we shall speak in dealing, later on, with the pulse), 
called the cardiograph, be placed on the spot where the impulse is felt most 
strongly, the lever is seen to be raised during the systole of the ventricles, 
and to fall again as the systole passes away, very much as if it were placed 
on the heart directly. A tracing may thus be obtained (see Fig. 58), of 
which we shall have to speak more fully immediately (see § 122). If the 
button of the lever be placed, not on the exact spot of the impulse, but 
at a little distance from it, the lever will be depressed during the sys- 
tole. While at the spot of impulse itself the contact of the ventricle is 
increased during systole, away from the ^pot the ventricle retires from 
the chest-wall (by the diminution of its right-to-left diameter), and hence, 
by the mediastinal attachments of the pericardium, draws the chest-wall 
after it. 
§ 119. The sounds of the heart. When the ear is applied to the chest, 
either directly or by means of a stethoscope, two sounds are heard, the first 
a comparatively long, dull, booming sound, the second a short, sharp, sudden 
one. Between the first and second sounds the interval of time is very THE HEART. 
163 
short-too short to be measurable-but between the second and the suc- 
ceeding first sound there is a distinct pause. The sounds have been 
likened to the pronunciation of the syllables lubb, dup, so that the cardiac 
cycle, as far as the sounds are concerned, might be represented by: lubb, 
dup, pause. 
The second sound, which is short and sharp, presents no difficulties. It is 
coincident in point of time with the closure of the semilunar valves, and is 
heard to the best advantage over the second right costal cartilage close to its 
junction with the sternum-i. e., at the point where the aortic arch comes 
nearest to the surface, and to which sounds generated at the aortic orifice 
would be best conducted. Its characters are such as would belong to a 
sound generated by membranes like the semilunar valves being suddenly 
made tense and so thrown into vibrations. It is obscured and altered or 
replaced by a " murmur " when the semilunar valves are affected by disease, 
and may be artificially obliterated, a murmur taking its place, by passing a 
wire down the arteries and hooking up the aortic valves. There can be no 
doubt, in fact, that the second sound is due to the semilunar valves being 
thrown into vibrations at their sudden closure. The sound heard at the 
second right costal cartilage is chiefly that generated by the aortic valves, 
and murmurs or other alterations in the sound caused by changes in the 
aortic valves are heard most clearly at this spot. But even here the sound 
is not exclusively of aortic origin, for in certain cases in which the semi- 
lunar valves on the two sides of the heart are not wholly synchronous 
in action the sound heard here is double (" reduplicated second sound 
one being due to the aorta and one to the pulmonary artery. While 
the sound is listened to on the left side of the sternum at the same 
level, the pulmonary artery is supposed to have the chief share in pro- 
<lucing what is heard, and changes in the sound heard more clearly here 
than on the right side are taken as indications of mischief in the pulmo- 
nary valves. 
The first sound, longer, duller, and of a more " booming " character than 
the second, heard with greatest distinctness at the spot where the cardiac 
impulse is felt, presents many difficulties in the way of a complete explana- 
tion. It is heard distinctly when the chest-walls are removed. The cardiac 
impulse, therefore, can have little or nothing to do with it. In point of time 
it is coincident with the systole of the ventricles, and may be heard to the 
greatest advantage at the spot of the cardiac impulse-that is to say, at the 
place where the ventricles come nearest to the surface, and to which sounds 
generated in the ventricle would be best conducted. 
It is more closely coincident with the closure and consequent vibrations of 
the auriculo-ventricular valves than with the entire systole ; for, on the one 
hand, it dies away before the second sound begins, whereas, as we shall see, the 
actual systole lasts up to, if not beyond, the closure of the semilunar valves ; 
and, on the other hand, the auriculo-ventricular valve ceases to be tense and to 
vibrate as soon as the contents of the ventricle are driven out. This suggests 
that the sound is caused by the sudden tension of the auriculo-ventricular 
valves, and this view is supported by the facts that the sound is obscured, 
altered, or replaced by murmurs when the tricuspid or mitral valves are dis- 
eased, and that the sound is also altered, or, according to some observers, 
wholly done away with, when blood is prevented from entering the ven- 
tricles by ligature of the venae cavae. On the other hand, the sound has not 
the sharp character which one would expect in a sound generated by the 
vibration of membranes such as the valves in question, but in its booming 
qualities rather suggests a muscular sound. Further, according to some 
observers, the sound, though somewhat modified, may still be heard when 164 
THE VASCULAR MECHANISM. 
the large veins are clamped so that no blood enters the ventricle, and, indeed, 
may be recognized in the few beats given by a mammalian ventricle rapidly 
cut out of the living body by an incision carried below the auriculo-ven- 
tricular ring. Hence the view has been adopted that this first sound is a 
muscular sound. In discussing the muscular sound of skeletal muscle (see 
§ 78), we saw reasons to distrust the view that this sound was generated by 
the repeated individual simple contractions which made up the tetanus, and 
hence correspond in tone to the number of those simple contractions re- 
peated in a second, and to adopt the view that the sound was really due to a 
repetition of unequal tensions occurring in a muscle during the contraction. 
Now, the ventricular systole is undoubtedly a simple contraction, a prolonged 
simple contraction, not a tetanus, and therefore under the old view of the 
nature of a muscular sound, could not produce such a sound ; but, accepting 
the other view, and reflecting how complex must be the course of the systolic 
wave of contraction over the twisted fibres of the ventricle, we shall not find 
great difficulty in supposing that that wave is capable in its progress of pro- 
ducing such repetitions of unequal tensions as might give rise to a " muscular 
sound," and consequently in regarding the first sound as mainly so caused. 
Accepting such a view of the origin of the sound, we should expect to find 
the tension of the muscular fibres, and so the nature of sound dependent on 
the quantity of fluid present in the ventricular cavities, and hence modified 
by ligature of the great veins, and still more by the total removal of the 
auricles with the auriculo-ventricular valves. We may add that we should 
expect to find it modified by the escape of blood from the ventricles into the 
arteries during the systole itself, and might regard this as explaining why it 
dies away before the ventricle has ceased to contract. 
Moreover, seeing that the auriculo-ventricular valves must be thrown into 
sudden tension at the onset of the ventricular systole, which, as we have seen, 
is developed with considerable rapidity, not far removed at all events from 
the rapidity with which the semilunar valves are closed, a rapidity, there- 
fore, capable of giving rise to vibrations of the valves adequate to produce 
a sound, it is difficult to escape the conclusion that the closure of these valves 
must also generate a sound which in a normally beating heart is mingled 
in some way with the sound of muscular origin, although the ear cannot 
detect the mixture. 
If we accept this view, that the sound is of double origin, partly " muscu- 
lar," partly " valvular," both causes being dependent on the tension of the 
ventricular cavities, we can perhaps more easily understand how it is that 
the normal first sound is at times so largely, indeed we may say so com- 
pletely, altered and obscured in diseases of the auriculo-ventricular valves. 
Since the left ventricle forms the entire left apex of the heart, the mur- 
murs or other changes of the first sound heard most distinctly at the spot of 
cardiac impulse belong to the mitral valve of the left ventricle. Murmurs 
generated in the tricuspid valve of the right ventricle are heard more dis- 
tinctly in the median line below the end of the sternum. 
Endocardiac Pressure. 
§ 120. Since the heart exists for the purpose of exerting pressure on the 
blood within its cavities, by which pressure the circulation of the blood is 
effected, the study of the characters of this endocardiac pressure possesses 
great interest. Unfortunately, the observation of this pressure is attended 
with great difficulties. The ordinary mercury manometer which is so useful 
in studying the pressure in the arteries fails us when applied to the heart. 
It is true that a long canula, or tube open at the end, filled with sodium THE HEART. 
165 
carbonate solution, may be introduced into the jugular vein and so slipped 
down into either the right auricle or the right ventricle, or may be similarly 
introduced into the carotid artery and with care slipped down through the 
aorta, past the semilunar valves, into the left ventricle, and having been 
thus introduced may, like the ordinary canula used in studying arterial 
pressure (§ 104), be brought into connection with a mercury manometer. 
In this way, as in the case of an artery, a graphic record may be obtained of 
the changes of pressure taking place in either of the above three cavities. 
But the changes in the ventricular cavities are so great and rapid, that the 
inertia of the mercury, an evil in the case of an artery, comes so largely into 
play that the curve described by the float on the mercury is far from being 
an accurate record of the changes of pressure in the cavity. 
The mercury manometer may, however, be made to yield valuable results 
by adopting the ingenious contrivance of converting the ordinary manometer 
into a maximum or a minimum instrument. 
The principle of the maximum manometer, Fig. 53, consists in the introduction 
into the tube leading from the heart to the mercury column of a (modified cup- 
[Feg. 53. 
The Maximum Manometer of Goltz and Gaule. At e a connection is made with the tube lead- 
ing to the heart. When the screw-clamp k is closed, the valve v comes into action, and the in- 
strument, in the position of the valve shown in the figure, is a maximum manometer. By revers- 
ing the direction of v it is converted into a minimum manometer. When k is opened, the varia- 
tions of pressure are conveyed along a, and the instrument then acts like an ordinary manometer. 
and-ball) valve, opening, like the aortic semilunar valves, easily from the heart, 
but closing firmly when fluid attempts to return to the heart. The highest pres- 
sure is that which drives the longest column of fluid past the valve, raising the 
mercury column to a corresponding height. Since this column, once past the 
valve, cannot return, the mercury remains at the height to which it was raised 
by it and thus records the maximum pressure. By reversing the direction of the 
valve, the manometer is converted from a maximum into a minimum instrument. 
The maximum manometer applied to the cavity of either ventricle or of 
the right auricle, gives a record of the highest pressure reached within that 
cavity, and the minimum manometer similarly shows the lowest pressure 
reached, during the time that the instrument is applied. 
The maximum manometer thus employed shows that the maximum pres- 166 
THE VASCULAR MECHANISM. 
sure in the left ventricle is distinctly greater than the mean pressure in the 
aorta (the ordinary mercury manometer having previously given the para- 
doxical result, due to the inertia of the mercury, that the mean pressure in 
the left ventricle might be less than in the aorta), that the maximum pres- 
sure in the right ventricle is less than in the left, and in the right auricle is 
still less. In the dog, for example, the pressure in the left ventricle reaches 
a maximum of about 140 mm. (mercury), in the right ventricle of about 60 
mm., and in the right auricle of about 20 mm. 
But the chief interest attaches to the minimum pressure observed ; for the 
minimum manometer records a negative pressure in the cavities of the heart- 
i. e., shows that the pressure in them may fall below that of the atmosphere. 
Thus in the left ventricle (of the dog) a minimum pressure varying from 
-52 to -20 mm. may be reached, the minimum of the right ventricle 
being from -17 to -16 mm., and of the right auricle from -12 to -17 mm.1 
Part of this diminution of pressure in the cardiac cavities may be due, as 
will be explained in a later part of this work, to the aspiration of the thorax 
in the respiratory movements. But even when the thorax is opened, and 
artificial respiration kept up, under which circumstances no such aspiration 
takes place, a negative pressure is still observed, the pressure in the left ven- 
tricle still sinking as low as -24 mm. Now, what the instrument actually 
shows is that at some time or other during the number of beats which took 
place while the instrument was applied (and these may have been very 
few) the pressure in the ventricle sank so many mm. below that of the 
atmosphere. Since the negative pressure is observed when the heart is 
beating quite regularly, each beat being exactly like the others, we may 
infer that a negative pressure occurs at some period or other of each cardiac 
cycle. But the instrument obviously gives us no information as to the exact 
phase of the beat in which the negative pressure occurs ; to this point as well 
as to the importance of this negative pressure we shall return presently. 
§ 121. The difficulties due to the inertia of the mercury may be obviated 
by adopting the method of Chauveau and Marey, which consists in intro- 
ducing in a large animal, such as a horse, through a bloodvessel into a cavity 
of the heart a tube ending in an elastic bag (Fig. 54, A) fashioned something 
like a sound, both tube and bag being filled with air, and the tube being 
connected with a recording "tambour." 
A tube of appropriate curvature, A, b, Fig. 54, is furnished at its end with an 
elastic bag or "ampulla," a. When it is desired to explore simultaneously both 
auricle and ventricle, the sound is furnished with two ampullae with two small 
elastic bags, one at the extreme end and the other at such a distance that when 
the former is within the cavity of the ventricle the latter is in the cavity of the 
auricle. Such an instrument is spoken of as a "cardiac sound." Each "ampulla" 
communicates by a separate air-tight tube with an air-tight tambour (Fig. 54, B) 
on which a lever rests, so that any pressure on the ampulla is communicated to 
the cavity of its respective tambour, the lever of which is raised in proportion. 
When two ampullae are used the writing points of both levers are brought to bear 
on the same recording surface exactly underneath each other. The tube is care- 
fully introduced through the right jugular vein into the right side of the heart 
until the lower (ventricular) ampulla is fairly in the cavity of the right ventricle, 
and consequently the upper (auricular) ampulla in the cavity of the right auricle. 
Changes of pressure on either ampulla then cause movements of the corresponding 
lever. When the pressure, for instance, on the ampulla in the auricle is increased, 
the auricular lever is raised and describes on the recording surface an ascending 
curve; when the pressure is taken off the curve descends; and so also with the 
ventricle. 
The "sound " may in a similar manner be readily introduced through the 
1 These numbers are to be considered merely as instances which have been observed, 
and not as averages drawn from a large number of cases. THE HEART. 
167 
carotid artery into the left ventricle and the changes taking place in that chamber 
also explored. 
When this instrument is applied to the right auricle and ventricle some 
such record is obtained as that shown in Fig. 55, where the upper curve is a 
Fig. 54. 
Marey's Tambour, with Cardiac Sound: A. A simple cardiac sound such as may be used for 
exploration of the left ventricle. The portion a of the ampulla at the end is of thin India-rubber, 
stretched over an open framework with metallic supports above and below. The long tube b 
serves to introduce it into the cavity which it is desired to explore. 
B. The tambour. The metal chamber m is covered in an air-tight manner with the India- 
rubber c, bearing a thin metal plate m' to which is attached the lever I moving on the hinge h. 
The whole tambour can be placed by means of the clamp cl at any height on the upright s'. The 
India-rubber tube t serves to connect the interior of the tambour either with the cavity of the 
ampulla of A or with any other cavity. Supposing that the tube t were connected with b, any 
pressure exerted on a would cause the roof of the tambour to rise and the point of the lever 
would be proportionately raised. 
tracing taken from the right auricle and the lower curve from the right 
ventricle of the horse, both curves being taken simultaneously on the same 
recording surface. 
In these curves the rise of the lever indicates pressure exerted upon the 
corresponding ampulla, and the upper curve from the right auricle shows 
the sudden brief pressure (b~) exerted by the sudden and brief auricular 
systole. The lower curve from the right ventricle shows that the pressure 
exerted by the ventricular systole begins almost immediately after the auricu- 
lar systole, increases very rapidly indeed, so that the lever rises in almost a 
straight line up to c', is continued for some considerable time, and then 
falls very rapidly to reach the base line. But it may be doubted whether 
the instrument can be trusted to tell much more than this. The pressure 
recorded by each lever is the pressure exerted on the ampulla, and this 
may continue to be exerted after all blood has been discharged from the 
cavity, the walls of the emptied cavity closing round and pressing on the 
ampulla. But, as we shall presently see, it is of great interest to determine, 
not only the force and duration of the pressure exerted by the ventricular 
systole, but also whether or no the fibres continue contracted and exert- 
ing pressure for an appreciable time after the blood has been forced out 
of the cavity. The figure, moreover, it need hardly be said, does not by 168 
THE VASCULAR MECHANISM. 
itself give any information as to the relative amounts of pressure ex- 
erted by the auricle and ventricle respectively. 
In the curve the auricular lever rises about 
half as high as the ventricular lever; but we 
must not infer from this that the auricular 
stroke is half as strong as the ventricular 
stroke; the former is arranged so as to move 
much more readily, to be much more sensitive 
than the latter. The instrument, it is true, 
may be experimentally graduated, and may 
then be used to determine the actual amount 
of pressure ; but for this purpose is not wholly 
satisfactory. We may add that the irregu- 
larities seen on the ventricular curve during 
the ventricular systole, and on the auricular 
curve at the same time, have given rise to 
much debate, and need not be discussed here. 
On the whole, the method, though useful for 
giving a graphic view of the series of events 
within the cardiac cavities during a cardiac cycle, the short auricular 
pressure, the long-continued ventricular pressure, lasting nearly half the 
whole period, and the subsequent pause when both parts are at rest or in 
diastole, cannot with safety be used for drawing more detailed conclusions. 
Perhaps the least untrustworthy method of recording the changes of 
endocardiac pressure is that recently introduced by Roy and Rolleston, 
though difficulties present themselves in the interpretation of the curves 
obtained by it. 
By means of a short canula introduced through a large vessel, or directly, as 
a trocar, through the walls of the ventricle (or auricle), the blood in the cavity is 
brought to bear on an easily moving piston. The movements of the piston are 
recorded by a lever, and the evils of inertia are met by making the piston and 
lever work against the torsion of a steel ribbon, the length of which, and conse- 
quently the resistance offered by which, and hence the excursions of the piston, 
can be varied at pleasure. 
The curves obtained by this method vary according to circumstances. 
We may take as fair examples two curves from the left ventricle, one (Fig. 
56, A) of a rapidly beating, and the other (Fig. 56, B) of a slowly beating 
heart. 
§ 122. In attempting to interpret these curves with the view of learning 
the changes of pressure taking place in the heart, it is desirable to study 
them in connection with the tracing of which we have already spoken 
(Fig. 57), taken by means of a light lever placed on the exposed ventricle, 
and which, as we have seen, is a curve of the changes taking place in the 
front-to-back diameter of the ventricle; or we may use what is very nearly 
the same thing, viz., a cardiographic tracing (Fig. 58) ; that is to say, a 
tracing of the cardiac impulse which is a curve of changes in the pressure 
exerted by the apex of the heart on the chest-wall. 
Fig. 55. 
Simultaneous Tracings from the 
Right Auricle and Ventricle of the 
Horse. (After Chauveau and Marey.) 
Various forms of cardiograph have been used to record the cardiac impulse. In 
some the pressure of the impulse, as in the sphygmograph, is transmitted directly 
to a lever which writes upon a travelling surface. In others the impulse is, by 
means of an ivory button, brought to bear on an air-chamber, connected by a 
tube with a tambour, as in Fig. 54 ; the pressure of the cardiac impulse com- 
presses the air in the air-chamber, and through this the air in the chamber of the 
tambour by which the lever is raised. In such delicate and complicated move- 
ments as those of the heart, however, the use of long tubes filled with air is liable 
to introduce various errors. THE HEART. 
169 
We may begin our study of these curves at any point in the cycle; let 
it be the point b' in Fig. 56. From this point the curve rises very abruptly, 
Fig. 56. 
Curves of Endocardiac Pressure-from Left Ventricle of Dog: A, a quickly beating, B, a 
more slowly beating, heart. The letters in this and the succeeding Figs. 57, 58, are explained in 
the text. 
almost in the vertical line, to a maximum at c, and the same sudden large rise 
to a maximum occurs in the front-to-back diameter of the ventricles (Fig. 
Fig. 57. 
(See Fig. 52.) 
57) and in the pressure of the apex against the chest wall (Fig. 58). There 
can be no doubt that this corresponds to the first part of the systole of the 
ventricles. By the sudden onset of the contraction of the ventricular 
fibres pressure is brought to bear on the contents of the ventricle, and 
there being as yet no escape for the blood, by the increasing contraction 
of the fibres the pressure becomes greater and greater. At the point c a 170 
THE VASCULAR MECHANISM. 
change take place in all three curves ; the rise is converted into a fall, 
which, however, is very gradual as far as d. In the case of the front-to- 
back diameter curve (Fig. 57) we may 
interpret this as meaning that while the 
continued contraction of the muscular 
fibres still maintains that change in the 
form of the ventricle by which the front- 
to-back diameter is increased, that same 
diameter is somewhat lessened by a dim- 
inution of the volume of the ventricles 
due to the escape of blood into the great arteries; and the cardiographic 
tracing admits of a similar interpretation-the apex relaxes its pressure on 
the chest-wall. We may extend the same interpretation to the pressure 
curve (Fig. 56). Somewhere about c the pressure in the (left) ventricle 
has become higher than the pressure in the aorta, and in consequence blood 
escapes from the former into the latter. Whether the exact moment of the 
opening of the valves is absolutely identical with the turn of the curve 
at c, the curve beginning to fall at the moment when the area of high pres- 
sure in the ventricle is made continuous with the area of lower pressure 
in the aorta, or whether it occurs a little before c, the still increasing 
contraction of the ventricular fibres still increasing the pressure on the 
column of blood as it begins to move from the cavity of the ventricle into 
the aorta, may be left for the present undecided. The sudden fall from d 
to a admits of only one interpretation, and that in all the curves; this can 
only be due to the sudden relaxation of the muscular fibres of the ventricle, 
whereby the front-to-back diameter suddenly diminishes, the apex suddenly 
ceases to press on the chest-wall, and the pressure which the ventricular 
walls were previously exerting on the fluid in the canula introduced 
into its cavity also suddenly ceases. From b' to d, then, the ventricular 
walls are still contracting ; during the whole of this time the real systole is 
being continued, but gives place at d to a rapid relaxation which ushers in 
or forms the first part of the sequent diastole. Some little time after the 
beginning of this systole, somewhere about c, as we have seen, blood 
begins to escape from the ventricle into the aorta ; this escape is certainly 
completed by the time d is reached, and we have reason to think that it is 
really completed some little time before. The entrance into the aorta of 
the column of blood ejected by the ventricle distends that vessel, and the 
distention passes on, as we have seen, along the arterial track as the pulse. 
If, now, we measure the time during which the aorta, even near the heart, 
is being distended by the injection of the ventricular contents, we find this 
to be appreciably less than the time from c to d, during which the systole 
of the ventricle is still going on, though the contents have already begun 
to escape at about c. This means that the ventricle, though empty, remains 
contracted for some little time after its contents have left the cavity. It is 
possible that the point c' in the three figures under discussion, where the 
descent of the lever changes in rate, becoming less rapid, corresponds to 
the end of the outflow from the ventricle; but this is not certain, and, 
indeed, the exact interpretation of this part of the curve is especially 
difficult. 
The escape from the ventricle is rapid and forcible; the flow ceases sud- 
denly. Hence, as we have already stated, § 116, owing to the column of 
blood tending to move on by virtue of its inertia after the propelling force 
has ceased to act, a negative pressure makes its appearance behind the 
column of blood discharged from the ventricle, and as soon as the column 
is lodged in the aorta leads to a reflux toward the ventricle. This reflux 
Fig. 58. 
Cardiogram from Man THE HEART. 
171 
would of itself have the effect of closing the valves even were the aorta 
a rigid tube. But the aorta is extensible and elastic and the effects of the 
movement of the column of fluid are combined with the effects of the move- 
ment of the arterial walls; the elastic action of the arterial walls, in a 
manner which we shall discuss later on in dealing with the pulse, also leads to 
a reflux. It has been urged that the reflux due to the negative pressure of 
the mere movement of the column of blood being more rapid, occurs 
independently of and earlier than the reflux due to the elastic recoil, the 
former closing the valves, the latter securing their complete closure. Be 
this so or no the valves are probably closed almost immediately after the 
escape of the ventricular contents, though observers are not agredd upon 
this point, some urging that the valves are not closed until so late a period 
as the point d, just as relaxation is about to begin. In the curves we 
are now considering, a notch, followed by a rise, or at least a more or less 
abrupt change in the course of the curve at c', is sometimes observed in that 
part of the curve which intervenes between the first large rise and the final 
sudden fall; and this secondary rise has been taken to indicate the closure 
of the semilunar valves. Sometimes two such notches and peaks are seen, 
and the occurrence of the two has been attributed to a want of synchronism 
in the closure of the pulmonary and aortic semilunar valves, the latter 
closing some little time before the former. But it is by no means clear that 
these notches and peaks are thus due to the closure of the valves; they 
may possibly have another origin, they are not always present, and, indeed, 
it does not seem certain that the closing of the valves should necessarily 
make an impress on the ventricular curve. 
§ 123. In the performance of the ventricle, then (and what has been said 
of the left ventricle applies also to the right ventricle), there appear to be 
four stages: 
1. A rapid "getting up" of pressure within the ventricle, all the 
valves being as yet closed ; this continues until the pressure within the 
ventricle, becoming greater than that in the aorta, throws open the aortic 
valves. 
2. The escape of the contents of the ventricle into the aorta, the contrac- 
tion of the ventricular walls still continuing. 
3. Further maintenance of the contraction for some little time after the 
main body, at all events, of the contents have passed the aortic valves; by 
this the complete emptying of the ventricle seems assured. 
4. Sudden and rapid relaxation of the ventricular walls. 
These four events together make up a large portion, and in a quickly 
beating heart the greater portion, of the whole cardiac cycle. 
Meanwhile, that is, during the time from b' to a, blood has been flowing 
from the great veins into the auricle; during the interval from b' to d none 
of this can pass into the ventricle since this is still contracted, but with the 
commencement of relaxation from d onward there is no longer any obstacle ; 
on the contrary, as we shall see, an inducement for the blood to pass from 
the auricle into the ventricle. 
For a brief time, as we have seen, there is probably an unbroken flow 
from the great veins (pulmonary or venae cavae) through the auricle into the 
ventricle, leading to a steady but slight increase of the front-to-back diam- 
eter, to a slight pressure of the apex on the chest-wall, and to a slight 
increase of intra-ventricular pressure, especially shown in the curve of the 
slowly beating heart of the horse (Fig. 55). In Fig. 57 the sudden rise due 
to the ventricular systole is preceded by a rise b followed by a fall, forming 
thus, as it were, a shoulder on the curve. This has been interpreted as indi- 
cating the sharp transient auricular systole; the sudden injection of the 172 
THE VASCULAR MECHANISM. 
auricular contents into the ventricle increases the front-to-back diameter of 
the ventricle, and the momentum of the rapid stroke being considerable, the 
lever is in each case carried too far forward, so that the rise is followed by a 
fall, producing a notch. A similar though somewhat different shoulder is 
also seen in the cardiogram, Fig. 58. In the curve of ventricular pressure 
taken by means of the cardiac sound (Fig. 55) there is a similar temporary 
increase b' in the ventricular pressure coincident with the auricular stroke b, 
and in the " piston " pressure-curve of the rapidly beating heart (Fig. 56, A) 
there is a similar shoulder b just preceding the rise of the ventricular systole. 
The meaning of the last curve is, however, doubtful, for in the similar curve 
of the more slowly beating heart (Fig. 56,2?) it occurs immediately after the 
relaxation of the ventricle, some time before the occurrence of the auricular 
systole, and in many curves taken by the same method is absent altogether. 
The exact meaning, therefore, of the shoulder b in the other curves must be 
left at present undecided. 
§ 124. We have still to consider the negative pressure shown by the 
minimum manometer. This instrument, as we have said, merely shows that 
the pressure in the ventricle (or auricle) becomes negative at some phase 
or other of the cardiac cycle, but does not tell us in which phase it occurs. 
Now there are two ways in which such a negative pressure might originate. 
In the first place, as we have just seen, a negative pressure makes its appear- 
ance in the rear of the column of blood driven from the ventricle into the 
aorta with great suddenness and rapidity. But this negative pressure, as we 
have also seen, follows the column into the aorta past the semilunar valves, 
and in part, at all events, determines the closure of the semilunar valves. 
Hence if this is the negative pressure which the minimum manometer 
records, it ought to be shown not only when the end of the tube connected 
with the manometer is in the cavity of the ventricle, but also when the tube 
is slipped out of the ventricle just past the semilunar valves. When the 
tube, however, is in the latter situation the manometer does not show the 
same marked negative pressure that it does when the tube is in the ventricle ; 
the negative pressure which occurs in the aorta at each beat is sufficient to 
produce such an effect on the minimum manometer as is produced when the 
instrument is in the ventricle. Hence we infer that the negative pressure 
shown by the minimum manometer is not produced in this way. We may, 
moreover, conclude that the semilunar valves are closed before this negative 
pressure makes its appearance in the ventricle ; otherwise, however produced, 
it would be transmitted from the interior of the ventricle through the open 
valves to the root of the aorta beyond. 
But there is another event which might give rise to a negative pressure. 
The relaxation of the ventricular walls is, as the curves (Figs. 56, 57, 58) 
show, a rapid process, something quite distinct from the mere filling of the 
ventricular cavities with blood from the auricles; and, though some have 
objected to the view, it may be urged that this return of the ventricle from 
its contracted (and emptied) condition to its normal form would develop a 
negative pressure. This return is probably simply the total result of the 
return of each fibre or fibre cell to its natural condition, though some have 
urged that the extra quantity of blood thrown into the coronary arteries at 
the systole helps to unfold the ventricles somewhat in the way that fluid 
driven between the two walls of a double-walled collapsed ball or cup will 
unfold it. 
Accepting the return of the ventricles to their normal form as the cause 
of the negative pressure (and it may be remarked that the return of the 
thick-walled left ventricle naturally exerts a greater negative pressure than 
the thin-walled right ventricle), it is obvious that the negative pressure will THE HEART. 
173 
assist the circulation by sucking the blood which meanwhile has been accu- 
mulated in the auricle, from that cavity into the ventricle, the auriculo-ven- 
tricular valves easily giving way. At the same time this very flow from the 
auricle will at once put an end to the negative pressure, which obviously can 
be of brief duration only. It may be further urged in support of this view, 
that even when the thorax is opened, so that the respiratory movements can 
no longer act toward producing a negative pressure in the auricle and great 
veins, a minimum manometer placed in the right auricle shows frequently 
no pressure at all (that is, a pressure equal to that of the atmosphere) 
and sometimes a decidedly negative pressure. Seeing that the blood under 
these circumstances is being driven along the great veins by a pressure which 
though low is always above that of the atmosphere, we may conclude that 
the negative pressure produced in the ventricle is the cause of this lowering 
of the pressure in the auricle, though it is unable to make itself felt along 
the great veins. 
§ 125. The duration of the several phases. We may, first of all, distin- 
guish certain main phases: (1) The systole of the auricles. (2) The systole, 
proper, of the ventricles, during which their fibres are in a state of contrac- 
tion, lasting to d in Figs. 56, 57, 58. (3) The diastole of the ventricles- 
that is to say, the time intervening between their fibres ceasing to contract 
and commencing to contract again. To these we may perhaps add (4) The 
pause or rest of the whole heart, comprising the period from the end of the 
relaxation of the ventricles to the beginning of the systole of the auricles; 
during this time the walls are undergoing no active changes, neither con- 
tracting nor relaxing, their cavities being simply passively filled by the 
influx of blood. 
The mere inspection of almost any series of cardiac curves, however 
taken-those, for instance, which we have just discussed-will show, apart 
from any accurate measurements, that the systole of the auricles is always 
very brief, that the systole of the ventricles is always very prolonged-always 
occupying a considerable portion of the whole cycle-and that the diastole of 
the whole heart, reckoned from the end either of the systole or of the relaxa- 
tion of the ventricle, is very various, being in quickly beating hearts very 
short and in slowly beating hearts decidedly longer. 
When we desire to arrive at more complete measurements, we are obliged 
to make use of calculations based on various data; and these give only 
approximate results. Naturally, the most interest is attached to the dura- 
tion of events in the human heart. 
The datum which perhaps has been most largely used is the interval 
between the beginning of the first and the occurrence of the second sound. 
This may be determined with approximate correctness, and is found to vary 
from 0.301 to 0.327 second, occupying from 40 to 46 per cent, of the whole 
period, and being fairly constant for different rates of heart-beat. That is 
to say, in a rapidly beating heart it is the pauses which are shortened, and 
not the duration of the actual beats. 
The observer, listening to the sounds of the heart, makes a signal at each event 
on a recording surface, the difference in time between the marks being measured 
by means of the vibrations of a tuning-fork recorded on the same surface. By 
practice it is found possible to reduce the errors of observation within very small 
limits. 
Now, whatever be the exact causation of the first sound, it is undoubtedly 
coincident with the systole of the ventricles, though possibly the actual com- 
mencement of its becoming audible may be slightly behind the actual begin- 
ning of the muscular contractions. Similarly the occurrence of the second 174 
THE VASCULAR MECHANISM. 
sound, which, as we have seen, is certainly due to the closure of the semi- 
lunar valves, has been taken to mark the close of the ventricular systole. 
And on this supposition the interval between the beginning of the first and 
the occurrence of the second sound has been regarded as indicating approxi- 
mately the duration of the ventricular systole-i. e., the period during which 
the ventricular fibres are contracting. 
The determination of the separate duration of each of the three periods of 
the ventricular systole-viz., the getting up of the pressure, the discharge of 
the contents, and the remaining emptied but contracted-is subjected to so 
much uncertainty that it need not be insisted on here; it may, however, be 
said that, roughly speaking, each phase occupies probably about 0.1 second. 
In a heart beating 72 times a minute, which may be taken as the normal 
rate, each entire cardiac cycle would last about 0.8 second, and taking 0.3 
second as the duration of the ventricular systole, the deduction of this would 
leave 0.5 second for the whole diastole of the ventricle, including its relaxa- 
tion, the latter occupying about or somewhat less than 0.1 second. In the 
latter part of this period there occurs the systole of the auricles, the exact 
duration of which it is difficult to determine, it being hard to say when it 
really begins, but which, if the contraction of the great veins be included, 
may perhaps be taken as lasting, on an average, 0.1 second. The " passive 
interval," therefore, during which neither auricle nor ventricle is undergoing 
contractions, lasts about 0.4 second, and the absolute pause or rest during 
which neither auricle nor ventricle is contracting or relaxing, about 0.3 
second ; if, however, a longer period be allotted to the ventricular systole, 
these periods must be proportionately shortened. The systole of the ventricle 
follows so immediately upon that of the auricles, that practically no interval 
exists between the two events. 
The duration of the several phases may, for convenience sake, be arranged 
in a tabular form as follows: 
Second. 
Systole of ventricle before the opening of the semilunar 
Second. 
valves, while pressure is still getting up (probably rather 
less than 
0.1 
Escape of blood into aorta (about) 
0.1 
Continued contraction of the emptied ventricle (possibly 
rather more than) 
0.1 J 
Total systole of the ventricle (probably rather more than) 
0.3 
Diastole of both auricle and ventricle, neither contracting, 
or "passive interval " (probably rather less than) 
0.4 ] 
Systole of auricle (about or less than) 
0.1 j 
Diastole of ventricle, including relaxation and filling, up to 
the beginning of the ventricular systole (probably rather 
less than) 
0.5 
Total cardiac cycle 
0.8 
Summary. 
§ 126. We may now briefly recapitulate the main facts connected with 
the passage of blood through the heart. The right auricle during its dias- 
tole, by the relaxation of its muscular fibres, and by the fact that all back- 
ward pressure from the ventricle is removed by the closing of the tricuspid 
valves, offers but little resistance to the ingress of blood from the veins. On 
the other hand, the blood in the trunks of both the superior and inferior 
vena cava is under a pressure, which, though diminishing toward the heart, 
remains higher than the pressure obtaining in the interior of the auricle ; the 
blood in consequence flows into the empty auricle, its progress in the case of 
the superior vena cava being assisted by gravity. At each inspiration this 
flow (as we shall see in speaking of respiration) is favored by the diminution THE HEART. 
175 
of pressure in the heart and great vessels caused by the respiratory move- 
ments. Before this flow has gone on very long, the diastole of the ventricle 
begins, its cavity dilates, the flaps of the tricuspid valve fall back, and blood 
for some little time flows in an unbroken stream from the vense cavae into 
the ventricle. In a short time, however, probably before very much blood 
has had time to enter the ventricle, the auricle is full; and forthwith its 
sharp sudden systole takes place. Partly by reason of the backward pres- 
sure in the veins, which increases rapidly from the heart toward the capil- 
laries, and which, at some distance from the heart, is assisted by the presence 
of valves in the venous trunks, but still more from the fact that the systole 
begins at the great veins themselves and spreads thence over the auricle, the 
force of the auricular contraction is spent in driving the blood, not back 
into the veins, but into the ventricle, where the pressure is still exceedingly 
low. Whether there is any backward flow at all into the great veins, or 
whether by the progressive character of the systole the flow of blood con- 
tinues, so to speak, to follow up the systole without break, so that the stream 
from the veins into the auricle is really continuous, is at present doubtful; 
though a slight positive wave of pressure synchronous with the auricular 
systole, travelling backward along the great veins, has been observed, at least 
in cases where the heart is beating vigorously. 
The ventricle thus being filled by the auricular systole, the play of the 
tricuspid valves described above comes into action, the auricular systole is 
followed by that of the ventricle, and the pressure within the ventricle, cut 
off from the auricle by the tricuspid valves, is brought to bear on the pul- 
monary semilunar valves and the column of blood on the other side of those 
valves. As soon as by the rapidly increasing shortening of the ventricular 
fibres the pressure within the ventricle becomes greater than that in the pul- 
monary artery, the semilunar valves open and the still continuing systole 
discharges the contents of the ventricle into that vessel. 
As the ventricle thus rapidly and forcibly empties itself, either the tran- 
sient negative pressure which makes its appearance in the rear of the ejected 
column of blood, or the elastic action of the aortic walls, leads to a reflux 
of blood toward the ventricle, the effect of which, however, is to close the 
semilunar valves and thus to shut off' the blood in the distended arteries from 
the emptied ventricle. Either immediately at or more probably some little 
time after this closing of the valves the ventricular systole ends and relax- 
ation begins ; then once more the cavity of the ventricle becomes unfolded 
and finally distended by the influx of blood, a negative pressure devel- 
oped by the relaxation probably aiding the flow from the auricle and 
great veins. 
During the whole of this time the left side has with still greater energy 
been executing the same manoeuvre. At the same time that the vena? cavae 
are filling the right auricle the pulmonary veins are filling the left auricle. 
At the same time that the-right auricle is contracting the left auricle is con- 
tracting, too. The systole of the left ventricle is synchronous with that of 
the right ventricle, but executed with greater force; and the flow of blood 
is guided on the left side by the mitral and aortic valves in the same wav 
that it is on the right by the tricuspid valves and the valves of the pul- 
monary artery. 
The Work Done. 
§ 127. rhe most important factor of all in the determination of the work 
of the vascular mechanism is the quantity of blood ejected from the ventricle 
into the aorta at each systole. The general result of some of these calcula- 
tions gives about 180 grammes (6 ozs.) as the quantity of blood which is 176 
THE VASCULAR MECHANISM. 
driven from each ventricle at each systole in a full-grown man of average 
size and weight, but this estimate is probably too high. 
In the dog the quantity has been experimentally determined, by allowing the 
heart to deliver its contents through one branch of the aorta, all others being 
ligatured or blocked, into a receiver, the contents of which are at intervals, by an 
ingenious contrivance, returned to the right auricle. The time taken to fill the 
receiver and the number of beats executed during that time being noted, the 
average quantity ejected at a beat is thus given. It is found to vary widely. 
Various methods have been adopted for calculating the average amount of 
blood ejected at each ventricular systole. The simplest method is to measure the 
capacity of the recently removed and as yet not rigid ventricle, filled with blood 
under a pressure equal to the calculated average pressure in the ventricle. On 
the supposition that the whole contents of the ventricle are ejected at each systole 
this would give the quantity driven into the aorta at each stroke. The other 
methods are very indirect. 
It is evident that exactly the same quantity must issue at a beat from 
each ventricle; for if the right ventricle at each beat gave out rather less 
than the left, after a certain number of beats the whole of the blood would 
be gathered in the systemic circulation. Similarly, if the left ventricle gave 
out less than the right, all the blood would soon be crowded into the lungs. 
The fact that the pressure in the right ventricle is so much less than that in 
the left (probably 30 or 40 mm. as compared with 200 mm. of mercury), is 
due, not to differences in the quantity of blood in the cavities, but to the fact 
that the peripheral resistance which has to be overcome in the lungs is so 
much less than that in the rest of the body. 
It must be remembered that though it is of advantage to speak of an 
average quantity ejected at each stroke, it is more than probable that that 
quantity may vary within very wide limits. Taking, however, 180 grammes 
as the quantity, in man, ejected at each stroke at a pressure of 250 mm.1 of 
mercury, which is equivalent to 3.21 metres of blood, this means that the 
left ventricle is capable at its systole of lifting 180 grammes 3.21 metres 
high, i. e., it does 578 gramme-metres of work at each beat. Supposing the 
heart to beat 72 times a minute, this would give for the day's work of the 
left ventricle nearly 60,000 kilogramme-metres. Calculating the work of 
the right ventricle at one-fourth that of the left, the work of the whole heart 
would amount to 75,000 kilogramme-metres, which is just about the amount 
of work done in the ascent of Snowdon by a tolerably heavy man. 
A calculation of more practical value is the following: Taking the 
quantity of blood as of the body-weight, the blood of a man weighing 
75 kilos would be about 5760 grammes. If 180 grammes left the ventricle 
at each beat, a quantity equivalent to the whole blood would pass through 
the heart in 32 beats, i. e., in less than half a minute. 
The Pulse. 
§ 128. We have seen that the arteries, though always distended, undergo 
at each systole of the ventricle a temporary additional distention, so that 
when a finger is placed on an artery, such as the radial, an intermittent pres- 
sure on the finger, coming and going with the beat of the heart, is felt, and 
when a light lever is placed on the artery, the lever is raised at each beat, 
falling between. 
This intermittent expansion which we call the pulse, corresponding to 
the jerking outflow of blood from a severed artery, is present in the arteries 
only, being, except under particular circumstances, absent from the veins 
and capillaries. The expansion is frequently visible to the eye, and in some 
1A high estimate is purposely taken here. THE PULSE. 
177 
cases, as where an artery has a bend, may cause a certain amount of loco- 
motion of the vessel. 
The temporary increase of pressure which is the cause of the temporary 
increase of expansion makes itself felt, as we have seen, in the curve of 
arterial pressure taken by the mercury manometer; but the inertia of the 
mercury prevents the special characters of each increase becoming visible. 
In Fick's spring manometer (Fig. 59), in which the increase of pressure 
unfolds a curved spring and so moves a lever, the inertia is much less, and 
satisfactory tracings may be taken by this instrument. Other instruments 
have also been devised for recording the special characters of each increase 
Fig. 59. 
Fick's Spring Manometer. The flattened tube in the form of a hoop is firmly fixed at one end, 
while the other free end is attached to a lever. The interior of the tube, filled with spirit, is 
brought, by means of a tube containing sodium carbonate solution, into connection with an 
artery, in much the same way as in the case of the mercury manometer. The increase of pressure 
in the artery being transmitted to the hollow hoop, tends to straighten it, and correspondingly 
moves the attached lever. 
of pressure or of the expansion of the artery which is the result of that 
increase. The easiest and most common method of registering the expansion 
of an artery is that of simply bringing a light lever to bear on the outside 
of the artery. 
A lever specially adapted to record a pulse tracing is called a sphygmo- 
graph, the instrument generally comprising a small travelling recording 
surface on which the lever writes. There are many different forms of sphyg- 
mograph, but the general plan of structure is the same. Fig. 60 represents 
in a diagrammatic form the essential parts of the sphygmograph, known as 
Dudgeon's [and Fig. 61, Marey's, which is in more common use]. The 
instrument is generally applied to the radial artery because the arm affords 
a convenient support to the fulcrum of the lever, and because the position of 
12 178 
THE VASCULAR MECHANISM. 
the artery, near to the surface and with the support of the radius below so 
that adequate pressure can be brought to bear by the lever on the artery, is 
Fig. 60. 
Diagram of a Sphygmograph ('Dudgeon's). Certain supporting parts are omitted so that the mul- 
tiplying levers may be displayed, a is a small metal plate which is kept pressed on the artery by the 
spring b. The vertical movements of a cause to-and-fro movements of the lever c about the fixed 
point d. These are communicated to and magnified by the lever e, which moves round the fixed 
point/. The free end of this lever carries a light steel marker which rests on a strip of smoked 
paper <7. The paper is placed beneath two small wheels and rests on a roller which can be rotated 
by means of clock-work contained in the box h. The paper is thus caused to travel at a uniform 
rate. The screw graduated in ounces (Troy) is brought to bear on the spring b by means of a cam 
and by this the pressure put on the artery can be regulated. The levers magnify the pulse-move- 
ments fifty times. 
favorable for making observations. It can, of course, be applied to other 
arteries. When applied to the radial artery some such tracing as that 
[Fig. 61. 
Marey's Sphygmograph • B, B is where the sphygmograph is applied to the arm; K, spring 
which rests upon radial artery ; V, screw for adjusting marking lever L; H, clock-work ; P, smoked 
paper upon which tracing is made ; r, small spring for causing descent of lever after raising.] 
shown in Fig. 62 is obtained. At each heart beat the lever rises rapidly and 
then falls more gradually in a line which is more or less uneven. THE PULSE. 
179 
§129. We have now to study the nature and characters of the pulse in 
greater detail. 
We may say at once, and indeed have already incidentally seen, that 
the pulse is essentially due to the action of physical causes; it is the 
Fig. 62. 
Pulse Tracing from the Radial Artery of Man The vertical curved line L, gives the tracing 
which the recording lever made when the blackened paper was motionless. The curved inter- 
rupted lines show the distance from one another in time of the chief phases of the pulse-wave, 
viz., x = commencement and A end of expansion of artery; p, pre-dicrotic notch, d, dicrotic 
notch. C, dicrotic crest. D, Post dicrotic crest. /, the post-dicrotic notch These are explained 
in the text later on. 
physical result of the sudden injection of the contents of the ventricle into 
the elastic tubes called arteries; its more important features may be ex- 
[Fig. 63. 
Apparatus of Marey for showing Mode in which Pulse is Propagated in the Arteries: B is a 
rubber pump, with valve attachment, to prevent a regurgitant current: I, V, I", are levers resting 
on a gum tube, at intervals of 20 cm. of tubing: C, drum upon which tracing is made ; H, clock- 
work to revolve drum.] 
plained on physical principles and may be illustrated by means of an arti- 
ficial model [Fig. 63]. 
If two or more levers be placed on the arterial tubes of an artificial 
model, Fig. 63, one near to the pump, and the others equal distances apart, 
with a considerable length of tubing between the proximal and distal levers, 
and the levers be made to write, on a vertical line, on a recording surface, 180 
THE VASCULAR MECHANISM. 
so that their curves can be more easily compared, the following facts may be 
observed when the pump is set to work regularly. 
At each stroke of the pump, each lever rises until it reaches a maxi- 
mum (Fig. 64, la, 2a, etc.) and then falls again, thus describing a curve. 
Fig. 64. 
Pulse-curve described by a series of Sphygmographic Levers, placed at intervals of 20 cm. 
from each other along an elastic tube into which fluid is forced by the sudden stroke of a pump. 
The pulse-wave is travelling from left to right, as indicated by the arrows over the primary (a) 
and secondary (b, c) pulse-waves. The dotted vertical lines drawn from the summit of the sev- 
eral primary waves to the tuning-fork curve below, each complete vibration of which occupies 
1-50 second, allow the time to be measured which is taken up by the wave in passing along 20 cm. 
of the tubing. The waves a' are waves reflected from the closed distal end of the tubing ; this is 
indicated by the direction of the arrows. It will be observed that in the more distant lever VI, 
the reflected wave, having but a slight distance to travel, becomes fused with the primary wave. 
(From Marey.) 
The rise is due to the expansion of the part of the tube under the lever, 
and the fall is due to that part of the tube running after the expansion to 
its previous calibre. The curve is, therefore, the curve of the expansion 
(and return) of the tube at the point at which the lever rests. We may 
call it the pulse-curve. It is obvious that the expansion passes by the lever THE PULSE. 
181 
in the form of a wave. At one moment the lever is at rest; the tube 
beneath it is simply distended to the normal amount indicative of the mean 
pressure which at the time obtains in the arterial tubes of the model; at the 
next moment the pulse expansion reaches the lever, and the lever begins to 
rise ; it continues to rise until the top of the wave reaches it, after which 
it falls again until finally it comes to rest, the wave having completely 
passed by. 
It may perhaps be as well at once to warn the reader that the figure which 
we call the pulse-curve is not a representation of the pulse-wave itself; it is 
simply a representation of the movements, up and down, of the piece of the 
wall of the tubing at the spot on which the lever rests during the time that 
the wave is passing over that spot. We may roughly represent the wave 
in the diagram Fig. 65 in which the wave shown by the dotted line is 
passing over the tube (shown in a condition of rest by the thick double 
line) in the direction from H to C. It must, however, be remembered 
Fig. 65. 
A rough Diagrammatic Representation of a Pulse-wave passing over an Artery. 
that the wave thus figured is a much shorter wave than is the pulse-wave 
in reality (that being, as we shall see, about 6 metres long), i. e., occu- 
pies a smaller length of the arterial system from the heart H toward the 
capillaries C. 
The curves below, X, Y, Z, represent, in a similarly diagrammatic fash- 
ion, the curves described, during the passage of the wave, by levers placed 
on the points x, y, z. At Z the greater part of the wave has already passed 
under the lever, which during its passage has already described the greater 
part of its curve, shown by the thick line, and has only now to describe the 
small part, shown by the dotted line, corresponding to the remainder of the 
wave from Z to H. At Y the lever is at the summit of the wave. At X 
the lever has only described a small part of the beginning of the wave, viz., 
from C to x, the rest of the curve, as shown by the dotted line, having yet 
to be described. 
But to return to the consideration of Fig. 64. 
§ 130. The rise of each lever is somewhat sudden, but the fall is more 
gradual, and is generally marked with some irregularities which we shall 
study presently. The rise is sudden because the sharp stroke of the pump 
suddenly drives a quantity of fluid into the tubing and so suddenly expands 
the tube; the fall is more gradual because the elastic reaction of the walls 
of the tube, which brings about the return of the tube to its former calibre 182 
THE VASCULAR MECHANISM. 
after the expanding power of the pump has ceased, is more gradual in its 
action. 
These features, the suddenness of the rise or up-stroke, and the more 
gradual slope of the fall or down-stroke, are seen also in natural pulse-curves 
taken from living arteries (Figs. 62, 66, etc.). 
Indeed, the difference between the up-stroke 
and the down-stroke is even more marked 
in the latter than in the former, the delivery 
of blood from the ventricle being more rapid 
than the issue of water from a pump as ordi- 
narily worked. 
It may here be noted that the actual size 
of the curve, that is, the amount of excursion 
of the lever, depends in part (as does also to 
a great extent the form of the curve) on the 
amount of pressure exerted by the lever on 
the tube. , If the lever only just touches the 
tube in its expanded state, the rise will be 
insignificant. If, on the other hand, the 
lever be pressed down too firmly, the tube 
beneath will not be able to expand as it 
otherwise would, and the rise of the lever 
will be proportionately diminished. There 
is a certain pressure which must be exerted 
by the lever on the tube, the exact amount depending on the expansive 
power of the tubing and on the pressure exerted by the fluid in the tube, 
in order that the tracing may be best marked. This is shown in Fig. 66, in 
which are given three tracings taken from the same radial artery with the 
same instrument; in the lower curve the pressure of the lever is too great, 
in the upper curve too small, to bring out the characters seen most distinctly 
in the middle curve with a medium pressure. 
§ 131. It will be observed that in Fig. 64, curve I., which is nearer the 
pump, rises higher, and rises more rapidly than curve II., which is further 
away from the pump; that is to say, at the lever further away from the 
pump the expansion is less and takes place more slowly than at the lever 
nearer the pump. Similarly in curve IV. the rise is still less, and takes 
place still less rapidly than in II., and the same change is seen still more 
marked in V. as compared with IV. In fact, if a number of levers were 
placed at equal distances along the arterial tubing of the model and the 
model were working properly, with an adequate peripheral resistance, we 
might trace out step by step how the expansion, as it travelled along the 
tube, got less and less in amount and at the same time became more gradual 
in its development, the curve becoming lower and more flattened out, until 
in the neighborhood of the artificial capillaries there was hardly any trace 
of it left. In other words, we might 
trace out step by step the gradual dis- 
appearance of the pulse. 
The same changes, the same gradual 
lowering and flattening of the curve, 
may be seen in natural pulse-tracings, 
as for instance in Fig. 67, which is a 
tracing from the dorsalis pedis artery, 
compared with the tracing from the ra- 
dial artery, Fig. 66, taken from the same individual with the same instru- 
ment on the same occasion. This feature is, of course, not obvious in all 
Fig. 66. 
Pulse-tracings from the same Ra- 
dial Artery under Different Pressures 
of the Lever. (The letters are ex- 
plained in a later part of the text.) 
Fig. 67. 
Pulse-tracing from Dorsalis Pedis, taken 
froiil the same individual as Fig. 66. THE PULSE. 
183 
pulse-curves taken from different individuals with different instruments and 
under varied circumstances; but if a series of curves from different arteries 
were carefully taken under the same conditions it would be found that the 
aortic tracing is higher and more sudden than the carotid tracing, which, 
again, is higher and more sudden than the radial tracing-the tibial tracing 
being in turn still lower and more flattened. The pulse-curve dies out by 
becoming lower and lower and more and more flattened out. 
And a little consideration will show us that this must be so. The systole 
of the ventricle drives a quantity of blood into the already full aorta. The 
sudden injection of this quantity of blood expands the portion of the aorta 
next to the heart, the part immediately adjacent to the semilunar valves 
beginning to expand first, and the expansion travelling thence on to the end 
of this portion. In the same way the expansion travels on from this portion 
through all the succeeding portions of the arterial system. For the total 
expansion required to make room for the new quantity of blood is not pro- 
vided by that portion alone of the aorta into which the blood is actually 
received ; it is supplied by the whole arterial system ; the old quantity of 
blood which is replaced by the new in this first portion has to find room for 
itself in the rest of the arterial space. As the expansion travels onward, 
however, the increase of pressure which each portion transmits to the suc- 
ceeding portion will be less than that which it received from the preceding 
portion. For the whole increase of pressure due to the systole of the ven- 
tricle has to be distributed over the whole of the arterial system ; the general 
mean arterial pressure is, as we have seen, maintained by repeated systoles, 
and any one systole has to make its contribution to that mean pressure ; the 
increase of pressure which starts from the ventricle must, therefore, leave 
behind at each stage of its progress a fraction of itself; that is to say, the 
expansion is continually growing less, as the pulse travels from the heart 
to the capillaries. Moreover, while the expansion of the aorta next to the 
heart is, so to speak, the direct effect of the systole of the ventricle, the expan- 
sion of the more distant artery is the effect of the systole transmitted by the 
help of the elastic reaction of the arterial tract between the heart and the 
distant artery ; and since this elastic reaction is slower in development than 
the actual systole, the expansion of the more distant artery is slower than 
that of the aorta, the up-stroke of the pulse-curve is less sudden, and the 
whole pulse-curve is more flattened. 
The object of the systole is to supply a contribution to the mean pressure, 
and the pulse is an oscillation above and below' that mean pressure-an oscil- 
lation which diminishes from the heart onwards, being damped by the elastic 
walls of the arteries, and so, little by little, converted into mean pressure 
until in the capillaries the mean pressure alone remains-the oscillations 
having disappeared. 
§ 132. If in the model the points of the two levers at different distances 
from the pump be placed exactly one under the other on the recording 
surface, it is obvious that, the levers being alike except for their position 
on the tube, any difference in time between the movements of the two 
levers will be shown by an interval between the beginnings of the 
curves they describe, the recording surface being made to travel suffi- 
ciently rapidly. 
If the movements of the two levers be thus compared, it will be seen that 
the far lever (Fig. 64, II.) commences later than the near one (Fig. 64, I.) ; 
the further apart the two levers are, the greater is the interval in time 
between their curves. Compare the series I. to VI. (Fig. 64). This means 
that the wave of expansion, the pulse-wave, takes some time to travel along 
the tube. In the same way it would be found that the rise of the near lever 184 
THE VASCULAR MECHANISM. 
began some fraction of a second after the stroke of the pump, meaning that 
time is required for the transmission of the wave. 
The velocity with which the pulse-wave travels depends chiefly on the 
amount of rigidity possessed by the tubing. The more extensible (with cor- 
responding elastic reaction) the tube, the slower is the wave; the more rigid 
the tube becomes, the faster the wave travels ; in a perfectly rigid tube, what 
in the elastic tube would be the pulse, becomes a mere shock travelling 
with very great rapidity. The width of the tube is of much less influence, 
though according to some observers the wave travels more slowly in the wider 
tubes. 
The rate at which the normal pulse-wave travels in the human body 
has been variously estimated at from 10 to 5 metres per second. In all proba- 
bility the lower estimate is the more correct one ; but it must be remembered 
that the rate may vary very considerably under different conditions. Accord- 
ing to all observers the velocity of the wave in passing from the groin to the 
foot is greater than in passing from the axilla to the wrist (6 metres against 
5 metres). This is probably due to the fact that the femoral artery with its 
branches is more rigid than the axillary and its branches. So also in the 
arteries of children, the wave travels more slowly than in the more rigid 
arteries of the adult. The velocity is also increased by circumstances which 
heighten and decreased by those which lessen the mean arterial pressure, 
since with increasing pressure the arterial walls become more and with 
diminishing pressure less rigid. Probably, also, the velocity of the pulse- 
wave depends on conditions of the arterial walls which we cannot adequately 
describe as mere differences in rigidity. In experimenting with artificial 
tubes it is found that different qualities of India-rubber give rise to very 
different results. 
Care must be taken not to confound the progress of the pulse-wave- 
i. e., of the expansion of the arterial wall-with the actual onward move- 
ment of the blood itself. The pulse-wave travels over the moving blood 
somewhat as a rapidly moving natural wave travels along a sluggishly flow- 
ing river. Thus while the velocity of the pulse-wave is 6 or possibly even 
10 metres per second, that of the current of the blood is not more than 
half a metre per second even in the large arteries, and is still less in the 
smaller ones. 
§ 133. Referring again to the caution given above not to regard the pulse- 
curve as a picture of the pulse-wave, we may now add that the pulse-wave is 
of very considerable length. If we know how long it takes for the pulse- 
wave to pass over any point in the arteries and how fast it is travelling, we 
can easily calculate the length of the wave. In an ordinary pulse-curve the 
artery, owing to the slow return, is seen not to regain the calibre which it had 
before the expansion, until just as the next expansion begins-that is to say, 
the pulse-wave takes the whole time of a cardiac cycle, viz., T8^ths second to 
pass by the lever. Taking the velocity of the pulse-wave as 6 metres per 
second the length of the wave will be T%ths of 6 metres-or nearly 5 metres. 
And even if we took a smaller estimate, by supposing that the real expan- 
sion and return of the artery at any point took much less time, say j^ths 
second, the length of the pulse-wave would still be more than 2 metres. But 
even in the tallest man the capillaries furthest from the heart, those in the 
tips of the toes, are not 2 metres distant from the heart. In other words, the 
length of the pulse-wave is much greater than the whole length of the arte- 
rial system, so that the beginning of each wave has become lost in the small 
arteries and capillaries sometime before the end of it has finally passed away 
from the beginning of the aorta. 
We must now return to the consideration of certain special features in THE PULSE. 
185 
the pulse, which from the indications they give or suggest of the condition 
of the vascular system are often of great interest. 
S 134. Dicrotism. In nearly all pulse-tracings, the curve of the expansion 
and recoil of the artery is broken by two, three, or several smaller elevations 
and depressions; secondary waves are imposed upon the fundamental or 
primary wave. In the sphygmographic tracing from the carotid, Fig. 68, 
Fig. 68. 
Pulse-tracing from Carotid Artery of Healthy Man (from Moens): ar, commencement of expan- 
sion of artery. A, summit of the first rise. C, dicrotic secondary wave. B, pre-dicrotic second- 
ary wave; p. notch preceding this. D, succeeding secondary wave. The curve above is that of a 
tuning-fork with ten double vibrations in a second. 
and m many of the other tracings given, these secondary elevations are 
marked, as B, C, D. When one such secondary elevation only is conspicu- 
ous, so that the pulse-curve presents two notable 
crests only, the primary crest and a secondary 
one, the pulse is said to he "dicrotic" ; when two 
secondary crests are prominent, the pulse is often 
called "tricrotic"; where several, " polycrotic." 
As a general rule, the secondary elevations appear 
only on the descending limb of the primary wave, as 
in most of the curves given, and the curve is then 
spoken of as " katacrotic." Sometimes, however, 
the first elevation or crest is not the highest, but ap- 
pears on the ascending portion of the main curve; 
such a curve is spoken of as " anacrotic," Fig. 69. 
Of these secondary elevations the most frequent, conspicuous, and impor- 
tant is the one which appears some way down on the descending limb, and 
is marked Con Fig. 68 and on most of the curves here given. It is more or 
less distinctly visible on all sphygmograms, and may be seen in those of the 
aorta as well as of other arteries. Sometimes it is so slight as to be hardly 
discernible ; at other times it may be so marked as to give rise to a really 
double pulse (Fig. 70), i. e., a pulse which can be felt as double by the 
Fig. 69. 
Anacrotic Sphygmograph- 
tracing from the Ascending 
Aorta. (Aneurism.) 
Fig. 70. 
Two Grades of Marked Dicrotism in Radial Pulse of Man. (Typhoid fever.) 
finger ; hence it has been called the dicrotic elevation or the dicrotic wave, 
the notch preceding the elevation being spoken of as the " dicrotic notch." 
Neither it nor any other secondary elevations can be recognized in the 
tracings of blood-pressure taken with a manometer. This may be explained, 186 
THE VASCULAR MECHANISM. 
as we have said (§ 128) by the fact that the movements of the mercury 
column are too sluggish to reproduce these finer variations; but dicrotism 
is also conspicuous by its absence in the tracings given by more delicately 
responsive instruments. Moreover, when the normal pulse is felt by the 
finger, most persons find themselves unable to detect any dicrotism. But 
that it does really exist in the normal pulse is shown by the fact that it 
appears in a most unmistakable manner in the tracing obtained by allow- 
ing the blood to spurt directly from an opened small artery, such as the 
dorsalis pedis, upon a recording surface. 
Less constant and conspicuous than the dicrotic wave, but yet appearing 
in most sphygmograms, is an elevation which appears higher up on the 
descending limb of the main wave ; it is marked B in Fig. 68, and on several 
of the other curves, and is frequently called the pre-dicrotic wave; it may 
become very prominent. Sometimes other secondary waves, often called 
" post-dicrotic," are seen following the dicrotic wave, as at D in Fig. 68, and 
some other curves; but these are not often present, and usually, even when 
present, inconspicuous. 
AV hen tracings are taken from several arteries, or from the same artery 
under different conditions of the body, these secondary waves are found to 
vary very considerably, giving rise to many characteristic forms of pulse- 
curve. Were we able with certainty to trace back the several features of the 
curves to their respective causes, an adequate examination of sphygmo- 
graphic tracings would undoubtedly disclose much valuable information 
concerning the condition of the body presenting them. Unfortunately, the 
problem of the origin of these secondary waves is a most difficult and com- 
plex one; so much so, that the detailed interpretation of asphygmographic 
tracing is still in most cases extremely uncertain. 
§ 135. The chief interest attaches to the nature and meaning of the di- 
crotic wave. In general, the main conditions favoring dicrotism are (1) a 
highly extensible and elastic arterial wall, (2) a comparatively low mean 
pressure, leaving the extensible and elastic reaction of the arterial wall free 
scope to act, and (3) a sufficiently vigorous and sufficiently rapid stroke of 
the ventricle, and the discharge of a large quantity of blood into the aorta. 
The development of the dicrotic wave may probably be explained as follows : 
At each beat the time during which the contents of the left ventricle are 
injected into the aorta is, as we have seen (§ 125), very brief. The expan- 
sion of the aorta is very sudden, and the cessation of that expansion is also 
very sudden. 
Now, when fluid is being driven with even a steady pressure through an 
elastic tube or a system of elastic tubes, levers placed on the tube will 
describe curves indicating variations in the diameter of the tube, if the inflow 
into the tube be suddenly stopped, as by sharply turning a stop-cock ; and a 
comparison of levers placed at different distances from the stop-cock will 
show that these variations of diameter travel down the tube from the stop- 
cock in the form of waves. The lever near the stop-cock will first of all fall, 
but speedily begin to rise again, and this subsequent rise will be followed by 
another fall, after which there may be one or more succeeding rises and falls- 
that is, oscillations-with decreasing amplitudes, until the fluid comes to rest. 
The levers further from the stop-cock will describe curves similar to the 
above in form but of less amplitude, and it will be found that these occur 
somewhat later in time, the more so the further the lever is from the stop- 
cock. Obviously these waves are generated at or near the stop-cock, and 
travel thence along the tubing. 
We may infer that at each beat of the heart similar waves would be 
generated at the foot of the aorta upon the sudden cessation of the flow from THE PULSE. 
187 
the ventricle, and would travel thence along the elastic arteries. The facts 
that each beat is rapidly succeeded by another, and that the flow which sud- 
denly ceases is also, by the nature of the ventricular stroke, suddenly gen- 
erated, may render the waves more complicated, but will not change their 
essential nature. 
The exact interpretation of the generation of these waves is perhaps not 
without difficulty, but two factors seem of especial importance. In the first 
place, as we have already more than once said, when a rapid flow is suddenly 
stopped a negative pressure makes its appearance behind the column of fluid. 
In a rigid tube this simply tends to a reflux of fluid. In an elastic tube its 
effects are complicated by the second factor, the elastic action and inertia of 
the walls of the tube. Upon the sudden cessation of the flow, the expansion 
of the tube, or as we may at once say, of the aorta, ceases, the vessel begins to 
shrink, and the lever placed on its walls falls, as from A onward in the pulse- 
curve. This shrinking is in part due to the elastic reaction of the walls of 
the aorta, but is increased by the "suction " action of the negative pressure 
spoken of above. In thus shrinking, however, under these combined causes, 
the aorta, through the inertia of its walls, overshoots the mark, it is carried 
beyond its natural calibre-i. e., the diameter it would possess if left to itself 
with the pressure inside and outside equal; it shrinks too much and conse- 
quently begins again to expand. This secondary expansion (taking for 
simplicity sake a pulse-curve in which the so-called pre-dicrotic wave, B, is 
absent or inconspicuous) causes the secondary rise of the lever up to C-that 
is, the dicrotic rise. In thus expanding again the aorta tends to draw back 
toward the heart the column of blood which by loss of momentum had come 
to rest, or, indeed, under the influence of the negative pressure spoken of 
above, was already undergoing a reflux. In this secondary expansion, more- 
over, the aorta is by the inertia of its walls, aided by that of the blood, again 
carried, so to speak, beyond its mark, so that no sooner has it become ex- 
panded and filled with fluid to a certain extent than it again begins to shrink 
as from C onward. And this shrinking may in a similar manner to the first 
be followed by a further expansion and shrinking, giving rise to a post- 
dicrotic wave, or it may be to post-dicrotic waves. And the successive 
changes thus inaugurated at the root of the aorta travel as so many waves 
along the arterial system, diminishing as they go. It will be observed that 
for the development of these waves a certain quality in the walls of the tubing 
is necessary. The tube must be such as possesses when at rest an open lumen ; 
the walls must be of such a kind that the tube remains open when empty- 
i. e., when the atmospheric pressure is equal inside and outside-so that when 
it shrinks too much it expands again in striving to retain its natural calibre. 
This we have seen to be a characteristic of the arteries. A collapsible 
tube of thin membrane will not show the phenomena ; such a tube when the 
stop-cock is turned collapses and empties itself, continuing to be collapsed 
without any effort to expand again. 
In the above explanation no mention has been made of the closing of the 
semilunar valves; we shall have to speak of these a little later on in refer- 
ring to the pre-dicrotic wave, and shall see that, under the view we have 
just given, the closing of the semilunar valves is to be regarded rather as 
the effect than the cause of the dicrotic wave. Many authors, however, give 
an interpretation of the dicrotic wave different from that detailed above. 
Thus, it is held that the primary shrinking from A onward, being brought 
to bear on the column of blood already come to rest, in face of the great 
pressure in front, drives the blood back against the semilunar valves, thus 
closing them, and that the impact of the column of blood against the valves 
starts a new wave of expansion, which reinforcing the natural tendency of 188 
THE VASCULAR MECHANISM. 
the elastic walls to expand again after their primary shrinking, produces 
tho dicrotic wave C. On this view, it is the blood driven back from the 
valves which expands the artery ; on the view given above, it is the ex- 
panding artery which draws the blood back toward the valves. 
Moreover, quite other views have been or are held concerning this 
dicrotic wave. According to many authors, it is what is called a " re- 
flected " wave. Thus, when the tube of the artificial model bearing two 
levers is blocked just beyond the far lever, the primary wave is seen to be 
accompanied by a second wave, which at the far lever is seen close to, 
and often fused into, the primary wave (Fig. 64, VI. a'), but at the near 
lever is at some distance from it (Fig. 64, I. a'), being the further from 
it the longer the interval between the lever and the block in the tube. The 
second wave is evidently the primary wave reflected at the block and travel- 
ling backward toward the pump. It thus, of course, passes the far lever be- 
fore the near one. And it has been argued that the dicrotic wave of the 
pulse is really such a reflected wave, started either at the minute arteries 
and capillaries, or at the points of bifurcation of the larger arteries, and 
travelling backward to the aorta. But if this were the case, the distance 
between the primary crest and the dicrotic crest ought to be less in arteries 
more distant from than in those nearer to the heart, just as in the artificial 
scheme the reflected wave is fused with a primary wave near the block (Fig. 
64, VI. 6 a. a'), but becomes more and more separated from it the further 
back toward the pump we trace it (Fig. 64, I. 1 a. a'). Now this is not the 
case with the dicrotic wave. Careful measurements show that the distance 
between the primary and dicrotic crests is either greater, or certainly not 
less, in the smaller or more distant arteries than in the larger or nearer 
ones. This feature indeed proves that the dicrotic wave cannot be due to 
reflection at the periphery, or, indeed, in any way a retrograde wave. Be- 
sides, the multitudinous peripheral division would render one large periph- 
erally reflected wave impossible. Again, the more rapidly the primary 
wave is obliterated, or at least diminished, on its way to the periphery, the 
less conspicuous should be the dicrotic wave. Hence increased extensibility 
and increased elastic reaction of the arterial walls which tend to use up 
rapidly the primary wave, should also lessen the dicrotic wave. But as a 
matter of fact these conditions, as we have said, are favorable to the promi- 
nence of the dicrotic wave. 
On the other hand, these and the other conditions which favor dicrotism 
in the pulse are exactly those which would favor such a development of 
secondary waves as has been described above, and their absence would be 
unfavorable to the occurrence of such waves. Thus dicrotism is less ma rked 
in rigid arteries (such as those of old people) than in healthy elastic ones; 
the rigid wall neither expands so readily nor shrinks so readily, and hence 
does not so readily give rise to such secondary waves. Again, dicrotism is 
more marked when the mean arterial pressure is low than when it is high ; 
indeed, dicrotism may be induced when absent, or increased when slightly 
marked by diminishing, in one way or another, the mean pressure. Now', 
when the pressure is high, the arteries are kept continually much expanded, 
and are therefore the less capable of further expansion ; that is to say, are, 
so far, more rigid. Hence the additional expansion due to the systole is not 
very great; there is a less tendency for the arterial walls to swing backward 
ami forward, so to speak, and hence a less tendency to the development of 
secondary waves. When the mean pressure is low, the opposite state of 
things exists; supposing, of course, that the ventricular stroke is adequately 
vigorous (the low pressure being due, not to diminished cardiac force, but to 
diminished peripheral resistance), the relatively empty but highly distensible THE PULSE. 
189 
artery is rapidly expanded, and, falling rapidly back, enters upon a second- 
ary (dicrotic) expansion, and even a third. 
Moreover, the same principles may be applied to explain why sometimes 
dicrotism will appear marked in a particular artery while it remains little 
marked in the rest of the system. In experimenting with an artificial tubing 
such as the arterial model, the physical characters of which remain the 
same throughout, both the primary and the secondary waves retain the same 
characters as they travel along the tubing, save only that both gradually 
diminish toward the periphery ; and in the natural circulation, when the 
vascular conditions are fairly uniform throughout, the pulse-curve, as a rule, 
possesses the same general characters throughout, save that it is gradually 
" damped off." But suppose we were to substitute for the first section of the 
tubing a piece of perfectly rigid tubing ; this at the stroke of the pump, on 
account of its being rigid, would show neither primary nor secondary ex- 
pansion, but the expanding force of the pump's stroke would be transmitted 
through it to the second elastic section, and here the primary and secondary 
waves would at once become evident. This is an extreme case, but the same 
thing would be seen to a less degree in passing from a more rigid, that is, 
less extensible and elastic section, to a less rigid, more extensible and elastic 
section ; the primary and secondary expansions, in spite of the general damp- 
ing effect, would suddenly increase. Similarly in the living body a pulse- 
curve which, so long as it is travelling along arteries in which the mean 
pressure is high, and which are therefore practically somewhat rigid, is not 
markedly dicrotic, may become very markedly dicrotic when it comes to a 
particular artery in which the mean pressure is low (and we shall see pres- 
ently that such a case may occur), and the walls of which are therefore for 
the time being relatively more distensible than the rest. 
Lastly, we may recall the observation made above (§ 130), that the curve 
of expansion of an elastic tube is modified by the pressure exerted by the 
lever employed to record it, and that hence, in the same artery and with the 
same instrument, the size, form, and even the special features of the curve 
vary according to the amount of pressure with which the lever is pressed 
upon the artery. Accordingly the amount of dicrotism apparent in a pulse 
may be modified by the pressure exerted by the lever. In Fig. 64, for 
instance, the dicrotic wave is more evident in the middle than in the upper 
tracing. 
§ 136. The pre-dicrotic wave (marked B on Fig. 68, and on several 
other of the pulse-curves), which precedes the dicrotic wave and is still 
more variable than that wave, being sometimes slight or even invisible and 
sometimes conspicuous, has given rise to much controversy. In the inter- 
pretation of the dicrotic wave given in the preceding paragraph it was 
stated that the negative pressure developed on the cessation of the flow in the 
rear of the column of blood, led by itself to a reflux toward the ventricle ; 
and it has been suggested that this reflux meeting and closing the semi- 
lunar valves starts a small wave of expansion before the larger dicrotic 
wave has had time to develop itself. On this view the semilunar valves 
would be actually closed before the occurrence of the secondary dicrotic 
expansion of the arterial walls, though the larger, more powerful reflux of 
this later event must render the closure more complete, and in doing so 
possibly gives rise to the second sound. According, however, to the second 
view given in the same paragraph, which regards the reflux due to the 
shrinking of the artery in face of the great pressure in front as firmly 
closing the semilunar valves, and as thus starting the secondary dicrotic 
wave of expansion, the firm closing of the semilunar valves must take place 
before the beginning, not during the development of the dicrotic wave ; it 190 
THE VASCULAR MECHANISM. 
is still possible, however, even on this view, as on the other, to suppose that 
an antecedent reflux, due to the negative pressure succeeding the cessation 
of flow from the ventricle, closes the valves and starts the pre-dicrotic wave. 
But the matter is one not yet beyond the stage of controversy. 
§ 137. In an anacrotic pulse the first rise is not the highest, but a 
second rise (B, Fig. 69) which follows and is separated from it by a notch 
is higher than, or at least as high as, itself. Such an anacrotic wave, though 
it may sometimes be produced temporarily in healthy persons, is generally 
associated with diseased conditions, usually such in which the arteries are 
abnormally rigid. In describing the ventricular systole, we spoke of the 
pressure within the ventricle as reaching its maximum just before the 
opening of the semilunar valves ; and this is apparently the normal event; 
but there are curves which seem to show that after the first sudden rise of 
pressure which opens the valves, followed by a brief lessening of pressure, 
which appears on the curve as a notch, the pressure may again rise, and 
that to a point higher than before. And a similar curve is sometimes de- 
scribed by the front-to-back diameter of the ventricle. The systole opens 
the valve as it were with a burst; this is followed by a slight relapse, 
and then the systole, strengthening again, discharges the whole of the ven- 
tricular contents into the aorta and so brings about a tardy maximum ex- 
pansion. And what is thus started in the aorta travels onward over the 
arterial system. It is difficult to see how these anacrotic events can be pro- 
duced, except by a certain irregularity in the ventricular systole; and, 
indeed, the anacrotic pulse is frequently associated with some disease or 
defect of the ventricle. 
§138. Venous pulse. Under certain circumstances the pulse may be 
carried on from the arteries through the capillaries into the veins. Thus, 
as we shall see later on, when the salivary gland is actively secreting, the 
blood may issue from the gland through the veins in a rapid pulsating 
stream. The nervous events which give rise to the secretion of saliva, lead 
at the same time, by the agency of vasomotor nerves, of which we will pre- 
sently speak, to a dilatation of the small arteries of the gland. When 
the gland is at rest the minute arteries are, as we shall see, somewhat con- 
stricted and narrowed, and thus contribute largely to the peripheral resist- 
ance in the part; this peripheral resistance throws into action the elastic 
properties of the small arteries leading to the gland, and the remnant of 
the pulse reaching these arteries is, as we before explained, finally de- 
stroyed. When the minute arteries are dilated, their widened channels 
allow the blood to flow more easily through them and with less friction ; the 
peripheral resistance which they normally offer is thus lessened. In conse- 
quence of this the elasticity of the walls of the small arteries is brought 
into play to a less extent than before, and these small arteries cease to do 
their share in destroying the pulse which comes down to them from the 
larger arteries. As in the case of the artificial model, where the " periph- 
eral " tubing is kept open, not enough elasticity is brought into play to 
convert the intermittent arterial flow into a continuous one, and the pulse 
which reaches the arteries of the gland passes on through them and through 
the capillaries, and is continued on into the veins. A similar venous pulse 
is also sometimes seen in other organs. 
Careful tracings of the great veins in the neighborhood of the heart 
show elevations and depressions, which appear due to the variations of intra- 
cardiac (auricular) pressure, and which may, perhaps, be spoken of as 
constituting a " venous pulse," though they have a quite different origin 
from the venous pulse just described in the salivary gland ; but at present 
they need further elucidation. In cases, however, of insufficiency of the 191 
THE VASCULAR MECHANISM. 
tricuspid valves, the systole of the ventricle makes itself distinctly felt in 
the great veins; and a distention travelling backward from the heart 
becomes very visible in the veins of the neck. This is sometimes spoken of 
as a venous pulse. 
Variations of pressure in the great veins, due to the respiratory move- 
ments, are also sometimes spoken of as a venous pulse ; the nature of these 
variations will be explained in treating of respiration. 
The Regulation and Adaptation of the Vascular Mechanism. 
§139. So far the facts with which we have had to deal, with the excep- 
tion of the heart's beat itself, have been simply physical facts. All the 
essential phenomena which we have studied may be reproduced on a dead 
model. Such an unvarying mechanical vascular system would, however, 
be useless to a living body whose actions were at all complicated. The 
prominent feature of a living mechanism is the power of adapting itself to 
changes in its internal and external circumstances. 
The vascular mechanism in all animals in which it is present is capable 
of local and general modifications, adapting it to local and general changes 
of circumstance. These modifications fall into two great classes: 
1. Changes in the heart's beat. These, being central, have, of course, a 
general effect; they influence or may influence the whole body. 
2. Changes in the peripheral resistance, due to variations in the calibre of 
the minute arteries, brought about by the agency of their contractile mus- 
cular coats. These changes may be either local, affecting a particular vas- 
cular area only, or general, affecting all or nearly all the bloodvessels of 
the body. 
These two classes of events are chiefly governed by the nervous system. 
It is by means of the nervous system that the heart's beat and the calibre 
of the minute arteries are brought into relation with each other, and with 
almost every part of the body. It is by means of the nervous system acting 
either on the heart or on the small arteries, or on both, that a change of cir- 
cumstances affecting either the whole or a part of the body is met by com- 
pensating or regulative changes in the flow of blood. It is by means of the 
nervous system that an organ has a more full supply of blood when at work 
than when at rest, that the tide of blood through the skin rises and ebbs 
with the rise and fall of the temperature of the air, that the work of the 
heart is tempered to meet the strain of overfull arteries, and that the arterial 
gates open and shut as the force of the central pump waxes and wanes. The 
study of these changes becomes, therefore, to a large extent a study of 
nervous actions. 
The circulation may also be modified by events not belonging to either of 
the above two classes. Thus, in this or that peripheral area, changes in the 
capillary walls and the walls of the minute arteries and veins may lead to 
an increase of the tendency of the blood corpuscles to adhere to the vascular 
walls, and so, quite apart from any change in the calibre of the bloodvessels, 
may lead to increase of the peripheral resistance. This is seen in an extreme 
case in inflammation, but may possibly intervene to a less extent in the 
ordinary condition of the circulation, and may also be under the influence 
of the nervous system. Further, any decided change in the quantity of 
blood actually in circulation must also influence the working of the vascular 
mechanism. But both these changes are unimportant compared with the 
other two kinds of changes. Hence, the two most important problems for 
us to study are, 1, how the nervous system regulates the beat of the heart, 
The Regulation of the Beat of the Heart. 192 
TIIE VASCULAR MECHANISM. 
and 2, how the nervous system regulates the calibre of the bloodvessels. We 
will first consider the former problem. 
The Development of the Normal Beat. 
§ 140. The heart of a mammal, or of a warm-blooded animal, generally 
ceases to beat within a few minutes after being removed from the body in 
the ordinary way, the hearts of newly-born animals continuing, however, to 
beat for a longer time than those of adidts. Hence, though by special pre- 
caution and by means of an artificial circulation of blood, an isolated 
mammalian heart may be preserved in a pulsating condition for a much 
longer time, our knowledge of the exact nature and of the causes of the 
cardiac beat is as yet very largely based on the study of the hearts of cold- 
blooded animals, which will continue to beat for hours, or under favorable 
circumstances even for days, after they have been removed from the body 
with only ordinary care. We have reason to think that the mechanism by 
which the beat is carried on varies in some of its secondary features in differ- 
ent kinds of animals; that the heart, for instance, of the eel, the snake, the 
tortoise, and the frog, differ in some minor details of behavior, both from 
each other and from the bird and the mammal; but we may, at first at all 
events, take the heart of the frog as illustrating the main and important 
truths concerning the causes and mechanism of the beat. 
In studying closely the phenomena of the beat of the heart it becomes necessary 
to obtain a graphic record of various movements. 
1. In the frog or other cold-blooded animal, a light lever may be placed directly 
on the ventricle (or on an auricle, etc.), and changes of form, due either to disten- 
tion by the influx of blood or to the systole, will cause movements of the lever, 
which may be recorded on a travelling surface. The same methods, as we have 
seen, may be applied to the mammalian heart. 
2. Or, as in Gaskell's method, the heart may be fixed by a clamp carefully ad- 
justed around the auriculo-ventricular groove, while the apex of the ventricle and 
some portion of one auricle arc attached by threads to horizontal levers placed 
respectively above and below the heart. The auricle and the ventricle each in its 
systole pulls at the lever attached to it, and the times and extent of the contrac- 
tions may thus be recorded. 
3. A record of endocardiac pressure may be taken in the frog or tortoise, as in 
the mammal, by means of an appropriate manometer. And in these animals, at 
all events, it is easy to keep up an artificial circulation. A canula is introduced 
into the sinus venosus, and another into the ventricle through the aorta. Serum 
or dilute blood (or any other fluid which it may be desired to employ) is driven by 
moderate pressure through the former; to the latter is attached a tube connected 
by means of a side piece with a small mercury manometer. So long as the exit-tube 
is open at the end fluid flows freely through the heart and apparatus. Upon closing 
the exit-tube at its far end the force of the ventricular systole is brought to bear on 
the manometer, the index of which registers in the usual way the movements of the 
mercury column. Newell Martin has succeeded in applying a modification of this 
method to the mammalian heart. 
4. The movements of the ventricle may be registered by introducing into it 
through the auriculo-ventricular orifice a so-called "perfusion" canula. Figs. 71 
and 72, I., with a double tube, one inside the other, and tying the ventricle on to 
the canula at the auriculo-ventricular groove, or at any level below that which may 
be desired. The blood or other fluid is driven at an adequate pressure through the 
tube a. enters the ventricle, and returns by the tube 1>. If L be connected with a 
manometer as in method 3, the movements of the ventricle may be registered. 
5. In the apparatus of Roy, Fig. 72, II., the exit-tube is free, but the ventricle 
(the same method may be adopted for the whole heart) is placed in an air-tight 
chamber filled with oil, or partly with normal saline solution and partly with oil. 
By means of the tube 6 the interior of the chamber a is continuous with that of a 
small cylinder c in which a piston d, secured by a thin flexible animal membrane THE VASCULAR MECHANISM. 
193 
works up and down. The piston again bears on a lever e by means of which 
its movements may be registered. When the ventricle con- 
tracts, and by contracting diminishes in volume, there is a 
lessening of pressure in the interior of the chamber; this is 
transmitted to the cylinder, and the piston correspondingly 
rises, carrying with it the lever. As the ventricle subsequently 
becomes distended the pressure in the chamber is increased, and 
the piston and lever sink. In this way variations in the volume 
of the ventricle may be recorded, without any great interference 
with the How of blood or fluid through it. 
The heart of the frog, as we have just said, will con- 
tinue to beat for hours after removal from the body, even 
after the cavities have been cleared of blood, and, indeed, 
when they are almost empty of all fluid. The beats thus 
carried out are in all important respects identical with 
the beats executed by the heart in its normal condition 
within the living body. Hence we may infer that the 
beat of the heart is an automatic action ; the muscular 
contractions which constitute the beat are due to causes which arise spon- 
taneously in the heart itself. 
In the frog's heart, as in that of the mammal, §115, there is a distinct 
sequence of events which is the same whether the heart be removed from, or 
Fig. 71. 
A Perfusion Camila. 
Fig 72. 
Purely Diagrammatic Figures of-I. Perfusion canula tied into frog's ventricle: a, entrance; 
b, exit-tube ; A, wall of ventricle ; B, ligature. 
II. Roy's apparatus modified by Gaskill , a, chamber filled with saline solution and oil, con- 
taining the ventricle A tied on to perfusion canula f; b, tube leading to cylinder c, in which 
moves piston d, working the lever e. 
be still in its normal condition within, the body. First comes the beat of 
the sinus venosus, preceded by a more or less peristaltic contraction of the 
large veins leading into it, next follows the sharp beat of the two auricles 
together, then comes the longer beat of the ventricle, and lastly, the cycle is 
completed by the beat of the bulbus arteriosus, which does not, like the 
mammalian aorta, simply recoil by elastic reaction after distention by the 
ventricular stroke, but carries out a distinct muscular contraction passing in 
a wave from the ventricle outward. 
When the heart in dying ceases to beat, the several movements cease, 
as a rule, in an order the inverse of the above. Omitting the bulbus arte- 
11; 194 
THE VASCULAR MECHANISM. 
riosus, which sometimes exhibits great rhythmical power, we may say that 
first the ventricle fails, then the auricles fail, and lastly the sinus venosus 
fails. 
The heart after it has ceased to beat spontaneously remains for some 
time irritable-that is, capable of executing a beat, or a short series of 
beats, when stimulated either mechanically, as by touching it with a blunt 
needle or electrically by an induction shock or in other ways. The artificial 
beat so called forth may be in its main features identical with the natural 
beat, all the divisions of the heart taking part in the beat, and the sequence 
of events being the same as in the natural beat. Thus when the sinus is 
pricked the beat of the sinus may be followed by a beat of the auricles and 
of the ventricle; and even when the ventricle is stimulated, the directly 
following beat of the ventricle may be succeeded by a complete beat of the 
whole heart. 
Under certain circumstances, however, the division directly stimulated 
is the only one to beat; when the ventricle is pricked for instance it alone 
beats, or when the sinus is pricked it alone beats. The results of stimulation, 
moreover, may differ according to the condition of the heart and according 
to the particular spot to which the stimulus is applied. 
With an increasing loss of irritability, the response to stimulation ceases 
in the several divisions in the same order as that of the failure of the natural 
beat-the ventricle ceases to respond first, then the auricles, and lastly the 
sinus venosus, which frequently responds to stimulation long after the other 
divisions have ceased to make any sign. 
It would appear as if the sinus venosus, auricles, and ventricle formed a 
descending series in respect to their irritability and to the power they possess 
of carrying on spontaneous rhythmic beats, the sinus being the most, potent. 
This is also seen in the following experiments: 
In order that the frog's heart may beat after removal from the body with 
the nearest approach in rapidity, regularity, and endurance to the normal 
condition, the removal must be carried out so that the excised heart still 
retains the sinus venosus intact. 
When the incision is carried through the auricles so as to leave the sinus 
venosus behind in the body, the sinus venosus beats forcibly and regularly, 
having suffered hardly any interruption from the operation. The excised 
heart, however, remains in the majority of cases for some time motionless. 
Stimulated by a prick or an induction-shock, it will perhaps give one, two, or 
several beats, and then come to rest. In the majority of eases, however, the 
animal having previously been in a vigorous condition, it will after a while 
recommence its spontaneous beating, the systole of the ventricle following 
that of the auricles; but the rhythm of beat will not be the same as 
that of the sinus venosus left in the body, but will be slower, and the beats 
will not continue to go on for so long a time as will those of a heart still re- 
taining the sinus venosus. 
When the incision is carried through the auriculo-ventricular groove, so 
as to leave the auricles and sinus venosus within the body, and to isolate the 
ventricle only, the results are similar but more marked. The sinus and 
auricles beat regularly and vigorously, with their proper sequence, but the 
ventricle, after a few rapid contractions due to the incision acting as a stimu- 
lus, generally remains for a long time quiescent. When stimulated, how- 
ever, the ventricle will give one, two, or several beats, and after a while, in 
many cases at least, will eventually set up a spontaneous pulsation with an 
independent rhythm ; and this may last for some considerable time, but the 
beats are not so regular and will not go on for so long a time as will those 
of a ventricle to which the auricles are still attached. THE VASCULAR MECHANISM. 
195 
If a transverse incision be carried through the ventricle at about its upper 
third, leaving the base of the ventricle still attached to the auricles, the por- 
tion of the heart left in the body will go on pulsating regularly, with the 
■ordinary sequence of sinus, auricles, ventricle, but the isolated lower two- 
thirds of the ventricle will not beat spontaneously at all, however long it be 
left. Moreover, in response to a single stimulus such as an induction-shock 
•or a gentle prick it gives, not, as in the case of the entire ventricle when 
stimulated at the base or of the ventricle to which the auricles are attached, 
a series of beats, but a single beat. 
Lastly, to complete the story, we may add that when the heart is bisected 
longitudinally, each half continues to beat spontaneously, with an indepen- 
dent rhythm, so that the beats of the two halves are not necessarily syn- 
chronous, and this continuance of spontaneous pulsations after longitudinal 
bisection may be seen in the conjoined auricle and ventricle, or in the iso- 
lated auricles, or in the isolated but entire ventricle. Moreover, the auricles 
may be divided in many ways and yet many of the segments will continue 
beating; small pieces even may be seen under the microscope pulsating, 
feebly, it is true, but distinctly and rhythmically. 
In these experiments, then, the various parts of the frog's heart also 
form, as regards the power of spontaneous pulsation, a descending series: 
sinus venosus, auricles, entire ventricle, lower portions of ventricle, the last 
■exhibiting under ordinary circumstances no spontaneous pulsations at all. 
§ 141. Now we have seen (§ 139) that these parts form to a certain 
extent a similar descending series as regards the presence of ganglia; at 
least so far that the ganglia are very numerous in the sinus venosus, that 
they occur in the auricles, and that while Bidder's ganglia are present at 
the junction of the ventricle with the auricles, ganglia are wholly absent 
from the rest of the ventricle. Hence, on the assumption (which we have 
already, § 96, seen reason to doubt) that the nerve cells of ganglia are sim- 
ilar in general functions to the nerve cells of the central nervous system, the 
view very naturally presents itself that the rhythmic spontaneous beat of 
the heart of the frog is due to the spontaneous generation in the ganglionic 
nerve cells of rhythmic motor impulses, which, passing down to the muscular 
fibres of the several parts, cause rhythmic contractions of these fibres, 
the sequence and coordination of the beating of the several divisions of the 
heart being the result of a coordination between the several ganglia in re- 
gard to the generation of impulses. Under this view the cardiac muscular 
fibre simply responds to the motor impulses reaching it along its motor nerve 
fibre in the same way as the skeletal muscular fibre responds to the motor 
impulses reaching it along its motor nerve fibre ; in both cases the muscular 
fibre is, as it were, a passive instrument in the hands of the motor nerve, or 
rather of the nervous centre (ganglion or spinal cord) from which the 
motor nerve proceeds. And the view, thus based on the fact of the frog's 
heart, has been extended to the hearts of (vertebrate) animals generally. 
There are reasons, however, which show that this view is not tenable. 
For instance, the lower two-thirds, or lower third, or even the mere tip 
of the frog's ventricle--that is to say, parts which are admitted not to con- 
tain nerve cells-may, by special means, be induced to carry on for a con- 
siderable time a rhythmic beat, which in its main features is identical with 
the spontaneous beat of the ventricle of the intact heart. If such a part 
of the frog's ventricle be tied on to the end of a perfusion canula (Fig. 71), 
the portion of the ventricular cavity belonging to the part may be ade- 
quately distended and at the same time be "fed" with a suitable fluid, 
such as blood, made to flow through the canula ; it will then be found that 
the portion of ventricle so treated will, after a preliminary period of qui- 196 
THE VASCULAR MECHANISM. 
escence, commence to beat, apparently spontaneously, and will continue so 
beating for a long period of time. It may be said that in this case the dis- 
tention of the cavity and the supply of blood or other fluid acts as a stim- 
ulus ; but if so the stimulus is a continuous one, or at least not a rhythmic 
one, and yet the beat is most regularly rhythmic. 
Then, again, the reluctance of the ventricle to execute spontaneous 
rhythmic beats is to a certain extent peculiar to the frog. The ventricle of 
the tortoise, for instance, the greater part of the substance of which is as 
free from nerve cells as is that of the frog, will beat spontaneously when 
isolated from the auricles with great ease and for a long time. Further, a 
mere strip of this ventricular muscle tissue, if kept gently extended and 
continually moistened with blood or other suitable fluid, will continue to 
beat spontaneously with very great regularity for hours, or even days, 
especially if the series be started by the preliminary application of induc- 
tion-shocks rhythmically repeated. 
In connection with this question we may call attention to the fact that 
the cardiac muscular fibre is not wholly like the skeletal muscular fibre; in 
many respects the contraction or beat of the former is in its very nature 
different from the contraction of the latter ; the former cannot be considered, 
like the latter, a mere instrument in the hands of the motor nerve fibre. 
The features of the beat or contraction of cardiac muscle may be studied on 
the isolated and quiescent ventricle, or part of the ventricle, of the frog. 
When such a ventricle is stimulated by a single stimulus, such as a single 
induction-shock or a single touch with a blunt needle, a beat may or may 
not result. If it follows it resembles, in all its general features at least, 
a spontaneous beat. Between the application of the stimulus and the first 
appearance of any contraction is a very long latent period, varying accord- 
ing to circumstances, but in a vigorous fresh frog's ventricle being about 
0.3 second. The beat itself lasts a variable but considerable time, rising 
slowly to a maximum and declining slowly again. Of course, when the beat 
is recorded by means of a light lever placed on the ventricle, what the 
tracing shows is really the increase in the front-to-back diameter of the 
ventricle during the beat-that is to say, one of the results of the contrac- 
tion of the cardiac fibres-and gives, in an indirect manner only, the extent 
of the contraction of the fibres themselves; and the same is the case with 
the other methods of recording the movements of the whole ventricle. We 
may, however, study in a more direct way the contraction of a few fibres 
by taking a slip of the ventricle (and for this purpose the tortoise is prefer- 
able to the frog) and suspending it to a lever after the fashion of a muscle- 
nerve preparation. We then get a curve of contraction, characterized 
by a long latent period, a slow long-continued rise, and a slow long-con- 
tinued fall-a contraction, in fact, more like that of a plain muscular fibre 
than of a skeletal muscular fibre. In the tortoise the contraction is partic- 
ularly long, the contraction of even the skeletal muscles being long in that 
animal; it is less long, but still long, in the frog; shorter still, but yet long 
as compared with the skeletal muscles, in the mammal. 
The beat of the ventricle, then, is a single but relatively slow, prolonged 
contraction wave sweeping over the peculiar cardiac muscle-cell, passing 
through the cement substance from cell to cell along the fibre, from fibre to 
fibre along the bundle, and from bundle to bundle over the labyrinth of the 
ventricular walls. 
Like the case of the skeletal muscle, this single contraction is accom- 
panied by an electric change, a current of action. The intact ventricle at 
rest is, as we have already said (§ 66), isoelectric, but each part just as it is 
about to enter into a state of contraction becomes negative toward the rest. 197 
THE VASCULAR MECHANISM. 
Hence, when the electrodes of a galvanometer are placed on two points, 
A, B, of the surface of the ventricle, a diphasic variation of the galvanom- 
eter needle is seen just as a beat, natural or excited, is about to occur. Sup- 
posing that the wave of contraction reaches A first, this will become nega- 
tive toward the rest of the ventricle, including B, but when the wave some 
time afterward reaches B, B will become negative toward the rest of the 
ventricle, including A. Compare §67. 
The beat of the auricles, that of the sinus venosus, and that of thebulbus 
arteriosus are similar in their main features to that of the ventricle, so that 
the whole beat may be considered to be a wave of contraction sweeping 
through the heart from sinus to bulbus; but the arrangement of fibres is 
such that this beat is cut up into sections in such a way that the sinus, the 
auricles, the ventricle, and the bulbus have each a beat, so to speak, to them- 
selves. In a normal state of things these several parts of the whole beat 
follow each other in the sequence we have described, but under abnormal 
conditions the sequence may be reversed, or one section may beat while the 
others are at rest, or the several sections may beat out of time with each 
other. 
So far the description of the contraction which is the foundation of the 
beat differs from that of a skeletal muscle in degree only; but now comes an 
important difference. When we stimulate a skeletal muscle with a strong 
stimulus we get a large contraction ; when we apply a weak stimulus we get 
a small contraction ; within certain limits (see § 77) the contraction is pro- 
portional to the stimulus. This is not the case with the quiescent ventricle 
or heart. When we apply a strong induction-shock we get a beat of a cer- 
tain strength ; if we now apply a weak shock, we get either no beat at all or 
quite as strong a beat as with the stronger stimulus. That is to say, the 
magnitude of the beat depends on the condition of the ventricle (or heart), 
and not on the magnitude of the stimulus. If the stimulus can stir the ven- 
tricle up to beat at all, the beat is the best which the ventricle at the time 
can accomplish ; the stimulus either produces its maximum effect or none at 
all. It would seem as if the stimulus does not produce a contraction in the 
same way that it does when it is brought to bear on a skeletal muscle, but 
rather stirs up the heart in such a way as to enable it to execute a spon- 
taneous beat, which, without the extra stimulus, it could not bring about. 
And this is further illustrated by the fact that when a ventricle is beating 
rhythmically, either spontaneously or as the result of rhythmic stimulation, 
the kind of effect produced by a new stimulus thrown in will depend upon 
the exact phase of the cycle of the beat at which it is thrown in. If it is 
thrown in just as a relaxation is taking place, a beat follows prematurely, 
before the next beat would naturally follow, this premature beat being obvi- 
ously produced by the stimulus. But if it be thrown in just as a contraction 
is beginning, no premature beat follows ; the ventricle does not seem to feel 
the stimulus at all. There is a period during which the ventricle is insen- 
sible to stimuli, and that however strong ; this period is called the " refrac- 
tory " period. (There is, it may be mentioned, a similar refractory period 
in skeletal muscle, but it is of exceedingly short duration.) From this it 
results that, when a succession of stimuli repeated at a certain rate are sent 
into the ventricle, the number of beats does not correspond to the number of 
stimuli; some of the stimuli falling in refractory periods are ineffective and 
produce no beat. Hence, also, it is difficult if not impossible to produce a 
real tetanus of the ventricle, to fuse a number of beats into one. And there 
are other facts tending to show that the contraction of a cardiac muscular 
fibre, even when induced by artificial stimulation, is of a peculiar nature, 
and that the analogy with the contraction of a skeletal muscular fibre, 198 
THE VASCULAR MECHANISM. 
induced by motor impulses reaching it along its nerve, does not hold 
good. 
These and other considerations, taken together with the facts already 
mentioned, that portions of cardiac muscular tissue in which ganglionic cells 
are certainly not present, can in various animals be induced, either easily or 
with difficulty, to execute rhythmic beats which have all the appearance of 
being spontaneous in nature, lead us to conclude that the beat of the heart 
is not the result of rhythmic impulses proceeding from the cells of the ganglia 
to passive muscular fibres, but is mainly the result of changes taking place 
in the muscular tissue itself. And here we may call attention to the peculiar 
histological features of cardiac muscular tissue ; though so far differentiated 
as to be striated, its cellular constitution and its " protoplasmic " features, 
including the obscurity of the striation, show that the differentiation is in- 
complete. Now, one attribute of undifferentiated primordial protoplasm is 
the power of spontaneous movement. 
§142. We have, moreover, evidence that it is the muscular tissue, and 
not the arrangement of ganglia and nerves, which is primarily concerned in 
maintaining the remarkable sequence of sinus beat, auricle beat, and ven- 
tricle beat. This is perhaps better seen in the heart of the tortoise than in 
that of the frog. 
In this animal the nerves passing from the sinus to the ventricle may be 
divided, or the several ganglia may be respectively removed, and yet the 
normal sequence is maintained. On the other hand, we find that interference 
with the muscular substance of the auricle, when carried to a certain extent, 
prevents the beat of the auricle passing over to the ventricle, so that the 
sequence is broken after the auricle beat. If, for instance, the auricle be cut 
through until only a narrow bridge of muscle be left connecting the part of 
the auricle adjoining the sinus with the part adjoining the auriculo-ventric- 
ular ring, or if this part be compressed with a clamp, a state of things may 
be brought about in which every second beat only, or every third beat only, 
of the sinus and auricle is followed by a beat of the ventricle ; and then, if 
the bridge be still further narrowed or the clamp screwed tighter, the ven- 
tricle does not at all follow in its beat the sequence of sinus and auricle, 
though it may after a while set up an independent rhythm of its own. 
This experiment suggests, and other facts support, the view that the normal 
sequence is maintained as follows: The beat begins in the sinus (including 
the ends of the veins) ; the contraction wave, beginning at the ends of the 
veins, travels over the muscular tissue of the sinus, and reaching the auricle 
starts a contraction in that segment of the heart; similarly the contraction 
wave of the auricular beat reaching the ventricle starts a ventricular beat, 
which in turn in like fashion starts the beat of the bulbus. And in hearts 
in a certain condition it is possible by stimulation to reverse this sequence, 
or to produce, by alternate stimulation, an alternation of a normal and a 
reversed sequence ; thus in the heart of the skate, in a certain condition, 
mechanical stimulation of the bulbus by indicating a beat of the bulbus 
will start a sequence of the bulbus. ventricle, auricle, and sinus, and similar 
stimulation of the sinus will produce a normal sequence of sinus, auricle, 
ventricle, and bulbus. 
It would, perhaps, be premature to insist that the nervous elements do 
not intervene in any way in the maintenance of this sequence; but the evi- 
dence shows that they are not the main factors, and we have at present no 
satisfactory indications of the way in which they do or may intervene. 
Two questions naturally suggest themselves here. The first is, Why does 
the cardiac cycle begin with the sinus beat? We have previously (§ 141) 
given the evidence that the sinus has a greater potentiality of beating than 199 
THE VASCULAR MECHANISM. 
the other parts; in and by itself it beats more readily and with a quicker 
rhythm than the other parts. When we ask the further question, Why has 
it this greater potentiality? the only answer we can at present give is that 
it is inborn in the substance of the sinus. The problem is somewhat of the 
same kind as why the heart of one animal beats so much quicker than that 
of another. All we can say at present is that the rate is the outcome of the 
molecular constitution of tissue, without being able to define that molecular 
constitution. 
The second question is, Why does not the contraction wave starting at the 
sinus spread as a continuous wave over the whole heart? why is it broken 
up into sinus beat, auricle beat, ventricle beat? We may here call to mind 
the fact mentioned in § 140 of the existence, more or less marked in all 
hearts and well seen in the heart of the tortoise, of a muscular ring or collar 
between the sinus and the auricle, and of a similar ring between the auricle 
and ventricle. The muscular tissue in these rings seems to be of a somewhat 
different nature from the muscular tissue forming the body of the sinus, or 
of the auricle, or of the ventricle. If we suppose that this tissue has a low 
conducting power, it may offer sufficient resistance to the progress of the 
contraction to permit the sinus, for example, to carry out or to be far on in 
the development of its beat before the auricle begins its beat (and thus bisect, 
so to speak, the beat which would otherwise be common to the two), and yet 
not offer so much resistance as to prevent the contraction wave passing ulti- 
mately on from the sinus to the auricle. We may in the tortoise by careful 
clamping or section of the auricle in its middle, by which an obstacle to the 
contraction wave is introduced, bisect the single auricular beat into two beats, 
one of the part between the sinus and the obstacle and another between the 
obstacle and the ventricular. We may thus consider the breaking up the 
primitive unbroken peristaltic wave of contraction from sinus to bulbus to be 
due to the introduction of tissue of lower conducting power at the junctions 
of the several parts. 
We do not say that this is the complete solution of the problem, but 
it at least offers an approximate solution; and here as elsewhere we 
have no satisfactory evidence of nervous elements being main factors in 
the matter. 
In the above we have dealt chiefly with the heart of the cold-blooded 
animal, but as far as we know the same conclusions hold good for the 
mammalian heart also. 
The question now arises, If the ganglia are not the prime cause of the 
heart's rhythmic beat, or of the maintenance of the normal sequence, what 
purposes do they serve? But before we even attempt to answer this 
question we must deal with the nervous mechanisms by which the beat 
of the heart, thus arising spontaneously within the tissues of the heart 
itself, is modified and regulated to meet the requirements of the rest of 
the body. 
The Government, of the Heart-beat by the Nervous System. 
§ 143. It will be convenient to begin with the heart of the frog, which, 
as we have seen, is connected with the central nervous system through 
and therefore governed by the two vagi nerves, each of which though 
apparently a single nerve contains, as we shall see, fibres of different origin 
and nature. 
If while the beats of the heart of a frog are being carefully registered an 
interrupted current of moderate strength be sent through one of the vagi, 
the heart is seen to stop beating. It remains for a time in diastole, perfectly 200 
THE VASCULAR MECHANISM. 
motionless and flaccid ; all the muscular fibres of the several chambers are 
for the time being in a state of relaxation. The heart has been inhibited 
by the impulses descending down the vagus from the part of the nerve 
stimulated. 
If the duration of the stimulation be short and the strength of the current 
great, the standstill may continue after the current has been shut off; the 
beats when they reappear are generally at first feeble and infrequent, but 
soon reach or even go beyond their previous vigor and frequency. If the 
duration of the current be very long, the heart may recommence beating 
while the stimulation is still going on, but the beats are feeble and infrequent 
though gradually increasing in strength and frequency. The effect of the 
stimulation is at its maximum at or soon after the commencement of the 
application of the stimulus, gradually declining afterward ; but even at the 
end of a very prolonged stimulation the beats may still be less in force or in 
frequency, or in both, than they were before the nerve was stimulated, and 
on the removal of the current may show signs of recovery by an increase in 
force and frequency. The effect is not produced instantaneously ; if on the 
curve the point be exactly marked when the current is thrown in, as at on, 
Fig. 73, it will frequently be found that one beat at least occurs after the 
Fig 73. 
Inhibition of Frog's Heart by Stimulation of Vagus Nerve: on marks the time at which the 
interrupted current was thrown into the vagus, off when it was shut off'. The time-marker below 
marks seconds. The beats were registered by suspending the ventricle from a clamp attached to 
the aorta and attaching a light lever to the tip of the ventricle. 
current has passed into the nerve; the development of that beat has taken 
place before the impulses descending the vagus have had time to affect the 
heart. 
The stimulus need not necessarily be the interrupted current; mechanical, 
chemical, or thermal stimulation of the vagus will also produce inhibition ; 
but in order to get a marked effect it is desirable to make use of not a single 
nervous impulse but a series of nervous impulses ; thus it is difficult to obtain 
any recognizable result by employing a single induction-shock of moderate 
intensity only. As we shall see later on, "natural" nervous impulses 
descending the vagus from the central nervous system, and started there, 
by afferent impulses or otherwise, as parts of a reflex act, may produce 
inhibition. 
The stimulus maybe applied to any part of the course of the vagus from 
high up in the neck right down to the sinus ; indeed, very marked results are 
obtained by applying the electrodes directly to the sinus, where, as we have 
seen, the two nerves plunge into the substance of the heart. The stimulus 
may also be applied to either vagus, though in the frog and some other 
animals, one vagus is sometimes more powerful than the other. Thus, it 
not infrequently happens that even strong stimulation of the vagus on one 201 
THE VASCULAR MECHANISM. 
side produces no change of the rhythm, while even moderate stimulation 
of the nerve on the other side of the neck brings the heart to a stand- 
still at once. 
If during the inhibition the ventricle or other part of the heart be stimu- 
lated directly-for instance, mechanically by the prick of a needle-a beat 
may follow ; that is to say, the impulses descending the vagus, while inhibit- 
ing the spontaneous beats, have not wholly abolished the actual irritability 
of the cardiac tissues. 
With a current of even moderate intensity, such a current, for instance, 
as would produce a marked tetanus of a muscle-nerve preparation, the stand- 
still is complete, that is to say, a certain number of beats are entirely 
dropped ; but with a weak current the inhibition is partial only, the heart 
does not stand absolutely still but the beats are slowed, the intervals between 
them being prolonged, or weakened only without much slowing, or both 
slowed and weakened. Sometimes the slowing and sometimes the weaken- 
ing is the more conspicuous result. 
It sometimes happens that, when in the frog the vagus is stimulated in the 
neck, the effect is very different from that just described ; for the beats are 
increased in frequency, though they may be at first diminished in force. 
And, occasionally, the beats are increased both in force and in frequency ; 
the result is augmentation, not inhibition. But this is due to the fact that 
in the frog the vagus along the greater part of its course is a mixed nerve 
and contains fibres other than those of the vagus proper. 
§ 144. If we examine the vagus nerve closely, tracing it up to the 
brain, we find that just as the nerve has pierced the cranium, just where it 
passes through the ganglion (G. K, Fig. 74), certain fibres pass into it from 
the sympathetic nerve of the neck, Sy., of the further connections of which 
we shall speak presently. 
This being the case, we may expect that we should get different results 
according as we stimulated (1) the vagus in the cranium before it was joined 
by the sympathetic, (2) the sympathetic fibres before they join the vagus, 
and (3) the vagus trunk containing the real vagus and the sympathetic fibres 
added. What we have previously described are the ordinary results of 
stimulating the mixed trunk, and these, as we have said, are not wholly 
constant, though usually and in the main most distinct inhibitory results 
follow. 
If we stimulate the sympathetic in the neck, as at Sy., Fig. 74, cutting 
the nerve below, so as to block all impulses from passing downward, and only 
allow impulses to pass up to the vagus ami thence down the mixed vagus 
trunk to the heart, we get very remarkable results. The beat of the heart, 
instead of being inhibited, is augmented ; the beats are increased either in 
frequency or in force, or most generally both in frequency and in force. 
The effect is, perhaps, best seen when the heart before stimulation is beating 
slowly and feebly ; upon stimulation of the cervical sympathetic the beats 
at once improve in vigor and frequency ; indeed, a heart which, for one 
reason or another, has almost ceased to beat may, by proper stimulation of 
the sympathetic, be called back into vigorous activity. 
If, on the other hand, we stimulate the vagus before it has been joined 
by the sympathetic fibres (and to insure the result not being marred by any 
escape of the stimulating current on to the sympathetic fibres it is necessary 
to stimulate the vagus within the cranium), we get pure and constant inhib- 
itory results-the beats are for a time wholly abolished, or are slowed, or are 
weakened, or are both slowed and weakened. 
Obviously, then, the heart of the frog is supplied through the vagus by 
two sets of fibres coming from the central nervous system, the one by the 202 
THE VASCULAR MECHANISM. 
vagus proper and the other by the cervical sympathetic nerve, and these two 
sets have opposite and antagonistic effects upon the heart. We find upon 
examination that we can make the following statements concerning them : 
The one set, those belonging to the vagus proper, are inhibitory; they 
weaken the systole and prolong the diastole, the effect with a strong stimula- 
Fig. 74. 
Diagrammatic Representation of the Course of Cardiac Augmentor Fibres in the Frog: V. r., 
roots of vagus (and ixth) nerve. G. F., ganglion of same. Or., line of cranial wall. Vg.. vagus 
trunk, ix., ninth, glosso-pharyngeal nerve. S. V. C„ superior vena cava. Sy., sympathetic nerve 
in neck. G. c., junction of sympathetic ganglion with vagus ganglion sending i. c. intra-cranial 
fibres passing to Gasserian ganglion. The rest of the fibres pass along the vagus trunk. G1, 
splanchnic ganglion connected with the first spinal nerve. G», splanchnic ganglion of the sec- 
ond spinal nerve. An. V., annulus of Vieussens. A.sb., subclavian artery. G111, splanchnic gan- 
glion of the third spinal nerve. III., third spinal nerve, r.c., ramus communicans. 
The course of the augmentor fibres is shown by the thick black line. They may be traced from 
the spinal cord by the anterior root of the third spinal nerve, through the ramus communicans 
to the corresponding splanchnic ganglion Gin and thence by the second ganglion Gn the annulus 
of Vieussens, and the first ganglion G1 to the cervical sympathetic Sy. and so by the vagus trunk 
to the superior vena cava S. F. C. 
tion being complete, so that the heart is for a time brought to a standstill. 
Sometimes the slowing, sometimes the weakening, is the more prominent. 
When the nerve and the heart are in good condition it needs only a slight 
stimulus, a weak current, to produce a marked effect; and it may be men- 
tioned that the more vigorous the heart, the more rapidly it is beating, the 
easier it is to bring about inhibition. Although, as we have said, the effect THE VASCULAR MECHANISM. 
203 
is at its maximum soon after the beginning of stimulation, a very prolonged 
inhibition may be produced by prolonged stimulation ; indeed by rhythmi- 
cal stimulation of the vagus the heart may be kept perfectly quiescent for a 
very long time and yet beat vigorously upon the cessation of the stimulus. 
In other words, the mechanism of inhibition-that is, the fibres of the vagus 
and the part or substance of the heart upon which these act to produce in- 
hibition, whatever that part or substance may be-is not readily exhausted. 
Further, the inhibition when it ceases is, frequently at all events, followed 
by a period of reaction, during which the heart for a while beats more vig- 
orously and rapidly than before. Indeed, the total effect of stimulating the 
vagus fibres is not to exhaust the heart, but rather to strengthen it; and by 
repeated inhibitions carefully administered, a feebly beating heart may be 
nursed into vigorous activity. 
The other set, those joining the vagus from the sympathetic, are "aug- 
mentor " or " accelerating " fibres; the latter name is the more common, but 
the former is more accurate, since the effect of stimulating these fibres is to 
increase not only the rapidity but the force of the beat; not only is the 
diastole shortened, but the systole is strengthened, sometimes the one result 
and sometimes the other being the more prominent. In contrast with the 
case of the vagus fibres, a somewhat strong stimulation is required to produce 
an effect; the time required for the maximum effect to be produced is also 
remarkably long. Moreover, at all events, in the case of a heart in which 
the circulation is not maintained, and which is therefore cut off from its 
normal nutritive supply, the augmentor fibres are far less easily exhausted 
than are the inhibitory fibres. Hence, when in such a heart both sets of 
fibres are stimulated together, as when the vagus trunk in the neck is stimu- 
lated, the first effects produced are those of inhibition ; but these on con- 
tinued stimulation may become mixed with those of augmentation, and 
finally the latter alone remain. Lastly, the contrast is completed by the 
fact that the augmentation resulting from the stimulation of the sympathetic 
is followed by a period of reaction in which the beats are feebler; in other 
words, augmentation is followed by exhaustion ; and, indeed, by repeated 
stimulation of these sympathetic fibres a fairly vigorous bloodless heart may 
be reduced to a very feeble condition. 
By watching the effects of stimulating the sympathetic nerve at various 
points of its course we may trace these augmentor fibres from their junction 
with the vagus down the short sympathetic of the neck through the first 
splanchnic or sympathetic ganglion connected with the first spinal nerve, G1 
(Fig. 74), through one or both the loops of the annulus of Vieussens, An. V, 
through the second ganglion connected with the second spinal nerve, Gu, 
to the third ganglion connected with the third spinal nerve, GU1, and thence 
through the ramus communicans or visceral branch of that ganglion, r. c., 
to the third spinal nerve, III., by the anterior root of which they reach the 
spinal cord. 
§ 145. Both sets of fibres may then be traced to the central nervous 
system; and we find accordingly that the heart may be inhibited or aug- 
mented by nervous impulses which are started in the nervous system either 
by afferent impulses as part of a reflex act or otherwise, and which pass to 
the heart by the inhibitory or by the augmenting tract. 
Thus, if the medulla oblongata, or a particular part of the medulla oblon- 
gata which is specially connected with the vagus nerve, be stimulated, the 
heart is inhibited ; if, for instance, a needle be thrust into this part, the heart 
stands still. This region in question may be stirred into action in a " reflex " 
manner by afferent impulses reaching it from various parts of the body. Thus, 
if the abdomen of a frog be laid bare, and the intestines be struck sharply 204 
THE VASCULAR MECHANISM. 
with the handle of a scalpel, the heart will stand still in diastole with all 
the phenomena of vagus inhibition. If the nervi mesenterici, or the connec- 
tions of these nerves with the spinal cord, be stimulated with the interrupted 
current, cardiac inhibition is similarly produced. If in these two experiments 
both vagi are divided, or the medulla oblongata is destroyed, inhibition is 
not produced, however much either the intestine or the mesenteric nerves be 
stimulated. This shows that the phenomena are caused by impulses ascend- 
ing along the mesenteric nerves to the medulla, and so affecting a portion of 
that organ as to give rise by reflex action to impulses which descend the 
vagi as inhibitory impulses. The portion of the medulla thus mediating 
between the afferent and efferent impulses may be spoken of as the cardio- 
inhibitory centre. 
Reflex inhibition through one vagus may be brought about by stimula- 
tion of the central end of the other. In general the alimentary tract seems 
in closer connection with the cardio-inhibitory centre than other parts of the 
body; and if the peritoneal surface of the intestine be inflamed, very gentle 
stimulation of the inflamed surface will produce marked inhibition. But 
apparently stimuli, if sufficiently powerful, will through reflex action pro- 
duce inhibition, whatever be the part of the body to which they are applied. 
Thus, crushing a frog's foot will stop the heart, and adequate stimulation of 
most afferent nerves will produce some amount of inhibition. 
The details of the reflex chain and the portion of the centre concerned in 
the development of augmenting impulses have not been worked out so fully 
as in the case of inhibitory impulses, but there can be little doubt that the 
former, like the latter, are governed by the central nervous system. 
§ 146. So far we have been dealing with the heart of the frog, but the 
main facts which we have stated regarding inhibition and augmentation of 
the heart beat apply also to other vertebrate animals, including mammals; 
and, indeed, we meet similar phenomena in the hearts of invertebrate 
animals. 
If in a mammal the heart be exposed to view by opening the thorax, and 
the vagus nerve be stimulated in the neck, the heart may be seen to stand 
still in diastole, with all the parts flaccid and at rest. If the current em- 
ployed be too weak, the result as in the frog is not an actual arrest, but a 
slowing or weakening of the beats. If a light lever be placed on the heart 
a graphic record of the standstill or of the slowing, of the complete or incom- 
plete inhibition, may be obtained The result of stimulating the vagus is 
also well shown on the blood-pressure curve, the effect of complete cardiac 
inhibition on blood-pressure being most striking. If, while a tracing of 
arterial pressure is being taken, the beat of the heart be suddenly arrested, 
some such curve as that represented in Fig. 75 will be obtained. It will be 
observed that two beats follow the application of the current marked by the 
point a, which corresponds to the signal x on the line below. Then for a 
space of time no beats at all are seen, the next beat b taking place almost 
immediately after the shutting off the current at y. Immediately after the 
last beat following a there is a sudden fall of the blood-pressure. At the 
pulse due to the last systole the arterial system is at its maximum of disten- 
tion ; forthwith the elastic reaction of the arterial walls propels the blood for- 
ward into the veins, and, there being no fresh fluid injected from the heart, 
the fall of the mercury is unbroken, being rapid at first, but slower afterward, 
as the elastic force of the arterial walls is more and more used up. With the 
returning beats the pressure correspondingly rises in successive leaps until 
the normal mean pressure is regained. The size of these returning leaps of 
the mercury may seem disproportionately large, but it must be remembered 
that by far the greater part of the force of the first few strokes of the heart THE VASCULAR MECHANISM. 
205 
is expended in distending the arterial system, a small portion only of the 
blood which is ejected into the arteries passing on into the veins. As the 
arterial pressure rises, more and more blood passes at each beat through the 
capillaries, and the rise of the pressure at each beat becomes less and less, 
until at last the whole contents of the ventricle pass at each stroke into the 
veins, and the mean arterial pressure is established. To this it may be 
added that, as we have seen, the force of the individual beats may be some- 
what greater after than before inhibition. Besides, when the mercurv 
Fig. 75. 
Tracing showing the Influence of Cardiac Inhibition on Blood-pressure. From a Rabbit, x, 
the marks on the signal line when the current is thrown into, and y, shut off from the vagus. The 
time-marker below marks seconds, the heart, as is frequently the case in the rabbit, beating very 
rapidly. 
monometer is used, the inertia of the mercury tends to magnify the effects 
of the initial beats. 
In the mammal inhibition may be brought about by impulses passing 
along fibres which, starting in the medulla oblongata, run down over the 
vagus nerve and reach the heart by the cardiac nerves. It would appear, 
however, that the inhibitory fibres do not belong to the vagus proper, but 
leave the central nervous system by the spinal accessory nerve. Thus if the 
roots of the spinal accessory be divided, those of the vagus proper being left 
intact, the spinal accessory fibres in the vagus trunk degenerate, and when 
this takes place stimulation of the vagus trunk fails to produce the ordinary 
inhibitory effects. In the mammal, as in the frog, inhibition may be brought 
about not only by artificial stimulation of the vagus trunk, but by stimula- 
tion in a reflex manner or otherwise of the cardio-inhibitory centre. Thus 
the fainting which often follows upon a blow on the stomach is a repetition 
of the result just mentioned as obtained on the frog by striking the stomach 
or stimulating the nervi mesenterici. So also the fainting, complete or 
partial, which accompanies severe pain or mental emotion, is an illustration 
of cardiac inhibition by the vagus. In fact, cardiac inhibition so far from 
being a mere laboratory experiment enters repeatedly into the every-day 
working of our own organism as well as that of other living beings. 
Indeed there is some reason for thinking that the central nervous system 
by means of the cardiac inhibitory fibres keeps as it were a continual rein on 
the heart, for, in the dog at least, section of both vagi causes a quickening 
of the heart's beat. 
In the dog the augmentor fibres (Fig. 76) leave the spinal cord by the 
anterior roots of the second and third dorsal nerves, possibly also to some 
extent by the fourth and fifth, pass along the rami communicantes of those 
nerves to the ganglion stellatum, first thoracic ganglion, or respectively to 
one or other of the ganglia forming part of the thoracic, splanchnic, or 
sympathetic chain immediately below, and thence upward through the 206 
THE VASCULAR MECHANISM. 
annulus of Vieussens, passing along one or other or both loops, to the inferior 
cervical ganglion. Their further course to the heart is along the nerves 
springing either from the inferior cer- 
vical ganglion or from the loop of 
Vieussens directly. Their exact path 
from the ganglia in fact seems to vary 
in different individuals. 
The path of the augmentor fibres 
has not been worked out so fully in 
other mammals as in the dog, but it 
is most probable that in all cases they 
leave the spinal cord by the anterior 
roots of the second and third dorsal 
nerves (possibly also by the fourth 
and fifth) and, passing up the sympa- 
thetic chain to the ganglion stellatum 
and annulus of Vieussens, proceed to 
the heart by nerves branching off 
from some part or other of the annu- 
lus or from the lower and middle 
cervical ganglia. 
Fig. 76. 
Diagrammatic Representation of the Cardial 
Inhibitory and Augmentor Fibres in the Dog. 
The upper portion of the figure represents the 
inhibitory, the lower the augmentor fibres. 
r.Vg., roots of the vagus; r.Sp.Ac., roots of the 
spinal accessory; both drawn very diagram- 
matically. G.J., ganglion jugulare; G.Tr. Vg., 
ganglion trunci vagi; Sp.Ac., spinal accessory 
trunk; ext.Sp.Ac., external spinal accessory; 
■i.Sp.Ac, internal spinal accessory; Vg., trunk 
of vagus nerve; n.c., branches going to heart; 
C.Sy., cervical sympathetic; G.C., lower cervi- 
cal ganglion : A.sb., subclavian artery ; An. E, 
annulus of Vieussens; G.StATh.^, ganglion 
stellatum or first thoracic ganglion: G.Th.*, 
G.Th3, G.ThA, second, third, and fourth tho- 
racic ganglia: D.II., D.IIL, DIV., D.V., second 
third, fourth, and fifth thoracic spinal nerves; 
r.c., ramus communicans; n.c., nerves (car- 
diac) passing to heart (superior vena cava) from 
cervical ganglion and from the annulus of 
Vieussens. 
The inhibitory fibres, shown by black line, 
run in the upper (medullary roots) of the spinal 
accessory, by the internal branch of the spinal 
accessory, past the ganglion trunci vagi, along 
the trunk of the vagus, and so by branches to 
the superior vena cava and the heart. 
The augmentor fibres, also shown by black 
line, pass from the spinal cord by the anterior 
roots of the second and third thoracic nerves 
(possibly also from fourth and fifth as indicated 
by broken black line), pass the second and first 
(stellate) thoracic ganglia by the annulus of 
Vieussens to the lower cervical ganglion, from 
whence, as also from the annulus itself, they 
pass along the cardiac nerves to the superior 
vena cava. 
The effects of stimulating these augmentor fibres in the mammal are, in 
general, the same as those witnessed in the frog. In the mammal, as in the 
frog, impulses along these augmentor fibres may be originated in the central 207 
THE VASCULAR MECHANISM. 
nervous system, and that probably in various ways. That palpitation of the 
heart which is so conspicuous an effect of certain emotions is probably due 
to the sudden positive action of augmenting impulses, though it may possibly 
be due, in part at least, to sudden withdrawal of normal, continuous, tonic, 
and inhibitory impulses. 
In the mammal, then, as in the frog, the heart is governed by two sets 
of nerves, the one antagonistic to the other. In the dog the roots of the 
spinal accessory nerve, by which inhibitory fibres leave the central nervous 
system, consists entirely of medullated fibres. Among these are fibres of 
fine calibre, 2//-3/Z in diameter, which may be traced down the trunk of 
the vagus, along the branches going to the heart, right down to the heart 
itself. There can be little doubt that these medullated fibres of fine calibre 
are the inhibitory fibres of the vagus, and indeed there is evidence which 
renders it probable that the inhibitory fibres of the heart are always medul- 
lated fibres of fine calibre, which continue as medullated fibres right down 
to the heart, but eventually lose their medulla in the heart itself. 
The anterior roots of the second and third dorsal nerves, and the (white) 
rami communicantes belonging to them, which, as we have just seen, con- 
tain in the dog augmentor fibres, also consist exclusively of medullated 
fibres. But the nerves which convey the augmenting impulses from the 
lower cervical ganglion, or from the annulus of Vieussens to the heart, 
eonsist of non-medullated fibres. Hence, the augmentor fibres must have 
lost their medulla, and become continuous with non-medullated fibres some- 
where in their course along the sympathetic chain. It is probable that the 
change occurs in the ganglion stellatum apd lower cervical ganglion, and 
it is further probable that the change is effected by the medullated fibre 
passing into one of the ganglion cells, and so losing its medulla, the im- 
pulses which it conveys passing out of the nerve cell by one or more of the 
other processes of the cell which are continued on as non-medullated fibres. 
Cf. § 94. 
In the dog then these two sets of nerve fibres, antagonistic to each other 
in function, differ in structure, the augmentor fibres early losing their 
medulla, and hence being over a large part of their course non-medullated 
fibres, whereas the inhibitory fibres are medullated fibres which, though they 
may pass by or through ganglia (as the ganglion jugulare and ganglion 
trunci vagi), do not lose their medulla in these ganglia, but remain as 
medullated fibres right down to the heart. And this difference in structure 
appears to hold good for all mammals, and is possibly true for vertebrates 
generally. 
§ 147. The question, What is the exact nature of the change brought 
about by the inhibitory and augmenting impulses respectively on their 
arrival at the heart? or, in other words, by virtue of what events produced 
in the heart itself do the impulses of one kind bring about inhibition, 
of the other kind augmentation ? is a very difficult one, which we can- 
not attempt to discuss fully here. We may, if we please, speak of an 
" inhibitory mechanism " placed in the heart itself, but we have no exact 
knowledge of the nature of such a mechanism. Still less do we possess 
any satisfactory information as to an augmenting mechanism. It has been 
suggested that some of the ganglia in the heart serve as such an inhibi- 
tory (or augmenting) mechanism ; but there is evidence that the inhibitory 
impulses produce their effect by acting directly on the muscular fibres, or at 
all events do not produce their effect by acting exclusively on any gan- 
glia. One evidence of this kind is supplied by the action of the drug 
atropine. 
If, either in a frog or a mammal, or other animal, after the vagus fibres 208 
THE VASCULAR MECHANISM. 
have been proved by trial to produce upon stimulation the usual inhibitory 
effects, a small quantity of atropine be introduced into the circulation ( when 
the experiment is conducted on a living animal, or be applied in a weak 
solution to the heart itself when the experiment is conducted, as in the case 
of a frog, on an excised heart, or after the circulation has ceased), it will 
after a short time be found not oidy that the stimulation, the application of 
a current, for instance, which previously when applied to the vagus pro- 
duced marked inhibition, now produces no inhibition, but even that the 
strongest stimulus, the strongest current applied to the vagus will wholly 
fail to affect the heart, provided that there be no escape of current on to the 
cardiac tissues themselves; under the influence of even a small dose of 
atropine, the strongest stimulation of the vagus will not produce standstill 
or appreciable slowing or weakening of the beat. 
Now it might be supposed that the atropine produces this remarkable 
effect by acting on some ganglionic or other mechanism intervening between 
the vagus fibres and the cardiac muscular tissue ; but we have evidence 
that the atropine acts either on the muscular tissue itself or on the very 
endings of the nerves in the muscular fibres. We have said (§ 140) that a 
properly prepared strip of the ventricle of the tortoise will execute for a 
long time spontaneous rhythmic contractions, it will go on "beating" for 
a long time. A strip of the auricle will exhibit the same phenomena even 
still more readily. If now, while such a strip from the auricle is satisfac- 
torily beating, a gentle interrupted current be passed through it, it will 
stop beating; the current inhibits the spontaneous beats; a very gentle 
interrupted current must be used, otherwise the effect is obscured by the 
more direct stimulating action of the current. If now the strip be gently 
bathed with a weak solution of atropine no such inhibitory effect is pro- 
duced by the interrupted current; the beats go on regardless of the action 
of the current. The interruption of this experiment is that in the first case 
the interrupted current stimulated the fine termination of the inhibitory 
fibres in the muscular strip, and that in the second case the atropine pro- 
duced some effect either on these fine fibres, or on their connections with the 
muscular substance or on the actual muscular substance itself, by virtue of 
which they ceased to act. But if this be so, if the same inhibitory effects 
are produced alike by stimulating the vagus trunk and stimulating the 
very endings of the nerves in the muscles of the heart, if not the actual 
muscular tissue itself, then there is no need to suppose the existence of any 
special inhibitory mechanism placed between the fibres in the vagus branches 
and the cardiac muscular tissue. 
The action of atropine on the heart is, so to speak, complemented by the 
action of muscarine, the active principle of many poisonous mushrooms. If 
a small quantity of muscarine be introduced into the circulation, or applied 
directly to the heart, the beats become slow and feeble, and if the dose 
be adequate the heart is brought to a complete standstill. The effect is 
in some respects like that of powerful stimulation of the vagus, but the 
standstill is much more complete, the effect is much more profound. Now 
if, in a frog, the heart be brought to a standstill by a dose of muscarine, the 
application of an adequate quantity of atropine will bring back the beats 
to quite their normal strength. The one drug is, as far as the heart is con- 
cerned (and, indeed, in many other respects), the antidote of the other. 
And, as in the case of atropine, so in the case of muscarine, there is evi- 
dence that the drug acts not on any ganglionic mechanisms, but on the car- 
diac tissue itself. 
The conclusion that inhibition is the result of changes in the cardiac tis- 
sue itself may serve to explain why in inhibition sometimes the slowing, THE VASCULAR MECHANISM. 
209 
sometimes the weakening is the more prominent. When the inhibitory 
impulses, by reason of particular fibres being affected or otherwise, are 
brought to bear chiefly on those parts of the heart, such as the sinus, which 
possessing higher rhythmic potentiality (see § 142) determine the sequence 
and set the rate of rhythm, it is the rate which is most markedly affected. 
When, on the other hand, the inhibitory impulses fall chiefly on the parts 
possessing lower rhythmic potentiality, the most marked effect is a diminu- 
tion in the force of the contractions. 
There is no adequate evidence then that the cardiac ganglia act as an 
inhibitory mechanism in the sense that they produce important changes in 
the nature of the impulses reaching them along vagus inhibitory fibres before 
those impulses pass on to the muscular tissue. We may add that there is 
similarly no adequate evidence that an^ of the ganglia act as an "aug- 
menting" mechanism. We have previously seen 141, 142) reasons 
for thinking the ganglia are not centres for the origination or regula- 
tion of the spontaneous beats. The question then arises, What are their 
functions? To this question we cannot at present give a wholly satisfac- 
tory answer. 
The inhibitory fibres remain, as we have seen, medullated fibres until they 
reach the heart, but it would appear that they lose their medulla somewhere 
in the heart before they actually reach the muscular tissue, and it is probable 
that the loss takes place in connection with some of the cardiac ganglia 
much in the same way that the augmenting fibres lose their medulla in the 
ganglia of the sympathetic chain ; but we do not know what is the physi- 
ological effect or the purpose of this loss of the medulla, and we cannot 
suppose that this is the sole or even chief use of the ganglia. Coincident 
with the loss of the medulla an increase of fibres frequently takes place, 
more than one non-medullated fibre leaving a nerve cell into which one 
medullated fibre enters ; and we may suppose that this mode of branching 
has purposes not fulfilled by the mere division of a fibre. Then again, 
bearing in mind the nutritive or "trophic" function of the spinal gan- 
glia alluded to in § 96, we may suppose that the cardiac ganglia are in 
some way concerned in the nutrition of the cardiac nerve fibres. But 
our knowledge is not yet sufficiently ripe to allow exact statements to 
be made. 
Other Influences Regulating or Modifying the Beat of the Heart. 
§ 148. Important as is the regulation of the heart by the nervous system, 
it must be borne in mind that other influences are or may be at work. The 
beat of the heart may, for instance, be modified by influences bearing directly 
on the nutrition of the heart. The tissues of the heart, like all other tissues, 
need an adequate supply of blood of a proper quality ; if the blood vary in 
quality or quantity the beat of the heart is correspondingly affected. The 
excised frog's heart, as we have seen, continues to beat for some considerable 
time, though apparently empty of blood. After a while, however, the beats 
diminish and disappear; ami their disappearance is greatly hastened by 
washing out the heart with a normal saline solution, which when allowed 
to flow through the cavities of the heart readily permeates the tissues on 
account of the peculiar construction of the ventricular walls. If such a 
" washed out " quiescent heart be fed with a perfusion canula, in the manner 
described (§ 141), with diluted blood (of the rabbit, sheep, etc.), it may be 
restored to functional activity. A similar but less complete restoration may 
be witnessed if serum be used instead of blood ; and a heart fed regularly 
with fresh supplies of blood oi' even of serum may be kept beating for a 210 
THE VASCULAR MECHANISM. 
very great length of time. In treating of the skeletal muscles we saw that 
in their case the exhaustion following upon withdrawal of the blood-stream 
might be attributed either to an inadequate supply of new nutritive mate- 
rial and oxygen, or to an accumulation in the muscular substance of the 
products of muscular metabolism, or to both causes combined. And the 
same considerations hold good for the nervous and muscular structures 
of the heart, though the subject has not yet been sufficiently well worked 
out to permit any very definite statements to be made. It seems prob- 
able, however, that an important factor in the matter is the accumulation 
in the muscular fibres and in the surrounding lymph of carbonic acid, and 
especially of the substances which give rise to the acid reaction. 
When the frog's heart is thus " fed " with various substances the interest- 
ing fact is brought to light that some substances, such, for instance, as very 
dilute lactic acid, lead to increased expansion, and others, such, for instance, 
as very dilute solutions of sodium hydrate, to diminished expansion, that is 
to continued contraction, of the quiescent ventricle. It would appear that 
the muscular fibres of the ventricle over and above their rhythmic contrac- 
tions are capable of varying in length, so that at one time they are longer, 
and the ventricle when pressure is applied to it internally dilates beyond the 
normal, while at another time they are shorter, and the ventricle, with the 
same internal pressure, is contracted beyond the normal. Further, in the 
frog at least, when the pause between two beats is lengthened the relaxa- 
tion of the ventricle goes on increasing, so that apparently the ventricle 
when beating normally is already somewhat contracted when a new beat 
begins. In other words, the ventricle possesses what we shall speak of in 
reference to arteries as tonicity or tonic contraction, and the amount of 
this tonic contraction, and in consequence the capacity of the ventricle, 
varies according to circumstances. We have, moreover, evidence that 
inhibitory impulses diminish and augmenting impulses increase this tonic 
contraction. 
When the frog's ventricle is thus artificially fed with serum or even with 
blood, the beats, whether spontaneous or provoked by stimulation, are apt to 
become intermittent and to arrange themselves into groups. This intermit- 
tence is possibly due to the serum or blood being unable to carry on nutrition 
in a completely normal manner, and to the consequent production of abnor- 
mal chemical substances ; and it is probable that cardiac intermittences seen 
during life have often a similar causation. Various chemical substances in 
the blood, natural or morbid, may thus affect the heart's beat by acting on 
its muscular fibres or its nervous elements, or both, and that probably in 
various ways, modifying in different directions the rhythm or the individual 
contractions, or both. 
The physical or mechanical circumstances of the heart also affect its beat; 
of these perhaps the most important is the amount of the distention of its 
cavities. The contractions of cardiac muscle, like those of ordinary muscle 
(see § 79), are increased up to a certain limit by the resistance which they 
have to overcome; a full ventricle will, other things being equal, contract 
more vigorously than one less full; though, as in ordinary muscle, the limit 
at which resistance is beneficial maybe passed, and an overfull ventricle will 
fail to beat at all. 
Under normal conditions the ventricle probably empties itself completely 
at each systole. Hence an increase in the quantity of blood in the ventricle 
would augment the work done in two ways : the quantity thrown out would 
be greater, and the increased quantity would be ejected with greater force. 
Further, since the distention of the ventricle is (at the commencement of 
the systole at all events) dependent on the auricular systole, the work of the VASOMOTOR ACTIONS. 
211 
ventricle (and so of the heart as a whole) is in a measure governed by the 
auricle. 
An interesting combination of direct mechanical effects and indirect ner- 
vous effects is seen in the relation of the heart's beat to blood-pressure. When 
the blood-pressure is high, not only is the resistance to the ventricular systole 
increased, but other things being equal, more blood flows (in the mamma- 
lian heart) through the coronary artery. Both these events would increase 
the activity of the heart, and we might expect that the increase would be 
manifest in the rate of the rhythm as well as in the force of the individual 
beats. As a matter of fact, however, we do not find this. On the contrary 
the relation of heart-beat to pressure may be put almost in the form of a 
law, that the " rate of the beat is in inverse ratio to the arterial pressure," 
a rise of pressure being accompanied by a diminution, and fall of pressure 
with an increase, of the pulse-rate. This, however, only holds good if the 
vagi be intact. If these be previously divided, then in whatever way the 
blood-pressure be raised-whether by injecting blood or clamping the aorta 
or increasing the peripheral resistance, through that action of the vasomotor 
nerves which we shall have to describe directly-or in whatever way it be 
lowered, no such clear and decided inverse relation between blood-pressure 
and pulse-rate is observed. It is inferred, therefore, that increased blood- 
pressure causes a slowing of the pulse, when the vagi are intact, because the 
cardio-inhibitory centre in the medulla is stimulated by the high pressure, 
either directly by the pressure obtaining in the bloodvessels of the medulla, 
or in some indirect manner, and the heart in consequence to a certain extent 
inhibited. 
Changes in the Calibre of the Minute Arteries. Vasomotor 
Actions. 
§ 149. All arteries contain plain muscular fibres, for the most part cir- 
cularly disposed, and most abundant in, or sometimes almost entirely con- 
fined to, the middle coat. Moreover, as the arteries become smaller the 
muscular element, as a rule, becomes more and more prominent as compared 
with the other elements, until, in the minute arteries, the middle coat consists 
almost entirely of a series of plain muscular fibres wrapped around the in- 
ternal coat. Nerve fibres, of whose nature and course we shall presently 
speak, are distributed largely to the arteries and appear to end chiefly in 
fine plexuses around the muscular fibre, but their exact terminations have 
not as yet been clearly made out. By mechanical, electrical, or other 
stimulation, this muscular coat may, in the living artery, be made to con- 
tract. During this contraction, which has the slow character belonging to 
the contractions of all plain muscles, the calibre of the vessel is diminished. 
The veins also, as we have seen, possess muscular elements, but these vary 
in amount and distribution very much more in the veins than in the arteries. 
Most veins, however, are contractile, and may vary in calibre according to 
the condition of their muscular elements. Veins are also supplied with 
nerves. It will be of advantage, however, to consider separately the little 
we know concerning the changes in the veins, and to confine ourselves at 
present to the changes in the arteries. 
If the web of a frog's foot be watched under the microscope, any indi- 
vidual small artery will be found to vary considerably in calibre from time 
to time, being sometimes narrowed and sometimes dilated ; and these changes 
may take place without any obvious changes either in the heart-beat or in 
the general circulation ; they are clearly changes of the artery itself. Dur- 
ing the narrowing, which is obviously due to a contraction of the muscular 212 
THE VASCULAR MECHANISM. 
coat of the artery, the capillaries fed by the artery and the veins into which 
these lead become less filled with blood and paler. During the widening, 
which corresponds to the relaxation of the muscular coat, the same parts are 
fuller of blood and redder. It is obvious that, the pressure at the entrance 
into any given artery remaining the same, more blood will enter the artery 
when relaxation takes place, and consequently the resistance offered by the 
artery is diminished, and less when contraction occurs and the resistance is 
consequently increased ; the blood flows in the direction of least resistance. 
The extent and intensity of the narrowing or widening, the constriction 
or dilatation which may thus be observed in the frog's web, vary very largely. 
Variations of slight extent, either more or less regular and rhythmic or 
irregular, occur even when the animal is apparently subjected to no disturb- 
ing causes, and may be spoken of as spontaneous; larger changes may follow 
events occurring in various parts of the body ; while as the result of experi- 
mental interference the arteries may become either constricted, in some cases 
almost to obliteration, or dilated until they acquire double or more than 
double their normal diameter. This constriction or dilation may be 
brought about not only by treatment applied directly to the web, but also 
by changes affecting the nerve of the leg or other parts of the body. Thus, 
section of the sciatic nerve is generally followed by a widening which may 
be slight or which may be very marked, and which is sometimes preceded 
by a passing constriction ; while stimulation of the peripheral stump of the 
divided nerve by an interrupted current of moderate intensity generally 
gives rise to constriction, often so great as almost to obliterate some of the 
minute arteries. 
Obviously, then, the contractile muscular elements of the minute arteries 
of the web of the frog's foot are capable by contraction or relaxation of 
causing decrease or increase of the calibre of the arteries ; and this condition 
of constriction or dilation may be brought about through the agency of the 
nerves. Indeed, not only in the frog, but also, and still more so, in warm- 
blooded animals, have we evidence that in the case of nearly all, if not all, 
the arteries of the body, the condition of the muscular coat, and so the calibre 
of the artery, is governed by means of nerves; these nerves have received 
the general name of vasomotor nerves. 
§ 150. If the ear of a rabbit, preferably a light-colored one, be held up 
before the light, a fairly conspicuous artery will be seen running up the 
middle line of the ear accompanied by its broader and more obvious veins. 
If this artery be carefully watched it will be found, in most instances, to be 
undergoing rhythmic changes of calibre, constriction alternating with dila- 
tation. At one moment the artery appears as a delicate, hardly visible, pale 
streak, the whole ear being at the same time pallid. After a while the 
artery slowly widens out, becomes broad and red, the whole ear blushing, 
and many small vessels previously invisible coming into view. Again the 
artery narrows and the blush fades away; and this may be repeated at 
somewhat irregular intervals of a minute, more or less. The extent and 
regularity of the rhythm are usually markedly increased if the rabbit be 
held up by the ears for a short time previous to the observation. Similarly 
rhythmic variations in the calibre of the arteries have been observed in 
several places, e. g., in the vessels of the mesentery and elsewhere; probably 
they are widely spread. 
Sometimes no such variations are seen ; the artery remains constant in a 
condition intermediate between the more extreme widening and extreme 
narrowing just described. In fact, we may speak of an artery as being at 
any given time in one of three phases. It may be very constricted, in which 
case its muscular fibres are very much contracted ; or it may be dilated, in VASOMOTOR ACTIONS. 
213 
which case its muscular fibres are relaxed ; or it may be moderately con- 
stricted, the muscular fibres being contracted to a certain extent, and 
remaining in such a condition that they may, on the one hand, pass into 
stronger contraction, leading to marked constriction, or, on the other hand, 
into distinct relaxation, leading to dilatation. We have reason to think, as 
we shall see, that many arteries of the body are kept habitually, or at least 
for long periods together, in this intermediate condition, which is frequently 
spoken of as tonic contraction, or tonus, or arterial tone. 
§ 151. If, now, in a vigorous rabbit, in which the heart is beating with 
adequate strength and the whole circulation is in a satisfactory condition, 
the cervical sympathetic nerve be divided on one side of the neck, remark- 
able changes may be observed in the bloodvessels of the ear of the same side. 
The arteries and veins widen, they together with the small veins and the 
capillaries become full of blood, many vessels previously invisible come into 
view, the whole ear blushes, and if the rhythmic changes described above 
were previously going on, these now cease ; and, in consequence of the extra 
supply of warm blood, the whole ear becomes distinctly warmer. Now these 
changes take place, or may take place, without any alteration in the heart- 
beat or in the general circulation. Obviously the arteries of the ear have, 
in consequence of the section of the nerve, lost the tonic contraction which 
previously existed ; their muscular coats, previously somewhat contracted, 
have become quite relaxed, and whatever rhythmic contractions were pre- 
viously going on have ceased. The more marked the previous tonic 
contraction, and the more vigorous the heart-beats, so that there is an 
adequate supply of blood to fill the widened channels, the more striking 
the results. Sometimes, as when the heart is feeble, or the pre-existing 
tonic contraction is slight, the section of the nerve produces no very obvious 
change. 
If, now, the upper segment of the divided cervical sympathetic nerve- 
that is, the portion of the nerve passing upward to the head and ear-be 
laid upon the electrodes of an induction machine and a gentle interrupted 
current be sent through the nerve, new changes take place in the blood- 
vessels of the ear. A short time after the application of the current, for in 
this effect there is a latent period of very appreciable duration, the ear grows 
paler and cooler, many small vessels previously conspicuous become again 
invisible, the main artery shrinks to the thinnest thread, and the main veins 
become correspondingly small. When the current is shut off from the nerve 
these effects still last some time, but eventually pass oft*; the ear reddens, 
blushes once more, and indeed may become even redder and hotter, with 
the vessels more filled with blood than before. Obviously the current has 
generated in the cervical sympathetic nerve impulses which, passing upward 
to the ear and finding their way to the muscular coats of the arteries of the 
ear, have thrown the muscles of those coats into forcible contractions, and 
have thus brought about a forcible narrowing of the calibre of the arteries 
-a forcible constriction. Through the narrowed constricted arteries less 
blood finds its way, and hence the paleness and coldness of the ear. If the 
impulses thus generated be very strong, the constriction of the arteries may 
be so great that the smallest quantity only of blood can make its way through 
them, and the ear may become almost bloodless. If the impulses be weak, 
the constriction induced may be slight only; and, indeed, by careful manipu- 
lation the nerve may be induced to send up to the ear impulses only just 
sufficiently strong to restore the moderate tonic constriction which existed 
before the nerve was divided. 
We infer from these experiments that among the various nerve fibres 
making up the cervical sympathetic, there are certain fibres which, passing 214 
THE VASCULAR MECHANISM. 
upward to the head, become connected with the arteries of the ear, and that 
these fibres are of such a kind that impulses generated in them and passing 
upward to the ear lead to marked contraction of the muscular fibres of the 
arteries, and thus produce constriction. These fibres are vasomotor fibres 
for the bloodvessels of the ear. From the loss of tone, so frequently follow- 
ing section of the cervical sympathetic, we may further infer that, normally 
during life, impulses of a gentle kind are continually passing along these 
fibres upward through the cervical sympathetic, which impulses, reaching 
the arteries of the ear, maintain the normal tone of those arteries. But, as 
we said, the existence of this tone is not so constant, and these tonic impulses 
are not so conspicuous as the artificial constrictor 
impulses generated by stimulation of the nerve. 
§ 151. The above results are obtained what- 
ever be the region of the cervical sympathetic 
which we divide or stimulate from the upper cervi- 
cal ganglion to the lower. We may, therefore, de- 
scribe these vasomotor impulses as passing upward 
from the lower cervical ganglion along the cervical 
sympathetic to the upper cervical ganglion, from 
which they issue by branches which ultimately 
find their way to the ear. But these impulses do 
not start from the lower cervical ganglion ; on 
the contrary, by repeating the experiments of di- 
vision and stimulation in a series of animals, we 
may trace the path of these impulses from the 
lower cervical ganglion (Fig. 77) through the 
annulus of Vieussens to the ganglion stellatum or 
first thoracic ganglion, and thence either along the 
ramus communicans (visceral branch) to the an- 
terior root of the second dorsal nerve, and thus to 
the spinal cord, or lower down along the thoracic 
sympathetic chain, and thence by other rami com- 
municantes to some other of the upper dorsal 
nerves, and thus to the spinal cord. The path 
taken by these vasomotor impulses for the ear is 
in fact very similar to that of the augmentor fibres 
for the heart (cf. Fig. 76), from the spinal coni 
up to the annulus of Vieussens and to the lower 
cervical ganglion ; but there they part company. 
We can thus trace these impulses along the cer- 
vical sympathetic to the anterior roots of certain 
dorsal nerves, and through these to a particular 
part of the spinal cord, where we will for the pres- 
ent leave them. We may accordingly speak of 
vasomotor fibres for the ear as passing from the 
dorsal spinal cord to the ear along the track just 
marked out; stimulation of these fibres at their 
origin in the spinal cord or at any part of their 
course (along the anterior roots of the second, 
third, or other upper dorsal nerves, visceral 
branches of those nerves, ganglion stellatum or 
upper part of thoracic sympathetic chain, annulus 
of Vieussens, etc.) leads to constriction in the 
bloodvessels of the ear of that side ; and section 
of these fibres at any part of the same course tends to abolish any previously 
Fig. 77. 
Diagram illustrating the 
Paths of Vaso-constrictor Fi- 
bres along the Cervical Sym- 
pathetic and (part of) the Ab- 
dominal Splanchnic : Aur., 
artery of ear; G. C.s., superior 
cervical ganglion ; Abd. Spl., 
upper roots of and part of ab- 
dominal splanchnic nerve; V. 
M. C., vasomotor centre in Ine- 
dulla. The other references 
are the same as in Fig. 76, g 
146. The paths of the constric- 
tor fibres are shown by the 
arrows. The dotted line in 
the spinal cord, Sp. C., is to in- 
dicate the passage of constric- 
tor impulses down the cord 
from the vasomotor centre in 
the medulla. VASOMOTOR ACTIONS. 
215 
existing tonic constriction of the bloodvessels of the ear, though this effect 
is not so constant or striking as that of stimulation. 
§ 153. We must now turn to another case. In dealing with digestion we 
shall have to study the submaxillary salivary gland. We may for the 
present simply say that this is a glandular mass well supplied with blood- 
vessels, and possessing a double nervous supply. On the one hand it receives 
fibres from the cervical sympathetic, Fig. 78, v. sym. (in the dog, in which 
the effects which we are about to describe are best seen, the vagus and cervical 
Fig. 78. 
Diagrammatic Representation of the Submaxillary Gland of the Dog, with its Nerves and 
Bloodvessels. The dissection has been on an animal lying on its back, but since all the parts 
shown in the figure cannot be seen from any one point of view, the figure does not give the exact 
anatomical relations of the several structures. 
sm. old. The submaxillary gland, into the duct (sm. d.) of which a canula has been tied. The 
sublingual gland and duct are not shown, n.l., n.l'. The lingual branch of the fifth nerve, the 
part n.l. is going to the tongue, ch.t., ch.t'., ch.t". The chorda tympani. The part ch. t". is pro- 
ceeding from the facial nerve ; at ch. t'. it becomes conjoined with the lingual n.l'., and afterward 
diverging passes as ch. t. to the gland along the duct; the continuation of the nerve in company 
with the lingual n.l. is not shown, sm. gl. The submaxillary ganglion with its several roots. 
a. car. The carotid artery, two small branches of which, a. sm. a. and r. sm. p., pass to the anterior 
and posterior parts of the gland, v. sm. The anterior and posterior veins from the gland, falling 
into v. j., the jugular vein. v. sym. The conjoined vagus and sympathetic trunks, g. cer. s. The 
upper cervical ganglion, two branches of which, forming a plexus (a.f.) over the facial artery, 
are distributed (n. sym.sm.) along the two glandular arteries to the anterior and posterior portions 
of the gland. 
The arrows indicate the direction taken by the nervous impulses during reflex stimulation of 
the gland. They ascend to the brain by the lingualand descend by the chorda tympani. 
sympathetic are enclosed in a common sheath so as to form what appears to 
be a single trunk), which reach the gland in company with the arteries sup- 
plying the gland (n. sym. sm.). On the other hand, it receives fibres from a 
small nerve called the chorda tympani (ch. f.), which, springing from the 
seventh cranial (facial) nerve, crosses the tympanum of the ear (hence the 
name) and, joining the lingual branch of the fifth nerve, runs for some dis- 
tance in company with that nerve, and then ends partly on the tongue, 
and partly in a small nerve which, leaving the lingual nerve before reach- 
ing the tongue, runs along the duct of the submaxillary gland, and is lost in 
the substance of the gland ; a small branch is also given off to the sub- 
lingual gland. 216 
THE VASCULAR MECHANISM. 
Now when the chorda tympani is simply divided no very remarkable 
changes take place in the bloodvessels of the gland, but if the peripheral 
segment of the divided nerve, that still in connection with the gland, be 
stimulated, very marked results follow. The small arteries of the gland 
become very much dilated and the whole gland becomes flushed. (As we 
shall see later on, the gland at the same time secretes saliva copiously, but 
this does not concern us just now.) Changes in the calibre of the blood- 
vessels are of course not so readily seen in a compact gland as in a thin, 
extended ear; but if a fine tube be placed in one of the small veins by 
which the blood returns from the gland, the effects on the bloodvessels of 
stimulating the chorda tympani become very obvious. Before stimulation 
the blood trickles out in a thin slow stream of a dark venous color; 
during stimulation the blood rushes out in a rapid full stream, often with a 
distinct pulsation and frequently of a color which is still scarlet and arte- 
rial in spite of the blood having traversed the capillaries of the gland ; the 
blood rushes so rapidly through the widened bloodvessels that it has not 
time to undergo completely that change from arterial to venous which nor- 
mally occurs while the blood is traversing the capillaries of the gland. 
This state of things may continue for some time after the stimulation 
has ceased, but before long the flow from the veins slackens, the issuing 
blood becomes darker and venous, and eventually the circulation becomes 
normal. 
Obviously the chorda tympani contains fibres which we may speak of as 
" vasomotor," since stimulation of them produces a change in, and brings 
about a movement in the bloodvessels ; but the change produced is of a 
character the very opposite to that produced in the bloodvessels of the ear 
by stimulation of the cervical sympathetic. There stimulation of the nerve 
caused contraction of the muscular fibres, constriction of the small arteries ; 
here stimulation of the nerve causes a widening of the arteries, which 
widening is undoubtedly due to relaxation of the muscular fibres. Hence 
we must distinguish between two kinds of vasomotor fibres, fibres the stim- 
ulation of which produces constriction, vaso-constrictor fibres, and fibres 
the stimulation of which causes the arteries to dilate, vaso-dilator fibres, 
the one kind being the antagonist of the other. 
The reader can hardly fail to be struck with the analogy between these 
two kinds of vasomotor fibres on the one hand, and the inhibitory and aug- 
mentor fibres of the heart on the other hand. The augmentor cardiac fibres 
increase the rhythm and the force of the heart-beats; the vaso-constrictor 
fibres increase the contractions of the muscular fibres of the arteries ; the 
one works upon a rhythmically active tissue, the other upon a tissue whose 
work is more or less continuous, but the effect is in each case similar-an 
increase of the work. The inhibitory cardiac fibres slacken or stop the 
rhythm of the heart and diminish the beats; the vaso-dilator fibres di- 
minish the previously existing contraction of the muscular fibres of the 
arteries so that these expand under the pressure of the blood. 
§ 154. But we must return to the vasomotor nerves. The cervical sympa- 
thetic contains vaso-constrictor fibres for the ear, and we may now add for 
other regions, also of the head and face. Thus the branches of the cervical 
sympathetic, going to the submaxillary gland of which we just spoke (Fig. 
78, n. sym. sm.'), contain vaso-constrictor fibres for the vessels of the gland ; 
stimulation of these fibres produces on the vessels of the gland an effect 
exactly the opposite of that produced by stimulation of the chorda tympani. 
But to this particular point we shall have to return when we deal with the 
gland in connection with digestion. A more important fact for our present 
purpose is that the cervical sympathetic appears to contain only vaso-con- VASOMOTOR ACTIONS. 
217 
stricter fibres ; if we put aside as exceptional and doubtful the result of 
certain observers who obtained vaso-dilator effects in the mouth and face, 
we may say that in no region to which the fibres of the cervical sympathetic 
are distributed can any vaso-dilator action be observed as the result of 
stimulation of the nerve at any part of its course. In the chorda tympani, 
on the other hand, the vasomotor fibres are exclusively vaso-dilator fibres, 
and this is true both of the part of the nerve ending in the submaxillary 
and sublingual glands and the rest of the ending of the nerve in the tongue. 
Stimulation of the chorda tympani (as far as the vasomotor functions of 
the nerve are concerned, for it has, as we shall see, other functions) at any 
part of its course, from its leaving the facial nerve to its endings in the 
tongue or gland, produces only vaso-dilator effects, never vaso constrictor 
effects. 
With many other nerves of the body the case is different. In the frog, 
division of the sciatic nerve leads to a widening of the arteries of the web 
of the foot of the same side, and stimulation of the peripheral end of the 
nerve causes a constriction of the vessels, which, if the stimulation be strong, 
may be so great that the web appears for the time being to be devoid of 
blood. Also, in a mammal, division of the sciatic nerve causes a similar 
widening of the small arteries of the skin of the leg. Where the condition 
of the circulation can be readily examined, as, for instance, in the hairless 
balls of the toes, especially when these are not pigmented, the vessels are 
seen to be dilated and injected, and a thermometer placed between the 
toes shows a rise of temperature amounting, it may be, to several degrees. 
If, moreover, the peripheral end of the divided nerve be stimulated, the 
vessels of the skin become constricted, the skin grows pale, and the temper- 
ature of the foot falls. And very similar results are obtained in the fore- 
limb by division and subsequent stimulation of the nerves of the brachial 
plexus. 
The quantity of blood present in the bloodvessels of the mammal, though it 
may sometimes be observed directly, has frequently to be determined indirectly. 
The temperature of passive structures subject to cooling influences, such as the 
skin, is largely dependent on the supply of blood ; the more abundant the supply 
the warmer the part. Hence, in these parts variations in the quantity of blood 
may be inferred from variations of temperature ; but in dealing with more active 
structures there are obviously sources of error in the possibility of the treatment 
adopted, such as the stimulation of a nerve giving rise to an increase of temper- 
ature due to increased metabolism, independent of variations in blood-supply. 
The quantity of blood may also be determined by the plethysmograph. In this 
instrument a part of the body, such as the arm, is introduced into a closed chamber 
filled with fluid, ex. gr., a large glass tube, the opening by which the arm is intro- 
duced being secured with a stout caoutchouc membrane. An increase or decrease 
of blood sent into the arm will lead to an increase or decrease of the volume.of the 
arm, and this will make itself felt by an increase or diminution of pressure in the 
fluid of the closed chamber, which may be registered and measured in the usual 
way. We shall have to speak again of a modification of this instrument when we 
are dealing with the kidney. 
So far the results are quite like those obtained by division and stimulation 
of the cervical sympathetic, and we might infer that the sciatic nerve and 
brachial plexus contain vaso-constrictor fibres for the vessels of the skin of 
the hind-limb and fore-limb, vaso-dilator fibres being absent. But some- 
times a different result is obtained : on stimulating the divided sciatic nerve 
the vessels of the foot are not constricted, but dilated-perhaps widely dilated. 
And this vaso-dilator action is almost sure to be manifested when the nerve 
is divided, and the peripheral stump stimulated some days after division, by 
which time commencing degeneration has begun to interfere with the irrita- 218 
THE VASCULAR MECHANISM. 
bility of the nerve. For example, if the sciatic be divided, and some days 
afterward, by which time the flushing and increased temperature of the foot 
following upon the section has wholly or largely passed away, the peripheral 
stump be stimulated with an interrupted current, a renewed flushing and 
rise of temperature is the result. We are led to conclude that the sciatic 
nerve (and the same holds good for the brachial plexus) contains both vaso- 
constrictor and vaso-dilator fibres, and to interpret the varying results as due 
to variations in the relative irritability of the two sets of fibres. The con- 
strictor fibres appear to predominate in these nerves, and hence constriction 
is the more common result of stimulation ; the constrictor fibres also appear 
to he more readily affected by a tetanizing current than the dilator fibres. 
When the nerve after division commences to degenerate, the constrictor 
fibres lose their irritability earlier than the dilator fibres, so that at a certain 
stage a stimulus, such as the interrupted current, while it fails to affect the 
constrictor fibres, readily throws into action the dilator fibres. The latter, 
indeed, in contrast to ordinary motor nerves (§ 81), retain their irritability 
after section of the nerve for very many days. The result is, perhaps, even 
still more striking if a mechanical stimulus, such as that of " crimping" the 
nerve by repeated snips with the scissors, be employed. Exposure to a low 
temperature again seems to depress the constrictors more than the dilators ; 
hence, when the leg is placed in ice-cold water stimulation of the sciatic, even 
when the nerve has been but recently divided, throws the dilator only into 
action and produces flushing of the skin with blood. Rhythmical stimula- 
tion, moreover, of even a freshly divided nerve produces dilatation. And 
there are other facts which support the same view that the sciatic nerve 
(and brachial plexus) contains both vaso-constrictor and vaso-dilator fibres 
which are differently affected by different circumstances. We may point 
out that the case of the vagus of the frog is a very analogous one; in it are 
both cardiac inhibitory (true vagus) and cardiac augmentor (sympathetic) 
fibres, but the former, like the vaso-constrictor fibres in the sciatic, are 
predominant, and special means are required to show the presence of the 
latter. 
In the splanchnic nerve (abdominal splanchnic), which supplies fibres to 
the bloodvessels of so large a part of the abdominal viscera, there is abun- 
dant evidence of the presence of vaso constrictor fibres, but the presence of 
vaso-dilator fibres has not clearly been shown. Division of this nerve leads 
to a widening of the bloodvessels of the abdominal viscera, stimulation of 
the nerve to a constriction ; and, as we shall see, since the amount of blood- 
vessels thus governed by this nerve is very large indeed, interference either 
in the one direction or the other with its vasomotor functions produces very 
marked results, not only on the circulation in the abdomen, but on the whole 
vascular system. 
In nerves going to muscles vaso-dilator fibres predominate; indeed, in 
these the presence of any vaso-constrictor fibres at all has not at present been 
satisfactorily established. When a muscle contracts there is always an 
increased flow of blood through the muscle ; this may be in part a mere 
mechanical result of the change of form, the shortening and thickening of 
the fibres opening out the minute bloodvessels, but is not wholly, and prob- 
ably not even largely, thus produced. A notable feature of vasomotor 
fibres is that, in very many cases at all events, their action is not affected by 
small or moderate doses of urari such as render the motor nerves of striated 
muscle powerless. Thus, in a frog placed under the influence of a moderate 
amount of urari, stimulation of a nerve going to a muscle will produce 
vasomotor effects unaccompanied and unobscured by any contraction of the 
striated fibres. By placing a thin muscle of a frog, such as the mylo-hyoid, VASOMOTOR ACTIONS. 
219 
under the microscope, and watching the calibre of the small arteries and the 
circulation of the blood through them while the nerve is being stimulated, 
the widening of the bloodvessels as the result of the stimulation may be ac- 
tually observed. This experiment appears not to succeed in a mammal; and 
it has been suggested that when a muscle contracts some of the chemical 
products of the metabolism of the muscle may, by direct action on the mi- 
nute bloodvessels apart from any nervous agency, lead to a widening of those 
bloodvessels ; this, however, is doubtful. With regard to the vaso-constrictor 
fibres, the only evidence that they exist in muscles is that when the nerve of 
a muscle is divided the bloodvessels of the muscle widen, somewhat like 
bloodvessels of the ear after division of the cervical sympathetic. This sug- 
gests the presence of vaso-constrictor fibres carrying the kind of influence 
which we called tonic, leading to an habitual moderate constriction ; it can- 
not, however, be regarded by itself as conclusive evidence ; but we must not 
discuss the matter here. 
Speaking generally, then, most if not all the arteries of the body are sup- 
plied with vasomotor fibres running in this or that nerve, the fibres being 
either vaso-constrictor or vaso-dilator, and some nerves containing one kind 
of fibres only, some both in varying proportion. Almost every nerve in the 
body, therefore, may be looked upon as influencing a certain set of blood- 
vessels, as governing a vascular area, the area being large or small, and the 
government being exclusively constrictor or exclusively dilator, or mixed. 
The Course of Vaso-constrictor and Vaso-dilator Fibres. 
§ 155. Both the vaso-constrictor and the vaso-dilator fibres have their 
origin in the central nervous system, the spinal cord, or the brain, but the 
course of the two sets appears to be very different. 
In the mammal, as far as we know at present, all the vaso-constrictor 
fibres for the whole body take their origin in the middle region of the spinal 
cord, or rather leave the spinal cord by the nerves belonging to this middle 
region. Thus in the dog the vaso-constrictor fibres, not only for the trunk 
but for the limbs, head, face, and tail, leave the spinal cord by the anterior 
roots of the spinal nerves reaching from about the second dorsal to the fourth 
lumbar nerve, both inclusive. Running in the case of each nerve root to 
the mixed nerve trunk they pass along the visceral branch, white ramus- 
communicans, to the chain of splanchnic ganglia lying in the thorax and 
abdomen-the so-called thoracic and abdominal sympathetic chain (Fig. 77\ 
From these ganglia they reach their destination in various ways. Thus 
those going to the head and neck pass chiefly through the second and third 
dorsal and partly through the fourth, fifth and first dorsal nerves, thence 
upward through the annulus of Vieussens to the lower cervical ganglion,, 
and thence, as we have seen, up the cervical sympathetic. Those for the 
abdominal viscera pass off in a similar way to the abdominal splanchnic 
nerves (Fig. 77, abd. spl.f Those destined for the fore limbs pass along in the 
fourth to the ninth dorsal nerves, both inclusive, chiefly in the seventh, and 
sometimes a few in the tenth, and so reach the brachial plexus ; while those 
for the leg pass through the eleventh dorsal to the third lumbar, both inclusive, 
a few passing through the tenth dorsal and the fourth lumbar, and finally 
to the sciatic plexus. Those for the tail pass through in the first to 
third lumbar inclusive. The constrictor fibres of the skin of the trunk 
probably reach the spinal nerves in which they ultimately run in a similar 
manner. All the vaso-constrictor fibres, whatever their destination, leave 
the spinal cord by the anterior roots of spinal nerves, and then, passing 
through the appropriate visceral branches, join the thoracic or abdominal 220 
THE VASCULAR MECHANISM. 
chain of splanchnic ganglia. In these ganglia the fibres undergo a remark- 
able change. Along the anterior root and along the visceral branch they 
are medullated fibres, but long before they reach the bloodvessels for which 
they are destined they become non-medullated fibres ; they appear to lose 
their medulla in the system of splanchnic ganglia. We may add that in the 
anterior roots and along the visceral branches, white rami communicantes, 
these fibres are invariably of small diameter, not more than 1.8 m to 3.6 p.. 
§ 156. The course of the vaso-dilator fibres appears to be a somewhat 
different one, some apparently accompanying the vaso-constrictor fibres, and 
others running an independent course, though the details have as yet been 
fully worked out in the case of only a few of the fibres. It is chiefly in the 
nerves belonging to the cranial and sacral regions of the central nervous 
system, whence, as we have seen, no vaso-constrictor fibres are known to issue, 
that the course of the vaso-dilator fibres has been successfully traced. Thus 
the vaso-dilator fibres for the submaxillary gland running in the chorda 
tympani may be traced, as we have seen, back to the facial or seventh nerve ; 
and the continuation of the chorda tympani along the lingual nerve to the 
tongue contains vaso-dilator fibres for that organ; when the lingual is stim- 
ulated, the bloodvessels of the tongue dilate owing to the stimulation of the 
conjoined corda tympani fibres. The ramus tympanicus of the glosso-pharyn- 
geal nerve contains vaso-dilator fibres for the parotid gland, and it appears 
probable that the trigeminal nerve contains vaso-dilator fibres for the eye 
and nose and possibly for othei' parts. In the anterior roots of the sacral 
nerves run vaso-dilator fibres which pass into the so-called nervi erigentes, the 
nerves stimulation of which by leading to a widening of the arteries of the 
penis, brings about the erection of that organ, the effect being assisted by a 
simultaneous hindrance to the venous outflow. Though vaso-dilator fibres 
are, as we have seen, present in the nerves of the limbs, and probably also 
in those of the trunk, the investigation of their several paths is rendered 
very difficult by the concomitant presence of vaso-constrictor fibres. There 
are some reasons for thinking that the vaso-dilator fibres in these nerves pur- 
sue a direct course from the spinal cord through the anterior spinal roots, 
and thus afford a contrast with the constrictor fibres of the same nerves, 
which, as we have seen, take a roundabout course, passing into the splanch- 
nic system before they join the nerve trunk. Our information, however, 
is too imperfect to allow any very positive statement to be made. Accept- 
ing this view, however, we may say that while all the vaso-constrictor fibres, 
as far as we know, come from a particular, though considerable, part of the 
spinal cord and pass into the splanchnic system on their way to their sev- 
eral destinations, the vaso-dilator fibres arise from all parts of the spinal 
cord as well as from the medulla oblongata, and pursue a more or less direct 
course to their destination. 
Further, while the vaso-dilator fibres, as they leave the central nervous 
system, are, like the vaso-constrictor fibres, fine medullated fibres, unlike 
the vaso-constrictors they retain their medulla for the greater part of their 
course, and only lose it near their termination in the tissue whose blood- 
vessels they supply. 
Lastly, while the vaso-constrictor fibres, as in the case of the cervical 
sympathetic, of the abdominal splanchnic, and of the nerves of the skin, 
and probably in all cases, are normally in a state of moderate activity (so 
long as they remain in connection with the central nervous system), the 
moderate activity maintaining that moderate constriction which we spoke 
of above as " tone," the vaso-dilators appear to possess no such continued 
activity. Section of vaso-constrictor fibres leads to loss of tone, diminution 
of constriction, lasting, as we shall see, for some considerable time; but VASOMOTOR ACTIONS. 
221 
section of vaso-dilators, according, at all events, to most observers, does not 
lead to analogous constriction or diminution of dilatation ; all that is observed 
is a transient increase of dilatation due probably to the section acting as a 
transient stimulus to the nerve at the place of section. But before we study 
the use made by the central nervous system of vasomotor nerves, it will be 
best to consider briefly some features of 
The Effects of Vasomotor Actions. 
§157. A very little consideration will show that vasomotox' action is a 
most important factor in the circulation. In the first place the whole flow 
of blood in the body is adapted to and governed by what we may call the 
general tone of the arteries of the body at large. In a normal condition 
of the body a very large number of the minute arteries of the body are in 
a state of tonic, i. e., of moderate, contraction, and it is the narrowing due to 
this contraction which forms a large item of that peripheral resistance which 
we have seen to be one of the great factors of blood-pressure. The normal 
general blood-pressure, and, therefore, the normal flow of blood, is in fact 
dependent on the " general tone " of the minute arteries. 
In the second place local vasomotor changes in the condition of the 
minute arteries changes-i. e., of any particular vascular area-have very 
decided effects on the circulation. The changes, though local themselves, 
may have effects which are both local and general, as the following con- 
siderations will show: 
Let us suppose that the artery A is in a condition of normal tone, is mid- 
way between extreme constriction and dilatation. The flow through A is 
determined by the resistance in A, and in the vascular tract which it sup- 
plies in relation to the mean arterial pressure, which is again dependent on 
the way in which the heart is beating and on the peripheral resistance of 
all the small arteries and capillaries, A included. If, while the heart and 
the rest of the arteries remain unchanged, A be constricted, the peripheral 
resistance in A will increase, and this increase of resistance will lead to an 
increase of the general arterial pressure. Since, as we have seen (§ 108), it 
is arterial pressure which is the immediate cause of the flow from the arteries 
to the veins, this increase of arterial pressure will tend to drive more blood 
from the arteries into the veins. The constriction of A, however, by in- 
creasing the resistance, opposes any increase of the flow through A itself, in 
fact will make the flow7 through A less than before. The whole increase of 
discharge from the arterial into the venous system will take place through 
the arteries in which the resistance remains unchanged, that is, through 
channels other than A. Thus, as the result of the constriction of an artery 
there occur (1) diminished flow through the artery itself, (2) increased gen- 
eral arterial pressure, leading to (3) increased flow through the other arteries. 
If, on the other hand, A be dilated, while the heart and other arteries remain 
unchanged, the peripheral resistance in A is diminished. This leads to a 
lowering of the general arterial pressure, which in turn tends to drive less 
blood from the arteries into the veins. The dilatation of A, however, by 
diminishing the resistance, permits, even with the lowered pressure, more 
blood to pass through A itself than before. Hence the diminished flow tells 
all the more on the rest of the arteries in which the resistance remains 
unchanged. Thus, as the result of the dilatation of any artery, there occur 
(1) increased flow of blood through the artery itself, (2) diminished general 
pressure, and (3) diminished flow through the other arteries. Where the 
artery thus constricted or dilated is small, the local effect, the diminution or 
increase of flow7 through itself, is much more marked than the general effects, 222 
THE VASCULAR MECHANISM. 
the change in blood-pressure and the flow through other arteries. When, 
how'ever, the area, the arteries of which are affected, is large, the general 
effects are very striking. Thus if while a tracing of the blood-pressure is 
being taken by means of a manometer connected with the carotid artery, 
the abdominanl splanchnic nerves be divided, a conspicuous but steady fall 
of pressure is observed, very similar to but more marked than that which is 
seen in Fig. 99. The section of the abdominal splanchnic nerves causes 
the mesenteric and other abdominal arteries to dilate, and these being very 
numerous, a large amount of peripheral resistance is taken away, and the 
blood-pressure falls accordingly; a large increase of flow into the portal 
veins takes place, and the supply of blood to the face, arms, and legs is pro- 
portionately diminished. It will be observed that the dilatation of the arteries 
is not instantaneous but somewhat gradual, as shown by the pressure sinking 
not abruptly but with a gentle curve. 
The general effects on blood-pressure by vasomotor changes are so 
marked that the manometer may be used to detect vasomotor actions. 
Thus, if the stimulation of a particular nerve or any other operation leads 
to a marked rise of the mean blood-pressure, unaccompanied by any 
•changes in the heart-beat, we may infer that constriction has taken place in 
the arteries of some considerable vascular area; and similarly, if the effect 
be a fall of blood-pressure, we may infer that constriction has given way to 
■dilatation. 
Vasomotor Functions of the Central Nervous System. 
§ 158. The central nervous system, to which we have traced the vaso- 
motor nerves, makes use of these nerves to regulate the flow of blood through 
the various organs and parts of the body ; by the local effects thus produced 
it assists or otherwise influences the functional activity of this or that tissue; 
by the general effects it secures the well-being of the body. 
The use of the vaso-dilator nerves, which is more simple than that of the 
vaso-constrictors since it appears not to be complicated by the presence of 
habitual tonic influences, is frequently conspicuous as part of a reflex act. 
Thus, when food is placed in the mouth, afferent impulses, generatedin the 
nerves of taste, give rise in the central nervous system to efferent impulses, 
■which descend the chorda tympani and other nerves to the salivary glands, 
and, by dilating the bloodvessels, secure a copious flow of blood through the 
glands, while, as we shall see later on, they excite them to secrete. The 
centre of this reflex action appears to lie in the medulla oblongata, and may 
be thrown into activity not only by impulses reaching it along the specific 
nerves of taste, but also by impulses passing along other channels; thus, 
emotions started in the brain by the sight of food or otherwise may give rise 
to impulses passing down along the central nervous system itself to the 
medulla oblongata, or even in tbe stomach may send impulses up the vagus 
nerve, or stimulation of one kind or another may send impulses up almost 
any sentient nerve, and these various impulses reaching the medulla may, by 
reflex action, throw into activity the vaso-dilator fibres of the chorda tym- 
pani and other analogous nerves, and bring about a flushing of the salivary 
glands, while at the same time they cause the glands to secrete. 
The vaso-dilator fibres of the nervi erigentes may be thrown into activity 
in a similar reflex way, the centre in this case being placed in the sacral or 
lumbar portion of the spinal cord, though it is easily thrown into activity 
by impulses descending down the spinal cord from the brain that such a 
centre does exist, is shown by the fact that when in a dog the spinal cord is 
completely divided in the dorsal region, erection of the penis may readily be VASOMOTOR ACTIONS. 
223 
brought about by stimulation of the sentient surfaces. And other instances 
might be quoted in which vaso-dilator fibres appear to be connected with a 
" centre " soon after their entrance into the nervous system. 
If, as seems probable (§ 153), the bloodvessels of a muscle dilate by vaso- 
motor action whenever the muscle is thrown into contraction, either in a 
reflex or voluntary movement, the vaso-dilator fibres of the muscle would 
seem to be thrown into action by impulses arising in the spinal cord not far 
from the origin of the ordinary motor impulses and accompanying those 
motor impulses along the motor nerve. 
§ 159. The case of the vaso-constrictor fibres is somewhat more com- 
plicated on account of the existence of tonic influences ; since the same fibres 
may, on the one hand, by an increase in the impulses passing along them, 
be the means of constriction, and, on the other hand, by the removal or 
diminution of the tonic influences passing along them, be the means of dila- 
tation. We have already traced all the vaso-constrictor fibres from the 
middle region of the spinal cord to the splanchnic system in the thorax and 
abdomen, from whence they pass (1) by the abdominal splanchnic and by the 
hypogastric nerves to the viscera of the abdomen and pelvis (concerning the 
vasomotor nerves of the thoracic viscera we know at present very little) ; 
(2) by the cervical sympathetic or cervical splanchnic, as it might be called, 
to the skin of the head and neck, the salivary glands and mouth, the eyes 
and other parts, and probably the brain, including its membranes; (3) by 
the brachial and sciatic plexuses to the skin of the fore and hind limbs, and 
by various other nerves to the skin of the trunk. The chief parts of the 
body supplied by vaso-constrictor fibres appear to be the skin, with its ap- 
pendages, and the alimentary canal, with its appendages, glandular and 
other ; the great mass of skeletal muscles appears to receive an insignificant 
supply of vaso-constrictor fibres. 
If, now, in an animal, the spinal cord be divided in the lower dorsal 
region, the skin of the legs becomes flushed, their temperature frequently 
rises, and there is a certain amount of fall in the general blood-pressure as 
measured, for instance, in the carotid ; and this state of things may last for 
some considerable time. Obviously, the section of the spinal cord has cut 
off the usual tonic influences descending to the lower limbs; in consequence 
the bloodvessels have become dilated, thus causing the general peripheral 
resistance to become proportionately diminished, and the general blood- 
pressure to fall. The tonic vaso-constrictor impulses for the lower limbs, 
therefore, have their origin in the central nervous system higher up than 
the lower dorsal region of the spinal cord. 
If the spinal cord be divided between the roots of the fifth and sixth 
dorsal nerves (that is to say, at the level where the path of the splanchnic 
fibres from the cord seems to divide (see Fig. 77) those issuing above pass- 
ing upward to the fore limbs and head, and those issuing below passing to 
the abdomen and lower limbs), the cutaneous bloodvessels of the lower limbs 
dilate, as in the former case, and on examination it will be found that the 
bloodvessels of the abdomen are also largely dilated ; at the same time the 
blood-pressure undergoes a very marked fall; it may, indeed, be reduced to 
a very few millimetres of mercury. Obviously, the tonic vaso-constrictor 
impulses passing to the abdomen and to the lower limbs take origin in the 
central nervous system higher up than the level of the fifth dorsal nerve. 
If the section of the spinal cord be made above the level of the second 
dorsal nerve, in addition to the above-mentioned results the vessels of the 
head and face also become dilated ; but, in consequence of the fall of general 
blood-pressure just mentioned, these vessels never become so full of blood, 
the loss of tone is not so obvious in them as after simple division of the 224 
THE VASCULAR MECHANISM. 
cervical sympathetic, since the latter operation produces little or no effect on 
the general blood-pressure. 
Obviously, then, the tonic vaso-constrictor impulses, which passing to the 
skin and viscera of the body maintain that tonic narrowing of so many small 
arteries by which the general peripheral resistance, and so the general blood- 
pressure, is maintained, proceed from some part of the central nervous 
system higher up than the upper dorsal region of the spinal cord. And, 
since exactly the same results follow upon section of the spinal cord in the 
cervical region right up to the lower limit of the medulla oblongata, we infer 
that these tonic impulses proceed from the medulla oblongata. 
On the other hand, we may remove the whole of the brain right down to 
the upper parts of the medulla, and yet produce no flushing, or only a slight 
transient flushing, of any part of the body, and no fall at all, or only a slight 
transient fall, of the general blood-pressure. We, therefore, seem justified 
in assuming the existence in the medulla oblongata of a nervous centre, 
which we may speak of as a vasomotor centre, or the medullary vasomotor 
centre, from which proceed tonic vaso-constrictor impulses, or which regu- 
lates the emission and distribution of such tonic vaso-constrictor impulses or 
influences over various parts of the body. 
§ 160. The existence of this vasomotor centre may, moreover, be shown 
in another way. The extent or amount of the tonic constrictor impulses 
proceeding from it may be increased or diminished, the activity of the centre 
may be augmented or inhibited by impulses reaching it along various afferent 
nerves ; and provided no marked changes in the heart-beat take place at the 
same time, a rise or fall of general blood-pressure may be taken as a token 
of an increase or decrease of the activity of the centre. 
In the rabbit there is found in the neck, lying side by side with the 
cervical sympathetic nerve and running for some distance in company with 
it, a slender nerve which may be ultimately traced down to the heart, and 
which if traced upward is found to come off somewhat high up from the 
vagus, by two or more roots, one of which is generally a branch of the 
superior laryngeal nerve. This nerve (the fibres constituting which are, in 
the dog, bound up with the vagus, and do not form an independent nerve) 
appears to be exclusively an afferent nerve ; when, after division of the nerve 
the peripheral end, the end still in connection with the heart, is stimulated, 
no marked results follow. The beginnings of the nerve in the heart are 
therefore quite different from the endings of the inhibitory fibres of the 
vagus, or of the augmentor fibres of the splanchnic (sympathetic) system ; 
the nerve has nothing to do with the nervous regulation of the heart (see 
139 et seq.). If, now, while the pressure in an artery such as the carotid is 
being registered, the central end of the nerve-i. e., the one connected with 
the brain-be stimulated with the interrupted current, a gradual but marked 
fall of pressure (Fig. 79) in the carotid is observed, lasting, when the period 
of stimulation is short, some time after the removal of the stimulus. Since 
the beat of the heart is not markedly changed, the fall of pressure must be 
due to the diminution of peripheral resistance occasioned by the dilatation 
of some arteries. And it is probable that the arteries thus dilated are 
chiefly, if not exclusively, those arteries of the abdominal viscera which are 
governed by the abdominal splanchnic nerves; for if these nerves are 
divided on both sides previous to the experiment, the fall of pressure when 
the nerve is stimulated is very small-in fact, almost insignificant. The 
inference we draw is as follows : The afferent impulses, passing upward along 
the nerve in question, have so affected some part of the central nervous 
system that the influences which, in a normal condition of things, passing 
along the abdominal splanchnic nerves keep the minute arteries of the VASOMOTOR ACTIONS. 
225 
abdominal viscera in a state of moderate tonic constriction, fail altogether, 
and those arteries in consequence dilate just as they do when the abdominal 
Fig. 79. 
Tracing showing the Effect on Blood-pressure of Stimulating the Central End of the Depressor 
Nerve in the Rabbit. On the time-marker below the intervals correspond to seconds. At x an 
interrupted current was thrown into the nerve. 
splanchnic nerves are divided, the effect being possibly increased by the 
similar dilatation of other vascular areas. Since stimulation of the nerve 
of which we are speaking always produces a fall, never a rise, of blood-pres- 
sure-the amount of fall, of course, being dependent on circumstances, such 
as the condition of the nervous system, state of blood-pressure, etc.-the 
nerve is known by the name of the depressor nerve. As we shall point out 
later on, by means of this afferent nerve from the heart the peripheral resist- 
ance is, in the living body, lowered to suit the weakened powers of a laboring 
heart. 
This gradual lowering of blood-pressure by diminution of peripheral 
resistance affords a marked contrast to the sudden lowering of blood-pres- 
sure by cardiac inhibition. (Compare Fig. 79 with Fig. 75.) 
§ 161. But the general blood-pressure may be modified by afferent im- 
pulses passing along other nerves than the depressor, the modification 
taking on, according to circumstances, the form either of decrease or of 
increase. 
Thus, if, in an animal placed under the influence of urari (some anaes- 
thetic other than chloral, etc., being used), the central stump of the divided 
sciatic nerve be stimulated, an increase of blood-pressure (Fig. 80) almost 
Fig. 80. 
Effect on Blood-pressure Curve of Stimulating Sciatic Nerve under Urari. (Cat.) x marks 
the moment in which the current was thrown into the nerve. Artificial respiration was car- 
ried on, and the usual respiratory undulations are absent. 
exactly the reverse of the decrease brought about by stimulating the de- 
pressor, is observed. The curve of the blood-pressure, after a latent period 
during which no changes are visible, rises steadily without any corre- 
sponding change in the heart's beat, reaches a maximum, and after a 
15 226 
THE VASCULAR MECHANISM. 
while slowly falls again, the fall sometimes beginning to appear before the 
stimulus has been removed. There can be no doubt that the rise of pres- 
sure is due to the constriction of certain arteries; the arteries in question 
being those of the abdominal splanchnic area certainly, and possibly those 
of other vascular areas as well. The effect is not confined to the sciatic ; 
stimulation of any nerve containing afferent fibres may produce the same 
rise of pressure, and so constant is the result that the experiment has been 
made use of as a method of determining the existence of afferent fibres in 
any given nerve and even the paths of centripetal impulses through the 
spinal cord. 
If, on the other hand, the animal be under the influence not, of urari, 
but of a large dose of chloral, instead of a rise of blood-pressure, a fall 
quite similar to that caused by stimulating the depressor is observed when 
an afferent nerve is stimulated. The condition of the central nervous 
system seems to determine whether the effect of afferent impulses on the 
central nervous system is one leading to an augmentation of vaso-constrictor 
impulses and so to a rise, or one leading to a diminution of vaso-constrictor 
impulses and so to a fall of blood-pressure. 
§ 162. We have used the words " central nervous system " in speaking 
of the above ; we have evidence, however, that the part of the central nervous 
system acted on by the afferent impulses is the vasomotor centre in the 
medulla oblongata, and that the effects in the way of diminution (depressor) 
or of augmentation (pressor) are the results of afferent impulses inhibiting 
or augmenting the tonic activity of this centre or of a part of this centre 
especially connected with abdominal splanchnic nerves. The whole brain may 
be removed right down to the medulla oblongata, and yet the effects of stimu- 
lation in the direction either of diminution or of augmentation may still be 
brought about. If the medulla oblongata be removed, these effects vanish 
too, though all the rest of the nervous system be left intact. Nay, more, by 
partially interfering with the medulla oblongata, we may partially diminish 
these effects and thus mark out, so to speak, the limits of the centre in ques- 
tion within the medulla itself. Thus, in an intact animal under urari, stimu- 
lation of the sciatic nerve with a stimulus of a certain strength will produce 
a rise of blood-pressure up to a certain extent. After removal of the whole 
brain right down to the medulla oblongata, the same stimulation will produce 
the same rise as before; the vasomotor centre has not been interfered with. 
Directly, however, in proceeding downward, the region of the centre in ques- 
tion is reached, stimulation of the sciatic produces less and less rise, until at 
last when the lower limit of the centre is arrived at no effect at all on blood- 
pressure can be produced by even strong stimulation of the sciatic or other 
afferent nerve. In this way the lower limit of the medullary vasomotor 
centre has been determined in the rabbit at a horizontal line drawn about 
4 or 5 mm. above the point of the calamus scriptorius, and the upper limit 
at about 4 mm. higher up-i. e., about 1 or 2 mm. below the corpora quadri- 
gemina. When transverse sections of the brain are carried successively 
lower and lower down, an effect on blood-pressure in the way of lowering it 
and also of diminishing the rise of blood-pressure resulting from stimulation 
of the sciatic, is first observed when the upper limit is reached. On carrying 
the sections still lower, the effects of stimulating the sciatic become less and 
less, until when the lower limit is reached no effects at all are observed. The 
centre appears to be bilateral, the halves being placed not in the middle line, 
but more sideways and rather nearer the anterior than the posterior surface. 
It may perhaps be more closely defined as a small prismatic space in the 
forward prolongation of the lateral columns after they have given off their 
fibres to the decussating pyramids. This space is largely occupied by a mass VASOMOTOR ACTIONS. 
227 
of gray matter, called by Clarke the antero-lateral nucleus, and containing 
large multipolar cells ; but it is by no means certain that this group of nerve 
cells really acts as the centre in question. 
§ 163. The above experiments appear to afford adequate evidence that, in 
a normal state of the body, the integrity of the medullary vasomotor centre 
is essential to the production and distribution of those continued constrictor 
impulses by which the general arterial tone of the body is maintained, and 
that an increase or decrease of vaso-constrictor action in particular arteries, 
or in the arteries generally, is brought about by means of the same medullary 
vasomotor centre. But we must not, therefore, conclude that this small por- 
tion of the medulla oblongata is the only part of the central nervous system 
which can act as a centre for vaso-constrictor fibres ; and, as we have seen, 
there is no evidence at present that the vaso-dilator fibres are connected 
with either this or any other one centre. In the frog reflex vasomotor effects 
may be obtained by stimulating various afferent nerves after the whole 
medulla has been removed, and, indeed, even when only a comparatively 
small portion of the spinal cord has been left intact and connected, on the 
one hand, with the afferent nerve which is being stimulated, and, on the 
other, with the efferent nerves in which run the vasomotor fibres whose action 
is being studied. In the mammal such effects do not so readily appear, but 
may with care and under special conditions be obtained. Thus in the dog, 
when the spinal cord is divided in the dorsal region, the arteries of the hind 
limbs and hinder part of the body, as we have already said (§ 158), become 
dilated. This one would naturally expect as the result of their severance 
from the medullary vasomotor centre. But if the animal be kept in good 
condition for some time, a normal or nearly normal arterial tone is after a 
while re-established ; and the tone thus regained may, by afferent impulses 
reaching the cord below the section, be modified in the direction certainly 
of diminution-i. e., dilatation-and possibly, but this is by no means so cer- 
tain, of increase-i. e., constriction. Dilatation of various cutaneous vessels 
of the limbs may be readily produced by stimulation of the central stump 
of one or another nerve. 
These remarkable results, which, though they are most striking in connec- 
tion with the lower part of the spinal cord, hold good apparently for other 
parts also of the spinal core, naturally suggest a doubt whether the explana- 
tion just given above of the effects of section of the medulla oblongata is a 
valid one. When we come to study the central nervous system, we shall 
again and again see that the immediate effect of operative interference with 
these delicate structures is a temporary suspension of nearly all their func- 
tions. This is often spoken of as "shock" and may be regarded as an 
extreme form of inhibition. An example of it occurs in the above experi- 
ment of section of the dorsal cord. For some time after the operation the 
vaso-dilator nervi erigentes (which, as far as we know, have no special connec- 
tion with the medullary vasomotor centre) cannot be thrown into activity as 
part of a reflex action ; their centre remains for some time inactive. After 
a while, however, it recovers, and erection of the penis through the nervi 
erigentes may then still be brought about by suitable stimulation of sensory 
surfaces. Hence the question may fairly be put whether the effects of cut- 
ting and injuring the structures which we have spoken of as the medullary 
vasomotor centre, are not in reality simply those of shock, whether the vas- 
cular dilatation which follows upon sections of the so-called medullary vaso- 
motor centre, does not come about because section of or injury to this region 
exercises a strong inhibitory influence on all the vasomotor centres situated 
in the spinal cord below. Owing to the special function of the medulla 
oblongata in carrying on the all-important work of respiration, a mammal 228 
THE VASCULAR MECHANISM. 
whose medulla has been divided cannot be kept alive for any length of time. 
We cannot, therefore, put the matter to the simple experimental test of extir- 
pating the supposed medullary vasomotor centre and seeing what happens 
when the animal has completely recovered from the effects of the operation ; 
we have to be guided in our decision by more or less indirect arguments. 
And against the argument that the effects are those of shock, we may put 
the argument, evidence for which we shall meet with in dealing with the 
central nervous system, that when one part of the central nervous system is 
removed or in any way placed hors de combat, another part may vicariously 
take on its function ; in the absence of the medullary vasomotor centre, its 
function may be performed by other parts of the spinal cord which in its 
presence do no such work. 
And we may, in connection with this, call attention to the fact that the 
dilatation or loss of tone which follows upon section of the cervical sympathetic 
(and the same is true of the abdominal splanchnic) is not always, though it 
may be sometimes, permanent; in a certain number of cases it has been 
found that after a while, it may not be until after several days, the dilatation 
disappears and the arteries regain their usual calibre; on the other hand, in 
some cases no such return has been observed after months or even years. 
This recovery, when it occurs, cannot always be attributed to any regenera- 
tion of vasomotor fibres in the sympathetic, for it is stated to have been 
observed when the whole length of the nerve including the superior cervical 
ganglion has been removed. When recovery of tone has thus taken place, 
dilatation or increased constriction may be occasioned by local treatment; the 
ear may be made to blush or pale by the application of heat or cold, by 
gentle stroking or rough handling and the like ; but neither the one nor the 
other condition can be brought about by the intervention of the central 
nervous system. So also the spontaneous rhythmic variations in the calibre 
of the arteries of the ear of which we speak, though they cease for a time 
after division of the cervical sympathetic, may in some cases eventually reap- 
pear and that even if the superior cervical ganglion be removed; in other 
cases they do not. And the analogous rhythmic variations of the veins of 
the bat's wing have been proved experimentally to go on vigorously when 
all connection with the central nervous system has been severed ; they may 
continue, in fact, in isolated pieces of the wing provided that the vessels are 
adequately filled and distended with blood or fluid. From these and other 
facts, even after making allowance for the negative cases, we may conclude 
that what we have spoken of as the tone of the vessels of the face, though 
influenced by and in a measure dependent on the central nervous system, 
is not simply the result of an effort of that system. The muscular walls of 
the arteries are not mere passive instruments worked by the central nervous 
system through the vasomotor fibres; they appear to have an intrinsic tone 
of their own, and it seems natural to suppose that when the central nervous 
system causes dilatation or constriction of the vessels of the face, it makes use, 
in so doing, of this intrinsic local tone. It has been supposed that this 
intrinsic tone is dependent on some local nervous mechanism ; in the ear at 
least no such mechanism has yet been found ; and, indeed, as we have said 
above (§ 176), no such peripheral nervous mechanism is really necessary. In 
the case both of a vessel governed by vaso-dilator fibres and one governed 
by vaso-constrictor fibres, we may suppose a certain natural condition of the 
muscular fibres which we may call a condition of equilibrium. In a vessel 
governed only by vaso-dilator fibres, if there be such, this condition of 
equilibrium is the permanent condition of the muscular fibre, from which it 
is disturbed by vaso-dilator impulses, but to which it speedily returns. In 
a vessel governed by vaso-constrictor fibres, and subject to tone, the muscu- VASOMOTOR ACTIONS. 
229 
lar fibre is habitually kept on the constrictor side of this equilibrium, and, 
as in the cases quoted above, may strive of itself toward some amount of 
active constriction even when separated from the central nervous system. 
But to return to the medullary vasomotor centre. Without attempting 
to discuss the matter fully, we may say that, after all due weight has been 
attached to the play of inhibitory impulses or " shock " as a result of opera- 
tive interference, there still remains a balance of evidence in favor of the 
view that the region of the medulla of which we are speaking does really 
act as a general vasomotor centre in the manner previously explained, and 
plays an important part in the vasomotor regulation of the living body. 
It is not, however, to be regarded as the single vasomotor centre, whence 
alone can issue tonic-constrictor impulses or whither afferent impulses from 
all parts of the body must always travel before they can affect the vaso- 
motor impulses passing along this or that nerve. We are rather to suppose 
that the spinal cord along its whole length contains, interlaced with the 
reflex and other mechanisms by which the skeletal muscles are governed, 
vasomotor centres and mechanisms of varied complexity, the details of 
whose functions and topography have yet largely to be worked out; and 
though, as we have seen, the medullary centre is essentially a centre of im- 
pulses issuing along vaso-constrictor fibres, it is possible that there are ties 
between it and vaso-dilator fibres also. As in the absence of the sinus 
venosus, the auricles and ventricle of the frog's heart may still continue to 
beat, so in the absence of the medulla oblongata these spinal vasomotor 
centres provide for the vascular emergencies which arise. As, however, in 
the normal entire frog's heart, the sinus, so to speak, gives the word and 
governs the work of the whole organ, so the medullary vasomotor centre 
rules and co-ordinates the lesser centres of the cord, and through them pre- 
sides over the chief vascular areas of the body. By means of these vaso- 
motor central mechanisms, by means of the head centre in the medulla, and 
the subsidiary centres in the spinal cord, the delicate machinery of the circu- 
lation which determines the blood-supply, and so the activity of each tissue 
and organ, is able to respond by narrowing or widening arteries to the ever- 
varying demands and to meet by compensating changes the shocks and 
strains of daily life. 
§ 164. We may sum up the history of vasomotor actions somewhat as 
follows: 
All, or nearly all-or, as far as we know, all-the arteries of the body are 
connected with the central nervous system by nerve fibres, called vaso- 
motor fibres, the action of which varies the amount of contraction of the 
muscular coats of the arteries, and so leads to changes in calibre. The 
action of these vasomotor fibres is more manifest and probably more 
important in the case of small and minute arteries than in the case of 
larger ones. 
These vasomotor fibres are of two kinds: the one kind, vaso-constrictor 
fibres, are of such a nature or have such connections at their central origin 
or peripheral endings that stimulation of them produces narrowing, con- 
striction of the arteries; and during life these fibres appear to be the means 
by which the central nervous system exerts a continued tonic influence on 
the arteries and maintains an arterial " tone." The other kind, the vaso- 
dilator fibres, are of such a kind or have such connections that stimulation 
of them produces widening, dilation of the arteries. There is no adequate 
evidence that these vaso-dilator fibres serve as channels for tonic dilating 
impulses or influences. 
The vaso-constrictor fibres leave the spinal cord by the anterior roots of 
the nerves coming from middle regions only of the spinal cord (in the dog. 230 
THE VASCULAR MECHANISM. 
and probably in other mammals from about the first dorsal to the fifth 
lumbar nerve), pass into the splanchnic ganglia connected with those nerves 
(thoracic and abdominal chain of sympathetic ganglia), where the fibres lose 
their medulla, and proceed to their destination as non-medullated fibres, 
either still in so-called sympathetic nerves, such as splanchnic, cervical 
sympathetic, hypogastric, etc., or along recurrent branches of the splanchnic 
system, to join the spinal nerves of the arm, leg, and trunk. 
In the intact organism the emission and distribution along these vaso- 
constrictor fibres of tonic-constrictor impulses, by which general and local 
arterial tone is maintained and regulated, is governed by a limited portion 
of the medulla oblongata known as the medullary vasomotor centre; and 
when some change of conditions or other natural stimulus brings about a 
change in the activity of the vaso-constrictor fibres of one or more vascular 
areas, or of all the arteries supplied with vaso-constrictor fibres, this same 
medullary vasomotor centre appears in such cases to play the part of a 
centre of reflex action. Nevertheless, in cases where the nervous connections 
of this medullary vasomotor centre with a vascular area are cut off by an 
operation, as by section of the cord, other parts of the spinal cord may act 
as centres for the vaso-constrictor fibres of the area, and possibly these 
subordinate centres may be to a certain extent in action in the intact 
organism. 
The vaso-dilator fibres appear to take origin in various parts of the 
central nervous system and to proceed in a direct course to their destination 
along the (anterior) roots and as part of the trunks and branches of various 
cerebro-spinal nerves; they do not lose their medulla until they approach 
their termination. They do not appear to serve as channels of tonic dilating 
influences; they are thrown into action generally as part of a reflex action, 
and their centre in the reflex act appears in each case to lie in the central 
nervous system not far from the centre of the ordinary motor fibres which 
they accompany. 
The effects of the activity of the vaso-dilator fibres appear to be essentially 
local in nature; when any set of them come into action the vascular area 
which these govern is dilated. And the vascular areas so governed are 
relatively so small that changes in them produce little or no effect on the 
vascular system in general. 
The effects of changes in the activity of the vaso-constrictor fibres are 
both local and general, and may be also double in nature. By an inhibition 
of tonic-constrictor impulses a certain amount of dilation may be effected; 
by an augmentation of constrictor impulses, constriction, it may be of con- 
siderable extent, may be brought about. When the vascular area so affected 
is small, the effects are local, more or less blood is distributed through the 
area; when the vascular area affected is large, the inhibition of constriction 
may lead to a marked fall, and an augmentation of constriction to a marked 
rise of general blood-pressure. 
§ 165. We shall have occasion later on again and again to point out 
instances of the effects of vasomotor action, both local and general, but we 
may here quote one or two characteristic ones. " Blushing" is one. Ner- 
vous impulses started in some parts of the brain by an emotion produce a 
powerful inhibition of that part of the medullary vasomotor centre which 
governs the vascular areas of the head supplied by the cervical sympathetic, 
and hence has an effect on the vasomotor fibres of the cervical sympathetic 
almost exactly the same as that produced by section of the nerve. In conse- 
quence the muscular walls of the arteries of the head and face relax, the 
arteries dilate, and the whole region becomes suffused. Sometimes an emo- 
tion gives rise not to blushing, but to the opposite effect, viz., to pallor. THE CAPILLARY CIRCULATION. 
231 
In a great number of cases this has quite a different cause, being due to a 
sudden diminution or even temporary arrest of the heart's beats; but in 
some cases it may occur without any change in the beat of the heart, and is 
then due to a condition the very converse of that of blushing, that is, to an 
increased arterial constriction ; and this increased constriction, like the dila- 
tation of blushing, is effected through the agency of the central nervous 
system and the cervical sympathetic. 
The vascular condition of the skin at large affords another instance. 
When the temperature of the air is low the vessels of the skin are con- 
stricted and the skin is pale; when the temperature of the air is high the 
vessels of the skin are dilated and the skin is red and flushed. In both 
these cases the effect is mainly a reflex one, it being the central nervous 
system which brings about augmentation of constriction in the one case and 
inhibition in the other, though possibly some slight effect is reproduced by 
the direct action of the cold or heat on the vessels of the skin simply. More- 
over, the vascular changes in the skin are accompanied by corresponding 
vascular changes in the viscera (chiefly abdominal) of a reverse kind. When 
the vessels of the skin are dilated those of the viscera are constricted, and 
vice versa, so that a considerable portion of the whole blood ebbs and flows, 
so to speak, according to circumstances from skin to viscera and from viscera 
to skin. By these changes, as we shall see later on, the maintenance of the 
normal temperature of the body is in large measure secured. 
When food is placed in the mouth the bloodvessels of the salivary glands, 
as we have seen, are flushed with blood as an adjuvant to the secretion of 
digestive fluid ; and as the food passes along the alimentary canal, each 
section in turn, with the glandular appendages belonging to it, welcomes its 
advent by flushing with blood, the dilatation being sometimes, as in the case 
of the salivary gland, the result of the activity chiefly of vaso-dilator fibres, 
but sometimes the result of the cessation of constrictor impulses and some- 
times the result of the two combined. So also when the kidney secretes 
urine, its vessels become dilated, and in general, wherever functional activity 
comes into play, the metabolism of tissue which is the basis of that activity 
is assisted by a more generous flow of blood through the tissue. 
§ 166. Vasomotor nerves of the veins. Although the veins are provided 
with muscular fibres and are distinctly contractile, and although rhythmic 
variations of calibre due to contractions may be seen in the great veins 
opening into the heart, in the veins of the bat's wing, and elsewhere, and 
similar rhythmic variations, also possibly due to active rhythmic contrac- 
tions, but possibly also of an entirely passive nature, have been observed in 
the portal veins, very little is known of any nervous arrangements govern- 
ing the veins. When in the frog the brain and spinal cord are destroyed, 
very little blood comes back to the heart as compared with the normal 
supply, and the heart in consequence appears almost bloodless and beats 
feebly. This has been, by some, regarded as more than can be accounted for 
by mere loss of arterial tone, and accordingly interpreted as indicating the 
existence of a normal tone in the veins dependent on the central nervous 
system. When the latter is destroyed, the veins become abnormally distended 
and a large quantity of blood becomes lodged and hidden, as it were, in them. 
The Capillary Circulation. 
§ 167. We have already some time back (§ 106) mentioned some of the 
salient features of the circulation through the capillaries, viz., the difficult 
passage of the corpuscles (generally in single file, though sometimes in the 
larger channels two or more abreast) and plasma through the narrow chan- 232 
THE VASCULAR MECHANISM. 
nels, in a stream which though more or less irregular is steady and even, 
not broken by pulsations, and slower than that in either the arteries or the 
veins. We have further seen that the capillaries vary very much in width 
from time to time; and there can be no doubt that the changes in their 
calibre are chiefly of a passive nature. They are expanded when a large 
supply of blood reaches them through the supplying arteries, and, by virtue 
of their elasticity, shrink again when the supply is lessened or withdrawn ; 
they may also become expanded by an obstacle to the venous outflow. 
On the other hand, as we have also stated, there is a certain amount of 
evidence that, in young animals at all events, the calibre of a capillary 
canal may vary, quite independently of the arterial supply or the venous 
outflow, in consequence of changes in the form of the epithelioid cells, allied 
to the changes which in a muscle-fibre or muscle-cell constitute a contrac- 
tion ; and though the matter requires further investigation, it is possible that 
these active changes play an important part in determining the quantity of 
blood passing through a capillary area; but there is as yet no satisfactory 
evidence that they, like the corresponding changes in the arteries, are 
governed by the nervous system. 
Over and above these changes of form, the capillaries and minute vessels 
are subject to changes and exert influences by virtue of which they play an 
important part in the work of the circulation. Their condition determines 
the amount of resistance offered by their channels to the flow of blood 
through those channels, and determines the amount and character of that 
interchange between the blood and the tissues which is the main fact of the 
circulation. 
If the web of the frog's foot, or, better still, if some transparent tissue of 
a mammal be watched under the microscope, it will be observed that, while 
in the small capillaries the corpuscles are pressed through the channel in 
single file, one after the other, each corpuscle as it passes occupying the 
whole bore of the capillary, in the larger capillaries (of the mammal), and 
especially in the small arteries and veins which permit the passage of more 
than one corpuscle abreast, the red corpuscles run in the middle of the 
channel, forming a colored core, between which and the sides of the vessels 
all around is a colorless layer, containing no red corpuscles, called the 
" plasmatic layer " or " peripheral zone." This division into a peripheral 
zone and an axial stream is due to the fact that in any stream passing through 
a closed channel the friction is greatest at the sides, and diminishes toward 
the axis. The corpuscles pass where the friction is least, in the axis. A 
quite similar axial core is seen when any fine particles are driven with a 
sufficient velocity in a stream of fluid through a narrow tube. As the veloc- 
ity is diminished the axial core becomes less marked and disappears. 
In the peripheral zone, especially in that of the veins, are frequently 
seen white corpuscles, sometimes clinging to the sides of the vessel, some- 
times rolling slowly along, and in general moving irregularly, stopping for 
a while and then suddenly moving on. The greater the velocity of the flow 
of blood, the fewer the white corpuscles in the peripheral zone, and with a 
very rapid flow they, as well as the red corpuscles, may be all confined to 
the axial stream. The presence of the white corpuscles in the peripheral 
zone has been attributed to their being specially lighter than the red corpus- 
cles, since when fine particles of two kinds, one lighter than the other, are 
driven through a narrow tube, the heavier particles flow in the axis and the 
lighter in the more peripheral portions of the stream. But, besides this, the 
white corpuscles have a greater tendency to adhere to surfaces than have 
the red, as is seen by the manner in which the former become fixed to the 
glass slide and cover-slip when a drop of blood is mounted for microscopical THE CAPILLARY CIRCULATION. 
233 
examination.. They probably thus adhere by virtue of the amoeboid move- 
ments of their protoplasm, so that the adhesion is to be considered not so 
much a mere physical as a physiological process, and hence may be expected 
to vary with the varying nutritive conditions of the corpuscles and of the 
bloodvessels. Thus while the appearance of the white corpuscles iu the 
peripheral zone may be due to their lightness, their temporary attachment 
to the sides of the vessels and characteristic progression is the result of their 
power to adhere ; and as we shall presently see, their amoeboid movements 
may carry them on beyond mere adhesion. 
§ 168. These are the phenomena of the normal circulation, and may be 
regarded as indicating a state of equilibrium between the blood on the one 
hand and the bloodvessels with the tissues on the other; but a different state 
of things sets in when that equilibrium is overthrown by causes leading to 
what is called inflammation or to allied conditions. 
If an irritant, such as a drop of chloroform or a little diluted oil of 
mustard, be applied to a small portion of a frog's web, tongue, mesen- 
tery, or some other transparent tissue, the following changes may be ob- 
served under the microscope; they may also be seen in the mesentery or 
other transparent tissue of a mammal. The first effect that is noticed is a 
dilatation of the arteries, accompanied by a quickening of the stream. The 
irritant, probably by a direct action on the muscular fibres of the arteries, 
has led to a relaxation of the muscular coat and hence to a widening; and 
we have already (§ 112), explained how such a widening in a small artery 
may lead to a temporary thickening of the stream. In consequence of the 
greater flow through the arteries, the capillaries become filled with corpus- 
cles, and many passages, previously invisible or nearly so on account of 
their containing no corpuscles, now come into view. The veins at the 
same time appear enlarged and full. If the stimulus be very slight, this 
may all pass away, the arteries gaining their normal constriction and the 
capillaries and veins returning to their normal condition ; in other words, 
the effect of the stimulus in such a case is simply a temporary blush. 
Unless, however, the chloroform or mustard be applied with especial care 
the effects are much more profound, and a series of remarkable changes 
sets in. 
In the normal circulation, as we have just said, white corpuscles may 
be seen in the peripheral, plasmatic zone, but they are scanty in number, 
and each one after staying for a little time in one spot suddenly gets free, 
sometimes almost by a jerk, as it were, and then rolls on for a greater or 
less distance. In the area now under consideration a large number of white 
corpuscles soon gather in the peripheral zones, especially of the veins and 
venous capillaries (that is, of the larger capillaries which are joining to 
form veins), but also, to a less extent, of the arteries ; and this takes place 
although the vessels still remain dilated and the stream still continues 
rapid though not so rapid as at first. Each white corpuscle appears to ex- 
hibit a greater tendency to stick to the sides of the vessels, and though 
driven away from the arteries by the stronger arterial stream, becomes 
lodged, so to speak, in the veins. Since new white corpuscles are continu- 
ally being brought by the blood-stream on to the scene, the number of them 
in the peripheral zones of the veins increases more and more, and this may 
go on until the inner surface of the veins and venous capillaries appears 
to be lined with a layer of white corpuscles. The small capillaries, too, 
contain more white corpuscles than usual, and even in the arteries these 
are abundant, though not forming the distinct layer seen in the veins. The 
white corpuscles, however, are not the only bodies present in the peripheral 
zone. Though in the normal circulation blood-platelets (see § 33) cannot 234 
THE VASCULAR MECHANISM. 
be seen in the peripheral zone, and hence must be confined (on the view, 
which has the greater support, that these bodies are really present in quite 
normal blood) to the axial stream, they make their appearance in that 
zone with the changes which we are now describing. Indeed, in many cases 
they are far more abundant than the white corpuscles, the latter appearing 
imbedded at intervals in masses of the former. Soon after their appearance 
the individual platelets lose their outline and run together into formless 
masses. 
§ 169. This much, the appearance of numerous white corpuscles and 
platelets in the peripheral zones may take place while the stream, though 
less rapid than at the very first, still remains rapid ; so rapid at all events 
that, owing to the increased width of the passages, in spite of the obstruc- 
tion offered by the adherent white corpuscles, the total quantity of blood 
flowing in a given time through the inflamed area is greater than normal. 
But soon, though the vessels still remain dilated, the stream is observed 
most distinctly to slacken and then a remarkable phenomenon makes its 
appearance. The white corpuscles lying in contact with the walls of the 
veins or of the capillaries are seen to thrust processes through the walls ; 
and, the process of a corpuscle increasing at the expense of the rest of the 
body of the corpuscle, the whole corpuscle, by what appears to be an ex- 
ample of amoeboid movement, makes its way through the wall of the ves- 
sel into the lymph space outside ; the perforation appears to take place in 
the cement substance joining the epithelioid plates together. This is the 
migration of the white corpuscles to which we alluded in § 32, and takes 
place chiefly in the veins and capillaries, not at all or to a very slight ex- 
tent in the arteries. Through this migration the lymph spaces around the 
vessels in the inflamed area become crowded with white corpuscles. At the 
same time the lymph in the same spaces not only increases in amount but 
changes somewhat in its chemical characters; it becomes more distinctly 
and readily coagulable, and is sometimes spoken of as " exudation fluid," 
or by the older writers as " coagulable lymph." This turgescence of the 
lymph spaces, together with the dilated, crowded condition of the blood- 
vessels, gives rise to the swelling which is one of the features of inflam- 
mation. 
If the inflammation now passes off the white corpuscles cease to emi- 
grate, cease to stick for any length of time to the sides of the vessels, the 
stream of blood through the vessels quickens again, and the vessels them- 
selves, though they may remain for a long time dilated, eventually regain 
their calibre, and a normal circulation is re-established. The migrated cor- 
puscles move away from the region, along the labyrinth of lymph spaces, 
and the surplus lymph also passes away along the lymph spaces and lym- 
phatic vessels. 
$ 170. The condition of things, however, instead of passing off may go 
on to a further stage. More and more white corpuscles, arrested in their 
passage, crowd the channels and block the way, so that though the vessels 
remain dilated the stream becomes slower and slower, until at last it stops 
altogether and " stagnation " or " stasis " sets in. The red corpuscles are 
driven in, often in masses, among the white corpuscles and platelets, the 
distinction between axial stream and peripheral zone becoming lost; and 
arteries, veins, and capillaries, all distended, sometimes enormously so, are 
filled with a mass of mingled red and white corpuscles and platelets. 
When actual stagnation occurs the red corpuscles run together so that 
their outlines are no longer distinguishable; they appear to become 
fused into a homogeneous red mass. And it may now be observed that, 
not only white corpuscles but also red corpuscles make their way through THE CAPILLARY CIRCULATION. 
235 
the distended and altered walls of the capillaries, chiefly, at all events, at 
the junctions of the epithelioid plates, into the lymph spaces beyond. This 
is spoken of as the diapedesis of the red corpuscles. 
This latter " stagnation " stage of inflammation may be the prelude to 
further mischief and indeed to the death of the inflamed tissue, but it, too, 
like the earlier stages, may pass away. As it passes away the outlines of 
the corpuscles become once more distinct, those on the venous side of the 
block gradually drop away into the neighboring currents-little by little 
the whole obstruction is removed, and the current through the area is 
re-established. 
The slowing and final arrest of the blood current described above is not 
due to any lessening of the heart's beat; the arterial pulsations, or at least 
the arterial flow, may be seen to be continued in full force down to the 
affected area, and there to cease very suddenly. It is not due to any con- 
striction of the small arteries increasing the peripheral resistance, for these 
continue dilated, sometimes exceedingly so. It must, therefore, be due to 
some new and unusual resistance occurring in the area itself, and there can 
be no doubt that this is to be found in an increased tendency of the 
corpuscles, especially of the white corpuscles, to stick to the sides of the 
vessels. The increase of adhesiveness is not caused by any change con- 
fined to the corpuscles themselves; for if, after a temporary delay, one 
set of corpuscles has managed to pass away from the affected area, the 
next set of corpuscles brought to the area in the blood-stream is sub- 
jected to the same delay and the same apparent fusion. The cause of 
the increased adhesiveness must, therefore, lie in the walls of the blood- 
vessels or in the tissue of which these form a part. That the increased 
adhesion is due to the vascular walls and not primarily to the corpuscles 
themselves is further shown by the fact that if in the frog, an artificial 
blood of normal saline solution to which milk has been added be substituted 
for normal blood, a stasis may by irritants be induced in which oil-globules 
play the part of corpuscles, and by their aggregation bring about an arrest 
of the flow. 
We are driven to conclude that there exist in health certain relations 
between the blood, on the one hand, and the walls of the vessel on the other, 
by which the tendency of the corpuscles to adhere to the bloodvessels is kept 
within certain limits; these relations consequently determine the normal 
flow, with its axial stream and peripheral zone, and the normal amount of 
peripheral resistance; in inflammations these relations, in a manner we 
cannot as yet fully explain, are disturbed so that the tendency of the 
corpuscles to adhere to the sides of the vessel is largely and progressively 
increased. Hence the tarrying of the corpuscles in spite of the widening 
of their path, and finally their agglomeration and fusion in the distended 
channels. 
We may add that the changes occurring in the vascular walls also at least 
facilitate the migration of the corpuscles, and modify the passage from the 
blood to the tissue of the fluid parts of the blood, the lymph of inflamed 
areas being richer in proteids than normal lymph. 
We must not, however, pursue this subject of inflammation any further. 
We have said enough to show that the peripheral resistance (and consequently 
all that depends on that peripheral resistance) is not wholly determined by the 
varying width of the minute passages, but is also dependent on the vital 
condition of the tissue of which the walls of the passage form a part. When 
the tissue is in health, a certain resistance is offered to the passage of blood 
through the capillaries and other minute vessels, and the whole vascular 
mechanism is adapted to overcome this resistance to such an extent that a 236 
THE VASCULAR MECHANISM. 
normal circulation can take place. When the tissue becomes affected, the 
disturbance of the relations between the tissue and the blood may, as in the 
later stages of inflammation, so augment the resistance that the passage of 
the blood becomes at first difficult and ultimately impossible. And it is 
quite open to us to suppose that under certain circumstances the reverse of 
the above may occur in this or that area, conditions in which the resist- 
ance may be lowered below the normal and the circulation in the area 
quickened. Thus the vital condition of the tissue becomes a factor in the 
maintenance of the circulation ; and it is possible, though not yet proved, 
that these vital conditions are directly under the dominion of the nervous 
system. 
§ 171. Changes in the peripheral resistance may also be brought about by 
changes in the character of the blood, especially by changes in the relative 
amount of gases present. When a stream of defibrinated blood is artificially 
driven through a perfectly fresh excised organ such as the kidney, it is found 
that the resistance to the flow of blood through the organ, measured for in- 
stance by the amount of outflow in relation to the pressure exerted, varies 
considerably owing to changes taking place in the organ, and may be in- 
creased by increasing the venous character and diminished by increasing 
the arterial character of the blood. Remarkable changes in the resistance 
are also brought about by the addition of small quantities of certain drugs 
such as chloral, atropine, etc., to the blood. 
These changes have been attributed to the altered blood acting on the 
walls of the vessels, inducing, for instance, constriction or widening of the 
small arteries, or it may be affecting the capillaries, for it has been asserted 
that the epithelioid plates of the capillaries vary in form according to the 
relative quantities of carbonic acid and oxygen present in the blood. But 
this is not the whole explanation of the matter, since similar variations in 
resistance are met with when blood is driven through fine capillary tubes of 
inert matter. In such experiments it is found that the resistance to the flow 
increases with a diminution of the oxygen carried by the red corpuscles, 
and is modified by the addition to the blood of even small quantities of 
certain drugs. 
It is obvious then that in the living body the peripheral resistance, being 
the outcome of complex conditions, may be modified in many ways. Ex- 
perience teaches us that, even in dealing with non-living inert matter, the 
flow of fluid through capillary tubes may be modified on the one hand by 
changes in the substance of which the tubes are composed, and on the other 
hand by changes in the chemical nature (even independent of the specific 
gravity) of the fluid which is used. In the living body both the fluid, the 
blood, and the walls of the minute vessels being both alive, are incessantly 
subject to change ; the changes in the one, moreover, are capable of reacting 
upon and inducing changes in the other; and, lastly, the changes both of 
the one and of the other may be primarily set going by events taking place 
in some part of the body far away from the region in which these changes 
are modifying the resistance to the flow. 
Changes in the Quantity of Blood. 
§ 172. In an artificial scheme changes in the total quantity of fluid in 
circulation will have an immediate and direct effect on the arterial pressure, 
increase of the quantity heightening and decrease diminishing it. This 
effect will be produced partly by the pump being more or less filled at each 
stroke, and partly by the peripheral resistance being increased or diminished 
by the greater or less fulness of the small peripheral channels. The venous 237 
CHANGES IN THE QUANTITY OF BLOOD. 
pressure will under all circumstances be raised with the increase of fluid, but 
the arterial pressure will be raised in proportion only so long as the elastic 
walls of the arterial tubes are able to exert their elasticity. 
In the natural circulation the direct results of change of quantity are 
modified by compensatory arrangements. Thus experiment shows that 
when an animal with normal blood-pressure is bled from one carotid, the 
pressure in the other carotid sinks so long as the bleeding is going on,1 and 
remains depressed for a brief period after the bleeding has ceased. In a 
short time, however, it regains or nearly regains, the normal height. This 
recovery of blood-pressure, after hemorrhage, is witnessed so long as the 
loss of blood does not amount to more than about 3 per cent, of the body- 
weight. Beyond that a large and frequently a sudden dangerous permanent 
depression is observed. 
The restoration of the pressure after the cessation of the bleeding is too 
rapid to permit us to suppose that the quantity of fluid in the bloodvessels 
is repaired by the withdrawal of lymph from the extra-vascular elements of 
the tissues. In all probability the result is gained by an increased action of 
the vasomotor nerves, increasing the peripheral resistance, the vasomotor 
centres being thrown into increased action by the diminution of their blood- 
supply. When the loss of blood has gone beyond a certain limit, this vaso- 
motor action is insufficient to compensate the diminished quantity (possibly 
the vasomotor centres in part become exhausted), and a considerable de- 
pression takes place; but at this epoch the loss of blood frequently causes 
anaemic convulsions. 
Similarly when an additional quantity of blood is in jected into the vessels, 
no marked increase of blood-pressure is observed so long as the vasomotor 
centre in the medulla oblongata is intact. If, however, the cervical spinal 
cord be divided previous to the injection, the pressure, which on account of 
the removal of the medullary vasomotor centre is very low, is permanently 
raised by the injection of blood. At each injection the pressure rises, falls 
somewhat afterward, but eventually remains at a higher level than before. 
This rise is stated to continue until the amount of blood in the vessels above 
the normal quantity reaches from 2 to 3 per cent, of the body-weight, be- 
yond which point it is said no further rise of pressure occurs. 
These facts seem to show, in the first place, that when the volume of the 
blood is increased, compensation is effected by a lessening of the peripheral 
resistance by means of a vaso-dilator action of the vasomotor centres, so 
that the normal blood-pressure remains constant. They further show that a 
much greater quantity of blood can be lodged in the bloodvessels than is 
normally present in them. That the additional quantity injected does re- 
main in the vessels is proved by the absence of extravasations and of any 
considerable increase of the extra-vascular lymphatic fluids. It has already 
been insisted that, in health, the veins and capillaries must be regarded as 
being far from filled, for were they to receive all the blood which they can, 
even at a low pressure, hold, the whole quantity of blood in the body would 
be lodged in them alone. In these cases of large addition of blood the 
extra quantity appears to be lodged in the small veins and capillaries (espe- 
cially of the internal organs), which are abnormally distended to contain 
the surplus. 
We learn from these facts the two practical lessons, first, that blood- 
pressure cannot be lowered directly by bleeding, unless the quantity removed 
be dangerously large, and secondly, that there is no necessary connection 
1 Chiefly in consequence of the free opening in the vessel from which the bleeding is 
going on cutting off a great deal of the peripheral resistance and so leading to a general 
lowering of the blood-pressure. 238 
THE VASCULAR MECHANISM 
between a high blood-pressure and fulness of blood or plethora, since an 
enormous quantity of blood may be driven into the vessels without any 
marked rise of pressure. 
A Review' of Some of the Features of the Circulation. 
§ 173. The facts dwelt on in the foregoing sections have shown us that 
the factors of the vascular mechanism may be regarded as of two kinds: 
one constant or approximately constant, the other variable. 
The constant factors are supplied by the length, natural bore, and distri- 
bution of the bloodvessels, by the extensibility and elastic reaction of their 
walls, and by such mechanical contrivances as the valves. By the natural 
bore of the various bloodvessels is meant the diameter which each would 
assume if the muscular fibres were wholly at rest, and the pressure of fluid 
within the vessel were equal to the pressure outside. It is obvious, however, 
that even these factors are only approximately constant for the life of an 
individual. The length and distribution of the vessels change with the 
growth of the whole body or parts of the body, and the physical qualities 
of the walls, especially of the arterial walls, their extensibility, and elastic 
reaction change continually with the age of the individual. As the body 
grows older the once supple and elastic arteries become more and more stiff 
and rigid, and often in middle life, or it may be earlier, a lessening of 
arterial resilience which proportionately impairs the value of the vascular 
mechanism as an agent of nutrition, marks a step toward the grave. The 
valvular mechanisms, too, also show signs of age as years advance, and 
more or less marked and increasing imperfections diminish the usefulness of 
the machine. 
The chief variable factors are, on the one hand, the beat of the heart, and, 
on the other, the peripheral resistance, the variations in the latter being 
chiefly brought about by muscular contraction or relaxation in the minute 
arteries, but also, though to what extent has not yet been accurately deter- 
mined, by the condition of the walls of the minute vessels according to 
which the blood can pass through them with less or with greater ease, as 
well as by the character of the circulating blood. 
§ 174. These two chief variables, the beat of the heart and the width 
of the minute arteries, are known to be governed and regulated by the cen- 
tral nervous system, which adapts each to the circumstance of the moment, 
and at the same time brings the two into mutual interdependence ; but the 
central nervous system is not the only means of government; there are other 
modes of regulation, and so other safeguards. 
Thus while undoubtedly the two prominent governors of the heart are 
the inhibitory fibres of the vagus and the augmentor fibres from the splanch- 
nic system, the one slowing the rhythm and weakening the stroke, the 
other quickening the rhythm and strengthening the stroke, other causes may 
vary the beat in the absence of any action of either of these two nerves. 
Mere distention of the ventricle, by increasing the tension of the ventricular 
fibres, and so increasing the force of the contraction of each fibre (see § 
148), brings about a more forcible beat. As we shall see in dealing with 
respiration, a powerful inspiration leads to a larger flow of blood into the 
heart, and forthwith the ventricle, out of its very fulness, gives stronger 
beats for the time. So also when by valvular disease or otherwise an un- 
usual obstacle is presented to the outflow from the ventricle, increased vigor 
in the strokes of the distended organ strives to compensate the mischief. As, 
however, in the case of the skeletal muscle, the tension, if too great, may be 
injurious. In a similar manner the auricle, by a stronger or a weaker con- SOME FEATURES OF THE CIRCULATION. 
239 
traction, may distend the ventricle to a greater or to a less extent, and so 
produce a stronger or weaker ventricular systole. 
§ 175. Still more efficient, perhaps, as a direct governor of the heart's 
beat is the quality and quantity of blood passing in the mammal through 
the coronary arteries and regulating the nutrition of the cardiac substance. 
In the absence of all interference by inhibitory and augmentor fibres the 
heart will continue beating with a certain rhythm and force, determined by 
the metabolism going on certainly in its muscular, and possibly to a certain 
extent also in its nervous elements. We have seen that the energy set free 
in an ordinary skeletal muscle, in response to a stimulus, may vary from 
nothing to a maximum according to the metabolism going on-according to 
the nutritive vigor of the muscular fibres. The spontaneous rhythmic beat 
of the cardiac substance may be regarded as the outcome of a metabolism 
more highly pitched, more elaborate, of a higher order than that which 
simply furnishes the ordinary skeletal fibre with mere irritability toward 
stimuli. All the more, therefore, may the beat be expected to be influenced 
by any change in the metabolism of the cardiac substance, and so by any 
change in the blood which furnishes the basis for that metabolism. Hence 
the beat of the heart, quite apart from extrinsic nervous influences, may 
vary largely in consequence of changes in its own metabolism, which in turn 
may result from alterations in the blood-supply, or may have a deeper origin, 
and be due to the fact that the cardiac substance, owing to failure in its 
molecular organization (a failure which maybe temporary or permanent), is 
unable to avail itself properly of the nutritive opportunities afforded by a 
normal quantity of normal blood. 
§ 176. As is well known, the beat of the heart may become temporarily 
or permanently irregular; that many hearts go on beating day after day, 
year after year, without any such irregularity is a striking proof of the com- 
plete balance which usually obtains between the several factors of which we 
are speaking. Sometimes such cardiac irregularities, those of a transient 
nature and brief duration, are the results of extrinsic nervous influences. 
Some events taking place in the stomach, for instance, give rise to afferent im- 
pulses which ascending from the mucous membrane of the stomach along cer- 
tain afferent fibres of the vagus to the medulla oblongata, so augment the 
action of the cardio-inhibitory centre as to stop the heart for a beat or two, 
the stoppage being frequently followed by a temporary increase in the rapid- 
ity and force of the beat. Such a passing failure of the heart-beat, in its sud- 
den onset, in its brief duration, and in the reaction which follows, very 
closely resembles the temporary inhibition brought about by artificial stimu- 
lation of the vagus. But these characters are not essential to cardiac inhibi- 
tion. For it must be remembered that the central nervous system possesses, 
in the form of natural nervous impulses of various origin, a means of stimu- 
lation far finer, more delicate and more varied than anything we can effect 
by our rough means of induction coils and electrodes. Thus in many cases 
of fainting, the heart-beats, instead of stopping abruptly, gradually die away 
or fade away it may be to an absolute brief arrest, but more frequently 
merely down to a feebleness which is insufficient to supply the brain with a 
quantity of blood adequate to maintain consciousness, and then in many 
cases, at all events, are resumed, or recover strength gradually and quietly 
without any boisterous reaction. In all probability all cases of simple faint- 
ing from emotion, pain, digestive troubles, etc., as distinguished from the 
syncope of actual heart disease, are instances of vagus inhibition, and though 
we cannot accurately reproduce their varied phases by direct stimulation of 
the vagus trunk, we may approach them more nearly by producing reflex 
inhibition, as by mechanical irritation of the abdomen. (See § 145.) 240 
THE VASCULAR MECHANISM. 
Whether definite temporary irregularity is ever brought about by means 
of the augmentor fibres we have at present no clear evidence ; but cases do 
occur of palpitation without previous stoppage, cases in which a few hurried 
strong beats come on, pass off, and are followed by feebler beats; and these 
may possibly be due to some transient influence of augmentor fibres thrown 
into activity as part of a reflex act or otherwise. And though we have no 
direct experimental evidence, it is very probable that the acceleration or 
augmentation of the beat, or a combination of the two, which so often follows 
emotion, is carried out by augmentor fibres. 
In all probability, however, irregularity in the heart-beat is much more 
frequently the result of intrinsic events, or the product of a disordered nutri- 
tion of the cardiac substance. The normal nutrition sets the pace of the 
normal rhythm. We cannot explain how this is affected ; nor can we explain 
why in one individual the normal pace is set so low as 50 or even 30 beats 
a minute, and in another as high as 90 a minute or even more, while in most 
persons it is about 70 a minute. The slower or the quicker the pace, though 
not normal to the species, must be considered as normal to the individual, 
for it may be kept up through long years in an organism capable of carry- 
ing on a normal man's duties and work. So long as we cannot explain these 
differences we cannot hope to explain how it is that a disordered nutrition 
brings about an irregular heart-beat, either the more regular irregularity of 
a " dropping " pulse, that is, a failure of sequence rather than an irregularity, 
or a more actively irregular rhythm, such as that accompanying a dilated 
ventricle. We may, however, distinguish two kinds of irregularity : one in 
which, in spite of all favorable nutritive conditions, the cardiac substance 
cannot secure, even perhaps for a minute, a steady rhythm ; and another in 
which the rhythm, though normal under ordinary circumstances, is, so to 
speak, in a condition of unstable equilibrium, so that a very slight change 
in conditions, too much or too little blood, or some small alteration in the 
composition of the blood, or the advent of some, it may be slight, nervous 
impulse, augmentor or inhibitory, develops a temporary irregularity. 
§177. No one thing, perhaps, concerning the heart is more striking 
than the fact that a heart which has gone on beating for many years, with 
only temporary irregularities, and those few and far between, a heart which 
must, therefore, have executed with long-continued regularity, many mill- 
ions of beats, should suddenly, apparently without warning, after a brief, 
flickering struggle, cease to beat any more. But we must remember that 
each beat is an effort-an effort, moreover, which, as we have seen 
(§ 141), is the best that the heart can make at the moment; the accom- 
plishment of each beat is, so to speak, a hurdle which has to be leaped 
-one of the long series of hurdles which make up the steeple-chase of life. 
At any one leap failure may occur ; so long as failure does not occur, so 
long as the beat is made, and a fair proportion of the ventricular contents 
are discharged into the great vessels, the chief end is gained, and whether 
the leap is made clumsily or well is, relatively considered, of secondary im- 
portance. But if the beat be not made, everything almost (provided 
that the miss be due, not to vagus inhibition, but to intrinsic events) is 
unfavorable for a succeeding beat; the mysterious molecular changes, by 
which the actual occurrence of one beat prepares the way for the next, are 
missing, the favorable influences of the extra rush of blood through the 
coronary arteries due to a preceding beat are missing also, and even the dis- 
tention of the cardiac cavities, at first favorable, speedily passes the limit 
and becomes unfavorable. And these untoward influences accumulate 
rapidly as the first miss is followed by a second and by a third. In this 
way a heart, which has been brought into a state of unstable equilibrium SOME FEATURES OF THE CIRCULATION. 
241 
by disordered nutrition (as, for instance, by imperfect coronary circulation, 
such as seems to accompany diseases of the aortic valves leading to regur- 
gitation from the aorta into the ventricle, in which cases sudden death is 
not uncommon), which is able just to accomplish each beat, but no more, 
which has but a scanty saving store of energy, under some strain or other 
untoward influence, misses a leap, falls, and is no more able to rise. Doubt- 
less in such cases could adequate artificial aid be promptly applied in time, 
could the fallen heart be stirred even to a single good beat, the favorable 
reaction of that beat might bring a successor, and so once more start the 
series ; but such a period of grace, of potential recovery, is a brief one. 
Even a coarse skeletal muscle, when cut off from the circulation, soon loses 
its irritability beyond all recovery, and the heart cut off from its own influ- 
ence on itself runs down so rapidly that the period of possible recovery is 
measured chiefly by seconds. 
§ 178. Turning now to the minute arteries and the peripheral resist- 
ance which they regulate, we may call to mind the existence of the two 
kinds of mechanism, the vaso-constrictor mechanism, which, owing to the 
maintenance by the central nervous system of a tonic influence, can be 
worked both in a positive constrictor and in a negative dilator direc- 
tion, and the vaso-dilator mechanism, which, as far as we know, exerts its 
influence in one direction only, viz., to dilate the bloodvessels. The latter 
dilator mechanism seems, as we have seen, to be used in special instances 
only, as seen in the cases of the chorda tympani and nervi erigentes; the 
use of the former constrictor mechanism appears to be more general. Thus 
the relaxation of the cutaneous arteries of the head and neck, which is the 
essential feature in blushing, seems due to mere loss of tone, to the removal 
of constrictor influences previously exerted through the vaso-constrictor 
fibres of the cervical sympathetic. Though probably dilator fibres pass 
directly along the roots of the cervical and of certain cranial nerves to the 
nerves of the head and neck, we have no evidence that these come into 
play in blushing; as we have seen, blushing may be imitated by mere sec- 
tion of the cervical sympathetic. So also the " glow " and redness of 
the skin of the whole body-i. e., dilatation generally of the cutaneous 
arteries-which is produced by external warmth, is probably another in- 
stance of diminished activity of tonic constrictor influences; though the 
result, that the dilatation produced by warming an animal in an oven is 
greater than that produced by section of nerves, seems to point to the 
dilator fibres for the cutaneous vessels which, as we have seen, probably 
exist in the sciatic and brachial plexuses and possibly in all the spinal 
nerves, also taking part in the action. A similar loss of constrictor action 
in the cutaneous vessels may be the result of certain emotions, whether going 
so far as actual blushing of the body, or merely producing a " glow." The 
effect of cold, on the other hand, and of certain emotions, or of emotions 
under certain conditions, is to increase the constrictor action on the cutane- 
ous vessels, and the skin grows pale. It may be worth while to point out 
that in both the above cases, while both the cold and warmth produce their 
effect chiefly at all events through the central nervous system, and very 
slightly, if at all, by direct action on the skin, their action on the central 
nervous system is not simply a general augmentation or inhibition of the 
whole vasomotor centre. On the contrary, the cold, while it constricts the 
cutaneous vessels, so acts on the vasomotor centre as to inhibit that portion 
of the vasomotor centre which governs the abdominal splanchnic area; 
while less blood is carried to the colder skin, by the opening up of the 
splanchnic area more blood is turned on to the warmer regions of the body, 
and the rise of blood-pressure which the constriction of the cutaneous vessels 242 
THE VASCULAR MECHANISM. 
tended to produce, and which might be undesirable, is thereby prevented. 
Conversely when warmth dilates the cutaneous vessels, it at the same time 
constricts the abdominal splanchnic area, and prevents an undesirable fall 
of pressure. 
The warm and flushed condition of the skin, which follows the drink- 
ing of alcoholic fluids, is probably in a similar manner the result of an in- 
hibition of that part of the vasomotor centre which governs the cutaneous 
arteries ; and it is probable also that except for the local effect of the fluid 
on the gastric mucous membrane, whereby some amount of blushing of the 
gastric bloodvessels takes place as a reflex act, this effect on the vessels of 
the skin is accompanied by an inverse constrictor action in the splanch- 
nic area. This last point, however, has not been proved experimentally 
and may not occur, since the influence of the alcohol is at the same time to 
increase the heart's action, and thus to obviate the fall of pressure which 
would certainly occur were the cutaneous and splanchnic vascular areas 
to be dilated at the same time. This effect of the alcohol on the heart 
may be a direct action of the alcohol on the cardiac substance, being car- 
ried thither by the blood ; but the effect, in being an augmentation of the 
force, and acceleration of the pace of the heart-beat of a temporary character, 
followed by a reaction in the direction of feebleness and slowness, so strik- 
ingly resemble the effects of artificially stimulating the cardiac augmentor 
fibres, that it is at least probable that the alcohol does act upon the cardiac 
augmentor mechanism. 
§ 179. The influence on the body of exercise illustrates both the manner 
in which the two vascular factors, the heart-beat and the peripheral resist- 
ance, are modified by circumstances, and the mutual action of these on each 
other. 
When the body passes from a condition of comparative rest and quiet to 
one of exertion and movement, the metabolism of the skeletal muscles (and 
of the nervous system) is increased and more heat is generated in them. 
We know for certain that the increased metabolism throws into the blood of 
the veins coming from the muscles an increased amount of carbonic acid, 
and it is probable, but not so certain, that it also loads the blood with lactic 
acid and other metabolic products ; at the same time, there is an increased 
consumption of oxygen ; the blood of the body tends to become less arterial 
and more venous. In dealing with respiration, we shall see that the influ- 
ence thus exerted on the blood leads to an increase in the respiratory move- 
ments, and we shall further see that the more vigorous working of the 
respiratory pump, since it promotes the flow of blood to and through the 
heart and lungs, quickens and strengthens the heart-beats. Possibly this 
mere mechanical effect of the more vigorous breathing is sufficient by itself 
to account for the increase in the frequency and vigor of the heart's action, 
but it is more than probable that it is the changed condition of the blood, 
which, while it hurries on the respiratory pump, also stimulates the vascular 
pump, either by a direct action on the cardiac substance, or through the 
medium of the central nervous system and the augmentor fibres. If, as 
experiments seem to show, the increased vigor of the respiratory movements 
compensates, or even over-compensates, the tendency of the whole blood to 
become more venous, so that during exercise the blood, which is distributed 
by the aorta, actually does not contain more carbonic acid and less oxygen 
than the rest, but even the reverse, then these efforts must be due to some of 
the products of muscular metabolism other than carbonic acid. 
The same changed condition of blood, while it thus excites the heart, di- 
lates the cutaneous vessels, as is clearly shown by the warm flushed skin, and 
at the same time throws into activity the perspiratory mechanism with which 243 
SOME FEATURES OF THE CIRCULATION. 
we shall hereafter have to deal. There can be no doubt, as we shall see 
later on, that the perspiration which accompanies muscular exercise is 
brought about by means of the central nervous system, and we may almost 
with certainty conclude that the dilatation of the cutaneous arteries is also 
brought about by means of the central nervous system, and most probably 
by means of an inhibition of that part of the vasomotor centre which main- 
tains under ordinary circumstances a greater or less tonic constriction of the 
cutaneous arteries; how far this may be assisted by the special action of 
vaso-dilator fibres we do not know. 
This widening of the cutaneous arteries diminishes largely the peripheral 
resistance, and so tends to lower the blood-pressure. Moreover, with each 
effort of each skeletal muscle the minute arteries of that muscle are dilated, 
so that during exercise, and especially during vigorous exercise calling into 
action many skeletal muscles, there must be in the body at large a very con- 
siderable widening of the minute arteries distributed to the various muscles, 
and in consequence a very considerable diminution of the peripheral resist- 
ance. These two diminutions of peripheral resistance, cutaneous and mus- 
cular, would tend to lower the blood-pressure-a result which would be most 
injurious, since the increased metabolism of the muscles demands a more 
rapid circulation in order to get rid of the products of metabolism, and for 
a rapid circulation a high blood-pressure is in most cases necessary, and in all 
cases advantageous. The evil is, in part at all events, met by the increased 
force and frequency of the heart's beats, for, as we have said again and 
again, the mean blood-pressure is the product of the heart-beat working 
against the peripheral resistance, and may remain constant when one factor 
is increased or diminished, provided that the other factor be proportionately 
diminished or increased. It is possible, then, that the mere increase in the 
heart's beats are, during exercise, sufficient to neutralize the diminution of 
peripheral resistance, or even to raise the blood-pressure above the normal; 
and, indeed, we find, as a matter of fact, that during exercise there is such 
an increase of the mean blood-pressure. But it is more than probable that 
much valuable labor of the heart is economized by neutralizing the imminent 
fall of blood-pressure in another manner. It would appear that while that 
part of the vasomotor centre which governs the cutaneous vascular area is 
being inhibited, that part which governs the abdominal splanchnic area is, 
on the contrary, being augmented. And in this way a double end is gained. 
On the one hand, the mean blood-pressure is maintained or increased in a 
more economical manner than by increasing the heart beats, and, on the 
other hand, the blood during the exercise is turned away from the digestive 
organs, which at the time are, or ought to be, at rest, and therefore requiring 
comparatively little blood. These organs certainly, at all events, ought not 
during exercise to be engaged in the task of digesting and absorbing food, 
and the old saying, " after dinner sit a while," may serve as an illustration of 
the working of the vascular mechanism with which we are dealing. The 
duty which some of the digestive organs have to carry out in the way of 
excretion of metabolic waste products is during exercise probably taken on 
by the flushed and perspiring skin. It is true that at the beginning of a 
period of exercise, before the skin, so to speak, has settled down to its work, 
an increased flow of urine, dependent on or accompanied by an increased flow 
of blood through the kidney, may make its appearance ; but in this case, as 
we shall see later on in dealing with the kidney, the flow of blood through 
the kidney may be increased in spite of constriction of the rest of the 
splanchnic area, and, besides, such an initial increase of urine speedily gives 
way to a decrease. 
§ 180. The effect of food on the vascular mechanism affords a marked 244 
THE VASCULAR MECHANISM. 
contrast to the effect of bodily labor. The most marked result is a widening 
of the whole abdominal splanchnic area, accompanied by so much constric- 
tion of the cutaneous vascular area, and so much increase of the heart's beat, 
as is sufficient to neutralize the tendency of the widening of the splanchnic 
area to lower the mean pressure, or perhaps even sufficient to raise slightly 
the mean pressure. 
Any large widening of the cutaneous area, especially if accompanied by 
muscular labor and the incident widening of the arteries of the muscles, 
would tend so to lower the general blood-pressure (unless met by a wasteful 
use of cardiac energy) as injuriously to lessen the flow through the active 
digesting viscera. A moderate constriction of the cutaneous vessels, on the 
other hand, by throwing more blood on the abdominal splanchnic area with- 
out tasking the heart, is favorable to digestion, and is probably the physio- 
logical explanation of the old saying, " If you eat till you're cold, you'll 
live to be old." 
In fact, during life there seems to be a continual give-and-take between 
the bloodvessels of the somatic and those of the splanchnic divisions of the 
body; to fill the one, the other is proportionately emptied, and vice versa. 
§ 181. We have seen (§ 160) that certain afferent fibres of the vagus 
forming in the rabbit a separate nerve, the depressor nerve, are associated 
with the vaso-constrictor nerves and the vasomotor centre in such away that 
impulses passing centripetally along them from the 'heart lower the blood- 
pressure by diminishing the peripheral resistance, probably inhibiting the 
tonic constrictor influences exerted along the abdominal splanchnic nerves, 
and so, as it were, opening the splanchnic flood-gates. We do not possess 
much exact information about the use of these afferent depressor fibres in 
the living body, but probably when the heart is laboring against the blood- 
pressure which is too high for its powers, the condition of the heart starts 
impulses which, passing along the depressor fibres up to the medulla 
oblongata, temper down, so to speak, the blood-pressure to suit the cardiac 
strength. 
We have, moreover, reason to think that not only does the heart thus 
regulate the blood-pressure by means of the depressor fibres, but also that 
the blood-pressure, acting, as it were, in the reverse direction, regulates the 
heart-beat; a too high pressure, by acting directly on the cardio-inhibitory 
centre in the medulla oblongata (either directly-that is, as the result of the 
vascular condition of the medulla itself-or indirectly, by impulses reaching 
the medulla along afferent nerves from various parts of the body) may 
send inhibitory impulses down the vagus, or so slacken or tone down the 
heart-beats. 
In the following sections of this work we shall see repeated instances, 
similar to or even more striking than the above, of the management of the 
vascular mechanism by means of the nervous system, and we, therefore, need 
dwell no longer on the subject. 
We may simply repeat that at the centre lies the cardiac muscular fibre, 
and at the periphery the plain muscular fibre of the minute artery. On 
these two elements the central nervous system, directed by this or that 
impulse reaching it along afferent nerve fibres, or affected directly by this or 
that influence, is during life continually playing, now augmenting, now 
inhibiting, now the one, now the other, and so, by help of the elasticity of 
the arteries and the mechanism of the valves, directing the blood-flow ac- 
cording to the needs of the body. BOOK II. 
THE TISSUES OF CHEMICAL ACTION. THEIR RESPECTIVE 
MECHANISMS. NUTRITION. 
CHAPTER I. 
THE TISSUES AND MECHANISMS OF DIGESTION. 
§ 182. The food in passing along the alimentary canal is subjected to 
the action of certain juices supplied by the secretory activity of the epithe- 
lial cells which line the canal itself or which form part of its glandular 
appendages. These juices, viz., saliva, gastric juice, bile, pancreatic juice, 
and the secretions of the small and large intestines, poured upon and min- 
gling with the food produce in it such changes that, from being largely insol- 
uble, it becomes largely soluble, or otherwise modify it in such a way that 
the larger part of what is eaten passes into the blood, either directly by 
means of the capillaries of the alimentary canal, or indirectly by means of 
the lacteal system, while the smaller part is discharged as excrement. 
Those parts of the food which are thus digested, absorbed, and made use 
of by the body, are spoken of as food-stuffs (they have also been called 
alimentary principles'), and may be conveniently divided into four great 
classes: 
1. Proteids. We have previously (§ 15) spoken of the chief characters 
of this class, and have dealt with several members in treating of blood and 
muscle. We may here repeat that in general composition they contain in 
100 parts by weight " in round numbers " rather more than 15 parts of 
nitrogen, rather more than 50 parts of carbon, about 7 parts of hydrogen, 
and rather more than 20 parts of oxygen; though essentially the nitro- 
genous bodies of food and of the body, they are made up of carbon to the 
extent of more than half their weight. 
The nitrogenous body gelatin, which occurs largely in animal food, and 
some other bodies of less importance, while more closely allied to proteid 
bodies than to any other class of organic substances, differ considerably from 
proteids in composition and especially in their behavior in the body ; they 
are not of sufficient importance to form a class by themselves. 
2. Fats, frequently but erroneously called hydrocarbons. These vary 
very widely in chemical composition, ranging from such a comparatively 
simple fat as butyrin to the highly complex lecithin (§ 69) ; they all possess, 
in view of the oxidation of both their carbon and their hydrogen, a large 
amount of potential energy. 
3. Carbohydrates, or sugars and starches. These possess, weight for 
weight, relatively less potential energy than do fats; they already contain 
in themselves a large amount of combined oxygen, and when completely 
oxidized give out, weight for weight, less heat than do fats. 
245 246 
THE TISSUES AND MECHANISMS OF DIGESTION. 
4. Saline or mineral bodies, and water. These salts are for the most part 
inorganic salts, and this class differs from the three preceding classes inas- 
much as the usefulness of its members to the body lies not so much in the 
amount of energy which may be given out by their oxidation as in the 
various influences which, by their presence, they exercise on the metabolic 
events of the body. 
These several food-stuffs are variously acted upon in the several parts of 
the alimentary canal, and we may distinguish, as the food passes along the 
digestive tract, three main stages : digestion in the mouth and stomach, 
digestion in the small intestine, and digestion in the large intestine. In 
many animals the first stage is, to a large extent, preparatory only to the 
second, which in all the animals is the stage in which the food undergoes 
the greatest change; in the third stage the changes begun in the previous 
stages are completed, and this stage is especially characterized by the absorp- 
tion of fluid from the interior of the alimentary canal. 
It will be convenient to study these stages more or less apart, though not 
wholly so, and it will also be convenient to consider the whole subject of 
digestion under the following heads : 
First, the characters and properties of the various juices and the changes 
which they bring about in the food eaten. 
Secondly, the nature of the processes by means of which the epithelium 
cells of the various glands and various tracts of the canal are able to manu- 
facture so many various juices out of the common source, the blood, and the 
manner in which the secretory activity of the cells is regulated and subjected 
to the needs of the economy. 
Thirdly, the mechanisms, here as elsewhere, chiefly of a muscular nature, 
by which the food is passed along the canal and most efficiently brought 
into contact with the several juices. 
Fourthly and lastly, the means by which the nutritious digested material 
is separated from the undigested or excremental material and absorbed into 
the blood. 
The Characters and Properties of Saliva- and Gastric Juice. 
Saliva. 
§183. Mixed saliva, as it appears in the mouth, is a thick, glairy, gen- 
erally frothy and turbid fluid. Under the microscope it is seen to contain, 
beside the molecular debris of food, bacteria and other organisms (fre- 
quently cryptogamic spores), epithelium scales, mucous corpuscles and 
granules, and the so-called salivary corpuscles. Its reaction in a healthy 
subject is alkaline, especially when the secretion is abundant. When the 
saliva is scanty, or when the subject suffers from dyspepsia, the reaction of 
the mouth may be acid. Saliva contains but little solid matter, on an aver- 
age probably about 5 per cent., the specific gravity varying from 1002 to 
1006. Of these solids, rather less than half, about 2 per cent., are salts 
(including at times a minute quantity of potassium sulphocyanate). The 
organic bodies which can be recognized in it are globulin and serum albumin 
(see §§ 16, 17), found in small quantities only-other obscure bodies occur- 
ring in minute quantity, and mucin; the latter is by far the most conspicu- 
ous organic constituent, the glairiness or ropiness of mixed and other kinds 
of saliva being due to its presence. 
Mucin. If acetic acid be cautiously added to mixed saliva, the viscidity 
of the saliva is increased, and on further addition of the acid a semi-opaque 
ropy mass separates out, leaving the rest of the saliva limpid. This ropy SALIVA AND GASTRIC JUICE. 
247 
mass, which is mucin, if stirred carefully with a glass rod, shrinks, becoming 
opaque, clings to the glass rod, and may be thus removed from the fluid. If 
the quantity of mucin be small and the, saliva be violently shaken or stirred 
while the acid is being added, the mucin is apt to be precipitated in flakes, 
and may then be separated by filtration. It may be added that the precipi- 
tation of mucin by acid is greatly influenced by the presence of sodium 
chloride and other salts ; thus, after the addition of sodium chloride, acetic 
acid, even in considerable excess, will not cause a precipitate of mucin. 
Mucin, thus prepared and purified by washing with acetic acid, swells 
out in water without actually dissolving; it will, however, dissolve into a 
viscid fluid readily in dilute (0.1 percent.) solutions of potassium hydrate, 
more slowly in solutions of alkaline salts. In order to filter a mucin solu- 
tion, great dilution with water is necessary. 
Mucin is precipitated by strong alcohol and by various metallic salts ; it 
may also be precipitated by dilute mineral acids, but the precipitate is then 
soluble in excess of the acid. 
Mucin gives the three proteid reactions mentioned in § 15, but it is a 
very complex body, more complex even than proteids, for by treatment with 
dilute mineral acids and in other ways, it may be converted into some form 
of proteid (acid-albumin when dilute mineral acid is used), while at the 
same time there is formed a body which appears to be carbohydrate and 
resembles a sugar in having the power of reducing cupric sulphate solutions. 
Solutions of mucin, moreover, on mere keeping are apt to lose their viscidity 
and to become converted into a proteid not unlike the body peptone, which, 
as we shall see, is the result of gastric digestion, and into a reducing body. 
Several kinds of mucin appear to exist in various animal bodies, but they 
seem all to agree in the character that they can by appropriate treatment 
be split up into a proteid of some kind and into a carbohydrate or allied 
body. 
§ 184. The chief purpose served by the saliva in digestion is to moisten 
and soften the food and to assist in mastication and deglutition. In some 
animals this is its only function. In other animals and in man it has a 
specific solvent action on some of the food-stuffs. Such minerals as are 
soluble in slightly alkaline fluids are dissolved by it. On fats it has no effect 
save that of producing a very feeble emulsion. On proteids it has also no 
specific action, though pieces of meat, cooked or uncooked, appear greatly 
altered after they have been masticated for some time; the chief alteration, 
however, which thus takes place is a change in the haemoglobin and a 
general softening of the muscular fibres by aid of the alkalinity of the saliva. 
Of course, when particles of food are retained for a long time in the mouth, 
as in the interstices or in cavities of the teeth, the bacteria or other organ- 
isms which are always present in the mouth may produce much more pro- 
found changes, but these are not the legitimate products of the action of 
saliva. The characteristic property of saliva is that of converting starch 
into some form of sugar. 
Action of saliva on starch. If to a quantity of boiled starch, which is 
always more or less viscid and somewhat opaque or turbid, a small quantity 
of saliva be added, it will be found after a short time that an important 
change has taken place, inasmuch as the mixture has lost its previous 
viscidity and become thinner and more transparent. In order to under- 
stand this change the reader must bear in mind the existence of the follow- 
ing bodies, all belonging to the class of carbohydrates: 
1. Starch, which forms with water not a true solution but a more or less 
viscid mixture, and gives a characteristic blue color with iodine. The for- 
mula is C6H10O5, or more correctly (C6H10O5)w since the molecule of starch 248 
THE TISSUES AND MECHANISMS OF DIGESTION. 
is some multiple (n being not less than 5) of the simpler formula. A kind 
of starch known as soluble starch, while giving a blue color with iodine, 
forms, unlike ordinary starch, a clear solution. 
2. Dextrins, differing from starch in forming a clear solution. Of these 
there are at least two ; one, erythrodextrin, often spoken of simply as dextrin, 
giving a port-wine red color with iodine, and second, achroodextrin, which 
gives no color at all with iodine. The formula for dextrin is the same as 
that for starch, but has a smaller molecule and might be represented by 
(C6H10O5)"'. 
3. Dextrose, also called glucose or grape-sugar, giving no coloration with 
iodine, but characterized by the power of reducing cupric and other metallic 
salts; thus, when dextrose is boiled with a fluid known as Fehling's fluid, 
which is a solution of hydrated cupric oxide in an excess of caustic alkali 
and double tartrate of sodium and potassium, the cupric oxide is reduced 
and a red or yellow deposit of cuprous oxide is thrown down. This reaction 
serves with others as a convenient test for dextrose. Neither starch nor that 
commonest form of sugar known as cane-sugar gives the reaction ; whether 
the dextrins do is doubtful. The formula for dextrose is C6H12O6; it is more 
simple than that of starch or dextrin and contains an additional H2O for 
every C6. Unlike starch and dextrin it can be obtained in a crystalline 
form, either from aqueous solutions (it being readily soluble in water), in 
which case the crystals contain water of crystallization, or from its solutions 
in alcohol (in which it is sparingly soluble), in which case the crystals have 
no water of crystallization. Solutions of dextrose have a marked dextro- 
rotatory power with rays of light. 
4. Maltose, very similar to dextrose, and like it capable of reducing 
cupric salts. The formula is somewhat different, being C12H220n. Besides 
this, it differs from dextrose chiefly in its smaller power-i. e., a given weight 
will not convert so much cupric oxide into cuprous oxide as will the same 
weight of dextrose-and in having a stronger rotatory action on rays of light. 
Like dextrose it can be crystallized, the crystals from aqueous solutions con- 
taining water of crystallization. 
Now, when a quantity of starch is boiled with water we may recognize 
in the viscid imperfect solution, on the one hand, the presence of the starch 
by the blue color which the addition of iodine gives rise to; and on the 
other hand, the absence of sugar (maltose, dextrose), by the fact that when 
boiled with Fehling's fluid no reduction takes place and no cuprous oxide 
is precipitated. 
If, however, the boiled starch be submitted for a while to the action of 
saliva, especially at a somewhat high temperature such as 35° or 40° C., it 
is found that the subsequent addition of iodine gives no blue color at 
all or very much less color, showing that the starch has disappeared or 
diminished ; on the other hand, the mixture readily gives a precipitate of 
cuprous oxide when boiled with Fehling's fluid, showing that maltose or 
dextrose is present. That is to say, the saliva has converted the starch into 
maltose or dextrose. The presence of the previously absent sugar may also 
be shown by fermentation and by the other tests for sugar. Moreover, if an 
adequately large quantity of starch be subjected to the charge, the sugar 
formed may be isolated, and its characters determined. When this is done 
it is found that while some dextrose is formed the greater part of the sugar 
which appears is in the form of maltose. As is well known, starch may, by 
the action of dilute acid, be converted into dextrin, and by further action 
into sugar; but the sugar thus formed is always wholly dextrose, and not 
maltose at all. The action of saliva in this respect differs from the action 
of dilute acid. SALIVA AND GASTRIC JUICE. 
249 
While the conversion of the starch by the saliva is going on the addition 
of iodine frequently gives rise to a red or violet color instead of a pure blue, 
but when the conversion is complete no coloration at all is observed. The 
appearance of this red color indicates the presence of dextrin (erythrodex- 
trin) ; the violet color is due to the red being mixed with the blue of still 
unchanged starch. 
The appearance of dextrin shows that the action of the saliva on the 
starch is somewhat complex; and this is still further proved by the fact 
that even when the saliva has completed its work the whole of the starch 
does not reappear as maltose or dextrose. A considerable quantity of the 
other dextrin (achroodextrin) always appears and remains unchanged to the 
end ; and there are probably several other bodies also formed out of the 
starch, the relative proportions varying according to circumstances. The 
change, therefore, though perhaps we may speak of it in a general way as 
one of hydration, cannot be exhibited under a simple formula, and we may 
rest content for the present with the statement that starch when subjected 
to the action of saliva is converted chiefly into the sugar known as maltose 
with a comparatively small quantity of dextrose and to some extent into 
achroodextrin (erythrodextrin appearing temporarily only in the pro- 
cess), other bodies on which we need not dwell being formed at the 
same time. 
Raw unboiled starch undergoes a similar change but at a much slower 
rate. This is due to the fact that in the curiously formed starch grain the 
true starch, or granulose, is invested with coats of cellulose. This latter 
material, which requires previous treatment with sulphuric acid before it 
will give the blue reaction on the addition of iodine, is apparently not acted 
upon by saliva. Hence the saliva can only get at the granulose by travers- 
ing the coats of cellulose, and the conversion of the former is thereby much 
hindered and delayed. 
§ 185. The conversion of starch into sugar, and this we may speak of as 
the amylolytic action of saliva, will go on at the ordinary temperature of the 
atmosphere. The lower the temperature the slower the change, and at about 
0° C. the conversion is indefinitely prolonged. After exposure to this cold 
for even a considerable time the action recommences when the temperature 
is again raised. Increase of temperature up to about 35°-40°, or even a 
little higher, favors the change, the greatest activity being said to be 
manifested at about 40°. Much beyond this point, however, increase of 
temperature becomes injurious, markedly so at 60° or 70° ; and saliva 
which has been boiled for a few minutes not only has no action on 
starch while at that temperature, but does not regain its powers on 
cooling. By being boiled, the amylolytic activity of saliva is permanently 
destroyed. 
The action of saliva on starch is most rapid when the reaction of the 
mixture is neutral or nearly so; it is hindered or arrested by a distinctly 
acid reaction. Indeed, the presence of even a very small quantity of free 
acid, at all events of hydrochloric acid, at the temperature of the body, not 
only suspends the action but speedily leads to permanent abolition of the 
activity of the juice. The bearing of this will be seen later on. 
The action of saliva is hampered by the presence in a concentrated state 
of the product of its own action-that is, of sugar If a small quantity of 
saliva be added to a thick mass of boiled starch, the action will after a while 
slacken, and eventually come to almost a standstill, long before all the starch 
has been converted. On diluting the mixture with water, the action will 
recommence. If the products of action be removed as soon as they are 
formed, dialysis for example, a small quantity of saliva will, if sufficient time 250 
THE TISSUES AND MECHANISMS OF DIGESTION. 
be allowed, convert into sugar a very large, one might almost say an indef- 
inite, quantity of starch. Whether the particular constituent on which the 
activity of saliva depends is at all consumed in its action has not at present 
been definitely settled. 
On what constituents do the amylolytic virtues of saliva depend ? 
If saliva, filtered and thus freed from much of its mucin and from other 
formed constituents, be treated with ten or fifteen times its bulk of alcohol, 
a precipitate is formed containing, besides other substances, all the proteid 
matters. Upon standing under the alcohol for some time (several days), the 
proteids thus precipitated become coagulated and insoluble in water. Hence, 
an aqueous extract of the precipitate, made after this interval, contains very 
little proteid material; yet it is exceedingly active. Moreover, by other 
more elaborate methods there may be obtained from saliva solutions which 
appear to be almost entirely free from proteids and yet are intensely amylo- 
lytic. But even these probably contain other bodies beside the really active 
constituent. Whatever the active substance be in itself, it exists in such 
extremely small quantities that it has never yet been satisfactorily isolated ; 
and indeed the only clear evidence we have of its existence is the manifesta- 
tion of its peculiar powers. 
The salient features of this body, this amylolytic agent, which we may call 
ptyalin, are then : 1st, its presence in minute and almost inappreciable quan- 
tity. 2d, the close dependence of its activity on temperature. 3d, its perma- 
nent and total destruction by a high temperature and by various chemical 
reagents. 4th, the want of any clear proof that it itself undergoes any 
change during the manifestations of its power-that is to say, the energy 
necessary for the transformation which it effects does not come out of itself; 
if it is at all used up in its action, the loss is rather that of simple wear and 
tear of a machine than that of a substance expended to do work. 5th, the 
action which it induces is probably of such a kind (splitting up of a mole- 
cule with assumption of water) as is effected by that particular class of agents 
called " hydrolytic." 
These features mark out the amylolytic active body of saliva as belong- 
ing to the class of ferments;1 and we may henceforward speak of the 
amylolytic ferment of saliva. The fibrin-ferment (§ 20) is so called 
because its action in many ways resembles that of the ferment of which 
we are now speaking. 
§ 186. Mixed saliva, whose properties we have just discussed, is the 
result of the mingling in various proportions of saliva from the parotid, 
submaxillary, and sublingual glands with the secretion from the buccal 
glands. These constituent juices have their own special characters, and 
these are not the same in all animals. Moreover, in the same individual 
the secretion differs in composition and properties according to circum- 
stances ; thus, as we shall see in detail hereafter, the saliva from the sub- 
maxillary gland secreted under the influence of the chorda tympani nerve 
is different from that which is obtained from the same gland by stimulating 
the sympathetic nerve. 
1 Ferments may, for the present at least, be divided into two classes, commonly called 
organised and unorganized. Of the former, yeast may be taken as a well-known example. 
The fermentative activity of yeast which leads to the conversion of sugar into alcohol, 
is dependent on the life of the yeast-cell. Unless the yeast-cell be living and functional, 
fermentation does not take place; when the yeast-cell dies fermentation ceases; and no 
substance obtained from the fluid parts of yeast, by precipitation with alcohol or other- 
wise, will give rise to alcoholic fermentation. The salivary ferment belongs to the latter 
class ; it is a substance, not a living organism like yeast. It may be added, however, 
that possibly the organized ferment, the yeast for instance, produces its effect by means 
of an ordinary unorganized ferment which it generates, but which is immediately made 
away with. 251 
SALIVA AND GASTRIC JUICE. 
In man pure parotid saliva may easily be obtained by introducing a fine canula 
into the opening of the Stenonian duct, and submaxillary saliva, or rather a 
mixture of submaxillary and sublingual saliva, by similar catheterization of the 
Whartonian duct. In animals the duct may be dissected out and a canula 
introduced. 
Parotid saliva in man is clear and limpid, not viscid ; the reaction of the 
first drops secreted is often acid, the succeeding portions, at all events when 
the flow is at all copious, are alkaline; that is to say, the natural secretion 
is alkaline, but this may be obscured by acid changes taking place in the 
fluid which has been retained in the duct, possibly by the formation of an 
excess of carbonic acid. On standing the clear fluid becomes turbid from a 
precipitate of calcic carbonate, due to an escape of carbonic acid. It con- 
tains globulin and some other forms of albumin, with little or no mucin. 
Potassium sulphocyanate may also sometimes be detected, but structural 
elements are absent. 
Submaxillary saliva, in man and in most animals, differs from parotid 
saliva in being more alkaline, and from the presence of mucin more viscid; 
it contains salivary corpuscles, that is bodies closely resembling if not iden- 
tical with leucocytes, and, often in abundance, amorphous masses. The so- 
called chorda saliva in the dog, that is to say, saliva obtained by stimulating 
the chorda tympani nerve (of which we shall presently speak), is under 
ordinary circumstances thinner and less viscid, contains less mucin and 
fewer structural elements than the so-called sympathetic saliva, which is 
remarkable for its viscidity, its structural elements, and for its larger total 
of solids. 
Sublingual saliva is more viscid and contains more salts (in the dog 
about 1 per cent.) than the submaxillary saliva. 
The action of saliva varies in intensity in different animals. Thus in 
man, the pig, the guinea-pig, and the rat, both parotid and submaxillary 
and mixed saliva are amylolytic; the submaxillary saliva being in most 
cases more active than the parotid. In the rabbit, while the submaxillary 
saliva has scarcely any action, that of the parotid is energetic. The saliva 
of the cat is much less active than the above; that of the dog is still less 
active, indeed is almost inert. In the horse, sheep, and ox, the amolytic 
powers of either mixed saliva or of any one of the constituent juices are 
extremely feeble. 
Where the saliva of any gland is active, an aqueous infusion of the same 
gland is also active. The importance and bearing of this statement will be 
seen later on. From the aqueous infusion of the gland, as from saliva itself, 
the ferment may be approximately isolated. In some cases at least some 
ferment may be extracted from the gland even when the secretion is itself 
inactive. In fact, a ready method of preparing a highly amylolytic liquid 
tolerably free from proteid and other impurities is to mince finely a gland 
known to have an active secretion, such, for instance, as that of a rat, to 
dehydrate it by allowing it to stand under absolute alcohol for some days, 
and then, having poured off most of the alcohol and removed the remainder 
by evaporation at a low temperature, to cover the pieces of gland with strong 
glycerin. Though some of the ferment appears to be destroyed by the 
alcohol, a mere drop of such a glycerin extract rapidly converts starch 
into sugar. 
§ 187. There is no difficulty in obtaining what may be fairly considered 
as a normal saliva; but there are many obstacles in the way of determining 
the normal characters of the secretion of the stomach. When no food is 
Gastric Juice. 252 
THE TISSUES AND MECHANISMS OF DIGESTION. 
taken the stomach is at rest and no secretion takes place. When food is 
taken, the characters of the gastric juice secreted are obscured by the food 
with which it is mingled. The gastric membrane may, it is true, be arti- 
ficially stimulated, by touch, for instance, and a secretion obtained. This 
we may speak of as gastric juice, but it may be doubted whether it ought 
to be considered as normal gastric juice. And indeed, as we shall see, even 
the juice which is poured into the stomach during a meal, varies in compo- 
sition as digestion is going on. Hence the characters which we shall give 
of gastric juice must be considered as having a general value only. 
Gastric juice, obtained in as normal a condition as possible from the 
healthy stomach of a fasting dog by means of a gastric fistula, is a thin, 
almost colorless fluid with a sour taste and odor. 
In the operation for gastric fistula an incision is made through the abdominal 
walls, along the linea alba, the stomach is opened, and the lips of the gastric 
wound securely sewn to those of the incision in the abdominal walls. Union 
soon takes place, so that a permanent opening from the exterior into the inside 
of the stomach is established. A tube of proper construction, introduced at the 
time of the operation, becomes firmly secured in place by the contraction of 
healing. Through the tube the contents of the stomach can be received, and the 
mucous membrane stimulated at pleasure. 
When obtained from a natural fistula in man, its specific gravity has 
been found to differ little from that of water, varying from 1.001 to 1.010, 
and the amount of solids present to be correspondingly small. In animals 
pure gastric juice seems to be equally poor in solids, the higher estimates 
which some observers have obtained being probably due to admixture with 
food, etc. 
Of the solid matters present about half are inorganic salts, chiefly alka- 
line (sodium) chlorides, with small quantities of phosphates. The organic 
material consists of pepsin, a body to be described immediately, mixed with 
other substances of undetermined nature. In a healthy stomach gastric juice 
contains a very small quantity only of mucin, unless some submaxillary 
saliva has been swallowed. 
The reaction is distinctly acid, and the acidity is normally due to free 
hydrochloric acid. This is shown by various proofs, among which we may 
mention the conclusive fact that the amount of chlorine present in gastric 
juice is more than would suffice to form chlorides with all the bases present, 
and that the excess, if regarded as existing in the form of hydrochloric acid, 
corresponds exactly to the quantity of free acid present. Lactic and butyric 
and other acids when present are secondary products, arising either by their 
respective fermentations from articles of food, or from the decomposition of 
their alkaline or other salts. In man the amount of free hydrochloric acid 
in healthy juice may be stated to be about 0.2 per cent., but in some animals 
it is probably higher. 
§ 188. On starch gastric juice has no amylolytic action ; on the con- 
trary, when saliva is mixed with gastric juice any amylolytic ferment which 
may be present in the former is at once prevented from acting by the acidity 
of the mixture. Moreover, in a very short time, especially at the tempera- 
ture of the body, the amylolytic ferment is destroyed by the acid, so that 
even on neutralization the mixture is unable to convert starch into sugar. 
On dextrose healthy gastric juice has no effect. And its power of con- 
verting cane-sugar seems to be less than that of hydrochloric acid diluted to 
the same degree of acidity as itself. In an unhealthy stomach, however, 
containing much mucus, the gastric juice is very active in converting cane- 
sugar into dextrose. This power seems to be due to the presence in the 
mucus of a special ferment, analogous to, but quite distinct from, the SALIVA AND GASTRIC JUICE. 
253 
ptyalin of saliva. An excessive quantity of cane-sugar introduced into 
the stomach causes a secretion of mucus, and hence provides for its own 
conversion. 
On fats gastric juice has at most a limited action. When adipose tissue 
is eaten, the chief change which takes place in the stomach is that the pro- 
teid and gelatiniferous envelopes of the fat-cells are dissolved, and the fats 
set free. Though there is experimental evidence that emulsion of fats to a 
certain extent does take place in the stomach, the great mass of the fat of 
a meal is not so changed. 
Such minerals as are soluble in free hydrochloric acid are for the most 
part dissolved ; though there is a difference in this and in some other re- 
spects between gastric juice and simple free hydrochloric acid diluted with 
water to the same degree of acidity as the juice, the presence either of the 
pepsin or of other bodies apparently modifying the solvent action of the 
acid. 
The essential property of gastric juice is the power of dissolving proteid 
matters and of converting them into a substance called peptone. 
Action of gastric juice on proteids. The results are essentially the same, 
whether natural juice obtained by means of a fistula, or artificial juice, i. e. 
an acid infusion of the mucous membrane of the stomach, be used. 
Artificial gastric juice maybe prepared in any of the following ways : 
1. The mucous membrane of a pig's or dog's stomach is removed from the mus- 
cular coat, finely minced, rubbed in a mortar with pounded glass and extracted 
with water. The aqueous extract filtered and acidulated (it is in itself somewhat 
acid), until it has a free acidity corresponding to 0.2 per cent, of hydrochloric 
acid, contains but little of the products of digestion, such as peptone, but is fairly 
potent. 
2. The mucous membrane similarly prepared and minced is allowed to digest 
at 35° C. in a large quantity of hydrochloric acid diluted to 0.2 per cent. The 
greater part of the membrane disappears, shreds only being left, and the some- 
what opalescent liquid can be decanted and filtered. The filtrate has powerful 
digestive (peptic) properties, but contains a considerable amount of the products 
of digestion (peptone, etc.) arising from the digestion of the mucous membrane 
itself.1 
3. The mucous membrane, similarly prepared and minced, is thrown into acom- 
f>aratively large quantity of concentrated glycerin, and allowed to stand. The mem- 
>rane may be previously dehydrated by being allowed to stand under alcohol, but 
this is not necessary, and a too prolonged action of the alcohol injures or even de- 
stroys the activity of the product. The decanted clear glycerin, in which a com- 
paratively small quantity of the ordinary proteids of the mucous membrane are 
dissolved, if added to hydrochloric acid of 0.2 per cent, (about 1 c.c. of the glycerin 
to 100 c.c. of the dilute acid are sufficient), makes an artificial juice tolerably free 
from ordinary proteids and peptone, and of remarkable potency, the presence of 
the glycerin not interfering with the results. 
Before proceeding to study the action of gastric juice on proteids, it will 
be useful to review very briefly the chief characters of the more important 
members of the group. 
The more important proteids which we have thus far studied are: 1. 
Fibrin, insoluble in water and not really soluble (i. e., without change) in 
saline solutions. 2. Myosin, insoluble in water, but soluble in saline solu- 
tions, provided these are not too dilute or too concentrated. 3. Globulin 
(including paraglobulin, fibrinogen, etc.), insoluble in water, but readily 
soluble in even very dilute saline solutions. 4. Albumin, serum-albumin, 
soluble in water in the absence of all salts. 5. Acid-albumin, into which 
globulins and myosin are rapidly converted by the action of dilute acids, 
1 These, however, may be removed by concentration at 40° C. and subsequent dialysis. 254 
THE TISSUES AND MECHANISMS OF DIGESTION. 
the particular acid-albumin into which the myosin of muscle is changed, 
being sometimes called syntonin. If the reagent used be not dilute acid, 
but dilute alkali, the product is called alkali-albumin. The two bodies, 
acid-albumin and alkali-albumin, are very parallel in their characters, and 
may readily be converted, the one into the other, by the use of dilute alkali 
or dilute acid respectively. Their most important common characters are 
insolubility in water and in saline solutions and ready solubility in dilute 
acids and alkalies. 6. Coagulated proteids. As we have seen, when fibrin 
suspended in water, serum-albumin in solution, acid-albumin or alkali- 
albumin suspended in water, or paraglobulin suspended in water or dis- 
solved in a dilute saline solution, are heated to a temperature which for the 
whole group may be put down at about 75° to 80° C., each of them be- 
comes coagulated, ami after the change is insoluble in water, saline solu- 
tions, dilute acids, etc., in fact in everything but very strong acids. Myosin 
and fibrinogen undergo a similar change at a lower temperature, viz., about 
56° C. We may, for present purposes, speak of all these proteids thus 
changed under the one term of coagulated proteids. 
To the above list we may now add two other proteids, viz.: 7. A kind 
of albumin which forms the great bulk of the proteid matter present in raw 
" white of egg," and which, since it differs in minor characters from the 
albumin of blood and of the tissues, is called egg-albumin. 8. The peculiar 
proteid casein, an important constituent of milk. This may perhaps be 
regarded as a naturally occurring alkali-albumin, since it has many resem- 
blances to the artificial alkali-albumin ; but for several reasons it is desir- 
able to consider it as an independent body. 
Egg-albumin, like serum-albumin, becomes coagulated at a temperature 
of about 75° to 80° C., and though casein as it naturally exists in milk is 
not coagulated on boiling, when separated out in a special way, and sus- 
pended in water in which it is insoluble, it becomes coagulated at about 75° 
to 80° C. 
It will be observed that all these proteids form, as regards their solubili- 
ties, a descending series in the following order : coagulated proteids ; fibrin ; 
acid-albumin with alkali-albumin, and casein ; myosin, globulin ; serum- 
albumin with egg-albumin. 
We must now return to the action of gastric juice. 
If a few shreds of fibrin, obtained by whipping blood, after being thor- 
oughly washed and boiled and thus by the boiling coagulated, be thrown 
into a quantity of gastric juice, and the mixture be exposed to a tempera- 
ture of from 35° to 40° C., the fibrin will speedily, in some cases in a few 
minutes, be dissolved. The shreds first swell up and become transparent, 
then gradually dissolve, and finally disappear with the exception of some gran- 
ular debris, the amount of which, though generally small, varies according to 
circumstances. If raw, that is, unboiled, uncoagulated fibrin be employed 
the same changes may be observed, but they take place much more rapidly. 
If small morsels of coagulated albumin, such as white of egg, be treated 
in the same way, the same solution is observed. The pieces become trans- 
parent at their surfaces ; this is especially seen at the edges, which gradually 
become rounded down; and solution steadily progresses from the outside 
of the piece inward. 
If any other form of coagulated albumin (e. g., precipitated acid- or 
alkali-albumin, suspended in water and boiled) be treated in the same way, 
a similar solution takes place. The readiness with which the solution is 
effected, will depend, cateris paribus, on the smallness of the pieces, or rather 
on the amount of surface as compared with bulk, which is presented to the 
action of the juice. SALIVA AND GASTRIC JUICE. 
255 
Gastric juice then readily dissolves coagulated proteids which other- 
wise are insoluble, or soluble only, and that with difficulty, in very strong 
acids. 
When proteids which are soluble in water, or in dilute acid, are treated 
with gastric juice, no visible change takes place ; but nevertheless, it is found 
on examination that the solutions have undergone a remarkable change, the 
nature of which is easily seen by contrasting it with the change effected by 
dilute acid alone. If raw white of egg, largely diluted with water and 
strained, be treated with a sufficient quantity of dilute hydrochloric acid, the 
opalescence or turbidity which appeared in the white of egg on dilution (and 
which is due to the precipitation of various forms of globulin accompanying 
the egg-albumin in the raw white) disappears, and a clear mixture results. 
If a portion of the mixture be at once boiled, a large deposit of coagulated 
albumin occurs. If, however, the mixture be exposed to 50° to 55° C. 
for some time, the amount of coagulation which is produced by boiling a 
specimen becomes less, and, finally, boiling produces no coagulation whatever. 
By neutralization, however, the whole of the albumin (with such restrictions 
as the presence of certain neutral salts may cause) may be obtained in the 
form of acid-albumin, the filtrate after neutralization containing no proteids 
at all (or a very small quantity). Thus the whole of the albumin present 
in the white of egg may be, in time, converted, by the simple action of dilute 
hydrochloric acid, into acid-albumin. Serum-albumin similarly treated 
undergoes in course of time a similar conversion into acid-albumin, and we 
have already seen (§ 59) that solutions of myosin or of any of the globulins 
are with remarkable rapidity converted into acid-albumin. Thus simple 
dilute hydrochloric acid of the same degree of acidity as gastric juice, merely 
converts these proteids into acid-albumin, the rapidity of the change differ- 
ing with the different proteids, being in some cases very slow, and requiring 
a relatively high temperature. 
If the same white of egg or serum-albumin be treated with gastric juice 
instead of simple dilute hydrochloric acid, the events for some time seem the 
same. Thus after a while boiling causes no coagulation, while neutraliza- 
tion gives a considerable precipitate of a proteid body, which being insoluble 
in water and in sodium chloride solutions and soluble in dilute alkali and 
acids, at least closely resembles acid-albumin. But it is found that only a 
portion of the proteid originally present in the white of egg or serum-albumin 
can thus be regained by precipitation. Though the neutralization be carried 
out with the greatest care it will be found, on filtering off the neutralization 
precipitate, that is, the acid-albumin, that the filtrate, as shown on employ- 
ing the various tests for proteid (see § 15) or on adding an adequate quan- 
tity of strong alcohol, still contains a very considerable quantity of proteid 
matter ; and, on the whole, the longer the digestion is carried on, the greater 
is the proportion borne by the proteid remaining in solution to the precipi- 
tate thrown down on neutralization ; indeed, in some cases at all events, all 
the proteid matter originally present remains in solution, and there is no 
neutralization precipitation at all, or at most a wholly insignificant one. 
§ 189. The proteid matter, thus remaining in solution after neutraliza- 
tion differs from all the proteids which we have hitherto studied, inasmuch 
as, though existing in a neutral solution, it is not coagulated by heat, like 
the egg-albumin or serum-albumin from which it has been produced; the 
solution, after the neutralization precipitate has been filtered off, remains 
quite clear when boiled. The only other solutions of proteids which do not 
coagulate on boiling are solutions of acid- or alkali-albumin ; but these solu- 
tions must be acid or alkali respectively ; the acid-albumin or alkali-albumin 
is insoluble in a neutral solution, and when simply suspended in water is 256 
THE TISSUES AND MECHANISMS OF DIGESTION. 
readily coagulated at a temperature of 75° C. This new proteid matter of 
which wTe are speaking is soluble in neutral solutions, indeed in distilled 
water, and can under no circumstances be coagulated by heat. 
Upon examination we find that the new proteid matter thus left in solu- 
tion consists of at least two distinct proteid bodies. If to the solution ammo- 
nium sulphate be added, part of the proteid matter is precipitated while part 
is still left in solution. The proteid body thus thrown down is called albu- 
mose (there are several varieties of albumose, but these need not now detain 
us). It approaches albumin in nature by reason of the fact that it will not 
diffuse through membranes ; that it differs, however, widely from that proteid 
is shown by its solutions not coagulating on boiling. The body which is not 
thrown down by ammonium sulphate is called peptone; it differs from albu- 
mose in being diffusible, for it will pass through membranes. The diffusion 
is not nearly so rapid as that of salts, sugar, and other similar substances ; 
indeed solutions of peptones may be freed from salts by dialysis. But it is 
very marked as compared with that of other proteids; these pass through 
membranes with the greatest difficulty, if at all. Peptone is insoluble in 
alcohol, and may be precipitated from its solutions by the addition of an 
adequate quantity of this reagent; but for this purpose a very large excess 
of alcohol is needed, otherwise much of the peptone remains in solution. It 
may be kept under alcohol for a long time without undergoing change, 
whereas other proteids are more or less slowly coagulated by alcohol. A 
useful test for peptone is furnished by the fact that a solution of peptone, 
mixed with a strong solution of caustic potash, gives on addition of a mere 
trace of cupric sulphate in the cold a pink color, whereas other proteids give 
a violet color. In applying this test, however, care must be taken not to add 
too much cupric sulphate, since in that case a violet color, deepening on 
boiling, that is, the ordinary proteid reaction (see § 15), is obtained. 
There are reasons for thinking that there are several kinds, or at least 
more than one kind, of peptone ; but we may for the present regard the sub- 
stance as one. For a long time albumose was confounded with peptone, and 
many of the commercial forms of " peptone" consist largely of albumose; 
indeed, the two are closely allied and have many reactions in common, the 
most striking differences being that peptone is diffusible, while albumose is 
not, or hardly at all, and that peptone is not, like albumose, precipitated by 
ammonium sulphate. The amount of albumose appearing in a digestion 
experiment, relative to the amount of true peptone, depends on the activity 
of the juice, and other circumstances. We may regard albumose as a less 
complete product of digestion than peptone. 
The precipitate thrown down by neutralization after the action of gastric 
juice on egg- or serum-albumin resembles, in its general characters, acid- 
albumin. Since, however, it probably is distinguishable from the body or 
bodies produced by the action of simple acid on muscle or white of egg, it is 
best to reserve for it the name ofparapeptone, which was originally applied to it. 
Thus the digestion by gastric juice of solutions of egg-albumin or serum- 
albumin results in the conversion of all the proteids present into peptone, 
albumose, and parapeptone, of which the first may be considered as the final 
and chief product, and the other two as intermediate products, occurring in 
varying quantity, possibly not always formed, and probably of secondary 
importance. When fibrin, either raw or boiled, or any form of coagulated 
proteid is dissolved and seems to disappear under the influence of gastric 
juice, the same products, peptone, albumose, and parapeptone make their 
appearance. The same bodies result when myosin or any of the globulins 
are subjected to the action of the juice ; and acid-albumin or alkali-albumin 
is similarly converted into albumose and peptone. SALIVA AND GASTRIC JUICE. 
257 
It is obvious that the effect of the action of the gastric juice is to change 
the less soluble proteid into a more soluble form, the change being either 
completed up to the stage of peptone, the most soluble of all proteids, or 
being left in part incomplete. This will be seen from the following tabular 
arrangement of proteids according to their solubilities: 
Soluble in distilled water. 
Aqueous solutions not coagulated on boiling : 
Diffusible Peptone. 
Not diffusible Albumose. 
Aqueous solutions coagulated on boiling Albumin. 
Readily soluble in dilute saline solutions (NaCI 1 per 
cent.) 
Insoluble in distilled water. 
Soluble only in stronger saline solutions (INaUl 5 to 10 
percent.) 
Globulins. 
Myosin. 
Insoluble in dilute saline solutions. 
Acid-albumin. 
Alkali-albumin. 
Casein. 
Readily soluble in dilute acid (IIC1 0.1 percent.) in 
the cold 
Soluble with difficulty in dilute acid, that is at high 
temperature (60° C.) and after prolonged treatment 
only 
Fibrin. 
Insoluble in dilute acids, soluble only in strong acids . Coagulated proteid. 
Milk when treated with gastric juice is first of all " curdled." This is the 
result partially of the action of the free acid, but chiefly of the special action 
of a particular constituent of gastric juice, of which we shall speak here- 
after. The curd consists of a particular proteid matter mixed with fat; and 
this proteid matter is subsequently dissolved with the same appearance of pep- 
tone, albumose, and parapeptone as in the case of other proteids. In fact, the 
digestion by gastric juice of all the varieties of proteids consists in the con- 
version of the proteid into peptone, with the concomitant appearance of a 
certain variable amount of albumose and parapeptone. 
§ 190. Circumstances affecting gastric digestion. The solvent action of 
gastric juice on proteids is modified by a variety of circumstances. The 
nature of the proteid itself makes a difference, though this is determined 
probably by physical rather than by chemical characters. Hence in making 
a series of comparative trials the same proteid should be used, and the form 
of proteid most convenient for the purpose is fibrin. If it be desired simply 
to ascertain whether any given specimen has any digestive powers at all, it 
is best to use boiled fibrin, since raw fibrin is eventually dissolved by dilute 
hydrochloric acid alone, probably on account of some pepsin previously 
present in the blood becoming entangled with the fibrin during clotting. 
But in estimating quantitatively the peptic power of two specimens of 
gastric juice under different conditions, raw fibrin prepared by Griitzner's 
method is most convenient. 
Portions of well-washed fibrin are stained with carmine and again washed to 
remove the superfluous coloring matter. A fragment of this colored fibrin thrown 
into an active juice on becoming dissolved, gives up its color to the fluid. 
Hence if the same stock of colored fibrin be used in a series of experiments, and 
the same bulks of fibrin and of fluid be used in each case, the amount of fibrin 
dissolved may be fairly estimated by the depth of tint given to the fluid. Fibrin 
thus colored with carmine may be preserved in ether. 258 
THE TISSUES AND MECHANISMS OF DIGESTION. 
Since, if sufficient time be allowed, even a small quantity of gastric juice 
will dissolve at least a very large if not an indefinite quantity of fibrin, we 
are led to take, as a measure of the activity of a specimen of gastric juice, 
not the quantity of fibrin which it will ultimately dissolve, but the rapidity 
with which it dissolves a given quantity. 
The greater the surface presented to the action of the juice, the more 
rapid the solution ; hence minute division and constant movement favor diges- 
tion. And this is probably, in part at least, the reason why a fragment of 
spongy filamentous fibrin is more readily dissolved than a solid clump of 
boiled white of egg of the same size. Neutralization of the juice wholly 
arrests digestion ; fibrin may be submitted for an almost indefinite time to 
the action of neutralized gastric juice without being digested. If the neu- 
tralized juice be properly acidified, it may again become active; when gas- 
tric juice, however, has been made alkaline, and kept for some time at a 
temperature of 35° C., its solvent powers are not only suspended b'ut actually 
destroyed. Digestion is most rapid with dilute hydrochloric acid of 0.2 per 
cent, (the acidity of natural gastric juice). If the juice contains much more 
or much less free acid than this, its activity is distinctly impaired. Other 
acids, lactic, phosphoric, etc., may be substituted for hydrochloric; but they 
are not so effectual, and the degree of acidity most useful varies with the 
different acids. The presence of neutral salts, such as sodium chloride, in 
excess is injurious. The action of mammalian gastric juice is most rapid at 
35°-40° C.; at the ordinary temperature it is much slower, and at about 
0° C. ceases altogether. The juice may be kept, however, at 0° C. for an 
indefinite period without injury to its powers. The gastric juice of cold- 
blooded vertebrates is relatively more active at low temperatures than that 
of warm-blooded mammals or birds. 
At temperatures much above 40° or 45° the action of the juice is im- 
paired. By boiling for a few minutes the activity of the most powerful 
juice is irrevocably destroyed. The presence in a concentrated form of the 
products of digestion hinders the process of solution. If a large quantity 
of fibrin be placed in a small quantity of juice, digestion is soon arrested; 
on dilution with the normal hydrochloric acid (0.2 per cent.), or if the mix- 
ture be submitted to dialysis to remove the peptones formed, and its acidity 
be kept up to the normal, the action recommences. By removing the prod- 
ucts of digestion as fast as they are formed, and by keeping the acidity up 
to the normal, a given amount of gastric juice may be made to digest a very 
large quantity of proteid material. Whether th'e quantity is really un- 
limited is disputed ; but in any case the energies of the juice are not rapidly 
exhausted by the act of digestion. 
§ 191. Nature of the action. All these facts go to show that the digestive 
action of gastric juice on proteids, like that of saliva on starch, is a ferment- 
action ; in other words, that the solvent action of gastric juice is essentially 
due to the presence in it of a ferment-body. To this ferment-body, which 
as yet has been only approximately isolated, the name of pepsin has been 
given. It is present not only in gastric juice, but also in the glands of the 
gastric mucous membrane, especially in certain parts and under certain 
conditions which we shall study presently. The glycerin extract of gastric 
mucous membrane, at any rate of that which has been dehydrated, contains 
a minimal quantity of proteid matter, and yet is intensely peptic. Other 
methods, such as the elaborate one of Briicke, give us a material which, 
though containing nitrogen, exhibits none of the ordinary proteid reactions, 
and yet in concert with normal dilute hydrochloric acid is peptic in a very 
high degree. We seem, therefore, justified in asserting that pepsin is not a 
proteid, but it would be hazardous to make any dogmatic statement con- SALIVA AND GASTRIC JUICE. 
259 
cerning a substance obtained in so small a quantity at a time that its 
exact chemical characters have not yet been ascertained. At present 
the manifestation of peptic powers is our only safe test of the presence 
of pepsin. 
In one important respect pepsin, the ferment of gastric juice, differs from 
ptyalin, the ferment of saliva. Saliva is active in a perfectly neutral 
medium, and there seems to be no special connection between the ferment 
and any alkali or acid. In gastric juice, however, there is a strong tie 
between the acid and the ferment, so strong that some writers speak of 
pepsin and hydrochloric acid as forming together a compound, pepto- 
hydrochloric acid. 
In the absence of exact knowledge of the constitution of proteids, we 
cannot state distinctly what is the precise nature of the change into pep- 
tone ; the various proteids differ from each other in elementary composition 
quite as widely as does peptone from any of them. Judging from the 
analogy with the action of saliva on starch, we may fairly suppose that 
the process is at bottom one of hydration; and this view is further sug- 
gested by the fact that peptone closely resembling, if not identical with, 
that obtained by gastric digestion, may be obtained by the action of the strong 
acids, by the prolonged action of dilute acids especially at a high tempera- 
ture, or simply by digestion with superheated water in a Papin's digester, 
that is to say, by means of agents which, in other cases produce their 
effects by bringing about hydrolytic changes; beyond this we cannot at 
present go. 
§ 192. All proteids, as far as we know, are converted by pepsin into pep- 
tone. Concerning the action of gastric juice on other nitrogenous sub- 
stances more or less allied to proteids, but not truly proteid in nature, our 
knowledge is at present imperfect. Mucin, nuclein, and the chemical basis 
of horny tissues are wholly unaffected by gastric juice. The gelatiniferous 
tissues are dissolved by it; and the bundles and membranes of connective 
tissue are very speedily so far affected by it that at a very early stage of 
digestion the bundles and elementary fibres of muscles which are bound 
together by connective tissue fall asunder; moreover, both prepared gelatin 
and the gelatiniferous basis of connective tissue in its natural condition, 
that is without being previously heated with water, are by it changed into 
a substance so far analogous with peptone that the characteristic property 
of gelatinization is entirely lost. Chondrin and the elastic tissues undergo 
a similar change. 
§ 193. Action of gastric juice on milk. It has long been known that an 
infusion of calves' stomach, called rennet, has a remarkable effect in rapidly 
curdling milk, and this property is made use of in the manufacture of 
cheese. Gastric juice has a similar effect: milk when subjected to the 
action of gastric juice is first curdled and then digested. If a few drops of 
gastric juice be added to a little milk in a test-tube, and the mixture ex- 
posed to a temperature of 40° C.,the milk will curdle into a complete clot in 
a very short time. If the action be continued the curd or clot will be ulti- 
mately dissolved and digested. IMilk contains, besides a peculiar form, or 
peculiar forms of albumin, fats, milk-sugar, and various salines, the peculiar 
proteid casein. In natural milk casein is present in solution, and " curd- 
ling" consists essentially in the soluble casein being converted (or more 
probably as we shall see presently, split up) into an insoluble modification 
of casein, which as it is being precipitated carries down with it a great deal 
of the fat and so forms the "curd." Now casein is readily precipitated 
from milk upon the addition of a small quantity of acid, and it might be 
supposed that the curdling effect of gastric juice was due to its acid reaction. 260 
THE TISSUES AND MECHANISMS OF DIGESTION. 
But this is not the case, for neutralized gastric juice, or neutral rennet, is 
equally efficacious. 
The curdling action of rennet is closely dependent on temperature, being 
like the peptic action of gastric juice favored by a rise of temperature up to 
about 40° C. Moreover the curdling action is destroyed by previous boiling 
of the juice or rennet. These facts suggest that a ferment is at the bottom 
of the matter; and, indeed, all the features of the action support this view. 
Moreover, as a matter of fact, a curdling ferment may be extracted by 
glycerin and by the other methods used for preparing ferments. The fer- 
ment, however, is not pepsin, but some other body ; and the two may be 
separated from each other. 
It might be thought that the rennet-ferment, rennin we may call it, acted 
by inducing a fermentation in the sugar of milk, giving rise to lactic acid 
which precipitated the casein by virtue of its being an acid. But this view 
is disproved by the following facts which show that the ferment produces its 
curdling effect by acting directly on the natural casein itself. Casein maybe 
precipitated unchanged, that is, capable of redissolving in water (the presence 
of calcic phosphate being assumed) by saturating milk with neutral saline 
bodies (such as sodium chloride or magnesium sulphate); and by being pre- 
cipitated and redissolved more than once may be obtained largely free from 
fat ami wholly free from milk-sugar. Such solutions of isolated casein freed 
from milk-sugar may be made to curdle like natural milk by the addition 
of rennin, showing that the milk-sugar has nothing to do with the matter. 
Moreover, the precipitate thrown down from milk by dilute acids, lactic acid 
included, is itself unaltered or very slightly altered casein, not curd, and 
with care may be so prepared as to be redissolved into solutions which 
curdle with rennin, like solutions of casein prepared by means of neutral 
salts. 
When isolated casein is curdled by means of rennin, two proteids, it is 
stated, make their appearance, one which is soluble and allied to albumin, 
and another which is insoluble and forms the curd. Curdling, therefore, 
according to this result appears to be the splitting up by a ferment of a more 
complex body ; and it is interesting to observe, as perhaps throwing light on 
the somewhat analogous formation of the fibrin, that this curdling action 
will not take place if calcic phosphate be wholly absent from the mixture. 
The calcic phosphate appears to play a peculiar part in determining the 
insolubility of the curd, for there is evidence that in the absence of calcic 
phosphate the ferment has power to attack the casein and split it up, but 
that both products remain in solution ; if calcic phosphate be present, the 
one, viz., the curd,1 becomes insoluble. 
Rennin is abundant in the gastric juice and in the gastric mucous mem- 
brane of ruminants, but is also found in the gastric juice of other animals, 
and either it, or what we shall presently have occasion to speak of as the 
antecedent of the ferment or zymogen, is present also in the mucous membrane 
of the stomach of most animals. A very similar if not identical ferment has 
also been found in many plants. 
The Act of Secretion of Saliva and Gastric Juice and the 
Nervous Mechanisms which Regulate it. 
§ 194. The saliva and gastric juice whose properties we have studied, 
though so different from each other, are both drawn ultimately from one 
1 It might be useful, in order to distinguish the curd from the natural soluble 
casein, to call the former tyrein (rvpos, cheese), and so reserve the name of casein for 
the latter. SECRETION OF SALIVA AND GASTRIC JUICE. 
261 
common source, the blood, and they are poured into the alimentary canal, 
not in a continuous flow, but intermittently as occasion may demand. The 
epithelial cells which supply them have their periods of rest and of activ- 
ity, and the amount and quality of the fluids which the cells secrete are 
determined by the needs of the economy as the food passes along the canal. 
We have now to consider how the epithelial cell manufactures its special 
secretion out of the materials supplied to it by the blood, and how the cell 
is called into activity by the presence of food, it may be as in the case of 
saliva at some distance from itself, or by circumstances which do not bear 
directly on itself. In dealing with these matters in connection with the 
digestive juices, we shall have to enter at some length into the physiology 
of secretion in general. 
The question which presents itself first is: By what mechanism is the 
activity of the secreting cells brought into play ? 
While fasting, a small quantity only of saliva is poured into the mouth ; 
the buccal cavity is just moist and nothing more. When food is taken, or 
when any sapid or stimulating substance, or indeed a body of any kind, is 
introduced into the mouth, a flow is induced which may be very copious. 
Indeed the quantity secreted in ordinary life during 24 hours has been 
roughly calculated at as much as from 1 to 2 litres. An abundant secretion 
in the absence of food in the mouth may be called forth by an emotion, as 
when the mouth waters at the sight of food, or by a smell, or by events oc- 
curring in the stomach, as in some cases of nausea. Evidently in these in- 
stances some nervous mechanism is at work. In studying the action of this 
nervous mechanism, it will be of advantage to confine our attention at first 
to the submaxillary gland. 
$ 195. The submaxillary gland is supplied with two sets of nerves. 
These are represented in Fig. 81, which is a very diagrammatic rendering 
of the appearances presented when the submaxillary gland is prepared for 
an experiment in a dog, the animal being placed on its back and the 
gland exposed from the neck. The one set, and that the more important, 
belongs to the chorda tympani nerve (ch. f/'). This is a small nerve, which 
branches off from the facial or seventh cranial nerve in the Fallopian canal 
before the nerve issues from the skull. Whether it really belongs to the 
facial proper has been doubted ; in man the fibres which form it are either 
fibres coming not from the roots of the facial proper, but from the portio 
intermedia Wrisbergi, or, according to some, fibres which, though joining 
the facial in the Fallopian canal, are ultimately derived from another 
(the fifth) cranial nerve. Leaving the facial nerve, the chorda tympani 
passes through the tympanic cavity or drum of the ear (hence the name) 
and joins or rather runs in company (ch. t.') with the lingual or gustatory 
branch of the fifth nerve. Some of the fibres run on with the lingual right 
down to the tongue (these are not shown in the figure), but many leave the 
lingual as a slender nerve (ch. t.), which, reaching Wharton's duct or duct 
of the submaxillary gland (sm. d.) runs along the duct to the gland. As 
the nerve courses along the duct nerve-cells make their appearance among 
the fibres, and these are especially abundant just after the duct enters the 
hilus of the gland. The fibres may be traced into the gland for some dis- 
tance, but, as we have said, their ultimate ending has not yet been definitely 
made out. Along its whole course up to the gland, the fibres of the chorda 
are very fine medullated fibres, but they lose their medulla in the gland. 
The other set of nerve-fibres reaches the gland along the small arteries 
of the gland. These are non-medullated fibres mixed with a few medullated 
fibres, and may be traced back to the superior cervical ganglion. From 
thence they may be traced still further back down the cervical sympathetic 262 
THE TISSUES AND MECHANISMS OF DIGESTION. 
to the spinal cord, following apparently the same tract as the vaso-constrictor 
fibres, treated of in § 152. 
§ 196. If a tube be placed in the duct, it is seen that when sapid sub- 
stances are placed on the tongue, or the tongue is stimulated in any other 
way, or the lingual nerve is laid bare and stimulated with an interrupted 
current, a copious flow of saliva takes place. If the sympathetic be divided, 
stimulation of the tongue or lingual nerve still produces a flow. But if the 
small chorda nerve be divided, stimulation of the tongue or lingual nerve 
produces no flow. 
Evidently the flow of saliva is a nervous reflex action, the lingual nerve 
serving as the channel for the afferent and the small chorda nerve for the 
efferent impulses. If the trunk of the lingual be divided above the point 
where the chorda leaves it, as at n. I.', Fig. 81, stimulation of the (front part 
Fig. 81. 
Diagrammatic Representation of the Submaxillary Gland of the Dog, with its Nerves and 
Bloodvessels. The dissection has been on an animal lying on its back, but since all the parts 
shown in the figure cannot be seen from any one point of view, the figure does not give the exact 
anatomical relations of the several structures. 
sm. gid. The submaxillary gland, into the duct (sm. d.) of which a canula has been tied. The 
sublingual gland and duct are not shown, n.l., n.l'. The lingual branch of the fifth nerve, the 
part n.l. is going to the tongue, ch.t., ch.t'., ch.t". The chorda tympani. The part ch.t". is proceeding 
from the facial nerve ; at ch. t'. it becomes conjoined with the lingual n.l'., and afterward diverging 
passes as ch. t. to the gland along the duct; the continuation of the nerve in company with the 
lingual n.l., is not shown, sm. gl. The submaxillary ganglion with its several roots, a. car. The 
carotid artery, two small branches of which, a. sm. a. and r. sm.p., pass to the anterior and poste- 
rior parts of the gland, v. sm. The anterior and posterior veins from the gland, falling into v.j., 
the jugular vein. v. sym. The conjoined vagus and sympathetic trunks, g.cer.s. The upper cer- 
vical ganglion, two branches of which, forming a plexus (aj.) over the facial artery, are dis- 
tributed (n. sym. sm.) along the two glandular arteries to the anterior and posterior portions of the 
gland. 
The arrows indicate the direction taken by the nervous impulses during reflex stimulation of 
the gland. They ascend to the brain by the lingual and descend by the chorda tympani. 
of) tongue produces, under ordinary circumstances, no flow. This shows 
that the centre of the reflex action is higher up than the point of section ; it 
lies in fact in the brain. 
In the angle between the lingual and the chorda, where the latter leaves the 
former to pass to the gland, lies the small submaxillary ganglion (represented dia- 
grammatically in Fig. 81, sm. gl.\ This consists of small masses of nerve-cells SECRETION OF SALIVA AND GASTRIC JUICE. 
263 
lying on the small bundles of nerve-fibres which spread out like a fan from the 
lingual and chorda tympani nerves (ch. t.) toward the ducts of the submaxillary and 
sublingual glands. It has been much debated whether this ganglion can act as a 
centre of reflex action in connection with the submaxillary gland, but no conclusive 
evidence that it does so act has as yet been shown ; it probably belongs in reality to 
the sublingual gland. 
Stimulation of the glosso-pharyngeal is even more effectual than that of 
the lingual. Probably this indeed is the chief afferent nerve in ordinary 
secretion. Stimulation of the mucous membrane of the stomach (as by food 
introduced through a gastric fistula) or of the vagus may also produce a flow 
of saliva, as indeed may stimulation of the sciatic, and probably of many 
other afferent nerves. All these'cases are instances of reflex action, the 
cerebro-spinal system acting as a centre. We may further define the centre 
as a part of the medulla oblongata, apparently not far removed from the vaso- 
motor centre. When the brain is removed down to the medulla oblongata, 
that organ being left intact, a flow of saliva may still be obtained by 
adequate stimulation of various afferent nerves; when the medulla is de- 
stroyed no such action is possible. And a flow of saliva may be produced 
by direct stimulation of the medulla itself. When a flow of saliva is ex- 
cited by ideas, or by emotions, the nervous processes begin in the higher 
parts of the brain, and descend thence to the medulla before they give rise 
to distinctly efferent impulses ; and it would appear that these higher parts 
of the brain are called into action when a flow of saliva is excited by 
distinct sensations of taste. 
Considering, then, the flow of saliva as a reflex act, the centre of which 
lies in the medulla oblongata, we may imagine the efferent impulses passing 
from that centre to the gland either by the chorda tympani or by the sym- 
pathetic nerve. Although it would perhaps be rash to say that in this 
relation the sympathetic nerve never acts as an efferent channel, as a matter 
of fact we have no satisfactory experimental evidence that it does so ; and 
we may, therefore, state that, practically, the chorda tympani is the sole 
efferent nerve. Section of that nerve, either where the fibres pass from the 
lingual nerve and the submaxillary ganglion to the gland, or where it runs 
in the same sheath as the lingual, or in any part of its course from the main 
facial trunk to the lingual, puts an end, as far as we know, to the possibility 
of any flow being excited by stimuli applied to the sensory nerves, or to the 
sentient surfaces of the mouth or of other parts of the body. 
The natural reflex act of secretion may be inhibited, like the reflex action 
of the vasomotor nerves, at its centre. Thus when, as in the old rice ordeal, 
fear parches the mouth, it is probable that the afferent impulses caused by 
the presence of food in the mouth cease, through emotional inhibition of 
their reflex centre, to give rise to efferent impulses. 
§ 197. In life, then, the flow of saliva is brought about by the advent to 
the gland along the chorda tympani of efferent impulses, started chiefly by 
reflex actions. The inquiry thus narrows itself to the question : In what 
manner do these efferent impulses cause the increase of flow? 
If in a dog a tube be introduced into Wharton's duct, and the chorda be 
divided, the flow, if any be going on, is from the lack of efferent impulses 
arrested. On passing an interrupted current through the peripheral portion 
of the chorda, a copious secretion at once takes place, and the saliva begins 
to rise rapidly in the tube; a very short time after the application of the 
current the flow reaches a maximum which is maintained for some time, and 
then, if the current be long continued, gradually lessens. If the current be 
applied for a short time only, the secretion may last for some time after the 
current has been shut off. The saliva thus obtained is but slightly viscid, 264 
THE TISSUES AND MECHANISMS OF DIGESTION. 
and under the microscope a very few salivary corpuscles, and occasionally 
only, amorphous lumps of peculiar material, probably mucous in nature, are 
seen. If the gland itself be watched, while its activity is thus roused, it 
will be seen (as we have already said, § 153) that its arteries are dilated and 
its capillaries filled, and that the blood flows rapidly through the veins in a 
full stream and of bright arterial hue, frequently with pulsating movements. 
If a vein of the gland be opened, this large increase of flow, and the lessen- 
ing of the ordinary deoxygenation of the blood consequent upon the rapid 
stream will be still more evident. It is clear that excitation of the chorda 
largely dilates the arteries; the nerve acts energetically as a vaso-dilator 
nerve. 
Thus stimulation of the chorda brings about two events: a dilatation of 
the bloodvessels of the gland, and a flow of saliva. This question at once 
arises, Is the latter simply the result of the former or is the flow caused by 
some direct action on the secreting cells, apart from the increased blood- 
supply ? In support of the former view we might argue that the activity of 
the epithelial secreting cell, like that of any other form of protoplasm, is 
dependent on blood-supply. When the small arteries of the gland dilate, 
while the pressure in the arteries on the side toward the heart is (as we have 
previously seen when treating generally of blood-pressure, § 109) corre- 
spondingly diminished, the pressure on the far side in the capillaries and 
veins is increased ; hence the capillaries become fuller, and more blood passes 
through them in a given time. From this we might infer that a larger 
amount of nutritive material would pass away from the capillaries into the 
surrounding lymph-spaces, and so into the epithelial cells, the result of 
which would naturally be to quicken the processes going on in the cells, and 
to stir these up to greater activity. But even admitting all this it does not 
necessarily follow that the activity thus excited should take on the form of 
secretion. It is quite possible to conceive that the increased blood-supply 
should lead only to the accumulation in the cell of the constituents of the 
saliva, or of the raw materials for their construction, and not to a discharge 
of the secretion. A man works better for being fed, but feeding does not 
make him work in the absence of any stimulus. The increased blood-supply, 
therefore, while favorable to active secretion, need not necessarily bring it 
about. Moreover, the following facts distinctly show that it need not. 
When a canula is tied into the duct and the chorda is energetically stimu- 
lated, the pressure acquired by the saliva accumulated in the canula and in 
the duct may exceed for the time being the arterial blood-pressure, even that 
of the carotid artery ; that is to say, the pressure of fluid in the gland out- 
side the bloodvessels is greater than that of the blood inside the bloodvessels. 
This must, whatever be the exact mode of transit of nutritive material 
through the vascular walls, tend to check that transit. Again, if the head 
of an animal be rapidly cut off, and the chorda immediately stimulated, a 
flow of saliva takes place far too copious to be accounted for by the empty- 
ing of the salivary channels through any supposed contraction of their walls. 
In this case secretion is excited in the gland though the blood-supply is 
limited to the small quantity still remaining in the bloodvessels. Lastly, if 
a small quantity of atropine be injected into the veins, stimulation of the 
chorda produces no secretion of saliva at all, though the dilatation of the 
bloodvessels takes place as usual ; in spite of the greatly increased blood- 
supply no secretion at all takes place. These facts prove that the secretory 
activity is not simply the result of vascular changes, but may be called forth 
independently ; they further lead us to suppose that the chorda contains two 
sets of fibres, one which we may call secretory fibres, acting directly on the 
secreting structures only, and the other vaso-dilator fibres, acting on the SECRETION OF SALIVA AND GASTRIC JUICE. 
265 
bloodvessels only, and further that atropine, while it has no effect on the 
latter, paralyzes the former just as it paralyzes the inhibitory fibres of the 
vagus. Hence when the chorda is stimulated there pass down the nerve, in 
addition to impulses affecting the blood-supply, impulses affecting directly 
the protoplasm of the secreting cells, and calling it into action, just as similar 
impulses call into action the contractility of the substance of a muscular 
fibre. Indeed, the two things, secreting activity and contracting activity, 
are very parallel. 
Since the chorda acts thus directly on the secreting cells, we should 
expect to find an anatomical connection between the cells and the nerve ; and 
some authors have maintained that the nerve-fibres may be traced into the 
cells. But, save perhaps in the case of certain glands of invertebrates 
(so-called salivary glands of Blatta), the evidence, as we have said, is as yet 
not convincing. 
§ 198. When the cervical sympathetic is stimulated, the vascular effects, 
as we have already said (§ 154), are the exact contrary of those seen when 
the chorda is stimulated. The small arteries are constricted, and a small 
quantity of dark venous blood escapes by the veins. Sometimes, indeed, 
the flow through the gland is almost arrested. The sympathetic, therefore, 
acts as a vaso-constrictor nerve, and in this sense is antagonistic to the 
chorda. 
As concerns the flow of saliva brought about by stimulation of the sym- 
pathetic, in the case of the submaxillary gland of the dog the effects are 
very peculiar. A slight flow results, and the saliva so secreted is remark- 
ably viscid, of higher specific gravity, and richer in corpuscles and in the 
above-mentioned amorphous lumps than is the chorda saliva. This action 
of the sympathetic is little or not at all affected by atropine. 
In the submaxillary gland of the dog, then, the contrast between the 
effects of chorda stimulation and those of sympathetic stimulation are very 
marked ; the former gives rise to vascular dilatation with a copious flow of 
fairly limpid saliva poor in solids, the latter to vascular constriction with a 
scanty flow of viscid saliva richer in solids. And in other animals a simi- 
lar contrast prevails, though with minor differences. Thus, in the rabbit 
both chorda saliva and sympathetic saliva are limpid and free from mucus, 
though the latter contains more proteids; in the cat, chorda saliva is more 
viscid than sympathetic saliva; but in both these cases, as in the dog, stimu- 
lation of the chorda causes a copious flow with dilated bloodvessels, and stimu- 
lation of the sympathetic a scanty flow with vascular constriction. We shall 
return again presently to these different actions of the two nerves; mean- 
while we have seen enough of the history of the submaxillary gland to learn 
that secretion in this instance is a reflex action, the efferent impulses of 
which directly affect the secreting cells, and that the vascular phenomena 
may assist, but are not the direct cause of the flow. 
§ 199. We have dwelt long on this gland because it has been more 
fruitfully studied than any other. But the nervous mechanisms of the other 
salivary glands are in their main features similar. Thus the secretion of 
the parotid gland, like that of the submaxillary, is governed by two sets of 
fibres; one of cerebro-spinal origin, running along the auriculo-temporal 
branch of the fifth nerve, but originating possibly in the glosso-pharyngeal, 
and the other of sympathetic origin coming from the cervical sympathetic. 
Stimulation of the cerebro-spinal fibres produces a copious flow of limpid 
saliva, free from mucus ; stimulation of the cervical sympathetic gives rise 
in the rabbit to a secretion also free from mucus, but rich in proteids and of 
greater amylolytic power than the cerebro-spinal secretion ; in the dog little 
or no secretion is produced, though, as we shall see later on, certain changes 266 
THE TISSUES AND MECHANISMS OF DIGESTION. 
are brought about in the gland itself. In both animals the cerebro-spi- 
nal fibres are vaso-dilator, and the sympathetic fibres vaso-con stricter in 
action. 
§ 200. The secretion of gastric juice. Though a certain amount of gastric 
juice may sometimes be found in the stomachs of fasting animals, it may be 
stated generally that the stomach, like the salivary glands, remains inactive, 
yielding no secretion, so long as it is not stimulated by food or otherwise. 
The advent of food into the stomach, however, at once causes a copious flow 
of gastric juice; and the quantity secreted in the twenty-four hours is pro- 
bably very considerable, but we have no trustworthy data for calculating the 
exact amount. So also when the gastric mucous membrane is stimulated 
mechanically, as with a feather, secretion is excited; but to a very small 
amount even when the whole interior surface of the stomach is thus repeat- 
edly stimulated. The most efficient stimulus is the natural stimulus, viz., 
food ; though dilute alkalies seem to have unusually powerful stimulating 
effects; thus the swallowing of saliva at ouce provokes a flow of gastric 
juice. 
During fasting the gastric membrane is of a pale gray color, somewhat 
dry, covered with a thin layer of mucus, and thrown into folds; during 
digestion it becomes red, flushed, and tumid, the folds disappear, and minute 
drops of fluid appearing at the mouths of the glands, speedily run together 
into small streams. When the secretion is very active, the blood flows from 
the capillaries into the veins in a rapid stream without losing its bright 
arterial hue. The secretion of gastric juice is, in fact, accompanied by 
vascular dilatation in the same way as is the secretion of saliva. 
§ 201. Seeing that, unlike the case of the salivary secretion, food is 
brought into the immediate neighborhood of the secreting cells, it is exceed- 
ingly probable that a great deal of the secretion is the result of the working 
of a local mechanism ; and this view is supported by the fact that when a 
mechanical stimulus is applied to one spot of the gastric membrane the 
secretion is limited to the neighborhood of that spot and is not excited in 
distant parts. This local mechanism may be nervous in nature or the effect of 
the stimulus may perhaps be conveyed directly from cell to cell, from the 
mouth of the gland to its extreme base, without the intervention of any 
nervous elements; but the vascular changes at least would seem to imply 
the presence of a nervous mechanism. 
The stomach is supplied with nerve fibres from the two vagi nerves and 
from the solar plexus of the splanchnic system. The two vagi after forming 
the oesophageal plexus on the oesophagus are gathered together again as two 
main trunks which run along the oesophagus-the left in the front, the right 
at the back-to the stomach. The left or anterior nerve is distributed to the 
smaller curvature and the front surface of the stomach, forming a plexus in 
which nerve-cells are present; and branches pass on to the liver and proba- 
bly to the duodenum. The right or posterior iferve is distributed to the 
hinder surface of the stomach, but only to the extent of about one-third of 
its fibres; about two-thirds of the fibres pass on to the solar plexus. The 
fibres of the vagus nerves thus distributed to the stomach are for the most 
part non-medullated fibres; by the time the vagus reaches the abdomen it 
consists almost exclusively of non-medullated fibres, medullated fibres being 
very few; the large number of medullated fibres which the nerve contains 
in the upper part of the neck pass off* into the laryngeal, cardiac, and other 
branches. 
From the solar plexus nerves, arranged largely in plexuses, pass in com- 
pany with the divisions of the coeliac artery, coronary artery of the stomach 
and branches of the hepatic artery, to the stomach. Though the two abdo- SECRETION OF SALIVA AND GASTRIC JUICE. 
267 
minal splanchnic nerves which join the solar plexus (semilunar ganglia) are 
chiefly composed of medullated fibres, the nerves which pass from the plexus 
to the stomach are to a large extent composed of non-medullated fibres. 
All these nerves, both branches of the vagi and those from the solar plexus, 
lie at first in company with the arteries on the surface of the stomach be- 
neath the peritoneum. From thence they pass inward, still in company 
with arteries, and form, on the one hand, a plexus containing nerve-cells 
between the longitudinal and circular muscular coats corresponding to what 
in the intestine we shall have to speak of as the plexus of Auerbach, whence 
fibres are distributed to the two muscular coats; and, on the other hand, a 
plexus in the submucous coat, also containing nerve-cells, corresponding to 
what is known in the intestine as Meissner's plexus. From this latter plexus 
fibres pass to the mucous membrane; some of these end in the mascularis 
mucosie; whether any are connected with the gastric glands, and if so how, 
is not at present known. 
There are no facts which afford satisfactory evidence that any part of this 
arrangement of nerves supplies such a local nervous mechanism as was sug- 
gested above. The importance, however, of such a local mechanism what- 
ever its nature, and the subordinate value of any connection between the 
gastric membrane and the central nervous system, is further shown by the 
fact that a secretion of quite normal gastric juice will go on after both vagi, 
or the nerves from the solar plexus going to the stomach have been divided, 
and, indeed, when all the nervous connections of the stomach are so far as 
possible severed. And all attempts to provoke or modify gastric secretion 
by the stimulation of the nerves going to the stomach have hitherto failed. 
On the other hand, in cases of gastric fistula, where by complete occlusion 
of the oesophagus stimulation by the descent of saliva has been avoided, the 
mere sight or smell of food has been seen to provoke a lively secretion of 
gastric juice. This must have been due to some nervous action ; and the 
same may be said of the cases where emotions of grief or anger suddenly 
arrest the secretion going on or prevent the secretion which would otherwise 
have taken place as the result of the presence of food in the stomach. So 
that much has yet to be learned in this matter. 
§ 202. The contrast presented between the scanty secretion resulting from 
mechanical stimulation and the copious flow which actual food induces is 
interesting because it seems to show that the secretory activity of the cells is 
heightened by the absorption of certain products derived from the portions 
of food first digested. This is well illustrated by the following experiment 
of Heidenhain. This observer, adopting the method employed for the intes- 
tine, of which we shall speak later on, succeeded in isolating a portion of the 
fundus from the rest of the stomach ; that is to say, he cut out a portion of 
the fundus, sewed together the cut edges of the main stomach, so as to form 
a smaller but otherwise complete organ, while by sutures he converted the 
excised piece of fundus into a small independent stomach opening on to the 
exterior by a fistulous orifice. When food was introduced into the main 
stomach secretion also took place in the isolated fundus. This at first sight 
might seem the result of a nervous reflex act; but it was observed that the 
secondary secretion in the fundus was dependent on actual digestion taking 
place in the main stomach. If the material introduced into the main stomach 
were indigestible or digested with difficulty, so that little or no products of 
digestion were formed and absorbed into the blood, such ex. gr. as pieces 
of ligamentum nuchae, very little secretion took place in the isolated fundus. 
We quote this now as bearing on the question of a possible nervous mechan- 
ism of gastric secretion, but we shall have to return to it under another 
aspect. 268 
THE TISSUES AND MECHANISMS OF DIGESTION. 
The Changes in a Gland constituting the Act of Secretion. 
§ 203. We have now to consider what are the changes in the glandular 
cells and their surroundings which cause this flow of fluid possessing specific 
characters into the lumen of an alveolus, and so into a duct. It will be con- 
venient to begin with the pancreas. 
The thin extended pancreas of a rabbit may, by means of special precau- 
tions, be spread out on the stage of a microscope and examined with even 
high powers, while the animal is not only alive but under such conditions 
that the gland remains in a nearly normal state, capable of secreting vigor- 
ously. It is possible under these circumstances to observe even minutely the 
appearances presented by the gland when at rest and loaded, and to watch 
the changes which take place during secretion. 
When the animal has not been digesting for some little time, and the gland 
is therefore " loaded," the outlines of the individual cells are very indistinct, 
the lumen of the alveolus is invisible or very inconspicuous, and each cell is 
crowded with small refractive spherical granules, forming an irregular gran- 
ular mass which hides the nucleus and leaves only a very narrow clear outer 
zone next to the basement membrane, or it may be hardly any such zone at 
all. (Fig. 82, A.) 
The blood-supply, moreover, is scanty, the small arteries being constricted 
and the capillaries imperfectly filled with corpuscles. 
If, however, the same pancreas be examined while it is in a state of activity, 
either from the presence of food in the stomach or from the injection of some 
stimulating drug, such as pilocarpine, a very different state of things is seen. 
The individual cells (Fig. 82, B) have become smaller and much more dis- 
Fig. 82. 
A Portion of the Pancreas of the Rabbit. (Kuhne and Sheridan Lea.) A at rest, B in a state 
of activity, a the inner granular zone, in which A is larger, and more closely studded with fine 
granules, than in B, in which the granules are fewer and coarser, b the outer transparent zone, 
small in A, larger in B, and in the latter marked with faint striae, c the lumen, very obvious in 
B, but indistinct in A. d an indentation at the junction of two cells, seen in B, but not occurring 
in A. 
tinct in outline and the contour of the alveolus, which previously was even, 
is now wavy, the basement membrane being indented at the junction of the 
cells, also the lumen of the alveolus is now wider and more conspicuous. 
In each cell the granules have become much fewer in number and, as it were, 
have retreated to the inner margin, so that the inner granular zone is much 
narrower and the outer transparent zone much broader than before; the 
latter, too, is frequently marked at its inner part by delicate strire running 
into the inner zone. At the same time the bloodvessels are largely dilated 
and the stream of blood through the capillaries is full and rapid. 
With care the change from the one state of things to the other may be SECRETION OF SALIVA AND GASTRIC JUICE. 
269 
watched under the microscope. The vascular changes can, of course, be 
easily appreciated, but the granules may also be seen to diminish in number. 
Those at the inner margin seem to be discharged into the lumen, and those 
nearer the outer margin to travel inward through the cell-substance toward 
the lumen, the faint stride spoken of above, apparently, at all events, being 
the marks of their paths. Obviously, during secretion, the granules with 
which the cell-substance was " loaded " are " discharged " from the cell into 
the lumen of the alveolus. What changes these granules may undergo dur- 
ing the discharge we shall consider presently. 
Sections of the prepared and hardened pancreas of any animal tell nearly 
the same tale as that thus told by the living pancreas of the rabbit. In 
sections, for instance, of the pancreas of a dog which has not been fed, and 
therefore has not been digesting, for some hours (twenty-four or thirty), the 
cells are seen to be crowded with granules (which, however, are usually 
shrunken and irregular owing to the influence of the hardening agent), 
leaving a very narrow outer zone. In similar sections of the pancreas of a 
dog which has been recently fed, six hours before for example, and in which, 
therefore, the gland has been for some time actively secreting, the granules 
are far less numerous, and the clear outer zone accordingly much broader 
and more conspicuous. With osmic acid these granules stain well, and are 
preserved in their spherical form, so that the cell thus stained maintains 
much of the appearance of a living cell. But with carmine, haematoxylin, 
etc., the granules do not stain nearly so readily as does the cell-substance of 
the cells, so that a discharged cell stains more deeply than does a loaded cell 
because the staining of the " protoplasmic " cell-substance is not so much 
obscured by the unstained granules ; besides which, however, the actual cell- 
substance stains probably somewhat more deeply in the discharged cell. It 
may be added that in the discharged cell the nucleus is conspicuous and well 
formed ; in the loaded cell it is generally in prepared sections, more or less 
irregular, possibly because in these it is less dense and more watery than in 
the discharged cell, and so shrinks under the influence of the reagents 
employed. 
These several observations suggest the conclusion that in a gland at rest 
the cell is occupied in forming by means of the metabolism of its cell-sub- 
stance and lodging in itself (§ 30) certain granules of peculiar substance 
intended to be a part and probably an important part of the secretion. 
This goes on until the cell is more or less completely " loaded." In such a 
cell the amount of actual living cell-substance is relatively small, its place 
is largely occupied by granules, and in itself has been partly consumed in 
forming the granules. During the act of secretion the granules are dis- 
charged to form part of the secretion, other matters, including water, as we 
shall see, making up the whole secretion; and the cell would be proportion- 
ately reduced in size were it not that the act of the discharge seems to stimu- 
late the cell-substance to a new activity of growth, so that the new cell-sub- 
stance is formed; this, however, is in turn soon in part consumed in order to 
form new granules. And what is thus seen with considerable distinctness 
and ease in the pancreas, is seen with more or less distinctness in other 
glands. 
§ 204. When we study an albuminous gland, the parotid gland, for in- 
stance, in a living state, we find that the changes which take place during 
activity are quite comparable to those of the pancreas. During rest (Fig. 
83, A), the cells are large, their outlines very indistinct, in fact almost in- 
visible, and the cell-substance is studded with granules. During activity 
(Fig. 83, J?) the cells become smaller, their outlines more distinct, and the 
granules disappear, especially from the outer portions of each cell. After 270 
THE TISSUES AND MECHANISMS OF DIGESTION. 
prolonged activity, as in Fig. 83, C, the cells are still smaller, with their 
outlines still more distinct, and the granules have disappeared almost 
entirely, a few only being left at the extreme inner margin of each cell, 
abutting upon the conspicuous, almost gaping lumen of the alveolus. And 
Fig. 83. 
Changes in the Parotid during Secretion. (Langley.) The figure, which is somewhat dia- 
grammatic, represents the microscopic changes which may be observed in the living gland. A. 
During rest. The obscure outlines of the cells are introduced to show the relative size of the 
cells; they could not be readily seen in the specimen itself. B. After moderate stimulation. C. 
After prolonged stimulation. The nuclei are diagrammatic, and introduced to show their ap- 
pearance and position. 
upon special examination it is found that the nuclei are large and round. 
In fact, we might almost take the parotid, as thus studied, to be more truly 
typical of secretory changes than even the pancreas. For the demarcation 
of an inner and outer zone is not a necessary feature of a secreting cell at 
rest. What is essential is that the cell-substance manufactures material, 
which for a while, that is during rest, is deposited in the cell, generally in 
the form of granules but not necessarily so, and that during activity this 
material is used up, the disappearance of the granules, when these are 
visible, being naturally earliest and most marked at the outer portions of 
each cell, and progressing inward toward the lumen, the whole cell becoming 
smaller and, as it were, shrunken. 
In the cells of the parotid gland and other albuminous cells the granules 
seen in the living or fresh cell differ from the granules seen in the pancreatic 
cell, inasmuch as they are easily dissolved or broken up by the action of 
alcohol, chromic acid, and the other usual hardening reagents, and hence in 
hardened specimens have disappeared. In consequence, in sections of har- 
dened and prepared albuminous glands the difference between resting or 
loaded and active or discharged cells may appear not very conspicuous ; 
and this is especially the case in the parotid gland of the rabbit when the 
Fig. 84. 
Sections of the Parotid of the Rabbit. (After Heidenhain.) A. At rest. B. After stimulation 
of the cervical sympathetic. Both sections are from hardened gland. 
activity has been called into play by stimulation of the auriculo-temporal 
nerve. When, however, either in the rabbit or the dog the cervical sym- 
pathetic is stimulated, though the stimulation gives rise in the rabbit to SECRETION OF SALIVA AND GASTRIC JUICE. 
271 
little secretion of saliva and in the dog to none at all, a marked effect on 
the gland is produced, and changes in the same direction as those already 
described may be observed. During rest the cells of the parotid as seen in 
sections of the gland hardened in alcohol (Fig. 84, J.) are pale, transpa- 
rent, staining with difficulty, and the nuclei possess irregular outlines as if 
shrunken by the reagents employed. After stimulation of the sympathetic 
the protoplasm of the cells becomes turbid (Fig. 84, B), and stains much 
more readily, while the nuclei are no longer irregular in outline, but round 
and large, with conspicuous nucleoli, the whole cell at the same time, at 
least after prolonged stimulation, becoming distinctly smaller. 
§ 205. In a mucous gland the changes which take place are of a like 
kind, though apparently somewhat more complicated, owing probably to 
the peculiar characters of the mucin which is so conspicuous a constituent 
of the secretion. 
If a piece of resting loaded submaxillary gland be teased out, while 
fresh and warm from the body, in normal saline solution, the cell-substance 
of the mucous cells (Fig. 85, a) is seen to be crowded with granules or 
spherules, which may fairly be compared with the granules of the pancreas, 
though perhaps less dense and solid than these. 
If a piece of a gland which has been secreting for some time, and is 
therefore a discharged gland, be examined in the same way (Fig. 85, 6), 
the granules are far less numerous and largely confined to the part of the 
cell nearer the lumen, the outer part of the cell around the nucleus consist- 
ing of ordinary " protoplasmic " cell-substance. The distinction, however, 
between an inner " granular zone " next 
to the lumen and an outer " clear zone " 
next to the basement membrane is less 
distinct than in the pancreas, partly be- 
cause the granules do not disappear in so 
regular a manner as in the pancreas, and 
partly because the outer zone of the mu- 
cous cell, as it forms, is less homogeneous 
than that of the pancreatic cell. 
The "granules" or "spherules" of 
the mucous cell are moreover of a pecu- 
liar nature. If the fresh cell, showing 
granules (either many as in the case of a 
loaded or few as in the case of a dis- 
charged cell), be irrigated with water or 
with dilute acids or dilute alkalies, the 
granules swell up (Fig. 85, a', b', into a 
transparent mass, giving the reactions of 
mucin, traversed by a network of " pro- 
toplasmic " cell-substance. In this way is 
produced an appearance very similar to 
that shown in sections of mucous glands 
hardened and stained in the ordinary 
way. 
In the loaded mucous cell in hardened 
and stained preparations (Fig. 86, a) 
there is seen a small quantity of protoplasmic cell-substance gathered 
round the nucleus at the outer part of the cell next to the basement mem- 
brane ; the rest of the cell consists of a network of cell-substance, the in- 
terstices being filled with transparent material, which, unlike the network 
itself and the mass of cell-substance round the nucleus, does not stain with 
Fig. 85. 
Mucous Cells from a fresh Submaxil- 
lary Gland of Dog. (Langley.) a and b 
isolated in 2 per cent, salt solution: a, 
from loaded gland; b, from discharged 
gland (the nuclei are usually more ob- 
scured by granules than is here repre- 
sented). On teasing out a fresh fragment 
in 2 to 5 per cent, salt solution, the cells 
usually become broken up so that iso- 
lated cells are rarely obtained entire; iso- 
lated cells are common if the gland be 
left in the body for a day after death ; a', 
b', treated with dilute acid: a', from 
loaded ; b', from discharged gland. 272 
THE TISSUES AND MECHANISMS OF DIGESTION. 
carmine or with certain other dyes. The discharged cell in similar prepara- 
tions (Fig. 86, 6) differs from the loaded cell in the amount of transparent 
Fig. 86. 
Alveoli of Dog's Submaxillary Gland Hardened in Alcohol and Stained with Carmine. (Lang- 
ley.) The network is diagrammatic, a, from a loaded gland, b, from a discharged gland ; the 
chorda tympani having been stimulated at short intervals during five hours. 
non-staining material being much less and chiefly confined to the inner part 
of the cell, while the protoplasmic cell-substance around the now large and 
well-formed nucleus is not only, both relatively and absolutely, greater in 
amount, but stains still more deeply than in the loaded cell. 
It would appear, therefore, that in the mucous cell, as in the pancreatic 
cell, the cell-substance forms and deposits in itself certain material in the 
form of granules. During secretion these granules disappear and presum- 
ably form part of the secretion. 
§ 206. The " central " or "chief" cells of the gastric gland also ex- 
hibit similar changes. In such an animal as the newt these cells may, though 
with difficulty, be examined in the living state. They are then found to be 
studded with granules when the stomach is at rest. During digestion these 
granules become much less numerous and are chiefly gathered near the 
lumen, leaving in each cell a clear outer zone. And in many mammals 
the same abundance of granules in the loaded cell, the same paucity of 
granules for the most part restricted to an inner zone in the discharged cell, 
may be demonstrated by the use of osmic acid (Fig. 87). 
When the stomach is hardened by alcohol these changes, like the similar 
changes in an albuminous cell, are obscured by the shrinking of the 
" granules," or by their swelling up and becoming diffused through the rest 
of the cell-substance ; so that, though in sections so prepared very striking 
differences are seen between loaded and discharged cells, these are unlike 
those seen in living glands. In specimens taken from an animal which has 
not been fed for some time, the central cells of the gastric glands are pale, 
finely granular, and do not stain readily with carmine and other dyes. 
During the early stages of gastric digestion, the same cells are found some- 
what swollen, but turbid and more coarsely granular ; they stain much more 
readily. At a later stage they become smaller and shrunken, but are even 
more turbid and granular than before, and stain still more deeply. This is 
true not only of the central cells in the cardiac glands, but also of the cells 
of which the pyloric glands are built up. In a loaded cell very little 
staining takes place, because the amount of living staining cell-substance 
is small relatively to the amount of material with which it is loaded and SECRETION OF SALIVA AND GASTRIC JUICE. 
273 
which does not stain readily. In the cell which after great activity has 
discharged itself, the cell is smaller, but what remains is largely living 
cell-substance, some of it new, and all stain- 
ing readily. It would appear also, that dur- 
ing the activity of the cell some substances, 
capable of being precipitated by alcohol, make 
their appearance, and the presence of this ma- 
terial adds to the turbid and granular aspect 
of the cell ; possibly, also, this material con- 
tributes to the staining. A similar material 
seems to make its appearance in the cells of 
albuminous glands. 
In the ovoid or border cells no very charac- 
teristic changes make their appearance. During 
digestion they become larger, more swollen, as 
it were, and in consequence bulge out the base- 
ment membrane, but no characteristic disap- 
pearance of granules can be observed. In the 
living state, the cell-substance of these ovoid 
cells appears finely granular, but in hardened 
and prepared sections has a coarsely granular, 
" reticulate " look, which is perhaps less marked 
in the swollen active cells than in the resting 
cells. 
§ 207. All these various secreting cells, 
then-pancreatic cell, mucous cell, albuminous 
cell, and central gastric cell-exhibit the same 
series of events, modified to a certain extent in 
the several cases. In each case the " proto- 
plasmic" cell-substance manufactures and 
lodges in itself material destined to form part 
of the juice secreted. In the fresh cell this 
material may generally be recognized under 
the microscope by its optical characters as 
granules; these, however, are apt to become 
altered by reagents. But we must guard our- 
selves against the assumption that the mate- 
rial which can thus be recognized is the only 
material thus stored up; we may in future, 
by chemical or other means, be able to differ- 
entiate other parts of the cell-body as being also material similarly 
stored up. 
During activity, while the gland is secreting, this material, either 
unchanged or after undergoing change, is wholly or partially discharged 
from the cell. The cell, in consequence of having thus got rid of more 
or less of its load, consists to a larger extent of actual living cell-sub- 
stance, this being in many cases increased by rapid new-growth, though 
the bulk of the discharged cell may be less than that of the loaded cell. 
This activity of growth continues after the act of secretion, but the dis- 
charged cell soon begins again the task of loading itself with new secretion 
material for the next act of secretion. 
Thus in most cases there is, corresponding to the intermittence of secre- 
tion, an alteration of discharge and loading; but it must be borne in mind 
that such an alteration is not absolutely necessary, even in the case of inter- 
mittent secretion. We can easily imagine that the discharge, say of 
Fig. 87. 
Gastric Gland of Mammal (Bat) 
during Activity. (Langley.) c, the 
mouth of the gland with its cylin- 
drical cells, n, the neck, contain- 
ing conspicuous ovoid cells, with 
their coarse protoplasmic net- 
work. f, the body of the gland. 
The granules are seen in the cen- 
tral cells to be limited to the inner 
portions of each cell, the round 
nucleus of which is conspicuous. 274 
THE TISSUES AND MECHANISMS OF DIGESTION. 
"granules" during secretion, should stir up the cell to an increased activ- 
ity in forming granules, and that the formative activity should cease 
when the secretory activity ceased. In such a case the number of new 
granules formed might always be equal to the number of old granules 
used up, and the active cell, in spite of its discharge, would possess as 
many granules-that is to say, as large a load-as the cell at rest. And 
in the central gastric cells of some animals it would appear that such a 
continued balancing of load and discharge does actually take place, so 
that no distinction in granules can be observed between resting and 
active cells. 
§ 208. We spoke just now of the material stored up in the cell and 
destined to form part of the secretion as undergoing change before it was 
discharged. In the mucous cell we have seen that the material deposited in 
the living cell has at first the form of granules. These granules, however, 
are easily converted into a transparent material lodged in the spaces of the 
cell-substance, which material, even if not exactly identical with, at least 
closely resembles, the mucin found in the secretion ; and apparently in the 
act of secretion the granules do undergo some such change. In the case 
of some other glands, moreover, we have chemical as well as optical 
evidence that the material stored up in the cells is, in part at least, not 
the actual substance appearing in the secretion, but an antecedent of that 
substance. 
An important constituent of pancreatic juice is, as we shall see later on, 
a body called trypsin, a ferment very similar to pepsin, acting on proteid 
bodies and converting them into peptone and other substances. Though in 
many respects alike, pepsin and trypsin are quite distinct bodies, and differ 
markedly in this, that while an acid medium is necessary for the action of 
pepsin, an alkaline medium is necessary for the action of trypsin ; and accord- 
ingly the pancreatic juice is alkaline in contrast to the acidity of gastric juice. 
Trypsin can, like pepsin (§ 191), be extracted with glycerin from substances 
in which it occurs; glycerin extracts of trypsin, however, need for the 
manifestation of their powers the presence of a weak alkali, such as a 1 
per cent, solution of sodium carbonate. 
Now, trypsin is present in abundance in normal pancreatic juice; but a 
loaded pancreas, one which is ripe for secretion, and which if excited to 
secrete would immediately pour out a juice rich in trypsin, contains no 
trypsin or a mere trace of it; nay, even a pancreas which is engaged in the 
act of secreting contains in its actual cells an insignificant quantity only of 
trypsin, as is shown by the following experiment: 
If the pancreas of an animal, even of one in full digestion, be treated, 
while still warm from the body, with glycerin, the glycerin extract, as judged 
of by its action on fibrin in the presence of sodium carbonate, is inert or nearly 
so as regards proteid bodies. If, however, the same pancreas be kept for 
twenty-four hours before being treated with glycerin, the glycerin extract 
readily digests fibrin and other proteids in the presence of an alkali. If the 
pancreas, while still warm, be rubbed up in a mortar for a few minutes with 
dilute acetic acid, and then treated with glycerin, the glycerin extract is 
strongly proteolytic. If the glycerin extract obtained without acid from the 
warm pancreas, and therefore inert, be diluted largely with water and kept 
at 35° C. for some time, it becomes active. If treated with acidulated instead 
of distilled water, its activity is much sooner developed. If the inert glycerin 
extract of warm pancreas be precipitated with alcohol in excess, the precipi- 
tate, inert as a proteolytic ferment when fresh, becomes active when exposed 
for some time in an aqueous solution, rapidly so when treated with acidulated 
water. These facts show that a pancreas taken fresh from the body, even SECRETION OF SALIVA AND GASTRIC JUICE. 
275 
during full digestion, contains but little ready-made ferment, though there is 
present in it a body which, by some kind of decomposition, gives birth to the 
ferment. We may remark incidentally, that though the presence of an alkali 
is essential to the proteolytic action of the actual ferment, the formation of 
the ferment out of its forerunner is favored by the presence of a small quan- 
tity of acid ; the acid must be used with care, since the trypsin, once formed, 
is destroyed by acids. To this body, this mother of the ferment, which has 
not at present been satisfactorily isolated, but which appears to be a complex 
body, splitting up into the ferment, which, as we have seen, is at all events 
not certainly a proteid body, and into an undeniably proteid body, the name 
of zymogen has been applied. But it is better to reserve the term zymogen 
as a generic name for all such bodies as, not being themselves actual fer- 
ments, may by internal changes give rise to ferments-for all " mothers of 
ferment," in fact-and to give to the particular mother of the pancreatic 
proteolytic ferment the name trypsinogen. 
Evidence of a similar kind shows that the gastric glands, both the car- 
diac and the pyloric glands, while they contain comparatively little actual 
pepsin, contain a considerable quantity of zymogen of pepsin, or pepsinogen; 
and there can be little doubt but that this pepsinogen is lodged in the cen- 
tral cells of the cardiac glands and in the somewhat similar cells which line 
the whole of the pyloric glands. 
§ 209. The act of secretion itself. The above discussion prepares us at 
once for the statement that the old view of secretion, according to which the 
gland picks out, separates, secretes (hence the name secretion), and so filters, 
as it were, from the common store of the blood the several constituents of the 
juice, is untenable. According to that view the specific activity of any one 
gland was confined to the task of letting certain constituents of the blood 
pass from the capillaries surrounding the alveolus through the cells to the 
channels of the ducts, while refusing a passage to others. We now know 
that certain important constituents of each juice, the pepsin of gastric juice, 
the mucin of saliva and the like are formed in the cell, and not obtained 
ready made from the blood. A minute quantity of pepsin does exist, it is 
true, in the blood, but there are reasons for thinking that this has made its 
way back into the blood, either being absorbed from the interior of the 
stomach or, as seems more probable, picked up directly from the gastric 
glands; and so with some of the other constituents of other juices. The 
chief or specific constituents of each juice are formed in the cell itself. 
But the juice secreted by any gland consists not only of the specific sub- 
stances such as mucin, pepsin, or other ferment, or other bodies, found in it 
alone, but also of a large quantity of water, and of various other substances, 
chiefly salines, common to it, to other juices, and to the blood. And the 
question arises, Is the water, are the salts and other common substances, fur- 
nished by the same act as that which supplies the specific constituents? 
Certain facts suggest that they are not. For instance, as mentioned some 
time ago, in the submaxillary gland of the dog stimulation of the chorda 
tympani produces a copious flow of saliva, which is usually thin and limpid, 
whilst stimulation of the cervical sympathetic produces a scanty flow of thick 
viscid saliva. That is to say, stimulation of the chorda has a marked effect 
in promoting the discharge of water, while stimulation of the sympathetic 
has a marked effect in promoting the discharge of mucin. To this we may 
add the case of the parotid of the dog. In this gland stimulation of a 
cerebro-spinal nerve, the auriculo-temporal, produces a copious flow of limpid 
saliva, while stimulation of the sympathetic produces itself little or no secre- 
tion at all; but when the sympathetic and the cerebro-spinal nerves are 
stimulated at the same time, the saliva which flows is much richer in solid 276 
THE TISSUES AND MECHANISMS OF DIGESTION. 
and especially in organic matter than when the cerebro-spinal nerve is stimu- 
lated alone. And we have already seen that in this gland the microscopic 
changes following upon sympathetic stimulation are more conspicuous than 
those which follow upon cerebro-spinal stimulation. 
These and other facts have led to the conception that the act of secretion 
consists of two parts, which in one case may coincide, in another may take 
place apart or in different proportions. On the one hand, there is the dis- 
charge of water carrying with it common soluble substances, chiefly salines, 
derived from the blood ; on the other hand, a metabolic activity of the cell- 
substance gives rise to the specific constituents of the juice. To put the 
matter broadly, the latter process produces the specific constituents, the 
former washes these and other matters into the duct. It has been further 
supposed that two kinds of nerve fibres exist: one governing the former 
process and, in the case of the submaxillary gland for instance, prepon- 
derating, though not to the total exclusion of the other kind, in the chorda 
tympani; the other governing the latter process and preponderating in the 
branches of the cervical sympathetic. These have been called respectively 
" secretory " and " trophic " fibres ; but these terms are not desirable. It 
may here be remarked that even the former process is a distinct activity of 
the gland and not a mere infiltration. For, as we have seen in the case of 
the salivary glands, when atropine is given, not only do the specific constitu- 
ents cease to be ejected as a consequence of stimulation of the chorda, but 
the discharge of water, in spite of the bloodvessels becoming dilated, is also 
arrested : no saliva at all leaves the gland. And what is true of the salivary 
glands as regards the dependence of the flow of water on something else 
besides the mere pressure of the blood in the bloodvessels appears to hold 
good with other glands also. Indeed, it has been suggested that the very 
discharge of water is due to an activity of the cell; the hypothesis has been 
put forward that changes in the cell give rise to the formation in the cell of 
substances which absorb water from the blood or lymph on the one side 
and give it up on the other side into the lumen of the alveolus. Such an 
hypothesis cannot be regarded as proved ; but the mere putting it forward 
raises a doubt as to the validity of the distinction on which we have been 
dwelling; and other considerations point in the same direction. For in- 
stance, if the common soluble salts present in a juice, as distinguished from 
the specific constituents, were merely carried into the juice by the rush, so 
to speak, of water, we should expect to find the percentage of these salts 
either remaining the same or perhaps decreasing when the juice was secreted 
more rapidly and in fuller volume. But under these circumstances the per- 
centage very frequently increases; and in general we find that under various 
circumstances the proportion of salts secreted to the quantity of water 
secreted may vary considerably. Obviously, while something determines 
the quantity of water passing into the alveolus, something else determines 
how much of common soluble salts that water contains, and still something 
else determines to what extent that water is also laden with specific constitu- 
ents and other organic bodies. The whole action is too complicated to be 
described as consisting merely of the two processes mentioned above, but the 
time has not yet come for clear and definite statements. Everything, how- 
ever, tends to show that the cell is the prime agent in the whole business, 
though we cannot at present define the nature of the several changes in the 
cell, nor can we say how those changes are exactly related to each other, 
to changes of the blood-pressure in the bloodvessels, or, we may add, to 
changes taking place in the lymph-spaces which lie between the blood and 
the cell. 
We may perhaps add that since in certain cutaneous secreting glands SECRETION OF SALIVA AND GASTRIC JUICE. 
277 
the alveolus, or what corresponds to the alveolus, is wrapped round with 
plain muscular fibres, the contraction of which appears to force the secretion 
outward, the idea has been suggested that in glands, such as we are now 
considering, the cell-substance, making use of " protoplasmic " contraction 
instead of actual muscular contraction, may force part of the cell contents 
into the lumen of the alveolus. Such a mode of secretion would be com- 
parable to the ejection of undigested material, or "excretion," by an amoeba. 
But we have no satisfactory evidence in favor of this view. 
§ 210. Throughout the above we have spoken as if the secretion were 
furnished exclusively by the cells of the alveoli or secreting portion of the 
gland, as if the epithelial cells lining the ducts, or conducting portion of 
the gland, contributed nothing to the act. In the gastric glands the slender 
cells lining the mouths of the glands (which correspond to ducts) and cover- 
ing the ridges between, are mucous cells secreting into the stomach gener- 
ally a small, but under abnormal conditions a large amount of mucus, 
which has its uses but is not an essential part of the gastric juice. In the 
salivary glands we can hardly suppose that the long stretch of character- 
istic columnar epithelium which reaches from the alveoli to the mouth of 
the long main duct serves simply to furnish a smooth lining to the conduct- 
ing passages; but we have as yet no clear indications of what the function 
of this epithelium can be. 
§ 211. Before we leave the mechanism of secretion there are one or 
more accessory points which deserve attention. 
In treating just now of the gastric glands we spoke as if pepsin were the 
only important constituent of gastric juice, whereas, as we have previously 
seen, the acid is equally essential. The formation of the free acid of the 
gastric juice is very obscure, and many ingenious but unsatisfactory views 
have been put forward to explain it. It seems natural to suppose that it 
arises in some way from the decomposition of sodium chloride drawn from 
the blood; and this is supported by the fact that when the secretion of 
gastric juice is actively going on, the amount of chlorides leaving the blood 
by the kidney is proportionately diminished ; but nothing definite can at 
present be stated as to the mechanism of that decomposition. 
In the frog, while pepsin free from acid is secreted by the glands of the 
lower portion of the oesophagus, an acid juice is afforded by glands in the 
stomach itself, which have accordingly been called oxyntic (d^u>£iv,to sharpen, 
acidulate) glands; but these oxyntic glands appear also to secrete pepsin. 
In the mammal the isolated pylorus secretes an alkaline juice; in fact, the 
appearance of an acid juice is limited to those portions of the stomach in 
which the glands contain both "chief" or "central" and "ovoid" or "bor- 
der" cells. Now, from what has been previously said, there can be no 
doubt that the chief cells do secrete pepsin. On the other hand, there is no 
evidence whatever of the formation of pepsin by the " border " or " ovoid " 
cells, though this was once supposed to be the case, and these cells were 
unfortunately formerly called "peptic" cells. Hence it has been inferred 
that the border cells secrete acid ; but the argument is at present one of ex- 
clusion only, there being no direct proof that these cells actually manufacture 
the acid. 
The rennin appears to be formed by the same cells which manufacture 
the pepsin, that is, by the chief cells of the fundus generally, and to some 
extent by the cells of the pyloric glands. We may add that we have evi- 
dence of the existence of a zymogen of rennin analogous to the zymogen of 
pepsin or of trypsin. 
§ 212. Seeing the great solvent power of both gastric and pancreatic juice 
the question is naturally suggested, Why does not the stomach digest itself? 278 
THE TISSUES AND MECHANISMS OF DIGESTION. 
After death, the stomach is frequently found partly digested, viz., in cases 
when death has taken place suddenly on a full stomach. In an ordinary 
death, the membrane ceases to secrete before the circulation is at an end. 
That there is no special virtue in living things which prevents their being 
digested is shown by the fact that the leg of a living frog or the ear of 
a living rabbit introduced into the stomach of a dog, through a gastric 
fistula, is readily digested. It has been suggested that the blood-current 
keeps up an alkalinity sufficient to neutralize the acidity of the juice 
in the region of the glands themselves; but will not explain why the 
pancreatic juice, which is active in an alkaline medium, does not digest the 
proteids of the pancreas itself, or why the digestive cells of the bloodless 
actinozoon or hydrozoon do not digest themselves. We might add, it does 
not explain why the amoeba, while dissolving the protoplasm of the swal- 
lowed diatom, does not dissolve its own protoplasm. We cannot answer this 
question at all at present, any more than the similar one, why the delicate 
protoplasm of the amoeba resists during life the entrance into itself by 
osmosis of more water than it requires to carry on its work, while a few 
moments after it is dead water enters freely by osmosis, and the effects of 
that entrance become abundantly evident by the formation of bullae and 
the breaking up of the protoplasm. 
The Properties and Characters of Bile, Pancreatic Juice, and 
Succus Entericus. 
§ 213. In the living body the food, subjected to the action first of the 
saliva and then of the gastric juice, undergoes in the stomach changes which 
we shall presently consider in detail, and the food so changed is passed on 
into the small intestine, where it is further subjected to the action of the bile 
secreted by the liver, of pancreatic juice secreted by the pancreas, and pos- 
sibly to some extent, though this is by no means certain, of a juice secreted 
by the intestine itself and called succus entericus. It will be convenient to 
study the minute structure of the liver in connection with other functions 
of the liver more important, perhaps, than that of the secretion of bile, 
namely, the formation of glycogen, and other metabolic events occurring 
in the hepatic cells; we have already studied the structure of the pan- 
creas ; and the structure of the intestine will best be considered by itself. 
We, therefore, turn at once to the properties and characters of the above- 
named juices. 
Bile. 
Though bile, after secretion in the lobules of the liver, is passed on 
along the hepatic duct, it is in the case of most animals not poured at once 
into the duodenum, but taken by the cystic duct to the reservoir of the gall- 
bladder. Here it remains until such time as it is needed, when a quantity 
is poured along the common bile duct into the intestine. 
The quality of bile varies much, not only in different animals, but in 
the same animal at different times. It is, moreover, affected by the length 
of the sojourn in the gall-bladder; bile taken direct from the hepatic duct, 
especially when secreted rapidly, contains little or no mucus; that taken 
from the gall-bladder, as of slaughtered oxen or sheep, is loaded with mucus. 
The color of the bile of carnivorous and omnivorous animals, and of man, 
is generally a bright golden red ; of herbivorous animals, a yellowish green 
or a bright green or a dirty green, according to circumstances, being much 
modified by retention in the gall-bladder. The reaction is neutral or alka- 
line. The following may be taken as the average composition of human BILE, PANCREATIC JUICE, AND SUCCUS ENTERICUS. 
279 
bile taken from the gall-bladder, and therefore containing much more 
mucus as well as, relatively to the solids, more water than bile from the 
hepatic duct. 
In 1000 parts. 
Water 859.2 
Solids: 
Bile Salts 91.4 
Fats, etc 9.2 
Cholesterin 2.6 
Mucus and Pigment 29.8 
Inorganic Salts 7.8 
140.8 
The entire absence of proteids is a marked feature of bile : pancreatic 
juice, as we shall see, contains a considerable quantity; saliva, as we have 
seen, a small quantity; normal gastric juice probably still less, and bile none 
at all. Even the bile which has been retained some time in the gall-bladder, 
though rich in mucus, contains no proteids. 
The constituents which form, apart from the mucus, the great bulk of 
the solids of bile, and which deserve chief attention, are the pigments and 
the bile-salts; of these we shall speak immediately. 
With regard to the inorganic salts actually present as such sodium salts 
are conspicuous, sodium chloride, amounting to 0.2 or more per cent., 
sodium phosphate to nearly as much, the rest being earthy phosphates 
and other matters in small quantity. The presence of iron, to the 
extent of about 0.006 per cent., is interesting, since, as we shall see, 
there are reasons for thinking that the pigment of bile, itself free from 
iron, is derived from iron-holding haemoglobin; some, at least, of the 
iron set free during the conversion of haemoglobin into bile pigment, 
which probably takes place in the liver, finds its way into the bile. 
Bile also appears to contain a small quantity, at all events occasionally, 
of other metals, such as manganese and copper; metals introduced into 
the body are apt to be retained in the liver and eventually leave it by 
the bile. 
The small quantity of fat present consists in part of the complex body, 
lecithin. 
The peculiar body, cholesterin, which though fatty-looking (hence the 
name "bile fat") is really an alcohol with the composition C26HnO, is con- 
spicuous by its quantity and constancy. It forms the greater part of most 
gall-stones, though some are composed chiefly of pigment. Insoluble in 
water and cold alcohol, though soluble in hot alcohol and readily soluble 
in ether, chloroform, etc., it is dissolved by the bile-salts in aqueous solution 
and hence is present in solution in bile Its physiological functions are 
obscure. 
The ash of bile consists largely of soda, derived partly from the sodium 
chloride and partly from the bile-salts, of sulphates derived chiefly if not 
wholly from the latter, and of phosphates partly ready formed and in part 
derived from the lecithin. 
§ 214. Pigment# of bile. The natural golden-red color of normal human 
or carnivorous bile is due to the presence of bilirubin. This, which is also 
the chief pigmentary constituent of gall-stones, and occurs largely in the 
urine of jaundice, may be obtained in the form either of an orange-colored 
amorphous powder or of well-formed rhombic tablets and prisms. Insoluble 
in water, and but little soluble in ether and alcohol, it is readily soluble 
in chloroform and in alkaline fluids. Its composition is C16H18N2O3. Treated 280 
THE TISSUES AND MECHANISMS OF DIGESTION. 
with oxidizing agents, such as nitric acid yellow with nitrons acid, it displays 
a succession of colors in the order of the spectrum. The yellowish golden- 
red becomes green, this a greenish-blue, then blue, next violet, afterward 
a dirty red, and finally a pale yellow. This characteristic reaction of bili- 
rubin is the basis of the so-called Gmelin's test for bile-pigments. Each 
of these stages represents a distinct pigmentary substance. An alkaline 
solution of bilirubin, exposed in a shallow vessel to the action of the air, 
turns green, becoming converted into biliverdin (ClfiH20N2O5 or C16H18N2O4, 
Maly), the green pigment of herbivorous bile. Biliverdin is also found at 
times in the urine of jaundice, and is probably the body which gives to bile 
which has been exposed to the action of gastric juice, as in biliary vomits, 
its characteristic green hue. It is the first stage of the oxidation of bili- 
rubin in Gmelin's test. Treated with oxidizing agents biliverdin runs 
through the same series of colors as bilirubin, with the exception of the 
initial golden-red. 
§ 215. The bile-salts. These consist, in man and many animals, of sodium 
glycocholate and taurocholate, the proportion of the two varying in different 
animals. In man both the total quantity of bile-salts and the proportion 
of the one bile-salt to the other seem to vary a good deal, but the glycocholate 
is said to be always the more abundant. In ox-gall sodium glycocholate is 
abundant and taurocholate scanty. The bile-salts of the dog, cat, bear, and 
other carnivora consist exclusively of the latter. 
Insoluble in ether, but soluble in alcohol and in water, the aqueous so- 
lutions having a decided alkaline reaction, both salts may be obtained by 
crystallization in fine acicular needles. They are exceedingly deliquescent. 
The solutions of both acids have a dextro-rotary action on polarized light. 
Preparation. Bile, mixed with animal charcoal, is evaporated to dryness and 
extracted with alcohol. If not colorless, the alcoholic filtrate must be further 
decolorized with animal charcoal, and the alcohol distilled off. The dry residue 
is treated with absolute alcohol, and to the alcoholic filtrate anhydrous ether is 
added as long as any precipitate is formed. On standing the cloudy precipitate 
becomes transformed into a crystalline mass at the bottom of the vessel. If the 
alcohol be not absolute, the crystals are very apt to be changed into a thick syrupy 
fluid. This mass of crystals has been often spoken of as bilin. Both salts are 
thus precipitated, so that in such a bile as that of the ox or man bilin consists both 
of sodium glycocholate and sodium taurocholate. The two may be separated by 
precipitation from their aqueous solutions with sugar of lead, which throws down 
the former much more readily than the latter. The acids may be separated from 
their respective salts by dilute sulphuric action, or by the action of lead acetate and 
sulphydric acid. 
On boiling with dilute acids (sulphuric, hydrochloric), or caustic potash 
or baryta water, glycocholic acid is split up into cholalic (cholic) acid and 
glycin. Taurocholic acid may similarly be split up into cholalic acid and 
taurin. Thus: 
Glycocholic acid. 
C'29H43NO6 
Cholalic acid. 
Glycin. 
CH.2.NH2(CO.OH). 
Taurocholic acid 
c26h45nso7 
+ h2o = 
Cholalic acid. 
C,4H4nOs I 
Taurin. 
C.2H4.NH2.SO3H. 
+ h2o = 
Both acids contain the same non-nitrogenous acid, cholalic acid ; but this 
acid is in the first case associated or conjugated with the important nitro- 
genous body glycin, or amido-acetic acid, which is a compound formed from 
ammonia and one of the " fatty acid " series, viz., acetic ; and in the second 
case with taurin, or amido-isethionic acid, that is a compound into which 
representatives of ammonia, of the ethyl group, and of sulphuric acid enter. BILE, PANCREATIC JUICE, AND SUCCUS ENTERICUS. 
281 
The decomposition of the bile acids into cholalic acid and taurin or glycin 
respectively takes place naturally in the intestine, the glycin and taurin 
being probably absorbed, so that from the two acids, after they have served 
their purpose in digestion, the two ammonia compounds are returned into 
the blood. Each of the two acids, or cholalic acid alone, when treated with 
sulphuric acid and cane-sugar, gives a magnificent purple color (Petten- 
kofer's test), with a characteristic spectrum. A similar color may, however, 
often be produced by the action of the same bodies on albumin, amyl alcohol, 
and some other organic bodies. 
§ 216. Action of bile on food. In some animals at least bile contains a 
ferment capable of converting starch into sugar ; but its action in this respect 
is wholly subordinate. 
On proteids bile has no direct digestive action whatever, but being, gen- 
erally at least, alkaline, and often strongly so, tends to neutralize the acid 
contents of the stomach as they pass into the duodenum, and, as we shall 
see, so prepares the way for the action of the pancreatic juice. To peptic 
action it is distinctly antagonistic ; the presence of a sufficient quantity of 
bile renders gastric juice inert toward proteids. Moreover, when bile, or a 
solution of bile-salts, is added to a fluid containing the products of gastric 
digestion, a precipitate takes place, consisting of parapeptone (when present), 
peptone, pepsin, and bile-salts. The precipitate is redissolved in an excess 
of bile or solution of bile-salts; but the pepsin, though redissolved, remains 
inert toward proteids. This precipitation actually does take place in the 
duodenum, and we shall speak of it again later on. 
With regard to the action of bile on fats, the following statements may 
be made : 
Bile has a slight solvent action on fats, as seen in its use by painters. It 
has by itself a slight but only slight emulsifying power ; a mixture of oil 
and bile separates after shaking rather less rapidly than a mixture of oil and 
water. With fatty acids bile forms soap. It is, moreover, a solvent of solid 
soaps, and it would appear that the emulsion of fats is under certain circum- 
stances at all events facilitated by the presence of soaps in solution. Hence 
bile is probably of much greater use as an emulsion agent when mixed with 
pancreatic juice than when acting by itself alone. To this point we shall 
return. Lastly, the passage of fats through membranes is assisted by wetting 
the membranes with bile or with a solution of bile-salts. Oil will pass to a 
certain extent through a filter-paper kept wet with a solution of bile-salts, 
whereas it will not pass or passes with extreme difficulty through one kept 
constantly wet with distilled water. 
Bile possesses some antiseptic qualities. Out of the body its presence 
hinders various putrefactive processes; and when it is prevented from flow- 
ing into the alimentary canal, the contents of the intestine undergo changes 
different from those which take place under normal conditions, and leading 
to the appearance of various products, especially of ill-smelling gases. 
These various actions of bile seem to be dependent on the bile-salts and 
not on the pigmentary or other constituents. 
Pancreatic Juice. 
§ 217. Natural healthy pancreatic juice obtained by means of a tem- 
porary pancreatic fistula differs from the digestive juices of which we have 
already spoken in the comparatively large quantity of proteids which it con- 
tains. Its composition varies according to the rate of secretion, for, with the 
more rapid flow, the increase of total solids does not keep pace with that of 
the water, though the ash remains remarkably constant. 282 
THE TISSUES AND MECHANISMS OF DIGESTION. 
By an incision through the linea alba the pancreatic duct (or ducts) can easily 
be found either in the rabbit or in the dog, and a canula secured in it. There is 
no difficulty about a temporary fistula ; but with permanent fistula? the secretion is 
apt to become altered in nature, and to lose many of its characteristic properties. 
Some, however, have succeeded in obtaining permanent fistulae without any impair- 
ment of the secretion. 
Healthy pancreatic juice is a clear, somewhat viscid fluid, frothing when 
shaken. It has a very decided alkaline reaction, and contains few or no 
structural constituents. 
The average amount of solids in the pancreatic juice (of the dog) obtained 
from a temporary fistula is about 8 to 10 per cent.; but in even thoroughly 
active juice obtained from a permanent fistula is not more than about 2 to 5 
per cent., 0.8 being inorganic matter ; and this is probably the normal 
amount. The important constituents of quite fresh juice are albumin, a 
peculiar form of proteid allied to myosin, giving rise to a sort of clotting, a 
small amount of fats and soaps, and a comparatively large quantity of sodium 
carbonate, to which the alkaline reaction of the juice is due, and which seems 
to be peculiarly associated with the proteids. 
Since, as we shall presently see, pancreatic juice contains a ferment acting 
energetically on proteid matters in an alkaline medium, it rapidly digests 
its own proteid constituents, and, when kept, speedily changes in character. 
The myosin-like clot is dissolved, and the juice soon contains a peculiar 
form of alkali-albumin (precipitable by saturation with magnesium sul- 
phate), as well as small quantities of leucin, tyrosin, and peptone, which 
seem to be the products of self-digestion and are entirely absent from the 
perfectly fresh juice. 
§ 218. Action on food-stuffs. On starch pancreatic juice acts with great 
energy, rapidly converting it into sugar (chiefly maltose). All that has 
been said in this respect concerning saliva might be repeated in the case of 
pancreatic juice, except that the activity of the latter is far greater than that 
of the former. Pancreatic juice and the aqueous infusion of the gland are 
always capable of converting starch into sugar, whether the animal from 
which they were taken be starving or well fed. From the juice, or, by the 
glycerin method from the gland itself, an amylolytic ferment may be 
approximately isolated. 
On proteids pancreatic juice also exercises a solvent action, so far similar 
to that of gastric juice that by it proteids are converted into peptone. If a 
few shreds of fibrin are thrown into a small quantity of pancreatic juice, 
they speedily disappear, especially at a temperature of 35° C., and the mix- 
ture is found to contain peptone. The activity of the juice in thus converting 
proteids into peptone is favored by increase of temperature up to 40° or 
thereabouts, and hindered by low temperatures ; it is permanently destroyed 
by boiling. The digestive powers of the juice in fact depend, like those of 
gastric juice, on the presence of a ferment which, as we have already said, 
may be isolated much in the same way as pepsin is isolated, and to which 
the name trypsin has been given. 
The appearance of fibrin undergoing pancreatic digestion is, however, 
different from that undergoing peptic digestion. In the former case the 
fibrin does not swell up, but remains as opaque as before, and appears to 
suffer corrosion rather than solution. But there is a still more important 
distinction between pancreatic and peptic digestion of proteids. Peptic 
digestion is essentially an acid digestion ; we have seen that the action only 
takes place in the presence of an acid, and is arrested by neutralization. 
Pancreatic digestion, on the other hand, may be regarded as an alkaline 
digestion ; the action is most energetic when some alkali is present, and the 283 
BILE, PANCREATIC JUICE, AND SUCCUS ENTERICUS. 
activity of an alkaline juice is hindered or delayed by neutralization and 
arrested by acidification, at least with mineral acids. The glycerin extract 
of pancreas is under all circumstances as inert in the presence of free mineral 
acid as that of the stomach in the presence of alkalies. If the digestive 
mixture be supplied with sodium carbonate to the extent of 1 per cent., 
digestion proceeds rapidly, just as does a peptic mixture when acidulated 
with hydrocholoric acid to the extent of 0.2 per cent. • Sodium carbonate of 
1 per cent, seems in fact to play in tryptic digestion a part altogether com- 
parable to that of hydrochloric acid of 0.2 per cent, in gastric digestion. 
And just as pepsin is rapidly destroyed by being heated to about 40° with 
a 1 per cent, solution of sodium carbonate, so trypsin is rapidly destroyed 
by being similarly heated with dilute hydrochloric acid of 0.2 per cent. 
Alkaline bile, which arrests peptic digestion, seems, if anything, favorable 
to tryptic digestion. 
Corresponding to this difference in the helpmate of the ferment, there is 
in the two cases a difference in the nature of the products. In both cases 
peptone is produced, and such differences as can be detected between pan- 
creatic and gastric peptones are relatively small; but in pancreatic digestion 
the by-product is not, as in gastric digestion, a kind of acid-albumin, but, 
as might be expected, a body having more analogy with alkali-albumin. 
Moreover, before the alkali-albumin is actually formed, the fibrin becomes 
altered and takes on characters intermediate between those of alkali-albumin 
and of ordinary albumin ; and when fresh raw, i. e., unboiled, fibrin is acted 
upon by pancreatic juice, one or more globulins appear as initial products. 
Further, there are evidences that differences of even a more profound 
nature than the above exist between pancreatic and gastric digestion. One 
of these is the appearance in the pancreatic digestion of proteids of two 
remarkable nitrogenous crystalline bodies, leucin and tyrosin. When fibrin 
(or other proteid) is submitted to the action of pancreatic juice, the amount 
of peptone which can be recovered from the mixture falls far short of the 
original amount of proteids, much more so than in the case of gastric juice ; 
and the longer the digestive action, the greater is this apparent loss. If a 
pancreatic digestive mixture be freed from the alkali-albumin by neutraliza- 
tion and filtration, the filtrate yields, when concentrated by evaporation, a 
crop of crystals of tyrosin. If these be removed the peptone may be pre- 
cipitated from the concentrated filtrate by the addition of a large excess of 
alcohol and separated by filtration. The second filtrate, upon being concen- 
trated by evaporation, yields abundant crystals of leucin and traces of 
tyrosin. Thus, by the action of the pancreatic juice, a considerable amount 
of the proteid which is being digested is so broken up as to give rise to 
products which are no longer proteid in nature. From this breaking up of 
the proteid there arise leucin, tyrosin, and probably several other bodies, 
such as fatty acids and volatile substances. 
As is well known, leucin and tyrosin are the bodies which make their 
appearance when proteids or gelatin are acted on by dilute acids, alkalies, or 
various oxidizing agents. Leucin is a body which, in an impure state, crys- 
tallizes in minute round lumps with an obscure radiate striation, but when 
pure forms thin glittering flat crystals. It has the formula C6H13NO2 or 
C5H)0.NH2(CO.OH), and is amido-caproic acid. Now, caproic acid is one 
of the " fatty acid " series, so that leucin may be regarded as a compound of 
ammonia with a fatty acid. Tyrosin, C9HnNO3, on the other hand, belongs 
to the " aromatic " series ; it is a phenyl compound, and hence allied to ben- 
zoic acid and hippuric acid. So that in pancreatic digestion the large com- 
plex proteid molecule is split up into fatty acid and aromatic molecules, 
some other bodies of less importance making their appearance at the same 284 
THE TISSUES AND MECHANISMS OF DIGESTION. 
time. We infer that the proteid molecules are in some way built up out of 
" fatty acid " and " aromatic " molecules, together with other components, 
and we shall later on see additional reasons for this view. 
Among the supplementary products of pancreatic digestion may be men- 
tioned the body indol (C8H7N), to which apparently the strong and pecu- 
liarly fecal odor which sometimes makes its appearance during pancreatic 
digestion is due. Indol, however, unlike the leucin and tyrosin, is not a 
product of pure pancreatic digestion, but of an accompanying decomposition 
due to the action of organized ferments. A pancreatic digestive mixture 
soon becomes swarming with bacteria, in spite of ordinary precautions, when 
natural juice or an infusion of the gland is used. When isolated ferment 
is used and atmospheric germs are excluded, or when pancreatic digestion 
is carried on in the presence of salicylic acid or thymol, which prevent the 
development of bacteria and like organisms but permit the action of the 
trypsin, no odor is perceived and no indol is produced. 
After long-continued digestion, especially when accompanied by putre- 
factive decomposition, the amount of proteids which are carried beyond the 
peptone stage and broken up may be very great. 
On the gelatiniferous elements of the tissues as they actually exist in the 
tissue previous to any treatment, pancreatic juice appears to have no solvent 
action. The fibrillae and bundles of fibrillae of ordinary untouched con- 
nective tissue are not digested by pancreatic juice, which in this respect 
affords a striking contrast to gastric juice. But when they have been pre- 
viously treated with acid or boiled, so as to become converted into actual 
gelatin, trypsin is able to dissolve them, apparently changing them much in 
the same way as does pepsin. Trypsin, unlike pepsin, will dissolve mucin. 
Like pepsin, it is inert toward nuclein, horny tissues, and the so-called 
amyloid matter. 
On fats pancreatic juice has a twofold action. In the first place it emul- 
sifies fats. If hog's lard be gently heated until it melts and be then mixed 
with pancreatic juice before it solidifies on cooling, a creamy emulsion last- 
ing for almost an indefinite time is formed. So also when olive oil is shaken 
up with pancreatic juice, the separation of the two fluids takes place very 
slowly, and a drop of the mixture under the microscope shows that the 
division of the fat is very minute. An alkaline aqueous infusion of the 
gland has similar emulsifying powers. In the second place pancreatic juice 
splits up neutral fats into their respective acids and glycerin. Thus, palmitin 
(or tripalmitin) (C15H3l.CO.O)3.C3H5 is with the assumption of 3H,0 split up 
into three molecules of palmitic acid 3(C]5H31.CO.OH) and one of glycerin 
(C3H5) (OH3) ; and so with the other neutral fats. If perfectly neutral fat 
be treated with pancreatic juice, especially at the body-temperature, the 
emulsion which is formed speedily takes on an acid reaction, and by appro- 
priate means not only the corresponding fatty acids, but glycerin may be 
obtained from the mixture. When alkali is present, the fatty acids thus set 
free form their corresponding soaps. Pancreatic juice contains fat and is 
consequently apt after collection to have its alkalinity reduced, and an 
aqueous infusion of a pancreatic gland (which always contains a consider- 
able amount of fat) very speedily becomes acid. 
Thus pancreatic juice is remarkable for the power it possesses of acting 
on all the food-stuffs, on starch, fats, and proteids. 
The action on starch, the action on proteids, and the splitting up of 
neutral fats appear to be due to the presence of three distinct ferments, and 
methods have been suggested for isolating them. The emulsifying power, on 
the other hand, is connected with the general composition of the juice (or of 
the aqueous infusion of the gland), being probably in large measure depend- BILE, PANCREATIC JUICE, AND SUCCUS ENTERICUS. 
285 
ent on the alkali and the alkali-albumin present. The proteolytic ferment, 
trypsin, as ordinarily prepared seems to be proteid in nature and capable of 
giving rise by digestion to peptone ; but it may be doubted, as in the case 
of pepsin and other ferments, whether the pure ferment has yet been isolated. 
There are no means of distinguishing the amylolytic ferment of the pan- 
creas from ptyalin. The term pancreatin has been variously applied to 
many different preparations from the gland, and its use had, perhaps, better 
be avoided. 
The action of pancreatic juice or of the infusion or extract of the gland, 
on starch, is seen under all circumstances, whether the animal be fasting or 
not. The same may probably be said of the action on fats. On proteids 
the natural juice, when secreted in a normal state, is always active. The 
glycerin extract or aqueous infusion of the gland, on the contrary, as we 
have already explained ($ 208), is active in proportion as the trypsinogen 
has been converted into trypsin. 
Succus Entericus. 
§ 219. When in a living animal a portion of the small intestine is liga- 
tured, so that the secretions coming down from above cannot enter its canal, 
while yet the blood-supply is maintained as usual, a small amount of secre- 
tion collects in its interior. This is spoken of as the succus entericus, and is 
supposed to be furnished by the glands of Lieberkuhn, of which we shall 
presently speak. 
Succus entericus may be obtained by the following method, known as that of 
Thiry modified by Vella. The small intestine is divided in two places at some 
distance (30 to 50 cm.) part. By fine sutures the lower end of the upper section 
is carefully united with the upper end of the lower section, thus, as it were, cut- 
ting out a whole piece of the small intestine from the alimentary tract. In suc- 
cessful cases union between the cut surfaces takes place, and a shortened but other- 
wise satisfactory canal is re-established. Of the isolated piece, the two ends are 
separately brought through incisions in the abdominal wall, and their mouths care- 
fully fastened in such a manner that each mouth of the piece opens on to the ex- 
terior. During the process of healing two fistulas are thus established, one leading 
to the beginning of and the other to the end of a short piece of intestine quite iso- 
lated from the rest of the alimentary canal; by means of these openings a small 
quantity of fluid can be obtained. 
The quantity secreted is said to be considerably increased by the administration 
of pilocarpine. 
Succus entericus obtained from the dog by the above method is a clear 
yellowish fluid having a faintly alkaline reaction and containing a certain 
quantity of mucus. It is said to convert starch into sugar and proteids into 
peptone (the action being very similar to that of pancreatic juice), to split 
up neutral fats, to emulsify fats, and to curdle milk. It is also said to con- 
vert rapidly cane-sugar into grape-sugar, and by a fermentative action to 
convert cane-sugar into lactic acid, and this again into butyric acid, with 
the evolution of carbonic acid and free hydrogen. 
According to the above results succus entericus is to be regarded as an 
important secretion acting on all kinds of food. But even at its best its 
actions are slow and feeble. Moreover, many observers have obtained nega- 
tive results, so that the various statements are conflicting. Besides, we have 
no exact knowledge as to the amount to which such a secretion takes place 
under normal circumstances in the living body. We may, therefore, con- 
clude that, at present at all events, we have no satisfactory reasons for sup- 
posing that the actual digestion of food in the intestine is, to any great 
extent, aided by such a juice. 286 
THE TISSUES AND MECHANISMS OF DIGESTION. 
Of the possible action of other secretions of the alimentary canal, as of 
the caecum and large intestine, we shall speak when we come to consider the 
changes in the alimentary canal. 
§ 220. Gall-stones. Concretions, often of considerable size, known as gall- 
stones, are not unfrequently formed in the gall-bladder, and smaller concre- 
tions are sometimes formed in the bile passages. In man two kinds of 
gall-stones are common. One kind consists almost entirely of cholesterin, 
sometimes nearly free from any admixture with pigment, sometimes more or 
less discolored with pigment. Gall-stones of this kind have a crystalline 
structure, and when broken or cut show frequently radiate and concentric 
markings. The other kind consists chiefly of bilirubin in combination with 
calcium. Gall-stones of this kind are dark-colored and amorphous. Less 
common than the above are small, dark-colored stones, having often a mul- 
berry shape, consisting not of bilirubin itself, but of one or other derivative 
of bilirubin. Gall-stones consisting almost entirely of inorganic salts, calcic 
carbonates and phosphates, are also occasionally met with. In the lower 
animals, in oxen for instance, bilirubin gall-stones are not uncommon, but 
cholesterin gall-stones are rare. 
A gall-stone appears always to contain a more or less obvious "nucleus," 
around which the material of the stone has been deposited, and which may 
be regarded as the origin of the stone ; the real cause of the formation of the 
stone lies, however, in certain changes in the bile, by which the cholesterin, 
or bilirubin, or other constituent ceases to remain dissolved in the bile. But 
we cannot discuss this matter here. 
The Secretion of Pancreatic Juice and of Bile. 
§ 221. The secretion of pancreatic juice. Although in some cases, as 
that of the parotid of the sheep, the flow of saliva is continuous or nearly so, 
in most animals, as in man, the intermittence of the secretion is very nearly 
absolute. While food is in the mouth saliva flows freely, but between meals 
only just sufficient is secreted to keep the mouth moist, and probably the 
greater part of this is supplied not by the larger salivary but by the small 
buccal glands. The flow of pancreatic juice, on the other hand, is much 
more prolonged, being in the rabbit continuous, and in the dog lasting for 
twenty hours after food. But this contrast between the secretion of saliva 
and that of pancreatic juice is natural, since the stay of food in the mouth, 
even during a protracted feast, is relatively short, whereas the time during 
which the material of a meal is able in some way or other to affect the pan- 
creas is very prolonged. 
The flow, though continuous or nearly so, is not uniform. In the dog the 
flow of pancreatic juice begins immediately after food has been taken, and 
rises to a maximum which may be reached within the first or, as in the case 
furnishing the diagram given in Fig. 88, the second hour, but which more 
commonly is not reached until the third or fourth hour. This rise is then 
followed by a fall, after which there is a secondary rise, reaching a second 
maximum at a very variable time, but generally between the fifth and 
seventh hours. This second maximum, however, is never so high as the 
first. 
The second rise may be due to material absorbed from the intestines 
being carried in the circulation to the pancreas, and so directly exciting the 
gland to activity, much in the same way as, in the case of the stomach, the 
absorption of digested material promotes the flow of gastric juice (see $ 202) ; 
and a similar absorption may contribute to the first rise also, but it is more 
probable that so marked and sudden a rise as this is carried out by some SECRETION OF PANCREATIC JUICE AND OF BILE. 
287 
nervous mechanism. The details of this mechanism have, however, not as 
yet been satisfactorily worked out. 
The pancreas derives its nerves, which reach it along its bloodvessels, 
from the solar plexus of the splanchnic system, but the ultimate origin of 
the fibres have not been traced out; some of them, however, certainly come 
through the plexus from the right vagus. 
Stimulation of the medulla oblongata, or of the spinal cord, will call forth 
secretion in a quiescent gland, or increase a secretion already going on. 
From this we may infer the existence of a reflex mechanism, though we 
cannot as yet trace out satisfactorily the exact path of either the afferent or 
the efferent impulses; all we can say is, that the latter do not reach the 
Fig. 88. 
Diagram illustrating the Influence of Food on the Secretion of Pancreatic Juice. (N. O. 
Bernstein.) The abscissae represent hours after taking food ; the ordinates represent in c.c. the 
secretion in ten minutes. A marked rise is seen at B immediately after food was taken, with a 
secondary rise between the fourth and fifth hours afterward. Where the line is dotted the obser- 
vation was interrupted. On food being again given at C, another rise is seen, followed in turn 
by a depression and a secondary rise at the fifth hour. A very similar curve would represent the 
secretion of bile. 
pancreas by the vagus, since stimulation of the medulla is effective after 
section of both vagi. 
A secretion already going on may be arrested by stimulation of the cen- 
tral end of the vagus, and the stoppage of the secretion, which has been 
observed as occurring during and after vomiting, is probably brought about 
in this way. This effect, which, however, is not confined to the vagus, stimu- 
lation of other afferent nerves, such as the sciatic, producing the same effect, 
maybe regarded (in the absence of any proof that the result is due to reflex 
constriction of the pancreatic bloodvessels unduly checking the blood-supply) 
as an inhibition of a reflex mechanism at its centre in the medulla, or in 
some other part of the central nervous system, much in the same way as 
fear inhibits at the central nervous system the secretion of saliva following 
food in the mouth (§ 196). But if so, then we must regard the secretion of 
pancreatic juice as closely resembling that of saliva, inasmuch as it is called 288 
THE TISSUES AND MECHANISMS OF DIGESTION. 
forth by a reflex act. Yet it is stated that, unlike the case of saliva, the 
secretion of pancreatic juice continues after all the nerves going to the gland 
have been divided, an operation which would do away with the possibility 
of reflex action. Such an experiment, however, cannot be regarded as 
decisive, since it is almost impossible to be sure of dividing all the nerves. 
No evidence has yet been brought forward to prove the existence of any 
double nervous mechanism similar to that of chorda fibres and sympathetic 
fibres in the salivary gland. All that can be said is that, when the gland is 
stimulated to secrete, the bloodvessels are dilated as in the salivary gland ; 
and we have already (§ 203) dwelt on the histological changes which accom- 
pany secretion. We may add that when the gland is stimulated to increased 
secretion, the increase is not merely an increase of water; the discharge of 
solids is increased even more than the discharge of water, so that the per- 
centage of solids in the juice increases. 
The quantity of pancreatic juice secreted, in the case of man, in twenty- 
four hours has been calculated at 300 c.c., but such a calculation is of very 
uncertain value. 
We have seen (§ 197) that in the salivary glands the pressure which may 
be exerted by the fluid in the ducts is very considerable, exceeding it may 
be even the blood-pressure in the carotid artery. In this respect the pan- 
creas differs from the salivary glands. When, in a rabbit, a canula con- 
nected with a vertical tube or a manometer is placed in the pancreatic duct, 
the column of fluid does not rise above a height corresponding to a pressure 
of about 17 mm. of mercury. But at this pressure the gland becomes (edem- 
atous on account of the juice secreted passing back through the walls of 
the ducts and alveoli into the connective tissue; a much higher pressure is 
needed to render a salivary gland cedematous ; and whether the low pressure 
observed in the pancreas is due to the ease with which oedema takes place, 
or to the actual secretion not being able to reach a higher pressure, cannot 
be stated with certainty. 
§ 222. The secretion of bile. The act of secretion of bile by the liver 
must not be confounded with the discharge of bile from the bile-duct into 
the duodenum. When the acid contents of the stomach are poured over 
the orifice of the biliary duct, a gush of bile takes place. Indeed, stimu- 
lation of this region of the duodenum with a dilute acid at once calls 
forth a flow, though alkaline fluids so applied have little or no effect. 
When no such acid fluid is passing into the duodenum no bile is, under 
normal circumstances, discharged into the intestine. The discharge is due 
to a contraction of the muscular walls of the gall-bladder and ducts, accom- 
panied by a relaxation of the sphincter of the orifice; both acts are prob- 
ably of a reflex nature, but the details of the mechanism have not been 
worked out. 
The secretion of bile, on the other hand, as shown by the result of bili- 
ary fistuke, is continuous ; it appears never to cease. When no food is 
taken the bile passes from the liver along the hepatic and then back along 
the cystic duct (the flow being aided probably by peristaltic contractions 
of the muscular fibres of the duct) to the gall-bladder, where it is tem- 
porarily stored ; hence in starving animals, when no discharge is excited by 
food, the gall-bladder becomes greatly distended with bile. But the secre- 
tion, though continuous, is not uniform. The rate of secretion varies, and 
is especially influenced by food ; it is seen to rise rapidly after meals, 
reaching its maximum, in dogs, in from four to eight hours. There seems 
to be an immediate, sudden rise when food is taken, then a fall, followed sub- 
sequently by a more gradual rise up to the maximum, and ending in a final 
fall to the lowest point. The curve of secretion, in fact, resembles that of SECRETION OE PANCREATIC JUICE AND OF BILE. 
289 
the secretion of pancreatic juice in having a double rise; and as in that 
case so in this, it is very probable that the first rise is in part the result of 
nervous action, and it is also possible that nervous influences intervene in 
the second, more lasting rise ; but, as we shall see presently, even nervous 
influences may affect the liver in a very indirect manner, and our knowl- 
edge as to any direct action of the nervous system on the liver is at present 
very imperfect. 
The liver receives its chief nervous supply from the solar plexus, and to a 
great extent through that part of the solar plexus called the hepatic plexus, 
which embraces the portal vein, hepatic artery, and bile-duct, as these plunge 
into the liver at the porta. The solar plexus is fed by the two abdominal 
splanchnic nerves, major and minor, by other smaller nerves from the lower 
parts of the splanchnic (sympathetic) chain, and by the terminal portion 
of the right vagus nerve. Small branches from the left vagus, rami hepatici, 
also pass directly to the liver from the termination of that nerve on the 
stomach, finding their way also through the porta. The fibres thus entering 
the liver from the several sources are, for the most part, non-medu Hated 
fibres ; with these, however, are mixed a certain number of medullated 
fibres. 
As to the functions of these nerves in reference to the secretion of bile, 
we may say at once that no satisfactory or exact statement can at present be 
made. 
§ 223. It must be remembered, however, that the liver is so peculiarly 
related to the other organs of digestion, and its vascular arrangements so 
special that, with regard to it, as compared with many other organs, an in- 
trinsic nervous mechanism must occupy a more or less subordinate position. 
The blood-supply of the pancreas for instance is dependent chiefly on the 
width for the time being of the pancreatic arteries; it will be affected, of 
course, by the general arterial pressure and by any circumstances which 
affect the outflow by the pancreatic veins, and therefore by the condition of 
the portal venous system of which those veins form a part; but in the main, 
the amount of blood bathing the alveoli of the pancreas will depend on 
whether the pancreatic arteries are constricted or dilated. The quality of 
the blood reaching the pancreas, being arterial blood drawn direct from the 
arterial foundation, will be modified only by such circumstances as modify 
the general mass of the blood. 
Very different is the case of the liver. The supply of arterial blood 
coming direct through the hepatic artery is small compared with the mass 
pouring through the vena portae ; it moreover, as we shall see, is distributed 
in capillaries among the small interlobular branches of the vena portae and 
has become venous, indeed merged with the portal blood, before it reaches 
the actual lobules. The supply of blood for the liver is mainly that 
through the vena portae; and this supply is not, like an arterial supply, a 
fairly uniform one, modified chiefly by the vasomotor events of the organ 
itself, but is dependent on what happens to be taking place in the alimen- 
tary canal and in abdominal organs other than the liver itself. When no 
food is being digested and the alimentary canal is at rest, the vessels of that 
canal, as we have already said in speaking of the stomach, are like those 
of the pancreas and salivary glands, in a state of tonic constriction ; a 
relatively small quantity of blood passes through them ; hence the flow 
through the vena portae is relatively inconsiderable, and the pressure in 
that vessel is low. When digestion is going on all the minute arteries of 
the stomach, intestine, spleen and pancreas are dilated, and general arterial 
pressure being by some means or other maintained (see § 180), a relatively 
large quantity of blood rushes into the vena portae and the pressure in that 
19 290 
THE TISSUES AND MECHANISMS OF DIGESTION. 
vessel becomes much increased, though, of course, remaining lower than 
the general arterial pressure. Moreover, during digestion, peristaltic move- 
ments of the muscular coats of the alimentary canal are, as we have seen, 
active; and these movements, serving as aids to the circulation (see § 110), 
help to increase the portal How. Further, the spleen, as we shall see in 
speaking of that organ, is in many animals richly provided with plain mus- 
cular fibres, and in such cases seems, especially during digestion, to act as a 
muscular pump driving the blood onward, with increased vigor, along the 
splenic veins to the liver. So that even were the liver not connected with 
the central nervous system by a single nervous tie, the tide of blood through 
the liver would ebb and flow according to the absence or presence of food 
in the alimentary canal. 
An increase of blood-supply does not, of course, necessarily mean an 
increase of secretory activity. As we have seen, § 197, in the presence of 
atropine the secretion of saliva may stand still in spite of dilated blood- 
vessels and the consequent rush of blood ; but we may safely assert that, 
other things being equal, a fuller blood-supply is favorable to activity. 
Apparently a mere change in the quantity of blood bathing an alveolus 
will not start in the cells the changes which constitute the act of secretion, 
any more than an increase in the blood bathing a muscular fibre will neces- 
sarily set going a contraction ; but unless there be some counteracting 
influence at work, a fuller and richer lymph around a cell will naturally 
lead to the cell taking up more material from the lymph, and so will increase 
the cell's store of energy. Hence, especially in the hepatic cell, which 
appears to be always at work, always undergoing metabolism of such a 
kind as to give rise to bile, we might fairly expect the greater flow through 
the portal vein to quicken the flow through the bile-duct. 
Ami, as a matter of fact, we do find vaso-constrictor action dominant 
over the secretion. In the various experiments which have been made to 
ascertain the action of the nervous system on the secretion of bile, it has 
always been found that stimulation of the medulla oblongata, or of the 
spinal cord, or of the abdominal splanchnic nerves, stops, or at least 
checks the flow of bile. Now the effect of these stimulations is, as we 
have already seen more than once, a powerful constricting action on the 
abdominal bloodvessels ; by such stimulation the blood-supply of the liver 
is materially diminished, and in consequence the secretory activity is slack- 
ened or arrested. 
But there is something besides the mere quantity of blood to be consid- 
ered in this relation. The blood which passes from the alimentary canal at 
rest is ordinary venous blood, laden simply with carbonic acid and the ordi- 
nary products of the metabolism of the muscular and mucous coats of the 
canal. When digestion is going on the portal blood is laden, as we shall 
see, with some, at all events, of the products of digestion, with sugar prob- 
ably and with various proteid bodies. And it is quite possible or even prob- 
able that some of these bodies in the portal blood reaching the hepatic cells, 
stir them up to secretory activity ; indeed, this view may be regarded as sup- 
ported by the fact that proteid food increases the quantity of bile secreted, 
whereas fatty food, which as we shall see passes chiefly, if not wholly, not 
by the portal vein but by the lymphatics, and which is probably largely 
disposed of in some way or other before it can reach the liver, has no such 
effect. 
Hence we may infer that at all events the second increase of the flow 
of bile which occurs during the later stages of digestion, may be to a 
large extent the direct effect of blood, laden with digestive products, 
passing from the stomach and intestines, especially the latter, to the liver 291 
SECRETION OF PANCREATIC JUICE AND OF BILE. 
by the portal vein, quite independent of any direct nervous action on 
the liver itself; and, indeed, it is possible that the first rise also may be 
partly due to the increased flow of blood from the stomach, aided by the 
absorption from that organ of a certain amount of digested material. 
Since, however, there is no evidence of any decrease in blood-supply, or 
in the rate of absorption, corresponding to the fall between the two rises, 
some influences other than those which we are discussing must be at work 
in the matter. 
§ 224. The blood-supply of the liver being thus, quite apart from any 
nervous supply of its own, so closely dependent on what is going on in the 
alimentary canal, it will be convenient to say a few words more concerning 
the vasomotor nerves of that canal. As we have already said, in speaking 
of the vascular system, § 155, the vaso-constrictor fibres for the stomach and 
intestines, large and small, issuing from what we may call the vaso-constrictor 
region of the cord, pass for the most part through the two abdominal splanch- 
nic nerves, major and minor, a small number only passing out below the 
roots of those nerves. When these splanchnic nerves are divided the 
vessels of the canal are dilated, when they are centrifugally stimulated the 
vessels are constricted. Whether there be any distinct vaso-dilator fibres for 
all or any part of the canal, and if so what course they take, is not known. 
When no food has for some time been taken, the mucous membrane of the 
stomach as seen through a gastric fistula is pale; the bloodvessels are con- 
stricted. And, as far as we know, a similar condition obtains throughout 
the small and large intestines. When food is taken the mucous membrane 
of the stomach becomes flushed ; its vessels become dilated. This appears 
to be the result of an inhibition of the previously existing tonic constric- 
tion ; at least we have no evidence supporting any other explanation. 
Apparently the presence of food in the stomach starts in the mucous mem- 
brane influences which, ascending to the central nervous system, inhibit the 
vasomotor centre for the abdominal splanchnic nerves or such part of that 
centre as governs the vaso-constrictor fibres of the stomach. By what path 
such afferent impulses reach the central nervous system is not as yet defi- 
nitely settled ; but possibly by the vagus nerve, if it be true, as stated, that 
centripetal stimulation of that nerve, while it raises the general blood-pres- 
sure by increasing, in a reflex manner, vaso-constriction in other regions, 
leads to a dilatation of the gastric vessels. So also it is probable that as the 
food reaches succeeding sections of the alimentary canal, those in turn in 
a similar manner become flushed with blood. In the frog there is some 
evidence that vaso-constrictors leaving the spinal cord by consecutive spinal 
nerves, govern the bloodvessels of consecutive sections of the alimentary canal. 
All this flushing of the canal with blood leads, we repeat, to an increased 
flow of blood at a higher pressure through the portal vein. Whether there 
be any additional mechanism set to work, such as, for instance, which some 
observations suggest, a rhythmical peristaltic contraction of the portal 
vein, by which the blood is still more rapidly hurried to the liver, and 
whether the increased venous supply through the portal vein is accompanied 
by a corresponding increase of the lesser supply of arterial blood through 
the hepatic artery, is not known. It may, perhaps, be here remarked that 
there is no need for any increase of arterial blood, since the blood from the 
alimentary canal, owing to its more rapid passage through the minute vessels, 
is probably like the corresponding blood in the veins of an active salivary 
gland (though probably also not to the same extent) less venous than usual 
during digestion, in spite of the extra quantity of carbonic acid thrown into 
it by the increased metabolism of the muscular coat during the peristaltic 
movements. 292 
THE TISSUES AND MECHANISMS OF DIGESTION. 
§ 225. It is interesting to observe that the pressure under which the bile 
is secreted is relatively low, like that of the pancreatic juice, not high like 
that of the saliva ; it is much lower than the arterial pressure in the same 
animal, whereas in the case of saliva (§ 197) the pressure is greater than the 
blood-pressure in the carotid artery. But, in the case of bile, since the blood 
which flows through the hepatic lobules is, mainly, venous portal blood, we 
have to compare the pressure of the secretion, not with arterial pressure but 
with the venous pressure in the portal system ; and in the dog it has been 
found that while the pressure of the bile secreted stood at about 200 mm. of 
a solution of sodium carbonate-that is, about 15 mm. mercury-the blood- 
pressure in a branch of the superior mesenteric vein stood only at about 90 
mm. of the same solution-that is, about 7 mm. mercury. Now, the venous 
pressure in the mesenteric veins is higher, though only slightly higher, than 
that in the portal vein into which these pour their blood (the difference of 
pressure being the main cause why the blood Hows from the one into the 
other), and is, therefore, certainly higher than the pressure in the portal 
capillaries of the hepatic lobules. So that what is true of the salivary gland 
is also true, on a different scale, of the liver, viz., that the pressure exerted 
by the secretion is higher than the pressure of the blood in the vessels feed- 
ing the secreting cells. 
§ 226. If the pressure in the bile-duct be artificially increased, as by 
pouring fluid into the glass tube or manometer with which the canula in the 
duct is connected, a resorption of the secreted bile takes place; and resorp- 
tion will also take place within the body, when the pressure generated by the 
act of secretion itself reaches and is maintained at a sufficiently high level. 
Thus, when in the living body the bile-duct is ligatured, or becomes ob- 
structed by gall-stones or otherwise, fluid is accumulated on the near side of 
the ligature at a pressure which goes on increasing until resorption of the 
bile takes place, bile salts and biliary pigments are thrown back upon the 
system, and " jaundice" results. It would appear that in these cases re- 
sorption takes place through the interlobular bile-ducts and not through the 
hepatic cells or other structures within the lobules. The high pressure in 
the ducts does not lead to a reversal of the current in the hepatic cells (at 
most it slackens or possibly stops the current), but the bile secreted into the 
interlobular ducts escapes from these. It further appears that the escape is 
not into the bloodvessels but into the lymphatics; the bile salts, pigments, 
and other constituents are carried into the thoracic duct, and in an indirect 
manner only find their way into the blood stream. 
To complete the history of the secretion of bile we ought now to turn to 
the manufacture of the biliary constituents within the cells. But since the 
hepatic cells are also engaged in labors other and more important, perhaps, 
than that of secreting bile, it will be convenient to defer what we have to 
say on this point until we come to speak of the formation of glycogen and 
of the general metabolic events taking place in the liver. 
The Muscular Mechanisms of Digestion. 
§ 227. From its entrance into the mouth until such remnant of it as is 
undigested leaves the body, the food is continually subjected to movements 
having for their object the trituration of the food as in mastication, or its 
more complete mixture with the digestive juices, or its forward progress 
through the alimentary canal. These various movements may briefly be 
considered in detail: 
§ 228. Peristaltic movements. Putting aside the somewhat complicated 
pharyngeal part of deglutition, and taking the oesophageal movements by THE MUSCULAR MECHANISMS OF DIGESTION. 
293 
themselves, we find that these, together with the movements of the stomach 
and of the small and large intestines right down to the aims are more or less 
alike, and may be described under the general name of " peristaltic" move- 
ments. We have already in § 88 spoken of these, but it may be well to 
consider them briefly again under a general aspect, before dwelling on the 
special movements of the several parts of the alimentary canal. 
The muscular coat of the alimentary canal consists, as we have seen, of 
two layers, separated more or less distinctly by a sheet of connective tissue, 
an outer thinner longitudinal layer, and an inner thicker circular layer ; and 
a similar arrangement obtains in nearly all the muscular hollow tubes of the 
body, except the arteries, in which the muscular elements are present not so 
much for the purpose of driving the blood onward as for the sake of regula- 
ting the irrigation. 
The action of the circular coat is fairly simple. A contraction starting 
at any part, travels onward in the same direction, generally downward, that 
is to say, from a part nearer the mouth to a part nearer the rectum, for a 
greater or less distance, the circularly disposed bundles contracting in se- 
quence. The result is a narrowing or constriction of the tube which, travel- 
ling more or less slowly along the tube, drives the contents onward ; when a 
butcher empties the intestine of a slaughtered animal by squeezing it high 
up with his hand or with his thumb and finger, and carrying the squeezing 
action downward along the length of the intestine, he makes the passive in- 
testine do very much what the circular coat does actively, by contraction, 
in the living animal. 
The action of the longitudinal coat is perhaps not so clear ; but a con- 
traction of the longitudinal coat taking place in any segment of the tube 
would tend to draw the tube over the contents lying immediately above, or 
below, the segment, very much as a glove is drawn over a finger. And a 
succession of such contractions travelling along the tube would lead to a 
movement of the contents in the same direction. Were the circular coat 
absent a longitudinal coat might by itself possibly suffice to propel the con- 
tents along the tube. In the presence of the circular coat, the action of the 
longitudinal coat in any segment of the tube, if taking place immediately 
before the circular contraction, would, by filling the segment with contents, 
render the squeezing action of the circular coat more efficient; if taking- 
place immediately after the circular contraction, it would help in quicken- 
ing the return of the tube to its normal calibre, for the contraction of the 
longitudinal coat tends to shorten and widen the segment, and thus would 
prepare it for new contents. We can hardly imagine that the two coats 
would contract at the same time, since they would tend to neutralize each 
other's action. Indeed, we may probably go farther and assume that in 
each segment of the canal first the longitudinal coat contracts while the 
circular coat is relaxed, and that then the circular coat contracts while the 
longitudinal relaxes. When we come to deal with respiration we shall meet 
with a similar double antagonistic and successive action between inspiratory 
and expiratory muscles ; we shall further see reason to think that the pro- 
cesses which start the expiratory act tend to check or inhibit the inspiratory 
act and vice versa; and very possibly a like see-saw of stimulation and in- 
hibition obtains in the muscles of the alimentary canal. 
It must be remembered that the circular coat is always much thicker 
than the longitudinal coat; and we may infer that while the chief work of 
driving the contents onward falls on the former, the latter assists the work, 
either in the way which we have suggested or in some other way. 
In the small intestine the tube is hung loosely and much twisted, so that 
many loops are formed ; the contents, moreover, are largely fluid. Hence, 294 
THE TISSUES AND MECHANISMS OF DIGESTION. 
the steady onward movement, such as is seen when more solid contents pass 
along the straight and somewhat firmly attached oesophagus, is complicated 
by movements due to a loop being projected forward by the entrance of fluid 
from above, or being dragged down by the weight of its new contents, or, 
on the other hand, due to a loop being retracted by the driving onward of 
its contents and the emptying of itself, and the like. In this way a peculiar 
writhing movement of the bowel is brought about, and the phrase " peristal- 
tic movements " is generally used to denote this total effect of the contrac- 
tion of the muscular coats; it will, however, be best to restrict the meaning 
to the progressive contraction of the circular coat assisted, in most cases, by 
a similar progressive contraction of the longitudinal coat. 
§ 229. Mastication. This in man consists chiefly of an up-and-down 
movement of the lower jaw, combined in the grinding action of the molar 
teeth, with a certain amount of lateral fore-and-aft movement. The lower 
jaw is raised by means of the temporal, masseter, and internal pterygoid 
muscles. The slighter effort at depression brings into action chiefly the 
digastric muscle, though the mylo-hyoid and genio-hyoid probably share in 
the matter. Contraction of the external pterygoids pulls forward the con- 
dyles and thrusts the lower teeth in front of the upper. Contraction of the 
pterygoids on one side will also throw the teeth on to the opposite side. The 
lower horizontally placed fibres of the temporal serve to retract the jaw. 
During mastication the food is moved to and fro and rolled about by the 
movements of the tongue. These are affected by the muscles of that organ 
governed by the hypoglossal nerve. 
The act of mastication is a voluntary one, guided as are so many volun- 
tary acts, not only by muscular sense, but also by contact sensations. The 
motor fibres of the fifth cranial nerve convey motor impulses from the brain 
to the above-mentioned muscles; but paralysis of the sensory fibres of the 
same nerve renders mastication difficult by depriving the will of the aid of 
the usual sensations. 
§ 230. Deglutition. The food when sufficiently masticated is, by the 
movements of the tongue, gathered up into a bolus on the middle of the 
upper surface of that organ. The front of the tongue being raised-partly 
by its intrinsic muscles and partly by the stylo-glossus-the bolus is thrust 
back between the tongue and the palate through the anterior pillars of the 
fauces or isthmus faucium. Immediately before it arrives there the soft 
palate is raised by the levator palati, and so brought to touch the posterior 
wall of the pharynx, which, by the contraction of the upper margin of the 
superior constrictor of the pharynx, bulges somewhat forward. The elevation 
of the soft palate causes a distinct rise of pressure in the nasal chambers? 
this can be shown by introducing a water manometer into one nostril and 
closing the other just previous to swallowing. By the contraction of the 
palato-pharyngeal muscles which lie in the posterior pillars of the fauces the 
curved edges of those pillars are made straight, and thus tend to meet in 
the middle line, the small gap between them being filled up by the uvula. 
Through these manoeuvres the entrance into the posterior nares is blocked, 
while the soft palate is formed into a sloping roof, guiding the bolus down 
the pharynx. By the contraction of the stylo-pharyngeus and palato- 
pharyngeus, the funnel-shaped bag of the pharynx is brought up to meet 
the descending morsel, very much as a glove may be drawn up over the 
finger. 
Meanwhile in the larynx, as shown by the laryngoscope, the arytenoid 
cartilages and vocal cords are approximated, the latter being also raised so 
that they come very near to the false vocal cords; and the cushion at the 
base of the epiglottis covers the rima glottidis, while the epiglottis itself is THE MUSCULAR MECHANISMS OF DIGESTION. 
295 
depressed over the larynx. The thyroid cartilage is now, by the action of 
the laryngeal muscles, suddenly raised up behind the hyoid bone, and thus 
assists the epiglottis to cover the glottis. This movement of the thyroid can 
easily be felt on the outside. Thus, both the entrance into the posterior 
nares and that into the larynx being closed, the impulse given to the bolus 
by the tongue can have no other effect than to propel it beneath the sloping 
soft palate over the incline formed by the root of the tongue and the epi- 
glottis. The palato-glossi or constrictores isthmi faucium, which lie in the 
anterior pillars of the fauces, by contracting close the door behind the food 
which has passed them. 
When the bolus of food is large it is received by the middle and lower 
constrictors of the pharynx, which, contracting in sequence from above 
downward, thrust it into the oesophagus, along which it is driven by a similar 
series of successive contractions which we shall speak of immediately as 
peristaltic action. This comparatively slow descent of the food from the 
pharynx into the stomach may be readily seen if animals with long necks, 
such as horses and dogs, be watched while swallowing. When, however, the 
morsel is not large, or when the substance swallowed is liquid, the movement 
of the back part of the tongue may be sufficient not merely to introduce the 
food into the grasp of the constrictors of the pharynx, but even to propel it 
rapidly, to shoot it, in fact, along the lax oesophagus before the muscles of 
that organ have time to contract. In such a mode of swallowing the middle 
and lower constrictors take little or no part in driving the food onward, 
though they and the oesophagus appear to contract from above downward 
after the food has passed by them, as if to complete the act and to insure 
that nothing has been left behind. Deglutition in this fashion still remains 
possible after these constrictors have become paralyzed by section of their 
motor nerves. 
When a second act of deglutition succeeds the first with sufficient rapid- 
ity, the nervous changes which start the pharyngeal movements of the second 
act appear to inhibit the oesophageal movements of the first act; and when 
swallowing is repeated rapidly several times in succession, the oesophagus 
remains quiet and lax during the whole time until immediately after the last 
swallow, when a peristaltic movement closes the series. 
When the stethoscope is applied over the oesophagus, at different regions, 
a sound is heard during deglutition ; sometimes two sounds are heard. The 
first and most constant is coincident with the passage of the bolus, and is due 
to this and to the muscular sound of the contracting muscles. The later and 
less constant sound appears to be caused by a quantity of air-bubbles with 
which the bolus was entangled, lodged at the cardiac end of the oesophagus, 
being forced into the stomach by the sequent peristaltic contraction of the 
oesophagus. 
It will be seen from what has been said that deglutition, though a con- 
tinuous act, may be regarded as divided into three stages : The first stage 
is the thrusting of the food through the isthmus faucium ; this may be either 
of long or short duration. The second stage is the passage through the 
upper part of the pharynx. Here the food traverses a region common both 
to the food and to respiration, and in consequence the movement is as rapid 
as possible. The third stage is the descent through the grasp of the con- 
strictors. Here the food has passed the respiratory orifice, and in conse- 
quence its passage again becomes comparatively slow, except in case of 
fluids and small morsels, when, as we have seen, it may continue to be rapid. 
The passage along the oesophagus may, perhaps, be regarded as constituting 
a fourth stage ; but it will be more convenient to consider the oesophageal 
movements bv themselves. 296 
THE TISSUES AND MECHANISMS OF DIGESTION. 
The first stage in this complicated process is undoubtedly a voluntary 
act. The raising of the soft palate and the approximation of the poste- 
rior pillars may also be at times voluntary, since they have been seen, in a 
case where the pharynx was laid bare by an operation, to take place before 
the food had touched these parts; but the movement may take place 
without any exercise of the will and in the absence of consciousness. 
Indeed, the second stage, taken as a whole, though some of the earlier 
component movements are, as it were, on the borderland between the volun- 
tary and involuntary kingdoms, must be regarded as a reflex act. The 
third and last stage, whatever be the exact form which it takes, is undoubt- 
edly reflex ; the will has no power whatever over it, and can neither origi- 
nate, stop, nor modifiy it. 
Deglutition in fact as a whole is a reflex act; it cannot take place unless 
some stimulus be applied to the mucous membrane of the fauces. When 
we voluntarily bring about swallowing movements with the mouth empty, 
we supply the necessary stimulus by forcing with the tongue a small 
quantity of saliva into the fauces, or by touching the fauces with the tongue 
itself. 
In the reflex act of deglutition, caused in the ordinary way by the food 
coming in contact with the fauces, the afferent impulses originated in the 
fauces are carried up to the nervous centre by the glosso-pharyngeal nerve, 
by branches of the fifth, and by the pharyngeal branches of the superior 
laryngeal division of the vagus. The latter seem of special importance, 
since the act of swallowing, quite apart from the presence of food in the 
mouth, may be brought about by centripetal stimulation of the superior laryn- 
geal nerve. The efferent impulses descend the hypoglossal to the muscles 
of the tongue, and pass down the glosso-pharyngeal, the vagus through the 
pharyngeal plexus, the fifth, and the spinal accessory, to the muscles of the 
fauces and pharynx ; their exact paths being as yet not fully known, and 
probably varying in different animals. The laryngeal muscles are governed 
by the laryngeal branches of the vagus. 
The centre of the reflex act lies in the medulla oblongata. Deglutition 
can be excited, by tickling the fauces, in an animal rendered unconscious 
by removal of the brain, provided the medulla be left. If the medulla be 
destroyed, deglutition is impossible. The centre for deglutition lies higher 
up than that of respiration, so that in the diseases or injuries involving 
the upper part of the medulla oblongata, the former act may be impaired 
or rendered impossible while the latter remains untouched. It has been 
said to form part of the superior olivary bodies, but this view is based on 
anatomical grounds only. We shall have to deal with this and similar 
matters in treating of the central nervous system. It is probable that, as 
is the case in so many other reflex acts, the whole movement can be called 
forth by stimuli affecting the centre directly, and not acting on the usual 
afferent nerves. 
$ 231. Movements of the oesophagus. These, as we have just said, are 
fairly simple. The circular contraction begun by the constrictors of the 
pharynx is continued along the circular coat of the oesophagus, and assisted 
by an accompanying contraction of the longitudinal coat, the direction being 
always, save in the abnormal action of vomiting, from above downward. 
It will be remembered that the muscular bundles of the oesophagus are 
composed of striated fibres in the upper part, and of plain unstriated fibre 
cells in the lower part, the transition occupying a different level in different 
animals. Nevertheless, as far as the peristaltic movement is concerned, the 
two kinds of fibres behave in the same way, except that the peristaltic wave, 
if we may so call it, travels more rapidly in the striated region. 297 
THE MUSCULAR MECHANISMS OF DIGESTION. 
These peristaltic movements of the cesophagus may, like those of the in- 
testine, be seen after removal of the organ from the body ; and, indeed, may 
continue to appear, upon stimulation, for an unusual length of time. They 
may, therefore, be carried out by the muscular elements, with or without the 
help of the nervous elements imbedded in them, apart from any action of 
the central nervous system. Nevertheless, in the living body, the move- 
ments of the oesophagus seem to be in a special way dependent on the central 
nervous system ; the contractions are not started and carried out by the 
walls of the tube alone, and so transmitted from section to section in the 
walls of the tube itself; but afferent impulses started in the pharynx and 
passing to the medulla oblongata, give rise to reflex efferent impulses which 
descend along nervous tracts to successive portions of the organ. If the 
oesophagus be cut across some way down, or if a portion of the middle region 
be excised, stimulation of the pharynx will produce a peristaltic contrac- 
tion, which travelling downward will not stop at the cut or excision, but 
will be continued on into the lower disconnected portion by means of the 
central nervous system. And it is stated that ordinary peristaltic contrac- 
tions of the lower part of the oesophagus can be readily excited by stimu- 
lation of the pharynx, but not by stimuli applied to its own mucous mem- 
brane. In the reflex act which thus brings about the peristaltic contraction 
of the oesophagus the afferent nerves are those of the pharynx, viz., the 
superior laryngeal nerve and pharyngeal branches of the vagus, branches 
of the fifth, and in some animals, at least, branches of the glosso-pharyngeal, 
but chiefly the first; and oesophageal movements can easily be excited by 
centripetal stimulation of the superior laryngeal. The centre lies in the 
medulla oblongata, being a part of the general deglutition centre ; and effer- 
ent impulses pass along fibres of the vagus, reaching the upper part of the 
oesophagus by the recurrent laryngeal nerves, and the lower part through 
the oesophageal plexus of the vagus. Section of the trunk of the vagus 
renders difficult the passage of food along the cesophagus, and stimulation 
of the peripheral stump causes oesophageal contractions. 
The force of this movement in the oesophagus is considerable ; thus in 
the dog a ball, pulling by means of a pulley against a weight of 250 
grammes, has been found to be readily carried down from the pharynx to 
the stomach. 
At the junction of the oesophagus with the stomach the circular fibres 
usually remain in a more or less permanent condition of tonic or obscurely 
rhythmic contraction, more particularly when the stomach is full of food, 
and thus serve as a sphincter to prevent the return of food from the stomach 
into the oesophagus. Upon the arrival of the bolus of food at the end of 
the oesophagus, the centre for this sphincter is inhibited and the orifice is 
thus opened up. Possibly the patency of the orifice is still further secured 
by a contraction of the longitudinal muscular fibres which radiate from the 
end of the oesophagus over the stomach. 
§ 232. Movements of the stomach. While the object of the oesophageal 
movement is simply to carry the swallowed bolus with all due speed to the 
stomach, and while the intestinal movement has, in like manner, simply to 
carry the intestinal contents onward, the twisted course of the looped path 
insuring all the mixing of the constituents of the contents that may be 
necessary, the movements of the stomach have a double object: on the one 
hand to provide an adequate exposure of the contents of the dilated chamber 
to the influence of the gastric juice, and on the other to propel the partially 
digested food, when ready, into the duodenum. We may, accordingly, dis- 
tinguish between what we may call the " churning " and the " propulsive " 
movements of the stomach. 298 
THE TISSUES AND MECHANISMS OF DIGESTION. 
When the stomach is empty all the muscular fibres, as we have said, 
longitudinal, circular, and oblique, fall into a condition which we may per- 
haps speak of as an obscure tonic contraction. The whole stomach is small 
and contracted, its cavity is nearly obliterated, and the mucous membrane, 
owing to the predominance of the circular coat, is, like the lining membrane 
of an empty artery, thrown into longitudinal folds. As more and more food 
enters the stomach all the coats become relaxed, with the exception of the 
pyloric sphincter, which remains at first permanently closed, and the less 
marked cardiac sphincter, which merely relaxes from time to time at each 
act of swallowing. No sooner, however, do the coats thus become relaxed 
than they set up obscure rhythmical peristaltic contractions, giving rise to 
the "churning" movements. These movements have been described as of 
such a kind that the contents flow in a main current from the cardia along 
the greater curvature to the pylorus, and back to the cardia along the lesser 
curvature, subsidiary currents mixing the peripheral portions of the con- 
tents with the more central; it may be doubted, however, whether any such 
regularity of flow is marked or constant, and it is not easy to see by what 
combination and sequence of contractions in the three coats, longitudinal, 
circular, and oblique, such a regular flow can be produced. But in any 
case, by such rhythmical contractions the food and gastric juice are 
rolled about and mixed together. These churning movements are fee- 
ble at first, even though the stomach be filled and distended by a large 
meal rapidly eaten ; they become more and more pronounced as digestion 
proceeds. 
Before digestion has proceeded very far the " propulsive " movements 
begin. These occur at intervals, and are repeated at first slowly but after- 
ward more rapidly. Each movement consists in a contraction of the circu- 
lar muscular fibres more powerful than any taking part in the churning move- 
ments, and leading to a circular constriction which, beginning apparently at 
about the obscurely defined groove which marks the beginning of the antrum 
pylori, travels down toward the pylorus, propelling the food onward. This 
movement is accompanied or rather preceded by a relaxation of, that is to 
say, in all probability an inhibition of the permanent contraction of, the 
sphincter pylori itself, in order that the gastric contents may pass into the 
duodenum. But the occurrence of this relaxation is determined by the na- 
ture of the gastric contents; for if the propulsive movement drives large 
undigested pieces toward the pylorus, the sphincter is apt to close again, the 
result of which is that the undigested morsels are carried back into the main 
body of the stomach. 
The combined effect, then, of the churning and of the propulsive move- 
ments is, after a certain part of the meal has been reduced to a thick fluid 
condition somewhat resembling pea soup and often called chyme, to strain 
off' this more fluid part into the duodenum, and to submit the remaining still 
solid pieces to the further action of the gastric juice. 
As digestion proceeds, more and more material leaves the stomach, which 
is thus gradually emptied, the last portions which are carried through being 
those parts of the food which are least digestible, and any wholly indigesti- 
ble foreign bodies which happen to have been swallowed ; the latter may 
perhaps never leave the stomach at all. The presence of food leads to the 
development of the movements ; but evidently it is not the mere mechanical 
repletion of the organ which is the cause of the movements, since the stomach 
is fullest at the beginning when the movements are slight, and becomes 
emptier as they grow more forcible. The one thing which does increase 
pari passu with the movements is the acidity, which is at a minimum when 
the (generally alkaline) food has been swallowed, and increases steadily on- THE MUSCULAR MECHANISMS OF DIGESTION. 
299 
ward. It has not, however, been definitely shown that the increasing acidity 
is the efficient stimulus giving rise to the movements. 
The movements of even a full stomach are said to cease during sleep. 
The nervous mechanism of the gastric movements had better be considered 
in connection with that of the intestinal movements. 
§ 233. Vomiting. In a conscious individual this act is preceded by feel- 
ings of nausea, during which a copious flow of saliva into the mouth takes 
place. This being swallowed carries down with it a certain quantity of air, 
the presence of which in the stomach, by assisting in the opening of the car- 
diac sphincter, subsequently facilitates the discharge of the gastric contents. 
The nausea is generally succeeded at first by ineffectual retching in which 
a deep inspiratory effort is made, so that the diaphragm is thrust down as low 
as possible against the stomach, the lower ribs being at the same time forcibly 
drawn in ; since during this inspiratory effort the glottis is kept closed, no 
air can enter into the lungs ; but some is drawn into the pharynx, and thence 
probably descends by a swallowing action into the stomach. When retching 
passes on to actual vomiting this inspiratory effort is succeeded by a sudden 
violent expiratory contraction of the abdominal walls, the glottis still being 
closed, so that the whole force of the effort is spent, as we shall see it is 
in defecation, in pressure on the abdominal contents. The stomach is, there- 
fore, forcibly compressed from without. At the same time, or rather imme- 
diately before the expiratory effort, by a contraction of its longitudinal fibres 
the oesophagus is shortened and the cardiac orifice of the stomach brought 
close under the diaphragm, while apparently by an inhibition of the circular 
sphincter, aided perhaps by a contraction of the fibres which radiate from 
the end of the oesophagus over the stomach, the cardiac orifice, which is 
normally closed, is somewhat suddenly dilated. This dilatation opens the way 
for the contents of the stomach, which, pressed upon by the contraction of 
the abdomen, and to a certain but probably only to a slight extent by the 
contraction of the gastric walls, are driven forcibly up the cesophagus. The 
mouth being widely open, and the neck stretched to afford as straight a 
course as possible, the vomit is ejected from the body. At this moment there 
is an additional expiratory effort which serves to prevent the vomit passing 
into the larynx. In most cases, too, the posterior pillars of the fauces are 
approximated, in order to close the nasal passage against the ascending stream. 
This, however, in severe vomiting is frequently ineffectual. 
Thus in vomiting there are two distinct acts: the dilatation of the cardiac 
orifice and the extrinsic pressure of the abdominal walls in an expiratory 
effort. Without the former the latter, even when distressingly vigorous, is 
ineffectual. Without the latter, as in urari poisoning, the intrinsic move- 
ments of the stomach itself are rarely efficient to do more than eject gas, 
and, it may be, a very small quantity of food or fluid. Pyrosis or water- 
brash is, however, probably brought about by this intrinsic action of the 
stomach. 
During vomiting the pylorus is generally closed, so that but little mate- 
rial escapes into the duodenum. When the gall-bladder is full, a copious 
flow of bile into the duodenum accompanies the act of vomiting. Part of 
this may find its way into the stomach, as in bilious vomiting, the pylorus 
then having evidently been opened. 
The nervous mechanism of vomiting is complicated and in many aspects 
obscure. The efferent impulses which cause the expiratory effort must come 
from the respiratory centre in the medulla ; with these we shall deal in 
speaking of respiration. The dilatation of the cardiac orifice is caused, in 
part at least, by impulses descending the vagi, since when these are cut real 
vomiting with discharge of the gastric contents, if it takes place at all, be- 300 
THE TISSUES AND MECHANISMS OE DIGESTION. 
conies difficult through want of readiness in the dilatation. Such intrinsic 
movements of the stomach as do take place and the movements of the 
oesophagus appear to be carried out by the usual nerves. The efferent im- 
pulses which cause the flow of saliva in the introductory nausea also descend 
along the usual nerves such as the chorda tympani. These various impulses 
may best be considered as starting from a vomiting centre in the medulla, 
having close relations with the respiratory centre. This centre may be ex- 
cited, may be thrown into action, in a reflex manner, by stimuli applied to 
peripheral nerves, as when vomiting is induced by tickling the fauces, or by 
irritation of the gastric membrane, or by obstruction of the intestine due to 
ligature, hernia, etc. That the vomiting in the last instance is due to nervous 
action, and not to any regurgitation of the intestinal contents, is shown by 
the fact that it will take place when the intestine is perfectly empty and 
may be prevented by section of the mesenteric nerves. The vomiting at- 
tending renal and biliary calculi is apparently also reflex in origin. Vom- 
iting in fact, as a rule, is a reflex action, the afferent impulses passing along 
one or other nerves, but most frequently along those connected with the 
alimentary canal, that is, along afferent fibres running in the vagus or in the 
splanchnic nerves. The centre, however, may be affected directly, as prob- 
ably in the cases of some poisons, and in some instances of vomiting from 
disease of the medulla oblongata. Lastly, it may be thrown into action by 
impulses reaching it from parts of the brain higher up than itself, as in cases 
of vomiting produced by smells, tastes, or emotions, or by the recollection 
of past events, and in some cases of vomiting due to cerebral disease. 
Many emetics, such as tartar emetic, appear to act directly on the centre, 
since, introduced into the blood, they will produce vomiting after a bladder 
has been substituted for the whole stomach. Others again, such as mustard 
and water, act in a reflex manner by irritation of the gastric mucous mem- 
brane. With others, again, which cause vomiting by developing a nauseous 
taste, the action involves parts of the brain higher than the centre itself. 
§ 234. Movements of the small intestine. These, as we have already said, 
are the typical peristaltic movements, simple except in so far as they are 
complicated by the existence of the pendent loops, the peculiar oscillating 
movements of which appear to be produced chiefly by the longitudinal fibres. 
The peristaltic movements, as a rule, take place from above downward, 
and a wave beginning at the pylorus may be traced a long way down. But 
contractions may, and in all probability occasionally do, begin at various 
points along the length of the intestine. A movement started by artificial 
stimulation some way down the intestine, may travel not only downward but 
also upward ; it has been disputed, however, whether in the living body any 
natural backward peristaltic movement really takes place. In the living 
body the intestines have periods of rest, alternating with periods of activity, 
the occurrence of the periods depending on various circumstances; the 
intensity of the movements also varies very considerably. 
§ 235. Movements of the large intestine. They are fundamentally the 
same as those of the small intestine, but distinct in so far as the latter cease 
at the ileo-caecal valve, at which spot the former normally begin ; they are 
simpler, inasmuch as the pendent loops are absent, and not so vigorous, since 
relatively to the diameter of the tube the amount of muscular fibre is less. 
Along the colon where the sacculi are well developed the movement may 
perhaps be described as almost intermittent from sacculus to sacculus, the 
contents of one sacculus being driven by the peristaltic contractions of its 
circular fibres into the next sacculus, which prepares to receive them by a 
relaxation of its circular and a contraction of its longitudinal fibres. 
Since the lips of the ileo-cjecal valve are placed transversely across the THE MUSCULAR MECHANISMS OF DIGESTION. 
301 
caecum, not only does distention of the caecum, by stretching the valve along 
the line of the lips, bring them into apposition, but the pressure exerted by 
the peristaltic movement has the same effect. In this way any return of the 
contents from the large to the small intestine is prevented. 
Arrived at the sigmoid flexure, the contents, now more or less solid feces, 
are supported by the bladder and the sacrum, so that they do not press on 
the sphincter ani. 
§ 236. Defecation. This is a mixed act, being superficially the result of 
an effort of the will, and yet carried out by means of an involuntary mechan- 
ism. Part of the voluntary effort consists in producing a pressure-effect, by 
means of the abdominal muscles. These are contracted forcibly as in ex- 
piration, but the glottis being closed and the escape of air from the lungs 
prevented, the whole force of the pressure is brought to bear on the abdo- 
men itself, and so drives the contents of the descending colon onward toward 
the rectum. The sigmoid flexure is by its position sheltered from this pres- 
sure ; a body introduced per anum into the empty rectum is not affected by 
even forcible contractions of the abdominal walls. 
The anus is guarded by the sphincter ani, which is habitually in a state 
of normal tonic contraction, capable of being increased or diminished by a 
stimulus applied, either internally or externally, to the anus. The tonic 
contraction is in part at least due to the action of a nervous centre situated 
in the lumbar spinal cord. If the nervous connection of the sphincter with 
the spinal cord be broken, relaxation takes place. If the spinal cord be 
divided somewhat higher up, for instance in the dorsal region, the sphincter, 
after the depressing effect of the operation, which may last several days, has 
passed off, regains and subsequently maintains its tonicity, showing that the 
centre is not placed higher up than the lumbar region of the cord. The 
increased or diminished contraction following on local stimulation is prob- 
ably due to reflex augmentation or inhibition of the action of this centre. 
The centre is also subject to influences proceeding from higher regions of the 
cord, and from the brain. By the action of the will, by emotions, or by 
other nervous events, the lumbar sphincter centre may be inhibited, and 
thus the sphincter itself relaxed ; or augmented, and thus the sphincter 
tightened. A second item, therefore, of the voluntary process in defecation 
is the inhibition of the lumbar sphincter centre, and consequent relaxation 
of the sphincter muscle. Since the lumbar centre may remain wholly effi- 
cient when separated from the brain, the paralysis of the sphincter which 
occurs in certain cerebral diseases is probably due to inhibition of this lum- 
bar centre, and not to paralysis of any cerebral centre. 
Thus a voluntary contraction of the abdominal walls, accompanied by a 
relaxation of the sphincter, might press the contents of the descending colon 
into the rectum and out at the anus. Since, however, as we have seen, the 
pressure of the abdominal walls is warded off the sigmoid flexure, such a 
mode of defecation would always end in leaving the sigmoid flexure full. 
Hence the necessity for these more or less voluntary acts being accompanied 
by an involuntary augmentation of the peristaltic action of the large intes- 
tine, sigmoid flexure, and rectum. 
In the movements of the rectum we can trace out more distinctly than in 
other regions of the alimentary canal the separate actions of the longitudinal 
and circular fibres. The former, by means of contractions travelling from 
above downward, shorten the rectum, and, since the anus affords a more or 
less fixed support, pull the rectum and its contents down ; the latter, by 
means of contractions travelling from above downward but taking place 
somewhat later, narrow the rectum and so squeeze the contents onward and 
outward. 302 
THE TISSUES AND MECHANISMS OF DIGESTION. 
Defecation then appears to take place in the following manner: The 
large intestine and sigmoid flexure becoming more and more full, stronger 
and stronger peristaltic action is excited in their walls. By this means the 
feces are driven into the rectum, and so, by a continuance of the movements, 
increasing in vigor, against the sphincter. Through a voluntary act, or 
sometimes at least by a simple reflex action, the lumbar sphincter centre is 
inhibited and the sphincter relaxed. At the same time the contraction of 
the abdominal muscles presses firmly on the descending colon, and thus, 
contractions of the levator ani assisting, the contents of the rectum are 
ejected. 
It must, however, be remembered that, while in appealing to our own con- 
sciousness, the contraction of the abdominal walls and the relaxation of the 
sphincter seem purely voluntary efforts, the whole act of defecation, includ- 
ing both of these seemingly so voluntary components, may take place in the 
absence of consciousness, and indeed, in the case of the dog at least, after 
the complete severance of the lumbar from the dorsal cord. In such cases 
the whole act must be purely reflex, excited by the presence of feces in the 
rectum. 
§ 237. The nervous mechanisms of gastric and intestinal movements. Both 
the stomach and intestines when removed from the body and thus wholly 
separated from the central nervous system may, by direct stimulation, be 
readily excited to movements; and indeed in the absence of all obvious 
stimuli movements which seem to be spontaneous may at times be observed. 
The movements of which we are speaking are orderly movements of a 
peristaltic nature, not mere local contractions of a few bundles of plain 
muscular fibres. The alimentary canal, therefore, like the heart, though to 
a less degree, possesses within itself such mechanisms as are requisite for 
carrying out its own movements; and, as in the case of the heart, there is 
no adequate evidence that the ganglia scattered in its muscular walls- 
namely, those forming the plexus of Auerbach-play any prime part in 
developing these movements. 
On the other hand, powerful movements of a peristaltic kind may be 
induced, not only as we have already seen in the oesophagus but also in the 
stomach, in the small intestines, and even in the large intestines by stimula- 
tion of the vagus nerve. 
The chief and usual cause of the movements of the stomach and intestines 
is the presence of food in their interior. But we do not know definitely the 
exact manner in which the food produces the movement. It may be that 
the food, by stimulating the mucous membrane, sends up afferent impulses, 
and that these give rise by reflex action to efferent impulses which descend 
the vagus fibres to successive portions of the canal, in a manner similar to 
that already described in reference to the oesophagus. If this be so the 
efferent impulses reach the stomach and upper part of the duodenum by the 
terminal portions of the two vagi, Fig. 89, J^.V., L.V., and reach the intes- 
tines by the portion of the right or posterior vagus, Fig. 89, Ji. V'., which 
passes into the solar plexus and thence by the mesenteric nerves. The 
afferent impulses from the stomach travel also apparently by the vagus ; the 
paths of those from the intestines have not yet been determined. 
But that such a reflex action through vagus fibres is not the only means 
by which the presence of food brings about the movements in question, is 
shown by the fact that these continue to be developed after section of both 
vagus nerves. Probably the whole action is a mixed one which we may 
picture to ourselves somewhat as follows: The alimentary canal possesses a 
power of spontaneous movement, feeble it is true, very inferior to that of the 
heart and very apt to be latent, but still existing. The presence of food in THE MUSCULAR MECHANISMS OF DIGESTION. 
303 
some way or other, by some direct action quite apart from the central nervous 
system, is able to increase this power so that, without any aid from the central 
nervous system, as after section of the vagi, adequate peristaltic move- 
ments can, under favorable circumstances, be carried out. Nevertheless, in 
Fig. 89. 
Diagram to illustrate the Nerves of the Alimentary Canal in the Dog.1 The figure is, for the 
sake of simplicity, made as diagrammatic as possible, and does not represent the anatomical relations. 
Oe. to Ret. The alimentary canal, oesophagus, stomach, small intestine, large intestine, rectum. 
L. V. Left vagus nerve, ending on front of stomach, r.l. Recurrent laryngeal nerve supplying 
upper part of oesophagus. R. L. Right vagus, joining left vagus in oesophageal plexus, oe. pl., sup- 
plying posterior part of stomach and continued as R'. V. to join the solar plexus, here represented 
bi/a single ganglion and connected with the inferior mesenteric ganglion (or plexus) m.gl. a. 
Branches from the solar plexus to stomach and small intestine and from the mesenteric ganglion 
to the large intestine. Spl. maj. Large splanchnic nerve arising from the thoracic ganglia and 
rami communicantes, r.c., belonging to dorsal nerves from the 6th to the 9th (or 10th). Spl. min. 
Small splanchnic nerve similarly arising from 10th and 11th dorsal nerves. These both join the 
solar plexus and thence make their way to the alimentary canal, c.r. Nerves from the ganglia, 
etc., belonging to 11th and 12th dorsal and 1st and 2d lumbar nerves, proceeding to the inferior 
mesenteric ganglia (or plexus), m. gl., and thence by the hypogastric nerve, n. hyp , and the hypo 
gastric plexus, pl. hyp., to the circular muscles of the rectum, l.r. Nerves from the 2d and 3d 
sacral nerves, S.2, S.3 (nervi erigentes), proceeding by the hypogastric plexus to the longitudinal 
muscles of the rectum. 
the normal course of events, satisfactory movements are still further secured 
by the reflex action through vagus fibres just described. Thus, in the dog, 
the act of swallowing food or even the mere smell of food has been observed 
to increase the movements of a piece of intestine isolated from the rest of 
1 It was not observed until too late that in the diagram of the nerves of the alimentary 
canal in the dog, twelve dorsal nerves had been represented. The figure, as stated, makes 
no pretence to anatomical exactness; but it would have been better to represent either 
thirteen or fifteen dorsal nerves. 304 
tup: tissues and mechanisms oe digestion. 
the alimentary canal but retaining its connections with the central nervous 
system. Under this view the peristaltic movements produced by centrifugal 
stimulation of the vagus in the neck are comparable not so much with the 
contraction of a skeletal muscle when its motor nerve is stimulated as with 
the beats which may be called forth in an inhibited or otherwise quiescent 
heart by stimulation of the cardiac augmentor fibres. 
Indeed, we may, perhaps, call the vagus fibres which pass to the stomach 
and intestines (and these, we may remark, are, like the cardiac augmentor 
fibres, non-meduHated fibres along the greater part of their course) aug- 
mentor fibres rather than motor fibres. We have all the more reason to do 
so since there exist companion, but antagonistic, inhibitory fibres. If while 
lively peristaltic action is going on in the bowels the splanchnic nerves be 
stimulated, the bowels are brought to rest, often in a very abrupt and 
marked manner. Inhibitory fibres, therefore, run in the splanchnic nerves 
(Fig. 89, Spl. maj. and min.), passing along them from the spinal cord to 
the abdominal plexuses, and thence to the alimentary canal. 
It will be noticed that the splanchnic nerves, while containing vaso-con- 
strictor, i. e., augmentor, fibres for the bloodvessels of the intestines, carry 
inhibitory fibres for the muscular coat; and probably the vagus, while con- 
taining augmentor fibres for the muscular coat, carries inhibitory dilator 
fibres for the bloodvessels. It may further be remarked that the vagus, 
while supplying augmentor fibres for the muscular mechanisms of the ali- 
mentary canal, carries, as we so well know, inhibitory fibres for the cardiac 
muscular mechanism. 
In the above statement we have purposely used the general term, peri- 
staltic movement; but, as we have seen, in the movements of the alimentary 
canal, two sets of muscles are concerned-the circular and the longitudinal. 
Now, in the rectum we are able to recognize that the two sets of muscles 
have quite distinct nervous supplies. The longitudinal coat is governed 
by nerve-fibres which, in the dog, leave the spinal cord in the anterior roots 
of the second and third sacral nerves (Fig. 89, S. 2, 8. 3), pass along the 
branches of those nerves frequently spoken of as the nervi erigentes, I. r., to 
the hypogastric plexus (pl. hyp.), and thence to the rectum. Stimulation 
of these nerves causes contractions of the rectum, which are confined to 
the longitudinal coat and, as we have said, pull the rectum down. The cir- 
cular coat is governed by fibres which leave the spinal cord by the anterior 
roots of the lower dorsal and first two lumbar nerves, Fig. 89 (coming from 
the lower part of that spinal region from which, as we have seen (§ 155) 
the vaso-constrictor fibres take origin), and, early losing their medulla, pass 
to the rectum by the inferior mesenteric ganglia, the hypogastric nerves, 
and hypogastric plexus (Fig. 89, m. gl., n. hyp., pl. hyp.). Stimulation of 
these fibres gives rise to contractions which are confined to the circular coat 
and squeeze out the contents of the rectum. A similar double nervous 
supply probably governs the longitudinal and circular coats along the 
whole alimentary canal; but the details of such a supply are at present 
unknown. 
Our knowledge, moreover, concerning the details of any special nervous 
mechanisms, by means of which the more complicated movements of the 
stomach, including the closing and opening of the sphincters, are carried out, 
is at present very imperfect. We cannot add to what we have incidentally 
said in speaking of vomiting. 
The movements of the rectum, including the sigmoid flexure, appear 
to be much more closely dependent on the central nervous system than 
are those of the rest of the alimentary canal. As we have said, the move- 
ments of both large and small intestine are rather assisted and aug- THE MUSCULAR MECHANISMS OF DIGESTION. 
305 
merited than primarily called forth by impulses descending from the central 
nervous system along the vagus fibres. As the large intestine, however, 
passes into the rectum, government by the vagus is replaced by government 
through the lumbar cord and the nerves just previously mentioned ; and 
this government appears to be not so much mere augmentation as the actual 
carrying out of the movements through reflex action. Hence, this is the 
part of intestinal movement which fails in diseases of the central nervous 
system, the failure leading to obstinate constipation, if not to actual dif- 
ficulty of defecation. The presence of feces in the sigmoid flexure no 
longer stirs up the reflex mechanism for their discharge ; meanwhile the 
more independent movements of the higher parts of the canal continue to 
drive the contents onward, and hence the feces accumulate in the sigmoid 
flexure and colon awaiting the delayed action of the imperfect reflex 
mechanism. With regard to the exact manner in which the presence of 
food acts as a stimulus, it may be worth while to remark that, although in 
the stomach, as we have seen, mere fulness is not the efficient cause of 
the movements, since these become more active as digestion proceeds and 
the bulk of the contents diminishes, yet in the intestine distention of the 
bowel up to certain limits most distinctly increases the vigox' of the move- 
ments just as distention of the cardiac cavities within certain limits im- 
proves the cardiac stroke. This is well seen in obstruction of the bowels, 
in which case the bowel distended above the obstruction is frequently 
thrown into violent peristaltic movements. This effect is in part at least 
due to the distention extending the muscular fibres, and so in a direct 
manner promoting their contraction (see § 77), but may be in part due 
to augmentor impulses excited in a reflex manner. Probably in an in- 
testine isolated from the central nervous system food provokes peristaltic 
movements much more by causing distention and so stretching the muscular 
coats than by acting as a stimulus to the mucous membrane, either through 
chemical action or in any similar way. 
§ 238. Next to the presence of food in the interior of the alimentary 
canal a deficient oxygenation of the blood supplied to the walls of the 
canal or the sudden cutting off of the supply of blood may be regarded 
as the most powerful provocatives of peristaltic action. When the aorta is 
clamped, or when the respiration is seriously interfered with, peristaltic 
movements become very pronounced. Thus, in death by asphyxia or suf- 
focation, an involuntary discharge of feces, which is in part at least the 
result of increased peristaltic action, is not an unfrequent result; and 
the marked peristaltic movements which are so frequently seen in an animal 
when the abdomen is laid open immediately after death appear to be due to 
the cessation of the circulation and the consequent failure in the supply of 
blood to the walls of the alimentary canal, and not, as has been suggested, 
to the contact with air of the peritoneal surface. Since it is blood which 
brings oxygen to the tissues, failure in the supply of blood is tantamount 
to failure in the supply of oxygen ; but the blood current brings other 
things besides oxygen and also takes things away; and the failure of this 
action also, probably, as well as failure in the supply of oxygen, provokes 
the movements in question. 
The movements thus produced are to some extent the result of the defi- 
cient supply of blood acting directly on the walls of the canal, though in 
asphyxia at all events this effect may be increased by the too venous blood 
stimulating the central nervous system, and thus sending augmentor impulses 
down the vagus. 
With regard to the mode of action of the drugs which promote peri- 
1 4 i /» 1 » » it x 1 1 i fit /uzivit Ixzxmzx 4- /i 4-1» 4- ...1, 11 x. « ~ 306 
THE TISSUES AND MECHANISMS OF DIGESTION. 
nicotin, appear to act directly on the walls of the canal, others, such as 
strychnia, produce their effect chiefly by acting through the central nervous 
system. 
The Changes which the Food Undergoes in the Alimentary 
Canal. 
§ 239. Having studied the properties of the digestive juices as exhibited 
outside the body, and the various mechanisms by means of which the food 
introduced into the body is brought under the influence of those juices, we 
have now to consider what, as matters of fact, are the actual changes which 
the food does undergo in passing along the alimentary canal; what are the 
steps by which the contents of the canal are gradually converted into feces. 
The events which lead to this conversion are twofold. On the one hand the 
digestive juices do bring about, inside the alimentary canal, changes which 
in the main are the same as those observed in laboratory experiments out- 
side the body and described in previous sections, though the results are 
somewhat modified by the special conditions which obtain within the body. 
On the other hand, absorption, that is to say, the passage from the interior 
of the canal into the bloodvessels and lymphatics, of digested material in 
company with water is going on along the whole length of the canal, and 
especially in the small and large intestines. It will be convenient to con- 
fine ourselves at present to the study of the first class of events, the changes 
effected in the canal, merely noting the disappearance of this or that prod- 
uct, and deferring the difficult problem of how absorption takes place to a 
subsequent and separate discussion. 
In the mouth the presence of the food, assisted by the movements of the 
jaw, causes, as we have seen, a flow of saliva. By mastication, and by the 
addition of mucous saliva, the food is broken into small pieces, moistened, 
and gathered into a convenient bolus for deglutition. In man some of the 
starch is, even during the short stay of the food in the mouth, converted 
into sugar; for if boiled starch free from sugar be even momentarily held 
in the mouth and then ejected into water (kept boiling to destroy the 
ferment), it will be found to contain a decided amount of sugar. In many 
animals no such change takes place. The viscid saliva of the dog serves 
almost solely to assist in deglutition ; and even the longer stay which food 
makes in the mouth of the horse is insufficient to produce any marked con- 
version of the starch it may contain. During the rapid transit through the 
oesophagus no appreciable change takes place. 
The amount of absorption of digested material, or even of simple water, 
from the mouth or (esophagus must always be insignificant. 
The Changes in the Stomach. 
§ 240. The arrival of the food, the reaction of which is either naturally 
alkaline, or is made alkaline, or at least is reduced in acidity, by the addi- 
tion of saliva, causes a flow of gastric juice. This, already commencing 
while the food is yet in the mouth, increases as the food accumulates in the 
stomach, and as, by the churning gastric movements, one part after another 
of the food is brought into contact with the mucous membrane. 
The characters of the juice appear to change somewhat as the act of 
digestion proceeds. The amount of pepsin in the gastric contents increases 
for some time after food is taken, and probably the actual secretion increases 
also. The acidity of the gastric contents is at first very feeble; indeed in 
man, in some cases at least, for some little time after the beginning of a meal 
no free acid is present, and during this period the conversion of starch into 307 
THE CHANGES IN THE ALIMENTARY CANAL. 
sugar may continue. This condition, however, is temporary only ; very 
soon the contents become acid, arresting the action of and ultimately de- 
stroying the amylolytic ferment; and, since the rate of secretion of acid 
appears to be fairly constant, the contents of the stomach, unless fresh alka- 
line food be taken, become more acid as digestion goes on. 
The gross effect of gastric digestion is to break up and partly to dissolve 
the larger lumps of masticated food into a thick grayish soup-like liquid 
called chyme, with which are still mixed in variable quantity larger and 
smaller masses of less changed food. This is the result, partly of the solu- 
tion of proteid matters, partly of the solution of the gelatiniferous connec- 
ti ve tissue holding the proteid elements together. In a fragment of meat, 
for instance, the muscular fibres, through the solution of the connective 
tissue binding them together, fall asunder, the sarcolemma is dissolved, and 
the fibres themselves split up sometimes longitudinally but most frequently 
by transverse cleavage into discs, and are ultimately more or less reduced 
partly into a granular mass, partly to actual solution. In a piece of tissue 
containing fat, the connective tissue binding the fat cells together and the 
envelopes of the fat cells are dissolved, so that the fat, fluid at the tempera- 
ture of the body, is set free from the individual cells and runs together into 
larger and smaller masses. In vegetable tissue the proteid elements are in 
part dissolved and, though there is no evidence that in man cellulose is dis- 
solved in the stomach, the whole tissue is softened and to a certain extent 
disintegrated. Milk is curdled and the curd subsequently more or less dis- 
solved. 
The thick soup-like acid chyme consists accordingly partly of substances 
which have entered into actual solution, partly of mere particles or droplets 
of proteid, fatty or other nature, and partly of masses small or great which 
may be recognized under the microscope as more or less changed portions of 
animal or vegetable tissue. The amount of material actually dissolved is in 
most specimens of chyme exceedingly small. When the solid parts are 
removed by filtration the clear filtrate contains beside salts, pepsin and free 
hydrochloric acid (the constituents of the gastric juice), a small amount of 
sugar, of parapeptone and of peptone. The sugar is often absent, the para- 
peptone is not always present, and the amount of peptone (or albumose) is 
always small. 
During gastric digestion the chyme thus formed is from time to time 
ejected through the pylorus, accompanied by even large morsels of solid 
less-digested matter. This may occur immediately when water alone is 
taken and within a few minutes of food having been taken ; but the larger 
escape from the stomach probably does not in man begin until from one to 
two, and lasts from four to five, hours after the meal, becoming more rapid 
toward the end, and such pieces as are the least broken up by the gastric 
juice and movements being the last to leave the stomach. 
The time taken up in gastric digestion probably varies in the same 
animal not only with different articles of food, but also with varying con- 
ditions of the stomach and of the body at large. In different animals it 
varies very considerably, being from twelve to twenty-four hours in the dog 
after a full meal, while the stomachs of rabbits are never empty but always 
remain largely filled with food, even during starvation. In man the stomach 
probably becomes empty between the usual meals. 
The total amount of change which the food undergoes in the stomach, 
that is the share taken by the stomach in the whole work of digestion, 
seems to vary largely in different animals, and in the same animal differs 
according to the nature of the meal. In a dog fed on an exclusively meat 
diet, a very large part of the digestion is said to be carried out by the 308 
THE TISSUES AND MECHANISMS OF DIGESTION. 
stomach, very little work apparently being left for the intestines; that is to 
say, the larger part of the meal is reduced in the stomach to actual solution 
and a considerable quantity is probably absorbed directly from the stomach. 
In such cases the amount of peptone found in the stomach during the diges- 
tion of the meal is found to be fairly constant, from which it may be in- 
ferred that the peptone is absorbed as soon as it is formed. There is also 
evidence that fat may to a certain extent undergo in the stomach changes 
leading to emulsion, similar to those which, as we shall see, are carried out 
in the small intestine. 
But such cases as these cannot be regarded as typical cases of gastric 
digestion, and in man, at all events, living on a mixed diet the work of the 
stomach appears to be to a large extent preparatory only to the subsequent 
labors of the intestine. It is true that our information on this matter is 
imperfect, being chiefly drawn from the study of cases of gastric or duodena! 
fistula, in which probably the order of things is not normal, or being in 
large measure deductions from experiments on animals, whose economy in 
this respect must be largely different from our own ; but we are probably 
safe in concluding that, in ourselves, the chief effect of gastric digestion is 
by means of the disintegration spoken of above to reduce the lumps of food 
to the more uniform chyme and so to facilitate the changes which take place 
in the small intestine. During the disintegration some of the proteid in the 
meal is converted into peptone; and the peptone so formed is probably 
absorbed at once ; but much proteid remains unchanged or at least is not 
converted into peptone, and the fats and starches undergo in themselves 
very little change indeed. 
In the act of swallowing, no inconsiderable quantity of air is carried 
down into the stomach, entangled in the saliva or in the food. This may 
be returned in eructations. When the gas of eructation or that obtained 
directly from the stomach is examined, it is found to consist chiefly of 
nitrogen and carbonic acid, the oxygen of the atmospheric air having been 
largely absorbed. In most cases the carbonic acid is derived by simple 
diffusion from the blood, or from the tissues of the stomach, which similarly 
take up the oxygen. In many cases of flatulency, however, it may arise 
from a fermentative decomposition of the sugar which has been taken as 
such in food or which has been produced from the starch, the gas being 
either formed in the stomach or passing upward from the intestine through 
the pylorus. 
The enormous quantity of gas which is discharged through the mouth 
in cases of hysterical flatulency, even on a perfectly empty stomach, 
and which seems to consist largely of carbonic acid, presents difficulties 
in the way of explanation ; it is possible that it may be simply diffused 
from the blood, but it is also possible that in many cases it is derived 
from air which the patient has hysterically swallowed, the oxygen having 
been removed, in the stomach, by absorption and replaced by carbonic 
acid. 
In, the Small Intestine. 
§ 241. The semi-digested acid food, or chyme, as it passes over the 
biliary orifice, causes, as we have seen (§ 223), gushes of bile, and at the 
same time the pancreatic juice flows into the intestine freely. These two 
alkaline fluids, especially the more strongly and constantly alkaline pan- 
creatic juice, tend to neutralize the acidity of the chyme, but the con- 
tents of the duodenum do not become distinctly alkaline until some distance 
from the pylorus is reached. The rapidity with which the change in the 
reaction is completed is not the same in all animals, and in the same animal THE CHANGES IN THE ALIMENTARY CANAL. 
309 
appears to vary according to the nature of the food and various circum- 
stances. In man, living on a mixed diet, the contents have probably 
become distinctly alkaline before they have passed far down the duode- 
num. On the other hand, in dogs the contents of the small intestine 
have been observed to be acid throughout, and that not only when fed 
on starch and fat, which might, by an acid fermentation of which we 
shall presently speak, give rise to an acid reaction, but even when fed on 
meat. 
The conversion of starch into sugar, which, as we have seen, is sooner or 
later arrested in the stomach, is resumed with great activity and indeed com- 
pleted by the pancreatic juice, possibly assisted by the succus entericus, the 
presence of bile being said to increase the activity of the pancreatic amylo- 
lytic ferment. The conversion begins as soon as the acidity of the chyme 
is sufficiently reduced and continues along the intestine; portions, however, 
of still undigested starch may be found in the large intestine, and even at 
times in the feces. 
The pancreatic juice, as we have seen, emulsifies fats, and also splits them 
into their respective fatty acids and glycerin. The fatty acids thus set free 
become converted by means of the alkaline contents of the intestine into 
soaps; but towhat extent saponification thus takes place is not exactly 
known. Undoubtedly soaps have to a small extent been found both in 
portal blood and in the thoracic duct after a meal ; but there is no proof 
that any large quantity of fat is introduced in this form into the circula- 
tion. On the other hand, the presence of neutral fats in the lacteals, and 
to a slight extent in portal blood, is a conspicuous result of the digestion of 
fatty matters; and in all probability saponification in the intestine is a sub- 
sidiary process, the effect of which is rather to facilitate the emulsion of 
neutral fats than to introduce soaps as such into the blood, for the pres- 
ence of soluble soaps favors the emulsion of neutral fats. Hence a rancid 
fat, i. e., a fat containing a certain amount of free fatty acid, forms an emul- 
sion with an alkaline fluid more readily than does a quite neutral fat. A 
drop of rancid oil let fall on the surface of an alkaline fluid, such as a solu- 
tion of sodium carbonate of suitable strength, rapidly forms a broad ring 
of emulsion, and that even without the least agitation. As saponification 
takes place at the junction of the oil and alkaline fluid, currents are set up 
by which globules of oil are detached from the main drop and driven out 
in a centrifugal direction ; the intensity of the currents and the consequent 
amount of emulsion depend on the concentration of the alkaline medium 
and on the solubility of the soaps which are formed. Now7 the bile and 
pancreatic juice supply just such conditions as the above for emulsionizing 
fats ; they both together afford an alkaline medium, the pancreatic juice 
gives rise to an adequate amount of free fatty acid, and the bile in addition 
brings into solution the soaps as they are formed. So that we may speak 
of the emulsion of fats in the small intestine as being carried on by the 
bile and pancreatic juice acting in conjunction ; and as a matter of fact 
the bile and pancreatic juice do largely emulsify the contents of the small 
intestine, so that the grayish turbid chyme is changed into a creamy-look- 
ing fluid, which has been sometimes called chyle. It is advisable, how- 
ever, to reserve this name for the contents of the lacteals. Many of the 
fats present in food-for instance, butter-already contain some fatty acids 
when eaten ; for these fats the initial action of the pancreatic juice is less 
necessary. 
This mutual help of bile and pancreatic juice in producing an emulsion 
explains to a certain extent the controversy which long existed between those 
who maintained that the bile and those who maintained that the pancreatic 310 
THE TISSUES AND MECHANISMS OF DIGESTION. 
juice was necessary for the digestion and absorption of fatty food. That the 
pancreatic juice does produce in the intestine such a change as favors the 
transference of neutral fats from the intestine into the lacteals is shown by 
the fact that in diseases affecting the pancreas much fatty food frequently 
passes through the intestine undigested and great wasting ensues; but it 
cannot be maintained that the pancreatic juice is the sole agent in this 
matter, since in animals in which the pancreatic ducts have been success- 
fully ligatured chyle is still found in the lacteals. On the other hand, that 
the bile is of use in the digestion of fat is shown by the prevalence of fatty 
stools in case of obstruction of the bile-ducts ; and though the operation of 
ligaturing the bile-ducts and leading all the bile externally through a fistula 
of the gall-bladder is open to objection, since it in some way or other so 
exhausts the animal as indirectly to affect digestion, still the results of experi- 
ments in which the resorption of fat was distinctly lessened (the quantity of 
fat in the lacteals falling from 3.2 to 0.2 per cent.) by the ligature and fistula 
obviously point to the same conclusion. That in man the succus entericus 
possesses a wholly insufficient emulsifying power is shown by the observa- 
tion of a case in which the duodenum opened on the surface by a fistula 
in such a way that the lower part of the intestine could be kept free from 
the contents of the upper part containing the bile and pancreatic juice 
and matters proceeding from the stomach. Fats introduced into the lower 
part, where they could not be acted upon either by the bile or by the pan- 
creatic juice, were but slightly digested. Without denying the possible 
assistance of the succus entericus, or even of gastric juice, we may con- 
clude that the digestion of fat is in the main carried out by the conjoint 
action of bile and pancreatic juice. 
§ 242. We have seen (§ 216) that the addition of bile to a digesting 
mixture gives rise to a precipitate. This is partly a coarse, flocculent pre- 
cipitate, consisting of parapeptone with some amount of bile acids, and 
partly of a finer, more granular precipitate, which is longer in falling down 
and consists chiefly of bile acids with a variable amount of peptone ; the 
latter is redissolved on the further addition of bile, even though the reaction 
of the mixture remain acid. In the upper part of the duodenum the inner 
surface, if examined while digestion is going on, is found to be lined by a 
colored flocculent and granular material, which is probably a precipitate 
thus formed ; the purpose of this precipitation is probably to delay the 
passage of the undigested parapeptone along the duodenum. Moreover, 
apart from this precipitation, bile arrests the action of pepsin, even while 
the reaction of the mixture still remains acid ; and as soon as an alkaline 
reaction is established the pepsin is apparently destroyed by the trypsin, so 
that with the flow of bile and pancreatic juice into the duodenum the pro- 
cesses which have been going on in the stomach come to an end. In fact, it 
would seem that the juices of the various districts of the alimentary canal 
are mutually destructive; thus, while pepsin in an acid solution destroys the 
active constituents of saliva and of pancreatic juice (probably also those of 
the succus entericus), it is in its turn antagonized or destroyed by the bile 
and the other alkaline juices of the intestine. Hence, pancreatic juice intro- 
duced through the mouth must lose its powers in the stomach, and can only 
be of use as an alkaline medium containing certain proteid matters. On 
the other hand, if, as we have reason to believe, the contents of the stomach 
as they issue from the pylorus still contain a large quantity of undigested 
proteids, these must be digested by the pancreatic juice (with or without the 
assistance of the succus entericus), the action of which seems to be assisted, 
or at least not hindered, by bile. And in dogs fed through a duodenal fis- 
tula, so that all gastric digestion is excluded, proteids are completely digested THE CHANGES IN THE ALIMENTARY CANAL. 
311 
and give rise to quite normal feces. To what stage the pancreatic digestion 
is carried, whether peptone is practically the only product, or whether the 
pancreatic juice in the body, as out of the body, carries on its work in the 
more destructive form, whereby the proteid material subjected to it is so 
broken down as to give rise to appreciable quantities of leucin and tyrosin, 
is at present not exactly known. Leucin and tyrosin have been found in 
the intestinal contents, and may therefore be formed during normal diges- 
tion, but whether an insignificant quantity or a considerable quantity of the 
proteid material of food is thus hurried into a crystalline form cannot be 
definitely stated. The extent to which the action is carried is probably dif- 
ferent in different animals, and probably varies also according to the nature 
of the meal and the condition of the body. Possibly when a large and un- 
necessary quantity of proteid material is taken at a meal, together with 
other substances, no inconsiderable amount of the proteids undergo this pro- 
found change, and, as we shall see, rapidly leave the body as urea without 
having been used by the tissues, their contribution to the energy of the 
body being limited to the heat given out during the changes by which 
they are converted into urea. To this apparently wasteful use of proteids 
we shall return in speaking of what is called the " luxus consumption " 
of food. 
§ 243. In dealing with the action of pancreatic juice we drew attention 
(§ 218) to the difference between the results of pure tryptic digestion and 
those obtained when bacteria or other microorganisms were allowed to be 
present. We saw that indol, for example, was the product of the action of 
these organisms, not of trypsin. Now indol is formed in varying quantity 
during the digestion which actually takes place in the intestine, some of it at 
times appearing in the urine as indigo-yielding substance (indican). More- 
over, bacteria and other microorganisms are present in the intestinal con- 
tents. Hence, we must regard the changes taking place in the intestine not 
as the pure results of the action of the several digestive juices, but as those 
results modified by or mixed with the results of the action of microorganisms. 
We spoke above (§ 216) of bile as being antiseptic, but this must be under- 
stood as meaning not that the presence of bile arrests the action of all micro- 
organisms within the intestine, but that it modifies their action, keeping it 
within certain limits and along certain lines. 
Concerning the exact nature and extent of the changes thus due to micro- 
organisms, our knowledge is at present very imperfect. The proteids and 
the carbohydrates seem to be the food-stufls on which these organisms pro- 
duce their chief effect. Out of the proteids they give rise not only to indol, 
but to several other compounds, among which may be mentioned phenol 
(C6HBO), of which a small quantity may be recognized in the feces, the rest 
being absorbed and appearing in the urine in the form of certain phenol- 
compounds, such as phenyl-sulphuric acid. Out of proteids they may also 
form the peculiar poisonous bodies called ptomaines, which appear in the 
ordinary putrefaction of proteids. But their most conspicuous effects are 
those on the carbohydrates. As the food descends the intestine, the presence 
of lactic acid becomes more and more obvious ; indeed, in some cases the 
naturally alkaline reaction of the intestinal contents may in the lower part 
of the intestine be changed into an acid one by the presence of lactic acid. 
Now, lactic acid may be formed out of sugar by means of a special organism 
inducing what is spoken of as the lactic acid fermentation. And we have 
every reason to believe that in even normal digestion a certain quantity of 
sugar, either eaten as such or arising from the amylolytic conversion of 
starch, does not pass away from the intestine into the blood as sugar, but 
undergoes this fermentation into lactic acid. To what extent this change 312 
THE TISSUES AND MECHANISMS OF DIGESTION. 
takes place we do not know ; the amount probably varies according to the 
amount of carbohydrates eaten, the condition of the alimentary canal, and 
other circumstances. It may be under certain circumstances simply a part 
of normal digestion ; under other circumstances it may be excessive and 
give rise to troubles. 
That fermentative changes may occur in the small intestine is further 
indicated by the facts that the gas there present may contain free hydrogen, 
and that chyme, after removal from the intestine, continues at the tempera- 
ture of the body to produce carbonic acid and hydrogen in equal volumes. 
This suggests the possibility of the sugar of the intestinal contents under- 
going the butyric acid fermentation, during which, as is well known, car- 
bonic anhydride and hydrogen are evolved. By this change the sugar is 
removed from the carbohydrate group into the fatty acid group ; it is thus, 
so to speak, put on its way to become fat. We shall see hereafter that sugar 
may be somewhere in the body converted into fat; this conversion, however, 
takes place chiefly if not wholly in the tissues, and such change as may take 
place in the alimentary canal is to be regarded as suggestive rather than as 
important. 
The hydrogen thus occurring in the intestine may also arise from the 
proteid decompositions spoken of above. However arising it may act as a 
reducing agent-reducing sulphates, for instance-and thus giving rise to 
sulphides and to sulphuretted hydrogen; as a reducing agent it assists in 
the formation of the fecal and urinary pigments. 
Thus, during the transit of the food through the small intestine, by the 
action of the bile and pancreatic juice, and possibly to some extent of the 
succus entericus, assisted by various microorganisms, the proteids are largely 
dissolved and converted into peptone and other products, the starch is 
changed into sugar, the sugar possibly being in part further converted into 
lactic and other acids, and the fats are largely emulsified and to some extent 
saponified. These products as they are formed pass into either the lacteals 
or the portal bloodvessels, so that the contents of the small intestine, by the 
time they reach the ileo-caecal valve, are largely but by no means wholly 
deprived of their nutritious constituents. So far as water is concerned, the 
secretion of water into the small intestine maintains such a relation to the 
absorption from it that the intestinal contents at the end of the ileum, 
though much changed, are about as fluid as in the duodenum. 
§ 244. The contents, whether alkaline or not, in the ileum now become 
once more distinctly acid. This, however, is not caused by any acid secre- 
tion from the mucous membrane; the reaction of the intestinal walls in the 
large as in the small intestine is alkaline. It must, therefore, arise from 
acid fermentations going on in the contents themselves ; and that fermenta- 
tions do go on is shown by the appearance of marsh gas as well as hydrogen 
in this portion of the alimentary canal. The character and amount of fer- 
mentation probably depend largely on the nature of the food, and probably 
also vary in different animals. 
Of the particular changes which take place in the large intestine we 
have no very definite knowledge ; but it is exceedingly probable that in the 
voluminous caecum of the herbivora a large amount of digestion of a pecu- 
liar kind goes on. We know that in herbivora a considerable quantity of 
cellulose disappears in passing through the alimentary canal, and even 
in man some is digested. It seems probable that this cellulose digestion 
takes place in the large intestine, and is the result of fermentative changes 
In the Large Intestine. THE LACTEALS AND THE LYMPHATIC SYSTEM. 
313 
carried out by means of microorganisms, marsh gas being one of the prod- 
ucts formed at the same time. 
Be this as it may, whether digestion, properly so called, is all but com- 
plete at the ileo-caecal valve, or whether important changes still await 
the chyme in the large intestine, one great characteristic of the work 
done in the colon is absorption. By the abstraction of all the soluble con- 
stituents, and especially by the withdrawal of water, the liquid chyme 
becomes as it approaches the rectum converted into the firm solid feces, and 
the color shifts from the bright orange, which the gray chyme gradually 
assumes after admixture with bile, into a darker and dirtier brown. 
The Feces. 
§ 245. These consist in the first place of the indigestible and undi- 
gested constituents of the meal ; shreds of elastic tissue, hairs and other 
horny elements, much cellulose and chlorophyll from vegetable, and some 
connective tissue from animal food, fragments of disintegrated muscular 
fibre, fat-cells, and not unfrequently undigested starch-corpuscles. The 
amount of each must of course vary very largely according to the nature 
of the food, and the digestive powers, temporary or permanent, of the indi- 
vidual. In the second place, to these must be added substances not dis- 
tinctly recognizable as parts of the food, but derived from the secretions of 
the alimentary canal. The feces contain mucus in variable amount, some- 
times albumin, cholesterin, butyric and other fatty acids, lime and magnesia 
soaps, coloring matters, and inorganic salts, especially earthy phosphates, 
crystals of ammonio-magnesia phosphates being very conspicuous. The re- 
action is generally but not always acid. They also contain a ferment similar 
in its action to pepsin, and an amylolytic ferment similar to that of saliva 
or of pancreatic juice. The bile salts are represented by a small quantity 
of cholalic acid, or some product of that body, and sometimes a very small 
quantity of taurin. The glycin and most or all of the taurin have been ab- 
sorbed from the intestine, and the cholalic acid has been partly absorbed 
and partly decomposed. The fact that the feces become " clay-colored " 
when the bile is cut off from the intestine shows that the bile-pigment is at 
least the mother of the fecal pigment; and a special pigment, which has been 
isolated and called stercobilin, is said to be identical with the substance called 
urobilin, which may be formed from bilirubin. As other special constitu- 
ents of the feces may be mentioned excretin, a somewhat complex nitrogen- 
ous body, whose exact chemical nature is at present uncertain, and skatol 
(C9H9N), a nitrogenous body which like indol is derived from the decompo- 
sition of proteids by means of microorganisms, and which is the chief cause 
of the fecal odor, since only a small quantity of indol remains in the feces. 
These odoriferous bodies are derived directly from the food; at the same 
time it is quite possible that other specific odoriferous substances may be 
secreted directly from the intestinal wall, especially from that of the large 
intestine. 
The Lacteals and the Lymphatic System. 
§ 246. We have seen that absorption does, or at least may, take place 
from the stomach. We have also stated that a large absorption, especially 
of water, occurs along the whole large intestine. Nevertheless, it is during 
the transit of food along the small intestine that the largest and most im- 
portant part of the digested material passes away from the canal, partly into 
the lacteals, partly into the portal vessels. The portal vessels are simply 
parts of the general vascular system ; the lacteals, into which, we may at 314 
THE TISSUES AND MECHANISMS OF DIGESTION. 
once say, the greater part of the fat passes, are similarly parts of the general 
lymphatic system, being in fact the lymphatic vessels of the alimentary canal, 
and especially of the small intestine. The only reason for the special name 
of lacteals is that, unlike the lymphatic vessels of other parts of the body, 
the lymphatics of the intestine contain at times a fluid of a milky white 
appearance. Hence for the better understanding of absorption by the lac- 
teals it will be desirable to study at some length the whole subject of the 
lymphatic system. 
The lymphatic vessels may be said to begin in minute passages, possess- 
ing special characters, known as lymph-capillaries. Broadly speaking, these 
lymph-capillaries are found, in the mammal, in all parts of the body in 
which connective tissue is found ; and they have special connections with 
those minute spaces in connective tissue which we have already more than 
once spoken of as lymph-spaces. 
These lymph-capillaries, which, as we shall see, are frequently arranged 
in plexuses, are continuous with other passages also minute but of a differ- 
ent and more regular structure, the lymphatic vessels proper, which are 
gathered into larger and larger vessels, all running like the bloodvessels in 
a bed of connective tissue, until at last all the lymphatic vessels of the body 
join either the great thoracic duct which opens by a valvular orifice into 
the venous system at the junction of the left jugular and subclavian veins, 
or the small right lymphatic trunk which similarly opens into the junction 
of the right jugular and subclavian veins. Serous cavities may be regarded 
as large lymph-spaces connected with lymphatic vessels. 
The Nature and Movements of Lymph (including Chyle). 
§ 247. We are led to regard the multitudinous spaces, both small and 
great, of connective tissue all over the body, including among these the 
"serous cavities," as forming the beginning or roots of the lymphatic sys- 
tem. Into these spaces certain parts of the plasma of the blood transude 
and so become lymph (to what extent the epithelioid lining of the large 
serous cavities plays in regulating the transudation of serous fluid, i. e., of 
lymph, into those cavities we do not know) ; from these spaces the lymph is 
continually flowing through the lymph-capillaries into the lymphatic vessels, 
and so by the thoracic duct and right lymphatic trunk back into the blood 
system. 
The amount of lymph occupying the lymph-spaces, lymph-capillaries, 
and minute lymphatic vessels of any region varies from time to time accord- 
ing to circumstances. A hand, for instance, which has been kept hanging 
down for some time becomes swollen and the skin tense ; if it be raised the 
swelling lessens and the skin becomes loose ; and a similar temporary swell- 
ing of the skin of the limbs, and of the skin generally, is frequently the 
result of active exercise. Such a swelling is partly due to the bloodvessels 
being dilated, or to the return flow along the veins being retarded, so that 
the blood capillaries become distended with blood, but is much more largely 
owing to the lymph-spaces and lymphatic vessels of the skin and under- 
lying structures being unusually filled with lymph. On the other hand 
the skin may become shrivelled and dry from a deficiency of lymph in 
the lymph spaces and vessels. Under even normal circumstances the 
quantity of lymph in the tissues may vary considerably, and under abnor- 
mal circumstances a very large amount of lymph may greatly distend 
the spaces of the connective tissue of the skin and other structures, giving 
rise to oedema or dropsy. Obviously there are agencies at work in the 
body by which the appearance of lymph in the spaces or its removal THE NATURE AND MOVEMENTS OF LYMPH. 
315 
thence along the lymph-channels, or both, may be either increased or 
diminished. 
The Characters of Lymph. 
§ 248. As it slowly flows from its origin in the tissues to the mouth of 
the thoracic duet (we may for simplicity's sake omit the right lymphatic 
trunk) the lymph is subjected to the influence of the lymphatic glands, and 
is possibly affected by the walls of the lymph-vessels. Moreover, the lymph 
coming from one tissue differs more or less in certain characters from the 
lymph arising in another tissue, just as the venous blood of one organ differs 
from the venous blood of another organ ; and these differences may be exag- 
gerated by the activity of theone or other tissue. Of these differences by 
far the most striking is that between the lymph coming from the alimentary 
canal during active digestion and known as chyle, and the lymph coming 
from other parts of the body. When digestion is not going on, and when, 
consequently, no considerable absorption of material from the alimen- 
tary canal into the lacteals is taking place, the fluid flowing along the 
lacteals is lymph, not differing from the lymph of other regions to any 
marked degree. 
The food, accordingly, which flows along the thoracic duct in an animal 
which has not been fed for some considerable time may be taken as illustrat- 
ing the general characters of lymph. The contents of the thoracic duct 
may be obtained by laying bare the junction of the subclavian and jugular 
(in the dog the junction of the axillary and jugular) veins, and introdu- 
cing a canula into the duct as it enters into the venous system at that point. 
The operation is not unattended with difficulties. 
Lymph, so obtained, is a clear transparent or slightly opalescent fluid 
which left to itself soon clots. The clotting is not so pronounced as that of 
blood, but clotting is caused as in blood by the appearance of fibrin. The 
fibrin which is formed, though scanty (0.5 per cent.), is identical apparently 
with that of blood, and, as far as we know, all that has been said previously, 
(§§ 14-23), concerning the nature of clotting blood applies equally well to 
lymph. 
Examined with the microscope lymph contains a number of corpuscles, 
lymph-corpuscles, which in all their characters as far as is at present known 
are identical with white blood-corpuscles; they vary in size from 5/z to 15/2, 
and the smaller corpuscles are much more abundant in lymph than in 
blood. Like the white corpuscles of blood they exhibit amoeboid move- 
ments. Their number varies in different animals, and, apparently, in the 
same animal, according to circumstances; on the whole perhaps it may be 
said that lymph-corpuscles are about as numerous in lymph as white cor- 
puscles in blood. Even when every care is taken to avoid admixture with 
blood, lymph, and especially chyle, not unfrequently contains a certain 
number of red blood-corpuscles; sometimes these are sufficient to give 
the lymph (or chyle) a reddish tinge. They have been observed within 
the living lymphatic vessels, even within small ones, and have probably 
in some manner or other made their way from the blood into the lymph 
channels. 
$ 249. The chemical composition of lymph, even when taken in each 
case from the thoracic duct, varies a good deal. The total solids are 
much less than in blood, amounting in general to not more than 5 or 6 
per cent. Hence the venous blood of a vascular area contains rather 
more solids than the arterial blood of the same area, since the blood in 
giving rise to the lymph during its passage through the capillaries from the 
arteries to the veins has parted with relatively more water than solid matter. 316 
THE TISSUES AND MECHANISMS OF DIGESTION. 
The proteids amount on the average to about 3 or 4 per cent., that is to 
say, to about half as much as in blood, the particular proteids present being 
the same as in blood, viz., albumin, paraglobulin and antecedents of fibrin. 
In lymph, as distinguished from chyle, the quantity of fat is small, and con- 
sists of the usual neutral fats and the soaps of their fatty acids, together 
with lecithin ; cholesterin may also be present. A certain amount of sugar 
(dextrose) appears to be always present, and several observers have found 
an appreciable quantity of urea. The ash of lymph like that of blood-serum 
contains a considerable quantity of sodium chloride, while phosphates and 
potash are scanty; it also contains iron, apparently in too great a quantity 
to be accounted for by the few red corpuscles which may be present. From 
lymph a certain amount of gas can be extracted, consisting chiefly or almost 
exclusively of carbonic acid, with a small quantity of nitrogen, the amount 
of oxygen present being exceedingly small. The importance of this we shall 
see when we come to study respiration. 
Broadly speaking, we may say that all the substances present in blood- 
plasma are present also in lymph, but are accompanied by a larger quantity 
of water. 
§ 250. Lymph may also be obtained from separate regions of the body, 
as from the lower or upper limbs, for instance, by introducing a fine canula 
into a lymphatic vessel. In its general features the lymph so obtained re- 
sembles that taken from the thoracic duct. Analyses of the lymph distend- 
ing the subcutaneous connective tissue in cases of dropsy show that this con- 
tains much less solid matter than normal lymph taken from the thoracic duct 
or larger lymphatic vessels. From this it has been inferred that the lymph 
normally existing in the lymph-spaces, lymph-capillaries and minute ves- 
sels contains an excess of water ; and, indeed, it has been asserted that the 
percentage of solids increases in passing from the smaller to the larger ves- 
sels; but this cannot be regarded as distinctly proved. The number of cor- 
puscles, however, as we have already said, appears to be increased in passing 
through the lymphatic glands. It has also been stated that the lymph in 
the finer lymph-vessels clots even less firmly than that in the thoracic duct. 
From this we may infer that some of the leucocytes in the adenoid tissue 
of the follicles of a lymphatic gland find their way into the lymph-sinus, 
and so into the efferent lymphatics, and that some of the fibrin factors are 
added to the lymph, or, at least, that some changes favorable to clotting are 
brought about. 
§ 251. The large serous cavities of the peritoneum, pericardium, etc., 
are parts of the lymphatic system ; indeed, pericardial or other serous fluid 
has all the general characters of lymph. We have already said (§ 20), that 
these fluids, when taken fresh from the body, clot (this is, at least, the case 
in most animals); the clot, when examined microscopically, is found to con- 
sist of colorless corpuscles like those of lymph or of blood entangled in the 
meshes of fibrin. Both in their proteid and other chemical constituents these 
serous fluids resemble lymph. Analyses of the accumulations of fluid occa- 
sionally occurring in these cavities show that they contain sometimes less and 
sometimes more solid matter than ordinary lymph. The aqueous humor of 
the eye contains very little solid matter; and the cerebro-spinal fluid is so 
peculiar that it had better be considered by itself in connection with the 
nervous system. 
§ 252. Chyle. In fasting animals the fluid flowing along the lacteals, 
as may be seen by inspection of the mesentery, is clear and transparent; it 
is lymph, differing, as we have said, in no essential respects from the lymph 
flowing along other lymphatic vessels. Shortly after a meal containing fat 
(and every meal does contain some fat), the lymph becomes white and THE NATURE AND MOVEMENTS OF LYMPH. 
317 
opaque like milk, the more so the richer the meal is in fat; it is then called 
chyle. Owing to the relatively large quantity of this milky fluid which for 
some time after a meal continues to be poured into the thoracic duct, the 
contents of that duct also become milky, and are also called chyle. In the 
thoracic duct the chyle of the lacteals is more or less mixed with lymph from 
other lymphatic vessels, but the former is so preponderating that the contents 
of the duct may be taken as illustrating the nature of chyle. 
Chyle differs from lymph in one important respect, and one only : where- 
as lymph ordinarily contains a small quantity only of fat, chyle contains a 
very large amount. The actual amount of fat present in the chyle of the 
thoracic duct varies, as may be expected, very considerably, according to the 
nature of the meal, the stage of digestion, and various circumstances. Five 
per cent, is a very common amount; in the dog it has been found to vary 
from 2 to 15 per cent. The increase in fat is chiefly if not exclusively due 
to an increase in the neutral fats ; though whether the small quantity of soaps 
and of lecithin present is greater than in lymph has not been distinctly as- 
certained. Cholesterin is probably present in greater amount than in lymph, 
since it probably conies from the bile poured into the intestines during diges- 
tion ; but this is not certain. How far the nature of the fat, that is, the 
proportion of the various kinds of fat, of stearin, etc., varies with the fats 
present in the meal has not been definitely ascertained. 
The condition of the fat in chyle is peculiar. Some of it exists, like the 
fat in milk, in the form of fat-globules of various sizes, but all small. A 
very considerable quantity, however, is present in the form of exceedingly 
minute spherules or granules, far smaller than any globules to be seen in 
milk ; these exhibit active " Brownian movements." The fat present in this 
form is spoken of as the " molecular basis " of chyle, and is very distinctive 
of chyle. In the emulsified contents of the intestine, often called chyle, the 
fat is finely divided, and to a large extent into small globules, but there is 
nothing corresponding to this molecular basis; the fat does not assume this 
condition until it has passed out of the intestine into the lacteals. Lymph 
examined with the microscope shows besides the white corpuscles only very 
few oil-globules and nothing of this molecular basis. Just as in fact lymph 
is, broadly speaking, blood minus its red corpuscles, so chyle is lymph plus 
a very large quantity of minutely divided neutral fat. 
The total amount of lymph or of chyle which enters the blood system 
through the thoracic duct, though it probably varies considerably, is prob- 
ably also always very large. It has been calculated that in a well-fed animal 
a quantity equal at least to that of the whole blood may pass through the 
thoracic duct in twenty-four hours, and of this it is supposed that about half 
comes through the lacteals from the alimentary canal, and therefore to a 
large extent from food, and the remainder from the body at large. These 
calculations are based on uncertain data, and cannot, therefore, be taken as 
of exact value, but we may use them for the sake of an illustration. Thus, 
in a man of average weight, that is, about 154 kilos., the quantity of blood 
(§ 38) being of the body weight is about 12 kilos. The quantity of lymph 
or chyle, therefore, discharged into the blood in an hour would be according 
to this calculation half a kilo., or something less than half a litre; and since 
the flow must vary considerably in the twenty-four hours would be sometimes 
less, and therefore sometimes even more, than this. 
The Movements of Lymph. 
§ 253. Making every allowance for the uncertainty of the calculations 
detailed in the preceding paragraph, it is obvious that the lymph must flow 318 
THE TISSUES AND MECHANISMS OF DIGESTION. 
with a not inconsiderable rapidity (if we take about half the above estimate, 
the rate will be about 5 c.c. per minute) through the thoracic duct, and 
therefore must also be continually streaming into that duct along the various 
lymphatic channels from the manifold lymph-spaces of the body. This 
onward progress of the lymph is determined by a variety of circumstances. 
In the first place, the remarkably widespread presence of valves in the 
lymphatic vessels causes every pressure exerted on the tissues in which they 
lie to assist in the propulsion forward of the lymph. Hence all muscular 
movements increase the flow. If a canula be inserted in one of the larger 
lymphatic trunks of the limb of a dog, the discharge of lymph from the 
canula will be more distinctly increased by movements, even passive move- 
ments, of the limb than by anything else. When we come to speak of the 
entrance of chyle into the lacteal radicles of the villi, we shall see that, at 
all events according to one view, the muscular fibres of the villus act as a 
kind of muscular pump, driving the chyle past the valved end of the lacteal 
radicle into the lymphatic canals below. In addition to the presence of valves 
along the course of the vessels, the opening of the thoracic duct into the venous 
system is guarded by a valve, so that every escape of lymph or chyle from 
the duct into the veins becomes itself a help to the flow. In the second 
place, we have already seen that the blood-pressure in the capillaries and 
minute vessels is considerably greater than that in the large veins, such as 
the jugular; in fact, this difference of pressure is the cause of the flow of 
blood from the capillaries to the heart. Now the lymph in the lymphatic 
spaces outside the capillaries and minute vessels undoubtedly stands at a 
lower pressure than the blood inside the capillaries; otherwise the transuda- 
tion from the blood into the tissues would be checked; but the difference is 
probably much less than the difference between the pressure in the capillaries 
and that in the large venous trunks. So that the lymph in the lymph-spaces 
of the tissues may be considered as standing at a higher pressure than the 
blood in the venous trunks, for instance, in the jugular vein. That is to say, 
the lymphatic vessels as a whole form a system of channels leading from a 
region of higher pressure, viz., the lymph-spaces of the tissues, to a region 
of lower pressure, viz., the interior of the jugular and subclavian veins. This 
difference of pressure will, as in the case of the bloodvessels, cause the lymph 
to flow onward in a continuous stream. Further, this flow, caused by the 
lowness of the mean venous pressure at the subclavian vein, will be assisted 
at every respiratory movement, since at every inspiration the pressure in the 
venous trunks becomes, as we shall see in dealing with respiration, negative, 
and thus lymph will be sucked in from the thoracic duct, while the increase 
of pressure in the great veins during expiration is warded off from the duct 
by the valve at its opening. In the third place, the How may be increased 
by rhythmical contractions of the walls of the lymphatics themselves, which, 
as we have seen, are remarkably muscular; and the peculiar interlacing of 
the muscular fibres above each valve suggests that the walls here act after 
the fashion of a tiny heart and by a rhythmical systole drive on the fluid, 
which by the action of the valve below collects at the spot. We have, how- 
ever, no experimental proof of this; for, though rhythmic variations have 
been observed in the lacteals of the mesentery, it is maintained that these 
are simply passive, i. e., caused by the rhythmic peristaltic action of the in- 
testine, each contraction of the intestine filling the lymph-channels more 
fully, and are not due to contractions of the walls of the lacteal vessels them- 
selves. In some of the lower animals, for instance in the frog, the muscular 
walls of the vessels are developed at places into distinctly contractile pro- 
pulsive organs, spoken of as lymph-hearts. Lastly, it is at least open for 
us, on the strength of the analogy that osmosis may give rise to increased 319 
THE NATURE AND MOVEMENTS OF LYMPH. 
pressure on one side of a diffusion septum, to suppose that the very processes 
which give rise to the appearance of lymph in the lymph-spaces of the tis- 
sues, tend themselves to promote the flow of lymph. We have at least, un- 
der all circumstances, one or other of these causes at work, promoting a 
continual flow from the lymphatic roots to the great veins. They are to- 
gether sufficient to drive, in man, the lymph from the lower limbs and trunk, 
against the effects of gravity, into the veins of the neck. In the upper limb 
the influences of gravity, owing to the varied movements of the limb, are 
as often favorable to as opposed to the natural flow of the lymph ; but, as 
we have already said, a long-continued unfavorable action of gravity, espe- 
cially in the absence of the aid of movements in the skeletal muscles, as when 
the arm hangs down motionless for some time, leads to accumulation of lymph 
at its origin in the lymph-spaces. The strength of the causes combining to 
drive on the lymph is strikingly shown in animals when the thoracic duct 
is ligatured ; in such cases a very great distention of the lymphatic vessels 
below the ligature is observed. 
§ 254. Although the phenomena of disease and, perhaps, general con- 
siderations render it probable that the nervous system governs in some way 
the stream of lymph, regulating, it may be, not only the flow along the 
definite lymph-canals, but also the transit of plasma into the lymph-spaces 
and the escape of lymph thence into the definite canals, our knowledge on 
these points is very imperfect. We have no proof that the muscular fibres 
in the walls of the lymphatic vessels are governed by nerves, or that the 
lymph-spaces are influenced directly by nervous action ; and most of the 
attempts to demonstrate any direct action of the nervous system on the 
lymphatics have hitherto failed. 
§ 255. T1ie passage of material, namely, of water containing certain sub- 
stances in solution, from the interior of the bloodvessel, where they form part 
of the plasma, into the lymph-capillary where they are called lymph, consists 
of two steps : the passage from the bloodvessel into the lymph-space, and the 
passage from the lymph-space into the lymph-capillary ; for, as we have seen, 
it is only in particular places that the lymph-capillary immediately surrounds 
the bloodvessel. Once arrived in the lymph-capillary the lymph finds an 
open path along the rest of the lymphatic system, but the connection between 
the lymph-space and the lymph-capillary is, as we have seen, peculiar and at 
least not a free and open one. 
The passage of material from the bloodvessel into the lymph-space we 
speak of as transudation. What can we say as to the nature of this process ? 
There are two known physical processes with which we may compare it: 
diffusion through a membranous or other porous partition, and filtration 
through a similar partition. Diffusion, though influenced by fluid pressure, 
is not the direct result of fluid pressure, but may, on the contrary, be the 
cause of differences of pressure on the two sides of the partition, and may 
work against fluid pressure. When a strong solution and a weak solution 
of salt are separated by a diffusion septum, diffusion takes place whether the 
columns of fluid be at the same level on the two sides of the septum or at 
different levels; and if the columns be at the same level to start with, that 
of the stronger solution soon comes to exceed the other in height, on account 
of the osmotic flow of water from the weaker into the stronger solution. 
Filtration, on the other hand, is the direct result of pressure; without dif- 
ference of pressure filtration does not take place; and, the filter remaining 
of the same nature and in the same condition, the amount of filtrate is 
dependent on the amount of pressure. May we speak of the process of 
transudation as a simple process of diffusion or a simple process of filtration ? 
that is to say, can all the phenomena of transudation be explained as simply 320 
THE TISSUES AND MECHANISMS OF DIGESTION. 
the results of one or other of these physical processes? Diffusion by itself 
will not account for the results; for the proteids of the blood-plasma are 
indiffusible or very nearly so, and yet the lymph contains a considerable 
quantity of these proteids. We have no satisfactory knowledge of the exact 
composition of lymph as it exists in the lymph-spaces. In the lymph of the 
larger lymph-trunks the diffusible saline substances are present in about the 
same proportion, and the indiffusible proteids to about or less than half as 
much as in blood-serum ; and we may perhaps assume that the lymph in the 
lymph-spaces contains relatively less proteids, but has otherwise the same 
composition as blood-plasma. Mere diffusion would not give rise to a fluid 
of such a nature. Can we speak of transudation, then, as a filtration ? The 
blood is undoubtedly flowing through the capillaries and other small vessels 
under a certain pressure; we have seen (§ 105) that the pressure is roughly 
speaking about 30 mm. Hg.; and it would be possible to select such a filter 
or porous partition as would at about this pressure permit the passage of a 
certain quantity of the inorganic and crystalline constituents of blood-plasma 
to pass through in company with a relatively smaller quantity of the proteids 
and a large quantity of the water, the red and white corpuscles being ex- 
cluded. Such a filtrate would be more or less of the nature of lymph ; and 
so far we might be justified in speaking of the transudation of lymph as a 
process of filtration. But the transit through the living wall of the blood- 
vessel is affected by circumstances in a manner so different from the manner 
in which the same circumstances affect a transit through an ordinary lifeless 
filter, that we gain but little and may be led into error by speaking of the 
process as a filtration. Substances in solution or otherwise, pass through a 
filter when the pressure is sufficient to drive them through the passages 
furnished by the interstices existing in the substance of the filter. In the 
case of an ordinary filter the substance of the filter is within limits perma- 
nent, and the passages correspondingly constant. The living wall of a 
capillary, however, is not a constant unchanging thing. The epithelioid 
plates and other elements which constitute it are alive, and being alive are 
continually undergoing change and are especially subject to change; more- 
over, as we have seen (§§ 22, 23), the vascular walls appear to be continu- 
ally acting upon and being acted upon by the blood. Hence a change in 
the blood tends to cause changes in them ; and these changes may materially 
affect in one direction or another their action as filters. In an ordinary filter 
increase of pressure necessarily entails increase of filtration ; in a living 
filter it may or may not, and the same increase of pressure may according 
to circumstances produce very different results as regards the transudation 
of lymph. 
Thus it seems reasonable to suppose, as we have suggested (§ 197), that, 
other things being the same, an increase of blood-pressure should necessarily 
increase the transudation of lymph. Hence when a small artery dilates, 
since the pressure in the still smaller branches and capillaries of that artery 
is, as we have more than once pointed out, increased, more lymph appears in 
the lymph-spaces ; indeed it is one of the main purposes of the widening of 
small arteries to supply the elements of the tissue with more lymph, that is, 
with more food. But it does not therefore follow that under all circum- 
stances widening of the artery should increase the passage of lymph ; some- 
thing may occur to counteract the natural effect of the increased pressure in 
the bloodvessels. An instance of this seems to be afforded by the case of 
the submaxillary gland, when the chorda nerve is stimulated while the gland 
is under the influence of atropine. As we have seen, though the arteries 
dilate, no secretion takes place ; and we cannot explain the absence of a flow 
into the alveoli by supposing that the extra amount of lymph which would THE NATURE AND MOVEMENTS OF LYMPH. 
321 
in normal circumstances form part of the secretion, and in the case of a 
fairly copious secretion would be considerable, now passes away by the lym- 
phatics without reaching the cells of the alveoli, for in such cases no extra 
flow in the lymphatics leading from the gland has been observed, and there 
is no accumulation of lymph in the connective tissue of the gland. Ap- 
parently, for some reason or other, in spite of the increased pressure in the 
blood vessels, more lymph than usual does not pass into the lymph-spaces. 
Then again, as we shall presently have occasion to point out, an increase 
of pressure in the bloodvessels produced by obstruction to the venous out- 
flow is much more efficient in promoting an increase of transudation, at all 
events an abnormal increase, than is an increase of arterial pressure; and 
the difference between the two cases appears to be too great to be accounted 
for on the ground that an obstruction to the venous outflow raises the pres- 
sure within the capillaries and small vessels more readily and to a higher 
degree than does the widening of the arteries. Moreover, that obstruction 
to venous outflow does not produce its effects in the way of transudation 
simply and merely by raising the capillary pressure is shown by the fact that 
the same amount of obstruction may or may not give rise to excessive trans- 
udation, according to the condition of the blood or other circumstances. 
For instance, though the obstruction produced by ligaturing a vein fre- 
quently causes excessive transudation, it does not always cause it, and the 
femoral vein of a dog may be ligatured without any excessive transudation 
taking place; yet if, after the ligature, certain changes be induced in the 
blood, excessive transudation occurs in the leg the vein of which has been 
ligatured, but not elsewhere. Pointing toward the same conclusion is the 
fact that excessive transudation more readily occurs when a vein is plugged 
by a thrombus arising from abnormal conditions of the vascular system than 
when a vein is simply ligatured. And in general we may say, and this is a 
point to which we shall return, that two things chiefly determine the amount 
of transudation : the pressure of the blood in the bloodvessels, and the con- 
dition of the vascular walls in relation to the blood, the latter being at least 
as important as the former. 
Another aspect of the matter, moreover, deserves attention. In filtration 
the movement takes place through the filter in one direction only, whereas 
in the living body, the passage of material through the capillary wall takes 
place in two opposite directions. In all the tissues, though more perhaps in 
certain tissues than in others, the passage from the bloodvessel into the 
lymph-space is accompanied by a passage from the lymph-space into the 
blood; while food for the tissue passes in one direction, waste products pass 
in the other. In a secreting gland the greater part of the lymph coming 
from the bloodvessels, the water and other matters, pass away into the lumen 
of the alveolus after undergoing changes in the cell; but even in such a case 
there is some return from the cells into the bloodvessels, carbonic acid, for 
instance, if nothing else, is given up by the cells to the blood; and in such 
organs as a muscle or the liver, the backward stream of material from the 
tissue to the blood is extensive and important. Moreover, this backward 
stream works against pressure; indeed, as may be seen in a muscle, it is 
when the bloodvessels are dilated and the pressure in the capillaries and 
small vessels highest, as during and after the contraction of the muscle, that 
the passage from the tissue into the blood is most energetic. Many of the 
waste products of the tissue are, it is true, diffusible, and we might be tempted 
to say that while the lymph which feeds the tissue traverses the vascular 
wall by filtration in the direction of pressure, the waste products return to 
the blood against pressure by diffusion ; but such a view cannot at present 
be regarded as proved ; and if it be true, as is maintained by some, that 322 
THE TISSUES AND MECHANISMS OF DIGESTION. 
lymph, including the proteids, may at times be reabsorbed from the tissue 
into the bloodvessels, it is distinctly contradicted. Finally, the introduction 
of certain substances (termed lymphagogues) into the blood, is followed by an 
increase in the quantity of lymph without any related change in the blood- 
pressure. We shall have to return to this question when we come to deal 
with the secretion of urine; but meanwhile we may adopt the conclusion, 
which is especially supported by the phenomena of disease, that while dif- 
fusion and filtration play their respective parts, diffusible substances pass- 
ing in and out of the blood more readily than indiffusible substances and an 
increase of pressure tending to promote transudation, the condition of the 
vascular wall so profoundly influences the transit of material as to render 
the process very complex. We may probably regard it as too complex to 
be compared even with filtration through a filter capable of widely changing 
in texture from time to time, and as more nearly resembling the process of 
secretion. 
Concerning the passage of the lymph from the confined lymph-spaces 
into the open gangways of the lymph-capillaries we know very little. If, as 
some think, the cavity of the lymph-capillary is shut off on all sides and 
completely by a continuous lining of sinuous epithelioid plates, then the 
passage from the lymph-space into it must be regarded as a sort of repetition 
of the passage from the blood-capillary into the lymph-space, as a second 
transudation. But if, as others think, and as on the whole seems more 
probable, the lymph-spaces open at places directly into the lymph-capillaries, 
the passage is a simply mechanical affair determined by the freedom of these 
openings. 
In either case the flow from the lymph-spaces will be facilitated by all 
events which promote, and checked by those which hinder the flow of lymph 
along the lymph-capillaries and the other lymphatic channels. 
We may here remark as influencing the quantity of lymph in the lymph- 
spaces and vessels that the quantity of lymph taken up from the lymph- 
spaces by the actual elements of the tissue may vary considerably. We 
remarked in § 30 on the peculiar relations of living tissue to water, and 
there are reasons for thinking that the very substance of a cell or a fibre 
(a muscular fibre, for instance) may hold in itself a larger quantity of water 
at one time than at another. The water thus taken up or given out, and the 
substances which may be carried in solution by that water, come from and 
go to the lymph. The condition of the tissue determines by itself the 
amount of lymph in the lymph-spaces. 
§ 256. Under the influence of all these several actions the lymph in the 
various lymph-spaces of the body varies in amount from time to time, but 
under normal circumstances never exceeds certain limits. Under patho- 
logical conditions those limits may be exceeded, and the result is known as 
oedema or dropsy. Similar excessive accumulations of lymph may occur not 
in the ordinary lymph-spaces, but in those larger lymph-spaces, the serous 
cavities, any large excess of fluid in the peritoneal cavity being known as 
ascites. 
The possible causes of oedema are, on the one hand, an obstruction to the 
flow of lymph from the lymph-spaces, and on the other hand an excessive 
transudation, the lymph gathering in the lymph-spaces faster than it can be 
carried away by a normal flow ; with the former the lymphatic system itself, 
with the latter chiefly the vascular system, is concerned. As a matter of 
fact, however, oedema is almost always, if not always, due to abnormal con- 
ditions of the vascular system, and is the result not of hindered outflow, but 
of excessive transudation. 
Owing to the numerous anastomoses of the lymph-vessels and the conse- THE NATURE AND MOVEMENTS OF LYMPH. 
323 
quent establishment of collateral streams, obstruction in the lymph-passages 
themselves rarely if ever gives rise to oedema ; and it may be here remarked 
that owing to the same free collateral communication between the lymph- 
vessels the labyrinthine passages of the lymphatic glands do not offer the 
serious obstacle to the onward flow of the general lymph-stream, as might at 
first sight be supposed. Nor have we at present any knowledge which would 
lead us to suppose that any physiological changes in the walls of the lym- 
phatic vessels or of the lymph-capillaries, or in the lymph-spaces, by giving 
rise in some way to obstacles to the flow of lymph, ever lead to an accumu- 
lation of lymph in the latter. 
One kind of oedema we have already touched upon in speaking of the 
capillary circulation (§ 169), viz., the "inflammatory" oedema. In this 
kind of oedema, owing to changes in the vascular walls, a larger amount of 
transudation passes into the lymph-spaces, and that transudation is richer in 
proteid matters, and contains a larger amount of the fibrin factors, or at all 
events, is much more distinctly coagulable than ordinary lymph, as well as 
crowded with migrating corpuscles. Allied to this inflammatory oedema is 
the increase of lymph, also apparently changed somewhat in character, 
which appears as " effusion " in the serous cavities when these are inflamed, 
as in pleurisy and peritonitis. 
One of the most common forms of oedema is an oedema of primarily, 
though not wholly, mechanical origin-oedema arising from obstruction to 
the venous flow; under these circumstances more lymph passes into the 
lymph-spaces than the lymph-vessels are able to carry away. If the femoral 
vein be tied the leg may become oedematous, and, as we have said, oedema is 
a common result of the plugging or obstruction of veins through disease ; 
the oedema which is so common an accompaniment of heart disease, involv- 
ing obstruction to the return of venous blood to the right side of the heart, 
and the ascites which follows upon hindrance to the portal flow, are instances 
of oedema of this kind. We have already remarked on the relation of 
transudation to blood-pressure, and in venous obstruction the rise of pressure 
within the small bloodvessels is distinguished from that due to arterial dila- 
tation by being accompanied with a want of adequate renewal of the blood ; 
this probably affects the epithelioid lining of the bloodvessels in such a way 
as to increase the transudation. And, indeed, as is seen in case of heart dis- 
ease with prolonged or repeated venous obstruction, the oedema, as time goes 
on and the tissues become impaired, is more easily excited and with greater 
difficulty removed, though the actual amount of obstruction, the actual 
increase of pressure in the small vessels, remains the same, or at least is not 
proportionately increased. 
Still another kind of oedema is one due to changes taking place in the 
blood, quite apart from variations of blood-pressure. This kind of oedema 
is seen in some diseases of the kidney, in " Bright's disease," for instance. 
In such cases the blood contains less proteids, and indeed less solids, is more 
watery and of lower specific gravity than is normal. But the oedema is not 
in these cases to be explained on the view that the more watery blood passes 
more readily through the capillary walls, for it may be shown experimentally 
that the mere thinning of the blood, as by the injection of normal saline 
solution into the bloodvessels, will not at once lead to oedema, at least in the 
limbs and trunk, and it is these which in Bright's disease especially become 
oedematous. In all probability the oedema of Bright's disease, if it be really 
due to the abnormal character of the blood, is produced by the abnormal 
blood so acting on the bloodvessels that these allow a transudation greater 
than the normal. Finally, oedema may be due to abnormal conditions of 
the tissues themselves. 324 
THE TISSUES AND MECHANISMS OF DIGESTION. 
But these are pathological questions into which we must not enter here. 
We have touched upon them because they illustrate the important processes 
taking place in the lymph-spaces, and, as we have more than once insisted, 
the lymph in the lymph-spaces is the middleman of all the tissues, and hence 
facts illustrating the laws which govern the flow of lymph into and out of 
the lymph-spaces are of fundamental physiological importance. 
§ 257. Lymph-hearts. In the frog and other amphibia and in reptiles 
the flow of lymph into the venous system is assisted by rhythmically pul- 
sating muscular lymph-hearts, which present many curious analogies with 
the blood-heart. The frog possesses four lymph-hearts. Of these, two 
belonging to the hind limbs are placed one on each side of the coccyx near 
its end, and, being covered only by aponeurosis and the skin, may without 
dissection be seen beating. Two anterior ones are placed on the transverse 
processes of the third vertebra, and are covered from view by the shoulder 
girdle. Each lymph-heart is a more or less oval sac lying in one of those 
lymph sacs or cavities lined with sinuous epithelioid plates, which, as we 
have said, are present in the frog. It is continued at one end, by an ori- 
fice guarded with valves, into a small vein which opens, in the case of the 
posterior heart, into a crural vein, and in the case of the anterior hearts into 
a jugular vein. The wall consists of muscular fibres arranged in a plexi- 
form manner and supported by a considerable amount of connective tissue. 
These fibres are striated and branched and are intermediate in character 
between cardiac and skeletal muscular fibres. Nerve-fibres terminate in 
these muscular fibres, and the muscular wall, unlike that of the blood-heart, 
is supplied with capillary bloodvessels. The interior is lined with epithelioid 
plates of sinuous outline, and this lymphatic lining is continued along a 
number of openings or pores, by which the cavity of the heart opens into 
the surrounding lymph-space. When the heart contracts the contents are 
driven into the vein, the lymphatic pores being closed by the approximation 
of the contracting muscular fibres; when the heart dilates the fluid in the 
vein is prevented from returning by the valves at its mouth, while the lymph 
enters readily from the surrounding space through the now open pores. In 
the frog regular lymphatic vessels are scanty ; hence these lymph-hearts 
become of considerable importance in promoting the flow of lymph. The 
lymph-hearts of reptilia are similar in structure and function. In the frog, 
in which they have been chiefly studied, the action of the lymph-hearts is in 
a measure dependent on the spinal cord. The posterior lymph-hearts belong- 
ing to the hind limbs are connected by means of the delicate tenth pair of 
spinal nerves with a region of the cord opposite the sixth or seventh vertebra 
in such a way that section of the nerve or destruction of the particular region 
of the cord suspends or destroys their activity. The anterior pair are simi- 
larly connected with a region of the spinal cord opposite the third vertebra. 
Each pair therefore seems to have a " centre " in the spinal cord ; but it is 
probable, though observers are not wholly agreed, that the hearts, after 
destruction of their spinal centre, ultimately resume their rhythmic beats, 
so that the dependence of their activity on the spinal centre is not an abso- 
lute one. Like the heart of the blood-system, the lymph-hearts may be in- 
hibited, and that in a reflex manner, the inhibition centre being moreover 
in the medulla oblongata. If a frog be carefully observed, the activity of 
the lymph-hearts will be found to vary largely, and these variations appear 
to be in part due to nervous influences, so that in this way the movement 
of lymph, and hence the processes of absorption, are in this animal directly 
dependent on the nervous system. ABSORPTION FROM THE ALIMENTARY CANAL. 
325 
Absorption from the Alimentary Canal. 
§ 258. We may now return to consider the absorption of the products 
of digestion, that is to say, the passage of these bodies from the interior of 
the alimentary canal, where they are really outside the body proper, into 
the body itself. For simplicity's sake we may consider digestion in a broad 
way as the conversion of practically non-diffusible proteids and starch into 
more diffusible peptone and highly diffusible sugar, and as the emulsifying, 
or division into minute particles of fats. We have reason to believe that 
some of the sugar may be changed into lactic acid, or even into butyric or 
other acids, that some of the proteids are carried beyond the peptone condi- 
tion into leucin and other bodies, and that some of the fat may be saponified ; 
and it may be that some of the proteid material of the food passes into the 
body as albumose or even as parapeptone, or in some other little changed 
condition. But we may probably with safety, for present purposes, assume 
that the greater part of the proteid is absorbed as peptone, that carbohy- 
drates are mainly absorbed as sugar, and that the greater part of the fat 
passes into the body as emulsified but otherwise unchanged neutral fat; and 
we may neglect the other conditions of digested food as subsidiary, and as 
far as absorption is concerned, unimportant. 
We have seen that two paths are open for these products of digestion, 
one by the capillaries of the portal system, the other by the lacteals. It 
cannot be a matter of indifference which course is taken. For if the prod- 
ucts pass by the lacteals they fall into the general blood-current after hav- 
ing undergone only such changes as they may experience in the lymphatic 
system ; while if they pass into the portal vein they are subjected to certain 
powerful influences of the liver (which we shall study in a future chapter) 
before they find their way to the right side of the heart, and if the substances 
be prevented from reaching the liver, as when in the dog the portal vein is 
connected with the inferior vena cava and they are thus at once transmitted 
to the general circulation, serious results ensue. We may, therefore, con- 
sider first which of the two paths is, as a matter of fact, taken by the several 
products, and subsequently study the mechanism of absorption in the two 
cases. 
The Course taken by the Several Products of Digestion. 
§ 259. From what lias already been said we have been led to regard the 
villi as the most active organs of absorption, and the structure of a villus 
leads us further to conclude that the diffusible peptones and sugar pass, 
together with the water in which they are dissolved, into the superficially 
placed capillary network of the villus and so into the portal system, while 
the merely emulsified fat, unable to traverse the wall of the capillary, passes 
on to the deep-seated lacteal radicle, and so finds its way into the lymphatic 
system. And the results of observation and experiment, as far as they go, 
support this view. 
Fats. After a meal containing fat the lymph of the lacteals contains fat, 
and is now called chyle ; and the richer the meal in fat the more conspicuous is 
the fat in the lymph-vessels. We cannot, however, prove that all the fat of a 
meal absorbed from the alimentary canal is poured by the thoracic duct into 
the venous system. If a meal containing a known quantity of fat be given 
to a dog and the small quantity of fat present in the feces corresponding to 
the meal be subtracted from that amount, we can determine the amount of 
fat absorbed, for we have no evidence whatever that any appreciable amount 
of fat undergoes a destructive decomposition in the alimentary canal. Col- 
lecting by means of a canula inserted into the thoracic duct the whole of 326 
THE TISSUES AND MECHANISMS OF DIGESTION. 
the chyle during and after the meal, so long as it remains milky, showing 
that fat is being absorbed, we can ascertain the quantity of absorbed fat, 
which would, but for the operation, have passed into the venous system. 
When this has been done, a very remarkable deficit, amounting it may be to 
40 or 50 per cent., has been observed ; that is to say, of every 100 parts of 
fat which disappear from the alimentary canal only about 60 parts find their 
way through the thoracic duct into the venous system. 
Are we then to conclude that the missing quantity finds its way into the 
portal system? Now the portal blood does, during digestion, contain a cer- 
tain quantity of fat; indeed, the serum is said at times to appear milky from 
the presence of fat. But the whole circulating blood during the digestion 
of a fatty meal contains, for a while, the fat poured into it by the thoracic 
duct; and it has been ascertained in the dog that the blood of the portal 
vein during digestion contains not more but less fat than the blood of the 
carotid artery, so that the fat which appears in the portal blood during 
digestion is, for the most part at least, not fat absorbed by the capillaries of 
the alimentary canal, but fat absorbed by the lacteals. Moreover, when the 
chyle of the thoracic duct is diverted through a canula, and not allowed to 
flow into the blood, the quantity of fat in the portal blood, as in the blood 
at large, is very small indeed. Lastly, when a villus of an intestine in full 
digestion of fat is treated with osmic acid, fat cannot be recognized by the 
microscope within the capillaries or other bloodvessels, though it abounds 
outside them in the substance of the villus and in the lacteal radicle. 
We may probably, therefore, infer with safety that all or at least very 
nearly all the fat absorbed from the intestine takes the path of the lacteals. 
As to the deficit mentioned above, that is as yet without explanation. It 
may be that in some way, on its course, in the lymphatic glands, for instance, 
the fat is taken away from the chyle, hidden so to speak somewhere away 
from both chyle and blood; but on this point we have no exact information. 
§ 260. Water and salts. If, in an animal, the rate of flow of lymph or 
chyle through a canula placed in the thoracic duct be watched, and water 
or, to avoid the injurious effect of simple water on the mucous membrane, 
normal saline solution, be then injected in not too great quantity into the 
intestine, no marked increase in the flow of chyle through the canula is 
observed. From this we may infer that the water of the intestinal contents 
is absorbed not into the lacteals but into the portal system. If, however, a 
very large quantity of the normal saline solution be injected so as to distend 
the intestine, then the flow of chyle is increased to some extent. It would 
appear, therefore, that while under normal conditions the water passes from 
the intestine mainly into the portal blood, some of it may under some cir- 
cumstances pass into the lacteals. 
With regard to the course taken by ordinary saline matters we possess 
no detailed information. When special salts, such as potassium iodide and 
others, easily recognized by appropriate tests, are introduced into the intes- 
tine, they may be speedily detected both in the blood and in the contents of 
the thoracic duct; but whether, in such cases, these salts find their way into 
the thoracic duct by the lacteal radicle of the villi, or pass into the lymph 
stream at some later part of its course, we do not know. Nor can we with 
regard to such a salt as sodium chloride, state absolutely that it passes mainly 
with the water into the portal blood, though we may fairly suppose this to 
be the case. 
§ 261. Sugar. Both blood and chyle contain, normally, a certain small 
amount of sugar; and careful inquiries show that the percentage of sugar 
in chyle and in general blood is fairly constant, neither being to any marked 
extent increased by even amylaceous meals; on the other hand, a meal con- ABSORPTION FROM THE ALIMENTARY CANAL. 
327 
taining sugar or starch does temporarily increase the quantity of sugar in 
the portal blood. From this we may infer that such portions of the sugar 
of the intestinal contents as are absorbed as sugar pass exclusively by the 
portal vein. We may, however, here call attention to the difficulties attend- 
ing an argument of this kind. In the first place the quantitative determi- 
nation of a small amount of sugar in so complex a fluid as blood is attended 
with great difficulties and uncertainties. In the second place a very large 
quantity of blood is at any one moment streaming through the capillaries of 
the alimentary canal; and we may perhaps speak of the quantity which 
passes through them during the whole period of digestion as being enormous. 
Hence though each 100 c.c. in passing through the capillaries might take up 
a quantity of sugar so small as to fall almost within the limit of errors of 
observation, yet the whole quantity absorbed during the hours of digestion 
might be considerable; or to put it in another way, an error of observation, 
unavoidable with our present means of analysis, on a sample of blood taken 
from the portal vessels might lead to a wholly unwarranted conclusion that 
sugar was or was not being absorbed. Making every allowance, however, for 
these difficulties, the increase of sugar which has been observed in the portal 
blood during digestion seems too great to permit of any other conclusion than 
that sugar is really absorbed from the alimentary canal by the bloodvessels. 
When, however, a large quantity of sugar dissolved in a large quantity of 
water is present in the intestine, the sugar in the chyle is said to be increased. 
In such a case the excess of water, as stated above, passes into the lacteals, 
and in so doing appears to carry some of the sugar with it. 
In this connection it should be remembered that the sugar resulting from 
digestion is for the most part maltose, while that in the portal blood is dex- 
trose, the former being changed into the latter probably while passing through 
the intestinal epithelium. 
§ 262. Proteids. The difficulties attending the experimental determina- 
tion of the path taken by proteids are greater even than in the case of sugar, 
for the exact quantitative estimation of peptone in blood (and we are assuming 
that proteids are mainly absorbed as peptone) is a task of the greatest diffi- 
culty-one compared with which that of estimating sugar appears almost 
easy. Bearing this in mind we may state that all observers are agreed that 
peptone is absent from chyle or at least that its presence cannot be satisfac- 
torily proved. On the other hand, while some observers have succeeded in 
finding peptone in the portal blood after food, but not during fasting, many 
have failed to demonstrate the presence of peptone in the blood either of the 
portal vein or of the vessels at large even after a meal containing large 
quantities of proteids. Of course, as we argued in speaking of the absorp- 
tion of sugar, the quantity of peptone passing into the portal blood at any 
moment might be small, and yet a considerable quantity might so pass during 
the hours of digestion. We may suppose, moreover, that that which does 
pass is immediately converted, possibly by some ferment action, into one or 
other of the natural proteids of the blood, or otherwise disposed of; and, 
indeed, peptone injected carefully and slowly into a vein disappears from the 
blood, though little or even none passes out by the kidney. And the view 
that peptone is so changed, possibly in the very act of absorption, is sup- 
ported not only by the statement that peptone may be found in the practi- 
cally bloodless wall, that is, mucous membrane, of the intestine removed from 
a dead animal even when it appears to be absent from the blood, but also 
and especially by the following observation. If an artificial circulation of 
blood be kept up in the mesenteric arteries supplying a loop of intestine re- 
moved from the body, the loop may be kept alive for some considerable time. 
During this survival a considerable quantity of peptone placed in the cavity 328 
THE TISSUES AND MECHANISMS OF DIGESTION. 
of the loop will disappear, t. e., will be absorbed, but cannot be recovered 
from the blood which is being used for the artificial circulation, and which 
escapes from the veins after traversing the intestinal capillaries. The disap- 
pearance is not due to any action of the blood itself, for peptone introduced 
into the blood before it is driven through the mesenteric arteries in the ex- 
periment may be recovered from the blood as it escapes from the mesenteric 
veins. It would seem as if the peptone were changed before it actually gets 
from the interior of the intestine into the interior of the capillaries. 
But the argument that the absence of peptone from the blood is no proof 
that the peptone is not absorbed into the blood may also be applied to the 
chyle, and thus leaves us unable to draw a conclusion as to the path of the 
proteids. The following indirect proof that peptone does not pass into the 
chyle has been offered, but it too is open to objection. We shall see here- 
after that the absorption of proteid material leads to an increase in the 
elimination of urea by the kidneys. So marked is this increase, that unless 
there be clearly some other causes at work leading to an increase of urea, 
such as fever for instance, an increase of urea in the urine following upon 
the administration of proteid food may be taken as a proof that the proteid 
food has been digested and absorbed. Now if in a dog the thoracic duct be 
successfully ligatured so that the chyle cannot pass as usual into the blood, 
and the dog be fed on proteid food, as free as possible from fat, so as not 
unnecessarily to load the obstructed lacteals, an increase in the urea of the 
urine is observed as usual. Obviously in such a case the proteid food is 
absorbed, and obviously also does not pass into the blood through the tho- 
racic duct (the success of the ligature having been proved by post-mortem 
examination). But the experiment, though as far as it goes supporting, does 
not rigorously prove, the view that the proteids are absorbed by the capilla- 
ries of the alimentary canal; for the thoracic duct and lymphatics below 
the ligature were found largely distended, and lymph and chyle appear to 
have escaped from the vessels; hence it is possible that some at least of the 
proteids were absorbed by the lacteals of the intestine, but finding their 
usual path blocked made their way into the blood stream. 
We may, therefore, say that the results of experiment, while they do not 
definitely prove, give some support to, and at least do not contradict, the 
view which we a little while ago put forward as probable, namely, that pro- 
teids, transformed into diffusible peptones, pass into the bloodvessels and 
not into the lacteals. 
But, if this view be provisionally accepted, it must be on the understand- 
ing that it is probable only ; and it may be that proteids do not take the 
same paths and are not absorbed in the same condition in all animals. The 
experiments just related were performed on dogs, that is to say, on carnivo- 
rous animals whose (natural) food contains a considerable quantity of fat, 
and whose lacteals might, therefore, be considered as preoccupied in the 
absorption of fat. The food of herbivora on the other hand contains a 
relatively small amount of fat; and if in these animals all the proteids and 
carbohydrates are absorbed by the bloodvessels, there is comparatively little 
left for the lacteals to do. Yet in these animals the lacteals and lymphatics 
are well developed. In the villus of a herbivorous guinea-pig or rabbit, 
though the reticular tissue is very scanty as compared with that present in 
the villus of a dog, the lacteal chamber is, relatively to the diameter of the 
villus, not merely as large as, but much larger than, in the dog. It is diffi- 
cult to suppose that this wide chamber is intended solely for the absorption 
of the relatively small amount of fat present in vegetable food. The ques- 
tion which we are discussing is clearly at present to be regarded as by no 
means settled. ABSORPTION FROM THE ALIMENTARY CANAL. 
329 
The Mechanism of Absorption. 
§ 263. I1 he absorption of fats. We have now to consider the manner in 
which these several substances pass into either the lacteal radicle or the 
capillary bloodvessels. It will be convenient to begin with the absorption 
of the fats. 
We have seen reason (§ 241), to think that the fats, remaining chiefly as 
neutral fats, are emulsified in the intestine by means of the bile and pan- 
creatic juice, the small quantity of soap which is formed probably serving 
simply the purpose of facilitating the emulsification. 
The neutral fats so emulsified pass in the first instance into the bodies of 
the columnar cells of the villi. It has, it is true, been maintained by some 
that they pass between the cells and not into them ; but the evidence is 
distinctly against this view. Since no such collections of fat globules are 
seen in the cubical cells of the glands of Lieberkuhn we infer that these 
have nothing to do with the absorption of fat. 
How the fat enters into the substance of the cell we do not know. We 
may presume that the striated border plays some part, but what part we do 
not know. Though, as we have seen, the rods making up the border appear 
able to move, to change their form, we have no evidence that the fat is in- 
troduced into the cells by means of any movements of these rods. We may 
imagine that the globules pass into the cell-substance by help in some way 
of these rods through amoeboid movements comparable with the ingestive 
movements of the body of an amoeba ; but we have no positive evidence 
to support this view. We said that bile promotes the passage of fat 
through membranes, possibly by in some way promoting a closer contact 
between the particles of fat and the substance of the membrane; but even 
if bile has this effect on the surface of the cells, its action in this respect can 
be subsidiary only. 
When fatty acids are ingested neutral fats appear in the chyle, indicating 
a synthesis of fatty acids into neutral fats in the epithelium of the villi. 
Within the columnar cell the fat may be seen, both in osmic acid prepa- 
rations and in fresh living cells, to be disposed in globules of various sizes, 
some large and some small, each globule placed in a space of the proto- 
plasmic cell-substance. It does not follow that the fat actually entered the 
cell exactly in the form of these globules; it may be that the fat passes the 
striated border in very minute spherules which, reaching the body of the 
cell, run together into larger globules; but whether this is so or not we do 
not know. 
From the columnar cell the fat passes into the spaces of the reticular 
tissue of the villus. It has, it is true, been contended that it passes along 
the substance of the bars of the reticulum ; but in carefully prepared osmic 
acid specimens of a villus in active digestion of fatty food, the fat may be 
distinctly recognized as largely filling up, still in the form of globules of 
various sizes, the spaces in the meshes of the reticulum which are not occu- 
pied by the leucocytes or allied wandering cells. The bases of the columnar 
cells, through the gaps in the basement membrane, directly abut upon the 
labyrinth of spaces; and the fat once out of the base of the cell is free in 
the spaces of the labyrinth. How it issues from the cell we do not exactly 
know ; possibly by a process analogous to the excretion of solid matters by 
an amoeba. 
From the labyrinth of spaces of the reticulum of the villus the fat passes 
into the cavity of the lacteal radicle; anol it is worthy of note that in the 
passage it undergoes a change. In the interior of the intestine, in the sub- 
stance of the columnar cell, and apparently in the labyrinth of the reticu- 330 
THE TISSUES AND MECHANISMS OF DIGESTION. 
lum, it is simply emulsified fat consisting ot globules small and large; within 
the lacteal radicle it consists partly of the same easily recognized globules 
but partly of the extremely divided " molecular basis " (§ 252) ; it is now 
no longer emulsified fat but chyle. How and by what means this extremely 
minute division of the globular fat into the "molecular basis" takes place 
we do not know ; nor do we know the exact manner in which the fat passes 
from the spaces of the reticulum into the interior of the radicle. 
We may here, perhaps, remark that the contents of the lacteal radicle 
consist not exclusively of fat, but of fat accompanied by the proteid and 
other substances which go to make up the chyle. Proteid and other sub- 
stances besides fat are also present in the lymph which occupies in part the 
labyrinth of the body of the villus, and are derived, like the lymph else- 
where, from the blood of adjacent capillaries ; at least, they are in part so 
derived, though it may be not wholly, for, as we have just seen, the passage 
of proteid material from the intestine into the substance of the villus past 
the capillaries, though not proved, must still be considered as possible. 
The spaces of the reticulum of the villus are more or less occupied by 
wandering cells of which we spoke under the general term of leucocytes. 
These do not all present the same appearances and most probably are not 
all of the same kind. 
Some of these leucocytes wander not only through the labyrinth of the 
reticulum but pass into the epithelium between the cells, and may project 
processes into or even make their way eventually into the interior of the 
intestine; or following the reverse course may wander from between the 
epithelial cells into the body of the villus; some of them, moreover, un- 
doubtedly contain fat. Hence the view has been suggested that these leuco- 
cytes are important agents, indeed the chief agents in the absorption of fat. 
It has been supposed that they, receiving the globules of fat into their cell- 
substance, in fact eating the fat exactly after the manner of an amoeba, 
either while projecting between the columnar cells, in which case they carry 
their burden of fat through the epithelium into the villus, or while wander- 
ing in the labyrinth of the villus bear it away bodily into the lymphatic 
system. But the number of leucocytes really containing any appreciable 
quantity of fat is too small to account for the amount of fat absorbed ; 
Nor is the abundance of leucocytes in the mucous membrane during the 
period of digestion a sure proof that they are concerned in absorption, but 
rather an indication only that active changes of some kind are going on, 
since after the administration of a saline such as magnesium sulphate, which 
produces effects the very reverse of absorption, these leucocytes are present 
in unusual numbers. Moreover, under some circumstances, as in the villi 
of a new-born puppy after a meal of milk, they are absent even when 
digestion of fat is rapidly going on and the lacteals are filling with fat. In 
fact, what we stated above concerning the presence of fat in the bodies of 
the columnar cells shows that leucocytes can have little to do in transferring 
fat from the interior of the intestine into the body of the villus; and there 
are no adequate reasons for attributing to them any real share in the trans- 
ference of fat from the body of the villus into the lacteal chamber. 
§ 264. The lacteal chamber opens at the base of the villus into the 
valved lymphatic vessels lying below, and in these the flow of lymph (chyle) 
is being promoted by the various causes detailed in § 253. The pressure, 
for instance, exerted by the peristaltic contractions of the intestine helps to 
empty the lymphatic vessel into which a lacteal chamber opens and so pro- 
motes the emptying of the latter. In addition to this the plain muscular 
fibres of the villus supply a special muscular pump for the emptying and 
filling of the lacteal chamber. These fibres and small bundles of fibres, ABSORPTION FROM THE ALIMENTARY CANAL. 
331 
though running in various directions and varying in number and arrange- 
ment in different animals, take on the whole a longitudinal direction 
parallel to the long axis of the villus. It has been supposed that in con- 
tracting and shortening the villus they compress the lacteal and thus empty 
it, and that when they relax and the villus elongates again, the emptied 
chamber fills once more. But a different interpretation of their action has 
been offered somewhat as follows. When the muscular fibres contract they 
shorten the villus. In thus becoming shorter the body of the villus becomes 
proportionately broader, since probably no great change of bulk in the 
reticulum takes place; in this broadening the part to give way will be the 
lacteal chamber, which thus becomes broader and larger. When the mus- 
cular fibres relax, the reticulum, the bars of which have been put on the 
stretch in a lateral direction, by elastic reaction brings back the villus to its 
former length, and the lacteal chamber elongates and narrows. On this 
view the muscular contraction expands and so fills, while the relaxation 
narrows and so empties, the lacteal chamber. Whichever view we adopt, we 
may at least conclude that contractions and relaxations of the muscular 
fibres in some way or other alternately fill and empty the lacteal chamber, 
and in all probability, at all events during digestion, rhythmical contractions 
of these fibres are continually going on. When the villus is shortened by 
the contraction of the muscular fibres, the columnar cells are compressed, 
becoming longer and narrower; when the muscular fibres relax and the 
villus elongates, the columnar cells return to their previous form. The 
alternating changes of form to which the columnar cells are thus subjected, 
and the alternating changes of pressure taking place in the reticulum, may 
also serve to promote the passage of material through the one and through 
the other. 
§ 265. The absorption of diffusible substances and of water. On the pro- 
visional assumption which we have made that the proteids are converted 
into peptone, we may consider, for the present at all events, peptone, sugar, 
and soluble salts as together forming a class distinguished from fats by their 
being diffusible, some more so than others. And we have made the further 
provisional assumption that these pass into the bloodvessels and not into the 
lacteals. 
The network of capillary bloodvessels is spread immediately beneath the 
basement membrane, and all the material which enters the lacteal chamber 
has to run the gauntlet of the meshes of this network. During digestion 
the capillaries of the intestine are filled and distended, so that at a time 
when absorption is taking place these meshes between the capillaries are 
unusually narrow. From the interior of these capillaires, as elsewhere, 
transudation is taking place; these capillaries supply the lymph which 
helps to fill up the labyrinth of the reticulum and the lacteal chamber. 
But to a much greater extent than elsewhere (cf. § 255) this current of 
transudation from within the capillary to without is accompanied by a 
reverse current from without to within. The diffusible substances in 
question pass from the intestine through the layer of epithelial cells, 
through the attenuated reticular lymph-space between the basement mem- 
brane and the capillary wall, and through the capillary wall into the blood 
current. Their passage consists of two stages: that through the epithelial 
cells from the intestine to the lymph-space, and that from the lymph-space 
into the bloodvessels. These two stages may be expected to differ, seeing 
that the structures concerned are different; but we may at first consider 
them as one, and speak of the passage from the intestine into the blood as 
a single event. 
In speaking of these substances as diffusible, we are using the term in 332 
RESPIRATION. 
reference to the well-known passage of such substances through thin mem- 
branes or porous partitions. When a strong solution of sugar or of common 
salt is separated by a thin membrane (vegetable parchment, dead urinary 
bladder, dead intestine, etc.) from a weak solution of sugar or of salt, the 
sugar or salt passes with a certain rapidity from the stronger to the weaker 
solution, and water passes from the weaker solution to the stronger ; if, to 
begin with, simple water be substituted for the weaker solution, the effect is 
at first still more striking. Peptone passes in the same manner, but as we 
have seen, much more slowly. The process is spoken of as a physical one, 
since it is not accompanied necessarily by any chemical change in the dif- 
fusing substance, nor is there any necessary change in the membrane or 
partition. The rate at which a substance diffuses, and the total amount of 
diffusion which can take place, are determined by certain qualities of the 
substance (which we may call physical, though they depend on the chemical 
nature of the substance) in relation to certain qualities of the membrane; 
thus two salts may diffuse through the same membrane at different rates, 
with different rates in the associated current of water, the osmotic current 
as it is called, from the weaker to the stronger solution ; and the same sub- 
stance may pass at different rates through different membranes. By a num- 
ber of observations, in which various substances in solution and several 
known membranes or partitions have been employed, a certain number of 
" laws of diffusion " have been established. 
Now if, by the statement that diffusible substances pass by diffusion into 
the blood-capillaries of the intestine, we are led to expect that the passage 
takes place exactly according to the laws established by observations on 
ordinary membranes, we should be led into error; for the disappearance of 
these substances from the interior of the intestine does not take place accord- 
ing to the laws which regulate their disappearance from one side of an 
ordinary diffusion septum. This can be ascertained by introducing solutions 
of the substances, of various strength, into a loop of intestine, isolated in the 
living animal by the method described in § 219, and watching their disap- 
pearance by analysis of the contents of the loop. For instance, sodium 
sulphate passes through an ordinary diffusion septum with a rapidity rather 
greater than that of dextrose, whereas dextrose disappears from the intestine 
distinctly more rapidly than sodium sulphate; peptone, which diffuses very 
slowly indeed through an ordinary diffusion septum, disappears rapidly 
(though not so rapidly as dextrose) from the intestine ; and when the details 
of the disappearance from the intestine of weak solutions of two salts which 
diffuse through an ordinary membrane at differents rates, which have, as it 
is said, different osmotic equivalents, are studied, these details are quite dif- 
ferent from those of ordinary diffusion. The more the matter is studied, the 
more decidedly apparent becomes the difference between ordinary diffusion 
and the absorption of diffusible substances from the intestine. 
CHAPTER II. 
The Structure of the Lungs and Bronchial Passages. 
RESPIRATION. 
§ 266. One particular item of the body's income, viz., oxygen, is pecu- 
liarly associated with one particular item of the body's waste, viz., carbonic 
acid, inasmuch as the means which are applied for the introduction of the THE MECHANICS OF PULMONARY RESPIRATION. 
333 
former are also used for the getting rid of the latter. Both are gases, and 
the ingress of the one as well as the egress of the other is far more depend- 
ent on the simple physical process of diffusion than on any active vital pro- 
cesses carried on by means of tissues. Oxygen passes from the air into the 
blood mainly by diffusion, and mainly by diffusion also from the blood into 
the tissues; in the same way carbonic acid passes mainly by diffusion from 
the tissues into the blood and from the blood into the air. Whereas, as we 
have seen in the secretion of the digestive juices the epithelial cell plays an 
all-important part, in respiration the entrance of oxygen from the lungs into 
the blood, and from the blood into the tissue, and the passage of carbonic 
acid in the contrary direction, are affected, if at all, in a wholly subordinate 
manner, by the behavior of the pulmonary, or of the capillary epithelium. 
What we have to deal with in respiration, then, is not so much the vital 
activities of any particular tissue, as the various mechanisms by which a 
rapid interchange between the air and the blood is effected, the means by 
which the blood is enabled to carry oxygen and carbonic acid to and from 
the tissues, and the manner in which the several tissues take oxygen from 
and give carbonic acid up to the blood. We have reasons for thinking that 
oxygen can be taken into the blood, not only from the lungs but also to a 
certain small extent from the skin, and, as we have seen, from the alimen- 
tary canal also ; and carbonic acid certainly passes away from the skin, and 
through the various secretions, as well as by the lungs. Still the lungs are 
so eminently the channel of the interchange of gases between the body and 
the air, that in dealing at present with respiration, we shall confine our- 
selves entirely to pulmonary respiration, leaving the consideration of the 
subsidiary respiratory processes till we come to study the secretions of which 
they respectively form part. 
The Mechanics of Pulmonary Respiration. 
§ 267. The lungs are placed, in a state which is always one of disten- 
tion, sometimes greater, sometimes less, in the air-tight thorax, the cavity 
of which they, together with the heart, great bloodvessels, and other organs, 
completely fill. By the contraction of certain muscles the cavity of the 
thorax is enlarged. The lungs must follow this enlargement and be them- 
selves enlarged, otherwise the pleural cavities would be enlarged ; but this 
is impossible so long as the walls are intact. The enlargement of the lung 
consists chiefly in an enlargement or expansion of the pulmonary alveoli, 
the air in which becomes, by the expansion, rarefied. That is to say, the 
pressure of the air within the lungs becomes less than that of the air out- 
side the body, and this difference of pressure causes a rush of air through 
the trachea into the lungs until an equilibrium of pressure is established 
between the air inside the lungs and that outside. This constitutes inspir- 
ation. On relaxation of the respiratory muscles (the muscles whose con- 
tractions have brought about the thoracic expansion), the elasticity of the 
lungs and chest-walls, aided, perhaps, to some extent by the contraction of 
certain muscles, causes the chest to return to its original size ; in consequence 
of this the pressure within the lungs becomes greater than that outside, and 
thus air rushes out of the trachea until equilibrium is once more established. 
This constitutes expiration ; the inspiratory and expiratory act together form- 
ing a respiration. The fresh air introduced into the upper part of the pul- 
monary passages by the inspiratory movement contains more oxygen and less 
carbonic acid than the old air previously present in the lungs. By diffusion 
the new or tidal air, as it is frequently called, gives up its oxygen to, and 
takes carbonic acid from, the old or stationary air, as it has been called, 334 
RESPIRATION. 
and thus when it leaves the chest in expiration has been the means of' both 
introducing oxygen into the chest and of removing carbonic acid from 
it. In this wav, by the ebb and flow of the tidal air, and by diffusion 
between it and the stationary air, the whole air in the lungs is being con- 
stantly renewed through the alternate expansions and contractions of the 
chest. 
§ 268. In ordinary respiration the expansion of the chest never reaches 
its maximum; by more forcible muscular contraction, by what is called 
labored inspiration, an additional thoracic expansion can be brought about, 
leading to an inrush of a certain additional quantity of air before equilib- 
rium is established. This additional quantity is often spoken of as comple- 
mented air. In the same way in ordinary respiration the contraction of the 
chest never reaches its maximum. By calling into use additional muscles, 
by a labored expiration an additional quantity of air, the so-called reserve 
or supplemental air, may be driven out. But even after the most forcible 
expiration, a considerable quantity of air, the residual air, still remains in 
the lungs. The natural condition of the lungs in the chest is, in fact, one 
of partial distention. The elastic pulmonary tissue is always to a certain 
extent on the stretch ; it is always, so to speak, striving to pull asunder 
the pulmonary from the parietal pleura ; but this it cannot do, because 
the air can have no access to the pleural cavity. When, however, the 
chest ceases to be air-tight, when by a puncture of the chest-wall or dia- 
phragm air is freely introduced into the pleural chamber, the elasticity 
of the lungs pulls the pulmonary away from the parietal pleura and the 
lungs collapse, driving out by the windpipe a considerable quantity of 
the residual air. Even then, however, the lungs are not completely 
emptied, some air still remaining in them ; this is probably air impris- 
oned in the infundibula by collapse of the bronchioles, which, as we have 
seen, have flaccid and not rigid walls. If, in a living animal, the pres- 
sure of the atmosphere continue to have access to the outside of a lung, 
the air thus imprisoned is gradually absorbed and the lung becomes solid. 
The same result may occur from the pressure of fluid accumulated in the 
pleural cavity. 
It need hardly be added that when the pleura is punctured and air can 
gain free admittance from the exterior in the pleural chamber, since the re- 
sistance to the entrance of the air into the pleural chamber is far less than 
the resistance to the entrance into the lungs, the effect of the respiratory 
movements is simply to drive air in and out of that chamber, instead of in 
and out of the lung. There is, in consequence, no renewal of the air within 
the lungs under those circumstances. If there be a sufficient obstacle to 
the entrance of air into the pleural chamber, such as a fold of tissue block- 
ing up the opening, the expansion of the chest may still lead to a distention 
of the lungs, and in this way, in some cases, puncture of the chest-walls has 
not seriously interfered with respiration. The parietal and pulmonary pleura 
are, in normal circumstances, separated by a very thin layer only of fluid, 
so that we may, perhaps, speak of them as being in a state of " adhesion," 
such as obtains between two wet membranes superimposed. And it has been 
suggested that this adhesion, having to be overcome before the two surfaces 
can separate, assists in preventing the entrance of air into the pleural cavity 
after puncture of the thorax ; but it has not been clearly shown that this is 
really of importance in the matter. 
§ 269. Before birth the lungs contain no air ; they are in the condition 
called atelectatic. The walls of the alveoli, the epithelial lining of which 
is, at that time, well developed, consisting of distinctly nucleated cells with 
granular cell-substance, are in contact, the cavity of the alveolus not having THE MECHANICS OF PULMONARY RESPIRATION. 
335 
as yet come into existence; the walls of the bronchioles are similarly in a 
collapsed condition, with their walls touching ; the more rigid bronchia, like 
the trachea, possess some amount of lumen, which, however, is occupied by 
fluid. When the chest expands with the first breath taken, the pressure of 
the inspired air has to overcome the" adhesion ''obtaining between the walls 
of the alveoli, thus in contact with each other and also those of the bronchi- 
oles. The force spent in thus opening out and unfolding, so to speak, the 
alveoli and bronchioles is considerable, and in the expiration succeeding the 
first inspiration most of the air thus introduced remains, the force exerted 
by the chest in returning to its previous dimensions after the breathing in 
and the elastic action of the alveoli being insufficient to bring the walls of 
the alveoli again into contact. Succeeding breaths unfold the lungs more 
and more, until all the alveoli and bronchioles are opened up, and then the 
whole force of the expiratory act is directed to driving out the previously 
inspired air. 
It is not, however, until some time after birth that the lungs pass into 
that further distended state of which we spoke above. In a newly-born 
animal there is no negative pressure obtaining in the pleural cavities; the 
lungs, when at rest, are not on the stretch, and opening the thorax does 
not lead to collapse of the lungs. The state of things obtaining later on 
is established, not at once, but gradually, and is apparently brought about 
by the thorax growing more rapidly, and so becoming relatively more 
capacious than the lungs. The distention of the lungs in the adult may 
be familiarly described as being due to the chest being too large for the 
lungs. 
§ 270. In man the pressure exerted by the elasticity of the lungs alone 
amounts to about 5 or 7 mm. of mercury. This is estimated by tying 
a manometer into the windpipe of a dead subject and observing the rise 
of mercury which takes place when the chest-walls are punctured. If we 
took 7.6 mm. as the pressure this would be just of the pressure of 
the atmosphere. If the chest be forcibly distended beforehand, a much 
larger rise of the mercury is observed, amounting, in the case of a dis- 
tention corresponding to a very forcible inspiration, to 30 mm. In the 
living body this mechanical elastic force of the lungs may be assisted by 
the contraction of the plain muscular fibres of the bronchi ; the pres- 
sure, however, which can be exerted by these probably does not exceed 
1 or 2 mm. 
When a manometer is introduced into a lateral opening of the windpipe 
of an animal, the mercury will fall, indicating a negative pressure, as it is 
called, during inspiration, and rise, indicating a positive pressure, during 
expiration, both fall and rise being slight and varying according to the 
freedom with which the air passes in and out of the chest. When a man- 
ometer is fitted with air-tight closure into the mouth, or better, in order to 
avoid the suction-action of the mouth, into one nostril, the other nostril and 
the mouth being closed, and efforts of inspiration and expiration are made, 
the mercury falls or undergoes negative pressure with inspiration, and rises 
or undergoes positive pressure during expiration. It has been found in this 
way that the negative pressure of a strong inspiratory effort may vary from 
30 to 74 mm., and the positive pressure of a strong expiration from 62 to 
100 mm. 
The total amount of air which can be given out by the most forcible 
expiration following upon the most forcible inspiration, that is, the sum of 
the complemental, tidal, and reserve airs, has been called the " vital capa- 
city "extreme differential capacity " is a better phrase. It may be meas- 
ured by a modification of a gas-meter called a spirometer; and though it 336 
RESPIRATION. 
varies largely, the average may be put down at 3000-4000 c.c. (200 to 
250 cubic inches). 
Of the whole measure of vital capacity, about 500 c.c. (30 cubic inches) 
may be put down as the average amount of tidal air, the remainder being 
nearly equally divided between the complemental and reserve airs. The 
quantity left in the lungs after the deepest expiration amounts to about 
1400 to 2000 c.c. 
Since the respiratory movements are so easily affected by various circumstances, 
the simple fact of attention being directed to the breathing being sufficient to cause 
modifications both of the rate and depth of the respiration, it becomes very diffi- 
cult to fix the volume of an average breath. Thus various authors have given 
figures varying from 53 c.c. to 792 c.c. The statement made above is the mean 
of observations varying from 177 to 699 c.c. 
§ 271. Graphic records of respiratory movements. These may be ob- 
tained in various ways. 
The simplest, readiest, and perhaps the most generally useful method is that of 
recording the movements of the column of air. This may be effected by intro- 
ducing a T-piece into the trachea, one cross-piece being left open and the other 
connected with a Marey's tambour or with a receiver, which in turn is connected 
with a tambour (see Fig. 54 and Fig. 90). The movements of the column of air 
in the trachea are transmitted to the tambour, the consequent expansions and 
contractions of which are transmitted to the recording drum by means of a lever 
resting on it. 
If, a receiver being used, the open end of the I- be closed, the animal breathes 
into and out of the receiver, and the movements of the tambour are greatly in- 
creased. This has the disadvantage that the air in the receiver soon becomes 
unfit for further respiration. A similar increase of the movements of the lever 
of the tambour may be obtained by connecting a piece of India-rubber tubing to 
the open end of the I-. By increasing the length of this tube, or slightly con- 
tracting it, the movements of the lever may be increased without very seriously 
interfering with the breathing of the animal. 
In another method the movements of the chest are recorded. When a small 
animal, such as a rabbit, is used, the whole animal may be placed in an air-tight 
box, breathing being carried on by means of a tube inserted into the trachea and 
carried through an air-tight orifice in the wall of the box. By another orifice 
and tube the air in the box is brought into connection with a tambour, which 
accordingly registers the changes of pressure in the air of the box produced by 
the movements of the chest (and body), and thus indirectly the movements of 
the chest. In man and larger animals the changes in the girth of the chest may 
be conveniently recorded by means of Marey's pneumograph. This consists of a 
hollow elastic cylinder, or a cylinder with elastic ends, the interior of which is 
connected with a tambour. By means of a strap attached to each end of the 
cylinder the instrument can be buckled round the chest like a girdle. When the 
chest expands, the ends of the cylinder are pulled out, and the air within the 
chamber rarefied; in consequence the lever of the tambour connected with its 
interior is depressed; conversely, when the chest contracts, the lever is elevated. 
The pneumatograph of Fick is somewhat similar. Or changes in one or other 
diameter of the chest maybe recorded by what maybe called the "calipers" 
method, as in the recording stethometer of Burdon-Sanderson. This consists of 
a rectangular framework constructed of two rigid parallel bars joined at right 
angles to a cross-piece. The free ends of the bars, the distance between which 
can be regulated at pleasure, are armed, the one with a tambour, the other simply 
with an ivory button. The tambour bears on the metal plate of its membrane 
(m', Fig. 54) a small ivory button in place of the lever. When it is desired to 
record the changes occurring in any diameter of the chest, e. (/., an antero-pos- 
terior diameter from a point in the sternum to a point in the back, the instrument 
is made to encircle the chest somewhat after the fashion of a pair of calipers, the 
ivory button at one free end being placed on the spine of a vertebra behind and 
the tambour at the other on the sternum in front in the line of the diameter which THE MECHANICS OF PULMONARY RESPIRATION. 
337 
Fig. 90. 
Apparatus for taking Tracings of the Movements of the Column of Air in Respiration. The 
recording apparatus shown is the ordinary cylinder recording apparatus. The cylinder A, covered 
with smoked paper, is by means of the friction- plate B put into revolution by the spring clock- 
work in C, regulated by Foucault's regulator D. By means of the screw E, the cylinder can be 
raised or lowered, and by means of the screw Fits speed may be increased or diminished. 
The tracheotomy tube t fixed in the trachea of an animal is connected by India-rubber tubing 
a with a glass T-piece inserted into the large jar G. From the other end of the T-piece proceeds 
a second piece of tubing b, the end of which can be either closed or partially obstructed at pleas- 338 
RESPIRATION. 
is being studied. The distance between the free ends of the instrument being 
carefully adjusted so that the button of the tambour presses lightly on the sternum, 
any variations in the length of the diameter in question will, since the framework 
of the tambour is immobile, give rise to variations of pressure within the tambour. 
These variations of the "receiving" tambour, as it is called, are conveyed by a 
flexible tube containing air to a second or "recording" tambour, the lever of 
which records the variations on a travelling surface. For the purpose of measuring 
the extent of the movements the instrument must be experimentally graduated. 
Other forms of calipers may, of course, be used. 
By still another method the variations in intra-thoracic pressure, by means of 
which the movements of the chest walls produce the movement of air in the 
lungs, may be recorded. This may be effected by introducing carefully, to the 
total exclusion of air, into a pleural cavity or into the pericardial cavity, a canula 
connected by a rigid tube with a manometer. With each inspiration a negative 
pressure, or rather an increase of the existing negative pressure, is produced, the 
mercury, or fluid, in the manometer returning at each expiration. An easier 
method of recording this intra-thoracic pressure is to introduce into the oesophagus 
an elistic sound (similar to the cardiac sound, Fig. 54) connected with a tambour. 
The (esophagus within the thorax, like the heart an I great vessels, as we shall see, 
is affected as well as the lungs by the variations of intra-thoracic pressure brought 
about by the respiratory movements. 
In yet another method the movements of the diaphragm, which, as we shall see, 
serve as the prime agent in bringing about the enlargement of the thoracic cavity, 
are recorded. This may be done by inserting, through an incision in the abdominal 
wall, a flat clastic bag between the diaphragm and abdominal organs. When in 
inspiration the diaphragm descends, it exerts on the bag a pressure which, by 
means of a tube, may be communicated to a tambour. Or a needle may be thrust 
through the chest wall so as to rest upon or transfix the diaphragm, and the head 
of the needle outside the body connected by a thread or otherwise with a lever; each 
upward and downward movement of the head of the needle, corresponding to the 
downward and upward movements of the diaphragm, is registered by the lever. 
Various modifications of these several methods have been adopted by various 
observers. They all, however, leave much to be desired. A very ingenious method 
of registering the contractions of the diaphragm has recently been introduced. Tn 
the rabbit two slips of muscular fibres forming part of the diaphragm, one on each 
side of the ensiform cartilage, are so disposed and possess such attachments that 
one or both of them maybe isolated without injury to either nerves or bloodvessels, 
and arranged so that while one end of the slip is securely fixed to the chest-wall as 
a fixed point, the other end can by a thread be brought to bear on a lever. The 
slip, even when thus arranged, appears to contract rhythmically in complete unison 
with the contractions of the whole of the rest of the diaphragm: it serves, so to 
speak, as a sample of the diaphragm; and hence its contractions, like those of the 
whole diaphragm, may be taken as a record of respiratory movements. 'The record 
has to be corrected for variations in the position of the fixed point. 
§ 272. In these various ways curves are obtained, which, while differing 
in detail, exhibit the same general features, and more or less resemble the 
curve shown in Fig. 91. 
As the figure shows, inspiration begins somewhat suddenly and advances 
rapidly, being followed immediately by expiration, which is carried out at 
first rapidly, but afterward more and more slowly. Such pauses as are seen 
usually occur between the end of expiration and the beginning of inspira- 
ure by means of the screw clamp c. From the jar proceeds a third piece of tubing d, connected 
with a Marey's tambour m (see Fig. 54), the lever of which Z writes on the recording surface. 
When the tube b is open the animal breathes freely through this, and the movements in the air 
of G and consequently in tlie tambour are slight. On closing the clamp c, the animal breathes 
only the air contained in the jar, and the movements of the lever of the tambour become conse- 
quently much more marked. 
Below the lever is seen a small time-marker n connected with an electro-magnet, the current 
through which, coming from a battery by the wires x and y, is made and broken by a clock-work 
or metronome. THE MECHANICS OF PULMONARY RESPIRATION. 
339 
tion. In normal breathing hardly any such pause exists, but in cases where 
the respiration becomes infrequent, pauses of considerable length may be 
observed. As we shall see in detail hereafter, the several parts of the whole 
act vary much, under various circumstances, in relation to each other. 
Sometimes expiration, sometimes inspiration is prolonged ; and either inspi- 
[Fig. 91. 
Tracing of Thoracic Respiratory Movements obtained by means of Marey's Pneumograph. 
A whole respiratory phase is comprised between a and a; inspiration, during which the lever 
descends, extending from a to b, and expiration from b to a. The undulations at c are caused by 
the heart's beat.] 
ration or expiration may be slow or rapid in its development. At times the 
chest may remain for a while at the height of inspiration, thus making a 
pause between inspiration and expiration. 
In what may be considered as normal breathing the respiratory act is re- 
peated about seventeen times a minute, the duration of the inspiration as 
compared with that of the expiration (and such pause as may exist) being 
about as ten to twelve ; but the rate varies very largely, and in this, as in 
the volume of each breath, it is very difficult to fix a satisfactory average, 
the figures given varying from twenty to thirteen a minute. It varies 
according to age and sex. It is influenced by the position of the body, 
being quicker in standing than in lying, and in lying than in sitting. Mus- 
cular exertion and emotional conditions affect it deeply. In fact, almost 
every event which occurs in the body may influence it. We shall have to 
consider in detail hereafter the manner in which these influences are brought 
to bear. 
When the ordinary respiratory movements prove insufficient to effect the 
necessary changes in the blood, their rhythm and character become changed. 
Normal respiration gives place to labored respiration, and this in turn to 
dyspnoea, which, unless some restorative event occurs, terminates in asphyxia. 
These abnormal conditions we shall study more fully hereafter. 
The Respiratory Movements. 
§ 273. When the movements of the chest during normal breathing are 
watched, or when a graphic record is taken by one or other of the methods 
just described, it is seen that during inspiration an enlargement takes place 
in the antero-posterior diameter, the sternum being thrown forward, and at 
the same time moving upward. The lateral width of the chest is also in- 
creased. The vertical increase of the cavity is not so obvious from the out- 
side, though when the movements of the diaphragm are watched by means 
of an inserted needle or otherwise, it is clear that the upper surface of that 
organ descends at each inspiration, the anterior walls of the abdomen bulging 
out at the same time. In the female human subject, the movement of the 
upper part of the chest is very conspicuous, the breast rising and falling 
with every respiration; in the male, however, the movements are almost en- 340 
RESPIRATION. 
tirely confined to the lower part of the chest. In labored respiration all 
parts of the chest are alternately expanded and contracted, the breast rising 
and falling as well in the male as in the female. We have now to consider 
these several movements in greater detail, and to study the means by which 
they are carried out. 
§ 274. Inspiration. There are two chief means by which the chest is 
enlarged in normal inspiration, viz., the descent of the diaphragm and the 
elevation of the ribs. The former causes that movement in the lower part 
of the chest and abdomen so characteristic of male breathing, which is hence 
called diaphragmatic; the latter causes the movement of the upper chest 
characteristic of female breathing, which is called costal. These two main 
factors are assisted by less important and subsidiary events. 
Even in the female human subject, the share taken in respiration by the 
diaphragm is an important one ; in the male the diaphragm must be regarded 
as the chief respiratory agent, and in some animals its use, for this purpose, 
is so prominent that the movements of the ribs may in normal breathing be 
almost neglected. In the rabbit for instance, in normal breathing, almost 
all the respiratory work is done by the contractions of the diaphragm. 
The descent of the diaphragm is effected by means of the contraction of 
its muscular fibres. When at rest the diaphragm presents a convex surface 
to the thorax ; when contracted it becomes much flatter, and in consequence 
the level of the chest-floor is lowered, the vertical diameter of the chest being 
proportionately enlarged. In descending, the diaphragm presses on the 
abdominal viscera, and so causes a projection of the flaccid abdominal walls. 
From its attachments to the sternum and the false ribs, the diaphragm, while 
contracting, naturally tends to pull the sternum and the upper false ribs 
downward and inward, and the lower false ribs upward and inward, toward 
the lumbar spine. In normal breathing, this tendency produces little effect, 
being counteracted by the accompanying general costal elevation, and by 
certain special muscles to be mentioned presently. In forced inspiration, 
however, and especially where there is any obstruction to the entrance of air 
into the lungs, the lower ribs may be so much drawn in by the contraction 
of the diaphragm that the girth of the trunk at this point is obviously 
diminished. 
§ 275. The elevation of the ribs is a much more complex matter than the 
descent of the diaphragm. If we examine any one rib, such as the fifth, we 
find that while it moves freely on its vertebral articulation, it inclines when 
in the position of rest in an oblique direction from the spine to the sternum ; 
hence it is obvious that when the rib is raised, its sternal attachment must 
not only be carried upward but also thrown forward. The rib may in fact 
be regarded as a radius, moving on the vertebral articulation as a centre, 
and causing the sternal attachment to describe an arc of a circle in the 
vertical plane of the body; as the rib is carried upward from an oblique 
to a more horizontal position, the sternal attachment must of necessity be 
carried further away in front of the spine. Since ail the ribs have a down- 
ward slanting direction, they must all tend, when raised toward the hori- 
zontal position, to thrust the sternum forward, some more than others, 
according to their slope and length. The elasticity of the sternum and 
costal cartilages, assisted by the articulation of the sternum to the clavicle 
above, permits the front surface of the chest to be thus thrust forward as 
well as upward, when the ribs are raised. By this action, the antero-pos- 
terior diameter of the chest is enlarged. 
Since the ribs form arches which increase in their sweep as one pro- 
ceeds from the first downward as far at least as the seventh, it is evident that 
when a lower rib such as the fifth is elevated so as to occupy or to approach THE MECHANICS OF PULMONARY RESPIRATION. 
341 
toward the position of the one above it, the chest at that level will become 
wider from side to side, in proportion as the fifth arch is wider than the 
fourth. Thus the elevation of the rib increases not only the antero-posterior 
but also the transverse diameter of the chest. Further, on account of the 
resistance of the sternum, the angles between the ribs and their cartilages 
are, in the elevation of the ribs, somewhat opened out, and thus also the 
transverse as well as the antero-posterior diameter, somewhat increased. In 
more than one way, then, the elevation of the ribs enlarges the dimensions 
of the chest. 
§ 276. The ribs are raised by the contraction of certain muscles. Of 
these the external intercostals are perhaps the most important. Even in the 
case where two ribs, such as the fifth and sixth, are isolated from the rest of 
the thoracic cage, by section of the structures occupying the intercostal spaces 
above and below, the contraction of the external intercostal muscle of the 
intervening space raises the two ribs, thus bringing them toward the posi- 
tion in which the fibres of the muscle have the shortest length, viz., the hori- 
zontal one. This elevating action is, in the entire chest, further favored by 
the fact that the first rib is less movable than the second, and so affords a 
comparatively fixed base for the action of the muscles between the two, the 
second in turn supporting the third, and so on, while the scaleni muscles in 
addition serve to render fixed, or to raise, the first two ribs. So that in 
normal respiration, the act may probably be described as beginning by a 
contraction of the scaleni. The first two ribs being thus raised or at least 
fixed, the contraction of the series of external intercostal muscles acts at a 
great disadvantage. 
While the elevating, i. e., inspiratory, action of the external intercostals 
is admitted by nearly all authors, the function of the internal intercostals has 
been much disputed. Some regard their action as wholly inspiratory ; others 
maintain, what is perhaps the more commonly adopted view, that while those 
parts of them which lie between the sternal cartilages act like the external 
intercostals as elevators, i. e., as inspiratory in function, those parts which 
lie between the osseous ribs act as depressors, i. e., as expiratory in func- 
tion. 
In the well-known model consisting of two rigid bars representing the 
ribs, moving vertically by means of their articulations within an upright 
representing the spine, and connected at their free ends by a piece repre- 
senting the sternum, it is undoubtedly true that stretched elastic bands 
attached to the bars in such a way as to represent respectively the external 
and internal intercostals, viz., sloping in the one case downward and forward, 
and in the other downward and backward, do, on being left free to contract, 
in the former case elevate and in the latter depress the ribs. Such a model, 
however, does not fairly represent the natural conditions of the ribs, which 
are not straight and rigid, but peculiarly curved and of varying elasticity, 
capable moreover of rotation on their own axes, and having their move- 
ments determined by the characters of their vertebral articulations. The 
mechanical conditions in fact of these muscles are so complex, that a deduc- 
tion of their actions from simple mechanical principles, or from the direc- 
tion of the fibres, must be exceedingly difficult and dangerous. Actual ex- 
periments on the cat and dog tend to show that in these animals the contrac- 
tion of the internal intercostals, along their whole length, takes place, in 
point of time, alternately with that of the diaphragm, and thus offer an 
argument in favor of these muscles being expiratory in function. 
Next in importance to the external intercostals come the levatores 
costarum, which, though small muscles, are able, from the nearness of their 
costal insertions to the fulcrum, to produce considerable movement of the 342 
RESPIRATION. 
sternal ends of the ribs. The external intercostals and the levatores costaruin 
with the scaleni may fairly be said to be the elevators of the ribs, i. e., the 
chief muscles-of costal inspiration in normal breathing. 
It must be added, however, that some observers deny that either set of 
intercostal muscles take any important part in raising the ribs. They hold 
that the chief if not the only use of these muscles is by their contraction to 
render the intercostal spaces firm and the whole thoracic cage rigid, so that 
the thorax is moved as a whole by the other muscles mentioned, and the 
intercostal spaces do not give way during the respiratory movements. 
Additional space in the transverse diameter is afforded probably by the 
rotation of the ribs on an antero-posterior axis; but this movement is quite 
subsidiary and unimportant. When the chest is at rest, the ribs are some- 
what inclined with their lower borders directed inward as well as downward. 
When they are drawn up by the action of the intercostal muscles, their 
lower borders are everted. Thus their flat sides are presented to the thoracic 
cavity, which is thereby slightly increased in width. 
§ 277. Labored inspiration. When respiration becomes labored, other 
muscles are brought into play. The scaleni are strongly contracted, so as 
distinctly to raise or at least give a very fixed support to the first and second 
ribs. In the same way the serratus posticus superior, which descends from 
the fixed spine in the lower cervical and upper dorsal regions to the second, 
third, fourth, and fifth ribs, by its contractions raises those ribs. In labored 
breathing a function of the lower false ribs, not very noticeable in easy 
breathing, comes into play. They are depressed, retracted, and fixed, thereby 
giving increased support to the diaphragm, and directing the whole energies 
of that muscle to the vertical enlargement of the chest. In this way the 
serratus posticus inferior, which passes upward from the lumbar aponeurosis 
to the last four ribs, by depressing and fixing those ribs becomes an adjuvant 
inspiratory muscle. The quadratus lumborum and lower portions of the 
sacro-lumbalis may have a similar function. 
All these muscles may come into action even in breathing which, though 
deeper th in usual, can hardly perhaps be called labored. When, however, 
the need for greater inspiratory efforts becomes urgent, all the muscles which 
can, from any fixed point, act in enlarging the chest, come into play. Thus 
the arms and shoulder being fixed, the serratus magnus passing from the 
scapula to the middle of the first eight or nine ribs, the pectoralis minor 
passing from the coracoid to the front parts of the third, fourth, and fifth 
ribs, the pectoralis major passing from the humerus to the costal cartilages, 
from the second to the sixth, and that portion of the latissimus dorsi which 
passes from the humerus to the last three ribs, all serve to elevate the ribs 
and thus to enlarge the chest. The sterno-mastoid and other muscles passing 
from the neck to the sternum, are also called into action. In fact, every 
muscle which by its contraction can either elevate the ribs or contribute to 
the fixed support of muscles which do elevate the ribs, such as the trapezius, 
levator anguli scapulae, and rhomboidei by fixing the scapula, may, in the 
inspiratory efforts which accompany dyspnoea, be brought into play. 
§ 278. Expiration. In normal easy breathing, expiration is in the main 
a simple effect of elastic reaction. By the inspiratory effort the elastic tissue 
of the lungs is put on the stretch ; so long as the inspiratory muscles continue 
contracting, the tissue remains stretched ; but directly those muscles relax, 
the elasticity of the lungs comes into play and drives out a portion of the 
air contained in them. Similarly the elastic sternum and costal cartilages 
are by the elevation of the ribs put on the stretch ; they are driven into a 
position which is unnatural to them. When the intercostal and other ele- 
vator muscles cease to contract, the elasticity of the sternum and costal car- THE MECHANICS OF PULMONARY RESPIRATION. 
343 
tilages cause them to return to their previous position, thus depressing the 
ribs and diminishing the dimensions of the chest. When the diaphragm 
descends, in pushing down the abdominal viscera, it puts the abdominal walls 
on the stretch ; and hence, when at the end of inspiration the diaphragm 
relaxes, the abdominal walls return to their place, and by pressing on the 
abdominal viscera, push the diaphragm up again into its position of rest. 
Expiration, then, during easy breathing is, in the main, simple elastic reac- 
tion ; but there is probably some, though possibly in most cases a very slight, 
expenditure of muscular energy to bring the chest more rapidly to its former 
condition. This is, as we have seen, supposed by many to be afforded by the 
internal intercostals acting as depressors of the ribs. If these do not act in 
this way, we may suppose that the elastic return of the abdominal walls is 
accompanied and assisted by a contraction of the abdominal muscles. The 
triangularis sterni, the effect of whose contraction is to pull down the costal 
cartilages, may also be regarded as an expiratory muscle. 
When expiration becomes labored, the abdominal muscles become im- 
portant expiratory agents. By pressing on the contents of the abdomen, 
they thrust them and therefore the diaphragm also, up toward the chest, the 
vertical diameter of which is thereby lessened, while by pulling down the 
sternum and the middle and lower ribs they lessen also the activity of the 
chest in its antero-posterior and transverse diameters. They are, in fact, 
the chief expiratory muscles, though they are doubtless assisted by the ser- 
ratus posticus inferior and portions of the sacro-lumbalis, since when the 
diaphragm is not contracting, the depression of the lower ribs which the 
contraction of these muscles causes, serves only to narrow the chest. As 
expiration becomes more and more forced, every muscle in the body which 
can either by contracting depress the ribs or press on the abdominal viscera, 
or afford fixed support to muscles having those actions, is called into play. 
§ 279. Facial and laryngeal respiration. The thoracic respiratory move- 
ments are accompanied by associated respiratory movements of other parts 
of the body, more particularly of the face and of the glottis. 
In normal healthy respiration the current of air which passes in and out 
of the lungs, travels, not through the mouth but through the nose, chiefly 
through the lower nasal meatus. The ingoing air, by exposure to the vas- 
cular mucous membrane of the narrow and winding nasal passages, is more 
efficiently warmed than it would be if it passed through the mouth ; and at 
the same time the mouth is thereby protected from the desiccating effect of 
the continual inroad of comparatively dry air. 
During each inspiratory effort the nostrils are expanded, probably by the 
action of the dilatores naris, and thus the entrance of air facilitated. The 
return to their previous condition during expiration is effected by the elas- 
ticity of the nasal cartilages, assisted perhaps by the compressores naris. 
This movement of the nostrils, perceptible in many people even during tran- 
quil breathing, becomes very obvious in labored respiration. 
When the mouth is closed, the soft palate, which is held somewhat tense, 
is swayed by the respiratory current, but entirely in a passive manner, and 
it is not until the larynx is reached by the ingoing air that any active move- 
ments are met with. When the larynx (the details of which we shall have 
to deal with at a later part of this work) is examined with the laryngoscope, 
it is frequently seen that, while during inspiration the glottis is widely open, 
with each expiration the arytenoid cartilages approach each other so as to 
narrow the glottis, the cartilages of Santorini projecting inward at the same 
time. Thus, synchronous with the respiratory expansion and contraction of 
the chest, and the respiratory elevation and depression of the alse nasi, there 
is a rhythmic widening and narrowing of the glottis. Like the movements 344 
RESPIRATION. 
of the nostril, this respiratory action of the glottis is much more evident in 
labored than in tranquil breathing. Indeed, in the latter case it is fre- 
quently absent. The manner in which this rhythmic opening and narrow- 
ing is effected will be described when we come to study the production of 
the voice. Whether there exists a rhythmic contraction and expansion of 
the trachea and bronchial passages, especially the smaller and more exclu- 
sively muscular ones, effected by means of the plain muscular tissue of those 
organs and synchronous with the respiratory movements of the chest, is un- 
certain. 
Changes of the Air in Respiration. 
§ 280. During its stay in the lungs, or rather during its stay in the bron- 
chial passages, the tidal air (by means of diffusion chiefly) effects exchanges 
with the stationary air ; in consequence the expired air differs from inspired 
air in several important particulars. 
The temperature of expired air is variable, but under ordinary circum- 
stances is higher than that of the inspired air. At an average temperature 
of the atmosphere, for instance at about 20° C., the temperature of expired 
air is in the mouth 33.9°, in the nose 35.3°. When the external tempera- 
ture is low, that of the expired air sinks somewhat, but not to any great 
extent, thus at -6.3° C. it is 29.8° C. When the external temperature is 
high, the expired air may become cooler than the inspired, thus at 41.9° it 
has been found to be 38.1°. The expired air takes its temperature from 
that of the body, that is, of the blood, and this as we shall see later on, while 
generally higher may, at times, be lower than that of the atmosphere. The 
exact temperature of the expired air in fact depends on the relative tempera- 
tures of the blood and inspired air, and on the depth and rate of breathing. 
The change in temperature takes place not in the lungs but in the upper 
passages, and chiefly in the nose and pharynx. 
§ 281. The expired air is loaded with aqueous vapor. The point of 
saturation of any gas, that is, the utmost quantity of water which any given 
volume of gas can take up as aqueous vapor, varies with its temperature, 
being higher with the higher temperature. For its own temperature expired 
air is, according to most observers, saturated with aqueous vapor. The 
moisture, like the warmth, is imparted not in the depths of the lung but in 
the upper passages. The inspired air as it passes into the bronchia is already 
saturated with moisture. 
§ 282. The expired air contains about 4 or 5 per cent, less oxygen, and 
about 4 per cent, more carbonic acid than the inspired air, the quantity of 
nitrogen suffering but little change. Thus 
Oxygen. 
Nitrogen. 
Carbonic acid. 
Inspired air contains • . 
. . 20.81 
79.15 
0.04 
Expired " " 
. . 16.033 
79.587 
4.38 
The quantity of nitrogen in the expired air is sometimes found to be 
slightly greater than as in the above table, but sometimes equal to, and 
sometimes less than, that of the inspired air. 
In a single breath the air is richer in carbonic acid (and poorer in 
oxygen) at the end than at the beginning of the breath. Hence, the longer 
the breath is held, the greater the (artificial) pause between inspiration and 
expiration, the higher the percentage of carbonic acid in the expired air. 
Thus, by increasing the interval between two expirations to 100 seconds, the 
percentage may be raised to 7.5. When the rate of breathing remains the 345 
CHANGES OF THE AIR IN RESPIRATION. 
same, by increasing the depth of the breathing the percentage of carbonic 
acid in each breath is lowered, but the total quantity of carbonic acid 
expired in a given time is increased. Similarly, when the depth of breath- 
ing remains the same, by quickening the rate the percentage of carbonic 
acid in each breath is lowered, but the quantity expired in a given time is 
increased. 
Taking, as we have done, the amount of tidal air passing in and out of 
the chest of an average man at 500 c.c., such a person will expire about 
22 c.c. of carbonic acid at each breath ; this, reckoning the rate of breath- 
ing at 17 a minute, would give over 500 litres of carbonic acid for the day's 
production. Actual determinations, however, give a rather smaller total 
than this ; thus, in a series of experiments of which we shall have to speak 
hereafter, the total daily excretion of carbonic acid in an average man was 
found to be 800 grins., i. e., rather more than 400 litres (406), containing 
218.1 grms. carbon and 581.9 grins, oxygen, the oxygen which actually dis- 
appeared from the inspired air at the same time being about 700 grms. 
This amount, it should be said, represents, owing to the manner in which the 
experiment was conducted, the gases given out and taken in, not by the 
lungs only, but by the whole body; but the amount of carbonic acid given 
out by other channels than the lungs is, as we shall see, very slight (10 grms. 
or even less), so that 800 grms. may be taken as the average production of 
carbonic acid by an average man. The quantity, however, both of oxygen 
consumed and of carbonic acid given out, is subject to very wide variations; 
thus, in the observations of which we are speaking, the daily quantity of 
carbonic acid varied from 686 to 1285 grms., and that of the oxygen from 
594 to 1072 grms. These variations and their causes will be discussed when 
we come to deal with the problems of nutrition. 
§ 283. When the total quantity of tidal air given out at any expiration 
is compared with that taken in at the corresponding inspiration, it is found 
that, both being dried and measured at the same temperature and pressure, 
the expired air is less in volume than the inspired air, the difference amount- 
ing to about ^th to ^th of the volume of the latter. Hence, when an 
animal is made to breathe in a confined space, the air is absolutely diminished 
in volume. The approximate equivalence in volume between inspired and 
expired air arises from the fact that the volume of any given quantity of 
carbonic acid is equal to the volume of the oxygen consumed to produce it; 
the slight falling short of the expired air is due to the circumstances that all 
the oxygen inspired does not reappear in the carbonic acid expired, some 
having formed within the body other combinations. 
§ 284. Besides carbonic acid, expired air contains various substances 
which may be spoken of as impurities, many of an unknown nature, and 
all in small amounts. Traces of ammonia have been detected in expired 
air, even in that taken directly from the trachea, in which case its presence 
could not be due to decomposing food lingering in the mouth. When 
the expired air is condensed by being conveyed into a cooled receiver, 
the aqueous product is found to contain organic matter, which from the 
presence of microorganisms introduced in the inspired air, is very apt 
rapidly to putrefy. The organic substances thus shown to be present in the 
expired air are the cause in part of the odor of breath. It is probable that 
some of them are of a poisonous nature, either poisonous in themselves as 
coming direct from and produced in some way or other in the pulmonary 
apparatus, or poisonous as being the products of putrefactive decomposition ; 
for various animal substances and fluids give rise by decomposition to dis- 
tinct poisonous products known as ptomaines, and it is possible that some of 
the constituents of expired air are of an allied nature. In any case the 346 
RESPIRATION. 
substances present have a deleterious action, for an atmosphere containing 
simply 1 per cent, of carbonic acid (with a corresponding diminution of 
oxygen) has very little effect on the animal economy, whereas an atmos- 
phere in which the carbonic acid has been raised to 1 per cent, by breath- 
ing is highly injurious. In fact, air rendered so far impure by breathing 
that the carbonic acid amounts to 0.08 per cent, is distinctly unwholesome, 
not so much on account of the carbonic acid as of the accompanying impuri- 
ties. Since these impurities are of unknown nature and cannot be estimated, 
the easily determined carbonic acid is usually taken as an indirect measure 
of their presence. We have seen that the average man loads at each breath 
500 c.c. of air with carbonic acid to the extent of 4 per cent. He will 
accordingly at each breath load 2 litres to the extent of 1 per cent.; and 
in one hour, if he breathe 17 times a minute, will load rather more than 
2000 litres to the same extent. At the very least, then, a man ought to be 
supplied with this quantity of air hourly, and if the air is to be kept fairly 
wholesome, that is with the carbonic acid reduced below 0.01 per cent., he 
should have even more than ten times as much. 
The Respiratory Changes in the Blood. 
§ 285. While the air in passing in and out of the lungs is thus robbed 
of a portion of its oxygen and loaded with a certain quantity of carbonic 
acid, the blood as it streams along the pulmonary capillaries undergoes 
important correlative changes. As it leaves the right ventricle it is venous 
blood of a dark purple or maroon color; when it falls into the left auricle 
it is arterial blood of a bright scarlet hue. In passing through the capil- 
laries of the body from the left to the right side of the heart it is again 
changed from the arterial to the venous condition. We have to inquire, 
What are the essential differences between arterial and venous blood, by 
what means is the venous blood changed into arterial in the lungs, and the 
arterial into venous in the rest of the body, and what relations do these 
changes in the blood bear to the changes in the air which we have already 
studied? 
The facts that venous blood at once becomes arterial in appearance on 
being exposed to or shaken up with air or oxygen, and that arterial blood 
becomes venous in appearance when kept for some little time in a closed 
vessel, or when submitted to a current of some indifferent gas such as 
nitrogen or hydrogen, prepare us for the statement that the fundamental 
difference between venous and arterial blood is in the relative proportion 
of the oxygen and carbonic acid gases contained in each. From both 
a certain quantity of gas can be extracted by means which do not other- 
wise materially alter the constitution of the blood ; and this gas when 
obtained from arterial blood is found to contain more oxygen and less 
carbonic acid than that obtained from venous blood. This is the real 
differential character in the two bloods; all other differences are either, 
as we shall see to be the case with the color, dependent on this, or are 
unimportant and fluctuating. 
If the quantity of gas which can be extracted by the mercurial air-pump 
from 100 volumes of blood be measured at 0° C. and a pressure of 700 mm., 
it is found to amount in round numbers to 60 volumes. 
The vacuum produced by the ordinary mechanical air-pump is insufficient to 
extract all the gas from blood. Hence it becomes necessary to use a mercury pump 
capable of producing a large Torricellian vacuum. In the form of mercurial pump 
which bears Ludwig's name (Fig. 92), two large globes of glass, one fixed and the 
other movable, are connected by a flexible tube : the fixed globe is made to com- THE RESPIRATORY CHANGES IN THE BLOOD. 
347 
municate by means of air-tight stopcocks alternately with a receiver containing the 
blood and with a receiver to collect the gas. When the movable globe filled 
with mercury is raised above the fixed one, the mercury from the former runs 
Fig. 92. 
Diagrammatic illustration of Ludwig's Mercurial Gas-pump: A and B are two glass globes con- 
nected by strong India-rubber tubes, a and b, with two similar glass globes, A' and B'. A is further 
connected by means of the stopcock c with the receiver Ccontaining the blood (or other fluid) to 
be analyzed, and B by means of the stopcock d and the tube e with the receiver D for receiving the 
gases. A and B are also connected with each other by means of the stopcocks / and g, the latter 
being so arranged that B also communicates with B' by the passage g'. A' and B1 being full of 
mercury, and the cocks k,f, g, and d being open, but c and g' closed, on raising A' by means of the 
pulley p the mercury of A' fills A, driving out the air contained in it into B, and so out through 
e. When the mercury has risen above g,f is closed, and g1 being opened, B' is in turn raised 
until B is completely filled with mercury, all the air previously in it being driven out through 
e. Upon closing d and lowering B' the whole of the mercury in B falls in B', and a vacuum con- 
sequently is established in B. On closing g', but opening g,f, and k, and lowering A', a vacuum 
is similarly established in A and in the junction between A and B. If the cock c be now opened, 
the gases of the blood in C escape into the vacuum of A and B. By raising A' after the closure 
of c and opening of d, the gases so set free are driven from A into B, and by the raising of B' 
from B, through e into the receiver D, standing over mercury. 
into and completely fills the latter, the air previously present being driven out. 
After adjusting the cocks, the movable globe is then depressed thirty inches 
below the fixed one, in which the consequent fall of the mercury produces an 
almost complete vacuum. By turning the proper cock this vacuum is put into 348 
RESPIRATION. 
connection with the receiver containing the blood, which thereupon becomes pro- 
portionately exhausted. By again adjusting the cocks and once more elevating 
the movable globe, the gas thus extracted is driven out of the fixed globe into 
a receiver. The vacuum is then once more established and the operation re- 
peated as long as gas continues to be given off from the blood. 
A modified form of pump working on the same principles as that of Ludwig, 
but involving the use of only one globe to be made vacuous and one movable 
reservoir for mercury, has been constructed by PHiiger. It presents several advan- 
tages over the one just described, the chief being that (1) non-defibrinated blood 
may be used for the extraction of gases, (2) the vacuum into which the gases are 
evolved is large, (3) this vacuum is kept dry by being connected laterally with a 
vacuous chamber containing sulphuric acid. The details of this construction are, 
however, complicated, and the greatest care is required in its use to avoid break- 
Fig. 93. 
Diagram of Alvergniat's Pump. 
age. Of later years a simplified form of pump lias been introduced for laboratory 
work, ft was first used by Gr6hant and Paul Bert, and is now frequently called an 
Alvergniat's pump, from the name of its present maker. Fig. 93 gives a diagram- 
matic representation of its construction. 
A is a glass bulb some five inches in diameter, blown on to a glass tube a below 
and on to a vertical tube b above. The lower end of a is connected by a thick- 
walled India-rubber tube with a reservoir for mercury B, which can be raised 
and lowered by means of a string passing over a pulley c. The vertical tube b 
is thickened at one place, and into this thickened portion a three-way tap d is 
ground. The upper end of b is prolonged (above the three-way tap) into a fine 
point. This point passes by a tight joint through the bottom of a vessel e, which 
can be partly filled with mercury, and over which a receiver f filled with mercury 349 
THE RESPIRATORY CHANGES IN THE BLOOD. 
for the collection of the gases, can be inverted. A tube g fused on laterally to 
one opening of the three-way tap d places the latter in connection with a thick- 
walled WoulfFs bottle C containing a layer of strong sulphuric acid. The second 
tubulure of this bottle is similarly connected by an elastic tube with the vessel 
/>, into which blood or other fluid may be introduced by means of the tap h. 
All the movable joints of the apparatus are protected by India-rubber tubes 
into which water can be poured, and a metal casing around the tap d, which 
may also be tilled with water, similarly prevents the possibility of any leakage 
here. 
The pump is used as follows: By placing the tap d in the position shown in the 
figure and raising B, the bulb A may be filled with mercury up to the top, the con- 
tained air being expelled through the upper end of b. By a slight turn of the tap 
all connection between A and either the tube g or the upper part of b may be cut 
off, and on lowering B a vacuum is established in the bulb A and part of the tube 
o. A may now be connected by the tap d with the tube g. and hence with C and 
D, and, h being closed, a partial vacuum is established in C and D. By means of 
the tap d the air in A may be cut off from g, and on raising B and placing the plug 
of d as shown in the figure this air may be expelled through the upper end of b. 
By slightly turning d and lowering B a vacuum is again established in A, and. as 
before, a further portion of air in C and I) maybe allowed to pass over into A and 
the vacuum in D and C increased. In this way all the air in D can be extracted, 
the final stages being facilitated by the admission of a little water into D. the last 
traces of air being driven over into A by the rush of vapor from the water. A 
known volume of blood having been collected over mercury in a small tube is now 
allowed to enter D through the tap h and yields up its gases to the vacuum. A 
repetition of the processes by which the air in D was originally extracted will now 
remove the gases which have been given off from the known volume of blood, the 
only difference being that now the tube / filled with mercury is inverted in the 
trough e over the upper end of the tube b. In this way the gases originally in 1) 
are not allowed to escape into the air, as was the case when the apparatus was 
being originally made vacuous, but are collected in f for subsequent analysis. 
During the extraction of the gases from the blood the bulb D is immersed in a 
vessel of warm water, to facilitate the exit of the gases and, by causing the for- 
mation of large quantities of aqueous vapor, to sweep the gases rapidly over into 
A. The sulphuric acid chamber C dries the vacuum before the admission of the 
blood into D, and hence makes it more perfect and causes the most complete and 
rapid evolution of gases from the blood. 
The average composition of the gas thus obtained from each of the two 
kinds of blood (the arterial blood being taken from a large artery, and the 
venous blood from the right side of the heart) is, stated in round numbers, 
as follows: 
From 100 vols. may be obtained- 
Of oxygen. 
Of carbonic acid. 
Of nitrogen. 
Of arterial blood, 20 vols. 
40 vols. 
1 to 2 vols. 
Of venous blood, 8 to 12 vols. 
46 vols. 
1 to 2 vols. 
all measured at 760 mm. and 0° C. 
That is to say, venous blood, as compared with arterial blood, contains 8 
to 12 per cent, less oxygen and 6 per cent, more carbonic acid. It must be 
remembered, however, that while arterial blood from whatever artery taken 
has always nearly the same proportion of gases, or at all events the same 
amount of oxygen, the amount of oxygen in venous blood, even when taken 
from the same vein, may vary a good deal, still more so when it is taken 
from different veins. The reason of this we shall see hereafter. 
It will be convenient to consider the relations of each of these gases sep- 
arately. 350 
RESPIRATION. 
§ 286. When a liquid such as water is exposed to an atmosphere contain- 
ing a gas such as oxygen, some of the oxygen will be dissolved in the water, 
that is to say, will be absorbed from the atmosphere. The quantity which 
is so absorbed will depend on the pressure of the oxygen in the atmosphere 
above ; the greater the pressure of the oxygen, the larger the amount which 
will be absorbed. If the pressure of the whole atmosphere remain the same, 
at 760 mm. of mercury for instance (the ordinary atmospheric pressure), the 
pressure of the oxygen may be increased or diminished by increasing or 
diminishing the proportion of oxygen in the atmosphere. So that with an 
atmosphere remaining at any given pressure the quantity of oxygen absorbed 
will depend on the quantity present in that atmosphere. If, on the other 
hand, water, already containing a good deal of oxygen dissolved in it, be ex- 
posed to an atmosphere containing little or no oxygen, the oxygen will escape 
from the water into the atmosphere. The oxygen, in fact, which is dissolved in 
the water, like the oxygen in the atmosphere above, stands at a certain 
pressure, the amount of pressure depending on the quantity dissolved; and 
when water containing oxygen dissolved in it is exposed to any atmosphere, 
the result, that is, whether the oxygen escapes from the water into the atmo- 
sphere or passes from the atmosphere into the water, depends on whether 
the pressure of the oxygen in the water is greater or less than the pressure 
of the oxygen in the atmosphere. Hence, when water is exposed to oxygen, 
the oxygen either escapes or is absorbed until equilibrium is established be- 
tween the pressure of the oxygen in the atmosphere above and the pressure 
of the oxygen in the water below. This result is, as far as mere absorption 
and escape are concerned, quite independent of what other gases are present 
in the water or in the atmosphere. Suppose a half-litre of water was lying 
at the bottom of a two-litre flask, and that the atmosphere in the flask above 
the water was one-third oxygen ; it would make no difference, as far as the 
absorption of oxygen by the water was concerned, whether the remaining 
two-thirds of the atmosphere was carbonic acid or nitrogen or hydrogen, 
or whether the space above the water was a vacuum filled to one-third with 
pure oxygen. Hence, it is said that the absorption of any gas depends on 
the partial pressure of that gas in the atmosphere to which the liquid is ex- 
posed. This is true not only of oxygen and water, but of all gases and 
liquids which do not enter into chemical combination with each other. 
Different liquids will, of course, absorb different gases with differing readi- 
ness; but with the same gas and the same liquid, the amount absorbed will 
depend directly on the partial pressure of the gas in the overlying space. 
It should be added that the process is much influenced by temperature. 
Hence, to state the matter generally, the absorption of any gas by any liquid 
will depend on the nature of the gas, the nature of the liquid, the pressure 
of the gas, and the temperature at which both stand. 
Now it might be supposed, and indeed was once supposed, that the oxygen 
in the blood was simply dissolved by the blood. If this were so, then the 
amount of oxygen present in any given quantity of blood exposed to any 
given atmosphere ought to rise and fall steadily and regularly as the par- 
tial pressure of oxvgen in that atmosphere is increased or diminished ; the 
absorption (or escape) of oxygen ought to follow what is known as the 
Henry-Dalton law of pressures. But this is found not to be the case. If 
we expose blood containing little or no oxygen to a succession of atmospheres 
containing increasing quantities of oxygen, we find that at first there is a 
very rapid absorption of the available oxygen, and then this somewhat sud- 
denly ceases or becomes very small; and if, on the other hand, wTe submit 
The Relations of Oxygen in the Blood. 351 
THE RESPIRATORY CHANGES IN THE BLOOD. 
arterial blood to successively diminishing pressures, we find that for a long 
time very little oxygen is given off, and then suddenly the escape becomes 
very rapid. The absorption of oxygen by blood does not follow the general 
law of absorption according to pressure. The phenomena, on the other hand, 
suggest the idea that the oxygen in the blood is in some particular combi- 
nation with a substance or some substances present in the blood, the combi- 
nation being of such a kind that it holds good during a lowering of pressure 
down to a certain limit, and that then dissociation readily occurs ; we may 
add that this limit is very closely dependent on temperature. It is, how- 
ever, not to be supposed that as the pressure is lowered, no oxygen what- 
ever is given off from the substance until a certain point is reached, and 
that at that point the whole store is in an instant dissociated, no more re- 
maining to be given off The case is rather that while pressure is being 
lowered down to a certain point, no appreciable dissociation takes place, and 
that then having begun it increases rapidly with each further lowering of 
pressure until the whole of the oxygen is given off. During the narrow 
range, between the first beginning to give off oxygen and the completion of 
the giving off, the compound of the oxygen with the substance or substances 
may be spoken of as partly, that is more or less, dissociated. What is the 
substance or what are the substances with which the oxygen is thus pecu- 
liarly combined? 
If serum free from red corpuscles be used in such absorption experiments, 
it is found that, as compared with the entire blood, very little oxygen is 
absorbed, about as much as would be absorbed by the same quantity of 
water ; and such as is absorbed does follow the law of pressure. In natural 
arterial blood the quantity of oxygen which can be obtained from serum is 
exceedingly small ; it does not amount to half a volume in one hundred 
volumes of the entire blood to which the serum belonged. It is evident 
that the oxygen which is present in blood is in someway or other peculiarly 
connected with the red corpuscles. Now, the distinguishing feature of the 
red corpuscles is the presence of haemoglobin. We have already seen (§ 24) 
that this constitutes 90 per cent, of the dried red corpuscles. There can be 
d priori little doubt that this must be the substance with which the oxygen 
is associated, and to the properties of this body we must therefore direct our 
attention. 
§ 287. Hemoglobin. When separated from the other constituents of the 
serum, haemoglobin appears as a substance, either amorphous or crystalline, 
readily soluble in water (especially in warm water) and in serum. 
Since haemoglobin is soluble in serum, and since the identity of the crystals 
observed occasionally within the corpuscles with those obtained in other ways shows 
that the haemoglobin as it exists in the corpuscle is the same thing as that which is 
artificially prepared from blood, it is evident that some peculiar relationship between 
the stroma and the haemoglobin must, in natural blood, keep the latter from being 
dissolved by the serum. Hence, in preparing haemoglobin it is necessary first of 
all to break up this connection and to set the haemoglobin free from the corpuscles. 
This maybe done by the addition of water, of ether, of chloroform, and of bile-salts, 
or by repeatedly freezing and thawing: blood so treated becomes " laky" (c/ § 24). 
It is also of advantage previously to remove the alkaline serum as much as possible, 
so as to operate only on the red corpuscles. The stroma and haemoglobin being thus 
separated, a solution of haemoglobin is the result. The alkalinity of the solution, 
when present, beinsr reduced by the cautious addition of dilute acetic acid, and the 
solvent power of the aqueous medium being diminished by the addition of one- 
fourth its bulk of alcohol, the mixture, set aside in a temperature of 0° C. in order 
still further to reduce the solubility of the haemoglobin, readily crystallizes, when the 
blood used is that of the dog, cat, horse, rat, guinea-pig, etc. In the case of the 
dog, indeed, it is simply sufficient to add ether carefully to the blood until it just 
becomes "laky," and then to let it stand in a cool place; the mixture soon becomes 352 
RESPIRATION. 
a mass of crystals. The crystals may be separated by filtration, redissolved in water, 
and recrystallized. 
Haemoglobin from the blood of the rat, guinea-pig, squirrel, hedgehog, 
horse, cat, dog, goose, and some other animals, crystallizes readily, the crys- 
tals being generally slender, four-sided prisms belonging to the rhombic 
system and often appearing quite 
acicular. [Figs. 94, 95, 96.] The 
crystals from the blood of the guinea- 
pig are octahedral, but also belong to 
the rhombic system; those of the 
squirrel are six-sided plates. The 
blood of the ox, sheep, rabbit, pig, 
and man crystallizes with difficulty. 
Why these differences exist is not 
known ; but the composition and the 
amount of water of crystallization 
vary somewhat in the crystals ob- 
tained from different animals. In 
the dog the percentage composition 
of the crystals has been determined 
as C. 53.85, H. 7.32, N. 16.17, O. 
21.84, S. 0.39, Fe. 0.43, with 3 to 4 
per cent, of water of crystallization. 
It will thus be seen that haemoglo- 
bin contains, in addition to the other 
elements usually present in proteid substances, a certain amount of iron ; 
that is to say, the element iron is a distinct part of the haemoglobin 
[Fig. 94. 
Tetrahedral from Blood of the Pig.] 
[Fig. 95. 
[Fig. 96. 
Hexagonal Crystals from Blood of Squirrel. 
On these six-sided plates, prismatic crystals, 
grouped in a stellate manner, not infrequently 
occur. (After Funke)]. 
Prismatic, from Human Blood.] 
molecule, a fact which of itself renders haemoglobin remarkable among the 
chemical substances present in the animal body. 
§ 288. The crystals when seen in a sufficiently thick layer under the 
microscope have the same bright scarlet color as arterial blood has to the 
naked eye ; when seen in a mass they naturally appear darker. An aqueous 
solution of haemoglobin, obtained by dissolving purified crystals in distilled THE RESPIRATORY CHANGES IN THE BLOOD. 
353 
water, has also the same bright arterial color. A tolerably dilute solution 
placed before the spectroscope is found to absorb certain rays of light in a 
peculiar and characteristic manner. A portion of the red end of the spec- 
trum is absorbed, as is also a much larger portion of the blue end ; but what 
is most striking is the presence of two strongly marked absorption bands, 
lying between the solar lines 1) and E. (See Fig. 97.) Of these the one 
toward the red side, sometimes spoken of as the band (a), is the thinnest, but the 
most intense, and in extremely dilute solutions (Fig. 97,1) is the only one vis- 
ible ; its middle lies at some little distance to the blue side of D. Its position 
may be more exactly defined by expressing it in wave-lengths. As is well 
known, the rays of light which make up the spectrum differ in the length of 
their ways, diminishing from the red end, where the waves are longest, to the 
blue end, where they are shortest. Thus, Frauenhofer's line I) corresponds to 
rays having a wave-length of 589.4 millionths of a millimetre. Using the same 
unit, the centre of this absorption band, a, of haemoglobin corresponds to the 
wave-length 578 ; as may be seen in Fig. 97, where, however, the numbers 
of the divisions of the scale indicate only lOO,OOOths of a millimetre. The 
other, sometimes called b, much broader, lies a little to the red side of E, 
its blueward edge, even in moderately dilute solutions (Fig. 97, 2), coming 
close up to that line ; its centre corresponds to about wave-length 539. Each 
band is thickest in the middle and gradually thins away at the edges. 
These two absorption bands are extremely characteristic of a solution of 
haemoglobin. Even in very dilute solutions both bands are visible (they 
may be seen in a thickness of 1 cm. in a solution containing 1 grm. of 
haemoglobin in 10 litres of water), and that when scarcely any of the ex- 
treme red end and very little of the blue end is cut off. They then appear 
not only faint but narrow. As the strength of the solution is increased the 
bands broaden and become more intense ; at the same time both the red end, 
and still more the blue end, of the whole spectrum are encroached upon (Fig. 
97, 3). This may go on until the two absorption bands become fused together 
into one broad band (Fig. 97, 4). The only rays of light which pass through 
the haemoglobin solution are those in the green between the blueward edge 
of the united bands and the general absorption, which is now rapidly ad- 
vancing from the blue end, and those in the red between the united bands 
and the general absorption at the red end. If the solution be still further 
increased in strength, the interval on the blue side of the united bands 
becomes absorbed also, so that the only rays which pass through are the 
red rays lying to the red side of D; these are the last to disappear, and 
hence the natural red color of the solution as seen by transmitted light. 
Exactly the same appearances are seen when crystals of haemoglobin are 
examined with a micro-spectroscope. They are also seen when arterial 
blood itself (diluted with saline solutions, so that the corpuscles remain in 
as natural a condition as possible) is examined with the spectroscope, as 
w7ell as when a drop of blood, which from the necessary exposure to air is 
always arterial, is examined with the micro-spectroscope. In fact, the spec- 
trum of haemoglobin is the spectrum of normal arterial blood. 
§ 289. When crystals of haemoglobin, prepared in the way described 
above, are subjected to the vacuum of the mercurial air-pump, they give off 
a certain quantity of oxygen, and at the same time they change in color. 
The quantity of oxygen given off is definite, 1 grm. of the crystals giving off 
1.59 c.c. of oxygen measured at 760 mm. Hg. and 0° C. In other words, 
the crystals of haemoglobin, over and above the oxygen which enters inti- 
mately into the composition of the molecule (and which alone is given in 
the elementary composition previously stated), contain another quantity of 
oxygen, which is in loose combination only, and which may be dissociated from 354 
RESPIRATION. 
them by subjecting them to a sufficiently low pressure. The change of color 
which ensues when this loosely combined oxygen is removed, is character- 
Fig. 97. 
The Spectra of Oxy-haemoglobin in Different Grades of Concentration, of (reduced) Haemo- 
globin, and of Carbonic-oxide-haemoglobin. (After Preyer and Gamgee): 1 to 4, solution of oxy- 
haemoglobin containing-(1) less than 0.01 per cent., (2) 0.09 per cent., (3) 0.37 per cent., (4) 0.8 per 
cent. 5, solution of (reduced) haemoglobin containing about 0.2 percent. 6, solution of carbonic- 
oxide-haemoglobin. In each of the six cases the layer brought before the spectroscope was 1 cm. 
in thickness. The letters (A, a, etc.) indicate Frauenhofer's lines, and the figures wave-lengths 
expressed in lOO.OOOths of a millimetre. THE RESPIRATORY CHANGES IN THE BLOOD. 
355 
istic ; the crystals become darker and more of a purple hue, and at the same 
time dichroic, so that while the thicker parts are purple, the thin edges 
appear greenish. 
An ordinary solution of haemoglobin, like the crystals from which it is 
formed, contains a definite quantity of oxygen in a similarly peculiar loose 
combination; this oxygen it also gives up when subjected in the air-pump to 
sufficiently low pressure, becoming at the same time of a purplish hue. This 
loosely combined oxygen may also be removed by passing a stream of 
hydrogen or other indifferent gas through the solution; the stream of 
hydrogen acts like an oxygen-vacuum to the haemoglobin and thus disasso- 
ciation is effected. Carbonic acid gas is unsuitable for this purpose, since, 
as we shall see, being an acid, it acts in another way on the haemoglobin. 
The oxygen may also be removed from the haemoglobin not only by physical 
but also by chemical means, as by the use of reducing agents. Thus if a 
few drops of ammonium sulphide or of an alkaline solution of ferrous sul- 
phate kept from precipitation by the presence of tartaric acid, be added to 
a solution of haemoglobin, or even to an unpurified solution of blood cor- 
puscles such as is afforded by the washings from a blood-clot, the oxygen in 
loose combination with the haemoglobin is immediately seized upon by the 
reducing agent. This may be recognized at once by the characteristic 
change of color; from a bright scarlet the solution becomes of a purplish- 
claret color, when seen in any thickness, but greenish when sufficiently 
thin ; the color of the reduced solution is exactly like that of the crystals 
from which the loose oxygen has been removed by the air-pump. 
Examined by the spectroscope, this reduced solution, or solution of re- 
duced hcemoglobin, as we may now call it, offers a spectrum (Fig. 97, 5) very 
different from that of the unreduced solution. The two absorption bands 
have disappeared, and in their place there is seen a single, much broader, 
but at the same time much fainter band, whose middle occupies a position 
about midway between the two absorption bands of the unreduced solution, 
though the redward edge of the band shades away rather further toward the 
red than does the other edge toward the blue; its centre corresponds to 
about .wave-length 555. At the same time the general absorption of the 
spectrum is different from that of the unreduced solution ; less of the blue 
end is absorbed. Even when the solutions become tolerably concentrated, 
many of the bluish-green rays to the blue side of the single band still pass 
through. Hence the difference in color between haemoglobin which retains 
the loosely combined oxygen,1 and haemoglobin which has lost its oxygen 
ami become reduced. In tolerably concentrated solutions, or tolerably thick 
layers, the former lets through the red and orange-yellow rays, the latter the 
red and the bluish-green rays. Accordingly, the one appears scarlet, the 
other purple. In dilute solutions, or in a thin layer, the reduced haemo- 
globin lets through so much of the green rays that they preponderate over 
the red, and the resulting impression is one of green. In the unreduced 
haemoglobin or oxy-haemoglobin, the potent yellow which is blocked out in 
the reduced haemoglobin makes itself felt, so that a very thin layer of oxy- 
haemoglobin, as in a single corpuscle seen under the microscope, appears 
yellow rather than red. 
It must be remembered that when we speak of reduced haemoglobin (or 
more briefly haemoglobin) with a purple color and a characteristic one- 
handed spectrum, we mean haemoglobin which has lost all its loosely asso- 
ciated oxygen. If a quantity of oxy-haemoglobin be exposed to an insufti- 
1 For brevity's sake we may call the haemoglobin containing oxygen in loose combina- 
tion, oxy-hemoglobin. and the haemoglobin from which this loosely combined oxygen has 
been removed, reduced haemoglobin or simply haemoglobin. 356 
RESPIRATION. 
ciently low pressure, or to the action of an insufficient quantity of the 
reducing action, it gives up a part only of its oxygen ; it is only partly 
reduced. Such a partly reduced solution still shows the two bands of oxy- 
haemoglobin. 
§ 290. When the haemoglobin solution (or crystal), which has lost its 
oxygen by the action either of the air-pump or of a reducing agent or by the 
passage of an indifferent gas, is exposed to air containing oxygen, an absorp- 
tion of oxygen at once takes place. If sufficient oxygen be present, the 
haemoglobin seizes upon sufficient oxygen to obtain its full complement, each 
gramme taking up in combination 1.59 c.c. of oxygen ; if there be an in- 
sufficient quantity of oxygen the haemoglobin still remains partly reduced; 
or perhaps we may say that a part only of the haemoglobin gets its allowance 
while the remainder continues reduced. If the amount of oxygen be suffi- 
cient, the solution (or crystal), as it takes up the oxygen, regains its bright 
scarlet color and its characteristic absorption spectrum, the single band being 
replaced by the two. Thus if a solution of oxy-haemoglobin in a test-tube, 
after being reduced by the action of a drop or two of ammonium sulphide 
solution and thus showing the purple color and the single band, be shaken 
up with air, the bright scarlet color at once returns, and when the fluid is 
placed before the spectroscope, it is seen that the single faint broad band of 
the reduced haemoglobin has wholly disappeared, and that in its place are 
the two sharp thinner bands of the oxy-haemoglobin. If left to stand in the 
test-tube the quantity of reducing agent still present is generally sufficient 
again to rob the haemoglobin of the oxygen thus newly acquired, and soon 
the scarlet hue fades back again into the purple, the two bands giving place 
to the one. Another shake and exposure to air will, however, again bring 
back the scarlet hue and the two bands ; and once more these may disappear. 
In fact, a few drops of the reducing fluid will allow this game of haemoglobin 
taking oxygen from the air and giving it up to the reducer to be played over 
and over again ; at each turn of the game the color shifts from scarlet to 
purple and from purple to scarlet, while the two bands exchange for the one 
and the one for the two. 
§291. Color of venous and arterial blood. Evidently we have in these 
properties of haemoglobin an explanation of at least one-half of the great 
respiratory process, and they teach us the meaning of the change of 
color which takes place when venous blood becomes arterial or arterial 
venous. 
In venous blood, as it issues from the right ventricle, the oxygen present 
is insufficient to satisfy wholly the htemoglobin of the red corpuscles; the 
haemoglobin is, to a large extent, reduced, hence the purple color of venous 
blood. When ordinary venous blood, diluted without access of oxygen, is 
brought before the spectroscope, the two bands of oxy-haemoglobin are seen. 
This is explained by the fact that in partly reduced haemoglobin, which we 
may conveniently regard as a mixture of oxy-haemoglobin and (reduced) 
haemoglobin, the two sharp bands of the former are always much more 
readily seen than the much fainter band of the latter. Now in ordinary 
venous blood there is always some loose oxygen, removable by diminished 
pressure or otherwise ; the haemoglobin is only partly reduced, there is always 
some, indeed a considerable quantity, of oxy-haemoglobin as well as (reduced) 
haemoglobin. It is only under special circumstances, as, for instance, after 
death by what we shall presently speak of as asphyxia, that all the loose 
oxygen of the blood disappears ; and then the two bands of oxy-haemoglobin 
vanish too. If even only a small quantity of oxygen be present, so distinct 
are the two bands that a solution of completely reduced haemoglobin may be 
used as a test for the presence of oxygen ; if oxygen be present in any fluid RESPIRATORY CHANGES IN THE BLOOD. 
357 
to which the reduced haemoglobin is added, the single band immediately 
gives way to the two bands of oxy-haemoglobin. 
As the venous blood passes through the capillaries of the lungs, this 
reduced haemoglobin takes from the pulmonary air its complement of oxygen, 
all or nearly all the haemoglobin of the red corpuscles becomes oxy-haemo- 
globin, and the purple color forthwith shifts into scarlet. For careful 
observations show that the haemoglobin of arterial blood is saturated or 
nearly saturated with oxygen ; it probably falls short of complete saturation 
by about 1 vol. of oxygen in 100 vols. of blood. By increasing the pressure 
of the oxygen, an additional quantity may be driven into the blood, but this, 
after the haemoglobin has become completely saturated, is effected by simple 
absorption. The quantity so added is extremely small compared with the 
total quantity combined with the haemoglobin. 
Passing from the left ventricle to the capillaries of the tissues the oxy- 
haemoglobin gives up some of its oxygen to the tissues, becoming, in part, 
reduced haemoglobin, and the blood in consequence becomes once more 
venous, with a purple hue. Thus the red corpuscles by virtue of their 
haemoglobin are emphatically oxygen-carriers. Undergoing no intrinsic 
change in itself, the haemoglobin combines in the lungs with oxygen, which 
it carries to the tissues; these, more greedy of oxygen than itself, rob it of 
its charge, and the reduced haemoglobin hurries back to the lungs in the 
venous blood for another portion. The change from venous to arterial blood 
is then in part (for as we shall see there are other events as well) a peculiar 
combination of haemoglobin with oxygen, while the change from arterial to 
venous is, in part also, a reduction of oxy-hsemoglobin ; and the difference of 
color between venous and arterial blood depends almost entirely on the fact 
that the reduced haemoglobin of the former is of purple color, while the oxy- 
haemoglobin of the latter is of a scarlet color. 
There may be other causes of the change of color, but these are wholly 
subsidiary and unimportant. When a corpuscle swells, its refractive power 
is diminished, and in consequence the number of rays which pass into and 
are absorbed by it are increased at the expense of those reflected from its 
surface; anything therefore which swells the corpuscles, such as the addition 
of water, tends to darken blood, and anything, such as a concentrated saline 
solution, which causes the corpuscles to shrink, tends to brighten blood. 
Carbonic acid has apparently some influence in swelling the corpuscles, and 
therefore may aid in darkening the venous blood. 
§ 292. We have spoken of the combination of haemoglobin with oxygen 
as being a peculiar one. The peculiarity consists in the facts that the oxy- 
gen may be associated and dissociated, without any general disturbance of 
the molecule of haemoglobin, and that dissociation may be brought about 
very readily. Haemoglobin combines in a wholly similar manner with other 
gases. If carbonic oxide (monoxide) be passed through a solution of 
haemoglobin, a change of color takes place, a peculiar bluish tinge making 
its appearance. At the same time the spectrum is altered; two bands are 
still visible, but on accurate measurement it is seen that they are placed more 
toward the blue end than are the otherwise similar bands of oxy-haemoglobin 
(see Fig. 97, 6) ; their centres corresponding respectively to about wave- 
lengths 572 and 533, while those of oxy-haemoglobin, as we have seen, cor- 
respond to 578 and 539. When a known quantity of carbonic oxide gas is 
sent through a haemoglobin solution, it will be found on examination that a 
certain amount of the gas has been retained, an equal volume of oxygen ap- 
pearing in its place in the gas which issues from the solution. If the solution 
so treated be crystallized, the crystals will have the same characteristic color 
and give the same absorption spectrum as the solution ; when subjected to 358 
RESPIRATION. 
the action of the mercurial pump, they will give off a definite quantity of 
carbonic oxide, 1 gramme of the crystals yielding 1.59 c.c. of the gas. In 
fact haemoglobin combines loosely with carbonic oxide just as it does with 
oxygen ; but its affinity with the former is greater than with the latter. 
While carbonic oxide readily turns out oxygen, oxygen cannot so readily 
turn out carbonic oxide. Indeed, carbonic oxide has been used as a means 
of driving out and measuring the quantity of oxygen present in any given 
blood. This property of carbonic oxide explains its poisonous nature. 
When the gas is breathed, the reduced and the unreduced haemoglobin of 
the venous blood unite with the carbonic oxide, and hence the peculiar, 
bright cherry-red color observable in the blood and tissues in cases of poison- 
ing by this gas. The carbonic-oxide haemoglobin, however, is of no use in 
respiration ; it is not an oxygen-carrier, nay more, it will not readily, though 
it does so slowly and eventually, give up its carbonic oxide for oxygen, when 
the poisonous gas ceases to enter the chest and is replaced by pure air. The 
organism is killed by suffocation, by want of oxygen, in spite of the blood 
not assuming any dark venous color; to adopt a phrase which has been 
used, the corpuscles are paralyzed. 
Haemoglobin smilarly forms a compound, having a characteristic spec- 
trum, with nitric oxide, more stable even than that with carbonic oxide. 
It has been supposed by some that the oxygen thus associated with haemo- 
globin is in the condition known as ozone; but the arguments urged in sup- 
port of this view are inconclusive. 
Products of the Decomposition of Haemoglobin. 
§ 293. Although a crystalline body, haemoglobin diffuses with great diffi- 
culty. This arises from the fact that it is in part a proteid body ; it consists 
of a colorless proteid, associated with a colored substance, which may be 
separated out from the haemoglobin, though not in the exact condition in 
which it naturally exists in the compound; this substance when separated 
out appears as a brownish-red body known as hamatin. All the iron be- 
longing to the haemoglobin is in reality attached to the haematin. A solution 
of haemoglobin, when heated, coagulates, the exact degree at which the 
coagulation takes place depending on the amount of dilution ; at the same 
time it turns brown from the setting free of the haematin. If a strong solu- 
tion of haemoglobin be treated with acetic (or other) acid, the same brown 
color, from the appearance of haematin, is observed. The proteid constitu- 
ent, however, is not coagulated, but by the action of the acid passes into 
the state of acid-albumin. On adding ether to the mixture, and shaking, 
the haematin is dissolved in the supernatant acid ether, which it colors a 
dark red, and which, examined with the spectroscope, is found to possess a 
well-marked spectrum, the spectrum of the so-called acid haematin of Stokes 
(Fig. 98, 6). The proteid in the water below the ether appears in a coagu- 
lated form owing to the action of the ether. In a somewhat similar manner 
alkalies split up haemoglobin into a proteid constituent and haematin. 
The exact nature of the proteid constituent of haemoglobin has not as 
yet been clearly determined. It was supposed to be globulin (hence the 
name haematoglobulin, contracted into haemoglobin), but though belonging 
to the globulin family, has characters of its own ; it is possibly a mixture of 
two or more distinct proteids. It has been provisionally named globin and 
is said to be free from ash. 
§ 294. Haematin when separated from its proteid fellow, and purified, 
appears as a dark-brown amorphous powder, or as a scaly mass with a RESPIRATORY CHANGES IN THE BLOOD. 
359 
Haematin in alkaline solution. 
Haematin more concentrated. 
Reduced Haematin in alkaline 
solution. 
Haematin in acid solution. 
Acid haematin in ethereal so- 
lution (Stokes's acid haema- 
tin). 
Methaemoglobin. 
Spectra of some Derivatives of Haemoglobin. (After Gamgee.) Compare with Fig. 97. 
Fig. 98. 360 
RESPIRATION. 
metallic lustre, having the probable composition of C32, H34, N4, Fe, O5? It 
is fairly soluble in dilute acid or alkaline solutions, and then gives charac- 
teristic spectra (Fig. 98, 1, 2, 5). 
An interesting feature in haematin is that its alkaline solution is capable 
of being reduced by reducing agents, the spectrum changing at the same 
time (Fig. 98, 3), and that the reduced solution will, like the haemoglobin, 
take up oxygen again on being brought into contact with air or oxygen. 
This would seem to indicate that the oxygen-holding power of haemoglobin 
is connected exclusively with its haematin constituent. 
By the action of strong sulphuric acid haematin may be robbed of all its 
iron. It still retains the feature of possessing color, the solution of iron-free 
haematin being a dark rich brownish-red ; but is no longer capable of com- 
bining loosely with oxygen. This indicates that the iron is in some way 
associated with the peculiar respiratory functions of haemoglobin ; though 
it is obviously an error to suppose, as was once supposed, that the change 
from venous to arterial blood consists essentially in a change from a ferrous 
to a ferric salt. 
Though not crystallizable itself, haematin forms with hydrochloric acid a 
compound, occurring in minute rhombic crystals, known as hoemin crystals. 
When blood is left until it decomposes, the haemoglobin is very apt to 
become changed into a peculiar body known as methoemoglobin, in the spec- 
trum of which a very conspicuous band is seen in the red between C and D 
(see Fig. 98, 4). The same change may be brought about by the action 
of weak acids, such as carbonic acid, by ozone, and by other agents such as 
nitrites and potassium permanganate. When a stream of carbonic acid is 
driven through blood or through a solution of haemoglobin the band in the 
red characteristic of methaemoglobin soon makes its appearance. Methae- 
moglobin differs but little, if at all, in elementary composition from haemo- 
globin ; it is maintained that it contains the same quantity of oxygen as 
oxy-haemoglobin but in a more stable condition, more intimately associated 
with the molecule. 
In conclusion, the condition of oxygen in the blood is as follows: Of the 
whole quantity of oxygen in the blood, only a minute fraction is simply 
absorbed or dissolved according to the law of pressure (the Henry-Dalton 
law). The great mass is in a state of combination with the haemoglobin, 
the connection being of such a kind that while the haemoglobin readily 
combines with the oxygen of the air to which it is exposed, dissociation 
readily occurs at low pressures, or in the presence of indifferent gases, or by 
the action of substances having a greater affinity for oxygen than has haemo- 
globin itself. The difference between venous and arterial blood, as far as 
oxygen is concerned, is that while in arterial blood the haemoglobin holds 
nearly its full complement of oxygen and may be spoken of as nearly 
wholly oxy-haemoglobin, in venous blood the haemoglobin is to a large but 
variable extent reduced ; and the characteristic colors of venous and arte- 
rial blood are in the main due to the fact that the color of reduced haemo- 
globin is purple, while that of oxy-haemoglobin is scarlet. 
The Relations of the Carbonic Acid in the Blood. 
§ 295. The presence of carbonic acid in the blood appears to be deter- 
mined by conditions more complex in their nature and at present not so well 
understood as those which determine the presence of oxygen. The carbonic 
acid is not simply dissolved in the blood ; its absorption by blood does not 
1 This formula is the old one of Hoppe-Seyler. 361 
THE RESPIRATORY CHANGES IN THE LUNGS. 
follow the law of pressures. It exists in association with some substance or 
substances in the blood, and its escape from the blood is a process of disso- 
ciation. We cannot, however, speak of it as being associated, to the same 
extent as is the oxygen, with the haemoglobin of the red corpuscles. So 
far from the red corpuscles containing the great mass of the carbonic acid, 
the quantity of this gas which is present in a volume of serum is according 
to some observers actually greater than that which is present in an equal 
volume of blood, i. e., an equal volume of mixed corpuscles and serum ; 
that is to say, the carbonic acid is much more largely associated with 
the serum (or, in the living blood, with the plasma) than with the red cor- 
puscles. 
When serum is subjected to the action of the mercurial pump, by far the 
greater part of the carbonic acid is given off; but a small additional quan- 
tity (2 to 5 vols. per cent.) may be extracted by the subsequent addition of 
an acid. This latter portion may be spoken of as " fixed " carbonic acid in 
distinction to the larger "loose" portion which is given off to the vacuum. 
When, however, the whole blood is subjected to the vacuum until the car- 
bonic acid ceases to be given off, the subsequent addition of acid is said not 
to set free any further quantity ; so that when serum is mixed with corpus- 
cles all the carbonic acid may be spoken of as " loose " ; and it is stated that 
the excess of carbonic acid in a quantity of serum over that present in the 
same bulk of entire blood, corresponds to the fixed portion in serum which 
has to be driven off by an acid. Moreover, even those who maintain that 
the quantity of carbonic acid in entire blood is less than that in an equal 
volume of serum, admit that the carbonic acid exists in some way or other 
at a higher pressure in and is more readily given off from entire blood than 
from serum. It seems probable that the carbonic acid exists associated with 
some substance or substances in the serum, or rather plasma, but that the 
conditions of its association (and those of its dissociation) are determined by 
the action of some substance or substances present in the corpuscles. It has 
been suggested that the association of the carbonic acid in the plasma is with 
one or other of the proteids of the plasma; but it has also been suggested 
that the association is one with sodium as sodium bicarbonate, and further 
than the haemoglobin of the corpuscles plays a part in promoting the disso- 
ciation of the sodium bicarbonate or even the carbonate, and thus keeping 
up the carbonic acid of the entire blood. Other observers, however, main- 
tain that the plasma does not hold this exclusive possession of the carbonic 
acid, but that a considerable quantity of this gas is in some way associated 
with the red corpuscles. Indeed, further investigations are necessary before 
the matter can be said to have been placed on a satisfactory footing. 
The Relations of the Nitrogen in the Blood. 
§ 296. The small quantity of this gas which is present in both arterial 
and venous blood seems to exist in a state of simple solution. 
The Respiratory Changes in the Lungs. 
§ 297. We have already seen that the blood in passing through the 
lungs takes up a certain variable quantity (from 8 to 12 vols. per cent.) of 
oxygen. We have further seen that the quantity so taken up, putting aside 
the insignificant fraction simply absorbed, enters into direct but loose com- 
bination with the haemoglobin. In drawing a distinction between the oxy- 
The Entrance of Oxygen. 362 
RESPIRATION. 
gen simply absorbed and that entering into combination with the haemo- 
globin, it must not be understood that the latter is wholly independent of 
pressure. On the contrary, all chemical compounds are in various degrees 
subject to dissociation at certain pressures and temperatures; and the existence 
of the somewhat loose compound of oxygen and haemoglobin is dependent on 
the partial pressure of oxygen in the atmosphere to which the haemoglobin 
is exposed. Not only will a solution of haemoglobin or a quantity of blood 
either absorb oxygen, and thus undergo association or undergo dissociation 
and give off oxygen according as the partial pressure of oxygen in the 
atmosphere to which it is exposed is high or low, but also the amount taken 
up or given off will depend on the degree of the partial pressure; the haemo- 
globin as we have seen may be partially as well as wholly reduced. The 
law, however, according to which absorption or escape thus takes place is 
quite different from that observed in the simple absorption of oxygen by 
liquids. The association or dissociation is further especially dependent on 
temperature, a high temperature favoring dissociation, so that at a high 
temperature less oxygen is taken up than would be taken up (or, as the 
case may be, more given off than would be given off) at a lower tempera- 
ture, the partial pressure of the oxygen in the atmosphere remaining the 
same. 
Moreover, in the blood we have to deal not with luemoglobin in simple 
solution, in which the molecules are dispersed uniformly through the solvent, 
but with the luemoglobin segregated into minute isolated masses, bottled up 
as it were in the individual corpuscles. The haemoglobin of each corpuscle 
is separated from its fellows by a layer, thin it may be but still a distinct 
layer, of colorless, haemoglobinless plasma. As the corpuscle makes its way 
through the narrow capillary paths of a pulmonary alveolus, it is separated 
from the air of the alveolus by a thin layer of plasma as well as by the film 
of the conjoined capillary and alveolar walls; and a like layer of plasma 
separates it from its fellows as it journeys in company with them through 
the wider passages of the arteries and veins. Through this layer of plasma, 
which containing no haemoglobin can hold oxygen in simple solution only, 
the oxygen has to pass on its way to and from the corpuscle ; and every cor- 
puscle may be considered as governing, as far as oxygen is concerned, a zone 
of plasma immediately surrounding itself. The corpuscle takes its oxygen 
directly from this zone and gives up its oxygen directly to this zone; and 
the pressure at which at any moment the oxygen exists in this zone will 
depend on the pressure of oxygen outside the zone, in the air of the pul- 
monary alveolus, for instance, and on the smaller or greater amount of 
oxygen associated with the luemoglobin of the corpuscle. 
The evidence, so far as it goes, seems to show that blood absorbs oxygen 
in the same way as an aqueous solution of luemoglobin of the same concen- 
tration ; the zone of plasma spoken of above as surrounding each corpuscle 
behaves as far as regards the passage of oxygen to and from the corpuscles 
in no essentially different respect from the way the molecules of water 
belonging to a molecule of dissolved luemoglobin behave in regard to 
the absorption or the giving off of oxygen by an aqueous solution of 
luemoglobin. 
The film of the conjoined capillary and alveolar wall is a thin membrane 
soaked with lymph and wet; we cannot speak of it as actually secreting a 
liquid secretion into the alveolus, for the cavity of the alveolus is filled with 
air which, though saturated with moisture, is air, not a liquid; still enough 
passes through the film to keep it continually moist. Through this film the 
oxygen has to make its way in order to gain access to the plasma and so to 
the corpuscle; it makes its way dissolved in the fluid, that is the lymph, THE RESPIRATORY CHANGES IN THE LUNGS. 
363 
which keeps the film moist. This film, moreover, is composed of living 
matter, and the considerations which a little while back (§ 265) we urged 
concerning the diffusion through a living membrane of solid substances in 
solution, hold good also for the diffusion of gases in solution. 
We have now to consider the question, Are the conditions in which 
haemoglobin and oxygen exist in ordinary venous blood as it flows to the 
lungs, of such a kind that the venous blood in passing through the pul- 
monary capillaries will find the partial pressure of the oxygen in the pul- 
monary alveoli sufficient to bring about the association of the additional 
quantity of oxygen whereby the venous is converted into arterial blood ? 
§ 298. In man, as we have seen, expired air contains about 16 per cent, 
of oxygen. The air in the pulmonary alveoli must contain less than this, 
since the expired air consists of a tidal air mixed by diffusion with the 
stationary air. How much less it contains we do not exactly know, but 
probably the difference is not very great. At the ordinary atmospheric 
pressure of 760 mm. 16 per cent, is equivalent to a partial pressure of 122 
mm. The question, therefore, stands thus, Will venous blood, exposed at 
the temperature of the body to a partial pressure of less than 122 mm. (less 
than 16 per cent.) of oxygen take up sufficient oxygen (from 8 to 12 vols. 
per cent.) to convert it into arterial blood? Numerous experiments have 
been made (chiefly but not exclusively on the dog) to determine on the one 
hand the oxygen-pressure of both arterial and venous blood (i. e., the partial 
pressure of oxygen in an atmosphere exposed to which the arterial blood 
neither gives up nor takes in oxygen, and the same for venous blood), and 
on the other hand the behavior at the temperature of the body or at ordi- 
nary temperatures of blood or of solutions of haemoglobin (for the two, as we 
have just said, behave in this respect very much alike) toward an atmos- 
phere in which the partial pressure of oxygen is made to vary. Without 
going into detail, we may state that these experiments show that the partial 
pressure of oxygen in the lungs is amply sufficient to bring about, at the 
temperature of the body, the association of that additional amount of oxy- 
gen by which venous blood becomes arterial. When a solution of haemo- 
globin or when blood is successively exposed to increasing oxygen pressures, 
as the partial pressure of oxygen is gradually increased, the curve of absorp- 
tion rises at first very rapidly but afterward more slowly; that is to say, 
the later additions of oxygen at the higher pressures are proportionately 
less than the earlier ones at the lower pressures. And this is consonant with 
what appears to be the fact, that the haemoglobin of arterial blood, though 
nearly saturated with oxygen, i. e., associated with almost its full comple- 
ment of oxygen, is not quite saturated. When arterial blood is thoroughly 
exposed to air it takes up rather more than 1 vol. per cent, of oxygen ; and 
that appears to represent the difference between exposing blood to pure air, 
such as enters or ought to enter the mouth in inspiration, and exposing 
blood to the air as it exists in the pulmonary alveoli. The greater relative 
absorption at the lower pressures has a beneficial effect, inasmuch as it still 
permits a considerable quantity of oxygen to be absorbed even when the 
partial pressure of oxygen in the air in the lungs is largely reduced, as in 
ascending to great heights. 
Observations made both with dog's blood and ox's blood seem to show 
that arterial blood ceases to take up oxygen and begins to give off oxygen ; 
in other words, that dissociation begins to take place when the partial 
pressure of the oxygen in the atmosphere to which it is exposed sinks to 
about 60 mm. of mercury; that is to say, when the whole atmospheric 
pressure is reduced from 760 mm. to about 300 mm., or when the percentage 
of oxygen in the atmosphere is reduced by decidedly more than half. And 364 
RESPIRATION. 
this accords with the observation that in man, when the oxygen of inspired 
air is gradually diminished without any other change in the air, symptoms 
of dyspnoea do not make their appearance until the oxygen sinks to 10 per 
cent, in the inspired air, and must therefore be less than this in the pul- 
monary alveoli. We may remark that at ordinary altitudes, even taking 
into account the diminution the oxygen undergoes before it reaches the pul- 
monary alveoli, the partial pressure of the oxygen in the atmosphere leaves 
a wide margin of safety. But at an altitude of 5500 metres (17,000 feet), 
at which the pressure of the whole atmosphere stands at about the limit 
given above of 300 mm., the partial pressure of the oxygen will be such 
that the venous blood cannot take up the quantity of oxygen proper to con- 
vert it into arterial blood, since at this limit arterial blood begins to give off 
oxygen. We may add that it is at this altitude that breathing becomes 
especially difficult; but to this we shall return. 
§ 299. The statements made so far refer to ordinary breathing, but the 
question may be asked, What happens when the renewal of the air in the 
pulmonary alveoli ceases, as when the trachea is obstructed? In such a 
case the oxygen in the alveoli is found to diminish rapidly, so that the 
partial pressure of oxygen in them soon falls below the oxygen-pressure of 
ordinary venous blood. But in such a case the blood is no longei* ordinary 
venous blood ; instead of being moderately it is largely and increasingly 
reduced ; instead of containing a comparatively small amount, it contains 
a large and gradually increasing amount of reduced haemoglobin. And as 
the reduction continues to increase, the oxygen-pressure of the venous blood 
also continues to decrease; it thus keeps below that of the air in the lungs. 
Hence, apparently, even the last traces of oxygen in the lungs may be taken 
up by the blood and carried away to the tissues. Whether or not the pul- 
monary tissue and the capillary walls, because of their being living structures, 
have any influence upon the transmission of oxygen must still be considered 
an unsettled question. 
The Exit of Carbonic Acid. 
§ 300. It seems natural to suppose that the carbonic acid would escape 
by diffusion from the blood of the alveolar capillaries into the air of the 
alveoli. But in order that diffusion should thus take place, the carbonic 
acid pressure of the air in the pulmonary alveoli must always be less than 
that of the venous blood of the pulmonary artery, and ought not to exceed 
that of the blood of the pulmonary vein. There are, however, many prac- 
tical difficulties in the way of an exact determination of the carbonic acid 
pressure of the pulmonary alveoli (for, though it must be greater than that 
of the expired air, it is difficult to say how much greater), and of the car- 
bonic acid pressure of the blood at the same time, so as to be in a position 
to compare the one with the other. In the case of oxygen there is always 
present in the lungs a surplus of the gas, a portion only being absorbed at 
each breath ; in the case of carbonic acid the whole quantity comes direct 
from the blood, and any modifications in breathing seriously affect the 
amount given out. Thus, when the breath is held for some time the per- 
centage of carbonic acid in the expired air reaches 7 or 8 per cent., but we 
cannot take this as a measure of the normal percentage of carbonic acid in 
the pulmonary alveoli, since by the mere holding of the breath the carbonic 
acid in the blood, and hence in the pulmonary alveoli, is increased beyond 
the normal. 
The difficulties of the problem seem, however, to have been overcome by 
an ingenious experiment in which there is introduced into the bronchus of 365 
THE RESPIRATORY CHANGES IN THE TISSUES. 
the lung of a dog a catheter, round which is arranged a small bag; by the 
inflation of this bag, the bronchus, whenever desired, can be completely 
blocked up. Thus, without any marked disturbance of the general breath- 
ing, and therefore without any marked change in the normal proportions of 
the gases of the blood, the experimenter is able to stop the ingress of fresh air 
into a limited portion of the lung. At the same time he is enabled by means 
of the catheter to withdraw a sample of the air of the same limited portion 
and by analysis to determine the amount of carbonic acid which it contains, 
or in other words, the partial pressure of the carbonic acid. The blood 
passing through the alveolar capillaries of this limited portion of the lung 
naturally possesses the same carbonic acid pressure as the rest of the venous 
blood flowing through the pulmonary artery-a pressure which, though 
varying slightly from moment to moment, will maintain a normal average. 
On the supposition that carbonic acid passes simply by diffusion from the 
pulmonary blood into the air of the alveoli, because the carbonic acid 
pressure of the latter is normally lower than that of the former, one would 
expect to find that the air in the occluded portion of the lung would con- 
tinue to take up carbonic acid until an equilibrium was established between 
it and the carbonic acid pressure of the venous blood. Consequently, if 
after an occlusion, say of some minutes (by which time the equilibrium 
might fairly be assumed to have been established), the carbonic acid pressure 
of the air of the occluded portion were determined, it ought to be found to 
be equal to, and not more than equal to, the carbonic acid pressure of the 
venous blood of the pulmonary artery. And this is the result which has 
been arrived at: it has been found that the pressures of the carbonic acid 
of the occluded air and of the venous blood of the right side of the heart 
are just about equal. Hence the evidence, so far as it goes, is distinctly in 
favor of the view7 that the escape of carbonic acid from the blood into the 
pulmonary alveoli is simply due to diffusion, and that there is no need to 
seek for any further explanation. 
As far then as can be seen at present, both the entrance of oxygen and 
the exit of carbonic acid by which venous blood is converted into arterial 
are the simple physical results of the exposure of the blood in the pulmon- 
ary capillary to the air of the pulmonary alveoli. 
§ 301. In passing through the several tissues the arterial blood becomes 
once more venous. The oxy-hiemoglobin becomes considerably reduced, and 
a quantity of carbonic acid passes from the tissues into the blood. The 
amount of change varies in the various tissues, and in the same tissue may 
vary at different times. Thus, in a gland at rest, as we have seen, the venous 
blood is dark, showing that the haemoglobin is to a large extent in the reduced 
condition ; when the gland is active, the venous blood in its color, and in 
the extent to which the haemoglobin is in the condition of oxy-haemoglobin, 
resembles closely arterial blood. The blood therefore which issues from a 
gland at rest is more " venous " than that from an active gland ; though 
owing to the more rapid flow of blood which, as we saw in an earlier sec- 
tion, accompanies the activity of the gland, the total quantity of oxygen 
taken up from and of carbonic acid discharged into the blood from the 
gland in a given time may be greater than the latter. The blood, on the 
other hand, which comes from an active, i. e., a contracting muscle, is, in 
spite of the more rapid flow, not only richer in carbonic acid, but also, 
though not to a corresponding amount, poorer in oxygen than the blood 
which flows from a muscle at rest. 
The Respiratory Changes in the Tissues. 366 
RESPIRATION. 
In all these cases the great question which conies up for our consideration 
is this: Does the oxygen pass from the blood into the tissues, and does the 
oxidation take place in the tissues, giving rise to carbonic acid, which passes 
in turn away from the tissues into the blood ? or do certain oxidizable redu- 
cing substances pass from the tissues into the blood, and there become oxidized 
into carbonic acid and other products, so that the chief oxidation takes 
place in the blood itself? 
There are, it is true, reducing oxidizable substances in the blood, but 
these are small in amount, and the quantity of carbonic acid to which they 
give rise when the blood containing them is agitated with air or oxygen, is 
so small as-scarcely to exceed the errors of observation. 
We may add that the oxidative power which the blood itself removed 
from the body is able to exert on substances which are undoubtedly oxidized 
in the body is so small that it may be neglected in the present considera- 
tions. If grape-sugar be added to blood or to a solution of haemoglobin, the 
mixture may be kept for a long time at the temperature of the body with- 
out undergoing oxidation. Even within the body a slight excess of sugar 
in the blood over a certain percentage wholly escapes oxidation, and is dis- 
charged unchanged. Many easily oxidized substances, such as pyrogallic 
acid, pass largely through the blood of a living body and are discharged in 
the urine without being oxidized; though perhaps in some of these cases, 
what appears to be an absence of oxidation is really an oxidation followed 
by a subsequent equivalent reduction taking place in the urine or elsewhere. 
The organic acids, such as citric, even in combination with alkaline bases, 
are only partially oxidized ; when administered as acids, and not as salts, 
they are hardly oxidized at all. It is, of course, quite possible that the 
changes which the blood undergoes when shed might interfere with its 
oxidative action, and hence the fact that shed blood has little or no oxidiz- 
ing power, is not a satisfactory proof that the unchanged blood within the 
living vessels may not have such a power. But did oxidation take place 
largely in the blood itself, one would expect even highly diffusible substances 
to be oxidized in their transit; whereas if we suppose the oxidation to take 
place in the tissues, it becomes intelligible why such diffusible substances as 
those which the tissues in general refuse to take up largely should readily 
pass unchanged from the blood through the excreting organs. 
On the other hand, it will be remembered in speaking of muscle, we 
drew attention (§ 61) to the fact that a frog's muscle removed from the body 
(and the same is true of the muscles of other animals) contains no free 
oxygen whatever; none can be obtained from it by the mercurial air-pump. 
Yet such a muscle will not only when at rest go on producing and discharging 
a certain quantity, but also when it contracts evolve a very considerable 
quantity, of carbonic acid. Moreover, this discharge of carbonic acid will 
go on for a certain time in muscles under circumstances in which it is im- 
possible for them to obtain oxygen from without. Oxygen, it is true, is 
necessary for the life of the muscle; when venous instead of arterial blood is 
sent through the bloodvessels of a muscle, the irritability speedily disappears, 
and unless fresh oxygen be administered the muscle soon dies. The muscle 
may, however, during the interval in which irritability is still retained after 
the supply of oxygen has been cut off, continue to contract vigorously. The 
supply of oxygen, though necessary for the maintenance of irritability, is not 
necessary for the manifestation of that irritability, is not necessary for that ex- 
plosive decomposition which develops a contraction. A frog's muscle will 
continue to contract and to produce carbonic acid in an atmosphere of 
hydrogen or nitrogen, that is, in the total absence of free hydrogen, both 
from itself and from the medium in which it is placed. THE RESPIRATORY CHANGES IN THE TISSUES. 
367 
Thus, on the one hand, the muscle seems to have the property of taking 
up and fixing in some way or other the oxygen to which it is exposed, of 
storing it up in its own substance in such a condition that it cannot be re- 
moved by simple diminished pressure (so that the pressure of oxygen in the 
muscular substance may be considered as always nil} and yet has not entered 
into any distinct combination which we can speak of as an oxidation, but is 
still available for such a purpose. On the other hand, the muscular substance 
is always undergoing a decomposition of such a kind that carbonic acid is 
set free, sometimes, as when the muscle is at rest, in small, sometimes, as 
during a contraction, in large quantities. The oxygen present in this car- 
bonic acid, as an oxidation product, comes from the previously existing store 
of which we have just spoken. The oxygen taken in by the muscle, what- 
ever be its exact condition immediately upon its entrance into the muscular 
substance, in the phase which has been called " intra-molecular," sooner or 
later enters into a combination, or perhaps we should rather say, enters into 
a series of combinations. We have previously urged (§ 30) that all living 
substance may be regarded as incessantly undergoing changes of a double 
kind, changes of building up and changes of breaking down. In the end- 
products of the breaking down, in the carbonic acid given out by muscle, 
for instance, we can recognize an oxidation product; but we do not know 
exactly at what stage or exactly in what way the oxygen is combined with 
the carbon. We may imagine that the oxygen as it comes from the blood 
is caught up, so to speak, by and disappears in the building-up process, and 
that through those processes it is made part of complex decomposable sub- 
stances whose decomposition ultimately gives rise to the carbonic acid ; but, 
as far as actual knowledge goes, we cannot as yet trace out the steps taken 
by the oxygen from the moment it slips from the blood into the muscular 
substance to the moment when it issues united with carbon as carbonic acid. 
But if the oxygen-pressure of the muscular tissue be thus always nil, 
oxygen will be always passing over from the blood corpuscles, in which it is 
at a comparatively high pressure, through the plasma, through the capillary 
walls, the lymph-spaces, and the sarcolemma, into the muscular substance, 
and as soon as it arrives there will be in some manner or other hidden away, 
leaving the oxygen-pressure of the muscular substance once more nil. Con- 
versely, the carbonic acid produced by the decomposition of the muscular 
substance will tend to raise the carbonic acid pressure of the muscle until it 
exceeds that of the blood ; whereupon carbonic acid will pass from the 
muscle into the blood, its place in the muscular substance being supplied by 
gas freshly generated. There will always in fact be a stream of oxygen 
from the blood to the muscle and of carbonic acid from the muscle to the 
blood. The respiration of the muscle, then, does not consist in throwing into 
the blood oxidizable substances, there to be oxidized into carbonic acid and 
other matters; but it does consist in the assumption and storing up of oxy- 
gen somehow or other in its substance, in the building up by help of that 
oxygen of explosive decomposable substances, and in the carrying out of 
decompositions whereby carbonic acid and other matters are discharged first 
into the substance of the muscle and subsequently into the blood. 
§ 302. Our knowledge of the respiratory changes in muscle is more com- 
plete than in the ease of any other tissue ; but we have no reason to suppose 
that the phenomena of muscle are exceptional. On the contrary, all the 
available evidence goes to show that in all tissues the oxidation takes place 
in the tissue, and not in the adjoining blood. It is a remarkable fact that 
lymph, serous fluids, bile, urine, and milk contain a mere trace of free or 
loosely combined oxygen, but a very considerable quantity of carbonic acid. 
And we may probably assert with safety with regard to all the tissues, that 368 
RESPIRATION. 
in the tissues themselves, in the lymph which bathes their lymph-spaces, and 
in the secretions which some of them pour forth, free oxygen is either wholly 
absent or so scanty that their oxygen-pressure may be regarded as nil, while 
carbonic acid is so abundant that the pressure of carbonic acid in them may 
be regarded as exceeding that of venous blood. An exception seems to be 
presented by the case of the lymph flowing along the larger lymphatic 
vessels, for in this the amount of carbonic acid, while usually higher than 
that of arterial blood, is lower than that of the general venous blood ; but 
this probably is due to the fact that the lymph in its passage onward is 
largely exposed to arterial blood in the connective tissues and in the lym- 
phatic glands, where the production of carbonic acid is slight as compared 
to that going on in muscles. All the facts point to the conclusion that it is 
the tissues, and not the blood, which become primarily loaded with carbonic 
acid, the latter simply receiving the gas from the former by diffusion, except 
the (probably) small quantity which results from the metabolism of the 
blood corpuscles; and that the oxygen which passes from the blood into the 
tissues is at once taken up and placed under such conditions that it is no 
longer removable by diminished pressure. 
We have seen that in muscle the production of carbonic acid is not 
directly dependent on the consumption of oxygen. The muscle produces 
carbonic acid in an atmosphere of hydrogen. What is true of muscle is 
true also of other tissues and of the body at large. It was shown long ago 
that animals might continue to breathe out carbonic acid in an atmosphere 
of nitrogen or hydrogen ; and this has more recently been illustrated by the 
remarkable experiment that a frog kept at a low temperature will live for 
several hours, and continue to produce carbonic acid, in an atmosphere abso- 
lutely free from oxygen. The carbonic acid produced during this period 
was made by help of the oxygen inspired in the hours anterior to the com- 
mencement of the experiment. The oxygen then absorbed was stowed away 
from the haemoglobin into the tissues, it was made use of to build up the 
explosive compounds, whose explosions later on gave rise to the carbonic 
acid. Or, to adopt a simile which has been suggested, the oxygen helps to 
wind up the vital clock ; but once wound up the clock will go on for a period 
without further winding. The frog will continue to live, to move, to pro- 
duce carbonic acid for a while without any fresh oxygen, as we know of old 
it will without any fresh food ; it will continue to do so till the explosive 
compounds which the oxygen built up are exhausted ; it will go on until the 
vital clock has run down. 
§ 303. To sum up, then, the results of respiration in its chemical aspects. 
As the blood passes through the lungs, the low' oxygen pressure of the venous 
blood permits the entrance of oxygen from the air of the pulmonary alve- 
olus, through the thin alveolar walls, through the thin capillary sheath, 
through the thin layer of blood-plasma to the red corpuscle, and the reduced 
haemoglobin of the venous blood becomes wholly, or all but wholly, oxy- 
haemoglobin. Hurried to the tissues, the oxygen, at comparatively high pres- 
sure in the arterial blood, passes largely into them. In the tissues the oxy- 
gen-pressure is always kept at an exceedingly low pitch by the fact that they, 
in some wray at present unknown to us, pack away at every moment into 
some stable combination each molecule of oxygen which they receive from 
the blood. With its oxy-haemoglobin largely but not wholly reduced, the 
blood passes on as venous blood. To what extent the haemoglobin is reduced 
will depend on the activity of the tissue itself. The quantity of haemoglobin 
in the blood is the measure of limit of the oxidizing power of the body at 
large; but within that limit the amount of oxidation is determined by the 
tissue, and by the tissue alone. THE NERVOUS MECHANISM OF RESPIRATION. 
369 
We cannot trace the oxygen through its sojourn in the tissue. We only 
know that sooner or later it comes back combined in carbonic acid (and 
other matters not now under consideration). Owing to the continual pro- 
duction of carbonic acid, the pressure of that gas in the extra-vascular 
elements of the tissue is always higher than that in the blood ; the gas 
accordingly passes from the tissue into the blood, and the venous blood 
passes on not only with its haemoglobin more or less reduced, i. e., with its 
oxy gen-pressure decreased, but also with its carbonic acid pressure increased. 
Arrived at the lungs, the blood finds the pulmonary air at a lower carbonic 
acid pressure than itself. The gas accordingly streams through the thin 
vascular and alveolar walls until the pressure without the bloodvessel is 
equal to the pressure within. At the same time the blood finds in the air of 
the pulmonary alveoli a supply of oxygen, more than adequate to convert, 
not entirely but nearly so, the reduced haemoglobin back again to oxy-haemo- 
globin. Thus the air of the pulmonary alveoli, having given up oxygen to 
the blood and taken up carbonic acid from the blood, having in consequence 
a higher carbonic acid pressure and a lower oxygen pressure than the tidal 
air in the bronchial passages, mixes rapidly with this by diffusion. The 
mixture is further assisted by ascending and descending currents ; and the 
tidal air issues from the chest at the breathing out poorer in oxygen and 
richer in carbonic acid than the tidal air which entered at the breathing in. 
The Nervous Mechanism of Respiration. 
§ 304. Breathing is an involuntary act. Though the diaphragm and all 
the other muscles employed in respiration are voluntary muscles, i. e., mus- 
cles which can be called into action by a direct effort of the will, and though 
respiration may be modified within very wide limits by the will, yet we 
habitually breathe without the intervention of the will; the normal breath- 
ing may continue, not only in the absence of consciousness, but even after 
the removal of all the parts of the brain above the medulla oblongata. 
We have already seen how complicated is even a simple respiratory act. 
A very large number of muscles are called into play. Many of these are 
very far apart from each other, such as the diaphragm and the nasal muscles ; 
yet they act in harmonious sequence in point of time. If the lower inter- 
costal muscles contracted before the scaleni, or if the diaphragm contracted 
alternately with the other chest-muscles, the satisfactory entrance and exit of 
air would be impossible. These muscles, moreover, are coordinated also in 
respect of the amount of their several contractions ; a gentle and ordinary 
contraction of the diaphragm is accompanied by gentle and ordinary con- 
tractions of the intercostals, and these are preceded by gentle and ordinary 
contractions of the scaleni. A forcible contraction of the scaleni, followed 
by simply a gentle contraction of the intercostals, would perhaps hinder 
rather than assist inspiration, and at all events would be waste of power. 
Further, the whole complex inspiratory effort is often followed by a less 
marked but still complex expiratory action. It is impossible that all these 
so carefully coordinated muscular contractions should be brought about in any 
other way than by coordinate nervous impulses descending along efferent 
nerves from a coordinating nervous Centre. By experiment we find this to 
be the case. 
When in a rabbit the trunk of a phrenic nerve is cut, the diaphragm on 
that side remains motionless, and respiration goes on without it. When both 
nerves are cut, the whole diaphragm remains quiescent, though the costal 
respiration becomes excessively labored. 
When an intercostal nerve is cut, no active respiratory movements are 370 
RESPIRATION. 
seen in the intercostal muscles of the corresponding space, and when the 
spinal cord is divided below' the origin of the seventh cervical spinal nerve, 
that is, below the exits of the roots of the phrenic nerves, costal respiration 
ceases, though the diaphragm continues to act, and that with increased vigor. 
When the cord is divided just below the medulla, all thoracic movements 
cease, but the respiratory actions of the nostrils and glottis still continue. 
These, however, disappear when the facial and recurrent laryngeal nerves 
are divided. We have already stated that after removal of the brain above 
the medulla, respiration still continues very much as usual, the modifications 
which ensue from the loss of the brain being unessential. Hence, putting 
all tiiese facts together, it is clear that the respiratory movements are, as we 
suggested, brought about by coordinated impulses which, developed in the 
central nervous system and starting in the first instance in the medulla, find 
their way along the several efferent nerves. The proof is completed by the 
fact that the removal of. or extensive injury to, the medulla alone is, save in 
exceptional cases which we will discuss presently, at once followed by the 
cessation of all respiratory movements, even though the rest of the nervous 
system including every muscle and every nerve concerned be left intact. 
Nay more, if only a small portion of the medulla, a tract whose limits have 
not been clearly defined, but which may be described as lying below' the 
vasomotor centre in the immediate neighborhood of the nuclei of the vagus 
nerves, be removed or injured, respiration ceases, and death at once ensues. 
Hence this portion of the nervous system was called by Flourens the vital 
knot, or ganglion of life, noeud vital. We shall speak of it as the respiratory 
centre. 
§ 305. The nature of this centre must be exceedingly complex ; for w hile 
even in ordinary respiration it gives rise to a whole group of coordinate ner- 
vous impulses of inspiration followed in due sequence by a smaller but still 
coordinate group of expiratory impulses of an antagonistic nature, in labored 
respiration fresh and larger impulses are generated, though still in coordina- 
tion with the normal ones, the expiratory events being especially augmented ; 
and in the cases of more extreme dyspnoea and asphyxia impulses overflow', 
so to speak, from it in all directions, though only gradually losing their 
coordination, until almost every muscle in the body is thrown into contrac- 
tions. 
We must not, however, conceive of this centre as one of such a kind that 
the impulses leave it fully coordinated and equipped so that nothing remains 
for them but to travel, unchanged, along the several efferent nerve-fibres to 
their several muscular destinations. On the contrary, we have reason to 
think that the respiratory motor nerves, like other motor nerves, are con- 
nected, just as they are about to issue from the spinal cord, with a nervous 
machinery in which nerve-cells playa part-a point which we shall consider 
more fully in treating of the spinal cord ; we have reason to think that the 
respiratory impulses starting from the respiratory centre pass into and are 
modified by secondary spinal nervous mechanisms before they issue along the 
motor nerve roots. Indeed, observations show that, under particular condi- 
tions, and especially in young animals, respiratory movements may be carried 
out in the entire absence of the medulla oblongata. Thus if, in a kitten or 
puppy or young rabbit, after division of the spinal cord below the medulla 
artificial respiration be kept up, and then pauses be made in the artificial 
respiration, during these pauses not only may what appear to be respiratory 
movements be induced in a reflex manner, by pinching or by blowing on the 
skin, but, especially if the excitability of the spinal cord be heightened by 
small doses of strychnine, even spontaneous efforts of breathing may occa- 
sionally be observed. These are the exceptional instances mentioned above. THE NERVOUS MECHANISM OF RESPIRATION. 
371 
Since in such cases the rhythmically repeated movements of the respiratory 
muscles are sometimes accompanied by rhythmic movements of the fore and 
hind limbs not respiratory in nature, it may be doubted whether these 
experiments really prove the existence of distinct respiratory centres in the 
spinal cord ; and at most they merely show that the respiratory nervous 
mechanism is not entirely confined, as was once thought, to the centre in the 
medulla, but also embraces other subsidiary mechanisms, which may perhaps 
be spoken of as centres, in the spinal cord below. It has, indeed, been main- 
tained by some that these lower spinal centres are the chief centres, and that 
the medullary centre acts merely in the way of regulating these; but it is diffi- 
cult to reconcile this view with the experience that interference with the 
medulla, limited entirely to the medulla, so often leads to the entire aboli- 
tion of the respiratory movements. This matter is not at present thoroughly 
worked out, but we shall probably not greatly err in regarding the respira- 
tory nervous system as in many ways analogous to the vasomotor nervous 
system, with its head centre in the medulla and secondary centres elsewhere, 
and in continuing to speak of the centre in the medulla as being " the respira- 
tion centre," while admitting that it works through other nervous machinery 
placed lower down in the spinal cord, and that this subordinate machinery 
may in exceptional cases carry out, though inadequately, the work of the 
chief centre. 
§ 306. Admitting then the existence of this medullary respiratory centre 
the question naturally arises, Are we to regard its rhythmic action as due 
essentially to changes taking place in itself, or as due to afferent nervous 
impulses or other stimuli which affect it in a rhythmic manner from with- 
out? In other words, Is the action of the centre automatic or purely 
reflex? We know that the centre maybe influenced by impulses proceed- 
ing from without, and that the breathing may be affected by the action of 
the will, or by an emotion, or by a dash of cold water on the skin, or in a 
hundred other ways; but the fact that the action of the centre may be thus 
modified from without, is no proof that the continuance of its activity is de- 
pendent on extrinsic causes. 
In attempting to decide this question we naturally turn to the pneumo- 
gastric as being the nerve most likely to serve as the channel of afferent 
impulses setting in action the respiratory centre. If both vagus nerves be 
divided, respiration still continues, though in a modified form. This proves 
distinctly that afferent impulses ascending those nerves are not the efficient 
cause of the respiratory movements. We have seen that when the spinal 
cord is divided below the medulla, the facial and laryngeal movements still 
continue. This proves that the respiratory centre is still in action, though 
its activity is unable to manifest itself in any thoracic movement. But 
when the cord is thus divided, the respiratory centre is cut off from all sen- 
sory impulses, save those which may pass into it from the cranial nerves of 
sensory function ; and that these sensory cranial nerves are not specially 
concerned in developing the activity of the respiratory centre is shown by 
the fact that the division of these cranial nerves by themselves, when the 
medulla and spinal cord are left intact, does not do away with the continu- 
ance of respiration. One cranial nerve, as we shall see, is especially con- 
cerned in respiration, viz., the vagus nerve; but if, after removal of the 
brain above the medulla, both vagus nerves be divided, respiration still 
goes on ; indeed, the respiratory impulses proceeding from the centre are, 
though in a peculiar way, exaggerated. Hence, though we cannot put the 
matter to an experimental test by dividing every sensory nerve in the body, 
while leaving the motor nerves of respiration intact, such an operation being 
practically impossible, we may infer that the respiratory impulses proceed- 372 
RESPIRATION. 
ing from the respiratory centre are not simply afferent impulses reaching the 
centre along afferent nerves ami transformed by reflex action in that centre. 
They evidently start de novo from the centre itself, however much their 
characters may be affected by afferent impulses, reaching that centre at 
the time of their being generated. The action of the centre is automatic, 
not simply reflex. 
$ 307. AVe find, on inquiry, that the activity of the centre is profoundly 
influenced by two classes of events. These, as we might expect, are, on the 
one hand, events producing changes in the quality of the blood distributed 
to the medulla by the left ventricle, especially as regards its gases, that is to 
say, events modifying the interchange taking place in the lungs ; and, on the 
other hand, nervous impulses started in various ways and reaching the centre 
along various nerves or nervous tracts. It will be convenient to consider the 
latter first. 
Afferent nervous impulses may affect the centre in many various ways. 
The whole act of breathing or of taking a breath is a double act, consist- 
ing of an inspiration and an expiration, and nervous impulses may especially 
affect the one or the other. One mode of breathing may differ from another 
in the depth of the individual breath, in the volume of air taken in and given 
out; and nervous impulses may increase or may diminish the depth of a 
breath, the volume of air respired. One mode of breathing again differs 
from another in the rapidity with which one breath succeeds another, that 
is, in the rate of rhythm ; and nervous impulses may slow or may quicken 
the rate of rhythm. Then, again, combination of affects so numerous and 
varied as almost to baffle description, may result from the influence of vari- 
ous nervous impulses. Emotions may affect a single breath or a long 
series of breaths, may quicken the rhythm while making each breath 
Fig. 99. 
Effect on Respiration of Section of One Vagus. The vagus was divided at the point marked 
x. The curve was obtained by means of a tambour connected with a receiver into which the 
animal (rabbit) breathed, as shown in Fig. 90, the lever falling in inspiration as air is sucked out 
of the tambour and rising in expiration as the air returns. Inspiration begins at a and ends at 
b. Expiration begins at b and ends at c. The lever gradually falls between c and a, owing to the 
escape of air from the apparatus. 
more shallow, or may, at the same time, make each breath deeper, or may 
slow the rhythm in either the one or the other manner, and may bear chiefly 
on inspiration or on expiration. Moreover, there is not an afferent nerve 
in the body which by means of afferent impulses passing along it may 
not be the instrument of influencing the respiratory centre. Of all the 
automatic centres in the body the respiratory centre is the one whose in- 
dependence is most obscured by the repeated effects of afferent nervous 
impulses. THE NERVOUS MECHANISM OF RESPIRATION. 
373 
Certain afferent nerves, however, appear to be more closely connected 
with it than others; and of these the most conspicuous and important are 
the two vagus nerves, which we have already mentioned in this connection. 
Their importance is well illustrated by the following experiments: If one 
vagus be divided in an ordinary way, without any special precautions, the 
respiration is either not materially changed, or if affected becomes slower 
(Fig. 99). If both be divided (Fig. 100) it becomes very slow, the pauses 
between expiration and inspiration being markedly prolonged. The cha- 
racter of the respiratory movement, too, is markedly changed ; each respira- 
tion is fuller and deeper, so much so, indeed, that, according to some observ- 
ers, what is lost in rate is gained in extent, the amount of carbonic acid pro- 
Fig. 100. 
Effect on Respiration of Section of Both Vagus Nerves. The curve was obtained in the same 
way as Fig. 99. The second vagus nerve was divided at x. 
duced and oxygen consumed in a given period remaining after division of 
the nerves about the same as when these were intact; but it is undesirable 
to insist too much on the exactness of this compensation. 
Fig. 101. 
Quickening of Respiration by Gentle Stimulation of the Central End of the Vagus Trunk. The 
curve was obtained in the same way as Figs. 99,100. Stimulation of the vagus began at x and 
ended at y. 
When after division of both vagus nerves in the neck, the medulla being 
intact, the central stump, that connected with the central nervous system, of 
one of them is stimulated with a gentle interrupted current, the effects are 
not always the same; one of two results may follow, and that whichever of 
the two nerves be used. In a certain number of cases, and these may, per- 
haps, be regarded as the more typical ones, the respiration, which from the 374 
RESPIRATION. 
division of the nerves had become slow, is quickened again (Fig. 101), and 
with care, by a proper application of the stimulus, the normal respiratory 
rhythm may for a time be restored. Upon the cessation of the stimulus the 
slower rhythm returns. If the current be increased in strength, the rhythm 
may in some cases be so accelerated that inspiration begins before the expira- 
tion of the preceding breath is completed (Fig. 102), and this may go on 
Fig. 102. 
Stimulation of Vagus Leading to Inspiratory Increase: This curve, unlike the preceding, was 
obtained by inserting a needle through the body wall, so as to rest on the diaphragm, and attach- 
ing a lever to the needle. (See §271.) The lever rises with each contraction of the diaphragm, 
so that inspiration begins at a and ends at b, expiration begins at b and ends at c, the interval be- 
tween c and a corresponding to the pause. Stimulation of the vagus begins at x. It will be seen 
that upon stimulation the inspiratory rises of the lever begin long before the preceding expira- 
tions are complete. 
until at last the diaphragm is brought into a condition of prolonged tetanus, 
and a standstill of respiration in an extreme inspiratory phase is the result. 
On the other hand, in a certain number of cases the result is of an opposite 
character. Even though the respiration be already slowed by division of 
the nerves, stimulation produces a still further slowing, the pauses between 
each expiration and the succeeding inspiration are prolonged (cf. Fig. 103), 
and in a certain number of cases actual standstill is brought about, but a 
standstill of a kind the opposite of the one just described, since the dia- 
phragm which in that case was in prolonged tetanus is in this case completely 
relaxed and remains for some time in the condition in which it is at the close 
of an ordinary breath. In a certain number of cases, and these are not 
uncommon, the result is intermediate between the two above extremes ; the 
diaphragm stands still in a prolonged contraction in a position which is inter- 
mediate between the height of inspiration and expiration. 
These results suggest the conclusion that the vagus nerve (we are dealing 
now with the main trunk of the nerve) contains afferent fibres of two kinds 
connected with the respiratory centre; one kind augmenting the action of 
the centre somewhat in the same way as the augmentor cardiac fibres aug- 
ment the beat of the heart, and the other kind having an inhibitory effect. 
Apparently sometimes the one and sometimes the other kind is, according to 
circumstances, most provoked by the stimulation, much in the same way 
as stimulation of the vagus in the frog, which, as we have seen, § 144, is 
the channel for both inhibitory and augmentor cardiac impulses, produces 
sometimes inhibition, sometimes augmentation of the heart-beat. To affect 
the heart of course the stimulation of the vagus must be centrifugal, 
directed toward the periphery, whereas to affect the respiration it must be 
centripetal, applied to the part of the nerve connected with the brain ; and THE NERVOUS MECHANISM OF RESPIRATION. 
375 
while the usual effect on the heart of ordinary stimulation of the vagus is 
inhibition, augmentation only occurring in special cases, the most common 
effect on respiration is augmentation, though inhibition is not unfrequently 
seen. When the experiment is conducted on an animal under the full 
influence of chloral, stimulation of the vagus generally produces inhibition 
of respiration, probably because the chloral renders the respiratory centre 
more susceptible to inhibitory influences. 
§ 308. We said just now " the action of the centre " ; but the respiratory 
centre is a double one ; it gives rise to inspiratory and to expiratory afferent 
impulses, and these are antagonistic the one to the other. If inspiratory 
and expiratory impulses issued from the centre at the same time and in 
equal potency, there could be no breathing at all, they would neutralize 
each other's effects; and, indeed, any amount of inspiratory impulse is 
antagonistic to a simultaneous expiratory impulse, and vice versa. Hence, 
for the adequate services of the respiratory centre we might expect to find 
that each kind of afferent impulse ascending the vagus affected the centre 
in a double and opposite way, inhibiting expiration while augmenting inspira- 
tion, or inhibiting inspiration while augmenting expiration. If we allow 
ourselves to speak of the whole respiratory centre as consisting of two parts, 
one the inspiratory part, or inspiratory centre concerned in the issue of 
inspiratory impulses, and the other the expiratory part, or expiratory centre 
concerned in the issue of expiratory impulses, we may suppose that these 
centres are so related to each other that afferent impulses, reaching the 
medulla, which augment or inhibit the one, necessarily inhibit or augment 
the other. We need perhaps hardly add that of these two centres we should 
expect to find the inspiratory centre the dominant and the most responsive 
one; in normal breathing it comes almost alone into obvious use, since, as 
we have seen, the expiratory muscles have then a very slight task only, the 
chest being emptied chiefly by elastic reaction ; and, speaking generally, 
breathing in is the first consideration-we breathe out mostly because we 
have already breathed in. 
There are many facts which support this view of the double antagonistic 
action of afferent respiratory impulses. If the central end of the superior 
laryngeal branch of the vagus be stimulated the effects are much more con- 
stant than those of stimulating the main vagus trunk. Whether the main 
trunk of the nerve be previously severed or not, the result of centripetal 
stimulation of the superior laryngeal branch is always in the direction of a 
slowing of the respiration (Fig. 103) ; and this may by proper stimulation 
be carried so far that a complete standstill of respiiation in the phase of 
rest is brought about. While the main trunk of the vagus contains fibres 
of two kinds, both augmentory and inhibitory of inspiration, the superior 
laryngeal branch appears to contain one kind only, those which inhibit 
inspiration. If now while this experiment is being conducted on a rabbit 
the abdomen be watched, it will be seen that the inhibition of inspiration is 
accompanied by a contraction of the abdominal muscles, that is by an effort 
at expiration ; the stimulation of the nerve while inhibiting respiration pro- 
vokes, to a certain extent, expiration. 
§ 309. That the trunk of the vagus is the channel of these two kinds 
of impulses, of a mutually antagonistic character, is further shown by apply- 
ing what may be considered as natural stimuli to the endings of the nerve 
in the lungs ; and the results so obtained have an especial value since the 
artificial stimulation of a nerve-fibre, at a part of its course by means of an 
electric current is at best a rough process, by which we cannot hope to do 
more than approximate to the results actually taking place in the living 
body when the nerve is stimulated at its endings by natural stimuli; and 376 
RESPIRATION. 
the approximation is perhaps less in the case of the exquisitely sensitive 
respiratory centre than in many other cases. 
Fig. 103. 
Slowing of Respiration by Stimulation of Superior Laryngeal Nerve. This curve was obtained 
in the same way as Eigs. 99,100,101, and the letters have the same meaning as in those figures. 
Stimulation begins at x and ends at y. 
If in an animal in which a careful graphic record of the respiratory 
movements is being taken, the trachea be suddenly closed at the summit of 
Fig. 104. 
Effects of Distention and Collapse of Lung. (Head.) Both curves are described by a lever 
attached, as stated in |271, to a slip of the diaphragm of a rabbit. A contraction of the diaphragm 
(inspiration) raises the lever; during relaxation of the diaphragm the lever fails. In A, the 
trachea is closed at x, the height of inspiration ; a pause follows during which the lever gradually 
sinks until an inspiration (a very powerful one) sets in. In B, the trachea is closed at the end of 
expiration, x; there follow powerful inspirations. 
an inspiration, the result is a pause before the succeeding inspiration follows, 
that is to say, a partial or temporary inhibition of inspiration ; and if during 377 
THE NERVOUS MECHANISM OF RESPIRATION. 
such an experiment on a rabbit a curve be taken by means of the isolated 
slip of the diaphragm, § 271, it will be seen (Fig. 104, A) that the slip elon- 
gates somewhat; that is to say, previously in a state of slight tonic contrac- 
tion, it changes in the direction of expiration. If, on the other hand, the 
trachea be suddenly closed at the end of an expiration (Fig. 104, B), when 
the lungs have returned to their emptied condition, the result is an increase 
of the sequent inspirations, that is to say, an augmentation of inspiratory 
impulses. If the chest or if the lung only be gently inflated, a temporary 
cessation of all inspiration may be produced, accompanied sometimes by an 
attempt at expiration. If, on the other hand, air be sucked out of the 
Fig. 105. 
Effects of Repeated Inflations. Positive Ventilation. (Head.) The lower curve is described, 
as in Fig. 104, by a lever attached to a slip of the diaphragm. The upper curve shows the infla- 
tions from x to y, which were made without any attempt to draw the air out at each inflation : 
each rise on this curve denotes an inflation. It will be observed that as the inflations are con- 
tinued the respiratory movements of the diaphragm are gradually " knocked down." 
chest, or if one lung be made to collapse by puncture of one pleural cham- 
ber, a prolonged inspiration is the frequent result, the diaphragm being 
thrown into a prolonged inspiratory tetanus. If the lungs are repeatedly 
inflated, without any means being taken to draw out the air after each infla- 
tion (Fig. 105), a procedure which we may speak of as positive ventilation, 
the result is that the inspiratory efforts are diminished, and if the ventila- 
tion is continued may cease altogether. If, on the other hand, air is repeat- 
edly sucked out of the lungs without any corresponding inflations (negative 
ventilation), the inspiratory efforts are increased (Fig. 106), and the increase 
may be such as to bring the diaphragm to a state of tetanus. And in 
general, though several complications occur which we cannot discuss here, 
the results of inflation of the lungs on the one hand and of suction or col- 
lapse of the lungs on the other hand, show that the mere inflation or per- 
haps rather the mere distention of the lung tends to inhibit inspiratory and 
usher in expiratory impulses, while collapse of the lung tends to inhibit 
expiratory and to develop inspiratory impulses, the effect on the inspiratory 
impulses, as might be expected from the dominance of the inspiratory por- 
tion of the centre, being more marked than the effect on the expiratory 
impulses. That the instrument by which these effects are produced is the 
vagus nerve is shown by the fact that they are no longer distinctly recog- 
nizable when both vagus nerves are divided. And that the results are due 
to the mere mechanical expansion and collapse of the lung in insufflation 
and collapse, and not to any chemical influences exerted by the larger 
amount or smaller amount of air present in the lung in the two cases 
increasing or diminishing the absorption of oxygen and escape of carbonic 
acid, is shown by the fact that the results remain in their main features 
the same when some indifferent gas, such as hydrogen, is used for inflation 
instead of air or oxygen. We infer therefore that the expansion of the 
pulmonary alveoli in some way or other so stimulates the endings in the 378 
RESPIRATION. 
lung of the pulmonary branches of the vagus, that impulses are generated 
which ascending the vagus trunk inhibit the inspiratory processes in the 
respiratory centre ; and that conversely collapse of the lung similarly gen- 
erates impulses which are augmentative of inspiratory impulses. And, 
assuming on the strength of analogy the existence in the vagus of two sets 
of fibres we may say that expansion stimulates the endings of the fibres 
which inhibit inspiration and concurrently tend to augment expiration, 
while collapse stimulates the fibres which inhibit expiration and augment 
inspiration. The respiratory pump may thus be looked upon as a self-regu- 
Fig. 106. 
Effects of Repeated Suctions of the Lungs. Negative Ventilation. (Head.) The curve corre- 
sponds exactly to Fig. 105, except that the lungs are subjected to repeated suctions without 
corresponding inflations. The result is that the inspirations are repeated in such a way as to 
lead almost to an inspiratory tetanus of the diaphragm. 
lating mechanism; the expansion of the lungs which is the result of the 
efferent inspiratory impulses tends to check the issue of these impulses and 
to inaugurate the sequent expiration ; and the return of the lungs in expi- 
ration tends to set going the succeeding inspiration. 
The regulative influence exerted by impulses normally ascending the 
vagus nerves is further shown by the following striking experiment : As we 
have already seen, the brain above the medulla may be removed without 
any extraordinary change in the respiration taking place. We have also 
seen that when both vagus nerves are divided the respiration is slower and 
deeper, but is otherwise regular. If, however, after the removal of the brain 
above the medulla both vagus nerves are divided, if the respiratory centre 
be cut off at one and the same time from impulses passing down from the 
higher parts of the brain and from impulses ascending the vagus nerves, 
the result is that the respirations take on the form of a series of long con- 
tinued inspiratory spasms. It would seem as if there were a tendency in the 
respiratory centre to go off into tetanic inspiratory explosions, that this ten- 
dency is held in check by impulses from the brain when the vagus nerves 
are divided, and by impulses along the vagus nerves when the brain is re- 
moved, but meets with no adequate checks when impulses from both sources 
are cut off at the same time. 
$ 310. Hypotheses have been put forward to explain the changes in the 
respiratory centre which lead to the rhythmic discharge of inspiratory and 
expiratory impulses and the further changes which result from the advent 
of augmenting and inhibitory impulses; but these as yet remain mere hy- 
potheses, and it would not be profitable to discuss them here. We may add THE NERVOUS MECHANISM OF RESPIRATION. 
379 
that, though the analogy of the cardiac nervous mechanism, in which we can 
anatomically distinguish between augmentor and inhibitory fibres, justifies 
us in speaking of augmentor and inhibitory and respiratory fibres as exist- 
ing in the vagus nerve, we are not as yet able to distinguish them by ana- 
tomical methods. We may further add that, so exquisitely sensitive is the 
respiratory centre to these afferent impulses stimuli too slight to produce 
any appreciable effect when applied to afferent nerves connected with 
an ordinary centre, such as a spinal reflex centre, may produce marked 
effects on the respiratory centre. For instance, the feeble electric current 
which is developed when the cut end of a divided vagus is replaced in the 
wound, the circuit between the cut end and the longitudinal surface of the 
nerve being closed through the blood or lymph of the wound, is often suffi- 
cient to develop inhibitory impulses. Again, when the connection of the 
respiratory centre with the lungs through the vagus nerves is abolished, not 
by section of the nerves, but by freezing both nerves at some part of the 
course of each nerve (an operation which, while completely blocking the 
passage of impulses along the nerve-fibres, does not itself act as a stimulus), 
the effect on the respiratory movements is much more in the direction of in- 
creasing and prolonging the inspiratory act than that of slowing the rhythm. 
Hence it would appear that what we have previously described as the result 
of dividing both vagus nerves is partly due to the blocking of natural im- 
pulses and partly to the section of the nerves, and possibly to electric cur- 
rents developed as suggested above, acting as stimuli and thus giving rise to 
artificial impulses. 
§ 311. The double or alternate respiratory action of the vagus nerves, on 
which we have dwelt above, may be taken as, in a general way, illustrative 
of the manner in which other afferent nerves and various parts of the cere- 
brum are enabled to influence respiration. As we have already said, and, 
indeed, know from daily experience, of all the apsychical nervous centres 
the respiratory centre is the one which is most frequently and most deeply 
affected by nervous impulses from various quarters. Besides the changes 
brought about by the will (and when we breathe voluntarily we probably 
make use, to some extent, of the normal nervous machinery of respiration, 
working through this, rather than sending independent volitional impulses 
direct to the diaphragm and other respiratory muscles), we find that 
emotions and painful sensations alter profoundly the character of the 
respiratory movements. And though these effects may be partly indirect 
(the emotion modifying the heart-beat or the tonus arteries, and so in- 
fluencing the flow of blood through the respiratory centre), they are 
chiefly due to the direct action of nervous impulses reaching that centre 
from higher parts of the brain. So, also, impulses from almost every sen- 
tient surface or passing along almost every sensory nerve may modify res- 
piration in one direction or another. The influence in this way of stimuli 
applied to the skin is well known to all ; but, perhaps, next to the vagus 
the nerve most closely connected with the respiratory centre is the fifth 
nerve, branches of which guard the nasal respiratory channels; the 
slightest stimulation of the nostrils at once affects the breathing and most 
frequently arrests it. The effects of stimuli of various strengths brought 
to bear on various nerves are very varied. Sometimes the result is an 
increase of inspiration, and that either by a quickening of the rhythm 
or by an increase of the individual breaths or by a combination of the 
two. Sometimes the result is inhibition of inspiration, accompanied or 
not by an increase of expiration, and sometimes, as when the stimulation 
causes a cough, the expiratory results may be out of all proportion to the 
modifications of inspiration. While in the case of some nerves, for instance, 380 
RESPIRATION. 
as we have seen, the superior laryngeal, and, it is said, the splanchnic nerves, 
the effects are exclusively, or, at least, chiefly inhibitory of inspiration and 
augmentative of expiration, that is, expiratory, and in others, perhaps, chiefly 
augmentative of inspiration, or inspiratory, in the case of most nerves the 
effect may be, according to circumstances, either in the one direction or the 
other. Perhaps, as a rule, weak stimuli tend to augment, and strong to in- 
hibit inspiration ; but the effects of artificially stimulating sensory nerves 
are complicated and often confused, because powerful afferent impulses by 
giving rise to pain may, through impulses generated by the pain itself and 
descending to the medulla from the brain, act in an indirect as well as in a 
direct manner ; and the prominence of the indirect painful impulses will, in 
any experiment, depend on the anaesthetic used. We may say, however, that in 
all cases the effect is very largely determined by the condition at the time 
being of the respiratory centre itself; and that, in turn, is determined not 
only by things which affect its nutrition, such as the character of the blood 
circulating in it, but also by the nature and amount of the other afferent 
impulses which are playing upon it at the same time. Thus, as we shall pres- 
ently see, the effect of a stimulus applied to the vagus, when the respiratory 
centre is inadequately supplied with arterial blood, is not the same when the 
centre has its normal supply of normal blood. So also a stimulus which, 
applied to the vagus or to another nerve in an intact animal, simply 
quickens inspiration, applied in an animal whose cerebral hemispheres 
have been removed will call forth a prolonged tetanic inspiratory gasp. 
The respiratory centre responds, in fact, in the most intricate and varied 
manner to nervous impulses proceeding from all parts of the body, and 
thus delicately adjusts the working of the respiratory pump to the needs 
of the economy. 
§ 312. The complicated nature of the respiratory centre is further shown 
by the fact that it appears to consist of two lateral halves which normally 
work in unison and yet may be made to work independently. If the me- 
dulla oblongata be carefully divided in the middle line respiration may con- 
tinue to go on in quite a normal fashion. If, however, one vagus be then 
divided, the respiratory movements, both costal and diaphragmatic, on the 
side of the body on which division of the vagus has taken place, become 
slower than those on the other side, so that the two sides are no longer 
synchronous ; and a stimulus confined to one vagus affects the respiratory 
movements of that side of the body only. So, also, a section of a lateral 
half of the cord below the medulla stops the respiratory movements on that 
side alone. 
§ 313. Besides these nervous influences, however, there is another cir- 
cumstance which, perhaps, above all others affects the respiratory centre, and 
that is the condition of the blood in respect to its respiratory changes ; the 
more venous (less arterial) the blood, the greater is the activity of the respira- 
tory centre. When by reason either of any hindrance to the entrance of air 
into the chest or other interference with the due interchange between the 
blood and the pulmonary air or of a greater respiratory activity of the tissues, 
as during muscular exertion, the blood becomes less arterial, more venous, 
i. e., with a smaller charge of oxygen and more heavily laden with carbonic 
acid, the respiration from being normal becomes labored. We may speak of 
normal breathing as eupnoea, and say that this, when the blood is insufficiently 
arterialized, passes into dyspnoea, an intermediate stage in which the respira- 
tory movements are simply exaggerated being known as hyperpncea. The 
modifications of breathing thus caused by deficient arterialization of blood 
are especially characterized by an increase in the total energy of the respira- 
tory impulses generated, and in this respect differ from the modifications 381 
THE NERVOUS MECHANISM OF RESPIRATION. 
resulting from interference with the nervous arrangements such as those fol- 
lowing upon section of the vagus nerves, in which case, as we have seen, the 
rhythm is much more profoundly affected than the amount. In dyspnoea 
the breathing is frequently quicker as well as deeper, there is an increase in 
the sum of efferent respiratory impulses, and the expiratory impulses, which 
in normal respiration are very slight, acquire a pronounced importance. As 
the blood becomes in cases of obstruction less and less arterial, more and 
more venous, the discharge from the respiratory centre becomes more and 
more vehement, ami instead of confining itself to the usual tracts and pass- 
ing down to the ordinary respiratory muscles, overflows into other tracts and 
puts into action other muscles, until there is, perhaps, hardly a muscle in the 
body which is not made to feel its effects. The muscles which are thus more 
and more thrown into action are especially those tending to carry out or to 
assist expiration ; and at last, if no relief is afforded, the violent but still 
definite respiratory movements give way to general convulsions of the whole 
body, which, however, have to a certain extent an expiratory character. 
With the onset of these convulsions dyspnoea is said to have passed into 
asphyxia. By the violence of these convulsions the whole nervous system 
becomes exhausted, the convulsions cease, and death is ushered in through a 
few infrequent and long-drawn breaths; but to this matter we shall return. 
The effect of venous blood, then, is to augment all those natural explosive 
decompositions of the substance of the central nervous system which give 
rise to respiratory impulses ; it increases their amount and also quickens their 
rhythm. The latter change, however, is much less marked than the former, 
the respiration being much more deepened than hurried, and the several 
respiratory acts are never so much hastened as to catch each other up, and 
so to produce an inspiratory tetanus like that resulting from stimulation of 
the vagus. On the contrary, especially as exhaustion begins to set in, the 
rhythm becomes slower than proportionate to the weakening of the individual 
movements. 
§ 314. The question naturally arises, Does this condition of the blood 
affect the substance of the central nervous system, that is to say, the respira- 
tory centre in the medulla (and the subsidiary spinal nervous mechanisms) 
directly, or does it produce its effect by stimulating the peripheral ends of 
afferent nerves in various parts of the body, and, by the generation there of 
afferent impulses, indirectly modify the action of the central nervous system? 
Without denying the possibility that the latter mode of action may help in 
the matter, as regards not only the vagus, but all afferent nerves, the follow- 
ing facts seem to show that the main effect is produced by the direct action 
of the blood on the central nervous system, and, indeed, on the medullary 
respiratory centre itself. If the spinal cord be divided below the medulla 
oblongata, and both vagi be cut, want of proper aeration of the blood still 
produces an increased activity of the respiratory centre, as shown by the 
increased vigor of the facial respiratory movements. If the supply of blood 
be cut off from the medulla by ligature of the carotid and intervertebral 
arteries dyspnoea is produced, though the operation produces at first no 
change in the blood generally, but simply affects the respiratory condition 
of the medulla itself by cutting off its blood-supply, the immediate result 
of which is an accumulation of carbonic acid and a paucity of available 
oxygen in the nervous substance of that region. If the blood in the carotid 
artery in an animal be warmed above the normal, a dyspnoea is produced 
which, though apparently not quite identical with the dyspnoea caused by 
imperfect arterialization of the blood, shows that the too high temperature 
of the blood directly affects the activity of the respiratory centre. We may 
conclude, therefore, that the condition of the blood affects respiration by 382 
RESPIRATION. 
acting directly on the respiratory centre. Moreover, it is the medullary 
centre which, at all events in adult animals, is affected by the too venous 
blood, since after the division of the spinal cord below the medulla, dyspnoeic 
thoracic respiratory movements and convulsions do not follow upon exclusion 
of air. They are, however, stated to occur in new-born animals, indicating 
that the subsidiary mechanisms in the upper spinal cord, of which we spoke 
in § 306. may be also affected by the too venous blood ; but the doubts which 
we previously urged hold good in these cases also. 
While the respiratory centre is thus being affected by the too venous 
blood, it is, until exhaustion begins to set in, more irritable, more easily and 
largely affected by afferent impulses than in its normal condition. During 
dyspnoea a stimulus which applied to the vagus or to some other sensory nerve 
under normal conditions would produce little or no effect, may start very 
powerful respiratory movements. 
§ 315. Deficient aeration produces two effects in blood : it diminishes the 
oxygen and increases the carbonic acid. Do both of these changes affect 
the respiratory centre, or only one, and if so, which ? When an animal is 
made to breathe an atmosphere containing nitrogen only, the exit of car- 
bonic acid by diffusion is not affected, and the blood, as is proved by actual 
analysis, contains no excess of carbonic acid. Yet all the phenomena of 
dyspnoea are present, and if the experiment be continued, convulsions ensue 
and the animal dies in asphyxia. In this case the result can only be 
attributed to the deficiency of oxygen. On the other hand, if an animal be 
made to breathe an atmosphere rich in carbonic acid, but at the same time 
containing abundance of oxygen, though the breathing becomes markedly 
deeper and also somewhat more frequent, there is no culmination in a con- 
vulsive asphyxia, even when the quantity of carbonic acid in the blood, as 
shown by direct analysis, is very largely increased. On the contrary, the 
increase in the respiratory movements may after a while pass off' the animal 
becoming unconscious, and appearing to be suffering rather from a narcotic 
poison than from simple dyspncea ; the excess of carbonic acid in the blood 
appears to affect other parts of the central nervous system, and especially 
portions of the brain, more profoundly than it does the respiratory centre. 
It has been maintained by some that while a deficiency of oxygen promotes 
inspiratory movements, an excess of carbonic acid stimulates the expiratory 
movements, the nervous mechanisms being so arranged that a lack of oxygen 
leads to an effort to get more of it, and a too great load of carbonic acid to 
an effort to get rid of it; but the facts are opposed to the existence of any 
such teleological adaptation. It is obvious, however, that a lack of oxygen 
and an excess of carbonic acid affect the respiratory centre in very different 
ways, and that in ordinary cases of interference with the interchange in the 
lungs, as in deficient aeration, it is the lack of oxygen which plays the 
principal part in developing the abnormal respiratory movements. We 
may infer that it too is chiefly concerned in regulating the more normal 
respiration, but cannot as yet say what is the exact share so be attributed to 
the carbonic acid. 
We may here point out that it is not to be supposed that each breath is 
determined by the condition of the blood flowing through the capillaries of 
the medulla at the moment preceding that breath ; it is not to be imagined 
that each breath is the result of the lack of oxygen felt immediately before. 
On the contrary, as we have previously urged, the respiratory centre, like 
the cardiac substance, is an automatic centre; the respiratory impulses issue 
from it in rhythmic series as a result of the molecular changes of the 
metabolism going on in its substance ; and whatever affects that rhythm, 
whether few or many beats be influenced, produces its result by modifying THE NERVOUS MECHANISM OF RESPIRATION. 
383 
that metabolism. A lack of oxygen in the blood, or a nervous impulse 
along an afferent fibre, both affect the centre by modifying its metabolism ; 
but each probably affects it in a different way. It is beyond our present 
knowledge to explain how either the one or the other acts. We may imagine 
that a lack of oxygen on the other hand has a more profound effect in modi- 
fying the whole complex series of metabolic changes, the whole chain of 
building up and breaking down processes, thus in some way or other ren- 
dering the whole edifice, so to speak, more unstable; and that an afferent 
augmenting impulse (and possibly an excess of carbonic acid) acts rather 
after the fashion of what we are accustomed to call a stimulus, and fires off 
a larger amount of the already stored up explosive compounds. And we 
may further imagine that the special feature of the substance of the respira- 
tory centre is that the metabolism is so arranged as to be thus, unlike that of 
other living substances, rendered unstable and more explosive, not simply 
diminished or deadened by a lack of oxygen. But these as yet are matters 
of speculation. 
We may, perhaps, add that, under various nutritive conditions, the sensi- 
tiveness of the metabolism of the respiratory centre to lack of oxygen may 
vary widely. Thus, while undoubtedly under the normal nutritive condi- 
tions afforded by the ordinary supply of normal blood to the medulla, lack 
of oxygen in that blood at once provokes increased respiratory movements, 
it need not do so under other nutritive conditions of the medulla. By trans- 
fusion a large proportion of the haemoglobin-holding blood may in an animal 
be gradually replaced by haemoglob in less normal saline solution. In such a 
case the amount of oxygen brought to the medulla by the diluted blood 
must be greatly diminished, and yet if the change be made sufficiently 
slowly, no conspicuous dyspnoea is produced ; under the new strange nutri- 
trive conditions of the diluted blood the medulla is not affected in the same 
way as before by lack of oxvgen. 
§316. There are reasons for thinking that conditions of the blood, other 
than variations in the amount of oxygen and carbonic acid, may also mate- 
rially affect the working of the respiratory centre. It is a matter of common 
experience that muscular exertion, especially if at all excessive, increases 
the respiratory movements; violent exercise soon puts a man " out of 
breath." This increased activity of the respiratory centre is in large meas- 
ure at all events caused by the character of the blood which during and for 
some little time after the movements is carried to the medulla, and not by 
any nervous impulses sent up to the medulla from the contracting muscles. 
This is shown by the fact that if in an animal the spinal cord be divided in 
the dorsal or lumbar region and the hind limbs be powerfully tetanized, the 
respiratory movements are increased ; the animal pants as it would do if it 
had been running. In such a case the only connection between the hind 
limbs and the respiratory centre is through the blood ; it must be some 
change in the blood caused by the muscular contractions which affects the 
medulla when the blood passes from the hind limbs to be distributed by the 
heart to the medulla. Now when a muscle contracts, its consumption of 
oxygen and production of carbonic acid, especially the latter (§ 60), are in- 
creased ; the blood leaving the muscle is more venous than usual. Hence, 
when many muscles are contracting powerfully, the blood carried to the 
right side of the heart is more venous than usual ; and we might expect 
that it is this unusually venous blood failing to be adequately arterialized in 
the lungs, and hence reaching the medulla from the left side of the heart in 
a more venous, less completely arterialized, condition than usual, which stirs 
up the respiratory centre to increased activity. 
On examination, however, it is found that the blood leaving the left side 384 
RESPIRATION. 
of the heart in such cases is not less arterialized, but, if anything, more 
arterialized than usual. The increased respiratory movements induced by 
the changed blood soon prove sufficient, or even more than sufficient, to give 
the blood the extra quantity of oxygen and to remove the extra quantity of 
carbonic acid. Obviously the blood coming from the tetanized muscles 
affects the respiratory centre by virtue of some quality which, unlike that 
due to the deficiency of oxygen or excess of carbonic acid, is not imme- 
diately affected by the passage through the lungs. Whether the quality in 
question be dependent on an excess of sarcolactic acid, or on some other 
product or products of muscular metabolism, we do not as yet know. But 
the fact that substances in the blood may so affect the respiratory centre is 
interesting, since it shows by how many safeguards the working of the re- 
spiratory centre is carefully adapted to the needs of the economy. 
Thus a change in the circumstances surrounding an animal body, or a 
change in the body itself, may, in one or more of several ways-by acting as 
a stimulus to some afferent nerves and so sending up afferent nervous im- 
pulses to the respiratory centre, or by interfering with the interchange of 
gases in the lungs, or by otherwise altering the proportion of the gases pres- 
ent in the blood reaching the respiratory centre, or by generating or increas- 
ing in that blood some substance or substances tending to affect the nutrition 
of the respiratory centre-affect the working of the all-important breathing 
mechanism. And the affection so wrought has generally an adaptative cha- 
racter ; it generally tends to protect the organism against the evil effects of 
the change. 
§ 317. Apnoea. When we attempt to hold our breath, we find that we 
can do this for a limited time only ; sooner or later a breath must come; but, 
as is well known, the time during which we can remain without breathing 
may on occasion be much prolonged, if we first of all take a series of deep 
breaths. It is probable, though perhaps not distinctly proved, that when 
we breathe voluntarily, or when by an act of the will we hold the respiratory 
apparatus in any one respiratory phase, the nervous impulses, generated by 
the will, do not pass down by a direct and independent course to the respira- 
tory muscles, but that the will makes use or modifies the activity of the 
medullary and spinal nervous respiratory mechanisms. The breath sooner 
or later inevitably follows because at last the natural impulses proceeding 
from the respiratory centre become too imperious to be any longer held in 
check by the impulses of volition passing down to the centre from the brain. 
The fact that a series of deep breaths, a thorough ventilation of the lungs, 
postpones the victory of the unconscious centre, shows that such a ventila- 
tion in some way delays the development of the natural respiratory impulses. 
A similar but still more marked delay may often be seen in an animal 
under artificial respiration. If in a rabbit artificial respiration is carried on 
very vigorously for a while, and then suddenly stopped, the animal does not 
immediately begin to breathe. For a variable period no respiratory move- 
ments at all take place, and breathing when it does begin occurs gently and 
normally, only passing into dyspnoea if the animal is unable to breathe of 
itself; and even then the transition is quite gradual. Evidently during this 
period the respiratory centre is in a state of complete rest, no explosions are 
taking place, no respiratory impulses are being generated, and the quiet 
transition from this condition to that of normal respiration shows that the 
subsequent generation of impulses is attended by no great disturbance. Not 
only is the centre at rest, but it is less irritable than the normal; impulses 
along the vagus or other nerves which otherwise would produce respiratory 
explosions are now ineffectual. This state of things is known as that of 
apnoea, the converse of dyspnoea; and the longer pause in breathing men- THE NERVOUS MECHANISM OF RESPIRATION. 
385 
tioned above as possible after unusual ventilation of the lungs may be re- 
garded as a brief apnoea. 
Now it seemed natural to suppose that such a state of rest of the respira- 
tory centre was brought about by the more than necessarily ample supply of 
oxygen afforded by the previous increased inspiratory movements; and, 
indeed, it was maintained that apnoea was the result of too great, just as 
dyspnoea is the result of too little, arterialization of the blood reaching the 
respiratory centre. It was argued that owing to the increased vigor of the 
artificial respiratory movements the haemoglobin of the arterial blood, which 
in normal breathing is not quite saturated with oxygen, became almost com- 
pletely so, and that at the same time the quantity of oxygen simply dissolved 
in the blood became largely increased and its tension largely augmented. 
But there are reasons which render such a view untenable. In the first 
place there is no direct and satisfactory proof that in apnoea the arterial 
blood is overloaded with oxygen as supposed ; indeed, during the course of 
apnoea before it has come to an end the blood becomes distinctly less arterial, 
more venous than usual, In the second place apnoea if not entirely impossi- 
ble, is much more difficult to bring about when both vagus nerves are divided, 
and if it does occur after section of the vagus nerves has not the same cha- 
racters as ordinary apnoea. Now, when artificial respiration is being carried 
on, section of the vagus nerves can have no effect on the quantity of oxygen 
taken up by the blood in the lungs. But the vagus nerves are the channel 
of impulses affecting the respiratory centre, and this relation of the apnoea 
to the vagus nerves suggests another and different interpretation of apnoea. 
As we have seen, expansion of the lung by acting in some way or other on 
the pulmonary terminations of the vagus nerve sends up along that nerve 
impulses which inhibit inspiration. And it is argued that repeated forcible 
inflations of the lungs produce apnoea by generating potent inhibitory im- 
pulses, which by a kind of summation of their effects in the medulla stop for 
a while the generation of respiratory impulses in the respiratory centre. 
This conclusion, moreover, is strongly supported by the fact that an apnoea 
may be produced, so long as the vagus nerves are intact, by forcible artificial 
respiration with hydrogen instead of atmospheric air ; in other words, the 
inhibitory impulses generated in the vagus nerves by the inflation are suffi- 
cient wholly to neutralize the development of respiratory impulses which 
the deficient arterialization of the blood would otherwise have produced. 
The exact nature and development of such a summation of inhibitory im- 
pulses, especially in the presence of correlative augmentative impulses called 
forth by the corresponding successive collapses of the lungs, is too complex a 
matter to be dwelt on here. Moreover, an apnoea may be produced, though, 
as we have said, with difficulty after section of both vagus nerves; but in 
this case air and not hydrogen must be used for inflation, the use of the 
latter, in contrast to the result when the nerves are intact, leading to dysp- 
noea. The subject cannot as yet be considered as fully cleared up. That 
apnoea as ordinarily produced is in some way the result of inhibitory im- 
pulses generated by the inflations can, however, hardly be doubted. 
$ 318. Secondary respiratory rhythm-Cheyne-Stokes respiration. A re- 
markable abnormal rhythm of respiration, first observed by Cheyne but 
afterward more fully studied by Stokes, and hence called by their combined 
names, occurs in certain pathological cases. The respiratory movements 
gradually decrease both in extent and rapidity until they cease altogether, 
and a condition of apnoea, lasting it may be for several seconds, ensues. 
This is followed by a feeble respiration, succeeded in turn by a somewhat 
stronger one, and thus the respiration returns gradually to the normal, or 
may even rise to hyperpnoea or slight dyspnoea, after which it again declines 
25 386 
RESPIRATION. 
in a similar manner. A secondary rhythm of respiration is thus developed, 
periods of normal or slight dyspnoeic respiration alternating by gradual 
transitions with periods of apnoea. The cause of the phenomena is not thor- 
oughly understood. Whether the waning and waxing of the respiratory 
movements be due to corresponding rhythmic changes in the nutrition of the 
respiratory centre itself, or to a rhythmic increase and decrease of inhibitory 
impulses playing upon that centre from other parts of the body, for instance 
from higher regions of the brain, has not yet been settled. It frequently appears 
in connection with a fatty condition of the heart, but has been met with in 
various maladies. Closely similar phenomena have been observed during 
sleep, under perfectly normal conditions; and this fact is rather in favor of 
the latter of the two explanations just given. The phenomena present a 
striking analogy with the " groups" of heart-beats so frequently seen in the 
frog's ventricle placed under abnormal circumstances. 
The Effects of Changes in the Composition and Pressure 
of the Air Breathed. 
§ 319. The preceding sections have shown us that the respiratory 
mechanism is arranged to work satisfactorily when the lungs are adequately 
supplied with air of the ordinary composition of, and at the ordinary pres- 
sure of, the atmosphere. We have further seen that the mechanism can 
adapt itself within certain limits to changes in the composition and pressure 
of the air supplied. We may now consider briefly what takes place when 
those limits are overstepped. The most striking effects are seen when, on 
account of occlusion of the trachea, or by breathing in a confined space, or 
for other reasons, a due supply of air not being obtained, normal respira- 
tion gives place, through an intermediate phase of dyspnoea, to the condition 
known as asphyxia; this, unless remedial measures be taken, rapidly proves 
fatal. 
Asphyxia. As soon as the blood becomes less arterial, more venous than 
normal, the respiratory movements become deeper and at the same time more 
frequent; both the inspiratory and expiratory phases are exaggerated, the 
supplementary muscles spoken of (§ 276) are brought into play, and the rate 
of the rhythm is hurried. These effects, as we have seen, are chiefly to be 
ascribed to the deficiency of oxygen in the blood. 
As the blood continues to become more and more venous the respiratory 
movements continue to increase both in force and frequency, a larger num- 
ber of muscles being called into action and that to an increasing extent. 
Very soon, however, it may be observed that the expiratory movements are 
becoming more marked than the inspiratory. Every muscle which can in 
any way assist in expiration is in turn brought into play; and at last almost 
all the muscles of the body are involved in the struggle. The orderly ex- 
piratory movements culminate in expiratory convulsions, the order and 
sequence of which are obscured by their violence and extent. That these 
convulsions, through which dyspnoea merges into asphyxia, are due to a 
stimulation (by the venous blood) of the medulla oblongata, is proved by 
the fact that they fail to make their appearance when the spinal cord has 
been previously divided below the medulla, though they still occur after 
those portions of the brain which lie above the medulla have been removed 
It is usual to speak of a "convulsive centre" in the medulla, the stimula- 
tion of which gives rise to these convulsions; but if we accept the existence 
of such a centre we must at the same time admit that it is connected by the 
closest ties with the normal expiratory division of the respiratory centre, 
since every intervening step may be observed between a simple slight ex- EFFECTS OF CHANGES IN COMPOSITION OF AIR. 
387 
piratory movement of normal respiration and the most violent convulsion of 
asphyxia. An additional proof that these convulsions are carried out by 
the agency of the medulla is afforded by the fact that convulsions of a 
wholly similar character are witnessed when the supply of blood to the 
medulla is suddenly cut off by ligaturing the bloodvessels of the head. In 
this case the nervous centres, being no longer furnished with fresh blood, 
become rapidly asphyxiated through lack of oxygen, and expiratory con- 
vulsions quite similar to those of ordinary asphyxia, and preceded like them 
by a passing phase of dyspnoea, make their appearance. Similar " anaemic " 
convulsions are seen after a sudden and large loss of blood from the body at 
large, the medulla being similarly stimulated by the lack of arterial blood. 
In ordinary fainting, which is loss of consciousness due to an insufficient 
supply of blood to the brain, the diminution of blood-supply is not great 
enough to produce these convulsions. 
Such violent efforts speedily exhaust the nervous system ; and the convul- 
sions after being maintained for a brief period suddenly cease and are fol- 
lowed by a period of calm. The calm is one of exhaustion ; the pupils, 
dilated to the utmost, are unaffected by light; touching the cornea calls 
forth no movement of the eyelids, and, indeed, no reflex actions can any- 
where be produced by the stimulation of sentient surfaces. All expiratory 
active movements have ceased ; the muscles of the body are flaccid and 
quiet; and though from time to time the respiratory centre gathers sufficient 
energy to develop respiratory movements, these resemble those of quiet 
normal breathing, in being, as far as muscular actions are concerned, almost 
entirely inspiratory They occur at long intervals, like those after section 
of the vagi; and like them are deep and slow. The exhausted respiratory 
centre takes some time to develop an inspiratory explosion ; but the impulse 
when it is generated is proportionately strong. It seems as if the resistance 
which had in each case to be overcome was considerable, and the effort in 
consequence, when successful, productive of a large effect. 
Very soon these inspiratory efforts become less frequent ; their rhythm 
becomes irregular; long pauses, each one of which seems a final one, are 
succeeded by several somewhat rapidly repeated inspirations. The pauses 
become longer, and the inspiratory movements shallower. Each inspiration 
is accompanied by the contraction of accessory muscles, especially of the 
face, so that each breath becomes more and more a prolonged gasp. The 
inspiratory gasps spread into a convulsive stretching of the whole body ; and 
with extended limbs, and a straightened trunk, with the head thrown back, 
the mouth widely open, the face drawn, and the nostrils dilated, the last 
breath is taken in. 
Thus we are able to distinguish three stages in the phenomena which 
result from a continued deficiency of air: 1. A stage of dyspnoea, charac- 
terized by an increase of the respiratory movements both of inspiration and 
expiration. 2. A convulsive stage, characterized by the dominance of the 
expiratory efforts, and culminating in general convulsions. 3. A stage of 
exhaustion, in which lingering and long-drawn inspirations gradually die 
out. When brought about by sudden occlusion of the trachea these events 
run through their course in about four or five minutes in the dog and in 
about three or four minutes in the rabbit. The first stage passes gradually 
into the second, convulsions appearing at the end of the first minute. The 
transition from the second stage to the third is somewhat abrupt, the con- 
vulsions suddenly ceasing early in the second minute. The remaining time 
is occupied in the third stage. 
The duration of asphyxia varies not only in different animals, but in the 
same animal under different circumstances. Newly born and young animals 388 
RESPIRATION. 
need much longer immersion in water before death by asphyxia occurs than 
do adults. Thus while in a full-grown dog recovery from drowning is 
unusual after one and a half minutes, a newborn puppy has been known to 
bear an immersion of as much as fifty minutes. The cause of the difference 
lies in the fact that in the quite young or rather just-born animal the respi- 
ratory changes of the tissues are much less active. These consume less 
oxygen, and the general store of oxygen in the blood has a less rapid de- 
mand made upon it. The respiratory activity of the tissues may also be 
lessened by a deficiency in the circulation ; hence bodies in a state of syn- 
cope at the time when the deprivation of oxygen begins can endure the loss 
for a much longer period than can bodies in which the circulation is in full 
swing. There being the same store of oxygen in the blood in each case, the 
quicker circulation must of necessity bring about the speedier exhaustion of 
the store. So also ansesthetics may diminish the effects and delay the final 
results: large doses of anaesthetics may prevent the exaggerated and con- 
vulsive movements. In many cases of drowning, death is hastened by the 
entrance of water into the lungs. 
By training, the respiratory centre may be accustomed to bear a scanty 
supply of oxygen for a much longer time than usual before dyspnoea sets in, 
as is seen in the case of divers. 
The phenomena of slow asphyxia, where the supply of air is gradually 
diminished, are fundamentally the same as those resulting from a sudden 
and total deprivation. The same stages are seen, but their development takes 
place more slowly. 
§ 320. Deficiency of air results not only in a diminution of the oxygen 
but also in an increase of the carbonic acid of the blood. We have seen, 
however (§ 315), that the phenomena of asphyxia are in the main due to the 
former, and that the accumulation of carbonic acid in the blood has subsid- 
iary effects only. 
If the percentage of oxygen in the inspired air be increased instead of 
diminished, the total pressure of the atmosphere remaining the same, the 
partial pressure of the oxygen alone being changed, no marked results 
follow. We have already seen (§ 297) that the percentage of oxygen in the 
ordinary atmosphere leaves a wide margin of safety, and that (§ 317) the 
phenomena of apnoea are in the main at least to be explained as the result 
not of an increase in the oxygen of the blood but of nervous impulses ascend- 
ing the vagus nerves. We have no satisfactory evidence that, provided the 
respiratory mechanism is in good working order, an increase of oxygen in 
the inspired air even to a whole atmosphere seriously modifies the respiratory 
act; and it may be doubted whether any effect is produced even when the 
mechanism is impaired. 
§ 321. The composition of the atmosphere, the pressure remaining the 
same, may be modified by the introduction of foreign gases. To some of 
these the respiratory mechanism is indifferent; for instance, hydrogen may 
be substituted for nitrogen without any change in the respiration, provided, 
of course, that the oxygen is not diminished. Other gases may produce 
poisonous effects, either by interfering with some of the respiratory processes 
or in other ways. Thus carbon monoxide, by combining with the haemo- 
globin of the red corpuscles, and so preventing the corpuscles from acting 
as oxygen-carriers, produces asphyxia through deficiency of oxygen. Sul- 
phuretted hydrogen interferes with the oxygenation of the blood by acting 
as a reducing agent. Some gases while allowing the ordinary respiratory 
changes of the blood to go on as usual produce toxic effects by acting on one 
or other of the tissues. Thus, as we have seen, an excess of carbonic acid 
in the blood seems to have a special effect on the central nervous system and 389 
EFFECTS OF CHANGES IN ATMOSPHERIC PRESSURE. 
so acts as a narcotic poison. The peculiar effects of nitrous oxide (laughing 
gas) are similarly due to the direct action of the gas in the blood on the 
central nervous system. Some gases are irrespirable and may interfere with 
respiration, even causing suffocation, on account of their causing spasm of 
the glottis, and this is said to be, to a certain extent, the case with an atmos- 
phere which is wholly or largely composed of carbonic acid. 
§ 322. The effects of changes in atmospheric pressure. Diminution of 
pressure. The partial pressure of the oxygen in the inspired air may be 
changed, not only by altering the composition of the air entering at the 
ordinary atmospheric pressure, but also by altering the total pressure of the 
atmosphere without changing its composition. The results of the latter are, 
however, complicated ; we have then to deal not merely with the effects on 
the interchange of gases in the lungs but with the effects on the whole 
organism. All the complicated machinery of the body is adapted and ar- 
ranged to work under what we may call ordinary atmospheric pressure, that 
is to say, within the limits of 760 mm. mercury at the sea level and about 
500 mm., corresponding to an altitude of 6000 feet, this being the range of 
ordinary human dwellings. Any great increase or decrease of pressure be- 
yond these limits will affect not only the exit of carbonic acid from and the 
entrance of oxygen into the blood, but, in varying degree, all the physical 
and chemical processes of the body. A gross instance of this is seen when 
an animal is suddenly subjected to a great diminution of pressure, as when 
it is placed in the receiver of an air-pump and the receiver rapidly ex- 
hausted. The animal is soon thrown into fatal convulsions, which are in 
part, but only in part, due to the liberation of gas from the blood within 
the bloodvessels ; the gas so set free mechanically interferes with the circula- 
tion, as by obstructing the play of the cardiac valves, or by plugging the 
smaller bloodvessels, and thus helps to bring the machine to a standstill. 
The free gas found in the vessels upon examination after death is said to be 
composed chiefly of nitrogen, the carbonic acid and the oxygen, which pro- 
bably were also set free, having been reabsorbed before the examination was 
made. 
But, quite apart from gross effects of this kind, it is very obvious that the 
organism must in many ways suffer from a diminution of pressure. The 
complex and delicately balanced vascular system is constructed to work at 
the ordinary atmospheric pressure. The force of the heart-beat and the 
tonic contraction of the small arteries are, so to speak, pitched to meet the 
influence exerted on the outside of the bloodvessels by the ordinary pressure 
of the atmosphere ; and any great diminution of that pressure must produce 
a greater or less disarrangement of the vascular mechanism until it is coun- 
terbalanced by some compensating changes. And a little reflection will 
supply many other instances. 
We have already called attention (§ 297) to the fact that, the total pres- 
sure of the atmosphere remaining the same, the partial pressure of the oxygen 
in the inspired air may be reduced as low as about 76 mm. (10 per cent.) 
without seriously modifying the respiration. In order to attain this dimi- 
nution of the partial pressure of the oxygen without changing the composi- 
tion of the atmosphere, the total pressure of the atmosphere must be reduced 
to the limit of 300 min., corresponding to an altitude of 17,000 feet. Now 
it is a matter of common experience that in ascending a mountain "distress" 
is felt long before such an altitude is reached. The distress felt on such 
occasions is probably due not so much, if indeed at all directly, to the 
diminution of oxygen as to a general disarrangement of the organism and 
perhaps more particularly of the vascular system. The nose-bleeding which 
is so frequent an occurrence under the circumstances show's that the minute 390 
RESPIRATION. 
bloodvessels more directly exposed to the diminution of pressure are pro- 
foundly affected by it; and what is true of them is, probably, in various 
ways and to different degrees, true of the whole vascular system. The 
breathlessness which is so marked a feature on these occasions seems due 
not so much to the fact that the blood which reaches the respiratory nervous 
centres is deficient in oxygen, as to the fact that the troubled vascular sys- 
tem fails to deliver to those centres their blood in an adequate fashion. 
It is a feature of the vascular system, and indeed of the other mechan- 
isms of the body in which nervous factors intervene, that they possess the 
power of adapting themselves to changed conditions; and as is well known, 
the human organism somewhat rapidly becomes accustomed to these mode- 
rate altitudes. Practice and custom have far less effect, though they have 
some, on the more fundamental processes depending on the actual supply of 
oxygen ; and it is at the extreme altitudes, where in addition to the other 
troubles a deficiency of oxygen definitely makes itself felt, that the body 
seems to fail in adapting itself to the new circumstances. 
The addition of these troubles not directly respiratory in nature, when 
the supply of oxygen is diminished by a diminution of the total pressure, 
perhaps explains why though an adequate lowering of pressure will produce 
asphyxia, that asphyxia is somewhat different from the ordinary asphyxia 
due to deprivation of air or oxygen. Convulsions which are essential to 
ordinary asphyxia are at times wholly absent; the nervous system under 
the peculiar conditions does not respond to the stimulus of the lack of 
oxygen ; and other nervous symptoms, such as a rapid onset of feebleness 
amounting almost to paralysis, are apt to make their appearance. 
§ 323. The effects of increase of atmospheric pressure. These are in 
many ways remarkable. Up to a pressure of several atmospheres of air, the 
only symptoms which present themselves are those somewhat resembling 
narcotic poisoning. The animal becomes sleepy and stupid, the result pro- 
bably not so much of respiratory changes, as of the effects of the increased 
pressure on the whole organism to which we have just alluded. At a pres- 
sure, however, of 15 atmospheres of air, or what amounts to the same thing, 
of 3 atmospheres of oxygen, and upward, a very remarkable phenomenon 
presents itself. The animals die of asphyxia and convulsions, exactly in the 
same way as when oxygen is deficient. Corresponding with this it is found 
that the production of carbonic acid is diminished. That is to say, when 
the pressure of the oxygen is increased beyond a certain limit, the oxida- 
tions of the body are diminished, and with a still further increase of oxygen 
are arrested altogether. The oxidation of phosphorus is perhaps analo- 
gous; at a high pressure of oxygen phosphorus will not burn. Not only 
animals, but plants, bacteria, and organized ferments, are similarly killed by 
too great a pressure of oxygen. 
The Relations of the Respiratory System to the Vascular ani> 
Other Systems. 
§ 324. Many events in the body show the influence which the respiratory 
movements exert on the circulation. When the brain of a living mammal 
is exposed by the removal of the skull, a rhythmic rise and fall of the cere- 
bral mass, a pulsation of the brain, quite distinct from the movements caused 
by the pulse in the arteries of the brain, is observed ; and upon examination 
it will be found that these movements are synchronous with the respiratory 
movements, the brain rising up during expiration and sinking during inspira- 
tion. They disappear when the arteries going to the brain are ligatured, or 
when the venous sinuses of the dura mater are laid open so as to admit of a RESPIRATORY UNDULATIONS. 
391 
free escape of the venous blood. They evidently arise from the expiratory 
movements in some way hindering and the inspiratory movements assisting 
the return of blood from the brain. We have already (§ 105) stated that 
during inspiration the pressure of blood in the great veins may become 
negative, i. e., may sink below the pressure of the atmosphere ; and a punc- 
ture of one of these veins may cause death by air being actually drawn into 
the vein and thus into the heart during an inspiratory movement When 
the veins of an animal are laid bare in the neck and watched, the so-called 
pulsus venosus may be observed in them, that is, they swell up during expira- 
tion and diminish again during inspiration. And indeed a little considera- 
tion will show that the expansion and contraction of the chest must have a 
decided effect on the How of blood through the thoracic portion of, and thus 
indirectly on that through the whole of, the vascular system. 
This is well illustrated by the eflects of respiration on arterial blood- 
pressure. We have seen, while treating of the circulation, that the arterial 
blood-pressure curves are marked by undulations, which, since their rhythm 
is synchronous with that of the respiratory movements, are evidently in 
some way connected with respiration. Similar undulations may be observed 
in the pulse-tracings taken from man. 
Fig. 107. 
Comparison of Blood-pressure Curve with Curve of Intra-thoracie Pressure. (Dog.) a is the 
blood-pressure curve taken by means of a mercury manometer ; it shows the respiratory undula- 
tion, the slower beats on the descent being very marked, b is the curve of intra-thoracic pressure 
obtained by connecting one limb of a manometer with the pleural cavity. Inspiration begins at 
i, expiration at e. With the beginning of inspiration (i) the expansion of the chest causes a 
marked fall of the mercury in the intra-thoracic manometer : but the effect soon diminishes, since 
the lessening of intra-thoracie pressure does not bear on the manometer alone, but on the lungs also; 
and as the lungs expand more and more the fall in the mercury becomes less and less until toward 
the end of inspiration the curve becomes very nearly a straight line. Conversely, the return of 
the chest at the beginning of expiration .e) produces at first a marked rise of the mercury in the 
manometer ; but this soon ceases as the air leaves the chest and the lungs shrink, whereupon the 
mercury falls slowly. 
When these undulations of the blood-pressure curve are compared care- 
fully with the respiratory movements or with the variations of intra-thoracic 
pressure, what is most commonly observed is that while the blood-pressure, 
on the whole, rises during inspiration and falls during expiration, neither the 
rise nor the fall is exactly synchronous with either inspiration or expiration. 
Fig. 107 shows two tracings from a dog taken at the same time, one, a, being 
the ordinary blood-pressure curve from the carotid, and the other, b, repre- 
senting the condition of the intra-thoracic pressure as obtained by carefully 
bringing a manometer into connection with the pleural cavity. On com- 
paring the two curves it is evident that neither the rise nor the fall of arterial 
pressure coincides exactly either with inspiration or with expiration. At 392 
RESPIRATION. 
the beginning of inspiration (i) the arterial pressure is seen to be falling; it 
soon, however, begins to rise, but does not reach the maximum until some 
time after expiration (e) has begun; the fall continues during the remainder 
of expiration, and passes on into the succeeding inspiration. This suggests 
the idea that, while inspiration tends to increase and expiration to diminish 
the blood-pressure, there are causes at work which in each case delay the 
effect. 
Extended observations, however, show that such a relation as that shown 
in the figure, though frequent, is not constant. In fact, the effects of the 
respiratory movements on blood-pressure are found to vary very widely 
according as the respiration is quick or slow, easy and shallow, or labored 
and deep, and especially as the air enters into the chest readily or with diffi- 
culty. Moreover, respiratory undulations of blood-pressure are seen not 
only with natural but also with artificial respiration ; in the latter the 
mechanical conditions are to a large extent the reverse of those of the former, 
and might fairly be expected to affect the circulation in a different way. 
The causation of these respiratory undulations is, in fact, complex. The 
respiratory act affects the vascular system in several different ways, and the 
general effect varies according as one or other influence is predominant. 
These several actions are sufficiently interesting and important to deserve 
discussion. 
§ 325. The heart and great bloodvessels are, like the lungs, placed in the 
air-tight thoracic cavity, and are subject like the lungs to the pumping action 
of the respiratory movements. Were there no lungs present in the chest, 
the whole force of the expansion of the thorax in inspiration would be 
directed to drawing blood from the extra-thoracic vessels toward the heart, 
and conversely in expiration the effect of the return of the thorax to its 
previous dimensions would be to drive the blood thus drawn in back again 
from the heart toward the extra-thoracic vessels. And, even in the presence 
of the lungs, some of this effect is still felt. The main purpose and the main 
result of the expansion of the chest in inspiration is, of course, to draw air 
into the lungs; by that expansion the air in the pulmonary alveoli is rare- 
fied and brought to a lower pressure than that of the atmosphere outside the 
chest; and the difference of pressure thus set up leads to an inrush of 
inspired air until an equilibrium of pressure is established between the air 
in the lungs and that outside the chest. Before, however, the inspired air 
can fill a pulmonary alveolus the elastic walls of the alveolus have to be dis- 
tended, and that distention is effected by means of the pressure which causes 
the inspired air to enter. Part of the atmospheric pressure, in fact, which 
causes the entrance of the air into the lung is spent in overcoming the elas- 
ticity of the pulmonary passages and cells. So that while by the inrush of 
inspired air the difference of pressure between the air inside the pulmonary 
alveoli and that outside the chest, brought about by the thoracic expansion, 
is completely neutralized, the difference between the pressure to which the 
parts lying within the thorax, but outside the lungs, are exposed and that 
outside the chest is not so completely neutralized. The pressure on these 
parts always falls short of the pressure of the atmosphere by the amount of 
pressure necessary to counterbalance the elasticity of the pulmonary passages 
and alveoli. Consequently, any structure lying within the thorax, but out- 
side the lungs, is never, even at the conclusion of an inspiration, when the 
lungs are filled with air, subject to a pressure as great as that of the atmos- 
phere. And, since the fraction of the atmospheric pressure which is thus 
spent in distending the lungs increases as the lungs become more and more 
stretched, it follows that the fuller the inspiration the greater is the differ- 
ence between the pressure on structures within the thorax, but outside the RESPIRATORY UNDULATIONS. 
393 
lungs, and the ordinary pressure of the atmosphere. Now, we have seen 
that the pressure necessary to counterbalance the elasticity of the lungs, 
when they are completely at rest (in the pause between expiration and in- 
spiration), is in man about 5 to 7 mm. of mercury, and that when the lungs 
are fully distended, as at the end of a forcible inspiration, the pressure rises 
to as much as 30 mm. of mercury. Hence, at the height of a forcible in- 
spiration the pressure exerted on the heart and great vessels within the 
thorax is 30 mm. less than the ordinary atmospheric pressure of 760 mm., 
and even when the chest is completely at rest, at the end of an inspiration, 
the pressure on the heart and great vessels is slightly (by about 5 mm. of 
mercury) below that of the atmosphere. We may add that any obstacle to 
the free ingress of the inspired air, any difficulty in the full expansion of 
the pulmonary alveoli, of course increases the negative pressure to which 
the thoracic structures outside the lungs are subjected by the expansion of 
the chest. Hence, when the trachea is closed a very large part of the thoracic 
expansion is directed to increasing the negative pressure around the heart 
and great bloodvessels. 
During an inspiration, then, the pressure around the heart and great 
bloodvessels becomes considerably less than that of the atmosphere on the 
vessels outside the thorax. During expiration this pressure returns toward 
that of the atmosphere, but in ordinary breathing never quite reaches it. 
It is only in forcible expiration that the pressure on the thoracic vascular 
organs reaches or exceeds that of the atmosphere. But if during inspira- 
tion the pressure bearing on the right auricle and the venae cavse become 
less than the pressure which is bearing on the jugular, subclavian, and other 
veins outside the thorax, this must result in an increased flow from the lat- 
ter into the former. Hence, during each inspiration a larger quantity of 
blood enters the right side of the heart. This probably leads to a stronger 
stroke of the heart, and at all events causes a larger quantity to be ejected 
by the right ventricle; this causes a larger quantity to escape from the left 
ventricle, and thus more blood is thrown into the aorta, and the arterial 
pressure proportionately increased. During expiration the converse takes 
place. The pressure on the intra-thoracic bloodvessels returns to the 
normal, the flow of blood from the veins outside the thorax into the vense 
cavse and right auricle is no longer assisted, and in consequence less blood 
passes through the heart into the aorta, and arterial pressure falls again. 
During forced expiration the intra-thoracic pressure may be so great as to 
afford a distinct obstacle to the flow from the veins into the heart. 
The effect of the respiratory movements on the arteries is naturally differ- 
ent from that on the veins. During inspiration the diminution of pressure 
in the thorax around the aortic arch tends to expand the aortic arch and 
thus to check the onward flow of blood and to diminish the pressure of blood 
within the aorta. During expiration the increase of pressure outside the 
aortic arch of course tends to increase also the blood-pressure within the 
aorta, acting in fact just in the same way as if the coats of the aorta them- 
selves contracted. Thus, as far as arterial blood-pressure is concerned, the 
effects of the respiratory movements on the great veins and great arteries 
respectively are antagonistic to each other; the effect on the veins being to 
increase arterial pressure during inspiration and to diminish it during expira- 
tion, while the effect on the arteries is to diminish arterial pressure during 
inspiration and to increase it during expiration. But we should naturally 
expect the effect on the thin-walled veins to be greater than that on the 
stout, thick-walled arteries, so much so that the direct effect on the arteries 
may be neglected. That is to say, we should expect the blood-pressure to 
rise during inspiration and to fall during expiration. This, as we have seen, 394 
RESPIRATION. 
is frequently the case, and, indeed, when the breathing is deep and labored, 
and especially during violent and sudden respiratory movements, the influ- 
ence in this direction on the blood-pressure curve of the pumping action of 
the chest is unmistakable. 
In attempting, however, to estimate the effect of the respiratory move- 
ments on blood-pressure we must bear in mind what is taking place in the 
abdomen. In inspiration the descent of the diaphragm compresses the 
abdominal viscera, and so, while at the very first it drives a quantity of 
blood onward along the inferior vena cava, subsequently hinders the upward 
flow from the abdomen and lower limbs; at the same time by compressing 
the abdominal aorta, it tends to raise the pressure in the thoracic aorta and 
its branches, while lowering that of the abdominal aorta and its branches. 
The effect of easy expiration would be the converse of this; but in forced 
expiration the pressure of the contracting abdominal muscles would, as in 
inspiration, first tend to drive the blood onward along the vena cava, but sub- 
sequently to hinder the flow both along the vena cava and the aorta. The 
effect of the abdominal movements therefore is mixed and variable, and their 
influence on the blood-pressure in the femoral artery must be different from 
that on the radial artery or other branch of the thoracic aorta. It is dif- 
ficult to predict what in all cases the effect would be; and the matter can- 
not be settled by eliminating the movements of the diaphragm through sec- 
tion of the phrenic nerves, since in such a case the whole working of the 
respiratory pump is materially affected. 
§ 326. In addition to the influence thus exerted by the thoracic move- 
ments on the great veins leading to and the great arteries leading from the 
heart, we have to consider the behavior of the pulmonary vessels themselves 
under the varying thoracic pressure. These, like the veme cavae and aorta, 
tend to expand under the influence of the inspiratory expansion of the chest, 
and thus to become fuller of blood, very much as they would if the whole 
lung were placed under a large cupping-glass. The first effect of this in- 
creased filling of the pulmonary vessels would be to retain for a while a 
certain quantity of blood in the lungs and thus to lessen the amount falling 
into the left auricle. But this would be temporary only, and the widening 
of the pulmonary vessels would speedily produce an exactly contrary effect, 
namely, an increased flow through the lungs due to the diminished resistance 
offered by the widened passages. Conversely, the first effect of expiration 
would be an increased flow into the left auricle due to the additional quan- 
tity of blood driven onward by the partial collapse of the pulmonary ves- 
sels, followed by a more significant diminished flow caused by the greater 
resistance now offered by the narrower vascular channels. Thus the effect 
of inspiration in this way would be first to diminish the flow into the left 
auricle and so into the left ventricle, but afterward, for the rest of the 
inspiration until the beginning of expiration, to increase the flow into the 
ventricle; while conversely the effect of expiration would be first, for a 
brief period, to increase, and afterward, during the rest of the movement, 
to diminish the flow of blood into the left ventricle. Further, while this 
may be considered as the effect on the pulmonary vessels, large and small 
taken altogether, the influence both of the thoracic negative pressure during 
inspiration, and the return in a positive direction during expiration, will bear 
more on the thin-walled pulmonary veins than on the stouter pulmonary 
artery ; that is to say, as inspiration becomes established, there will be a 
diminution of pressure in the pulmonary veins greater than that in the pul- 
monary artery, and this will be an additional influence favoring the flow into 
the left ventricle; during expiration a similar difference of effect will be 
felt in the contrary direction. During the increase of flow into the ventricle RESPIRATORY UNDULATIONS. 
395 
the quantity of blood ejected at each stroke will increase, and each stroke 
will (§ 148) be increased in vigor, in consequence of which the arterial 
pressure will rise. Conversely, during the decrease of flow into the ventricle 
the arterial pressure will fall. Hence the general effect of the movements 
of the chest on the pulmonary vessels will be during the beginning of inspira- 
tion to continue the lowering of arterial pressure which was taking place 
during expiration, but subsequently to raise the arterial pressure ; and con- 
versely, at the beginning of expiration to continue the rise of arterial pres- 
sure which was taking place during inspiration, but subsequently to lower 
the arterial pressure. In ordinary breathing, as we have seen, what may be 
considered as the normal relations of blood-pressure to the respiratory move- 
ments are precisely of this kind. 
§ 327. Effects of the respiratory movements, however, are seen not only 
in natural but also in artificial respiration. When, for instance, in an animal 
under urari, artificial is substituted for natural respiration, undulations of 
the blood-pressure curve, synchronous with the respiratory movements, are 
still observed (Fig. 108), though generally less in extent than those seen 
under natural conditions. 
Now in artificial respiration, the mechanical conditions under which the 
thoracic viscera are placed as regards pressure, are the exact opposite of 
those existing during natural respiration, for when air is blown into the 
trachea to distend the lungs, the pressure within the chest is increased 
instead of diminished. Under these circumstances, applying the consid- 
erations laid down in the preceding paragraph with regard to natural 
respiration, we should expect to find that while the first effect of an arti- 
ficial inspiration would be to drive an additional quantity of blood out 
of the lungs into the left ventricle, and thus to raise arterial pressure, 
this would be in turn followed by a fall of arterial pressure due to the 
increased resistance offered both to the passage of blood through the 
lungs and to the entrance of blood through the venae cavae into the 
right auricle. Conversely, the effect of the succeeding expiration would 
be an initial continuance of the fall of arterial pressure succeeded by a 
rise. In other words, we should expect to find in artificial respiration 
effects exactly the reverse of those which we find in normal respiration ; 
and, indeed, in many curves of blood-pressure taken during artificial 
respiration this is the case. 
Both in natural and in artificial respiration, however, the features of the 
blood-pressure curve vary according as the breathing is hurried or slow, 
shallow or deep, and according to the facility with which air enters the 
chest, so much so that at times the blood-pressure curves of natural and 
artificial respiration may closely resemble each other. And a little con- 
sideration would lead us to expect this. 
We have seen that the rise in arterial pressure which marks the respir- 
atory undulation is, in the main, due to a temporary greater amount of 
blood thrown into the aorta by the left ventricle, and that, correspondingly, 
the fall of pressure completing the undulation is in the main due to a tem- 
porary lessening of the amount so thrown. Though the causes discussed in 
§ 325 undoubtedly make themselves prominent in labored and violent re- 
spiratory movements, we may conclude that in ordinary respiration, both 
natural and artificial, the main events producing the respiratory undula- 
tions are those discussed in § 326. We may restate the conclusions of 
that discussion by saying that the respiratory movements affect the amount 
of flow of blood into the left ventricle, and so the discharge of blood from 
the left ventricle into the aorta, in two main ways. In the first place, 
through the widening or narrowing of the pulmonary vessels they alter 396 
RESPIRATION. 
the capacity of the vessels to hold blood for the time being. In the 
second place, in consequence of the difference of resistance, occasioned by 
the widening or narrowing, they alter the rate of flow through the pul- 
monary vessels. The first factor is a brief and passing one; the extra 
room due to widening is soon filled up, the narrowed vessels soon dis- 
charge the quantity which they can no longer hold. But the second 
factor is a more lasting one; so long as in the respiratory movement the 
vessels remain widened or narrowed so long is the rate of flow increased or 
diminished. These two factors produce opposite effects, and hence the total 
result of any particular kind of respiration will depend on their relative 
prominence. With quickly repeated respiratory movements the first factor 
comes to the front; when the respiratory movements are more slowly re- 
peated and more slowly carried out the second factor is the more potent. 
Hence it comes about that in quickly repeated artificial respiration where 
the first factor is predominant, and the prominent effect of each inflation 
is to diminish the capacity of, and so to empty the pulmonary vessels and 
to increase the flow into the ventricle whereby the pressure rises in infla- 
tion, that is in inspiration, the blood-pressure curve stimulates that of a 
slowly repeated natural respiration, where the pressure also rises in in- 
spiration, but where, the second factor being predominant, the rise of 
pressure brought about by each inspiration is due mainly to the more 
rapid flow through the widened pulmonary vessels. And other illustra- 
tions of a like kind could be given. 
§ 328. Besides the mechanical effects of the respiratory movements the 
vascular system is influenced by respiration through the changes in the gases 
of the blood. 
Changes in the blood may affect, on the one hand, the vasomotor system 
and, on the other hand, the heart. They may further affect the heart either 
directly by acting on the cardiac tissues, or indirectly by means of the inhib- 
itory and augmentor cardiac nerves. They may also, probably, affect the 
peripheral vessels, not only through vasomotor nerves, but by acting 
directly on the walls of the smaller vessels. We have indications of an 
action of respiration on the cardio-inhibitory system, even in normal quiet 
respiration. One striking feature of the respiratory undulation in the blood- 
pressure curve of the dog1 is the fact that the pulse-rate is quickened during 
the rise of the undulation and becomes slower during the fall. (See Fig. 107.) 
A similar influence may be seen in pulse-tracings taken from man. The 
quickening of the beat might be considered as itself partly accounting for 
the rise of pressure, or, on the other hand, it might be urged that the in- 
creased flow of blood which causes the rise of pressure, at the same time 
leads to the quickening of the beat, were it not for one fact, viz., that the 
difference is at once done away with, without any other essential chaiige in 
the undulations, by section of both vagus nerves. Evidently the slower pulse 
during the fall is caused by a coincident stimulation of the cardio-inhibitory 
centre in the medulla oblongata, the quicker pulse during the rise being due 
to the fact that, during that interval, the centre is comparatively at rest. 
We have here indications that, while the respiratory centre in the medulla 
oblongata is at work, sending out rhythmic impulses of inspiration and ex- 
piration, the neighboring cardio-inhibitory centre is, as it were, by sympathy, 
thrown into an activity of such a kind that its influence over the heart waxes 
with each expiration and wanes with each inspiration. We cannot as yet 
explain exactly the manner in which the activity of the one centre influ- 
ences that of the other; it may be that during the expiratory phase the 
1 In the rabbit, the respiratory undulations, though well marked, present a very small 
difference of pulse-rate in the rise and fall. RESPIRATORY UNDULATIONS. 
397 
blood reaching the medulla is not quite so well arterialized, especially as far 
as the escape of carbonic acid is concerned, as during the inspiratory phase, 
and that the cardio-inhibitory centre is sufficiently sensitive to appreciate the 
slight difference; but of this we cannot be sure. 
§ 329. When through interference with the pulmonary interchange the 
blood sent out from the left ventricle becomes and continues to be less 
arterialized than usual, the effects on both the heart and the vasomotor 
system become conspicuous. The rhythm of the heart-beats is most distinctly 
slowed. This, under ordinary circumstances, when the vagus nerves are 
intact, is probably in part the result of vagus inhibition, the venous blood, 
as suggested above, stimulating the cardio-inhibitory centre in the medulla. 
But the slowing is not wholly caused in this way, for it is still conspicuous in 
an animal placed under urari and with both vagus nerves divided. Compare 
curves 3 and 4 with 1 and 2 in Fig. 108. How this slowing is brought about 
is not very clear. When venous blood is sent through an excised heart, the 
beat is, it is true, slowed, but it is also and still more conspicuously weak- 
ened. Now when the blood becomes too venous, as is shown in Fig. 108, 
even after the action of the vagus nerves has been eliminated by section and 
also by urari, the slowing is out of proportion to the weakening, since, as we 
shall presently see, the blood-pressure rises; and though that rise is chiefly 
due to vasomotor constriction, still it could not take place if the cardiac stroke 
were very notably weakened. It may be that the venous blood stimulates 
the cardiac augmentor mechanism in such a way as to bring about an aug- 
mentation of the cardiac stroke rather than a quickening of the rhythm ; 
but this has not been definitely proved. In any case a slow beat, with such 
a maintenance of the strength of the cardiac strokes as permits the contin- 
uance for some considerable time of a high blood-pressure, is met when the 
arterialization of the blood is interfered with. Sooner or later, however, the 
deficiency of oxygen in the blood diminishes the store of explosive compounds 
in the cardiac muscular substance, the beats lessen in force, often showing a 
temporary increase in frequency, and soon become irregular. 
$ 330. The effects of deficient arterialization on the vasomotor system 
are well shown when in an animal placed under a moderate dose of urari so 
as to eliminate the complications due to contractions of the skeletal muscles, 
with both vagi divided so as to insure the elimination of inhibitory impulses 
from the medulla, artificial respiration is suspended. Soon after the respira- 
tion is stopped, a very large but steady rise of pressure is observed. (See Fig. 
108.) The rise so witnessed is very similar to that brought about by power- 
fully stimulating a number of vaso-constrictor nerves; and there can be no 
doubt that it is due to the venous blood stimulating the vasomotor centre in 
the medulla, and thus causing constriction of the small arteries of the body, 
especially those of the splanchnic area, since, as we shall see, in speaking of 
the skin, a too venous blood leads to a widening of the cutaneous arteries. 
We say "stimulating the medullary vasomotor centre" because, though we 
must admit that, since a rise of pressure follows upon dyspnoea when the 
spinal cord has been previously divided below the medulla, the venous blood 
may stimulate other vasomotor centres in the spinal cord and possibly even 
act directly on local peripheral mechanisms, yet the fact that the rise of 
pressure is much less under these circumstances shows that the medullary 
centre plays the chief part. As we have just said, the effect of this vaso- 
constriction in raising the pressure, if not assisted by an increase, at all 
events is not neutralized by an adequate decrease of the cardiac stroke. 
Upon the cessation of the artificial respiration, the respiratory undulations of 
course cease also, so that the blood-pressure curve rises at first steadily in 
almost a straight line broken only by the heart-beats; yet after a while new 398 
RESPIRATION. 
undulations, the so-called Traube or Traube-Hering curves, make their ap- 
pearance (Fig. 108, 2, 3), very similar to the previous ones, except that their 
curves are larger and of a more sweeping character. These new undula- 
tions, since they appear in the absence of all thoracic or pulmonary move- 
ments, passive or active, and are witnessed even when both vagi are cut, 
must be of vasomotor origin ; the rhythmic rise must be due to a rhyth- 
mic constriction of the small arteries, and this probably is caused by a 
rhythmic discharge from vasomotor centres, and especially from the medul- 
lary vasomotor centre. The undulations are maintained as long as the blood- 
pressure continues to rise. With the increasing venosity of the blood, 
however, both the vasomotor centres and the heart become enfeebled; the 
undulations disappear, and the blood-pressure rapidly sinks. 
Fig. 108. 
Blood-pressure Curves during a Suspension of Breathing. (Traube-Hering Curves.) The 
curves, 1, 2, 3, 4, 5, are portions selected from one long continuous tracing forming the record of 
a prolonged observation, so that the several curves represent successive stages of the same exper- 
iment. Each curve is placed in its proper position relative to the base line, which, to save space, 
is omitted ; and it is obvious that, starting from the stage represented by 1, the blood-pressure rises 
in stages 2, 3, and 4, but falls again in stage 5. Curve 1 is taken from a period when artificial res- 
piration was being kept up, and the undulations visible are those the nature of which have been 
discussed; the vagus nerves having been cut the pulsations on the ascent and descent of the 
undulations do not differ. When the artificial respiration was suspended these undulations dis- 
appeared, and the blood-pressure rose steadily, while the heart beats became slower. Soon, as 
shown in curve 2, new undulations appeared. A little later the blood-pressure was still rising, 
the heart-beats still slower, but the undulations still more obvious (curve 3). Still later (curve 4) 
the pressure was still higher, but the heart-beats were quicker and the undulations flatter. The 
pressure then began to fall rapidly (curve 5), and continued to fall until some time later artificial 
respiration was resumed. • 
We may here incidentally remark that the occurrence of long, slow un- 
dulations is not dependent on the cessation of the respiratory movements, 
and on an abnormally venous condition of the blood. They are sometimes 
(Fig. 109) seen in animals whose breathing is fairly normal. We need 
not discuss them any further now, and have introduced them chiefly to 
illustrate the fact that the vasomotor nervous system is apt to fall into a 
condition of rhythmic activity. It has been suggested that the normal 399 
RESPIRATORY UNDULATIONS. 
respiratory undulations may be due to a rhythmic rise and fall of the 
activity of the vasomotor centre, synchronous, like that of the cardio- 
inhibitory centre, with the respiratory movements. There can, however, be 
no doubt that the respiratory variations in blood-pressure are due to the 
mechanical conditions discussed above, and that vasomotor influences inter- 
vene but little if at all. 
Fig. 109. 
Blood-pressure Curve of a Rabbit, Recorded on a Slowly Moving Surface, to show Traube- 
Hering Curves. (The curve was described not by means of a mercury manometer, but by an in- 
strument similar to but not identical with Fick's spring kymograph.) In each heart-beat the 
upward and downward strokes are very close together, but may be easily distinguished by the 
help of a lens. The undulations of the next order are those of respiration. The wider sweeps 
are the Traube-Hering curves, of which two complete curves and portions of two others are 
shown. Each Traube-Hering curve comprises about nine respiratory curves, and each respiratory 
curve about the same number of heart-beats. 
§ 331. The further general effects, similar to the above, on the vascular 
system of deficient arterialization of the blood may be studied by taking a 
blood-pressure tracing from the carotid or other artery of an animal while 
the interference with respiration is pushed on to a fatal asphyxia. During 
the first and second stages of the asphyxia the blood-pressure rises rapidly, 
attaining a height far above the normal. During the third stage it falls 
even more rapidly, repassing the normal and becoming nil as death ensues. 
If the animal, no urari having been given, is breathing of itself, and if, as 
usually is the case, the asphyxia is brought about by occlusion of the trachea, 
so that the mechanical effects of the respiratory movements are exaggerated 
by the air being unable to enter the chest, the respiratory undulations of 
the pressure-curve due to the mechanical causes discussed above are, espe- 
cially during the first stage, extensive, abrupt, and irregular, the inspiratory 
movements being accompanied by a conspicuous fall of pressure. When 
the animal has been previously placed under urari, so that the respiratory 
impulses cannot manifest themselves by any muscular movements, the rise 
of the pressure-curve, as we have already said, is at first steady and un- 
broken, but after a variable period Traube's curves make their appearance. 
As during the third stage the pressure sinks, these undulations pass away. 
The heart-beats are at first somewhat quickened, but speedily become 
slow, at the same time, as we have seen, not notably losing force, so that the 
pulse-curves on the tracing are exceedingly bold and striking. But the 
boldness of the curve of the mercury manometer is, it must be remembered, 
partly the mere result of the slowness of the rhythm ; the mercury has time 
to fall largely between each two beats. (Fig. 108, 3 and 4.) Even while 
the blood-pressure is sinking, and when the cardiac stroke is now certainly 
lessening in vigor, the slowness of the cardiac rhythm is still sufficient to 
maintain somewhat these characters of the curve. The strokes at last, how- 400 
RESPIRATION. 
ever, rapidly fail in strength and become irregular, though the heart con- 
tinues to beat for some seconds after the respiratory movements have ceased. 
If the chest of an animal be opened under artificial respiration, and 
asphyxia brought on by cessation of the respiration, it will be seen that the 
heart during the second and third stages becomes completely gorged with 
venous blood, all the cavities as well as the large veins being distended to 
the utmost. If the heart be watched to the close of the events, it will be 
seen that the feebler strokes which come on toward the end of the third stage 
are quite unable to empty its cavities; and when the last beat has passed 
away its parts are still choked with blood. The veins spurt out when 
pricked; and it may frequently be observed that the beats recommence 
when the over-distention of the heart's cavities is relieved by puncture of 
the great vessels. When rigor mortis sets in after death by asphyxia, the 
left side of the heart is more or less emptied of its contents; but not so the 
right side. Hence, in an ordinary post-mortem examination in cases of 
death by asphyxia, while the left side is found comparatively empty, the 
right appears gorged. 
The various phenomena of asphyxia are probably brought about in the 
following way: 
The increasingly venous character of the blood augments the action of 
the vasomotor centres, both the medullary centre and the subsidiary centres 
in the spinal cord, and thus leads to a constriction of the small arteries, 
especially of the splanchnic area. This is the chief cause of the markedly 
increased blood-pressure; though the venous blood may possibly also act 
directly on peripheral vasomotor mechanisms, or what is more likely, may 
increase the peripheral resistance in the capillaries themselves, since there 
are reasons for thinking (§ 171) that venous blood rich in carbonic acid 
meets with more friction, and passes less easily through the capillaries than 
does blood less venous in character. 
This increased peripheral resistance and the high blood-pressure to which 
it gives rise, while tending to increase the distention of the left ventricle and 
so indirectly helping to augment the force of the heart's beat, soon becomes 
a direct obstacle to the heart emptying itself of its contents. On the other 
hand, the labored respiratory movements favor the flow of venous blood 
toward the heart, which in consequence becomes more and more full. This 
repletion is, moreover, assisted by the marked infrequency of the beats which 
is soon developed. This in turn depends in part on the cardio-inhibitory 
centre in the medulla being stimulated by the venous blood, but, as we 
have previously seen, cannot be wholly accounted for in this way. The 
increased resistance in front, the augmented supply from behind, and the 
long pauses between the strokes, all concur in distending the heart more 
and more. 
When the large veins have become full of blood, the inspiratory move- 
ments can no longer have their usual etfect in facilitating the venous flow 
into the right auricle. The chief effect of the chest movement, as far as the 
circulation is concerned, is to widen and so to increase the capacity of the 
pulmonary vessels, and at the same time to diminish the pressure around the 
large arteries; hence the marked sinking of the blood-pressure during each 
inspiratory movement. 
The distention of the cardiac cavities, at first favorable to the heart-beat, 
as it increases becomes injurious; and the cardiac tissues after a while be- 
come enfeebled by the action of the venous blood, so that the strokes of the 
heart become weaker and irregular. 
On account of this increasing feebleness of the heart's beat, accompanied 
by more or less irregularity, the blood-pressure, in spite of the continued RESPIRATORY UNDULATIONS. 
401 
arterial constriction, begins to fall, since less and less blood is pumped into 
the arterial system ; the boldness of the pulse-curves at this stage is chiefly 
due to the infrequency of the strokes. As the quantity which passes from 
the heart into the arteries becomes less second by second, the pressure gets 
lower and lower, the descent being assisted by the exhaustion of the vaso- 
motor centre, until almost before the last beats it has sunk to zero. Thus at 
the close of asphyxia, while the heart and venous system are distended with 
blood, the arterial system is less than normally full. 
$ 332. While changes occurring primarily in the respiratory system thus 
affect the vascular system, conversely changes occurring primarily in the 
vascular system affect the respiratory system. Two kinds of change in the 
vascular system bearing on two parts of the respiratory system deserve 
special attention. 
In the first place the respiratory mechanism may be affected by changes 
in the blood-supply to the respiratory centre in the medulla. We have 
already seen (§ 314) that the sudden cutting off of the supply of blood to 
the medulla gives rise to dyspnoeic respiratory movements and may lead to 
expiratory convulsions. That is an extreme case; but, short of that, the 
activity of the respiratory centre, the extent and character of the respiratory 
explosions which take place in it, may be varied according as the constricted 
or dilated condition of the small arteries branching off* from the basilar 
artery or of the basilar artery itself allows a scanty or a full flow of blood 
through the medulla. And it is possible that some forms of dyspnoea may 
be brought about in this way. 
Much more common and important, however, is the second kind of 
change, that affecting the circulation through the lungs. In the normal 
organism an adequate supply of arterial blood to the tissues is secured by an 
adequate renewal of the air in the pulmonary alveoli, and an adequately 
rapid flow of blood through the pulmonary capillaries. When, as by ob- 
struction in the pulmonary arteries, or by failure of the cardiac valves, or, 
and perhaps especially, by an insufficient cardiac stroke, the stream of blood 
from the lungs into the left ventricle is lessened either in amount or in 
rapidity, less oxygen is carried to the tissues, including the nervous tissue of 
the medulla, and dyspnoea or " want of breath " follows. When the circu- 
lation through the lungs is in full healthy swing, the haemoglobin of the red 
corpuscles is as we have seen saturated or nearly saturated with oxygen. If 
owing to a slower stream the red corpuscles tarry longer in their passage 
along the walls of the pulmonary alveoli they cannot thereby take up a 
compensating addition of oxygen, indeed, it is doubtful if they can take up 
any additional oxygen at all. The blood falling under these circumstances 
into the left ventricle and sent thence over the body is not more arterial 
than usual; at the same time the amount of blood sent out at each heart- 
stroke is less, often much less, than the normal; and the medulla as well as 
other tissues suffer in consequence from a deficiency of oxygen. The de- 
ficient supply to the medulla manifests itself in dyspnoeic or at least in 
labored breathing, which sometimes, through the mechanical influences 
discussed above, has the happy result of improving the pulmonary circula- 
tion and so produces compensating effects. When the pulmonary artery is 
suddenly plugged with a clot the primary and urgent symptom is "want 
of breath," though air enters freely into the chest; and " cardiac dyspnoea " 
is a common symptom of cardiac disease. 
$ 333. Other systems of the body are also related to the respiratory sys- 
tem, though by ties less striking than those which bind to it the vascular 
system. We have seen that deficient arterialization of the blood stirs up the 
muscles of the alimentary canal to increased activity, and we shall presently 402 
RESPIRATION. 
see that the same condition has a notable effect in promoting the perspira- 
tion ; it probably has a similar influence over other secretions. On the 
other hand, as we have seen (§ 316), there are reasons for thinking that the 
activity of the respiratory centre and so the energy of the whole respiratory 
act is influenced by chemical changes, other than the decrease of oxygen and 
increase of carbonic acid, brought about in the blood by the activity of the 
skeletal muscles. 
The closeness and the intricacy of the ties which thus connect the respira- 
tory system with almost all parts of the body may be illustrated by consider- 
ing the effects of muscular work on the body, and the conditions which, 
apart from the capacity of the muscles themselves and of the motor nervous 
apparatus which puts them to work, determine the power of the body to do 
work. During work, especially arduous work, the muscular contractions rob 
the blood of much oxygen and load it with much carbonic acid. This 
change in the blood would itself increase the activity of the respiratory centre 
and the energy of the respiratory movements, and might be sufficient to 
secure such an increase of these movements that the deficiency of oxygen and 
increase of carbonic acid should never overstep certain limits. But, as we 
have said, apparently other products of muscular metabolism act so potently 
in stimulating the respiratory centre, that the respiratory movements are 
more than sufficient to compensate the changes in the gases of the blood. 
The efficacy of the augmented respiratory movements is much increased by 
a concomitant increase in cardiac activity and a swifter or fuller stream of 
blood through the lungs; indeed, unless backed up by the cardiac increase, 
the mere increase of the pulmonary ventilation might prove inadequate. 
Hence the capacity for arduous muscular labor is determined not by the 
respiratory mechanism alone, nor by the vascular system alone, but by 
both, and especially by both working together in harmony and concert. 
The increased ventilation would be idle unless it were accompanied by a 
quicker circulation, and the quicker circulation would similarly be of com- 
paratively little use unless accompanied by increased ventilation. To a 
bystander the working of the respiratory pump is much more obvious than 
that of the vascular system, and indeed the subject himself is much more 
directly conscious of changes in the former than of changes in the latter. 
Hence when the organism ceases to be able to meet the demands which the 
labor is making upon it, the subject is said to be "out of breath," though in 
a large number of cases the failure lies much more at the door of the vas- 
cular than of the respiratory system. And, as a rule, it may perhaps be 
said that when two men differ in their capacity for strenuous work, such as 
running a race, the difference, though it is often familiarly spoken of as one 
of "wind " or power of breathing, is in reality not a difference in ventilat- 
ing capacity but a difference in the power of the heart to keep up to and 
work in harmony with the increased respiratory movements. 
Thus there are two main factors in respiration, the respiratory mechanism 
proper, and the circulation, the one bringing the air to the blood and the 
other the blood to the air. We may remind the reader that there is also a 
third factor, and that one of great moment, the amount of haemoglobin, that 
is, the number of red corpuscles, in the blood. The amount of oxygen taken 
up from the lungs depends not only on the strokes of the respiratory and the 
vascular pumps, but also on the richness of the blood in red corpuscles. A 
body which from loss of blood or from disease is anaemic is thrown out of 
breath by very slight exertion, not so much because the respiratory or the 
vascular pump is weak, but because, through lack of oxygen-carriers, with 
their best efforts the combined pumps can only deliver to the tissues, 
including the medulla, an inadequate supply of oxygen. And fat persons, 403 
MODIFIED RESPIRATORY MOVEMENTS. 
whose store of haemoglobin in proportion to their body weight is always 
below par, are proverbially " scant of breath." 
Modified Respiratory Movements. 
§ 334. The respiratory mechanism with its adjuncts, in addition to its 
respiratory function, becomes of service, especially in the case of man, as a 
means of expressing emotions. The respiratory column of air, moreover, in 
its exit from the chest, is frequently made use of in a mechanical way to 
expel bodies from the upper air-passages. Hence arise a number of pecu- 
liarly modified and more or less complicated respiratory movements, sighing, 
coughing, laughter, etc., adapted to secure special ends which are not dis- 
tinctly respiratory. They are all essentially reflex in character, the stimulus 
determining each movement, sometimes affecting a peripheral afferent nerve 
as in the case of coughing, sometimes working through the higher parts of 
the brain as in laughter and crying, sometimes possibly as in yawning and 
sighing, acting on the respiratory centre itself. Like the simple respiratory 
act, they may with more or less success be carried out by a direct effort of 
the will. 
Sighing is a deep and long-drawn inspiration, chiefly through the nose, 
followed by a somewhat shorter, but correspondingly large expiration. 
Yawning is similarly a deep inspiration, deeper and longer continued 
than a sigh, drawn through the widely open mouth, and accompanied by a 
peculiar depression of the lower jaw and frequently by an elevation of the 
shoulders. 
Hiccough consists in a sudden inspiratory contraction of the diaphragm, 
in the course of which the glottis suddenly closes, so that the further entrance 
of air into the chest is prevented, while the impulse of the column of air 
just entering, as it strikes upon the closed glottis, gives rise to a well-known 
accompanying sound. The afferent impulses of the reflex act are conveyed 
by the gastric branches of the vagus. The closure of the glottis is carried 
out by means of the inferior laryngeal nerve. See Voice. 
In sobbing a series of similar convulsive inspirations follow each other 
slowly, the glottis being closed earlier than in the case of hiccough, so that 
little or no air enters into the chest. 
Coughing consists in the first place of a deep and long-drawn inspiration 
by which the lungs are well filled with air. This is followed by a complete 
closure of the glottis, and then comes a sudden and forcible expiration, in 
the midst of which the glottis suddenly opens, and thus a blast of air is 
driven through the upper respiratory passages. The afferent impulses of 
this reflex act are, in most cases, as when a foreign body is lodged in the 
larynx or by the side of the epiglottis, conveyed by the superior laryngeal 
nerve ; but the movement may arise from stimuli applied to other afferent 
branches of the vagus, such as those supplying the bronchial passages and 
stomach and the auricular branch distributed to the meatus externus. Stimu- 
lation of other nerves also, such as those of the skin by a draught of cold 
air, may develop a cough. 
In sneezing the general movement is essentially the same, except that the 
opening from the pharynx into the mouth is closed by the contraction of the 
anterior pillars of the fauces and the descent of the soft palate, so that the 
force of the blast is driven entirely through the nose. The afferent impulses 
here usually come from the nasal branches of the fifth. When sneezing, 
however, is produced by a bright light, the optic nerve would seem to be the 
afferent nerve. 
Laughing consists essentially in an inspiration succeeded, not by one, but 404 
THE ELIMINATION OF WASTE PRODUCTS. 
by a whole series, often long continued, of short spasmodic expirations, the 
glottis being freely open during the whole time, and the vocal cords being 
thrown into characteristic vibrations. 
In crying, the respiratory movements are modified in the same way as in 
laughing; the rhythm and the accompanying facial expressions are, how- 
ever, different, though laughing and crying frequently become indistin- 
guishable. 
CHAPTER III. 
THE ELIMINATION OE WASTE PRODUCTS. 
§ 335. We have traced the food from the alimentary canal into the blood, 
and, did the state of our knowledge permit, the natural course of our study 
would be to trace the food from the blood into the tissues, and then to follow 
the products of the activity of the tissues back into the blood and so out 
of the body. This, however, we cannot as yet satisfactorily do; and it will 
be more convenient to study first the final products of the metabolism of the 
body, and the manner in which they are eliminated, and afterward to return 
to the discussion of the intervening steps. 
Our food consists of certain food-stuffs, viz., proteids, fats, and carbo- 
hydrates, of various salts, and of water. In their passage through the blood 
and tissues of the body, the proteids, fats, and carbohydrates are converted 
into urea (or some closely allied body), carbonic acid, and water, the nitrogen 
of the urea being furnished by the proteids alone. Many of the proteids 
contain sulphur, and also have phosphorus attached to them in some com- 
bination or other, and some of the fats taken as food contain phosphorus; 
these elements ultimately undergo oxidation into phosphates and sulphates, 
and leave the body in that form in company with the other salts. 
Broadly speaking, then, the waste products of the animal economy are 
urea, carbonic acid, salts, and water. These leave the body by one or other 
of three main channels, the lungs, the skin, and the kidney. Some part, it 
is true, leaves the body by the bowels, for, as we have seen, the feces contain, 
besides undigested portions of food, substances which have been secreted into 
the bowel, and are, therefore, waste products; but the amount of these is so 
small that they may be neglected. 
The lungs serve as the channel for the discharge of the greater part of 
the carbonic acid, and a considerable quantity of water; this discharge we 
have just studied. Through the skin there leave the body a comparatively 
small quantity of salts, a little carbonic acid, and a variable but on the whole 
large quantity of water. 
The kidneys discharge all or nearly all the urea and allied bodies, the 
greater portion of the salts, and a large amount of water, with an insignifi- 
cant quantity of carbonic acid. They are especially important, since by 
them practically all the nitrogenous waste leaves the body ; and to them we 
will turn first. 
The Composition and Characters of Urine. 
§ 336. These are so fully dwelt upon in special works that we may coniine 
ourselves here to salient points. The healthy urine of man is a clear yellowish 405 
THE COMPOSITION AND CHARACTERS OF URINE. 
slightly fluorescent fluid of a peculiar odor, saline taste, and acid reaction, 
having a mean specific gravity of 1020, and generally holding in suspension 
a little mucus. The mucus, when present, comes from the urinary passages, 
as do also the occasional epithelial cells. All the rest of the urine may be 
considered as the secretion of the kidney. 
The urine, as we have said, is the chief channel by which solid matters 
leave the body, a small quantity only passing by the skin and practically 
none by the lungs. Hence, neglecting for the present the skin, we may say 
that all the substances taken into the body sooner or later leave the body by 
the urine, save the few substances which may be retained permanently within 
the body and the substances which make up the body at the moment of its 
death. We accordingly find that the urine contains a large number of sub- 
stances, the exact amount of each substance present in a given quantity of 
urine varying, in the case of every substance somewhat, and in the cases of 
many substances very largely, from time to time. The composition of urine 
is not only complex but extremely variable. 
Moreover, a little consideration will show that the several substances 
present in urine must have very different histories. Some of the con- 
stituents of urine appear in it in the exact form in which they were intro- 
duced into the mouth ; they have been simply absorbed from the alimentary 
canal into the blood and excreted by the kidney without undergoing change ; 
they are derived directly and without change from the food. 
Others again are the products of changes which the food has undergone in 
the body ; and these changes may be slight or may be extensive, and may 
take place on the one hand in the alimentary canal, or during a brief transit 
of the substance in the blood-stream, or even in the urine itself, may, so to 
speak, be superficial; or on the other hand may take place in the very depths 
of the tissues and be closely associated with the very life of the tissues. We 
shall, however, have to return to these matters later on, and may here briefly 
consider what substances are, normally and abnormally, present in urine, and 
the chief features of the fluid itself. 
§ 337. Besides water, the constituents of urine are : 
Nitrogenous crystalline bodies. Neglecting the small proportion of these 
bodies which, especially in the case of flesh-eaters, are introduced into the 
economy with the food, as kreatin and the like, and so pass into the urine 
with no or with comparatively little change, we may on the whole regard the 
substances of this class as the products of the changes which the proteid 
matters (and allied substances such as gelatin and the like) present in food 
have undergone either while the food was simply food, still in the alimentary 
canal, for instance, or after the food had been built up into the tissues of the 
body. 
Of these by far the most important, in the urine of man and mammalia, 
is the body urea (N2H4CO). It is the chief form in which, in these animals, 
nitrogen leaves the body. We shall have to discuss the relations and forma- 
tion of urea later on, but meanwhile we will simply state that it has remark- 
able double connections with two great groups. On the one hand, it is 
related to the ammonia group, and by hydration is readily converted into 
ammonium carbonate (N2H4CO+2H2O = (NH4)2COS). On the other hand, 
it is related to the great cyanogen group, ammonium cyanate and urea being 
isomeric, and the former by simple heating being converted into the latter 
(NH4.CNO = N2H4CO). 
Though a base, forming salts with acids, such as nitrates, oxalates, etc., 
urea occurs in urine in a free and independent condition. 
Closely allied to urea, occurring apparently as a by-product of the 
same line of metabolism, is uric acid (C5H4N4O3) which is found always in 406 
THE ELIMINATION OF WASTE PRODUCTS. 
the urine of man, occurring in small but variable quantity. In the urine of 
some animals, such as birds and reptiles, it occurs in abundance, and indeed 
in these replaces urea as the chief nitrogenous excretion. Uric acid is a 
more complex body than urea, one molecule of uric acid splitting up, under 
the influence of certain reagents, into two molecules of urea and a com- 
pound of oxalic acid. Its decomposition products, however, under different 
reagents are very numerous and complex, though urea occurs among them 
frequently and characteristically. Uric acid may be synthetically produced 
out of urea and glycin (glycocol). 
It is a weak dibasic acid, and occurs in normal human urine, not as a 
free acid but as an acid salt, being combined with potassium and sodium, 
and to a less extent with calcium and ammonium. In quite normal urine 
these salts are soluble in the urine, even after the fluid has cooled down to 
the ordinary temperature of the air ; but not infrequently the urates, soluble 
in the urine at the temperature at which it leaves the body, are precipitated 
when the fluid cools, forming the well-known "deposit of urates." On 
further standing the salts are apt to be decomposed and thus to give rise to 
crystals of uric acid. 
Besides urea and uric acid the urine contains small but variable quanti- 
ties of more or less nearly allied bodies, such as kreatinin, xanthin, hypo- 
xanthin, and guanin. Concerning these we will at present only say that 
kreatinin is a hydrated form of the body kreatin which we spoke of (§ 62) 
as a constituent of muscles. Kreatin by hydration is readily converted into 
kreatinin, and kreatinin by dehydration into kreatin ; kreatin introduced 
into the alimentary canal or into the blood appears in the urine as kreat- 
inin ; and in flesh-eaters some at least of the kreatinin of the urine is derived 
directly from the kreatin present in the meat eaten as food; but we shall 
discuss the subject of kreatin later on. 
Besides the above, such bodies as leucin, taurin, cystin, allantoin, and 
ammonium oxalurate are occasionally found in urine, but cannot be regarded 
as constituents of normal urine. 
In the urine of man hippuric acid appears to be always present in small 
quantities, and in the urine of herbivora occurs in large quantities. In these 
latter it is derived more or less directly, by changes of which we shall have 
to speak in a succeeding chapter, from constituents of the food-containing 
bodies belonging to the aromatic group (benzoic acid series) ; but the small 
quantity present in man and other carnivora appears to come from the 
metabolism of proteid matter which, as we have already seen, contains an 
aromatic constituent. Another member of the aromatic group, tyrosin, is 
occasionally present in urine ; and as more regular constituents of normal 
urine may be mentioned certain phenol compounds, such as phenylsulphuric 
acid, the phenol constituents of which are derived from the action of micro- 
organisms in the alimentary canal (see § 243) ; these substances, though they 
no longer contain nitrogen, take origin from bodies of the aromatic series. 
Similar changes are also the source of indigo compounds (indican) in the 
urine, derived from indol (see § 219). 
§ 338. Inorganic salts. These for the most part exist in urine in natural 
solution, the composition of the ash almost exactly corresponding with the 
results of the direct analysis of the fluid; in this respect urine contrasts 
forcibly with blood, the ash of which is largely composed of inorganic sub- 
stances, which previous to the incineration existed in peculiar combination 
with proteid and other complex bodies. In the ash of urine there is rather 
more sulphur than corresponds to the sulphuric acid directly determined ; 
this indicates the existence in urine of some sulphur-holding complex body. 
And there are traces of iron, pointing to some similar iron-holding sub- THE COMPOSITION AND CHARACTERS OF URINE. 
407 
stance. But otherwise, all the substances found in the ash exist as salts in 
the natural fluid. 
The chief bases are sodium, potassium, calcium and magnesium in the 
form of chlorides, phosphates and sulphates. The exact way in which the 
several bases and acids are combined is to some extent a matter of uncer- 
tainty ; but sodium chloride is certainly present and in considerable quan- 
tity ; it is the most abundant and important inorganic constituent. A large 
portion of the phosphoric acid seems to exist as acid sodium phosphate, the 
rest as soluble calcium ami magnesium phosphates. The remaining chief 
salts, occurring, however, in smaller quantity, are potassium and sodium 
sulphate and calcium chloride. 
Ammonia occurs in small quantity, alkaline carbonates are frequently 
found, traces of nitrates are at all events occasionally present, as also indica- 
tions of salicylates and of sulpho-cyanates. 
The phosphates are derived partly from the phosphates taken as such in 
food, partly from the phosphorus or phosphates peculiarly associated with 
the proteids, and partly from the phosphorus of certain complex fats such 
as lecithin. When urine becomes alkaline (and, as we shall presently see, 
it may do so by changes taking place in itself) the calcic and magnesic 
phosphates are converted into basic salts which, being insoluble, are pre- 
cipitated, the sodium phosphate remaining in solution. When the alka- 
linity, as is frequently the case, is due to ammonia, ammonio-magnesium 
phosphate is formed and is apt to appear in crystals. The sulphates are 
derived partly from the sulphates taken as such in food and partly from the 
sulphur of the proteids. The carbonates, when occurring in large quantity, 
generally have their origin in the oxidation of such salts as citrates, tartrates, 
etc. The bases present depend largely on the nature of the food taken. 
Thus with a vegetable diet, the excess of the alkalies in the food reappears 
in the urine; with an animal diet, the earthy bases in a similar way come 
to the front. 
§ 339- Non-nitrog enous bodies. These exist in very small quantities, 
and many of them are probably of uncertain occurrence. Some of these 
are organic acids, the most constant perhaps being oxalic acid ; to this may 
be added glycerin-phosphoric, lactic, formic, acetic, butyric and possibly 
succinic acids. Inosit has also been said to occur normally. It has been 
maintained that minute quantities of sugar (dextrose) are invariably pres- 
ent in even healthy urine; this, however, has not as yet been placed beyond 
all doubt. The nature of the substances which give to urine its character- 
istic odor has not been made out; probably there are more such bodies than 
one. 
§ 340. Pigments. Urine is always colored, the tint varying from a light 
to a dark yellow with an admixture of brown. In the course of twenty- 
four hours, a not inconsiderable quantity of pigment must leave the body 
by the urine; but the nature of the normal pigment or pigments of urine 
is at present obscure and the subject of much controversy. The matter 
is apparently further complicated by the presence in urine of what have 
been called " chromogens," that is to say, bodies which are not colored 
themselves but which readily give rise to pigments upon oxidation ; and it 
is probable that some of these " chromogens " of the urine are reduction 
products of the respective pigments, the reduction taking place in the 
urine after secretion, or during or even before secretion. There is frequently 
present in urine, especially in cases of fever, a pigment which has been 
isolated and determined, which has a characteristic spectrum, and which, 
being maintained by some to be a derivative of bilirubin, has been called 
urobilin. It is not this urobilin, how'ever, which gives to urine its ordinary 408 
THE ELIMINATION OF WASTE PRODUCTS. 
color. Some observers, on the other hand, maintain that normal urine does 
contain and, in part at least, owes its normal color to a somewhat similar 
but different body, which in consequence they have called " normal " uro- 
bilin. It is in fact not possible, at the present moment, to make definite and 
satisfactory statements as to whether urine contains one or more than one 
normal pigment, as to its or their nature, as to whether they are derived 
from bile pigment or directly from the hsematin or haemoglobin or in other 
ways, or as to the several steps by which they are produced. There are also 
abnormal coloring matters present on occasion, such for instance as the pecu- 
liar red coloring matter occurring sometimes in the urine of acute rheuma- 
tism, which has been called uroerythrin ; but our knowledge concerning 
these is very imperfect. 
§ 341. Ferments and other bodies. Even normal urine has frequently 
been found to contain a small quantity, hardly amounting to more than a 
trace, of proteid material, apparently an albumin; but the normal presence 
of even this small quantity has been disputed. Urine, however, certainly 
contains ferment bodies. 
When urine is treated with many times its volume of alcohol, a granular 
or flocculent precipitate is thrown down, consisting chiefly of phosphates, 
together with some other substances or probably several other substances, in 
very small quantities. An aqueous solution of the precipitate, which may 
be freed from the phosphates, is both amylolytic and proteolytic. Ferments 
may also and more readily be extracted from urine by allowing shreds of 
fibrin to soak in the urine for a few hours, and then removing and washing 
them. The ferments become entangled in the fibrin in such a way as not 
to be easily removed by washing. The washed shreds will convert starch 
into sugar; and when treated with dilute hydrochloric acid digest them- 
selves, showing the presence of pepsin. By this method it has been ascer- 
tained that an amylolytic ferment and pepsin are present in quantities which 
vary in the twenty-four hours according to the meals. Rennin has also been 
found, and, at times at least, trypsin. From this it appears that some of the 
ferments of the alimentary canal escape from the body by the urine, being 
probably reabsorbed directly from the respective gland ; the quantity which 
thus escapes is insignificant. 
A small quantity of gas, about 15 vols. per cent., can be extracted by the 
mercurial pump from urine received direct from the body without exposure 
to air. The gas so obtained consists chiefly of carbonic acid, nitrogen being 
very scanty, and oxygen occurring in very small quantities or being wholly 
absent. The meaning of this we have already touched upon in speaking of 
respiration (see § 302). 
§ 342. The quantities in which these multifarious bodies, all of which, 
as we have seen, we may perhaps regard as constituents of normal urine, are 
present in different specimens of urine, vary within very wide limits, being 
dependent on the nature of the food taken and on the conditions of the body. 
The amount not of water only, but of many of the other several constituents, 
varies widely and indeed rapidly, so that the percentage composition of urine 
will vary from hour to hour if not from minute to minute. The causes which 
determine these variations in the nature and amount of urine we shall study 
later on. Meanwhile what may be called the average composition of human 
urine is shown in the following table in which the acids and bases are put down 
separately. URINARY CONSTITUENTS. 
409 
Amounts of the Several Urinary Constituents Passed in 
Twenty-Four Hours. (After Parkes.) 
By an average 
man of 66 kilos. 
Per 1 kilo 
of body-weight. 
Water 
. . 1500.000 grms. 
23.0000 grms. 
Total solids 
1.1000 
Urea 
. • 33.180 
0.5000 
Uric acid 
. . 0.555 
0.0084 
Hippuric acid 
. . 0.400 
0.0060 
Kreatinin 
. . 0.910 
0.0140 
Pigment, and other substances . 
. . 10.000 
0.1510 
Sulphuric acid 
. . 2.012 
0.0305 
Phosphoric acid 
. . 3.164 
0.0480 
Chlorine 
. . 7.000 
0.1260 
(8.21) 
Ammonia 
. . 0.770 
Potassium 
. . 2.500 
Sodium 
. . 1 1.090 
Calcium 
. . 0.260 
Magnesium 
. . 0.207 
72.000 
§ 343. The acidity of urine. The healthy urine of man is acid, owing 
to the presence of acid sodium phosphate, the absence of free acid being 
shown by the fact that sodium hyposulphite gives no precipitate. The 
amount of acidity is about equivalent to 2 grammes of oxalic acid in 
twenty-four hours, but the degree of acidity at any one time varies much 
during the day, being in an inverse ratio to the amount of acid secreted by 
the stomach ; thus it decreases after food is taken, and increases again as 
gastric digestion comes to an end. It varies with the nature of the food ; 
with a vegetable diet the excess of alkalies in the food, being secreted by 
the urine, leads to alkalinity, or at least to diminished acidity, whereas this 
effect is wanting with an animal diet, in which the alkalies are less abundant, 
earthy bases preponderating. Hence the urine of carnivora is generally very 
acid, while that of herbivora is alkaline. The latter, when fasting, are for 
the time being carnivorous, living entirely on their own bodies, and hence 
their urine becomes under these circumstances acid. 
The natural acidity increases for some time after the urine has been dis- 
charged, owing to the formation of fresh acid, apparently by some kind of 
fermentation. This increase of acid frequently causes a precipitation of 
urates, which the previous acidity, even after the cooling of the urine, had 
been insufficient to throw down. After a while, however, the acid reaction 
gives way to alkalinity. This is caused by a conversion of the urea into 
ammonium carbonate through the agency of a specific " organized " ferment. 
This ferment as a general rule does not make its appearance except in urine 
exposed to the air; it is only in unhealthy conditions that the fermentation 
takes place within the bladder, and in such cases is due either to micro- 
organisms introduced into the bladder from without, during the use of in- 
struments for instance, or to the action of an unorganized ferment, secreted, 
apparently by the walls of the bladder. 
§ 344. Abnormal constituents of urine. The structural elements found in 
the urine under various circumstances are blood, pus and mucous corpuscles, 
epithelium from the bladder and kidney, and spermatozoa. To these may 
be added the so-called " casts " which are either " epithelial casts," that is to 
say, cylinders of more or less altered epithelial cells shed from the tubules, or 
structureless, "fibrinous" casts, which are cylinders of peculiar material 
moulded in the lumina of the tubules; the exact nature of this material is 410 
THE ELIMINATION OF WASTE PRODUCTS. 
at present a matter of doubt; it is not always the same, but appears not to 
be fibrin. 
The most common and important abnormal constituents of urine are 
albumin, giving rise to albuminuria, and sugar, giving rise to glycosuria or 
diabetes. The soluble proteids, generally spoken of as " albumin," in the 
urine differ in different cases. The exact determination of their nature is a 
matter of some difficulty, since, as we have seen, we have in differentiating 
the various proteids to trust largely to their behavior as regards precipita- 
tion upon the addition of certain saline bodies; and the presence of saline 
bodies in the natural urine introduces complications. It would appear, 
however, that the proteids usually present are serum-albumin and globulin ; 
these are not, however, as a rule, if ever, present in the same relative pro- 
portions as in blood-plasma; and either the one or the other may be 
present by itself. A form of albumose (§ 189), called hemi-albumose, is 
sometimes found, and, indeed, probably very many distinct kinds of pro- 
teids are from time to time present. If egg-albumin be injected into the 
blood it appears in the urine as egg-albumin, and peptone similarly injected 
appears as peptone. 
The sugar which is found in the urine of diabetes is undistinguish- 
able from ordinary dextrose; but whether it is absolutely identical with 
that body, or whether the sugar in all cases of diabetic urine is ex- 
actly the same in character, cannot, perhaps, as yet, be regarded as defi- 
nitely settled. 
When blood is mingled with urine in the kidney and in the urinary 
passages the constituents of the former are, of course, added to those of 
the latter; and when, as sometimes happens, chyle from the lacteals makes 
its way into the kidneys, the urine contains the fats and other constituents 
of chyle. Fats, however, may be present without the urine being distinctly 
" chylous." 
Cholesterin, bile-acids, bile-pigments, and one or other of a large 
number of bodies arising from a disordered metabolism of the body, such 
as leucin, tyrosin, acetone (in cases of diabetes), oxalic acid, taurin, cystin, 
and many others are also found more or less frequently ; some of these, 
indeed, have been regarded as normal constituents. Besides these the urine 
serves as the chief channel of elimination for various bodies, not proper 
constituents of food, which may happen to have been taken into the sys- 
tem. Thus various minerals, alkaloids, salts, pigmentary and odoriferous 
matters, may be passed unchanged. Many substances thus occasionally 
taken undergo, however, changes in passing through the body : the most 
important of these, since the changes which they undergo throw light 
on the metabolic processes of the body, will be considered in a succeeding 
chapter. 
The Secretion of Urine. 
§ 345. The kidney consists of tw.o parts, so distinct in structure that 
it seems impossible to resist the conclusion that their functions are differ- 
ent, and that the mechanism by which the urine secreted is of a double 
kind. On the one hand, the tubuli uriniferi, with their characteristic epi- 
thelium, seem obviously to be actively secreting structures comparable to 
the secreting alveoli of the salivary and other glands. On the other hand, 
the Malpighian capsules, with their glomeruli, are organs of a peculiar 
nature, with an almost insignificant epithelium, and their structure irresist- 
ibly suggests that they act rather as what may be called, in a general way, 
a filtering than as a truly secreting mechanism. Hence has arisen the view THE SECRETION OF URINE. 
411 
which frequently bears the name of Bowman, since he was the first to put it 
forward, that certain constituents only of the urine are secreted after the fash- 
ion of other secreting glands by the tubuli uriniferi, and that the rest of the 
constituents, including a great deal of the water, with such highly soluble and 
diffusible salts as pre-exist in adequate quantity in the blood, are, as it were, 
filtered off by the glomeruli of the Malpighian capsules. We shall see, 
later on, reason to doubt whether we are justified in applying the term 
" filtration," which has a definite physical meaning, to the process by which 
water and other substances pass from the bloodvessels of the glomerulus into 
the lumen of the tubule; for that process is, as we shall find, peculiar and 
complex. But such a doubt need not prevent us from recognizing that the 
whole act of secretion of urine consists of two parts, one of which is much 
more closely dependent on the flow of blood through the kidney than is the 
ordinary process of secretion, such as has hitherto come before us, and another 
part which seems to bear the same relation to the flow of blood as does ordi- 
nary secretion. 
That the work of the kidney is, to an unusual degree, dependent on the 
flow of blood through it, seems suggested by the vascular arrangements; for 
these are extremely favorable to a full and rapid stream of blood through 
the organ. The short and relatively broad renal artery comes off direct 
from the abdominal aorta, where the blood-pressure is extremely high ; the 
renal vein opens directly into the vena cava, where the blood-pressure is ex- 
tremely low. Between the mouth of the renal artery and the mouth of the 
renal vein the difference of pressure is very great, indeed ; and, as we have 
seen in treating of the vascular system, it is the difference of pressure be- 
tween two points of the vascular tract which is the actual cause of the flow 
o€ blood from the one point to the other. The difference of pressure, in- 
deed, which drives the blood through the limited area of the kidney is the 
same difference of pressure which drives the blood along the abdominal 
aorta down both legs back again to the vena cava. 
This free and abundant supply of blood is regulated, is either increased 
or diminished, according to the needs of the moment, by the vasomotor sys- 
tem ; this is shown by experimental and other results, which it will be profit- 
able to study in some detail. Before entering into these details, however, it 
will be well to call attention to the fact that when vasomotor events modify 
the flow of blood through an organ, they produce their effects in one direc- 
tion or another by working on arterial blood-pressure. Thus, as we shall see, 
when stimulation or section of a nerve increases the flow of blood through the 
kidney, it does so by increasing the pressure in the small vessels of the kid- 
ney, including the capillary loops of the glomeruli. In such a case the walls 
of the glomerular loops, through which the passage of materials to form 
(part of) the urine takes place, are subjected to two influences-on the one 
hand, to a fuller, more rapid flow of blood past them, and, on the other, to 
an increase of the pressure which that blood, as it passes along, exerts on 
them. We shall have, subsequently, to discuss the share taken by these two 
influences in determining and modifying the passage of material through 
the walls of the glomerular loops; and this will bear on the question of 
filtration, to which we have above alluded ; but, for the present, it will be 
convenient to deal with the effects of variation in blood-pressure apart from 
this secondary question. 
§ 346. The vasomotor mechanisms of the kidney. It may be shown ex- 
perimentally that the kidney is supplied with a vasomotor mechanism as 
well developed, perhaps, as that of any part of the body. By means of a 
modification of the plethysmograph (Figs. 110, 111), we can readily observe 
the variations which take place in the volume of the kidney. 412 
THE ELIMINATION OF WASTE PRODUCTS. 
The instrument consists of two parts, one of which (Fig. 110), called the oncom- 
eter,1 is applied to the organ about to be studied, while the other (Fig. Ill), 
called the oncograph, is the recording part of the apparatus. Any diminution in 
the volume of the organ (Fig. 110. A"), kidney, spleen, etc., as the case may be, 
diminishes the pressure on the fluid in the chamber a; some of the fluid in the 
chamber M (Fig. Ill) accordingly passes through the tube A" (Fig. Ill) and the 
tube T (Fig. 110) to the chamber a ; the piston 1) accordingly falls and with it the 
lever H. Similarly an increase in the volume of the organ causes the lever to rise. 
Fig. 110. 
Renal Oncometer. (Seen in section, semi-diagrammatic.) K, kidney; V, vessels and nerves 
imbedded in fat, etc., entering hilus of organ; O, C, and I, C, outer and inner metal capsules 
screwed together by the screw S, and holding between them the edge of the membrane M, which 
applies itself to the surface of the kidney and forms with the metal capsule two chambers, a and 
B. one of which (B) is closed by a plug filling the opening B, while the other (a) communicates 
by a tube 7'with the recording instrument. The other opening, C (which is closed by a small 
tap), is for the purpose of filling the chamber a with warm oil after the kidney has been placed 
in the box, the other chamber B having been previously partly filled, the quantity introduced 
into it depending upon the size of the kidney. 
The volume of the kidney may be increased by a swelling of its con- 
stituent cells and other structural elements, by an accumulation of lymph in 
its lymph-spaces, and by a distention of its bloodvessels. Compared with 
the third, the two former causes are in health so insignificant and prob- 
lematical that they may be disregarded. Further, the distention of the 
bloodvessels will in general depend on the constriction or dilatation of the 
renal arteries and their ramifications, for distention due to venous obstruction 
will only occur in special cases. Hence variations in the volume of the kid- 
ney may be taken as a measure of variations in its vascular supply-increase 
of volume indicating dilated renal vessels, and decrease of volume indicating 
constriction of the renal vessels. 
1 From oncos, bulk. THE SECRETION OF URINE. 
413 
When by means of the instrument just described a tracing is taken of 
the volume of a kidney in what maybe considered a normal condition, some 
such result as that shown in Fig. 112 is obtained. 
Fig. 111. 
Semi-diagrammatic Sectional View of Oncograph. (Half natural size.) K, tube connecting 
instrument with oncometer; D, piston floating on oil contained in the cavity M; the oil is pre- 
vented from escaping by the side of the piston by the delicate flexible membrane E, which does 
not interfere with the movements of the piston ; H, recording lever connected with the piston by 
a needle G passing through the guides F, F'. The screw C is for the purpose of clamping the 
edge of the membrane between the two ring-shaped surfaces at N, while the side tube L is for the 
purpose of filling the instrument. 
The volume of the kidney is seen to be so delicately responsive to 
changes in the mean arterial pressure that the curve reproduces almost 
exactly a blood-pressure curve, showing not only the respiratory undulations 
BLOOD PRESSURE 
Fig. 112. 
KIDNEY CURVE 
Blood-pressure Tracing, and Curve from Renal Oncometer. (Natural size.) The blood-pres- 
sure abscissa line has been raised 2.75 cm. (the actual medium blood-pressure having been 115 
mm. Hg.). The time-curve gives interruptions recurring every three seconds. 
but even the rise and fall due to the individual heart-beats. With each rise 
of mean arterial pressure more blood is driven into the renal vessels and the 
kidney swells; with each fall of pressure less blood enters and the kidney 
shrinks. On other tracings taken in the same way may often be seen (not 414 
THE ELIMINATION OF WASTE PRODUCTS. 
shown in Fig. 112) the wider variations corresponding to the Traube-Hering 
curves; but it will be observed that in these the kidney shrinks with the 
rise of pressure and swells with the fall. For as we have seen (§ 331) the 
rise in the Traube-Hering undulation is due to an augmentation of peripheral 
resistance caused by the constriction of minute arteries; and this constriction 
occurs in the kidney as elsewhere; the renal arterioles take their share in 
producing the result, and in consequence of their constriction the kidney 
shrinks. Similarly the relaxation of the renal vessels contributes to bring 
about the sequent fall. 
§ 347. In the course of a discussion in an earlier part of this work 
(§ 157) on the local and general effects of arterial constriction and dilata- 
tion, we saw that the local blood-pressure in and flow of blood through the 
capillaries and other minute vessels of this or that vascular area may be 
increased- 
1. By an increase of the general blood-pressure brought about-(a) by an 
increased force, frequency, etc., of the heart's beat, (6) by the constriction of 
the small arteries supplying areas other than the area in question. 
2. By a relaxation of the artery (or arteries) supplying the area itself, 
which, while diminishing the pressure in the artery itself, increases the 
pressure in the capillaries and small veins which the artery supplies. It 
need hardly be added that this local relaxation must not be accompanied 
by a too great dilatation elsewhere. 
The same local blood-pressure and flow of blood may similarly be 
diminished- 
1. By a constriction of the artery of the area itself (and its branches), 
which, while increasing the pressure on the cardiac side of the artery, dimin- 
ishes the pressure in the capillaries and veins which are supplied by the 
artery. This again must not be accompanied by a too great constriction 
elsewhere. 
2. By a lowering of the general blood-pressure, brought about-(a) by 
diminished force, etc., of the heart's beat, (6) by a general dilatation of the 
small arteries of the body at large, or by a dilatation of vascular areas other 
than the area in question. 
Applying these considerations to the bloodvessels of the kidney, we 
should expect to find the following: 
A rise in general blood-pressure, and that means a rise of pressure in the 
abdominal aorta at the mouth of the renal artery, will cause a greater flow 
of blood through, and so an expansion of the kidney, provided that the renal 
arteries themselves are not unduly constricted at the same time. This is 
well shown, as we have seen, in the curve given above, where the increase 
of pressure due to each heart-beat, as well as that due to each respiratory 
movement, being of central origin and not due to arterial constriction and 
being unaccompanied by any compensating constriction of the renal artery, 
leads to expansion of the kidney, that is, to a greater flow of blood through 
the kidney. 
If, however, the rise of general blood-pressure be due to events which at 
the same time cause a constriction of the renal arteries, the flow through the 
kidney may not only not be increased, but even be diminished ; the kidney 
may shrink instead of expanding. Thus if dyspnoea be brought about, as 
by stopping artificial respiration during an experiment, the kidney at once 
shrinks ; the too venous blood stimulates the vasomotor centre, and probably 
also by direct action on the bloodvessels leads to a general arterial constric- 
tion and so to a rise of blood-pressure; but the renal vessels are involved in 
this constriction, so much so that their constricted condition more than coun- 
terbalances the general rise of blood-pressure, and less blood flows through 415 
THE SECRETION OF URINE. 
the renal vessels. So also when the medulla or spinal cord is directly stimu- 
lated by induction shocks (the animal being under urari so as to eliminate 
the complications due to contractions of the skeletal muscles) the renal ves- 
sels share so fully in the arterial constriction which results that, in spite of 
the great rise of mean pressure which is induced, less blood than normal 
passes through the renal vessels, and the kidney shrinks. Or if the abdom- 
inal splanchnic nerves be stimulated, since, as we shall see, these carry vaso- 
constrictor fibres for the kidney, in spite of the rise of blood-pressure which 
follows, the kidney shrinks on account of the great constriction of the renal 
vessels. 
On the other hand, if a rise of blood-pressure be for any reason not ac- 
companied by a compensating constriction of the renal arteries, that rise, 
whether it be brought about by general constriction of arteries other than 
the renal or by an increase of the cardiac delivery, causes the kidney to 
swell, showing a greater flow of blood. Such a condition of things may be 
induced by section of the nerves of the renal plexus, whereby the paths of 
all vaso-constrictor impulses to the kidney are blocked. After this has been 
done, a rise of general pressure, whether by dyspnoea, or by direct stimulation 
of the spinal cord, or by stimulation of the abdominal splanchnic nerves, 
leads to a greater flow through the renal vessels and an increased expansion 
of the kidney. 
A rise of general blood-pressure, then, may be accompanied by either a 
shrinking or a swelling of the kidney, by either a greater or lesser flow of 
blood through the kidney, according to the concomitant condition of the 
renal vessels; or, indeed, may under certain circumstances be accompanied 
by no change at all in the renal circulation, the local effects exactly counter- 
balancing the general ones. 
Conversely, in a similar way, a fall of blood pressure leads to a lesser 
flow through the renal vessels and a shrinkage of the kidney unless it be 
accompanied by a dilatation of the renal vessels out of proportion to the 
general fall. Thus when the spinal cord is divided below the medulla the 
fall of general blood-pressure is, as we have seen (§ 159), very marked, 
being due to an abolition for the time being of wonted constrictor impulses. 
The pressure in the aorta falls rapidly, and at the same time, owing to the 
more open pathway through the region of peripheral resistance in the body 
generally, the pressure in the vena cava is increased ; the difference of pres- 
sure between the mouth of the renal artery in the aorta and the mouth of 
the renal vein in the vena cava is so largely reduced that in spite of the 
concomitant relaxed condition of the renal vessels themselves the flow of 
blood through the kidney is largely diminished. 
It will of course be understood that, the general blood-pressure remain- 
ing the same, the flow through the kidney will at once be, on the one hand, 
increased by dilatation and, on the other, decreased by constriction of the 
renal vessels themselves. The constricted or dilated condition of the renal 
vessels can by themselves produce but little effect on the pressure either in 
the aorta or in the vena cava ; and the difference between the pressure at 
the mouth of the renal artery and that at the mouth of the renal vein re- 
maining the same, the more open passages of the dilated renal vessels must 
lead to a fuller, and the narrower passages of the constricted renal vessels to 
a scantier, flow through the kidney. 
§ 348. By means of the oncometer, watching the shrinking and swelling 
of the kidney and thus judging of the flow of blood through it, the results 
being always interpreted with reference to the general blood-pressure on the 
lines of the above discussion, the paths of vasomotor impulses to the kidney 
have been approximately made out. Vaso-constrictor fibres for the kidney 416 
THE ELIMINATION OF WASTE PRODUCTS. 
are supplied from what we have previously (§ 155 and elsewhere) spoken of 
as the vaso-constrictor region of the spinal cord. They issue from the spinal 
cord by the anterior roots of a large number of the spinal nerves taking 
origin from this region, and may be traced (in the dog) as high up as the 
sixth dorsal, a few perhaps even to the fourth dorsal, and as low down as the 
second lumbar (fourth lumbar if only thirteen nerves be counted as dorsal) ; 
but most seem to pass by the eleventh, twelfth, and thirteenth dorsal nerves. 
Passing through the corresponding ganglia of the splanchnic (sympathetic) 
chain, these fibres reach the solar plexus and thus the renal plexus by the 
abdominal splanchnic nerve; those, however, coming from some of the lower 
nerves apparently do not contribute to the splanchnic nerve, but take a sepa- 
rate course. Centrifugal stimulation of these anterior roots produces shrink- 
ing of the kidney, all the more marked and distinct in the case of the 
eleventh, twelfth, and thirteenth dorsal roots because the effect on the kidney 
is then not so much masked by vasomotor effects on other organs. Stimu- 
lation of the higher roots also produces shrinking of the kidney, but less 
marked, since in these cases the stimulation bears at the same time largely 
on vaso-constrictor fibres for other abdominal organs, and so by raising the 
general blood-pressure tends to neutralize the local effect on the kidney. 
And even the very decided shrinking of the kidney which results from the 
stimulation of the splanchnic trunk itself is less than would take place if 
the stimulation affected the vessels of the kidney only. 
§ 349. We stated in § 154 that by the method of slowly repeated rhyth- 
mical stimulation the presence of vaso-dilator fibres in the sciatic nerve 
might be detected, though these are largely mixed with vaso-constrictor 
fibres; and slow rhythmical stimulation of the anterior roots of the above- 
mentioned lower dorsal nerves leads, not, as does ordinary rapidly inter- 
rupted stimulation, to shrinking, but to swelling of the kidney, showing that 
these roots contain vaso-dilator fibres as well as vaso-constrictor fibres. The 
higher (anterior) roots also appear to contain some renal vaso-dilator fibres ; 
but the effect of stimulating them by the slow rhythmic method is more 
masked by a concomitant dilatation of the vessels of the other abdominal 
organs, the roots in question containing vaso-dilator as well as vaso-con- 
strictor fibres for those organs; this leads to a fall of general blood-pressure 
whereby the tendency of the kidney to swell is counteracted. 
§ 350. It is obvious then that by means of this vasomotor mechanism 
the flow of blood through the kidney is governed by the central nervous 
system in such a way that afferent impulses, started in this or that region or 
surface, and passing up to the central nervous system, may lead either to 
constriction or to dilatation of the renal vessels; and to such actions of this 
kind we shall presently return. Meanwhile we wish to call attention to the 
fact that the volume of the kidney is remarkably sensitive to chemical 
changes taking place in the blood. The injection into the blood of even a 
small quantity of water causes a transient shrinking of the kidney followed 
by a more lasting expansion. The injection of urea and some other diuretics 
produces the same effect to a more marked degree, leading especially to a 
swelling which lasts for some considerable time, while the injection of normal 
saline solution, and especially of such diuretics as sodium acetate, causes an 
expansion from the very first, the primary shrinking being absent. It is, 
moreover, worthy of note that these effects of diuretics and of chemical 
changes in the blood are observed even after all the renal nerves have 
apparently been completely severed. Hence the changes in volume caused 
by the presence of these substances in the blood must be due to the sub- 
stances acting either upon some peripheral vasomotor mechanism, or, even 
more directly, on the bloodvessels themselves. It may be added that they THE SECRETION OF URINE. 
417 
will produce considerable effects in the kidney itself without appreciably 
modifying the general blood-pressure. 
§ 351. If, while the kidney is in the oncometer, and the various experi- 
ments on section and stimulation of nerves and the like are being carried 
on, a canula be tied in the ureter, the secretion of urine may be watched at 
the same time. It will then be seen that the flow of urine through the end 
of the canula is not equable, and does not either increase or decrease in an 
even manner. On the contrary, it will frequently be found that a sort of 
gush of urine takes place, several drops following each other in rapid suc- 
cession, followed by a cessation of flow ; and if the ureter be watched it 
will be seen that the gushes of urine are synchronous with waves of 
peristaltic contraction sweeping down the ureter. Obviously the urine 
collects, to a certain extent, in the pelvis of the kidney, and is driven 
thence by muscular action from time to time; to this point we shall return 
later on. 
Making every allowance, however, for these irregularities of flow, we may 
take the rate of flow from the end of the canula as a measure of the rate 
of secretion ; and it is found that as a general rule increased flow of urine 
is coincident with swelling of the kidney, that is with a greater flow of 
blood through it, and diminished or arrested flow of urine is coincident 
with shrinking of the kidney, that is, with a diminished flow of blood 
through it. 
A striking instance of this is afforded by the expriment of dividing in 
the dog the spinal cord below the medulla. The blood-pressure then, as we 
know, falls rapidly, owing to the removal of constrictor impulses from the 
small arteries and the general diminution of peripheral resistance which 
follows upon so many small arteries becoming dilated ; and though the renal 
arteries probably share in the general relaxation, yet, owing to the fall of 
pressure in the aorta conjoined as this is by a corresponding rise of pressure 
in the vena cava, the flow of blood through the kidney is largely dimin- 
ished. We find that after the operation the secretion of urine is greatly 
diminished; indeed, in most cases the flow from the end of a canula is 
almost arrested. In fact, we may almost make the general assertion that, 
when in the dog the blood-pressure falls to about 30 mm. Hg. or less, the 
secretion of urine is for the time stopped. These and other results support 
the view stated above that the secretion of urine is in quite a special way 
dependent on the flow of blood through the kidney ; and we may further 
conclude that the secretion which is so particularly influenced by the flow 
of blood is that special kind of secretion, allied to filtration, which takes 
place through the glomeruli, and not the more ordinary kind of secretion 
by means of the epithelium of the tubuli uriniferi. But before we proceed 
to discuss how the increased flow of blood increases the glomerular flow of 
urine, we must turn to consider the functions of the epithelium of the 
tubuli. 
Secretion by the Renal Epithelium. 
§ 352. The glomerular mechanism is, after all, a small portion only of 
the whole kidney, and the epithelium over a large part of the course of the 
tubuli uriniferi bears most distinctly the characters of an active secreting 
epithelium. These facts would lead us d priori to suppose that the flow of 
urine is in part the result of an active secretion comparable to that of the 
salivary or other glands which we have already studied. And we have ex- 
perimental and other evidence that such is the case. 
In the first place, a flow of urine may be artificially excited even when 
the natural flow has been arrested by diminution of blood-pressure. Thus 418 
THE ELIMINATION OF WASTE PRODUCTS. 
if, when the urine lias ceased to flow in consequence of a section of the 
medulla oblongata, certain substances, such as urea, urates, sodium acetate, 
and the like, be injected into the blood, a more or less copious secretion is at 
once set up. This secretion is, or at least may be, unaccompanied by any 
rise of general blood-pressure sufficient to account for the increased secretion 
as the mere result of an increased flow of blood. It is true (as we have 
seen, § 350) that the injection of these substances leads to an expansion of 
the kidney, an expansion which is probably due to a local dilatation of the 
small renal arteries ; but the flow of urine which is observed in these cases 
is too great to be accounted for by any increase of flow of blood which the 
local dilatation may bring about; and hence we conclude that the increase 
of secretion is of a different kind from that which follows upon mere increase 
of blood-How. It seems much more reasonable to suppose that the presence 
of the above substances in the blood excites the renal epithelial cells to an 
unwonted activity, causing them to pour into the interior of the tubules a 
copious secretion, just as the presence of pilocarpine in the blood will cause 
the salivary cells to pour forth their secretion into the lumen of their ducts ; 
and that this activity of the epithelial cells is accompanied, also as in the 
case of the submaxillary and other glands, by a vascular dilatation, which, 
though adjuvant and beneficial, is not the distinct cause of the activity. 
This view is further supported by the following remarkable experiment, 
which goes far to show that of the various substances which, having found 
their way into the blood, are thrown out by the kidney, some pass into the 
urine through the glomeruli, while others are distinctly secreted by the 
tubuli uriniferi, the discharge of the latter being accompanied by a general 
activity of the secreting cells, as shown by the How of water taking place at 
the same time. 
In the amphibia the kidney has a double vascular supply; it receives 
arterial blood from the renal artery, but there is also poured into it venous 
blood from another source. The femoral vein divides at the top of the thigh 
into two branches, one of which runs along the front of the abdomen to 
meet its fellow in the middle line and form the anterior abdominal vein, 
while the other passes to the outer border of the kidney and branches in the 
substance of that organ, forming the so-called renal portal system. Now' 
the glomeruli, in some species at least of these animals, are supplied exclu- 
sively by the branches of the renal artery, the renal vena portse only serv- 
ing to form the capillary plexus around the tubuli uriniferi, which is also 
supplied by the efferent vessels of the glomeruli. From this it is obvious 
that if the renal artery be tied, the blood is shut off entirely from the 
glomeruli; and actual observation of the kidney has, in the animals in ques- 
tion, shown that under these circumstance there is no reflux from the capil- 
lary network surrounding the tubules back to the glomeruli ; thus the kidney 
by this simple operation is transformed into an ordinary secreting gland 
devoid of any special filtering mechanism. Such a kidney may be used to 
ascertain what substances are excreted by the glomeruli, and what by the 
tubules in some other part of their course. It is found that urea injected 
into the blood gives rise to a secretion of urine when the renal arteries 
are tied; this substance, therefore, is secreted by the epithelium of the tu- 
bules, and in being so secreted gives rise at the same time to a flow of water 
through the cells into the interior of the tubules. Sugar and peptones, on 
the other hand, which injected into the blood readily pass through the un- 
touched kidney and appear in the urine, do not pass through a kidney the 
renal arteries of which have been tied, even when a diuretic such as urea 
is given at the same time in order to secure a flow of urine. These sub- 
stances, therefore, are excreted by the glomeruli. THE SECRETION OF URINE. 
419 
The validity of this experiment, which may be accepted as indicating a 
marked difference between glomerular secretion on the one hand and epithe- 
lial or tubular secretion on the other, depends on the absence of any collateral 
circulation whereby the glomeruli may be supplied with blood after ligature 
of the renal artery. In these animals anastomoses occur between the renal 
arteries and the arteries of the generative organs; and unless the renal 
artery be so tied as to avoid these collateral communications the results 
of the experiment are different. 
Additional evidence in favor of the secretory activity of the epithelial 
cells is afforded by the following observation. Into the veins of animals in 
which the urinary flow had been arrested by section of the spinal cord below 
the medulla a quantity of the blue coloring material known as sodium 
sulphindigotate1 is injected. This substance is rapidly excreted on the one 
hand by the liver in the bile, and on the other hand by the kidney. By 
varying the quantity injected, killing the animals at appropriate times after 
the injection of the material, and examining the kidneys microscopically 
and otherwise, it may be ascertained that the pigment so injected passes from 
the blood into the renal epithelium, and from thence into the channels of 
the tubules. There being no stream of fluid through the tubules, owing to 
the arrest of urinary flow by means of the preliminary operation, the pig- 
ment travels very little way down the interior of tubules, and remains very 
much where it was cast out by the epithelial cells. There are no traces 
whatever of the pigment having passed by the glomeruli; and the cells 
which appear most distinctly to take up and eject it are those lining such 
portions of the tubules (viz., the first and second convoluted tubules, zigzag 
tubules, and ascending limbs of the loops of Henle) as from their micro- 
scopic features have been supposed to be the actively secreting portions of 
the entire tubules. The following observation which has been made is of a 
peculiarly interesting character. After injecting a certain quantity of pig- 
ment, and allowing such a time to elapse as might be judged from previous 
experiments would suffice for the passage of the material through the epithe- 
lium to be pretty well completed, a second quantity was injected. It was 
found that the excretion of this second quantity was most incomplete and 
imperfect. It seems as if the cells were exhausted by their previous efforts, 
just as a muscle which has been severely tetanized will not respond to a 
renewed stimulation. 
The above observation may be objected to on the ground that this color- 
ing matter does not occur as a constituent of the blood either in health or 
disease, and especially that the absence of any concomitant discharge of 
fluid from the cells excites suspicion that the process observed was not really 
one of secretion ; for the injection of such substances as urea or urates into 
the blood does cause a copious flow of fluid, and indeed thus prevents the 
microscopic tracking out of their passage, which in the case of urates might 
otherwise be done much in the same way as with the sodium sulphindigotate. 
Moreover, other observers have maintained that the sodium sulphindigotate 
does like ordinary carmine pass through the glomeruli. But their results 
may probably be explained by the glomeruli having been damaged by a too 
rapid or too abundant injection ; and in the case of the amphibian kidney, 
when sodium sulphindigotate is injected after ligature of renal arteries, no 
urine is found in the bladder, but the pigment can be traced through the 
epithelium of the secreting portions of the tubules. Without insisting too 
much on the value of the sodium sulphindigotate experiments, they may be 
taken as fairly supporting the view which we are considering. We may add 
1 Sometimes called indigo-carmine, though this name is more properly applied to a 
crude, impure preparation of potassium sulphindigotate. 420 
THE ELIMINATION OF WASTE PRODUCTS. 
that in birds, the urine of which contains little water, urates may be detected 
in the epithelium of the tubules, but not in the capsules. 
Though much remains to be cleared up, we may, for the present, con- 
clude that the secretion of urine does consist of two separate and distinct 
acts; secretion by the glomeruli, which we may for brevity's sake speak of 
as glomerular secretion, and secretion by the epithelium of the tubuli, which 
we may speak of similarly as tubular secretion. But these forms of secre- 
tion, especially the former, but to a certain extent the latter also, differ from 
the secretion of such a gland as the salivary, and both deserve some special 
consideration. 
§ 353. The nature of glomerular secretion. We have seen that the ex- 
pansion of the kidney which has for its accompaniment an increased flow 
of urine is one brought about by the renal artery and its various branches 
becoming dilated, under such circumstances that the difference between the 
blood-pressure in the aorta at the mouth of the renal artery and the blood- 
pressure at the vena cava at the mouth of the renal vein is at the same time 
increased, or at all events is not diminished. 
In dealing with the vascular system we saw that relaxation of a small 
artery, taking place without any marked change in the general blood-pres- 
sure and in neighboring arteries, leads to a fuller and more rapid stream of 
blood through the capillaries supplied by the artery, and that at the same 
time the pressure in the capillaries themselves is increased; owing to the 
decrease of peripheral resistance through the widening of the artery, the 
great fall of pressure (see § 105) so characteristic of the peripheral region 
is shifted from the arterial side of the capillaries toward the venous side and 
to the capillaries themselves. 
Hence, as we have already said, when the renal artery dilates two things 
happen in the loops of the glomeruli: a fuller, more rapid stream of blood 
passes through them, and that blood as it Hows through them is exerting a 
greater pressure than before on their walls. How does each of the events 
stand toward the secretion of urine? 
We have not at present the means of inducing a fuller and more rapid 
How without increasing the pressure; but we may easily obtain increase of 
pressure without the fuller and more rapid flow. If we hinder or obstruct 
the outflow through the renal vein we at once increase the pressure in the 
glomerular loops as in the other capillaries of the kidney. Now, when the 
blood-pressure in the glomeruli is thus raised by partial obstruction to the 
venous outflow, the flow of urine so far from being increased is diminished. 
Obviously, then, the passage of water and material through the walls of the 
glomerular loops, to go to form the urine, is not the result of mere pressure, 
and cannot, therefore, be spoken of properly as a process of filtration. 
(Of. § 255.) And we may here draw a comparison between the passage of 
water and material through the wall of a capillary in an ordinary situation 
to form lymph and the passage through the wall of the glomerular loop to 
form urine or part of urine. The former, as we have seen (§ 255), appears 
to be directly dependent on pressure, though influenced as we have also 
seen in a very material way by the condition of the vascular wall; and 
hindrance to venous outflow, so inefficient in promoting a flow of urine, is 
as we have seen especially favorable to the transudation of lymph. In the 
former case the substances which pass through the capillary wall may be 
described as the constituents of the blood generally, proteids as well as salts 
and other soluble and diffusible matters. Through the wall of the glomer- 
ular loop there pass, so long as that wall is sound and intact, neither 
albumin nor globulin nor fibrin factor, but only water accompanied by 
some, and apparently a selection of some, of the soluble diffusible constitu- THE SECRETION OF URINE. 
421 
ents of the blood ; for, as we have said, the presence of proteids in normal 
urine is contested, and, at most, there is present a very small quantity only 
(which, moreover, may come from the tubular epithelium). This difference 
in the material which passes through may be referred to the differences in 
the nature of the partition. The transudation of lymph takes place through 
the capillary wall ; between the blood on one side and the lymph in the 
lymph-space on the other is only the thin film of conjoined epithelioid 
plates. But the corresponding wall of the glomerular loop is covered over 
and wrapped around, so to speak, by an adherent layer of cells, which 
though reduced and thin are still epithelial cells; the materials which go 
to form urine have to pass through these cells as well as through the film 
of epithelioid plates. It seems to be this layer of cells which determines 
what shall pass and what shall not. 
Obviously the passage through this epithelium is of a peculiar nature. 
The necessary condition for the due accomplishment of the passage is as we 
have seen a full and rapid stream of (arterial) blood ; the high pressure 
which accompanies that full and rapid stream, though probably under 
normal circumstances an adjuvant, is by itself helpless. Thus when the 
pressure is raised by venous obstruction, in which case the high pressure is 
accompanied by a slow stream or by actual arrest of the flow, even the 
passage of mere water is retarded. Seeing that many of the constituents 
of urine are diffusible substances certainly preexisting in the blood, inor- 
ganic salines for instance, and seeing that diffusible abnormal constituents 
of blood, such as peptone and sugar, pass into the urine not by the tubular 
epithelium but by the glomeruli, we might expect that diffusion, in contrast 
to filtration (see § 265), played an important part in the passage; and a 
full rapid stream would undoubtedly favor diffusion. But diffusion by 
itself will not explain matters. Egg-albumin differs very slightly as regards 
diffusibility from serum-albumin, and yet while at the most a minute quan- 
tity only of the latter passes into the urine in normal circumstances, the 
former when injected into the blood at once makes its way into the urine, 
presumably by the glomeruli. On the other hand urea is an eminently diffus- 
ible body, and yet if we can trust the experiments on the amphibian kidney, 
the main mass at all events of the urea of the urine passes by the epithelium 
of the tubules. 
The important part played by the epithelium is shown when the epithe- 
lium is deranged. If the renal artery be temporarily ligatured or other- 
wise obstructed, so that the glomeruli are shut off from the blood-supply for 
some little time, the secretion of urine is stopped ; on reestablishment of the 
circulation the secretion of urine slowly returns, and the urine is then found 
to be albuminous, remaining so for some little time. The serum-albumin 
and globulin which could not pass through the intact epithelium, can pass 
through when the epithelium is damaged by interference with its nutrition. 
The appearance of albumin in the urine (albuminuria) is a not infrequent 
symptom of kidney disease, and its presence in other than minute quantities 
indicates imperfections in the glomerular epithelium. But even under un- 
healthy conditions that epithelium still governs to a certain extent the pas- 
sage of material; for the proteids of the blood-plasma do not pass through 
bodily or in a proportion which corresponds either to the relative proportion 
in which they exist in the plasma or to the relative ease (or difficulty) with 
which they pass through membranes. Though the "albumin" of albumin- 
ous urine frequently consists of both serum-albumin and globulin, these do 
not necessarily occur in the same proportion as in blood ; they vary in urine 
much more than they do in blood ; and indeed the one or the other may be 
absent; moreover, fibrin factors are very rarely found. 422 
THE ELIMINATION OF WASTE PRODUCTS. 
Hsemoglobinuria, or the presence of haemoglobin in urine, may be brought 
about by injecting into the bloodvessels laky blood, or some substance such 
as pyrogallic acid, which will " break up" the corpuscles of the blood. Now, 
in such cases there is evidence that the haemoglobin passes through the 
glomeruli; minute disc-like masses of haemoglobin, the so-called " menisci," 
are, by appropriate methods of preparation, found in situ in the capsules. 
Such a passage is very far removed from being a process of diffusion. 
We may conclude, then, that the passage of material through the glom- 
eruli, like the transudation of lymph, and even to a more marked extent, is 
a complex affair in which the ordinary physical processes of diffusion and 
filtration may play their part, but are not masters of the situation. 
§ 354. The work of the epithelium of the tubules. As we have said, the 
structural features of the epithelial cells of the tubules seem to justify the 
conclusion that they exercise a secretory,activity comparable with that of a 
salivary or a gastric gland. But their work is in many ways peculiar. In 
the case of the salivary, gastric, and pancreatic glands there can be no doubt 
that the specific constituents of the several secretions, mucin, pepsin, trypsin, 
and the like, are manufactured in the alveolar cells out of antecedents of 
some nature or other. The evidence, as we have seen, is all against the 
view that these glands merely withdraw, secrete in the old sense of the word, 
from the blood these substances preexisting in the blood. When the salivary 
glands are extirpated, or the pancreas or the stomach removed, there is no 
accumulation in the blood of the specific constituents of the corresponding 
secretions. So also when the liver is extirpated there is no accumulation in 
the blood of either bile acids or bile pigment. With regard to the kidney 
and the most important constituent of urine, namely, urea, the case is differ- 
ent. If the kidneys in a mammal be extirpated, or if the kidneys by disease 
or by ligature of the ureters be so damaged as to be unable to carry on their 
work, an accumulation takes place in blood, not as was once thought, of some 
antecedent of urea, such as kreatin, but of urea itself. In the case of birds 
and reptiles which excrete not urea, but chiefly uric acid, the accumulation 
is one of uric acid. Obviously in secreting urea the work of the epithelium 
of the tubules is largely, if not exclusively, confined to simply picking the 
urea out of the blood and pushing it, so to speak, into the lumina of the 
tubules. We might, perhaps, say exclusively, for there is no evidence that 
any urea at all is actually manufactured in the kidney. 
But even this mere picking up the urea is after all not a simple process; 
the epithelial cell of the tubule is not a mere passive sieve of peculiar struc- 
ture, especially adapted to strain off the urea from the blood. As we have 
already seen, when urea or uric acid is injected into the blood the result is 
not a mere increase in the proportions of urea (or uric acid) present in the 
urine which is being secreted. The injection leads to an increased flow of 
urine, the whole activity of the cell is stirred up, and other constituents, not 
at the moment like the urea existing in excess in the blood, are discharged 
into the lumina of the tubules together with the urea. 
How the urea, which is in this peculiar manner taken out of the blood, 
comes to make its appearance in the blood is a problem in which the kidney 
is not concerned, and with which we shall deal in treating of the metabolic 
events of the body generally. 
§ 355. In the case of some other constituents of the urine we have evi- 
dence that the cells do something more than simply pick the constituent out 
of the blood. Hippuric acid, as we have seen, occurs in small quantity in 
the urine of man, and in larger amount in the urine of herbivora. Now, 
hippuric acid may be formed by the combination, with dehydration, of ben- 
zoic acid and glycin (C7H6O2+C2H5NO2-H,O-C9H9NO3) ; and benzoic THE SECRETION OF URINE. 
423 
acid introduced into the alimentary canal or injected into the blood reap- 
pears in large measure in the urine as hippuric acid. Somewhere in the 
body the benzoic acid meets with and combines with glycin. And we have 
experimental proof that the combination may and probably does take place 
in the kidney. 
If a circulation of blood be kept up through the bloodvessels of the kid- 
ney freshly removed from a living animal, and benzoic acid and glycin be 
added to the blood as it is about to enter into the kidney, hippuric acid will 
be found in the blood issuing from the kidney, especially if the same blood 
be passed through the kidney several times ; the blood used must be blood 
containing oxy-haemoglobin, carbonic-oxide haemoglobin not producing the 
effect. The mere mixing with the blood itself is insufficient; and if the 
blood be sent not through a kidney just removed from the living body, but 
through one taken from a dead body or one which has been left to itself for 
some time after removal from a living body, the synthesis will not be effected. 
To carry out the combination by means of the kidney which has been re- 
moved from the body, the kidney must retain for a while its own life, it must 
be a "surviving" kidney. Nor is it absolutely necessary to bring the ben- 
zoic acid and glycin to the kidney by means of a blood-stream. If a "sur- 
viving " kidney be divided rapidly into small pieces and the benzoic acid 
rapidly mixed with the pieces, hippuric acid is formed. Nor is it necessary 
to furnish the glycin. If benzoic acid alone be used, hippuric acid is formed 
all the same. Glycin, as we have previously said, cannot be recognized as 
a normal constituent of any of the tissues; nevertheless, as we have seen in 
speaking of glycocholic acid in the bile, and as we shall see later on, glycin 
must make a momentary appearance in various metabolic processes of the 
body, being immediately on its appearance converted into something else, so 
that it never remains as glycin. It apparently is formed in the kidney, and 
is thus momentarily available for the conversion of benzoic into hippuric 
acid. 
It seems probable, therefore, that, with regard to this particular con- 
stituent of urine, hippuric acid, the cells of the tubules have the power of 
effecting a combination between the benzoic acid brought to them by the 
blood and the glycin which they furnish by means of their own metabolism, 
and in this way produce hippuric acid. 
Not only benzoic acid, but many other bodies taken into the system, 
reappear in the urine combined with glycin, and in their cases also the 
combination probably takes place through the activity of the cells of the 
tubules of the kidney. Moreover, other changes than the assumption of 
glycin, the various changes which many chemical substances taken into the 
system undergo before reappearing in the urine, probably also take place to 
a large extent in the kidney, and are also carried out by means of the 
epithelium of the tubules. 
What other constituents of normal urine are produced in this or a similar 
manner we do not as yet definitely know. The pigment urobilin, which, as 
we have seen, is supposed to be a derivative from bilirubin, may be brought 
ready-formed from the liver or may have the finishing touches given to it in 
the kidney itself; and the other normal or abnormal urinary pigments pos- 
sibly arise either directly from haemoglobin or indirectly from that body 
through the biliary pigment by a transformation taking place in the cells of 
the tubules. There is also evidence in frogs that acid sodium phosphate is 
furnished by the cells of the tubules. 
In conclusion, then, we may say that the activity of the epithelium of the 
kidney appears especially modified, as compared with other secreting glands, 
to meet the special object which the kidney has to secure. The purpose of 424 
THE ELIMINATION OF WASTE PRODUCTS. 
the kidney is not to provide a fluid, urine, which can be made use of for the 
needs of the body, but to cast out waste matters from the body. Hence its 
secretory activity is limited largely to the mere discharge of matters which 
reach it preexistent in the blood, though in several cases it gives the final 
shape to the excreted substance before it passes into the ureter. 
§ 356. We may illustrate the preceding discussions by briefly passing in 
review some of the more usual ways in which the secretion of urine is in 
ordinary life modified. 
In the preceding section the composition of urine was illustrated by the 
daily output of the several constituents, rather than by a percentage account 
of any specimen of urine, for the reason that the composition of urine varies 
within extremely wide limits. This is especially the case as regards the pro- 
portion of water to solids. One urine may be of high specific gravity with 
a small amount of water relatively to the solids, while another may have so 
little color and such a low specific gravity as to appear hardly more than 
water. The reason of these extreme differences lies in the fact that the 
kidney is not only the channel by which waste solids leave the body, but 
also an important outlet for the discharge of the stream of water which, in 
order that the various processes of the body may be duly carried on, is con- 
tinually passing through the system. It is frequently of advantage to the 
body to discharge through the kidney a large amount of water, more or less 
irrespective of the solid matters which are, so to speak, washed away with 
it; and hence the advantage of the glomerular mechanism so specially 
adapted for the discharge of water. 
As we shall see presently, to the skin also falls the duty of discharging 
large quantities of water. The respiratory organs also, as we have seen, 
serve for the discharge of water; but the amount which the latter put out 
can only be varied by the inconvenient method of increasing or diminishing 
the whole act of breathing. Hence we find special relations between the 
skin and the kidneys, correlating the work of the one to that of the other as 
regards this particular work of the discharge of water. 
When the body is exposed to cold the discharge of water from the skin in 
the form of sweat is checked, and the cutaneous vessels are constricted. At 
the same time the bloodvessels of the abdominal viscera, including the kid- 
neys are dilated, but not out of proportion to the constriction of the cuta- 
neous vessels, for the general blood-pressure does not fall, but if anything 
rises somewhat. Thus there is established just the state of things which is 
favorable to a fidl and rapid stream of blood through the renal glomeruli, 
and an increased flow of urine results. It is possible, we may, perhaps, say 
probable, that the nervous system affords a special tie between the skin and 
the kidney, so that under the circumstances in question the renal arteries are 
dilated even more than those of the other abdominal viscera ; but this has 
not been proved experimentally. It is also possible that by another reflex 
mechanism of the central nervous system the skin may work upon the kid- 
ney, not by the vasomotor nerves alone, but also by nerves governing the 
secretory activity of the tubules; but we have no satisfactory indications of 
any such mechanisms, and it seems more probable that the connections 
should be with the glomerular mechanism, since the chief object at all events 
is to get rid of water. 
Conversely, when the body is exposed to warmth the skin perspires freely 
and the cutaneous vessels are widely dilated ; and conversely also the renal 
and other abdominal vessels are constricted, so that a slow and small stream 
of blood trickles through the glomeruli, and the urine which is secreted is 
scanty. 
§ 357. Even more important than its relations to the skin are the rela- 425 
THE SECRETION OF URINE. 
tions of the kidney to the water absorbed by the alimentary canal; this is 
especially seen when large quantities of fluid are drunk. The whole of the 
water thus introduced into the alimentary canal passes into the blood, for in 
a healthy organism no amount of fluid drunk, unless it throws the economy 
out of order, can affect the amount of water present in the feces. But the 
addition to the blood of even a very large quantity of fluid does not, as we 
have seen, by its mere quantity (§ 172), increase the general blood-pressure, 
and therefore cannot in this way produce what it undoubtedly does produce, 
an increased flow of urine. 
The fluid so absorbed may act on the kidney in two ways. On the one 
hand, as we have seen (§ 350), the injection of water into the blood produces 
a local dilatation of the renal vessels, as indicated by the swelling of the 
kidney. Thus the absorption of mere water from the alimentary canal may 
stir up to greater activity the glomerular mechanism, and in so doing may 
be assisted by the presence of various substances absorbed from the alimen- 
tary canal with the water, for some of these also may similarly lead to 
dilatation of the renal vessels. 
On the other hand, some or other of the chemical bodies thus passing into 
the blood with the water drunk may excite the secretory activity of the 
tubules, and that either by acting directly on the epithelium as they are car- 
ried through the kidney in the blood of the renal arteries, or indirectly 
through some intervention of the central nervous system. 
Our knowledge is at present too scanty to enable us to decide which of 
these two methods is the one usually employed by the organism ; but the 
inordinate flow of urine, so poor in solids as to be little more than water, 
which may be directed through the kidney by means of an adequate " drink- 
ing bout," would lead us to conclude that in such cases the organism, striv- 
ing, though too often in vain, to free itself from the evils to which it is being 
subjected, has recourse rather to the simpler glomerular mechanism than to 
the more expensive tissue-wasting activity of the tubules; and the urine in 
such cases is probably discharged chiefly by the method of dilating the renal 
vessels and thus throwing the poisoned blood into the glomeruli. 
When, however, fluid is taken simply as a proper accompaniment of solid 
food, the increase of urine which results has probably another origin. As 
we have already said, and as we shall point out more fully later on, the 
absorption of proteid material, which is a constituent and generally a con- 
spicuous constituent of every meal, leads to a formation of urea ; and urea, 
as we have seen reason to believe, directly stimulates the epithelium of the 
tubules to secretory activity. And what seems prominently true of urea is 
probably true of many other products of digestion ; so that the increased 
flow of urine which follows an ordinary meal accompanied by not more 
than the ordinary amount of fluid, is the result of the labors of the epithe- 
lium of the tubules as well as of the fuller stream of blood through the 
glomeruli. 
§ 358. What has just been said concerning the influence on the kidney 
of food ami water may be applied also to the action of substances which 
being especially efficacious in promoting a flow of urine when taken into the 
body are called "diuretics." The several actions of various diuretics are 
very varied, and it would be out of place to discuss them fully. We may, 
however, say that while the action of some appears simple that of others is 
complex. 
Such agents as sodium acetate and potassium nitrate probably produce 
their effect chiefly by acting directly on the kidney, including, as we have 
seen (§ 350), local vascular dilatation and so working on the glomeruli, but 
probably at the same time also stirring up, after the fashion of urea, the 426 
THE ELIMINATION OF WASTE PRODUCTS. 
epithelium of the tubules to secretory activity, the accompanying fuller 
stream of blood through the whole kidney being, as in the case of the 
salivary and other glands, a useful adjuvant. 
The diuretic effect of such an agent as digitalis is probably more com- 
plex. By increasing the cardiac stroke, and at the same time constricting 
many small vessels, digitalis raises the general blood-pressure; but the tend- 
ency of the increased blood-pressure to increase the flow of urine may be 
counterbalanced by the constriction of the renal vessels themselves. And 
while it is a matter of common experience that digitalis is very effective as 
a diuretic in cardiac disease, there is great doubt whether it really acts as a 
diuretic in health; in cardiac disease it probably raises the blood-pressure 
by improving the cardiac stroke and not by constriction of the bloodvessels. 
But even in the absence of cardiac disease, digitalis has been found in 
certain cases to act as a powerful diuretic, and in these cases either it must 
act directly on the tubular epithelium or its effects in constricting the renal 
arteries must be less than its effects on other small arteries or must pass off 
before the influence of the heightened blood-pressure has disappeared. 
§ 359. Quite removed from the intervention of chemical substances in 
the blood and yet most striking is the influence on the kidney of the central 
nervous system. The potent influence of emotions in promoting the secre- 
tion of urine is proverbial, and the general features of " nervous" urine, the 
water increased out of proportion to the solid constituents, especially seen 
in the " urina hysterica," which is hardly more than simple water, often dis- 
charged in enormous quantity, at once suggests the view that impulses origi- 
nating in the brain and passing down to the kidney along the vaso-dilator 
fibres, of whose existence evidence was given in $ 349, lead to dilated blood- 
vessels and great play of glomerular activity, without perhaps producing 
any other direct effect on the economy ; though possibly the same emotions 
by constricting the cutaneous and, it may be, other vessels may raise the 
general blood-pressure and so help the dilated renal vessels. 
The Discharge of Urine. 
§ 360. The urine, like the bile, is secreted continuously ; the flow may 
rise and fall, but in health never absolutely ceases for any length of time. 
The cessation of renal activity, the so-called suppression of urine, entails 
speedy death. The minute streams passing continuously-now more rapidly, 
now more slowly-along the collecting and discharging tubules, are gathered 
into the renal pelvis, whence the fluid is carried along the ureters into the 
bladder partly by pressure and gravity, and from time to time partly, as we 
have already said (§ 351), by the peristaltic contractions of the muscular 
walls of the ureter. 
If in a living animal a ureter be laid bare and stimulated, mechanically 
or otherwise, at a part of its course, waves of peristaltic contraction may be 
seen to pass in both directions from the spot stimulated-up toward the 
kidney and down toward the bladder. In the absence of artificial stimula- 
tion spontaneous waves of contraction make their appearance, sometimes 
repeated with tolerable regularity (about every twenty seconds in the rabbit), 
sometimes occurring in groups with longer pauses between. These sponta- 
neous contractions invariably pass in one direction, from the kidney to the 
bladder, and their frequency and vigor seem to be determined by the activity 
of the secretion of urine. But they are not directly called forth by the 
urine, either mechanically distending the tube or chemically stimulating the 
inner surface, for regularly recurring contractions may be observed in a THE DISCHARGE OF URINE. 
427 
kidney and ureter removed from the body, or even in an isolated excised 
piece of the ureter. 
The rhythmically repeated contractions arise spontaneously in the mus- 
cular coat of the ureter much in the same way as the similar cardiac con- 
tractions arise in the muscular substance of the heart; and it may here be 
mentioned in support of what was urged in § 141 with regard to the heart- 
beats not being started by nerve-cells, that rhythmically repeated spontane- 
ous peristaltic contractions have been observed in isolated pieces of ureter 
taken from the middle of its course, in which no nerve-cells and indeed no 
distinct nerve-fibres could be observed. 
In the living body these spontaneous movements, beats they might be 
called, are subordinated to the flow of urine into the pelvis ; the more active 
the secretion of urine, the more frequent and vigorous are the beats of the 
pelvis and ureter ; but the exact mechanism by which the secretion and the 
movements are maintained in harmony has not yet been cleared up. 
Micturition. 
§ 361. In the urinary bladder the urine is collected, its return into the 
ureters being prevented by the oblique entrance into the bladder and valvu- 
lar nature of the orifices of those tubes, and its discharge from thence in 
considerable quantity is effected from time to time by a somewhat complex 
muscular mechanism, of the nature and working of which the following is 
a brief account: 
The involuntary muscular fibres forming the greater part of the vesical 
walls are arranged, partly in a more or less longitudinal direction and 
partly in a circular manner. After it has been emptied the bladder is con- 
tracted and thrown into folds ; as the urine gradually collects, the bladder 
becomes mere and more distended. The escape of the fluid is in part pre- 
vented by the resistance offered by the elastic fibres in the walls of the ure- 
thra, which help to keep the urethral channel closed. But this is not all, 
for observation shows that fluid is retained within the bladder up to a pres- 
sure of twenty inches of water, so long as the bladder is governed by an 
intact spinal cord, but gives way to a pressure of six inches only when the 
lumbar spinal cord is destroyed or the vesical nerves are severed. This 
affords very strong evidence that the obstruction at the neck of the bladder 
to the exit of urine depends on some tonic muscular contraction maintained 
by a reflex or automatic action of the lumbar spinal cord. And it has been 
maintained that it is the circularly disposed fibres specially developed 
around the neck of the bladder which are the subjects of this tonic contrac- 
tion and thus the chief cause of retention ; hence the name sphincter vesica?. 
The continuity of these fibres, however, with the rest of the circular fibres 
of the bladder suggests that they probably do not act as a sphincter, but 
that their use lies in their contracting after the rest of the vesical fibres, and 
thus finishing the evacuation of the bladder. The resistance in question is 
supplied by a tonic contraction not of the circular fibres of the bladder 
itself, but of the muscular fibres-partly plain, partly striated-surround- 
ing the prostatic portion of the urethra, and constituting the sphincter vesicce 
externus or prostat icus, or sphincter of Henle. It is stated that artificially 
excited contractions of these fibres will resist a pressure of fluid in the 
bladder. 
When the bladder has become full, we feel the need of making water, the 
sensation being heightened, if not caused, by the trickling of a few drops of 
urine from the full bladder into the urethra. We are then conscious of an 
effort; during this effort the bladder is thrown into a long-continued con- 428 
THE ELIMINATION OF WASTE PRODUCTS. 
traction of an obscurely peristaltic nature, the force of which is more than 
sufficient to overcome the resistance offered by the urethra, and the urine 
issues in a stream, the sphincter vesicae externus being at the same time either 
relaxed after the fashion of the sphincter ani, or at least overcome. In its 
passage along the urethra, the exit of the urine, at all events of the last por- 
tions, is forwarded by irregularly rhythmic contractions of the bulbo-caver- 
nosus or ejaculator urinae muscle, the contractions of which compress the 
urethra; and the whole act is further assisted by pressure on the bladder 
exerted by means of the abdominal muscles, very much the same as in 
defecation. 
In the case of the rectum we w'ere able (§ 237) to distinguish between the 
actions of the longitudinal and of the circular coats, and we said that the 
two coats had distinct nervous supplies (Fig. 89). The bladder has, as we 
have said, a similar nerve supply, and it is very probable, but not yet dis- 
tinctly proven, that this like double supply has a like double action. Stimu- 
lation of the branches coming from the sacral nerves, at the same time that 
it throws the longitudinal coat of the rectum into contractions, brings about 
in the dog, in which the longitudinal fibres of the bladder are much more 
pronounced than the circular, powerful vesical contractions. Moreover, 
stimulation of the sacral nerves on one side produces unilateral contraction 
of the bladder. From this we may infer that the sacral nerves govern the 
longitudinal coat. Stimulation of the hypogastric nerves carrying fibres 
from the dorsal and upper lumbar cord (see Fig. 109), while throwing the 
circular coat of the rectum into strong contractions, gives rise to vesical con- 
tractions, but these are by no means so marked as those which appear when 
the sacral nerves are stimulated. We may probably conclude that the more 
important fibres in the fundus of the bladder, which are for the most part 
longitudinal, are to be regarded as governed chiefly by the sacral nerve- 
fibres, while the circular muscular fibres around the neck of the bladder, 
whose contraction completes, as it were, the emptying of the bladder, are 
those on which the hypogastric nerve-fibres have the chief influence. 
§ 362. We said just now " when the bladder has become full," but this 
must not be understood to mean "when the bladder has received a certain 
quantity of fluid." On the contrary, it is a matter of common experience 
that we feel the desire to make water sometimes when a large quantity and 
sometimes when a small quantity of urine has accumulated in the bladder. 
We have evidence that the bladder possesses to a very high degree that ob- 
scure continuous contraction which we speak of as " tone and, further, that 
the amount of its tone is exceedingly variable, the organ, quite independ- 
ently of distinct efforts at micturition, being at one time contracted and at 
another flaccid and distended. When it is in a contracted state, a small 
quantity of fluid may exert the same effect on the vesical walls as a larger 
quantity when the bladder is flaccid. Hence, while the determining cause 
of the desire to make water is the pressure of the urine upon the vesical 
walls, the quantity needed to produce the necessary fulness is dependent on 
the amount of tonic contraction of the muscular fibres existing at the time. 
And we have evidence that this tone is regulated by the nervous system. 
§ 363. Micturition as sketched above seems at first sight, and especially 
when we appeal to our own consciousness, a purely voluntary act. A volun- 
tary effort throws the muscular fibres of the bladder into contractions, an 
accompanying voluntary effort lessens the tone of the sphincter externus, 
probably by inhibiting its centre in the spinal cord, while other voluntary 
efforts throw the ejaculator and abdominal muscles into contractions, and 
the resistance of the urethra being thereby overcome, the exit of the urine 
naturally follows. THE DISCHARGE OF URINE. 
429 
There are facts, however, which prevent the acceptance of so simple a 
view. In the first place, in cases of urethral obstruction, where the bladder 
cannot be emptied when it reaches its accustomed fulness, the increasing 
distention sets up fruitless but powerful contractions of the vesical walls, 
contractions which are clearly involuntary in nature, which wane or disap- 
pear, and return again and again in a rhythmic manner, and which may be 
so strong and powerful as to cause great suffering. It seems that the fibres 
of the bladder, like all other muscular fibres, have their contractions aug- 
mented in proportion as they are subjected to tension. Justus a previously 
quiescent ventricle of a frog's heart may be excited to a rhythmic beat by 
distending its cavity with blood, so the quiescent bladder may, quite inde- 
pendently of the will, be excited, by the distention of its cavity, to a peristaltic 
action which in normal cases is never carried beyond a first effort, since 'with 
that the bladder is emptied and the stimulus is removed, but which in cases 
of obstruction is enabled clearly to manifest its rhythmic nature. 
In the second place it has been shown that quite normal micturition may 
take place in a dog in which the lumbar region of the spinal cord has been 
completely and permanently separated by section from the upper dorsal 
region. In such a case there can be no exercise of volition, and the whole 
process appears as a reflex action. When under these circumstances the 
bladder becomes full (and otherwise apparently the act fails) any slight 
stimulus, such as sponging the anus or slight pressure on the abdominal 
walls, causes a complete act of micturition ; the bladder is entirely emptied, 
and the stream of urine toward the end of the act undergoes rhythmical 
augmentations due to contractions of the ejaculator urime. These facts can 
only be interpreted on the view that there exists in the lower spinal cord (of 
the dog) what we may speak of as a micturition centre capable of being 
thrown into action by appropriate afferent impulses, the action of the centre 
being such as to cause a contraction of the walls of the bladder and of the 
ejaculator urinse, and at the same time to suspend the tone of the sphincter 
vesicse externus. Clinical experience also goes to show the existence of a 
similar micturition centre in man, placed higher up in the cord than the 
corresponding " genital " centre governing the genital organs. 
Moreover, we have, in the case both of man and of other animals, experi- 
mental and other evidence that contraction of the bladder is frequently 
brought about by reflex action. Thus the pressure within the bladder when 
observed for any length of time is found to be subject to considerable and 
manifold variations. Over and above passive changes in pressure due to the 
respiratory movements, through which the bladder is pressed upon at each 
descent of the diaphragm, active contractions, of a strength inadequate to 
bring about micturition, are from time to time observed. These in some 
instances appear to be spontaneous, or to be the result of emotions, but they 
may be readily induced in a reflex manner by stimulating various sentient 
surfaces or sensory nerves. And common experience affords many instances 
where vesical contractions thus brought about in a reflex manner acquire 
strength adequate to empty the bladder. 
Observation of vesical pressure may be most conveniently carried out by intro- 
ducing into the bladder a catheter connected with a water manometer and a regis- 
tering apparatus, and so arranged as to allow fluid to be driven into or received 
from the bladder at pleasure. 
§ 364. Involuntary micturition obviously of reflex nature has fre- 
quently been observed in cases of paralysis from disease of or injury to the 
spinal cord ; and the involuntary micturition which is common in children, 
as the result of irritation of the penis and genital organs, and which some- 430 
THE ELIMINATION OF WASTE PRODUCTS. 
times occurs in the adult as the result of emotions, or at least sensory im- 
pressions, appears to be the result of reHex action. In these several cases 
we may fairly suppose that the centre in the spinal cord is affected by affer- 
ent impulses reaching it along various sensory nerves or descending from 
the brain. Hence we are led to the conception that when we make water 
by a conscious effort of the will, what occurs is not a direct action of the will 
on the muscular walls of the bladder, but that impulses started by the will 
descend from the brain after the fashion of afferent impulses and thus in a 
reHex manner throw into action the micturition centre in the spinal cord. 
We may draw an analogy between the micturition apparatus and the re- 
spiratory mechanism. We saw reasons in the latter case to think that when 
the will interfered with the respiratory movements, it did so by acting upon 
the nervous mechanism in the central nervous system and not by acting di- 
rectly on the muscular fibres of the diaphragm and other respiratory muscles. 
And the case of the plain muscular fibres of the bladder seems even stronger 
than that of respiratory muscles so largely skeletal in nature. We might 
also draw an analogy with the heart. We are not able to throw into action, 
by any direct effort of the will, the cardiac augmentor mechanism. Were 
we able to do so powerfully and suddenly, we might throw into violent ac- 
tion a weakly beating heart much in the same way that we empty an ob- 
scurely contracting bladder. Nor is this view negatived by the fact that 
paralysis of the bladder, or rather inability to make water either voluntarily 
or in a reflex manner, is a common symptom of cerebral or spinal disease 
or injury. Putting aside the cases in which the reflex act is not called forth 
because the appropriate stimulus has not been applied, the failure in mic- 
turition under these circumstances may be explained by supposing that the 
shock of the spinal injury or some extension of the disease has rendered the 
spinal centre unable to act. 
The so-called incontinence of urine in children is simply an easily ex- 
cited and frequently repeated reflex micturition. In cases of cerebral or 
spinal disease a form of incontinence is frequently met with which seems to 
be of a different nature. The bladder becoming full, but, owing to a failure 
in the mechanism of voluntary or reflex micturition, being unable to empty 
itself by a complete contraction, a continued dribbling of urine takes place 
through the urethra, the fulness of the bladder being sufficient to overcome 
the resistance at the neck of the urethra. It is probable, however, that 
even in these cases the flow is partly caused by obscure, unfelt, intrinsic 
contractions of the bladder. 
§ 365. Whether, under normal conditions, the urine undergoes any 
notable change during its stay in the bladder has been much debated. 
Experiments show that poisonous substances injected into the bladder with 
all due care to avoid any abrasion of the epithelium are absorbed and pro- 
duce their usual effects. It has also been stated that if a solution of' urea 
be injected into the bladder after ligature of both ureters, and allowed to 
stay for some hours, part of the urea disappears. But at present there is 
no very decided proof that under ordinary conditions either the water or 
other constituents of urine are to any appreciable extent absorbed by the 
bladder. 
Under abnormal conditions, as in inflammation or irritation of the blad- 
der, the urine may have undergone marked changes during its stay in the 
bladder, one of the most common being a change of some of the urea into 
ammonium carbonate, by which the urine also becomes alkaline. Under 
abnormal conditions also, the mucus of the urine, which in a healthy man 
is insignificant, though in some animals, for instance the horse, it occurs in 
considerable quantity, is largely increased during the stay in the bladder. 431 
THE NATURE AND AMOUNT OF PERSPIRATION. 
Since there are in man no goblet cells in the vesical epithelium (in the frog 
they are present) or mucous glands in the walls of the bladder, this mucus 
must be supplied by an abnormal metabolism of the ordinary epithelial 
cells. 
The Nature and Amount of Perspiration. 
§366. The quantity of matter which leaves the human body by way 
of the skin is very considerable. Thus, it has been estimated that while 
0.5 gramme passes away through the lungs per minute, as much as 0.8 
gramme passes through the skin. The amount, however, varies extremely; 
it has been calculated, from data gained by enclosing the arm in a caout- 
chouc bag, that the total amount of perspiration from the whole body in 
twenty-four hours might range from 2 to 20 kilos; but such a mode of 
calculation is obviously open to many sources of error. 
Of the whole amount thus discharged, part passes away at once as 
watery vapor mixed with volatile matters, while part may remain for a 
time as a fluid on the skin ; the former is frequently spoken of as insensible, 
the latter as sensible, perspiration or sweat. The proportion of the insen- 
sible to the sensible perspiration will depend on the rapidity of the secre- 
tion in reference to the dryness, temperature, and amount of movement of 
the surrounding atmosphere. Thus, supposing the rate of secretion to 
remain constant, the drier and hotter the air, and more rapidly the strata 
of air in contact with the body are renewed, the greater is the amount of 
sensible perspiration, which is, by evaporation, converted into the insensible 
condition ; -'and, conversely, when the air is cool, moist, and stagnant, a large 
amount of the total perspiration may remain on the skin as sensible sweat. 
Since, as the name implies, we are ourselves aware of the sensible perspira- 
tion only, it may, and frequently does, happen that we seem to ourselves to 
be perspiring freely, when, in reality, it is not so much the total perspira- 
tion which is being increased as the relative proportion of the sensible per- 
spiration. The rate of secretion may, however, be so much increased that 
no amount of dryness or heat, or movement of the atmosphere, is sufficient 
to carry out the necessary evaporation, and thus the sensible perspiration 
may become abundant in a hot, dry air. And, practically, this is the usual 
occurrence, since, certainly, a high temperature conduces, as we shall point 
out presently, to an increase of the secretion, and it is possible that mere 
dryness of the air has a similar effect. 
The amount of perspiration given off is affected not only by the condition 
of the atmosphere, but also by the circumstances of the body. Thus it is 
influenced by the nature and quantity of food eaten, by the amount of fluid 
drunk, by the character of exercise taken, by the relative activity of the 
other excreting organs, more particularly of the kidney, by mental condi- 
tions, and the like. Variations may also be induced by drugs and by dis- 
eased conditions. How these various influences produce their effects we shall 
study immediately. 
The fluid perspiration, or sweat, when collected, is found to be a clear, 
colorless fluid of a distinctly salt taste, with a strong and distinctive odor, 
varying according to the part of the body from which it is taken. Besides 
accidental epidermic scales, it contains no structural elements. 
Sweat, as a whole, is furnished partly by the sweat-glands and partly by 
the sebaceous glands, for, as we shall see, the small amount which simply 
transudes through the epidermis, apart from the glands, may be neglected. 
Now, the secretions from these two kinds of glands differ widely in nature, 
and the characters of the sweat, as a whole, will vary according to the rela- 432 
THE ELIMINATION OF WASTE PRODUCTS. 
tive proportion of the two kinds of secretion. The secretion of the sebaceous 
glands appears to be fairly constant, the larger variations of the total sweat 
depending chiefly on the varying activity of the sweat-glands. Hence, when 
sweat is scanty, the constituents of the sebum influence largely the characters 
of the sweat; when, on the contrary, the sweat is very abundant, these may 
be disregarded, and the sweat may be considered as the product of the sweat- 
glands. 
We are not able at present to make a complete statement as to what 
bodies occur exclusively in the sebum and what in the secretion of the 
sweat-glands. The former consists, very largely, of fats and fatty acids, 
and appears to contain some form or forms of proteids; but we have 
reason to think that the sweat-glands secrete, in some quantity, some 
forms of fat, and especially volatile fatty acids. 
When sweat is scanty, the reaction is generally acid, but when abund- 
ant, is alkaline ; and when a portion of the skin is well washed the 
sweat which is collected immediately afterward is usually alkaline. From 
this we may infer that the secretion of the sweat-glands is naturally 
alkaline, but that when mixed, sweat is acid ; the acidity being due to fatty 
(or other) acids of the sebum. In the horse, which is singular in hair- 
covered animals for its frequent profuse sweating, the sweat is said to be 
always acid and to contain a considerable quantity of some form of 
proteid. These features are probaby due to the large admixture of 
sebum from the numerous sebaceous glands connected with the hairs. 
Taking ordinary sweat, such as may be obtained by enclosing the arm 
in a bag, we may say that in man the average amount of solids is from 1 
to 2 per cent., of which about two-thirds consist of organic substances. 
The chief normal constituents are: (1) Sodium chloride, with small 
quantities of other inorganic salts. (2) Various acids of the fatty series, 
such as formic, acetic, butyric, with probably propionic, caproic, and 
caprylic. The presence of these latter is inferred from the odor ; it is 
probable that many various volatile acids are present in small quanti- 
ties. Lactic acid, which has been reckoned as a normal constituent, is 
stated not to be present in health. (3) Neutral fats and cholesterin ; 
these have been detected even in places, such as the palm of the hand, 
where sebaceous glands are not present. (4) The evidence goes to show 
that neither urea nor any ammonia compound exists in the normal secre- 
tion to any extent, though some observers have found a considerable quan- 
tity of urea (calculated at 10 grms. in the twenty-four hours for the 
whole body). Apparently a small amount of nitrogen leaves the body 
through the skin, but this is probably supplied by the sebum or by the 
epidermis. 
In various forms of disease the sweat has been found to contain, some- 
times in considerable quantities, blood, albumin, urea (particularly in 
cholera), uric acid, calcium oxalate, sugar (in diabetic patients), lactic 
acid, indigo (or indigo-yielding bodies, giving rise to " blue " sweat), bile, 
and other pigments. Iodine and potassium iodide, succinic, tartaric, and 
benzoic (partly as hippuric) acids have been found in sweat when taken 
internally as medicines. 
Cutaneous Respiration. 
§ 367. A frog whose lungs have been removed will continue to live for 
some time; and during that period will continue, not only to produce car- 
bonic acid, but also to consume oxygen. In other words, the frog is able to 
breathe without lungs, respiration being carried on efficiently by means of THE NATURE AND AMOUNT OF PERSPIRATION. 
433 
the skin. In mammals and in man this cutaneous respiration is, by reason 
of the thickness of the epidermis, restricted within very narrow limits; and, 
indeed, it has been questioned whether it can be spoken of at all as a true 
respiration. When the body remains for some time in a closed chamber to 
which the air passing in and out of the lungs has no access (as when the 
body is enclosed in a large air-tight bag fitting tightly round the neck, or 
where a tube in the trachea carries air to and from the lungs of an animal 
placed in an air-tight box), it is found that the air in the chandler loses 
oxygen and gains carbonic acid. The amount of carbonic acid which is thus 
thrown off by the skin of an average man in twenty-four hours amounts to 
about 10 grms., or according to some observers to (no more than) about 4 
grins., increasing with a rise of temperature and being very markedly aug- 
mented by bodily exercise. It is stated that the amount of oxygen con- 
sumed is about equal in volume to that of the carbonic acid given off' but 
some observers make it rather less. It may be doubted, however, whether 
the carbonic acid comes direct from the blood; it may come from decom- 
position taking place in the sweat-of carbonates, for instance. Similarly 
the oxygen which disappears may be simply used in oxidizing some of the 
constituents of the sweat. It is evident that the loss which the body suffers 
through the skin consists, besides a small quantity of sodium chloride, chiefly 
of water. 
When an animal, a rabbit for instance, is covered over with an imper- 
meable varnish, such as gelatin, so that all exit or entrance of gases or 
liquids by the skin is prevented, death shortly ensues. This result cannot 
be due, as once thought, to arrest of cutaneous respiration, seeing how insig- 
nificant and doubtful is the gaseous interchange by the skin as compared 
with that by the lungs. Nor are the symptoms at all those of asphyxia, but 
rather of some kind of poisoning, marked by a very great fall of tempera- 
ture, which, however, seems to be the result not of diminished production of 
heat, but of an increase of the discharge of heat from the surface. The ani- 
mal may be restored, or at all events its life may be prolonged with the 
abatement of the symptoms, if the great loss of heat which is evidently 
taking place be prevented by covering the body thickly with cotton-wool or 
keeping it in a warm atmosphere. The symptoms have not as yet been 
clearly analyzed, but they seem to be due in part to a pyrexia or fever pos- 
sibly caused by the retention within or reabsorption into the blood of some 
of the constituents of the sweat, or by the products of some abnormal meta- 
bolism, and in part to a dilatation of the cutaneous vessels caused by the 
application of varnish ; owing to the dilated condition of the cutaneous ves- 
sels the loss of heat through the skin is abnormally large, even though the 
varnish may not be a good conductor. 
§ 368. Absorption by the skin. Although under normal circumstances 
the skin serves only as a channel of loss to the body, it has been maintained 
that it may, under particular circumstances, be a means of gain, and the 
little which we have to say on this matter may perhaps be said here. Cases 
are on record where bodies are said to have gained in weight by immersion 
in a bath, or by exposure to a moist atmosphere during a given period, in 
which no food or drink was taken, or to have gained more than the weight 
of the food or drink taken ; the gain in such cases must have been due to 
the absorption of water by the skin. Direct experiments, however, throw 
doubt on these statements, for they show that under ordinary circumstances 
such a gain by the skin is slight, being apparently due to mere imbibition 
of water by the upper layers of the epidermis. 
Absorption of various substances takes place very readily by abraded 
surfaces where the dermis is laid bare or covered only by the lowest layers 434 
THE ELIMINATION OF WASTE PRODUCTS. 
of epidermis, but it has been debated whether substances in aqueous solution 
can be absorbed by the skin when the epidermis is intact, the evidence on 
this point being contradictory. In the case of the skin of the frog an ab- 
sorption of water and of various soluble substances certainly takes place. 
In the case of the sound human skin there are no a prior i reasons why water 
carrying substances dissolved in it should not pass inward through the 
corneous as well as the other layers of the epidermis, the amount so passing 
depending, among other things, upon the condition of the skin ; and com- 
mon experience seems to show that it does. Nevertheless, the results of 
actual experiment are conflicting. Some observers maintain that soluble 
non-volatile substances are not absorbed, and that volatile substances, such 
as iodine, which may be detected in the system after a bath containing them, 
are absorbed not by the skin, but by the mucous membrane of the respira- 
tory organs, the substance making its way to the latter by volatilization from 
the surface of the bath. Others, again, have found evidence of absorption, 
especially with volatile substances, even when care has been taken to avoid 
all errors ; and the greater weight may perhaps be given to these, since they 
accord with common experience. The conflict of experimental results, how- 
ever, at least shows that we do not fully understand the conditions under 
which such absorption takes place. 
There is, moreover, evidence that even solid particles can pass through 
an intact skin. The lymphatics m the skin of a newborn infant have been 
found crowded with the particles of the peculiar fatty secretion which covers 
the skin at birth ; and solid particles rubbed into even the sound skin may, 
especially when applied in a fatty vehicle, as e. g., in the well-known mer- 
cury ointment, find their way into the underlying lymphatics. The wander- 
ing leucocytes which are at times found among the epidermic cells may 
perhaps take part in this transport. 
The Mechanism of the Secretion of Sweat. 
§ 369. In dealing with the manner in which various circumstances 
affect the amount of sweat secreted we may, as we have already said, con- 
sider the sweat as a whole to be supplied by the sweat-glands alone. For 
though it seems evident that some amount of fluid must pass by simple 
transudation through the ordinary epidermis of the portions of skin inter- 
vening between the mouths of the glands, yet on the whole it is probable 
that the portion which so passes is a small fraction only of the total quantity 
secreted by the skin ; and direct experiment shows that even the simple 
evaporation of water is much greater from those parts of the skin in which 
the glands are abundant, than from those in which they are scanty. We 
have as yet no evidence that the sebaceous glands vary in activity ; their 
very peculiar form of secretion, if we may speak of it as a secretion, is not 
adapted to sudden changes, and at all events we have as yet no evidence 
that circumstances rapidly and largely modify the amount of sebum dis- 
charaed by healthy sebaceous glands. 
The secreting activity of the skin, like that of the other glands, is usually 
accompanied and aided by vascular dilatation. In one of the early experi- 
ments on division of the cervical sympathetic it was observed that, in the 
case of the horse, the vascular dilatation of the face on the side operated on 
was accompanied by increased perspiration. Indeed, the connection between 
the state of the cutaneous bloodvessels and the amount of perspiration is a 
matter of daily observation. When the vessels of the skin are constricted, 
the secretion of the skin is diminished ; when they are dilated, it becomes 
abundant. In this way, as we shall later on point out, the temperature of THE MECHANISM OF THE SECRETION OF SWEAT. 
435 
the body is largely regulated. When the surrounding atmosphere is warm, 
the cutaneous vessels are dilated, the amount of sweat -secreted is increased, 
and the consequently augmented evaporation tends to cool down the body. 
On the other hand, when the atmosphere is cold, the cutaneous vessels are con- 
stricted, perspiration is scanty, and less heat is lost to the body by evaporation. 
The analogy with the other secreting organs which we have already 
studied leads us, however, to infer that there are special nerves directly 
governing the activity of the sudoriparous glands, independent of varia- 
tions in tbe vascular supply. And not only is this view suggested by many 
facts, such as the profuse perspiration of the death-agony, of various crises 
of disease, and of certain mental emotions, and the cold sweats occurring in 
phthisis and other maladies, in all of which the skin is anaemic rather than 
hyperaemic, but we have direct experimental evidence of a nervous mechan- 
ism of perspiration as complete as the vasomotor mechanism. 
If in the cat1 the peripheral stump of the divided sciatic nerve be stimu- 
lated with the interrupted current, drops of sweat may readily be observed 
to gather on the hairless sole of the foot of that side. The sweating is not 
due to any increase of blood-supply, for it may be observed when the cuta- 
neous vessels are thrown into a state of constriction by the stimulus, or even 
when the aorta or crural artery is clamped previous to the stimulation, and 
indeed may be obtained by stimulating the sciatic nerve of a recently ampu- 
tated leg. Moreover, when atropine has been injected, the stimulation pro- 
duces no sweat, though vasomotor effects follow as usual. The analogy 
between the sweat-glands of the foot and such a gland as the submaxillary 
is in fact very close, and we are justified in speaking of the sciatic nerve as 
containing secretory fibres distributed to the sudoriparous glands of the foot. 
Similar results may be obtained with the nerves of the fore limb. And in 
ourselves a copious secretion of sweat may be induced by tetanizing through 
the skin the nerves of the limbs or the face. 
If a cat in which the sciatic nerve has been divided on one side be exposed 
to a high temperature in a heated chamber, the limb the nerve of which has 
been divided remains dry, while the feet or the other limbs sweat freely. 
This result shows that the sweating which is caused by exposure of the body 
to high temperatures is brought about by the agency of the central nervous 
system, and not by a local action on the sweat-glands ; for the foot of the 
limb whose nerve has been divided is equally exposed to the high tempera- 
ture. A high temperature it is true up to a certain limit increases the irrita- 
bility of the epithelium of the sweat-glands and predisposes it to secrete, just 
as it promotes action in the case of a muscle or nerve or other forms of living 
substance. Thus stimulation of the sciatic in the cat produces a much more 
abundant secretion in a limb exposed to a temperature of 35° or somewhat 
above, than in one which has been exposed to a distinctly lower temperature, 
and in a limb which has been placed in ice-cold water hardly any secretion 
at all can be gained ; but apparently mere rise of temperature without nerve- 
stimulation will not give rise to a secretory activity of the glands. The 
sweating caused by a dyspnoeic condition of blood, and such appears to be 
the sweat of the death-agony, is similarly brought about by the agency of 
the central nervous system. When an animal with the sciatic nerve divided 
on one side is made dyspnoeic, no sweat appears in the hind limb of that side 
though abundance is seen in the other feet. 
1 The cat sweats freely in the hairless soles of the feet but not on any part of the body 
covered with hairs. The dog also sweats in the same regions but not so freely as the cat; 
indeed, sweating is often absent, the ducts being stopped by growth of the corneous epi- 
dermis. Rabbits and other rodents appear not to sweat at all. The snout of the pig 
sweats freely; and the often profuse sweating of the horse, a singular event among hair- 
covered animals, is known to all. 436 
THE ELIMINATION OF WASTE PRODUCTS. 
Sweating may be brought about as a reflex act. Thus when the central 
stump of the divided sciatic is stimulated sweating is induced in the other 
limbs, and in ourselves the introduction of pungent substances into the mouth 
will frequently give rise to a copious perspiration over the side of the face. 
We are thus lead to speak of sweat centres, analogous to the vasomotor cen- 
tres, as existing in the central nervous system ; and as in the case of vaso- 
motor centres, a dispute has arisen as to whether there is a dominant sweat 
in the medulla oblongata or whether such centres are more generally dis- 
tributed over the whole of the spinal cord. 
It does not at present appear certain whether the sweating caused by heat 
is carried out by direct action of the heated blood on the sweat centres, or 
by the higher temperature stimulating the skin and so sending up afferent 
impulses which produce the effect in a reflex manner; but in the case of 
dyspnoea at least we may fairly suppose that the action of the venous blood 
is chiefly if not exclusively on the nerve centres. Some drugs, such as pilo- 
carpine, which cause sweating, appear to produce their effect chiefly by a 
local action on the glands, since the action continues after the division of 
the nerves (though pilocarpine apparently has as well some slight action on 
the nerve centres) and the antagonistic action of atropine is similarly local. 
Picrotoxine and strychnine appear to produce their sweating action chiefly 
if not exclusively by acting on the central nervous system, while nicotine 
seems to act both centrally and peripherally. 
§ 370. The sweat-fibres for the hind foot (in the cat) appear to leave the 
spinal cord by the roots of the twelfth thoracic to the third lumbar nerve 
inclusive, pass along the rami communicantes to the abdominal sympathetic, 
and thus reach the sciatic nerve. They thus follow very much the course 
of the vaso-constrictor fibres of the lower limb. The sweat-nerves for the 
fore-foot leave the spinal cord by the roots of the fourth to the ninth or 
tenth, chiefly in the sixth, seventh, and eighth dorsal nerve, inclusive, pass 
into the thoracic sympathetic, thence into the ganglion stellatum, and so 
join the brachial plexus by the fine branches passing from the ganglion to 
the spinal nerves. The course to the fore-foot is finally along the median 
and ulnar nerves respectively. In the horse the sweat-fibres for the side of 
the face and in the pig those for the snout appear to run in branches of the 
fifth nerve and not in the facial; in the latter animal at least some of these 
fibres reach the fifth nerve from the cervical sympathetic, but apparently 
not all. 
§ 371. The fact mentioned above that in the horse, after section of the 
cervical sympathetic nerve on one side of the neck, profuse sweating is apt 
to break out on that side of the face, has suggested the idea that this nerve 
conveys inhibitory impulses to the sweat-glands of the head and face, and 
that when it is divided the sweat fibres running in the fifth nerve, having 
nothing to counteract them, set up sweating. But it is probably sufficient 
in this case to suppose that the glands predisposed to activity by the higher 
temperature brought about by the section of the sympathetic dilating the 
bloodvessels, are more easily excited by any stimulus working upon them 
through the fifth nerve. And though the idea of a double nervous mechan- 
ism, augmenting and inhibitory, governing the activity of the sweat-glands, 
is a tempting one, there are at present no satisfactory reasons for adopting it. THE HISTORY OF GLYCOGEN. 
437 
CHAPTER IV. 
THE METABOLIC PROCESSES OF THE BODY. 
§ 372. We have followed the food through its changes in the alimen- 
tary canal, and have seen it enter into the blood, either directly or by the 
intermediate channel of the lacteals, in the form of peptone (or otherwise 
modified albumin), sugar, lactic acid, and fats, accompanied by various 
salts and water. We have further seen that the waste products which 
leave the body are urea, carbonic acid, salts and water. We have now to 
attempt to connect together the food and the waste products; to trace out 
as far as we are able the various steps by which the one is transformed into 
the other. There remains the further task to inquire into the manner in 
which the energy set free in this transformation is distributed and made 
use of. 
The master tissues of the body are the muscular and nervous tissues; 
all the other tissues may be regarded as the servants of these. And we 
may fairly presume that, besides the digestive and excretory tissues which 
we have already studied, many parts of the body are engaged either in 
further elaborating the comparatively raw food which enters the blood, in 
order that it may be assimilated with the least possible labor by the master 
tissues, or in so modifying the waste products which arise from the activity 
of the master tissues that they may be removed from the body as speedily 
as possible. There can be no doubt that manifold intermediate changes 
of this kind do take place in the body ; but our knowledge of the matter 
is at present very imperfect. In a few instances only can we localize these 
metabolic actions and speak of distinct metabolic tissues. In the majority 
of cases we can only trace out or infer chemical changes, without being 
able to say more than that they do take place somewhere; and in conse- 
quence, perhaps somewhat loosely, speak of them as taking place in the 
blood. 
How little we know concerning the metabolism of the master tissues 
themselves was shown when we were dealing with these tissues in an earlier 
part of this work; but success in the study of these can hardly be ex- 
pected until our knowledge is increased as regards the changes which the 
blood undergoes before it reaches and after it leaves the muscle or the 
nerve. The fact that a large part of the absorbed food is carried through 
the liver before it is thrown on the general circulation leads us to suppose 
that in this large organ important metabolic processes are carried on ; and 
observation with experiment confirms this view. Important as the secre- 
tion of bile may be, the other metabolic functions of the liver are of still 
greater importance. 
The History of Glycogen. 
§ 373. If the liver of a well-fed animal be removed immediately after 
death, rapidly divided into small pieces, thrown into boiling water, rubbed 
up and boiled, a decoction may be obtained which after careful neutraliza- 
tion and filtration will be tolerably free from proteid matter. Such a 
decoction is remarkably opalescent, milky in fact in appearance, much more 
so than a similar decoction from muscle or other tissue, and remains opal- 
escent even after repeated filtration. Treated with iodine, the solution 
turns a brownish-red, port-wine red color, not unlike that given by dextrin 438 
THE METABOLIC PROCESSES OF THE BODY. 
when iodine is added ; the color disappears on warming, but reappears on 
cooling provided that not too much proteid matter has been left in the 
solution. Treated with Fehling's fluid or other tests for sugar, the solution 
is found to contain a small and variable, but only a small, quantity of sugar. 
If the solution be exposed, preferably in a warm room, to the action of 
saliva or of some other amylolytic ferment, or be boiled with dilute acid, 
the opalescence disappears; and the now clear transparent solution gives 
no longer the port-wine reaction with iodine. Tested, moreover, with Fehl- 
ing's fluid or by other means it is now found to contain a considerable 
quantity of sugar. 
If alcohol be added to the opalescent solution until the mixture con- 
tains 60 per cent, of the alcohol (previous concentration by evaporation 
being desirable) a white amorphous precipitate is thrown down. This pre- 
cipitate, removed by filtration, boiled with an alcoholic solution of potash 
in which it is insoluble, but which dissolves and destroys any proteids 
which may be present, treated with ether to remove fatty impurities, and 
washed with alcohol, may be obtained in a pure condition. It then appears 
as a white amorphous powder, fairly soluble in water, but always giving 
rise to a milky opalescent solution unless an excess of alkali be present, in 
which case the opalescence may be slight or absent. 
The opalescent solution of this purified material gives a port-wine reac- 
tion with iodine, but no reaction whatever with Fehling's fluid or the other 
sugar tests. Treated with an amylolytic ferment or boiled with dilute acid, 
the solution, like the raw decoction of liver, loses its opalescence and its 
port-wine reaction with iodine, but now gives an abundant evidence of the 
presence of sugar, dextrose if boiling with acid has been employed, malt- 
ose chiefly if an amylolytic ferment has been used. If quantitative deter- 
mination be employed it will be found that the amount of sugar obtained is 
proportionate to the amount of the white powder acted upon ; in other 
words the substance forming an opalescent solution is converted into sugar, 
the solution of which is clear. Obviously the substance is a body allied to 
the starch ; and this is confirmed by the elementary composition, which is 
found to be CBH|UO5 or some multiple of this. 
Hence this body is called glycogen. And it is obvious from what has 
been stated above that the liver of a well-fed animal at the moment of 
death contains a considerable quantity of glycogen either in a free state or 
in such a condition that it is set free by subjecting the liver to the action 
of boiling water. We may add that it occurs in the liver in the hepatic 
cells, for the reaction of a port-wine color given under certain conditions 
by the hepatic cells is due to the presence of glycogen in them. 
§ 374. If the liver, instead of being treated immediately upon the 
death of the animal, is allowed to remain in the body of the dead animal 
for several hours, especially in a warm place, before a decoction is made 
of it, the decoction will be found to have little or no opalescence, to be 
quite clear, to give little or no port-wine reaction with iodine, but to 
contain a very considerable quantity of sugar. As we have said above, 
the decoction even of a liver taken immediately after death generally con- 
tains some little sugar, and the quantity of sugar in the liver appears, as a 
rule, to increase steadily after death, the amount of glycogen diminishing 
at the same time. The glycogen then present in the liver at the moment 
of death is gradually after death by some action or other converted into 
sugar. 
The action is that of some agency whose activity is destroyed by the tem- 
perature of boiling water; hence the directions repeatedly given above to 
throw the liver into boiling water. This naturally suggests the presence in THE HISTORY OF GLYCOGEN. 
439 
the liver of an amylolytic ferment. But not only have attempts to isolate 
from the liver an amylolytic ferment failed, in the hands of most observers 
at least, but the exact nature of the sugar which appears shows that the 
change is not effected by an ordinary amylolytic ferment. In the case of 
the amylolytic ferment of saliva, pancreatic juice, intestinal juice, and indeed 
of all other amylolytic animal fluids, the sugar into which starch or glyco- 
gen is converted is maltose. Now, the sugar which appears in the liver after 
death is dextrose, identical, as far at least as can at present be made out, 
with ordinary dextrose. We are led, therefore, to infer that the change of 
glycogen into sugar which appears to go on after death is carried out by 
some action of the liver, probably of the hepatic cell itself, which is done 
away with by a temperature of 100° C., but which is not the action of a 
ferment capable of being isolated. 
§ 375. We have used above the phrase "well-fed" animal because the 
amount of glycogen present in the liver of an animal at any one time is 
very variable, and especially dependent on the amount and nature of the 
food previously taken. When all food is withheld from an animal the glyco- 
gen in the liver diminishes, rapidly at first, but more slowly afterward. Even 
after some days' starvation a small quantity is frequently still found ; but in 
rabbits, at all events, the whole may eventually disappear. 
If an animal, after having been starved until its liver maybe assumed to 
be free, or almost free, from glycogen, be fed on a diet rich in carbohydrates 
or on one consisting exclusively of carbohydrates, the liver will in a short 
time be found to contain a very large quantity of glycogen. Obviously the 
presence of carbohydrates in food leads to an accumulation of glycogen in 
the liver; and this is true both of starch and of dextrin and of the various 
forms of sugar-cane, grape, and milk sugar. The effect may be quite a 
rapid one, for glycogen has been found in tbe liver in considerable quantity 
within a few hours after the introduction of sugar into the alimentary canal 
of a starving animal. 
If an animal similarly starved be fed on an exclusively meat diet a cer- 
tain amount of glycogen is found in the liver. This appears to be especially 
the case with dogs (probably with other carnivorous animals also), and in 
earlier works on the subject the constant presence of glycogen in the livers 
of dogs fed on meat was regarded as an important indication of the forma- 
tion within the body of non-nitrogenous from nitrogenous material. But in 
the first place, the quantity of glycogen thus stored up in the liver as the 
result of a meat diet is much less than that which follows upon a carbohy- 
drate diet; and in the second place, ordinary meat, especially horseflesh on 
which dogs in such experiments are usually fed, contains in itself (§ 62) a 
certain amount either of glycogen or some form of sugar. Moreover, when 
animals are fed not on meat, but on purified proteid, such as fibrin, casein, 
or albumin, the quantity of glycogen in the liver becomes still smaller, 
though, according to most observers, remaining greater than during starva- 
tion. We may infer, therefore, that part of the glycogen which appears in 
the liver after a meat diet is really due to carbohydrate materials present in 
the meat. Part, however, would appear to be the result of the actual pro- 
teid food ; and we have similar evidence that gelatin taken as food leads to 
the formation of some glycogen in the liver. But in this respect these nitro- 
genous substances fall far short of carbohydrate material. 
With regard to fats, all observers are agreed that these lead to no accu- 
mulation of glycogen in the liver; an animal fed on an exclusively fatty 
diet has no more glycogen in its liver than a starving animal. 
Hence of the three great classes of food-stuffs the carbohydrates stand 
out prominently as the substances which taken as food lead to an accumu- 440 
THE METABOLIC PROCESSES OF THE BODY. 
lation of glycogen in the liver. We may remark that the greatest accumu- 
lation of glycogen is effected not by a pure carbohydrate diet, but by a 
mixed diet rich in carbohydrates. A quantity of carbohydrate mixed with 
a certain proportion of proteid gives rise to a larger amount of glycogen in 
the liver than the same quantity of carbohydrate given by itself; and it is 
possible that the presence of an appropriate quantity of fat still further 
assists the accumulation. But this result probably depends, in part at 
least, on the fact that, though differences may be met with in different ani- 
mals, a mixture of the several classes of food-stuffs is more readily digested, 
resulting in more nutritive material being thrown upon the blood, than is a 
meal consisting exclusively of one kind of food-stuff alone. 
As far as we know at present the glycogen which thus appears in the 
liver as the result of feeding either with any of the various forms of car- 
bohydrates or with proteids, or with other substances, is of the same kind 
and presents the same characters; at least we have no evidence to the con- 
trary. 
The storing-up of glycogen in the liver is also influenced by other circum- 
stances than the taking of food. For instance, in the frog an increase of 
glycogen takes place during the winter months. In the summer months the 
liver of a frog will be found to contain very little glycogen (Fig. 114, C), 
Fig. 113. 
Section of Liver of Frog. (Langley.) The figure shows the tubular structure of the liver. At 
a, a tubule is seen in transverse, at b in longitudinal section ; I, lumen of tubule. The liver was 
that of a winter frog, and the cells show an inner zone of proteid granules ; the outer zone was 
chiefly occupied by glycogen. 
unless the animal has been unusually well fed ; whereas a liver examined in 
midwinter (Figs. 113, 114, A) will be found to contain a considerable quan- 
tity, even though no food has been taken for months. In such a case the 
material for the formation of the glycogen in the liver must have been fur- 
nished by some part of the body of the frog, and could not, as may be the case 
when a meal leads immediately to an increase of glycogen, be supplied THE HISTORY OF GLYCOGEN. 
441 
directly from the food. It seems as if in the summer the frog lives up to its 
capital of hepatic glycogen, spending it as fast almost as it is made, but that 
during the winter a quantity is funded to provide for the demands of the 
late winter and early spring. 
This winter storage of hepatic glycogen in the frog seems closely depen- 
dent on temperature. If a winter frog, whose liver is presumably more or 
less loaded with glycogen, be exposed for some time to a temperature of 20° 
or a little higher, the liver will afterward be found to contain little or no 
glycogen (Fig. 114, B) ; and conversely, if a summer frog be exposed to 
untimely cold, glycogen, though not in any great quantity, begins to be 
stored up in the liver. 
§ 376. Before we attempt to discuss further how food and other circum- 
Fig. 114. 
Three Phases of the Hepatic Cells of the Frog. (Langley.) A, cells rich in glycogen. Taken froma 
frog during winter. The cells are large and proteid granules are massed around the lumen, the ho- 
mogeneous outer zones of the cells are largely composed of glycogen, which is present in con- 
siderable abundance. The outer zones contain numerous fat globules, shown as dark spots; 
but, as stated in the text, these fat globules vary much. B, cells poor in glycogen. Taken from a 
winter frog ■which had been kept at 22° C. for ten days. The cells contain very little glycogen, and 
the proteid granules are dispersed throughout the cell. In a summer frog well fed on proteids the 
cells would present a very similar appearance. C, starved cells. Taken from a summer frog after 
a long fast. The cells are small and almost free from glycogen. The proteid granules are dis- 
persed throughout the cell. All the specimens were hardened in 1 per cent, osmic acid, and are 
drawn to the same, or nearly to the same scale. 
stances thus affect the glycogen in the liver, it will be desirable to take up 
the matter which we left on one side, viz., the consideration of the histo- 
logical changes occurring in the hepatic cells under various conditions. It 
will be convenient to begin with the cells of the more distinctly tubular 
gland of the frog. 
In a frog which has not been subjected to any special treatment the cell 
substance of the hepatic cell (cf. Fig. 114, A) will generally be found to 
contain lodged in itself three kinds of material, the presence of which, if 
not directly recognizable in the fresh cell, may be demonstrated by the 
use of various reagents. In the first place, oil globules of variable size 442 
THE METABOLIC PROCESSES OF THE BODY. 
and in variable amount are scattered throughout the cell; sometimes, as 
we have already said, these are extremely abundant; but there is otherwise 
nothing very special about these fat globules in the hepatic cell to demand 
any discussion concerning them apart from the general discussion on the 
formation of fat, into which we shall enter later on. 
In the second place, a number of small discrete granules may be seen 
lodged in the cell substance. These appear to be of a proteid nature and 
are generally most abundant on the inner side of the cell near the lumen of 
the bile passage. The presence of these granules is closely dependent on the 
activity of the digestive processes. They diminish when digestion is going 
on and accumulate again afterward. Putting aside certain details, we may 
say that these granules behave very much like the granules in an albumin- 
ous salivary cell, a pancreatic cell, or a chief gastric cell; and we may prob- 
ably safely conclude that they, like the granules in these cells, are in some 
way concerned in the formation of the secretion ; that is, in their case, bile. 
In the third place, the cell contains, more especially in its outer parts 
nearer the bloodvessel, away from the lumen of the bile passage, a variable 
quantity of material which differs from the ordinary cell substance in being 
hyaline and refractive, and hence glassy-looking, and in staining port-wine 
red with iodine, instead of brownish-yellow, as does ordinary cell substance. 
This material is, though with some little difficulty, soluble in water, and by 
this means may be dissolved out from the cell. When this is done the places 
which it occupied appear as vacuoles or gaps of various sizes limited by bars 
of the cell substance, which thus take on the form of a network, the meshes 
of which are wider and more conspicuous in the outer part of the cell, in 
which the hyaline material was previously most abundant. In the inner 
part of the cell where the hyaline material was scanty the cell substance is 
more dense, and even in the outer part a shell of more dense, less reticulate 
cell substance affords a definite outline to the cell. There can be no doubt 
that this hyaline material is either actual glycogen, such as may be ex- 
tracted from the liver, or, as seems more probable from its definite solu- 
bility, glycogen in some more or less loose combination with some other 
body, a combination, however, of such a kind that the iodine reaction makes 
itself felt. 
§ 377. The above may be taken as a general description of a cell in an 
ordinary condition. The question now comes before us, What changes are 
brought about by various foods or by the absence of food ? 
If a frog be largely fed on a diet containing large quantities of carbo- 
hydrates, the liver will be found rich in glycogen, and the cells will present 
the following characters: The cell is relatively large (c/. Fig. 114, A), and, 
as it were, swollen ; the cell substance is largely occupied by the hyaline 
material just spoken of, especially in its outer parts, so that in sections pre- 
pared and mounted in the ordinary way in which the glycogen has been 
dissolved out, the greater part of the cell consists of a loose open network of 
bars of stained cell substance with wide meshes; a certain quantity of more 
solid, generally granular-looking cell substance occupies the part of the cell 
nearest the lumen, and a thin shell of cell substance forms an envelope for 
the rest of the cell. The nucleus is large and distinct, but though changes in 
the nucleus accompanying changes in the cell substance have been described, 
they are not sufficiently important to detain us now. When such a cell is seeu 
in a perfectly fresh state, the hyaline refractive material (which, we need 
hardly say, gives a marked reaction with iodine) often hides the nucleus and 
the greater part of the cell substance proper. 
If, on the other hand, the frog be fed on a proteid diet free from carbo- 
hydrates-for instance, on fibrin-the liver contains little or no glycogen, THE HISTORY OF GLYCOGEN. 
443 
and the hepatic cells are not only much smaller, but present an appearance 
very different from the above (c/. Fig. 114, B). Little or no hyaline mate- 
rial is visible, the cells give little or no port-wine reaction with iodine, but 
only the usual brown-yellow proteid reaction, and in specimens prepared and 
mounted in the ordinary way the cell substance appears densely granular 
throughout. 
Lastly, if the frog be starved, and if to the effects of starvation there be 
added those of exposure to a high temperature (25° C.), by which, as we have 
seen, the hepatic cells are markedly affected, the liver is found to be free 
from glycogen and the hepatic cells to be extremely small (c/. Fig. 114, C), 
only half the size or even less of those of the well-fed frog, but otherwise 
much like the cells in a frog fed on proteid material. 
§ 378. In the mammal changes in the hepatic cells similar to those just 
described as occurring in the frog have also been observed. When the 
animal is fed on a diet rich in carbohydrates, and when, therefore, as we 
have seen, the liver abounds in glycogen, the hepatic cells (Fig. 115) are 
larger (so large that they have by some authors been described as com- 
pressing the lobular capillaries) and loaded with the same refractive hya- 
line material staining port-wine red with iodine. When this material is 
dissolved out a coarse open network of cell substance is displayed. The 
Fig. 115. 
Fiq. 116. 
Section of Mammalian Liver rich in Gly- 
cogen. (Langley.) Osmic acid specimen, gly- 
cogen not dissolved out. 
Section of Mammalian Liver containing 
little or no Glycogen. (Langley.) Osmic 
acid specimen. The granules are not well 
preserved in some of the cells. 
most marked point of difference between the mammalian and frog's hepatic 
cell under these conditions is that in the former, the hyaline glycogenic sub- 
stance is gathered at first centrally around the nucleus (not more on the outer 
side, as is the case in the frog) and spreads from the centre toward the pe- 
riphery, always leaving on the extreme outside a somewhat thick shell of 
cell substance, which in hardened and prepared specimens may strikingly 
simulate a thickened cell-wall. We may add that in an animal thus fed the 
whole liver is very large, and, as it were, swollen ; it is also soft and tears easily. 
In an animal fed on proteids alone, for instance on fibrin, the liver fre- 
quently contains some glycogen and the hepatic cells contain a small quantity 
of hyaline glycogenic material. As in the corresponding case in the frog, 
the cells are comparatively small, and the cell substance appears finely and 
uniformly granular. 
In a starved mammal the liver is small, dense to the touch, and tough; 
it contains a trace only of glycogen or none at all; the cells (Fig. 116) are 
small, as it were shrunken, and the cell substance, which gives no port-wine 
reaction, or a mere trace only, with iodine, is still more finely granular. 
§ 379. The microscopic appearances just described show, and indeed 444 
THE METABOLIC PROCESSES OF THE BODY. 
general considerations lead us to the same conclusion, that the processes 
taking place in an hepatic cell are very complex. In the first place, the con- 
stituents of bile are being formed and discharged into the bile passages after 
the fashion of ordinary secreting glands. In the second place, a formation 
of glycogen is also taking place, and we shall have presently to consider 
briefly the relations of the one process to the other. In the third place, as 
is especially indicated by the somewhat peculiar effects on the hepatic cell of 
food exclusively proteid in nature, other processes, similar perhaps to the 
formation of glycogen, but not resulting in the storage of any carbohydrate 
material, and dealing possibly with proteid substances, also take place. 
Hence the exact interpretation of all the changes which may be observed 
becomes exceedingly difficult. 
Leaving the processes of the first and third kind wholly on one side for 
the present, and confining our attention entirely to the glycogen, it is obvi- 
ous that the hepatic cell manufactures the glycogen in some way or other 
and lodges it in its own substance for the time very much in the same way 
that a secreting cell manufactures and lodges in itself for a time material 
for the secretion which it is about to pour forth. There is this difference, 
that in the one case the material of the secretion, after undergoing, as we 
have seen, more or less change, is cast out into the lumen of the alveolus, 
whereas in the other case the glycogen, which must undergo change, since 
it may be made to disappear rapidly from the hepatic cell, is not when 
changed cast out into the bile passages ; it must therefore be sent back again 
to the blood. 
§ 380. We say " manufacture the glycogen in some way or other," and 
we have now to inquire what we know concerning the nature and the several 
steps of this manufacture. 
We have already seen that the presence of glycogen in the liver is 
especially favored by a carbohydrate diet; and in our studies on digestion 
we have seen reason to think that a very large part at all events of the 
carbohydrate material of a meal is absorbed as sugar by the capillaries of 
the intestine and carried as sugar to the liver in the portal blood. Hence, 
it seems only reasonable to conclude that the glycogen which makes its 
appearance in the liver after an amylaceous meal arises from a direct con- 
version of the sugar carried to the liver by the portal vein, the sugar 
becoming through some action of the hepatic cell substance dehydrated into 
glycogen, or animal starch, as it has been called, the process being a reverse 
of that by which in the alimentary canal starch is hydrated into sugar 
through the action of the salivary and pancreatic ferments. Vegetable cells 
can undoubtedly convert both starch into sugar and sugar into starch ; and 
there are no d priori arguments or positive facts which would lead us to 
suppose that the activity of animal living substance cannot accomplish the 
latter as well as the former of these changes. We are quite ignorant, it is 
true, of the exact way in which either the hydration or the dehydration is 
effected by living substances; but we are equally ignorant of the exact way 
in which an amylolytic ferment effects the hydration of starch into sugar, 
which it carries out with so much apparent ease. It is not a great assump- 
tion to suppose that the continually changing living substance, which in its 
changes is continually giving out energy, has the power of acting on mole- 
cules of starch or of sugar in contact with or even only near to itself, and 
so of hydrating starch into sugar or of dehydrating sugar into starch. 
The latter process may be a more difficult one than the former, but not one 
beyond the power of the living substance. We may fairly suppose that a 
quantity of sugar in solution present in a vacuole, for instance, of the hepatic 
cell substance can be, by some action of the cell substance, converted into 445 
THE HISTORY OF GLYCOGEN. 
glycogen in a solid form, filling up the vacuole. Again, as we have inci- 
dentally mentioned, sugar injected into the jugular vein readily gives rise to 
sugar in the urine; but a very considerable quantity can be slowly injected 
into the portal vein without any appearing in the urine. This suggests the 
idea that the liver, so to speak, catches the sugar as it is passing through 
the hepatic capillaries and at once dehydrates it into glycogen. 
Similar consideration may also be applied to the case mentioned above of 
the appearance of glycogen in the hepatic cells of winter (fasting) frogs. 
We have reason to think that sugar makes its appearance as a product of 
the metabolism of various tissues. The sugar thus arising finding its way 
into blood may be made use of at once elsewhere, converted speedily, for 
instance, into carbonic acid and so got rid of. But we can readily imagine 
that under certain circumstances, as for instance when the activities of the 
animal were lessened by a low temperature, it was not so made use of and 
remained in the blood. If so, it would in the course of the circulation be 
carried to the liver, and might be at once taken up by the hepatic cells and 
converted into glycogen ; and these might be so active that the blood was 
never at any time allowed to remain loaded with sugar to such an extent as 
to permit a loss through the urine. 
§ 381. Upon such a view, the carbohydrate taken as food would be 
converted into glycogen by the agency of the hepatic cell, without at any 
time becoming an integral part of the living substance of the cell. Such a 
view may be the true one ; but it is open for us to look at the matter in 
another light. We may push still further the analogy between the glycogen 
of the hepatic cell and the material with which a secreting cell is loaded. In 
dealing with secretions we saw reasons for regarding such a body as mucin 
to be a product of the metabolism of the cell substance of the mucous cell; 
and we may similarly regard glycogen, or sugar readily convertible into 
glycogen, or at least some or other carbohydrate material, as a normal pro- 
duct of the metabolism of the hepatic cell. We may thus conceive of the 
hepatic cells as being continually engaged in giving rise to carbohydrate 
material in the form either of sugar or of some other body ; and we may 
suppose that under certain circumstances, as in the absence of adequate food, 
the carbohydrate material thus formed is at once discharged into the blood 
of the hepatic vein for the general use of the body, but that under other 
circumstances, as when an amylaceous meal has been taken, the immediate 
wants of the economy being covered by the carbohydrates of the meal, the 
carbohydrate products of the hepatic metabolism are stored up as glycogen. 
Under such a view the sugar of the meal is used up somewhere in the body, 
and the glycogen to the storage of which in the liver it gives rise comes 
direct from the hepatic substance. And a similar explanation may be given 
of the storing-up of the glycogen in the liver under such circumstances as 
those of the winter frog previously mentioned. 
We do not possess at present experimental or other evidence of so clear 
a kind as to enable us to decide dogmatically between these two views. We 
have seen that proteid food, though in this respect falling far below carbo- 
hydrated food, does or may give rise to a certain amount of glycogen in the 
liver; and gelatin seems to have the same effect. Further, in certain cases 
of the disease diabetes, of which we shall have to speak presently, and which 
is characterized by the presence of a large amount of sugar in the blood, 
sugar continues to be formed in large quantity, even when the diet is re- 
stricted to proteid and fatty matters, all carbohydrates being excluded. 
Now in diabetes we have reason to believe that the large quantity of sugar 
in the blood is accompanied by a large deposition of glycogen in the liver, 
and indeed in other tissues ; for in the few cases which have been examined 446 
THE METABOLIC PROCESSES OF THE BODY. 
sufficiently soon after death, and in which owing to the suddenness of death 
there was- no opportunity for stored-up glycogen to disappear, a very large 
quantity of glycogen has been found in the liver or some other organs. 
Hence the phenomena of diabetes may be taken as showing, in a much more 
striking manner than do any experiments, that proteid material taken as 
food may give rise to hepatic glycogen. 
§ 382. We may now turn to another question, the answer of which is in 
a measure dependent on the one which we have just discussed. What is the 
use and purpose of this hepatic glycogen ? What ultimately becomes of the 
glycogen thus for a while stored up in the liver? 
One view which has been put forward is as follows: We have evidence, 
as we shall presently learn, that a great deal of the fat of the body is not 
taken as such in the food, but is constructed anew in the body out of other 
substances. Both carbohydrates and proteids, taken in excess or under 
certain circumstances, lead to an accumulation of fat; and we have reason 
to believe that carbohydrates, on the one hand, and the carbon-holding por- 
tions of various proteids, on the other, may by some process or other be con- 
verted into fat. And it has been suggested that the glycogen in the liver is 
a phase of a constructive fatty metabolism, that it is material on its way to 
become fat. 
Another view, one which has already been suggested while we were dealing 
with the manner of formation of glycogen, makes use of the formation of fat 
for the purposes of analogy only. Seeing that adipose tissue serves as a 
storehouse of fat which is not wanted by the body at the moment, but may 
be wanted presently, the question readily presents itself, May not the hepatic 
glycogen have an analogous function? May we not regard the presence of 
glycogen in the liver as in large measure due to the fact that it is deposited 
there simply as a store of carbohydrate material, being accumulated when- 
ever amylaceous material is abundant in the alimentary canal, and being 
converted into sugar ami so drawn upon by the body at large to meet the 
general demands for carbohydrate material during the intervals when food is 
not being taken ? And we can accept this view without being able to say 
definitely what becomes of the sugar thus thrown into the hepatic blood. 
It was formally believed that this sugar underwent an immediate and direct 
oxidation as it was circulating in the blood, but we have already dwelt 
(§ 360) on the objections to such a view. It is sufficient for us at the present 
to admit that the sugar is made use of in some way or other. 
Now, many considerations lead us to believe that a certain average com- 
position is necessary for that great internal medium the blood, in order that 
the several tissues may thrive upon it to the best advantage, one element of 
that composition being a certain percentage of sugar. It would appear that 
some at least if not all of the tissues are continually drawing upon the blood 
for sugar, and that hence a certain supply must be kept up to meet this 
demand. On the other hand an excess of sugar in the blood itself would 
be injurious to the tissues. And as a matter of fact we find that the quan- 
tity of sugar in the blood is small but constant; it remains about the same 
when food is being taken as in the intervals between meals. If sugar be 
injected into the jugular vein in too large quantities or to rapidly, a certain 
quantity appears in the urine, indicating an effort of the system to throw off 
the excess and so bring back the blood to its average condition. The Main- 
tenance of such a constant percentage of sugar would obviously be pro- 
vided for, or at least largely assisted by the liver acting as a structure where 
the sugar might at once and without much labor be packed away in the 
form of the less soluble glycogen, at those times when, as during an amyla- 
ceous meal, sugar is rapidly passing into the blood, and there is danger of THE HISTORY OF GLYCOGEN. 
447 
the blood becoming loaded with far more sugar than is needed for the time 
being; and it may be incidently noted that a larger quantity of sugar may 
be injected into the portal than into the jugular vein without any reappear- 
ing in the urine, apparently because a large portion of it is in such a case 
retained in the liver as glycogen. At those times, on the other hand, when 
we may suppose that sugar ceases to pass into the blood from the alimentary 
canal, the average percentage in the blood is maintained by the glycogen 
previously stored up becoming reconverted into sugar, and being slowly dis- 
charged into the hepatic blood. 
Moreover, this view, that the glycogen of the liver is a reserve fund of 
carbohydrate material, is strongly supported by the analogy of the migra- 
tion of starch in the vegetable kingdom. We know that the starch of the 
leaves of a plant, whether itself having previously passed through a glucose 
stage or not, is normally converted into sugar, and carried down to the roots 
or other parts, where it frequently becomes once more changed back again 
into starch. 
§ 383. Glycogen is found in other parts of the body than the liver, and a 
study of the facts relating to the presence of glycogen in other tissues will 
help us to a true conception of the purposes of the hepatic glycogen. Next 
to the liver, the skeletal muscles are perhaps the most conspicuous glycogen- 
holders. So frequently is glycogen found in muscle that it may be regarded 
as an ordinary though not an invariable constituent of that tissue; indeed 
it may almost be considered as a constituent of all contractile tissues. The 
quantity varies very largely both in the different muscles of the same animal 
and corresponding muscles of different animals. It disappears, according 
to some observers, readily upon starvation, even before the hepatic glycogen 
is exhausted ; but all observers are not agreed on this point, and in some 
muscles, at least, it appears to be retained for a very long time. It is said 
to be increased in quantity when the nerve of the muscle is divided, and the 
muscle thus brought into a state of quiescence. On the other hand it 
diminishes or even disappears, being apparently converted into dextrose, 
when the muscle enters into rigor mortis. Some observers have found that 
it diminishes during tetanus, and maintain that it, after conversion into dex- 
trose, is used up in the act of contraction, forming through its oxidation the 
immediate supply of the energy set free in the contraction. But even grant- 
ing that the glycogen in a muscle may be diminished during prolonged 
labor, it cannot be admitted that the oxidation or other chemical change of 
glycogen is a necessary part of the ordinary metabolism of a muscular con- 
traction, since many muscles wholly free from glycogen are perfectly well 
able to carry on long-continued contractions. 
Another view of the use of glycogen in muscle is suggested by the fact 
that undeveloped embryonic muscles are peculiarly rich in glycogen. In a 
young embryo, at the time when the muscular substance, though undergoing 
striation, is still largely " protoplasmic" in nature, the quantity of glycogen 
present is enormous ; it frequently amounts to 40 per cent, of the dry mate- 
rial. At this period the hepatic cells are immature and very little glycogen 
is present in them. Later on, as the muscles become more wholly striated, 
the glycogen largely disappears from the muscle, and very soon afterward 
begins to be stored up in the liver. 
The meaning of this can hardly be mistaken. The glycogen in the imma- 
ture muscle is a store of carbohydrate material, laid down on the spot, and 
ready at once to be used in what we may probably call the fierce metabolic 
struggle by which the simple protoplasmic cell substance of the rudiment 
of the muscular fibre is transformed into the highly differentiated striated 
contractile substance. And we shall probably not err in considering the 448 
THE METABOLIC PROCESSES OF THE BODY. 
glycogen of the mature muscle to hold a similar position ; it is carbohydrate 
material stored up on the spot, a local branch, so to speak, of the great carbo- 
hydrate bank. It is destined to become part of the contractile substance, 
and as such will contribute to the energy set free in a muscular contrac- 
tion ; but its energy is only available in this way after it has undergone the 
necessary metabolism and become part of muscular substance; it cannot be 
fired off in a contraction while it lies as raw glycogen, or even as dextrose, 
in the interstices of the muscular fibre. 
§ 384. Glycogen may also be found in considerable quantity in the pla- 
centa. Here, as we shall see in the latter part of this work, it is laid down 
in epithelial cells which lie on the boundary between the maternal and the 
foetal tissues. And here, too, there can be little doubt that it is a store of 
carbohydrate material for the nourishment of the foetus. 
It has also been found in leucocytes, in cartilage corpuscles, especially in 
those large rapidly growing and rapidly multiplying cartilage corpuscles 
which lie in the outer zone of endochondral ossification, and in other situa- 
tions. In cases of diabetes, where the body is overloaded with carbohy- 
drate material, it has been found in considerable quantity in the testis, in 
the brain, and elsewhere. Its occurrence in these situations, and under these 
circumstances, may be regarded as additional evidence of the truth of the 
view which we have expounded above, that the main purpose of the deposi- 
tion of glycogen is to afford a store, either general or local, of carbohydrate 
material, which can be packed away without much trouble so long as it re- 
mains glycogen, but which can be drawn upon as a source of soluble circu- 
lating sugar whenever the needs of this or that tissue demand it. It thus 
forms a very complete analogue to the vegetable starch, and fitly earns the 
name of animal starch. 
We have some reasons for thinking that there are several varieties of 
glycogen, and that the glycogen which exists in muscle is not quite identi- 
cal with that which occurs in the liver. Indeed, there seem to be interme- 
diate stages between glycogen and starch, or dextrin. The physiological 
value of these differences has not yet, however, been clearly determined, 
and, with this caution, we may continue to speak of glycogen as a single 
substance. 
Diabdes. 
§ 385. Natural diabetes is a disease characterized by the appearance 
of a large quantity of sugar in the urine, due, as we have already said, 
to the presence of an abnormal quantity of sugar in the blood. Into 
the pathology of the various forms of this disease it is impossible to enter 
here; but a temporary diabetes, the appearance for a while of a large 
quantity of sugar in the urine, may be artificially produced in animals in 
several ways. 
If the medulla oblongata of a well-fed rabbit be punctured in the region 
which we have previously described (§ 162) as that of the vasomotor centre 
(the area marked out as the " diabetic area," agreeing very closely with that 
defined as the vasomotor area), though the animal need not necessarily be 
in any other way obviously affected by the operation, its urine will be found, 
in an hour or two, or even less, to be increased in amount and to contain a 
considerable quantity of sugar. A little later the quantity of sugar will 
have reached a maximum, after which it declines, and in a day or two, or 
even less, the urine will again be perfectly normal. The better fed the 
animal, or, more exactly, the richer in glycogen the liver, at the time of 
the operation, the greater the amount of sugar. If the animal be pre- THE HISTORY OF GLYCOGEN. 
449 
viously starved so that the liver contains little or no glycogen, the urine 
will, after the operation, contain little or no sugar. It is clear that the 
urinary sugar of this form of artificial diabetes comes from the glycogen 
of the liver. The puncture of the medulla causes such a change in the 
liver that the previously stored-up glycogen disappears, and the blood 
becomes loaded with sugar, much, if not all, of which passes away by 
the urine. In the absence of any proof to the contrary, we may assume 
that in this form of artificial diabetes, the glycogen previously present 
in the liver becomes converted into sugar, just as we know that it does 
become so converted by post-mortem changes. The glycogenic function 
of the liver is, therefore, subject to the influences of the nervous system, and, 
in particular, to the influence of a region of the cerebro-spinal centre, which 
we already know as the vasomotor centre, or at least of a part of that region. 
Before we attempt to discuss this nervous influence we must say a few 
words on the nerves of the liver. 
§ 386. The liver is supplied with nerves from the hepatic plexus, which 
passes into the liver at the porta and, running in the portal canal with the 
hepatic artery and portal vein, is distributed to various parts of the organ. 
This plexus, which is the only nerve supplying the liver, consists partly of 
medullated and partly of non-medullated fibres, and is an extension of the 
great solar plexus, already often mentioned. Into that plexus, as we have 
already seen, the right (posterior) vagus sends the greater part of its fibres, 
and in that plexus both the abdominal splanchnic nerves, major and minor, 
end, on both sides of the body. The left (anterior) vagus forms slight con- 
nections only with the solar plexus, but sends off a very distinct branch 
directly to the hepatic plexus. The liver, therefore, has nervous connec- 
tion with the central nervous system by both vagus nerves and by the 
(abdominal) splanchnic nerves. Besides this, other nerve-fibres find their 
way through the splanchnic sympathetic chain, or possibly otherwise, to 
the solar plexus from the spinal cord without taking part in either of the 
splanchnic nerves; and these may, perhaps, join the hepatic plexus. 
Concerning the destination of the fibres of the hepatic plexus within 
the liver we know little or nothing definitely. Some, undoubtedly, supply 
the hepatic artery and its branches ; but we cannot at present say what 
proportion of the whole number of fibres end in this way. Some, again, 
are destined for the bile-ducts, and before the plexus passes into the liver 
it sends fibres to the gall-bladder; these probably end in the muscular 
coats of these organs. Whether any of the nerve-fibres end in the re- 
markably muscular coats of the portal vein, or whether, as theoretical 
reasons would, perhaps, lead us to suppose, some are connected with the 
hepatic cells, we do not for certain know, though some observers have 
claimed to have traced nerve-fibres directly into the hepatic cells. 
§ 387. With regard to the exact nature of the influence started by the 
puncture of the medulla, and the path by which that influence reaches the 
liver, our information is at present very imperfect. One thing seems clear, 
viz., that the influence in question is not carried down by tbe main vagus 
trunks; for not only has the section of both these nerves in the neck no 
marked eflect in the way of producing diabetes, but the " diabetic punc- 
ture" of the medulla oblongata is as efficient after division of both vagus 
nerves as before. Seeing how close to, or almost indentical with, the vaso- 
motor centre is the diabetic centre, if we may use the phrase, it seems nat- 
ural to suppose that the undue conversion of glycogen into sugar which 
follows the puncture is the result of some vasomotor disturbance in the 
liver, for instance, dilatation of the hepatic artery. But we have no clear 
proof that this is the true explanation. 450 
THE METABOLIC PROCESSES OF THE BODY. 
§ 388. A temporary diabetes may be brought about by the administra- 
tion of the substance phloridzin. This, however, is a glucoside, and part 
of the sugar which appears in the urine, after a dose of it, may come direct 
from the drug itself; but the quantity of sugar discharged is too great to 
be accounted for in this way, and similar diabetic effects are produced by 
the administration of phloretin, a derivate of phloridzin, not a glucoside, 
and not giving rise to sugar by its own decomposition. The sugar which 
appears in the urine after a dose of this substance seems to come in part 
at least from the hepatic store of glycogen when that is present; but the 
drug will give rise to sugar in the urine of starving animals, from whose 
livers (and other tissues) glycogen is presumably absent. 
Artificial diabetes is also a prominent symptom of urari poisoning. This 
is not due to the artificial respiration, which is had recourse to in order to 
keep the urarized animals alive; because, though disturbance of the respir- 
atory functions sufficient to interfere with the hepatic circulation may pro- 
duce sugar in the urine, artificial respiration may with care be carried on 
without any sugar making its appearance. Moreover, urari causes diabetes 
in frogs, although in these animals respiration can be satisfactorily carried 
on without any pulmonary respiratory movements. The exact way in 
which this form of diabetes is brought about has not yet been clearly made 
out. 
A very similar diabetes is seen in carbonic-oxide poisoning; and is one 
of the results of a sufficient dose of morphia, of amyl nitrite and of some 
other drugs. 
There can be no doubt that in diabetes, arising from whatever cause, 
the sugar appears in the urine because the blood contains more sugar than 
usual. The system can only dispose (either by oxidation, or as seems more 
probable in other ways) of a certain quantity of sugar in a certain time. 
Sugar injected into the jugular vein reappears in the urine whenever the 
injection becomes so rapid that the percentage of sugar in the blood reaches 
a certain (low) limit. Sugar in the urine means an excess of sugar in the 
blood. How in natural diabetes that excess arises has not at present been 
clearly made out. It may be that some forms of diabetes resemble the 
artificial diabetes just described as resulting from puncture of the medulla, 
and arise from a too rapid conversion of the hepatic glycogen, or from 
carbohydrate material failing to be stored up as glycogen, or from an 
excessive manufacture of carbohydrate material by the hepatic cells. All 
forms of diabetes, however, cannot be satisfactorily explained in this way ; 
and it has been suggested, though adequate proof has not yet been supplied, 
that the sugar of diabetes is of a peculiar nature and accumulates in the 
blood because it is unable to undergo those changes, whatever they be, 
which befall the normal sugar of the blood. We cannot here discuss the 
subject in detail; but there is much to be said in favor of the view that the 
sources of the excess of sugar in the blood may be various, and hence that 
several distinct varieties of diabetes may exist. In severe cases of diabetes 
the aberrant nature of the metabolism which is going on in some or other 
of the tissues of the body is shown by the appearance of abnormal sub- 
stances in the urine. Thus acetone is frequently present, and the fatal issue 
of certain cases has been attributed to poisoning by that substance; oxybu- 
tyric acid and other various organic, chiefly volatile, acids are also some- 
times present. But in respect to these and other abnormal bodies we are 
not at present clear whether they are, like the sugar itself, the products of 
an abnormal metabolism which is the root of the disease, or whether they 
are secondary products, that is to say, products of the general disordered 
metabolism induced by the constant presence in the blood of an excess of SPLEEN. 
451 
sugar. We have already, in discussing the formation of glycogen, called 
attention to the fact that in severe cases of diabetes the sugar must have a 
non-amylaceous source; and the fact that the urea is increased (and that 
too in some cases in ratio with the sugar) in diabetes, suggests that the 
sugar may arise from proteids which have been split up into a nitrogenous 
(urea) and a non-nitrogenous moiety, and so points out the way in which 
proteids may be a source of glycogen. 
As a sort of converse to diabetes we may mention that the administra- 
tion of arsenic in sufficient doses or for an adequate time prevents an accu- 
mulation of glycogen in the liver and apparently in the body generally, 
whatever be the diet used. The presence of the metal in the hepatic cell 
seems to prevent the cell substance from manufacturing glycogen either 
from carbohydrate material brought to it, or out of its own substance. As 
another kind of converse we may also state that the administration of 
glycerin, especially through the alimentary canal, diminishes the effect of 
the diabetic puncture, or of morphia or of other poisoning, in hurrying on 
the hepatic store of glycogen into sugar, and thus diminishes the sugar in 
the urine; the presence of the glycerin in the hepatic cell appears to be in 
some way a hindrance to the conversion of the glycogen into sugar. Now 
glycerin injected into the alimentary canal of a normal animal leads to an 
increase of glycogen in the liver; and the view very naturally suggests 
itself that this increase arising from the glycerin is to be explained by the 
glycerin inhibiting in some way a normal conversion of the glycogen store 
into sugar which is continually going on, and thus increasing for the time 
that store. 
Spleen. 
§ 389. The movements of the spleen. After a meal the spleen increases 
in size, reaching its maximum about five hours after the taking of food ; it 
remains swollen for some time, and then returns to its normal bulk. In 
certain diseases, such as the pyrexia attendant on certain fevers or inflam- 
mations, and more especially in ague, a somewhat similar temporary enlarge- 
ment takes place. In prolonged ague a permanent hypertrophy of the 
spleen, the so-called ague-cake, occurs. 
The turgescence of the spleen seems to be due to a relaxation both of the 
small arteries and of the muscular tissue of the capsule and of the trabeculae; 
to be, in fact, a vascular dilatation accompanied by a local inhibition of the 
tonic contraction of the other plain muscular fibres entering into the struc- 
ture of the organ, the latter, at all events in some animals, being probably 
the more important of the two. And the condition of the spleen, like that 
of other vascular areas, appears to be regulated by the central nervous sys- 
tem, the digestive turgescence being fairly comparable to the flushed con- 
dition of the pancreas and of the gastric membrane during their phases of 
activity. 
The application of the plethysmographic method to the spleen, carried 
out in the way which we described in speaking of the kidney (§ 346), 
enables us to study more exactly the variations in volume which the organ 
undergoes. 
A "spleen curve" (Fig. 117) taken in the same way as a " kidney curve" 
does not, in the dog at all events, show variations in the volume of the spleen 
corresponding with the pulse waves. The kidney curve, as we have seen, 
(§ 346), gives clear indications of each heart-beat, but the spleen curve 
shows, besides the larger waves of which we shall speak directly, only undu- 
lations due to the respiratory movements ; and these, always very slight, are 452 
THE METABOLIC PROCESSES OF THE BODY. 
sometimes not visible. In other words, the spleen does not expand with the 
increase of blood-pressure occurring in the splenic arteries after each heart- 
beat ; this may be due to the muscular coat resisting expansion. Moreover, 
when the supply of blood to the spleen is wholly and suddenly cut off, as by 
clamping the aorta, the spleen curve sinks very slowly, showing that the 
spleen is diminishing in volume not suddenly but very slowly. The path- 
way of the blood through the splenic reticulum is peculiar; and increase or 
decrease in the volume of the spleen means more or less blood held in the 
spleen-pulp, not necessarily a greater or less flow of blood through the 
organ. 
Of special interest are the large slow variations of volume which, besides 
the respiratory undulations, the spleen curve usually shows, as seen in the 
figure. Rhythmic contractions and expansions, though not always present, 
frequently make their appearance, each contraction with its fellow expansion 
lasting in the cat and dog about a minute, and recurring with great regu- 
Fig. 117. 
Normal Spleen Curve from Dog. (Roy.) The upper curve is the spleen curve showing the 
rhythmic contractions and expansions; the smaller waves are due to the respiratory movements. 
The lower curve is the blood-pressure curve, and the point a of the spleen curve corresponds 
in time to the point b of the blood-pressure curve. The marks on the time-curve below indicate 
seconds. 
larity for a long time ; and besides these the volume varies widely from time 
to time. There can be little doubt but that the rhythmic variations in 
volume are due in these animals to rhythmic contractions, with intervening 
relaxations, of the muscular trabeculae and capsule; the slower variations 
are also probably due to the same cause. In many animals the contractility 
of the splenic tissue is shown by the white lines of constriction which appear 
when the electrodes of an induction machine in action are drawn over its 
surface; and similiar lines may be produced by mechanical stimulation with 
the point of a needle. So that the spleen in these animals may be considered 
as a muscular organ, now expanding to receive a larger quantity of blood 
and now contracting to drive the blood on to the liver. When the muscular 
elements are scanty in or absent from the capsule and trabeculae, the expan- 
sion and contraction of the whole organ must depend alone or chiefly on 
variations in the width of the supplying arteries. We have evidence, more- 
over, that the muscular activity of the spleen, whether of the muscular cap- 
sule and trabeculae and arteries combined, or of the latter alone, is under the 
dominion of the nervous system. A rapid contraction of the spleen may be 
brought about in a direct manner by stimulation .of the splanchnic or vagus 
nerves, or in a reflex manner by stimulation of the central end of a sensory 
nerve ; it may also be caused by stimulation of the medulla oblongata with THE FORMATION OF THE CONSTITUENTS OF BILE. 
453 
a galvanic current or by means of asphyxia. Though the matter has not 
yet been fully worked out, we have already sufficiently clear indications 
that the flow of blood through the spleen is, through the agency of the 
nervous system, varied to meet changing needs. At one time a small 
quantity of blood is passing through or is being held by the organ, and the 
metabolic changes which it undergoes in the transit are comparatively 
slight. At another time a larger quantity of blood enters the organ, and is 
let loose, so to speak, into the splenic pulp, there to undergo more profound 
changes, and afterward to be ejected by the rhythmic contractions of the 
muscular trabeculae. 
It is further obvious that these changes going on in the spleen must have 
an important influence on the changes going on in the liver; it cannot be of 
indifference to the latter organ, whether a relatively small quantity of blood, 
relatively little changed, reaches it from the spleen, or whether it receives a 
relatively large quantity of blood, profoundly altered by the changes which 
it has undergone in the spleen-pulp. 
§ 390. The chemical constituents of the spleen. Besides the chemical 
bodies which one would expect to find in a vascular, muscular organ full of 
blood, the spleen contains bodies, lodged apparently in the spleen-pulp, which 
give it special chemical characters. One of the most important of these is 
a special proteid of the nature of alkali-albumin, holding iron in some way 
peculiarly associated with it. The occurrence of this ferruginous proteid, 
accompanied as it is by several peculiar but at present little-understood pig- 
ments, rich in carbon, which are partly present in the cells spoken of above 
and partly deposited in the branched cells of the reticulum, appears to be 
connected with the changes undergone by the haemoglobin which we shall 
presently discuss. The inorganic salts of the spleen, or at least those of its 
ash, are remarkable for the large amount of both soda and phosphates and 
the small amount of potash and chlorides which they contain, thus differing 
from those of blood corpuscles on the one hand, and from those of blood- 
serum on the other. But perhaps the most striking feature of the spleen- 
pulp is its richness in the so-called extractives. Of these the most common 
and plentiful are succinic, formic, acetic, butyric, and lactic acids, inosit, 
leucin, xanthin, hypoxantbin, and uric acid. Tyrosin apparently is not 
present in the perfectly fresh spleen, though leucin is; both are found when 
decomposition has set in. The constant presence of uric acid is remarkable, 
especially since it has been found even in the spleen of animals, such as the 
herbivora, whose urine contains none. 
The richness of the spleen in these extractives is an indication of the 
importance of the metabolic events with which the organ has to do ; but it 
will be more profitable to discuss what goes on in the spleen in connection 
with the metabolic changes in the other parts of the body, in the liver for 
instance, than to attempt to lay down any so-called " functions " of the 
spleen. When we confine our attention to the spleen itself we learn very 
little ; thus the whole organ may be successfully removed without any very 
obvious changes in the economy resulting. We may return, therefore, to 
the discussion of the formation of the bilirubin of bile, and of the changes 
undergone by haemoglobin, with which, as we shall see, the spleen is con- 
nected, and which, moreover, has to do with the formation of other pig- 
ments. 
The Formation of the Constituents of Bile. 
§391. Bile pigments. After extirpation of the liver no accumulation 
of bile pigment or bile salts takes place in the blood. This is well shown in 454 
THE METABOLIC PROCESSES OF THE BODY. 
frogs, which survive the operation for some considerable time; but the same 
results have been obtained in birds (geese and ducks). There can be no 
doubt, therefore, that these substances are formed in the liver, and not sim- 
ply withdrawn from the blood by the liver in some such way as we have 
seen reason to think urea is withdrawn from the blood by the kidney. 
When the plasma of circulating blood is made to contain haemoglobin 
detached from the corpuscles, bile pigment frequently makes its appearance 
in the urine. The presence of free haemoglobin may be obtained by inject- 
ing into the veins a solution of haemoglobin or blood made " laky " by freez- 
ing and thawing or by the addition of a small quantity of bile salts, or by 
simply injecting into the veins a quantity of distilled water or a small 
quantity of ether or chloroform or of bile salts, all of which tend to " break 
up" red corpuscles and set free haemoglobin. A similar result occurs in 
poisoning by certain drugs, such as toluylendiamine. Under these circum- 
stances not only does bile pigment, bilirubin, make its appearance in the 
urine, but the quantity of bilirubin secreted by the liver is increased. 
Obviously the presence of dissolved haemoglobin in the plasma of the blood, 
and, presumably more especially of the blood reaching the liver by the 
portal vein, leads to an increased formation of bilirubin, which take place 
in such a manner that the whole of the bilirubin so formed does not pass 
into the bile but part is retained in or thrown back into the circulation and 
appears in the urine. 
We have already mentioned the chemical connection between haemoglo- 
bin and bilirubin. Haemoglobin after the detachment of its proteids com- 
ponent becomes haematin (C32H32N4FeO4). By treatment with sulphuric 
acid or otherwise (§ 294), haematin may be deprived of its iron ; and this 
iron-free haematin (sometimes called haematoporphyrin) is said to have the 
composition C32H32N4O5, differing from bilirubin only in its oxygen and hy- 
drogen (C32H32N4O5+2H2O-O=C32H36N4O6).1 Moreover, in old blood-clots 
in the body the haemoglobin of the clot becomes in time transformed into 
an iron-free body which has been called haematoidin, but which both in com- 
position and reactions appears to be identical with bilirubin. 
These several facts lead us to the conclusion that the bilirubin of the 
bile is simply some of the haemoglobin of the blood transformed by the 
throwing off of its proteid and its iron components. It is natural to sup- 
pose that the transformation takes place in, and is effected by, the agency of 
the hepatic cells ; and this view is supported by the fact that the hepatic 
cells are characterized by containing certain peculiar iron compounds. 
When all the blood is carefully washed out of the liver by injection through 
the bloodvessels, by which means the remaining bile is got rid of at the same 
time, the hepatic substance is found to contain a small quantity of iron, 
sufficient to give the cells a diffused dark color when treated with ammo- 
nium sulphide; the exact amount appears to vary largely, but the causes of 
the variation have not been determined. That this iron is in organic com- 
bination is indicated by the fact that with potassium ferrocyanide and sul- 
phocyanide the blue or red action is not observed until after treatment with 
hydrochloric acid. Apparently there are several such compounds, of a pro- 
teid or of a nuclein (§ 29) nature, from some of which the iron is more easily 
removed than others, and these compounds appear to be present in both the 
cell substance and the nucleus. It will be remembered (§ 213) that bile 
contains a distinct quantity of iron, which probably has its origin in the 
iron thus set free from haemoglobin and retained in the hepatic cell; but it 
does not follow that all the iron thus set free makes its way into the bile; 
and, indeed, the quantity of iron discharged in the bile in twenty-four hours 
1 Doubling the formula for bilirubin given in ? 214. THE FORMATION OF THE CONSTITUENTS OF BILE. 
455 
is much smaller than the quantity calculated to be set free in the formation 
out of the haemoglobin of the quantity of bilirubin discharged during the 
same period. Apparently the iron compounds of the hepatic cell have some 
other work than the simple discharge of iron into the bile. 
The fact mentioned above, that the presence of free haemoglobin in the 
blood leads not only to an increase of bilirubin in the bile, but also to its 
presence in the urine, offers some difficulties; for if the bilirubin be formed 
out of haemoglobin by and in the hepatic cell, one would expect to find that 
the whole of it passed into the bile, and that it could not appear in the 
blood and so in the urine unless reabsorption from the bile passages, due to 
obstruction took place; and there is no evidence of any sufficient obstruc- 
tion occurring in these cases. Indeed the presence of bilirubin in the urine 
in these cases has been urged by some as an argument that bilirubin is 
formed in the blood or at least elsewhere than in the liver, and is simply 
excreted by the liver. Not only, however, as stated above, is there no ac- 
cumulation of bile in the blood after extirpation of the liver, but that ope- 
ration prevents the appearance of bilirubin in the urine as a consequence of 
the presence of free haemoglobin in the blood. The phenomena in question, 
therefore, do not disprove that the bilirubin is formed in the liver; they 
may be taken, however, to show that that formation, viewed as a secretory 
act, is peculiar, since the hepatic cell appears under certain circumstances to 
discharge its product of secretion into the blood or lymph as well as into 
the bile passages. 
§ 392. We may assume then that the hepatic cell has the power of split- 
ting up the haemoglobin brought to it, and of discharging part as bilirubin 
while it retains for a time the iron component in some organic combination; 
and, if we further assume that it works upon the entire haemoglobin we may 
presume that makes some subsequent use of the proteid component. But 
are we justified in assuming that the whole work is done by the hepatic 
cells? Are we to conclude that bilirubin is manufactured by some act of 
the hepatic cells which includes not only the conversion of haemoglobin into 
bilirubin, but also the extraction of the haemoglobin from the red corpus- 
cles as these are streaming slowly through the lobular hepatic capillaries in 
close contact with the hepatic cells? Now, as far as we know at present, 
haemoglobin can only be set free by means of a disintegration of the corpus- 
cles ; we have no instances of a corpuscle parting with some of its haemo- 
globin and proceeding on its way otherwise unchanged ; and we have no 
histological evidence of any disintegration of red corpuscles in the liver cor- 
responding to the formation of bile. Nor can we draw any conclusion from 
the result of a comparative enumeration of red corpuscles in the portal and 
hepatic blood, for these are too insecure to rest any conclusion upon. On 
the other hand, as we have just seen, the presence in the plasma of the blood 
of haemoglobin in a free condition is peculiarly potent in exciting the forma- 
tion of bilirubin. The evidence, therefore, is very strong for the view that 
as far as the formation of the greater part at least of the bilirubin is con- 
cerned, the action of the hepatic cell is limited to converting into bilirubin 
the free haemoglobin offered to it by the portal blood. 
By what means, under normal conditions, is the presence of that free 
haemoglobin secured ? We have seen reason to conclude from histological 
appearances that a certain number of red corpuscles undergo change in the 
spleen-pulp; and it seems natural to infer that one duty of the spleen is to 
set free haemoglobin from the corpuscles and thus, through the splenic veins 
and so the portal vein, to supply the liver with material for bilirubin. But 
this cannot be the only source, since the secretion of bile continues after 
extirpation of the spleen. There must, therefore, be other regions of the 456 
THE METABOLIC PROCESSES OF THE BODY. 
body in which a similar change of red corpuscles is going on ; it has been 
suggested that the red marrow of bones is one of these; but further infor- 
mation on these points is needed. 
We may then go so far as to say that the bilirubin of the bile is derived 
from the haemoglobin of the blood, and that the later stages of the trans- 
formation, including the discharge of the iron of the haematin component, 
take place in and by means of the hepatic cell ; but much beyond this is at 
present uncertain. It must be remembered too that, though after extirpation 
of the liver no accumulation of bilirubin takes {dace, showing that the 
bilirubin is formed by the liver, yet the whole change from red corpuscles 
to bilirubin may occasionally take place quite apart from the liver, as 
shown by the presence of haematoidin in old blood-clots. 
$393. The formation of the bile-acids. About this we know still less. 
Taking glycocholic and taurocholic acids as the typical bile-acids, recognizing 
($ 215) that these arise from the union of cholalic acid with glycin and 
taurin respectively, and remembering that taurin is found in several tissues, 
and that glycin (see $ 355) though not an actual constituent of any of the 
tissues, must certainly arise in tissue metabolism, we may conclude that the 
chief work in this respect of the hepatic cell is to provide cholalic acid, and 
to effect the combination with glycin and taurin, though possibly some 
amount of either one or the other of these bodies may be furnished by the 
hepatic substance itself. As to how cholalic acid arises out of the metabolism 
of the hepatic cell, we know no more than we do about the formation of 
kreatin in muscle or of pepsin in a gastric cell. We are equally ignorant 
about the origin of glycin and taurin, and cannot explain why in one animal 
glycocholic and in another taurocholic acid is prominent in the bile, though 
the two bodies, as shown especially by the presence of sulphur in the taurin, 
are widely different. It has been observed that the presence of bile in the 
intestines seems to excite the liver to increased biliary action ; since the bile- 
acids are rapidly changed in the intestine and the cholalic acid speedily 
altered, it seems probable that the increased biliary activity is due to the 
absorption of the glycin and taurin respectively. From which we may 
conclude that the presence of these bodies stirs up the hepatic cell to an in- 
creased formation of cholalic acid. 
§ 394. As a general rule, the formation of bile-acids runs parallel with 
the formation of bile-pigment, an increase or decrease of bile meaning an 
increase or decrease of both constituents. But there are some facts which 
seem to show that the two actions may be dissociated. The condition or 
symptom known as " jaundice" is essentially an excess of bilirubin in the 
blood, whereby the tissues such as the skin and the fluids such as the urine 
are colored with the yellow pigment. In most of the maladies in which 
jaundice is a symptom, there is evidence of an obstruction to the flow of 
bile through the bile passages; and the presence of bile in the blood, and 
hence in the tissues at large, is in such cases due to the fact that the bile 
after secretion by the hepatic cells is reabsorbed from the bile-ducts (see 
§ 226). 
But in certain cases where jaundice is a prominent symptom, no evidence 
of any obstruction whatever to the flow of bile can be obtained. This is the 
case in the jaundice of yellow fever and of a peculiar allied malady known 
as "acute yellow atrophy of the liver." Now in these cases there is no evi- 
dence of an accumulation in the blood or elsewhere of bile-acids as there is 
of bile pigment. And in the obscure malady known as simple or idiopathic 
jaundice, in which though the anatomical conditions are unknown there is 
at least no sign of obstruction, the urine though loaded with bile pigment is 
said to contain no bile-acids. ON UREA AND ON NITROGENOUS METABOLISM. 
457 
§ 395. The question may be asked, Is the secretion of bile independent 
of or in some way or other connected with the glycogenic activity of the 
cells? To this we cannot at present give a definite answer. In some of the 
invertebrata the cells in the organ, called a liver, which manufacture gly- 
cogen, are distinct from those which secrete bile or other digestive juices; 
and it might be inferred that in the vertebrate the two actions, though 
taking place, as they certainly do, in the same cell, take place apart and 
distinct. There are facts which seem to indicate that the two are intimately 
connected ; but we have as yet no exact knowledge concerning the matter. 
It has been urged that the portal blood is chiefly concerned with the forma- 
tion of glycogen, and the blood of the hepatic artery with the secretion of 
bile ; but there is no adequate support of this view. It must be remembered, 
moreover, that, in addition to the formation of glycogen and the secretion 
of bile, other metabolic events, especially affecting proteid or at least nitro- 
genous constituents of the body, are also taking place; and to these we 
must now turn. 
On Urea and on Nitrogenous Metabolism in General. 
§ 396. We have seen that nitrogenous proteid material in some form or 
other enters into the composition of all the tissues of the body, and we have 
further seen that it is so conspicuously and constantly present wherever 
living subtances are manifesting vital energies as to justify the conclusion 
that the changes which it undergoes are in some way essential to the mani- 
festation of those energies. We have seen, it is true, reason to think that in 
some tissues at least, in muscle for instance, a large part of the energy set 
free during activity preexisted as latent energy and had its immediate source 
not in proteid (nitrogenous) but in some other constituents of muscle; and 
indeed, as we shall see later on, the greater part of the whole energy of the 
body must be regarded as the energy of carbon compounds and not of 
nitrogen compounds; but this is quite consistent with the view that proteid 
material in some way or other essentially intervenes in, we may perhaps go 
so far as to say directs, the changes by which in the body energy is set free 
in the peculiar way which we speak of as living. 
We have seen that at all events the greater part of the proteid material 
of the food enters the blood as proteid material either as peptone or in some 
other form, and is carried as proteid material to the tissues. 
We have seen that the nitrogen of proteid material leaves the body so 
largely in the form of urea, that the other nitrogenous excretions may for 
the time be left out of consideration. 
And lastly we have seen reason to think that this urea which leaves the 
body in urine is brought to the kidney as urea in the blood, the kidneys 
themselves apparently having no special power of forming urea out of some- 
thing which is not urea, but only contributing to the general stock of urea 
by virtue of their own proteid metabolism. We have now to study the 
little we know concerning the steps by which the proteid material of the food 
and of the body is converted into this urea of the blood, which is the source 
of the urea of the urine. 
§ 397. In the first place we may take it for granted that the urea carried 
to the kidney in the blood had an antecedent in something which was not 
urea. We can hardly suppose that the proteid constituent of living sub- 
stance, when in the course of its metabolism it ceases to be proteid, breaks 
up at once into urea and into non-nitrogenous bodies. All we have learned 
goes to show that what we call metabolism is not a single abrupt change, 
but consists essentially in a series of changes; and we may safely conclude 458 
THE METABOLIC PROCESSES OF THE BODY. 
that proteid material in becoming urea passes through phases in which the 
nitrogen exists in chemical combinations distinct from proteid material on 
the one hand and urea on the other. 
In the second place it is extremely probable that the series of changes 
by which proteid material becomes urea is not the same in all the tissues 
and on all occasions. We should naturally expect to find the proteid 
material following different lines of metabolism in different places or under 
different circumstances, the different lines all converging to the same body, 
urea, because for some reasons or other urea appears to be, in the main, the 
most convenient form in which the nitrogen can leave the blood and the 
body. 
We should accordingly expect to find, on the one hand, various nitroge- 
nous bodies resulting from proteid metabolism in various parts of the body, 
and, on the other hand, arrangements by means of which these various 
bodies were reduced to the common form urea, preparatory to their dis- 
charge from the body by the kidney. And actual observation as far as it 
goes supports this view, though our knowledge of the whole matter is very 
imperfect. 
§ 398. We may turn our attention first to the metabolism of the skeletal 
muscles, since these represent, as far as mere quantity is concerned, by far 
the greater part of the proteid capital of the body. We may safely infer 
that they furnish a large part of the urea of the urine; though undoubtedly 
a small mass of tissue might by reason of its more rapid metabolism work 
over a greater quantity of proteid material than a much larger mass with a 
slower metabolism ; yet we have no reason to think that the proteid metab- 
olism of skeletal muscle, obscure though it is in its nature, is so slow as to 
neutralize the probable effect of the great bulk of muscle existing in the 
body. 
In dealing with the chemistry of muscle (§ 62) we saw that urea, save in 
the exceptional instances of certain cartilaginous fishes, was conspicuous by 
its absence from the extract of muscle, whereas, a very appreciable quantity 
of kreatin was invariably present, and, indeed, was the prominent nitroge- 
nous crystalline constituent of that extract. It seems difficult to resist the 
conclusion that kreatin is the main normal nitrogenous product of the metab- 
olism of skeletal muscles. If we accept this view, then, upon the fact of 
the presence of kreatin in, and the absence of urea from, the muscle itself, 
we may base the conclusion that while the muscle produces kreatin as an 
antecedent of urea, the kreatin so produced is converted into urea in some 
part of the body other than the muscle itself. Kreatin, as we have already 
seen, may be easily split up, and we may probably with safety assume is split 
up, somewhere in the body, into urea and sarcosin. But sarcosin does not 
appear in the urine as such ; hence, the conversion of kreatin into (part of) 
the urea of the urine entails as well the further conversion of sarcosin into 
urea. Now sarcosin, as we have seen, is methyl-glycin ; we may regard it 
for our present purposes as simple glycin, and hence the total conversion of 
kreatin into urea entails the conversion of glycin into urea. This, however, 
does not offer any additional difficulty, since we know from direct observa- 
tion that glycin introduced into the alimentary canal does not reappear as 
such in the urine, but produces a corresponding increase in the urea of the 
urine; from which we infer that glycin absorbed from the alimentary canal 
is, somewhere in the body, converted into urea. We shall speak of this con- 
version later on, and shall then see that, as far as urea is concerned, glycin 
(amido-acetic acid) and sarcosin (methyl glycin, methyl-amido-acetic acid) 
undergo the same change, the amide moiety in each case being converted 
into urea, while the non-nitrogenous moiety is oxidized and thrown off ON UREA AND ON NITROGENOUS METABOLISM. 
459 
Meanwhile, we may state the conclusion at which we have provisionally 
arrived, namely, that the nitrogenous metabolism of muscle probably 
gives rise to kreatin, which, in some part of the body other than muscle, 
is probably split up into urea, ready for excretion, and into sarcosin which 
also, somewhere in the body, is further converted into urea. And bearing 
in mind the large mass of the skeletal muscles, we may further conclude 
that a large portion of the urea leaving the body by the urine is formed in 
this way. 
§ 399. We must not, however, leave this statement without referring to 
a difficulty. Kreatinin, as we have seen, is so frequently found in urine as 
to be regarded as a normal constituent, at all events, of human urine ; and 
kreatinin is, as we have seen, the urinary form, so to speak, of kreatin ; the 
one body easily changes into the other by the assumption or removal of 
H2O. This suggests the question, Is not the kreatinin of urine the repre- 
sentative of the kreatin of the muscles, which is thus exerted directly 
without undergoing the change into urea just discussed ? In answer to 
this we may say, in the first place, that the quantity of kreatinin in the 
urine, though variable, is small; we may put the average at about 1 grm. 
in twenty-four hours. Now, muscle contains from 0.2 to 0.4 per cent, of 
kreatin ; and this, taking the total muscle of the body (to say nothing of 
other sources of kreatin, which we shall mention presently) at about 30 
kilos, would give 60 to 120 grms. of kreatin as present in the muscles of the 
body at any one moment. We can hardly suppose that the metabolism of 
muscle is so slow as out of this stock only to provide the 1 grm. of kreat- 
inin in twenty-four hours. Moreover, the kreatin in urine vanishes during 
starvation, is very markedly increased by a diet of flesh which contains 
kreatin, and is not increased either by muscular exercise (which, however, 
would only indirectly affect nitrogenous metabolism of muscle), or by such 
conditions, fever, for instance, as notably increase the urea of urine by in- 
creasing the nitrogenous metabolism of muscle. We infer, therefore, that 
the normal presence of kreatinin in urine is due to the direct administration 
of kreatin present in a (normal) flesh diet, and has nothing to do with the 
muscular metabolism of the individual who is secreting the kreatinin in his 
urine. 
The fact, however, that the kreatin present in the muscle of the food and 
absorbed from the alimentary canal does not undergo a change into urea, but 
is excreted as kreatinin, that is, virtually as kreatin, warns us to be careful 
in adopting the conclusion arrived at above, that the kreatin produced by 
muscular metabolism in the living body is a conspicuous antecedent of the 
urea of the urine. It is difficult to see why kreatin passing into the blood 
of the capillaries of the muscle should be changed into urea, while that which 
passes into the capillaries of the portal system is not; for reasons which will 
be apparent presently, we should rather expect that the latter being more 
directly exposed to the influence of the liver would be more readily and 
more completely converted than the former. Indeed, the question forces 
itself upon us, Is kreatin, after all, the natural main product of the nitro- 
genous metabolism of muscle? It is possible that in the normal metab- 
olism of the living muscle the nitrogen leaves the muscular substance 
and passes into the blood in another form, as some substance not kreatin, 
and that it is as the muscle dies that kreatin is formed, just as the solid 
myosin is unknown to living fibre but makes its appearance in a dying 
one? We have no positive evidence, however, that this is so, and, mean- 
while, may continue to suppose that kreatin is formed, and that, in conse- 
quence, kreatin is a conspicuous antecedent of the urea of the urine; but 
we must not regard this as proved. 460 
THE METABOLIC PROCESSES OF THE BODY. 
§ 400. Our knowledge of the metabolism of the nervous tissues is, as 
we have seen, very imperfect 70), but the presence of the kreatin in the 
central nervous system leads us to infer that the nitrogenous metabolism of 
the living substance of nerve-cells and of the axis-cylinder of nerve-fibres 
is, in its broad features, identical with that of muscle substance. The mass, 
however, of the nerve-cells and axis-cylinder of the body, all put together, 
is small, compared with the mass of skeletal muscle; moreover, the energy 
set free by the metabolism of a mass of nervous matter though " higher " 
in quality is less in quantity than that set free by the metabolism of an 
equal mass of muscle, or, in other words, its metabolism is less rapid. 
Hence, we may probably consider the metabolism of the nervous system 
as a mere addition to that of the muscular system, at least as regards 
the point on which we are now dwelling. The amount of nitrogenous 
metabolism taking place in connective tissue, cartilage, bone, and the skin 
is probably still less, and, for our present purposes, needs no special dis- 
cussion. 
§ 401. The nitrogenous metabolism of the glands, however, more par- 
ticularly that of the liver, does deserve special consideration ; and we may 
at once turn to a quite different aspect of the question in hand. 
When the rate of discharge of urea from the body is observed during a 
period of some, length, especially under-varied circumstances, the direct 
effect of nitrogenous food becomes more striking. We have already said, 
and shall again return to the point, that muscular contraction does not di- 
rectly increase the output of urea; the discharge of urea, for instance, is 
not necessarily increased by even great bodily labor. The introduction, 
however, of even a small quantity of proteid material into the alimentary 
canal at once increases the urea of the urine; and in the curve of the dis- 
charge of urea in the twenty-four hours each meal is followed by a con- 
spicuous rise. The absorption of proteid material from the alimentary 
canal is followed by an immediate proportionate increase in the quantity 
of urea which is secreted by the kidneys, and that, as we have seen, means 
an increase in the urea brought to the kidney by the renal artery. What 
is the origin of this additional urea? 
Two views present themselves. On the one hand, since some portion of 
the proteid material of every meal, at all events of every necessary meal, 
goes to repair the proteid waste continually going on in the parts of the body 
where proteid metabolism is taking place, we may suppose that the presence 
of an extra quantity of proteid material thrown upon the blood from the 
food acts as a stimulus to the tissues, to the muscles, for instance, as well as 
others, stirs them up to increased nitrogenous metabolism and thus produces 
an increase of energy, chiefly if not exclusively in the form of heat, accom- 
panied by an increase of the antecedents of urea and so of urea. In other 
words, the increase of urea in question is the result of an increase in the 
general nitrogenous metabolism of the body. 
On the other hand, we may suppose that in order to prevent the whole 
body being encumbered with it, this excess of proteid food material is, in 
some special part of the body, split up into a nitrogenous and a non-nitro- 
genous moiety, and that, while the latter is stored up as fat or glycogen, the 
former is at once converted into urea and got rid of. We have already 
(§ 218) seen that a step in this direction may take place while the food is as 
yet in the alimentary canal; we have seen that pancreatic juice may carry 
part of the proteids on which it acts beyond the stage of albumose and pep- 
tone, and reduce that part into leucin, tyrosin, and other bodies. We do not 
know, as we have already said, to what extent this more profound digestion 
by pancreatic juice does actually take place in the living body ; it may take ON UREA AND ON NITROGENOUS METABOLISM. 
461 
place to a very slight extent and it may under certain circumstances take 
place to a considerable extent. But in any case it illustrates the way in 
which a somewhat similar disruption of proteid material, a disruption which 
may be broadly described as a splitting up of the proteid into a nitrogenous 
and a non-nitrogenous moiety, may take place somewhere in the body and so 
lead to the sudden formation of some antecedent of urea. The antecedent 
may be leucin or may be some other body or bodies. 
In support of this view may be urged the fact that such bodies as leucin, 
glycin, asparagin, and many others when introduced into the alimentary 
canal are transformed into urea. When these bodies are administered in 
not too great quantities they do not reappear in the urine, but the urea is 
proportionately increased. 
§ 402. We have seen reason to think that the proteids of a meal are 
absorbed not by the lacteals but by the portal bloodvessels, and such bodies 
as leucin probably take the same course. This being so, all these bodies pass 
through the liver and are subjected to such influences as maybe exerted by 
the hepatic cells. Now, we have no positive evidence that the liver does or 
can exert such an action on proteid material itself as to separate a relatively 
simple nitrogen compound from the remaining constituents, leaving these to 
form a body rich in carbon ; we have no positive proof that the increase of 
proteid metabolism just spoken of as leading to an increase of urea takes 
place in the liver rather than in the tissues at large; we may go so far per- 
haps as to suspect that it is largely or wholly confined to the liver, but we 
have no convincing demonstration. We have, however, a convergence of 
evidence that the last stage of the process, namely, the conversion into 
urea of some product of proteid metabolism, which though allied to is not 
exactly urea, does occur in the liver. In the first place, a large quantity 
of urea seems to be present in the liver of mammals ; in this respect the liver 
presents a strong contrast to the muscles; in the liver of birds the urea is 
represented by urates. Moreover, when a stream of fresh blood is passed 
several times through the liver of an animal recently killed, the percentage 
of urea in the blood so used is found to be decidedly increased. This, how- 
ever, does not prove that urea is formed in the liver, since the increased 
quantity of urea in the blood which had been circulated might have been 
simply urea which had been washed out from the liver, where it had pre- 
viously been staying. Still as far as it goes it is suggestive. In the second 
place, in certain cases of a form of disease of the liver known as acute yellow 
atrophy in which the hepatic cells are so changed that their functional 
activity is largely diminished, the urea of the urine not only undergoes a 
very marked decrease but appears to be replaced to a very large extent by 
leucin. This fact suggests that leucin (and not for instance kreatin) is the 
chief immediate product of the nitrogenous metabolism of the body, and 
that the leucin thus produced is in a normal state of things converted into 
urea by the liver. And in this connection it may be remarked that not 
only is leucin found in nearly all the tissues after death, especially in the 
glandular tissues, but also appears with striking readiness in almost all 
decompositions of proteids, and is, moreover, a product of decomposition of 
gelatiniferous substances. Without going, however, so far as to conclude 
that leucin is the chief antecedent of urea, we may take the above observa- 
tion as indicating that the normal liver has, in some way or other, the power 
of converting leucin into urea. If this be so we may also venture to suppose 
that when such bodies as leucin, glycin, etc., introduced into the alimentary 
canal appear in the urine as urea the transformation has taken place in the 
liver. The body tyrosin which so often accompanies leucin, belonging as it 
does to the aromatic series, stands on a different footing from leucin and the like. 462 
THE METABOLIC PROCESSES OF THE BODY. 
§ 403. The transformation, however, of leucin into urea raises a new 
point of view. Leucin, as we know, is amido-caproic acid ; and, with our 
present chemical knowledge, we can conceive of no other way in which leu- 
cin can be converted into urea than by the complete reduction of the former 
to the ammonia condition (the caproic acid residue being either elaborated 
into a fat or oxidized into carbonic acid) and by a reconstruction of the latter 
out of the ammonia so formed. We have a somewhat parallel case in gly- 
cin, which is amido-acetic acid ; here, too, a reconstruction of urea out of 
an ammonia phase must take place. Moreover, when ammonium chloride 
is given to a dog a very large portion reappears as urea, i. e., there is an 
increase in the urea of the urine corresponding to a large portion of the 
nitrogen contained in the ammonium chloride. And in the case of other 
animals also, indeed of man himself, there is evidence that somewhere in the 
body ammonia may be converted into urea. Hence in all these cases where 
ammonia or ammonia compounds are changed into urea, the last step at all 
events is one of synthesis; and this suggests the possibility that in the ordi- 
nary proteid metabolism also, the downward katabolic series of changes 
may finish off with a synthetic effort, the last stage of the former being the 
appearance of an ammonia compound which is subsequently reconstructed 
into urea. 
This synthesis, like the transformation of leucin and other bodies, prob- 
ably takes place in the liver; and in support of this view we have a certain 
amount of experimental evidence. Birds maybe kept alive after total extir- 
pation of the liver for a longer time than can mammals; and when in geese 
the liver is removed the uric acid (representing in these animals the urea of 
the mammal) is largely decreased, while the ammonia of the urine is largely 
increased. After the removal of the liver also, leucin, glycin, and other 
amides or amido-acids administered by the alimentary canal no longer 
increase the uric acid of the urine, as they do in the intact animal. In these 
animals, the synthesis of ammonia compounds into uric acid, which is parallel 
to the synthesis into urea occurring in the mammal, seems to take place in 
the liver, and we may infer is in some way or other effected by the hepatic 
cells. 
As to the exact way in which ammonia, either as such or in form of an 
amide or amido-acid changes into urea, we have no certain knowledge. 
Ammonium carbonate, we know, is readily formed out of urea by simple 
hydration, and we may imagine that the living organism can carry out the 
reverse process and dehydrate ammonium carbonate into urea. There is, 
however, a certain amount of evidence that not ammonium carbonate but 
ammonium carbamate is the immediate antecedent of urea; and, indeed, out 
of the body, by electrolyzing a solution of ammonium carbamate with alter- 
nating currents, a certain amount of urea may be artificially produced. But 
this is a matter too obscure to be discussed here. 
$ 404. Uric acid. This, like urea, is a normal constituent of human 
urine, and, like urea, has been found in the blood, in the liver, and in the 
spleen ; it is a conspicuous constituent of an extract of the latter organ. 
In some animals, such as birds and most reptiles, it takes the place of urea. 
In various diseases the quantity in the urine is increased ; and at times, as 
in gout, uric acid accumulates in the blood, and a deposit of urates takes 
place in the tissues. Since by oxidation a molecule of uric acid can be 
split up into two molecules of urea, and a molecule of some carbon acid, 
uric acid is commonly spoken of as a less oxidized product of proteid metab- 
olism than urea. But there is no evidence whatever to show that the 
former is a necessary antecedent of the latter ; on the contrary, all the facts 
known go to show that the appearance of uric acid is the result of a metab- ON UREA AND ON NITROGENOUS METABOLISM. 
463 
olism slightly diverging from that leading to urea ; indeed, it is probable 
that the divergence occurs toward the end of the series of changes, for urea 
given by the mouth to birds appears in the urine as uric acid, and, con- 
versely, uric acid given to mammals appears in the urine as area. We 
have no evidence to prove that the cause of the divergence lies in an insuf- 
ficient supply of oxygen to the organism at large; on the contrary, uric acid 
occurs in the rapidly breathing birds as well as in the more torpid reptiles. 
Nor can the fact that in the frog, again, urea replaces uric acid be explained 
by reference to that animal having so large a cutaneous in addition to its 
pulmonary respiration. The final causes of the divergence are to be 
sought rather in the fact that urea is the form adapted to a fluid, and 
uric acid to a more solid excrement. Nor is there in man or the mam- 
mal any satisfactory physiological or clinical evidence that an increase of 
uric acid is the result of deficient oxidation. The absolute amount of uric 
acid discharged by man and its proportion to the urea passed at the same 
time varies a good deal. There is no positive evidence that the quantity 
excreted is necessarily increased by nitrogenous diet, unless some disorder 
supervenes ; indeed, it is asserted that both absolutely and relatively to the 
urea the quantity excreted is greater upon a mixed diet than upon a highly 
proteid one. Alkalies in the food seem undoubtedly to diminish it, and 
alcohol, at least in excess, to increase it. 
So far from considering uric acid as a less oxidized antecedent of urea, 
we ought, perhaps, rather to regard its appearance as a result of a synthesis 
in which urea or some allied body takes part. As we have said, uric acid 
may be formed synthetically by heating together urea and glycin ; and it 
has more recently been similarly prepared from various allied bodies. As 
to where or how such a synthesis is effected in the living body, we know 
little or nothing for certain, and can only make conjectures. The constant 
presence of uric acid in the spleen, however, and the frequently noted con- 
nection between a rise and fall of uric acid in the urine and variations in 
the volume and therefore presumably in the activity of the spleen, suggest 
that the change may be brought about in that organ ; but it must be remem- 
bered that in birds and reptiles the formation of uric acid seems to be 
effected in the same organs as that of urea and in an analogous manner ; and 
the arguments which we have used concerning the formation of urea in the 
liver of mammals, maybe applied to the formation of uric acid in the livers 
of birds and reptiles. It is more probable, therefore, that in the mammal 
the turn to uric acid rather than urea is given in the liver, the spleen, how- 
ever, possibly playing its part also in the matter. 
§405. Of the meaning of the appearance in the tissues of such bodies 
as xanthin, hypoxanthin, guanin, and the like, and of the exact nature of 
the metabolism which gives rise to them or which they themselves undergo, 
we know little or nothing. The presence of these several bodies may be 
taken as illustrating the complex and varied nature of proteid metabolism 
to which we referred above. Urea is the chief end-product of proteid 
metabolism, but that end is probably reached in several ways; so that prob- 
ably a very large number of nitrogenous chemical substances make a mo- 
mentary appearance in the body. Some of these fail to become urea, and 
either without or after further change make their appearance in the urine. 
But we do not know whether their appearance is accidental, the result of 
imperfect chemical machinery, or whether they, though small in quantity, 
serve some special ends in the economy. Perhaps sometimes or with some 
of them it is the one case, at other times or with others it is the other case. 
When proteid material undergoes outside the body, either by the action 
of trypsin or as the result of decomposition or under the influence of chem- 464 
THE METABOLIC PROCESSES OF THE BODY. 
ical agents, that change by which it is converted into leucin, the leucin 
which appears in some considerable quantities is accompanied by tyrosin, 
which appears in smaller quantities, as well as by other bodies. The almost 
constant appearance of tyrosin as a result of the decomposition of proteid 
material leads one, as we have previously said, to the conception that some 
representative of the aromatic series enters into the constitution of proteid 
substance; and it is possible that the hippuric acid of flesh-eating animals 
derives its benzoic acid constituent from this aromatic radicle of proteid 
matter. Tyrosin itself does not appear in the body as a normal product of 
proteid metabolism, and we are therefore led to infer that in proteid meta- 
bolism the aromatic radicle takes on some other form. Whether, as in 
tyrosin, the aromatic (phenyl) nucleus is associated with an ammonia repre- 
sentative or no, we do not know. But if it is, then, since neither tyrosin nor 
any similar body is a constituent of normal urine, the ammonia constituent 
is somewhere dissociated from the phenyl one; and while the former con- 
tributes to the stock of urea, the latter is either discharged by the urine as 
hippuric acid, having as we have seen effected in the kidney a new associa- 
tion with the ammonia representative, glycin, or leaves the body as one or 
other of the urinary phenyl compounds, or possibly may be oxidized some- 
where into carbonic acid and water. Our knowledge on this point is 
limited, but we have ventured to refer to the point since it further illus- 
trates the complexity of proteid metabolism. 
§ 406. In speaking of urea (§ 337) we alluded to its relations to the 
cyanogen compounds. Bearing in mind the peculiarly large amount of 
energy set free as heat during the isomeric transformation of many cyanogen 
compounds, as well as the large store of potential energy existing in cyanogen 
itself, the heat of combustion of which is very large, and contrasting these 
properties with those of ammonia and the ammonia compounds, we cannot 
help being tempted toward the view that in the actual living structure the 
nitrogen exists in the form of cyanogen compounds, and that in the passage 
to dead nitrogenous waste, during which energy is set free, the cyanogen 
compound changes to the amide or other ammonia representative. And there 
are several facts which lend support to such a view, such as the presence of 
sulphocyanates in saliva and urine, which we may look upon as a sort of 
leakage of cyanogen factors, the artificial production of kreatinin out of 
cyamide and sarcosin, and other facts. But the matter, though it deserves 
to be borne in mind, is too obscure to be dwelt on here. 
§ 407. We may now briefly sum up the varied discussions which have 
occupied us in the present section. 
Urea is the main end-product of proteid metabolism. Unlike hippuric 
acid and some other constituents of urine, urea is simply excreted by the 
kidneys, being brought to them in the blood, they apparently, beyond the 
simple act of excretion, doing no more than merely contributing to the 
stock of urea in so far as they are masses of proteid material undergoing 
proteid metabolism as part of their general life. What are the immediate 
antecedents of urea we do not clearly know ; but it is probable that they 
are not one but several and indeed possibly many. We have reason to 
think that urea may be formed out of amides or amido-acids, or out of 
ammonia itself by a synthetic process; and we have indications that this 
synthesis is effected in the liver by the agency of the hepatic cells. But we 
do not know whether this synthesis bears only on particular nitrogen-hold- 
ing substances of food or of the body, or whether it comes into play in the 
normal metabolism of proteid material. If the kreatin which is so con- 
spicuous a constituent of muscular and nervous structures is a stage in the 
direct line to urea, then the synthesis would affect only the sarcosin which STRUCTURES AND PROCESSES OF OBSCURE NATURE. 
465 
the kreatin in becoming urea sets free. But it is by no means clear that 
kreatin is such a stage. 
The evidence as far as it goes tends to show that the metabolism of pro- 
teid is very complex and varied, that a large number of nitrogen-holding 
substances make a momentary appearance in the body, taking origin at this 
or that step in the downward stairs of katabolic metabolism and changing 
into something else at the next step, and that the presence in various parts 
of the body and even in the urine, in small quantities, of so many varied 
nitrogenous crystalline substances, forming a large part of what are known 
as extractives, has to do with this varied metabolism. Possibly the transfor- 
mations by which nitrogen thus passes downward take place to a certain 
extent in such organs as the liver and the spleen, which are remarkably rich 
in these extractives. 
On Some Structures and Processes of Obscure Nature. 
§ 408. The thyroid body. Certain structures of obscure nature, but 
probably connected in some way or other with some of the metabolic pro- 
cesses in the body, are often spoken of under the undesirable name of " duct- 
less glands." Such are the thyroid body or gland, the pituitary body, the 
thymus, and the suprarenal capsules. These differ from each other so essen- 
tially that the only plea which can be urged in favor of considering them 
together is convenience and our ignorance of their respective functions. 
The thyroid body is the one of the group most deserving to be called a 
gland, since it, like the lungs, arises as a two-lobed diverticulum from the 
ventral surface of the anterior part of the alimentary canal, and at first, 
like the lungs also, behaves as if it were about to become a double racemose 
gland. The connection with the throat, however, which should have become 
a duct, is soon obliterated, and the two lobes, united with each other by an 
isthmus across the trachea, lose all traces of any branching ducts within 
them, and become transformed into masses of isolated ductless alveoli bound 
together with connective tissue. 
Hence, when a section is taken through a hardened and prepared lobe 
of an adult thyroid, what is seen is a limiting capsule of connective tissue 
sending into the interior numerous septa, which surround ami separate from 
each other round or oval spaces, the sections of the isolated alveoli. These 
are of variable size, some being visible to the naked eye, and each is lined 
by a single layer of low columnar or cubical nucleated cells resting on a 
basement membrane, leaving a large cavity, which in fresh specimens is 
filled with a glairy fluid. The cells present no special characters. 
The septa of connective tissue, fairly rich in elastic elements, but remark- 
ably free from adipose tissue, contain numerous bloodvessels derived from 
the superior and inferior thyroid arteries, the branches of which, relatively 
large and frequently anastomosing, end for the most part in capillary net- 
works round the alveoli; from these capillaries and those of the septa the 
blood is gathered into veins also relatively large, which, forming plexuses 
on the surface of the organ, end in the superior middle and inferior thy- 
roid veins. The thyroid body is thus furnished with an abundant supply 
of blood. 
The septa also contain a very large number of lymphatic vessels, which, 
both on the surface of the organ and along the septa, are arranged in plex- 
uses of anastomosing trunks of considerable size. Small nodules of adenoid 
tissue are also found in the septa. 
The nerves of the thyroid body are also abundant. They are, in man, 
derived chiefly from the cervical sympathetic nerve, passing off from the 466 
THE METABOLIC PROCESSES OF THE BODY. 
middle and lower cervical ganglia; their exact terminations within the 
organ is not known. Fine filaments are also said to be given off to it from 
the external branch of the superior laryngeal nerve. 
The " accessory " thyroid bodies often found are of the same nature as 
the main body. 
Very frequently, so frequently in the adult as to be of almost normal 
occurrence, the alveoli contain not simple glairy fluid but a more solid clear 
material called " colloid this generally appears in the centre of an alveolus 
and may fill up the whole lumen ; occasionally more or less changed epi- 
thelial cells may be seen lying between it and the layer of cells resting on 
the basement membrane. Extravasations of blood into the alveoli are also 
not uncommon. 
The thyroid body is very apt to become enlarged, sometimes enor- 
mously so, and is then spoken of as goitre. The enlargement may be due 
simply to an increase in the number of otherwise fairly normal alveoli and 
septa. But very often a number of alveoli become more or less confluent, 
forming a cyst; and at times the whole gland appears to be composed of a 
number of cysts of varying size, frequently loaded with " colloid" material. 
There is also a form of goitre in which the enlargement is chiefly or even 
exclusively due to an increase in the vascular supply, the bloodvessels being 
abnormally distended ; and this apparently may occur without any structural 
changes in the walls of the bloodvessels. Sometimes, however, the arteries 
undergo aneurismal enlargements, with changes in their coats. 
'rhe glairiness of the fluid contents of the alveoli has generally been 
attributed to the presence of mucin, and this body has also been said to 
have been found within the lymphatic vessels running in the septa; but 
some observers have urged that the material in question is not true mucin, 
but a peculiar form (or forms) of proteid substance. The " colloid " material 
so frequently appearing has also been regarded as allied to mucin, but its 
exact nature has not as yet been satisfactorily determined. Besides these 
special substances the alveoli or cysts also contain serum-albumin and 
globulin. The " extractives " of the thyroid appear to contain kreatin or 
kreatinin in not inconsiderable quantities, xanthin, and lactic (paralactic) 
acid ; guanin is said to be absent. In large and old cysts cholesterin is 
sometimes present; and when, as often happens, extravasations of blood into 
the cysts have taken place, haemoglobin, or at a later stage hsematoidin 
(bilirubin) has been found. 
§ 409. The large supply of blood to the thyroid suggests the idea that 
the organ is the seat of some of the subsidiary metabolic processes to which 
we referred in the last section, and this view is supported by the presence of 
the extractives just mentioned ; but we have no detailed knowledge of what 
actually goes on. 
The presence of the peculiar mucin-like body in the alveoli, and the ten- 
dency to " colloid formation " further suggests some relation of the organ to 
the formation or distribution of mucin ; and this view has derived a certain 
support from some experimental results, but these, though numerous, have 
proved neither uniform nor accordant. When in certain animals (monkeys, 
dogs, and other carnivora, and the same has been observed in man) the gland 
is extirpated, even with the greatest care, the operation is frequently followed 
by the occurrence of peculiar nervous symptoms, such as muscular twitchings 
and tremors, spasms, and even tetanic convulsions (more especially observed 
in young animals), accompanied or succeeded by irregularity or failure of 
voluntary movements; subsequently there may ensue varied symptoms 
which may be described under the general term of disordered nutrition, 
ending eventually in death. In a certain number of cases, however, in the STRUCTURES AND PROCESSES OF OBSCURE NATURE. 
467 
above kinds of animal, no serious symptoms follow, even the total extirpation 
of the organ producing no marked effect; and in rabbits and other her- 
bivorous animals removal is said never to be followed by any of the above 
results. It has been urged that the symptoms when seen are the effects not 
of the mere absence of the organ, but of mischief set up by the operation in 
adjoining structures, more especially in the laryngeal nerves and vagus 
trunks ; but this does not seem a valid explanation. If, as suggested above, 
certain metabolic processes are normally going on in the organ, we may fairly 
suppose that, in the absence of the organ, the interruption of the normal 
sequence of chemical change would throw upon the circulation certain 
strange substances which, acting like a poison, might produce the nervous 
symptoms, throw into disorder the nutrition of various tissues, and finally 
bring about death. We may further explain the cases where symptoms are 
absent by supposing that, for some reason or other, " things have taken a 
different turn," the particular poisonous substance have not made their 
appearance, but innocuous ones have taken their place; and we know how 
slight a change in chemical composition may turn a poison into an inert 
body. This, of course, remains a mere supposition until we can state what 
the exact metabolic processes are, and name the substances which work the 
mischief; but it seems more reasonable to accept such a provisional supposi- 
tion, than to conclude that the thyroid may be removed without producing 
any effect whatever on the organism. An animal without a thyroid may 
appear perfectly well, because the circumstances to which it is exposed do 
not happen to test the imperfection from which it is really suffering, just as 
a man's inability to swim may not be apparent until he happens to fall into 
the water. The animals which do succumb to the operation of removal of 
the organ are, for some reason or other, put to the test, and are found want- 
ing. The very discordance of the experimental results points the physio- 
logical moral that the phenomena which we are as yet able to observe form, 
as it were, a mere surface covering intricate processes at present wholly, or 
nearly wholly, hidden from us. 
The above experimental results receive additional interest and at the 
same time support from clinical experience. The connection between goitre 
and cretinism-the latter disease being, broadly speaking, a result of dis- 
ordered nutrition telling largely on the nervous system-has long been 
recognized ; and attention has also been called to some tie between disease 
of the thyroid and a morbid condition, known as myxoedema, in a certain 
number of cases of which mucin or a mucin-like body has been found in 
great excess in the skin and in other tissues. In monkeys the removal of 
the thyroid has, in some cases, been followed, besides the symptoms men- 
tioned above, some of which resemble those of myxoedema, by an accumu- 
lation of mucin or a mucin-like body in the skin and various tissues. It is 
very difficult not to connect this with the formation in the thyroid of colloid 
material in the contents of the alveoli. But we know so little about the 
nature of mucin and its allies, about their real relations to more ordinary 
proteid substances, and about the part which they play in physiological pro- 
cesses, that any views as to the exact connection between the presence of 
mucin in the tissues at large and changes taking place in the thyroid must 
be at present to a large extent speculation. 
The large vascular supply of the thyroid, and the phenomena of a 
disease known as exophthalmic goitre, in which vascular enlargement 
of the thyroid is associated with cardiac symptoms, and other vascular 
disturbances, especially of the head, have suggested that, apart from 
metabolic processes, the circulation in the thyroid may, perhaps in a 
more or less mechanical way, be connected with and influence the cir- 468 
THE METABOLIC PROCESSES OF THE BODY. 
culation in the brain. But the exact nature of this influence has not 
been made clear. 
§ 410. The pituitary body. The lower, posterior, lobe of this organ 
resembles the thyroid body (the upper, anterior, lobe is of quite distinct 
nature, being really a part of the central nervous system). 
Concerning the processes which take place in these alveoli, and the pur- 
poses of the organ as a whole, we know absolutely nothing. 
§411- The supra-renal bodies. A (mammalian) supra-renal body, when 
cut across, is seen to consist of two distinct phrts, an outer thicker cortical 
part, of yellowish color, striated radially, and an inner thicker medullary 
part of dark color. At the depression on the anterior surface called the 
hilus, whence issues the comparatively large supra-renal vein, the cortex 
thins away so that the medulla comes to the surface. These two parts, cor- 
tex and medulla, are not, like the cortex and medulla of a lymphatic gland, 
different arrangements of the same material, but are of essentially different 
nature, and, indeed, are of different origin. The medulla is derived from, is 
a modification of, sympathetic ganglia, while the cortex is derived from 
masses of mesoblastic cells surrounding the great bloodvessels; and in some 
animals the two form wholly separate bodies. The so-called accessory supra- 
renals are composed of cortex alone. 
The whole organ is surrounded by a capsule of connective tissue, free 
from muscular fibres, and not very rich in elastic elements. From the cap- 
sule septa pass inward and form a framework, the cavities of which are filled 
by cells or groups of cells differing in nature and differently arranged in 
the cortex and in the medulla. The middle larger part of the cortex is 
composed of somewhat long solid columns of polyhedral cells, lodged in 
corresponding meshes of the framework. The columns, which are three or 
four cells thick and several cells in length, though somewhat irregular and 
varying in size, do not anastomose, being wholly separated from each other 
by the bars of connective tissue, and possess no central cavity or lumen. 
The bloodvessels, which are abundant in these bars of connective tissue, do 
not penetrate the columns. The cell substance of the cells is of a yellow- 
ish color, often containing yellowish oil globules, and possesses a clear round 
nucleus. 
In the outer part of the cortex immediately underneath the capsule is a 
thin zone in which the groups of cells are not columnar, but rounded and 
irregular; and again in the inner part of the cortex abutting on the medulla 
is another thin zone, in which the columnar arrangement is lost, the cells 
being here disposed in a network of thin cords, and the individual cells to a 
large extent separated from each other by delicate continuations of the 
coarser connective-tissue septa. Hence, the main median part of the cortex, 
which from the prominent columnar arrangement appears striated radially, 
is often called the zona fasciculata, the thin outer part the zona glomerulosa, 
and the thin inner part the zona reticularis; but as far as the essential cha- 
racters of the cells are concerned, all the three zones are alike. 
The medulla also consists of cells or groups of cells lying in the meshes 
of a connective-tissue framework, but the cells are of a different nature from 
those of the cortex. They are irregular and often branched, and their cell 
substance, though it sometimes contains pigment, is generally clear and 
transparent. The medulla, moreover, is further distinguished from the 
cortex by the abundant supply of bloodvessels and of nerves. 
The cells of the medulla and of the inner zone (zona reticularis) of the 
cortex are very apt to undergo change after death, and to become diffluent. 
The arteries which come from the aorta and from the renal and phrenic 
arteries pass into the organ on the surface, and traversing the cortex, supply- STRUCTURES AND PROCESSES OF OBSCURE NATURE. 
469 
ing as they go both capsule and cortex with a moderate number of vessels, 
end in the medulla the connective-tissue bars of which bear numerous large 
venous sinuses, into which the capillaries pour their blood, and from which 
the blood is gathered up into the supra-renal vein. 
A large number of nerves, consisting chiefly of medullated fibres from 
the solar plexus, the renal plexus, the phrenic nerve, and the vagus, pass 
into the supra-renal body at the hilus and on the under surface, and forming 
numerous plexuses, coarse and fine, some carrying small groups of nerve 
cells, end chiefly in the medulla, though some pass on to the cortex. The 
ultimate endings are not yet known. 
The lymphatics are fairly numerous, and form plexuses in the capsule 
and in the connective tissue of the framework; it is stated that the lym- 
phatic vessels surrounding the groups of cells in the cortex communicate 
with spaces between the cells. 
§ 412. Besides the ordinary proteid and other chemical constituents, the 
supra-renal body contains some substance or substances possessing striking 
color reactions, giving a dark-blue or dark-green color with ferric chloride, 
and a carmine-red tint with various oxidizing agents. This substance (whose 
nature is not exactly known, and which is confined to or most abundant in 
the medulla) is not soluble in the ordinary solvents of pigments, such as 
alcohol, ether, chloroform, etc., but is readily soluble in dilute acids. 
Among the extractives, hippuric and benzoic acid, and taurocholic acid 
or taurin have been found, but it is not certain that these are normal con- 
stituents. 
§ 413. Some of the histological features of the supra-renal bodies, 
namely, the groups of cells and their abundant blood-supply, suggest, on 
the one hand, that important metabolic processes take place in them, some 
of which are probably connected with the history of the pigments of the 
body at large. On the other hand, the unusually large nerve-supply, and 
the derivation of part of the body from the sympathetic ganglia, suggest 
peculiar nervous connections. And the organ has often served as a starting- 
point for speculations in these two directions; but our exact knowledge con- 
cerning them is very limited. The results of experiment have taught us 
little; extirpation, for example, has been often followed by the death of the 
animal operated upon, but the cause of the death in such cases is by no 
means clear. 
One fact, gained by clinical experience, is the only real item of know- 
ledge which we possess. Disease of the supra-renal bodies, apparently 
tubercular in nature and beginning in the medulla, is so often associated 
with a change in the color or with an increase of the pigment of the skin- 
"bronzed skin," "Addison's disease "-that some connection between the two 
must exist; but the several links of the chain areas yet unknown. It is 
tempting to associate the increase of pigment in the bronzed skin with the 
chromogen or color-yielding substance spoken of above; but we have no 
warrant for doing so, such, for instance, as any indication of ties between 
the supra-renal bodies and changes either in haemoglobin itself or in bilirubin, 
which two bodies we have reason to regard more particularly as mothers of 
pigment. Moreover the bronzed skin is only one of the symptoms of Addi- 
son's disease, failure of nutrition and nervous symptoms being also present. 
§ 414. The thymus. This, though it arises in the embryo as a paired 
outgrowth from the epithelial walls of a pair of visceral clefts, and thus 
begins as an epithelial structure into which mesoblastic elements subse- 
quently intrude, soon puts on such characters as to appear essentially a 
lymphatic structure, and, indeed, might be regarded as a part of the lymph- 
atic system. 470 
THE METABOLIC PROCESSES OF THE BODY. 
It consists of a capsule of connective tissue, plain muscular fibres being 
absent, and septa or trabeculre of the same nature which divide the organ 
into a number of irregular more or less cylindrical anastomosing follicles 
or lobules, and send finer radiating septa into the interior of each lobule. 
These lobules present the same characters throughout the whole mass of 
the organ, there not being, as in a lymphatic gland, any distinction between 
a cortex and a medulla of the whole body. The words are, however, 
applied to each lobule, to distinguish the central from the peripheral part 
of the lobule itself. Both the central medulla and the peripheral cortex 
of each lobule consist of a framework of reticular connective tissue, which 
in the cortex is identical with or closely allied to adenoid tissue, but in the 
medulla is coarser and more open and to a larger extent composed of 
branched anastomosing epithelioid cells. The meshes of the cortex are 
crowded with leucocytes, but these are much less abundant in and more 
easily fall out of the medulla, so that in sections the medulla appears more 
transparent than the cortex. It will be observed that this arrangement is 
almost the reverse of that obtaining in the alveolus of a lymphatic gland, 
in which the finer gland substance with its adenoid tissue crowded with leu- 
cocytes is placed in the centre, surrounded by the more open network of 
the lymph sinus. 
The bloodvessels of the thymus running along the septa form capillary 
networks which, though closer and more abundant in the cortical than in 
the medullary portions of the lobules, have no such special arrangements 
as obtains in lymphatic glands. 
Lymphatic vessels, abundant in the capsule and septa, are undoubtedly 
in connection with the substance of the lobules. 
The medullary substance frequently contains bodies, known as " con- 
centric capsules," nests of concentrically disposed nucleated flattened epi- 
thelial or epithelioid cells. They appear to arise from a proliferation of 
the epithelioid cells lining small bloodvessels, and have been supposed to 
be connected with the degenerative changes by which, with obliteration of 
the vessels, the whole organ dwindles away soon after birth. 
§ 415. From the thymus there may be extracted by means of saline 
solution a form of globulin or a proteid allied to globulin which, like the 
corresponding bodies from lymphatic glands or from leucocytes, seems to 
have some special relations to the formation of fibrin. Thus, as has already 
been said (§ 22), a sojution of this globulin-like body from the thymus, 
injected into the veins will give rise to extensive intra-vascular clotting. 
The thymus, like the other bodies on which we are now dwelling, is 
also rich in extractives. Thus xanthin, hypoxanthin, leucin, lactic, suc- 
cinic and other acids have been found in it. 
But of what really takes place in the body we have no exact knowledge. 
Since the thymus is best developed before birth, disappearing after birth at 
a rate which varies much in different individuals and still more in different 
kinds of animals, and being eventually replaced by fat and connective 
tissue, it is obvious that its chief functions are in some way associated with 
events taking place before birth or in early life. 
The History of Fat. Adipose Tissue. 
§ 416. Globules of fat of various sizes make their appearance in the 
very elements of most of the tissues, in muscular fibres, in epithelial cells, 
in nerve cells, in leucocytes, and so on ; and the medulla of medullated 
nerves consists largely of a peculiar fatty material. Besides this certain 
cells of connective tissue at various times, and in various places, become so THE HISTORY OF FAT. ADIPOSE TISSUE. 
471 
loaded with fat that groups of the cells become practically masses of fat. 
Connective tissue thus loaded with fat is called adipose tissue; and masses 
of adipose tissue of all manner of sizes and of shapes adapted to the several 
situations are found in various parts of the body. Many of the internal 
organs, more especially the kidneys, are wrapped in adipose tissue; but the 
largest deposit is one lying in the subcutaneous tissue, sometimes called the 
"panniculus adiposus"; and a "fat" body is distinguished from a "lean" 
body chiefly, though by no means exclusively, by the amount of subcuta- 
neous adipose tissue. 
Of all the tissues of the body adipose tissue is the most fluctuating in 
bulk; within a very short space of time a large amount of adipose tissue 
may disappear, and within an almost equally short time the quantity 
present in a body may be several times multiplied. When too much or too 
little food is given it is the subsequent adipose tissue which first and most 
rapidly increases or decreases in bulk. 
§ 417. A small piece of adipose tissue, examined under a low power, 
appears to be made up almost entirely of rounded masses of highly refrac- 
tive material, closely packed together. These rounded masses, which stain 
an intense black with osmic acid and give other reactions of fat, are 
arranged in irregular lobules ; between the lobules, and between the indi- 
vidual rounded masses, may be seen a small amount of fibrillated connec- 
tive tissue carrying bloodvessels. 
When the tissue has been hardened and stained, and the fat has been 
removed by solvents, what was previously only visible as a rounded mass 
of fat is now seen, under higher powers, to be a cell, but a cell nearly the 
whole of the cell substance of which has become transformed into a single 
large vacuole. Over the greater part of the circumference of the cell the 
cell substance is reduced to a mere thin shell or envelope, or cell mem- 
brane, but at one part a thicker disc-like remnant is seen, and in this is 
placed a rounded or oval, often flattened nucleus. Between these fat-cells 
may be seen a few bundles of connective tissue forming a scanty loose net- 
work, the rounded meshes of which are occupied by the fat-cells, the matrix 
of the bundles appearing at places continuous with, or adherent to, the 
envelopes of the cells; ordinary connective-tissue corpuscles are also here 
and there present, though rarely visible between the larger, 50,a to 130/z, 
fat-cells. In injected specimens it is further seen that the connective-tissue 
meshwork carries small bloodvessels, which form capillary networks around 
the groups of fat-cells and even around individual cells. After death, upon 
cooling, the fat in the fat-cells may solidify in crystals. 
It is obvious that a fat-cell is a cell belonging to connective tissue, in the 
cell substance of which fat has been collected to such an extent that the cell, 
which increases largely in bulk during the process, is almost wholly trans- 
formed into a large vacuole filled with fat, the cell substance being reduced 
to a thin envelope of the vacuole, thickened at one part where the nucleus, 
thrust on one side by the gathering fat, is placed. Adipose tissue is a col- 
lection of such fat-cells held together by a meagre quantity of vascular con- 
nective tissue. 
By studying the development of adipose tissue in the embryo or else- 
where, we may trace out the steps of the formation of the fat-cells. In the 
embryo, in a situation where adipose tissue is about to be formed, the con- 
nective tissue is seen to contain a number of small nucleated cells, rounded 
or somewhat irregular in form, the cell substance of which at first presents 
no special characters, and contains not more than what may be called the 
ordinary amount of fat globules or spherules. Very soon, however, these 
minute drops or specks increase in number, the cell substance at the same 472 
TIIK METABOLIC PROCESSES OF THE BODY. 
time increasing in bulk while remaining round or becoming more distinctly 
so, and the smaller drops run together into larger ones [Fig. 118]. This 
goes on ; the fat increasing in quantity coalesces more and more, and the 
cell, as a whole, becomes larger and larger, the cell substance at first keep- 
ing up in bulk with the increasing fat, but subsequently ceasing to increase, 
being apparently used up in the formation of the fat. Thus the original 
small "protoplasmic" cell is at last transformed into the larger fat-cell, all 
the fat having run together into a vesicle the envelope of which, thickened 
on one side to carry the nucleus, is furnished by the remnant of the cell 
substance. In some cases, the nucleus instead of being pushed early on one 
side, remains central though the collection of fat has become considerable; 
it is, however, eventually displaced. The whole process appears very similar 
to the deposition of mucin in the cells of a mucous gland 205) ; and we 
[Fig. 118. 
Deposition of Fat in Connective-tissue Cells: f, a cell with a few isolated fat-droplets in its 
protoplasm; f, a cell with a single large and several minute drops ; f, fusion of two large drops: 
g, granular or plasma cell, not yet exhibiting any fat deposition ; c. t., flat connective-tissue cor- 
puscles; c. c., network of capillaries.] 
may by analogy infer that the fat-cell becomes a fat-cell by the cell manu- 
facturing fat in some way or other, and depositing the fat so formed in 
the interstices of its substance. The most striking superficial distinctions 
seem to be that in the mucous cell the granules or spherules remain discrete 
within the cell, being separated by bars of cell substance, whereas in the fat- 
cell the globules, as they form, run together until at last they unite into a 
single mass; and further that while in the mucous cell, even when most 
heavily loaded, a relatively large amount of active cell-substance still re- 
mains, in the fat-cell a mere remnant is left and that chiefly surrounding the 
displaced nucleus. 
Some observers are of opinion that the cells belonging to connective tissue 
which thus become fat-cells of adipose tissue belong exclusively to the kind 
which we spoke of as plasma cells, but this is doubtful. Others again, while 
admitting that the cells which become fat-cells resemble in appearance or- 
dinary connective-tissue corpuscles and may like them be branched, believe 
them nevertheless to constitute a special kind of connective-tissue corpuscle, 
being led to this view by the fact, that though adipose tissue is very generally 
distributed throughout the connective tissue of the body, it is apt to appear 
in particular situations, rather than in others, and in some tracts of connec- 
tive tissue never under normal circumstances makes its appearance. Others 
again maintain that, under favorable circumstances, any connective tissue 
corpuscle may become a fat-cell. THE HISTORY OF FAT. ADIPOSE TISSUE. 
473 
The fat in the interior of bones forming the yellow marrow appears to 
have the same general structure and to be formed in the same way as the 
rest of the adipose tissue. 
$ 418. The fat thus deposited in a fat-cell sooner or later disappears. It 
is not injected bodily into the surrounding lymph-spaces of the connective 
tissue, but passes away either into the blood stream or into the lymphatics 
by some processes not as yet fully understood. The shell of cell substance 
which forms the envelope of the fat-cell is probably of a differentiated nature, 
and may have properties which assist the escape of the fat; but on this 
point we have no exact knowledge. The disappearance of the fat appears 
to take place in two different ways. On the one hand, and this perhaps is 
the more ordinary method, the fat gradually disappears, little by little, and 
the rounded distended vesicle gradually assumes the characters of a connec- 
tive-tissue corpuscle, even of a branched one. On the other hand, especially 
when the disappearance is rapid and total, the space previously occupied by 
fat becomes filled with a clear fluid resembling lymph, the fat vesicle being 
transformed into a lymph vesicle. This condition, however, is temporary 
only, the lymph is subsequently absorbed and the vesicle shrinks. At times 
the emptying of the cell, whether by the one method or the other, is followed 
by a rejuvenescence of the cell, the nucleus by division gives rise to several 
nuclei, and the cell divides into new cells, each of which may, under appro- 
priate conditions, develop again into a fat-cell. 
§ 419. The fat thus lodged in adipose tissue varies somewhat in composi- 
tion in various animals, but is chiefly composed of olein, palmitin, and stearin 
in varying proportions, with small quantities of the glycerin compounds of 
such fatty acids as butyric, capronic, caprylic, etc., together with a little 
lecithin and cholesterin. The " fat " of one animal, that is, the fat thus con- 
tained in adipose tissue, differs from the fat of another animal partly by the 
presence of more or less of one or more of these less abundant fats, but 
chiefly by the proportion in which the three main fats, olein, palmitin, and 
stearin, are respectively present in the mixed fat. The melting-points of 
these three fats being different, the melting-point of the fat of the body will 
differ according to the relative proportions in which the three are present. 
Thus the subcutaneous fat of man melts at from 15° to 22° or higher, the 
fat round the kidney being firmer and not melting until 25° ; the fat of the 
dog melts at about 22°, that of the goose at about 25°, of the ox at about 
40°, and of the sheep at 50°, the less resistant fat of the man and dog con- 
taining relatively more olein than that of the ox or of the sheep. 
§ 420. When we come to consider the question, By what processes does 
the fat make its appearance in the fat-cell ? we are brought face to face with 
much the same kind of problem as that which occupied us in dealing with 
glycogen. On the one hand we may suppose that the fat is brought to the 
fat-cell as fat and is in some way taken up by the cell and deposited in the 
cell substance with little or no change. On the other hand, we may suppose 
that the fat is manufactured by the fat-cell, in some such way as mucin or 
pepsin is manufactured by a mucous or a gastric cell, out of and by means 
of its cell substance, and that the process of fattening, or of producing fat in 
fat-cells, consists essentially in feeding and so building up the cell substance 
which subsequently breaks down into fat, and does not consist merely in 
bringing fat within reach of the cell. Which of these views is the true one, 
or how far are both these operations carried on in the animal body? 
In support of the latter view it may be urged that, not only the more 
complex living substance, but, as we have more than once urged, the simpler 
proteid constituent of living substance, obviously contains what we may call 
a fatty radicle, so that we might expect fat to be formed out of its metab- 474 
THE METABOLIC PROCESSES OF THE BODY. 
olism. And as a matter of fact not only in adipose tissue, but in every 
part of the body, living substance is continuously giving rise to and tempo- 
rarily depositing in itself some amount of fat, and in what is known as fatty 
degeneration there seems to be evidence of the formation of fat out of 
proteid material. 
On the other hand, we have traced the fats taken as food, and found that 
they pass with comparatively little change from the alimentary canal, chiefly 
through the intermediate passage of the lacteals, into the blood, from which 
they rapidly disappear after a meal. We might infer from this that an 
excess of fat thus entering the blood would naturally be disposed of by 
being simply stored up in the available adipose tissue without any further 
change ; we can imagine that the fat not immediately wanted by the 
economy passes in some way from the blood to the connective tissue (the 
white blood-corpuscles which appear loaded with fat after a meal possibly 
acting as intermediaries), and that the connective-tissue corpuscles swallow 
the fat brought to them after the fashion of an amoeba, not digesting it but 
simply keeping it in store until it is wanted elsewhere. 
What do experiments teach on this matter? 
In the first place, it is evident that in an animal fattened on ordinary fat- 
tening food, only a small fraction of the fat stored up in the body can possi- 
bly come direct from the fat of the food. Long ago in opposition to the 
views of Dumas and his school, who taught that all construction of organic 
material, that all actual manufacture of living substance or even of its 
organic constituents, was confined to vegetables and unknown in animals, 
Liebig show'ed that the butter present in the milk of a cowr was much greater 
than could be accounted for by the scanty fat present in the grass or other 
fodder she consumed. He also urged as an argument in the same direction, 
that the wax produced by bees, which though having a different composition 
from fat may be used as an analogy, is out of all proportion to the wax or 
allied bodies contained in their food, consisting as this does chiefly of sugar. 
And it has since been shown in many ways that, in fattening animals, the 
fat accumulated in the body cannot be accounted for by the fat which has 
been taken in the food. It has been proved by direct analysis. Thus of 
two young pigs, as much alike as possible, of the same litter, one was killed 
and analyzed, the amount of fat in the body being among other things deter- 
mined. The other was fattened for a certain length of time on food whose 
composition was known, and then killed and analyzed. It was found that 
for every 100 parts of fat in the food 472 parts of fat were stored up in the 
body during the fattening period. It is clear that fat may be formed in the 
body out of something which is not fat. 
§ 421. There are two possible sources of this manufactured fat. The 
carbohydrates of the food form one source. In treating of digestion (§ 243), 
we referred to the possibility of carbohydrates during digestion in the alimen- 
tary canal becoming by fermentation converted into butyric acid ; and we 
suggested that higher and more complex members of the same fatty acid series 
might be obtained out of carbohydrates by somewhat analogous changes, 
carried on, however, not in the alimentary canal by means of foreign organ- 
ized ferments, but in the tissues through the activity of the tissues them- 
selves. We cannot as yet trace out the steps nor can we definitely point to 
any particular tissues other than the fat-cells themselves as the seats of any 
such changes. But there can be no doubt that the carbohydrate material 
does in some way or other give rise to fat. A carbohydrate diet is the kind 
of diet most efficacious in producing an accumulation of fat in the body ; 
sugar or starch, in some form or other, is always a large constituent of 
ordinary fattening foods. 475 
THE HISTORY OF FAT. ADIPOSE TISSUE. 
Another source of fat is to be found in the proteids. We have seen that 
the urea of the urine practically represents the whole of the nitrogen which 
passes through the body. Now in any given quantity of urea, the amount 
of carbon is far less than that found in the quantity of proteid containing 
the same amount of nitrogen. Thus the percentage composition of the two 
being respectively, 
Carbon. 
Hydrogen. 
Oxygen. 
Nitrogen. 
Sulphur. 
Urea 
. . . . 20.00 
6.66 
26.67 
46.67 
Proteid . . . 
. ... 53 
7.30 
23.04 
15.53 
1.13 
100 grms. of urea contain about as much nitrogen as 300 grins, of proteid ; 
but the 300 grms. of proteid contain 139 grms. (159 - 20) more carbon than 
do the 100 grms. urea. Hence the 300 grms. of proteid in passing through 
the body and giving rise to 100 grms. of urea, would leave behind 139 grms. 
of carbon, in some combination or other ; and this surplus of carbon, if the 
needs of the economy did not demand that it should be immediately con- 
verted into carbonic acid and thrown off from the body, might be deposited 
somewhere in the form of fat. It has been calculated that in this way 100 
grms. of proteid food might furnish 24 grms. of fat. We have already seen, 
in treating of the action of the pancreatic juice (§ 218), that there is evi- 
dence of a fatty element (viz., leucin, which is amido-caproic acid, and so 
belongs to the fatty acid series) being thrown off from the complex proteid 
compound in the very process of digestion ; and though, as we have said, we 
have no proof that this action of pancreatic juice takes place largely in the 
normal body, its value as an example is none the less important. 
Some observers have pushed this view of the production of fat out of 
proteids so far as to insist that all the fat formed in the body arises in this 
way out of proteid material, and that when carbohydrate food gives rise to 
the formation of fat it does so by shielding from oxidation the carbon moiety 
of the proteid food taken at the same time, thus permitting it to be stored 
up as fat. The carbohydrate itself, they argue, never becomes fat but its 
presence allows fat to be formed out of proteid material. This view has 
obviously a very important economical bearing, since, if it be true, it is use- 
less to increase the carbohydrate material of food for the purpose of fatten- 
ing, unless a sufficient proportion of proteid material be given at the same 
time. 
The view, however, has been proved to be untenable by several investiga- 
tions carried out on different animals. It has been shown than an animal 
rapidly fattened on a diet consisting of proteids with much carbohydrate will 
store up far more fat than can possibly be accounted for by the proteids of 
the diet. Thus a dog, the fat in whose body had been reduced to a minimum 
by starvation, was fed for a period on measured quantities of proteids and 
carbohydrates, and killed. The amount of fat found after death in his body, 
making full allowance for the fat which remained after the starvation and 
for the fat accompanying the proteids in the meat given as food, was found 
to be far more than could be supplied by the carbon in the proteids of the 
food, even supposing that every jot of those proteids which did not go to 
make up the increase of the proteid " flesh " of the body taking place during 
the fattening was used for the purpose of forming fat. Similar experiments 
on geese and pigs have led to similar results; and if fat be formed in this 
way in the bodies of carnivora and omnivora, we may be sure that the same 
holds good for the bodies of herbivora. We may therefore conclude that fat 
can be constructed in the body on the one hand out of proteid material and 
on the other hand by some direct conversion of carbohydrates. 476 
THE METABOLIC PROCESSES OF THE BODY. 
§ 422. It is clear then that a construction of fat does occur in the body 
somewhere. What limits can we place on the degree to which this construc- 
tion is carried ? When the food contains sufficient actual fat to account for 
the fat stored up in the body, does any construction of fat take place? In 
the first place we find that when the food contains abnormal fats such as are 
not present in the body, spermaceti for instance, or erucin (from rape-seed 
oil), these fats are not to be found, or are found in very small quantity, in 
the fat which is stored up in the body as a consequence of a large supply of 
that food. In the second place we may call to mind the statement previously 
made, that the composition of fat varies in different animals. The fat of a 
man differs from the fat of a dog, even if both feed on exactly the same food, 
fatty or otherwise. Were the fat which is taken as food stored up as adi- 
pose tissue directly and without change, recourse being had to other sources 
of food for the construction of fat only in cases where the fat in the food was 
deficient, we should expect to find that the nature of the fat of the body 
would vary greatly with the food. So far from this being the case, direct 
experiments show that the fat of the dog is, as far as composition is con- 
cerned, very largely independent of the food, that the normal constituents 
of fat make their appearance very much as usual, and in very much their 
appropriate proportion, though their proportion in the food may largely vary, 
and though some of them may be wholly absent. Thus in one experiment 
the fat of the body contained considerable quantities of stearin after a diet 
free from stearin, and in another preserved the normal amount of olein after 
a diet free from olein. 
Of course it is quite possible that in such cases as these, though the 
stearin, or the olein, when absent from the food, was in some way or other 
constructed anew, yet at the same time those constituents which were present 
were simply stored up; and the small quantity of erucin present in the fat 
of the body after feeding on erucin must have been directly stored up. So 
also, when an animal is rapidly fattened on a diet consisting of a small 
quantity of proteid and a large quantity of fat, the amount of fat stored up 
may be too great to have come from the proteids of the diet, in which case we 
may infer that it was the actual fat of the food simply deposited in the fat-cells 
of the body. But even in this case, as more distinctly in the others, it is also 
open for us to suppose that all the fat taken as food was in some way or other 
disposed of, and that all the new fat which made its appearance was constructed 
anew. And the latter view is more perhaps in harmony with the histological 
facts previously mentioned, as well as supported by other considerations. 
At the present, however, we may be content with the following conclu- 
sions: 1. Fat is actually formed in the animal body, and the fat present at 
any moment in the body is not exclusively, if at all, fat merely stored up 
from the fat of the food. 2. The carbon elements of the newly-formed fat 
may be supplied either from carbohydrate food, or from the carbon surplus of 
proteid food, or from fats taken as food which are not the natural constitu- 
ents of the body-fat. 3. The fat stored up appears as fat granules or drops 
deposited in the cell substance of certain cells, and the increase of the fat in 
the cells is accompanied first by a growth, and subsequently by a consump- 
tion of the cell substance; but, as in the analogous case of glycogen, there 
is no complete evidence to show whether the fat granules which appear are 
simply deposited by the cell substance in a more or less mechanical manner, 
without their forming an integral portion of that cell substance, the chief 
stages of the manufacture of the fat having been gone through elsewhere, or 
whether they arise from a breaking up, a functional metabolism of the cell 
substance of the fat-cell itself; the latter view is on the whole, however, the 
more probable. TIIE MAMMARY GLAND. 
477 
§ 423. Since milk is a secretion, and indeed an excretion, the mammary 
gland ought not to be classed as a metabolic tissue, in the limited meaning 
we are now attaching to those words. Yet the metabolic phenomena giving 
rise to the secretion of milk are so marked and distinct, have so many 
analogies with the purely metabolic events which take place in adipose 
tissue, and so strikingly illustrate metabolic events in general, that it will 
be more convenient to consider the matter here, rather than in any other 
connection. 
The mammary gland, formed like a sweat gland, of which it maybe con- 
sidered an extreme development, by an ingrowth of the Malpighian layer of 
the epidermis, is a compound racemose gland, constructed after the general 
plan of such a gland and thus composed of branching ducts, ending in 
secreting alveoli. 
§ 424. The appearances presented by the alveoli differ widely according 
as the gland is one which is being used for suckling, or is one in a resting or 
dormant condition, that is to say, before any pregnancy at all has taken 
place or in the interval between two suckling periods. In the suckling gland 
each alveolus consists of a basement membrane, presenting the usual charac- 
ters, lined with a single layer of cells leaving a wide lumen ; but the appear- 
ances presented by the cells differ from time to time according to circum- 
stances and are not the same in all the alveoli at the same time. We may, 
however, distinguish two conditions which, since they seem to correspond to 
the loaded and discharged conditions of an ordinary gland, we may call the 
loaded and the discharged phase respectively, conditions intermediate between 
the two being met with. 
In the discharged phase the alveolus is lined by a layer of low cubical 
or even flattened cells, so that the relatively large area of the alveolus is 
almost wholly occupied by the lumen in which some of the constituents of 
the milk may still be retained. Each cell consists of granular cell substance 
in which is placed a rounded or oval nucleus. Sometimes the free edge of 
the cell is jagged and uneven as if a portion of the free border had been 
torn away. 
In a fully loaded phase the appearances are very different. The alveolus 
is now lined with a layer of tall columnar cells projecting unevenly into the 
lumen, the outline of which is correspondingly irregular and the area of 
which is much reduced. While the broader base of each cell rests on the 
basement membrane, the other end, conical or irregular, stretches toward the 
centre of the lumen. Instead of one nucleus, two or even more are now 
present, one well formed and normal being placed nearer the base, and the 
others, often showing signs of breaking or degeneration, nearer the free end. 
Sometimes constrictions are seen whereby the free peripheral portion of the 
cell, including one or more of the nuclei, is apparently being separated from 
the basal portion in which the remaining nucleus is lodged ; and occasionally 
portions or fragments of cells, nucleated or nucleusless, may be seen lying 
in the cavity of the alveolus. In the cell substance, especially toward the 
free border of the cell, are numerous oil globules of various sizes as well as 
granules or particles of other nature : some of the larger oil globules may be 
seen projecting from the surface as if about to be extruded from the cell ; 
and in the cavity of the alveolus oil globules with a thinner or thicker coat- 
ing of cell substance are frequently present. 
Between such a fully loaded phase and a completely discharged phase 
various intermediate conditions may be observed, the cells being of greater 
or less height, containing one nucleus only or more than one, the cell sub- 
The Mammary Gland. 478 
THE METABOLIC PROCESSES OF THE BODY. 
stance occupied with a few or with many oil globules and other granules, 
and the free border more or less jagged. 
§ 425. The dormant resting mammary gland, that for instance of an 
animal which has never been pregnant, is much smaller than a suckling 
gland, owing to the alveoli being both smaller and less numerous. Each 
alveolus, moreover, is not a cavity lined with a single layer of epithelium, but 
a solid cylinder or mass of comparatively small, rounded or polyhedral 
cells. So long as pregnancy does not occur the growth of these is exceed- 
ingly slow, and the products of such metabolism as goes on in them are 
carried away by the blood, so that under normal circumstances no secretion 
takes place. 
When pregnancy occurs rapid growth of the mamma takes place, numer- 
ous new alveoli being formed by budding, but all for a time remaining solid 
cylinders of cells. At the approach of the birth of the offspring, the 
central cells undergo metabolic changes, especially a fatty transformation, 
and either before or after birth are cast off, leaving a single layer to line the 
alveoli and to carry on the work of secretion as described above. It is 
generally supposed that these shed cells supply the so-called " colostrum cor- 
puscles" characteristic of the first milk, of which we shall speak presently. 
At the end of lactation an absorption of some of the alveoli takes place, 
and in old age still further absorption goes on with great diminution of the 
lumina. 
§ 426. The connective tissue, joining together the lobules of various 
sizes, surrounding the lobulesand running in between the projecting blind 
ends of the alveoli within the lobules, is rich in bloodvessels, which form 
capillary networks round the alveoli; it also carries a considerable number 
of lymphatic vessels which arise in lymph-spaces around the alveoli and else- 
where. Leucocytes are numerous in the spaces of this connective tissue, 
and some of them may make their way through the basement membrane 
and between the secreting cells into the cavities of the alveoli and so appear 
in the milk. 
§ 427. The nature of milk. Human milk has a specific gravity of from 
1028 to 1034, and when quite fresh possesses a slightly alkaline reaction. 
It speedily becomes acid ; and cow's milk, even when quite fresh, is some- 
times slightly acid, the change of reaction taking place during the stagna- 
tion of the milk in the mammary ducts. 
The constituents of milk are : 
1. Proteids, viz., casein, and an albumin, agreeing in its general features 
with ordinary serum-albumin, but which, since it is said to differ somewhat 
in its solubilities and rotary power from serum-albumin, has been called 
lactalbumin. The casein, as we have seen (§ 193), undergoes through the 
action of rennin a change whereby insoluble casein (tyrein) makes its ap- 
pearance and the milk is curdled. Casein may, however, be precipitated in 
an unchanged form by saturating milk with neutral salts, or by the careful 
addition of acetic acid to diluted milk, or by first adding to the diluted milk 
a slight quantity of acetic acid and then passing through it a stream of 
carbonic acid. In the filtrate the presence of the lactalbumin, which occurs 
in small and variable quantities, may be shown by coagulation with heat, or 
by precipitation with potassium ferrocyanide, etc. In the process of curdling 
the casein, as stated in § 193, appears to be not simply changed into tyrein, 
but to be split up into tyrein and into another proteid, which unlike the 
lactalbumin is not coagulated by heat and which appears to be allied to 
peptone or albumose. This or a similar peptone-like body has also been 
found in small quantities even in milk which has not curdled ; it has been 
called lactoprotein. The lactalbumin, though coagulated by heat when 479 
THE MAMMARY GLAND. 
isolated, is not so coagulated as it exists in the natural milk, the alkalinity 
of the milk, which is increased by boiling, preventing this. Similarly 
casein, though coagulated by heat when simply suspended in water after 
being precipitated, is not coagulated by heat when it exists in a natural con- 
dition in milk ; in these respects casein behaves like alkali-albumin, which it 
resembles in other features also. Hence milk when boiled does not coagu- 
late as a whole, though in the superficial layers exposed to the air changes 
take place by which a film or skin, derived chiefly from the albumin but 
partly from the casein, appears on the surface ; if this be removed a fresh 
portion undergoes the same change. 
2. Fats. These are, in the main, palmitin, stearin, and olein ; but other 
fats, supplied by butyric and other fatty acids in combination with glycerin, 
accompany the above in small quantities. In this respect the fat of milk 
resembles that of adipose tissue. Lecithin and cholesterin are also present 
in very small quantity, as well as a yellow coloring matter. The fat present 
in milk differs in different animals as to the relative proportion of olein, 
palmitin, and stearin, and as to the kinds and relative amount of the other 
scantier fats. 
The mixture of these fats, fluid at ordinary temperatures, is present in 
natural milk in the form of globules of various sizes but for the most part 
exceedingly small (in man from 2 jj. to 5 //). JMilk is in fact a typical emul- 
sion, and it is the presence of the casein in the milk which brings about the 
emulsion. 
On standing a great deal of the fat collects on the top of the milk in the 
form of cream, but in this, as in the butter which is formed from it, the 
globules are still discrete, so long at least as the butter is "fresh." By the 
use of a centrifugal machine nearly the whole of the fat may be separated 
from the plasma. 
3. Milk sugar or lactose. This is very apt to undergo fermentation into 
lactic acid, through the agency of an organized ferment; the milk thus 
becomes sour, and the casein is precipitated in a flocculent form when the 
acid is produced in sufficient quantity. Since the change will take place 
even when every care is taken to exclude germs from the atmosphere having 
access to the milk, the organized ferments must be present in the milk in the 
ducts of the gland. 
4. Salts. Though traces of urea and kreatinin have been noted by some 
observers, the extractives of milk, beyond the lecithin and cholesterin al- 
ready mentioned, are insignificant. The salts are of more importance; 
these are chiefly calcic phosphate, of whose function in the process of curd- 
ling we spoke in § 193, and potassic and sodic chlorides, with a small 
quantity of magnesic phosphate. Sulphates appear to be absent. A small 
quantity of an iron salt is present, and traces of sulphocyanide have 
been observed. Besides the phosphorus in the actual form of phosphates, 
milk contains a further considerable quantity of phosphorus in the pro- 
teids and in the nuclein, as well as some sulphur in the former. The 
inorganic constituents of milk may, broadly speaking, be said to differ 
distinctly from those of blood, and to much more nearly resemble those 
of the entire body. 
The composition of milk in the same animal varies widely from time to 
time, and besides undergoes marked changes during the period of lactation. 
The relative general composition of human milk and that of the cow, the 
mare, and the bitch may perhaps be shown by the following table; but it is 
difficult to draw an average, since the individual analyses given differ so 
much ; the figures given for casein and fat in the milk of the bitch may be 
unusually high. 480 
THE METABOLIC PROCESSES OF THE BODY. 
Average Composition of Milk in Different Animals. 
Woman. 
Cow. Mare. 
Bitch. 
Casein, etc 
o 
4 2.5 
10 
Fats 
2.75 
4 2 
10 
Sugar 
... 5 
4.4 5 
3.5 
Salts 
0.25 
0.6 0.5 
0.5 
Total Solids .... 
10 
13 10 
24 
W ater 
90 
87 90 
76 
The quantity of milk secreted by a woman in twenty hours at the height 
of lactation has been calculated at 700 to 800 c.c. A good milch cow will 
yield about 10 litres of milk per diem. 
§ 428. Colostrum. This is the name given to the milk secreted at the 
beginning of a period of lactation, just before and for some days after par- 
turition. This milk differs from the subsequent milk in microscopical cha- 
racters and in chemical composition. 
When ordinary milk is examined under the microscope hardly anything 
is seen besides the fat globules except a very few imperfect cells or portions 
of cells, consisting of cell substance more or less loaded with fat and con- 
taining sometimes a more or less altered nucleus. A few minute granules, 
thought by some to be particles of suspended casein or nuclein, are, however, 
also visible. 
Colostrum, on the other hand, contains a large number of cells or cor- 
puscles, which have been called " colostrum corpuscles." Some of these 
closely resemble leucocytes, others are either cells of about the same size, 
round or irregular, and possessing a nucleus, often misshapen, or are merely 
portions of cell substance without a nucleus. In all of them the cell sub- 
tance may be loaded with fat globules or may be fairly free from fat. Some 
of these cells appear to be undergoing disintegration ; some may at a favor- 
able temperature exhibit slow amoeboid movements, and must then at least 
be regarded as living. 
Colostrum also differs from ordinary milk in containing not only a large 
quantity of albumin (lactalbumin), but also a decided amount of globulin. 
In consequence of this, colostrum differs from milk, inasmuch as it is dis- 
tinctly coagulated by heat. 
As stated above, during the rapid growth by which the gland is enlarged 
preparatory to lactation, the alveoli are at first solid masses of cells with 
little or no lumen, and a lumen is established subsequently by the discharge 
of the central cells. It is usually supposed that the cells so discharged, 
some undergoing much, others comparatively little, change, supply the colos- 
trum corpuscles just spoken of, and at the same time furnish the globulin 
and excess of albumin also characteristic of colostrum. But this is not cer- 
tain. The alveoli at this time contain peculiar cells resembling colostrum 
corpuscles except that they are free from fat; and it is suggested that these 
being discharged and taking up fat in amoeboid fashion become colostrum 
corpuscles. Some regard tbe colostrum corpuscles as simply leucocytes 
which have similarly taken up fat. 
§ 429. The mammary gland is present both in the female and the male 
child at birth ; and in both sexes at and for a few days after birth is thrown, 
in common with all the other secreting glands, into secretory activity, and a 
small quantity of milk, the " witches' milk," so called by the Germans, is 
discharged from the nipple. The milk resembles in all essential features 
the milk of lactation. In both sexes this initial activity soon passes off, the 
gland in the female further developing at puberty, but in the male remain- 
ing, save in exceptional cases, in its infantile condition or somewhat retro- 
grading. THE MAMMARY GLAND. 
481 
§ 430. The secretion of milk. From what has been already said, it is 
obvious that the secretion of milk, while resembling the secretion of the 
other secreting glands which we have studied in being essentially an activity 
of the epithelial cells lining the alveoli, nevertheless presents certain inter- 
esting features special to itself. If the account given in § 424 be a true 
one, morphological changes in the cells are more prominent than in the case 
of other glands; and we may interpret the appearances there related some- 
what as follows: When the discharged gland with its low epithelium begins 
the work of loading, the cells distinctly " grow." Their cell substance in- 
creases in bulk and, elongating, projects into the lumen of the alveolus. At 
the same time the nucleus divides as if the cell were about to give birth to 
new cells; but at first, at all events, no division of the cell substance takes 
place, and the new nuclei lie imbedded in a common cell body. The cell 
substance meanwhile puts on secretory activity ; it deposits in itself material 
to form milk. The deposit of fat is conspicuous and easily recognized, but 
we may fairly infer that the other less easily distinguished proteid and car- 
bohydrate materials are deposited in the cell substance in a similar fashion. 
Then follows the ejection of the prepared material, and this may take place 
in one of two ways. The oil globules of fat may be extruded from the cell 
substance much in the same way that an amoeba extrudes its excrement, and 
possibly other constituents of milk may be ejected by a similar method. 
But, besides this, the deferred cell division now takes place in a somewhat 
imperfect fashion, so that portions of the old cell carrying nuclei with them 
come asunder from the rest of the cell in which a nucleus is left, and lie 
loose in the lumen of the alveolus; portions of cell substance free from 
nuclei appear also to be cast off. Here, in the lumen of the alveolus, they 
rapidly undergo change ; the cell substance is altered and dissolved, and its 
load of prepared material, probably undergoing in the act some further 
change, is set free, the nuclei also undergoing change and becoming ulti- 
mately broken up. Hence the constituents of milk are provided for, not 
only as in other glands by the material with which the cell loads itself and 
subsequently discharges into the lumen of the alveolus, but also by the 
actual substance of part of the cell itself. The characteristic nuclein of the 
milk has thus its origin in all probability in the shed nuclei of the secreting 
cells, and we may perhaps infer that the still more characteristic casein 
exists in milk in the form of casein and not of some other proteid in conse- 
quence of this intervention of the actual cell substance in the formation of 
the milk. 
§ 431. The secretion of milk then would appear to illustrate, even more 
fully and clearly than do other glands, the truth on which we have so often 
insisted, that a secretion is eminently the result of the metabolic activity of 
the secreting cell. The blood is the ultimate source of milk, but it becomes 
milk only through the activity of the cell, and that activity consists largely 
in a metabolic manufacture by the cell, and in the cell, of the common things 
brought by the blood into the special things present in the milk. Experi- 
mental results tell the same tale. Thus the quantity of fat present in milk 
is largely and directly increased by proteid, but not increased-on the con- 
trary diminished-by fatty food. This effect on the mammary gland in 
particular is in accordance with what we shall presently learn to be the 
general effect on the body of proteid in contrast to that of fatty food ; 
proteid food seems to increase the general metabolic activity of the body, 
while fatty food tends to lessen it. Moreover the proteid food seems actually 
to furnish the fat; and we have already suggested a manner in which pro- 
teids may give rise to fat. That the fat of the milk need not necessarily 
come from the fat of the food is shown by the following experiment: A bitch 482 
NUTRITION. 
fed on meat for a given period gave off more fat in her milk than she could 
possibly have taken in her food ; and this moreover took place while she was 
gaining in weight and " laying on fat," so that she could not have supplied 
the mammary gland with fat by simply transferring fat from the store pre- 
viously existing in the adipose tissue of her body ; she apparently obtained 
the fat ultimately from the proteids of her food. And the histological facts 
given above favor the view that the formation of fat out of proteids in such 
cases takes place in the cells of the alveoli. The experimental then as well 
as the histological evidence goes to show that the fat of milk is formed in the 
cell and by the cell, and is not simply gathered out of the blood. 
The casein in a similar way seems to be formed by the action of the cell. 
It cannot be gathered out of the blood, since the blood contains no real 
casein ; it must be formed in the gland. Some observers have maintained 
that when milk is kept at 35°, the casein is increased through some ferment 
action taking place in the milk itself; but this seems not to be the case, and 
the formation of casein must be regarded as the result of the action of the 
cell. Even the albumin present appears to be not the ordinary serum- 
albumin simply passed from the blood through the cell into the lumen of 
the alveolus, but the slightly different lactalbumin. We may perhaps regard 
the albumin as less difficult to manufacture than the casein ; and we may 
explain the fact that relatively to the albumin the casein is less at the very 
beginning and especially toward the end of lactation, by supposing that the 
cell has in the first case not got into full working order, and in the second case 
is waning in power. The peptone-like body in milk, though small in quantity, 
is a further indication of the proteid metabolism taking place in the cell. 
That the milk sugar, lactose, also is formed in and by the cell, is indicated 
by the facts that it is found in no other part of the body, and that its pres- 
ence in milk is not dependent on carbohydrate food, for it is maintained in 
abundance in the milk of carnivora when these are fed exclusively on meat, 
as free as possible from any kind of sugar or glycogen. A glycogen-like 
body has moreover been described as existing in the cells, and it is suggested 
that this body is the antecedent of the lactose. 
We thus have evidence in the mammary gland of the formation, by the 
metabolic activity of the secreting cell, of the representatives of the three 
great classes of food-stuffs, proteids, fats, and carbohydrates. 
§ 432. That both the secretion and ejection of milk are under the con- 
trol of the nervous system is shown by common experience, but the exact 
nervous mechanism has not yet been fully worked out. While the erection 
of the nipple ceases when the spinal nerves which supply the breast are 
divided, the secretion continues, and is not arrested even when the sym- 
pathetic as well as the spinal nerves are cut. 
CHAPTER V. 
NUTRITION. 
The Statistics of Nutrition. 
§ 433. The preceding chapter has shown us how wholly impossible it is 
at present to master the metabolic phenomena of the body, by attempting to 
trace out, forward or backward, the several changes undergone by the indi- 
vidual constituents of the food, the body, or the waste products. Another THE STATISTICS OF NUTRITION. 
483 
method is, however, open to us-the statistical method. We may ascertain 
the total income and the total expenditure of the body during a given 
period, and by comparing the two maybe able to draw conclusions concern- 
ing the changes which must have taken place in the body while the income 
was being converted into the output. Many researches have been carried 
out by this method ; but valuable as are the results which have been thereby 
gained, they must be received with caution, since in this method of inquiry 
a small error in the data may, in the process of calculation and inference, 
lead to wrong conclusions. The great use of such inquiries is to suggest 
ideas, but the views to which they give rise need to be verified in other 
ways before they can acquire real worth. 
Composition of the animal body. The first datum we require is a know- 
ledge of the composition of the body, as far as the relative proportion of the 
various tissues is concerned. In the human body the proportions by weight 
of the chief tissues, in the fresh state, are probably somewhat as follows: 
* 
Adult man. 
Newborn baby. 
Per cent. 
Per cent. 
Skeleton 
15.9 
17.7 
Muscles 
41.8 
22.9 
Thoracic viscera 
1.7 
3.0 
Abdominal viscera .... 
7.2 
11.5 
Fat 
Skin 
18.2) 
6.9 ] 
20.0 
Brain 
1.9 
15.8 
An analysis of a cat has given the following result: 
Per cent. 
Muscles and tendons 45.0 
Bones 14.7 
Skin . 12.0 
Mesentery and adipose tissue 3.8 
Liver 4.8 
Blood (escaping at death) 6.0 
Other organs and tissues 13.7 
One point of importance to be noticed in these analyses is that the 
skeletal muscles form nearly half the body ; we have already seen (§ 38) 
that about a quarter of the total blood in the body is contained in them, and 
have already (§ 398) insisted that a large part of the metabolism of the 
body is carried on in the muscles. Next to the muscles we must place the 
liver, for though far less in bulk than them, it is subject to a very active 
metabolism ; this is suggested by the fact that it alone may hold about a 
quarter of the whole blood, and is also indicated by the numerous facts 
brought before us in the preceding chapter. 
§ 434. The starving body. Before attempting to study the influence of 
food, it will be useful to ascertain what changes occur in the body when all 
food is withheld. A cat of known weight was starved for thirteen days. 
At the beginning of the period the body was presumed to have the compo- 
sition above given ; at the close of the period a direct analysis of the body 
was made. From this it appeared that during the hunger period the cat 
hand lost 734 grammes of solid material, of which 248.8 were fat and 118.2 
muscle, the remainder being derived from the other tissues. The percent- 
ages of dry solid matter lost by the more important tissues during the period 
were as follows: 484 
NUTRITION. 
Per cent. 
Adipose tissue 97.0 
Spleen 63.1 
Liver 56.6 
Muscles 30.2 
Blood ; 17.6 
Brain and spinal cord 0.0 
Thus, the loss during starvation fell most heavily on the fat, indeed nearly 
the whole of this disappeared. Next to the fat, the glandular organs, the 
tissues which we have seen to be eminently metabolic, suffered most. Then 
come the muscles, that is to say, the skeletal muscles, for the loss in the 
heart was very trifling; obviously this organ, on account of its importance 
in carrying on the work of the economy, was spared as much as possible; it 
was, in fact, fed on the rest of the body. The same remark applies to the 
brain and spinal cord; in order that life might be prolonged as much as pos- 
sible, these important organs were nourished by material drawn from less 
noble organs and tissues. The blood suffered proportionately to the general 
body-waste, becoming gradually less in bulk, but retaining the same specific 
gravity; of the total dry proteid constituents of the body, 17.3 per cent, was 
lost, which agrees very closely with the 17.6 per cent, dry material (almost 
wholly proteid) lost by the blood. It is worthy of remark that the tissues 
in general become more watery than in health. Similar observations on 
other animals have led to similar results, the chief discordance being that 
in some cases the bones have suffered considerable loss, in others compara- 
tively little. We might be inclined to infer from these data the conclu- 
sions that metabolism is most active in the adipose tissue, next in such 
metabolic tissues as the hepatic cells and spleen-pulp, then in the muscles, 
and so on ; but we have no warrant for these conclusions. Because the 
Joss of cardiac and nervous tissue was so small, we must not therefore 
infer that their metabolism was feeble; they may have undergone rapid 
metabolism, and yet have been preserved from loss of substance by 
their drawing upon other tissues for their material. The great loss of 
adipose tissue is obviously to be explained by the fact that that tissue 
is essentially a storehouse of material, and the similarly great, though less, 
loss in the spleen and liver indicates, as, indeed, the facts recorded in the 
previous chapter suggest, that these organs, too, serve in part as store- 
houses. 
During the starvation period, the urine contained, in the form of urea 
(and that practically represents all the nitrogen of the urine), 27.7 grammes 
of nitrogen. Now, the amount of muscle which was lost during the period 
contained about 15.2 of nitrogen. Thus, more than half the nitrogen of 
the output during the starvation period must have come ultimately from the 
metabolism of muscular tissue. This fact we have already used in discus- 
sing the history of urea, and shall have occasion to make further use of it 
hereafter. The amount of urea excreted per diem has been observed, in 
some cases, to fall very rapidly during the first day or two of starvation, 
and then to diminish gradually, though often showing considerable irregu- 
larities. In other cases no such large initial fall has been observed. It is 
most marked in animals which have been well fed before the beginning of 
the starvation, especially in those which have had a rich nitrogenous diet; 
and the discharge, in these cases, of an extra quantity of urea in the first 
day or two is obviously connected with that immediate effect of food on 
the excretion of urea, to which we have already (§ 401) referred, and to 
which we shall have to return in speaking of what is known as " luxus- 
consumption." THE STATISTICS OF NUTRITION. 
485 
Comparison of Income and Output of Material. 
§ 435. Method. We have now to inquire how the elements of food are 
distributed in the excreta, in order that, from the manner of the distribu- 
tion, we may infer the nature of the intermediate stages which take place 
within the body. By comparing the ingesta with the excreta, we shall 
learn what elements have been retained in the body, and what elements 
appear in the excreta which were not present in the food; from these we 
may infer the changes which the body has undergone through the influence 
of the food. 
In the first place, the real income must be distinguished from the ap- 
parent one by the subtraction of the feces. We have seen that by far the 
greater part of the feces is undigested matter, i. e., food which, though 
placed in the alimentary canal, has not really entered into the body. The 
share in the feces taken up by matter which has been excreted from the 
blood into the alimentary canal, is so small that it may be neglected; cer- 
tainly, with regard to nitrogen, the whole quantity of this element, which 
is present in the feces, may be regarded as indicating simply undigested 
nitrogenous matter. 
The income, thus corrected, will consist of so much nitrogen, carbon, hy- 
drogen, oxygen, sulphur, phosphorus, saline matters, and water, contained in 
the proteids, fats, carbohydrates, salts, and water of the food, together with 
the oxygen absorbed by the lungs, skin, and alimentary canal. The output 
may be regarded as consisting of (1) the respiratory products of the lungs, 
skin, and alimentary canal, consisting chiefly of carbonic acid and water, 
with small quantities of hydrogen and carburetted hydrogen, these two latter 
coming exclusively from the alimentary canal ; (2) of perspiration, consist- 
ing chiefly of water and salts, for the dubious excretion (see § 366) of urea 
by the skin may be neglected, and the other organic constituents of sweat 
amount to very little; and (3) of the urine, which is assumed to contain all 
the nitrogen really excreted by the body, besides a large quantity of saline 
matters, and of water. Where great accuracy is required, the total nitrogen 
of the urine ought to be determined ; it is maintained, however, that no 
errors of serious importance arise when the urea alone, as determined by 
Liebig's'method (which was largely used in the researches forming the basis 
of the present discussion), is taken as the measure of the total quantity of 
nitrogen in the urine, since, in this method, other nitrogenous bodies besides 
urea are precipitated, and so contribute to the quantitative result. It has 
been, and, indeed, still is, debated whether the body may not suffer loss of 
nitrogen by other channels than by the urine and feces, whether nitrogen 
may not leave the body by the skin, or, indeed, in a gaseous state, by the 
lungs. The balance of the conflicting evidence seems, however, in favor 
of the view that no such loss takes place. It would appear that though 
nitrogen, the pivot, so to speak, of the chemical changes of living beings, 
forms so large a portion of the atmosphere, and, moreover, is physically 
diffused through the bodies of both plants and animals, free nitrogen is of 
no chemical use to either of them. It enters into and remains in their 
bodies as an inert substance, and the nitrogen which leaves a plant or ani- 
mal, in a gaseous state, is simply a part of the same inert supply, and does 
not come from the breaking up of the nitrogenous substances of the body 
or of the food. 
Of these elements of the income and output, the nitrogen, the carbon, 
and the free oxygen of respiration are by far the most important. Since 
water is of use to the body for merely mechanical purposes, and not solely 
as food in the strict sense of the word, the hydrogen element becomes a 486 
NUTRITION. 
dubious one ; the sulphur of the proteids and the phosphorus of the fats are 
insignificant in amount; while the saline matters stand on a wholly differ- 
ent footing from the other parts of food, inasmuch as they are not sources 
of energy, and pass through the body with comparatively little change. 
The body-weight must, of course, be carefully ascertained at the beginning 
and at the end of the period, correction being made where possible for the 
feces. 
It will be seen that the labor of such inquiries is considerable. The 
urine, which must be carefully kept separate from the feces, requires daily 
measurement and analysis. Any loss by the skin, either in the form of 
sweat, or, in the case of woolly animals, of hair, must be estimated or ac- 
counted for. The food of the period must be, as far as possible, uniform in 
character, in order that the analyses of specimens may serve faithfully for 
calculations involving the whole quantity of food taken ; and this is espe- 
cially the case when the diet is a meat one, since portions of meat differ so 
much from each other. But the greatest difficulty of all lies in the estima- 
tion of the carbonic acid produced and the oxygen consumed. In some of 
the earlier researches this factor was neglected, and the variations occurring 
were simply guessed at, through which very serious errors were introduced. 
No comparison of income and output can be considered satisfactory unless 
at least the carbonic acid produced be directly measured by means of a res- 
piration chamber. And in order that the comparison should be really com- 
plete, the water given off by the skin and lungs must be directly measured 
also ; but this seems to be more difficult than the determination of the car- 
bonic acid. 
In the plan originally adopted by Regnault and Reiset, and followed by some 
other observers, the animal experimented on is allowed to breathe a limited and 
measured atmosphere. The carbonic acid, as fast as it is formed, is fixed and 
removed by a strong solution of caustic potash, and the normal percentage of 
oxygen in the atmosphere is maintained by a supply of this gas from a gas-holder. 
In this way both the oxygen consumed and the carbonic acid produced are directly 
determined, while the continual supply of fresh oxygen prevents any evil effects 
due to breathing a confined portion of air. In order, however, to avoid all possible- 
errors arising from a too restricted atmosphere, a different method has been adopted 
by Pettenkofer and Voit. Their apparatus consists essentially of a large chamber, 
capable of holding a man comfortably. Ry means of a steam-engine a current of 
pure air, measured by a gasometer, is drawn through the chamber. Measured 
portions of the outgoing air are from time to time withdrawn and analyzed ; and 
from the data afforded by these analyses the amounts of carbonic acid (and other 
gases) and of water given off bj7 the occupant of the chamber during a given time 
are determined. The oxygen consumed is not determined directly; but if the 
total amounts of carbonic acid ami of water given out by the lungs and skin are 
ascertained, and the amount of urine and feces known, the quantity of oxygen 
consumed may be arrived at by a simple calculation. For evidently the difference 
between the terminal weight plus all the egesta and the initial weight plus all the 
ingesta can be nothing else than the weight of the oxygen absorbed during the 
period. This method in turn, however, is also open to objections, since minute 
errors in the analyses of the small samples of air employed for the determinations 
attain considerable dimensions when these are multiplied so as to give the changes 
in the whole mass of air passed through the apparatus. It seems, moreover, unde- 
sirable to leave the quantity used of so important an element as oxygen to be deter- 
mined by indirect calculations. 
Let us imagine, then, an experiment of this kind to have been completely 
carried out; that the animal's initial and terminal weights have been accu- 
rately determined ; the composition of the food satisfactorily known to con- 
sist of so much proteid, fat, carbohydrates, salts, and water, and to contain THE STATISTICS OF NUTRITION. 
487 
so much nitrogen and carbon ; the weight of the feces and the nitrogen they 
contain ascertained ; the nitrogen of the urine determined ; the carbonic 
acid and water given off by the whole body carefully measured, and the 
amount of oxygen absorbed calculated-what interpretation can be placed 
on the results ? 
Let us suppose that the animal has gained w in weight during the period. 
Of what does w consist ? Is it fat or proteid material which has been laid 
on, or simply water which has been retained, or some of one and some of the 
other? Let us further suppose that the nitrogen of the urine passed during 
the period is less, say by x grammes, than the nitrogen in the food taken, 
after deduction, of course, of the nitrogen in the feces. This means that 
x grammes of nitrogen have been retained in the body; and we may with 
reason infer that they have been retained in the form of proteid material. 
We may even go further, and say that they are retained in the form of flesh, 
i. e., of muscle. In this inference we are going somewhat beyond our tether, 
for the nitrogen might be stored up as some proteid constituent of the hepatic 
cells or of some other tissue; indeed, it might be for the while retained in 
the form of some nitrogenous crystalline body. But this last event is 
unlikely; and if we used the word " flesh " to mean nitrogen-holding living 
substance (proteid) of any kind, we may without fear of any great error reckon 
the deficiency of x grammes nitrogen as the storing up of a grammes flesh. 
There still remain w- a grammes of increase to be accounted for. Let us 
suppose that the total carbon of the egesta has been found to be y grammes 
less than that of the ingesta; in other words, that y grammes of carbon have 
been stored up. Some carbon has been stored up in the flesh with the 
nitrogen just considered ; this we must deduct from y, and we shall then 
have y' grammes of carbon to account for. Now there are only two prin- 
cipal forms in which carbon can be stored up in the body-as glycogen or as 
fat. The former is, even in most favorable cases, inconsiderable, and we 
therefore cannot err greatly if we consider the retention of y' grammes carbon 
as indicating the laying on of b grammes fat. If a + b are found equal to 
w, then the whole change in the economy is known ; if w- (a + 6) leaves a 
residue c, we infer that in addition to the laying on of flesh and fat some 
water has been retained in the system. If w- (a + b) gives a negative 
quantity, then water must have been given off at the same time that flesh 
and fat were laid on. In a similar way the nature of a loss of weight can be 
ascertained, whether of flesh or fat or of water, and to what extent of each. 
The careful comparison, the debtor and creditor account of income and out- 
put, enables us, with the cautions rendered necessary by the assumptions just 
now mentioned, to infer the nature and extent of the bodily changes. The 
results thus gained ought, of course, if an account is kept of the water taken 
in and given out, to agree with the amount of oxygen consumed, and also to 
tally with the conclusions arrived at concerning the retention or the reverse 
of water. 
Having thus studied the method, and seen its weaknesses as well as its 
strength, we may briefly review the results which have been obtained by its 
means. 
§ 436. Nitrogenous metabolism. When a meal of lean meat, as free as 
possible from fat, is given to a dog which has previously been deprived of 
food for some time, and whose body therefore, is greatly deficient in flesh, it 
might be expected that the larger part of the food would be at once stored 
up to supply pressing deficiencies, and that only the smaller part would be 
immediately worked off as urea corresponding to the nitrogenous metabolism 
going on in the body at the time, increased somewhat by the labor thrown 
on the economy by the very presence of the food. This, however, is not the 488 
NUTRITION. 
case as far as the nitrogen of the meal is concerned ; the larger portion 
passes off as urea at once, and only a comparatively small quantity is 
retained. If the diet be continued, and we are supposing the meals given 
to be large ones, the proportion of the nitrogen which is given off in the form 
of urea goes on increasing until at last a condition is established in which 
the nitrogen of the egesta exactly equals that of the ingesta. This condition, 
which is spoken of as " nitrogenous equilibrium," is attained in dogs with an 
exclusively meat diet only when large quantities of food are given, and it is 
not easily maintained for any length of time. The exact quantity of meat 
required to attain nitrogenous equilibrium varies with the previous condition 
of the dog; equilibrium is frequently attained when 1500 or 1800 grammes 
of meat are given daily. 
Thus the most striking effect of a purely nitrogenous diet is largely to 
increase the nitrogenous metabolism of the body; and we shall see later on 
that it increases the metabolism not only of the nitrogenous but also of the 
other constituents of the body. 
The establishment of nitrogenous equilibrium does not mean that a body- 
equilibrium is established, that the body-weight neither increases nor dimin- 
ishes. On the contrary, when the meal necessary to balance the nitrogen is 
a large one, the body though it is neither gaining nor losing nitrogen may 
gain in total weight; and the increase is proved by calculation from the 
income and output, and indeed by actual examintion of the body, to be due 
to the laying on of fat. The amount so stored up may be far greater than 
can possibly be accounted for by any fat still adhering to the meat given as 
food. We are therefore driven to the conclusion that the proteid food is 
split into a urea moiety and a fatty moiety, that the urea moiety is at once 
discharged, and that such of the fatty moiety as is not made use of directly 
by the body is stored up as adipose tissue. And this disruption of the pro- 
teid, as we have already (§ 401) suggested, explains at the same time why 
the meat diet so largely and immediately increases the urea of the egesta. 
The characteristic effect of proteid food to increase the metabolism of the 
body is shown on other animals besides the dog, and not only by means of 
calculations of what is supposed to take place in the body, but also by direct 
analysis. Thus the analysis of the body of a pig, which had been fed on a 
known diet, compared with the analysis of that of another pig of the same 
litter, killed at the time when the first was put on the fixed diet, gave as a 
result that of the dry nitrogenous material of the food only about 7 per cent, 
was laid up as dry proteid material during the fattening period, though the 
amount of proteid food was low. This contrasts strongly with the amount 
of fat stored up during the same period (see § 420). Similar observations 
carried out on sheep showed that in these animals the storing up of nitro- 
genous material was even less, only about 4 per cent, of that given in the 
food. 
Every quantity of proteid material taken into the alimentary canal thus 
appears to affect proteid metabolism in two ways. On the one hand, it ex- 
cites a rapid proteid metabolism giving rise to an immediate, and generally 
large, increase of urea; on the other hand, it serves to maintain the more 
regular normal proteid metabolism continually taking place in the body, 
and so contributes to the normal regular discharge of urea. It seems very 
natural to suppose that the proteid which plays the first of these two parts is 
not really built up into the tissues, does not become actual living substance, 
but undergoes the changes which give rise to urea outside the actual living 
substance in the blood or elsewhere ; and we have seen that under the influ- 
ence of the pancreatic juice some of the proteid food may undergo the greater 
part of such a change while it is as yet within the alimentary canal. Hence 489 
THE STATISTICS OF NUTRITION. 
has arisen the very natural distinction to which we have already alluded 
between " tissue proteids " or " morphotic proteids " which are actually built 
up into the living substance of the tissues and give rise to urea through the 
metabolism of living substance, and " circulating proteids " or " floating pro- 
teids" which do not at any period of their career within the body become 
an integral part of the living substance and by their metabolism set free 
energy not in the way of vital manifestations, but in the form of heat only. 
We shall latter on consider what is the exact meaning which we ought to 
attach to the words " becoming part of the living substance ;" and hence 
shall defer until then any discussion of the appropriateness of these phrases 
and of the validity of the distinction which they formulate. 
It was once thought, as we shall presently see, erroneously, that the ex- 
clusive purpose of proteid food was to supply the proteid tissues, and that 
all the energy set free in the body in vital manifestations, such as movement 
and the like as distinguished from heat, had its origin in proteid metabolism, 
the metabolism of fats and carbohydrates giving rise to heat only. Hence 
when it first became known that a certain proportion of proteid food appar- 
ently underwent a metabolism giving rise to heat only, without becoming 
part of the tissues, this seemed to be a wasteful expenditure of precious 
material; and the metabolism of this portion of proteid food was accordingly 
spoken of as a " luxus-consumption," a wasteful consumption. 
Before leaving this subject we may call attention to a possible analogy 
between the history of proteids and that of fats and carbohydrates. The 
uniform composition of the bleed, which the body seems ever striving to 
maintain, probably applies to its proteids as well as to its other constituents. 
We have seen that a surplus of non-nitrogenous materials in the blood is 
withdrawn from the circulation and stored up as fat or glycogen, and it is 
possible that an excess of proteids might similarly be stored up in some tis- 
sue or tissues, in the hepatic cells for instance, though from the facts pre- 
viously mentioned it is obvious that the power of storage is far less than in 
the case of fats and carbohydrates. Such a store of proteid matter would 
represent a sort of circulating proteid, but nevertheless for its final metab- 
olism might have to form an integral part of some living tissue unit. 
§ 437. The effects of fatty and of carbohydrate food. Unlike those of 
proteid food, the effects of fats and carbohydrates cannot be studied alone. 
When an animal is fed simply on non-nitrogenous food, death soon takes 
place; the food rapidly ceases to be digested, and starvation ensues. We 
can therefore only study the nutritive effects of these substances when they 
are taken together with proteid material. 
When a small quantity of fat is taken, in company with a fixed moderate 
quantity of proteid material, the whole of the carbon of the food reappears 
in the egesta. No fat is stored up; some even of the previously existing fat 
•of the body may be consumed. As the fat of the meal is increased, a point 
is soon reached at which carbon is retained in the body as fat. So also with 
starch or sugar; when the quantity of this is small, there is no retention of 
carbon ; as soon, however, as it is increased beyond a certain limit, carbon is 
stored up in the form of fat or, to a smaller extent, as glycogen. Fats and 
carbohydrates, therefore, differ markedly from proteid food in that they are 
not so distinctly provocative of metabolism. This is exceedingly well shown 
in the results obtained on the pig previously mentioned. It was found that 
472 units of fat were laid on for every 100 units of fat taken as such in 
the food (which consisting of barley-meal, etc. contained a very small amount 
of actual fat), while for every 100 units of the total dry non-nitrogenous food 
including fat, starch, cellulose, etc., no less than 21 units were retained in 
the body in the form of fat. No clearer proof than this could be afforded 490 
NUTRITION. 
that fat is formed in the body out of something which is not fat. In § 421 
we have already discussed this formation of fat out of carbohydrates. 
As one might imagine, the presence of fat or carbohydrates in the food 
is found to decrease the amount of proteid material necessary to establish 
nitrogenous equilibrium. For instance, with a diet of 800 grms. meat and 
150 grms. fat, the nitrogen in the egesta became equal to that in the ingesta 
in a dog, in whose case 1800 grms. meat had to be given to produce the same 
result in the absence of fat or carbohydrates. 
On the other hand, it was found that, with a fixed quantity of fatty or 
carbohydrate food, an increase of the accompanying proteid led not to a 
storing up of the surplus carbon contained in the extra quantity of proteid, 
but to an increase in the consumption of carbon. Proteid food increases 
not only proteid but also non-nitrogenous metabolism. This explains how 
an excess of proteid food may, by the increase of general metabolism, actu- 
ally reduce the fat of the body. 
We have at present no exact information concerning the nutritive differ- 
ences between fats and carbohydrates, beyond the fact that in the final com- 
bustion of the two, while carbohydrates require sufficient oxygen to combine 
with their carbon only, there being already sufficient oxygen in the carbo- 
hydrate itself to form water with the hydrogen present, fats require in addi- 
tion oxygen to combine with some of their hydrogen. Hence in herbivora, 
living largely on carbohydrates, a larger portion of the oxygen consumed 
reappears in the carbonic acid of the egesta than in carnivora, in which 
animals, living chiefly on proteids and fats, more of it leaves the body com- 
bined with hydrogen to form water. This relation of the oxygen to the car- 
bonic acid is often expressed as the quotient of the volume of the carbonic 
acid expired divided by the volume of the oxygen consumed, the " respira- 
tory quotient," 2, which is in herbivora about 0.9 and in carnivora about 
0.6 or 0.7. When an herbivorous animal starves, it feeds on its own fat, and 
under these circumstances the respiratory quotient falls to the carnivorous 
standard; and indeed many circumstances affect this respiratory quotient. 
The carbohydrates are notably more digestible than the fats, but on the other 
hand the fats contain more potential energy in a given weight. As to the 
nutritive difference between starch and sugar, we know nothing very definite; 
it has been thought, however, that cane-sugar is rather more fattening than 
starch. 
§ 438. The effects of gelatin as food. It is a matter of common experi- 
ence that gelatin will not supply the place of proteids as a constituent of 
food. Animals fed on gelatin together with fat or carbohydrates die very 
much in the same way as when they are fed on non-nitrogenous material 
alone. Nevertheless it would appear, as might be expected, that the presence 
of gelatin in food is not without effect. This nitrogenous equilibrium is 
established at a lower level of real proteid food when gelatin is added. In 
a dog, moreover, fed on a diet of gelatin and fat, the excess of nitrogen in 
the excreta over that in the ingesta is less than when the same dog is fed on 
a diet of fat alone ; that is to say, the gelatin has sheltered from metabolism 
some proteid constituents of the body; and the consumption of fat seems 
also to be lessened by the presence of gelatin. These facts become intelli- 
gible if we suppose that gelatin is rapidly split up into a urea and a fat 
moiety in the same way that we have seen a certain quantity of proteid 
material to be. It is this direct destructive metabolism of proteid matter 
which gelatin can take up; it seems, however, unable to imitate the other 
function of proteid matter, and to take part in the formation of living sub- 
stance ; or in the phraseology of a preceding paragraph (§ 436), it can take THE STATISTICS OF NUTRITION. 
491 
the place of circulating but not of tissue proteid. What is the cause of this 
difference we cannot at present say. 
§ 439. Peptone as food. Since proteids are at least largely, as we have 
seen (§ 262), converted into and absorbed as peptone, and since, as we have 
also seen, the peptone appears during the very act of absorption to be recon- 
verted into some other form of proteid matter, possibly serum-albumin, it 
might seem natural to suppose that peptone given as food would, as far as 
metabolism is concerned, play the same part as other proteids. Neverthe- 
less, some observers have maintained with regard to both peptones and the 
allied albumoses that, like gelatin, these bodies " can take the place of cir- 
culating but not of tissue proteid." On the whole, however, the evidence 
goes to show that animals can " lay on flesh " when the proteid in their food 
consists entirely of peptone or albumose. A difficulty appertaining to 
digestion prevents any large substitution of peptone for ordinary proteids, 
since as might be expected diarrhoea is apt to be set up. 
§ 440. The effects of salts as food. All food contains, besides the sub- 
stances possessing potential energy, which we have just studied, certain saline 
matters, organic and inorganic, having in themselves little or no such poten- 
tial energy, but yet either absolutely necessary or highly beneficial to the 
body. These must have important functions in directing the metabolism of 
the body ; the striking distribution of them in the tissues, the preponderance 
of sodium and chlorides in blood-serum and of potassium and phosphates in 
the red corpuscles, for instance, must have some meaning ; but at present we 
are in the dark concerning it. The element phosphorus seems no less im- 
portant, from a biological point of view, than carbon or nitrogen ; it is as 
absolutely essential for the growth of a lowly being like penicillium as for 
man himself. We find it probably playing an important part as the con- 
spicuous constituent of lecithin and other complex fats belonging to the 
nervous system ; we find it prominent in the peculiar body nuclein ; we find 
it peculiarly associated with the proteids, but we cannot explain its role. The 
element sulphur, again, is only second to phosphorus, and we find it as a con- 
stituent of nearly all proteids; but we cannot foretell the exact changes 
which would take place in the economy if all the sulphur of the food were 
withdrawn. In the kreatin of the epidermis and its appendages, hairs, etc., 
sulphur is probably undergoing excretion, though its presence in kreatin may 
have to do with the peculiar physical characters of corneous epithelium. 
We know that the various saline matters are essential to health ; that 
when they are not present in proper proportions nutrition is affected. Dogs 
fed on food freed as much as possible from all saline matters, but otherwise 
abundant, with a proper proportion of the food-stuffs, soon exhibit symptoms 
showing that the metabolism of their tissues, especially of their central 
nervous system, is going wrong; they sutler from weakness, soon amounting 
to paralysis, and are often carried off by convulsions. And more or less 
similar derangements of nutrition follow the absence or a deficiency of indi- 
vidual salts. During starvation these various salts continue to be discharged 
from the body ; in some way or other they are carried along in the metabolic 
stream, and their presence is in some way essential to the various metabolic 
processes; hence, they need to be always present in daily food. In what 
way it is that they thus direct metabolism we do not know ; we are aware 
that the properties and reactions of various proteid substances are closely 
dependent on the presence of certain salts, but beyond this we know very 
little. The inorganic salts are those the nutritive value of which has been 
chiefly studied by experiment, but we have reason to believe that the organic 
salts, or extractives, which are present in greater or less quantity in all food 
of both vegetable and animal origin, are no less essential to the proper meta- 492 
NUTRITION. 
bolic activities of the body. The undoubted connection of scurvy with the 
lack of fresh vegetable food, other conditions helping, may perhaps turn in 
part on this, for the evidence that the disease is due to the deficiency of 
potash alone is not conclusive. 
Lastly, water has an effect on metabolism, as shown, among other things, 
by the fact that when the water of a diet is increased, the urea is increased 
to an extent beyond that which can be explained by the increase of fluid 
increasing the facilities of mere excretion. 
The Energy of the Body. 
The Income of Energy. 
§ 441. Broadly speaking, the animal body is a machine for converting 
potential into actual energy. The potential energy is supplied by food ; 
this the metabolism of the body converts into the actual energy of heat 
and mechanical labor. We have in the present section to study what is 
known of the laws of this conversion, and of the distribution of the energy 
set free. 
Neglecting all subsidiary and unimportant sources of energy, we may say 
that the income of animal energy consists in the oxidation of food into its 
waste products-viz., the oxidation of proteids, fats, and carbohydrates into 
urea, carbonic acid, and water. A principle laid down by the chemist teaches 
that the potential energy of any body, considered in relation to any chemical 
change which it may undergo, is the same when the final result is the same, 
whether that result be gained at one leap or by a series of steps ; that, for 
instance, the energy set free by the oxidation of 1 grm. of fat into carbonic 
acid and water is the same, whatever the changes forward or backward which 
the fat undergoes before it finally reaches the stage of carbonic acid and 
water; and similarly, that the energy available for the body in 1 grm. of 
dry proteid is the energy given out by the complete combustion of that 1 
grm., less the energy given out by the complete combustion of that quantity 
of urea to which the 1 grm. of proteid gives rise in the body. Taking this 
as our guide, we can readily calculate the amount of potential energy con- 
tained in an average twenty-four hours' diet, and thus obtain the average 
daily income of energy. For the potential energy of most of the substances 
used as food has been determined by direct calorimetric observations; and 
the several determinations, though they vary somewhat, agree sufficiently 
closely to serve as data for the calculations in question. 
The total combustion of the following substances has given for one 
gramme of each substance the following results expressed in calories-that 
is, in gramme-degree units of heat: 
Meat, free from fat, 5103 and 5324. Fibrin, 5511. Egg-albumin, 5579. 
Thus, taking round numbers, we may say that 1 grm. of proteid material 
contains 5000 or 5500 calories of potential energy, according as we use the 
lower or higher determinations. 
Fat beef or mutton, 9069, 9365, 9423. Butter, 7267 or 9192. Again, 
in round numbers, we may say that 1 grm. of fat contains about 9000 
calories. 
Arrowroot (nearly pure starch), 3912. Starch, 4123. Cellulose, 4146. 
Dextrose, 3692. Cane sugar, 3866. Here again, taking round numbers, 
we shall not be far wrong in saying that the potential energy of 1 grm. of 
carbohydrate material is about 4000 calories. 
The combustion of 1 grm. of urea sets free an amount of energy which 
has been determined by one observer as 2206, by another as 2465 calories. THE ENERGY OF THE BODY. 
493 
We have seen (§ 421) that 1 grm. of proteid gives rise in the body to I 
grm. urea. Hence, to obtain the energy of 1 grm. proteid material avail- 
able for the economy, we must deduct from its potential energy one-third 
the potential energy of 1 grm. urea-that is, in round numbers, 700 or 800 
calories. This will give us 5000 - 700, or 5500 - 800, that is, 4300 or 4700 
calories, according as we take the lower or higher data ; or we may take as 
a mean 4500 calories. The data, then, so far, are as follows: 
1 gramme proteid 4500 calories. 
1 gramme fat 9000 " 
1 gramme carbohydrate 4000 " 
The average diet of an average man-that is, the average amount of 
each food-stuff respectively taken daily-may be determined experimentally 
or statistically. Thus, a man may determine by a series of trials the diet 
on which, while neither losing or gaining weight and maintaining " nitro- 
genous equilibrium " (§ 436), he enjoys good health. Or an average may 
be struck of a large number of diets used by various people. We shall 
have something to say of this latter statistical method when we come to 
speak of diet. For the present purpose we may use one arrived at experi- 
mentally, which we will speak of as Ranke's diet, since it was determined 
by a physiologist of that name from observations on himself. It was com- 
posed of 1000 grms. proteid, 100 grms. fat, 240 grms. carbohydrate. Such 
a diet would give 
100 grammes proteid (4500) 450,000 calories. 
100 grammes fat (9000) 900,000 " 
240 grammes carbohydrate (4000) 960,000 " 
2,310,000 li 
If we translate the units of heat into units of work, the 2,310,000 gramme- 
degree, or 2310 kilogramme-degree calories will give us about 980,000, or, 
in round numbers, somewhere about one million kilogramme-metres. 
We may, in passing, call attention to the fact that the proteids supply a 
relatively small part of the total energy, and that the share contributed by 
the large mass of carbohydrates is not much greater than that belonging to 
the much smaller quantity of fat. In the average diet obtained by the sta- 
tistical method, in which the data are largely drawn from public institutions, 
the (cheaper) carbohydrates are still further increased at the expense of the 
(dearer) fats, a change which may tend to reduce somewhat the total energy ; 
but this does not materially affect the broad results just given. 
The Expenditure. 
§ 442. There are two ways only in which energy is set free from the 
body: mechanical labor and heat. The body loses energy in producing 
muscular work, as in locomotion and in other kinds of labor, in the move- 
ments of the air in respiration and speech, and, though to a hardly recog- 
nizable extent, in the movements of the air or contiguous bodies by the 
pulsations of the vascular system. The body loses energy in the form of 
beat by conduction and radiation, by respiration and perspiration, and by 
the warming of the urine and feces. All the internal work of the body, all 
the mechanical labor of the internal muscular mechanisms with their accom- 
panying friction, all the molecular labor of the nervous and other tissues, 
is converted into heat before it leaves the body. The most intense mental 494 
NUTRITION. 
action, unaccompanied by any muscular manifestations, the most energetic 
action of the heart or of the bowels, with the slight exceptions mentioned 
above, the busiest activity of the secreting or metabolic tissues, all these end 
simply in augmenting the expenditure in the form of heat. 
A normal daily expenditure in the way of mechanical labor can be easily 
determined by observation. Whether the work take on the form of walk- 
ing, or of driving a machine, or of any kind of muscular toil, a good day's 
work may be put down at about 150,000 kilogramme-metres. 
The normal daily expenditure in the way of heat cannot be so readily 
determined. Direct calorimetric observations on the whole body are attended 
with so many difficulties, except in the case of small animals, that their value 
is uncertain ; and observations made by placing a part only of the body, an 
arm or leg for example, in the calorimeter, and from the data thus gained 
calculating the heat produced by the whole body, are subject to many ad- 
ditional sources of error. 
The calorimeters usually employed in chemical operations, in measuring, for 
instance, the heat given out in chemical changes, are unsuitable for experiments 
on living animals. Such are the mercury calorimeter, in which the chemical action 
to be studied is made to take place in the midst of a mass of mercury, from the 
consequent expansion of which through the heat taken up the amount of' heat given 
out is calculated, or the ice calorimeter in which in a similar way the heat given out 
is calculated by the amount of ice melted. The latter has been used for physio- 
logical purposes, but an animal surrounded by ice is under such abnormal conditions 
that the results are of little value. The methods usually adopted by physiologists 
are as follows: 
In one method, the water calorimeter, the animal is placed in a metal chamber 
surrounded by a.jacket filled with water. The heat given out by the animal warms 
the water in the jacket, and the amount given out is calculated upon the increase 
of the temperature of the water. By supplying the animal with air through a long 
spiral tube passing through the water-jacket, the heat given out in the expired air 
is prevented from being lost. 
This method may be employed in a simpler form, when the heat given out by a 
part of the body, the arm or leg for instance, is all that has to be determined. 
The part is then merely placed in a bath of water, from the changes of tempera- 
ture of which the amount given out is calculated. And this modification of the 
method may with due precautions be employed for the whole body. 
In Rosenthal's calorimeter the chamber in which the body or part of the body 
is placed is surrounded by. not a water-jacket, but an air-jacket, which thus serves 
as an air calorimeter. The instrument consists essentially of three concentric 
copper cylinders ; the inner one contains the animal (or other source of heat); the 
other one serves merely as a casing to protect those inside from changes of temper- 
ature due to currents of air and the like; and the middle one encloses an air-space 
between itself and the inner one. There are special arrangements for closing the 
cylinders after the introduction of the animal, and for supplying the animal with 
air for breathing purposes. With the air-jacket, or space between the inner or 
middle cylinders, are connected a manometer and a thermometer. When an animal 
(or other source of heat) is placed in the inner cylinder, the temperature and the 
pressure of the air in the air-jacket are increased ; and from the amounts of increase 
measured by the thermometer and the manometer the amount of heat given out 
from the animal is calculated. 
The calorimeters of D'Arsonval and Rubner are constructed on very similar 
principles. 
Various attempts have been made to acertain the amount of heat given 
out by the body in an indirect manner, as for instance by calculating the 
heat given out by the oxidation of the food. As trustworthy as any is the 
plan of simply subtracting the normal daily mechanical expenditure from 
the normal daily income. Thus 150,000 kilogramme-metres subtracted from 
one million kilogramme-metres gives 850,000 kilogramme-metres as the daily 495 
THE ENERGY OF THE BODY. 
expenditure in the form of heat; i. e., between one-fifth and one-sixth of the 
total income is expended as mechanical labor, the remaining four-fifths or 
five-sixths leaving the body in the form of heat. The results given by direct 
calorimetric observations and by other calculations give somewhat higher 
figures than these ; and indeed these may probably be taken as under rather 
than over the true amount. In any case they are to be regarded as furnishing 
nothing more than a rough average, the exact amount varying according to 
the size, the weight, and the condition of the individual, as well as according 
to variations in circumstances. 
§ 443. The energy of mechanical work. We have already in treating of 
muscle and elsewhere partly discussed this subject, but may here say the rest 
that has to be said. 
The older writers, even after it had been proved that the animal body was 
constructive, as far as the formation of fat was concerned, still held to the 
distinction between nitrogenous or plastic and non-nitrogenous or respiratory 
food. Put broadly, this view' was that all the nitrogenous food went to build 
up the proteid tissues, the muscular flesh and the like, and that the nitro- 
genous egesta arose solely from the functional metabolism of these tissues, 
while the non-nitrogenous food was used with equal exclusiveness for respira- 
tory or calorific purposes, being either directly oxidized in the blood, or, if 
present in excess, stored up as fatty tissue. According to this view the two 
classes of income corresponded exactly to the two forms of expenditure. 
We have already urged several objections against this view. We have seen 
that in the blood itself very little oxidation takes place ; that it is the active 
tissue, and not the passive blood-plasma, which is the seat of oxidation. We 
have further seen that proteid food may undoubtedly be, in the above sense, 
respiratory and incidentally give rise to the storing up of fat. One division 
of the view is thereby overthrown. We have now to inquire whether the 
other division holds good, whether muscle and the other proteid tissues are 
fed exclusively on the proteid material of food, and whether muscular 
energy comes exclusively from the metabolism of the proteid constituents of 
muscle. We have already seen (§ 63) that when the muscle itself is exam- 
ined, we find no proof of nitrogenous waste, but, on the other hand, clear 
evidence of the production of non-nitrogenous bodies, such as carbonic acid. 
And when we ask the question, Does muscular exercise proportionately 
increase the urea given off by the body as a whole? for this according to the 
theory in question it certainly ought to do, the evidence we can obtain, 
though somewhat varying, gives on the whole a decidedly negative answer. 
In the majority of observations no marked change at all in the amount 
was met with ; indeed, in some cases there was a distinct decrease, followed 
by an increase on the following days. Some observers, however, found a 
very marked increase, and this was especially the case when the subject 
under observation took a large amount of food and performed very severe 
labor. On the whole, the various results obtained by different observers 
justify the conclusion that exercise by itself, even when severe, does not 
necessarily increase the amount of urea excreted, but that conditions may 
obtain in which such an increase undeniably occurs. We may draw the 
further conclusion that experiments of this kind do not supply the right 
method for determining the point at issue. It must be remembered that it 
is not the muscles alone which feel the influence of the labor ; the circulation 
and indeed the whole body are affected by it. If we suppose a large part 
or even only some part of the urea to come from other than muscular metab- 
olism, from changes in the hepatic cells for instance, we should expect that 
these changes, and with them the amount of urea discharged, would be 
influenced by labor, especially by severe labor. 496 
NUTRITION. 
In no case has a direct relation between the amount of labor and 
amount of urea been observed. More than this, the following experience 
lands us in an absurdity, if we suppose the whole energy of muscular work 
to arise from proteid metabolism. Two observers performed a certain 
amount of work (an ascent of a mountain) on a non-nitrogenous diet, and 
estimated the amount of urea passed during the period. Assuming the 
urea to represent the oxidation of so much proteid matter, which oxidation 
represented in turn so much energy set free, they found that, whereas the 
actual work done amounted to 129.026 and 148.656 kilogramme-kilometres 
for each observer respectively, the total energy available from proteid 
metabolism during the period was in the case of the first 68.69, and of the 
second 68.376 kilogramme-kilometres. That is to say, the energy set free 
by the proteid metabolism of the muscles engaged in the work was far less 
than the amount necessary to accomplish the work actually done, to say 
nothing of its having to provide as well for the movements of respiration 
and circulation. Their muscular energy, therefore, must have had other 
sources than proteid metabolism. 
That, on the contrary, the production of carbonic acid is at once and 
largely increased by muscular exercise is beyond all doubt. One hour's 
hard labor will increase fivefold the quantity of carbonic acid given off 
within the hour. And in an experiment directed to this point it was 
found that a man in twenty-four hours consumed 954 grammes oxygen and 
produced 1284 grammes carbonic acid when doing work, as against 708 
grammes oxygen consumed and 911 grammes carbonic acid produced when 
remaining at rest, the quantity of urea secreted being in the first case 37 
grammes, in the second 37.2 grammes. 
It is evident that the conclusions arrived at by the statistical method 
entirely corroborate those gained by an examination of muscle itself, viz., 
that during muscular contraction the explosive decomposition which takes 
place bears chiefly, if not exclusively, on the non-nitrogenous constituents 
of the muscle, and that it is the non-nitrogenous products which alone 
escape from the muscle and from the body, any nitrogenous products which 
result being retained within the muscle, or at least within the body. We 
must, therefore, reject the second as well as the first division of the views 
under discussion ; not only is the muscle not fed exclusively on proteid 
material, but also its energy does not arise from an exclusively proteid 
metabolism. 
Animai Heat. 
§ 444. The sources and distribution of heat. We have already seen 
that the conception of the non-nitrogenous portions of food being solely 
calorifacient or respiratory proves to be unfounded when we attempt to 
trace the history of the food on its way through the body. The same view 
is still more strikingly shown to be inadequate when we study the manner 
in which the heat of the body is produced. We may, indeed, at once 
affirm that the heat of the body is generated by the chemical changes, 
which we may speak of generally as those of oxidation, undergone not by 
any particular substances, but by the tissues at large. Wherever metab- 
olism is going on, or, to be more exact, wherever destructive metabolism, 
katabolism, is going on, heat is being set free. In growth and in repair, in 
the deposition of new material, in the transformation of lifeless pabulum 
into living tissue, in the constructive metabolism, the anabolism of the body, 
and in the smaller synthetic processes of which we spoke in dealing with urea 
(§ 403), heat is undoubtedly to a certain extent being absorbed and ren- THE ENERGY OF THE BODY. 
497 
dered latent; the energy of the construction may be, in part at least, sup- 
plied by the heat present. But all this, and more than this, viz., the heat 
present in a potential form in the substances themselves so built up into the 
tissue, is lost to the tissue during its destructive metabolism ; so that the 
whole metabolism, the whole cycle of changes from the lifeless pabulum 
through the living tissue back to the lifeless products of vital action, is 
eminently a source of heat. 
Of all the tissues of the body the muscles, not only from their bulk, 
forming as they do so large a portion of the whole frame, but also from the 
characters of their metabolism, must be regarded as the chief sources of 
heat. 
In treating (§ 65) of the thermal changes in muscle we have seen that 
in the total energy expended in a muscular contraction, the ratio of that 
which appears as heat to that which appears as external work is variable. 
If we take a proportion which is somewhat higher than the mean of the 
range there given (one-fifth to one-twenty-fifth), and assume that the energy 
involved in the work done in a muscular contraction is about one-tenth 
of the total energy expended, the rest going out as heat, then, upon the 
calculation that the total external work of the body is about one-fifth of 
the total energy set free in the body, it is clear that the heat given out by 
the muscles, even if we consider only the heat given out when they are con- 
tracting, must form a very large part of the total heat given out by the 
body. And even if, as recent researches indicate, the muscular machine 
works more economically than we have hitherto supposed, the amount of 
heat given out by the skeletal muscles must still remain very large. More- 
over, to the skeletal muscle we must add the heart which, never resting, 
does in the twenty-four hours, as we have seen (§127), no inconsiderable 
amount of work, and must give rise to no inconsiderable amount of heat. 
But the skeletal muscles, though frequently, are not continually contract- 
ing ; they have periods, at times long periods, of rest; and during these 
periods of rest, metabolism, of a subdued kind it is true, but still a metab- 
olism involving an expenditure of energy is going on. This quiescent 
metabolism must also give rise to a certain amount of heat ; and if we add 
this amount, which in the present state of our knowledge we cannot exactly 
gauge, to that given out during the movements of the body, it is very clear, 
even in the absence of exact data, that the metabolism of the muscles must 
supply a very large proportion of the total heat of the body. They are 
par excellence the thermogenic tissues. 
Next to the muscles in importance come the various secreting glands. 
In these the secreting elements, at the periods of secretion at all events, are 
in a state of metabolic activity, which activity as elsewhere must give rise 
to heat. In the case of the salivary gland of the dog the temperature of 
the saliva secreted during stimulation of the chorda has been found to be 
as much as 1° or 1.5° higher than that of the blood in the carotid artery at 
the same time, and in all probability the investigation of other secreting 
glands would lead to similar results. Of all these various glands the liver 
deserves special attention on account of its size and large supply of blood, 
and because it appears to be continually at work. If there be any truth 
in the views urged in the preceding chapter touching the large and varied 
metabolic work of the liver, we must conclude that a very large amount of 
heat is set free in this organ ; and that holds good even if we make a large 
allowance for the various synthetic anabolic processes which may take place 
and by which heat would be absorbed and made latent. We find, indeed, 
that the blood in the hepatic vein is the warmest in the body. Thus in the 
dog a temperature of 40.73° C. has been observed in the hepatic vein, 
32 498 
NUTRITION. 
while that of the vena cava inferior was 38.35° to 39.58°, and that of the 
right heart 37.7°. The fact that the blood of the hepatic vein is warmer 
than that of either the portal vein or the aorta, shows that the increased 
temperature is not due simply to the liver being far removed from the sur- 
face of the body. 
The brain, too, may be regarded as a source of heat, since its tempera- 
ture is higher than that of the arterial blood with which it is supplied ; 
though from the smaller quantity of blood passing through its vessels, as 
well as from the changes in it being less massive, it cannot, in this respect, 
compare with either the liver or the muscles as a source of heat to the 
body. 
The blood itself cannot be regarded as a source of any considerable 
amount of heat, since, as we have so frequently urged, the oxidations oi 
other metabolic changes taking place in it are comparatively slight. The 
heat evolved by the indifferent tissues, such as bone, cartilage, and connec- 
tive tissue, may be passed over as insignificant; and we cannot even regard 
the adipose tissue as a seat of the production of heat, since the fat of the 
fat-cells is in all probability not oxidized in situ, but simply carried away 
from its place of storage to the tissue which stands in need of it, and it is in 
the tissue that it undergoes the metabolism by which its latent energy is set 
free. Some amount of heat is also produced by the changes which the food 
undergoes in the alimentary canal before it really enters the body. . 
Hence, taking a survey of the whole body, we may conclude that since 
metabolism is going on to a greater or less extent everywhere, heat is every- 
where being generated ; but that, looked at from a quantitative point of view, 
the muscles and the glandular organs must be regarded as the main sources 
of the heat of the body, the muscles being, in all probability, the more im- 
portant of the two. 
§ 445. But heat, while being thus continually produced, is as continually 
being lost, by the skin, the lungs, the urine, and the feces. The blood pass- 
ing from one part of the body to the other, and carrying warmth from the 
tissues where heat is being rapidly generated, to the tissues or organs where 
heat is being lost by radiation, conduction, or evaporation, tends to equalize 
the temperature of the various parts, and thus maintains a " constant bodily 
temperature." 
When the production of heat is not great as compared with the loss there 
is no great accumulation of heat within the body, the temperature of which 
consequently is but slightly raised above that of surrounding objects. Thus 
the temperature of the frog, for instance, is rarely more than 0.04° to 0.05° 
above that of the atmosphere, though in the breeding season the difference 
may amount to 1°. Such animals, and they comprise all classes except birds 
and mammals, are spoken of as cold-blooded ; they have been also called 
poikilothermic, that is, of varied temperature. Exceptions among them are 
not uncommon. Some fish, such as the tunny, are warmer than the water 
in which they live, ami in a species of python (P. bivittatua) a difference of 
as much as 12° has been observed. In a beehive the temperature may rise 
at times as much as to 40°. In the so-called warm-blooded animals, birds 
and mammals, the loss and production of heat are so balanced that the tem- 
perature of the body remains constant at, in round numbers, 35° or 40°, 
whatever be the temperature of the air ; hence these have been called homoio- 
thermic, of constant temperature. The temperature of man is about 37° ; 
in some birds it is as high as 44° (Hirundo), and in the wolf it is said to be 
as low as 35.24°. 
This temperature is with slight variations maintained throughout life. 
After death the generation of heat rapidly diminishes, and the body THE ENERGY OF THE BODY. 
499 
■speedily becomes cold ; but for some short time immediately following upon 
systemic death, a rise of temperature may be observed, due to the fact that, 
while the metabolism of the tissue is still going on, the loss of heat is some- 
what checked by the cessation of the circulation. The onset of pronounced 
rigor mortis causes a marked accession of heat, and when occurring after 
certain diseases may give rise to a very considerable elevation of tempera- 
ture. 
This mean bodily temperature of warm-blooded animals is, during health, 
maintained, with slight variations of which we shall presently speak, within 
a very narrow margin, a rise, or indeed a fall of much more than a degree 
above or below the limit given above being indicative of some failure in the 
organism, or of some unusual influence being at work. It is evident, there- 
fore, that the mechanisms which coordinate the loss with the production of 
heat must be exceedingly sensitive. It is obvious, moreover, that the mech- 
anisms may act when the bodily temperature is tending to rise, by either 
checking the production or by augmenting the loss of heat; conversely when 
the bodily temperature is tending to fall, they may act by either increasing 
the production or by diminishing the loss of heat. As the regulation of tem- 
perature by variations in the loss of heat is better known than regulation by 
variations in production, it will be best to consider the former first. 
§ 446. Regulation by variations in loss. Heat is lost to the body by the 
warming of the feces and of the urine, by the warming of the expired air, by 
the evaporation of the water of respiration, by conduction and radiation 
from the skin, and by the evaporation of the water of perspiration. It has 
been calculated that the relative amounts of the loss by these several chan- 
nels are as follows: In warming the feces and urine about 3, or, according 
to others, 6 per cent. By respiration about 20, or, according to others, 
about only 9 per cent., leaving 77, or alternately 85, per cent, for conduction 
and radiation and evaporation by the skin. 
The two chief means of loss, then, which are at all susceptible of any 
great amount of variation, and which can be used to regulate the tempera- 
ture of the body, are the skin and the lungs. 
The more air passes in and out of the lungs in a given time, the greater 
will be the loss in warming the expired air, and in evaporating the water of 
respiration. In such animals as the dog, which do not perspire freely by the 
skin, respiration is a most important means of regulating the temperature ; 
and in the dog a very close connection may be observed between the produc- 
tion of heat and respiratory activity. The changes which give rise to this 
loss take place before the inspired air reaches the pulmonary alveoli; both 
the warming and the evaporation are effected in the nasal and pharyngeal, 
and to some extent in the bronchial passages. Some observers have main- 
tained that the left side of the heart is warmer than the right, and hence 
have argued that chemical changes leading to a considerable development of 
heat take place in the pulmonary capillaries. It would appear, however, 
that the right ventricle, owing to its lying nearer to the liver, the high tem- 
perature of which has already been mentioned, is, in reality, rather hotter 
than the left. And, indeed, we have no satisfactory evidence of any large 
amount of heat being produced by any pulmonary metabolism. 
The great regulator, however, is undoubtedly the skin ; and this has a 
more or less double action. In the first place, it regulates the loss of heat 
by means of the vasomotor mechanism. The more blood passes through 
the skin, the greater will be the loss of heat by conduction, radiation, and 
evaporation. Hence any action of the vasomotor mechanism which, by 
causing dilatation of the cutaneous vascular areas, leads to a large flow 
of blood through the skin, will tend to cool the body ; and, conversely, any 500 
NUTRITION. 
vasomotor action which, by constricting the cutaneous vascular areas, or by 
dilating the splanchnic vascular areas, causes a smaller flow through the skin, 
and a larger flow of blood through the abdominal viscera, will tend to heat 
the body. In the second place, besides this, the special nerves of perspiration 
will act directly as regulators of temperature, increasing the loss of heat 
when they promote, and lessening the loss when they cease to promote, the 
secretion of the skin. The working of this heat-regulating mechanism is 
well seen in the case of exercise. Since every muscular contraction gives 
rise to heat, exercise must increase for the time being the production of heat; 
yet the bodily temperature rarely rises so much as a degree centigrade, if at 
all. By exercise the respiration is quickened, and the loss of heat by the 
lungs increased. The circulation of blood is also quickened, and the cuta- 
neous vascular areas becoming dilated, a larger amount of blood passes 
through the skin. Added to this, the skin perspires freely. Thus a large 
amount of heat is lost to the body, sufficient to neutralize the addition 
caused by the muscular contraction, the increase which the more rapid flow 
of blood through the abdominal organs might tend to bring about being 
more than sufficiently counteracted by their smaller supply for the time. 
The sense of warmth which is felt during exercise in consequence of the 
flushing of the skin, is, in itself, a token that a regulative cooling is being 
carried on. In a similar way the application of external cold or heat defeats 
its own ends, either partially or completely. Under the influence of external 
cold, the cutaneous vessels are constricted, and the splanchnic vascular areas 
dilated, so that the blood is withdrawn from the colder and cooler regions to 
the hotter and heat-producing organs. This vascular change may be used 
to explain the fact that stripping naked in a cold atmosphere often gives rise 
to a distinct increase in the mean temperature of the blood, as indicated by 
a thermometer placed in the mouth, though possibly the effect may be partly 
due to an actual increase of the production of heat. Under the influence of 
external warmth, on the other hand, the cutaneous vessels are dilated, a rapid 
discharge of heat takes place ; and if the circumstances be such that the body 
can perspire freely, and the perspiration be readily evaporated, the tempera- 
ture of the body may remain very near to the normal, even in an excessively 
hot atmosphere. Thus, more than a century ago, two observers were able 
to remain with impunity in a chamber heated even to 127° C. (260° Fahr.), 
and with ease in one so hot that it became painful for them to touch the 
metal buttons of their clothing. It is unnecessary to give any more exam- 
ples of this regulation of temperature by variations in the loss of heat; they 
all readily explain themselves. 
§ 447. The production of heat, its variations and regulation. As we 
have already said, the exact determination of the amount of heat produced 
in the living body is attended with great difficulties; still, certain conclu- 
sions have been arrived at based partly on direct calorimetric observations, 
the more recent ones with improved calorimeters being especially valuable, 
and partly on what seem to be trustworthy deductions from observed chemi- 
cal changes. 
The rate of production of heat in a living body is determined by a 
variety of circumstances. In the first place, what may be called the general 
rate of metabolism, and so of the production of heat, varies in different 
kinds of animals. Of two animals of the same bulk and weight placed under 
the same circumstances, one " living faster" than the other, metabolizes its 
living substance more rapidly, and so produces heat more rapidly. Thus 
direct calorimetric observations, as far as they at present go, show that a 
man, on the average, produces more heat, per kilo, per hour, than does a 
dog, and a dog more than a rabbit. Probably every species has what may 501 
THE ENERGY OF THE BODY. 
be called its specific coefficient, and every individual his personal coefficient 
of heat-production, the coefficient being the expression of the inborn qualities 
proper to the living substance of the species and of the individual. 
A larger living body will naturally produce more heat than a smaller 
living body of the same nature, since the larger body possesses, so to speak, 
a greater number of heat-producing units. But this is neutralized by an 
opposing tendency. The smaller body, having relatively to its bulk a 
larger amount of surface, loses heat at a more rapid rate than does the 
larger body; and, therefore, to maintain the balance between loss and pro- 
duction, so as to secure the same constant bodily temperature (and, as we 
have just seen, the bodily temperature of warm-blooded animals is remarka- 
bly uniform), it must produce heat, per unit of its body, at a more rapid 
rate. As a rule, the greater loss of heat owing to the relatively greater 
surface is so marked that of two animals having the same constant bodily 
temperature, of two species of mammals, or of two individuals of the same 
race, we should expect the smaller one to produce a relatively larger amount 
of heat. And direct calorimetric observations show that this is so. The 
struggle for existence has raised what we have just called the specific or 
personal coefficient of the smaller animal. 
From what we have seen concerning the immediate effects of a meal, we 
should be inclined to expect that food would temporarily increase the pro- 
duction of heat; and not only is this view confirmed by common experience 
and by our own sensations, but direct calorimetric observations afford ex- 
perimental proof of its truth. In the dog it has been found that the rate of 
production increases after a meal, reaching its maximum from the sixth to 
the ninth hour, and then declining to a level which may be regarded as that 
secured by the general metabolism of the body, and which appears to be 
maintained with remarkable constancy until after long starvation the 
economy begins to break down. 
Labor, muscular work, has a powerful influence in increasing the pro- 
duction of heat. As we have seen, of the total heat produced in the body, 
a certain portion must always be attributed to muscular contractions, which 
even in the most quiet body are always going on ; in an ordinary active 
body a considerable quantity of heat must be thus generated. Hence, the 
more active the body the greater the production of heat. As we stated 
before (§ 85), in a contraction the proportion of the energy set free to do 
work to that set free as heat appears to vary under different circumstances ; 
and the increase of heat due to labor probably varies in a corresponding 
way. The details of this relation have yet to be worked out, but we may at 
least conclude that, when a man pushes his daily labor beyond the 150,000 
kilogramme-metres, the additional energy thus leaving his body as work done 
is not taken out of the 850,000 kilogramme-metres given in § 442 as the 
average daily output of heat, but the total setting free of energy and the 
total production of heat is at the same time increased. And it need hardly 
be said that the figures in question give only an average estimate for a man 
of average build and weight, taking an average amount of average food, 
and doing an average amount of work. 
§ 448. The production of heat thus determined by these several influ- 
ences, some of which are themselves regulated by the nervous system, is 
further regulated in a remarkable manner. For it is not solely by variations 
in the loss of heat that the constant temperature of the warm-blooded animal 
is maintained. Variations in the amount of heat actually generated in the 
body constitute an important factor, not only in the maintenance of the 
normal temperature, but also in the production of the abnormally high or 
low temperatures of various diseases. Many considerations have long led 502 
NUTRITION. 
physiologists to suspect the existence of a nervous mechanism, by which 
afferent impulses arising in the skin or elsewhere might, through the central 
nervous system, originate efferent impulses, whose effect would be to in- 
crease or to diminish the metabolism of the muscles or other organs, and 
thus to increase or diminish the amount of heat generated for the time 
being in the body. The existence, in fact, of a metabolic or thermogenic 
nervous mechanism, comparable in many respects to the vasomotor mechan- 
ism or to the various secreting nervous mechanisms, seems in itself a priori 
probable. And we have experimental evidence that such a mechanism does 
really exist. 
The warm-blooded animal is distinguished from the cold-blooded animal 
by the fact that when it is exposed to cold or heat it does not, like the latter, 
become colder or hotter, as the case may be, but, within certain limits, main- 
tains its normal temperature. If the maintenance of the temperature of the 
warm-blooded animal during exposure to cold is assisted by an increased 
production of heat, and is not due simply to a diminished loss, there should be 
evidence of an increased metabolism during that exposure. We ought to find, 
under these circumstances, an increased production of carbonic acid and an 
increased consumption of oxygen, since it is to these products, rather than 
to the nitrogenous factors, on the peculiarities of which as uncertain signs- 
of metabolism we have already insisted, we must look for indications of the 
rise or fall of metabolic activity. 
Taking the consumption of oxygen, and, though with less confidence,, 
the production of carbonic acid, as a measure of metabolic activity and so- 
of heat-production, it has been shown that a marked contrast in this respect 
exists between cold-blooded and warm-blooded animals exposed to changes 
of temperature. In the cold-blooded animal, cold diminishes and heat in- 
creases the metabolic activity of the body; as the temperature to which the 
animal is subjected rises or falls, so the consumption of oxygen and produc- 
tion of carbonic acid is increased or lessened. The body of a cold-blooded 
animal behaves in this respect like a mixture of dead substances in a chem- 
ist's retort; heat promotes and cold retards chemical action in both cases. 
Very different is the behavior of a warm-blooded animal. In this case, within 
a lower and a higher limit, cold increases and heat diminishes the bodily 
metabolism, as shown by the increased or diminished consumption of oxygen 
and production of carbonic acid as the temperature falls or rises. In these 
animals there is obviously a mechanism of some kind, counteracting, and 
indeed overcoming, the more direct effects which alone obtain in cold- 
blooded animals. And that this mechanism is of a nervous nature is indi- 
cated by the following facts : 
When a warm-blooded animal is poisoned byurari,the temperature falls 
and the metabolism, measured by the consumption of oxygen and the pro- 
duction of carbonic acid, sinks also; and that the latter is the cause, not the 
effect, of the former is shown by the fact that the metabolism continues to 
fall though loss of heat be prevented by surrounding the animals with wrap- 
pings of cotton-wool. In such a urarized animal, exposure to higher tem- 
peratures augments and exposure to lower temperatures diminishes metab- 
olism; the urarized warm-blooded animal, in fact, behaves like a cold-blooded 
animal. Similar, but perhaps not such striking or so constant results, are 
gained by division of the medulla oblongata. After this operation the tem- 
perature of the body sinks, and the fall, though partly due to increased loss 
of heat by the skin, caused by the dilated condition of the cutaneous vessels, 
is also accompanied by diminished metabolism, and is, therefore, in part due 
to diminished production of heat. And when an animal is in this condition, 
exposure to higher temperatures increases and exposure to lower tempera- THE ENERGY OF THE BODY. 
503 
tures diminishes the bodily metabolism. We can best explain these results 
by supposing that, under normal conditions, the muscles, which as we have 
seen contribute so largely to the total heat of the body, are placed, by means 
of their motor nerves and the central nervous system, in some special con- 
nection with the skin, so that a lowering of the temperature of the skin leads 
to an increase, while a heightening of the temperature of the skin leads to a 
decrease of the muscular metabolism. Further, the centre of this thermo- 
taxic reflex mechanism appears to be placed somewhere in the nervous system 
above the spinal cord. When urari is given, the reflex chain is broken at 
its muscular end; when the spinal cord is divided, the break is nearer the 
centre. 
We may add that the muscular metabolism which thus helps to regulate 
temperature need not involve visible muscular contractions. At the same 
time, the heat given out by the muscles will be temporarily increased at 
every contraction which may occur. Thus, the shivering which follows 
exposure to cold distinctly helps to warm the body; indeed, some observers 
have been led to think that, in man, this visible effect of cold plays a more 
important part in his heat regulation than the invisible actions which we 
have just described. We may also add that the regulative nervous mechan- 
ism may apparently be overborne by an exposure to too great heat or cold. 
When, for instance, the cold to which the animal is exposed becomes exces- 
sive, the reaction of the thermotaxic nervous system is powerless against the 
direct action on the tissues of the depressing influences, and the metabolism, 
together with the temperature, sinks. 
§ 449. In a number of experiments it has been shown that injuries to, 
such as those caused by puncture or galvanic cautery, or electrical stimula- 
tion of, limited portions of the more central portions of the brain may give 
rise to a great increase of the temperature of the body without producing 
any other marked symptom. The increase is shown by the increase of met- 
abolism, increased production of carbonic acid, and increased consumption 
of oxygen, as well as by direct calorimetric observations, to be due to an 
increased production of heat. This naturally suggests that the portions of 
the brain in question contain the hypothetical heat centre just mentioned, 
the lesion on stimulation exciting the centre to activity by direct action on 
it, instead of in the usual reflex manner. The matter has not, however, as 
yet been clearly worked out; and indeed observers are not agreed as to the 
exact parts of the brain injury to which, or stimulation of which, produces 
the effect. 
§ 450. By regulative mechanisms of the kind just discussed the tem- 
perature of the warm-blooded animal is maintained within very narrow 
limits. In ordinary health the temperature of a man varies between 36° 
and 38°, the narrower limits being 36.25° and 37.5°, when the thermometer 
is placed in the axilla. In the mouth the reading of the thermometer is 
somewhat (0.25° to 1.5°) higher; in the rectum it'is still higher (about 
0.9°) than in the mouth. The temperature of infants and children is 
slightly higher and much more susceptible of variation than that of adults, 
and after forty years of age the average maximum temperature (of health) 
is somewhat lower than before that epoch. A diurnal variation, independent 
of food or other circumstances, has been observed, the maximum ranging 
from 9 A.M. to 6 p.m. and the minimum from 11 p.m. to 3 a.m. Meals cause 
sometimes a slight elevation, sometimes a slight depression, the direction of 
the influence depending on the nature of the food-alcohol seems always to 
produce a fall. Exercise and variations of external temperature, within 
ordinary limits, cause a very slight change, on account of the compensating 
influences which have been discussed above. The rise from even active 504 
NUTRITION. 
exercise does not amount to 1° ; when labor is carried to exhaustion a de- 
pression of temperature may be observed. In travelling from very cold to 
very hot regions a variation of less than a degree occurs, and the tempera- 
ture of inhabitants of the tropics is practically the same as of those dwell- 
ing in arctic regions. 
§ 451. Many of the maladies of the body are characterized by an in- 
crease of the bodily temperature known as " fever " or " pyrexia," the 
thermometer very frequently rising to 39° or 40°, not unfrequently to 41°, 
and at times reaching 43° or even 44° ; but these higher temperatures can- 
not long be borne without the organism failing. And, as we have said, any 
increase in man of the bodily temperature beyond 38°, or even beyond 37.5°, 
indicates some disturbance. In most cases the rise of temperature has a 
definite objective cause, some local inflammation or suppuration, or, as in 
specific fevers, the presence in the economy of some " materies morbi," of 
the nature of an organized germ or of some other nature. We cannot here 
discuss the connection between local inflammation or the specific poison and 
the high temperature, but we have increasing evidence that the high tem- 
perature of fever is due, not merely to a diminution of the loss of heat, 
though this may be a factor, but also, and indeed chiefly, to an increased 
production of heat. In fever the production of carbonic acid and the con- 
sumption of oxygen, that is to say, the metabolic changes of the tissues, are 
increased. The urea also is increased, and that in such a way as to confirm 
the view already expressed that much of the heat comes from such a metab- 
olism of the skeletal muscles as, unlike an ordinary contraction, directly 
involves the nitrogenous elements. The inordinate metabolism of the body 
at large thus characteristic of fever is shown by the wasting which it entails. 
Calorimetric observations also show in a direct manner that the production 
of heat is increased. Of course, mere increased production alone would be 
insufficient to raise the temperature of the body, for it might be met, up to 
a very high limit, by a compensating increase of loss of heat; but in fever 
this compensation is wanting, and it is perhaps this absence of due regu- 
lation which is most characteristic of the febrile condition. 
In some maladies the bodily temperature falls distinctly below the normal 
average, reaching for instance 35°, or even lower. In such cases there can 
be little doubt that the condition is due to diminished metabolism and 
diminished heat production. 
One of the most marked phenomena of starvation is the fall of tempera- 
ture, which becomes very rapid during the last days of life The lowered 
metabolism diminishes the production of heat, and the lowered temperature 
in turn still further diminishes the metabolism. Indeed, the low tempera- 
ture is a powerful factor in bringing about death, for life may be much 
prolonged by wrapping a starving animal in some bad conductor, so as to 
economize the bodily heat. 
§ 452. Effects of great heat. As we said above, the regulative heat 
mechanism is unable to withstand the strain of too great an external heat or 
too prolonged an exposure to a great but less degree of heat. The tempera- 
ture of the body then rises above the normal; and it has been observed that 
the temperature is more easily raised by warmth than depressed by cold, at 
least when neither is very intense. When either in this way by external 
warmth or through pyrexia the temperature of the body is raised some 6° 
or 7° above the normal, to 45° or thereabouts, death speedily ensues. The 
chain of events thus leading to death has not been as yet clearly made out, 
and most likely the events do not take exactly the same course in all cases; 
but we shall probably not go far wrong in attributing death to the fact that 
the high temperature hurries on the metabolism of the several tissues, of 505 
THE ENERGY OF THE BODY. 
some more than others, at such a spendthrift rate that their capital is soon 
exhausted. We have seen (§ 314) that too warm blood produces dyspnoea 
and soon exhausts the metabolic capital of the respiratory centre. Too 
warm blood similarly hurries on the beats of the heart; an explosion of the 
contractile substance is each time prematurely brought on before a sufficient 
quantity of explosive substance is accumulated, each stroke becomes more 
and more feeble as the rate is quickened, the beats become irregular and 
finally cease. Either of these two events alone and certainly both together 
are enough to bring the working of the bodily mechanism to an encl ; but 
other tissues beside the heart and the respiratory centre are suffering in the 
same way, notably the rest of the central nervous system. This, too, is 
being hurried on unduly in its inner changes, so that not only consciousness 
is lost and other objective manifestations of nervous action go wrong or fail, 
but the regulative grasp of the central nervous system on the tissues of the 
body at large is loosened, and tumult takes the place of order. Whether 
this or that sign of disorder comes to the front, whether, for instance, 
convulsions take place, would appear to depend upon the exact turn 
taken by the abnormal events. In heat-stroke, more commonly known as 
sun-stroke, the essential condition of which seems to be a rapid rise of the 
temperature of the body, owing to a sudden failure of the thermotaxic 
mechanism, the symptoms vary. Sometimes the heart suddenly gives 
way, at other times the respiratory centre seems to be more directly 
affected; sometimes convulsions make their appearance, but more com- 
monly death takes place through a comatose condition of the brain, an 
initial phase of excitement of the central nervous system being not unfre- 
quently witnessed. 
Mammalian muscle, it will be remembered (§ 82), becomes rigid at 
about 50° ; but death probably always occurs before that higher tempera- 
ture is reached by the blood, so that a sudden rigor mortis from heat 
(rigor caloris) cannot be regarded as a factor in death from exposure to 
too great heat. 
§ 453. Effects of great cold. The effects of a too great lowering of the 
temperature of the body, which is, generally, the result of too great ex- 
ternal cold, and rarely, if ever, arises from internal causes lowering the 
metabolism and thus the production of heat, are, in their origin, the reverse 
of those of a too high temperature. The metabolism of the tissues is low- 
ered ; and not only are the katabolic changes, which lead to the setting free 
of energy thus affected, but the anabolic changes also share in the depres- 
sion. Thus " living substance" falls to pieces less readily, but is also made 
up less readily ; and could this slackening of metabolism be carried on in 
the several tissues at a rate proportionate to the rate at which each tissue 
lives, life might thus be brought to a peaceful end by gradual arrest of the 
life of each part of the whole body. And, indeed, in some cases, where the 
lowering of the temperature takes place gradually, something like this does 
occur even in warm-blooded animals. The diminished metabolism tells first 
and chiefly on the central nervous system, especially on the brain, and more 
particularly on those parts of that organ which are concerned in con- 
sciousness. The intrinsic lowering of the cerebral metabolism is further 
assisted by a slowing of the heart-beat and of the breathing; drowsiness is 
succeeded by a condition very like to, if not identical with, that known 
as sleep, which we shall study later on, but by a sleep which insensibly 
passes into the sleep of death. In some cases, however, especially those 
in which the lowering of the temperature is sudden and rapid, disorders of 
the nervous system intervene and convulsions like those of asphyxia are 
produced. 506 
NUTRITION. 
On Nutrition in General. 
§ 454. It may now be profitable to take a brief survey of the various 
conclusions at which we have arrived concerning the problems of nutrition. 
We have seen that the several tissues, using lymph as a medium, live 
upon the blood, taking up from the blood the materials for, and returning 
to the blood the products of, their metabolism. The blood itself we have 
also seen to be replenished with food from the alimentary canal and with 
oxygen from the lungs, and to be freed from waste products by means of 
the excretory organs. In this double action the raw material of the food,, 
on the one hand, undergoes between its being placed in the mouth and 
its taking part in the metabolism of the tissue which ultimately uses 
it, many intermediate changes carried on in various parts of the body,, 
and the waste products similarly undergo intermediate changes between 
leaving the tissue and appearing in the urine, the sweat, or the expired 
air. 
We have further seen reason to think that the metabolic events of the 
body take place, in the main, in the tissues, not in the blood stream on its 
way between the heart and the tissues. Changes proper to the blood 
itself take place in the blood ; the corpuscles, red and white, with the 
plasma undergo, like the rest of the body, their proper metabolic cycles,, 
and in this sense blood may be called a tissue, if there is any advantage 
in using the phrase; but, apart from these intrinsic blood changes, as far 
as we can see at present the metabolism undergone, during their transit 
along the blood channels, by the substances which are merely carried in 
the blood from place to place, is an insignificant part of the total metab- 
olism of the body. 
By metabolism of a tissue we understand the total chemical changes 
taking place in the tissue; and we divide these changes into those which 
either directly or indirectly are concerned in the building up (anabolic), 
and those which are, in like manner, concerned in the breaking down 
(katabolic) of the living substance. We shall explain presently what we 
mean by the words " directly " and " indirectly," used in this connection. 
And we may here repeat the caution (§ 30) that though, for convenience 
sake, we use the phrase " living substance," what is really meant by the 
words is not a thing or body of a particular chemical composition, but 
matter undergoing a series of changes. 
§ 455. Since the several tissues originate through a differentiation of 
the simpler primordial protoplasm, we may infer that we have a right to 
speak of a general plan of metabolism common to all the tissues, modified 
in various particulars in various tissues. It is more reasonable, for instance, 
to suppose that there is such a general plan common to both muscle and 
gland, than to suppose that the metabolism of the one differs wholly from, 
or only accidentally resembles that of the other. And we may profitably 
take the nutrition of muscle as exemplifying, in the midst of the features 
special to the muscle, the general plan of vital metabolism. The muscle, in 
a normal state of things, lives ultimately on the proteids, fats, carbohydrates, 
salts, and water of the food, and on the oxygen of the inspired air, but 
lives directly on the blood which brings these things to it. Taking the pro- 
teids first, we may ask the question, How does the blood supply the muscle 
with proteids? 
The blood contains three classes of proteids: 1, serum-albumin ; 2, para- 
globulin, and 3, fibrinogen. With regard to the function of these three 
kinds of proteids in the nutrition of muscle, our only conclusions at present 
are indirect onces, based chiefly on the results of experiments as to the rel- 507 
ON NUTRITION IN GENERAL. 
ative value of these substances in maintaining or restoring the irritability 
of muscle. It is found that when the washed-out frog's heart (§ 148) is fed 
with defibrinated blood, the restoration is as good as with whole blood ; and 
that while the effects of globulin are uncertain, and while peptone and albu- 
mose appear to act in an injurious manner, the restorative effects of serum- 
albumin are marked. From these results we may provisionally infer that 
the muscle in its (total) anabolic changes takes up and so lives upon the 
serum-albumin of the blood. But this conclusion must be regarded as pro- 
visional only, and indeed uncertain. For we must remember that the blood 
supplies not only the food (including oxygen) for the muscle, but also the 
conditions under which the muscle can live and avail itself of the food 
offered to it. The complex actions through which a certain quantity of 
proteid and other material is built up into living muscular substance, need 
for their execution a favorable medium, need certain physical and chemical 
conditions; and it may be that the favorable influence of serum-albumin is 
simply due to its presence in some way assisting the transformation into 
living substance of raw material still remaining in the muscular fibres and 
not to its supplying new raw material. 
Dextrose is, as we have repeatedly said, always present in the blood in 
small quantity, and appears to be the only carbohydrate contituent of blood- 
plasma. Experiments carried out on a large animal, such as the horse or 
cow', have shown that the venous blood coming from a muscle contains less 
dextrose than the arterial blood going to the muscle, and that the difference 
is much increased by throwing the muscle into contraction. From this we 
may provisionally conclude that dextrose is an essential part of the food of 
the muscle. 
The blood, as we have seen, also contains a certain amount of fat; and 
if we push the analogy between the whole body and the muscle, we may 
infer that the muscle takes up fat as food for itself from the blood. But 
we have no experimental evidence in favor of this. Moreover, we have 
seen that fat and carbohydrate are, in the animal body, more or less trans- 
ferable. We have distinct proof that the body can transform carbohydrate 
into fat; and it is very probable that it can transform fat into carbohydrate. 
Seeing how much more easily a soluble diffusible carbohydrate like sugar 
can be carried from place to place by the fluids of the body than can im- 
miscible fats, it seems reasonable to suppose that when the body has to draw 
upon its store of fat in the cells of adipose tissue, the fat, on leaving the fat- 
cell, is transformed into sugar, its carbon, so to speak, being dealt out to the 
tissues in the form of dextrose. Indeed, we may, perhaps, dwelling on the 
fact that a muscle, though itself essentially of proteid build, turns over 
(§ 85) in its daily work so much more carbon than nitrogen, entertain the 
view that what muscle wants as food is a certain amount of proteid plus an 
additional quantity of carbon in some form or other, and that dextrose is a 
convenient form in which the additional carbon can be supplied. And we 
may hold this view without prejudice to any opinion that the carbon so 
brought, while being built up into the living substance, may be again ar- 
ranged as fat, and in the course of the metabolism of the muscle may be 
later on separated from the living substance and deposited in the fibre as 
globules of fat. But our knowledge is at present insufficient to decide 
whether this view is true or not. 
The various salts brought to the muscle by the plasma, though they supply 
no energy, are as essential to the life of muscle as the energy-holding proteid 
or carbon compound ; and experiments made with regard to some of them, 
calcic salts, for instance, show that their presence or absence materially affects 
the maintenance or restoration of irritability. Some of these probably play 508 
NUTRITION. 
the part only of securing by their presence favorable conditions for the due 
metabolic processes, somewhat after the way in which the presence of calcic 
phosphate determines the curdling of milk ; but some we probably ought to 
regard as actually entering into the processes themselves. Of these matters, 
however, we know very little. 
§ 456. The end-products of muscular metabolism are, as we have seen, 
carbonic acid, lactic acid, and kreatin,or some other nitrogenous bodies, and 
we have already (§ 85) said all we have to say concerning the formation of 
these products. We may, however, briefly consider here the question, What 
is the relation of these various metabolic processes to the structural elements 
of the tissue? When we say that the muscular fibre is continually under- 
going metabolism do we mean that very jot and tittle of the fibre is under- 
going change and that at the same rate? We can hardly suppose this. It 
seems unlikely, for instance, that the metabolism of the fibrillar substance is 
identical with that of the interfibrillar substance, whatever be the view we 
take as to the properties or meaning of the two substances. Further, if we 
accept the suggestion made in § 85 as to a contractile substance, which, 
though having peculiar qualities, being peculiarly related to and having 
peculiar connections with the rest of the fibre, may in a broad way be com- 
pared with the glycogen of an hepatic cell, we can conceive that this contrac- 
tile substance may be manufactured without the whole of it at least having 
been at any time an integral part of what we may in a stricter sense call the 
real living substance of the fibre. We should thus be led to regard the 
metabolic events occurring in muscle as falling into two classes at least: 
those taking place in the living more permanent framework, and those bear- 
ing on the formation and destruction of the contractile substance lodged in 
that living framework. Further, if we suppose that the metabolism by 
which the muscles supply so much of the heat of the body, and which, as we 
have seen, may and does go on independently of contractions, is not a me- 
tabolism of the same contractile substance differing from the metabolism of 
a contraction in being so ordered that all the energy goes out as heat, none 
being employed to effect a change of form, but is a metabolism of some other 
" thermogenic " substance, we should have to add a third class to the other 
two. These, of course, are at present matters of speculation ; but on the 
whole, the evidence we can gather tends, and perhaps increasingly tends, to 
show that in muscle there does exist such a framework of what we may call 
more distinctly living substance which rules the histological features of the 
fibre, and whose metabolism though high in quality does not give rise to 
massive discharges of energy, and that the interstices, so to speak, of this 
framework are occupied by various kinds of material related in different 
degrees to the framework and therefore deserving to be spoken of as more 
or less living, the chief part of the energy set free by muscle coming directly 
from the metabolism of some or other of this material. And the same view 
may be extended to other tissues. Both the framework and the intercalated 
material undergo metabolism, and have, in different degrees, their anabolic 
and katabolic changes ; both are concerned in the life of the living substance 
but one more directly than the other, and this is what was meant by the 
terms "directly" and "indirectly," used in §454. Such a mode of expres- 
sion seems preferable to the more common one, based on the analogy of a 
firearm, of the muscle fibre firing off the contractile material; in the firearm 
there are no such connections between the machine and the charges as obtain 
in the living mechanism. We may perhaps further be led by this to dis- 
tinguish between growth as bearing on the framework, and mere temporary 
nutrition as bearing on the accumulation and expenditure of the lodged 
material. We may add that since some of the material so lodged in the 509 
ON NUTRITION IN GENERAL. 
framework will consist of substances which have not yet undergone metabo- 
lism, but are either about to be worked up into the framework itself, or are 
about to be transformed in a more direct way into some product of metabo- 
lism, or are substances whose presence is in some way necessary for the car- 
rying on of metabolic processes in which they themselves take no bodily 
part, we must recognize a continuity without any sharp break between this 
material which we regard as part of the tissue, and the lymph which simply 
bathes the tissue and flows through the interstices. Hence such phrases as 
" tissue proteid " and " floating proteid " (§ 436), are undesirable if they are 
understood to imply a sharp line of demarcation between the "tissue" and 
the blood or lymph, though useful as indicating two different lines or degrees 
of metabolism. 
§ 457. The products of muscular metabolism pass into the lymph bath- 
ing the fibre, and so, either by a direct path into the capillaries or by a more 
circuitous course through the general lymphatic system, into the blood. The 
fate of the carbonic acid we have fully treated of in dealing with respiration ; 
the little we know concerning the nitrogenous product or products has been 
stated in dealing with urea ; the third recognized product is lactic acid, 
sarcolactic acid. Did any considerable amount of oxidation take place in 
the blood stream while the blood is flowing along the larger channels, sub- 
ject only to the influence of the vascular walls, we might fairly expect that 
the lactic acid discharge from the muscles would be subjected to oxidizing 
influences while still within the blood stream of the larger channels. We 
have, however, no satisfactory evidence of any lactic acid being oxidized in 
this way. On the contrary, there is a certain amount of experimental and 
other evidence that lactic acid present in the blood is somehow or other dis- 
posed of by the liver; and that if the liver fail to do its duty, lactic acid 
may appear in the urine. It is tempting to suppose that it might there by 
a synthetic effort be converted into glycogen, the liver thus utilizing some 
of the muscular waste product, but the experimental and other evidence is 
all against this view. In the absence of actual knowledge we infer that it 
is in the liver oxidized into carbonic acid and water, thus adding its con- 
tribution to the supply of heat, or prepared in some way for oxidation else- 
where. Probably such a change is not confined to the liver, but takes place 
in other organs, such as the spleen. Thus the kind of action on which we 
dwelt in treating of urea, namely, that the products of the metabolism of 
one organ are carried to other organs for further elaboration and possible 
utilization, applies to the non-nitrogenous as well as to the nitrogenous pro- 
ducts of muscular metabolism; and if a muscle gives rise to other non-nitro- 
genous products than carbonic and lactic acid these are probably disposed of 
in some such way as the lactic acid. In speaking of glycogen in the winter 
frog (§ 380) we said that possibly the glycogen so stored up might arise 
from sugar brought to the liver from other tissues. If that be so, we should 
further expect that some at least of that sugar, either as such or as some 
allied substance, would come from the skeletal muscles which form so large 
a part of the body of the frog; and if so, we must conclude that under the 
special circumstances obtaining in the winter frog the muscles discharge into 
the blood a non-nitrogenous product not in the form either of carbonic or 
lactic acid. It is perhaps, however, more probable that the sugar in ques- 
tion comes from a metabolism of the fat stored up in the" fatty bodies " and 
elsewhere. 
§ 458. As far as we can see at present the plan of nutrition thus briefly 
sketched out for muscle holds good for the other tissues as well, the chief or 
at least the most conspicuous differences bearing on the nature and proper- 
ties of and the changes undergone by the material formed by and held by 510 
NUTRITION. 
the more distinctly structural work. Thus the mucin of the salivary mucous 
cell finds its analogue either in the contractile substance itself, or more prob- 
ably in some early nitrogenous product of the explosion of the contractile 
substance, such as may correspond to the myosin of rigid muscle. The 
metabolism of the hepatic cell seems, as we have seen, to be especially cha- 
racterized by its returning to the blood a body, viz., sugar, still containing a 
considerable amount of energy, available for use in other parts of the body. 
And this suggests the question whether in the normal metabolism of mus- 
cular substance a similar something, still holding a considerable quantity 
of energy, some proteid substance for instance, may not be returned to the 
blood; so that the metabolism of muscle is imperfectly described in saying 
that the results are carbonic and lactic acids and an antecedent of urea. If 
this be so, then muscles may be of other use to the body at large than as 
mere contractile machines, just as the liver has other uses than the produc- 
tion of bile. And the same considerations may be applied to the other 
tissues as well. 
§ 459. Whether the chief product of the metabolism of any tissue be a 
proteid substance, or a fat, or a carbohydrate, proteid substance is the pivot, 
so to speak, of the metabolism, and nitrogenous bodies always appear as the 
products of metabolism. This is strikingly seen in the nutrition of plants 
where, as far as mere bulk or weight is concerned, the active metabolizing 
tissue is insignificant compared with the mass of products of metabolism 
heaped up in the form of starch or cellulose or some allied carbohydrate. 
The protoplasm of a vegetable cell soon becomes a mere film bearing a heavy 
burden of heaped-up metabolic products and eventually disappears; and 
of that film only a part corresponds to what we spoke of above as the living 
framework of the muscle. Yet that scanty proteid-built framework is more 
or less directly concerned in the production of the carbohydrate material 
and the various conversions which that material undergoes. Proteid, nitro- 
gen, changes are entangled with the carbon changes; and since the products 
of metabolism in the plant are not as in the animal cast out of the organism, 
but for the most part heaped up within it, we find the plant storing up in 
parts, where if they serve no useful purpose they at least do no harm, nitro- 
genous products of metabolism, such as those known as vegetable alkaloids, 
many of which by their amide nature betray their kinship to the animal 
nitrogenous product urea. 
§ 460. The rate at which in the adult, leaving aside for the present the 
special nutrition of the young, nutrition is carried on, and the characters of 
the nutrition, are dependent on a variety of circumstances. Each tissue has 
of course a line of nutrition of its own which circumstances may favor or 
hinder, but cannot change in nature ; the nutrition of the hepatic cell cannot 
be altered to that of the muscular fibre. The same tissue, moreover, has in 
different racesand different individuals specific and individual characters of 
nutrition ; the flesh of a dog is not the same as that of a man, the muscle of 
one man lives differently from that of another, the metabolism per unit of 
body weight is, as we have seen, greater in the smaller organism, and so on. 
Within the limits and subject to the conditions, however, thus fixed by 
race and personality, general influences produce general variations in nutri- 
tion. The rate of nutrition of a tissue, for instance, is dependent on the food, 
on the amount and nature of the food material brought to the tissue by the 
blood. We have seen that proteid food, in contrast to carbon food, markedly 
increases the metabolism of the body. Since this increase tells not only on 
the nitrogenous but also on the carbon metabolism (§ 437), it cannot be the 
result of a mere luxus consumption of the proteid food itself; and unless we 
suppose that the presence of the excess of proteid material either in the ali- ON NUTRITION IN GENERAL. 
511 
mentary canal, or while passing through the capillaries of some organ, such 
as the liver, acts as a stimulus to some reflex nervous machinery through 
whose action the metabolism of certain or of all the tissues is hurried on, we 
must conclude that it is the direct access of proteid material to the tissues 
themselves which stirs them up to increased metabolic activity. That pro- 
teid food should do this, and not carbohydrate or fat, seems to be connected 
with the fact just dwelt on that proteid material is the pivot of metabolism. 
§ 461. In the preceding chapters of this work we have had abundant 
evidence that the metabolism of the tissues is subject to the government of 
the central nervous system; the contraction of a muscle, the secretory activity 
of a gland, the increased or diminished production of heat, all afford in- 
stances of nervous impulses affecting metabolism. In most of these instances 
the changes induced fall within the downward, katabolic phase and have a 
downward character; thus, when a muscle contracts, the result is a conver- 
sion of more complex bodies into simpler bodies; and the same, as far as we 
can see, is true of most other cases. But it is open for us to suppose that 
nervous impulses might affect the upward, anabolic phase and have a con- 
structive influence. There are no reasons for regarding such an action as 
impossible; and indeed some phenomena, such as those of inhibitory nerves 
and the antagonism between these and augmentor nerves, pointedly suggest 
some such view. Thus, we may suppose that an inhibitory impulse produces 
such changes in the cardiac muscular substance that the upward constructive 
processes are assisted and the downward disruptive processes checked, whereby 
the setting free of energy is checked, and so the beats hindered or stopped, 
the inhibitory effect being followed by a period of rebound in which the 
savings of the inhibited period are spent in increased action. Conversely we 
may suppose that an augmentor impulse hinders the anabolic and assists the 
katabolic changes, and conversely also, when it has done its work, leaves the 
tissue with diminished capital manifested by feebler beats or by the absence 
of the power to beat. And similarly in the case of the respiratory centre 
and other tissues. When we have to study the origination of visual im- 
pulses in the retina we shall come upon a view that a wave of light may 
affect what we shall call a visual substance either by promoting anabolic 
constructive changes or by increasing katabolic destructive changes accord- 
ing to its wave length. There is then evidence, to a certain extent, for the 
view on which we are dwelling ; but, without discussing the matter any fur- 
ther, we may say that the conception, though suggestive, has not yet been 
•demonstrated, and so far can only be spoken of as probable. 
§ 462. One value perhaps of such a view lies in the fact that it warns us 
against assuming that a nervous impulse can only produce disruptive kata- 
bolic changes such as are seen in muscular contraction or in secretion. The 
effects of stimulating a nerve going to a muscle or a salivary gland are strik- 
ing and obvious, and the behavior of a muscle or a gland as far as contraction 
and secretion are concerned is, within certain limits, under experimental con- 
trol. But there are certain phenomena, seen chiefly in the course of disease, 
and lying, to a very small extent only, within the control of experiment, 
which seem to show that the central nervous system governs the metabolic 
changes, the nutrition, not only of muscle and gland, but of various other 
tissues, in a deeper and more general way than that of simply promoting (or 
hindering) contraction or secretion. Thus, as we have seen (§ 81), when the 
connection between a muscle and the central nervous system is severed, the 
muscle eventually wastes and loses its vitality; when all the nerves going to 
the submaxillary gland are severed, the gland, instead of being, as in the 
normal condition, intermittingly active and quiescent, pours forth a continu- 
ous "paralytic" secretion and eventually degenerates and wastes. When in 512 
NUTRITION. 
a rabbit the fifth nerve is divided in the skull the loss of sensation in those 
parts of the face of which it is the sensory nerve is followed by nutritive 
changes. Very soon, within twenty-four hours, the cornea becomes cloudy; 
and this is the precursor of an inflammation which may involve the whole 
eye and end in its total disorganization. At the same time the nasal cham- 
bers of the side operated on are inflamed, and very frequently ulcers make 
their appearance on the lips and gums. And similar results have been seen 
in other animals, including man. If the operation be conducted in a young 
animal, which subsequently lives to maturity, the head may become bilater- 
ally unsymmetrical, as shown especially by the skull. Again, division of 
both vagus nerves is very apt to be followed by inflammation of both lungs, 
by fatty degeneration of the heart, and so by death. 
In several of these instances the effect is a mixed one, and the problem 
complicated. Thus, in the case of division of the fifth nerve, seeing how 
delicate a structure the eye is, and how carefully it is protected by the mech- 
anisms of the eyelids and tears, it seems reasonable to suppose that the in- 
flammation in question might simply be the result of the irritation caused 
by dust and contact with foreign bodies, to which the eye, no longer guided 
and protected by sensations, these being destroyed by the section of the nerve, 
became subject. In the same way the ulcers on the lips and gums might be 
explained as injuries inflicted by the teeth on those structures in their insen- 
sitive condition. And some observers maintain that the inflammation of the 
eye may be greatly lessened or altogether prevented if the organ be carefully 
covered up, and in all possible ways protected from the irritating influences 
of foreign bodies. Other observers, however, have failed to prevent the 
inflammation in spite of every care. So, also the inflammation of the lungs 
following upon division of both vagus nerves seems to be due, not to any 
direct nutritive action of the pulmonary branches of the vagus on the pul- 
monary tissue, but to food accumulating in the pharynx, owing to the paral- 
ysis of the oesophagus and larynx, and then passing into the air passages, 
and so settingup inflammation. Death in these cases is, moreover, often the 
simple result of inanition caused by the paralysis of the oesophagus allowing 
no food to reach the stomach. The phenomena of the paralytic secretion of 
saliva are also of a complicated nature. 
But even without insisting on such instances as the above, various other 
phenomena of disease seem to indicate such an influence of the nervous 
system on nutrition as we are discussing. As examples we might mention 
the rapid and peculiar degeneration of and loss of contractility in the skeletal 
muscles in certain affections of the spinal cord, the changes in the muscles 
being more rapid and profound than in the nerves; the phenomena of bed- 
sores, especially the so-called acute bedsores of cerebral apoplexy; some at 
least of the cases of vesical affections attendant on spinal cerebral diseases 
or injuries ; the more rapid atrophy and loss of contractility in muscles which 
follow upon contusions of nerves as compared with the effects of simple sec- 
tion of nerves; the occurrence of certain eruptions, such as lichen, zona, 
ecthyma, etc., in various spinal or cerebral diseases, and indeed the general 
phenomena, and especially the topography of the eruption, of a large number 
of cutaneous diseases. Lastly, but not least, we might quote the general 
process of inflammation. These are examples of disordered nutrition. To 
them we might add as instances of altered but yet orderly nutrition the 
remarkable connections observed between changes in the form of the fingers 
and growth of the nails and hairs, and certain internal maladies, such, for 
instance, as the "clubbed fingers" of phthisical and other patients, and the 
like. We might also call attention to the influence of light on the nutrition 
of animals. The experience of blind people and blind animals indicates ON DIET. 
513 
some special connection between visual sensations and the nutrition of the 
skin ; and this can hardly be other than a nervous connection. The effects 
of prolonged darkness on nutrition in general and the experimental results 
which show that the total metabolism of the body is influenced by light, also 
suggest some nervous action. The influence of cold, again, in determining 
the growth of hair points in the same direction. 
Making every allowance for the intervention in the production of the 
phenomena quoted above of such factors as common actions of the nervous 
system already well known to us, such as vasomotor changes, making every 
allowance for the consequences of the failure or bluntness of sensation and 
the absence of those beneficial after-results of muscular activity which we 
pointed out in § 86, recognizing, moreover, that changes in one organ may 
affect the condition of other distant organs by changes induced in the com- 
position or qualities of the blood, there still remains a residue which seems 
distinctly to point to the conclusion that the influence of the nervous system 
is not limited to such changes of the muscles as belong to the production of 
contractions or the generation of heat, but bears on the whole nutrition of 
the muscle. Similar considerations lead us also to conclude that the influ- 
ence of the nervous system bears on the whole nutrition of the glands, of the 
bloodvessels, of the skin, and of the connective tissue in general, in fact of 
nearly the whole body. 
On Diet. 
§ 463. An ordinary man living an ordinary life will need for the main- 
tenance of vigorous health a certain amount of food of a certain kind ; this 
we may take as a normal diet. 
Presuming that the experience of man has led him to adopt what is good 
for him, we may ascertain approximately the normal diet by means of the 
statistical method, by examining the nature and amount of the daily food of 
a very large number of individuals. The most valuable data for this pur- 
pose are those gained by inquiries among persons who choose their own food ; 
the results gained from the diets used in prisons or other institutions, or 
among bodies of men such as the army, though more readily arrived at, are 
open to the objection that the diets in question are determined in part by the 
theoretical opinions of those whose duty it is to fix the diet. Putting together 
the various statistical results thus obtained, and selecting the quantities which 
seem to be most commonly used rather than attempting to strike a strict 
average or take a strict mean, we find that in an ordinary diet for the twenty- 
four hours the several food-stuffs are: 
Proteids from 100 to 130 grammes. 
Fats " 40 to 80 " 
Carbohydrates " 450 to 550 " 
to these we must add 
Salts 30 grammes. 
Water 2800 " 
The total (available) potential energy of the lower estimate is 2610, of the 
higher 3505 (kilogramme-degree) calories, calculated, in round numbers, on 
the data of § 441. With such a statistical diet we may compare an experi- 
mental diet, that is to say a diet arrived at through a series of trials on an 
individual man whose body might be taken to be an average one, that diet 
being considered a normal one in which the body, maintaining vigorous 514 
NUTRITION. 
health, neither gained nor lost in weight, and remained, moreover, in nitro- 
genous equilibrium with the nitrogen of the egesta equal to that of the ingesta. 
To make sure that under such a diet the body was remaining of the same 
composition, there ought to be evidence of a carbon equilibrium also, other- 
wise during the period of the experiment fat might be replaced by water 
(see § 435; ; but this is unlikely, and we may therefore accept the method 
as a fair one. It has given in the hands of two different observers the 
following somewhat different results, the diet A being that already quoted 
in § 441: 
A B 
Proteids 
Fats 
100 " 56 
Carbohydrates 
240 " 500 
Salts 
25 " 
W ater 
2600 " 
The total (available) potential energy is respectively 2310 and 3035 calories. 
On the whole, the diets gained by the two methods agree very largely. 
To put down a single column of figures as " the normal diet" would be to 
affect a vain and delusive accuracy. If we desire, for theoretical purposes, 
to select some one set of figures rather than others, we might be influenced by 
the considerations that the lower amount of proteids in the experimental 
diet was nearer the mark than the higher amount of some of the statistical 
diets, and further that, where cost is not of moment, the substitution of fat 
for an excess of carbohydrates is desirable. We should be thus led to take 
the experimental diet A as on the whole the best or most " normal " one, and 
that is the one which we employed in the calculations of § 441. It will be 
observed that the potential energy of this diet is less than that of any of the 
others, and, as we said while then speaking of it, may be considered low; 
but there was no evidence that it was insufficient. Still it must be remem- 
bered that neither it nor any of the others is to be regarded as distinctly 
proved to be the real normal diet. Against the experimental diet we may 
urge that the number of experiments have been few, and conducted on a few 
individuals only at most, and that a larger number of experiments, with a 
variety of combinations of different amounts of the several food-stuffs, might 
lead to a different result; that, for instance, with certain amounts of fats and 
carbohydrates, the amount of proteid needed to maintain healthy bodily 
equilibrium, including nitrogenous equilibrium, might be reduced much be- 
low the 100 grammes, especially if particular kinds of proteids, fat, or carbo- 
hydrates were used, and especial attention (see § 440) were paid to the salts. 
And, indeed, a considerable number of observations have been made tending 
to show that a man of average size and weight may continue in nitrogenous 
equilibrium and in good health with a daily ration of much less than 100 
grammes proteid, with as little as 40 grammes for example. To this we 
shall have to refer in speaking of a vegetable diet. Against the statistical 
diet, on the other hand, we may urge that instinct is not an unerring guide, 
and that the choice of a diet is determined by many other circumstances 
than the physiological value of the food. 
§ 464. Taking, however, some such diet as the above to be the approxi- 
mately true normal diet, we may call attention to the fact that the normal 
diet is made up of each of the three great food-stuffs, carbohydrates being in 
excess. We may here remark incidentally that the diets of both the car- 
nivora and herbivora agree with that of omnivora in containing all three 
food-stuffs; they differ from each other as to the relative proportions only. 
As we have seen, the body may be maintained in equilibrium on proteid food ON DIET. 
515 
alone; but an exclusively proteid diet is not only bought dearly in the 
market, but also paid for dearly within the economy ; we are, of course, 
now speaking of man. To obtain the necessary carbon out of the carbon 
moiety of proteid unnecessary labor is thrown on the economy, and the system 
tends to become blocked with the amides and other nitrogenous waste arising 
but of the nitrogen moiety simply thrown off to secure the carbon. 
Fats and carbohydrates are much more akin to each other than is either 
to proteid ; and if, on the one hand, as (§ 455) seems possible or even prob- 
able, the fat of the food and of the body is converted into sugar either on 
its way to become built up into the tissue, or in the course of the changes 
taking place outside the real living framework of the tissue by which it is 
reduced to carbonic acid, and that, on the other hand, carbohydrates can 
furnish the fat whose presence in the body is necessary, we might expect that 
carbohydrate alone without fat might with proteid form a normal diet. But 
on this point experience is probably to be trusted ; and we may infer that in 
every normal diet some fat at least must be added to the starches and the 
sugars. 
The advantage of this mixture is probably felt while the food is as yet 
within the alimentary canal. What we have learned concerning digestion 
leads us to regard it as a complicated process, and we cannot readily imagine 
that the proteolytic, amylolytic, and adipolytic changes run their several 
courses, especially in the small and large intestine, apart from and irrespec- 
tive of each other. We are rather led to suppose that the accompaniment 
of one set of changes, in some indirect manner, favors the others; and it is 
for that reason probably that we take our food-stuffs not separately, but 
mixed in the same meal, often on the same plate, and even in the same mouth- 
ful. But apart from this the two food-stuffs, fats and carbohydrates, must 
play different parts in the economy, so that the one cannot be wholly substi- 
tuted for the other; and though, beyond the fact that the one seems to be a 
source of energy and the other not, we do not as yet know the true physio- 
logical function of the hydrogen of the fat as compared with that of the 
differently disposed hydrogen of the carbohydrate, we may perhaps infer 
that the difference of use within the body of the two kinds of food-stuffs 
bears not so much on their ultimate consumption to supply energy, as on 
the various complicated processes which they undergo and arrangements in 
which they take part before the end of their work is reached. We have 
had a hint that the carbohydrate more rapidly supplies the heat-giving 
metabolism than does the fat; and this suggests an advantage to the economy 
in receiving daily a certain portion of the more tardy material, while at the 
same time it may be taken to mean that the fat before it is used to give rise 
to energy has first to be converted into sugar, and so takes more time in its 
work. 
The main carbohydrate of every diet is starch, and as far as we can learn 
at present, the starch which is so large a part of the cereals and vegetables 
consumed by man is the same body in all of them ; for the use of such bodies 
as inulin is so insignificant that it may be neglected. Man, however, con- 
sumes no inconsiderable quantity of sugar, chiefly cane sugar. Since the 
starch of a meal does not become available for the economy until it has been 
converted into sugar, we might be inclined to infer that it was a matter of 
indifference whether the carbohydrate of a diet were supplied as starch or 
as sugar. But besides the fact that any large deficit of starch in a diet 
might seriously interfere with the general course of digestion, especially if as 
urged above the several digestive processes are more or less dependent on 
each other, it must be remembered that the sugar into which starch is 
changed by digestion is maltose, while cane sugar appears to be either 516 
NUTRITION. 
absorbed as cane sugar or at most only inverted. Moreover, if our labora- 
tory experiments truly represent the digestion taking place in the living 
body, only part of the starch (§ 194) is changed into maltose, while part 
becomes some variety of dextrine or of starch. Our knowledge of sugars 
and of their fate in the economy is too imperfect for us to be able to state 
the effects on the body of digested starch as compared with those of cane 
sugar or milk sugar ; but that these are or may be different is shown by the 
experience of medical practice. In many cases the total effect on the body 
of a diet from which cane sugar is as much as possible eliminated, though 
starch be allowed, is very different from that of one of which cane sugar 
forms an appreciable part. 
Concerning cellulose, which in herbivora appears certainly to serve as 
a source of energy and to be a real food-stuff, our knowledge will not allow 
us to decide whether it has any special uses of its own, or whether the body 
is simply led to utilize and make the best of what is a necessary accompani- 
ment of the starch of vegetable food. 
Concerning the salts present in a diet, we need only repeat what was said 
in § 440, that these, though affording of themselves little or no energy, are 
as essential a part of a diet as the energy-giving food-stuffs, inasmuch as they 
in some way or other direct metabolism and the distribution of energy. 
And this is true not only of the inorganic salines, such as chlorides and 
phosphates, but also of the so-called extractives. As we have seen, the 
presence of these bodies, both the simpler inorganic and the more complex 
organic salts, in the blood or in the extra-vascular juices or lymph of the 
tissues is essential to or directs or modifies the metabolic activity of the sev- 
eral tissues. The beneficial effects, as components of special diets, of such 
things as beef-tea and meat extract, which consist chiefly of salts and ex- 
tractives, with a very small quantity of albumose or other forms of proteid,. 
and the effects either beneficial or deleterious of drugs, both turn in common 
upon their taking a part of some kind or other in, it may be upon their 
interference with, metabolic processes. The salts and extractives of a diet 
may be looked upon as necessary daily medicines, and a medicine as a more 
or less extraordinary variation in these elements of a diet. 
Alcohol, to the use of which as a component of an ordinary diet special 
interest for various reasons attaches, comes in this class. For though 
observations show that the greater part of a moderate dose of alcohol is 
oxidized within the body, and so serves as a source of energy, man has 
recourse to alcohol not for the minute quantity of energy which is supplied 
by itself, but for its powerful influence on the distribution of the energy 
furnished by other things. That influence is a very complex one and can- 
not be fully discussed here. It is stated that moderate or small doses of 
alcohol diminish the consumption of oxygen and production of carbonic 
acid, that is to say, diminish the total result of the metabolism of the body, 
while larger but still not intoxicating doses have a contrary effect and in- 
crease the total metabolism. But such a statement affords no sound basis 
for any conclusion as to the general physiological effect of alcohol, or as to 
its usefulness as part of an ordinary diet; it does not justify such a conclu- 
sion, for example, as that alcoholic drinks, taken in moderation, by diminish- 
ing metabolism economize the resources of the body. The prominent physio- 
logical problem of dietetics is not either to increase or diminish the metab- 
olism of the body, but to direct that metabolism into proper channels; and 
whether in each particular case a given dose of alcohol gives a right or a 
wrong turn to the physiological processes of the body depends on the partic- 
ular circumstances of the case. For the action of all these bodies of which 
we are now speaking, in contrast with the actions of the food-stuffs proper,. ON DIET. 
517 
is not only complex but variable; so complex and variable that simple ex- 
perience is at present a more trustworthy guide than speculative physiology. 
We may add that the physiological action of alcoholic drinks is still further 
complicated by the fact that most such drinks contain, beside ethylic alcohol, 
various other allied substances, whose action is even more potent than that 
of the ethylic alcohol itself, and whose presence very markedly determines 
the total effect of the drink. Such articles of diet as tea and coffee stand 
upon very much the same footing as alcohol. 
The quantity of fluid which a man drinks or should drink daily, or 
more correctly the quantity of water which he should daily add to the 
dry solids of his diet, must vary widely according to circumstances. It 
will differ according as he is perspiring freely or not, according to the 
nature of the dry solids of the diet, whether largely carbohydrate or not, 
and so on. A lower limit, below which excretion is impeded, and a higher 
limit, above which digestion and metabolism are injuriously affected, prob- 
ably exist; but we have as yet no adequate data which will enable us to fix 
either of them. 
§ 465. In the selection of articles of food to supply the food-stuffs and 
other constituents of a normal diet, regard must, of course, be had in the 
first place to the amount of potential energy present in the material. The 
articles chosen for the daily fare must contain between them so much pro- 
teid, fat, and carbohydrate, representing so much available energy. But 
it is no less important that the potential energy in the material should be 
really available for the economy. The material must have such qualities 
that it is digested within the alimentary canal, and further that its digestion 
and absorption do not give rise to trouble either in the alimentary canal or 
in that secondary digestion carried on by means of the various metabolic 
events which we have discussed in preceding sections. A really nutritious 
substance is one which not only contains in itself an adequate supply of 
energy, but is of such a nature that its energy can be appropriated by the 
economy with ease, or at least with as little trouble as possible. We have 
approximate data for determining how far an estimate of the relative use- 
fulness of various articles of food must be corrected, by allowing for the pro- 
portion of each which after an ordinary meal merely passes through the 
alimentary canal, and the energy of which is not in any way available for 
the body's use. Thus, a number of observations carried out on healthy 
individuals gave, in the case of the following articles of food, the following 
figures as the percentage, reckoned in each case on dry material, which 
could be recovered from the feces, and was, therefore, not digested and not 
used by the body : Meat, 5 per cent.; eggs, 5 per cent.; milk, 9 per cent.; 
bread (white), 4 per cent. ; black bread, 15 per cent.; rice, 4 per cent.; 
maccaroni, 4 per cent.; maize, 7 per cent.; peas, 9 per cent.; potatoes, 11 
per cent. It must, however, be remembered that the actual correction to be 
made in any case will depend on the mode of cooking of the material, on 
the character of the meal of which it forms part, and on the individual 
capabilities of the consumer, the latter, too, varying under different circum- 
stances. 
The above refers to what may be called rough digestibility, but besides 
this there are other circumstances to be considered. The same food-stuff 
in two articles of food, though actually digested, that is to say taken up by 
the alimentary canal, may, even while still within the alimentary canal, 
undergo changes in the one case differing from those in the other. A pro- 
teid may for instance in one case tend to be entirely converted into peptone, 
or to break up into leucin, etc., or in other cases to undergo other changes; 
and a carbohydrate may in one case be absorbed as maltose, and in another 518 
NUTRITION. 
give rise to lactic acid. Indeed, when we speak of the digestibility or the 
indigestibility of this or that article of food, we do not in many cases so 
much mean the relative amount of the substance taken up in some way or 
other by the alimentary canal as the characters advantageous or otherwise 
of the changes which it undergoes in being so taken up. 
Hence the purely chemical statement of the amount of potential energy 
present in an article of food is no safe guide of the physiological value of 
the substance. A chunk of cheese stands very high on, generally at the 
top of, a table of the nutritive value of articles of food drawn up on ex- 
clusively chemical principles-according to the units of energy present in 
a unit of the material-but it is very low down in a corresponding physio- 
logical table. And similarly a dish of old peas has a very different physio- 
logical function from a plate of fresh meat, even when both contain the 
same amount of nitrogen. 
In thus correcting for digestion the nutritive value of a diet it must 
also be borne in mind that the alimentary canal, while chiefly a receptive 
organ, is also to some extent (§ 245) an excretory organ ; a free passage 
through the canal is needed not only for carrying off undigested matter but 
also for getting rid of excreted matter; and the presence of the former, up 
to certain limits, assists the discharge of the latter. Were it possible to 
prepare a diet every jot and tittle of which could be digested and absorbed, 
the use of such a diet would probably bring about disorder in the economy, 
through the absence of a sufficiently rapid discharge of the matters ex- 
creted into the alimentary canal. Hence cellulose and like substances, 
even when unutilized through absorption, are not without their use, and 
experience shows that digestion may be promoted by eating undigestible 
things. 
§ 466. The several food-stuffs of a diet may be drawn from the animal 
or from the vegetable kingdom. Vegetable proteids appear to undergo the 
same changes in the alimentary canal as do animal proteids, and the main 
effects on the body of proteids from the two sources seem to be the same. 
Our knowledge at present, however, is too imperfect to enable us to decide 
whether the functions of the two are exactly the same, whether the body 
behaves exactly the same upon a diet in which the proteids are exclusively 
of vegetable origin, as upon a diet in which, otherwise the same, the pro- 
teids are partly of animal origin also. Nor have we much better know- 
ledge of the relative nutritive value of vegetable and animal fats. And as 
we have already said, we possess little or no exact knowledge as to the part 
played by those extractives in respect to the amount and nature of which 
animal food strikingly differs from vegetable food. In attempting, there- 
fore, a judgment from a purely physiological point of view as to the value 
of an exclusively vegetarian diet compared with a diet of both animal and 
vegetable origin, we can do little more at present than inquire whether the 
former supplies the several food-stuffs in adequate quantity, in proper pro- 
portion, and in such a form as to be economically utilized by the body. 
The careful examination during three separate periods of several days 
each of the ingesta and egesta of a man, 28 years old, weighing 57 kilos, 
who had for three years lived on an exclusively vegetable diet, viz., bread, 
fruit, and oil, gave the following results : 
The daily diet consisted on the average of 719 grms. solid matter and 
1084 grms. water. It contained 
Proteids 54 grammes, containing 8.4 N. 
Fats 22 grammes. 
Carbohydrates 557 grammes (about } sugar and 2 starch). 
(Cellulose) 16 grammes. ON DIET. 
519 
The daily feces weighed, when fresh, 333 grms. containing 75 grms. solid 
matter, and were therefore both bulky and watery. There were present in 
the feces, fat 7 grms., starch 17 grms., cellulose 9 grms., showing that 30 
per cent, of the fat, 6 per cent, of the starch, and 56 per cent, of the cellu- 
lose had not been utilized by the body. The subject had really lived on 
fat, 15 grms., carbohydrates 540 grms. (and cellulose 7 grms.). The feces 
contained no less than 3.46 nitrogen. If we reckon the whole of this as 
proteid, this would give 22 grms. of undigested proteid, so that there has 
been a waste of 41 per cent, of the proteids, leaving only 32 grms. avail- 
able for real use in the body; and, indeed, a very small portion only of this 
nitrogen can be regarded as really discharged from the body itself. The 
total solids of the feces must be reckoned as partly excreta but chiefly un- 
digested food. If we regard the 75 grms. of solid feces as entirely undi- 
gested food, the whole solid food available for the body must be reduced 
from 719 grms. to 644 grms. 
The urine of the day contained 5.33 grms. nitrogen ; this added to the 
3.46 grms. nitrogen in the feces gives 8.79 grms. nitrogen in the total egesta 
as compared with the 8.4 grms. nitrogen of the food, indicating a slight 
loss of nitrogenous material from the body ; but if we suppose that all the 
nitrogen in the feces was not in the form of undigested food we may neglect 
this; and indeed the subject of the observation was in apparently good 
health and stationary weight. 
Compared with either of the normal diets given in § 463, the above diet 
is striking for the low amount of proteids and of fats and the relative ex- 
cess of carbohydrates. But though such a diet may be taken as perhaps 
fairly typical of the daily food of a rigid vegetarian, a much more richly 
proteid diet may be obtained from sources still strictly vegetable. Thus 
the diet, entirely vegetable in nature, of an average Japanese laborer of 
about the same weight as the individual whose data we have just given has 
been estimated to consist of proteids 102 grms., fat 17 grms., carbohydrates 
578 grms. And the diet of a Roumanian peasant, living chiefly on beans 
and maize with the addition of fat of some kind, has been calculated to 
furnish no less than proteids 182 grms., fat 93 grms., carbohydrates 968 
grms.; but the real nutritive value of such a diet must need very large cor- 
rection indeed. 465.) 
The examination of the diet of an individual living with a fair nitro- 
genous equilibrium and apparently good health on a modified vegetable 
diet-that is to say, one which included milk and eggs-gave the follow- 
ing: Proteids, 74 grms.; fat, 58 grms.; carbohydrates, 490 grms., a diet 
which differs from the normal diet almost solely in the lesser amount of 
proteids, one-third of which, by the by, was supplied by the animal mate- 
rial, eggs and milk. In another instance, nitrogenous equilibrium and 
fairly good health were secured, for some weeks at all events, on a vege- 
table diet yielding proteids, about 100 grms.; fat, 70 grms.; carbohydrates, 
400 grms.; but in this nearly the whole of the fat was furnished by the 
animal product butter, and Liebig's extract was freely used. 
Confining ourselves, however, to the more strictly vegetarian diet, we 
may conclude in the first place that, unless the daily food be very large in 
amount, the proteid element of such a diet falls considerably below the 100 
or more grms. given in the normal diet. But we cannot authoritatively say 
that such a reduction is necessarily an evil; for, as we stated above (§ 463), 
our knowledge will not, at present, permit us to make an authoritative 
exact statement as to the extent to which the proteid may be reduced with- 
out disadvantage to the body when accompanied by adequate provision of 
the other elements of food ; and this statement holds good whether the body 520 
NUTRITION. 
be undertaking a small or large amount of labor. A second feature of such 
a diet is the marked reduction of the fat and its replacement by carbohy- 
drates. Although here again we cannot make a distinctly authoritative 
statement, the evidence which we possess bears clearly in the direction that 
such a reduction is a marked disadvantage. A third and very characteristic 
feature of the strictly vegetarian diet is the relatively large amount of un- 
digested food lost to the body and discharged as feces. Even when the diet 
is scanty, so that the proteid element is low, the amount of feces relatively 
to the total food is high ; and when a more normal proteid contribution is 
secured by ample meals the feces becomes exceedingly voluminous. Indeed 
when, leaving man, we compare the herbivorous with the carnivorous mam- 
mal, we find that the former is almost as clearly distinguished from the latter 
by its frequent and abundant feces as by the anatomical features of its or- 
ganization. We have already urged that, since the feces serve as a means 
of excretion of the real waste products of metabolism, a certain amount of 
vehicle to carry these away is of advantage or even necessary ; but there are 
no facts at present known to us which show that the larger intestinal cur- 
rent of the purely vegetable diet effects any such good as can compensate 
for the obvious waste of labor incurred in the transport and management, 
to say nothing of the opportunities of mischief offered by a mass of mate- 
rial more subject to the dominion of foreign organisms than even to that of 
the body itself, though these opportunities are less than with a correspond- 
ing mass of animal origin. With respect to these three features, then, the 
strictly vegetarian diet seems, on physiological grounds, inferior to one of 
a mixed nature. There are, as we said, other aspects, still of a physiologi- 
cal kind, to be considered, such as the relative digestibility of vegetable 
articles of food, the relative metabolic value of the food-stuffs of vegetable 
origin, and the influence of animal extractives ; but any fuller discussion of 
these points would be out of place here. 
§ 467. We have treated the diet discussed above as a normal diet, suit- 
able for man under ordinary or general circumstances. Ought such a diet 
to be modified for the various exigencies of life, such as labor, age, climate, 
and the like ? 
We shall discuss the influence of age in the concluding portions of this 
work. 
We may be inclined, at first sight, to assume that the total amount of 
the diet should vary with the weight, that is, the size, of the individual; 
and, indeed, in discussions on nutrition, statements concerning metabolism 
and amounts of food are often given in terms of per kilo of body weight. 
In a broad sense, it may be true that a small man needs less food than 
a large one; but it must be remembered that, as we saw in speaking of 
animal heat, the smaller organism, having the relatively larger surface, 
carries on a more rapid metabolism per unit of body weight, and so needs 
relatively more food. And, moreover, the influence of size is probably far 
less than the influence exerted by the inborn individual characters of the 
organism, giving rise to what we may call the personal equation of metab- 
olism. The smaller metabolism of woman, leading to the use of scantier diet, 
as compared with that of man, is to be regarded in this light rather than 
with reference to the average lesser weight of woman. The relative metab- 
olism of the two sexes may be illustrated by the case of an active man and 
his wife, both of about the same age and weight, the man being rather the 
heavier and the woman rather the older, who in carrying out together an 
experiment on the relative values of vegetable and animal food, both lived 
for some time on the same kind of diet, and found that nutritive equilibrium 
was, in the one case and in the other, maintained when ON DIET. 
521 
Proteids. 
Fats. 
Carbohydrates. 
The man consumed daily about . 
. . 100 
70 
400 
The wife " " " . 
. . 60 
67 
340 
The most striking difference is in the proteids. 
§ 468. With regard to climate, the chief considerations attach to tem- 
perature. When the body is exposed to a low temperature the general metab- 
olism of the body is increased owing to a regulative action of the nervous 
system (§ 448). We might infer from this that more food is necessary in 
cold climates ; and, since the increase in the metabolism appears to manifest 
itself chiefly in a greater discharge of carbonic acid, and therefore to be espe- 
cially a carbon metabolism, we might infer that the carbon elements of 
food should be especially increased. When the body is exposed to high tem- 
peratures, the same reflex mechanism tends to lower the metabolism ; but the 
effects in this direction are much less clear than those of cold, and soon 
reach their limits; the bodily temperature is maintained constant under the 
influence of surrounding warmth not so much by diminished production as 
by increased loss. We may infer from this that in warm climates not less, 
but, if anything rather more, food than in temperate climates is necessary in 
order to supply the perspiration needed for the greater evaporation and dis- 
charge of heat by the skin. 
In both cold and warm climates, however, man trusts much more to vari- 
ations in his clothing and immediate surroundings to protect him against 
cold or to guard him from heat than to any marked variations in his normal 
diet. In the former he may, perhaps, be expected to eat somewhat more, 
since, in spite of wrappings, the skin still feels in part the cold, and thus the 
nervous mechanism for the increase of metabolism is, to a certain extent, set 
to work. And since the metabolism thus increased appears to affect espe- 
cially the carbon of the body, he may further be expected to increase the fats 
rather than the carbohydrates of his food, seeing that the former supply 
him with the most energy for their weight. But it is very doubtful whether 
what he might thus be expected to gain over a corresponding increase in 
carbohydrates is not more than counterbalanced by the increased labor of 
digestion ; and the habits of the dwellers in arctic climates cannot safely be 
taken as guides in this matter, for their reputed love of fat is probably the 
result of that being their most available form of carbon. Indeed, the evi- 
dence that the increase of metabolism provoked by cold bears exclusively 
on carbon constituents is so uncertain that it may be doubted whether any 
change in the normal diet, beyond some increase in the whole, should be made 
to meet a cold climate. Similar reasons would lead one to infer that man 
in the warmer climate would maintain, on the whole, the same normal diet, 
the only change being, perhaps, to increase it slightly, possibly throwing 
the increase chiefly on the carbohydrates with the special view of further- 
ing perspiration. 
§ 469. A special diet for the purpose of fattening-that is to say, for the 
accumulation of adipose tissue out of proportion to the rest of the body-is 
not needed in the case of man. The power to store up fat in adipose tissue 
is much more dependent on certain inborn qualities of the organism which 
we cannot at present define than on the kind of food ; of two bodies living 
on the same diet, and under the same circumstances, one will become fat 
while the other will remain lean ; and it is an object of the agriculturist to 
develop by breeding and selection a " constitution " which will store up the 
most fat on the cheapest diet. In fattening animals the chief care, when the 
selection of the kind of animal has been made, is to provide adequate carbo- 
hydrate food, which, as we have seen, is the chief fattener ; and the object of 
the farmer in rearing stock for the butcher is mainly to convert cheap vege- 522 
NUTRITION. 
table carbohydrate into dear animal fat. Further aids in fattening may be 
found in providing repose for the body of such a kind that, while sufficient 
energy is expended to secure adequate digestion and absorption of food, all 
causes leading to an increase of metabolism by which energy is set free and 
leaves the body are avoided as much as possible. 
To avoid fat rather than to increase it is often an object of human care. 
This may be effected by diminishing fats and carbohydrates, but also, in a 
very marked manner, by relatively increasing the proteids. Proteid food, 
as we have seen, augments the whole metabolism of the body, hurrying on 
the destruction not only of proteid but of carbon food ; and a tendency to 
corpulency may be counteracted by a diet in which fats and carbohydrates 
are much restricted, and proteids are largely increased. When, as in what 
is known as the Banting method, the diet is almost exclusively proteid, the 
nitrogenous overwork entails dangers on organisms which do not possess the 
power of ridding themselves freely of the large amount of nitrogenous waste 
which such a diet produces. A less severe method in which the fats and 
carbohydrates are diminished only, not entirely done away with, and the 
proteids only moderately increased, is less open to objection ; and such a diet, 
assisted by other hygienic conditions, has proved successful. 
An increase of daily food, largely proteid in nature, given under cir- 
cumstances such as a large amount of passive exercise and skin stimulation, 
known as " massage," which will not only favor digestion but also promote 
metabolism in general, may be given with favorable results. In this way 
an enormous metabolism may be excited, and yet so carried on that the 
body gains both in flesh and in fat. Thus, in one case, the patient with an 
initial weight of 45 kilos, and a daily nitrogenous metabolism calculated as 
28 grins, proteid, reached in the course of about fifty days a weight of 60 
kilos, the daily nitrogenous metabolism being raised on one occasion to 182 
grms. proteid, with an average on the whole period of 150 grms. During 
the treatment no less than 8420 grms. of proteid were taken as food. 
§ 470. With regard to labor, since, as we have seen, the energy expended 
as work done is not taken out of and away from the amount set free as heat, 
the two forms of energy being so related that an increase of work done is 
accompanied by a greater or less increase of heat set free, it is obvious that 
a man who is doing a hard day's muscular work needs a larger income of 
energy for the day than does an idle man. What we have learnt concerning 
muscular metabolism further shows us that the additional energy needed is 
not necessarily to be supplied by an increase in the proteid components of 
the diet; the energy of muscular contraction does not come, as was once 
thought, from proteid metabolism (§ 443). The fact that it is the carbon 
metabolism which is augmented in muscular work may suggest that the 
extra food for extra work should be exclusively carbon compounds; and if, 
as seems probable, the carbohydrates are more readily and directly available 
for the functional metabolism of muscle than are the fats, we might be further 
led to recommend an increase in carbohydrates to form a diet especially suited 
for labor. But several considerations should make us hesitate before we come 
to such a conclusion. A muscle is not a machine within the body which can 
be loaded and fired off irrespective of the rest of the body. In the perform- 
ance of muscular labor, the condition of the muscle, the amount of energy 
available in the muscle itself, is of course of prime importance; but, and 
this perhaps especially holds good in severe labor, of great importance also, 
we might almost say of no less importance, is, as we have urged 333), the 
power of the body as a whole to avail itself of the energy latent in the mus- 
cle. The power of doing work hangs not on the muscle alone, but on the 
heart, the lungs, the nervous system, and, indeed, on the whole body. It is ON DIET. 
523 
very doubtful whether we ever, even in supreme efforts, draw upon more 
than a portion of the capital of energy lodged in the muscle itself; fatigue 
is far more a nervous than a muscular condition, and even the distinctly 
muscular fatigue is as we have seen (§ 86), partly at least the result of the 
accumulation of products and not alone the using up of available energy. 
In choosing a diet for muscular labor we must have in view not the muscle 
itself but the whole organism. And though it is possible that future research 
may suggest minor changes in the various components of a normal diet such 
as would lessen the strain during labor on this or that part of the body, on 
the muscles as well as on other organs, our present knowledge would rather 
lead us to conclude that what is good for the organism in comparative rest 
is good also for the organism in arduous work, that the diet, normal for the 
former condition, would need for the latter a limited total increase but no 
striking change in its composition. In preparing the body for some com- 
ing arduous labor, in " training," as it is called, an increase of proteid food, 
for the purpose of hurrying on the general metabolism of the body, and 
thus of making " new Mesh " and renovating the body, so to speak, in view 
of the strain to be put upon it, may perhaps suggest itself; but even this is 
doubtful. 
The principles of such a conclusion with regard to muscular work may 
be applied with still greater confidence to nervous or mental work. The 
actual expenditure of energy in nervous work is relatively small, but the 
indirect influence on the economy is very great. The closeness and intri- 
cacies of the ties which bind all parts of the body together are very clearly 
shown by the well-known tendencies of so-called brain work to derange the 
digestive and metabolic activities of the body; and if there be any diet 
especially suited for intellectual labor it is one directed not in any way 
toward the brain, but entirely toward lightening the labors of and smoothing 
the way for such parts of the body as the stomach and the liver. BOOK III. 
THE CENTRAL NERVOUS SYSTEM AND ITS INSTRUMENTS. 
CHAPTER I. 
THE SPINAL CORD. 
On Some Features of the Spinal Nerves. 
§ 471. We have called the muscular and nervous tissues the master 
tissues of the body; but a special part of the nervous system, that which we 
know as the central nervous system, the brain and spinal cord, is su- 
preme among the nervous tissues and is master of the skeletal muscles as 
well as of the rest of the body. We have already (Book I., Chapter III.) 
touched on some of the general features of the nervous system, and have 
now to study in detail the working of the brain and spinal cord. We have 
to inquire what we know concerning the laws which regulate the discharge 
of efferent impulses from the brain or from the cord, and to learn how that 
discharge is determined, on the one hand, by intrinsic changes originating, 
apparently, in the substance of the brain or of the cord, and, on the other 
hand, by the nature and amount of the afferent impulses which reach them 
along afferent nerves. 
As we shall see, the study of the spinal cord cannot be wholly separated 
from that of the brain, the two being very closely related. Nevertheless, it 
will be of advantage to deal with the spinal cord by itself as far as we can. 
The medulla oblongata or spinal bulb1 we shall consider as part of the brain. 
But before we speak of the spinal cord itself, it will be desirable to say a 
few words concerning the spinal nerves, that is to say, the nerves which 
issue from the spinal cord. 
We have already seen (§ 92) that each of the spinal nerves arises by two 
roots, an anterior root attached to the ventral or anterior surface, and a pos- 
terior root attached to the dorsal or posterior surface of the cord. We have 
further seen that the latter bears a ganglion, a " ganglion of the posterior 
root" or "spinal ganglion," as we have (§ 93) studied the structure of this 
ganglion. 
We stated at the same time that while the trunk of a spinal nerve con- 
tained both efferent and afferent fibres, the efferent fibres were gathered up 
into the anterior root and the afferent fibres into the posterior root; but we 
gave no proof of this statement. 
§ 472. Before we proceed to do so, it will be as well to say a few words 
on the terms " efferent " and " afferent." By efferent nerve-fibres we mean 
nerve-fibres which in the body usually carry impulses from the central ner- 
1 The term medulla oblongata is not only long, but presents difficulties, since the word 
medulla is now rarely used to denote the whole spinal cord (medulla spinalis) but is gen- 
erally used to denote the peculiar coat of a nerve-fibre, the white substance of Schwann. 
In using instead the word bulb or, if necessary, spinal bulb there is little fear of confu- 
sion with any other kind of bulb. The adjective is in not uncommon use, in such phrases 
as "bulbar paralysis." 
525 526 
THE SPINAL CORD. 
vous system to peripheral organs. Most efferent nerve-fibres carry impulses 
to muscles, striated or plain, and the impulses passing along them give rise 
to movements ; hence they are frequently spoken of as " motor " fibres. But 
all efferent fibres do not end in or carry impulses to muscular fibres ; we have 
seen for instance that some efferent fibres are secretory. Moreover, all the 
nerve-fibres going to muscular fibres do not serve to produce movement; 
some of them, as in the case of certain vagus fibres going to the heart, are 
inhibitory and may serve to stop movement. 
By "afferent" nerve-fibres we mean nerve fibres which in the body 
usually carry impulses from peripheral organs to the central nervous sys- 
tem. A very common effect of the arrival at the central nervous system of 
impulses passing along afferent fibres is that change in consciousness which 
we call a " sensation ;" hence afferent fibres or impulses are often called 
" sensory " fibres or impulses. But as we have already in part seen, and as 
we shall shortly see in greater detail, the central nervous system may be 
affected by afferent impulses, and that in several ways, quite apart from the 
development of any such change of consciousness as may be fairly called a 
sensation. We shall see reason for thinking that afferent impulses reaching 
the spinal cord, and, indeed, other parts of the central nervous system, may 
modify reflex or automatic or other activity without necessarily giving rise 
to a " sensation." Hence it is advisable to reserve the terms " efferent " and 
" afferent " as more general modes of expression than " motor " or " sensory." 
We have seen in treating of muscle and nerve, that the changes pro- 
duced in the muscle serve as our best guide for determining the changes 
taking place in a motor nerve; when a motor nerve is separated from its 
muscle (§ 70) the only change which we can appreciate in it is an electrical 
change. Similarly in the case of an afferent nerve, the central system is our 
chief teacher ; in a bundle of afferent fibres isolated from the central nervous 
system, in a posterior root of a spinal nerve for instance, the only change 
which we can appreciate is an electrical change. To learn the characters of 
afferent impulses we must employ the central nervous system. But in this 
we meet with difficulties. In studying the phenomena of motor nerves we 
are greatly assisted by two facts. First, the muscular contraction by which 
we judge of what is going on in the nerve is a comparatively simple thing, 
one contraction differing from another only by such features as extent or 
amount, duration, frequency of repetition and the like, and all such differ- 
ences are capable of exact measurement. Secondly, when we apply a stim- 
ulus directly to the nerve itself, the effects differ in degree only from those 
which result when the nerve is set in action by natural stimuli, such as the 
will. When we come, on the other hand, to investigate the phenomena of 
afferent nerves, our labors are for the time rendered heavier, but in the end 
more fruitful, by the following circumstances : First, when we judge of what 
is going on in an afferent nerve by the effects which stimulation of the nerve 
produces in some central nervous organ, in the way of exciting or modify- 
ing reflex action, or modifying automatic action, or affecting consciousness, 
we are met on the very threshold of every inquiry by the difficulty of clearly 
distinguishing the events which belong exclusively to the afferent nerve from 
those which belong to the central organ. Secondly, the effects of applying 
a stimulus to the peripheral end-organ of an afferent nerve are very different 
from those of applying the same stimulus directly to the nerve trunk. This 
may be shown by the simple experience of comparing the sensation caused 
by bringing any sharp body into contact with a nerve laid bare in a wound 
with that caused by contact of an intact skin with the same body. These 
and like differences reveal to us a complexity of impulses, of which the 
phenomena of motor nerves gave us hardly a hint. ON SOME FEATURES OF THE SPINAL NERVES. 
527 
We shall further see in detail later on that our consciousness may be 
affected in many different ways by afferent impulses; we must distinguish 
not only sensory from other afferent impulses, but also different kinds of 
sensory impulses from each other. Certain afferent nerves are spoken of as 
nerves of special sense, and the nature of the afferent impulses passing along 
these special nerves, together with the modifications of consciousness caused 
by arrival of these impulses at the central nervous system, constitute by them- 
selves a complex ami difficult branch of study. In some of the problems 
connected with the central nervous system we shall have to appeal to the 
results of a study of these special senses; but, on the other hand, a knowl- 
edge of the central nervous system is necessary to a proper understanding 
of the special senses; and on the whole it will be more convenient to study 
the former before the latter. 
§ 473. The proof that the afferent and efferent fibres which are both 
present in the trunk of a spinal nerve are parted at the roots, the efferent 
fibres running exclusively in the ventral or anterior root, and the afferent 
fibres exclusively in the dorsal or posterior root, is as follows: 
When the anterior root is divided the muscles supplied by the nerve 
cease to be thrown into contractions, either by the will or by reflex action, 
while the structures to which the nerve is distributed retain their sensibil- 
ity. During the section of the root, or when the proximal stump connected 
with the spinal cord is stimulated, no sensory effects are produced. When 
the distal stump is stimulated the muscles supplied by the nerve are thrown 
into contractions. When the posterior root is divided the muscles supplied 
by the nerve continue to be thrown into action by an exercise of the will, or 
as part of a reflex action, but the structures to which the nerve is distributed 
lose the sensibility which they previously possessed. During the section of 
the root, and when the approximal stump is stimulated, the sensory effects 
are produced. When the distal stump is stimulated no movements are 
called forth. These facts demonstrate that sensory impulses pass exclusively 
by the posterior root from the peripheral to the central organs, and that 
motor impulses pass exclusively by the anterior root from the central to the 
peripheral organs; and as far as our knowledge goes the same holds good 
not only for sensory and motor but also for afferent and efferent impulses. 
An exception must be made to the above general statement, an account 
of the so-called " recurrent sensibility " which is witnessed in conscious mam- 
mals under certain circumstances. It sometimes happens that when the 
distal stump of the divided anterior root is stimulated, signs of pain are wit- 
nessed. These are not caused by the concurrent muscular contractions or 
cramp which the stimulation occasions, for they persist after the whole trunk 
of the nerve has been divided some little way below the union of the roots 
above the origins of the muscular branches, so that no contractions take 
place. They disappear when the posterior root is subsequently divided, and 
they are not seen if the mixed nerve-trunk be divided close to the union of 
the roots. The phenomena are probably due to the fact that bundles of 
sensory fibres of the posterior root, after running a short distance down the 
mixed trunk, turn back and run upward in the anterior root (being distrib- 
uted probably to the pia mater) and by this recurrent course give rise to 
the recurrent sensibility. 
§ 474. Concerning the ganglion on the posterior root, we may say de- 
finitely that we have no evidence that it can act as a centre of reflex action, 
nor have we any evidence that it can spontaneously give origin to efferent 
impulses and thus act as an automatic centre, as can the central nervous 
system itself. The bodies of the nerve-cells behave somewhat differently 
from the axis-cylinders at some distance from the cells, though, as we have 528 
THE SPINAL CORD. 
seen, these are in reality processes of the nerve-cells ; thus the nerve-cells in 
the ganglion appear to be more sensitive to certain poisons than are the 
nerve-fibres of the nerve-trunk. But beyond this, our knowledge concern- 
ing the function of the ganglion is almost limited to the fact that it is in 
some way intimately connected with the nutrition of the nerve. As we have 
already (§ 81) said, when a mixed nerve-trunk is divided the peripheral 
portion degenerates from the point of section downward toward the periph- 
ery. The central portion does not so degenerate, and if the length of nerve 
removed be not too great, the central portion may grow downward along the 
course of the degenerating peripheral portion, and thus regenerate the nerve. 
This degeneration is observed when the mixed trunk is divided in any part 
of its course from the periphery to close up to the ganglion. When the pos- 
terior root is divided between the ganglion and the spinal cord, the portion 
attached to the spinal cord degenerates, but that attached to the ganglion 
remains intact. When the anterior root is divided, the proximal portion in 
connection with the spinal cord remains intact, but the distal portion between 
the section and the junction with the other root degenerates; and in the 
mixed nerve-trunk many degenerated fibres are seen, which, if they be care- 
fully traced out, are found to be motor (efferent) fibres. If the posterior 
root be divided carefully between the ganglion and the junction with the 
anterior root, the small portion of the posterior root left attached to the 
peripheral side of the ganglion above the section remains intact, as does also 
the rest of the root from the ganglion to the spinal cord, but in the mixed 
nerve-trunk are seen numerous degenerated fibres, which when examined are 
found to have the distribution of sensory (afferent) fibres. Lastly, if the 
posterior ganglion be excised, the whole posterior root degenerates, as do also 
the sensory (afferent) fibres of the mixed nerve-trunk. Putting all these 
facts together, it would seem that the growth of the efferent and afferent 
fibres takes places in opposition directions, and starts from different nutri- 
tive or " trophic " centres. The afferent fibres grow away from the ganglion 
either toward the periphery or toward the spinal cord. The efferent fibres 
grow outward from the spinal cord toward the periphery. This difference 
in their mode of nutrition is frequently of great help in investigating the 
relative distribution of efferent and afferent fibres. When a posterior root 
is cut beyond the ganglion, or the ganglion excised, all the afferent nerves 
degenerate, and in the mixed nerve-branches these afferent fibres, by their 
altered condition, can readily be traced. Conversely, when the anterior 
roots are cut, the efferent fibres alone degenerate, and can be similarly recog- 
nized in a mixed nerve-tract. When the anterior is divided some few fibres 
in it do not, like the rest, degenerate, and when the posterior root is divided, 
a few fibres in the anterior root are seen to degenerate like those of the pos- 
terior root; these appear to be the fibres which give to the anterior root its 
" recurrent sensibility." In the case of certain spinal nerves at all events, 
it has also been ascertained that when the posterior root is divided, while 
most of the fibres in the part of the root thus cut off from the ganglion but 
left attached to the cord degenerate, some few do not. These few appear to 
have their trophic centre not in the ganglion, but in some part of the spinal 
cord itself; we shall refer to these later on. 
This method of distinguishing nerve-fibres by the features of their de- 
generation, called the "degeneration method," or sometimes, from the name 
of the physiologist who introduced it, the " Wallerian method," has proved 
of great utility. Thus in the vagus nerve, which is composed not only of 
fibres which spring from the real vagus root, but also of fibres proceeding 
from the spinal accessory roots, the two may be distinguished by section of 
the vagus and spinal accessory roots respectively. We shall presently see THE STRUCTURE OF THE SPINAL CORD. 
529 
that this method may be applied to the differentiation of tracts of fibres in 
the brain and spinal cord. 
The Structure of the Spinal Cord. 
§ 475. Lying within the vertebral canal the spinal cord is protected by 
its " membranes," the dura mater, the arachnoid membrane and the pia 
mater. The consideration of the arrangement of these membranes and of 
the structure of the dura mater and arachnoid we will leave until we come 
to speak of the vascular and lymphatic supplies of the central nervous sys- 
tem ; the histology of the pia mater may more fitly come with that of the 
spinal cord itself. 
Along its whole length from its junction with the bulb at its termination 
in the filum terminale the spinal cord, while possessing certain general fea- 
tures, is continually changing as to special features. It will be convenient 
to study first the general structure of some particular part, for instance the 
middle of the thoracic (dorsal)1 region, and afterward to point out the special 
features which obtain in the several regions. 
A transverse vertical section of either a fresh or a hardened and pre- 
pared spinal cord at the thoracic region possesses an outline which is, roughly 
speaking, circular. In the middle of the anterior or ventral surface is a ver- 
tical fissure, the ventral or anterior fissure (Fig. 119, A. F.), running some 
way across the thickness of the cord from the ventral toward the dorsal sur- 
face. Opposite to it on the posterior or dorsal surface is a corresponding, 
deeper but narrower, dorsal or posterior fissure (Fig. 119, P. K) which, how- 
ever, as we shall see, differs materially in nature from the anterior fissure, 
and ought to be called a septum rather than a fissure. Between the two fis- 
sures the substance of the cord is reduced to a narrow isthmus uniting the 
two lateral halves, which in a normal cord are like each other in every 
respect. In the middle of the isthmus lies the section of a small canal, the 
central canal (Fig. 119, c. c.), which is all that remains of the relatively wide 
neural canal of the embryo. 
Each lateral half consists of an outer zone of white matter surrounding, 
except at the isthmus, an inner more or less crescentic, or comma-shaped 
mass of gray matter. The convexity of each crescent is turned toward the 
median line of the cord, the two crescents being placed back and back and 
joined together by the isthmus just spoken of. The somewhat broader ante- 
rior extremity of the crescent, or head of the comma, is called the anterior 
cornu or horn; and the narrower posterior extremity of the crescent, or tail 
of the comma, is called the posterior cornu or horn. The part by which each 
horn is joined on to the middle part of the crescent is called the cervix, ante- 
rior and posterior respectively. The isthmus joining the backs of the two 
crescents, like the crescents themselves, consists, for the most part, of gray 
matter, the band running posterior or dorsal to the central canal being called 
the posterior gray commissure (Fig. 119, p. g. c.), and the band running ante- 
rior or ventral to the canal being called the anterior gray commissure (Fig. 
119, a. g. c.fi The posterior fissure touches the posterior gray commissure, 
1 It is very desirable to use the terms "dorsal" and "ventral" for the parts of the 
eerebro-spinal axis which lie respectively near the dorsal or back part, and the ventral or 
belly part of the body, instead of the terms posterior and anterior; but if this is done 
the use of the word dorsal to denote the region of the cord between the lumbar and cer- 
vical regions is apt to lead to confusion ; hence the introduction of the word thoracic. If 
this use of dorsal and ventral be adhered to, before and behind, above and below, may 
conveniently be used to denote nearer the head and nearer the tail (or coccyx) respect- 
ively; anterior and posterior may also be used in the same sense except in the case of 
anterior and posterior fissure and horn, which terms seem too much honored by time to 
be thrown aside. 530 
THE SPINAL CORD. 
but the anterior gray commissure is separated from the bottom of the ante- 
rior fissure by a band of white matter, called the anterior white commissure, 
Fig. 119. 
A Transverse Dorso-ventral Section of the Spinal Cord (Human) at the Level of the Sixth 
Thoracic (Dorsal) Nerve. (Sherrington.)' Magnified 15 times. One lateral half only is shown. 
The large conspicuous nerve-cells (drawn from actual specimens) are shaded black to render 
their relative size, shape and position more obvious; the outline of the gray matter has been 
made thick and dark in order to render it conspicuous. A. F., antetror fissure; P. F., posterior 
fissure; c. c., central canal; c. g. 8., central gelatinous substance; A. r., anterior root: P. r„ lateral 
(or intermediate) bundle; P. F., median bundle or posterior root of spinal nerve; p' p" fibres of 
posterior root passing, p' indirectly through the substance of Rolando, p" directly into gray mat- 
ter; a. g. c., anterior gray commissure; p. g. c., posterior gray commissure; a. c., anterior white 
commissure; ant. col., anterior column ; lat. col., lateral column ; post, col., posterior column; s.g., 
the substance of Rolando; 8., septum marking out the external posterior column or column of 
Burdach, c. p., from the median posterior column, or column of Goll, m. p. 1, cells of the anterior 
horn ; 3, posterior column, or vesicular cylinder, or column of Clarke, the area of the cylinder 
defined by a dotted line; 4, cells of the intermedio-lateral tract or lateral tract or lateral horn; 6, 
cells of the posterior horn ; 7, cells of the anterior cervix ; y, a tract of fibres passing from the 
vesicular cylinder to the lateral column. 
1 For this and many succeeding figures 1 am deeply indebted to my friend and former 
pupil, Dr. Sherrington, who has kindly prepared the figures for me from his original 
drawings. THE STRUCTURE OF THE SPINAL CORD. 
531 
or more simply, the white commissure, or sometimes the anterior commissure 
(Fig. 119, a. c.). 
If the section be taken at the level of the origin of a pair of spinal 
nerves, it will be seen that the anterior or ventral root, piercing the white 
matter opposite the head of the comma in several distinct bundles (Fig. 
119, A. r.), plunges into the anterior cornu, while the posterior or dorsal 
root (Fig. 119, P. r., P. r'.), having the appearance of a single undivided 
bundle, passes, in part at least, into the posterior horn. Both roots are dis- 
persed lengthways along the cord, the hinder roots of one nerve being close 
to the foremost roots of the nerve below, but it is only the anterior roots 
which are dispersed sideways. The compact bundle of the posterior root 
divides, with tolerable sharpness, the white matter in each lateral half of 
the cord into (1) a posterior portion lying between the posterior fissure and the 
posterior root, which portion since, as we shall see, runs in the form of a col- 
umn along the length of the cord, is called the posterior column, and (2) into 
a portion lying to the outside of the posterior root between it and the ante- 
rior fissure, called the antero-lateral column. This latter may be considered 
as further divided, by the entrance of the anterior roots into a lateral column 
(Fig. 119, lat. col.) between the posterior root and the most external bundle 
of the anterior root, and into an anterior column (Fig. 119, ant. col.), between 
the anterior fissure and the most external bundle of the anterior root. The 
part traversed by the bundles of the anterior root, as they make for the 
anterior horn, accordingly belongs to the anterior column ; but some writers 
speak of the anterior column as lying between the anterior fissure and the 
nearest bundle of the anterior root, thus making the region of the anterior 
root belong to neither anterior nor lateral column. And indeed the dis- 
tinction between the anterior and the lateral column is. to a great extent, 
artificial. 
§ 476. The "white matter" consists exclusively of medullated fibres 
supported partly by connective tissue and partly by a peculiar tissue known 
as neuroglia, of which we shall presently speak. The fibres are of various 
sizes, but many of them are large, and in all of them the medulla is conspic- 
uous. They run for the most part longitudinally, so that in transverse sec- 
tions of the cord nearly the whole of the white matter appears under the 
microscope to be composed of minute circles, the tranverse sections of the 
longitudinally-disposed fibres, imbedded in the supporting structures. The 
gray matter " also contains medullated fibres, but these are for the most 
part exceedingly fine fibres possessing a medulla which appears to differ 
from that of an ordinary nerve-fibre, since it does not stain readily with 
osmic acid, but is rendered visible by special modes of preparation such as 
that known as Weigert's. Hence these fine fibres are not apparent in ordi- 
nary carmine or other specimens, and indeed their presence was for a long 
time overlooked. Besides these fine medullated fibres, if we may call them 
such, the gray matter contains what the white matter does not, nerve-cells 
with branching processes, naked axis-cylinders, and delicate filaments aris- 
ing from the division of axis-cylinders or from the branching of nerve-cells, 
all these various structures being imbedded in neuroglia. Owing to the re- 
lative abundance of the white refractive medulla, the white matter possesses 
in fresh specimens a characteristic, opaque white color ; hence the name. 
The gray matter from the relative scantiness of medulla has no such opaque 
whiteness, is much more translucent, and in fresh specimens has a gray or 
rather pinkish-gray color, the reddish tint being due to the presence partly 
of pigment and partly of blood, for the bloodvessels are much more abun- 
dant in the gray matter than in the white. 
The pia mater which closely invests the cord all around consists of con- 532 
THE SPINAL CORD. 
nective tissue fairly rich in elastic elements and abundantly supplied with 
bloodvessels; it is indeed essentially avascular membrane and furnishes the 
nervous elements of the cord with their chief supply of blood. It sends in 
at intervals partitions or septa of the same nature as itself radiating toward 
the central gray matter. The narrow posterior fissure is completely tilled up 
by a large septum of this kind, indeed, as we have said, is in reality not a 
fissure but a large septum ; but the anterior fissure is too wide for such an 
arrangement; the whole membrane dips down into this fissure, following the 
surface of the cord and being reflected at the bottom. From these primary 
septa, secondary finer septa still composed of ordinary fibrillated connective 
tissue, carrying bloodvessels, branch off"; but these are soon merged into the 
peculiar supporting tissue, called, as we have said, neuroglia. This consists 
in the first place of small branching cells, lying in various planes. The 
branching is excessive, so that the body of the cell is reduced to very small 
dimensions, indeed at times almost obliterated, the nucleus disappearing 
while the numerous branches are continued as long, fine filaments or fibres 
pursuing a devious but for the most part a longitudinal course. In the 
second place these cells and fibres or filaments are imbedded in a homo- 
geneous ground substance. Relatively to the fibres and ground substance 
the bodies of the cells (which are called Deiter's cells), especially bodies such 
as bear obvious nuclei, are very scanty ; hence in sections, especially in 
transverse sections, of the cord the neuroglia has often a dotted or punctated 
appearance, the dots being the transverse sections of the fine longitudinally- 
disposed fibres imbedded in the ground substance. Examined chemically, 
the neuroglia is found to be composed not like connective tissue of gelatin, 
but of a substance which appears to be closely allied to keratin, the chief 
constituent of horny epidermis, hairs and the like, and which has therefore 
been called neurokeratin (see also § 68). And indeed this neuroglia, though 
like connective tissue a supporting structure, is not, like connective tissue, 
of mesoblastic, but of epiblastic origin. The walls of the neural canal of 
the embryo which are transformed into the spinal cord of the adult consist 
at first of epithelial, epiblastic cells ; and while some of these cells become 
nervous elements, others become neuroglia. The epithelial cells which are 
destined to form neuroglia become exceedingly branched, while their orig- 
inally protoplasmic cell substance becomes transformed to a large extent 
into neurokeratin. 
The neuroglia fills up the spaces between the radiating larger septal pro- 
longations of the pia mater and the finer branched septa which starting from 
the larger ones carry minute bloodvessels into the interior of the white 
matter. In these spaces it is so arranged as to form delicate tubular canals, 
of very variable size, running for the most part in a longitudinal direction. 
Each of these tubular canals is occupied by and wholly filled up with 
medullated nerve-fibre of corresponding size. A medullated nerve-fibre of 
the white matter of the spinal cord resembles a medullated nerve-fibre of a 
nerve (§ 68) in being composed of an axis-cylinder and a medulla; but it 
possesses no primitive sheath or neurilemma. This is absent and indeed is 
not wanted ; the tubular sheath of neuroglia affords in the spinal cord (and, 
as we shall see, in the central nervous system generally) the support which in 
a nerve is afforded by the neurilemma. Nodes are, according to most authors, 
absent, but some say they are present. 
The white matter of the cord consists then of a more or less solid mass of 
neuroglia, having the structure just described, which is permeated by minute 
canals, some exceedingly fine and carrying very fine 2 // fibres, others larger 
and carrying fibres up to the size of 15 /a This mass is further broken up 
into areas by the smaller and larger vascular connective-tissue septa with the 533 
THE STRUCTURE OF THE SPINAL CORI). 
edges and endings of which the neuroglia is continuous. Most of the nerve- 
fibres, as we have said, run longitudinally and in a transverse section of the 
cord are cut transversely; but, as we shall see, fibres are continually passing 
into and out of the white matter, and in so doing take a more or less trans- 
verse course; these, however, are few compared with those which run in a 
longitudinal direction. On the outside of the cord below the pia mater the 
neuroglia is developed into a layer of some thickness from which nerve-fibres 
are absent; this is often spoken of as an inner layer of the pia mater; but 
being neuroglia and not connective tissue is of a different nature from the 
pia mater proper. A layer of this superficial neuroglia also accompanies 
the larger septa, and a considerable quantity is present in the large septum 
called the posterior fissure. 
The pia mater carries not only bloodvessels but also lymphatics; of these, 
however, we shall speak when we come to deal with the vascular arrange- 
ments of the whole of the central nervous system. 
§ 477. In the gray matter we may distinguish the larger, more conspicu- 
ous nerve-cells and the rest of the gray matter in which these cells lie. We 
have already (§ 95) described the general features of these larger nerve-cells, 
and shall have presently to speak of their special characters and grouping. 
Meanwhile the most important point to remember about them besides the 
fact that they vary largely in form and size is that while one process may or 
does become an axis-cylinder of a nerve-fibre, the others rapidly branch, and 
breaking up into fine nerve-filaments are lost to view in the rest of the gray 
matter. 
These larger nerve-cells form, however, a part only, and in most regions 
of the cord the smaller part, of the whole gray matter. In a transverse sec- 
tion from the thoracic region (Fig. 119) a few only of these larger nerve-cells 
are seen in the whole section, and though they appear more numerous insec- 
tions from the cervical and especially from the lumbar regions (Figs. 121, 
122), yet in all cases they occupy the smaller part of the area of the gray 
matter. The larger part of the gray matter consists, besides a neuroglia 
supporting the nervous elements, of nerve-filaments running in various direc- 
tions and forming, not a plexus properly so called, but an interlacement of 
extreme complexity. These filaments are, on the one hand, the fine medul- 
lated fibres spoken of above as being recognized with difficulty, and, on the 
other hand, non-medullated filaments ranging from fairly wide and con- 
spicuous naked axis-cylinders down to fibrils of extreme tenuity, the latter 
arising apparently either from the division of axis-cylinders and nerve-fibres 
passing into or out of the gray matter or from the continued branching of 
processes of nerve-cells. By the modes of preparation now available it has 
been shown that the fine medullated fibres, so far from being rare, are in 
certain parts of the gray matter so abundant as even to preponderate over 
the non-medullated fibres or fibrils. Lastly, besides the conspicuous nerve- 
cells spoken of above, which, though of various sizes, may all perhaps be 
spoken of as large, a very large number of other cells of small size, some of 
which at all events must be regarded as true nerve-cells, are present in the 
gray matter. 
The neuroglia in which all these structures, nerve-cells, fine medullated 
nerve-fibres, naked axis-cylinders,and fine filaments are imbedded, is identical 
in its general characters with that of the white matter, but, as naturally fol- 
lows from the nature of the nervous elements which it supports, is differently 
arranged. Instead of forming a system of tubular channels it takes on the 
form of a sponge-work with large spaces for the larger nerve-cells and fine 
passages for the nervous filaments. At the junction of the gray matter with 
ihe white matter, the neuroglia of the one is continuous with that of the 534 
THE SPINAL CORD. 
other, and the connective-tissue septa of the latter run right into the former ; 
the outline of the gray matter is not smooth and even, but broken by tooth- 
like processes due to the septa. Since, as we have just said, some of the true 
nerve-cells are very small, and since the nerve-filaments like the neuroglia 
fibres are very fine and take like them an irregular course, it often becomes 
very difficult in a section to determine exactly which is neuroglia and which 
are nervous elements. The neuroglic cells may, however, be distinguished 
perhaps from the smaller nerve-cells by their nuclei not being so conspicuous 
or so relatively large as in a nerve-cell, and by their staining differently. 
The gray matter, then, may be broadly described as a bed of'neuroglia, 
containing a certain number of branching nerve-cells, for the most part 
though not exclusively large and conspicuous, but chiefly occupied by what 
is not so much a plexus as an intricate interweaving of nerve-filaments 
running apparently in all directions. Some of these filaments are fairly con- 
spicuous naked axis cylinders, and a few are easily recognized medullated 
fibres of ordinary size ; but by far the greater number are either exceedingly 
fine medullated fibres, whose medulla is only made evident by special modes 
of preparation, or delicate fibrils devoid of medulla. With the nervous web 
formed by these filaments the branching processes of the nerve-cells, on the 
one hand, and the divisions of nerve-fibres passing into or out of the gray 
matter on the other hand, appear to be continuous. It may be added that 
the gray matter is well supplied with bloodvessels, these being in it, as stated 
above, relatively much more numerous than in the white matter. 
§ 478. The central canal is lined by a single layer of columnar epithelial 
cells, which are generally described as bearing cilia ; but it is not certain 
that the processes which may be seen projecting from the surfaces of the 
cells are really cilia. These epithelial cells rest not on a distinct basement 
membrane, but on a bed of neuroglia, free apparently, or nearly so, from 
nervous elements which surrounds the central canal and is sometimes spoken 
of as the substantia gelatinosa centralis (Fig. 119, c. g. s.). The attached 
basis of the epithelial cells are branched or taper to a filament, and become 
continuous with the branched cells or fibres of the neuroglia below. As we 
said above, the neuroglia elements are transformed epithelial cells ; and the 
continuity of the cells, which retaining the characters of epithelial cells form 
a lining to the canal, with the cells which have become branched and lost 
their epithelial characters, indicates the epithelial origin of the latter. 
The central canal with the surrounding area of neuroglia forms the 
central part of the isthmus uniting the two lateral halves of the cord. Pos- 
terior (dorsal) to this central mass lies the posterior gray commissure (Figs. 
119, 121, 122, p. g. c.) composed chiefly of fine filaments running trans- 
versely, and anterior (ventral) to it lies first the thinner anterior gray com- 
missure (Figs. 119, 121, 122 a. g. c.) of a similar nature, and then the rela- 
tively thick white commissure (Figs. 119, 121, 122, a. c.) which is formed by 
medullated fibres crossing over from one side of the cord to the other, and 
thus constitutes a decussation of fibres along the whole length of the cord. 
On each side the central mass of neuroglia of which we are speaking 
gradually merges into the central gray matter of the corresponding lateral 
half. 
The end or head (caput), as it is frequently called, of the posterior horn 
is occupied not by ordinary gray matter, but by a peculiar tissue, the sub- 
stantia gelatinosa of Rolando, which forms a sort of cap to the more ordinary 
gray matter, but differs in size and shape in different regions of the cord. 
(Cf. Figs. 119, 120, 121, s. g.) In carmine and some other modes of prepa- 
ration it is frequently stained more deeply than is the ordinary gray matter, 
and in such preparations is very conspicuous. It may be described as con- 535 
THE STRUCTURE OF THE SPINAL CORD. 
sisting of a somewhat peculiar neuroglia traversed by fibres of the posterior 
root, and containing a large number of cells which, for the most part small, 
the cell-bodies being small relatively to the nuclei, are not all alike, some 
being probably nervous and others not. It takes origin from the cells form- 
ing the immediate walls of the embryonic medullary canal. In the embryo 
this canal is relatively wide, though compressed from side to side, and in 
transverse sections of the medullary tube appears at a certain stage as a 
narrow oval slit placed vertically and reaching almost from the dorsal to the 
ventral surface. The dorsal part of this long slit is later on closed up by 
the coming*together of the walls and the obliteration of the greater part of 
the cavity, leaving the ventral part to form a circular canal, which by the 
development of the anterior columns assumes the central position. During 
this closure of the dorsal part of the canal a mass of the cells lining the 
canal is cut from the rest on each side, and during the subsequent growth 
takes up a position at the end of the posterior horn. Hence, though it never 
apparently contains any cavity, the substance of Rolando may be regarded 
as an isolated portion of the walls of the medullary canal, which has under- 
gone a development somewhat different from that of the portion which 
remains as the lining of the central canal. Traces of this origin may be 
seen even in the adult. Thus, in the lower end of the cord, in what we shall 
speak of presently as the conus medullaris, the central canal widens out dor- 
sally, and in section (Fig. 120, A) presents on each side a bay x stretching 
Fig. 120. 
Diagram to illustrate the Nature of the Substance of Rolando. The figures are purely dia- 
grammatic and are not drawn to the same scale. In all three figures the gray matter is shaded 
with fine lines and the white matter with dots. A, transverse section of the lower end of the 
conus medullaris in man; e, epithelium lining the medullary canal; x, lateral expansion of the 
canal; B, transverse section of the spinal cord of the calf in the lower thoracic region ; r, sub- 
stance of Rolando: c, central canal; C, transverse section through mid-thoracic region of cord in 
man. 
out toward the position of the posterior horn. At this region of the cord, 
though both white and gray matter are developed on the ventral surface, the 
posterior columns do not meet on the dorsal surface, but leave the central 
canal covered only by tissue which perhaps may be called neuroglia, but is 
of peculiar nature and origin. In the calf, in a part of the dorsal region the 
substance of Rolando is not confined to the tip of the posterior horn, but is 
continued to meet its fellow in the middle line. (Fig. 120, B.) If we imagine 
the dorsal portion of the canal of A to be cut off from the ventral portion, 
its cavity to be obliterated, and the lining epithelium with some of the sur- 
rounding elements to undergo a special development, the condition in B is 
reached by the growth of the posterior columns. From B the transition to 
the normal state of things as in Fig. 120, c, is a very slight one. The extreme 
dorsal tip of the horn, being of a more open texture than the substance of 
Rolando, is sometimes called the zona spongiosa. 
§ 479. The grouping of the nerve-cells. The nerve-cells, at all events the 
cells which are large enough to to be easily and without doubt recognized to 536 
THE SPINAL CORD. 
be nerve-cells, form, as we have seen, only a part of the gray matter, and in 
some parts of the cord, in the thoracic region for instance, are so sparse that 
in a section of the spinal cord in this region thin enough to show its histo- 
logical features satisfactorily, the bodies of a few only of such cells are 
visible (Fig. 119); the greater part of the gray matter consists not of the 
bodies of conspicuous nerve-cells, but of a mass of fibres and fibrils passing 
apparently in all directions. In the cervical (Fig. 121), and especially in 
the lumbar (Fig. 122) regions, the nerve-cells are both absolutely and rela- 
tively more abundant; but even in a section taken from the lumbar region 
the nerve-cells, all put together, form the smaller part of the whole area of 
gray matter. Moreover, in respect of the number of cells, all the sections 
of even the same region of the cord are not alike. Seeing that the cord may 
be considered as growing out of the fusion of a series of paired ganglia, each 
ganglion corresponding to a nerve (c/. § 92), we may fairly expect to find the 
fusion not complete, so that the nerve-cells would appear more numerous 
opposite a nerve than in the middle between two nerves. In some of the 
lower animals this arrangement is most obvious, and there are some reasons 
for thinking that even in man the nerve-cells are metamerically increased at 
the level of each nerve. 
Even when casually observed, it is obvious that the nerve-cells are not 
scattered in a wholly irregular manner throughout the gray matter, being, 
for instance, much more conspicuous in the anterior horn than elsewhere; 
and more careful observation allows us to arrange them to a certain extent 
in groups. 
The cells of the anterior horn are for the most part large and conspicuous, 
67/z to 135/z in diameter, branch out in various directions, and present an 
irregular outline in sections taken in different planes. We have reason to 
think that every one of them possesses an axis-cylinder process, which, in 
the case at all events of most of the cells, passing out of the gray matter 
becomes a fibre of the adjacent anterior root. They are obvious and con- 
spicuous in all regions of the cord, though much more numerous and indi- 
vidually larger in the cervical and lumbar enlargements than in the thoracic 
region. We may further, with greater or less success, divide them into 
separate groups. 
In the cervical and lumbar regions a fairly distinct group of cells is seen 
lying on the median side of the gray matter close to the anterior column 
(Figs. 121, 122, 1). This may be called the median group. It appears also 
in the thoracic region (Fig. 119, 1) ; indeed, the question arises whether all 
the cells of the anterior horn in this region do not belong to this group. The 
other cells so conspicuous in the lumbar and cervical enlargements, and there- 
fore probably in some way associated with the limbs, may be spoken of as 
forming altogether a lateral group; but we may, though with some uncer- 
tainty, subdivide them into two or three groups. Thus in the lumbar region 
a group of cells (Fig. 122, 2 lying near the lateral margin of the more 
dorsal part or base of the horn may be distinguished, as a lateral sub-group, 
from the cells occupying the ventral lateral corner of the horn and forming 
a ventral or anterior sub-group (Fig. 122, 2 co) ; and the same distinction, 
though with less success, may be made in the cervical region (Fig. 121). 
Further, we may perhaps in both regions distinguish a group of cells placed 
more in the very middle of the horn as a central sub group (Figs. 121, 122, 
2 But, in all cases, the separation of these cells, which we have spoken 
of as a whole as lateral cells, into minor groups, is far less distinct than the 
separation of the median group from these lateral cells, especially if we 
admit that in the thoracic region the median group is alone clearly repre- 
sented. THE STRUCTURE OF THE SPINAL CORD. 
537 
Fig. 121. 
Transverse Dorso-ventral Section of Spinal Cord (Human) at the Level of the Sixth Cervical 
Nerve. (Sherrington.) This is drawn on the same scale as Fig. 119, that is, magnified fifteen times 
r.f. I., lateral reticular formation ; r.f. p., posterior reticular formation; p', fine fibres of lateral 
bundle of the posterior root; p", p"', fibres of median bundle of posterior root entering gray matter 
from external posterior column ; x, gray matter of posterior horn ; Sp. a., bundles of fibres belong- 
ing to the spinal accessory nerve ; in the lateral reticular formation they are seen cut transversely ; 
b, is the natural septum of connective tissue marking out the cerebellar tract C. T. from the crossed 
pyramidal tract C. P. T.; z. s., zona spongiosa; 2 x , 3, y> lateral cells of the anterior horn ; 5, cells 
in the region of the lateral reticular formation. The other letters of reference are the same as in 
Fig. 119. 538 
THE SPINAL CORD. 
In the thoracic region a group of rather smaller cells is seen at the base 
of the anterior horn, near to the junction with the isthmus (Fig. 119, 7). 
In the cervical and lumbar region these cells are very scanty (Figs. 121, 
122, 7). 
The cells of the posterior horn contrast strongly with those of the ante- 
rior horn in being few, and for the most part small. They are branched ; 
and though we have reason to believe that, like the cells of the anterior 
horn, they possess each an axis-cylinder process, this is not easily deter- 
mined by actual observation ; the processes do not run out to join the poste- 
rior root, as do the corresponding processes in the anterior horn, and there- 
fore are not so readily seen. These cells occur in all regions of the cord, 
and appear to be arranged in two or more groups. The lateral margin of 
the posterior horn, at about the middle or neck of the horn, is along the 
whole length of the cord, but especially in the cervical region, much 
broken up by bundles of fibres passing in various directions and forming 
an open network, called the lateral reticular formation (Figs. 121, 122, r. f. 
lat.f In all regions of the cord a number of cells are found associated 
with this reticular formation, forming the group of the lateral reticular for- 
mation (Figs. 121, 122, 5). In all regions of the cord, also a group of cells 
(Figs. 119, 121, 122, 6) is found in that part of the horn where, a little 
ventral to the substance of Rolando, the uniform field of gray matter is 
broken up into a kind of network by a number of bundles of white fibres 
running in various directions. The network has also been called a reticu- 
lar formation, and has received the name of posterior reticular formation 
(Figs. 121, 122, r. f p.) to distinguish it from the lateral reticular forma- 
tion just mentioned ; the two, however, in some regions (see Fig. 119) join 
each other, and thus cut off a ventral portion of the posterior horn con- 
taining nerve-cells from a dorsal portion, x in Figs. 121, 122, in which no 
obvious or conspicuous nerve-cells are present. 
The groups of cells just mentioned, with the restrictions and modifications 
spoken of, occur along the whole length of the cord ; but the group of cells 
to which we must now call attention is almost confined to a special region 
of the cord, or at least is but feebly represented elsewhere. In the thoracic 
region, especially in the lower thoracic region (we shall return to the limits 
of the group later on), at the base of the posterior horn (Fig. 119, 3), just 
ventral to the curve formed by the posterior gray commissure as this bends 
dorsally to join the posterior horn, is seen on each side of the cord a con- 
spicuous group of cells known as Clarke's column, or the posterior vesicular 
column or vesicular cylinder. The cells composing this group, though vary- 
ing in size at different levels, are rather large cells, and are for tlie most 
part fusiform, with their long axis placed lengthways along the cord, so 
that in transverse sections they often appear to have a rather small round 
body. They are surrounded by, and as it were imbedded in, a mass of fine 
fibres, the area of which is indicated by a dotted line in Fig. 118. 
Also conspicuous in the thoracic region is another group of cells lying 
on the outer side of the middle of the gray matter at about the junction of 
the anterior and posterior horns. This is known as the intermedio-lateral 
tract, and is sometimes called the lateral horn (Fig. 119, 4). The cells com- 
posing it are somewhat small spindle-shaped cells with their long axis 
placed transversely. The group is conspicuous, as we have said, in the 
thoracic region ; it may be recognized in the lumbar region (Fig. 121, 4), 
but in the cervical region becomes confused with the most dorsally placed 
or lateral sub-group of the anterior horn. We shall, however, have to re- 
turn to these groups of cells when we come to speak of the differences 
between the spinal regions of the cord. THE STRUCTURE OF THE SPINAL CORD. 
539 
§ 480. The tracts of white matter. At first sight the white matter of the 
cord appears to be of uniform nature. We can use the nerve-roots to 
delimitate the anterior, posterior, and lateral columns, but we appear to 
Fig. 122. 
Transverse Dorso-ventral Section of the Spinal Cord (Human) at the Level of the Third Lum- 
bar Nerve. (Sherrington.) This is drawn to the same scale as Figs. 119, 120, and in the same 
way, except that the outline of the gray matter is not exaggerated, Pr'. median; Pr. interme- 
diate; Pr". lateral bundles of posterior roots. The region comprised under m.t. is the marginal 
zone of Lissauer's zone. The other letters of reference are the same as in Figs. 119,121. The 
three figures, 119,121,122, are intended to illustrate the main differential features of the cervical, 
thoracic, and lumbar cord. 
have no criteria to distinguish parts in each column. In the cervical and 
upper thoracic regions of the cord, a septum (Fig. 119, s.) in the posterior 
column, somewhat more conspicuous than the other septa, has enabled 
anatomists to distinguish an inner median portion, the median posterior 540 
THE SPINAL CORD. 
column, commonly called the postero-median column or column of Goll (Fig. 
119. m. pf, from an outer lateral portion, the external posterior column, com- 
monly called the postero-external column or column of Burdach (Fig. 119, 
e. p.f the lateral part of which, nearer the gray matter, has, for reasons 
which we shall see later on, been called the posterior root-zone. But beyond 
this neither the irregular septa nor other features will enable us to distin- 
guish one part of the white matter as different in nature from another. 
Nor have we better success when with the scalpel we attempt to dissect 
out the white matter into separate strands. Nevertheless we have convin- 
cing evidence that the white matter is arranged in strands, or tracts, or 
columns, which have different connections at their respective ends, which 
behave differently under different circumstances, which we have every 
reason to believe carry out different functions, but which cannot be sepa- 
rated by the scalpel, because each of them is more or less mixed with fibres 
of a different nature and origin. The evidence for the existence of these 
tracts is twofold. 
One kind of evidence is embryological in nature. When a nerve-fibre 
is being formed in the embryo, either in the spinal cord or elsewhere, the 
essential axis-cylinder is formed first and the less essential medulla is 
formed later. Now when the developmental history of the spinal cord is 
studied it is found that, in the several regions of the cord, all the fibres of 
the white matter do not put on the medulla at the same time. On the con- 
trary, in certain tracts, the medulla of the fibres makes its appearance early, 
in others later. By this method it becomes possible to distinguish certain 
tracts from others. 
Another kind of evidence is supplied by facts relating to the degenera- 
tion of the fibres of the white matter. We have seen (§474) that the 
degeneration of a nerve-fibre is the result of the separation of the fibre 
from its trophic centre, and that while the trophic centre of the afferent 
fibres is in the ganglion on the posterior root, that of the efferent fibres is 
in some part of the spinal cord. • In the case of the efferent fibres the 
degeneration might be spoken of as descending from the spinal cord to the 
muscles or other peripheral organs. In the case of the afferent fibres of 
the trunk of the nerve, the degeneration is also one descending from the 
ganglion down to the skin or other peripheral organ. When, however, the 
section is carried through the posterior root of a spinal nerve, the degener- 
ation takes place in the part of the nerve between the section and the spinal 
cord ; it runs up from the section to and into the spinal cord, and may, 
therefore, be called an ascending degeneration. Thus we may say that 
when a nerve-trunk or when a nerve-root is cut completely across, all the 
fibres, which are thereby separated from their trophic centres, degenerate. 
When the nerve-trunk is divided, all the fibres below the section undergo 
descending degeneration. If the anterior root be cut across, all the fibres 
of the root below the section undergo descending degeneration. If the 
posterior root be cut across, all the fibres of the root above the section un- 
dergo ascending degeneration with the exception of certain fibres which do 
not degenerate at all, and of which we shall speak later on. 
When the spinal cord is cut across, for instance in the dorsal region, all 
the fibres of the white matter do not degenerate either in the part of the 
cord above the section or in the part below. Some fibres, and indeed some 
tracts of fibres degenerate, and some do not. Further, some tracts degen- 
erate in the cord above the section, and thus undergo what has been called 
an ascending degeneration ; other tracts degenerate in the cord below the 
section, and thus undergo what has been called a descending degeneration. 
These terms must, however, be used with caution. When a nerve-trunk is THE STRUCTURE OF THE SPINAL CORD. 
541 
cut across, the degeneration actually descends, in the sense that the progress 
of the degenerative changes may be traced downward ; they begin at the 
section and travel downward at a rate sufficiently slow to permit a difference 
being observed between the progress of degeneration at a spot near the sec- 
tion and that of one further off After section of or injury to the spinal cord, 
however, it is not possible to trace any such progress either upward or down- 
ward ; in the tracts both above and below the section or injury, degenera- 
tion either begins simultaneously along the whole length of the degenerating 
tract, or progresses along the tract so rapidly that no differences can be 
observed, as far as the stage of degeneration is concerned, between parts near 
to and those far from the section and injury. When, for instance, the cord 
is divided in the cervical region, subsequent examination of the tracts of 
the so-called descending degeneration shows that the degeneration is as far 
advanced in the lumbar region far away from the section as in the cervical 
region just below the section. Applied to the spinal cord, therefore, the 
term descending degeneration means simply degeneration below the seat of 
injury or disease, ascending degeneration means simply degeneration above 
the seat of injury or disease. We may add that the histological features of 
the degeneration of fibres in the spinal cord are not wholly identical with 
those of the degeneration of fibres in a nerve-trunk. Thus, the neurilemma 
with its nuclei being absent from the fibres of the cord, no proliferation of 
nuclei takes place; the axis-cylinder and medulla simply break up, are ab- 
sorbed, and disappear. 
Similar degenerations, ascending or descending, or both, are seen when 
the section is not carried right through the whole cord, but particular parts 
of the cord are cut through or deeply injured. And similar degenerations 
occur as the consequences of disease set up in parts of the cord. 
In this way the results of sections of or of other injuries to or of diseases 
of the spinal cord, have enabled us to mark out certain tracts of the white 
matter as undergoing degeneration and others as not, and, moreover, certain 
tracts as undergoing descending and others as undergoing ascending degen- 
eration. Further, the delimitation of tracts of white matter by the process 
of degeneration agrees so well with the results of the embryological method 
as to leave no doubt that the white matter does consist of tracts which differ 
from each other in nature and in function. 
The several tracts thus indicated vary in different regions of the cord. 
They may be broadly described as follows: 
I. Descending tracts, that is to say, tracts which undergo a descending 
degeneration in the sense noted above. 
The most important and conspicuous is a large tract (Fig. 123, cr.P.} 
occupying the posterior part of the lateral column, coming close upon the 
outer margin of the posterior horn, and for the most part not reaching the 
surface of the cord. We shall have to return to this tract more than once, 
and may here simply say that it is most distinctly marked out by both the 
embryological and the degeneration methods, that it may be traced along 
the whole length of the cord from the top of the cervical region to the end 
of the sacral region, and that it enters the cord from the brain through the 
structures called the pyramids of the bulb, which we shall study later on. 
These pyramids cross over or decussate as they are about to pass into the 
cord, forming what is known as the decussation of the pyramids, and the 
tract of fibres in question shares in this decussation. Hence this tract is 
called the crossed pyramidal tract or more simply the pyramidal tract. 
A smaller, less conspicuous descending tract occupies the median portion 
of the anterior column (Fig. 123, d.P.\ This is not only much smaller but 
also much more variable than the crossed pyramidal tract, is not present in 542 
THE SPINAL CORD. 
the lower animals, being found in man and the monkey only and being better 
developed in man than in the monkey, and reaches a certain way only down 
the spinal cord, generally coming to an end in the thoracic region. It, too, 
comes down from the pyramid, and is a continuation of that part of the pyra- 
mid which, unlike the rest, does not decussate in the bulb; thus the tract 
which coming down from the left side of the brain runs in the left pyramid 
Fig. 123. 
Diagram to illustrate the General Arrangement of the Several Tracts of White Matter in the 
Spinal Cord. (Sherrington.) The section is taken at the level of the fifth cervical nerve. The 
relations of the tracts in different regions of the cord are shown in Fig. 1'27. The ascending tracts, 
tracts of ascending degeneration, are shaded with dots, the descending tracts, tracts of descending 
degeneration, are shaded with lines; the shading in each case put on one side of the cord, only 
the reference letters being placed on the other side. cr.P. crossed pyramidal tract, or more 
shortly, pyramidal tract ; d.P. direct pyramidal tract shaded on the side opposite to that on 
which cr.P. is shaded, in order to indicate the difference of the two as to crossing; C.b. cerebellar 
tract; s.lr. and c.r. together indicate the median posterior tract, or tract of fibres of the posterior 
roots, cr. representing, as is explained more fully in the text, the cervical and s.lr. the sacral, 
lumbar, and dorsal roots; asc.a.l. the antero-lateral ascending tract; desc.l. the antero-lateral de- 
scending tract. The area, not shaded, marked x, is the small descending tract or rather patch 
mentioned in the text as observed in certain regions of the cord, in the external posterior column 
r.z. The small area at the tip of the posterior horn, marked L, is the posterior marginal zone of 
Lissauer's zone. 
in the bulb, passes down into the left anterior column of the cord. Hence 
this smaller tract is called the direct pyramidal tract. 
These two are the most conspicuous and important descending tracts, but 
names have been given to two other descending tracts. One, known as the 
antero-lateral descending tract, is a large tract placed in the antero-lateral 
column, and seen in section (Fig. 123, desc. I.) as an elongated area stretch- 
ing from the pyramidal tract toward the anterior column and reaching at 
times as far as the anterior fissure. The area is large, however, because the 
tract is very diffuse, that is to say, the fibres with descending degeneration, 
or fibres which degenerate below the section or injury, are very largely 
mixed up with fibres which do not degenerate; in this respect this tract 
contrasts with the pyramidal tract, which is to a much greater extent com- 
posed of fibres with descending degeneration, though even in it there are 
a considerable number of fibres which do not degenerate. Indeed, this 
antero-lateral descending tract is so diffuse that it hardly deserves to be 
called a tract. 
The other is a small, narrow, comma-shaped tract (Fig. 123, rr), situated 
in the middle of the external posterior column which has been observed in 
the cervical and upper thoracic regions, and has been called the "descend- 
ing " comma tract. But the degeneration reaches a short way only below THE STRUCTURE OF THE SPINAL CORI). 
543 
the section or injury, and the group of fibres thus degenerating can hardly 
be considered as forming a tract comparable to the other tracts. The area 
probably represents fibres of the posterior root which take a descending course 
soon after their entrance into the cord. 
II. Ascending tracts, that is to say, tracts in which the degeneration takes 
place above the section or injury. 
A conspicuous ascending tract of a curved shape (Fig. 123, C.b.) occu- 
pies the outer dorsal part of the lateral column lying to the outside of the 
crossed pyramidal tract, between it and the surface of the cord. It appears 
to begin in the upper lumbar region, being said to be absent from the lower 
lumbar and sacral cord, and may be traced upward increasing in size through 
the thoracic and cervical cord to the bulb. In the bulb it may be traced 
into the restiform body or inferior peduncle of the cerebellum, and so to the 
cerebellum ; for the restiform body serves, as we shall see, in each lateral 
half of the brain, as the main connection of the cerebellum with the bulb 
and spinal cord. Hence this tract is called the cerebellar tract. 
A second important ascending tract occupies the median portion of the 
posterior columns (Fig. 123, cr., s.lr.), and so far coincides with what we 
described above as the median posterior column, in the upper regions of the 
cord, that it may be called the median posterior tract; it extends along the 
whole length of the spinal cord, varying at different levels in a manner which 
we shall presently study, and ending above in the bulb. 
A third ascending tract, called the ascending antero-lateral tract, or tract 
of Gowers, occupies (Fig. 123, asc. a. I.) the outer ventral part of the lateral 
column. It has somewhat the form of a comma, with the head filling up 
the angle left between projecting portions of the cerebellar and pyramidal 
tracts, and the tail stretching away ventrally along the outer margin of the 
lateral column outside the antero-lateral descending column, the end of the 
tail often reaching to the anterior roots. It may be traced along the whole 
length of the cord, but it is not so distinct and compact a tract as the two 
ascending tracts just mentioned ; the fibres with ascending degeneration, that 
is to say, the fibres degenerating above the section or seat of injury, are very 
largely mixed with fibres of a different nature and origin. 
We may further remark that these several tracts differ from each other, 
in some cases markedly, as to the diameter of their constituent fibres. Thus 
the cerebellar tract is composed almost exclusively of remarkably coarse 
fibres. The median posterior tract, on the contrary, is made up of fine fibres 
of very equable size, while the fibres of the antero-lateral ascending tract 
are of a size intermediate between the other two. The pyramidal tract, on 
the other hand, is made up of fibres of almost all sizes mixed together. 
The tracts then which are thus marked out are, as descending tracts, the 
crossed and the direct pyramidal tracts, with the less distinct or important 
antero-lateral descending tract; and, as ascending tracts, the cerebellar tract, 
the median posterior tract, and the less distinct antero-lateral ascending 
tract. If we suppose all these tracts taken away there is still left a consider- 
able area of white matter, namely, nearly the whole of the external posterior 
column, the external anterior column, including the region traversed by the 
bundles of the anterior roots, and that part of the lateral column which lies 
between the antero-lateral descending tract and the crossed pyramidal tract 
on the outside and the gray matter on the inside. From this area of white 
matter we may put on one side at present the external posterior column, 
because, as we shall see, this column is largely composed of the fibres of 
the posterior root which pass through this column, especially through the 
lateral part of it near the gray matter, on their way to their ultimate desti- 
nation; hence the alternative name of posterior root-zone. Wemaysimi- 544 
THE SPINAL CORI). 
larly leave for the present the small zone of white matter composed of very 
fine fibres known as the posterior marginal zone or Lissauer's zone (Fig. 123, 
L.), lying dorsal to the tip of the posterior horn and in the lower regions 
reaching to the outside of the cord ; for this, too, belongs to the fibres of the 
posterior root. Leaving these parts out of consideration we may say as 
regards the rest of the white matter that the present state of our knowledge 
will not allow us to divide it into special tracts. All this area is largely 
composed of fibres which do not undergo either ascending or descending 
degeneration as the result of section, injury, or disease. It has been sug- 
gested that these fibres either have no trophic centre at all or have double 
ones, one above and one below, on either of which they can in case of need 
lean ; so that when the fibre is divided at any level, the upper portion is still 
nourished from some centre above, and the lower from some centre below. 
At all events, whether this be the true explanation or not, the fibres in this 
part of the white matter cannot be differentiated into tracts by a study of 
their degeneration. Fibres of this kind, which we can speak of neither as 
ascending nor as descending, also occur in the external posterior column 
mingled with the fibres of the posterior root. And we may repeat the cau- 
tion, that even in the several ascending and descending tracts just described, 
especially in those which we spoke of as less distinct or as more diffuse, many 
fibres are present which undergo neither ascending nor descending degen- 
eration. 
§ 481. It may be as well perhaps to insist here once more, that when 
these several tracts or the fibres running in the tracts are spoken of as ascend- 
ing or descending, what is meant is that the degeneration takes place above 
the section or seat of injury or disease in the one case, and takes place below 
in the other. It has been supposed by many that the nervous impulses which 
these fibres severally carry, travel in the same direction as that taken by the 
degeneration, that the ascending tracts carry impulses from below upward, 
that is to say, carry impulses which arising from peripheral organs pass to 
various part of the spinal cord or of the brain, that they are, in other words, 
channels of afferent impulses, and that conversely the descending tracts carry 
efferent impulses. To this view is often added as a corollary, that the tracts 
which do not degenerate at all carry impulses both ways, and hence cannot 
be considered as either afferent or efferent channels, but simply as communi- 
cating channels. Upon this it may be remarked that impulses do not neces- 
sarily travel in the same direction as the degeneration; when a spinal 
nerve-trunk is divided the afferent fibres as well as the efferent fibres both 
degenerate in a descending direction toward the periphery, though the 
former carry impulses in the other direction. Hence the direction of degen- 
eration is no proof of the direction in which impulses travel; moreover, as 
we have seen, degeneration does not actually travel along the fibres of the 
spinal cord in the same way that it does along the fibres of a nerve-trunk. 
It may be that the descending tracts do carry impulses in a descending di- 
rection, that is, efferent impulses, and that the ascending tracts serve to carry 
afferent impulses; but the proof that they do thus respectively act must be 
supplied from other facts than those of degeneration. Moreover, we shall 
have to return to these ascending and descending tracts and to study their 
behavior along the length of the cord before we can use the facts concerning 
them as a basis for any discussion as to their functions. 
§ 482. The connections of the nerve-roots. If we regard the spinal cord, 
and apparently we have a right to do so, as resulting from the fusion of a 
series of segments or metameres, each segment, represented by a pair of spinal 
nerves, being a ganglionic mass, that is to say, a mass containing nerve-cells 
with which nerve-fibres are connected, we should expect to find that the THE STRUCTURE OF THE SPINAL CORD. 
545 
fibres of a spinal nerve soon after entering in or before issuing from the 
spinal cord are connected with nerve-cells lying in the neighborhood of the 
attachment of the nerve to the cord. We should, we say, expect to find 
this; but owing to the difficulty of tracing individual nerve-fibres through 
the tangled mass of the substance of the cord, our actual knowledge of the 
termination of the fibres of the posterior root and origin of the fibres of the 
anterior root is at present far from complete. 
With regard to the anterior root, there can be no doubt that a very large 
proportion of the fibres in the root are continuations of the axis-cylinders of 
cells in the anterior horn. The fibres which can thus be traced are of large 
diameter and appear to be chiefly if not exclusively motor fibres for the 
skeletal muscles. In the frog a laborious enumeration, on the one hand, of 
the number of fibres in the anterior roots, and, on the other hand, of the 
number of cells of the anterior horn in the areas corresponding to the 
nerve-roots has, it is true, shown a very remarkable agreement in the number 
between the two. We might be inclined from this to conclude that all the 
fibres of an anterior root start directly from cells in the anterior horn, and 
that all the cells in the anterior horn end in fibres of the nearest anterior 
root. But several considerations prevent us from trusting too much to this 
observation especially in the case of the higher animals. The anterior root 
contains other fibres than motor fibres for the skeletal muscles, vasomotor 
fibres for instance, secretory fibres and others ; and it is a priori unlikely 
that these should have origin from the same cells as the motor fibres of the 
skeletal muscles. Moreover, as a matter of fact, some of the fibres have 
been traced through the anterior horn, on the one hand, toward the posterior 
horn, and, on the other hand, toward the lateral column ; others again are 
found to pass through the anterior horn of their own side to the bottom of 
the anterior fissure where, crossing over to the other side and thus forming 
part of the anterior white commissure, they appear to ascend to the anterior 
horn of the other side. We cannot at present make any positive statement 
as to the real origin and exact nature of these fibres which thus upon enter- 
ing the cord pass by the cells in the anterior horn without joining them, 
though those which cross by the anterior white commissure are supposed to 
take origin in the cells of the anterior horn of the other side; it is sufficient 
for our present purposes to remember that while a large number of the fibres 
of the anterior root, presumably those supplying the skeletal muscles, take 
origin in the cells of the anterior horn, shortly before they issue from the 
cord, others have some other origin. And similarly we have reason to think 
that all the cells in the anterior horn do not send out axis-cylinder processes 
to join the anterior roots of the same side. We may, however, regard a 
large number at all events of the cells of the anterior horn, at the level of 
as well as a little below and a little above the level of the exit of any par- 
ticular anterior root, as constituting a sort of nucleus of origin for the larger 
number of the fibres, and those most probably the skeletal motor fibres, of 
that anterior root. 
The posterior root enters the cord not in several bundles laterally scat- 
tered as does the anterior root, but in a more compact mass. This mass, 
however, consists of at least two distinct bundles, which upon their entrance 
into the cord, take different courses. One bundle, the larger one, lying to 
the inner or median side of the other, consisting of relatively coarse fibres, 
and called the median bundle (Fig. 121, Pr'), passes obliquely into the lateral 
part of the external posterior column, which, as we have said, is in conse- 
quence often spoken of as the posterior root-zone. Here the fibres changing 
their direction run longitudinally for some distance upward (some, however, 
certainly in the upper cervical region, and probably in other regions, run a 546 
THE SPINAL CORD. 
short distance downward), but eventually either go, as we shall see, to form 
the median posterior tract or make their way back into the gray matter at 
the base of the posterior horn and thus join the vesicular cylinder, though 
some are said to be continued on through the gray matter into the anterior 
horn. The other smaller bundle placed to the outside of the former, and 
called the lateral bundle (Fig. 121, Pr), may be again divided into an inter- 
mediate bundle (Fig. 122, Pr) lying next to the median bundle, and into a 
still more lateral bundle (Fig. 122, Pr"'). The former, consisting also of 
coarse fibres, plunges directly through the substance of Rolando at the ex- 
tremity of, and so into the gray matter of the horn, where the fibres chang- 
ing their direction run in part at least longitudinally in the gray matter in 
bundles known as " the longitudinal bundles of the posterior horn " (Figs. 
121, 122, r.f p.), some of which appear to pass on to the anterior horn. 
The small, most external or lateral portion of the lateral bundle, consisting of 
fine fibres and sometimes spoken of as the lateral bundle, on entering the cord 
at once ascends for some distance, and thus forms the thin layer of fine fibres, 
the posterior marginal zone or Lissauer's zone, indicated in Fig. 122 by m. t., 
which lies between the actual extremity of the horn and the surface of the 
cord, and in the upper regions of the cord (cf. Fig. 121,7/) runs some way 
upward on the lateral margin of the horn between the gray matter and the 
crossed pyramidal tract. As it ascends this layer continually gives off fibres 
to the gray matter of the posterior horn in the cells of which they appear 
to end. 
Thus, while part of the median bundle does not join the gray matter at 
all but goes to form the median posterior tract, the rest of that bundle and 
all the other fibres of the root, sooner or later, join the gray matter either 
of the posterior horn or of some other part. 
§ 483. The specialfeatures of the several regions of the spinal cord. The 
cord begins below in the slender filament called the filwm terminale, which 
lying in the vertebral canal, in the midst of the mass of nerve-roots called 
the cauda equina, rapidly enlarges at about the level of the first lumbar ver- 
tebra into the conus medullaris. This may be regarded as the beginning of 
the lower portion of a fusiform enlargement of the cord known as the lumbar 
swelling, which reaches as high as about the attachment of the roots of the 
twelfth or eleventh thoracic nerve at the level of the eighth thoracic ver- 
tebra, the broadest part of the swelling being about opposite the third lumbar 
nerve. Above the lumbar swelling, through the thoracic region the some- 
what narrowed cord retains about the same diameter until it reaches the 
level of the first or second thoracic nerve opposite the seventh cervical ver- 
tebra where a second fusiform enlargement, the cervical swelling, broader and 
longer than the lumbar swelling, begins. The broadest part of the cervical 
swelling is about opposite to the fifth or sixth cervical nerve; from thence 
the diameter of the cord becomes gradually somewhat less until it begins to 
expand into the bulb, but even in the highest part is greater than in the 
thoracic region. The sectional area of the cord increases therefore from 
below upward, but not regularly, the irregularity being due to the lumbar 
and cervical swellings. The extremity of the filum terminale is said to con- 
sist entirely of neuroglia closely invested by the membranes, even the central 
canal being absent. A little higher up the central canal begins, and nerve- 
cells with nerve-fibres make their appearance in the neuroglia; thus a kind 
of gray matter covered by a thin superficial layer of white matter is estab- 
lished. We have already referred to the peculiar features of the lower end 
of the conus (§ 477) ; but higher up the canal becomes central and small, 
the posterior columns are developed, and the gray matter contains more 
nervous elements and relatively less neuroglia, becomes in fact ordinary gray THE STRUCTURE OF THE SPINAL CORD. 
547 
matter. From thence onward to very near the junction with the bulb, where 
transitional features begin to come in, the spinal cord may be said to have 
the general structure previously described. 
The sectional area of the white matter increases in absolute size and on 
the whole in a steady manner from below upward. In other words, in a 
section at any level, the number of longitudinal fibres forming the white 
matter is greater than the number at a lower level, and less than the number 
at a higher level; for any difference which may exist in the diameter of the 
individual fibres is insufficient to explain the differences in the total sectional 
Fig. 124. 
Diagram showing the United Sectional Areas of the Spinal Nerves, proceeding from Below 
Upward. In this, as in the succeeding figures, 125, 126, 127,128,129, all of which refer to man, the 
left-hand side represents the bottom of the cord and the right-hand the top of the cord, the nu- 
merals indicating successively the sacral, lumbar, thoracic, and cervical nerves. The several 
figures are not drawn to the same scale. 
area of the white matter. If we were to measure in man the sectional area 
of each of the spinal nerves as it joins the cord, and to add them together, 
passing along the cord from below upward, the results put in the form of a 
curve would give us some such figure as that shown in Fig. 124; the area 
gained by adding together the sectional areas of the nerves increases in a 
fairly steady manner from below upward. The curve of the sectional area 
of the white matter of the cord taken from below upward would be very 
similar, but if anything more regular. It must be understood, however, 
Fig. 125. 
Diagram showing the Variations in the Sectional Area of the Gray Matter of the Spinal Cord 
along its Length. 
that the dimensions of the areas would not be the same in the two cases. 
The sectional area of the white matter at the top of the cervical region, though 
greater than anywhere lower down, is far less than the united sectional area 
of all the nerves below that level. The white matter is not formed by all 
the fibres from the nerves which join the spinal cord continuing to run along 
the cord up to the brain ; as we have seen, some at least of the fibres end in 
the gray matter. Nevertheless, the white matter in passing up the cord ap- 548 
THE SPINAL COKD. 
pears to receive a permanent addition at the entrance of each nerve. We 
may infer that each nerve has a representative of itself starting from the 
level of its entrance and running up to some part of the brain. Whether 
the fibres thus representative of the nerve are continuations of the very 
fibres of the nerve itself, or are new fibres starting from some relay of 
gray matter, with which the fibres of the nerve are also connected, is another 
question. 
§ 484. The gray matter in contrast to the white matter shows great varia- 
tions in area along the length of the cord (Fig. 125). From the entrance 
of the coccygeal nerve upward the area increases very rapidly, reaching a 
maximum at about the level of the fifth lumbar nerve. It then rapidly de- 
creases to about the level of the eleventh thoracic nerve, maintains about the 
same dimensions all through the thoracic region, and begins to increase again 
at about the level of the second thoracic nerve. Its second maximum is 
reached at about the level of the fifth or sixth cervical nerve, after which 
the area again becomes smaller, remaining, however, at the upper cervical 
region much larger than in the thoracic region. 
Fig. 126. 
Diagram showing the Relative Sectional Areas of the Spinal Nerves as they Join the Spinal 
Cord. 
The meaning of these variations becomes clear when weturn to Fig. 126, 
which shows in a similar diagrammatic manner the sectional areas of the 
several spinal nerves. It will be observed that the increase and decrease of 
the sectional area of the gray matter follow very closely the increase and 
decrease of the quantity of nerve, that is to say, neglecting differences in the 
diameter of the fibres, in the number of nerve-fibres passing into the cord. 
The sectional areas of the first and second sacral, fourth and fifth lumbar 
nerves are very large, and opposite to these the sectional area of the gray 
matter of the cord is very large also ; the enlargement of gray matter which 
is the essential cause of the lumbar swelling is correlated to the large number 
of fibres which enter and leave the cord at this region to supply chiefly the 
lower limbs. Similarly the enlargement of gray matter which is the essen- 
tial cause of the cervical swelling is correlated to the large number of fibres 
which enter and leave this region of the cord to supply chiefly the upper 
limbs. In the thoracic region, where the number of fibres entering and 
leaving the cord is relatively less, the sectional area of the gray matter is 
also less. Since the attachments of the several spinal nerves are not exactly 
equidistant from each other along the length of the cord, the sectional area 
is not an exact measure of bulk ; the total bulk of gray matter, for instance, 
belonging to two nerves which enter the cord close together is less than that 
of two nerves giving rise to the same sectional area of gray matter as the 
former two but entering the cord far apart from each other. Still the error 
which may be introduced by taking sectional area to mean bulk is, for pres- 
ent purposes at all events, so small that we may permit ourselves to say that 
in the successive regions of the spinal cord the bulk of gray matter in any 
segment is greater or less according to the size of the nerve (or pair of 
nerves, right and left) belonging to that segment. 
From this anatomical fact we appear justified in drawing the conclusion THE STRUCTURE OF THE SPINAL CORD. 
549 
that at all events a great deal of the gray matter of the spinal cord may be 
considered as furnishing a nervous mechanism, with which the efferent fibres 
of each spinal nerve just before they leave the cord, and the afferent fibres 
soon after they join the cord, are more immediately connected. It may be 
that the whole of the gray matter is thus directly connected with and thus 
rises and falls with the fibres of the nerves; or it may be that there is a sort 
of cord of gray matter, which maintains a uniform bulk along the whole 
length of the cord and serves as a basis which is here more and there less 
swollen by the addition of the gray matter more immediately connected with 
the fibres of the nerves. This question the method which we are now using 
cannot settle. 
§ 485. Owing to these different rates of increase of the gray and white 
matter respectively along the length of the cord, we find that in sections of 
the cord taken at different levels the appearances presented vary in a very 
distinct manner. This is strikingly shown by comparing Figs. 119, 121 and 
122. At the level of the third lumbar nerve (Fig. 122) the gray matter is 
very large, reaching, as we have seen, its maximal sectional area at about 
this point, so that although the area of white matter is not very great the 
whole area of the cord is considerable. 
At the level of the sixth thoracic nerve (Fig. 119), in spite of the white 
matter having very decidedly increased, the gray matter has shrunk to such 
very small dimensions that the total sectional area of the cord has markedly 
diminished. 
At the level of the sixth cervical (Fig. 121) the gray matter has again 
increased, reaching here, as we have seen, its second maximum ; the white 
matter has also further increased, and that indeed very considerably, so that 
the total area of the cord is much greater than in any of the lower regions. 
Further details of the varying size of the white matter and of the gray 
matter at different levels are also shown in the series given in Fig. 127. In 
these, combined with the three figures just referred to, it will be observed 
that the serial increase and decrease of the gray matter does not affect all 
parts of the gray matter alike, so that the outline of the gray matter changes 
very markedly in passing from below upward. In the coccygeal region each 
lateral half is a somewhat irregular oval, and in the sacral region (Fig. 127, 
Sac.) the differentiation into anterior and posterior horns is still very indis- 
tinct. In the lumbar region the two horns are sharply marked out, though 
both the posterior and anterior horns are broad and more or less quadrate. 
In the thoracic region the decrease of gray matter has affected both horns, 
so that both are pointed and slender, while the junction between them has 
not undergone so much diminution, so that what has been called the lateral 
horn is relatively conspicuous. In the cervical region the returning increase 
bears much more on the anterior horn, which again becomes large and broad, 
than on the posterior horn, which still remains slender and pointed. Taking 
the form of the gray matter in the thoracic region as the more typical form 
of the gray matter we may say that while the increase in the lumbar swell- 
ing bears equally on the anterior and posterior horns, that in the cervical 
region bears chiefly on the anterior horns. 
Now we have no reason to suppose that either afferent impulses reach the 
lumbar spinal cord in greater numbers from the lower limbs, or along any 
of the nerves joining this part of the cord, or that those which do reach it 
are of a more complex nature than is the case with the afferent impulses 
reaching the cervical cord along the nerves of the upper limbs. The increase 
of gray matter in the posterior horns is therefore not correlated to any in- 
crease in the number or complexity of the afferent impulses reaching the 
cord ; and we may provisionally conclude that at least a large part of the 550 
THE SPINAL CORD. 
gray matter in the posterior horn is not specially concerned in any elabo- 
ration or transformation of afferent impulses immediately upon their arrival 
at the cord. Indeed, we have seen that while there is ample evidence to 
connect the nerve-cells, and therefore presumably the gray matter in general 
of the anterior horn with the efferent motor fibres of the anterior root, there 
is no corresponding evidence as to any large immediate connection of the 
afferent fibres of the posterior root with the nerve-cells, or indeed any other 
part of the gray matter of the posterior horn. We may add that, as we 
shall point out later on, so essential is the concurrence of appropriate affer- 
ent impulses to the due carrying out of complex coordinate motor or effer- 
ent impulses, that we can scarcely expect to find any increase in the nervous 
mechanisms devoted to the purely motor function of carrying out motor 
impulses without a corresponding increase in the nervous mechanisms 
belonging to the afferent impulses, by means of which these motor impulses 
Fig. 127. 551 
THE STRUCTURE OF THE SPINAL CORD. 
Diagram illustrating some of the Features of the Spinal Cord at Different Levels. (Sherring- 
ton.) All the figures are drawn to scale, and represent the cord magnified four times. They show 
the difference at different levels in the shape and size of the cord, in the outline of the gray 
matter, and in the relative position of the anterior and posterior fissures, and also show the varia- 
tions at different levels of the several " tracts" of the white matter. 
C2 at the level of the second cervical nerve, C6 of the fifth cervical, C8 of the eighth cervical. 
D« of the second thoracic, D5of the fifth thoracic, L} of the first lumbar, £5 of the fifth lumbar 
and Sac. of the second sacral nerve. 
The shading of the tracts is the same as in Fig. 123; but in the median posterior column of 
D2 the areas of fibres coming from the sacral nerves s.r. and lumbar nerves l.r. are distinguished 
from the area d.r. of fibres belonging to the thoracic nerves. In C8 no distinction is made between 
any of these sets of fibres; in £5 only fibres of sacral nerves are represented; in £1 D8 D& the 
more dorsal small portion corresponds in sacral fibres and the next to lumbar, or lumbar thoracic 
nerves. 
are guided and coordinated. Hence, were the nervous mechanisms re- 
stricted to the posterior horns, we should expect to find a greater parallel- 
ism than does actually exist between them and the anterior horns. 
§ 486. The changes in the area of gray matter illustrated by the state- 
ments and diagrams given above refer to the gray matter as a whole-that 
is, not only to nerve-cells, but also to strands and networks of nerve-fibres 
and nerve-fibrils, and indeed include to a certain extent neuroglia. We 
have seen (§ 479) that we are able to distinguish certain large and con- 
spicuous nerve-cells in the gray matter, and to arrange these into groups. 552 
THE SPINAL CORD. 
The gray matter contains many other small nerve-cells, which we are not 
able at present to name or arrange, but whose existence must always be 
borne in mind. Confining ourselves now, however, to the groups of larger, 
more conspicuous nerve-cells, we find that, broadly speaking, the chief dif- 
ferences which can be observed in the cells of the anterior horn along the 
length of the cord are that in the thoracic region the nerve-cells of the ante- 
rior horn are few and relatively small, while in the cervical and lumbar 
region, especially in the latter, they are numerous and large. It is not easy, 
even if possible, to distinguish in the thoracic region the several groups of 
cells marked in Figs. 121 and 122 as 2cc,/5, the median group (Figs. 121, 
122, 1), indeed, seems to be the only group present in the mid-thoracic 
region (Fig. 119, 1). The group of the posterior horn (Figs. 119, 121, 122, 
6) appears to be about the same in all regions. 
With two other groups of nerve-cells striking differences are seen in dif- 
ferent regions. The vesicular cylinder, for instance (Fig. 119, 3), is most 
conspicuous in the thoracic region. It may be said to reach from the 
seventh or eighth cervical nerve to the third lumbar nerve, being perhaps 
most developed in the lower thoracic and upper lumbar region. It is 
absent in the cervical region above the seventh or eighth cervical nerve, and 
in the lumbar region below the third lumbar nerve; but a similar group of 
cells is present opposite the second and third cervical nerves; a group of 
more doubtful likeness is seen in the sacral region below, and the column is 
said to have a representative in the bulb above the spinal cord proper. It 
seems natural to infer that the cells forming this vesicular cylinder are 
connected neither with the ordinary somatic motor fibres governing the 
skeletal muscles, nor with the ordinary afferent sensory somatic fibres 
coming from the skin and elsewhere, but in some way with some special 
sets of fibres; on this point, however, no authoritative statement can as yet 
be made. 
The lateral horn or intermedio-lateral tract (Fig. 119, 4) is also most con- 
spicuous in the thoracic region. In the lumbar region it is lost or traced 
with great difficulty, and in the cervical region seems to be merged into the 
most dorsally placed division of the lateral group of cells of the anterior 
horn. It is possible that this group represents in the limbless thoracic 
region the cells which are developed into the great lateral group of the ante- 
rior horn in the regions of the limbs. 
§ 487. The white matter, as we have seen, increases in sectional area 
with considerable regularity from below upward. If, instead of a diagram 
of the increase of the whole white matter we construct in a similar way 
diagrams of the anterior, posterior, and lateral columns respectively, we 
find that while the sectional area of the lateral column (Fig. 128) increases 
with some considerable regularity from below upward, though not so regu- 
larly as does the whole area of white matter, both the anterior (Fig. 129) 
and the posterior (Fig. 130) columns agree to a certain extent with the gray 
matter in showing a decided increase in both the lumbar and the cervical 
swellings. We may, provisionally at least, infer from this that, while con- 
siderable portions of both the anterior and the posterior columns are, like 
the adjoining gray matter, in some way or other concerned in the exit and 
entrance of efferent and afferent fibres, the larger portion of the lateral 
column is concerned in the transmission of impulses to and fro, between the 
local mechanisms below, immediately connected with the several spinal 
nerves, and the brain above. This conclusion seems incidentally confirmed 
(though these diagrams must not be strained to carry detailed inferences) by 
the sudden increase of the lateral column above the lumbar swelling, as if 
the large mass of nervous mechanism for the lower limbs concentrated in THE STRUCTURE OF THE SPINAL CORD. 
553 
this region demanded a sudden increase in the number of fibres connecting 
it with the brain above. 
This more or less continuous increase of the lateral column partly ex- 
plains the change of form in the general outline of the transverse section 
of the cord which is observed in passing upward from the lower to the 
higher regions. In the coccygeal, sacral, and lumbar regions the outline, 
though varying somewhat, chiefly owing to the disposition of the gray 
matter, is on the whole circular. ' In the thoracic region, especially in the 
upper part, the increase of the lateral columns increases the side-to-side 
Fig. 128. 
Diagram showing the Variations in the Sectional Area of the Lateral Columns of the Spinal 
Cord along its Length. 
Fig. 129. 
Diagram showing the Variations in the Sectional Area of the Anterior Columns of the Spinal 
Cord along its Length. 
Fig. 130. 
Diagram showing the Variations in the Sectional Area of the Posterior Columns of the Spinal 
Cord along its Length. 
diameter so much that the section becomes oval, and in the cervical region 
this increase of the side-to-side diameter out of proportion to the dorso-ven- 
tral diameter is very marked. The actual outline of the whole transverse 
section is, however, determined also to a certain extent by the changes of 
form of the gray matter. 
The cord, moreover, undergoes along its length a change which is not 
very clearly indicated in the diagrams (Figs. 129, 130). By comparing the 
series of transverse sections given in Fig. 127, it will be seen that the rela- 
tive position of the central canal shifts along the length of the cord. In the 
sacral and lumbar regions the central canal is nearly at the centre of the 
circle of outline, and the posterior and anterior fissures are nearly of equal 
depth. Even in the upper lumbar region, and still more in the thoracic re- 
gion, the position of the central canal is shifted nearer to the ventral sur- 
face, so that the posterior fissure becomes relatively longer, deeper than the 554 
THE SPINAL CORD. 
anterior. This shifting goes on through the cervical region up to about the 
level of the second cervical nerve, where it is arrested by the beginning of 
the changes through which the spinal cord is transformed into the far more 
complicated bulb. 
This lengthening of the posterior fissure indicates an increase in the 
dorso-ventral diameter of the posterior columns, and this, not being accom- 
panied by a compensating diminution of the side-to-side diameter, shows in 
turn that the posterior columns undergo an increase in passing upward. 
From this we may add to the provisional conclusion just arrived at with 
regard to the lateral columns, the further conclusion that some part of the 
posterior columns also is concerned in transmitting impulses, in a more or 
less direct manner, between the various regions of the cord below and the 
brain above. The anterior columns do not increase in the same marked 
manner, though over and above the increase due to the lumbar and cervical 
swellings a continued increase may be observed, especially in the upper cer- 
vical region ; it is in this upper region that the direct pyramidal tract is 
best developed. 
§ 488. The provisional conclusions at which we have arrived are further, 
to a certain extent at least, confirmed and extended by a study of the 
behavior at the several regions of the cord of the special tracts of white 
matter described in § 479. 
The pyramidal tract, that is to say, the crossed pyramidal tract entering 
the spinal cord above from the pyramid, is very large in the cervical region, 
having the form and situation shown in Fig. 127, CjGG. From thence 
downward it diminishes in size, the diminution being especially rapid in the 
lumbar swelling (Fig. 127, L^), where the tract, being no longer covered in 
by the cerebellar tract, comes to the surface of the cord ; but it may be 
traced by the degeneration method down as far as the coccygeal region, and 
indeed appears to be coexistent with the entrance of spinal nerves into the 
cord. Diminution of the tract means a lessening of the number of fibres; 
and since we cannot suppose that any of the fibres come suddenly to an end 
in the tract itself, we are led to infer that along the cord, from above down- 
ward, fibres are successively leaving the tract and passing to some other part 
of the cord. We seem further justified in concluding that the fibres which 
thus successively leave the tract go to join the series of local nervous mech- 
anisms with which the spinal nerves communicate, as we have seen reason 
to believe, upon their entrance into the cord. Indeed, as we shall see later 
on, we have reason to think that the nervous mechanisms which the fibres in 
question join are those belonging to the motor fibres of the anterior roots. 
This pyramidal tract does not begin in the pyramid, but may be traced 
through the lower parts of the brain right up to special areas in the cortex 
or surface of the cerebral hemispheres; and very strong reasons maybe 
brought forward in support of the view that the fibres of this tract are 
fibres which carry impulses from the cortex to successive portions of the 
spinal cord and there give rise to efferent impulses which pass to appro- 
priate skeletal muscles. The tract, therefore, is not only a descending 
tract by virtue of the mode of degeneration, but may be spoken of in a 
broad sense as a tract of efferent impulses descending from the cerebral 
cortex; and, indeed, it is maintained that it is the channel of the partic- 
ular kind of efferent impulses which we shall speak of as voluntary or voli- 
tional impulses. We may add that as the tract passes along a path, which 
we shall subsequently describe, from the cerebral cortex through the lower 
parts of the brain to the pyramid, it gives off fibres to mechanisms con- 
nected with several of the cranial nerves, much in the same way that it 
gives off fibres to the spinal nerves. 555 
THE STRUCTURE OF THE SPINAL CORD. 
We may, therefore, picture to ourselves this pyramidal tract as starting 
in the form of a broad sheaf of fibres from a certain district on the surface 
of one of the cerebral hemispheres. Putting aside for the present any pos- 
sible increase of the number of fibres by division of fibres (though we have 
reason to think that this does, to a certain extent, occur), we may regard 
the tract as being at its maximum at its beginning in the cortex. As it 
descends to the decussation of the pyramids in the bulb it loses a certain 
number of fibres, which pass off' to the cranial nerves. Having crossed and 
entered into the lateral column of the cord it continues to give off fibres to 
the spinal nerves, probably to the anterior root of each in succession, and 
so goes on its way down the cord continually diminishing until the last 
remaining fibres are given off to the last coccygeal nerve. 
When degeneration is set up along this tract, as may be done by in- 
juries to particular areas of the cerebral cortex, the main mass of degene- 
rated fibres, after crossing over from one side of the cerebro-spinal axis to 
the other in the decussation of the pyramids at the lower end of the bulb, 
during its further progress down the spinal cord, keeps to the side to which 
it has crossed right down to the end. Hence, as we have said, it is called 
the crossed pyramidal tract. The main mass of fibres, the degeneration of 
which has been started by injury to the left side of the brain, crosses over to 
the right side of the spinal cord and runs down the lateral column of the 
right side to the end of the cord. Nevertheless, some fibres appear to cross 
over again in the spinal cord and then to run along the same side as the side 
of the brain injured-along the left side in the case just mentioned. Such 
fibres are spoken of as " recrossed fibres." 
The direct pyramidal tract (Fig. 127, dP\ except that it does not cross 
at the decussation of the pyramids, is otherwise similar to the crossed pyra- 
midal tract, and, indeed, is a part of the same strand to which the crossed 
tract belongs. When degeneration in this tract is started by injury to par- 
ticular areas of the cerebral cortex, say on the left half of the brain, the 
degeneration may be traced through the left anterior pyramid, and so to the 
left median anterior column of the spinal cord. The direct tract is never so 
extensive or marked as the crossed tract, does not reach so far down, is much 
more variable both in length and in sectional area, and, as we have said, is 
almost confined to man. Diminishingas it descends it may be said to cease 
in the middle thoracic region (Fig. 127, D-a D^. Taking an average, we may 
say that, of the whole strand running in the pyramids above the decussa- 
tion, about three-fourths of the fibres go to form the crossed and about one- 
fourth to form the direct tract. We shall see later on that the impulses 
coming down along the united tract in the brain may, broadly speaking, 
be said to cross over wholly from one side to the other before they reach 
the skeletal muscles, so that the impulses passing along the fibres in, say, 
the left pyramid, reach the muscles of the right limbs and right side of the 
body, whether the fibres cross over at the decussation to form the crossed 
or remain on the same side to form the direct pyramidal tract. We are, 
therefore, led to infer that the fibres in the direct tract, as they pass down 
the cord, cross over in the cord itself before they make connections with the 
fibres of the anterior roots. Probably the crossing is effected by means of 
some of the decussating fibres which form the anterior white commissure. 
A part only, indeed a small part, of the commissure can serve this purpose; 
most of the fibres of the commissure, and in the lower regions of the cord, 
where the direct tract no longer exists, all the fibres, must have some other 
functions. Some of the fibres of this great pyramidal tract leave the tract, 
as we have said, to join some of the cranial nerves before the pyramids of 
the bulb are reached ; and the impulses passing along these fibres also cross 556 
THE SPINAL CORD. 
over to the opposite side before they issue along the cranial nerves. Hence 
we infer that these fibres decussate above the decussation of the pyramids 
just as those of the direct tract decussate below it. So that of the whole 
strand as it leaves the cerebral cortex, while the main mass of fibres crosses 
over at the decussation of the pyramids, the rest of the fibres cross the 
middle line in succession from the level of the third cranial nerve to the 
level of the lower limit of the direct tract; below the decussation of the 
pyramids the crossing takes place by means of the anterior commissure of 
the cord, above the decussation by means of what we shall later on learn to 
speak of as the raphe of the bulb, or by structures corresponding to this 
higher up. 
§ 489. The cerebellar tract (Fig. 127, C-b.) is, as we have seen, a tract of 
ascending degeneration ; the degeneration in it makes its appearance above 
the section or seat of other injury of the cord. If begins somewhat sud- 
denly at the level of the second lumbar nerve region, being absent at least 
as a distinct tract below ; injury of the cord at the level of the middle and 
lower lumbar nerves leads to no marked tract of degeneration (though pos- 
sibly scattered single fibres may degenerate), while injury higher up does. 
The tract lies, as we have said, close to the surface of the cord in the poste- 
rior part of the lateral column just outside the crossed pyramidal tract, 
and while varying somewhat in the shape of its section from level to level, 
remains throughout a somewhat narrow crescentic patch. At the top of 
the spinal cord, it passes, as we have said, from the lateral columns into 
the restiform bodies of the bulb, and so to certain parts of the cere- 
bellum. 
When the section or lesion is limited to one side of the cord, the degen- 
eration is similarly limited to the same side, and that along its whole course 
up to the cerebellum ; there is no evidence of any of the fibres decussating 
in the cord. 
The area of the tract increases from below upward. This has been deter- 
mined by the embryological method, by noting the appearance of the me- 
dulla in the fibres, as well as by comparing the extent of the degeneration 
following upon a section high up in the cord with that following upon a sec- 
tion lower down From this we infer that the fibres composing the tract 
must start successively from other parts of the cord along its length-that 
is to say, the tract must be fed by fibres coming from other structures in the 
cord. On the other hand, it is found that the degenerated area following 
upon a section or injury diminishes as it is traced upward ; when, for instance, 
a section is made in the mid-thoracic region, the area of degeneration in the 
tract is greater immediately above the section than it is higher up, say in 
the cervical region. From this we are led to infer that though the tract is 
successively fed along its course by fibres coming from other parts of the 
cord, some of the fibres entering the tract, though like their companions 
undergoing an ascending degeneration, do not like them continue in the 
tract right up to the cerebellum, but pass off to other parts of the cord 
on their way upward. This, however, is equivalent to saying that the 
tract is not a pure or homogeneous one, but consists of at least two sets of 
fibres, only one of which is continued on to the cerebellum and strictly 
deserves the name of "cerebellar." It may, perhaps, here be mentioned 
that while the fibres composing the tract are, as a whole, conspicuously 
coarse, large fibres, with these there are mingled, especially in the thoracic 
region, a number of much finer fibres ; but these apparently undergo a 
descending, not an ascending, degeneration, and do not, therefore, really 
belong to the tract; they may be fibres which have strayed from the pyram- 
idal tract. THE STRUCTURE OF THE SPINAL CORD. 
557 
We have as yet no very clear evidence as to the origin of the fibres which 
compose the tract. Unlike the case of the median posterior tract of which 
we have next to speak, no degeneration, at least in the lumbar and thoracic 
regions, appears in the tract after section merely of the roots of the nerves; 
to produce the degeneration the cord itself must be injured. From this we 
may infer that the tract is not fed directly by the fibres of the posterior roots. 
Some observers maintain that the tract is fed by fibres coming from the 
vesicular cylinder and point out that both the tract and the column begin at 
the same level somewhat suddenly ; but the want of parallelism between the 
course of the tract and that of the cylinder along the length of the cord, the 
latter being as we said conspicuous in the thoracic region while the tract 
steadily increases upward, is distinctly opposed to such a view. From the 
fact that the degeneration taking place in it is an ascending one, it is sup- 
posed that the tract is the channel for ascending, that is to say, in a broad 
sense, afferent impulses. And considerable interest attaches to the fact that 
these impulses should be carried, not to the cerebrum but to the cerebellum. 
Our knowledge on this point, however, is very imperfect, and what can be 
said in the matter had better be said later on. 
§ 490. The median posterior tract is the other conspicuous tract of ascend- 
ing degeneration ; it also is supposed to be a channel for ascending afferent 
impulses; and this view is rendered almost certain by the intimate relations 
of the tract to the fibres of the posterior roots. 
In dealing so far with the tracts of degeneration in the spinal cord we 
have always spoken of the degeneration as being the result of lesions of the 
spinal cord itself. Experiments on animals, however, and clinical experience 
have shown that division or injury of the fibres of the posterior roots is fol- 
lowed by tracts of degeneration in the spinal cord, though no damage what- 
ever may have been done to the substance of the cord itself. These tracts 
make their appearance in the median posterior columns, the exact path and 
limits of the degeneration differing with the different spinal nerves. The 
results of the division of different groups of nerves are so instructive that we 
may dwell upon them in detail. 
If the posterior roots of two or three lumbar nerves (on one side) be 
divided, an examination of the cord, after an interval long enough to allow 
degeneration to be well established, will bring to light the following features : 
The divided roots will be found to have degenerated right up to their entrance 
into the cord. A section of the cord opposite the entrance of the lowest 
divided root will show no degeneration of the cord beyond that of the bundles 
of fibres passing in. A little higher up degeneration will be observed in the 
external posterior column close to the posterior horn; and as we ascend we 
find that this degeneration first spreads over a large portion of the external 
posterior column, and then invades the median posterior column ; the de- 
generation does not affect the whole of the median posterior column but 
leaves intact a small dorsal portion, roughly triangular in shape, at the angle 
between the fissure and the dorsal surface of the cord, as well as some portion 
of the more ventral part of the column nearest the gray commissure. Still 
a little higher up we should find that degenerated fibres had disappeared 
from the external portion of the external posterior column close to the gray 
matter, though still existing in the more median part of that column as well 
as in the median posterior column to the extent just indicated. Still a little 
higher up the whole of the degeneration would have disappeared from the 
external posterior column, but the tract of degeneration in the median pos- 
terior column would remain, the extent of degeneration being dependent on 
the number of roots which had been divided. Lastly, by carrying the sec- 
tions still higher up the cord we should be able to trace this tract in the 558 
THE SPINAL CORD. 
median posterior column right up to the bulb, where it would come to an 
end. 
If we divided some of the thoracic nerves instead of the lumbar we should 
obtain very similar results : a degeneration of the external posterior columns 
a little above the entrance of the roots, spreading across the column toward 
the median line, and wholly disappearing at a certain height above, accom- 
panied by a degeneration of a part of the median posterior column, reaching 
from a little distance above the entrance of the divided nerve-roots right up 
to the bulb. This latter tract of degeneration would, however, not occupy 
the same position as that consequent upon division of the lumbar nerves; its 
position would be more ventral, nearer the gray commissure, and rather more 
lateral. Compare Fig. 127, D2, where Ir. indicates the degeneration due to 
section of the lumbar nerves, and dr. that of the thoracic nerves. If we 
divided some of the cervical posterior roots we should get similar results, 
with the difference that the tract of degeneration in the median posterior 
columns would occupy a position still more ventral and still more lateral 
(Fig. 127, c. r.), while if we divided the sacral nerves the tract of degen- 
eration would be dorsal and median to the tract belonging to the lumbar 
nerves, and would occupy more or less of the triangle left below that tract 
(Fig. 127, D2 s. r.). The degeneration, it will be understood, is in all cases 
confined to the same side of the cord as that of the divided roots. We may 
add, in order to complete the story of the effects of division of the posterior 
roots, that the section leads to degeneration of the marginal zone (Lissauer's 
tract), but this degeneration reaches for a certain distance only up the cord 
and then disappears. It will be remembered that this zone is fed by fibres 
(of fine calibre) belonging to the external or lateral bundle of the posterior 
roots. 
These results may be interpreted as follows: The (great majority of the) 
fibres of the posterior root, cut off from their ganglion by the division, 
degenerate centripetally toward the spinal cord. We have previously seen 
that many of the fibres of the root pass into the external posterior column 
and run up in that column for some distance. The degeneration observed in 
this column for some distance above the entrance of the divided roots shows 
that the fibres run lengthways for some distance is this column, while the 
disappearance of the degeneration a little higher up similarly shows that the 
fibres eventually leave the column. The appearance of degeneration in the 
median posterior column shows that some of these fibres have passed into 
that column from the external posterior column, and the continuation of that 
degeneration right up to the bulb indicates that these fibres pursue an 
unbroken course in that column along the whole length of the cord. The 
area of degeneration, or more exactly the number of degenerated fibres in 
the continued tract of degeneration in the median posterior column is much 
less than that in the temporary or short tract of degeneration in the ex- 
ternal posterior column. This shows that some only of the fibres passing 
into the external posterior column go on to join the median posterior column 
and so reach the bulb; the rest obviously take another path, and we have 
already seen reason to think that many of these end in the gray matter of 
the cord. Hence of all the fibres joining the cord in a posterior root, while 
some, and these we may add are chiefly fine fibres, entering the gray matter 
directly or passing into the posterior marginal zone, soon make such con- 
nections that the degeneration due to the section of the roots spreads no 
further, a large number, and these chiefly coarse fibres, before they make any 
such connection pass into and occupy for some length of the cord the external 
posterior column. We may here remark that though these fibres are spread 
over the greater part of this column, they do not form the whole of the THE STRUCTURE OF THE SPINAL CORD. 
559 
column; they are mixed up with fibres of a different nature and origin. Of 
these fibres of the posterior root which thus run in the external posterior 
column while still dependent for their nutritive activity on the ganglion of 
the root, some, indeed the greater part, leave the tract and make such con- 
nections in the gray matter that their degeneration ceases; others, forming 
the smaller part, pass into the median posterior column, and taking up a 
definite position in that column pursue an unbroken course to the bulb. 
All the fibres, therefore, of the posterior roots do not end in the gray 
matter soon after their entrance into the cord. A representative of each root 
is carried right up to the bulb by means of the median posterior column ; of 
the axis-cylinders which leave the ganglion on the root, a certain relatively 
small number pursue an unbroken course for some little distance through 
the external posterior column, and for the rest of their way through the 
median posterior column, along the whole length of the cord above the 
entrance of the root until they find an ending in the gray matter of the bulb. 
Further, each spinal nerve has this representative of its posterior root placed 
in a definite position in the posterior median column, the arrangement being 
such, as shown in Fig. 127, that the lower (sacral) nerves find their place in 
the more dorsal and median part of the column, while the nerves above are 
successively placed in positions more and more ventral and external. 
As far as our knowledge goes at present we are led to believe that this 
median posterior tract is very largely made up of fibres having this origin. 
It affords a channel by which afferent impulses are carried straight up the 
cord from the nerve-trunk without making connections on the way. We 
may repeat that the path is confined to the same side of the cord along its 
whole length ; there is no crossing over to the other side. 
In the above description we have spoken only of the result following sec- 
tion of the posterior roots outside the cord; but it will be understood that 
similar results follow upon section or of injury to or disease of the cord itself 
affecting the posterior columns or the bundles of the roots as they enter the 
cord. When such a lesion occurs there may be observed in the region of the 
cord above the lesion a degeneration of the external posterior column, reach- 
ing some little distance up, and a more limited degeneration of a part of the 
median posterior column stretching right up to the bulb. The position and 
form of the tract of the degeneration in the median posterior column will 
depend on the level of the lesion along the length of the cord, according as 
it interrupts the ascending representatives of the sacral nerves only, or of 
the lumbar and sacral nerves, or of the dorsal and cervical nerves as well. 
A complete section or hemi-section of the cord will produce results corre- 
sponding to the division on both sides or on one side of all the nerves below 
the section. 
We may add that while, according to some observers, the strand of fibres 
belonging to a particular root or group of roots having once taken up its 
position in the median posterior column remains unchanged until it reaches 
the bulb, according to others it diminishes in area, some of its fibres making 
connections in the cord itself. 
§ 491. The antero-lateral ascending tract (Fig. 127, asc. a. I.) is less well 
known than either of the two preceding; it is also more diffuse, that is to 
say, the fibres undergoing degeneration are more largely mixed with fibres 
of a different nature ami origin. It appears to extend down the cord to a 
lower level than the cerebellar tract, but its lower limit has not yet been 
accurately determined. Since the degeneration taking place in it is an 
ascending one, it has been inferred that it serves as the path for afferent 
and indeed for sensory impulses. Degeneration in it is seen only after 
section or injury of the substance of the cord itself, not after division of the 560 
THE SPINAL COKD. 
posterior roots. If, then, it is to be regarded as a channel of afferent im- 
pulses passing into it from the posterior roots, those impulses must pass into 
it along those fibres of the posterior root which find secondary trophic cen- 
tres in some part of the gray matter; in this respect this tract resembles the 
cerebellar tract, and differs from the median posterior tract. The latter 
is the direct continuation up the cord to the bulb of such fibres as are still 
trusting for their nutritive activity to the cells of the ganglion on the pos- 
terior root; the fibres of both the former trust for their nutritive activity to 
some part of the gray matter of the cord, and presumably to the nerve- 
cells of that gray matter. A further resemblance between the antero-lateral 
ascending and cerebellar tracts must be admitted, if future researches con- 
firm the opinion of those who hold that the former like the latter, at the top 
of the cord, pass along the restiform body to the cerebellum. Indeed, under 
such a view it would appear probable that the antero-lateral tract is simply 
a more diffuse and outlying part of the cerebellar tract. 
§ 492. We may now briefly pass in review, somewhat as follows, the chief 
facts which we have learned concerning the structure of the spinal cord, 
always keeping in view their physiological meaning. 
The important feature of the spinal cord is the presence of what we have 
called " gray matter," and all our knowledge goes to show that the important 
powers of the spinal cord, by which it differs from a thick multiple nerve, 
and by virtue of which we speak of it as a nervous centre orseriesof centres, 
are in some way or other associated with this gray matter. 
With this gray matter the fibres of the spinal nerves are connected. The 
greater part of the fibres of the anterior root certainly end in or rather take 
origin from the gray matter close to the attachment of the root, and the rest 
most probably join the gray matter at no great distance. The fibres of the 
posterior root run, as we have seen, for some little distance in the white 
matter, but if we except the special bundle which runs in the median pos- 
terior tract right up the cord to the bulb without joining the spinal gray- 
matter at all, we may say that the fibres of the posterior root also join the 
gray matter not far from the attachment of the root. 
Morphological reasons lead us, as we have seen, to regard the spinal cord 
as a series of segments, each segment corresponding to a pair of nerves; 
and even in the spinal cord of man we may recognize a segmental ground- 
work, obscured though this is by fusion and overlaid by the several commis- 
sural tracts. Each segment of this groundwork we may conceive of as a 
central mass of gray matter, connected on each side with an anterior and a 
posterior root, thus constituting a segmental nervous mechanism capable of 
carrying out certain functions. 
Such a segment has been compared to a ganglion, but it differs strikingly 
from a ganglion, whether of the posterior root or of the splanchnic system, 
both in structure and in function. A ganglion and the gray matter of a 
spinal segment both contain nerve-cells, and so far resemble each other ; but 
there the resemblance for the most part ends. In a ganglion the constituent 
nerve-cell is a development of the axis-cylinder of a fibre into a nucleated 
cell-body which lies on the course of the fibre, and may, as in a splanchnic 
ganglion, be placed just where one fibre divides into two or more. We have 
clear evidence that the cell, that is to say, the nucleus with the adjacent cell 
substance, exercises an important influence on the nutrition, and so on the 
functional activity of the nerve-fibre, it acts, as we have seen, as a "trophic 
centre." There are also reasons for thinking that the cell substance is more 
sensitive, more readily responsive to changes in its circumstances than is the 
axis-cylinder at some distance from the cell. But we have no satisfactory 
evidence that the cell can automatically originate nervous impulses in itself THE STRUCTURE OF THE SPINAL CORD. 
561 
as the outcome of its own intrinsic changes. Nor have we any evidence 
that the cell can exert any marked transforming power over the impulses 
passing along the fibre ; the impulses which travel away from the cell do not 
appear to differ markedly from those which travel toward it. The several 
instances in which there seemed to be evidence that splanchnic ganglia acted 
as centres either of reflex or of automatic action, have, as we have seen, 
broken down ; and it is not even suggested that the ganglia of the posterior 
roots possess any such powers. The gray matter of the spinal cord, on the 
other hand, as we have already seen, and as we shall see more in detail, is 
especially characterized by the possession of reflex and automatic as well as 
of other powers. 
In structure, moreover, such a spinal segment differs strikingly from a 
ganglion and exhibits features unknown in ganglia. In a ganglion the 
nerve-fibres may divide, and in a small peripheral ganglion the division may 
give rise to very delicate fibrils; but the fibres or fibrils resulting from the 
division leave the ganglion to follow their appropriate courses; the division 
serves for dispersion only. In the spinal cord, on the other hand, both 
efferent and afferent fibres divide in such a way that their divisions are lost 
to view in the gray matter ; division here seems to serve the purpose of union. 
The efferent fibres of the anterior root may be traced back as a process of a 
cell in the anterior horn. That cell gives off other processes, but no one of 
these processes is continued as an axis-cylinder process stretching across the 
gray matter until it becomes a fibre of the posterior root, or as anything like 
such an axis-cylinder process. On the contrary, all the processes, except 
the axis-cylinder process, divide into branches, and appear to end in ner- 
vous fibrils lost to view in the gray matter. Conversely, though our know- 
ledge of the junction of the posterior fibres with the gray matter is much more 
imperfect than that of the junction of the anterior fibres, what we do know 
leads us to believe that the fibres of the posterior root, either by the media- 
tion of cells, or by direct division of the axis-cylinder without the media- 
tion of cells, similarly break up into fibrils and are similarly lost in the 
gray matter. All the evidence goes to show that the anterior and posterior 
roots are functionally continuous; this functional continuity is, however, 
effected not by a gross continuity of axis-cylinders, but in a peculiar man- 
ner through the division of branches of nerve-cells or of axis-cylinders into 
the nervous tangle which forms such a special feature of the gray matter of 
the cord. We may, perhaps, venture to regard the gray matter of the seg- 
mental groundwork, of which we are now alone speaking, as constituting a 
nervous network or web, formed certainly in part by the rapidly dividing 
branches of nerve-cells, and probably in part by the divisions of directly 
dividing nerve-fibres. 
In any ordinary section of the spinal cord the gray matter presents to 
view much more than this nervous groundwork. To say nothing of the in- 
dubitable neuroglia and the obscure structures, including small cells, which 
are claimed now to be neuroglia, now to be nervous in nature, the gray 
matter in every section shows numerous distinct nerve-fibres crossing it in 
various directions; of these fibres a few are ordinary medullated fibres,some 
are non-medullated fibres, that is to say, are naked axis-cylinders, while 
others, and these the more numerous, are the peculiar medullated fibres of 
small diameter spoken of in § 476. A large number of these fibres, indeed 
all the larger ones, though they go to make up what we call gray matter, 
are not continuous with, and do not belong to, the groundwork or nervous 
web, at all events do not form part of the groundwork seen in the same 
section as themselves. They are simply fibres traversing the groundwork, in 
spaces of the neuroglia bed, on their way up or down the cord, or across the 
36 562 
THE SPINAL CORD. 
cord from one part to another. It may be that some of the finer medullated 
fibres do really enter into the groundwork, and so contribute to the nervous 
web; but our knowledge is too imperfect to afford a clear decision on this 
point. Our inability to define its exact limits need not, however, prevent 
our recognizing the existence of the groundwork. 
The prominence in this groundwork of the larger nerve-cells has led to 
the conception that the powers of the spinal segment are exercised by these 
nerve-cells to the exclusion of the other elements of the nervous web. But 
such a view has not been adequately proved. What we do know is that the 
nuclei and cell-bodies of the cells of the anterior horn exercise an important 
influence on the nutrition of the fibres of the anterior root which proceed 
from them, and possibly also influence the nutrition of the other branches 
of the cells forming part of the groundwork ; and these cells are probably 
so conspicuous a feature of every section of the spinal cord because of the 
important task intrusted to them of maintaining in due order the nutrition 
of the long stretch of motor fibres reaching from them to the muscular 
fibres or other peripheral organs. The fibres of the posterior root are not so 
obviously connected with the conspicuous cells of the gray matter; indeed, 
as we have said, it may be doubted, though the view is maintained by some, 
whether any cell intervenes to secure the continuity of a posterior fibre with 
the groundwork, a division of the axis-cylinder serving this purpose ; and 
this becomes intelligible when we bear in mind that the posterior fibres are 
governed as far as their nutrition is concerned by the nerve-cells of the gan- 
glion on the posterior root, which ought probably to be considered as much 
a part of the spinal cord as the cells of the anterior horn. The nerve-cell 
of the ganglion is adequate to secure the due nutrition of the nerve-fibre 
until it joins the groundwork, and probably helps to maintain the nutrition 
of the groundwork itself. 
Hence we may perhaps, until fresh evidence to the contrary is brought 
forward, incline to the view that the powers of the gray matter do not 
depend on the conspicuous cells alone or even chiefly, but on the peculiar 
molecular constitution and nature of the whole groundwork. The nuclei of 
the cells of the anterior horn with the cell substance adjacent to each and 
the cells of the ganglia on the posterior root probably govern the nutrition, 
and so the functional activity of the groundwork as well as of the issuing 
and entering fibres; but there appears to be as yet no convincing evidence 
of any other peculiar powers confined to the cells and absent from other 
parts of the groundwork. We may add that, in accordance with this view, 
the other cells of the gray matter, such as those of the vesicular cylinder, 
are to be regarded as of importance for governing the nutrition of fibres, 
commissural and others, starting from the spinal segment, and of the part 
of the groundwork from which by their mediation the fibres start, rather 
than for determining the functions of the groundwork of the segment or of 
the fibres receiving impulses from it. 
§ 493. The segmental groundwork of gray matter belonging to each pair 
of spinal nerves is so fused with that of all the other pairs as to form along 
the whole length of the cord a mass of gray matter which appears, under 
certain circumstances at all events, to be continuous in the sense that impulses 
may pass in all directions along it. But each spinal segment is in addition 
connected by means of tracts of white matter with parts more or less distant. 
The crossed pyramidal tract is such a longitudinal commissural tract, con- 
necting apparently each spinal segment in succession with a certain part of 
the cortex of the cerebrum. We have reason to think, as we shall see later 
on, that impulses descending this or that fibre or group of fibres of this tract 
give rise to the issue of motor impulses along this or that fibre or group of THE STRUCTURE OF THE SPINAL CORD. 
563 
fibres of an anterior root. We do not at present know what is the exaet 
manner by which the fibre in the pyramidal tract is connected with the fibre 
of the anterior root. It seems certain, however, that the connection is not 
in the form of a fibre isolated from the rest of the gray matter, containing, 
so to speak, the pyramidal fibre into a cell of the anterior horn whence the 
fibre of the anterior root issues. Most probably the pyramidal fibre makes 
connections with the segmental groundwork spoken of above, whether with 
or without the intervention of a cell we cannot at present tell. The direct 
pyramidal tract is a like tract of less extent downward, and the less known 
antero-lateral descending tract is probably of a similar nature. 
The cerebellar and antero-lateral ascending tracts are in like manner to 
be regarded as longitudinal commissures between the successive spinal seg- 
ment below and some part of the brain above. We have reason to think 
that these tracts convey upward impulses of a nature which may be called 
afferent, and are, therefore, in some way probably connected with the pos- 
terior roots. We do not know as yet the exact nature of the connection; 
but probably in those cases also the commissural fibres are united not directly 
to the posterior fibres, but indirectly by means of the segmental groundwork. 
And since these tracts do not degenerate after section of the posterior roots, 
but only after section or other lesion of the cord itself, we may infer that 
their junction with the groundwork is effected by means of trophic cells, by 
means of some or other of the cells spoken of a little while before. 
The median posterior tract seems to be a commissural tract of a nature 
different from any of the above. Through it a certain part of each posterior 
root is brought into connection, not with its own spinal segment, but with the 
bulb above, and so with the brain, which thus receives direct representatives 
of each afferent spinal nerve. If, however, as some maintain, the bundle in 
this tract starting from a spinal nerve below diminishes as it proceeds upward, 
throwing off fibres to pass elsewhere, though always carrying some fibres 
right up to the bulb, we must add to the above the further view that this 
tract connects also each posterior root, not with its own segment, but with 
other more or less distant segments. 
§ 494. All the evidence which we possess goes to show that each strand of 
each of these tracts runs isolated, that is to say, makes no connections with 
adjoining structures at any part of its course, from its beginning or end in the 
brain and its end or beginning in its appropriate spinal segment, or in the 
case of the median posterior tract from its beginning in the ganglion of a 
posterior root and its end in the bulb or in some distant spinal segment. In 
the crossed pyramidal tract, for instance, we have reason to think that one 
or more fibres run a quite unbroken and isolated course from the cortex 
of the cerebrum through various parts of the brain, along the whole length 
of the cord until they reach the lowermost spinal segmental mechanism. 
These tracts serve in no way to connect one segmental mechanism with 
another. The segmental mechanisms are, however, connected together; and 
the connections between them seem to be of two kinds. In the first place, 
as we have already suggested, the segmental pieces of gray matter are so 
fused together as to form what appears to be a continuity of gray matter from 
one end of the cord to the other. Though we cannot actually track our 
way histologically through, and are still less aware of the physiological na- 
ture of the labyrinth of nerve-cells, fibres, and fibrils which make up what 
we have called the groundwork, we may with considerable probability assume 
that the passage of nervous impulses along it is determined as much by the 
condition of the material as by its anatomical disposition ; that, for instance, 
the restrictions to the flow of an impulse are brought about much more fre- 
quently by the refusal of the molecules of nervous matter to take up the 564 
THE SPINAL CORL). 
molecular disturbance which is the essence of the impulse; that is to say, by 
molecular resistance than by actual breaks of continuity in the nervous 
matter. Indeed, we have some reasons for thinking that actual structural 
continuity of nervous material is not essential to functional continuity ; that 
a nerve-fibril, for instance, may produce its due effect on another nerve-fibril 
or on a nerve-cell, if sufficiently in contact with it, though the microscope 
fails to demonstrate actual continuity. 
But besides the gray matter there are areas of white matter which do not 
belong either to the nerve-roots as these are making their way into the gray 
matter, or to any of the tracts which we have mentioned. These comprise 
the strands of fibres which do not undergo either ascending or descending 
degeneration when parts of the spinal cord are injured or diseased. The 
area of white matter left when all the various tracts of ascending and de- 
scending degeneration detailed above are taken out, seems at all events in the 
higher parts of the cord (Fig. 127), relatively small, and future observations 
may continue to still further reduce it; but it must be remembered that none 
of the above mentioned tracts are " pure; " they are all more or less mixed 
up, and some largely mixed up, with fibres which do not degenerate. Our 
knowledge is at present too scanty to allow us to make any statement with 
confidence concerning the function either of the fibres forming the white 
matter not yet marked out into tracts, or of the fibres scattered among the 
acknowledged tracts. But we may, at all events provisionally, assume that 
these fibres serve in the main as commissures connecting the successive seg- 
mental mechanisms with each other; we may conclude that changes taking 
place in one segmental mechanism can by means of these fibres produce 
correlated changes in some other distant segmental mechanism, without 
calling into action any of the gray matter of the intervening segmental 
mechanisms. 
The commissures which we may suppose to be thus furnished by white 
matter are longitudinal commissures connecting the segmental mechanisms 
of the same lateral half of the spinal cord with each other. A transverse 
connection between the two lateral halves is afforded in some measure by the 
anterior white commissure. We shall see, however, later on, reasons for 
thinking that many impulses besides those passing along the anterior com- 
missure cross from one side of the cord to the other ; and these, whether they 
pass along distinct fibres or along the general groundwork, must travel by 
the gray matter of the isthmus forming the anterior and posterior gray com- 
missures. 
Thus, as far as we can see at present, the spinal cord consists of a series 
of segmental mechanisms with their respective afferent and efferent roots (the 
gray matter of the several segments being continuous along the cord), of 
encephalic ties of white matter between the several segments and the brain, 
of longitudinal commissural tracts connecting together the several segmental 
mechanisms, and of transverse commissures running largely in the gray 
matter. 
The Reflex Actions of the Spinal Cord. 
§ 495. In the preceding portions of this work we have repeatedly seen 
that though we can learn much concerning the working of an organ or tissue 
or part of the body by studying its behavior when isolated from the rest of 
the body, all the conclusions thus gained have to be checked by a study of 
the behavior of the same organ or part while it is still an integral part of the 
intact body. All the several organs and tissues are so bound together by 
various ties that the actions of each depend on the actions of the rest; and to THE REFLEX ACTIONS OF THE SPINAL CORD. 
565 
say that the life of eaeh part is a function of the life of the whole, is no less 
true than to say that the life of the whole is a function of the life of each 
part. This is especially borne in upon us when we come to study the actions 
of the central nervous system. We may, on anatomical grounds, separate 
the spinal cord from the brain ; but when we come to consider the respective 
functions of the two, we are brought face to face with the fact that in actual 
life a large part of the work of the brain is carried out by means of the 
spinal cord, and conversely the spinal cord does its work habitually under 
the influence of, if not at the direct bidding of, the brain. We may gain 
certain conclusions by studying the behavior of the spinal cord isolated from 
the brain, or of parts of the spinal cord isolated from each other; but we 
must be even more cautious than when we were dealing with other parts of 
the body, and must greatly hesitate to take it for granted that the work 
which we can make the spinal cord or a part of the spinal cord do, when 
isolated from the brain, is the work which is actually done in the intact body 
when the brain and spinal cord form an unbroken whole. Moreover, this 
caution becomes increasingly necessary when in our studies we pass from the 
simpler nervous system of one animal to the more complex nervous system 
of another; for it is by the complexity of their central nervous systems, 
more than by anything else, that the "highest" animals are differentiated 
from those " below'" them. When we compare a rabbit, a dog, a monkey, 
and a man, the differences in the vascular, digestive, and respiratory systems 
of the four, striking as they may appear, sink into insignificance compared 
with the differences exhibited by their respective central nervous systems. 
We need caution when from the results of experiments on dogs or rabbits 
we draw conclusions as to the digestion or circulation of man, but we need 
far greater caution when from the behavior of the isolated spinal cord of 
one of these animals we infer the behavior of the intact spinal cord of man. 
A further difficulty meets us when an experimental investigation entails 
operative interference with the central nervous system. Removal or section 
of, or other injury to parts of the brain or spinal cord is very apt to give rise 
in varying degree to what is known as " shock." The cutting or tearing or 
other lesion of any considerable mass of nervous substance affects the activity, 
not only of the structures immediately injured, but of other, it may be far 
distant structures. The nature of " shock " is not as yet thoroughly under- 
stood, but may perhaps, in part at all events, be explained by regarding the 
lesion as a very powerful stimulus, which, partly by way of inhibition but 
still more by way of exhaustion, depresses or suspends for a while normal 
functions, and thus gives rise to temporary diminution or loss of conscious- 
ness, of volition, of reflex movements, and other nervous actions. Thus a 
section through the spinal cord, even when made with the sharpest instru- 
ment and with the utmost skill, so as to avoid all bruising as much as possi- 
ble, may for a while suspend all reflex activity of the cord, or indeed all the 
obvious activities of the whole central nervous system. We may add that 
such a "shock" of the central nervous system may also be produced by 
sudden lesions not bearing directly on the central nervous system, as, for 
instance, by extensive injury to a limb. 
Moreover, in many cases in which the effects of experimental interference 
have been watched for some considerable time, days, months, or years after 
the operation, it has been observed, on the one hand, that phenomena which 
are conspicuous in the early period may eventually disappear, and, on the 
other hand, that activities which are at first absent may later on make their 
appearance ; movements, for instance, which are at first frequent after a while 
die away, and conversely, movements which at first seemed impossible are 
later on easily achieved. We have to distinguish or to attempt to distin- 566 
THE SPINAL CORD. 
guish between the temporary and the lasting effects of the operation, includ- 
ing among the former not only those of ordinary "shock," but others of 
slower development or longer duration. In many instances where a part of 
the central nervous system is by section or otherwise suddenly separated 
from the rest, the phenomena suggest that the separated part is at first pro- 
foundly influenced as to its activities by the withdrawal of various influences 
which previously were being exerted upon it by the rest of the system, but 
later on accommodates itself to the new conditions, and learns, so to speak, 
to act without the help of those influences. And indeed it is possible that 
some of the effects of even immediate "shock " may be due, not as suggested 
above, to the action of an inhibitory or exhausting stimulus, but to the 
sudden cessation of habitual influences. 
Still, in spite of all these difficulties, it is possible not only to ascertain 
the working of an isolated portion of the central nervous system, but even to 
infer from the results some conclusions as to the share taken by that portion 
in the working of the entire and intact system. There can be no doubt, for 
instance, that the spinal cord can, quite apart from the brain, carry out 
various reflex actions, and, moreover, it does carry out actions of this kind 
when in the intact organism it is working in contact with the brain. Indeed, 
the carrying out of various reflex actions seems to be one of the most impor- 
tant functions of the spinal cord, so much so that though the brain or, at 
least, parts of the brain can also and do develop reflex actions, the spinal 
cord offers the best field for the study of these actions. We have already 
(§ 97) touched on the general features of reflex actions, and elsewhere have 
incidentally dwelt on particular instances; we may therefore confine our- 
selves now to certain points of special interest. 
§ 496. Reflex movements are perhaps best studied in the frog and other 
cold-blooded animals, since in these the actions of the cord are less dependent 
on, and hence less obscured by the working of, the other parts of the central 
nervous system. They obtain, however, in the warm-blooded mammal also, but 
in these special preparations are necessary to secure their full development. 
In the frog the shock, which, as we have said, follows upon division of the 
spinal cord and for awhile suspends reflex activity,soon passes away; within 
a very short time after the bulb, for instance, has been divided the most com- 
plicated reflex movements can be carried on by the frog's spinal cord when 
the appropriate stimuli are applied. With the mammal the case is very 
different. For days even after division of the spinal cord the parts of the 
body supplied by nerves springing from the cord below the section may ex- 
hibit very feeble reactions only. In the dog, for instance, after division of 
the spinal cord in the lower dorsal region, the hind limbs hang flaccid and 
motionless, and pinching the hind foot evokes as a response either slight 
irregular movements or none at all. Indeed, were our observations limited 
to this period we might infer that the reflex actions of the spinal cord in the 
mammal were but feeble and insignificant. If, however, the animal be kept 
alive for a longer period, for weeks, or better still for months, though no 
union or regeneration of the spinal cord takes place, reflex movements of a 
powerful, varied, and complex character manifest themselves in the hind 
limbs and hinder parts of the body; a very feeble stimulus applied to the 
skin of these regions promptly gives rise to extensive and yet coordinate 
movements. Indeed, the more the matter is studied, the stronger is the evi- 
dence that the reflex movements carried out by isolated portions of the 
spinal cord of the mammal are hardly less definite, complete, and purpose- 
ful than those witnessed in the frog. It is worthy of attention, as bearing 
out the remarks made above on the great differentiation of the central ner- 
vous system in the higher animals, that the reflex phenomena in mammals THE KEFLEX ACTIONS OF THE SPINAL CORD. 
567 
vary very much not only in different species but also in different individuals 
and in the same individual under different circumstances. Race, age, and 
previous training seem to have a marked effect in determining the extent 
and character of the reflex actions which the spinal cord is capable of 
carrying out; and these seem also to be largely influenced by passing cir- 
cumstances, such as whether food has been recently taken or not. It has 
been asserted that the isolated spinal cord of the rabbit, which has been 
the subject of so many experiments, is, as compared with that of the dog 
and many other mammals, singularly deficient in the power of carrying out 
complex reflex movements. 
In studying reflex actions in man we are met with the difficulty that we 
never have to deal with a portion of the spinal cord separated from the rest 
of the central nervous system under the favorable circumstances of experi- 
mental investigation. In man, we must be content to examine reflex actions 
either while the whole nervous system is intact, or when a portion of the cord 
has been wholly or partially separated by some more or less diffuse disease 
or by some accident involving more or less crushing of the nervous struc- 
tures. Hence, the caution already given, as to drawing inferences concern- 
ing man from the results of experiments on animals, acquires still greater 
force. 
§ 497. Confining ourselves at first to the results of experiments on 
animals we may say that in both cold-blooded and warm-blooded animals 
the salient feature of ordinary reflex actions is their purposeful character, 
though every variety of movement may be witnessed, from a simple spasm 
to a most complex manoeuvre. And in all reflex movements, both simple 
and complex, we can recognize certain determining influences which more 
or less directly contribute to the shaping of this purposeful character. 
Thus the features of any movement taking place as part of a reflex action 
are in part determined by the characters of the afferent impulses. Simple 
nervous impulses generated by the direct stimulation of afferent nerve-fibres 
generally evoke as reflex movements merely irregular spasms in a few 
muscles; whereas the more complicated differentiated sensory impulses gen- 
erated by the application of the stimulus to the skin, readily give rise to 
large and purposeful movements. It is easier to produce a complex reflex 
action by a slight pressure on or other stimulation of the skin than by even 
strong induction-shocks applied directly to a nerve-trunk. If in a brainless 
frog, the area of skin supplied by one of the dorsal cutaneous nerves be sepa- 
rated by section from the rest of the skin of the back, the nerve being left 
attached to the piece of skin and carefully protected from injury, it will be 
found that slight stimuli applied to the surface of the piece of skin easily 
evoke reflex actions, whereas the trunk of the nerve may be stimulated with 
even strong currents without producing anything more than irregular move- 
ments. In ordinary mechanical and chemical stimulation of the skin it is 
not a single impulse but a series of impulses which passes upward along the 
sensory nerve, the changes in which may be compared to the changes in a 
motor nerve during tetanus. In every reflex action, in fact, the central 
mechanism may be looked upon as being thrown into activity through a 
summation of the afferent impulses reaching it. Hence while a reflex action 
is readily called forth by even feeble induction-shocks applied to the skin 
if they be repeated sufficiently rapidly, a solitary induction-shock is inef- 
fectual unless it be strong enough to cause in the skin or nerves changes 
of an electrolytic nature sufficient to give rise of themselves to a series of 
impulses. 
§ 498. When a muscle is thrown into contraction in a reflex action, the 
pitch of the sound which it gives forth does not vary with the stimulus, but 568 
THE SPINAL CORD. 
is constant, being the same as that given forth by a muscle thrown into con- 
traction by the will. From which we infer, even bearing in mind the dis- 
cussion in § 78 concerning the nature of the muscular sound, that in a re- 
flex action the afferent impulses do not simply pass through the centre in 
the same way that they pass along afferent nerves, but are profoundly modi- 
fied. And in accordance with this we find, as we shall see, that a reflex 
action takes up an amount of time, the greater part of which is spent in the 
carrying out of the central changes, and which though variable is always 
much longer, and may be very much longer, than that taken up by the mere 
passage of a nervous impulse along a corresponding length of nerve-fibre. 
The term reflex action is therefore an unsuitable one. The afferent impulse 
is not simply reflected or turned aside into an efferent channel; on its arrival 
at the centre, it starts changes of a different nature from and more complex 
than its own ; and the issue of efferent impulse is the result of those more 
complex changes, not the mere continuation of the simpler afferent impulse. 
In other words, the interval between the advent at the central organ of 
afferent, and the exit from it of efferent impulses, is a busy time for the ner- 
vous substance of that organ ; during it many processes, of which we have 
at present very little exact knowledge, are being carried on. 
§ 499. The character of the movement forming part of a reflex action 
is also influenced by the intensity of the stimulus. A slight stimulus, such 
as gentle contact of the skin with some body, will produce one kind of 
movement; and a strong stimulus, such as a sharp prick applied to the same 
spot of skin, will call forth quite a different movement. When a decapi- 
tated snake or newt is suspended and the skin of the tail slightly touched 
with the finger, the tail bends toward the finger ; when the skin is pricked 
or burnt, the tail is turned away from the offending object. And so in 
many other instances. It must be remembered, of course, that a difference 
in the intensity of the stimulus entails a difference in the characters of the 
efferent impulses; gentle contact gives rise to what.we call a sensation of 
touch, while a sharp prick gives rise to pain, consciousness being differently 
affected in the two cases because the afferent impulses are different. Hence 
the instances in question are in reality fuller illustrations of the dependence, 
to which we called attention above, of the characters of a reflex movement 
on the characters of the afferent impulses. 
Further, as we have already pointed out (§ 97), while the motor impulses 
started by a weak stimulus applied to an afferent nerve are transmitted along 
a few, those started by a strong stimulus may spread to many efferent nerves. 
Granting that any particular afferent nerve is more especially associated with 
certain efferent nerves than with any others, so that the reflex impulses gen- 
erated by afferent impulses entering the cord by the former pass with the 
least resistance down the latter, we must evidently admit further that other 
efferent nerves are also, though less directly, connected with the same affer- 
ent nerve, the passage into the second efferent nerve meeting with a greater 
but not an insuperable resistance. When a frog is poisoned with strychnine, 
a slight touch on any part of the skin may cause convulsions of the whole 
body ; that is to say, the afferent impulses passing along any single afferent 
nerve may give rise to the discharge of efferent impulses along any or all 
of the efferent nerves. This proves that a physiological, if not an ana- 
tomical, continuity obtains between all parts of the spinal cord which are 
concerned in reflex action, that the nervous network intervening between 
the afferent and efferent fibres forms along the whole length of the cord a 
functionally continuous field. This continuous network, however, we must 
suppose to be marked out into tracts presenting greater or less resistance to 
the progress of the impulses into which afferent impulses, coming along this TIIE KEFLEX ACTIONS OF THE SPINAL CORD. 
569 
or that afferent nerve, are transformed on their advent at the network ; and 
accordingly the path of any series of impulses in the network will be deter- 
mined largely by the energy of the afferent impulses. And the action of 
strychnine may be, in part, explained by supposing that it reduces and 
equalizes the normal resistance of this network, so that even weak impulses 
travel over all its tracts with great ease. 
§ 500. Further, the movement, forming part of a reflex action, varies in 
character according to the particular part of the body to which the stimulus 
is applied. The reflex actions developed by stimulation of the internal vis- 
cera are different from those excited by stimulation of the skin. We have 
reason to think that the contraction of, or other changes in a skeletal muscle 
may produce, by reflex action, contractions of other muscles ; and such 
reflex actions also differ from those started by stimulation of the skin. In 
reflex actions started by applying a stimulus to the skin the movement's vary 
largely, according to the particular area of the skin which is affected. Thus, 
pinching the folds of skin surrounding the anus of the frog produces differ- 
ent effects from those witnessed when the flank or toe is pinched ; and, 
speaking generally, the stimulation of a particular spot calls forth partic- 
ular movements. In the case of the simple reflex movements, it appears to 
be a general rule that a movement started by the stimulation of a sensory 
surface or region on one side of the body is developed on the same side of 
the body, and if it spreads to the other side, still remains most intense on the 
same side; the movement on the other side, moreover, is symmetrical with 
that on the same side. It has been maintained that " crossed " or diagonal 
reflex movements, as where stimulation of one fore-foot leads to movements 
of the opposite hind limb, do not occur unless some portion of the bulb be 
left attached to the spinal cord. Seeing that locomotion of four-footed 
animals is largely effected by diagonal movements of the limbs, one would 
rather have expected to find the spinal cord itself provided with mechan- 
isms to assist in carrying them out; and, indeed, it is affirmed that in the 
case of cold-blooded animals and of many young mammals, after a division 
of the spinal cord below the bulb, a gentle stimulation will provoke a diagonal 
movement, slight pressure on one fore foot, for example, giving rise to move- 
ments in the opposite hind leg ; a strong stimulus, however, will produce an 
ordinary one-sided movement. Again, when in a dog the cord has been 
divided in the lower thoracic region so that the hind limbs depend on the 
lumbar cord alone, a rhythmically repeated drawing up and letting down 
of the hind limbs is witnessed when these are allowed to hang down ; and 
these movements, which appear to be of a reflex nature excited by the 
pendent position of the limbs, are often seen to alternate regularly in the 
two limbs, the right leg being extended while the left leg is being drawn up 
and vice versa. It may further be observed that if the foot of one pendent 
limb be pinched while the other limb is passively flexed the flexion of the 
limb which is pinched is accompanied by an extension of the other limb. 
In these respects, however, different animals, as already urged, differ from 
each other. 
§ 501. From these and similar phenomena we may infer that the nervous 
network spoken of above is, so to speak, mapped out into nervous mechan- 
isms by the establishment of lines of greater or less resistance, so that the 
disturbances in it generated by certain afferent impulses are directed into 
certain efferent channels. It may be added that though conspicuously pur- 
poseful movements seem to need the concurrent action of several segments of 
the cord, and as a rule, the greater the length of the cord involved the more 
complex and the more distinctly purposeful the movement, still the move- 
ments evoked by even a segment of the cord may be purposeful in character ; 570 
THE SPINAL CORD. 
hence we must conclude that every segment of the nervous network is 
mapped out into mechanisms. But the arrangement of these mechanisms, 
especially of the more complex ones, is not a fixed and rigid one. We can- 
not always predict exactly the nature of the movement which will result 
from the stimulation of any particular spot, because the result will vary 
according to the condition of the spinal cord, especially in relation to the 
strength and character of the stimulus. Moreover, under a change of cir- 
cumstances a movement quite different from the normal one may make its 
appearance. Thus when a drop of acid is placed on the right flank of a 
brainless frog, the right foot is almost invariably used to rub off the acid ; 
in this there appears nothing more than a mere " mechanical" reflex action. 
If, however, the right leg be cut off, or the right foot be otherwise hindered 
from rubbing off the acid, the left foot is, under the exceptional circum- 
stances, used for the purpose. This at first sight looks like an intelligent 
choice. A choice it evidently is ; and were there many instances of choice, 
and were there any evidence of a variable automatism, like that which we 
call " volition," being manifested by the spinal cord of the frog, we should 
be justified in supposing that the choice was determined by an intelligence. 
But, as we shall have occasion later on to point out, a frog, deprived of its 
brain so that the spinal cord only is left, makes no spontaneous movements 
at all. Such an entire absence of spontaneity is wholly inconsistent with the 
possession of intelligence. Then again the above experiment, if not the only 
instance, is, at all events, by far the most striking instance of choice on the 
part of a brainless frog. We are, therefore, led to conclude that the phe- 
nomena must be explained in some other way than by being referred to the 
working of an intelligence. Moreover, this conclusion is supported by the 
behavior of other animals. Thus similar vicarious reflex movements may 
be witnessed in mammals, though not perhaps to such a striking extent as 
in frogs. In dogs, in which partial removal of the cerebral hemispheres has 
apparently heightened the reflex excitability of the spinal cord, the remark- 
able scratching movements of the hind leg which are called forth by stimu- 
lating a particular spot on the loins or side of the body, are executed by 
the leg of the opposite side, if the leg of the same side be gently held. 
In this case the vicarious movements are effectual, the leg not being, as 
in the case of the frog, crossed over so as to bear on the spot stimu- 
lated, and cannot be considered as betokening intelligence. Again, the 
" mechanical " nature of reflex actions is well illustrated by the behavior 
of a decapitated snake. When the body of the animal in this condition 
is brought into contact at several places at once with an arm or a stick, 
complex reflex movements are excited, the obvious purpose as well as 
effect of which is to twine the body round the object. A decapitated 
snake will, however, with equal and fatal readiness twine itself round a 
red-hot bar of iron, which is made to touch its skin in several places at 
the same time. 
§ 502. In considering the nature of the events in the spinal cord which 
determine the behavior of the frog in the instance just mentioned we must 
bear in mind that the movements in question are " coordinated ; " that is to 
say, not only are many distinct muscles brought into play, but certain rela- 
tions are maintained between the amount, duration, and exact time of occur- 
rence of the contraction of each muscle and those of the contractions of its 
fellow muscles sharing in the movement. In the absence of such coordina- 
tion the movement would become irregular and ineffectual. We shall have 
occasion later on in dealing with voluntary movements to point out that 
the coordination and hence the due accomplishment of a voluntary move- 
ment is dependent on certain afferent impulses passing up from the con- THE REFLEX ACTIONS OF THE SPINAL CORD. 
571 
trading muscles to the central nervous system, and guiding the discharge 
of the efferent impulses which call forth the contractions. When these 
afferent impulses affect consciousness we speak of them as constituting a 
" muscular sense ; " it is, as we shall see, by the " muscular sense" that we 
become aware of and can appreciate the condition of our muscles. But we 
have reason to think that the afferent impulses which constitute the basis of 
the muscular sense, whatever be their exact nature, in order to play their 
part in bringing about the coordination of a voluntary movement need not 
pass right up to the brain and develop a distinct muscular " sense," but may 
produce their effect by working on the nervous mechanisms of the spinal 
cord with which the motor fibres carrying out the movement are connected. 
In other words, the coordination of a voluntary movement takes place in 
the part of the spinal cord which carries out the movement, and not in the 
brain, though the latter may be conscious of the whole movement including 
its coordination. 
But if the spinal cord possesses mechanisms for carrying out coordinated 
movements, which in the case of voluntary movements are discharged by 
nervous impulses descejiding from the brain, we may infer that in reflex 
actions the same mechanisms are brought into action though they are dis- 
charged by afferent impulses coming along afferent nerves instead of by 
impulses descending from the brain. The movements of reflex origin, in all 
their features except their exciting cause, appear identical with voluntary 
movements ; the two can only be distinguished from each other by a know- 
ledge of the exciting cause. And it seems unreasonable to suppose that the 
spinal cord should possess two sets of mechanisms in all respects identical, 
save that the one is discharged by volitional impulses from the brain and the 
other by afferent impulses from afferent nerves. 
We are led therefore to the conclusion that in a reflex action two kinds 
of afferent impulses are concerned : the ordinary afferent impulses which dis- 
charge the nervous mechanism within the cord and so provoke the move- 
ment, and the afferent impulses which connect that nervous mechanism with 
the muscles about to be called into play, and which take part in the coordi- 
nation of the movement provoked. The nature of these latter afferent im- 
pulses is at present obscure ; but if we admit, as we seem compelled to do, that 
the character of a reflex action is determined by them as well as by the affer- 
ent impulses which actually discharge the mechanism, it seems possible that 
a fuller knowledge of these coordinating afferent impulses may afford an 
adequate explanation of the fact that when, as in the case of the frog in 
question, the usual set of muscles cannot be employed by the nervous 
mechanism, recourse is had to another. 
We have avoided the introduction of the word " consciousness " as un- 
necessarily complicating the question ; and it would be out of place to discuss 
psychological problems here. We may remark, however, that since we have 
no objective proofs of consciousness outside ourselves, and only infer by 
analogy that such and such an act is an outcome of consciousness on account 
of its likeness to acts which are the outcome of our own consciousness, we 
conclude that the brainless frog possesses no active consciousness like our 
own, because absence of spontaneous movements seems to be irreconcilable 
with the existence of an active consciousness whose very essence is a series 
of changes. Consciousness, as we recognize it, seems to be necessarily ope- 
rating as, or to be indissolubly associated with the presence of, an incessantly 
repeated internal stimulus; and we cannot conceive of that stimulus failing 
to excite mechanisms of movement which, as in the case of the brainless 
frog, are confessedly present. We may, however, distinguish between an 
active continuous consciousness, such as we usually understand by the term, 572 
THE SPINAL CORD. 
and a passing and momentary condition, which we may speak of as con- 
sciousness, but which is wholly discontinuous from an antecedent or from a 
subsequent similar momentary condition ; and indeed we may suppose that 
the complete consciousness of ourselves, and the similarly complete con- 
sciousness which we infer to exist in many animals, has been gradually 
evolved out of such a rudimentary consciousness. We may, on this view, 
suppose that every nervous action of a certain intensity or character is ac- 
companied by some amount of consciousness, which we may, in a way, com- 
pare to the light emitted when a combustion, previously giving rise to 
invisible heat, waxes fiercer. We may thus infer that when the brainless 
frog is stirred by some stimulus to a reflex act, the spinal cord is lit up by a 
momentary flash of consciousness coming out of darkness and dying away 
into darkness again ; and we may perhaps further infer that such a passing 
consciousness is the better developed the larger the portion of the cord in- 
volved in the reflex act and the more complex the movement. But such a 
momentary flash, even if we admit its existence, is something very different 
from consciousness as ordinarily understood, is far removed from intelli- 
gence, and cannot be appealed to as explaining the " choice " spoken of 
above. 
$ 503. Lastly, the characters of a reflex movement are, as we need 
hardly say, dependent on the intrinsic condition of the cord. The action 
of strychnine just alluded to is an instance of an apparent augmentation 
of reflex action best explained by supposing that the resistances in the cord 
are lessened. There are probably, however, cases in which the explosive 
energy of the nervous substance is positively increased above the normal. 
Conversely, by various influences of a depressing character, as by various 
anaesthetics or other poisons, reflex action may be lessened or prevented ; 
and this again may arise either from an increase of resistance or from a 
diminution in the actual discharge of energy. So also various diseases may 
so affect the spinal cord as to produce, on the one hand, increased reflex ex- 
citability, so that a mere touch may produce a violent movement, and, on the 
other hand, diminished reflex excitability, so that it becomes difficult or im- 
possible to call forth reflex action. 
§ 504. When we come to study the reflex actions of man we should at 
first perhaps be inclined to infer that, since in him the spinal cor'd is so 
largely used as the instrument of the brain, the independent reflex actions of 
the cord, at least such as affect skeletal muscles, are in him of much less im- 
portance than they appear to be in animals, and experience seems to sup- 
port this view. But it must be remembered that in his case, as we have 
already stated (§ 496), we lack the guidance of experimental results; we 
are obliged to trust to the entangled phenomena of disease or to a study of 
the behavior of the cord while it is still a part of an intact nervous system ; 
and each of these methods presents difficulties of its own. The movements, 
which in the intact human body we can recognize as indubitable reflex 
actions, are, as a rule, simple and unimportant. They are, in by far the 
greater number of instances, occasioned by stimulation of the skin or of the 
mucous membrane, for the most part involve a few muscles only, and 
rarely indicate any very complex coordination. The flexion, followed by 
extension, of the leg, which is called forth by tickling the sole of the foot, 
or the winking of the eye when the cornea or conjunctiva is touched, may 
perhaps be regarded as the type of these movements. A very common 
form of reflex action is that in which a muscle or group of muscles is 
thrown into contraction by stimulation of the overlying or neighboring 
skin, as when the abdominal muscles contract upon stroking the skin of 
the abdomen or the testicle is retracted upon stroking the inside of the THE KEFLEX ACTIONS OF THE SPINAL CORD. 
573 
thigh. A reflex movement may occur as the result of stimulation of an 
organ of special sense, parts of the central nervous system other than the 
spinal cord serving as the centre. A sound or a flash of light readily pro- 
duces a start, a bright light makes the eye wink and may cause the person 
to sneeze (the greater coordination manifest in this act being due to the 
fact that the complex respiratory mechanism is brought into play (§ 334), 
and reflex movement may result from a taste or smell. A special form of 
reflex action, or at least an action resembling a reflex action, is called forth 
by sharply striking certain tendons; for instance, striking the tendon below 
the patella gives rise to a sudden extension of the leg, known as the "knee- 
jerk"; but it will be best to discuss these " tendon reflexes," or " muscle 
reflexes," as they are called, later on in another connection. 
On the whole the reflex movements carried out by the intact nervous 
system of man are, we repeat, scanty and comparatively simple; but we 
are not justified in inferring from this that the human spinal cord, left to 
itself, is incapable of doing more; that owing to the predominant activity 
of the brain it has lost the powers possessed by the spinal cord in the lower 
animals. For it may be that the cord, when joined to the brain, is through 
various influences proceeding from the latter in a different condition from 
that in which it is when separated from the brain ; indeed, we have reason 
to think that this is so; and we may here remark that in the lower animals, 
as in man, the development of reflex movements is difficult and uncertain 
in the presence of the brain. 
When we turn to the teaching of disease, however, we again find that 
reflex movements carried out by the cord or by parts of the cord are, on 
the whole, scanty and simple. 
In some stages of certain diseases of the spinal cord extensive reflex 
movements are witnessed; but these are not purposeful, coordinated move- 
ments, such as have been described above as occurring in frogs and mam- 
mals after experimental interference, but rather mere exaggerations of the 
simpler reflex movements witnessed when the nervous system is intact. In 
cases of paraplegia (such being the term generally used when disease or 
injury has cut off the cord, generally the lower part of the cord, from the 
brain, so that the will cannot bring about movements in, and the mind 
derives no sensation from, the parts below the lesion, the legs for instance), 
it sometimes happens that contact with the bedclothes or other external 
objects sets up from time to time rhythmically repeated movements, the 
legs being alternately drawn up and thrust out again. And an exaggera- 
tion of the " knee-jerk " or other " tendon reflexes " is a very common symp- 
tom in certain spinal diseases. It is rarely, if ever, that reflex movements 
of a really complicated character are observed. Moreover, clinical experi- 
ence shows that in man, when a portion of the cord is isolated, reflex 
actions carried out by means of that portion, so far from being exagger- 
ated, are much more commonly exceedingly feeble or absent altogether. In 
the cases in which the physiological continuity of the lower with the upper 
part of the cord has been broken by disease, by some growth invading the 
nervous structures, or by some changes of the nervous structures themselves, 
we may attempt to explain the absence from the lower part or coordinate 
reflex activity, such as is seen in the lower animal, as due to the disease not 
only affecting the powers of the actually diseased part, but influencing the 
whole cord below, and either by inhibition, of which we shall speak pres- 
ently, or in some other way depressing its functions. But the same absence 
of complex reflex movements is also often observed in cases in which the 
cord has been severed by accident, and, indeed, though accidental injuries to 
the human cord generally produce more profound and extensive mischief 574 
THE SPINAL CORD. 
than that which results in animals from experimental interference, clinical 
experience tends, on the whole, to support the view that in man the more 
complete subordination of the spinal cord to the brain has led to the dying- 
out of the complex reflex actions which are so conspicuous in the lower 
animals. This, however, cannot be regarded as distinctly proved. 
When we come to study voluntary movements, we shall see reason to 
think that in man, as in the lower animals, the will in carrying out these 
movements makes use of complex nervous mechanisms situated in the 
spinal cord-nervous mechanisms into the working of which, as urged 
above, afferent impulses enter largely ; and it seems improbable that these 
spinal mechanisms should be capable of being thrown into action by the 
will only. In the act of walking, for instance, it is highly probable that 
the movements of the legs are the direct results of the action of nervous 
mechanisms in the lumbar cord brought into play by the will, being thus, 
in an indirect manner only, the products of volitional impulses; and even 
in man, though clinical experience only affords us instances of this machin- 
ery working apart from the brain in a damaged condition and under unfa- 
vorable circumstances, so that the resemblance of the movements observed 
to the complete act of walking is but feeble, still it seems similarly probable 
that under more favorable circumstances the lumbar cord separated from 
the brain might as part of a reflex act carry out the movements in a more 
complete and coordinate manner. 
§ 505. We have dwelt above chiefly on reflex actions, in which the 
efferent impulses cause contractions of skeletal muscles, since these are 
undoubtedly the most common and the most prominent forms of reflex 
action ; but it must not be forgotten that the efferent impulses of reflex 
origin may produce contractions of other muscles, as well as other effects, 
such as secretion, for instance. On several of these we have dwelt, from 
time to time in previous parts of this work, and it will be unnecessary to 
repeat them here. But it may be worth while to point out that the spinal 
cord, by serving as a reflex centre for innumerable ties which correlate the 
nutritive or metabolic activities of the several tissues to events taking place 
in other parts of the body, plays a conspicuous part in securing the welfare 
of the whole body. In dealing (§462) with the general problems of nutri- 
tion, we stated that an orderly nutrition appears to be in some way depend- 
ent on nervous influences. Many of these nervous influences appear to 
issue from the spinal cord, either as parts of a reflex act or as the outcome 
of some automatic processes. When in a dog the lumbar cord is wholly 
separated from the rest of the cord by section, the nutrition of the hind 
limbs and the general health of the animal may, with care, be maintained 
in a very satisfactory condition ; but if that small separated piece of the 
cord be destroyed, death inevitably ensues before long, in spite of every care 
and precaution, being brought about apparently by the disordered nutrition 
of the hind limbs and other parts supplied by nerves coming from the lum- 
bar cord. In man extensive injuries to the spinal cord are followed by bed- 
sores and other results of impaired nutrition ; and, indeed, death is gen- 
erally brought about in this way in cases of paraplegia caused by accidental 
crushing or severance of the cord. 
§ 506. Inhibition of reflex action. The reflex actions of the spinal cord, 
like other nervous actions, may be totally or partially inhibited, that is to 
say, may be arrested or hindered in their development by impulses reaching 
the centre while it is already in action. Thus, if the body of a decapitated 
snake be allowed to hang down, slow rhythmic pendulous movements, which 
appear to be reflex in nature, soon make their appearance; all these may 
be for a while arrested by slight stimulation, as by gently stroking the tail. THE REFLEX ACTIONS OF THE SPINAL CORD. 
575 
We have already seen that the action of such nervous centres as the respi- 
ratory and vasomotor centres, which frequently, at all events, is of a reflex 
nature, may be either inhibited or augmented by afferent impulses. The 
micturition centre in the mammal, which is also largely a reflex centre, may 
be easily inhibited by impulses passing downward to the lumbar cord from 
the brain, or upward along the sciatic nerves. In the case of dogs, whose 
spinal cord has been divided in the thoracic region, micturition set up as a 
reflex act by simple pressure on the abdomen or by sponging the anus is at 
once stopped by sharply pinching the skin of the leg. And it is a matter 
of common experience that in man micturition maybe suddenly checked by 
an emotion or other cerebral event. The erection centre in the lumbar 
cord, also in large measure a reflex centre, is similarly susceptible of being 
inhibited by impulses reaching it from various sources. And, indeed, 
many similar instances of the inhibition of reflex movements might readily 
be quoted. 
Several apparent instances of the inhibition of reflex acts are not really 
such ; in these cases all the nervous processes of the act may take place in 
their entirety and yet fail to produce their effect on account of a failure in 
the muscular part of the act. Thus, when we ourselves by an effort of the 
will stop the reflex movements which otherwise would be produced by 
tickling the soles of the feet, we achieve this to a large extent by throwing 
voluntarily into action certain muscles, the contractions of which antagonize 
the action of the muscles engaged in carrying out the reflex movements. 
But it may be doubted, even in these cases, whether inhibition is always or 
wholly to be explained in this way; and certainly in very many instances 
of reflex inhibition no such muscular antagonism is present, and the reflex 
act is checked at its nervous centre. 
When the brain of a frog is removed, and the effects of shock have 
passed away, reflex actions are developed much more readily and to a much 
greater degree than in the entire animal, and in mammals also reflex excita- 
bility has been observed to be increased by removal of the cerebral hemi- 
spheres. This suggests the idea that in the intact nervous system the brain 
is habitually exerting some influence on the spinal cord, tending to prevent 
the normal development of the spinal reflex actions. And we learn by ex- 
periment that stimulation of certain parts of the brain has a remarkable 
effect on reflex action. If a frog, from which the cerebral hemispheres have 
been removed (the optic lobes, bulb, and spinal cord being left intact), be 
suspended by the jaw, and the toes of the pendent leg be from time to time 
dipped into very dilute sulphuric acid, a certain average time will be found 
to elapse between the dipping of the toe and the resulting withdrawal of the 
foot. If, however, the optic lobes or optic thalami be stimulated, as by 
putting a crystal of sodium chloride on them, it will be found on repeating 
the experiment, while these structures are still under the influence of the 
stimulation, that the time intervening between the action of the acid on the 
toe and the withdrawal of the foot is very much prolonged. That is to say, 
the stimulation of the optic lobes has caused impulses to descend to the cord, 
which have there so interfered with the nervous processes engaged in carry- 
ing out reflex actions as greatly to retard the generation of efferent impulses, 
or, in other words, has inhibited the reflex action of the cord. And similar 
results may be obtained in mammals by stimulating certain parts of the 
corpora quadrigemina, which bodies are homologous to the optic lobes of 
frogs. From this it has been inferred that there is present in this part of 
the brain a special mechanism for inhibiting the reflex actions of the spinal 
cord, the impulses descending from this mechanism to the various centres of 
reflex action being of a specific inhibitory nature. But, as we have already 576 
THE SPINAL CORD. 
seen, impulses of an ordinary kind, passing along ordinary sensory nerves, 
may inhibit reflex action. We have quoted instances where a slight stim- 
ulus, as in the pendulous movements of the snake, and where a stronger 
stimulus, as in the case of the micturition of the dog, may produce an in- 
hibitory result; we may add that in the frog adequately strong stimuli 
applied to an afferent nerve will inhibit, i. e., will retard or even wholly 
prevent, reflex action. If the toes of one foot are dipped into dilute sul- 
phuric acid at a time when the sciatic of the other leg is being powerfully 
stimulated with an interrupted current, the period of incubation of the reflex 
act will be found to be much prolonged, and in some cases the reflex with- 
drawal of the foot will not take place at all. And this holds good, not only 
in the complete absence of the optic lobes and bulb, but also when only a 
portion of the spinal cord, sufficient to carry out the reflex action in the 
usual way, is left. There can be no question here of any specific inhibitory 
centres, such as have been supposed to exist in the optic lobes. But if it is 
clear that inhibition of reflex action may be brought about by impulses 
which are not in themselves of a specific inhibitory nature, we may hesitate 
to accept the view that a special inhibitory mechanism in the sense of one 
giving rise to nothing but inhibitory impulses is present in the optic lobes 
of frogs, and after removal of the brain that the exaltation of reflex ac- 
tions which is manifest is due to the withdrawal of such a specific inhibitory 
mechanism. 
The presence of the brain does obviously produce an effect which may 
be broadly spoken of as inhibitory, and a specific action of the brain, in an 
effort of the will, may stop or inhibit a specific reflex action ; but we must 
not in these matters be led too much away by the analogy of the special 
and limited cardiac inhibitory mechanism. There we have apparently to 
deal with fibres, whose exclusive duty it is to convey inhibitory impulses 
from the bulb to the cardiac muscle, and inhibition of the heart, at least 
through nervous influences, is exclusively carried out by them. But already, 
in studying the nervous mechanism of respiration, we have seen reason to 
think that afferent impulses passing along the same nerves and probably 
along the same fibres may, according to circumstances, now inhibit, now 
augment the respiratory centre, and have thus been led to speak of in- 
hibitory impulses, that is, impulses producing an inhibitory effect, apart 
from specific inhibitory fibres. In the complex working of the central 
nervous system we may still more expect to come across similar instances of 
the same channels serving as the path either of inhibition or of augmenta- 
tion. In all probability actions or processes which we may speak of as 
inhibitory do play, as indeed we shall see, an important part in the whole 
work of the central nervous system ; in all probability many of the phe- 
nomena of nervous life are the outcome of a contest between what we may 
call inhibitory and exciting or augmenting forces; but in all probability, 
also, we ought rather to seek for the explanation of how vagus impulses 
inhibit the beat of the heart by reference to the inhibitory phenomena of 
the central nervous system, than to attempt to explain the latter by the 
little we know of the former. At present, however, we must be content with 
the fact that experiments on animals show that the brain not only by some 
action or other may inhibit particular spinal reflex movements, but also 
habitually exercises a restraining influence on the reflex activity of the 
whole cord, though we are unable to state clearly how this inhibition is 
carried out. 
We say " experiments on animals," because though we know, as stated 
above, by an appeal to our own consciousness, that an action of the brain, 
an effort of the will, may stop a particular reflex act, we have no evidence THE REFLEX ACTIONS OF THE SPINAL CORD. 
577 
that in man separation of the cord from the brain leads, as in animals, to 
heightened reflex activity. In diseases, or injuries to the cord, reflex actions 
are, as we have said, sometimes exaggerated, but it is possible, and indeed 
probable, that the increase is due to the morbid processes producing a 
greater irritability of the cord itself, and not to the withdrawal of any in- 
hibitory influences. In many cases, in perhaps the greater number, no 
exaggeration but a diminution or even absence of reflex activity is observed ; 
so much so that could we trust explicitly to clinical experience, we should 
be inclined to conclude that the scantiness of spinal reflex action in man 
was due not to any preoccupation of the cord by influences proceeding from 
a dominant brain, but to an inherent paucity of spinal reflex mechanisms. 
But we have already said all we have at present to say on this point. 
§ 507. The time required for reflex actions. When one eyelid is stimulated 
with a sharp electrical shock, both eyelids blink. Hence, if the length of 
time intervening between the stimulation of the right eyelid and the move- 
ment of the left eyelid be measured, this will give the total time required 
for the various processes which make up a reflex action. It has been found 
to be from 0.0662 to 0.0578 second. Deducting from these figures the time 
required for the passage of afferent and efferent impulses along the fifth and 
facial nerves to and from the bulb, and for the latent period of the con- 
traction of the orbicularis muscle, there would remain 0.0555 to 0.0471 
second for the time consumed in the central operations of the reflex act. 
The calculations, however, necessary for this reduction, it need not be said, 
are open to sources of error; moreover, the reflex act in question is carried 
out by the bulb and not by the spinal cord proper. Blinking thus produced 
is a reflex act of the very simplest kind ; but, as we have seen in the pre- 
ceding pages, reflex acts differ very widely in nature and character ; and we 
accordingly find, as indeed we have incidentally mentioned, that the time 
taken up by a reflex movement varies very largely. This, indeed, is seen in 
blinking itself. When the blinking is caused not by an electric shock 
applied to the eyelid, but by a flash of light falling on the retina, in which 
case complex visual processes are involved, the time is distinctly prolonged; 
moreover, the results in different experiments in which light serves as the 
stimulus are not nearly so uniform as when the blinking is caused by stimu- 
lation of the eyelid. 
In general it maybe said that the time required for any reflex act varies 
very considerably with the strength of the stimulus employed, being less for 
the stronger stimuli; this we should expect, seeing that the efferent impulses 
of the reflex act are not simply afferent impulses transmitted through the 
central organ, but result from internal changes in the central organ started 
by the afferent impulse or impulses; and these internal changes will nat- 
urally be more intense and more rapidly effected when the afferent impulses 
are strong. It is stated that when the movement induced is on the same side 
of the body as the surface stimulation of which starts the act, the time 
taken up is less than when the movement is on the other side of the body, 
allowance being made for the length of central nervous matter involved in 
the two cases; that is to say, the central operations of a reflex act are prop- 
agated more rapidly along the cord than across the cord. The rapidity of 
the act varies, of course, with the condition of the spinal cord, the act being 
greatly prolonged when the cord becomes exhausted ; and a similar delay 
has been observed in cases of disease. The time thus occupied by purely 
reflex actions must not be confounded with the interval required when the 
changes taking place in the central nervous system are of a more complicated 
nature, and more or less distinctly involve mental operations; of the latter 
we shall speak later on. 578 
THE SPINAL CORD. 
The Automatic Actions of the Spinal Cord. 
§ 508. We speak of an action of an organ or of a living body as being 
spontaneous or automatic when it appears to be not immediately due to any 
changes in the circumstances in which the organ or body is placed, but to 
be the result of changes arising in the organ or body itself and determined 
by causes other than the influences of the circumstances of the moment. 
Some automatic actions are of a continued character; others, like the beat 
of the heart, are repeated in regular rhythm ; but the most striking auto- 
matic actions of the living body, those which we attribute to the working of 
the will and which we call voluntary or volitional, are characterized by their 
apparent irregularity and variableness. Such variable automatic actions 
form the most striking features of an intact nervpus system, but are conspic- 
uously absent from a spinal cord when the brain has been removed. 
A brainless frog placed in a condition of complete equilibrium in which 
no stimulus is brought to bear on it, protected, for instance, from sudden 
passing changes in temperature, from a too rapid evaporation by the skin 
and the like, remains perfectly motionless until it dies. Such apparently 
spontaneous movements as are occasionally witnessed are so few and seldom, 
that we can hardly do otherwise than attribute them to some stimulus, inter- 
nal or external, which has escaped observation. In the mammal (dog) after 
division of the spinal cord in the dorsal region regular and apparently spon- 
taneous movements may be observed in the parts governed by the lumbar 
cord. When the animal has thoroughly recovered from the operation the 
hind limbs rarely remain quiet for any long period ; they move restlessly in 
various ways; and when the animal is suspended by the upper part of the 
body, the pendent hind limbs are continually being drawn up and let down 
again with a monotonous rhythmic regularity, suggestive of automatic 
rhythmic discharges from the central mechanisms of the cord. In the 
newly born mammal too, after removal of the brain, movements apparently 
spontaneous in nature are frequently observed. But all these movements, 
even when most highly developed, are very different from the movements, 
irregular and variable in their occurrence though orderly and purposeful in 
their character, which we recognize as distinctly voluntary. Even admit- 
ting that some of the movements of the brainless mammal may resemble 
voluntary movements in so far as they are due to changes taking place in 
the spinal cord itself independent of the immediate influence of any stimulus, 
we are not thereby justified in speaking of the spinal cord as developing a 
will in the sense that we attribute a will to the brain. 
§ 509. In the ease of the beat of the heart, the automatic rhythmic dis- 
charge of energy appears to be exclusively the outcome of the molecular 
nutritive changes taking place in the cardiac substance. The beat may be 
modified, as we have seen, by nervous impulses reaching the cardiac sub- 
stance along certain nerves ; but the actual existence of the beat is wholly 
independent of these extraneous influences; the rhythmic discharge con- 
tinues when they are entirely absent. The automatic rhythmic discharge of 
respiratory impulses from the respiratory centre is also dependent on the 
intrinsic molecular changes of the centre, these being, as we have seen, 
largely determined by the character of the blood streaming through it; but 
in this case extrinsic nervous impulses, reaching the centre along the vagus 
and other nerves, play a much more important part than do similar impulses 
in the case of the heart. They act so continually on the centre and enter so 
largely into its working, that we are compelled to regard the activity of the 
centre as fed, if we may use the word, not only by the intrinsic molecular 
nutritive processes of the centre itself, but also by the extrinsic nervous in- 579 
THE AUTOMATIC ACTIONS OF THE SPINAL CORD. 
fluences which flow into the centre from without. The automatism of the 
spinal cord as a whole resembles, in this respect, that of the respiratory cen- 
tre rather than that of the heart. It has for its basis doubtless the intrinsic 
molecular changes of the gray matter, on whose remarkable constitution we 
dwelt in a previous section ; the metabolic events of this substance are so 
ordered as to give rise to discharges of energy; but the discharge appears to 
be also intimately dependent on the inflow into the gray matter of afferent 
impulses and influences. The normal discharges of efferent impulses from 
the cord undoubtedly take place under the influences of these incoming im- 
pulses ; and it may be doubted whether the gray matter of the cord would 
be able in the absence of all afferent impulses, to generate any sustained 
series of discharges out of its merely nutritive intrinsic changes. The auto- 
matic activity of the cord is fed not only by intrinsic nutritive events, but 
also by extrinsic influences. 
In this feature we may, however, find perhaps the reason why the auto- 
matic activity of the spinal cord is so limited as compared with that of the 
brain. In spite of certain striking but superficial characters of which we 
shall speak later on, the gray matter of the brain presents no histological 
features so different from those of the gray matter of the cord as to justify 
us in concluding that the one is capable and the other incapable of devel- 
oping the impulses which we call volitional out of the molecular nutritive 
changes of its substance. We are, therefore, led to the conclusion that the 
fuller automatic activity of the brain is due to the intrinsic changes of its 
substance being so much more largely assisted by the influx of various 
afferent impulses and influences, notably those of the special senses. To this 
question, however, we shall have to return later on. 
§ 510. In treating of the vascular system we saw that the central ner- 
vous system exercised through the vasomotor nerves such an influence on the 
muscular coats of the bloodvessels as to maintain what we spoke of as 
"tone," section of vaso-constrictor fibres leading to " loss of tone." We 
saw further that arterial tone, though normally dependent on the general 
vasomotor centre in the bulb, could be kept up by the cord itself, that, for 
instance, a tone of the bloodvessels of the hind limbs could be maintained 
by the isolated dorso-lumbar cord. This maintenance of arterial tone may 
be spoken of as one of the "automatic" functions of the spinal cord. We 
have also seen that plain muscular fibres, other than those of the arteries, 
notably the fibres forming sphincters, such as the cardiac and pyloric sphinc- 
ters of the stomach, the sphincter of the bladder, and especially the sphinc- 
ter of the anus, also possess tone, and that the tone of these sphincters is 
also dependent on the spinal cord, or on some part of the central nervous 
system. We need not repeat the discussions concerning these mechanisms 
and other instances of the spinal cord exercising an automatic influence 
over various viscera; we have referred to them here, since they serve as 
an introduction to a question which been much debated, and which has 
many collateral and important bearings, namely, the question whether the 
spinal cord exercises an automatic function in maintaining a tone of the 
skeletal muscles. 
The question is not one which can be settled off-hand by a simple 
experiment. Most observers agree that the section of a motor nerve does 
not produce any clearly recognizable immediate lengthening of a muscle 
supplied by the nerve, in the same way that section of a vaso-constrictor 
nerve undoubtedly gives rise to a relaxation of the muscular fibres in the 
arteries governed by it; and it has been inferred from this that skeletal 
tone does not exist. But there are several facts to be taken into con- 
sideration before we can come to a just decision. 580 
THE SPINAL CORD. 
The skeletal muscles have been described as being placed " on the stretch " 
in the living body. If a muscle be cut away from its attachments at each 
end, it shortens ; if it be cut across, it gapes. In other words, the muscle in 
the living body possesses a latent tendency to shorten, which is continually 
being counteracted by its disposition and attachments. In studying mus- 
cular contraction we saw (§ 85) that the shortening of a contraction is fol- 
lowed by a relaxation or return to the former length, both the contraction 
and relaxation being the result of molecular changes in the living muscular 
substance. We have now to extend our view and to recognize that, apart 
from the occurrence of ordinary contractions, molecular changes are by means 
of nutritive processes continually going on in the muscle in such a way that 
the muscle, though continually on the stretch, does not permanently lengthen, 
but retains the power to shorten upon removal or lessening of the stretch, 
and conversely though possessing this power of shortening permits itself to 
lengthen when the stretch is increased. In this way the muscle is able to 
accommodate itself to variations in the amount of stretch to which it is from 
time to time subjected. When a flexor muscle, for instance, contracts, the 
antagonistic extensor muscle is put on an increased stretch and is correspond- 
ingly lengthened ; when the contraction of the flexor passes off the extensor 
returns to its previous length ; and so in other instances. Thus by virtue of 
certain changes within itself a muscle maintains what may be called its nat- 
ural length in the body, always returning to that natural length both after 
being shortened and after being stretched. In this the muscle does no more 
than do the other tissues of the body which, within limits, retain their 
natural form under the varied stress and strain of life; but the property is 
conspicuous in the muscle ; and its effects in skeletal muscles correspond so 
closely to those of arterial tone, that we may venture to speak of it as a skel- 
etal tone. Indeed, the molecular changes at the bottom of both are probably 
the same. 
These changes are an expression of the life of the muscle ; they disappear 
when the muscle dies and enters into rigor mortis ; and, moreover, during 
life they vary in intensity so that the "tone " varies in amount according to 
the nutritive changes going on. We have seen reason to believe that the 
nutrition of a muscle, as of other tissues, is governed in some way by the 
central nervous system. We saw, in treating of muscle and nerve (§ 81), 
that the irritability of a muscle is markedly affected by the section of its 
nerve, i. e., by severance from the central nervous system ; and again (§ 462), 
in speaking of the so-called trophic action of the nervous system, we referred 
to changes in the nutrition of muscles occasioned by diseases of the nervous 
system. And experience, especially clinical experience, shows that the nu- 
tritive changes which determine tone are very closely dependent on a due 
action of the central nervous system. When we handle the limb of a healthy 
man, we find that it offers a certain amount of resistance to passive move- 
ments. This resistance, which is quite independent of, that is to say, which 
may be clearly recognized in the absence of all distinct muscular contrac- 
tions of volitional or other origin, is an expression of muscular tone, of the 
effort of the various muscles to maintain their "natural" length. In many 
cases of disease this resistance is felt to be obviously less than normal ; the 
limb is spoken of as " limp " or " flabby," or as having " a want of tone." In 
other cases of disease, on the other hand, this resistance is markedly increased; 
the limb is felt to be stiff or rigid; more or less force is needed to change it 
from a flexed to an extended, or from an extended to a flexed condition ; and 
in the range of disease we may meet with very varying amounts of increased 
resistance, from a condition which is only slightly above the normal to one 
of extreme rigidity. In some cases the condition of the muscle is such as at 581 
THE AUTOMATIC ACTIONS OF THE SPINAL CORD. 
first sight seems much more comparable to a permanent ordinary contraction 
than to a mere exaggeration of normal tone ; but all intermediate stages are 
met with, and indeed these extreme cases may be taken as indicating that 
the molecular processes which maintain what we are now calling tone are at 
bottom of the same nature as those which carry out a contraction ; they 
serve to show the fundamental identity of the skeletal tone with the more 
obvious arterial tone. 
Clinical experience then shows that the central nervous system does 
exert on the skeletal muscles such an influence as to give rise to what we may 
speak of as skeletal tone-changes in the central nervous system, leading in 
some cases to diminution or loss of tone, in other cases to exaggeration of 
tone, manifested often as conspicuous rigidity. The question why the 
changes take one direction in one case and another in another is one of great 
difficulty (the occurrence of extreme rigidity being especially obscure), and 
cannot be discussed here. We have called attention to the facts simply 
because they show the existence of skeletal tone and its dependence on the 
central nervous system. This conclusion is confirmed by experiments on 
animals, and these also afford proof that in animals the spinal cord can by 
itself, apart from the brain, maintain the existence of such a tone. In a 
frog, after division of the cord below the brain, the limbs during the period 
of shock are flabby and toneless ; but after a while, as the shock passes off, 
tone returns to the muscles, and the limbs offer when handled a resistance 
like that of the limbs of an entire frog. When the animal is suspended the 
hind limbs do not hang perfectly limp and helpless, but assume a definite 
position ; and that this position is due to some influence proceeding from the 
spinal cord is shown by dividing the sciatic nerve on one side ; the hind limb 
on that side now hangs quite helpless. This more pendent position shows 
that some of the flexors have lengthened in consequence of the section of 
the nerve, and this result may be taken as refuting the argument, quoted 
above against the existence of tone, which is based on the statement that a 
muscle cannot be observed to lengthen after section of its nerve. It may be 
here remarked that if the brainless frog, whose hind limbs are more or less 
pendent when the body is suspended, be placed on its belly, the hind limbs 
are brought into a flexed position under the body by means of obvious mus- 
cular contraction; and from this it might be inferred that the maintenance 
of the position of the pendent limb was also the result of a feeble contrac- 
tion. But no obvious contractions can be observed in the latter case, as in 
the former; and when in the former the limb has once been brought into the 
flexed position, that position, like the pendent position, is maintained without 
obvious contractions. As we said above, "tone" may pass into something 
which appears to be identical with a contraction, but where no obvious con- 
tractions are observed it seems preferable to speak of the state of the 
muscle as one of tone. 
In the dog, after division of the cord in the thoracic region, the hind 
limbs during the period of shock are limp and toneless. In the warm- 
blooded animal, as we have said, the effects of shock are much more lasting 
than in the cold-blooded animal; and in the dog the tone of the skeletal 
muscle returns much more slowly than in the frog. Indeed, when the divis- 
ion of the cord has taken place low down the skeletal tone returns very 
slowly, and may be manifested very feebly, or even be absent altogether. 
But under favorable circumstances, when a sufficient length of cord has been 
left, a fairly normal tone is reestablished. In man, in accordance with the 
facts previously mentioned (§ 504), skeletal tone, which has been lost through 
the continuity of the cord being broken by disease or accident, appears 
rarely if ever to return fully in the regions below the lesion. 582 
THE SPINAL CORD. 
We may therefore on the whole of the evidence conclude that the main- 
tenance of skeletal tone is one of the functions of the cord ; but we may here 
repeat that the condition of the cord, on which depends the issue from the 
cord along the efferent cords of the influences, whatever their nature, which 
produce tone in the muscle, may be, and indeed is, in its turn dependent on 
afferent impulses. In the case of the frog quoted above the tone of the pen- 
dent limbs disappears or is greatly lessened when the posterior roots of the 
sciatic nerves are divided, though the anterior roots be left intact. In the 
absence of the usual stream of afferent impulses passing into it, the cord 
ceases to send forth the influences which maintain the tone. Hence the 
maintenance of tone presents many analogies with a reflex action, especially 
when we remember that, as stated above, tone passes insensibly into con- 
traction ; and it may be a mere matter of words whether we speak of the 
maintenance of tone as an automatic or as a reflex action of the cord. 
We may, however, distinguish the part played by the afferent impulses in 
assisting the cord to a condition in which it is capable of maintaining tone 
from the part played by an afferent impulse in causing a reflex action ; in 
the former the action of the afferent impulses seems analogous to that of a 
supply of arterial blood in maintaining an adequate irritability of the ner- 
vous substance, in the latter the afferent impulses lead directly to a discharge 
of energy. And it is convenient to distinguish the two things by different 
names. 
§ 511. The close connection between tone and reflex action is illustrated 
by the so-called "tendon phenomena," which, on the one hand, are considered 
as cases of ordinary reflex action, and, on the other hand, have been regarded 
as exemplifying a special influence of the spinal cord on the irritability of 
the muscle. It is well known that when the leg is placed in an easy posi- 
tion, resting for instance on the other leg, a sharp blow on the patellar tendon 
will cause a sudden jerk forward of the leg brought about by a contraction 
of the quadriceps femoris; it is necessary or at least desirable for a good 
development of the jerk, that the tendon (and muscle) should be somewhat 
on the stretch. Similarly the muscles of the calf may be thrown into action 
by tapping the tendo Achillis, pqt somewhat on the stretch by flexion of the 
foot; and in some cases the same muscles may be made to execute a series of 
regular rhythmatic contractions, called " clonic " contractions, by suddenly 
pressing back the sole of the foot, so as to put them on the stretch. These, 
and other instances of a like kind, at first sight appear to be, and indeed are 
by many observers maintained to be, cases of reflex action, due to afferent 
impulses started in the tendon ; hence they have been frequently spoken of 
as " tendon-reflex." Other observers maintain that they are not reflex, but 
due to direct stimulation of the muscles, the vibrations set up in the more or 
less tense tendon being transmitted to the muscles, and so throwing the latter 
into contractions. The chief arguments against their being reflex are that 
the interval between the tap and the contraction is very short (0.03 or 0.04 
second), shorter than the ordinary interval of a reflex action (§ 507), and 
that the movement persists after section of the nerves of the tendon. The 
first argument is perhaps not a very strong one, and the second may be met 
by supposing that in such a case at least, if not always, the reflex act really 
begins in the muscle, being started in it by the vibrations transmitted to it 
along the tendon. 
But even if we admit that the movements are purely muscular, started 
and carried out in the muscle without the help of the usual reflex chain of 
afferent impulses, spinal centre, and efferent impulses, we must at the same 
time admit that they are closely dependent on the integrity of the spinal 
cord and of the connections between the cord and the muscle. In the case THE AUTOMATIC ACTIONS OF THE SPINAL COED. 
583 
of animals they disappear when the spinal cord is destroyed, or the nerves 
going to the muscles are severed, or even when the posterior roots only are 
divided. The measure of their development both in animals and in man is 
also closely dependent on the condition of the spinal cord and of the central 
nervous system generally. They may be increased or diminished, augmented 
or inhibited by a coincident voluntary effort directed toward some other end, 
or by the coincident development of a sufficiently distinct sensation. In 
general it may be said that whatever favors the activity of the spinal cord 
tends to increase them, and whatever depresses the activity of the spinal coni 
tends to diminish them. They are diminished or wanting in certain diseases 
of the spinal card (e. 7., locomotor ataxia) and exaggerated in others ; so much 
so indeed that they have become of practical clinical importance as a means 
of diagnosis. Whether we regard them as instances of ordinary reflex 
action, or consider that they are carried out by the muscle itself and that 
the cord intervenes only so far as to increase, maintain, or diminish the irri- 
tability of the muscular substance, it remains good that they are prominent 
whenever the conditions increase the reflex or other excitability of the cord, 
and diminish or disappear when the conditions lower or abolish that excita- 
bility. 
§ 512. Disease in man reveals other actions of the spinal cord which 
bear features different from those of an ordinary reflex movement, and yet 
have been described as reflex in nature. For instance, certain affections of 
the cord are characterized by the legs becoming rigid in extreme extension, 
the rigidity of the straightened limbs being often so great that when a by- 
stander lifts up one leg from the bed the other leg is raised at the same 
time. The rigidity is due to the extensor muscles being thrown into a state 
of contraction, which is so uniform and long continued that it may be 
spoken of as a "tonic" contraction ; such a tonic rigidity may, however, be 
replaced by a series of rhythmic, " clonic " contractions. It has sometimes 
been observed that the limbs when flexed are supple and free from rigidity, 
but that rigidity sets in so soon as they are brought into the position of ex- 
tension, the leg becoming suddenly fixed and straight somewhat in the way 
that a clasp-knife springs back when opened It seems clear that the pecu- 
liar contraction is carried out by means of the spinal cord, but the whole 
action, though it is often spoken of as ,a " muscle-reflex," is very unlike an 
ordinary reflex movement. In an ordinary movement an extensor is 
brought into action when a limb is flexed-not when it is already ex- 
tended ; and if in a reflex act the condition of the muscle about to be 
thrown into action determines in any way the discharge of impulses from 
the reflex centre, we should expect that the stretching of an extensor 
muscle by flexion-not its relaxation by extension-would determine the 
discharge of extensor impulses. In the case of the diseases in question 
.just the opposite seems to take place ; the position which appears to deter- 
mine the development of the remarkable contraction is precisely that in 
which the strain upon the extensors is at its minimum. It may be doubted, 
therefore, whether the word reflex should be used to denote such phenom- 
ena ; but the phenomena themselves deserve attention, especially, perhaps, 
as showing how in the disorders of the gray matter of the cord due to dis- 
ease impulses or influences which are latent only in health become actual 
and effective. 
It remains for us to speak of the part played by the spinal cord, as the 
instrument of the brain, in the execution of voluntary movements and in the 
development of conscious sensations ; but it will be best to consider these 
matters in connection with the brain itself, to the study of which we must 
now turn. 584 
THE BRAIN. 
CHAPTER II. 
THE BRAIN. 
On Some General Features of the Structure of the Brain. 
§ 513. It would be out of place to attempt to give here a complete de- 
scription of the structure of the brain ; but certain features must be kept 
fresh in the mind as a basis for physiological discussion ; and to these we 
must now turn our attention, a general acquaintance with the topographical 
anatomy of the brain being presupposed.1 
Like the spinal cord, the brain consists of " white matter," in which the 
nervous elements are almost exclusively medullated fibres, and of " gray 
matter," in which nerve-cells and other nervous elements are also present; 
but the gray matter of the brain is much more variable in structure than 
that of the spinal cord, and possesses features peculiar to itself; these we 
shall study later on. 
For physiological purposes the brain may be conveniently divided into 
parts corresponding to the divisions which appear in it in the embryo. At 
an early stage in the life of the embryo, that part of the medullary tube 
which is about to become the brain differs from that which is about to 
become the spinal cord, in that the central canal, which in the latter is of 
fairly uniform bore along its whole length, is in the former alternately 
widened and narrowed, so that the tube forms a series of vesicles, the cerebral 
vesicles, succeeding each other lengthways. At first these vesicles are three 
in number, called respectively fore-brain, mid-brain, and hind-brain ; but the 
fore-brain, after having developed on each side a lateral vesicle, the optic 
vesicle, subsequently transformed into the retina and optic nerve, gives rise 
in front of itself to a pair of vesicles placed side by side, or rather to a single 
vesicle with a deep median furrow, the vesicle of the cerebrum, containing a 
cavity divided by a median partition into two cavities, lying side by side, 
which open into the cavity of the original fore-brain by a Y-shaped opening. 
This embryonic chain of vesicles is developed into the adult brain by unequal 
growth of the walls and unequal expansion of the cavities, certain features 
being also impressed upon it by the bend on the longitudinal axis, which 
takes place in the region of the mid-brain and is known as the cranial 
flexure. 
§ 514. In the hind part of the binder vesicle or hind-brain, the ventral, 
basal portion or floor is thickened to form the bulb, while the greater part of 
the dorsal portion or roof does not thicken at all, is not transformed into 
nervous elements, but remains as a single layer of epithelium, adherent to 
the pia mater overlying it, and so forms a thin covering to the lozenge- 
shaped cavity of the vesicle, now known as the fourth ventricle. 
In the front part of the same hind-brain, on the contrary, the roof and 
sides are enormously developed into the conspicuous cerebellum overhanging 
the front part of the fourth ventricle, while the floor is also thickened into 
the pons Varolii. 
This thickening of the pons is largely made up, on the one hand, of hori- 
zontal nerve-fibres, which run transversely from each side of the cerebellum 
into the pons, or from one side of the cerebellum to the other, and, on the 
other hand, of longitudinal fibres, which run forward from the bulb and are 
1 Figs. 131 and 146, which will be found in succeeding sections, may with advantage 
be consulted in reading this section, though not specially referred to in the text. STRUCTURE OF THE BRAIN. 
585 
wrapped round by and interlaced with the others. At the front margin of 
the pons these longitudinal fibres, augmented in number, appear as two thick 
strands, the crura cerebri, forming the floor of the mid-brain, the roof of 
which is thickened into the corpora quadrigemina, and the cavity of which 
is reduced to a narrow tubular passage, the aqueduct of Sylvius, or iter a tertio 
ad quartum ventriculum. 
At the level of the fore-brain the crura cerebri, diverging rapidly from 
each other as they pass forward, leave the median portion of the floor of the 
vesicle now known as the third ventricle very thin, but form, especially behind 
and ventrally, thick lateral walls, which are further increased in thickness 
by the development on each side of a mass largely composed of gray matter, 
known as the optic thalamus. The roof of the third ventricle, like that of 
the fourth ventricle, is not developed into nervous elements, but remains 
extremely thin, and consists of nothing more than a single layer of epi- 
thelium. 
§ 515. In front of the third ventricle each diverging crus cerebri spreads 
out in a small radial fashion into the corresponding half of the paired vesicle 
of the cerebellum now developed into the preponderant cerebral hemispheres, 
the two cavities of which are now known as the lateral ventricles. The 
growth of the cerebral hemispheres is not only much greater than that of the 
rest of the brain, but also takes place in a special manner. At their first 
appearance the cerebral hemispheres lie wholly in front of the fore-brain or 
vesicle of the third ventricle, but in their subsequent growth, while expand- 
ing in nearly all directions, they extend especially backward. Thus, in the 
adult brain, on the dorsal surface they not only completely cover up the 
third ventricle but also overlap the mid-brain, reaching so far back as to 
cover the front border of the cerebellum, while on the ventricle surface, 
though in the middle line they leave exposed the floor or ventral portions 
of the walls of the third ventricle, at the sides they are seen to reach as far 
backward as on the dorsal surface. The median furrow on the dorsal sur- 
face which separates each hemisphere from its fellow is at first shallow, but 
rapidly deepens, so that as the hemispheres grow they become separated 
from each other by a narrow, deep longitudinal fissure, into which, as we 
shall see, a fold of the dura mater dips. This fissure is not only deep ver- 
tically-i. e., from the dorsal surface ventrally-but at the front of the 
brain runs backward in the middle line almost as far as the level of the 
third ventricle, so as completely to separate from each other the anterior 
parts of each hemisphere, known as the anterior lobes; at the back of the 
brain also it similarly runs forward in the middle line for a considerable 
distance, so as to separate from each other the posterior lobes. Hence the 
two great masses of the cerebral hemisphere are united with each other, not 
along their whole length, but for about a third of that length, the isthmus 
or bridge thus connecting them lying at some depth below the dorsal sur- 
face at the bottom of the longitudinal fissure, in about the middle third of 
its length. 
At its first appearance each lateral ventricle is of a more or less oval 
form, its walls are of uniform thickness, and it lies in front of the third ven- 
tricle. During the growth of the hemispheres it acquires a peculiar shape 
and becomes divided into an anterior cornu or horn stretching into the anterior 
portion, a posterior horn stretching into the posterior portion, and a descend- 
ing horn, which curves laterally and ventrally into the middle portion of the 
hemisphere; owing to the great backward extension of the hemispheres the 
lateral ventricles come to lie not only in front of but also at the side of, and 
indeed, to a certain extent, above or dorsal to the third ventricle; and dur- 
ing the growth of the parts the originally wide Y-shaped opening which 586 
THE BRAIN. 
placed the hind ends of the two lateral ventricles in communication with the 
front of the third ventricle becomes narrowed into a slit-like passage of sim- 
ilar form, the foramen of Monro, which still opening into the front of the 
third ventricle, now leads on each side from a point rather in front of the 
middle of the lateral ventricle. 
As the hemisphere enlarges, the growth of the walls of the vesicle is not 
uniform in all parts. At an early period there may be observed in the ven- 
tral wall or floor of the vesicle a thickening, which assuming a special, more 
or less semilunar, form and projecting into the cavity becomes the body 
known as the corpus striation. As development proceeds the corpus striatum 
on each side becomes attached to the optic thalamus, lying behind and to the 
median side of itself, the radiating fibres of the crus cerebri passing between 
the two, and also as we shall see dividing the corpus striatum into two bodies, 
called the nucleus caudatus and nucleus lenticularis. A notable result of this 
growth and change of position of the hemispheres and of the coalescence of 
the corpus striatum with the optic thalamus is that the latter body,.though 
really belonging to the third ventricle, comes to project somewhat into the 
lateral ventricle; a strip of the upper surface of the optic thalamus, along 
its outer, lateral edge, forms a portion of the floor of the lateral ventricle in 
the median region on each side of the third ventricle. Besides this special 
development of the corpus striatum, the walls of each vesicle, with the 
exception of the median part by which the two vesicles coalesce with each 
other, become (we are now speaking of the higher mammals) thickened 
much in the same way all over, the surface being folded so as to give rise to 
convolutions or gyri separated by furrows or sulci; and the thickening 
taking place in such a way as to give the ventricle its peculiar shape. The 
median coalesced part undergoes a different and peculiar change. This part, 
which at first lies in front of the third ventricle, through the changes 
brought about by the growth of the hemispheres so shifts its position as to 
lie immediately over, dorsal to third ventricle, very much as if this part of 
the cerebral vesicles had been folded back over the fore-brain. .In the 
junction itself we may distinguish a dorsal and a ventral portion. The 
dorsal portion is developed into a system of transverse commissural fibres 
passing across from one hemisphere to the other. In the median region these 
fibres form a thick compact band, called the corpus callosum, which may be 
exposed to view at the bottom of the longitudinal fissure, while on each side 
they spread away in all directions to nearly all parts of the surface of the 
hemispheres, passing over and helping to form the roof of the lateral ven- 
tricles. The band is not flat but curved ventralward; hence in a longitudi- 
nal vertical section of the brain taken in the middle line it presents a curved 
form with the concavity directed ventralward. While this dorsal portion of 
the junction is developed at the sides as well as in the middle line, the ven- 
tral portion is developed in the median region only, and that in a special 
way, so that it forms below, ventral to, the corpus callosum au arched plate, 
in the shape of a triangle with the apex directed forward, called the fornix, 
which lies immediately above the thin epithelial roof of the third ventricle. 
In front, the narrower apical portion of the fornix lies at some little distance 
below, ventral to, the corpus callosum, and here the junction between the 
two vesicles is reduced to a thin sheet, the septum lucid um; but behind, the 
broader basal portion of the fornix is arched up so as to lie immediately 
under and touch the corpus callosum. Hence the septum lucidum has the 
form of a more or less triangular vertical sheet, broad in front and narrow- 
ing behind, separating the two lateral ventricles. The sheet may be con- 
ceived of as being double and formed by the apposition of two layers, one 
belonging to each ventricle ; between these two layers is developed a narrow STRUCTURE OF THE BRAIN. 
587 
closed cavity containing fluid, called the fifth ventricle. But while the lateral 
ventricles open by the foramen of Monro into the third ventricle, and the 
third ventricle is continuous by means of the aqueduct with the fourth ven- 
tricle, which again passes into the central canal of the spinal cord, the whole 
series being developed out of the same embryonic neural canal, the fifth ven- 
tricle communicates with none of them ; it is a cavity of different origin. 
The corpus callosum or dorsal portion of the junction between the vesi- 
cles spreads out, as we have said, laterally along its whole length, and thus 
forms a broad band joining the two hemispheres together; the middle por- 
tion spreads out in a more or less straight direction, though curving over the 
ventricle upward and downward to reach various parts of the hemisphere, 
while the front and hind ends bend round on each side forward and back- 
ward to reach the anterior and posterior parts. Thus through the corpus 
callosum the thick wall of one ventricle is made continuous with that of the 
other. The disposition of the fornix or ventral portion of the junction is 
very different. At its apex in front the fornix bifurcates into two bands, 
known as the pillars of the fornix, which on each side become continuous 
with, and take a peculiar course in the walls of the third ventricle. In like 
manner behind, the angles of the base of the fornix are continuous with the 
walls of the lateral ventricles, that is to say, with the thick mass of the 
hemispheres, being also prolonged as two special strands of fibres called the 
crura of the fornix. But along each side of the triangle, between the attach- 
ments in front and behind, the substance of the fornix is not continued into 
the substance of the corresponding hemisphere; the edge of the fornix 
appears on each side to lie loose on the dorsal surface of the optic thalamus, 
which here forms the median portion of the floor of the lateral ventricle; 
between the optic thalamus below and the fornix above there seems to be a 
narrow slit by which the cavity of the lateral ventricle communicates with 
parts outside itself. In reality, however, there is no actual breach of con- 
tinuity though there is a breach of nervous substance. The slit is bridged 
over by a layer of epithelium, by means of which the edge of the fornix is 
made continuous with the upper surface of the optic thalamus, and the 
median wall of the lateral ventricle made complete. But this layer of 
epithelium has the following peculiar relations to the pia mater covering the 
brain: 
We have said that the roof of the third ventricle, like that of the fourth 
ventricle, consists only of a layer of epithelium devoid of nervous elements. 
We have further seen that the fornix and the hind part of the corpus cal- 
losum with which it is continuous overlie the third ventricle, the free base 
of the fornix with the rounded hind end of the corpus callosum above form- 
ing together the hind border of the junction or bridge between the two hemi- 
spheres. The pia matter covering the dorsal surface of the brain, passing 
forward under this curved border, spreads over the top of the third ventricle, 
becoming adherent to the layer of epithelium just referred to, and thus 
forms a vascular sheet called the velum interposition, which serves as the 
actual roof of the third ventricle, immediately below, ventral to, the fornix ; 
it cannot be seen without previously removing the fornix. At the lateral 
edge of the fornix, on each side, this same vascular sheet of pia mater pro- 
jects from beneath the fornix into the lateral ventricle, carrying with it the 
layer of epithelium which, as we said, made the edge of the fornix actually 
continuous with the rest of the walls of the lateral ventricle; the part of the 
pia mater thus seen projecting beyond the edge of the fornix when the 
lateral ventricle is laid open is called the choroid plexus. To this peculiar 
intrusion of the pia mater, by which the nutrition of the brain is assisted, 
we shall return when we come to speak of the vascular arrangements of the 588 
THE BRAIN. 
brain. Meanwhile we may point out, that while this vascular ingrowth seems 
to make the cavity of the third ventricle continuous with that of the lateral 
ventricle on each side, and all three with the exterior of the brain, it really 
does not do so. The cavity of the third ventricle is made complete by the 
layer of epithelium forming its roof, and the cavity of the lateral ventricle 
is made complete by the layer of epithelium passing from the lateral edge of 
the fornix over the choroid plexus to the other parts of the wall of the 
ventricle. To pass along this line from the actual cavity of the lateral into 
that of the third ventricle one must first pierce the epithelium covering the 
choroid plexus, thus gaining access to the pia mater of the plexus and of the 
velum, and then again pierce the epithelium coating the under surface of 
the vellum and forming the roof of the third ventricle It is only by the 
foramen of Munro that a real communication exists between the cavity of 
the lateral and that of the third ventricle. 
Thus by the large growth and backward extension of the cerebral hemi- 
spheres, the third ventricle comes to form, as it were the front end of the 
cerebro-spinal axis, the crura cerebri expanding on each side of the third 
ventricle into the cerebral hemispheres which cover up the ventricle on the 
dorsal surface, but leave its wall exposed on the ventral surface. Attached 
to the dorsal surface of the third ventricle at its hind end, ventral to and 
somewhat projecting beyond the base of the fornix, lies the pineal gland 
with its attachments, the remnants of a once-important median organ ; and 
attached to the ventral surface of the ventricle, at the apex of a funnel- 
shaped projection, the infundibulum, lies the pituitary body, also a remnant 
of important ancestral structures. 
$516. We may then divide the whole brain into a series of parts corre- 
sponding to the main divisions of the embryonic brain. At the front lie 
the cerebral hemispheres, with the lateral ventricles, developed out of the 
cerebral vesicles; and with these are associated the corpora striata, the term 
cerebral hemisphere being sometimes used so as to include these bodies, and 
sometimes so as to exclude them. Next come, corresponding to the original 
fore-brain, the parts forming the walls of the third ventricle, conspicuous 
among which are the optic thalami; for these bodies, though they appear 
to intrude into the lateral ventricles, belong properly to the third ventricle. 
In the mid-brain which follows, the cavity, now the tubular passage of the 
aqueduct, is roofed in by the two pairs, anterior and posterior, or corpora 
quadrigemina, the dimensions of which are not very great; but a thick 
floor is furnished by the crura cerebri. In each crus we must distinguish 
between a dorsal portion called the tegmentum, in which a large quantity 
of gray matter is present, and in which a great complexity in the arrange- 
ment of fibres exists, and a ventral portion, the pes or crusta, which is a 
much more uniform mass of longitudinally disposed fibres. As the crura 
passing forward diverge into the cerebral hemisphere on each side, the teg- 
mentum ceases at the hinder end and ventral parts of the optic thalamus; 
it is the pes which supplies the mass of fibres radiating into each cerebral 
hemisphere. In a view of the ventral surface of the brain, the base of the 
brain as it is frequently called, the crura may be seen emerging from the 
anterior border of the pons. This we have spoken of as the thickened floor 
of the front part of the hind-brain, but in reality it encroaches a little on 
the mid-brain, the hind part of the corpora quadrigemina being in the same 
dorso-ventral plane as the front part of the pons. (See Fig. 131.) In the 
main, however, the pons belongs to the fore part of the hind-brain, the roof 
and sides of which are developed, as we have said, into the cerebellum. 
This superficially resembles the cerebral hemispheres in its large size, and 
in the special development of its surface, which is formed of gray matter THE BULB. 
589 
folded in a remarkable manner and often spoken of as cortex. The cere- 
bellum, though the lateral portions, called the hemispheres, project above 
the median portion, called the vermis, is, unlike the cerebrum, a single 
mass; each lateral half, however, sends down ventrally a mass of fibres 
which, running transversely, partly end in the pons and partly are continued 
across the pons into the other lateral half; this mass of fibres, thus con- 
stituting, as we have said, a considerable part of the pons, forms on each 
side, just as it leaves the cerebellum to enter the pons, a thick strand, 
called the middle peduncle of the cerebellum. From the cerebellum there 
also proceeds backward into the bulb on. each side a thick strand of fibres, 
the inferior peduncle of the cerebellum or restiform body; and a third 
strand, the superior peduncle of the cerebellum, passes forward on each 
side into the region of the corpora quadrigemina. As the cerebellar 
peduncles converge behind the corpora quadrigemina the angle between 
them is filled up by a thin sheet of nervous matter, the valve of Vieussens, 
which thus for a little distance backward forms a roof for the front part of 
the fourth ventricle, just where the lozenge-shaped cavity is narrowing to 
become the aqueduct. Behind the cerebellum and pons comes the bulb, 
which, as we have said, is the thickened floor of the hind part of the hind- 
brain, the roof of the cavity being here practically wanting. 
Of these several divisions the first division, that of the cerebral hemi- 
spheres, including the corpora striata, stands apart from the rest by reason 
both of its origin and the character of its development. As we shall see, 
this anatomical distinction corresponds to a physiological difference. 
Of the other parts of the brain the crura cerebri deserve special atten- 
tion. We may regard these as starting in the cord, but largely augmented 
in the bulb; they traverse the pons, where they are still further increased, 
and passing beneath the corpora quadrigemina, with which as well as with 
the cerebellum they make connections, end partly in the region of the optic 
thalami and walls of the third ventricle, but to a great extent in the cere- 
bral hemispheres. We may, in a certain sense, consider the rest of the 
brain as built upon and attached to these fundamental basal or ventral 
strands. 
§ 517. Connected with the brain are a series of paired nerves, the 
cranial nerves. The first and second pair, the olfactory nerves and the 
optic nerves, differ in their origin and mode of development from all the 
rest so fundamentally as to cause regret that they are included in the same 
category. We shall consider these by themselves in due course. The re- 
maining pairs, from the third pair to the twelfth, forming a much more 
homogeneous category, we shall also consider in their proper place. We 
must now turn to study in greater detail some of the structural features of 
the brain, and we may with advantage begin with the bulb. 
The Bulb. 
§ 518. The spinal cord, as it ascends to the brain, becomes changed 
into the more complex bulb, partly by a shifting of the course of the tracts 
of white fibres, partly by an opening up of the narrow central canal into 
the wide and superficial fourth ventricle, but chiefly by the development of 
new gray matter. 
When the anterior ventral aspect of the bulb is examined (Fig. 181, C.), 
it will be seen that the anterior columns of the cord are interrupted for 
some distance in the median line by bundles of fibres (Py. dec.) which, 
appearing to rise up from deeper parts, cross over from side to side and so 
confuse the line of the anterior fissure. This is the decussation of the pyra- 590 
THE BRAIN. 
Outlines of Parts of the Brain. A, Dorsal; B, Lateral; C, Ventral Aspect. Natural size. (Sherrington.) Ant. col., anterior; lat. col., lateral ; post, col., poste- 
rior column of spinal cord : m. p., median posterior column (fasciculus gracilis); e. p., external posterior column (fasciculus cuneatus); Py. de., decussation of pyra- 
mids; Py., pyramid ; ol., olivary body; A. c„ ala cinerea; T. ac., tuberculum acusticnm ; str., striae acusticfe •, E. t., eminentia teres; F. a, fovea anterior; I. c., locus 
caeruleus; f. a. e., external arcuate fibres; Via., valve of Vieussens; R., restiform body; Po., pons; S. P., superior peduncle; P., part of middle peduncle of cere- 
bellum ; SP. P., the same two peduncles cut across; in B the limits of each are marked by dotted lines cut across. OR., crus cerebri; Q. P., posterior: Q. A., ante- 
rior corpus quadrigeminum ; F., lateral fillet; Op., optic tract; C. g. I., lateral; C.g.m., median corpus geniculatum ; Per., pulvinar of optic thalamus; P. n., 
pineal gland: c. m., corpora mammillaria or albicantia; p. r. C. i., posterior roots; a. r. 0. i., anterior roots of first cervical nerve; sp. ac., spinal roots of spinal 
accessory ; XII. to III., cranial nerves; V. x , motor root; V. (3, sensory root of fifth; 4, fourth ventricle. The tranverse lines indicate the levels of the sections in 
Figs. 132-137. 
Fig. 131. THE BULB. 
591 
mids, above which the place of the anterior columns of the spinal cord is 
taken by two larger, more prominent columns, the pyramids of the bulb 
(Py.), which are continued forward to the hind margins of the pons. On 
the other side of, lateral to, each pyramid lies a projecting oval mass, the 
olivary body or inferior olive (ol.) separating the pyramid from a column of 
white matter, the restiform body (P), which, occupying the lateral region of 
the bulb, when traced backward appears to continue the line of the lateral 
column of the cord, and when traced forward is seen to run up to the cere- 
bellum as the inferior peduncle of that organ. On the posterior dorsal 
aspect no such decussation is seen. The two posterior columns of the cord 
diverge from each other, leaving between them a triangular space, the 
calamus scriptorius, which is the hind part of the lozenge-shaped shallow 
cavity of the fourth ventricle. As the cord passes into the bulb, the poste- 
rior column, as a whole, grows broader, and the division into a median pos- 
terior and an external posterior column becomes very obvious and distinct 
by the appearance of a conspicuous furrow separating the two. At some 
distance, however, in front of the point of divergence of the columns or 
apex of the calamus scriptorius, the furrow becomes less marked, and it 
eventually fades away. In its course the furrow takes such a line that the 
median posterior column, forming the immediate lateral boundary of the 
fourth ventricle, has the appearance of a strand broad behind but thinning 
away in front, while the external posterior column, also broadening as it 
advances forward, seems to be wedged in between the median posterior 
column on its median edge and the restiform body on its lateral edge; 
hence the former is here called the fasciculus (or funiculus) gracilis (m. p.), 
and the latter the fasciculus (or funiculus) cuneatus (e. p.). Further forward 
both columns seem to merge with each other and with fibres which curve 
round to form part of the restiform body ; the relations, however, of these 
two columns to each other and to the other parts of the bulb, as well as 
the nature of the other several changes by which the cord is transformed 
into the bulb, are disclosed by transverse vertical (dorso-ventral) sections, 
to the study of which we must now turn. 
A section (Fig. 132, 1) taken at the hind margin of the decussation, at 
which level the first cervical nerve takes origin, when compared with a sec- 
tion of the cord at the level of the second cervical nerve (cf Fig. 127, C2), 
shows that certain changes are already taking place in the gray matter. The 
anterior horns are not much altered, but the posterior horns are, as it were, 
pushed out laterally and dorsally so that the posterior columns, which as yet 
retain their previous great depth, become very much broader than they are 
lower down, encroaching, so to speak, on the lateral columns. At the same 
time the substance of Rolando (s. g.), forming the head or caput of the horn, 
has enlarged into a more or less globular form, and lies near the surface of 
the cord though separated from it by a compact tract of longitudinal fibres 
( F. a.), which, as we shall see, belongs to the fifth cranial nerve. A con- 
siderable development of the reticular formation (f. ret.) at the side of the 
gray matter ventral to the posterior horn has also taken place, and this with 
the shifting of the position of the posterior horn has driven the lateral horn 
(I. h.) nearer to the anterior horn. From this lateral horn a root of the 
eleventh spinal accessory cranial nerve (XL) may be seen taking origin. 
Further, a great increase of gray matter round the central canal may also 
be observed. 
These changes, however, are of degree only; what seems to be an abso- 
lutely new feature is the presence of bundles of fibres (Py. dec.) which start- 
ing from the anterior column of one side cross over to and are apparently 
lost in the gray matter of the neck of the anterior horn of the other side ; in 592 
THE BRAIN. 
so crossing the fibres push aside the bottom of the anterior fissure. When 
the course of these fibres is investigated, either by simple microscopic obser- 
vation, or still better by the method of degeneration, it is found that they 
may be traced from the anterior column of one side, across the anterior 
commissure, through the neck of the anterior horn to the lateral column of 
the opposite side, and to that part of the lateral column which we have pre- 
viously described as the crossed pyramidal tract. 
In a section a little higher up (Fig. 132, 2), these decussating fibres form 
on each side a large strand which starts from a part of the anterior column, 
now becoming distinctly marked off as the pyramid (Py.), and is apparently 
lost in the reticular formation, but in reality passes on to the crossed pyram- 
idal tract of the lateral column. This strand, as it crosses over, completely 
cuts off the head of the anterior horn from the more central gray matter, 
and forms with its fellow a large area of decussating fibres between the 
bottom of the anterior fissure and the central gray matter. When a surface 
view of the bulb is examined the decussation is seen to be effected by alter- 
nate bundles, passing now from right to left, now from left to right; and in 
transverse sections we find correspondingly that the anterior fissure appears 
bent now to the left and now to the right, according as the section cuts 
through a bundle passing from left to right or from right to left. 
In sections still higher up (Fig. 132, 3 and 4) this conspicuous strand of 
fibres crossing obliquely from side to side will be no longer seen ; decussating 
fibres are seen dorsal to the anterior fissure, but these, of which we shall speak 
presently, are of different nature and origin. The fibres which in sections 
below were seen in the act of crossing are now gathered into masses of longi- 
tudinal fibres, the pyramids (Py.) one on each side of the anterior fissure, 
each with a sectional area of a rounded triangular form clearly marked out 
from the surrounding structures; the section is taken above the decussation 
of the pyramids. Or, tracing the changes from below upward, we may say 
that the decussation is now complete; on each side the whole of the crossed 
pyramidal tract of the spinal cord has, in the region of the bulb below7 the 
level of the present sections, crossed over to the other side, and joining with 
the direct pyramidal tract of the anterior column of the cord of the same 
side has become the pyramid of the bulb. In other words, the decussation 
of the pyramids is, as we have already hinted, the passing off from each 
pyramid and the crossing over to the opposite side of the cord of those fibres 
which are destined to become the crossed pyramidal tract of the spinal cord 
of the opposite side, while the rest of the pyramid pursues its course on the 
same side as the direct pyramidal tract. 
§ 519. In the spinal cord the bottom of the anterior fissure is separated 
from the central canal by nothing more than the anterior white commissure 
and a narrow7 band of gray matter, composed of the anterior gray commissure 
and of part of the central gelatinous substance. During the decussation of 
the pyramids, the decussating fibres push, as it were, the central canal with 
its surrounding gray matter to some distance from the bottom of the anterior 
fissure. In sections above the decussation the bottom of the fissure does not 
again approach the central canal, but continues to be removed to some dis- 
tance from it, and, as we pass upward, to an increasing distance, by the 
interposition of tissue which consists largely of decussating fibres. These, 
however, though they seem to continue on the decussation of the pyramids, 
are shown by the embryological and degeneration methods to have no con- 
nection with the pyramids, but belong to another system of decussation. As 
wre have seen (§ 477) the anterior commissure along the whole length of the 
cord contains decussating fibres. Some of these in the upper part of the cord 
are fibres crossing from the direct pyramidal tract of one side to the gray THE BULB. 
593 
Fig. 132. 
Transverse Dorso-ventral Sections of the Bulb (Man) at Different Levels. (Sherrington.) Fig. 
132 and Figs. 133-137 form a series of transverse dorso-ventral sections of the brain taken at dif- 
ferent levels from the hind end of the bulb to the front of the third ventricle; the several levels 
are shown by the lines drawn in Fig. 131. They are all magnified twice. The details are shown, 
for the sake of simplicity, in diagrammatic fashion ; the white matter is left unshaded, the course 
of the fibres being indicated in a few important instances only; the gray matter is shaded for- 
mally, the nerve-cells being indicated in the case only of the nuclei of the cranial nerves. The 
want of complete bilateral symmetry which is often met with in such sections is indicated in 
several of the figures. 
1. At the hind limit of the decussation of the pyramids ; 2. In the middle of the decussation; 
3. At the upper end of the decussation ; 4. Just below the point of the calamus scriptorius; 5. Just 
above the point; 6. Through the middle of the ala cinerea. 
Py. Pyramids; Py. dec. decussation of the pyramids; Supra Py. dec. superior decussation; f. a. i. 
internal arcuate fibres; f. a. e. external arcuate fibres ; Cb. position Qf cerebellar tract; R. resti- 594 
THE BRAIN. 
matter of the other side, and so may be regarded as part of the whole pyram- 
idal tract; but others are of different origin ; and even in the region of 
the actual decussation of the pyramids some of the fibres which cross over 
do not belong to the pyramidal tract. This system of decussating fibres 
becomes increasingly prominent above the decussation of the pyramids, and 
through it the ventral area of the bulb between the central canal and the 
anterior fissure is much increased. The fibres as they cross form a middle 
line of partition, the raphe (Fig. 132, 4, 5, r) which increases in depth in the 
upper parts of the bulb, and on each side of the raphe help to breakup the 
gray matter (which previously formed the anterior horns) into what is 
called the reticular formation. We shall return to this presently, but may 
here call attention to a special development of these decussating fibres 
which is seen just above the decussation of the pyramids. In a section at 
this level (Fig. 132, 3) a strand of fibres (Supra Py. dec.) may be seen to 
start chiefly from the gracile nucleus but also to some extent from the cune- 
ate nucleus, to sweep round the central gray matter, and to decussate ven- 
tral to this between it and the bottom of the anterior fissure. This is called 
the superior decussation, or, for reasons which we shall see later on, the sen- 
sory decussation. 
§ 520. We must now turn to the posterior fissure and its relations to 
the fourth ventricle. We saw that at the beginning of the pyramidal de- 
cussation, the posterior horns had been thrown backward and outward so as 
to increase the posterior columns. The posterior fissure is still of great 
depth, so that by the increase of depth and maintenance of depth the poste- 
rior column, the lateral limit of which is still sharply marked out by the 
swollen head of the posterior horn as well as by the highest posterior root- 
lets of the first cervical nerve, acquires at this level its maximum of bulk. 
From this point forward the depth of the posterior fissure and the dorso- 
ventral diameter of the posterior columns diminishes. The head of the 
horn (Fig. 132, 2) is thrown still further outward into the lateral regions; 
developments of gray matter at the base and to some extent at the neck of 
the horn (of these we shall speak presently) encroach (Fig. 132, 3) dorsally 
on the white matter of the columns; and the central gray matter appears 
to rise dorsally at the expense of the posterior fissure, in coincidence with 
the development described above as taking place on the ventral side of the 
canal. 
Still a little further forward, in a section, for instance (Fig. 132, 4), a 
little way behind the apex of the calamus scriptorius, the central gray matter, 
which still forms a rounded mass around the central canal, is brought yet 
nearer to the posterior fissure. 
In a section yet a little further forward (Fig. 132,5) carried through the 
hinder narrower part of the fourth ventricle itself, it is seen that the central 
canal has opened out on the dorsal surface, and that the gray matter, which 
form body or inferior peduncle of the cerebellum; e. p. external posterior column, fasciculus 
cuneatus ; m. p. median posterior column, fasciculus gracilis; r. raphe ; I. h. lateral horn ; m. p. n. 
nucleus of the median posterior column or gracile nucleus; e. p. n. nucleus of the external pos- 
terior column or cuneate nucleus ; e. p. n. (m) median division and e.p. n. (I) lateral division of 
the same; oZ. olivary body ; ol. a. median accessory, and ol. e. lateral accessory olive; in.ol. inter- 
olivary layer: a. I. n. lateral (antero-lateral) nucleus; n. a. arcuate nucleus; a. c. remnant of an- 
terior horn ; f. -ret. reticular formation ; s. g. substance of Rolando ; a. r. c. I. anterior root, and p. r. 
c. I. posterior root of first cervical nerve; XI. root of spinal accessory nerve; XII. twelfth or hy- 
poglossal nerve; n. XII. nucleus of the same in 6; the nucleus may be traced, however, through 
2, 3, 4, 5, in connection with the fibres of the nerve ; s. X. sensory or main part of the glosso- 
pharyngeal-vago-accessory nucleus; X. m. motor nucleus of the vagus, or nucleus ambiguus; IX. 
a. ascending root of the glosso-pharyngeal nucleus; V. a. ascending root of the fifth nerve; 4th. 
fourth ventricle ; the ependyma or lining is indicated by a thick dark line; and in 5 and 6. the 
tooth-like section of the projecting obex is shown. THE BULB. 
595 
iii previous sections surrounded it, is now exposed to the surface on the 
floor of the ventricle, the median posterior columns being thrust aside. In 
a still more forward section (Fig. 132, 6) this gray matter in correspondence 
with the increasing width of the ventricle occupies a still wider area, thrust- 
ing still further aside the narrowing upper ends of the two posterior 
columns. 
During these successive changes the large, wide posterior (both external 
posterior and median posterior) columns of the cervical spinal cord and 
beginning bulb, are reduced to small dimensions and in the end disappear; 
but before we speak of the course and fate of the tracts of fibres constitu- 
ting these columns we must turn to the important changes of the gray 
matter. 
§ 521. A transverse section through the lower end of the decussation 
(Fig. 132, 1) shows, as we have said, few differences as regards the gray 
matter from one taken at the level of the second cervical nerve. The 
changes noticeable are mainly the changes in position of the posterior horns, 
the increase of central gray matter around the central canal, the approach 
of the lateral horn, from which spring the roots of the spinal accessory 
nerve, to the anterior horn, and an increase of the reticular formation in the 
bay ventral to the posterior horn. 
In the middle of the decussation (Fig. 132, 2) the decussating fibres are 
cutting the head of the anterior horn away from the base of the horn and 
the central gray substance, and the isolated head is diminished in size, being 
separated from the surface of the cord by an increasing thickness of white 
matter. The lateral horn and origin of the spinal accessory root do not 
share in this isolation, but are driven back again dorsally toward the poste- 
rior root to join the reticular formation which is increasing in area, while 
the lateral column of white matter is diminishing in bulk by the withdrawal 
of the pyramidal tract. 
Still a little further forward, the anterior horn seems at first sight to 
have wholly disappeared (Fig. 132, 3 and 4), but its disappearance is coin- 
cident with an increase of the reticular formation in the position of the 
lateral columns, as well as with the growth of tissue mentioned above between 
the anterior fissure and the central gray matter. In fact, between the ante- 
rior pyramids on the ventral side and the largely increased and laterally ex- 
panded gray matter on the dorsal side, a large area of peculiar tissue now 
extends on each side for a considerable distance from the middle line of the 
raphe, encroaching on what was the lateral column of white matter; and a 
corresponding area of similar tissue may be traced from this level through 
the higher parts of the bulb up into the pons and crura cerebri. The tissue 
consists of nerve-fibres running transversely, longitudinally, and in other 
directions, so as to form a network,the bars of which are often curved ; and 
with these fibres are found branched nerve-cells in considerable number, 
some of them small, both fibres and cells being as elsewhere imbedded in 
neuroglia. Though differing from the ordinary gray matter of the cord by 
the more open character of its network, it may be considered as a form of 
gray matter. We may consider it as being in reality the gray matter of the 
apparently lost anterior horn broken up and dispersed by the passage of a 
large number of fibres and bundles of fibres, especially of the decussating 
fibres spoken of in § 518, which since they curve through this area from the 
middle line laterally are called arcuate or arciform fibres, internal arcuate 
fibres (Fig. 132, 6, f. a. i.) to distinguish them from the external arcuate 
fibres (f. a. e.) of which we shall speak presently, Fragments of more 
compact gray matter also belonging probably to the anterior horn are seen 
at intervals in this area (Fig. 132, 6, ac.) and elsewhere. We have seen that 596 
THE BRAIN. 
nearly all the way along the cord the gray matter of the neck of the poste- 
rior horn is similarly broken up by bundles of fibres into what we there called 
the reticular formation (Figs. 121, 122, r. f. p. and r. f. /.) ; and this area in 
the bulb though it possesses characters of its own is also called the reticular 
formation. In the more lateral portion of this formation, the network is 
more open and irregular, the fibres are finer, and the nerve-cells are more 
abundant than in the median portion where the nerve-cells, except in the 
immediate neighborhood of the raphe, are less numerous or even absent, and 
the fibres are coarser. These two parts are sometimes distinguished as the 
outer or lateral and the inner or median formation. In the middle line the 
fibres distinctly interlace and decussate in an oblique manner, some running 
nearly vertically in the dorso-ventral plane, thus constituting, as we have 
said, a thick raphe, which, however, at its edges gradually mergesin to the 
more open network. 
§ 522. Within the area, bounded by the pyramids ventrally, the ex- 
panded gray matter dorsally, the raphe in the middle line, and the white 
matter laterally, certain distinct compact masses of gray matter make their 
appearance, as we pass upward toward the pons. 
One of the most important of these gives rise to the olivary body, or in- 
ferior olive, which, as we have seen, projects as an oval mass (Fig. 131, ol.) 
on each side of the pyramids, reaching from a level which is somewhat 
higher up than the lower limit of the pyramids, almost but not quite to the 
pons. The olivary body, as a whole, consists partly of white matter, that is, of 
fibres, and partly of gray matter, sometimes called the olivary nucleus. This 
latter is disposed in the form of a hollow flask or curved bowl, with deeply 
folded or plaited walls, having a wide open mouth directed inward toward 
the middle line, and forward toward the pons (Fig. 132, 4, 5, 6, ol.). The 
flask is filled within by white matter, and covered up on its outside with 
white matter as well as traversed by fibres. The gray matter thus forming 
this flask-shaped nucleus consists of small rounded nerve-cells, lying in a 
bed of tissue which is partly ordinary neuroglia, and partly a fine nervous 
network. 
Lying to the median side of the olivary body, immediately dorsal to the 
anterior pyramid is another small mass of gray matter, in the form of a disc, 
appearing in transverse sections as a thick bent rod, in some sections consist- 
ing of two parts (Fig. 132, 4, ol. a.). This is the accessory olivary nucleus. A 
very similar body lies dorsal to the olivary nucleus, in the lateral reticular 
formation ; this is also called an accessory olivary nucleus, being distinguished 
(Fig. 132, 6, ol. e.) by the name outer accessory nucleus from the above-men- 
tioned inner accessory nucleus. It will be observed in these transverse sec- 
tions that the inner accessory nucleus is separated from the olivary nucleus 
by a bundle of white fibres (Fig. 132, 4, 5, 6, XII.) which, running ventrally 
from the gray matter in the dorsal region, comes to the surface between the 
anterior pyramids and the olivary body. This is the hypoglossal or twelfth 
cranial nerve. 
On the surface of the anterior pyramid itself is seen on each side a small 
mass of gray matter (Fig. 132, 5, 6, n. a.), which since it appears to be con- 
nected with a system of superficial transverse fibres, which we shall describe 
directly as the external arcuate fibres (Fig. 132, 3, 4, 5, 6,/. a. e.), is called 
the arcuate nucleus. It seems to belong to the same group as the accessory 
olives. 
Lastly, a small somewhat diffuse collection of gray matter is seen in sec- 
tions as a rounded mass of irregular form placed laterally to the reticular 
formation (Fig. 132, 4, 5, 6, a. I. n.). This, which at its first appearance 
seems to be budded off' from the general mass of gray matter (Fig. 132, 3, THE BULB. 
597 
a. I. nJ) and which is probably a detached portion of the base of the anterior 
horn or of the lateral region of the gray matter, is called the lateral or 
antero-lateral nucleus. 
Hence, besides the diffuse reticular formation, this ventral part of the 
bulb contains more sharply defined collections of gray matter in the olivary 
nucleus, aud the other bodies just mentioned. 
§ 523. We must now turn to the dorsal part of the bulb. Here in the 
first place we must distinguish between the portions of gray matter which 
are more immediately connected with the cranial nerves taking origin from 
this part of the bulb, and the portions which have no such obvious connec- 
tion. In the spinal cord, the anterior horns supply, as we have seen, the 
origins of the successive anterior motor nerves ; but in the transformation of 
the cord into the bulb the anterior horns have been broken up or displaced ; 
and the parts of the anterior horns serving as the nuclei of origin for motor 
nerves have been translated from the ventral to the more dorsal regions. 
Hence, it is in the more dorsal part of the gray matter that we have to seek 
for the nuclei of origin not only of afferent but also of motor cranial nerves. 
It will be convenient to consider all these nuclei of origin of cranial nerves 
by themselves, and we may here confine ourselves to the gray matter of other 
nature. We may, however, say that these nuclei from that of the third 
nerve backward are more or less closely associated with the gray matter 
immediately surrounding the central canal. This central gray matter, in the 
narrow sense of the term, is marked out somewhat low down (Fig. 132, 3) 
by the fibres of the sensory decussation which sweep round it; it appears in 
sections higher up as a fairly distinct region (Fig. 132, 4) ; and it is this 
part of the gray matter which is exposed on the floor of the fourth ventricle 
when the central canal (Fig. 132, 5, 6) opens out into that space. We say 
exposed ; but in reality the true gray matter is covered by a superficial layer 
of tissue of a peculiar nature (indicated in Fig. 132, 5, 6, by a thick black 
line) similar to that which is found at the hind end of the conus medullaris 
in the spinal cord. 
We saw that at the level of the first cervical nerve coincident with the 
horizontal flattening out of the posterior horns the posterior columns assumed 
very large dimensions. In this region (Fig. 132, 1) they consist entirely of 
white matter-that is, of longitudinal fibres. 
At a little higher level, however, at the level of the middle of the decus- 
sation for example, an islet of gray matter (Fig. 132, 2, m. p. nJ) makes its 
appearance in the median posterior column. A little further forward, at the 
level of the established pyramids, it will be seen (Fig. 132, 3) that this islet 
is the hind end of an invasion from the more centrally placed gray matter, 
and that at the same time there has taken place a similar inroad of gray 
matter into the external posterior column (Fig. 132, 3, e. p. nJ) ; indeed, a 
slight extension of gray matter into the external posterior column may be 
seen even before this (Fig. 132, 2, e. p. nJ). It will further be observed that 
these gray masses have so largely encroached on the white matter that both 
the median posterior or fasciculus gracilis and the external posterior column 
or fasciculus cuneatus, instead of being simply tracts of white fibres, as they 
were in the hinder part of the bulb and in the cord, have now become 
columns of gray matter covered by a relatively thin layer of white fibres. 
These columns of gray matter are now called respectively the median poste- 
rior nucleus or nucleus fasciculi gracilis, or, more shortly, the gracile nucleus; 
and the external posterior nucleus, or nucleus fasciculi cuneati, or the cuneate 
nucleus. From the ventral aspect of these nuclei a larger number of fibres 
pass ventrally, with a more or less curved course, to form, as we have seen 
(§ 518), the superior decussation and to pursue certain paths through the 598 
THE BRAIN. 
reticular formation, of which we shall speak later on. It is at this level and 
for some little distance above (Fig. 131, 4, 5) that these nuclei acquire their 
greatest development. Further forward (Fig. 132, 6), when the fourth ven- 
tricle has opened out and the nuclei of the cranial nerves are becoming con- 
spicuous, and the posterior columns have been thrust aside laterally, both 
these nuclei have diminished in size; still further forward they become still 
smaller, and toward the pons they gradually disappear. 
The mass of gelatinous substance, forming at the level of the first cervical 
nerve the swollen caput of the horn close to the surface but separated from 
it by a band of fibres ( Va.) of fine calibre, to which we have already referred 
as belonging to the fifth cranial nerve, increases in bulk at a somewhat 
higher level (Fig. 132, 2, 3, s. y.) and forms on the surface a slight projec- 
tion, called the tubercle of Rolando. It soon, however, becomes thrust ven- 
trally by the divergence of the posterior columns, and more and more covered 
up by the fibres which are going to form the increasing restiform body (Fig. 
132, 4, 5, 6, R). Retaining this position the islet of gelatinous substance 
diminishes in size further forward (Fig. 133,«. g.fi and eventually disappears. 
§ 524. The fibres of the bulb. It is obvious, from what has already been 
said, that the arrangement into posterior, lateral, and anterior columns, so 
clear and definite in the spinal cord, becomes broken up in the bulb ; indeed 
it will be best, in treating of the bulb, not to attempt to trace out these 
Fig. 133. 
Through the Bulb just Behind the Pous. ^Sherrington.) Taken in the line 110, Fig. 131. Py. 
pyramids; R. restiform body ; Cbm. cerebellum; F. fillet; f. a. e. external,/, a. i. internal arcuate 
fibres; t. bundle of fibres from olive to the lenticular nucleus ; I. posterior longitudinal bundles; 
n.f. t. nucleus of the fasciculus teres; s. o. superior olive; n. c. e. nucleus centralis (the marks 
within it are sections of bundles of fibres by which it is traversed); 8. g. substance of Rolando; 
V. a. ascending root of fifth nerve; VII. a. nucleus of the seventh nerve; VIII. auditory nerve, 
chiefly the dorsal or cochlear root; VIII. oc median nucleus; VIII. (3. lateral nucleus; VIII. y. 
accessory nucleus of auditory nerve ; IX. fibres of root of ninth nerve passing through ascending 
root of fifth nerve. 
columns, but to speak of the course of the several tracts into which these 
columns may be divided. 
The direct and cross pyramidal tracts of the cord unite to form, as we 
have seen, the pyramid of the bulb, and so pass on to the pons. We need 
say nothing more at present concerning this important pyramidal strand, 
except that, as we trace it down from the pons to the spinal cord, it gives off 
to the bulb itself fibres which make connections with the motor fibres of the 
cranial nerves proceeding from this region. 
Concerning the course taken by the other less conspicuous " descending " THE BULB. 
599 
tract, the antero-lateral descending tract, our knowledge is very imperfect; 
nothing definite can be said at present. 
The cerebellar tract, occupying near to the surface a position which in the 
series of sections (Fig. 132, Cb.) appears now rather more ventral, now more 
dorsal, eventually passes into the restiform body, of which it forms a large 
part, and thus reaches the cerebellum. The antero-lateral ascending tract 
possibly also takes the same course, but this is not as yet certain. 
The median posterior tract or column, becoming the fasciculus gracilis, 
ends in the gracile nucleus ; and in a similar manner the external posterior 
column, or fasciculus cuneatus, ends in the median and lateral masses of the 
cuneate nucleus. As we have seen, the white matter of these columns dim- 
inishes as the nuclei increase; and the nuclei after absorbing, so to speak, the 
white matter diminish in turn ; the ascending degeneration observed in these 
columns stops at these nuclei. It is a suggestive fact that as these nuclei 
diminish forward the restiform body increases in bulk. 
The remaining fibres of the cord, belonging partly to the anterior column 
and partly to the lateral column, not gathered into any of the above-men- 
tioned tracts, appear to end, chiefly at all events, in the reticular formation 
of the bulb itself, though some are carried on to the higher parts of the 
brain. 
§ 525. Thus of the various tracts or strands of the spinal cord two only 
are known definitely and certainly to pass as conspicuous unbroken strands 
through the bulb to or from higher parts ; namely, the pyramidal tract to 
the cerebrum and the cerebellar tract to the cerebellum. All, or nearly all, 
the rest of the longitudinal fibres of the cord reaching the bulb end, as far 
as we know at present, in some part or other of the bulb; and we may infer 
that some or other nerve-cells of the bulb serve as relays to connect these 
fibres of the cord with other parts of the brain. 
The gracile and cuneate nuclei stand out conspicuously as relays of this 
kind, and through them the posterior columns of the cord make secondary 
connections on the one hand with the cerebellum, and on the other hand 
with various regions of the cerebrum. We have said (§ 519) that fibres 
passing ventrally from the gracile and cuneate nuclei sweep in a curved 
course through the reticular formation as the internal arcuate fibres (Fig. 
132,/. a. i.). The hindmost of these form the superior decussation already 
referred to, as seen in sections of the fore part of and in front of the pyram- 
idal decussation (Fig. 132, 3, Supra Py. dec.). After decussating ventral 
to the central canal, these fibres form an area called the inter-olivary layer 
(Fig. 132, 4, in. ol.), lying dorsal to the pyramids between two of the olivary 
nuclei. This layer may be regarded as the hind end or beginning on each 
side of a remarkable longitudinal strand called the fillet (Figs. 131, B. F, 
133, F), of the connections of which in the front part of the brain we shall 
speak hereafter. Thus these two nuclei are the source of fibres which cross 
to the other side of the bulb, and reaching the inter-olivary layer dorsal to 
the pyramids run up to higher parts of the brain by the fillet. We may 
add that the formation of the fillet is also probably assisted by fibres from a 
tract which lies just dorsal to the inter-olivary layer, and is derived from the 
anterior columns of the cord. Besides its fibres of descending degeneration 
the anterior column contains fibres of ascending degeneration, and these rise 
dorsally in the bulb to form the tract in question. Though the whole tract 
is of some length, the component fibres appear to be short. 
The gracile and cuneate nuclei give rise also to other fibres which, though 
also sweeping ventrally and crossing to the other side, do not, when they 
reach the inter-olivary region, assume a longitudinal direction, as do the 
fibres forming the fillet, but as external arcuate fibres (Fig. 132,/. a. e.) 600 
THE BRAIN. 
pursue a course which is at first ventral along the side of the anterior fissure, 
and then lateral over the ventral surface of the pyramid and olivary nucleus, 
by which path they reach the lateral surface of the bulb, and so the restiform 
body and cerebellum. In this way the two nuclei in question contribute to 
the restiform body of the opposite side of the bulb. These external arcuate 
fibres, which as they sweep round the ventral surface of the pyramid traverse 
the arcuate nucleus, though they vary much in individual brains, form a 
considerable portion of the white matter seen on the ventral and lateral sur- 
faces of the bulb; it is by them that the olivary nucleus is covered up. 
The cuneate and gracile nuclei, besides this crossed and somewhat round- 
about connection with the restiform body of the opposite side, are believed 
to have more direct connection with the restiform body of the same side by 
means of fibres which pass by a more or less direct lateral path from them to 
it. Accepting this view, we may say that the two nuclei are connected with 
the opposite side of the cerebellum by external arcuate fibres, and with the 
same side of the cerebellum by the other fibres just mentioned. In any case, 
the connection between the two nuclei and the cerebellum is large and im- 
portant. 
Thus the important strand of fibres which is called in the bulb the resti- 
form body, and higher up the inferior peduncle of the cerebellum, is con- 
nected with the spinal cord in two chief ways; directly by means of the 
cerebellar tract and indirectly by means of the cuneate and gracile nuclei, 
which, as we have said, diminish in bulk forward as the restiform body 
increases. By the relay of the gracile nucleus it is brought into connection 
with the median posterior column along the whole length of the cord, and so 
with that division of the posterior roots which (§ 490) in each of the several 
spinal nerves goes to form that column. By the relay of the cuneate nucleus 
it is brought into connection with such parts of the external posterior column 
as end in that nucleus, and thus probably with other fibres of the posterior 
roots of the upper spinal nerves. And if we admit that the cerebellar tract 
is connected, by the relay of the vesicular cylinder or by other nerve-cells, 
with the rest of the posterior roots of the spinal nerves, we may conclude 
that the restiform body is, by means of these relays, a continuation of the 
spinal posterior roots. 
The restiform body and so the cerebellum is also specially connected with 
the olivary body of the opposite side; for when in young animals one side 
of the cerebellum is removed, the olivary body of the opposite side atrophies. 
The course of the fibres maintaining this connection is not as yet accurately 
known, but they probably pass from the olivary nucleus of one side through 
the inter-olivary layer, and so laterally through the reticular formation of 
the other side. Lastly, we may add that a tract which is sometimes included 
in the restiform body as its median or inner division has quite a different 
origin from any of the above ; the fibres which compose it come, as we shall 
see, from the auditory nerve. 
The further connections of the bulb with the cerebrum it will be best to 
leave until we come to deal with the structural arrangement of the rest of 
the brain. 
Meanwhile enough has been said to show that the bulb differs very mate- 
rially in structure from the spinal cord. The gray matter of the bulb is far 
more complex in its nature than is that of any part of the cord ; and the 
arrangement of the several strands and tracts of fibres is far more intricate. 
The structural features on the whole perhaps suggest that the main functions 
of the bulb are twofold: on the one hand, it seems fitted to serve as a head- 
centre governing the spinal cord, the various reins of which, with the ex- 
ceptions noted, it holds, as it were, in its hands; on the other hand, it ap- THE GRAY MATTER. 
601 
pears no less adapted to act as a middleman between parts of the spinal cord 
below and various regions of the brain above. As we shall see, experiment 
and observation give support to these suggestions. 
The Disposition and Connections of the Gray and White 
Matter of the Brain. 
THE GRAY MATTER. 
§ 526. As we pass up from the bulb to the higher parts of the brain, the 
differentiation of the gray matter into more or less separate masses, which 
we have seen begin in the bulb, becomes still more striking. We have to 
distinguish a large number of areas or collections of gray matter, more or 
less regular in form and more or less sharply defined, from the surrounding 
white matter; to such collections the several terms corpus, locus, nucleus, 
and the like, have from time to time been given. These areas or collections 
vary greatly in size, in form, and in histological characters ; they differ from 
each other in the form, size, features, and arrangement of the nerve-cells, in 
the characters of the nervous network of which the nerve-cells form a part, 
and especially perhaps in the extent to which the more distinctly gray mat- 
ter is traversed and broken up by bundles of white fibres. Guided by the 
analogy of the spinal cord, as well as by the results of experiments and 
observations directed to the brain itself, we are led to believe that the com- 
plex functions of the brain are intimately associated with this gray matter; 
and a full knowledge of the working of the brain will carry with it a know- 
ledge of the nature and meaning of the intricate arrangement of the cerebral 
gray matter. At present, however, our ignorance as to these things is great; 
and, though various theoretical classifications of the several collections of 
gray matter have been proposed, it will perhaps be wisest to content our- 
selves here with a very broad and simple arrangement. We will divide the 
whole gray matter of the brain into four categories only: 1. The central 
gray matter lining the neural canal; and this we may consider the nuclei 
of the cranial nerves, some of which are closely associated with it. 2. The 
superficial gray matter of the roof of some of the main divisions of the brain, 
such as that of the cerebral hemispheres, and of the cerebellum. 3. The 
intermediate gray matter more or less closely connected with the crura 
cerebri. 4. Other collections and areas of gray matter. We will, more- 
over, confine ourselves at present for the most part to their general features 
and topography, reserving what we have to say concerning their histological 
characters for another occasion. 
§ 527. The ventricles of the brain, like the central canal of the spinal 
cord, of which they are a continuation, are lined by an epithelium which is 
in general a single layer of columnar cells said to be ciliated throughout, 
though it is often difficult to demonstrate the cilia. Beneath this epithelium 
lies a layer of somewhat peculiar neuroglia, forming with the epithelium, as 
we have said (§ 523), the ependyma, which, well developed in the floor of the 
fourth ventricle and in the walls of the third ventricle and of the aqueduct, 
is thin and scanty in the lateral ventricles. Beneath, and more or less con- 
nected with the ependyma in the sides and floor of the third ventricle, is a 
fairly conspicuous layer of gray matter, which is well developed in the parts 
of the floor exposed on the ventral surface of the brain, and known as the 
lamina terminalis, the anterior and posterior perforated spaces, the tuber 
1. The Central Gray Matter and the Nuclei of the Cranial Nerves. 602 
THE BRAIN. 
cinereum, etc. This layer is not continued forward into the lateral ventri- 
cles of the cerebral hemispheres, but it is well developed backward along the 
aqueduct (Figs. 136, 137), and in the floor of the fourth ventricle, and through 
the bulb becomes, as we have seen (§ 523), continuous with the central gray 
matter of the cord. The nerve-cells of this gray matter are on the whole 
small and in many places scant. 
§ 528. The several roots of the cranial nerves from the third nerve 
backward may be traced within the brain substance to special collections of 
gray matter, called the nuclei of the cranial nerves, some of which lie close 
upon the central gray matter, while others are placed at some distance from 
it. The optic nerve and what is sometimes called the olfactory nerve, namely, 
the olfactory bulb and tract, may advantageously be dealt with apart, since 
these two nerves are not, like the other cranial nerves, simple outgrowths 
from the walls of the original neural canal, but are in reality elongated 
vesicles, budded off from the neural canal, the cavities of which have been 
obliterated. We may add that part of the retina, and of the gray matter 
of the olfactory tract, may perhaps be considered as corresponding to the 
nuclei of which we are speaking, the retinal and proper olfactory fibres being 
connected with them very much as the fibres of the remaining cranial nerves 
are connected with their respective nuclei. In the brain the segmental reg- 
ularity of the nerve-roots so conspicuous in the spinal cord is very greatly 
obscured. We shall have something to say on this point later on ; but at 
present we may be content to treat the several nerves in a simple topographical 
manner. They may be seen in a ventral view of the brain (Fig. 131, C.), 
leaving the brain at various levels by what is called their "superficial 
origin the third nerve issuing in front of the pons, and the last or hypo- 
glossal stretching back to the hind end of the bulb. Part, indeed, of the 
eleventh nerve, the spinal accessory nerve properly so-called, makes con- 
nections with the spinal cord below the bulb as far back as the sixth or 
seventh cervical nerve, or even lower; but this part may by these connec- 
tions be distinguished from the remaining part of the nerve, as well as from 
all other cranial nerves. The nuclei to which the nerve-roots may be traced 
within the brain substance, sometimes spoken of as the " deep origin," range 
in position from the hinder part of the bulb to the hind end of' the third 
ventricle. The nucleus of the hypoglossal nerve begins in the bulb just 
above the decussation of the pyramids, the nucleus of the third nerve ends 
beneath the hind end of the floor of the third ventricle; and all the rest of 
the nuclei may be broadly described as placed-between these limits in various 
parts of the floor of the central canal or in adjoining structures, though part 
of one nucleus, namely, that of the fifth nerve, can be traced, as we shall see, 
back into the spinal cord as far as the second cervical nerve, and probably 
extends still further. Fig. 138 is a diagram showing in a roughly approxi- 
mate manner the nuclei of the several nerves as they would appear in a bird's- 
eye view of the floor of the aqueduct and fourth ventricle looked at on the 
dorsal aspect. 
$ 529. The twelfth or hypoglossal nerve. The nucleus of this nerve, which 
it will be convenient to take first (Fig. 138, XII.}, is a long column of gray 
matter lying in the bulb parallel to, and very close to, the median line. It 
reaches from the hinder part of the fourth ventricle at about the level of the 
hind end of the auditory nucleus, as far back as beyond the hind end of 
the olivary body. At its extreme hind end or beginning (Fig. 132, 2) 
it occupies a ventral position and is a part of the anterior horn ; thence 
it gradually rises dorsally (Fig. 132, 3, 4, 5), but so long as the central 
canal remains closed, continues to occupy a distinctly ventral position in 
reference to the central canal; in its front part it is, by the opening up THE GRAY MATTER. 
603 
of the fourth ventricle, brought into an apparently more dorsal position 
(Fig. 132, 6). 
The nucleus consists mainly of large nerve-cells with distinct axis-cylinder 
processes, which though pursuing a somewhat irregular course may be traced 
into the fibres of the nerve. These, starting from the ventral surface of the 
nucleus along its length, run ventrally through the reticular formation, and 
making their way in a series of bundles, between the olivary nucleus on the 
lateral side and the pyramid and median accessory olive on the median side, 
gain the surface along the groove which separates the pyramid from the 
olivary body. 
§ 530. The ninth or glosso-pharyngeal, tenth or vagus, and eleventh or 
spinal accessory nerves. It will be advantageous to consider these three 
nerves together. 
In the spinal accessory nerves we must distinguish, as we have said, two 
parts: the "spinal accessory" proper, formed by the roots which come off 
from the cervical spinal cord, reaching as far down as the sixth or seventh 
cervical nerve, and the " bulbar accessory," whose roots come off from the 
bulb just below the vagus. 
The spinal accessory proper takes origin in the group of cells lying in the 
extreme lateral margin of the anterior horn, from whence the fibres proceed 
directly outward through the lateral column, and issue from the cord along 
a line immediate between the anterior and posterior roots; the upper roots 
undergo, with the portion of the lateral horn from which they spring, the 
shifting spoken of in § 518. 
The bulbar accessory starts from an elongated nucleus in the bulb which 
is common to it, to the vagus, and to the glosso-pharyngeal; hence we have 
taken these three nerves together. This (Fig. 138) stretches further for- 
ward than the hypoglossal nucleus, reaching the level of the transverse fibres 
called striae acustieae (str.), but does not extend so far behind. 
In transverse sections of the bulb, which pass a little below and a little 
above the point of the calamus scriptorius (Fig. 132, 4, 5), two nuclei or 
collections of cells are seen in the gray matter round the central canal. 
The more ventral one is the hypoglossal nucleus, the more dorsal one the 
beginning or hind part of the combined accessory-vago-glosso-pharyngeal 
nucleus. 
When a little further forward the central canal opens out into the fourth 
ventricle (by which change the hypoglossal nucleus (Fig. 132, 6 n. xii.) is 
brought nearer to the dorsal surface in the floor of the fourth ventricle) this 
combined nucleus, increasing in breadth, is thrown to the side and assumes a 
more lateral position, lying now on the side of, but still somewhat dorsal to, 
the hypoglossal nucleus, between it and the now diminishing gracile nucleus. 
In this position the nucleus appears to consist of two parts, a median and 
lateral, the median part having conspicuous nerve-cells of moderate size, the 
lateral part having but few cells and those of small size. From this level 
the nucleus runs forward, maintaining nearly the same position in the ffooi* 
of the fourth ventricle, but gradually becoming thinner, and ends, as we have 
said, at about the level of the striae acustieae on the dorsal surface correspond- 
ing on the ventral surface to a level a little behind the hind margin of the 
pons. 
From this combined nucleus, but chiefly from the median part, fibres 
sweep in a ventral and lateral direction through the dorsal part of the retic- 
ular formation, pass ventral to, or in some cases through, the gelatinous sub- 
stance and the strand of fibres connected with the fifth nerve (Fig. 132, F. 
a), and reach the surface of the bulb on its lateral aspect in a line between 
the olivary and restiform bodies (Fig. 131, C.). Along this line maybe seen 604 
THE BRAIN. 
(Fig. 131, C.), a series of roots ; of these the lowest, the accessory roots, spring 
from the hind part, the highest, the glosso-pharyngeal roots, from the front 
part (and it is these especially which pierce the gelatinous substance-Fig. 
133, IX. a), and the intermediate, the vagus roots, from the middle part of 
the combined nucleus. Hence we may speak of the hind part of the whole 
nucleus as being the accessory nucleus, the middle part as the vagus nucleus, 
and the front part as the glosso-pharyngeal nucleus. 
All the fibres, however, of the roots of these three nerves do not take 
origin from the nucleus in question ; some of the fibres start in a different 
way. In sections of the bulb above the decussation of the pyramid a 
patch of gray matter is seen lying in the lateral part of the reticular 
formation (Fig. 132, X. m.), about midway between ventral and dorsal sur- 
faces. What is thus disclosed by sections is a column of gray matter, the 
"nucleus ambiguus " (Fig. 138, no.), stretching about as far forward and 
backward as the combined accessory-vago-glosso-pharyngeal nucleus, but 
placed distinctly more ventrally and somewhat more laterally. (In Fig. 
138, it and the combined nucleus are represented on different sides of 
the diagram to avoid confusion through the overlapping of the shad- 
ing.) From it fibres curve round (Fig. 132, X. m.) to join the accessory- 
vago-glosso-pharyngeal roots, but especially the vagus roots. It may, 
therefore, be considered as a second nucleus of the vagus (and possibly 
of the other) roots. 
But there is yet a third source of some of the fibres of the nerves of which 
we are speaking. In sections through the bulb there may be seen just ven- 
tral to and a little lateral to the combined nucleus (Fig. 132, 4, 5, 6, IX. a.) 
the circular section of a longitudinal bundle of fibres. In the hinder sections 
(Fig. 132, 4) the bundle is a very thin one, and still further back it is lost to 
view, though there are reasons for thinking that some of the fibres are con- 
tinued back into the cervical cord, as far as the origin of the fourth cervical 
nerve or even beyond ; in the more forward sections (Fig. 132, 5 and 6) it 
increases in diameter and may be traced forward to the front end of the com- 
bined nucleus into which it merges. It is a bundle of fibres which, starting 
successively in the lateral gray matter of the cervical cord and higher up in 
the reticular formation of the bulb, run longitudinally forward ; the bundle 
at first increases in size by the addition of fresh fibres at each step; but fur- 
ther forward the fibres leave the bundle to pass into the roots of the nerves 
of which we are speaking, especially of the glosso-pharyngeal, and the bundle 
eventually ends in front by passing into the glosso-pharyngeal roots. The 
gray matter from which these fibres take origin does not form a defined com- 
pact area, is not, therefore, a nucleus in the sense in which we are now using 
the term, but is diffused among the rest of the gray matter along a consider- 
able length. The fibres are, nevertheless, fibres of nerve-roots, and the bundle 
is called the ascending root of the glosso-pharyngeal, the term ascending being 
used since it is customary to trace such structures from below upward, that 
is, from behind forward ; though since the fibres in question are probably 
afferent fibres carrying impulses backward from the nerves to the gray 
matter, " descending " would be the more appropriate word. The bundle 
has also been called the fasciculus solitarius; and, since its position has 
been supposed to correspond to that of the area marked out experimentally 
as the respiratory centre (§ 304), it has been spoken of as the respiratory 
bundle. 
The roots of these three nerves, then, the bulbar accessory, the vagus, 
and the glosso-pharyngeal-all leaving the surface of the brain along the 
line between the olive and the restiform body, and all so far alike that it 
is impossible upon mere inspection to say where in the series the fibres of THE GRAY MATTER. 
605 
the middle nerve, the vagus, begin and end-spring from three sources, the 
combined nucleus, the nucleus ambiguus, and the ascending root. 
§ 531. The eighth or auditory nerve. This nerve differs from the other 
nerves which we are now considering, in being a nerve of special sense ; its 
arrangements are complicated. In a view of the base of the brain (Fig. 
131, C) the nerve is seen to leave the surface of the brain from the ventral 
surface of the fore part of the restiform body at the hind margin of the pons 
as two strands or roots, one of which winds round the restiform body so as to 
reach its dorsal surface, while the other appears to sink into the substance of 
the bulb to the median side of the restiform body ; and in a transverse sec- 
tion of the bulb (Fig. 133), just behind the pons, the two roots may be seen 
embracing the restiform body, one passing on its dorsal and the other on its 
ventral side. The former is called the dorsal root (Fig. 133), or sometimes 
the lateral root, or, since it reaches back or lower down than the other, the 
posterior or inferior root; the latter is called the ventral root (Fig. 135), or 
sometimes the median root, or, since it reaches further forward or higher up 
than the other, the anterior or superior root. When we come to study the 
ear we shall find that one division of the auditory nerve is distributed to the 
cochlea alone, and is called the nervus cochlearis, the rest of the nerve being 
distributed to the utricle, saccule and semicircular canals as the nervus ves- 
tibularis. As we shall see, there are reasons for thinking that the vestibular 
nerve carries up to the brain from the semicircular canals impulses other 
than those, or besides those, which give rise to sensations of sound, whereas, 
the cochlear nerve appears to be exclusively concerned in hearing; and in 
some structural details these two divisions of the auditory nerve differ from 
each other. Hence it is important to note that according to careful inves- 
tigations the cochlear nerve is the continuation of the dorsal root and the 
vestibular nerve the continuation of the ventral root. With these roots of 
the auditory nerve proper also issues, a little in front of the ventral root, the 
small nerve called the portio intermedia Wrisbergi, which goes to join the 
facial nerve. 
The auditory nucleus, as a whole, is a broad mass, having in transverse 
sections of the bulb a somewhat triangular form, lying in the lateral parts 
of the floor of the fourth ventricle, reaching in front somewhat beyond the 
level of the striae acusticae, and overlapping behind the front parts of the 
nucleus ambiguus and the combined accessory-vago-glosso-pharyngeal nu- 
cleus : it extends laterally some distance outside the former nucleus. 
The nucleus, however, consists of two distinct parts, a median or inner 
nucleus (Fig. 138, VIII., ml), characterized by the presence of small cells, 
and a lateral or outer nucleus (Fig. 138, VIII., /.), the cells of which are 
much larger, some of them being very large. The lateral nucleus is placed 
somewhat deeper than, ventral to, the median nucleus ; it also extends farther 
forward (Figs. 133 and 134, VIII., (i), so that the front end of the whole 
nucleus is furnished by the lateral nucleus alone, which at its front end 
occupies a more dorsal position than at its hind end. 
Moreover, this auditory nucleus thus placed in the floor of the fourth 
ventricle is not the whole of the nucleus of the auditory nerve. At the 
convergence of the dorsal and ventral roots on the ventral surface of the 
restiform body is placed a group of cells, forming a swelling which, in its 
general appearance and in the characters of its cells is not unlike a gan- 
glion on the posterior root of a spinal nerve. This is called the accessory 
nucleus. 
When we trace the fibres of the nerve centralward into the brain, we find 
that a large number, at least, of the fibres of the dorsal root, cochlear nerve 
(Fig. 133), end, according to most observers, in the cells of the accessory 606 
THE BRAIN. 
nucleus or in nerve-cells lying dorsal to the accessory nucleus, and especially 
in a group of cells giving rise to the tuberculum acusticum, which, small in 
man, is conspicuous in some animals. Hence, the farther part of this dorsal 
root, as it winds round the lateral and dorsal surface of the restiform body, 
consists largely, if not wholly, of fibres which are derived not directly from 
the trunk of the nerve, but indirectly through the relay of the accessory 
nucleus or of other cells. Reaching the dorsal surface of the restiform body, 
these fibres appear on the floor of the fourth ventricle as the strice acusticce 
(Fig. 131, str.), and end partly in the median nucleus, partly in other regions 
of the bulb. The exact determination, however, of the endings of this root 
Fig. 134. 
Through the widest part of the Fourth Ventricle. (Sherrington.) Taken in the line 111, Fig. 
131. Py., pyramidal fibres cut transversely; tr. P., the superficial (ventral) transverse fibres of the 
pons. The shaded part of the pons (gr. P.) indicates gray matter mingled with the deeper trans- 
verse fibres. F., the fillet; Tp., the trapezium ; C. R. the restiform body or inferior peduncle of 
the cerebellum, cut across obliquely : S. P., the superior peduncles of the cerebellum ; r., raphe ; 
s. o., superior olive; C. IP corpus dentatum of the cerebellum ; Rf. n., the nucleus of the roof. 
s. g., tubercle of Rolando ; V. S„ section through sulcus in the vermis superior of the cerebellum , 
t., bundle from the olive to the lenticular nucleus • VIII., the eighth or auditory nerve, its ventral 
or vestibular root proceeding from VIII. 0. the front part of the lateral auditory nucleus ; VII. n., 
the nucleus of the seventh or facial nerve; VI., the nucleus of the sixth nerve ; VII. g., fibres of 
the seventh nerve cut across as they sweep round the nucleus of the sixth before issuing from 
the pons at VII.; 4th, the fourth ventricle, here roofed in by the cerebellum ; the shading of 
the central gray matter immediately surrounding the ventricle is, for the sake of simplicity 
omitted. 
is a matter of considerable difficulty; some observers regard the accessory 
nucleus as homologous; not with the Gasserian and with the spinal ganglia, 607 
THE GRAY MATTER. 
but with the other, true, cranial nuclei; and in any case we must probably 
consider the median division of the auditory nucleus, not as a nucleus in the 
sense in which we are now using it, but rather as a secondary connection 
within the bulb. 
When we trace the ventral root, vestibular nerve (Fig. 134), inward, we 
find that it makes, according to most observers, no connections at all with the 
accessory nucleus; and that it passes (Fig. 134, VlHyio the median side of the 
restiform body, between it and the ascending root of the fifth nerve, and so 
reaches the lateral division of the nucleus, in the large cells of which most 
at least; of its fibres are said to end, and which, therefore, may be regarded 
as the nucleus of the ventral root. On this point, however, all authors are 
not agreed. The lateral auditory nucleus, with the fibres proceeding to and 
from it, lying as they do to the median or inner side of the restiform body 
proper, are sometimes spoken of as the median or inner division of the resti- 
form body; and from the nucleus a considerable number of fibres pass up 
with the restiform body into the cerebellum as a continuation of this 
" median division of the restiform body." Some authors maintain that these 
fibres are continued straight on from the nerve to the cerebellum ; but the 
more recent investigations seem to show that they all make connections with 
the nerve-cells of the lateral nucleus on their way. These fibres constitute 
a connection between the auditory (vestibular) nerve and the cerebellum, 
the physiological significance of which we shall see later on ; we may, per- 
haps, compare it to the connection between the posterior roots of the spinal 
nerves and the cerebellum through (the vesicular cylinder and) the cere- 
bellar tract. 
The other central connections of the lateral nucleus are, like those of 
the accessory and of the median nucleus, complicated and obscure. But we 
may call attention to a set of fibres which, starting apparently in the acces- 
sorynucleus, run directly transverse in the ventral region of the tegmentum 
just dorsal to the transverse fibres of the pons, forming what is called the 
trapezium (Fig. 134, Tp.\ 
Lastly, we may add that the fibres of the peculiar portio intermedia 
appear to take origin from the accessory nucleus. 
§ 532. The seventh or facial nerve. The nucleus (Fig. 138, VII., and 
Figs. 133, 134, VIL nf of this nerve (it being borne in mind that the motor 
fibres for the orbital region (the orbicular muscle, etc.), though they run in 
the trunk of this nerve, really belong to the third nerve and take origin 
from the hind part of the nucleus of the third nerve) is narrower in front 
than behind, reaches from the level of the striae acusticae some distance into 
the region of the pons, and occupies in the midst of the reticular formation, 
a little dorsal of the patch of gray matter called the upper olive, a position 
corresponding closely to that of the nucleus ambiguus. The cells of the 
nucleus are large, and possess well-marked axis-cylinder processes, which 
are gathered up at the dorsal surface of the nucleus to form the root. This, 
rising up dorsally, describes a loop (Fig. 134, VII g.) round the nucleus of 
the sixth or abducens nerve, running forward for some little distance dorsal 
to that nucleus, and then descends again ventrally, passing to the lateral 
side of its own nucleus, between it and the ascending root of the fifth ( F.a.) ; 
it thus gains the surface of the brain at the hinder margin of the pons, 
lateral to the abducens, opposite the front end of the groove between the 
olivary body and the restiform body. As it thus encircles the nucleus of 
the abducens, it looks as if it were receiving fibres from that body ; but the 
evidence goes to show that these fibres simply pass through the nucleus, and 
do not take origin from any of its cells. 
§ 533. The sixth or abducens nerve. This nerve starts from a compact oval 608 
THE BRAIN. 
nucleus (Fig. 138, VI.), lying at the level of the hinder part of the pons, 
and therefore of the front part of the fourth ventricle, in the central gray 
matter of the floor of the ventricle, or rather just between it and the reticular 
formation, a little on one side of the median line (Fig. 134, VI.). A slight 
swelling of the floor of the fourth ventricle, eminentia teres, marks its posi- 
tion (Fig. 138, e. t.). The nucleus contains fairly large nerve-cells, with dis- 
tinct axis-cylinder processes. These are gathered at the median side of the 
nucleus to form the thin root, which, passing ventrally and laterally, at some 
little distance from the median raphe, through the reticular formation, runs 
backward above the pyramidal bundles of the pons, and finally comes to 
the surface at the hinder edge of the pons, opposite the front end of the 
pyramid (Fig. 131, C.). 
Fig. 135. 
Through the Pons at the Exit of the Fifth Nerve. (Sherrington.) In the line 112, Fig. 131. 
C. R. remains of restiform body; S. P. superior peduncle of the cerebellum ; F. m. median; F. I. 
lateral fillet; T. R., tegmental reticular formation; tr. P. superficial transverse fibres of the pons ; 
I. posterior longitudinal bundles ; V. s. superior vermis (sections of three folia are shown, one 
being detached; between them the intervening sulci laid open by the section are seen) ; I7a. 
valve of Vieussens or anterior velum ; r. raphe; Py. pyramidal fibres ; yr. P. gray matter of the 
pons ; s. o., superior olive ; t., placed on the left side indicates the position of a bundle of longi- 
tudinal fibres which may be traced forward into the subthalamic regions ; V. m. motor nucleus; 
V. s„ sensory nucleus; and V. roots of the fifth nerve. 4th, fourth ventricle; shading of cen- 
tral gray matter omitted as in Fig. 134. 
§ 534. The fifth or trigeminal nerve. This nerve, as it comes to the sur- 
face on the ventral aspect of the pons (Fig. 131, C.), near the front edge, 
at some distance from the median line, consists of two parts, a smaller motor THE GRAY MATTER. 
609 
root and a larger sensory root, the latter bearing the large ganglion of Gas- 
ser ; and the origin of the nerve is in many ways complex. Both roots 
may be traced in an oblique direction (Fig. 135, F.), inward and toward 
the dorsal surface, through the pons to the reticular formation beneath the 
floor of the front part of the fourth ventricle, the smaller motor root taking 
up a position median to the larger sensory root. 
Here the motor root comes into connection with a collection of nerve- 
cells (Figs. 138 and 135, F. m.\ which may be regarded as its nucleus; but 
this is not the whole nucleus of the motor root. From the level of the 
nucleus there stretches forward as far as the level of the anterior corpora 
quadrigemina a bundle of longitudinal fibres which, since it is usually traced 
from the front backward until it passes into the root of the nerve, is spoken 
of as the descending root of the fifth nerve. 
This descending root begins as a few scattered bundles of fibres at the 
level of the anterior corpora quadrigemina, in the peripheral lateral part 
of the central gray matter surrounding the aqueduct, dorsal, and lateral 
(Fig. 137, F. d.), to the nucleus of the third nerve (Fig. 137, III. n.). 
From thence the fibres pass backward, augmenting in number, and soon 
form a compact bundle, semilunar in transverse section, lying lateral to the 
fourth nerve as this is rising dorsally (Fig. 136, F. d.) ; still increasing in 
Fig. 136. 
Through the Fore Part of the Pons. (Sherrington.) In the line 113, Fig. 131. Py. pyramidal 
fibres ; F. C. fibres from the frontal cortex ; S. P. superior peduncle of the cerebellum; Fm. me- 
dian portion; Fl. lateral portion of the fillet; I. posterior longitudinal bundles; P. C. Q. posterior 
corpora quadrigemina ; y. fibres which become detached from the fillet, and further forward form 
(the innermost) part of the pes of the crus; I. c. locus cseruleus; n. P. Q. nucleus of the posterior 
corpora quadrigemina-the outline is made too sharp; IV. bundles of the fourth nerve decussat- 
ing, IV. n. its nucleus ; V. d. descending root of the fifth nerve; Aq. the aqueduct; c. g. the region 
of central gray matter. 
number in their course backward, they gradually assume a more ventral 
position as the aqueduct opens into the fourth ventricle. All along its 
course this descending root has attached to it large (70m or more in diam- 
eter), sparse spheroidal nerve-cells of striking appearance ; these, however, 
seem too few to give origin to all of the fibres, and there are some reasons 610 
THE BRAIN. 
for connecting this root with the collection of gray matter called " locus cse- 
ruleus." (Fig. 136, I. c.). 
We may probably regard this descending root as belonging to the motor 
division of the nerve ; but it is stated that many of the fibres of this root 
pass into the sensory root, eventually finding their way, according to some 
observers, into the ophthalmic branch. 
The sensory root may be similarly traced into a nucleus, the sensory 
nucleus (Figs. 138 and 135, V. s.) lying lateral to the motor nucleus, and 
connected with this is the striking tract of fibres, to which already we have 
so frequently alluded, and which is called the ascending root of the fifth 
nerve. 
This ascending root begins as a bundle or bundles of few fibres which 
may be traced backward as far, at least, as the level of the second cervical 
nerve, and is soon conspicuous in transverse sections (Figs. 132 et seq., V. a.) 
as a semilunar patch of white matter forming a sort of cap on the outside of 
the swollen caput of the posterior horn, between this structure and the 
longitudinal fibres which are beginning to form the restiform body on the 
Fig. 137. 
Through the Crus and Anterior Corpora Quadrigemina. (Sherrington). One-half only is shown 
in the line 114, Fig. 131. Py. the pyramidal portion of the pes; Fr. the region of the pes occupied 
by fibres from the frontal portion of the cortex; Pr. O. the region occupied by fibres coming from 
the occipital portion of the cortex ; y. fibres coming from the fillet; Op. the optic tract; F. the 
fillet, I. the lateral portion, m. the median portion; I. the posterior longitudinal bundle; B. a. the 
brachium of the anterior corpus quadrigeminum ; x. fibres from the posterior commissure of the 
cerebrum ; r. raphe ; S. n. substantia nigra; R. n. red nucleus; C. g. I. lateral, and C. g. m. median 
corpus geniculatum ; Pvr. pulvinar of optic thalamus ; A. Q. n. nucleus or gray matter of anterior 
corpus quadrigeminum ; III. n. nucleus of III. third nerve ; III.' rootlets from the dorsal part of 
III. n. the nucleus of the third nerve, which cross the median line to emerge with rootlets derived 
from the nucleus of the opposite side; s. m. superficial layer of fibres of the ant. corp, quad.; d. 
m. deep layer; Aq. aqueduct surrounded by cerebral gray matter. 
surface. Passing upward, and continually augmenting in bulk, the root 
clings, as it were, to the gelatinous substance of the caput of the posterior 
horn, and sinks with it inwardly and ventrally as this becomes covered up 
first by the restiform body and subsequently by the issuing trunk of the 
great eighth or auditory nerve (Figs. 133, 134). Passing still forward, be- 
yond the disappearing gelatinous substance, the root, still growing larger 
and divided into several distinct bundles, runs into the reticular formation 
of the pons and, reaching the level of the sensory nucleus, suddenly bends 
rounds and joins the sensory root. 611 
THE GRAY MATTER. 
Fig. 138. 
Diagram to illustrate the Position of the Nuclei of the Cranial Nerves. (Sherrington.) The 
brain is supposed to be viewed from the dorsal aspect, the cerebral hemispheres and cerebellum 612 
THE BRAIN. 
This ascending root differs from the descending root in not having con- 
spicuously attached to it any collection of nerve-cells; in this respect it 
resembles the ascending root of the glosso-pharyngeal, and we may add part 
of the posterior root of an ordinary spinal nerve, the fibres of which, as we 
have seen, pass into the gray matter without being obviously connected with 
nerve-cells. In its lower part at least it consists of extremely fine fibres, and 
indeed looks very much like a continuation of the bulb of the marginal 
(Lissauer's) zone of the spinal cord. 
§ 535. The fourth or trochlear nerve. The nucleus of this nerve (Fig. 
138, IV.} is a column of somewhat large multipolar cells on each side of the 
median line below the aqueduct (Fig. 136, IV. n.}, reaching from the level 
of the junction of the anterior and posterior corpora quadrigemina to the 
hinder level of the latter body. 
The root, starting from the lateral surface of the nucleus, does not take 
at first a ventral direction, but sweeps laterally and dorsally in the outer 
layers of the central gray matter (Fig. 136), and so curving round to the 
dorsal surfaces reach the valve of Vieussens, where in the median line it 
decussates with its fellow in the substance of the valve; such a decussation 
at a distance from the nucleus of origin is exceptional in the cranial nerves. 
Leaving the surface of the brain in the valve, it takes a superficial course, 
curving (Fig. 131, B} laterally and ventrally, and makes its appearance in 
a ventral view of the brain at the front edge of the pons, on the lateral edge 
of the crus (Fig. 131, C.}. 
§ 536. The third or oculo-motor nerve. The nucleus of this nerve (Fig. 
138, III., 137, III. n.} is a column of, for the most part, fairly large multip- 
olar cells lying on each side close to the median line, in the gray matter of 
the central canal, just dorsal to a bundle of fibres which we shall speak of 
as the longitudinal posterior bundle; it reaches from the level of the pos- 
terior commissure in the third ventricle to the level of the junction of the 
anterior and posterior corpora quadrigemina. In a section taken through 
its middle (Fig. 137) the nucleus is seen to give off fibres which run verti- 
cally toward the ventral surface, traversing the tegmentum and a body (Bn.} 
having been cutaway. The nuclei are represented as if seen through transparent material. On 
the right side, the corpus striatum and optic thalamus have been cut away horizontally to some 
little depth in order to show their internal structure. L. lateral, E. P. external posterior, and M. 
P. median posterior column of the cord ; I. P. inferior peduncle, S. P. superior peduncle, and P. 
middle peduncle of the cerebellum, all cut across. The dotted curved lines, upper and lower, on 
the right half of the figure to which the dotted line P. V. outside the figure points, mark the 
upper and lower boundaries of the pons on the ventral aspect. The outline of the fourth ven- 
tricle is shown by a bold thick line. In the floor of the ventricle are shown, on the right half: 
fp. fovea posterior; Th. trigonum hypoglossi; T. ac. trigonum acusticum ; e. t. eminentia teres: 
s. m. striae medullares on acusticae ; /. a. fovea anterior; I. c. locus caeruleus; I. g. valve of Vieus- 
sens; Qp. posterior, and Qa. anterior corpus quadrigeminum ; Pg. pineal gland ; Nr. the outline 
of the red nucleus; 3, the third ventricle, in which C. indicates the middle or soft commissure; 
F. p. a. the pillars of the fornix, behind which is indicated in the cavity of the third ventricle the 
hollow of the infundibulum ; C. C. g. the genu of the corpus callosum, between which and the 
fornix the cavity often called the fifth ventricle is indicated ; F. portion of convolution of frontal 
hemisphere cut across. On the left side are shown : C. S. corpus striatum : 0. 1. optic thalamus; 
Pv. pulvinar; T. a. tuberculum anterius; ch. s. choroidal sulcus marking the place of reflection 
of the choroidal plexus. On the right side are exposed : N. C. head of, Nc. end of tail of nucleus 
caudatus; Cip'., Cip". the two parts of the globus pallidus, and Pt. putamen of the nucleus lenti- 
cularis ; N. a. anterior nucleus; N. med. medium nucleus, N. tat. lateral nucleus, and Pv'. pulvinar 
of the optic thalamus; Cia. front limb, Cig. knee or genu, Cip. hind limb of internal capsule; Ce. 
external capsule ; Cl. claustrum. The numerals III. to XII. indicate the nuclei of the respective 
cranial nerves, all shown on the left side with the exception of the accessory vago-glosso-pharyn- 
geal IX.. X., IX., which to avoid confusion is placed on the right side. V. is the motor nucleus 
of the fifth nerve with the descending root, V. a. the sensory nucleus of the same with the long 
ascending root VIII. m. median nucleus, VIII. I. lateral nucleus of the auditory nerve: n. a. nu- 
cleus ambiguus. The ascending root of the ninth nerve is seen at the hind end of the combined 
nucleus of IX.,X., XI. tup: gray matter. 
613 
which we shall presently speak of as the " red nucleus," but apparently 
making no connections with these structures, and pierce the median edge of 
the pes, emerging (Fig. 131, C.) on the surface to the median side of each 
crus. As we shall see later on, this nerve is now exclusively efferent, what- 
ever it may have been in more primitive beings. We shall also see later on 
that impulses starting from the cerebrum of one side pass to the nerve of the 
other side, that is to say, decussate ; and this is also the case with the other 
efferent cranial nerves. The fibres which appear to take origin from the 
nerve-cells of the nucleus do not cross over after emerging from the nucleus, 
but keep to the same side; there is no distant decussation as in the case just 
noted of the fourth nerve. There are, however, fibres (Fig. 137, III'.') which 
leaving the nucleus cross the median raphe from one side to the other, and 
these possibly are the paths for the decussation of the impulses; but they 
may be fibres passing from the crus across the raphe to the nucleus. This 
nerve has special relations with the optic tract, but of these we shall speak 
when we come to deal with the functions of the nerves. 
§ 537. In attempting to understand the nature and relations of these 
cranial nerves, it must be borne in mind that, while morphological studies 
lead us to believe that, as the vertebrate body has been developed out of an 
invertebrate ancestry, so the brain of the vertebrate has arisen by a series 
of modifications from the nervous structures placed at the head and around 
the mouth of an invertebrate, the same studies teach us that such an evo- 
lution has been accomplished by means of profound changes. We have, for 
instance, reason to think that the mouth of the vertebrate does not corre- 
spond to the mouth of the invertebrate, but is a new structure, whose appear- 
ance has been accompanied by a considerable dislocation of parts. We must 
accordingly expect to find the indications of a segmental arrangement greatly 
obscured on the one hand by transposition, and on the other by fusion. 
The twelfth or hypoglossal nerve is one whose nature seems fairly simple. 
It is in function exclusively an efferent nerve. The large cells, with con- 
spicuous axis-cylinder processes, which characterize its nucleus, are exactly 
like those of the anterior horn of the spinal cord which give origin to the 
fibres of an anterior root. The nucleus, moreover, in its position corresponds 
to part of the anterior horn of the spinal cord, if we take into account the 
shifting involved in the decussation of the pyramids, and in the new develop- 
ments of the bulb. If we compare Fig. 132 with any section of the cord, we 
see that the hypoglossal nerve corresponds to an anterior root of the spinal 
cord, but that the fibres, after leaving the cells from which they take their 
origin, traverse in the former a large tract and in the latter case a small 
tract of tissue. Whether the whole nerve corresponds to the fibres of several 
segments fused together or to those of one segment spread out longitudinally 
is, for our present purposes, of secondary importance. 
Recognizing the hypoglossal nerve as the homologue of a spinal anterior 
root, we may go on to claim the nuclei of the third and fourth nerves as 
similar groups of cells of the anterior horn, giving rise to anterior roots. 
The position of the nuclei, the character of the cells, the function of the 
fibres, all support this view. The case is perhaps not so clear as that of the 
hypoglossal nerve, since there are reasons for thinking that these nerves 
have undergone in the course of evolution greater changes than has the 
hypoglossal nerve ; still these reasons do not oppose the above conclusion. 
The nucleus of the exclusively motor sixth nerve does not exactly corre- 
spond to those of the third and fourth in position; but we may probably 
place it in the same series with them. Thus we have in succession the third, 
fourth, sixth, and twelfth nerves, with their respective nuclei, as the anterior 
roots of nerves of their several segments. 614 
THE BRAIN. 
In the fifth nerve the dislocation and fusion spoken of above has intro- 
duced difficulties. The motor nucleus, with the fibres of the motor root to 
which it gives origin, has by some been considered as homologous to the 
series just described ; but it is at once obvious that we cannot look upon this 
great fifth nerve as corresponding to one spinal nerve, with its anterior and 
posterior root, great as the superficial resemblance seems to be. The features 
of the remarkable ascending root forbid this. The fibres of this root may be 
traced back, as we have said, to the very beginning of the bulb, and, indeed, 
into the spinal cord beyond ; as far as can be ascertained, they are not in an 
obvious and direct manner connected with nerve-cells along their course ; 
but the bundle of fibres clings, as we have seen, to the gelatinous substance 
of the posterior horn of the spinal cord and to the continuation of this along 
the bulb, and the fibres are lost in this structure. The root, therefore, as we 
have said, corresponds very closely to part at least of the posterior root of a 
spinal nerve, and though the matter has not yet been experimentally proved, 
we may infer that the trophic centres of these fibres are to be found in the 
cells of the Gasserian ganglion. 
But if the ascending root be of the nature of a posterior root (and we 
may incidentally remark that the term ascending has been unhappily 
chosen, since, if it be an afferent root, the direction of the impulses which 
it carries will be a descending one, namely, from the entrance in the pons 
toward the hinder parts), we can hardly suppose that it belongs to a single 
segment, or is the complement of the motor root alone ; in it, most prob- 
ably, the posterior fibres of several segments are blended together. Further, 
we may, perhaps, infer that the other fibres of the sensory root which end 
directly in what we have called the sensory nucleus, are in nature quite dis- 
tinct from the fibres of the ascending root; and, if so, difficulties arise as to 
the nature and homologies of the nucleus in question. These, however, we 
must not discuss here, nor can we enter into the question of the nature of 
the descending root, concerning the fibres of which, as we have said, author- 
ities differ as to whether they pass into the motor or sensory root. We have 
said enough to show that this fifth nerve is extremely complex, and that its 
apparent conformity to a simple spinal nerve is in reality misleading. 
The fibres of the vagus, glosso-pharyngeal, and bulbar accessory, taken 
together, are partly efferent, partly afferent. The combined nucleus of these 
three nerves, the cells of which are small and devoid of conspicuous axis- 
cylinder processes, is usually regarded as a sensory nucleus, and in the dia- 
gram (Fig. 138) is shaded accordingly. It may, perhaps, be compared to 
the sensory nucleus of the fifth. Thus, the ascending root, or fasciculus soli- 
tarius, presents many analogies with the ascending root of the fifth, and we 
are led to regard this as, like it, a gathering of certain afferent fibres of the 
posterior roots of several segments ; in its case also the term ascending is 
misleading. But there are many difficulties in connection with this nucleus, 
as with the fifth. We must not enter into a detailed discussion concerning 
them, but may remark that we have here perhaps to deal with complexities 
due to the fact that certainly many vagus and glosso-pharyngeal fibres, and 
probably some of those of the fifth, are splanchnic in function. 
The nucleus ambiguus contains large conspicuous cells and we may prob- 
ably regard it as a motor nucleus, especially of the vagus fibres. We may 
also, perhaps, place it and the nucleus of the seventh nerve in the same cate- 
gory, and further class with them the motor nucleus of the fifth, looking 
upon all three as so many detached portions of gray matter, corresponding 
to some part of the anterior horn of the spinal cord. Whether they are 
exactly homologous to the hypoglossal nucleus, and their fibres to simple 
anterior roots, is not so clear. THE GRAY MATTER. 
615 
Lastly, the auditory nerve, both from its character as a nerve of special 
sense and from the remarkable features of its nuclei, is even more difficult. 
Most probably it results from the fusion of more roots than one; but it is 
impossible at present to obtain a clear conception of the nature of the whole 
nerve. 
§ 538. The whole of the surface of each cerebral hemisphere for some 
little depth inward consists of gray matter, possessing special characters; 
this is called the cortical gray matter, or the cortex cerebri, or shortly and 
simply the cortex. As we shall see, by its histological and still more by its 
physiological features, it stands apart from all other kinds of gray matter. 
The whole of the surface of the cerebellum is also covered with gray 
matter, which, while possessing features of its own, so far resemble the cere- 
bral cortex, in its histological characters that it too has been spoken of as 
cortex, as the cortex cerebelli. By its functional manifestations, however, 
it differs widely from the cerebral cortex ; and since there are many advan- 
tages in being able to use the word cortex in connection with the cerebrum 
only, it is desirable not to speak of a cerebellar cortex but to employ the 
term "superficial gray matter of the cerebellum." 
The third ventricle and the hinder part of the fourth ventricle are not 
roofed in by nervous material, and possess no superficial gray matter at all. 
In the corpora quadrigemina, which form the roof of the aqueduct or cavity 
of the mid-brain, gray matter is present and possesses, in the case of the 
anterior corpora quadrigemina at least, characters to a certain extent analo- 
gous to those of the cortex and to the cerebellar superficial gray matter ; 
but it will be best to consider the gray matter of these bodies as belonging 
to another category. 
2. The Superficial Gray Matter. 
3. The Intermediate Gray Matter of the Crural System. 
§ 539. We have seen (§ 516) that the crura cerebri form the promi- 
nent part of a system of longitudinal fibres stretching from each cerebral 
hemisphere to the bulb and to the spinal cord. This system of fibres, upon 
which we may consider the various parts of the brain to be, as it were, 
founded, we may speak of as the crural system. It is, it is true, not one 
continuous strand, but a number of different strands, having different begin- 
nings and endings ; but these all contribute to the crura and are so far alike 
as to justify us in considering them as a system. The cortical gray matter 
of each hemisphere is, as we shall see, connected with various parts of this 
system, and in one sense we may regard this system as beginning in the cor- 
tex of each hemisphere, and ending in the spinal cord. But certain masses 
of gray matter in the hemisphere not strictly cortical, and several important 
masses and areas of gray matter lying between the hemisphere and the cord, 
are connected with the system; and these we may speak of as the "inter- 
mediate gray matter of the crural system." 
Corpus striatum and optic thalamus. Of all these several collections of 
gray matter, the largest, most conspicuous, and perhaps the most important 
are the two masses in the front part of the system known as the corpus stria- 
tum and optic thalamus. The former is, as we have seen (§ 515), a develop- 
ment of the wall of the cerebral vesicle, the latter a development of the wall 
of the vesicle of the third ventricle. They are, therefore, of different origin ; 
although in thecourseof the growth of the brain they become closely attached 
to each other, they are at the outset quite separate and distinct. Moreover, 
as we shall see, they differ from each other so essentially, in their nature and 
relations, that they cannot be considered as homologous bodies; and the term 616 
THE BRAIN. 
" basal ganglia " often applied to them is, therefore, unfortunate. Neverthe- 
less, it will render the description of their topographical relations easier, if 
for a little while we consider them together. 
When the lateral ventricle is laid open from above, part of the corpus 
striatum is seen projecting into the cavity of the ventricle. In front the 
projecting part is broad, forming the lateral wall and part of the floor of 
the ventricle, and to its median side lies the cavity of the ventricle, sepa- 
rated from its fellow by the septum lucidum. Further back the projecting 
part, becoming gradually narrower, assumes a more lateral position and 
passes into the descending horn. In this part of its course there lies on its 
median side, separated from it by a narrow band called the tcenia semicir- 
cularis or stria terminalis, the optic thalamus, a narrow strip of the sur- 
face of which is seen projecting outside the edge of the choroid plexus. 
If now, both lateral ventricles be laid open by removal of the corpus cal- 
losum, and the fornix with the velum interposition and choroid plexus be 
taken away, so as fully to expose the third ventricle, and also, in order to 
obtain a better view, the whole of the hinder part of the cerebrum contain- 
ing the posterior horns of the lateral ventricle, be completely cut away, it 
is seen (Fig. 138) that the two optic thalami ( 0. T.) present themselves as 
two large oval bodies, placed obliquely athwart the diverging crura cerebri 
and converging in front to form the immediate walls of the third ventricle. 
In front and to the sides of the optic thalami are seen the corpora striata 
(C. 8.) forming anteriorly the lateral walls of the two lateral ventricles, 
and diverging behind to allow of the interposition of the optic thalami. 
On each side of the brain, then, these two bodies, the corpus striatum and 
optic thalamus, appear as two masses of gray matter placed on the crus 
cerebri as this, diverging from its fellow, begin to spread out into the cere- 
bral hemisphere, the corpus striatum being placed somewhat in front of the 
optic thalamus. The relations of the two bodies, moreover, are such that 
while the optic thalamus alone forms the wall of the third ventricle to 
which it properly belongs, and the corpus striatum forms part of the wall 
of the lateral ventricle to which it in turn properly belongs, the optic thala- 
mus also projects into and seems to form part of the wall of the lateral 
ventricle, though at its origin it had nothing to do with the cerebral vesicle. 
We spoke just now of these bodies as being placed on the crura cerebri, 
but though their dorsal surfaces thus project from the dorsal surface of the 
diverging crura, a large portion of each body is, so to speak, imbedded in 
the substance of the diverging crus, and what is seen in the above surface 
view is only a part of each body, and, indeed, in the case of the corpus 
striatum, only a small part. In order to understand the nature and rela- 
tions of these two important bodies we must study sections taken through a 
cerebral hemisphere in various planes (Figs. 139-14G). 
Each crus is made up, as we have seen, of a dorsal portion or tegmentum 
consisting largely of gray matter, and a ventral portion or pes consisting 
exclusively of longitudinally disposed fibres. The tegmentum ends partly 
in structures lying ventral to the thalamus, partly in the thalamus itself; 
and we may for the present leave this part of the crus out of consideration. 
The fibres of the pes, while continuing their oblique course forward and 
outward, soon rise dorsally by the side of the thalamus and hence, in a 
transverse dorso-ventral section at the level of the hind part of the thala- 
mus (Fig. 139), are seen leaving their previous position ventral to the sub- 
stantia nigra (8n) and passing (Cip) by the side of the thalamus on their 
way to the central white matter of the hemisphere. In this part of their 
course they form a thick strand separating the thalamus (In.) from a large 
mass of gray matter which, roughly triangular in section, is divided by THE GRAY MATTER. 
617 
partitions of white matter into three parts (Gp, Gp", Pt), and of which 
we shall speak directly as the nucleus lenticularis. 
Fig. 139. 
Diagrammatic Outline of a Transverse Dorso-ventral Section through the Right Hemisphere 
(Man), at Level Posterior to the Knee of the Internal Capsule. (Sherrington.) Natural size 
Nc, nucleus caudatus; in the upper part of the figure the section of the nucleus is through the 
narrower portion which succeeds the wider front end or head ; in the lower part of the figure the 
section passes through the tail of the nucleus near its end, and this portion of it has for the sake 
of clearness been sundered from the gray matter at Na, nucleus amygdalae, more distinctly than 
in reality is the case. Gp', Gp", globus pallidus, seen here in two segments, and Pt, putamen of 
nucleus lenticularis ; an, the anterior; in, the inner ; and In, the lateral nucleus of the optic thala- 
mus ; at ll is seen the " latticed layer " lying next to Cip, the posterior limb of the internal capsule 
aud containing many strands of fibres which mingle with it. In the thalamus between the ante- 
rior and internal nuclei on the one hand and the lateral nucleus on the other is a layer shaded 
less deeply in the figure, representing the internal medullary lamina of the thalamus, consisting 
largely of white matter. Other collections of white matter within the thalamus are Vb, the bun- 
dle of Vicq. d'Azyr, and F, the lower end of the anterior pillar of the fornix ; F, the upper end 
of the anterior pillar of the fornix, below cc the corpus callosum ; C.sb, corpus subthalamicum; 
forming a fairly continuous mass with the thalamus; Sn, substantia nigra; cl, claustrum; ce, 
external capsule; Ga, terminal portion of anterior commissure; In, the insula or island of Reil, 
Iv, lateral ventricle; l.v.d.. descending horn of lateral ventricle; V. 3, in the position of the third 
ventricle; the outlines of the cavities are made diagrainmatieally distinct by thick black lines; 
Op optic tract; P P, parietal lobe; T, temporal lobe. 
If instead of taking a transverse we take a longitudinal dorso-ventral 618 
THE BRAIN. 
(or, as it is called, sagittal) section (Fig. 145) we find that the fibres form- 
ing the strand in question do not continue parallel to each other as they rise 
dorsally but diverge in a radiating manner, forming the so-called corona 
radiata. If again we take horizontal sections at proper levels (Figs. 138, 
144), we find that this strand or rather thick band of dorsally directed radi- 
ating fibres not only stretches (Cip} between the thalamus and the gray mass 
just spoken of, but reaching further forward passes (Cia) between the same 
gray mass on the lateral side and another gray mass (Ac) on the median side, 
the latter from its position being evidently the part of the corpus striatum 
which projects into the lateral ventricle. The same horizontal sections further 
teach us that the front part of the band (Cm) is bent at an angle upon the 
hind part (Cip). 
It appears, then, from these sections that the fibres of the pes as they rise 
up dorsally into the hemisphere spread out in the form of a fan bent upon 
itself. This fan-like expansion of the pes is called the internal capsule, the 
angle formed by the bend being called its genu or knee (Cig}, the part in 
front of the knee the/roni limb, and the part behind the knee the hind limb. 
And horizontal sections at levels more dorsal than those given in Figs. 138- 
144 would show that the fibres composing this fan-like internal capsule, as 
they rose dorsally, curved away in various directions to reach nearly all parts 
of the surface of the hemisphere. We may add that though the internal 
capsule is mainly composed of fibres which thus stretch all the way from the 
cerebral cortex to the pes of the crus, it also contains other fibres of which 
we shall speak later on. 
§ 540. The gray mass separated from the thalamus by the hind limb of 
the internal capsule is called as a whole the nucleus lenticularis, since in hori- 
zontal section it presents a certain though distant resemblance to a lens. Of 
the three divisions into which it is split up by the partitions of white matter, 
the two median ones Gp, Gp",are spoken of together as the globus pallidus, 
the name being given to them on account of their paler color. The third, 
lateral division Pt, is called the putamen. The use of these two names for 
the two different parts of the one body, appears to be justified by the different 
connections and features of the two parts. 
The gray mass which in a horizontal section (Fig. 138, Nc} is separated 
from the nucleus lenticularis by the front limb of the external capsule, and 
which projects into the lateral ventricle, is called the nucleus caudatus. The 
nucleus caudatus and the nucleus lenticularis form together the corpus stria- 
tum ; the former, since it projects into the lateral ventricle, being the part of 
the corpus striatum seen when the lateral ventricle is laid open,is sometimes 
spoken of as the intra-ventricular portion of the whole body, while the 
nucleus lenticularis, which is wholly hidden in the hemisphere and in no 
part projects into the lateral ventricle, is called the extra-ventricular por- 
tion. 
But only a part, indeed only a relatively small part, of the nucleus cauda- 
tus is disclosed in such a horizontal section ; to learn the somewhat peculiar 
form and relations of the whole nucleus a number of sections of a hemi- 
sphere taken in different planes must be studied ; and these will at the same 
time explain why the nucleus is called " caudatus." These teach us that the 
nucleus has somewhat the form of a comma (Fig. 142). The thick, rounded 
head forms the lateral wall of the front part of the lateral ventricle ; thence 
the body passes backward, narrowing rapidly and diverging somewhat later- 
ally ; in its course it arches over the nucleus lenticularis, curving so much 
that the end of the tail, sweeping round the hinder border of that body and 
changing its direction, runs eventually ventral to it. In a horizontal section 
taken at a certain depth, such as that represented in Fig. 138, only a portion 619 
THE GRAY MATTER. 
of the head or body (No) in the front part of the figure, and a transverse 
section of the end of the tail (JVc) in the hind part of the figure are seen ; all 
the intervening portion of the nucleus lies above the plane of the section. 
In a transverse, dorso-ventral section taken somewhat anteriorly through the 
front limb of the capsule (Fig. 140), the head or body of the nucleus cauda- 
Fig. 140. 
Diagrammatic Outline of a Transverse Dorso-ventral Section through the Right Hemisphere 
(Man) at a Level Anterior to Fig. 139. (Sherrington.) Natural size. No, nucleus caudatus; Gp', 
Gp", globus pallidus, seen here in two segments, and Pt, putamen of nucleus lenticularis; OT, 
optic thalamus, with ca, anterior commissure, in close relation to cia, anterior limb of internal 
capsule; ce, external capsule; op, optic tract; cc, corpus callosum ; f, fornix; Iv, a space that in its 
upper part belongs to the lateral ventricle, in its lower was filled by the fold of subarachnoid tis- 
sue and pia mater, the side fringe of which, covered with epithelium, forms the choroid plexus; 
this fold was detached in the making of the section and was removed; In, the insula ; F, frontal 
lobe; P, parietal lobe; T, temporal lobe. For greater clearness the cortical gray matter, which 
is shaded in Fig. 139, is in this figure left unshaded. 
tus (jVc), which has not yet reached its greatest dimensions, is seen lying 
dorsal to the nucleus lenticularis, separated from it by the white mass of the 
front limb (cia) of the capsule, though this is somewhat broken up by strands 
of gray matter passing from one nucleus to the other. In a transverse dorso- 620 
THE BRAIN. 
ventral section, taken still more anteriorly through the frontal lobe (Fig. 
141), the head of the nucleus caudatus is seen at about it greatest size, and 
the diminishing nucleus lenticularis (Nl), represented by the putamen alone, 
is becoming fused with it, the two nuclei being separated by a small quantity 
of white matter of the internal capsule, and that largely broken up by 
bundles of gray matter, giving rise to a striated appearance. In a similar 
section still further forward, the nucleus lenticularis would be absent, the 
head of the nucleus caudatus appearing by itself. Returning to the hinder 
part of the hemisphere, we find in a dorso-ventral section taken through the 
hind limb of the capsule (Fig. 139) that while the nucleus lenticularis is 
Fig. 141. 
Diagrammatic Outline of a Transverse Dorso-ventral Section of Right Hemisphere (Man) 
through the Frontal Lobe. (Sherrington.) Natural size. Nc, head of nucleus caudatus, and Nl, 
the front end of the putamen of the nucleus lenticularis becoming fused with it; cc, corpus cal- 
losum, cut through at its front bend or rostrum, so that both dorsal and ventral portions are 
shown ; between these is seen the fifth ventricle or cavity in the septum lucidum SI; Iv, lateral 
ventricle; Cl, claustrum ; F, frontal lobe. Cortical gray matter, as in Fig. 140, left unshaded. 
here at its greatest size, the head of the nucleus caudatus (No), lying dorsal 
to the nucleus lenticularis and separated from it by a considerable thickness 
of internal capsule, has much diminished; the same section, moreover, 
shows ventral to the nucleus lenticularis and clinging to the descending 
horn of the lateral ventricle (Z.v.d.), the extreme tip of the tail of the nucleus 
caudatus (Ac) soon about to fuse with the small mass of gray matter called 
the nucleus amygdala (Na). A sagittal (longitudinal dorso-ventral) section 
taken at some distance from the median line (Fig. 142) shows the curved 
course of the larger portion of the nucleus caudatus, the extreme head as 
well as the latter part of the tail lying out of the plane of the section ; and THE GRAY MATTER. 
621 
a similar section taken nearer the middle line (Fig. 145) shows how the 
nucleus in the middle portion is broken up by bands of fibres of the internal 
capsule traversing it, and thus contributing to the striated appearance; the 
same section also shows that the globus pallidus, as well as the putamen, 
becomes continuous with the nucleus caudatus. 
Fig. 142. 
Diagrammatic Outline of a Sagittal Section taken through the Right Hemisphere (Man), Seen 
from the Mesial Surface. (Sherrington.) Half natural size. The plane of the section is not 
truly sagittal, but slightly inclined. Nc, the caudate nucleus exposed to the left of the letters Nc 
in nearly its entire anterior extent, to right of the letters in a considerable part of its posterior 
extent. It forms an arch of gray matter over the gray matter of Pt, the putamen, and Gp, the 
globus pallidus of the lenticular nucleus; Na, the amygdaloid nucleus ; Ci, Ci, Ci, the internal 
capsule; Ca, the anterior commissure; cc, the hinder limit of fibres of the splenium corporis 
eallosi, P, the parietal lobe; T, the temporal. 
Thus, when we speak of the corpus striatum as a whole we mean a large 
mass of gray matter lying lateral to the optic thalamus, reaching nearly as 
far back as that body and stretching much further forward, as far forward in 
fact as does the lateral ventricle; but it is important to remember that it is 
divided into two masses or nuclei, which are fused together, and that im- 
perfectly at the very front only. These two nuclei are, the one the comma- 
shaped nucleus caudatus, the bulk of which is placed forward projecting into 
the lateral ventricle, and which on the whole is the more dorsal portion of 
the whole body, the other the irregularly shaped nucleus lenticularis, the 
bulk of which is placed further back than the lateral ventricle, by the side 
of the optic thalamus, and which on the whole is the more ventral portion of 
the whole body. It is no less important to remember that the radiating 
fibres, which we call the internal capsule, pass in the hinder region of the 
whole body between the thalamus and the nucleus lenticularis, forming the 
hind limb of the capsule, and in the front region between the nucleus cau- 
datus and the nucleus lenticularis, forming the front limb of the capsule, the 
front and hind limbs being bent on each other so as to form an angle, the 
so-called knee. 
§ 541. The optic thalamus as a whole is a somewhat oval mass of gray 622 
TIIE BRAIN. 
matter lying, as we have said, athwart the diverging crus, in which it is 
partly imbedded. Its curved median side covered with a thin layer of cen- 
tral gray matter forms the lateral wall of the third ventricle (Figs. 138, 139, 
144), and in a longitudinal vertical section of the brain taken in the line of 
the middle of the third ventricle (Fig. 143, O-T.} is seen occupying the space 
between the fornix and hind end (splenium) of the corpus callosum above 
and the diverging crus below. Its more or less straight lateral border abuts 
Fig. 143. 
View of Right Half of Brain of Man as Disclosed by a Longitudinal Section in the Median 
Line through the Longitudinal Fissure. (Sherrington.) Half natural size. The bulb, seen in 
longitudinal section at B, passes into the pons, P, and into the crus cerebri, which last is cut obliquely 
across as it diverges into the hemisphere and passes out of the section. A part of the ventral 
surface of the crus is shown in the shaded part marked CR. At GL the central canal of the spinal 
cord is seen opening out into the fourth ventricle (4th), overhung by the cerebellum (bisected in 
the middle line), and passing on by the aqueduct beneath the posterior, QP, and anterior, QA, 
corpora quadrigemina into the third ventricle (3). The posterior corpus quadrigeminum is continuous 
behind with the valve of Vieussens, attached to the superior peduncle of the cerebellum, and seen 
in a longitudinal section overhanging the front part of the fourth ventricle. The corpora quad- 
rigemina appear relatively small because the section passes in the median line in the depression 
between the right and left bodies of the two pairs ; and immediately in front of them is the sec- 
tion of the mesially placed pineal gland P, which overhangs the opening of the aqueduct into the 
third ventricle, and the right arm of which, running in the lateral wall of the third ventricle, is 
shown by an unshaded tract. The roof of the third ventricle is seen to be furnished by the arch 
of the fornix F, shown unshaded in longitudinal section. Posteriorly the body of the fornix passes 
into the diverging right posterior pillar, where J1 is shaded, and is lost to view under the over- 
hanging rounded hind end or splenium (Sp) of the corpus callosum. In front the body of the fornix 
is seen passing just behind the transverse section of the anterior commissure A into the diverging 
right anterior pillar f, which is lost to view as it stretches in the lateral wall of the ventricle toward 
the corpus mammillare or albicans M. The small white cross immediately behind f indicates the 
position of the foramen of Monro. The bulging median surface of the optic thalamus OT, is seen 
forming the lateral wall of the hinder (and owing to the cranial flexure, the more dorsal) part of 
the third ventricle, and on this, below the area of the pineal gland, is seen, unshaded, the section 
of the soft or middle commissure C. Between the pineal gland (P) and the splenium (Sp) is seen 
the hind end or pulvinar of the thalamus projecting into the so-called transverse fissure of the 
brain, shown shaded in the figure, by which the pia mater, passing on beneath the posterior 
part of the cerebrum and above the cerebellum, gains access to the third ventricle, the position of 
the velum being shown by the thin black line stretching from the splenium to the fornix. The 
front (and more ventral) part of the third ventricle is seen to end in the infundibulum, attached 
to which is the pituitary body II, seen in section at L. In front of the infundibulum is seen the THE GRAY MATTER. 
623 
on the internal capsule (Figs. 138,139,144). Its dorsal surface, as we have 
already seen, also forms part of the wall of the third ventricle and is free ; 
but there lies close above it the prolongation of the pia mater, forming the 
velum interpositum with its choroid plexus (§ 515), which creeps in over it 
beneath the projecting hind end of the corpus callosum and the fornix 
(Fig. 143). Its ventral surface is fused with the crus ; indeed the tegmental 
or dorsal portion of the crus may be said to end in it and in certain struc- 
tures lying ventral to the thalamus, in what is called the " subthalamic 
region" (Fig. 139), while the fibres of the pes pass first ventral and then 
lateral to it to form the internal capsule. 
The gray matter of the whole body is more or less distinctly divided by 
sheets of white matter, as seen both in horizontal and in vertical sections 
(Figs. 136, 139, 144), into three parts which have received the name of 
nuclei, namely, the median or inner nucleus (Fig. 139 in), which with the 
thin layer of central gray matter forms the side wall of the third ventricle; 
the larger lateral nucleus (In), which abuts upon the internal capsule; and 
the smaller anterior nucleus (an), which lies on the dorsal surface of the 
front part of the body, and which thus at its front end appears to project 
into the lateral ventricle. 
These three nuclei form, however, not the whole of the optic thalamus, 
but only the larger front portion; behind them lies the important portion 
called the pulvinar, into which the hind part of the median nucleus merges; 
this is partly imbedded in the crus ventrally, and in the hemisphere laterally, 
and is partly free, coming to the surface beneath the hind end of the corpus 
callosum. In a median longitudinal section of the brain (Fig. 143) it is the 
pulvinar which forms the cushion-like (hence the name) end of the thalamus 
beneath the overhanging splenium of the corpus callosum, by the side of the 
pineal gland ; and in the horizontal view (Fig. 138, Per), in which the 
hemispheres are supposed to have been removed, the same pulvinar is seen 
projecting over the crus by the side of the anterior corpus quadrigeminum. 
The buried portion of the pulvinar is exposed in a transverse section taken 
through the anterior corpus quadrigeminum (Fig. 137); the extreme end of 
this part of the pulvinar (Per) is here seen lying dorsal and lateral to the 
pes of the crus, immediately above two masses of gray matter, the corpora 
geniculata (Cgl. Cgm.), of which we shall speak later on. One of these, 
the lateral corpus geniculatum (C. g. I.), is especially connected with the 
optic tract (op), and, as we shall see hereafter, the pulvinar itself is also 
connected with the optic tract, and is an important part of the central appa- 
ratus of vision. 
§ 542. The substantia nigra, the red nucleus, and other gray matter of the 
tegmentum. Nerve-cells and groups of nerve-cells, or areas of gray matter, 
too small to deserve special names, are scattered throughout the tegmentum 
optic nerve cut across at the optic decussation OP, stretching from which to the anterior commissure 
is the lamina terminalis. Stretching between the corpus callosum cc (seen in longitudinal section 
with a striated appearance, and ending in front at the rostrum R and behind at the splenium Sp) 
dorsally and the fornix ventrally is seen (unshaded) the septum lucidum SL, but the greater part 
of this has been cut away in order to disclose the right lateral ventricle, in the wall of which is 
seen the bulging nucleus caudatus NC. Above the corpus callosum is seen the mesial surface of 
the right hemisphere forming the right lateral wall of the longitudinal fissure. On this mesial 
surface appears immediately above the corpus callosum the arched gyrus fornicatus GF, defined 
above by the calloso-marginal fissure f. cm. The whole of the surface seen in the frontal region in 
front of the calloso-marginal fissure, though divided by fissures, is called the marginal convolution. 
In the middle parietal region a block of the cerebral substance has been removed in order to 
show the position of the central fissure or fissure of Rolando, f. c, and immediately below this is 
seen a part of PA. C., the paracentral lobule. In the occipital region, PR. C., is the precuneus or 
quadrate lobule, and C, the cuneus, while at G. L. is seen a part of the lingual lobule. T. i. is a part 
of the inferior temporo-occipital convolution, the greater part of which is hidden from view by the 
pons and crus. 624 
THE BRAIN. 
along its course. But, besides these and the nuclei of the third and fourth 
cranial nerves, of which we have already spoken, certain larger collections 
of gray matter deserve attention. A conspicuous mass of gray matter, cir- 
cular in transverse section, placed in the midst of the tegmentum on each 
side but somewhat near the middle line, and stretching from the hinder 
margin of the third ventricle beneath the anterior corpus quadrigeminum 
(Figs. 137, 138), is, from the red tint it possesses, called the red nucleus, 
nucleus, or locus ruber. It is traversed by fibres of the third nerve as these 
make their way ventrally from the nucleus to the surface. 
We must consider also as belonging to the tegmentum a large area of 
gray matter, somewhat lens-shaped in section (Fig. 137, Sn), which lies 
between the pes and tegmentum, sharply marking off the one from the other. 
From its dark appearance, due to the abundance of black pigment, it is 
called the substantia nigra or locus niger. It acquires its largest dimensions 
at about the middle of the length of the crus, coming to an end in front 
(Fig. 139, Sn) and fading away behind (Fig. 136), as the crus passes beneath 
the posterior corpora quadrigemina. These two, the red nucleus and the 
substantia nigra, are perhaps the most important collections of gray matter 
in the tegmentum, but we may add that at the front of the crus as the sub- 
stantia nigra comes to an end there is seen in a somewhat similar position 
ventral to the hind part of the optic thalamus a collection of gray matter 
called the corpus subthalamicum (Fig. 139, C. sb). 
At the hinder part of the crus, as it is about to plunge into the pons, 
while the pes, now decreasing relatively in size, still continues to be ordinary 
white matter composed of longitudinal bundles of medullated fibres, the 
tegmentum takes on more and more the structure which in speaking of the 
bulb we called reticular formation, and which, as we saw, deserves to be con- 
sidered as a kind of gray matter. 
The gray matter of the pons. When the conjoined crura as we trace them 
backward plunge beneath the pons, the longitudinal fibres of the pes of each 
crus are, as we have said, soon split up into bundles and scattered among the 
transverse fibres belonging to the pons itself. Dorsal to this system of trans- 
verse and longitudinal fibres forming the pons proper, between it on the ven- 
tral surface and the central gray matter with the posterior corpora quadri- 
gemina on the dorsal surface, is a region which may be called tegmental, 
since it is a continuation of the tegmentum of the crus. In the front part 
of the pons (Fig. 136), where the posterior corpora quadrigemina still form 
the dorsal roof of the section, this tegmental area, which is much broken up 
by certain strands of longitudinal fibres, of which we shall speak later on, 
contains scattered nerve-cells, and is largely composed of reticular formation. 
In this is placed on each side a group of nerve-cells, the locus cceruleus (Fig. 
136, I. c.), to which we have already referred (§ 534) as probably serving in 
part as the origin of the descending root of the fifth nerve ( V. d.), just ven- 
tral to which it lies. This acquires larger dimensions further back, in the 
front part of the fourth ventricle (Fig. 138, I. c.) between the levels repre- 
sented in Figs. 135 and 136, and is a collection of large spindle-shaped 
nerve-cells; it has a bluish tint when its black pigment is seen shining 
through the surrounding more or less transparent material, hence the name. 
In the hinder parts of the pons (Figs. 134,135), where the cerebellum is 
seen overhanging the open fourth ventricle, the reticular formation of the 
tegmental area is still more conspicuous. The only special collection of 
gray matter in this region to which we need call attention is one which, con- 
sisting, like the olivary body of the bulb (or inferior olive), of a wall of 
gray matter surrounding and surrounded by white matter, is called the 
upper olive (Figs. 134, 135, s. o.). THE GRAY MATTER. 
625 
The ventral part of the pons, or the pons proper, unlike the pes of the 
crus, contains, mixed with the fibres, a very considerable quantity of gray 
matter. This is fairly abundant in the front part of the pons (Fig. 136) below 
the corpora quadrigemina, but increases even more behind this (Figs. 125, 
135). Hence, though the pons proper is largely built up of transverse and 
longitudinal fibres, and though it contains no compact aggregations of gray 
matter receiving special names, it does contain scattered throughout it a very 
large quantity of gray matter, far more, indeed, than is present in the teg- 
mental portion ; the gray matter of the pons-that is, of the pons proper- 
must be regarded as forming a very important part of the gray matter of 
the crural system, and of no little physiological significance. 
Behind the pons the crural system is continued into the bulb, with whose 
structure we have already dealt. 
4. Other Collections of Gray Matter. 
§ 543. Of these, three deserve chief attention, and may be classed 
together, though they differ in nature. 
The gray matter of the corpora quadrigemina. On each side of and some- 
what dorsal to the central gray matter of the aqueduct, which, as we have 
seen, is well developed, especially on the ventral side, collections of gray 
matter form the chief part of the corpora quadrigemina, both anterior and 
posterior. 
The gray matter of the anterior corpora quadrigemina (Fig. 137, A. Q. n.) 
is more distinctly marked off from, and separated by a wider tract of white 
matter from, the central gray matter of the aqueduct than is that of the pos- 
terior corpora quadrigemina (Fig. 137, nPQ) ; it is, moreover, of a different 
nature. Indeed the two pairs of bodies have quiet different relations, are of 
different nature, and perform different functions. 
Corpora geniculata. The two optic nerves, as we shall see in detail later 
on, give rise, through the optic decussation, to the two optic tracts. Each 
optic tract (Figs. 131, 137, Op) winds round the crus cerebri on its ventral 
surface to reach the substance of the hemisphere in the region below the 
optic thalamus, and as it does so is described as dividing into a lateral and 
median portion. The lateral portion just as it sweeps round the far edge, 
that is the outer or lateral edge, of the crus bears a rounded swelling (Fig. 
131, B and C, Cgl.), the lateral or outer corpus geniculatum, the interior of 
which consists largely of gray matter (Fig. 137, Cgl). The median portion 
similarly bears another like swelling occupying a more median position, the 
median or inner corpus geniculatum (Fig. 131, A and B, Cgm), the interior 
of which (Fig. 137, Cgm) also consists of gray matter. It is to be regretted 
that these two bodies should bear the same name, for they are different in 
their origin, in their connections, and in their functions. The lateral body 
is said to be derived from the fore-brain, that is from the vesicle of the third 
ventricle, has definite connections with the retinal optic fibres, and is dis- 
tinctly concerned in vision ; the median body is derived from the mid-brain, 
is not definitely connected with the retinal fibres, and appears to be in no 
way concerned in vision. We shall, however, return later on to the connec- 
tions and probable functions of these bodies. 
Corpus dentatum of the cerebellum. In the midst of the mass of white 
matter which is formed in the interior of the cerebellum by the confluence 
of the three peduncles is found (Fig. 134, C I)) an area of gray matter 
arranged, like the olivary body of the bulb, as a sharply folded or plaited 
band in the shape of a flask or bowl. As in the similar olivary body the 
gray matter of the flask is covered up by and its interior filled up with white 626 
THE BRAIN. 
matter; the mouth of the flask is, on each side, directed toward the median 
line ; the fibres pass chiefly to the superior peduncle. 
Fig. 144. 
p 
Outline of Horizontal Section of Brain, to show the Internal Capsule. Natural size. The sec- 
tion is taken at a level more ventral than shown in Fig. 138. The gray matter of the cortex and 
claustrum is left unshaded, but that of the corpus striatum and optic thalamus is shaded ; OT, 
optic thalamus, showing the median, lateral, and anterior nuclei; NL, nucleus lenticularis, show- 
ing the putamen large, and the inner division of the globus pallidus very small; NC, nucleus 
caudatus, the large head in front of, and the diminishing tail behind, the thalamus ; O, the knee 
of the internal capsule. From " Eye" to " Die/" marks the position of the pyramidal tract as a 
whole, and the several letters indicate broadly the relative positions of the several constituents 
of the tract, named according to the movements with which they are concerned: thus Eye, move- 
ments of the eyes ; Hd, of the head ; Tg, of the tongue; mth, of the mouth ; Shi, of the shoulder; 
Elb, of the elbow; Dig, of the hand ; Abd, of the abdomen; Hip, of the hip; Kn, of the knee; 
Dig, of the foot; S, the temporo-occipital tract; oc, fibres to the occipital lobe; op, optic radiation. 
At this level the fibres of the frontal tract, in the fore limb of the capsule in front of the pyram- 
idal tract, run almost horizontally, parallel with the plane of the section. (Cf. Fig. 146, Fron). cc, 
the rostrum of the corpus callosum, Spl, the splenium of the same, both cut across horizontally. 
The thick dark line indicates the boundary of the cavities of the anterior and descending horns 
of the lateral ventricle and of the third ventricle, the two ventricles being laid open into one by 
the removal of the velum and choroid plexus, etc. The oval outline in the fore part of this cavity 
indicates the fornix. Lateral to the nucleus lenticularis is seen in outline the claustrum, the cor- 
tex of the island of Reil and the operculum or convolution overlapping the island of Reil. Pis 
inserted to show which is the hind part of the section. 627 
THE ARRANGEMENT OF THE FIBRES OF THE BRAIN. 
There are also other collections of gray matter in the central white mat- 
ter of the cerebellum, one of which, called the " nucleus of the roof," is con- 
nected with the two inferior peduncles. 
THE ARRANGEMENT OF THE FIBRES OF THE BRAIN. 
§ 544. The systems, tracts, and bundles of fibres in which the white 
matter of the brain is arranged, may be distinguished from each other, partly 
through mere mechanical separation by means of the scalpel, partly by being 
traced out with the help of the microscope, but, as in the spinal cord, much 
more fully and completely by differences of development, and by the method 
of degeneration. 
Fig. 145. 
Outline of a Sagittal Section through the Hemisphere-Man. (Sherrington.) The section is 
taken not far to the right of the median plane and is one-half linear or natural size. The gray 
matter of the corpus striatum and thalamus is shaded. Nc, Nc, the caudate nucleus; Pt, the 
putamen, and Gp, the globus pallidus of the lenticular nucleus; O T, the optic thalamus; CI, the 
internal capsule with a streaked appearance revealing approximately the direction taken by fibre- 
bundles passing into it from the portion of corona radiata over it. In these sets of bundles may 
be broadly distinguished a frontal system, fron, a pyramidal system, P Y(subdivisible into cranial 
(cran.), brachial (brack.), dorso-lumbar (dors. him.), and lumbo-sacral (lum. sac.) parts, and a tem- 
poro-occipital system, sens.; the situation of the genu of the internal capsule is indicated by g. 
CR, the crus cerebri; Oc, the so-called optic radiations passing into the occipital lobe; cc, the 
splenial end of the corpus callosum; v, v, v. the lateral ventricle cut across in three different 
places; F, the fornix in cross-section ; Op, the optic tract in cross-section. Part of the cerebellum 
is seen in outline to the right. 
We have seen that a marked feature of the brain is presented by the two 
crura cerebri which, running forward from the hind parts of the brain, 
spread out into each cerebral hemisphere. We have also seen that the crus 
in the wide sense of the word consists of two parts, a dorsal part, the teg- 
mentum, and a ventral part, the pes or crusta, and that these two parts differ 
very strikingly from each other in structure and in relations. The pes con- 
sists exclusively of bundles of longitudinal fibres, and we may trace these 
from the cerebral hemispheres into the pons and some of them beyond the 
pons into the bulb and spinal cord. The tegmentum is more complex in 
structure; it consists of gray matter, and of fibres and bundles of fibres 
having various relations both with the collections of gray matter lying 
within itself and with surrounding structures. It too has connections with 
the parts lying in front of it, and with the parts lying behind it; we may 
trace it, too, backward through the pons into the bulb and forward to the 
optic thalamus. If we allow ourselves to conceive of the optic thalamus as 628 
THE BRAIN. 
constituting the front ending of the tegmentum, we may arrange a large part 
of the brain into two main regions-into a tegmental region stretching from 
the optic thalamus through the dorsal portion of the pons to the dorsal por- 
tion of the bulb, and into a region, which we may call the pedal region, 
stretching from the internal capsule through the ventral portion of the pons 
to the ventral portion of the bulb. 
The fibres of the brain, as a whole, may be broadly classified into longi- 
tudinal tracts connecting parts of the brain with succeeding parts and into 
transverse or commissural tracts between one lateral half and the other, and 
into tracts connected with the several cranial nerves. Taking the longitu- 
dinal fibres first, we may in accordance with the division just explained into 
a pedal and a tegmental region, consider these as forming, on the one hand, 
a pedal and, on the other hand, a tegmental system. 
Both systems begin, as we shall see, in the cortex of the cerebral hemi- 
spheres. We shall have to deal with the topography of the cortex later on, 
but may here say that the first broad division of the whole surface of a 
hemisphere is into four main regions : frontal, parietal, occipital, and tem- 
poral (Figs. 139, 140,144). 
LONGITUDINAL FIBRES OF THE PEDAL SYSTEM. 
§ 545. The pyramidal tract. We have already 488) said that the 
pyramidal tract of the spinal cord may be traced to a particular region of 
the cerebral cortex. We shall study the details of this region, which is 
often spoken of as the " motor area " later on, but may here say that broadly 
speaking it is parietal in position and corresponds to the parts of the cortex 
gathered round the fissure of Rolando. Fibres passing from the gray matter 
of the cortex of this region to the white matter below, and so contributing 
their share to the central white matter of the hemisphere, converge (Figs. 
145, 146) to form part of the internal capsule, namely, that part which in a 
horizontal section (Fig. 144, Eye to Dig) occupies the knee and stretches for 
more than half, or nearly two-thirds, along the hind limb of the capsule, 
between the optic thalamus on the inside and the nucleus lenticularis on the 
outside. From the knee and hind limb of the capsule they pass by the side 
of and ventral to the optic thalamus (Fig. 139, 146), and so contribute to 
form the beginning of the crus cerebri. In thus converging to take up their 
position in the capsule and in their further passage to the crus the fibres 
follow a course of somewhat complicated curvature. As we trace the cap- 
sule from more dorsal to more ventral levels, we find it continually changing 
in form ; the exact shape of the capsule shown in Fig. 144 only holds good 
for the level at which the section was taken ; it differs somewhat from that 
shown in Fig. 138 taken at a slightly different level, and sections still more 
dorsal or still more ventral would present still greater differences. When 
we examine a series of horizontal sections, taken in succession from the 
dorsal to the ventral regions, we find that the knee shifts its position and 
changes in the width of its angle, that the two limbs vary in direction, in 
size, and in shape, and that at last the bent, flattened capsule passes into the 
more or less rounded crus by the rapid disappearance of the fore limb, and 
the consequent extinction of the angle; so that in one sense it is the hind 
limb which becomes the crus, and the fibres of the fore limb may be said to 
pass into the crus through the ventral portion of the hind limb. Hence it 
is obvious that the fibres of the pyramidal tract, like the other fibres of the 
capsule, are continually changing their direction as they pass through the 
capsule. Moreover, while the fibres from the different parts of the " motor 
area " assume definite positions in relation to each other as they pass into LONGITUDINAL FIBRES OF THE PEDAL SYSTEM. 
629 
the capsule, their relative positions are not constant, but vary somewhat. To 
this point, however, we shall return when we come to speak of the function 
of this tract. 
Fig. 146. 
Outline of a Transverse Dorso-ventral Section of the Right Half of the Brain. Natural size. 
(Sherrington.) The section, which is taken at the level of the knee of the capsule, and is therefore 
intermediate between those shown in Figs. 139 and 140, is introduced to illustrate the course of the 
constituents of the pyramidal tract. OT, optic thalamus; Nc, nucleus caudatus-the head only 
appears in this section; Pt, putamen ; Gp", Gp,' the two parts of the globus pallidus of the nucleus 
lenticularis; C, the claustrum ; CE, the external capsule ; In, the island of Reil; ca, the anterior 
commissure, shaded to render it distinct and the fibres from the temporo-sphenoidal lobe which 
pass into it being indicated by broken lines; Op, the optic tract; I v d, the end of the descending 
horn of the lateral ventricle; F, the fornix ; F', the end of the anterior pillar of the fornix in the 
base of the thalamus; cc, corpus callosum; OP, anterior part of the occipital lobe, fc is the central 
fissure, or fissure of Rolando. The course of the fibres of the pyramidal tract connected respectively 
with the trunk, leg, and arm and hence with spinal nerves, and of those connected with the face 
and hence with cranial nerves, is shown by broken lines. These are all seen converging into the 
internal capsule, CI. This figure should in respect to the course of these fibres be compared with 
the horizontal section shown in Fig. 144, and the sagittal figure shown in Fig. 145. S indicates the 
course of the most anterior and dorsal part of the temporo-occipital tract. The fine dotted lines 
converging to the corpus callosum, c c, indicate the course of the callosal fibres. 
In the crus these fibres run exclusively in the pes and form a compact 
strand (Fig. 137, Py) occupying the central and larger portion of the pes 
between a small median portion on the inside and a lateral portion on the 630 
THE BRAIN. 
outside. Maintaining this position along the crus they enter the pons, but 
here the previously compact strand is split up by the interlacing transverse 
fibres of the pons into a number of scattered bundles, which, however, as a 
whole, still keep their central position. They form the greater part of, but 
not all, the bundles seen cut transversely in transverse sections of the pons 
(Figs. 135, 136). Further backward they become the pyramid of the bulb, 
and so give rise in the spinal cord to the direct and crossed pyramidal tracts. 
These fibres from the motor area of the cortex of the cerebrum are thus the 
source of the pyramidal tracts of the spinal cord, and hence the whole 
strand of fibres from the cortex downward has been called the pyramidal 
tract. We have said (§ 488) that we have reasons for thinking that the 
pyramidal tract in the spinal cord makes connections through the gray 
matter of the anterior horn with the anterior roots of all the spinal nerves 
in succession ; and similarly we have reason to think that along its course in 
the crus, in the pons, and in the bulb, before it reaches the cord, the tract 
also makes connections with the nuclei of those cranial nerves which are 
motor in function. During the passage of the tract through the internal 
capsule the fibres destined for cranial nuclei occupy the knee, while those 
belonging to the spinal cord run in the hind limb. Some authors limit the 
term pyramidal tract to the spinal moiety, since this alone forms the pyra- 
mid ; but this is undesirable. 
This tract is well marked out by the degeneration method, and the de- 
generation in it is a descending one, the trophic centres of the fibres being 
cells in the gray matter of the cortex. Removal of or injury to the cortex 
of the whole motor area gives rise to a degeneration along the whole tract, 
and removal of or injury to part of the area gives rise to degeneration of 
some of the strands. The tract is also well marked out by the embryologi- 
cal method ; the fibres belonging to it acquire their medulla at times different 
from those of other fibres. 
Anterior or frontal cortical. Fibres from the gray matter of the cortex 
in front of the motor area also pass to the internal capsule, but occupy the 
fore limb (Fig. 145, fronf Thence they pass to the crus, of which they 
form the small inner, median portion of the pes (Fig. 137, Fr.f and from 
the crus pass into the pons; in transverse sections of the pons they are seen 
as scattered bundles (Fig. 136, F. C.) to the median side of the pyramidal 
fibres. But here they seem to end ; the degeneration of the tract is a de- 
scending one, and ceases here. Most probably the fibres end in the nerve- 
cells of the gray matter, which, as we hav.e seen, is abundant in the pons. 
It is also probable that through these nerve-cells the fibres of this tract are 
connected with transverse fibres passing along the middle cerebellar pedun- 
cle into the cerebellum of the opposite side ; but this has not been definitely 
proved. 
Posterior or temporo-occipital cortical. Fibres from the gray matter of 
parts of the cortex behind the motor area also converge to the internal cap- 
sule, forming the hinder end of the hind limb behind the pyramidal tract 
(Fig. 144, S). These fibres also contribute to form the crus cerebri, passing 
into the pes, of which they occupy the outer lateral portion (Fig. 137, Pr. 0.). 
From the crus they pass into the pons, where, like the fibres of the preceding 
tract, they appear to end, and probably in a like manner. This tract has 
been described as one of ascending degeneration, but in all probability like 
the preceding is one of descending degeneration. 
The above three tracts of fibres may, therefore, all be regarded as start- 
ing from or having their trophic centres in the cortical gray matter of the 
hemispheres, as all helping to form, first, the internal capsule and then the 
pes of the crus cerebri. But while the pyramidal tract passes, in part, to the 631 
LONGITUDINAL FIBRES OF THE TEGMENTAL SYSTEM. 
spinal cord, the other two cease at the pons, and probably through the gray 
matter of the pons make connections with the cerebellum. Further, while 
the pyramidal tract coming from the middle region of the cortex occupies a 
middle position in the capsule and a middle position in the crus, the system 
from the front part of the cortex occupies a front position in the capsule 
and an inner or median position in the crus, and the system from the hind 
part of the cortex, a hind position in the capsule and an outer or lateral 
position in the crus. As the three systems pass from the cortex through the 
capsule to form the pes of the crus, their positions in relation to each other 
are shifted from one plane into another. As the fibres spread out from the 
pes through the capsule to all parts of the cortex, or, put in another way, as 
they converge from the cortex through the capsule to the pes, they form a 
fan, the corona radiata, which is not only curved, but the constituent parts 
of which cross each other. 
Besides these three systems all passing from various regions of the cortex 
to the crus, there is yet a fourth strand contributed to the pes by the cere- 
bral hemisphere, though not starting in the cortex. From the nucleus 
caudatus fibres pass down to the crus, and take up a position in the pes 
dorsal to the tract just mentioned, occupying a lens-shaped area immediately 
ventral to the substantia nigra, and probably passing into the substantia 
nigra itself. These cannot be traced further down than the pons, where 
they appear to end, though possibly some terminate higher up in the sub- 
stantia nigra. This tract has a descending degeneration, and may be re- 
garded as a tract analogous to the front and hind cortical tracts, though it 
begins not in the cortex but in the nucleus caudatus; it is not, however, 
a very pure tract, many fibres of the pyramidal tract passing into it in the 
pes. 
These are the main tracts of the pedal system. For, though the nucleus 
lenticularis gives off fibres to the internal capsule, our knowledge of the 
further course of these is at present imperfect, and though there seem to be 
longitudinal fibres connecting the bulb, the pons, and the pes at various 
levels, these are not numerous, and at all events do not form conspicuous 
strands. 
LONGITUDINAL FIBRES OF THE TEGMENTAL SYSTEM. 
§ 546. Cortical fibres. Although the fibres of the pedal system form, as 
we have seen, the greater part, they do not form the whole, of the internal 
capsule. Fibres coming from all or nearly all parts of the cortex, though 
they help to form the internal capsule, do not go on to form the pes, but 
pass to the optic thalamus (Fig. 139, 1.1.) and appear to end in the gray 
matter of that body. In their passage through the capsule the fibres of this 
nature from the frontal and parietal regions of the cortex, occupy the ex- 
treme end of the anterior limb in front of the strand of the fibres of the 
pedal system (Fig. 144, Th.). The fibres from the occipital and tem- 
poral regions-those from the occipital regions being the most numerous, 
and indeed being very conspicuous-occupy the extreme hind end of the 
hind limb of the capsule, behind the temporo-occipital division of the pedal 
system (Fig. 144, Op.). Since, as we shall see, we have reason to associate 
the occipital region of the cortex with vision, the fibres thus radiating to (or 
from) the thalamus through the extreme hind limb of the capsule from (or 
to) the occipital cortex have been called the optic radiation. 
All the above tracts of fibres, though joining the thalamus and not pass- 
ing on to the pes, take part in the formation of the internal capsule. But 
a considerable number of fibres coming from the temporo-occipital region, 632 
THE BRAIN. 
and especially from the temporal region, pass to the thalamus without join- 
ing the capsule; they pass ventral to and behind the pes as this plunges into 
the hemisphere to become the capsule, and so reach the thalamus. 
We may here perhaps diverge for a moment to point out the contrast 
between the optic thalamus and the corpus striatum, or at least the nucleus 
caudatus. The former does not contribute to the pedal system, the latter 
supplies a marked contribution. The former receives fibres from all parts 
of the cortex ; there are no such special contributions from the cortex to the 
latter. And this difference accords with the experience that when parts of 
the cortex are removed, or are congenitally absent, no degeneration or want 
of development is observed in the corpus striatum, while degeneration or 
want of development is observed in the optic thalamus as well as in parts of 
the pedal and tegmental systems. Hence, while we may regard the optic 
thalamus as an intermediate mass of gray matter receiving fibres from the 
cortex, and connecting the cortex with lower parts of the tegmental region, 
the corpus striatum appears rather to be analogous to the cortex itself, to 
be a special modification of the cortex, sending fibres down into the pedal 
system, but itself receiving no special tracts of fibres from the cortex. Indeed 
we may probably regard the corpus striatum as the oldest part of the super- 
ficial gray matter of the hemisphere, the more ordinary cortex being a later 
development. 
The tegmentum proper, lying ventral to the hind end of and behind the 
thalamus, in which region, as we have seen, the conspicuous red nucleus is 
situated, is thus, by reason of its connection with the thalamus, indirectly 
connected with the cortex. But besides this, it has direct connections of its 
own with the cortex. Some of the fibres of the optic radiation, as well as 
fibres from the temporal and occipital regions described above as sweeping 
round the base of the internal capsule, are said to pass not to the thalamus, 
but to the tegmentum. Other fibres from the frontal and parietal regions 
traversing the lenticular nucleus in the sheets of white matter splitting the 
nucleus into parts are also said to reach the tegmentum either by piercing 
through or by sweeping round the internal capsule. On their path these 
fibres are, according to some observers, joined by fibres coming from the len- 
ticular nucleus itself, and possibly from the caudate nucleus, which fibres, 
on the view that these nuclei are modified cortex, may also be considered as 
cortical. Thus the fore part of the tegmental region is brought into ample 
connection with the cerebral hemisphere partly by fibres joining the thala- 
mus, partly by fibres passing directly to the tegmentum proper. 
The mode of degeneration of these cortical fibres of the tegmental system 
is at present a matter of dispute. Nor is the general nature of the fibres 
conclusively determined, though it is generally supposed that they carry 
impulses from the thalamus and parts of the tegmentum to the cortex. 
$ 547. In the tegmentum from the subthalamic region to the bulb the 
reticular formation is, as we have seen, more or less abundant; this, and the 
occurrence of various bundles of fibres, gives the region great complexity ; 
and we must confine ourselves here to touching on one or two important 
longitudinal strands which traverse it. 
The superior peduncle of the cerebellum is one of the most important of 
these. This, on each side consists of a bundle of fibres which, taking origin 
chiefly from the gray matter of the nucleus dentatus and the smaller neighbor- 
ing collection of gray matter, and also in part from the superficial gray mat- 
ter, leaves the cerebellum in front of and to the median side of the restiform 
body and passes forward toward the corpora quadrigemina to converge with 
its fellow. At first the two peduncles are superficial and dorsal in position 
(Figs. 134, 135, S. Pf and the space between them is bridged over by the LONGITUDINAL FIBRES OF THE TEGMENTAL SYSTEM. 
633 
valve of Vieussens (Fig. 135, Via); but, still converging, they soon sink 
ventrally beneath the posterior corpora quadrigeinina and at the level of the 
junction between the anterior and posterior corpora quadrigeinina meet and 
decussate ventral to those bodies in the ventral region of the tegmentum 
(Fig. 136, S. P.). Beyond the decussation they are continued forward in 
the tegmentum ventral to the anterior corpora quadrigeinina as two strands, 
one on each side, which appear to end in the red nuclei. 
In this way the peduncles connect certain parts of the gray matter of the 
cerebellum with the tegmental region, and more particularly with the red 
nucleus, and thus indirectly with the structures with which that region is 
itself connected. 
The fillet. This, as we have seen (§ 525), takes origin in the bulb, in the 
inter-olivary layer between the inferior olives, from fibres which are derived 
through the supra-pyramidal or sensory decussation from the gracile and 
cuneate nuclei. From this origin it passes forward on each side as a flat 
band into the tegmental region of the pons, receiving accessions from the 
superior olive and other collections of gray matter, and dividing there into 
two strands, the median (Figs. 135,136, Fin) and lateral (Figs. 135, 136, Fl, 
and Fig. 131, B F) fillet. The lateral division ends partly in the gray 
matter of the posterior corpus quadrigeminuni, and partly in the white 
matter underlying (Fig. 137, dm) the anterior corpus quadrigeminum; the 
median division passing further forward appears partly to end in the gray 
matter of the anterior corpus quadrigeminum, but partly to be continued on 
to the subthalamic region of the tegmentum ventral to the thalamus, thence 
to the thalamus, and so to the cortex. 
The longitudinal posterior bundles. In a transverse section through the 
fore part of the pons at the level of the posterior corpora quadrigeinina a 
rather conspicuous bundle of longitudinal fibres (called the longitudinal 
posterior bundle) is seen on each side, cut transversely, in the dorsal region 
of the tegmentum just ventral to the nucleus of the fourth nerve (Fig. 136,I). 
Traced backward from the aqueduct beneath the fourth ventricle, it becomes 
less conspicuous (Fig. 135, I), though maintaining its position dorsal to the 
reticular formation, and at the hind end of the bulb appears to be a con- 
tinuation forward of those fibres, "ground fibres," of the anterior column of 
the cord which probably serve as successive short longitudinal commissures 
between the segments of the cord. While the somewhat analogous fillet 
runs ventral to the reticular formation, this posterior longitudinal bundle 
runs always dorsal to that structure. It may be traced forward as far as 
the nucleus of the third nerve, as is seen in transverse sections lying imme- 
diately ventral to that group of cells (Fig. 137, I), but its further connections 
forward have not as yet been determined. It is relatively more prominent 
in the lower than in the higher animals, and its fibres acquire their medulla 
relatively early. It is supposed to be connected with the nuclei of the nerves 
governing the muscles of the eye, and so to be concerned in the movements 
of that organ. 
Tracts from the corpora quadrigemina. From each corpus quadrigeminum 
there passes obliquely forward and downward on each side a band of fibres, 
connected with the gray matter of the corpus and known as the brachium. 
The anterior brachium (Fig. 137, Ba), as we shall see in dealing with the 
optic nerve, joins the lateral corpus geniculatum and helps to form the optic 
tract, but some of its deeper-lying fibres proceed to the occipital cortex form- 
ing part of the fibres which we have (§ 545) described as passing from the 
occipital cortex to and past the thalamus. The posterior brachium passes to 
the median corpus geniculatum ; having received fibres from and probably 
given fibres up to that body, it is continued on to the tegmentum, and, ac- 634 
THE BRAIN. 
cording to some authors, through the tegmentum by the hind part of the 
hind limb of the internal capsule to the temporal region of the cortex, 
mingling in its course with fibres from the thalamus. 
Transverse or So-called Commissural Fibres. 
§ 548. The two chief masses are those, on the one hand, belonging to 
the cerebrum, and those, on the other hand, belonging to the cerebellum. 
In the cerebrum the most imposing mass of transverse fibres form the 
corpus callosum. Starting from the cortex in nearly all parts of the hem- 
isphere, the fibres converge toward the thick body of the corpus callosum 
placed in the middle line, and thence diverge to nearly all parts of the 
cortex of the hemisphere on the other side, interlacing in their course 
with the cortical fibres of the pedal and tegmental systems. It is sup- 
posed that by means of these fibres, each part of the cortex of one hemi- 
sphere is brought into connection with the corresponding part of the other 
hemisphere. 
Besides these callosal fibres from one hemisphere to another, the white 
matter of each hemisphere contains fibres called " association fibres," passing 
from one convolution to another of the same hemisphere. 
The small anterior white commissure though it is placed in the front part 
of the third ventricle (Fig. 143, A) and, in part of its course, lies along the 
thalamus (Fig. 140, Cd), is really a commissure of particular parts of the 
cerebral hemispheres. A portion, very small in man, belongs to the olfactory 
tract; the rest takes origin on each side in a limited portion of the cortex 
(Fig. 139, Ca), which we shall later on speak of as the temporo-sphenoidal 
convolution, and in which callosal fibres are deficient, whence it arches for- 
ward through the globus pallidus, past the thalamus (Figs. 146, ca, 140, Cd) 
to the front part of the third ventricle. It may be remarked that this com- 
missure is still found in those lower animals which do not possess an obvious 
corpus callosum. 
The small posterior commissure may be regarded as simply a commissure 
between the two thalami, but it also helps to unite the tegmentum of the two 
sides, and some fibres are said to pass on each side into the hemisphere. The 
middle or soft commissure of the third ventricle (Fig. 138, c), though it con- 
tains transverse fibres, is, in the main, a collection of gray matter, indeed, a 
part of the central gray matter. 
The fornix, together with, at all events, part of the septum lucidum 
which joins it with the corpus callosum, must also be regarded as a com- 
missural structure. But its relations are peculiar; for while, behind, the 
diverging posterior pillars begin in the cerebral hemispheres, namely, in the 
walls of the descending horn of the lateral ventricle on each side, in front 
the anterior pillars or columns, leaving the cerebral hemispheres, pass along 
the lateral walls of the third ventricle (Fig. 143,/), and apparently end 
in the gray matter of the corpora albicantia. Whether the band of fibres, 
known as Vicq d'Azyr's bundle (Fig. 139, Vb), which running in the lat- 
eral wall of the third ventricle leads dorsally from each corpus albicans up 
to the anterior nucleus of the thalamus, is really to be considered as a 
continuation of the fornix is disputed ; it may more probably be regarded 
as a part of the system spoken of above as connecting the cortex with the 
thalamus. 
In the cerebellum true commissural fibres are supplied by the middle 
peduncles; but by no means all the fibres of these peduncles are of this 
nature. The fibres of the middle peduncle, in contrast to those of the 
superior peduncle which starts chiefly from the nucleus dentatus, or other SUMMARY. 
635 
internal gray matter, and to those of the inferior peduncle which start 
chiefly from the superficial gray matter of the vermis, appear to start from 
the superficial gray matter of the whole surface, from that of the median 
vermis as well as from that of the lateral hemispheres; they thus form the 
greater part of the central white matter. Sweeping down into the pons, 
they form the transverse fibres of that body, interlacing with the longitu- 
dinal fibres of the crural system and intermingling with the abundant gray 
matter. 
Of these transverse fibres of the pons, a certain number are truly com- 
missural ; they make no connections with cells in the pons, but continue 
their way unbroken across it; they start in the superficial gray matter of 
one side of the cerebellum and end in the superficial gray matter of the other 
side, the parts of the gray matter thus united being probably corresponding 
parts. The most ventrally placed transverse fibres of the pons which form 
a superficial layer of white matter, free from gray matter (Fig. 134, tr. P.) 
are probably of this nature, as are also the transverse fibres placed most 
dorsally, just ventral to the tegmental region. 
A large number of the transverse fibres are not of this nature. They 
cross from one side of the cerebellum to the opposite side of the pons, but 
end in the pons apparently in the nerve-cells of the gray matter; and it is 
supposed that by these nerve-cells they are brought into connection with the 
longitudinal fibres of the pedal system and thus with the cerebrum. They 
are transverse appendages of the pedal system, not true commissural fibres, 
though they do cross the median line. 
It is further supposed that other fibres of the middle peduncle reaching 
the pons do not cross the median line, but keeping to the same side and 
changing their direction, take a longitudinal upward course either with or 
without the intervention of nerve-cells, and so make their way to the teg- 
mentum. But this is not certain. 
We must also consider as commissural structures the numerous fibres 
crossing or serving to form the median raphe in the bulb. This raphe, with 
similar commissural fibres, is present in the tegmental portion of the pons, and, 
indeed, in the tegmentum itself. 
Fibres also cross from one side to the other in connection with the 
cranial nerves, but these, as well as all the tracts specially connected with 
the cranial nerves, including the olfactory and optic nerves, had better be 
considered by themselves. 
Summary. 
§ 549. It may perhaps appear from the foregoing that the brain consists 
of a number of isolated masses of gray matter, and some large, some small, 
connected together by a multitude of ties of white matter arranged in per- 
plexing intricacy; and the addition of numerous collections of gray matter 
and strands of white matter, of which we have made no mention, would still 
further increase the perplexity. Nevertheless, a systematic arrangement may 
be recognized, at least, to a certain extent. 
The least conspicuous, but perhaps in point of origin the oldest part of the 
brain, seems to be what we have called the central gray matter. This seems 
to serve chiefly as a bed for the development of the nuclei of the cranial 
nerves. 
Next to the central gray matter and more or less associated with it comes 
what is called the tegmental region, of which the reticular formation, com- 
ing into prominence in the bulb and continued on to the subthalamic region, 
forms, as it were, the core. Belonging to the tegmental system are numerous 636 
THE BRAIN. 
masses of gray matter from the conspicuous optic thalamus and the red 
nucleus in front to the several nuclei of the bulb behind. This complex 
tegmental system, which may, perhaps, be regarded as a more or less con- 
tinuous column of gray matter, comparable to the gray matter of the spinal 
cord, serves as a sort of backbone to the rest of the central nervous system. 
With the spinal cord it is connected by various ties, besides being, as it 
were a continuation of the spinal gray matter, and around it are built up 
the great mass of the cerebrum, and the smaller but still larger mass of 
the cerebellum ; the less important corpora quadrigemina we may for sim- 
plicity's sake neglect. 
At the hind end we find various parts of the spinal cord becoming con- 
nected with this tegmental system, either passing into it and becoming, as 
far as our present knowledge goes, lost in it, or supplying strands or fibres 
which passing into it become through it connected with other parts. Thus 
the anterior column of the cord exclusive of the direct pyramidal tract, 
the lateral column exclusive of the crossed pyramidal and cerebellar 
tracts (and possibly the antero-lateral ascending tract), together with part 
of the posterior column, appear to join the tegmental system, while part 
of the posterior column, after the relay of the gracile and cuneate nuclei, 
passes through the system as the fillet destined for various structures. 
At the front end we find all parts of the cerebral cortex (though some 
regions, namely, the temporo-occipital, to a greater extent than others), con- 
nected with the thalamus and other parts of the tegmental system ; and the 
corpus striatum may possibly possess like connections. 
The relations of the cerebellum to this system are notable. On the one 
hand the cerebellum is directly connected with the system, partly by fibres 
which pass from the bulb to join the restiform body or inferior peduncle, 
partly by the superior peduncles which are in a measure lost in the tegmen- 
tum, and partly probably by fibres of the middle peduncles also making 
connections with the tegmentum. On the other hand, the cerebellum forms 
around the tegmental system a great junction between the spinal cord and 
the cerebrum. To the spinal cord it is joined in a direct manner by the 
cerebellar tract and possibly by the antero-lateral ascending tract, and in 
an indirect manner by the relay of the gracile and cuneate nuclei. To all 
parts of the cerebral cortex it appears to be joined by those conspicuous 
strands of the pedal system which end in the pons, and there make connec- 
tions with the fibres of the middle peduncle. And we may here perhaps 
remark that while this connection between the cerebrum and cerebellum is 
wholly a crossed one, each cerebral hemisphere being joined with the oppo- 
site half of the cerebellum, the connections between the spinal cord and the 
cerebellum are largely uncrossed ones, that by the cerebellar tract being 
wholly uncrossed, and that with the posterior column by the relay of the 
gracile and cuneate nuclei being in part uncrossed. 
Thus the cerebral cortex has a double hold, so to speak, on the rest of 
the central nervous system, first through the tegmental system, and secondly 
through the cerebellar junction. But in addition to this there is another 
tie between the cerebral cortex and the whole length of the cerebro-spinal 
axis, or at least between it and the whole series of motor mechanisms in 
succession from the nucleus of the third nerve to the nucleus, if we may so 
call it, of the anterior root of the coccygeal nerve, namely, the great pyram- 
idal tract, which thus appears as a something superadded to all the rest of 
the central nervous system. 
When the cerebral hemispheres are removed this pyramidal tract falls 
away, as does also the pedal system leading from the cerebrum to the pons, 
but there still remains the tegmental system with its cerebellar and other WITHOUT CEREBRAL HEMISPHERES. 
637 
adjuncts, and this, as we shall see, constitutes a nervous machinery capable 
of carrying out exceedingly complicated acts. 
On the Phenomena Exhibited by an Animal Deprived of its 
Cerebral Hemispheres. 
§ 550. The cerebral hemispheres, as we have more than once insisted, 
seem to stand apart from the rest of the brain. In the case of some ani- 
mals it is possible to remove the cerebral hemispheres and to keep the ani- 
mal not only alive, but in good health for a long time-days, weeks, or 
even months after the operation. In such case we are able to study the 
behavior of an animal possessing no cerebral hemispheres, and to compare 
it with that of an intact animal. Such an experiment is best carried out 
on a frog. In this animal it is comparatively easy to remove the cerebral 
hemispheres, including the parts corresponding to the corpora striata, leav- 
ing behind intact and uninjured the optic thalami with the optic lobes (or 
representatives of the corpora quadrigeminal, the small cerebellum, and the 
bulb. If the animal be carefully fed and attended to, it may be kept alive 
for a very long time-for more than a year, for instance. 
The salient fact about a frog lacking the cerebral hemispheres is that, as 
in the case of a frog deprived of its whole brain, the signs of the working 
of an intelligent volition are either wholly absent or extremely rare. The 
presence of the bulb and the middle parts of the brain (for so we may con- 
veniently call the cerebral structures lying between the cerebral hemispheres 
and the bulb) insures the healthy action of the vascular, respiratory, and 
other nutritive systems ; food placed in the mouth is readily and easily swal- 
lowed ; the animal when stimulated executes various movements ; but if it 
be left entirely to itself, and care be taken to shield it from adventitious 
stimuli, either it remains perfectly and permanently quiescent, or the appar- 
ently spontaneous movements which it carries out are so few and so limited 
as to make it very doubtful whether they can fairly be called volitional. 
Such a frog, for instance, after being kept alive for some time and made to 
exhibit the phenomena of which we are about to speak, has been placed on 
a table with a line drawn in chalk around the area covered by its body, and 
left to itself has subsequently been found dead without having stirred out- 
side the chalked circle. 
We must here, however, repeat the caution laid down in § 495, as to the 
ultimate effects of an operation on the central nervous system. The longer 
the frog is kept alive and in good health after the removal of the cerebral 
hemispheres, the greater is the tendency for apparently spontaneous move- 
ments to show themselves. For days, or even weeks, after the operation 
there may be no signs whatever of the working of any volition ; but after 
the lapse of months, movements, previously absent, of such a character as 
to suggest that they ought to be called voluntary, may make their appear- 
ance. To this point we shall return, but may, in the meanwhile, state that 
even in their most complete development such movements do not negative 
the view that the frog in the absence of the cerebral hemispheres is want- 
ing in what we ordinarily call a " will." 
§ 551. We have seen that a frog from which the whole brain has been 
removed, and the spinal cord only left, appears similarly devoid of a "will;" 
but the phenomena presented by a frog possessing the middle portions of the 
brain differ widely from those presented by a frog possessing a spinal cord 
only. We may, perhaps, broadly describe the behavior of a frog from which 
the cerebral hemispheres only have been removed by saying that such an 638 
THE BRAIN. 
animal, though exhibiting no spontaneous movements, can by the application 
of appropriate stimuli be induced to perform all, or nearly all, the move- 
ments which an entire frog is capable of executing. It can be made to swim, 
to leap, and to crawl. Left to itself, it assumes what may be called the natu- 
ral posture of a frog, with the fore limbs erect and the hind limbs flexed, so 
that the line of the body makes an angle with the surface on which it is 
resting. When placed on its back it immediately regains this natural pos- 
ture. When placed on a board it does not fall from the board when the 
latter is tilted up so as to displace the animal's centre of gravity; it crawls 
up the board until it gains a new position in which its centre of gravity is 
restored to its proper place. Its movements are exactly those of au entire 
frog, except that they need an external stimulus to call them forth. They 
differ, moreover, fundamentally from those of an entire frog in the following 
important feature: they inevitably follow when the stimulus is applied ; they 
come to an end when the stimulus ceases to act. By continually varying the 
inclination of a board on which it is placed, the frog may be made to con- 
tinue crawling almost indefinitely; but directly the board is made to assume 
such a position that the body of the frog is in equilibrium, the crawling 
ceases ; and if the position be not disturbed the animal will remain impassive 
and quiet for an almost indefinite time. When thrown into water the 
creature begins at once to swim about in the most regular manner, and will 
continue to swim until it is exhausted, if there be nothing present on which 
it can come to rest. If a small piece of wood be placed on the water the 
frog will, when it comes in contact with the wood, crawl upon it and so 
come to rest. If disturbed from its natural posture, as by being placed on 
its back, it immediately struggles to regain that posture; only by the appli- 
cation of continued force can it be kept lying on its back. Such a frog, if 
its flanks be gently stroked, will croak; and the croaks follow so regularly 
and surely upon the strokes that the animal may almost be played upon like 
a musical, or, at least, an acoustic, instrument. Moreover, provided that the 
optic nerves and their arrangements have not been injured by the operation, 
the movements of the animal appear to be influenced by light; if it be urged 
to move in any particular direction, it seems in its progress to avoid obstacles, 
at least such as cast a strong shadow ; it turns its course to the right or left, 
or sometimes leaps over the obstacle. In fact, even to a careful observer, the 
differences between such a frog and an entire frog which was simply very 
stupid or very inert would appear slight and- unimportant, except in this, 
that the animal without its cerebral hemispheres is obedient to every stimulus, 
and that each stimulus evokes an appropriate movement; whereas, with the 
entire animal it is impossible to predict whether any result at all, and, if so, 
what result, will follow the application of this or that stimulus. Both may 
be regarded as machines; but the one is a machine and nothing more; the 
other is a machine governed and checked by a dominant volition. 
Now, such movements as crawling, leaping, swimming, and, indeed, as we 
have already urged, to a greater or less extent all bodily movements are 
carried out by means of coordinate nervous motor impulses, influenced, 
arranged, and governed by coincident sensory or afferent impulses. Muscu- 
lar movements are determined by afferent influences proceeding from the mus- 
cles and constituting the foundation of the muscular sense; they are also 
directed by means of afferent impulses passing centripetally along the sensory 
nerves of the skin, the eye, the ear, and other organs. Independently of the 
particular afferent impulses, which, acting as a stimulus, call forth the move- 
ment, very many other afferent impulses are concerned in the generation 
and coordination of the resultant motor impulses. Every bodily move- 
ment, such as those of which we are speaking, is the work of a more or less WITHOUT CEREBRAL HEMISPHERES. 
639 
complicated nervous mechanism, in which there are not only central and 
efferent but also afferent factors. And, putting aside the question of con- 
sciousness, with which we have here no occasion to deal, it is evident that in 
the frog deprived of its cerebral hemispheres all th^se factors are present, 
the afferent no less than the central and the efferent. The machinery for all 
the necessary and usual bodily movements is present in all its completeness. 
We may regard the share, therefore, which the cerebral hemispheres take in 
executing the movements of which the entire animal is capable, as that of 
putting this machinery into action or of limiting its previous activity. The 
relation which the higher nervous changes concerned in volition bear to this 
machinery may be compared to that of a stimulus, always bearing in mind 
that the effect of a stimulus on a nervous centre may be either to start 
activity or to increase or to curb or to stop activity already present. We 
might almost speak of the will as an intrinsic stimulus. Its operations are 
limited by the machinery at its command. We may infer that in the frog 
the action of the cerebral hemispheres in giving shape to a bodily movement 
is that of throwing into activity particular parts of the nervous machinery 
situated in the lower parts of the brain and in the spinal cord ; precisely the 
same movement may be initiated in the absence of the cerebral hemispheres 
by applying such stimuli as shall throw precisely the same parts of that 
machinery into the same activity. 
Very marked is the contrast between the behavior of such a frog which, 
though deprived of its cerebral hemispheres, still retains the other parts of 
the brain, and that of a frog which possesses a spinal cord only. The latter 
when placed on its back makes no attempt to regain its normal posture; in 
fact, it may be said to have completely lost its normal posture, for even when 
placed on its belly it does not stand with its fore feet erect, as does the other 
animal, but lies flat on the ground. When thrown into water, instead of 
swimming it sinks like a lump of lead. When pinched or otherwise stimu- 
lated it does not crawl or leap forward; it simply throws out its limbs in 
various ways. When its flanks are stroked it does not croak; and when a 
board on which it is placed is inclined sufficiently to displace its centre of 
gravity it makes no effort to regain its balance, but falls off the board like a 
lifeless mass. Though, as we have seen, the various parts of the spinal cord 
of the frog contain a large amount of coordinating machinery, so that the 
brainless frog may, by appropriate stimuli, be made to execute various pur- 
poseful coordinate movements, yet these are very limited compared with 
those which can be similarly carried out by a frog possessing the middle and 
lower parts of the brain in addition to the spinal cord. It is evident that a 
great deal of the more complex machinery of this kind, especially all that 
which has to deal with the body as a whole, and all that which is concerned 
with equilibrium and is specially governed by the higher senses, is seated 
not in the spinal cord but in the brain. We do not wish now to discuss the 
details of this machinery; all we desire to insist upon at present is that in 
the frog the nervous machinery required for the execution, as distinguished 
from the origination, of bodily movements even of the most complicated kind, 
is present after complete removal of the cerebral hemispheres, though these 
movements are such as to require the cooperation of highly differentiated 
afferent impulses. 
§ 552. In warm-blooded animals the removal of the cerebral hemi- 
spheres is attended with much greater difficulties than in the case of the 
frog. Nevertheless, in the bird the operation may be carried out with 
approximate success. Pigeons, for instance, have been kept alive for five 
or six weeks after complete removal of the cerebral hemispheres, with the 
exception of portions of the crura and corpora striata immediately sur- 640 
THE BRAIN. 
rounding the optic thalami ; these parts were left in order to ensure the 
intact condition of the latter bodies. 
When the immediate effects of the operation have passed off, and for 
some time afterward; the appearance and behavior of the bird are strik- 
ingly similar to those of a bird exceedingly sleepy and stupid. It is able 
to maintain what appears to be a completely normal posture, and can bal- 
ance itself on one leg, after the fashion of a bird which has in a natural 
way gone to sleep. Left alone in perfect quiet,it will remain impassive and 
motionless for a long time. When stirred it moves, shifts its position ; and 
then, on being left alone, returns to a natural, easy posture. Placed on its 
side or its back it will regain its feet; thrown into the air, it flies with con- 
siderable precision for some distance before it returns to rest. It frequently 
tucks its head under its wings, and at times may be seen to clean its feath- 
ers; when its beak is plunged into corn, it eats. It maybe induced to 
move not only by ordinary stimuli applied to the skin, but also by sudden 
loud sounds, or by flashes of light; in its flight it will, though imperfectly, 
avoid obstacles, and its various movements appear to be to a certain extent 
guided not only by touch but also by visual impressions. 
In a certain number of cases this sleepy, drowsy condition passes off and 
is succeeded by a phase in which the bird, apparently spontaneously, without 
the intervention of any obvious stimulus, moves rapidly about. It does 
not fly, that is to say, it does not raise itself from the ground in flight, but 
walks about incessantly for a long while at a time, the periods of activity 
alternating with periods of repose. It seems, from time to time, to wake up 
and move about, and then to go to sleep again ; and it has been observed 
that during the night it appears to be always asleep. It is obvious, there- 
fore, that the sleepy, quiescent condition is not due simply to the absence 
of the cerebral hemispheres, but is a temporary effect of the operation, and 
that spontaneous movements, that is to say, movements not started by any 
obvious stimulus, may occur after removal of the cerebral hemispheres. 
But the movements so witnessed differ from those of an intact bird. They 
are, it is true, varied ; and the variations are in part dependent on external 
circumstances, the bird being guided by tactile, and as we have said, visual 
sensations, or, to be more exact, by impressions made upon the sensory 
nerves of the skin and on the retina; but they do not show the wide varia- 
tions of voluntary movements. The bird never Hies up from the ground, 
never spontaneously picks up corn, and its aimless, monotonous, restless 
walks, resembling the continuous swimming of the frog thrown into the 
water after being deprived of its cerebral hemispheres, forcibly suggest that 
the activity is the outcome of some intrinsic impulse generated in the 
nervous machinery in some way or other, but not by the working of a con- 
scious intelligence as in the impulse which we call the will. 
Still we must not shut our eyes to the fact that spontaneous movements, 
whatever their exact nature, are manifested by a bird in the absence of the 
cerebral hemispheres, and become the more striking the more complete the 
recovery from the passing effects of the mere operation. Could such birds 
be kept alive for any considerable time, possibly further developments 
might be witnessed, and indeed cases are on record where birds have been 
kept alive for months after the operation, and have shown spontaneous 
movements of a still more varied character than those just described ; but 
in such cases the removal of the hemispheres has not been complete, por- 
tions of the ventral regions being left behind ; and, though a mere remnant 
left around the optic thalami can hardly be regarded as a sufficient cause 
for the spontaneity of which we are speaking, a larger mass, still more or 
less retaining its normal structure, might have a marked effect. And we 641 
WITHOUT CEREBRAL HEMISPHERES. 
may here perhaps remark that all these facts seem to point to the conclu- 
sion that what may be called mechanical spontaneity, sometimes spoken of 
as "automatism,'' differs from the spontaneity of the "will" in degree 
rather than in kind. Looking at the matter from a purely physiological 
point of view (the only one which has a right to be employed in these 
pages), the real difference between an automatic act and a voluntary act is 
that the chain of physiological events between the act and its physiological 
cause is in the one case short and simple, in the other long and complex. 
We have seen that a frog lacking its cerebral hemispheres, viewed from one 
standpoint, appears in the light of a mechanical apparatus, on which each 
change of circumstances produces a direct, unvarying, inevitable effect. 
And yet it is on record that such a frog, if kept alive long enough for the 
most complete disappearance of the direct effects of the operation, will 
bury itself in the earth at the approach of winter, and is able to catch and 
swallow flies and other food coming in its neighborhood, although in other 
respects it shows no signs of an intelligent volition, and answers with un- 
erring mechanical certainty to the play of stimuli. We may add that in 
some fishes the removal of their cerebral hemispheres, which in these ani- 
mals form a relatively small part of the whole brain, produces exceedingly 
little change in their general behavior. 
These, however, are not the considerations on which we wish here to 
dwell; we have quoted the behavior of the bird deprived of its cerebral 
hemispheres mainly to show that in this warm-blooded animal, as in the 
more lowly cold-blooded frog, the parts of the brain below or behind the 
cerebral hemispheres constitute a nervous machinery by which all the ordi- 
nary bodily movements may be carried out. The bird, like the frog, suffers 
no paralysis when the cerebral hemispheres are removed ; on the contrary, 
though its movements have not been studied so closely as those of the frog, 
the bird without its cerebral hemispheres seems capable of executing at all 
events all the ordinary bodily movements of a bird. And in the bird as in 
the frog, the afferent impulses passing into the central nervous system, 
whether they give rise to consciousness or not, play an important part not 
only in originating but in guiding and coordinating the efferent impulses 
which stir the muscles to contract, the coordination being effected partly in 
the spinal cord, but largely and indeed chiefly in the parts of the brain 
lying behind the cerebral hemispheres. It is further worthy of notice that 
spontaneity of movement of the kind which we have described is much 
more prominent in the more highly developed bird than in the more lowly 
frog. The cerebral hemispheres are not the only part of the central nerv- 
ous system which has undergone a greater development in the bird ; the 
other parts of the brain have also acquired a far greater complexity than 
in the frog. 
§ 553. In the mammal the removal of the cerebral hemispheres is still 
more difficult than in the bird ; the animal cannot be kept alive for more 
than a few hours; but in some mammals it is possible to observe during 
those few hours phenomena kindred to those witnessed in the bird 
and in the frog. The rabbit or rat, from which the whole of both hemi- 
spheres has been removed with the exception of the parts immediately 
surrounding the optic thalami, can stand, run, and leap. Placed on its 
side or back it at once regains its feet. Left alone it generally remains as 
motionless and impassive as a statue, save now and then when a passing 
impulse seems to stir it to a sudden but brief movement; but sometimes it 
seems subject to a more continued impulse to move, in which case death 
usually follows very speedily. Such a rabbit will remain for minutes together 
utterly heedless of a carrot or cabbage-leaf placed just before its nose, though 642 
THE BRAIN. 
if a morsel be placed within its mouth it at once begins to eat. When stirred 
it will with ease and steadiness run or leap forward ; and obstacles in its 
course are very frequently, with more or less success, avoided. In some cases 
the animal (rat) has been described as following by movements of the head a 
bright light held in front of it (provided that the optic nerves and tracts 
have not been injured during the operation), as starting when a shrill and 
loud noise is made near it, and as crying when pinched, often with a long 
and seemingly plaintive scream. So plaintive is the cry which it thus gives 
forth as to suggest to the observer the existence of passion ; this, however, is 
probably a wrong interpretation of a vocal action ; the cry appears plaintive 
simply because, in consequence of the completeness of the reflex nervous 
machinery and the absence of the usual restraints, it is prolonged. 
Without insisting too much on such results as these, and allowing full 
weight to the objection which may be urged, that in some of these cases 
parts of the cerebral hemispheres surrounding the optic thalami were left, 
there still remains adequate evidence to show that a mammal such as a 
rabbit, in the same way as a frog and a bird, may in the complete or all but 
complete absence of the cerebral hemispheres maintain a natural posture, 
free from all signs of disturbance of equilibrium, and is able to carry out 
with success, at all events all the usual and common bodily movements. 
And as in the bird and frog, the evidence also shows that these movements 
not only may be started by, but in their carrying out are guided by and 
coordinated by afferent impulses along afferent nerves, including those of the 
special senses. But in the case of the rabbit it is even still clearer than in 
the case of the bird that the effects of these afferent impulses are different 
from those which result when the impulses gain access to an intact brain. 
The movements of the animal seem guided by impressions made on its retina, 
as well as on other sensory nerves; we may perhaps speak of the animal as 
the subject of sensations; but there is no satisfactory evidence that it pos- 
sesses either visual or other perceptions, or that the sensations which it ex- 
periences give rise to ideas. Its avoidance of objects depends not so much 
on the form of these as on their interference with light. No image, whether 
pleasant or terrible, whether of food or of an enemy, produces an effect on 
it, other than that of an object reflecting more or less light. And we may 
infer that it lacks the possession of an intelligent will. But it must always 
be remembered that some of the phenomena are due to the operation pro- 
ducing other results than the mere absence of the part removed. We must 
bear in mind that in all the above experiments, while the positive phenomena, 
the things which the animal continues able to do, are of great value, the 
negative phenomena, the things which the animal can no longer do, are of 
much less, indeed of doubtful value. The more carefully and successfully 
the experiments are carried out, the narrower become what we may call the 
" deficiency phenomena," the phenomena which are alone and directly due 
to something having been taken away. Were it possible to keep the rabbit 
alive long enough for the mere effects of the operation to pass completely 
away, we should not only probably witness, as in the case of the bird, a 
greater scope of movement and more frequent spontaneity, but possibly find 
a difficulty in describing the exact condition of the animal. 
§ 554. Hitherto attempts to witness similar phenomena in more highly 
organized mammals, such as the dog, have failed ; these animals do not 
recover from the operation of removing the whole of both their hemispheres 
sufficiently to enable us to judge whether they, like the frog, the bird, and 
the rabbit, can carry out coordinate bodily movements in the absence of the 
hemispheres, or whether in them this part of the brain, so largely developed, 
has usurped functions which in the lower animals belong to other parts. THE MACHINERY OF COORDINATED MOVEMENTS. 
643 
Our knowledge is largely confined to the experience that when in a dog the 
cerebral convolutions are removed piecemeal at several operations, the 
animal may be kept alive and in good health for a long time, many months 
at least, even after these parts of the brain have been reduced to very small 
dimensions, and that under these circumstances the animal is not only able 
to carry out with some limitations his ordinary bodily movements, but also 
exhibits a spontaneity obviously betokening the possession not merely of a 
conscious volition but of a certain amount of intelligence. Unless we are 
willing to believe that a mere fragment, so to speak, of the hemispheres can 
take on most extended powers, such an experience seems to show that in the 
dog as in the rabbit and in the bird, the development of so-called higher 
functions is not limited to the cerebral hemispheres, that the middle and 
lower portions of the brain in the higher animals as compared with the 
lower do not increase in bulk merely as the instruments of the hemispheres, 
but like the hemispheres acquire more and more complex functions. We 
may perhaps go so far as to ask the question whether the volition and intel- 
ligence which such a dog exhibits is not as much the product of the parts 
lying behind the hemispheres as of the stump left in the front. 
If we can thus say little about the condition of a dog without the cerebral 
hemispheres, we can say still less about the monkey, which in all matters 
touching the cerebral nervous system serves as our best, indeed our only 
guide for drawing inferences concerning man ; but in all probability the 
monkey in this respect bears somewhat the same relation to the dog that the 
dog bears to the bird. 
In short, the more we study the phenomena exhibited by animals pos- 
sessing a part only of their brain, the closer we are pushed to the conclusion 
that no sharp line can be drawn between volition and the lack of volition, 
or between the possession and absence of intelligence. Between the muscle- 
nerve preparation at the one limit and our conscious willing selves at the 
other there is a continuous gradation without a break ; we cannot fix on 
any linear barrier in the brain or in the general nervous system, and say, 
" Beyond this there is volition and intelligence, but up to this there is none." 
This, however, is not the question with which we are now dealing. What 
we want to point out is that in the higher animals, including at least some 
mammals, as in the frog, after the removal of the cerebral hemispheres, even 
though conscious volition and intelligence appear to be largely, if not en- 
tirely lost, the body is still capable of executing all the ordinary movements 
which the animal in its natural life is wont to perform, in spite of these 
movements necessitating the cooperation of various afferent impulses; and 
that therefore the nervous machinery for the execution of these movements 
lies in some part of the brain other than the cerebral hemispheres. We 
have reason for thinking that it is situated in the structures forming the 
middle- and hind-brain ; as we shall see, interference with these parts pro- 
duces at once remarkable disorders of movement. 
The Machinery of Coordinated Movements. 
§ 555. We may now direct our attention for a while to some considerations 
concerning the nature of this complex nervous machinery for the coordina- 
tion of bodily movements, and especially concerning the part played by 
afferent impulses. Most of our knowledge on this point has been gained 
by a study of animals not deprived of, but still possessing, their cerebral 
hemispheres, or by deductions from the data of our own experience; but it 
is possible in most cases to eliminate from the total results the phenomena 
which are due to the working of a conscious intelligence. Some of the most 644 
THE BRAIN. 
striking facts bearing on this matter have been gained by studying the effects 
of operative interference with certain parts of the internal ear, known as the 
semicircular canals. 
When in a pigeon the horizontal membranous semicircular canal is cut 
through, the bird is observed to be continually moving its head from side to 
side. If one of the vertical canals be cut through, the movements are up 
and down. The peculiar movements may not be witnessed when the bird is 
perfectly quiet, but they make their appearance whenever it is disturbed or 
attempts in any way to stir. When the injury is confined to one canal only, 
or even to the canals of one side of the head only, the condition after a 
while passes away ; when the canals of both sides have been divided, it 
becomes much exaggerated, lasts much longer, and in some cases is said to 
remain permanently. After such injuries it is found that these peculiar 
movements of the head are associated with what appears to be a great want 
of coordination of bodily movements. If the bird be thrown into the air, 
it flutters and falls down in a helpless and confused manner; it appears to 
have lost the power of orderly flight. If placed in a balanced position, it 
may remain for some time quiet, generally with its head in a peculiar pos- 
ture : but directly it is disturbed, the movements which it attempts to execute 
are irregular and fall short of their purpose. It has great difficulty in pick- 
ing up food and in drinking; and in general its behavior very much resem- 
bles that of a person who is exceedingly dizzy. 
It can hear perfectly well, and therefore the symptoms cannot be regarded 
as the result of any abnormal auditory sensations, such as a "roaring" in the 
ears. Besides any such stimulation of the auditory nerve as the result of the 
section would speedily die away, whereas these phenomena may last for a 
very considerable time. 
The movements are not occasioned by any partial paralysis, by any want 
of power in particular muscles or group of muscles ; though removal of 
the canals of one side has been described as leading to diminished muscular 
force on the same side of the body, the mere diminution of force is insuffi- 
cient to explain the phenomena. Nor, on the other hand, are the movements 
due to any uncontrollable impulse; a very gentle pressure of the hand suf- 
fices to stop the movements of the head, and the hand in doing so experi- 
ences no strain. The assistance of a very slight support enables movements 
otherwise impossible or most difficult to be easily executed. Thus, though 
when left alone the bird has great difficulty in drinking or picking up corn, 
it will continue to eat with ease if its beak be plunged into water or into a 
heap of barley ; the slight support of the water or the grain seems sufficient 
to steady its movements. In the same way it can, even without assistance, 
clean its feathers and scratch its head, its beak and foot being in these ope- 
ations guided by contact with its own body. 
The amount of disorder thus induced differs in different birds, and some 
movements are more affected than others. As a general rule, it may be said 
that the more complex and intricate a movement, the fuller and more del- 
icate the coordination needed to carry it out successfully, the more markedly 
is it disordered by the operation ; thus, after injury of the canals, while a 
pigeon cannot fly, a goose is still able to swim. 
In mammals (rabbits) section of the canals also produces a certain 
amount of loss of coordination, but much less than that witnessed in birds; 
and the movements of the head are not so marked, peculiar oscillating 
movements of the eyeballs, differing in direction and character according 
to the canal or canals operated upon, becoming, however, prominent. In 
the frog no deviations of the head are seen, but there is some loss of coordi- 
nation in the movements of the body. In fishes no effect at all is produced. THE MACHINERY OF COORDINATED MOVEMENTS. 
645 
Injury to the bony canals alone is insufficient to produce the symptoms; 
the membranous canals themselves must be divided or injured. The cha- 
racteristic movements of the head may, however, be brought about in a bird 
without opening the bony canal, by suddenly heating or cooling a canal, 
especially its ampullar terminations, or by the making or breaking of a con- 
stant current directed through the canal. 
There can be no doubt that these characteristic movements of the head 
are the result of afferent impulses started in the nervous endings of the 
auditory nerve over the ampulla of the canal and conveyed to the brain 
along that nerve. And that injury to or other stimulation of each of the 
three canals should produce in each case a different movement of the head, 
the direction of the movement being different according to the plane in 
which the canal lies, shows that these impulses are of a peculiar nature. 
This is further illustrated by the following experiment: If the horizontal 
canal be carefully laid bare, and the membranous canal opened so as to ex- 
pose the endolymph, blowing gently over the opened canal with a fine glass 
canula will produce a definite movement of the head, which is turned to the 
one side or to the other, according as the current of air drives the endolymph 
toward or away from the ampulla. From this it is inferred that a movement 
of the endolymph over, or an increased pressure of the endolymph on, the 
nervous endings in the ampulla gives rise to afferent impulses which in 
some way determine the issue of efferent impulses leading to the movement 
of the head. It is further suggested that since the planes of the three canals 
lie in the three axes of space, any change in the position of the head must 
lead to changes in the pressure of the endolymph on the walls of the ampul- 
lae or to movements of endolymph over those walls, and so must give rise 
to impulses passing up the auditory nerve; and that since every change of 
position will affect the three canals differently (whereas, the changes of pres- 
sure of the endolymph involved in a "wave of sound" will affect all three 
ampullae equally), those impulses will differ according to the direction of 
the change. A still further extension of this view supposes that since in 
any one position of the head the pressure of the endolymph will differ in 
the three ampullae, mere position of the head, as distinguished from change 
of position, is adequate to generate afferent impulses differing in the differ- 
ent positions. 
Let us now for a while turn aside to ourselves and examine the coordina- 
tion of the movements of our own bodies. When we appeal to our own con- 
sciousness we find that our movements are governed and guided by what 
we may call a sense of equilibrium, by an appreciation of the position of our 
body and its relations to space. When this sense of equilibrium is dis- 
turbed we say we are dizzy, and we then stagger and reel, being.no longer 
able to coordinate the movements of our bodies or to adapt them to the 
position of things around us. What is the origin of this sense of equilib- 
rium ? By what means are we able to appreciate the position of our 
body ? There can be no doubt that this appreciation is in large measure 
the product of visual and tactile sensations ; we recognize the relations of 
our body to the things around us in great measure by sight and touch ; we 
also learn much by our muscular sense. But there is something besides 
these. Neither sight nor touch nor muscular sense can help us when, placed 
perfectly flat and at rest on a horizontal rotating table, with the eyes shut 
and not a muscle stirring, we attempt to determine whether or not the table 
and we with it are being moved, or to ascertain how much it and we are 
turned to the right or to the left. Yet under such circumstances we are 
conscious of a change in our position, and some observers have been even 
able to pass a tolerably successful judgment as to the angle through which. 646 
THE BRAIN. 
they have been moved. There can be no doubt that such a judgment is 
based upon the interpretation by consciousness of afferent impulses which 
are dependent on the position of the body, but which are not afferent im- 
pulses belonging to sensations of touch or sight, or taking part in the mus- 
cular sense. And it is urged with great plausibility that the afferent im- 
pulses in question are those which we have just referred to as started in the 
semicircular canals. 
If we admit the existence of such ampullar impulses, if we may venture 
so to call them, and recognize them as contributing largely not only to our 
direct perception of the position of the head and thus of the body, but also 
in a more indirect way to what we have called the sense of equilibrium, we 
should expect to find that when they are abnormal the sense of equilibrium 
is disturbed, and that in consequence a failure of coordination in our move- 
ments results. And the loss of coordination which we described above as 
resulting from injury to the semicircular canals has accordingly been attrib- 
uted to a deficiency or disorder of normal ampullar impulses. 
But we must here distinguish between two things. It seems clear that 
when the membranous canals are injured or otherwise stimulated, afferent 
impulses are generated which, on the one hand, may produce peculiar move- 
ments of the head, and, on the other hand, seem able when the injury is large 
to cause a loss of coordination of bodily movements. But it does not neces- 
sarily follow from this that in a normal condition of things afferent impulses 
are continually passing up to the brain from the semicircular canals, and 
that the loss of coordination which follows upon injury to the canals is due 
to these normal impulses being deficient or altered. It may be that such 
normal impulses do not exist, and that the loss of coordination is the result 
of the central machinery for coordination being interfered with by quite new 
impulses generated by the injury to the canal with the consequent loss of 
endolymph acting as a stimulus to the endings of the nerve. For the expe- 
rience quoted above, though it proves that afferent impulses other than those 
of sight, touch, and the muscular sense do reach the brain and afford a basis 
for a judgment as to the position of the body, does not by itself prove that 
those impulses come from the semicircular canals; the arrangement of the 
canals is undoubtedly suggestive ; but it is quite possible that the afferent 
impulses in question may be generated by one or other of various changes, 
vasomotor and others, of the tissues of the body which are involved in a 
change of position. And if it be true, as affirmed by some observers, that 
both auditory nerves may be completely and permanently severed without 
any effect on the coordination of movements, it is obvious that the incoordi- 
nation which follows upon section of the semicircular canals- is due to some 
special irritation setup by the operation, and not to the mere absence of any 
normal ampullar impulses. On the other hand, if the effects are those of 
irritation, it is difficult to understand how they can, as according to certain 
observers they certainly do, become permanent. It has, however, been 
strongly urged that in such cases of permanent incoordination, the operation 
has set up secondary mischief in the brain, in the cerebellum for instance, 
with which, as we have seen (§ 531), the vestibular auditory nerve makes 
special connections, and that the permanent effects are really due to the dis- 
ease going on here; and we have reason, as we shall see, to think that the 
cerebellum is concerned in the coordination of movements. It cannot, 
therefore, be regarded as settled that the canals are the source of normal 
impulses, or that our conscious appreciation of the position of the head and 
so of the body in space is based on such impulses. But such a view is not 
disproved ; and in any case it remains true that injury to the canals does in 
some way or other, either by generating new impulses or by altering pre- THE MACHINERY OF COORDINATED MOVEMENTS. 
647 
existing ones, so modify the flow of afferent impulses into the machinery of 
coordination as to throw that machinery out of gear. 
§ 556. We have dwelt on these phenomena of the semicircular canals 
because they illustrate in a striking manner the important part played by 
afferent impulses in the coordination of movements. We saw reason to 
think (§ 502) that even in an ordinary reflex movement carried out by the 
spinal cord or by a portion of the cord, afferent impulses, other than those 
which excite the movement are at work, determining such coordination as 
is present. In such a case the coordinating afferent impulses are relatively 
simple in character and start chiefly at all events in the muscles concerned. 
In an animal possessing the lower parts of the brain, though deprived of the 
cerebral hemispheres, the coordinating afferent impulses, in accordance with 
the greater diversity and complexity of the movements which the animal is 
able to execute, are far more potent and varied. Besides afferent impulses 
from the muscles, forming the basis of what we have called the muscular 
sense, afferent impulses from the skin, forming the basis of the sense of touch 
in the wide meaning of that word, other afferent impulses of obscure cha- 
racter from the viscera and various tissues, and the peculiar afferent ampullar 
impulses of which we have just spoken, important special afferent impulses 
borne along the nerves of sight and hearing come into play. The frog, the 
bird, and even the mammal, deprived of the cerebral hemispheres, though it 
may show little signs or none at all of having a distinct volition, is, as we 
have urged, indubitably affected by visual and auditory impressions, and 
whether we admit or not that such an animal can rightly be spoken of as 
being conscious, we cannot resist the conclusion that afferent impulses started 
in its retina or internal ear produce in its central nervous system changes 
similar to those which in a conscious animal form the basis of visual and 
auditory sensations, and we must either call these changes sensations or find 
for them some new word. Whatever we call them, and whether conscious- 
ness is distinctly involved in them or not, they obviously play an important 
part as factors of the coordination of movements. Indeed, when we appeal 
to the experience of ourselves in possession of consciousness, we find that 
though various sensations clearly enter into the coordination of our move- 
ments, we carry out movements thus coordinated without being distinctly 
aware of these coordinating factors. In every movement which we make 
the coordination of the movement is dependent on the impulses or influences 
which form the basis of the muscular sense, yet we are not distinctly con- 
scious of these impulses ; it is only, as we shall see, by special analysis that 
we come to the conclusion that we do possess what we shall call a muscular 
sense. So again, taking the matter from a somewhat different point of view, 
many of our movements, markedly, as we shall see, those of the eyeballs, are 
coordinated by visual sensations, and when we sing or when we dance to 
music our movements are coordinated by the help of sensations of sound. In 
these cases distinct sensations in the ordinary sense of the word intervene; 
if we cannot see or cannot hear, the movement fails or is imperfect; yet 
even in these cases we are not directly conscious of the sensations as coordi- 
nating factors; it needs careful analysis to prove that the success of the 
movement is really dependent on the sound or on the sight. These and 
other facts suggest the view that the point at which the various afferent im- 
pulses which form the basis of the sensations of a conscious individual enter 
into the coordination mechanism is or may be some way short of the stage 
at which the complete conversion of the impulse into a perfect sensation 
takes place. The events which constitute what we may call visual impulses, 
as these leave the retina to sweep along the optic nerve are, we must admit, 
very different from those which in the appropriate parts of the brain con- 648 
THE BRAIN. 
stitute what we may call conscious vision ; and probably between the begin- 
ning and the end there are progressive changes. It is probable, we say, that 
these visual events may affect the coordinating mechanism at some stage of 
their progress before they reach their final and perfect form. If this be so 
we may further conclude that though, when the whole nervous machinery is 
present in its entirety, the afferent impulses which take part in coordination 
must inevitably at the same time give rise to conscious sensations, they 
might still effect their coordinating work when, owing to their imperfec- 
tion or lack of the terminal part of the nervous machinery, the impulses 
failed to receive their final transformation, and conscious sensations were 
absent. In other words, the coordinating influences of sensory or afferent 
impulses are not essentially dependent on the existence of a distinct con- 
sciousness. 
§ 557. We have raised this point partly for the sake of illustrating the 
working of the coordination machinery in the absence of the cerebral hemi- 
spheres, but also in order to aid in the interpretation of the subjective condi- 
tion which we speak of as giddiness or dizziness or vertigo. The condition 
of the pigeon after an injury to the semicircular canals is comparable to that 
of a person who is giddy or dizzy, and, indeed, vertigo is the subjective 
expression of a disarrangement of the coordination machinery, especially 
of that concerned in the maintenance of bodily equilibrium. It may be 
brought about in many ways. When a constant current of adequate strength 
is sent through the head from ear to ear, we experienced a sense of vertigo ; 
our movements then appear to a bystander to fail in coordination, in fact to 
resemble those of a pigeon whose semicircular canals have been injured ; 
and, indeed, the effects are probably produced in the same way in the two 
cases. In what is called Meniere's disease attacks of vertigo seem to be asso- 
ciated with disease in the ear, being attributed by many to disorder of the 
semicircular canals, and cases have been recorded of giddiness as well as 
deafness resulting from disease of the auditory nerve. Visual sensations are 
very potent in producing vertigo. Many persons feel giddy when they look 
at a waterfall; and this is a case in which both the sense of giddiness and 
the disarrangement of coordination is the result of the action of a pure sen- 
sation and nothing else. In the well-known intense vertigo which is caused 
by rapid rotation of the body visual sensation plays a part when the rotation 
is carried on with the eyes open, but only a part; for vertigo may be induced, 
though not so readily, by rotation with the eyes completely shut. In the 
latter case it has been suggested that the vertigo is caused by abnormal am- 
pullar impulses, but these can only contribute to the result, which is in the 
main caused by direct disturbance of the brain. When the rotation is car- 
ried out with the eyes open, the vertigo which is felt when the rotation ceases 
is partly caused by the visual sensations, on account of the behavior of the 
eyeballs, ceasing to be in harmony with the rest of the sensations and affer- 
ent impulses which help to make up the coordination. The rotation sets up 
peculiar oscillating movements of the eyeballs, which continue for some time 
after the rotation has ceased ; owing to these movements of the eyeballs the 
visual sensations excited are. such as would be excited if external objects 
were rapidly moving, whereas all the other sensations and impulses which 
are affecting the central nervous system are such as are excited by objects at 
rest. In a normal state of things the visual and the other sensations and 
impulses which go to make up the coordinating machinery are in accord 
with each other in reference to the events in the external world which are 
giving rise to them ; after rotation they are for a time in disaccord, and the 
coordinating machinery is in consequence disarranged. 
When we interrogate our own consciousness, we find that we are not dis- THE MACHINERY OF COORDINATED MOVEMENTS. 
649 
tinctly conscious of this disaccord ; the visual sensations are so prepotent in 
consciousness that we really think the external world is rapidly whirling 
round ; all that we are further conscious of is the feeling of giddiness and our 
inability to make our bodily movements harmonize with our visual sensations. 
So that even in the cases where the loss of coordination is brought about by 
distinct sensations, what we really appreciate by means of our consciousness 
is the disarrangement of the coordinating machinery. It is the apprecia- 
tion of this disorder which constitutes the feeling of vertigo ; both the feel- 
ing of giddiness and the disordered movements are the outcome, one sub- 
jective and the other objective, of the same thing. It is not because we 
feel giddy that we stagger and reel; our movements are wrong because the 
machinery is at fault, and it is the faulty action of the machinery which also 
makes us feel giddy. 
We may here perhaps remark that it is an actually disordered condition 
of the coordinating mechanism which gives rise to the affection of conscious- 
ness which we call giddiness, not a mere curtailing of the mechanism or any 
failure on its part to make itself effective. Complete blindness limits the 
range of activity of the machinery but leaves the remainder intact, and no 
giddiness is felt. So again in certain diseases of the nervous system the 
muscular sense is interfered with over considerable regions of the body, and 
in these regions coordination fails or is imperfect, but the central machinery 
is not thereby affected, though its area of usefulness is limited, and no giddi- 
ness is experienced ; and so in other instances. 
§ 558. Forced movements. So far we have dwelt on disorders of the co- 
ordinating machinery brought about by the action of various afferent im- 
pulses. We have now to call attention to some peculiar phenomena which 
result from operative interference with parts of the brain, and which in some 
instances at least may be taken to illustrate how this complex machinery 
works when some of its inner wheels are broken. 
All investigators who have performed experiments on the brain have ob- 
served, as the result of injury to various parts of it, remarkable movements 
which have the appearance of being irresistible, compulsory, forced. They 
vary much in the extent to which they are developed ; some are so slight as 
hardly to deserve the name, while others are strikingly intense. One of the 
most common forms is that in which the animal rolls incessantly round the 
longitudinal axis of its own body. This is especially common after section 
of one of the crura cerebri, or of the middle and inferior peduncles of the 
cerebellum, or after unilateral section of the pons, but has also been wit- 
nessed after injury to the bulb and corpora quadrigemina. Sometimes the 
animal rotates toward and sometimes away from the side operated on. An- 
other form is that in which the animal executes " circus movements," i. e., 
continually moves round and round in a circle of longer or shorter radius, 
sometimes toward and sometimes away from the injured side. This may be 
seen after several of the above-mentioned operations, and in one form or an- 
other is not uncommon after various unilateral injuries to the brain. There 
is a variety of the circus movement, the " clock-hand movement," said to 
occur frequently after lesions of the posterior corpora quadrigemina, in 
which the animal moves in a circle, with the longitudinal axis of its body 
as a radius and the end of its tail for a centre. And this form again may 
easily pass into a simple rolling movement. In yet another form the animal 
rotates over the transverse axis of its body, tumbles head over heels in a 
series of somersaults; or it may run incessantly in a straight line backward 
or forward until it is stopped by some obstacle. These latter forms of forced 
movements are sometimes seen after injury to the corpus striatum even when 
a very limited portion of the gray matter is affected. And many of these 650 
THE BRAIN. 
forced movements may result from injuries which appear to be confined to 
the cerebral cortex. 
When the phenomena are well developed, every effort of the animal brings 
on a movement of this forced character. Left to itself and at rest the ani- 
mal may present nothing abnormal, its posture and attitude may be quite 
natural; but when it is excited to move or when it attempts of itself to move, 
it executes not a natural movement, but a forced one, turning round or roll- 
ing over as the case may be. In severe cases the movement is continued 
until the animal is exhausted ; when the exhaustion passes off the animal 
may remain for some little time quiet, but some stimulus, intrinsic or ex- 
trinsic, soon inaugurates a fresh outbreak, to be again followed by exhaustion. 
In some of the milder forms, that for instance of the circus movement 
with a long radius, the curved character of the progression appears simply 
due to the fact that in the effort of locomotion volitional impulses do not 
gain such ready access to one side of the body as to the other, the injury 
having caused some obstacle or other. Hence the contractions of the mus- 
cles of one side (the left, for instance) of the body are more powerful than 
the other, and in consequence the body is continually thrust toward the other 
(the right) side. As is well known, we ourselves, when our walk is not 
guided by visual sensations, tend to describe a circle of somewhat wide 
radius, the deviation being due to a want of bilateral symmetry in our limbs ; 
and the above circus movement is only an exaggeration of this. 
But the other more intense forms of forced movements are more compli- 
cated in their nature. No mere blocking of volitional impulses will explain 
why an animal whenever it attempts to move rolls rapidly over or rushes 
irresistibly forward or backward. It is not possible with our present know- 
ledge to explain how each particular kind of movement is brought about; 
and, indeed, the several kinds are probably brought about in different ways, 
for they differ so greatly from each other that we only class them together 
because it is difficult to know where to draw the line between them. But 
we may regard the more intense forms as illustrating the complex nature of 
what we have called the coordinating machinery, the capabilities of which 
are, so to speak, disclosed by its being damaged. Such gross injuries as are 
involved in dividing cerebral structures or in injecting corrosive substances 
into this or that part of the brain, must, of necessity, partly by blocking the 
way to the impulses which in a normal state of things are continually pass- 
ing from one part of the brain to another, partly by generating new unusual 
impulses, seriously affect the due working of the general coordinating machin- 
ery. The fact that an animal can, at any moment, by an effort of its own 
will, rotate on an axis or run straight forward, shows that the nervous 
mechanism for the execution of those movements is ready at hand in the 
brain, waiting only to be discharged; and it is easy to conceive how such a 
discharge might be affected either by the substitution for the will of some 
potent intrinsic afferent impulse or by some misdirection of volitional im- 
pulses. Persons who have experienced similar forced movements as the 
result of disease report that they are frequently accompanied, and seem to 
be caused, by disturbed visual or other sensations; thus they attribute their 
suddenly falling forward to the occurrence of the sensation that the ground 
in front of them is suddenly sinking away beneath their feet. Without 
trusting too closely to the interpretations the subjects of these disorders give 
of their own feelings, and remembering what was said above concerning ver- 
tigo, we may at least conclude that the unusual movements are in many 
cases due to a disorder of the coordinating mechanism, brought about by 
strange or disordered sensory impulses. And this view is supported by the 
fact that many of these forced movements are accompanied by a peculiar THE MACHINERY OF COORDINATED MOVEMENTS. 
651 
and wholly abnormal position of the eyes, which alone might perhaps explain 
many of the phenomena. 
§ 559. The phenomena presented by animals deprived of their cerebral 
hemispheres show that this machinery of coordination is supplied by cerebral 
structures lying between the cerebral hemisphere above and the top of the 
spinal cord below. But when we ask the further question, How is this 
machinery related to the various elements which go to make up this part 
of the brain? the only answers which we receive are of the most imperfect 
kind. 
In the case of the frog we can, after removal of the cerebral hemispheres, 
make an experimental distinction in the parts left between the optic thalami 
with the optic nerves and tracts, the optic lobes, and the bulb with the rudi- 
mentary cerebellum. When the optic thalami are removed, as might be 
expected, the evidence of visual impressions modifying the movements of 
the animal disappears; and it is stated that apparently spontaneous move- 
ments are much more rare than when the thalami are intact. When the 
optic lobes as well as the cerebral hemispheres are removed, the power of 
balancing is lost; when such a frog is thrown off its balance by inclining 
the plane on which it is placed, it slips back or falls down ; the special co- 
ordinating mechanism for balancing must, therefore, in this animal have a 
special connection with the optic lobes. But after removal of these organs 
the animal is still capable of a great variety of coordinate movements; 
unlike a frog retaining its spinal cord only, it can swim and leap, it main- 
tains a normal posture, and when placed on its back immediately regains 
the normal posture. The cerebellum of the frog is so small, and in re- 
moving it injury is so likely to be done to the underlying parts, that it 
becomes difficult to say how much of the coordination apparent in a frog 
possessing cerebellum and bulb is to be attributed to the former or to the 
latter; probably, however, the part played by the former is small. 
In the case neither of the bird nor of the mammal have we any exact 
information as to the behavior of the animal after removal of the parts 
behind the hemispheres, in addition to the hemispheres themselves. Our 
knowledge is confined to the results of the ablation or of the stimulation of 
parts, the cerebellum for instance, in animals in which the rest of the brain 
has been left intact. Observations of this kind have disclosed many inter- 
esting facts, besides the forced movements just referred to, but they have not 
led to, and indeed could hardly be expected to lead to, any clear views as to 
the point which we are now discussing. It does not follow that every part, 
injury or stimulation of which interferes with coordinated movements, or 
gives rise to definite, forced, or other movements, is to be considered as part 
of the machinery under consideration. The corpora striata and cerebral 
hemispheres form, as we have seen, no part of the machinery, yet injury to 
them may disorder the machinery; and the fact that removal of or injury 
to the cerebellum disorders the machinery is no proof by itself that the cere- 
bellum is an essential part of the machinery. 
If we may trust to deductions from structural arrangements, we might 
be inclined to infer that the anatomical relations of the tegmental region 
from the bulb upward point to its serving as the foundation of the machin- 
ery in question. Behind it has full connections with various parts of the 
cord, while in front by means of the optic thalami and anterior corpora 
quadrigemina, if not by other ways as well, it is so far associated with the 
optic nerves that the path seems open for visual impulses to gain access to 
it. To this foundation, however, we must add the cerebellum, on account of 
its relations to it, to the cord, and to the bulb through the restiform bodies, 
including its ties with the auditory nerve. And if we add the cerebellum 652 
THE BRAIN. 
we must also probably add the pons. We may exclude the pes of the crus, 
since this is composed exclusively of fibres,bringing the cerebral hemispheres, 
including the corpora striata, into connection with the pons, bulb, and cord, 
and so with the coordinating machinery itself, as well as with other parts of 
the nervous system. And observation, as far as its goes, supports this deduc- 
tion from anatomical relationships. We will, however, defer what else we 
have to say on this point until after we have discussed the carrying out of 
voluntary movements. 
Ox Some Histological Features of the Brain. 
§ 560. The white matter of the brain, as we have already said, like that 
of the spinal cord, consists of medullated fibres of various sizes imbedded 
in neuroglia and supported by septa of connective tissue derived from the pia 
mater. Save that cells, or even groups or rows of cells, for the most part 
small cells, about many of which it may be debated whether they are nerve- 
cells or neuroglia cells, are frequently seen between the fibres and bundles of 
fibres, the white matter of the brain seems essentially identical with that of 
the spinal cord. 
The gray matter of the brain in general also corresponds to the gray 
matter of the cord in consisting of branching nerve-cells, fine medullated 
fibres of peculiar nature, non-medullated fibres and fibrils, with a few ordi- 
nary medullated fibres, all supported in neuroglia. 
The " central " gray matter is extremely like that of the cord except 
that the nervous elements are imbedded in a relatively larger quantity of 
neuroglia. Immediately underneath the epithelium lining the several ven- 
tricles and the aqueduct, the neuroglia is especially developed, forming a 
distinct layer which may be regarded as a continuation of the central gelat- 
inous substance of the spinal cord, and which, with the epithelium overlying 
it, forms what is known as the ependyma. The " nuclei " of the cranial 
nerves are, as we have seen, comparable to the groups of nerve-cells in the 
spinal cord. 
A great deal of gray matter of the brain may be spoken of as more 
" diffuse " or '• scattered," more broken up by bundles of fibres than is the 
case in the spinal cord. The " reticular formation " of the bulb and of the 
tegmental region is an extreme form of this diffuse gray matter. And even 
in such collections of indubitable gray matter as the corpus striatum, optic 
thalamus, and the like, the pure gray matter, if we may use the term, is 
much more interrupted and broken up by conspicuous bundles of white 
fibres than is the case in any region of the spinal cord. In the corpora 
quadrigemina the gray matter is broken up by sheets or bundles of white 
matter. 
The nerve-cells of the several collections of gray matter are not all alike ; 
they present in different regions differences in size, form, and in other cha- 
racters. The cells of the nucleus caudatus, for instance, are rather small and 
often round or spindle-shaped, while those of the optic thalamus are large, 
branched, and rich in pigment. The cells of the substantia nigra are spindle- 
shaped, of moderate size, and so loaded with black pigment (in man) as to 
justify the name; those of the locus cseruleus are very large and spherical, 
with just so much pigment as to give a bluish tint. But our knowledge of 
the finer histological details of the various masses of gray matter is at present 
too imperfect to afford any basis whatever for physiological deductions ; and 
it will be hardly profitable to dwell upon them. Two regions of gray matter 
alone call for special description, the cortex cerebri and the superficial gray 
matter of the cerebellum. ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 
653 
The Superficial Gray Matter of the Cerebellum. 
§ 561. The surface of the cerebellum is increased by being folded or 
plaited into leaf-like folds, and each of these primary folds is similarly 
folded into a number of secondary, also leaf-like, folds or lamellae. Each 
of these lamellae consists of a central core of white matter, the fibres of 
which pass inward to and contribute to form the central white matter of the 
cerebellum, and of a superficial layer of gray matter. A section through 
a lamella perpendicular to the surface shows that the gray matter consists 
essentially of two layers: a layer lying next to the white matter formed by 
densely crowded small cells, called the nuclear layer, and between this and 
the superficial pia mater a much thicker layer of peculiar nature, called the 
molecular layer. Between these two layers, and connected, as we shall see, 
with both of them, lies a row of very large and remarkable cells, called the 
cells of Purkinje, the bodies of which abut on the nuclear layer, and the 
long branches of which traverse the molecular layer ; these cells so placed 
may be said to constitute a third layer. Before proceeding further, we may 
here remark that a section of the lamellae, that is, one of the secondary, not 
one of the primary, folds, while still remaining a vertical section (that is, 
perpendicular to the surface), may be carried through the lamella in different 
planes, and that, of these several planes, the sections taken in two of them 
are especially instructive, namely, the one taken in what we may call the 
longitudinal plane, passing from the top of the lamella to its base, and the 
one taken at right angles to the former, in what we may call the transverse 
plane. The nuclear layer and the molecular layer present the same broad 
features in both longitudinal and transverse sections, but the long-branched 
processes of the cells of Purkinje since they run in the transverse plane are 
adequately seen in transverse sections only ; longitudinal sections show only 
their profiles. 
The molecular layer is of a peculiar nature. In many modes of prepara- 
tion and in many sections it appears chiefly composed of a granular or dotted 
ground substance; hence the name molecular, as if it were an aggregation 
of molecules. The dots, however, are sections of fine fibrils, some of which 
are neuroglia fibrils but others are undoubtedly nervous. The layer consists 
in fact partly of nervous elements, and here perhaps even more than else- 
where it is extremely difficult to say with regard to many of the elements 
whether they are neuroglial or nervous in nature. A considerable portion 
of the whole area of the molecular layer is taken up by the conspicuous 
branched processes of the cells of Purkinje; and scattered about lie numer- 
ous small cells, some of which are neuroglia cells, but some of which are 
undoubtedly nerve-cells. The most conspicious feature of the layer, how- 
ever, is the presence in large numbers of the fine fibrils ; but before we speak 
of these it will be desirable to turn to the cells of Purkinje and the nuclear 
layer. 
The cell of Purkinje possesses a large (40 m by 30//) flask-shaped body, 
surrounding a large, conspicuous, clear, rounded nucleus; it has much the 
appearance of a large ganglion cell. The base of the flask rests on the nuclear 
layer, and from it there proceeds a single axis-cylinder process which, passing 
through the nuclear layer somewhat obliquely, and in its passage acquiring 
a medulla, joins the central white substance as a medullated fibre. The 
cells, as we have said, form a single layer only, but since this covers the nu- 
clear layer over the whole of the lamella, a considerable number of the fibres 
of the white central matter, though only a very small fraction of the whole, 
are thus derived from these cells of Purkinje. The narrowed neck of the 
flask running outward in the molecular layer divides in an arborescent 654 
THE BRAIN. 
fashion into a large number of branches which, spreading out laterally in 
the transverse plane and stretching as far as the surface, ramify through the 
molecular* layer, and are eventually lost to view as exceedingly fine fibrils. 
Some observers maintain that some of the fine processes are continuous 
with processes of the small nerve-cells of the molecular layer, but this is not 
admitted by all. In any case the fibrillar terminations of these cells of 
Purkinje contribute to the fine fibrils of the molecular layer. 
The nuclear layer in ordinary stained specimens has the appearance of 
a mass of nuclei closely crowded together in a bed of reticular nature; and 
since the nuclei usually stain deeply, the layer stands out in strong contrast 
to the much less deeply stained molecular layer. Careful examination with 
special modes of preparation shows, however, that while some of the nuclei 
are nuclei belonging to neuroglia and bloodvessels, the majority belong to 
small nerve-cells of a peculiar nature. In these cells the nucleus is sur- 
rounded by cell substance, which, forming a thin layer immediately around 
the nucleus, is chiefly disposed as thin spreading branches, some of which end 
in a peculiar arborescence not unlike a muscle end-plate; these processes 
contribute with the neuroglia to form the reticular-looking bed spoken of 
above. No process can be traced inward to the central white matter; but 
one of the processes gives off a branch, which passing vertically outward 
takes on the appearance of a delicate axis-cylinder process and runs, with- 
out dividing, into the molecular layer for a variable distance, sometimes 
reaching close to the surface, but at last divides at right angles into two 
fibrils, which run in the longitudinal plane in opposite directions for a con- 
siderable distance, and are ultimately lost to' view. Since these cells in the 
nuclear layer are very numerous and each gives rise in the above manner to 
longitudinal fibrils, the molecular layer is traversed by a multitude of fibrils, 
visible as such in longitudinal sections, but appearing as dots in transverse 
sections, in which the cells of Purkinje are best displayed. 
Besides these longitudinal fibrils proceeding from the cells of the nuclear 
layer, special modes of preparation similarly disclose numerous transverse 
as well as more or less oblique fibrils. Many of these appear to result from 
the branching of the small nerve-cells of the molecular layer, and some of 
those so arising descend to the layer of the cells of Purkinje and end around 
the bodies of those cells in remarkable nests of fibrils, without, however, 
actually making connections with them. 
The medullated fibres of the central white matter of a lamella pass on all 
sides into the nuclear layer; or, put in another way, medullated fibres pass- 
ing out of the nuclear layer at all points converge to form the central white 
matter. Some of these fibres, as we have seen, begin or end in the cells of 
Purkinje. None of them appear to join the cells of the nuclear layer, and 
we have no evidence that any of them end or begin in any way in the nu- 
clear layer. A certain number, however, may be seen to pass through the 
nuclear layer and between the cells of Purkinje into the molecular layer, 
where losing their medulla they divide and apparently contribute to the 
numerous fibrils of the molecular layer. The presumption, therefore, is that 
all the fibres of the white matter begin or end either in the cells of Pur- 
kinje or the fibrils of the molecular layer. 
The superficial gray matter of the cerebellum then resembles the gray 
matter of the spinal cord in so far as it consists of branching nerve-cells, 
nerve-fibres, and nerve-fibrils imbedded in neuroglia; but the disposition 
and features of the several factors are peculiar. We may take, perhaps, as 
the key of the structure the fibrils of the molecular layer ; this layer is rela- 
tively very thick, about 400 m, much thicker than the nuclear which, how- 
ever, varies in thickness, being generally thickest at the top of the fold; ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 
655 
hence the number of fibrils in it may be spoken of as enormous. These 
fibrils seem certainly to be connected on the one hand with the cells of the 
nuclear layer, and on the other hand with the scattered small cells of their 
own layer; but we have no evidence that these two sets of fibrils are con- 
tinuous with each other; on the contrary, it seems more probable that the 
two sets of cells represent two independent systems. We can hardly doubt 
that these fibrils are in functional connection with the medullated fibres of 
the central white matter; but we have no clear evidence that the system 
of scattered cells is continuous either with the cells of Purkinje, and so with 
the medullated fibres belonging to those cells, or with the medullated fibres 
which end independently in the molecular layer; and we have no evidence 
at all that the system of the cells of the nuclear layer is connected with 
either. We can hardly think otherwise than that the molecular changes 
which sweep to and fro along the tangle of these fibrils (whose nutrition is 
probably governed and hence whose functional activity is probably regulated 
by the nuclear and scattered cells respectively) are influenced by or originate 
the nervous impulses passing along the medullated fibres of the white matter; 
and hence we must conclude that either a continuity exists which has as yet 
escaped detection, or, what is quite possible if not probable, that one fibril 
can act upon another by simple contact or even at a distance. Further, 
while the cell of Purkinje, with its large cell-body and nucleus, its conspicu- 
ous axis-cylinder process and its other branched processes presents many 
analogies with motor cells, such as those of the anterior horn of the spinal 
cord, and raises the presumption that the impulses which move along its 
axis-cylinder process proceed outward from the cell as motor or at least as 
efferent impulses, we have no direct proof that this is so. And though it is 
tempting to suppose that the other medullated fibres, which like the fibres 
of a posterior root are lost in the gray matter, without the intervention of a 
conspicuous cell, carry efferent impulses, we have as yet no proof of this. 
All we can say is that the gray matter is connected in two different ways 
with at least two sets of fibres, which probably therefore, have different 
functions. 
We may here add the remark that the large body of the cell of Purkinje 
lies, as indeed do the other nervous elements, in an appropriate space in 
the bed of neuroglia. Between the surface of the cell and the wall of neur- 
oglia is a space, generally so narrow as to be potential rather than actual, 
but which may sometimes be considerable. Whether small or large it con- 
tains lymph, and the cavity in which the cell lies is in connection with the 
lymphatics of the brain. Each cell then lies in a lymph-space; but we 
merely mention the fact now ; we shall have to return to the matter when 
we come to deal with the lymphatic and vascular arrangements of the brain 
and spinal cord. 
The Cerebral Cortex. 
§ 562. While the superficial gray matter of the cerebellum does not 
differ strikingly as to its histological features in different regions, very con- 
siderable differences are observed in different regions of the cerebral cortex 
A general plan of structure may perhaps be recognized, but as we pass from 
one part of the cerebral surface to another we find modifications continually 
taking place. We must content ourselves here with attempting a descrip- 
tion of the general plan, followed by an indication of the more striking cha- 
racteristics of certain regions. 
The cortical gray matter, having an average thickness of about 3 mm., 
but varying considerably in different regions from 1.8 mm. in some parts of 656 
THE BRAIN. 
the occipital lobe to 4.2 mm. at the dorsal summit of the precentral convolu- 
tion, is, like other gray matter, composed of nerve-cells, and of nerve-fibres 
and fibrils supported by neuroglia. The nerve-cells, at least the conspicuous 
and easily recognized nerve-cells, are scattered, and appear, in sections, to be 
imbedded in and separated from each other by a not inconsiderable but 
variable quantity of somewhat peculiar ground substance, not unlike that 
which forms so large a part of the molecular layer of the cerebellum. Part 
of this ground substance, which apparently is not confined to any particular 
layer, but stretches throughout the thickness of the cortex, is undoubtedly 
neuroglial in nature, but part, and probably the greater part, is nervous in 
nature ; it is largely composed of fine fibrils traversing it in various direc- 
tions, the transverse sections of the fibrils giving it a characteristic dotted 
or " molecular " appearance; and the majority of these fine fibrils are prob- 
ably the continuations of branching nerve-cells or dividing nerve-fibres, the 
remainder being neuroglial fibrils. In this respect it resembles the molec- 
ular layer of the cerebellum, but it is, to a much greater extent than is that 
layer, traversed by medullated nerve-fibres, especially by fine medullated 
fibres like those seen in the gray matter of the spinal cord 476). 
The nerve-cells imbedded in this ground substance in more or less dis- 
tinct layers are of various kinds. The most conspicuous, abundant and cha- 
racteristic nerve-cells found in the cortex of all regions of the cerebellum 
are those which from their shape are called pyramidal cells. These vary 
very much in size and have been distinguished as "small pyramidal" cells 
averaging 12 p. in length by 8 p in breadth, and " large pyramidal " cells, 
sometimes called " ganglionic cells," of which the medium size is about 40 m 
in length by 20 m in breadth. Some of the latter, occurring in special 
regions, are of very large size, 120 p by 50 and have been called "giant 
cells." 
The features of a " large pyramidal " cell are very characteristic. Such a 
cell appears in a well-prepared vertical section of the cortex as an elongated 
isosceles triangle placed vertically, with the base looking toward the under- 
lying white substance and the tapering apex pointing to the surface. The 
cell substance is finely granulated or fibrillated, the fibrilhe sweeping round 
in various directions ; it not unfrequently contains pigment. In the midst of 
this cell substance rather near the base lies a large, clear, conspicuous round 
or oval nucleolated nucleus. At the base the cell substance is prolonged 
into a number of processes. One of these, generally starting from about the 
middle of the base, runs for some distance without dividing, and soon acquir- 
ing a medulla may be recognized as an axis-cylinder process; the fibre to 
which it gives origin sweeps with a more or less curved course into the sub- 
jacent white matter. In some instances the axis-cylinder process, by a 
T-division like that seen in a ganglion of a posterior root 93), gives rise 
to two fibres, one of which may take a horizontal direction ; in some regions 
of the cortex, the occipital for instance, the axis-cylinder process is said to 
give rise by division to several fibres. The other processes from the base, 
especially those from the angles of the triangle, rapidly branch into fine 
fibrils which are soon lost to view in the ground substance. The apex of 
the triangle is also prolonged into a process, which, giving off fine lateral 
branches, makes, as it were, straight for the surface, but ultimately branch- 
ing into fine fibrils is lost to view at some distance from the body of the cell. 
The cell lies in a cavity of the ground substance, which it appears normally 
to fill, but from the walls of which it sometimes shrinks, developing between 
itself and the wall of the cavity a space which may contain not only lymph, 
but occasionally leucocytes. In prepared specimens the retraction within its 
cavity of the artificially shrunken cell may be often observed. ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 
657 
The "small pyramidal" cells have much the same features; that is to 
say, the cells are characterized by their pyramidal form, though this is nat- 
urally not so distinct, by their vertical position, and by the possession of 
branching processes which are lost in the molecular ground substance; the 
presence, however, of a midbasal axis-cylinder process has not been clearly 
demonstrated. 
Other nerve-cells are more like the ordinary nerve-cells of the spinal 
cord and of the internal cerebral gray matter; they are branched cells, of 
irregular, not of pyramidal, form, and for the most part small, 18 p. by 10 //. 
They may be characterized by the relatively large size, 7 of the nucleus, 
and do not possess an axis-cylinder process; at least, such a process has not 
yet been demonstrated. They are frequently spoken of as " angular " cells. 
Another kind of cell, the "fusiform cell," which is found in all regions 
of the cortex, has a characteristic spindle-shape, the cell substance being 
prolonged at the opposite poles into tapering, ultimately branched processes. 
The long axis of the cell is generally placed horizontally, following the cur- 
vature of the cortex, and being thus at the sides of the sulci vertical to the 
surface of the brain ; it is, however, at times inclined at various angles. 
Still another kind of cell, the "granule cell," or " nuclear cell," is one in 
which the nucleus is surrounded by a relatively small quantity of cell sub- 
stance, 9 p. by 7 //, more or less spherical in form in ordinary preparations, 
but probably breaking up into delicate branched processes. Cells of this 
kind are sparsely scattered throughout the cortex generally, but in particu- 
lar regions-e. g., the occipital-are crowded together into a layer, which in 
many respects resembles the nuclear layer of the cerebellum, and has been 
called the " granular " or " nuclear " layer. 
Lastly, throughout the cortex are found, besides indubitable nerve-cells 
and indubitable neuroglial cells, numerous small, somewhat irregular cells, 
concerning which it may be debated whether they are really nervous or 
simply neuroglial in nature. Moreover, in using the names given above for 
the various kinds of nerve-cells, it must be remembered that many transi- 
tional forms are observed ; cells, for instance, may be seen intermediate in 
form between pyramidal cells and "fusiform " or "angular" cells. 
The medullated nerve-fibres which take part in the cortex may be con- 
sidered provisionally as forming two categories. In the first place, fibres 
sweep up vertically into the cortex from the subjacent " central white 
matter," taking at first a curved course as they enter into the gray matter, 
and then appearing to run straight toward the surface. These are arranged 
in the deeper levels in bundles, leaving vertical columns of the gray matter 
between them ; but at more superficial levels the bundles spread out and are 
gradually lost to view. Besides these distinct vertical fibres and bundles of 
fibres of the ordinary medullated kind, which we have reason to think are 
the ends (or beginnings) on the one hand of fibres of the pedal and tegmen- 
tal systems, and on the other hand of fibres of the corpus callosum, or the 
other commissural fibres spoken of as " association " fibres (§ 548), an ex- 
ceedingly large number of fibres of the peculiar fine medullated kind run 
in various directions, forming a dense network in the ground substance of 
the gray matter between the cells. We may add that this system of fine 
medullated fibres is of late growth, and is not fully developed in man until 
two or three years after birth. Many of the medullated fibres, coarse as well 
as fine, take a horizontal direction parallel to the surface, and in certain re- 
gions are specially developed into a layer or into two layers so as to form a 
horizontal streak or streaks. 
The vascular pia mater invests closely, as we have said, the whole surface 
of the cortex, dipping down into the sulci; and from it, as in the case of the 
42 658 
THE BRAIN. 
spinal cord, processes carrying bloodvessels a,nd bearing lymph-spaces pass 
inward to supply the gray matter with blood. But while, as we shall see 
later on, the supply of bloodvessels to the gray matter is considerable, the 
truly connective-tissue elements of the pia mater processes are soon merged 
into neuroglia. Immediately beneath the pia mater forming the immediate 
surface of the cortex is a thin layer consisting of neuroglia only. 
§ 563. The nerve-cells of the above several kinds are arranged more or 
less distinctly in layers parallel to the surface, so that the whole thickness of 
the cortex may by means of them be, more or less successfully, divided into 
a series of zones, one above the other; and we may, as we have said, recog- 
nize on the one hand a general arrangement common to the whole surface, 
and on the other hand modifications existing in the several regions. The 
general arrangement may be said to be one of five layers or zones, usually 
counted from the surface inward. 
The fifth layer, lying next to the central white matter, fairly uniform in 
characters and thickness (about 1 mm.) over the greater part of the brain, 
is characterized by the presence of somewhat sparsely scattered " fusiform " 
cells, though other branched cells are present. It is broken up into vertical 
columns by the bundles of vertical fibres, and its demarcation from the white 
matter below is somewhat indistinct, owing to the fact that in the brain the 
white matter, especially that lying beneath the cortex, contains cells and 
small groups of cells lying between the bundles of fibres to a much greater 
extent than does the white matter of the spinal cord. 
The fourth layer, lying above the preceding, varies much more both in 
thickness (0.35 mm. to 0.15 mm.) and in its characters. The constituent 
cells are on the one hand large pyramidal cells, and on the other hand 
"granule" or "nuclear" cells. In some regions it may be subdivided into 
two layers, the small " nuclear '' .cells being so abundant as to form in the 
upper part of the layer a separate layer called the "granule" or "nuclear" 
layer. This fourth layer, like the preceding fifth layer beneath it, is split 
up into vertical columns by the bundles of vertical fibres, but to a less degree. 
It is marked in its lower part by a horizontal streak due to numerous, mostly 
fine, medullated fibres running horizontally. In the cortex of the island of 
Keil this horizontal layer is developed into a conspicuous sheet of medullated 
fibres, separating the fourth and fifth layers by a distinct interval of obvious 
white matter. This fifth layer of fusiform cells, thus detached from the rest 
of the cortex, is what is called the claustrum (Figs. 138, 139, c/.). 
In the third layer, the constituent cells are the characteristic pyramidal 
cells. These are for the most part large, though diminishing in size from 
below upward, and the layer has been called the " layer of large pyramidal 
cells," though in certain regions the largest pyramidal cells, and notably the 
giant-cells, are found in the preceding, fourth, layer. The cells are, on the 
whole, scattered somewhat sparsely, though frequently gathered into small 
groups, and among them occur small "nuclear" and other cells. The 
bundles of vertical fibres spread out rapidly in this layer, so that the 
columnar arrangement becomes lost, and many of the fibres undoubtedly 
become axis-cylinder processes of the pyramidal cells. Though the layer 
varies in thickness (1 mm. to 0.4 mm.), and in some of its features in different 
regious, the characteristic pyramidal cells are present over the whole surface 
of the hemisphere. In the lower part of the layer a second horizontal streak 
of closely interwoven horizontal fibres frequently makes its appearance. 
The second layer, generally a thin one, though varying from 0.25 mm. to 
0.75 mm. in thickness, is also formed by pyramidal cells, but is distinguished 
from the layer below by the absence of large and medium-sized cells and by 
the presence of numerous small cells closely packed together; it has been ON SOME HISTOLOGICAL FEATURES OF THE BRAIN. 
659 
called " the layer of small pyramidal cells." As we have said, these smaller 
pyramidal cells differ somewhat from the larger cells; and the cells in this 
layer are sometimes described as " angular." 
The firstand most superficial layer is characterized by the predominance 
of the molecular ground substance, the cells being few. far between, small, 
and irregular. The ground substance itself seems to be more largely neur- 
oglial in nature than in the other layers, and, as we said above, its extreme 
surface appears to be furnished by neuroglia alone. The layer is generally 
spoken of as the "peripheral" or "superficial layer," or sometimes as the 
" molecular" layer. The tapering vertical processes of the pyramidal cells 
may be traced into this layer, which indeed varies in thickness according to 
the abundance of pyramidal cells in the subjacent layers; numerous some- 
what fine medullated fibres also traverse it in a horizontal direction. 
§ 564. The general arrangement just described varies, as we have 
said, in different regions of the cerebral surface. We must content our- 
selves here with pointing out the characteristics of two or three important 
regions. 
The region ■which we have (§ 545) called the " motor area " or " region " 
is characterized on the one hand by the great thickness (1 mm.) of the third 
layer, that of large pyramidal cells, as well as by the number and size of 
the cells contained in it, and on the other hand, and especially, by the 
prominence in the fourth layer of remarkable clusters of very large pyram- 
idal cells, of the kind which are referred to above (§ 562) as being fre- 
quently called " ganglionic ; " it is in this region that " giant cells " are 
found in the fourth layer, namely, in the upper part of the precentral and 
at the summit of the postcentral convolution, and in the paracentral 
lobule, acquiring their greatest size at the top of the precentral con- 
volution. 
The occipital region is characterized by the prominence of the " granule " 
or " nuclear " cells. These not only form a distinct division of the fourth 
layer, but are also conspicuous in other layers, their arrangements being such 
that some authors have been led to divide the cortex of this region into seven 
or even eight layers. In the present state of our knowledge we may be con- 
tent with insisting that the great mark of this occipital region is the abun- 
dance of these small " nuclear " cells, together with other small " angular " 
cells, whereby the pyramidal cells seem to be made less conspicuous. It is 
worthy of notice, however, that in the third, but more especially in the 
fourth layer, a few cells of very large size are met with, which by their 
large branched cell substance and conspicuous axis-cylinder process resemble 
the large cells in the motor region; but it should be noted that while these 
large cells occur (at least in man and in the monkey, though not in some 
of the lower animals, as the rabbit) in very definite clusters in the motor 
region, they occur singly in the occipital region. In this occipital region 
the layer of horizontal fibres in the fourth layer is very conspicuous, and 
owing to the number of ordinary medullated fibres present, forms a white 
streak visible even to the naked eye. 
In the frontal region, in front of the motor region, the arrangement is 
more in accordance with what we have described as the general plan. The 
two pyramidal layers are well marked, as is also the fourth layer; but the 
layer of large pyramidal cells is much thinner than in the motor region, as 
is also, though to a less extent, the fourth layer, while the fifth layer, that of 
fusiform cells, is thicker than elsewhere. Small " nuclear " cells are perhaps 
more abundant in this region throughout all layers than in the motor region, 
but are far less conspicuous than in the occipital region. 
We may here remark that the transition in structure from one region to 660 
THE BRAIN. 
another is very gradual, not sharp and distinct, and is perhaps especially 
gradual in passing from the motor region hackward to the occipital region. 
It is not possible to recognize histologically the limit, for instance, of the 
motor region as determined experimentally. 
In special regions of the brain, for instance in the olfactory bulb, of which 
we shall speak later on, very great modifications of the general plan may be 
observed in the cortex. We cannot enter upon these, but may just refer to 
the cornu ammonis or hippocampus. At the ventral end of the temporal 
lobe the gyrus hippocampi, the structure of whose cortex follows the general 
plan, is thrust inward so as to project into the cavity of the descending horn 
of the lateral ventricle, forming the ridge-like prominence known by the 
above name. The substance of the cornu ammonis is therefore cortical sub- 
stance covered on the side of the ventricle by a thin prolongation of the 
central white matter, which is in turn covered by the ependyma lining the 
ventricle. A vertical section of this substance shows that while the fifth 
and fourth layers are reduced to small dimensions, the third layer, that.of 
large pyramidal cells, is well developed, though narrow. The cells are large 
and remarkably long, and the tapering processes are arranged so regularly 
as to give rise, especially in stained preparations, to a marked radiate ap- 
pearance. At the level of the second layer there occurs a large develop- 
ment of capillary bloodvessels and a scarceness of cells, giving rise to a 
"lacunar" appearance; and the first or molecular layer is of some consid- 
erable thickness. From the prominence of the pyramidal cells in this 
region, the third layer in the general plan of the cortex has sometimes been 
spoken of as the " formation of the cornu ammonis." 
§ 565. In the present state of knowledge it is impossible to come to any 
satisfactory conclusion concerning the meaning of the variety and arrange- 
ment of the cells and other constituents of the cortex. The cells with their 
branches, the nerve-fibres and the nerve-fibrils form a network of gray 
matter which we may compare with the gray matter of the spinal cord 
(§ 492), but which is obviously, as we might expect, far more complex than 
that is. We may conclude, and experimental observation confirms the con- 
clusion, that the large pyramidal cells with recognizable axis-cylinder pro- 
cesses serve as trophic centres for the fibres which appear to start from them. 
And we may, though with less confidence, explain the large size of these 
cells in the motor region, by the fact that they give rise to fibres of the 
pyramidal tract stretching a long way from their origin in the cell, and 
therefore demanding great nutritive activity on the part of the cell. We 
may perhaps also conclude that these fibres are efferent, motor fibres, des- 
tined to carry impulses from the cortex to the peripheral, or at least distant 
parts. And we may further, with distinctly less confidence, however, assume 
that the size of the cell is correlated to the energy which has to be expended 
in the discharge of efferent, motor impulses. If we accept these conclusions 
we must also bear in mind that such cells, 'with axis-cylinder processes con- 
tinued on as fibres, are not limited to, though most abundant in the motor 
region, but are found in all regions of the cortex ; and we must hence con- 
clude that impulses, which we must call efferent, proceed from all parts of 
the cortex. 
It is obvious, however, that the connection of the cortical network of 
gray matter with the fibres of the white matter is effected in part only, and 
that a small part, by the method of axis-cylinder processes definitely pro- 
longed from the cell substance of cells. A part, and probably a greater part, 
of the fibres sweeping up from the subjacent white matter, whether they be 
fibres of the pedal and tegmental systems or callosal or " association " fibres, 
end in the gray matter in some other way than by bodily being continued ON VOLUNTARY MOVEMENTS. 
661 
in the cell substance of cells; they plunge into and break up within the 
network, of which fibrils no less than cells form a conspicuous part; and we 
may here repeat the remark which we made in speaking of the cerebellum 
concerning the actual continuity of the elements of the network. Moreover, 
besides the vertical fibres obviously coming from the subjacent white matter, 
we have in this gray matter to deal with the fibres of horizontal and other 
directions, which may come from white matter not far off, but which may 
come from some neighboring gray matter; our present knowledge will not 
enable us to settle this point. 
In the spinal cord we were able to divide all the fibres into afferent and 
efferent respectively; though even here we met with some difficulty. Deal- 
ing with the cerebral cortex, which, as we have already seen, is certainly 
especially concerned in voluntary movements and in the development of full 
sensations, we may be tempted to consider the fibres connected with the gray 
matter as similarly divisible into motor and sensory; and we may go on to 
suppose that the fibres joining the cortex as axis-cylinder processes of recog- 
nizable cells are motor fibres, and that all the other fibres joining the gray 
matter in some other way are sensory fibres. But in doing so we are going 
beyond our tether; in all probability the nervous processes going on in the 
cortex are far too complex to permit such a simple classification of the 
functions of fibres as that into motor and sensory; and any attempt to 
arrange either fibres or regions of the cortex as simply motor or sensory is 
probably misleading. But we shall have to return to these matters when we 
deal with the functions of the cortex. 
On Voluntary Movements. 
§ 566. When we examine ourselves we recognize certain of our move- 
ments as " voluntary ;" we say that we carry them out by an effort of the 
" will." And when we witness the movements of other people or of animals 
we regard as also voluntary such of those movements as by their characters 
and by the circumstances of their occurrence seem to be carried out in the 
same way as our own voluntary movements. Even in the case of some of 
our own movements we are not always clear whether they are really volun- 
tary or no; and in the case of other people and of animals it is still more 
difficult to decide the question. It would be out of place to attempt to dis- 
cuss here how voluntary movements really differ from involuntary move- 
ments, or, in other words, what is the nature of the will; we must be content 
to take a somewhat rough use of the words "voluntary," "volitional," and 
" will " as a basis for physiological discussion. We may, however, remark that 
as far as the muscular side of the act, if we may use such an expression, is 
concerned, a voluntary movement does not differ in kind from an involun- 
tary movement. It is perfectly true that a skilled man may by practice 
learn to execute muscular manoeuvres which he would not have learned to 
execute had not intelligent volition been operative within him ; but our own 
experience teaches us that many more or less intricate movements which have 
undoubtedly been learned by help of the will may be carried out under cir- 
cumstances of such a kind that we feel compelled to regard them as, at the 
time, involuntary ; and it may at least be debated whether every movement 
which we can carry out by an effort of the will may not appear, under ap- 
propriate circumstances, as part of an involuntary act. In the case of the 
lower animals, in the frog deprived of its cerebral hemispheres, for instance, 
we have seen that voluntary differ from involuntary movements, not by 
their essential nature, but by the relation which their occurrence bears to 
circumstances. We have, therefore, to seek for the distinction between 662 
THE BRAIN. 
voluntary and involuntary, not in the coordination of the muscular and 
nervous components of a movement, but in the nature of the process which 
starts the whole act. 
The histories related in a preceding section, of various animals deprived 
of their cerebral hemispheres, while they have further shown the difficulty 
of drawing a sharp line between the presence and absence of volition, such as 
when we appeal to our own consciousness we seem able to draw, have taught 
us that in a broad sense the presence of volition is, in the higher vertebrata, 
dependent on the possession of the cerebral hemispheres; and wre have now 
to inquire what we know concerning the way in which the cerebral cortex- 
for this, as we have seen, is the important part of the cerebral hemisphere- 
by the help of other parts of the nervous system carries out a voluntary 
movement. 
§ 567. With this view we may at once turn to the results of experi- 
mental interference with the cortex. When the surface of the brain is laid 
bare by removal of the skull and dura mater, mechanical stimulation of the 
cortex produces little or no effect, thus affording a contrast with the results 
of mechanically stimulating other portions of the brain, or other nervous 
structures. And for a long time the cortex was spoken of as insensible to 
stimulation. When, however, the electric current is employed, either the 
make and break of the constant current or the more manageable inter- 
rupted current, very marked results follow. It is found that certain move- 
ments follow upon electric stimulation of certain regions or areas. The re- 
sults, moreover, differ in different animals. It will be convenient to begin 
with the dog, on which animal the observations of this kind were first con- 
ducted. 
When the surface of the dog's brain is viewed from the dorsal surface a 
short but deep sulcus it seen toward the front, running outward almost at 
right angles from the great longitudinal fissure; this is called the crucial 
sulcus (Fig. 147), the gyrus or convolution in front and behind it, and 
sweeping around its end, being called the sigmoid gyrus. It will hardly 
be profitable to discuss here either the homology of this sulcus or the 
names of the other sulci and convolutions of the dog's brain. We men- 
tion this sulcus because it is found that stimulation of the cortex in a region 
which may be broadly described as that of the neighborhood of this crucial 
sulcus gives rise to movements of various parts of the body, whereas no such 
movements result from stimulation of the extreme frontal region in front 
of the area around the crucial sulcus, or from stimulation of the occipital 
region behind this area. Certain exceptions may be made to this broad 
statement, but these it will be best to discuss in reference to the more 
highly developed monkey. 
The region of the cortex in the neighborhood of the crucial sulcus may 
then be termed an " excitable " or " motor " region, inasmuch as stimulation 
of this region leads to movements carried out by skeletal muscles, while 
stimulation of other regions does not. Further, stimulation of particular 
districts or areas of the region leads to particular movements carried out 
by particular muscles. For instance, stimulation of the more median 
parts of the gyrus behind the crucial sulcus (Fig. 147, C) leads to move- 
ments of the hind limb, whereas stimulation of the lateral part or outei* end 
of the same gyrus leads to movements of the fore limb, and we may here 
distinguish between an area, stimulation of which (Fig. 147, A') leads to 
flexion of the fore limb, and an area (Fig. 147, A), stimulation of which 
leads to extension of the same limb. In a similar way stimulation of other 
areas within the " motor " region leads to movements of this kind or of that 
kind of the tail, of the eyes, of the mouth, of other parts of the face, of the ON VOLUNTARY MOVEMENTS. 
663 
tongue, and so on. Obviously in the dog this region of the cortex has con- 
nections with the skeletal muscles which do not obtain between other regions 
of the cortex and those muscles; and further, the region in question is topo- 
graphically differentiated, so that certain areas or districts of this region are 
specially connected with certain skeletal muscles or groups of muscles. We 
may speak of a " localization of function " in this region as compared with 
Fig. 147. 
The Areas of the Cerebral Convolutions of the Dog, according to Hitzig and Fritsch. (1) B, 
The area for the muscles of the neck. (2) A, The area for the extension and adduction of the 
fore limb. (4) E, The area for the flexion and rotation of the fore limb. (4) C, The area for the 
hind limb. Running transversely toward and separating 1 and 2 from 3 and 4 is seen the crucial 
sulcus. (5) D, The facial area. 
other regions of the cortex, and in the several areas within the region as 
compared with each other. 
The muscles which are thus thrown into contraction are the muscles of 
the opposite side of the body. When " the fore-limb area," as we may call 
it, of the right hemisphere is stimulated, it is the left fore limb which is 
moved; and so with the other areas; it is only in exceptional cases, as in 
certain movements of the eyes, that the effect is bilateral; a movement 
confined to the same side as that stimulated is never witnessed. 
The results are most clear when the current employed as a stimulus is not 
stronger than is just sufficient to produce the appropriate movement (roughly 
speaking, a current just perceptible to the tongue of the operator is in ordi- 
nary cases a useful one), and when the cortex is in good nutritive condition. 
In any experiment the results obtained by the earlier stimulations, soon after 
the cortex has been exposed, are the best; after repeated stimulations the 
surface is apt to become hypertemic, and it is then frequently observed that 
the movements resulting from the stimulation of a particular area are not 
confined to the appropriate muscles, but spread to the corresponding muscles 
of the opposite side, then to muscles connected with other cortical areas, and 
at last to the muscles of the body generally ; at the same time the move- 
ments lose their distinctive purposeful character and the animal is thrown 
into convulsions of an epileptiform kind. It not unfrequently happens that 
an experiment has to be stopped in consequence of the onset of these epilep- 
tiform convulsions. The response of movement to stimulation may be ob- 
served while the animal is under the moderate influence of an anaesthetic, 
but a too profound anaesthesia lessens or annuls the effects. 664 
THE BRAIN. 
In order to carry out a closer analysis of the phenomena it is desirable 
to watch or record the contraction of a particular group of muscles, or per- 
haps better still a particular muscle, e. g., the area for extension of the hind 
limb may be studied by help of the extensor digitorum communis of the 
limb. When this is done, the following important facts may be observed : 
The area of cortex having been found which gives the best movements, and 
the stimulus being no stronger than is necessary, isolation of the area from 
its lateral surroundings by a circular incision carried to some little depth 
will not prevent the development of contractions in the muscle; but these 
do cease, even without the circular incision, if by a horizontal section the 
gray cortex is separated from the subjacent white matter. After removal 
of the cortex, stimulation of the white matter underlying the area produces 
the appropriate contraction; not only, however, is a stronger stimulus 
necessary, but also the latent period, that is the time intervening between 
the beginning of the application of the stimulating current and the begin- 
ning of the muscular contraction, is appreciably shortened. The appropriate 
contractions not only appear when the white matter immediately below the 
cortex is stimulated, but by making successive horizontal sections and stim- 
ulating each in turn, the effect may, so to speak, be traced through the cen- 
tral white matter of the hemisphere down to the internal capsule. We may 
conclude from these results, that when the current is applied to the surface 
of the cortex, certain parts of certain structures in the gray matter are 
stimulated, the process having a marked latent period, and that as the out- 
come of the changes induced in the gray matter, impulses pass along the 
fibres leading down from the gray matter to the internal capsule and so by 
the pedal system of fibres to the spinal cord and motor spinal roots. The 
anatomical considerations advanced in a previous section lead us to suppose 
that the fibres in question belong to the great pyramidal tract, on which we 
have so much insisted ; and, as we shall see, all our knowledge confirms this 
view. 
It must not, however, be supposed that the several areas stimulation of 
which produces each its distinctive movement, are in the dog sharply de- 
fined from each other; when the term area for extension of the hind limb 
is used, it must not be supposed that the area can be defined by an outline 
within which stimulation produces nothing but extension of the hind limb, 
and outside which stimulation never produces extension of the hind limb. 
All that is meant is that extension of the hind limb is the salient and strik- 
ing result of stimulating the area. When we study the various movements, 
and especially perhaps when we study, by help of a graphic record, the con- 
tractions of various individual muscles resulting from the stimulation of 
various parts of the motor region, we find not only that the areas for 
particular movements or particular muscles are very diffuse, but that the 
several areas largely overlap each other. If, for instance, we were to map 
out on the same diagram the several areas belonging to four or five muscles 
of different parts of the body, such as the extensors of the digits of the fore 
and of the hind limb, the flexors of the same, and the orbicular muscle of 
the eyelid, that is to say, the several areas within which in turn stimulation 
of the cortex produced contraction of the particular muscle, the overlap- 
ping would be so great that the whole figure would appear highly confused. 
In a similar way the excitable motor region as a whole would gradually 
merge into, be broken up into, the unexcitable frontal, occipital, and tem- 
poral regions, in front, behind, and below. In other words, the localization 
in the cortex of the dog is to a marked degree imperfect. 
In this respect the dog, corresponding to its position in the animal hier- 
archy, is intermediate between such animals as the rabbit, the bird, and the ON VOLUNTARY MOVEMENTS. 
665 
frog, on the one hand, and the more highly developed monkey on the other ; 
and that is one reason why we have taken the dog first and dwelt so long 
upon it. In the rabbit, a similar localization may be observed, but far less 
definite, far more diffuse ; it becomes still less in the bird, and is hardly 
recognizable in the frog. It will not be profitable to dwell on the details of 
the phenomena of these lower animals; but the phenomena of the monkey, 
leading up as they do to those of man, call for special notice. 
§ 568. When in a monkey, in an individual, for instance, belonging to 
the genus Macacus, the surface of the cerebrum is explored with reference 
to the effects of electric stimulation, it is found that when the current is 
applied to the precentral or ascending frontal and the post-central or 
ascending parietal convolutions which lie respectively in front of and behind 
the important central fissure or fissure of Rolando (cf. Fig. 148), movements 
of the fore limb follow. The "motor area for the fore limb" thus discov- 
ered is more circumscribed and definite than is the corresponding area in 
the dog. Its outline (Fig. 149) is roughly that of a truncated triangle bi- 
sected by the central fissure, with the broad base at some distance from the 
mesial line, and the truncated apex reaching on the lateral surface of the 
hemisphere to a well-marked bend in the lower part of the central fissure. 
Behind, it reaches as far as the intra-parietal fissure which somewhat sharply 
defines its hind border, and in front it ceases no less definitely at some little 
distance behind the precentral fissure. Further examination shows that the 
whole area is divided into areas corresponding to movements of particular 
parts of the forearm, and that these are arranged in a definite relation to 
each other. In the more dorsal part of the area, at the base of the triangle, 
stimulation produces movements of the shoulder (Fig. 149) ; if the elec- 
trodes be shifted ventrally, movements of the elbow make their appear- 
ance ; if still more ventrally, movements of the wrist come in, and these are 
in turn succeeded ventrally by movements of the digits generally, of the 
forefinger, and lastly of the thumb. A very striking experiment may be 
made by applying a current of suitable strength, first at the lower ventral 
border of the area, and then gradually advancing upward toward the mesial 
line; the thumb is moved first, then the forefinger, then the rest of the 
digits, then the wrist, next the elbow, and lastly the shoulder. Further, in 
certain parts of the area the resulting movement is flexion of the appropri- 
ate segment of the limb, in other parts extension, in certain parts abduc- 
tion, in other parts adduction, and so on. 
Similar exploration shows that the " area for the hind limb " lies on the 
median side of the area for the fore limb, stretching besides on to the mesial 
surface along the marginal convolution which forms the dorsal portion of 
the wall of the great longitudinal fissure ; it reaches as far back as the intra- 
parietal sulcus, and is succeeded in front by the " area for the trunk " (Fig. 
150). Within this general area for the hind limb we may similarly distin- 
guish special areas for the hip (Figs. 149, 150) in the front portion, for the 
knee and ankle behind this, and for the digits still further backward, the 
area for the great toe being, however, in front of the area for the other 
digits. 
In front of the areas for the limbs and trunk, on the median dorsal sur- 
face, dipping down into the mesial surface along the marginal convolution 
(Fig. 150) and reaching laterally on the lateral dorsal surface to the dorsal 
extremity of the precentral sulcus (Fig. 149), is the "area for the head," 
that is to say, for the movements of the head brought about by contractions 
of the muscles of the neck. 
Ventral to this again, in front of the precentral sulcus, is the "area for 
the eyes," that is to say, for contractions of the ocular muscles ; and behind 666 
THE BRAIN. 
the precentral sulcus, ventral to the arm area lies a small area for move- 
ments of the eyelids, brought about by contractions of the orbicularis mus- 
cle. Ventral to this again is the " area for the face," in which we may dis- 
tinguish an area for the mouth, that is an area stimulation of which pro- 
duces changes in the buccal orifice, opening, shutting, drawing to one side, 
etc., and an area for movements of the tongue. These two areas reach down- 
ward to the fissure of Sylvius and backward to the line of the intra-parietal 
sulcus. In front of them, occupying all the ventral part of the precentral 
convolution and reaching forward as far as the precentral sulcus, where it 
Fig. 148. 
Outline of Brain of Monkey (Macacus) to show Principal Sulci (Fissures) and Gyri (Convolu- 
tions). (Sherrington, after Horsley and Schafer.) Natural size. The brain figured is the same as 
that in Fig. 149, and the two figures should be consulted together. Over each sulcus, purposely 
printed very thick, the name is written in small capitals, over each gyrus in italics, x indicates 
the small depression, hardly to be called a sulcus, which is supposed to be homologous with the 
superior frontal sulcus of man ; and w, y, z, similarly indicate sulci whose homologies are not 
certain. For some synonyms see Figs. 152, 154. 
meets the area for the eyes, lies an area stimulation of which produces 
movements of the pharynx or larynx, as well as of the mouth or face, and 
which may be divided into areas for mastication, for swallowing, and for the 
production of the voice. 
We might speak of these several areas in another way by referring to the 
nerves concerned in carrying out the several movements, though in doing ON VOLUNTARY MOVEMENTS. 
667 
so we must remember that there is not an exact correspondence between the 
relative position of a muscle along the axis of the body or along the axis 
of a limb and the relative position along the cerebro-spinal axis of the 
nerve or nerves governing the muscle. We may, however, adopting this 
method, note that the sacral and lumbar nerves are represented by the 
most mesial portion of the whole motor area and by the hind division of 
this mesial portion ; that the lumbar and thoracic nerves are represented by 
Fig. 149. 
Left Hemisphere of the Cerebrum of Monkey (Macacus), Viewed from its Left Side and from 
Above. (Sherrington, after Horsley and Beevor.) Natural size. The figure shows the positions 
of the portions of the cortex concerned with movement of various parts, and with the senses of 
sight, smell, and hearing, The cortical area connected with the movements of the leg is shaded 
vertically across, that with the movements of the arm horizontally, and that with the movements 
of the trunk in a slanting direction ; the area connected with the movements of the head (neck), 
face, and eyes is dotted. The course of the chief fissures is indicated by single lines. 
the front division of the same mesial portion; that the upper thoracic with 
the lower cervical nerves belong to a region lying lateral to, and the upper 
cervical nerves to one lying in front of, the preceding area ; and, lastly, 
that the remaining lateral and ventral portions of the whole motor region 
appertain to the cranial nerves. But the topographical differentiation does 
not come out so clearly by this method as by that of taking for our guide 
distinctive movements of the several parts of the body. 
It will be observed that all these areas taken together, represented by 668 
THE BRAIN. 
the portion of Figs. 149, 150 shaded in one way or another, occupy chiefly 
the parietal region of the cerebral surface, though they also reach into the 
frontal region. Stimulation of the frontal region in front of this motor 
area or of the occipital region behind, whether on the lateral or on the 
mesial surface, or of the temporal region, whether also on the latter or on the 
mesial surface, or of the gyrus fornicatus (Fig. 150) connecting the frontal 
and occipital regions on the mesial surface, and running ventral to the 
marginal gyrus, does not give rise to movements; or, to be more exact, does 
not give rise to movements comparable to those just described as resulting 
from stimulation of various parts of the motor region. Movements do take 
place when certain parts of the occipital or of the temporal region are stim- 
ulated, but these are not only feeble and experimentally uncertain, but 
appear to be of a different nature from those resulting from stimulation of 
Fig. 150. 
Mesial Aspect of the Left Half of the Brain of Macacus, displayed by Section in the Median 
Sagittal Plane and Removal of the Cerebellum. (Sherrington, after Horsley and Beevor.) Natural 
size. The hatched and stippled parts of the surface show the regions of the cortex connected 
with movements of the foot, knee, hip, tail, trunk, and neck respectively. The several positions of 
the areas of cortex connected with vision and smell and with cutaneous sensation are indicated by 
the appropriate words. The plane of section has passed through the corpus callosum, cc., cc., cc., 
and through the anterior commissure, c., sparing the left pillar of the fornix, F; behind it has 
bisected the anterior part of the pons, laying open the aqueduct, Aq. (iter a tertio ad quartum 
ventriculum); Pons, the left half of the pons in frontal section; Op., the optic commissure cut 
across; in, the root of the third cranial nerve; FR., the frontal pole; Oc., the occipital pole; 
On., the cuneus ; Pen , the precuneus ; G fn, G fn, G fn, the gyrus fornicatus; the unlettered fissure 
seen to form the upper boundary of this gyrus in its supra-callosal part is the calloso-marginal; 
Po f, the parieto-occipital fissure. 
the motor region ; it will be convenient to speak of the nature and meaning of 
this kind of movement when we come to discuss the development of sensations. 
§ 569. It is obvious from the foregoing that the mechanism for the 
development of these movements of cerebral origin are far more highly 
differentiated in the monkey than in the dog. But even in the monkey 
(Macacus and allied forms) the differentiation is still very incomplete. If 
we explore, for instance, the area for the wrist, we find that its limits are 
ill-defined. In some parts of the area we obtain movements of the wrist 
only, but in other parts of the area stimulation produces not only move- 
ments of the wrist, but also of the shoulder or of the digits, or of the neck; 
and so with the other areas. ON VOLUNTARY MOVEMENTS. 
669 
If, however, not a Macacus or other ordinary monkey, but the more 
highly developed orang-outang be taken as the subject of experiments, the 
differentiation is found to be distinctly advanced ; the several areas are 
more sharply defined, and what is important to note, the respective areas 
tend to be separated from each other by portions of cortex stimulation of 
which gives rise to no movement at all. 
The opportunities of stimulating the cortex of man himself have been 
few and far between, and have, for the most part, been conducted under 
unfavorable circumstances; but as far as the results so obtained go, they 
show that the topographical distribution of areas for the several movements 
is carried out on the same plan as in the monkey (we are purposely confin- 
ing ourselves now to the results of artificial stimulation) ; and, moreover, 
justify the conclusion, which a, priori reasons would lead us to adopt, that 
in man the differentiation is advanced still further than in the monkey. 
Thus, when we survey a series of brains in succession, from the more 
lowly frog, through the bird, the rabbit, the dog, and other lower mammals 
up to the monkey, the anthropoid ape, and so to man himself, we find an 
increasing differentiation of the cerebral cortex, by which certain areas of 
the cortex are brought into special connection with certain skeletal or other 
muscles in such a way that stimulation of a particular portion of the gray 
matter gives rise to a particular movement, and to that alone. 
§ 570. In treating of the structure of the brain we spoke (§ 545) of 
the pyramidal tract as starting from the motor region of the cortex ; and it 
is obvious that the fibres of this tract must be concerned in the develop- 
ment of the movements which we have just described. When the move- 
ments are brought about by stimulation of the fibres in some part of their 
course, in the internal capsule, for instance, there can be no doubt that the 
stimulation starts impulses which travelling down the tract to the origin of 
certain cranial or spinal nerves, in some way give rise to coordinate motor 
impulses along the motor fibres of the nerves; and we may with reason 
speak of the impulses then passing along the tract as motor or efferent in 
nature. When the stimulus is applied direct to the cortex, we may assume 
that processes, started in the gray matter, eventuate in similarly efferent 
impulses along the fibres of the tract. All the evidence leads us to regard 
this tract as an efferent tract. 
When the spinal cord is divided in the lower dorsal region and the elec- 
trodes of an electrometer are brought into connection with the transverse 
cut surface and with some point of the longitudinal surface above, the 
electrometer gives evidence of currents of action (manifested as negative 
variations of a demarcation current or current of rest (§ 67) whenever the 
motor area of the hind limb is stimulated, but not when other parts of the 
cortex are stimulated. We have already said that stimulation of any part 
of the motor region may, under abnormal conditions, give rise to general 
epileptiform convulsions; when these occur during such an experiment as 
the above, currents of action manifest themselves in the lower dorsal cord, 
whether the stimulation giving rise to the convulsions be applied to the area 
for the hind limb or to any part of the motor region. It has been further 
observed that the currents of action developed within the spinal cord tally 
in a very exact manner with the muscular movements. The convulsions 
begin with a sustained " tonic " contraction of the muscles, and the elec- 
trometer shows a similar sustained current of action ; this is followed by 
rhythmic movements of the muscles, accompanied by corresponding rhyth- 
mic movements of the mercury of the electrometer. Without insisting too 
much on the exact interpretation of these results, we may take them as at 
least showing that, when the motor region of the cortex is excited, nervous 670 
THE BRAIN. 
impulses accompanied by " currents of action " pass downward along the 
fibres of the pyramidal tract. 
The results of stimulating the fibres of the tract in their course through 
the corona radiata and the internal capsule, and the results obtained by 
studying the degenerations following upon injury to or removal of the several 
parts of the cortical motor region, agree in marking out the paths taken by 
the several constituents of the tract through the central white matter of the 
hemisphere, the corona radiata, and the capsule. Comparing Figs. 149, 150, 
with Figs. 144, 145, and 146, it will be seen that the portions of the tract 
destined for the cranial nerves, and so for the movements of the eyes, the 
mouth, face, tongue, pharynx, and larynx, starting from the ventral parts of 
the more frontal district of the motor region, take up their position at the 
knee of the internal capsule; and the portion destined for those upper 
cervical nerves which carryout movements of the head through the muscles 
of the neck, starting from the extreme frontal and dorsal parts of the area, 
is also apparently directed to the knee of the capsule. The rest of the tract, 
starting from the part of the area lying at mice behind and mesial to the 
above, occupies in the capsule a position posterior to them in the hind limb 
of the capsule ; and it will be observed that the tract for the fore limb which 
begins on the lateral surface of the tracts for the trunk and hind limb, shifts 
its course in relation to theirs, so that in the capsule it is front of them, 
not lateral to them. It may further be observed that while in the tracts for 
the trunk and hind limb the same fore-and-aft order which obtains on the 
surface is reproduced in the capsule, even apparently to the strange prece- 
dence of the ankle over the knee, the order of the several elements in the 
fore-limb tract which is lateral on the surface becomes regularly fore and aft 
in the capsule. In the capsule the several elements are arranged in a linear 
order, corresponding broadly to that of the distribution of the muscles along 
the longitudinal axis of the body; on the cortex they are disposed in an 
order the cause of which is at present not very clear, but which is probably 
determined by the respective relations of the several parts of the motor 
region to the functional activity of the other parts of the cortex. In the 
shifting from the one order to the other, the several constituent fibres, as we 
have said, describe a somewhat peculiar course; and when we remember, as 
stated in § 545, that the order shown in Fig. 144 is only the order obtaining 
at one particular level of the capsule, and that from the dorsal beginnings 
of the capsule in the corona radiata to its ventral end in the pes the capsule 
is continually changing in form, and its fibres therefore continually shifting 
their relations to each other, the whole course of the several fibres of the 
tract from their origin in the cortex until they are gathered up into the 
central portion of the pes (Fig. 137, 7y) must be a very complicated one. 
When the area of one hemisphere is stimulated, the movement which 
results is in most cases seen on the other side of the body, and on that other 
side alone. Thus when the area for the fore limb on the left hemisphere 
is stimulated it is the right fore limb which is moved. This is in accordance 
with what we have learned of the pyramidal tract and its ultimate entire 
decussation before it reaches the motor nerves, the decussation either occur- 
ring massively as in the case of the crossed pyramidal tract, or in a more 
scattered manner along the upper part of the spinal cord in the case of the 
direct pyramidal tract; and, as we have seen, there is a similar decussation 
for such part of the pyramidal tract as is connected with the cranial nerves 
above the decussation of the pyramids. Except in the case of certain areas 
for movements naturally bilateral, of which we shall speak presently, the 
movement is normally on the crossed side, and on the crossed side only. 
Under abnormal conditions, however, the limb on the other side-that is, of ON VOLUNTARY MOVEMENTS. 
671 
the same side as the hemisphere stimulated-may move also. But such an 
abnormal movement of the same side has not the same characters as the 
proper movement of the crossed limb. Instead of being an orderly coordi- 
nate movement, it is a more simple, either tetanic or perhaps tonic, or 
rhythmic, clonic, contraction of the muscles. Obviously its mechanism is 
of a different nature from that by which the proper movement of the crossed 
limb is effected ; but it is important to bear in mind that a movement of 
the uncrossed limb may take place; and further, that the abnormal condi- 
tions continuing, similar movements of an uncoordinated character may 
spread to the hind limb and other parts of the crossed side, though the stimu- 
lation be still confined to the arm area, then to other parts of the uncrossed 
side, until, as we have said, the whole body is thrown into epileptiform con- 
vulsions. This feature must not be forgotten. In fact, it may be fairly 
insisted upon that while we may speak of a particular coordinate movement 
as being the normal outcome of an ordinary careful stimulation of a par- 
ticular area in a normal condition, it is no less true that diffuse uncoordinated 
movements, culminating in general epileptiform convulsions, are the natural 
outcome of the stimulation of any area in an abnormal condition. And in 
attempting to form any opinion of the nature of the first act we must bear 
the second in mind. 
As we said above, the movements resulting from cortical stimulation 
are most conveniently described in terms of parts of the body-of the arm, 
of the thumb, of the tongue, etc. The movements of the same part may 
be further distinguished by means of the nomenclature usually adopted in 
speaking of muscular movements, such as flexion, extension, abduction, 
adduction, etc.; so that, within the area bearing the name of some particular 
part-such as the wrist, for instance-we have to distinguish an area for the 
flexion and another for the extension of that joint; and in like manner in 
reference to other parts. But it will be readily understood that it is easier 
to map out the area for a particular part than to distinguish the areas cor- 
responding to the several movements of that part. Hence the nomenclature 
usually adopted in speaking of the motor region is one based on the parts of 
the body moved, rather than on the character of the movements. The more 
closely, however, the movements in question are studied, the more probable 
it appears that the localization which obtains in the cortex is essentially a 
localization corresponding not to parts of the body or to nerves or to mus- 
cles, but to movements. In considering this point it must be remembered 
how rude and barbarous a method of stimulation is that of applying electrodes 
to the surface of the gray matter compared with the natural stimulation which 
takes place during cerebral action ; the one probably is about as much like 
the other as is striking the keys of a piano at a distance with a broomstick 
to the execution of a skilled musician. Were it in our power to stimulate 
the cortex in any way at all approaching the natural method, we should, in 
all probability, arrive at two results : on the one hand, we should be able to 
produce at will a variety of movements of different degrees of complexity, 
some very simple, others very complex, and for these we should have to use 
names suggested by the characters and purpose of each movement, and by 
these alone; on the other hand, we should find very decided limits to the 
number and kind of movements which we could evoke, limits fixed in the 
case of each subject partly by inherited organization, partly by the training 
of the individual. 
Some such results of refined experimentation are, indeed, already fore- 
shadowed by the rude results of our present rough methods. The move- 
ments which usually follow stimulation of the motor region, and which we 
have described as flexion, etc., are, so to speak, the elementary factors of 672 
THE BRAIN. 
ordinary bodily movements, the detached and imperfect chords of a musical 
piece ; and in the following facts relating to their production we can recog- 
nize the influences of organization and habit. As we have said, stimulation 
of the motor area of one hemisphere produces movements, as a rule, which 
are limited to one side of the body, and that the opposite side. Now both 
in ourselves and in the higher animals a large number of bodily movements, 
especially of the limbs, are habitually unilateral ; and, putting aside the 
question why there should be two halves of the brain, and why tbe one half 
of the brain should be associated with the cross half of the body, we may 
recognize in the unilateral crossed movement resulting from stimulation of 
the cortex an accordance with natural habits. But some movements of the 
body are ordinarily bilateral; the two eyes, for instance, are ordinarily 
moved together, and the two sides of the trunk move together very much 
more frequently than do the two fore limbs or the two hind limbs. And 
in accordance with this we find that stimulation of the motor area for the 
eyes on either hemisphere produces movements of both eyes, and stimula- 
tion of the trunk area of one hemisphere is also very apt to produce bilat- 
eral action of the trunk muscles; in such instances the movements on both 
sides are quite normal movements. We may incidentally remark that re- 
moval of the trunk area leads to a good deal of bilateral degeneration, that 
is, to degeneration of strands in the pyramidal tracts of both sides, whereas 
such a bilateral degeneration is comparatively scanty after removal of the 
leg or arm area. 
That it is the movement and not the part moved which is, so to speak, 
represented on the cortex is further shown by the relative magnitude of 
the several cortical areas when they are mapped out according to parts 
of the body. The area for the arm, for instance (c/. Figs. 149, 150), is, 
so to speak, enormous compared to that of the trunk when the relative 
bulks of these two parts of the body are considered ; and within the arm 
area itself the space occupied by the thumb and forefinger and digits is, 
bulk for bulk, out of proportion to the space allotted to the shoulder; so 
also the area for the eyes, or for the mouth is out of proportion to the 
size of those organs. But these relative sizes of the respective areas be- 
come intelligible when we bear in mind relative mobility, nimbleness, 
and delicacy of execution ; in these respects the shoulder is far behind 
the thumb, while the eyes and mouth surpass most other parts of the 
body. 
We are brought yet a step further when we compare, in respect of the 
cortical motor region, animals of different grades of organization ; and the 
results thus obtained lead us to the conclusion that the motor region is cor- 
related not to movements in general, but to movements of a particular kind. 
Taking in series the rabbit, the dog, the monkey, and man, we find in pass- 
ing from one to the other, an increase in prominence and in differentiation 
of the motor region accompanied by an increase in the bulk of the pyram- 
idal tract; among the many striking differences between the brains of these 
several animals, these two features, the increasing complexity of the motor 
region and the increasing size of the pyramidal tract, are among the most 
striking. The size of the pyramidal tract is itself correlated to the com- 
plexity of the motor region, and, being the more easily determined, may be 
used as indicating both ; the difference in the size of the pyramidal tract in 
these animals is seen all along the whole length of the cord (Fig. 151). Now 
as regards mere quantity of movement, if we may use such an expression, 
the differences between these animals are of no great moment. If we 
were to take the amount of energy expended as movement in twenty-four 
hours per gramme of muscle present in the body in each of the four cases. ON VOLUNTARY MOVEMENTS. 
673 
we should certainly not find any correspondence between that and the size 
of the pyramidal tract. If, however, we take a particular kind of move- 
ment, what we may perhaps call skilled movement, that is, movement car- 
ried out by means of intricate changes in the central nervous system, we do 
find a remarkable parallelism in the above cases between the amount of such 
skilled movement entering into the daily life of the individual and the size 
of the pyramidal tract. In these two respects man is much above the monkey, 
and the monkey far above the dog. We may conclude then that the cortical 
Fig. 151. 
MAN 
DOG 
Diagram to illustrate the Relative Size of the Pyramidal Tract in the Dog, Monkey, and Man. 
(Sherrington.) The figure shows in outline the lateral half of the cord, at the level of the fifth 
thoracic nerve, in A, man ; B, monkey ; C, dog. Ais a reproduction of IP in Fig. 127 ; B and C 
are drawn of the same size as a. Py., shaded obliquely, the pyramidal tract; the depth of shad- 
ing indicates that the tract is more crowded with true pyramidal fibres as well as larger in A 
than in B, and in B than in C. In B, Py' is an outlying portion of the pyramidal tract separated 
from the rest by the cerebellar tract; Py. d., the direct pyramidal tract, present in man only. 
The gray matter seems relatively large in C because the section was taken from a very young 
puppy. 
MONKEY 
motor region is in some way especially concerned with the kind of movement 
which we have called " skilled." 
§ 571. These skilled movements are, to a large extent, though not 
exclusively, voluntary movements. We have, in a previous section, seen 
reason to believe that the cerebral cortex is in some way especially asso- 
ciated with the development of voluntary movements. Putting together 
this conclusion and the conclusions just arrived at, we are naturally led to 
the further conclusion that the cortical motor region, with the pyramidal 
tract belonging to it, plays an important part in carrying out voluntary 
movements. Do other facts support this view ; and if so, what light do' 
they throw on the question as to what part and what kind of part the 
motor region thus plays? 
In this connection we naturally desire to know what are the results of 
removing from an otherwise intact animal the whole motor region and more 
especially this or that particular portion of it. Before proceeding further, 
however, we may once more call attention to the caution given in § 495 and 
repeated in $ 553 ; indeed, when we consider the high organization and com- 
plex functions which obviously belong to the cortex, when we bear in mind 
that it appears to govern, and must therefore be bound by close ties to almost 
all the rest of the central nervous system, we must be prepared to find after 
removing a portion of cortex that the pure "deficiency" phenomena, those 
which result from the mere absence of a piece of the cortex, are largely 
obscured by the other effects of the operation. 
In the rabbit the results have been almost purely negative. When in this 
animal the part of the cortex which may be considered as the motor region 674 
THE BRAIN. 
is removed, nothing remarkable is observed in the movements of the animal 
We can hardly suppose that the operations of the central nervous system are 
the same in an injured as in an intact animal, and the differences induced 
ought to be betrayed by the movements of the body ; but at present they 
have escaped observation. 
In the dog the removal of an area is followed by a loss or diminution of 
voluntary movement in the corresponding part of the body. When, for 
instance, the area for the fore limb is removed from the left hemisphere, the 
right fore limb is completely or partially " paralyzed." In carrying out its 
ordinary movements the operated animal makes little or no use of its right 
fore limb. But this state of things is temporary only. After a while the 
animal regains power over the limb, and in successful cases recovery is so 
complete that it is impossible to point out in the limb any appreciable devia- 
tion from the normal use. And careful examination after death has shown 
not only that the area had been wholly removed, but also that there was no 
regeneration of the lost parts ; the removal of the cortex leads in such cases, 
as usual, to degeneration of the corresponding strand in the pyramidal tract 
right away from the cerebral surface to the endings of the strand in the 
cervical and dorsal spinal cord. Nor can it be urged in such cases that dif- 
fused remnants of the arm area had been left in the remaining parts of the 
motor region ; for the whole motor region has been removed, and yet the 
animal has recovered to such an extent that a casual observer could detect 
no differences between the movements of the two sides of the body. Closer 
examination did disclose certain imperfections of movement; but the opera- 
tion had involved injury to or produced changes in structures other than 
the motor region, and the imperfections might have been due to the addi- 
tional damage. Nor can it be urged that, in such a case, where one side is 
removed, the remaining hemisphere takes on double functions; for the 
greater part of the motor areas have been removed on both sides, and yet 
the animal's movements have been so far apparently complete that a casual 
observer would see nothing strange in them. Again, the whole motor region 
has been removed from one hemisphere in a young puppy, and some time 
later when the movements seemed to have recovered their normal condition, 
the removal of the motor region of the other hemisphere has produced merely 
a paralysis of the crossed side of the body, and that as before only of a tem- 
porary character. 
Two things have to be noted here. In the first place, the removal of an 
area does affect the movements which are brought about by stimulating that 
area, it leads to their disappearance or at least to great diminution of them ; 
and this affords an additional argument that the connection between the area 
and the movement is a real and important one. In the second place, the 
physiological effect is temporary only, though the anatomical results of the 
operation are permanent, for the cortex is never renewed, and the pyramidal 
tract degenerates along its whole length, never to be restored ; this shows 
that we have to deal here with events of a very complex character. When 
a particular movement results from stimulation of the appropriate cortical 
area, we may be sure that whatever takes place in the cortex and along the 
pyramidal tract, motor impulses, duly coordinated, pass along certain ante- 
rior roots to certain muscles; and we know that if we removed a sufficient 
length of each of those anterior roots that particular movement would be 
lost for the rest of the life of the individual. We may, therefore, infer that 
the events which, whatever be their exact nature, taking place in the cortex 
and along the pyramidal tract lead ultimately to the issue of motor impulses 
along the anterior roots, differ essentially from the events attending the 
transmission of ordinary motor impulses. ON VOLUNTARY MOVEMENTS. 
675 
In the case of the monkey, the results of removing parts of the cortical 
motor region have not been so accordant as in the case of the dog. The 
two animals agree perfectly in so far that the removal of a particular area 
leads, as an immediate result, to the loss of the corresponding movement; 
but while in some instances recovery of the movement has in the monkeyas 
in the dog after a while taken place, in other instances the "paralysis" has 
appeared to be permanent. As a rule the paralysis caused by a large lesion 
is not only more extensive, but also of longer duration than that caused by 
a small one; and natural bilateral movements, as of the eyes, reappear 
earlier than unilateral movements. The facts, however, within our know- 
ledge relating to the permanence of the effect are neither numerous nor exact 
enough to justify at present a definite conclusion. On the one hand, the 
positive cases where recovery has taken place are of more value than the 
negative ones, since in the latter the recovery may have been hindered by 
concomitant events of a nature which we may call accidental; and it is at 
least a priori most unlikely that the pyramidal tract mechanism, if we may 
use the expression, though it may differ in the monkey and the dog in degree 
of development, differs so essentially in kind that damage of it leads in the 
one case to permanent, and in the other to mere temporary, loss of function. 
We may add that we should further expect to meet in the monkey with 
more prominent and more lasting complications due to the subsidiary effects 
of the operation, and it may be doubted whether in any of the recorded 
experiments the animal has been allowed to live a sufficient time for these 
subsidiary events to have cleared away, leaving only what we have called 
the " deficiency" phenomena, due to the loss of the cortical area alone. On 
the other hand, it must be remembered that the movements of the monkey 
are more intricate in origin, more "skilled," than those of the dog; and it 
may be that differences in the characters of movements determine the possi- 
bility of their recovery. In illustration of this we may quote the expe- 
rience that, after the removal of the arm area in the monkey, a certain 
awkwardness in the movements of the thumb is one of the last effects of the 
operation. 
§ 572. Before we proceed, however, any further in the discussion, it will 
be of advantage to turn aside to what is known concerning the cortical 
motor region in man. As we have already said, theoretical considerations 
lead us to believe that the cortical motor region in man is disposed in accord- 
ance with the plan of the anthropoid ape as ascertained experimentally, but 
with the differentiation carried still further; and the few cases of experi- 
mental stimulation of the human cortex support this view. Our chief 
knowledge in this matter is derived from the study of disease; and in this 
the advantages of dealing with one of ourselves are largely counterbalanced 
by the disadvantages due to disease being so often anatomically diffuse and 
physiologically changeful and progressive. 
We said above that during experiments on animals stimulation of any 
part of the motor region may under abnormal conditions lead to general 
epileptiform convulsions. Now clinical study has shown that in man certain 
kinds of epileptic attacks are of similar cortical origin. In these cases it 
has been observed that the attack begins in a particular movement, by con- 
tractions of particular muscles or of the muscles of a particular region of 
the body, of the hand, foot, toe, thumb, etc., and then spreads in a definite 
order or " march " over the muscles of other regions until the whole body is 
involved. When in an experiment on an animal epileptiform convulsions 
supervene, they similarly start from the region of the body, the motor' area 
of which is beneath the electrodes at the time, and similarly spread by a 
definite " march " over the whole body. Hence, in the human epileptiform 676 
THE BRAIN. 
attacks of which we are speaking, it has been inferred that the immediate 
exciting cause of the attack is to be sought in events taking place in that 
part of the cortex which serves as the area for the movement which ushers 
in the attack. Further inquiry has not only confirmed this view, but has 
also shown that the topography of the cortical areas in man, as thus deter- 
mined, very closely follows that of the monkey. 
Other diseases of the cortex have been marked, among other symptoms, 
by loss or impairment of particular movements. In most of such cases the 
cortical lesion has been of such an extent as to involve a number of special 
areas at the same time, and so to lead to loss or impairment of movement 
over relatively considerable regions of the body, such as the whole of one 
arm; and in general the teaching of these cases of disease, while confirming 
the deductions from the monkey, and giving us some general idea of the 
topography of the human motor cortical region, has at present given us 
approximate results only. Figs. 154 and 155 show in broad diagrammatic 
manner the position and relative extent of the motor areas for the leg, arm, 
and face in man as far as has yet been ascertained. To assist the reader we 
give at the same time diagrams (Figs. 152 and 153) illustrating the nomen- 
clature of the surface of the human brain. 
One area is of special and instructive interest. Speech is an eminently 
" skilled " movement. We have seen that in the monkey the area for the 
mouth and tongue lies at the ventral end of the central fissure or fissure of 
Rolando, ventral to the arm area, and that the extreme ventral and front 
part of the motor region just above the fissure of Sylvius supplies an area 
which we marked as that of phonation (Fig. 149). In the monkey the area 
of phonation is determined by experimental stimulation; in man, in a 
similar position, on the third or lowest frontal convolution, sometimes called 
Broca's convolution, ventral to and in front of, and probably overlapping 
backward, the area which in Fig. 154 is marked "face," and which includes 
the mouth and tongue, clinical study has disclosed the existence of an area 
which may be spoken of as the area of "speech." Lesions of the cortex in 
this area cause a loss of or interference with speech, the condition being 
known as aphasia; to this we shall presently return. In Fig. 154 this area 
is shown in an approximate manner. 
The movements of speech are essentially bilateral movements. In the 
dog and monkey various bilateral movements maybe excited by stimulation 
of the appropriate area in either hemisphere; and analogy would lead us to 
suppose that in man the movements of speech would be connected with the 
speech area in both one and the other hemisphere. The results of lesions, 
however, show that it is in most cases especially the left hemisphere which 
is connected with speech; it is a lesion in the third frontal convolution of 
the left hemisphere, often associated with other lesions of the same hemi- 
sphere leading to paralysis of the right side of the body and face, which 
causes aphasia, it being only in exceptional cases that the condition results 
from a lesion of the corresponding area of cortex on the right hemisphere. 
In man, then, clinical study corroborates the conclusions deduced from 
the experimental investigation of the dog and of the monkey, but still leaves 
us in uncertainty as to the question what and what alone are the absolutely 
permanent effects of the loss of a cortical area and nothing else. On the 
one hand, in the cases in which recovery of a movement follows upon its 
loss or impairment, it is open for us to suppose that the lesion itself was tem- 
porary, and that with the cure of the malady the cortical area regained its 
normal condition. On the other hand, where the disease continues, the per- 
manency of the loss of any movement may be attributed to the disease 
doing more than merely suspend' the function of the cortical area. Aphasia, ON VOLUNTARY MOVEMENTS. 
677 
especially in young persons, has been followed by recovery, but in such 
cases it has been supposed that the dormant area on the right side has been 
awakened to activity by the loss of the left area ; and in support of this 
Fig. 152. 
Diagram of the Gyri (convolutions) and Sulci (fissures) on the Lateral Surface of the Right 
Hemisphere of Man. (Gowers.) 
Fig. 153. 
The same on the Mesial Surface. (Gowers.) In both figures the sulci are indicated by italic 
and the convolutions by roman type. The following list of some synonyms may perhaps be of 
use in connection with these figures and those of the brain of the monkey (Figs. 149 and 150). 
Gyri or convolutions. Precentral or anterior central = ascending frontal. Postcentral or pos- 
terior central = ascending parietal. Superior temporal = infra-marginal = first temporal. Tri- 
angular lobule = cuneus. Central lobe - island of Reil. Paracentral lobule = the mesial face 
of the superior frontal within the marginal gyrus. Cingulum = the part of the gyrus fornicatus 
which adjoins the corpus callosum. Gyrus hippocampi = uncinate gyrus, though the latter name 
is sometimes restricted to the front part of the hippocampal gyrus; the two may be considered 
as a continuation of the gyrus fornicatus, and the three together, forming a series, have been 
called " the great limbic lobe." 
Sulci or flssuers. Central = Rolandic, or of Rolando. Perpendicular ptari eto-occipital. 
Parietal = intra-parietal or sometimes inter-parietal. Tempor=o-sphenoidal lobe =mporal lobe. 
view cases have been recorded in which a first aphasia, due to a lesion on 
the left side has been followed by a second aphasia due to a subsequent lesion 678 
THE BRAIN. 
occurring on the right side. On the whole, perhaps, the evidence of clinical 
study tends to show that in man the loss of movement due to the destruction 
Fig. 154. 
The Lateral Surface of the Right Cerebral Hemisphere of Man in Outline, to illustrate the 
Cortical Areas. (Reduced from nature.) The position of the areas of the cortex concerned with 
movements of the face, arm, and leg, and with the senses of sight and hearing are approximately 
shown. The position of the area connected with speech (Broca's centre) is also shown for the sake 
of comparison of it with the position of the otherareas; the representation of speech in the cor- 
tex cerebri lies, however, in the left hemisphere chiefly. Oc. L, occipital lobe ; Ft. L, frontal lobe ; 
Te. L, temporal lobe ; Sy.f, the fissure of Sylvius; C.f, the central fissure (Rolandic); Cm.f, indi- 
cates the position of the posterior end of the calloso-marginal fissure. 
Fig. 155. 
The Mesial Surface of the Right Cerebral Hemisphere of Man in Outline, to illustrate the 
Cortical Areas. The areas shown are those connected witii the movements of the leg and with 
the senses of sight and smell. /Y. L, the frontal lobe of the hemisphere : Oc. L, the occipital 
lobe; Te. L, the temporal lobe ; Cm.f, the calloso-marginal fissure separating the marginal gyrus 
above from the gyrus fornicatus below; <y, marks the situation of the central fissure, the fissure 
itself not being apparent on the mesial aspect of the hemisphere. The corpus callosum and the 
anterior commissure are seen in cross section. 
by disease of an area is a permanent one, though actual demonstration of 
this is wanting. ON VOLUNTARY MOVEMENTS. 
679 
§ 573. We may now return to the discussion of the question, What is 
the part played by a motor area, and by the contribution from that area to 
the pyramidal tract in carrying out the movements with which the area is 
associated ? 
We may premise that the evidence points very distinctly to the con- 
clusion that whatever be the nature of the whole chain of events of which 
the cortical area seems to be a sort of centre, the fibres of the pyramidal 
tract serve as the channel of processes which we must regard as efferent in 
nature. It is perfectly true that in many cases at least the removal of a 
cortical area has led to diminished sensibility of the part in which move- 
ments are excited by stimulation of the area ; and there are many facts, of 
which we shall presently quote a very striking one, which go to show that 
the cortex of the motor region is largely influenced by sensory impulses from 
various parts of the body ; but we cannot suppose that the pyramidal tract 
is the channel by which such sensory impulses reach the cortex. As we 
have previously (§ 481) urged, the fact that the degeneration of the fibres 
in the tract is a descending one, and cannot be trusted by itself to prove 
that the direction in which the fibres carry impulses is only that from the 
cortex downward ; but this added to the fact that when the fibres of the 
tract are stimulated at any part of their course, movements, the signs of the 
occurrence of efferent centrifugal impulses, are produced, leaves no doubt 
that the tract is. one of efferent fibres. Hence we may infer that whatever 
be the nature of the events taking place in a motor area during the carry- 
ing out of a movement, the part played by the fibres of the pyramidal 
tract is that of carrying efferent impulses from the area to the muscles con- 
cerned. 
Let us consider first the movements of speech in man, the evidence touch- 
ing the connection of which with an area on the third frontal convolution 
appears so very clear. Speech is eminently a " skilled " movement; it 
involves the most delicate coordination of several muscular contractions, 
and we may certainly say of it that it has to be" learned." The whole chain 
of coordinated events by which the utterance of a sentence, a word, or any 
vocal sign is accomplished consists of many links, the breaking of any of 
which will lead to failure of one kind or another in the act. Something may 
go wrong in the glossal or other muscles, in the nerve-endings in those 
muscles, or in the fibres of the nerves, hypoglossal and others, between the 
central nervous system and the muscles, or something may go wrong in that 
part of the central nervous system, the bulb to wit, in which a certain amount 
of coordination is carried out just previous to the issue of the motor impulses. 
Damage done to any of these parts of the mechanism may lead to dumbness 
or to imperfect speech. In the latter case the imperfections have a certain 
character ; if we are at all able to gather the wish of the speaker, we recog- 
nize that he is attempting to utter the right words in the right sequence, but 
that his efforts are frustrated by imperfect coordination or imperfect mus- 
cular action ; his speech is " thick," the syllables are blurred and the like. 
Disease of the bulb at times leads to imperfect speech of this kind in which 
the imperfection may be recognized as due to the lack of proper coordina- 
tion of motor impulses. The affection of speech known as "aphasia," 
which is caused by lesions of the cortex, is of a different character, and the 
forms of imperfect speech caused by bulbar disease have justly been distin- 
guished from true aphasia by the use of other terms. Cases of complete 
aphasia in which all power of speech is lost, do little more than help us to 
ascertain the topographical position in the cortex of the "speech" area, but 
cases of partial aphasia are especially instructive. Without attempting to 
go into the details of the subject and into the many considerations which 680 
THE BRAIN. 
have to be had in mind in dealing with it, for there are different kinds of 
aphasia, we may venture to say that the striking feature of partial aphasia 
is the failure to say certain words or syllables, and the tendency to substi- 
tute some wrong word or syllable for the right one. The words or syllables 
which are uttered are rightly pronounced without defect of articulation ; 
and in many cases, though the right word cannot be produced as a direct 
effort of the will, it may be uttered under the influence of an emotion, or, 
indeed, sometimes as the result of some physical processes more complex 
than those involved in the mere volitional effort to say the word. An in- 
structive case is recorded of a man suffering from slight aphasia, who, after 
several failures to say the word "no" by itself, at last said, "I can't say 
no, sir." 
From the phenomena of partial aphasia we may draw the deduction 
that the cortical speech area does not carry out the whole of the coordina- 
tion of the impulses involved in articulation. That coordination is exceed- 
ingly complex, and we ought, perhaps, to recognize in it more than one de- 
gree or kind of coordination. The failure of articulation in disease of the 
bulb shows that a certain amount of coordination takes place there ; for the 
affections of speech due to bulbar disease are not the same as those resulting 
from the mere loss of this or that muscle or nerve. We must, of course, 
admit that some, possibly a great deal, of coordination of a certain kind 
takes place in the cortex, for the bulb cannot by itself be made to speak ; 
exactly how much, the knowledge at present at our disposal leaves a matter 
of great uncertainty; but it is sufficient for our present purpose to recog- 
nize that whatever may be the nature of the events taking place in the 
cortical area during the act of speech, those events make use of the machin- 
ery already provided in the bulb. The word spoken does not start, so to 
speak, ready made in the cortex ; it is not that a group of impulses start 
from the cortex with their coordination fully achieved, and pass along cer- 
tain nerve-fibres to certain muscles, making their way without change 
through the tangle of the bulb, as if this were merely a bundle of lines 
offering paths for, but exercising no influenceover the impulses. We must 
rather suppose that something takes place in the cortex of the third frontal 
convolution, as the result of which efferent impulses pass along the appro- 
priate fibres of the pyramidal tract to the bulb, and there start a series of 
events leading to the issue of the coordinated impulses by which the word is 
spoken. 
§ 574. We have no reason whatever to think that the cortical area for 
speech differs in its fundamental characters from other divisions of the motor 
region, and are justified in carrying on to other areas the deduction we have 
just drawn in connection with the speech area. With that end in view we 
may now turn back to the experimental results obtained on the dog, and it 
will make our discussion simpler if we take as an illustration some large area 
such as the fore-limb area. 
We have seen that stimulation of this area produces what we may, to 
start with, speak of simply as movements of the fore limb; and guided by 
the analogy of speech in man we may confidently conclude that when the 
dog voluntarily moves the fore limb, the act is carried out by means of 
events taking place in the fore-limb cortical area. The simplicity of the 
electrical phenomena resulting from cortical stimulation, which we described 
in § 658, might at first sight lead us to conclude that the whole matter was 
fairly simple; and, indeed, some writers appear to entertain the conception 
that in a voluntary movement such as that of the fore limb, all that takes 
place is that the " will " stimulates certain cells in the cortical area, causing 
the discharge of motor impulses along the pyramidal fibres connected with ON VOLUNTARY MOVEMENTS. 
681 
those cells, and that these motor impulses travel straight down the pyram- 
idal tract to the motor fibres of the appropriate nerves, undergoing possibly 
some change at the place in the cord where the pyramidal fibre makes junc- 
tion with the fibre of the anterior root, but deriving their chief if not their 
whole coordination from the cortex itself, that is to say, being coordinated 
at their very starting-point. That such a view is untenable and that the 
simplicity of the electrical phenomena is misleading is shown by the follow- 
ing two considerations among others : On the one hand, as was shown in a 
previous section, the coordination of movements may be carried out apart 
from the cortex, namely, in the absence of the hemispheres; and we can 
hardly suppose that there should be two quite distinct systems of coordina- 
tion to carry out the same movement, one employed when volition was the 
moving cause, and the other when something else led to the movement. On 
the other hand, the analogy of speech justifies us in concluding that the cor- 
tical processes do take advantage of coordination effected by the action of 
other parts of the nervous system. 
Bearing this in mind, we may recall attention to the remarkable effects 
which result from the removal of the area. These are twofold. In the first 
place, there is more or less complete paralysis of the limb ; all the movements 
of the limb are for a time ineffective. It is not that purely voluntary move- 
ments are alone, so to speak, cut out; the reflex and other movements are 
also impaired or temporarily abolished, and, as we have already said, in 
many cases at least the sensations of the limb are interfered with. These 
troubles are, of course, in part the effects of the mere operative interference 
belonging to what we spoke of in § 495, as being of the nature of shock. 
But, even giving full weight to this consideration, there remains the fact that 
the cortical area is associated with the various coordinating and other ner- 
vous mechanisms belonging to the limb by such close ties that these are 
thrown into disorder when it is injured. And side by side with this we may 
put the remarkable fact previously stated, that during an abnormal condi- 
tion of the cortical area stimulation of the area, instead of producing the 
appropriate movements confined to the limb, may give rise to movements 
of other parts culminating in epileptiform convulsions. 
In the second place, this paralysis is temporary only ; the voluntary move- 
ments are after a while regained, and that in spite of the fore-limb moiety of 
the pyramidal tract permanently degenerating along its whole length, neither 
it nor the cortical area ever being regenerated. This shows that whatever 
be the chain of events in the intact animal, it is possible for the " will " of 
the animal to get at the muscles and motor mechanisms of the fore limb by 
some other path than that provided by the appropriate cortical area and 
corresponding path of the pyramidal tract; and the facts previously recorded 
(§ 571) show that that other part is not the corresponding part of the pyram- 
idal system belonging to the other half of the hemisphere, and, indeed, is 
not any part at all of the whole pyramidal system. The " will," whatever 
be the processes by which it takes origin, and wherever be the place where 
they are carried on, is able in the absence of the pyramidal system to produce 
its effect on the motor fibres of the brachial nerves by working on other 
parts of the central nervous system. 
Hence while admitting, as we must do, that in the intact animal the cor- 
tical area and pyramidal tract play their part in carrying out voluntary 
movements, their action is not of that simple character supposed by the view 
referred to above. On the contrary, we are driven to regard them rather as 
links, important links, it is true, but still links in a complex chain. As we 
have already urged, we may probably speak of the changes taking place in 
the pyramidal fibres as being on the whole of the nature of efferent impulses; 682 
THE BRAIN. 
but we should be going beyond the evidence if we concluded that they were 
identical with the ordinary efferent impulses of motor nerves. And above 
all it must not be left unnoticed that the cortical area has close, if not direct,, 
connections of a sensory nature with the part in whose movements it is con- 
cerned. This is shown by the following remarkable results which may make 
their appearance when stimulation of the cortex is carried on while the animal 
(dog) is in a particular stage of the influence of morphine. If a submini- 
mal stimulus be found, that is, a current of such intensity that applied to a 
motor area it will produce no movement, but if increased ever so slightly 
will give a feeble constriction of the appropriate muscle, it may be observed 
that a slight stimulus, such as gently stroking the skin over the muscles in 
question, will render the previous subminimal stimulus effective, and so call 
forth a movement. Thus, if the area experimented on be that connected 
with the lifting of the forepaw, and the subminimal stimulus be applied to 
the area at intervals, after several applications followed by no movements, a 
gentle stroke or two over the skin of the paw will lead to the paw being- 
lifted the next time the stimulus is applied to the area. A similar result, 
but less sure and striking, may follow upon the stimulation of parts of the 
body other than the part corresponding to the area stimulated. Then, again, 
it has been observed that in certain other stages of the influence of morphine, 
the cortex and the rest of the nervous system are in such a condition that 
the application of even a momentary stimulus to an area leads not to a sim- 
ple movement, but to a long-continued tonic contraction of the appropriate 
muscles. Under these circumstances a gentle stimulus, such as stroking the 
skin or blowing on the face, applied immediately after the application of the 
electric stimulus to the area, suddenly cuts short the contraction, and brings 
the muscles at once to rest and normal flaccidity. 
These experiments show that the development of the processes in the 
cortex leading to the issue of what we have agreed to call efferent impulses 
along the pyramidal fibres is markedly affected by sensory impulses, and 
especially by sensory impulses started in the skin overlying and corresponding 
to the muscles put into movement. How these sensory impulses reach the 
cortex we do not exactly know; but we have no evidence to show that 
afferent, centripetal impulses can travel backward, so to speak, along the 
pyramidal fibres; and it is more reasonable to suppose that the sensory im- 
pulses in question reach the cortex by the ordinary paths of sensory impulses, 
which we shall presently discuss. We may therefore take the results of the 
experiments as showing how close is the connection of the motor area with 
the sensory mechanisms of the spinal cord and lower parts of the brain, and 
as illustrating the complexity of the chain of events by which the motor 
area brings about voluntary movements. 
§ 575. We have above used the general phrase " movements of the limb," 
since in the dog it is not easy to pick out certain movements as being particu- 
larly skilled movements. In the monkey such a distinction is easier. In 
this animal, as we have said, recovery of voluntary movement also takes 
place after removal of a cortical area, or at least has done so in many cases; 
and while the phenomena immediately following removal on the whole re- 
semble those witnessed in the dog, a certain order of recovery may be ob- 
served ; the more skilled movements are the last to return. When, for 
instance, the arm area is removed, the delicate movements of the hand, of 
the thumb and finger, are the last to be reestablished ; and a condition of 
things may be met with in which the animal after removal, say of the arm 
area in the left hemisphere, uses by preference the left hand at a time when, 
if prevented from using that hand, he is able to use the right; that is to say, 
the recovery in the right limb after the removal of the area on the left side ON VOLUNTARY MOVEMENTS. 
683 
is nearly but not quite complete; the " will " can gain access to the right 
hand, but not so easily as to the left hand, and this latter is used, though 
under ordinary circumstances it would not be used. 
When we turn to man, in whom the great development of the pyramidal 
system and differentiation of the cortical area is paralleled by the promi- 
nence of skilled and trained movements, the analogy of the phenomena of 
speech, if it be true, as clinical histories seem to show, that destruction by 
disease of the speech area of both sides causes permanent aphasia, would 
lead us to conclude that at least highly skilled voluntary movements are 
carried out by the pyramidal system and by that alone. But in reference to 
this it must be remembered that such a permanent aphasia may be due, not 
to mere loss of the pyramidal channel, not to the will being merely unable 
to gain access to lower coordinating mechanisms, but to the absence of the 
differentiated cortical gray matter, by reason of which absence the will can- 
not initiate the first processes of the act of speech ; it may be that were it 
able to do so, the processes so started might, in the absence of the pyramidal 
tract, find some other way to the bulbar mechanism, as in the case of the un- 
skilled movements of the dog. This point, however, clinical histories have 
not definitely settled. Moreover, in dealing with the phenomena of the 
nervous system of man, as revealed by disease, we meet in reference to the 
cerebral cortex the same difficulty that we dwelt upon in dealing with the 
spinal cord (§ 504). Lesions of the pyramidal system, of the internal capsule 
for instance, lead to the loss not only of skilled, but of all voluntary move- 
ments ; according to the character and position of the lesion this or that part 
of the body is wholly withdrawn from the influence of the will. And it is 
possible to maintain the thesis that man has become so developed as to his 
nervous system and the motor cortex, so accustomed to make use exclusively 
of the pyramidal system, that the will has lost the power, still possessed by 
lower animals, to gain access by some path other than the pyramidal one to 
the immediate nervous mechanisms of movement. The data for forming a 
satisfactory conclusion as to this point are so few and uncertain that it would 
be unprofitable to discuss the question here ; but we may venture to point out 
that, great as is the development of the cerebral cortex and the pyramidal 
system in man, that development is accompanied by a hardly less striking 
expansion of other parts of the brain not directly connected with the pyram- 
idal system which we have previously seen reason to associate with the 
coordination of movements, for example the cerebellum. And, indeed, it is 
clear that, admitting the pyramidal tract to be the ordinary channel by 
which volitional impulses pass to, or by which the will gains access to, the 
motor mechanisms immediately associated with the anterior roots of this or 
that spinal nerve, we must also admit that those volitional impulses passing 
along the pyramidal tract, or at least some of the processes constituting the 
will, are in connection with, and thus are influenced by the condition of, 
other parts of the brain. When, for instance, a gymnast executes a skilled 
voluntary movement in which all his four limbs and other parts as well per- 
haps of his body are involved, it is probably the case that changes of the 
nature of efferent impulses sweep down his pyramidal tract, and that these 
impulses, starting in a definite order from his cortex, that is to say, having 
undergone a certain amount of initial coordination at their very origin, meet 
with further coordination in the spinal gray matter, which serves as a set of 
nuclei of origin for the motor nerves concerned in the movement before they 
issue as ordinary motor impulses along the anterior roots. But this is not 
all. Should the gymnast's semicircular canals happen to be injured and his 
cerebellum thereby be troubled, or mischief fall on some other part of the 
brain which like this has no direct connection with either the pyramidal 684 
THE BRAIN. 
tract or the motor cortex, the movement fails through lack of coordination, 
though both the cortex, the pyramidal tract, and the spinal motor mechan- 
isms remain as they were before. Obviously the carrying out of a voluntary 
movement is a very complex proceeding, and the motor cortex with the 
pyramidal tract is only one part of the whole mechanism ; so far from the 
whole business being confined to these, it is perhaps no exaggeration to say 
that in each movement of the kind most parts of the whole brain have a 
greater or less share. 
The exact nature of the part played by the cortex and the pyramidal 
tract in voluntary movements our present knowledge is inadequate to define. 
When we pass in review a series of brains from the lower to the higher and 
see how the pyramidal system is, so to speak, grafted on to the rest of the 
brain, when we observe how the increasing differentiation of the motor cor- 
tex runs parallel to the increasing possession of skilled educated movements, 
we may perhaps suppose that a " short cut " from the cortex to the origins 
of the several motor nerves, such as is afforded by the pyramidal fibres, 
from the advantages it offers to the more primitive path from segment to 
segment along the cerebro-spinal axis has by natural selection been developed 
into being in man the chief and most important instrument for carrying out 
voluntary movements ; but, we repeat, it remains even in its highest develop- 
ment a link in a chain, and a knowledge of how the whole chain works is 
at present hidden from us. 
We must not here wander into psychological problems, but may repeat 
that in the above discussion we have used the word "will" in a general 
sense only. A man maybe brought into a condition, for instance in certain 
hypnotic phases, in which he can carryout all the various skilled movements 
which he has inherited or which he has learned ; and yet, according to some 
definitions of the word " will," those movements could not be said to be initi- 
ated by his will. It can hardly be doubted that in such cases the motor 
cortex and pyramidal tract play their usual part. But we may pass from 
such cases as these through others, until we come to cases where a skilled 
movement which has been learned and practised by the working of an in- 
telligent will, may continue to be carried out under circumstances which 
seem to preclude the intervention of any conscious will at all; and the 
transition from one case to another is so gradual, that it is impossible to sup- 
pose that there has been any shifting of the machinery employed for carry- 
ing out the movement. So that a volitional origin is not an essential feature 
of these so-called voluntary movements, and the machinery of the motor 
cortex and pyramidal tract is available for other things than pure voli- 
tional impulses. 
§ 576. The preceding discussion will enable us to be very brief concern- 
ing a question which has from time to time been much discussed, and which 
has acquired perhaps fictitious importance, viz., the question as to how 
volitional impulses leading to voluntary movements travel along the spinal 
cord. The conclusions at which we have arrived, namely, that in the normal 
carrying out of voluntary movements the chief part is played by efferent 
impulses passing along the pyramidal tract, carries with it the answer that 
volitional impulses travel in the spinal cord along the pyramidal tract. 
In the dog, in which the whole pyramidal tract crosses at the decussation 
of the pyramids, we should expect to find that a break in the pyramidal 
tract of one side of the cord at any point along its length caused loss of 
voluntary movement on the same side below the level of the break. And 
experiments as far as they go support this view. No one it is true has 
attempted to divide or otherwise cause a break in the pyramidal tract alone, 
leaving the rest of the cord intact; and, indeed, even if an injury were ON VOLUNTARY MOVEMENTS. 
685 
limited to the area marked out as the pyramidal tract, fibres other than 
pyramidal fibres would be injured at the same time, since the tract is never 
a " pure " one. But it has been found that a section of a lateral half of the 
cord, a lateral hemisection, or a section limited to the lateral column of one 
side, has for one of its principal effects loss of voluntary movement on the 
same side in the parts supplied by motor nerves leaving the cord below the 
level of the section. We say " one of its principal effects" because, besides 
the concomitant interference with sensations concerning which we shall speak 
presently, the loss of voluntary movement is not absolutely confined to the 
same side ; there is some loss of power on the crossed side, at least in a large 
number of cases. We must not lay stress on this crossed paralysis because 
it is probably one of the effects of the mere operation, not a pure " defi- 
ciency " phenomenon, and, indeed, appears soon to pass away. But taking 
into consideration what was said above concerning the effects of removing 
cortical areas, it is important to note that in the experience of many experi- 
menters the loss of voluntary power on the operated side diminishes after a 
while, and that the animal if kept alive and in good health long enough 
appears to regain almost full voluntary power over the affected parts. In 
such cases, as in other operations on the central nervous system, there is no 
regeneration of nervous tissue ; the two surfaces of the section unite by con- 
nective not nervous tissue, and the tracts which as the result of the section 
degenerate downward or upward are permanently lost. Hence, even if we 
admit that in the intact animal a voluntary movement is chiefly carried out 
by means of efferent impulses passing along the pyramidal tract right down 
to the motor mechanisms of the cord immediately connected with the motor 
nerves, we must also admit that the " will " under changed circumstances 
can find other channels for gaining access to the same mechanisms. 
It has been further observed that if in the dog a hemisection be made at 
one level, for instance in the lower thoracic region of the cord, and then, 
after waiting until the voluntary power over the hind limb of that side has 
returned, a second hemisection, this time on the other side, be made at a 
higher level, this second operation is followed by results similar to those of 
the first; there is loss of voluntary power on the side operated on, with some 
loss of power on the crossed side, and as in the first case this loss of power 
not only on the same but also on the crossed side may eventually disappear. 
This shows among other things that the recovery after the first operation was 
not due to the remaining pyramidal tract doing the work of both. Further, 
the hemisection may be repeated a third time, the third hemisection being- 
on the same side as the first, with at least a very considerable return of power 
over both limbs. That is to say, under such abnormal circumstances volun- 
tary impulses may, so to speak, thread their way in a zigzag manner from 
side to side along the mutilated cord until they reach the appropriate spinal 
motor mechanisms. Such an abnormal state of things does not, however, 
really militate against the view that under normal circumstances volitional 
impulses normally travel along the pyramidal tract; but it does show what, 
indeed, has already been shown by the phenomena of strychnine poisoning 
(§ 499) that in the central nervous system the passage of nervous impulses 
(using these words in the general sense of changes propagated along ner- 
vous material) is not rigidly and unalterably fixed by the anatomical distri- 
bution of tracts of fibres; in all such discussions as those in which we are 
engaged we must bear in mind that physiological conditions as well as ana- 
tomical continuity are potent in determining the passage of these impulses. 
$ 577. When we reflect on the great prominence of the pyramidal tract 
in the spinal cord of man as compared with that of the dog, we may justly 
infer not only that the pyramidal tract is under normal circumstances more 686 
THE BRAIN. 
exclusively the channel of volitional impulses in man than in such lower 
animals, but also, bearing in mind the discussion in a previous chapter 
(§ 504) concerning the activities of the spinal cord of man, that the potential 
alternatives presented by the spinal cord of the dog are greatly reduced in 
that of man. And such clinical histories of disease or accidental injury in 
man as we possess support this conclusion. Lesions confined to one-half of 
the cord, or even lesions confined to the lateral column of one-half, appear 
to lead to loss of voluntary power on the same side, and the same side only, 
in the parts below the level of the lesion ; and the same symptoms have been 
observed to accompany disease limited apparently to the pyramidal tract of 
one side. Moreover, though cases of recovery of power have been recorded, 
we have not such satisfactory evidence as in animals of the volitional im- 
pulses ultimately making their way along an alternative route; but here the 
same doubts may be entertained as were expressed in discussing the reflex 
acts of the cord in man. 
When we say that the loss of voluntary power is seen on the side of the 
lesion only, we should add that this statement appears to apply chiefly to the 
thoracic and lower parts of the cord. We have seen that in man, in the 
upper regions of the cord, the pyramidal tract is only partly crossed ; a vari- 
able but not inconsiderable number of the pyramidal fibres do not cross at 
the decussation of pyramids, but running straight down as the direct pyram- 
idal tract effect their crossing lower down in the cervical and upper thoracic 
regions. Hence, we should infer that a hemisection of, or a lesion confined 
to one side of, the cervical cord would affect the voluntary movements of the 
crossed side as well as of the same side, though not to the same extent. But 
we have no exact information as to this point. And, indeed, the purpose of 
the direct tract is not clear ; there is no adequate evidence for the view which 
has been held that these direct fibres are destined for the upper limbs and 
upper part of the body ; since they are the last to cross we should, a priori, 
be inclined to suppose that they were distributed to lower rather than higher 
parts. 
§ 578. We may now briefly summarize what we know concerning volun- 
tary movements. And it will be convenient to trace the events in order 
backward. 
Certain muscles are thrown into a contraction which even in the briefest 
movements is probably of the nature of a tetanus. In almost every move- 
ment more than one muscle, as defined by the anatomists, is engaged, and in 
many movements a part of several muscles is employed, and not the whole 
of each. It is perhaps partly owing to the latter fact that a muscle which 
has become tired in one kind of movement, may show little or no fatigue 
when employed for another movement, though we must bear in mind that in 
a voluntary movement fatigue is much more of nervous than of muscular 
origin. 
Besides the active muscles, if we may so call them, which directly carry 
out the movement, the metabolism of which supplies the energy given out as 
work done, other muscles, some of which are antagonistic to the active mus- 
cles and some of which may be spoken of as adjuvant, enter into the whole 
act. In flexion, for instance, of the forearm on the arm it is not the flexor 
muscles only but the extensors also which are engaged. According to the 
immediately preceding position and use of the arm, and according to the 
kind and amount of flexion which is to be carried out, the extensors will be 
either relaxed, that is to say, inhibited, or thrown into a certain amount of 
contraction. And in some of the more complicated voluntary movements 
the part played by adjuvant muscles is considerable. Hence, in a voluntary 
movement the will has to gain access not only to the active muscles, but also ON VOLUNTARY MOVEMENTS. 
687 
to the antagonistic and adjuvant muscles; and every voluntary movement, 
even one of the simplest kind, is a more or less complex act. 
The impulses which lead to the contraction of the active muscles reach 
the muscles along the fibres of the anterior roots (we may for the sake of 
simplicity take spinal nerves alone, neglecting the peculiar cranial nerves), 
and such evidence as we possess goes to show that the impulses governing 
the antagonistic and adjuvant muscles travel by the anterior roots also; the 
question whether the inhibition of the antagonistic muscles when it takes 
place is carried out by inhibitory impulses passing as such along the fibres, 
or simply by central inhibition of previously existing motor impulses, need 
not be considered now. These anterior roots are connected, as we have 
seen, with the gray matter of the cord, and in each hypothetical segment 
of the cord we may recognize the existence of an area of gray matter which, 
though we cannot define its limits, we may, led by the analogy of the cranial 
nerves, call the nucleus of the nerve belonging to the segment; and we may 
further recognize in such a nucleus what we may call its efferent and its 
afferent side. 
Every voluntary movement, even the simplest, is, as we have repeatedly 
insisted, a coordinated movement, and in its coordination afferent impulses 
play an important part. The study of reflex actions (§ 502) has led us to 
suppose that each spinal segment presents a nervous mechanism in which a 
certain amount of coordination is already present, in which efferent impulses 
are adjusted to afferent impulses. But the results obtained by stimulating 
separate anterior nerve-roots show that, in the case of most muscles at all 
events, the especially active muscles of the limbs for instance, each muscle is 
supplied by fibres coming from more than one nerve-root, that is to say, the 
spinal nucleus, or at least the spinal motor mechanism for any one muscle, 
extends over two or three segments. Hence a fortiori in a voluntary move- 
ment, involving as this does in most cases more than one muscle, the spinal 
mechanism engaged in the act spreads over at least two or three segments, 
thus allowing of increased coordination. In that coordination the impulses 
serving as the foundation of muscular sense play an important part, but 
other afferent impulses, such as those from the adjoining skin, also have their 
share in the matter ; and it is worthy of notice that not only is the skin over- 
lying a muscle served, broadly speaking, by nerve-roots of the same segment 
as the muscle itself, afferent in one case, efferent in the other, but in the 
parts of the body where coordination is especially complex, in the fingers 
for instance, not only is each muscle supplied from more than one segment, 
but also each piece of skin is supplied in the same way by the posterior roots 
of more than one nerve. 
In the case of the frog it is clear that in reflex movements a large amount 
of coordination is carried out by these various spinal mechanisms ; and as we 
have urged, we may safely infer that in the voluntary movements of the frog, 
the will makes use of this already existing coordination, whatever be the 
exact path by which in this animal the will gains access to the spinal mechan- 
isms. In the dog we may conclude that in voluntary movements the spinal 
mechanisms, with coordinating functions, are also set in action, in this case 
by impulses passing straight from the cortex to the mechanisms by the pyram- 
idal tract, though apparently, in the absence of the pyramidal tract, the 
will can work upon the mechanisms by changes travelling through other 
parts of the cerebro-spinal axis. And in the monkey and man, subject to 
the doubts already expressed as to the potentialities of the human spinal 
cord, we may probably also infer that in each voluntary movement some, 
perhaps we may say much, of the coordination is carried out by the spinal 
mechanism set into action through impulses along the pyramidal tract. We 688 
THE BRAIN. 
may probably further infer that a careful adjustment obtains between the 
beginnings of the pyramidal tract in the cortex and its endings in the cord, 
so that the topography of " areas " or " foci " in the cortex above is an image 
or projection of the spinal mechanisms below. 
The complex character, on which we insisted just now, of almost every 
voluntary movement necessitates that in every such movement a large area 
of spinal mechanism is involved. But this is not all. The movements of 
any part of the legs, for instance, are not determined, nor is the coordination 
of the movements affected, simply by what is going on in the legs and the 
part of the spinal cord belonging to them. The discussion in a previous 
section has shown that much of the coordination of the body is carried out 
by the middle portions of the brain, and on these the motor area must have 
its hold as on the spinal mechanisms. 
The details of the nature of that hold are at present unknown to us ; but 
it must be remembered that not all the fibres passing down from the motor 
region, not all those even proceeding from the densest and most clearly 
defined motor areas, are pyramidal fibres. With the pyramidal fibres are 
mingled fibres having other destinations, and some of these probably pass 
to the thalamus and so join the great tegmental region. Moreover, the 
motor region must have close ties with other regions of the cortex whence, as 
we have seen (§ 545) fibres pass to the pons to make connections with the 
cerebellum. On the other hand, as we have seen (§ 525) the cerebellum is 
especially connected with what we may fairly consider the afferent side of 
the spinal cord and bulb. These facts must merely be taken as indicating the 
possibilities by which the motor region is kept in touch with the great coordi- 
nating mechanism ; it would be venturesome at present to say much more. 
In an ordinary voluntary movement an intelligent consciousness is an 
essential element. But many skilled movements initiated and repeated by 
help of an intelligent conscious volition may, when the nervous machinery 
for carrying them out has acquired a certain facility (and in all the higher 
processes of the brain we must recognize that, in nervous material at all 
events, action determines structure, meaning by structure molecular arrange- 
ment and disposition), be carried out under appropriate circumstances with 
so little intervention of distinct consciousness that the movements are then 
often spoken of as involuntary. All the arguments which go to show that 
the distinctly conscious voluntary skilled movement is carried out by help 
of the appropriate motor area go to show that the motor area must play its 
part in these involuntary skilled movements also. So that distinct con- 
sciousness is not a necessary adjunct to the activity of a motor area. And 
it is worthy of notice that some of these, in their origin, purely voluntary 
skilled movements, which by long-continued training have become almost 
as purely involuntary, are hampered rather than assisted by being "thought 
about." 
The word " training " suggests the reflection that the physiological inter- 
pretation of becoming easy by practice is that new paths are made, or the 
material of old paths made more mobile by effort and use. We have already 
urged (§ 494) that the gray matter of the spinal cord is a network, in which 
the passage of impulses is determined by physiological conditions rather than 
anatomical continuity, and the same considerations may with still greater 
force be applied to the brain. We must suppose that training promotes the 
growth and molecular mobility of the motor area and of all its connections. 
There are doubtless limits to the changes which can be effected, but within 
these limits the will, blundering at first in the maze of the nervous network, 
gradually establishes easy paths ; though even to the end it blunders-in 
trying to carry out one movement it often accomplishes another. VISUAL AND OTHER SENSATIONS. 
689 
Lastly, without attempting to enter into psychological questions, we may 
at least say that the birthplace of what we call the " will," is not conter- 
minous with the motor area ; the will arises from a complex series of events, 
some of which take place in other regions of the cortex, and probably in 
other parts of the brain as well. With these parts the motor area has ties 
concerned not in the carrying out of volition, but in the generation of the 
will. So that, looking round on all sides, it is obvious, as we have said, that 
the motor area is a mere link in a complex chain. It is, moreover, a link of 
such a kind, that while the changes which the breaking of it makes in the 
daily life of a lowly animal, such as the dog, in whom the experience of the 
individual adds relatively little to the nervous and psychical storehouse 
transmitted from his ancestors, can hardly be appreciated by a bystander, 
those which the breaking of it makes in the daily life of a man, whose brain 
at any moment is not only a machine fitted for present and future work but 
a closely packed record of his past life, are obvious not only to the individual 
himself, but to his fellows. 
On the Development within the Central Nervous System of 
Visual and of some other Sensations. 
Visual Sensations. 
§ 579. In the chain of events through which some influence brought to 
bear on the periphery of a sensory nerve gives rise a sensation, we are able, 
with more or less success, to distinguish between those events which are 
determined by the changes at the periphery and those which are the expres- 
sion of changes induced in the central nervous system. Thus when certain 
rays of light proceeding from an object and falling upon the eye give rise to 
visual perception of the object, two sets of events happen : the rays of light, 
by help of the mechanisms of the eye, partly dioptric, partly nervous, give 
rise to certain changes in the fibres of the optic nerve, which we may call 
visual impulses ; and these visual impulses reaching the brain along the optic 
nerve give rise to visual sensations and so to visual perception of the object. 
We shall later on, under the heading of "the senses," deal chiefly with the 
peripheral events, and have now to consider some points connected with the 
central events, to learn what we know concerning how the various sensory 
impulses travelling along the several kinds of sensory nerves behave within 
the central nervous system. In doing so we shall have from time to time to 
refer to peripheral events, but only occasionally, and never in any great 
detail. It will be convenient to begin with the special sense of sight, and 
we must first briefly call attention to a few points which we shall have to 
study in fuller detail hereafter. 
The eye is so constructed that images of external objects are brought to 
a focus on the retina, the stimulation of which by light starts the visual im- 
pulses along the fibres of the optic nerve; and the distinctness with which, 
by means of the visual sensations arising out of these visual impulses, we 
perceive external objects is dependent on the sharpness of the retinal images. 
The eye is further so constructed that, in any position of the eye, the rays 
of light proceeding from a portion only of the external world fall upon the 
retina; or in other words, in any one position of the eye only a portion of 
the external world is visible at the same time. The portion so seen is spoken 
of as the visual field for that position. 
The image thrown on the retina is an inverted one, so that the top of an 
actual object is represented by the lower, and the bottom by the upper, part 690 
THE BRAIN. 
of the retinal image ; similarly the actual left-hand side of the retinal image 
corresponds to the right-hand side of the actual object, and the right-hand 
side to the left-hand side. Hence the right-hand half of the visual field 
corresponds to the left-hand side of the retina, and the left-hand half to the 
right-hand side. 
The eye can be moved in various directions, and since in the visual field 
the portion of external nature which can be seen at the same time differs 
with each different position, a large range of vision is thus secured ; and this 
can be further increased by movements of the head. Moreover, we nor- 
mally make use of two eyes; our normal vision is binocular, and the visual 
field of the right eye differs from that of the left eye. There is one striking 
difference which must always be borne in mind. A section carried through 
the eye in a vertical and front-to-back plane, through what we shall learn 
to call the optic axis (Fig. 156, o. x.) (the exact details of the plane may be 
left for the present), will divide the retina into two lateral halves, and in 
each retina one-half will be on the nasal side next to the nose, and the other 
half will be on the malar or temporal side, next to the cheek or temple. It 
must be remembered that the nasal halves and temporal halves of the two 
retinas do not occupy corresponding positions in space. The temporal half 
of the left retina is on the left side of its own eye, whereas the temporal half 
of the right retina is not on the left, but on the right, side of its eye; and 
so with the nasal halves. Now, in the right eye, the right-hand side of the 
visual field corresponds to the nasal half of the retina, and the left-hand side 
of the visual field to the temporal half of the retina, whereas in the left eye 
the right-hand side of the visual field corresponds to the temporal half of 
the retina, and the left-hand side to the nasal half. This is shown in Fig. 
156, where the left-hand visual field and the retinal area concerned are 
shown shaded in each eye. 
When we look at an object with the two eyes, though two retinal images 
are produced, one in one eye and one in the other, we perceive one object 
only, not two. This is the essential fact of binocular vision ; when certain 
parts of each retina are stimulated at the same time we are conscious of one 
sensation only, not two; and the parts of the two retinas which, stimulated 
at the same time, give rise to one sensation are spoken of as " corresponding 
parts." From the structure and relations of the two eyes it follows that the 
temporal side of the right, and the nasal side of the left, eye are such cor- 
responding parts, while the nasal side of the right eye corresponds to the 
temporal side of the left eye. But the whole of each retina is not employed 
in binocular vision. Owing to the position of the two eyes in relation to the 
nose, it comes about that an object held very much on one side, to the left- 
hand side for instance, while it is capable of producing an image on the 
extreme nasal side of the left eye, and can be seen, therefore, by that eye, 
cannot produce an image on the temporal side of the right eye; the nose 
blocks the way. It is, therefore, not seen by the right eye, and the vision 
of it is monocular by the left eye only. In Fig. 156 it may be seen that the 
left visual field of the left eye (L. F. L.) extends more to the left, and is 
larger than the left visual field of the right eye (L. F. R.\ and that the right 
retinal area, corresponding to the left visual field, extends further along the 
nasal side of the left side (a') than it does along the temporal side of the 
right eye (a'), the difference being due to the presence of the nose (F'.). 
And similar conditions obtain with regard to the extreme right-hand side 
of the visual field. 
§ 580. After these preliminary statements, we may now turn to con- 
sider some anatomical facts concerning the ending of the optic nerve in the 
brain. VISUAL AND OTHER SENSATIONS. 
691 
Fig. 156. 
Diagram to illustrate the Nervous Apparatus of Vision in Man. (Sherrington.) L, the left eye ; 
H, the right eye ; o, x., the optic axis; F, the outline of the face between the eyes; Op T, the right 692 
THE BRAIN. 
The optic nerve of each eye consists of nerve-fibres coining from all 
parts of the retina of that eye; but the two optic nerves meet, ventral to 
the floor of the third ventricle, cross each other at the optic chiasma (Fig. 
156, Op. De.}, and are thence continued on under the name not of optic 
nerves, but of optic tracts ( Op. T.}. The decussation of fibres which takes 
place in the chiasma has peculiar characters. At their decussation (we are 
speaking now of man) the fibres in the optic nerve belonging to the tem- 
poral half of the eye in which the nerve ends pass into one optic tract, 
namely, the optic tract of the same side, while the fibres belonging to the 
nasal half pass into another optic tract, namely the optic tract of the oppo- 
site side. Thus the fibres of the temporal half, of the right eye and of the 
nasal half of the left eye pass into the right optic tract, and the fibres of 
the nasal half of the right eye and of the temporal half of the left eye pass 
into the left optic tract. Compare Fig. 156, in which the fibres forming the 
right optic tract are shaded, while those forming the left optic tract are left 
unshaded. Now the nasal half of one retina and the temporal half of the 
other retina are "corresponding" parts. Hence, while each optic tract 
contains fibres belonging to half of each eye, the two halves thus repre- 
sented in each tract are corresponding halves. 
The amount and character of the decussation taking place in the optic 
chiasma differs in different animal types, the difference having relation to 
the amount of binocular vision, which in turn depends on the position of 
the eyes in the head, that is, on the prominence of the face between the 
eyes. In the fish, for instance, with laterally placed eyes, no binocular 
vision at all is possible, and the decussation is complete; the whole optic 
nerve of each eye crosses over to the other optic tract. Between this and 
the arrangement in man, just described, various stages obtain in various 
animals. 
The chiasma also contains at its hinder part fibres which have no con- 
nection with the optic nerves or the eyes, but are simply commissural tracts 
passing from one side of the brain, namely, from the median corpus genicu- 
latum (§ 543) along one optic tract, through the chiasma to the other optic 
tract, and so to the median corpus geniculatum of the other side of the 
brain. These fibres are spoken of as the inferior or posterior (optic) com- 
missure or arcuate commissure, or Gudden's commissure. It was once 
thought that in a similar way fibres passed from one retina along one optic 
nerve through the front part of the chiasma to the other optic nerve, and 
to the other retina, forming an anterior (optic) commissure; but this seems 
to be an error. 
§ 581. The optic vesicle is, as we have seen, budded off from the fore- 
brain or forerunner of the third ventricle, and the optic chiasma is 
optic tract (shaded) supplying, through Op De, the optic decussation, the temporal side of the 
retina of the right eve and the nasal side of the retina of the left eye, L.L.L and L.F.R, the 
left visual fields of the left and right eye respectively; the two fields and the parts of the two 
retinas whose excitation produces vision over the fields are shaded, the object a in the field of the 
right side giving rise to an image at a', and a on the left side an image at a'. The right optic tract 
is represented as ending in GL, the lateral corpus geniculatum; in Pv, the pulvinar; and in AQ, 
the anterior corpus quadrigeminum, all three stippled; op rad, the optic radiation from these 
bodies to R.Oc, the right occipital lobes whose stippled cortex indicates the " visual area;" d, 
the " direct " tract to the cortex; c c, corpus callosum, cut across at the splenium ; I. v. d, descend- 
ing horn of the lateral ventricle; the left side has been utilized to indicate at F, shaded with 
lines, the cortical motor area for the eyes ; fm. c. indicates the path from it to iii, iv, vi, the nuclei 
of the third, fourth, and sixth nerves; p. b,the posterior longitudinal bundle, shown as a broken 
line; NC, the nucleus caudatus; LN, the nucleus lenticularis; and TH, optic thalamus shown in 
outline; cia, the front limb; cig, the knee, and cip, the hind limb of the internal capsule. The 
outlines of the fourth ventricle, 4th Vn, and of the posterior corpora quadrigemina are shown by 
dotted lines, that of the bulb is shown by a fine line; p, the pineal gland. VISUAL AND OTHER SENSATIONS. 
693 
attached to and forms part of the floor or ventral wall of that ventricle. 
In a view of the basal or ventral surface of the brain the diverging optic 
tracts are seen to separate the anterior perforated space and lamina cinerea 
in front from the posterior perforated space, tuber cinereum with the in- 
fundibulum, and corpora albicantia behind, all these being parts of the 
floor of the third ventricle. From the gray matter in this floor, fibres 
forming what is sometimes spoken of as Meynert's commissure, belonging 
neither to the optic nerves nor to the inferior commissure, join the optic 
tracts, eventually leaving them to pass to the pes. Hence the whole of the 
optic tract is by no means derived from the optic nerve; the fibres just 
mentioned and the inferior commissure form parts of the optic tract not 
connected with the retina. 
Each optic tract crosses obliquely, being in crossing firmly attached to 
the ventral surface of the crus cerebri of the same side (Fig. 131, C), and 
is soon lost to view, being covered up by the temporo-sphenoidal lobe of 
the hemisphere. When this is removed the tract is seen to sweep dorsally 
round the crus toward the dorsal aspect, and as we have already (§ 543) 
said, to become connected on the further side of the crus with the two cor- 
pora geniculata, lateral and median. We may say at once that the median 
corpus geniculatum has no connection with that part of the tract which is 
derived from the optic nerve, and is not concerned in vision, but is con- 
nected with that part of the tract, sometimes called the median part, which 
goes to form the inferior commissure. We may confine our attention to 
that part of the tract which consists exclusively of fibres coming from the 
retinas of the two eyes, for it is this part, and this part only, which is con- 
cerned in vision. 
§ 582. This ends in three main ways, as shown diagrammatically in 
Fig. 156 In the first place, part of the tract ends in the lateral corpus 
geniculatum {GL.\ formed of alternating layers of white and gray matter, 
the gray matter containing in some parts large nerve-cells, and in others 
small nerve-cells. In these cells of one kind or another, many of the fibres 
appear to end. In the second place, a very large number of fibres passing 
the corpus geniculatum on its ventral and lateral surfaces spread out into 
the pulvinar (PF.). In the third place, others in considerable number, 
taking a more median direction, reach the anterior corpus quadrigeminum 
(AQ.). These two sets also, like the first, end apparently in the nerve-cells 
of the respective bodies. Thus, the really optic fibres of the optic tract 
end in one of three collections of gray matter, the lateral corpus genicula- 
tum, the pulvinar, or the anterior corpus quadrigeminum. Further, we 
have reasons for thinking that a considerable part at all events of the gray 
matter of these three bodies is associated with and, in a certain sense, 
dependent on the fibres of the optic nerves ; the reasons are as follows: We 
know that when a nerve-fibre is cut away from its trophic centre it degen- 
erates; but the division and the loss of the peripheral degenerating portion 
has no obvious effect on the trophic centre; when a spinal nerve, for in- 
stance, is divided below the spinal ganglion, though the nerve below the 
section degenerates, the ganglion and the piece of nerve in connection with 
it remain very much as before; we have it, however, in our power to bring 
about changes of a deeper and wider character, a cessation of growth 
amounting to atrophy, by operative interference with nervous structures 
before they are fully developed. Thus, in an adult animal, a section of an 
optic nerve or removal of the eye leads to degeneration in the optic nerve 
and optic tract; the optic fibres have their trophic centre in certain cells of 
the retina, of which we shall speak in treating of vision, and cut away from 
that centre they degenerate; by this means the nature of the optic decussa- 694 
THE BRAIN. 
tion in animals, and, indeed, in man, has been ascertained. But if the eves 
be removed (removal of both eyes being desirable on account of the cha- 
racters of the optic decussation) in a newborn animal, not only do both the 
optic nerves and the greater part of both optic tracts cease only to be fur- 
ther developed and degenerate, but the bodies mentioned above, the two 
lateral corpora geniculata, the pulvinar on each side, and the two anterior 
corpora quadrigemina, do not fully develop; certain parts of them undergo 
atrophy. The development of these nervous structures seems therefore to 
be largely dependent on their functional connection with the eyes by means 
of the optic tracts and nerves. 
The same method confirms the view expressed above, that the median 
corpus geniculatum has no connection with vision. When the eyes of new- 
born animals are extirpated, neither the median corpora geniculata nor the 
posterior corpora quadrigemina show any signs of atrophy, and the part of 
the optic tract which does not degenerate is the inferior commissure con- 
necting the two median corpora geniculata. Obviously these parts are asso- 
ciated with functions of the brain other than those of sight. The lateral 
corpora geniculata, the pulvinar, and the anterior corpora quadrigemina, are, 
we may repeat, alone to be regarded as the chief central parts in which the 
optic nerve ends. We may also repeat that owing to the peculiarity of the 
optic decussation each optic nerve thus finds its endings in both sides of the 
brain. 
While the optic chiasma is, as we have seen, helping to form the floor of 
the third ventricle, it gives off fibres to the posterior perforated spot. Some 
of these have been supposed to pass directly in the wall of the ventricle to 
the nucleus of the third (oculo-motor) nerve, and to serve as a channel for 
afferent impulses, causing constriction of the pupil ; but to this we shall 
return in dealing hereafter with the movements of the pupils. 
§ 583. Though the above three bodies are undoubtedly the chief endings 
of the optic nerve, three primary visual centres, if we may so call them, it 
is also believed that some fibres of the optic tract, making connections with 
neither of these three bodies, pass by the crus cerebri straight to certain parts 
of the cerebral hemisphere (Fig. 156, d); but this fourth ending is by no 
means so clearly established as are the other three. 
And undoubtedly the main connection of the cerebral hemisphere with 
the optic tract is not a direct one, but an indirect one, through the three 
bodies in question. A number of fibres proceeding from the occipital cortex 
and reaching the thalamus through the hind limb of the internal capsule 
formed what was called the "optic radiation." These fibres beginning (or 
ending) in the cortex of the occipital region, end (or begin) (Fig. 156, op. rad.) 
to a large extent in the pulvinar and in the lateral corpus geniculatum, but 
also in the anterior corpus quadrigeminum, reaching it by the anterior 
brachium (§ 547). When even in a grown animal the occipital cortex is 
destroyed, not only these fibres, but also parts of the pulvinar and external 
corpus geniculatum, undergo degeneration, and there is some change in the 
anterior corpus quadrigeminum. When the same cortex is destroyed in a 
newborn animal the same parts atrophy ; and in such cases the optic tract 
and nerve, which are but little affected by the operation in the adult animal, 
are also involved in the atrophy. We may add that removal of both eyes 
in the newborn animal is said to lead, besides the atrophy of the three bodies 
inquestion, to diminished occipital lobe due to lack of white matter. We 
may therefore conclude that in the complex act of vision two orders of cen- 
tral apparatus are involved ; we may speak of two kinds of centres for 
vision-the primary or lower visual centres supplied by the three bodies of 
which we are speaking, and a secondary or higher visual centre supplied by VISUAL AND OTHER SENSATIONS. 
695 
the cortex in the occipital region of the cerebrum. And experimental results 
accord with this view. 
Before we proceed to discuss those results, one or two preliminary obser- 
vations may prove of use. 
In the first place, as we have previously urged, the interpretation of the 
results of an experiment, in which we have to judge of sensory effects, are 
far more uncertain than when we have to judge of motor effects, that is, of 
course, when the experiment is conducted on an animal. We can estimate 
the motor effect quantitatively, we can measure and record the contraction 
of the muscle, but in estimating a sensory effect we have to depend on signs, 
our interpretation of which is based on analogies which may or may not be 
misleading. We are on safer ground when we can appeal to man himself 
in the experiments instituted by disease; but the many advantages thus 
secured are often more than counterbalanced by the diffuse characters or 
the complex concomitants of the lesion. In dealing with sensory effects we 
must expect and be content for the present with conclusions less definite 
and more uncertain even than those gained by the study of motor effects. 
In the second place, in dealing with vision, it will be desirable to know 
the meaning which we are attaching to the words which we employ. By 
blindness, that is, " complete " or "total " blindness, we mean that the move- 
ments and other actions of the body are in no way at all influenced by the 
amount of light falling on the retina. Of partial or incomplete or imperfect 
vision, using the word vision in its widest sense, there are many varieties ; and 
we may illustrate some of the defects of the visual machinery, regarded as a 
whole, with its central as well as its peripheral parts, by referring to certain 
defects of vision due to changes in the eye itself. The eye may fall into such 
a condition that the mind can only appreciate, and that to a varying degree, 
the difference between light and darkness; the mind is aware that the retina 
(or it may be part of the retina) is being stimulated to a less or greater 
degree, but cannot perceive that one part of the retina is being stimulated 
in a different way from another part; a sensation of light is excited, but not 
a set of visual sensations corresponding to the sets of pencils of luminous 
rays, which, reflected or emanating from external objects in a definite order, 
are falling upon the eye. The eye again may fall into another condition, in 
which such sets of visual sensations are excited, but on account of dioptric 
imperfections or for other reasons the several sensations are not adequately 
distinct; the mind is aware through the eye of the existence of " things," 
but cannot adequately recognize the character of those things; the visual 
images are blurred and indistinct. And a large number of gradations are 
possible between the extreme condition in which only those objects which 
present the strongest contrast with their surroundings are visible to a condi- 
tion which only just falls short of normal vision. Imperfections of this kind, 
of varying degree, may result from failure, not in the peripheral apparatus, 
not in the retina or optic nerve or other parts of the eye, but in the central 
apparatus; the retinal image may be sharp, the retina and the optic fibres 
may be duly responsive, but from something wrong in some part or other of 
the brain the visual sensations excited by the visual impulses may fail in 
distinctness, and that in varying degree; imperfections of vision, whether of 
central or peripheral origin, in which visual sensations fail in distinctness, 
are generally spoken of under the not wholly unexceptionable name of 
amblyopia. 
If one optic nerve be divided, total blindness of one eye will result; but 
if one optic tract be divided, it follows from what has been said above that 
half-blindness in the corresponding halves of both eyes will result. If, for 
instance, the right optic tract (Fig. 1-54, Op. T.) be divided, the left visual 696 
THE BRAIN. 
fields of both eyes will be blotted out. The same condition will be brought 
about by failure in the optic tract at its central ending, provided of course 
the mischief be confined to the ending of the one tract. Such a half-blind- 
ness or half-vision is spoken of as hemianopsia or hemianopia or hemiopia; 
the words left and right are generally used in reference to the visual field ; 
thus, left hemianopsia is the blotting out of both left visual fields through 
failure of the right optic tract. 
If, instead of the whole optic nerve being divided, certain bundles only 
were cut across, partial blindness would be the result, a portion of the visual 
field would be blotted out, and mischief limited to a few bundles of one optic 
tract would lead to corresponding blots in the corresponding halves of the 
visual fields of both eyes. 
Further, an affection of half the retina or of a limited area in the retina 
might occur of such a character as to lead, not to complete, but to partial 
blindness, to a hemi-amblyopia or to a partial amblyopia. The part of the 
retina so affected might be central, or peripheral, or a quadrant, or any patch 
of any size, form, and relative position. And we may further imagine it at 
least possible that mischief in the brain might be so limited as to produce 
any of the above partial effects, though the retina, optic nerve, and optic 
tracts all remained intact. 
The above visual imperfections we have illustrated by changes in the 
peripheral apparatus, but there is a kind of imperfection which we may still 
call a visual imperfection, though it is of purely central origin. In a normal 
state of things a visual sensation excited in the brain is or maybe linked on 
to a chain of physical events; we often, then, speak of it as a visual idea. 
When we see a dog the visual sensation, or rather the group of sensations 
making up the visual perception of the dog, does not exist by itself, apart 
from all the other events of the brain ; it joins and affects them, and among 
the events which it so affects may be and often are psychical events; the 
visual perception "enters into our thoughts" and modifies them. Between 
the visual impulse, as it travels along the optic nerve or tract, and its ulti- 
mate psychical effect, a whole series of events intervene ; and we may take 
it for granted that the chain may be broken or spoiled at any of its links, at 
the later as well as at the earlier ones. We may therefore consider it possi- 
ble that the break or damage may occur at the links by which the fully 
developed visual sensation joins on to psychical operations. We may sup- 
pose that an object is seen and yet does not affect the mind at all, or affects 
it in an abnormal way. 
These foregoing considerations emphasize the difficulty and uncertainty of 
interpreting the visual condition of an animal which has been experimented 
upon. When, for instance, after an operation, an animal ceases to be influ- 
enced in its previous normal manner by the visual effects of external objects, 
a most careful psychical analysis is often necessary to enable us to judge 
whether the newly introduced disregard of this or that object is due to the 
mere visual sensation being blurred or blunted, or to some failure in the 
psychical appreciation of the sensations; and in most cases such an analysis 
is beyond our reach. The greatest caution is needful in drawing conclusions 
from experiments of this kind, especially from such as appear to have been 
hastily carried out or hastily observed ; and we must be content here to 
dwell on some of the broader features only of the subject. 
§ 584. Since we have in this matter to trust so much to analogies with our 
own experience, we may turn at once to the monkey as being more instruc- 
tive than any of the lower animals. We have already said that electrical 
excitation of the occipital cortex behind the motor region may produce 
movements, but that these movements are in character different from those VISUAL AND OTHER SENSATIONS. 
697 
caused by stimulation of the motor region itself. In the monkey stimulation 
of parts of the occipital region, the occipital lobe, and the angular gyrus, for 
instance, may give rise to movements of the eyes, of the eyelids, and of the 
head, that is of the neck, all the movements so produced being such as are 
ordinarily connected with vision. It will not be profitable to enter here into 
the details concerning the exact topography of the excitable parts or of the 
special characters of the movements so called forth. But it is important to 
note that these movements are unlike the movements excited by stimulation 
of the appropriate motor area, inasmuch as their occurrence is far less cer- 
tain, they need a stronger stimulus to bring them out; when evoked they are 
feeble, being easily antagonized by appropriate stimulation of the motor 
area, and they have a much longer latent period. They are not due to any 
indirect stimulation of the motor area, through "association" fibres con- 
necting the spot stimulated with the motor area, or otherwise, since they 
persist after removal of the motor area. Movements of this kind may also 
be witnessed in the dog. They are obviously the result of impulses trans- 
mitted in some direct manner from the cortex to some parts below, and may 
be taken as an indication that the parts of the cortex in question are in some 
way connected with vision. The exact manner, however, in which they are 
brought about is at present obscure. The explanation of their genesis which 
is frequently offered, namely, that the stimulation so affects the cortical gray 
matter as to give rise to visual sensations, and that the movements express 
these sensations, does not seem satisfactory. For, if it be possible that the 
gross changes which the electric current sets going in the cortical gray matter 
can reproduce the psychical events which take place in that gray matter in 
the normal action of the brain, we should expect stimulation of any and 
every part of the cortex to call forth some movement or other, since it cannot 
be doubted that every part of the cortex is in some way or other engaged in 
psychical operations, and that every psychical phase tends to express itself 
in movement: whereas, outside the motor region, with the exceptions we 
are now discussing, the cortex is, as we have seen, " inexcitable," and even 
within the motor region itself the excitable substance is scattered, with in- 
creasing segregation as we advance along the animal scale, among inexcitable 
substance. When we speak of the region or substance as inexcitable, we do 
not mean that the electric current produces no effect; we only mean that 
the effect is not manifested by movement; the real difference between the 
excitable motor region and the inexcitable rest of the cortex is probably 
that in the several motor areas the current, playing upon the beginnings of 
the pyramidal fibres, is able to inaugurate simple motor impulses or some- 
thing like them, whereas elsewhere the molecular changes induced by the 
current are too confused to reach their normal expression. There can be no 
doubt, of course, that molecular changes in this or that part of the brain, 
set going by processes other than actual visual impulses along the optic 
nerves, may give rise to visual sensations; and, as we shall see in dealing 
with the senses, the subject of such "subjective" sensations is unable to 
distinguish them from sensations of "objective" origin ; but it is at least 
unlikely that the coarse disturbances started by a tetanizing current should 
take such a definite form. Moreover, the view in question is disproved by 
the experimental result that the same movements are brought about when 
the cortex is pared away and the electrodes are applied to the subjacent 
white matter. This result suggests the existence of efferent tracts or bundles 
of a special kind, differing from those of the pyramidal kind, though like 
them making connections with the ocular and other muscles; we have, 
however, as yet no other evidence of such tracts existing. 
§ 585. The results of removal of the cortex support the same general 698 
THE BRAIN. 
conclusion, though there is much discordance among the various observers, 
both as to the particular results and especially as to their interpretation. 
One broad fact comes out in all the observations, namely, that the removal 
of or injury to the hind region of the cortex always produces some disturb- 
ance of vision, and produces disturbance of vision more surely and to a 
greater extent than does injury to or removal of any other region of the 
cortex ; but beyond this broad fact there is much dispute, and we must be 
content here with a very brief statement. 
In the monkey, some observers have found that removal of the occipital 
lobe on one side (the region marked " vision '' in Figs. 149 and 150) caused 
hemiopia, the effect on the visual fields being a crossed one; when the right 
lobe was removed there was blindness in the left visual fields, that is, in the 
right halves of the retinas of both eyes ; in other words, the visual impulses 
passing along the right optic tract failed to produce their usual effect, so 
that the animal disregarded objects on its left-hand side. We may remark 
that the decussation of the optic nerves in the monkey is very similar to that 
in man. When both occipital lobes were removed, total blindness resulted. 
But, and this is most important, not only was the hemiopia caused by the 
removal of one lobe transient, but also according to some observers, the lost 
vision returned after the total removal of both lobes, though some impair- 
ment might be noticed long afterward, so long in fact as the animal was kept 
alive. 
In the hands of other observers destruction of the angular gyrus of one 
side (Fig. 148) has led to hemiopia, failure in the left (or right) visual field, 
indicating failure in the central endings of the right (or left) optic tract, 
being caused by removal of the right (or left) gyrus, and destruction of both 
angular gyri has led to total blindness, not only the hemiopia, but the total 
blindness being, however, apparently transitory. And cases have been ob- 
served in which the transient blindness due to removal of the occipital lobes 
has been succeeded by permanent hemiopia upon the subsequent removal of 
the angular gyrus. Indeed, the general, but not uniform, tendency of the 
many experiments which have been made is to connect, in the monkey, both 
the occipital lobe and the angular gyrus with vision. 
In the dog, removal of portions of the occipital cortex have also led to 
partial and transient blindness, or, according to some, to permanent blind- 
ness; but the difficulties of judging the visual condition of a dog are very 
considerable, and his vision is so different from that of a man, so much less 
binocular, for instance, than his, that it would not be profitable to relate at 
length the results obtained in the dog or to discuss the conclusions which 
have been derived from them. We will only say that some observers have 
been led to think that the lateral part of the retina is connected with the 
lateral part of the visual occipital area, the front part with the front part, 
and so on, the retina being, as it were, projected on to the occipital cortex ; 
but the facts are not clear enough to make it worth while to dwell upon 
them here. 
In man, clinical histories so far conform to the results of experiments on 
the monkey as to associate the occipital cortex, and more particularly the 
cuneus (see Figs. 152 and 153), with vision. They have, however, raised a 
point on which we have not yet touched. In the experiments on the mon- 
key quoted above, the result (putting aside transient effects due probably to 
"shock ") of interference with one side of the brain was hemiopia; and this 
is what we might expect from the anatomical relations; the optic tract goes 
straight to the tegmental masses of its own side, and the optic radiation 
passes from those masses to the occipital cortex of the same side; there is 
no decussation, save of the fibres of the optic nerve, as they pass into the VISUAL AND OTHER SENSATIONS. 
699 
optic tract at the chiasma. Clinical histories teach the same lessons as these 
experiments on animals; lesions limited to the occipital lobe have for a 
symptom hemiopia; and this is said to be especially the result of mischief 
limited to the apex of the occipital lobe, that is, to the cuneus. But experi- 
ments on monkeys have been made in which destruction of one angular 
gyrus has produced, not hemiopia, but crossed blindness or crossed ambly- 
opia, that is to say, has affected the whole of the retina of one eye, and that 
the crossed eye, the eye of the same side not being, or being supposed not to 
be, at all affected ; similar results have also been stated to follow upon 
removal of one occipital lobe. And a few clinical cases have been recorded 
in which disease, especially of the angular gyrus, seemed to affect the vision 
of the whole of the crossed eye. (It must be remembered that the angular 
gyrus of man corresponds to a part only of the whole angular gyrus of the 
monkey. (Cf Fig. 148 with Fig. 152.) Some authors have, in accordance 
with this, put forward the theory that the occipital lobe serves as a cortical 
centre for the optic tract of its own side only, and so for one-half of each 
retina, while in front of this on the angular gyrus is a centre in which both 
optic tracts are represented. But the clinical histories bearing on this point 
cannot be regarded as wholly satisfactory ; and with reference to the experi- 
mental results we may once more insist, and the warning applies perhaps 
with particular force to these experiments on vision, on the danger of con- 
founding those immediate effects of operative interference, which are of the 
nature of " shock " in the wide sense of that word, with those pure " de- 
ficiency " phenomena which are alone the outcome of the loss of the part 
removed. It is difficult to resist the conclusion that much of the transitory 
blindness which is observed in these experiments belongs to the former 
category, that the effect is transient because it is of the nature of shock, and 
not because the loss of faculty is supplied by some other cortical area being 
subsequently substituted for the one removed. In the dog, injury to the 
frontal region of the cortex unaccompanied by any secondary mischief in 
the occipital region has led to impaired vision ; and this was probably an 
instance of "shock," for we have no other reason to connect the frontal 
region of the cortex with vision. We must be very careful in drawing the 
conclusion that, because an operation produces transient blindness, the part 
operated on has a direct share in vision ; and we may well hesitate to 
accept the view that the whole retina is represented in the crossed hemi- 
sphere. 
In conclusion we may say that, when all the many results which have 
been arrived at by experiment or by clinical observation are duly weighed, 
it will be felt that while the evidence for the occipital lobe, especially the 
cuneus, being concerned in the matter is convincing, we cannot in the pres- 
ent state of our knowledge dogmatically exclude the angular gyrus, and that 
hence the only clear and consistent statement which can be made with any 
confidence is the broad and simple one that the hind region of the cortex is 
in some way intimately concerned in vision. 
§ 586. Such an attitude becomes all the more necessary when we ask 
ourselves the question, What is it which actually takes place in the cortex 
during vision ? Are we to conceive of it as if a visual impulse set going 
along the fibres of the optic tract underwent no essential change until it 
reached the cortex, as if it there suddenly developed into a " visual sensa- 
tion ?" We can hardly suppose this. Between the cortex and the optic 
tract, the lower visual centres, the tegmental masses, intervene; and we can 
hardly suppose that interference with these bodies produces the same effect 
on vision as simple section of the optic tract. We have seen in a previous 
section that the frog and the bird certainly, and according to some observers 700 
THE BRAIN. 
also the rabbit, are in the absence of the cerebral hemispheres not totally 
blind, their movements being guided by retinal impressions; and cases are 
recorded of the dog being obviously still guided in some measure by retinal 
impressions after the occipital lobes had been wholly or almost wholly 
removed. And, though this is a matter at present outside exact knowledge, 
and though it is perhaps possible for simple afferent impulses to determine 
even complex movements without the intervention of "consciousness," we 
are probably justified in assuming that the simple visual impulses, travelling 
along the fibres of the optic tract, undergo important transformations in the 
tegmental masses, and that the changes which are propagated along the 
fibres of the optic radiation constitute something quite different from the 
impulses along the optic tract or nerve. 
Judging from the analogy of the motor region we may probably assume 
that in vision the cortical events are psychical in nature, and that the func- 
tion of the optic radiation is to furnish what we may call crude visual sensa- 
tions for further psychical elaboration. 
Nor need this view compel us to suppose that injury to or removal of 
the cortex must produce only psychical blindness or psychical impairment 
of vision, though this point has probably not been sufficiently held in view 
during the various experiments, sufficient care not having been taken to 
determine how far the blindness was purely psychical. Bearing in mind the 
degeneration following upon lesions of the occipital cortex, and the far- 
reaching effects of any operation on the brain, we may suppose that injury 
to the cortex affects the lower centres as well; and some of the transient 
impairment of vision, on which we have just dwelt, may, perhaps, be ex- 
plained as the effect of the cortical injury on the lower centres. 
Although the matter is thus in many of its details at present outside our 
exact knowledge, we may probably conclude that in the complex act of 
complete vision, while part, especially the more psychical part, is carried 
out in the cortex, more particularly of the occipital region, part is accom- 
plished in the lower centres, the tegmental masses. As to the several func- 
tions of the three masses, we know almost absolutely nothing. Electric 
stimulation, and, it is said, mechanical stimulation also, of the anterior cor- 
pora quadrigemina in mammals, or the optic lobes in lower animals, calls 
forth movements of the eyes, and of various parts of the body ; and removal 
of them causes blindness and in some cases loss of coordination of move- 
ments. Our knowledge on these points is not very exact; but from the 
above facts as well as from the connections of the anterior corpora quad- 
rigemina with the parts of the brain behind we may suppose that these 
bodies are more especially concerned with the part visual impulses play in 
determining the coordination of movements. We must remember, however, 
that all three masses are connected with the cortex, and probably all three 
play a part in vision even of the highest psychical kind. 
Sensations of Smell. 
§ 587. In many animals in whom the sense of smell is acute, a portion 
of the cortex, known as the "pyriform lobe" or " hippocampal lobule," and 
which is anatomically continuous with the front end of the hippocampal 
gyrus (the part to which the name uncinate gyrus is often restricted), ac- 
quires relatively large dimensions. This and the anatomical relations just 
mentioned would lead us to suppose that a part of the cortex which is con- 
tinuous with the front end of the hippocampal gyrus is in some way con- 
nected with smell. The argument from comparative anatomy, however, is 
one which must be used with caution ; since, besides the great difficulty of VISUAL AND OTHER SENSATIONS. 
701 
determining the homologies of parts of the brain in different animals, rela- 
tive increase in the part in question might be correlated to other things than 
the power of smell, and might be determined by circumstances having no 
relation to smell. 
The experimental evidence, though, on the whole, it gives support to the 
view, is conflicting ; and when the difficulty of determining whether a 
"dumb animal " can or cannot smell is borne in mind, this will not be won- 
dered at. The observation that electrical stimulation of the region in ques- 
tion gives rise to movements of the nostrils, which have been interpreted as 
sniffing in response to subjective olfactory sensations, cannot have much 
weight; and while some observers have found that the removal of this 
part of the brain destroys the sense of smell, others have obtained negative 
results. 
The few clinical histories which bear upon the matter are, perhaps, more 
trustworthy. .These seem to show that a lesion involving the cortex of this 
region, but leaving the olfactory bulb and tract, as well as other parts of 
the brain, intact, may destroy or greatly impair smell. And we may, per- 
haps, give particular weight to the cases in which epileptiform attacks, pre- 
ceded by an " aura" in the form of a peculiar smell, have been associated 
with disease limited to this region ; for the phenomena of "aura" seem to 
be connected with cortical processes. 
Though the evidence on the whole goes to show that the cortex at the 
front end of the hippocampal gyrus is especially connected with smell, and 
we have so marked it (in Fig. 155), yet the whole matter stands on a some- 
what different footing from the sense of sight. In man the relations of 
smell to the other operations of the brain (though, as we shall see in deal- 
ing with the senses, somewhat peculiar) are far more limited than are those 
of vision, and the psychical development of simple olfactory sensations is 
extremely scanty. 
§ 588. This special sense, though so closely associated with smell, stands, 
together with the special sense of hearing, on a different footing from the 
two preceding special senses, since the nerves concerned belong to the cate- 
gory of ordinary cranial nerves, and we lack, in reference to them, the ana- 
tomical leading which is offered to us in the case of the optic and olfactory 
nerves. 
We shall see in dealing with the senses that the fifth nerve and the 
glosso-pharyngeal nerve have been considered as nerves of taste, but that 
the matter is one subject to controversy; the gustatory function of the fifth 
is attributed to the peculiar chorda tympani nerve, and other questions have 
been raised. Whatever view we take, however, the nerves of taste are ordi- 
nary cranial nerves, and we have no anatomical guidance as to the fibres of 
either of the above two nerves making special connections with any part of 
the cortex. Though sensations of taste enter largely into the life of ani- 
mals, and, indeed, of man himself, we have no satisfactory indications which 
will enable us to connect this special sense with any part of the cortex ; the 
view, indeed, has been put forward that some part of the cortex in the 
lower portion of the temporal lobe, not far from the centre for smell, serves 
as a centre for taste; but the arguments in favor of this view are not, as 
yet at least, convincing. 
Sensations of Taste. 
Sensations of Hearing. 
§ 589. The cochlear division of the eighth or auditory nerve may be 
assumed to be a nerve of the special sense of hearing, and of that alone; 702 
THE BRAIN. 
the vestibular division serves, as we have seen, for other functions than those 
of hearing (§ 555), but, as we shall urge in dealing with the senses, is not to 
be regarded as wholly useless for the purposes of that sense. The cochlear 
division we have traced (§ 531) into the bulb, and the vestibular division 
into the lateral auditory nucleus (which, perhaps, may be regarded as a 
continuation or segmental repetition forward of the cuneate nucleus or of 
part of that nucleus), and into the cerebellum, the cerebellar continuation 
being probably the part of the nerve which serves for coordinating func- 
tions. The connections of the auditory nerve with the cerebral hemisphere 
belong to the same category as those of other afferent cranial, and we may 
add spinal, nerves; we have no very clear anatomical guide toward any 
particular part of the cortex. 
When we turn to the empirical results furnished by experiment and 
clinical observations, we find that these, though even less definite and less 
accordant than in the case of the senses of sight and smell, point to part of 
the first or superior temporal (temporo-sphenoidal) convolution (Figs. 149, 
152, and 154) lying in the temporal lobe just ventral to the Sylvian fissure, 
as being especially concerned in hearing in some such way as the occipital 
lobe is concerned in vision. 
Electrical stimulation of this region of the cortex gives rise to " pricking 
of the ears," and other movements such as are frequently connected with 
auditory sensations; but such phenomena are in this instance, perhaps, to 
be depended upon even less than in other similar instances. While some 
observers maintain that this convolution, the operation including other por- 
tions of the temporal lobe as well, may be removed from a monkey without 
producing any certain signs of deafness, other observers have found that 
removal of it on one side affected the hearing of the ear on the opposite side, 
and removal on both sides brought the animal into a condition in which, 
without being, perhaps, absolutely deaf, it reacted toward sound in a very 
imperfect manner indeed, very different from its normal behavior. The 
scanty clinical histories bearing on this matter are not very decisive; for 
though deafness has been observed in connection with disease affecting the 
superior temporal convolution, the lesion has usually invaded other parts as 
well, and the deafness has been associated with other symptoms, notably 
aphasia. An auditory " aura " has, however, at times been observed in con- 
nection with disease of this region, as also a peculiar psychical failure, 
known as " word-deafness," in which, though sounds are heard-that is to 
say, auditory sensations are felt, it may be even as usual-the perception or 
psychical appreciation of the sounds is lacking, and a spoken word is not 
recognized. 
Lastly, we may add that though, as we said, the anatomical leading is 
not definite, observers have found that in newborn animals, on the one 
hand, destruction of the part of the cortex probably corresponding to the 
region mentioned above leads to atrophy of the median corpus geniculatum, 
and to some extent of the posterior corpus quadrigeminum; and, on the 
other hand, destruction of the internal ear leads to an atrophy of part of 
the lateral fillet of the opposite crossed side, which may be traced to the 
posterior corpus quadrigeminum ; and thence to the median corpus genicu- 
latum ; and section of the lateral fillet on one side leads, among other re- 
sults, to atrophy of the strise acusticae and tuberculum acusticum (§ 531) of 
the crossed side. This suggests that the path of auditory impulses is along 
the cochlear nerve to the lateral fillet of the crossed side, and so by the pos- 
terior corpus quadrigeminum and median corpus geniculatum to the cortex 
of the temporal lobe of that crossed side, the two latter bodies bearing toward 
hearing a relation somewhat like that borne toward sight by the anterior CUTANEOUS AND SOME OTHER SENSATIONS. 
703 
corpus quadrigeminum and lateral corpus geniculatum. But the matter 
needs further investigation. 
There remains the special sense of touch, but this we had better consider 
in connection with sensations in general. 
On the Development of Cutaneous and some other Sensations. 
§ 590. The sensations with which we have just dealt arise through im- 
pulses passing along special nerves or parts of special nerves, the optic nerve, 
the olfactory nerve, etc. We have now to deal with sensations arising 
through impulses along the nerves of the body generally. These are of 
several kinds. In the first place, there are sensations which we may speak 
of as " cutaneous sensations," the impulses giving rise to which are started 
in the skin covering the body, or in the so-called mucous membrane lining 
certain passages. These sensations, which, as we shall see in dealing with 
the senses, are dependent on the existence of special terminal organs in or 
near the skin, are sensations of " touch," in the narrow meaning of that 
word, by which we appreciate contact with and pressure on the skin, and 
the sensations of " temperature," which again we may, as we shall see, 
divide into sensations of " heat " and sensations of "cold." These sensations 
may be excited in varying degree by impulses passing along any nerve 
branches of which are supplied to the skin. Then there are the sensations 
constituting the " muscular sense,'' to which we have already referred, and 
these again maybe excited in any nerve having connections with the skeletal 
muscles. 
As we shall see in dealing with the senses, when a nerve is laid bare and 
its fibres are stimulated directly either by pressure, such as pinching, or by 
heat, or by cold, or in other ways, the sensations which are caused do not 
enable us to appreciate whether the stimulation is one of contact or pressure, 
or of temperature, or of some other kind; we only experience a "feeling," 
which at all events when it reaches a certain intensity we speak of as " pain." 
And we have reason to think that at least from time to time impulses along 
various nerves gives rise to sensations which have been spoken of as those of 
" general sensibility," by which in addition to other sensations, such as those 
of touch and of the muscular sense, we become aware of changes in the con- 
dition and circumstances of our body. When the stimulation of the skin 
exceeds a certain limit of intensity, the sense of touch or temperature is lost 
in, that is to say, is not appreciated as separate from, the sense of pain ; and 
under abnormal circumstances acute sensations of pain are started by 
changes in parts, for example tendons, the condition of which under normal 
circumstances we are not conscious of appreciating through any distinct 
sensation, though it may be that these parts do normally give rise to feeble 
impulses contributing to " general sensibility." It may therefore be debated 
whether "pain " is a phase of all sensations, or of general sensibility alone, 
or a sensation sui generis. We shall have something further to say on this 
matter when we treat of the senses; meanwhile it will be convenient for 
present purposes if we consider that the sensations we have to deal with just 
now are the sensations of touch and of temperature, those of the muscular 
sense, and those of general sensibility, including those of pain. 
§ 591. The fairly convincing evidence that the occipital cortex has special 
relations with vision, and the less clear evidence that other regions have 
special relations with smell and hearing, suggest that special parts of the 
cortex have special relations with the sensations now under consideration. 
But in the cases of the senses of sight and smell we had a distinct anatomical 
leading; and we have seen how uncertain is the evidence where such an 704 
THE BRAIN. 
anatomical leading fails, as in hearing and taste. In the case of sensations 
of the body at large, the anatomical leading similarly fails. Moreover, any 
attempt to push the analogy of sight raises the following question : If there 
were two optic nerves on each side of the head, would there be two cortical 
areas, one for each nerve, in each hemisphere, or one visual area only? And 
again, if the optic nerve were the instrument for some sense in addition to 
that of sight, would there be two cortical areas, one for each sensation, or 
one area only serving as the cortical station, so to speak,of the whole nerve? 
If we push the analogy of sight it is open for us, since we cannot give a 
definite answer to the above question, to suppose either that there is one area 
for touch, another area for temperature, and so on, each for the whole body, 
or that there is an area for sensations of all kinds for each afferent nerve, or, 
that there is an intricate arrangement which supplies all the combinations of 
the two which are required for the life of the individual. Of the three 
hypotheses the latter is the more probable; but if so, it is by its very nature 
almost insusceptible of experimental proof, especially when we bear in mind 
what we have already said touching the difficulty of judging the sensations 
of animals. If the judgment of visual sensations is difficult, how much more 
difficult must be the judgment of sensations of touch and temperature? 
Indeed, sensations of pain are the only sensations of which we can form a 
quantitative judgment in animals; and our method of judging even these, 
namely, by studying the movements or other effects indirectly produced, is a 
most imperfect one. 
We can learn, therefore, almost absolutely nothing in this matter from 
experimental stimulation of the cortex in animals. As we have previously 
(§ 584) urged, the absence of movements when parts of the cortex other than 
the motor regions are stimulated is no evidence that the stimulation does not 
give rise to psychical events into which sensations enter; and movements 
follow stimulation of the motor area, not because that area is wholly given 
up to motor events, but because from the histological arrangement the 
stimulus gets ready access to relatively simple motor mechanisms. That the 
motor region has close connections with sensory factors is not only almost 
certain on theoretical grounds, but is shown in many ways, for example by 
the experiment, described in § 574, of exalting the sensitiveness of a motor 
area by generating peripheral sensory impulses. 
Nor can the effects on sensation of removal of parts of the cortex be in- 
terpreted with clearness and certainty. In the monkey removal or destruc- 
tion of the gyrus fornicatus (Figs. 148 and 150) on the mesial surface of 
the brain, ventral to the calloso-marginal sulcus which forms on the mesial 
surface the ventral limit of the motor region (an operation of very great dif- 
ficulty), has brought the whole of the opposite side of the body to a condition 
which has been described as an anaesthesia, that is, a loss of all cutaneous 
tactile sensations, and an analgesia, that is, a loss of sensations of pain, the 
condition being accompanied by little or no impairment of voluntary move- 
ments and, though apparently diminishing as time went on, lasting until the 
death of the animal some weeks afterward. Again, removal of the contin- 
uation of the gyrus fornicatus into the gryus hippocampi has, in other in- 
stances, led to a more transient anaesthesia also of the whole or greater part 
of one side of the body. And it is asserted that removal of no other region 
of the cortex interferes with cutaneous ami painful sensations in so striking 
and lasting a manner as does the removal of parts or of the whole of this 
mesial region. 
These results, however, do not accord with clinical experience which, 
though scanty, seems, as far as it goes, to show that in man, when mischief 
apparently limited to the cortex produces loss of sensation, it is the parietal CUTANEOUS AND SOME OTHER SENSATIONS. 
705 
lobe corresponding to the motor region which is affected; but there appears 
to be no record of any case of a cortical lesion affecting sensation without 
affecting movement. We have previously called attention to the fact that 
the temporary loss or impairment of movement which follows removal of an 
area is frequently, if not always, accompanied by an impairment of cutane- 
ous sensations in the limb or part " paralyzed and side by side with this 
we may put the experience that in the human epileptiform attacks of cortical 
origin, the seizure is at times ushered in by peculiar sensations, called the 
" aura," in the part movements of which inaugurate the march of convul- 
sive movements. But these things do not show that the cortical area is the 
seat of sensationsthey rather illustrate what we said concerning the 
complexity of the chain of which the events in the cortical area are links, 
and the close tie between sensory factors and the characteristic elements of 
the motor region. 
In the dog, while removal of almost any considerable portion of the 
cortex affects sensation, removal of parts of the frontal region producing, 
perhaps, less effect than removal of parts in other regions, the loss or im- 
pairment of sensation appears to be transient, though having a duration 
broadly proportionate to the extent of cortex removed ; and when a very 
large portion of the cortex is removed, some imperfection appears to remain 
to the end. We have already referred to the case of a dog from which the 
greater part of both cerebral hemispheres had been removed, but which 
remained capable of carrying out most of the ordinary bodily movements, 
and that apparently in a voluntary manner; in this case the " blunting" of 
cutaneous sensations was perhaps more striking than the imperfection of 
movement. It will be worth while to consider the condition of this dog a 
little closely, on account of the light which it throws on the problem which 
we are now discussing. 
Clinical experience shows that in man the integrity of the cerebral hemi- 
spheres, and of the connection of the hemispheres with the rest of the central 
nervous system, is essential to the full development of sensations; and that 
in this respect, each hemisphere is related to the crossed side of the body. A 
very common form of paralysis or " stroke " is that due to a lesion of some 
part of one hemisphere (the exact position of the lesion need not concern us 
now), frequently caused by rupture of a bloodvessel, in which the patient 
loses all power of voluntary movement and all sensations on the crossed side 
of his body (including the face); he is said to be suffering from hemiplegia, 
" one-sided stroke." Not only do voluntary impulses fail to reach the mus- 
cles of the affected side, but sensory impulses, such as those which started, 
for instance, in the skin, would, under normal conditions, lead to sensations 
of touch, of heat or cold, or of pain, fail to affect consciousness, when they 
originate on the affected side; the patient cannot on that side feel a rough 
surface, or a hot body, or the prick of a pin. For the sake of clearness we 
suppose the loss of movement and sensation to be complete, but it might, of 
course, be partial. Such a case shows, we repeat, that the integrity of the 
cerebral hemisphere, and of the connection of that hemisphere, we may say 
of the cortex of that hemisphere, with the other parts of the nervous system, 
is essential to the development of the sensations; but it does not prove that 
the cortex of the hemisphere is the " seat " of the sensations, it does not prove 
that the afferent and sensory impulses started in the skin undergo no mate- 
rial change until they reach the cortex and are then suddenly converted 
into sensations ; it only proves that in the complex chain of events by which 
sensory impulses give rise to full conscious sensations the events in the cortex 
furnish an indispensable link. And the phenomena of the dog in question, 
on the one hand, illustrate how complex the chain is, and, on the other hand, 706 
THE BRAIN. 
suggest that the completeness of the loss of sensation in the hemiplegic man 
is not a pure " deficiency " phenomenon, but is due to the lesion affecting the 
chain of events in some way or other besides merely removing the link fur- 
nished by means of the cortex. For, as we previously urged, the dog in 
question, however curtailed its psychical life may have been, seemed to a 
casual observer to feel and move much as usual. Neglecting visual and 
auditory sensations with which we are not now dealing, it needed careful 
observation to ascertain that some of the animal's movements fell short, the 
failure being apparently due to the lack of adequately energetic coordinating 
sensory impulses ; a stronger stimulus than usual had to be applied to the 
skin in order to call forth the usual movements and other tokens that the 
stimulus was " felt." As we have before urged, it is impossible to suppose 
that the mere stump of cerebrum left in this case could have taken on all the 
functions of the lost hemispheres; and making, as we have previously done, 
full allowance for the differentiation between man and dog, we must conclude 
that in the more general sensations with which we are now dealing, as with 
the more special visual sensations, the full development of a complete sensa- 
tion is a complex act of more stages than one between the afferent impulse 
along the afferent nerve and the affection of consciousness which we subject- 
ively recognize as "the sensation ;" the cortical events are only some among 
several. It follows that any analogy between the cortical events which play 
their part in a sensation and the cortical events which immediately precede 
the issue of impulses from the motor region along the fibres of the pyram- 
idal tract is misleading; the highly differentiated motor localization does 
not justify us in concluding that there exists a similar topographical distri- 
bution of sensation. 
§ 592. We may now attack the problem in a different way, and instead 
of beginning with the cortex begin with afferent impulses started along affer- 
ent nerves from their peripheral endings, and attempt to trace them central- 
ward. And first we may call to mind what anatomical guidance we possess 
(§482). • 
The fibres of posterior roots, the channels of afferent impulses, end in 
the spinal cord in at least two main ways. One set is continued on, not 
broken by any relays, as the median posterior tract, and by this tract repre- 
sentatives of all the spinal nerves are connected with the gracile nucleus in 
which (§ 523) the median posterior column ends. The other fibres of a pos- 
terior root appear to end in the gray matter not far from their entrance; 
but from the gray matter there starts the cerebellar tract, which, though not 
conclusively proved to be, may be assumed to be, an efferent tract. We may 
therefore probably suppose that afferent impulses along certain of the fibres 
of the posterior root make their way upward along the cerebellar tract, and 
there are some reasons for regarding the vesicular cylinder and the cells 
which represent this where it is not conspicuous in the regions of the cord, 
as a relay between the two systems of fibres. There are also the more scat- 
tered fibres of the ascending antero-lateral tract (§ 480), which probably is 
also an afferent tract, and therefore probably also connected with the poste- 
rior roots; but, as we have seen, our knowledge of this tract is imperfect, 
though, if, as some urge, it ends in the restiform body, we may perhaps con- 
sider it as similar at least to the cerebellar tract, and treat the two as one. 
Thus there seem to be at least two main recognized paths, in the form of 
tracts of fibres, for afferent impulses along the cord ; one along the median 
posterior column, the other along the lateral column in the cerebellar tract. 
The latter passes straight up to the cerebellum by the restiform body, trav- 
elling along the same side of the cord, and any crossing of impulses passing 
along this tract must take place before they enter the tract; we have, how- CUTANEOUS AND SOME OTHER SENSATIONS. 
707 
ever, no anatomical guidance for such a crossing. The other path, along the 
median posterior tract, comes to end in the gracile nucleus; it has, indeed, 
been urged that the gracile nucleus is thus connected chiefly with the lower 
limbs and lower part of the body, and that the analogous posterior root 
fibres from the upper limbs and neck pass similarly into the cuneate nucleus, 
or at least into the median division of that nucleus, but this cannot be con- 
sidered as proved. Moreover, both the posterior columns, median and ex- 
ternal, bring to these nuclei fibres which have started from some relay in the 
gray matter lower down, and which are not fibres coming straight without 
any relay from the posterior roots; these, however, we cannot distinguish 
from each other in their course beyond the nuclei. From the gracile and 
cuneate nuclei the path onward is a double one, one broad, one narrow. The 
broad path, the one having most fibres and presumably carrying most im- 
pulses, leads to the cerebellum by the restiform body; and here the path, 
previously continued exclusively along the same side of the cord, becomes 
partly crossed though remaining partly uncrossed, the sensory decussation in 
the bulb being the crossed and the other fibres passing from the nuclei 
straight to the restiform body being the uncrossed one (§ 525) ; the uncrossed 
one we may, perhaps, look upon as really an upper part of the cerebellar 
tract. The narrow path is the fillet (§ 547), by which some of the fibres 
from the nuclei are continued on toward the cerebrum. This path is a 
crossed one, the crossing taking place in the sensory decussation, and it car- 
ries relatively few impulses, the chief increase in the size of the fillet as it 
passes onward being due to fibres coming from structures other than the 
gracile and cuneate nuclei. 
Hence of the sensory impulses travelling along continuous tracts in the 
spinal cord, these tracts apparently keeping always to the same side, the 
great majority pass to the cerebellum ; and of these again the greater num- 
ber, all those along the cerebellar tract, and some of those passing through 
the gracile and cuneate nuclei, remain uncrossed to the end. The only path 
by which all these impulses thus passing to the cerebellum can gain access 
to the cortex of the cerebrum, is by some or other of the ties between the 
cerebellum and the cerebral cortex. The relatively few impulses which pass 
along the fillet are for the most part carried to the middle parts of the brain, 
for only a small portion of the fillet passes to the cortex (§ 547), and it is 
not clear that this part of the fillet comes from the gracile and cuneate 
nuclei, so that most of these impulses can gain access to the cortex only by 
the relays of these middle parts of the brain. 
Very striking, indeed, are these constant relays along the path of sensory 
impulses; in this respect the sensory impulses offer a strong contrast to the 
motor impulses. But a still more complex system of relays has to be men- 
tioned ; for yet a third path is open for sensory afferent impulses along the 
cord. We must admit the possibility of afferent impulses travelling along 
the network of the gray matter, their path being either absolutely confined 
to the gray matter, or leaving the gray matter at intervals, and joining it 
again by means of those longer or shorter commissural or internuncial 
fibres which unite the longitudinal segments of gray matter, and form no 
inconsiderable portion of the whole white matter of the cord. We have 
seen (§ 499) that under abnormal circumstances impulses pass freely in all 
directions along the gray matter, and we may conclude that under normal 
circumstances they can pass along it under restrictions and along lines de- 
termined by physiological conditions. The fibres in the white matter which 
do not show either descending or ascending degeneration are probably, as we 
have said (§ 494), internuncial fibres, connecting segments of gray matter 
in a longitudinal direction ; and, though we have no exact knowledge touch- 708 
THE BRAIN. 
ing this matter, we may suppose that some of these convey impulses upward, 
and others downward. 
If, as some maintain, the fibres of the ascending antero-lateral tract end 
not in the cerebellum, but in the gray matter of the bulb, or higher up, we 
have a fourth path for sensory impulses, which, after the primary relay in 
the segmental gray matter, pass straight up to the bulb. 
§ 593. How do experimental results and clinical histories accord with 
such an anatomical programme? 
We may first call attention to an experiment, which, though somewhat 
old, carried out on rabbits, and confined to one region only of the cord, the 
lower thoracic, has nevertheless a certain value on account of its affording 
more or less distinctly quantitative and measurable results. We have seen 
(§ 161) that afferent impulses started in afferent fibres, in those, for instance, 
of the sciatic nerve, so affect the vasomotor centre in the bulb as to cause a 
rise of blood-pressure, at least in an animal under urari. Those afferent 
impulses must pass by some path or other from the roots which supply the 
sciatic nerves with afferent fibres along the thoracic and cervical cord to the 
bulb. If the path be blocked, the stimulation of the sciatic nerve will fail 
to produce the usual rise of blood-pressure. Now in a rabbit the amount of 
rise of blood-pressure following upon the stimulation of one sciatic nerve 
with a certain strength of current having been ascertained, it is found that 
a much less rise of blood-prtssure or noneat all follows the same stimulation 
after division of certain parts of the cord in the mid or upper thoracic 
region ; that is to say, the section of the cord has partially or completely 
blocked the path of the afferent impulses. Further, the block is conspicuous 
when the lateral column is divided, and is not increased by other parts of 
the cord being divided at the same time; when both lateral columns are 
divided, the block is almost complete. And further, supposing one sciatic, 
say the right, is the one which is stimulated, a block occurs both when the 
lateral column of the same, right, side and when that of the crossed, left, 
side is divided, but is greater when the division is on the crossed than when 
it is on the same side. We may infer that the impulses, which reach the 
lumbar cord by the roots of the sciatic nerve, travel up the cord, or give 
rise within the lumbar cord to events which we may compare to nervous 
impulses, and which travel up the cord in such a manner that in the lower 
thoracic region they pass almost exclusively along the fibres of the lateral 
column, some having kept to the same side of the cord, but more having 
crossed over to the opposite side before reaching the thoracic region. 
This result was obtained in rabbits, and the experiment was carried out 
in the lower thoracic region only; the conclusions to be drawn from it hold 
good for that animal only, and for that part only of its cord. Moreover, 
the experiment only tests the path of such impulses as reach and affect the 
vasomotor centre in the bulb. It is, however, exceedingly probable that the 
impulses which, generated in sensory nerves, affect the vasomotor centre are 
impulses which, in the conscious animal, give rise to sensations of pain ; in 
an intact animal changes in the vasomotor centre occasioned by the stimula- 
tion of sensory nerves are accompanied by signs of more or less pain. And 
indeed this is confirmed by the fact that similar results were obtained when, 
the experiment being conducted in a similar way, signs of pain instead of 
variations in blood-pressure were taken as the tokens of the blocking of 
impulses. Hence, assuming this, we may regard the experiment as indicat- 
ing that the impulses which form the basis of painful sensations pass by the 
lateral columns in the lower thoracic region of the cord of the rabbit, and 
therefore, though this is a further assumption, by the same columns along the 
whole length of the cord. We further may infer that while some of the CUTANEOUS AND SOME OTHER SENSATIONS. 
709 
impulses keep to the same side of the cord, others, aud indeed the greater 
number, cross to the opposite side. 
These conclusions entail assumptions, but the main interpretation of the 
whole experiment entails a still greater assumption. The testing of the 
influence of the sciatic stimulation was carried out soon after the section of 
the cord, and yet we have assumed that the block of the impulses was due 
to a pure deficiency phenomenon, the absence of a usual path. But we 
have no right to do this. It is possible that the section produced, in some 
way or other, a depressing or inhibitory effect lower down in the cord, 
affecting structures other than the lateral columns; all our experience, in- 
deed, of the effects of operations on the cord would lead us to expect this. 
It is further possible that a section of the lateral column might produce this 
depressing effect, while sections of other parts did not, or might produce 
more effect than the former. It is possible, for instance, that the section of 
the thoracic lateral column inhibited, for the period during which the ex- 
periment was carried out, the gray matter of the lumbar cord, and that the 
block really took place in this gray matter. Until the uncertainties thus 
attending the interpretation are removed the experiment is not valid as a 
proof that the lateral columns are the paths of afferent impulses; it would, 
however, still serve to indicate that the afferent impulses reaching the cord 
along the sciatic nerve crossed over to a large extent before they came under 
the influence of the inhibition, since we have no evidence to show that such 
an inhibitory action of the section would be exerted chiefly on the crossed 
side. 
Again, we have seen that the afferent impulses affecting the vasomotor 
centre gain access to that centre without the help of the parts of the brain 
above the bulb ; the existence of the vasomotor centre was made out 162), 
by combining stimulation of a sciatic nerve with a series of operations con- 
sisting in making successive transverse sections of the bulb from above 
downward ; and it was not until the sections reached the vasomotor centre 
that the blood-pressure effects of the sciatic stimulation wyere modified. 
Hence, if the experiment be taken as showing that not only afferent im- 
pulses affecting the vasomotor centre, but other afferent impulses also travel 
by the lateral columns, it would also seem to show that these other impulses 
pass in like manner to the bulb, and gain access to the cortex through the 
bulb. This increases a difficulty which presents itself even when the afferent 
impulses affecting the vasomotor centre are alone considered. If the experi- 
ment means anything, it means that the impulses having in some way or 
other reached the lateral column, travel up that column by some continuous 
path, and indeed is generally taken as having that meaning. But if we put 
aside the very doubtful view that the ascending antero-lateral tract ends in 
the bulb, there is no continuous afferent tract in the lateral column ending 
in the bulb ; the only definite continuous afferent tract in the lateral column 
of which we have any clear knowledge, namely, the cerebellar tract, ends 
not in the bulb but in the cerebellum. And if we attempt to get out of 
the difficulty by supposing that those impulses at least which affect the 
vasomotor centre, after travelling for some distance in the cerebellar tract, 
leave that tract for some path leading to the bulb (and the cerebellar tract 
does probably give off as well as receive fibres along its course), we practi- 
cally admit that the experiment does not prove the existence of a continuous 
path. 
A further difficulty is raised by the fact that, according to the interpreta- 
tion which we are discussing, the section of the lateral column breaks the 
paths of what we may consider two kinds of impulses: those, the larger 
number, which have already crossed from one side of the cord to the other, 710 
THE BRAIN. 
and those which have remained on the same side. For, as we have already 
said, we have evidence, in man at least and some other animals, that afferent 
impulses cross completely over somewhere or other on their path before 
they are developed into full sensations; and we have also evidence, though 
less strong, that they cross not long after their entrance into the cord. But, 
if we suppose this to be the case in the rabbit also, it follows that in the 
experiment in question the impulses which were blocked on their passage 
along the lateral column of the same side, whatever the way by which they 
reached that lateral column, were pursuing a path which would eventually 
have led them to the other side of the cord. Hence, the section of the 
lateral column, in breaking their path, broke not a continuous path keeping 
to the lateral column up the length of the cord, but a path which soon left 
the lateral column to pass elsewhere. The experiment, therefore, as far as 
the impulses passing up the same side are concerned, does not prove that 
they pursue a continuous path along the lateral column ; and if so, what be- 
comes of the validity of the experiment as regards the impulses crossing over 
from the other side? for the experiment in itself makes no distinction between 
the two. 
We may add, however, that though the point has not been specially 
investigated, it is possible that in the rabbit, in whose hind limbs bilateral 
movements are so predominant, there is associated with the movements a 
bilateral arrangement for sensations, and that those impulses which remain 
along the same side of the cord as the nerve in which they originate, are 
carried up to the brain without any crossing at all. 
§ 594. The results of this vasomotor experiment then, though they are 
frequently quoted, do not when closely considered afford adequate proof that 
afferent impulses pursue a continuous path along the lateral columns of the 
cord, and, moreover, the facts brought to light by the experiment show but 
little accord with the anatomical programme. We have dwelt on it so long 
because it is more or less illustrative of the many difficulties attending the 
interpretation of experiments of this kind ; and it is in this respect all the 
more valuable because the actual experimental results are sharp and clear. 
We may pass over more rapidly the numerous experiments on the lower 
mammals, such as rabbits and dogs, in which other indications of sensation 
have been made use of, chiefly those which are the signs of painful sensa- 
tions ; these have been carried out in various regions of the cord, but chiefly 
in the thoracic region, and in them a like uncertainty of interpretation is 
further increased by the want of exactness and agreement in the results. 
If we content ourselves with making no distinction between the different 
kinds of afferent impulses, and in the case of these animals it woidd hardly 
be profitable to attempt to make a distinction, we may say that the several 
experiments so far agree that they point to the lateral columns as being the 
chief paths of afferent, sensory impulses, or to speak more exactly, to the 
passage of these impulses being especially blocked by section of the lateral 
columns. Some observers find that in the dog and other lower mammals a 
section of the lateral column on one side, or at least a hemisection of the 
cord, produces " loss of sensation " on the opposite side greater than on the 
same side, or confined to the opposite side and even accompanied by an 
exaltation of sensation, a hyperaesthesia, on the same side. Other observers 
again, and these certainly competent observers, find that, in the dog, section 
of one side affects sensations on both sides, and, indeed, chiefly on the same 
side. We may perhaps once more repeat the warning of the difficulty attend- 
ing the quantitative and qualitative determination of sensations in such 
an animal as the dog; and may remark that in all these cases of unilateral 
section the increased blood-supply due to failure of the normal vaso-con- CUTANEOUS AND SOME OTHER SENSATIONS. 
711 
strictor tone must influence the peripheral development of sensory im- 
pulses. 
In these experiments, as in those on voluntary movements, it is most 
important to distinguish between immediate or temporary and more lasting 
effects; and observers have found that the loss of sensation following a 
hemisection of the cord, like the loss of voluntary movement, is temporary 
only, and eventually disappears, though the recovery is slower and less 
complete than is the case with movements. As with voluntary movement 
(§576) so with sensation, recovery, though less complete than that of 
movement, is possible when a hemisection on one side has been at a later 
date followed by a hemisection on the other side. We may, therefore, 
repeat in reference to sensations the remarks which we made in reference 
to movement. There is, however, an important difference between the two 
cases: in respect to movement we have evidence that under normal condi- 
tions the pyramidal tract plays an important part and that any other path 
for volitional impulses is more or less an alternative one, whereas in respect 
to sensation we have no anatomical or other distinct proof of any such 
normal path. 
The experiments on monkeys are in like manner neither accordant nor 
decisive; and even in these animals with their more varied signs of sensa- 
tions, the interpretation of these signs is beset with fallacies. Some observ- 
ers have found that a hemisection (in the thoracic region) produced loss 
of sensation on the crossed side, accompanied by little or no loss on the 
same side; other observers again have failed to obtain after a hemisection 
satisfactory proof of any such marked loss on the crossed side. Further, large 
portions of the lateral column, the more internal parts adjacent to the gray 
matter being left, have been removed without any very obvious and certainly 
without any lasting defects of sensation on the one side or on the other. 
§ 595. The clinical histories of diseases of the spinal cord in man bring 
to light in a fairly clear manner a fact of some importance, namely, that 
the several impulses which form the bases of the several kinds of sensa- 
tions, of touch, heat, cold, and pain, and of the muscular sense, are trans- 
mitted along the cord in different ways and presumably by different struc- 
tures, for disease may impair one of these sensations and leave the others 
intact. Thus cases of spinal disease are recorded, in which on one side of 
the body or in one limb ordinary tactile sensations seemed to be little im- 
paired, and yet sensations of pain were absent; when a needle was thrust 
into the skin no pain was felt, though the patient was aware that the needle 
had been pressed upon the skin at a particular spot; and conversely, in 
other cases pain has been felt upon the insertion of a needle, though mere 
contact with or pressure on the skin could not be appreciated. Again, 
cases are recorded in which the skin was sensitive to touch or pain, but not 
to variations of temperature; it is further stated that cases have been met 
with in which cold could be appreciated, but not heat, and vice versa; and 
there are some facts which point to sensations of pain being more closely 
associated with those of heat, and tactile sensations with those of cold, than 
those of pain with those of touch or those of heat with those of cold. Cases 
of spinal disease are also recorded in which the muscular sense appeared to 
be affected apart from other sensations. We shall return to these matters 
later on in dealing with the senses; we refer to them now simply as show 
ing that disease, limited as far as can be ascertained to the spinal cord, may 
affect the several sensations separately, and therefore as suggesting that the 
several kinds of impulses, forming the bases of the several kinds of sensa- 
tions, are transmitted in different ways and follow different "paths" along 
the spinal cord. 712 
THE BRAIN. 
Clinical histories, moreover, agree, at least to a large extent, in showing 
that when the lesion is confined to one-half of the cord, the sensations 
affected in the parts below the level of the lesion are chiefly or even exclu- 
sively those of the crossed side. But there is not entire accordance, espe- 
cially as to the crossing being complete. And with regard to the muscular 
sense there is a distinct conflict of opinion ; the majority of cases seem to 
show that in unilateral disease or injury to the cord, the muscular sense in 
company with the voluntary movements fails on the same side; but cases 
have been recorded in which the muscular sense in company with other 
sensations seemed to be affected on the crossed side ; it must be remem- 
bered, however, that it is very difficult to appreciate a deficiency of muscu- 
lar sense mingled with deficiencies in other sensations, and we should a 
priori expect the muscular sense to run parallel with motor impulses. 
When, however, we appeal to clinical histories or indications as to the 
several paths within the spinal cord taken by these several impulses, the 
answer is a most uncertain one, as indeed might be expected from the too 
often diffuse character of the lesions of disease; and it is perhaps not too 
much to say that no satisfactory deductions at all can be made. 
§ 596. Whether, then, we turn to experiments on animals or to the studv 
of disease, the teachings with regard to sensation, in contrast to those with 
regard to voluntary movement, are in the highest degree uncertain and 
obscure. A few general reflections will perhaps help us to appreciate the 
value of such facts as we possess. 
We have seen reason to think that in every movement, whether voluntary 
and of cortical origin, or involuntary and started either as a simple spinal 
reflex or through the working of some part or other of the brain, the motor 
impulses, which sweep down the motor fibres to the muscles, issue mar- 
shalled and coordinated from the gray matter of the cord (for the sake of 
clearness we may omit the cranial nerves), from what we have called the 
motor mechanisms of the cord. Analogy would lead us to suppose that 
the afferent impulses, forming the bases of the several kinds of sensations, 
similarly left the afferent fibres to join the gray matter of the cord in what 
w7e may call the sensory mechanism. And such anatomical leading as we 
possess seems to support this view; with the exception of the median pos- 
terior tract, to which we will return immediately, all the fibres of a poste- 
rior root seem to end in the gray matter not very far from the entrance of 
the root. We have seen that a coordinate reflex movement may be carried 
out by at least a few segments of the cord ; that a reflex movement may be 
started by stimuli of various kinds and therefore presumably by afferent 
impulses of various kinds; and that impulses forming the basis of the 
muscular sense are essential to the coordination of the movement. All 
our knowledge goes to show that in reflex movement carried out by a few 
segments of the cord, the whole chain of events between the arrival of the 
afferent impulses along the posterior root and the issue of efferent impulses 
along the anterior root may be carried out by gray matter, and gray matter 
alone. We may further infer that, while on the one hand the same pro- 
cedure might obtain not through a few segments only but along the whole 
length of the cord, there would be an advantage, especially in respect to 
the rapidity of transmission, in employing internuncial tracts of fibres be- 
tween the several segments, the advantage being greater the more distant 
the segments which have to work together. 
We might further suppose that it would be of advantage to possess some 
direct path between the cerebral cortex and the spinal sensory mechanism 
immediately connected with the posterior root, such as is afforded by the 
pyramidal tract between the cortex and the spinal motor mechanism CUTANEOUS AND SOME OTHER SENSATIONS. 
713 
immediately connected with the anterior root. But no anatomical evi- 
dence of such a tract is forthcoming; and, as we have before remarked, 
along all the tracts which seem to be sensory in nature, in contrast to what 
takes place in the motor tracts, relays of gray matter are continually being 
interpolated. 
The median posterior tract, since it gathers up representatives of succes- 
sive nerves, presents itself as the nearest approach to such a sensory homo- 
logue of the pyramidal tract, though it ends in the bulb and is not con- 
tinued directly on to the cortex. And possibly it does play a somewhat 
analogous part, in so far as it serves as a special connection between the 
brain and the whole series of spinal nerves. But we are wholly ignorant as 
to what it really does; and whatever be the exact nature of the part which 
it plays, it probably has relations not to one kind of sensation only, but to 
all the different kinds of sensation. It has indeed been supposed by some 
to be especially a tract for the impulses of the muscular sense; but neither 
experiment nor clinical study affords adequate proof of this view. The con- 
dition known as locomotor ataxia, the salient feature of which is loss or 
impairment of muscular sense, is associated with disease of the posterior 
root and of its entrance into the cord, not with disease confined exclusively 
to the median posterior column. Moreover, the tract cannot carry all the 
impulses of muscular sense, since some of them must pass at once into the 
gray matter to take part in the coordination of reflex movements, and must 
therefore travel by fibres which do not form this tract. Similarly there is 
no adequate proof of the tract being an exclusive channel for tactile or for 
painful sensations. 
We may also, perhaps, urge similar considerations with regard to the 
cerebellar tract, which, though starting from a relay of gray matter, is thence 
onward to the cerebellum a continuous tract. This tract also has been sup- 
posed to carry impulses of a particular kind, and more particularly those of 
muscular sense. There is lessd priori objection to this view, since the tract 
starts from the gray matter, where the impulses of muscular sense may have 
already done their, so to speak, local work, and ends in the cerebellum, 
which, as we have seen, seems especially connected with the coordination of 
movements. But with respect to this tract also neither experiment nor 
clinical study affords any clear and decisive proof that it is solely or even 
especially concerned with the muscular sense. 
With regard to the antero-lateral ascending tract our knowledge is too 
imperfect to justify us in supposing that it is the special or exclusive channel 
for any one kind of sensation, or indeed in drawing any conclusions at all 
concerning it. 
But when we subtract from the white matter of the cord these con- 
tinuous tracts of ascending degeneration of presumably sensory or afferent 
function, and the continuous tracts of descending degeneration, which we 
may confidently speak of as motor or at least efferent, there are left only the 
fibres which we have (§ 494) supposed to be longitudinal commissural or 
internuncial fibres between successive segments. We are thus driven back 
to our former conclusion, that sensory impulses pass either by the gray matter 
alone, or by a series of steps as it were, by relays of gray matter connected 
by internuncial tracts of fibres, whose length we cannot ascertain, but which 
may be short. That such internuncial tracts intervene is rendered probable 
on the one hand by the fact that section of the white matter, leaving the 
gray matter untouched, does affect sensations, and on the other hand by the 
fact that the several kinds of sensation appear to travel along the cord by 
separate paths, or at least may be separately blocked. It is of course, as we 
have already urged, possible that the effect of a section of a tract of fibres 714 
THE BHAIN. 
may not be the mere block due to loss of continuity, but some action on the 
gray matter with which the fibres are connected, whereby that gray matter 
fails of its usual functions and ceases to carry onward the sensory impulses 
reaching it from below ; it is also possible that this or that lesion of disease 
may, directly or indirectly, affect particular parts of the gray matter or 
affect the gray matter in a particular way, so that a certain kind of sensory 
impulse and none other is blocked. On the other hand, we have reason to 
think that the rate at which impulses travel along the gray matter is very 
slow compared with that along nerve-fibres ; ami in the struggle for life 
rapidity of transmission of nervous impulses is of great importance. Hence 
the view that the internuncial fibres intervene has more to commend it; it 
is, moreover, to a certain extent supported by clinical histories. But, if we 
accept this view, we must at the same time admit that, in animals at least, 
the lines provided by the internuncial tracts are not rigid-that within limits 
and under circumstances alternative routes are possible. 
§ 597. We may here, perhaps, raise once more, and this time more point- 
edly than before, the doubt whether we are justified in assuming, as we 
generally do assume, that the events which take place in the fibres connect- 
ing relays of gray matter within the central nervous system are exactly the 
same as those which take place in the fibres of nerves outside the central 
system during the passage of what we call a nervous impulse. jMost of our 
knowledge of a nervous impulse has been gained by the study of the motor 
nerve of a muscle-nerve preparation. Our knowledge of the processes in af- 
ferent nerves is much more imperfect; but there are many facts which at least 
suggest that the molecular events constituting an afferent impulse along an 
afferent nerve are different from, and probably more complicated than, those 
constituting an efferent impulse along an efferent nerve. And, with regard 
to the processes taking place in fibres within the central nervous system, we 
have hardly any exact experimental knowlege at all. It has been main- 
tained by many observers that not only the gray matter, but also the tracts 
of white matter in the spinal cord, while they are capable of conveying im- 
pulses in one direction or the other, are incapable of being so excited by 
artificial stimuli as to generate new impulses. These observers maintain that 
when movements or signs of sensation follow the direct stimulation of various 
parts of the cord, the effects are due to issuing motor fibres or entering sen- 
sory fibres having been stimulated, and not to a stimulation of the intrinsic 
substance of the parts themselves; they propose accordingly to call these 
parts " kinesodic " and " testhesodic " respectively, that is to say, serving as 
paths for motor or sensory impulses without being themselves either motor 
or sensory. The evidence on the whole goes to show that this view is a mis- 
taken one; that the various tracts of the spinal cord, like the pyramidal 
tract, and indeed other parts of the brain, are excitable toward artificial 
stimuli. The question cannot, however, be considered as definitely closed; 
and the very fact that it has been raised illustrates the point on which we 
are now dwelling. We may further quote, in similar illustration of the 
same point, the following remarkable fact which was observed in the series 
of experiments referred to in $ 576 on the effects of repeated hemisection of 
the spinal cord in dogs: The animal had partially recovered voluntary 
movements in his hind limbs after a third hemisection of the thoracic cord, 
and yet when, at his death, a strong tetanizing current was directed through 
the bulb and cervical cord, no movements of the hind limbs followed; the 
impulses started by artificial stimulation could not pass the bridge which 
sufficed for volitional impulses of natural origin. It is not too much to say 
that our experimental knowledge as to the events which accompany the 
activity of the structures within the central nervous system is almost entirely CUTANEOUS AND SOME OTHER SENSATIONS. 
715 
limited to the recognition of the "currents of action " referred to in § 570. 
We are already going beyond our tether when we assume on the strength of 
this that the processes started in the fibres of the pyramidal tract by artifi- 
cial stimulation are irf all respects identical with those started in the fibres 
of a motor nerve. We are going still more beyond our tether when we 
assume that the processes started in the same pyramidal fibres as the out- 
come of natural events in the motor cortex are of the same kind. But these 
assumptions are trifles compared with the assumption that the events taking 
place in the fibres of the optic radiation passing from the pulvinar to the 
occipital cortex are identical with the events taking place in the fibres of 
the optic tract on the way to the pulvinar, or that the events travelling along 
the spinal cord to the brain as the result of a prick of the little finger are 
identical with those which the prick has started in the fibres of the ulnar 
nerve. Of the latter events we know a little ; of the former events we know 
next to nothing. And we may here ask the question, What is the meaning 
of these continual relays of gray matter along the sensory tract, unless it be 
that at each relay some transformation, some further elaboration of the im- 
pulses take place, until what were the relatively, but only relatively, simple 
impulses along the fibres of the peripheral nerve are by successive steps 
changed in the complex events which we call a conscious sensation ? This 
is what we had in mind when we gave (§ 565) a note of warning concerning 
the danger of considering all the events in the central nervous system as either 
motor or sensory in nature. It is perhaps not an exaggeration to represent 
the views of some observers as if they supposed that afferent impulses, say 
tactile impulses, that is, impulses eventually giving rise to tactile sensations, 
travelled unchanged from the skin to the cortex, and there suddenly blos- 
somed into sensations. If such a view were true, undoubtedly the chief task 
of physiology, almost the only one, would be to ascertain the tract along 
which these impulses passed. But if, on the other hand, the views just now 
urged have any real foundation, the question of tracts or paths sinks into 
insignificance compared with the almost untouched problems as to what are 
the several changes by which simple impulses are developed into full sensa- 
tions, and when and how the changes are effected. 
§ 598. Seeing how unsatisfactory is our present knowledge with regard 
to the tracts or paths of sensations in the relatively simple spinal cord, it 
would be useless to attempt any discussion as to their paths in the much 
more complex brain. If it be probable that the passage is effected by relays 
of gray matter in the former, the same method is much more probable in the 
latter ; and if neither experiment nor clinical study throws much light on 
the path up to the bulb, these cannot be expected to give much help in 
the maze of gray matter and fibres by which the bulb is joined to the cortex. 
The several defined areas or collections of gray matter, and the several 
strands and tracts of fibres which we briefly described in a previous section, 
must have of course a meaning; but it may be doubted whether we have 
even so much as a correct glimpse of that meaning in any one case, if we 
except those which are in immediate connection with the cranial nerves and 
their nuclei. Seeing that the thalamus appears, on the one hand, to be con- 
nected with all or nearly all parts of the cortex, and, on the other hand, to 
serve as the front of the tegmental system, it is tempting to suppose that it 
plays an important part in sensations pertaining to the body generally, as 
part of it, the pulvinar, certainly does with reference to the special sense of 
sight; but we have no decisive indications as to what part it plays. And 
the part which it plays, whatever that may be, is not an exclusively sensory 
one, since both experimental and morbid lesions of the thalamus are apt to 
produce disorders of movement as well as other efferent effects. We ought 716 
THE BRAIN. 
perhaps to say the parts which it plays, for it is a complex body, having 
many ties and probably performing many duties. 
The conspicuous fillet again, seeming as it does to be a special internun- 
cial tract connecting what appear to be more particularly afferent or sensory 
parts of the bulb, such as the gracile and cuneate nuclei, with various parts 
of the middle brain and probably with the cortex, presents itself as a 
probable path of sensations of one kind or another from the body at large, 
the " narrow path " of the anatomical programme (§ 592) ; but in reference 
to this too, beyond its probable connection with the auditory sensations 
(§ 589), we lack evidence. 
A conspicuous part of the brain, namely, the cerebellum, naturally ar- 
rests our attention on account of its large connections with what appear to 
be afferent structures ; in the anatomical programme we called it " the broad 
path." By the cerebellar tract it has an uncrossed grip upon what is prac- 
tically the whole length of the spinal cord ; by the other constituents of the 
inferior peduncle it has a like uncrossed grip upon what appear to be affer- 
ent structures in the bulb, the gracile and cuneate nuclei, as well as on the 
eighth (vestibular) nerve and probably representatives of other afferent 
cranial nerves ; it has further a crossed grip through the gracile and cuneate 
nuclei on the afferent posterior columns of the whole cord. It is, of course, 
possible that the cerebellar tract, though in itself uncrossed, lays its hand, 
by means of the vesicular cylinder for instance, on impulses which have 
already crossed from the posterior roots of the other side; for as we have 
seen the evidence as a whole shows that sensory impulses do cross over ; but 
neither has the crossing of the impulses been definitely proved, nor has the 
path of the crossing been clearly demonstrated ; while, on the contrary, the 
fibres of the auditory nerve which pass to the cerebellum, and which, as we 
have suggested (§ 531), may be compared to an outlying part of the cere- 
bellar tract, certainly continue uncrossed into the peduncle of the same 
side. We may conclude, therefore, that the ties of the cerebellum with 
the posterior roots are both crossed and uncrossed. And we may regard 
this double grip of the cerebellum on the cord, this grip on both sides of 
it, as an additional evidence that the ties of the cerebellum with the 
spinal cord are not merely for the purpose of serving as the channel for 
the impulses of muscular sense, but are the means by which the cere- 
bellum transforms or elaborates sensory impulses, not of muscular sense 
alone or chiefly, but probably of all kinds, in order that they may take 
part in cerebral operations, of which the coordination of bodily move- 
ments may be one, but probably is only one, of several or even of many. 
Some Other Aspects of the Functions of the Brain. 
§ 599. It is difficult to say anything definite concerning the transmis- 
sion of sensory impulses and the development of sensations; it is still more 
difficult to say anything definite, beyond what has been already incidentally 
said, concerning the parts played in the work of the brain by the various 
aggregations of gray matter and tracts of fibres forming the middle part of 
the brain. Neither experiment nor clinical study has as yet afforded any 
clear or sure leading. 
To what has already been said about the cerebellum, we may add the 
following; 
Electrical stimulation of the surface of the cerebellum, in the monkey 
and in other animals, has led to movements of the eyes, and of other 
parts of the head; but we cannot from such results draw any satisfactory 
inferences. OTHER ASPECTS OF THE FUNCTIONS OF THE BRAIN. 
✓ 
717 
The removal of various parts of the cerebellum, especially of the 
medium parts, has led to a want of coordination in bodily movements ; 
and an unsteady gait, due to a like want of adequate coordination, is a 
frequent symptom of cerebellar disease. But the incoordination which 
makes its appearance immediately after removal of, or injury to, the cere- 
bellum may eventually disappear, even though large portions have been 
removed ; and many cases of cerebellar disease have been recorded in 
which the most perfect coordination of movements was retained. Hence, 
the results of experimental and clinical study, while on the whole support- 
ing the conclusion that the cerebellum has in some way to do with coor- 
dination, throw little or no light on the exact nature of the part which the 
organ plays in the complex process, but perhaps rather show that we are 
at present wholly ignorant of how coordination is brought about. 
Many hypotheses have been put forward as to the work carried out by 
the cerebellum, but none of these can be said to have an adequate basis. 
And, indeed, if there be any value in the reflections we have repeatedly 
made in previous pages, the physiologist ought not to use the words " func- 
tions of the cerebellum." From a physiological point of view it is, so to 
speak, a matter of accident that various structures, the seats of various 
physiological processes, have, from morphological causes, been gathered 
together into the body which anatomists call the cerebellum. The task 
of the physiologist is to unravel the ties binding these various cerebellar 
structures with other parts of the central nervous system, and so with 
various parts of the body at large. 
We must content ourselves here with calling attention to two or three 
broad and suggestive facts concerning its structure and connections. 
In the first place, one striking fact about the cerebellum is the very large 
development of commissural fibres connecting together the superficial gray 
matter of the two hemispheres for the greater part of their extent, and 
passing, not only through the pons (§ 548) as part of the middle peduncle, 
but also through the median vermis. This great commissure is second only 
to the great callosal commissure of the cerebrum ; and from the fact that 
median lesions of the cerebellum, those which do most damage to this com- 
missure, are the most effective in causing incoordination and forced move- 
ments, we may infer that it in some way plays an important part in co- 
ordination. 
A second striking fact is one on which we have already just dwelt, the 
connection, chiefly an uncrossed one, through the inferior peduncle, with the 
afferent structures of the bulb and spinal cord. We may now add that the 
fibres of this peduncle passing into the centre of the white matter of the 
cerebellar hemisphere of the same side enclose the gray matter of the nu- 
cleus dentatus and appear largely to end in that body, though some pass 
on to the vermis. 
A third striking fact is the connection, this being, as far as we know, 
wholly a crossed one, through the pons and pes, with the cerebral cortex 
both of the extreme frontal region and of the temporo-occipital region 
and possibly or even probably with more scattered cortical elements of the 
parietal (motor) region. This connection is one between cortex and cortex, 
or, at least, between cerebral cortex and cerebellar superficial gray matter, 
for the fibres of the middle peduncle passing from the gray matter of the 
pons which serves as a relay end in the surface of the lateral hemisphere 
of the cerebellum. The frontal cortical fibres passing to the pes have a 
descending degeneration, that is, from the cortex to the pons, and we may 
probably assume that the similar temporo-occipital fibres similarly degener- 
ate downward to the pons (§ 545). From this it has been inferred that 718 
THE BRAIN. 
this cerebro-cerebellar connection carries impulses from the cebral cortex 
to the cerebellum ; and it has been further inferred that these impulses 
are of the nature of motor impulses. As we have more than once urged, 
the character of degeneration, that is, whether " ascending " or " descend- 
ing," is not a satisfactory proof of the direction taken by impulses; but it 
is, perhaps, of more importance to remember that, as we have also urged, 
we have no right to assume that the impulses passing along such a tract as 
the one in question must be either sensory or motor, or indeed that 
such a tract serves as an instrument for producing effects in one direction 
only. 
That during life the fibres of which we are speaking serve as an important 
chain by which cerebral cortex and cerebellum affect each other, there 
can be but little doubt; but we are wholly in the dark as to what really 
takes place along the fibres. We have seen (§ 506) reason to think that the 
brain may and does exert an inhibitory influence over the spinal cord ; and 
the mechanical certainty with which an animal deprived of its cerebral 
hemispheres responds to stimuli, in contrast to the uncertainty attending the 
result of stimuli applied to an intact animal, as well as all the experience of 
our own daily life, shows that the cerebral cortex can work in an inhibitory 
manner on other parts of the brain ; the remarkable "forced movements" 
on which we dwelt in a previous section seem, in some instances, to be the 
result of the abrupt snap of some inhibitory bond. Conversely all the ex- 
perience of our daily life, many of the phenomena of the condition known 
as hypnotism and of allied conditions, as well as various experimental re- 
sults such as that quoted in § 574, where a sensory impulse seems to inhibit 
the activity of a motor area, show that the cortex may itself in turn be in- 
hibited by other parts of the central nervous system. But we have at present 
no satisfactory indications as to the paths of inhibitory impulses or as to how 
inhibition is brought about; nor have we any proof that the cerebro-cerebellar 
tract is an inhibitory one in either direction. 
We may add that some of the fibres of the middle peduncle appear to 
be neither commissural nor connected with the cortical fibresin the pes, but 
to end in other ways; and tracts have been described as continuing on- 
ward, some of the cerebellar fibres of the middle peduncle on the one hand 
upward toward the cerebrum, and, on the other hand,downward toward the 
spinal cord. It has been further urged that these tracts are efferent in 
function. 
Lastly, we may call attention to the superior peduncles. These, which, 
as we have seen, appear to come largely from the gray matter of the nucleus 
dentatus and to end in the tegmentum, largely in the red nucleus, may be 
regarded as constituting through the relay of the front part of the tegmen- 
tum another tie, presumbly of a different nature from the foregoing, between 
the cerebellum and the cortex; indeed, it used to be called processus a 
cerebello ad cerebrum. It is an obviously crossed tract (Fig. 136, SP); it 
connects one nucleus dentatus, and so presumably by that relay the fibres 
of the inferior peduncle ending in that body, and perhaps other fibres pro- 
ceeding from the superficial gray matter of one side of the cerebellum, with 
the red nucleus and other parts of the tegmentum of the crossed side, and 
thus with the cortex of the crossed side. It has been supposed that the 
direction of impulses passing along it is from the cerebrum to the cere- 
bellum, but we have no clear proof of this ; indeed, as to what it does, we 
have no satisfactory evidence either experimental or clinical. 
We may here incidentally remark that, in consequence of afferent tracts 
being traced to or toward the tegmentum, and of the sharp contrast pre- 
sented between the tegmentum and the conspicuously motor pyramidal tract OTHER ASPECTS OF THE FUNCTIONS OF THE BRAIN. 
719 
in the pes, the view has gained ground that the tegmentum is essentially a 
sensory structure. But there does not appear to be adequate evidence either 
clinical or experimental for such a conclusion. The thalamus, which we have 
regarded as the front so to speak of the tegmentum, cannot, as we have 
already urged (§ 598), be considered exclusively or especially sensory. And 
many of the ties of the tegmentum, such as the fibres from the corpora striata 
ending in the substantia nigra, for this may be considered as properly be- 
longing to the tegmentum, are of the kind which we may suppose to be 
efferent or motor. Indeed, we may probably regard the whole tegmentum 
as being broadly the analogue in the forward segments of the cerebro-spinal 
axis of both the anterior and posterior gray matter of the spinal segments 
behind. 
Though we are thus in the dark concerning what goes on in the cere- 
bellum, it may be worth while to call attention once more to the remarkable 
characters of the superficial gray matter ($ 561). The many points of re- 
semblance between it and the cerebral cortex cannot but suggest that the 
processes taking place in it have some analogies with cortical events. And 
it is, at least, a fact of some significance that congenital deficiency, or 
atrophy of the cerebral hemisphere of one side, is frequently accompanied 
by a corresponding deficiency of the crossed cerebellar hemisphere. 
§ 600. Both the anterior and posterior corpora quadrigemina are com- 
plex in structure ; not only do they differ from each other, but also in each 
the gray matter differs in different parts, both as to its nature and appear- 
ance and as to its connections with tracts of fibres. If we have little right 
to speak of the " functions of the cerebellum," we have even less right to 
speak of the ''functions of the corpora quadrigemina" or of either pair of 
them. The anterior pair, as we have seen, has to do in some way with 
vision ; but we have reason to think that a part only of the whole body is 
thus concerned ; and there is some foundation for the view that of this part 
one portion belongs, so to speak, to the optic tract, and another portion to 
the cortical fibres of the optic radiation. Possibly still another part is con- 
cerned in bringing, as we have (§ 7586) suggested, visual impulses to bear on 
the coordination of movements. 
Stimulation of the surface of the posterior pair, besides giving rise to 
movements of various parts of the body, has in monkeys and some other 
animals the singular effect of producing a vocal utterance in the form of a 
cry or bark. But we cannot make much use of these results for the pur- 
pose of drawing conclusions as to what share these bodies take in the whole 
work of the brain. In the frog, the optic lobes correspond to the two pair 
of corpora quadrigemina together ; and the cry just mentioned may, perhaps, 
be put side by side with the fact that in the frog the optic lobes seem to 
furnish a mechanism for croaking; when the optic lobes are destroyed, the 
reflex croaking mentioned in § 551 is done away with. The probable con- 
nection of the posterior corpora quadrigemina with hearing is also interest- 
ing in this connection ; but we have no satisfactory evidence of any special 
ties between the bodies in question and either the cortical area for phonation 
or the vocal mechanism in general; the occurrence of the cry remains so far 
an isolated fact. 
In frogs, in which the cerebellum is very small, the optic lobes seem to 
be particularly concerned in the coordination of movements. When the 
brain is removed by means of a section behind the optic lobes the animal 
loses the power of balancing itself (§ 551), which it possesses when the sec- 
tion passes in front of the optic lobes : and injury to the optic lobes pro- 
duces incoordination of movement and often "forced movements." It has 
been maintained that the loss of coordination is in these cases due to re- 720 
THE BRAIN. 
moval of or injury to the central gray matter in the walls of the third ven- 
tricle, and not to mere removal of or injury to the optic lobes ; but the 
whole evidence goes to show that in the frog and in the bird the optic lobes 
do play a part in the coordination of movement, though lesions of the cen- 
tral gray matter around the third ventricle, or, indeed, of the thalamus or 
other parts of the tegmentum, may give rise to loss of coordination or to 
" forced movements." 
In the mammal removal of or injury to the posterior corpora quadri- 
gemina does not cause blindness, but may, like a lesion of the anterior pair, 
give rise to loss of coordination or to forced movements ; the effect, how- 
ever, is, in most instances, very temporary. The connection of the anterier 
pair with vision suggests a clew as to how this pair takes part in coordina- 
tion ; but as to how the posterior pair could intervene in the matter we have 
hardly so much as a hint; for, even if we admit a connection between them 
and the sense of hearing, and, remembering that a loud sound would often 
cause a person to reel, further admit that purely auditory impulses, as dis- 
tinct from what we have called ampullar impulses, may take part in the 
general coordination of bodily movements and in the maintenance of equi- 
librium, as they certainly do in the special coordination of laryngeal move- 
ments, still we are not much nearer an understanding of the matter. We 
may add that section of the lateral fillet, which appears as a conspicuous tie 
between the posterior corpora quadrigemina and the parts of the nervous 
system behind them, does not appear to have any marked effect in produ- 
cing incoordination. 
In fine, beyond the broad facts on which we dwelt in a previous section, 
namely, that we maintain our equilibrium and carryout complex movements 
involving often several parts of our body, through what we call coordination, 
that afferent impulses supply important factors of this coordination, and 
that the cerebellum, through the vestibular nerves in part at all events, 
together with other portions of the middle brain, are in some way its chief 
instruments, we as yet know very little. We have certainly no adequate 
knowledge as to how either pair of corpora quadrigemina exactly intervene 
in the matter, or, indeed, as to what other parts they play in the general 
work of the brain. 
With regard to other tracts of fibres or areas of gray matter we have 
nothing to say, except as regards those which are more or less immediately 
connected with certain of the cranial nerves, such for instance as the nerves 
for movements of the eyes, and these it will be best to consider when we have 
to deal with the nerves themselves. 
§ 601. Besides the somatic functions which in previous discussions we 
have chiefly had in view, the brain as a whole undoubtedly carries out 
splanchnic functions ; concerning these, however, we must be very brief. 
Of the respiratory and vasomotor functions of the bulb we have already 
treated in their appropriate places, and we have referred (§ 449) to the 
experimental evidence that a lesion of the corpus striatum or of the front 
part of the optic thalamus has a remarkable influence on the development 
of heat in the body. We have further seen that the higher parts of the 
brain, acting through the bulb, exercise powerful influences on respiration, 
on the vasomotor system, and on the beat of the heart. Daily experience 
affords abundant instances of actions such as these, as well as of the influence 
of the brain on other organic functions. We can bring our will to bear on 
the mechanism of micturition (§ 364) which is almost wholly, and on the 
mechanism of defecation (§ 236) which is largely, splanchnic in nature. 
These movements, however, are not skilled movements; and as we explained 
in dealing with them, the action of the brain as regards them seemed limited 721 
OTHER ASPECTS OF THE FUNCTIONS OF THE BRAIN. 
to augmenting or inhibiting the activity of spinal centres. We should, there- 
fore, hardly expect them to be specially represented in the cortical motor 
region. But emotions have a much wider and more powerful influence over 
the splanchnic functions than has the will, and have the power of affecting 
the work of certain organs, for instance the heart and secreting glands, which 
the will is unable to touch. And since we have every reason to believe that 
the cortex is closely associated with the emotions, we may naturally infer 
that elements of the cortex supply a link in the chain through which an 
emotion influences this or that splanchnic activity ; we may, accordingly, 
expect to find that stimulation of some part or other of the cortex produces 
splanchnic effects. The results of experimental investigation, however, are 
both scanty and discordant; but the greater weight should perhaps be 
attached to the positive results. Thus, some observers find that stimulation 
of the cortex, the locality being in the dog some part of the sigmoid gyrus, 
produces movements of the bladder ; and they trace the path of this influence 
through the front part of the thalamus and the tegmentum to the bulb and 
so to the cord, excluding the cerebellum, which other observers believed to 
be concerned in the matter. Some observers again find that stimulation of 
the cortex produces a flow of " chorda saliva," while others maintain that 
the secretion, when it does occur, is an indirect and not a direct effect of the 
cortical stimulation ; and it may be remarked that the cortical area, which 
is claimed to be a " salivation area," lying in the dog on the convolutions 
dorsal to and in front of the Sylvian fissure, is not either the area connected 
with the facial nerve, or that allotted to taste or smell. 
Similarly, stimulation of parts of the cortex has in the hands of various 
observers led to movements or to arrest of movements of the intestines, to 
changes in the beat of the heart, and to various vasomotor and other effects ; 
but it will not be profitable to enter into any further details. We may, 
however, add the remark that when the cortical motor area for a limb is 
removed, or suffers a lesion, the temporary paralysis which is thereby caused 
is accompanied by a rise of temperature in the limb; this may be at times 
very great indeed-in the monkey, for instance, the hand or foot on the 
paralyzed side may be as much as 10° C. higher than that of the other side. 
The effect is partly due to vasomotor paralysis, but, especially considering 
that the muscles of the limb are relatively quiescent and so producing less 
heat than usual, cannot be due to that alone. The remarkable result may 
be taken as still further illustrating the complexity of the processes connected 
with the cortical motor area ; the area is in some way associated with the 
vascular arrangements and nutrition of the muscles with whose movements 
it is concerned. 
§ 602. There remain yet a few words to be said about the cortex. We 
regard, and justly so, the spontaneous intrinsic activity of the brain as the 
most striking feature of its life. The nearest approach to it which we find 
elsewhere in the body is perhaps the rhythmic beat of the heart. The 
analogy between the " regular automatism" of the one, and the " irregular 
automatism " of the other is a striking one; and indeed our knowledge of 
the relatively simple spontaneity of the heart has probably influenced to a 
large extent our conceptions of the complex spontaneity of the brain. In 
the heart the rhythmic discharge of energy is chiefly determined by in- 
trinsic chemical changes, by the metabolism of the cardiac substance; the 
influence of external circumstances, apart from those which provide an 
adequate supply of proper blood, is wholly subsidiary and serves only to 
raise or to lower the intrinsic changes from time to time, as occasion may 
demand. And the analogy of the heart has perhaps led us to exaggerate 
the part played in the brain by the like intrinsic chemical metabolism. 722 
THE BRAIN. 
(We are here, of course, viewing the action of the brain from the only 
standpoint admissible in these pages, the purely physiological one ; but such 
a mode of treatment does not prejudge other points of view.) Some writers 
use expressions which seem to imply the conception that the nervous 
changes forming the basis of the psychical and other processes of the brain 
are chiefly the direct outcome of the chemical metabolism of the gray 
matter and especially of the nerve-cells. They speak of "the discharge of 
energy " from these cells in the same way that we can speak of the dis- 
charge of energy from a cardiac fibre. But, to say nothing of the low rate 
of nervous metabolism as measured in terms of chemical energy, we have 
no experimental or other evidence of nervous substance in any part of the 
body being, like the cardiac substance, the seat of an important metabolism 
carried on irrespective of influences other than purely nutritive ones. In 
the case of nerve-cells interpolated along nerves composed of fibres of the 
same kind, as in the sporadic ganglia, all the instances where the nerve-cells 
were supposed to initiate active processes have, on examination, broken 
down ; as we have seen, the ganglia of the heart do not supply the moving 
cause of the heart-beat. It is only in the central nervous system, where 
nerve-cells, as part of gray matter, are found at the meeting of nerve-fibres 
of different kinds, that we have any evidence of " discharge of energy " from 
the cells. 
As we pointed out (§ 510) in speaking of the spinal cord, the discharge 
of efferent impulses from the central nervous system, though it undoubt- 
edly must have a certain chemical basis, namely, the metabolism of the 
nervous substance, is, in the first place, dependent on the advent of afferent 
impulses. But this, if true of the spinal cord, is still more true of the 
brain, which receives or may receive not only all the impulses which reach 
it through the cord, but especially potent and varied impulses directly 
through the cranial nerves. All life long, the never-ceasing changes of the 
external world continually break as waves on the peripheral endings of the 
afferent nerves; all lifelong, nervous impulses, now more now fewer, are con- 
tinually sweeping inward toward the centre; and the nervous metabolism, 
which is the basis of nervous action, must be at least as largely dependent 
on these influences from without, as on the mere chemical supply furnished 
by the blood. 
We have developed this point because of the influence it must have on 
our conceptions of the physiological processes taking place in the cortex. 
If we accept the view just laid down, we must regard the supereminent 
activity of the cortex and the characters of the processes taking place in it 
as due not so much to the intrinsic chemical nature of the nervous substance 
which is built up into the cortical gray matter as to the fact that impulses 
are continually streaming into it from all parts of the body, that almost all 
influences brought to bear on the body make themselves felt by it. To put 
the matter in a bald way we may ask the question, What would happen in 
the cortex if, its ordinary nutritive supply remaining as before, it were cut 
adrift from afferent impulses of all kinds? We can hardly doubt but that 
volitional and other physical processes would soon come to a standstill and 
consciousness vanish. This is, indeed, roughly indicated by the remarkable 
case of a patient whose almost only communication with the external world 
was by means of one eye, he being blind of the other eye, deaf of both ears, 
and suffering from general anaesthesia. Whenever the sound eye was closed, 
he went to sleep. It is further indirectly illustrated by the following ex- 
perimental result. We have seen (§ 567) that a vertical incision carried 
through the depth of the gray matter around an area does not prevent 
stimulation of the surface of the area producing the usual movements. OTHER ASPECTS OF THE FUNCTIONS OF THE BRAIN. 
723 
But after such an incision the animal sutlers a paralysis of the movements 
connected with the area, like that resulting from the removal of the gray 
matter of the area; and the operation is said to be followed by degenera- 
tive changes in the area, and degeneration of the pyramidal fibres starting 
from it. Some of this effect may be due to nutritive changes brought about 
by injury to the pia mater ami division of bloodvessels; but it cannot be 
wholly accounted for in this way; it appears as if the life of the area is 
curtailed when its nervous ties are broken. 
We may conclude then that we are not justified in speaking of con- 
sciousness of volition, or other psychical processes, even admitting that these 
fail when the cortex is removed, as being functions of the cortex in the same 
way that we speak of the functions of other organs; they are rather func- 
tions of the connections of the cortex with the other parts of the central 
nervous system. 
We should add that they are also functions of the connections of the 
several parts of the cortex with each other. All our knowledge goes to 
show that psychical processes are dependent on, or are in some way asso- 
ciated with the cortex ; but whatever classification of psychical functions 
we adopt, we are wholly unable to make out any localization of functions, 
such as we can make out for movements, visual sensations and the like. 
Even taking the broad and elementary division into " the emotions" and 
"the intellect," we cannot satisfactorily allot either division to any particu- 
lar part of the hemisphere. In dogs, removal of particular parts of the 
hemispheres has indeed been observed to change the character of the ani- 
mal, converting for instance a vicious, morose dog into a mild and inoffen- 
sive one; and removal of the front part of the hemisphere seems to have 
frequently a marked effect in rendering the animal more impressionable and 
excitable; he becomes much more demonstrative and " gushing " in his 
behavior than before. But these are mere bints, and the clinical histories 
of disease in man do not enable us to say much more. Such knowledge as 
we do possess rather tends to show that the psychical processes in proportion 
as they become more complex involve a greater number of nervous factors, 
and, therefore, have for their material basis a greater width of nervous area, 
or in other words their localization becomes less definite. Thus, while we 
may localize the beginning of a psychical process, a visual sensation for 
instance, and one of its terminal acts, such as the issue of impulses along 
the pyramidal tract, we cannot put our finger on the seat of the interme- 
diate transactions. These even in the simplest process must be complex, 
and must involve many factors. Our simplest conceptions of the external 
world are based on a combination of visual sensations and tactile sensations. 
It being granted that the visual sensation, in one phase of its development, 
is connected with certain changes in some spot of the occipital cortex, there 
must be some tie between this and the corresponding nervous seat of the 
tactile sensation wherever that may be, and further ties between these and 
other parts of the cortex. Hence, as we said, the psychical process is a 
function of connections. 
Many of these ties are most probably furnished by the association fibres 
passing from one part of the cortex to a neighboring part. We must also 
probably admit that impulses, or, to use a more general word, processes, may 
travel laterally along the tangle of the cortical gray matter, for this, like 
the gray matter of the spinal cord, seems to form a physiological continuity, 
no more broken by the fissures than is the cord by its segmental arrange- 
ment ; and we know nothing as to the limits which must be placed on the 
distance to which such processes may travel from their focus of origin. 
Further, seeing how completely in the dark we are as to the reason why we 724 
THE BRAIN. 
possess two hemispheres, and especially seeing that, as shown by speech, the 
whole of each hemisphere is not identical in action with the whole of the 
other, we may perhaps suppose that the fibres of the corpus callosum, which 
form so large a part of the central white matter of the hemisphere, have 
other duties than that of merely keeping the points of one hemisphere in 
touch with the corresponding points of the other hemisphere. But, when 
we have made every allowance for all these direct intercortical connections, 
we are driven to the conclusion that the indirect ties between one part of 
the cortex and another through the lower parts of the brain are of no less, 
perhaps of greater importance. This, indeed, is shown by the relations of 
the motor region. We have already urged, that even as regards the mere 
carrying out of a skilled movement (and we may add, whether that be vol- 
untary or involuntary in the ordinary common use of the words) the motor 
region must have other ties with the part moved than merely the efferent 
tie of the pyramidal fibres; it must have sensory afferent ties, and the 
course of these, including perhaps even those which belong to the muscular 
sense, we may regard as an indirect one along the spinal cord and middle 
parts of the brain, though the details are as yet unknown to us. It must, 
moreover, as we have also seen, have ties, at least in many cases, with parts 
other than the part moved, for instance with the general coordinating 
machinery. And the ease with which some not very obvious change will 
permit the stimulation of a limited motor area to start epileptiform convul- 
sions shows how many and close are the ties in another direction. Further, 
when we go beyond the final phases of the process in the motor cortex to 
those which precede the issue of the efferent impulses, we find the ties mul- 
tiplying. For instance, since our movements are so largely guided by visual 
sensations, there must be ties between the motor cortex and the central 
visual apparatus, it may be of the occipital cortex, but it may also be of 
the lower visual centres. As we insisted, the motor area is only a link in a 
complex chain ; and what we can see, dimly though it be, in reference to 
the cortical motor processes, probably holds good for those other cortical 
processes as well, of whose nervous genesis we know at present nothing. 
Hence even the higher psychical events cannot truly be spoken of as func- 
tions of the cortex, meaning that they are simply the outcome of molecular 
changes in the cortical gray matter ; they are rather to be regarded as the 
outcome of complex processes in which the parts of the brain below the 
cortex play a part no less important than that of the cortex itself. If so, 
the fibres passing down from the cortex to the middle brain have functions 
by which they take part even in our psychical life, functions for which 
neither the words motor nor sensory are fitting. 
On the Time taken up by Cerebral Operations. 
§ 603. We have already seen (§ 507) that a considerable time is taken 
up in a purely reflex act, such as that of winking, though this is perhaps 
the most rapid form of reflex movement. When the movement which is 
executed in response to a stimulus involves cerebral operations a still longer 
time is needed ; and the interval between the application of the stimulus 
and the commencement of the muscular contraction varies according to the 
nature of the mental labor involved. 
The simplest case is that in which a person makes a signal immediately 
that he perceives a stimulus-ex. gr., closes or opens a galvanic circuit the 
moment that he feels an induction-shock applied to the skin, or sees a flash 
of light, or hears a sound. By arrangements similar to those employed in 
measuring the velocity of nervous impulses, the moment of the application 725 
ON THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 
of the stimulus and the moment of the making of the signal are both re- 
corded on the same travelling surface, and the interval between them is 
carefully measured. This interval, which has been called the " reaction 
period " or " reaction time," may be divided into three stages : 1. The time 
during which afferent impulses are generated in the peripheral sense organs 
and transmitted along the afferent nerves to the central nervous system ; this 
may be called the "afferent stage." 2. The time during which, through 
the operations of the central nervous system, the afferent impulses are trans- 
formed into efferent impulses; this maybe called the "central stage." 
3. The time taken up by the passage of the efferent impulses along the 
efferent nerves and the transformation of the nervous impulses into mus- 
cular contractions ; this may be called the " efferent stage." In the efferent 
stage the events are comparatively simple, and though not absolutely con- 
stant, do not vary largely ; we are able to form a fairly satisfactorily esti- 
mate of its duration, ami so of the share in the whole reaction period which 
may be allotted to it. The events of the afferent stage are much more com- 
plex, and the estimates of its duration, being arrived at in an indirect 
manner and chiefly based upon calculations of the whole reaction time, are 
very uncertain. Hence all attempts to estimate the length of the " central " 
stage, the " reduced reaction period," as it is sometimes called, by subtract- 
ing the efferent and afferent stages, must be subject to much error. But a 
good deal may be learned by studying the variations under different cir- 
cumstances of the reaction period as a whole. 
Taking first of all the cases in which the events of the central stage are 
simple, such as those where the subject has merely to make a signal upon 
feeling a sensation, we find that the length of the reaction period is dependent 
on the intensity of the stimulus, being shorter with the stronger stimulus. 
But variations in the strength of the stimulus, especially in the case of 
minimal stimuli, have a much more striking effect in determining the cer- 
tainty of the reaction than in affecting the length of the period. Thus, when 
the signal is made in response to some visual sensation, upon seeing an elec- 
tric spark, for instance, if the spark be a very weak one the subject of the 
experiment often fails to make a signal at all, though he may rarely fail if 
the spark be a strong one. 
Some of the most marked variations in the length of the reaction period 
are determined by the individuality of the subject. Thus, with the same 
stimulus applied under the same circumstances, the reaction period of one 
person will be found very different from that of another. 
The length of the reaction period varies also according to the nature and 
disposition of the peripheral organs stimulated. In general, it may be said 
that cutaneous sensations produced by the stimulus of an electric shock 
applied to the skin (the signal, for instance, being made by the right hand 
when the shock is felt by the left hand) are followed by a shorter reaction 
period than are auditory sensations, while the period of these is in turn 
shorter than that of visual sensations produced by luminous objects; on the 
other hand, the shortest period of all is said to be that of visual sensations 
produced by direct electrical stimulation of the retina. Roughly speaking, 
wre may say that the reaction period is for cutaneous sensations one-seventh, 
for hearing one-sixth, and for sight one-fifth of a second. 
Practice materially shortens the reaction period ; indeed, after long prac- 
tice, making the signal, at first a distinct effort of the will, takes on the cha- 
racters of a reflex act, with a correspondingly shortened interval. Lastly, 
we may add that in the same individual and with the same stimulus, the 
length of the period will vary according to circumstances, such as the time of 
year, the weather, and the like, as well as according to the condition of the 726 
THE BRAIN. 
individual, whether fresh or fatigued, fasting or replete, having taken more 
or less alcohol, and the like. 
The reaction period of vision has long been known to astronomers. It 
was early found that when two observers were watching the appearance of 
the same star a considerable discrepancy existed between their respective 
reaction periods, and that the difference, forming the basis of the so-called 
" personal equation," varied from time to time, according to the personal 
condition of the observers. 
§ 604. The events taking place in the central stage are, of course, com- 
plex, and this stage may be subdivided into several stages. Without at- 
tempting to enter into psychological questions, we may at least recognize 
certain elementary distinctions. The afferent impulses started by the stimu- 
lus, whatever be their nature, when they reach the central nervous system 
undergo changes, and as we have seen, probably complex changes, before 
they become sensations ; and further changes, now of a more distinctly 
psychical character, are necessary before the mind can duly appreciate the 
characters of these sensations and act accordingly. Then come the psychical 
processes through which these appreciated sensations, or perceptions, or 
apperceptions, as they are sometimes called, determine an act of volition. 
Lastly, there are the executive processes of volition, the processes which, 
psychical to begin with, end in the issue of coordinate motor impulses, or, 
in other words, start the distinctly physiological processes of the efferent 
stage. We may thus speak of the time required for the perception of the 
stimulation, of the time required for the action of the will, and of the time 
required for the complex psychical processes which link these two together. 
Accepting this elementary analysis, it is obvious that the total length of the 
central stage may be varied by differences in the length of each of these 
parts; and a more complete analysis would, of course, open the way for 
further distinctions. Hence, by studying the variations of the whole reac- 
tion time under varying forms of psychical activity, we may form an esti- 
mate of time taken up by various psychical processes. 
We may take as an instance the case in which the subject of the experi- 
ment has to exercise discrimination. The mode of making the signal being 
the same, and the stimulus being of the same order in each trial-that is to 
say, visual, or cutaneous, or auditory, etc.-and general circumstances 
remaining the same, two different stimuli are employed, and the subject is 
required to make a signal in response to the one stimulus, but not to the 
other; the subject has to discriminate between the psychical effects of the 
two stimuli. Suppose, for example, the stimulus is the sound of a spoken or 
sung vowel, and the subject is required to make a signal when a is spoken 
or sung, but not when o is spoken or sung. If the subject's whole reaction 
period be determined (1) in the usual way, with either a or o spoken (and 
the result will be found not to differ materially whether a or o be used), the 
subject knowing that only a or only o will bespoken, and then be determined 
again (2) when he has to discriminate in order that he may make the signal 
when a is spoken, but not when o is spoken, he not knowing which is about 
to be spoken, the whole reaction period will be found to be distinctly longer 
in the second case. The experiment may be varied by making use of all the 
vowel sounds taken irregularly as the stimulus, the subject responding by a 
signal to one only, as arranged beforehand. And, of course, other orders 
of stimulus may be used, either visual, the signal being made when a red 
light is shown but not when other colors are shown, or tactile, the signal 
being made when one part of the body is touched but not when other parts 
are touched, and the like. 
In such experiments, where the subject has to distinguish, to discrimi- 727 
ON THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 
nate, between two or more events, the prolongation of the reaction period is 
obviously due to the longer time required for the psychical processes taking 
place during what we have called the central stage. In the two cases, one 
without and the other with discrimination, not only are the afferent and 
efferent stages the same in both, but we have no reason to suppose that in 
the central stage there is any difference between the two cases as to the time 
taken up bv the transformation of simple sensory impulses into perceptions, 
or as to that taken up by the will in gaining access to the motor apparatus 
and so starting the processes of the efferent stage; the delay takes place in 
the psychical processes intervening between these two parts, and the amount 
of delay is the measure of the time needed for the processes involved in the 
discrimination. This " discrimination period " has been found to differ in 
the same individual according to the sensation employed, visual, auditory, 
etc., and according to the kind of difference in the sensation which has to 
be discriminated, for instance in visual sensations between colors or between 
objects in different parts of the field of vision. In a series of observations 
made in this way, the discrimination period, i. e., the prolongation of the 
simple reaction period due to having to discriminate, was found to range 
from 0.011 second to 0.062 second. 
Another series of observations may be made in the following way : The 
signal being one made with the hand, the simple reaction period for a 
stimulus is determined with the signal given by the right hand. Two kinds 
of stimuli are then employed, both of the same order, two vowel sounds for 
instance, and the subject is directed to respond to one vowel with the right 
hand and to the other with the left hand. It is found, the subject being 
right-handed, that the reaction period is greater when the signal is made 
with the left hand. In this case the delay takes place not in the recognition 
of the effects of the stimulus, nor in the processes through which the will is 
formed upon that recognition ; these are the same in the two cases; it takes 
place in the processes by which the will is brought to bear on the nervous 
motor apparatus for making the signal, on the cortical origin, for example, 
of the pyramidal tract; these processes take a longer time in the case of the 
unaccustomed left hand than in the case of the usual right hand. In this 
way we obtain a measure, so to speak, of the volitional side of psychical 
processes. 
In a somewhat similar way we may obtain a measure of the time re- 
quired for perception. A strong sensation following too closely upon a weak 
one will prevent the psychical recognition of the weaker one. If, for 
instance, two or three letters in white on a black background be presented 
to the eye, and a large white surface be presented afterward at an interval 
which is made successively shorter and shorter, it is found that when the 
interval is made very brief indeed the letters cannot be perceived at all. 
In proportion as the interval is prolonged, the recognition of the letters in- 
creases, until at an interval of about 0.05 second they are fully and clearly 
recognized. That is to say, the time required for perception is in such a 
case of about that length. 
The duration of all these psychical processes, as of the simple reaction 
period itself, varies of course under different circumstances, and the dis- 
crimination period may be conveniently used for measurements of the vary- 
ing effects of circumstances. Practice shortens the discrimination period as 
it does the simple reaction period. One of the most powerful influences is 
that of attention. And it is stated that the shortening of the period is 
greater when the attention is concentrated on the making of the signal than 
when it is more especially directed to recognition of the stimulus; in other 
words, the volitional processes are more amenable than are the perceptive 728 
THE BKA1N. 
processes to the psychical action which we call attention. On the other 
hand, the period is distinctly prolonged if the observer be distracted by con- 
comitant sensations. For example, the period for discriminating between 
two visual sensations is prolonged if powerful auditory sensations be excited 
at the same time. 
The same method of measurement may be used in other ways and under 
other circumstances with reference to psychical processes. It must be re- 
membered, however, that all such observations are open to many fallacies 
and need particular caution. It not unfrequently happens that false results 
are obtained ; for instance, the subject, expecting the stimulus to be brought 
to bear upon him and straining his attention, makes the signal before the 
stimulus actually comes off. And the interpretation of the results obtained 
are in many cases very difficult; but it would be out of place to dwell upon 
these matters any further here. 
The Lymphatic Arrangements of the Brain and Spinal Cord. 
§ 605. The membranes of the brain and spinal cord. The cerebro-spinal 
canal is lined by a tough lamellated membrane, composed of connective 
tissue with a small amount of elastic network, called the dura mater, which, 
somewhat closely adherent to the walls of the cranial cavity, is separated 
from those of the vertebral canal by a considerable space, containing blood- 
vessels, especially large venous sinuses, and some fat. It may be considered 
as a development of the periosteum lining the cerebro-spinal cavity. It 
sends tubular sheaths for some distance along the several cranial and spinal 
nerves ; and forms between the cerebral hemispheres, in the longitudinal 
fissure, a conspicuous sickle-shaped vertical fold, the falx cerebri, as well as 
a smaller horizontal or oblique fold between the cerebellum and cerebrum 
known as the tentorium. 
The vascular pia mater is closely attached to the surface of the brain and 
spinal cord, dipping down, as we have seen, into the ventral or anterior fissure 
of the cord as well as into the fissures of the brain. Sheath-like investments 
of pia mater are continued along the several nerves as they leave the cerebro- 
spinal cavity; and in the vertebral canal an imperfect partition half-way 
between the dorsal and ventral surfaces of the cord is furnished by a mem- 
brane of connective tissue which, continuous along its whole length with the 
pia mater, is attached to and fused with the dura mater at intervals only, 
namely, between the successive nerve-roots. Since its outer edge has thus a 
toothed appearance, this membrane is called the ligamentum denticulatum. 
Between the pia mater next to the brain and cord and the dura mater next 
to the bony walls is a cavity, which is divided into two by a thin membrane, 
the arachnoid, composed of interwoven bundles of connective tissue. The 
space between the arachnoid and the dura mater is called the subdural space, 
and the space between the arachnoid and the pia mater is called the sub- 
arachnoid space. When the brain is exposed by removing the roof of the 
skull and slitting open the dura mater, the subdural space is laid bare, and 
the arachnoid is seen stretching over the pia mater; in the vertebral canal 
the arachnoid lies close to the dura mater, so that usually, when the dura 
mater is slit open and turned back, the arachnoid is carried with it and the 
cavity exposed is that of the subarachnoid space. The arachnoid, like the 
dura mater and the pia mater, is continued for some distance over the nerves 
as they leave the cerebro-spinal cavity; so that each nerve at its exit is sur- 
rounded by a tubular prolongation of the subdural space, and within this a 
similar tubular prolongation of the subarachnoid space. 
The subdural space is broken up to a slight extent only by bridles carry- 729 
THE LYMPHATIC ARRANGEMENTS OF THE BRAIN. 
ing nerves and bloodvessels, especially venous sinuses, between the pia mater 
and dura mater, and over the surface of the brain by villus-like projections 
of the arachnoid called Pacchionian glands, some of which pierce the venous 
sinuses of the dura mater. It is lined throughout, both on its dural and on 
its arachnoid wall, by an epithelium of Hat epithelioid cells, and may be 
compared to a serous cavity, such as that of the peritoneum. Like the serous 
cavities it contains normally a small quantity only of fluid, and its size is 
potential rather than actual. 
The subarachnoid space on the other hand is, especially in certain regions, 
such as the dorsal portions of the vertebral canal and the base of the brain, 
much broken up by bridles of connective tissue passing from it to the pia 
mater, as well as by a network of sponge-like arrangement of bundles of 
connective tissue lying immediately beneath itself, and giving it, when viewed 
from below, a honeycomb or fenestrated appearance. The under surface of 
the membrane itself, as well as all the trabeculae of the sponge-work and the 
bridles, are covered with an epithelium of flat epithelioid cells, which is con- 
tinued also over the pia mater and the ligamentum denticulatum, and lines 
the tubular sheath-like prolongations of the space along the issuing nerve- 
roots. The subarachnoid space, therefore, like the subdural space, may be 
regarded as a serous or large lymphatic space, but it is an actual not a mere 
potential space ; it always contains an appreciable quantity of fluid, which, 
however, is not ordinary lymph, but is furnished in a particular way and 
deserves special study. To understand the nature and origin of this cerebro- 
spinal fluid, as it is called, we must turn to some special arrangements of the 
pia mater. 
§ 606. The pia mater proper, consisting of interwoven bundles of con- 
nective tissue with some elastic fibres and a considerable number of 
connective-tissue corpuscles, serves, as we have said, as the bearer of blood- 
vessels to the nervous structures which it invests. The small arteries, as 
they pass into the nervous substance by way of the septa, are surrounded by 
perivascular lymphatic canals, with which spaces in the neuroglial ground- 
work, both of the brain and spinal cord, especially spaces surrounding the 
larger nerve-cells, are continuous. As is the case with other tissues, so with 
the central nervous system, the several elements of the tissue are bathed 
with lymph derived from the blood ; and this, oozing through the spaces into 
the perivascular canals and the other lymphatic vessels of the pia mater, 
makes its way into the subarachnoid space ; but the fluid in the subarachnoid 
space has other sources besides. 
The roof of the fourth ventricle is, as we have said (§ 514), reduced to a 
single layer of non-nervous columnar epithelium, which appears as a mere 
lining to the pia mater overlying it. In the hinder part of the ventricle this 
roof is perforated by a distinct narrow oval orifice, the foramen of Majendie. 
By this orifice, which passes right through both the pia mater and the 
underlying layer of epithelium, the cavity of the fourth ventricle, and so the 
whole series of cavities derived from the original medullary canal, the lateral 
and third ventricles, the aqueduct, and the central canal of the spinal cord, 
are made continuous with the subarachnoid space. There are also other less 
conspicuous communications between the subarachnoid space ami the fourth 
ventricle. Hence the cerebro-spinal fluid is made common to all these cavi- 
ties, and is furnished not only by the pia mater investing the outside of the 
brain and spinal cord, but also, and indeed probably to a larger extent, by 
the epithelium lining the several cavities of the cerebro-spinal axis, especially 
perhaps by those portions of that epithelium which coat the processes of pia 
mater projecting into those cavities at certain places. 
We saw previously (§ 515) that a large fold of the pia mater, carrying in 730 
THE BRAIN. 
with it the thin non-nervous epithelium which alone represents at the place 
the original wall of the medullary canal, is thrust inward at the transverse 
fissure of the brain, beneath the fornix, to form the velum interpositum, thus 
supplying a roof to the third ventricle, and that it thence.projects into each 
lateral ventricle as the choroid plexus of each side, reaching from the fora- 
men of Monro in front along the edge of the fornix to thetipof the descend- 
ing horn. The velum being a fold of the pia mater consists theoretically of 
two layers, and between the upper dorsal layer and the lower ventral layer 
lies a thin bed of connective tissue carrying arteries forward from the hind 
edge of the corpus callosum, and similarly carrying veins backward ; these 
vessels supply the choroid plexus with an abundant supply of blood. In the 
choroid plexus the folded pia mater is developed into a number of villus- 
like processes, the primary processes bearing secondary ones. Each process 
consists, like a villus, of a basis of connective tissue, in which the bloodvessels 
end in close-set capillary loops, covered with an epithelium. The epithelium, 
though continuous with the rest of the epithelium lining the lateral ventricle, 
and thus, as we have said, shutting off the lateral from the third ventricle 
(except at the foramen of Monro), and though like it derived from the wall 
of the original medullary canal, is different in structure. Over the ventricle 
generally the epithelium consists of ordinary short columnar, apparently 
ciliated cells, with more or less transparent cell substance; the cells over the 
choroid plexus are cubical, often irregular in form, and their cell substance 
is loaded with granules, some of which are pigmentary. They have very 
much the appearance of "active" secreting cells; and indeed a branched 
process of the plexus may be compared to an everted alveolus of a secreting 
gland, with the epithelium outside and the bloodvessels within. It cannot 
be doubted that these cells play an important part in secreting into the cavity 
of the ventricle fluid which, passing thence by the foramen of Monro into 
the third and so into the fourth ventricle, finds its way by the foramen of 
Majendie into the subarachnoid space. 
As the velum overhangs the third ventricle it sends down vertically two 
longitudinal linear fringes, which, resembling in structure the choroid plex- 
uses of the lateral ventricle, are called the choroid plexuses of the third ven- 
tricle. From the roof of the fourth ventricle there hangs down on each side 
a similar linear fringe, the choroid plexus of the fourth ventricle, which is 
especially developed at its front end beneath the overhanging cerebellum. 
These subsidiary choroid processes doubtless assist in furnishing cerebro- 
spinal fluid, but their share is small compared with that of the main choroid 
plexuses of the lateral ventricle. 
$ 607. The cerebrospinal fluid. The specimens of cerebro-spinal fluid 
which have been examined as to their composition are not quite comparable 
with each other, since while some (such as those obtained from cases where 
a fracture of the base of the skull has placed the subarachnoid space at the 
base of the brain, where it is largely developed, in communication with the 
external meatus, and the fluid escapes by the ear) may be regarded as 
normal, others (such as those obtained from cases of hydrocephalus where 
the ventricles contain an unusual quantity of fluid, or from cases of spinal 
malformations) must be considered as abnormal. In most of the more com- 
plete analyses, the fluid examined has belonged to the latter class; and the 
following statements apply, strictly speaking, to them alone. 
With this caution, we may say that cerebro-spinal fluid is a transparent 
colorless or very slightly yellowish fluid, of faint alkaline reaction, free from 
histological elements. The specific gravity is about 1010 or less, the amount 
of solids being on an average 1 per cent. Of these by far the greater part, 
0.8 or 0.9 per cent., is supplied by salts, the total quantity of which as well THE LYMPHATIC ARRANGEMENTS OF THE BRAIN. 
731 
as the relative amount of the several constituents being about the same as 
obtain in blood and lymph. The comparative deficiency of solids is due to 
the scantiness of the proteids, which rarely exceed 0.1 per cent. These are 
chiefly globulin and a form of albumose, or even peptone; albumin is said 
to be generally absent. The fluid, save apparently in exceptional cases, does 
not clot, and contains neither fibrogenous factors nor fibrin ferment. It 
very frequently contains a substance which like dextrose reduces Fehling's 
solution but which is not sugar; it appears to be pyrocatechin or a closely 
allied body. 
Seeing that a fluid of such a composition is of a different nature from 
ordinary lymph, furnished entirely in the ordinary way, we might be inclined 
to infer that probably a very large part of the whole mass of the fluid is 
furnished by the secreting epithelium of the choroid plexus. But it must 
be borne in mind, that the foregoing analyses refer chiefly to fluid appearing 
under abnormal circumstances, and it would be hazardous to draw any wide 
inference from them. We have little or no exact experimental evidence as 
to how much fluid is actually secreted by the choroid plexuses; and if the 
fluids which have been analyzed do represent a mixture of ordinary lymph 
supplied through the pia mater with the peculiar secretion of the choroid 
plexus and cerebro-spinal canal, some further change beyond the mere min- 
gling of the two fluids is needed to explain the remarkable absence of albumin 
which has been so strongly insisted upon by various authors. 
§ 608. We may fairly suppose that during life the fluid is continually 
being supplied, from the one source or the other ; but we have no very exact 
knowledge as to the rate at which it is furnished. In the dog, the fluid has 
been observed to escape at a rate varying very largely under different cir- 
cumstances, and ranging from 1 c.c. in forty minutes to as much as 1 c.c. in 
six minutes, the total quantity discharged in twenty-four hours varying from 
36 c.c. to 240 c.c. In the cases of fracture of the base of the skull men- 
tioned above, a very considerable flow7 has been frequently observed ; but it 
may be doubted whether the abnormal circumstances of such cases have not 
raised the secretion above normal. The rate of flow was found in the dog 
to be much increased by the injection of substances (normal saline solution) 
into the blood, but to be relatively little influenced by artificial heightening 
of arterial pressure. This has been put forward as indicating that the fluid 
is chiefly furnished as a secretion and not as an ordinary transudation of 
lymph ; but it cannot be regarded as affording a valid argument. The pres- 
sure under which the fluid exists is also very variable ; it is closely dependent 
on the vascular arrangements of which we shall have to speak presently. 
In the dog the average pressure has been estimated at about 10 mm. of 
mercury. 
If the fluid is thus continually formed it must always find a means of 
escape. This is probably supplied by the tubular prolongations of the sub- 
arachnoid space along the nerve-roots; these are continuous with the lym- 
phatic vessels of the nerves, and so with the lymphatics of the body generally ; 
and in the skull, the passages of this kind along the cranial nerves, especially 
along the two optic nerves into the orbits, afford a ready means of escape. 
It is also urged that some of the fluid escapes through the Pacchionian 
glands directly into the blood of the venous sinuses. In a dead body fluid 
introduced into the subarachnoid space through an opening over the bulb, 
disappears at even a very low pressure with great rapidity. The circum- 
stances then are, however, not the same as in life; and the few experiments 
which have been made seem to show7 that, during life, a somewhat high 
pressure is required to secure the escape of fluid introduced in addition to 
that naturally secreted. Thus it is stated that when in a dog normal saline 732 
THE BRAIN. 
solution is introduced into the subarachnoid cavity at the lower end of the 
spinal cord very little resorption takes place so long as the pressure remains 
as low as about 10 c.c. of mercury; as the pressure is increased beyond this 
resorption quickly increases. But it may be doubted whether the resorption 
of added fluid is a fair test of the escape of fluid naturally present; and the 
experiment is of value rather as showing simply that there are means of 
escape than as affording a measure of the rate of escape. Besides, the 
immediate effects of applying pressure at the caudal end of the spinal cord 
are not the same as those of applying the pressure within the skull. 
The rate of possible escape is not without importance as regards the 
mechanical importance of the cerebro-spinal fluid. Thus it has been urged 
that when an extra quantity of blood is driven into the skull, any injurious 
intercranial compression is prevented, not only by the transference of a cor- 
responding quantity of cerebro-spinal fluid through the foramen of Majendie 
from the cranium into the spinal canal, the walls of which are less rigidly 
complete, but also by the direct escape of the fluid from the cavity of the 
skull along the cranial nerves in the manner described. It has also been 
urged that the fluid at the base of the skull, in the large subarachnoid spaces 
of which it gathers in larger quantity than elsewhere, acts as a sort of pro- 
tective water cushion to the delicate cerebral substance, and that, in general, 
the presence of the fluid is mechanically useful to the welfare of the brain, 
removal of the fluid by aspiration being said to lead to hemorrhage from the 
pia mater and to various nervous disorders. But our knowledge as to the 
part which the fluid plays is at present very imperfect; and its very peculiar 
chemical characters suggest that it has some chemical functions. 
The Vascular Arrangements of the Brain and Spinal Cord. 
§ 609. The bloodvessels reach the nervous structures by means of the 
pia mater. In the spinal cord arteries coming from the vertebral, inter- 
costal, and other arteries, and travelling along the nerve-roots join the pia 
mater, and then through the fissures and septa reach all parts of the cord ; 
but, as we have previously remarked, the capillary network is much denser, 
and, therefore, the blood-supply much greater in the gray than in the white 
matter. The veins, also gathered up along the septa and fissures into the 
pia mater, those coming from the gray matter forming, before they reach the 
external pia mater, a conspicuous longitudinal vein on each side of the poste- 
rior gray commissure, pass from the pia mater to the large venous sinuses 
of the dura mater and so to adjoining veins. 
In the brain two important features of the distribution of the arteries 
deserve special attention. In the first place, the quadruple supply by the 
right and left vertebral and internal carotid arteries is made one by remark- 
able anastomoses forming the circle of Willis. The right and left vertebral 
arteries entering the vertebral canal at the level of the sixth cervical ver- 
tebraand running forward toward the brain, join beneath the ventral surface 
of the bulb to form the single median basilar artery. This, after giving off 
branches to the bulb, cerebellum, and pons, divides into the right and left 
posterior cerebral arteries. Each internal carotid entering the skull reaches 
the base of the brain in the region of the floor of the third ventricle, and, 
passing ventral to and athwart the optic tract, gives off the large and impor- 
tant middle cerebral artery along the fissure of Sylvius, and, then, turning 
forward and toward the median line, passes dorsal to the optic nerve to end 
in the anterior cerebral artery. Just, however, as it gives off the middle 
artery, it sends backward, inclining to the middle line, a relatively large 733 
THE VASCULAR ARRANGEMENTS OF THE BRAIN. 
branch, the posterior communicating artery, which joins the posterior cere- 
bral near the origin of this from the basilar artery. Moreover, the two 
anterior cerebral arteries, soon after they have crossed the optic nerves, just 
as they are about to run straight forward along the frontal lobes, are joined 
together by a short, wide branch, the anterior communicating artery. In 
this way the vertebral arteries through the basilar artery join with the carotid 
arteries to form around the optic chiasma beneath the floor of the third ven- 
tricle an arterial circle, the circle of Willis. 
Blood can pass along this circle in various ways-from the basilar artery 
along the right posterior communicating artery to the right internal carotid, 
and so by the right anterior cerebral artery and anterior communicating 
artery to the left side of the circle, and similarly from the basilar artery 
along the left side to the right, or from the right or from the left carotid 
through the circle, to the right hand or to the left hand in each case. Since 
the channel of the circle is a fairly wide one, the passage in various direc- 
tions is an easy one ; all the vessels radiating from the circle, including the 
basilar artery and its branches, can be supplied by the carotids alone, or by 
the vertebrais alone, or even by one carotid or one vertebral alone. In this 
way an ample supply of blood to the brain is secured in the face of any 
hindrance to the flow of blood along any one of the four channels. 
In what may, perhaps, be considered the usual arrangement, the calibre 
of the posterior communicating arteries is rather smaller than the other parts 
of the circle, so that, other things being equal, most of the vertebral blood 
will pass by the posterior cerebral arteries, while the carotid blood passes to 
the middle and anterior cerebral arteries ; but many variations are met with. 
We may also here, perhaps, call to mind the fact that the left carotid com- 
ing off from the top of the aorta offers a shorter path for the blood than 
does the right carotid which comes off from the innominate artery. 
Another special feature of the arterial supply to the brain is that the 
three large cerebral arteries-posterior, middle, and anterior-are distrib- 
uted almost exclusively to the cortex and to the subjacent white matter, 
while the deeper parts of the hemisphere, the nucleus caudatus, thalamus, 
and the like, with the capsule and other adjoining white matter, are supplied 
by smaller arteries coming direct from the circle of Willis, or from the very 
beginnings of the three cerebral arteries. It is stated that these two sys- 
tems make no anastomoses with each other; but this appears to vary much 
in different individuals. We may add that the anterior cerebral artery 
supplies the cortex of the dorsal aspect of the frontal lobe as well as the 
front and middle portions of the whole mesial surface of the hemisphere; 
while the middle cerebral, always large, is distributed to the side of the 
brain, that is, the parietal lobe, with the ventral part of the frontal lobe 
and the dorsal part of the temporal lobe; the posterior cerebral sup- 
plying the rest of the cortex, that is to say, the occipital lobe, including the 
hind part of the mesial surface of the hemisphere, together with the ventral 
part of the temporal lobe. The distribution of these arteries, therefore, does 
not correspond to functional divisions, for while the middle cerebral supplies 
a large part of the motor region, it does not supply the whole of it, and does 
supply parts outside of it. Though the small arteries as they run in the pia 
mater on the surface of the cortex anastomose freely, there is very little 
anastomosis between the small arteries which, leaving the pia mater, dip 
down into the substance of the brain ; hence, when these latter arteries are 
blocked, the nutrition of the part of the cortex supplied by them is apt to 
be impaired. 
§610. The venous arrangements of the brain have very special cha- 
racters. 734 
THE BRAIN. 
Along the upper convex border of the sickle-shaped fold of dura mater, 
the falx cerebri, is developed a large venous sinus, the superior longitudinal 
sinus. This, triangular in section, increasing in calibre from before back- 
ward, is a sinus, not a vein ; its walls are formed of nothing but connective 
tissue lined with epithelium, muscular elements being entirely absent. 
Though its channel is broken by bridles of connective tissue passing across 
it, it possesses no valves, and, indeed, these are absent from all the sinuses 
and veins of the brain. Most of the blood returning from the cortex and 
subjacent white matter is carried into this sinus by veins, the mouths of 
which are for the most part directed forward, that is to say, against the 
direction of the blood stream. Along the lower concave border of the falx 
is a similar sinus, the inferior longitudinal sinus, which, however, is small, 
and into which relatively few veins open. 
From the deeper parts of the brain, and especially from the choroid 
plexus, blood is conveyed by the veins of Galen along the velum interposi- 
tum to the transverse fissure, where the veins of Galen join the inferior longi- 
tudinal sinus to form the straight sinus. This, running along the line formed 
by the intersection of the vertical falx with the (more or less) horizontal 
tentorium, joins the end of the superior longitudinal sinus to form the reser- 
voir or cellar, called the torcular Herophili, from which the lateral sinus, 
passing on each side along the convex border of the tentorium and gather- 
ing veins from the cerebellum and hind regions, as well as from the base of 
the brain, delivers the blood into the internal jugular vein. 
It should be added that veins from the aiose and, through the ophthalmic 
veins, from the face join the veins and sinuses of the brain, and that the so- 
called emissary veins pass through the cranium from the scalp to the superior 
longitudinal and lateral sinuses. 
The channels for the venous blood of the brain are therefore not veins, 
but sinuses; not so much tubes for maintaining a uniform current, as longi- 
tudinal reservoirs, which, while affording an easy onward path, can also be 
easily filled and easily emptied, and in which the blood can move to and fro 
without the restrictions of valves. This arrangement is correlated to the 
peculiar surroundings of the brain, which is not, like other organs, protected 
merely by skin or other extensible or elastic tissue, but is encased by a fairly 
complete inextensible envelope, the skull. As a consequence of this, when 
at any time an extra quantity of blood is sent from the heart to the brain 
room must be made for it by the increased exit of the fluids already present. 
For any pressure on the brain-substance beyond a certain limit is injurious 
to its welfare and activity, as is seen in certain maladies, where blood pass- 
ing by rupture of the bloodvessels out of its normal channels remains effused 
on the surface of the brain or elsewhere, and thus taking up the room of the 
proper brain-substance leads, by " compression," as it is called, to paralysis, 
loss of consciousness, or death. Some room may, as we have seen (§ 608), be 
provided by the escape of cerebro-spinal fluid from the skull. But, within 
the limits of the normal cerebral circulation, the characteristic venous sinuses 
especially serve to regulate the internal pressure; they form temporary reser- 
voirs from which a comparatively large quantity of blood can be rapidly dis- 
charged from the cranium, the flow7 from the sinuses being greatly assisted 
by the low or negative pressure obtaining in the veins of the neck at each 
inspiratory movement of the chest. 
§ 611. The supply of blood to the brain seems at first sight not to cor- 
respond to the importance of this the chief organ of the body. In the rabbit 
it would appear that hardly more than one per cent, of the total quantity of 
the blood of the body is present at any one time in the brain, a quantity but 
little more than half that which is found in the kidneys; and while the THE VASCULAR ARRANGEMENTS OF THE BRAIN. 
735 
weight of blood in the brain at anyone time amounts to about five per cent, 
of the total weight of the organ, being about the same as in the muscles, in 
the kidney it amounts to nearly twelve per cent., and in the liver to as much 
as nearly thirty per cent. Making every allowance for the relative small 
size and functional importance of the rabbit's brain, the blood-supply of 
even the human brain must still be small ; and making every allowance for 
rapidity of current, the interchange between the blood and the nervous ele- 
ments must also be small. In other words, the metabolism of the brain-sub- 
stance is of importance not so much on account of its quantity as of its special 
qualities. 
The circulation in the brain may be studied by help of various methods. 
A manometer may be connected with the peripheral end of the divided inter- 
nal carotid artery, a second manometer being attached in the usual way to 
the central portion. Since the peripheral manometer records the blood-pres- 
sure in the circle of Willis transmitted along the peripheral portion of the 
carotid artery, variations of pressure in the circle of Willis may thus be 
studied ; and a comparison of the peripheral with the central manometer 
will indicate what general changes are taking place in the circulation 
through the brain. Thus a fall of pressure in the peripheral manometer 
unaccompanied by any corresponding fall in the central manometer would 
show that the " peripheral resistance " in the brain was being lowered, in 
other words, that the vessels were being dilated. 
In another method, in the dog, the outflow of venous blood from the 
lateral sinus through the posterior facial vein has been measured. The 
freedom with which blood passes along the sinuses justifies the assumption 
that the outflow through the open vein gives an approximate measure of the 
rate of flow under natural conditions; still the results are only approximate, 
and besides, the continued loss of blood introduces error. 
A third method is a plethysmographic one. The skull is made to serve 
as the box of the plethysmograph or oncometer (§ 346) ; a small piece of 
the roof having been removed by the trephine, a membrane is fitted to the 
hole, and the movements of the membrane are recorded by help of a piston 
and lever or directly by a lever. In young subjects, the fontanelle, or por- 
tion of the cranium not yet ossified, may be utilized as a natural membrane, 
and its movements recorded in a similar manner. When the instrument is 
fitted to the hole in a water-tight manner, this method records variations in 
internal pressure; and we may take it for granted, unless otherwise indi- 
cated, that greater or less pressure is due to more or less blood passing to 
the brain. But the amount of pressure brought to bear on the recording 
instrument will also depend on the readiness with which the cerebro-spinal 
fluid escapes from the cavity of the skull; if there be a hindrance to the 
escape, or, on the other hand, an increased facility of escape, the same in- 
crease of supply of blood will produce in one case a less, in the other a 
greater movement of the lever. If the membrane be attached loosely to 
tiie hole so as to allow free escape of the cerebro-spinal fluid, the lever 
practically resting on the surface of the cerebral hemisphere, the method 
records variations in the dorso-ventral diameter of the hemisphere, and 
these may be taken as measuring variations in the volume of the brain and 
so in the blood-supply. In neither form, however, does the method by itself 
give us all the information which we want. An increase of blood in the 
brain, and therefore an expansion of the brain, and so a movement of the 
recording instrument, may result either from a fuller arterial supply or 
from hindrance to the venous outflow ; the former condition is, at least in 
most cases, favorable to, the latter always and distinctly injurious to, the 
activity of the nervous structures; hence the teachings of the lever must 736 
THE BRAIN. 
be corrected by a simultaneous observation of the general arterial pressure 
and of the blood-pressure in the veins of the neck. Moreover, the argument 
which we used (§ 353) in reference to the kidney may be applied here and 
probably with equal force, namely, that the value of the blood stream for 
the nutrition of the tissue is dependent not alone on the amount of blood- 
pressure but also and especially on the rapidity of the flow; indeed, this 
second factor is of particular importance in view of the need of supplying 
the nervous elements with an adequate interchange of gases. Now of the 
rapidity of flow the plethysmographic method can give us indirect informa- 
tion only. 
§ 612. By one or other or all of these methods certain important facts 
have been made out. The volume of the brain as determined by the amount 
of blood present in it, is continuously undergoing changes brought about by 
various causes. Each heart-beat makes itself visible on the cerebral as on 
the renal plethysmographic tracing, and as we have seen in speaking of res- 
piration, the diminution of pressure in the great veins of the neck during in- 
spiration leads to a shrinking, and the reverse change during expiration to 
a swelling of the brain. The plethysmograph also shows variations, larger 
and slower than the respiratory undulations, and brought about by various 
causes, such as the position of the head in relation to the trunk, movements 
of the limbs, modifications of the respiratory movements, and apparently 
phases of activity of the brain itself, as in waking and sleeping ; undulations 
corresponding to the Traube-Hering variations (§ 330) of blood-pressure may 
not unfrequently be observed. 
All the various methods show that the flow through the brain is largely 
determined by a vasomotor action of some kind or another. And this we 
might indeed infer from ordinary experience. When the head is suddenly 
shifted from the erect to a hanging position, there must be a tendency for 
the blood to accumulate in the cranial cavity, and conversely when the head 
is suddenly shifted from a hanging to an erect position, there must be a 
tendency for the supply of blood within the cranium to be for a while less 
than normal. Either change of position, and especially perhaps the latter, 
would lead to cerebral disturbances, which in turn would in ourselves be 
revealed by affections of our consciousness. That a perfectly healthy and 
especially young organism, whose vasomotor mechanisms are at once effective 
and delicately responsive, can pass swiftly from one position of the head to 
the other without inconvenience, whereas those in whom the vasomotor 
mechanisms have by age or otherwise become imperfect are giddy when 
they attempt such rapid changes, is in itself adequate evidence of the im- 
portance of the vasomotor arrangements affecting the circulation through 
the brain. The several methods agree in showing that increased general 
arterial pressure, such as that, for instance, induced by stimulation of a 
sensory nerve, leads to a greater flow of blood to the brain ; the volume of 
the brain is increased and the venous outflow by the lateral sinus is quick- 
ened. Conversely, a lowering of arterial pressure leads to a lessened flow 
of blood to the brain. 
Seeing that the cerebral arteries have well-developed muscular coats, the 
basilar artery in fact being conspicuous in this respect, one would be led to 
suppose that the brain possessed special vasomotor nerves of its own ; and 
recognizing the importance of blood-supply to rapid functional activity one 
would perhaps anticipate that by special vasomotor action, the supply of 
blood to this or that particular part of the brain might be regulated apart 
from changes in the general supply. The various observations, however, 
which have hitherto been made have failed to demonstrate with certainty 
any such special vasomotor nerves or fibres directly governing cerebral THE VASCULAR ARRANGEMENTS OF THE BRAIN. 
737 
vessels. It would be hazardous to insist too much on this negative result, 
especially since the observations have been chiefly directed to the nerves of 
the neck, the experimental difficulties of investigating the presence of vaso- 
motor fibres in the cranial nerves being very great. Still it may be urged 
and indeed has been urged that the flow of blood through the brain is so 
delicately responsive to the working of the general vasomotor mechanism 
just because it has no vasomotor nerves of its own. In such au organ as 
the kidney, an increase of general blood-pressure, as we have more than 
once insisted, may or may not lead to a greater flow through the kidney 
according as the vessels of the kidney itself, through the action of the renal 
vasomotor nerves, are dilated or constricted ; and as we have seen, a con- 
striction of the renal vessels may be one of the contributors to the increased 
general pressure. In the brain, on the other hand, an increase of general 
arterial pressure seems always to lead to increase of flow. Thus in the 
Traube-Hering undulations just mentioned, the expansions of the brain 
are coincident with the rises of the general pressure, whereas in the normal 
kidney and in other organs the local Traube-Hering undulation reverses the 
general one, the shrinkings are synchronous with the rises of pressure, the 
local constriction being one of the factors of the general rise. It is argued 
that in the absence of vasomotor nerves of their own, the cerebral vessels 
are wholly, so to speak, in the hands of the general vasomotor system, so 
that when the blood-pressure is high owing to a large vaso-constriction in 
the abdominal viscera, more blood must necessarily pass to the brain, and 
when again the blood-pressure falls through the opening of the splanchnic 
flood-gates (§ 159) less blood necessarily flows along the cerebral vessels. 
And indeed one may recognize here a sort of self-regulating action ; for 
diminishing the supply of blood to the vasomotor centre in the bulb acts, as 
we know, as a powerful stimulus in producing vaso-constriction, and so leads 
to a rise of blood-pressure; but this very rise of blood-pressure drives more 
blood to the brain, including the bulb, and thus the injurious effects to the 
brain threatened by an anaemic condition are warded off by the very be- 
ginning of the anaemia itself. All these advantages are, however, quite 
compatible with the coexistence of special vasomotor mechanisms. 
§ 613. Moreover, the flow of blood to, and consequent change in the bulk 
of, the brain, and indeed the flow of blood through the brain, as measured 
by the venous outflow, may be modified independently of changes in the 
general blood-pressure. For instance, stimulation of the motor region of the 
cortex quickens the venous outflow, without producing any marked change 
in the general blood-pressure ; this feature becomes very striking at the onset 
of epileptiform convulsions when these make their appearance. It is diffi- 
cult not to connect such a result of functional activity with some special 
vasomotor nervous arrangement comparable to that so obvious in the case 
of a secreting gland. Again, it has been observed that certain drugs have 
an effect on the volume of the brain, quite incommensurate with their effect 
on the vasomotor system ; thus in particular the injection into the general 
blood-stream of a weak acid produces a large and immediate expansion of 
the brain, while the introduction of a weak alkali similarly gives rise to 
similar considerable shrinking. It is suggested that these effects are pro- 
duced by the acid or alkali acting directly on the muscular coats of the 
minute arteries and so leading to relaxation or contraction respectively. In 
treating of the chemistry of nervous substance (§ 70) we stated that "the 
gray matter of the central nervous system is said to be slightly acid during 
life and to become more acid after death." Recent observations go to show7 
that the gray matter of the cortex is faintly alkaline during life and under 
normal conditions, but becomes acid after death or when its blood-supply is 738 
SIGHT. 
interfered with; and it has been urged that nervous gray matter like mus- 
cular substance develops acidity during activity as well as upon death, the 
acidity being probably due in each case to some form of lactic acid. And 
just as it has been suggested that the dilatation of the minute arteries of a 
skeletal muscle, accompanying or following the contraction of the muscle, 
is brought about by the acid generated during the contraction causing a 
relaxation of the muscular coats of the minute arteries, so it has been sug- 
gested that a similar acidity, the product of nervous activity, similarly leads 
in nervous tissue to a dilatation of the vessels of the part. The existence of 
special vasomotor mechanisms would, however, afford a more satisfactory 
explanation of these and other phenomena; in spite of the negative results 
so far obtained, the matter is obviously one needing further investigation. 
Meanwhile we have abundant evidence that, however brought about, the 
flow of blood through the brain, and probably through particular parts of 
the brain, is varied in accordance with the needs of the brain itself and the 
events taking place elsewhere in the body. 
CHAPTER III. 
SIGHT. 
§ 614. A ray of light falling on the retina gives rise to what we call a 
sensation of light; but in order that distinct vision of any object may be 
gained, an image of the object must be formed on the retina, and the bettei* 
defined the image the more distinct will be the vision. Hence, in studying 
the physiology of vision, our first duty is to examine into the arrangements 
by which the formation of a satisfactory image on the retina is effected ; 
these we may call briefly the dioptric mechanisms. We shall then have to 
inquire into the laws according to which rays of light impinging on the 
retina give rise to sensory impulses, and those according to which the im- 
pulses thus generated give rise in turn to sensations. Here we shall come 
upon the difficulty of distinguishing between the unconscious or physical 
and the conscious or psychical factors. And we shall find our difficulties 
increased by the fact, that in appealing to our own consciousness we are apt 
to fall into error by confounding primary and direct sensations with states 
of consciousness which are produced by the weaving of these primary sen- 
sations with other operations of the central nervous system, or, in familiar 
language, by confounding what we see with what we think we see. These 
two things we will briefly distinguish as visual sensations and visual judg- 
ments ; and we shall find that both in vision with one eye, but more 
especially in binocular vision, visual judgments form a very large part of 
what we frequently speak of as our sight. 
§ 615. [The eyeball is of as pheroidal shape. It consists of two seg- 
ments of different-sized spheres. The larger segment is situated posteriorly, 
and constitutes about five-sixths of the walls of the eyeball. From its free 
margin projects the smaller segment, which is that of a smaller sphere. The 
posterior segment is composed of a whitish, opaque, firm wall, consisting of 
three coats or tunics-the sclerotic, the choroid, and retina. The anterior 
segment is continuous with the sclerotic coat. (Fig. 157.) It is a trans- 
parent, elastic, convex organ, called the cornea. The cornea consists of three 
layers-an anterior and posterior elastic lamina, having between them a SIGHT. 
739 
layer which is the proper tissue of the organ. This middle layer is com- 
posed of about sixty superimposed laminae of fusiform fibrous cells. In the 
interstices between the laminae are found tubular spaces, which contain a 
transparent fluid. The anterior and posterior elastic laminae are structure- 
less and highly elastic. When separated from the proper corneal tissue they 
have a great tendency to curl up, suggesting that they are active agents in 
the retention of a proper curvature of the cornea. The cornea is covered 
on its anterior surface by the conjunctival mucous membrane, which consists 
of three or four layers of pavement epithelial cells; the deeper layers of 
cells are oblong, and placed perpendicularly. The conjunctiva at this point 
has no perceptible basement membrane. The posterior surface of the cornea 
is covered by a transparent serous membrane, which consists of a simple 
layer of polygonal pavement epithelial cells resting on an elastic mem- 
Fig. 157. 
Diagram of a Horizontal Section of the Eyeball a, outer or sclerotic coat; d, the cornea ; b, 
middle or choroidal coat; m, ciliary ligament; s, ciliary process ; e, ciliary muscle, and/, iris ; c, 
inner coat of retina, continuous with the optic nerve behind, with a dark layer outside it; g, lens; 
t, suspensory ligament of the lens ; h, vitreous body • n, hyaloid membrane ; i, posterior chamber ; 
o, canal of Petit; r, sinus circularis iridis; I, optic nerve. The dotted line through the centre is 
the longitudinal axis of the ball. 
brane. This is called the membrane of Demours. The cornea has no 
bloodvessels, and therefore derives its nutriment by diffusion. 
The sclerotic coat is so named on account of the firmness of its texture and 
of its hardness. It forms the outer tunic of the posterior segment. It is 
whitish, opaque, smooth, excepting at the points of attachment of the muscles 
of the eyeball. It is composed of white fibrous tissue, arranged more or less 
in bundles, which interlace each other in various directions. Anteriorly the 
interlacements are in a generally transverse direction ; posteriorly the direc- 
tion is longitudinal. This coat also contains yellow elastic fibres and 
fusiform nucleated yells. It is continuous anteriorly with the cornea, and 
posteriorly with the perineurium of the optic nerve. At the internal border 
of the junction with the cornea is a venous sinus called the sinus circularis 
iridis or canal of Schlemm. The optic nerve pierces it about 2.6 mm. internal 
to the antero-posterior axis of the eyeball. At this point the coat is perfo- 740 
SIGHT. 
rated by minute openings for the passage of the nerve-filaments. One of 
these openings, which is relatively large, gives passage to the arteria centralis 
retinae. Surrounding this point of entrance of the optic nerve are many 
small openings for the passage of the ciliary nerves and vessels. The in- 
ternal surface of the sclera contains some 
pigment granules. It is separated from 
the choroid coat by a delicate flocculent 
cellular tissue, called the lamina fusca. 
The choroid coat is a vascular membrane 
containing some pigment granules. The 
external portion is composed principally 
of bloodvessels and nerves. Between the 
vessels are found numerous stellate pig- 
ment-cells, which form a fibrous network. 
The internal surface, where it adjoins the 
pigment layer of the retina, also contains 
pigment-cells. Posteriorly it is pierced 
by the optic nerve; anteriority it is con- 
tinuous with the ciliary processes, and is 
separated from the sclerotic coat by the 
ciliary muscle. 
The ciliary processes are arranged in the 
form of a ring. They consist of about sixty 
to eighty somewhat conical-shaped bodies, 
situated with their bases internally. (Fig. 
158.) They are placed posterior to the 
iris, and are attached by their thickened 
or internal extremities to the suspensory ligament of the lens. 
The ciliary muscle arises from the point of junction of the sclerotic coat 
and the cornea. It consists of two portions-a radiating or meridional and 
a circular layer. The radiating fasciculi are situated externally and have a 
meridional direction. (Fig. 159.) From this layer numerous fasciculi inter- 
Fig. 158. 
Inner View of the Front of the 
Choroid Coat with its Ciliary Processes, 
and the Back of the Iris, a, anterior 
piece of the choroid coat; b, ciliary pro- 
cesses; c, iris; d, sphincter of the pupil; 
e, bundles of fibres of the dilator of the 
pupil. 
Fig. 159. 
Section of the Ciliary Region of the Eye in Man. a, meridional muscular fasciculi of the 
musculus ciliaris ; b, deeper-seated radiating fasciculi; c, c. c, annular plexus ; d, annular muscle 
of Muller; f, muscular lamina on the posterior surface of the iris; g, muscular plexus at the 
ciliary border of the iris; e, annular tendon of the musculus ciliaris; h, ligamentum pectinatum. 
lace between the fasciculi of the circular layer, which occupies an internal 
position to the radiating layer. The ciliary muscle is inserted into the 
external surface of the anterior portion of the choroid coat, the fibres extend- 
ing somewhat posterior to the anterior margin of the retina. This muscle is 
a very important factor in the mechanism of accommodation. THE EYEBALL. 
741 
The iris is a fibro-muscular curtain which is suspended between the 
cornea and the crystalline lens. It is attached by its circumference to the 
internal wall of the sinus c. iridis. In its centre is a round perforation called 
the pupil, which is susceptible of considerable variations in size. This mem- 
brane is composed of a fibro-connective tissue having a general radiating 
direction from the pupillary border. Within this tissue are found pigment- 
cells and unstriated muscular tissue. The muscular tissue element consists 
of radiating and circular fasciculi. (Fig. 160.) The circular fasciculi form 
Fig. 160. 
Muscular Structure of the Iris of a White Rabbit, a, sphincter of the pupil; b, b, radiating 
fasciculi of dilator muscle; c, c, connecting tissue with its corpuscles. 
a sphincter at the pupillary margin ; the radiating fasciculi radiate from 
the sphincter to the circumference. At the circumference of the iris the 
membrane lining the anterior chamber forms fibrous processes, which are 
termed the ligamentum. iridis pectinatum. The posterior surface is covered 
with a pigmentary layer, which is a continuation of the pigment layer of 
the retina. 
The retina or third coat consists of two portions: the pigmentary mem- 
brane and terminal elements of the optic nerve. The pigmentary membrane 
or external layer, which has been called the system of the uvea, covers the 
whole of the internal surface of the ciliary processes, the iris, and the 
choroid. It consists of a single layer of hexagonal nucleated pigment-cells 
of a dark-brown color. From the internal surface of this membrane delicate 
fibres are continued between the cellular elements of the nervous layer. It 
is frequently dissected with the choroid coat, and spoken of as one of its 
laminae. The color of the iris in different individuals is dependent upon the 
density of the fibro-connective tissue anterior to the uvea, and to the amount 
of pigment-granules in it. In persons with dark eyes the pigment in this 
tissue is relatively more abundant. 
The internal or nervous layer of the retina is composed essentially of the 
terminal nerve elements of the optic nerve. Externally it is covered with 
the pigmentary layer; internally it is lined by a homogeneous transparent 
structure called the hyaloid membrane. The structure of the retina is one 
of great complexity. It consists of nine distinct layers, seven of which are 
layers of nerve elements. All of these layers are bound together and sup- 
ported by a connective tissue which contains bloodvessels. This layer ex- 742 
SIGHT. 
tends from the entrance of the optic nerve to a point where the annular 
fasciculi of the ciliary muscle are found ; at this point the nervous elements 
cease to exist, and the layer has an irregular dentated margin called the 
ora serrata. Beyond this the nervous layer is continued as a mere fibrous 
extenuation. 
The optic nerve pierces the sclerotic and the choroid coats, and the pig- 
mentary membrane of the retina, when it rapidly divides into vast numbers 
of fibres, which consist alone of the axis-cylinders or their ultimate fibrillae. 
Fig. 161. 
Fig. 162. 
Fig. 163. 
Fig. 161.-Diagrammatic Representation of the Connections of the Nerve-fibres in the Retina. 
1, membrana limitans interna; 2, optic nerve-fibre layer; 3, layer of ganglion cells; 4, internal 
granulated or molecular layer; 5, internal granule layer; G, external granulated or molecular 
layer; 1, external granule layer; 8, membrana limitans exterior; 9, bacillary layer, or layer of 
rods and cones. 
Fig. 162.-Rod and Cone from the Retina of Man, preserved in a two per cent, solution of 
perosmic acid to show the fine fibres of the surface, and the different lengths of the internal seg- 
ment. The outer segment of the cone is broken up into disks, which, however, are still adherent 
to one another ; at the base of the cone are seen a few fine hairs. ( + 1000 diameters.) 
Fig. 163.-Diagrammatic Representation of the Connective Tissue and Radial Fibres of Muller 
of the Retina as seen near the Ora Serrata. The numbers correspond to those of the several 
layers of the retina shown in Fig. 161. 
This layer of fibres is continuous over nearly the whole of the internal sur- 
face, and is called the second or optic nerve-fibre layer. On its internal sur- 
face, between it and the hyaloid membrane, is a delicate structure called the 
first layer, or membrana limitans interna. The third or ganglion layer is com- 
posed of multipolar ganglion cells, similar to those found in the cerebral 
substance. In the posterior portion of the retina these ganglion cells are in 
several layers; at the macula lutea there are as many as eight, and at the 
anterior portion of the retina there is but a single layer. From each of these THE EYEBALL. 
743 
cells fibres are continued to the fifth or internal granule layer, which consists 
of granular cells with nuclei. Between the third and fifth layers is a layer 
of vesicular matter containing nerve-fibrils of extreme minuteness. This 
layer is the fourth or internal granulated or molecular layer. The sixth or 
external granulated or molecular layer consists of parallel interlaced fibres, 
containing nuclei and smooth cells. The seventh or external granule layer is 
very similar to the fifth. The eighth layer consists of a delicate membrane 
of connective tissue, called the membrana limitans externa. The ninth or 
bacillary layer, or layer of rods and cones, or Jacob's membrane, is composed 
of two elements, the rods and cones. These layers are supported by con- 
nective tissue and a peculiar neuroglial structure known as the radial fibres 
of Muller. (Fig. 163.) The rods are cylindrical bodies, each ending exter- 
nally in a truncated, flattened extremity, and internally as an attenuated 
fibre, which probably communicates witii the deeper layer of ganglion cells. 
The cones, as their name indicates, are conical-shaped bodies. Each consists 
of two portions, a conical body having projecting from its apex a rod-like 
segment, which appears in all respects like the rods. This segment is called 
the cone rod. The terminal extremities of the cone rods do not extend as 
far externally as the extremities of the rods. The rods and cones have 
been demonstrated to consist of two segments or limbs, which are com- 
posed of filaments, granular matter, and nuclei. The outer limb of the 
rods contains a pinkish pigment known as " visual purple," which is 
extremely sensitive to light. 
The optic nerve, where it pierces the coats of the eye, projects somewhat 
beyond the surface of the retina, as a papilla; here the essential nerve ele- 
ments of the retina are absent, and luminous 
rays are unperceived; hence, it is called the 
blind spot. About 2.6 mm. external to the 
point of entrance of the optic nerve, and in 
the exact centre of the retinal surface corre- 
sponding to the antero-posterior axis of the eye, 
is the " yellow spot of Sommerring," or macula 
lutea. (Fig. 164.) It is an elliptical-shaped spot, 
having its long diameter transverse. In the 
centre of the macula lutea is a depression called 
the fovea centralis. At this point the nervous 
layer of the retina is very much modified in 
the composition of the different layers. The 
nervous layer is much thicker than at any other 
part of the membrane. The ganglion (third) 
and external granulated (sixth) layers are even 
more thickened. The ganglion layer consists of 
six or eight laminae of cells. The rods of the 
ninth layer are absent, and are replaced by 
cones. In the fovea centralis the internal gran- 
ulated (fourth), the internal granule (fifth), and the optic nerve-fibre 
(second) layers are wanting. The ganglion cell (third), the external gran- 
ulated (sixth), and the external granule (seventh) layers are increased in 
thickness. The ganglion layer of cells in the fovea consists of three 
lamime. In all portions of the nervous layer the rods greatly predomi- 
nate in number over the cones, excepting in the macula lutea, where they 
are entirely absent. The retina is much thicker posteriorly, becoming thin- 
ner as it extends forward, the nervous layer gradually disappearing in the 
anterior portion of the membrane. 
The interior of the eyeball is divided into two portions by the crystalline 
Fig. 164. 
Objects on the Inner Surface 
of the Retina. In the centre of 
the ball is the yellow limbus 
luteus. here represented by shad- 
ing, and in its middle the dark 
spot. To the inner side is the 
nerve, with its accompanying 
artery. (After Sommerring.) 744 
SIGHT. 
lens and its suspensory ligament. The anterior portion contains the aqueous 
humor, the posterior contains the vitreous body. 
The crystalline lens measures about 7 mm. in transverse diameter, and 
about 4 mm. antero-posterior diameter. It is a transparent biconvex body, 
somewhat flattened anteriorly. It consists of a number of segments which 
radiate from the centre, similar to the segments of an orange. These seg- 
ments are composed of superimposed laminae of varying density. The most 
superficial are soft and gelatinous; the deeper are relatively bard, so that 
they form a kernel or nucleus. The laminae are made up of parallel fibres, 
with an undulating course, the convexities and concavities of the adjoining 
fibres fitting accurately into each other. The lens is covered with a capsule 
consisting of a transparent, elastic, fragile membrane, which has a tendency 
to curl up, with its external surface innermost. 
The suspensory ligament of the lens is formed by a continuation of the 
hyaloid membrane which lines the vitreous body. The hyaloid membrane is 
a delicate transparent structure situated between the vitreous body and mem- 
brana limitans interna of the retina. It is continued in front of the ora 
serrata, where it divides into two layers. The posterior is attached to the 
posterior portion of the capsule of the lens; the anterior portion gradually 
becomes thicker as it extends forward behind the ciliary processes, and is 
attached to the anterior surface of the capsule. This thickened portion of 
the membrane, which is corrugated where it has attached the ciliary pro- 
cesses, is called the zone of Zinn. These two layers constitute the suspensory 
ligament. Between them is a triangular canal, with its base corresponding 
to the crystalline lens. This is called the canal of Petit. 
The vitreous body is contained within the cavity formed by the hyaloid 
membrane and the posterior surface of the lens. It consists of a clear, color- 
less, albuminous fluid, having an extremely delicate interlacement of fibres 
extending in all directions through it. These fibres are not discernible in 
the adult, but can readily be seen in the foetus. 
The aqueous humor is contained within the space formed by the posterior 
surface of the cornea and the anterior surface of the lens. The space, which 
is divided into two chambers by the iris, is filled with a clear, colorless, 
limpid fluid containing saline and proteid substances in solution. This fluid 
constitutes the aqueous humor. 
The anterior external portion of the eyeball, comprising the surface of the 
cornea and about 6 or 8 mm. of the sclerotic coat, is covered by the conjunc- 
tival mucous membrane.] 
Dioptric Mechanisms. 
The Formation of the Image. 
§ 616. The eye is a camera, consisting of a series of surfaces and media 
arranged in a dark chamber, the iris serving as a diaphragm ; and the object 
of the apparatus is to form on the retina a distinct image of external objects. 
That a distinct image is formed on the retina may be ascertained by removing 
the sclerotic from the back of an eye, and looking at the hinder surface of 
the transparent retina while rays of light proceeding from any external object 
are allowed to fall on the cornea. 
§ 617. A dioptric apparatus in its simplest form consists of two media 
separated by a (spherical) surface ; and the optical properties of such an ap- 
paratus depend upon (1) the curvature of the surface, (2) the relative refrac- 
tive power of the media. The eye consists of several media, bounded by 
surfaces which are approximately spherical, but of different curvature. The DIOPTRIC MECHANISMS. 
745 
surfaces are all centred on a line called the optic axis, which meets the retina 
at a point somewhat above and to the inner (nasal) side of the fovea cen- 
tralis. In passing from the outer surface of the cornea to the retina the 
rays of light traverse in succession the cornea, the aqueous humor, the lens, 
and the vitreous humor. Refraction takes place at all the surfaces bound- 
ing these several media, but particularly at the anterior surface of the 
cornea, and at both the anterior and posterior surfaces of the lens. Since 
the anterior and posterior surfaces of the cornea are parallel, or very nearly 
so, the rays of light would suffer little or no change of direction in passing 
through the cornea, if it were bounded on both sides by the same medium. 
The direction of the rays of light in the aqueous humor would, therefore, 
remain the same if the cornea were made exceedingly thin ; if, in fact, its 
two surfaces were made into one, forming a single anterior surface to the 
aqueous humor; or, which comes to the same thing in the end, since the 
refractive power of the substance of the cornea is almost exactly the same 
as that of the aqueous humor, the refraction at the posterior surface of the 
cornea may be neglected altogether. Thus the two surfaces of the cornea 
are practically reduced to oue. The lens varies in density in different parts, 
the refractive power of the central portions being greater than that of the 
external layers ; but the refractive power of the whole may, without any 
serious error, be assumed to be uniform. The refractive power of the vitreous 
humor is almost exactly the same as that of the aqueous humor. 
§ 618. Thus the apparently complicated natural eye may be simplified 
into a "diagrammatic eye," in which the refracting surfaces are reduced to 
three, viz.: (1) the anterior surface of the cornea, (2) the anterior surface of 
the lens separating the lens from the aqueous humor, and (3) the posterior 
surface of the lens separating the lens from the vitreous humor. The media 
will similarly be reduced to two: the substance of the lens and the aqueous 
or vitreous humor. This "diagrammatic eye" is of great use in the various 
calculations which become necessary in studying physiological optics; for 
the magnitudes which are derived by calculation from it represent the cor- 
responding magnitudes in an average natural eye with sufficient accuracy to 
serve for all practical purposes. The values adopted by Listing for the 
constants of this "diagrammatic eye," and to him we are indebted for the 
introduction of it, are as follows: 
Radius of curvature of cornea 8 mm. 
" of anterior surface of lens 10 " 
" of posterior " " 6 " 
Refractive index of aqueous or vitreous humor 
Mean refractive index of lens || 
Distance from anterior surface of cornea to anterior surface of lens 4 mm. 
Thickness of lens 4 " 
rhe calculated position of the principal posterior focus, i. e., the point at 
which all rays falling on the cornea parallel to the optic axis are brought to 
a focus, is in the diagrammatic eye 14.6470 mm. behind the posterior surface 
of the lens, or 22.6470 mm. behind the anterior surface of the cornea. 
That is to say, the fovea centralis must occupy this position in order that a 
distinct image of a distant object may be formed upon it. It must be under- 
stood that these values refer to the eye when at rest, i. e., when it is not 
undergoing any strain of accommodation. 
Accommodation. 
§ 619. When an object, a lens, and a screen to receive the image are so 
arranged in reference to each other that the image falls upon the screen in 746 
SIGHT. 
exact focus, the rays of light proceeding from each luminous point of the 
object are brought into focus on the screen in a point of the image corre- 
sponding to the point of the object. If the object be then removed further 
away from the lens, the rays proceeding in a pencil from each luminous point 
will be brought to a focus at a point in front of the screen, and, subsequently 
diverging, will fall upon the screen as a circular patch composed of a series 
of circles, the so-called diffusion circles, arranged concentrically round the 
principal ray of the pencil. If the object be removed, not further from, but 
nearer to the lens, the pencil of rays will meet the screen before they have 
been brought to focus in a point, and consequently will in this case also 
give rise to diffusion circles. When an object is placed before the eye, so 
that the image falls into exact focus on the retina, and the pencils of rays 
proceeding from each luminous point of the object are brought into focus 
in points on the retina, the sensation called forth is that of a distinct image. 
When, on the contrary, the object is too far away, so that the focus lies in 
front of the retina, or too near, so that the focus lies behind the retina, and 
the pencils fall on the retina not as points, but as systems of diffusion circles, 
the sensation produced is that of an indistinct and blurred image. In order 
that objects both near and distant may be seen with equal distinctness by 
the same dioptric apparatus, the focal arrangements of the apparatus must 
be accommodated to the distance of the object, either by changing the refrac- 
tive power of the lens or by altering the distance between the lens and the 
screen. 
§ 620. That the eye does possess such a power of accommodation is shown 
by every-day experience. If two needles be fixed upright some two feet or 
so apart into a long piece of wood, and the wood be held before the eye so 
that the needles are nearly in a line, it will be found that if attention be 
directed to the far needle, the near one appears blurred and indistinct, and 
that conversely, when the near one is distinct, the far one appears blurred. 
By an effort of the will we can at pleasure make either the far one or the 
near one distinct; but not both at the same time. When the eye is arranged 
so that the far needle appears distinct, the image of that needle falls exactly 
on the retina, and each pencil from each luminous point of the needle unites 
in a point upon the retina; but when this is the case the focus of the near 
needle lies behind the retina, and each pencil from each luminous point of 
this needle falls upon the retina in a series of diffusion circles. Similarly, 
when the eye is arranged so that the near needle is distinct, the image of 
that needle falls upon tbe retina in such a way, that each pencil of rays 
from each luminous point of the needle unites in a point on the retina, while 
each pencil from each luminous point of the far needle unites at a point in 
front of the retina, and then diverging again falls on the retina in a series 
of diffusion circles. If the near needle be gradually brought nearer and 
nearer to the eye, it will be found that greater and greater effort is required 
to see it distinctly, and at last a point is reached at which no effort can make 
the image of the needle appear anything but blurred. The distance of this 
point from the eye marks the limit of accommodation for near objects. 
Similarly, if the person be short-sighted, the far needle may be moved 
away from the eye, until a point is reached at which it ceases to be seen 
distinctly, and appears blurred. In the one case the eye, with all its power, 
is unable to bring the image of the needle sufficiently forward to fall on the 
retina; the focus lies permanently behind the retina. In the other the eye 
cannot bring the image sufficiently backward to fall on the retina ; the focus 
lies permanently in front of the retina. In both cases the pencils of rays 
from the needles strike the retina in diffusion circles. 
§ 621. The same phenomena may be shown with greater nicety by DIOPTRIC MECHANISMS. 
747 
what is called Scheiner's Experiment. If two smooth holes be pricked in a 
card, at a distance from each other less than the diameter of the pupil, 
and the card be held up before one eye, with the holes 
horizontal, and a needle placed vertically be looked at 
through the holes, the following facts may be observed : 
When attention is directed to the needle itself, the image 
of the needle appears single. Whenever the gaze is 
directed to a more distant object, so that the eye is no 
longer accommodated for the needle, the image appears 
double and at the same time blurred. It also appears 
double and blurred when the eye is accommodated for 
a distance nearer than that of the needle. When only 
one needle is seen, and the eye therefore is properly ac- 
commodated for the distance of the needle, no effect is 
produced by blocking up one hole of the card, except 
that the whole field of vision seems dimmer. When, 
however, the image is double on account of the eye be- 
ing accommodated for a distance greater than that of 
the needle, blocking the left-hand hole causes a disap- 
pearance of the right-hand or opposite image, and 
blocking the right-hand hole causes the left-hand image 
to disappear. When the eye is accommodated for a 
distance nearer than that of the needle, blocking either 
hole causes the image on the same side to vanish. The 
diagram will explain how these results are brought 
about. 
Let a (Fig. 165) be a luminous point in the needle, 
and a e, af the extreme right-hand and left-hand rays 
of the pencil of rays proceeding from it, and passing 
respectively through the right-hand e, and left-hand/, 
holes in the card. (The figure is supposed to be a 
horizontal section of the eye.) When the eye is accom- 
modated for a, the rays e and / meet together in the 
point c, the retina occupying the position of the plane 
n n; the luminous point appears as one point, and 
the needle will appear as one needle. When the 
eye is accommodated for a distance beyond a, the retina may be consid- 
ered to lie1 no longer at n n, but nearer the lens, at m m for example; the 
rays a e will cut this plane at p, and the rays af at q; hence the luminous 
point will no longer appear single, but will be seen at two points, or rather 
as two systems of diffusion circles, and the single needle will appear as two 
blurred needles. The rays passing through the right-hand hole e, will cut 
the retina at p-i. e., on the right-hand side of the optic axis ; but, as we 
shall see in speaking of the judgments pertaining to vision, the image on the 
right-hand side of the retina is referred by the mind to an object on the left- 
hand side of the person ; hence the affection of the retina at p, produced by 
the rays a e falling on it there, gives rise to the image of the spot a at P, and 
similarly the left-hand spot q corresponds to the right-hand Q. Blocking 
the left-hand hole, therefore, causes a disappearance of the right-hand image, 
and vice versa. Similarly, when the eye is accommodated for a distance 
nearer than the needle, the retina may be supposed to be removed to 11, and 
the right-hand a e and left-hand a f rays, after uniting at c, will diverge 
Fig. 165. 
Diagram of Scheiner's 
Experiment. 
1 Of course in the actual eye, as we shall see, accommodation is effected by a change-in 
the lens, and not by an alteration in the position of the retina; but for convenience sake, 
we may here suppose the retina to be moved. 748 
SIGHT. 
again, and strike the retina at p' and q'. The blocking of the hole e will 
now cause the disappearance of the image q on the left-hand side of the 
retina, and this will be referred by the mind to the right-hand side, so that 
Q will seem to vanish. 
If the needle be brought gradually nearer and nearer to the eye, a point 
will be reached within which the image is always double. This point marks 
with considerable exactitude the near limit of accommodation. With short- 
sighted persons, if the needle be removed further and further away, a point 
is reached beyond which the image is always double; this marks the far 
limit of accommodation. 
The experiment may also be performed with the needle placed horizon- 
tally, in which case the holes in the card should be vertical. 
The adjustment of the eye for near or far distances may be assisted by 
using two needles, one near and one far. In this case, one needle should be 
vertical and the other horizontal, and the card turned round so that the holes 
lie horizontally or vertically according to whether the vertical or horizontal 
needle is being made to appear double. 
§ 622. In what may be regarded as the normal eye, the so-called emme- 
tropic eye [Fig. 166], near the limit of accommodation is about 10 or 12 cm. 
[Fig. 166. 
Emmetropic Eye. Parallel Rays focussed on the Retina.] 
and the far limit may be put for practical purposes at an infinite distance. 
The " range of distinct vision," therefore, for the emmetropic eye is very 
great. In the myopic, or short-sighted eye [Fig. 167] the near limit is 
brought much closer (5 or 6 cm.) to the cornea ; and the far limit is at a 
variable, but not very great distance, so that the rays of light proceeding 
from an object not many feet away are brought to a focus, not on the retina, 
[Fig. 167. 
Myopic Eye. Rays coming from a distance focussed too soon.] 
but in the vitreous humor. The range of distinct vision is therefore, in the 
myopic eye very limited. In the hypermetropic [Fig. 168], or long-sighted 
eye, the rays of light coming from even an infinite distance are, in the pas- 
sive state of the eye, brought to a focus beyond the retina. The near limit 
of accommodation is at some distance off, and a far limit of accommodation 
does not exist. The presbyopic eye, or the long sight of old people, resembles 
the hypermetropic eye in the distance of the near point of accommodation, 
but differs from it inasmuch as the former is an essentially defective condi- 
tion of the accommodation mechanism, whereas in the latter the power of DIOPTRIC MECHANISMS. 
749 
accommodation may be good, and yet, from the internal arrangements of the 
eye, be unable to bring the image of a near object on to the retina. When 
a normal eye becomes presbyopic, the far limit may remain the same, but 
since the power of accommodating for near objects is weakened or lost, the 
change is distinctly a reduction of the range of distinct vision. In the nor- 
mal emmetropic eye, when no effort of accommodation is made, the principal 
focus of the eye lies on the retina, in the myopic eye in front of it, and in 
the hypermetropic eye behind it. 
[Fig. 168. 
Hypermetropic Eye. Kays coming from a distance focussed too late.] 
§ 623. Mechanism of accommodation. In directing our attention from a 
far to a very near object, we are conscious of a distinct effort, and feel that 
some change has taken place in the eye; when we turn from a very near to 
a far object, if we are conscious of any change in the eye, it is one of a dif- 
ferent kind. The former is the sense of an active accommodation for near 
objects ; the latter, when it is felt, is the sense of relaxation after exertion. 
Since the far limit of an emmetropic eye is at an infinite distance, no 
such thing as active accommodation for far distances need exist. The only 
change that will take place in the eye in turning from near to far objects 
will be a mere passive undoing of the accommodation previously made for 
the near object. And that no such active accommodation for far distance 
takes place is shown by the facts-that the eye, when opened after being 
closed for some time, is found not in medium state, but adjusted for distance ; 
that when the accommodation mechanism of the eye is .paralyzed by atropine 
or nervous disease, the accommodation for distant objects is unaffected ; and 
that we are conscious of no effort in turning from moderately distant to far 
distant objects. The sense of effort often spoken of by myopic persons as 
being felt when they attempt to see things at or beyond the far limit of their 
range seems to arise from a movement of the eyelids, and not from any in- 
ternal changes taking place in the eye. 
§ 624. What, then, are the changes which take place in the eye when 
we accommodate for near objects ? It might be thought, and, indeed, once 
was thought, that the curvature of the cornea was changed, becoming more 
convex, with a shorter radius of curvature, for near objects. Young, how- 
ever, showed that accommodation took place as usual when the eye (and 
head) is immersed in water. Since the refractive powers of aqueous humor 
and water are very nearly alike, the cornea with its parallel surfaces, placed 
between these two fluids, can have little or no effect on the direction of the 
rays passing through it when the eye is immersed in water. And accurate 
measurements of the dimensions of an image on the cornea have shown that 
these undergo no change during accommodation, and that, therefore, the 
curvature of the cornea is not altered. Nor is there any change in the form 
of the bulb ; for any variation in this would necessarily produce an altera- 
tion in the curvature of the cornea, and pressure on the bulb would act in- 
juriously by rendering the retina anaemic and so less sensitive. In fact, 
there are only two changes of importance which can be ascertained to take 
place in the eye during accommodation for near objects. 750 
SIGHT. 
One is that the pupil contracts. When we look at near objects, the 
pupil becomes small ; when we turn to distant objects, it dilates. This, 
however, cannot have more than an indirect influence on the formation of 
the image ; the chief use of the contraction of the pupil in accommodation for 
near objects is to cut off the more divergent circumferential rays of light. 
§ 625. The other and really efficient change is that the anterior surface of 
the lens becomes more convex. If a light be held before the eye, three reflected 
images may, with care and under proper precautions, be seen by a bystander: 
[Fig. 169. 
Diagram of Images reflected from the Eye.] 
one a very bright one caused by the anterior surface of the cornea (a), a 
second less bright, by the anterior surface of the lens (6), and a third very 
dim, by the posterior surface of the lens (c) ; when the images are those of 
an object, such as a candle, in which a top and bottom can be recognized, 
the two former images are seen to be erect, but the third inverted. When 
the eye is accommodated for near objects, no change is observed in either 
the first or the third of these images ; but the second, that from the ante- 
rior surface of the lens, is seen to become distinctly smaller, showing that 
the surface has become more convex. When, on the contrary, vision is 
directed from near to far objects, the image from the anterior surface of the 
lens grows larger, indicating that the convexity of the surface has dimin- 
ished, while no change takes place in the curvature either of the cornea or 
of the posterior surface of the lens. And accurate measurements of the 
size of the image from the anterior surface of the lens have shown that the 
variations in curvature which do take place are sufficient to account for the 
power of accommodation which the eye possesses. 
The observation of these reflected images is facilitated by the simple instrument 
introduced by Helmholtz and called a pliakoscope. It consists of a small, dark 
chamber, with apertures for the observed and observing eyes; a needle is fixed at 
a short distance in front of the former, to serve as a near object, for which accom- 
modation has to be made; and a lamp or candle is so disposed as to throw an image 
on each of the three surfaces of the observed eye. Since the distance between two 
images is more readily appreciated than is a simple change of size of a single image, 
two prisms are employed so as to throw a double image of the lamp on each of the 
three surfaces [Fig. 169, B. C]. When the anterior surface of the lens becomes 
more convex the two images reflected from that surface approach each other (C ), 
when it becomes less convex they retire from each other (B). 
These observations leave no doubt that the essential change by which 
accommodation is effected is an alteration of the convexity of the anterior 
surface of the lens. And that the lens is the agent of accommodation is 
further shown by the fact that after removal of the lens, as in the operation 
for cataract, the power of accommodation is lost. In the cases which have 
been recorded, where eyes from which the lens had been removed seemed DIOPTRIC MECHANISMS. 
751 
still to possess some accommodation, we must suppose that no real accommo- 
dation took place, but that the pupil contracted when a near object was 
looked at, and so assisted in making vision more distinct. 
§ 626. This increase of the convexity of the lens has been supposed to 
be due to a compression of the circumference of the lens by a contraction of 
the iris ; but this is disproved by the fact that accommodation may take 
place in eyes from which the iris is congenitally absent. It has also been 
attributed to vasomotor changes, to increased fulness of the vessels of the iris 
or ciliary processes surrounding the lens; but this also is disproved by the 
fact that accommodation may be effected after death, in an eye which is 
practically bloodless, by stimulating the ciliary ganglion or ciliary nerves 
with an interrupted current or by other means. The real nature of the 
mechanism seems to be as follows. 
§ 627. The lens when examined after removed from the eye is found to 
be a body of considerable elasticity. When the curvature of the anterior 
surface of the lens is determined, as may be done by appropriate means, in 
its natural position in the eye at rest, and then again determined after the 
lens has been removed from the eye, the anterior surface is found to be more 
convex in the latter than in the former case. There seems to be in the eye 
in its natural condition, some agency at work keeping the anterior surface 
of the lens somewhat flattened. The suspensory ligament, attached to the 
choroid and ciliary processes behind, and passing over the front of the lens, is 
just such a structure as would produce this effect. In the natural position of 
the choroid this ligament is tense, and tends to flatten the front of the lens. 
When the choroid is pulled forward, the ligament becomes slack and the 
lens bulges out forward. Further, the ciliary muscle attached on the one 
hand to a fairly fixed region, the junction of the sclerotic and cornea, and 
on the other to the looser and more movable choroid, would naturally, when 
thrown into contraction, pull forward the choroid and so slacken the suspen- 
sory ligament, and hence permit the elastic lens to bulge out forward. And 
we have experimental evidence, carried out on lower animals, that stimula- 
tion of the ciliary ganglion or of its so-called radix brevis does lead, on the 
one hand, to a contraction of the ciliary muscle and pulling forward of the 
choroid, and, on the other hand, to an increased curvature of the anterior 
surface of the lens. Hence we may conclude that accommodation for near 
objects consists essentially in a contraction of the ciliary muscle, which, by 
pulling forward the choroid coat and the ciliary process, slackens the sus- 
pensory ligament, and allows the lens to bulge forward by virtue of its elas- 
ticity, and so to increase the convexity of its anterior surface [Figs. 170 and 
171]. 
Accommodation is in most cases a voluntary act; since, however, the 
change in the lens is always accompanied by movements in the iris, it will 
be convenient to consider the latter before we discuss the nervous mechan- 
ism of the whole act. 
§ 628. Movements of the pupil. Though by making the efforts required 
for accommodation we can at pleasure contract or dilate the pupil, it is not 
in our power to bring the will to act directly on the iris by itself. This 
fact alone indicates that the nervous mechanism of the pupil is of a peculiar 
character, and such indeed we find it to be. The pupil is contracted (1) 
when the retina (or optic nerve) is stimulated, and when light falls on the 
retina, the brighter the light the greater being the contraction, (2) when we 
accommodate for near objects. The pupil is also contracted when the eye- 
ball is turned inward, when the aqueous humor is deficient, in the early 
stages of poisoning by chloroform, alcohol, etc.; in nearly all stages of 
poisoning by morphine, physostigmine, and some other drugs; and in deep 752 
SIGHT. 
slumber. The pupil is dilated (1) when stimulation of the retina (or optic 
nerve) is diminished or arrested as in passing from a bright into a dim light 
or into darkness, (2) when the eye is adjusted for far objects. Dilatation 
also occurs when there is an excess of aqueous humor, during dyspnoea, 
during violent muscular efforts, as the result of a stimulation of sensory 
nerves, as an effect of emotions, in the later stages of poisoning by chloro- 
form, etc., and in all stages of poisoning by atropine and some other drugs, 
and in pathological conditions of the nervous system. 
[Fig. 170. 
Diagram to illustrate Accommodation. C, cornea ; S, sclerotic ; F, C, N, vertical plane of the 
cornea ; B, C, D, axis of the eye ; s, s, canal of Schlemm ; p, angle formed by the iris and cornea, 
or margin of anterior chamber ; m, position of iris and curvature of lens in an eye converged for 
parallel rays, distant vision, or negative accommodation; n, position of iris and curvature of lens 
required for near objects or for positive accommodation.] 
[Fig. 171. 
Emmetropic Eye. The dotted lines show how accommodation for the diverging rays of near 
objects is effected by the bulging of the lens. The dotted lines indicate the change in the 
lens and the effect on the light rays.] 
§ 629. Contraction of the pupil is caused by contraction of the circular 
fibres or sphincter of the iris. Dilatation is caused by contraction of the 
radial fibres of the iris; for though the existence of radial fibres has been 
denied by many observers, the preponderance of evidence is clearly in favor 
of their being really present. 
Considering how vascular the iris is, it does not seem unreasonable to 
interpret some of the variations in the condition of the pupil as the results 
of simple vascular turgescence or of depletion brought about by vasomotor 
action or otherwise, the small or contracted pupil corresponding to the di- 
lated and filled and the large or dilated pupil to the constricted and emptied 
condition of the bloodvessels. Thus slight oscillations of the pupil may be 
observed synchronous with the heart-beat and others synchronous with the 
respiratory movements. But the variations in the pupil seem too marked to 
be merely the effects of vascular changes, and indeed that constriction of the 
pupil cannot be wholly the result of turgescence, nor dilatation wholly the 
result of depletion of the vessels of the iris, is shown by the facts that both 
these events may be witnessed in a perfectly bloodless eye, and that the 
movements of the pupil when brought about by agents which also affect the DIOPTRIC MECHANISMS. 
753 
bloodvessels begin some time before the changes in the calibre of the blood- 
vessels, and indeed may be over before these have arrived at their maximum. 
Moreover, the fibres of the sympathetic, which, as we shall see, are con- 
cerned in causing dilatation of the pupil, run a somewhat different course 
from those which govern the bloodvessels of the eye. We may, therefore, 
adhere to the view that the main changes of the pupil in the direction of 
narrowing and widening are brought about by contractions of the plain 
muscular fibres in the iris. 
§ 630. Muscular contractions leading to changes of the pupil may be 
observed in the eye removed from the body, and indeed in the extirpated 
iris. The plain muscular fibres of the iris, like other plain muscular fibres, 
are remarkably sensitive to variations in temperature. Besides this there 
seems to be, in certain animals at least, a connection within the eye between 
the iris and retina of such a kind that light falling into an extirpated eye 
will lead to a narrowing of the pupil. Putting aside, however, such excep- 
tional events we may lay down the broad principle that contraction of the 
pupil, brought about by light falling on the retina, is a reflex act, of which 
the optic is the afferent nerve, the third or oculo-motor the efferent nerve, 
and the centre some portion of the brain lying below the corpora quadri- 
gemina in the front part of the floor of the aqueduct of Sylvius. This is 
proved by the following facts: When the optic nerve is divided, the falling 
of light on the retina no longer causes a contraction of the pupil; when 
the third nerve is divided, stimulation of the retina or of the optic nerve no 
longer causes contraction ; but direct stimulation of the peripheral portion 
of the divided third nerve causes extreme contraction of the pupil. If the 
region of the brain spoken of above as a centre be carefully stimulated con- 
traction of the pupil will take place even in the absence of light and after 
division of the optic nerve. After removal of the same centre stimulation 
of the retina is ineffectual in narrowing the pupil. But if the centre and its 
connections with the optic nerve and third nerve be left intact and in thor- 
oughly sound condition, contraction of the pupil will occur as a result of 
light falling on the retina, though all other nervous parts be removed. 
§ 631. The nervous centre is not a double centre with two completely 
independent halves, one for each eye ; there is a certain amount of functional 
communion between the two sides, so that when one retina is stimulated both 
pupils contract. It might be imagined that this cerebral centre acted as a 
tonic centre, whose action was simply increased, not originated, by the stimu- 
lation of the retina; but this is disproved by the fact that if the optic nerve 
be divided subsequent section of the third nerve produces no further dilatation. 
In considering the movements of the pupil, however, we have to deal not 
only with a narrowing of the pupil thus brought about in a reflex way by 
contraction of the circular sphincter fibres, and with the absence of such a 
narrowing, but also with active dilatation due to a contraction of the radial 
dilator fibres, and this renders the whole matter much more complex than 
might be supposed to be the case from the simple statement just made. 
§ 632. rhe iris is supplied, in common with the ciliary muscle and 
choroid, by the short ciliary nerves (Fig. 172, s. c.) coming from the oph- 
thalmic or lenticular (ciliary) ganglion (/. c.) which is connected by its roots 
with the third nerve (r. 6.), the cervical sympathetic nerve (sym.), and 
with the nasal branch of the ophthalmic division of the fifth nerve (r. I.). 
The short ciliary nerves are, moreover, accompanied by the long ciliary 
nerves (/. c.) coming from the same nasal branch of the ophthalmic division 
of the fifth nerve. What are the uses of these several nerves in relation to 
the pupil ? 
§ 633. If the cervical sympathetic in the neck be divided, all other por- 754 
SIGHT. 
tions of the nervous mechanism being intact, a contraction of the pupil (not 
always very well marked) takes place, and if the peripheral portion (i. e., the 
upper portion still connected with the 
head) be stimulated, a well-developed 
dilatation is the result. The sympa- 
thetic has, it will be observed, an effect 
on the iris the opposite of that which it 
exercises on the bloodvessels; when it 
is stimulated the pupils are dilated 
while the bloodvessels are constricted. 
This dilating influence of the sympa- 
thetic may, as in the case of the vaso- 
motor action of the same nerve, be 
traced back down the neck to the upper 
thoracic ganglion and thence along the 
rami communicantes and roots of the 
lower cervical and first dorsal or two 
first dorsal spinal nerves, to a region 
in the lower cervical and upper dorsal 
cord (called by some authors the cen- 
trum cil io-spinale inferius), and from 
thence up through the medulla oblon- 
gata to a centre, which appears to be 
placed in the floor of the front part 
of the aqueduct of Sylvius not far 
from and apparently on either side 
of the centre for contraction of the 
pupil. 
§ 634. The dilatation of the pupil 
which is witnessed in dyspnoea, and 
that which results from stimulation of 
sensory nerves and from emotions, ap- 
pears to be brought about by the 
action of the sympathetic, the venous 
blood or the sensory impulses or the 
emotional impulses so affecting the 
dilating centre as to augment the dilat- 
ing impulses proceeding from it along 
the sympathetic. The existence of the 
subordinate centre in the cervical or 
dorsal cord, spoken of just now, is sup- 
posed to be indicated by the fact that 
after division of the medulla oblongata, and consequent severance of the 
efferent paths from the centre in the aqueduct of Sylvius, dilatation of the 
pupil may still be brought about, in some animals at least, by dyspnoea or 
by adequate stimulation of sensory nerves. A question is raised here in 
fact somewhat similar to that raised in connection with the medullary respi- 
ratory centre (p. 370) ; and here as there we may probably conclude that 
the independent action of such a spinal centre is of subordinate importance. 
§ 635. The pupil then seems to be under the dominion of two antagon- 
istic mechanisms: one a contracting mechanism, reflex in nature, the third 
nerve serving as the efferent and the optic as the afferent tract; the other a 
dilating mechanism, apparently tonic in nature, but subject to augmentation 
from various causes, and of this the cervical sympathetic is the efferent 
channel. Hence, when the third or optic nerve is divided, not only does 
Fig. 172. 
Diagrammatic Representation of the 
Nerves Governing the Pupil. II. optic 
nerve; I. g. lenticular ganglion ; r. b. its 
short root from III.: oc. m. third or oculo- 
motor nerve : sym. its sympathetic root; r. I. 
its long root from V. ophthm. the nasal branch 
of the ophthalmic division of the fifth nerve . 
s. c. the short ciliary nerves from the lenticu- 
lar ganglion; I. c. the long ciliary nerve from 
the nasal branch of the ophthalmic division 
of the fifth nerve. DIOPTRIC MECHANISMS. 
755 
contraction of the pupil cease to be manifest, but active dilatation occurs, on 
account of the tonic dilating influence of the sympathetic being left free to 
work. When, on the other hand, the sympathetic is divided, this tonic 
dilating influence falls away, and contraction results. When the optic or 
third nerve is stimulated, the dilating, effect of the sympathetic is overcome, 
and contraction results; and when the sympathetic is stimulated, any con- 
tracting influence of the third nerve which may be present is overcome, and 
dilatation ensues. 
§ 636. But there are considerations which show that the matter is still 
more complex than this. A small quantity of atropine introduced into the 
eye or into the system causes a dilatation of the pupil. This might be 
attributed to a paralysis of the third nerve, and, indeed, it is found that 
after atropine has produced its effects the falling of light on the retina no 
longer causes contraction of the pupil. A difficulty, however, is introduced 
by the fact that when the third nerve is divided, and when, therefore, the 
contracting effects of stimulation of the retina are placed entirely on one 
side, and there is nothing to prevent the sympathetic producing its dilating 
effects to the utmost, dilatation is still further increased by atropine. When 
physostigmine is introduced into the eye or system, contraction of the pupil 
is caused, whether the third nerve be divided or not; and when the dose is 
sufficiently strong the contraction is so great that it cannot be overcome by 
stimulation of the sympathetic. The dilatation which is caused by a suffi- 
cient dose of atropine may be greater than that which can ordinarily be 
produced by stimulation of the sympathetic, and the contraction caused by 
a sufficient dose of physostigmine may be greater than that which is ordi- 
narily produced in a reflex manner by stimulation of the optic nerve, or 
even than that produced by direct stimulation of the third nerve. Evi- 
dently these drugs act either directly on the plain muscular fibres of the iris 
or on some local mechanism, the one in such a way as to cause dilatation, 
the other in such a way as to cause contraction. Such a local mechanism 
cannot, however, lie in the ophthalmic ganglion, for both drugs continue to 
produce these effects in a most marked degree after the ganglion has been 
excised. We must suppose, therefore, that the mechanism if it exists is 
situated in the iris itself or in the choroid, where, indeed, ganglionic nerve- 
cells are abundant. The movements of the iris in the extirpated eye, spoken 
of just now, may perhaps be attributed to the same local mechanism. Fur- 
ther it is stated that with stimulation of the sympathetic, the latent period, 
i. e., the period intervening between the beginning of stimulation and the 
beginning of the movement of the iris, is much greater than with stimu- 
lation of the third nerve, indicating that the former acts through a local 
mechanism but the latter more directly on the muscular fibres. The whole 
question, however, of this local mechanism, and of the exact mode of action 
of the various drugs and of the changes in the body which lead to contrac- 
tion or dilatation respectively of the pupil, needs fuller discussion than we 
can afford to give to it here. We may add that the local action of atropine 
in contrast to any action on the cerebral centre is well illustrated by apply- 
ing atropine to one eye locally. The pupil of that eye dilates widely ; in 
consequence more light falls on the retina, and this so affects the cerebral 
centre, which as we have seen is not strictly unilateral but in communion 
with its fellow, that increased constricting impulses pass from both centres, 
and these, though ineffectual in the atropinized eye, lead in the untouched 
eye to an increased narrowing of the pupil. 
§ 637. The share of the fifth nerve in the work of the iris seems to be in 
part a sensory one; the iris is sensitive, and the sensory impulses which are 
generated in it pass from it along the fibres of the fifth nerve. 756 
SIGHT. 
We may sum up the nervous mechanism of the pupil then somewhat as 
follows : The salient and most frequently repeated event, the contraction of 
the pupil upon exposure to light, is a reflex act, the centre of which is 
placed in the brain; and the correlative widening of the pupil upon dimi- 
nution of light is due to the tonic action of the sympathetic making itself 
felt upon the waning of its antagonist. The contraction of the pupil in the 
earlier stages of the action of alcohol and chloroform and in slumber is 
probably due to an increased action of the contracting centre, but the 
narrow pupil caused by such drugs as morphine and physostigmine is due, 
chiefly at least, to a local action. The dilating effects of such drugs as 
atropine are also largely due to a local action, but in the widened pupil of 
the later stages of alcohol poisoning and of dyspnoea we can probably trace 
the effects of an exhaustion of the cerebral contracting centre, assisted pos- 
sibly by an increased activity of the dilating centre. 
§ 638. There remains a word to be said concerning the contraction of 
the pupil which takes place when the eye is accommodated for near objects, 
and when the pupil is turned inward (the two being closely allied, since the 
eyes converge to see near objects), and the return to the more dilated condi- 
tion when the eye returns to rest and regains the accommodation for far 
objects. These are instances of what are called "associated movements." 
Two movements are thus spoken of as " associated " when the special central 
nervous mechanism employed in carrying out the one act is so connected by 
nervous ties of some kind or other with that employed in carrying out the 
other, that when we set the one mechanism in action we unintentionally set 
the other in action also. The ciliary muscles which bring about accommo- 
dation are governed in this action by fibres that may be traced, through the 
ciliary nerves and lenticular ganglion, along the third or oculo-motor nerve, 
to a centre which lies (in dogs) in the hind part of the floor of the third 
ventricle, and which is especially connected with the most anterior bundles 
of the roots of the third nerve. This centre is under the command of our 
will; when we wish to accommodate for near objects we throw it into action, 
and, when in action, it calls also into action by "association" the centre for 
the contraction of the pupil; when the action of the accommodation centre 
ceases and the eye falls back to the condition of rest, in which it is accom- 
modated for far objects, the action of the pupil-contracting centre ceases 
also, and the pupil therefore widens. 
§ 639. The mechanism of accommodation may also be affected in a local 
manner. And the drugs which have a special action on the pupil, such as 
atropine and Calabar bean, also affect the mechanism of accommodation. 
Atropine paralyzes it, so that the eye remains adjusted for far objects; and 
physostigmine throws the eye into a condition of forced accommodation for 
near objects. This double action has been explained by the supposition that 
while atropine paralyzes, physostigmine throws into tonic or tetanic contrac- 
tion, on the one hand, the circular muscles of the iris and, on the other, the 
ciliary muscles; but the phenomena, on inquiry, appear too complicated to 
be explained in so simple a manner. 
We can accomodate at will; but few persons can effect the necessary 
change in the eye unless they direct their attention to some near or far 
object, as the case may be, and thus assist their will by visual sensations. By 
practice, however, the aid of external objects may be dispensed with ; and 
it is when this is achieved that the pupil may be made to dilate or contract 
at pleasure, accommodation being effected without the eye being turned to 
any particular object. DIOPTRIC MECHANISMS. 
757 
Imperfections in the Dioptric Apparatus. 
§ 640. The emmetropic eye may be taken as the normal eye. The myopic 
and hypermetropic eyes may be considered as imperfect eyes, though the 
former possesses certain advantages over the normal eye. An eye might be 
myopic from too great a convexity of the cornea or of the anterior surface 
of the lens, or from permanent spasm of the accommodation-mechanism, or 
from too great a length of the long axis of the eyeball. The last appears to 
be the usual cause. Similarly, most hypermetropic eyes possess too short a 
bulb. Moreover, in the strongly-marked myopic eye there is frequently hy- 
pertrophy of the longitudinal (meridional) fibres of the ciliary muscle, often 
spoken of exclusively as the ciliary muscle, and atrophy or absence of the 
circular fibres; in the hypermetropic eye, on the other hand, the circular 
fibres are well developed and the meridional fibres scanty. The presbyopic 
eye is, as we have seen, an eye normally constituted in which the power of 
accommodation has been lost or is failing through increasing weakness of the 
ciliary muscle or a loss of elasticity in the lens, or through the parts be- 
coming rigid. 
§ 641. Spherical aberration. In a spherical lens the rays which impinge 
on the circumference are brought to a focus sooner than those which pass 
nearer the centre, and the rays proceeding from a luminous point are no 
longer brought to a single focus at one point, but form a number of foci at 
different distances. Hence when rays are allowed to fall on the whole of 
the lens, the image formed on a screen placed in the focus of the more central 
rays is blurred by the diffusion-circles caused by the circumferential rays 
which have been brought to a premature focus. In an ordinary optical in- 
strument spherical aberration is obviated by a diaphragm which shuts off 
the more circumferential rays. In the eye the iris is an adjustable diaphragm ; 
and when the pupil contracts in near vision the more divergent rays proceed- 
ing from a near object which tend to fall on the circumferential parts of 
the lens are cut off. As, however, the refractive power of the lens does not 
increase regularly and progressively from the centre to the circumference, 
but varies most irregularly, the purpose of the narrowing of the pupil cannot 
be simply to obviate spherical aberration ; and indeed the other optical im- 
perfections of the eye are so great that such spherical aberrations as are 
caused by the lens produce no obvious effect on vision. 
§ 642. Astigmatism. We have hitherto treated the eye as if its dioptric 
surfaces were all parts of perfect spherical surfaces. In reality this is rarely 
the case, either with the lens or with the cornea. Slight deviations do not 
produce any marked effect, but there is one deviation, known as regular 
astigmatism, which, present to a certain extent in most eyes, very largely 
developed in some, frequently leads to very imperfect vision. This defect is 
due to the dioptric surface being not spherical but more convex along one 
meridian than another, more convex, for instance, along the vertical than 
along the horizontal meridian. When this is the case the rays proceeding 
from a luminous point are not brought to a single focus at a point, but 
possess two linear foci, one nearer than the normal and corresponding to the 
more convex surface, the other further than the normal focus and corre- 
sponding to the less convex surface. If the vertical meridians of the sur- 
face be more convex than the horizontal, then the nearer linear focus will 
be horizontal and the further linear focus will be vertical, and vice versa. 
(This can be shown much more effectually on a model than in a diagram in 
which we are limited to two dimensions.) Now, in order to see a vertical 
line distinctly, it is much more important that the rays which diverge from 
the line in a series of horizontal planes should be brought to a focus prop- 758 
SIGHT. 
erly than those which diverge in the vertical plane of the line itself; and 
similarly, in order to see a horizontal line distinctly, it is much more im- 
portant that the rays which diverge from the line in a series of vertical 
planes should be brought to a focus properly than those which diverge in 
the horizontal plane of the line itself. Hence a horizontal line held before 
an astigmatic dioptric surface, most convex in the vertical meridians, will 
give rise to the image of a horizontal line at the'nearer focus, the vertical 
rays diverging from the line being here brought to a linear horizontal focus. 
Similarly, a vertical line held before the same surface will give rise to an 
image of a vertical line at the further focus, the horizontal rays diverging 
from the vertical line being here brought to a linear vertical focus. In 
other words, with a dioptric surface most convex in the vertical meridians 
horizontal lines are brought to a focus sooner than are vertical lines. 
Most eyes are thus more or less astigmatic, and generally with a greater 
convexity along the vertical meridians. If a set of horizontal or vertical 
lines be looked at, or if the near point of accommodation be determined by 
Scheiner's experiment (p. 747), for the needle placed first horizontally and 
then vertically, the horizontal lines or needle will be distinctly visible at a 
shorter distance from the eye than the vertical lines or needle. Similarly, 
the vertical line must be further from the eye than a horizontal one if both 
are to be seen distinctly at the same time. The cause of astigmatism is, in 
the great majority of cases, the unequal curvature of the cornea; but some- 
times the fault lies in the lens, as was the case with Young. 
When the curvature of the cornea or lens differs not in two meridians 
only but in several, irregular astigmatism is the result. A certain amount 
of irregular astigmatism exists in most lenses, thus causing the image of a 
bright point, such as a star, to be not a circle but a radiate figure. 
§ 643. Chromatic aberration. The different rays of the spectrum are 
of different refrangibility, those toward the violet end of the spectrum 
being brought to a focus sooner than those near the red end. This in optical 
instruments is obviated by using compound lenses made up of various kinds 
of glass. In the eye we have no evidence that the lens is so constituted as 
to correct this fault; still the total dispersive power of the instrument is so 
small that such amount of chromatic aberration as does exist attracts little 
Fig. 173. 
Diagram illustrating Chromatic Aberration, hh is the dioptric surface; hv represents the blue 
and h r the red rays ; V is the focal plane of the blue, A'of the red rays. 
notice. Nevertheless, some slight aberration may be detected by careful 
observation. When the spectrum is observed at some distance the violet end 
will not be seen in focus at the same time as the red. If a luminous point 
be looked at through a narrow orifice covered by a piece of violet glass, 
which while shutting out the yellow and green allows the red and blue rays 
to pass through, there will be seen alternately an image having a blue 
centre with a red fringe, or a red centre with a blue fringe, according as the 
image of the point looked at is thrown on one side or other of the true 
focus. Thus supposing/(Fig. 173) to be the plane of the mean focus of 
J., the violet rays will be brought to a focus in the plane F, and the red VISUAL SENSATIONS. 
759 
rays in the plane A'. If the rays be supposed to fall on the retina between 
Land/ the diverging or blue rays will form a centre surrounded by the 
still converging red rays; whereas if the rays fall on the retina between / 
and E, the converging red rays will form a centre with the still diverging 
blue rays forming a fringe around them. If the rays fall on the retina at 
f, the two kinds of rays will be mixed together; as will be seen from the 
figure, the circumferential converging red ray h r as it cuts the plane of 
the retina is, in ordinary vision, accompanied by the diverging violet ray 
h v, and thus by a sort of compensation, we see together even the rays which 
differ most in refraction. 
§ 644. Entoptic phenomena. The various media of the eye are not uni- 
formly transparent; the rays of light in passing through them undergo local 
absorption and refraction, and thus various shadows are thrown on the retina, 
of which we become conscious as imperfections in the field of vision, espe- 
cially when the eye is directed to a uniformly illuminated surface. These 
are spoken of as entoptic phenomena, and are very varied, many forms 
having been described. 
The most common are those caused by the presence of floating bodies in 
the vitreous humor, the so-called muscat volitantes. These are readily seen 
when the eye is turned toward a uniform surface, and are frequently very 
troublesome in looking through a microscope. They are especially obvious 
when divergent rays fall upon the eye. They assume the form of rows and 
groups of beads, of single beads, of streaks, patches, and granules, and may 
be recognized by their almost continual movement, especially when the head 
or eye is moved up and down. When an attempt is made to fix the vision 
upon them, they immediately float away. Tears on the cornea, temporary 
unevenness on the anterior surface of the cornea after the eyelid has been 
pressed on it, and imperfections in the lens or its capsule, also give rise to 
visual images. Not unfrequently a radiate figure corresponding to the ar- 
rangement of the fibres of the lens makes its appearance. 
Imperfections in the margin of the pupil appear in the shadow of the iris 
which bounds the field of vision; and the movements of the iris in one eye 
may be rendered visible by looking at a bright point or luminous surface 
through a pin-hole in a card placed close in front of the eye, in the 
anterior focus in fact, and then alternately closing and opening the other 
eye; the field of the first may be observed to contract when light enters and 
to expand when the light is shut off from the second. The media of the eye 
are fluorescent: a condition which favors the perception of the ultra-violet 
rays. If a white sheet or white cloud be looked at in daylight through a 
Nicol's prism, a somewhat bright double cone or double tuft, with the apices 
touching, of a faint blue color, is seen in the centre of the field of vision, 
crossed by a similar double cone of a somewhat darker yellow color. These 
are spoken of as Haidinger's brushes; they rotate as the prism is rotated, 
and are supposed to be due to the unequal absorption of the polarized light 
in the yellow spot. The prism must be frequently rotated, as when the 
prism remains at rest the phenomena fade. Lastly, the optical arrange- 
ments have a further imperfection in that the dioptric surfaces are not truly 
centred on the optic axis. 
Visual Sensations. 
§ 645. Light falling on the retina excites sensory impulses, and these 
passing up the optic nerve to certain parts of the brain, produce changes in 
certain cerebral structures, and thus give rise to what we call a sensation. In 
a sensation we ought to be able to distinguish between the events through 760 
SIGHT. 
which the impact of the rays of light on the retina is enabled to generate 
sensory impulses, and the events, or rather series of events, through which 
these sensory impulses (for, judging by the analogy of motor nerves, we have 
no reason to think that they undergo any fundamental changes in passing 
along the optic nerve), by the agency of the cerebral arrangements, develop 
into a sensation. Such an analysis, however, is, at present at least, in most 
particulars, quite beyond our power; and we must therefore treat of the sen- 
sations as a whole, distinguishing between the peripheral and central phe- 
nomena, on the rare occasions when we are able to do so. 
The Origin of Visual Impulses. 
§ 646. Of primary importance to the understanding of the way in which 
luminous undulations give rise to those nervous changes which pass along 
the optic nerve as visual impulses, is the fact that the rays of light produce 
their effect by acting not on the optic nerve itself but on its terminal organs. 
They pass through the anterior layers of the retina apparently without in- 
ducing any effect; it is not till they have reached the region of the rods and 
cones that they set up the changes concerned in the generation of visual im- 
pulses; and the impulses here generated travel back to the layer of fibres 
in the anterior surface of the retina and thence pass along the optic nerve. 
That the optic fibres are themselves insensible to light and that visual im- 
pulses begin in the region of rods and cones is shown by the phenomena of 
the blind spot and of Purkinje's figures respectively. 
§ 647. Blind spot. There is one part of the retina on which rays of light 
falling give rise to no sensations ; this is the entrance of the optic nerve, and 
the corresponding area in the field of vision is called the blind spot. If the 
visual axis of one eye, the right for instance, the other being closed, be fixed 
on a black spot in a white sheet of paper, and a small black object, such as 
the point of a quill pen dipped in ink, be moved gradually sideways over 
the paper away to the outside of the field of vision, at a certain distance the 
black point of the quill will disappear from view. On continuing the move- 
ment still further outward the point will again come into view and continue 
in sight until it is lost in the periphery of the field of vision. If the pen be 
used to make a mark on the paper at the moment when it is lost to view, 
and at the moment when it comes into sight again, and if similar marks be 
made along the other meridians as well as the horizontal, an irregular out- 
line will be drawn circumscribing an area of the field of vision within which 
rays of light produce no visual sensation. This is the blind spot. The 
dimensions of the figure drawn vary, of course, with the distance of the paper 
from the eye. If this distance be known, the size as well as the position of 
the area of the retina corresponding to the blind spot may be calculated 
from the diagrammatic eye (p. 745). The position exactly coincides with 
the entrance of the optic nerve, and the dimensions (about 1.5 mm. diameter) 
also correspond. While drawing the outline as above directed the indica- 
tions of the large branches of the retinal vessels as they diverge from the 
entrance of the nerve can frequently be recognized. The existence of the 
blind spot is also shown by the fact that an image of light, sufficiently small, 
thrown upon the optic nerve by means of the ophthalmoscope, gives rise to 
no sensations. 
The existence of the blind spot proves that the optic fibres themselves are 
insensible to light; it is only through the agency of the retinal expansion 
that these can be stimulated by luminous vibrations. 
§ 648. Purkinje's figures. If one enters a dark room with a candle, and 
while looking at a plain (not parti-colored) wall, moves the candle up and VISUAL SENSATIONS. 
761 
down, holding it on a level with the eyes by the side of the head, there will 
appear in the field of vision of the eye of the same side, projected on the 
wall, an image of the retinal vessels, quite similar to that seen on looking 
into an eye with the ophthalmoscope. The field of vision is illuminated with 
a glare, and on this the branched retinal vessels appear as shadows. In this 
mode of experimenting the light enters the eye through the cornea, and an 
image of the candle is formed on the nasal side of the retina; and it is the 
light emanating from this image which throws shadows of the retinal vessels 
on to the rest of the retina. A far better method is for a second person to 
concentrate the rays of light, with a lens of low power, on to the outside of 
the sclerotic just behind the cornea ; the light in this case emanates from the 
illuminated spot on the sclerotic and passing straight through the vitreous 
humor throws a direct shadow of the vessels on to the retina. Thus the rays 
passing through the sclerotic at b, Fig. 174, in the direction b v, will throw a 
shadow of the vessel v on to the retina at fl; this will appear as a dark line 
at B in the glare of the field of vision. This proves that the structures in 
which visual impulses originate must lie behind the retinal vessels, otherwise 
the shadows of these could not be perceived. 
If the light be moved from b to a, the shadow on the retina will move 
from fl to a, and the dark line in the field of vision will move from B to A. 
If the distance B A be measured when the whole image is projected at a 
Fig. 174. 
Fig. 175. 
Diagram illustrating the Formation of 
Purkinje's Figures when the Illumination 
is Directed through the Sclerotic. 
Diagram illustrating the Formation of 
Purkinje's Figures when the Illumination 
is Directed through the Cornea. 
known distance, k B from the eye, k being the optical centre,1 then, knowing 
the distance k fl in the diagrammatic eye, the distance fl a can be calculated. 
But if the distance fl a be thus estimated, and the distance b a be directly 
measured, the distances fl v, a v, b »,av can be calculated, and if the appearance 
in the field of vision is really caused by the shadow of > falling on fl, these 
distances ought to correspond to the distances of the retinal vessels > from the 
1 For the properties of the optical centre we must refer the reader to the various 
treatises on optics. The optical centre of a lens is the point through which all the princi- 
pal rays, of the various pencils of rays falling on the lens, pass. The diagrammatic eye 
of Listing (p. 745) has two optical centres, but these may, without serious error, be further 
reduced for practical purposes to one lying in the lens near its posterior surface, at about 
15 mm. distance from the retina. 762 
SIGHT. 
sclerotic b on the one hand, and from that part of the retina £ where visual 
impressions begin, on the other. H. Muller found that the distance /? * thus 
calculated corresponded to the distance of the retinal vessels from the layer 
of rods and cones. Thus Purkinje's figures prove in the first place that the 
sensory impulses which form the commencement of visual sensations origi- 
nate in some part of the retina behind the retinal vessels, i. e., somewhere 
between them and the choroid coat; and H. Muller's calculations go far to 
show that they originate at the most posterior or external part of the retina, 
viz., the layer of rods and cones. It must be admitted, however, that H. 
Muller's results were not sufficiently exact to allow any great stress to be 
placed on this argument. 
In the second method of experimenting the image always moves in the 
same direction as the light, as it obviously must do. In the first method, 
where the light enters through the cornea, the image moves in the same 
direction as the light when the light is moved from right to left, provided 
the movement does not extend beyond the middle of the cornea, but in the 
opposite direction to the light when the latter is moved up and down. In 
Fig. 175, which represents a horizontal section of an eye, if a be moved to «, 
b will move to /?, the shadow on the retina c to f, and the image d to d. If, 
on the other hand, a be supposed to move above the plane of the paper, b 
will move below, in consequence c will move above, and d will appear to 
move below, i. e., d will sink as a rises. 
It is desirable in these cases to move the light to and fro, especially 
in the first method, as the retina soon becomes tired, and the image 
fades away. Some observers can recognize in the axis of vision a faint 
shadow corresponding to the edge of the depression of the fovea centralis. 
§ 649. The retinal vessels may also be rendered visible by looking 
through a small orifice, such as a pin-hole in a card placed close to the eye, 
at a bright field such as the sky, and moving the orifice very rapidly from 
side to side or up and down. If the movement be from side to side the ves- 
sels which run vertically will be seen ; if up and down, the horizontal vessels. 
The fine capillary vessels are seen more easily in this way than by Purkinje's 
method. The same appearances may also be produced by looking through 
a microscope from which the objective has been removed and the eye-piece 
only left (or in which, at least, there is no object distinctly in focus in the 
field), and moving the head rapidly from side to side or backward and for- 
ward. Or the microscope itself may be moved ; a circular movement of the 
field will then bring both the vertically and horizontally directed vessels into 
view at the same time. 
§ 650. The photo-chemistry of the retina. In seeking to understand how 
it is that rays of light falling upon the region of the rods and cones can give 
rise to sensory, visual impulses in the optic nerve, we may adopt one or other 
of two views. On the one hand, we may suppose that the vibrations of the 
ether are able, through the means of the retinal apparatus of the rods and 
cones for example, to give rise in some way or other to molecular vibrations 
which are the beginning of the nervous impulses in the optic nerve. No 
satisfactory explanation of how such a change can be brought about has 
been offered, and indeed the difficulties of such a conception are very great. 
On the other hand, we may more naturally turn to a chemical explanation. 
We are familiar with the fact that rays of light are able to bring about the 
decomposition of very many chemical substances, and we accordingly speak 
of these substances as being sensitive to light. All the facts dwelt on in this 
book illustrate the great complexity and corresponding instability of the 
composition of protoplasm. And we might reasonably suppose that proto- 
plasm itself would be sensitive to light; that is to say, that rays of light VISUAL SENSATIONS. 
763 
falling on even undifferentiated protoplasm might set up a decomposition of 
that protoplasm, and so inaugurate a molecular disturbance ; in other words, 
that light might act as a direct stimulus to protoplasm. As a matter of fact, 
however, such evidence as we at present possess goes to show that native 
undifferentiated protoplasm is, as a rule, not sensitive to light (that is, to 
those particular waves which, when they fall on our retina give rise in us to 
the sensation of light}, though in the case of some lowly organisms, whose 
protoplasm exhibits very little differentiation and in particular contains no 
pigment, a sensitiveness to light has been observed. Nor can we be surprised 
at this indifference of protoplasm when we reflect that what we may call pure 
protoplasm is remarkable for its transparency, that is to say, the rays of light 
pass through it with the slightest possible absorption. But in order that 
light may produce chemical effects, it must be absorbed ; it must be spent 
in doing the chemical work. Accordingly, the first step toward the forma- 
tion of an organ of vision is the differentiation of a portion of protoplasm 
into a pigment at once capable of absorbing light and sensitive to light, 
i. e., undergoing decomposition upon exposure to light. An organism, a por- 
tion of whose protoplasm had thus become differentiated info such a pig- 
ment, would be able to react toward light. The light falling on the organ- 
ism would be in part absorbed by the pigment, and the rays thus absorbed 
would produce a chemical action and set free chemical substances which 
before were not present. We have only to suppose that the chemical sub- 
stances are of such a nature as to act as a stimulus to the protoplasm of 
other parts of the organism (and we have manifold evidence of the exqui- 
site sensitiveness of protoplasm in general to chemical stimuli), in order to 
see how rays of light falling on the organism might excite movements in it, 
or modify movements which were being carried on, or might otherwise affect 
the organism in whole or in part. 
Such considerations as the foregoing may be applied to even the complex 
organ of vision of the higher animals. If we suppose that the actual ter- 
minations of the optic nerve are surrounded by substances sensitive to light, 
then it becomes easy to imagine how light falling on these sensitive sub- 
stances should set free chemical bodies possessed of the property of acting 
as stimuli to the actual nerve-endings, and thus give rise to visual impulses 
in the optic fibres. We say "easy to imagine," but we are at present far 
from being able to give definite proofs that such an explanation of the 
origin of visual impulses is the true one, probable and enticing as it may 
appear. 
§ 651. One of the most striking features in the structure of the retina is 
the abundance of black pigment in the retinal, or as it is sometimes called 
choroidal, epithelium. It is difficult to suppose that the sole function of 
this pigment is to absorb the superfluous rays of light, and that the rays 
thus absorbed are put to no use, but simply wasted. And indeed it has been 
shown that the pigment is sensitive to light; but the changes in it induced 
by light are excessively slow. Moreover, its presence cannot be of funda- 
mental importance, since vision is not only possible but fairly distinct with 
albinos, in which this pigment is absent. 
•Then again, in the vast majority of vertebrate animals, the outer limbs 
of the rods are suffused with a purplish-red pigment, the so-called visual 
purple, which is so eminently sensitive to light that images of external objects 
may by appropriate means be photographed in it on the retina. When the 
eye of a frog or of a rabbit is examined in an ordinary way, with full expos- 
ure to light, the retina appears colorless. But if the eye be kept in the dark 
for some time before it is examined, the retina, if removed rapidly, will be 
found to be of a beautiful purplish-red color. Upon exposure to light the 764 
SIGHT. 
color changes to yellow and then fades away, leaving, however, the retina 
not only white, but more opaque than it was before. Upon examination 
with the microscope it is found that the purple color is confined exclusively 
to the rods and to the outer limbs of the rods, the inner limbs being wholly 
devoid of it. 
§ 652. The color of the rods is due to the presence of a distinct pigment, 
the " visual purple," diffused through the substance of the outer limbs ; and 
this may be extracted from the rods by dissolving these in an aqueous solution 
of bile-salts. A clear purple solution is thus obtained, which is capable of 
being bleached by the action of light, and in its general features and be- 
havior is similar to the pigment as it naturally exists in the retina. 
Visual purple is found, as we have said, exclusively in the outer limbs of 
the rods; it has never yet been found in the cones, and it is accordingly 
absent from the retinas (such as those of snakes), which are composed of 
cones only, and from the macula lutea and fovea centralis of the retinas of 
man and the ape. The intensity of the coloration varies in different animals, 
and the retinas even of some animals possessing rods (bat, dove, hen) seem 
to be wholly devoid of the visual purple; it is generally well marked in 
retinas in which the outer limbs of the rods are well developed. Its absence 
or presence is not dependent on nocturnal habits, since the' intense color of 
the retina of the owl is in strong contrast to the absence of color in the bat 
It has been found in the retina of the embryo. 
§ 653. The visual purple is bleached not only by white, but also bi 
monochromatic, light. Of the various prismatic rays the most active are th< 
greenish-yellow rays, those to the blue side of these coming next, the leas 
active being the red. Now it is precisely the greenish-yellow rays which an 
most readily absorbed by the color itself. A natural colored retina or a solu 
tion of visual purple gives a diffuse spectrum without any defined absorption 
bands, and according to the amount of coloring material through which tin 
light passes, absorption is seen either to be limited to the greenish-yellow 
part of the spectrum or to spread thence toward the blue, and, to a muck 
less extent, toward the red. Thus the various prismatic rays produce h 
photo-chemical effect on the visual purple in proportion as they are absorber 
by it. Under the action of light the visual purple, whether in solution o. 
in its natural condition in the rods, passes through a purplish-orange to a 
yellow, and finally becomes colorless; and we appear to be justified in 
speaking of a "visual yellow" and "visual white" as products of tin 
photo-chemical changes undergone by the visual purple. 
For the restoration of the visual purple, after it has been destroyed by 
light, the maintenance of the circulation of the blood through the tissues of 
the eye is not essential. The choroidal epithelium has by itself, provided 
that it still retains its tissue life, the power of regenerating the purple. If 
a portion of the retina of an excised eye be raised from its epithelial bed, 
bleached, and then carefully restored to its natural position, the purple will 
return if the eye be kept in the dark. The choroidal epithelium may, in 
fact, be spoken of as a " purpurogenous " membrane. 
§ 654. If the image of some bright object, such as a lamp or a window, 
be thrown on to the retina, either of an eye in its natural position or of one 
recently excised, care having been taken to keep the retina for some time 
previous away from any rays of light, the portion of the retina on which 
the rays have fallen will be found to be bleached, the rest of the retina re- 
maining purple. In fact, an "optogram " of external objects may thus be 
obtained ; and if the retina be removed and treated with a four per cent, 
solution of potash alum before the choroidal epithelium has had time to 
obliterate the bleaching effects, the retina may remain permanently in that VISUAL SENSATIONS. 
765 
condition; the photo-chemical effect may, as the photographers say, be 
" fixed." 
It seemed very tempting, especially upon the first discovery of it, to sup- 
pose that this visual purple is directly concerned in vision. If we suppose 
that visual purple itself is inert toward the endings of the optic nerve, but 
that either visual yellow or visual white, i. e., some product of the action of 
light on visual purple, may act as a stimulus to those endings, the way seems 
opened to understanding how rays of light can give rise to sensory impulses 
in the optic nerve. Unfortunately visual purple is absent from the cones, 
and from the fovea centralis, which, as we shall see, is the region of distinct 
vision ; it is further entirely wanting in some animals which undoubtedly 
see very well; and, lastly, animals, such as frogs, naturally possessing the 
pigment, continue to see very well, and even apparently to see colors, when 
their visual purple has been absolutely bleached, as it may be by prolonged 
exposure of the eyes to strong light. We cannot therefore, at present at 
least, explain the origin of visual impulses by the help of visual purple. At 
the same time its history suggests that some substances, sensitive like it to 
light, but unlike it, colorless and therefore escaping observation, may exist, 
and by photo-chemical changes be the means of exciting the optic nerves. 
And, as we shall see later on, one theory of color vision is based on the 
assumption that vision is carried on in some way or other by changes in 
what may be called the visual substances present in the retina, these sub- 
stances being used up and regenerated as vision is going on. 
But even admitting as probable the existence of these sensitive visual sub- 
stances, the changes in which lead to stimulation of the real endings of the 
retinal nervous mechanism, we cannot at present state anything definite con- 
cerning those nerve-endings or the manner of their stimulation. It may be 
that even the outer limbs of the rods and cones, in spite of the apparent 
break of continuity between the outer and inner limbs, are really nervous 
in nature. It may be, on the other hand, that the outer limbs are either 
purely dioptric in function, or are associated with the sensitive visual sub- 
stances in such a way that the purely nervous structures must be considered 
as extending no further at least than the inner limbs. We cannot as yet 
make any definite statement in the one direction or the other. 
§ 655. In connection with the origin of visual impulses, we may per- 
haps call attention to the remarkable changes which the cells of the retinal 
pigment epithelium undergo under the influence of light. When an eye has 
been shut off from all light for some little time the pigment is concentrated 
in the bodies of the cells, and the remarkable filamentous processes of the 
cells, with the pigment granules or crystals which they carry, extend a slight 
distance only between the limbs of the rods and cones (about one-third down 
the length of the outer limbs of the rods). Under the influence of light 
these processes, loaded with pigment, thrust themselves a much longer way 
down toward the external limiting membrane ; in consequence a considerable 
quantity of pigment is found massed between the outer and even the inner 
limbs of the rods and cones; indeed, the outer limbs of the rods swelling at 
the same time become jammed, as it were, between the masses of pigment, 
causing the epithelial layer to adhere very closely to the layer of rods and 
cones. 
§ 656. The retina and optic nerve, like other nervous structures, develop 
electric currents, which may be spoken of as currents of rest and currents 
of action. They may be shown by placing one electrode on the retina of a 
bisected eye, or on the cornea of a whole one, and the other on the optic 
nerve, or hind part of the eyeball, or even on some distant part of the body. 
They are also manifested by the isolated retina itself. The phenomena 766 
SIGHT 
appear somewhat complicated by the appearance now of positive, now of 
negative, variations; but this fact comes out clearly, that the incidence of 
light on the irritable retina develops an electric change, the magnitude of 
which is to a certain extent proportionate to the intensity of the light acting 
as a stimulus. The changes accordingly diminish and cease to appear as 
the retina gradually loses its irritability after death. We may add that 
these electric phenomena appear to be quite independent of the condition 
of the visual purple. 
Simple Sensations. 
§ 657. Relations of the sensation to the stimulus. If we put aside for the 
present all questions of color, we may say that light, viewed as a stimulus 
affecting the retina, varies in intensity, that is, in the energy of the luminous 
vibrations as manifested by their amplitude, and in duration, that is, in the 
length of time a succession of waves continues to fall upon the retina. The 
effect of the light will also depend on the extent of retinal surface exposed 
to the luminous vibrations at the same time. Taking a luminous point, in 
order to eliminate the latter circumstance, we may make the following state- 
ments : 
The sensation has a duration much greater than that of the stimulus, and 
in this respect is comparable to a muscular contraction caused by such a 
stimulus as a single induction-shock. The sensation of a flash of light, for 
instance, lasts for a much longer time than that during which luminous 
vibrations are falling on the retina. Hence, when two stimuli, such as two 
flashes of light, follow each other at a sufficiently short interval, the two 
sensations are fused into one; and a luminous point moving rapidly round 
in a circle gives rise to the sensation of a continuous circle of light. This 
again is quite comparable to muscular tetanus. The interval at which fusion 
takes place, that is, the interval between successive stimuli which must be 
exceeded in order that successive distinct sensations may be produced, varies 
according to the intensity of the light, being shorter with the stronger light; 
with a faint light it is about second, with a strong light or second. 
This may be shown by rotating rapidly before the eye a disc arranged with 
alternate black and white sectors of equal width. With a faint illumination 
the flickering, indicative of the successive sensations from the white sectors 
not being completely fused, ceases when the rotation becomes so rapid that 
each pair of black and white sectors takes only second in passing before 
the eye. When a brighter illumination is used the rapidity must be in- 
creased before the flickering disappears. That part of the sensation which 
is recognized as lasting after the cessation of the stimulus is frequently 
spoken of as the " after-image." 
Though the duration of the after-image is longer with the stronger light 
(that caused by looking even momentarily at the sun lasting for some time), 
the commencement of the decline of the sensation begins relatively earlier, 
hence the greater difficulty in the complete fusion of successive sensations 
with the stronger light. The interval at which fusion takes place differs 
with different colors, being shortest with yellow, intermediate with red, and 
longest with blue. 
The duration of a stimulus necessary to call forth a sensation is exceed- 
ingly short; thus the shortest possible flash, such as that of an electric 
spark, gives rise to a sensation of light. 
Objects in motion when illuminated by a single electric spark appear 
motionless, the stimulus of the light reflected from them ceasing before they 
can make an appreciable change in their position. When a moving body is VISUAL SENSATIONS. 
767 
illuminated by several rapid flashes in succession, several distinct images 
corresponding to the positions of the body during the several flashes are 
generated ; the images of the body corresponding to the several flashes fall 
on different parts of the retina. 
§ 658. The intensity of the sensation varies with the luminous intensity 
of the object; a wax candle appears brighter than a rushlight. The ratio, 
however, of the sensation to the stimulus is not a simple one. If the lumi- 
nosity of an object be gradually increased from a very feeble stage to a very 
bright one, it will be found that though the corresponding sensations like- 
wise gradually increase, the increments of the sensations due to increments 
of the luminosity gradually diminish; and at last an increase of the lumi- 
nosity produces no appreciable increase of sensation ; a light, when it reaches 
a certain brightness, appears so bright that we cannot tell when it becomes 
any brighter. Hence it is much easier to distinguish a slight difference of 
brightness between two feeble lights than the same difference between two 
bright lights; we can easily tell the difference between a rushlight and 
a wax candle ; but two suns, or even two bright lamps, one of which differed 
from the other merely by just the number of luminous rays which a wax 
candle emits in addition to those sent forth by a rushlight, would appear to 
us to have exactly the same brightness. In a darkened room an object 
placed before a candle will throw what we consider a deep shadow on a 
sheet of paper or any white surface. If, however, sunlight be allowed to 
fall on the paper at the same time from the opposite side the shadow is no 
longer visible. The difference between the total light reflected from that 
part of the paper where the shadow was, and which is illuminated by the 
sun alone, and that reflected from the rest of the paper which is illuminated 
by the candle as well as by the sun, remains the same ; yet we can no longer 
appreciate that difference. 
On the other hand, if using two rushlights we throw two shadows on a 
white surface and move one rushlight away until the shadow caused by it 
ceases to be visible, and, having noted the distance to which it had to be 
moved, repeat the same experiment with two wax candles, we shall find 
that the wax candle has to be moved just as far as the rushlight. In fact, 
it is found by careful observation that, within tolerably wide limits, the 
smallest difference of light which we can appreciate by visual sensations is 
a constant fraction (about T^oth) of the total luminosity employed. The 
same law holds good with regard to the other senses as well. The smallest 
difference in length we can detect between two lines, one an inch long and 
the other a little less than an inch, is the same fraction of an inch that the 
smallest difference in length we can detect between a line a foot long and 
one a little less than a foot, is of a foot. Put in a more general form, then, 
the law, which is often called Weber's law,1 is as follows: When a stimulus 
is continually increased, the increase of stimulus necessary to call forth the 
smallest appreciable increase of sensation always bears the same proportion 
to the whole stimulus. 
§ 659. Distinction and fusion of sensations. When light falls on a large 
portion of the retina the total sensation produced is greater in amount than 
when a small portion only of the retina is affected ; a large piece of white 
paper produces a greater total effect on our consciousness than a small one, 
though, if the surfaces be uniformly and equally illuminated, the intensity of 
the sensation is in each case the same ; the small piece of paper appears as 
bright or as " white" as the large one. If the images of two luminous ob- 
jects fall on the retina at sufficient distances apart, the consequent sensations 
1 From which Fechner, by an assumption, obtained a mathematical expression or for- 
mula, which is sometimes incorrectly spoken of as Fechner's law. 768 
SIGHT. 
are distinct, and the intensity of each sensation will depend solely upon the 
luminosity of the corresponding object. If, however, the two objects are 
made to approach each other, a point will be reached at which the two sen- 
sations are fused into one. When this occurs the intensity of the total 
sensation produced will be greater than that of either of the sensations 
caused by the single objects. A number of luminous points scattered over 
a wide surface would appear each to have a certain brightness ; each would 
give rise to a sensation of a certain intensity. If they were all gathered 
into one spot, that spot would appear far brighter than any of the previous 
points; the intensity of the sensation would be greater. We may therefore 
suppose the retina to be divided into areas corresponding to sensational 
units. If the images from two luminous objects fall on separate visual areas, 
if we may so call them, two distinct sensations will be produced; if, on the 
contrary, they both fall on the same visual area, one sensation only will be 
produced. Where the sensations are separate, the intensity of the one 
(with exceptions hereafter to be mentioned) is not affected by the presence of 
the other; but where they become fused the intensity of the united sensa- 
tions is greater than either of, though not equal to the sum of, the single 
sensations. The existence of these sensational units is the basis of distinct 
vision. When we speak of the smallest size visible or distinguishable, we 
are referring to the dimensions of the retinal areas corresponding to these 
sensational units. The retinal area must be carefully distinguished from the 
sensational unit, for the sensation is, as we have seen, a process whose arena 
stretches from the retina to certain parts of the brain, and the circumscrip- 
tion of the sensational unit, though it must begin as a retinal area, must also 
be continued as a cerebral area in the brain, the latter corresponding to, 
and being as it were the projection of, the former. With most people two 
stars appear as a single star when the distance between them subtends an 
angle of less than 60 seconds; and the best eyes generally fail to distinguish 
two parallel white streaks when the distance between the two, measured 
from the middle of each, subtends an angle of less than 73 seconds. Some, 
however, can distinguish objects 50 seconds distant from each other. An 
angle of 73 seconds in an object corresponds in the diagrammatic eye (see 
p. 745) to the length of 5.36 /z in the retinal image,1 and one of 50 seconds 
to 3.65 /z. 
§ 660. In the human eye 50 cones may be counted along a line of 200 /z 
in length drawn through the centre of the yellow spot; this would give 4 /z 
for the distance between the centres of two adjoining cones in the yellow 
spot, the average diameter of a cone at its widest part being 3 /z, and there 
being slight intervals between neighboring cones. Hence, if we take the 
centre of a cone as the centre of an anatomical retinal area, these anatomical 
areas correspond very fairly to the physiological visual areas as determined 
above. That is to say, if two points of the retinal image are less than 4 /z 
apart, they may both lie within the area of a single cone; and it is just 
when they are less than about 4 /z apart that they cease to give rise to two 
distinct sensations. It must be remembered, however, that the fusion or 
distinction of the sensations is ultimately determined by the brain and not 
by the retina. Two points of the retinal image less than 4 /z apart might 
lie both within the area of a single cone; but the reason why, under such 
circumstances, they give rise to one sensation only is not because one cone- 
fibre only is stimulated. Two points of a retinal image might lie, one on 
the area of one cone and another on the area of an adjoining cone, and still 
be less than 4 /z apart; in such a case two cone-fibres would be stimulated. 
1 By m is meant the micromillimeter, one one-thousandth of a millimeter. VISUAL SENSATIONS. 
769 
and yet only one sensation would be produced. So also in the less sensitive 
peripheral parts of the retina two points of the retinal image might stimu- 
late two cones a considerable distance apart, and yet give rise to one sensa- 
tion only. 
In the case where the two points lie entirely within the area of a single 
cone, it is exceedingly probable that, even if the adjacent cones or cone- 
fibres in the retina are not at the same time stimulated, impulses radiate 
from the cerebral ending of the excited cone into the neighboring cerebral 
endings of the neighboring cones ; in other words, the sensation-area in the 
brain does not exactly correspond to and is not sharply defined like the 
retinal area, but gradually fades away into neighboring sensation-areas. 
We may imagine two points of the retinal image so far apart that even the 
extreme margins of their respective cerebral sensation-areas do not touch 
each other in the least; in such a case there can be no doubt about the two 
points giving rise to two sensations. We might, however, imagine a second 
case where two points were just so far apart that their respective sensation- 
areas should coalesce at their margins, and yet that, in passing from the 
centre of one sensation-area to the centre of the other, we should find on 
examination a considerable fall of sensation at the junction of the two 
areas ; and in a third case we might imagine the two centres to be so close 
to each other that in passing from one to the other no appreciable diminu- 
tion of sensation could be discovered. In the last case there would be but 
one sensation, in the second there might still be two sensations if the mar- 
ginal fall were great enough, even though the areas partially coalesced. 
Thus, though the mosaic of rods and cones is the basis of distinct vision, the 
distinction or fusion of two vision impulses is ultimately determined by the 
disposition and condition of the cerebral centres. Hence the possibility of 
increasing by exercise the faculty of distinguishing two sensations, since by 
use the cerebral sensation-areas become more and more differentiated. This, 
however, is even more strikingly shown in touch than in sight. 
Color Sensations. 
§ 661. When we allow sunlight reflected from a cloud or sheet of paper 
to fall into the eye, we have a sensation which we call a sensation of white 
light. When we look at the same light through a prism, and allow differ- 
ent parts of the spectrum to fall in succession into the eye, we have sensa- 
tions which we call respectively sensations of red, orange, yellow, green, blue, 
violet, etc., light. In other words, rays of light falling on the retina give 
rise to different sensations, according to the wave-lengths of the rays. 
Though we speak of the spectrum as consisting of a few colors, such as red, 
orange, etc., there are an almost infinite number of intermediate tints in the 
spectrum itself; and we perceive in external nature a large number of colors, 
such as purple, brown, gray, etc., which do not correspond to any of the 
color sensations gained by regarding the successive parts of the spectrum. 
We find, however, on examination, that certain distinct color sensations, not 
corresponding to any of the colors of the spectrum may be obtained by the 
fusion of the sensations caused by two or more of the prismatic colors. 
Thus purple, which is not present in the spectrum, may be at once produced 
by fusing the sensations of blue and red in proper proportions. Moreover, 
many of the various tints and shades of nature may be imitated by fusing a 
particular color sensation with the sensation of white, or by allowing a cer- 
tain quantity of light of a particular color to fall sparsely over the area of 
the retina, which is at the same time protected from the access of any other 
light, i. e., as we say, by mixing the color with black. Thus the browns 770 
SIGHT 
of nature result from various admixtures of yellow, red, white, and black ; 
and a small quantity of white light, scattered over a large area of the retina, 
i. e., white largely mixed with black, forms a gray. In fact, the qualities of 
a color depend (1) on the nature of the prismatic color or colors, i.e., on the 
wave-lengths of the constituent rays, falling on a given area of the retina ; 
(2) on the amount of this colored light which falls on the area of the retina 
in a given time; and (3) on the amount of white light falling on the same 
area at the same time. When rays corresponding to a prismatic color fall 
upon the retina unaccompanied by any white light, the color is said to be 
"saturated and a color is spoken of as more or less saturated according 
as it is mixed with less or more white light. When we are led to describe a 
color as being of such a tint or hue, we are guided by the first of the above 
conditions. But we have no common phrases by which we distinguish the 
second of the above conditions from the third. The word " pale," it is true 
is most frequently used to express a color very slightly saturated ; but the 
words " rich " or "deep" are used sometimes as meaning highly saturated, 
sometimes as meaning simply that a large quantity of light of the particular 
hue is passing into the eye. So also with the phrase " brightthis we 
often use when a large amount of colored and white light fall at the same 
time on the same retinal area, but we sometimes also use it to express the 
mere intensity of the sensation. 
The best method of fusing color sensations is that adopted by Maxwell, of 
allowing two different parts of the spectrum to fall on the same part of the retina 
at the same time. The use of the pure prismatic colors eliminates errors which 
arise when pigments, the colors of which are not pure, but mixed, are employed. 
And where pigments are used, it is the sensations to which the pigments give rise 
which must be mixed and not the pigments themselves. Thus while the sensations 
gained by looking at gamboge yellow and indigo respectively when fused give rise 
to a sensation of white, gamboge and indigo themselves when mixed appear green. 
The color of the mixed pigment is due to the fact that the rays which reach the eye 
from the mixture are those which are least absorbed by the two pigments. The 
gamboge absorbs the blue rays very largely, but the green to a much less extent ; 
while the indigo absorbs the red and yellow rays very largely, but also absorbs very 
little of the green. Hence green is the predominant hue of the mixture. When 
pure pigments, i. e., pigments corresponding as closely as possible to the prismatic 
colors, are used, satisfactory results may be gained, either by using the reflected 
image of one pigment, and arranging so that it falls on the retina at the same spot 
as the direct image of the other pigment, or by allowing the image of one pigment 
to fall on the retina before the sensation produced by the other has passed away. 
The first result is easily reached by Helmholtz's simple method of placing two 
pieces of colored paper a little distance apart on a table, one on each side of a glass 
plate inclined at an angle. By looking with one eye down on the glass plate the 
reflected image of the one paper may be made to coincide with the direct image of 
the other, the angle which the glass plate makes with the table being adjusted to 
the distance between the two pieces of paper. In the second method, the "color 
top " is used ; sectors of the colors to be investigated are placed on a disc made to 
rotate very rapidly, and the image of one color is thus brought to bear on the 
retina so soon after the image of another that the two sensations are fused into 
one. 
§ 662. When the sensation corresponding to the several prismatic colors 
are fused together in various combinations, the following remarkable results 
are brought about: 
1. When red and yellow in certain proportions are mixed together the 
result is a sensation of orange, quite indistinguishable from the orange of the 
spectrum itself Now the latter is produced by rays of certain wave-lengths, 
whereas the rays of red and of yellow are respectively of quite different 
wave-lengths. The orange of the spectrum cannot be made up by any mix- VISUAL SENSATIONS. 
771 
ture of the red and the yellow of the spectrum in the sense that the red and 
yellow rays can unite together to form rays of the same wave-lengths as the 
orange rays; the three things are absolutely different. It is simply the 
mixed sensation of the red and yellow which is so like the sensation of orange ; 
the mixture is entirely and absolutely a physiological one. In the same 
way we may by appropriate mixtures produce the sensations corresponding 
to other parts of the spectrum. Now we must suppose that rays of different 
wave-lengths give rise to different sensory impulses; that, for instance, the 
sensory impulses generated by orange rays are different from those generated 
by red and by yellow rays. Hence we are led by the fact of mixed sensa- 
tions being identical with other apparently simple sensations to infer that 
the sensory impulses which any ray originates are either themselves of a 
complex character, or in becoming converted into sensations give rise to 
complex or mixed sensations; that, for instance, the impulse or sensation 
which a ray in the middle of the orange gives rise to, is not a simple impulse 
or sensation answering exclusively to the color of that ray, but that the ray 
gives rise either to a complex impulse which becomes converted into a com- 
plex sensation, or to a simple impulse which eventually develops into a 
mixed or complex sensation, into the composition of which in each case 
other orange tints and shades of red and yellow enter. 
§ 663. 2. When certain colors are mixed together in pairs in certain 
definite proportions, the result is white. These colors are: 
Red (near a),1 and Blue-Green (near F) ; 
Orange (nearC), and Blue (between F and G) ; 
Yellow (near I)), and Indigo-Blue (near G); 
Green-Yellow (near E), and Violet (between G and M), 
and are said to be " complementary " to each other. To these might be 
added the peculiar non-prismatic color purple, which with green also 
gives white. 
3. If we select arbitrarily any three colors corresponding to any three 
parts of the spectrum sufficiently far apart-say, for instance, red, green, 
and blue-we can, by a proper adjustment of the proportions of each, 
produce white. Further, these three colors can be taken in such pro- 
portions as with a proper addition, if necessary, of white to produce the 
sensations of all other colors.2 That is to say, given three standard sen- 
sations, all the other sensations may be gained by the proper mixture of 
these. 
§ 664. It is obvious from the foregoing that our real color sensations are 
much fewer in number than those which we appear to have when we look on 
the colors of the spectrum or of nature; that rays of light awake in us cer- 
tain simple sensations, which mixed in various proportions reproduce all our 
sensations. And the question arises, What is the nature or what are the 
characters of these simple sensations ? 
When we examine our own sensations of light we find that certain of 
these seem to be quite distinct in nature from each other, so that each is 
something sui generis, whereas we easily recognize all other sensations as 
various mixtures of these. Thus red and yellow are to us quite distinct; 
we do not recognize anything common to the two; but orange is obviously 
a mixture of red and yellow. The sensations caused by different kinds of 
1 These letters refer to Frauenhofer's lines. 
2 A few highly saturated colors cannot be so reproduced, but a mixture of any one of 
them with white can. We may, perhaps, therefore speak of these saturated colors as 
being reproduced by a proper combination of the three arbitrarily selected colors, with 
the subtraction of white. 772 
SIGHT. 
light, which thus appear to us distinct, and which we may speak of as 
" fundamental sensations," are white, black, red, yellow, green, blue. Each 
of these seems to us to have nothing in common with any of the others, 
whereas in all other colors wTe can recognize a mixture of two or more of 
these. 
This result of common experience suggests the idea that these fundamental 
sensations are the primary or simple sensations, spoken of above as those out 
of which all other sensations may be supposed to be compounded. And a 
theory has been proposed to reconcile the various facts of color vision with 
the supposition that we possess these six fundamental sensations. This theory, 
known as that of Hering, is somewhat as follows : The six sensations readily 
fall into three pairs, the members of each pair having analogous relations 
to each other. White and black naturally go together, the one being the 
antagonistic or correlative of the other. There is a similar connection 
between red and green, the one being the complementary of the other, and 
between yellow and blue, which are similarly complementary. We saw 
reason, a short time back (p. 765), for believing that vision originates in 
the changes taking place in certain visual substances (or a visual substance) 
in the retina. And the theory of which we are speaking supposes that there 
exist in the retina, or at least somewhere in the vistial apparatus, three dis- 
tinct visual substances which are continually undergoing a double metab- 
olism, one constructive, of assimilation or building up, and the other 
destructive, of dissimilation or breaking down. One of these substances 
is further of such a nature that when dissimilation is in excess of assimila- 
tion we have a sensation of white, and when assimilation is in excess a sen- 
sation of black. With a second substance excess of dissimilation provokes 
red, of assimilation green ; and with the third substance, yellow and blue 
respectively. When in the latter two substances dissimilation and assimila- 
tion are exactly equal, no effect is produced ; but with the first substance 
this condition produces in us the effect of gray. Further, these substances 
are of such a kind that while the first or white-black substance is influenced 
by rays along the whole range of the spectrum, the two other substances are 
differently influenced by rays of different wave-length. Thus in the part of 
the spectrum which we call red, the rays promote a rapid dissimilation of 
the red-green substance with comparatively slight effect in either direction 
on the yellow-blue substance; hence our sensation of red. In that part of 
the spectrum which we call yellow the rays effect a marked dissimilation of 
the yellow-blue substance, but their action on the red-green substance is 
equal in the direction of both assimilation and dissimilation ; hence our sen- 
sation of yellow. The green rays, again, promote assimilation of the red- 
green substance, leaving the assimilation of the yellow-blue substance equal 
to the dissimilation, and similarly blue rays cause assimilation of the yellow- 
blue substance, and leave the red-green substance neutral. Finally, at the 
extreme blue end of the spectrum the rays once more provoke dissimila- 
tion of the red-green substance. When orange rays fall on the retina, 
there is an excess of dissimilation of both the red-green and the yellow- 
blue substance ; when greenish-blue rays are perceived there is an excess 
of assimilation of both these substances; and other intermediate tints cor- 
respond to variable amounts of dissimilation or assimilation of two or more 
of these substances. 
§ 665. When all the rays together fall on the retina, the red-green and 
yellow-blue substances remain in equilibrium, but the white-black substance 
is violently dissimilated ; and we say the light is white. 
Another theory (known as the Young-Helmholtz theory, because it was 
introduced by Young and more fully elaborated by Helmholtz) strives to VISUAL SENSATIONS. 
773 
reduce the matter to still further simplicity. Starting from the fact men- 
tioned a short time since, that all color sensations, including the sensation of 
white, may be obtained by the appropriate mixture of three standard sensa- 
tions, this theory teaches that our visual apparatus is so constituted as, when 
excited, to give rise to three primary sensations, and that these primary sen- 
sations are called forth in different degrees by different rays of light, so that 
each ray gives rise to a different mixture of the three. Several sets of three 
such primary sensations might be chosen, which would satisfy the conditions 
of giving rise, by appropriate mixture, to all sensations of color, including 
white; but for reasons into which we cannot enter fully here, the sensations 
which may thus be taken as primary sensations appear to correspond to our 
sensations of red, green, and blue or violet. Such a view of three primary 
■color sensations is represented in the diagram (Fig. 176). Thus the red 
Fig. 176. 
Diagram of Three Primary Color Sensations 1, is the so-called " red2, "green,1' and 3, 
violet " primary color sensation. R, 0. Y, etc., represent the red, orange, yellow, etc., color of 
the spectrum, and the diagram shows by the height of the curve in each case, to what extent 
the several primary color sensations are respectively excited by vibrations of different wave- 
lengths. 
primary sensation, excited to a certain extent by the rays at the extreme red 
end, is most powerfully affected by the rays at a little distance from the end, 
the rays from this point onward toward the blue end producing less and less 
effect. The curve of the green primary sensation begins later, and reaches 
its maximum in the green of the spectrum, while the blue or violet primary 
sensation is still later, and only reaches its maximum toward the blue end 
of the spectrum. Each ray calls forth each sensation, but to a different 
degree, and the total result of each ray, or of each group of rays, is deter- 
mined by the proportionate amount of the three sensations. Thus the sen- 
sation of orange (0 in the figure) is brought about by a mixture of a great 
deal of the primary red with much less of the primary green, and hardly 
any of the primary blue; the orange sensation is converted into a yellow 
sensation by diminishing the primary red and largely increasing the primary 
green, the primary blue undergoing also some slight increase. And similarly 
with all the other sensations. When each of the primary sensations is ex- 
cited to a maximum, as when ordinary light falls on the retina, the result is 
a sensation of white. According to this theory, black is simply the absence 
of sensation from the visual apparatus. 
In the view, as originally put forward by Young, the three primary sen- 
sations were supposed to be represented by three sets of fibres, each set of 
fibres being differently affected by different rays of light, and the impulses 
passing to the brain along each set awakening a distinct sensation. No such 
distinction of fibres can be found in the retina; but an anatomical basis of 
this kind is not necessary for the theory ; we can easily conceive of the same 774 
SIGHT. 
fibre transmitting three distinct kinds of impulses ; or we may suppose that 
the visual substances are three in number instead of six, the changes in each 
substance provoking a primary sensation. 
§ 666. Such are the two main theories of color vision ; and much may be 
said in favor of both of them ; at the same time both of them present many 
difficulties. To discuss them fully is a task beyond the limits of this book, 
and to discuss them in any but a full manner would be unsatisfactory. We 
must be satisfied, therefore, with the foregoing simple statement of the two 
views. Independently of any theory, however, we may remember (1) 
that all the sensations which we experience under the action of light of 
whatever kind may be reduced to six-white, black, red, yellow, green, and 
blue; and (that these may be all reproduced by various mixtures of three 
standard sensations, if black be allowed to indicate the absence of all sen- 
sation. These are matters of fact; what is at present debated is whether 
the six fundamental sensations are the outcome of three primary sensa- 
tions or whether they represent six distinct conditions of the visual 
apparatus. 
§ 667. Color-blindness. Persons vary much in their power of appre- 
ciating and discriminating color, i. e., in the intensity and accuracy of their 
color sensations. Some people regard as similar, colors which to most peo- 
ple are glaringly distinct; the former are said to be " color-blind." The 
most common form of color-blindness is that of persons unable to distinguish 
green and red from each other. As in the case of Dalton, they tell a red 
gown lying on a green grass plot, or a red cherry among the green leaves, 
by its form, and not by its color. They confound not only red, green, and 
certain forms of brown, but also rose, purple, and blue. Such persons are 
often spoken of as " red blind." On the Hering theory they lack the red- 
green visual substance ; hence, all the color sensations they possess must be 
those of yellow and blue free from all mixture of red or green ; and such 
accounts as have been given of their sensations by those persons who are 
" red blind " in one eye, but possess normal vision with the other, accord 
with this conclusion. On the Young-Helmholtz theory, such persons lack 
the primary red sensation ; and hence the sensations which they have must 
be mixtures of green and blue alone, our yellow appearing to them a bright 
green, and our green-blue a kind of gray. 
All such red-blind people ought, on either theory, to be less affected than 
are persons with normal eyes, by the red end of the spectrum ; this ought 
with them to be shortened and obscure. In a certain number of persons 
who confound red and green, this is the case ; but in some instances no such 
lack of appreciation of the red end of the spectrum can be ascertained. 
Such cases have been supposed to be green blind, that is, lacking the pri- 
mary sensation of green. According to the Hering theory green blindness 
apart from red blindness is impossible, the only two possible color defects 
being red-green and blue-yellow blindness. And the existence of distinct 
green blindness has been held to contradict that theory. On the other 
hand, the Hering theory admits the possibility of total color-blindness, i. e., 
the inability to see anything but white and black ; and this on the Young- 
Helmholtz theory, is impossible, since for vision to exist at all, one of the 
three primary sensations must be present; a man to see at all must see 
things in various shades of either red, or of green, or of violet, though he 
may confound this single-colored vision with the normal vision of white of 
different intensities. But, indeed, a full examination of color-blindness 
rather increases than diminishes the difficulties of deciding between the two 
rival theories. 
§ 668. Influence of the pigment of the yellow spot. In the macula lutea, VISUAL SENSATIONS. 
775 
which part of the retina we use chiefly for vision, images falling on other 
parts of the retina being said to give rise to " indirect vision," the yellow 
pigment absorbs some of the greenish-blue rays. Hence, the sensation 
which we receive from objects which we are in the habit of calling white 
is that which, if this pigment were absent, we should receive from objects 
more or less yellow. We may use this feature of the yellow spot for the 
purpose of making the spot, so to speak, visible to ourselves, by an experi- 
ment suggested by Maxwell. A solution of chrome alum, which only trans- 
mits red and greenish-blue rays, is held up between the eye and a white 
cloud. The greenish-blue rays are absorbed by the yellow spot, and here 
the light gives rise to a sensation of red ; whereas in the rest of the held of 
vision the sensation is that ordinarily produced by the purplish solution. 
The yellow spot is consequently marked out as a rosy patch. This very soon, 
however, dies away. 
In speaking of sensation as a function of the stimulus (p. 766), we re- 
ferred to white light only ; but the different colors are unequal in the rela- 
tions borne by the intensity of the stimulus to the amount of sensation 
produced. Thus the more refrangible blue rays produce a sensation more 
readily than the yellow or red rays. Hence, in dim lights, as those of 
evening and moonlight, the blues preponderate, and the reds and yellows 
are less obvious. So also when a landscape is viewed through a yellow 
glass, the yellow hue suggests to the mind bright sunlight and summer 
weather, although the actual illumination which reaches the eye is dimin- 
ished by the glass. Conversely, when the same landscape is viewed through 
a blue glass the idea of moonlight or winter is suggested. 
The theory of three primary color sensations may be used to explain 
why any colored light, if made sufficiently intense, appears white. Thus a 
violet light of moderate intensity appears violet because it excites the pri- 
mary sensation of violet much more than those of green and red. If the 
stimulus be increased the maximum of violet stimulation will be reached, 
while the stimulation of green will continue to be increased and even that 
of red to a slight degree. The result will be that the light appears violet 
mixed with green, that is blue. If the stimulus be still further increased 
while the green and violet are both excited to the maximum, the red stimu- 
lation may be increased until the result is violet, green, and red in the pro- 
portions which make white light. And so with light of other colors. 
§ 669. After-images. We have already seen that in vision the sensation 
lasts much longer than the stimulus. Under certain circumstances, such as 
particular conditions of the eye, an intense stimulus, etc., the sensation is so 
prolonged that it is spoken of as an after-image. Thus, if the eye be directed 
to the sun, the image of that body is present for a long while after; and, if, 
on early waking, the eye be directed to the window for an instant and then 
closed, an image of the window with its bright panes and darker sashes, the 
various parts being of the same color as the object, will remain for an appre- 
ciable time. These images, which are simply continuations of the sensation, 
are spoken of as positive after-images. They are best seen after a momentary 
exposure of the eye to the stimulus. 
When, however, the eye has been for some time subject to a stimulus, the 
sensation which follows the withdrawal of the stimulus is of a different kind ; 
what is called a negative after-image, or negative image, is produced. If, after 
looking steadfastly at a white patch on a black ground, the eye be turned to 
a white ground, a gray patch is seen for some little time. A black patch on 
a white ground similarly gives rise on a gray ground to a negative image in 
the form of a white patch. This may be explained as the result of exhaus- 
tion. When the white patch has been looked at steadily for some time, that 776 
SIGHT. 
part of the retina on which the image of the patch fell becomes tired ; hence, 
the white light, coming from the white ground subsequently looked at, which 
falls on this part of the retina, does not produce so much sensation as in 
other parts of the retina; and the image, consequently, appears gray. And 
so, in the other instance, the whole of the retina is tired, except at the patch ; 
here the retina is for a while most sensitive, and hence the white negative 
image. 
When a red patch is looked at, the negative image is a green-blue, that 
is, the color of the negative image is complementary to that of the object. 
Thus, also, orange produces a blue, green a pink, yellow an indigo-blue, neg- 
ative image, and so on. This, too, can be explained as a result of exhaus- 
tion on either hypothesis of color vision. When the colored patch is looked 
at, one of the three primary color sensations is much exhausted, and the 
other two less so, in varying proportions, according to the exact nature of 
the color of the patch ; and the less exhausted sensations become prominent 
in the after-image. Thus, the red patch exhausts the red sensation, and the 
negative image is made up chiefly of green and blue sensations, that is, 
appears to be greenish-blue, or bluish-green, according to the tint of the red. 
On the other hypothesis, we may suppose that, owing to the continued effect 
of looking at the red patch, dissimilation of the red-green substance 
becomes less and less, leading to a prominence and, indeed, to an actual in- 
crease of the process of assimilation of the same substance ; hence, the sensa- 
tion of green dominating in the negative image. 
Similarly, when the eye, after looking at a colored patch, is turned to a 
colored ground, the effects may easily be explained by reference to the com- 
parative exhaustion of the color sensations excited by the patch and the 
ground respectively ; if a yellow ground be chosen after looking at a green 
object, the negative image will appear of a reddish-yellow, and so on. 
§ 670. The theory of three primary sensations does not so readily ex- 
plain why negative images should make their appearance without any sub- 
sequent stimulation of the retina. When the eyes are shut and all access 
of light, even through the eyelids, carefully avoided, the field of vision is not 
absolutely dark ; there is still a sensation of light, the so-called " proper 
light " of the retina. If a white patch on a black ground be looked at for 
some time, and the eyes then shut, a negative (black) image of the spot 
will be seen on the ground of the " proper light " of the retina, having in 
its immediate neighborhood a specially bright corona. So, also, if a window 
be looked at and the eyes then closed, the positive after-image with bright 
panes and dark sashes gives rise to a negative after-image with bright sasbes 
and dark panes ; and similar effects appear with colors. These and similar 
facts have been largely used in support of the Hering theory. When the 
eye has been looking at red, and so has caused dissimilation of the red- 
green substance, mere rest, as on shutting the eyes, favors assimilation of the 
same substance and thus leads to a sensation of green. And the rhythmic 
oscillations from one color to its correlative and back again, frequently 
observed under these conditions and which point to assimilation and dissimi- 
lation alternately gaining the upper hand, are not without analogies in other 
common instances of protoplasmic metabolism. 
Visual Perceptions. 
§ 671. Hitherto we have studied sensations only, and have considered 
an external object, such as a tree, as simply a source of so many distinct 
sensations, differing from each other in intensity and kind (color). In the 
mind these sensations are coordinated into a perception. We are not only VISUAL PERCEPTIONS. 
777 
conscious of a number of sensations of bright and dim lights, of green, 
brown, black, etc., but these sensations are so related to each other and by 
virtue of cerebral processes so fashioned into a whole, that we " see a tree." 
We sometimes, in illustration of such an effect, speak of an image or pic- 
ture in the mind corresponding to the physical image on the retina. 
When we look upon the external world, a variety of images are formed 
at the same time on the retina, and give rise to a number of contempora- 
neous visual sensations. The sum of these sensations constitutes " the field of 
vision," which varies, of course, with every movement of the eye. This 
field of vision, being in reality an aggregate of sensations, is, of course, a 
subjective matter ; but wTe are in the habit of using the same phrase to denote 
the sum of external objects which give rise to the aggregate of visual sensa- 
tions ; in common language the field of vision is " all that we can see " in 
any position of the eye, and we have a field of vision for each eye sepa- 
rately and for the two eyes combined. 
§ 672. Using for the present the words in their subjective sense, we may 
remark that we are able to assign to each constituent sensation its place 
among the aggregate of sensations constituting the field of vision ; we can, 
as we say, localize the sensation. We can say whether it belongs to (what 
we regard as) the right hand or left hand, the upper or the lower part of 
the field of vision. We are able to distinguish the relative positions of 
any two distinct sensations; and the relative positions, together with the re- 
lative intensities and qualities (color) of the sensations arising from any 
object determine our perception of the object. It need hardly be remarked 
that this localization is purely subjective. We simply determine the posi- 
tion of the sensation in the field of vision (which is itself a wholly subjective 
matter) ; we do not determine the position of the object. The connection 
between the position of the object in the external world and the position of 
the sensation in the field of vision cannot be determined by visual observa- 
tion alone. All the information which can be gained by the eye is limited 
to the field of vision, and provided that the relative position of the sensa- 
tions in the field of vision remained the same, the actual position of exter- 
nal objects might, as far as vision is concerned, be changed without our being 
aware of it. 
As a matter of fact the field of vision in one important particular does 
not correspond to the field of external objects. The image on the retina is 
inverted ; the rays of light proceeding from an object which by touch we 
know to be on what we call our right hand, fall on the left-hand side of the 
retina. If, therefore, the field of vision correspond to the retinal image, 
the object would be seen on the left hand. We, however, see it on the right 
hand, because we invariably associate right-hand tactile localization with 
left-hand visual localization ; that is to say, our field of vision, when inter- 
preted by touch, is a re-inversion of the retinal image. 
The dimensions of the field of vision of a single eye are about 145 degrees 
for the horizontal and 100 degrees for the vertical meridian, the former 
being distinctly greater than the latter. The horizontal dimension of the 
field of vision for the tw7o eyes is about 180 degrees. By movements of the 
eyes, however, even apart from those of the head, the extent may be con- 
siderably increased. 
The satisfactory perception of external objects requires distinct vision ; 
and of this, as we have already said, the formation of a distinct image on 
the retina is an essential condition. We can receive visual sensations of all 
kinds with the most imperfect dioptric apparatus, but our perception of an 
object is precise in proportion to the clearness of the image on the retina. 
§ 673. Region of distinct vision. If we take two points, such as two black 778 
SIGHT. 
dots, only just so far apart that they can be seen distinctly as two when 
placed near the axis of vision, and then, keeping the axis fixed, move the 
two points out into the circumferential parts of the field of vision, it will be 
found that the two soon appear as one. The two sensations become fused, as 
they would do if brought nearer to each other in the centre of the field. 
The further away from the centre of the field, the further apart must two 
points be in order that they may be seen as two. In other words, vision is 
much more distinct in the centre of the field than toward the circumference. 
Practically the region of distinct vision may be said to be limited to the 
macula lutea, or even to the fovea centralis ; by continual movements of the 
eye we are constantly bringing any object which we wish to see in such a 
position that its image falls on this region of the retina. 
The diminution of distinctness does not take place equally from the 
centre to the circumference along all meridians. The outline described by 
a line uniting the points where two spots cease to be seen as two when moved 
along different radii from the centre is a very irregular figure. 
The sensations of color are much more distinct in the centre of the retina 
than toward the circumference. If the visual axis be fixed and a piece of 
colored paper be moved toward the outside of the field of vision, the color 
undergoes changes and is eventually lost, red disappearing first, and blue 
last, the object remaining visible, though with very indistinct outlines, when 
its color can no longer be recognized. A purple color becomes blue, and a 
rose color a bluish white. In fact, there seems to be a certain amount of 
red-blindness in the peripheral parts of all retinas. 
Modified Perceptions. 
§ 674. Since our perception of external objects is based on the distinct- 
ness of the sensations which go to form the perception, it might be expected 
that when an image of an object is formed on the retina the sensory impulses 
would correspond to the retinal image, the sensations correspond to the sen- 
sory impulses and the perception correspond to the sensations, and that, 
therefore, the mental condition resulting from our looking at any object or 
view would correspond exactly to the retinal image. We find, however, that 
this is not the case. The sensations and probably even the simple sensory 
impulses produced by an image react upon each other, and these reactions 
modify our perceptions, independently of the physical conditions of the 
retinal image. There arise certain discrepancies between the retinal image 
and the perception, some having their source in the retina, some in the brain, 
and others being of such a nature that it is difficult to say where the 
irrelevancy is introduced. 
§ 675. Irradiation. A white patch on a dark ground appears larger, 
and a dark patch on a white ground smaller, than it really is (Fig. 177). 
This is especially so when the object is somewhat out of focus, and may in 
this, case be partly explained by the diffusion circles which, in each case, 
encroach from the white upon the dark. But over and beyond this, any 
sensation coming from a given retinal area occupies a larger share of the 
field of vision, when the rest of the retina and central visual apparatus are 
at rest, than when they are simultaneously excited. It is as if the neighbor- 
ing, either retinal or cerebral, structures were sympathetically thrown into 
action at the same time. 
§ 676. Contrast. If a white strip be placed between two black strips, 
the edges of the white strip near the black will appear whiter than its 
median portion ; and if a white cross be placed on a black background, the 
centre of the cross will appear sometimes so dim, compared with the parts 779 
VISUAL PERCEPTIONS. 
close to the black, as to seem shaded. This occurs even when the object is 
well in focus; the increased sensation of light which causes the apparent 
greater whiteness of the borders of the cross is the result of the "con- 
trast " with the black placed adjoining it. Still more curious results are 
seen with colored objects. If a small piece of gray paper be placed on a 
sheet of green paper, and both covered with a sheet of thin tissue paper, the 
gray paper will appear of a pink color, the complementary of the green. 
Fig. 177. 
This effect of contrast is far less striking, or even wholly absent, when the 
small piece of paper is white instead of gray, and generally disappears when 
the thin covering of tissue paper is removed. It also vanishes if a bold, 
broad black line be drawn round the small piece of paper, so as to isolate it 
from the ground color. If a book or pencil be placed vertically on a sheet 
of white paper, and illuminated on one side by the sun and the other by a 
candle, two shadows will be produced, one from the sun, which will be illu- 
minated by the yellowish light of the candle, and the other from the candle, 
which will in turn be illuminated by the white light of the sun. The former 
naturally appears yellow; the latter, however, appears not white but blue; 
it assumes, by contrast, a color complementary to that of the candle-light 
which surrounds it. If the candle be removed, or its light shut off by a 
screen, the blue tint disappears, but returns when the candle is again allowed 
to produce its shadow. If, before the candle is brought back, and vision be 
directed through a narrow blackened tube at some part falling entirely 
within the area of what will be the candle's shadow, the area, which in the 
absence of the candle appears white, will continue to appear white when the 
candle is made to cast its shadow, and it is not until the direction of the 
tube is changed so as to cover part of the ground outside the shadow, as well 
as part of the shadow, that the latter assumes its blue tint. 
§ 677. Filling up the blind spot. Though, as we have seen, that part of 
the retina which corresponds to the entrance of the optic nerve is quite in- 
sensible to light, we are conscious of no blank in the field of vision. When 
in looking at a page of print we fix the visual axis so that some of the print 
must fall on a blind spot, no gap, is perceived. We could not expect to see 
a black patch, because what we call black is the absence of the sensation of 
light from structures which are sensitive to light; we must have visual 
organs to see black. But there are no visual organs in the blind spot, and 
consequently we are in no way at all affected by the rays of light which fall 
on it. There is in our subjective field of vision no gap corresponding to the 
gap in the retinal image. We refer the sensations coming from two points 
of the retina lying on opposite margins of the blind spot to two points lying 
close together, since we have no indication of the space which separates 
them. Concerning the effects which are produced when an object in the 
field of view passes into the region of the blind spot there has been much 780 
SIGHT. 
discussion. In ordinary vision, of course, the existence of the blind spot is 
of little moment since it is outside the region used for distinct vision, and 
besides, the image of an object does not fall on the blind spots of both eyes 
at the same time. [See Fig. 179.] 
§678. Ocular spectra. So far from our perceptions exactly correspond- 
ing to the arrangements of the luminous rays which fall on the retina, we 
may have visual sensations and perceptions in the entire absence of light. 
Any stimulation of the retina or of the optic nerve sufficiently intense will 
give rise to a visual sensation. Gradual pressure on the eyeball causes a 
sensation of rings of colored light, the so-called phosphenes ; a sudden blow 
on the eye causes a sensation of flashes of light, and the seeming identity of 
the visual sensations so brought about with visual sensations produced by 
light is well illustrated by the statement once gravely made in a German 
court of law by a witness who asserted that on a pitch-dark night he recog- 
nized an assailant by help of the flash of light caused by the assailant's hand 
coming in violent contact with his eye. Electrical stimulation of the eye 
or optic nerve will also give rise to visual sensations. 
The sensations which may arise without any light falling on the retina 
need not necessarily be undefined ; on the contrary they may be most clearly 
defined. Complex and coherent visual images or perceptions may arise in 
the brain without any corresponding objective luminous cause. These so- 
called ocular spectra or phantoms, which are the result of an intrinsic stimu- 
lation of some (probably cerebral) part of the visual apparatus, have a dis- 
tinctness which gives them an apparent objective reality quite as striking as 
that of ordinary visual perceptions. They may occasionally be seen with 
the eyes open (and therefore while ordinary visual perceptions are being 
generated) as well as when the eyes are closed. They sometimes become so 
frequent and obtrusive as to be distressing, and form an important element 
in some kinds of delirium, such as delirium tremens. 
§ 679. Appreciation of apparent size. By the eye alone we can only esti- 
mate the apparent size of an object, we can only tell what space it takes in 
the field of vision, we can only perceive the dimensions of the retinal image, 
and therefore have a right only to speak of the angle which the diameter of 
the object subtends. The real size of an object must be determined by other 
means. But our perception of even the apparent size of an object is so 
modified by concurrent circumstances that in many cases it cannot be relied 
on. The apparent size of the moon must be the same to every eye, and yet 
while some persons will be found ready to compare the moon in the mid- 
heavens with a three-penny piece, others will liken it to a cart-wheel; that 
Fig. 178. 
is to say, the angle subtended by the moon seems to the one to be about 
equal to that subtended by a three-penny piece held at the distance from the 
eye at which it is most commonly looked at, and to the other about equal to 
that subtended by a cart-wheel similarly viewed at the distance at which it 
is most commonly looked at. If a line such as A C, Fig. 178, be divided 
into two equal parts, A B, B C, and A B be divided by distinct marks into 
several parts, as is shown in the figure, while B Cis left entire, the distance 
A B will always appear greater than CB. So also, if two equal squares be 
marked, one with horizontal and the other with vertical alternate dark and 
light bands, the former will appear higher, and the latter broader, than it 
really is. Hence short persons affect dresses horizontally striped in order to BINOCULAR VISION. 
781 
increase their apparent height, and very stout persons avoid longitudinal 
stripes. Two perfectly parallel lines or bands, each of which is crossed by 
slanting parallel short lines, will appear not parallel, but diverging or con- 
verging according to the direction of the cross-lines. 
Again, when a short person is placed side by side with a tall person, the 
former appears shorter and the latter taller than each really is. The moon 
on the horizon appears larger than when at the zenith, because in the first 
position it can be most easily compared with terrestrial objects. The absence 
of comparison may, however, contribute to an opposite effect, as when a 
person looks larger in a fog; being seen indistinctly, he is judged to be 
further off than he really is, and so appears larger than he naturally would 
do at the distance at which he is supposed to be. So, conversely, distant 
mountains, when seen distinctly in a clear atmosphere appear small, because, 
on account of their distinctness, they are judged to be nearer than they really 
are. Indeed, our daily life is full of instances in which our direct percep- 
tion is modified by circumstances. Among those circumstances previous 
experience is one of the most potent, and thus simple perceptions become 
mingled with what are in reality judgments, though frequently made uncon- 
sciously. But this intrusion of past experience into present perceptions and 
sensations is most obvious in binocular vision, to which we now turn. 
Binocular Vision. 
Corresponding or Identical Points. 
§ 680. Though we have two eyes, and must therefore receive from every 
object two sets of sensations, our perception of an object is under ordinary 
circumstances a single one; we see one object, not two. But putting either 
eye into an unusual position, as by squinting, we can render the perception 
double; we see two objects where one only exists. From which it is evi- 
dent that singleness of perception depends on the image of the object fall- 
ing on certain parts of each retina at the same time, these parts being so 
related to each other that the sensations from each are blended into one per- 
ception ; and it is also evident that the movements of the eyeballs are 
adapted to bring the image of the object to fall on these " corresponding " 
or "identical" parts, as they are called, of each retina. 
When we look at an object with one eye the visual axis of that eye is 
directed to the object, and when we use two eyes the visual axes of the two 
eyes converge at the object, the eyeballs moving accordingly. The corre- 
sponding points of the two retinas are those on which the two images of the 
object fall when the visual axes converge at the object. Thus in Fig. 179, 
if xx, x x' be the two visual axes, x x' being the centres of the foveae centrales 
of the two eyes, then, the object y, x, z being seen single, the point y on the 
one retina will " correspond " to or be " identical " with the point y' on the 
other, and the point x in the one to the point x' in the other. Hence a point 
lying anywhere on the right side of one retina has its corresponding point 
on the right side of the other retina, and the points on the left of one corre- 
spond with those on the left of the other. Thus, while the upper half of the 
retina of the left eye corresponds to the upper half of the retina of the right 
eye and the lower to the lower, the nasal side of the left eye corresponds 
with the malar side of the right, and the malar of the left with the nasal 
side of the right. 
The blending of the two sensations into one only occurs when the two 
images of an object fall on these corresponding points of the two retinas. 
Hence it is obvious that in single vision with two eyes the ordinary move- 782 
SIGHT. 
ments of the eyeballs must be such as to bring the visual axes to converge 
at the object so that the two images may fall on corresponding points. 
When the visual axes do not so converge, and when, therefore, the images do 
not fall on corresponding points, the two sensations are not blended into one 
perception and vision becomes double. 
Fig. 179. 
Diagram illustrating Corresponding or Identical Points: L, the left, R, the right eye ; n, nodal 
point; o, optic nerve (blind spot); x, fovea, x' y' z' in the right eye are corresponding points to x y 
z in the left eye. v I, visual axis. The two figures below are projections of^L the left and R the 
right retina, f, fovea; o, blind spot, a and c on the temporal side of L correspond to a' and c' on the 
nasal side of R. vmhm, lines separating quadrants. 
Movements of the Eyeballs. 
§ 681. The eye is virtually a ball placed in a socket, the bulb and the 
orbit forming a ball-and-socket joint. In its socket-joint the optic ball is 
capable of a variety of movements, but it cannot by any voluntary effort be 
moved out of its socket. It is stated that by a very forcible opening of the 
eyelids the eyeball may be slightly protruded ; but this trifling locomotion 
may be neglected. By disease, however, the position of the eyeball in the 
socket may be materially changed. 
Each eyeball is capable of rotating round an immobile centre of rotation, 
which has been found to be placed a little (1.77 mm.) behind the centre of 
the eye ; but the movements of the eye round the centre are limited in a pecu- BINOCULAR VISION. 
783 
liar way. The shoulder-joint is also a ball-and-socket joint; and we know that 
we can not only move the arm up and down round a horizontal axis passing 
through the centre of rotation of the head of the humerus, and from side to 
side round a vertical axis, but we can 
also rotate it round its own longitudi- 
nal axis. When, however, we come to 
examine closely the movements of the 
eyeball we find that though we can 
move it up and down round a horizon- 
tal axis, as when with fixed head we 
direct our vision to the heavens or to 
the ground, and from side to side, as 
when we look to left or right, and 
though by combining these two move- 
ments we can give the eyeball a variety 
of inclinations, we cannot, by a volun- 
tary effort, rotate the eyeball round 
its longitudinal visual axis. The ar- 
rangement of the muscles of the eye- 
ball will permit of such a movement, 
but we cannot by any direct effort of 
will bring it about by itself. In cer- 
tain movements of the eye rotation of 
the eyeball does take place and by 
bringing about these movements we 
can indirectly cause rotation ; but wre 
cannot rotate the eyeball except thus 
indirectly as a part of these movements. 
If, when vision is directed to any 
object the head be moved from side to 
side, the eyes do not move with it; they 
appear to remain stationary, very 
much as the needle of a ship's compass remains stationary when the head 
of the ship is turned. The change in the position of the visual axes to which 
the movement of the head would naturally give rise is met by compensating 
movements of the eyeballs; were it not so, steadiness of vision would be 
impossible. 
§ 682. There is one position of the eyes which has been called the primary 
position. It corresponds to that which may be attained by looking at the 
distant horizon with the head vertical and the body upright; but its exact 
determination requires special precautions. The visual axes are then parallel 
to each other and to the median plane of the head. All other positions of 
the eyes are called secondary positions. 
§ 683. Muscles of the eyeball. The eyeball is moved by six muscles, the 
recti inferior, superior, internus, and ext emus, and the obliqui inferior and 
superior. It is found by calculation from the attachments and directions of 
the muscles, and confirmed by actual observation, that the six muscles may 
be considered as three pairs, each pair rotating the eye round a particular 
axis. The relative attachments and the axes of rotation are diagrammati- 
cally shown in Fig. 180. The rectus superior and the rectus inferior rotate 
the eye round a horizontal axis, which is directed from the upper end of the 
nose to the temple; the obliquus superior and obliquus inferior round a 
horizontal axis directed from the centre of the eyeball to the occiput; and 
the rectus internus and rectus externus round a vertical axis (which, being 
at right angles to the plane of the paper, cannot be shown in the diagram), 
Fig. 180. 
Diagram of the Attachments of the Mus- 
cles of the Eye and of their Axes of Rota- 
tion, the latter being represented by dotted 
lines. 
The axis of rotation of the rectus externus 
and internus, being perpendicular to the 
plane of the paper, cannot be shown. (After 
Fick.) 784 
SIGHT. 
passing through the centre of rotation of the eyeball parallel to the median 
plane of the head when the head is vertical. Thus the latter pair acting 
alone would turn the eye from side to side, the other straight pair acting 
alone would move the eye up and down, while the oblique muscles acting 
alone would give the eye an oblique movement. The rectus externus acting 
alone would turn the eye to the malar side, the internus to the nasal side, 
the rectus superior upward, the rectus inferior downward, the oblique superior 
downward and outward, and the inferior upward and outward. The recti 
superior and inferior in moving the eye up and down also turn it somewhat 
inward and at the same time give it a slight amount of rotation ; but this 
is corrected if the oblique muscles act at the same time; and it is found 
that the rectus superior acting with the obliquus inferior moves the eye up- 
ward, and the rectus inferior with the obliquus superior downward in a 
vertical direction. In oblique movements also, the obliqui are always asso- 
ciated with the recti. Hence the various movements of the eyeball may be 
arranged as follows: 
Straight 
movements. 
Elevation. Rectus superior and obliquus inferior. 
Depression. Rectus inferior and obliquus superior. 
Adduction to t> . • * 
nasal side. Rectus intemus. 
Adduction to t> A t 
malar side. Rectus extennis. 
Elevation with Rectus superior and internus with obliquus 
adduction. inferior. 
Depression Rectus inferior and internus with obliquus 
with adduction. superior. 
Elevation with Rectus superior and externus with obliquus 
abduction. inferior. 
Depression Rectus inferior and externus with obliquus 
with abduction. superior. 
Oblique 
movements. 
§ 684. Coordination of visual movements. Thus even in the movements 
of a single eye a considerable amount of coordination takes place. When 
the eye is moved in any other than the vertical and horizontal meridians, 
impulses must descend to at least three muscles, and in such relative energy 
to each of the three as to produce the required inclination of the visual 
axis. But the coordination observed in binocular vision is more striking 
still. If the movements of any person's eyes be watched it will be seen that 
the two eyes move alike. If the right eye moves to the right, so does also 
the left; and, if the object looked at be a distant one, exactly to the same 
extent; if the right eye looks up, the left eye looks up also, and so in every 
other direction. Very few persons are able by a direct effort of the will to 
move one eye independently of the other; though some, and among them 
one distinguished both as a physiologist and an oculist, have acquired this 
power. In fact, the movements of the two eyes are so arranged that in the 
various movements the images of any object should fall on the corresponding 
points of the two retinas, and that thus single vision should result. We 
cannot by any direct effort of our will place our eyes in such a position that 
the rays of light proceeding from any object shall be brought to a focus on 
parts of the two retinas which do not correspond, and thus give rise to two 
distinct visual images. We can bring the visual axes of the two eyes from 
a condition of parallelism to one of great convergence, but we cannot, with- 
out special assistance, bring them from a condition of parallelism to one of 
divergence. The stereoscope will enable us to create a divergence. If in a 
stereoscopic picture the distance between the pictures be increased very 
gradually so as carefully to maintain the impression of a single object, the BINOCULAR VISION. 
785 
visual axes may be brought to diverge. Similarly if a distant object be 
looked at with a prism before one eye, and the image of the object be kept 
carefully single, while the prism is turned very slowly up or down, then on 
suddenly removing the prism a double image is for a moment seen ; show- 
ing that the eye before which the prism was placed has moved in disaccord- 
ance with the other. The double image, however, in a few seconds after the 
removal of the prism becomes single, on account of the eyes coining into 
accordance. 
It is only when loss of coordination occurs, as in various diseases and in 
alcoholic or other poisoning, that the movements of the two eyes cease to 
agree with each other. It is evident, then, that when we look at an object 
to the right, since we thereby abduct the right eye and adduct the left, we 
throw into action the rectus externus of the right eye and the rectus intern us 
of the left, and similarly when we look to the left we use the rectus externus 
of the left and the rectus internus of the right eye. On the other hand, 
when we look at a near object, and therefore converge the visual axes, we 
use the recti interni of both eyes ; and when we look at a distant object, and 
bring the axes from converging toward parallelism, we use the recti externi 
of both eyes. In the various movements of the eye there is therefore, so to 
speak, the most delicate picking and choosing of the muscular instruments. 
Bearing this in mind, it cannot be wondered at that the various movements 
of the eye are dependent for their causation on visual sensations. In order 
to move our eyes we must either look at or for an object; when we wish to 
converge our axes we look at some near object, real or imaginary, and the 
convergence of the axes is usually accompanied by all the conditions of near 
vision, such as increased accommodation and contraction of the pupil. And 
so with other movements. The close association of the movements of the 
eye may be illustrated by the following case: Suppose the eyes, to start with, 
accommodated for the far distance, and that it is desired to direct attention to a 
nearer point lying in the visual line of the right eye. In this case no move- 
ment of the right eye is required ; all that is necessary is for the left eye to 
be turned to the right, that is, for the rectus internus of the left eye to be 
thrown into action. But in ordinary movements the contraction of this 
muscle is always associated with either the rectus externus of the right eye, 
as when both eyes are turned to the right, or the rectus internus of that eye, 
as in convergence ; the muscle is quite unaccustomed to act alone. This 
would lead us to suppose that in the case in question the contraction of the 
rectus internus of the left eye is accompanied by a contraction of both recti 
externus and internus of the right eye, keeping that eye in lateral equilib- 
rium. And the peculiar oscillating movements seen in the right eye, as well 
as the sense of efforts in the right eye which is felt by the person, show this 
to be the case. 
§ 685. Such a complex coordination requires for its carrying out a dis- 
tinct nervous machinery, and we have reasons for thinking that such a 
machinery exists in certain parts of the corpora quadrigemina or in the 
underlying structures. In the nates there appears to be a common centre 
for both eyes, stimulation of the right side producing movements of both 
eyes to the left, of the left side movements to the right; while stimula- 
tion in the middle line behind causes a downward movement of both 
eyes with convergence of the axes and in the front an upward move- 
ment with return to parallelism, both accompanied by the naturally asso- 
ciated movements of the pupil. Stimulation of various parts of the nates 
causes various movements, depending on the position of the spot stimu- 
lated. After an incision in the middle line, stimulation of the nervous 
centre on one side produces movements in the eye of the same side only. 786 
SIGHT. 
The Horopter. 
§ 686. When we look at any object we direct to it the visual axis, so that 
when the object is small, the " corresponding" parts of the two retinas, on 
which the two images of the object fall, lie in their respective fovese cen- 
trales. But while we are looking at the particular object, the images of other 
objects surrounding it fall on the retina 
surrounding the fovea, and thus go to 
form what is called indirect vision. And 
it is obviously of advantage that these 
images also should fall on " correspond- 
ing " parts in the two eyes. Now for any 
given position of the eyes there exists in 
the field of vision a certain line or sur- 
face of such a kind that the images of 
the points in it all fall on corresponding 
points of the retina. A line or surface 
having this property is called a horopter. 
The horopter is, in fact, the aggregate 
of all those points in space which are 
projected on to corresponding points 
of the retina; hence its determination 
in any particular case is simply a mat- 
ter of geometrical calculation. In some 
instances it becomes a very complicated 
figure. The case whose features are 
most easily grasped is a circle drawn 
in the plane of the two visual axes 
through the point of the convergence of the axesand the optic centres of the 
two eyes. It is obvious from geometrical relations that in Fig. 181 the 
images of any point in the circle will fall on corresponding points of the 
two retinas. When we stand upright and look at the distant horizon, the 
horopter is (approximately, for normal emmetropic eyes) a plane drawn 
through our feet, that is to say, is the ground on which we stand ; the 
advantage of this is obvious. 
Fig. 181. 
Diagram illustrating a Simple Horopter. 
When the visual axes converge at C, the 
images a, a of any point A on the circle 
drawn through C and the optical centres 
k, k will fall on corresponding points. 
Visual Judgments. 
§ 687. Binocular vision is of use to us, inasmuch as the one eye is able 
to fill up the gaps and imperfections of the other. For example, over and 
above the monocular filling up of the blind spot, of which we spoke on page 
779, since the two blind spots of the two eyes, being each on the nasal side, 
are not " corresponding " parts, the one eye supplies that part of the field 
of vision which is lacking in the other. And other imperfections are sim- 
ilarly made good. But the great use of binocular vision is to afford us 
means of forming visual judgments concerning the form, size, and distance 
of objects. 
§ 688. Judgment- of distance and size. The perceptions which we gain 
simply and solely by our field of vision concern two dimensions only. We 
can become aware of the apparent size of any part of the field correspond- 
ing to any particular object, and of its topographical relations to the rest of 
the field, but no more. Had we nothing more to depend on, our sight would 
be almost valueless as far as any exact information of the external world 
was concerned. By the association of visual sensations with sensations of 
touch, and with sensations derived from the movements of the eyeballs re- 
quired to make any such part of the field as corresponds to a particular VISUAL JUDGMENTS. 
787 
object distinct, we are led to form judgments, i. e., to draw conclusions con- 
cerning the external world by means of an interpretation of our visual per- 
ceptions. Looking before us, we say we see a certain object of a certain 
color nearly in front of us or much on our right hand or much on our 
left; that is to say, we judge such an object to be in such a position because 
from the constitution of our brain, strengthened by all our experience, we 
associate such a part of our field of vision with such an object. The sub- 
jective visual complex sensation or perception is to us a symbol of the 
external object. 
Even with one eye we can, to a certain extent, form a judgment, not only 
as to the position of the object in a plane at right angles to our visual axis, 
but also as to its distance from us along the visual axis. If the object is 
near to us, we have to accommodate for near vision ; if far from us, to relax 
our accommodation mechanism so that the eye becomes adjusted for distance. 
The muscular sense (of which we shall speak presently) of this effort enables 
us to form a judgment whether the object is far or near. Seeing the narrow 
range of our accommodation, and the slight muscular effort which it entails, 
all molecular judgments of distance must be subject to much error. Every- 
one who has tried to thread a needle without using both eyes knows how 
great these errors may be. When, on the other hand, we use two eyes, we 
have still the variations in accommodation, and, in addition, have all the 
assistance which arises from the muscular effort of so directing the two 
eyes on the object that single vision will result. When the object is near, 
we converge our visual axes ; when distant, we bring them back toward 
parallelism. This necessary contraction of the ocular muscles affords a 
muscular sense, by the help of which we form a judgment as to the distance 
of the object. Hence, when by any means the convergence which is neces- 
sary to bring the object into single vision is lessened, the object seems to 
become more distinct, when increased, to move toward us-as may be seen 
in the stereoscope. 
§ 689. The judgment of size is closely connected with that of distance. 
Our perceptions, gained exclusively from the field of vision, go no further 
than the apparent size of the image, i. e., of the angle subtended by the 
object. The real size of the object can only be gathered from the apparent 
size of the image when the distance of the object from the eye is known. 
Thus perceiving directly the apparent size of the image, we judge the dis- 
tance of the object giving the image, and upon that come to a conclusion as 
to its size. And, conversely, when we see an object, of whose real size we 
are otherwise aware, or are led to think we are aware, our judgment of its 
distance is influenced by its apparent size. Thus when in our field of vision 
there appears the image of a man, knowing otherwise the ordinary size of a 
man, we infer, if the image be very small, that the man is far off. The 
reason of the image being small may be because the man is far off, in which 
case our judgment is correct; it may be, however, because the image has 
been lessened by artificial dioptric means, as when the man is looked at 
through an inverted telescope, in which case our judgment becomes a delu- 
sion. So also an image on a screen when gradually enlarged seems to come 
forward, when gradually diminished seems to recede. In these cases the 
influence on our judgment of the muscular sense of binocular adjustment, or 
monocular accommodation, is thwarted by the more direct influence of the 
association between size and distance. 
§ 690. Judgment of solidity. When we look at a small circle, all parts 
of the circle are at the same distance from us, all parts are equally distinct 
at the same time, whether we look at it with one eye or with two eyes. 
When, on the other hand, we look at a sphere, the various parts of which are 788 
SIGHT. 
at different distances from us, a sense of the accommodation, but much more 
a sense of the binocular adjustment, of the convergence or the opposite of the 
two eyes, required to make the various parts successively distinct, makes us 
aware that the various parts of the sphere are unequally distant; and from 
that we form a judgment of its solidity. As with distance of objects, so with 
solidity, which is at bottom a matter of distance of the parts of an object, we 
can form a judgment with one eye alone; but our ideas become much more 
exact and trustworthy when two eyes are used. And we are much assisted 
by the effects produced by the reflection of light from the various surfaces 
of a solid object; so much so, that raised surfaces may be made to appear 
depressed, or vice versa, and flat surfaces either raised or depressed, by 
appropriate arrangements of shadings and shadow. 
Fig. 182. 
§ 691. Binocular vision, moreover, affords us a means of judging of the 
solidity of objects, inasmuch as the image of any solid object which falls on 
the right eye cannot be exactly like that which falls on the left, though 
both are combined in a single perception of the two eyes. Thus, when we 
look at a truncated pyramid placed in the middle line before us, the image 
which falls on the right eye is of the kind represented in Fig. 182, R, while 
that which falls on the left eye has the form of Fig. 182, L; yet the percep- 
tion gained from the two images together corresponds to the form of which 
Fig. 182, B, is the projection. Whenever we thus combine in one perception 
two dissimilar images, one of the one and the other of the other eye, we 
judge that the object giving rise to the images is solid. 
This is the simple principle of the stereoscope, in which two slightly dis- 
similar pictures, such as would correspond to the vision of each eye sep- 
arately, are, by means of reflecting mirrors, as in Wheatstone's original 
instrument, or by prisms, as in the form introduced by Brewster, made to 
cast images on corresponding parts of the two retinas, so as to produce a 
single perception. Though each picture is a surface of two dimensions only, 
the resulting perception is the same as if a single object, or group of objects, 
of three dimensions had been looked at. 
It might be supposed that the judgment of solidity which arises when two 
dissimilar images are thus combined in one perception was due to the fact 
that all parts of the two images cannot fall on corresponding parts of the 
two retinas at the same time, and that therefore the combination of the two 
needs some movement of the eyes. Thus, if we superimpose R on L (Fig. 
182), it is evident that when the bases coincide the truncated apices will not, 
and vice versa; hence, when the bases fall on corresponding parts, the apices 
will not be combined in one image, and vice versa; in order that both may 
be combined, there must be a slight, rapid movement of the eyes from the 
one to the other That, however, no such movement is necessary for each 
particular case is shown by the fact that solid objects appear as such when 
illuminated by an electric spark, the duration of which is too short to permit 
of any movement of the eyes. If the flash occurred at the moment that 
the eyes were binocularly adjusted for the bases of the pyramids, the two THE PROTECTED MECHANISMS OF THE EYE. 
789 
apices not falling on exactly corresponding parts would give rise to two 
perceptions, and the whole object ought to appear confused. That it does 
not, but, on the contrary, appears a single solid, must be the result of cere- 
bral operations, resulting in what we have called a judgment. 
§ 692. Struggle of the two fields of vision. If the images of two surfaces, 
one black and the other white, are made to fall on corresponding parts of the 
eye, so as to be united into a single perception, the result is not always a 
mixture of the two impressions, that is, a gray, but, in many cases, a sensa- 
tion similar to that produced when a polished surface, such as plumbago, is 
looked at: the surface appears brilliant. The reason, probably, is because 
when we look at a polished surface, the amount of reflected light which falls 
upon the retina is generally different in the two eyes ; and hence we associate 
an unequal stimulation of the two retinas with the idea of a polished surface. 
So, also, when the impressions of two colors are united in binocular vision, 
the result is, in most cases, not a mixture of the two colors, as when the same 
two impressions are brought to bear together at the same time on a single 
retina, but a struggle between the two colors, now one and now the other, 
becoming prominent, intermediate tints, however, being frequently passed 
through. This may arise from the difficulty of accommodating at the same 
time for the two different colors (see p. 758); if two eyes, one of which is 
looking at red and the other at blue, be both accommodated for red rays, 
the red sensation will overpower the blue, and vice versa. It may be, how- 
ever, that the tendency to rhythmic action, so manifest in other simpler 
manifestations of protoplasmic activity, makes its appearance also in the 
higher cerebral labors of binocular vision. 
The Protected Mechanisms of the Eye. 
§ 693. The eyeball is protected by the eyelids, which are capable of move- 
ments called respectively opening and shutting the eye. The eye is shut by 
the contraction of the orbicularis muscle, carried out either as a reflex or 
voluntary act by means of the facial nerve. The eye is opened chiefly by 
the raising of the upper eyelid through the contraction of the levator pal- 
pebrae carried out by means of the third nerve. The upper eyelid is also 
raised and the lower depressed, the eye being thus opened, by means of 
plain muscular fibres existing in the two eyelids and governed by the cer- 
vical sympathetic. The shutting of the eye, as in winking, is in general 
effected more rapidly than the opening. 
The eye is kept continually moist partly by the secretion of the glands 
in the conjunctiva, and of the Meibomian glands, but chiefly by the secre- 
tion of the lachrymal gland. Under ordinary circumstances the fluid thus 
formed is carried away by the lachrymal canals into the nasal sac and thus 
into the cavity of the nose. When the secretion becomes too abundant to 
escape in this way it overflows on to the cheeks in the form of tears. 
If a quantity of tears be collected, they are found to form a clear, faintly 
alkaline fluid, in many respects like saliva, containing about 1 per cent, of 
solids, of which a small part is proteid in nature. Among the salts present 
sodium chloride is conspicuous. 
§ 694. The nervous mechanism of the secretion of tears, in many re- 
spects, resembles that of the secretion of saliva. A flow is usually brought 
about either in a reflex manner by stimuli applied to the conjunctiva, the 
nasal mucous membrane, tongue, optic nerve, etc., or more directly by 
emotions. Venous congestion of the head is also said to cause a flow. The 
efferent nerves belong either to the cerebro-spinal system (the lachrymal and 790 
HEARING, SMELL, AND TASTE. 
orbital branches of the fifth nerve) or arise from the cervical sympathetic, 
the afferent nerves varying according to the exciting cause. 
The act of blinking undoubtedly favors the passage of tears through the 
lachrymal canals into the nasal sac, and hence when the orbicularis is para- 
lyzed tears do not pass so readily as usual into the nose; but the exact 
mechanism by which this is effected has been much disputed. According 
to some authors, the contraction of the orbicularis presses the fluid onward 
out of the canals, which upon the relaxation of the orbicularis dilate and 
receive a fresh quantity. Others maintain that a special arrangement of 
muscular fibres keeps the canals open even during the closing of the lids, so 
that the pressure of the contraction of the orbicularis is able to have full 
effect in driving the tears through the canals. 
CHAPTER IV. 
HEARING, SMELL, AND TASTE. 
§ 695. As in the eye, so in the ear, we have to deal first with a nerve 
of special sense, the stimulation of which gives rise to a special sensation ; 
secondly, with terminal organs through which the physical changes proper 
to the special sense are enabled to act on the nerve; and thirdly, with sub- 
sidiary apparatus, by which the usefulness of the sense is increased. The 
central connections of the auditory nerve are such that whenever the audi- 
tory fibres are stimulated, whether by means of the terminal organs in the 
usual way or by the direct application of stimuli, electrical, mechanical, etc., 
the result is always a sensation of sound. Just as stimulation of the optic 
fibres produces no other sensation than that of light, so stimulation of the 
auditory fibres produces no other sensation than that of sound.1 The ter- 
minal organs of the auditory nerve are of two kinds : the complicated organ 
of Corti in the cochlea, and the epithelial arrangements of the maculae and 
cristae acusticae in other parts of the labyrinth. Waves of sound falling on 
the auditory nerve itself produce no effect whatever ; it is only when by the 
medium of the endolymph they are brought to bear on the delicate and 
peculiar epithelium cells which constitute the peripheral terminations of the 
nerve, that sensations of sound arise. Such delicate structures are for the 
sake of protection naturally withdrawn from the surface of the body where 
they would be subject to injury. Hence, the necessity of an acoustic appa- 
ratus, forming the middle and external ear, by which the waves of sound 
are most advantageously conveyed to the terminal organs. 
Hearing. 
§ 696. [The ear, or organ of hearing, is composed of three parts, called 
the external, middle, and internal ear. 
The external ear consists of an outer projecting portion, called the pinna, 
and the auditory canal, or meatus auditorius externus. (Fig. 183.) The 
pinna is a somewhat oblong funnel-shaped organ, the smaller portion of the 
funnel being attached to the skull by ligamentous tissue, the larger portion 
serving to collect and convey the sonorous undulations to the meatus. It is 
composed of cartilage covered by integument. Its surface is irregularly 
1 It will be seen later on that there are reasons for thinking that impulses passing along 
the auditory nerve may give rise to other effects than auditory sensations. HEARING. 
791 
curved and depressed. The outer projecting rim is the helix; anterior to 
the helix is a second elevation, called the antihelix, which describes a curve 
partially around a deep depression which leads to the meatus, called the 
Fig. 183. 
Vertical Section of the Meatus Auditorius and Tympanum. (Scarpa.) a, cartilaginous part 
of the meatus; b, osseous portion ; c, membrana tympani; d, cavity of the tympanum; e, Eusta- 
chian tube. 
Fig. 185. 
Fig. 184. 
Fig. 184.-Inner view of the Membrana 
Tympani in the Foetus, with the Malleus at- 
tached. a, membrane or drum of the tym- 
panum ; b, malleus; c, band of circular fibres 
at the circumference ; d, inferior, and e, supe- 
rior tympanic artery; /, tympanic bone. 
Fig. 185.-Plan of the Ossicles in Position in the Tympanum, with their Muscles, a, cavity of 
the tympanum; b, membrana tympani; c, Eustachian tube; d, malleus; e, incus; f, stapes- 
g, laxator tympani muscle ; h, tensor tympani; i, stapedius. 
concha. Between the helix and antihelix is the fossa of the helix. The 
antihelix bifurcates at its superior portion, and encloses the fossa of the anti- 
helix. Projecting posteriorly from the anterior portion of the concha is a 792 
HEARING, SMELL, AND TASTE. 
papillary prominence called the tragus; posterior to this, separated by a 
fissure, is the antitragus, which is a continuation of the helix. On the 
inferior portion of the pinna is a soft pendulous portion, termed the 
lobule. The meatus leads from the concha to the middle ear, from which it 
is separated by the tympanic membrane. Its direction is forward, inward, 
and slightly upward ; its lowrer surface being longer than the upper, on ac- 
count of the obliquity of the position of the tympanic membrane. The canal 
consists of an external membrano-cartilag inous portion, which is continuous 
with the pinna, and an internal osseous portion formed by the mastoid bone. 
In the external portion of the canal are found numerous hairs and seba- 
ceous glands ; in the internal portion are found the ceruminous glands, which 
secrete a peculiar substance commonly known as the earwax. 
§ 697. The middle ear or tympanum is an irregular flattened cavity, 
situated in the petrous portion of the temporal bone, and lined with a mu- 
cous membrane. It is separated from the meatus by a membranous dia- 
phragm, which is the tympanic membrane; and from the internal ear by an 
osseo-membranous partition, which formsa common wall for both. Through 
the Eustachian tube it communicates with the pharynx. On its posterior 
wall are seen orifices of the mastoid cells. The tympanic membrane is a 
semi-transparent oval membrane, concave on its external and convex on its 
Fig. 186. 
Fig. 187. 
Natural size. 
Interior of the Osseous Labyrinth. (After Sommerring.) V, vestibule; ar, aqueduct of the 
vestibule; o, fovea hemielliptiea; r, fovea hemispherical S, semicircular canals; s, superior: p, 
posterior; i, horizontal; a, a, a, the ampullar extremity of each ; C, cochlea; ac, aqueduct of the 
cochlea ; sr, osseous zone of the lamina spiralis, above which is the scala vestibuli, communicat- 
ing with the vestibule ; st, scala tympani below the spiral lamina. 
internal surface, where it has attached the long process of the malleus, one 
of the ossicles. It is placed in an oblique position, sloping downward, for- 
ward, and inward at an angle of about 45°. Its circumference is attached 
to a groove in the temporal bone. In the foetus this portion of the bone 
exists as a separate piece, called the tympanic bone (Fig. 184), but it after- 
ward becomes ossified to the temporal. The tympanic membrane consists 
of three layers-the external, middle, and internal. The external is a 
continuation of the integument covering the meatus ; the internal is a con- 
tinuation of the mucous membrane lining the tympanum ; the middle layer, 793 
HEARING. 
which is the most important, is tense, strong, and fibrous, made up of cir- 
cular and radiating fibres, with a small amount of elastic tissue intermixed. 
§ 698. In the internal wall of the tympanum are two small openings- 
the Jenera ovalis and fenestra rotunda-which communicate with the laby- 
rinth. The fenestra rotunda is closed by a membrane. Extending between 
the tympanic membrane and the fenestra ovalis are the ossicles, consisting 
of three small bones, which form a system of levers. These ossicles are 
termed, from their resemblance to particular objects, the malleus, incus, and 
stapes. (Fig. 185.) The malleus consists of a head, neck, long and short 
process, and handle. The head articulates with the roof of the tympanum 
and in a depression of the incus; the handle is directed downward and 
attached by its whole length to the tympanic membrane; the long process 
( processus gracilis) is directed forward and has attached the insertion of the 
laxator tympani muscle; the short process, which is at the base of the long 
process, has attached the insertion of the tensor tympani muscle. The incus 
consists of a body, a long and short process. The body of the incus has a 
depression in which articulates the head of the malleus; the short process 
is attached to the posterior wall of the tympanum ; the long process (len- 
ticular process) is placed almost vertically, and at its end is a rounded 
Fig. 188. 
Representation of the Semicircular Canals Enlarged. (From a model in University College 
Museum.) a, superior vertical; b, posterior or inferior vertical; and c, horizontal canal; d, com- 
mon opening of the two vertical canals ; e, part of the vestibular cavity ; f, opening of the aque- 
duct of the vestibule. 
process (the os orbiculare), which articulates with the head of the stapes. 
The stapes consists of a head, neck, two crura, and a base. The head arti- 
culates with the long process of the incus; the neck serves as a point of in- 
sertion of the stapedius muscle; the crura diverge from the neck and unite 
with the oval base at its greatest diameter. The base is fixed in the fe- 
nestra ovalis by attachments formed by the lining membranes of both the 
tympanum and internal ear. These ossicles are connected with each other 
and to the walls of the tympanum by ligaments, and at their articulations 
they are furnished with cartilages and synovial membranes. They are en- 
veloped by prolongations of the mucous membrane lining the tympanum. 
§ 699. The internal ear or labyrinth is the most essential portion of the 
auditory apparatus. It consists of three portions-the vestibule, semicircular 794 
HEARING, SMELL, AND TASTE. 
canals, and cochlea-and is situated within the petrous portion of the tem- 
poral bone. Within the osseous labyrinth is a membranous labyrinth to 
which the auditory nerve is distributed. The vestibule is an irregular 
chamber which serves as a common means of communication between the 
tympanum and the semicircular canals and cochlea. On its external wall 
is the fenestra ovalis, closed by the base of the stapes. On its internal wall 
is a depression called the fovea hemispherica, which is perforated by minute 
openings for the passage of auditory nerve-filaments. Above and posterior 
to this depression is another, the fovea hemielliptica. Posterior to the fovea 
hemispherica is the orifice of the aqueductus vestibuli. In the posterior 
wall are five openings leading to the semicircular canals. Anteriorly, it 
communicates with the cochlea by the aperturce scales, vestibuli cochleae. The 
semicircular canals are three in number-superior, posterior or inferior, and 
horizontal. They form the greater portion of a circle, and communicate 
with the vestibule by five openings, one of which is common to the superior 
and horizontal canals. The superior canal is situated vertically and at 
right angles with the posterior surface of the petrous bone ; the posterior 
canal is also vertical and parallel with the posterior surface of the petrous 
bone; the inferior canal is placed horizontally and at right angles to the 
others. At the commencement of each of these canals is a dilated portion, 
called the ampulla. 
§ 700. The cochlea occupies the anterior portion of the labyrinth. Its 
base, which corresponds to the internal auditory meatus, is perforated by 
many minute orifices for the passage of filaments of the cochlear branch of 
the auditory nerve. The cochlea consists of a central axis, or modiolus, 
which has a spiral canal wound around it. This canal makes two and a 
half complete turns, and terminates in the apex of the cochlea in an ex- 
pansion termed the infundibulum. (Fig. 189.) The modiolus is somewhat 
Fig. 189. 
Section through the Cochlea. (Breschet.) a, axis with its canals; b, infundibulum or en- 
larged upper end of the axis; c, septum of the cochlea; d, membrane of Corti; e, membrane of 
Reissner ; f, hiatus or hellicotrema ; st, scala tympani; sv, scala vestibuli. 
cone-shaped, and forms the internal wall of the canal, being perforated in its 
centre and sides by apertures for the passage of the filaments of the audi- 
tory nerve. The canal is divided into two passages or scalce by a septum 
called the lamina spiralis, which is partly osseous and partly membranous. 
The osseous portion projects from the modiolus, midway across the canal; 
it consists of two laminae, between which the nerve-filaments run. The 
membranous portion extends from the external margin of the osseous lamina HEARING. 
795 
to the external wall of the canal. It consists of two layers; the superior is 
the membrane of Corti, or membrana tectoria; the inferior the membrana 
basilaris, which is attached externally to the planum semilunare. These 
membranes are placed parallel with each other and contain between them 
the organ of Corti, which rests on the basilary membrane. (Fig. 190.) 
§ 701. The scala vestibuli1 communicates below with the vestibule by the 
apertures scales vestibuli cochleae; the lower passage, or scala tympani, com- 
municates with the tympanum by the fenestra rotunda. These scalse com- 
municate at the apex of the cochlea by an opening termed the hiatus or 
helicotrema, which exists in consequence of a deficiency of the last half turn 
of the lamina spiralis. 
The osseous portion of the lamina spiralis has on its superior external 
portion a denticulated cartilaginous substance called the lamina denticulata. 
From the superior surface of the spiral lamina, and internal to the lamina 
denticulata, is a delicate membrane extending upward and outward at an 
angle of about 45 degrees to the external wall of the scala. This is called 
the membrane of Reissner. It divides the scala into two passages, the lower 
of which is the ductus cochlearis. This duct ends in the apex of the cochlea 
in a coeca, and communicates at the base with the saccule by the ductus 
reuniens; it contains the essential portion of the auditory apparatus of the 
cochlea, and is a part of the membranous labyrinth. 
Fig. 190. 
A Diagram of a Section of the Tube of the Cochlea, enlarged. (Modified from Henle.): ST, 
scala vestibuli; ST, scala tympani; CC, canal of the cochlea; 1, membrane of Reissner; 2,coch- 
lear branch of the auditory nerve; 3, lamina spiralis ossea ; 4, planum semilunare; a, lamina 
denticulata ; b, sulcus spiralis; c, tympanic lip of the sulcus spiralis; d, inner rods of Corti; e, outer 
rods of Corti;/, lamina reticularis; i, inner hair-cells ; mb, membrana basilaris; me, membrane of 
Corti; p, outer hair-cells; sm, central space between the rods. 
§ 702. The organ of Corti rests upon the basilary membrane. It consists 
of the inner and outer hair-cells, and two rows of elongated cells, placed 
parallel with each other, having an inclining position so that their free ex- 
tremities rest against each other and thus form the arch of Corti, which covers 
the central space. (Fig. 190.) These rows are called the inner and outer 
rods or pillars of Corti. From the superior extremity of both the inner and 
1 The upper scala is divided into two parts by a membranous partition, the upper of 
■which is called the scala vestibuli; the other, ductus cochlearis (Fig. 190). 796 
HEARING, SMELL, AND TASTE. 
outer rods, finger-like processes project externally. At their bases corre- 
sponding to the central space are single rows of nucleated cells. On the 
Fig. 191. 
Fig. 191.-Petrous Bone partly Removed to Show the Membranous Labyrinth in Place. 
(Breschet.) a, small sac: b, its otolith ; c, ductus reunions ; d, large sac or utricle ; e, its otolith; 
f, ampullary enlargements on a semicircular tube ; g, semicircular tube. 
Fig. 192.-Distribution of Nerves to the Membranous Labyrinth. (Breschet.) a, nerve to the 
saccule ; b, nerve entering the ampullary enlargement on a semicircular tube; c, branch of the 
nerve entering the large sac or utricle. 
Fig. 193. 
Diagram of the mode of Termination of the Auditory Nerve in the Ampullee and Sacculi. 1, 
wall of the ampulla ; 2, structureless basement-membrane; 3, doubly contoured nerve-fibres; 4, 
axis-cylinder traversing the basement-membrane ; 5, plexiform union of fine nerve-fibres with 
interspersed nuclei: 6, fusiform cells, with nucleus and dark fibre in their interior; 7, supporting 
cells ; 8, auditory hairs. HEARING. 
797 
internal side of the inner rod is a single row, and on the external side of the 
outer rods are three rows of elongated ciliated cells. Extending across the 
top of the organ of Corti, from the inner hair-cells to the external wall of 
the canal, is a very delicate structure called the reticular membrane. The 
auditory nerve-filaments probably terminate in the ciliated cells, being 
intimately connected with the cilia. 
§ 703. The osseous labyrinth is lined by a fibro-serous membrane which 
secretes a watery fluid called the perilymph. The perilymph fills the scalar 
of the cochlea, and surrounds the ductus cochlear is and the membranous 
portions of the labyrinth, which are situated in the vestibule and semicircu- 
lar canal. 
§ 704. The membranous labyrinth is a closed sac consisting of the semi- 
circular canals, a vestibular portion, and the ductus cochlearis of the cochlea. 
The semicircular canals are of the same form as the osseous canals, and are 
contained within them. The vestibular portion consists of an expanded 
body, the utricle, and a smaller body, the saccule. The utricle is situated at 
the fovea hemielliptica ; the semicircular canals open on its internal surface. 
The saccule lies at the fovea hemispherica; it is connected with the ductus 
cochlearis by the ductus reunions. In the walls of the saccule and utricle 
are two calcareous bodies called the otoliths. The walls of the ampulla, 
according to Bowman, also contain some grains of a similar substance. The 
walls of the membranous labyrinth consist of a fibrous tissue, lined by pave- 
ment nucleated epithelial cells, having a structureless basement-membrane. 
These epithelial cells are much modified at the place of entrance of the 
fibres of the auditory nerve. The vestibular branches of the auditory nerve 
are distributed to the ampullae, utricle, and saccule. (Fig. 192.) In the 
utricle and the saccule the fibres terminate in oval plates, called the maculce 
acusticce, which are more or less colored by the deposition of yellow pigment. 
In the ampullae the fibres terminate in elevations called the cristce acusticce. 
After the nerve-filament pierces the membranous wall at these points, the 
axis-cylinder alone penetrates the basement-membrane ; it then forms a 
plexus of delicate nerve-fibres with nuclei, and finally terminates in fusiform 
epithelial cells which have terminal cilia called the auditory hairs. (Fig. 
193.) These ciliated cells are supported by columnar epithelium. 
The membranous labyrinth is lined by polygonal nucleated epithelium, 
which secretes the endolymph which fills the sac.] 
§ 705. Waves of sound can and do reach the endolymph of the laby- 
rinth by direct conduction through the skull. Since, however, sonorous 
vibrations are transmitted with great difficulty from the air to solids and 
liquids, and most sounds come to us through the air, some special apparatus 
is required to transfer the aerial vibrations to the liquids of the internal ear. 
This apparatus is supplied by the tympanum and its appendages. 
§ 706. The concha. The use of this, as far as hearing is concerned, is to 
collect the waves of sound coming in various directions, and to direct them 
on to the membrana tympani. In ourselves of moderate service only, in 
many animals it is of great importance. 
§ 707. The membrana tympani. It is a characteristic property of 
stretched membranes that they are readily thrown into vibration by aerial 
waves of sound. The membrana tympani, from its peculiar conformation, 
being funnel-shaped with a depressed centre surrounded by sides gently con- 
vex outward, is peculiarly susceptible to sonorous vibrations, and is most 
readily thrown into corresponding movements when waves of sound reach it 
The Acoustic Apparatus. 798 
HEARING, SMELL, AND TASTE. 
by the meatus. It has, moreover, this useful feature, that unlike other 
stretched membranes, it has no marked note of its own. It is not thrown 
into vibrations by waves of a particular length more readily than by others. 
It answers equally well within a considerable range to vibrations, of very 
different wave-lengths. Had it a fundamental tone of its own, we should 
be distracted by the prominence of this note in most of the sounds we hear. 
When sounds impinge on the solids of the head, as when a watch is held 
between the teeth, the membrana tympani is still functional. Vibrations are 
conveyed from the temporal bone to it and hence pass in the usual way, iu 
addition to those transmitted directly from the bone to the perilymph. 
§ 708. 7Vie auditory ossicles. The malleus, the handle of which de- 
scending forward and inward, is attached to the membrana tympani, and the 
incus, whose long process is connected by means of its os orbiculare or len- 
ticular process and the stapes to the fenestra ovalis, form together a body 
which rotates round an axis, passing through the short process of the incus, 
the bodies of the incus and malleus, and the processus gracilis of the malleus. 
[Fig. 194.] When the malleus is carried inward, the incus moves inward 
[Fig. 194. 
The Ossicles in Position. Magnified four times. (After Hensen.) The figure represents a sec- 
tion through tympanum in the line of the long axis of the malleus and incus; the short process 
of the incus, p'b', has been cut through. 
T.C, the tympanic cavity ; mbr, handle of malleus; u, umbo; p.b, short process of the malleus 
shown in dotted outline as pushing outward the membrana flaccida; T.l, the attachment of the 
tendon of the tensor tympani; Ig, the attachment of the external ligament of the malleus; Ig.s, 
the superior ligament of the malleus; t.f, the teeth of the incus ; p'l. the long process shaft of the 
incus; St, the stapes.] 
too, and when the malleus returns to its position, the incus returns with it, 
the peculiar saddle-shaped joint with its catch-teeth permitting this move- 
ment readily, but preventing the stapes being pulled back when the mem- 
brana tympani with the malleus is for any reason pushed outward more 
than usual; the joint then gapes, so as to permit the malleus to be moved 
alone. Various ligaments, the superior or suspensory, anterior, and external, 
also serve to keep the malleus in place. The whole series of ossicles may 
be regarded as a single-armed lever, moving on the ligamental attachment 
of the short process of the incus to the posterior wall of the tympanum, the 
weight being brought to bear at the end of the long process of the incus, 
and the power at the end of the handle of the malleus. The long, malleal HEARING. 
799 
arm of this lever is about 9$ mm., the short, stapedial, mm. in length ; 
hence, the movements of the stapes are less than those of the tympanum ; 
but the loss in amplitude is made up by a gain of force, which is in itself 
an obvious advantage. 
Thus every movement of the tympanic membrane is transmitted through 
this chain of ossicles to the membrane of the fenestra ovalis, and so to the 
perilymph of the labyrinth ; the vibrations of the tympanic membrane are 
conveyed with increased intensity, though with diminished amplitude, to the 
latter. That the bones thus move en masse has been proved by recording 
their movements in the usual graphic method. A very light style attached 
to the incus or stapes is made to write on a travelling surface; when the 
membrana tympani is thrown into vibrations by a sound the curves described 
by the style indicate that the chain of bones moves with every vibration of 
the tympanum. On the other hand, the comparatively loose attachments of 
the several bones is an obstacle to the molecular transmission of sonorous 
vibrations through them. Moreover, sonorous vibrations can only be trans- 
mitted to or pass along such bodies as either are very long compared to the 
length of the sound-waves, or, as in the case of membranes and strings, have 
one dimension very much smaller than the others. Now the bones in ques- 
tion are not especially thin in any one dimension, but are in all their dimen- 
sions exceedingly small compared with the length of the vibrations of even 
the shrillest sounds we are capable of hearing; hence, they must be useless 
for the molecular propagation of vibrations. 
[Fig. 195. 
[Fig. 196. 
Fig. 195.-Diagram of the Outer Wall of the Tympanum (Right Ear) as Seen from the Mesial 
Side, showing Insertion of Tensor Tympani. Magnified twice. (After Schwalbe.) m.t, mem- 
brana tympani; m.b, handle of M, the malleus; 7, the incus; E.t, Eustachian tube; T.T, tensor 
tympani, the tendon of which is attached to the handle of the malleus; Ig.a, the anterior, and 
Zq.s, the superior, ligament of the malleus; ch.t, the chorda tympani nerve passing through the 
tympanic cavity. 
Fig. 196.-The Stapes in Position. Much magnified. (Schwalbe.) 1, the end of the shaft of the 
incus ; 2, its expansion or os orbiculare ; 2', the articular cartilage of the same ; 3, the capitulum 
of the stapes ; 3', its articular cartilage ; 4, the hoops of the stapes; 5, the foot-plate of the stapes ; 
5', its articular cartilage; 6, the membrane of the fenestra ovalis; ST, the tendon of the stape- 
dius muscle attached to the capitulum of the stapes.] 
§ 709. The tensor tympani muscle even in a quiescent state is of use in 
preventing the membrana tympani being pushed out far. (Fig. 195.) When 
it contracts it renders the membrana tympani more tense, and hence has 
been supposed to act as a damper, lessening the amount of vibration of the 
membrane in the case ot too powerful sounds ; it is said to be readily thrown 800 
HEARING, SMELL, AND TASTE. 
into contraction at the commencement of a sound or noise, but to return to 
rest during the continuance of a musical note. Efferent impulses reach it 
through fibres of the fifth nerve, and its activity is regulated by a reflex 
action. In some persons the muscle seems to be partly under the dominion 
of the will, since a peculiar crackling noise which these persons can produce 
at pleasure appears to be caused by a contraction of the tensor tympani. 
The so-called laxator tympani is considered to be not a muscle at all, but 
a part of the ligamentous supports of the malleus. 
§ 710. The stapedius muscle by pulling upon the head of the bone (Fig. 
196) is supposed to regulate the movements of the stapes, and especially to 
prevent its base being driven too far into the fenestra ovalis during large or 
sudden movements of the membrana tympani. It is governed by fibres 
from the facial nerve. 
§711. The Eustachian tube. This serves to maintain an equilibrium of 
pressure betwen the external air and that within the tympanum, and to 
serve as an exit for the secretions of that cavity. Were the tympanum per- 
manently closed the vibrations of the membrana tympani would be inju- 
riously affected by variations of pressure occurring either inside or outside. 
The Eustachian tube is undoubtedly open during swallowing, but it is still 
disputed whether it remains permanently open or is opened only at inter- 
vals ; probably it is, at most times, neither widely open nor closely shut. 
§ 712. Each vibration communicated by the stapes to the perilymph 
travels as a wave over the vestibule, the semicircular canals, and other parts 
of the labyrinth; and from the perilymph is transmitted through the mem- 
Auditory Sensations. 
Fig. 197. 
Diagram of the Organ of Corti. (After Retzius.) i.r, inner rod of Corti; o.r, outer rod of Corti. 
i.h.c, inner hair-cells ; n.c, the group of nuclei beneath it; o. Ac, outer hair-cell, or cell of Corti, 
of the first row ; c.D, its twin cell of Deiters-four rows of these twin cells are shown. 
n.aud, the auditory nerve perforating the tympanic lip, Lt, and lost to view among the nuclei 
beneath the inner hair-cell; i.sp.n, the inner spiral strand of nerve-fibrillse; t.sp.n, the spiral 
strand of the tunnel; o.sp.n, the outer spiral strand belonging to the first row of outer hair-cells; 
the three succeeding spiral strands belonging to the three other rows are also shown. Nerve- 
fibrillae are shown stretching radially across the tunnel. 
H.c, Hensen's cells; Cl.c, Claudius's cells; m.b, basilar membrane; t.l, lymphatic epithelioid 
lining of the basilar membrane on the side toward the scala tympani; Ig.sp, spiral ligament; c', 
cells lining the spiral groove, overhung by l.v, the vestibular lip; m.t, the tectorial membrane-a 
fragment of it is seen torn from the rest and adherent to the organ of Corti just outside the outer- 
most row of outer hair-celis. 
branous wall to the endolymph. From the vestibule it passes on into the 
scala vestibuli of the cochlea, and descending the scala tympani, ends as an 
impulse against the membrane of the fenestra rotunda. In the regions of 
the maculae and cristae the vibrations of the endolymph are supposed to throw HEARING. 
801 
into corresponding vibrations tbe so-called auditory hairs. In the cochlea 
the vibrations of the perilymph are supposed to throw into vibrations the 
basilar membrane with the superimposed organ of Corti, consisting of the 
rods of Corti with the inner and outer hair-cells. (Fig. 197.) The vibra- 
tions thus transmitted to these structures give rise to nervous impulses in 
the terminations of the auditory nerves, and these impulses reaching certain 
parts of the brain produce what we call auditory sensations. We are accus- 
tomed to divide our auditory sensations into those caused by noises and those 
caused by musical sounds. It is the characteristic of the latter that the 
vibrations which constitute them are periodical; they occur apd recur at 
regular intervals. When no marked periodicity is present in the vibrations, 
when the repetition of the several vibrations is irregular, or the period so 
complex as not to be readily appreciated, the sensation produced is that of 
a noise. There is, however, no abrupt line between the two. Between a 
pure and simple musical sound produced by a series of vibrations, each of 
which has exactly the same wave-length, and a harsh noise in which no 
consecutive vibrations may be alike, there are numerous intermediate stages. 
§ 713. In both noises and musical sounds we recognize a character 
which we call loudness. This is determined by the amplitude of the vi- 
brations ; the greater the disturbance of the air (or other medium) the 
louder the sound. In a musical sound we recognize also a character 
which we call pitch. This is determined by the wave-length of the vi- 
brations ; the shorter the wave-length, the larger the number of consec- 
utive vibrations which fall upon the ear in a second, the higher the pitch. 
We are able to speak of a whole series of tones or musical sounds of differ- 
ent pitch, from the lowest to the highest audible tone. And even in many 
noises we can, to a certain extent, recognize a pitch, indicating that among 
the multifarious vibrations there is a periodicity of certain groups of vibra- 
tions. 
§ 714. Lastly, we distinguish musical sounds by their quality ; the same 
note sounded on a piano and on a violin produce very different sensations, 
even when a series of vibrations having in each case the same period of 
repetition is set going. This arises from the fact that the musical sounds 
generated by most musical instruments are not simple but compound vibra- 
tions. When the note C in the treble, for instance, is struck on the piano, 
and we analyze the total sound, we find that it can be resolved partly into 
a series of vibrations with a period characteristic of the pure tone of the 
treble C, and partly into other series of vibrations with periods character- 
istic of the C in the octave above, of the G above that, of the C in the next 
octave, and of the E above that. And the sensation which we associate with 
the sound of the treble C on the piano is determined by the characters of 
the complex vibration arising out of these several constituent simple vibra- 
tions. Almost all musical sounds are thus composed of what is called a 
" fundamental tone " accompanied by a number of " overtones." And the 
overtones varying in number and relative prominence in different instruments, 
give rise to a difference in the sensation caused by the whole tone. So that 
while the fundamental tone determines the pitch of the sound, the quality 
of the sound is determined by the number and relative prominence of the 
overtones. In a somewhat similar way we distinguish the quality of 
noises, such as a banging, crackling, or rustling noise, by an appreciation 
of sudden or irregular changes in the amplitude and period of the con- 
stituent vibrations. 
§715. Since we have a very considerable appreciation, capable by 
exercise of astonishing enlargement, of the loudness, pitch, and quality 
of a wide range of noises and musical sounds, it is clear that, within the 802 
HEARING,, SMELL, AND TASTE. 
limits of hearing, each vibration or series of vibrations must produce its 
effect on the auditory nerves, according to the measure of its intensity and 
period. Out of those effects, out of the sensory impulses to which the several 
vibrations thus give rise, are generated our sensations of the noise or of the 
sound. 
The vibrations of a musical sound (and since noises are so imperfectly 
understood, we may, with benefit, chiefly confine ourselves to musical 
sounds), as they pass through the air (or other medium) are not discrete; 
the vibrations corresponding to the fundamental tone and overtones do 
not travel as so many separate waves ; they all together form one com- 
plex disturbance of the medium ; and it is as one composite wave that 
the sound falls on the membrana tympani, and passing through the audi- 
tory apparatus, breaks on the terminations of the auditory nerve. And 
when two or more musical sounds are heard at the same time, the same 
fusion of the waves occurs. Since we can distinguish several tones reach- 
ing our ear at the same time, it is clear that we must possess in our 
minds or in our ears some means of analyzing these composite waves of 
sound which fall on our acoustic organs, and of sorting out their con- 
stituent vibrations. 
§716. There is at hand a simple and easy physical method of analyzing 
composite sounds. If a person standing before an open piano sings out any 
note, it will be observed that a number of the strings of the piano will be 
thrown into vibration, and on examination it will be found that those strings 
which are thus set going correspond in pitch to the fundamental tone and to 
the several overtones of the note sung. The note sung reaches the strings 
as a complex wave, but these strings are able to analyze the wave into its 
constituent vibrations, each string taking up those vibrations and those vibra- 
tions only which belong to the tone given forth by itself when struck. If we 
suppose that each terminal fibril of the auditory nerve is connected with an 
organ so far like a piano-string that it will readily vibrate in response to a 
series of vibrating impulses of a given period and to none other, and that 
we possess a number of such terminal organs sufficient for the analysis of all 
the sounds which we can analyze, and that each terminal organ so afiected 
by particular vibrations gives rise to a sensory impulse and thus to a sensa- 
tion of a distinct character-if we suppose these organs to exist, our appre- 
ciation of sounds is in a large measure explained. In the organ of Corti 
we find structures, the arrangement of which irresistibly suggests to us that 
these are the organs we are seeking. We have only to suppose that of the 
long series of rods of Corti, varying regularly as these do from the bottom 
to the top of the spiral, in length and in the span of their arch, each pair 
will vibrate in response to a particular tone, and the whole matter seems 
explained. But the more the subject is inquired into, the more complex and 
difficult it appears; and we are obliged to conclude that the part played by 
the rods of Corti is only a subordinate part of the function of the whole 
organ of Corti. 
In the first place, it is difficult to see how the rods of Corti, even if they 
are thrown into vibration, can originate sensory impulses, for the fibrils of 
the auditory nerve terminate in the inner and outer hair-cells, and it is in 
these cells, and not along the course of these fibrils as they pass under and 
between the rods of Corti, that the sensory impulses must begin. In the 
second place, the variation in length of the fibres along the series is insuffi- 
cient for the work assigned to them. Moreover, they appear not to be elastic. 
Lastly, they are wholly absent in birds, who very clearly can appreciate 
musical sounds. This last fact proves indubitably that the rods in question 
are not absolutely essential for the recognition of tones. In the face of these HEARING. 
803 
difficulties it has been suggested that the basilar membrane, which is present 
in birds as well as in mammals, and which, being tense radially but loose 
longitudinally, i. e., along the spiral of the cochlea, may be considered as 
consisting of a number of parallel radial strings, each capable of inde- 
pendent vibrations, is the sought-for organ of analysis ; for it may be shown 
mathematically that a membrane so stretched in one direction only is capa- 
ble of vibrating in such a manner. And the radial dimensions of the 
basilar membrane give a much greater range of difference than do the rods 
of Corti, diminishing in man downward from 0.495 mm. at the top to 
0.04125 mm. near the bottom of the spiral, whereas the difference in length 
of the latter is simply that between 0.048 and 0.085 mm. for the inner and 
between 0:019 and 0.085 mm. for the outer fibres. According to this view, 
a particular simple vibration reaching the scala tympani of the cochlea 
throws into sympathetic vibrations a small portion of the basilar membrane, 
the vibrations of which in turn so affect the structures overlying it that sen- 
sory impulses are generated. The sensory impulses reaching the brain give 
rise to a corresponding sensation of a particular tone. 
The remarkable reticular membrane which has such peculiar relations 
with the hair-cells, and through them with the basilar membrane, must, one 
might imagine, have some special function ; but it is impossible at present 
to assign to it any satisfactory duty. The structural arrangements seem, if 
anything, to indicate, that when a segment of the basilar membrane is 
thrown into vibrations, the overlying hair-cells, reticular membrane, and 
rods of Corti vibrate en masse with it. But this renders the whole matter 
still more difficult. Indeed the whole subject is in the highest degree ob- 
scure, and the most we can say is that the organ of Corti as a whole seems 
to be in some way connected with the appreciation of tones, but that at 
present it is very hazardous to attempt to explain how it acts, or to assign 
particular functions to particular parts. The distinction between the inner 
and outer hair-cells seems to be very parallel to that between the rods and 
cones of the retina; but even this analogy may be a fallacious one. 
It has been observed that among the auditory hairs of the Crustacea, 
some will vibrate to particular notes ; but the auditory hairs of the mammal 
are far too much of the same length to permit the supposition that they can 
act as organs of analysis. 
If the organ of Corti is the means by which we appreciate tones, it is 
evident that by it also we must be able to estimate loudness, for the quality 
of a musical sound is dependent on the relative intensity, as well as on the 
nature, of the overtones. And since noise is at best but confused music, the 
cochlea must be a means of appreciating noises as well as sounds. But this 
would leave nothing whatever for the rest of the labyrinth to do in respect 
to the appreciation of sound save so far as the difference in structure between 
the hair-cells of Corti, with their short, thick rods, and the hair-bearing 
structures in the maculae and cristse, with their thin, delicate hairs, may 
possibly indicate a difference of function, the latter being more susceptible 
to the irregular vibrations of noises. That the vestibule and semicircular 
canals are, however, concerned in hearing is shown by its being the only 
auditory organ in the ichthyopsida, unless we suppose that in the higher 
vertebrates its function has been wholly transferred to the cochlea. That 
the semicircular canals may have duties apart from hearing we shall show 
later on. 
§ 717. Concerning the function of the other parts of the internal ear we 
know very little. The otoliths have been supposed to intensify the vibrations 
of the endolymph ; but since apparently they are lodged in a quantity of 
mucus it is probable that they really act as dampers. A similar damping 804 
HEARING, SMELL, AND TASTE. 
action has been suggested for the membrane of Corti (membrana tectoria) 
overhanging the fibres and hair-cells; and some writers have supposed that 
muscular fibres present in the planum semilunare may by tightening the 
basilar membrane serve as a sort of accommodation mechanism. 
It must, however, be borne in mind that even making the fullest allow- 
ance for the assistance afforded us by the organ of Corti, the appreciation 
of any sound is ultimately a mental act. The analysis of the vibrations by 
the fibres of Corti or the basilar membrane is simply preliminary to a 
synthesis of the sensory impulses so generated into a complex sensation. 
We do not receive a distinct series of specific auditory impulses resulting in 
a specific sensation for every possible variation in the wave-length of sonor- 
ous vibrations any more than we receive a distinct series of specific visual 
impulses for every possible wave-length of luminous vibrations. In each 
case we have probably a number of primary sensations, from the various 
mingling of which, in different proportions, our varied complex sensations 
arise; the difference between the eye and the ear being that whereas in the 
former the number of primary sensations appears to be limited to three or 
at most to six, in the latter, thanks to the organ of Corti, the number is 
very large; what the exact number is we cannot at present tell. Our ap- 
preciation for a sound is at bottom an appreciation of the combined effect 
produced by the relative intensities to which the primary auditory sensations 
are, with thejielp of the organ of Corti, excited by the sound. 
§ 718. Whatever be the explanation of the manner in which our distinct 
auditory sensations arise, the range and precision of our appreciation of 
musical sounds is very great. Vibrations with a recurrence below 30 a sec- 
ond 1 are unable to produce a sensation of sound ; if the waves are powerful 
enough we may feel them, but we do not hear them if the vibrations are sim- 
ple, and such as would give rise to a pure tone; if the fundamental tone is 
accompanied by overtones we may hear these, and are thus apt to say we 
hear the former when in reality we only hear the latter. The note of the 
16-feet organ pipe, 33 vibrations a second, gives us the sensation of a droning 
sound. A tone of forty vibrations is, however, quite distinct. In the other 
direction it is possible to hear a note caused by 38,000 vibrations a second, 
though the limit for most persons is far lower-about 16,000. Some persons 
hear grave sounds more easily than high ones and vice versa. This may be 
so pronounced as to justify the subjects being spoken of as deaf to grave or 
high tones respectively. The range in different animals is very different. 
The power of distinguishing one note from another varies, as is well 
known, in different individuals, according as they have or have not a " mu- 
sical ear." A well-trained ear can distinguish the difference of a single or 
even of a half vibration a second, and that through a long range of notes. 
The range of an ordinary appreciation of tones lies between 40 and 4000 
vibrations a second, i. e., between the lowest bass C (Q 32 vibrations) and 
the highest treble C (C5 4224 vibrations) of the piano; tones above and 
below these, even when audible, being distinguished from each other with 
great difficulty. 
§ 719. When two consecutive sounds follow each other at a sufficiently 
short interval the sensations are fused into one. In this respect auditory 
sensations are of shorter duration than ocular sensations. When ocular 
sensations are repeated ten times in a second they become fused (p. 766), 
whereas the ticks of a pendulum beating 100 in a second are readily audible 
as distinct sounds. When two tuning-forks not quite in tune are struck to- 
gether the interference of the vibrations gives rise to an alternating rise and 
1 By some authors the limit is placed as low as 24 or even 15 a second. 805 
HEARING. 
fall of the sound, known as "beats." When the beats follow each other as 
rapidly as 132 in a second they cease to be recognized, that is to »ay, the 
sensations which they cause become fused. Before they disappear they give 
a peculiar disagreeable roughness to the sound. The pleasure given by 
musical sounds depends largely on the absence of this incomplete fusion of 
sensations. 
§ 720. Corresponding to entoptic phenomena there are various entotic 
phenomena, sensations or modifications of sensations originating in the 
tympanum or in the labyrinth ; moreover, sensations of sound may rise in the 
auditory nerve or in the brain itself, without any vibration whatever falling 
on the labyrinth. 
Auditory Judgments. 
§ 721. In seeking for the cause of our visual sensations we invariably 
refer to the external world. The sensation caused by direct stimulation of 
the optic nerve or retina by a blow or a galvanic current, we identify with 
that caused by a flash of light. A sensation arising from any stimulation 
of the left side of our retina we regard as caused by some object on the 
right-hand side of our external visible world. In a similar way, but to a 
less extent, we project our auditory sensations into the world outside us, and 
when the auditory nerve is affected we seek the cause in vibrations starting 
at a greater or less distance from us. We do not think of the sound as 
originating in the ear itself. 
This mental projection of the sound is much more complete when the ear 
is stimulated by vibrations reaching it through the membrana tympani than 
when the vibrations are conducted by the solids of the head directly to the 
perilymph of the labyrinth. When the meatus externus is filled with fluid 
and the vibrations of the membrana tympani are in consequence interfered 
with the apparent outwardness of sounds is to a very large extent lost; 
sounds, however caused, seem under these circumstances to arise in the ear. 
Hence it would seem that the vibrations of the membrana tympani, or pos- 
sibly the action of the muscles attached to the ossicula, give rise to obscure 
sensations of which, by themselves, we are not distinctly conscious, but which 
nevertheless lead us to judge that the sounds heard by means of the tym- 
panum come from outside the ear. 
§ 722. Our judgment of the distance of sounds is very limited. A sound 
whose characters we know appears to us near when it is loud and far off 
when it is faint. A blindfold person will be unable to distinguish between 
the difference of intensity produced on the one hand by a tuning-fork being 
held before him, first with the broad edge of the fork toward him and then 
with the narrow edge, and the difference on the other hand caused by the 
removal of the tuning-fork to a distance. We can, on the whole, better 
appreciate the distance of noises than of musical sounds. 
§ 723. Our judgment of the direction of sounds is also very limited. 
Our chief aid in this is the position in which we have to place the head in 
order that we may hear the sound to the best advantage. If a tuning-fork 
be held in the median vertical plane over the head, though it is easy to rec- 
ognize it as being in the median plane, it becomes very difficult when the 
eyes are shut to say what is its position in that plane, i. e., whether it is more 
toward the front or back of the head. In this respect, too, our appreciation 
is more accurate in the case of noises than of musical sounds, with the excep- 
tion of those given out by the human voice, the direction of which can be 
judged better than even that of a noise. 806 
HEARING, SMELL, AND TASTE. 
Smell. 
§ 724. [The nasal fossae are two irregular cavities which communicate 
anteriorly with the air through the anterior nares and posteriorly with the 
pharynx through the posterior nares. The fossae are partially divided into 
upper, middle, and lower air-passages or chambers by the superior, middle, 
and inferior turbinated bones. They are lined by the Schneiderian or 
pituitary mucous membrane, which is continuous anteriorly with the integu- 
ment and posteriorly with the mucous membrane of the pharynx ; and with 
the membrane lining the ducts and sinuses connected with the fossae. At the 
position of the distribution of the olfactory nerve-filaments it is much 
thicker, more vascular, pigmented, and lined by columnar nucleated epithe- 
lial cells; the remaining portion of the membrane covering the fossae, ex- 
cepting near the anterior nares, is lined by columnar ciliated epithelium. 
This membrane contains racemose mucous glands, which secrete mucus for 
the purpose of keeping the membrane constantly moist, which is a condition 
essential to perfect olfaction. 
§ 725. The olfactory tract is a prolongation of the cerebrum, which ter- 
minates anteriorly in a bulbous expansion, the olfactory ganglion. It consists 
principally of gray matter. This ganglion rests upon the cribriform plate 
Fig. 198. 
Fig. 199. 
Fig. 198.-Vertical Section of Right Nasal Fossa, 
showing Outer Side of Fossa. 1, olfactory tract; 2, ol- 
factory nerves; 3, middle turbinated bone ; 4, lower 
turbinated bone ; 5, branches from the fifth nerve. 
Branches of the fifth are also shown in the anterior 
portion. (After Arnold.) 
Fig. 199.-Cells of the Olfactory Mucous Membrane, (a, b, c, after Schultze ; cl, e,f, after Lock- 
hart Clarke.) 
of the ethmoid bone, and in this position sends about twenty filaments, which 
consist of gray matter alone, through the cribriform plate to be distributed to 
the pituitary membrane of the upper third of the septum nasi, the upper 
portion of the root of the nose, the superior, and a portion of the middle 
turbinated bones. (Fig. 198.) The whole surface corresponding to the dis- 
tribution of the olfactory nerve is colored brownish by the pigment in the 
epithelial cells of the mucous glandsand membrane. This pigmented region 
is called the regio olfactoria, and is the essential portion of the nasal fossae 
concerned in olfaction. 
$ 726. According to Schultze, the epithelium of the regio olfaetoria is 
of two kinds: The first (Fig. 199, a) consists of yellow nucleated proto- SMELL. 
807 
plasmic cells, which have a cylindrical body terminating at its free extremity 
as a squared truncated surface ; the other extremity of the body is stretched 
out as a filamentous prolongation, which expands into a triangular plate as 
it approaches the submucous tissue. From the base of this plate a number 
of filaments are given off, which are prolonged into the submucous tissue. The 
second variety of epithelial cells (c) is found at the borders of the reg io 
olfactoria. They are similar to those just described, excepting that their 
free surface is covered with cilia. Between the epithelial cells the olfac- 
tory nerves terminate. These terminal filaments (6,/) are long, delicate 
structures, which have a number of fusiform expansions along their course; 
in the largest expansion is found an oval nucleus. The terminal filaments 
are called the olfactory cells. As yet no connection between the subepi- 
thelial and interepithelial nerve-filaments has been demonstrated. The 
epithelial cells (d and e) in the above figure are shown connected with 
the subepithelial tissue. The fifth nerve supplies the fossae with sensory 
filaments.] 
§ 727. Odorous particles present in the inspired air passing through 
the lower nasal chambers diffuse into the upper nasal chambers, and falling 
on the olfactory epithelium produce sensory impulses which, ascending to 
the brain, give rise to sensations of smell. We may presume that the sen- 
sory impulses are originated by the contact of the odorous particles with 
the peculiar rod-shaped olfactory cells described by Max Schultze ; but we 
are as much in the dark about this matter as about the development of 
visual sensory impulses in the rods and cones or of auditory sensory im- 
pulses in the organ of Corti. 
The susidiary apparatus of smell is exceedingly meagre. By the forced 
nasal inspiration, called sniffing, we draw air so forcibly through the nostrils 
that currents pass up into the upper as well as the lower nasal chambers; 
and thus a more complete contact of the odorous particles with the olfactory 
membrane than that supplied by mere diffusion is provided for. 
We have every reason to think that any stimulus applied to the olfactory 
nerve will produce the sensation of smell; but the proof of this is not so 
clear as in the case of the optic and auditory nerves. We are, however, sub- 
ject to sensations of smell not caused by objective odors. The olfactory 
membrane is the only part of the body in which odors as such can give rise 
to any sensations; and the sensations to which they give rise are always 
those of smell. The mucous membrane of the nose is, however, also an in- 
strument for the development of afferent impulses other than the specific 
olfactory ones. Chemical stimulation of the olfactory membrane by pungent 
substances such as ammonia gives rise to a sensation distinct from that of 
smell, a sensation which affords us no information concerning the chemical 
nature of the stimulus, and which is indistinguishable from the sensations 
produced by chemical stimulation of other parts of the nasal membrane as 
well as of other surfaces equally sensitive to chemical action. It is probable 
that these two kinds of sensations thus arising in the olfactory membrane 
are conveyed by different nerves, the former by the olfactory, the latter by 
the fifth nerve. 
§ 728. For the development of smell it appears necessary that the odor- 
ous particles should be conveyed to the nasal membrane in a gaseous medium, 
or, at least, that the surface of the membrane should not be exposed at the 
same time to the action of fluids. Thus, when the nostril is filled with rose- 
water, the odor of roses is not perceived ; and simply filling the nostrils with 
distilled water suspends for a time all smell, the sense returning gradually 
after the water has been removed ; the water apparently acts injuriously on 
the delicate olfactory cells. 808 
HEARING, SMELL, AND TASTE. 
Each substance that we smell causes a specific sensation, and we are not 
only able to recognize a multitude of distinct odors, but also to distinguish 
individual odors in a mixed smell. 
As in the previous senses, we project our sensation into the external 
world; the smell appears to be not in our nose, but somewhere outside us. 
We can judge of the position of the odor, however, even less definitely that 
we can of that of a sound. 
The sensation takes some time to develop after the contact of the stim- 
ulus with the olfactory membrane, and may last very long. When the 
stimulus is repeated the sensation very soon dies out; the sensory terminal 
organs speedily become exhausted. Mental associations cluster more 
strongly around sensations of smell than around any other impressions 
we receive from without. And reflex effects are very frequent, many 
people fainting in consequence of the contact of a few odorous particles 
with their olfactory cells. 
Apparently the larger the surface the more intense the sensation ; 
animals with acute scent having a proportionately large area of olfactory 
membrane. The quantity of material required to produce an olfactory 
sensation may be, as in the case of musk, almost immeasurably small. 
When two different odors are presented to the two nostrils, an oscilla- 
tion of sensation similar to that spoken of in binocular vision (p. 788) takes 
place. 
§ 729. The assertion that the olfactory nerve is the nerve of smell has 
been disputed. Cases have been recorded of persons who appeared to have 
possessed the sense of smell, and yet in whom the olfactory lobes were found 
after death to be absent. Direct experiments on animals, however, show 
that loss of the olfactory lobes entails loss of smell. On the other hand, it 
is stated that section or injury of the fifth nerve causes a loss of smell 
though the olfactory nerve remains intact; but in these cases it has not 
been shown that the olfactory membrane remains intact, and it is quite pos- 
sible that, as in the case of the eye, changes may take place in the nasal mem- 
brane as the result of the injury to the fifth nerve, sufficient to prevent its 
performing its usual functions. 
Taste. 
§ 730. [The peripheral organs concerned in the sense of taste are local- 
ized in the mucous membrane covering the dorsum of the tongue, the fauces, 
soft palate, and uvula, and possibly a portion of the upper part of the 
pharynx. This membrane is analogous in structure to other membranes of 
its type, except on the dorsum of the tongue, where its structure is similar 
to that of the integument. At this position it consists of a corium, with a 
papillary and a superficial epithelial layer. 
The structure of the corium is similar to that of the skin, but is thinner 
and less compact. It serves as a point of insertion of the muscular fibres of 
the tongue. 
§ 731. Thejoapz'Zte are thickly distributed over the whole dorsal surface,but 
more particularly marked in the anterior two-thirds. They project as minute 
prominences, which give the tongue a roughened, characteristic appearance. 
The papillae are of two kinds, the simple and compound. The simple papillae 
are similar to those found in the skin ; they are found scattered over the 
whole dorsal surface between the compound papillae. They are most nu- 
merous in the posterior portion of the organ. The compound papillae are 
of three varieties: the papillae maximae or circumvallatae, the papillae mediae 
or fungiformes, and the papillae minimae or filiformes. TASTE. 
809 
The papillae, circwnvallatce (Fig. 200), which are the largest, are about 
eight or ten in number and form a V-shaped row at the junction of the 
middle and posterior two-thirds of the tongue. They consist of a central, 
broad papilla, surrounded by an annular ring or wall of about the same 
elevation and separated from the central papilla by a circular fissure. The 
central papilla, as well as the surrounding wall, is covered by simple papillae. 
Each of them receives one or more capillary loops and nerve-filaments. 
Fig. 200. 
Vertical Section of the Circumvallate Papillae. (From Kolliker.) A, the papillae; B, the sur- 
rounding wall; a, the epithelial covering; b, the nerves of the papilla and wall spreading toward 
the surface; c, the secondary papillae. 10/i. 
The papillae fungiformes (Fig. 201) are found principally on the tip and 
sides of the tongue, although scattered sparsely over the whole of the anterior 
two-thirds. These are so named from their fungiform shape, being expanded 
at their free extremity and projecting on a short, thick pedicle. They are 
covered by simple papillae, and contain plexuses of vessels and nerves. 
Fig. 201. 
Surface and Section of the Fungiform Papillae. (From Kolliker, and after Todd and Bowman.) 
A, the surface of a fungiform papilla, partially denuded of its epithelium, ; a, epithelium. B, 
section of a fungiform papilla with the bloodvessels injected ; a, artery; v, vein ; c, capillary loops 
of simple papillae in the neighboring structure of the tongue. 
The papillae filiformes (Fig. 202) are by far the most numerous, and are 
found thickly distributed over the entire surface of the anterior two-thirds 
of the tongue. They are minute, conical in shape, and generally arranged 
in bipenniform rows, which are more or less parallel with the two rows of 
papillae circumvallatse. Their free surface is covered with simple papillae. 
The epithelium covering them is greatly modified and appears in the form 
of hair-like processes. (Fig. 202.) These processes are bathed in mucus, 
are movable, and have a general inclination pointing backward. The exist- 
ence of these hair-like processes on the filiform papilla; suggests that this 
variety of papillae is intimately connected with the tactile sensibility of the 
tongue, and not with gustation. In carnivora and herbivora these processes 
are of a horny structure, and perform an active function in the attrition 
and prehension of food. In man their special function appears, through 810 
HEARING, SMELL, AND TASTE. 
their intimate connection with the tactile sense, to guide the tongue in its 
variable and complicated movements. 
Fig. 202. 
A, vertical section near the middle of the dorsal surface of the tongue; a, a, fungiform papillee; 
b, filiform papillae, with their hair-like processes; c, similar ones deprived of their epithelium, 
magnified 2 diameters; B, filiform compound papillae; a, artery; v, vein ; c, capillary loops of the 
secondary papillae; b, line of basement-membrane ; d, secondary papillae, deprived of e, e, the 
epithelium; f, hair-like processes of epithelium capping the simple papillae, magnified 25 diam- 
eters; g, separated nucleated particles of epithelium, magnified 300 diameters. 1, 2, hairs found 
on the surface of the tongue; 3, 4, 5, ends of hair-like epithelial processes, showing varieties in 
the imbricated arrangement of the particles, but in all a coalescence of the particles toward the 
point; 5 encloses a soft hair, magnified 160 diameters. (After Todd and Bowman.) 
Fig. 203. 
Gustatory Bulbs from the Lateral Gustatory Organ of the Rabbit. (Magnified 450 diameters.) 
§ 732. The ultimate terminations of the gustatory nerves are yet envel- 
oped in obscurity. According to Engelmann, the glosso-pharyngeal nerves TASTE. 
811 
terminate in flask-shaped organs which are termed the gustatory bulbs or 
taste buds. (Fig. 203.) These bulbs are found principally in the papillary 
surface of the wall of the circumvallate papillae. They are also found in 
the fungiform papillae, but are less numerous. They consist of a flask-shaped 
fundus, which rests upon the subepithelial tissue, and a mouth which opens 
upon the surface of the mucous membrane. The mouth is known as the 
gustatory pore. The fundus of the flask is composed of two varieties of cells ; 
the outer or investing cells are fusiform, nucleated, and granular, placed 
parallel and arranged concentrically in a direction from the base to the 
neck ; they thus form a wall which encloses elongated nucleated cells with 
filamentous processes, which extend through the gustatory pore and project 
as very finely pointed or truncated extremities. These inner cells are called 
the gustatory cells, and are supposed to be the essential terminal elements 
concerned in gustation. Their relation to the gustatory nerves has not as 
yet been plear ly demonstrated, but they are evidently connected with the 
ganglionic plexuses of nerve-fibres at the papillary bases. The gustatory 
nerves are also supposed to terminate in the epithelium of the papillae.] 
§ 733. The word taste is frequently used when the word smell ought to 
be employed. We speak of "tasting" odoriferous substances, such as an 
onion, wines, etc., when in reality we only smell them as we hold them in our 
mouth ; this is proved by the fact that the so-called taste of these things 
is lost when the nose is held, or the nasal membrane rendered inert by 
a catarrh. 
The terminal organs of the sense of taste thus more strictly defined are 
the endings of the glosso-pharyngeal and lingual nerves in the mucous mem- 
brane of the tongue and palate, those nerves serving as the special nerves 
of taste. Whether the so-called gustatory buds can be regarded as specific 
organs of taste appears doubtful. The subsidiary apparatus is confined to 
the tongue and lips, which by their movements assist in bringing the sapid 
substances into contact with the mucous membrane of the mouth. 
Though we can hardly be said to project our sensation of taste into the 
external world, we assign to it no subjective localization. When we place 
quinine in our mouth, the resulting sensation of taste gives us no informa- 
tion as to where the quinine is, though we may learn that by concomitant 
general sensations arising in the buccal mucous membrane. 
§ 734. We recognize a multitude of distinct tastes, which may be broadly 
classified into acid, saline, bitter, and sweet tastes. Sapid substances have 
the power of producing these sensations by virtue of their chemical nature. 
But other stimuli will also give rise to sensations of taste. When the tongue 
is tapped, a taste is felt; and when a constant current is passed through the 
mouth, an alkaline or, in some persons, a bitter metallic taste is developed 
when the anode, and an acid taste when the kathode, is placed on the tongue. 
It is probable that in these cases the terminal organs are indirectly affected 
by the current. When hot or pungent substances are introduced into the 
mouth, sensations of general feeling are excited, which obscure any strictly 
gustatory sensations which may be present at the same time. 
Though analogy would lead us to suppose that a stimulus applied to any 
part of the course of the real gustatory fibres of either the glosso-pharyngeal 
or lingual nerves would give rise to a sensation of taste and nothing else, the 
proof is not forthcoming, since both these nerves are mixed nerves contain- 
ing other afferent fibres as well as those of taste. 
When the constant current is used as a means of exciting taste, gustatory 
sensations are found to be developed in the back, edges, and tip of the 
tongue, the soft palate, the anterior pillar of the fauces, and a small tract 
of the posterior part of the hard palate. They are absent from the anterior 812 
FEELING AND TOUCH. 
and middle dorsal and under surface of the tongue, the front portion of the 
hard palate, the posterior pillars of the fauces, the gums, and the lips. 
Sapid substances are unsuitable as a test for this purpose, on account of their 
rapid diffusion. Bitter substances produce most effect when placed on the 
back, and sweet substances when placed on the tip, of the tongue; but the 
tasting power of the tip of the tongue varies very much in different indi- 
viduals, and in many seems almost entirely absent. It is said that acids are 
best appreciated by the edge of the tongue. 
§ 735. It is essential for the development of taste that the substance to 
be tasted should be dissolved, and the effect is increased by friction. The 
larger the surface the more intense the sensation. The sensation takes some 
time to develop, and endures for a long time, though this may be in part 
due to the stimulus remaining in contact with the terminal organs. A tem- 
perature of about 40° is the one most favorable for the production of the 
sensation. At temperatures much above or below this, taste is much im- 
paired. The nerves of taste are, as we have said, the glosso-pbaryngeal 
and the lingual or gustatory. The former supplies the back of the tongue, 
and section of it destroys taste in that region. The latter is distributed to 
the front of the tongue, and section of it similarly deprives the tip of the 
tongue of taste. There is no reason for doubting that the gustatory fibres 
in the glosso-pharyngeal are proper fibres of that nerve ; but it has been 
urged by many that the gustatory fibres of the lingual are derived from the 
chorda tympani, and that those fibres of the lingual which come from the 
fifth are employed exclusively in the sensations of touch and feeling; the 
evidence in favor of this view is, however, inconclusive. 
CHAPTER V. 
FEELING AND TOUCH. 
General Sensibility and Tactile Perceptions. 
§ 736. We have taken the foregoing senses first in the order of discussion 
on account of their being eminently specific. The eye gives us only visual 
sensations, the ear only auditory snsations. The sensations are produced in 
each case by specific stimuli; the eye is only afiected by light and the ear 
only by sound. Moreover, the information they afford us is confined to the 
external world ; they tell us nothing about ourselves. The various visual 
sensations which arise in our retina are referred by us not to the retina itself, 
but to some real or imaginary object in the world without (including as part 
of the external world such portions of our own bodies as are visible to our- 
selves). Such also, with diminishing precision, is the information gained by 
hearing, taste, and smell. 
All the other afferent nerves of the body, centripetal impulses along which 
are able to affect our consciousness, are the means of conveying to us in- 
formation concerning ourselves. The sensations, arising in them from the 
action of various stimuli, are referred by us to appropriate parts of our own 
body. When any body comes in contact with our finger, we know that it is 
our finger which lias been touched ; from the resultant sensations we not only 
learn the existence of certain qualities in the object touched, but we also are 813 
GENERAL SENSIBILITY AND TACTILE PERCEPTIONS. 
led to connect the cognizance of these qualities with a particular part of our 
own body. 
§ 737. Like the more specific senses previously studied, the sensations of 
which we are now speaking, and which may be referred to under the name of 
touch, using that word for the present in a wide meaning, require for their 
production terminal organs ; and the chief but not exclusive organ of touch 
is to be found in the epidermis of the skin and certain underlying nervous 
structures. For the development of specific tactile sensations these terminal 
organs are as essential as are the terminal organs of the eye for sight or of 
the ear for hearing. Contact of the skin with a hard or with a hot body 
gives rise to a distinct sensation, whereby we recognize that we have touched 
a hard or a hot body. But the application of either body or of any other 
stimulus to a nerve-trunk gives rise to a sensation of general feeling only, 
corresponding to the simple sensation of light which is produced by direct 
stimulation of the optic nerve. We have no more tactile perception of a 
body which is in contact with a nerve-trunk than we could have visual 
perception of any luminous object, the rays proceeding from which were 
strong enough to excite sensory impulses when directed on to the optic 
nerve instead of on to the retina, supposing such a thing to be possible. It is 
further characteristic of these ordinary nerves of general feeling, that the 
sensations caused by any stimulation of them beyond a certain degree de- 
velop that state of consciousness which we are in the habit of speaking of 
as " pain." Putting aside the general feeling which many parts of the eye 
possess, a very strong luminous stimulation of the retina is required to pro- 
duce a sensation of pain, if indeed it can be at all brought about; whereas 
a very moderate stimulation of the skin, and almost every stimulation of an 
ordinary nerve-trunk, is said by us to be painful. 
Though the skin is the chief organ of touch, the mucous membrane 
lining the various passages of the body also serves as an instrument for 
the same sense, but only for a short distance from the respective orifices. 
We can recognize hard or hot bodies with our lips or mouth, but a hot 
liquid, when it has reached the oesophagus or stomach, simply gives rise to 
a sensation of pain ; we cannot distinguish the sensation caused by it from 
the sensation caused by a draught of a too acid fluid. 
From parts and tissues of the body other than the skin and the portions 
of mucous membrane just mentioned we have obscure sensations of general 
feeling, by which we are made vaguely aware of the general condition of our 
body, though our judgments in this matter are chiefly influenced by what 
we shall have to speak of directly as a muscular sense. In all parts of the 
body, however, on occasions all too frequent, this general feeling may become 
prominent as pain. 
§ 738. The stimuli which, when applied to the skin, give rise to tactile 
perceptions are of two kinds only : (1) mechanical, that is, the contact of 
bodies exerting varying degrees of pressure; and (2) thermal, i. e., the rais- 
ing or lowering of the temperature of the skin by the approach or contact 
of hot or cold bodies. We can judge of the weight and of the temperature 
of a body, because we can, through touch, perceive how much it presses when 
allowed to rest on our skin or how hot it is. But we can through touch de- 
rive no other perceptions and form no other judgments. An electric shock 
sent through the skin will give rise to a sensation, but the sensation is an 
indefinite one, because the electric current acts not on the terminal organs 
of touch, but on the fine nerve-branches of the skin. We cannot distinguish 
the sensation so caused from a mechanical prick of similar intensity ; we 
cannot perceive that the sensation is caused by an electric current. Sim- 
ilarly, certain chemical substances, such as a strong acid, will give rise to a 814 
FEELING AND TOUCH. 
sensation, but we cannot perceive the acid, we can form no judgment of 
its nature such as we could if we tasted it; and if the acid does not per- 
meate the skin so as to act directly and chemically on the fine nerve-fibres, 
we cannot distinguish the acid from any other liquid giving rise to the 
same simple contact impressions. The terminal organs of the skin are 
such as are only affected by pressure or by temperature. Conversely, 
pressure or a variation in temperature brought to bear on a nerve-trunk, 
instead of on the terminal organs, produces no specific tactile sensations of 
pressure or temperature, but merely general sensations of feeling rapidly 
rising into pain. 
Tactile Sensations. 
Sensations of Pressure. 
§ 739. As with visual, so with tactile, and, indeed, with all other sensa- 
tions, the intensity of the sensation maintains that general relation to the 
intensity of the stimulus which we spoke of at p. 767 as being formulated 
under Weber's law. We can distinguish the difference of pressure between 
one and two grammes as readily as we can that between ten and twenty or 
one hundred and two hundred. 
When two sensations follow each other in the same spot at a sufficiently 
short interval, they are fused into one ; thus, if the finger be brought to bear 
lightly on a rotating card having a series of holes in it, the holes cease to be 
felt as such when they follow each other at a rapidity of about 1500 in a 
second. The vibrations of a cord cease to be appreciable by touch when 
they reach the same rapidity. When sensations are generated at points of 
the skin too close together they become fused into one; but to this point we 
shall return presently. 
§ 740. The sensation caused by pressure is at its maximum soon after 
its beginning, and thenceforward diminishes. The more suddenly the pressure 
is increased, the greater the sensation ; and if the increase be sufficiently 
gradual, even very great pressure may be applied without giving rise to any 
sensation. A sensation in any spot is increased by contrast when the sur- 
rounding areas are not subject to pressure. Thus, if the finger be dipped 
into mercury, the pressure will be felt most at the surface of the fluid ; and 
if the finger be drawn up and down, the sensation caused will be that of a 
ring moving along the finger. 
All parts of the skin are not equally sensitive to pressure; small differ- 
ences of simple pressure are more readily appreciated when brought to bear 
on the palmar surface of the finger, or on the forehead, than on the arm or 
on the sole of the foot. In making these determinations, all muscular move- 
ments should be avoided in order to eliminate the muscular sense, of which 
we shall speak presently; and the area stimulated should be as small and 
the surfaces in contact as uniform as possible. In a similar manner, small, 
consecutive variations of pressure, as in counting a pulse, are more readily 
appreciated by certain parts of the skin than by others; and the minimum 
of pressure which can be felt differs in different parts. In all cases, varia- 
tions of pressure are more easily distinguished when they are successive than 
when they are simultaneous. 
§ 741. When the temperature of the skin is raised or lowered in any 
spot, we receive sensations of heat and cold respectively ; and by these sen- 
sations of the temperature of our skin, we form judgments of the temper- 
Sensations of Temperature. TACTILE SENSATIONS. 
815 
ature of bodies in contact with it. Bodies of exactly the same temperature 
as the region of the skin to which they are applied produce no such thermal 
sensations, though we can, from the very absence of sensation, form a judg- 
ment as to their temperature; and good conductors of heat appear respec- 
tively hotter and colder than bad conductors raised to the same temperature. 
§ 742. We may consider the skin as having at any given time and in 
any given spot a normal temperature at which the sensation of temperature 
is at zero; for under ordinary circumstances we are not directly conscious 
of the temperature of our skin ; it is only when the normal temperature at 
the spot is raised or lowered that we have a sensation of heat or cold re- 
spectively. This normal temperature may be at the same time different in 
different parts of the body; thus, at a time when neither the forehead nor 
the hand are giving rise to any sensation of temperature, we may, by putting 
the hand to the forehead, frequently feel the former hot or cold because the 
normal temperatures of the two parts differ. The normal temperature in 
any spot may also vary from time to time. Thus, when the hand is placed 
in a warm medium for some time, the sensation of warmth ceases; a new 
normal temperature is established with the zero of sensation at a higher level, 
a depression or elevation of this new temperature giving rise, however, as 
before, to sensations of heat and cold respectively. That it is the changed 
condition, and not the change itself, of which we are conscious, is shown by 
the fact that when a portion of the skin is cooled, by brief contact with a 
cold metal, for instance, we are still conscious of the spot being cold after the 
cooling agent has been removed-that is, at a time when a cooled spot is in 
reality being heated by the surrounding warmer tissues. 
§ 743. The change in temperature of the skin necessary to produce a sen- 
sation must have a certain rapidity; and the more gradual the change the 
less intense the sensation. The repeated dipping of the hand into hot water 
produces a greater sensation than when the hand is allowed to remain all 
the time in the water, though in the latter case the temperature of the skin 
is most affected. The effects of contrast are also seen in these sensations as 
in those of pressure. 
We can with some accuracy distinguish variations of temperature, espe- 
cially those lying near the normal temperature of the skin. These sensations, 
in fact, follow Weber's law, though apparently sensations of slight cold are 
more vivid than those of slight heat, the range of most accurate sensation 
seeming to lie between 27° and 33°. 
The regions of the skin most sensitive to variations in temperature are not 
identical with those most sensitive to variations in pressure. Thus the 
cheeks, eyelids, temples, and lips are more sensitive than the hands. The 
least sensitive parts are the legs, and front and back of the trunk. 
§ 744. The simplest view which can be taken with regard to the dis- 
tinction between pressure sensations and temperature sensations, and which is 
suggested by the facts just mentioned, is to suppose that two distinct kinds 
of terminal organs exist in the skin, one of which is affected only by pressure, 
and the other only by variations in temperature; and that the two kinds of 
peripheral organs are connected with different parts of the central sensory 
organs by separate nerve-fibres. Certain pathological cases have been 
quoted as showing not only that this is the case, but that the two sets of 
fibres pursue different courses in the spinal cord. Thus in certain diseases 
or injuries to the brain or spinal cord, hypersesthesia as regards temperature 
has been observed unaccompanied by an augmentation of sensitiveness to 
pressure; and, conversely, instances have been seen where the patient could 
tell when he was touched, but could not distinguish between hot and cold. 
On the other hand, there are facts which show a close dependence between 816 
FEELING AND TOUCH. 
the sensations of pressure and temperature. When each stimulus is brought 
to bear on a very limited area, the two sensations are frequently confounded, 
especially in those regions of the body where sensations are not acute. So, 
also, a penny cooled down nearly to zero, and placed on the forehead will 
be judged by most people to be as heavy or even heavier than two pennies 
of the temperature of the forehead itself; and, conversely, a body warmer 
than the skin will often appear heavier than a body of the same weight, but 
of the same temperature as the skin. Moreover, cases have been recorded 
where a hot body, such as a heated spoon, was felt, though the application of 
the same spoon at the temperature of the body produced no sensations, and 
yet the heated spoon was not recognized as a hot body, but appeared to be 
simply something touching the skin. It may be argued that these instances 
show nothing more than the changes in the skin, whatever they be, which 
give rise to sensations of pressure, are modified by the temperature of the 
skin for the time being, whereby the judgment as to the pressure which is 
being exerted is rendered faulty ; but they may also be taken to indicate 
that variations in pressure and temperature affect the same terminal organs, 
and the same nerve-fibres, though affecting them in a different way, and 
generating nervous impulses so far different that they give rise to different 
sensations. And we may here note that we certainly cannot speak of nerves 
of warmth in the same sense in which we speak of nerves of sight or of 
hearing. A stimulus (of whatever kind) applied to an optic or auditory 
nerve, if adequate, gives rise, as we have seen, to a sensation of light or of 
sound; a stimulus, on the other hand, applied to the trunk of a cutaneous 
nerve, gives rise only to general feeling or pain ; though the nerve certainly 
contains fibres by which sensations of pressure and of temperature reach the 
brain, the general feeling which stimulation of the trunk causes is akin 
neither to sensations of pressure nor to those of warmth. 
§ 745. The rapidity with which hot or cold bodies brought into contact 
with the skin give rise to sensations of temperature, suggests that the ter- 
minal apparatus for generating these sensations, whatever be its nature, is 
placed in the epidermis, and indeed as near as possible to the surface. Pres- 
sure, on the other hand, can be readily transmitted through even a thick 
layer of skin. And those who maintain the existence of different terminal 
organs for pressure and temperature, regard the nerve-endings in the epider- 
mis as the latter, and the corpuscular tactus, end-bulbs, and allied organs as 
the former. But the evidence we possess concerning this matter is at present 
inconclusive. 
Tactile Perceptions and Judgments. 
§ 746. When a body presses on any part of our skin, or when the 
temperature of the skin at that point is raised, we are not only conscious of 
pressure or of heat, but perceive that a particular part of our body has been 
touched or heated. We refer the sensations to their place of origin, and we 
thus by touch perceive the relations to ourselves of the body which gives 
rise to the tactile sensations, in the same way as in our visual perception of 
external objects we refer to external nature the sensations originating in 
certain parts of the retina. When we are touched on the linger and on the 
back we refer the sensations to the finger and to the back respectively, and 
when we are touched at two places on the same finger at the same time we 
refer the sensations to two points of the finger. In this way we can localize 
our sensations, and are thus assisted in perceiving the space relations of ob- 
jects with which we come in contact. 
§ 747. This power of localizing pressure sensations varies in different TACTILE PERCEPTIONS AND JUDGMENTS. 
817 
parts of the body. The following table, from Weber, gives the distance at 
which two points of a pair of compasses must be held apart, so that when 
the two points are in contact with the skin the two consequent sensations 
can be localized with sufficient accuracy to be referred to two points of the 
body, and not confounded as one: 
Tip of tongue 1.1 mm. 
Palm of last phalanx of finger 2.2 " 
Palm of second " " 4.4 " 
Tip of nose 6.6 " 
White part of lips 8.8 " 
Back of second phalanx of finger 11.1" 
Skin over malar bone 15.4 " 
Back of hand 29.8 " 
Forearm 39.6 " 
Sternum 44.0 " 
Back 66.0 " 
And an analogous distribution has been observed in reference to the locali- 
zation of sensations of temperature. As a general rule, it may be said that 
the more mobile parts are those by which we can thus discriminate sensations 
most readily. The lighter the pressure used to give rise to the sensations, 
the more easily are two sensations distinguished ; thus two points which, when 
touching the skin lightly, appear as two, may, when firmly pressed, give rise 
to one sensation only. The distinction between the sensations is obscured bv 
neighboring sensations arising at the same time. Thus, two points brought 
to bear within a ring of heavy metal pressing on the skin, are readily con- 
fused into one. And it need hardly be said that these tactile perceptions, 
like all other perceptions, are immensely increased by exercise. 
§ 748. Our "field of touch," if we may be allowed the expression, is 
composed of tactile areas or units, in the same way that our field of vision 
is composed of visual areas or units. The tactile sensation is, like the visual 
sensation, a symbol to us of some external event, and we refer the sensation 
to its appropriate place in the field of touch. All that has been said (p. 768) 
concerning the subjective nature of the limits of visual areas, applies equally 
well, mutatis mutandis, to tactile areas. When two points of the compasses 
are felt as two distinct sensations, it is not necessary that two, and only two, 
nerve-fibres should be stimulated; all that is necessary is that the two cere- 
bral sensation-areas should not be too completely fused together. The im- 
provement by exercise of the sense of touch must be explained not by an 
increased development of the terminal organs, not by a growth of new nerve- 
fibres in the skin, but by a more exact limitation of the sensational areas in 
the brain, by the development of a resistance which limits the radiation 
taking place from the centres of the several areas. 
§ 749. By a multitude of simultaneous and consecutive tactile sensations 
thus converted into perceptions we are able to make ourselves acquainted 
with the form of external objects. We can tell by variations of pressure 
whether a surface is rough or smooth, plane or curved, what variations of 
surface a body presents, and how far it is heavy or light; and from the in- 
formation thus gained we build up judgments as to the form and nature of 
objects, judgments, however, which are most intimately bound up with visual 
judgments, the knowledge derived by one sense correcting and completing 
that obtained by the other. As in other senses, so in this, our sensations 
may mislead us and cause us to form erroneous judgments. This is well 
illustrated by the so-called experiment of Aristotle. It is impossible in an 
ordinary position of the fingers to bring the radial side of the middle finger 818 
FEELING AND TOUCH. 
and the ulnar side of the ring finger to bear at the same time on a small 
object, such as a marble. Hence, when with the eyes shut we cross one 
finger over the other, and place a marble between them so that it touches 
the radial side of the one and the ulnar side of the other, we recognize that 
the object is such as could not under ordinary conditions be touched at the 
same time by these two portions of our skin, and therefore judge that we 
are touching not one but two marbles. Upon repetition, however, we are 
able to correct our judgment, and the illusion disappears. 
§ 750. Distinct tactile sensations are, as we have seen, produced only 
when a stimulus is applied to a terminal organ. When sensations or affect- 
ions of general sensibility other than the distinct tactile sensations are 
developed in the termination of a nerve, we are still able, though with less 
exactitude, to refer the sensation to a particular part of the body. Thus, 
when we are pricked or burned, we can feel where the prick or burn is. 
When a sensory nerve-trunk is stimulated, the sensation is always referred 
to the peripheral terminations of the nerve. Thus a blow on the ulnar nerve 
at the elbow is felt as a tingling in the little and ring fingers corresponding 
to the distribution of the nerve, and sensations started in the stump of an 
amputated limb are referred to the absent member. When cold is applied 
to the elbow it is felt as cold in the skin of the elbow ; but a cooling of the 
ulnar nerve at this spot, since stimulation of a nerve-trunk gives rise to 
general sensations only, simply gives rise to pain which is referred to the 
ulnar side of the hand and arm. 
§ 751. When we come into contact with external bodies we are con- 
scious not only of the pressure exerted by the object on our skin, but also of 
the pressure which we exert on the object. If we place the hand and arm 
Hat on a table, we can estimate the pressure exerted by bodies resting on the 
palm of the hand, and so come to a conclusion as to their weights; in this 
case we are conscious only of the pressure exerted by the body on our skin. 
If, however, we hold the body in the hand, we not only feel the pressure of 
the body, but we are also aware of the muscular exertion required to sup- 
port and lift it. We possess a muscular sense; and we find by experience 
that when we trust to this muscular sense as well as to sensations of pres- 
sure, we can form much more accurate judgments concerning the weight of 
bodies than when we rely on sensations of pressure alone. When we want 
to tell how heavy a body is, we are not in the habit of allowing it simply to 
press on the hand laid flat on a table; we hold it in our hand and lift it up 
and down. We appeal to our muscular sense to inform us of the amount 
of exertion necessary to move it, and by help of that, judge of its weight. 
And in all the movements of our body we are guided, even to an astonish- 
ing degree of accuracy, as is well seen in the discussions concerning vision, 
by an appreciation, more or less distinctly conscious, of the amount of the 
contraction to which we are putting our muscles. In some way or other we 
are made aware of what particular muscles or groups of muscles are being 
thrown into action, and to what extent that action is being carried. We are 
also conscious of the varying condition of our muscles, even when they are 
at rest; the tired and especially the paralyzed limb is said to " feel" heavy. 
In this way the state of our muscles largely determines our general feeling 
of health and vigor, of weariness, ill health and feebleness. 
It has been suggested that since muscle possesses little or no general 
sensibility, comparatively little pain being felt for instance when muscles are 
cut, our muscular sense is chiefly derived from the traction of the contract- 
The Muscular Sense. 819 
THE VOICE. 
ing muscle on its attachments ; and undoubtedly in many instances of cramp, 
the pain is chiefly felt at the joints; and, as we know, Pacinian bodies are 
abundant around the joints. Afferent nerves, however, having a different 
disposition from the ordinary motor nerves which terminate in end-plates, 
have been described as present in muscle ; and analogy would lead us to sup- 
pose that these afferent fibres, though possessing a low general sensibility, 
might be easily excited in a specific manner by a muscular contraction ; but 
further investigations are necessary before these can be accepted as the true 
nerves of the muscular sense. 
§ 752. In favor of the view that the muscular sense is peripheral and 
not central in origin may be urged the fact that the sense is felt when the 
muscles are thrown into contraction by direct galvanic stimulation instead 
of by the agency of the will. Many authors, while admitting the existence 
of a muscular sense of peripheral origin, contend that we also possess and 
are very largely guided in our movements by what might be called a 
" neural " sense of central origin. That is to say, the changes in the central 
nervous system involved in initiating and carrying out a movement of the 
body so affect our consciousness that we have a sense of the effort itself. 
It has been observed that when the posterior roots are divided, move- 
ments become less orderly, as if they lacked the guidance of a muscular 
sense; and although the impairment of the movements may be due in part 
to the coincident loss of tactile sensations, it is probable that it is increased 
by the loss of the muscular sense. There is a malady or rather a condition 
attending various diseased states of the central nervous system called loco- 
motor ataxia, the characteristic feature of which is that, though there is no 
loss of direct power over the muscles, the various bodily movements are 
effected imperfectly and with difficulty, from want of proper coordination. 
In such diseases the pathological mischief is frequently found in the posterior 
columns of the spinal cord and the posterior roots of the spinal nerves, that 
is, in distinctly afferent structures ; and the phenomena seem in certain cases 
at least to be due to inefficient coordination caused by the loss both of the 
muscular sense and of ordinary tactile sensations. The patients walk with 
difficulty, because they have imperfect sensations both of the condition of 
their muscles and of the contact of their feet with the ground. In many 
of their movements they have to depend largely on visual sensations ; hence 
when their eyes are shut, they become singularly helpless. In other cases 
again ataxia may be present without any impairment of touch ; but a dis- 
cussion of the varied phenomena of this class of maladies cannot be entered 
into here. 
CHAPTER VI. 
SPECIAL MUSCULAR MECHANISMS, 
The Voice. 
§ 753. [The larynx is a membrano-cartilaginous chamber, broader above 
than below, and situated in the anterior median portion of the neck. It 
consists of a number of cartilages, which are articulate with each other, 
connected by ligaments, moved by a number of muscles, and lined by a 
mucous membrane. 820 
SPECIAL MUSCULAR MECHANISMS. 
The principal cartilages are the thyroid, cricoid, the two arytenoid, and 
the epiglottis. 
The thyroid cartilage is the largest and consists of two quadrilateral 
plates or alae, which are continuous with each other in front, where they 
form the prominence called the pomum Adami. The posterior borders of 
the thyroid cartilage serve as a point of attachment of the stylo-pharyngeus 
and palato-pharyngeus muscles. The upper part of each of these borders 
terminates in a superior cornu, which articulates with the hyoid bone ; the 
lower portion terminates in the inferior cornu, which articulates with the 
cricoid cartilage. The upper border between the cornua is connected with 
Fig. 204. 
Median Section of Mouth, Nose, Pharynx, and Larynx, a, septum of nose, below it section 
of hard palate ; b, tongue; c, section of velum pendulum palati; d, d. lips ; u, uvula ; r, anterior 
arch or pillar of fauces: i, posterior arch ; t, tonsil; p, pharynx; h, hyoid bone; k, thyroid carti- 
lage ; n, cricoid cartilage ; s, epiglottis ; v, glottis ; 1, posterior opening of the nares; 3, isthmus 
faucium ; 4, superior opening of larynx ; 5, passage into oesophagus; 6, mouth of right Eustachian 
tube. 
the hyoid bone by the thyro-hyoid membrane. The lower border is con- 
nected with the cricoid cartilage by the thyro-cricoid membrane at the 
median line, and at the sides by the crico-thyroid muscles. 
The cricoid cartilage is situated below the thyroid cartilage with its broad 
portion posteriorly. At the upper part of its broad portion are two smooth 
surfaces on which the arytenoid cartilages articulate. 
The arytenoid cartilages are pyramidal in form and articulate on the 
upper surface of the cricoid. Each cartilage has an external, posterior, and 
internal (median) surface, an apex and a base. The apex is pointed back- 
ward and inward, and is surmounted by a small cartilaginous tubercle, called 
the cartilage of Santorini (Fig. 207). The base, which articulates with the 
cricoid cartilage, presents at its external internal angle a projection called 
the processus vocalis. At the posterior internal angle is a second projection, 
called the processus muscularis. 
§ 754. The superior opening of the larynx is formed anteriorly by the 
epiglottis, posteriorly by the apices of the arytenoid cartilages, and laterally 
by the aryteno-epiglottidean folds stretching between these points. The in- THE VOICE. 
821 
ferior opening corresponds to the inferior border of the cricoid cartilage. 
Between these points is the cavity of the larynx, which has stretching across 
its sides the vocal cords. The vocal cords consist of two pairs ; the supe- 
rior or false vocal cords are membrano-ligamentous bands, which extend 
from the receding angle of the thyroid to the external surfaces of the ary- 
tenoid cartilages ; the inferior or true vocal cords (chordce vocales) are mem- 
brano-ligamentous bands which stretch across the cavity of the larynx from 
the receding angle of the thyroid to the processus vocales of the arytenoid 
cartilages. Between the borders of the true and false vocal cords is an 
elliptical opening, the ventricle, which leads to a space running upward and 
behind the false vocal cords, called the sacculus laryngis. The mucous 
Fig. 205. 
Fig. 206. 
Fig. 207. 
Fig. 205.-View of the Larynx and part of the Trachea from Behind with the Muscles Dissected 
h, the body of the hyoid bone; e, epiglottis ; t, the posterior borders of the thyroid cartilage; c, the 
median ridge of the cricoid; a, upper part of the arytenoid; 8, placed on one of the oblique fasci- 
culi of the arytenoid muscle ; b, left posterior crico-arytenoid muscle ; ends of the incomplete car- 
tilaginous rings of the trachea; I, fibrous membrane crossing the back of the trachea; n, muscular 
fibres exposed in a part. (From Quain's Anatomy.) 
biG. 206. View of the Larynx from Above. 1, aperture of glottis; 2, arytenoid cartilages; 3, 
vocal cords; 4, posterior crico-arytenoid muscles; 5, lateral crico-arytenoid muscle of right side, 
that of left side removed; 6, arytenoid muscle ; 7, thyro-arytenoid muscle of left side, that of right 
side removed ; 8, thyroid cartilage ; 9, cricoid cartilage; 13, posterior crico-arytenoid ligament. With 
the exception of the arytenoid muscle, this diagram is a copy from Mr. Willis's figure. 
I ig. 207.-V iew of the I pper Part of the Larynx as seen by means of the Laryngoscope during 
the Utterance of a Grave Note, c, epiglottis; s, cartilages of Santorini; a, arytenoid cartilages; 
z, base of the tongue; ph, posterior wall of the pharynx. 
membrane lining this sac contains a great number of follicular glands 
which discharge a mucous secretion for the purpose of lubricating the true 
vocal cords. 
Between the true vocal cords is an opening which is called the rima glot- 
tidis. The form of the glottis varies very much both in the inspiratory and 
expiratory acts, and in the act of phonation. 
§ 755. ihe muscles of the larynx are divided anatomically into the in- 
trinsic and extrinsic. The former are nine in number, four of them being 822 
SPECIAL MUSCULAR MECHANISMS. 
in pairs. They are the essential muscles concerned in the movements of the 
arytenoid cartilages and chordae vocales. The extrinsic muscles connect the 
larynx with adjacent parts, and are for the most part concerned in the 
elevation and depression of the organ. 
The larynx is lined with a mucous membrane which is continuous above 
with that lining the pharynx and mouth, and below with that lining the 
trachea. Above the chorda vocales it is lined with pavement epithelium, 
excepting at the lower anterior portion, where it is ciliated ; below the 
chordae vocales the epithelium is of a ciliated columnar variety. The mu- 
cous membrane contains many mucous glands, which are pretty uniformly 
distributed; they are, however, very abundant in the part of the membrane 
lining the sacculus laryngis.] 
§ 756. A blast of air, driven by a more or less prolonged expiratory 
movement, throws into vibrations two elastic membranes-the chordae 
vocales. These impart their vibrations to the column of air above them, 
and so give rise to the sound which we call the voice. Since the sound is 
generated in the vocal cords, we may speak of them and of those parts of 
the larynx which decidedly affect their condition as constituting the essential 
vocal apparatus; w hile the chamber above the vocal cords, comprising the 
ventricles of the larynx with the false vocal cords, the pharynx and the 
cavity of the mouth, the latter varying much in form, constitutes a sub- 
sidiary apparatus of the nature of a resonance tube, modifying the sound 
originating in the vocal cords. In the voice, as in other sounds, we distin- 
guish : 1, Loudness. This depends on the strength of the expiratory blast. 
2, Pitch. This depends on the length and tension of the vocal cords. 
Their length may be regarded as constant, or varying only with age. It 
consequently determines the range only of the voice, and not the particular 
note given out at any one time. The shrill voice of the child is deter- 
mined by the shortness of the cords in infancy, and the voices of a soprano, 
tenor, and baritone are all dependent on the respective length of their 
vocal cords. Their tension is, on the contrary, variable ; and the chief 
problems connected with the voice refer to variations in the tension of the 
vocal cords. 3, Quality. This depends on the number and character of 
the overtones accompanying any fundamental note sounded, and is deter- 
mined by a variety of circumstances, chief among which is the physical 
quality of the cords. 
§ 757. The vocal cords, attached in front to the thyroid cartilage, end 
behind in the processus vocales of the arytenoid cartilages. Hence a dis- 
tinction has been drawn between the rima vocalis, i. e., the opening bounded 
laterally by the vocal cords, and the rima respiratoria, or space between the 
arytenoid cartilages behind the processus vocales; these names, however, 
are not free from objections. In quiet breathing (Fig. 208, B) the two 
form together a V'shaped space, which, as we have seen (p. 343), in deep 
inspiration is widened into a rhomboidal opening by the divergence of the 
processus vocales (Fig. 208, C). When a note is about to be uttered, the 
vocal cords are by the approximation of the processus vocales brought into 
a position parallel to each other, and the whole rima is narrowed (Fig. 
208, A). By their parallelism and by the narrowness of the interval be- 
tween them the cords are rendered more susceptible of being thrown into 
vibration by a moderate blast of air. The problems we have to consider 
are, first, by what means are the cords brought near to each other or drawn 
asunder as occasion demands ; and, secondly, by what means is the tension 
of the cords made to vary. We may speak of these two actions as narrow- 
ing or widening of the glottis, and tightening or relaxation of the vocal 
cords. THE VOICE. 
823 
& 758. Narrowing of the glottis. The change of form of the glottis is 
best understood when it is borne in mind that each arytenoid cartilage is, 
when seen in horizontal section (Fig. 208), somewhat ot the form of a tri- 
angle with an internal or median, an external, and a posterior side, the pro- 
cessus vocalis being placed in the anterior angle at the junction of the 
median and external sides. When the cartilages are so placed that the pro- 
cessus vocales are approximated to each other, and the internal surfaces o 
Fig. 208. 
The Larynx as Seen by Means of the Laryngoscope in Different Conditions of the Glottis. 
(From Quain's Anatomy, after Czermak.) A, while singing a high note; B, in quiet breathing; 
C, during a deep inspiration. The corresponding diagrammatic figures A', B', C, illustrate the 
changes in position of the arytenoid cartilages and the form of the rima vocalis and rima re- 
spiratoria in the above three conditions. I, the base of the tongue; e, the upper free part of the 
epiglottis; e', the tubercle or cushion of the epiglottis; ph, part of the anterior wall of the 
pharynx behind the larynx; w, swelling in the aryteno-epiglottidean fold caused by the carti- 
lage of Wrisberg; s, swelling caused by the cartilage of Santorini; a, the summit of the ary- 
tenoid cartilage; cv, the true vocal cords; cvs, the false vocal cords; tr, the trachea with its 
rings ; b, the two bronchi at their commencement. 
the cartilages nearly parallel, the glottis is narrowed. When on the con- 
trary the cartilages are wheeled round on the pivots of their articulations, so 
that the processus vocales diverge, and the internal surfaces of the cartilages 
form an angle with each other, the glottis is widened. 
§ 759. There are several muscles forming together a group which has 
been called by Henle the sphincter of the larynx. These are : 1, the thyro- 
ary-epiglotticus, proceeding from the inner surface of the thyroid cartilage 
and from the arytenoid epiglottidean ligament, and sweeping round the outer 
ridge of the arytenoid cartilage of its own side to be inserted into the pro- 
cessus muscularis of the arytenoid cartilage of the other side; 2, the thyro- 
arytenoideus externus, passing from the reentrant angle of the thyroid carti- 
lage to be inserted into the outer edge of the arytenoid cartilage of the same 824 
SPECIAL MUSCULAR MECHANISMS. 
side ; 3, the thyro-arytenoideus iuternus, passing from the angle of the thyroid 
cartilage to the processus vocalis and outer side of the arytenoid cartilage; 
4, the arytenoideus (posticus), passing transversely from one arytenoid carti- 
lage to another. All these muscles, when they act together, grasp round the 
glottis and tend to close it up; and each of them, acting alone, has, with the 
exception of the last named (arytenoideus), the same effect. In addition to 
these, the crico-arytenoideus lateralis, which passes from the lateral border 
of the cricoid cartilage upward and backward to the outer angle of the ary- 
tenoid. by pulling this outer angle forward throws the processus vocalis 
inward, and so also narrows the glottis. 
§ 760. Widening of the glottis. The crico-arytenoidexis posticus, passing 
from the posterior surface of the cricoid cartilage to the outer angle of the 
arytenoid cartilage behind the attachment of the lateral crico-arytenoideus, 
pulls back this outer angle, and so causing the processus vocalis to move 
outward, widens the glottis. The arytenoideus posticus, acting alone, has a 
similar effect. 
§ 761. Tightening of the vocal cords. The crico-thyroideus pulls the 
thyroid downward and forward, and so increases the distance between that 
cartilage and the arytenoids when the latter are fixed. Supposing, then, the 
arytenoideus and crico-arytenoideus posticus to fix the arytenoids, the effect 
of the contraction of the crico-thyroideus would be to tighten the vocal 
cords. 
§ 762. Slackening of the vocal cords. This is effected by the whole 
sphincter group just mentioned, but more especially by the thyro-arytenoidei 
extemus and iuternus; these, acting alone, supposing the arytenoid cartilages 
to be fixed, would pull the thyroid cartilage upward and backward, and so 
shorten the distance between the processus vocalis and that body. 
§ 763. Thus almost every movement of the larynx is effected not by one 
muscle only, but by several, or at least by more than one, acting in concert. 
The movements which give rise to the voice are preeminently combined and 
coordinate movements. When we remember how a very slight variation in 
the tension of the vocal cords must give rise to a marked difference in the 
pitch of the note uttered, and yet what a multitude of fine differences of 
pitch are at the command of a singer of even moderate ability, it appears 
exceedingly probable that the various muscular combinations required to 
produce the possible variations in pitch are of such a kind that frequently a 
part only, possibly a few fibres only, of a particular muscle may be thrown 
into contraction, while all the rest of the muscle remains quiet. Taking 
into view, moreover, the great range of pitch possessed by even common 
voices, as compared with the possible variations of tension of which the 
vocal cords in their natural length are capable, it has been suggested that 
some of the fibres of the thyro-arytenoideus interims, which, passing either 
from the thyroid or from the arytenoid, appear to end in the vocal cords 
themselves, may, by fixing particular points of the cords, so to speak, " stop" 
them ; and by thus artificially shortening the length actually thrown into 
vibration, produce higher notes than the cords in their natural length are 
capable of producing. It has also been suggested that the processus vocales 
may overlap each other, and thereby shorten the length of cord available 
for vibration. 
§ 764. These various muscles are supplied by the vagus nerve, or rather 
by spinal accessory fibres running in the vagus trunk. The superior laryn- 
geal is the afferent nerve supplying the mucous membrane, but it also con- 
tains the motor fibres distributed to the crico-thyroid muscle ; hence, when 
this nerve is divided on one side the corresponding vocal cord is relaxed and 
high notes become impossible. It is worthy of notice that this, the chief THE VOICE. 
825 
tensor, and therefore the most important, muscle of the larynx, has a sepa- 
rate and distinct nervous supply. According to some authors the aryte- 
noideus posticus also receives its nervous supply from this nerve ; but this 
is denied by others. 
The inferior laryngeal or recurrent branch supplies all the other muscles. 
When this nerve is divided the voice is lost, since the approximation and 
parallelism of the vocal cords can no longer be effected. When in a living 
animal both recurrent nerves are divided, the glottis is seen to become im- 
mobile and partially dilated, the vocal cords assuming the position in which 
they are found in the body after death, and which may be considered as the 
condition of equilibrium between the dilating and constricting muscles. 
During forcible inspiration the glottis passes from this condition in the 
direction of more complete dilatation ; during forcible expiration, the change 
is one of constriction. When the peripheral portion of one recurrent nerve 
is stimulated, the vocal cord of the same side is approximated to the middle 
line ; when both nerves are stimulated, the vocal cords are brought together 
and the glottis is narrowed. Though the nerve is distributed to both dilating 
and constricting muscles, the latter overcome the former when the nerve is 
artificially stimulated. In the complete closure of the glottis, which is so 
important a part of the act of coughing (p. 403), the group of muscles 
which we have spoken of as constituting a sphincter is thrown into forcible 
contractions by the recurrent laryngeal nerve. 
§ 765. Though fundamentally a voluntary act, the utterance of a given 
note is not affected by the direct passage of simple volitional impulses down 
to the laryngeal muscles. So complex and coordinate a movement as that 
of sounding even a simple and natural note requires a coordinating nervous 
mechanism in which, as in other complex muscular actions, afferent impulses 
play an important part. Auditory sensations, if not as important for an 
accurate management of the voice as are visual sensations for the movements 
of the eye, are yet of prime importance. This is recognized when we say 
that such and such a one whose power over his laryngeal muscles is imperfect 
" has no ear." 
A person may speak or sing in two kinds of voice. In the one the 
sounds are full and strong, and the resonance chamber which is supplied by 
the trachea, bronchi, and indeed by the whole chest, is thrown into power- 
ful and palpable vibrations; hence this voice is spoken of as the chest- 
voice. The other kind of voice, called the falsetto, is thin and poor, deals 
chiefly with high notes, and is not accompanied by the same conspicuous 
vibrations of the chest. Much controversy has taken place as to the exact 
manner in which these two voices are respectively produced. The prevailing 
opinion teaches that in the chest-voice the vocal cords are somewhat thick, 
their substance being thrust inward toward the median line by the contrac- 
tion of the thyro-arytenoidei externi muscles, and the opening between them, 
sometimes so narrow as to be almost linear, extends along their whole length. 
In the falsetto voice, on the other hand, the vocal cords are said to be thin 
and membranous, and the note to be given forth by a vibration, not of the 
whole width of the cords, as in the chest-voice, but of the extreme edges 
only, the lateral parts, though not absolutely at rest, vibrating with a differ- 
ent rhythm. Though the whole larynx in the falsetto voice is stretched in 
the antero-posterior direction, and the vocal cords correspondingly elongated, 
the rima vocalis does not extend along their whole length ; at their posterior 
part the cords are in contact, and indeed, according to some authors, the 
high falsetto notes are produced by a sort of "stopping" of the cords. The 
sense of effort which accompanies the falsetto voice indicates that the changes 
in the larynx which bring it about are effected by some special muscular 826 
SPECIAL MUSCULAR MECHANISMS. 
manoeuvres, as is also suggested by the fact that the ease with which falsetto 
notes can be uttered is readily increased by practice. The change from the 
chest to the falsetto voice is an abrupt one, and the combined range may be 
very extensive, as in the case of persons who can carry on a duet, singing 
alternately, for instance, in a tenor (chest) and a soprano (falsetto) voice. 
§ 766. The ventricles of Morgagni are apparently of use in giving the 
vocal cords sufficient room for their vibrations, and perhaps supply a secre- 
tion by which the vocal cords are kept adequately moist. The purpose of 
the false vocal cords is not exactly known. Some authors think that in the 
falsetto voice they are brought down into contact with, and thus serve to 
stop, the true vocal cords. 
At the age of puberty a rapid development of the larynx takes place, 
leading to a change in the range of the voice. The peculiar harshness of 
the voice when it is thus " breaking" seems to be due to a temporary con- 
gested and swollen condition of the mucous membrane of the vocal cords 
accompanying the active growth of the whole larynx. The change in the 
mucous membrane may come on quite suddenly, the voice " breaking," for 
instance, in the course of a night. 
Speech. 
§ 767. Every sound, every note (for all vocal sounds when considered 
by themselves are musical sounds), caused by the vibrations of the vocal 
cords, besides its loudness due to the force of the expiratory blast, and its 
pitch due to the tension of the cords, has a quality of its own, due to the 
number and relative prominence of the overtones which accompany the 
fundamental tone. Some of these features which make up the quality are im- 
posed on the note by the nature of the vocal cords, but still more arise from 
various modifications which the relative intensities of the overtones undergo 
through the resonance of the cavity of the mouth and throat. Whenever 
we hear a note sounded by the larynx we are able to recognize in it features 
which enable us to state that one or other of the " vowels " is being uttered. 
Vowel sounds are in fact only extreme cases of quality, extreme prominence 
of certain overtones brought about by the shape assumed by the buccal and 
pharyngeal passages and orifices, as the vibrations pass through them. Each 
vowel has its appropriate and causative disposition of these parts. When i 
(ee in feet) is sounded, the sounding-tube of the upper air-passages is made 
as short as possible, the larynx is raised and the lips are retracted, the whole 
cavity of the mouth taking on the form of a broad flask with a narrow neck. 
During the giving out of e (a in fat) the shape of the mouth is similar, but 
somewhat longer. For the production of a (as in father) the mouth is widely 
open, so that the buccal cavity is of the shape of a funnel with the apex at 
the pharynx. With o, the buccal cavity is.again flask-shaped, with the 
mouth more closed than in a, but the lips, instead of being retracted as in i 
and e, are somewhat protruded, so that the sounding-tube is prolonged. The 
greatest length of the tube is reached in u (oo), in which the larynx is 
depressed and the lips protruded as much as possible. While the two latter 
vowels are being uttered, the general form of the buccal cavity is that of a 
flask with a short neck and a small opening, the orifice being smaller for u 
than for o. 
§ 768. Each of these various " vowel " forms of the mouth possesses a 
note of its own, one toward which it acts as a resonance chamber. Thus, if 
several tuning-forks of various pitch be held while sounding before a mouth 
which has assumed the particular form necessary for sounding U, it will be 
Vowels. SPEECH. 
827 
found that the resonance will be particularly great with the fork having the 
pitch of the bass 6-flat. Similarly, other and higher notes will be intensi- 
fied when the mouth is moulded to utter the other vowels. And it is the 
experience of singers that each vowel is sung with peculiar ease on a note 
having a prominent overtone corresponding to the tone proper to the mouth 
when moulded to utter the vowel. The precise nature of the vowel sounds 
is, however, still disputed. 
As the vibrations are travelling through the pharyngeal and buccal cavi- 
ties, the posterior nares are closed by the soft palate ; and it may be shown, 
by holding a flame before the nostril, that no current of air issues from the 
nose when a vowel is properly said or sung. When the posterior nares are 
not effectually closed the sound acquires a nasal character. The same hap- 
pens when the anterior nares are closed, as when the nose is held between 
the fingers, the nasal chamber then forming a cavity of resonance. 
Consonants. 
§ 769. Vowels are, as their name implies, the only real vocal sounds; it 
is only on a vowel that a note can be said or sung. Our speech, however, is 
made up not only of vowels but also of consonants, i. e., of sounds which are 
produced not by the vibrations of the vocal cords but by the expiratory blast 
being in various ways interrupted or otherwise modified in its course through 
the throat and mouth. 
The distinction between the two is, however, not an absolute one, since, 
as we have seen, the characters of the several vowels depend on the form of 
the mouth, and in the production of some consonants (B, D, M, N, etc.) 
vibrations of the vocal cords form a necessary though adjuvant factor. 
Consonants have been classified according to the place at which the 
characteristic interruption or modification takes place. Thus it may occur: 
1. At the lips, by the movement or position of the lips in reference to 
each other or to the teeth, giving rise to labial consonants. 
2. At the teeth, by the movement or position of the front part of the 
tongue in reference to the teeth or the hard palate, giving rise to dental 
consonants. 
3. In the throat, by the movement or position of the root of the tongue 
in reference to the soft palate or pharynx, giving rise to guttural consonants. 
Among the dentals again may be distinguished the dentals commonly so 
called, such as T, the sibilants such as S, and the lingual L, all differing in 
the relative position of the tongue, teeth, and palate. 
Consonants may also be classified according to the character of the move- 
ments which give rise to them. Thus they may be either explosive or con- 
tinuous. 
1. Explosives. In these the characters are given to the sound by the sud- 
den establishment or removal of the appropriate interruption. Thus, in 
uttering the labial P, the lips are first closed, then an expiratory current of 
air is driven against them, and upon their being suddenly opened, the sound 
is generated. Similarly, the dental T is generated by the sudden removal 
of the interruption caused by the approximation of the tip of the tongue to 
the front of the hard palate, and the guttural K by the sudden removal of 
the interruption caused by the approximation of the root of the tongue to 
the soft palate. 
The labial B differs from P, inasmuch as it is accompanied by vibrations 
of the vocal cords (that is, a vowel sound is uttered at the same time), and 
these vibrations continue after the removal of the interruption. Hence, B is 828 
SPECIAL MUSCULAR MECHANISMS. 
often spoken of. as being uttered with voice and P without voice ; and D and 
G (hard) with voice bear the same relation to T and K without voice. 
The continuous consonants may further be divided into- 
2. Aspirates. In these the sound is generated by a rush of air through a 
constriction formed by the partial closure of the lips, or by the raising of 
the tongue against the hard or soft palate, etc. Thus, F is sounded when 
the lips are brought into partial, and not as in P and B into complete approxi- 
mation, and a current of air is driven through the narrowed opening. F is 
uttered without any accompanying vibration of the vocal cords, i. e., without 
voice. With voice it becomes V. 
The sibilant S is formed by a rush of air past an obstruction caused by 
the partial closure of the teeth, the front of the tongue being depressed at 
the same time; and S accompanied with vibrations of the vocal cords 
becomes Z. 
In Sh the dorsal surface of the tongue is raised so as to narrow the 
passage between that organ and the palate for a considerable portion of its 
length. 
Th is formed by placing the tongue between the two partially open rows 
of teeth ; and the hard and soft Th bear to each other the same relation as 
do P and B. 
L is produced when the passage is closed in the middle by pressing the 
tip of the tongue against the hard palate and the air is allowed to escape at 
the sides of the tongue. 
When the constriction in an aspirate is formed by the approximation of 
the root of the tongue to the soft palate, we have the guttural CH (as in 
loch) without voice and GH (as in lough) with voice. 
3. Resonants or nasals. In these, all of which must have vibrations of 
the vocal cords as a basis, the usual passage through the mouth is closed 
either in a labial, dental, or guttural fashion, and the peculiar character is 
given to the sound by the nasal chambers acting as a resonance cavity. Thus 
in M, the passage is closed by the approximation of the lips, in N by the 
approximation of the tongue to the hard palate, and in NG by the approxi- 
mation of the root of the tongue to the soft palate. 
4. The. various forms of R are often spoken of vibratory, the charac- 
teristic sounds being caused by the vibration of some or other of the parts 
forming a constriction in the vocal passage. Thus the ordinary R is pro- 
duced by vibrations of the point of the tongue elevated against the hard 
palate, the guttural R by the vibrations of the uvula or other parts of the 
walls of the pharynx ; and in some languages there seems to be an R pro- 
duced by the vibrations of the lips. 
H is caused by the rush of air through the widely opened glottis. When, 
in sounding a vowel, the sound coincides with a sudden change in the posi- 
tion of the vocal cords from one of divergence to one of approximation, the 
vowel is pronounced with the spiritus asper. When the vocal cords are 
brought together before the blast of air begins, the vowel is pronounced with 
the spiritus lenis. The Arabic H is produced by closing the rima vocalis, 
the epiglottis and false vocal cords being depressed, and sending a blast of air 
through the rima respiratoria. 
On many of the above points, however, there are great differences of 
opinion, the discussion of which as well as of other more rare consonantal 
sounds would lead us too far away from the purpose of this book. The 
following tabulated statement must, therefore, be regarded as introduced for 
convenience only. LOCOMOTOR MECHANISMS. 
829 
Explosives. Labials, without voice P. 
with voice B. 
Dentals, without voice T. 
with voice I). 
Gutturals, without voice K (hard C). 
with voice G (hard). 
Aspirates. Labials, without voice F. 
with voice V. 
Dentals, without voice L, S (soft C), Sh. Th (hard), 
with voice Z, Zh (in azure, the French j), 
Th (soft). 
Gutturals, without voice CH (as in loch). 
with voice GH as in lough). 
Resonants. Labial, M. 
Dental, N. 
Guttural, NG. 
Vibratory. Labial, not known in European speech. 
Dental, R (common). 
Guttural, R (guttural). 
§ 770. Whispering is speech without any employment of the vocal cords, 
and is effected chiefly by the lips and tongue. Hence, in whispering the 
distinction between consonants needing and those not needing voice, such as 
B and P, becomes for the most part lost. 
Locomotor Mechanisms. 
§ 771. The skeletal muscles are for the most part arranged to act on the 
bones and cartilages as on levers, examples of the first kind of lever being 
rare, and those of the third kind, where the power is applied nearer to the 
fulcrum than is the weight, being more common than the second. This 
arises from the fact that the movements of the body are chiefly directed to 
moving comparatively light weights through a great distance, or through a 
certain distance with great precision, rather than to moving heavy weights 
through a short distance. The fulcrum is generally supplied by a (perfect 
or imperfect) joint, and one end of the acting muscle is made fast by being 
attached either to a fixed point, or to some point rendered fixed for the time 
being by the contraction of other muscles. There are few movements of the 
body in which one muscle only is required ; in the majority of cases several 
muscles act together in concert; nearly all our movements are coordinate 
movements. Where gravity or the elastic reaction of the parts acted on does 
not afford a sufficient antagonism to the contraction of a muscle or group of 
muscles, the return to the condition of equilibrium is provided for by the 
action either elastic or contractile of a set of antagonistic muscles; this is 
seen in the case of the face. 
§ 772. The erect posture, in which the weight of the body is borne by 
the plantar arches, is the result of a series of contractions of the muscles of 
the trunk and legs, having for their object the keeping the body in such a 
position that, the line of gravity falls within the area of the feet. That this 
does require muscular action is shown by the facts, that a person when stand- 
ing perfectly at rest in a completely balanced position falls when he becomes 
unconscious, and that a dead body cannot be set on its feet. The line of 
gravity of the head falls in front of the occipital articulation, as is shown 
by the nodding of the head in sleep. The centre of gravity of the combined 
head and trunk lies at about the level of the ensiform cartilage, in front of 
the tenth dorsal vertebra, and the line of gravity drawn from it passes 
behind a line joining the centres of the two hip-joints, so that the erect body 830 
SPECIAL MUSCULAR MECHANISMS. 
would fall backward were it not for the action ef the muscles passing from 
the thighs to the pelvis, assisted by the anterior ligaments of the hip-joints. 
The line of gravity of the combined head, trunk and thighs falls, moreover, 
a little behind the knee-joints, so that some, though little, muscular exertion 
is required to prevent the knees from being bent. Lastly, the line of gravity 
of the whole body passes in front of the line drawn between the two ankle- 
joints, the centre of gravity of the whole body being placed at the end of 
the sacrum ; hence some exertion of the muscles of the calves is required to 
prevent the body falling forward. 
§ 773. In walking, there is in each step a moment at which the body 
rests vertically on the foot of one, say the right, leg, while the other, the left 
leg, is inclined obliquely behind with the heel raised and the toe resting on 
the ground. The left leg, slightly flexed to avoid contact with the ground, 
is then swung forward like a pendulum, the length of the swing or step being 
determined by the length of the leg; and the left toe1 is brought to the 
ground. On this left toe as a fulcrum, the body is moved forward, the centre 
of gravity of the body describing a curve the convexity of which is upward 
and the left leg necessarily becoming straight and rigid. As the body moves 
forward, a point will be reached similar to that with which we supposed the 
step to be started, the body resting vertically on the left foot, and the right 
leg being directed behind in an oblique position. The movement on the left 
foot, however, carries the body beyond this point, and in doing so swings the 
right leg forward until it is the length of a step in advance of its previous 
position, and its toe in turn forms a fulcrum on which the body, and with it 
the left leg, is again swung forward. Hence in successive steps the centre 
of gravity, and with it the top of the head, describes a series of consecutive 
curves, with their convexities upward, very similar to the line of flight of 
many birds. 
Since in standing on both feet the line of gravity falls between the two 
feet, a lateral displacement of the centre of gravity is necessary in order to 
balance the body on one foot. Hence in walking the centre of gravity de- 
scribes not only a series of vertical, but also a series of horizontal curves, 
inasmuch as at each step the line of gravity is made to fall alternately on 
each standing foot. While the left leg is swinging, the line of gravity falls 
within the area of the right foot, and the centre of gravity is on the right 
side of the pelvis. As the left foot becomes the standing foot, the centre of 
gravity is shifted to the left side of the pelvis. The actual curve described 
by the centre of gravity is, therefore, a somewhat complicated one, being 
composed of vertical and horizontal factors. The natural step is the one 
which is determined by the length of the swinging leg, since this acts as a 
pendulum ; and hence the step of a long-legged person is naturally longer 
than that of a person with short legs. The length of the step, however, may 
be diminished or increased by a direct muscular effort, as when a line of 
soldiers keep step in spite of their having legs of different lengths. Such a 
mode of marching must obviously be fatiguing, inasmuch as it involves an 
unnecessary expenditure of energy. 
In slow walking there is an appreciable time, during which, while one 
foot is already in position to serve as a fulcrum, the other, swinging, foot has 
not yet left the ground. In fast walking this period is so much reduced that 
one foot leaves the ground the moment the other touches it; hence there is 
practically no period during which both feet are on the ground together. 
When the body is swung forward on the one foot acting as a fulcrum 
with such energy that this foot leaves the ground before the other, swinging, 
1 This indicates perhaps what should be done rather than the actual practice; most 
people put the heel to the ground first, the contact with the toe coming later. 831 
LOCOMOTOR MECHANISMS. 
foot has reached the ground, there being an interval during which neither 
foot is on the ground, the person is said to be running, not walking. 
In jumping, this propulsion of the body takes place on both feet at the 
same time; in hopping, it is effected on one foot only. 
§ 774. The locomotion of four-footed animals is necessarily more com- 
plicated than that of man. The simple walk, such as that of the horse, is 
executed in four times, with a diagonal succession ; thus, right fore-leg, left 
hind-leg, left fore-leg, right hind-leg. In the amble, such as that of the 
camel, the two feet of the same side are put down at one and the same time, 
this movement being followed by a similar movement of the other two legs; 
it corresponds, therefore, very closely to human walking. In the trot, which 
corresponds to human running, the two diagonally opposite feet are brought 
to the ground at the same time, and the body is propelled forward on them. 
Concerning this, however, as well as concerning the still more complicated 
gallop and canter, observers are not agreed, and much discussion has arisen. 
The other problems connected with the action of the various skeletal 
muscles of the body are too special to be considered here. BOOK IV. 
THE TISSUES AND MECHANISMS OF REPRODUCTION. 
CHAPTER I. 
ORGANS OF REPRODUCTION. 
§ 775. Many of the individual constituent parts of the body are capa- 
ble of reproduction-i. e., they can give rise to parts like themselves; or 
they are capable of regeneration-i. e., their places can be taken by new 
parts more or less closely resembling themselves. The elementary tissues 
undergo during life a very large amount of regeneration. Thus, the old 
epithelial scales which fall away from the surface of the body are suc- 
ceeded by new scales from the underlying layers of the epidermis; old 
blood-corpuscles give place to new ones ; worn-out muscles, or those which 
have failed from disease, are renewed by the accession of fresh fibres; 
divided nerves grow again ; broken bones are united ; connective tissue 
seems to disappear and reappear almost without limit; new secreting cells 
take the place of the old ones which are cast off; in fact, with the excep- 
tion of some cases, such as cartilage, and these doubtful exceptions, all 
those fundamental tissues of the body, which do not form part of highly 
differentiated organs, are, within limits fixed more by bulk than by any- 
thing else, capable of regeneration. That regeneration by substitution of 
molecules, which is the basis of all life, is accompanied by a regeneration 
by substitution of mass. 
In the higher animals regeneration of whole organs and members, even 
of those whose continued functional activity is not essential to the well- 
being of the body, is never witnessed, though it may be seen in the lower 
animals ; the digits of a newt may be restored by growth, but not those of 
a man. And the repair which follows even partial destruction of highly 
differentiated organs, such as the retina, is in the higher animals very im- 
perfect. 
In the higher animals the reproduction of the whole individual can be 
effected in no other way than by the process of sexual generation, through 
which the female representative element or ovum is, under the influence 
of the male representative or spermatozoon, developed into an adult indi- 
vidual. 
We do not purpose to enter here into any of the morphological problems 
connected with the series of changes through which the ovum becomes the 
adult being; or into the obscure biological inquiry as to how the simple, 
all-but-structureless ovum contains within itself, in potentiality, all its future 
developments, and as to what is the essential nature of the male action. 
These problems and questions are fully discussed elsewhere ; they do not 
833 834 
ORGANS OF REPRODUCTION. 
properly enter into a work on physiology, except under the view that all 
biological problems are, when pushed far enough, physiological problems. 
We shall limit ourselves to a brief survey of the more important physio- 
logical phenomena attendant on the impregnation of the ovum and on the 
nutrition and birth of the embryo. 
§ 776. [ The female organs of generation are anatomically divided into the 
internal and external organs. The latter comprise the labia majora and 
minora, the clitoris, the hymen, the meatus urinarius, the vulvo-vaginal 
glands, and the mucous and sebaceous glands which are distributed in 
the mucous membrane covering the parts. The external organs play a 
very subsidiary part in the function of reproduction, and they will be passed 
by with this brief notice. 
The internal organs comprise the vagina, uterus, Fallopian tubes, and 
ovaries. 
The vagina is a musculo-membranous canal, about four to six inches 
long, directed obliquely upward and backward, and extending from the 
hymen to the cervix uteri, where it is attached at a point a short distance 
Fig. 209. 
Diagrammatic View of the Uterus and its Appendages, as seen from Behind. (From Quain.) 
Half natural size. The uterus and upper part of the vagina have been laid open by removing 
the posterior wall; the Fallopian tube, round ligament, and ovarian ligament have been cut 
short, and the broad ligament removed on the left side ; u, the upper part of the uterus; c, the 
cervix opposite the os internum ; the triangular shape of the uterine cavity is shown, and the 
dilatation of the cervical cavity with the rugse, termed arbor vitae ; v, upper part of the vagina ; 
od, Fallopian tube or oviduct; the narrow communication of its cavity with that of the cornu of 
the uterus on each side is seen; I, round ligament; lo, ligament of the ovary ; o, ovary ; i, wide 
outer part of the right Fallopian tube ; fl, its fimbriated extremity ; po, parovarium ; h, one of the 
hydatids frequently found connected with the broad ligament. 
above the os uteri. Its walls consist of an external coat of longitudinal 
muscular fibres, a middle erectile coat, and an internal mucous coat. The 
mucous membrane is continuous below with that covering the external geni- 
tals, and above with the mucous membrane lining the uterus. The anterior 
and posterior surfaces are marked by longitudinal folds or raphe, from 
which a number of transverse folds are given off. This membrane is pro- 
vided with mucous glands, and is thickly covered with sensitive papillae. 
The uterus is a flattened, pyriform, muscular organ (Fig. 209). Ana- 
tomically it is divided into the fundus, neck, and cervix. The neck indicates 
the point of division between the lower constricted portion, which is the 
cervix, and the upper expanded portion, the fundus. The cervix, which 
extends from the neck to the end of the organ, projects into the vagina, at 
which point it is marked by a transverse fissure, called the os uteri. 835 
ORGANS OF REPRODUCTION. 
The cavity of the uterus is somewhat triangular in shape, and very much 
flattened antero-posteriorly. The inferior angle of the cavity is continuous 
with the canal running through the cervix to the 
vagina. The superior angles are called the cornua ; 
at the bottom of each is an orifice of a Fallopian 
tube. The uterus is composed of three coats; a 
serous (formed by the peritoneum), a muscular, and 
a mucous coat. The mucous coat is continuous with 
that lining the Fallopian tubes and vagina. It is 
covered with columnar ciliated epithelium, and, if 
examined with a lens, the openings of the mucous 
follicles will be seen to be very profusely distributed 
over the surface. If a vertical section is made, as 
in Fig. 210, the tubules will be seen to be arranged 
perpendicularly to the surface, having a wavy 
course. In the impregnated uterus they become 
much swollen and enlarged. The mucous mem- 
brane lining the cervix, on account of its peculiar 
appearance, is called the arbor vitce uterinus. 
The Fallopian tubes (Fig. 209) are about four 
inches in length, and extend from the cornua of the 
uterus to the ovaries, where they end in enlarged 
expanded extremities, the margins of which are 
covered by long, slender processes, one of them 
being connected to the ovary. This portion of the 
tube is called the fimbriated extremity. The tubes 
are composed of a serous, muscular, and mucous 
layer. The mucous membrane is covered with 
ciliated columnar epithelium. 
The ovaries (Fig. 209) are flattened, ovoidal 
bodies, which are situated one on each side of the 
uterus, and enclosed in the folds of the broad liga- 
ments. They are each connected with the uterus 
by a ligament, and with the Fallopian tube by one 
of its fimbrise. They each consist of a fibrous coat 
(tunica albuginea) which encloses the stroma of the organ. (Fig. 211.) The 
stroma is composed of a soft, vascular fibrous tissue, having imbedded in it a 
number of small bodies, called Graafian vesicles, which are in divers stages 
of development. These vesicles commence their development in the deeper 
portions of the ovary, and as they approach maturity gradually make their 
way to the surface, where they project as prominences, and, their capsule 
finally rupturing, discharge their contents into the Fallopian tube. Each 
vesicle consists of an external coat formed by the ovary, an internal coat or 
capsule, and within this a layer of cells, which constitutes the membrana 
granulosa. The interior of the vesicle consists of an albuminous fluid, in 
which is suspended the ovule. 
§ 777. The male generative organs!. The same physiological interest is 
not centred in the male organs of generation as in those of the female, 
the principal interest being concentrated upon the organs which secrete the 
male fluid by which the ovule is impregnated. Our remarks will, therefore, 
be almost entirely confined to the organs concerned in the secretion of this 
fluid. 
The male organs comprise the penis, or organ of copulation, the pros- 
tate and Cowper's glands, the testicles, and vasa deferentia and vesiculae 
seminales. 
Fig. 210. 
Section of the Lining 
Membrane of a Human 
Uterus at the Period of Com- 
mencing Pregnancy, show- 
ing the arrangements and 
other peculiarities of the 
glands, d, d, d, with their ori- 
fices, a, a, a, on the internal 
surface of the organ. Twice 
the natural size. 836 
ORGANS OF REPRODUCTION. 
The prostate gland surrounds the neck of the bladder and commence- 
ment of the urethra (Fig. 211). It secretes a milky fluid, which is con- 
veyed by the prostatic ducts to the floor of the urethra. Cowper's glands 
are two small glands which are situated between the layers of the deep 
perineal fascia at the anterior part of the membranous urethra. They 
secrete a viscid fluid, which is conveyed by ducts to the floor of the 
urethra. 
The testes or testicles are two small, flattened, ovoidal glands, which are 
situated in a musculo-membranous pouch, called the scrotum, and suspended 
by the spermatic cords. Each testicle consists of two parts: the gland 
proper and the epididymis. The gland (Fig. 213) is composed of an outer 
Fig. 211. 
View of a Section of Prepared Ovary of the Cat. (After Schron.) X 6. 1, outer covering and 
free border of the ovary; 1', attached border; 2, the ovarian stroma, presenting a fibrous and 
vascular structure ; 3, granular substance lying external to the fibrous stroma ; 4, bloodvessels ; 
5, ovigerms in their earliest stages occupying a part of the granular layer near the surface; 6, 
ovigerms which have begun to enlarge and to pass more deeply into the ovary; 7, ovigerms 
round which the Graafian follicle and tunica granulosa are now formed, and which have passed 
somewhat deeper into the ovary and are surrounded by the fibrous stroma ; 8, more advanced 
Graafian follicle with the ovum imbedded in the layer of cells constituting the proligerous disc; 
9, the most advanced follicle containing the ovum ; 9', a follicle from which the ovum has acci- 
dentally escaped; 10, corpus luteum. 
fibrous coat, the tunica albuginea, this being covered by a serous membrane, 
the tunica vaginalis. The substance of the gland consists of a number of 
pyramidal lobular divisions, which are situated with their bases toward the 
surface. Each lobule is composed of several convoluted tubuli seminiferi, 
and are separated from adjoining lobules by prolongation of fibrous tissue 
from the tunica albuginea. The tubules are composed of a homogeneous 
basement membrane, which is lined by granular nucleated epithelium. In 
the apices of the lobules they have a straight course, and form the vasa recta. 
They then enter the fibrous tissue of the mediastinum (Fig. 213), and form 
a plexus of tubes called the rete testis, which end in the upper part of the 
mediastinum as the vasa efferentia, and these becoming very much convoluted 
from the globus major or head of the epididymis. The tubules of the globus 
major unite to form a single tube, which is very much convoluted, and con- 
stitutes the body and globus minor of the epididymis, and is then continued 
from the globus minor to the base of the bladder as the excretory duct or 
vas deferens. 
The vas deferens, commencing at the globus minor, ascends in the pos- 
terior part of the spermatic cord through the spermatic canal into the pelvis, 
where it runs to the base of the bladder and becomes enlarged, sacculated, 837 
ORGANS OF REPRODUCTION 
and narrowed, and joins with the duct of the vesicula seminalis to form a 
common ejaculatory duct. The walls of the vas deferens are composed of 
fibrous and muscular tissue, which is lined by a mucous membrane with 
columnar epithelium. 
The vesiculce seminales are two elongated sacculated bodies, placed ex- 
Fig. 212. 
Fig. 213. 
Fig. 212.-The Base of the Male Bladder, with the Vesiculee Seminales and Prostat Hand. 
(After Haller.) 1, the urinary bladder; 2, the longitudinal layer of muscular fibres; 3, pros- 
tate gland ; 4, membranous portion of the urethra; 5, the ureters ; 6, bloodvessels ; 7, lef,.S, right 
vas deferens ; 9, left seminal vesicle in its natural position; 10, ductus ejaculatorius of the left 
side traversing the prostate gland; 11, right seminal vesicle injected and unravelled; 12,13, 
blind pouches of vesiculae ; 14, right ductus ejaculatorius traversing the prostate. 
Fig. 213.-a, lobules ; b, vasa recta; c, mediastinum ; d, vasa efferentia ; e, body of epididymis; 
J, rete testes ; g, globus minor ; h, vas deferens ; i, tunica albuginea and its interlobu'ar reflec- 
tions ; I, globus major. 
Fig. 214. 
A, Spermatozoa from the Human Vas Deferens. (After Kolliker.) 1, magnified 350 diameters ; 
2, magnified 800 diameters; a, from the side ; b, from above. B, Spermatic Cells and Spermatozoa 
of the Bull undergoing Development. (After Kolliker.) 450; 1. 1, spermatic cells, with one or 
two nuclei, one of them clear; 2,3, free nuclei, with spermatic filaments forming; 4, the fila- 
ments enlongated and the body widened; 5, filaments nearly developed. 
ternal to the vasa deferentia. The structure of the seminal vesicles is simi- 
lar to that of the vasa deferentia, consisting of a fibro-muscular wall lined 
with a mucous membrane, which is covered by granular, nucleated, polygonal 
epithelium cells. These organs serve as receptacles for the seminal fluid 
secreted by the testes, and at the same time produce a secretion of their own 838 
MENSTRUATION. 
which is added to it. The ejaculatory ducts, which are formed by the 
union of the ducts of the vasa deferentia and vesiculie seniinales, open into 
the prostatic portion of the urethra. Their coats are thinner, but have 
essentially the same structure as the vasa deferentia, with which they are 
continuous. 
§ 778. The seminal fluid is a complex secretion, being composed of the 
anatomical elements of spermatozoa, which are formed in the testes, and of 
the secretions of the vasa deferentia, vesicular seniinales, the prostrate and 
Cowper's glands, and the mucous glands of the urethra. The seminal fluid 
is of a thick, whitish striated appearance, and, if examined microscopically, 
is seen to contain innumerable bodies which are in active motion. These 
are the sperniatozoids, and are the essential male elements concerned in the 
fecundation of the ovule. Each of these bodies (Fig. 214) consists of a 
flattened, ovoidal head, having at its base a tapering caudate appendage in 
active vibratile motion. These anatomical elements were at first considered 
animalcula, but they are now looked upon as free mases of protoplasm with 
ciliary appendages, which endow them with the power of migration. 
The spermatozoa are developed from the nuclei of vesicles which are 
formed in the tubules of the testes. The nuclei are metamorphosed into 
the heads of the spermatozoa, the ciliary appendages being afterward devel- 
oped as a sort of outgrowth. Different stages of the development and other 
interesting features are shown in the above figure.] 
CHAPTER IT. 
MENSTRUATION. 
§ 779. From puberty, which occurs at from 13 to 17 years of age, to 
the climacteric, which arrives at from 45 to 50 years of age, the human 
female is subject to a monthly discharge of ova from the ovaries, accompanied 
by special changes, not only in those organs but also in the Fallopian tubes 
and uterus, as well as by general changes in the body at large, the whole 
[Fig. 215. 
[Fig. 216. 
Fig. 215.-Section of Graafian Follicle of a Mammal. (After Von Barr.) 1, stroma of the ovary 
with bloodvessels ; 2, peritoneum; 3 and 4, layers of the external coat of the Graafian follicle ; 5, 
membrana granulosa: 6, fluid of the Graafian follicle; 7, granular zone, or discus proligerus, con- 
taining the ovule (8).] 
Fig. 216.-Ovula of the Sow. (After Barry.) 1, germinal spot; 2, germinal vesicle; 3, yolk ; 4, 
zona pellucida; 5, discus proligerus; 6, adherent granules or cells.] 
constituting " menstruation." The essential event in menstruation is the 
escape of an ovum from its Graafian follicle (Fig. 215). The whole ovary 
at this time becomes congested, and the ripe follicle bulges from its surface. 
The most projecting portion of the wall of the follicle, which has previously MENSTRUATION. 
839 
become excessively thin, is now ruptured, and the ovum, which having left 
its earlier position, is lying close under the projecting surface of the follicle, 
escapes, together with the cells of the discus prroligerus (Fig. 216), into the 
Fallopian tube. How the entrance of the ovum into the Fallopian tube is 
secured is not exactly known. Some maintain that the ovary is grasped by 
the trumpet-shaped fimbriated mouth of the Fallopian tube, itself turgid 
and congested, the movement necessary to bring this about being effected 
by the plain muscular fibres present in the mouth of the tube. Others, re- 
jecting this view, and asserting that the turgescence of the tube does not 
occur until after the ovum has become safely lodged in the tube, suggest 
Fig. 217. 
Successive Stages of the Formation of the Corpus Luteum in the Graafian Follicle of the Sow 
(as seen in Vertical Section). At a is shown the state of the follicle immediately after the expul- 
sion of the ovule, its cavity being filled with blood, and no ostensible increase of its epithelial 
lining having yet taken place; at b a thickening of this lining has become apparent; at c it 
begins to present folds, which are deepened at d, and the clot of blood is absorbed pari passu, and 
at the same time decolorized; a continuance of the same process, as shown at e,f, g, h, forms the 
corpus luteum, with its delicate cicatrix. 
that the ovum is carried in the proper direction by currents in the peritoneal 
cavity set up by the action of the ciliated epithelium lining the tube, cur- 
rents whose direction and strength seem, as shown by experiment, to be 
adequate to carry into the uterus particles present in the peritoneal fluid. 
Arrived in the tube, the ovum travels downward, very slowly, by the action 
probably of the cilia lining the tube, though possibly its progress may 
occasionally be assisted by the peristaltic contractions of the muscular walls. 
The stay of the ovum in the Fallopian tube may extend to several days. 
There is an effusion of blood into the ruptured follicle, which is subsequently 
followed by histological changes in the coats of the follicle resulting in a 
corpus luteum1 (Fig. 217). The discharge of the ovum is accompanied not 
1 [The following tabular statement by Dalton expresses the principal differences be- 
tween the corpus luteum of the non-pregnant and pregnant female: 
Corpus Luteum of Menstruation. 
Corpus Luteum of Pregnancy. 
At the end of three 
Three-quarters of an inch in diam- 
weeks. 
eter: central clot reddish; convo- 
luted wall pale. 
One Month. 
Smaller; convoluted wall bright yel- 
Larger; convoluted with bright yel- 
low ; clot still reddish. 
low; clot still reddish. 
Two Months. 
Reduced to the condition of an insig- 
Seven-eighths of an inch in diameter; 
nificant cicatrix. 
convoluted wall bright yellow; clot 
perfectly decolorized. 
Six Months. 
Absent. 
Still as large as at end of second 
month; clot fibrinous; convoluted 
wall paler. 
One-half an inch in diameter; central 
Nine Months. 
Absent. 
clot converted into radiating cica- 
trix ; the external wall tolerably 
thick and convoluted, but without 
any bright yellow color.] 840 
MENSTRUATION. 
only by a congestion or erection of the ovary and Fallopian tube, but also 
by marked changes in the uterus, especially in the uterine mucous mem- 
brane. While the whole organ becomes congested and enlarged, the mucous 
membrane, and especially the uterine glands, are distinctly hypertrophied. 
The swollen internal surface is thrown into folds which almost obliterate the 
cavity; and a hemorrhagic discharge, often considerable in extent, constitut- 
ing the menstrual or catamenial flow, takes place from the greater part of 
its surface. The blood as it passes through the vagina becomes somewhat 
altered by the acid secretions of that passage, and when scanty coagulates 
but slightly; when the flow, however, is considerable, distinct clots may 
make their appearance. The swollen and-hypertrophied mucous membrane 
then undergoes a rapid degeneration, and is shed, passing away sometimes in 
distinct masses, forming the latter part of the menstrual flow. The loss of 
the mucous membrane is so complete, that the bases only of the uterine 
glands are left, and from the epithelial cells lining these the regeneration of 
the new membrane is said to take place. It is not certain that menstrua- 
tion, in the human subject at all events, is always accompanied by a dis- 
charge of an ovum; indeed cases have been recorded in which menstruation 
continued after what appeared to be complete removal of both ovaries. And 
it seems probable also that under certain circumstances, e. g., coitus, a dis- 
charge of an ovum may take place at other times than at the menstrual 
period. Since, however, the time during which both the ovum and the 
spermatozoon may remain in the female passages alive and functionally 
capable is considerable, probably extending to some days, coitus effected 
either some time aftei' or some time before the menstrual escape of an ovum 
might lead to impregnation and subsequent development of an embryo ; 
hence the fact that impregnation may follow upon coitus at some time after 
or before menstruation is no very cogent argument in favor of the view that 
such a coitus has caused an independent escape of an ovum. The escape of 
the ovum is said to precede, rather than coincide with or follow, the cata- 
menial flow. If no spermatozoa come in contact with the ovum it dies, the 
uterine membrane returns to its normal condition, and no trace of the dis- 
charge of an ovum is left, except the corpus luteum in the ovary. 
780. It is obvious that in these phenomena of menstruation we have 
to deal with complicated reflex actions affecting not only the vascular 
supply, but, apparently in a direct manner, the nutritive changes of the 
organs concerned. Our studies on the nervous action of secretion render it 
easy for us to conceive in a general way how the several events are brought 
about. It is no more difficult to suppose that the stimulus of the enlarge- 
ment of a Graafian follicle causes nutritive as well as vascular changes in 
the uterine mucous membrane, than it is to suppose that the stimulus of food 
in the alimentary canal causes those nutritive changes in the salivary glands 
or pancreas which constitute secretion. In the latter case we can to some 
extent trace out the chain of events ; in the former case we hardly know 
more than that the maintenance of the lumbar cord is sufficient, as far as 
the central nervous system is concerned, for the carrying on of the work. 
In the case of a dog in which the spinal cord had been completely divided 
in the dorsal region while the animal was as yet a mere puppy, " heat " or 
menstruation took place as usual. IMPREGNATION. 
841 
CHAPTER III. 
IMPREGNATION. 
§ 781. In coitus the discharge of the semen containing the spermatozoa 
is most probably effected by means of the peristaltic contractions of the vesi- 
culte seminales and vasa deferentia, assisted by rhythmical contractions of 
the bulbo-cavernosus muscle, the whole being a reflex act, the centre of 
which appears to be in the lumbar spinal cord. In the dog emissions of 
semen can be brought about by stimulation of the glans penis after complete 
division of the spinal cord in the dorsal region. The emission of semen is 
preceded by an erection of the penis. This we have already seen (p. 274) is 
in part at least due to an increased vascular supply brought about by means 
of the nervi erigentes; it is probable, however, that the condition is further 
secured by a compression of the efferent veins of the corpora cavernosa by 
means of smooth muscular fibres present in those bodies. The semen being 
received into the female organs, which are at the time in a state of turges- 
cence resembling the erection of the penis, but less marked, the spermatozoa 
find their way into the Fallopian tubes, and here (probably in its upper 
part) come into contact with the ovum. In the case of some animals im- 
pregnation may take place at the ovary itself. The passage of the sperma- 
tozoa is most probably effected mainly by their own vibratile activity; but 
in some animals a retrograde peristaltic movement travelling from the uterus 
along the Fallopian tubes has been observed ; this might assist in bringing 
the semen to the ovum, but inasmuch as these movements are probably 
parts of the act of coitus, and impregnaton may be deferred till some time 
after that event, no great stress can be laid upon them. 
As the result of the action of the spermatozoa on the ovum, the latter, 
instead of dying as when impregnation fails, awakes to great nutritive 
activity accompanied by remarkable morphological changes; it enlarges 
and develops into an embryo. 
§ 782. [Preceding the time of the occurrence of the entrance of the 
spermatozoon into the egg, certain histological changes have been observed 
to occur, and in order thoroughly to understand these, as well as the changes 
which follow in the ovum, it will first be necessary to review the histology 
of the egg. 
The ovule is a minute cell, the wall being formed by a structureless, 
transparent membrane, called the zona pellucida, or vitelline membrane. 
Within this is the yolk, or vitellus, which consists of a granular semi-fluid 
mass, having suspended in it a nucleus or germinal vesicle which contains a 
nucleolus or germinal spot. The germinal vesicle consists of a very delicate, 
transparent, homogeneous membrane which encloses a fluid with granules, 
suspended in which is an eccentric nucleolus of a granular and fibrillated 
structure. 
§ 783. Previous to the occurrence of the impregnation of the ovule a 
very interesting series of changes have been observed to take place. Accord- 
ing to Balfour, the first interesting point to be noticed is the migration of 
the germinal vesicle toward the cell wall. The vesicular wall then becomes 
wavy and gradually disappears, while at the same time the nucleolus or 
germinal spot has undergone metamorphosis, so that what remains of these 
structures is a spindle-shaped mass. One extremity of this mass gradually 
projects through the cell wall and is thrown off as a polar vesicle. From the 
other remaining portion a second polar vesicle is formed, the part of the 842 
IMPREGNATION. 
mass then remaining in the ovule being permanent, and is called the female 
pronucleus. The next change observed is the appearance of a zone, of 
radial striae around the pronucleus and its migration to the centre of the egg. 
The spermatozoon then penetrates the wall of the ovule, probably at the 
point of the formation of the polar vesicles. The tail of the spermatozoon 
becomes absorbed, and the head is metamorphosed into the male pronucleus. 
From the male pronucleus a number of radiating striae are given off" in all 
directions, and it then migrates toward the female pronucleus, and afterward 
fuses with it, forming a single or cleavage nucleus. 
§ 784. Cleavage or segmentation of the vitellus then begins (Fig. 218), 
by which process the nucleus thus formed divides into two parts, each taking 
Fig. 218. 
Diagrams of the Various Stages of Cleavage of the Yolk. (After Dalton.) 
with it half of the vitelline mass. These two divide into four, and these four 
into eight, and so on indefinitely until an agglomerate mass of nucleated 
cells results, each of which contains a part of the cleavage nucleus. This 
mass of cells is called the mulberry mass, and the 
cells constituting it arrange themselves about the in- 
terior of the zona pellucida and form the blastodermic 
vesicle or membrane. This membrane then splits up 
into two layers, the external and internal, a third 
or middle layer being afterward formed between 
them. 
Immediately after the formation of the two layers 
of blastoderm, an opaque rounded collection of small 
cells occurs, called the area germinativa or embryonic 
spot. (Fig. 219.) This spot then becomes elongated, 
and in its longitudinal axis the first trace of the em- 
bryo appears as a faint line, termed the primitive trace, 
this being in the midst of a clear elongated mass of 
cells, the area pellucida, which is itself surrounded by 
a more opaque zone. 
§ 785. In front of the primitive trace two folds are formed from which 
a groove is prolonged backward in a line with the primitive trace. These 
folds gradually extend along the entire length of the groove, and from the 
lamina dorsales, which, by growing, project more and more above the 
groove, and, gradually approaching each other, coalesce and enclose the 
neural canal, which will afterward contain the cerebro-spinal axis. At 
about the same period, corresponding to the development of the dorsal 
laminae similar laminae are given off from the under surface of the blasto- 
derm. These are the lamina ventrales, which, by gradually enlarging and 
finally coalescing, enclose the abdominal cavity. Beneath the floor of the 
groove above described a delicate, whitish collection of cells appears. This 
is the chorda dorsalis or notochord, around which are afterward developed 
the bodies and processes of the vertebra. 
Fig. 219. 
Impregnated Egg, with 
Commencement of For- 
mation of Embryo. (Af- 
ter Dalton.) Showing the 
area germinativa or em- 
bryonic spot, the area 
pellucida, and the prim- 
itive groove or trace. IMPREGNATION. 
843 
During this period other changes have also taken place. The cephalic 
and caudal extremities have become fixed and form the cephalic and caudal 
flexures; and the epabryoalso being curved upon itself laterally, the vitelline 
mass appears separated from it by a constriction. This constriction gradu- 
ally increasing, finally separates the vitelline mass as a vesicular body, it 
being connected with the body of the embryo by the vitelline duct. (Fig. 
220.) The vesicular body thus formed is called the umbilical vesicle. This 
at first communicates with the intestinal cavity, but as development proceeds 
the duct of communication becomes closed, and the vesicle is merely attached 
by a pedicle, and finally disappears altogether. At the time of the develop- 
ment of the bloodvessels, vessels appear on the surface of the umbilical 
vesicle, constituting the vascular area, the chief vessels being the omphalo- 
mesenteric arteries and veins. The vessels of the vascular area absorb the 
nutritive material contained within the vesicle and convey it to the embryo 
for its sustenance. 
Fjg. 220. 
Diagrammatic Section showing the Relation in a Mammal and in a Man between the Primi- 
tive Alimentary Canal and the Membranes of the Ovum. The stage represented in this diagram 
corresponds to that of the fifteenth or seventeenth day in the human embryo, previous to the ex- 
pansion of the allantois : c, the villous chorion ; a, the amnion; a', the place of convergence of 
the amnion and reflection of the false amnion, a" a", or outer or corneous layer; e, the head and 
trunk of the embryo, comprising the primitive vertebrae and cerebro-spinal axis ; i, i, the simple 
alimentary canal in its upper and lower portions ; the yolk-sac or umbilical vesicle; vi, the vitel- 
line duct: w, the allantois connected by a pedicle with the anal portion of the alimentary canal. 
§ 786. Shortly after the occurrence of the commencement of the forma- 
tion of the umbilical vesicle, double folds, formed of the external layer of 
the blastoderm, are given off from the cephalic and caudal extremities, and 
laterally, which curve around over the dorsal surface of the embryo, where 
they meet and coalesce, and, their point of junction becoming absorbed, form 
the amniotic cavity. (Figs. 220, 221, 222, and 223.) The outer layer of 
the fold, or false amnion, gradually expands and covers the whole of the in- 
ternal surface of the vitelline membrane, which it ultimately replaces; the 844 
IMPREGNATION. 
inner layer, or true amnion, is continuous with the skin of the embryo at the 
umbilicus, and closely envelops it. The amniotic cavity or sac thus formed 
becomes filled with the liquor amnii, which gradually increases in quantity 
as pregnancy advances, up to about the fifth or sixth month, when the 
quantity gradually decreases up to the time of labor. 
Fig. 221. 
Fig. 222. 
a, chorion with villi. The villi are shown to be best developed in the part of the chorion to 
which the allantois is extending; this portion ultimately becomes the placenta, b, space between 
the two layers of the amnion ; c, amniotic cavity; d, situation of the intestine, showing its con- 
nection with the umbilical vesicle; e, umbilical vesicle situation of heart and vessels; g,allan- 
tois. (After Todd and Bowman.) 
§ 787. At about the time of the commencement of the development of 
the amnion a new organ, the allantois, appears as a pyriform mass of cells at 
a point immediately posterior to the vitelline duct and projecting through 
the same opening. (Fig- 223.) This mass of cells undergoes rapid growth, 
spreading itself between the true and false amniotic folds, finally completely 
Fig. 223. 
Fig. 224. 
Fig. 223.-Diagram of Fecundated Egg. (After Dalton.) a, umbilical vesicle ; b, amniotic 
cavity; c, allantois. 
Fig. 224.-Fecundated Egg with Allantois nearly Complete, a, inner layer of amniotic fold; 
b, outer layer of ditto ; c, point where the amniotic folds come in contact. The allantois is seen 
penetrating between the outer and inner layers of the amniotic folds. This figure, which repre- 
sents only the amniotic folds and the parts within them, should be compared with Figs. 221 and 
222, in which will be found the structures external to these folds. 
enclosing the embryo and amnion (Fig. 224), becoming at the same time 
adjoined to the false amnion, when it is developed into the true chorion. Dur- 
ing the process of the development of the allantois, it has become very vas- 
cular ; at first there are two arteries and two veins, afterward one of the 
veins disappears. These vessels constitute the umbilical vessels, forming part 
of the umbilical cord which connects the allantois with the embryo. During IMPREGNATION. 
845 
the development of the allantois it presents three distinct anatomical por- 
tions : a portion which becomes constricted off, as it were, from the rest and 
forms the urinary bladder ; the outer portion forms the chorion, the inter- 
mediate portion forming the umbilical cord. 
Fig. 225. 
Fig. 226. 
Fig. 225.-Entire Human Ovum of Eighth Week; sixteen lines in length (not reckoning the 
tufts); the surface of the chorion partly smooth and partly rendered shaggy by the growth of tufts. 
Fig. 226.-Portion of One of the Fcetal Villi, about to form part of the Placenta ; highly magni- 
fied. a, a, its cellular covering; b, b, b, its looped vessels ; c, c, its basis of connective tissue. 
During the development of the embryo up to this time, the first chorion 
was formed by villosities formed on the vitelline membrane; and following 
that by villosities developed upon the false amnion. The allantois then 
becoming developed, completely covers the internal surface of the false 
amnion, which then gradually disappears as a distinct structure. The true 
chorion is then formed by the allantois, which becomes covered by a growth 
of a multitude of vascular shaggy tufts or villi (Fig. 225). These villi, at 
first, are distributed over the entire surface of the organ, but they soon com- 
mence disappearing, except at a small area corresponding to the attachment 
of the pedicle which connects the allantois with the embryo. At this point 
they become greatly increased in number, and also in size and vascularity. 
These villi are composed of a fibro-granular matrix, in which are numerous 
capillary loops, and are covered with a layer of epithelial cells (Fig. 226). 
This portion of the chorion forms the foetal portion of the placenta. 
No sooner, however, have these changes begun in the ovum than correla- 
tive changes, brought about probably by reflex action, but at present most 
obscure in their causation, take place in the uterus. The mucous membrane 
of this organ, whether the coitus resulting in impregnation be coincident 
with a menstrual period or not, becomes congested, and a rapid growth takes 
place, characterized by a rapid proliferation of the epithelial and subepithe- 
lial tissues. Unlike the case of menstruation, however, this new growth does 
not give way to immediate decay and hemorrhage, but remains, and may be 846 
IMPREGNATION. 
distinguished as a new temporary lining to the uterus, the so-called decidua. 
Into this decidua the ovum, on its descent from the Fallopian tube, in which 
it has undergone developmental changes, extending most probably as far, at 
least, as the formation of the blastoderm, if not further, is received; and in 
this it becomes imbedded, the new growth closing in over it. (Figs. 227, 228.) 
Meanwhile the rest of the'uterine structures, especially the muscular tissue, 
Fig. 227. 
Fig. 228. 
Fig. 227.-First Stage of the Formation of the Decidua Reflexa around the Ovum. 
Fig. 228.-More Advanced Stage of Decidua Reflexa. 
become also much enlarged ; as pregnancy advances a large number of new 
muscular fibres are formed. As the ovum continues to increase in size, it 
bulges into the cavity of the uterus, carrying with it the portion of the 
decidua which has closed over it. Henceforward, accordingly, a distinction 
is made in the now well-developed decidua between the decidua reflexa, or 
Fig. 229. 
Section of a Portion of a Fully Formed Placenta, with the part of the Uterus to which it is 
attached, a, umbilical cord ; b, b, section of uterus, showing the venous sinuses ; c, c, c, branches, 
of the umbilical vessels ; d, d, curling arteries of the uterus. 
that part of the membrane which covers the projecting ovum, and the 
decidua vera, or the rest of the membrane lining the cavity of the uterus, 
the two being continuous around the base of the projecting ovum. That part 
of the decidua which intervenes between the ovum and the nearest uterine THE NUTRITION OF THE EMBRYO. 
847 
wall is frequently spoken of as the decidua serotina. As the ovum develops 
into the foetus with its membranes, the decidua reflexa becomes pushed 
against the decidua vera; about the end of the third month, in the human 
subject, the two come into complete contact all over, and ultimately the dis- 
tinction between them is lost. In the region of the decidua serotina, the 
allantoic vessels of the foetus develop a placenta. 
§ 788. In the earliest stages of the development of the placenta, the 
delicate villous processes of the chorion insinuate themselves into the hyper- 
trophied follicles of the decidua serotina. The villi then undergo a rapid 
increase in size and vascularity, becoming branched into secondary and 
tertiary ramifications; while, at the same time, corresponding changes are 
taking place in the follicles, by which they become greatly increased in size 
and vascularity, and at the same time forming diverticula in which are im- 
bedded the ramifications of the villi. The villi and follicles thus grow 
simultaneously, and finally become blended with each other and are no 
longer separate structures. The follicular bloodvessels first form capillary 
plexuses; these vessels, however, become enlarged, forming frequent anasto- 
moses, and finally coalescing to form venous sinuses (Fig. 229), in which are 
bathed the foetal villi. There is no continuity established between the 
maternal and foetal blood; the interchange of nutritive material necessary 
for the growth and development of the foetus takes place through the delicate 
walls of the villi.] 
For further account of the various changes by which these events are 
brought about, as well as of the history of the embryo itself, we must refer 
the reader to anatomical treatises. 
CHAPTER IV. 
THE NUTRITION OF THE EMBRYO. 
§ 789. During the development of the chick within the hen's egg the 
nutritive material needed for the growth first of the blastoderm, and subse- 
quently of the embryo, is supplied by the yolk, while the oxygen of the air, 
passing freely through the porous shell, gains access to all the tissues both of 
the embryo and the yolk, either directly or by the intervention of the allan- 
toic vessels. The mammalian embryo, during the period which precedes the 
extension of the allantoic vessels into the cavities of the uterine wall to 
form the placenta, must be nourished by direct diffusion, first from the con- 
tents of the Fallopian tube and subsequently from the decidua; and its 
supply of oxygen must come from the same sources. All analogy would 
lead us to suppose that, from the very first, oxidation is going on in the 
blastodermic and embryonic structures; but the amount of oxygen actually 
withdrawn from without is probably exceedingly small in the early stages, 
seeing that nearly the whole energy of the metabolism going on is directed 
to the building up of structures, the expenditure of energy in the form of 
either heat or external work being extremely small. The marked increase 
of bulk which takes place during the conversion of the mulberry mass into 
the blastodermic vesicle shows that at this epoch a, relatively speaking, large 
quantity of water at least, and probably of nutritive matter, must pass from 
without into the ovum ; and subsequently, though the blastoderm and em- 848 
THE NUTRITION OF THE EMBRYO. 
bryo may for some time draw the material for their continued construction 
at first hand from the yolk-sac or umbilical vesicle, both this and they con- 
tinue probably until the allantois is formed to receive fresh material from 
the mother by direct diffusion. 
§ 790. As the thin-walled allantoic vessels come into closer and fuller 
connection with the maternal uterine sinuses, until at last in the fully 
formed placenta the former are freely bathed in the blood streaming 
through the latter, the nutrition of the embryo becomes more and more 
confined to this special channel. The blood of the foetus flowing along 
the umbilical arteries effects exchanges with the venous blood of the 
mother, and leaves the placenta by the umbilical vein richer in oxygen 
and nutritive material and poorer in carbonic acid and excretory prod- 
ucts than when it issued from the foetus. 
As far as the gain of oxygen and the loss of carbonic acid are concerned 
these are the results of simple diffusion. Venous blood, as we have already 
seen, always contains a quantity of oxy-hsemoglobin, and the quantity of this 
substance present in the blood of the uterine veins is sufficient to supply all 
the oxygen that the embryo needs ; the blood of the foetus, containing less 
oxygen than even the venous blood of the mother, will take up a certain 
though small quantity. The foetal blood travelling in the umbilical artery 
must, in proportion to the extent of the nutritive changes going on in the 
embryo, possess a higher carbonic tension than that in the umbilical vein or 
uterine sinus; and by diffusion gets rid of this surplus during its stay in 
the placenta. The blood in the umbilical arteries and veins is, therefore, 
relatively speaking, venous and arterial respectively, though the small ex- 
cess of oxy-haemoglobin in the blood of the umbilical vein is insufficient to 
give it a distinctly arterial color, or to distinguish it as sharply from the 
more venous blood of the umbilical artery, as in ordinary arterial from or- 
dinary venous blood. Thus, the foetus breathes by means of the maternal 
blood, in the same way that a fish breathes by means of the water in which 
it dwells. 
The blood of the foetus is very poor in haemoglobin, corresponding to its 
low oxygen consumption. When the mother is asphyxiated the foetus is 
asphyxiated too, the oxygen of the latter passing back again into the blood 
of the former; and the asphyxia thus produced in the foetus is much more 
rapid than that which results when the oxygen is used up by the tissues of 
the foetus alone, as when the umbilicus is ligatured and the foetus not allowed 
to breathe. 
If oxygen and carbonic acid thus pass by diffusion to and from the 
mother and the foetus, one might fairly expect that diffusible salts, proteids, 
and carbohydrates would be conveyed to the latter, and diffusible excretions 
carried away to the former, in the same way ; and if fats can pass directly 
into the portal blood during ordinary digestion, there can be no reason for 
doubting that this class of food-stuffs also would find its way to the foetus 
through the placental structures. We do know from experiment that dif- 
fusible substances will pass both from the mother to the foetus, and from 
the foetus to the mother; but we have no definite knowledge as to the exact 
form and manner in which, during normal intra-uterine life, nutritive mate- 
rials are conveyed to or excretions conveyed from the growing young. The 
placenta is remarkable for the great development of cellular structures, 
apparently of an epithelial nature, on the border-land between maternal and 
foetal elements; and it has been suggested that these form a temporary di- 
gestive and secretory (excretory) organ. But we have no exact knowledge 
of what actually does take place in these structures. From the cotyledons 
of ruminants may be obtained a white creamy-looking fluid, which from THE NUTRITION OF THE EMBRYO. 
849 
many features of its chemical composition might almost be spoken of as a 
" uterine milk.'' 
Speaking broadly, the foetus lives on the blood of its mother, very much 
in the same way as all the tissues of any animal live on the blood of the body 
of which they are the parts. 
§791. For a long time all the embryonic tissues are " protoplasmic" in 
character; that is, the gradually differentiating elements of the several tis- 
sues remain still imbedded, so to speak, in undifferentiated protoplasm ; 
and during this period there must be a general similarity in the metabolism 
going on in various parts of the body. As differentiation becomes more and 
more marked, it obviously would be an economical advantage for partially 
elaborated material to be stored up in various foetal tissues, so as to be ready 
for immediate use when a demand arose for it, rather than for a special call 
to be made at each occasion upon the mother for comparatively raw mate- 
rial needing subsequent preparatory changes. Accordingly, we find the 
tissues of the foetus at a very early period loaded with glycogen. The 
muscles are especially rich in this substance, but it occurs in other tissues 
as well. The abundance of it in the former may be explained partly by the 
fact that they form a very large proportion of the total mass of the foetal 
body, and partly by the fact that, while during the presence of the gly- 
cogen they contain much undifferentiated protoplasm, they are exactly the 
organs which will ultimately undergo a large amount of differentiation, 
and, therefore, need a large amount of material for the metabolism which 
the differentiation entails. It is not until the later stages of intra-uterine 
life, at about the fifth month, when it is largely disappearing from the 
muscles, that the glycogen begins to be deposited in the liver. By this 
time histological differentiation has advanced largely, and the use of the 
glycogen to the economy has become that to which it is put in the ordinary 
life of the animal; hence we find it deposited in the usual place. Besides 
being present in the foetal, glycogen is found also in the placental, structures ; 
but here probably it is of use, not for the foetus, but for the nutrition and 
growth of the placental structures themselves. We do not know how much 
carbohydrate material finds it way into the umbilical vein ; and we cannot, 
therefore, state what is the source of the foetal glycogen ; but it is at least 
possible, not to say probable, that it arises, in part at all events, from a 
splitting up of proteid material. 
§ 792. Concerning the rise and development of the functional activities 
of the embryo our knowledge is almost a blank. We know scarcely any- 
thing about the various steps by which the primary fundamental qualities 
of the protoplasm of the ovum are differentiated into the complex phenom- 
ena which we have attempted in this book to expound. We can hardly 
state more than that while muscular contractility becomes early developed, 
and the heart probably, as in the chick, beats even before the blood-corpus- 
cles are formed, movements of the foetus do not, in the human subject, 
become pronounced until after the fifth month ; from that time forward they 
increase and subsequently become very marked. They are often spoken of 
as reflex in character; but only a preconceived bias would prevent them 
from being regarded as largely automatic. The digestive functions are 
naturally, in the absence of all food from the alimentary canal, in abeyance. 
Though pepsin may be found in the gastric membrane at about the fourth 
month, it is doubtful whether a truly peptic gastric juice is secreted during 
intra-uterine life ; trypsin appears in the pancreas somewhat later, but an 
amylolytic ferment cannot be obtained from that organ till after birth. The 
date, however, at which these several ferments make their appearance in the 
embryo appears to differ in different animals. The excretory functions of 
54 850 
THE NUTRITION OF THE EMBRYO. 
the liver are developed early, and about the third month bile pigment and 
bile salts find their way into the intestines. The quantity of bile secreted 
during intra-uterine life accumulates in the intestine, and especially in the 
rectum, forming, together with the smaller secretion of the rest of the canal, 
and some desquamated epithelium, the so-called meconium. Bile salts, both 
unaltered and variously changed, the usual bile pigments, and cholesterin, 
are all present in the meconium. The distinct formation of bile is an indi- 
cation that the products of foetal metabolism are no longer wholly carried 
off by the maternal circulation ; and to the excretory function of the liver 
there are now added those of the skin and kidney. The substances escaping 
by these organs find their way into the allantois or into the amnion, accord- 
ing to the arrangement of the foetal mem- 
branes in different classes of animals; in 
both these fluids urea or allied bodies have 
been found as well as the ordinary saline 
constituents; the latter may or may not 
have been actually secreted. From the 
allantoic fluid of ruminants the body allan- 
toin has been obtained, and human and 
other amniotic fluids have been found to 
[Fig. 230. 
Diagram of the Foetal Circulation. 1, the umbilical 
cord, consisting of the umbilical vein and two umbilical 
arteries, proceeding from the placenta (2); 3, the umbi- 
lical vein dividing into three branches, two (4, 4) to be 
distributed to the liver, and one (5), the ductus venosus, 
which enters the inferior vena cava (6); 7, the portal 
vein, returning the blood from the intestines, and unit- 
ing with the right hepatic branch ; 8, the right auricle; 
the course of the blood is denoted by the arrow proceed- 
ing from 8 to 9, the left auricle; 10, the left ventricle; 
the blood following the arrow to the arch of the aorta 
(11), to be distributed through the branches given off by 
the arch to the head and upper extremities; the arrows 
12 and 13 represent the return of the blood from the head 
and upper extremities through the jugular and sub- 
clavian veins to the superior vena cava (14), to the right 
auricle (8), and in the course of the arrow through the 
right ventricle (15), to the pulmonary artery (16); 17, the 
ductus arteriosus, which appears to be a proper con- 
tinuation of the pulmonary artery; the offsets at each 
side are the right and left pulmonary arteries cut off. 
The ductus arteriosus joins the descending aorta (18,18), 
which divides into the common iliacs, and these into 
the internal iliacs, which become the umbilical arteries 
(19), and return the blood along the umbilical cord to 
the placenta, and the external iliacs (20), which are con- 
tinued into the lower extremities. The arrows at the 
termination of these vessels mark the return of the 
venous blood by the veins to the inferior vena cava.] 
contain urea. It is maintained by some, however, that the fluid in the 
amnion is secreted by the mother, and that hence the substances present in 
it are of maternal origin. 
§ 793. About the middle of intra-uterine life, when the foetal circula- 
tion [Fig. 230] is in full development, the blood flowing along the umbilical 
vein is carried chiefly by the ductus venosus into the inferior vena cava and 
so into the right auricle. Thence it is directed by the valve of Eustachius 
through the foramen ovale into the left auricle, passing from which into the 
left ventricle it is driven into the aorta. Part of the umbilical blood, how- 
ever, instead of passing directly to the inferior cava, enters by the portal 
vein into the hepatic circulation, from which it returns to the inferior cava THE NUTRITION OF THE EMBRYO. 
851 
by the hepatic veins. The inferior cava also contains blood coming from 
the lower limbs and lower trunk. Hence the blood which passing from the 
right auricle into the left auricle through the foramen ovale is distributed 
by the left ventricle through the aortic arch, though chiefly blood coming 
direct from the placenta, is also blood which on its way from the placenta 
has passed through the liver, and blood derived from the tissues of the lower 
part of the body of the foetus. The blood descending as foetal venous blood 
from the head and limbs by the superior vena cava does not mingle with 
that of the inferior vena cava, but falls into the right ventricle, from which 
it is discharged through the ductus arteriosus (Botalli) into the aorta, below 
the arch, whence it flows partly to the lower trunk and limbs, but chiefly by 
the umbilical arteries to the placenta. A small quantity only of the con- 
tents of the right ventricle finds its way into the lungs. Now the blood 
which conies from the placenta by the umbilical vein direct into the right 
auricle is, as far as the foetus is concerned, arterial blood; and the portion 
of umbilical blood which traverses the liver probably loses at this epoch 
very little oxygen during its transit through that gland, the liver being at 
this period a simple excretory rather than an actively metabolic organ. 
Hence the blood of the inferior vena cava, though mixed, is on the whole 
arterial blood ; and it is this blood which is sent by the left ventricle through 
the arch of the aorta into the carotid and subclavian arteries. Thus the 
head of the foetus is provided with blood comparatively rich in oxygen. 
The blood descending from the head and upper limbs by the superior vena 
cava is distinctly venous; and this passing from the right ventricle by the 
ductus arteriosus is driven along the descending aorta, and together with 
some of the blood passing from the left ventricle around the aortic arch falls 
into the umbilical arteries and so reaches the placenta. The foetal circula- 
tion then is so arranged that while the most distinctly venous blood is driven 
by the right ventricle back to the placenta to be oxygenated, the most dis- 
tinctly arterial (but still mixed) blood is driven by the left ventricle to the 
cerebral structures, which have more need of oxygen than have the other 
tissues. Contrary to what takes place afterward, the work of the right ven- 
tricle is in the foetus greater than that of the left; and, accordingly, that 
greater thickness of the left ventricular walls, so characteristic of the adult, 
does not become marked until close upon birth. 
In the later stages of pregnancy the mixture of the various kinds of 
blood in the right auricle increases preparatory to the changes taking place 
at birth. But during the whole time of intra-uterine life the- amount of 
oxygen in the blood passing from the aortic arch to the medulla oblongata 
is sufficient to prevent any inspiratory impulses being originated in the 
medullary respiratory centre. This, during the whole period elapsing be- 
tween the date of its structural establishment, or rather the consequent full 
development of its irritability, and the epoch of birth, remains dormant ; 
the oxygen-supply to the protoplasm of its nerve-cells is never brought so 
low as to set going the respiratory molecular explosions. As soon, however, 
as the intercourse between the maternal and umbilical blood is interrupted 
by separation of the placenta or by ligature of the umbilical cord, or when, 
as by the death of the mother, the umbilical blood ceases to be replenished 
with oxygen by the maternal blood, or when in any other way blood of 
sufficiently arterial quality ceases to find its way by the left ventricle to the 
medulla oblongata, the supply of oxygen in the respiratory centre sinks, 
and when the fall has reached a certain point an impulse of inspiration is 
generated and the foetus for the first time breathes. This action of the 
respiratory centre may be assisted by adjuvant impulses reaching the centre 
along various afferent nerves, such as those started by exposure of the body 852 
THE NUTRITION OF THE EMBRYO. 
to the air, or to cold; but these are subordinate, not essential. A retarded 
first breath may be hurried on by dashing water on the face of the new- 
born infant; but, on the other hand, the foetus, upon the cessation of the 
placental circulation, will make its first respiratory movements while it is 
still invested with the intact membranes and thus sheltered from the air and 
indeed from all external stimuli. 
§ 794. Before this first breath is taken the pulmonary alveoli contain 
no air, and the lungs when thrown into water sink at once ; they are then 
said to be "atelectatic." After the first breath, the alveoli contain air and 
the lungs float when thrown into water. A striking difference, however, 
exists between the lungs of a newborn infant and those of an older person. 
When the pleural cavity of the former is opened, the lungs do not collapse, 
no air is driven out by the trachea ; that partial distention of the lungs, and 
negative thoracic pressure, appears not to be established immediately upon 
birth. That portion of the residual air in the lungs of the adult which, 
remaining after the most forcible expiration, is still driven from the lungs 
upon the pleural cavity being laid open, and which might be called " col- 
lapse air," is wanting in the newborn infant. When the change from one 
condition to the other is effected is not at present known ; it may possibly 
arise from the growth of the chest outstripping that of the lungs. 
When the first breath is taken, as under normal circumstances it is, with 
free access to the atmosphere, the lungs become filled with air, the scanty 
supply of blood which at the moment was passing from the right ventricle 
along the pulmonary artery returns to the left auricle brighter and richer 
in oxygen than ever was the foetal blood before. With the diminution of 
resistance in the pulmonary circulation caused by the expansion of the 
thorax, a larger supply of blood passes into the pulmonary artery instead 
of into the ductus arteriosus, and this derivation of the contents of the 
right ventricle increasing with the continued respiratory movements, the 
current through the latter canal at last ceases altogether, and its channel 
shortly after birth becomes obliterated. Corresponding to the greater flow 
into the pulmonary artery, a larger and larger quantity of blood returns 
from the pulmonary veins into the left auricle. At the same time the cur- 
rent through the ductus venosus from the umbilical vein having ceased, the 
flow from the inferior cava has diminished ; and the blood of the right 
auricle finding little resistance in the direction of the ventricle, which now 
readily discharges its contents into the pulmonary artery, but finding in the 
left auricle, which is continually being filled from the lungs, an obstacle to 
its passage through the foramen ovale, ceases to take that course. Any re- 
turn of blood from the now vigorous and active left auricle into the right 
auricle is prevented by the valve which, during the later stages of intra- 
uterine life, has been growing up in the left auricle over the foramen ovale. 
At birth the edge of this valve is to a certain extent free, so that, in case of 
an emergency, as when the pulmonary circulation is obstructed, a direct 
escape of blood into the left auricle from the over-burdened right auricle 
can take place. Eventually, in the course of the first year, adhesion takes 
place, and the separation of the two auricles becomes complete. With its 
larger supply of blood and greater work the left ventricle acquires the 
greater thickness characteristic of it during life. Thus the foetal circula- 
tion, in consequence of the respiratory movements to which its interruption 
gives rise, changes its course into that characteristic of the adult. PARTURITION. 
853 
CHAPTER V. 
PARTURITION. 
§ 795. In spite of the increasing distention of its cavity, the uterus 
remains quiescent, as far as any marked muscular contractions are concerned, 
until a certain time has been run. In the human subject the period of 
gestation generally lasts from 275 to 280 days', i. e., about 40 weeks, the 
general custom being to expect parturition at about 280 days from the last 
menstruation. Seeing that in many cases it is uncertain whether the ovum 
which develops into the embryo left the ovary at the menstruation preceding 
or succeeding coitus, or, as some have urged, independent of menstruation, 
by reason of the coitus itself, an exact determination of the duration of 
pregnancy is impossible. 
Tn the cow the period of gestation is about 280 days, in the mare about 350, 
sheep about 150 days, dog about 60 days, rabbit about 30 days. 
§ 796. The extrusion of the foetus is brought about partly by rhythmical 
contractions of the uterus itself and partly by a pressure exerted by the 
contraction of the abdominal muscles, similar to that described in defeca- 
tion. The contractions of the uterus are the first to appear, and their first 
effect is to bring about a dilatation of the os uteri; it is not until the later 
stages of labor, while the foetus is passing into the vagina, that the ab- 
dominal muscles are brought into play. 
§ 797. The whole process of parturition may be broadly considered a 
reflex act, the nervous centre being placed in the lumbar cord. In a dog, 
whose dorsal cord had been completely severed, parturition took place as 
usual; and the fact that, in the human subject, labor will progress quite 
naturally while the patient is unconscious from the administration of chloro- 
form, shows that in woman also the whole matter is an involuntary action, 
however much it may be assisted by direct volitional efforts. That the uterus 
is capable of being thrown into contractions through reflex action, excited 
by stimuli applied to various afferent nerves, is well known. The contraction 
of the uterus, which is so necessary for the prevention of hemorrhage after 
delivery, may frequently be brought about by exerting pressure or by dash- 
ing cold water on the abdomen, by the introduction of foreign bodies into 
the vagina, and especially by putting the child to the nipple. And we learn 
from experiments on animals that rhythmic contractions of the uterus re- 
sembling at least those of parturition, may be brought about in a reflex 
manner by stimulating various afferent nerves. Similar movements may be 
induced by direct stimulation of the spinal cord along its whole length, as 
well as of various parts of the brain ; but there are reasons for thinking that 
in these cases the impulses started in the brain and upper part of the spinal 
cord produce their effects by working upon what may be called a " parturi- 
tion " centre in the upper lumbar regions of the cord. And it would appear 
that the uterine contractions which are induced by such drugs as ergot, as 
well as those caused by asphyxia, are, at all events in part, brought about 
by the agency of the same lumbar centre. From this centre the paths for 
the efferent impulses appear (in the dog) to be two-fold; one along sympa- 
thetic tracts, by nerves passing from the inferior mesenteric ganglion to the 
hypogastric plexus, and the other along spinal tracts by branches of the 
sacral nerves to the same plexus. It is stated that the characters of the 854 
PARTURITION. 
movements induced by stimulating these two tracts are somewhat different, 
and moreover that the sympathetic tract is vaso-constrictor and the spinal 
tract vaso-motor in nature ; but the matter has not yet been fully worked out. 
§ 798. We are, however, hardly justified in considering the rhythmical 
contractions of the uterus during parturition as simple reflex acts excited by 
the presence of the foetus. We are utterly in the dark as to why the uterus, 
after remaining apparently perfectly quiescent (or with contractions so slight 
as to be with difficulty appreciated) for months, is suddenly thrown into 
action, and within, it may be, a few hours or even less get rid of the burden 
it has borne with such tolerance for so long a time; none of the various 
hypotheses which have been put forward can be considered as satisfactory. 
And until we know what starts the active phase, we shall remain in ignor- 
ance of the exact manner in which the activity is brought about. The 
peculiar rhythmic character of the contractions, each " pain " beginning 
feebly, rising to a maximum, then declining, and finally dying away alto- 
gether, to be succeeded after a pause by a similar pain just like itself, pain 
following pain like the tardy long-drawn beats of a slowly beating heart, 
suggests that the cause of the rhythmic contraction is seated, like that of the 
rhythmic beat of the heart, in the organ itself. And this view is supported 
by the fact that contractions of the uterus similar to those of parturition 
have been observed in animals even after complete destruction of the spinal 
cord ; and the movements induced by asphyxia seem in part, and those 
caused by some drugs such as ammonia seem to be wholly, due to an intrin- 
sic action of the uterus itself. Nevertheless, general evidence supports the 
conclusion that, in a normal state of things at all events, the contractions of 
the uterus, like those of the lymph-hearts, are largely dependent on the 
spinal cord. 
The occurrence of contractions in conseqence of an asphyxiated condition 
of the blood explains why, when pregnant animals are asphyxiated, an ex- 
trusion of the foetus frequently takes plaee. There is no evidence, however, 
that the onset of labor is caused by a gradual diminution of oxygen in the 
blood, reaching at last to a climax. Nor are there sufficient facts to connect 
parturition with any condition of the ovary resembling that of menstruation. 
The action of the abdominal muscles in parturition is, on the other hand, 
obviously a reflex act carried out by means of the spinal cord, the necessary 
stimulus being supplied by the pressure of the foetus in the vagina or by the 
contraction of the uterus. Hence the whole act of parturition may with 
reason be considered as a reflex one. 
Whether it be wholly a reflex or partly an automatic one, the act can 
readily be inhibited by the action of the central nervous system. Thus 
emotions are a very frequent cause of the progress of parturition being sud- 
denly stopped ; as is well known, the entrance into the bedroom of a stranger 
often causes for a time the sudden and absolute cessation of " labor " pains, 
which previously may have been even violent. Judging from the analogy 
of micturition, between which and parturition there are many points of 
resemblance, we may suppose that this inhibition of uterine contractions is 
brought about by an inhibition of the centre in the lumbar cord. 
§ 799. After the expulsion of the foetus, the foetal placenta separates 
from the uterine walls, and is, together with the remnants of the membranes, 
expelled after it. The uterus then falls into a firm tonic contraction similar 
to that of the emptied bladder, by which means hemorrhage from the vessels 
torn by the separation of the placenta is avoided. The lining membrane of 
the uterus is gradually restored, the muscular elements are reduced by a 
rapid fatty degeneration- and in a short time the whole organ has returned 
to its normal condition. THE PHASES OF LIFE. 
855 
CHAPTER VI. 
THE PHASES OF LIFE. 
§ 800. The child has at birth, on an average, rather less than one-third 
the maximum length, and about one-twentieth the maximum weight, to 
which in future years it will attain. 
The composition of the body of the newborn babe, as compared with that 
of the adult, will be seen from the following table, in which the details are 
more full than those given on p. 482. 
Weight of organ in percentage 
of body-weight. 
Weight of organ in 
adult, as compared 
with that of newborn 
babe taken as 1. 
Newborn babe. 
Adult. 
Eye 
. . . 0.28 
0.028 
1.7 
Brain 
. . . 14.34 
2.37 
3.7 
Kidneys.... 
. . . 0.88 
0.48 
12 
Skin 
. . . 11.3 
6.3 
12 
Liver 
. . . 4.39 
2.77 
• 13.6 
Heart 
. . . 0.89 
0.52 
15 
Stomach and intestine 2.53 
2.34 
20 
Lungs 
. . . 2.16 
2.01 
20 
Skeleton.... 
. . . 16.7 
15.35 
26 
Muscles, etc . . 
. . . 23.4 
43.1 
28 
Testicle .... 
. . . 0.037 
0.8 
60 
It will be observed that the brain and eyes are, relatively to the whole 
body-weight, very much larger in the babe than in the adult, as is also, 
though to a less extent, the liver. This disproportion is a very marked em- 
bryonic feature, and, as far as the brain and eye are concerned at least, has 
a morphological or phylogenic, as well as a physiological or teleological, sig- 
nificance. Inasmuch as the smaller body has relatively the larger surface, 
the skin is naturally proportionately greater in the babe. It is chiefly by 
the accumulation of muscle or flesh, properly so called, that the child ac- 
quires the bulk and weight of man, the skeletal framework, in spite of its 
being specifically lighter in its earlier cartilaginous condition, maintaining 
throughout life about the same relative weight. 
§ 801. The increase in stature is very rapid in early infancy, proceed- 
ing, however, by decreasing increments. During or shortly before puberty, 
there is again a somewhat sudden rise, with a subsequent more steady but 
diminishing increase up to about the twenty-fifth year. From thence to 
about fifty years of age the height remains stationary, after which there may 
be a decrease, especially in extreme old age. 
§ 802. The increase in weight is also very rapid at first, and proceeding, 
like the height, with diminishing increments, may continue till about the 
fortieth year. After the sixtieth year a decline of variable extent is gen- 
erally witnessed. It is a remaj'kable fact, however, that in the first few days 
of life, so far from there being an increase, there is an actual decrease of 
weight, so that, even on the seventh day the weight still continues to be less 
than at birth. 
§ 803. The saliva of the babe is active on starch, and its gastric juice, 
unlike that of many newborn animals, has good peptic powers, from which 
we may infer that its digestive processes in general are identical with those 
of the adult; but the feces of the infant contain, besides considerable quan- 856 
THE PHASES OE LIFE. 
tity of undigested food (fat, casein, etc.), unaltered bile-pigment, and unde- 
composed bile-salts. 
£ 804. The heart of the babe (see Table, p. 855) is, relatively to its 
body-weight, larger than the adult, and the frequency of the heart-beat 
much greater, viz., about 130 or 140 per minute, falling to about 110 in the 
second year, and about 90 in the tenth year. Corresponding to the smaller 
bulk of the body, the whole circuit of the blood system is traversed in a 
shorter time than in the adult (12 seconds as against 22) ; and, consequently, 
the renewal of the blood in the tissues is exceedingly rapid. The respiration 
of the babe is quicker than that of the adult, being at first about 35 per 
minute, falling to 28 in the second year, to 26 in the fifth year, and so on- 
ward. The respiratory work, while it increases absolutely as the body 
grows, is, relatively to the body-weight, greatest in the earlier years. It 
is worthy of notice that the absorption of oxygen is said to be relatively 
more active than the production of carbonic acid ; that is to say, there 
is a continued accumulation of capital in the form of a store of oxygen- 
holding explosive compounds. This, indeed, is the striking feature of 
infant metabolism. It is a metabolism directed largely to constructive ends. 
The food taken represents, undoubtedly, so much potential energy; but 
before that energy can assume a vital mode, the food must be converted into 
tissue; and, in such a conversion, morphological and molecular, a large 
amount of energy must be expended. The metabolic activities of the infant 
are more pronounced than those of the adult, for the sake, not so much of 
energies which are spent on the world without, as of energies which are for 
a while buried in the rapidly increasing mass of flesh. Thus, the infant re- 
quires, over and above the wants of the man, not only an income of energy 
corresponding to the energy of the flesh actually laid on, but also an income 
corresponding to the energy used up in making that living sculptured flesh 
out of the dead amorphous proteids, fats, carbohydrates, and salts, which 
serve as food. Over and above this, the infant needs a more rapid metab- 
olism to keep up the normal bodily temperature. This, which is no less, 
indeed, slightly (0.3°) higher, than that of the adult, requires a greater ex- 
penditure, inasmuch as the infant with its relatively far larger surface, and 
its extremely vascular skin, loses heat to a proportionately much greater 
degree than does the grown-up man. It is a matter of common experience 
that children are more affected by cold than are adults. 
This rapid metabolism is, however, not manifest immediately upon birth. 
During the first few days, corresponding to the loss of weight mentioned 
above, the respiratory activities of the tissues are feeble ; the embryonic 
habits seem as yet not to have been completely thrown off, and as was stated 
on p. 305, newborn animals bear with impunity a deprivation of oxygen 
which would be fatal to them later on in life. 
§ 805. The quantity of urine passed, though scanty in the first two days, 
rises rapidly at the end of the first week, and in youth the quantity of urine 
passed is, relatively to the body-weight, larger than in adult life. This may 
be, at least in quite early life, partly due to the more liquid nature of the 
food, but is also in part the result of the more active metabolism. For not 
only is the quantity of urine passed, but also the amount of urea and some 
other urinary constituents secreted, relatively to the body-weight, greater in 
the child than in the adult. The presence of uric, of oxalic, and, according 
to some, of hippuric acids in unusual quantities is a frequent characteristic 
of the urine of children. It is stated that calcic phosphates, and indeed 
the phosphates generally, are deficient, being retained in the body for the 
building up of the osseous skeleton. 
§ 806. Associated probably with these constructive labors of the growing THE PHASES OF LIFE. 
857 
frame is the prominence of the lymphatic system. Not only are the lym- 
phatic glands largely developed and more active (as is probably shown by 
their tendency to disease in youth), but the quantity of lymph circulation 
is greater than in later years. Characteristic of youth is the size of the 
thymus body, which increases up to the second year, and may then remain 
for a while stationary, but generally before puberty, has suffered a retro- 
gressive metamorphosis, and frequently hardly a vestige of it remains behind. 
The thyroid body is also relatively greater in the babe than in the adult; 
the spleen, on the other hand, which grows rapidly in early infancy, is not 
only absolutely, but also relatively, greater in the adult. It need hardly 
be said that the recuperative power of infancy and early youth is very 
marked. 
§ 807. It would be beyond the scope of this work to enter into the 
psychical condition of the babe or the child, and our knowledge of the de- 
tails of the working of the nervous system in infancy is too meagre to per- 
mit of any profitable discussion. It is hardly of use to say that in the 
young the whole nervous system is more irritable or more excitable than in 
later years; by which we probably to a great extent mean that it is less 
rigid, less. marked out into what, in preceding portions of this work, we 
have spoken of as nervous mechanisms. It may be mentioned that stimula- 
tion of the various cerebral areas, in newborn animals, does not give rise to 
the usual localized movements. The sense of touch, both as regards pres- 
sure and temperature, appears well developed in the infant, as does also the 
sense of taste, and, possibly, though this is disputed, that of smell. The 
pupil (larger in the infant than in the man) acts fully, and Bonders observed 
normal binocular movements of the eyes in an infant less than an hour old. 
The eye is (in man) from the outset fully sensitive to light, though of course 
visual perceptions are imperfect. As regards hearing, on the other hand, 
very little reaction follows upon sounds-i. e., auditory sensations seem to be 
dull during the first few days of life ; this may be partly, at least, due to 
absence of air from the tympanum and a tumid condition of the tympanic 
mucous membrane. As the child grows up his senses rapidly culminate, and 
in his early years he possesses a general acuteness of sight, hearing, and touch 
which frequently becomes blunted as his psychical life becomes fuller. 
Children, however, are said to be less apt at distinguishing colors than in 
sighting objects; but it does not appear whether this arises from a want of 
perceptive discrimination or from their being actually less sensitive to vari- 
ations in hue. A characteristic of the nervous system in childhood, the 
result, probably, of the more active metabolism of the body, is the necessity 
for long or frequent and deep slumber. 
§ 808. Dentition marks the first epoch of the new life. At about seven 
months the two central incisors of the lower jaw make their way through 
the gum, followed immediately by the corresponding teeth in the upper jaw. 
The lateral incisors, first of the lower and then of the upper jaw, appear at 
about the ninth month, the first molars at about the twelfth month, the 
canines at about a year and a half, and the temporary dentition is completed 
by the appearance of the second molars usually before the end of the second 
year. 
§ 809. About the sixth year the permanent dentition commences by the 
appearance of the first permanent molar beyond the second temporary 
molar ; in the seventh year the central permanent incisors replace their tem- 
porary representatives, followed in the next year by the lateral incisors. In 
the ninth year the temporary first molars are replaced by the first bicuspids, 
and in the tenth year the second temporary molars are similarly replaced 
by the second bicuspids. The canines are exchanged about the eleventh or 858 
THE PHASES OF LIFE. 
twelfth year, and the second permanent molars are cut about the twelfth or 
thirteenth year. There is then a long pause, the third or wisdom tooth not 
making its appearance till the seventeenth, or even twenty-fifth year, or in 
some cases not appearing at all. 
§ 810. Shortly after the conclusion of the permanent dentition (the 
wisdom teeth excepted) the occurrence of puberty marks the beginning of a 
new phase of life; and the difference between the sexes, hitherto merely 
potential, now becomes functional. In both sexes the maturation of the 
generative organs is accompanied by the well-known changes in the body at 
large; but the events are much more characteristic in the typical female 
than in the aberrant male. Though in the boy, the breaking of the voice 
and the rapid growth of the beard which accompany the appearance of act- 
ive spermatozoa, are striking features, yet they are, after all, superficial. 
The curves of his increasing weight and height, and of the other events of 
his economy, pursue for a while longer an unchanged course; the boy does 
not become a man till some years after puberty; and the decline of his 
functional manhood is so gradual that frequently it ceases only when disease 
puts an end to a ripe old age. With the occurrence of menstruation, on the 
other hand, at from thirteen to seventeen years of age, the girl almost at 
once becomes a woman, and her functional womanhood ceases suddenly at 
the climacteric in the fifth decennium. During the whole of the childbear- 
ing period her organism is in a comparatively stationary condition. While 
before the age of puberty, up to about the eleventh or twelfth year, the girl 
is lighter and shorter than the boy of the same age, in the next few years 
her rate of growth exceeds his; but she has then nearly reached her maxi- 
mum, while he continues to grow. Her curve of weight from the nineteenth 
year onward to the climacteric remains stationary, being followed subse- 
quently by a late increase, so that while the man reaches his maximum of 
weight at about forty, the woman is at her greatest weight about fifty. 
§ 811. Of the statical differences of sex, some, such as the formation 
of the pelvis and the costal mechanism of respiration, are directly connected 
with the act of childbearing, while others have only an indirect relation to 
that duty ; and indications, at least, of nearly all the characteristic differ- 
ences are seen at birth. The baby boy is heavier and taller than the baby 
girl, and the maiden of five breathes with her ribs in the same way as does 
the matron of forty. The woman is lighter and shorter than the man, the 
limits in the case of the former being from 1.444 to 1.740 metres of height, 
and from 39.8 to 93.8 kilos of weight, in the latter from 1.467 to 1.890 of 
height, and from 49.1 to 98.5 kilos of weight. The muscular system and 
skeleton are both absolutely and relatively less in woman, and her brain is 
lighter and smaller than that of man, being about 1272 grammes to 1424. 
Her metabolism, as measured by the respiratory and urinary excreta, is also 
not only absolutely but relatively to the body-weight less, and her blood is 
not only less in quantity, but also of lighter specific gravity, and contains a 
smaller proportion of red corpuscles. Her strength is to that of man as 
about 5 to 9, and the relative length of her step as 1000 to 1157. 
§ 812. From birth onward (and indeed from early intra-uterine life) the 
increment of growth progressively diminishes. At last a point is reached at 
which the curve cuts the abscissa line, and the increment becomes a decre- 
ment. After the culmination of manhood at forty and of womanhood at the 
climacteric, the prime of life declines into old age. The metabolic activity 
of the body, which at first was sufficient not only to cover the daily waste, 
but to add new material, later on is able only to meet the daily wants, and 
at last is too imperfect even to sustain in its entirety the existing frame. 
Neither as regards vigor and functional capacity, nor as regards weight and 859 
THE PHASES OF LIFE. 
bulk, do the turning-points of the several tissues and organs coincide either 
with each other or with that of the body at large. We have already seen 
that the life of such an organ as the thymus is far shorter than that of its 
possessor. The eye is in its dioptric prime in childhood, when its media are 
clearest and its muscular mechanisms most mobile, and then it for the most 
part serves as a toy ; in later years, when it could be of the greatest service 
to a still active brain, it has already fallen into a clouded and rigid old age. 
The skeleton reaches its limit very nearly at the same time as the whole 
frame reaches its maximum of height, the coalescence of the various epiphyses 
being pretty well completed by about the twenty-fifth year. Similarly the 
muscular system in its increase tallies with the weight of the whole body. 
The brain, in spite of the increasing complexity of structure and function to 
which it continues to attain even in middle life, early reaches its limit of 
bulk and weight. At about seven years of age it attains what may be con- 
sidered as its first limit, for though it may increase somewhat up to twenty, 
thirty, or even later years, its progress is much more slow after than before 
seven. The vascular and digestive organs as a whole may continue to 
increase even to a very late period. From these facts it is obvious that 
though the phenomena of old age are, at bottom, the result of the individual 
decline of the several tissues, they owe many of their features to the dis- 
arrangement of the whole organism produced by the premature decay or dis- 
appearance of one or other of the constituent bodily factors. Thus, for 
instance, it is clear that were there no natural intrinsic limit to the life of 
the muscular and nervous systems, they would nevertheless come to an end 
in consequence of the nutritive disturbances caused by the loss of the teeth. 
And what is true of the teeth is probably true of many other organs, with 
the addition that these cannot, like the teeth, be replaced by mechanical 
contrivances. Thus the term of life which is allotted to a muscle by virtue 
of its molecular constitution, and which it could not exceed were it always 
placed under the most favorable nutritive conditions, is, in the organism, 
determined by the similar life-terms of other tissues; the future decline of 
the brain is probably involved in the early decay of the thymus. 
§ 813. Two changes characteristic of old age are the so-called calcare- 
ous and fatty degenerations. These are seen in a completely typical form 
in cartilage, as for instance, in the ribs; here the protoplasm of the carti- 
lage-corpuscle becomes hardly more than an envelope of fat-globules, and 
the supple matrix is rendered rigid with amorphous deposits of calcic phos- 
phates and carbonates, which are at the same time the signs of past and the 
cause of future nutritive decline. And what is obvious in the case of carti- 
lage is more or less evident in other tissues. Everywhere we see a disposi- 
tion on the part of protoplasm to fall back upon the easier task of forming 
fat rather than to carry on the more arduous duty of manufacturing new 
material like itself; everywhere almost we see a tendency to the replacement 
of a structured matrix by a deposit of amorphous material. In no part of 
the system is this more evident than in the arteries; one common feature of 
old age is the conversion by such a change of the supple elastic tubes into 
rigid channels, whereby the supply to the various tissues of nutritive material 
is rendered increasingly more difficult, and their intrinsic decay proportion- 
ately hurried. 
S 814. Of the various tissues of the body the muscular and nervous are, 
however, those in which the functional decline, if not structural decay, be- 
comes soonest apparent. The dynamic coefficient of the skeletal muscles 
diminishes rapidly after thirty or forty years of life, and a similar want of 
power comes over the plain muscular fibres also; the heart, though it may 
not diminish, or even may still increase in weight, possesses less and less 860 
THE PHASES OF LIFE. 
force, and the movements of the intestine, bladder, and other organs, dimin- 
ish in vigor. In the nervous system, the lines of resistance, which, as we 
have seen, help to map out the central organs into mechanisms, and so to 
produce its multifarious actions, become at last hindrances to the passage of 
nervous impulses in any direction, while at the same time the molecular 
energy of the impulses themselves becomes less. The eye becomes feeble, 
not only from cloudiness of the media and presbyopic muscular inability, 
but also from the very bluntness of the retina ; the sensory and motor im- 
pulses pass with increasing slowness to and from the central nervous system, 
and the brain becomes a more and more rigid mass of protoplasm, the molec- 
ular lines of which rather mark the history of past actions than serve as 
indications of present potency. The epithelial glandular elements seem to 
be those whose powers are the longest preserved ; and hence the man who in 
the prime of his manhood was a " martyr to dyspepsia " by reason of the 
sensitiveness of gastric nerves and the reflex inhibitory and other results of 
their irritation, in his later years, when his nerves are blunted, and when, 
therefore, his peptic cells are able to pursue their chemical work undisturbed 
by extrinsic nervous worries, eats and drinks with the courage and success 
of a boy. 
§ 815. Within the range of a lifetime are comprised many periods of a 
more or less frequent recurrence. In spite of the aids of a complex civili- 
zation, all tending to render the conditions of his life more and more equable, 
man still shows in his economy the effects of the seasons. Some of these are 
the direct results of varying temperature, but some probably, such as the 
gain of weight in winter and the loss in summer, are habits acquired by 
descent. Within the year, an approximately monthly period is manifested 
in the female by menstruation, though there is no exact evidence of even a 
latent similar cycle in the male. The phenomena of recurrent diseases, and 
the marked critical days of many other maladies, may be regarded as point- 
ing to cycles of smaller duration than that of the moon's revolution, unless 
we admit the view urged by some authors that in these cases the recurrence 
is to be attributed rather to periodical phases in the disease-producing germ 
itself, than to variations in the medium of the disease. 
§ 816. Prominent among all other cyclical events is the fact that most 
animals possessing a well-developed nervous system, must, night after night, 
or day after day, or at least time after time, lay them down to sleep. The 
salient feature of sleep is the cessation of the automatic activity of the brain ; 
it is the diastole of the cerebral beat. But the condition is not confined to 
the cerebral hemispheres; all parts of the body either directly or indirectly 
take share in it. The phenomena of sleep are perhaps seen in their simplest 
form in the winter sleep of hibernation, to which especially cold-blooded 
animals, but also to some extent warm-blooded animals, are subject. In 
these cases the cold of winter slackens the vibrations and lessens the explo- 
sions of the protoplasm, not only of nervous but also of muscular and glandu- 
lar structures ; indeed the activity of the whole body is lowered, in some 
respects almost to actual arrest. At the same time that the labor of the cere- 
bral molecules becomes insufficient to develop consciousness, the respiratory 
centre is either wholly quiescent or discharges feeble impulses at rare inter- 
vals, and the heart beats with a slow', infrequent stroke, not by reason of any 
inhibitory restraint, but because its very substance in its slow molecular 
travail can gather head for explosions only after long pauses of rest. And 
such few and distant beats as do occur are amply sufficient to meet the needs 
of the feeble metabolism of the several tissues. The sleep of every day 
differs from the sleep of winter cold chiefly because the slackening of molec- 
ular activities is due in the former not to extrinsic but to intrinsic causes, 861 
THE PHASES OF LIFE. 
not to changes in the medium, but to exhaustion of the subject, and because 
the phenomena are largely confined to the cerebral hemispheres. It is true 
that the whole body shares in the condition. The pulse and breathing are 
slower, the intestine and other internal muscular mechanisms are more or less 
at rest, the secreting organs are less active, some apparently being wholly 
quiescent, and the sleeper on waking rubs his eyes to bring back to his con- 
junctiva its needed moisture. Indeed the whole metabolism and the depend- 
ent temperature of the body are lowered ; but we cannot say at present how 
far these are the indirect results of the condition of the nervous system, or 
how far they indicate a partial slumbering of the several tissues. 
§ 817. Thoracic respiration is said to become more prominent than dia- 
phragmatic respiration during sleep, and the Cheyne-Stokes rhythm of 
respiration (see p. 385) is frequently observed. During sleep the pupil is 
contracted, during deep sleep exceedingly so; and dilatation, often unaccom- 
panied by any visible movements of the limbs or body, takes place when 
any sensitive surface is stimulated ; on awaking also the pupils dilate. The 
eyeballs have been generally described as being during sleep directed upward 
and converging or, according to some authors, diverging; but others main- 
tain that in true sleep the visual axes are parallel and directed to the far 
distance. The eyes of children have been described as continally executing 
during sleep movements, often irregular and unsymmetrical and unaccom- 
panied by changes in the pupils. 
§ 818. We are not at present in a position to trace out the events which 
culminate in this inactivity of the cerebral structures. It has been urged 
that during sleep the brain is anaemic; but even if this anaemia is a constant 
accompaniment of sleep, it must, like the vascular condition of a gland or 
any other active organ, be regarded as an effect, or at least as a subsidiary 
event, rather than as a primary cause. Nor can the view which regards 
sleep as the result of a shifting of the mechanical arrangements of the cranial 
circulation be considered as satisfactory. The explanation of the condition 
is rather to be sought in purely molecular changes; and the analogy between 
the systole and the diastole of the heart, and the waking and sleeping of the 
brain, may be profitably pushed to a very considerable extent. The sleep- 
ing brain in many respects closely resembles a quiescent but still living ven- 
tricle. Both are, as far as outward manifestations are concerned, at rest, but 
both may be awakened to activity by an adequately powerful stimulus. 
Both, though quiescent, are irritable, in both the quiescence will ultimately 
give place to activity, and in both an appropriate stimulus applied at the 
right time will determine the change from rest to action. Just as a single 
prick will under certain circumstances awaken a ventricle, which for some 
seconds has been motionless, into a rhythmic activity of many beats, so a 
loud noise will start a man from sleep into a long day's wakefulness. And 
just as in the heart the cardiac irritability is lowest at the beginning of the 
diastole and increases onward till a beat bursts out, so is sleep deepest at its 
commencement after the day's labor ; thence onward slighter and slighter 
stimuli are needed to wake the sleeper. For, judging of the depth of ordi- 
nary nocturnal sleep by the intensity of the noise required to wake the 
sleeper, it may be concluded that, increasing very rapidly at first, it reaches 
its maximum within the first hour ; from thence it diminishes, at first rapidly, 
but afterwards more slowly. 
§ 819. We cannot, however, at present make any definite statements 
concerning the nature of the molecular changes which determine this rhythmic 
rise and fall of cerebral irritability. The fact that the products of proto- 
plasmic activity when they accumulate within the protoplasm appear to 
become in the end an obstruction to that activity, has suggested the idea that 862 
THE PHASES OF LIFE. 
the presence in the cerebral tissue of an excess of the products of nervous 
metabolism is the cause of sleep. Indeed lactic acid, the increase of which 
was supposed to be the cause of the acid reaction of muscular and nervous 
tissues after exercise, has been especially pointed to in this connection ; but, 
as we have seen, the acid reaction in question appears not to be due to any 
increased production of lactic acid. Besides, if the accumulation of meta- 
bolic products of any kind were the cause of sleep, it is not clear why we 
should ever have any hope of waking. More may be said in favor of the 
conception that during the waking hours the expenditure of oxygen exceeds 
the income, and that the quiescence, which we call sleep, conies from the 
exhaustion of the body's store of oxygen, more especially of that " intra- 
molecular" oxygen of which we spoke in dealing with the respiration of the 
tissues. But to this view must be added some hypothesis, such as the byplay 
of some inhibitory mechanism, whereby the respiratory centre is not roused 
to increased activity by this lack of oxygen, for, as we have seen, the breath- 
ing shares in the slumber of the body, though continuing to play with an 
amount of energy which permits a gradual restoration of the lost store of 
oxygen and so finally brings on the awakening which ends the sleep. And 
the necessity for such a complication indicates that the explanation is, at 
present at least, inadequate. 
The phenomena of sleep show very clearly to how large an extent an 
apparent automatism is the ultimate outcome of the effects of antecedent 
stimulation. When we wish to go to sleep we withdraw our automatic brain 
as much as possible from the influence of all extrinsic stimuli; and an in- 
teresting case is recorded of a lad whose connection with the external world 
was, from a complicated anaesthesia, limited to that afforded by a single eye 
and a single ear, and who could be sent to sleep at will by closing the eye 
and stopping the ear. 
§ 820. The cycle of the day is, however, manifested in many other ways 
than by the alternation of sleeping and waking, with all the indirect effects 
of these two conditions. There is a diurnal curve of temperature (see p. 537), 
apparently independent of all immediate circumstances, the hereditary im- 
press of a long and ancient sequence of days and nights. Even the pulse, so 
sensitive to all bodily changes, shows, running through all the immediate 
effects of the changes of the minute and the hour, the working of a diurnal 
influence which cannot be accounted for by waking and sleeping, by work- 
ing and resting, by meals and abstinence between meals. And the same 
may be said concerning the rhythm of respiration, and the products of pul- 
monary, cutaneous, and urinary excretion. There seems to be a daily curve 
of bodily metabolism, which is not the product of the day's events. Within 
the day we have the narrower rhythm of the respiratory centre with the 
accompanying rise and fall of activity in the vasomotor centres. And 
lastly, there stands out the fundamental fact of all bodily periodicity, that 
alternation of the heart's systole and diastole which ceases only at death. 
Though, as we have seen, the intermittent flow in the arteries is toned down 
in the capillaries to an apparently continuous flow, still the constantly re- 
peated cycle of the cardiac shuttle must leave its mark throughout the 
whole web of the body's life. DEATH. 
863 
CHAPTER VII. 
DEATH. 
§ 821. When the animal kingdom is surveyed from a broad stand- 
point, it becomes obvious that the ovum, or its correlative the spermatozoon, 
is the goal of an individual existence; that life is a cycle beginning in an 
ovum and coming round to an ovum again. The greater part of the actions 
which, looking from a near point of view at the higher animals alone, we 
are apt to consider as eminently the purposes for which animals come into 
existence, when viewed from the distant outlook whence the whole living 
world is surveyed, fade away into the likeness of the mere byplay of ovum- 
bearing organisms. The animal body is in reality a vehicle for ova; and 
after the life of the parent has become potentially renewed in the offspring, 
the body remains as a cast-off envelope whose future is but to die. 
Were the animal frame not the complicated machine we have seen it to 
be, death might come as a simple and gradual dissolution, the "sans every- 
thing " being the last stage of the successive loss of fundamental powers. 
As it is, however, death is always more or less violent; the machine comes 
to an end by reason of the disorder caused by the breaking down of one of 
its parts. Life ceases not because the molecular powers of the whole body 
slacken and are lost, but because a weakness in one or other part of the 
machinery throws its whole working out of gear. 
§ 822. We have seen that the central factor of life is the circulation of 
the blood, but we have also seen that blood is not only useless, but injurious, 
unless it is duly oxygenated : and w7e have further seen that in the higher 
animals the oxygenation of the blood can only be duly effected by means 
of the respiratory muscular mechanism, presided over by the medulla oblon- 
gata. Thus the life of a complex animal is, when reduced to a simple form, 
composed of three factors: the maintenance of the circulation, the access of 
air to the haemoglobin of the blood, and the functional activity of the respi- 
ratory centre ; and death may come from the arrest of either of these. As 
Bichat put it, death takes place by the heart, or by the lungs, or by the brain. 
In reality, however, when we push the analysis further, the central fact of 
death is the stoppage of the heart, and the consequent arrest of the circula- 
tion ; the tissues then all die, because they lose their internal medium. The 
failure of the heart may arise in itself, on account of some failure in its 
nervous or muscular elements, or by reason of some mischief affecting its 
mechanical working. Or its stoppage may be due to some fault in its inter- 
nal medium, such for instance as a want of oxygenation of the blood, which 
in turn maybe caused by either a change in the blood itself, as in carbonic- 
oxide poisoning, or by a failure in the mechanical conditions of respiration, 
or by a cessation of the action of the respiratory centre. The failure of 
this centre, and indeed that of the heart itself, may be caused by nervous 
influences proceeding from the brain, or brought into operation by means of 
the central nervous system ; it may, on the other hand, be due to an imper- 
fect state of blood, and this in turn may arise from the imperfect or perverse 
action of various secretory or other tissues. The modes of death are in 
reality as numerous as are the possible modifications of the various factors 
of life ; but they all end in a stoppage of the circulation, and the with- 
drawal from the tissues of their internal medium. Hence we come to con- 
sider the'death of the body as marked by the cessation of the heart's beat, 
a cessation from which no recovery is possible ; and by this we are enabled 864 
DEATH. 
to fix an exact time at which we say the body is dead. We can, however, 
fix no such exact time to the death of the individual tissues. They are not 
mechanisms, and their death is a gradual loss of power. In the case of the 
contractile tissues, we have apparently in rigor mortis a fixed term by 
which we can mark the exact time of their death. If we admit that after 
onset of rigor mortis recovery of irritability is impossible, then a rigid 
muscle is one permanently dead. In the case of the other tissues we have 
no such objective sign, since the rigor mortis of simple protoplasm manifests 
itself chiefly by obscure chemical signs. And in all cases it is obvious that 
the possibility of recovery, depending as it does on the skill and knowledge 
of the experimenter, is a wholly artificial sign of death. Yet we can draw 
no other sharp line between the seemingly dead tissue whose life has flickered 
down into a smouldering ember which can still be fanned back again into 
flame, and the handful of dust, the aggregate of chemical substances into 
which the decomposing tissue finally crumbles. 
Moreover, the failure of the heart itself is at bottom loss of irritability, 
and the possibility of recovery here also rests, as far as is known at present, 
on the skill and knowledge of those who attempt to recover. So that, after 
all, the signs of the death of the whole body are as artificial as those of the 
death of the constituent tissues. APPENDIX. 
ON THE CHEMICAL BASIS OF THE ANIMAL BODY. 
The animal body, from a chemical point of view, may be regarded as a mixture 
of various representatives of three large classes of chemical substances, viz., pro- 
teids, carbohydrates, and fats, in association with smaller quantities of various saline 
and other crystalline bodies. By proteids are meant bodies containing carbon, oxy- 
gen, hydrogen, and nitrogen in a certain proportion, varying within narrow limits, 
and having certain general features; they are frequently spoken of as albuminoids. 
By carbohydrates are meant starches and sugars and their allies. We have also 
seen that the animal body may be considered as an assemblage of protoplasm under 
various modifications and of numerous products of protoplasmic activity. We do 
not at present know anything definite about the molecular composition of active 
living protoplasm ; but when we submit protoplasm to chemical analysis, in which 
act it is killed, we always obtain from it a considerable quantity of the material 
spoken of as proteid. And many authors go so far as to speak of protoplasm as 
being purely proteid in nature; they regard the living protoplasm as proteid mate- 
rial, which, in passing from death to life, has assumed certain characters and pre- 
sumably has been changed in construction, but still is proteid matter ; they some- 
times speak of protoplasm as " living proteid " or "living albumin." It is worthy 
of notice, however, that even simple forms of protoplasm, like that constituting 
the body of a white corpuscle, forms of protoplasm which we may fairly consider 
as native protoplasm, when they can be obtained in sufficient quantity for chemical 
analysis, are found to contain some representatives of carbohydrates and fats as 
well as of proteids. We might, perhaps, even go so far as to say, that in all 
forms of living protoplasm, the proteid basis is found upon analysis to have some 
carbohydrates and some kind of fat associated with it. Further, not only does the 
normal food, which is eventually built up into protoplasm, consist of all three classes, 
but as we have seen in the sections on nutrition, protoplasm gives rise by metab- 
olism to members of the same three classes; and, as far as we know at present, 
carbohydrates and fats, when found in the body out of proteid food, are so formed 
by the agency of living protoplasm, by some living tissue. Hence there is at least 
some reason for thinking it probable that the molecule of protoplasm, if we may use 
such a phrase, is far more complex than a molecule of proteid matter, that it con- 
tains in itself residues, so to speak, not only of proteid, but also of carbohydrate and 
fatty material. 
Be this as it may, for no dogmatic statement can at present be made, when we 
examine the various tissues and fluids of the animal body from a chemical point of 
view we find present in different places, or at different times, several varieties and 
derivatives of the three chief classes; we find many forms of proteids, and bodies 
closely allied to proteids, in the forms of mucin, gelatin, etc. ; many varieties of 
fats ; and several kinds of carbohydrates. 
We find, moreover, many other bodies which we may regard as stages in the 
constructive or destructive metabolism of both native and differentiated protoplasm, 
and which are important not so much from the quantity in which they occur in the 
animal body at any one time as from their throwing light on the nature of animal 
metabolism ; these are such bodies as urea, other organic crystalline bodies, and the 
extractives in general. 
In the following pages the chemical features of the more important of these 
various substances which are known to occur in the animal body will be briefly 
considered, such characters only being described as possess or promise to possess 
865 866 
APPENDIX. 
physiological interest. The physiological function of any substance must depend 
ultimately on its molecular (including its chemical) nature ; and though at present 
our chemical knowledge of the constituents of an animal body gives us but little 
insight into their physiological properties, it cannot be doubted that such chemi- 
cal information as is attainable is a necessary preliminary to all physiological 
study. 
PROTEIDS. 
These form the principal solids of the muscular, nervous, and glandular tissues, 
of the serum of blood, of serous fluids, and of lymph. In a healthy condition, 
sweat, tears, bile, and urine contain mere traces, if any, of proteids. Their general 
percentage composition may be taken as 
0. 
H. 
N. 
C. 
S. 
From 
20.9 
to 23.5 
6.9 
to 7.3 
15.2 
to 17.0 
51.5 
to 54.5 
0.3] 
to 2.0 ] 
■ (Hoppe-Seyler. 
From 
50.0 
6.8 
15.4 
22.8 
0.4 j 
■ (Drechsel.) 
to 55.0 
to 7.3 
to 18.2 
to 24.1 
to 5.0 J 
These figures are obtained from a consideration of numerous analyses, slight differences in the 
various results being immaterial, where the purity of the substance operated upon cannot be def- 
initely determined. 
In addition to the above constituents, proteids leave on ignition a variable quantity of ash. In 
the case of egg-albumin the principal constituents of the ash are chlorides of sodium and potas- 
sium, the latter greatly exceeding the former in amount. The remainder consists of sodium and 
potassium, in combination with phosphoric, sulphuric, and carbonic acids, and very small quan- 
tities of calcium, magnesium, and iron, in union with the same acids. There is also a trace of 
silica.2 The ash of serum-albumin contains an excess of sodium chloride, but the ash of the pro- 
teids of muscle contains an excess of potash salts and phosphates. The nature of the connection 
of the ash with the proteid is still a matter of obscurity. Globin from haemoglobin is said to leave 
no ash on ignition. 
Proteids as met with in the animal body are all amorphous; some are soluble, 
some insoluble in water, and all are, for the most part, insoluble in alcohol and ether; 
they are till soluble in strong acids and alkalies, but in becoming dissolved mostly 
undergo decomposition. Their solutions possess a left-handed rotatory action on the 
plane of polarization, the amount depending on various circumstances, and being, 
with one exception, viz., peptones, changed by heating. 
Crystals into whose composition certain proteid (especially globulin)3 elements enter were 
long since observed in the seeds of many plants;. as yet they have not been obtained sufficiently 
isolated or in quantities large enough to permit any accurate analysis to be made. A method o'f 
isolating in quantity and recrystallizing these substances has, however,4 been indicated, and it 
seems probable that analysis of these may lead to interesting information on the subject of the 
constitution and combinations of proteids. 
The presence of proteids may be determined by the following tests: 
1. Heated with strong nitric acid, they or their solutions turn yellow, and this 
color is, on the addition of ammonia, or caustic soda or potash, changed to a deep 
orange hue. (Xanthoproteic reaction.) 
2. With Millon's reagent they give, when present in sufficient quantity, a pre- 
cipitate, which turns red on heating. If they are only present in traces, no precip- 
itate is obtained, but merely a red coloration of the solution. 
3. If mixed with some concentrated solution of sodic hydrate, and one or two 
drops of a solution of cupric sulphate, a violet color is obtained, which deepens in 
tint on boiling. 
The above serve to detect the smallest traces of all proteids. The two following 
tests may be used when there is more than a trace present, but do not hold for 
every kind of proteid. 
4. Render the fluid strongly acid with acetic or other acid, and add a few drops 
of a solution of ferrocyanide of potassium ; a precipitate shows the presence of 
proteids. 
5. Render the fluid, as before, strongly acid with acetic acid, add an equal volume 
of a concentrated solution of sodic sulphate, and boil. A precipitate is formed if 
proteids are present. 
1 Hdb. Phys. Path. Chem. Anal., Ed. iv. (1875), S. 223. 
2 See Gmelin, Hdb. Org. Chem., Bd. viii. S. 285. 
3 Vines, Journ. of Physiol., vol. iii. (1880), p. 93. 
4 Drechsel, Journ. f. prakt Chem., N. F., Bd. xix (1879), S 331. CHEMICAL BASIS OF THE ANIMAL BODY. 
867 
This last reaction is useful, not only on account of its exactness, but also because the reagents 
used produce no decomposition of other bodies which maybe present; and hence after filtration 
the same fluid may be further analyzed for other substances. Additional methods of freeing a 
solution from proteids are: acidulating with acetic acid and boiling, avoiding any excess of the 
acid ; precipitation by excess of alcohol; in the latter case the solution must be neutral or faintly 
acid. Hoppe-Seyler1 recommends the employment of a saturated solution of freshly precipitated 
ferric hydrate, in acetic acid ; this is added to the solution, and on boiling the whole of the pro- 
teids are precipitated as well as the ferric salt, the latter as a basic acetate. Briicke's method of 
removing the last traces of proteids from glycogen solution is also of use (see Glycogen). Precip- 
itation of the last traces of proteids by means of hydrated oxide of lead at a boiling tempera- 
ture2 may be also employed. 
[Solutions may entirely be freed from proteids by one of the following methods: 
(I) Add acetic acid to faint acidity, then tannic acid; (2) render acid with hydro- 
chloric acid, then add iodide of mercury and potassium ; (3) render decidedly acid 
with hydrochloric acid, then precipitate any proteids in solution with phospho- 
tungstic acid. ] 
Proteids may be conveniently divided into classes. 
Class I. Native Albumins. 
Members of this class, as their name implies, occur in a natural condition in 
animal tissues and fluids. They are soluble in water, are not precipitated by very 
dilute acids, by carbonates of the alkalies, or by sodium chloride. They are coagu- 
lated by heating in solution to a temperature of about 70° C. If dried at 40° C., 
the resulting mass is of a pale yellow color, easily friable, tasteless, inodorous, and 
soluble. 
1. Egg-albumin. 
Forms in aqueous solution a neutral, transparent, yellowish fluid. From this 
it is precipitated by excess of strong alcohol. If the alcohol be rapidly removed 
the precipitate may be readily redissolved in water; if subjected to longer action 
a coagulation occurs, and the albumin is then no longer thus soluble. Strong 
acids, especially nitric acid, cause a coagulation similar to that produced by heat 
or by the prolonged action of alcohol ; the albumin becomes profoundly changed 
by the action of the acid and does not dissolve upon removal of the acid. Mer- 
curic chloride, argentic nitrate, and lead acetate, precipitate the albumin, forming 
insoluble compounds of variable composition with it: the precipitants may be re- 
moved by means of sulphuretted hydrogen and the albumin again obtained, appar- 
ently unaltered, in solution. 
Strong acetic acid in excess gives no precipitate, but when the solution is con- 
centrated the albumin is transformed into a transparent jelly. A similar jelly is 
produced when a strong caustic potash is added to a concentrated solution of egg- 
albumin. In both these cases the substance is profoundly altered, becoming, in the 
one case, acid ; in the other, alkali-albumin. 
The specific rotatory power of egg-albumin in aqueous solution is, for yellow 
light-35.5°. Hydrochloric acid, added until the reaction is strongly acid, in- 
creases this rotation to-37.7°. The formation of the gelatinous compound with 
caustic potash is at first accompanied by an increase, but this is followed by a 
decrease of rotation. 
Preparation. White of hen's egg is broken up with scissors into small pieces, 
diluted with an equal bulk of water, and the mixture shaken strongly in a flask 
till quite frothy; on standing, the foam rises to the top, and carries all the fibres 
in whose meshwork the albumin was contained. The fluid from which the foam 
has been removed, is strained, and treated carefully with dilute acetic acid as long 
as any precipitate is formed; the precipitate is then filtered off, and the filtrate 
after neutralization purified by dialysis and then concentrated at 40° to its original 
bulk. 
This form of albumin resembles, to a great extent, the one previously described. 
The following may suffice as distinguishing features: 
1. The specific rotation of serum-albumin is-56°; that of egg-albumin is - 
35.5°, both measured for yellow light. 
2. Serum-albumin. 
1 Op. cit., S. 227. " Hofmeister, Zeitschr. f. Physiol. Chem., Bd. ii. (1878), S. 288. 868 
APPENDIX. 
2. Serum-albumin is not coagulated by being shaken up with ether; egg-albu- 
min is. 
3. Serum-albumin is not very readily precipitated by strong hydrochloric acid, 
and such precipitate as does occur is readily redissolved on further addition of the 
acid; the exact reverse of these two features holds good for egg-albumin. 
4. Precipitated or coagulated serum-albumin is readily soluble, egg-albumin is 
with difficulty soluble, in strong nitric acid. 
5. Egg-albumin, if injected subcutaneously or into a vein, appears unaltered in 
the urine j1 serum-albumin similarly injected does not thus normally pass out by the 
kidney. 
[6. Gautier states that 10 c.c. of the following solution, added to 2 c.c. of the 
solution to be tested, will precipitate egg-albumin but not serum-albumin : Caustic 
soda, sp. gr. 0.7, 250 c.c. ; 1 per cent, sulphate of copper, 50 c.c. ; glacial acetic 
acid, 700 c.c.] 
Serum-albumin is found not only in blood-serum, but also in lymph, both that 
contained in the proper lymphatic channels and that diffused in the tissues; in 
chyle, milk, transudations, and many pathological fluids. 
It is this form in which albumin generally appears in the urine. 
In addition to the above, Scherer2 has described two closely related bodies, to which he gives 
the names paralbumin and metalbumin. The first he obtained from ovarian cysts; its alkaline 
solutions are remarkable for being very ropy. It seems doubtful whether this body is a proteid ; 
it differs sensibly in composition from these. Haerlin8 gives as its composition, O. 26.8, II. 6.9. 
N. 12.8, C. 51.8, S. 1.7 per cent. It seems to be associated with some body like glycogen, capable of 
being converted into a substance giving the reactions of dextrose. Metalbumin, found in a drop- 
sical fluid, resembles the preceding, but is not precipitated by hydrochloric acid, or by acetic acid 
and ferrocyanide of potassium ; it is precipitated, but not coagulated, by alcohol; its solution is 
scarcely coagulated on boiling. 
Albumins are generally found associated with small but definite amounts of 
saline matter. A. Schmidt4 says that they may be freed from these by dialysis, 
and that they are then not coagulated on boiling. From this it might be inferred 
that the albumin and the saline matters were peculiarly related, and that the 
latter played some special part during the coagulation of the former by heat. 
Schmidt's observations, however, have not been conclusively corroborated by sub- 
sequent observers. 
CLASS II. Derived Albumins (Albuminates). 
When a native albumin in solution, such as serum-albumin, is treated for some 
little time with a dilute acid such as hydrochloric, the properties become entirely 
changed. The most marked changes are: (1) that the solution is no longer 
coagulated by heat; (2) that when the solution is carefully neutralized the whole 
of the proteid is thrown down as a precipitate; in other wTords, the serum-albumin 
which was soluble in water, or at least in a neutral fluid containing only a small 
quantity of neutral salts, has become converted into a substance insoluble in water 
or in similar neutral fluids. The body into which serum-albumin thus becomes 
converted by the action of an acid is spoken of as acid-albumin. Its characteristic 
features are that it is insoluble in distilled water, and in neutral saline solutions, 
such as those of sodic chloride, that it is readily soluble in dilute acids or dilute 
alkalies, and that its solutions in acids or alkalies are not coagulated by boiling. 
When suspended, in the undissolved state, in water, and heated to 70° C., it be- 
comes coagulated, and is then undistinguishable from coagulated serum-albumin, 
or indeed from any other form of coagulated proteid. It is evident that the sub- 
stance when in solution in a dilute acid is in a different condition from that in 
which it is when precipitated by neutralization. If a quantity of serum- or egg- 
albumin be treated with dilute hydrochloric acid, it will be found that the con- 
version of the native albumin into acid-albumin is gradual; a specimen heated 
to 70° C. immediately after the addition of the dilute acid, will coagulate almost 
as usual; and another specimen taken at the same time will give hardly any 
1. Acid-albumin. 
1 Stokvis, Rech. exp. sur les Condit, pathol. de PAlbuminurie, Bruxelles, 1867; also Lehmann, 
Arch. f. Pathol. Anat., Bd. xxx. (1864), S 593. 
2 Ann der Chern, und Pharm., Bd. Ixxxii., S. 135. 
3 Chem. Centralblatt, 1862, No. 56. 4 Pfliiger's Archiv, xi. (1875), S. 1. CHEMICAL BASIS OF THE ANIMAL BODY. 
869 
precipitate on neutralization. Some time later, the interval depending on the pro- 
portion of the acid to the albumin, on temperature, and on other circumstances, 
the coagulation will be less, and the neutralization precipitate will be considerable. 
Still later, the coagulation will be absent, and the whole of the proteid will be 
thrown down on neutralization. 
If finely-chopped muscle, from which the soluble albumins have been removed 
by repeated washing, be treated for some time with dilute (0.2 per cent.) hydro- 
chloric acid, the greater part of the muscle is dissolved. The transparent acid 
filtrate contains a large quantity of proteid material in a form which, in its gen- 
eral characters at least, agrees with acid-albumin. The acid solution of the pro- 
teid is not coagulated by boiling, but the whole of the proteid is precipitated on 
neutralization ; and the precipitate, insoluble in neutral sodic chloride solutions, 
is readily dissolved by even dilute acids or alkalies. The proteid thus obtained from 
muscle has been called syntonin, but we have at present no satisfactory test to dis- 
tinguish the acid-albumin (or syntonin) prepared from muscle from that prepared 
from egg- or serum-albumin. When coagulated albumin or other coagulated pro- 
teid or fibrin is dissolved in strong acids, acid-albumin is formed; and when fibrin 
or any other proteid is acted upon by gastric juice, acid-albumin is one of the first 
products; and these acid-albumins cannot be distinguished from acid-albumin pre- 
pared from muscle or native albumin. Though hydrochloric acid is perhaps the 
most convenient acid for forming acid-albumin, other acids may also be used for 
the purpose of preparing it. Acid-albumin is soluble not only in dilute alkalies, 
but also in dilute solutions of alkaline carbonates; its solutions in these are not 
coagulated by boiling. 
If sodic phosphate in excess is added to an acid solution of acid-albumin, the 
acid-albumin is precipitated ; this also occurs on adding sodic acetate or phos- 
phate. 
As special tests of acid-albumin maybe given: 1. Partial coagulation of its 
solution in lime-water on boiling. 2. Further precipitation of the same solution 
after boiling, on the addition of calcic chloride, magnesic sulphate, or sodic 
chloride. 
Dissolved in very dilute hydrochloric acid, acid-albumin (syntonin) prepared from 
muscle possesses a specific Isevo-rotatory power of-72° for yellow light, this being 
independent of the concentration.1 On heating the solution in a closed vessel in a 
water-bath the rotatory power rises to -84.8°. 
The body known as parapeptone, which makes its appearance during the peptic 
digestion of proteids, is closely allied to the substances just described. 
If serum- or egg-albumin or washed muscle be treated with dilute alkali instead 
of with dilute acid, the proteid undergoes a change quite similar to that which was 
brought about by the acid. The alkaline solution, when the change has become 
complete, is no longer coagulated by heat, the proteid is wholly precipitated on neu- 
tralization, and the precipitate, insoluble in water and in neutral sodic chlorine solu- 
tion, is readily soluble in dilute acids or alkalies. Indeed in a general way it may 
be said that acid-albumin and alkali-albumin are nothing more than solutions of the 
same substance in dilute acids and alkalies respectively. When the precipitate 
obtained by the neutralization of a solution of acid-albumin in dilute acid is dis- 
solved in a dilute alkali, it may be considered to become alkali-albumin ; and con- 
versely when the precipitate obtained from an alkali-albumin solution is dissolved in 
dilute acid, it may be regarded as acid-albumin. 
It is stated2 as a characteristic reaction of this modified or derived albumin that 
it is not precipitated when its alkaline solutions are neutralized in the presence of 
alkaline phosphates; solutions of acid-albumin, on the contrary, are said to be pre- 
cipitated on neutralization in the presence of alkaline phosphates, and this difference 
is considered to be a distinguishing feature of the two proteids. But doubt has been 
cast on this statement.3 
Alkali-albumin may be prepared by the action not only of dilute alkalies, but 
also of strong caustic alkalies on native albumins as well as on coagulated albu- 
2. Alkali-albumin. 
1 Hoppe-Seyler, Hdb. Phys. Path. Chem. Anal., Ed. iv. (1875), S. 246. 
2 Hoppe-Seyler, loc. cit., 8. 245. 
*Soyka. Ptiuger's Arch., Bd. xii. (1876), S. 347. 870 
APPENDIX. 
min and other proteids. The jelly produced by the action of caustic potash on white 
of egg, spoken of in Class I., 1, is alkali-albumin; the similar jelly produced by 
strong acetic acid is acid-albumin. One of the most productive methods of obtain- 
ing alkali-albumin is that introduced by Lieberkuhn,1 and consists in adding a strong 
solution of caustic potash to purified white of egg until the above-mentioned jelly 
is obtained. This is then cut into small pieces, and dialyzed until quite white. The 
lumps are then dissolved by heating on the water-bath, and the alkali-albumin pre- 
cipitated by the careful addition of acetic acid. 
Both alkali-and acid albumin are with difficulty precipitated by alcohol from 
their alkaline or acid solutions. The neutralization precipitates, however, become 
coagulated under the prolonged action of alcohol. 
The body "protein," described by Mulder, appears, if it exists at all, to be closely connected 
with this body. All subsequent observers have, however, failed to confirm his views. 
The rotatory power of alkali-albumin varies according to its source ; thus 
when prepared by strong caustic potash from serum-albumin, the rotation rises 
from -56° (that of serum-albumin) to -86° ; for yellow light. Similarly prepared 
from egg-albumin, it rises from -38.5° to -47° ; and if from coagulated white of 
egg, it rises to-58.8°. Hence the existence of various forms of alkali-albumin is 
probable. 
In addition to the methods given above, alkali-albumin may be also readily obtained by shak- 
ing milk with strong caustic soda solution and ether, removing the ethereal solution, precipitat- 
ing the remaining fluid with acetic acid and washing the precipitate with water, cold alcohol and 
ether. 
The most satisfactory method of regarding acid- and alkali-albumin is to con- 
sider them as respectively acid and alkali compounds of the neutralization precip- 
itate. We have reason to think that when the precipitate is dissolved in either an 
acid or an alkali, it does enter into combination with them. The neutralization pre- 
cipitate is in itself neither acid- nor alkali-albumin, but may become either, upon 
solution in the respective reagent. 
It is probable that several derived albumins exist,2 differing according to the proteid from 
which they are formed or possibly according to the mode of their.preparation, and that each of 
these may exist in its correlative forms of acid-and alkali-albumin; but the whole subject re- 
quires further investigation. 
Acid-albumin, prepared by the direct action of dilute acids on native albumin, or 
on muscle-substance, contains sulphur, as shown by the brown coloration which ap- 
pears when the precipitate is heated with caustic potash in the presence of basic 
lead acetate. Alkali-albumin, at all events as prepared by the action of strong caus- 
tic potash or soda, does not contain any sulphur; and the acid-albumin. prepared 
by the solution in an acid of the neutralization precipitate from such an alkali-albu- 
min solution, is similarly free from sulphur. 
3. Casein. 
This is the well-known proteid existing in milk. When freed from fat, and in 
the moist condition, it is a white, friable, opaque body. In most of its reactions 
it corresponds closely with alkali-albumin; thus it is readily soluble in dilute acids 
and alkalies, and is re-precipitated on neutralization; if, however, potassic phos- 
phate is present, as is the case in milk, the solution must be strongly acid before 
any precipitate is obtained. 
Various reactions have at different times been assigned to casein as distinguishing it from the 
closely allied body alkali-albumin. Later researches have, however, in most cases cast so much 
doubt on these differences that the identity or non-identity of casein and alkali-albumin must 
still be left an open question, the discussion of which would be out of place here. 
Casein, as occurring in milk, has had several reactions ascribed to it, as characteristic; but 
these lose their importance on considering that milk contains, in addition to casein, other sub- 
stances, such as potassic phosphate, and a number of bodies which yield acids by fermentation. 
The presence of potassic phosphate has an especial influence on the reactions of casein. In the 
entire absence of this salt, acetic acid in the smallest quantities, as also carbonic anhydride, gives 
a precipitate ; but if this salt is present, carbonic anhydride gives no precipitate, and acetic acid 
only one when the solution is acid from the presence of free acid, and not from that of acid 
potassic phosphate.3 
1 PoggendorfTs Annalen, Bd. Ixxxvi., 8. 118. 
2 Mbrner. Pfliiger's Arch., Bd. xvii. (1878), S. 468. 
3 See Kiihne, Lehrb. d. Physiol. Chem., 1868, S. 565. CHEMICAL BASIS OF THE ANIMAL BODY. 
871 
When prepared from milk by magnesic sulphate (see below), freed by ether 
from fats, and dissolved in water, casein possesses a specific rotatory power of -80° 
for yellow light; in dilute alkaline solutions, of-76°; in strong alkaline solutions, 
of --91°; in dilute hydrochloric acid, of -87°. 
' Casein has been asserted to occur in muscle, in serous fluids, and in blood-serum 
(serum-casein). In many cases it has probably been confounded with globulins 
(see Class III.); but blood-serum and muscle-plasma undoubtedly contain an 
alkali-albumin in addition to whatever globulin may be present, and the usual 
doubt exists as to the identity of this with true casein. Its presence maybe shown 
by adding dilute acetic acid to blood-serum which has been freed from globulin by 
a current of carbonic anhydride; a distinct precipitate is thrown down. A sub- 
stance similar to casein has also been described as existing in unstriated muscle and 
in the protoplasm of nerve-cells. 
Preparation. Dilute milk with several (10 to 15) times its bulk of water, add 
dilute acetic acid till a precipitate begins to appear, then pass a current of carbonic 
anhydride, filter, and wash the precipitate with water, alcohol, and ether; the com- 
plete removal of the fat carried down with the casein presents some difficulties. 
Magnesic sulphate added to saturation also precipitates casein from milk; the pre- 
cipitate as thus formed is readily soluble on the addition of water. 
Class III. Globulins. 
Besides the native albumins there are a number of native proteids which differ 
from the albumins in not being soluble in distilled water; they need for their solu- 
tion the presence of an appreciable, though it may be a small, quantity of a neutral 
saline body, such as sodic chloride. Thus they resemble the albuminates in not 
being soluble in distilled water, but differ from them in being soluble in dilute sodic 
chloride or other neutral saline solutions. Their general characters may be stated 
as follows: 
They are insoluble in water, soluble in dilute (1 percent.) solutions of sodic chlo- 
ride ; they are also soluble in dilute acids and alkalies, being changed on solution 
into acid- and alkali-albumin respectively, unless the acids and alkalies are exceed- 
ingly dilute. The saturation with solid sodic chloride of their solutions in dilute 
sodic chloride, precipitates most members of this class. 
1. Globulin (Crystallin). 
If the crystalline lens be rubbed up with fine sand, extracted with water and 
filtered, the filtrate will be found to contain at least three proteids. On passing a 
current of carbonic anhydride a copious precipitate occurs; this is globulin. 
The addition of dilute acetic acid to the filtrate from the globulin gives a precipitate of 
alkali-albumin;1 and the filtrate from this, if heated, gives a further precipitate, due to serum- 
albumin. 
In its general reactions globulin corresponds almost exactly with the next mem- 
bers of this class (paraglobulin and fibrinogen), but has no power to form or pro- 
mote the formation of fibrin in fluids containing the above-mentioned bodies, and 
possesses the following special features: 1. According to Lehmann, its oxygenated 
neutral solutions become cloudy on heating to 73° C., and are coagulated at 93° C. 
2. It is readily precipitated on the addition of alcohol. According to Hoppe- 
Seyler, if is not precipitated mi saturation with sodic chloride, resembling vitellin 
in this respect. 
According to Kiihne2 and Eichwald® a globulin with properties identical with those just 
given may be precipitated from dilute serum by the cautious addition of acetic acid. This body 
is stated by Weyl4 ro be the same as paraglobulin (fibrinoplastin), the latter differing from it only 
by a small admixture of fibrin-ferment. 
Preparation. Blood-serum is diluted tenfold with water, and a brisk current of 
carbonic anhydride is passed through it. The first-formed cloudiness soon becomes 
2. Paraglobulin {Fibi'inoplastin). 
1 But see also Pfl tiger's Arch., Bd. xiii. (1876), S. 631. 
2Lehrb. d. Physiol. Chern., 1868, S. 175. 
8Beitrage zur Chem. d. gewehebild. Subst., Berlin, 1873, H. 1. 
4Zeitsehr. f. Physiol. Chem., Bd. i. (1878), S. 79. 872 
APPENDIX. 
a flocculent precipitate, which is finally quite granular, and may easily be separated 
by decantation and filtration ; it should be washed on the filter with water contain- 
ing carbonic acid. 
It has usually been stated that paraglobulin may be separated from serum by 
saturation with sodic chloride. But Hammarsten1 has shown that this is only in 
part true, a considerable portion of the globulin remaining unprecipitated. The 
separation may, however, be completely effected by saturation with magnesic sul- 
phate. When determined by this method the amount of paraglobulin in serum is 
very considerable, amounting in some cases, according to Hammarsten, to as much 
as 4.565 per cent, (reckoned on 100 c.c. of serum). The quantity seems to vary in 
different animals, the precipitation being much more complete in serum from ox- 
blood than in that from the blood of horses. 
From its solution in dilute sodic chloride, paraglobulin may be precipitated by a 
current of carbonic anhydride or the addition of exceedingly dilute (less than 1 pro 
mi lie) acetic acid. If the acid is strong, the precipitated proteid becomes immedi- 
ately changed into acid-albumin (Class IL, 1). In pure water, free from oxygen, 
paraglobulin is insoluble, but on shaking with air or passing a current of oxygen, 
solution readily takes place; from this it may be reprecipitated by a current of 
carbonic anhydride. Very dilute alkalies dissolve this body without change; if, 
however, the strength of the alkali be raised even to 1 per cent, the paraglobulin is 
changed into alkali-albumin (Class II., 2). 
According to Kiihne and A. Schmidt the solutions of this body in water con- 
taining oxygen or in very dilute alkalies are not coagulated on heating. The 
sodic chloride solutions do, however, coagulate when heated to 68°-7O° C.,2 and 
if the substance itself be suspended in water and heated to 70° C. it is coagulated. 
Although insoluble in alcohol, its solutions are with difficulty precipitated by this 
reagent. 
Paraglobulin occurs not only in blood-serum, but it is also found in white cor- 
puscles, in the stroma of red corpuscles (to some extent at least), in connective 
tissue, the cornea, aqueous humor, lymph, chyle, and serous fluids. 
For the occurrence of globulin in urine see Edlefsen3 and Senator.4 
3. Fibrinogen. 
The general reactions of this body are identical with those of paraglobulin. The 
most marked difference between the two is the point at which coagulation of their 
solutions takes place. Hammarsten5 has shown that fibrinogen in a 1-5 per cent, 
solution of sodic chloride coagulates at from 52°-55° C., whereas, as stated above, 
paraglobulin (fibrinoplastin) coagulates first at from 68°-7O° C. This, however, 
is disputed by A. Schmidt, who holds that the substance coagulating at 52°-55° is 
not fibrinogen, but a sort of nascent fibrin. There is also a marked difference in 
the precipitability of the bodies by sodic chloride. (See below.) Other differences 
between the two may be thus enumerated: In precipitating fibrinogen by a cur- 
rent of carbonic anhydride the containing fluid must be much more strongly diluted, 
and the gas must pass for a much longer time. The precipitate thus obtained 
differs from that of paraglobulin in that it forms a viscous deposit, adhering more 
closely to the sides and bottom of the containing vessel; there is also no flocculent 
stage previous to the viscous precipitate. 
Fibrinogen occurs in blood, chyle, serous fluids, and in various transudations. 
'Hie relations of fibrinogen and paraglobulin to the formation of fibrin have been 
discussed in the text, p. 33. 
Preparation.6 Salted plasma, obtained by centrifugalizing blood whose coagula- 
tion is prevented by the addition of a certain proportion of magnesic sulphate, is 
mixed with an equal volume of a saturated (35.87 per cent, at 14° C.)' solution of 
sodic chloride; the fibrinogen is thus precipitated, while the paraglobulin remains 
in solution. The adhering plasma may be removed by washing with a solution of 
1 Pfliiger's Archiv, Bd. xvii. (1878), S. 446; Bd. xviii. (1878), S. 38. 
Hammarsten. op. cit. 
8Centralblatt f. med. Wiss., Jahrg. 1870. S. 367. Also Arch. f. klin. Med., Bd. vii. S. 69. 
4 Virchow's Archiv, Bd. lx. S. 476. 5 Upsala Lakarefbrenings forhandiingar. Bd. xi., 1876. 
6See Hammarsten, Nov. Act. Reg. Soc. Sei.. Upsala, Ser. iii. vol. x. (1875), p. 31. Also Pfliiger's 
Archiv, Bd. xix. (1879), S. 563, and Bd. xxii. (1880), S. 431. 
7 Poggiale, Ann. Chim. Phys. (3), vol. viii. p. 469. 873 
CHEMICAL BASIS OF THE ANIMAL BODY. 
sodic chloride, and the fibrinogen finally purified by being several times dissolved in 
and reprecipitated by sodic chloride. 
There is no proof that the whole of the substance thrown down by carbonic 
anhydride from diluted blood-serum is fibrinoplastic; indeed we know that a true 
globulin devoid of fibrinoplastic properties may be prepared from serum.1 Weyle2 
considers that there is only one globulin in serum, which he characterizes by the 
name of "serum-globulin, " and regards fibrinoplastin as a mixture of this body 
with a portion of fibrin-ferment. We know for certain (see p. 28) that the whole 
of the fibrinoplastic precipitate, used to cause the coagulation of a fibrinogenous 
fluid, does not enter into the composition of the fibrin produced ; we also know that 
such a precipitate may lose its fibrinoplastic powers without any marked change in 
its general reactions. It would seem advisable, therefore, to speak of the deposit 
produced by carbonic anhydride in dilute serum, or by saturation with sodic chloride 
in undiluted serum, as globulin, and to distinguish it as fibrinoplastic globulin when 
it is able to give rise to fibrin. Fibrinogen similarly might be spoken of as fibrin- 
ogenous globulin. The name crystallin, rather than globulin, might then be given 
to the substance obtained from the crystalline lens. 
4. Myosin. 
This is the substance which forms the chief proteid constituent of dead, rigid 
muscle; its general properties and mode of preparation have been already 
described at p. 84. In the moist condition it forms a gelatinous, elastic, clotted 
mass; dried, it is very brittle, slightly transparent, and elastic. From its solution 
in sodic chloride it is precipitated, either by extreme dilution or by saturation 
with the solid salt. When precipitated by dilution and submitted to the pro- 
longed action of water, myosin loses its property of being soluble in solutions of 
sodic chloride.3 The sodic chloride solution, if exposed to a rising temperature, 
becomes milky at 55° C., and gives a flocculent precipitate at 6(F C. This precip- 
itate is. however, no longer myosin, for it is insoluble in a 10 per cent, sodic chloride 
solution, and does not, until after many days' digestion, yield syntonin on treat- 
ment with hydrochloric acid (0.1 per cent.). It is, in fact, coagulated proteid (see 
Class V.). 
Myosin is excessively soluble in dilute acids and alkalies. Advantage may be 
taken of its solubility in the former to extract it from muscles.4 But if the 
reagents are at all concentrated, myosin undergoes in the act of solution a radical 
change, becoming in the one case acid-albumin or syntonin, in the other alkali- 
albumin (Class II.). 
Like fibrin, it can in some cases decompose hydrogen dioxide, and oxidize guaiacum with for- 
mation of a blue color. 
5. Vitellin. 
As obtained from the yolk of egg, of which it is the chief proteid constituent, 
vitellin is a white granular body, insoluble in water, but very soluble in dilute sodic 
chloride solutions; it surpasses myosin in this respect, for the solution maybe easily 
filtered. Its coagulation temperature is higher than that of myosin, lying, accord- 
ing to Weyl,5 between 70° C. and 80° C. Saturation with sodic chloride gives 
no precipitate ; in this respect it differs from most other members of this class. In 
yolk of egg vitellin is always associated with, and probably exists in combination 
with, the peculiar complex body lecithin. 
Denis, and after him Hoppe-Seyler, have shown that vitellin before the treatment requisite to 
free it from lecithin possesses properties quite different from other proteids. 
A theory has been advanced that vitellin is really a complex body like haemo- 
globin, and on treatment with alcohol splits up into coagulated proteid and leci- 
thin. When well purified it contains 0.75 percent, sulphur, but no phosphorus. 
Dilute acids or alkalies readily convert it in its uncoagulated form into a member 
of Class II. 
1 Kuhne and Eichwald, loc. cit. 2 Loc. cit. 
8 Weyl, Zeitschr. f. Physiol. Chem., Bd. i. (1878), S. 77. 
4 Danilewsky, Zeitschr. f. Physiol. Chem., Bd. v. (1881), S. 158. 5 Op. cit. 874 
APPENDIX. 
Fremy and Valenciennes' have described a series of proteids, viz., ichthin, ichthidin, etc. 
derived from fish and amphibia. They appear to be either identical with, or closely allied to, 
vitellin. 
Preparation. Yolk of egg is treated with successive quantities of ether as long 
as this extracts any yellow coloring matter; the residue is dissolved in moderately 
strong (10 per cent.) sodic chloride solution, and filtered. The filtrate on falling 
into a large excess of water is precipitated. In this state it is mixed with lecithin 
and nuclein, and in order to free it from these it was usually treated with alcohol.2 
This, as above stated, entirely changes the vitellin into a coagulated form. It 
seems probable that the separation of vitellin from the other bodies with which it 
is mixed in the yolk of egg may be effected by precipitating the sodic chloride 
solution by the addition of excess of water; the precipitate is then redissolved in 
10 per cent, solution of sodic chloride, and the process repeated as rapidly as pos- 
sible. 
6. Globin. 
Globin, stated by Preyer3to be the proteid residue of the complex body haemoglobin (see p. 
351), ought probably to be considered as an outlying member of this class. It is, however, not 
readily soluble either in dilute acids or sodic chloride solutions. It is said to be absolutely free 
from ash. 
Insoluble in water and dilute sodic chloride solutions; soluble, with difficulty, in 
dilute acids and alkalies, and more concentrated neutral saline solutions. 
Fibrin, as ordinarily obtained, exhibits a filamentous structure, the component 
threads possessing an elasticity much greater than that of any other known solid 
proteid. 
Class IV. Fibrin. 
If allowed to form gradually in large masses, the filamentous structure is not so noticeable, and 
it resembles in this form pure India-rubber. Such lumps of fibrin are capable of being split in 
any direction, and no definite arrangement of parallel bundles of fibres can be made out. 
At ordinary temperature fibrin is insoluble in water, being dissolved only at very 
high temperatures, and then undergoing a complete change in its characters. In 
hydrochloric solutions of 1-5 percent, fibrin swells up and becomes transparent, 
but is not dissolved.4 In this condition the mere removal of the acid by an excess 
of water, neutralization, or the addition of some salt, causes a return to the 
original state. If, however, the acid be allowed to act for many days at ordinary 
temperatures, or for a few hours at 40°-60° C., solution takes place, and the result- 
ing proteid is syntonin. In dilute alkalies and ammonia, fibrin is much more 
readily soluble, though in this case also the solution is greatly aided by warming; 
the resulting fluid contains no longer fibrin, but alkali-albumin. This property is 
not distinctly characteristic of fibrin, although it dissolves perhaps more readily in 
both dilute acids and alkalies than do coagulated proteids. None of these solu- 
tions can be coagulated on heating, which is intelligible when it is remembered 
that they no longer contain fibrin, but either acid- or alkali-albumin. In addition 
to the above, fibrin is soluble, though with difficulty and only after .a considerable 
time, in 10 percent, solutions of sodic chloride, potassic nitrate, or sodic sulphate, 
the solution being often accompanied by putrefactive changes. These solutions 
may be coagulated by a temperature of 60° C., and are precipitated by dilution with 
water or saturation with solid sodic chloride; in fact, by the action of the neutral 
saline solutions the fibrin has become converted into a body exceedingly like myosin 
or globulin.3 
On ignition of fibrin a residue of inorganic matter is always obtained ; it is, how- 
ever, considered that sulphur is the only one of these elements which enters essen- 
tially into its composition. In other respects fibrin corresponds entirely in general 
composition with the other proteids. 
Suspended in water and heated to 70° C., it loses its elasticity and becomes 
opaque; it is then indistinguishable from other coagulated proteids. 
1 Compt. Rend., T. xxxviii. pp. 469, 525. 
2 Weyl, op. cit„ 8. 74. 3 Die Bhitkrystalle (1871), S. 166. 
4 Complete solution may, however, take place if the fibrin, as is frequently the case, contains 
any adherent pepsin. 
6 Gautier, Compt. Rend., T. Ixxix. (1874), p. 227. CHEMICAL BASIS OF THE ANIMAL BODY. 
875 
A peculiar property of this body remains yet to be mentioned, viz., its power of decomposing 
hydrogen dioxide. Pieces of fibrin placed in this fluid, though themselves undergoing no change, 
soon become covered with bubbles of oxygen; and guaiacum is turned blue by fibrin in presence 
of hydrogen dioxide or ozonized turpentine. 
Preparation. By vigorously stirring blood with a bundle of twigs and then 
washing with water until it is quite white. If required perfectly pure and colorless 
it should be prepared from plasma free from corpuscles. If the blood, before stir- 
ring, be diluted with an equal bulk of water, the subsequent washing of the fibrin 
is much facilitated, and it may readily be obtained quite white. Any adherent fats 
may be removed by ether. 
When globulin, myosin, and fibrin are compared each with the other, it will be 
seen that they form a series in which myosin is intermediate between globulin and 
fibrin. 'Globulin is excessively soluble in even the most dilute acids and alkalies; 
fibrin is almost insoluble in these; while myosin, though more soluble than fibrin, 
is less soluble than globulin. Globulin again dissolves with the greatest ease in a 
very dilute solution of sodic chloride. Myosin, on the other hand, dissolves with 
difficulty; it is much more soluble in a 10 per cent, than in a 1 per cent, solution 
of sodic chloride; and even in a 10 per cent, solution the myosin can hardly be said 
to be dissolved, so viscid is the resulting fluid and with such difficulty does it filter. 
Fibrin again dissolves with great difficulty and very slowly in even a 10 per cent, 
solution of sodic chloride, and in a 1 per cent, solution it is practically insoluble. 
When it is remembered that fibrin and myosin are, both of them, the results of 
coagulation, their similarity is intelligible. Myosin is. in fact, a somewhat more 
soluble form of fibrin, deposited not in threads or filaments but in clumps and 
masses. 
Class V. Coagulated Proteids. 
These are insoluble in water, dilute acids and alkalies, and neutral saline solu- 
tions of all strengths. In fact, they are really soluble only in strong acids and strong 
alkalies, though prolonged action of even dilute acids and alkalies will effect some 
solution, especially at high temperatures. During solution in strong acids and 
alkalies a destructive decomposition takes place, but some amount of acid- or alkali- 
albumin is always produced. 
Very little is known of the chemical characteristics of this class. They are pro- 
duced by heating to 70° C. solutions of egg- or serum-albumin, globulins, suspended 
in water or dissolved in saline solutions; by boiling fora short time fibrin suspended 
in water or dissolved in saline solutions, or precipitated acid- and alkali-albumin sus- 
pended in water. They are readily converted at the temperature of the body into 
peptones by the action of gastric juice in an acid, or of pancreatic juice in an alka- 
line medium. 
All proteids in solutions are precipitated by an excess of strong alcohol. If the 
precipitant be rapidly removed they are again soluble in water, but if the precip- 
itated proteids are subjected for some time to the action of the alcohol they are, 
with the exception of peptones, coagulated and lose their solubility. It appears, 
however, that the proteids contained in the aleurone-grains of plants are exceed- 
ingly resistant to this coagulating action of alcohol.1 
It seems scarcely necessary to point out the distinction in the use of the word " coagulation " 
as applied to blood- or muscle plasma on the one hand and to the action of heat and alcohol upon 
proteids on the other. The difference is obvious when it is remembered that in the first case the 
coagulation leads to the formation of fibrin (Class iv.), or myosin (Class in.), and that these bodies 
may then further be coagulated by heat or alcohol as described above. 
Class VI. Peptones.2 
Very soluble in water, and not precipitated from their aqueous solutions by the 
addition of acids or alkalies, or by boiling. Insoluble in alcohol, they are precip- 
itated with difficulty by this reagent, and are unchanged m the process; they differ 
from all other proteids in not being coagulated by prolonged exposure to alcohol. 
They are not precipitated by cupric sulphate, ferric chloride, or, except in the in- 
stances to be mentioned presently, by potassic ferrocyanide, and acetic acid. In 
these points they differ from most other proteids. On the other hand, precipita- 
1 See Vines, Journ. of Physiol., vol. iii. p. 108. 
[2 The albumoses are so closely related to peptones that they are commonly considered together 
with peptones as a class.] 876 
APPENDIX. 
tion is caused by chlorine, iodine, tannin, mercuric chloride, nitrates of mercury and 
silver, and both acetates of lead ; also by bile-acids in an acid solution. In common 
with all proteids, these bodies possess a specific laevo-rotatory power over polarized 
light; but they differ from all other proteids in the fact that boiling produces no 
change in the amount of rotation. 
A solution of peptones, mixed with a strong solution of caustic potash, gives, 
on the addition of a mere trace of cupric sulphate, a pink color. An excess of the 
cupric salt gives a violet color, which deepens in tint on boiling, in fact the ordinary 
proteid reaction. Other proteids simply give the violet color. But the most cha- 
racteristic feature of peptones is their relatively great diffusibility, a property which 
they alone, of all the proteids, may be said to possess, since all other forms of pro- 
teids pass through membranes with the greatest difficulty, if at all. 
The diffusibility of peptones is, however, absolutely small as compared with that of crystalline 
bodies such as sodic chloride; in fact, solutions of peptones may be freed from salts by dialysis, a 
process employed in their preparation. 
Notwithstanding their probable formation in large quantities in the stomach and 
intestine, to judge from the results of artificial digestion, a very small quantity only 
can be found in the contents of these organs. They are probably absorbed as soon 
as formed. Another point of interest is their reconversion into other forms of pro- 
teids, since this must occur to a great extent in the body. We are, however, as yet 
ignorant of the manner in which this reverse change is effected. 
Preparation. All proteids with the exception of lardacein, yield peptones 
(and other products) on treatment with acid gastric or alkaline pancreatic juice 
most readily at the temperature of the human body. Peptones are likewise 
produced in the absence of pepsin and trypsin, by the action of dilute and mode- 
rately strong acids at medium temperatures, also by the action of distilled water at 
high temperatures under pressure. For various methods of preparing peptones, see 
Maly,1 Adamkiewicz,2 Henninger,3 and Pekelharing.4 
It appears possible to reobtain ordinary coagulable proteids from peptones by the action of 
either prolonged heating to 140°-170° C. or of dehydrating agents.6 
No difference in percentage composition between peptones and the proteid from 
which they are formed has, at present, been definitely established. 
We have used the word "peptones" in the plural number because we have 
reason to think that more than one kind of peptone exists. Meissner6 described 
three peptones, naming them respectively A- B- and C-peptone. lie distinguished 
them as follows: A-peptone is precipitated from its aqueous solutions by concen- 
trated nitric acid, and also by potassic ferrocyanide in the presence of even weak 
acetic acid. B-peptone is not precipitated by concentrated nitric acid, nor will 
potassic ferrocyanide give a precipitate unless a considerable quantity of strong 
acetic acid be added at the same time. C-peptone is precipitated neither by nitric 
acid nor by potassic ferrocyanide and acetic acid, whatever be the strength of the 
acetic acid. In place, however, of speaking of all these as peptones, it is better to 
consider C-peptone as the only real peptone, and the A- and B-peptones as not 
peptones at all. Nevertheless we have reason, from the researches of Kuhne, to 
speak of more than one peptone, viz., of a hemipeptone which is capable under the 
action of trypsin of being converted into leucin and tyrosin, and of an antipeptone 
which resists such a decomposition. The name antipeptone is given to the latter 
on account of this resistance which it offers toward trypsin; the name hemipep- 
tone, given to the former, signifies that this peptone is the twin or correlative half 
of anti peptone. 
We have seen (p. 255) that when any proteid is digested with pepsin, what we 
may preliminarily call a by-product makes its appearance. This by-product wdiich 
has many resemblances to acid-albumin or syntonin, appearing as a neutralization 
precipitate soluble in dilute acids and alkalies but insoluble in distilled water, is 
i Ptiiiger's Arch., Bd. ix. (1874), S. 585. 
2 Die Natur n. Nahwerth d. Peptons (1877), S. 33. 
3 De la Nature et du R61e physiologiques des Peptones, Paris, 1878. 
4I'fiuger's Arch., Bd. xxii. (1882), S. 185. 
6 Henninger, loc. cit.; Hofmeister, Zeitsch. f. physiol. Chem., Bd. ii. (1878), S. 206 ; Pekelharing, 
loc. cit. 
6 Zeitsch. f. rat. Med., Bde. vii., viii., x., xii. u. xiv. 877 
CHEMICAL BASIS OF THE ANIMAL BODY. 
generally spoken of as parapeptone. According to Finkler1 this neutralization 
precipitate is especially abundant if the pepsin be previously modified by exposure 
to a temperature of 40° to 60° C. The pepsin thus modified is spoken of by Fink- 
ler as "isopepsin." Many authors regard parapeptone, syntonin, and acid-albu- 
min as being the same thing. Meissner, however, gave the name parapeptone to a 
body which need not and probably does not make its appearance during normal 
natural digestion or during artificial digestion with a thoroughly active pepsin, but 
which is formed when proteids are subjected to the action of weak hydrochloric 
acid, either alone or in company with an imperfectly acting pepsin, and which in 
certain characters is quite distinct from ordinary syntonin or acid-albumin. Its 
distinguishing feature is that it cannot be changed into peptone by the action of 
even the most energetic pepsin, though it is really so converted under the influ- 
ence of trypsin; otherwise it very closely resembles syntonin. We have here an 
indication that the simple characters by which we have described acid-albumin 
may be borne by bodies having marked differences from each other. The re- 
searches'of Kithne2 have thrown an important light on these differences. The 
fundamental notion of Kiihne's view is that an ordinary native albumin or fibrin 
contains within itself two residues, which he calls respectively an anti-residue and 
a hemi-residue. The result of either peptic or tryptic digestion is to split up the 
albumin or fibrin, and to produce on the part of the anti-residue antipeptone, and 
on the part of the hemi-residue hemipeptone, the latter being distinguished from 
the former by its being susceptible of further change by tryptic digestion into 
leucin, tyTosin, etc. Antipeptone remains as antipeptone even when placed under 
the action of the most powerful trypsin, provided putrefactive changes do not 
intervene. 
Before the stage of peptone (whether anti- or hemi-) is reached, there is an in- 
termediate stage corresponding to the formation of syntonin. In both normal 
peptic and tryptic digestion antipeptone is preceded by an anti-albumose and 
hemipeptone by a hemi-albumose. Of these the anti-albumose is closely related 
to syntonin, and has hitherto been regarded as syntonin. The hemi-albumose 
has not been so frequently observed; it was, however, isolated by Meissner; it is 
apparently the body called by him A-peptone. It possesses several peculiar feat- 
ures. If its solutions are heated they partially coagulate at about 60°-63° C. : the 
precipitate is soluble at about 70° C. and is re precipitated as the temperature 
again falls. It also yields a precipitate with nitric acid and potassic ferrocyanide, 
and this also is soluble at the higher temperature, re precipitating on cooling. In 
these respects it closely resembles a proteid body observed by Bence-Jones in the 
urine of osteomalacia. It approaches myosin in being readily soluble in a 10 per 
cent, solution of sodic chloride. 
If, however, albumin be digested with insufficient or with imperfectly active 
pepsin, or simply with dilute hydrochloric acid at 40° C., anti-albumose is not 
formed, but in its place a body makes its appearance which Kithne calls anti-albu- 
inate.3 Its characteristic property is that it cannot be converted by peptic digestion 
into peptone, though it can be so changed by tryptic digestion. It is in fact the 
parapeptone of Meissner. 
It may perhaps be advisable, now that Meissner's parapeptone is cleared up, to 
reserve the name parapeptone for the initial products of both peptic and tryptic 
digestion, and to speak of anti-albumose and hemi-albumose as being both para- 
peptones. But in this sense parapeptone will be an intermediate and not a collat- 
eral product of digestion. 
Meissner also described a particularly insoluble form of his parapeptone as dys- 
peptone, and another intermediate product as a metapeptone ; but further investi- 
gation of both these bodies, as well as his B-peptone, is necessary. Under the 
influence of dilute hydrochloric acid, anti-albumate becomes changed into a body 
which Kithne calls anti-albumid, and which seems identical with the very insoluble 
proteid described by Schiitzenberger as "hemiprotein," and probably with Meiss- 
ner's dyspeptone. The same body is produced at once in company with products 
belonging to the hemi-groun by the action of 3 to 5 per cent, sulphuric acid on 
native albumin or fibrin. The following tables show the relations and genesis of 
1 Pfliiger's Archiv, Bd. xiv. (1887), 8.128. 
2 Only a short account of these has as yet been published. Verhandl. d. Naturhist.-med. 
Verein, Heidelberg, Bd. i. Heft 4,1876. 
3 An albumate must not be confounded with an albuminate. 878 
APPENDIX. 
the bodies we have just described. The several products (antipepton^, etc.) are 
given in duplicate, on the hypothesis (which, though not proved, is probable) that 
the changes of digestion are essentially hydrolytic changes,1 accompanied by a re- 
duplication ; that, just as a molecule of starch splits up into at least two molecules 
of dextrose, or as a molecule of cane-sugar splits up into a molecule of dextrose 
and a molecule of levulose, so a molecule of anti-albumose, for instance, splits up 
into two molecules of antipeptone, and so on. But the whole scheme is, of course, 
only provisional. 
Action of Pepsin. 
Decomposition of Proteids by Digestion. 
Special action of Trypsin. 
Albumin. 
Antipeptone. 
Anti-albumose. 
Antipeptone. 
Hemi-albumose. 
Leucin. 
Hemipeptone. 
Tyrosin, 
Leucin. 
Hemipeptone. 
Tyrosin, 
etc. 
etc. 
Decomposition by Acids. 
By 0.25 per cent. HC1 at 40° C. 
Albumin. 
Anti-albumate. 
Hemi-albumose. 
Anti-albumid. 
Hemipeptone. 
Hemipeptone. 
By 3-5 per cent. H2SO< at 100° C. 
Albumin. 
Anti-albumid. 
Hemi-albumose. 
Hemipeptone. 
Hemipeptone. 
Leucin, Tyrosin, etc. 
Leucin, Tyrosin, etc. 
Class VII. Lardacein, or the so-called Amyloid Substance. 
The substance to which the above name is applied is found as a pathological 
deposit in the spleen and liver, also in numerous other organs, such as the blood- 
vessels, kidneys, lungs, etc. 
It is insoluble in water, dilute acids and alkalies, and neutral saline solutions. 
In centesimal composition it is almost identical with other proteids,2 viz. : 
0 and S. 
H. 
N. 
C. 
24.4 
7.0 
15.0 
53.6 
The sulphur in this body exists in the oxidized state, for boiling with caustic 
potash gives no sulphide of the alkali. The above results of analysis would lead 
at once to the ranking of lardacein as a proteid, and this is strongly supported by 
other facts. Strong hydrochloric acid converts it into acid-albumin, and caustic 
alkalies into alkali-albumin. On the other hand it exhibits the following marked 
differences from other proteids. It wholly resists the action of ordinary digestive 
fluids; it is colored red, not yellow, by iodine, and violet or pure blue by the joint 
action of iodine and sulphuric acid. From these last reactions it has derived one 
of its names, "amyloid," though this is evidently badly chosen ; for not only does 
it differ from the starch group in composition, but by no means can it be converted 
1 Henninger, loc. cit., p. 49. 
2 C. Schmidt, Ann. d. Chem. u. Pharm., Bd. ex. S. 250, and Friedreich u. Kekudl, Virchow's 
Archiv, Bd. xvi. S. 50. CHEMICAL BASIS OF THE ANIMAL BODY. 
879 
into sugar: this latter is one of the crucial tests for a true member of the carbo- 
hydrate group. According to Heschl1 and Cornil,2 anilin-violet (methyl-anilin) 
colors lardaceous tissue rosy red, but sound tissue blue. 
The colors mentioned above as being produced by iodine and sulphuric acid are much clearer 
and brighter when the reagents are applied to the purified lardacein. When the reagents are 
applied to the crude substance in its normal position in the tissues, the colors obtained are always 
dark and dirty looking. 
Purified lardacein is readily soluble in moderately dilute ammonia, and can. by 
evaporation, be obtained from this solution in the form of tough, gelatinous Hakes 
and lumps; in this form it gives feeble reactions only with iodine. If the excess 
of ammonia is expelled, the solution becomes neutral, and is precipitated by dilute 
acids. 
Preparation. The gland or other tissue containing this body is cut up into small 
pieces, and as much as possible of the surrounding tissue removed. The pieces are 
then extracted several times with water and dilute alcohol, and if not thus rendered 
colorless are repeatedly boiled with alcohol containing hydrochloric acid. The 
residue after this operation is digested at 40° C., with good artificial gastric juice in 
excess. Everything, except lardacein and small quantities of mucin, nuclein, 
keratin, together with some portion of the elastic tissue, will thus be dissolved and 
removed.3 From the latter impurities it may be separated by decantation of the 
finely-powdered substance. 
The chief products of the decomposition of proteids are ammonia, carbonic 
anhydride, leucin, and tyrosin. Several other bodies, for the most part, like leucin, 
amidated acids, such as aspartic acid, glutamic acid, etc., have also been obtained; 
also by tryptic digestion, hypoxanthin, and perhaps xanthin. But urea has never 
yet been derived by direct decomposition from proteid material, the statements to 
this effect having been based on errors. In spite of numerous researches, we cannot 
at present state definitely what is the real constitution of a proteid, or in what 
manner these several residues are contained in the undecomposed substance. It is 
unnecessary to give here any of the formulae, nearly all empirical, which have been 
made to represent a proteid; they all give with equal exactitude the percentage 
composition, but beyond this they are untrustworthy. Of the various attempts 
which have been made to assign to proteids some definite molecular structure, none 
appear, at the present stage of information, sufficiently reliable for general accept- 
ance. 
Among the most elaborate labors in this direction may be mentioned those of Hlasiwetz and 
Haberman. In their first publication,4 starting from the' general similarity of tbe products of 
decomposition of the proteids and carbohydrates, they tried to establish a definite relation bet ween 
the two classes of bodies. In this they were not successful, and in their second research5 they 
came to the conclusion that the carbohydrates take no part in the formation of the proteids. 
Other experiments in the same direction have been made by Schiitzenberger.6 He shows that 
albumin can be decomposed into carbonic anhydride and ammonia, and that the ratio of these two 
is the same as though urea had been the body on which he operated. From this he concludes that 
*• the molecule of albumin contains the grouping of urea and represents a complex ureide." In 
his second publication7 he confirms his previous results, stating that the ammonia, carbonic anhy- 
dride, and oxalic acid, produced by the decomposition of proteids, are so connected quantitatively 
as to be capable of derivation from varying proportions of urea and oxamide. He also obtained 
from the decomposition of proteids a nitrogenous residue which could be formulated as giving 
rise to all the amidated acidsand other bodies spoken of above. Thus, according to him, albumin, 
built up as a complex ureide, decomposes into ammonia, carbonic, oxalic, and acetic acids, and 
this nitrogenous body ; this last then gives rise to the other products of decomposition.8 
It will be noticed that in the general description of the various proteids distinc- 
tive reactions for each could not be given, but that varying solubilities were the 
chief means at our disposal for distinguishing them. They may be arranged 
according to their solubilities in the following tabular form: 
1 Wien. med. Wochenschr., No. 32, S. 714. 
2 Compt. Rend., T. Ixxx. (1875), p. 1288. 
# Kiihne und Rudneff, Virchow's Archiv, Rd. xxxiii. (1865), S. 66. 
4 Ann. d. Chem. u. Pharm., Bd. clix. S. 304. 6 Ibid., Bd. clxix. 8.150. 
6 Comptes Rendus, T. Ixxx. (1875), p. 232. Bull, de la Soc. Chim. xxiii., 161,193, 216, 242, 385, 
483, xxiv. 2 et 145. 
7 Comptes Rendus, T. Ixxxi. p. 1108. Bull, de la Soc. Chim. xxv. 147. 
8 See also Schiitzenberger, Ann. de Chim. et de Phys., T. xvi. (1879), p. 280. 880 
APPENDIX. 
Soluble in distilled water- 
Aqueous solution not coagulated on boiling .... Peptones. 
Aqueous solution coagulated on boiling Albumins. 
Insoluble in distilled water: 
Soluble in NaCl solution 1 per cent Globulins. 
Soluble in HC1 0.1 per cent, in the cold ) Acid- and Alkali- 
Insoluble in NaCl solution 1 per cent. . j albumin. 
Insoluble in HC1 0.1 per cent, in the cold, but soluble ) . 
at 60° C ...... \Fibinn. 
Insoluble in HC1 0.1 per cent, at 60° C. ; soluble in strong acids. 
Soluble in gastric juice Coagulated albumin. 
Insoluble " " Lardacein. 
Such a classification is, however, obviously a wholly artificial one, useful for tem- 
porary purposes, but in no way illustrating the natural relations of the several 
members. Nor is a division into "native" and "derived" proteids much more 
satisfactory. It is true that we may thus put together serum- and egg-albumin, 
with vitellin, myosin, and fibrin, on the one hand; and peptones, coagulated pro- 
teids, and acid- with alkali-albumin, on the other. But in what light are we to 
consider casein, seeing that, though a natural product, it has so many resemblances 
to alkali-albumin? Moreover, the system of classification must be useless wThich 
would place fibrinoplastic globulin and fibrinogen in the same class as fibrin, and 
yet we can hardly speak of either of the two former bodies as derived proteids. If 
the view be true that when fibrin is converted into peptone the large molecule of 
the former is split up, with assumption of water, into two smaller molecules of the 
latter, one belonging to the " anti " and the other to the " hemi " group, we might 
speculate on a possible classification of all proteids into hemi-proteids, anti- 
proteids, and holo-proteids. Thus serum- and egg-albumin, myosin, and fibrin 
would be undoubtedly holo-proteids, peptones either anti- or hemi-proteids, and 
we should have to distinguish probably in the heterogeneous group of derived 
albumins both anti-, hemi-, and holo-proteid members. It is possible, moreover, 
that fibrinoplastic and fibrinogenous globulin and casein may be natural hemi- or 
anti-proteids, and not holo-proteids. But we have at present no positive knowledge 
on these points. 
[Enzymes. 
Enzymes are unorganized soluble ferments, such as ptyalin, pepsin, trypsin, 
amylopsin, rennin, fibrin-ferment, etc., and have been considered in various parts 
of this work. ] 
These resemble the proteids in many general points, but exhibit among them- 
selves much greater differences than do the proteids. As regards their molecular 
structure nothing satisfactory is known. Their percentage composition approaches 
that of the proteids, and like these they yield, under hydrolytic treatment, large 
quantities of leucin and in some cases tyrosin. They are all amorphous. 
Nitrogenous Non-crystalline Bodies allied to Proteids. 
Mucin. (0, 35.75. H, 6.81. N, 8.50. C, 48.94.)1 
The characteristic component of mucus. Its exact composition is not yet known, 
the figures given above being merely an approximation. 
As occurring in the normal condition it gives to the fluids which contain it the 
well-known ropy consistency, and can be precipitated from these by acetic acid, 
alcohol, alum, and mineral acids; the latter, if in excess, redissolve the pre- 
cipitate. but this is not the case with acetic acid. In its precipitated form it is 
insoluble in water, but swells up strongly in it, and this effect is increased by the 
presence of many alkali salts. Alkalies and alkaline earths dissolve it readily. 
Its solutions do' not dialyse; they give the proteid reactions with Millon's 
reagent and nitric acid, but not that with sulphate of copper, and are precipitated 
by basic lead acetate only when neutral or faintly alkaline. According to Eichwald,2 
i Eichwald, Ann. d. Cheni. u. Pharm., Bd. cxxxiv. S. 198. 2 Op. cit. CHEMICAL BASIS OF THE ANIMAL BODY. 
881 
when heated with dilute mineral acids, mucin yields acid-albumin, and another body 
which in many of its properties closely resembles a sugar, inasmuch as it reduces 
solutions of cupric sulphate. Prolonged boiling with sulphuric acid gives leucin 
and about 7 per cent, of tyrosin. 
Preparation.1 Ox-gall or an aqueous extract of finely chopped submaxillary 
gland is acidulated with acetic acid; the precipitated mucin is then washed with 
water, dissolved in dilute sodic carbonate and finally precipitated with acetic acid. 
It may also be obtained from snails.'2 
Chondrin. (0,31.04. H, 6.76. N, 13.87. C, 47.74. S, 0.60 per cent.) 3 
This is usually regarded as forming the essential part of the matrix of hyaline 
cartilage, and is contained in the interstices of the fibres in elastic cartilage. A 
similar substance can be prepared from the cornea. Boiled with water, it dissolves 
slowly, forming an opalescent solution, which is precipitated by acetic acid, lead 
acetate, dilute mineral acids, alum, and salts of silver and copper; an excess of the 
last four reagents redissolves the precipitate. Solutions of this body gelatinize on 
standing, even if very dilute; the solid mass is insoluble in cold water, readily 
soluble in hot water, alkalies, and ammonia. 
The aqueous and alkaline solutions of chondrin possess a left-handed rotatory 
power on polarized light of-213.5°; in presence of excess of alkali this be- 
comes -552.0°, both measured for yellow light.4 
It seems, according to the observations of many, that chondrin can, by heating 
with hydrochloric acid, be converted into a body whose reactions resemble those of 
syntonin, and another substance, which like the similar product from mucin, so far 
resembles grape-sugar that it reduces cupric salts in alkaline solution ;5 it appears, 
however, to contain nitrogen. The existence of chondrin as a distinct substance 
has, however, been denied6 on the supposition that it is in all cases a mere mixture 
of other bodies. It is stated that a substance having all the reactions of the so- 
called chondrin, may at any time be produced by a mixture of mucin, glutin, and 
inorganic salts. The extreme similarity in the reactions of chondrin and mucin 
point to a close relationship between the two. The whole subject, however, requires 
more complete investigation. With alkalies or dilute sulphuric acid chondrin gives 
leucin, but no tyrosin or glycin. Whether chondrin exists as such in cartilage is 
uncertain; it seems probable that it does not, since its extraction from cartilage re- 
quires an amount of boiling with water much greater than that requisite to dissolve 
dried chondrin. 
Preparation. From cartilage by extracting with water and precipitating with 
acetic acid. 
[Chondrin is not probably a distinct substance, but a mixture of mucin and 
gelatin.] 
Gelatin or Glutin.7 (0, 23.21. H, 7.15. N, 18.32. C, 50.76. S, 0.56 
per cent.) 
This is the substance which is yielded when connective-tissue fibres are heated 
for several days with very dilute acetic acid, at a temperature of about 15° C., or 
by the prolonged action of water in a Papin's digester. The elastic elements of 
connective tissue are unaffected by the above treatment. 
As obtained in this way glutin is when heated a thin fluid, solidifying on cooling 
to the well-known gelatinous form. When dried it is a colorless, transparent, brittle 
body, swelling up, but remaining undissolved in cold water; heating, or the addition 
of traces of acids or alkalies, readily effects its solution. When dissolved in water 
it possesses a laevo-rotatory power of-130°, at 30° C. ; the addition of strong alkali 
or acetic acid reduces this to-112° or-114°, both measured for yellow light.8 Its 
solutions will not dialyse. 
Mercuric chloride and tannic acid are the only two reagents which yield insoluble 
1 Eichwald, op. cit. and Chem. Centralb., 1866, No. 14. Staedeler, Ann. d. Chem. u. Pharm., Bd. 
Cxi. S. 14. Landwehr, Zeitschr. f. physiol. Chem., Bd. v. (1881), S. 371. 
2 Landwehr, Zeitschr. f. physiol. Chem., Bd. vi. (1882), S. 75. 
31, v. Mering, Beitrag zur Chemie des Knorpels, 1873. 
4 Hoppe-Seyler, Hdb. phys. path. chem. Anal., Anti. 1875, S. 262. 
6 De Bary, Hoppe-Seyler's Untersuch., Hft. 1, S. 71. 
6 Morochowetz, Verh'and. Naturhist.-med. Ver., Heidelberg, Bd. i. (1876), Hft. 5. 
' Not to be confounded with the vegetable proteid "gluten." 
8 Hoppe-Seyler, Hdb. d. phys. path. chem. Anal., 4 Aufl. 1875, S. 222. 882 
APPENDIX. 
precipitates with this body. Its presence prevents the action of Trommer's sugar- 
test, since it readily dissolves the precipitated cuprous oxide. The proteid reactions 
of glutin are so feeble that they are probably due merely to impurities. Heated 
with sulphuric acid it yields ammonia, leucin, and glycin, but no tyrosin. 
[When gelatin is digested it undergoes alterations similar to those of fibrin and 
albumin, forming gelatoses and gelatin-peptones, but the results of recent investi- 
gation indicate that while gelatin is valuable as a food-stuff for furnishing energy, 
it is not of value for the growth of the nitrogenous tissues. 
It appears improbable that glutin exists ready-formed in connective-tissue fibres, 
since these do not swell up in water, and only yield glutin after prolonged treat- 
ment with boiling water; to which it may be added that while glutin is acted upon 
by trypsin, the connective-tissue fibres in their natural condition resist its action 
(see p. 284). When glutin is submitted for some time to the action of dilute hydro- 
chloric acid at 38° C., and the change is brought about even more readily by the 
action of pepsin, it loses its power of gelatinizing and is now diffusible through 
porous membranes: the name of gelatin-peptone has been given to the product 
thus obtained.1 
Elastin. (O, 20.5. H, 7.4. N, 16.7. C, 55.5 per cent.) 
This characteristic component of elastic fibres is left on the removal of all the 
glutin, mucin, etc., from such tissues as "ligamentum nuchae," advantage being 
taken of its not being altered when it is heated with water, even under pressure, 
with strong acetic acid, or with dilute alkalies. When moist it is yellow and elastic, 
but on drying becomes brittle. It is soluble in strong alkalies at boiling tempera- 
tures, and concentrated sulphuric and nitric acids dissolve it even in the cold ; it is 
also dissolved by the action of papaya juice. It is precipitated from solutions by 
tannic acid, but not by the addition of ordinary acids. Notwithstanding that it 
closely approaches the proteids in its percentage composition, and gives distinct 
although feeble proteid reactions, any very close relationship between the two ap- 
pears improbable, since elastin when treated with sulphuric acid, yields leucin (30-40 
per cent.) only and no tyrosin. 
Hilger2 has obtained a similar body from the shell membrane of snakes' eggs. 
Keratin.3 (0, 20.7-25.0. H, 6.4-7.0. N, 16.2-17.7. C, 50.3-52.5. S, 
0.7-5.0 per cent.) 
This body, though somewhat resembling the proteids in general composition, 
differs from them and also from the preceding bodies so widely in other properties, 
that its description is placed here for convenience rather than anything else. Hair, 
nails, feathers, horn, and epidermic scales consist for the most part of keratin. 
Heated with water in a digester at 150° C. keratin is partially dissolved with evo- 
lution of sulphuretted hydrogen; the solution then gives with acetic acid and fer- 
rocyanide of potassium a precipitate soluble in excess of the acid. Prolonged boiling 
with alkalies and acids, even acetic, dissolves keratin ; the alkaline solutions evolve 
sulphuretted hydrogen on treatment with acids. The sulphur in keratin is evi- 
dently very loosely united to the substance, and in all its reactions there appears to 
be a want of similarity between keratin and either proteids, mucin or gelatin. The 
most common of its products of decomposition are leucin (10 per cent.) and tyrosin 
(3.6 percent.), and some aspartic acid; no glycin is formed. What is generally 
known as keratin is probably a compound body, which has not yet been resolved 
into its components. 
Ewald and Kijhne4 have described a new body to which, since it occurs as a constituent of 
nervous tissue (both of nerves and of the central nervous system), and is yet closely identical with 
ordinary horny tissue, they give the name of neuro-keratin. It is prepared in quantity from the 
brain by extracting this tissue with alcohol and ether, and subjecting the residue to the action of 
pepsin and trypsin. The final residue is neuro-keratin, and amounts to 15 to 20 per cent, of the 
original tissue. 
Nuclein. C29H49N 9P3O22. 
Discovered by Miescher5 in the nuclei of pus corpuscles and in the yellow cor- 
puscles of yolk of egg. Other observers have subsequently obtained it from yeast, 
1 Hofmeister, Zeitschr. f. physiol. Chem., Bd. il. (1878), S. 299. 
2 Ber. d. Deutsch. Chem. Gesellsch., 1873. S. 166. But see also next reference. 
3 Lindwall, " Nagra bidrag till kann. om. Ker.. Upsala Lakarefs. forb. xvi. (1881), p. 546. 
* Verhand. Naturhist.-mea. Ver., Heidelberg. Bd. i. (1876), Heft 5. 
3 Med.-chem. Untersuch., Hoppe-Seyler, Heft 4, 1872, S. 441 u. 502. 883 
CHEMICAL BASIS OF THE ANIMAL BODY. 
from semen, from the nuclei of the red blood-corpuscles of birds and amphibia, 
from hepatic cells, and it is probably present in all nuclei. 
When newly prepared it is a colorless amorphous body, soluble to a slight ex- 
tent in water, readily soluble in many alkaline solutions; but its solubilities alter on 
keeping. If added gradually in sufficient quantity to a solution of caustic alkali 
it first neutralizes the solution and then renders it acid. It seems to possess an 
indistinct xantho-proteic reaction, but gives no reaction with Millon's fluid. It 
yields precipitates with several salts, e. g., zinc chloride, argentic nitrate, and cupric 
sulphate. 
Preparation,1 Since nuclein is very resistant to the action of pepsin, it may be 
obtained from the granular residue consisting chiefly of nuclei, which occurs after 
digesting pus with pepsin. The most remarkable feature of this body is its large 
percentage of phosphorus, 9.59 per cent. This phosphorus is readily separated 
by boiling with strong hydrochloric acid or caustic alkalies ; the same occurs when 
solutions of nuclein are acidulated and allowed to stand. 
Although not found as a constituent of any mammalian tissue, this substance 
composes the chief part of the exo-skeleton of many invertebrates. It may 
probably be regarded as the animal analogue of the cellulose of plants, and from 
this point of view it possesses considerable morphological interest. Both cellulose 
and chitin appear to yield some form of sugar when treated with strong acids. 
When purified, chitin is a white amorphous body, often retaining the shape of 
the tissue from which it has been prepared. It is insoluble in all reagents except 
strong mineral acids, the best solvents being sulphuric or hydrochloric acids. The 
immediate addition of water to these solutions reprecipitates the chitin in an un- 
altered form; but the prolonged action of sulphuric acid causes a decomposition 
resulting, according to some observers, in the formation of an amorphous ferment- 
able carbohydrate; and when hydrochloric acid is used an amidated carbohydrate 
is obtained to which the name of glycosamin3 (C6H13NO3) has been given. 
Preparation.* The cleansed exo-skeleton of a lobster is thoroughly extracted 
with dilute hydrochloric acid and then with caustic soda. To purify it finally it is 
submitted to prolonged boiling with a solution of potassic permanganate. 
Chitin. C15HMN2O10.2 
[Nucleo-albumin. 
This term is applied to a class of substances which seem to be composed of 
nuclein and an albuminous proteid. Casein is probably a nucleo-albumin, for when 
subjected to peptic digestion a residue of nuclein is obtained.] 
CARBOHYDRATES. 
Certain members only of this class occur in the human body; of these the most 
important and widespread are those known as glycogen and the two sugars, grape- 
sugar or dextrose (glucose), with which diabetic sugar seems to be identical,5 and 
maltose. Next to these comes milk-sugar. Inosit is another body of this class, 
although it differs in many important points from the preceding two. 
Sugars are often considered to be polyatomic alcohol. Several of them stand in peculiar rela- 
tion to mannit, and may be converted into that substance by the action of sodium amalgam.6 
[When solutions of sugars are warmed in the presence of phenyl-hydrazin and 
dilute acetic acid, amorphous or crystalline substances separate, which are termed 
osazones. These differ according to the different sugars.] 
Occurs in the contents of the alimentary canal to a variable extent dependent 
on the nature of the food taken. It is also a normal constituent of blood, chyle, 
and lymph. Concerning its presence in the liver, see p. 538. The amniotic fluid 
1. Dextrose (grape-sugar). C6H12O6+H2O. 
1 See Kossel, Zeitschr. f. physiol. Chem., Bd. iii. (1879), S. 284 ; iv. (1880), S. 290 ; vii. (1883), S. 7. 
" Untersuch. uber d. Nuclein u. ihre Spaltungsprod, Strassb., 1881. 
2 Ledderhose. Zeitschr. f. physiol. Chem., Bd. ii. (1878), S. 213. 
3 Ledderhose, loc. cit., Bd. iv. (1880), S. 139. 
4 Biitschli, Arch. f. Anat. u. Physiol., Jahrg., 1874, S. 362. 
6 The question, however, whether several varieties of sugar occurring in the animal body have 
not been confounded together under the common name of dextrose or glucose may be consid- 
ered at present an open one. 
6 Linnemann, Ann. d. Chem. u. Pharm., Bd. cxxiii., S. 136. 884 
APPENDIX. 
also contains this body. Bile in the normal condition is free from sugar, so also is 
urine, though this point has given rise to great dispute.1 The disease diabetes is 
characterized by an excess of dextrose in the fluids and tissues of the body (see d. 
448). 
When pure, dextrose is colorless and crystallizes from its aqueous solution in 
six-sided tables or prisms, often agglomerated into warty lumps. The crystals will 
dissolve in their own weight of cold water, requiring, however, some time for the 
process; they are very readily soluble in hot water. Dextrose is somewhat spar- 
ingly soluble in alcohol, and crystallizes from anhydrous alcohol in prisms free from 
water of crystallization; it is. moreover, insoluble in ether. 
The freshly prepared cold aqueous solution of the crystals possesses a dextro-rotatory power of 
+104° for yellow light. This, quickly on heating, more slowly on standing, falls to +56°, at 
which point it remains constant. 
Dextrose readily forms compounds with acids and many salts ; the latter are very unstable, 
decomposition rapidly ensuing on heating them. When its metallic compounds are decom- 
posed, the decomposition is, in many cases, accompanied by the precipitation of the metals, e. g., 
silver, gold, mercury, bismuth. Caustic alkalies readily decompose them, as also does ammonia. 
Dextrose is readily and completely precipitated by lead acetate and ammonia. 
An important property of this body is its power of undergoing fermentations. 
Of these the two principal are: 1. Alcoholic. This is produced in aqueous solu- 
tions of dextrose, under the influence of yeast. The decomposition is the follow- 
ing: CfiH,2O6 = 2C2H6O 4-2CO2, yielding (ethyl) alcohol and carbonic anhydride. 
Other alcohols of the acetic series are found in traces, as also are glycerin, succinic 
acid, and probably many other bodies. The fermentation is most active at about 
25° C. Below 5° 0. or above 45° C. it almost entirely ceases. If the saccharine 
solution contains more than 15 per cent, sugar it will not all be decomposed, as 
excess of alcohol stops the reaction. 2. Lactic. This occurs in the presence of 
decomposing nitrogenous matter, especially of casein, and is probably the result 
of the action of a specific ferment.2 The first stage is the production of lactic acid, 
C6H12O6= 2C3H6O3. In the second butyric acid is formed with evolution of hydro- 
gen and carbonic anhydride: 2C3H8O3 = C4H8O2+2CO2+4H. The above changes, 
the first of which is probably undergone by sugar to a considerable extent in the 
intestine, are most active at 35° C. ; the presence of alkaline carbonates is also 
favorable. It is, moreover, essential that the lactic acid should be neutralized as 
fast as it is formed, otherwise the presence of the free acid stops the process. 
The preparation, detection, and estimation of dextrose are so fully given in 
various books that they need not be detailed here. 
2. Maltose. C12H.,2O11+H,O. 
This form of sugar was first described by Dubrunfaut3 as a product of the action 
of malt extract on starch. Its existence was for a long time doubted until O'Sul- 
livan 4 repeated and confirmed the previous experiments. According to him it 
crystallizes in fine acicular crystals, possesses a specific rotatory power of + 150° and 
a reducing power which is only one-third as great as that of dextrose. It seems 
probable that this is the chief sugar obtained by the action not only of diastase 
but of ptyalin and pancreatic ferment upon starch and perhaps also upon glycogen ;5 
although some dextrose may at the same time be formed. Musculus and Gruber6 
have shown that maltose may also be formed by the action of dilute sulphuric acid 
on starch, and that it is capable of undergoing alcoholic fermentation. 
Preparation. See Musculus and Gruber (loc. cit.). 
3. Milk-sugar. C12HwOn-l-H2O. 
Also known as lactose. It is found in milk, and is characteristic of this secre- 
tion. It is said, however, to occur abnormally in the urine of lying-in women.7 
It yields, when pure, hard, colorless crystals, belonging to the rhombic system 
(four-sided prisms). It is less soluble in water than dextrose, requiring for solu- 
1 See Seegen, Der Diabetes Mellitus, 2 ed., S. 196. 
2 Lister. Path. Soe. Trans., vol. for 1878, p. 425, also Quart. Journ. of Micros. Sci., vol xviii. 
(1878), p. 177. 
3 Ann. Chim. Phys. (3) xxi. (1847), p. 178. 
4 Journ. Chem. Soe . Ser. 2, vol. x. (1872), p. 579. 
5 Musculus u. v. Mering, Zeitschr. f. physiol. Chem., Bd. ii. (1878), S. 403. 
6 Zeitschr. f. physiol. Chem., Bd. ii. (1878), S. 177. 7 Hofmeister, Ibid., Bd. I. (1877), S. 101. 885 
CHEMICAL BASIS OF THE ANIMAL BODY. 
tion six times its weight of cold, but only two parts of boiling, water ; it is entirely 
insoluble in alcohol and ether. It is fully precipitated from its solutions by the 
addition of lead acetate and ammonia. 
When freshly dissolved, its aqueous solution possesses a specific dextro-rotatory power of +93.1° 
for sodium light': this diminishes slowly on standing, rapidly on boiling, until it finally remains 
constant at 4-52.5°. The amount of rotation is independent of the concentration of the solution. 
Lactose unites readily with bases, forming unstable compounds; from its metallic compounds 
the metal is precipitated in the reduced state on boiling; it reduces copper salts as readily as 
dextrose, but to a less extent, viz., in the ratio of 70 : 100. 
Lactose is generally stated to. admit of no direct alcoholic fermentation; this 
may, however, sometimes be induced by the prolonged action of yeast. By boiling 
with dilute mineral acids lactose is converted into galactose, which readily undergoes 
alcholic fermentation and possesses a greater rotatory power than lactose. 
It mav be remarked here that though isolated lactose is incapable of direct alcoholic fermenta- 
tion,milk'itself may be fermented. Berthelot was unable in this direct alcoholic fermentation 
to detect any intermediate change of the lactose into any other fermentable sugar. 
Lactose is, however, directly capable of undergoing the lactic and butyric fer- 
mentation ; the circumstances and products are the same as in the case of dextrose 
(see above). The action is generally productive of a collateral small quantity of 
alcohol. 
Lactose is thus distinguished from dextrose by its smaller solubility in water, 
insolubility in alcohol, crystalline form, lower cupric oxide reducing power, and its 
incapability of undergoing direct alcoholic fermentation. 
Preparation. After the removal of the casein and other proteids of the milk, 
the mother-liquor is evaporated to the crystallizing point; the crystals are purified 
by repeated crystallization from warm water. 
4. Inosit. C6H12O6 + 2H2O. 
This substance occurs but sparingly in the human body; it was found originally 
by Scherer1 in the muscles of the heart. Cloetta showed its presence in the lungs, 
kidneys, spleen, and liver,2 and Muller in the brain.3 It occurs, also, in diabetic 
urine and in that of " Bright's disease," and is found in abundance in the vegetable 
kingdom. 
Pure inosit forms large efflorescent crystals (rhombic tables); in microscopic 
preparations it is usually obtained in tufted lumps of fine crystals. Easily soluble 
in water, it is insoluble in alcohol and ether. It possesses no action on polarized 
light, and does not reduce solutions of metallic salts. 
It admits of no direct alcoholic, but is capable of undergoing the lactic, fermen- 
tation ; according to Hilger,4 the acid formed is sarcolactic. It is unaltered by 
heating with dilute mineral acids. 
Preparation. It may be precipitated from its solutions by the action of basic 
lead acetate and ammonia; the lead is then removed by sulphuretted hydrogen, and 
the inosit precipitated with excess of alcohol. 
As a special test (Scherer's) may be mentioned the production of a bright violet 
color by careful evaporation to dryness on platinum foil, with a little ammonia and 
calcium chloride. 
5. Dextrin. C6H10O5. 
By boiling starch-paste with dilute acids, or by the action of ferments, the starch 
is converted into an isomeric body, to which, from its action on polarized light, the 
name dextrin has been given. It is soluble in water, but is precipitated by alcohol. 
It does not undergo alcoholic fermentation until after it has been changed into 
dextrose, nor can it reduce metallic salts. It yields a reddish port-wine color with 
iodine, which disappears on warming and does not return on cooling. Further 
action of acids or of ferments converts dextrin into dextrose. Dextrin is present 
in the contents of the alimentary canal after a meal containing starch, and has also 
been found in the blood. 
There is not the least doubt that several modifications of dextrin exist, and may 
be obtained by the action of acids and ferments on starch. Of these two of the 
1 Ann. d. Chem. u. Pharm., Bd. Ixxiii., S. 322. 2Ibid., Bd. xcix.. S. 289. 
3 Ibid., Bd. ciii., S. 140. 4 Ibid., Bd. clx., S. 333. 886 
APPENDIX. 
best known are those described by Briicke 1 under the name of erythrodextrin and 
achrobdextrin. the former giving a red color with iodine, the latter not yielding any 
color at all. Erythrodextrin may be readily converted into a sugar by the action of 
ferments, and thus is not found as a product of the complete action of ptyalin on 
starch. Achrobdextrin, on the other hand, is not thus converted by ferments, and 
therefore remains in solution, together with the sugar formed by the action of 
ptyalin on starch. Achrobdextrin may be converted into dextrose by boiling with 
dilute hydrochloric acid. 
6. Glycogen. C6H10O5. 
Belongs to the starch division of carbohydrates. Discovered by Bernard in the 
liver and other organs (see p. 437). 
Glycogen is, when pure, an amorphous powder, colorless and tasteless, readily 
soluble in water, insoluble in alcohol and ether. Its aqueous solution is generally, 
though not always, strongly opalescent, but contains no particles visible microscopi- 
cally ; the opalescence is much reduced by the presence of free alkalies. The same 
solution possesses, according to Hoppe-Seyler, a very strong dextro-rotatory power, 
about three times as great as that of dextrose ;2 it dissolves hydrated cupric oxide ; 
but this is not reduced on boiling. 
By the action of dilute mineral acids (except nitric) it is partially converted into 
a form of sugar very closely resembling, though probably differing somewhat from, 
true dextrose, and the same conversion is also readily effected by the action of amy- 
lolytic ferments. The sugar into which the glycogen of the liver is naturally con- 
verted after death (see p. 438) appears to be true dextrose;3 so also the sugar of 
diabetes. The result of the action of diastase, or salivary or pancreatic ferment, 
upon glycogen is, however, according to Musculus and v. Mering,4 a mixture of 
achrobdextrin and maltose; the quantity of dextrose making its appearance at the 
same time being very small. 
Opalescent solutions of glycogen usually become clear on the addition of caustic 
alkali; Vintschgau and Dietl5 have shown that this is accompanied on boiling by 
a change which converts a portion of the glycogen into a substance to which they 
gave the name of /3-glycogen-dextrin. (Kiihne® had previously described a body 
to which he gave the name glycogen-dextrin. That described by Vintschgau and 
Dietl differs slightly from Kiihne's body, hence the name. According to these 
authors one-fifth of the glycogen is at the same time changed into some other, at 
present undetermined, substance. Normal lead acetate gives a cloudiness, the basic 
salt a precipitate, in solutions of glycogen. 
As tests for this body may be used the formation of a port-wine color with 
iodine; this disappears on warming, but returns on cooling. The same color is 
produced by the action of iodine on dextrin, but this does not reappear on cooling 
after its disappearance by warming. 
Preparation of glycogen. The following is Briicke's7 method: The filtered or 
simply strained decoction of perfectly fresh liver or other glycogenic tissue is, when 
cold, treated alternately with dilute hydrochloric acid and a solution of the 
double iodide of potassium and mercury8 as long as any precipitate occurs. In 
the presence of free hydrochloric acid the double iodide precipitates proteid mat- 
ters so completely as to render their separation by filtration easy. The proteids 
being thus got rid of, the glycogen is precipitated from the filtrate by adding 
alcohol to the extent of between 60 and 70 per cent. Too much alcohol is to be 
avoided, since other substances as well as glycogen are thereby precipitated. The 
glycogen is now washed with alcohol, first of 60 and then of 95 percent., after- 
ward with ether, and finally with absolute alcohol. It is then dried over sulphuric 
acid. 
1 Sitzber. d. Wien. Akad., 1872, iii. Abth. Also, Vorlesungen, 2 Aufl., 1875, Bd. i., S. 224. 
2See Kulz, Pfliiger's Arch., Bd. xxiv. (1881), S. 85. 
3 Pfliiger's Arch., Bd. xix. (1879), S. 106, and xxii. (1880), S. 206. Also, Kulz, Ibid., Bd. xxiv. 
(1881), S. 52. 
4 Zeitsehr. f. physiol. Chern., Bd. ii. (1878), S. 403. 
5 Pfliiger's Arch'., Bd. xvii. (1878), S. 154. 
6Lehrb. d. physiol. Chem. (1868). 8. 63. 
7 Sitzungsber. d. Wiener Akad., Bd. Ixiii. (1871), ii. Abth. 
8 This may be prepared by precipitating potassic iodide with mercuric chloride and dissolving 
the washed precipitate in a hot solution of potassic iodide as long as it continues to be taken up. 
On cooling, some amount of precipitate occurs, which must be filtered off; the filtrate is then 
ready for use. CHEMICAL BASIS OF THE ANIMAL BODY. 
887 
[7. Animal Gum. 
This carbohydrate is found in milk and urine, and is prepared by the action of 
superheated water on the salivary glands. J 
8. Tunicin. (C6H10O5)n. 
This body is regarded by many observers as identical with the true cellulose of 
plants, while others have ascribed to it properties differing from those of cellulose 
sufficiently to justify its receiving a distinct name. It appears to be more resistant 
to the action of chemical reagents than plant cellulose. 
Lt constitutes the chief part of the integument of the ascidia or tunicata. As 
prepared from this source it is, when pure, quite white, and usually retains the shape 
of the tissue. It is unacted upon by any reagent except strong acids and alkalies, 
and by the action of the former it yields some form of sugar. 
FATS, THEIR DERIVATIVES AND ALLIES. 
The Acetic Acid Series. 
General formula, CnH2nO2 (monobasic). 
This, which is one of the most complete homologous series of organic chemistry, 
runs parallel to the series of monatomic alcohols. Thus formic acid corresponds to 
methyl alcohol, acetic acid to ethyl (ordinary) alcohol, and so on. The several acids 
may be regarded as being derived from their respective alcohols by simple oxida- 
tion; thus ethyl alcohol yields by oxidation acetic acid C2H6O fl- O2 = C2H4O2 + 
H2O. The various members differ in composition by CII2, and the boiling-points 
rise successively by about 19° C. Similar relations hold good with regard to their 
melting-points and specific gravities. The acid properties are strongest in those 
where n has the least value. The lowest members of the series are volatile liquids, 
acting as powerful acids; these successively become less and less fluid, and the 
highest members are colorless solids, closely resembling the neutral fats in outward 
appearance. Consecutive acids of the series present but very small differences of 
chemical and physical properties, hence the difficulty of separating them ; this is 
further increased in the animal body by the fact that exactly those acids which 
present the greatest similarities usually occur together. 
The free acids are found only in small and very variable quantities in various 
parts of the body; their derivatives, on the other hand, form most important con- 
stituents of the human frame, and will be considered further on. 
Formic Acid. CHO.OH. 
When pure is a strongly corrosive, fuming fluid, with powerful irritating odor, 
solidifying at 0° C., boiling at 100° C., and capable of being mixed in all propor- 
tions with water and alcohol. It has been obtained from various parts of the body, 
such as the spleen, thymus, pancreas, muscles, brain, and blood; in the latter its 
presence may be due to the action of acids on the haemoglobin. According to some 
authors,1 it occurs only in urine. 
Heated with sulphuric acid it yields carbonic oxide and water; with caustic 
potash it gives hydrogen and oxalic acid. 
Acetic Acid. C2H3O.OH. 
Is distinguished by its characteristic odor; its boiling-point is 117° C.; it solid- 
ifies at 5°, and is fluid at all temperatures above 15° C. It is soluble in all propor- 
tions in alcohol and water. 
It occurs in the stomach as the result of fermentative changes in the food, and 
is frequently present in diabetic urine. In other organs and fluids it exists only in 
minute traces. 
With ferric chloride it yields a blood-red solution, decolorized by hydrochloric acid. (It differs 
in this last reaction from sulphocyanide of iron.) Heated with alcohol and sulphuric acid, the 
characteristic odor of acetic ether is obtained. It does not reduce silver nitrate. 
1 Buliginsky, Hoppe-Seyler's Med. chem. Mittheilung, Heft 2, S. 240. Thudichum, Journ. 
Chem. Soc., vol. viii. p 400 888 
APPENDIX. 
Propionic Acid. C3H5O.OH. 
This acid closely resembles the preceding one. It possesses a very sour taste and 
pungent odor; it is soluble in water, boils at 141° C., and may be separated from 
its aqueous solution by excess of calcic chloride. 
It occurs in small quantities in sweat, in the contents of the stomach, and in 
diabetic urine when undergoing fermentation. It is similarly produced, mixed, 
however, with other products, during alcoholic fermentation, or by the decompo- 
sition of glycerin. It partially reduces silver nitrate solution on boiling. 
Butyric Acid. C4H7O.OH. 
An oily, colorless liquid, with an odor of rancid butter, soluble in water, alcohol, 
and ether, boiling at 162° C. Calcic chloride separates it from its aqueous solution. 
Found in sweat, the contents of the large intestine, feces, and in urine. It oc- 
curs in traces in many other fluids, and is plentifully obtained when diabetic urine is 
mixed with powdered chalk and kept at a temperature of 35° C. It exists as a 
neutral fat in small quantities in milk. 
An oily liquid, of penetrating odor and burning taste; soluble in 30 parts of 
water at 12° C. ; readily soluble in alcohol and ether. Boils at 175° C.; possesses, 
in free and combined form, a feeble right-handed rotation of the plane of polariza- 
tion. 
It is found in the solid excrements, and is formed readily by the decomposition, 
through putrefaction, of impure leucin, ammonia being at the same time evolved ; 
hence its occurrence in urine when that fluid contains leucin, as in cases of acute 
atrophy of the liver. 
Valerianic Acid. C5II9O.OH. 
Caproic Acid. C6HnO.OH. 
Caprylic Acid. C8H15O OH. 
Capric (Rutic) Acid. CloH19O.OH. 
These three occur together (as fats) in butter and are contained in varying pro- 
portions in the feces from a meat diet. The first is an oily fluid, slightly soluble in 
water; the others are solids and scarcely soluble in water; they are soluble in all 
proportions in alcohol and ether. They may be prepared from butter, and sepa- 
rated by the varying solubilities of their barium salts. 
Laurostearic Acid. C1?H23O.OH. 
Myristic Acid. C14H27O.OH. 
These occur as neutral fat in spermaceti, in butter, and other fats. They pre- 
sent no points of interest. 
Palmitic Acid. CWH31O.OH. 
Stearic Acid. CI8II35O.OH. 
These are solid, colorless when pure, tasteless, odorless crystalline bodies, the 
former melting at 62° C., the latter at 69.2° C. Tn water they are quite insoluble; 
palmitic acid is more readily soluble in cold alcohol than stearic; both are readily 
dissolved by hot alcohol, ether, or chloroform. Glacial acetic acid dissolves them 
in large quantity, the solution being assisted by warming. They readily form soaps 
with the alkalies, also with many other metals. The varying solubilities of their 
barium salts afford the means of separating them when mixed ; 1 this may also be 
applied to many others of the higher members of this series. 
These acids in combination with glycerin (see below), together with the analo- 
gous compound of oleic acid, form the principal constituents of human fat. As 
salts of calcium they occur in the feces and in " adipocere," and probably in chyle, 
blood, and serous fluids, as salts of sodium. They are found in the free state in 
decomposing pus, and in caseous deposits of tuberculosis. 
The existence of margaric acid intermediate to the above two is not now admitted, since 
Heintz2 has shown that it is really a mixture of palmitic and stearic acids. Margaric acid pos- 
sesses the anomalous melting-point of 59.9° C A mixture of 60 parts stearic and 40 of palmitic 
acid melts at 60.3° 
1 Heintz, Annal. d. Phys. u. Chern.. Bd. xclii., S. 588. s Op. cit. CHEMICAL BASIS OF THE ANIMAL BODY. 
889 
Many acids of this series occur as glycerin compounds in various fats. They are 
very unstable and readily absorb oxygen when exposed to the air. The higher 
members are decomposed on attempting to distil them. Their most peculiar prop- 
erty is that of being converted by traces of N02 into solid, stable metameric acids 
capable of being distilled. They bear an interesting relation to the acids of the 
acetic series, breaking up when heated with caustic potash into acetic acid and 
some other member of the same series, thus: 
Acids of the Oleic (Acrylic) Series. 
H(CnH2n-8)O2 (monobasic). 
Oleic acid. 
hc18hS3o2 + 
2KH0 
Potassic acetate. 
kc2h3o2 + 
Potassic palmitate. 
kc16h31o2 
+ H, 
Oleic Acid. C18H33O.OH. 
This is the only acid of the series which is physiologically important. It is found 
united with glycerin in all the fats of the human body. 
When pure it is, at ordinary temperatures, a colorless, odorless, tasteless, oily 
liquid, solidifying at 4° C. to a crystalline mass. Insoluble in water, it is soluble 
in alcohol and ether. It cannot be distilled without decomposition. It readily 
forms soaps with potassium and sodium, which are soluble in water; its com- 
pounds with most other bases are insoluble. It may be distinguished from the 
acids of the acetic series by its reaction with NO2 and by the changes it under- 
goes when exposed to the air. 
These may be considered as ethers formed by replacing the exchangeable atoms 
of hydrogen in the triatomic alcohol glycerin (see below) by the acid radicles of the 
acetic and oleic series. Since there are three such exchangeable atoms of hydrogen 
in glycerin, it is possible to form three classes of these ethers; only those, however, 
which belong to the third class occur as natural constituents of the human body: 
those of the first and second are of theoretical importance only. 
They possess certain general characteristics. Insoluble in water and cold alcohol, 
they are readily soluble in hot alcohol, ether, chloroform, etc.; they also dissolve 
one another. They are neutral bodies, colorless and tasteless when pure, are not 
capable of being distilled without undergoing decomposition, and yield as a result 
of this decomposition solid and liquid hydrocarbons, water, fatty acids, and a pecu- 
liar body, acrolein. (Glycerin contains the elements of one molecule of acrolein and 
two molecules of water.) 
They possess no action on polarized light. 
They may readily be decomposed into glycerin and their respective fatty acids by 
the action of caustic alkalies or of superheated steam. 
The Neutral Fats. 
Palmitin (Tri-palmitin). 
^160-36U /3 
O3. 
The following reaction for the formation of this fat is typical for all the others: 
Glycerin. 
C3HJ 0 
63 J°3 
Palmitic acid. 
+ 3C16§31° J O 
Palmitin. 
C,HS 1 n 
""(C»HnO)« i 
+ 3h}°- 
Palmitin is slightly soluble in cold alcohol, readily so in hot alcohol, or in ether; 
when pure it crystallizes in fine needles; if mixed with stearin, it generally forms 
shapeless lumps, although the mixture may at times assume a crystalline form, and 
was then regarded as a distinct body, namely, margarin. It possesses three differ- 
ent melting-points, according to the previous temperatures to which it has been 
subjected. It solidifies in all cases at 45° 0. 
Preparation. From palm oil, by removing the free palmitic acid with alcohol 
and crystallizing repeatedly from ether. 
Stearin (Tri-stearin), 
03. 
This is the hardest and least fusible of the ordinary fats of the body, is also the 
least soluble, and hence is the first to crystallize out from solutions of the mixed 890 
APPENDIX. 
fats. It crystallizes usually in square tables. It presents peculiarities in its fusing- 
points similar to those of palmitin. 
Preparation. From mutton suet, its separation from palmitin and olein being 
effected by repeated crystallization from ether, stearin being the least soluble. 
Olein (Tri-olein). (C|^5°)3 
O3. 
It is obtained with difficulty in the pure state, and is then fluid at ordinary 
temperatures. It is more soluble than the two preceding ones. It readily under- 
goes oxidation when exposed to the air, and is converted by mere traces of N02 
into a solid isomeric fat. Olein yields, on dry distillation, a characteristic acid, 
the sebacic, and is saponified with much greater difficulty than are palmitin and 
stearin. 
Preparation. From olive oil, either by cooling to 0° C. and pressing out the 
olein that remains fluid, or by dissolving in alcohol and cooling, when the olein 
remains in solution while the other fats crystallize out. 
r n 
Glycerin. 5 
O3. 
This principal constituent of the neutral fats may, as above stated, be looked 
upon as a triatomic alcohol. 
When pure, glycerin is a viscid, colorless liquid, of a well-known sweet taste. It 
is soluble in water and alcohol in all proportions, insoluble in ether. Exposed to 
very low temperature it becomes almost solid; it may be distilled in close vessels 
without decomposition, between 275°-280° C. 
It dissolves the alkalies and alkaline earths, also many oxides, such as those of 
lead and copper; many of the fatty acids are also soluble in glycerin. 
It possesses no rotatory power on polarized light. 
It is easily recognized by its ready solubility in water and alcohol, its insolubility 
in ether, its sweet taste, and its reaction with bases. The production of acrolein is 
also characteristic of glycerin. 
C3H8Os - 2H2O - C3H4O (Acrolein). 
Preparation. By saponification of the various oils and fats. It is also formed 
in small quantities during the alcoholic fermentation of sugar.1 
Soaps. These may be formed by the action of caustic alkalies on fats. The 
process consists in a substitution of the alkali for the radicle of glycerin, the latter 
combining with the elements of water to form glycerin. Thus: 
Tristearin. 
(C,«H„O),) o 
C,H5 J °» 
+ 8h}° = 
Potassic stearate. 
3^18^35^ | 0 
Glycerin. 
+ c»H4o 
+ H, J0'- 
Pancreatic juice can split up fats into glycerin and free fatty acids, and the bile is known to 
be capable of saponifying these fatty acids. The amount of soaps formed in the alimentary 
canal is, however, small and unimportant. 
Acids of the Glycolic Series. 
Running parallel to the monatomic alcohols (C„H2a+2O) is the series of diatomic 
alcohols or glycols (CnH„+2O2). Thus corresponding to ethyl alcohol is the dia- 
tomic alcohol, ethyl-glycol. As from the monatomic alcohols, so from the glycols, 
acids may be derived by oxidation; from the latter (glycols), however, two series 
of acids can be obtained, known respectively as the glycolic and oxalic series. 
The first stage of oxidation of the glycol gives a member of the glycolic series. 
Thus: 
Ethyl-glvcol. 
C2H6O2 
+ O2 = 
Glycolic acid. 
CJIA 
+ H20, or more generally 
CnH2n+ 2Oa + O2 - CnH2nO3 + H2O. 
1 Pasteur, Ann. d. Chem. u. Pharm., Bd. cvi., S. 338. CHEMICAL BASIS OF THE ANIMAL BODY. 
891 
By further oxidation a member of the glycolic series can be converted into a 
member of the oxalic series. Thus : 
Glycolic acid, 
c2h4o3 
+ O2 = 
Oxalic acid 
mo. 
+ H20, or more generally 
CnH2nO3 + O2 = CnH2n-2O4 + HSO. 
The acids of the glycolic series are diatomic but monobasic; but those of the 
oxalic series are diatomic and dibasic. 
The following table may be given to show the general relationship of alcohols 
and acids: 
Radicle. 
Alcohol. 
Acid. 
Glycols. 
Acid I. 
Acid II. 
Methyl (CH,) 
CH/OH) 
Formic. 
hcho2 
Acetic. 
Ethyl-glycol. 
C2H4(OH)2 
Propyl-glycol. 
C3H6(OH)2 
Butyl-glycol. 
C4H8(OH)2 
Carbonic. 
h2co3 
Glycolic. 
Oxalic. 
Ethyl (C2H5) 
C2H5(OH) 
hc2h3o2 
Propionic. 
hc2h3o3 
Lactic. 
h2c4o4 
Malonic. 
Propyl (C3H7) 
c3h7(0H) 
hc3h502 
Butyric. 
hc3h5o3 
Oxybutyric. 
h2c3h2o4 
Succinic. 
Butyl (C4H9) 
C4H9(OH) 
HC4H7O2 
ch5h7o3 
h2c4h4o4 
Lactic Acid. C3H6O3. 
Glycolic Acid Series. 
Next to carbonic acid, the most important member of this series, as far as physi- 
ology is concerned, is lactic acid. 
Lactic acid exists in four isomeric modifications, but of these only three have 
been found in the human body. These three all form syrupy, colorless fluids, solu- 
ble in all proportions in water, alcohol, and ether. They possess an intensely sour 
taste, and a strong acid reaction. When heated in solution they are partially dis- 
tilled over in the escaping vapor. They form salts with metals, of which those 
with the alkalies are very soluble and crystallize with difficulty. The calcium and 
zinc salts are of the greatest importance, as will be seenelater on. 
1. Ethylidene-lactic acid. This is the ordinary form of the acid, obtained as 
the characteristic product of the well-known "lactic fermentation." It occurs in 
the contents of the stomach and intestines. According to Heintz.1 it is found also 
in muscles, and according to Gscheidlen 2 in the ganglionic cells of the gray sub- 
stance of the brain. In many diseases it is found in urine, and exists in a large 
amount in this excretion after poisoning by phosphorus.3 
It may be prepared by the general methods of slowly oxidizing the corresponding glycol or by 
acting on monochlorinated propionic acid with moist silver oxide. In obtaining it from the pro- 
ducts of lactic fermentation, the crusts of zinc lactate are purified by several crystallizations, and 
the acid liberated from the compounds by the action of sulphuretted hydrogen. 
2. Ethylene-lactic acid. This acid is found accompanying the next to be 
described, in the watery extract of muscles.4 From this it is separated by taking 
advantage of the different solubilities in alcohol of the zinc salts of the two acids. 
It seems probable, however, that it has not yet been prepared in the pure state by 
this method. 
Wislicenus first obtained this acid by heating hydroxycyanide of ethylene with aqueous solu- 
tions of the alkalies. 
The same observer found it also in many pathological fluids. 
3. Sarcolactic acid. This acid has not yet been procured synthetically. As 
its name implies, it is that form of the acid which chiefly occurs in muscles, and 
1 Ann. d. Chem. u. Pharm., Bd. clvii., S. 320. 
2 Pfliiger's Archiv. Bd. viii. (1873-74), S. 171. 
3 Schultzen and Riess, Ueber acute Phosphorvergiftung. Chem. Centralb., 1869, 8. 681. 
4 Ann. d. Chem. u. Pharm., Bd. cxxviii., S. 6. 892 
APPENDIX. 
hence exists in large quantities in Liebig's "extract of meat." It is often found 
also in pathological fluids. This is the only acid of the series which possesses any 
power of rotating the plane of polarized light; it is otherwise indistinguishable 
from the preceding ethylidene-lactic acid, and is generally represented by the same 
formula. The free acid has dextro-, the anhydride laevo-rotatory action. The 
specific rotation for the zinc salt in solution is -7.65° for yellow light. 
[Fig. 231. 
[Fig. 232. 
Zinc Sarcolactate. (After Kiihne.)] 
Calcium Sarcolactate. (After Kiihne.)] 
The zinc and calcium salts [Figs. 231, 232] for sarcolactic acid are more soluble, 
both in water and alcohol, than those of ethylidene-lactic acid, but less so than 
those of ethylene lactic acid, and the same salts of ethylene-lactic acid contain 
more water of crystallization than those of the other two. 
Heintz1 has compared the above acids to the modifications capable of existing in tartaric 
acid" 
Hydracrylic acid, the fourth in this series of lactic acids, is distinguished by the nature of its 
decomposition on heating. It is never found as a constituent of animal bodies. 
Oxalic Acid Series. 
Oxalic Acid. H2C2d4. 
In the free state this acid does not occur in the human body. Calcic oxalate, 
however, is a not unfrequent constituent of urine, and enters into the composition 
[Fig. 233. 
Calcium Oxalate.] 
of many urinary calculi, the so-called mulberry calculus consisting almost entirely 
of it. It may occur in feces, and in the gall-bladder, though this is rarely observed. 
1 Op. cit. 
- See, further. Wislicenus, op. cit. Also Ann. d. Chem. u. Pharm., Bd. clxvi., S. 3; Bd. clxvii., 
S. 302, and Zeitschr. f. Chem., Bd. xiii., S. 159. 893 
CHEMICAL BASIS OF THE ANIMAL BODY. 
As ordinarily precipitated from solutions of calcic salts by amnionic oxalate, 
calcic oxalate is quite amorphous, but in urinary deposits it assumes a strong cha- 
racteristic crystalline form, viz., that of rectangular octohedra [Fig. 233]. In some 
cases it presents the anomalous forms of rounded lumps, dumb-bells, or square 
columns with pyramidal ends. It is insoluble in water, alcohol, and ether, also in 
ammonia and acetic acid. Mineral acids dissolve this salt readily, as also to a smaller 
extent do solutions of sodic phosphate or urate. All the above characteristics serve 
to detect this salt; its microscopical appearance, however, is generally of most use 
for this purpose. 
The pure acid is prepared either by oxidizing sugar with nitric acid, or decom- 
posing ligneous tissue with caustic alkalies. 
Succinic Acid. H2C4H4O4. 
This is the third acid of the oxalic series, being separated from oxalic acid by 
the intermediate malonic acid, H2C3H,O4. It occurs in the spleen, the thymus, 
and thyroid bodies, hydrocephalic and hydrocele fluids. 
According to Meissner and Shepard1 it is found as a normal constituent of urine. This is con- 
tested by SaTkowski,2 and also by v. Speyer. It seems probable, however, that since wines and 
fermented liquors contain succinic acid, and this latter passes unchanged into the urine, that it 
may thus be occasionally present in this excretion. 
Succinic acid crystallizes in large rhombic tables, also at times in the form of 
large prisms; they are soluble in 5 parts of cold water, and 2.2 of boiling, slightly 
soluble in alcohol, and almost insoluble in ether. The crystals melt at 180° C., 
and boil at 236° C., being at the same time decomposed into the anhydride and 
water. The alkali salts of this acid are soluble in water, insoluble in alcohol and 
ether. 
Preparation. Apart from the synthetic methods, it may readily be obtained by 
the fermentation of calcic malate, acetic acid being produced simultaneously. 
Its presence is recognized by the microscopic examination of its crystals, and 
its characteristic reaction with normal lead acetate. With this it gives a precip- 
itate, easily soluble in excess of the precipitant, but coming down again on warm- 
ing and shaking.3 
Cholesterin. (C26H44O.) 
This is the only alcohol which occurs in the human body in the free state. 
(The triatomic alcohol glycerin is almost always found combined as in the fats; 
and cetyl-alcohol, or aethal, is obtained only from spermaceti.) It is a white crys- 
talline body, crystallizing in fine needles from its solution in ether, chloroform, or 
benzol; from its hot alcoholic solutions it is deposited on cooling in rhombic 
tables [Fig. 234]. When dried it melts at 145° C., and distils in closed vessels at 
360° C. It is quite insoluble in water and cold alcohol: soluble in solutions of bile 
salts 
Solutions of cholesterin possess a left-handed rotatory action on polarized light, 
of-32° for yellow light, this being independent of concentration and of the nature 
of the solvent. 
Heated with strong sulphuric acid it yields a hydrocarbon; with concentrated 
nitric it gives cholesteric acid and other products. It is capable of uniting with 
acids and forming compound ethers. 
Cholesterin occurs in small quantities in the blood and many tissues, and is pres- 
ent in abundance in the white matter of the cerebro-spinal axis and in nerves. 
It is a constant constituent of bile, forming frequently nearly the whole mass of 
some gall-stones. It is found in many pathological fluids, hydrocele, the fluid of 
ovarian cysts, etc. 
Preparation. From gall-stones by simple extraction with boiling alcohol, and 
treatment with alcoholic potash to free from extraneous matter. 
As tests for this substance may be given : With concentrated sulphuric acid and 
a little iodine a violet color is obtained, changing through green to red or blue. 
This is applicable to the microscopic crystals. After dissolving in chloroform a 
blood-red solution is formed on the addition of an equal volume of concentrated 
1 Untersuch. uber d. Entsteh. d. Hippursaiire. Hanover, 1866. 
2 Pfluger's Archiv, Bd. ii. (1869), S. 367, and Bd. iv. (1871), S. 95. 
3 For further particulars see Meissner, op. cit., and Meissner and Solly, Zeitschr f. rat. (Med 3) 
B. xxiv., S. 97. 894 
APPENDIX. 
sulphuric acid; this solution if exposed to the air in an open dish turns blue, 
green, and finally yellow; the sulphuric acid under the chloroform has a green 
[Fig. 234. 
Cholesterin Crystals and Fatty Aggregations and Molecules Spontaneously 
Deposited in the Urine.] 
fluorescence. After evaporation to dryness with nitric acid, the residue turns red 
on treating with ammonia. 
This body is described here rather for the sake of convenience than from its possessing any 
close relationship to the substances immediately preceding. 
Complex Nitrogenous Fats. 
Lecithin. C^H^NPO,. 
Occurs widely spread throughout the body. Blood, bile, and serous fluid con- 
tain it in small quantities, while it is a conspicuous component of the brain, nerves, 
yolk of egg, semen, pus, white blood-corpuscles, and the electrical organs of the ray. 
When pure it is a colorless, slightly crystalline substance, which can be kneaded, 
but often crumbles during the process. It is readily soluble in cold, exceedingly so 
in hot alcohol; ether dissolves it freely though in less quantities, as also do chloro- 
form, fats, benzol, carbon disulphide, etc. It is often obtained from its alcoholic 
solution by evaporation, in the form of oily drops. It swells up in water and in 
this state yields a flocculent precipitate with sodium chloride. 
Lecithin is easily decomposed ; not only does this decomposition set in at 70° C., 
but the solutions, if merely allowed to stand at the ordinary temperature, acquire 
an acid reaction, and the substance is decomposed. Acids and alkalies, of course, 
effect this much more rapidly. If heated with baryta water it is completely decom- 
posed, the products being neurin, glycerin-phosphoric acid, and baric stearate. This 
may be thus represented : 
Lecithin. 
C^NFO, 
Stearic acid. 
2C18H36O2 
Glycerin-phosphoric 
acid. 
+ c3h9po6 + 
Neurin. 
c5h15no2. 
+ 3H2O = 
When treated in an ethereal solution with dilute sulphuric acid, it is merely split 
up into neurin and distearyl-glycerin-phosphoric acid. Hence, Diakonow 1 regards 
lecithin as the distearyl-glycerin-phosphate of neurin, two atoms of hydrogen in the 
1 Hoppe-Seyler's Med.-Chern. Untersuch., Heft ii. (1867), S. 221; Heft iii. (1868), S. 405. Centralbl. 
f. d. med. Wiss. (1868), Nr. 1, 7, u. 28. 895 
CHEMICAL BASIS OF THE ANIMAL BODY. 
glycerin-phosphoric acid being replaced by the radicle of stearic acid. It appears 
also that there probably exist other analogous compounds in which the radicles of 
oleic and palmitic acids take part. 
Preparation. Usually from the yolk of egg, where it occurs in union with 
vitellin. Its isolation is complicated, and the reader is referred to Hoppe-Seyler.1 
Glycerin-phosphoric Acid. C3H9PO6. 
Occurs as a product of the decomposition of lecithin, and hence is found in those 
tissues and fluids in which this latter is present; in leukaemia the urine is said to 
contain this substance. It has not been obtained in the solid form. It has been 
produced synthetically by heating glycerin and glacial phosphoric acid; it may be 
regarded as formed by the union of one molecule of glycerin with one of phosphoric 
acid, with elimination of one molecule of water. It is a dibasic acid ; its salts with 
barium and calcium are insoluble in alcohol, soluble in cold water. Solutions of its 
salts are precipitated by lead acetate. 
Protagon. (C160H308N5PO35?) 
A crystalline body containing nitrogen and phosphorus, obtained by Liebreich2 
from the brain substance and regarded by him as its principal constituent. The 
researches of Hoppe-Seyler and Diakonow tended to show that protagon was 
merely a mixture of lecithin and cerebrin. A repetition of Liebreich's experiments 
has, however, led Gamgee and Blankenhorn3 to confirm the truth of his results. 
Protagon appears to separate out from warm alcohol on gradual cooling in the form 
of very small needles, often arranged in groups; it is slightly soluble in cold, more 
soluble in hot alcohol, and ether. It is insoluble in water, but swells up and forms 
a gelatinous mass. It melts at 200° C., and forms a brown syrupy fluid. 
Preparation. Finely divided brain substance, freed from blood and connective 
tissue, is digested at 45° C. with alcohol (85 percent.) as long as the alcohol extracts 
anything from it. The protagon which separates out from the filtrate is well 
washed with ether to get rid of all cholesterin and other bodies soluble in ether, 
and finally purified by repeated crystallization from warm alcohol. 
Neurin (Cholin). C5H15NO2. 
Discovered by Strecker4 in pig's gall, then in ox-gall. It does not occur in the 
free state except as a product of the decomposition of lecithin. It is a colorless 
fluid, of oily consistence, possesses a strong alkaline reaction, and forms with acids 
very deliquescent salts. The salts with hydrochloric acid and the chlorides of 
platinum and gold are the most important. 
Neurin is a most unstable body, mere heating of its aqueous solution sufficing to 
split it up into glycol, trimethylamin and ethylene oxide. 
Preparation. From yoke of egg. For this see Diakonow.5 
Wurtz6 has obtained it synthetically, first by the action of glycol hydrochloride on trimethyl- 
amin, and then by that of ethylene oxide and water on the same substance. The above, together 
with the mode of its decomposition, point to the idea that neurin may be regarded as trimethyl- 
oxyethyl-ammonium hydrate, N(CH3)3(C2H6O)OH. 
Is found in the axis-cylinder of nerves, in pus-corpuscles, and largely in the 
brain. Tn former times many names were given to the substance when in an 
impure state, e. g., cerebric acid, cerebrote, etc. W. Muller7 first prepared it in 
the pure form, and constructed the above formula from his analysis; the mean of 
these is 0, 15.85 ; H. 11.2; N, 4.5; C, 68.45. Great doubts are, however, thrown 
upon its purity by the researches of later observers. According to Liebreich8 and 
Diakonow,9 it is a glucoside.10 
Cerebrin. C17HS3NO3 (?). 
1 Med.-Chem. Untersuch., Heft ii. (1867), S. 215. 
2 Ann. d. Chem. u. Pharm., Bd. cxxxiv., 8. 29. 
3Zeitschr. f. physiol. Chem., Bd. iii. (1879), S. 260, and Journ. of Physiol., vol. ii. (1879), p. 113. 
4 Ann. d. Chem. u. Pharm., Bd. cxxiii., S. 353; Bd. cxlviii., S. 76. 
6 Op. cit. (sub. Lecithin). 6 Ann. d. Chem. u. Pharm., Sup. Bd. vi„ S. 116 u. 127. 
7 Ann. d. Chem. u. Pharm., Bd. cv., 8. 361. 
8 Arch. f. pathoi. Anat., Bd. xxxix. (1867). 
9Centrallb. f. d. med. Wiss., 1868, No. 7. 
10See also Geogheghan, Zeitschr. f. physiol. Chem., Bd. iii. (1879), S. 332. 896 
APPENDIX. 
Cerebrin is a light, colorless, exceedingly hygroscopic powder, which swells up 
strongly in water, slowly in the cold, rapidly on heating. When heated to 80° C. 
it turns brown, and at a somewhat higher temperature melts, bubbles up, and 
finally burns away. It is insoluble in cold alcohol or ether ; warm alcohol dissolves 
it easily. Heated with dilute mineral acids, cerebrin yields a sugar-like body, pos- 
sessing left-handed rotation, but incapable of fermentation. 
Preparation. For this see W. Muller.1 
[Charcot's Crystals. These crystals [Fig. 235], first discovered by Charcot, 
have been obtained from the semen, the blood of leukmmics, the expectoration of 
[Fig. 235. 
Charcot's Crystals.] 
asthmatics, and the various tissues. It is not identical with ethylinimine or piper- 
azine, as was supposed.] 
NITROGENOUS METABOLITES. 
The Urea Group, Amides, and Similar Bodies. 
Urea. (NH2)2CO. 
The chief constituent of normal urine in mammalia and some other animals; 
the urine of birds also contains a small amount. Normal blood, serous fluids, 
lymph, and the liver all contain the same body in traces. It is not found in the 
muscles as a normal constituent, but may make its appearance there under certain 
pathological conditions. 
[Fig. 236. 
Urea Crystals separated by slow evaporation from Aqueous Solution. (After Funke.)] 
When pure it crystallizes from a concentrated solution in the form of long, thin, 
glittering needles. [Fig. 236.| If deposited slowly from dilute solutions, the form 
is that of four-sided prisms with pyramidal ends; these are always anhydrous. It 
possesses a somewhat bitter cooling taste, like saltpetre. It is readily soluble in 
water and alcohol, the solutions being neutral. In anhydrous ether it is insoluble. 
1 Op. cit. CHEMICAL BASIS OF THE ANIMAL BODY. 
897 
The crystals may be heated to 120° C. without being decomposed; at a higher tem- 
perature they are first liquefied and then decomposed, leaving no residue. Heated 
with strong acids or alkalies, decomposition ensues, the final products being car- 
bonic anhydride and ammonia. The same decomposition may also occur as the 
result of the action of a specific ferment on urea in an aqueous solution.1 Nitrous 
acid at once decomposes it into carbonic anhydride and free nitrogen. It readily 
forms compounds with acids and bases; of these the following are of importance: 
Nitrate of Urea. (NH2)2CO.HNO3. 
Crystallizes in six-sided or rhombic tables [Fig. 237]. Insoluble in ether and 
nitric acid, soluble in water, slightly soluble in alcohol. 
[Fig. 237. 
Crystals of Nitrate of Urea. (Krukenberg, after Kuhne.)] 
Oxalate of Urea. [(NH2)2CO]2,H2C2O4+H2O. 
Often crystallizes in long thin prisms [Fig. 238], but under the microscope is 
obtained in a form closely resembling the nitrate; it is slightly soluble in water, 
less so in alcohol. 
[Fig. 238. 
Crystals of Oxalate of Urea. (Krukenberg, after Kuhne.)] 
With mercuric nitrate urea yields three salts, containing respectively, four, 
three, and two equivalents of mercuric oxide to one of urea. The first is the pre- 
1 Musculus, Pfliiger's Archiv, Bd. xii. (1876), S. 214. Jakscb, Zeitsch. f. physiol. Chem., Bd. v. 
(1881), S. 395. 898 
APPENDIX. 
cipitatc formed in Liebig's quantitative determination of urea, and may be repre- 
sented by the formula: 2N2H4CO.Hg(NO3)23HgO. The exact constitution of 
these salts has not yet been determined. 
Preparation. Ammonic sulphate and potassic cyanate are mixed together in 
aqueous solution, and the mixture is evaporated to dryness. The residue, when 
extracted with absolute alcohol, yields urea. From urine, either by evaporating to 
dryness, having previously precipitated the urine with normal and basic lead acetate 
in succession and removed the lead by sulphuretted hydrogen, and then extracting 
with alcohol; or concentrating only to a syrup, and then forming the nitrate of 
urea ; this is washed with pure nitric acid and decomposed with baric compound. 
Detection in solution. Tn addition to the microscopic appearance of the crys- 
tals obtained on evaporation, the nitrate and oxalate should be formed and exam- 
ined. Another part should give precipitate with mercuric nitrate, in the absence 
of sodic chloride, but not in the presence of this last salt in excess. A third por- 
tion is treated with nitric acid containing nitrous fumes; if urea is present, nitrogen 
and carbonic anhydride will be obtained. To a fourth part nitric acid in excess 
and a little mercury are added, and the mixture is warmed. In presence of urea 
a colorless mixture of gases (N and CO2) is given off. A fifth portion is kept 
melted for some time, dissolved in water, and cupric sulphate and caustic soda are 
added ; a red or violet color, due to biuret, is developed. 
Quantitative determination. For this some special manual must be con- 
sulted.1 It will suffice here to point out that the determination is made either with 
a solution of mercuric nitrate of known strength (Liebig) ; by decomposing the 
urea by means of sodic hypobromite into nitrogen, carbonic anhydride, and water, 
and measuring the nitrogen (Knop) [N2H4CO'+3NaBrO =3NaBr + CO2+2H2O 
+N2], or by heating the urea with caustic baryta in a sealed tube, the urea being 
determined by the weight of baric carbonate formed (Bunsen). 
Urea is generally considered to be an amide of carbonic acid, i. e., carbamide. 
The amide of an acid is formed when water is removed from the ammonium salt 
of the acid; if the acid be dibasic and two molecules of water be removed, the 
result is often spoken of as a diamide. Tims if from ammonic carbonate (NH4)2 
CO3, two molecules of water, 2H2O, be removed, carbonic acid being a dibasic 
acid, the result is urea; thus: 
(NH4)2CO3-2H2O = (NH2)2CO, 
which may be written either according to the ammonia type as 
co 
H2 
h2 
N2 
or as 
CO 
nh2 
nh2 
two atoms of amidogen (NH2) being substituted for two atoms of hydroxyl 
(HO). 
This connection between carbonic acid and urea is shown by the fact that 
amnionic carbonate may be formed out of urea by hydration, as when urea is 
subjected to the specific ferment mentioned above. Regarded, then, as a diamide 
of carbonic acid, urea may be spoken of as carbamide. But the theoretical deri- 
vation of urea from amnionic carbonate by dehydration cannot be realized in 
practice, whereas urea can readily be formed from amnionic carbamate, and 
Kolbe is inclined to regard it, not as the diamide of carbonic acid, but as the 
amide of carbamic acid. Ammonium carbamate, CO2N2Hfi minus H2O, gives 
urea, CO, N,, Ht-which, if carbamic acid be written as CO, OH, NH2, may be 
written as CO, NH2, NH2, one atom of amidogen being substituted for one atom 
of hydroxyl, and not two, as when the substance is regarded as derived from car- 
bonic acid. Drechsel's experiments indicate a ready derivation of urea from 
ammonic carbamate. He has obtained urea by the electrolysis of a solution of 
this salt with rapidly alternating currents, thus removing the elements of water 
1 Neubauer and Vogel, Analyse des Harus, viii. Aufl., 1881, S. 264. 899 
CHEMICAL BASIS OF THE ANIMAL BODY. 
from the carbamate by such alternating processes of oxidation and reduction as 
may be supposed to take place in the body. The reaction is expressed as fol- 
lows : 
1. NH.CO O.NH1 + O = NH2.CO 0 NH2 + H2O. 
2. NH2.CO.ONH2 + H2 = NH2.CO.NH2 + H2O. 
Wanklyn1 and Gamgee,2 however, since urea, when heated with a large excess 
of potassic permanganate, gives off all its nitrogen in a free state, and not in the 
oxidized form of nitric acid, as do all other amides, conclude that it is not an amide 
at all, that it is isomeric only and not identical with carbamide. 
It is important to remember that urea is also isomeric with ammonic cyanate, 
( N 
C • and, indeed, was first formed artificially by Wohler (1823) from this 
body. We thus have three isomeric compounds, ammonium cyanate, urea, and 
carbamide, related to each other in such a way that urea may be obtained readily 
either from ammonium cyanate or from ammonic carbamate, and may with the 
greatest ease be converted into ammonic carbonate.3 Now urea is a much more 
stable body than ammonic cyanate, and in the transformation of the latter into the 
former energy is set free ; and it is worthy of notice that, though the presence of 
sulphocyanides in the saliva probably indicates the presence of cyanic residues in 
the body, the nitrogenous products of the decomposition of proteids belong chiefly 
to the class of amides, cyanogen compounds being rare among them. Pfliiger4 has 
called attention to the great molecular energy of the cyanogen compounds, and 
has suggested that the functional metabolism of protoplasm by which energy is set 
free may be compared to the conversion of the energetic unstable cyanogen com- 
pounds into the less energetic and more stable amides. In other words, ammonium 
cyanate is a type of living, and urea of dead nitrogen, and the conversion of the 
former into the latter is an image of the essential change which takes place when 
a living proteid dies. 
Compound ureas. The hydrogen atoms of urea can be replaced by alcohol and acid radicles. 
The results are compound ureas or ureides when the hydrogen is replaced by an acid radicle. 
Many of them are called acids, since the hydrogen from the amide group, if not all replaced as 
above, can be replaced by metal. Thus, the substitution of oxalyl (oxalic acid) gives parabanic 
acid, 
co 
H» or CO, NH2, N.C2O2; 
C2O2 
n2< 
of tartronyl (tartronic acid), dialuric acid, CO, NH?, N.C3H2O3; of mesoxalyl (mesoxalic acid), 
alloxan, CO, NIL, N.C3O3. These bodies are interesting as being also obtained by the artificial 
oxidation of uric acid. (See below.) 
Uric Acid. C5H4N4O3. 
The chief constituent of the urine in birds and reptiles ; it occurs only sparingly in 
this excretion in man and most mammalia. It is normally present in the spleen, 
and traces of it have been found in the lungs, muscles of the heart, pancreas, brain, 
and liver. Urinary and renal calculi often consist largely of this body or its salts. 
In gout, accumulations of uric acid salts may occur in various parts of the body, 
forming the so-called gouty concretions. 
It is when pure a colorless, crystalline powder, tasteless, and without odor. 
The crystalline form is very variable, but usually tends toward that of rhombic 
tables.5 When impure it crystallizes readily, but then possesses a yellowish or 
brownish color. In water it is very insoluble (1 in 14,000 or 15,000 of cold water); 
ether and alcohol do not dissolve it appreciably. On the other hand, sulphuric 
acid takes it up without decomposition, and it is also readily soluble in many 
salts of the alkalies, as in the alkalies themselves. Ammonia, however, scarcely 
dissolves it. 
1 Arch. f. Physiol., 1880, S. 550. 
2 Journ. Chem. Soc., 2, vol. vi. p. 25. 
3 The following literature is interesting in connection with the question of the cyanic or amide 
origin of urea-Drechsel: Ber. d. k. s. Gesell. d. Wiss., Leipzig, Sitz. 25 Juli, 1875 ; Arch. f. Physiol., 
1880, S. 505. v. Knieriem : Zt. f. Biol., Bd. x. (1874), S. 263. Munk : Zt. f. physiol. Chem., Bd. ii. 
(1878), S. 29. E. Salkowski: Centralbl. f. d. med. Wiss., 1875, No. 58; Ber. d. Deutsch. Chem. (re- 
sell., 1875, S. 116. Zeitsch. f. physiol. Chem., Bd. i. (1877), Sn. 1 u. 374 ; Bd. iv. (1880), Sn. 54 u. 103. 
Schmiedeberg: Arch. f. exp. Pathol., Bd. viii. (1877), S. 1. 
4 Pfliiger's Archiv., Bd. x. (1875), S. 337. 
6 See Ultzmann and K. B. Hoffmann, Atlas der Harnsedimente, Wien, 1872. 900 
APPENDIX. 
Salts of uric acid. Of these the most important are the acid urates of sodium, 
potassium and ammonium. The sodium salt crystallizes in manydifferent forms 
[Fig. 239], these not being characteristic, since they are almost the same for the 
corresponding compounds of the other two bases. It is very insoluble in cold water 
(1 in 1100 or 1200), more soluble in hot (1 in 125). It is the principal constituent 
of several forms of urinary sediment, and constitutes a large part of many calculi; 
the excrement of snakes contains it largely. The potassium resembles tlie sodium 
salt very closely, as also does the compound with ammonium ; the latter occurs gen- 
erally in the sediment from alkaline urine. [Fig. 240.] 
[Fig. 239. 
[Fig. 240. 
Fig. 239.-Urate of Soda, a a, from a gouty concretion ; b b, artificially prepared by adding 
liq. sodee to the amorphous urate deposit.] 
Fig. 240.-The Normal Deposit from Ammoniacal Urine, showing Crystals of Ammoniaco- 
Magnesian Phosphate, Amorphous Phosphate of Lime, and Spheres of Urate of Ammonia.] 
Preparation. Usually from guano or snake's excrement. From guano by boil- 
ing with caustic potash (1 part alkali to 20 of water) as long as ammonia is evolved. 
In the filtrate a precipitate of acid urate of potassium is formed by passing a 
current of carbonic anhydride; this salt is then washed, dissolved in a caustic 
potash, and decomposed by carefully pouring its solution into an excess of hydro- 
chloric acid. 
The presence of uric acid is recognized by the following tests: The substance 
having been examined microscopically, a portion is evaporated carefully to dryness 
with one or two drops of nitric acid. The residue will, if uric acid is present, be 
of a red color, which on the addition of ammonia turns to purple. This is the mu- 
rexide test, and depends on the presence of alloxan and alloxantin in the residue. 
Schiff1 has given a delicate reaction for uric acid. The substance is dissolved in 
sodic carbonate and dropped on paper moistened with a silver salt. If uric acid 
be present a brown stain is formed, due to the reduction of the carbonate of silver. 
An alkaline solution of uric acid can, like dextrose, reduce cupric sulphate, with 
precipitation of the cuprous oxide. 
Uric acid resists very largely the action of even strong acids and alkalies, exhibit- 
ing in this respect a marked difference from urea. It might therefore perhaps be 
supposed that urea residues do not pre-exist in uric acid ; nevertheless by oxida- 
tion uric acid does give rise not only to ordinary urea, but also, and at the same 
time, to the compound ureas (ureides) spoken of above. 'Phus, by oxidation with 
acids, 
Uric acid. 
c5h4n4o3 
Alloxan. 
C4N2H2O4 + 
Urea. 
CN2H4O. 
Now alloxan, as was stated above, is a compound urea, viz., mesoxalyl-urea, and 
by hydration can be converted into mesoxalic acid and urea, thus: 
+ h2o + o = 
1 Ann. d. Chem. u. Pharm., Bd. cix. S. 65. CHEMICAL BASIS OF THE ANIMAL BODY. 
901 
Alloxan. 
C4N2H2O4 
+ 2H,0 
Mesoxalic. 
= C2H2O5 + 
Urea. 
CN2H4O; 
and by the action of chlorine uric acid can be split up directly into a molecule of 
mesoxalic acid and two molecules of urea: 
Uric acid. 
c5h4n4o3 
Mesoxalic acid. 
= c3h2o5+ 
Urea. 
2CN2H4OC 
By oxidation with alkalies, uric acid is converted into allantoin and carbonic 
acid, 
+ Cl2 + 4H2O 
+ 2HC1. 
Uric acid. 
c5h4n4o3 
Allantoin. 
c4h8n4o3 
+ H2O 4- 0 = 
+ C02; 
and allantoin, by hydration, becomes allanturic or lantanuric acid and urea, 
Allantoin. 
c4h6n4o3 
Urea. 
CH4N2O 
Allanturic acid. 
+ c3h4n2o. 
+ h2o = 
Now allanturic acid is a compound urea, with a residue of glyoxylic acid. By 
other oxidations of uric acid, parabanic acid (oxalyl-urea), oxaluric acid (which is 
hydrated parabanic acid), and dialuric acid (tartronyl-urea) are obtained. In fact, 
all these decompositions of a molecule of uric acid lead to the production of urea 
and of a carbon acid of some kind or other. The relation of uric acid to urea, as 
illustrated by the above reactions, is brought very prominently into view by the 
synthesis of uric acid which has recently been performed.1 It is obtained by simply 
fusing together glycocine (amido-acetic acid) and urea at a temperature of 200- 
230° C. The converse formation of glycocine from uric acid with the simultaneous 
production of ammonia and carbonic anhydride has been known for some time. 
Since in this latter reaction the ammonia and carbonic anhydride are in the propor- 
tions in which they would be obtained from cyanic or cyanuric acid, uric acid has 
been regarded as built up from residues of cyanuric acid and glycin, just as hippuric 
acid is formed from glycin and benzoic acid. It was also at one time supposed that 
uric acid might be regarded as tartronyl cyanamide. 
N2 
C3H.A 
(CN)2 
h2 
If the existence of some cyanogen residue is thus assumed in the molecule of 
uric acid, then it must be supposed that before urea can be obtained from it a 
molecular change takes place by which a portion at least of the nitrogen of the uric 
acid is converted into the same condition as the rest of the nitrogen, viz., into the 
amide state. 
If this be so, since the metabolism of the animals in which uric acid replaces 
urea cannot be supposed to be fundamentally different from that of the urea-pro- 
ducing animals, we may infer that the antecedent of both uric acid and urea in the 
regressive metabolism of proteids is, as we suggested above, a body containing some 
at least of its nitrogen in the form of cyanogen.2 
Kreatin. C\H9N3O2. 
Occurs as a constant constituent of the juices of muscles, though possibly it 
may be formed during the process of extraction by the hydration of kreatinin. 
Kreatin is not a normal constituent of urine, but it is said to occur in traces in 
several fluids of the body. When found in urine its presence is probably due to 
the conversion of kreatinin, a constant constituent of urine, into kreatin during 
its extraction, since Dessaignes3 has shown that the more rapidly the separation is 
effected, the less is the quantity of kreatin obtained, and the greater the amount of 
kreatinin. 
In the anhydrous form it is white and opaque, but crystallizes with one molecule 
of water in colorless, transparent rhombic prisms [Fig. 241. a]. It possesses a some- 
1 Horbaczewski; Ber. d. Deutsch. Chem. Gesell., Jahrg. 1882, 8. 2678. 
2 See v. Knieriem, Zeitschr. f. Biol., Bd. xiii. (1877), S. 36. Schroder, Zeitschr. f. physiol. Chem., 
Bd. ii. (1878), 8. 228. 
3 Jahrb. Pharm. (3), Bd. xxxii. S. 41. 902 
APPENDIX. 
what bitter taste, is soluble in cold, extremely soluble in hot water, is less soluble in 
absolute than in dilute alcohol, and is soluble in ether. 
It is a very weak base, scarcely neutralizing the weakest acids. It forms crystal- 
line compounds with sulphuric, hydrochloric, and nitric acids. 
[Fig. 241. 
Crystals of Kreatin and Kreatinin. a, crystals of kreatin; b, crystals of kreatinin ; c, crystals of 
chloride of zinc and kreatinin.] 
Preparation. From extract of muscle by precipitating completely with basic 
lead acetate, and crystallizing out the kreatin, mixed with kreatinin. From this 
latter it is separated by the formation of the zinc-salt of kreatinin, kreatin not 
readily yielding a similar compound. 
Kreatin may be converted into kreatinin under the influence of acids, the transformation 
being one of simple dehydration. 
Kreatin may be decomposed into sarcosin (methyl-glycin) and urea: 
C4H9N3O2 + H20 = C3H7NO2 + CH4N2O; 
it may be formed synthetically1 by the action of sarcosin and cyanamide: 
c3h7no2 + ch2n3 = C4H9N3O. 
Sarcosin is glycin in which one atom of hydrogen has been replaced by the alcohol 
radicle methyl, thus: . 
Glycin 
c,h3o 
nh2 
0 
becomes 
C2H..(CH3)O 
nh2 
0; 
like glycin, sarcosin has not been found in free state in the body. 
Kreatinin. C4H7N3O. 
This, which is simply a dehydrated form of kreatin, occurs normally as a con- 
stant constituent of urine and of muscle extract. It crystallizes in colorless shining 
prisms [Fig. 241, 6], possessing a strong alkaline taste and reaction. It is readily 
soluble in cold water (1 in 11.5), also in alcohol, but is scarcely soluble in ether. It 
acts as a powerful base, forming with acids and salts compounds with crystallize 
well. Of these the most important is the salt with zinc chloride (C4H7N3O)2 
ZnCl2. It is formed when a concentrated solution of the chloride is added to a not 
too dilute solution of kreatinin. Since the compound is very little soluble in alcohol 
it is better to use alcoholic rather than aqueous solutions. It crystallizes in warty 
lumps composed of aggregated masses of prisms or fine needles. [Fig. 241, c. | 
Preparation. Either by the action of acids on kreatin. or from human urine by 
concentrating and precipitating with lead acetate ; in the filtrate from this a second 
1 Sitzungsber. d. Bayerisch. Akad., 1868, Heft 3, 8. 472. CHEMICAL BASIS OF THE ANIMAL BODY. 
903 
precipitate is caused by the addition of mercuric chloride, and consists of a com- 
pound of this salt with kreatinin. The mercury is removed by sulphuretted hydro- 
gen, and the kreatin purified by the formation of the zinc salt and washing with 
alcohol. 
Kreatinin-zinc chloride may be converted into kreatin, by the action of hydrated oxide of lead 
on its boiling aqueous solution. 
Allantoin. C4H6N4O3. 
The characteristic constituent of the allantoic fluid of the foetus; it occurs also 
in the urine of animals for a short period after their birth. Traces of it are some- 
times detected in this excretion at a later date. 
It crystallizes in small, shining, colorless prisms [Fig. 242], which are tasteless 
[Fig. 242. 
[Fig. 243. 
Fig. 242.-Crystals from Concentrated Urine of Calf, showing in Centre a Large Bundle of 
Prisms of Allantoin. (After Kuline.)] 
Fig. 243.-Hypoxanthin-silver-nitrate, C6H4N4O.AgNO3. (Krukenberg, after Kiihne.)] 
and odorless. They are soluble in 160 parts of cold, more soluble in hot water, 
insoluble in cold alcohol and ether, soluble in hot alcohol. Carbonates of the alka- 
lies dissolve them, and compounds may be formed of allantoin with metals but not 
with acids. 
Allantoin, as already stated, p. 901, is one of the products of the oxidation of 
uric acid, and by further oxidation gives rise to urea. 
Preparation. This is best carried out by the careful oxidation of uric acid, either 
by means of potassic permanganate or ferrocyanide, or by plumbic oxide. 
Hypoxanthin or Sarkin. C5H4N4O. 
Is a normal constituent of muscle, occurring also in the spleen, liver, and medulla 
of bones. In lukaemia it appears in the blood and urine. It crystallizes in fine 
[Fig. 244. 
Hypoxanthin-nitrate, CSH4N4O.HNO3. (Kuhne).] 
needles which are soluble in 300 parts of cold, more soluble in hot water, insoluble 
in alcohol, soluble in acids and alkalies. It forms crystalline compounds with acids 904 
APPENDIX. 
and bases [Figs. 243, 244, 245]. It is precipitated by basic acetate of lead, the pre- 
cipitate being soluble in a solution of the normal acetate. Its preparation from 
[Fig. 245. 
[Fig. 246. 
Hypoxanthin-hydrochloride, C5H4N4O.HCI. (Kuhne.)] 
[Fig. 247. 
Xanthin-hydrochloride, C5H4N4O2.HCI. 
(Kuhne.)J 
Xanth in-nitrate, C6H4N4O2.HNO3. 
(Kiihne.)] 
muscle-extract depends on its precipitation first by basic acetate of lead, and then 
by an ammoniacal solution of silver nitrate after the removal of kreatin. 
Both hypoxanthin and the next body, xanthin, can also be obtained from proteids by the 
action of putrefactive changes, of water at boiling temperature, of dilute hydrochloric acid (0.2 
per cent.) at 40° C , and by the action of gastric and pancreatic ferments.1 Chittenden has noticed 
a peculiar difference between fibrin and egg-albumin when submitted to the above processes; he 
finds that the latter does not yield hypoxanthin when treated with boiling water, with dilute 
hydrochloric acid, or gastric ferment, while the former does. Egg-albumin, on the other hand, 
yields hypoxanthin by the action of pancreatic ferment in alkaline solution, but not so readily as 
fibrin does. 
Xanthin. C5H4N4O2. 
First discovered in a urinary calculus, and called xanthic oxide. More recently 
it has been found as a normal, though scanty, constituent of urine, muscles, and 
several organs, such as the liver, spleen, thymus, etc. 
[Fig. 248. 
When precipitated by cooling from its hot, saturated, aqueous solution it falls 
in white flocks, but if the solution be allowed to precipitate slowly it is obtained in 
Crystals of Xanthin-silver-nitrate, C5H4N4O2.AgNO3. (Krukenberg, after Ktihne.)] 
1 Salomon, Zeitschr. f. physiol. Chern., Bd. ii. (1878-1879), S. 60. Krause, Inaug. Diss., Berlin, 
1878 Chittenden, Journ.of Physiol., vol. ii. (1879), p. 28. See also Drechsel, Ber. d. Deutsch. Chem. 
Gesell.. .Jahrg xiii. (1880), S. 240. Salomon, Ibid., S. 1160. Kossel, Zeitsch. f. physiol. Chem., Bd. v. 
(1881), Sn. 152 u. 267. CHEMICAL BASIS OF THE ANIMAL BODY. 
905 
small scales. When pure it is a colorless powder, very insoluble in water, requir- 
ing J 509 times its bulk for solution at 100° C. Insoluble in alcohol and ether, it 
readily dissolves in dilute acids and alkalies, forming crystallizable compounds. 
[Figs. 24G, 247, 248.] 
Hypoxanthin by oxidation becomes xanthin. Both these bodies as well as the 
following, guanin and carnin, are evidently closely allied to uric acid ; indeed, uric 
acid by the action of sodium-amalgam may be converted into a mixture of xanthin 
and hypoxanthin. 
Preparation. It is obtained from urine and the aqueous extract of muscles by 
a process similar to that for hypoxanthin. and is then separated from the latter by 
the action of dilute hydrochloric acid; this separation depends on the different 
solubilities of the hydrochlorides of the two bodies. For further information see 
Neubauer and Vogel.1 
Carnin. C7H8N4O3. 
Discovered by Weidel2 in extract of meat, of which it constitutes about 1 per 
cent. 
It crystallizes in white masses composed of very small, irregular crystals; it is 
soluble with difficulty in cold, more easily soluble in hot water, insoluble in alcohol 
and ether. Its aqueous solution is not precipitated by normal lead acetate, but is 
by the basic acetate of this metal. It unites with acids and salts, forming crystal- 
line compounds. 
Preparation. Is found in the precipitate caused in extract of meat by basic 
acetate of lead.3 
This body possesses an interesting relation to hypoxanthin, into which it maybe converted by 
the action either of nitric acid or, still better, of bromine. 
Guanin. C5H5N5O. 
First obtained from guano, but recently observed as occurring in small quanti- 
ties in the pancreas, liver, and muscle extract. 
[Fig. 249. 
[Fig. 250. 
Guanin Hydrochloride. C5H5N5O.HCI + HoO. 
(After Kuh ne.)] 
Guanin Nitrate. C6H6N5O.HNO3 + l^HaO. 
(After Ktihne.)] 
It is a white amorphous powder, insoluble in water, alcohol, ether, and ammo- 
nia. It unites with acids, alkalies, and salts to form crystallizable compounds. 
[Figs. 249, 250.] 
Preparation. From guano by boiling successively with milk of lime and caustic 
soda, precipitating with acetic acid, and purifying by solution in hydrochloric acid 
and precipitation by ammonia. 
1 Harn-Analyse, ed. viii. (1881), 8. 26. Also the literature quoted above on hvDOxanthin 
2 Ann. d. Chern, u. Pharm., Bd. clviii. S. 365. 
3 See Weidel, op. cit. 906 
APPENDIX. 
Guanin may, by the action of nitrous acid, be converted into xanthin. By 
oxidation it can be made to yield principally guanidine and parabanic acid, accom- 
panied, however, by small quantities of urea, xanthin, and oxalic acid. Capranica 
has given several reactions characteristic of this body.1 
Its separation from hypoxanthin and xanthin depends on its insolubility in 
water and behavior with hydrochloric acid. 
Kynurenic acid. C20HhN2O6+2H2O. 
Found in the urine of dogs, and first described by Liebig.2 When pure it crys- 
tallizes in brilliant white needles, insoluble in cold, soluble in hot alcohol. The only 
salt of this body which crystallizes well is that formed with barium. For prepara- 
tion and other particulars see Liebig,3 Schultzen, and Schmiedeberg.4 
Glycin. C2H2(NH2)O(OH). Also called Glycocol and Glycocin. 
Does not occur in a free state in the human body, but enters into the composi- 
tion of many important substances, c. g., hippuric and bile acids. It crystallizes 
into large, colorless, hard rhombohedra, which are easily soluble in water, insoluble 
in cold, slightly soluble in hot alcohol, insoluble in ether. It possesses an acid reac- 
tion, but a sweet taste. It has also the property of uniting with both acids and 
bases to form crystallizable compounds. In this it exhibits its amide nature, and 
that it is an amide is rendered evident from the methods of its synthetic prepara- 
tion ; thus mono-chloracetic acid and ammonia give glycin and ammonic chloride; 
C3H3C1.O2 + 2NH3 = C2H2(NH2)O(OH) + NH4C1. It is amido-acetic acid. 
Heated with caustic baryta it yields ammonia and methylamine. 
Preparation. From glutin by the action of acids or alkalies; from hippuric 
acid by decomposing it with hydrochloric acid at a boiling temperature and remov- 
ing by precipitation the simultaneously formed benzoic acid. 
In addition to entering into the composition of taurocholic acid taurin is found 
in traces in the juices of muscle and in the lungs. 
It crystallizes in colorless, regular six-sided prisms [Fig. 251]; these are readily 
soluble in water, less so in alcohol. The solutions are neutral, It is a very stable 
Taurin. C2H7NO3S. 
FFig. 251. 
Taurin Crystals.] 
compound, resisting temperatures of less than 240° C. ; it is not acted on by dilute 
alkalies and acids, even when boiled with them. It is not precipitated by metallic 
salts. 
Taurin is amido-isethionic acid; and may be synthetically prepared from isethi- 
onic (ethyl-sulphuric) acid by the action of ammonia; thus : 
ci?5 
so.+nh.=§H; 
so3+h2o. 
1 Zeitschr. f. phys. Chem., Bd. iv. (1880), S. 240. 
2 Ann. d. Chem. u. Pharm., Bd. Ixxxvi. S. 125, and Bd. cviii., S. 354. 
3 Op. cit. 4 Ann. d. Chem. u. Pharm., Bd. clxiv. S. 155. CHEMICAL BASIS OF THE ANIMAL BODY. 
907 
Preparation. As a product of the decomposition of bile, and is purified by 
removing any traces of bile acids by means of lead acetate, and then successively 
crystallizing from water. 
Leucin. C6H13NO2. 
Is one of the principal products of the decomposition of nitrogenous matter, 
either under the influence oi putrefaction or of strong acids and alkalies. It occurs, 
however, normally in the pancreas, spleen, thymus, thyroid, salivary glands, liver, 
etc., and is one of the products of the tryptic (pancreatic) digestion of proteids; 
in acute atrophy of the liver it is present in the urine in large quantity in company 
with tyrosin. 
As usually obtained in an impure form it crystallizes in rounded lumps [Fig. 252], 
which are often collected together and sometimes exhibit radiating striation. When 
[Fig. 252. 
Leucin Crystals.] 
pure, it forms very thin, white, glittering, flat crystals. These are easily soluble in 
hot water, less so in cold water and alcohol, insoluble in ether. They feel oily to 
to the touch, and are without smell and taste. Acids and alkalies dissolve them 
readily, and crystallizable compounds are formed. 
Carefully heated to 170° C. it sublimes, but at a higher temperature is decomposed, yielding 
amylamin, carbonic anhydride, and ammonia. In the presence of putrefying animal matter it 
splits up into valeric acid and ammonia. 
Leucin is amido-caproic acid, and may be represented thus: 
C6HnO 
NH2 
0. 
Preparation. From horn shavings by boiling with sulphuric acid, neutralizing 
with baryta and separating from tyrosin by successive crystallization. See also 
Kiihne,1 who prepares it by the action of pancreatic ferment (trypsin) on proteids. 
Scherer has given the following test for leucin. The suspected substance is 
evaporated carefully to dryness with nitric acid; the residue, if it is leucin, will be 
almost transparent and turn yellow or brown on the addition of caustic soda. If 
this be again very carefully concentrated with the alkali, an oily drop is obtained, 
which is quite characteristic of this substance. Leucin, if not too impure, may be 
easily recognized by its subliming on being heated; a characteristic color of amyl- 
amin is at the same time evolved. 
Is not found as a constituent of the animal body, but appears to be formed by 
the decomposition of proteids, notably during the germinative changes of the pro- 
teids in leguminous seeds.2 It is a crystalline body, and when boiled with acids or 
alkalies is readily converted into aspartic acid. 
Asparagin. C4H8N2O3. 
This acid has been obtained in small quantities among the products of the pan- 
creatic digestion of fibrin3 and vegetable glutin,4 although not occurring as a con- 
Aspartic (or asparaginic) Acid. C4H7NC4. 
1 Virchow's Archiv, Bd. xxxix., S. 130. 
2 Landwirthski u. Versuchs Stationen, Bd. xvii. 1. 
3 Radziejewski u. Salkowski. Ber. d. Deutsch, chem. Gesell., Jahrg. vii. (1874), S. 1050. 
4 V. Knieriem, Zeitschr. f. Biol., Bd. xi. (1875), S. 198. 908 
APPENDIX. 
stituent of any animal tissue or secretion. It is, on the other hand, found normally 
in plants, notably in beet-sugar molasses. It arises also as a constant product of 
the action of alkalies and other reagents on both vegetable and animal proteids, and 
of acids on gelatin.1 It thus possesses considerable interest in respect of its re- 
lation to the proteids. It crystallizes in rhombic prisms which are but sparingly 
soluble in cold water or alcohol, readily soluble in boiling water. Its acid solutions 
are dextro-rotatory, its alkaline laevo-rotatory and reduce Fehling's fluid. It forms 
a characteristic readily crystallizable compound with copper. Nitrous acid converts 
it into malic acid. 
Glutaminic Acid. C5H9NO4. 
The circumstances and conditions under which this body occurs are in general 
the same as for the aspartic acid, and hence as a product of proteid decomposition 
it acquires some importance. It has not, however, as yet been obtained by the 
action of pancreatic ferments on proteids, and in this it differs from the preceding 
body. 
It crystallizes in rhombic tetrahedra or octahedra ; is not very soluble in cold, but 
readily soluble in hot water; insoluble in alcohol and ether. Its acid solutions 
possess a strong dextro-rotatory power, and it reduces Fehling's fluid. 
Cystin. C3H7NSO2. 
Is the chief constituent of a rarely occurring urinary calculus in men and dogs. 
It may also occur in renal concretions and in gravel, and is occasionally found in 
urine. 
From calculi it is obtained, by extraction with ammonia, as colorless six-sided 
tables or rhombohedra [Fig. 253], which are neutral and tasteless. It is insoluble 
[Fig. 253. 
Cystin Crystals.] 
in water, alcohol, and ether, soluble in ammonia and the other alkalies, and also in 
mineral acids. The fact that this body is one of the few crystalline substances, 
occurring physiologically, which contain sulphur, renders its detection very easy. 
Apart from its insolubility in water, etc., it yields, with caustic potash and salts of 
either silver or lead, a brown coloration due to the presence of the sulphides of 
these metals. 
According to Dewar and Gamgee 2 cystin is amido-sulpho-pyruvic acid, and its formula is 
C3H6NSO2-pyruvic being lactic acid minus two atoms of hydrogen. 
1 Horbaczewski, Sitzb d. k. Akad. d. Wiss., Wien, 1880. 2 Abth., Juni Heft. 
2 Journ. of Anat, and Physiol., Nov., 1870, p. 143. CHEMICAL BASIS OF THE ANIMAL BODY 
909 
The Aromatic Series. 
This is not found as a normal constituent of the body, but owes its presence in 
urine to the fermentative decomposition of hippuric acid, whereby glycin and ben- 
zoic acid are formed: 
Benzoic Acid. HCTH5O2. 
Hippuric acid. 
C2H4(C7H5O)NO2 
Glycin. 
C2H5NO2 
Benzoic acid. 
+ C7H6O, 
+ h20 = 
The sublimed acid is generally crystallized in fine needles, which are light and 
glistening; any odor they possess is not due to the acid, but to an essential oil, with 
which they are mixed. When precipitated from solution the crystalline form is 
always indistinct. This acid is soluble in 200 parts cold or 25 partsof boiling water, 
but is easily soluble in alcohol or ether. It sublimes readily at 145° C. ; it also 
passes oft' in the vapors arising from its heated solutions. 
Preparation. Either as above from hippuric acid by fermentation, by boiling 
the hippuric acid with acids or alkalies, or by sublimation from gum-benzoin. 
Generally accompanies leucin, and is perhaps found normally in small quan- 
tities in the pancreas and spleen. It is also usually obtained in large quantities 
by the decomposition of proteid matter, either by putrefaction or the action of 
acids. 
Tyrosin. C9HnNO3. 
The researches of Radziejewski1 render it probable that tyrosin does not occur normally in 
any part of the human organism, except as a product of pancreatic digestion. 
All attempts to synthetize tyrosin were for some time fruitless, although evi- 
dence was obtained sufficient to indicate the probable existence in its molecule of 
some aromatic (phenyl) radicle.2 More recently the synthesis has been performed,3 
and we now have every reason for regarding tyrosin as para-hydroxy-phenyl a 
alanine. This synthesis, as well as that of uric acid, referred to above, is of con- 
siderable importance, since the more definite the knowledge which is possessed of 
the true molecular structure of the products of proteid decomposition, the more 
reason is there for expecting that the synthesis of a proteid itself may be realizable 
in the not very remote future. 
[Fig. 254. 
Tyrosin Crystals.] 
Tyrosin crystallizes in exceedingly fine needles which are usually collected into 
feathery masses [Fig. 254]. The crystals are snow-white, tasteless and odorless, 
almost, insoluble in cold water, readily soluble in hot water, acids, and alkalies, 
1 Archiv. f. path. Anat., Bd. xxxvi., S. 1. Zeitschr. f. anal. Chem., Bd. v., S. 466. 
2 Barth., Chem. Centralbl., 1865, S. 1029; 1869, S. 761; 1872, S. 830. Hiifner, Ibid., 1869, S. 139. 
Beilstein u. Kiihlberg, Ibid., 1872, S. 830. 
3Erlenmeyer u. Lipp., Ber. d. Deutsch. Chem. Gesell., Jahrg. xv. (1882), S. 1544. 910 
APPENDIX. 
insoluble in alcohol and ether. If crystallized from an alkaline solution tyrosin often 
assumes the form of rosettes composed of fine needles arranged radiately. 
Tyrosin does not sublime by heating, but is decomposed with an odor of phenol 
and nitrobenzol. On boiling with Millon's reagent it gives a reaction almost identi- 
cal with, but much more marked than, that for proteids (Hoffman's test). If 
tyrosin is treated on a watch-glass with one or two drops of strong sulphuric acid, 
then diluted with a little water, neutralized with calcic carbonate, and the solution 
filtered, a characteristic violet color is obtained on the addition of a drop of acid-free 
ferric chloride (Piria's test). 
Preparation. By means similar to those employed for leucin, the separation of 
the two depending on their widely differing solubilities. According to Kiihne's 
method,1 large quantities are easily obtained as the result of pancreatic digestion. 
Hippuric Acid. C9H9NO3. Or Benzoyl-glycin. C2H4(C7H5O)NO2. 
Ts found in considerable quantities in the urine of herbivora, and also, though 
to a much smaller amount, in the urine of man. It is formed in the body by the 
union with dehydration of glycin and benzoic acid. 
Crystallized from a saturated aqueous solution it assumes the form of fine 
needles; if from a more dilute solution, white, semitransparent four-sided prisms 
are obtained [Fig. 255]. These when pure are odorless, with a somewhat bitter 
[Fig. 255. 
Hippuric Acid Crystals.] 
taste. They are soluble in 600 parts of cold water, readily soluble in boiling water, 
readily soluble in alcohol, less so in ether. All the solutions redden litmus. 
Hippuric acid is monobasic, and forms salts which are readily soluble in water 
(except the iron salts); from these, if in sufficiently concentrated solutions, excess 
of hydrochloric acid precipitates the acid in fine needles. When heated with 
concentrated mineral acids it is resolved into benzoic acid and glycin. The same 
decomposition occurs in presence of putrefying bodies. Strong nitric acid pro- 
duces an odor of nitrobenzol. 
Preparation. Fresh urine of horses or cows is treated with milk of lime in 
order to form calcic hippurate and thus prevent the decomposition of the hippuric 
acid, filtered, and the filtrate evaporated to a small bulk ; the hippuric acid is then 
precipitated by adding an excess of hydrochloric acid ; the acid is then purified by 
several crystallizations from boiling water. 
When heated in a small tube, hippuric acid gives a sublimate of benzoic acid and 
amnionic benzoate, accompanied by an odor like that of new hay, while oily, red 
drops are observed in the tube. This is very characteristic and distinguishes it 
from benzoic acid. 
1 Op. cit. (sub. Leucin). CHEMICAL BASIS OF THE ANIMAL BODY. 
911 
This body is undoubtedly obtained as the result of the putrefactive decomposi- 
tion of proteids, notably in putrefactive pancreatic digestions.1 It may be obtained 
from the distillate of such digestive mixtures. It is also found in the contents of 
the alimentary canal under the same conditions which give rise to indol. When so 
occurring a portion of it may be obtained from the feces, while the rest reappears 
in the urine.2 
Phenylic (carbolic) Acid, or Phenol. C6II6O. 
Buliginsky 3 says the urine of many animals, of cows and horses always, contains a substance 
insoluble in alcohol, and not precipated by lead acetate and ammonia, which by the action of 
dilute mineral acids gives carbolic acid. The same acid applied to the body externally or inter- 
nally also passes into the urine.4 Similarly benzol (C6II6), when taken into the stomach, appears 
as a carbolic acid in the urine.5 
[Fig. 256. 
Crystals of Kynurenic Acid. (After Kiihne.)] 
[Fig. 257. 
The pure acid crystallizes in long, colorless prismatic needles; they melt at 35° C., 
and boil at 180° C. It is readily soluble in alcohol and ether, slightly soluble in 
water (1 part in 20). In most cases it acts as a weak acid, forming crystalline salts 
with the alkalies. With nitric acid it yields picric acid. Its solutions reduce silver 
and mercury salts. 
Crystals of Barium Kynurenate. (After Kiihne.)] 
1 Baumann, Zeitsch. f. physiol. Chem., Bd. i. (1877), S. 70. 
2 Salkowski, Ber. d. Deutsch. Chem. Gesell., Bd. ix. (1876), S. 1595. Centralb. f. d. med. Wiss. 
(1876\ S. 818. Ber. d. Deutsch. Chem. Gesell., Bd. x. (1877), S. 842. Virchow's Arch., Bd. Ixxii. 
(1878), S. 409. See also Centralbl. f. d. med. Wiss. (1878), Nos. 30, 31, 34, 42, and Zeitschr. f. physiol. 
Chem., Bd. ii. (1878), S. 241. 
3 Hoppe-Seyler, Med. chem. Untersuch., Heft ii. (1867), S. 234. 
4 Alm£n, Neues Jahrb. d. Pharm., Bd. xxxiv., S. 111. Salkowski, Pfluger's Archiv, Bd. v. 
(1871-72), S. 335. 
5 Schultzen u. Naunyn, Reichert u. DuBois Reymond's Archiv (1867), Heft iii.,S. 349. 912 
APPENDIX. 
Preparation. By the dry distillation of salicylic acid, also from the acid pro- 
ducts of the distillation of coal. It is obtained in the last portions of the distillate 
when preparing indol, and is separated by forming a compound with bromine, 
C6HBr3O3. 
[Kynurenic acid. This acid is obtained from the urine of dogs. It crystal- 
lizes in long needles or four-sided prisms [Fig. 256], and with barium forms peculiar 
triangular crystals [Fig. 257]. 
The Bile Series. 
Cholalic (or Cholic) Acid. H.CH24H39O5 + H20. 
Occurs in traces in the small intestine, in large quantities in the contents of the 
large intestine, and the feces of men, cows, and dogs. In icterus, the urine often 
contains traces of this acid. But its principal interest lies in its being the starting- 
point for the various bile acids (see below). The pure acid may be amorphous or 
crystalline, in the latter case crystallizing from hot alcoholic solutions in tetrahedra. 
These crystals are insoluble in water and ether. In the amorphous form it is some- 
what soluble in water and ether. Ileated to 200° 0., it is converted into water and 
dyslysin (C?4H43O3). 
This acid possesses, in the anhydrous condition, a specific rotatory power of 
+50° for the yellow light; when it crystallizes with H2O, the rotation is +35°. 
The rotatory power of the alkali salts is always less than the above, and when in 
solution in alcohol the rotation is independent of the concentration. For the 
alcoholic solution of the sodium salt the rotation is +31.4°. 
Preparation. By the decomposition of bile acids by means of acids, alkalies, or 
fermentative changes. 
Bayer 1 has examined the bile acids obtained from human bile, and has prepared from them 
cholalic acid. To this he assigns the formula If this be so, then cholalic acid of human 
bile would seem to be a body entirely different from that obtained from ox-bile, and analyzed by 
Strecker. Bayer's results, however, require further confirmation. 
Pettenkofer s test.2 
This well-known test for bile acids depends on the reaction of cholalic acid in 
presence of sugar and sulphuric acid. If to a solution of the acid a little sugar be 
added and then sulphuric acid, keeping the temperature below, but not much below, 
70° C., a beautiful reddish purple is obtained. If diluted with alcohol this solution 
gives a characteristic spectrum with two absorption bands, one between D and E, 
nearest to E, the other close to F on the red side of F. 
The reaction is much impeded by the presence of coloring matters; moreover, 
proteids and other bodies easily decomposed by sulphuric acid, such as amyl-alcohol 
and oleic, give a similar result; the coloring matter produced from these bodies 
does not, however, give the absorption bands described above.3 
Glycocholic Acid. C26H43NO6. 
This body was first obtained in the crystalline form and described by Gmelin 
(1826), who gave it the name of " cholic " acid. 
To avoid confusion it is now best to use the term " cholic " as a synonym for " cholalic," 
Demarcay, who first (1838) described the cholalic acid as a product of the decomposition of bile 
acids, having given it the name of cholic acid. The name cholalic acid is perhaps the best, since 
it indicates the method by which the bile acids are split up, viz., by treatment with alkali. 
This is the principal bile acid of ox-gall: it is also present in the bile of man, but 
has so far not been observed in that of carnivora. In icterus the urine may contain 
traces of this acid. 
It crystallizes in fine, glistening needles. These tire slightly soluble in cold water, 
readily so in hot water and alcohol, but insoluble in ether. They possess a bitter 
and yet sweet taste, and a strong acid reaction. 
The salts of this acid are readily soluble in water and crystallize well. The salts, 
as well as the free acid, exert right-handed polarization amounting to +29.0° for 
the acid, and + 25.7° for the sodium salt, both measured for yellow light. 
1 Zeitschr. f. physiol. Chem., Bd. ii. (1878-79), S. 358. 
2I'ettenkofer. Annalen d. Chem. u. Pharm , Bd. lii. (1844), 8. 90. 
8 For further information on this subject see : Bischoff, Zeitschr. f. rat. Med., Ser. 3, Bd. xxi.,S. 
126. Schulze. Ann. d. Chem. u. Pharm., Bd. Ixxi. (1849), S. 266. Schenk, Anatom.-physiol. Unter- 
such., Wien, 1872, S. 47. Adamkiewicz, Pfliiger's Arch., Bd. ix. (1874), S. 156. CHEMICAL BASIS OF THE ANIMAL BODY. 
913 
Glycocholic acid is a compound of glycin and cholalic acid ; thus: 
Cholalic acid. 
c24H40o5 + 
Glycin. 
c2nh5o 
-H,0 
Glycocholic acid. 
=c26h43no6. 
Prolonged boiling with dilute mineral acids or caustic alkalies decomposes glycocholic acid 
into glycin and cholalic acid; if dissolved in concentrated sulphuric acid and then warmed, 
glycocholic acid by the removal of one molecule of water yields cholonic acid, CseFtuNOs. The 
ba'rium salt of this last acid is insoluble in water, which fact is of importance, since cholonic 
acid possesses nearly the same specific rotatory power as glycocholic acid. 
Preparation. From ox-gall by evaporation to a syrup, decolorizing with animal 
charcoal, extracting with strong alcohol, and precipitating by a large excess of ether. 
Its separation from taurocholic acid depends on its precipitation by normal lead 
acetate, taurocholic acid not being precipitated by this reagent. 
Taurocholic Acid. C26H45NSO7. 
Occurs also in ox-gall, but is found especially plentiful in human bile and that 
of carnivora, notably of the dog. 
It crystallizes with difficulty in very fine needles, which are exceedingly deliques- 
cent. When dried it is an amorphous powder, with pure bitter taste, easily sol- 
uble in water and alcohol, insoluble in ether. All its salts are soluble in water, and 
are precipitated by basic lead acetate in the presence of free ammonia. The sodium 
salt dissolved in alcohol has a specific rotatory power of+24.5°; if dissolvedin 
water this rotation is less, and in this respect it resembles glycocholic acid. 
This acid is far more unstable than the preceding one, being decomposed if 
boiled with water. The products of decomposition are taurin and cholalic acid. 
Taurocholic acid is a compound of taurin and cholalic acid ; thus : 
Cholalic acid, 
CJI40O5 + 
Taurin. 
C2H7NO3S 
-ILO = 
Taurocholic acid. 
C26H45NO7S. 
Preparation. From the bile of dogs by a process similar to that for glycocholic 
acid. It is separated from traces of this latter and from cholalic acid by precipita- 
tion with basic lead acetate and ammonia.1 
Bile Pigments. 
It is found chiefly in the fresh bile of man and carnivora, to which it gives the 
characteristic dark golden-red color. It frequently constitutes a considerable part 
of some kind of gall-stones, not, however, as free bilirubin, but as a compound with 
earthy matter, chiefly chalk ; the gall-stones of oxen and pigs often contain 40 per 
cent, of this compound.2 These are, therefore, the best material from which to 
prepare bilirubin. 
Preparation. The gall-stones are treated with strong acetic or dilute hydro- 
chloric acid, to separate the earthy matter, and the residue is thoroughly washed 
with water and alcohol and dried. From this residue the prolonged action of hot 
chloroform extracts the bilirubin, which may either be obtained in the amorphous 
form by precipitation with alcohol of its solution in chloroform, or as well-defined 
crystals by the slow evaporation of the chloroform solution. 
The most usual form of the crystals is that of rhombic prisms ; they are readily 
soluble in chloroform and alkaline solutions only. 
By treatment with oxidizing agents, such as nitrous acid, bilirubin takes up 
oxygen and becomes biliverdin, the color at the same time changing to green. The 
possible oxidation does not end here, and if continued a series of products are ob- 
tained, each with a characteristic color, as in the well-known Gmelin's test.3 Of 
these only the final product of the oxidation has been obtained in a state of suffi- 
cient purity to enable any definite statements to be made of its characteristics.4 
This is the body known as Choletelin (see below). 
Bilirubin. C16H1SN2O3. 
1 Parke, Tubing. Med.-chem. Unters., Bd. i., S. 160. 
2 Maly, Sitzber. d. Wien. Akad., Bd. Ivii. (1868), ii. Abth., Febr. Hft. 
3 Tieamann u. Gmelin, Die Verdauung, 1826, S. 79. 
4Heynsiusu. Campbell, Pfliiger's Arch., Bd.iv. (1871), S.497. 
58 914 
APPENDIX. 
Biliverdin. CigH^N.^Oi.1 
This product of the oxidation of bilirubin gives the characteristic color to the bile 
of herbivora and to biliary vomits. It occurs also probably at times in the urine of 
jaundice and in the pigmentary matter of the placenta. It is found, or occurs in 
traces only, in gall-stones. 
Preparation. An impure product is obtained by precipitating ordinary herbiv- 
orous bile with baric chloride, washing the precipitate with water and alcohol, and 
decomposing it with hydrochloric acid. The biliverdin thus obtained is washed with 
ether and dissolved in alcohol. From its solution in the latter it is obtained as an 
amorphous green powder by slow evaporation. Pure biliverdin is best prepared by 
the slow oxidation in the air of bilirubin, dissolved in dilute caustic soda. 
It does not crystallize, and is insoluble in ether or chloroform ; readily soluble in 
alcohol. When oxidized it gives the same play of colors as does bilirubin, with the 
formation of the same final and intermediate products. 
Neither this body nor bilirubin gives any characteristic absorption bands. 
There seems now no reason for doubting that the bile pigments are derived ulti- 
mately from the coloring matter of the blood. 
Virchow has described 2 the gradual changes in old blood-clots, as of cerebral 
hemorrhage, which lead to the presence of the so-called haematoidin-crystals. 
Though these have not been obtained in sufficient quantities to enable their 
composition to be finally fixed by a chemical analysis,3 still the identity of their 
crystalline form with that of bilirubin, and the fact that they both give the same 
play of colors when oxidized, as in Gmelin's test, justify the assumption that 
haematoidin and bilirubin are identical.4 Moreover, the balance of experimental 
evidence distinctly supports the view that a liberation from the corpuscles of the 
coloring matter of the blood in the bloodvessels by an injection of chloroform, 
water, etc., leads generally to the appearance of bile-pigments in the urine.5 The 
occurrence of bilirubin crystals in the urine has frequently been observed after 
the operation of transfusion of blood in man. The chemical possibility of the con- 
version of haemoglobin into biliverdin is readily seen by a comparison of the formulae 
of haematin (see p. 464) and bilirubin. The former has, according to Hoppe-Sey- 
ler,6 the composition indicated in the formula 2(C34H35N4FeO5), while that of bili- 
rubin is C1BII1KN2O3. Although the conversion has not as yet been directly effected, 
the following facts are significant: If bilirubin is treated with sodium amalgam the 
substance known as hydrobilirubin (see below) is obtained. If haematin is dissolved 
in caustic soda and treated with sodium amalgam or in hydrochloric acid solution 
with zinc dust, a substance is obtained which is now recognized as identical with 
hydrobilirubin.7 This is the most direct chemical evidence of the relation of the 
coloring matters of the blood and bile. 
Choletelin. C16H18N2O6(?).8 
This substance is obtained as the final product of the oxidation of either bilirubin 
or biliverdin. It is best prepared by acting upon bilirubin with nitrous acid in 
presence of alcohol; the various colors of Gmelin's reaction are observed and the 
final reddish-yellow solution, if poured into water, yields a precipitate of choletelin. 
It is not crystalline and is soluble in alcohol, ether, and chloroform. When freshly 
prepared it seems to give an uncertain absorption band if examined in an acid solu- 
tion. On this account some observers 9 have been led to regard it as identical with 
hydrobilirubin (urobilin). There is, however, no doubt that they are quite distinct 
bodies.10 
1 Maly, Sitzb. d. Wien. Akad., Bd. Ixx. (1874), iii. Abth. 
2 Arch. f. path. Anat., Bd. 1., S. 383. 
3 Robin, Ann. d. Chem. u. Pharm., Bd. cxvi., S. 89. 
4 But see also Prever, Die Blutkrystalle, 1871, S. 187. 
s Tarchanoff, Pfluger's Arch., Bd. ix. (1874), S. 53. See also Bd. x. (1875), S. 208. 
* Physiologische Chemie, 1879, S. 395. 
7 Hoppe-Sevier, Med.-chem. Untersuch., Heft iv., 1871, S. 523. Ber. d. Deutsch, chem. Gesell., 
vii. (1874), S. 1065. 
3 Maly, Sitzb. d. Wien. Akad., Bd. Ivii. (1868); 2 Abth. Febr. und Bd. lix., 1869; 2 Abth. April. 
See also Heynsius and Campbell, loc. cit. 
9 Heynsius and Campbell, loc. cit. Stokvis, Centralbl. f. d. med. Wiss., No. 14 (1873), S.211. 
10 Maly, Centralbl. f. d. med. Wiss., No. 21 (1875), S. 321. Liebermann, Pfluger's Arch., Bd. xi. 
(1875), S. 181. CHEMICAL BASIS OF THE ANIMAL BODY. 
915 
This body was first described by Maly1 as resulting from the action of sodium 
amalgam on an alkaline solution of bilirubin. When the reaction is complete the 
solution is precipitated with hydrochloric acid, the precipitate dissolved in ammo- 
nia, again precipitated by acid, and the substance thus finally obtained is washed 
with water. It is readily soluble in alcohol, less so in ether. Its alkaline solutions 
are yellow, and these turn pink on the addition of acid. Both its acid and alka- 
line solutions, the latter especially on the addition of a few drops of chloride of 
zinc, give a characteristic absorption band between b and F.2 In the colors of its 
alkaline and acid solutions and the greenish fluorescence of its ammoniacal solution 
on the addition of chloride of zinc, and in its absorption spectrum, hydrobilirubin 
shows its close relation to the urobilin (see below), with which indeed it is now con- 
sidered to be identical. It is also identical with a body named stercobilin,3 which 
had previously been described as a product of the alteration of the bile-pigments in 
the alimentary canal occurring in feces. There is no difficulty in seeing how this 
change (hydrogenation) can be brought about in the intestine since it is known that 
a considerable quantity of hydrogen may make its appearance by fermentative pro- 
cesses in the intestine, and in its nascent state might readily produce the simple 
change which is known to occur when bilirubin is converted into hydrobilirubin. 
Hydrobilirubin. C32H4ON4O7. 
Pigments of Urine. 
Our knowledge of these bodies is at present limited and imperfect. Most prob- 
ably4 they are numerous, but only two appear sufficiently well characterized to 
deserve mention here. 
As stated above, this is now regarded as identical with hydrobilirubin. It was 
first described by Jaffe5 as a well-characterized normal urinary pigment and its 
identity with hydrobilirubin subsequently determined.6 
Normal urine contains only small quantities of urobilin, but there is present a 
substance (chromogen) which under the influence of acids, with absorption of 
oxygen, yields urobilin. The urine of fever frequently contains a considerable 
amount of actual urobilin as such. 
The properties described above for hydrobilirubin are identical with those of 
urobilin. Its preparation from urine is somewhat difficult, and for this some spe- 
cial manual must oe consulted.7 
Urobilin. C32H40N4O7. 
Ur oery thriii. 
Is considered to be the substance which gives to the urine of rheumatism its 
characteristic color. Very little is known of its chemical properties.8 It appears 
to be an amorphous reddish body with an acid reaction, slowly soluble in water, 
alcohol, and ether. When treated with caustic alkali it turns green. Urine con- 
taining this body takes on a characteristic reddish-yellow color on the addition of 
concentrated hydrochloric acid. 
Thudichum considers that normal urine contains only one pigment, which he calls uro- 
chrome.8 Maly is inclined to regard this as the same as urobilin.10 More recently Thudichum 
has upheld his former views.11 
The Indigo Series. 
A body was long ago described12 as occurring in the urine and sweat of men 
and other animals which yielded by the action of acids the blue coloring matter 
Indican. C8H7NSO4. 
1 Centralb, f. d. med. Wiss., No. 54,1871. Annal. d. Chem., Bd. clxiii. (1872), S. 77. 
2 Vierordt, Zeitschr. f. Biol., Bd. ix. (1873), S. 160. 
3 Vanlair and Masius, Centralbl. f. d. med. Wiss., No. 24, 1871. 
4 Vierordt, Die quantitativ Spectralanalyse, etc., Tubingen, 1876, S. 81. 
6 Centralb. f. d. med. Wiss., 1868, S. 243. Virchow's Arch., Bd. xlvii. (1869), S. 405. 
6 Maly, Ann. d. Chem. u. Pharm., Bd. clxiii. (1872), S. 77. 
7 Vide Neubauer and Vogel, Harnanalyse, ed. viii. (1881), S. 81. 
8 Heller's Arehiv (2) Bd. iii. (1854), S. 361. 
» Brit. Med Journ., N. 8., No. 201, 1864, p. 509. 
10 Maly, Ann. d. Chem. u. Pharm., loc. cit., 1872, S. 90. 
11 Journ. Chem. Soc., Ser. 2, vol. xiii (1875), pp. 397, 401. 
12 Schunk, Phil. Mag., vol. x. p. 73; xiv. p. 228; xv. pp. 29, 117, 183. Chem. Centralbl., 1856, S. 
50; 1857, S. 957; 1858, S. 225. Hoppe-Seyler, Arch. f. path. Anat., Bd. xxvii., S. 388. Jaffe, Pfliiger's 
Arch., Bd. iii. (1870), S. 448. 916 
APPENDIX. 
indigo as one of the products of its decomposition. Schunk considered this sub- 
stance to be identical with the indican known to occur in several plants (Indigofera, 
Isatis). Hoppe-Seyler,1 on the other hand, having regard to the greater ease with 
which the indican from plants undergoes decomposition, regarded them as most 
probably different substances. Baumann has shown 2 that the two are really differ- 
ent, and has confirmed his earlier statements in a more recent publication.3 
According to him, the indican obtained from urine is not a glucoside (so also 
Hoppe-Seyler) and yields sulphuric acid by the action of hydrochloric acid. He 
assigns to it the formula C8HBA.O.SO2.OH, and regards it as indoxy-sulphuric acid. 
The acid itself is not yet known in the free state, but it yields stable salts such as 
that of potassium. C8HBN.SO4K. It occurs largely in the urine as the result of 
the presence of indol in the alimentary canal. In this way Baumann and Brieger4 
were enabled to obtain large quantities by giving indol to a dog. For its prepara- 
tion their original paper must be consulted. 
When treated in aqueous solution with hydrochloric acid in presence of oxygen 
it yields indigo-blue. 
2C3H6NSO4K + 02 - 2C8H5NO + 2KHSO4. 
It is always estimated in urine by conversion into indigo-blue. 
Indigo. C8H5NO. 
It is formed, as stated above, from indican, and gives rise to the bluish color 
sometimes observed in sweat and urine. 
It may, by slow formation from indican, be obtained in fine crystals; these are 
insoluble in water, slightly soluble, with a faint violet color, in alcohol and ether. 
Chloroform also dissolves them to a slight extent. Indigo is soluble in strong sul- 
phuric acid, forming at the same time two compounds with this acid; these are 
soluble in water. It possesses a pure blue color; when pressed with a hard body 
a reddish copper-colored mark is left, and the crystals exhibit the same color if seen 
in reflected light. 
The soluble compounds with sulphuric acid give an absorption band in the spec- 
trum which lies close to the D line and to the red side of it. This may be used to 
detect indigo. 
Treated with reducing agents, indigo is decolorized, being reduced to indigo- 
white. The latter contains two atoms more hydrogen than indigo. 
Indol. C8H7N. 
To this body the specific odor of the feces is partly due. It is obtained as the 
final product of the reduction of indigo, and also by the distillation of protcid 
matter with caustic alkalies.5 
It often occurs among the products of the action of pancreatic ferment on pro- 
teids; its presence in such cases appears, however, to be due, not to the action of 
the trypsin, but to a simultaneous putrefaction under the influence of bacteria, 
etc.6 If the pancreatic digestion be carried on in the presence of salicylic acid, 
indol does not make its appearance. Indol is a crystalline body, soluble in boiling 
water, alcohol, and ether. It passes over in the steam when its aqueous solution 
is boiled. It is characterized by the following reactions: A strip of pine-wood 
moistened with hydrochloric acid is colored bright crimson when dipped into a 
solution of indol. Its alcoholic solution turns red when treated with nitrous acid, 
and its aqueous solution gives a copious red precipitate with the same reagent. It 
also yields a characteristic crystalline compound with picric acid. 
1 Handb. d path. chem. Anal., ed. iv. (1875), S. 191. 
2 Pfliiger's Arch , Bd. xiii. (1876), S. 301. Zeitschr. f. physiol. Chem., Bd. i. (1877-78), S. 60. 
3 Zeitschr f. physiol. Chem., Bd. iii. (1879), S. 254. 
4 Zeitschr. f. physiol. Chem., Bd. iii. (1879), S. 254. See also Ber. d. Deutsch, chem. Gesell., xii. 
(1879), Sn. 1098, 1192, 2166; and xiii. (1880), S. 408. 
5 Kuhne, Ber d. deutsch. chem. Gesell., viii. (1875), S. 206. 
6 Kuhne, Verhand. Heidelb. naturhist.-med. Ver., N.S., Bd. i., Hft. 3. Ber. d. Deutsch, chem. 
Gesell. (1875), S. 206. 917 
CHEMICAL BASIS OF THE ANIMAL BODY. 
Noticed by Brieger1 as one of the products of putrefactive changes in the small 
intestine. Secretan2 had previously described a similar substance as arising from 
the putrefaction of albumin. 
Skatol is crystalline and contains nitrogen ; it is more soluble in water than indol, 
and does not give rise to any red coloration with nitrous acid. 
Skatol readily passes into the urine when it occurs in the alimentary canal, and 
then gives a violet-red reaction with strong hydrochloric acid. 
v. Nencki3 prepares this substance by the putrefaction of a mixture of finely 
divided pancreas and muscle substance. After the addition of acetic acid the 
mass is distilled, when the skatol readily passes over. From the distillate it is 
precipitated by picric acid, and the precipitate when again distilled with ammonia 
gives off pure skatol, which may be finally purified by crystallization. 
Skatol. C9H9N (?). 
[PTOMAINES AND LEUKOMAINES. 
These substances comprise the so-called animal alkalies, and belong to a class 
of amines. They are called alkaloids because of their close resemblance in toxic 
properties to the same class of substances obtained from plants. The difference 
between ptomaines and leukomaines as classes is that the former are products of 
abnormal metabolism and the latter of normal metabolism. Ptomaines are usually 
highly toxic, while leukomaines are of feeble toxicity. It seems probable that in 
specific diseases the disease germs form ptomaines and that the peculiar patholog- 
ical states are due in a large measure to these substances acting as peculiar poisons.] 
1 Ber. d. Deutsch, chem. Gesell., Jahrg. x. (1877), S. 1027. 
2 Recherches sur putrefaction de I'albumine. Geneva, 1876. 
3 Centralbl. f. d. med Wiss., 1878, S. 849. INDEX. 
4 BERRATION, chronic, 758 
IL spherical, 757 
Absorption, from alimentary canal, 325 
of diffusible substances and water, 331 
of fats, 329 
Accommodation, 745, 751 
mechanism of, 749 
Acetic acid series, 887 
Acid-albumin, 84 
formation of, 255 
Acid, acetic, 887 
aspartic, 907 
benzoic, 909 
butyric, 888 
capric, 888 
caproic, 888 
caprylic, 888 
cholic, 912 
ethylene-lactic, 891 
ethylidene-lactic, 891 
formic, 887 
glutaminic, 908 
glycerin-phosphoric, 895 
glycocholic, 912 
hippuric, 910 
kynurenic, 906 
lactic, 891 
laurostearic, 888 
myristic, 888 
oleic, 889 
oxalic, 892 
palmitic, 888 
phenylic, 911 
propionic, 888 
sarcolactic, 891 
stearic, 888 
succinic, 892 
taurocholic, 913 
uric, 899 
salts of, 900 
valerianic, 888 
Acoustic apparatus, 798 
Adipose tissue, 470 
Afferent impulses, 526, 706 
After-images, 775 
negative, 775 
positive, 775 
Albumin, acid, 868 
alkali, 869 
derived, 868 
egg, 867 
native, 867 
nucleo-, 884 
serum, 865 
Albuminous glands, changes in, during 
secretion, 269 
Alkali albumin, 85 
Allantoin, 903 
Allantois, 842 
Alvergniat's pump, 349 
Amblyopia, 695 
Amides, 896 
Amoeba, characteristics of, 17 
Amoeboid movements, 121 
Anelectrotonus, 103 
Animal body, chemical basis of, 863 
Animal gum, 888 
heat, 496 
distribution of, 496 
production of, 500 
regulation by variations in loss, 
499 
sources of, 496 
temperature of body, 498 
Aphasia, 676 
complete, 679 
partial, 679 
Apnoea, 384 
Aqueous humor, 744 
Area for mastication, 666 
production of voice, 666 
speech, 676 
swallowing, 666 
Aromatic series, 909 
Arteries, changes in calibre of, 211 
Ascites, 322 
Asparagine, 907 
Aspartic acid, 907 
Asphyxia, 386 
Astigmatism, 757 
Atropine, action of, on heart, 208 
on pupil, 756 
Auditory judgments, 805 
sensations, 800 
Automatic actions, 131 
Automatism, irregular, 721 
regular, 721 
Benzoic acid, 909 
Bile, 278 
acids, 280 
formation of, 456 
action of, on food, 281 
antiseptic qualities of, 281 
characters of, 278 
composition of, 278 
formation of constituents of, 453 
919 920 
INDEX. 
Bile, influence on peptic digestion, 309 
pigments of, 279, 453, 913 
Gmelin's test for, 280 
resorption of, 291 
salts, 280 
tests for, 280 
Pettenkofer's, 280 
secretion of, 288 
series, 912 
Bilirubin, 279, 912 
Biliverdin, 280, 913 
Binocular vision, 781 
Bladder, muscles of, 427 
Blindness, 695 
color, 774 
Blind spot, filling up of, 779 
Blood, 24 
carbonic acid in, 360 
changes in quantity of, 236 
chemical composition of, 50 
clotting of, 26 
huffy coat in, 27 
causes of, 38 
corpuscles in, 37 
crassamentum in, 26 
effect of neutral salts on, 32 
effect of sodium chloride on, 28 
effect of temperature on, 31 
fibrin in, 28 
fibrin ferment in, 34 
fibrinogen in, 33 
in arteries, 37 
in veins, 36 
length of time of, 27 
paraglobulin in, 30 
plasmine in, 32 
rapidity of, 27 
serum in, 29 
serum-albumin in, 30 
color of, 37 
brightness of, due to reflection of 
light through corpuscles, 39 
venous and arterial, 356 
composition of, 50 
gases, 349 
methods of obtaining, 346 
haematin, 359 
features of, 359 
method of obtaining, 359 
haemin, 360 
crystals of, 360 
haemoglobin, 351 
characters of, 351 
method of obtaining, 352 
spectroscopic features of, 353 
varieties of crystals. 352 
composition of, methaemoglobin, 360 
corpuscles, 37 
average number of, in human 
blood, 40 
characteristics of, 37 
red, structure of, 40 
haemoglobin in, 40 
white, 43 
effect of deficient aeration on, 382 
platelets, 49 
Blood plaques, 49 
pressure, 137 
curves from carotid of rabbit, 140 
quantity of, its distribution in body, 54 
rate of circulation, 150 
relations of oxygen in, 350 
respiratory changes in, 346 
supply, influence of, on contractile tis- 
sues, 113 
of liver, 289 
Brain, 584 
absence of signs of volition and intelli- 
gence, 637 
and spinal cord, lymphatic arrange- 
ments of, 728 
membranes of, 728 
vascular arrangements of, 732 
arteries of, distribution and characters 
732 
blood-supply of, 734 
methods of investigating, 735 
bulb of, 589 
fibres of, 598 
central gray matter of, 601 
cerebro-spinal fluid, 731 
changes in gray matter of, 595 
commissural fibres of, 634 
connections of gray and white matter 
of, 601 
corpora geniculata, 625 
quadrigemina, 625 
gray matter of, 625 
corpus striatum, 625 
cortical fibres, 631 
cranial nerves, 589 
embryonic, 584 
fibres of, 627 
functions of, 716 
gracile and cuneate nuclei of, 597 
gray matter of, 601 
histological features of, 653 
intermediate gray matter of, 615 
longitudinal fibres of, 628 
nature and relations of the several 
nuclei of, 613 
olivary nucleus of, 596 
optic thalamus, 615 
posterior bundles of, 633 
cortical of, 630 
pyramidal tract of, 628 
splanchnic functions of, 720 
structure of, 584 
superficial gray matter of, 615 
superior or sensory decussation, 592 
Brain, tracts from corpora quadrigemina of, 
633 
venous arrangement of, 733 
white matter of, 530 
Broca's convolution, 676 
Burdach, columns of, 540 
Butyric acid, 889 
CALABAR bean, action of, on pupil, 756 
Calorimeters, 494 
Capillaries, circulation in, 143, 231 INDEX. 
921 
Capillaries, pressure of blood in, 142 
stagnation or stasis, 235 
vessels of muscle, 77 
Capric acid, 889 
Caproic acid, 889 
Caprylic acid, 889 
Carbohydrates, 88, 884 
Carbonic acid exhaled, 344, 364 
Cardiac contraction, features of, 196 
curves, discussion of, 168 
impulse, 162 
phases, duration of, 173 
Carnin, 899 
Casein, 259, 868 
Cell body, 126 
Cells, pyramidal, 656 
Central nervous system, vasomotor functions 
of, 222 
Cerebellar tract, course of, 556 
Cerebellum, corpus dentatum of, 625 
functions of, 716, 719 
gray matter of, 653 
histology of, 654 
superior pedicle of, 632 
Cerebral convolutions, areas of, 662 
of the dog, 662 
cortex, 655 
hemispheres, phenomena exhibited by 
animals deprived of, 637-643 
operations, time taken up by, 724 
Cerebrin, 98, 895 
Cerebro-spinal fluid, 730 
nerves, 124 
Cerebrum, under surface or base of, 123 
Changes of living tissue, 46 
Charcot's crystals, 896 
Chauveau and Lortet's haematachometer, 
153 
Chemical changes, 89 
Cheyne-Stokes respiration, 385 
Chitin, 883 
Cholalic acid, 912 
Cholesterin, 97, 279, 893 
Choletelin, 915 
Chondrin, 882 
Chorda saliva, 721 • 
tympani nerve, nature and action of, 
264 
Chordae vocales, 821 
Choroid coat, 740 
Chromatic aberration, 757 
Chyle, characters of, 316 
Chyme, 307 
Ciliary movement, 119 
Circles, diffusion, 746 
Circulation, causes of irregular heart-beat, 
238 
circumstances determining character of 
flow, 144 
effects of alcohol on, 241, 242 
of food on, 244 
foetal, 851 
hydraulic principles of, 144 
influence of exercise on, 242 
Clarke, columns of, 538 
Cochlea, 794 
Cold, sensations of, 703 
Color-blindness, 774 
sensations, 769 
Colostrum, 480 
Composition of bile, 279 
of blood, 50 
of gastric juice, 252 
of milk, 480 
of perspiration, 432 
of saliva, 246 
of starving body, 484 
of the animal body, 483 
of urine, 409 
Concha, 798 
Connective tissue, 134 
Constant current, action of, 100 
Contractile muscles, 85 
tissues, 54, 116 
Contraction, breaking, 101 
idio-muscular, 112 
making, 101 
tetanic, 57 
wave of, 78, 117 
Contrast of visual perceptions, 778 
Coordinated movements, machinery of, 643- 
652 
Cord, spinal, 529 
volitional impulses in, 585 
Cornea, 738 
Cornu ammonis, 660 
Corpora geniculata, 625 
quadrigemina, functions of, 719 
Corpus luteum, 840 
Corpuscles, connective tissue, 134 
number of, in human blood, 40 
of blood, 37 
Cortex, cerebral, 655, 721 
histology of, 655-661 
psychical processes in, 722 
Corti, organ of, 795 
pillars of, 795 
rods of, 795 
Cortical areas, removal of, 674 
motor region, 662-668 
Cramps, muscular, 118 
Cristae acusticae, 798 
Crystalline lens, 744 
Crystals, Charcot's, 896 
Currents, constant, 58 
electrotonic, 104 
induced, 58 
of action, 95 
of rest, 95 
Curves of endocardiac pressure, 168 
Cutaneous sensations, 703 
Cystin, 908 
DANIELL'S battery, 57 
Death, 863 
Decomposition of proteids by digestion, 879 
Defecation, 301 
Deglutition, 294 
Depressor nerve, 225 
Detection in solutions, S08 
Development of embryo, 841 922 
INDEX. 
Development of placenta, 846 
Dextrin, 248, 886 
Dextrose, 248, 884 
Diabetes, 448 
Diagrammatic eye, 745 
Diaphragm as a respiratory muscle, 339 
Dicrotism, 185 
Diet, normal, 513 
modifications of, 519 
Difference in respiration of sexes, 339 
Digestion, 245 
decomposition of proteids by, 879 
Dilatation of arteries, 134 
Dioptric apparatus, imperfections in, 757 
mechanisms, 744 
Discharge of energy, 722 
Diuretics, 425 
Divisions of ear, 790 
of spinal cord, 530 
Dubois-Reymond key, 60 
Ducts of mammary glands, 477 
Dyspnoea, 339, 380 
Exhalation of carbonic acid, 344, 364 
of organic matters, 346 
Exhaustion, causes of, 115 
Expiration, 342 
Eye, 738 
apparent size, 780 
ciliary region of, 740 
diagrammatic, 745 
emmetropic, 752 
horizontal section of, 739 
hypermetropic, 748 
images reflected from, 750 
mechanisms of, 789 
movements of, 781 
muscles of, 781 
myopic, 748 
physiological anatomy of, 738-744 
pigment cells of, 741 
presbyopic, 748 
FACIAL nerve, 607 
course of, 607 
Fallopian tubes. 832 
Faradization, 63 
Fats, complex nitrogenous, 892 
formation of, 474 
nature of, in adipose tissue, 472 
neutral, 890 
their derivatives and allies, 888 
Feces, 313 
Female generative apparatus, 834 
Fibres, afferent, 124 
efferent, 124 
sensory, 124 
vaso-constrictor, 216 
vaso-dilator, 216 
course of, 218 
Fibrin, 875 
in clotting of blood, 28 
Fibrinogen, 872 
Fick's spring manometer, 176 
Fifth nerve, 608 
branches of, 609 
Flatulence, 308 
Foetal circulation, 850 
Food, action of bile and pancreatic juice in 
small intestine, 308 
changes of. in large intestine, 312 
in mouth, 306 
in small intestine, 308 
in stomach, 306 
effects of fatty and carbohydrate, 489 
of gelatin as, 490 
salt as, 491 
peptone as, 491 
-stuffs, 245 
classes of, 245 
Formic acid, 888 
Fourth nerve, 612 
Fovea hemispherica, 794 
Functional activity, influence of, on con- 
tractile tissues, 114 
Ear, 790 
cochlea of, 794 
division of, 790 
Eustachian tube, 798 
external, 790 
internal, or labyrinth, 793 
meatus auditorius externus, 790 
membrane of Reissner, 796 
membrani tympani, 792 
membranous labyrinth, 798 
organ of Corti, 796 
osseous labyrinth of, 797 
ossicles of, 798 
perilymph, 797 
physiological anatomy of, 790 
stapedius muscle of, 800 
tensor tympani muscle of, 799 
Efferent impulses, 525 
Elastin, 883 
Electrical changes, 92 
stimuli, 57 
Electrodes, non-polarizable, 92 
Electrotonic currents, 104 
Electrotonus, 101 
variations of irritability during, 103 
Eleventh nerve, 603 
Embryo, nutrition of, 847 
Emmetropic eye, 752 
Energy, discharge of, 722 
expenditure of, 493 
income of, 492 
of mechanical work, 495 
Entoptic phenomena, 759 
Enzymes, 880 
Epiglottis, 820 
Epileptiform convulsions, 671, 675 
Epithelium of the ducts, functions of, 276 
Ethylene-lactic acid, 892 
Ethylidene-lactic acid, 892 
Eustachian tube, 800 
Eupnoea, 380 
Exhalation of aqueous vapor, 344 
HALEN, veins of, 734 
yj Gall-stones, 286 INDEX. 
923 
Gases in blood, 50 
Gastric and intestinal movements, nervous 
mechanisms of, 302 
digestion, circumstances affecting, 257 
acidity, 258 
temperature, 258 
juice, 251 
action of, on milk, 259 
on proteids, 252 
chemical characters of, 251 
formation of free acid of, 276 
nature of the action of, 258 
secretion of, 266 
Gelatin, 134, 882 
Generative organs, female, 834 
male, 833 
physical anatomy of, 834 
Globin, 874 
Globulins, 871 
Glottis, narrowing of, 823 
widening of, 824 
Glutamidic acid, 908 
Glutin, 882 
Glycerin, 891 
-phosphoric acid, 895 
Glycin, 906 
Glycogen, 88, 437, 887 
characters of, 438 
conversion of, into sugar by liver, 
438 
in muscle, 447 
in placenta, 448 
uses of, in liver, 446 
Glycolic acid series, 892 
Gmelin's test for bile, 280 
Goll, columns of, 540 
Graafian follicle, 938 
Gray matter, 128 
Grove's-battery, 57 
Guanin, 905 
Gudden's commissure, 693 
Gum, animal, 888 
Gustatory bulbs, 811 
mucous membrane, physiological anat- 
omy of, 808 
pore, 811 
Heart, endocardiac pressure, 164 
methods of determining, 165 
first sound of, 163 
impulse of, 162 
influences regulating beat of, 209 
main events occurring in ventricle dur- 
ing a beat, 171 
negative pressure, nature and cause of, 
172 
normal beat of, 157 
analysis of, 193 
development of, 192 
regulation of beat of, 191 
second sound of, 163 
sounds of, 162 
summary of events constituting a beat, 
174 
valves of, 158 
ventricular systoles, 159 
visible movements of, 157 
work done by, 175 
Heat, sensations of, 703 
Helmholtz's arrangement for equalizing the 
make and break shocks, 63 
phakoscope, 750 
Hemiamblyopia, 696 
Hemianopia, 696 
Hemianopsia, 696 
Hemiopia, 696 
Hippocampal lobule, 700 
Hippuric acid, 910 
formation of, 422 
Histohaematin, 86 
Horopter, 786 
Human brain, nomenclature of the surface 
of, 676, 
ovum, 836 
Hyaloid membrane of eye, 741 
Hydrobilirubin, 915 
Hypernoea, 380 
Hypnotic phases, 684 
Hypoglossal nerve, 602 
course of, 602 
Hypoxanthin, 903 
IMAGE, formation of, 744 
1 Imperfect speech, 679 
Impregnation, 841 
Impulse, nervous, 98 
Indican, 915 
Indigo, 916 
series, 915 
Indol, 284, 916 
luduction coil, 59, 71 
Inflammation, phenomena of, 232 
Inhibition, features of, 200 
of frog's heart by stimulation of vagus 
rierve, 200 
Inhibitory nerves, 132 
Inogen, 116 
Inosit, 886 
Inspiration, 340 
Intercostal muscles in respiration, 341 
Interfibrillar substance, 80 
Intestinal digestion, 308 
Hemacytometer of Gowers, 42 
Haemadromometer, Volkmann's, 151 
Haematachometer, Chauveau and Lortet's, 
153 
Haematin, 359 
Haemin, 360 
Haemoglobin, 351 
in red blood-corpuscles, 40 
Hearing, sensations of, 701 
Heart, 157 
-beat, augmentation of, 203 
government of, by the nervous sys- 
tem, 199 
reflex inhibition of, 203 
cardiac cycle of, 158 
sound and tambour, 166 
change of form of, 160 
duration of the several phases of the 
cardiac cycle, 173 924 
INDEX. 
Iris, 741 
Irradiation, 778 
Male generative apparatus, 835 
glands, 835 
prostate, 836 
semen, 838 
spermatozoa, 838 
testicles, 835 
tunica albuginea, 836 
vas deferens, 836 
vesiculae seminales, 837 
Maltose, 248, 885 
Mammary glands, 477 
changes occurring in, 478 
relation to nervous system, 482 
secretion of milk, 481 
structure, 477 
Manometer, mercury, 139 
venous, 146, 147 
Marey's tambour with cardiac sound, 167 
Mastication, 294 
Maximum manometer of Goltz and Gaule, 
165 
Maxwell's method of fusing color sensations, 
770, 775 
Mechanisms, dioptric, 744 
of accommodation, 749 
locomotor, 829 
Membrana tympani, 798 
Membrane of Reissner, 795 
Menstruation, 836 
Mercurial gas pump, Ludwig's, 346 
Metabolism, general features of, 506 
influence of nerves on, 510 
Metabolites, nitrogenous, 896 
Metamere, neural. 123 
Methaemoglobin, 360 
Meynert's commissure, 693 
Microorganisms, action of, in alimentary 
canal, 311 
Micturition, 427 
involuntary, 429 
nervous mechanism of, 428 
voluntary, 429 
Middle ear, 792 
Milk, composition of, 480 
constituents of, 478 
human, 478 
quantity secreted, 480 
secretion of, 481 
sugar, 885 
Millon's reagent, 29 
Morse key, 60 
Motor area, characteristics of, 659 
for leg, arm, and face in man, posi- 
tion and relative extent of, 676 
Movements of heart, 157 
of limbs, 682 
of pupil, 751 
skilled, 673 
Mucin, 246, 881 
Mucous gland, changes in during secretion, 
271 
Muscae volitantes, 759 
Muscle and nerve, degree of irritability of, 
111 
experiments with, 60 
phenomena of, 55 
JACOB'S membrane, 743 
Jaundice, 456 
Juice, gastric, 251 
action on proteids, 253 
artificial, 253 
composition of, 252 
pancreatic, 281 
action on food-stuffs, 282 
IT ATELECTROTONUS, 103 
Keratin, 883 
Kidneys, vaso-constrictor nerves of, 415 
vaso-dilator nerves of, 416 
vasomotor mechanisms of, 411 
Kinesodic, 714 
Kreatin, 88, 901 
Kreatinin, 902 
Kymograph, 142 
Ludwig's, 142 
Kynurenic acid, 906 
T ACTEALS, 313 
IJ Lactic acid, 892 
Laminae in a hardened lens, 744 
Lardacein, 879 
Large intestine, movements of, 300 
Larynx, 819 
cartilages of, 820 
cavity of, 821 
muscles of, 821 
vocal cords, 821 
Latent period, 70 
Laurostearic acid, 889 
Lecithin, 97, 894 
Leclanche battery, 57 
Leucin, 907 
Leucocythaemia, characterized by increase 
of white corpuscles, 49 
Leukomaines, 917 
Life, phases of, 855, 862 
Ligamentum nuchae, 134 
Limb, movements of, 682 
Liver, nerves of, 449 
Living body, study of, 17 
Load, influence of, 110 
Lobes of brain, 585 
Ludwig's kymograph, 142 
stromuhr, 152 
Lungs, 332 
atelectatic, 334 
capacity of, 336 
pressure exerted in breathing, 335 
Lymph, characters of, 315 
chemical composition of, 315 
-hearts, structure and functions, 324 
microscopic characters of, 215 
movements of, 317 
MACULjE acusticae, 798 
Magnetic interrupter, 62 INDEX. 
925 
Muscle case, 80 
chemistry of, 83 
contractions, simple, 65 
currents, 92 
curve, 65 
double, 73 
from the gastrocnemius of the frog, 
65 
single induction shock repeated 
rapidly, 73, 74, 75 
slowly, 73 
dead, 84 
during contraction, changes taking 
place in, 77 
energy of, 115 
Muscle fibres, microscopic changes, 81 
-nerve preparation, 57, 102 
as a machine, 107 
plasma, 85 
sartorius of frog, 78 
size and form of, 111 
stapedius, 800 
under polarized light, 81 
Muscular and nervous action, nature of, 115 
irritability, 55 
contraction, single, 57 
simple, 57 
irritability, 57 
sense, 808 
Myoglobulin, 86 
Myograph, pendulum, 69 
spring, 69 
Myosin, 84, 873 
Myosinogen, 86 
Myristic acid, 889 
Neurokeratin, 98 
Neutral fats, 890 
Nicol prism, 81 
Nitrate of urea, 897 
Nitrogenous metabolism, 487 
metabolites, 896 
non-crystalline bodies allied to proteids, 
881 
Nuclein, 883 
Nucleo-albumin, 884 
OCCIPITAL region, prominence of nu- 
clear cells in, 659 
Ocular spectra, 780 
(Edema, 322 
(Esophagus, movements of, 296 
Oleic series, acids of, 890 
Olein, 891 
Olfactory mucous membrane, cells of, 807 
sensations, 700 
Oncograph, 413 
Oncometer. 412 
Optic radiation, 694 
thalamus, 615 
Organic matter, exhalation of, 345 
Organs of Corti, 795 
of reproduction, 832 
Ovaries, 835 
Ovum, 838 
Oxalate of urea, 897 
Oxalic acid series, 892 
PALMITIC acid, 889 
Palmitin, 890 
Pancreas, changes in, during secretion, 269 
of rabbit, 269 
Pancreatic juice, 282 
action of, on fats and starch, 284 
on food-stuffs, 282 
characters of, 282 
secretion of, 286 
Papillae, circumvallate, 809 
filiform, 809 
fungiform, 809 
Paraglobulin, 871 
in the clotting of blood, 30 
Paralysis, crossed, 685 
Parapeptone, 256 
Parotid gland, nervous mechanism of, 265 
Parturition, 853 
Pendulum myograph, 68 
Pepsin, 258 
Peptogenous food, 277 
Peptones, 256, 875 
Perceptions, general sensibility and tactile, 
812 
modified, 778 
Peristaltic movements, 292 
influences bearing on, 305 
Perspiration, 431 
Pettenkofer's test for bile salts, 280, 912 
Phenylic acid, 911 
Physostigmine, action of, on pupil, 756 
Pituitary body, 468 
NASAL fossa, right, 806 
fossae, 806 
Nerve and muscle, electric currents of, 92 
cells, grouping of, 535 
of spinal cord, 129 
variations in, 552 
centre, 130 
chemistry of, 97 
eighth or auditory, 605 
eleventh or spinal accessory, 603 
fibres in retina, connections of, 742 
fifth or trigeminal, 608 
fourth or trochlear, 612 
ninth or glosso-pharyngeal, 603 
roots, connections of, 544 
seventh or facial, 607 
sixth or abducens, 608 
tenth or vagus, 603 
tetanization of, 99 
third or oculo-motor, 612 
twelfth or hypoglossal, 602 
Nerves, electric currents in, 99 
Nervous impulse, 98 
changes in nerve during passage 
of, 97 
measurement of velocity of, 70 
system, central, 128 
tissues, general features of, 122 
Neurin, 98, 895 926 
INDEX. 
Plasmine, in clotting of blood, 32 
Pons Varolii, 584 
gray matter of, 623 
Posterior column of spinal cord, 531 
Presbyopic eye, 748 
Pressure, arterial, 138 
in capillaries, 143 
in veins, 138 
Products of digestion, course taken bv fats, 
325 
by proteids, 327 
by sugar, 326 
by water and salts, 326 
Proprionic acid, 889 
Prostate gland, 834 
Protagon, 98, 895 
Proteid bodies, characters of the more im- 
portant, 253 
Proteids, 866 
classification of, 257 
coagulated, 875 
decomposition of, by digestion, 878 
general composition of, 28 
xanthoproteic test for, 28 
Protoplasm, 20 
Pseudopodia, 121 
Psychical processes, duration of, 725 
Ptomaines, 313, 345, 896 
Pulse, 176 
anacrotic, 185, 190 
katacrotic, 185 
methods of recording, 176 
venous, 190 
Pulse-curve, characters of, 181 
Pulse-curves described by sphygmographic 
levers, 177 
Pulse-tracing from an artificial model, 
179 
from carotid artery of healthy man, 
185 
Pulse-wave, anacrotic, causes of, 190 
changes of, along the arterial tract, 
182 
diacrotic, 185 
cause of, 185 
features of, 182 
length of, 185 
predicrotic, 189 
velocity of, 184 
Pupil, movements of, 751 
nerves governing, 753 
Purkinje, cells of, 653 
Purkinje's figures, 760, 762 
Pyramidal cells, 656 
Pyrexia, 504 
Pyriform lobe, 700 
Rapidity of circulation, 150 
in capillaries, 154 
in veins, 154 
of clotting of blood, 27 
Reaction of urine, 409 
Red-blind, 774 
blood-corpuscles, 37 
counting of, 42 
apparatus for the, 42 
diameter of, 37 
disintegration of, 42 
number of, 41 
shape of, 37 • 
structure of, 37 
study of, 37 
marrow in bone, a source of red cor- 
puscles, 43 
Reflex actions, 130 
nature of, 130 
of spinal cord, 564 
inhibition, 203 
Refraction, 745 
Regulation of heat. 499 
Reissner, membrane of, 795 
Rennin, 250 
Reproduction, 832 
corpus luteurn, 840 
of menstruation, 840 
of pregnancy, 840 
development of embryo, 842 
allantois, 844 
placenta, 847 
segmentation of ovum in, 
842 
vascular area, 843 
Graafian follicle in, 838 
impregnation of ovule, 841 
menstruation, 838 
organs of, 832 
Fallopian tubes, 835 
male, 835 
ovaries, 835 
uterus, 834 
vagina, 834 
ovule in, 841 
ovum in, 838 
spermatozoa in, 841 
Respiration, 332 
absorption of oxygen in, 361 
apnoea in, 384 
apparatus for taking tracings, 337 
asphyxia in, 336, 386 
phenomena of, 386 
carbonic acid exhaled, amount of, 344 
changes of air in, 344 
Cheyne-Stokes, 385 
complemental air in, 334 
dyspnoea in, 336, 380 
effect of breathing foreign gases, 388 
of changes in atmospheric pres- 
sure, 389 
of muscular exercise on, 383 
exhalation of aqueous vapor, 344 
of carbonic acid, 344, 364 
of organic matters, 345 
expiration, 342 
Quadrigeminal bodies, 625 
Quality of sound, 801 
Quantitative determination, 898 
Quantity of aqueous vapor exhaled, 344 
of blood, 53 
mode of estimating, 54 
of carbonic acid exhaled, 344 
of organic matters exhaled, 345 INDEX. 
927 
Respiration, expired air, impurities of, 345 
nature of, 345 
temperature of, 344 
facial and laryngeal, 343 
graphic records of movements of, 336 
influence of vagus nerves on, 372 
inspiration, 340 
labored inspiration, 342 
modified movements of, 403 
coughing, 403 
crying, 404 
hiccough, 403 
laughing, 404 
sighing, 403 
sneezing, 403 
sobbing, 403 
yawning, 403 
movements of diaphragm, 339 
of expiration, 342 
of inspiration, 339 
muscles of, 340 
nervous mechanism of, 369 
number of, 339 
relations of respiratory system to vas- 
cular and other systems, 390 
residual air in, 334 
ribs in, action of, 341 
function of, 341 
stationary air in, 833 
tidal air in, 333 
visible movements in, 339 
Respiratory centre, 370 
changes in blood, 346 
in the tissues, 365 
undulations, 391 
Retina, 741 
connective tissues of, 741 
inner surface of, 743 
of man, rod and cone from, 742 
photo-chemistry of, 762 
Ribs, action of, in respiration, 341 
function of, in respiration, 340 
Rigor mortis, 83 
Ritter-Valli law, 112 
Rods of Corti, 795 
Rolando, substantia gelatinosa of, 534 
tubercle of, 598 
Sarkin, 903 
Scala tympani, 794 
vestibuli, 795 
Scheiner's experiment, diagram of, 747 
Schlemm, canal of, 739 
Sclerotic coat, 739 
Secretion, general nature of, 273 
nature of the act of, 275 
Segmentation of ovum, 842 
Semicircular canals, 794 
Seminal fluid, 838 
' Seminiferous tubules, 836 
| Sensation, 759 
Sensations, auditory, 800 
cutaneous, 703 
distinction and fusion of, 766 
of cold, 703 
of color, 769 
primary, 773 
of hearing, 701 
of heat, 703 
of pressure, 814 
of smell, 700 
of taste, 701 
of temperature, 703, 814 
of touch, 703 
relation of, to stimulus, 766 
simple, 766 
tactile, 814 
visual, 689, 759 
Sense, muscular, 814 
Sensory impulses, 759 
Serous fluid, chemical character of, 316 
Serum-albumin in clotting of blood, 30 
Serum in clotting of blood, 29 
Seventh nerve, 607 
j Sexes, difference in respiration, 339 
Sight, 738 
Sixth nerve, 608 
Skatol, 917 
Skin, absorption by, 433 
cutaneous respiration, 432 
perspiration, amount of, 431 
composition of, 432 
secretion of sweat, mechanism of, 434 
Small intestine, contents of, 308 
movements of, 292, 300 
Smell, sensations of, 700 
Soaps, 891 
Somatic and splanchnic nerves, 123 
Sommerring, yellow spot of, 743 
Sound, 801 
pitch of, 801 
quality of, 801 
Sounds of the heart, 162 
Specific gravity of urine, 405 
Spectra, ocular, 780 
Spectroscopic analysis of haemoglobin, 
353 
Speech, 826 
areas for, 676 
imperfect, 679 
caused by bulbar disease, 680 
thick, 679 
Spermatozoa, 838 
Spherical aberration, 757 
SACRAL nerves of dog, 125 
Saline matters in blood, 51 
Saliva, 246 
action of, on starch, 247 
characters of mixed, 250 
of parotid, 251 
of sublingual, 251 
of submaxillary, 251 
chemical characters of, 246 
composition of, 246 
nature of amylolytic action of, 249 
secretion of, by means of chorda tym- 
pani nerve, 261 
Salts of bile, 280 
of uric acid, 900 
Santorini, cartilage of, 820 
Sarcolactic acid, 892 928 
INDEX. 
Sphygmograph, Dudgeon's, 177 
Marey's 177 
Spinal accessory nerve, 603 
cord, 525 
antero-lateral ascending tract of, 
559 
ascending tracts of, 543 
automatic actions of, 578 
central canal of, 530, 534 
cerebellar tract, 556 
columns of Burdach, 540 
of Clark, 539 
of Goll, 540 
complexity of reflex movements 
of, 570, 574 
cornu of, 530 
descending tracts of, 541 
features of, 548 
fissures of, 529 
gelatinous substance of Rolando, 
534 
gray matter, nature of, 560-563 
structure of, 523 
longitudinal, commissural tracts 
of, 563 
loss of tone of skeletal muscles in, 
579 
reflex actions of, 564 
inhibitions of, 574 
time required for, 577 
relative size and form of, 549 
reticular formation, 538 
rigidity of muscles through action 
of, 583 
structure of, 529 
tendon phenomena of, 582 
white matter, structure of, 530 
tracts of, 540 
disease, sensations of pain in, 711 
ganglia, 125 
nerves, 525 
Spleen, chemical constituents of, 453 
movements of, 451 
red corpuscles in, 43 
Spring manometer, Fick's, 176 
Stapedius muscle, 800 
Starch, 247 
Stearic acid, 889 
Stearin, 890 
Stellate nerve cells, 127 
Stimuli, characters of, 107 
Stimulus, nature and mode of application 
of, 107 
Stomach, gastric juice in, 266 
movements of, 297 
Submaxillary gland, 261 
nerves of, 261-263 
Succinic acid, 893 
Succus entericus, 285 
nature and action of, 285 
Supra-renal bodies, 467 
chemical constituents of, 468 
functions of, 469 
structure of, 468 
Sylvius, aqueduct of, 585 
Syntonin, 85 
1 TACTILE perceptions and judgments, 816 
. Tambour, Marey's 167 
Taste, 808 
buds, 811 
circumvallate papillae, 809 
filiform papillae, 809 
fungiform papillae, 809 
gustatory bulbs, 811 
sensations of, 701 
! Taurin, 906 
Taurocholic acid, 913 
Temperature, 498 
effects of great cold on, 505 
of great heat on, 504 
influence of food on, 501 
on contractile tissues, 113 
of muscular action on, 501 
of time of day on, 503 
of body, 497, 498 
of various animals, 497 
pyrexia, 504 
sensations of, 703 
Tensor tympani muscle of ear, 799 
Tenth nerve, 603 
Testicles, 836 
Tests for bile, 280 
Tetanic contractions, 57, 73 
j Tetanus, 73 
contractions in, 109 
Thalami optici, 623 
| Thermal changes, 90 
| Thermopile, 90 
Third nerve, 612 
Thoracic duct, 314 
Thymus, functions of, 469 
structure of, 470 
Thyroid body, 465 
functions of, 466 
structure of, 465 
I Tidal air, 333 
Tissues, contractile, 54 
Tongue, 808 
Tonic contraction, 118, 669 
Tracing from heart of cat, 161 
of respiration, 338 
Transudation, phenomena of, 319 
Traube-Hering variations, 736 
Trigeminal nerve, 608 
Trochlear nerve, 612 
Trypsin, 274 
Trypsinogen, 275 
Tunica albuginea, 836 
Tunicin, 885 
Twelfth nerve, 602 
Tyrosin, 909 
UMBILICAL vesicle, 843 
Urari, poisoning by, 56 
rise of blood-pressure in animals 
under, 708 
Urea, 896 
formation of, in liver, 459, 460 
group, 896 
nitrate of, 897 
oxalate of, 897 INDEX. 
929 
Urea, relations to cyanogen compounds, 464 
synthesis of, 406, 462 
Uric acid, 405, 462, 899 
salts of, 900 
Urine, 404 
abnormal constituents of, 409 
albumin, 410 
sugar, 410 
acidity of, 409 
amount of, 408 
composition of, 409 
ferments in, 408 
general characters of, 404 
hippuric acid in, 406 
inorganic salts in, 406 
non-nitrogenous constituents of, 407 
normal organic constituents of, 405 
pigments, 481, 915 
reaction of. 409 
relations of secretion of, to food and 
drink, 425 
secretion of, 410 
glomerular, 420 
specific gravity of, 405 
urea in, 405 
uric acids in, 405 
amount of, in, 405 
Urobilin, 915 
Uroerythrin, 915 
Uterus, 834 
os uteri, 835 
structure of, 833 
Utricles of ear, 797 
Vision, binocular, 781 
in man, apparatus of, 691 
region of distinct, 777 
Visual field, 689 
impulses, origin of, 760 
judgments, 786 
of distance, 786 
of size, 786 
of solidity, 787 
movements, coordination of, 784 
perceptions, 776 
contrast of, 778 
purple, 764 
sensations, 689, 759 
white, 764 
yellow, 764 
Vitellin, 873 
Vitreous body, 744 
Vocal cords, 821 
movements of, 823 
slackening of, 824 
tightening of, 824 
Volitional impulses in the cord, 684 
Volkmann's haemadromometer, 151 
Voluntary movements, 661, 679 
action of, on the lower animals, 662 
Vomiting, 299 
VVATERY vapor, exhalation of, 344 
VV Wave, contraction, 79 
White blood-corpuscles, 43 
chemical examination of, 45 
migration of, 48 
movements of, 47 
nuclei of, 44 
number of, 44 
origin of, 47 
proportion of, to red, 44 
size of, 44 
structure of, 44 
transformation into red, 49 
work of, 47 
White matter, 97, 128 
Willis, circle of, 733 
Work done by heart, 175 
VAGINA, 834 
V Vagus nerve, 603 
Valerianic acid, 889 
Various forms of stimuli, 58 
Vascular mechanism, 133 
Vas deferens, 836 
Vaso-constrictor fibres, course of, 218 
nerves, 124 
-dilator fibres, course of, 220 
-motor actions, 211 
effects of, 220 
Veins, blood-pressure in, 137 
circulation in, rapidity of, 154 
vasomotor nerves of, 231 
Velocity of blood in arteries, 150 
in capillaries, 154 
of the pulse wave, 154 
Venous pulse, 190 
sinuses, 734 
Vertigo, phenomena and causation of, 647 
Vesicuke seminales, 837 
Vibrating tuning-fork with Despretz signal 
70 
Vierordt haematachometer, 153 
Vision, 759 
59 
V A NTH IN, 904 
A Xanthoproteic test for proteids, 28 
yOUNG-HELMHOLTZ theory of color 
1 sensations, 772-775 
^INN, zone of, 744 
LA Zona pellucida, 841 
spongiosa, 535 
Zymogen, 275