FUNDAMENTALS OF HUMAN PHYSIOLOGY FUNDAMENTALS 
OF 
HUMAN PHYSIOLOGY 
BY 
R. G. PEARCE, B.A., M.D., 
Assistant Professor of Physiology, University of Illinois 
AND 
J. J. R. MACLEOD, M.B„ D.P.H., 
Professor of Physiology. Western Reserve University 
SEVENTY-ONE ILLUSTRATIONS AND EIGHT COLOR PLATES 
ST. LOUIS 
C. V. MOSBY COMPANY 
1916 Copyright, 1916, by the C. V. Mosby Company 
Press of 
The C. V. Mosby Company 
St. Louis PREFACE. 
The object kept in view in the preparation of the present vol- 
ume has been to give an elementary, yet comprehensive, review 
of the various facts and theories which go to form the modern 
science of human physiology. Jt is hoped that such a volume 
will be of use, not only to college students who may desire, for 
the sake of the knowledge itself, to learn something of the work- 
ings of the human body, but also to those who must know some 
physiology before they can properly proceed to study other 
sciences, such as pharmacology and hygiene. 
Although similar in scope with the sister volume designed 
primarily for the use of dental students, considerable altera- 
tions have been made so as to substitute, for those parts of the 
subject with which the dental student must have a special ac- 
quaintance, matters of more general interest. Thus, much less 
space is devoted to the subjects of salivary secretion and dental 
caries, while on the other hand more attention has been given 
to a brief description of the structure of the more important 
organs and tissues, and to other general facts of anatomy. The 
physiological action of a few of the better known drugs has also 
been indicated, and the chapters dealing with the physiology 
of the central nervous system have been somewhat simplified. 
In the first two chapters some of the essential facts bearing on 
the application of the laws of physical chemistry to life pro- 
cesses are discussed, and since a general knowledge of these 
laws is assumed, it is advised that students who may be unfamil- 
iar with them should consult some text in general chemistry. 
The authors desire to thank Prof. T. Wingate Todd and Mr. 
P. M. Spurney for their kind assistance in the preparation of 
the diagrams. 
The authors are also deeply indebted to Dr. Paul G. Hanzlik 
for his advice in connection with the adaptation of the book for 
the use of students of pharmacy. 
R. G. Pearce. 
J. J. R. Macleod. 
V CONTENTS. 
Chapter I. 
THE STRUCTURAL BASIS OF THE BODY. Page 
The Scope of Physiology-The Structural Basis of the Body-The 
Epithelial Tissue-The Connective Tissue-The Muscular Tis- 
sue-The Nervous Tissue-The Gross Structures of the Body- 
The Skin-The Subcutaneous Tissues-The Muscles-The Body 
Cavities-The Skeleton-The Bones of the Trunk-The Limbs 
-The Articulations 17 
y Chapter II. 
THE PHYSICO-CHEMICAL BASIS OF LIFE. 
The Chemical Basis of Animal Tissues-Water-Proteins-Lipoids- 
Carbohydrates-Physico-The Influence of Chemical Laws on 
Physiological Processes-Properties of Crystalloids-Osmotic 
Phenomena in Cells-Reactions of Body Fluids-Colloids-Gen- 
eral Nature of Enzymes or Ferments 33 
Chapter III. 
THE MUSCULAR SYSTEM. 
The General Properties of Muscular Tissues - Contractility - 
Irritability-The Simple Muscular Contraction-Tetanic Con- 
traction - Effect of Load-^Elasticity of Muscle - Chemical 
Changes Accompanying Contraction-Rigor Mortis 48 
Chapter IV. 
THE BLOOD. 
Introduction-Physical Properties-The Corpuscles-Erythrocytes 
-Haemoglobin-Enumeration of Blood Corpuscles-The Origin 
of the Erythrocytes-The White Cells-Leucocytes-Lympho- 
cytes-Estimation of the White Cells-Function of the Leuco- 
cytes-The Blood Platelets-The Blood Plasma 51 
VIII IX 
Chapter V. 
THE BLOOD. Page 
The Defensive Mechanism of the Blood-Coagulation of the Blood 
-Antibodies in the Blood-The Process of Inflammation- 
Toxins-Antitoxins-Ehrlich's Side Chain Theory of Immunity 
-Anaphylaxis-Phagocytosis-Opsonins-Vaccines-Serum Di- 
agnosis 58 
Chapter VI. 
THE LYMPH. 
Lymph Formation-Lymphagogues-Lymph Reabsorption-The 
Movement of Lymph 66 
Chapter VII. 
THE CIRCULATORY SYSTEM. 
Introduction-Anatomical Considerations-The Heart-The Blood 
Vessels-Physiological Properties of Heart Muscle-Character 
of Cardiac Contraction-The Sequence of the Heart Beat-The 
Action of Inorganic Salts on the Heart Beat-The Vascular 
Mechanism of the Heart-Definition of Terms-Events of the 
Cardiac Cycle-The Heart Sounds-Diseases of the Cardiac 
Valves .' 70 
Chapter VIII. 
THE CIRCULATION. 
The Blood Flow Through the Vessels-The Part the Heart Plays- 
The Part the Arteries Play-Arterial Blood Pressure-Factors 
Which Maintain the Blood Pressure-Velocity of Blood Flow- 
The Return of the Blood to the Heart-Circulation Time- 
Pulsatile Acceleration of the Blood Flow-The Pulse-The Cir- 
culation in the Lungs-The Influence of the Nervous System on 
the Circulation-The Nervous Control of the Heart-The Car- 
diac Nerves-Accelerator Nerves-Inhibitory Nerves-Relation 
of the Sympathetic and Vagus Nerves to the Heart-The Car- 
diac Center-The Cardiac Depressor Nerves-The Nervous Con- 
trol of the Blood Vessels-Vasomotor Nerves-Vasoconstrictor 
Nerves-Vasodilator Nerves-Vasomotor Reflexes-The Effect 
of Gravity on the Circulation-Haemorrhage-Chemical Control 
of Circulation-Asphyxia-Action of Drugs on Circulation 83 
CONTENTS. CONTENTS. 
X 
Chapter IX. 
THE RESPIRATION. Page 
Introduction-Internal Respiration-Oxidation in the Tissues-Re- 
lation of Oxidative Process to Activity-Physical Laws Govern- 
ing Solution of Gases-Haemoglobin-The Mechanism of the 
Respiratory Exchange-The Exchange of Carbon Dioxide-The 
External Respiration-Anatomical Considerations-The Mech- 
anism of Breathing-The Part the Diaphragm Plays-The Part 
the Thorax Plays-The Movements of the Lungs-Respiratory 
Sounds - Artificial Respiration - Volumes of Air Respired - 
Mechanism of Gaseous Exchange in Lungs 104 
Chapter X. 
THE RESPIRATION. 
The Nervous Control of the Respiration-Reflex Respiratory Move- 
ments - Chemical Control of Respiration - The Effect of 
Changes in the Respired Air on the Respiration-Mountain 
Sickness-Ventilation-The Voice-Mechanism of the Voice- 
Speech 121 
Chapter XI. 
ANIMAL HEAT AND FEVER. 
Animal Heat-Normal Temperature-Factors Concerned in Main- 
taining the Body Temperature-Regulation of Body Tempera- 
ture-Fever-Antipyretics 132 
Chapter XII. 
DIGESTION. 
Necessity and General Nature of Digestion-The Alimentary Canal 
-Anatomical Considerations-Blood Supply of the Alimentary 
Canal-The Mouth-The Teeth-The Salivary Glands-The 
Pharynx-The Stomach-The Small Intestines-The Large In- 
testines-The Liver and the Pancreas 138 XI 
CONTENTS. 
Chapter XIII. 
DIGESTION. Page 
Digestion in the Mouth-Salivary Secretion-The Nerve Supply of 
the Salivary Glands-The Reflex Nervous Control of Salivary 
Secretion-General Functions of Saliva-The Hygiene of the 
Mouth-Tartar Formation and Salivary Calculi-Mastication- 
Deglutition or Swallowing-The Act of Vomiting 150 
Chapter XIV. 
DIGESTION: IN THE STOMACH. 
The Secretion of Gastric Juice - The Active Constituents of 
Gastric Juice - The Movements of the Stomach - The Open- 
ing of the Pyloric Sphincter-Rate of Discharge of Food from 
the Stomach 163 
Chapter XV. 
DIGESTION: IN THE INTESTINE. 
Secretion of Bile and Pancreatic Juice-Functions and Composi- 
tion of Pancreatic Juice and Bile-Chemical Changes Produced 
by Intestinal Digestion-Bacterial Digestion in the Intestine- 
Products of Bacterial Digestion-Protection of Mucous Mem- 
brane of Intestine Against Autodigestion-Movements of the 
Intestines - Action of Cathartics - The Absorption of Food- 
Resume of Actions of Digestive Enzymes 174 
Chapter XVI. 
METABOLISM: ENERGY BALANCE. 
Introductory-General and Special Metabolism-Energy Balance- 
Caloric Value of Foods-Basal Heat Production-Influence of 
Food, Muscular Work, Atmosphere, and Size of Body 186 
Chapter XVII. 
METABOLISM: THE MATERIAL BALANCE OF THE BODY. 
Starvation-Normal Metabolism-Nitrogen Balance-Protein Spar- 
ers-The Irreducible Protein Minimum-Varying Nutritive 
Values of Different Proteins 194 CONTENTS. 
XII 
Chapter XVIII. 
THE SCIENCE OF DIETETICS. Page 
The Proper Amount of Nitrogen-Chittenden's Experiments-The 
Most Suitable Diet for Efficiency-Chemical Composition of 
the Common Foodstuffs 202 
Chapter XIX. 
SPECIAL METABOLISM. 
Metabolism of Proteins-Urea-Ammonia-Creatinin-Purin Bodies 
-Relative Importance of Proteins, Fats and Carbohydrates in 
Metabolism 211 
Chapter XX. 
SPECIAL METABOLISM. 
Metabolism of Fats-Metabolism of Carbohydrates-Metabolism 
of Inorganic Salts-Vitamines 218 
Chapter XXL 
THE DUCTLESS GLANDS. 
Introduction-Thyroid and Parathyroid Glands-Adrenal Glands- 
Pituitary Gland-Spleen-Thymus Gland 227 
Chapter XXII. 
THE FLUID EXCRETIONS. 
The Excretion of Urine-Composition of Urine-Organic Constitu- 
ents-Urea-Ammonia-Creatinin-Uric Acid-Inorganic Con- 
stituents-Abnormal Constituents-The Organs of Excretion 
-The Blood Supply of the Kidney-Nature of Urine Excretion 
Micturition-Drugs which act on the Kidney-The Secretions 
of the Skin-The Sweat Glands-The Sebaceous Glands-The 
Mammary Glands » 239 XIII 
CONTENTS. 
THE NERVOUS SYSTEM. Page 
The Functions and Structure of the Nervous System-Fundamental 
Elements of the Reflex Arc-Integration of the Nervous Sys- 
tem 251 
Chapter XXIV. 
THE NERVOUS SYSTEM. 
Reflex Action-The Nerve Structures Involved in the Reflexes of 
the Higher Animals-The Receptors of Pain, Touch, Tempera- 
ture-Local Anesthesia and Analgesia-The Afferent Fiber- 
Choice of Paths on Entering Spinal Cord-The Nerve Center 
-The Efferent Neurone-Types of Reflexes-Spinal Shock- 
The Essential Characteristics of Reflex Action-Muscular 
Tone and Reciprocal Action of Muscles-Symptoms Due to 
Lesions Affecting the Reflexes 256 
Chapter XXV. 
THE NERVOUS SYSTEM. 
The Brain Stem-The General Course and Functions of the Cranial 
Nerves-The Brain-Influence of the Brain on the Reflex Func- 
tions of the Spinal Cord-Functions of the Cerebrum-Cerebral 
Localization - Experimental and Clinical Observations - The 
Sensory Centers-The Mental Process-Aphasia-The Cerebel- 
lum-Relationship to Body Equilibrium-The Semicircular 
Canals-The Sympathetic Nervous System-General Character- 
istics-The Course of Some of the Most Important Pathways- 
Action of Drugs on the Central Nervous System 268 
Chapter XXVI. 
THE SPECIAL SENSES: VISION. 
Optical Apparatus of the Eye-Formation of Retinal Image- 
Changes in the Eye During Accommodation from Near Vision 
-The Function of the Pupil-Imperfections in the Optical 
System of the Eye-Long and Short-Sightedness-Astigma- 
tism, etc.-The Sensory Apparatus of the Eye-The Functions 
of the Retina-Blind Spot-Fovea Centralis-The Movements 
of the Eyeballs-Diplopia-Judgments of Vision-Color Vision 
-Color Blindness 285 
Chapter XXIII. XIV 
CONTENTS. 
Chapter XXVII. 
THE SPECIAL SENSES. Page 
Hearing-The Cochlea-How Sound Waves are Transmitted to 
this by Tympanic Membrane and Auditory Ossicles-Causes 
of Deafness-Taste-Nature of Receptors for Taste-The 
Location of the Four Fundamental Taste Sensations-Rela- 
tionship Between Chemical Structure and Taste-Association 
Between Taste, Common Sensation of Touch, and Smell- 
Action of Certain Drugs on Taste-Smell-Nature of the Re- 
ceptors of Smell (the Olfactory Epithelium) - Nature of 
Stimulus 297 
Chapter XXVIII. 
REPRODUCTION. 
Fertilization-The Accessory Phenomena of Reproduction in 
Man-Female Organs-Male Organs-Impregnation-Ovulation 
-Pregnancy-Birth 306 ILLUSTRATIONS. 
Fig. Page 
1. Diagram of a cell . 20 
2. Types of epithelial cells 21 
3. Types of epithelial cells 22 
4. Connective tissue cells from a chick embryo 23 
5. Fibers from ligamentum nuchac of the ox 23 
6. Segment of a transversely ground section from the shaft of a 
long bone . 24 
7. Fat cells treated with alcohol 25 
8. Cell from smooth muscle of intestine; cross section of smooth 
muscle of intestine 25 
9. Voluntary muscle fiber 25 
10. Large-sized nerve cell with processes 26 
11. The thoracic and abdominal cavities 28 
12. The human skeleton 29 
13. Dialyser 39 
14. Thoma-Zeiss Hsemocytometer 53 
15. White blood-corpuscles from man 55 
16. Position of the heart in the thorax 71 
17. Diagram of the heart and large vessels 72 
18. Diagram of the valves of the heart 73 
19. Cross section of small artery and vein 74 
20. Arterioles and capillaries from the human brain 75 
21. Dissection of heart to show auriculo-ventricular bundle 78 
22. Diagram showing relative pressure in auricle, vestricle and 
aorta 80 
23. Diagram of experiment to show how a pulse comes to disap- 
pear when fluid flows through an elastic tube when there is 
resistance to the outflow 85 
24. Apparatus for measuring the arterial blood pressure in man... 87 
25. Section of cat's lung 92 
26. Effect of stimulating vagus and sympathetic nerves on a frog's 
heart 94 
27. Diagram of structure of lungs showing larynx, bronchi, bron- 
chioles and alveoli Ill 
28. The position of the lungs in the thorax 113 
29. Hering's apparatus for demonstrating the action of the respira- 
tory pump 114 
30. Diagram to show movement of diaphragm during respiration.. 115 
XV ILLUSTRATIONS. 
XVI 
Fig. Page 
31. Position to be adopted for effecting artificial respiration 118 
32. Diagram of laryngoscope 128 
33. Position of the glottis preliminary to the utterance of sound... 128 
34. Position of open glottis 128 
35. The position of the tongue and lips during the utterance of the 
letters indicated 130 
36. Diagram of the alimentary tube and its appendages 141 
37. Scheme of a longitudinal section through a human tooth 143 
38. Section from the human maxillary gland 144 
39. The stomach and duodenum opened 145 
40. The mucosa of the stomach 146 
41. Longitudinal section of duodenum near pyloric end 148 
42. The microscopic structure of the liver 149 
43. Cells of parotid gland showing zymogen granules 150 
44. The nerve supply of the submaxillary gland 151 
45. The changes which take place in the position of the root of the 
tongue, the soft palate, the epiglottis and the larynx during 
the second stage of swallowing 158 
46. Diagrams of outline and position of stomach as indicated by 
skiagrams taken on man in the erect position at intervals 
after swallowing food 164 
47. Diagram of stomach showing miniature stomach separated 
from main stomach by a double layer of mucous membrane. 165 
48. Diagram of the time it takes for a capsule containing bismuth 
to reach the various parts of the large intestine 184 
49. Diagram of Atwater-Benedict respiratory calorimeter 189 
50. The thyroid gland 229 
51. Cretin, 19 years old 230 
52. Case of myxoedema 231 
53. Median sagittal section through pituitary of monkey 235 
54. Before and after onset of acromegalic symptoms 236 
55. The situation, direction, forms, and supports of the kidney.... 243 
56. Longitudinal section through the kidney 244 
57. Diagram of urinary system 247 
58. Schema of simple reflex arc 252 
59. Diagram of nervous system of segmented invertebrate 254 
60. Diagram of section of spinal cord showing tracts 259 
61. Under aspect of human brain 269 
62. Vertical transverse section of human brain 270 
63. Cortical centers in man 275 
64. The semicircular canals of the ear 282 
65. Formation of image on retina 287 
66. Section through the anterior portion of the eye 288 XVII 
ILLUSTRATIONS. 
Fig. Page 
67. A, spherical aberration; B, chromatic aberration 291 
68. Errors in refraction 292 
69. Semidiagrammatic section through the right ear 298 
70. Tympanum of right side with the auditory ossicles in place. .. . 300 
71. Schema to show the course of the taste fibers from tongue to 
brain 302 
COLOR PLATES. 
Plate I. Diagram of circulation Facing 76 
Plate Hi Dietetic chart Facing 207 
Plate III. Diagram of the uriniferous tubules, the arteries, and 
veins of the kidneys Facing 244 
Plate IV. The simplest reflex arc in the spinal cord Facing 257 
Plate V. Reflex arc through the spinal cord, in which an inter- 
mediary neurone exists between the afferent and 
efferent neurones Facing 259 
Plate VI. Course of the pyramidal fibers from the cerebral cor- 
tex to the spinal cord Facing 260 
Plate VII. Diagram of the dorsal aspect of the medulla and 
pons Facing 272 
Plate VIII. Diagrammatic view of the organ of Corti Facing 297 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
THE STRUCTURAL BASIS OF THE BODY. 
CHAPTER I. 
The Scope of Physiology.-Physiology is the study of the 
phenomena of living things, just as anatomy or morphology is a 
study of their structure. The study of anatomy is most logically 
pursued by starting with the simplest organisms and gradually 
proceeding through the more complex forms until man is 
reached. Except for certain fundamental functions, such as 
nutrition, which are common to all cells, this method is not 
the most suitable one to pursue in physiology, because in the low- 
est organisms all of the functions are crowded together in a lim- 
ited number of cells-indeed, it may be in one single cell. It is 
easier to study a function when it is performed by a tissue or 
organ that has been set apart for this particular purpose than 
when it is performed by cells that do many other things. Another 
reason for paying more attention to the functions of higher 
rather than lower animals is that the knowledge which we acquire 
may be more directly applicable in explaining the functions of 
man, and therefore in enabling us more readily to detect and 
rectify any abnormalities. 
During the embryonic development of one of the higher ani- 
mals, a single cell, the ovum, produces numerous other cells, 
which become more and more collected into groups, in many of 
which the cells undergo very marked changes in shape and 
structure, or produce materials, such as the skeleton or teeth, 
which show no cell structure whatsoever. Thus we have formed 
the tissues and organs, each having some particular function of 
17 18 
THE PHYSIOLOGICAL SYSTEMS. 
its own, although certain functions remain which are common 
to all. In other words, as the organism becomes more and more 
complex, there comes to be a division of labor on the part of the 
cells that comprise it. The conditions are exactly like those 
which obtain in the development of a community of men. In 
primeval communities there is little division of labor, every indi- 
vidual makes his own clothes, hunts his own food, manufactures 
and uses his own implements of war, but as civilization begins 
to appear, certain individuals specialize as hunters and fighters, 
others as makers of clothing, others as artisans. Although, in 
its first stages, this division of labor may be far from absolute, 
for every member of the community must still fight and take part 
in the building of his hut, yet it soon tends to become more and 
more so, until, as in the civilized communities of this twentieth 
century of ours, specialization has become the order of the day. 
A good example of a one-celled animal is the amoeba, which is 
often found floating in stagnant water, and which consists of 
nothing more than a mass of tissue, or protoplasm, as it is called, 
and yet this apparently simple structure can move from place to 
place, it can pick up and incorporate with its own substance par- 
ticles of food with which it comes in contact, it can store up as 
granules certain of these foodstuffs, and get rid of others that it 
does not require; it grows as a result of this incorporation, until 
at last it splits in two and each half repeats the cycle. In other 
words, this single cell shows all of the so-called attributes of life: 
movement, digestion and assimilation of food, growth and repro- 
duction. No one of these properties is necessarily confined to 
living structures alone, for some perfectly inanimate bodies may 
exhibit one or other of them, yet when all occur together, we 
consider the structure to be living. 
In the higher animals, these functions are performed by the 
so-called systems, such as the digestive, the circulatory, the res- 
piratory, the excretory, the motor, the nervous and the reproduc- 
tive, each system being composed of certain organs and tissues 
which are designed for the special purpose of carrying out some 
particular function or functions. One function, however, is com- 
mon to all of the organs and tissues, namely, that of nutrition, FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
19 
which includes the process by which the digested food is built up 
into the protoplasm of the cells, or assimilation, and that by 
which the resulting substances are broken down again, or disas- 
similation. It is by these processes that the energy of life is set 
free; the energy by which the tissues perform their functions, 
and which appears as body heat. Every cell in the animal body 
is therefore a seat of energy production, and at the same time 
each is a machine for converting this energy into some definite 
form of work. In this regard the animal machine differs from a 
steam engine, in which energy liberation occurs in the furnace, 
and conversion of this energy to movement occurs in the pis- 
tons. The furnace and the machinery of the animal body are 
located in the tissue cells, and the digestive, circulatory, respira- 
tory and excretory systems are provided for the purpose of 
transporting, to and from the living cells, the fuel (i. e., the 
food), along with' the oxygen to burn it and the gases produced 
by its combustion. These processes of assimilation and disas- 
similation constitute the study of metabolism, the practical side 
of which is included in the science of nutrition. 
The Structural Basis of the Body. 
As has been indicated above, the structural and physiological 
unit of the body is the cell, and the structure and function of 
any organ depends on the nature of the cells which compose it. 
The general characteristics of the cells present important sim- 
ilarities, whether it be a cell which forms the whole organism 
as in the case of the amoeba or a cell which forms an infinites- 
imal part of a higher organism. A cell may be defined as a 
small mass of protoplasm having a nucleus, and in general, we 
may regard protoplasm as any material which is endowed with 
life. It is the physical basis of life, and is not any specific sub- 
stance. The protoplasm of the muscle cell is, for example, quite 
different from that of the nerve cell. Indeed in any single cell 
there are at least two kinds of protoplasm-one composing the 
nucleus and the other, the cytoplasm. The nucleus is generally 
oval or spherical and lies near the center of the cytoplasm, THE EPITHELIAL TISSUES. 
20 
which forms the outer protoplasmic mass. The cytoplasm sur- 
rounds the nucleus and is more homogenous than the nucleus. 
There are often granules and small vacuoles in it which rep- 
resent stages in the metabolism of the cell. 
By multiplication and differentiation, the simple cell is finally 
represented in the body in a number of different forms. These 
Vacuoles. 
Chromatin network. 
Linin network. 
Spongioplasm. 
Hyaloplasm. 
Nuclear fluid. 
Nucleolus. 
Nuclear membrane. 
Chromatin net-knot. 
Cell-membrane. 
Centrosome. 
Exoplasm. 
Centrosphere. 
Foreign inclosures. Metaplasm. 
Fig. 1.-Diagram of a cell. (Bohm, Davidoff and Huber.) 
compose the elemental tissues of the body, the epithelial, the 
connective, the muscular and nervous tissue. We might also 
classify the blood and the lymph, as fundamental tissue. 
The Epithelial Tissue.-This consists almost entirely of cells 
with a very small amount of intercellular substance which holds 
the cells together. This tissue covers the internal and external 
free surfaces of the body, as the external layers of the -skin, 
mucous membranes of the mouth, the alimentary canal, the in- 
ternal surfaces of the body cavities and the secreting cells along 
the ducts of the various glands of the body. Because of its 
wide distribution and function, we find many varieties of epi- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
21 
thelial cells. Figs. 2 and 3 illustrate some of the varieties of this 
tissue. 
- Mucous. 
Goblet cell. 
Pavement cell. 
i 
- Pear-shaped 
cell. 
Pavement cells. 
Interstitial cells. 
Fig. 2.-Types of epithelial cells. (Hill's Histology.) A, Simple columnar 
cells from intestine ; B, Ciliated epithelium from trachea ; C, Epithelial cells 
from bladder. 
The Connective Tissue.-This group of tissues, in which are 
included the bones, cartilages and tendons, serves as a support- 
ing framework for the muscular, nervous and glandular organs. 
Its function is entirely one of support. The connective tissue 
cells are made up of two elements, i. e., cells and intercellular 22 
THE MUSCULAR TISSUES. 
substance. The intercellular substance is produced by the con- 
nective tissue cells and exists in many varieties throughout the 
body. In places it forms an elastic coat of fibrous material, as 
in the walls of the blood vessels; it is sometimes dense and 
Pavement cell. 
Pear-shaped cell. 
Interstitial cell. 
Corneum or 
horny layer. 
Stratum 
lucidum. 
Stratum 
granulosum. 
Malpighian 
or germ- 
inal layer. 
Fig. 3.-Types of epithelial cells. (Hill's Histology.) A, Section of blad 
der epithelium: B, Section of epidermis of skin from palm-surface of Anger 
Cj Section of striated epithelium from esophagus. 
fibrous in character, as in the fascia which covers the various 
organs. In bone the connective tissue substance is impreg- 
nated with inorganic salts which makes it very hard and firm. 
Fatty or adipose tissue consists of connective tissue cells in 
which are deposited large amounts of fat. Neuroglia is an- 
other form of connective tissue found in the nervous tissues 
(Figs. 4, 5, 6 and 7). 
The Muscular Tissue.-There are two kinds of muscular tis- 
sue, which differ in histological and physiological character- 
istics, viz.: smooth and striated muscle. The smooth or non- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
23 
striated kind is found in the muscle coats of the alimentary 
canal, the coats of the blood vessels, etc. It is made up of 
Fig. 4.-Connective-tissue cells from a chick embryo. (Hill's Histology.) 
Fig. 5.-a, Yellow elastic fibers from the teased ligamentum nuchee of the 
ox ; b, Cross-section of a portion of the ligamentum nuchse of the ox. The 
elastic fibers are grouped in bundles with a few intervening connective- 
tissue cells. (Hill's Histology.) 
masses of mononucleated spindle-shaped cells (Fig. 8). Close his- 
tological examination reveals a faint longitudinal striation in the 
cells. This form of muscle is not under voluntary control and 
is therefore sometimes called involuntary. 
The striated muscles constitute the tissue which composes 
the voluntary contractile part of the body. Heart muscle is 
striated but it differs a little from voluntary muscle in a num- 
ber of histological and physiological details. The skeletal mus- 
cles-so-called because they are attached to and move the bones 
--are composed of masses or bundles of striated fibers which 24 
THE MUSCULAR TISSUES. 
vary in diameter from .01 to .1 mm. and may sometimes reach 
the length of 40 mm. The fibers are inclosed in a sheat, known 
Outer circumferen- 
tial lamella. 
Haversian or con- 
centric lamelice. 
Haversian canal. 
Interstitial lamella. 
Inner circumferen- 
tial lamelice. 
Fig. 6.-Segment of a transversely ground section from the shaft of a 
long bone, showing all the lamellar systems. Metacarpus of man. (Bohm 
and Davidoff.) FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
25 
as the sarcolemma. This variety of muscle (Fig. 9) receives 
its name of striated from the fact that in microscopic prepara- 
tion its fibrils show a remarkable cross striation, the significance 
of which is not known. 
Connective-tissue cell. 
Fig. 7.-Fat cells as they appear in sections treated with alcohol. Alcohol 
dissolves the fat. (Hill's Histology.) 
Nucleus of fat cell. 
Nucleus 
Nucleus 
Fig. 8.-a. Cell from smooth muscle of intestine ; b, Cross section of smooth 
muscle of intestine. (Hill's Histology.) 
Artery 
Vein. 
Sarcolemma. 
Sarcostyles. 
Fig. 9.-Voluntary muscle fiber. The sarcoplasm has broken, showing 
the smooth sarcolemma. (Hill's Histology.) 
The Nervous Tissues.-Nerve tissue is composed of nerve 
cells and fibers arising from the nerve cell. Such a structure 
is known as a neurone. The neurone is imbedded and sup- 
ported by connective tissue, the neuroglia. The long fibers aris- 26 
THE GROSS STRUCTURES OF THE BODY. 
ing from the nerve cells receive their nourishment from the cell. 
These may be very long; for example, the nerve cells from 
which the nerves in our hands or feet arise, are located some- 
where in the spinal cord (see Fig. 10). The long process run- 
Brush-like telodendrion. 
Main dendrite. _ 
Secondary dendrite. 
Basal dendrite.. - 
Neuraxis with collaterals. 
Fig. 10.-Large-sized nerve cell with processes. (Bohm and Davidoff.) 
ning out from the cell is known as an axone, and the shorter 
more branching process, the dendrite. 
The Gross Structures of the Body. 
From the fundamental tissues the various parts and organs 
of the body are developed, just as the various buildings which 
form a city are built out of the same varieties of building ma- 
terial. The description of the various individual organs of the FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
27 
body will be found in the appropriate chapters. At present we 
will concern ourselves with a brief consideration of the gross 
architecture of the body. 
The Skin.-The body is everywhere covered by the skin. 
This tissue is built of an outer layer of epithelial tissue and 
an inner one of connective tissue. The hairs and nails are mod- 
ifications of epithelial tissue. In the connective tissues are 
found the blood vessels and nerves which supply the skin. 
The Subcutaneous Tissue.-Just beneath the skin is a layer 
composed of connective tissue. In some places this binds the 
skin directly to the bones and in others it separates the skin 
from the muscles. The subcutaneous fat is found in this layer 
and is called adipose tissue. The subcutaneous tissue is usually 
spoken of as fascia. 
The Muscles.-The skin and the superficial fascia form a pro- 
tective covering for the muscles, bones and internal organs. 
The muscles comprise the lean meat of the body and are formed 
by a mass of muscle cells which have been described above. The 
muscle bundles are held together by connective tissue, in which 
are found the blood vessels and the nerves which supply the 
muscles. The attachments of the muscles to the bones are known 
as the origin and the insertion of the muscles. The insertion 
usually refers to the attachment of the muscle on the bone which 
has the greatest freedom of movement, when the muscle con- 
tracts. The covering of the bone is composed of a fibrous con- 
nective tissue, called the periosteum. To this the muscles are 
attached either directly or indirectly, by means of a tough band 
called a tendon. Whenever a muscle contracts, the points of 
attachment are brought closer together. 
The Body Cavities.-The trunk has two compartments which 
are filled with the various organs composing the viscera. The 
upper chamber is called the thoracic cavity and holds the heart 
and the lungs; the lower chamber, which is separated from the 
supper by a sheet of muscle called the diaphragm, is the ab- 
dominal cavity. This contains, mainly, the viscera concerned 
in the digestion and absorption of food, and the organs of ex- 
cretion. 28 
THE BODY CAVITIES. 
Fig. 11.-The thoracic and abdominal cavities. (From a preparation by 
T. Wingate Todd.) FUNDAMENTALS OF HUMAN PHYSIOLOGY 
29 
The viscera are directly or in- 
directly attached to the posterior 
surfaces of the thoracic and ab- 
dominal cavities by means of 
sheets of connective tissue 
(called mesentaries) in which 
are found the blood vessels and 
the nerves that supply the vis- 
cera (see Fig. 11). The figures 
give the general idea of the 
structure and the position of the 
various viscera. 
The cranial cavity is formed 
by the bones of the skull, and is 
the receptacle for the brain. The 
cranial cavity is extended by 
means of a large opening in the 
base of the skull into the canal 
formed by the spinous processes 
of the vertebral column. This 
canal contains the spinal cord. 
The Skeleton.-The bones and 
cartilages of the body form the 
skeleton, which is the structural 
foundation for the soft tissues. 
We will consider it in three di- 
visions : the skull, the trunk, and 
the limbs. 
The Bones of the Skull.- 
The skull is composed of the 
bones of the cranium and the 
face. The cranium is the bony 
case which protects the brain. It 
is formed by the union at their 
edges of several flat-shaped bones, 
and it is attached to the ver- 
Fig. 12.-The human skeleton. (From 
a photograph by T. Wingate Todd.) 30 
THE SKELETON. 
tebral column in such a way that the spinal canal terminates in 
the large opening in the base of the skull called the foramen 
magnum (see Fig. 12). It is through this opening that the 
spinal cord leaves the skull. There are numerous other openings 
in the base of the skull for the entrance and exit of blood ves- 
sels and nerves. They include the bones of the upper jaws and 
the nasal bones. The lower jaw is the only unpaired bone in the 
face. 
The Bones of the Trunk.-The spinal column is composed 
of thirty-four or thirty-five irregular shaped bones, or verte- 
bras, bound together by ligaments and separated from one an- 
other by cartilagenous disks. The upper seven of the vertebrae, 
forming the neck, are joined in a manner which permits of a 
relatively large degree of motion. These are the cervical ver- 
tebra?. The next twelve vertebra? are the thoracic, and to them 
the ribs are attached. They have a fairly restricted degree of 
motion. Below are five larger vertebrae forming the lumbar por- 
tion of the column. When the body is bent forward or side- 
wise, the greatest degree of motion occurs in the joints between 
the lumbar vertebrae. The rest of the bones forming the ver- 
tebral column are more or less fused together to form the sa- 
crum, consisting of five bones, and the coccyx, with four or five 
bones representing the tail of lower animals. The posterior 
portion of each vertebrae consists of an arch of bone. This forms 
the spinal canal in which the spinal cord lies. The spinal nerves 
emerge between the bones of the vertebrae along the whole length 
of the column. 
There are twelve pairs of ribs. The upper ones are small but 
they increase progressively in length from above down until 
the seventh, below which they gradually decrease again. The 
upper six pairs are attached in front by means of cartilage to 
the sternum or breast bone. The next four pairs terminate in 
front in a cartilage connecting them with the pair directly above. 
The two lower pairs are attached only to the vertebrae, and are 
called the false or floating ribs. 
The bones of the shoulder girdle are paired. The large wing- 
shaped bone on the lateral posterior surface of the shoulder is FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
31 
the scapula and the smaller bone in front and attached to the 
breast bone is the clavicle or shoulder bone. The shoulder gir- 
dle furnishes support for the upper limbs. 
The bones forming the pelvic or hip girdle are also paired. 
The individual bones are fused together, however, and appear 
to form a right and left bone usually called the innominate. 
The pelvic arch forms the floor of the body cavity and it fur- 
nishes the support for the lower limbs. 
The Limbs.-The upper and lower limbs or more or less alike, 
inasmuch as both contain analogous bones. The bone of the 
upper arm is the humerus; the lower arm contains two long 
bones, the ulna and the radius. There are eight bones in the 
wrist called the carpals. The hand bones are the five meta- 
carpals, and the fourteen phalanges compose the finger bones. 
In the lower limb we have the femur or thigh bone, the tibia 
and the fibula, the bones of the lower leg. The ankle is made 
up of seven irregular shaped cubical bones known as the tar- 
sals. The foot contains five metatarsals and the fourteen pha- 
langes, the bones of the toes. 
A close study of the figures of the skeleton will give a much 
better idea of the structure of the skeleton than can be had from 
a description. 
Articulations.-The union of one bone with another is known 
as an articulation, of which there are several varieties in the 
body. When the bones are connected in a manner so that they 
are immovable, as in the bones of the cranium and hip, the 
union is known as a suture. An articulation allowing some 
movement of the bones is called a joint. The joints in the ver- 
tebral column allow only a limited amount of motion, whereas 
the joints of the limbs allow a large amount of motion. In 
the hip and shoulder we have what is known as a ball and socket 
joint. Here the upper end of the limb bones are rounded and 
fit into a socket of the shoulder or hip bone allowing a wide 
range of movement. At the elbow and knee there is a hinge 
joint which allows the lower segment of the limb to be flexed 
or extended on the upper one in one plane only. This form of 
joint connects the bones of the fingers and toes. The bones of 32 
ARTICULATIONS. 
the wrist and the ankle form a gliding joint, and are capable 
of little movement. The articulating surfaces of the joints are 
covered with a smooth membrane (synovial) which is bathed 
with a small amount of fluid which serves to lubricate the joint. 
The joints are held together by means of ligaments formed 
from tough fibrous tissue. When this connection is torn, with- 
out a displacement of the bones, the injury is called a sprain, 
and when the bones are actually displaced, there is a dislocation 
of the joint. CHAPTER II. 
THE PHYSICO-CHEMICAL BASIS OF LIFE. 
With the object of ascertaining to what extent the known laws 
of physics and chemistry can explain the fundamental processes 
that are common to all cells, we must make ourselves familiar, 
first of all, with the chemical and physical nature of the constitu- 
ents of the cell, and secondly with the physico-chemical laws 
which govern the reactions that take place between these con- 
stituents. The same laws will control the reactions which take 
place in the juices secreted by cells; for example, in the blood 
and in the secretions, such as the saliva. 
The Chemical Basis of Animal Tissues.-Certain substances 
are found in every living cell and in approximately equal quan- 
tities ; hence these may be considered the primary constituents of 
protoplasm. In general they consist of the proteins, lipoids, in- 
organic salts, water, and probably the carbohydrates. Protoplasm 
is the substance composed of these primary constituents. By its 
activity the protoplasm produces the secondary constituents of 
the cell, which are not the same in all cells, and which include the 
granules of pigment or other material, the masses of glycogen, 
the globules of fat or the vesicles of fluid which are found em- 
bedded in the protoplasm. 
By whatever process we attempt to isolate its constituents, we 
of course kill the cell, so that we can never learn by analysis what 
may have been the real manner of union of these substances in 
the living condition. All we can find out is the nature of the 
building material after the structure (the cell) into which it is 
built has been pulled to pieces. If the chemical process by which 
we disintegrate the cell is a very energetic one, for example, com- 
bustion, we always find the elements, carbon, hydrogen, nitrogen, 
oxygen, sulphur, phosphorus, sodium, potassium, calcium, chlo- 
33 34 
THE CHEMICAL BASIS OF THE CELL. 
rine, and usually traces of other elements, such as iodine, iron, 
etc. If the decomposition be less complete, definite chemical 
compounds are obtained, namely, water, proteins, lipoids, car- 
bohydrates, and the phosphates and chlorides of sodium, potas- 
sium and calcium. We shall proceed to consider briefly the main 
characteristics of each of these substances and their place in the 
animal economy. 
Water.-This is the principal constituent of active living 
organisms, and is the vehicle in which the absorbed foodstuffs 
and the excretory products are dissolved. It may be said indeed 
that protoplasm is essentially an aqueous solution, in which other 
substances of vast complexity are suspended. Water, on account 
of its very unique physical and chemical properties, is of prime 
importance in all physiological reactions. These properties are: 
its chemical inactivity at body temperatures; its great solvent 
power (it is the best known universal solvent) ; its specific heat, 
or capacity of absorbing heat; and, depending on this, the large 
amount of heat which it takes to change water into a vapor- 
latent heat of steam. These last mentioned properties are made 
use of in the higher animals for regulating the body temperature. 
Of great importance in the maintenance of the chemical bal- 
ance of the body are the electric phenomena which attend the 
solution of certain substances in water. This will be discussed 
later in connection with ionization. Water has also a very great 
surface tension. It is this property which determines the height 
to which water will rise in plants and in the soil, and which no 
doubt plays a role in the processes of absorption going on in 
various parts of the animal body. 
Proteins.-The great importance of proteins in animal life is 
attested by the fact that they are absolutely indispensable in- 
gredients of food. An animal fed on food containing no protein 
will die nearly as soon as if food had been withheld altogether. 
Proteins are complex bodies composed of carbon, hydrogen, oxy- 
gen, nitrogen, and, in nearly all cases, sulphur. Some may con- 
tain in addition phosphorus, iron, iodine, or certain other 
elements. The proportions in which the above elements are 
found in different proteins do not vary so much as the differences FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
35 
in the chemical behavior of the proteins would lead us to expect. 
In general the percentage composition by weight is: 
Carbon 53 per cent 
Hydrogen 7 per cent 
Oxygen 22 per cent 
Nitrogen 16 per cent 
Sulphur 1 to 2 per cent 
The essential differences in the structure of the molecules of 
different proteins have been brought to light.by studies of the 
products obtained by partially splitting up the molecule. We 
are able to do this by subjecting protein to the action of super- 
heated steam, or by boiling with acids or alkalies in various con- 
centrations, or by the action of the ferments of digestive juices 
or by bacteria. The cleavage produced by ferments or bacteria 
is much more discriminate than that brought about by strong 
chemical reagents; that is to say, the chemical groupings are not 
so roughly torn asunder by the biological as by the chemical 
agencies. 
At first the proteins break up into compounds still possessing 
many of the features of the protein molecule. These are the 
proteoses and peptones, which consist of aggregates of smaller 
molecules, capable of being further resolved into simple crystal- 
line substances. These have been called the building stones of the 
protein molecule, and although they differ from one another in 
many respects, they have one feature in common, namely, that 
each consists of an organic acid having one or more of its hydro- 
gen atoms substituted by the radicle, NH2. Such substances are 
called amino bodies, or amino acids. For example, the formula of 
acetic acid is CH3C00H. If for one of the H atoms there is sub- 
stituted the NH2 group, we have CH,NH2COOH, which is amino 
acetic acid, or glycocoll. 
That the large and complex protein molecule is really built up 
out of these amino bodies has been very conclusively shown by 
Emil Fischer, who succeeded in causing two or more of them to 
become united to form a body called a polypeptid. When several 
amino bodies were thus synthesized, the polypeptid was found to 36 
LIPOIDS. 
possess many of the properties of peptones, which we have just 
stated are the earliest decomposition products of protein. 
Proteins differ from one another, not only in the nature of the 
amino bodies of which they are composed (although certain of 
these are common to all proteins), but also in the manner in 
which the amino bodies are linked together. We shall see the 
practical value of knowing what are the amino bodies in a given 
protein when we come to the subject of dietetics (see p. 202). 
The proteins of the cell are classified into two groups. The 
first includes the simple proteins, such as egg and serum albumin; 
and the second, the compound proteins, from which non-protein 
groups can be split off. 
Lipoids.-These include all the substances composing a cell 
which are soluble in fat solvents. Besides fats and fatty acids, 
the most important of these substances are lecithin and choles- 
terol. 
Lecithin is widely distributed in the animal body, and is very 
important in the metabolism and in the physical structure of the 
cell. It consists chemically of glycerine, fatty acid, phosphoric 
acid, and a nitrogenous base called cholin. 
Cholesterol is another widely distributed lipoid. It is not in 
reality a fatty body, but rather resembles the terpenes. Lecithin 
and cholesterol are abundant in brain tissue, in the envelopes of 
erythrocytes, and in bile. 
The fats exist mainly as secondary constituents of the cell, 
being deposited in very large amounts in certain of the connective 
tissue cells of the body, in bone marrow and in the omental tis- 
sues. Chemically, the tissue fats are of three kinds: olein, pal- 
mitin, and stearin, each having a distinctive melting point. They 
are compounds of the tri-valent alcohol, glycerine, and one of the 
higher fatty acids, oleic, palmitic, or stearic acid. Besides those 
that are present in the animal tissues, fats made up of glycerine 
combined with various lower members of the fatty acid series 
occur in such secretions as milk. In order to understand the 
influence which fats have on general metabolism, it is important 
to remember that they differ from the carbohydrates in contain- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
37 
ing a very low percentage of oxygen and a relatively high per- 
centage of hydrogen and carbon. Thus, the empirical formula 
of palmitin is C51H98O6 or C3H5(C16H31O2)3, that of dextrose 
C6H12O6, and of protein C72H112N18O22S. 
The Carbohydrates are also mainly secondary cell constitu- 
ents, although it is becoming more and more evident that they 
are also necessary as primary constituents. In general they may 
be defined chemically as consisting of the elements C, H, and 0, 
the latter two being present in the molecule in the same propor- 
tion as in water; thus, the formula for dextrose is C6H1206. 
The basic carbohydrates are the simple sugars or monosac- 
charides, such as grape sugar or dextrose. When two molecules 
of monosaccharide become fused together with the elimination 
of a molecule of water (thus giving the formula C^H^On), a 
secondary sugar or disaccharide results. Cane sugar, lactose (or 
milk sugar) and maltose (or malt sugar) are examples. If sev- 
eral nonsaccharide molecules similarly fuse together, polysac- 
charides having the formula (C6H10Og)n are formed. These in- 
clude the dextrines or gums, glycogen or animal starch, the ordi- 
nary starches, and cellulose. Since so many molecules are fused 
together, it is not to be wondered at that there should be so many 
varieties of each of these classes of polysaccharides, for, as in the 
case of proteins, not only may the actual "building stones" of 
the molecule be different, but they may be built tQgether in very 
diverse ways. The polysaccharides may be hydrolyzed (i. e., 
caused to take up water and split up) into disaccharides, and 
these into monosaccharides by boiling with acids or by the action 
of diastatic and inversive ferments (see p. 47). 
The following formulae illustrate these facts: 
1. C6Hj2O6- a monosaccharide (dextrose). 
2. C12H220xl=a disaccharide (cane sugar) composed of: 
C6H206+C6H1206-h2o. 
3. (C6HxoO5)n = a polysaccharide (starch) composed of: 
n C6H12O6 - n H20 where n signifies that an indefinite number 
of molecules are involved in the reaction. 38 
PHYSICO-CHEMICAL LAWS. 
The Influence of Physico-Chemical Laws on Physiological 
Processes. 
Having learned of what materials the cell is composed, we may 
proceed to enquire into the chemical and physical reactions by 
which it performs its functions. The cell, either of plants or 
of animals, may be considered as a chemical laboratory, in which 
are constantly going on reactions, that are guided, as to 
their direction and scope, by the physical conditions under which 
they occur. A study of the material outcome of these reactions 
constitutes the science of metabolism, to which special chapters 
are devoted further on. At present, however, we must briefly 
examine the physico-chemical conditions existing in the cell 
which may give the directive influence to the reactions. Why 
should certain cells, like those which line the intestine, absorb 
digested food and pass it on to the blood, whilst others, like those 
of the kidney, pick up the effete products from the blood and 
excrete them into the urine? We must ascertain whether these 
are processes depending on purely physico-chemical causes, or 
whether they are a function of the living protoplasm itself, a 
vital action, as we may call it. In general it may be said that 
the aim of most investigations of the activities of cells is to find 
a physico-chemical explanation for them, and it is one of the 
achievements of modern physiology that some should have been 
thus explainable. A large number, however, do not permit of 
such an explanation, and this has induced certain investigators 
to believe that there are some animal functions which are strictly 
vital and can never be accounted for on a physical basis. The 
"physical" and the "vital schools" of physiologists are there- 
fore always with us. 
From the standpoint of physical chemistry, the cell may be 
considered as a collection of two classes of chemical substances, 
called crystalloids and colloids, dissolved in water, or in the lip- 
oids, or in each other, and surrounded by a membrane which is 
permeable towards certain substances but not towards others 
(semipermeable, as it is called). On a larger scale, the same gen- 
real conditions exist in all of the animal fluids, such as the blood, FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
39 
the lymph, the secretions and the excretions. We may therefore 
study the above laws with a view to applying them to both cells 
and body fluids. 
Properties of Crystalloids.-As their name implies, these 
form crystals under suitable conditions. When present in solu- 
tion they diffuse quickly throughout the solution, and can readily 
Fig. 13.-Dialyser made of tube of parchment paper suspended in a vessel 
of distilled water. The fluid to be dialysed is placed in the tube, and the 
distilled water must be frequently changed. 
pass through membranes, such as a piece of parchment, placed 
between the solution containing them and another solution. This 
process is called dialysis, and the apparatus used for observing 
it, a dialyser (see Fig. 13). Dialysis differs from filtration, the 
latter process consisting in the passage of fluids, and the sub- 
stances dissolved in them, through more or less pervious mem- 
branes as a result of differences of pressure on the two sides of 
the membrane. If instead of using a simple membrane, such as 
parchment, we choose one which does not permit the crystalloid 
itself to diffuse, but permits the solvent to do so-a semipermeable 
membrane, as it is called,-a very interesting property of dis- 
solved crystalloids comes to light, namely, their tendency to oc- 
cupy more room in the solvent, that is, to cause dilution by at- 
tracting the solvent through the membrane. Cell membranes are 
semipermeable, but they are too small and delicate for most ex- 
perimental purposes. For this purpose we use an artificial mem- 
brane composed of a precipitate of copper ferrocyanide sup- 
ported in the pores of an unglazed clay vessel. If a solution of 40 
ELECTROLYTES. 
crystalloid-say, cane sugar-be placed in such a semipermeable 
membrane and this then submerged in water, it will be found 
that the cane sugar solution quickly increases in volume, or if 
expansion be impossible, a remarkably high pressure will be 
developed. This is called osmotic pressure, and it is a measure 
of the tendency of dissolved crystalloids to expand in the solvent. 
It has been found that the laws which govern osmotic pressure 
are identical with those governing the behavior of gases. There- 
fore, osmotic pressure ought to be proportional to the number of 
molecules of dissolved crystalloid. This is the case for the sugars, 
but it is not so for the saline crystalloids, such as the alkaline 
chlorides, nitrates, etc., for these cause a greater osmotic pres- 
sure than we should expect from their molecular weights. Why 
is this? The answer is revealed by observing the behavior of 
the two classes of crystalloids towards the electric current. So- 
lutions of sugars or urea do not conduct the current any better 
than water, whereas solutions of saline crystalloids conduct very 
readily. The former are therefore called non-electrolytes and 
the latter electrolytes. It has been found that the reason for 
this is that molecules of electrolytes when they are dissolved 
break into parts called "ions," each ion being charged with 
electricity of a certain sign, i. e., positive or negative. When- 
ever an electric current is passed through the solution, the ions, 
hitherto distributed throughout the solution in pairs carrying 
electrical charges of opposite signs, now line themselves up so 
that the ions with one kind of charge form a chain across the 
solution along which that kind of electricity readily passes, and 
in so doing carries the ions with it. 
This splitting of electrolytes into ions is called dissociation or 
ionization. The ions which carry a charge of positive elec- 
tricity and which therefore travel towards the kathode or nega- 
tive pole, (since unlike electricities attract each other) are called 
kathions, and the negatively charged ions that travel to the anode, 
anions. Hydrogen and the metallic elements belong to the group 
of kathions; oxygen, the halogens and all acid groups, to the 
anions. These facts may be more clearly understood from the 
following equations: FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
41 
In water, or in a solution of a non-electrolyte, molecules of 
II2O or non-electrolyte may be represented as existing thus: 
H2O h2o h2o 
h2o h2o ii2o 
h2o h2o h2o 
In a solution of an electrolyte, the molecules split into ions 
thus: 
Na* Cl" Na* 'Cl" Na+ Cb 
Na* Cl- Na* Cb Na* Cb 
Na* Cb Na* Cb Na* Cb 
When an electric current passes through a solution of an 
electrolyte, the ions arrange themselves thus: 
Kathode- Anode* 
Na* Na* Na+ Cl- Cl- Cb 
Na* Na* Na+ Cl- Cl- Cb 
Na* Na* Na+ Cl- Cl- Cb 
To return to osmotic pressure, the ions influence this as if they 
were molecules, so that when we dissolve, say, sodium chloride 
in water, the osmotic pressure is almost twice what it should be, 
because every molecule has split into two ions. 
Osmotic Phenomena in Cells.-Over and over again we shall 
have to refer to these physico-chemical processes in explaining 
physiological phenomena. For the present it may make matters 
clearer if we consider how osmosis explains the behavior of cells 
when suspended in different solutions. The cell wall acts as a 
semipermeable membrane. Thus, if we examine red blood cor- 
puscles suspended in different saline solutions under the micro- 
scope, we shall observe that they shrink or crenate when the solu- 
tions are strong, and expand and become globular in shape when 
these are weak. The shrinkage is due to diffusion of water out 
of the corpuscle and the swelling, to its diffusion in; that is to 
say, in the former case the osmotic pressure of the surrounding 
fluid is greater than that of the corpuscular contents and vice 42 
REACTION OF BODY FLUIDS. 
versa in the latter case. In this way we have a simple and con- 
venient method of comparing the relative osmotic pressure of dif- 
ferent solutions. When the solution has a higher pressure, it is 
called hypertonic, when less, hypotonic, when the same, isotonic. 
It is evident that the body fluids must always be isotonic with the 
cell contents, and that we must be careful never to introduce 
fluids into the blood vessels that are not isotonic with the blood. 
A one per cent solution of common salt is almost isotonic with 
blood, and is accordingly used for intravenous or subcutaneous 
injections, or for washing out body cavities or surfaces lined with 
delicate membranes, such as the conjunctiva or nares. Such a 
solution is generally called a physiological or normal salt solu- 
tion. 
Reaction of Body Fluids.-Closely dependent upon these 
properties of ionization are the reactions which determine the 
acidity and alkalinity of the body fluids. When we speak of the 
degree of acidity or alkalinity of a solution in chemistry, we 
mean the amount of alkali or acid, respectively, which it is nec- 
essary to add in order that the solution may become neutral to- 
wards an indicator, such as litmus. This titrible reaction is how- 
ever a very different thing from the real strength of the acid or 
alkali; for example, we may have solutions of lactic and hydro- 
chloric acids that require the same amount of alkali to neutral- 
ize them, but the hydrochloric acid solution will have much more 
powerful acid properties (attack other substances, taste more 
acid, act much more powerfully as an antiseptic, etc.). The rea- 
son for the difference is the degree of ionization ; the strong acids 
ionize much more completely than the weak. As a result of this 
ionization, each molecule of the acid splits into H-ions and an 
ion composed of the remainder. To ascertain the real acidity 
we must therefore measure the concentration of H-ions. (These 
considerations also apply in the case of alkalies, only in this case 
OH-ions determine the degree of alkalinity.) This can be done 
accurately by measuring the speed at which certain chemical 
processes proceed, that depend on the concentration of H-ions. 
The conversion of cane sugar into invert sugar is a good process 
to employ for measuring the speed of reaction. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
43 
But even this refinement in technique does not enable us to 
measure the H-ion concentration-for now we must use this ex- 
pression when speaking of acidity or alkalinity-of such impor- 
tant fluids as blood and saliva, in which there is an extremely 
low H-ion concentration. If either of these fluids be placed on 
litmus papers, the red litmus turns blue, but all that this signifies 
is that the litmus is a stronger acid than those present in blood 
or saliva, so that it decomposes the bases with which they were 
combined and changes the color. If we employ phenolphthalein, 
which is a much feebler acid, then blood serum reacts neutral and 
saliva often acid. 
Methods have been devised to estimate the hydrogen-ion con- 
centration in the various fluids of the body. The details of this 
cannot be given here. 
Before leaving this subject, it is important to point out that 
the blood has an H-ion concentration which is practically the 
same as that of water, i. e., is as nearly neutral as it could be. It 
also has the power of maintaining this neutrality practically con- 
stant even when large amounts of acid or alkali are added to it. 
Although saliva and some other body fluids are not so nearly 
neutral as blood, yet they can also lock away much acid or 
alkali without materially changing the H-ion concentration. This 
property is due to the fact that the body fluids contain such salts 
as phosphates and carbonates, which exist as neutral and acid 
salts, and can change from the one state to the other without 
greatly altering the H-ion concentration, and yet, in so changing, 
can lock away or liberate H- or OH-ions. This has been called 
the "buffer" action, and is a most important factor in maintain- 
ing constant the neutrality of the animal body. 
Colloids.-These are substances which do not diffuse through 
membranes when they are dissolved. Thus if blood serum be 
placed in a dialyser which is surrounded by distilled water, all 
the crystalloids will diffuse out of it, leaving the colloids, which 
consist mainly of proteins. The physical reason for this failure 
to diffuse is the large size of the molecules, in comparison with 
the small size of those of the crystalloids. By causing a beam of 
light to pass through a colloidal solution and holding a micro- 44 
COLLOIDS. 
scope at right angles to this beam, the colloidal particles become 
evident, just as particles of dust become evident in a beam of 
daylight in a darkened room. 
Filters can be made of unglazed porcelain impregnated with 
gelatin in which the pores are so very minute that colloids can 
not pass through them, though water and inorganic salts do so. 
When blood serum is filtered through such a filter, the filtrate 
contains no trace of protein. The colloidal molecules can very 
readily be caused to fuse together, thus forming aggregates of 
molecules which become so large that they either confer an 
opacity on the solution or actually form a precipitate. 
A property of colloids which is closely related to the above is 
that of adsorption. This means the tendency for dissolved sub- 
stances to become condensed or concentrated at the surface of 
colloidal molecules. An example is the well known action of 
charcoal when shaken with colored solutions. It removes the pig- 
ment by adsorbing it. Adsorption is due to surface tension, 
which is the tension created at the surface between a solid and a 
liquid, or between a liquid and a gas. It is in virtue of surface 
tension that a raindrop assumes a more or less spherical shape. 
Since colloids exist as particles, there must be an enormous num- 
ber of surfaces throughout the solution, that is, an enormous sur- 
face tension. Now many substances, when in solution, have the 
power of decreasing the surface tension, and in doing so it has 
been found that they accumulate at the surface, that is to say, 
in a colloidal solution, at the surface of the colloidal molecules. 
The practical application of this is that it helps to explain the 
physical chemistry of the cell, the protoplasm of which is a col- 
loidal solution containing among other things proteins and 
lipoids. The lipoids depress the surface tension and therefore 
collect on the surface of the* cell and form its supposed mem- 
brane, whilst the proteins exist in colloidal solution inside. It is 
possibly by their solvent action on lipoids that ether and chloro- 
form so disturb the condition of the nerve cells as to cause anes- 
thesia. A knowledge of colloidal chemistry is coming to be of 
great importance in physiology. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
45 
General Nature of Enzymes or Ferments. 
To decompose proteins, fats or carbohydrates into simple mole- 
cules in the laboratory necessitates the use of powerful chemical 
or physico-chemical agencies. Thus, to decompose the protein 
molecule into amino bodies requires strong mineral acid and a 
high temperature. In the animal body similar processes occur 
readily at a comparatively low temperature and without the use 
of strong chemicals in the ordinary sense. The agencies which 
bring this about are the enzymes or ferments. These are all col- 
loidal substances (see p. 43), so that they are readily destroyed 
by heat and are precipitated by the same reagents as proteins. 
They are capable of acting in extremely small quantities. Thus, 
a few drops of saliva can convert large quantities of starch solu- 
tion into sugar. During their action, the enzymes do not them- 
selves undergo any permanent change, for even after they have 
been acting for a long time, they can still go on doing their work 
if fresh material be supplied upon which to act. These proper- 
ties are explained by the fact that they act catalytically, just as 
the oxides of nitrogen do in the manufacture of sulphuric acid. 
That is to say, they do not really contribute anything to a chemi- 
cal reaction, but merely serve as accelerators of reactions, which 
however would occur, though very slowly, in their absence. Thus, 
to take our example of starch again, if this were left for several 
years in the presence of water, it would take up some of the water 
and split into several molecules of sugar (p. 37). The enzyme 
ptyalin in saliva merely acts by hurrying up or accelerating the 
reaction so that it occurs in a few minutes. 
Enzymes differ from inorganic catalysers in the remarkable 
specificity of their action, there being a special enzyme for prac- 
tically every chemical change that occurs in the animal body. 
Thus, if we act on any of the sugars called disaccharides (cane 
sugar, lactose and maltose) with an inorganic catylytic agent, 
such as hydrochloric acid, they will split up into their constitu- 
ent monosaccharide molecules, whereas in the body, each disac- 
eharide requires a special or specific enzyme for itself. The en- 
zyme acting on one of them, in other words, will be absolutely 46 
ENZYMES. 
inert towards the others. This specificity of action is explained 
by supposing that each substance to be acted on (called the sub- 
strat) is like a lock to open which the proper key (the enzyme) 
must be fitted. 
Enzymes are peculiarly sensitive towards the chemical condi- 
tion of the fluid in which they are acting, more particularly its 
reaction. Thus the enzyme of saliva acts best in neutral reaction, 
whereas the enzyme of gastric juice acts only in the presence of 
acid, and those of pancreatic juice, in the presence of alkali. 
Enzymes may unfold this action either inside or outside of the 
cells which produce them. Thus, the enzymes produced in the 
digestive tract act outside the gland cells, but the enzyme of the 
yeast cell acts in the cell itself and is never secreted. The former 
are called extracellular enzymes and the latter intracellular. The 
activities of intracellular enzymes are much more liable to be 
interfered with by unfavorable conditions than those of extra- 
cellular enzymes. This is because the former become inactive 
whenever anything occurs to destroy the protoplasm of the cell 
in which they act. The living protoplasm is necessary to bring 
the substrat in contact with them. On this account enzymes used 
to be classified into organized and unorganized. We know that 
there really is no difference in the enzyme itself; the only differ- 
ence is with regard to the place of activity. The cells that com- 
pose the tissues of animals perform their various chemical activi- 
ties in virtue of the intracellular enzymes which they contain. 
These are, therefore, the chemical reagents of the laboratory of 
life. After the animal dies, the intracellular enzymes may go 
on acting for a time and digest the cells from within. This is 
called autolysis. 
Enzymes are classified into groups according to the nature of 
the chemical action which they accelerate. Thus: 
Hydrolytic enzymes-cause large molecules to take up water 
and split into small molecules. (Most of the digestive 
enzymes belong to this class.) 
Oxidative enzymes (oxydases)-encourage oxidation. 
Deamidating-remove nitrogen from proteins. 
Coagulative-convert soluble into insoluble proteins. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
47 
Each group is further subdivided according to the nature of 
the substrat on which the enzymes act; e. g., hydrolytic enzymes 
are subdivided into amylolases-acting on starch; invertases-• 
acting on disaccharides; proteases-acting on proteins; ureases 
-acting on urea, etc. 
When enzymes are repeatedly injected into the blood, or under 
certain other conditions, they have the power, like toxines, of 
producing antienzymes. As their name signifies, these are bodies 
which retard the action of enzymes. Thus, if some blood serum 
from an animal into which trypsin has been injected for some 
days previously be mixed with a trypsin solution, the mixture 
will digest protein very slowly, if at all, when compared with a 
mixture of the same amount of trypsin and protein (see also 
p. 180). CHAPTER III. 
THE MUSCULAR SYSTEM. 
The General Properties of Muscular Tissues.-The intimate 
nature of the physical changes taking place during the contrac- 
tion of a muscle are not understood, and the histological changes 
which occur have had various interpretations put on them. For 
a discussion of these a textbook on histology should be consulted. 
The physiological property which distinguishes muscular tis- 
sue from other forms of tissue is that of contractility. It is to 
this property that the forcible shortening of the muscles which 
produces movements is due. The shortening occurs in the long 
axis of the muscle and is accompanied by a compensatory thick- 
ening in the transverse diameter, which keeps the bulk of the 
muscle constant. After the period of active contraction the 
muscle remains in the contracted position unless it be pulled back 
into extension by some force. No isolated muscle can actively 
expand; it can only do so passively. Muscle does not possess the 
property of initiating the contraction. This depends on the ner- 
vous system acting on another property of muscle, namely, its 
irritability, that is, the ability of the muscle to react very quickly 
to a stimulus. The amount of stimulus which it requires is very 
small compared with the reaction brought about in the muscle. 
A muscle can be stimulated in other ways than through its 
nerve, namely, by mechanical, thermal, electrical, and chemical 
stimuli applied directly to it. By using these artificial stimuli 
on muscles excised from the body the properties of muscular 
contraction can be studied. 
A record of the contraction of a muscle of a frog may be made 
by excising it and attaching one end to a suitable clamp and the 
other end to a light lever the opposite end of which is arranged 
to trace on smoked paper placed on a rapidly revolving drum. 
If such a muscle be electrically excited, it will record its con- 
traction as a curve on the smoked surface of the paper, and show 
48 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
49 
a number of interesting details as to the properties of contracting 
muscles. 
The muscle does not begin to contract at the exact moment that 
the stimulus is applied. A very short latent period (.01 sec.) 
elapses between the stimulus and the beginning of the contrac- 
tion. During this time the muscle is undergoing some internal 
change which must precede the contraction. The period of active 
contraction is relatively short (.04 sec.) and the period of relax- 
ation somewhat longer (.05 sec.). The ordinary movements of 
the body cannot obviously be of the nature of a single muscular 
contraction, for they much exceed one-tenth of a second in dura- 
tion. They are in fact produced by a prolonged contraction of 
muscles caused by the fusions of several single contractions. This 
is known as tetanic contraction, and it can easily be produced 
in the. muscle preparation described above by giving it a series 
of electrical stimuli from an induction coil. If the stimuli 
be properly timed, a contraction curve somewhat higher and 
showing no relaxation phase will be produced. When the ex- 
citation is discontinued, the muscle returns to its normal length. 
The amount of load which the muscle lifts has a peculiar effect. 
Up to a certain point an increase in the load increases the effi- 
ciency of the muscle and the muscle will actually perform more 
work with a moderate load than with no load at all. After a 
certain load is reached, the efficiency of the muscle begins to 
diminish and further increase of the load decreases the work 
accomplished by the muscle. The principle involved here is made 
use of by fork and shovel manufacturers, who are careful to 
make their implements carry the load best suited to develop the 
maximal efficiency of the muscles of a normal average man. Al- 
lowing the laborer to choose his own shovel is not always the 
best for the laborer or for his employer. 
Another interesting fact is that a contracted muscle is more 
elastic than a relaxed muscle. Equal weights attached to a con- 
tracted and to a relaxed muscle will produce a greater elonga- 
tion in the contracted than in the relaxed muscle. It is this prop- 
erty which protects the muscle from sudden rupture when at- 
tempts are made to lift loads that are too heavy. 50 
CHEMISTRY OF MUSCULAR CONTRACTION. 
The Chemical Changes Which Accompany Muscular Contrac- 
tion are concerned in the liberation of energy by the oxidation 
of the organic foodstuffs and the converting of this energy into 
muscular energy. Just how this change is brought about is not 
known. During muscular activity a great amount of oxygen is 
required and a large amount of carbon dioxide is given off. It 
is very interesting, however, to know that the maximal exchange 
of these gases does not actually accompany but follows the mus- 
cular activity, thus indicating that a muscle becomes charged 
with energy, so to speak, during rest and discharges itself in 
much the same manner as a storage battery during a period of 
activity. If a muscle be made to contract till it becomes fatigued, 
a large amount of sarco-lactic acid accumulates in the tissue. This 
poisons the muscle and makes it unable to contract. If this be 
washed out with saline, the muscle will again contract for a time. 
Rigor mortis, or the rigidity which comes on after death, may 
be due to the development of sarco-lactic acid in the tissues be- 
cause they have become deprived of oxygen. CHAPTER IV; 
THE BLOOD. 
Introduction.-The individual cells forming the most simple 
types of life are nourished by substances which they obtain 
directly from the water in which the animal lives. In exchange 
for this food, they excrete into the water the waste materials of 
their metabolism. As the organism becomes more and more 
complex this direct interchange of materials becomes impossible, 
and the blood and lymph assume the task of delivering food to 
the tissues and of removing the waste materials. To accomplish 
this, these fluids come into close relation with the absorbing, elimi- 
nating, and general tissue elements of the body, the lymph being 
in immediate contact with the cells and the blood moving quickly 
from place to place. Therefore all the elements found in the 
tissues and all the waste materials produced by the body are 
present at some time in the blood. The blood may indeed be 
compared to the wholesaler of commerce, who handles all the 
materials for the support of life, and the lymph to the retailer, 
who distributes to the tissue cells the materials which they need. 
In short, it may be said that the blood replenishes the lymph for 
the losses which it incurs in supplying the tissues. 
Physical Properties.-Ordinary mammalian blood is an 
opaque, somewhat viscid fluid, varying in color from a bright 
red in arterial blood to a dark red in venous blood. Contact with 
air changes venous blood to arterial blood. Microscopical exami- 
nation shows that the blood is not perfectly homogenous, but 
consists of a clear fluid in which cells called corpuscles are sus- 
pended. 
The Corpuscles. 
There are three varieties of these: the red corpuscles (to which 
the color of blood is due), the white corpuscles and the blood 
platelets. 
51 52 
THE ERYTHROCYTES. 
Erythrocytes.--The red corpuscles, or erythrocytes, as they 
are called, are by far the most numerous, there being five mil- 
lion of them in a cubic millimeter of normal blood. Examined 
under the microscope, they are seen in man to be flattened, bi- 
concave, non-nucleated discs; but in the embryo, as well as in 
birds and reptiles, they have a nucleus. Each corpuscle consists 
of an envelope and a framework of protein and lipoid material 
containing a substance known as haemoglobin. 
Hemoglobin is a very complex body, belonging to the general 
class of compound proteins (see p. 36L Haemoglobin has the 
ability to unite with large amounts of oxygen, thus enabling 
the blood to carry the oxygen gathered in the lungs, to the dis- 
tant tissues. It consists of a combination of a simple protein, 
globin, and a pigment, haematin. Hcematin contains iron, which 
is responsible for the ability of oxygen to unite with the haemo- 
globin molecule. The combination of haemoglobin with oxygen 
is not very stable, and can be readily broken with the liberation 
of oxygen. Jt is for this reason that this molecule is adapted to 
carry oxygen to the tissues. The quantity of haemoglobin held 
by the corpuscle may vary and in some diseases, as in chloro- 
anaemia, for instance, it may be greatly diminished, so much so 
that the tissues may be unable to obtain the proper amount of 
oxygen. The amount of haemoglobin actually present in a sample 
of blood may be estimated by the intensity of the red color it 
gives to the blood. To estimate this intensity a drop of blood 
is received on blotting paper, the stain being then compared 
either with that produced by normal blood in various dilutions 
on the same paper, or with a standardized chart. From the con- 
centration of normal blood whose stain most nearly matches that 
of the unknown sample, we can determine the percentage of 
haemoglobin in the latter, or we can read this directly from the 
chart. 
Enumeration of the Blood Corpuscles.-The number of red 
or white cells present in a cubic millimeter of blood may be esti- 
mated by the use of a haemocytometer or blood-counter. This 
consists of two mixing capillary tubes, in one of which the blood 
is diluted one hundred times with saline solution, and in the FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
53 
other, ten times with 0.337% acetic acid. The former dilution is 
for counting red, and the latter, for counting white corpuscles. 
A drop of the diluted blood is then placed on a special glass 
slide which contains a counting chamber of such a depth that 
when a cover slip is put over a drop of fluid in the chamber, a 
column of fluid dne-tenth of a millimeter deep is obtained (Fig. 
14). The chamber is graduated with cross lines, so that each 
square represents a known fraction of a millimeter. The average 
number of corpuscles found in a number of squares, by actual 
count wflth a microscope, is multiplied by the factors of dilution 
employed, the product being the number of cells in a cubic milli- 
0.100mm. 
C. Zeiss 
Jena. 
Fig. 14.-Thoma-Zeiss Haemocytometer: M, mouthpiece of tube (G), by 
which blood is sucked into S; B, bead for mixing; a, view of slide from 
above; b, in section; c, squares in middle of B, as seen under microscope. 
meter of blood. The erythrocytes, which in health number about 
five million in a cubic millimeter, may decrease to less than a 
million in disease, such as pernicious anaemia, or after haemor- 
rhage. On the other hand, they may number six or seven million 
in people who live at high altitudes. The oxygen-carrying power 
of the blood is proportional to the percentage of haemoglobin, so 
that by estimating this and the number of corpuscles, a fair idea 
of the condition of the blood is obtained. 
The Origin of Erythrocytes.-It is interesting to inquire 
into the source of the blood cells, but although this has been the 
subject of many researches, it is by no means definitely settled 54 
THE ORIGIN OF ERYTHROCYTES. 
just what the process is or in what part of thebody the cells origi 
nate. Nor is it definitely known just where the worn out cells 
are dealt with. In the embryo certain cells are set apart to 
develop the vascular system. Some of these form the blood ves- 
sels and some the red corpuscles, but later in foetal life, the latter 
come from cells in the spleen, liver and red bone-marrow. At 
first the red corpuscles are nucleated, but towards the end of 
foetal life they begin to lose their nuclei, so that at birth there 
are very few nucleated red corpuscles remaining in the blood. 
After birth, the red corpuscles are formed in the red bone-mar- 
row of the flat bones. In these places special nucleated cells are 
found, which are called erythroblasts, and from these the ery- 
throcytes develop. After severe haemorrhage nucleated red cells 
may appear in the blood for a short time; the same is true in 
some forms of anaemia in which there occurs a very rapid destruc- 
tion accompanied with a very rapid formation of red cells. 
Since the life of an erythrocyte is necessarily limited, provision 
must be made for the destruction and elimination of the sub- 
stances of which they are composed. In the pigments of the bile 
we find the remains of part of the haemoglobin. The bile is 
secreted by the liver into the intestine (see p. 174), and in case 
the free outflow of bile is interfered with, the blood absorbs the 
pignient and the individual becomes yellow or is said to be jaun- 
diced. The bile pigments do not, however, contain all the ele- 
ments of the haemoglobin, for the iron is not excreted by the bile. 
It is, on the contrary, stored up by the liver to be used again 
in the formation of fresh haemoglobin. Some have thought that 
the function of the spleen is to destroy the red blood cells, the 
waste products of which are sent to the liver through the splenic 
vein. The evidence for this is the presence of pigment and iron- 
containing substances in the blood of this vein. 
Iron is an essential constituent in the haemoglobin molecule, 
and it is necessary that some be constantly supplied to the body 
in the food. But this amount need not be large, since the iron- 
containing substance can be used time and again in the manu- 
facture of new haemoglobin, and once the body has the requisite 
amount, little more need be added (see p. 223). FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
55 
The White Blood Cells.-In normal human blood there are 
about ten thousand cells in a cubic millimeter of blood, or about 
one to every five hundred red cells. In many ways they resemble 
the unicellular amoeba, for like it they have the power of making 
independent movement by extending tiny processes called pseu- 
dopodia in one direction and by retracting them in another. By 
virtue of this peculiar movement they are able to flow, as it 
were, between the endothelial cells of the capillaries and find 
their way into the tissue spaces. There are a number of forms 
of white cells differing from each other in size, in the character 
of their nucleus, and in the granules they contain. In general, 
they are classified in two main groups on morphological 
grounds, viz., leucocytes and lymphocytes. 
Small 
mononuclear. 
Large 
mononuclear. 
Polymorphic 
Polynuclear. 
Eosinophile. 
Fig. 15.-White blood-corpuscles from man. (Hill's Histology.) 
The leucocytes are the most numerous and compose about 65 
per cent of the total white cells. They are characterized by a 
lobed nucleus, the parts of which are connected by strands of 
chromatin material. To this class belong several sub-groups. 
The most important of these are the cells known as polymorpho- 
nuclear leucocytes. They comprise about 96 per cent of the 
leucocytes. Others are known as eosinophiles, since they have 
granules which have a marked affinity for acid stains. 
The lymphocytes, the second variety, are so-called, since 
they are supposed to be formed in the lymph glands of the body. 
They possess a single large round nucleus surrounded by a clear 
layer of protoplasm. There are two sub-groups in this class: 
the large mononuclear lymphocytes, which contain a rather abun- 
dant cytoplasm about the nucleus, and the small mononuclear 
lymphocytes, in which the amount of cytoplasm is very small. 56 
THE BLOOD PLASMA. 
The former comprise about 4 per cent, and the latter about 30 
per cent, of the white cells. 
Estimation of the White Cells.-The number of white cells 
found in the blood is estimated by the same principle that is em- 
ployed in the counting of the red cells (see p. 52). In certain 
diseases their number may vary greatly. The number is also in- 
creased after meals. A marked increase over normal is known 
as a leucocytosis. 
The Function of the Leucocytes.-In acute infections, as 
in appendicitis, pneumonia, and localized or general septic con- 
ditions in which pus is formed, there is usually a great increase 
in the number of the polymorphonuclear leucocytes. In more 
chronic infections, as in tuberculosis, the lymphocytes are found 
in greater number. In the parasitic diseases of animal origin, as 
tapeworm and hookworm, in some skin diseases, and in scarlet 
fever, the eosinophile leucocytes are more abundant. In the 
disease leucocythiemia the lymphocytes may be present in such 
great numbers that they impede the movement of blood by in- 
creasing its viscosity or thickness. The above observations sug- 
gest that leucocytes play an important role in the protection of 
the body from infective processes. This function will be dis- 
cussed later. Another important function they may have is the 
preparation of the peculiar proteins which are found in the 
blood plasma. 
The Blood Platelets.-These bodies are smaller than the 
erythrocytes, and number about 300,000 in a cubic millimeter 
of blood. When blood is sh^d they disintegrate very rapidly, 
and set free a substance which plays a part in the coagulation of 
the blood. Little is known concerning their chemical constitution 
or their physiological function. 
The Blood Plasma. 
The blood plasma is a very complex fluid containing all the va- 
ried substances associated with the function of the blood. Water 
composes 90 per cent of the plasma. The plasma proteins consti- 
tute the largest solid constituent (7 per cent), and include serum FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
57 
globulin, serum albumin, and fibrinogen. There are a number of 
bodies which contain nitrogen which are not proteins. These 
may be grouped into two classes, the first, represented by the 
amino acids and other nitrogenous bodies derived from the pro- 
tein of the food and from which the tissue cells are built, and the 
second group, represented by waste materials given off by the 
tissue cells. These include substances such as urea, uric acid, 
creatinin, and ammonia. The non-nitrogenous organic bodies are 
dextrose, of which 0.1 per cent is present in normal plasma, and a 
small quantity of fat. About 1 per cent of inorganic salts are 
found, the chief of which is sodium chloride, which constitutes 
60 per cent of the ash. Sodium carbonate is found in a little less 
degree. Besides these two we find small amounts of potassium, 
sodium and calcium chlorides and phosphates. An important 
group of substances known as hormones are excreted into the 
plasma by some of the glands of the body, and affect the meta- 
bolism of the tissues in a specific manner. Another group of 
bodies, the antitoxins, complements, and opsonins (see p. 61), 
are found in the blood. These are concerned in the protection of 
the body against infective organisms. CHAPTER V. 
THE BLOOD (Cont'd). 
The Defensive Mechanisms of the Blood. 
The Coagulation of the Blood.-Whenever a blood vessel is 
slightly cut, the blood, which at first comes very freely, soon 
ceases to flow because of the formation of a plug or clot of blood 
at the site of the injury. The process by which the blood spon- 
taneously forms the plug in the injured vessel is known as coagu- 
lation, or clot formation. It protects the body from fatal hem- 
orrhage in case of an ordinary wound. A clot is a semi-solid 
mass, which on microscopical examination is seen to consist of a 
meshwork of fibrils holding the blood corpuscles in their inter- 
spaces. If blood is collected in a basin and whipped with some 
twigs while it is clotting, the fibrils will collect on the twigs in 
stringy masses, and the blood will remain fluid. The stringy 
material is called fibrin. Obviously, fibrin cannot exist in the 
blood stream, else the blood would form a clot within the blood 
vessels; it is formed only when occasion demands, such as an in- 
jury to the bloQd vessel. There are a number of experiments 
which explain the process of coagulation. 
Thus, if blood is prevented from clotting by cooling it to 0.° 
Centigrade, and is then mixed with a saturated solution of salt, 
a white precipitate forms, which may be filtered off and dissolved 
in 0.1 per cent salt -water. This solution may be made to clot by 
the addition of a very little blood from which the fibrin has been 
removed. In other words, we have prepared a substance which 
under proper conditions forms the fibrin of the clot. This sub- 
stance is called fibrinogen, since it is the precursor of fibrin. 
Again, if blood be treated with sodium oxalate, it will not clot 
unless calcium salts be added in amount sufficient to precipitate 
completely all the oxalate and leave some in excess. In other 
58 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
59 
words, the presence of a soluble calcium salt is necessary in order 
to have the blood clot. Defibrinated blood will, however, cause 
the clotting of pure fibrinogen solutions even though all the cal- 
cium be removed from both solutions. 
In order to explain the above facts, we must assume that three 
substances are present in solution in the blood: fibrinogen, cal- 
cium salts, and another substance, which has been called throm- 
bogen. Under the proper conditions, thrombogen will combine 
with calcium salts to form thrombin, which in turn unites with 
fibrinogen to form fibrin, which is the substance forming the 
framework of the clot. 
The reason why the blood does not clot within the blood ves- 
sels is not definitely known. It is probable that the blood con- 
tains a substance which prevents the combination of thrombogen 
with calcium salts, and which we call anti-thrombin. Whenever 
a blood vessel is injured, the tissues and the blood platelets liber- 
ate a lipoid body called kephalin, which unites with the anti- 
thrombin and thus allows the formation of thrombin to take place 
at the site of the wound. The whole process may be graphically 
shown in the following schema: • 
Anti-thrombin -|- kephalin = inactive anti-thrombin. 
Thrombogen + calcium salts = thrombin. 
Thrombin fibrinogen = fibrin. 
Fibrin -f- corpuscles = clot. 
Because calcium salts are necessary to the coagulation of the 
blood, the administration of calcium lactate has been exten- 
sively used as a means to arrest haemorrhages or to increase the 
coagulation power of the blood. It is doubtful, however, if the 
calcium content of the blood ever sinks below that required for 
clot formation. Normal horse serum h'as also been recommended 
on empirical grounds in various conditions in which the coag- 
ulation power of the blood is deficient, as in the severe anaemias 
or following operations in which it is difficult to arrest haem- 
orrhage. 
Antibodies in the Blood.-The coagulation of the blood is 
only one of the measures which are developed in the blood for the 60 
THE DEFENSIVE MECHANISM OF THE BLOOD. 
protection of the animal. No less important in this regard are 
the destruction and removal of toxic and injurious substances 
from the body. 
All the infectious diseases are caused by the agency of micro- 
organisms. The greater number of these are microscopic plants 
known as bacteria and fungi; some, however, are unicellular ani- 
mals known as protozoa. It is especially against the bacteria that 
a method of defense exists in the body; the protozoal diseases, on 
the other hand-such as syphilis, malaria, sleeping sickness and 
those caused by amoeba in the mouth and alimentary tract-find 
relatively little resistance offered to their growth in the body, and 
their destruction therefore must be for the most part brought 
about by drugs. 
The Process of Inflammation, which in a general way is 
known by the common symptoms of fever, pain, swelling and 
redness, is a sign of an increased activity on the part of the tis- 
sues in an effort to destroy some foreign body which is poisonous 
to the cells. Microscopical examination of a section of inflamed 
tissue will show that the blood vessels are dilated, and that the 
tissue spaces are infiltrated with leucocytes. It suggests that the 
blood elements must have a very important part in the process. 
The study of this function of the body is one of the most inter- 
esting chapters of physiological science, and includes the ques- 
tions of immunity from disease and the cure of infectious pro- 
cesses. 
Many pathogenic organisms can be cultivated on artificial 
media and the products of their metabolism can then be studied. 
It has been found that they may be divided into two groups: the 
one group producing the soluble poisons, or true toxins, which 
are excreted from the cell; and the other producing toxic sub- 
stances, the endo-toxins, which are not excreted from the cell. 
We will first take up the manner in which the body deals with 
the toxins. 
Toxins.-If a culture of diphtheria or tetanus bacilli be fil- 
tered through a porcelain filter, the bodies of the bacilli are re- 
moved and the filtrate contains the soluble toxic principles which 
the bacilli have produced and excreted into the nutrient fluid. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
61 
Injections of a small amount of this filtrate into an animal will 
produce the same symptoms as are produced when a pure culture 
of the bacilli is injected. Each bacillus produces a specific kind 
of toxin. Diphtheria toxin acts primarily on the vascular sys- 
tem ; tetanus toxin, on the central nervous system. The chemical 
nature of the toxin molecule is unknown, since it has been impos- 
sible to separate it in pure form. It is probably closely related 
to the protein molecule, and on the other hand resembles the 
ferments in many of its actions (see p. 45). A peculiarity in 
the action of the toxins is that a relatively long period elapses 
between the injection of the toxin and the reaction of the body, 
whereas in the case of the alkaloids or vegetable poisons, the re- 
action appears very quickly. 
Antitoxin.-In spite of the very poisonous character of the 
toxin molecule, the body is provided with a means of defense 
against it, and is able to make itself still further immune to the 
action .of the toxin. Thus, if somewhat less than the fatal dose 
of diphtheria or tetanus toxin be injected into the body, certain 
symptoms will follow, and the animal will react to the toxin in 
such a way that a subsequent injection can be made larger with- 
out proving fatal. If successively increasing doses are given, the 
animal after some weeks will be able to withstand very large 
doses of the toxin. In other words, the body develops an im- 
munity towards the toxic agent; it produces an antibody which 
neutralizes the poison of the toxin. To this body we give the 
name of antitoxin. Since these antibodies are found in solution 
in the blood, it is possible to withdraw the blood from such an 
immune animal, and inject it into a non-immune animal, thus 
rendering the latter immune to the toxin. It is this principle 
that is used in the preparation of diphtheria and tetanus anti- 
toxins. The exact nature of the combination of the toxin and the 
antitoxin cannot be learned from chemical studies, but Ehrlich 
has given to the phenomenon a biological explanation based on 
the various known reactions of the bodies. 
Ehrlich's Side Chain Theory of Immunity.-Briefly summar- 
ized Ehrlich's theory is as follows: Each toxin molecule is 
made up of a central nucleus of chemical radicles similar to those 62 
found in organic compounds. To the main body of this mole- 
cule are attached at least two other radicles, or side chains. One 
of these has a great affinity for certain chemical constituents of 
the tissues of susceptible animals, and unites the toxin molecule 
to the tissue cell. This chain is known as the haptophore group. 
The other side chain, the toxophore group, exerts the injurious 
effect upon the tissue after the haptophore group has joined the 
toxin to the cell. For example, tetanus toxin owes its effect to 
the fact that nervous tissue contains a chemical substance which 
unites readily with the haptophore group of the tetanus toxin, 
and also substances that are readily attacked by the toxophore 
group of the toxin. The antitoxins are supposed to act by com- 
bining with the haptophore group, thus preventing the toxin 
from uniting with the cell. 
According to this theory the formation of antitoxins may be 
accounted for as follows: When a receptor, as we may term the 
portion of the cell which unites with the haptophore groups, is 
united to the toxin, the cell endeavors to adapt itself to the loss of 
this radicle by the production of another similar one. Since the 
general rule of nature is to respond to an action with an over- 
reaction, many more receptors are made than are actually needed 
to unite with the haptophore groups of the toxin present. The re- 
ceptors produced in such great number break away from the 
parent cell. These accordingly are stored up in the blood, and 
whenever any of the particular toxin for which they are adapted 
is present in the circulation, they unite with it* and thus prevent 
the toxin from uniting with the tissue cells. A body which pos- 
sesses a store of such antibodies is said therefore to be 
immune. 
Toxins are not the only substances which will produce specific 
antibodies. This property is a general characteristic of proteins. 
Any substance producing an antibody is known as an antigen. 
For example, if human blood be injected into a rabbit, and after 
several days some of the rabbit's blood serum is mixed with hu- 
man blood serum, a precipitate will form, whereas the blood of 
a normal rabbit will produce no such precipitate. The first in- 
jection of human blood serves to stimulate the rabbit cells to 
THE SIDE CHAIN THEORY. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
63 
form some substance which precipitates any human blood sub- 
sequently added. The reaction is specific, for the blood of any 
other species of animal will not be precipitated by blood from a 
rabbit sensitized with human blood, and the reaction offers a very 
accurate method of differentiating between human blood and 
other blood in medico-legal cases. The body thus formed is known 
as a precipitin. 
Anaphylaxis.-Again, if a rabbit be injected with some hu- 
man serum two or three weeks after a previous injection, the 
animal will go into a very profound state of shock. The blood 
pressure will be lowered, the heart's action weakened, and breath- 
ing interfered with. This condition is known as anaphylactic 
shock. The reaction is a general one for proteins and is specific 
for each protein used. The phenomenon is explained by assum- 
ing that the first injection, while producing the bodies which we 
referred to above as precipitins, also produces an excess of a fer- 
ment which is able to break down the foreign protein very quick- 
ly when the second injection takes places. The products of the 
broken protein molecule, as they are produced in the blood, are 
poisonous to the body and produce the phenomenon above de- 
scribed. 
Phagocytosis.-By far the greater number of pathogenic or- 
ganisms do not excrete a poisonous toxin into the surrounding 
medium, but they cause disease by directly attacking the tissues. 
The diphtheria bacillus does not enter the body, but only ex- 
cretes a soluble toxin which the body absorbs. When a disease 
involves the infection of the tissues themselves by a micro-or- 
ganism, other types of defense than those described above are 
used. This defense depends on the fact that some of the leu- 
cocytes of the blood and lymph have the ability to ingest and de- 
stroy foreign bodies which are present in the blood and tissues, 
in much the same way as the amoeba takes its food. This func- 
tion of the leucocytes to destroy foreign bodies is known as pha- 
gocytosis. In the changes which accompany the metamorphosis 
of certain forms of larva, the leucocytes are the agents which re- 
move those parts of the body which are no longer of service to 
the animal. Likewise the leucocytes of the blood can be shown 64 
OPSONINS. 
to ingest pathogenic bacteria and to destroy them. The exact 
function of the different varieties of white cells in the blood is 
not definitely known. In active inflammatory processes the poly- 
morphonuclear leucocytes are by far the most numerous. On 
the other hand, in cases of chronic infection, as in tuberculosis 
the number of lymphocytes is increased. Some of the forms of 
white cells do not take an active part in the ingestion of bacteria, 
and therefore cannot directly destroy them. Yet, in the defense 
of the organism, they take a part which is no less important' than 
that of the phagocyte. 
In very simple forms of life the cells of the alimentary tract 
both ingest and digest the food material. In higher forms the 
cells of the alimentary tract secrete the fluids which digest the 
food. In the one case the digestion is intra-cellular, and in the 
latter, extra-cellular. In the same way we find the blood leu- 
cocytes able both to destroy and to digest substances by intra- 
cellular action, and also sharing with other cells of the body the 
power to secrete substances into the blood plasma which have the 
power of destroying the organisms or toxic material. 
Opsonins.-Normal blood serum has a very strong destruc- 
tive influence on most species of bacteria, whether they are patho- 
genic or not. This ability is not possessed to the same extent by 
the blood plasma. The difference is explained by the fact that 
in the process of coagulation the white blood cells are broken 
down and liberate their bactericidal bodies. Extracts made of 
leucocytes have this same effect, but the reaction is much more 
rapid in the presence of blood plasma or serum. The co-oper- 
ation on the part of the plasma- or serum is explained by the 
presence of some substance in solution which enables the leu- 
cocytes to attack the bacteria more readily. 
That some such substances also aid in the phagocytic action of 
the leucocytes is indicated by the fact that the white cells ingest 
bacteria much more quickly in blood serum than in normal saline 
solution. These substances are known as opsonins, and are char- 
acteristic for each individual organism which stimulates their 
production. At the beginning of an infective process, in which 
the phagocytosis is very active, each leucocyte may be able to at- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
65 
tack only one or two bacteria; later in the disease, however, when 
the opsonic power has been increased for the infective agent, the 
leucocytes may be able to ingest a much larger number without 
injury to themselves. The opsonic index is a figure expressing 
the ratio of the number of pathogenic organisms of a certain 
kind that a normal leucocyte can ingest in serum, to that which 
the same leucocyte can ingest in the presence of the serum of a 
patient who is suffering from the infective agent. A high op- 
sonic index therefore indicates a relative immunity or high resist- 
ance to the disease in question. 
Vaccines.-The bactericidal power of the leucocytes for many 
bacteria can be greatly increased by the injection of dead bac- 
teria into the body. This fact is made use of in the prepara- 
tion of bacterial vaccines, which consist of suspensions of dead 
bacteria in physiologic salt solutions. Great care and skill must 
be used in the preparation of these vaccines, which should be 
used only as a therapeutic agent when bacterilogic examina- 
tion has demonstrated the infecting organism. The failure of 
vaccines to produce the desired effect, in many cases, may be 
because the infecting bacteria produce antibodies which in turn 
neutralize those produced by the body to protect it from the 
toxic action of the bacteria in question. In recent years inoc- 
ulations with typhoid vaccine have been extensively and suc- 
cessfully used in various armies as a prophylactic measure. 
Serum Diagnosis.-The determination of the presence of spe- 
cific antibodies in the body has been made a diagnostic method 
in the case of some diseases. For example, in typhoid fever, the 
blood soon acquires the ability to inhibit the movement of the 
typhoid bacillus. This phenomenon is the basis of the Widal test 
for typhoid. 
The serum diagnosis of syphilis, known as the Wassermann 
test, depends on the production of antibodies in the syphilitic 
blood, which, under the proper conditions, will bring about the 
destruction of blood corpuscles. CHAPTER VI. 
THE LYMPH. 
The blood circulates in closed tubules, so that the nourishment 
which is supplied the tissues and the effete products which re- 
sult from their activity must pass through the walls of the ves- 
sels. The fluid which is transuded from the capillaries and which 
surrounds the cells of the tissues is known as the lymph, and 
serves as the medium of exchange between the cells and the blood 
plasma. It is the middleman of exchange between the blood and 
the tissues. 'Lymph is a slightly yellow transparent fluid, closely 
resembling the blood plasma from which it is derived. To aid in 
returning the lymph to the blood, there is provided a special 
system of vessels called the lymphatics, which are very thin- 
walled capillary tubules lined with endothelial cells. These tu- 
bules lead to larger ones which, after passing through a lymph 
gland along their course, finally empty into a large vein-like ves- 
sel, the thoracic duct, lying alongside of the oesophagus in the 
thorax, and emptying into the left subclavian vein. A smaller 
lymphatic vessel, the right thoracic duct, empties into the right 
subclavian vein. 
The lymph obtained from the thoracic duct by means of a fine 
tube inserted into the vessel varies somewhat in nature. After a 
meal the fluid is like milk, because of the presence of droplets of 
fat which have been absorbed from the intestines. The lymphatics 
of the viscera appear as white lines in the mesentery and on this 
account are called lacteals. The lymph which is collected during 
a fast is very much like the blood plasma. Its specific gravity is 
less than that of blood, since it contains less protein material, but 
on the other hand its salt content is the same and it clots in much 
the same manner as blood. On microscopic examination there 
are found many colorless corpuscles, identical with those present 
in blood. Some of these corpuscles are formed within the lymph 
66 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
67 
glands through which the lymph vessels pass on their way to the 
subclavian vein. 
Lymph Formation.-Many physiologists have attempted to 
discover the precise mechanism by which the plasma passes 
through the capillary walls into the lymph spaces, but the com- 
plete knowledge of the process is not yet at hand. The relatively 
high blood pressure within the capillaries provides filtration 
pressure by which a fluid might be filtered through the capillary 
walls, and there is no doubt that such a process does occur, as, for 
example, after the capillary pressure has been increased by con- 
striction of the veins by a bandage, etc. Filtration, however, 
cannot explain all the known phenomena of lymph formation. 
Osmosis (p. 40) also plays a part as follows: The tissues use up 
the nutritional elements brought to them by the lymph. The 
diffusion pressure of the substances in the lymph is now reduced 
so that it becomes less than that present in the blood. Therefore 
substances within the blood must pass out through the capillary 
walls into the lymph, thus keeping the concentration of the fluid 
more or less constant. The waste products of the tissue pass into 
the lymph and, by increasing the molecular concentration of the 
lymph, draw water from the blood. Again, the breaking down of 
the large protein molecules into smaller ones, in the processes of 
tissue metabolism, will cause the molecular concentration of the 
tissues to rise, increasing the osmotic pressure. This causes water 
to be abstracted from the lymph, which in turn draws on the 
blood for water. 
Lymphagogues.-There are certain substances which affect 
the rate of lymph formation in a very peculiar way. These are 
called lymphagogues, and include extracts from many shell fish, 
leech extract, peptones, etc. When such substances are injected 
into the blood of an animal, there follows a great increase in the 
rate of lymph formation and lymph flow. Indeed some people 
are very susceptible to this action, and eating shell fish, oysters, 
and some fruits will cause their tissues to become swollen be- 
cause of an increased lymph formation. How these substances 
can effect the change by altering the physico-chemical constitu- 
tion of the blood plasma is not clear. Some investigators believe 68 
THE LYMPH. 
that they have a stimulating action on the endothelial cells lining 
the capillaries and thus produce an actual secretion of lymph. 
It is more probable, however, that they poison these cells in a 
way which increases their permeability and thus permits a freer 
filtration of lymph from the blood plasma. There are other 
facts nevertheless which support the theory of an actual secreting 
mechanism within the cells of the capillary walls, but they are 
too technical to consider here. They suggest that although the 
physico-chemical laws of diffusion, osmosis, filtration, etc., play 
the most important role in lymph formation, the cells of the 
capillary walls may themselves have an active part in the pro- 
cess. 
Lymph Reabsorption-Within the tissue spaces, and within 
the cells of the tissues, changes are continually taking place which 
alter the character of the lymph. Oxygen and food substances 
are removed from the lymph by the tissue cells, and wraste sub- 
stances, the result of the -tissue metabolism, are added to it. In 
the case of oxygen and carbon dioxide, the exchange is so reg- 
ulated as to keep constant the supply of these bodies in the 
lymph. The loss of any substance is quickly compensated for by 
the addition of new material from the blood. The solid waste 
matter excreted by the cell can also find its way directly from 
the cell through the lymph and into the blood plasma. It is 
probable that during periods of rest or of slight activity the 
lymphatics are of little importance in the exchange of the lymph. 
However, when the exudation of lymph becomes increased, as 
during exercise or following the use of some lymphagogue, or 
when there are substances in the lymph which the capillaries 
cannot absorb into the blood, the lymphatics become very im- 
portant in helping to remove the excess of lymph formed. 
The Movement of the Lymph.-The mechanism by which the 
lymph of the tissues is collected by the capillaries of the lymph- 
atic system is not understood any better than the mechanism of 
lymph formation, but no doubt the same laws apply to both pro- 
cesses.* The movement of the lymph along the lymphatic vessels 
is possible because of the presence of valves along the course of 
the vessels. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
69 
The process of lymph absorption is rather slow except when it 
is aided by the massage produced by the movements of the sur- 
rounding parts. The rapid action of poisons, or drugs intro- 
duced by a hypodermic syringe, is due to their absorption from 
the intra-cellular or lymph spaces directly into the blood. Col- 
ored solutions as india ink are absorbed by the lymphatics, and 
by using a substance like this it is possible to trace the lymphat- 
ics of a portion of the body. Micro-organisms, such as the strep- 
tococcus, which causes one of the familiar forms of what is known 
as blood poisoning, are taken up by the lymphatics, and it is 
easy to trace the channels traversed by the organism by the in- 
flamed lymphatic walls which appear as red lines under the skin. 
Since all these vessels pass through a lymphatic gland on their 
way to the subclavian vein, these glands are often very much 
swollen, and may even be destroyed as the result of the infection. 
It is probable that one of the functions of the lymph gland is to 
catch and render non-toxic, poisons which are being carried into 
the circulation by way of the lymphatics. One of the most 
dreaded diseases, carcinoma, is carried by the lymphatic system 
to other parts of the body. For this reason we most often see the 
metastatic growths of cancer in the region of the lymph glands 
which have caught the straying cancer cell and have been infect- 
ed by it. 
The increased exudation of lymph in the tissues which occurs 
in inflammatory conditions is no doubt of great advantage to the 
tissues, since, by this means, a greater supply of nourishtnent is 
provided for the repair of the damaged cells, and the defensive 
substances (antibodies, etc.) are brought into play. CHAPTER VII. 
THE CIRCULATORY SYSTEM. 
Introduction.-The circulatory system provides for the trans- 
portation of blood through the tissues, thus enabling each indi- 
vidual cell to obtain nourishment and to rid itself of the waste 
products of its activity. The system includes the heart, the 
blood vessels, and the lymphatics. 
From a mechanical standpoint, we may say that the heart con- 
sists of a pair of pumps; each pump consisting of two parts, an 
upper chamber, the auricle, and the lower one, the ventricle. 
Thin, membranous valves, called auriculo-ventricular, separate 
the upper and lower chambers and prevent the blood from flow- 
ing back into the auricle when the ventricle contracts. Connect- 
ed with the ventricles are the arteries, which conduct the blood 
away from the heart, to which it is returned by the great veins 
leading into the auricles. At the point where the arteries emerge 
from the heart are cup-shaped valves, called semilunar, which 
prevent the passage of blood from the arteries into the ventricles 
while the latter are relaxing. 
Anatomical Considerations. 
The heart is suspended at its base by the large arteries, and 
lies practically free in a sac of tough fibrous tissue called the 
pericardium. On each side are the lungs, with the diaphragm 
below, the chest wall in front, and the oesophagus behind 
(Fig. 16). The surface of the heart and the interior of the 
pericardial sac are bathed with a serous fluid, the pericardial 
fluid. The muscular fibers forming the walls of the four cham- 
bers of the heart are arranged so that their contraction dimin- 
ishes the size of the cavities and empties the heart of blood. 
From the study of the embryonic heart, and from comparative 
70 71 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
studies in the lower animals, we know that the heart has de- 
veloped from a single tube, the division of the auricles and 
the ventricles being a rather late stage in the development of the 
mammalian heart. The fact that the two auricles beat synchro- 
nously, followed by the contraction of the two ventricles, is signi- 
ficant of the development of the auricles from the proximal, and 
Fig. 16.-The position of the heart in the thorax. (T. Wingate Todd.) 
of the ventricles from the distal end of the primitive cardiac 
tube. 
The fibers of the auricles run transversely, beginning and end- 
ing in the fibrous tissue which separates the auricles from the 
ventricles. The musculature of the ventricles is somewhat hard- 
er to trace. There are layers that run transversely around the 
ventricles, and also layers which describe more or less of a spiral 
course from the base of the ventricles to the apex and then are 
reflected back in transverse layers, until they finally end in the 
papillary muscles, which are connected with fibrinous threads, 
the chordae tendineae, to the edge of the auriculo-ventricular 
valves. 
When the ventricles contract, this arrangement of muscular 72 
THE HEART. 
fibers causes the apex and the base of the heart to approach one 
another, and the transverse section is changed from an ellipse to 
a circle. The base of the heart, hung as it is to the large vessels 
in the thorax, appears to be fixed, and one would expect that the 
Superior 
Vena Cava 
Arch of Aorta 
Pulmonary 
Artery 
Right 
Pulmonary 
Vein 
Left 
Pulmonary 
Vein 
Right 
Auricle 
Left 
.Auricle 
Mi tral 
Vai ve 
Tricua; i 1 
Valve 
Semi-Luna 
Valves 
Left 
Ventricle 
Inferior 
Vena Cava 
Right Ventricle"-- 
Fig. 17.-Diagram of the heart and the large vessels. 
apex is the part which moves up and down. This is not the case, 
however, as is shown by experiment, and is explained by the fact 
that the blood, when it is forced from the ventricle during the FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
73 
cardiac contraction, exerts its force on the ape$ as well as on the 
blood in the arteries. This serves to fix the apex in the vertical 
position and to bring the base of the ventricles downwards 
during their contraction. In some individuals there is a 
visible pulsation at about the level of the fifth rib on the left side. 
This is called the apex beat, and is caused by the rotation of the 
apex in the transverse diameter and by the sudden change of the 
ventricle from a soft flabby condition into a firm one. 
Fig^l8.-Diagram of valves of the heart. The valves are supposed to be 
viewed from above, the auricles having been partially removed. A. aorta 
with semilunar valve ; B, pulmonary artery and valve ; C, tricuspid, and D, 
mitral valve ; E, right, and F, left coronary artery; G-, wall of right, and H, 
of left auricle; I, wall of right, and J, of left ventricle. (From Stewart's 
Physiology.) 
The walls of the auricles are relatively thin, as they are not 
required to do heavy work. The ventricular muscles, on the 
other hand, are well developed, that of the left ventricle being 
very strong and adapted to the heavy work it must perform. 
The valves guarding the opening between the auricles and 
ventricles are composed of thin membranes of fibrous tissue, cov- 
ered with endothelial cells similar to the lining of the heart and 
the blood vessels (Fig. 18). In acute rheumatism and tonsil- 
litis, the endothelial covering of the interior of the heart and of 
the valves is often inflamed, and permanent changes may take 
place which injure the valves and produce what is known as val- 
vular disease (>f the heart, The chordae tendineae connect the 74 
THE BLOOD VESSELS. 
free margins of the valves with the papillary muscles, which arise 
from the musculature of the ventricle like little knobs of tissue. 
This arrangement prevents the valves from being everted into 
the auricle during the contraction of the ventricle. The valves 
on the left side consist of two flaps and are called the mitral 
valves; those on the right side have three flaps and hence are 
called tricuspid valves. The valves guarding the arterial orifices 
consist of three cup-shaped membranes and are known as the 
semilunar valves, because of their crescent-shape when they are 
closed. Whenever the pressure in the arteries is greater than 
that in the ventricles, these valves are tightly closed, and prevent 
any blood entering the ventricle from the arteries. 
The Blood Vessels.-The blood vessels are divided into three 
classes: The arteries, which carry the blood away from the 
heart; the veins, which carry the blood back to the heart, and 
the capillaries, which connect the arteries and the veins. They 
Fig. 19.-Cross section of small artery and vein: A, artery; V. vein. 
(Hill's Histology.) 
are all tubular structures, lined with a delicate coat of epi- 
thelial cells. The walls of the arteries are relatively much 
stronger, and are made up of layers of fibrous and elastic tissue 
and layers of smooth muscle fibers (see Fig. 19). The elastic 
tissue plays a very important part in the circulation of the 
blodd, as its tendency to stretch makes the arteries less liable 
to be ruptured by any increase in pressure of the blood, and they 
can adapt themselves to any sudden change in the amount of FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
75 
blood which is forced into them. The contraction of the muscle 
found in the arterial walls lessens the size of the lumen. The 
importance of such an action will be shown later. As the ar- 
teries branch, the walls become thinner, although the muscular 
coat is found in the very terminal branches of the arteries, 
which are called the arterioles, and from which the capillaries 
Fig. 20.-Arterioles and capillaries from the human brain. Magnified 300 
times. 1, Small artery ; 2, Transition vessel; 3, Coarser capillaries ; 4, Finer 
capillaries ; a, Structureless membrane still with some nuclei, representative 
of the tunica adventitia; b, Nuclei of the muscular fiber-cells; c, Nuclei 
within the small artery, perhaps appertaining to an endothelium ; d. Nuclei 
in the transition vessels. (Gray's Anatomy.) 
arise. The walls of the capillaries consist of a single layer of 
epithelial cells, which being very thin, thus allows for the dif- 
fusion of the various elements of the blood into the lymph or 
vice versa. The lymph is contained in special vessels called 
lymphatics. Diffusion also occurs' between the tissues and the 
lymph and blood. 76 
PROPERTIES OF HEART MUSCLE. 
The veins have, in general, the same structure as the arteries. 
Their lumen is relatively larger and their walls much thinner 
than those of the arteries. 
As will be seen from the accompanying diagram (Plate I) the 
blood pumped from the two sides of the heart circulates through 
two distinct and separate systems of blood vessels. From the 
right ventricle the blood goes through the pulmonary artery to 
the lungs and is returned to the left auricle by the pulmonary 
veins, then tn the left ventricle, whence it is sent over the body 
through the aorta and its branches, to the capillaries imbedded 
in the tissues. From these it is returned through the veins to the 
venae cavae, which discharge it into the right auricle. We may 
say, therefore, that the circulatory system consists of two circles 
of tubing interposed in which are two force pumps, the valves 
of which are so disposed as to allow the blood to flow in one direc- 
tion only. 
The Physiologic Properties of Heart Muscle. 
The Character of Cardiac Contraction.-The contraction of 
our voluntary muscles is not due to a single stimulus sent from 
the brain through the nerves, but rather to a series of such stim- 
uli, which produce a more or less continued or tonic contraction 
of the muscle. If this were not the case, our movements would 
be very quick and jerky, similar to those made by a person suf- 
fering with St. Vitus dance. In the case of the heart muscle, 
however, each beat consists of a single complete muscular con- 
traction, and it is impossible to produce a tonic or continued con- 
traction in the heart such as can be produced in voluntary mus- 
cle by rapid successive stimuli. Another peculiarity of heart 
muscle is that each time it contracts it does so with all the force 
that it has at the moment. Skeletal muscle contracts with great- 
er or less intensity according to the strength of the stimulus it 
receives. 
Heart muscle, and in a lesser degree some other muscles, such 
as those of the intestinal tract and spleen, have the power of 
making automatic rhythmic contractions which follow each other 
in a definite sequence. This phenomenon in the case of cardiac Plate I-Diagram of Circulation. The blood circulates as follows: V.C. 
(vense cavse), R.A. (right auricle), R.V. (right ventricle), P.A. (pulmonary 
artery), P.V. (pulmonary vein, red), L.A. (left auricle), L.V. (left ventricle), 
A.A. and D.A. (ascending and descending aorta), H.V. and B. (capilaries of 
head, viscera and body generally), P.V. (portal vein, blue), Li. (liver). The 
small black vessels are the azyfios veins. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
77 
muscle is not dependent on the influence of the nerves, as can be 
shown by the fact that the heart removed from the body will con- 
tinue to beat for some time if it is properly nourished by perfus- 
ing blood through it under pressure. The cause of this prop- 
erty of automaticity is still unsettled, and there have been some 
very interesting discussions and arguments among physiologists 
concerning it. Some believe that the heart muscle has this prop- 
erty inherent in itself, and that it*originates the impulse which 
causes the contraction of the heart; while others'think that there 
are present in the heart-muscle cells of a nervous character whose 
special function it is to originate the beat. Experimental facts 
can be found in support of either theory, but the question is still 
in dispute. Heart muscle differs from other muscle in that each 
fiber consists of a single cell containing striated protoplasm. It 
may quite well be that this kind of muscle possesses some char- 
acteristics usually ascribed to nervous tissue, and that it does 
originate the stimuli which produce automatic movements. 
The Sequence of the Heart Beat.-Inspection of the beating 
heart of a recently killed turtle or frog shows that the heart beat 
begins by a contraction in the large veins where they join the 
auricles. From these vessels the beat spreads, as it were, to the 
auricles and then to the ventricles, beginning at the base and 
ending at the apex. It has been determined that the auricles 
possess the power of making rhythmical contractions to a greater 
degree than do the ventricles, and that the contraction of the 
ventricle is dependent upon stimuli arising in the auricle. For 
this reason the auricles have been called the pacemaker of the 
heart. The auricles are completely separated from the ventricles 
by a firm sheet of connective tissue save by a thin band of mus- 
cular tissue in one locality, known as the auriculo-ventricular 
bundle or the Bundle of His. This tissue is responsible for the 
conduction to the ventricle of the stimulus arising in the auricle 
at each heart beat. Disease of the Bundle of His produces 
changes in the rate of the ventricular contraction which may be 
detected by alteration in the regularity and reduction in the 
rate of the heart beat. Such a condition is known as heart- 
block. 78 
THE SEQUENCE OF THE HEART BEAT. 
It is of interest to know that there has been quite an advance 
recently in the knowledge of the conduction of the cardiac im- 
pulse from the auricles on to the ventricles. It has been known 
for a long time that when a muscle contracts, a small but definite 
electric current is set up between the relaxed and the contract- 
ing portions of the muscles. New methods of detecting and re- 
Fig. 21.-Dissection of heart to show auriculo-ventricular bundle (Keith) ; 
S, the beginning of the bundle, known as the A-V node; 2, the bundle dividing 
into two branches'; if, the branch running on the right side of the interven- 
tricular septum. (From Howell's Physiology.) 
cording the direction of the flow of such currents produced in 
the heart in man have shown that cases of heart block are by no 
means rare. The instrument used for this purpose is a highly 
sensitized galvanometer, and the tracings are known as electro- 
cardiograms. By this method it can be shown that in certain 
cases of heart disease the auricles beat twice to the ventricles 
once, or again that the auricles may beat very fast while the 
ventricles are beating very irregularly and slowly. 
The Action of Inorganic Salts on the Heart Beat.-A very 
interesting theory has recently been advanced concerning the 
cause of the heart beat. It will be remembered that the blood 
contains salts of sodium, potassium and calcium in solution. If 
these salts are replaced by other non-poisonous salts in the same 
concentration as the salts removed, the heart will not beat. If FUNDAMENTALS OF HUMAN PHYSIOLOGY 
79 
the heart is perfused with a solution of sodium chloride alone, 
the beat becomes very weak and finally stops. If, however, a 
small amount of calcium and potassium salts is added to the 
sodium chloride solution, the heart will again begin to beat, but it 
stops after a while in a state of relaxation, or diastole, if calcium 
chloride is removed from the solution, or in systole, or contrac- 
tion, if the potassium salts are removed. These experiments sug- 
gest that the salts of the blood offer a solution to the problem of 
the cause of the heart beat, the potassium favoring relaxation, 
and the calcium contraction. If the proper balance of these 
salts is present in the blood, it is conceivable that a regular se- 
quence of contraction and relaxation of cardiac muscle will take 
place because of the action of the salts. 
The Vascular Mechanism of the Heart. 
Definition of Terms.-A definition of the terms applied to 
the different phases of the heart's activity will help in the de- 
scription of the events which occur during one complete heart 
beat. The period of actual contraction of the heart is termed 
systole. This is divided into auricular and ventricular systole. 
The term sphygmic period is applied to that part of ventricular 
systole during which the blood is actually leaving the ventricles. 
The period of relaxation and rest of the cardiac muscles is called 
diastole. The cardiac cycle includes the time of systole and dias- 
tole of the heart. 
The Events of the Cardiac Cycle.-During diastole the blood 
flows in a steady stream from the great veins through the two 
auricles into the ventricles, the auriculo-ventricular valves being 
open. When the ventricles are as full as the weight and the pres- 
sure of the blood can make them, auricular systole begins. The 
auriculo-ventricular valves at this instant are floating in the 
blood which has collected in the ventricles, and are almost in the 
position of closure, but a narrow chink still remains between 
them, and through this, auricular systole forces blood under 
pressure into the ventricle, thus filling the ventricles completely. 
At the dead stop of auricular systole there are currents of blood 80 
THE CARDIAC CYCLE. 
reflected back along the sides of the ventricles which strike the 
under surface of the valves and completely close them. Ven- 
tricular systole now begins. The closed valves prevent the pass- 
age of blood back into the auricles, and the entire force of the 
ventricles is expended in forcing the blood out through the ar- 
terial openings. Whenever the pressure in the ventricles exceeds 
that in the arteries, the semilunar valves open and remain open 
till the force of the ventricle falls below the pressure of blood in 
the arteries. The time between the closing of the auriculo-ven- 
tricular valves and the opening of the semilunar valves is called 
the period of getting up power, or the pre-sphygmic period (Fig. 
22). 
Auricular 
Systole 
Ventricular .Systole 
Pause 
Ventricle 
Aor 
Auricle . 
Sound of heart 
Sound of heart 
Fig. 22.-Diagram showing relative pressure in auricle, ventricle and aorta. 
Seconds 
It is obvious that when the blood is leaving the ventricles the 
pressure must be less in the arteries than in the heart. Each ven- 
tricle pours out more blood into its artery than can pass through 
the capillaries in the same unit of time, and hence the arterial 
walls are stretched and the blood is put under their elastic ten- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
81 
sion. At the moment the ventricles exert less pressure than does 
the elastic recoil of the arteries on the blood, the semilunar valves 
are closed tightly by backward eddying currents in the arteries. 
Their closure prevents any return of blood into the ventricles. 
The blood, having attained' a certain momentum during the 
sphygmic period, is carried on by its inertia for a fraction of a 
second after the ventricle ceases to exert pressure on it, thus pro- 
ducing a partially relaxed artery just beyond the semilunar 
valves. This- momentum being lost, the blood, by the pressure 
which the stretched elastic wall of the arteries exerts on the 
blood, is forced back on to the semilunar valves and into the par- 
tially relaxed base of the aorta. The blood, being thus prevent- 
ed from returning to the heart, must continue to flow on into the 
capillaries, and this onward flow never ceases, because the next 
cardiac systole occurs before the arteries have ceased to exert 
all of their recoil pressure on the blood (see also p. 84). 
After the arterial valves close, the ventricles continue to relax, 
and the pressure within quickly falls below that which obtains 
in the partially filled auricles. At this moment the weight of the 
blood which has accumulated in the auricles during the systole, 
forces the valves of the auriculo-ventricular orifice open, and the 
ventricle again begins to fill. The period between the closure of 
the semilunar valves and the opening of the auriculo-ventricular 
valves is known as the post-sphygmic period, and is the begin- 
ning of the diastole of the ventricles. The above events com- 
prise those taking place in a complete cardiac cycle. 
The Heart Sounds.-If one applies his ear to the front of the 
chest, or better still uses a stethoscope, which physicians use to 
examine the sounds of the lungs and heart, two sounds will be 
heard during each cardiac cycle. The first sound is dull, low 
pitched, and long; the second sharp, high and short. Following 
the second sound is a short pause. It has been determined ex- 
perimentally that the first sound is caused partly by the closure 
and sudden tension of the auriculo-ventricular valves at the mo- 
ment of cardiac systole, and partly by the muscular contraction 
of the ventricle. Anything which interferes with the closure of 
the valves causes an alteration in the sound; for instance, if the 82 
THE HEART SOUNDS. 
valves are diseased there will be a leaking of blood back into the 
auricles during systole, and this will cause a distinct murmur to 
take the place of the sound. If the musculature of the heart is 
weakened, the sound is also modified. Hence the first sound of 
the heart is an important diagnostic sign in heart disease. The 
second sound of the heart is due to the sudden tension exerted 
on the semilunar valves at the moment the blood is forced back 
on them, following ventricular systole. This sound is also sub- 
ject to variations in heart disease, especially in disease of the 
valves themselves, in which case because of roughening they may 
offer resistance to the outrush of blood from the ventricles, or by 
not closing tightly, allow the passage of blood in the wrong direc- 
tion. In either ease the sound is changed in character and is a 
useful diagnostic sign. 
By using these heart sounds as signals of the events occurring 
within the heart, it is possible to calculate the time relations of 
the various phases of the cardiac cycle. The heart in the ordinary 
individual beats about seventy times a minute, so that we may 
say that the cardiac cycle is completed in about eight-tenths of a 
second. Systole of the auricles takes about one-tenth of a second, 
systole of the ventricle three-tenths of a second, and diastole 
about five-tenths of a second. 
Diseases of Cardiac Valves.-If the mitral valve is diseased, 
the blood may be retarded from flowing from the auricle into the 
ventricle. This condition is called mitral stenosis. If the valves 
cannot close tightly and thereby permit the blood to regurgitate 
into the auricle during ventricular systole, the condition is called 
mitral insufficiency. Disease of the semilunar valves is likewise 
divided into aortic stenosis and insufficiency, depending on the 
character of the functional change in the valves. CHAPTER VIII. 
THE CIRCULATION (Cont'd). 
The Blood Flow Through the Vessels. 
Introduction.-A clearer idea of the principles governing the 
circulation of blood through the vessels can be had if the laws 
governing the flow of water in a city water system are called to 
mind. For example, a water-works system is arranged by means 
of either special pumps or a standpipe, to furnish a stream of 
water at a constant rate and pressure into the city water mains. 
The water is first forced into one large pipe and from this de- 
livered to the consumer by means of much smaller pipes. By 
simple mathematical calculation it can be shown that the total 
cross-section area of the smaller pipes is many times that of the 
main pipe; for the sake of argument, let us say 800 times great- 
er. Therefore the average rate of flow of water in the smaller 
pipes must be 800 times less than in the main pipe, providing all 
the outlets are open. However, if only one-half of the distribut- 
ing pipes are in use, the flow of water would be only 400 times 
less than in the main pipe, and the resistance offered by the walls 
of the pipes to the flowing water is also halved. Thus the same 
amount of water is delivered in the same unit of time but under 
twice the pressure, since only one-half of the force used to deliv- 
er the water through all the pipes is used in delivering it through 
one half of them. In other words, it takes X force to overcome 
the resistance offered by Y, therefore X equals Y. When X re- 
mains constant and Y is halved, then X-Y/2 equals X/2, leav- 
ing X/2 as a remainder. To bring it home, there is less water 
delivered from the garden hose and it has far less pressure be- 
hind it when all the neighbors are also using the water, than 
there is when only a few outlets are in use. Likewise, if the 
amount and the pressure of water in the main pipe are varied 
by changing the force of the pumps or the level of water in the 
83 84 
THE BLOOD FLOW THROUGH THE VESSELS. 
stand pipe, the amount and pressure of water delivered are also 
varied in the same direction. 
The pumps or the standpipe correspond to the heart and the 
large arteries, the distributing pipes to the smaller arteries and 
capillaries. With these ideas in mind let us consider the part the 
heart and blood vessels play in maintaining the circulation. 
The Part the Heart Plays.-At each systole 60 to 90 c. c. of 
blood are forced into the aorta. Cardiac systole lasts about 0.3 
of a second, the diastole 0.5 second. Therefore the heart is rest- 
ing about 60 per cent of the time. By experiment it has been 
demonstrated that the left ventricle forces the blood out into the 
aorta with a pressure equivalent to the weight of a column of 
mercury from 160 to 190 mm. in height. The heart alone, how- 
ever, actually propels the blood through the arteries for only the 
time of its systole; during the diastole, as already explained, the 
blood would cease to flow entirely if it were not for the part 
which the large'arteries play in maintaining the circulation. 
The Part the Arteries Play.-If 100 c. c. of water are forced 
in 0.3 second into an ordinary metal pipe at intervals of 0.8 of a 
second, 100 c. c. must flow out from the opposite end in 0.3 sec- 
ond ; during 0.5 second no water will be flowing in the tube. Let 
us now replace the metal tube with an elastic rubber tube, the 
end of which is fitted with a nozzle filled with glass beads. Now 
if 100 c. c. of water are forced into the tube in 0.3 second, the 
rubber tube expands because the beads retard the free outflow of 
water and thus make it impossible for 100 c. c. of water to pass 
through them in the time allotted. After the water ceases to flow 
into the tube, the water stored up in the expanded portion con- 
tinues to flow out through the beads because of the elastic recoil 
of the rubber. If the resistance offered to the water and the 
expansile force of the tube be properly adjusted, a constant 
stream of water may be obtained from the outlet, in spite of the 
fact that an intermittent force is supplying the water (Fig. 23). 
The intermittent stream of the arteries is changed into the 
constant stream in the veins by a somewhat similar process. The 
walls of the arteries are composed in part of a layer of strong 
elastic tissue, and this expands to a greater or less degree at each 85 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
heart beat. The resistance which the arteries and the capillaries 
offer to the flow of blood prevents the passage of the entire sys- 
tolic output of the heart into the veins during the actual ven- 
tricular contraction. It is, therefore, necessary that the large 
arteries expand in order to make room for the blood. A part of 
the energy of the heart beat is stored up in the elastic coats of 
the arteries, and after closure of the semilunar valves, which 
guard the ventricular orifice, the blood in the distended arteries 
is forced on through the capillaries by the pressure of the ar- 
terial walls. 
Arterial Blood Pressure.-From the foregoing description we 
see that there are several factors which contribute to the main- 
Fig. 23.-Diagram of experiment to show how a pulse (produced by com- 
pressing the bulb B) comes to disappear when fluid flows through an elastic 
tube (F) when there is resistance (a) to the outflow. A, basin of water; 
B, bulb syringe; C and E, stop cocks; D, rigid tube; F, elastic tube; G, 
bulb filled with sponge. 
tenance of a constant.stream of blood through the capillaries: 
viz., the pumping action of the heart, the resistance of the ar- 
terioles and capillaries, the elastic recoil of the blood vessels, and 
the amount of blood itself. That the velocity and the pressure 
of the blood depend on these factors was first of all demonstrated 
in 1732 by Rev. Stephen Hales, who in a book published in that 
year reports having experimentally determined the blood pres- 
sure in the femoral artery of a horse. He found that the pres- 
sure was sufficient to raise the blood in a tube seven feet above 
the level of the heart, and he also observed that each beat of the 
heart and each respiratory movement affected the pressure of the 
blood. The pressure exerted by the blood on the vessel wall at 
the height of the systole of the ventricle is known as the systolic 
blood pressure, and that exerted by the elastic recoil of the 86 
ARTERIAL BLOOD PRESSURE. 
arteries on the blood during the diastole of the heart is known 
as the diastolic blood pressure. The average between these two 
pressures is called the average or mean arterial blood pressure. 
Since Hales' experiment better apparatus has been devised to 
measure the blood pressure in animals under different conditions. 
Determinations of the pressure existing in different portions 
of .the vascular system show that there is a steady decrease of 
pressure of the blood from the aorta to the entrance of the vena 
cava into the right auricle. It thus happens that the blood is al- 
ways flowing from a place of higher pressure to one of lower 
pressure. 
Methods which are of much practical importance in the diag- 
nosis of vascular diseases have been devised to determine the 
blood pressure in man. The principle of these methods consists 
in measuring the pressure required to shut off completely the 
blood supply in an artery. This is accomplished by placing a 
rubber sac encased in a leather band about the arm (Fig. 24). 
By means of tubing this sac is connected with a mercury gauge 
and an air pump. When the sac is pumped up with air, the ves- 
sels in the arm are compressed, and when the blood can no longer 
force its way under the obstruction, the pulse at the wrist disap- 
pears and at this moment the height of the mercury in the gauge 
is measured. This represents the systolic blood pressure. If de- 
sired, a similar measurement may be made in the arteries of the 
leg. 
To measure the diastolic pressure is more difficult. The method 
depends on the experimentally determined fact that when the 
pulse wave produced in the arteries by each systole of the heart, 
is of greatest amplitude, the pressure in the air sac or compress- 
ing band equals the lowest pressure present in the vessel between 
the pulses. 
Recently improvements have been made in the method of judg- 
ing the point of obliteration of the artery, and also the point of 
maximum pulsation, by listening to the sounds produced at each 
pulse wave when the artery is being compressed. 
The systolic blood pressure in the artery of the arm in healthy 87 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
young men varies from 110 to 130 mm. of mercury when it is 
determined in the sitting posture. When a person is lying down 
the pressure is a little less, and after hard exercise a little higher. 
The blood pressure under ordinary conditions is relatively con- 
stant, and is dependent on a delicate adjustment of the relation- 
ship existing between the force of the heart, the amount of blood 
Fig. 24.-Apparatus for measuring the arterial blood pressure in man. 
The pressure in the cuff is raised by means of the syringe until the pulse 
can no longer be felt at the wrist. This pressure is read off on the mercury 
manometer (systolic pressure). 
pumped at each beat, the resistance which the walls of the blood 
vessels offer to the flow of the blood, the size of the vascular sys- 
tem, and the amount of blood in the body. Since the amount of 
blood in the body is relatively constant, we may say that the 
factors which change are the heart and the blood vessels. How 88 
ARTERIAL BLOOD PRESSURE. 
these factors influence the blood pressure may be seen if we again 
compare the system to the city water supply. 
Factors Which Maintain Blood Pressure.-When the most 
water is being pumped into the mains, then the water has the 
greatest velocity and pressure. Likewise, when the heart is 
pumping most blood into the aorta, the velocity and the pressure 
of blood in the vessels are the greatest. If the amount of water 
remains constant, a uniform outflow through all the outlet tubes 
will be maintained, but if the number of outlet tubes be dimin- 
ished, then more water will have to flow, per minute of time, 
through the remaining tubes; hence the velocity and the pres- 
sure must be increased. 
The same conditions are present in the body. A narrowing of 
the arterioles throughout the body or in some extensive vascular 
area, causes the pressure and the velocity of the blood to be in- 
creased in the remaining vessels, provided, of course, the heart 
beat is unchanged. A dilation of the arterioles, on the other 
hand, results in a fall of pressure and a decrease in the velocity 
of the blood. In the same way also an increase or decrease in the 
action of the heart will result in an increase or decrease in the 
pressure and velocity of the blood. 
The dependence of these two factors, i. e., the heart and the 
vascular system, on the maintenance of the normal blood pres- 
sure, is seen in the fact that, with a fast heart and dilated blood 
vessels, the blood pressure may be exactly the same as when the 
heart is beating very slowly but the arterioles are all constricted. 
It is apparent, therefore, that the velocity of the blood in the 
vessels is dependent on the pressure of the blood and the extent 
of the vascular area at the time in question. 
The Velocity of the Blood.-By the velocity of the flow of 
blood we mean the actual time it takes for a particle of blood to 
pass between two points. If the rate were uniform throughout 
the vascular area, we could compute the time which a particle of 
blood would take to pass through the circulatory system. This 
is not the case, however, for the flow of blood is much swifter in 
the aorta than in the smaller vessels, and here again our analogy 
between the circulatory system and the city water system applies. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
89 
Just as the combined cross area of the small pipes leading from 
the main pipe of the water system is greater by many times than 
the area of the main pipe, so it has been estimated that the total 
cross section of the capillaries of the body is 800 times larger 
than that of the aorta. 
It has been estimated that the rate of blood flow in the aorta is 
about 320 mm. per second. The average rate of flow in the capil- 
laries must then be 800 times less than that in the aorta, or 0.4 
mm. per second. As the length of a capillary has been estimated 
to be about 0.5 mm., the blood takes about a second to pass 
through them into the veins. This has been verified by micro- 
scopic examination of the blood flow in the capillaries. 
The velocity of the blood must be altered whenever the size of 
the vascular area is changed, and since during a cardiac cycle 
exactly the same amount of blood is delivered into the right 
auricle as the left ventricle forces out into the aorta, it follows 
that the same amount must pass through the vascular area of the 
body in the same time. In other words, the amount of blood 
which flows in a given series of blood vessels in a given time is in- 
dependent of the size of the blood vessels. 
The Return of the Blood to the Heart.-We must now con- 
sider the nature of the force which propels the blood, and study 
what changes take place in the movement of the blood during its 
passage through the vessels. 
The blood is expelled from the left ventricle with consider- 
able force and at a high velocity. On its way through the body 
much of the energy given out by the contraction of the heart is 
used to overcome the resistance offered by the walls of the ves- 
sels and the capillaries. In consequence of this, the velocity and 
the pressure of the blood on the sides of the vessels are much re- 
duced. 
The blood is collected from the capillaries by the veins, and 
since the volume of the veins is less than the volume of the capil- 
laries its velocity is much increased. The relatively large caliber 
of the veins, however, offers little resistance to the flow of blood, 
and the energy remaining from that imparted to the blood by the 
heart has full power to make itself felt. Nevertheless, this is not 90 
CIRCULATION TIME. 
sufficient alone to force the blood onward and back to the heart, 
and we must seek other accessory factors to explain the venous 
return. 
The veins are equipped with cup-shaped valves which permit 
the passage of blood only in one direction, i. e., towards the heart. 
Every movement of a muscle therefore squeezes some of the blood 
onward. This massaging influence of the muscles is very im- 
portant. Its absence accounts for the fact that it is impossible 
to stand still for a long period of time without the limbs becom- 
ing very painful, especially in the case of varicose veins, where 
the valves of the veins are no longer functional, so that there is 
nothing to prevent the blood from returning to the more depend- 
ent positions. Another source of energy to the returning blood 
is the aspiratory effect of the thorax at each inspiration. This 
action will be considered in the study of the respiratory mechan- 
ism. 
Circulation Time.-The actual time which is taken for the 
blood to traverse the circulatory system has been variously esti- 
mated. Obviously such figures can give only average results, 
since the distance through which blood to the arm must flow is 
less than that to the legs. In general, it may be said that the 
blood makes a complete circulation in from 25 to 30 beats of the 
heart. The circulation through the lungs requires about one- 
fourth of this time. 
That the velocity of the blood flow through different vessels 
varies, is apparent from actual observations made on severing 
them and actually observing the rate of outflow. The following 
figures expressing the blood supply per minute to each hundred 
grams of organ have been determined experimentally: 
Leg 5 c. c. 
Head 20 " 
Stomach 21 " 
Intestines ... .31 " 
Spleen 58 " 
Liver (venous).... 59 c. c. 
Liver (arterial)... 25 " 
Brain 136 " 
Kidney 150 " 
Thyroid 560 " 
in the arteries differs from that in the veins and the capillaries 
The Pulsatile Acceleration of Blood Flow.-The flow of blood FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
91 
in that it is swifter and pulsatile in character. This pulsatile 
variation is due to the acceleration of the blood flow caused by 
each heart beat, and the reason that this is not seen in the capil- 
laries and veins is that the resistance which the walls of the 
capillaries and arterioles offer to the blood is so great that the 
cardiac factor, acting only for a brief time, is lost. The energy 
represented in the increased rate of flow, is spent in stretching the 
walls of, the arteries, which contract after the pulsatile wave has 
passed, and thus force the blood onward. 
The Pulse.-The pulsatile expansion of the arteries at each 
heart beat has been mentioned in connection with the factors 
which help to maintain the normal blood pressure. It is this 
also which produces the phenomenon which is known as the pulse. 
From time immemorial the physician has be6n accustomed to 
come to an idea concerning the condition of the circulation by 
feeling the pulse, for it represents changes in the arterial ten- 
sion occurring during each cardiac cycle. 
Since the pulse is not due to an actual movement of blood 
along the arteries, but rather to changes in tension producing 
an expansion of the vessel wall, it follows that the transmission 
of the wave may be much more rapid than the movement of 
blood. This may be explained by reference to the motion im- 
parted to a row of billiard balls when the one on the end is hit 
with the cue. The one hit actually moves very little, but imparts 
its energy of movement to the others, so that the ball at the end 
of the row moves away with some velocity, while the others move 
slowly. The wave of energy spreads in a fraction of a second 
from ball to ball. Any local change in the vessel may slow 
down the rate of transmission, and if there is a difference in the 
appearance of the pulse in the two arms or legs, it is indicative 
of some obstruction or change in one of the vessels. 
Other qualities of the pulse which may assist the physician in 
judging of the condition of the circulatory system are its rate 
and its compressibility. Its rate tells us how fast the heart is 
beating, and its compressibility gives a rough idea of the blood 
pressure. 
The Circulation Through the Lungs.-In general the same 92 
THE CARDIAC NERVES. 
conditions are present in the circulation of the blood through 
the lungs as are found in the systemic circulation. The right 
ventricle is far less powerful than the left, so that the pressure 
of the blood in the lung vessels is less than that in the systemic 
vessels. The respiratory movements also cause the size of the 
blood vessels in the lungs to vary in a marked degree. These 
changes in the capacity of the pulmonary blood vessels affect the 
systemic blood pressure. Thus, at the height of inspiration, the 
lungs may contain one-twelfth of the blood of the body, while 
Respiratory 
bronchiole. 
- Lung 
tissue. 
Alveolar 
duct. 
Fig. 25.-Section of cat's lung. (Bohm and Davidoff.) 
during expiration this amount may be lessened to one-fifteenth 
to one-eighteenth of the total. This condition makes it possible 
for the heart to be filled more rapidly during the later part of 
inspiration and the beginning of expiration, than at other times, 
and accounts for the rise of blood pressure observed at this time. 
The Influence of the Nervous System on the Circulation. 
Up to the present time we have considered the circulatory 
system as a purely automatic and mechanical apparatus for FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
93 
carrying blood to all parts of the body. It is necessary that 
this apparatus vary in its activity, not only according to the 
needs of the body as a whole, but also according to the needs of 
the various parts of the body. It would be poor economy for the 
heart to maintain through all parts of the body at all times a 
stream of blood which would be large enough for all emergencies. 
There must be some way of controlling the blood flow according 
to the needs of the body. This function is served primarily by 
the central nervous system, which is connected by means of 
nerves with the musculature of the heart and the blood vessels, 
and secondarily by secretions from the so-called ductless glands, 
the best known of which are the adrenal glands (see p. 233). 
The Nervous Control of the Heart. 
The Cardiac Nerves.-The heart is supplied with both sen- 
sory and motor nerves. Sensory nerves carry stimuli from the 
peripheral regions to the brain and are known as afferent nerves. 
Motor nerves, on the other hand, carry stimuli from the brain 
to the muscles or glands, and are known as efferent nerves. The 
efferent nerves of the heart are found in fibers coming from the 
spinal cord by way of the sympathetic system, and by the vagi 
or the tenth pair of cranial nerves (see p. 271). It must be 
clearly understood that the nerves merely regulate the heart 
beat, but have nothing to do with its occurrence. In other words, 
the heart continues to beat after all the nerves have been severed. 
The Accelerator Nerves.-To understand how the fibers 
reach the heart, the reader is referred to the general description 
of the sympathetic nervous system on page 282. The sympathetic 
fibers of the heart are found in the first and second spinal nerves 
of the thoracic region. After connecting vyith nerve cells situ- 
ated in the stellate ganglion, they go to the heart, where they 
end about the cardiac muscular fibers. 
Cutting the sympathetic fibers to the heart causes a slower beat 
and a prolonged diastole. On the other hand, stimulation of the 
nerves with an electric current increases the rate of the heart 
(Fig. 25). For the above reasons the sympathetic nerves to the 
heart are known as accelerator or augmentory nerves. 94 
THE CARDIAC NERVES. 
The Inhibitory Nerves.-The vagi are a pair of nerves arising 
on each side of the medulla, and running a course downwards 
through the neck into the thoracic and abdominal cavities. This 
pair of nerves supply fibers to the various organs of these regions 
including the heart, which receives branches from both vagi. 
It is possible by simple experiments to demonstrate the function 
of these fibers. 
For example, if the vagus on one side be cut, the heart rate 
will increase a little; if both vagi be cut, the beat is still 
more markedly quickened, and the increased discharge of blood 
from the heart produces a rise in the arterial blood pressure. 
By cutting these nerves we remove the influence which the 
central nervous system exerts through them on the heart rate. 
Since the heart beats faster after this operation, we must con- 
clude that this organ constantly receives stimuli from the brain 
Time in seconds 
I Flormal Ventricul 
I ar beat 
Stimulation of 
Vagus nerve 
formal 
Stimulation of 
Sympathetic 
Mormal 
Fig. 26.-Effect of stimulating vagus and sympathetic nerves on the frog's 
heart. This tracing was obtained by attaching the tip of the ventricle to a 
lever which recorded the movement of the heart on the smoked surface of 
a moving drum. 
through the vagi, and that these stimuli cause the heart to beat 
more slowly. Such a continued action of a nerve is known as a 
tonic influence. 
That the vagi can slow the heart or even stop it altogether is 
shown by stimulation of these nerves with an electric current of 
suitable strength (Fig. 26). If weak shocks are employed, the 
heart is slowed, the blood pressure falls somewhat, and the 
diastolic pressure becomes markedly decreased, because the ar- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
95 
teries have a greater period of time in which to empty between 
the beats. If somewhat stronger stimuli be used, the heart will 
stop beating entirely, and remain in the diastolic position for 
several seconds, during which the blood pressure will sink to 
zero. It is scarcely possible to kill an animal by stimulation 
of the vagus, however, since the heart will begin to beat after 
a short time in spite of the continued vagus stimulation. 
Relation of the Sympathetic and Vagus Nerves to the Heart. 
-The antagonistic action existing between the cardiac fibers of 
the sympathetic and vagus nerves allows the heart to respond 
quickly to any need that the body may demand of it. These 
demands are made through the brain, by various afferent or sen- 
sory nerves. This is brought about in the following way: 
The Cardiac Center.-In the medulla, the hind part of the 
brain, there is a collection of nerve cells from which the cardiac 
branches of the vagus arise. Near by also are located the cells 
from which the sympathetic nerves of the heart arise. Both of 
these nerve centers, for by this term are known the important 
cell stations of the brain, are supplied by extensive connections 
with afferent or sensory fibers coming from all parts of the body, 
the brain and even the heart. The centers become more or less 
active in response to impulses reaching them along the sensory 
fibers. 
The Cardiac Depressor Nerves.-One of the most important 
of the different cardiac nerves is that known as the cardiac depres- 
sor. It has its beginnings in filaments lying in the left ventricle 
and in the aorta, and runs to the medulla in the vagus trunk in 
most mammals, or as a separate nerve in the rabbit. The normal 
stimulus to the depressor nerve is a high blood pressure in the 
ventricles and aorta. The stimulus, thus set up, acts through the 
vagus center and the vagus nerve, and slows the heart. It also 
acts on the vasomotor center and causes the blood vessels to 
dilate. Both changes produce a fall in the blood pressure. The 
vagus nerve, besides the afferent vagus fibers, carries afferent or 
sensory nerves to the vagus center. This can be demonstrated 
by cutting one vagus and stimulating the central end, i. e., the 
end running to the brain. A marked slowing of the heart usually 96 
NERVOUS CONTROL OF BLOOD VESSELS. 
results. By acting through the vagus center and nerves, or 
through the sympathetic center and nerves, most of the sensory 
nerves of the body, if stimulated, can produce a reflex slowing 
or quickening of the heart beat. One cannot, however, always 
predict exactly what result will be obtained. The stimulation of 
the fifth nerve in the nasal cavity or in the mouth always causes 
a reflex slowing of the heart. Stimulation of the laryngeal nerve 
and the nerves of the peritoneum have a similar effect. It 
is also of interest to note that the act of swallowing will often 
cause a decrease in the rate of the heart through reflex vagus 
action. 
The relation of the blood pressure to the rate of the heart has 
been noted in connection with the cardiac depressor nerve 
(p. 95). Anythng which produces an increase in the pulse 
rate, other conditions being equal, will cause an increase in the 
blood pressure, and this acts reflexly to bring about a slowing 
of the heart. The reverse of this is likewise true. In this quick- 
ening or slowing of the heart, the vagi and the sympathetic 
nerves always act. In the adult the normal rate of the heart 
varies between 68 and 76 per minute. In children the rate is a 
little faster, and in infants it may be normally 130 or more. 
The Nervous Control of the Blood Vessels. 
During muscular activity the metabolism of the body may be 
increased five or six times, as can be judged from the amount of 
carbon dioxide given off by the lungs. Since this increase is due 
to the activity of the muscles, it is necessary that these obtain 
a greater supply of oxygen, and that they be able to rid them- 
selves of the carbon dioxide which is a waste product of their 
activity. Every other organ requires an increased blood supply 
when it becomes active, so that blood has to be diverted from the 
inactive to the active tissues, and the least important activities 
of the body have to be subordinated to the one which is most 
needed at the time in question. This action is brought about 
partly by the central nervous system, acting through its afferent 
and efferent nerves on the musculature of the blood vessels of FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
97 
the body, and partly by means of chemical substances which are 
produced at an early stage of the activity itself. 
The Vasomotor Nerves.-It was discovered in the middle of 
the past century by the French physiologist, Claude Bernard, 
that section of the cervical sympathetic nerve in the neck of the 
rabbit causes a marked dilation of the blood vessels of the ear, 
and that during stimulation of the nerve with an electric cur- 
rent, the blood vessels become very small, and the ear conse- 
quently colder. This experiment shows that the nervous system 
plays an important role in the control of the flow of blood through 
the tissues, and from it many important truths about the nervous 
control of the blood vessels may be deduced. If cutting a nerve 
will cause the blood vessels to dilate, and stimulating the same 
nerve with an electric current will cause the vessels to constrict 
to much less than their normal size, it follows that the blood 
vessels must be normally held in a state half way between ex- 
treme dilation and constriction by stimuli received from the 
nervous system. The nerve fibers which carry the stimuli, be- 
cause of their power of producing constriction of the blood ves- 
sels, are known as vasoconstrictor nerve fibers. They are com- 
parable in action to the accelerator nerves to the heart, since stim- 
ulation of either type of nerve tends to produce an increase in the 
blood pressure, the one by quickening the heart rate and the 
other by constricting the blood vessels and increasing the resist- 
ance to the flow of blood. 
The presence of the vasoconstrictor fibers in the sympathetic 
nerves is easily shown by the fact that stimulation of these nerves 
to any part of the body produces a marked diminution in the 
size of the part to which the nerves are connected. At the same 
time there is an increase in the general blood pressure, because 
the freedom of outflow of blood from the arterial system is some- 
what reduced. The large nerves which supply the limbs also 
contain vasoconstrictor nerves. These are derived from fibers 
coming from the ganglia of the sympathetic chain in the thorax 
and abdomen and joining with the roots of the spinal nerves in 
order that the fibers may be distributed along with the cerebro- 
spinal nerves to the part in question (see p. 282). 98 
VASOMOTOR NERVES. 
The vasomotor nerves to the kidney and the abdominal viscera 
are for the most part suppl ieel by the lower thoracic nerves. 
These sympathetic fibers are combined and enter the abdomen in 
what are known as the splanchnic nerves, which terminate about 
nerve cells in a ganglion behind the stomach, which is called the 
semilunar ganglion of the solar plexus. 
Vasodilator Nerves.-There is another class of efferent nerve 
fibers to the arteries, which are known as the vasodilator nerves 
When stimulated they bring about a dilatation of the arte- 
rioles, and allow a greater amount of blood to pass through the 
vessels. Vasodilator nerve fibers are found in all the spinal 
nerves, and they run to the blood vessels along with the nerve 
trunks supplying the various organs. Unlike the vasoconstrictor 
nerves, they do not seem to be continually exerting an influence 
or tonic action on the blood vessels. Because their action 
is hard to elicit, not so much is known of their normal 
functions as is known of the vasoconstrictor nerves. In some 
nerves, however, they predominate and their action is easily 
seen. Such is the case in the chorda tympani, a nerve coming 
from the seventh cranial nerve and supplying the submaxillary 
gland with fibers, which when stimulated bring about an increase 
in the flow of saliva and marked dilatation of the blood vessels 
of the gland (see p. 151). The arterioles normally may be sup- 
posed to be held in a state midway between dilatation and con- 
traction. Stimulation of the vasodilator nerves probably in- 
hibits the tonic action of the vasoconstrictor nerves and the mus- 
cles of the vessels are extended by the force of the arterial blood 
pressure. 
After section of the sciatic nerve, the constrictor fibers soon 
die, and the dilator fibers, which live for a time, may be shown 
to be present by the fact that the volume of the leg increases 
when the nerve is stimulated. 
Vasomotor Reflexes.-In the same manner that the heart is 
influenced by afferent stimuli reaching cardiac centers from peri- 
pheral parts of the body, we find afferent stimuli affecting the 
size of the blood vessels reflexly by way of the vasomotor center 
-located in the medulla near the vagus center-and the vaso- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
99 
motor nerves. Some of the afferent impulses cause dilation of 
the blood vessels, while others cause constriction. Perhaps the 
most important of the sensory nerves, which, when stimulated, 
produce a dilation of the blood vessels, is the cardiac depressor, 
which we mentioned in connection with the afferent nerves of 
the heart. It will be remembered that this nerve has sensory 
endings in the left ventricle and in the aorta, and that these are 
stimulated when the blood pressure in the arterial system reaches 
too great a height for the safety of the individual. The stimuli 
originating in the sensory endings of this nerve are carried to 
the cardiac center and are then transmitted to the heart through 
the vagus nerves. Besides the slowing of the heart which is thus 
produced, there also occurs a dilation of the peripheral vessels 
brought about by the action of the stimuli on the vasomotor 
center. This is easily demonstrated by electrically stimulating 
the cardiac depressor nerve after both vagi have been 
cut in the neck and the reflex vagus action thus removed. The 
fall of blood pressure which is obtained under these conditions 
is due to an inhibition of the constrictor center and a stimula- 
tion of the dilator center of the vasomotor nerves. 
The stimulation of many of the afferent or sensory nerves of 
the body is followed by a change in the blood pressure. Just 
what this change may be it is often impossible to predict. Strong 
sensory stimuli of short duration may produce a marked rise in 
blood pressure, the constrictor center being the most affected. 
On the other hand, if the stimuli are very strong or continued 
over a long period of time, the constrictor nerves may become 
exhausted, as it were, resulting in a dilation of the arteries and 
a fall in the general blood pressure. Like phenomena are often 
seen following fright, pain, grief, and excitement. The patient 
becomes suddenly pale, dizzy, and may faint, losing conscious- 
ness entirely. This is due to a fall in the arterial blood pressure 
produced by a temporary inhibition of the vasoconstrictor nerves 
and perhaps also by a slowing of the heart, due to vagus stimula- 
tion. If the person be standing, the blood naturally flows to the 
vessels of the abdominal viscera and dependent portions of the 
body, and the brain is thereby rendered bloodless. The treat- 100 
EFFECTS OF GRAVITY ON BLOOD FLOW. 
ment of these cases is to elevate the feet and abdomen and to 
lower the head. 
In case the depression of the blood pressure slowly develops 
because of the gradual onset of fatigue in the vasomotor and 
other nervous centers, a condition known as surgical shock super- 
venes. The treatment of this condition demands plenty of air, 
stimulants, saline or blood transfusion, and measures to main- 
tain the body temperature. 
The Pressure Effects of Gravity on the Blood Flow vary 
according to the posture of the body. In the upright position 
the blood vessels of the feet support a column of blood of rela- 
tively great height, but when the individual is lying down this 
ceases to be the case. In spite of this, by means of the delicate 
adjustments which the nervous system can bring about in the 
heart and the blood vessels, there is little difference in the pres- 
sure of the blood in the arteries in any position which the person 
may assume. The blood vessels and nerves soon lose this power 
if it is not continuously exercised. This is illustrated in patients 
who have been confined to their beds for a time. If they try to 
walk or to stand up suddenly, they become very dizzy and may 
faint, which means that the blood has left the vessels of the 
brain and is gathered by the force of gravity in the vessels of 
the dependent parts of the body. With a normal vasomotor 
mechanism, the vessels of the feet and viscera would quickly 
constrict to such an extent that the blood pressure would remain 
at its normal height in the vessels of the brain. 
The fact that stimulation of sensory nerves by the gross meth- 
ods of the laboratory results in very profound changes in the 
blood pressure and in the velocity of the circulation of the blood, 
suggests that normally the vasomotor and cardiac nerves play 
an important role in the proper distribution of blood in the 
various parts of the body. It may be supposed that normally the 
nerves of the vascular system function to control the blood flow 
through the various organs according to their respective needs. 
Whenever the work of an organ is increased, the blood flow like- 
wise is augmented in the part, while in the rest of the body the 
blood flow is diminished to a greater or less extent. The blood FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
101 
supply is continually changing according to the call of the vari- 
ous tissues for blood; now the muscles, now the digestive organs, 
now the brain demand more blood, and this is supplied in the 
proper amount by the nervous system commanding some arte- 
rioles to dilate and others to constrict. 
Haemorrhage.-The action of the vasomotor mechanism is 
beautifully shown in the case of hemorrhage. As blood is with- 
drawn, the vasomotor nerves are stimulated by the falling pres- 
sure in the brain. This brings about a more powerful tonic con- 
striction of the vessels through the action of vasoconstrictor 
nerves, the vascular area becomes smaller and smaller in size, 
and less blood is required to maintain the blood pressure. Be- 
cause of this mechanism a relatively large amount of blood can 
be lost without affecting the general blood pressure. 
The Regulation of the Blood Supply by Chemical Stimuli.- 
The caliber of the blood vessels may be influenced by other means 
than through their nervous mechanism. Acids in very small 
concentrations cause a vascular dilatation. For example, lactic 
acid and carbonic acid, both of which are formed during muscu- 
lar work, may produce a local dilatation of the blood vessels, the 
phenomenon thus constituting an automatic mechanism for deliv- 
ering more blood to a part when it is needed. On the other hand, 
the secretion of the adrenal and of a portion of the pituitary 
gland (see p. 233) produces a constriction of the vessels and 
thus tends to maintain the normal blood pressure. Recently it 
has been shown that during periods of excitement and sensory 
pain the amount of the adrenal secretions may be increased and 
the arterial blood pressure raised as a result of general vasocon- 
striction (p. 97). Because of its vasoconstricting properties, 
extract of the adrenal glands {"adrenalin" or " epinephrin"} 
is used in local anaesthetics, as in cocain solution, to prevent 
bleeding and to minimize the absorption of the cocain into the 
general circulation. 
Asphyxia.-Whenever the amount of oxygen which the blood 
must supply to the tissues falls below the minimum amount re- 
quired, a condition known as asphyxia develops. If the nervous 
centers are intact, any interference with the respiratory function, 102 
ACTION OF DRUGS ON THE CIRCULATION. 
as by obstruction of the respiratory passages, lack of ogygen in 
the atmosphere, or the presence of irrespirable gases in the at- 
mosphere-such as carbon monoxide, which reduces the oxygen 
capacity of the haemoglobin-interferes with the blood supply of 
the brain and will produce a train of phenomena in which the 
respiratory and circulatory changes are prominent. In ordinary 
asphyxia two factors may be involved, a deficiency of oxygen 
and an excess of carbon dioxide in the blood. The phenomena 
following each are essentially the same, and may be divided into 
three typical stages. In the first stage, that of hyperpnoea, the 
respirations are increased in rate and amplitude. This stage 
merges into the second, which consists of exaggerated expiratory 
efforts, and loss of consciousness; stimulation of the vascular cen- 
ters in the brain, causing general vasoconstriction accompanied 
with vagus slowing of the heart also occurs. The net result is a 
rise in blood pressure. In the third stage, the expiratory efforts 
give way to slow deep inspirations followed by expiratory con- 
vulsions. The pupils dilate widely, the heart becomes very weak 
from lack of oxygen and overwork, and death occurs from car- 
diac failure. The changes produced in the respiratory move- 
ments, as well as those of the vascular system, are caused by the 
direct stimulation of the respiratory (see p. 122) and vascular 
centers, by excess of carbon dioxide and by the lack of oxygen 
in the blood. 
The Physiological Action of Some Drugs on the Circulation. 
-Drugs which affect the circulation may do so either by a direct 
action on the muscles of the heart or blood vessels, or by stim- 
ulating the nervous mechanism which controls the movement of 
the blood. Of first importance in the list of such drugs are 
those of the digitalis group. These drugs, while they do not 
affect the normal circulation to any great extent, exert a very 
beneficial action in some cases of heart failure, partly because 
they stimulate the cardiac and vasomotor centers and partly 
by a direct tonic influence on the musculature of the heart. In 
general, this results in a slowing of the heart beat, some in- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
103 
crease in tone of the blood vessels and a more complete empty- 
ing of the ventricles. The latter effect predominates and the 
result is an increase in the amount of blood supplied to the 
tissues of the body. In cases where the circulation has been de- 
ficient and the tissues are improperly nourished, with the re- 
sult that fluid collects in them, causing what is known as oedema, 
the administration of digitalis is followed by an improvement 
in the heart's action and the excreting power of the kidneys with 
the subsequent loss of the oedema. Because drugs of this series 
have what is known as a cumulative action when given over a 
period of time, it is unwise to take them except on the advice 
of a physician. 
Strychnine has enjoyed considerable reputation as an efficient 
heart stimulant. It is extremely improbable that it has any 
such action in the doses prescribed therapeutically. 
Drugs belonging to the nitrite series, such as amyl nitrite, 
nitroglycerine and sodium nitrite, upon administration, produce 
a marked fall in the blood pressure which is due to the peri- 
pherial dilatation of the blood vessels. This is a direct action 
on the muscles and does not involve the nervous system. The 
use of an extract of the adrenal glands, commercially known as 
adrenalin or epinephrin, as an astringent in case of hgemorrhage 
is due to its action on the muscles of the arterioles. The intra- 
venous injection of adrenalin is followed by an icrease in the 
blood pressure brought about by the general constriction of the 
peripheral blood vessels. This action is made use of in the treat- 
ment of urticaria, a disease commonly known as hives, in which 
there is a dilatation of the blood vessels of the skin. CHAPTER IX. 
THE RESPIRATION. 
Oxygen is one of the essential substances required by every 
living organism, in the cells of which it combines with the carbon 
to form carbon dioxide, and with hydrogen to form water. All 
the phenomena accompanying the supply and utilization of oxy- 
gen and the excretion of carbon dioxide are included under the 
subject of respiration. 
In the simplest forms of life the exchange of oxygen and car- 
bon dioxide gas occurs directly with the air, but in more complex 
organisms this sort of exchange is impossible, since practically 
none of the cells composing the organism is in direct communi- 
cation with the air. Some sort of respiratory apparatus becomes 
necessary, so that each cell may be supplied with oxygen and 
have its carbon dioxide removed. In the higher animals this is 
accomplished through the agency of the blood, which is well 
adapted to transport the oxygen and carbon dioxide, first 
because it contains chemical bodies with which the gases can 
unite, and secondly because it comes in close contact with the 
tissue cells in the peripheral portions of the body, and with the 
atmospheric air in the capillaries of the lungs. The study of 
the respiratory function therefore includes the mechanism of 
the gas exchange between the tissues and the blood, or internal 
respiration, and also that between the lungs and the blood, or 
external respiration. 
Internal Respiration. 
The energy which the body expends in the performance of the 
functions of life, including the heat which is required to main- 
tain the body temperature, is produced in the cellular chemical 
reactions, in which the oxygen of the air combines with the 
104 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
105 
hydrogen and carbon of the foodstuffs to form water and carbon 
dioxide gas. 
Oxidation in the Tissues.-The actual mechanism which 
unites the oxygen with the carbon and hydrogen of the food- 
stuffs within the tissue cells, is not entirely known. In spite of 
the fact that the processes of combustion of hydrocarbon matter 
outside the body yield the same end products as the oxidations 
taking place within it, the two processes are not strictly analo- 
gous. An important point of difference lies in the fact that the 
intracellular materials-fats, proteins, and carbohydrates-are 
oxidized with relatively great rapidity at low temperatures 
(98.4°), whereas the same reactions outside the body require a 
very high temperature. 
Let us take as an example the cell of the yeast plant, in which 
there is a substance, under the influence of which, the sugar 
molecule becomes split up, at a temperature below that of the 
body, to produce carbon dioxide and water. Similar substances 
are present in the tissue cells of plants and animals; they are 
the ferments or enzymes (see p. 45), and they act as catalytic 
agents. The function of these bodies is to increase the velocity 
of many chemical reactions which otherwise proceed so slowly 
that they may be said in some cases not to exist. A class of 
these substances is present within the tissue cells, which at the 
demand of the tissues control the extent and the velocity of the 
union of oxygen with the hydrocarbons of the food. Such en- 
zymes are known as oxidases. 
What evidence have we, however, that this oxidation takes 
place within the tissues and not within the blood itself? It is 
conceivable that the substances that are to be oxidized are col- 
lected from the tissues by the blood, and that the oxygen combines 
with them in this fluid. It is quite possible that some oxidation 
takes place in the blood, for it is essentially a tissue and has 
a metabolism of its own, but this is not true for the oxidation 
which concerns the tissues, since this takes place in the tissues 
themselves, as can be shown by the following fact: The blood of 
a frog may be replaced with saline solution in which oxygen is 
dissolved under pressure, without killing the animal. It is 106 
OXIDATION IN THE TISSUES. 
hardly conceivable that oxidation similar to that occurring with- 
in the body can take place in a solution of sodium chloride. 
Relation of Oxidative Process to Activity.-Under ordinary 
conditions the blood has a supply of food and oxygen sufficient 
for the needs of the body. An excess of either does not intensify 
the oxidative process. An animal will give off the same amount 
of carbon dioxide in an atmosphere of pure oxygen as it will 
under ordinary conditions. This fact indicates that the oxida- 
tive processes are governed not by the supply of food or oxygen, 
but rather by the actual needs of the tissues. A muscle freshly 
removed from the body may be made to contract, and will give 
off carbon dioxide for some time in the entire absence of oxygen 
in the surrounding medium. Another feature of this experiment 
is that for a time after the muscle has ceased to contract, it will 
produce heat and take up a large amount of oxygen. Indeed 
the maximal intake of oxygen and output of heat often occurs 
after the actual period of work. In this respect the muscle can 
be likened to a storage battery which is charged by the actual 
expenditure of energy and delivers quickly the energy stored 
up when the circuit is closed. If the volume intake of oxygen 
and output of carbon dioxide is measured, it will be found that 
the amounts are greatly increased during periods of tissue activ- 
ity. Experiments have demonstrated that a muscle at full work 
will use up its own volume of oxygen in ten minutes. To supply 
such an amount of oxygen requires a very high degree of effi- 
ciency on the part of the distributing agent, the blood. 
Physical Laws Governing Solution of Gases.-A brief re- 
view of the physical laws governing the solution of gases in water 
will help us materially to understand the mechanism of the trans- 
portation of oxygen and carbon dioxide by the blood and the 
respiratory mechanism in general. 
Gases differ from solid and fluid materials in that the- parti- 
cles which compose them repel more than they attract each 
other, thus permitting the gas to diffuse throughout the atmos- 
phere. The repelling force exerted by the molecules of gas on 
the walls of the container produce the phenomena of gaseous 
pressure. It follows, therefore, that the pressure which a gas FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
107 
exerts varies with the number of molecules of the gas present 
in the atmosphere. The various gases diffuse out into space 
until this diffusion pressure is balanced by the force of gravity. 
The weight of a substance is the force which gravity exerts on 
it. The weight of a gas would therefore be an indication of the 
diffusion pressure of the gas in the atmosphere, and also indi- 
rectly of the number of molecules of gas present. The weight 
of a gas or the pressure of the gas, on the earth's surface is 
measured by an instrument called a barometer, in which the 
atmosphere is balanced against a long column of mercury, and 
the weight expressed in the number of millimeters of mercury 
which the atmosphere will support. At sea level and at 15.5° 
Centigrade, the pressure which the atmosphere exerts on the 
surface of any fluid is sufficient to support the weight of a col- 
umn of mercury 760 millimeters in weight. 
The solubility of a gas in a fluid is measured by the number of 
cubic centimeters of gas which one cubic centimeter of fluid will 
dissolve under standard conditions of temperature and pressure. 
Such a figure is known as the coefficient of solubility. For ex- 
ample, pure carbon dioxide gas under standard conditions of 
temperature and pressure (760 mm. pressure and 15.5 degrees 
Cent.) will dissolve to the amount of one c.- c. in one c. c. of water. 
Under like conditions only 0.04 c. c. of oxygen will be dissolved. 
The coefficient of solubility of carbon dioxide is therefore 1.0 and 
of oxygen 0.04. 
The amount of gas which will go into solution in water depends 
on three factors: the temperature of the water, the solubility of 
the gas in water, and the pressure which the gas exerts on the 
surface of the water. Asa rule, the higher the temperature of the 
water, the less gas will go into solution, or in other words, the 
solubility of a gas varies inversely with the temperature. 
If, in place of having pure gases over a fluid, a mixture of 
several gases be present, then we find the solubility of each of 
the gases varying directly with the pressure it exerts on the sur- 
face of the fluid. Suppose that in place of exposing a cubic centi- 
meter of water to oxygen at 760 mm. pressure, we expose it to 
oxygen at a pressure of 152 mm. mercury, which is the normal 108 
HEMOGLOBIN. 
pressure of oxygen in the air (V5 of an atmosphere) it would 
absorb 1/5 of .04 c. e. or .008 c. c. of oxygen. The presence of 
other gases does not enter into consideration, for according to 
Dalton-Henry's law, when two or more gases are mixed together, 
each of them produces the same pressure as if it separately occu- 
pied the entire space and the other gases were absent. When the 
fluid has taken up all the gas it can, an equilibrium becomes es- 
tablished between the gas in the atmosphere and the gas within 
the fluid. The pressure which the gas in the fluid exerts on the 
gas in the atmosphere is known as the tension of the gas, and 
equals the pressure of the gas in the outside atmosphere to which 
it is exposed. This can be easily measured. 
Since the pressure of the oxygen in the air in the lungs is less 
than that in the outside atmosphere, it is apparent that if the 
blood should carry the same amount of oxygen as water does, the 
amount would be very small indeed. Analysis of the amount of 
oxygen in arterial blood shows that it contains 40 times the 
amount per c. c. that water can dissolve under like conditions. 
For example, let us imagine human blood to be water. It would 
carry then only 1/40 of the volume of oxygen that it does, and 
the tissues of the body would need a vascular system the size of 
an elephant's in order to obtain as much oxygen as normally 
is supplied by the blood. Therefore it is obvious that the laws 
for simple solutions can apply only in a slight degree to the 
gases in the blood. They would account at the most for only 0.7 
per cent of the total oxygen and 2 per cent of the carbon dioxide 
found in the blood. 
Haemoglobin.-The extraordinary ability of the blood to 
carry oxygen and carbon dioxide lies in the presence of sub- 
stances capable of chemically uniting with and storing up large 
amounts of the gases. The iron-containing protein substance 
called haemoglobin, found in the red blood cells, carries the 
oxygen, and the alkalies and proteins of the blood carry most of 
the carbon dioxide. Analysis of samples of arterial and venous 
blood gives the following average figures, which represent the 
volumes of the gas found in one hundred volumes of blood. 109 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
Oxygen CO2 Nitrogen 
100 vol. arterial blood contains 20 40 1-2 
100 vol. venous blood contains 10-12 45-50 1-2 
The small amount of nitrogen present in the blood in spite of 
the large percentage found in the atmosphere (4/5 of the baro- 
metric pressure being due to nitrogen) is due to the absence of 
any chemical body within the blood plasma which will unite with 
nitrogen. Of the 20 volumes per cent of oxygen found in arterial 
blood only 0.7 per cent is in solution in the plasma. 
The Mechanism of the Respiratory Exchange.-The oxygen 
in the alveoli or air passages of the lungs comprises about 14 to 
15 per cent of the total air, and exerts on the cells of the respira- 
tory epithelium a pressure of about 100 mm. mercury, more or 
less. Venous blood when it reaches the lungs contains about 50 
per cent less oxygen than does arterial blood, and can take up 
from 6 to 8 e. c. of oxygen for every one hundred c. c. of blood. 
Haemoglobin solutions are almost completely saturated with oxy- 
gen at pressures of oxygen much less than 100 mm. of mercury. 
There are, therefore, very favorable conditions in the lungs for 
haemoglobin to take up oxygen from the air. It must be under- 
stood, however, that the haemoglobin does not obtain oxygen di- 
rectly from the air. The haemoglobin is held in the blood corpus- 
cles which are floating in the blood plasma. Between the plasma 
and the air in the lungs lie two thin'membranes, the capillary wall 
and the wall lining the air sac of the lung. The oxygen must first 
be dissolved by the fluid in the lung epithelium; from this the 
cells of the capillary walls take oxygen, and the plasma in turn 
takes the oxygen from the capillary cells. The plasma loses the 
oxygen thus obtained because the haemoglobin is very greedy for 
oxygen. There is accordingly a difference in the oxygen pressure 
in the plasma of the capillaries of the lungs sufficient to account 
for the absorption of oxygen by the haemoglobin of the blood. The 
blood leaving the lungs is delivered into the left heart, from 
which it is distributed over the body. Since oxidation takes 
place within the tissue cells, oxygen is being continually called 
for, and the lymph surrounding the cells must continually gain a 
fresh supply of oxygen from the plasma of the blood. This re- 110 
THE EXCHANGE OF CARBON DIOXIDE. 
duces the tension of oxygen in the plasma and causes an evolution 
of oxygen from the oxyhaemoglobin, which is taken up by the 
plasma to be passed on to the lymph and then on to the cell. 
There is thus a descending scale of pressure or tension of oxygen 
from the air of the lungs, where its pressure may amount to 100 
mm. of mercury, until it reaches the tissue elements, where the 
pressure may be considered zero. Under ordinary conditions the 
circulation is fast enough to prohibit the complete reduction of 
the oxyhaemoglobin. In case it is not, or in case the oxygen sup- 
ply is short, the phenomena of asphyxia develop (see p. 101). 
Effect of Carbon Dioxide on Oxyhaemoglobin.-As a result 
of the oxidative changes which take place within the cells, carbon 
dioxide is produced, and the tension of this gas rises in the tis- 
sues. It will be remembered in the discussion of the dissociation 
of oxyhaemoglobin, that the effect of increased tensions of carbon 
dioxide is to increase the rate of reduction of oxyhaemoglobin into 
oxygen and haemoglobin. Since there is a high tension of car- 
bon dioxide present in the tissues and at the site of the capil- 
laries, the effect on the reduction of oxyhaemoglobin is very 
marked, and has a great influence on the rate at which oxygen is 
supplied to the tissues. Just as there is a descending pressure 
of oxygen from the air in the lungs to the cell, so is there a de- 
crease in pressure from the carbon dioxide in the cells to the air 
of the lungs. This gas therefore passes through the lymph to the 
plasma and out of the plasma through the pulmonary epithelium 
by the simple process of diffusion. 
The Exchange of Carbon Dioxide.-Analysis of venous blood 
shows that 100 c. c. contains about 45 to 50 c. c. of carbon diox- 
ide, and that the gas exerts a pressure or tension of about 40 mm. 
mercury, which is equal to about five per cent of an atmosphere. 
Now water will dissolve under these conditions about 2^ c. c. of 
carbon dioxide per 100 c. c. This would leave the most of the 
carbon dioxide of the blood unaccounted for, in case the blood 
has the same solvent power for the gas that water has. The rest 
of the carbon dioxide therefore must be accounted for as being 
in chemical combination with the constituents of the plasma and 
corpuscles. The major part is probably held in the form of FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
111 
sodium carbonate and bicarbonate, the remainder being combined 
with the proteins of the plasma and the red corpuscles. The most 
satisfactory explanation of the manner in which carbon dioxide 
is dissociated from the above mentioned compounds in the blood, 
is that there are substances in the plasma, such as the blood pro- 
teins, which act as weak acids, and gradually drive off the carbon 
dioxide wh'en, as in the air in the lungs, its escape is rendered 
easy by a lowered carbon dioxide pressure outside the plasma. 
The External Respiration. 
Anatomical Considerations.-The constant call of the tissues 
for oxygen and the formation of the waste gas, carbon dioxide, 
demands a mechanism by which the blood can continually renew 
its supply of oxygen and excrete its excess of carbon dioxide. 
This exchange, as we have seen, is effected in the lungs, which 
are built up in the following way: 
The nasal and oral cavities lead to the pharynx, from which 
Fig. 27.-Diagram of structure of lungs showing larynx, bronchi, bron- 
chioles and alveoli. 
open two tubes: one posterior, the oesophagus, going to the ali- 
mentary tract, and the other, anterior the trachea, going to the 
lungis (Fig. 27). At the beginning of the trachea is placed the 
larynx, or the voice box, the opening of which is guarded by a 
flap of tissue, the epiglottis. Within the larynx are the vocal 
cords. The trachea, or windpipe, is a relatively large tube, about 
four and one-half inches long, which, after its entrance into the 112 
THE LUNGS. 
thorax, divides into two tubes, the bronchi, each of which subdi- 
vides again and again, the branches gradually growing smaller 
until they are mere twigs, and are known as bronchioles, or small 
bronchi. The lumen of the trachea and bronchi is maintained 
patent by cartilage plates, which are imbedded in the walls of the 
tubes. The bronchioles, however, have no such plates, their walls 
being composed of fibrous and elastic tissue, in which is a layer 
of smooth muscle. The whole system of tubes is lined with a 
layer of ciliated epithelium. (See Fig. 25, p. 92.) 
The bronchioles terminate in wide air sacs or cavities, the in- 
fundibuli, from the walls of which extend numerous minute cavi- 
ties, the alveoli. The walls of the alveoli are very thin but 
strong, and are composed of a layer of elastic tissue lined with a 
single layer of flattened epithelium. It is estimated that the epi- 
thelial surfaces of the alveoli, if they were spread out on a flat 
surface, would cover about 1,000 square feet. Such a large area 
exposed to the air of the lungs offers the best of facilities for the 
rapid exchange of the respiratory gases, and in fact the walls of 
the alveoli are the true respiratory membrane of the lung, for 
through them the exchange of gases between the air and the 
blood takes place. Below the epithelial cells of the alveoli lie the 
capillaries of the pulmonary artery in a regular meshwork; so 
numerous, indeed, are they that each individual erythrocyte is 
able to come in close contact with the air in the alveolus, separ- 
ated only therefrom by the lining of the alveolus, the wall of the 
artery, and the plasma of the blood. This arrangement makes 
possible the rapid exchange of gases which 'must take place with- 
in the lungs. 
The two lungs in company with the heart occupy the thoracic 
cavity, which is bounded above and on the sides by the ribs and 
their attached tissues, and below by the diaphragm, a muscular 
sheet of tissue which divides the body cavity into a thoracic and 
an abdominal portion (Fig. 28). The lungs are suspended at 
their roots, which are composed of the trachea and the pulmonary 
blood vessels, and they lie free in the thoracic cavity in close ap- 
position with the walls of the thorax. Covering the outside of 
the lungs and the inside of the thoracic cavity, which is in con- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
113 
tact with the lungs, is a thin endothelial membrane known as the 
pleura, the surface of which is kept moist by a secretion of 
lymph. This smooth membrane allows the surface of the lungs 
to move easily over the inner surface of the thorax during the 
changes in the size of the cavity which accompany the respiratory 
movements. 
Mechanism of Breathing.-Normal breathing has the object 
of bringing about a constant renewal of air in the lungs, and it 
is effected by movements of the thorax and diaphragm. When- 
ever the cavity of the thorax is enlarged, as in the act of inspira- 
tion, the lungs must increase in size to fill the space, and air is 
Fig. 28.-The position of the lungs in the thorax. (T. Wingate Todd.) 
pushed into the respiratory tubules and the air sacs by the pres- 
sure of the outside atmosphere. At expiration the reverse takes 
place, and the air is expelled. A very good conception of the 
mechanism by which this is brought about may be had by refer- 
ence to Fig. 29. Any increase in size of the bottle, as by pulling 
down the bottom rubber membrane, will cause air to expand the 
rubber sacs coming in by the tube passing through the cork of the 
bottle. When the size of the cavity is decreased by releasing the 114 
MECHANISM OF BREATHING. 
membrane, the reverse takes place and air is expelled from the 
rubber sacs. 
With every inspiration the thorax is increased in size in all 
diameters, from above downwards by the contraction of the 
diaphragm, and in the transverse diameter by the movement of 
the ribs. 
The Part Played by the Diaphragm.-The diaphragm is a 
circular sheet of muscle which divides the body cavity into two 
compartments, the upper being the thorax, the lower the abdom- 
Fig. 29.-Hering's apparatus for demonstrating the action of the respira- 
tory pump. The thorax is represented by a bottle, the diaphragm by a sheet 
of rubber forming its bottom, the trachea by a tube passing through the 
cork, and the lungs by two thin rubber bags. A thin piece of rubber tubing 
crosses the bottle. This represents the heart. The action of the diaphragm 
pumps air in and out of the lungs and water through the heart. The lungs 
and heart are thin rubber bags. (From Baird and Co.'s catalogue.) 
inal cavity. In the upper compartment are the lungs and heart 
with the accompanying blood vessels and air passages. The ab- 
dominal cavity contains the digestive organs and glands, as the 
liver, kidneys, spleen and reproductive organs. The peripheral FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
115 
edges of the diaphragm are attached to the lumbar vertebrae at 
the back, to the lower border of the ribs on the sides, and to the 
tip of the sternum in front. The muscular fibers radiate to- 
wards the center and end in a tendinous sheet of tissue called the 
central tendon of the diaphragm. When these fibers are relaxed, 
the diaphragm is pushed up into the thoracic cavity, forming a 
dome-shaped arch. This is caused by the pressure of the abdomi - 
nal organs, supported by the muscular walls of the abdomen, on 
its lower surface, a suction pressure on the upper surface of the 
diaphragm being maintained by the natural tendency of the 
lungs to contract. The central tendon is pulled downwards and 
the arched dome is flattened on contraction of the diaphragm, 
Fig. 30.-Diagram to show movement of diaphragm during respiration: 
I, expiration; II, normal inspiration; III, forced inspiration. 
thus increasing the size of the thoracic cavity (Fig. 30). An- 
other result of the lowering of the diaphragm is the slight pro- 
trusion of the abdomen due to the pressure exerted on the vis- 
cera. This type of breathing is therefore known as abdominal or 
diaphragmatic breathing. 116 
MECHANISM OF BREATHING. 
The Part Played by the Thorax.-The action of certain 
muscles attached to the ribs also produces an enlargement of the 
thoracic cavity. Each pair of corresponding ribs, which are ar- 
ticulated posteriorly with the vertebral column and anteriorly 
with the sternum, forms a ring directed obliquely from behind 
forwards and downwards. Any muscles whose action would 
bring about a raising of the anterior ends of the ribs, would 
therefore lessen the oblique position and increase the distance be- 
tween each pair of ribs, and also add to the antero-posterior 
diameter of the thorax. Each rib increases in length from 
above downwards, and as the ribs are raised, the lower longer 
rib occupies the place previously held by its shorter neighbor. 
This movement therefore causes the dome or apex of the thorax 
to become more flat and broad. And also the lower ribs are 
so articulated with the spinal column that they exhibit an up- 
ward rotary movement, which resembles that made by a bucket 
handle, and which increases the lateral or transverse diameter 
of the thorax. 
The muscles which are responsible for the inspiratory eleva- 
tion of the ribs are mainly the external intercostals, aided by 
other muscles of the thorax, some of which are called into use 
only when very powerful respiratory movements are necessary. 
Normal expiration is almost entirely a passive act. The re- 
coil of the stretched elastic tissue of the lungs, after the in- 
spiratory muscles have ceased to act, returns the diaphragm and 
thoracic cage to the expiratory position. This is aided somewhat 
by the actions of the internal intercostal muscles which lower the 
ribs. By increasing the size of the thoracic cavity, inspiration 
causes a corresponding increase in volume of the thoracic organs; 
viz., the lungs and the vascular structures, because the thorax is 
a closed cavity, and whenever it expands it must either produce a 
vacuum between the organs which fill it and its own walls, or the 
volume of the organs must increase. It is the latter process 
which mainly occurs, the result being that air is pushed into the 
lungs by the atmospheric pressure whenever the thoracic cavity 
is increased in size. 
The Movements of the Lungs.-The changes produced in FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
117 
the size of the thoracic cavity and the lungs during normal res- 
piration or in disease, are easily determined by noting the sounds 
which are produced by tapping or percussing with the fingers the 
thoracic walls during inspiration and expiration. A low-pitched 
resonant sound is elicited over the lungs containing air, whereas 
a high-pitched non-resonant or tympanitic hollow sound is heard 
over the solid viscera and abdominal organs. In disease when 
changes take place in the substance of the lungs, as in tubercu- 
losis, pneumonia, etc., alterations occur in the tone elicited on 
percussion. These alterations are of great diagnostic import- 
ance. In pleurisy, a condition in which the pleural surfaces are 
roughened, a friction rub or vibration, produced by the rubbing 
of the roughened surfaces of the pleura of the lungs on that of 
the thorax, may be detected by placing the ear over the affected 
area. The pain following a broken rib is caused by the irritation 
of the pleural membrane by the broken edge of the rib. It is al- 
leviated by making the ribs immovable by tightly strapping the 
thorax with adhesive plaster over the region of the pain. 
Respiratory Sounds.-Accompanying inspiration a rustling 
sound, described as a vesicular sound, may be heard over most of 
the lung area. It is produced by the dilatation of the alveoli and 
fine bronchi. Over the larger air passages a high, sharper tone 
is heard, called bronchial breathing. In diseases in which the 
alveoli are destroyed and the lungs are filled with fluid, etc., 
the bronchial breath sounds replace the vesicular sounds. 
Variations occur in the respiratory movements under various 
emotional and physical conditions. Any foreign or irritating 
body within the air passages will cause a cough. This consists 
in a forced expiration, during the first portion of which the glot- 
tis is closed. The irritating substance is likely, to be expelled by 
the sudden opening of the glottis. The presence of irritating 
substances in the nasal cavity gives rise to sneezing, a sudden 
and noisy expiration through the nasal passages preceded by a 
rapid and deep inspiration. In crying, inspirations are short and 
spasmodic, followed by prolonged expirations, whereas laughing 
is quite the reverse. Yawning, the expression of drowsiness or 
ennui, consists in long deep inspirations followed by a short ex- 118 
ARTIFICIAL RESPIRATION. 
piration. Hiccoughing is due to spasmodic contractions of the 
diaphragm, the peculiar sound being due to sudden closure of 
the glottis. 
Artificial Respiration.-In cases of suspended respiration in 
human beings caused by drowning, excess of anaesthesia, or other 
injury, artificial respiration is often necessary to restore normal 
breathing. The most efficient of these methods is described by 
Schafer, and is known as Schafer's method (Fig. 31). He de- 
scribes the method as follows: It consists of laying the subject 
in the prone posture, preferably on the ground, with a thick 
folded garment underneath the chest and epigastrium. The 
operator puts himself athwart or at the side of the subject, facing 
Fig. 31.-Position to be adopted for effecting artificial respiration. 
(Schafer.) 
his head (Fig. 31) and places his hands on each side over the 
lower part of the back (lower ribs). He then slowly throws the 
weight of his body forward to bear upon his own arms, and thus 
presses upon the thorax of the subject and forces the air out of 
the lungs. This being effected, he gradually relaxes the pressure 
by bringing his own body up again to a more erect position, but 
without moving his hands. These movements are repeated reg- 
ularly at a rate of twelve to fifteen per minute until normal res- 
piration begins. 
Volumes of Air Respired.-At each inspiration the lungs take 
in about 500 c. c. of air, which is given out again at expiration. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
119 
This is known as the tidal air. After the completion of the ordi- 
nary inspiration, it is possible by a forced inspiration to take 
1500 c. c. more air into the lungs. This amount is known as the 
complemental air. Likewise after a normal expiration about 1500 
c. c. more air can be expelled from the lungs. This is known as 
the supplemental air. In spite of forced expiration there will 
still remain within the lung about 1000 c. c. of air which fills the 
alveoli and air tubes, known as the residual air. This air remains 
in the air spaces after the forced expiration because the lungs 
cannot relax to their fullest extent, being held open by the suc- 
tion pressure of the thorax. In other words, the thoracic cavity 
is larger in the expiratory position by 1000 c. c. than the lungs 
are. That this is the case is shown by the immediate contraction 
of the lungs into a small volume when the thorax is opened, for 
then the atmospheric pressure becomes equalized on the out- 
side and inside of the lungs, and the elastic tissue contracts and 
forces out the residual air. From this it is obvious that the elas- 
tic recoil of the stretched lungs must always tend to pull the 
organ away from the chest wall and thus create a negative or suc- 
tion pressure within the thoracic cavity. Anything which de- 
stroys this relation makes breathing impossible, because the lungs 
are no longer held against the chest walls. It is for this reason 
that wounds in the chest are very dangerous. 
The trachea, bronchi, etc., require quite a little air to fill them, 
so that only a part of the tidal air reaches the alveoli. In other 
words, it is only a portion of the air we expire'that comes in 
contact with the respiratory epithelium and undergoes any 
change in composition. 
It is estimated that about 140 c. c. represents the actual vol- 
ume of the air tubes. This leaves 360 c. c. of air which reaches 
the alveoli. This amount is used to dilute the 1000 c. c. of 
residual air and 1500 c. c. of supplemental air already in the 
alveoli. In fact the function of breathing may be said to con- 
sist in continually diluting the alveolar air with a quantity of 
fresh air in order that its composition may remain more or less 
constant. 
The inspired or atmospheric air is a mixture of oxygen, car- 120 
GASEOUS EXCHANGE IN LUNGS. 
bon dioxide and nitrogen, and is relatively constant under ordi- 
nary conditions. The expired air varies somewhat according to 
the rate and depth of respiration. The following table gives the 
average percentage composition of inspired and expired air: 
Nitrogen 
Oxygen 
CO, 
Inspired air 
79 
20.96 
0.04 
Expired air 
... 79 + 
16.02 
4.38 
The above analysis shows that there is a marked difference be- 
tween the inspired and the expired air. It shows us further that 
of the oxygen taken up by the blood, only part appears again 
combined with carbon in the gas CO2. The retention of oxygen 
is due to the oxidation of substances which do not appear in the 
expired gases. This subject is fully discussed under the head of 
respiratory quotient in the chapter on metabolism (p. 194). 
Mechanism of Gaseous Exchange in Lungs.-We have seen 
that in the blood the pressure or tension of the oxygen is greater, 
whereas that of the CO2 is less than in the tissues. These rela- 
tions will account for the gas exchange which occurs between the 
blood and tissues if we apply the physical law of the diffusion of 
gases, which states that two gases under different pressures and 
separated by a membrane through which they may pass free- 
ly, will mix with each other until the tensions on both sides 
of the membrane are equal. Before this law can be applied to 
explain the exchange of gases between the blood and air within 
the lungs, we must prove that the tension of the oxygen is less, 
and of the CO2 greater in the venous blood than in the alveolar 
air. A consideration of these problems is included under the 
subject of external respiration. CHAPTER X. 
THE RESPIRATION (Cont'd). 
Nervous Control of Respiration.-Under normal conditions 
we breathe from 14 to 18 times a minute. According to the de- 
mand of the tissues for oxygen, we breathe fast or slow, but the 
respirations are rhythmic in time and under like conditions are 
equal in volume. The respiratory movements, unlike those of 
the heart are initiated by impulses transmitted to the respira- 
tory muscles from the central nervous system. These arise from 
the so-called respiratory centers in the medulla oblongata (p. 
268). Anatomically these centers cannot be sharply localized, 
but destruction of the portion of the medulla in which they exist 
causes an immediate cessation of respiratory movements. The 
centers are connected with the diaphragm by the phrenic nerves, 
and to the muscles of the ribs, larynx and nares by spinal or 
cranial nerves. Like all other nerve centers, the respiratory 
center is influenced by afferent impulses, the chief ones of 
which come from the lungs by way of the vagus, but there 
are many others. In fact all the sensory nerves of the body, 
as well as the higher centers of the brain, are able to influ- 
ence the respiratory center. Disease of the phrenic nerves 
causes paralysis of the diaphragm, and impairs the ventilation 
of the lungs. Likewise paralysis involving the spinal cord be- 
low the exit of the phrenic nerves may paralyze the nerves 
of the thoracic muscles, and throw the whole work of respiration 
on the diaphragm. 
If the vagus nerves of a dog or cat are cut in the neck, the 
respiration becomes deeper and slower, yet the volume of air re- 
spired per minute is not greatly altered. This change is due to 
the elimination of stimuli normally coming from the lungs by 
way of the vagi to the respiratory center, which serve to control 
the depth of respiration. It can be experimentally demonstrated 
121 122 
CHEMICAL CONTROL OF RESPIRATION. 
that the collapse of the alveoli of the lungs which occurs at the 
end of normal expiration, and the stretching of the alveolar walls 
which occurs at the end of normal inspiration, cause stimuli to be 
passed along the vagi to the center, and that these stimuli bring 
on the next phase of respiration. The breaking of the connection 
between the lungs and the alveoli destroys this influence and the 
respirations become deep and slow. 
In the absence of the vagi, the higher centers assume partial 
control of the regulation of the respiratory movements. If they 
also are destroyed, however, breathing becomes inadequate to 
maintain life, although the center itself is still able to keep up a 
modified, rhythmic respiration. 
Reflex Respiratory Movements.-The cutaneous nerves, es- 
pecially those of the face and abdomen, have a marked influence 
on respiration. These can be excited by heat or cold or pain; 
for instance, a cold bath will cause a deepening or quickening of 
the respiration. Another example is found in the forced ex- 
piratory effort made on inhalation of acid or sharp smelling sub- 
stances, which not only affect the olfactory nerves, but also the 
sensitive endings of the fifth nerve in the nasal mucous mem- 
brane. 
Chemical Control of Respiration.-In spite of this very effec- 
tive method of nervous control of the respiration, there is an- 
other no less important means of respiratory control, which de- 
pends on the ability of chemical substances in the blood to stim- 
ulate the respiratory center. The substances which most readily 
affect the center are acids, such as carbon dioxide (which in solu- 
tion forms a weak acid,) and lactic acid, which is formed under 
certain conditions in the body. Lack of oxygen, if it be consid- 
erable, also causes the center to show marked signs of activity. 
In the introductory chapter the physico-chemical properties of 
the blood and tissue fluids were discussed. It will be recalled 
that these are practically neutral fluids, that is, they show an al- 
most exact balance in the number of hydrogen and hydroxyl 
ions, a condition which determines the neutrality of a fluid. Any 
increase in the amount of carbon dioxide in the blood would form 
proportionately more carbonic acid, which yields hydrogen ions, 
and thus tend to destroy the neutral balance of the blood. This FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
123 
increase in the hydrogen ion concentration in the blood is suffi- 
cient to stimulate the respiratory center and augment the rate 
and depth of respiration in order to expel the carbon dioxide 
and thus reduce the acidity of the blood. All acids which yield 
hydtogen ions in solution have this effect on respiration when 
they are injected into the blood. Lactic acid, which is formed 
when the oxygen supply to the tissues is diminished or inade- 
quate, is perhaps the most important factor coming into play in 
the stimulation of the respiratory center which occurs during 
exercise. The carbon dioxide tension of the blood during exer- 
cise may be actually decreased owing to the increased ventilation 
of the lungs as a result of the presence of lactic acid in the blood. 
The increase in breathing due to lack of oxygen is not nearly 
so easily elicited as that caused by excess of acids. In fact, the 
percentage of oxygen may be diminished to about one-half of 
that found in the atmosphere before breathing is markedly af- 
fected. 
In disturbances of the gaseous exchange of the lungs, the re- 
spiratory center attempts to compensate for the change by in- 
creasing the number and the depth of the respirations. If the 
gas exchange be markedly insufficient, the breathing becomes 
very much exaggerated, and practically all possible respiratory 
muscles are called into play. This is the case during an attack 
of asthma, in which the muscles of the arms and abdomen are 
used by the patient in his efforts to obtain enough air. Difficult 
breathing of this kind is known as dyspnoea. If the gas exchange 
is very insufficient, the phenomenon of asphyxia sets in. 
The control of the respiration, therefore, may be said to be 
two-fold, dependent not only on the nerve supply of the respira- 
tory center from the afferent sensory and cerebral nerves, but 
also on the chemical constitution of the blood, which stimulates 
the center directly. Each factor plays an important part in the 
control of the respiratory movements. 
The bronchial muscles are supplied through the vagi with 
nerve fibers which produce dilatation and constriction of the 
bronchi. Just what the normal conditions are which call for the 
action of these nerves is not known. It is generally thought that VENTILATION. 
124 
asthma is caused by the constriction of the bronchioles by spasm 
of the bronchial muscles. Atropin, a drug which paralyzes cer- 
tain nerves, is of therapeutic use in this disease, since it paralyzes 
the nerve endings in the bronchial muscles. Adrenalin is also 
sometimes of use. 
The Effect of Changes in the Respired Air on the Respiration. 
A very slight increase in the percentage of carbon dioxide in the 
alveolar air is accompanied by a very marked quickening of re- 
spiration. On the other hand, the carbon dioxide content of the 
atmosphere may be increased to about one per cent without em- 
barrassing the respiratory function, except during muscular 
work, and it is only at concentrations of carbon dioxide of three 
or four per cent of an atmosphere that the respiratory function 
is seriously impaired. The reason for this is that the inspired 
air becomes greatly diluted before it reaches the alveoli, so that 
a slight increase-up to one per cent of carbon dioxide-in the 
atmosphere only quickens and deepens the respiration sufficient- 
ly to maintain the pressure of carbon dioxide at its normal level 
in the alveoli. 
An increase in the oxygen pressure has no such effect. In fact 
pure oxygen has scarcely any influence on the rate of breathing 
in the normal man. In persons suffering with heart failure or 
diseases in which the respiratory function of the lungs is im- 
paired, however, the presence of a high concentration of oxygen 
in the alveoli may make it possible for the oxygen-starved blood 
to obtain enough of this gas to saturate it and thus improve the 
general condition. The reason for these effects of oxygen is that 
under normal conditions the pressure of oxygen in the atmos- 
phere is more than sufficient to saturate the haemoglobin of the 
blood, so that an increase in the oxygen pressure will add only a 
small amount more of oxygen to that dissolved in the plasma al- 
ready. On the other hand, the oxygen pressure in the atmos- 
phere may be reduced to less than half that found at sea level 
without destroying life. This brings up the interesting question 
of mountain sickness. 
Mountain Sickness.-At an altitude of 5,000 meters (about 
16,000 feet) the air is reduced to a little over half an atmos- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
125 
phere, and the oxygen tension is therefore only about eleven per 
cent of an atmosphere in place of twenty per cent. Therefore, 
in order to supply the needed oxygen, respiration must become 
more rapid. This, however, by washing out the carbon dioxide, 
serves to reduce the tension of carbon dioxide in the alveoli and 
blood to such an extent that the action of this gas on the re- 
spiratory center is weakened, and breathing may be very slow 
or cease for a time, producing a condition known as apnea. The 
lack of oxygen weakens the heart, the slightest muscular move- 
ments are accomplished with difficulty, and the individual suf- 
fers from nausea, vertigo, headache and general weakness. After 
living for some time at such altitudes a person becomes accus- 
tomed to the rarity of the atmosphere and in some manner is 
able to compensate for the lessened oxygen in the air. 
Ventilation.-The disagreeable odor of a crowded room and 
the symptoms which accompany it are well known and are usual- 
ly attributed to the rebreathing of air. In support of this the 
historical incident of the Black Hole of Calcutta, in which many 
people perished from lack of air, is often cited. We have already 
seen that atmospheres up to one per cent of carbon dioxide, or 
containing less than half of the normal percentage of oxygen, 
can be respired with no ill effects. But the percentage of carbon 
dioxide in the worst ventilated room does not, as a rule, rise above 
five-tenths per cent, or at most over one per cent, of an atmos- 
phere. That this amount affects our body metabolism is impos- 
sible, since the carbon dioxide in the alveolar air is kept at a 
constant level of from five to six per cent by the control which the 
respiratory center exercises on the respiratory movements. 
Moreover perfectly normal respiration can take place in a room 
where the oxygen content is so low that a match will not burn. 
Because of these facts it was suggested at one time that a toxic 
substance might be present in the expired air, but this has not 
been confirmed by subsequent investigators. In spite of the fact 
that there is a normal percentage of oxygen and carbon dioxide, a 
room may be unbearably close if it is too warm and the air is 
saturated with moisture. So long as the body can radiate its heat 
quickly into the atmosphere, the room does not feel stuffy, but 126 
VENTILATION. 
when evaporation is slow, because of saturation of the air, and 
heat is no longer given off quickly by the body, the individuals 
in the room become very uncomfortable. An electric fan, which 
distributes the air evenly over the room and thus quickens the 
removal of the warm moist air immediately surrounding the 
body, adds much to the comfort of the person. In addition to 
insisting upon fresh air in public offices and private houses, 
it is necessary that the ventilating engineer should pay heed 
to something besides the percentage of oxygen and carbon 
dioxide in the room. He should also direct his efforts towards 
cooling and increasing the circulation of the air that surrounds 
the bodies of the individuals, by setting the air in motion by 
means of fans. 
The conditions of temperature, the moisture, and the windless 
atmosphere found in public rooms and homes diminish the heat 
loss of the body and thus the heat production, which means that 
the activity of the occupants must be less. A reasonable tem- 
perature with a relatively low percentage of moisture, and ordi- 
nary care in providing fresh air, will maintain the proper hy- 
gienic conditions of a room. 
The temperature of the blood exerts an influence on the 
respiratory centers, which is reflected in the rate and depth of 
the respiration. The increased respiratory rate observed in 
fever may be reduced by any measures which reduce the tem- 
perature of the blood. If this be advisable, cool baths, per- 
fusions and the antipyretic drugs are of value. There are a 
number of drugs which affect the respiratory system either by 
direct action on the respiratory organs or by influencing the 
nervous mechanism concerned in respiration. The drugs which 
produce a sensation of nausea are often used to increase the 
bronchial secretions. As was shown on p. 61, the mechanism 
depends on the reflex stimulation, through the vomiting center, 
of secretory fibers of the nerves going to the bronchi. The use 
of ammonium chloride in cough syrups to increase and liquefy 
the secretions is purely empirical. A number of drugs depress 
the respiratory center, such as the opium series. For this rea- 
son they are used extensively in cough syrups. Any drug which FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
127 
decreases the supply of oxygen in the blood will likewise stim- 
ulate the respiratory rhythm. Carbon monoxide gas belongs to 
this group. 
Caffein acts as an excitant on the respiratory center, and is 
often of use in cases where the respiration is depressed; for 
instance, in poisoning by opium, alcohol and other narcotics. 
The Voice. 
The voice-producing mechanism in man consists of the trachea, 
through which the air is blown from the lungs; the larynx, the 
modified upper portion of the trachea, which contains the vocal 
cords; and the pharynx, and upper air passages. The larynx 
forms the entrance into the trachea. It is composed of a number 
of cartilaginous plates which are united in a manner to form a 
box. Stretched from front to back on each side across the upper 
portion of the larynx are thin sharp-edged membranes, the vocal 
cords. The attachments of the muscles to the cartilages and the 
articulations of the several cartilages with each other, are so ar- 
ranged as either to tighten or loosen the tension, or increase or 
decrease the opening between the edges of the cords. The cleft 
between the cords is called the glottis. The length of the vocal 
cords varies from 11 to 15 mm., being longer in men than in 
women and children. Branches of the vagus and the spinal ac- 
cessory nerves supply the muscles of the larynx with motor 
nerves. The sensory nerves, arising in the epithelium of the 
larynx, are also branches of the vagus. Mechanical stimulation 
of the mucous membrane of the larynx or electrical stimulation 
of the superior laryngeal nerve will cause a cough or a forced 
expiratory movement. 
The Changes Which Occur in the Position of the Vocal Cords 
during the production of certain sounds may be studied by the 
use of the laryngoscope, the principle of which is shown in Fig. 
32. The view obtained from such an instrument is shown in 
Figs. 33 and 34. The base of the tongue appears at the top; be- 
low this is the edge of the epiglottis, the flap of tissue guarding 
the entrance to the larynx; and below in the middle line are seen 128 
THE VOICE. 
the true vocal cords as white shining membranes. Just above 
these, on either side, are two pink flaps of tissue, the false vocal 
cords. These secrete a fluid which moistens the true cords. 
\ Tongue 
\ depressor 
The Production of the Voice.-If the vocal cords be put in 
a state of tension and the aperture between them be narrowed, 
causing them to offer a resistance to the passage of air issuing 
Fig. 34.-Position of open glottis. 
1, tongue; e, epiglottis; ae, ary-epi- 
glottidean fold ; c, cartilage of Wris- 
berg ; ar, arytenoid cartilage; o, 
glottis; v, ventricle of Morgagni; 
ti, true vocal cord; ts, false vocal 
cord. (From Stewart's Physiology.) 
Fig. 33.-Position of the glottis 
preliminary to the utterance of 
sound, rs, true vocal cord; ar, ary- 
tenoid cartilage; b, pad of the epi- 
glottis. (From Stewart's Physi- 
ology. ) 
from the lungs, they may be made to vibrate and to produce 
sounds. It has been experimentally determined that a pressure 
of expired air of from 140 to 240 mm. of water is required to FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
129 
produce a sound of the ordinary pitch and loudness, while in 
loud shouting much greater pressures are necessary. 
The sound of the voice, like any other sound, may vary in 
pitch, loudness and quality. The range of pitch of the voice is 
generally about two octaves, the pitch itself being determined 
primarily by the lengths of the cords. This accounts for the 
high-pitched voice of children, in whom the cords are short, and 
the low pitch of the voice in men, in whom they are long. In 
singing, three registers can be distinguished, the head, middle 
and chest registers. The deeper notes of the singer come from 
the chest register, and are produced by the vibrations of the en- 
tire cords, whereas in the upper registers only the inner edge of 
the cords vibrate. 
The intensity or loudness of a vocal sound depends upon the 
amplitude of the vibrations of the vocal cords, and this is pro- 
portional to the strength of the expiratory blast. The pitch of 
a note rises and falls somewhat with the intensity of the pres- 
sure of the air, and for this reason high notes are usually loud 
notes. The quality of the voice, like that of a musical instru- 
ment, depends on the overtones, or harmonics, that it produces. 
For example, when a stretched string is made to vibrate, it not 
only vibrates as a whole, but portions of it vibrate independent- 
ly and give off separate tones which are known as overtones. 
Since the tone which the string produces by the vibration of its 
entire length is the loudest and lowest in pitch, it is picked out 
as the fundamental tone. The fundamental tones of instru- 
ments may be exactly the same, but the tones yet differ from one 
another because of the number and the intensity of the over- 
tones. 
Speech. 
The pure, musical tones produced by the vocal cords are modi- 
fied by changes in the character of the air passages above them. 
The various combinations which are produced give rise to sounds 
which make up speech. Many of the simple combinations are 
found in all languages, but every language is characterized by 
certain sounds which are peculiar to it. 130 
SPEECH. 
The sounds produced in speech may be divided into two 
groups, the vowels and the consonants. The vowel sounds are 
continuous and are formed in the lower air passages with the 
Fig. 35.-The position of the tongue and lips during the utterance of the 
letters indicated. 
help of the glottis. The consonants are produced by more or 
less complete interruptions of the outflowing air in different 
portions of the vocal tract. 
All the vowels can be produced in the whispered voice, that is, FUNDAMENTALS OF HUMAN PHYSIOLOGY 
131 
they can be produced without the actual vibration of the vocal 
cords. The mouth cavity, however, assumes the same position in 
the case of the whispered vowel as it does for the spoken vowel. 
By changing the shape of the air passages, the various vowel 
tones are produced. In Fig. 35 are seen the various positions of 
the tongue and palate for the production of the different vowels. 
When vowels are being uttered, the soft palate closes the en- 
trance to the nasal cavity. 
The consonants are named according to the position at which 
the interruption of the air current takes place. The labials are 
formed at the lips: p, b; the dentals, between the' tongue and the 
teeth: t, d. The gutterals, k, g, ch, arise between the posterior 
portion of the arched tongue and the soft palate; and the Ger- 
man r is produced with the help of vibrations of the uvula. 
Sounds like m, n, ng, are termed nasal consonants, since they 
are sounded through the nasal cavity (see Fig. 35). CHAPTER XI. 
ANIMAL HEAT AND FEVER. 
In considering the problem of animal heat, it is essential to 
bear clearly in mind the distinction between amount and inten- 
sity of heat. The former is measured in calories (see p. 185), 
and the latter in degrees of temperature. To measure the tem- 
perature of a man a maximal thermometer with the Fahrenheit 
or Centigrade scale is placed in some protected part of the body, 
as the mouth, the axilla or the rectum. It is found by such meas- 
urement that the temperature varies according to the site of ob- 
servation and the time of day. It varies between 36.0° C. 
(96.8° F.) and 37.8° C. (100.0° F.) in the rectum; between 36.3° 
C. (97.3° F.) and 37.5° C. (99.5° F.) in the axilla; and between 
36.° C. (96.8° F.) and 37.25° C. (99.3° F.) in the mouth. These 
variations indicate that the temperature is higher in the deeper 
than in the superficial parts of the body; in other words, that 
the visceral blood is warmer than that of the surface of the body. 
The variations of temperature due to the time of day are most 
evident when it is taken in the rectum, and they amount in health 
to a little over 1° C. or a little below 2° F., the highest tempera- 
ture occurring about 3 p. m., and the lowest about 3 a. m. This 
is called the diurnal variation and it may become much greater 
in febrile diseases. 
Animals whose temperature behaves as above described are 
called warm-blooded in contrast to those animals, called cold- 
blooded, in whom it is only a degree or two above that of the 
air, with which it runs parallel. Such animals include fishes, 
amphibians, snakes, etc. Between the cold and the warm-blooded 
animals is a group in which the animal is warm-blooded in sum- 
mer and cold-blooded in winter. These are the hibernating ani- 
mals, such as the hedgehog, the marmot, the bat, etc. In this 
connection it is interesting to note that the human infant be- 
132 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
133 
haves more or less like a cold-blooded animal for some time im- 
mediately following birth, during which period it must there- 
fore be carefully protected from cooling, for, if its temperature 
be allowed to fall to any considerable extent, it is not likely to 
survive. It takes several months before the heat regulating 
mechanism becomes so developed that the infant can withstand 
any considerable degree of cold. 
Factors Concerned in Maintaining the Body Temperature.- 
The' body temperature is a balance between heat production and 
heat loss. Heat is produced by combustion of the organic food- 
stuffs in the muscles, the amount which each foodstuff thus pro- 
duces being the same as when it is burned outside the body, 
except in the case of protein, when allowance must be made for 
the incomplete combustion of this substance in the animal body 
(see p. 187). The muscles are therefore the furnaces of the ani- 
mal body, the fuel being the organic foodstuffs. Heat is lost 
from the body mainly from the skin, but partly also from the 
lungs and in excreta. Heat loss from the skin is brought about 
by the utilization of several physical processes, namely: (1) by 
conduction along objects which are in contact with the skin or 
through the air; (2) by convection, that is, by being carried 
away in currents of air which move about the body; (3) by radi- 
ation; (4) by evaporation of sweat. This last is the means by 
which most heat can be lost, because it takes a large amount of 
latent heat to vaporize the sweat (see p. 34). 
Heat loss from the lungs is mainly due to vaporization of 
water, with which the expired air is saturated. A small amount 
is also absorbed in warming the air itself. The heat lost in the 
urine and faeces is almost negligible. 
The Regulation of the Body Temperature.-It is plain that 
a very sensitive regulatory mechanism must exist in order that 
the production and loss of heat may be so adjusted as to keep the 
body temperature practically constant. When heat loss becomes 
excessive, then must heat production be increased to maintain the 
balance, and vice versa when heat loss is slight. The conditions 
are to a certain extent comparable with those obtaining in a 
house heated by a furnace and radiators and provided with a 134 
ANIMAL HEAT AND FEVER. 
thermo-regulator, which, being activated by the temperature of 
the rooms, acts on the furnace so as to raise or lower its rate of 
combustion. . 
In the animal body the thermo-regulator is the nervous sys- 
tem. Whenever the temperature of the blood changes from the 
normal, a nerve center called the thermogenic becomes acted on 
with the result that it transmits impulses to the muscles, which, 
by increasing or diminishing their tone (see p. 265), cause a 
greater or a less heat-production. But the center does more than 
the thermo-regulator of a house, for it controls the agencies of 
heat-loss. Thus, when the blood temperature tends to rise, the 
thermogenic center causes more heat to be lost from the skin and 
lungs in the following ways: (1) It acts on the blood vessels of 
the skin, causing them to dilate so that more blood is brought to 
the surface of the body to be cooled off. (2) It excites the sweat 
glands, so that more heat has to be utilized to evaporate the sweat. 
(3) It quickens the respirations, so that more air has to be 
warmed and saturated with moisture. The degree to which these 
cooling processes are used varies in different animals. Thus in 
the dog, since there are no sweat glands over the surface of the 
body (they are confined to the pads of the paws), increase in the 
respiration is the chief method of cooling, hence the panting 
on warm days. 
In the case of man, civilization has stepped in to assist the 
reflex control of heat loss, as by the choice of clothing and the 
artificial heating of rooms. Desirable though this voluntary 
control of heat-loss from the body may be, there can be little 
doubt that it is often overdone to the detriment of good health. 
Living in overheated rooms during the cooler months of the 
year suppresses to a very low degree the heat loss from the body 
and thereby lowers the tone and heat production of the muscular 
system. The food is thereby incompletely metabolized and is 
stored away as fat; the superficial capillaries are constricted and 
the skin becomes bloodless. But it is not looks alone that suffer, 
but health as well, for, by having so little to do, the heat-regulat- 
ing mechanism gets out of gear so that when it is required to 
act, as when the person goes outside, it may not do so promptly FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
135 
enough, with the result that the body temperature falls some- 
what, and catarrhs, etc., are the result. There can be little doubt 
that much of the benefit of open-air sleeping is due to the con- 
stant stimulation of the metabolic processes which it causes. 
The importance of the evaporation of sweat in bringing about 
loss of heat in man partly explains why climate should have so 
important an influence on his well-being. It is not so much the 
temperature of the air, as its relative humidity, that is of impor- 
tance ; that is, the degree, expressed in percentage, to which the 
air is saturated with moisture at the temperature of observation. 
Thus, a relative humidity of 75 per cent at 15° C. means that 
the air contains 75 per cent of the total amount of moisture which 
it would contain if it were saturated with moisture at a tempera- 
ture of 15° C. A high relative humidity at a high temperature 
makes it impossible for much sweat to be evaporated, with the 
result that the body cannot cool properly, and the body tempera- 
ture is likely to rise unless muscular activity be reduced to a 
minimum. This explains why it is impossible to do much muscu- 
lar work in hot humid atmospheres. On the other hand, if the 
relative humidity is low, the temperature may rise to an extraor- 
dinary degree (even above that of the body itself) without caus- 
ing fever, provided always that the body is not so covered with 
clothing that evaporation of sweat is impossible. 
At low temperatures of the air, relative humidity has an effect 
which is exactly opposite to that which it has at high tempera- 
tures, for now it affects, not the evaporation of sweat, but the 
heat conductivity of the air itself. Cold moist air conducts 
away heat much more rapidly than cold dry air. Hence, a tem- 
perature many degrees below zero on the dry plains of the West 
may be much more tolerable to man than a much higher tem- 
perature along the shores of the Great Lakes. 
Fever.-Any rise of temperature above the normal limits 
constitutes fever. When of slight degree, as it is in many semi- 
acute diseases, its detection demands frequent observation, so 
as to allow for the normal diurnal variation of the body tempera- 
ture. For example, if the temperature were recorded in the 
morning in such a patient, a slight degree of fever might quite 136 
ANIMAL HEAT AND FEVER. 
easily be missed, because at this time the normal temperature is 
low. In acute infectious diseases, the afternoon temperature 
may rise to 106° F. oi; 41° C., or even above this, without prov- 
ing fatal. A temperature of 113° F. or 45° C. has been observed, 
but lasting for only a short time. Fever is always higher in in- 
fants and young children than in adults. 
As to the causes of fever, two possibilities exist: either (1) that 
heat production has been increased, or (2) that heat loss has 
been diminished, or, of course, both factors may operate simul- 
taneously. To go into this unsolved problem is unnecessary here; 
suffice it to say that there can be no doubt that disturbance in 
the thermogenic center is the underlying cause of fever, and 
that it is the avenues of heat loss by the skin rather than the 
sources of heat supply in the muscles that are first of all acted 
on. The cold sensation down the back, the shivering, the goose 
skin, are the familiar initial symptoms of fever, and when the 
fever comes to an end, excessive sweating sets in and this, in part 
at least, explains the fall in temperature. Increased combustion 
in the muscles no doubt occurs during the height of the fever 
and accounts for the great wasting, but that this is not the only 
cause of the rise in temperature is evidenced by the fact that 
severe muscular exercise does not in itself cause fever, even 
although there may be much more combustion going on in the 
body (see p. 191). 
Certain drugs called antipyretics lower the temperature in 
fever. The most important of these are acetanilide, salicylates 
(aspirin), phenacetin, and quinine. The first three mentioned 
act on the thermogenic center, whereas quinine seems to act 
directly on the combustion processes in the muscles. The body 
temperature is raised by cocaine and by the toxic products of 
bacterial growth. Even cultures which have been attenuated by 
keeping them for some time at high temperatures have this 
effect, and it is believed by many that fever is of the nature of 
a protective mechanism to destroy or attenuate the invading 
bacteria. There is bacteriological as well as clinical support for 
this view, thus, certain pathogenic organisms (such as the strep- 
tococcus of erysipelas) cannot live at a temperature above 41° C., 137 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
and cholera patients are much more likely to survive if the dis- 
ease be accompanied by a moderate degree of fever. 
Heat stroke, or sun stroke, is due to an increase in body tem- 
perature that is above the limits of safety. When sweating and 
the other processes by which heat is lost from the body are act- 
ing properly it is remarkable how high an air temperature may 
be borne without danger; for example, in dry air a man can sit 
for some minutes in an oven at 100°C. while his dinner cooks 
beside him (Leonard Hill). But if anything should interfere 
with heat loss, or if heat .production be excessive, as during mus- 
cular exercise, there is always danger of heat stroke. Free move- 
ment of the air is probably the most important way for safe- 
guarding against deficient heat loss. It is almost certainly on 
account of the absence of such air movement, coupled with a 
high relative humidity, that discomfort ' is experienced in hot, 
stuffy atmospheres, for the faulty heat loss causes a slight rise 
in body temperature. This slight degree of hyperpyrexia low- 
ers the resistance of the organism to infection. CHAPTER XII. 
DIGESTION. 
Necessity and General Nature of Digestion: The Alimentary 
Canal. 
The never-ceasing process of combustion that goes on in the 
animal body, as well as the constant wear and tear of the tissues, 
makes it necessary that the supply of fuel and of building mate- 
rial be frequently renewed. For this purpose food is taken. This 
food is composed of fats and carbohydrates, which are mainly 
fuel materials, of inorganic salts and water, which are neces- 
sary to repair the worn tissues and of proteins which are both 
fuel and repair materials, and are therefore the most important 
of the organic foodstuffs. The blood transports the foodstuffs 
from the digestive canal to the tissues. In the digestive canal the 
foodstuffs are digested by hydrolyzing enzymes (see p. 46), 
which are furnished partly in the secretions of the digestive 
glands and partly from the numerous micro-organisms that 
swarm in the intestinal contents. The enzymes, as we have seen, 
are very discriminative in their action, for not only is the enzyme 
for protein without action on a fat or carbohydrate, but each of 
the different stages in protein break-down requires its own pe- 
culiar enzyme. It becomes necessary therefore that the enzymes 
be mixed with the food in proper sequence, and to render this 
possible the digestive canal is found to be divided into special 
compartments, such as the mouth, the stomach, the small intes- 
tines, etc., each provided with its own assortment of enzymes 
and with some mechanism by which the food, when it has been 
sufficiently digested, can be passed on to the next stage. 
Such correlation between the different stages of digestion 
necessitates the existence, in the different levels of the gastro- 
intestinal tract, of mechanisms which are specially developed to 
138 139 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
bring about the right secretion at the right time. These mech- 
anisms are of two essentially different types, a nervous reflex 
control, and a chemical or "hormone" control. The nervous con- 
trol is exercised through a nerve center, which is called into ac- 
tivity by afferent stimuli proceeding from sensory nerve endings 
or receptors (see p. 256) that are especially sensitized so as to 
be stimulated by some property of food (its taste or smell, or 
its consistency or chemical nature). This type of control 
exists where prompt response of the glandular secretion is impor- 
tant, as in the mouth and in the early stages of digestion in the 
stomach. The hormone control consists in the action directly on 
the gland cells of substances which have been absorbed into the 
blood from the mucous membrane of the gastro-intestinal tract. 
The production of these substances depends upon the nature of 
the contents of the digestive tube. This is a more sluggish proc- 
ess of control than the nervous, but it is sufficient for the cor- 
relation of most of the digestive functions. 
These considerations point the way to the scheme which we 
must adopt in studying the process of digestion; we must explain 
how each digestive juice comes to be secreted, what action it has 
on the foodstuffs, and what it is, after each stage in digestion is 
completed, that controls the movement onward of the food to 
the next stage. And when we have followed each foodstuff to 
its last stage in digestion, we may then proceed to study the 
means by which the digested foodstuffs are absorbed into the cir- 
culating fluids, and in what form they are carried to the tissues. 
On account of the varying nature of their food we find that 
the digestive system differs considerably in different groups of 
animals. In the omnivora, such as man, the digestive canal be- 
gins with the mouth cavity, in which the food is broken up me- 
chanically and is mixed with the saliva in sufficient amount to 
render it capable of being swallowed. The saliva, by containing 
starch-splitting ferment, also initiates the digestive process. The 
food is then carried by way of the oesophagus to the stomach, in 
the near or cardiac end of which it collects and becomes 
gradually permeated by the acid gastric juice. It is then caught 
up, portion by portion, by the peristaltic waves of the 140 
THE ALIMENTARY CANAL. 
further or pyloric end of the stomach and, after being thor- 
oughly broken down by this movement and partially digested 
by the pepsin of gastric juice, is passed on in portions into 
the duodenum, where it meets with the secretions of the pancreas 
and liver. These secretions, acting along with auxiliary juices 
secreted by the intestine itself, ultimately bring most of it into a 
state suitable for absorption. What the digestive juices leave 
unacted on bacteria attack, especially in the caecum, so that by 
the time the food has gained the large intestine it has been di- 
gested as far as it can be. In its further slow movement along 
the large intestine the process of absorption of water proceeds 
rapidly. 
Disturbances in the digestive process may be due not only to 
possible inadequacy in the secretion of one or other of the diges- 
tive juices, but also to disturbances in the movements of the 
digestive canal. Such disturbances will not only prevent the 
forward movement of the food at the proper time, but, by failing 
to agitate the food, they will prevent its thorough admixture 
with the digestive juices, so that the enzymes which these con- 
tain will not become properly mixed with the food. 
The Alimentary Canal. 
Anatomical Considerations.-The alimentary canal or tract 
may be considered as an involuted portion of the skin, whose 
function is to prepare and to absorb material for the nourish- 
ment of the body. The canal forms a tube which communicates 
with the exterior at the mouth and the anal opening. It is 
lined throughout with mucous membrane composed of epithelial 
tissue overlying a submucous coat of loose connective tissue. 
The outer coats are made up of fibrous connective tissue and 
circular and longitudinal layers of smooth muscles. Imbedded 
in the coats are many blood vessels, lymphatics and nerves. 
Numerous glands which pour their digestive juices directly into 
the canal are also found in the walls of the canal; good exam- 
ples of these are the gastric or stomach glands. Other glands, 
too large to be imbedded in the walls, are connected with the 
canal by means of ducts, through which the glandular secre- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
141 
tions nnd their way. The salivary glands, the pancreas and the 
liver are examples of this type. 
The total length of the canal is about 28 feet; the great length 
being possible since the tube is greatly coiled in the abdominal 
cavity. The canal is functionally and structurally divided into 
MOUTH 
JALIVARY 
GLANDS 
'PHARYNX 
LIVER 
-GULLET 
STOMACH 
-PANCREAS 
LARGE 
INTESTINE 
SMALL/ 
INTESTINE 
Fig. 36.-Diagram of rhe alimentary tube and its appendages. (After 
Testut.) 
ANUS 
several organs, viz.: the mouth, oesophagus, stomach, small and 
large intestines, and rectum. The mouth contains the teeth and 
the tongue. The mouth posteriorly opens into the pharynx, 
which also communicates above with the nasal passages. The 
pharynx, below the level of the mouth terminates at the opening 142 
THE ALIMENTARY CANAL. 
of two tubes, the respiratory opening or larynx anteriorly, and 
the oesophageal, posteriorly. The oesophagus passes through the 
thorax, penetrates the diaphragm, and then terminates in a di- 
lated sac,- the stomach. The stomach at its lower pole or pyloric 
region narrows and is continued as a greatly coiled and long 
tube, the small intestines, which in turn leads into a larger and 
shorter tube, the large intestines. This finally terminates in the 
rectum or lower bowel and emerges to the exterior through the 
anal orifice. 
The Blood Supply of the Alimentary Canal.-The stomach 
and the intestines are attached to the posterior body wall by 
means of a sheet of tissue called a mesentary, in which are found 
the blood vessels, lymphatics and nerves which supply the ali- 
mentary canal. The arterial blood supply is derived from ves- 
sels coming directly from the descending aorta. The branches 
of these finally end in a capillary network in the walls of the 
intestinal canal. The venous blood which emerges from this 
capillary network is collected by the veins of the mesentary 
which lead to the large vessel going to the liver, the portal vein. 
This vein, on entering the liver, breaks up into a capillary 
system which is continued into the liver veins; these empty into 
the vena-cava at the level of the diaphragm. The liver also re- 
ceives arterial blood through the hepatic artery, which is a 
branch of the aorta. 
The vagus (the tenth cranial nerve) which courses through 
the neck and the thorax finally ends in the abdomen, and along 
with the sympathetic nerve fibers, contained in the splanchnic 
nerves, innervates the intestinal canal and accessory glands. 
The Mouth.-The cavity of the mouth is bounded in front 
and on the sides by the lips, and the cheeks, above by the pal- 
ate, and below by tissues of the lower jaw. The cavity contains 
the tongue and teeth. The tongue is a thick muscular organ 
covered with mucous membrane which is endowed with both 
tactile and taste sensibility. Its function is to mix and roll the 
food between the teeth, and to aid in the production of speech. 
In health it is moist and of a red color. In disease it may be 
covered with a thick fur caused by profuse bacterial growth. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
143 
The Teeth are found implanted on the borders of the upper 
and lower jaw bones. The bones are covered with a tissue, 
known as the gum, which encircles the lower portion of each 
-Enamel. 
-Pulp cavity. 
-Dentin. 
- Cementum. 
Fig. 37.-Scheme of a longitudinal section through a human tooth. In 
the enamel are seen the "lines of Retzius." (Hill's Histology, after Bohm 
and Davidoff.) 
tooth. Two sets of teeth are developed during life. The first 
set is the milk or baby teeth. These develop shortly after the 144 
THE ALIMENTARY CANAL. 
/ 
eighth month usually and are lost during childhood, when the 
second set of teeth or the permanent ones appear. The teeth 
differ from each other in form according to their use. The 
front teeth, or the incisors, are sharp for the purpose of cutting 
and tearing. The back teeth have large bases or crowns for the 
purpose of grinding and crushing the food. 
A tooth consists of three parts-the crown, or the exposed 
portion; the neck, a narrow constriction at the edge of the gum, 
and the root or roots, by which the tooth is fixed to the jaw bone. 
The tooth itself is composed of a hard outer covering surround- 
ing .a central pulp cavity, which contains a blood vessel and a 
nerve. The outer covering consists of a very firm, hard sub- 
stance of fine texture, the enamel. This is the protective cover- 
Parietal cell. 
Acinus. 
Parietal cell. 
Intralobular duct. 
Interlobular duct. 
Fig. 38.-Section from the human submaxillary gland. (Hill's Histology.) 
ing of the tooth. Beneath this is dentine, a much softer and 
less resistant material than the enamel. When the enamel is 
broken the dentine soon suffers. The minerals composing the 
teeth are the carbonates and the phosphates of calcium. 
The ducts of the salivary glands open into the mouth cavity. 
The saliva is secreted by three pairs of glands, the parotid, sub- 
maxillary, and sublingual. The parotid glands lie just in front 
' of the ears and behind the ramus of the lower jaw. It is these 
glands which are usually infected in the disease called mumps. 
The submaxillary and the sublingual glands lie beneath the 
tongue in the tissues of the floor of the mouth. The structure 
of the salivary glands is fundamentally the same as that of most FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
145 
secreting glands. Fig. 38 shows a cross section of the submax- 
illary gland. Notice that the secreting cells surround a central 
tube, which finally joins a larger one and this in turn unites to 
form the excretory duct of the gland. 
The Pharynx.-The mouth cavity leads posteriorly into the 
pharynx. On either side of the opening, on what is termed the 
fauces of the pharynx, are found masses of lymphoid or glan- 
dular tissue, the tonsils. These are often the seat of infections, 
and together with the adenoids, which are similar tissue found 
on the posterior wall of the pharynx, may be hypertrophied to 
such an extent as to hinder breathing. This condition demands 
the removal of the tissue. 
The lower end of the pharynx terminates in two tubes; the 
anterior one leads to the lungs and the posterior is the oeso- 
Cardiac Orifice 
(Esophagus 
Fundus 
Small Curvature 
Hepatic Duct 
Cystic Duct. 
Great Curvature 
Pylorus 
Ductus Communis 
Choledochus 
Duct of Wirsung 
Duodenum 
Fig. 39.-The stomach and duodenum opened. (Buchanan's Anatomy.) 
phagus which, passing through the neck and chest, ends below 
the diaphragm in the stomach. The inner or mucous mem- 
brane of the oesophagus is lined with epithelial tissue which con- 
tains many small glands. The outer coat of the tube contains 
muscular and fibrous connective tissue. 
The Stomach.-The stomach is an expanded sac-like portion 146 
THE ALIMENTARY CANAL. 
of the alimentary canal. The end joining the oesophagus is 
known as the cardiac portion, and that joining the small intes- 
tines, the pyloric portion of the stomach. The entire inner 
surface of the stomach is lined with a thick mucous membrane, 
which is crowded with the opening of glands, the secretion of 
which constitutes the gastric juice. These glands are simple 
tubular structures which are closely packed side by side in the 
Fig. 40.-The mucosa of the stomach. (Gray's Anatomy.) 
mucous membrane. The glands at the cardiac portion differ 
a little from those in the pyloric region. It is generally thought 
that the cardiac glands are responsible for the secretions of the 
hydrochloric acid and the pyloric glands for the pepsin which 
is found in the gastric juice. 
The muscular coats of the stomach number three. The inner FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
147 
coat is made up of strips of muscle which run more or less ob- 
liquely; the middle coat consists of a sheet of circular muscle 
evenly distributed over the whole stomach save at the pyloric 
end. This coat is much better developed and stronger. The 
outer layer of muscle runs longitudinally. Outside the muscle 
coats is a firm connective tissue coat. In it are found the larger 
blood vessels and nerves and lymphatics which supply the struc- 
ture of the stomach. 
As mentioned above, the circular coat of muscle is highly de- 
veloped in the pyloric region of the stomach. It forms the 
pyloric sphincter between the stomach and the beginning of the 
intestines. Contraction of this sphincter prevents any material 
from leaving the stomach. 
The Small Intestines-a much coiled tube about twenty 
feet in length, begins at the pyloric end of the stomach and ends 
in the large intestines. The coats of the intestines resemble those 
of the stomach in structure. The outer surface is made of tough 
fibrous connective tissue. Below this are the two layers of mus- 
cle tissue,' an outer longitudinal and an inner circular one. In- 
side the muscle coats is a layer of loose connective tissue called 
the submucous coat. In it are found numerous blood and lym- 
phatic vessels. The mucous membrane or inner coat of the 
intestines is pink, soft and very vascular. The submucous and 
mucous coats appear to be too large to fit the intentine and 
are therefore rolled up into ridges in the forms of ereseentric 
folds which are called valvula conniventes. These folds serve 
to retard the passage of food, thus favoring its more thorough 
digestion, and they also increase the absorbing surface of the 
canal. Histological examination also shows that there are still 
more minute foldings in the membrane, for the epithelial surface 
is not smooth, but is everywhere thrown up into minute pro- 
cesses, resembling the pile of velvet, which are called the villi. 
The outer surfaces of these are covered with columnar epithe- 
lium, but the underlying submucous coat forms delicate pro- 
cesses, in which are seen a network of blood vessels and a cen- 
tral lymphatic vessel. 
The entrance of the small intestine into the large is guarded 148 
THE ALIMENTARY CANAL. 
by a structure called the ileo-colic valve. Its presence delays 
the passage of food from small to large intestine and prevents 
the backward passage of food into the small intestine. 
The Large Intestine forms the terminal portion of the ali- 
mentary canal. It is about five feet long and varies in diameter 
from two and a half to one and a half inches. It lies like an 
~ Villus. 
Mucosa. 
Crypt of 
- Lieber- 
kuhn. 
M uscularis 
mucosa. 
. Glands of 
I Brunner 
[ in the sub- 
mucosa. 
Circular 
- muscle 
layer. 
Longitudi- 
nal muscle 
layer. 
Fig. 41.-Longitudinal section of duodenum near pyloric end, showing 
gland of Brunner and mucosa of the intestines. (Hill's Histology.) 
- Serous coat. 
inverted "U" in the body and thus forms an ascending, a trans- 
verse and descending portion. Just below the entrance of the 
small intestine is a somewhat dilated portion called the caecum, 
from which arises the vermiform appendix. This becomes in- 
flamed in appendicitis. The coats of the large intestine resem- 
ble those of the small intestine. The outer longitudinal layer FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
149 
of muscle is not as well developed, nor is the mucous membrane 
thrown into ridges as it is in the case of the small intestine. The 
lower portion of the large intestine, which serves for the storage 
of fecal material, is called the rectum. 
The Liver and the Pancreas.-Besides the glands which lie 
in the coats of the alimentary canal, there exist two large glands 
which empty into the small intestine through ducts. These are 
the liver and the pancreas. The liver is the largest gland of the 
Fig. 42.-The microscopic structure of the liver. (Highly magnified.) 
A. Lobule, showing the intralobular plexus; B, Lobule showing the hepatic 
cells. (Buchanan's Anatomy.) 
body, and is situated in the upper right hand portion of the ab- 
dominal cavity (see Fig. 55). Its structure and function are 
very complex. It secretes a fluid, the bile, which finds its way 
into the intestine through the bile duct. During digestion the 
bile is poured directly from the liver into the intestine. In the 
intervals the bile is stored up in the bile or gall bladder. Fig. 
42 gives the gross and microscopical structure of the liver. 
The pancreas is a tubular gland resembling the salivary glands 
in structure. Its duct empties, in company with the bile duct, 
into the intestine. CHAPTER XIII. 
DIGESTION (Cont'd). 
Digestion in the Mouth. 
Salivary Secretion.-In the mouth, besides its preparation 
for swallowing, by mastication, etc., the food, mainly on account 
of its taste and smell, stimulates sensory nerve endings which, 
by acting on nerve centers, set agoing several of the digestive 
secretions. The first of these is the secretion of the salivary 
glands. On account of their ready accessibility to experimental 
investigation, very extended studies have been made of the sali- 
vary glands, and from these studies some of the most important 
physiological truths, concerning the nature of the nervous con- 
trol of glands in general, have been drawn. Of the three salivary 
Fig. 43.-Cells of parotid gland showing zymogen granules: A, after pro 
longed rest; B, after a moderate secretion; C, after prolonged secretion 
(Langley.) 
glands in man, the parotid secretes a watery saliva usually con- 
taining the enzyme, ptyalin, and the submaxillary and subling- 
ual secrete a sticky saliva containing mucin, usually along with 
some ptyalin. When the glands are not secreting, the cells that 
compose them are engaged in preparing material to be secreted. 
By microscopical examination, this material is seen in the pi'OtO- 
150 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
151 
plasm of the cells (Fig. 43) as granules, which are extremely 
small in the serous gland cells, but much larger in the mucous. 
In both types of gland the granules so crowd the cell that the 
nucleus becomes indistinct and the cell itself much swollen. 
After the gland has been active, the granules disappear, being 
evidently discharged from the cell into the duct of the gland. 
The granules are believed to represent the precursors of the 
ptyalin or mucin of saliva-hence their name of "zymogen" or 
"mother of ferment" granules-rather than these substances 
themselves. Watery or saline extracts of the glands contain 
neither mucin nor ptyalin, nor does the addition of acetic acid to 
a mucous gland cause any precipitate of mucin; indeed, it has 
an entirely opposite action, it causes the granules to swell. 
Fig. 44.-The nerve supply of the submaxillary gland: Lj, lingual nerve; 
c. t., chorda tympani; g. gland. Wharton's duct is ligated and it will be 
noticed that the chorda leaves the lingual nerve, just before this crosses the 
duct; thus forming the submaxillary triangle. (Claude Bernard.) 
The Nerve Supply of the Salivary Glands.-The salivary 
glands are supplied with two sets of nerve fibers. One of these 
arises directly from the brain and is carried in what is known 
as the parasympathetic or cerebral autonomic nerves. For ex- 
ample, the fifth cranial nerve contains fibers which go to the 
parotid gland, and the seventh nerve, fibers which supply the 
sublingual and submaxillary glands. Tbe other set of nerves 152 
SALIVARY SECRETION. 
is derived from the sympathetic system proper (see p. 282). 
These have their origin in the spinal cord and ascend through 
the neck to finally terminate about the cells of the salivary 
glands. In both sets of nerves there are two kinds of fibers; 
the vasomotor, which controls the size of the blood vessels and 
therefore the blood supply, and the second, which exercises 
a secretory influence on the gland. On account of the as- 
sociation of secretory and vasodilator fibers, in the cerebral 
nerves, stimulation leads to the secretion of large quantities of 
saliva, the amount of which, as well as its percentage of organic 
and* inorganic constituents, varies with certain limits with the 
strength of the stimulus. Although secretory activities also be- 
come excited when the sympathetic nerve is stimulated, as is 
revealed by histological examination of the gland, there is only 
a slight flow of saliva from the duct because of the concomitant 
curtailment of the blood supply. In so far as actual secretion of 
saliva is concerned, the net result of stimulation of either nerve 
is therefore dependent upon whether dilatation or constriction 
of the blood vessels of the gland occurs, and this might lead us 
to conclude that the secretion is secondary to changes in the 
blood supply; in other words, that it is unnecessary to assume 
the independent existence of specific secretory nerve impulses. 
That such secretory fibers do exist, however, is established by 
many facts. Two of these are: (1) The vessels still dilate but 
no secretion occurs after a certain amount of atropin has been 
allowed to act on the gland. This alkaloid paralyzes the secre- 
tory nerve fibers, but has no action on those concerned in vaso- 
dilation. (2) If the secretions were jnerely the result of in- 
creased blood supply, in other words, were a filtrate from the 
blood, the pressure in the duct would at all times be less than 
that in the blood vessels; but this is not the case, for during stim- 
ulation of the cerebral nerves the duct pressure may rise far 
above that of the blood vessels. 
But it must never be lost sight of that although both kinds of 
fibers do exist, they are very closely associated in their action. 
The Reflex Nervous Control of Salivary Secretion.-The 
structural differences between the parotid and submaxillary FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
153 
glands suggest that their functions may not be the same; that 
their respective secretions must be required for different pur- 
poses. To put this supposition to the test, it becomes necessary 
to adopt some means by which the conditions calling forth 
the secretion of each gland may be separately studied. This can 
be accomplished by a small surgical operation in which the ducts 
are transplanted so as to discharge through fistula? in the cheek, 
the secretion being easily collected, by allowing it to flow into a 
funnel which is tied in place. 
In general,two distinct types of stimuli may call forth secretion 
of one or other gland, namely: (1) direct stimulation of sensory 
nerve endings in the mouth, and (2) psychological stimuli in- 
volving more or less of an association of ideas. 
Of the stimuli which cause secretion by acting on sensory nerve 
endings in the mouth, some influence the parotid, others, the sub- 
maxillary gland, and different stimuli produce different effects. 
Even for pure mechanical stimulation of the buccal mucosa, a 
marked degree of discrimination is shown; thus, smooth clean 
pebbles may be rolled around in the mouth and yet cause no 
saliva to be secreted, whereas dry sand will immediately cause 
the parotid to discharge enormous quantities of thin watery 
juice. Similarly dry bread crumbs invoke copious parotid secre- 
tion, bread itself having little effect; water, ice, etc., are inert, 
but if they contain a trace of acid an abundant secretion is in- 
stantly poured out. It is plain in all these cases that the pur- 
pose of the secretion is to assist in the removal or neutralization 
of the substance which is present in the mouth. The thick 
mucous secretion of the submaxillary and sublingual glands 
seems to depend more on the chemical nature of the food than on 
its mechanical state, boiled potatoes, hard boiled eggs, meat, etc., 
causing the secretion of a thick slimy saliva, which by coating 
the food assists swallowing. The relish for the food seems to be 
of little account in influencing the secretion of saliva, for noxious 
substances, or those that are acid, or very salty, call forth much 
more secretion than do savory morsels. Although mere mechani- 
cal stimulation is not in itself an adequate stimulus, yet move- 
ment of the lower jaw is quite effective, as for example in chew- 154 
FUNCTION OF THE SALIVA. 
ing, or when the mouth is kept open, as by a gag in a dental 
operation. 
The stimulus does not, however, require to be applied to the 
buccal mucosa itself; it may be psychic or associational, and 
here again a remarkable discrimination is evident, although the 
response is not so predictable as when the stimulus is local. 
Thus, when dry bread or sand is shown to a dog to which previ- 
ously these substances have been given by mouth, salivation fol- 
lows, but this is not the case when moist bread or pebbles are 
offered. Appetite plays an important part in this psychic reflex, 
for when dry food is shown to a fasting animal, salivation is 
marked, but may cause no secretion when it is offered to a well- 
fed animal. It is possible in this case, however, that there may 
be inhibition of the glandular activities on account of the pres- 
ence of food products in the blood. Perhaps the most interesting 
fact of all is that even a fasting animal will after a time fail to 
salivate if he be repeatedly shown food which causes a secretion, 
but which he is not permitted to get. The response is immedi- 
ately established again, however, if some food, or indeed some 
other object, be placed in the mouth. A hungry animal will even 
salivate when he hears some sound which by previous experience 
he has learned to associate with feeding time. The psychic 
reflexes are evidently dependent upon an association of ideas (a 
nervous integration, see p. 254; they are conditioned reflexes, 
and are therefore the result of a certain degree of education. 
They are easily rendered ineffective by confusing the usual asso- 
ciations. 
General Functions of Saliva.-These observations indicate 
that a very important function of the saliva is what we may call 
a mechanical one, namely, either to flood the mouth cavity with 
fluid and so to wash away objectionable objects in it, or to lubri- 
cate the food with mucin and so facilitate swallowing. The sol- 
vent action of saliva is also important for the act of tasting (see 
p. 301). Its chemical activities in many animals seem to be lim- 
ited to the neutralizing properties of the alkali which is present 
in it, but in man and the herbivora it also contains a certain 
amount of a diastatic enzyme, ptyalin, which can quickly con- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
155 
vert cooked starches into dextrines and maltose. Even when this 
action is most pronounced, however-for it varies considerably 
in different individuals-it cannot proceed to any extent in the 
mouth cavity, partly on account of the short time food remains 
here, and partly because many starches, as in biscuits, are taken 
more or less in a raw state. In some animals, such as the dog, 
the saliva has no diastatic action whatever. Although there can 
therefore be little diastatic digestion in the mouth, a good deal 
may go on in the stomach, for the saliva that is swallowed along 
with the food does not become destroyed by the gastric juice 
until some thirty minutes after the food has gained the stomach. 
Although mastication of the food and its preparation for 
swallowing are undoubtedly the main functions of the mouth cav- 
ity, another exists which is of very great importance for proper 
digestion; this is the stimulation of the taste nerve endings, 
and, for foods with a flavor, of those of the olfactory nerve in the 
posterior nares. Such stimulation not only gratifies the appetite, 
but it serves as the adequate stimulus to set agoing the secretion 
of the gastric juice. Without any relish for food, digestion as 
a whole materially suffers, and for this reason unpalatable food 
is always more or less indigestible. 
Recent investigations point to another function of the saliva. 
Pepsin, a ferment which is important in the digestion of the 
proteins and which is found in the juice secreted by the glands 
of the stomach, is readily absorbed by starch when in the col- 
loidal state as it is generally eaten. In this condition the fer- 
ment is not free to act upon the proteins and digestion is de- 
layed. If the saliva be allowed to partially digest the starch 
into sugar before the food reaches the stomach, the colloidal state 
is changed by the action of the ptyalin of the saliva, and ab- 
sorption of the ferment does not occur. 
The Hygiene of the Mouth.-The relation of the saliva and 
mouth hygiene to dental disease and tooth decay, and to many 
infections such as chronic rheumatism and visceral disease, 
has been an important subject of research in recent years. It 
is certain that because of improper care, conditions arise in the 
mouth which lead to the decay of the teeth and to more serious 156 
interference to the health in general. The prevention of such 
processes depends on keeping the mouth clean. For this pur- 
pose we have the mechanical cleaning of the teeth with a brush, 
and the use of various dental powders and pastes. These act 
mostly in a mechanical manner. 
Numerous mouth washes are also prescribed on account of 
their cleansing, neutralizing and antiseptic properties. When 
the reaction of the saliva is acid to litmus an alkaline mouth 
wash is required, whereas in other conditions, an astringent 
acid wash is prescribed. 
Tartar Formation and Salivary Calculi.-Under certain con- 
ditions a precipitate, varying in color from pale yellow to almost 
black, collects on the teeth, particularly on the lower incisors 
and molars. This precipitate is called tartar, and it may be 
either hard (as on the incisors) or soft (as on the molars). Its 
chemical composition varies considerably, but may be given as 
follows: 
TARTAR FORMATION. 
I 
II 
Water and organic matter 
32.24 per cent 
31.48 per cent 
Magnesium phosphate 
0.98 per cent 
4.91 per cent 
Calcium phosphate 
63.08 per cent 
72.73 per cent 
Calcium carbonate 
3.7 per cent 
(Talbot) 
The organic matter consists of epithelial scales, other extran- 
eous matter and leptothrix chains. The place and manner of 
deposition shows clearly that the tartar is largely derived from 
the saliva, the chemical explanation of the precipitation being 
probably as follows: Saliva, as it is produced in the gland, con- 
tains calcium bicarbonate, which is soluble in water, and is pre- 
vented from changing into the insoluble carbonate by the pres- 
ence of free carbon dioxide in solution. When the saliva is dis- 
charged into the mouth some of the carbon dioxide escapes from 
it so that the bicarbonate changes to carbonate and becomes pre- 
cipitated. The precipitate carries down with it phosphates as well 
as any organic debris or mico-organisms that may be present. 
The precipitation of calcium carbonate may even take place 
in the salivary ducts (Wharton's), thus forming salivary calculi, FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
157 
which may reach the size of a pea or larger. Such calculi may 
contain as much as 3.8 per cent of organic matter, the remainder 
being largely calcium carbonate. The following table gives the 
composition of three such calculi: 
I 
II 
III 
Calcium carbonate 
81.2 per cent 
79.4 per cent 
80.7 per cent 
Calcium phosphate 
4.1 per cent 
5.0 per cent 
4.2 per cent 
Magnesium phosphate ... 
Organic matter and other 
present 
soluble solids 
13.3 per cent 
13.3 per cent 
13.4 per cent 
Water 
1.3 per cent 
2.3 per cent 
1.7 per cent 
(Talbot) 
Mastication.-By the movements of the lower jaw on the 
upper, the two rows of teeth come together so as to serve for bit- 
ing or crushing the food. The resulting comminution of the food 
forms the first step in digestion. The up and down motion 
of the lower jaw results in biting by the incisors, and after the 
mouthful has been taken, the side to side movements enable the 
grinding teeth to crush and break it up into fragments of the 
proper size for swallowing. The most suitable size of the mouth- 
ful is about five cubic centimeters, but this varies greatly with 
habit. After mastication, the mass weighs from 3.2 to 6.5 gram, 
about one-fourth of this weight being due to saliva. The food 
is now a semi-fluid mush containing particles which are usually 
less than 2 millimeters in diameter. Some, however, may measure 
7 and even 12 millimeters. 
Determination of the proper degree of fineness of the food is a 
function of the tongue, gums and cheeks, for which purpose the 
mucous membrane covering them is supplied with very sensitive 
touch nerve endings (see p. 256). The sensitiveness of the 
tongue, etc., in this regard explains why an object which can 
scarcely be felt by the fingers seems to be cpiite large in the 
mouth. If some particles of food that are too large for swallow- 
ing happen to be carried backward in the mouth, the tongue re- 
turns them for further mastication. 
The saliva assists in mastication in several ways: (1) by dis- 
solving some of the food constituents; (2) by partially digesting 158 
MASTICATION. 
some of the starch; (3) by softening the mass of food so that it 
is more readily crushed; (4) by covering the bolus with mucus 
so as to make it more readily transferable from place to place. 
The secretion of saliva is therefore stimulated by the chewing 
movements, and its composition varies according to the nature 
of the food (p. 152). In some.animals, such as the cat and dog, 
Fig. 45.-The changes which take place in the position of the root of the 
tongue, the soft palate, the epiglottis and the larynx during the second stage 
of swallowing. The thick dotted line indicates the position during swal- 
lowing. 
there is no mastication, the food being merely coated with sa- 
liva and then swallowed. In man the ability thus to bolt the 
food can readily be acquired, not however without some detri- 
ment to the efficiency of digestion as a whole. Soft starchy food 
is little chewed, the length of time required for the mastication 
of other foods depending mainly on their nature, but also to a 
certain degree on the appetite and the size of the mouthful. 
It cannot be too strongly insisted upon that the act of masti- 
cation is of far more importance than merely to break up and FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
159 
prepare the food for swallowing. It causes the food to be moved 
about in the mouth so as to develop its full effect on the taste 
buds; the crushing also releases odors which stimulate the ol- 
factory epithelium. On these stimuli depend the satisfaction 
and pleasure of eating, which in turn initiate the process of gas- 
tric digestion (see p. 163). Thus it has been observed in chil- 
dren with gastric fistulae that the chewing of agreeable food 
caused the gastric juice to be actively secreted, which, however, 
was not the case when tasteless material was chewed. 
Deglutition or Swallowing.-After being masticated the food 
is rolled up by the tongue, acting against the palate, into a bolus, 
and this, after being lubricated by saliva, is moved, by elevation 
of the front of the tongue, towards the back of the mouth. 
About this time a slight inspiratory contraction of the dia- 
phragm occurs-the so-called respiration of swallowing-and 
the mylohyoid muscle of the floor of the mouth quickly con- 
tracts with the consequence that the bolus passes between the 
pillars of the fauces into the pharynx. This marks the beginning 
of the second stage, the first event of which is that the bolus, by 
stimulating sensory nerve endings, acts on nerve centers situated 
in the medulla oblongata so as to cause a coordinated series of 
movements of the muscles of the pharynx and larynx and an in- 
hibition for a moment of the respiration (p. 121). The movements 
alter the shape of the pharynx and of the various openings into 
it in such a manner as to compel the bolus of food to pass into 
the oesophagus: (see Fig. 45) thus, (1) the soft palate becomes 
elevated and the posterior wall of the pharynx bulges forward 
so as to shut off the posterior nares, (2) the posterior pillars of 
the fauces approximate so as to shut off the mouth cavity, and 
(3) in about a tenth of a second after the mylohyoid has con- 
tracted, the larynx is pulled upwards and forwards under the 
root of the tongue, which by being drawn backwards becomes 
banked up over the laryngeal opening. This pulling up of the 
larynx brings the opening into it near to the lower half of the 
dorsal side of the epiglottis, but the upper half of this structure 
projects beyond and serves as a ledge to guide the bolus safely 
past this critical part of its course. (4) To further safeguard 160 
DEGLUTITION. 
any entry of food into the air passages, the laryngeal opening is 
narrowed by approximation of the true and false vocal cords. 
The force which propels the bolus, so far, is mainly the con- 
traction of the mylohyoid, assisted by the movements of the root 
of the tongue. When it has reached the lower end of the 
pharynx, however, the bolus readily falls into the oesophagus, 
which has become dilated on account of a reflex inhibition of the 
constrictor muscles of its upper end. This so-called second stage 
of swallowing is therefore a complex coordinated movement ini- 
tiated by afferent stimuli and involving reciprocal action of 
various groups of muscles: inhibition of the respiratory muscles 
and of those that constrict the oesophagus, and stimulation of 
those that elevate the palate, the root of the tongue and the 
larynx. It is purely an involuntary process. 
The last stage of .deglutition consists in the passage of the 
swallowed food along the oesophagus. The way in which this is 
done depends very much on the physical consistence of the food. 
A solid bolus, that more or less fills the oesophagus, excites a 
typical peristaltic wave, which is characterized by a dilatation 
of the oesophagus immediately in front of, and a constriction 
over and behind the bolus. This wave travels down the oesopha- 
gus at such a rate that it reaches the cardiac sphincter in about 
five or six seconds. On arriving here the cardiac sphincter, 
ordinarily contracted, relaxes for a moment so that the bolus 
passes into the stomach. The peristaltic wave travels much more 
rapidly in the upper portion of the oesophagus than lower down 
because of differences in the nature of the muscular coat, this 
being of the striated variety above, and of the non-striated, be- 
low. The purpose of more rapid movement in the upper portion 
is no doubt that the bolus may be hurried past the regions, 
where, by distending the oesophagus, it might interfere with the 
function of neighboring structures, such as the heart. The peris- 
taltic wave of the oesophagus, unlike that of the intestines (see 
p. 181), is transmitted by nerves, namely, by the oesophageal 
branches of the vagus, one of the most important of the nerves 
arising directly from the brain. If these be severed, but the mus- 
cular coat left intact, the oesophagus becomes dilated above the FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
161 
level of the section and contracted below, and no peristaltic wave 
can pass along it; on the other hand, the muscular coat may be 
severed (by crushing, etc.) but the peristaltic wave will jump 
the breach, provided no damage has been done to the nerves. 
The propagation of the wave by nerves indicates that the sec- 
ond and third stages of deglutition must be rehearsed, as it were, 
in the nerve centers from which arise the fibers to the pharynx 
and the different levels of the cesophagus. The afferent stimuli 
which initiate this process arise, not as might be expected, in 
the oesophagus itself, but in the pharynx, and they are carried 
to the brain by the fifth, superior laryngeal and vagus nerves; 
thus, a foreign body placed directly in the cesophagus does not 
begin to move until the pharynx is stimulated, as by touching it. 
The Act of Vomiting.-This is usually preceded by a feeling 
of sickness or nausea and is initiated by a very active secretion 
of saliva. The saliva, mixed with air, accumulates to a consider- 
able extent at the lower end of the oesophagus and thus 
distends it. A forced inspiration is now made, during the 
first stage of which the glottis is open so that the air enters 
the lungs, but later the glottis closes so that the in- 
spired air is sucked into the oesophagus, which, already 
somewhat distended by saliva, now becomes markedly so. The 
abdominal muscles then contract so as to compress the stomach 
against the diaphragm and, simultaneously, the cardiac sphincter 
relaxes, the head is held forward and the contents of the stomach 
are ejected through the previously distended oesophagus. The 
compression of the stomach by the contracting abdominal mus- 
cles is assisted by an actual contraction of the stomach itself, 
as has been clearly demonstrated by the X-ray method. After 
the contents of the stomach itself have been evacuated, the 
pyloric sphincter may also relax and thus permit the contents 
(bile, etc.) of the duodenum to be vomited. 
The act of vomiting is controlled by a center located in the 
medulla, and the afferent fibers to this center may come from 
many different regions of the body. Perhaps the most potent 
of them come from the sensory nerve endings of the fauces and 
pharynx. This explains the tendency to vomit when the mucosa 162 
DEGLUTITION. 
of this region is mechanically stimulated. Other afferent im- 
pulses come from the mucosa of the stomach itself, and.these are 
stimulated by swallowing certain drugs called emetics, import- 
ant among which are strong salt solution, mustard water, zinc 
sulphate, etc. When some poisonous substance has been swal- 
lowed, the immediate treatment is to give one of these emetics 
and thus cause the poison to be vomited. Certain other emetics, 
particularly tartar emetic and apomorphine, act on the vomiting 
center itself, and can therefore act when given subcutaneously, 
center of the brain itself, and can therefore act when given sub- 
cutaneously with a hypodermic syringe. Afferent vomiting im- 
pulses also arise from the abdominal viscera, thus explaining the 
vomiting which occurs in strangulated hernia, and in other irri- 
tative lesions involving this region. CHAPTER XIV. 
DIGESTION (Cont'd). 
Digestion in the Stomach. 
The Secretion of Gastric Juice.-After passing the cardiac 
sphincter, the food collects in the fundus of the stomach. When 
it is solid in consistency it becomes disposed in definite layers, 
the first swallowed near the mucosa, the last swallowed in the 
center. When, as is usual in man, the food is more or less fluid, 
it collects in the most dependent part of the body of the stomach 
and the layer formation is less evident (see Fig. 46). Within a 
few minutes of the entry of the first portion of food, the glands 
of the gastric mucosa begin to secrete their digestive juices. The 
immediate exciting cause of this secretion is not the contact of 
food with the mucosa-although this acts later-but is a ner- 
vous stimulus transmitted to the stomach through the vagus 
nerve1 and coming from a nerve center situated in the medulla. 
The activities of this gastric center are called into operation by 
afferent impulses in the nerves that terminate in the taste buds 
and olfactory epithelium. The process of gastric secretion is 
therefore initiated in the mouth, and the stimulus that is re- 
sponsible for it is the good taste and the flavor of the food. Just 
as in the case of the salivary glands, the food, in order to excite 
the secretion, need not actually enter the mouth, for a psychologi- 
cal stimulus may also act on the gastric center. Thus, the sight 
or smell of savory food, or even the hearing of some sound that is 
known by experience to be associated with the gratification of 
the appetite can call it forth. These important facts were first 
of all revealed by observations through a gastric fistula (arti- 
ficial opening) in the case of a boy who, because of stricture of 
the oesophagus, was unable to take food by the mouth. This 
lAfter the vagi are cut, this secretion of gastric juice does not occur. 
163 164 
DIGESTION IN THE STOMACH. 
boy had to be fed through the gastric fistula, but it was noticed 
that when he was allowed to chew food for which he had a relish 
and then spit it out, gastric secretion occurred. This observa- 
tion suggested to Pavlov the establishment of analogous condi- 
tions in dogs, with the modification that, besides the fistula in 
the stomach, another was made in the oesophagus. The animal 
could therefore swallow interminably without ever becoming 
satisfied, because the food escaped by the oesophageal fistula. 
Fig. 46.-Diagrams of outline and position of stomach as indicated by skia- 
grams taken on man in the erect position at intervals after swallowing food 
impregnated with bismuth subnitrate. A, moderately full; B, practically 
empty. The clear space at the upper end of the stomach is due to gas, and 
it will be noticed that this "stomach bladder'' lies close to the heart. (T 
Wingate Todd.) 
Nevertheless the gastric juice flowed abundantly, provided this 
"sham feeding" was with appetizing food. Stones, bread, acid or 
irritating substances, although they might cause much saliva to 
be secreted and swallowed (see p. 153), had no influence whatso- 
ever on the flow of gastric juice. The only adequate stimulus 
was gratification of the appetite. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
165 
In passing, it may be well to call attention to the practical 
importance of these observations in connection with the feeding 
of debilitated persons; by frecpient feeding with appetizing food 
the nutritional condition is likely to improve much more rapidly 
than by occasional stuffing with uncongenial mixtures, however 
rich these may be in calories and nitrogen. 
The secretion is therefore well named the appetite juice, and 
it lasts sometimes for nearly two hours after sham feeding has 
been discontinued. Yet this is only about one-half as long as the 
time during which gastric juice is secreted when the food is ae- 
Fig. 47.-Diagram of stomach showing miniature stomach (8) separated 
from the main stomach (V) by a double layer of mucous membrane. A.A. is 
the opening of the pouch on the abdominal wall. (Pavlov.) 
tually permitted to enter the stomach. In order to investigate 
the cause of the continued secretion, it was necessary to devise 
some means by which the gastric juice could be collected, un- 
mixed with food, while normal digestion was in progress. Ab 
there is no duct, the only means by which this could be done was 
by isolating a portion of the stomach as a pouch with an opening 
exteriorly through which the secretions collecting in it could be 
removed. An operation for making such a pouch, or "miniature 
stomach," as it is called, without injuring any of the nerves of 166 
DIGESTION IN THE STOMACH. 
the stomach has been devised by Pavlov (see Fig. 47). By sim- 
ultaneously collecting the secretions from the main stomach and 
the miniature stomach after sham feeding, it was found that they 
ran strictly parallel with each other, in amount as well as in 
strength of secretion. The secretion in the miniature stomach 
therefore accurately mirrors the secretion occurring in the main 
stomach, and so permits us to study this during the actual diges- 
tion of food. 
By introducing food directly into the main stomach through 
a fistula, it was found, by observations on the secretions from the 
miniature stomach, that very little secretion occurred until after 
some time, provided of course that precautions had been taken, 
as by experimenting on a sleeping animal, not to excite the appe- 
tite juice. There was found to be great discrimination in the 
nature of the adequate stimulus for this local secretion; mechani- 
cal stimulation of the gastric mucosa, contact with alkaline fluids, 
such as saliva, or with white of egg, failed to produce any secre- 
tion ; water had a slight effect, milk still more, whereas a marked 
secretion occurred when a decoction of meat or meat extract, or 
a solution containing the half-digested products of peptic diges- 
tion (such as Witte's peptone) was placed in the main stomach. 
It was further observed, when meat was directly placed in the 
stomach, that the juice which collected in the pouch increased, 
both in quantity and in strength, after the first hour, and that 
it continued to flow even after four hours, thus' indicating that 
the primary stimulus had come from the extractives in the meat, 
further stimulation being due to the proteose and peptones 
liberated as the protein of the meat became digested. 
This local stimulation is independent of the medullary nerve 
center that controls secretion of the appetite juice, for it still oc- 
curred after both vagi had been divided or even after destruc- 
tion of the sympathetic nerve plexuses in the abdomen. It might, 
however, still be a nervous reflex involving the local nerve struc- 
tures (plexus of Auerbach) in the walls of the stomach, although 
this is not so probable as that it is dependent upon some chemical 
excitation of the gland cells by substances appearing in the blood 
as a result of absorption from the stomach. This "hormone" FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
167 
(see p. 227) is not merely absorbed food, for no gastric secretion 
occurred when solutions of meat extract, or of peptone were in- 
jected intravenously. It must therefore be some substance which 
is absorbed into the blood from the mucous membrane of the 
stomach, and which is produced in this as a result of the action 
of the gastric contents on its cells. In confirmation of this view 
it has been shown that boiled extracts of the mucous membrane 
of the pyloric region of the stomach (made with water or weak 
acid or solutions of peptone or dextrin) cause some gastric juice 
to be secreted when they are injected in small quantities every 
ten minutes into a vein, similar injections of the extracting fluids 
themselves being without effect. 
We are now provided with the necessary facts from which to 
draw a completed account of the mechanism of gastric secretion. 
The satisfaction of taking food causes appetite juice to flow and 
this soon digests some of the protein. The products of this diges- 
tion, along with the extractive substances of the food, after some 
time (which is probably quite short in the case of man), gain the 
pylorus, where they act on the mucosa to produce some hormone, 
which becomes absorbed into the blood and stimulates further 
secretion of the juice. As digestion proceeds juice therefore con- 
tinues to be secreted. The appetite juice sets the process agoing; 
it initiates gastric digestion. 
The Active Constituents of Gastric Juice.-When there is no 
food in the stomach, a certain amount of mucous secretion 
is present in it, and most of the gland cells are filled with zymo- 
gen granules (see p. 151). An extract (made with glycerine) 
of the mucosa in this resting condition exhibits no digestive 
powers; but if the mucosa be first of all macerated with weak 
hydrochloric acid, the extract becomes highly active, because it 
contains large amounts of the proteolytic ferment pepsin. Other 
cells in the stomach produce the necessary hydrochloric acid. 
It may be concluded, therefore, that during the process of secre- 
tion the zymogen granules are activated by hydrochloric 
acid and converted to pepsin. In conformity with this, it 
has been found that the secretion of a pouch of stomach pre- 
pared from the pyloric region possesses no digestive activity, 168 
DIGESTION IN THE STOMACH. 
since in this region no hydrochloric acid is secreted. The activa- 
tion of the zymogen can also be accomplished by tissue extracts 
and by the products of micro-organismal growth. Because of 
such growth in the stomach contents, it is often found, in dis- 
eased conditions in which there is no acid secretion, that active 
pepsin, nevertheless, is present. Accompanying the pepsin, if 
indeed not identical with it, the gastric juice contains the milk- 
curdling ferment, rennin. It also contains a fat-splitting fer- 
ment, lipase, whose activities are, however, limited to emulsified 
fats. 
The most remarkable constituent of the gastric secretion is 
hydrochloric acid, which in some animals, such as the dog, may 
attain a percentage of 0.6, being usually about 0.4 in the case of 
man. It is derived from the parietal cells of the glands in the 
cardiac region of the stomach, none being present in the secre- 
tion of the pyloric region, where there are no parietal cells. 
The source of the acid is of course the blood, for although this 
is practically neutral, yet it contains, on the one hand, substances 
such as sodium bicarbonate which readily yield hydrogen ions, 
and on the other, chlorides which, by dissociation, make chlorine 
ions readily available. Although it is thus possible, in the light 
of modern physico-chemical teaching, to formulate an equation 
for the reaction, yet we are at a loss to explain why just at this 
particular place (i. e., in the gland cells of the stomach) in the 
animal body, and nowhere else, the Cl- and H-ions should be 
picked out of the blood and secreted as HC1. 
Little as we know about the cause and mechanism of the secre- 
tion of hydrochloric acid, we do know something regarding its 
value and use in the process of digestion, and in general we may 
state that this is partly regulatory and partly digestive. It is 
regulatory in that it serves as the exciting cause of subsequent 
events in the digestive process, and digestive not only in that it 
actually assists in the break-down of protein, but also because it 
may cause a certain amount of acid hydrolysis of sugar after 
enough has been secreted so that some is free. Its action 
on protein is, however, the most important, for it initiates pro- 
teolytic break-down by producing so-called acid-protein on FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
169 
which the pepsin-itself also dependent, as we have seen, on a 
preliminary activation by acid-then unfolds its action. As the 
protein becomes progressively broken down, the proteose and 
peptone which are produced absorb still more of the acid, so 
that it is some considerable time after gastric digestion has 
started before any acid is allowed to exist in the free state. 
It is only after there is some free acid that it can hydrolyse 
sugars or perform another important function, namely, act as an 
antiseptic. In this regard, however, it must be remembered that 
it is only towards certain organisms that such antiseptic action 
is displayed, for there may be bacteria in the gastric contents 
even in cases of excessive secretion of hydrochloric acid. The 
undoubted tendency for intestinal putrefaction to increase when 
there is a deficient secretion of hydrochloric acid is probably de- 
pendent more upon the delay in digestion which this occasions, 
than upon any specific antiseptic power of hydrochloric acid. 
During the time that elapses before a sufficiency of hydrochloric 
acid has accumulated to perform this function, bacterial fermen- 
tation occurs in the stomach contents. Carbohydrates are broken 
down by this process, at first into simple sugars and then into 
lactic acid, which may come to be present in considerable amount 
before the fermentation process is terminated. For these reasons 
we find that there is relatively much more lactic acid detectable 
in the gastric contents removed by the stomach tube at an early 
stage in gastric digestion than later. 
The so-called acid albumin which results from the action of 
the acid, becomes attacked by the pepsin, which still further 
breaks it down into so-called proteose and peptones, which do 
not coagulate by heat and which become progressively more dif- 
fusible through animal membranes. Although pepsin is capable 
of carrying the digestive process far beyond the stage of pep- 
tones, this does not occur in the comparatively short time (about 
six hours) during which the food remains in the stomach. Slight 
as is this action of pepsin in the stomach, it nevertheless appears 
to be of considerable importance for the subsequent digestion of 
protein by the other proteolytic ferments, trypsin and erepsin 
(see p. 178), which operate in the small intestine. Thus, a given 170 
DIGESTION IN THE STOMACH. 
amount of blood serum becomes digested much farther in a 
given time by a given amount of trypsin if it receives a prelim- 
inary digestion by means of pepsin, than when it is acted on by 
trypsin alone, and erepsin will cause no digestion of most pro- 
teins unless these are first of all acted on by either pepsin or 
trypsin. But peptic digestion is not essential for life, for sev- 
eral cases are now on record in which individuals have thrived 
after the stomach has been removed. 
The milk curdling action of gastric juice is due partly to the 
hydrochloric acid and partly to pepsin. Curiously enough the 
curdled milk undergoes little further change until it reaches 
the small intestine. 
The lipase in gastric juice can act only on emulsified fat and 
in a neutral or alkaline reaction. Fat digestion cannot therefore 
be an important gastric process. 
It has been supposed that there is a certain specific adaptation 
between the chemical nature of the food and the amount and 
strength of the gastric secretion. For example, it has been 
found, by observations on the gastric juice flowing from a minia- 
ture stomach (see Fig. 47), that feeding with bread causes a 
maximal secretion during the first hour, whereas with an equiva- 
lent amount of flesh the maximum occurs during the first and 
second hours, and with milk it is delayed till the third or fourth. 
In proteolytic power the bread juice is much the strongest of the 
three, but it contains a lower percentage of acid than the others. 
The Movements of the Stomach.-Solid food after being 
swallowed accumulates in the body of the stomach, where on ac- 
count of an absence of movements it is not uniformly acted on 
by the gastric juice, its outer layers only becoming digested. In 
the case of man, however, some of the food, because of its 
semi-fluid nature, passes beyond the so-called transverse band 
and into the pyloric region, in which waves of contraction make 
their appearance. Starting very faintly at this point, these 
waves travel towards the pylorus and become gradually more 
marked until they may become so deep as practically to cut off a 
portion of the pyloric region from the rest of the stomach. This 
last portion of the pylorus, sometimes called the pyloric canal, FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
171 
gradually contracts on the food which has been forced into it, 
thus tending to eject it through the pyloric sphincter, or, if this 
is closed, to cause it to pass back again as an axial stream into 
the proximal part of the pylorus, which has been called the 
pyloric vestibule. These waves occur every fifteen to twenty 
seconds, three or four being present in the pyloric vestibule at 
one time. They become more marked as digestion proceeds, and 
are accompanied by a gradual diminution in size of the body 
of the stomach (see Fig. 46). Their function, besides carrying 
the food towards the outlet of the stomach, is to keep it properly 
mixed with the gastric juice. 
The Opening of the Pyloric Sphincter.-The mere pressure 
with which the contents of the vestibule are thus driven, with 
each peristaltic wave, against the pyloric sphincter does not 
alone succeed in opening it; for half an hour after feed- 
ing with protein, for example, no food may pass the sphincter, 
although during this time there may have been well over a hun- 
dred peristaltic waves. Nor is it the consistency of the food 
which controls the opening. It must therefore be some chemical 
property which the food acquires during its stay in the stomach. 
This has definitely been shown by Cannon to be the presence of 
free acid. By measuring the length of the skiagram shadow in 
the intestines after feeding cats with bismuth-impregnated foods 
rendered acid or alkaline, it could be clearly shown that acid 
hastened the initial discharge, whereas alkalies retarded it, and 
observations through a fistula in the vestibule showed that any 
delay in the appearance of acid in the contents was associated 
with a delay in the opening of the sphincter. 
But the sphincter does not remain open; it quickly closes 
after a little chyme, as the half digested food is called, has passed 
through it. This closure is due to stimulation of afferent nerve 
endings in the duodenum by the free acid in the chyme. The 
sphincter remains closed so long as there is any free acid in the 
duodenum. Whenever this acidity has become neutralized by 
the alkali present in the bile and pancreatic juice, the acid on 
the stomach side again becomes operative and the sphincter 
opens. 172 
DIGESTION IN THE STOMACH. 
The pyloric sphincter is thus under the control of a nerve 
reflex, called the duodenal reflex, which transmits influences 
that tend to relax the sphincter when the afferent nerve fibers 
from the stomach side are excited by acid, but which cause it still 
more powerfully to contract when the acid acts on afferent fibers 
having their terminations in the duodenum. When both afferent 
paths are simultaneously stimulated, the duodenal predominates 
over the gastric, so that the sphincter remains closed until the 
acidity of the chyme in the duodenum has all been neutralized, 
and this seems to be true however faint the acidity may be on 
the duodenal side and however strong on the stomach side. The 
reflex arc is situated in the walls of the stomach and duodenum, 
for it operates after complete isolation of these from the central 
nervous system. It is a function of the nerve plexus found pres- 
ent in the walls-the myenteric plexus. 
Rate of Discharge of Food from the Stomach.-The acidity 
of the gastric contents, as we have just seen, must attain a cer- 
tain degree before it becomes an adequate stimulus for the open- 
ing of the pyloric sphincter, and consequently the rate at which 
the different foodstuffs leave the stomach is to a large extent 
proportional to their power of combination with the acid. Pro- 
teins combine with large amounts of acid, so that their initial 
discharge is delayed and their subsequent passage slow. Car- 
bohydrates absorb but little acid, so that they begin to leave early 
and the stomach is soon emptied of them. The passage of fats is 
peculiar; when taken alone, which, however, is scarcely ever the 
case, they seem to bring about a partial relaxation of the pyloric 
sphincter, so that bile and pancreatic juice regurgitate into the 
stomach and some fat may pass out; but the subsequent dis- 
charge into the intestines is very slow, so slow indeed that each 
discharged portion seems to become completely absorbed before 
any further discharge occurs. When fats are mixed with other 
foods, they materially delay the discharge. These effects are 
no doubt due in part to the inhibitory influence which fats have 
on gastric secretion; and in part to the liberation of fatty acid in 
the duodenum by the action of pancreatic lipase. This fatty acid 173 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
seems to be liberated so quickly that it is not immediately neu- 
tralized by alkali. 
Water alone begins to leave the stomach almost immediately 
after it is taken, because in this case the sphincter opens before 
an acid reaction has been acquired, and remains open on account 
of there being no acid in the duodenum to effect its closure. 
Water does not remain for a sufficient time in the stqmaeh to 
excite any gastric secretion, and consequently it readily car- 
ries infection into the intestine. The discharge of raw egg al- 
bumin is peculiar. Like water it begins to pass the pylorus im- 
mediately after ingestion, its reaction for some time being alka- 
line; it becomes acid later, so that the discharge becomes inter- 
mittent because of the duodenal reflex. The consistency of food 
itself does not affect the rate of discharge unless hard particles 
are present in it, when a marked retardation occurs. 
It is well known that the gastric contents are but slowly dis- 
charged into the duodenum when there is excessive gas accu- 
mulation in the stomach. This is due to the atony of the stomach 
which accompanies pathological gas accumulation. CHAPTER XV. 
DIGESTION (Cont'd). 
Intestinal Digestion: The Movements of the Intestines 
Absorption. 
The Secretion of Bile and Pancreatic Juice.-Besides caus- 
ing reflex closure of the pyloric sphincter, the contact of the 
chyme, which is the name given to the semi-digested food as it 
leaves the stomach, with the duodenal mucosa inaugurates the 
processes of intestinal digestion by exciting the secretion of bile 
and pancreatic juice. Neither of these juices is secreted into the 
intestine during fasting; but both begin to flow very soon after 
taking food, and they gradually increase in amount for about 
three hours, and then rapidly decline. The bile at first comes 
mainly from the gall bladder, in which it has accumulated dur- 
ing fasting. When the gall bladder supply has been exhausted, 
the bile comes directly from the liver without entering the gall 
bladder. This direct secretion becomes more and more marked 
as digestion proceeds. 
Bile is partly an excretory product of the liver, and is thus 
being constantly secreted into the bile ducts. On account of its 
value as a digestive fluid it is not, however, allowed to run to 
waste, but is stored up in the gall bladder until food arrives in 
the duodenum, when the bile is immediately discharged as above 
described. 
The sudden discharge of bile from the gall bladder is depen- 
dent upon a nerve reflex excited by the contact of the acid chyme 
with the duodenum. The increased secretion of bile which occurs 
later in digestion, like the secretion of pancreatic juice, is, how- 
ever, independent of nerves, for it has been found that it occurs 
when acid is placed in the duodenum after all the nerves, but 
not the blood vessels of the duodenum, have been cut. The 
only way by which such a result can be explained is 
174 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
175 
by assuming that the acid causes some chemical sub- 
stance to be added to the blood, which then carries it to the pan- 
creas and liver, upon the cells of which it exercises a stimulating 
influence. This explanation was shown to be correct by 
studying the effect which is produced on the secretion of pan- 
creatic juice and bile by intravenous injections of decoctions of 
intestinal mucosa made with weak acid and subsequently neu- 
tralized. An immediate secretion resulted. The acid extract 
contains some hormone whose production, in the normal 
process of digestion, is evidently occasioned by the contact of the 
acid chyme with the duodenal mucosa. This hormone is called 
secretin but we know very little of its exact chemical nature. It 
is not a ferment, for it withstands heat; it is not a protein, for it 
can be extracted by boiling the mucous membrane with weak 
acids after treatment with alcohol. It is readily oxidized in the 
presence of alkalies, and is of the same nature in all animals. 
It is useless to give secretin as a drug with the hope that it will 
stimulate pancreatic secretion, for it is not absorbed from the 
lumen of the intestine. 
Although most abundant in the mucosa of the duodenum and 
jejunum, secretin is also present in the mucosa of the lower end 
of the small, and to a lesser degree, in that of the large intestine. 
Soap solutions act like acid in producing secretin. A fatty meal, 
therefore, excites the flow of much pancreatic juice and bile, be- 
cause the fatty acid which is split off unites with alkali and 
forms soap. 
It may be that the first portion of pancreatic juice to be se- 
creted after a meal, is the result, not of secretin formation, but 
of reflex nervous stimulation of the pancreas. In comparison 
with the hormone control the nervous control is, however, quite 
unimportant in pancreatic secretion, for there is no necessity in 
the intestine, as in the mouth, or to a less degree in the stomach, 
for a quick response to the stimulus which is set up by the pres- 
ence of food. The histological changes produced in the gland 
cells of the pancreas by secretory activity are much the same as 
in the parotid glands. 
Functions of the Bile and Pancreatic Juice.-These two 176 
INTESTINAL DIGESTION. 
juices are very closely associated in their activities. This fact 
is perhaps most strikingly demonstrated in the digestion and ab- 
sorption of fat; for, in the absence of either secretion, large 
amounts of unabsorbed fat appear in the faeces. Both juices 
contain relatively large amounts of alkali, which neutralizes the 
acidity of the chyme. In the pancreatic juice alone, for example, 
there is a sufficient concentration of sodium carbonate to neu- 
tralize the acid in an equal volume of gastric juice. Whenever 
the chyme becomes alkaline the pepsin present in it ceases to act 
and conditions thus become suitable for the activities of the pan- 
creatic enzymes. Besides its neutralizing action, the bile causes 
the chyme to assume a somewhat greater consistency, by pre- 
cipitating incompletely peptonized protein, as well as pepsin. 
The precipitate becomes redissolved when excess of bile has be- 
come mixed with the chyme and the significance of the precipita- 
tion may be that it causes a temporary delay in the movement of 
the chyme along the duodenum, thus allowing it to become prop- 
erly mixed with pancreatic juice before it moves further along 
the intestine. 
Composition, Properties and Functions of the Bile.- 
Water 85.9 
Total Solids 14.1 
of which: 
Bile Salts 9.14 
r^pcithin and Cbolestprnl 1 10 
Mucinoid Substance 
Pigment 
Organic 
2.98 
Inorganic Salts 0.78 
The bile is a greenish-yellow fluid of sticky consistency and 
bitter taste. Its most interesting constituents are the bile salts, 
which are complex organic substances, having an important func- 
tion to perform in assisting the lipase and amylopsin of pan- 
creatic juice in their digestive activities. Otherwise the bile con- 
tains no digestive enzymes. The cholesterol is not a readily FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
177 
soluble substance, so that it is apt to become precipitated in the 
bile duct and cause gall stones. The distention of the ducts 
by the gall stones may cause great pain (biliary colic). The 
formation of gall stones is encouraged by inflammatory processes 
of the mucous membrane of the ducts. When bile fails to reach 
the intestine, because of blocking the ducts, either by gall stones 
or by inflammatory swelling of the mucous membrane, the di- 
gestion, especially of fats* is much interfered with, and the fasces 
become foul smelling and pale in color. 
The Composition and Properties of Pancreatic Juice.-The 
pancreatic juice contains three important enzymes: lipase (act- 
ing on fats), amylopsin (acting on starch), and trypsinogen 
(acting on protein). Lipase and amylopsin are secreted in an 
active condition, but trypsinogen is without any action until it 
has become changed into trypsin. This does not occur until the 
pancreatic juice has reached the intestine, when the activation 
is brought about by a ferment present in the intestinal juice 
(secretion of Lieberkuhn's follicles) called enterokinase. The 
intestinal juice contains this activator only when there is some 
trypsinogen present in the intestine. There is no enterokinase, 
for example, in the juice that is secreted as a result of mechan- 
ical stimulation of the intestinal mucosa, but it immediately ap- 
pears when some pancreatic secretion is brought in contact with 
the mucosa. 
Enterokinase is not t'he only substance which can activate 
trypsinogen; the addition to the pancreatic juice of calcipm 
salts, or the contact of the juice with leucocytes, as in granula- 
tion tissue, or even mere standing of the juice, has a similar ac- 
tivating effect. If the pancreatic juice, in escaping from the duct, 
should run over granulation tissue, as occurs when a fistula 
(i. e., an opening made by surgical operation) of the duct is 
made, it becomes activated and unless precautions are taken it 
will excoriate the wound. Should it escape into the peritoneum, 
as when a cyst bursts, it also becomes activated. 
It will be remembered that the amount of gastric juice secreted 
varies with different foods, being relatively more abundant on a 
diet of bread than on one of milk, or even meat (p. 166). Simi- 178 
INTESTINAL DIGESTION. 
lar quantitative differences exist in the secretion of pancreatic 
juice and this is probably to be explained by the varying quanti- 
ties of acid chyme coming in contact with the duodenal mucosa. 
Chemical Changes Produced by Intestinal Digestion.-In the 
lower portion of the duodenum and in the jejunum, the digestive 
enzymes of the pancreatic juice act on the food in full intensity. 
The trypsin rapidly hydrolyzes the proteins to peptone, which if 
it is not immediately absorbed may become further broken down 
to amino acids and aromatic compounds. The lipase hydrolyses 
fat to glycerine and fatty acid, which are absorbed, the former 
as such, the latter, after combining with alkali to form soap, or, 
if no alkali be available, with bile salts to form compounds which 
like soap are soluble in water. Amylopsin converts into mal- 
tose any starch or dextrines which the ptyalin of saliva has failed 
to act on. The maltose thus formed, and the other disaccharides, 
cane sugar and lactose, although soluble in water, do not become 
absorbed into the blood as such but become further hydrolyzed 
by the action of so-called inverting enzymes, of which there is one 
for each disaccharide (see p. 37). These inverting enzymes are 
more plentiful in extracts of the mucosa than in the intestinal 
juice itself, from which we conclude that it is only after they 
have been absorbed into the cells of the intestines that the disac- 
charides are inverted. The process, in other words, is an in- 
tracellular one. 
One other enzyme exists in the intestinal juice, namely,erepsin. 
It acts on partially hydrolyzed proteins and on caseinogen, so 
as to hydrolyze them completely into the amino acids. 
Erepsin is a widely distributed enzyme in the animal body, be- 
ing present in practically every tissue, although it is absent from 
blood plasma. It is present in much greater concentration in ex- 
tracts of the intestinal mucosa than in the succus entericus, so 
that, like the inverting enzymes, it possibly displays its action 
while the protein is being absorbed as proteoses and peptones. 
It serves as the last barrier against the entry into the blood of 
protein in any other form than as a mixture of amino acids. Less 
completely digested protein is poisonous when added to the blood 
(p. 63). FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
179 
Most of the food is now in a suitable condition for absorption. 
Before we proceed to study the nature of this process, however, 
there are one or two further digestive changes that we must con- 
sider. 
The Digestive Function of Intestinal Bacteria.-On account 
of the antiseptic action of free hydrochloric acid, there is, ordi- 
narily, no bacterial growth in the stomach, but the neutraliza- 
tion of acid by the pancreatic juice and bile in the intestine pro- 
vides a perfect medium for such growth. The extent and nature 
of the bacterial growth varies very greatly according to the na- 
ture of the diet. 
There can be no doubt that the micro-organisms are a valuable 
aid to digestion in the case of most animals, especially of those 
whose diet includes cellulose. Indeed, in such animals as the 
herbivora special provision is made to encourage bacterial growth 
by the great length of the large intestine, for without bacteria, 
digestion of cellulose is impossible. Thus if newly-hatched chicks 
be fed with sterilized grain they succumb in about two weeks, 
but if a small amount of the excrement of the fowl be mixed 
with the grain, they thrive as ordinarily. On the other hand, if 
the food contains no cellulose, animals may develop and grow 
with sterile intestinal contents; thus guinea pigs have been re- 
moved from the uterus under aseptic conditions and kept in a 
sterile place on sterilized milk and have thrived and grown as 
normal guinea pigs. The organisms in the intestine of man are 
probably much more useful than harmful. No doubt they are 
parasites, but they are useful parasites; they work for their liv- 
ing, not only by assisting when necessary in the digestion of 
food but also by destroying certain substances which, if absorbed, 
would have a toxic action on the host. Thus cholin, a substance 
produced by the digestion of lecithin, is distinctly poisonous, but 
it really never gets into the blood because the bacteria destroy it. 
In the case of man bacterial digestion occurs in both the small 
and the large intestines, and there are varieties of bacteria capa- 
ble of acting on all the foodstuffs. They may break up the sugars 
into lactic acid or even further so as to form CO2 and H. It has 
been claimed that this formation of lactic acid in the intestine is 180 
INTESTINAL DIGESTION. 
of benefit to the health of man because when it occurs other bac- 
teria which are more harmful than useful become destroyed. To 
encourage this growth of lactic acid bacteria, it has been recom- 
mended that large quantities of sour milk should be taken. It is 
undoubtedly true that such treatment is of benefit in many per- 
sons who suffer from excessive intestinal putrefaction, but that 
such treatment should prolong the life of otherwise healthy indi- 
viduals is visionary. As in herbivora, there are also bacteria in 
man which break up cellulose, producing methane and CO2. After 
diets containing much vegetable matter, therefore, a large 
amount of gas is likely to accumulate in the intestines. From 
fats, the intestinal bacteria produce lower fatty acids, which 
tend to cause the contents in the lower portion of the small in- 
testines to become acid in reaction. 
Although capable of hydrolyzing native protein from the very 
start, bacteria act most readily on protein that has been partially 
digested by the proteolytic enzymes of the stomach and intes- 
tines. The products of this action are more or less characteristic 
because of the peculiar manner in which the aromatic groups of 
the protein molecule are attacked, producing from it such sub- 
stances as phenol, skatol, indol, etc., to which the characteristic 
odor of the fasces is due. When protein has been adequately di- 
gested in the stomach, it is so rapidly acted on by the trypsin 
(and erepsin) of the small gut and is so quickly absorbed that 
bacteria have no chance to act on it. When protein has been in- 
adequately digested in the stomach, however, the trypsin fails to 
digest it quickly enough, so that bacterial putrefaction sets in 
which may be quite marked in the small intestine, although much 
more so in the colon. Even when they do not find a suitable sub- 
strat in the food, the bacteria attack the proteins of the intes- 
tinal secretions themselves, which accounts for the well-known 
occurrence of this process during starvation. 
The Immunity of the Walls of the Digestive Organs Toward 
the Enzymes Which Act within Them.-The immunity of the 
mucosa of the stomach and intestines seems to be due in main to 
the presence in the cells of the mucosa of anti-enzymes, that is, of 
substances which can inhibit the action of the various enzymes FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
181 
(antipepsin, antitrypsin, etc.). As we should expect, very strong 
anti-enzymes can be prepared from tapeworms and other intes- 
tinal worms. It is by virtue of possessing these, that the worms 
are not digested. The immunity of the gland cells and ducts, as 
of the pancreas, to the proteolytic enzymes which they produce 
is possibly to be explained in another way, namely, by the ex- 
istence of the enzyme as an inactive precursor (e. g., trypsino- 
gen) until after the secretion has been carried to a region whose 
walls contain the specific anti-body. A certain degree of im- 
munity to the destructive action of the intestinal bacteria on the 
mucous membrane may be conferred by the mucin, which is quite 
abundant, at least in the empty stomach and in the large intes- 
tine. The relatively poor growth of bacteria which occurs on 
inoculating ftecal matter in culture media-although many bac- 
teria can be seen by microscopic examination to be present-is 
probably to be explained by their having been killed by the 
mucin. 
The Movements of the Intestines. 
The Movements of the Small Intestine have two functions: (1) 
to macerate and mix up the food and (2) to move it along to- 
wards the lower end of the gut. These two functions are sub- 
served by two different types of movement, the so-called pendular 
and the peristaltic. The pendular movements are rendered evi- 
dent by allowing the intestine to float out in a bath of isotonic 
saline (p. 42), when the various loops sway from side to side like 
a pendulum. By closer examination it can be seen that the move- 
ments are produced by faint waves of contraction of both muscu- 
lar coats, which sweep with considerable rapidity along the gut. 
When the waves arrive at a part of the intestine containing any 
solid substance, they become accentuated, and this becomes most 
marked at the middle of the solid mass of food, thus tending, on 
account of the contraction of the circular fibers, to divide the 
mass into two. These movements are therefore somtimes called 
segmenting movements. Their function is evidently to break up 
the food masses and thus mix the food with the digestive juices. 
This can be very well shown in skiagram shadows of the ab- 182 
INTESTINAL DIGESTION. 
domen some time after taking food mixed with bismuth. A 
column of food can be seen to divide into several segments, each 
of which in a few seconds breaks into two, the neighboring 
halves then joining together, and the process repeating itself. 
Two varieties of peristaltic waves are usually described, both 
of which are characterized by a marked constriction preceded by 
a distinct dilatation of the gut, which may extend for a consid- 
erable distance down it (two feet). The one variety of wave 
travels slowly (% cm. per minute), and has the function of car- 
rying along the food; the other travels very rapidly (peristaltic 
rush), and is evidently for the purpose of hurrying along irri- 
tating substances. 
Besides being set up by the presence of food in the intestine, 
these waves may be influenced through the nervous system; stim- 
ulation of the vagus excites them, whereas stimulation of the 
sympathetic brings about a marked inhibition, in which the whole 
gut becomes profoundly relaxed with the exception of the ileo- 
colic sphincter, which contracts. This influence of the splanch- 
nic may be excited reflexly, as by pain or fear. 
The Movements of the Large Intestine are more difficult to 
study than those of the small intestine. They vary considerably in 
different animals, as indeed is to be expected when we remember 
that the function of this part of the alimentary tract depends 
upon the nature of the food. In herbivora, for example, food 
may lie in the capacious caecum for days, and even in carnivora, 
in which this part of the gut is rudimentary, it may remain for 
twenty-four hours. In man the conditions seem to be intermedi- 
ate between those in the herbivora and carnivora, and the move- 
ments are believed to be as follows: As the semi-fluid food en- 
ters the caecum through the ileo-caecal valve and collects in the 
caecum and proximal colon, it excites the occurrence of waves of 
constriction, which start probably about the hepatic flexure and 
travel in a central direction towards the caecum, into which the 
food is thus forced back. 
Occasionally the arrival of the wave at the caecum starts a 
true peristaltic wave, which travels distally, getting feebler as 
it proceeds, and which may carry some of the contents into the FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
183 
transverse colon. Here the contents assume more or less of the 
consistency of faeces, and more powerful peristaltic waves make 
their appearances so that the solid masses are carried on towards 
the rectum. These waves are sufficiently energetic to keep the 
descending colon comparatively empty, and the faecal masses 
gradually accumulate in 'the sigmoid flexure and rectum until 
evacuated by the act of defaecation. 
The act of defecation is accomplished by the simultaneous 
peristaltic contraction of the rectum and the opening of the in- 
ternal sphincter of the anus. Cathartics are medicines which 
accelerate or bring about a passage of the intestinal contents 
along the alimentary tract and cause the emptying of the bowel. 
They produce this effect either by directly exciting and accel- 
erating the intestinal peristalsis, or indirectly, by lessening the 
normal absorption or increasing the secretions of the intestinal 
glands, and so keeping the contents of the intestine fluid and 
voluminous. 
Examination of the accompanying diagram (Fig. 48) will show 
how' long food takes to pass along the various parts of the gastro- 
intestinal tract. 
The Absorption of Food. 
As has been explained, the whole object of digestion is to break 
up the large molecules of which food is composed into smaller 
ones so that they can be absorbed into the blood or lymph which 
circulates in the mucous membrane of the intestines. Except un- 
der unusual circumstances, no absorption occurs until the small 
intestine is reached. Here sugars are absorbed into the blood 
as dextrose, and proteins as amino acids, whilst fats are ab- 
sorbed into the lymphatic vessels, as fatty acids and glycerine. 
These substances are absorbed in solution, which would lead us 
to expect that, because of the water absorbed along with them, 
the contents of the small intestine would be more solid at its 
lower than at its upper end; but this is not the case, for the 
digestive juices which have been secreted make up for the loss 
of water. It is in the large intestine that the water is finally 
absorbed. 184 
THE ABSORPTION OF FOOD. 
Attempts have been made to explain the mechanism of ab- 
sorption in terms of the known laws of filtration, osmosis, surface 
tension, and imbibition, but little further progress has been 
made than to establish the fact that although these processes 
may play a role, they are not alone re- 
sponsible. Thus, if blood serum be placed 
in an isolated loop of intestine, it will 
become entirely absorbed, even although 
identical in all the above properties with 
the blood of the animal. That osmosis 
does have some influence, however, is evi- 
denced by the well-known effect of a 
strong saline solution in the intestine; it 
attracts water from the blood, thus di- 
luting the intestinal contents and stim- 
ulating peristaltic contractions. It is in 
this way that saline cathartics act. 
Regarding the absorption of fats, it 
is now definitely known that these are 
first of all split into fatty acid and gly- 
cerine by the action of the lipase of pan- 
creatic juice. The fatty acid then unites 
with alkali to form a soap, or with bile 
salts to form a soluble compound. In ei- 
ther case, the dissolved fatty acid passes , 
into the intestinal epithelium, into which is also absorbed the 
glycerine, the two re-uniting after their absorption so as to form 
neutral fat again. The neutral fat then passes into the central 
lacteal of the villus, whence it is transported by the abdominal 
lymphatics to the thoracic duct, which discharges it into the 
subclavian vein on the left side of the root of the neck. 
Hunger sensations coincide with stomach contractions, but 
these differ from those which occur during digestion. Thirst is 
due to dryness of the throat. It is temporarily relieved by 
moistening the throat, but unless liquid is swallowed permanent 
thirst develops because the tissues become dry. 
Fig. 48.-Diagram of time 
it takes for a capsule con- 
taining bismuth to reach 
the various parts of the 
large intestine. Resume of Actions of Digestive Enzymes. 
Secretion 
Enzyme or 
Adjuvant 
Agency 
Action 
Saliva 
Ptyalin 
Alkalies 
Converts boiled starch into maltose. 
Favors action of ptyalin. 
Gastric juice.. 
Pancreatic 
juice 
Bile 
Pepsin 
HC1 
Lipase 
Trypsinogen . . 
Lipase . ..'.... 
Amylopsin . .. 
Alkali 
Bile salts .... 
Alkali 
(1) Converts metaproteins (acid albu- 
min, etc.) into proteoses and pep- 
tones. 
(2) Clots milk. 
(1) Produces metaproteins. 
(2) Acts as antiseptic. 
(3) Stops action of ptyalin. 
Acts on emulsified fats. 
Inactive until acted on by enterokinase. 
Splits neutral fat into fatty acid and 
glycerine. 
Converts all starches into maltose. 
(1) Helps to neutralize HC1 of chyme. 
(2) Combines with fatty acid to form 
soaps. 
(1) Augment the action of lipase and 
and amylopsin. 
(2) Precipitate pepsin and peptones in 
chyme. 
(3) Combines with fatty acids. 
(1) Helps to neutralize HC1 of chyme. 
(2) Combines with fatty acid to form 
Intestinal 
juice 
Enterokinase. . 
Erepsin 
Inverting 
enzymes 
soaps. 
Converts trypsinogen into trypsin, 
which splits proteins into amino 
bodies. 
Converts caseinogen and peptones into 
simple amino bodies. 
One for each disaccharide, splitting 
them into monosaccharides. 
Bacteria 
Acting on 
carbohydrates 
Acting on 
(Both the last two enzymes are more 
plentiful in the epithelium than 
in the intestinal juice.) 
(1) Digest cellulose. 
(2) Splits monosaccharides into lactic 
and lower acids. 
fats 
Acting on 
proteins . ... 
Split higher, into lower fatty acids. 
Split off aromatic groups, as phenol, 
cresol, etc. 
(Besides these specific actions, bacteria 
may perform many of the diges- 
tive functions of the juices.) CHAPTER XVI. 
METABOLISM. 
The Energy Balance. 
Introductory.--The object of digestion, as we have seen, is 
to render the food capable of absorption into the circulatory 
fluids, the blood and lymph. The absorbed food products are 
then transported to the various organs and tissues of the body, 
where they may be either used or stored away against future 
requirements. After being used, certain substances are produced 
as waste products, and these pass back into the blood to be car- 
ried to the organs of excretion, by which they are expelled from 
the body. By comparison of the amount of these excretory prod- 
ucts with that of the constituents of food, we can tell how much 
of the latter has been retained in the body, or lost from it. This 
constitutes the subject of general metabolism. On the other 
hand, we may direct our attention, not to the balance between 
intake and output, but to the chemical changes through which 
each foodstuff must pass between its absorption and excretion. 
This is the subject of special metabolism. In the one case we 
content ourselves with a comparison of the raw material which 
is acquired and the finished product which is produced by the 
animal factory; in the other, we seek to learn something of the 
particular changes to which each crude product is subjected be- 
fore it can be used for the purpose of driving the machinery of 
life or of repairing the worn out parts of the body. 
In drawing up such a balance sheet of general metabolism, we 
must select for comparison substances which are common to both 
intake and output. In general the intake comprises, besides oxy- 
gen, the proteins, fats and carbohydrates, and the output, carbon 
dioxide, water and the various nitrogenous constituents of urine. 
This dissimilarity in chemical structure between the substances 
ingested and those excreted limits us, in balancing the one 
186 187 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
against the other, to a comparison of the smallest fragments 
into which each can be broken. Such fragments are the ele- 
ments, and of these carbon and nitrogen alone can be measured 
with accuracy in both intake and output. From the balance 
sheets of intake and output of carbon and nitrogen, and from 
information obtained by observing the ratio between the 
amounts of oxygen consumed by the animal and of carbonic 
acid (C02) excreted, we can draw far-reaching conclusions 
regarding the relative amounts of protein, fat and carbohydrate 
which have participated in the metabolism. 
As has already been stated, the essential nature of the meta- 
bolic process in animals is one of oxidation, that is, one by 
which large unstable molecules are broken down to those that 
are simple and stable. During this process of katabolism, as 
it is called, the potential energy locked away in the large mole- 
cules becomes liberated as actual or kinetic energy, which takes 
the form of movement and heat. It therefore becomes of im- 
portance to compare the actual energy which an animal ex- 
pends in a given time with the energy which has meanwhile 
been rendered available by metabolism. This is called the 
energy balance. We shall first of all consider this and then 
proceed to examine somewhat more in detail the material bal- 
ance of the body. 
Energy Balance. 
The unit of energy is the large calorie (written C), which is 
the amount of heat required to raise the temperature of one kilo- 
gramme of water through one degree (Centigrade) of tempera- 
ture.1 We can determine the caloric value by allowing a meas- 
ured quantity of a substance to burn in compressed oxygen in 
a steel bomb which is placed in a known volume of water at a 
certain temperature. Whenever combustion is completed, we 
ascertain the increase in temperature of the water in degrees 
(Centigrade), and mulitply this by the volume of water in 
iThe distinction between a calorie and a degree of temperature must be 
clearly understood. The former expresses quantity of actual heat energy; 
the latter merely tells us the intensity at Which the heat energy is being given 
out. 188 
THE ENERGY BALANCE. 
liters. Measured in such a calorimeter, as this apparatus is 
called, it has been found that the number of calories liberated 
by burning one gramme of each of the proximate principles of 
food is as follows: 
~ x Starch 4.1 
Carbohydrates _ 
Sugar 4.0 
Protein 5.0 
Fat 9.3 
The same number of calories will be liberated at whatever rate 
the combustion proceeds, provided it results in the same end 
products. When a substance, such as sugar or fat, is burned in 
the presence of oxygen, it yields carbon dioxide and water, which 
are also the end products of the metabolism of these foodstuffs 
in the animal body; therefore, when a gramme of sugar or fat 
is rapidly burned in a calorimeter, it releases the same amount 
of energy as when it is slowly oxidized in the animal body. But 
the case is different for proteins, because these yield less com- 
pletely oxidized end-products in the animal body than they yield 
when burned in oxygen; so that, to ascertain the physiological 
energy-value of protein, we must deduct from its physical heat 
value (calories) the physical heat-value of the incompletely ox- 
idized end-products of its metabolism. It is obvious that we can 
compute the total available energy of our diet by multiplying 
the quantity of each foodstuff by its caloric value. 
In order to measure the energy which is actually liberated in 
the animal body, we must also use a calorimeter, but of some- 
what different construction from that used by the chemist, for 
we have to provide for long continued observations and for an 
uninterrupted supply of oxygen to the animal. Animal calor- 
imeters are also usually provided with means for the measure- 
ment of the amounts of carbon dioxide (and water) discharged 
and of oxygen absorbed by the animal during the observation. 
Such respiration calorimeters have been made for all sorts of ani- 
mals, the most perfect for use on man having been constructed - 
in America (see Fig. 49). As illustrating the extreme accuracy 
of even the largest of these, it is interesting to note that the act- 
ual heat given out when a definite amount of alcohol or ether is FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
189 
burned in one of them exactly corresponds to the amount as meas- 
ured by the smaller bomb calorimeter. All of the energy liber- 
ated in the body does not, however, take the form of heat. A 
variable amount appears as mechanical work, so that to measur e 
in'calories all of the energy which an animal expends, one must 
add to the actual calories given out, the caloric equivalent of 
Fig. 49.-Diagram of Atwater-Benedict Respiration Calorimeter. As the 
animal uses up the O2, the total volume of air shrinks. This shrinkage is indi- 
cated by the meter, and a corresponding amount of O2 is delivered from the 
weighed O3-cylinder. The increase in weight of bottles II and III gives the 
CO2. 
the muscular work which has been performed by the animal 
during the period of observation. This can be measured by 
means of an ergometer, a calorie corresponding to 425 kilo- 
grammeters2 of work. That it has been possible to strike an 
accurate balance between the intake and the output of energy 
of the animal body, is one of the achievements of modern experi- 
mental biology. It can be done in the case of the human ani- 
2 A kilogrammeter is the product of the load in kilograms multiplied by 
the distance in meters through which it is lifted. 190 
THE ENERGY BALANCE. 
mal; thus, a man doing work on a bicycle ergometer in the Bene- 
dict calorimeter gave out as actual heat, 4,833 C., and did work 
equalling 602 C., giving a total of 5,435 C. By drawing up 
a balance sheet of his intake and output of food material during 
this period, it was found that the man had consumed an amount 
capable of yielding 5,459 C., which may be considered as ex- 
actly balancing the actual output. 
Having thus satisfied ourselves as to the extreme accuracy of 
the method for measuring energy output, we shall now consider 
some of the conditions which control it. To study these we must 
first of all determine the basal heat production, that is, the small- 
est energy output which is compatible with health. This is as- 
certained by allowing the man to sleep in the calorimeter and 
then measuring his calorie output while he is still resting in bed 
in the morning, and fifteen hours after the last meal. When 
the results thus obtained on a number of individuals are calcu- 
lated so as to represent the calorie output per kilogram of 
body weight in each case, it will be found that 1 C. per kilo per 
hour is discharged. That is to say, the total energy expenditure 
in 24 hours in a man of 70 kilos, which is a good average weight, 
will be 70 X 24 = 1,680 C. 
When food is taken the heat production rises, the increase over 
the basal heat production amounting, for an ordinary diet, to 
about ten per cent. Besides being the ultimate source of all the 
body heat, food is therefore a direct stimulant of heat production. 
This specific dynamic action, as it is called, is not, however, the 
same for all groups of foodstuffs, being greatest for proteins and 
least for carbohydrates. Thus, if a starving animal is given an 
amount of protein which is equal in caloric value to the calorie 
output during starvation, the calorie output will increase by 30 
per cent, whereas with carbohydrates it will increase only by 
6 per cent. Evidently, then, protein liberates much free heat 
during its assimilation in the animal body; it burns with a hot- 
ter flame than fats or carbohydrates, although, as in the case 
of fats, at least, before it is completely burnt, it may not yield so 
much energy. This peculiar property of proteins accounts for 
their well-known heating qualities. It explains why protein com- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
191 
poses so large a proportion of the diet of peoples living in cold 
regions, and why it is cut down in the diet of those who dwell 
near the tropics. Individuals maintained on a low protein diet 
may suffer intensely from the cold. 
If we add to the basal heat production of 1,680 C. another 
168 C. (or 10 per cent) on account of food, the total 1,848 C. 
nevertheless falls far short of that which we know must be liber- 
ated when we calculate the available energy of the diet. What 
becomes of the extra fuel? The answer is that it is used for 
muscular work. Thus it has been found that if the observed 
person, instead of lying down in the .calorimeter, is made to sit 
in a chair, the heat production is raised by 8 per cent, or if 
he performs such movements as would be necessary for ordinary 
work (writing at a desk), it may rise 29 per cent, that is to say, 
to 9'0 C. per hour. Allowing 8 hours for sleep and 16 hours for 
work, we can thus account for 2,168 C., the remaining 300 odd 
C. which is required to bring the total to that which we know, 
from statistical tables of the diets of such workers, to be the 
actual daily expenditure, being due to the exercise of walking. 
If the exercise be more strenuous, still more calories will be ex- 
pended; thus, to ascend a hill of 1,650 feet at the rate of 2.7 
miles an hour requires 407 extra calories. Field workers may 
expend, in 24 hours, almost twice as many calories as those en- 
gaged in sedentary occupations. 
Another factor which controls the energy output is the cool- 
ing influence of the atmosphere. When this is marked, more 
heat must be liberated in order to maintain the body temperature 
(see p. 132). In other words, the necessary heat loss must be 
compensated by an increased heat production, just as we must 
burn more coal to keep the house at a given temperature on a 
cold, than on a warm, day. This adjustment of energy liberation 
to the rate of cooling at the surface of the body explains, among 
other things, why it should be that small animals give out much 
more energy, per unit of body weight, than those that are larger. 
The small animal has relatively the greater surface area, just 
as two cubes of equal weight when brought together have a com- 
bined weight which is double that of either cube, but a surface 192 
THE ENERGY BALANCE. 
area which is less than double (two surfaces having been brought 
together). Greater tendency to surface cooling explains why 
small animals should so much more quickly succumb to cold 
than those that are larger, and why slim persons should feel 
the cold more keenly than those that are stout. 
Other things, such as diet, external temperature, etc., being 
the same, it is therefore surface area and not body weight which 
determines the energy production, a fact which is clearly dem- 
onstrated by finding that the calorie output for different animals 
is constant when it is calculated for each square meter of sur- 
face. Thus, a horse produces only 14.5 C. per kg. of body 
weight in 24 hours, whereas a mouse produces 452 C., but if 
we calculate according to square meter of surface the dif- 
ferences practically vanish. These facts, however, do not apply 
when the differences in size are due to age. This has been 
most strikingly demonstrated in the case of man, for it has been 
found that the calorie requirement per unit of surface is very 
distinctly greater in the early years of life than later. Thus, tak- 
ing the discharge of carbon dioxide as a criterion of the energy 
discharge, the following results have been obtained from indi- 
viduals sitting down: 
Carbon dioxide discharged, per 
Average age 
Average weight 
square 
meter of surface 
(years) 
. (kilograms) 
Wales 
and 
hour (grams) 
9 2/3 
28 
29.9 
12 1/2 
34 
26.5 
15 1/2 
51 
23.5 
19 1/2 
60 
21.8 
25 
68 
18.5 
35 
68 
16.9 
45 
77 
16.3 
58 
85 
Females 
14.2 
8 
22 
26.6 
12 
36 
t , 
20.1 
15 
49 
16.0 
17 2/3 
54 
14.8 
30 
54 
16.3 
45 
67 
17.9 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
193 
This table shows us clearly that over and above the greater 
combustion necessary on account of their relatively greater sur- 
face, children require calories for growth. They must be fed 
more liberally than adults, otherwise they starve. The table 
further shows that boys must be more liberally fed than girls of 
equal age and body weight, probably because of their greater 
restlessness. It is on account of these greater food requirements 
that children are the first to die in famine. 
Recent work has shown that the above conclusions are not 
strictly warranted by the facts, for there appear to be other 
factors than surface and mass of the body affecting the energy 
requirement of the growing organism. CHAPTER XVII. 
METABOLISM (Cont'd). 
The Material Balance of the Body. 
We must distinguish between the balances of the organic and 
the inorganic foodstuffs. From a study of the former we shall 
gain information regarding the sources of the energy production 
whose behavior under various conditions we have just studied. 
From a study of the inorganic balance, although we shall learn 
nothing regarding energy exchange-for such substances can 
yield no energy-we shall become acquainted with several facts 
of extreme importance in the maintenance of nutrition and 
growth. 
To draw up a balance sheet of organic intake and output re- 
quires an accurate chemical analysis of the food and of the 
excreta (urine and expired air). Furnished with such analyses 
we proceed to ascertain the total amount of nitrogen and carbon 
in the excreta in a given time and to calculate, from the known 
percentage of nitrogen in protein, how much protein must have 
undergone metabolism. We then compute how much carbon this 
quantity of protein would account for, and we deduct this from 
the total carbon excretion. The remainder of carbon must have 
come from the metabolism of fats and carbohydrates, and al- 
though we cannot tell exactly its source, yet we can arrive at a 
close approximation by observing the respiratory quotient (R. 
Q.), which is the ratio of the volume of carbon dioxide exhaled 
CO2 
to that of oxygen retained by the body in a given time, i. e., 
O2 
When carbohydrates are the only foodstuff undergoing metabol- 
ism, the quotient is one, that is to say, the CO2 excretion and O2 
intake are equal in volume. The reason for this is that a molecule 
of carbohydrate consists of C along with H and 0 in the same 
194 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
195 
proportions as they exist in water; therefore oxygen is required 
to oxidize the C., but not the H., and, since equimolecular quan- 
tities of all gases occupy equal volumes (at the same tempera- 
ture and pressure), the volume of CO2 produced equals the vol- 
ume of C. required to produce it. The conditions are other- 
wise in the case of fats and proteins, for besides C. these mole- 
cules contain an excess of H., so that 0. is required to oxidize 
some of the H., as well as all of the C. A greater volume of 02 
is therefore absorbed during their combustion than the volume 
of C02 that is produced, and R. Q. is about 0.7. By observing 
this quotient, therefore, we can approximately determine the 
source from which the non-protein carbon excretion is derived. 
Having in the above manner computed how much of each of the 
proximate principles has undergone metabolism, we next pro- 
ceed to compare intake and output with a view to finding 
whether there is an equilibrium between the two, or whether re- 
tention or loss is occurring. 
Starvation.-In order to furnish us with a standard condition 
with which we may compare others, we will first of all study the 
metabolism during starvation. When an animal is starved, it 
has to live on its own tissues, but in doing so, it saves its protein 
so that the excretion of nitrogen falls after a few days to a low 
level, the energy requirements being meanwhile supplied, as 
much as possible, from stored carbohydrate and fat. Although 
always small in comparison with fat, the stores of carbohydrate 
vary considerably in different animals. They are much larger 
in man and the herbivora than in the carnivora. During the 
first few days of starvation it is common, in the herbivora, to 
find that the excretion of nitrogen is actually greater than it was 
before starvation, because the custom has become established in 
the metabolism of these animals of using carbohydrates as the 
main fuel material, so that when this fuel is withheld, as in 
starvation, proteins are used more than before and the nitrogen 
excretion becomes greater. We may say that the herbivorous 
animal has become carnivorous. The same thing may occur in 
man when the previous diet was largely carbohydrate. 
During the greater part of starvation, however, most of the 196 
STARVATION. 
energy required to maintain life is derived from fat, as little 
as possible being derived from protein. This type of metabolism 
lasts until all the available resources of fat have become ex- 
hausted, when a more .extensive metabolism of protein sets in 
with the consequence that the nitrogen excretion rises. This is 
really the harbinger of death-it is often called the premortal 
rise in nitrogen excretion. It means that all the ordinary fuel 
of the animal economy has been used up, and that it has become 
necessary to burn the very tissues themselves in order to obtain 
sufficient energy to maintain life. Working capital being all 
exhausted, an attempt is made to keep things going for a little 
longer time by liquidation of permanent assets. But these assets, 
as represented by protein, are of little real value in yielding the 
desired energy because, as we have seen, only 4.1 calories are 
available against 9.3, obtainable from fats. These facts explain 
why during starvation a fat man excretes daily less nitrogen 
than a lean man, and why the fat man can stand the starvation 
for a longer time. 
Not only is there this general saving of protein during star- 
vation, but there is also a discriminate utilization of what has 
to be used by the different organs according to their relative 
activities. This is very clearly shown by comparison of the loss 
of weight which each organ undergoes during starvation. The 
heart and brain, which must be active if life is to be maintained, 
lose only about 3 per cent of their original weight, whereas the 
voluntary muscles, the liver and the spleen lose 31, 54 and 67 
per cent, respectively. No doubt some of this loss is to be ac- 
counted for as due to the disappearance of fat, but a sufficient 
remainder represents protein to make it plain that there must 
have been a mobilization of this substance from tissues where it 
was not absolutely necessary, such as the liver and voluntary 
muscles, to organs, such as the heart, in which energy transfor- 
mation is sine qua non of life. The vital organs live at the ex- 
pense of those whose functions are accessory. 
When we compare the excretion of carbon dioxide from day 
to day during starvation, it will be found to remain practically 
constant, when calculated for each kilogram of body weight. The FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
197 
same is true for the calorie output. Certain unusual substances 
such as creatin also make their appearance in the urine, and 
there is an increase in the excretion of ammonia, indicating that 
larger quantities of free acid are being set free in the organism. 
Starvation ends in death in an adult man in somewhat over 
four weeks, but much sooner in children, because of their more 
active metabolism. At the time of death the body weight may 
be reduced by 50 per cent. The body temperature does not 
change until within a few days of death, when it begins to fall, 
and it is undoubtedly true that if means be taken to prevent cool- 
ing of the animal at this stage, life will be prolonged. 
Normal Metabolism.-Apart from the practical importance 
of knowing something about the behavior of an animal during 
starvation, such knowledge is of great value since it furnishes a 
standard with which to compare the metabolism of animals under 
normal conditions. Taking again the nitrogen balance as indi- 
cating the extent of protein wear and tear in the body, let us 
consider first of all the conditions under which equilibrium may 
be regained. It would be quite natural to suppose that if an 
amount of protein containing the same amount of nitrogen as 
is excreted during starvation were given to a starving animal, 
the intake and output of nitrogen would balance. We are led 
to make this assumption because we know that any business bal- 
ance sheet showing an excess of expenditure over income could 
be met by such an adjustment. But it is a very different matter 
with the nitrogen balance sheet of the body; for, if we give the 
starving animal just enough protein to cover the nitrogen loss, 
we shall cause the excretion to rise to a total which is practically 
equal to the starvation amount plus all that we have given as 
food, and although by daily giving this amount of protein there 
may be a slight decline in the excretion, it will never come near 
to being the same as that of the intake. Such feeding will pro- 
long life for a few days only. 
To strike equilibrium we must give an amount of protein whose 
nitrogen content is at least two and one-half times that of the 
starvation level. For a few days following the establishment of 
this more liberal diet, the nitrogen excretion will be far in ex- 198 
NORMAL METABOLISM. 
cess of the income, but it will gradually decline until it corre- 
sponds to the intake. Having once gained an equilibrium, we 
may raise its level by gradually increasing the protein intake. 
During this progressive raising of the protein intake, it will be 
found, at least in the carnivora (cat and dog), that for a day 
or so immediately following each increase in protein intake, 
a certain amount of nitrogen is retained by the body. The ex- 
cretion of nitrogen, in other words, does not immediately be- 
come adjusted so as to correspond to the intake. The amount 
of nitrogen thus retained is too great to be accounted as. a re- 
tention of disintegration products of protein; it must there- 
fore be due to an actual building up of new protein tissue, that 
is, growth of muscles. 
Such results undoubtedly obtain in the cat, and less mark- 
edly in the dog. In man and the herbivorous animals, this is 
not the case, for in these we can never give a sufficiency of pro- 
tein alone to maintain nitrogen equilibrium; there will always 
be an excess of excretion over intake. But indeed it scarcely re- 
quires any experiment to prove this, for it is self-evident when 
we consider that there are only 400 C. in a pound of lean meat, 
and there are few who could eat more than 4 pounds a day, an 
amount which however would only furnish about half of the re- 
quired calories. A person fed exclusively on flesh is therefore 
being partly starved, although he may think that he is eating 
abundantly and be quite comfortable and active. This fact has 
a practical application in the so-called Baiting cure for obesity, 
which consists essentially in limiting the diet to flesh and green 
vegetables, allowing only a very small quota of carbohydrates 
or fats. 
Protein Sparers.-Very different results are obtained when 
carbohydrates or fats are freely given with the protein. Nitrogen 
equilibrium can then be regained on very much less protein; so 
we speak of fats and carbohydrates as being "protein sparers." 
Carbohydrates are much better protein sparers than fats; indeed 
they are so efficient in this regard that it is now believed that 
carbohydrates are essential for life, so that when the food con- 
tains no carbohydrates, a part of the carbon of protein is FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
199 
converted into this substance. This important truth is supported 
by evidence derived from other fields of investigation (e. g., the 
behavior of diabetic patients, where the power to use carbohy- 
drates is much depressed). The marked protein-sparing action 
of carbohydrates is illustrated in another way, namely, by the 
fact that we can greatly diminish the protein break-down during 
starvation by giving carbohydrates. In this way we can indeed 
reduce the daily nitrogen excretion to about one-third what it is 
in complete starvation. 
The Protein Minimum.-In the case of man living on an 
average diet, although the daily nitrogen excretion is about 15 
grams, it can be lowered to about 6 grams, provided that, 
in place of the protein that has been removed from the diet, 
enough carbohydrate is given to bring the total calories 
up to the normal daily requirement. If an excess of 
carbohydrate over these energy requirements be given, the 
protein may be still further reduced and yet equilibrium main- 
tained. To do this, however, it is not the amount of carbohy- 
drate alone that determines the ease with which the irreducible 
protein minimum can be reached; the kind of protein itself makes 
a very great difference. This has been very beautifully shown 
by one investigator, who first of all, determined his nitrogen ex- 
cretion while living on nothing but starch and sugar, and then 
proceeded to see how little of differnt kinds of protein he had 
to take in order to bring himself into nitrogenous equilibrium. 
He found that he had to take the following amounts: 30 gm. meat 
protein, 31 gm. milk protein, 34 gm. rice protein, 38 gm. potato 
protein, 54 gm. bean protein, 76 gm. bread protein, and 102 gm. 
Indian corn protein. The organism is evidently able to satisfy 
its protein demands when it takes meat protein much more 
readily than with vegetable proteins. 
To understand why proteins should vary so much in their 
nutritive value, we must examine their ultimate structure very 
closely. When the protein molecule is disintegrated, as by diges- 
tion, it yields a great number of nitrogen-containing acids, the 
amino acids, as well as several bases and aromatic substances. 
The most important of these acids are glycin, alanin, serin, valin, 200 
NORMAL METABOLISM. 
leucin, prolin, aspartic and glutamic acids, the bases being lysin, 
histidin and arginin and the aromatic bodies, phenylalanin, tyro- 
sin and tryptophan. These substances constitute the available 
"units" or "building stones" of protein molecules, but in no 
two proteins are the materials used exactly in the same propor- 
tions, some proteins having a preponderance of one or more and 
an absence of others, just as in a row of houses there may be no 
two that are exactly alike, although for all of them the same 
building materials were available. Albumin and globulin are 
the most important proteins of blood and tissues, so that the 
food must contain the necessary units for their construction. If 
it fails in this regard, even to the extent of lacking only one of 
them, the organism will either be unable to construct that pro- 
tein, and will therefore suffer from partial starvation, or it will 
have to construct for itself this missing unit, a process which it 
can accomplish for some but not all of the units. 
It is therefore apparent that those proteins are most valu- 
able as foods that contain an array of units which can be reunited 
to form all the varieties of protein entering into the structure 
of the body proteins. Naturally, the protein which most nearly 
meets the requirement is meat protein, so that we are not sur- 
prised to find that less of it than of any other protein has to be 
taken to gain nitrogen equilibrium. Casein, the protein of milk, 
although it does not contain one of the most important units, 
namely, glycin, is almost as good as meat protein, because the 
organism is itself able to manufacture glycin. When, on the 
contrary, proteins (such as zein from corn) are given, in which 
certain units are missing, starvation inevitably ensues. But it 
does not do so if the missing units, (which in the case of zein is 
tryptophan) are added to the diet. 
These most important facts have been ascertained by experi- 
ments carried out in New Haven by Osborne and Mendel. 
Young albino rats, just weaned, were fed on a basal diet con- 
sisting of the sugar, fat and salts of milk to which was added 
the protein whose nutrition value it was desired to study. The 
rats were weighed from day to day, and the results plotted 
as a curve-the curve of growth. A gradually rising curve FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
201 
was obtained when casein or the albumin of milk or eggs, or 
the edestin of hemp seed, or the glutenin of wheat was fed, 
but this was not the case with the gliadin of wheat or, as 
above mentioned, with zein of coin. It will be seen, there- 
fore, that of the two proteins in wheat one, glutenin, contains 
all the necessary units for building up the growing tissues, but 
that in the other protein, gliadin, some essential unit is absent; 
by analysis this was found to be lysin. By adding lysin. to 
gliadin a normal curve of growth resulted, thus showing that 
this was really the missing unit. The result was made even more 
spectacular by feeding a batch of young rats on gliadin alone, 
so that they remained undeveloped and stunted, and then adding 
lysin to their diet, when they very quickly made up for lost time, 
and soon reached, if not quite, yet almost as good a development 
as their more fortunate brothers who had been fed on glutenin 
or casein from the first. 
The animal economy itself can therefore produce certain of 
the amino bodies-thus, as we have seen, it can produce glycin- 
this power being much more developed, in the case of herbivor- 
ous, as compared with carnivorous animals. In the vegetable 
food on which oxen live, several of the prominent amino bodies 
of muscle protein are missing, but they are constructed in the 
organism by altering the arrangement of the molecules of those 
amino bodies which are present, so that a protein is built up 
which is very like that present in the tissue of the carnivorous 
animals. Even in the case of the herbivora, however, there are 
limitations to the power of forming new amino bodies. Trypto- 
phan, for example7 cannot be formed in this way. THE SCIENCE OF DIETETICS. 
CHAPTER XVIII. 
In order that a proper assortment of amino bodies may be 
assured in the' diet, protein is taken in excess of the quan- 
tity necessary to repair the tissues. It has been thought 
by some that the surplus thus taken by the average indi- 
vidual is much more than need be, and that an unnecessary strain 
is thus thrown on the organs which have to dispose of the excess. 
It has been claimed by the adherents of this view that many of 
the obscure symptoms-headaches, muscular and back pains, 
sleepiness, etc.-that city folk are liable to suffer from, are due 
to the presence in the blood of unnecessary by-products of ex- 
cessive protein metabolism. Such opinions seemed to receive 
very weighty indorsement some years ago when Chittenden pub- 
lished a long series of observations showing that men in various 
callings in life, could perform their daily work quite satisfac- 
torily and apparently maintain their health after reducing the 
protein of their diets to less than half of the usual amount. No 
direct benefit could be claimed for this reduction except that 
some of the men believed that they felt better and fitter and 
more inclined for work, an improvement which admits of no 
quantitative measurement because of the psychological elements 
involved. Although these observations were conducted with 
all the care and accuracy of the highly trained scientist, they 
have been considered quite inadequate to justify the claim that 
man takes too much protein. The observations have, neverthe- 
less, been of immense value in compelling a careful review of 
the evidence that the proportion of protein which habit has pre- 
scribed as being the proper one for us to take, is really the most 
suitable for our daily needs. 
There are, however, differences in the protein content of the 
diet according to the race and environment. This has been as- 
202 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
203 
certained by compiling the stardard diet for a community, that 
is, measuring the exact quantities of protein and carbohydrate 
in the diets which the people are accustomed to live on, and aver- 
aging the results. One remarkable outcome of such statistical 
work has been to show that for peoples living under approxi- 
mately the same conditions as regards climate and amount of 
daily muscular work, the average daily requirement of calories, 
carbon and nitrogen works out pretty much the same, although 
there may be some diversity in the proportions of protein and 
carbohydrate. The following table shows this: 
Type of individuals. Protein 
Fat 
gm. 
Carbo. 
gm. 
Total Cal. 
C. 
gm. 
N. 
gm. 
gm. 
Average workman in 
Germany, 20 years age. 
118 
56 
500 
3,045 
328 
18.8 
German soldier in the 
field 
151 
46 
522 
3,190 
340 
24 
British soldier in peace... 
133 
115 
429 
3,400 
21.3 
Russian soldier in war (Man- 
churian campaign) . ... 
187 
27 
775 
4,900 
30 
Professional man 
100 
100 
240 
2,324 
230 
16 
Such figures can be compiled with tolerable accuracy because 
the diet is under control. It is of course more difficult to collect 
sufficiently accurate data regarding the diets of civilians, but it 
is safe to say that the average city dweller in temperate zones 
derives his daily requirement of 15 gm. nitrogen in 95 gm. 
of protein, which also yields 60 gm. of the required 250 gm. car- 
bon. This deficit he might supply either from fats or carbohy- 
drates, the actual proportion depending on availability and price. 
It should be particularly noted that the proportion of protein is 
very much increased whenever strenuous muscular work has to 
be performed. Now the question is, do such statistical studies 
substantiate Chittenden's claim that the protein which we are 
accustomed to consume could profitably be reduced? They cer- 
tainly do not. Let us for a moment consider the health condition 
and physical development of communities such as the Bengalis 
of Lower Bengal, who live largely on rice and take only a little 
less in the way of protein than the amount Chittenden would 204 
DIETETICS. 
have us take. Their body weight, chest measurement and muscular 
development are distinctly inferior to those of the natives of 
Eastern Bengal, who, nevertheless, belong to the same race as 
the lower Bengalis, but differ from them in taking more protein 
in their food. Not only this, but the lower Bengalis are in every 
sense of the word half starved, and are very prone to disease, 
especially of the kidneys, the very type of disease to which we 
are told excessive protein consumption must predispose. Dia- 
betes is also very prevalent amongst these people, probably be- 
cause of the enormous quantities of sugar-yielding food (car- 
bohydrates) which they are compelled to eat in order to pro- 
vide sufficient calories for life. Mentally, they are a very in- 
ferior race. This, then, is an experiment on a much grander 
scale than Chittenden's, and what of the results? It is for- 
tunate that most of Chittenden's subjects "through force of 
circumstances" have returned to their old dietetic habits. 
Exactly concordant results have been obtained when attempts 
have been made to reduce the protein in the dietaries of public 
institutions such as prisons, alms houses, etc. There has invari- 
ably been a distinct increase in the sick list, especially of such 
diseases as pneumonia, tuberculosis, etc. And if we seek for 
evidence of an opposite nature, we do not find that excessive 
protein ingestion is fraught with any evil consequences to the 
community. Thus the Eskimo takes five times more protein than 
the Bengali and two and one-half times more than the European, 
yet he is peculiarly free from "uric acid" diseases; and his 
physical endurance and his power of withstanding cold are ex- 
traordinary. 
There are a great many secondary factors, such as availability, 
taste, etc., that determine the average diet of a community, but 
the main determining factors are instinct and experience. In 
the struggle for supremacy of one race over another, we may 
assume that adequacy of diet has been a determining factor, 
and that the average which is taken usually represents that 
which conduces to the greatest efficiency. 
We have dealt at some length on these questions because of 
their great practical importance, and because they show us that FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
205 
in the matter of the protein content of our diet, as in that of all 
other animal functions, there comes into play the principle of 
the "factor of safety." We have two lungs, although it is quite 
possible to live with one only, two kidneys, although one will 
usually suffice; and so with our food; we could get along for 
some time with about half of the protein which we take, but at 
the constant risk of a deficiency, for should physical exhaustion 
occur, a reserve of building stones ought to be available to re- 
store the tissue which has been c6nsumed. Instead of the excess 
of protein throwing a strain on the organism, the contrary is 
the case, for it is indisputably a greater strain for the tissues to 
have to construct new building stones than to use those sup- 
plied ready made in the food. 
Another deduction which we may draw from these observa- 
tions is that more protein should be taken when its source is 
mainly vegetable food than when it is animal. On the other 
hand, there is nothing to indicate that one kind of animal pro- 
tein possesses any advantages over another; flesh protein, milk 
protein, egg protein are practically of equal dietetic value, and 
with regard to which varieties of meats-whether light or dark 
-are most nutritious, all we can say is that any differences that 
may be thought to exist are not due to differences in the chemical 
nature of the proteins which they contain, but depend on their 
flavor and digestibility. There are more fads and fancies about 
which meats are nutritious and which are not so than would fill 
a volume, but after all the whole question is one of flavor. Man 
digests best what he likes best, and he thrives best when digestion 
is good. A sound knowledge of the principles of dietetics is 
of no less importance for the pharmacist than the physician; but 
there are no simple rules by which the most suitable diet for 
each individual can be prescribed. Many factors besides the 
nutritive values of the food must be considered, and the old 
adage should never be forgotten, that "one man's food is an- 
other man's poison." 
Very practical conclusions may be drawn from these observa- 
tions regarding the most suitable diet for the city dweller. It is 
evident that we are now-a-days in possession of a sufficient 206 
DIETETICS. 
amount of scientific information regarding both the daily require- 
ments of the body and the ability of the various foodstuffs to 
fulfill these requirements, to compute, from the market prices of 
foods, how much it should take per diem for an individual, or a 
family of individuals, to live healthfully and economically. The 
day will surely come when, through the medium of schools and 
the press, everyone will know what we may call the fundamentals 
of dietetics, namely: (1) that a man of sedentary occupation 
(the ordinary city clerk) requires daily 2,-600 calories, and a 
laboring man, at least 3,000 calories. (2) That at least 5 per 
cent of the calories should be provided in protein food of animal 
origin (meats, milk) with 10 per cent or more as other protein 
(bread, oatmeal, etc.). 
To enable the housewife to purvey the necessary food to meet 
these requirements, she must therefore become familiar with the 
caloric value and the percentage of protein in the different 
classes of protein foods, and of the caloric values of other great 
staples of diet. Canned foods will no doubt some day have 
printed on the label: '1 This can contains .... calories, of which 
.... per cent are in proteins of grade " And this is no 
utopian idea; it is practical common sense. The adoption of 
such a scheme is far more likely to be the solution of the problem 
of the high cost of living than anything else, for, indeed, it is not 
so much the high cost of living as it is the cost of high living 
that troubles us. We demand business efficiency in our manufac- 
turing organizations, and yet we are inclined to ridicule as im- 
practical any attempts at nutritive efficiency in the animal organ- 
ization which is our own body. Not only the principles of 
dietetics, but the details as well are now so thoroughly under- 
stood that their application in the feeding of the masses is only 
a matter of education. Dietery impostures of the meanest de- 
scription, often hiding behind a "bluff" of scientific knowledge, 
are of course the most serious enemies we shall have to face in 
spreading the knowledge. It will be the duty of physicians, of 
pharmacists, and of the educated classes to offset this commercial 
brigandage by spreading the gospel of food efficiency. 
As illustrating the food efficiency, in relationship to cost we Whole milk. 
Skim milk. 
Cream. 
Cheese. 
Rutter. 
Egg. 
Average meat (raw) 
Average mutton (raw) 
Average pork (raw) 
Fish-flounder (raw) 
Bacon. 
Wheat bread 
I Oats. 
Rice. 
Ash and water. 
Protein of 2nd Quality. 
Protein of 1st Quality. 
Eat. 
Carbohydrate. 
Calories. 
Plate II-Dietetic chart, showing the percentage amounts of the various 
proximate principles (indicated by the shaded areas) and the calories (indi- 
cated in red) yielded by burning 1 lb. of the commoner foodstuffs. The num- 
bers to the left represent the caloric values and the names to the right, the 
food in question. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
207 
may take the following table from the menu of a well-known 
restaurant company: 
Cost 
Calories 
Calories 
Cost 
in cents 
per portion 
Total 
% in 
protein 
for 5 cents 
in cents 
per 1000 
calories 
Bread 
.. 5 
933 
12 
933 
5 
Apple pie 
Boston pork 
5 
343 
5 
337 
15 
and beans .... 
.. 15 
868 
12 
276 
18 
Ham sandwich . 
5 
212 
20 
198 
30 
Corn beef hash. 
.. 15 
538 
14 
170 
30 
Beef stew 
.. 15 
641 
25 
199 
32 
Club sandwich . 
...' 25 
438 
20 
82 
61 
Sliced pineapple 
5 
36 
8 
36 
138 
Mayonaise 
.. 20 
53 
16 
13 
35 
(Lusk) 
The above table is not by any means from a cheap restaurant. 
By economy and judicious purchasing it is possible even in New 
York to purchase, for 8 cents, 1,000 calories having the proper 
proportion of protein, so that a working man may easily cover 
his dietetic requirements for 25 cents a day, exclusive of the 
cost of cooking. All he spends above this is for personal taste 
and relish. 
Chemistry of the Commoner Foodstuffs. 
The accompanying diagram (Plate II) indicates the composi- 
tion of some of the commoner foods and is self-explanatory. There 
are certain foodstuffs concerning which a little more detail may 
however be advisable. 
Wheat Flour, besides a large amount of starch, contains two 
proteins, glutein and gliadin. When the flour is mixed with 
water and then kneaded, it forms dough, because the proteins 
change into a sticky substance called gluten. As dough the flour 
is not a suitable food, because the digestive juices cannot pene- 
trate it. To render it digestible the dough must be made porous 
and this is accomplished by causing bubbles of carbon dioxide 
gas to develop in it, either by mixing it with baking powder 
which is composed of a bicarbonate and an organic acid (tar- 208 
DIETETICS. 
taric) or by keeping it in a warm place with yeast, which fer- 
ments the sugar that is present. The sugar is developed from 
the starch by the action of the diastase (see p. 154) present in 
the yeast. 
When the yeast has been allowed to act for some time, or if 
baking powder was used, when the gas formation has ceased, suit- 
able portions (loaves) of dough are placed in the oven. The heat 
causes the inclosed bubbles of gas to expand so that the whole 
mass becomes aerated and further increase of temperature acts 
on the proteins and starches on the surface coagulating the for- 
mer and converting the latter into dextrins. The crust is thus 
formed. Brown bread is made from wheat from which all the 
husk has not been removed. There are two possible advantages 
of this over white bread, namely, the husks act as a mild laxative 
and they seem to contain traces of vitamines (see p. 224). 
Other Cereals.-These include maize or Indian corn, oatmeal 
and rice, and differ from wheat in that their proteins do not form 
gluten when mixed with water. They cannot therefore be formed 
into bread unless they be mixed with some wheat flour. They are 
relatively rich in ash, and maize contains a large proportion of 
fat. When rice composes a large proportion of the diet, as is the 
case in tropical countries, the unpolished variety should be used 
to supply the vitamines. When the diet is a mixed one, however, 
danger of an insufficiency of vitamines cannot exist. As has 
been already explained, the protein of cereals is not of first qual- 
ity, because it does not contain all of the amino acids (building 
stones) of tissue proteins. 
Milk and Milk Preparations.-Whole milk is as nearly as 
possible a perfect food, for its protein is of the first quality and 
it contains a sufficiency of fats and carbohydrates for the growth 
of the tissues. Where muscular exercise must also be performed, 
carbohydrates should be added to the milk, and this is best ac- 
complished by the use of cereals. Milk is an economical food, 
for one quart nearly equals in nutritive value a pound of steak 
or eight or nine eggs, and is easily digested and assimulated, but 
somewhat constipating. The chief protein of milk is caseinogen 
(phospho protein) and is characterized by being precipitated FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
209 
by weak acids and by the action of gastric juice. When milk sours 
some of the milk sugar, or lactose, becomes converted by bacterial 
action into lactic acid and this precipitates caseinogen. When 
an extract of the mucous membrane of the stomach is added to 
milk and the mixture kept warm, the clot which forms is called 
casein. By separating the casein and allowing it to stand for 
some time ferments, derived from moulds and bacteria, act on 
it to produce cheese. The cheese, besides casein, contains much 
fat and mineral matter. Cheddar cheese is especially rich in fat. 
Cheese is a very concentrated article of diet and when taken in 
moderation is thoroughly digested and assimilated. 
Cream consists of the milk fats with some of the constituents 
of milk. It is the most easily assimilated of all the fats and is 
hence very nutritious. When sweetened, flavored and frozen it 
forms ice cream, which should not be regarded, as it usually is, 
as a luxury, but as a highly nutritious food. It should not there- 
fore surprise the indulgent parent when a child refuses food 
after visiting the corner pharmacy. On standing, cream ripens 
(undergoes change due to bacterial growth), and the fat can 
be made to separate as butter. There is no foodstuff that con- 
tains more calories than butter, and it also contains certain vi- 
tamines. The fluid from which the butter separates, butter- 
milk, contains practically no fat and is acid to the taste because 
of bacterial action on the lactose producing lactic acid. Its in- 
fluence on the nature of bacterial growth in the intestines has 
already been referred to. 
Eggs.-The only point we need emphasize is the much 
greater percentage of fat substances (lipoids) in the yolk than 
in the white. One dozen eggs equals in food value two pounds 
of meat. Eggs are therefore more costly than milk. 
Meats.-The building stones of the protein molecule of meat, 
for reasons which are obvious, are more nearly identical with 
those of the tissues of man than are those of any other food. The 
carbohydrate is however insufficient in amount, for which rea- 
son we take potatoes with meat. The flavors of different meats 
depend largely on the extractive substances which they contain. 
These include creatin and purin substances. When a decoction 210 
DIETETICS. 
of meat is evaporated to small bulk, after precipitating all of the 
protein, meat extract is prepared, which, like coffee or tea, has 
no nutritive value but acts as a mild stimulant (caffein and thein 
are chemically very closely related to the purin bodies of meat 
extract). Clear soups are mainly dilute solutions of meat ex- 
tractives, but in beef tea, if properly made, there is much 
meat protein. 
Other Foods and Condiments.-Although green vegetables 
and salads consist very largely of water, they are very important 
articles of diet, because they contain cellulose, which serves to 
increase the bulk of the intestinal contents-to serve as ballast, 
as it were-and prevent constipation by keeping the intestinal 
musculature active. Some vegetables, such as spinach, are 
especially important since they contain iron. Salads have a 
further importance because of the oil taken with them. The rel- 
ishes and the condiment flavors are by no means insignificant 
adjuncts of diet, for they give the relish to food without which 
digestion is likely to be inefficient. This most important prop- 
erty of diet has been sufficiently insisted upon elsewhere. CHAPTER XIX. 
SPECIAL METABOLISM. 
But we must now return to the more theoretical aspects of our 
subject. We will proceed to trace out very briefly the interme- 
diary stages in metabolism through which proteins, fats and car- 
bohydrates have to pass in order to yield the energy required 
to drive the animal machine and to supply material with which 
to repair the broken-down tissues. 
Metabolism of Proteins.-We must follow the amino acids 
after their absorption into the blood until they ultimately reap- 
pear, the nitrogen among the nitrogenous constituents of urine 
and the carbon as part of the carbon dioxide of expired air. In 
order to do this it is necessary for us to become familiar with 
the nature and source of the urinary substances which contain 
nitrogen, and to consider some of the most important chemical 
relationships of these substances, so that we may understand how 
they become formed in the body. The substances in question are: 
urea, ammonia, creatinin, the purin bodies, and undetermined 
nitrogenous substances. Urea and ammonia may be considered 
together. 
Urea and Ammonia.-There is no doubt that it is as ammonia 
that the nitrogen of the amino acids is set free in the organism. 
The free ammonia would, however, be highly poisonous, so that 
it immediately becomes combined with acid substances to form 
harmless neutral salts. The acid which is ordinarily used for 
this purpose is carbonic, of which there is always plenty in the 
blood and tissue juices. The ammonium carbonate thus formed 
becomes changed into urea by removal of the elements of water 
from the molecule, thus: 
OH ONH4 NH2 NH2 
2NH3 + Cq/ = co - h2o = co^ - h2o = co 
OH ONH4 ONH4 \'H2 
Ammonia Carbonic Ammonium Ammonium Urea 
acid carbonate carbamate 
211 212 
METABOLISM OF PROTEINS. 
The conversion of ammonium carbonate occurs largely in the 
liver. Our evidence for this is: (1) If solutions containing 
ammonium carbonate be made to circulate through an excised 
liver, urea is formed. (2) If this organ be seriously damaged, 
either experimentally or by disease, less urea and more ammonia 
a'ppear in the urine. We see therefore that urea is formed in 
order to prevent the poisonous action of ammonia. But the am- 
monia may be more usefully employed; instead of being com- 
bined with carbonic acid in order that it may be got rid of, it 
may be employed to neutralize, and thus render harmless, any 
other acids that make their appearance. Thus, it may be em- 
ployed to neutralize the acids which sometimes result during the 
metabolism of fat, as in the disease diabetes; or the lactic acid 
that appears in the muscles during strenuous muscular exercise; 
or the acids produced on account of inadequate oxygenation. 
Taking acids by the mouth has a similar effect; thus the am- 
monia excretion rises after drinking solutions containing weak 
mineral acids. 
Ammonia is, of course, not the only alkali which is available 
in the organism for the purpose of neutralizing acids. The 
fixed alkalies, sodium and potassium, are also used. Thus, when 
we greatly increase the proportion of these, as by taking alkaline 
drinks, or by eating vegetable foods, the ammonia excretion 
diminishes. 
Urea is an inert substance, capable of uniting with acids to 
form unstable salts (urea nitrate and oxalate), and like other 
amino acids, being decomposed by nitrous acid so as to yield 
free nitrogen. This latter reaction is used for the quantitative 
estimation of urea, the evolved nitrogen being proportional to 
the amount of urea, thus: 
NH2 
CC)/ + 2 HNO2 = 2 CO2 + 2 N2 + 2 H2O 
^NH2 
Certain bacteria are capable of causing urea to take up 2 mole- 
cules of water so as to form ammonium carbonate, a process FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
213 
really the reverse of that which occurs in the organism and rep- 
resented by the above formulae. This change occurs in urine and 
accounts for the ammoniacal odor which develops when this fluid 
is allowed to stand. 
Creatinin.-This is very closely related to creatin, which is the 
most abundant extractive substance in muscle, and which yields 
urea when it is boiled with weak alkali. These chemical facts 
would lead us to expect that some relationship must exist be- 
tween the creatin of muscle and the creatinin and urea of urine, 
but, so far, it has been impossible to show what this relationship 
is. One very important fact has, however, been brought to light, 
namely, that creatin makes its appearance in the urine when 
carbohydrate substances are not being oxidized in the body, as 
in starvation, and in the disease diabetes. This is one reason for 
the growing belief that carbohydrates are something more than 
mere energy materials (see p. 216). The excretion of creatinin 
is so remarkably independent of the amount of protein in the 
food that it is believed to represent more especially the end prod- 
uct of the protein break-down of the tissues themselves, in con- 
trast to urea, which partly represents the cast-off nitrogen of the 
protein of the food. 
Purin Bodies.-These are of particular interest because they 
include uric acid, about which more nonsense has been written 
than about any other product of animal metabolism. The so- 
called uric acid diathesis is very largely a medical myth-a cloak 
for ignorance. Uric acid is the end oxidation product of the 
purin bodies, which include the hypoxanthin and xanthin of 
muscle and their amino derivatives, the adenin and guanin of 
nuclein. 
These relationships are seen in the following formulae: 
Oxy purins of muscle 
Hypoxanthin C5H4N4O 
Xanthin C5H4N4O2 
Amino purins of nuclein. . 
Adenin C5H4N4NH 
Guanin C6H4N4ONH 
Uric acid C5H4N4O, 
There are therefore two sources for uric acid in the animal 214 
PROTEIN METABOLISM. 
body, namely, the muscles and the nuclei of the cells. This ex- 
plains why the uric acid excretion increases after strenuous mus- 
cular work, and why it is much above the normal when cellular 
break-down is very excessive, as in the disease called leucocythe- 
mia, in which there is an excess of leucocytes in the blood (see 
p. 55). Another source of uric acid is the food when it con- 
tains either muscle (flesh) or glands (sweetbreads), for a large 
proportion (about half) of the ingested purins do not become 
destroyed in their passage through the organism, but become 
oxidized to uric acid, which is excreted in the urine. This is 
called the exogenous in contrast to purin produced in the tissues, 
which is called endogenous. 
There is only a trace of uric acid in the urine of mammals, but 
in birds and reptiles most of the nitrogen is present in this form. 
The reason is that in these animals it is important to have semi- 
solid, instead of fluid excreta, so that the urea which results from 
protein metabolism becomes converted into uric acid, which, 
either free or as salts, is relatively insoluble. Uric acid is chemi- 
cally a diureide, that is to say, it consists of two urea molecules 
linked together by a chain of carbon atoms. The chain of carbon 
atoms is furnished by substances not unlike lactic acid and the 
synthesis occurs in the liver. If this organ be removed from the 
circulation in birds, such as geese, in which the operation is 
comparatively easy, a very large part of the uric acid in the urine 
becomes replaced by ammonium lactate. 
The relative insolubility of uric acid and its salts, which we 
have already referred to, makes it apt to become precipitated in 
urine, especially on standing. It forms the orange reddish de- 
posit, so frequently observed in summer, when on account of per- 
spiration the urine does not contain as much water as usual. 
Such deposits do not therefore indicate that there is an excess of 
uric acid in the blood, but merely that enough water is not being 
excreted to dissolve the usual amount of urates. Sometimes the 
urate becomes deposited in the joint cartilages, particularly in 
those of the great toe, causing local swelling and redness and 
great pain. This is gout, and it may be most effectually treated 
by drinking large quantities of alkaline fluids, and eliminating FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
215 
from the dietary such foodstuffs as meats and sweetbreads, which 
yield exogenous purins. As we have said, there is no reason to 
believe that any other diseases besides gout are due to an ex- 
cess of uric acid in the blood. 
Besides the above there are traces of other nitrogenous sub- 
stances in the urine, such as: 
1. Hippuric acid, which, as its name signifies, is very abun- 
dant in the urine of the horse and other herbivora, and which is 
the excretory product of the aromatic substances which the food 
of these animals contains. 
2. Cystin, an amino acid containing sulphur. 
3. Pigments and mucin. 
The exact significance of the end products of nitrogenous met- 
abolism has been very beautifully demonstrated by Folin, of 
Harvard. The observations were made on several men who lived 
for some days on a diet rich in protein (but containing no purin- 
containing foodstuffs), and then on one which was very poor in 
protein. The problem was to see how each of the nitrogenous 
constituents behaved during the two periods, both absolutely 
and in relation to the total amount of nitrogen excreted. In or- 
der to show the latter relationship the results are given, as in the 
following table, not as urea, etc., but as urea-nitrogen, etc.: 
On the protein-rich 
diet 
On the protein- 
poor diet 
Quantity of urine 
. .1170 c. c. 
385 c. c. 
Total nitrogen 
16.8 gm. 
3.6 gm. 
Urea-nitrogen 
14.7 gm. (87.5) 
2.2 gm. (61.7) 
Ammonia-nitrogen 
0.49 gm. (3.0) 
0.42 gm. (11.3) 
Uric acid-nitrogen 
0.18 gm. (1.1) 
0.09 gm. (2.5) 
Creatinin-nitrogen 
0.58 gm. (3.6) 
0.60 gm. (17.2) 
Undetermined nitrogen. 
0.85 gm. (4.9) 
0.27 gm. (7.3) 
The figures in parentheses represent the percentage which the 
nitrogen of each substance furnishes of the total amount of nitro- 
gen excreted. It will be seen that urea decreases on the poor 
diet relatively more than total nitrogen, thus indicating that it 
comes partly from proteins in the food (exogenous) and partly 216 
PR()TEIN METAB()LISM. 
from the organism itself (endogenous). This result leads us to 
infer that most of the amino substances of protein foods which 
are not required as building stones for the tissues are broken 
down so as to yield ammonia, which is excreted as exogenous urea 
in the urine, but that the amino acids that are really appropri- 
ated by the tissues, although they may also produce some urea 
(endogenous), cause other end-products to be formed. The most 
important of these endogenous bodies is evidently creatinin, for, 
as will be seen from the above table, this substance is excreted in 
the same absolute amount during both the starvation and the 
protein-rich periods. 
Direct evidence that this conclusion is correct has been ob- 
tained by examination of the blood and muscles for amino bodies, 
ammonia and urea. The results have shown that the amino 
acids absorbed from the intestine are carried through the liver 
into the systemic blood, which transports them to the muscles, 
where those that are not required for building up the tissues 
are broken down into ammonia and a carbonaceous residue, which 
is then burned just exactly as if it were carbohydrate or fat. 
The useless ammonia becomes converted into urea in the manner 
already described, either in the muscles themselves, or by being 
carried to the liver, which, as we have seen, possesses to a very 
high degree the power of producing urea. 
The Relative Importance of Proteins, Fats and Carbohy- 
drates in Metabolism.-The metabolism of fats and carbohy- 
drates, with regard both to their importance as builders of living 
tissues and the type of their metabolism, is very different from 
that of proteins. That carbohydrates and fats are less impor- 
tant in the animal economy than proteins is evidenced by the 
fact that we can live perfectly well on protein food alone, but 
not on either of the others. This does not, however, justify us 
in concluding that carbohydrates and fats are merely materials 
which are oxidized by the tissues for the purpose of producing 
energy, fuel as it were, and which can be dispensed with. They 
are more than this, for nd* cell, in however starved a condition 
it may be, is entirely free from either of them, thus indicating 
that they must have been produced out of protein itself. Pro- teins are no doubt the most important ingredients of cells, but 
fats and carbohydrates are indispensable also. 
As reserve materials, striking differences exist among the 
three foodstuffs. Proteins are of little value in this regard for, as 
we have seen, very little, if any, can become laid down in the 
tissues when excess is taken as food; on the contrary, all that is 
not required is thrown out of the body, and when the food sup- 
ply is cut off, as in starvation, the protein is spared as much as 
possible (see p. 195). Carbohydrates are very readily depos- 
ited as a starch-like substance, called glycogen, and this reserve 
is the first to be called on, not only in starvation, but also when 
muscular work is performed. It may be considered as the most 
immediately available material for combustion in the organism, 
but the limits of its storage are restricted in man to some hun- 
dreds of grams, which, as we have seen, soon become used 
up in starvation. Fat is pre-eminently the storage material, and 
the supply may serve in man to furnish, along with a little pro- 
tein, enough fuel for several weeks' existence. 
The relative importance of the three foodstuffs is shown in the 
extent to which each is used in the metabolism during muscular 
exercise. When there is an abundant store of glycogen, the 
energy is entirely derived from this source; when there is little 
glycogen but much fat, it is fat that is burned, and when neither 
of these is abundant but much protein is being taken with the 
food, or the animal is reduced to living on its own tissues, as in 
starvation, it is protein. In other words, the type of metabolism 
occurring during muscular work is the same as that which imme- 
diately preceded it; the only change is in the extent of the com- 
bustion, not in the nature of the fuel employed. 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
217 CHAPTER XX 
SPECIAL METABOLISM (Cont'd). 
Metabolism of Fats.--Fats are absorbed by the lacteals and 
discharged into the blood of the left subclavian vein through 
the thoracic duct. They are carried to various parts of the body 
and gain entry into the cells, in the protoplasm of which they 
become deposited. This process occurs extensively in the sub- 
cutaneous connective tissues, between the muscles, and retroperi- 
toneally around the kidney (the suet). The fat which is thus 
deposited possesses more or less the same qualities as the fat of 
the food. Thus, when the only fat taken over a long period of 
time is one with a very low melting-point, such as oil, the fat 
deposited in the tissues is likely to be oily in character, whereas 
it is stiff after feeding with a high melting-point fat, such as 
mutton fat. This similarity between the tissue fat and that of 
the food becomes very striking when the animal has been sub- 
jected to a preliminary period of starvation and then fed for 
some weeks with a large excess of the particular fat and as little 
carbohydrate and protein as possible. Fat in the food is of 
course not the only source of the fat in the tissues. It is also 
formed out of carbohydrates, a fact which is well known to 
farmers, who fatten their stock by feeding them with maize 
and other starchy grains, and to physicians, who reduce their 
corpulent patients by restricting carbohydrate foods. The fat 
thus deposited has the chemical characteristics of the fat which 
is peculiar to that animal. It is almost certain that there is ordi- 
narily no formation of fat out of protein in the higher animals. 
The fat thus deposited in the tissues may remain for a long 
time, but ultimately it is again taken up by the blood and car- 
ried to whatever active tissue requires it as fuel. Before being 
thus burnt, it splits into glycerine and fat acid (see p. 178). The 
fat acid possibly undergoes some preliminary change in the 
218 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
219 
liver; in any case, the long chain of carbon atoms of which we 
have seen the fat molecule to be composed (see p. 36) becomes 
oxidized (burnt), not all at once but piece by piece, two carbon 
atoms being split off at a time. If the fat acid chain originally 
contained an even number of carbon atoms, the oxidation 
process may stop short when there are yet four carbon 
atoms in the chain, thus producing oxybutyric acid 
(CH3CHOHCH2COOH). This imperfect metabolism of fat oc- 
curs in severe cases of diabetes and often causes death. It also 
occurs in carbohydrate starvation, and indicates, more clearly 
than any thing else, that even carbohydrates are essential for life. 
Metabolism of Carbohydrates.-It will be remembered that 
these include the starches and the sugars, and that during diges- 
tion they are all hydrolyzed to dextrose or hevulose, as which 
they are absorbed into the blood of the portal vein. This ab- 
sorption is rapid, so that a striking increase in the percentage 
of sugar occurs in the blood of the portal vein shortly after the 
food has been taken. Most of this excess of sugar does not imme- 
diately gain entry to the blood of the systemic circulation, how- 
ever, because it is retained by the liver. For this purpose the 
liver cells convert the sugar into the starch-like substance, glyco- 
gen, which becomes deposited in their protoplasm as irregular 
colloidal masses, which stain with iodine and carmine. The liver 
does not manage in this way to remove all of the excess of sugar 
from the portal blood, so that, even in a healthy animal, there 
is a distinct postprandial increase of sugar, or hyperglycaemia, as 
it is called, in the systemic blood. If too much sugar passes the 
liver it causes so marked a postprandial hyperglycaemia that 
some sugar escapes into the urine, thus causing glycosuria. This 
is one of the early symptoms of diabetes, and its occurrence 
furnishes us with a warning that less carbohydrates should be 
given in the food. If the warning be heeded, the severer form 
of the disease will very probably be staved off. 
The glycogen deposited in the liver stays there until the per- 
centage of sugar in the systemic blood begins to fall below the 
normal level (which in man is about 0.1 per cent), when it 
becomes reconverted into sugar, which is added to the blood. 220 
METABOLISM OF CARBOHYDRATES. 
The reason why the sugar in the systemic blood tends to fall is 
that the tissues, especially the muscles, are using it up as fuel. 
If so much sugar is taken* that the storage capacity of the liver 
is overstepped, the excess of sugar is carried by the systemic 
blood to the tissues, where much of it may be changed into fat. 
The glycogenic function of the liver, as the above process is 
called, is analogous to the starch-forming function of many 
plants, such as potatoes. Of the sugar which is formed in the 
green leaves of these plants, some is immediately used for build- 
ing up other substances, the remainder being converted into 
starch, which becomes deposited in the roots, etc., until it is 
required (as during the second year's growth), when it is grad- 
ually reconverted into sugar. 
Besides carbohydrates it is known that proteins form glyco- 
gen; fats, however, cannot form it. In severe cases of diabetes 
it is therefore usual to find that although carbohydrate foods 
are entirely withheld, dextrose continues to be eliminated in the 
urine. It may come partly from the protein of the food and 
partly from that of the tissues. 
The adjustment between the rate at which the glycogen of the 
liver becomes converted into dextrose and the percentage of 
sugar in the systemic blood is effected partly through the nervous 
system and partly by means of substances called chemical mes- 
sengers of hormones (see p. 227) secreted into the blood from the 
ductless glands, such as the pancreas and the adrenals. The 
very first symptoms of diabetes, which we have seen to consist in 
an excessive postprandial rise in the systemic blood-sugar and a 
consequent glycosuria, must therefore be due to defects in one 
or other of these regulatory mechanisms. It is therefore of great 
interest to know that glycosuria can be induced in the lower 
animals by stimulation of the nerves of the liver or by interfer- 
ing with the function of the pancreas or the adrenal glands. The 
nerves of the liver may be stimulated either directly or through 
a nerve center located in the medulla oblongata (see p. 258). 
Complete removal of the pancreas is followed in a few hours 
by a very acute form of diabetes, which is invariably fatal in a 
few weeks, whatever the treatment may be. Injection of extract FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
221 
of the adrenal gland (adrenalin) causes a transient hyperglycas- 
mia and glycosuria. 
These laboratory discoveries have in their turn caused clinical 
investigators to pay close attention to the nature of the causes 
of diabetes. It has been found, as a result, that oft-repeated 
overstimulation of the nervous system-nerve strain, as it is 
called-greatly predisposes to this disease. For example, it has 
been found that a considerable proportion of students who un- 
derwent a severe examination for a university degree had sugar 
in the urine which was passed immediately after leaving the 
examination room. Even more interesting was the observation 
of that of a number of men waiting on the side lines as reserves 
in one of the large football games, about one-half of them passed 
sugar, due to nervous excitation of the glycogenic function. Be- 
sides these types of nerve strain, nervous glycosuria may also be 
brought on by fright and terror. This has perhaps been most 
definitely shown by frightening a tom-cat by allowing a dog to 
bark at it; the cat shortly afterward passed urine containing 
much sugar. Now, whereas occasional attacks of such nervous 
glycosuria are harmless, yet their repeated occurrence undoubt- 
edly weakens the ability of the liver to control properly the per- 
centage of sugar in the blood, with the consequence that post- 
prandial hyperglycamiia becomes more and more marked and 
takes longer to disappear, so that there comes to be a permanent 
increase in the percentage of sugar in the blood. This persistent 
excess of sugar acts as a poison and causes deterioration of many 
of the tissues, and if unchecked will lead to severe diabetes. 
It is for these reasons that diabetes is relatively common 
amongst locomotive engineers and ship captains; it is also 
said to be distinctly on the increase amongst business men. A 
most important element in the treatment of diabetes is therefore 
removal of the possible causes of nerve strain. Rest and quiet 
and freedom from worry, coupled with removal of sufficient 
amounts of carbohydrates from the diet so as to keep the urine 
free of sugar, is the correct treatment. One common symptom 
of diabetes is loosening of the teeth. When this is observed the 
urine passed an hour or so after lunch should be examined for 222 
METABOLISM OF INORGANIC SALTS. 
sugar. Properly conducted treatment will often cause the teeth 
to tighten up again. 
A very common cause of death in diabetes is coma, which is 
due to the poisoning of the animal by acid substances (oxy- 
butyric acid) resulting from the imperfect oxidation of fat 
(see p. 219). While these acid substances are gradually accumu- 
lating in the blood, the organism attempts to neutralize them by 
diverting ammonia from its normal course into urea (see p. 211) ; 
hence the ammonia content in the urine is very high in severe 
cases of diabetes. Along with these acids and ammonia, acetone 
also appears in the urine and breath, so that one can often diag 
nose a severe.case of diabetes by the smell of these substances 
in the breath. Diabetes is therefore a disease which the dentist 
should always be on the lookout for. 
Metabolism of the Inorganic Salts.-Being already com- 
pletely oxidized, inorganic salts cannot yield any energy during 
their passage through the animal body but nevertheless they are 
essential to life. They are used not only for the building up of 
bones and teeth, but also for the proper carrying out of the 
metabolic processes. In this respect they are like the lubricant 
of a piece of machinery, the organic foodstuffs being like the fuel. 
Their indispensability is very clearly shown by the fact that 
animals die sooner when they are fed on food from which all 
traces of inorganic salts have been extracted than when they are 
deprived of food altogether. This result shows us that during 
the metabolism of organic foods substances must be produced 
which act as poisons in the absence of inorganic salts. Some of 
these poisonous substances are no doubt acid in reaction because 
life can be prolonged for some time by merely adding sodium 
carbonate to the salt-free food. But salts not having any 
neutralizing powers are also necessary to keep the animal 
alive. 
The chief salts which we take with our food are the chlorides, 
carbonates and organic acid salts (e. g., citrates, tartarates, etc.) 
of sodium and potassium and of calcium. We also take some iron 
and traces of iodine. All of these are already present in suffi- 
cient amount in the ordinary foodstuffs, except sodium chloride, FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
223 
or common salt. This we must add to our food. The extent to 
which the addition of common salt is made varies very strikingly 
according to the nature of the organic food. When this is 
mainly vegetable in origin, much common salt is required, the 
reason being apparently that vegetables contain large quantities 
of potassium salts which would be harmful unless a proper pro- 
portion of sodium is also taken. The demand for sodium by 
herbivorous animals often inclines these to wander for hundreds 
of miles from their feeding grounds to salt licks. Here they take 
enough sodium chloride to last them for some time. The carniv- 
orous animals do not visit salt licks unless it be for the purpose 
of preying on the herbivorous visitors. The salt hunger from 
which they suffer compels the herbivora to go to the salt licks 
even in the face of this danger of destruction by the carnivora. 
The same relationship between the desire for salt and the diet 
is seen in man, the salt consumption per capita being much 
greater in rural than in urban communities. 
Usually enough iron is taken either in meats or in certain vege- 
tables, as spinach. The body is very careful of its supply of 
iron (which is the most important constituent of haemoglobin), 
but if it loses it more quickly than the loss can be made good 
from the food, anemia results and it becomes necessary to pre- 
scribe iron salts as medicine. 
Similarly with calcium, there is usually enough in the food 
even of growing animals to meet the demands which bone and 
teeth formation entails. Rickets is not usually due to a defi- 
ciency of calcium in the food, but to a depraved condition of the 
general nutrition, making it impossible for the available calcium 
to be properly used. Good food, air and exercise, rather than 
drugs, is the correct treatment for rickets. 
Our knowledge of just what each particular inorganic salt does 
in the metabolism of an animal is not yet very far developed, but 
some most important discoveries have been made in this connec- 
tion during recent years. Thus, by observing the isolated beat- 
ing heart of the frog or turtle it has been found that a certain 
proportion of sodium, calcium and potassium salts is essential 
to the maintenance of a proper beat. With sodium chloride 224 
VITAMINES. 
alone the beat soon stops, with excess of potassium an immediate 
paralysis occurs, and with excess of calcium an immediate rigor 
or permanent contraction. Analogous results are obtained with 
other muscles. Salts in certain proportions may even cause 
processes of cell division to start in the ova of some of the lower 
animals. In other words, a process of embryo development 
which is usually induced by impregnation by the male 
elements may be made to start by the action of salts. 
Vitamines.-Another class of bodies called vitamines is of 
great importance as adjuncts of diet. Without them metabolism 
becomes upset, and serious symptoms make their appearance with 
perhaps death as the ultimate result; and this happens even al- 
though the protein, fat, carbohydrate and inorganic salts of the 
diet be in proper proportion. The first indication of the import- 
ance of vitamines was furnished by observations on a disease 
called Beri-Beri, which occurs among peoples of tropical coun- 
tries, and is- characterized by severe neuralgic pains, muscular 
weakness and paralysis; symptoms which are due to inflamma- 
tion of the nerves (neuritis). It was noted that it occurred most 
frequently in the case of people whose main article of diet was 
polished rice, but was infrequent in the case of those using the 
unpolished grain. The difference between these two grades of 
rice is that the one (the unpolished) still contains some of the 
brownish husk; the other is free of it. This observation suggested 
the experiment of adding some of the ground-up rice husks to the 
polished rice diet of those suffering from the disease, with the 
result that the symptoms soon disappeared. Moreover, when 
unpolished rice was supplied, in place of polished rice, to natives 
among whom Beri-Beri was very prevalent, the disease disap- 
peared entirely. Other foodstuffs contain this vitamine, so that 
Beri-Beri does not occur with mixed diets. 
In order to learn something more about these remarkable sub- 
stances it was necessary to seek for some animal in which symp- 
toms similar to those of Beri-Beri could be induced by feeding 
with polished rice. Pigeons were found most suitable. When 
these birds are kept exclusively on such a diet, they develop the 
most alarming symptoms of neuritis (paralysis, weakness, etc.), FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
225 
which however disappear in a few hours, not only when unpol- 
ished rice or rice polishings (or husks) are given, but also when 
meat, or beans, or a small piece of yeast is mixed with the rice. 
Attempts have naturally been made to isolate the substance 
which is responsible for this remarkable action, and indeed some 
success can already be reported. For example, it has been pos- 
sible to separate from rice polishings and from yeast small traces 
of crystalline substances having a most powerful action in pre- 
venting neuritis. 
Even such success in investigating the cause of Beri-Beri in 
rice-feeders would scarcely warrant us in asserting that vita- 
mines are essential constituents of our own varied diets. To show 
that they are, however, has been no very difficult task. Thus, it 
is known that although young rats thrive admirably on milk diet, 
they fail to do so on one of artificial milk, that is, of milk made 
in the laboratory by mixing together, in proper proportions, the 
same proteins, fats, carbohydrates and salts that occur in milk. 
In this chemical mixture, something is wanting which exists only 
when the ingredients of milk are compounded by the mammary 
glands. The addition to synthetic milk of desiccated milk from 
which most of the proteins had been removed bestowed on it full 
nutritive value. 
The practical importance of this observation in the feeding of 
infants, we need not insist on. Suffice it to say that it is quite 
possible that prolonged boiling of milk, as for its sterilization, 
may deprive it of vitamines and thus render the child liable to 
such diseases as rickets and infantile scurvy, or at least interfere 
materially with its proper development and growth. Among the 
symptoms thus produced, especially in the case of infantile 
scurvy, ulcers may develop on the gums, or the teeth may become 
loosened. Change of diet may in a few days restore perfect 
health, or even the addition of a few teaspoonfuls of orange or 
lemon juice to the original diet may suffice. It is often miracu- 
lous how quickly such treatment may change a fretful, pain- 
stricken child to one of perfect health and cheerfulness. 
Innumerable other examples of the wonderful influence of 
these mysterious vitamines in nutrition might be cited. The 226 
VITA MINES. 
practical point to bear in mind is that, however correctly our 
diet may be composed with regard to caloric and chemical re- 
quirements, it is likely to be unsuitable unless it contains a cer- 
tain, though perhaps extremely minute, amount of the drug-like 
substances called vitamines. CHAPTER XXI. 
THE DUCTLESS GLANDS. 
Introductory.-We have no more than touched the very 
fringe of the subject of metabolism, and yet we have learned 
enough to impress us with the fact that although the chemical 
processes occurring in the body are extremely complicated, they 
are nevertheless under perfect control. We must now learn 
something regarding the nature of this control. 
If we take such a metabolic process as that which carbohy- 
drates undergo, we should expect that the conditions which deter- 
mine whether glycogen shall be formed or broken down would 
be chemical in nature. We should expect, in other words, that 
some change in the chemical composition of the blood-either its 
reaction or the amount of sugar in it, or the appearance in it of 
some decomposition product of sugar-would determine whether 
or not glycogen should be mobilized as sugar. In muscular work, 
for example, sugar is required by the contracting muscles, and 
we find that the glycogen stores in the liver become very quickly 
depleted to meet the demand. The question is, how do the mus- 
cles transmit their requirements to the liver so as to cause this 
organ to mobilize the dextrose ? Our natural assumption would 
be that the active muscles cause some change to occur in the 
blood and that it is this change which excites the liver cells. 
Such a control of the metabolic activities of one tissue by prod- 
ucts of the activity of another, transmitted between them by 
way of the blood, is known as hormone control. We have already 
become acquainted with it in connection with the control of cer- 
tain of the digestive glands, particularly the pancreas (see 
p. 175), and it is no doubt very largely by such a mechanism 
that a given metabolic process becomes active or supressed, as 
occasion demands. 
227 228 
THE DUCTLESS GLANDS. 
The hormones in such cases are in part the intermediary prod- 
ucts of metabolism, but besides these hormones others must exist 
to call forth or regulate the activities of tissues which are not 
immediately concerned in general metabolism but rather with 
special processes, such as the excitability of the nervous system 
(e. g., adrenalin), the behavior of the reproductive glands (e. g., 
in the secretion of milk), the growth of certain tissues (e. g., of 
subcutaneous tissues, of hairs) or the atrophy of others, (e. g., 
of the uterus after pregnancy is terminated). For such hor- 
mones, special manufacturing centres are provided in the duct- 
less glands. The thyroid and thymus glands in the neck, the 
pituitary in the brain, the spleen and adrenal glands in the ab- 
domen are good examples. None of these has any duct, but they 
discharge the products of their activity-internal secretion- 
into the blood stream, by which it is carried to the tissue or organ 
on which it acts. Internal secretions may also be produced by 
certain cells of the digestive glands, as, for example, the so-called 
Isles of Langerhans of the pancreas, and likewise there are cer- 
tain organs, such as the ovaries and testes, whose main functions 
are of a special nature, but which also possess the power of pro- 
ducing very powerful internal secretions. • 
We shall confine our attentions for the present, however, to 
the strictly ductless glands. Their function is ascertained ex- 
perimentally either by removing the gland by operation or by in- 
jecting an extract of it and then observing the behavior of the 
animal. Much can also be learned by observing patients in whom 
the gland is diseased. 
The Thyroid and Parathyroid Glands.-The thyroid gland 
consists of two oval lobes situated one on either side of the 
trachea just below the larynx or voice box, and connected to- 
gether over the trachea by an isthmus of thyroid tissue. Em- 
bedded in the substance of each lobe of the gland on the poste- 
rior surface are the two very small parathyroid glands. Minute 
examination shows the thyroid glands to be composed of vesicles 
lined by low columnar epithelium and filled with a clear glossy 
substance called colloid. The parathyroids have an entirely dif- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
229 
ferent structure, being composed of elongated groups of poly- 
hedral cells with no colloid material. 
The functions of the two glands are probably essentially dif- 
ferent, the thyroid having to do with the general nutrition of the 
Fig. 50.--The thyroid gland. (Gray's Anatomy, after Spalteholz.) 
animal, and the parathyroid with the condition of the nervous 
system. They lie so close together, however, that it is very diffi- 
cult to study their separate functions. The importance of the 
glands is indicated by the relatively large blood supply. 230 
THE DUCTLESS GLANDS. 
When the thyroid is not properly developed in children, the 
condition is known as cretinism (Fig. 51). The child fails to 
grow in height, although its bones may thicken. The cranial 
bones soon fuse together, so that the growth of the brain is hin- 
Fig. 51.-Cretin, 19 years old. The treatment with thyroid extract was 
started too late to be of benefit. (Patient of<Dr. S. J. Webster.) 
dered and the mental powers fail to develop. The child becomes 
idiotic, and although it may live for years, it will remain, even 
at thirty years of age, a stunted, pot-bellied, ugly creature with 
the intelligence of an infant. The cause of this failure to de- 231 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
velop is undoubtedly bound up in some way with the deficiency 
of the thyroid, for if the cretin be given the extract of this gland, 
its condition will immediately improve, and indeed, if taken 
early enough, it may quickly make up for lost time and grow 
both physically and mentally as it ought to. 
Atrophy of the thyroid gland in older persons causes myxoe- 
dema. (Fig. 52). The symptoms of this are very charaeteris- 
A. 
B. 
Fig. 52.-A, Case of myxoedema; B, Same after seven months' treatment. 
(Tigerstedt.) 
tic, being most commonly seen in women. The skin is dry 
and often of a yellowish color, the hair falls out, the subcutaneous 
tissues grow excessively, so that the hands, the feet and the face 
become large and puffy, and the speech indistinct, because of the 
thickening of the lips. The metabolism also becomes very slug- 
gish, so that the intake of food and the excretion of nitrogen in 
the urine become diminished, and the temperature subnormal. If 
unchecked, mental symptoms become apparent, first of all, a 
dulling of the intellect with sleepiness and lethargy, and later, 
muscular twitchings and tremors. Just as in cretinism, so in 232 
THE THYROID GLAND. 
myxoedema, administration of thyroid extract causes these symp- 
toms to disappear, so that in a month or so the patient may have 
returned to his or her normal condition, to maintain which, how- 
ever, the thyroid extract must continue to be given. 
When the gland is removed surgically, either in lower animals 
or in man, very acute symptoms ending in death usually super- 
vene. These include a peculiar form of muscular tremor called 
tetany, passing into actual convulsions, which, by involving the 
respiratory muscles, ultimately cause dyspnoea and death. It 
is, however, probable that these nervous symptoms are due to the 
unavoidable removal of the parathyroid glands. The tetany is 
removed by giving calcium salts. These conditions associated 
with deficiency of the thyroid are grouped together as hypothy- 
roidism. 
Even in healthy individuals thyroid extract taken by mouth 
excites a more active- metabolism, and may cause increased heart 
activity. One result of this increased metabolism is disappear- 
ance of subcutaneous fat and increased appetite, thus rendering 
the administration of moderate doses of thyroid extract a not 
uncommon method of treatment for obesity. Such treatment 
should never be attempted except under the control of a physi- 
cian, for it is very easy to take too much of the extract and cause 
palpitation and nervous excitement. 
When the thyroid (and parathyroid) glands become excess- 
ively active in man, the condition is called hyperthyroidism, and 
the symptoms are very like those above described as produced 
by taking thyroid extract. To be exact, they are palpitation 
wasting of the muscles and consequent weakness, extreme ner- 
vousness and protrusion of the eyeballs. On account of this last 
mentioned symptom the condition is usually called exophthalmic 
goitre. This acute and often fatal disease is to be distinguished 
from chronic goitre, in which there are very few general symp- 
toms, but great enlargement of the thyroid gland, indeed an en- 
largement which may be so pronounced as practically to obliter- 
ate the neck and sometimes so compress the trachea as to inter- 
fere with breathing. The cases of chronic goitre occur in the 
same districts in which the exophthalmic variety is common, these FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
233 
being, in this country, the shores of the great inland lakes and 
the river valleys, but not in districts bordering on the sea. They 
are also common in certain districts in Switzerland and Eng- 
land. It is of interest that in the lake and river districts in 
this country the thyroids of over ninety per cent of all dogs are 
more or less hypertrophied. 
The above remarkable influence of the thyroids on metabolism 
is in some way dependent upon the colloid material which fills 
the vesicles of the gland. This colloid contains a peculiar sub- 
stance called iodothyrin, because it contains iodine, an element 
which is not found present in any other part of the animal body. 
The Adrenal Glands.-As their name signifies, these are situ- 
ated one on either side just above the kidneys. Each gland is 
yellowish in color, and is seen on microscopic examination to be 
composed of a medullary and a cortical portion. The medulla 
consists of irregular collections of cells containing granules 
which stain deeply brown with chromic acid and are therefore 
called chromophile granules. Similar chromophile granules may 
exist in other parts of the body. The great splanchnic nerve, 
which it will be remembered arises from the sympathetic chain 
in the thorax (see p. 282), makes very intimate connection with 
the adrenal medulla, for which reason and because of the fact 
that it is developed from the same embryonic tissue as the sym- 
pathetic system of nerves, the medulla of the adrenal gland is 
believed to be closely bound up with the functions of the sympa- 
thetic nervous system. The cortex is composed of rows of col 
unjnar cells which do not contain chromophile granules. Small 
though they be, the adrenal glands are essential to life, for their 
removal causes extreme muscular weakness and a fall in blood 
pressure followed by death within twenty-four hours. When 
they are the seat of disease (tuberculous), symptoms of extreme 
muscular prostration, accompanied by vomiting and a peculiar 
bronzing of the skin, set in and grow steadily worse until at last 
the patient succumbs. This is called Addison's disease. 
The most striking proof of their importance is obtained by in- 
jecting an extract of the medulla of the adrenal gland into a 
vein. It causes an immediate rise in blood pressure, which is 234 
THE ADRENAL GLANDS. 
more or less proportional to the strength of the extract. The 
rise is accompanied by a slowing of the heart, due to the reflex 
stimulation of the vagus centre excited by the rising blood pres- 
sure. When this reflex slowing is rendered impossible by cutting 
the vagi, the rise in blood pressure following the injection may 
be enormous. The active substance in the extract is called adren- 
alin, suprarenin, adrenin or epinephrin. It is a comparatively 
simple chemical body, having the formula: 
CH 
/\ x 
(HO) C C-CH (OH) CH2-NHCH3, 
(HO)C CH 
^CH 
and existing in two varieties which differ from one another ac- 
cording to the direction toward which the plane of polarized light 
is rotated. The variety rotating to the left is, by many times, 
stronger in its physiological actions than that which rotates to 
the right. The discovery of its chemical structure has made it 
possible for chemists to prepare suprarenin synthetically, and 
also to prepare a series of related substances having less marked 
though similar properties. These are closely related to certain 
of the bpdies which appear during the putrefaction of meat. 
By careful studies of the action of the suprarenin, or related 
substances, it has been found that the rise in blood pressure, above 
referred to, is due to stimulation of the muscle fibers in the walls 
of the blood vessels. It is on this account that a weak solution of 
suprarenin is used to stop haemorrhage, as after removing polypi 
from the nose, or in bleeding from the gums, as after tooth ex- 
traction. The muscle of arteries is by no means the only struc- 
ture on which adrenalin acts; indeed it stimulates every structure 
which is capable of being stimulated by the sympathetic nervous 
system (see p. 282). Thus, it causes the pupil to dilate, saliva 
to be secreted (p. 151), the movements of the intestine to be in- 
hibited (p. 182), whereas it has no action on the.blood vessels FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
235 
of the lungs or brain, which do not possess vasomotor nerves. 
This similarity between the results which follow suprarenin in- 
jection and stimulation of the sympathetic system is particularly 
significant when we call to mind the fact that the medulla of the 
adrenal gland is developed from the same embryonic tissue as 
the sympathetic system. The clotting power of the blood is 
diminished after injections of suprarenin. 
The Pituitary Gland.-This occupies the Sella Turcica of the 
base of the cranium and is composed of three portions or lobes. 
The anterior lobe consists of large epithelial cells and is really 
Fig. 53.-Median sagittal section through pituitary of monkey; semidia- 
grammatic (Herring): a, Optic chiasma; b, third ventricle; c, g, pars in- 
termedia ; d, epithelium of pars intermedia extending round neck of pars 
nervosa; e, pars glandularis seu epithelialis; f, intraglandular cleft, lying 
between pars glandularis (e) and pars intermedia (g) ; h, pars nervosa. 
(Howell's Physiology.) 
an isolated outgrowth from the epiblast of the upper end of the 
alimentary canal. Its complete excision causes death in a few 
days, but if only a part is removed, a condition called hypo- 
pituitarism develops, of which adiposity and sexual impotence 236 
THE PITUITARY GLAND. 
are the main symptoms. When this lobe becomes excessively 
active in man (because of hypertrophy), it causes a peculiar 
growth of the bones, particularly of the lower jaw, thus making 
the person look as if he were very powerful. This disease is 
called acromegaly (Fig. 54), and besides the changes in the 
bones, there is frequently considerable metabolic disturbance, 
causing a mild form of diabetes. When the hypertrophy of the 
anterior lobe occurs in youth, most of the bones of the body may 
be affected, thus causing the condition known as giantism. 
The intermediary lobe is also composed of columns of epithe- 
lial cells, but there is often some colloidal material between the 
Fig. 54.-A, To show the appearance before the onset of acromegalic symp- 
toms : B, The appearance after seventeen years of the disease. (Aftei 
Campbell Geddes.) 
columns. This colloid differs from that of the thyroid in con- 
taining no iodine. 
The posterior lobe is really a downgrowth from the brain, and 
is composed of neuroglia mixed with some of the epithelial cells 
of the intermediary lobe. This lobe can be excised without caus- 
ing any evident change in the animal, but nevertheless it must FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
237 
have some important functions to perform, because extracts of it, 
when injected intravenously, have very pronounced effects, viz.: 
(1) a rise in blood pressure; (2) a very striking diuretic action 
(i. e., causes urine to be excreted) ; (3) secretion of milk. The 
active principle of these extracts has not as yet been isolated, 
although the extracts can be considerably concentrated, thus 
yielding the trade preparation called pituitrin. 
It is particularly interesting to note that although the anterior 
lobe does not yield any active extract, yet its excision is fatal. 
On the other hand, the posterior lobe can be removed with im- 
punity, although extracts of it have profound physiological 
effects when they are injected into normal animals. 
The Spleen.-Notwithstanding the fact that this is the larg- 
est of the ductless glands, it is the one whose functions are the 
least well understood. It can be excised without causing any 
evident disturbance, and extracts of it when injected intraven- 
ously do not have any characteristic effects. It becomes very 
much enlarged in certain diseases, namely: (1) in leucocythe- 
mia, a form of anaemia, which is characterized by a great increase 
in*the leucocytes of the blood (see p. 55) ; (2) in typhoid fever 
(enteric fever) ; (3) in malaria. It becomes contracted after 
taking quinine. Under the microscope it is seen to be composed 
of a sponge of fibrous tissue, the spaces being filled with blood, 
which flows freely into them from arterioles in whose walls 
lymphoid tissue is abundant. Here and there, this lymphoid 
tissue becomes collected in nodules, which are large enough to 
be seen by the naked eye and are called Malpighian corpuscles. 
In the blood of the spleen, partly broken down erythrocytes 
are often visible. Sometimes, also, cells like those found in red 
bone marrow and having to do with the manufacture of new red 
corpuscles make their appearance. 
Taking all these facts together, it is believed that the spleen 
has the following functions: (1) manufacture of leucocytes; 
(2) manufacture of erythrocytes; (3) destruction of erythro- 
cytes; (4) removal from the blood of certain poisons. 
The Thymus Gland.-The thymus gland, situated at the root 
of the neck, is quite large at birth, but its size gradually dimin- 238 
THE THYMUS GLAND. 
ishes as the animal grows. By the time that puberty is reached, 
it has almost disappeared. It is composed of peculiarly arranged 
lymphoid tissue, having nests of epithelial cells embedded in it. 
It seems to bear some relationship to the generative glands, for 
its removal in young male animals hastens the growth of the 
testes. CHAPTER XXII. 
THE FLUID EXCRETIONS. 
The Excretion of Urine. 
The Composition of the Urine.-The waste substance result- 
ing from the processes of metabolism in the tissues are eliminated 
from the body in a gaseous, fluid, or solid state. With the excep- 
tion of the carbon dioxide and water of the expired air, and cer- 
tain substances which are excreted into the intestines or appear 
in the secretions of the skin glands, the metabolic products are 
eliminated in the urine. 
The composition of the urine is therefore rather complex and 
varies greatly with the nature of the food and the amount of 
water taken. By careful analysis of the urine from a number of 
individuals on ordinary diet, the average amount of the various 
constituents in what may be considered a normal urine can be 
estimated. Normal human urine is a clear yellow fluid, a little 
heavier than water, having a specific gravity of 1.016 to 1.02. If 
tested with litmus paper it usually shows an acid reaction, 
mainly due to the presence of acid salts, such as sodium dihy- 
drogen phosphates, but partly also to acid substances derived 
from proteins. Herbivorous animals secrete an alkaline urine, 
which is no doubt caused by the presence of the large amount of 
alkaline earths and the relatively small amount of protein mat- 
ter in their diet. Human urine becomes alkaline in reaction 
when vegetables are the main ingredients of the diet. 
The character of most of the urinary constituents and the man- 
ner by which they are derived from the foodstuffs have been de- 
scribed in the chapter on metabolism, and in the following ac- 
count only a brief review of their physical and chemical nature 
is necessary. 
The Organic Substances of the Urine.-These comprise a 
number of nitrogenous compounds. The following figures, ob- 
239 240 
THE URINE. 
tained from the results of the analysis of a number of normal 
average urines, show how the nitrogen is distributed among these 
compounds. 
Urea 85 to 90% 
Ammonia 2 to 4% 
Creatinin 3% 
Uric acid 1 to 2% 
Unclassified nitrogen 5 to 6% 
Urea.-From the above figures it is seen that the greater part 
of the nitrogen eliminated by man appears as urea. The relative 
amount of urea eliminated depends very largely on the diet, be- 
ing 90 per cent or more of the total nitrogen excretion on a full 
protein diet, and 60 per cent or less during starvation. The total 
amount excreted is about 30 grams per 100 grams of protein in 
the diet. 
Chemically urea has the following formula: 
nh2 
oc^ 
nh2 
If prepared pure it forms long colorless needles or four-sided 
prisms. It is very soluble in water. Hot alkalies, such as sodium 
hydroxide, decompose it into ammonia and carbon dioxide. The 
same reaction occurs in case of bacterial decomposition by the 
micrococcus urea, and accounts for the ammoniacal odor of urine 
after standing in the air. The significance of urea in regard to 
protein metabolism and the method of its formation are dis- 
cussed on page 211. 
Ammonia.-This, combined with chlorine or other acid radi- 
cles, is normally found in small amounts in the urine. It is one 
of the important agencies in maintaining the neutrality of the 
tissues, since with acids it forms ammonia salts, which are neu- 
tral in reaction and which are eliminated in the urine. 
Creatinin.-The amount of this substance found in the urine FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
241 
is very constant from day to day, and is independent of the diet. 
It is largely a product of the metabolism of the body tissues. 
Uric Acid.-Uric acid is a purine body and its relationship 
to the other purines, and its mode of formation and significance 
are fully discussed in the chapter on metabolism (p. 213). It is 
relatively insoluble in water, and when allowed to crystalize it 
forms small rhombic crystals. It can unite with an alkali, such 
as sodium hydroxide, to form two salts: a neutral or diurate of 
sodium ('C5H2N4O3Na2) and the biurate or acid urate of sodium 
(C5H2N4O3HNa). The biurates are neutral in reaction and con- 
stitute the urates normally found in the blood and urine. They 
exist in two isomeric forms (a and the b). The b is more solu- 
ble than the a form. It may be that the deposition of urate tar- 
tar on the teeth, and the deposits of urates in the joints of a pa- 
tient suffering with gout, are due to the change of the b form 
into the less soluble a type. 
There are a number of other nitrogenous bodies in the urine 
which are included in the item of unclassified nitrogen in the 
above analysis. The most important of these is urinary indican, 
which is derived from the indol produced in the intestines by the 
action of bacteria on the amino acid tryptophane. The yellow 
color of the urine is produced by a pigment called urochrome, 
which is believed to be derived from the pigments in the blood. 
The Inorganic Constituents of the Urine.-The urinary 
salts are chiefly the chlorides, sulphates and phosphates of so- 
dium, potassium, calcium and magnesium. The potassium and 
sodium salts are found in greatest abundance, since they form 
the main inorganic constituent of the food, and moreover the 
greater portion of the salts of the heavier metals, as calcium, iron, 
bismuth, mercury, etc., is excreted by the intestines. There is 
very little retention of salts by the body except during the for- 
mation of bone, so that the amount of the inorganic constituents 
of urine varies from day to day with the diet. The chlorides are 
formed for the most part from the inorganic chlorides of the 
food; the phosphates and the sulphates are derived from the sul- 
phur and phosphorus of the nucleo-protein molecules. If the 
urine is neutral or alkaline in reaction, there is apt to be a de- 242 
the organs of excretion. 
posit of calcium or magnesium phosphate. This will dissolve 
when the urine is rendered faintly acid. 
Abnormal Constituents of the Urine.-Many of the sub- 
stances found in the blood occur in minute traces in the urine. 
When any of these bodies are increased to an unusual amount 
in the urine, they become what we may term pathological con- 
stituents. The bodies most commonly affected are the proteins 
and sugars. The finding of a protein, such as albumin, in more 
than the faintest trace, is an indication of nephritis or Bright's 
disease. The presence of albumin may be detected by heating in 
a test tube a slightly acidulated sample of urine. 
Normal urine contains the faintest trace of the blood sugar 
dextrose, but in abnormal conditions, as in the disease diabetes 
or after a meal rich in sugars, a large amount of dextrose ap- 
pears in the urine as a result of an increase in the sugar of the 
blood. The condition probably represents the inability of the 
tissues to make use of their carbohydrate food in the proper man- 
ner, and the kidney therefore excretes the sugar as if it were a 
waste material. 
The Organs of Excretion, The Kidneys. 
Lying upon the posterior wall of the abdominal cavity at 
the level of the lower ribs and on each side of the vertebral col- 
umn are the kidneys, the organs of urine excretion. Each kidney 
is of the nature of a tubular gland of a very complex structure, 
anatomically adapted to bring a large amount of blood at a high 
pressure in close relation with the excreting epithelial cells which 
line the walls of the gland tubules. The tubules empty into a 
pouch-shaped sac on the inner edge of the kidney, the pelvis 
of the kidney, and this is connected with the urinary bladder by 
means of a small tube, the ureter. 
The uriniferous tubules may be divided into the excretory 
portion and the collecting portion. The tubules arise in the 
outer part of the kidney, in the region called the cortex, as a 
body called the Malpighian corpuscle. This corpuscle consists 
of the dilated end of a tubule which is invaginated to form a FUNDAMENTALS OF HUMAN PHYSIOLOGY 
243 
cup-shaped vessel, within the cup of which lies a tuft of cap- 
illaries. The capillaries compose the structure known as the 
TERMINAL 
PART OF 
PROSTATE 
VEIN 
ORIGIN OF 
DUCTUS 
CHOLEDOCHUS 
FIBROUS 
CAPSULE 
PELVIS OF 
KIDNEY 
'/ LEFT 
/ SPERMATIC 
' ARTERY 
-AND VEIN 
-CELLULO- 
FIBROUS 
SUBPERITONEAL 
LAMINA 
RIGHT 
SPERMATIC 
VEIN 
Fig. 55.-The situation, direction, forms, and supports of the kidney. 
(Gray's Anatomy, after Soppey.) 
glomerulus, and the tubular part is known as the capsule of 
Bowman. 
From Bowman's capsule a short neck leads into what is known 
as the convoluted tubule, which is a very tortuous vessel lined 
with large epithelial cells. This structure lies in the cortex 
of the kidney and is nourished by the blood which has already 
been through the glomerular capillaries. A loop of the tubule 
leads down into the center or medullary portion of the kidney 
and back again to the cortex, where the cortex again becomes 
very tortuous, and finally empties, in company with many other 
similar vessels, into a common collecting tubule, which leads to 
the pelvis of the kidney. 244 
THE KIDNEYS. 
The Blood Supply of the Kidney is very large compared 
with that of the other organs of the same size. The renal arteries 
come from the aorta and distribute their blood directly to the 
Fig. 56.-Longitudinal section through the kidney: 1, Cortex; 1', me- 
dullary rays; 1", labyrinth; 2, medulla; 2', papillary portion of medulla; 
2", boundary layer of medulla; 3, transverse section of tubules in the 
boundary layer; 4, fat of renal sinus; 5, artery; *, transverse medullary 
rays; A, branch of renal artery; C, renal calyx; U, ureter (after Tyson and 
Henle). 
glomeruli and the inner medullary portions of the kidney. The 
vessels of the glomerulus are collected into an afferent vein, which 
again breaks up into capillaries to supply the remaining struc- 
tures of the cortical portions of the kidney (Plate III). 
The Nerves of the Kidney.-The kidney is very richly sup- 
plied with vasomotor nerve fibers, which are carried to it in the 
splanchnic nerves. Whether there are nerve fibers in either the 
vagus or splanchnic nerves which have a secretory influence on 
the kidney cells, is at present an unsettled question. 
The Nature of Urine Excretion.-In spite of repeated at- 
tempts to explain the nature of urine excretion, there remain Plate III-Diagram of the uriniferous tubules (black), the arteries (red) 
and the veins (blue) of the kidney. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
245 
many steps in the process which are not fully understood. The 
constituents of the urine are formed by other organs than the 
kidney, and are present in the blood plasma. The function of 
the kidney is to remove these substances from the blood. Many 
bodies are present in the blood plasma which are not found in the 
urine, and again some of the urinary constituents are found in 
far greater concentration in the urine than in the blood plasma. 
To explain these facts, Ludwig, a famous physiologist of the 
nineteenth century, formulated what is known as the mechanical 
theory of urine excretion. Impressed by the peculiar relation- 
ship of Bowman's capsule and the glomerular capillaries, he con- 
cluded that the Malpighian corpuscle is a filtering apparatus 
which separates, in dilute solution, a portion of all the diffusible 
substances of the blood. The absence of such diffusible sub- 
stances as sugar in normal urine and its presence in the blood in 
a relatively large amount, he believed to be due to the ability of 
the epithelium of the tubules to reabsorb these substances from 
the dilute urine. Likewise, the high concentration of salts and 
nitrogenous bodies, such as urea, he explained by reabsorption 
of water through the tubules into the blood. In support of this 
theory Ludwig demonstrated that the urine excretion varied 
directly with the blood flow and the blood pressure of the kid- 
ney. In other words, the greater the supply of blood and the 
greater its pressure, the more rapidly will the watery solution 
of the urine be filtered from the blood. He was not able, how- 
ever to bring any satisfactory proof of the reabsorption of water 
or other substances by the epithelium of the urinary tubules. 
Indeed, most experiments show that this does not occur. 
It is impossible to explain all the facts of urinary excretion 
by simple physical laws. For example, urea and dextrose are 
both found in the blood and both obey the same physico-chemical 
laws; nevertheless the one is excreted in the urine and the other 
is retained in the blood. Furthermore, when certain pigments 
are injected into the blood, they are excreted by the kidney cells, 
but do not appear in those of other parts of the body. 
That an increase in the pressure of blood in the renal vessels 
has a very marked accelerating effect on the excretion of urine, 246 
THE KIDNEYS. 
is not necessarily evidence that the increased blood supply is the 
cause of the excretion. That other factors are concerned is demon- 
strated by the action of drugs which cause an increase in renal ex- 
cretion. For example, digitalis, a drug stimulating the circulatory 
apparatus, causes a marked diuresis in cases of a weak heart 
where the pressure has been totally inadequate to maintain a 
urine excretion, but has little or no action on the normal kidney. 
On the other hand, sodium sulphate injected into the blood 
causes a diuresis without marked change in rate of blood flow 
or blood pressure by direct stimulation of the renal epithelium. 
In almost every case, moreover, an increase in the excretion of 
urine is followed by an increase in the amount of oxygen used 
up by the kidney. It is a general law that every increase in cell 
activity is accompanied by an increase in the amount of oxygen 
used by the organ, and the increased blood flow accompanying 
most forms of diuresis is readily explained on the basis of the 
physiological need of the tissue for water and oxygen. If physi- 
cal laws were sufficient to explain all the phenomena of excre- 
tion, there would be no need for oxygen in increased amounts 
during periods of increased urine formation. A conception of 
the actual amount of work which the cells must do to excrete the 
urine may be obtained by comparing the osmotic pressure of the 
urine with that of the blood. The osmotic pressure of the blood 
is only half that of the urine, and for each one thousand cubic 
centimeters excreted, it is sufficient to call for the expenditure, 
on the part of the renal cells, of a force capable of lifting a 
pound through one thousand feet. 
We may conclude that the nature of the excretory mechanism 
cannot be explained by the physico-chemical laws as we now 
know them, i. e., the phenomena of osmosis, filtration, absorption, 
etc., but rather that it must be due to a vital action on the part 
of the renal cells. It is this vital function of the cells which 
enables them to remove one substance from the blood and to leave 
another which is identically the same so far as physico-chemical 
properties are concerned. 
Micturition.-The urine discharged from the collecting 
tubules of the kidney into the pelvis, is carried to the urinary FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
247 
bladder through the ureters (Fig. 57). The muscular coats of 
the ureter have a movement similar to that of the digestive canal 
and by peristaltic waves force the urine down through the ureter 
into the bladder. The urine thus collected by the bladder is 
retained for a time and is at intervals ejected through the urethra 
by the act of micturition. This consists of strong contraction of 
the bladder walls, together with the contraction of the diaphrag- 
matic and abdominal muscles, the effect of which is to reduce the 
(Kidney \ 
Ureter 
Bl idd er 
U reit/hro. 
Fig. 57.-Diagram of urinary system. 
size of the bladder cavity and to expel the urine with pressure, 
through the urethra. 
The act is under nervous control, the motor nerves being de- 
rived from nerve cells found in the lumbar region of the cord. 
The stimuli here produced co-ordinate the muscular movements 
of the act. The afferent or sensory stimuli which initiate the 
act are excited by the distention of the bladder, or by the pass- 
age of a few drops of urine into the first portion of the urethra. 
These stimuli pass to the center in the cord and are returned to 248 
THE FUNCTION OF THE SKIN. 
the muscles of the bladder also causing the sphincter, which closes 
the bladder to be relaxed. In the voluntary act the motor nerves 
are stimulated by impulses from the higher centers. 
Diuretics are substances which stimulate the excretion of 
urine. They may act in several ways; either by increasing the 
blood pressure and the blood supply to the kidney when these 
are deficient, as is the case with digitalis, or by producing some 
change in the kidney cells or the blood which brings about an 
increase ih the secretion. Taking large quantities of water in- 
creases the flow of urine by altering the osmotic pressure of the 
blood. Substances as urea, caffein, sodium sulphate, pituitrin, 
diuretin, etc., probably act directly on the kidney epithelium as 
irritants, bringing about a more active excretion. 
The Function of the Skin. 
The skin serves a double function, that of protecting the body 
from the outside environment, and that of excreting essential 
fluids from its glands. Contrary/ to general belief, the glands 
of the skin do not excrete the waste substances of the body, or 
at least do so only to a very limited degree. Their functions are : 
to regulate the internal heat of the body (sweat glands) ; to lubri- 
cate its surface and hairs (sebaceous glands) ; and to provide the 
best form of nourishment for the newborn animal (mammary 
glands). 
The Sweat Glands.-These are simple coiled tubular struct- 
ures, found practically everywhere in the cutaneous tissue of the 
body, being especially numerous in certain parts, as in the palms 
of the hands and the soles of the feet. The excreting cells line 
the lower portions of the tubules, and are composed of granular, 
columnar epithelium. The glands are richly supplied with nerve 
fibers. 
The amount of sweat given off in a day varies greatly, since 
it is influenced by many things, as heat, moisture, exercise, cloth- 
ing, etc. (see p. 134). The perspiration of which we are uncon- 
scious amounts to a considerable number of grams (700 to 900 
grams) in a day. Although it is very difficult to obtain pure FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
249 
sweat unmixed with the secretions of the other glands of the 
skin, we know that it consists for the most part of water, having 
a specific gravity of about 1.004. The salty taste is due to inor- 
ganic salts and to the impurities which the sweat dissolves on the 
surface of the skin. There is only a trace of urea and related 
substances, and probably the sweat glands never aid the kidneys 
in the excretion of these bodies. 
The most important function of the sweat glands is to control 
the temperature of the body by regulating the rate of its heat 
loss. Dry air is a poor conductor of heat, and to vaporize water 
requires a large amount of heat. As the water of the sweat is 
evaporated, the body loses heat rapidly. This principle is practi- 
cally applied by the housewives of tropical countries. The water 
is placed in porous pots and the rapid evaporation on the out- 
side of the pot cools the water within. 
The secretion of sweat, like the secretion of saliva, is under 
the control of the central nervous system, as can be demonstrated 
by electrically exciting the nerves supplying the paw of a cat or 
dog. Following such stimulation drops of sweat are found on 
the paw. The secretion is not due to an increased blood flow, as 
can be shown by stimulating the nerves in a limb severed from 
its blood supply, in which case a few drops of sweat will still 
appear. A center in the brain and subsidiary centers in the 
spinal cord have been found which, when stimulated, produce 
a secretion of sweat. 
Some drugs have the peculiar action of exciting the secretion 
of sweat, either reflexly through the nerve center or by stimula- 
tion of the nerve endings about the cells of the glands. To the 
former class belong such drugs as strychnine and picrotoxin, and 
to the latter, pilocarpin. Atropin, on the other hand, inhibits 
the secretion by paralyzing the secretory nerve mechanism. An 
increase in the external temperature will cause a secretion of 
sweat only when the sensory and motor nerves of the part are 
both functional. To stimulate the sweat nerves, heat therefore 
must act reflexly through the sensory nerves and the centers of 
the brain or spinal cord. 
The Sebaceous Glands.-Besides the sweat glands there are 250 
numerous other glands in the skin. These are associated with 
the hairs, and are called sebaceous glands. They secrete an oily 
semiliquid material which affords protection to the hair and the 
skin. Its oily nature prevents the hair from becoming too brittle, 
and protects the skin from moisture. 
The Secretion of Milk.-The mammary glands are modified 
sebaceous glands which secrete a nutrient fluid, milk. The 
glands are much better developed in the female than in the male, 
and are excited to physiological activity at the birth of a child. 
Human milk is a white or yellowish fluid, without odor and with 
a peculiar sweet taste. It contains protein substances called 
caseinogen, lact-albumin, and lact-globulin; also a sugar called 
lactose or milk sugar, and fats and inorganic matter, as the chlo- 
rides of sodium, potassium and calcium. Human milk is by far 
the best food for the infant, and should be replaced by other 
food only when absolutely necessary. 
THE SECRETION OF MILK CHAPTER XXIII. 
THE NERVOUS SYSTEM. 
The General Functions and Structure of the Nervous System. 
-When a unicellular organism, such as the amoeba, is stimulated 
it responds by a movement because its protoplasm possesses 
among its other properties those of excitability, conductivity and 
contractility. In the case of multicellular organisms, some cells 
are set aside for the assimilation of food, others for movement, 
others to receive stimuli from the outside, others to compose 
tougher protective tissues on the surface, and still others, in many 
animals, to compose definite organs of offense. This location 
of specific functions in a certain group of cells makes it neces- 
sary, for the welfare of the organism as a whole, that some means 
of communication be provided between the different parts of the 
animal, for otherwise the cells which are occupied, say, in ab- 
sorbing food, would be unable to move away when some destruc- 
tive agency approached them, and indeed the moving (muscle) 
cells could never know when they ought to become active. In 
some of the lower organisms these messages are carried by chemi- 
cal substances present in the fluids that bathe the cells. These 
belong to the group of hormones which we have already studied 
in connection with the ductless glands (see p. 227). The re- 
sponses mediated in this way are, however, too slow for the quick 
adaptation which it is necessary that the organism should un- 
dergo in its battle for life. If it had to depend on such a mech- 
anism alone, the organism would already be within the clutches 
of its enemy before it could make any attempt to defend itself. 
Some more sensitive mechanism, both for receiving and 
for transmitting impulses throughout the organism, becomes nec- 
essary. This is furnished by the nervous system, which, in its 
simpler form, consists of a cell on the surface of the animal so 
specialized that it responds to changes in the environment. This 
251 252 
GENERAL STRUCTURE OF THE NERVOUS SYSTEM 
receptor cell, as it is called, is prolonged inside the animal as a 
fiber, the nerve fiber, which passes to effector cells specialized 
either as muscle fibers or gland cells. When a stimulus acts on 
the receptor cell it therefore sets up a nerve impulse which causes 
effector cells to become active, so that the animal either moves 
away or prepares to defend itself by secreting some poisonous 
substance or making some defensive movement. There are, how- 
ever, very few, even of the lowliest organisms, which have so 
simple a nervous system as this, for the nerve fibers from differ- 
ent receptors usually join together to form a nerve plexus and 
they do not run directly to the effector cell, but to another cell, 
v 
Fig. 58.-Schema of simple reflex arc: r, receptor in an epithelial mem- 
brane ; a, afferent fiber; s, synapsis; c, nerve cell of center; e, efferent fiber; 
m, effector organ. 
the central nerve cell, which is specialized as a junctional or dis- 
tributing center, and which then transmits the impulse by a fiber 
of its own to the proper effector organs. 
Thus we have the essential elements of the so-called reflex arc 
(Fig. 58),dhat is, a receptor connected with a nerve fiber called 
afferent running to a central nerve cell which is again connected 
with a nerve fiber called efferent, which passes to some effector 
organ. In certain of the lower organisms these nerves and nerve 
cells are continuous throughout, but in the higher animals the 
fibers originating from each cell do not actually join with those FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
253 
of others, but only come in close contact with them. They are 
contiguous but not continuous, and the nerve impulses pass from 
one to another by contact rather than by transmission through 
continuous tissue. 
Every nerve cell gives off at least one process called the axon, 
and it is this which forms the axis cylinder of the nerve fiber. 
There are usually other processes, but they differ from the axon 
in that they- branch freely and do not run for any distance from 
the cell. They are called dendrites. The axon may also occasion- 
ally give off a branch, often called a collateral, but it is not until 
it has reached the effector organ or some other nerve cell that 
the branching is pronounced. It now breaks up into a mass of 
fine branches. When these occur at a second nerve cell, they 
closely encircle the cell, forming a basket-like structure around 
it. This is called a synapsis. The nerve impulse can travel from 
the fiber through its synapsis on to the nerve cell which this sur- 
rounds, but it cannot travel in the opposite direction. This valve- 
like action at the synapsis explains why a nerve impulse travels 
along a reflex arc in one direction only. Each nerve cell with 
its axon and dendrites is called a neurone. Reflex arcs are there- 
fore composed of two or more neurones, and the nervous system 
is built up of great numbers of reflex arcs. 
The nerve cells which constitute the centers are usually col- 
lected in groups called ganglia. In the segmented invertebrates, 
such as the worms and crustaceans, there is one such ganglion 
for each segment, each ganglion being connected with its neigh- 
bors by nerve fibers, thus forming a chain, along the ventral 
aspect of the animal, and also having numerous nerve fibers con- 
necting it with the various receptors and effectors of the segment 
(Fig. 59). At the head end of the animal several of these gang 
lia become fused together to form a larger ganglion, which lies 
just behind the gullet and from which two fibers pass around the 
gullet to unite in front of it in a large ganglion, which usually 
shows three lobes. These larger head ganglia receive the affer- 
ent nerve fibers from the adjacent projicient sense organs, 
namely, the eyes, the ears, the organ of smell, and the antennae 
or feelers; these being really receptors which have become 254 
GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 
highly specialized for the purpose of receiv- 
ing impressions from a distance. Many of 
the efferent fibers which arise from the cells 
of the head ganglia go to the muscles which 
move the head end of the animal, others, how- 
ever, do not run directly to effectors, but they 
run down the nerve chain to make synaptic 
connection with the cells of some of the seg- 
mental ganglia. This connection of the cells 
of the head ganglia with those supplying the 
segments enables the former to exercise a dom- 
inating influence over the activities of the lat- 
ter, the purpose being that approaching dan- 
gers may have a greater influence in deter- 
mining the response of the animal than stim- 
uli that are merely local. When, for example' 
some sight or sound of an approaching enemy 
is received by the head ganglia, these will 
transmit impulses down the ganglion chain 
which so influence the various nerve cells as to 
produce, in all of them, a co-ordinated action 
for the purpose of getting the animal out of 
danger. Even should some local stimulus be 
acting on one or more of the segments, the 
stimulus which is received through the head 
ganglia will obtain the upper hand and annul 
or inhibit the local influence. The part will 
become subservient to the whole. This illus- 
trates the integration of the nervous system, 
which, as we pass to higher animals, we shall 
find to become more and more developed and 
intricate. 
So far, however, the nervous reaction is 
purely of the nature of a reflex; but in the 
higher animals other factors, namely, memory 
and volition, come to exercise a dominating in- 
fluence on the nature of the response. The 
Fig. 59. - Dia- 
gram of nervous 
system of segment- 
ed invertebrate; a, 
sup racesophageal' 
ganglion; b, sub- 
oesophageal gang- 
lion ; oe, oesopha- 
gus or gullet. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
255 
afferent stimulus arriving, let us suppose- at nerve cells controll- 
ing the movements of the leg, may fail'to cause a response of the 
corresponding muscles because of impulses meanwhile trans- 
mitted from higher memory centers, for the animal may have 
learned by experience that such a movement as the local stim- 
ulus would in itself call forth, is hurtful to its own best inter- 
ests. This experience will have become" stored away as a mem- 
ory in the higher (memory) nerve centers, so that whenever the 
local stimulus comes to be repeated, impulses are discharged 
from these memory centers to the local nerve center and the re- 
flex response does not occur, or is much modified in nature. For 
storing away these memories and for related psychological proc- 
esses of volition, etc., the anterior portions of the nervous system 
in the vertebrates become very highly developed so as to consti- 
tute the brain, and the simple chain of ganglia of the inverte- 
brates comes to be replaced by the spinal cord. 
As we ascend the scale of the vertebrates, the brain becomes 
more and more developed, until in the higher mammalia, such 
as man, very few reflex actions can occur independently of the 
higher centers which are located in it. In other words, the reflex 
arc now involves, not one nerve center, but several, and of these 
the most important are located in the brain. CHAPTER XXIV. 
THE NERVOUS SYSTEM (Cont'd). 
The Nerve Structure Involved in the Reflexes of the Higher 
Mammals.-In general, as already mentioned, these include a 
receptor, an afferent fiber, a nerve center, an efferent fiber and 
an effector organ. 
The Receptor.-The receptor exists as one of the sensory 
nerve terminators situated in the skin (extero-ceptors) or in the 
deep tissues, such as the joints, the muscles or the viscera 
(proprio-ceptors). Many receptors are highly specialized so as 
to respond only to one kind of stimulus, and each special kind 
of receptor is located where it will be of most use. Thus, there 
are special receptors for sensations of heat, others for cold, others 
for touch, others for pain. The pain receptors are distributed 
more or less uniformly over the body. They are present in the 
deeper structures, such as the teeth, the joints and the serous 
coverings of the viscera. Sometimes, as on the cornea and in the 
pulp of the teeth, they are the only kind of receptor present. 
The touch receptors are collected in small areas called "touch 
spots," which are much more numerous on the tip of the tongue, 
the lips, or the tips of the fingers than on the skin of the legs, 
the arms or the back of the trunk. The frequency of touch spots 
on the tip of the tongue makes a foreign body in the mouth ap- 
pear to be larger than when we feel it with the fingers. The 
touch spots on the finger tips may acquire great acuity of per- 
ception by education, as in the case of a blind person, who has 
to use his fingers for reading. The remarkable irregularity of 
distribution of touch spots may be very beautifully shown by 
finding out how far apart the points of a pair of calipers must 
be from each other in order to be distinguished as separate. 
This distance is not more than 3 mm. for the tips of the fingers, 
Reflex Action. 
256 Plate IV-The simplest reflex arc in the spinal cord. (After Kblliker.) 
The afferent fiber in the posterior root (in black) gives off collaterals, which 
end by synapses around the cells of the anterior horn (in red), the axons 
of which form the efferent fibers of the anterior roots. (From Howell's 
Physiology.) FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
257 
but it is over 60 mm. for the skin of the back of the neck. The 
temperature receptors are still more definitely located in areas, 
some being specialized for heat and others for cold. These so- 
called heat and cold spots are most frequent on the portions of 
the body that are covered by clothing, for example, the skin of 
the thorax, than on those that are exposed, for example, the face. 
They are fairly frequent on the skin of the dorsum of the hand, 
where their existence can be very easily demonstrated by slowly 
drawing a pencil gently over the skin. At certain places the 
point of the pencil feels hot, at others cold, and in others it 
causes no temperature sensation whatsoever. 
All varieties of receptors are present on the skin of the hand, 
but in certain diseases of the nerves or spinal cord, one kind of 
receptor may become inactive, thus causing, when the absent sen- 
sation is that of pain, the condition called analgesia, which must 
be distinguished from that of anesthesia, when all sensations are 
paralyzed. In analgesia a pin prick causes only a sensation of 
touch. When the nerves of the arm are cut and the cut ends 
then sutured together so that the nerve fibers regenerate, the skin 
sensations do not all return at the same time. Those of pain and 
of extreme degrees of heat and cold return in from six to twenty- 
six weeks, wfiereas those of touch and the finer degrees of tem- 
perature do not return until after one or two years. The power 
of localizing the point of application of the stimulus is also late 
in returning; thus, if we touch the finger of such a person and 
ask him to tell us where, he may indicate some spot that is quite 
a distance away from the one actually touched. Certain drugs, 
such as cocaine, have the power, when applied locally, of ren- 
dering all the receptors insensitive. 
The Afferent Fiber.-Another name for this is the sensory 
nerve, because it carries the sensations received by the receptors 
up to the nerve center. All afferent fibers enter the spinal cord by 
the posterior nerve roots, on each of which, it will be remem- 
bered, is situated a ganglion, the posterior root ganglion. The 
cells of this ganglion are connected with the afferent fibers by 
a short branch running at right angles to the latter (Plate IV). 
The function of the cells is to maintain the nutrition of the affer- 258 
REFLEX ACTION. 
ent fibers, for if these be divided before they reach the ganglion, 
the peripheral or far away end undergoes degeneration, whereas 
if the cut be made between the ganglion and the cord, degenera- 
tion occurs central-wards, that is, towards and into the cord. This 
degeneration always occurs in the portion of the nerve fiber which 
has been disconnected from the nerve cell. It therefore furn- 
ishes us with a ready method for finding out whether the fiber 
is running towards or away from the brain. In the former case, 
the fiber is said to be ascending, and it degenerates above the 
section; in the latter case, it is descending and it degenerates 
below the section. Since degenerated nerve fibers give charac- 
teristic staining reactions, we are thus furnished with a means 
of finding out what becomes of the afferent fibers after they 
enter the cord. 
To further trace the course and connections of the afferent fib- 
ers in the cord, we must therefore cut the posterior roots between 
the ganglion and spinal cord and after a few weeks kill the 
animal and make microscopic examination of the cord, stained 
in special ways. If we take a series of such sections above the 
level at which the posterior roots have been cut, we shall find 
that opposite the point of entry of the cut root, the degenerated 
fibers occupy an area near the tip of the posterior horn of grey 
matter. As we examine sections taken higher and higher up, 
the degenerated area will be found to shift gradually towards 
the median fissure, occupying, first of all, the so-called postero- 
lateral column, and later the postero-median (Fig. 60). When 
we get to the medulla oblongata or "bulb," the degenerated 
areas disappear because the fibers have terminated by forming 
synapses around the cells of the two large ganglia which form 
the bulgings seen on the posterior aspect of this structure. The 
fresh relay of nerve fibers do not degenerate after section of the 
posterior roots, but by other means of investigation they have 
been found to become collected into a bundle called the fillet, 
which crosses, or decussates, to the other side of the medulla and 
runs up through the pons varolii and crura cerebri, some of the 
fibers ending near the optic thalamus, whilst others run on to 
the grey matter of the motor areas of the cerebrum. Plate V-Reflex arc through the spinal cord, in which an intermediary 
neurone (in blue) exists between the afferent and efferent neurones. (From 
Howell's Physiology.) Fundamentals of human physiology. 
259 
The posterior root fiber, shortly after entering the cord, gives off 
a branch at right angles (called a collateral), or in its course up 
the cord it may give off several collaterals, their destination 
being the grey matter of the cord, in which they terminate by 
synapses around nerve cells. Certain of these may be cells of 
the anterior horn. These cells give rise to the efferent fibers, 
which leave the spinal cord by the anterior or motor roots (see 
ventral 
Fig. 60.-Diagram of section of spinal cord, showing tracts. (After Kol- 
liker) ; g, posterior median, and b, postero-lateral columns; p.c., crossed 
pyramidal, and p.d., direct pyramidal tracts; f, cerebellar tract. (After 
Howell.) 
Plate IV). Other collaterals run to intermediary cells, which 
then communicate with the anterior horn cells (Plate V). 
The Nerve Center and Intermediary Neurones.-When 
the entering nerve impulse travels by a collateral to an anterior 
horn cell, we have the simplest type of reflex action, namely, one 
involving a receptor, a sensory nerve fiber, the posterior root, a 
collateral, the' anterior horn cell, the anterior root, a motor nerve 
fiber and an effector organ. But such a simple reflex seldom 260 
REFLEX ACTION. 
occurs in the higher animals. The afferent impulse when it en- 
ters the cord is more likely to travel up the posterior columns 
and then, as already outlined, to the cerebrum, where it is trans- 
mitted to the large pyramidal nerve cells of the grey matter. 
From the pyramidal cells spring the fibers of the pyramidal 
tracts, which, as they pass downward through the white matter of 
the cerebrum, crowd closer and closer together until, by the time 
the basal ganglia are reached (optic thalamus on the inside, 
and corpus striatum on the outside), they form a narrow bun- 
dle which occupies the middle /portion of the strip of white mat- 
ter, which lies between these ganglia. This white matter is 
called the internal capsule (Fig. 62), and it is of very great 
clinical interest because, being in the neighborhood of a large 
artery (branch of middle cerebral), which sometimes bursts in 
elderly people, it is apt to become torn up by extravasated 
blood, thus destroying the pyramidal fibers and causing paraly- 
sis. This is what occurs in apoplexy. Below the internal capsule 
the fibers run into the crura cerebri, then into the pons, thence 
into the medulla oblongata, in the front of which they form a dis- 
tinct bulging called the pyramid; hence their name pyramidal 
fibers (see Fig. 61). 
In the lower portion of the medulla, a most interesting thing 
occurs, namely, three-fourths of the fibers cross to the oppo- 
site side, thus constituting the decussation of the pyramids 
(Plate VI). These crossed fibers run down in the lateral columns 
of the spinal cord as the crossed pyramidal tracts. The pyra- 
midal fibers which do not cross in the medulla form the direct 
pyramidal tracts of the cord, and they gradually cross in the 
cord itself. The pyramidal fibers end by synapsis around the 
cells of the anterior horn, so that all fibers from the cerebrum 
ultimately cross to the opposite side before they reach the anterior 
horn cells, for which reason it happens that a lesion involving 
the'pyramidal tract anywhere above the decussation, such as a 
haemorrhage in the internal capsule above referred to, always 
causes paralysis of the opposite side of the body (hemiplegia). 
These facts regarding the course of the pyramidal fibers have 
been ascertained by microscopic examination of sections from Plate VI -Course of the pyramidal fibers from the cerebral cortex to the 
spinal cord : 1, fibers to nuclei of cranial nerves ; 3, fibers which do not cross 
in the medulla (direct pyramidal tract) ; 4 and 5, fibers which cross in medulla 
(crossed pyramidal tract). (After Howell.) FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
264 
various levels of the spinal cord some time after destruction of 
the Rolandic area of the cerebrum (see p. 275). The pyramidal 
fibers are degenerated and they occupy the areas indicated 
in Fig. 6D. Since the degeneration occurs below the destruction, 
it is called descending degeneration, in contradistinction to as- 
cending degeneration, which we saw to follow section of the 
posterior roots between their ganglia and the cord (see p. 258). 
To sum tip, the sensory impulse on entering the spinal' cord 
by the posterior root, by traversing a collateral, may take the 
shortest possible pathway to the efferent nerve cell of the an- 
terior horn, or it may avoid this and travel up the posterior 
columns of the cord to the medulla, thence by the fillet to the 
cerebral cortex of the opposite side, and thence down the pyra- 
midal tracts to the anterior horn cells. In this long cerebral 
route there are at least three places where the impulse must pass 
by means of a synapsis from nerve fibers on to nerve cells, and 
then along the nerve fibers arising from these. These three 
places are: (1) in the medulla, (2) in the cerebral cortex, (3) 
in the anterior horn. 
This long cerebral route, as it is called, is by no means the 
only one along which afferent impulses may travel to the brain. 
Some may be carried by collaterals to certain cells of the grey 
matter of the cord, and from these cells fibers may run up the 
cord to the cerebellum or lesser brain. These cerebellar tracts 
are located in the lateral columns of the cord outside the crossed 
pyramidal tracts (see Fig. 60). They do not degenerate when 
the posterior roots are cut, but do so after section of the cord 
itself (this distinguishing them from the fibers in the posterior 
columns). The impulses which they transmit to the cerebellum 
have to do with certain subconscious sensations concerned in 
the maintenance of the tone of the muscles. There are also 
certain pathways in the white matter of the cord which trans- 
mit descending impulses from the cerebellum. 
The main bundles of ascending and descending fibers in the 
spinal cord are charted in Fig. 60,^which should be carefully 
studied. 
The Efferent Fiber, or Neurone.-As already explained 262 
REFLEX ACTION. 
the cell of this neurone is located in the anterior horn of grey 
matter of the cord. These anterior horn cells are distinguished 
from the other nerve cells of the grey matter by their large size 
and angular shape, and they become greatly increased in num- 
ber in the portions-of the cord from which the nerves going to 
the extremities originate. The fibers springing from them pass 
out in the anterior roots. If the cells are destroyed or the an- 
terior roots cut, degeneration occurs below the lesion, and para- 
lysis of the effector organs (muscles) to which they run results, 
but this paralysis is very slight in degree unless the lesion af- 
fects several roots, or the cells of several adjacent levels of the 
cord. The reason for this is that the nerve cells of one level of 
the cord only partially supply a given muscle or group of mus- 
cles with nerve fibers, thus showing that even the small muscles 
receive their nerve fibers from several adjacent levels of 
the cord. The anterior horn cells sometimes become destroyed 
by disease, namely, in infantile paralysis (poliomyelitis anter- 
ior). The resulting paralysis is never recovered from. 
Types of Reflexes.-Having traced the paths through which 
reflexes occur in the higher animals, we may now proceed to 
consider certain typical forms of reflex action and the condi- 
tions which may cause them to become altered. We must first 
of all confine our attention to the characteristic reflexes of the 
so-called spinal animdl, for it is only after we have done so that 
it will be possible for us to determine what influence the brain 
has in modifying the spinal reflexes. The spinal animal (dog, 
for example) is prepared by cutting across the spinal cord some- 
where below the origin of the phrenic nerves. After the' imme- 
diate effects of the operation have been recoverd from, the 
regions of the animal's body lying below the level of the sec- 
tion of the cord, suffer from a condition called spinal shock. 
All reflex movements are absent, the sphincters are paralyzed so 
that incontinence of urine and faeces exists, and various "tro- 
phic" or nutritive changes occur in the skin (abscesses form, 
hair falls out, etc.). After some time, the length of which de- 
pends on the position of the animal in the animal scale, the 
sphincters regain their tone and the reflexes gradually reappear FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
263 
in the paralyzed region, the first to do so being the protective 
reflexes, of which the flexion reflex is the type. 
The flexion reflex is elicited by any stimulus which would cause 
pain in an animal capable of feeling. Such stimuli are called 
nocuous and the reflex response is always of such a nature- 
usually flexion-as to cause the injured part to be removed 
from further damage. The return of the flexion reflex is soon 
followed by that of the knee jerk, which is elicited by tapping 
the patellar tendon after putting it on the stretch by passively 
bending the knee joint. Somewhat later in many animals (e.g., 
dog) the scratch reflex appears, so-called because it consists of 
a scratching movement of the hind leg in response to mechanical 
irritation of the flank of the animal. It is a reflex of very great 
interest because it illustrates to what a remarkable degree the 
spinal cord, unaided by the brain, is capable of bringing about 
complicated and purposeful co-ordinated movement. Later still, 
in the lower animals, practically all the reflex movements which 
a normal animal exhibits may reappear. 
When the cord becomes severed in man, as by spinal fracture, 
spinal shock is extremely profound, and in order to keep the 
patient alive great care must be taken, on account of the incon- 
tinence of urine, to prevent infection of the bladder and kidneys 
and to protect the skin from, ulceration (bed sores). Even in 
such cases, however, many of the reflexes recover in the para- 
lyzed regions, but the recovery is slow and the limbs invariably 
atrophy. It is particularly important to note that the time of re- 
appearance of the reflexes bears a relationship to the degree of 
development of the cerebral hemispheres, thus rendering it evi- 
dent that spinal shock is due to a break in the nerve paths which 
lead to and from the brain. The higher the animal, the more 
frequently do all reflex acts involve a cerebral path instead of 
taking the short cuts available through the collaterals (see p. 
254. From usage, as it were, the cerebral paths become so well 
developed that when they are suddenly severed, the reflex action 
becomes impossible until the entering afferent impulse has 
learned to use the hitherto unused short cuts available through 
collaterals. When completely recovered from spinal shock, an 264 
REFLEX ACTION. 
animal, say a dog, in so far as voluntary movement is con- 
cerned, is entirely paralyzed in all portions of the body below 
the leve^ of the section of the cord. It cannot voluntarily move 
the affected parts, it cannot walk, it feels no pain or any other 
sensation below the lesion, and yet when appropriately stimu- 
lated, the paralyzed limbs may reflexly undergo various, often 
very complicated movements. 
The Essential Characteristics of Reflex Action.-As studied 
on a perfectly recovered spinal dog these are as follows: 
1. For a certain interval after applying the stimulus there 
is no response, the duration of this "latent period" depending 
partly on the nature of the reflex (short in the protective re- 
flexes, long in the scratch reflex) and partly on the strength of 
the stimulus. 
2. The response may persist for some time after the stimulus 
is removed (after response). 
3. The degree of the response is roughly proportional to the 
strength of the stimulus, except in certain of the protective re- 
flexes, such as the conjunctival, which consists in the closing of 
the eyelids when anything touches the eye. 
4. The response is often rhythmical in character, even though 
the stimulus be continuously applied. This is well seen in the 
scratch reflex. 
5. There are certain ways, apart from an alteration in the 
stimulus, by which we may cause a reflex movement to become 
increased or decreased. Thus, taking the flexion reflex as an 
example, the flexion may be /diminished: (1) By stimulating 
some other reflex movement which involves the same muscles, 
but which is antagonistic to flexion, e.g., by stimulating the 
opposite limb and causing the so-called/crossed extension reflex. 
(2) By causing strong afferent impulses to pass through other 
levels of the spinal cord, e. g., pinching the tail. A similar 
"interference" is well illustrated in the case of man by stimulat- 
ing the fifth nerve by firm pressure on the upper lip at a time 
when there is an inclination to sneeze. The sneezing, which is 
a reflex due to irritation of the mucosa of the nose, can usually 
be prevented. Expressing this phenomenon of reflex interfer- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
265 
ence in popular language, we may say that when the attention 
of a segment of the cord, or its extension in the brain is taken 
up by some other stimulus, a reflex already in action, or about 
to act, is depressed. Pain, such for example as toothache, may 
likewise be lessened by applying counter-irritation such as a 
blister to some neighboring skin area. (3) By means of certain 
drugs known as anesthetics, which depress the excitability of the 
nerve cells. (4) By fatigue. 
The reflex movement may be increased: (1) by applying a 
second stimulus to some other area of skin of the same hind leg 
or by applying electrical stimulation to the central end of one 
of its sensory nerves; (2) by raising the excitability of the 
nerve centers by certain drugs, such as strychnine; (3) by first 
of all causing the movement to disappear, though the stimulation 
causing it is maintained, by exciting some other part of the 
body (see above). When the reflex reappears it is much more 
pronounced than formerly. 
Muscular Tone and Reciprocal Action of Muscles.-Having 
learned some of the general characteristics of the reflex move- 
ments, we may now proceed to inquire into the method by which 
the spinal cord is enabled, by itself, so to direct the afferent im- 
pulses which enter it, that the nerve cells of the anterior horn 
discharge suitable impulses to bring about such complicated 
movements as have just been described. When a motor nerve 
or an anterior spinal root is stimulated, the muscles which con- 
tract are not grouped in such a way as to cause any purposeful 
or co-ordinated movement. Contractors, extensors, adductors 
and abductors are quite likely all to contract at once and by 
thus opposing one another to effect no definite movement. When 
such stimulation is extensive (e.g., involves a considerable num- 
ber of motor fibers), it is common to find that the extensor 
muscles predominate over the others, so that the limb becomes 
extended. Such is the case when some poisonous substance 
causes irritation of the nerve centers in the spinal cord. 
To cause a co-ordinated movement it is necessary that one 
group of muscles should become relaxed whilst their antagonistic 
group is undergoing contraction. Now, it might at first sight be 266 
REFLEX ACTION. 
imagined that this relaxation is merely a passive act, that is to 
say, that the uncontracting group of muscles do nothing more 
than remain quiescent and permit themselves to be stretched. 
But such is not the case; on the contrary, they become actively 
extended. This they are enabled to do because of the fact that, 
even when apparently relaxed, a muscle is really not so, but 
exists in a condition called tone, that is, in a slightly contracted 
state. This tone becomes greatly diminished during sleep, and it 
can be caused almost to disappear by deep anesthesia. It is for 
this purpose, as well as to abolish pain, that anesthetics are 
administered before attempting to reduce a dislocation. 
Tone is maintained by the nerve cells of the anterior horn of 
the spinal cord. When therefore an afferent impulse brings 
about flexion at the knee joint, it does so by exercising two 
diametrically opposite influences on the anterior horn cells: it 
stimulates those which preside over the flexor muscles and de- 
presses the tonic influence of those supplying the extensors. 
This tone-depressing action recalls the inhibitory influence which 
the vagus nerve exercises over the heart beat (see p. 93), and 
since it always occurs along with a contraction of antagonistic 
muscles it is called reciprocal inhibition. Certain poisons, par- 
ticularly strychnine and, tetanus toxin, cause this reciprocal 
action to break down so that all the muscles around a joint con- 
tract at the same time and produce an extension. Tetanus 
toxin is the poison produced by the tetanus bacillus, and its 
interference with the reciprocal inhibition of the muscles of the 
lower jaw causes lockjaw. 
Symptoms Due to Lesions Affecting the Reflexes.-From 
what we have learned regarding the functions of the spinal 
cord, it is easy for us to explain the following symptoms and 
conditions resulting from pathological destruction or stimula- 
tion of various parts of it: 
1. In destruction of the continuity of the afferent or efferent 
fibers of the reflex arc, the reflexes are absent. This occurs in 
chronic inflammation of the nerves (neuritis) and in the disease 
called- locomotor ataxia, in which the lesion consists of a de- 
structive pathological process involving the posterior columns 
X FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
267 
of the spinal cord. One of the first symptoms of locomotor 
ataxia is absence of the knee jerk, which, it will be remembered, 
is elicited by tapping the patellar tendon after putting it pas- 
sively on the stretch, either by sitting with the feet swinging on 
the edge of a table, or by crossing one knee over the other. Pains, 
called crises, are also usual in various parts of the body. Later 
symptoms are inability to stand without falling when the eyes 
are shut, inco-ordinated walking, in which the foot is lifted too 
high and is brought down to the ground again too violently, loss 
of sensation of the skin of the foot and leg, and changes in the 
pupillary reflexes of the eye (see p. 290). The joints also be- 
come swollen and the articular surfaces roughened so that a 
grating sensation is experienced when the joint is bent (Char- 
cot's joint). The condition gradually gets worse, so that the 
patient becomes bedridden. Death is usually due to complica- 
tions. 
2. Destruction of the anterior horn cells not only causes 
absence of reflex action, but is followed by marked atrophy of 
the affected muscles. It has been supposed that this points to 
a so-called trophic influence of these nerve cells, that is to say, 
a power of influencing nutrition. Such changes occur in infan- 
tile paralysis (poliomyelitis anterior). 
3. Stimulation of the above fibers may cause exaggeration of 
the reflexes, as in the earlier irritative stages of neuritis, in 
tumors pressing on the nerve roots, or when the membranes of 
the cord become inflamed, as in meningitis. 
4. Removal of impulses coming from the cerebrum by way of 
the pyramidal tracts causes exaggerated reflexes. Such occur 
in paralysis of both sides of the body in paraplegia, and on the 
paralyzed side in hemiplegia. 
In a paraplegic patient the weakest stimulus applied to the 
skin of the paralyzed portion of the body will call forth a wide 
spread and much exaggerated reflex contraction. CHAPTER XXV. 
THE NERVOUS SYSTEM (Cont'd). 
The Brain Stem, the Cranial Nerves, and the Brain. 
The Brain Stem.- The medulla, the pons Varolii, and the mid- 
brain (Figs. 61 and 62), compose the brain steam, which is really 
an upward extension of the grey matter, and of certain of the 
columns of the spinal cord, into'the base of the brain, with special 
nerve centers and especially large bundles of inter-connecting 
nerve fibers superadded. It is because of the crossing in various 
directions of these bundles of fibers that the structure of the 
medulla, pons and mesencephalon is so difficult to understand. 
The grey matter, as in the spinal cord, lies deep and the fibers are 
superficial. Of the latter, the pyramids and fillet, already de- 
scribed, are the most important, and their direction is longi- 
tudinal. The most prominent of the connecting or commisural 
nerve bundles are the upper, middle and lower peduncles of the 
cerebellum, or small brain, which, it will be remembered, lies 
over and at the side of the pons varolii and midbrain. The 
lower peduncles spring from the medulla and connect the spinal 
cord with the cerebellum. They form the lower edges of the 
fourth ventricle. The middle peduncles enter the sides of the 
pons, in which they cross at right angles with the pyramidal 
fibers (p. 260). They connect the cerebellum of one side with 
the cerebrum of the opposite side. The superior peduncles join 
the encephalon just under the posterior corpora quadrigemina, 
and the fibers composing them decussate to the other side to be- 
come connected with certain of the so-called basal ganglia. 
The basal ganglia are the optic thalamus and the corpora stri- 
ata, two large collections of nerve cells protruding into the third 
and lateral ventricles of the brain and having the internal capsule 
between them (see p. 260). The nerve cells composing these 
ganglia receive impulses from nerve fibers arriving at them both 
268 FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
269 
from below (coming from the spinal cord) or from above (com- 
ing from the cerebrum). They then transmit these impulses 
along their own nerve fibers, which may run to various other 
Fig. 61.-Under aspect of human brain. In the center line from below 
upwards are seen a section of the upper end of the spinal cord, and the 
medulla oblongata (m), with certain of the cranial nerves (as numbered). 
In front of this is the pons (p), with the large fifth nerve arising from it, 
and the middle peduncles of the cerebellum (M. Ped) running into the cere- 
bellum (A). The rounder bodies anterior to the pons are the corpora quad- 
rigemina (Cq), at the sides of which are the crura cerebri and the origins of 
the third and fourth nerves. The optic and olfactory nerves are in front. 
The under surfaces of the cerebrum (Cb) and cerebellum (A) constitute 
the remainder of the drawing. (From a preparation by P. M. Spurney.) 270 
THE CRANIAL NERVES. 
parts of the brain. The optic thalamus, as its name signifies, is 
intimately associated with the optic nerves. 
Another important collection of nerve cells occurs in the 
corpora quadrigemina. These exist as four rounded swellings, 
two on either side, just where the superior peduncles of the cere- 
bellum come together. Their nerve cells serve as distributing 
centers for visual and auditory impulses, carried to them through 
tracts of nerve fibers connected with the optic and auditory 
Fig. 62.-Vertical transverse section of human brain. Below is a section 
of the pons (P) showing the fibers which connect the brain stem and cere- 
brum radiating up through the internal capsule (IC), which is bounded 
mesially by the optic thalmus (T), and laterally by the corpus striatum (L). 
The third (III-V) and lateral ventricles (LV) of the brain are seen in the 
center (black). The thickness of the grey matter and the infolding of the 
surfaces, as convolutions, should be noted. (From a preparation by P. M. 
Spurney.) 
nerves. The corpora quadrigemina are usually more developed 
in the brain of the lower animals than in that of man. 
The Cranial Nerves.-On account of the introduction of 
the new structures described above there is no regularity in the FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
271 
arrangement of the grey matter in the brain stem as there is in 
the cord. Instead of forming horns, the grey matter is scat- 
tered in colonies or nuclei, many of which are centers for 
the fibers of the cranial nerves. Some of these fibers are, of 
course, afferent and some efferent. Since many of the cranial 
nerves are connected with the nose, mouth and teeth, it is im- 
portant for us to learn something concerning the location of 
their centers and the general function of the nerves. There are 
twelve pairs of cranial nerves, and the last ten of these originate 
from the grey matter of the medulla, pons or midbrain. The 
following list indicates the general functions of the nerves: 
1. Olfactory. 
nerve of smell. 
arises from fore- 
brain. 
2. Optic. 
3. Oculo motor. A 
nerve of sight. 
arises from fore- 
brain. 
4. Trochlear. C 
6. Abducens. ) 
nerves to the mus- 
cles of the eyeball. 
arise from midbrain. 
5. Trigeminal. 
sensory nerve of 
face. 
arises mainly in 
pons. 
7. Facial. 
main motor nerve of 
face muscles. 
arises in pons and 
medulla. 
8. Auditory. 
nerve of hearing and 
of semicircular 
canals. 
arises in pons. 
9. Glosgo-pharyn- 
geal. 
motor nerve of phar- 
ynx, sensory nerve 
of taste. 
arises mainly in 
medulla. 
10. Vagus. 
efferent and afferent 
nerve to various 
viscera. 
arises in medulla. 
11. Spinal accessory. 
mainly blends with 
vagus 
arises with vagus 
except spinal por- 
tion, which extends 
down into spinal 
cord. 
12. Hypoglossal. 
motor nerve for 
tongue muscles 
arises in medulla. 
It is important to note that, like the spinal nerves, many of 
the cranial nerves are composed of two roots, motor and sensory, 272 
THE BRAIN. 
each having its own center. This fact justifies the statement 
which we have already made that the brain stem is really an up- 
ward prolongation of the spinal cord, and just as we saw that 
each posterior root of the spinal cord is characterized by pos- 
sessing a ganglion, so also is there a ganglion in the sensory 
divisions of the cranial nerves. This ganglion, however, is often 
difficult to find. The nerve cells which compose it unite with 
the fibers of the sensory root by a T-shaped junction, and the 
fibers terminate by synapsis around the cells of the sensory 
nuclei. The ganglion of the fifth nerve is the Gasserian. Those 
for the eighth are the ganglia found in the cochlea and internal 
auditory meatus (Scarpa's ganglion). The ganglia of the ninth 
and tenth nerves are situated along the course of the nerves. 
The approximate position of the various ganglia will be best 
learned by consultation of the accompanying diagram (Plate 
VII). 
In the brain stem there are three sensory or afferent nuclei, a 
long, combined one for the ninth, tenth and eleventh nerves, ex- 
tending practically from the upper to the lower limits of the 
medulla, one for the eighth in the center of the pons, and a 
very long one for the fifth, extending from near the upper limit 
of the pons down into the spinal cord. The motor or efferent 
nuclei for the third, fourth, sixth and twelfth nerves are com- 
posed of cells shaped like those of the anterior horn of the spinal 
cord. They lie near the middle line and extend throughout 
the whole length of medulla and pons. The motor nuclei of the 
fifth, seventh, ninth, tenth and eleventh lie outside the above. 
The Brain.-The first question, which naturally arises is, 
what influence does the brain have on the reflex movements pro- 
duced through the spinal cord? These influences may be sum 
marized as follows: 
1. The brain enables the animal to will that a particular 
movement shall or shall not take place, irrespective of the stimu- 
lation of spinal reflexes. Much of this influence of the brain is 
of course voluntary in nature, but some of it is subconscious or 
involuntary. In general it may be said that the cerebrum, 
through the pyramidal tracts, usually exercises a damping or Plate VII--Diagram of the dorsal aspect of the medulla and pons showing 
the floor of the fourth ventricle with the nuclei of origin of the cranial 
nerves. (After Sherrington.) The sensory nuclei are colored red and are 
numbered on the left of the diagram, the motor, blue and numbered on the 
right. The peduncles of the cerebellum-8. (superior), M. (middle), and 
(inferior), are shown cut across. C.O., corpora quidrigemina. The above 
nuclei are of course present on both sides. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
273 
inhibitory influence on the spinal reflexes. It is for this reason 
that the reflex response to a certain stimulus is usually much 
more pronounced in a spinal, as compared with a normal animal. 
For example, it is impossible to bring about the scratch reflex 
in many normal dogs, whereas it is always present in spinal 
animals. , 
In man this restraining influence of the pyramidal tracts on 
spinal reflexes is very evident in the case of knee-jerk, which, 
it will be remembered, is the extension of the leg which occurs 
when the stretched patellar tendon is tapped. Ordinarily the 
kick is moderate in degree, but in patients whose pyramidal 
tracts are diseased, as in spastic paraplegia, it becomes very 
pronounced. 
2. The brain, being the receiving station for the projicient 
sensations (p. 285), sight, hearing and smell, adds greatly to 
the number of afferent pathways by which reflex actions can 
be excited. 
3. Since in higher animals all the afferent impulses usually 
travel through the brain (p. 260), many nerve centers become 
more or less involved in the reflex actions, so that a much higher 
degree of co-ordination than that seen in a spinal animal attends 
the muscular response. For example, some of these afferent 
impulses reach the cerebellum, whose function, as we shall see, 
is to strengthen some impulses and weaken others, so that a 
more perfect movement results. 
4. The animal becomes conscious not only of the nature and 
place of application of the sensory stimulus itself, but of the 
degree to which it has moved its muscles in response. 
The Functions of the Cerebrum. 
The complicated movements, such as those involved in the 
scratch reflex, which we have seen that a spinal animal can carry 
out in the paralyzed region after shock has passed away, become 
more and more numerous and complicated as the higher centers 
are.left in connection with the spinal cord. That is to say, the 
higher up in the cerebrospinal axis the section is made, the 
more capable does the part of the animal below the section be- 274 
THE CEREBRAL LOCALIZATION. 
come to peform complicated movements. The important centers 
in the medulla, pons and mesencephalon add their influence to 
those of the spinal cord itself, so that integration becomes more 
comprehensive. If the cut js made above the level of the pons, 
in other words, if the cerebral hemispheres alone be discon- 
nected from the rest of the cerebrospinal axis-decerebration, 
as it is called-we obtain an animal possessing all the reflex 
actions that are necessary for its bare existence, although it is 
of course incapable of feeling or, if the basal ganglion be also 
destroyed, of seeing or hearing. It becomes a mere automaton: 
it breathes, the blood circulation is normal, it can walk or run 
or swim, it swallows food if the reflex act of swallowing be 
stimulated by placing the food in the mouth, but it has not the 
sense to take food itself even when this is placed near it. All 
the mental processes are absent; it has no memory, no volition, 
no likes and dislikes. By seeing that it takes food, it has been 
possible to keep such a decerebrated /dog alive for eighteen 
months, and the lower we descend in the animal scale, the easier 
it becomes to perform the operation and to keep the animal alive. 
In higher animals, such as monkeys, however, life is impossible 
without the cerebrum, thus supporting the conclusion, which we 
have already drawn (see p. 255), that the cerebrum comes to 
be a necessary part of every reflex action in the higher aniAials. 
Cerebral Localization.-The various functions of the cere- 
brum are located in different portions of it. This localization 
of cerebral functions has been very extensively studied during 
recent years, partly by experimental work on the higher mam- 
malia and partly by clinical studies on man. Careful observa- 
tions are made of the behavior of the various functions of the 
animal either after removal or destruction of a portion of the 
cerebrum, or during its stimulation by the electric current. Im- 
portant additions to our knowledge of cerebral localization are 
also being made by correlating the symptoms observed in insane 
persons with the lesions which are revealed by post-mortem 
examination. 
It has been found that there are roughly three areas on the 
cerebrum with distinct and separate functions (Fig. 63). I. In the portions of the cerebrum which lie in front of the 
ascending frontal convolutions-prefrontal region-are located 
the centers of the intellect (thought, ideation, memory, etc.). 
This part of the cerebrum is accordingly by far the best de- 
veloped in man; it is much less so in the apes and monkeys, 
becomes insignificant in the dog, and still more so in the rabbit. 
It has been destroyed by accident in man with the result that 
all the higher mental powers vanished. 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
275 
Fig. 63.-Cortical centers in m^n. Of the three shaded areas bordering on 
the Rolandic fissure (Rol.), the most anterior is the precentral associational 
area, the middle one is the motor area (the position of the body areas are 
indicated on it), and the most posterior is the sensory area, to the cells of 
which the fillet fibers proceed. The centers for seeing and hearing are also 
shown. The unshaded portion in front of the Rolandic area is the precentral; 
the portions behind, the parietal and temperosphenoidal. 
II. The next portion includes roughly the region of the 
cerebrum bordering upon the Rolandic fissure (i. e., the ascend- 
ing frontal and ascending parietal convolutions). Here are 
located the highest centers for the movements of the various 
parts of the body. Microscopic examination of the grey matter 
reveals the presence of large triangular nerve cells, which com- 
municate by synapses (see p. 252) with the afferent fibers that 
carry the sensory impulses, whose course from the posterior THE CEREBRAL LOCALIZATION. 
276 
spinal roots we have already traced (p. 258). From each of 
these cells an efferent fiber runs to join the pyramidal tract 
(p. 260), and thus connect with the anterior horn cells of the 
spinal cord. 
In the Rolandic area, as it is called, is therefore situated the 
cerebral link in the chain of neurones (see p. 261) through 
which the ordinary movements of the body take place. Such 
movements may be set agoing, either by stimulation of the 
Rolandic nerve cells through afferent fibers-a pure reflex-or 
by impulses coming to them from the centers of volition situated 
in the prefrontal convolutions. Or, again, the nerve cell, at the 
same time that it receives a sensory impulse coming up from 
the spinal cord, may receive one from the prefrontal convolu- 
tions which may either interdict or greatly modify the reflex 
response. Every possible muscular group in the body has a 
center of its own in the Rolandic area, the determination of the 
exact location of these centers being one of the achievements 
of modern medical science. Thus, if we stimulate with a finely 
graded electric stimulus, say, the center of the thumb, it will 
be found that the thumb undergoes a slow, purposeful, co-ordi- 
nated movement; and so on for every other center. Or, if in- 
stead of stimulating, we cut away one of the centers and allow 
the animal to, recover from the immediate effects of the opera- 
tion, it will be found that all the more finely co-ordinated move- 
ments of the corresponding part of the body have disappeared, 
although gross reflex movements may be possible, because the 
spinal reflexes are still intact. If the entire Rolandic area on 
one side is removed, the muscles of the opposite side of the body, 
except those of the trunk, become completely paralyzed for 
some time, after which, however, particularly in the case of 
young animals, the paralysis becomes recovered from, thus in- 
dicating that some other portions of the brain have assumed 
the function of the destroyed centers. If the stimulus is a very 
strong one, the movements do not remain confined to the cor- 
responding muscle group, but they spread on to neighboring 
groups until ultimately the whole extremity or perhaps even 
all the muscles of that side of the body are involved. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
277 
These experimental results find their exact counterpart in 
clinical experience. Thus when some center becomes irritated 
by pressure on it of some tumor growing in the membranes of 
the brain (meningeal tumor), or by a piece of bone, as in de- 
pressed fracture of the skull, or by blood clot, convulsive at- 
tacks (known as Jacksonian epilepsy) are common. The 
first sign of such an attack is usually some peculiar sensation 
(aura) affecting the part of the body which corresponds to the 
irritated area; the muscles of this part begin to twitch and 
more muscles are involved, until ultimately all those of the cor- 
responding half of the body become contracted. There is, how- 
ever, no loss of consciousness, as there is in true epilepsy. 
ThQ evident cause of these symptoms has clearly indicated the 
proper treatment for such cases, namely, surgical removal of 
the cause of irritation. For this purpose a very careful study 
is first of all made of the exact group of muscles in which the 
convulsions originate; the location of the area on the cerebrum 
is thus ascertained and a trephine hole is made in the correspond- 
ing part of the cranium and through this hole the tumor or 
blood clot is removed. 
III. These so-called motor areas are of course also sensory 
areas in the sense that the afferent stimuli which come up from 
the spinal cord run to them. They are really sensori-motor 
centers. For some of the more highly specialized proficient 
sensations, such as vision and hearing (see p. 285), there are, 
however, special centers. These, along with an extensive field 
of associational or junctional grey matter, constitute the third 
main division of the cerebral cortex and occupy the greater 
part of the parietal, the temporosphenoidal and the occipital 
lobes. The visual is the most definite of these centers. Thus 
if the occipital lobe be removed or destroyed by disease on one 
side, the corresponding half of each retina becomes blind. It 
is by studying the exact nature of the involvement of vision in 
such cases that the physician is able to locate the position of a 
tumor, etc. 
The center for hearing is in the temporosphenoidal lobe, but 
its location is not very definite. 278 
THE MENTAL PROCESS. 
It will be seen, however, that the visual and auditory centers 
take up but a small part of this third division of the cerebrum, 
the most of it being occupied by associational areas. The nerve 
cells of these areas do not, like those of the motor and sensory 
centers, send libers which run as pyramidal or optic fibers to 
some lower nerve center, but only to other cerebral centers, 
which they serve to link together. They are specialized to serve 
as junction points for all the receiving and discharging centers 
of the cerebrum, so that all actions may be properly correlated or 
integrated. These junctional centers thus perform the great 
function of adapting every action of the entire animal to some 
definite purpose. Together with the nerve cells in the prefrontal 
areas, the associational cells represent the highest development 
of cerebral integration, so that we find the areas in which they 
lie becoming more and more pronounced, the higher we ascend 
the animal scale. 
The Mental Process.-The impression received by the visual 
center when a young animal looks for the first time at, say a 
bell, becomes stored away in nerve cells lying in or close to that 
center, and when the bell is moved sound memories are likewise 
stored in the auditory center. At first these remain as isolated 
memory impressions and the animal is unable to associate the 
sight with the sound of the bell But later, with repetition, the 
visual and the auditory centers become linked together, through 
nerve cells and fibers which occupy the associational areas, so 
that the invocation of one memory is followed by association 
with others. It is evident that the intricacy of this interlace- 
ment of different centers will, in large part, determine the in- 
tellectual development of the animal, and the possibility of his 
learning to judge of all the consecpiences that must follow every 
impression which^he receives or every act which he performs. 
In man these associational areas are very poorly developed at the 
time of birth, so that the human infant can perform but a few 
acts for itself. Everything has to be learned, and the learning 
process goes hand in hand with development of the associational 
areas, which proceeds through many years. On the other hand, 
most of the lower animals are born with the associational areas FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
279 
already laid down and capable of very little further increase., 
so that, although much more able than the human infant to 
fend for itself at birth, the lower animal does not afterwards 
develop mentally to the same extent. 
The practical application of these facts concerning the func- 
tions of different areas of the cerebrum is in the study of mental 
diseases. To serve as an example we may take aphasia. This 
means inability to interpret sights or sounds or to express the 
thoughts in language. In the former variety-called sensory 
aphasia-the patient can see or hear perfectly well, but fails 
to recognize that he has seen or heard the object before. Tie 
fails to recognize a printed word (word blindness) or to in- 
terpret it when spoken (word deafness). The lesion responsible 
for this condition is located in the associational areas and not 
in the centers themselves,. In the other variety, called motor 
aphasia, the patient understands the meaning of sounds or 
sights, of spoken or written words, but is unable to express his 
thoughts or impressions in language. The lesion in this case in- 
volves some of the centers concerned in the higher control of 
the muscles which are used in speech, and very commonly it is 
situated in the left side of the cerebrum. In all three forms of 
aphasia there is more or less decrease in the mental powers. 
Cerebellum. 
The afferent impulses set up by stimulation of the nerves of 
the skin in a spinal animal, and due therefore to changes in 
the environment, after entering the spinal cord travel to the 
various centers in the cord. Although complicated movements 
may result (e.g., the scratch reflex), there is an entire absence 
of the power of maintaining bodily equilibrium, and the animal 
cannot stand because the muscles are not kept in the degree of 
tone which is necessary to keep the joints properly stiffened. 
A similar inability to maintain the center of gravity of the 
body results from removal of the cerebellum, or small brain, 
which it will be remembered is situated dorsal to the medulla 
and pons, with which it is connected by three peduncles. The 
cerebellum consists of two lateral hemispheres and a median 280 
THE CEREBELLUM. 
lobe called the vermis. The remarkable infolding of the grey 
matter which composes its surface, and the large number of 
nuclei which lie embedded in its central white matter are struc- 
tural peculiarities of the cerebellum. 
The immediate results of removal of the cerebellum consist in 
extreme restlessness and inco-ordination of movements. The 
animal is constantly throwing itself about in so violent a man- 
ner that unless controlled it may dash itself to death. Gradually 
the excitement gets less, until after several weeks all that is 
noticed is that there is a condition of muscular weakness and 
tremor, and difficulty in maintaining the body equilibrium. 
Quite similar symptoms occur when the cerebellum is diseased 
in man (as by the growth of a tumor), the condition being 
called cerebellar ataxia, and being characterized by the uncer- 
tain gait which is like that of a drunken man. 
These observations indicate that the function of the cerebellum 
is to harmonize the actions of the various muscular groups, so 
that any disturbance in the center of gravity of the body may be 
subconsciously rectified by appropriate action of the various 
muscular groups. It evidently represents the nerve center hav- 
ing supreme control over other nerve centers, so that these may 
not bring about such movements as would disturb the equili- 
brium of the animal. 
In order that the cerebellum may perform this function it 
must, however, be informed of two things. In the first place, it 
must know the existing state of contraction of the muscles and 
the tightness of the various tendons that pull upon the joints, 
and in the second, it must know the exact position of the center 
of gravity of the body. 
Information of the condition of the muscles and tendons is 
supplied through the nerves of muscle sense, which run in 
every muscle nerve and are connected in the muscles with 
peculiar sensory nerve terminations called muscle spindles. 
When the muscles contract, or the tendons are put on the stretch, 
these spindles are compressed and sensory or afferent stimuli 
pass up the nerves of muscle sense, enter the cord by the pos- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
281 
terior roots and reach the cerebellum by way of the lateral col- 
umns (see p. 261). 
Information regarding the center of gravity of the body is 
supplied through the vestibular division of the eighth nerve, 
which, it will be recalled, is connected with the semicircular can- 
als and vestibule. In these structures are membranous tubes or 
sacs containing a sensory organ (called the crista or macula 
acoustica), which consists essentially of groups of columnar cells 
furnished with very fine hair-like processes at their free ends 
and connected at the other end with the fibers of the eighth 
nerve. The hair-like processes float in the fluid which is con- 
tained in the membranous canals or sacs. This fluid does not, 
however, completely fill these structures, so that it moves when- 
ever the head is moved. This movement affects the hair-like 
processes and thus sets up nerve impulses which are carried 
to the cerebellum. 
To make the hair cells of this receiving apparatus capable 
of responding to every possible movement of the head, it is, 
however, evident that there must be some definite arrangement 
of the tubes. This is provided for in the disposition of the semi- 
circular canals in three planes, namely, a horizontal and two 
vertical (Fig. 64). Taken together the three canals form a struc- 
ture which looks somewhat like a chair, the horizontal canals 
being the seat of the chair and the two vertical canals joining 
together to form its back and arms. The back of each chair is 
directed inwards so that they are back to back. At one end of 
each canal is a swelling, the ampulla, in which the sensory nerve 
apparatus above described is located. It is evident that when 
the head is moved in any direction the fluid in some of these 
canals will be set in motion. It is this movement of the fluid 
which stimulates the hair cells. That this is really the function 
of the semicircular canals is proved by the fact that if they are 
irritated or destroyed, grave disturbances occur in the bodily 
movements. This is what occurs in Meniere's disease, in which 
attacks of giddiness, often severe enough to cause the patient 
to fall, and accompanied by extreme nausea, are the chief symp- 
toms, the lesion being a chronic inflammation involving the 282 
THE SYMPATHETIC NERVOUS SYSTEM. 
semicircular canals. It is believed by some that the constant 
movements of the fluid in the semicircular canals is the cause 
of sea sickness. The unusual nature of these movements causes 
Fig. 64.-The semicircular canals of the ear, showing their arrangement 
in the three planes of space. (From Howell's Physiology.) 
confusion in the impressions transmitted to the cerebellum from 
the canals, but after a while the cerebellum may become accus- 
tomed to them and the sea sickness passes away. 
The Sympathetic Nervous System. 
Along with the vagus and one or two less prominent cere- 
brospinal nerves, the sympathetic constitutes the autonomic 
nervous system, so-called because it has to do with the innerva- 
tion of automatically acting structures, such as the viscera, the 
glands and the blood vessels. The characteristic structural fea- 
ture of the nerves of this system is that they are connected 
with nerve ganglia located outside the central nervous system. 
In these ganglia the nerve fibers run to nerve cells, around which 
they form synapses, thus permitting the nerve impulse to pass 
on to the cell, which then transmits it to its destination along its FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
283 
own axon (see p. 253). Before arriving at the ganglion in 
which the synapsis is formed, the fibers are called pregan- 
glionic; after they leave, they are called postganglionic. A 
preganglionic fiber may run through several ganglia before it 
becomes changed to a postganglionic fiber. In the case of the 
vagus and other cerebral autonomic nerves, the ganglia are 
often situated, as in the heart (see p. 92), at the end of the 
nerve, but in the case of the sympathetic itself, they are more 
numerous, and are mainly situated at the sides of the vertebral 
column, where, together with the connecting fibers, they form a 
chain-the sympathetic chain-which can easily be seen on 
opening the thorax and displacing the heart and lungs. 
Two fine branches connect each of the spinal nerves with 
the corresponding sympathetic ganglion. It is through one of 
these branches that the sympathetic chain receives its fibers 
from the spinal cord. Through the other, fibers run from the 
ganglion to the spinal nerve. Some of the sympathetic ganglia 
are situated at a distance from the spinal cord; the ganglia 
which compose the solar and hypogastric plexuses are examples. 
In the thorax, the uppermost ganglion is very large and is 
called the stellate ganglion. Its postganglionic fibers constitute 
the vasomotor nerves of the blood vessels of the anterior ex- 
tremity, and the sympathetic fibers to the heart. Some pregan- 
glionic fibers run through the stellate ganglion to pass up the 
neck as the cervical sympathetic, their cell station being in the 
superior cervical ganglion. They act on the pupil (dilating it), 
on the salivary glands (causing vasoconstriction and stimulating 
glandular changes), and on the blood vessels of the head, face 
and mucosa of the inside of the mouth. 
From about the fifth dorsal vertebra downwards, branches run 
from the sympathetic chain on each side to become collected 
into a large nerve called the great splanchnic, which passes down 
by the pillars of the diaphragm into the abdomen and runs to 
the ganglia of the coeliac plexus. This nerve supplies all of 
the blood vessels of the intestines and other abdominal viscera. 
Its action on these vessels has already been described (see p. 
97). It also carries nerve impulses for the control of the move- THE SYMPATHETIC NERVOUS SYSTEM. 
284 
ments of the stomach and intestines and for some of the digestive 
glands. In the abdomen the sympathetic chain gives off branches, 
which form the pelvic nerves and supply the blood vessels of 
the lower extremity. It is important to note that the connections 
between the sympathetic system and the cerebrospinal axis are 
limited to the spinal nerve roots between the second thoracic and 
the second lumbar. The results which follow stimulation of 
the sympathetic system are exactly like those which are pro- 
duced by injections of adrenalin (see p. 233). 
Depending partly on their mode of origin from the central 
nervous system and partly on the manner in which various drugs 
act on them, the autonomic system of nerves has been divided 
into: (1) sympathetic, (2) parasympathetic, and (3) enteric. 
The sympathetic include all fibers that arise from the thoracic 
portion of the spinal cord. Their endings are stimulated by 
adrenin and paralyzed by ergotoxin. The parasympathetic in- 
clude the fibers arising along with the cranial and pelvic nerves. 
They are stimulated by pilocarpin and muscarin and paralyzed 
by atropin. The vagus nerve to the heart is an example. The 
enteric constitute the extensive plexuses of nerves already de- 
scribed as present between the coats of the stomach and intes- 
tines. They have a regulatory function over the movements of 
these viscera, and they react toward drugs much like the para- 
sympathetics. Nicotin is a drug which acts on the synapses, 
and therefore affects all fibers of the autonomic system, although 
its action is relatively feeble in the case of the enteric group. CHAPTER XXVI. 
THE SPECIAL SENSES. 
The sensory nerve terminations, or afferent receptors, that 
are scattered over the skin are affected by stimuli which come in 
actual contact with the surface of the body. In order that the 
stimuli transmitted from a distance, such as those of light, sound 
and smell, or the projicient sensations as they are called, may 
be appreciated by the nervous system, specifically designed or- 
gans, called the organs of special sense, are required. These 
organs collect the stimuli in such a way as to cause them to act 
effectively on receptors which have been especially adapted to 
react to them. 
Although not really a projicient sensation, taste is conven- 
iently considered along with the above. 
Vision. 
Light is due to vibration of the ethereal particles that oc- 
cupy space. The vibrations occur at right angles to the rays 
of light, and these travel at high velocity in straight lines from 
the source of the light. The rate of vibration of the rays is not 
always the same, and on this difference depends the color of the 
light, red light vibrating much slower, and its waves being 
accordingly much longer, than those of violet light. The termi- 
nations of the optic nerve, the retina, have been specially 
developed to receive the light waves. But in order that a 
comprehensive picture of everything that is to be seen may be 
projected on the retina, an optical apparatus, consisting of the 
cornea and lens, is situated in front of it. The retina and the 
optical apparatus are built into a globe-the eyeball-which, 
pivoting on the attachment of the optic nerve, can be so moved 
that images from different parts of the field of vision may be 
285 286 
focused in turn on the retina. These movements are' effected 
by the so-called ocular muscles. 
There are, therefore, three functions involved in the act of 
seeing: (1) That of the retina, in reacting to light. (2) That of 
the cornea, etc., in focusing the light. (3) That of the ocular 
muscles, in moving the eyeball. 
The Optical Apparatus of the Eye. 
It will readily be seen that the eye is constructed on much 
the same principle as a photographic camera, the retina being 
like the sensitive plate. There is, however, an important dif- 
ference in the manner by which objects at varying distances are 
brought to a focus on the sensitive surface in these two cases: 
in the camera, it is done by adjusting the distance between the 
lens and the focusing screen; in the eye, it is done by varying 
the convexity of the lens. 
In order to understand how the optical apparatus works, it 
is necessary to know something about the refraction of light. 
When a ray of light passes from one medium to another, it be- 
comes bent or refracted. When it passes from air to water or 
glass, for example, it becomes refracted so that the angle which 
the refracted ray makes with the perpendicular to the surface 
is less than that of the entering ray. In other words, the ray 
becomes bent towards the perpendicular. The greater the dif- 
ference in density between the two media, the greater is the 
difference between the two angles. A figure expressing the ratio 
between these two angles is called the index of refraction. If 
the ray of light leaves the denser medium by a surface which 
is parallel with that by which it entered (as in passing through 
a pane of glass), it will be refracted back to its old direction, 
but if, as in a prism, it leaves the denser medium by a surface 
which forms an angle with that by which it entered, the original 
refraction will be exaggerated. If two prisms be placed with 
their broad ends together, parallel rays of light coming from a 
certain direction will be bent so that, on leaving the prisms, 
they meet somewhere behind them. Two prisms so arranged are 
virtually the same as a biconvex lens. It is plain that the 
VISION. FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
287 
focusing power of such a lens will depend on two things: first, 
its index of refraction, and, secondly, the curvature of its sur- 
faces. 
A considerable part of the actual refraction of the rays 
which enter the eye is accomplished at the curved surface of 
the cornea, a smaller degree of refraction taking place at the 
lens itself. The reason for this is that the refractive index 
from air to cornea is much greater than that between the lens 
and the humors of the eye in which the lens is suspended, these 
humors and the cornea having very much the same refractive 
indices. The entering rays are, therefore, refracted at two 
places in the eye, namely, at the anterior surface of the cornea 
and on passing through the lens. 
Fig. 65.-Formation of image on retina. O.A. is the optic axis. 
Accommodation of the Eye for Near Vision.-When the eye 
is at rest, its optical system is of such a strength that parallel 
rays, i. e., rays that are reflected from objects at a distance, are 
brought to a focus exactly on the retina. The picture thus 
formed is, however, upside down for the same reason that 
it is so on the screen of a camera (Fig. 65). When the object 
looked at is so near that the rays reflected from it are divergent 
when they enter the eye, it becomes necessary, if the image is 
still to be focused on the retina, that some adjustment take place 
in the optical system of the eye. This could happen in one 
of two ways, either by lengthening the distance between the 
lens and the retina (the method used in a camera), or by in- 
creasing the convexity of the lens. The former process cannot 288 
VISION. 
occur in the eye, but the second is rendered possible by bulging 
of the anterior surface of the lens. There are several ways by 
which this bulging of the lens can be proven to occur. Thus, 
if the eye of a person who is looking at some distant object be 
inspected from the side of the head, that is to say, in profile, 
it is easy to note the exact position of the iris, which, with the 
pupil in its center, hangs as a circular curtain just in front of 
the lens (Fig. 66). If the person is now told to regard some 
Fig. 66.-Section through the anterior portion of the eye: C, the cornea; 
I, the iris (note the circular muscular fibers cut across at the margin) ; L, 
the lens; Ci, the ciliary process; S., the suspensory ligament; Scl, the scler- 
otic or outer protective coat of the eye. (From a preparation by P. M. Spur- 
ney.) 
object held close to him, it will be seen that the iris is pushed 
forward nearer to the cornea. That this is really due to a bulg- 
ing of the anterior surface of the lens can be shown by placing 
a candle to one side and a little in front of the head and then, 
from the other side, viewing the images of the candle flame 
which are cast on the eye. It will be seen that one image occurs 
at the anterior surface of the cornea, and another, less distinct, 
at the anterior surface of the lens. This image from the lens FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
289 
will be seen to move forward-that is to say, closer to the image 
at the cornea-when the person shifts his gaze from a distant 
to a near object. By using optical apparatus for measuring the 
size of the images, the degree to which the convexity of the lens 
has increased, as a result of the bulging, can be accurately 
measured. 
This change in the convexity of the lens depends on the fact 
that it is composed of a ball of transparent elastic material, 
which is kept more or less flattened antero-posteriorly because of 
its being slung in a capsule which compresses it. The edges of 
the capsule are attached to a fine ligament (the suspensory liga- 
ment), which runs backwards and outwards to become inserted 
into the ciliary processes (Fig. 66). These processes exist as 
thickenings of the anterior portion of the choroid, or pigment 
coat of the eye, and they can be moved forwards by the action 
of a small fan-shaped muscle, called the ciliary muscle, which 
at its narrow end originates in the corneo-scleral junction, and 
runs back to be attached, by its wide end, to the ciliary pro- 
cesses. When this muscle is at rest, the ciliary processes lie at 
such a distance from the edges of the lens that the suspensory 
ligament is put on the stretch. When the ciliary muscle con- 
tracts, it pulls the ciliary processes forward, thus slackening 
the suspensory ligament and removing the tension on the capsule 
of the lens, with the result that the latter bulges because of its 
elasticity. The ability of the lens to become accommodated for 
near vision depends, therefore, first, on the elasticity of the 
lens, and secondly, on the action of the ciliary muscle. Inter- 
ference with either of these renders accommodation faulty. For 
example, the lens, along with the other elastic tissues of the 
body [e. g., the arteries (p. 86)], becomes less elastic in old 
age, thus accounting for the "long-sightedness" (or presbyopia) 
which ordinarily develops at this time. Paralysis of the ciliary 
muscle produces the same effect in even more marked degree, 
which explains the utter inability to bring about any accommo- 
dation after treating the eye with atropin, which is given for 
this purpose before testing the vision in order to find out the 
strength of lenses required to correct for errors in refraction. 290 
THE FUNCTION OF THE PUPIL. 
The Function of the Pupil.-Every optical instrument con- 
tains a so-called diaphragm, which is a black curtain having a 
central aperture whose diameter can be altered to any required 
size. The object of this is to prevent all unnecessary rays of 
light from entering the optical instrument, thus materially in- 
creasing the distinctness of the image. In the eye, this function 
is performed by the iris with the pupil in its center. The size 
of the pupil is altered by the action of two sets of muscle fibers 
in the iris. One of these runs in a circular manner around the 
inner edge of the iris; by contracting it causes constriction of 
the pupil, an event which occurs, along with the bulging of the 
lens, during accommodation for near vision. The other layer of 
fibers runs in a radial manner, and by contracting causes dila- 
tation of the pupil. This occurs in partial darkness, or when the 
eye is at rest (although not during sleep). The circular fibers 
are supplied by the third nerve, and the radial fibers by the 
sympathetic. Under ordinary conditions both muscles are in a 
state of tonic contraction (see p. 265), so that the actual size 
of the pupil at any moment is the balance between two opposing 
muscular forces. This renders its adjustment in size very sensi- 
tive. For example, it can become dilated either by stimulation 
of the sympathetic (which occurs when any irritative tumor 
affects the cervical sympathetic nerve), or by paralysis of the 
third nerve (as by giving atropin). Coilversely, constriction of 
the pupil may be the result of stimulation of the third nerve (as 
by a tumor at the base of the brain) or paralysis of the sympa- 
thetic. 
These local conditions acting on the afferent nerves to either 
pupil are not nearly so often called into play as conditions acting 
reflexly on both eyes at the same time. 
Certain of the afferent impulses which call these reflexes into 
play travel by the optic nerve to the nerve centers for the pupil, 
such for example as the stimulus set up by light falling on the 
retina. The afferent pathway concerned in the contraction of 
the pupil, which occurs in accommodation, must, on the other 
hand, be a different one because in the disease locomotor ataxia 
(see p. 266), the pupil contracts on accommodation, but does not FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
291 
do so when light is thrown into the eyes. The nerve centers for 
the pupil are very sensitive to general nervous conditions, thus 
accounting for the dilatation of the pupil which occurs during 
fright or other emotions, or pain. The pupils are contracted in 
the early stages of asphyxia or anesthesia, as in the early stages 
of nitrous oxide administration, but they become dilated when 
the anesthesia or asphyxia becomes profound. Their condition 
helps to serve as a gauge of the depth of anesthesia. 
Imperfections of Vision.-The optical system of the eye is 
not perfect. Some of these imperfections exist in every eye, 
whilst others are only occasional. The errors in every eye are 
those known as spherical and chromatic aberration. Spherical 
Fig. 67.-A, spherical aberration. The rays which strike the margins of 
the lens are brought to a focus before those striking near the center. B, 
Chromatic aberration. The ray of white light (W) is dissociated by the 
lens into the spectral colors, of which those at the red end (R) are not 
brought to a focus so soon as those at the violet end (V). 
aberration (Fig. 67), occurs because the edges of the lens have 
a higher refractive power than the center, so that the image on 
the retina is surrounded by a halo of overfocused rays. Chro- 
matic aberration is due to the fact that white light, on passing 
through the lens, suffers some decomposition into its constituent 
colored rays (the rainbow colors), of which certain ones (viz., 
those towards the violet end of the spectrum) -come to a focus 
sooner than others (viz., those towards the red end), thus creat- 
ing a colored edge on the focused image. These errors are 
greatly minimized, although not entirely removed, by the pupil, 
which cuts out the peripheral rays. 
The occasional errors are long-sightedness or hypermetropia, 292 
VISION. 
short-sightedness or myopia, and astigmatism (Fig. 68). Hyper- 
metropia is due to the eyeball being too short so that the focus 
of the image is behind the retina. The error is corrected by 
prescribing convex glasses. Myopia is due to the opposite con- 
Fig. 68.-Errors in refraction: E shows the formation of the image on 
the retina in the normal or emmetropic eye; H shows the condition in long- 
sight, or hypermetropia, where the eyeball is too short; M shows the condi- 
tion in short-sight, or myopia, where the eyeball is too long. 
dition, that is, the eyeball is too long, so that the focus occurs 
in front of it. Concave glasses correct it. Astigmatism is due to 
the lens or cornea being of unequal curvature in its different FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
293 
meridians. This causes the rays of light in one plane to be 
brought to a focus before those in other planes, so that the two 
hands of a clock, when they are at right angles to each'other, 
cannot be seen distinctly at the same instant, although they can 
be successively focused. A certain amount of astigmatism exists 
in every eye, but when it becomes extreme, it is necessary to 
correct it by prescribing glasses which are astigmatic in the 
opposite meridian to that of the eye. Such glasses are called 
cylindrical. 
Astigmatism may occur along with either myopia or hyper- 
metropia, and when any of these errors is only slight in degree, 
the patient may be able, by efforts of accommodation, to over- 
come the defect. The strain thus thrown on the ciliary muscle 
is, however, quite commonly the cause of severe headache. The 
correction of the errors should never be left to untrained per- 
sons, but a proper oculist should be consulted, since it is usually 
necessary to give atropin so that the accommodation may be 
paralyzed and the exact extent of the error measured. The use 
of improper glasses may aggravate the defect of vision and do 
much more harm than good. 
The Sensory Apparatus of the Eye. 
The Functions of the Retina.-The image which is formed on 
the retina by the optical system of the eye sets up nerve im- 
pulses which travel by the optic nerve to the visual center in 
the occipital lobes of the cerebrum (see p. 277), where they are 
interpreted. Microscopic examination of the retina has shown 
that it consists of several layers of structures, the innermost 
being of fine nerve fibers which arise from an adjacent layer of 
large nerve cells, and the outermost of peculiar rod or cone- 
shaped cells, called the rods and cones. Between the layer of 
large cells and the layer of rods and cones are several layers 
composed of other nerve cells and of interlacements of the pro- 
cesses of cells and nerve fibers. The rods and cones are the 
structures acted on by light, the other layers of the retina being 
for the purpose of connecting the rods and cones with the large 
nerve cells from which the fibers of the innermost layer arise. 294 
VISION. 
The fibers all converge to the optic disc, which is a little to the 
inside of the posterior pole of the eyeball. At this point the 
fibers of the nerve fiber layer bend backwards at right angles 
and run'into the optic nerve, thus crowding out the other layers 
and causing the existence of a blind spot, which can be readily 
demonstrated by closing one eye, say the left, and with the other 
regarding the letter B in the next line. Although the S is 
B S 
also distinctly visible in most positions, yet if the book be 
moved towards and away from the eye, the S will become in- 
visible at a certain distance corresponding to that at which the 
rays from it are impinging upon the blind spot. As we alter 
the distance of the book from the eye, the line of vision, or 
visual axis, being fixed on the B, the image of the S travels 
from side to side across the inner or nasal half of the retina, 
and at a certain position strikes the optic disc. Ordinarily we 
are unaware of the blind spot, partly because we have two eyes 
and, the blind spot being towards the nasal side of each retina, 
the image of an object does not fall on it in both eyes at the 
same time; and partly because we have learned to disregard it. 
The area or extent of the blind spot may become so increased, as 
by excessive smoking, that it becomes noticeable. 
At another portion of the retina called the fovea centralis, all 
the layers become thinned out except that of the rods and cones, 
especially the cones. This, as we should expect, is by far the 
most sensitive portion of the retina, and is indeed the portion on 
which we cause the image to be focused when we desire to see 
an object clearly. The remainder of the retina is only suffi- 
ciently sensitive to give us a general impression of what we are 
looking at. Thus when we view a landscape, we can see only a 
small portion clearly at one time, although we have a general 
impression of the whole. The portion which we see clearly is 
that which is focused on the fovea, and we keep moving our 
eyes in all directions so that every part of the landscape may in 
turn be properly seen. We see with the fovea what the rest of 
the retina informs us there is to be seen. FUNDAMENTALS HF HUMAN PHYSIOLOGY. 
295 
The Movements of the Eyeballs.-In order that we may be 
enabled to move our eyes so as to see objects in different posi- 
tions in the visual field, the eyeballs are provided with six little 
muscles, four recti and two obliques. These muscles are in- 
nervated by the third, fourth and sixth nerves (see p. 271). The 
images in the two eyes cannot of course fall on anatomically 
identical parts of the retinae, but they fall on parts that are 
physiologically identical. Thus, an object, say on the right of 
the field of vision, will cause an image to fall on the nasal side 
of the right retina and on the temporal side of the left retina. 
We do not, however, see two objects because by experience we 
have come to learn that these are corresponding points on the 
retinae. When an object is brought near to the eye, the two 
eyeballs must converge so as to bring the visual axes on to the 
corresponding points. This convergence of the eyeballs con- 
stitutes the third change occurring in the eyes during accom- 
modation for near vision, the other two being, as we have seen, 
bulging of the lens and contraction of the pupil. It is interest- 
ing that these three changes are controlled by the third nerve. 
If anything happens to throw one of the images on to some 
other portion of one retina, double vision is the result. This 
condition of diplopia, as it is called, can be brought about, vol- 
untarily, by pressing on one eyeball at the edge of the eye, or 
it may occur as a result of paralysis or incoordinate action of 
one or more of the ocular muscles. This occurs in certain in- 
toxications, as, for example, that produced by alcohol. 
Just as in the case of errors of refraction, e. g., astigmatism, 
slight degrees of diplopia may cause symptoms that are more 
distressing than when marked diplopia exists, because we try 
to correct for slight errors and the effort causes pain (headache) 
and fatigue, whereas with extreme errors we do not try to correct 
but, instead, we learn to disregard entirely the image in one 
eye. Whenever the incoordination of ocular movement is per- 
manent, as when due to shortening of one of the muscles, it is 
called strabismus. This condition is usually congenital, and can 
often be rectified by a surgical operation. 
Judgments of Vision.-Besides these purely physiological 296 
VISION 
problems of vision, there are many others of a physio-psycho 
logical nature. Such for example are the visual judgments of 
size, distance, solidity, and color. Judgments of size and dis- 
tance are dependent on: (1) the size of the retinal image, (2) 
the effort of accommodation necessary to obtain sharp defini- 
tion, and (3) the amount of haze which appears to surround the 
object. Judgment of solidity depends on the fact that the 
images produced on the two retinae are not exactly from the 
same point of view; they are like the two photographs of a 
stereoscopic picture. The brain on receiving these two slightly 
different pictures fuses them into one, but judges the solidity of 
the object from the differences in the two pictures. 
Judgment of color, or color vision, forms a subject of great 
complexity. It apparently depends on the existence in the re- 
tina of three varieties of cones, one variety for each of the three 
primary colors. The primary colors are red, green and violet; 
and by mixing them on the retina in equal proportions (as by 
rotating a disc or top on which they are painted as sectors) a 
sensation of white results; by using other proportions, any of the 
other colors of the spectrum may be produced. When one of 
these primary color receptors is absent from the retina, color 
blindness exists. Thus if the red or the green receptors are 
absent, the patient cannot distinguish between red and green 
lights. Such persons cannot be employed in railway or nautical 
work. Plate VIII-Diagrammatic view of the organ of Corti (Testut) : D, basilar 
membrane; A. B, inner and outer rods of Corti; (J, 6', 6," hair cells; 7. 7', 
supporting cells. (From Howell's Physiology.) CHAPTER XXVII. 
THE SPECIAL SENSES (Cont'd). 
Hearing. 
Like light, sound travels in waves, but not as transverse waves 
of the ether that fills space, but as longitudinal waves of con- 
densation and rarefaction of the atmosphere itself. The magni- 
tude of these waves is much greater and their rate of trans- 
mission much slower than the waves of light; therefore we see 
the flash of a gun long before we hear its sound. The several 
qualities of sound, such as pitch, loudness and quality or timber, 
depend respectively on the frequency, the magnitude and the 
contour of the waves. Sound waves are not appreciated by the 
ordinary nerve receptors but only by those of the cochlear 
division of the eighth nerve. These are connected, in the cochlea 
of the internal ear, with a highly specialized receptor capable 
of converting the sound waves into nerve impulses. The cochlea 
consists of a bony tube wound two and one-half times as a spiral 
around a central column, up the center of which runs the 
end of the cochlear nerve. A longitudinal section of the 
cochlea (Fig. 69), therefore shows us this spiral tube in sec- 
tion at several places, and it is noticed that there projects 
into it from the central column a ledge of bone having a C-shaped 
free margin. From the lower lip of the C, a membrane called 
the basilar membrane, stretches across the tube, which it thus 
divides into two canals, of which the upper is again divided into 
two by another membrane running from the upper surface of the 
bony ledge. 
The basilar membrane is a very important part of the mechan- 
ism for reacting to sound waves. Resting on it is a peculiar struc- 
ture called the Organ of Corti (Plate VIII), which in transverse 
sections of the cochlear canal is seen to be composed of two rows 
of long epithelia] cells set np on end like the rafters of a roof, 
297 298 
HEARING. 
with shorter "hair" cells leaning up against them, particularly 
on the side away from the central column. The sound waves which 
act on the basilar membrane are transmitted to the fluid which 
fills the uppermost of the three divisions of the cochlear tube (see 
Fig. 69) through a membrane covering an oval shaped opening 
(the oval window) in the bony partition separating the internal 
from the middle ear. After reaching the apex of the cochlea they 
pass through a small aperture in the basilar membrane into the 
Fig. 69.-Seffiidiagrammatic section through the right ear (Czermak) ; G, 
external auditory meatus; T, membrana tympani; P, tympanic cavity or 
middle ear with the auditory ossicles stretching across it and the Eustachian 
tube (E) entering it; o, oval window; r, round window; B, semicircular 
canals; cochlea; Vt, upper canal of cochlea; Pt, lower canal of cochlea. 
(From Howell's Physiology.) 
lowest canal, down which they travel to lose themselves against 
the membrane covering another opening (the round window) sit- 
uated near the oval window in the same partition of bone. As 
they pass along these canals the waves cause the basilar mem- 
brane to move or vibrate. The vibration affects the cells of the 
Organ of Corti, and so sets up nerve impulses which are trans- 
mitted to the cochlear nerve by means of nerve fibers which con- 
nect with each of the main cells of the Organ. A fine membrane FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
299 
(called Tectorial) rests on the top of the hair cells, and by rub- 
bing on them when they move, this membrane augments the 
action of the basilar membrane. 
We must now consider how the sound waves are brought 
from the outside to the oval window. The pinna of the ear col- 
lects the sound waves from the outside and directs them into the 
external auditory canal, at the inner end of wdiich they strike 
the drum of the ear or tympanic membrane. This membrane is 
stretched loosely in an oblique direction across the canal, and is 
composed partly of fibers which radiate to the edge of the 
membrane from the handle of the malleus, a process of one of 
the auditory ossicles, to which it is attached. Because of these 
properties, the tympanic membrane, unlike an ordinary drum, 
is capable of vibrating to a great variety of notes, and the 
vibrations cause the handle of the malleus to move in and out. 
Between the tympanic membrane and the cochlea is the middle 
ear, or tympanum, consisting of a cavity across which stretches 
the auditory ossicles composed of three small bones, the malleus, 
the incus and the stapes. Besides the long process or handle 
already described, the malleus consists of a rounded head sit- 
uated above and forming a saddle-shaped articulation with the 
head of the incus and a short process which runs from just be- 
low the head to the anterior wall of the tympanum. The incus 
is somewhat like a bicuspid tooth, the malleus articulating with 
the crown, and having two fangs, a short one passing backward 
and a long one vertically downwards. This process, at its lower 
end, suddenly bends inwards to form a ball and socket joint 
with a stirrup-shaped bone (the stapes), the foot piece of which 
is oval in shape and fits into the oval window already mentioned. 
The ossicles act together as a bent lever, the axis of rotation 
passing through the short process of the malleus in front and the 
short process of the incus behind. If perpendiculars be drawn 
from this axis to the tips of the handle of the malleus and the 
long process of the incus, it will be found that the latter is only 
two thirds the length of the former (Fig. 70). The amplitude 
of movement at the stapes will therefore be only two-thirds of 
that at the center of the tympanic membrane, but one and one- 300 
HEARING. 
half times stronger. The increase in force with which the 
movements of the tympanic membrane are conveyed to the oval 
window is still further magnified by the fact that the latter is 
only one-twentieth the size of the former. It is by these move- 
ments at the oval window that waves are set up in the fluid 
occupying the uppermost membranous tube of the cochlea and 
thus acting on the basilar membrane. The tympanic cavity or 
Fig. 70.-Tympanum of right side with the auditory ossicles in place (Mor- 
ris) : 1, incus (like bicuspid tooth) with one process (3) attached to wall of 
tympanum and the other running downwards to articulate at 9 and 8, the 
stapes; 10, head of malleus attached to tympanic membrane. (From How- 
ell's Physiology.) 
tympanum across which the chain of ossicles stretches is kept 
at atmospheric pressure by the Eustachian tube, which connects 
it with the posterior nares. 
Deafness may be due to the following causes: 
1. Rupture of the tympanic membrane. 
2. Ankylosis or stiffening of the joints between the ossicles FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
301 
and the ligaments which hold them in place in the tympanic 
cavity. Flexibility of the joints between the ossicles prevents 
sudden jars at the oval window, for the joint between the mal- 
leus and incus, being saddle-shaped, unlocks whenever abnormal 
or excessive movements are transmitted to the malleus. 
3. Blocking of the Eustachian tube. This is quite com- 
monly a result of adenoids or it may be due simply to a catarrh 
of the tube. The result of the block is that the pressure on the 
tympanic cavity falls below that of the atmosphere because of 
absorption of oxygen into the blood, and the tympanic mem- 
brane bulges inwards and becomes stretched so that it cannot 
vibrate properly to the sound waves. The deafness in this case 
is easily removed by reopening the Eustachian tube by forcing 
air into it. This can be done by attaching a large syringe bulb 
to one nostril, closing the other nostril, and while the patient is 
swallowing a mouthful of water, suddenly compressing the bulb. 
The auditory distress which is experienced by a person on 
going into compressed air (as into a caisson) is also due to dis- 
turbance in the tympanic pressure, for it takes a few moments 
before this reaches that on the outside. Blowing the nose usually 
removes the distress. 
In all these conditions, the patient hears perfectly when a 
tuning fork is applied to the skull or teeth. This is because the 
sound vibrations are then transmitted to the cochlea through 
the bones of the head. When the cochlea is diseased, however, 
the tuning fork cannot be heard either when it is sounded in the 
air or when it is applied to the skull or teeth. 
The Sense of Taste. 
Scattered over the mucous membrane of the tongue and buccal 
cavity, and extending back into the pharynx and even into the 
larynx, are the receptors of taste, or taste buds. They are most 
numerous in the grooves around the circumvallate papillae at 
the root of the tongue, and in the fungiform papillae. Each 
taste bud is composed of a mass of fusiform cells packed like a 
barrel filled with staves. The staves in the center project as 
hairs beyond those on the outside, and it is evidently by action 302 
TASTE. 
on these hairs that certain dissolved substances set up a stimulus 
of taste. This stimulus is then conveyed by fine nerve fibers 
which arborize around the taste cells, to the chorda tympani and 
lingual nerves in the anterior portion of the tongue and the 
glossopharyngeal in the posterior part. Through these nerves 
the sensations are carried to the combined afferent nucleus of 
the fifth and ninth nerves in the medulla oblongata (see Fig. 
71). 
Fig. 71.-Schema to show the course of the taste fibers from tongue to 
brain (Cushing). The dotted lines represent the course as indicated by Cush- 
ing's observations. The full black lines indicate another path by which the 
impulses may reach the brain. (From Howell's Physiology.) 
Substances cannot be tasted unless they are in solution, thus, 
quinine powder is tasteless. One of the functions of saliva is 
to bring substances into solution in order that they may be 
tasted. 
There are four fundamental taste sensations: sweet, saline, 
bitter and sour or acid. The ability to distinguish each of these 
tastes is not evenly distributed over the tongue, but occurs in 
definite areas. These can be mapped out by applying solutions, fundamentals of human physiology. 
303 
possessing one or another of these qualities, by means of a fine 
camel-hair brush, to different portions of the tongue previously 
dried somewhat with a towel. Bitter taste is absent from all 
parts of the tongue except the base, hence a mouthful of a weak 
solution of quinine sulphate has practically no taste until it is 
swallowed, when however it tastes intensely bitter. Sweet and 
sour tastes are most acute at the tip and sides of the tongue. 
Saline taste is more evenly distributed. 
This location of taste sensations is not a hard and fast one, 
for neighboring taste buds in, say, the bitter area at the root of 
the tongue may appreciate different tastes; thus, if a solution 
containing quinine and sugar be applied to one papilla, it may 
taste sweet, whereas when applied to a neighboring one, it tastes 
bitter. With weak solutions one taste may neutralize another; 
thus the addition of a small amount of salt to a weak sugar solu- 
tion may remove its sweet taste. This neutralization of one 
taste by another does not occur when the solutions are stronger; 
thus a mixture of acid and sugar, as in lemonade, causes stimu- 
lation of both "acid" and "sweet" taste buds. The stimula- 
tion of one kind of taste bud may cause other taste buds to be- 
come more acutely sensitive, which explains the sweetish taste of 
water after washing out the mouth with a solution of salt. 
Attempts have been made to correlate the chemical structure 
of organic substances with the taste which they excite, but with 
little success. Thus pure proteins have very little taste, whereas 
half-digested protein is intensely bitter; on the other hand, the 
pure amino acids, which form a large proportion of the de- 
composition products in such a digest, are sweet. In the case of 
acids and alkalies, however, it has been established that the acid 
taste is due to the H-ion and the alkaline to the OH-ion. Some 
acids, such as acetic, taste more acid than we should expect from 
their degree of dissociation into H-ions. This is because of 
their power of penetration into the cells of the taste buds. When 
platinum terminals from a battery are applied to the tongue, 
the positive pole tastes alkaline and the negative acid, because 
OH-ions accumulate at the former and H-ions at the latter. 
The Association of Taste, Touch and Smell.-The four 304 
TASTE. 
fundamental tastes do not nearly represent all the tastes and 
flavors with which we are familiar. The relish of an appetizing 
meal, the piquancy of condiments, the bouquet of a fine wine, 
would remain unappreciated were there no other nerve receptors 
than those described above. Two other types of nerve receptors 
are involved, namely, (1) those of common sensation, as in the 
case of acids, which add an astringent character to the sour 
taste, and (2) those of smell, as in wines and flavored foods. 
The importance of the sense of smell in "tasting" explains the 
loss of this ability during nasal catarrh or cold in the head. 
Under such conditions an apple and an onion may taste alike. 
Certain drugs when applied to the tongue affect taste sensa- 
tions in different degrees. Thus cocaine first of all paralyzes 
the receptors of common sensation so that pain is no longer 
felt and an acid loses all of its astringent qualities and merely 
tastes sour. A little later the bitter taste also disappears, then 
salt, then sour, but the saline taste remains even after the cocaine 
has developed its full effect. Another interesting drug acting 
on the taste sensations, is a substance present in the leaves of 
Gymnema sylvestre. When these leaves are chewed, the sweet 
and bitter tastes are absent, those of acid and of salt and ordi- 
nary sensation (astringency, etc.) being, however, unaffected. 
The Sense of Smell. 
In man the sense of smell is very feeble when compared with 
that of the lower animals, and it is of very unequal development 
in different individuals. It is, moreover, readily fatigued, as is the 
experience of every one who has been compelled to live in stuffy 
rooms. The receptors are represented by the columnar epithelium 
of the superior and middle turbinate bones and the adjacent 
parts of the nasal septum. This epithelium is composed of large 
columnar cells, each cell being connected with a nerve fiber which 
is one of the branches of a fusiform bipolar nerve cell lying im- 
mediately beneath the epithelium. The second branch of each 
nerve cell runs through the cribiform plate to join the olfactory 
bulb. After making connections with nerve cells here, the path- FUNDAMENTALS of human physiology. 
305 
way is continued along the olfactory tract to the hippocampal 
region of the brain. As we would expect, this portion of the 
brain is highly developed in those animals having a very acute 
sense of smell. 
The olfactory epithelium is kept constantly moist with fluid, 
and substances cannot; be smelled unless the odorous particles 
which they give off become dissolved in this fluid. These odor- 
ous particles diffuse into the upper nares from the air currents 
which, with each respiration, are passing backwards and for- 
wards along the lower nasal passages. There is no actual move- 
ment of air over the olfactory epithelium. 
Nature of Stimulus.-It is impossible to state just exactly 
what it is that emanates from an odorous body to excite the ol- 
factory sense. All we can say is that it does not require to be 
present in more than the merest trace in the air in order to un- 
fold its action. Thus even in the case of man, with his undevel- 
oped sense of smell, 0.000,000,000,04 of a gramme of mercaptan, 
suspended in a liter of air, can be smelled, and in the case of the 
dog, the dilution may no doubt be many thousand times greater. 
The sense of smell is the most important of the projicient sensa- 
tions in certain aquatic animals, and is very closely associated 
with the sexual functions of the animal. Just as in the case of 
taste, certain substances owe their peculiar odors to simultane- 
ous stimulation of the olfactory epithelium and the receptors of 
common sensation. Thus the pungency of acids, of ammonia, 
chlorine, etc., is due to stimulation of the endings of the fifth 
nerve. Attempts have been made to classify odors, as has been 
done for tastes, but with no success. CHAPTER XXVIII. 
REPRODUCTION. 
The most important function of an animal's life is the produc 
tion of a new individual which in all peculiarities of function and 
structure is essentially like the parent. The fundamental prob- 
lems of the process of reproduction which are of physiological 
importance, are those of fertilization and heredity. Fertiliza 
tion consists in the union of two parent cells to produce a new 
cell which is endowed with the power of growth and subdivision. 
Heredity refers to the phenomenon which directs the cell thus 
fertilized to develop into an individual like its parents. 
Since up to the present time most of our knowledge of these 
processes is based on anatomical data, we will discuss them very 
briefly and will pay more attention to what we may term the 
accessory phenomena of reproduction, which are of more practi- 
cal interest at present. 
Reproduction in the unicellular animals is a simple process. 
The parent cell divides exactly in halves and two daughter cells 
are produced. In the multicellular animals this type of repro- 
duction is impossible and the process is delegated to a portion 
of the animal's body known as the reproductive system. This 
system in pian includes the specialized tissues which produce the 
cells or eggs from which the new individual develops, and the 
accessory organs which are concerned in providing favorable 
conditions for the development of these cells. 
Fertilization.-A very simple type of fertilization is seen in 
unicellular animals, which ordinarily reproduce by simple divi- 
sion. After a series of simple divisions the cell becomes unable 
to develop more cells until after it has united with another cell 
to form one large cell. This process is termed conjugation. In 
higher forms, the development of the egg is always preceded by 
the phenomenon of fertilization, which is somewhat similar to 
306 307 
FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
that of conjugation in lower forms. In this process, cells of two 
types are concerned, the male, or sperm cell, or spermatozoon, 
and the female cell or ovum. The spermatozoon has the ability 
to move and to penetrate the ovum. The nuclear elements of 
both cells unite to form a new nucleus, which is then capable of 
undergoing- a long series of subdivisions. In changes which pre- 
cede fertilization, the nuclear material originally present in both 
male and female cells is reduced, and when the cells fuse, the re- 
sulting nucleus contains a normal quantity of nuclear material. 
The Accessory Phenomena of Reproduction in Man.- 
The beginning of the active sexual life in man is between the 
ages of fourteen and sixteen, and is called the age of puberty. 
In both boys and girls the whole body shows a marked develop- 
ment at this time. The growth of hair on the pubic regions and 
arm pits, and on the face of boys, the deepening of the male 
voice, and the development of the breasts in the female, are all 
accompanying phenomena of the development of puberty. In 
females this age is marked by the onset of menstruation, which 
consists of a periodic flow of mucus and blood from the uterus 
The flow lasts from four to five days, and recurs with great regu- 
larity about every four weeks. In males fully formed seminal 
fluid, containing live sperm cells, appears. 
The Female Organs of Reproduction.-These are the ovaries, 
oviducts, uterus and the vagina. The ovaries are paired bodies 
lying in the lower part of the abdominal cavity and held in posi- 
tion by the broad ligament. The cells from which the ova de- 
velop are imbedded in the fibrous tissue of the ovary. A number 
of these cells, better developed than their fellows, and surrounded 
by a layer of cells, which form a sort of follicle, lie near the sur- 
face of the ovary. These are the Graafian follicles, in which the 
ova develop till they are ripe, when they are extruded into the 
abdominal cavity by rupture of the follicle. In very close appo- 
sition to the ovaries is a tube, the oviduct, which leads to the 
uterus. The outer end of this tube is fimbriated, and it is fur- 
nished with cilia, the movements of which cause currents in the 
fluids of the abdominal cavity, and which direct the ova dis- 
charged from the follicle into the oviduct. The uterus is a pear- 308 
REPRODUCTION. 
shaped organ with muscular walls. It is about 7 cm. in length, 
and consists of an upper dilated portion, called the fundus, and 
a lower constricted portion, called the cervix. The cervix opens 
by a small aperture into the vagina, which is a membranous canal 
about 10 cm. long extending to the vaginal outlet at the external 
genitalia. 
The Male Organs of Generation are the testicles, vas deferens, 
seminal vesicles, the penis, the prostate gland, and a number of 
small glands along the urethra. 
The testicles consist of two parts, a portion of which is cellular 
and is concerned in the development of the spermatozoa; and a 
portion called the epididymis, containing the lower portion of 
the very long and convoluted duct, the vas deferens. This duct 
connects the testicles with the seminal vesicles, which lie at the 
base of the bladder and in close relation to the prostate gland. 
The seminal vesicles are united by a short duct with the urethra, 
which is the outlet for the excretions of both the kidney and the 
testicles. 
The spermatozoa are developed in the testicles and find their 
way to the seminal vesicles through the vas deferens. On their 
way they become mixed with a number of fluid secretions, the 
chief of which are derived from the seminal vesicles of the pros- 
tate gland and of the glands of Cowper. The resulting mixture 
is the seminal fluid. * 
Impregnation.-The seminal fluid containing the spermatozoa 
is deposited in the vagina during coitus. Attracted by the acid 
reaction of the secretions of the uterus or under an unknown in- 
fluence, the spermatozoa soon enter the uterine cavity through 
its opening into the vagina, and find their way to the oviduct, 
where they remain waiting for the ovum to appear. 
Ovulation.-At about the time of a menstrual period an ovum 
is discharged from a ripened Graafian follicle and finds its way 
into the oviduct by way of the fimbriated extremity of the tube, 
down which it is conducted to the uterus. It is a debated ques- 
tion as to what the exact relation between menstruation and ovu- 
lation may be. Whether ovulation precedes or follows menstrua- FUNDAMENTALS OF HUMAN PHYSIOLOGY. 
309 
tion is not known, but the weight of evidence favors the belief 
that menstruation serves to prepare the uterine walls for the 
reception of the fertilized ovum should one be discharged. In 
animals there are periods, called the rutting period, during 
which impregnation of the ovum with the spermatozoon is pos- 
sible. Preceding this period there occurs a swelling of the exter- 
nal genitalia and some discharge of mucus. This period probably 
corresponds to the menstrual period in woman, for there is much 
evidence to show that impregnation occurs most frequently fol- 
lowing the menses. 
Menstruation ceases during pregnancy and is generally absent 
during the period of lactation. It ceases altogether between the 
ages of about forty-five and fifty. After this time, which is 
known as the climacteric period, a woman is no longer capable 
of bearing children. 
The union of the spermatozoon and the ovum usually occurs 
in the oviduct. If the ovum is not fertilized it is cast off. If it 
is fertilized, a considerable thickening of the uterine mucous 
membrane takes place from the proliferation of its cells. When 
the ovum reaches the uterus, it becomes imbedded in the mucous 
membrane of the fundus of the uterus. This mucous membrane 
is very vascular and soon becomes fused with the outer layer of 
the ovum. 
Pregnancy.-At first the ovum receives its nourishment 
directly from the mucous membrane of the uterus, but as the 
ovum develops and becomes what we term an embryo, the part 
lying next to the uterine mucosa becomes very vascular; a similar 
process takes place in the uterine mucosa directly in contact with 
the embryo. By this process the placenta is formed, the organ 
through which the embryo obtains nourishment from the mother. 
The vascular system of the embryo is, however, entirely sepa- 
rate from the maternal vessels, and the blood of the mother 
never directly enters the embryo. The interchange between the 
two must be effected through the cells covering the vessels of the 
uterine and foetal portions of the placenta. In other words, the 
embryo may be said to live a parasitic yet entirely independent 
life, since through its placental vessels it exchanges its effete 310 
products for the oxygen and nourishment contained in the 
mother's blood. 
Birth.-While the ovum is being developed into a human 
being by division of the original cell of the fertilized ovum, the 
uterus becomes very much enlarged, and its walls increase in size 
by the growth of muscular tissue. At the end of approximately 
280 days from the date of impregnation of the ovum, the devel- 
opment is complete and birth takes place. This consists in the 
expulsion of the foetus by muscular contractions of the uterus. 
Directly the child is born, the placenta begins to separate from 
the uterine wall and is soon expelled. The child deprived of its 
placental nourishment must now begin an independent life. It 
must take in its own oxygen and give off carbon dioxide by its 
respiratory organs. It must take its food through the alimentary 
canal, and excrete its waste products through its kidneys. 
BIRTH. INDEX. 
Abducens or sixth nerve, 271 
Aberration, chromatic, 291 
spherical, 291 
Absorption, 183 
Accellerator nerves of heart, 93 
Accommodation, 287 
mechanism, 289 
pupil in, 290 
Acidity, 42 
of gastric juice, 168 
Acromegaly, 236 
Addison's disease and adrenals, 232 
Adrenalin, 234 
Adrenals (suprarenal capsules), 
232 
Adsorption, 44 
Afferent nerve paths, 257 
Albumin, 36 
Albuminuria, 242 
Alimentary canal, anatomy of, 140 
Amino bodies, 35 
Amoeba, 18 
Ammonia, 211 
in urine, 240 
Amlopsin, 177 
Anesthesia, 257 
Analgesia, 257 
Anaphylaxis, 263 
Animal heat, 132 
Antibodies in blood, 59 
Antienzymes, 47, 180 
Antipyretics, 136 
Antithrombin, 59 
Antitoxin, 61 
Apex beat of heart, 73 
Aphasia, 279 
Appetite, 154, 168 
Arterial blood pressure, 85 
Articulations, 37 
Asphyxia, 101 
Assimilation (see Metabolism) 
Association areas of cerebrum, 278 
fibers of cerebrum, 278 
Associative memory, 279 
Asthma, 123 
Astigmatism, 292 
Atmosphere and metabolism, 191 
Auditory areas of cerebrum, 278 
Auditory ossicles, 299 
Augmentor nerves of heart, 93 
Auricle, function of, 167 
Auriculo-ventricular valves, 74 
Auscultation of lungs, 117 
Autonomic nervous system, 282 
Bacteria digestion, 169, 179 
Beat of heart, 72, 76 
Beef tea, 209 
Beri-Beri, 224 
Bile, 174 
Binocular vision, 295 
Bladder, urinary, 247 
Blind spot, 294 
Blood, 51 
coagulation of, 58 
functions of, 51 
gases of, 108 
microscopic characters of, 51 
physical properties of, 51 
plates, 56 
plasma, 56 
Blood corpuscles, 51 
enumeration of, 52 
source of, 54 
311 312 
INDEX 
Blood flow, rate of, 90 
Blood pressure, 85 
Blood vessels, anatomy of, 74 
nervous control of, 97 
Body cavities, 27 
Body fat, source of, 218 
Brain, 268 
Bread, 207 
Breathing, mechanism of, 113 
Bright's disease, 242 
Bundle of His, 77 
Butter, 208 
Calcium, 223 
Calcium salts and coagulation of 
blood, 59 
Calorimeter, 189 
Calory, 188 
Capacity of lungs, 118 
Carbodydrates, 37 
food values of, 188 
metabolism of, 116 
relative metabolic importance, 
216 
Carbon dioxide: 
effect of oxyhaemoglobin, 110 
mechanism of exchange, 110 
production of, 104 
Cardiac cycle, events of, 79 
Cardiac muscle, 76 
Cardiac depressor nerve, 95 
Centers, vascular-nervous, 95 
Cerebellum, 279 
Cereals, 208 
Cerebrum, 273 
function of, in modifying re- 
flexes, 276 
localization in, 275 
relation to receptor system, 275 
sensory areas, 277 
Cheese, 209 
Chemical composition of body, 33 
Chemistry, of bile, 176 
of foods, 207 
of gastric juice, 167 
of pancreatic juice, 174 
of urine, 239 
Childbirth, 310 
Cholesterol, 36 
Chordae tendineae, 74 
Chyme, 171 
Ciliary muscle, 299 
Circulation, 83 
diagram of, 70 
influence of arteries, 84; 
of drugs, 101; of gravity, 100; 
of haemorrhage, 194; of ner- 
vous system, 92; of respiratory 
movements, 92 
renal, 244 
pulmonary, 91 
time of, 90 
venous, 89 
Circulatory system, anatomy, 70 
Circumvallate papillae, 301 
Clothing, 134 
Climate, effect of temperature, 135 
Coagulation of blood, 58 
Cocaine, 103 
Colloids, 43 
Complemental air, 119 
Condiments, 210 
Cones of retina, 293 
Connective tissue, 21 
Consciousness, 273 
Consonants, 130 
Contraction of muscle, 48 
tetanic contraction, 49 
Co-ordination, function of cerebel- 
lum, 279 
Cord, spinal, 257 
Cords, vocal, 127 
Cornea, 288 
Corpora quadrigemina, 269 
Corpuscles of blood, 52 313 
INDEX. 
Corti, organ of, 297 
Coughing, 117 
Cranial nerves, 271 
Creatinin, 240 
Cretinism, 230 
Cream, 209 
Crying, 217 
Crystalloids, 39 
Cystine, 215 
Deglutition, 159 
Dentrite, 253 
Determination of blood pressure, 
86 
Diabetes, 220 
Dialysis, 39 
Diaphragm, relation to breathing, 
114 
Diastole of heart, 80 
Diastolic blood pressure, 86 
Dietetics, 202 
Diet, suitability of, 205 
fundamentals of, 206 
Digestion: 
bacterial-intestine, 179 
of cellulose, 179 
necessity of, 138 
in intestine, 174, 179 
in mouth, 150 
in stomach, 163 
object of, 138 
resume of digestive ferments, 185 
Direct pyramidal tract, 260 
Disaccharides, 37 
Diuretics, 248 
Ductless glands, 227 
Dyspnea, 123 
Efferent nerve paths, 261 
Eggs, 209 
Electrolytes, 40 
Energy balance (see Metabolism) 
Enterokinase, 177 
Enzymes, 45 
Epithelial tissue, 20 
Erepsin, 178 
Erythrocytes, 52 
Eustachian tube, 300 
Excreta, endogenous and exogen- 
ous, 215 
Excretion, from lungs, 111 
renal, 239 
Exercise, muscular, and metabol- 
ism, 217 
Exogenous excreta, 215 
Expiration, 212 
Expired air, composition of, 119 
Expectorants, 126 
Eye (see Vision) 
Fat, chemical composition of, 36 
food value of, 188 
of body, source of, 218 
structure of, 37 
relative metabolic importance of, 
216 
metabolism of, 218 
Ferments, 45 
Fertilization, 306 
Fetus, nutrition of, 309 
Fever, 135 
Fibrin, source of, 58 
Fibrinogen, 58 
Flavor, 297 
Foods, common composition of, 207 
Fovea centralis, 294 
Gall bladder, 174 
stones, 177 
Ganglia, 253 
spinal, 252, 282 
sympathetic, 252, 282 
Ganglion, definition of, 253 
semilunar, 98, 283 
Gas, absorption of, by liquid, 106 
partial pressure of, 106 
Gases of blood, 109 
Gas exchanges, in lungs, 120 
in tissues, 105 
Gastric didgestion, 168 
Gastric juice, constituents of, 167 314 
INDEX. 
Gastric secretion, control of, 163 
Giantism, 236 
Glands, ductless, 227 
gastric, 162 
mammary, 250 
pancreatic, 174 
salivary, 150 
sebaceous, 250 
of skin, 248 
sweat, 248 
thyroid, 228 
Glomerulus, 243 
Glottis, 127 
Gluten, 207 
Glycogen, 219 
Glycosuria, 219 
Goiter, 231 
Graafian follicle, 307 
Growth, curve of, 200 
Hair-cells of cochlea, 254 
Hoptophore group, 62 
Hearing, 298 
Heart, anatomy of, 70 
augmentor, nerves of, 93 
heart block, 78 
cavities of, 70 
change in form of, 71 
contractions, maximal, 76 
influence of salts on, 78 
inhibitory center of, 95 
inhibitory nerves of, 93 
nerves of, 92 
passage of beat over, 77 
pace-maker of, 77 
physiological peculiarities of, 76 
position of, 70 
refractory period of, 70 
rhythmic action of, 76 
sounds of, 81 
valves of, 73 
vascular mechanism of, 79 
work of, 84 
Heart valves, 73 
Heat, animal, sources of, 132 
value of foodstuffs, 188 
Hematin, 52 
Hemorrhage, 101 
Hemoglobin, 52 
absorption of oxygen by, 109 
chemical nature of, 52 
estimation of, 52 
Hiccough, 117 
Hippuric acid, 215 
Hormones, 139, 227 
Hunger, 184 
Hydrogen ions, 42 
measurement of, 43 
Hydrochloric acid in gastric juice, 
167 
Hyperglycsemia, 219 
Hypothroidism, 231 
Hyperthyroidism, 231 
Immunity, Ehrlich's theory of, 61 
specific nature of, 62 
Immunization, 62 
Infection-resisting mechanism, 61 
Inflammation, 60 x 
Inhibitory nerves of heart, 93 
Inorganic salts, metabolism, 222 
Inspiration, 113 
Internal capsule, 260 
Internal secretion, 227 
Intestinal digestion, 128 
Intestinal juice, 128 
Intestine, large, movements of, 182 
Intestine, small, movements of, 181 
Ions, 40 
Ionization, 40 
Iron, 223 
Katabolic processes, 187 
Kephalin, 59 INDEX. 
315 
Kidney, blood flow through, 24 
blood supply of, 244 
minute structure of, 243 
nerve of, 243 
Knee jerk, 263 
Lactation, 250 
Lacteals, 66 
Lecithin, 36 
Lens, crystalline, 299 
Leucocytes, movements of, 55 
function of, 56 
Lipase, in gastric juice, 170 
in pancreatic juice, 177 
Lipoids, 36 
Liver, excretory function of, 173 
glycogenetic function of, 220 
Localizing power of retina, 295 
Locomotor ataxia, 266 
Lungs, changes of blood in, 120 
movements of, 117 
Lymph, movements of, 68 
formation of, 67 
glands, 69 
relation of, to blood, 66 
resorption of, 68 
vessels, 68 
Lymphagogues, 67 
Lymphocytes, 55 
Lymph nodes, 69 
Maintenance food, 202 
Malpighian capsule, 242 
Malpighian pyramids of kidney, 
242 
Mammary gland, 250 
Mastication, 157 
saliva and, 158 
Material balance of body, 194 
Measurement of arterial pressure, 
86 
Meat, 209 
extract, 210 
Menstruation, 307 
Mental process, 279 
Metabolism, general, 186 
influence of atmosphere, 190 
muscular work, 190 
surface area, 190 
basal heat production, 190 
specific dynamic action, 190 
Metabolism, special, 211 
carbohydrates, 219 
fats, 218 
inorganic salts, 222 
proteins, 211 
Middle ear, 298 
Milk, composition of, 208 
Micturition, 246 
Monosaccharides, 37 
Motor area of cortex, 274 
Mountain sickness, 124 
Mouth, digestion in, 154 
Mouth washes, 156 
Muscles, 22, 48 
Muscle sense, 280 
Muscular elasticity, 49 
Muscular energy, source of, 106 
Muscular tone, 265 
Muscular work, expenditure of 
energy, 202 
Myopia, 292 
Myxcedema, 230 
Nausea, 161 
Nerve impulse, 251 
Nerve paths, afferent, 268 
efferent, 262 
method of tracing, 257 
Nerve plexus, 252 
Nerve system, 251 
sympathetic, 282 
Nerve tissues, 23 
Neurones, intermediary, 259 
Nitrogen equilibrium, 187 
balance sheet, 197 
Nutrition {see Metabolism) 
Nutrition of embryo, 309 
Nutritive value of foods, 207 316 
INDEX. 
Obesity, treatment of, 198 
Oculomotor nerve, 271 
Opsonins, 64 
Optical defects, 291 
Optic thalami, 259 
Organ of Corti, 291 
Osmosis, 40 
Osmotic pressure, 40 
Oviduct, 307 
Ovulation, 308 
Ovum, 307 
Oxidase, 105 
Oxidation, in tissues, 105 
as source of animal heat, 106 
Oxygen, absorption of, by blood, 
108 
Oxyhaemoglobin, effect of CO, 
on, 110 
Pain, 257 
Pancreatic juice, 174 
composition of, 177 
Pancreatic secretin, 175 
Pancreatic secretion, control of, 
174 
Paralysis, 267 
Parathyroids, 228 
Pepsin, 167 
Pepsinogen, 167 
Peptone, 35 
Peristalsis, 182 
Perspiration, 248 
Phagocytosis, 64 
Physico-chemical laws, 38 
Pituitary body, 235 
Platelets, or plaques, of blood, 56 
Plasma, blood, 56 
Pons Varolii, 258 
Postsphygmic period, 80 
Precipitins, 62 
Pregnancy, 309 
Presbyopia, 292 
Pressure, arterial, 86 
intrathoracic, 115 
osmotic, 40 
Presphygmic period, 80 
Properties of body, physical and 
physiological, 33 
Proteins, chemical composition 
of, 34 
compound, 35 
insoluble, 36 
irreducible minimum, 199 
nutritive value of, 197 
relative metabolic importance 
of, 216 
requirement of body for, 203 
simple, 35 
sparers of, 198 
Proteose, 34 
Protoplasm, composition of, 33 
primary constituents of, 33 
secondary constituents of, 34 
Puberty, 307 
Pulmonary circulation, 92 
Pulse, use of, in diagnosis, 91 
tracings, 91 
wave, 224 
Purin bodies, 213 
Pyloric sphincter, control of, 171 
Pylorus, 170 
Pyramidal tracts, 260 
Range of voice, 129 
Rate of blood flow, 90 
of body fluids, 43 
Reason, faculty of, 279 
Reciprocal inhibition, 266 
Receptors, 62, 255 
Red blood corpuscles, 52 
Reflex animal, characteristics of, 
compared with normal, 263 
Reflex arcs, 252 
Reflex action, 264 
Reflex paths, 256 
Reflex time, 262 317 
INDEX. 
Reflexes, function of spinal cord 
in, 262 
types of, 262 
Renal secretion, 242 
Reproduction, sexual, 306 
Reproductory organs, accessory: 
female, 307 
male, 308 
Residual air, 119 
Respiration, 104 
artificial, 118 
control of, 123 
external, 111 
internal, 104 
nerves of, 222 
volume of air in, 120 
Respiratory center, 121 
exchange, 109 
movements, 115 
organs, 172 
quotient, 119, 194 
reflex, 121 
sounds, 117 
Rickets, 223 
Rolando, fissure of, 275 
Roots of spinal nerves, 258 
Saliva, function of, 154 
tartar formation, 156 
Salivary glands, 39 
secretion of, 150 
Scratch reflex, 263 
Salt hunger, 223 
Sea sickness, 282 
Sebaceous glands, 249 
Secretin, gastric, 166 
pancreatic, 174 
Secretion: 
control of, 139 
gastric, control of, 164 
milk, 250 
pancreatic, control of, 174 
salivary, control of, 154 
sebaceous, 240 
Secretory process: 
hormone control of, 139 
nervous control of, 139 
Semicircular canals, bony, 281 
Semilunar ganglion, 98 
Semilunar valves, 73 
Semipermeable membrane, 39 
Sensory areas of cortex, 277 
Serum diagnosis, 65 
Shivering, 136 
Shock, 100 
Sight, 285 
Skin, function of, 248 
Skeleton, 29 
Smell, 303 
Sneezing, 117 
Solutions, isotonic, 42 
hypertonic, 42 
hypotonic, 42 
Sound, loudness of, 129 
Sounds of heart, 81 
Special senses, 285 
Specific dynamic action of foods, 
190 
Tartar, 156 
Taste, 302 
Taste-buds, 302 
Tectorial membrane, 298 
Temperature of body, 132 
Temperature, effect of, on mus- 
cular contraction, 133 
Temperature sensation zero, 257 
Temperature sense, 257 
Temperature, bodily, regulation 
of, 133 
Tetany, 231 
Thorax, contents of, 113 
movements of, in respiration, 
113 
Thrombin, 59 
Thrombogen, 59 
Thymus, 238 
Thyroid gland, 229 318 
INDEX. 
Tidal air, 118 
Touch, sensations of, 256 
Toxins, bacterial, 60 
Toxophores, 61 
Trypsin, 74 
Trypsinogen, 177 
Urea, 211, 240 
Uric acid, 213, 240 
Urinary organs, 242 
Urinary salts, nitrogen, 211 
Urine, ammonia, 211 
excretion of, 239 
nature of excretory process, 243 
Vaccines, 65 
Vagus nerve, action of, on heart, 
93 
Valves of heart, 73, 82 
Varicose viens, 90 
Vasoconstrictor nerves, 98 
Vasodilator center, 95 
Vasodilator nerves, 98 
Vasomotor tone, 101 
Veins, blood in, 89 
Velocity of blood, 88 
Ventilation, 175 
Vision, 285 
color, 296 
stereoscopic, 296 
Visual defects, 290 
treatment of, 291 
Vital capacity, 119 
Vitamines, 224 
Vocal cords, false, 126 
relation of, to pitch, 127 
Voice, 126 
Vomiting, 161 
Vowels, 129 
Water, proportion of, in body, 34 
physiological properties of, 34 
Wheat flour, 207 
White blood-corpuscles, 55 
Xanthin bodies, 213 
Yawning, 117