ELEMENTARY 
HUMAN PHYSIOLOGY 
SIMPSON CORNELL UNIVERSITY 
NOTES OF LECTURES 
ON 
ELEMENTARY 
HUMAN PHYSIOLOGY 
BY 
SUTHERLAND SIMPSON 
D.Sc., M.D. (Edin.) F.R.S. (Edin.) 
ITHACA. N. Y. 
1 HE COMSTOCK PUBLISHING CO. 
1924 COPYRIGHT 1924 
SUTHERLAND SIMPSON 
ALL RIGHTS RESERVED 
PRINTED IN U.S.A. 
PRESS OF W. F. HUMPHREY 
GENEVA, N. Y. Fig- 1 NOTES OF LECTURES 
ON 
ELEMENTARY 
HUMAN PHYSIOLOGY 
INTRODUCTION 
The science of Biology has for its object the study of living 
things. It is divided into two great branches--vegetable biology 
or Botany and animal biology or Zoology, for plants as well as 
animals are alive. 
Further, the study of any living object, plant or animal, includes 
a knowledge first of its form or structure-Morphology-and 
second of the functions, uses and activities of the various organs 
or parts of which it is made up-Physiology. Morphology, or 
anatomy, and physiology to a large extent go hand in hand, for, 
in order to obtain a true knowledge of the functions of any organ 
or organism, we must first understand something about its struc- 
ture or anatomy. 
The physiology of man differs in no essential from that of the 
lower animals; as a matter of fact most of our real knowledge of 
the functions of the human body has been gained from experi- 
mental studies on lower animals. 
1 STRUCTURE OF HUMAN BODY 
As we shall see later, all the tissues of the body are derived from 
a single cell, the fertilized ovum. In the process of development 
differentiation of structure is accompanied by specialization of 
function, and the higher we go in the animal scale the more com- 
plex does the organism become, both in its anatomy and physiology. 
Probably no mechanism we know of approaches the human body 
in complexity of structure and function. 
Each part of the body having a special function of its own to 
perform is termed an Organ, e. g. stomach, liver, brain, etc. 
Groups of organs related in function are termed Systems; for 
example, each individual muscle is an organ, but all muscles, of 
whatever variety, have a common function, viz.-to produce 
movement of some kind or other, and so together they form the 
muscular system. The heart and the blood vessels (arteries, 
capillaries, veins), together with the lymphatics, are concerned 
with the continuous movement of the blood in a circle, and so form 
the circulatory system, etc. 
The body may be regarded as consisting of the following sys- 
tems,- 
Skeletal System consists of bones, cartilages and ligaments. 
Muscular System--Muscles whose function it is to shorten or 
contract and so to produce movement. 
Nervous System-Brain and spinal cord, cranial and spinal 
nerves, etc. 
Circulatory System-Heart, bloodvessels and lymphatics. 
Respiratory System--Air passages, windpipe and larynx, lungs, 
muscles of respiration. 
Alimentary System-Stomach, intestine, liver, pancreas, etc. 
Endocrinous System--Glands producing internal secretions such 
as suprarenal capsules, thyroid, parathyroids, pituitary, reproduc- 
tive glands, pancreas, etc. 
Excretory System-Skin, kidneys, liver, lungs. 
Reproductive System-Ovaries, testes, male and female organs 
of generation. 
When the different organs are examined by the microscope it is 
seen that all are made up of textures or tissues which consist of 
Cells, Intercellular substance and Fibers; therefore, the ultimate 
analysis shows the body to be composed of microscopic units termed 
cells, with intercellular substance and fibers derived from them. 
2 THE TISSUES OF THE BODY 
The important discoveries of Schleiden (1838) and Schwann 
(1839) showed that the tissues of plants and animals are not con- 
tinuous structures, but are made up of small units, of microscopic 
size, termed cells. The unit in biology, is the cell just as in 
chemistry it is the atom and in physics the electron. 
Structure of Typical Animal Cell-The cell in the human body 
may vary from 1 /300 to 1 /3000 of an inch in diameter. It consists 
of three portions, the cell protoplasm, the nucleus, and the centro- 
some and attraction sphere. 
1. The Cell Protoplasm or Cytoplasm is a soft-jelly-like material 
consisting of a dense network of fibrillae termed the spongioplasm, 
with a more fluid portion in its meshes called the hyaloplasm. 
The hyaloplasm may contain true granules and vacuoles, but the 
intersections of the spongioplasmic network often, under the micro- 
scope, present a granular appearance. The whole may or may not 
be surrounded by a definite cell membrane. 
2. The Nucleus is generally round or oval and usually occupies 
a position not far from the center of the cell. It is enclosed in a 
distinct envelope, the nuclear membrane. The contents of this 
membrane, like the cell protoplasm, consist of a network of coarse 
threads which readily stain with dyes and on this account is called 
the chromoplasm; it corresponds to the spongioplasm of the cell 
body, the strands of the nuclear network, however, being much 
coarser. The interstices of this spongework are filled with a more 
fluid, non-staining material termed the nuclear sap or nuclear 
hyaloplasm. A rounded, denser portion of the chromatin material, 
found somewhere within the nucleus, is called the nucleolus; 
there may be more than one. 
3. The Centrosome with its Attraction Sphere is situated in the 
cell protoplasm, near the nucleus. The former is a minute particle 
which appears to have an attractive influence on the protoplasmic 
fibrils and granules in its neighborhood, this giving rise to what is 
known as the attraction sphere. The centrosome initiates the 
complicated process of cell division known as mitosis or karyo- 
kinesis. 
The nucleus has an important influence on the nutrition of the 
cell body; any part of the cell protoplasm which is cut off 
FROM THE NUCLEUS UNDERGOES DEGENERATION. 
3 4 
HUMAN PHYSIOLOGY 
PROTOPLASM 
Protoplasm was called by Huxley the "physical basis of life" 
since it enters into the composition of every living tissue. It is a 
viscous and, usually, colorless substance, neutral or slightly alkaline 
in reaction. 
The Chemical Composition of living protoplasm is unknown 
since any attempt at analysis must necessarily kill it. The analysis 
of dead protoplasm shows that it contains at least 75% of water 
and 25% or less of solids. The solids consist of organic (protein 
essential) and inorganic constituents, and ultimate analysis shows 
that the human body contains the following chemical elements: 
Carbon, oxygen, hydrogen, nitrogen, sulphur, phosphorus, iron, 
sodium, potassium, lithium, calcium, magnesium, manganese, 
chlorine, fluorine, iodine, and silicon; several of these exist only 
in traces. 
Vital Properties of Protoplasm 
1. Power of Assimilation and Disassimilation-In living proto- 
plasm certain chemical changes are always going on. It has the 
power of building up into its own substance materials taken in 
from the outside in the form of food; at the same time disintegrative 
changes are taking place in it whereby large and complex molecules 
are being broken down into simpler ones, which are finally excreted. 
The first of these processes is termed assimilation or anabolism 
(upbuilding), the second disassimilation or katabolism (down- 
breaking), and the term metabolism is used to include the sum 
total of all these chemical rearrangements. 
In the child the anabolic changes predominate and growth takes 
place; in starvation, wasting disease and old age the katabolic 
changes are in excess, and the body loses weight, w'hile in the 
healthy adult, where the body weight does not change appreciably, 
the two balance each other. 
No inorganic or dead substance that we know of possesses this 
property. 
2. Excitability or Irritability-Living protoplasm is excitable or 
irritable, and by this is meant that it has the power of responding 
to stimulation or irritation applied from the outside. 
Changes in the surroundings may either attract or repel the 
protoplasm. Such attraction or repulsion may be caused by 
various chemical substances (chemiotaxis), by mechanical pressure 
(barotaxis), by light (phototaxis), by changes in temperature 
(thermotaxis) and by electricity (galvanotaxis). 
If the reaction is toward the stimulus it is said to be positive, 
and if away from it negative; thus we may have positive or negative 
chemiotaxis according as the protoplasm is attracted or repelled 
by the chemical substance, etc. A good example of positive ch em- THE TISSUES 
5 
iotaxis is seen in the movement of the white blood corpuscles to- 
wards the various microbes, or other foreign bodies, which may 
enter the tissues. 
Certain unicellular organisms, particularly plant cells, are at- 
tracted by light-positive phototaxis, and others are repelled by 
it-negative phototaxis, while some of the infusoria when brought 
between the poles of a glavanic battery may be seen to move 
towards the positive pole and others towards the negative pole. 
These phenomena explain the apparently voluntary movements 
of many unicellular organisms which, for example, appear to select 
certain kinds of food while in reality they are compelled to move 
towards it by chemical attraction. The apparently spontaneous, 
amoeboid movement of white blood corpuscles is probably the 
result of chemiotaxis. 
3. Reproduction. Living protoplasm has the property of re- 
producing itself. Cells, when they reach a certain size, either 
divide to form two smaller cells, or die and disappear. 
Cell division may either be direct or indirect. The latter method, 
termed mitosis or karyokinesis, is much the more common, and 
is a very complicated process. It appears to be initiated by the 
centrosome and the nucleus which in dividing passes through 
a series of remarkable changes a detailed description of which will 
be found in any work on histology. 
In amitotic or direct cell division, which is rare, the cell proto- 
plasm and nucleus divide into two parts directly, without any 
preliminary changes in the centrosome and nucleus. The result, 
as in mitotic division, is the production of two daughter cells 
from one original mother cell. 
The cause of cell division is not understood, but it may possibly 
be due to defective nutrition. As any spheroidal body grows 
bigger, the volume or mass increases more rapidly than the surface, 
hence as the cell becomes larger and larger, the surface, which rep- 
resents the feeding area, becomes smaller and smaller in relation to 
the mass of material to be nourished, and something of the nature of 
starvation of the cell protoplasm may take place. The alterations 
in the metabolism, so induced, may set up the changes in the 
nucleus and centrosome which lead to the division of the cell. 
The Ovum is a Typical Cell--All the tissues of the body are 
derived from the impregnated ovum. 
After union of male and female pronuclei to form nucleus of 
fertilized ovum, the cell (ovum) divides by mitosis into two 
daughter cells, then each of these into two, making four, and so on 
until, in the young embryo, three layers of cells are formed termed 
epiblast or ectoderm (outer layer), hypoblast or entoderm (inner 
layer) and mesoblast or mesoderm (middle layer). 6 
HUMAN PHYSIOLOGY 
Tissues derived from ectoderm-Epidermis (scarf-skin) and its 
derivatives, nail, hair, epithelium of sebaceous, sweat and mam- 
mary glands; epithelium of mouth and nose; brain, spinal cord, 
nerves and ganglia; parts of eye and ear. 
From Entoderm---Epithelium of pharynx, gullet, stomach, small 
and large intestine, pancreas, liver and gall bladder; epithelium of 
larynx, windpipe, lungs, and of thryoid and parathyroid glands; 
epithelium of urethra and urinary bladder. 
From Mesoderm--All the other tissues of the body, including 
connective tissues (bone, cartilage, etc.) muscles, blood-vessels, 
heart, etc. 
All tissues are composed of cells, intercellular substance and 
fibers. 
Amount of intercellular substance varies in different tissues,; 
ground substance (in cartilage) cement substance (in endothelium). 
In process of development from original cell great specialization 
in structure and function takes place. 
Tissues may be arranged into five great groups,-epithelium, 
CONNECTIVE TISSUES, MUSCLE, NERVE, BLOOD AND LYMPH. 
I. EPITHELIAL TISSUES 
Cells are arranged on free surfaces. Varieties: 
1. Simple Squamous or Pavement Epithelium-Flat nucleated 
cells united at margins by cement substance, forming a single layer. 
Outlines shown by treatment with silver nitrate. 
Found lining the great serous cavities (pericardial, pleural, 
peritoneal), heart, blood-vessels and lymphatics. Here frequently 
termed endothelium, where it forms a smooth frictionless surface. 
Also lines air cells of lungs and covers posterior surface of cornea 
and anterior surface of iris. 
2. Stratified Squamous Epithelium--Several layers or strata of 
cells. 
Found in epidermis of skin; lining mouth, nose, pharynx, gullet; 
covering conjunctiva and anterior surface of cornea. 
Has mainly a protective function. 
Surface cells have lost their nuclei and become transformed into 
a horny substance termed keratin, impervious to water. They 
are dead and are continually being shed and replaced by cells from 
deeper layers. 
3. Transitional Epithelium is really a special form of stratified 
squamous epithelium. Lines ureters, urinary bladder and part of 
urethra. 
4. Columnar Epithelium-So called because cells are shaped 
like columns. 
Found lining stomach, small and large intestine, ducts of glands 
opening into alimentary canal. THE TISSUES 
7 
Striated at free margin in intestine where it is mainly absorp- 
tive in function. 
"Goblet or chalice cells" secrete mucin. 
Low columnar or cubical epithelium. 
5. Ciliated Epithelium--Lines respiratory tract, fallopian tubes 
and uterus, ventricles and canals of cerebrospinal axis. 
May be arranged in single layer or in several layers. 
Free border provided with a series of hairlike processes termed 
cilia which, while cell is alive, show constant lashing movements. 
Nature and functions of ciliary movement. 
Effect of narcotic agents (alcohol, ether, chloroform, etc.) on 
movement. 
6. Glandular Epithelium-Found lining tubules and saccules of 
secreting glands, such as salivary glands, pancreas, liver. Sometimes 
called secretory epithelium. 
Cells differ in form in different glands, but are usually cubical or 
polygonal. 
All are characterized by presence of granules in protoplasm, 
which are abundant when cells are resting, and disappear when 
cells have been actively secreting. 
7. Sensory Epithelium--Cells of ectodermic origin which be- 
come modified to form the receptive structures in the organs of the 
special senses, hearing, taste, smell. 
Nerve fibers end in relation to these cells and carry impressions 
from them to the sensorium in the brain. 
II. CONNECTIVE TISSUES 
These form mainly inactive, supporting structures, binding 
organs and parts of organs together. 
They contain a comparatively large amount of intercellular 
material or ground substance in which cells lie scattered, singly or 
in groups. 
1. Mucoid Tissue-Embryonic connective tissue sometimes 
termed Wharton's jelly. 
Found in umbilical cord of foetus and in vitreous humour of eye 
in adult. 
2. Connective Tissue Proper-Several varieties--areolar tissue, 
tendon, ligament. 
Consists of fibers (white and yellow) and several kinds of con- 
nective tissue cells. 
In areolar tissue, which is found under the skin, the fibers are 
loosely arranged and are mostly of the white variety. This tissue 
is elastic and permits of the free movement of the skin on the parts 
below. 
A somewhat denser variety of this tissue forms sheaths for mus- 8 
HUMAN PHYSIOLOGY 
cles, tendons and nerves, and surrounds bone (periosteum) and 
cartilage (perichondrium). 
Tendon consists of dense bundles of parallel white fibers with 
rows of c. t. cells between. It contains few elastic fibers. 
In elastic tissue, such as the ligamentum nuchas, or outer 
coat of blood-vessels, yellow fibers predominate, with fine white 
fibers between the bundles. 
3. Adipose Tissue-Developed from young connective tissue 
by deposition of fat in c. t. cells which are then termed fat cells. 
Cell in envelope swollen and rounded with nucleus squeezed to 
one side. 
Cells are usually arranged in clusters which are vascular. 
Fatty tissue found in almost every part of body but most 
abundant under skin, around kidneys and heart, on omentum and 
mesentery, and in bone marrow. 
4. Cartilage--Three varieties. Found covering articular ends 
of bone (hyaline cartilage); between bodies of vertebras as 
intervertebral discs (white fibro-cartilage) ; in epiglottis, lobe 
of ear, and Eustachian tube (yellow elastic cartilage). 
In hyaline cartilage the cells, in groups of two or more, are sur- 
rounded by a capsule and imbedded in a hyaline (ground glass) 
matrix. 
Other forms are more closely related to fibrous tissue. 
5. Bone--Bone may be looked upon as a connective tissue in 
which the matrix or ground substance is solidified and hardened 
by the deposition in it of lime salts. The connective tissue cells 
which take on a bone forming character are termed osteoblasts. 
When they become enclosed in the layers or lamellae of calcified 
material which they lay down, they are termed bone cells or bone 
corpuscles, the spaces in which they lie are termed lacunae and the 
minute passages leading from one lacuna to another, canaliculi. 
Bone is said to be compact or spongy according to the density of 
its structure. In long bones the shaft consists of compact or dense 
bone while the spongy variety is found at the ends. The cavity in 
the center of the shaft is known as the medullary cavity and con- 
tains the bone marrow. 
Bone may be developed directly from membrane (e. g. the peri- 
osteum which surrounds it) or it may be preceded by cartilage. 
The growth in thickness is brought about by the deposition of 
layer after layer of compact bone underneath the periosteum, 
while the increase in length is due to the transformation of cartilage 
at the ends. At each end of a long bone there is a plate of cartilage 
termed the epiphysis and there is a continual growth of bone from 
these plates; when the bone reaches its full length growth stops, 
and the epiphyses are gradually replaced by osseous tissue. If, 
from any cause, the epiphyses is diseased, or damaged, or removed, THE TISSUES 
9 
no further increase in length will take place at that end, hence the 
surgeon's anxiety, in operations on joints, to preserve if possible 
the epiphyses; if these are removed in a growing person the result 
is a shortened limb. 
At the same time that the bone is growing thicker, by deposition 
on its outer surface from the periosteum, the medullary cavity is 
also increasing in size and this modeling is effected by osteoclasts, 
large connective tissue cells, whose function it is to absorb bone. 
This hollowing out prevents the bone from becoming too heavy 
but does not proceed so far as to unduly diminish its strength. 
Although bone is hard, and apparently resistant, it is easily 
moulded by prolonged pressure, e. g. the growth of a tumour 
pressing on a bone, will cause its absorption and lead to distortion, 
if given sufficient time. That growing bone can be greatly modified 
in shape by persistent pressure is shown by the deformed feet of 
Chinese women due to tight bandaging in childhood. 
Bone in the recent state contains about 50% of water. The 
solid constituents consist of organic matter, one part, and inorganic 
matter, two parts. The organic or animal matter is chiefly collagen 
which is converted into gelatin by boiling, while the inorganic matter 
is composed of earthy salts, mainly calcium phosphate, calcium 
carbonate, calcium fluoride with magnesium phosphate in small 
amount. 
All the inorganic constituents may be removed by immersing 
the bone in dilute nitric or hydrochloric acid for some weeks, and 
the organic matter may be got rid of by burning. After either 
operation the bone retains its shape, but in the one case it is soft 
and pliable, and in the other brittle and easily crumbled. 
In early life the bones contain more organic matter than in the 
adult and, consequently, are less rigid and not so readily fractured. 
As age advances, the proportion of inorganic matter increases, 
and the bones become more brittle. In extreme old age, the 
weight of the body alone may be sufficient to fracture the neck of 
the femur, for example. 
Since bone contains a large proportion of lime salts it is essential, 
especially in young children where bone formation is very active, 
that these be present in sufficient amount in the food to satisfy 
the demands of the tissues. No article of diet is so rich in calcium 
as milk and this is the best food for the young child on that account. 
There are three varieties of muscular tissue,'-striated, striped or 
voluntary muscle; non-striated, plain or involuntary muscle; cardi- 
ac or heart muscle. 
1. Striated Muscle forms the so-called flesh of the body and is 
found in the muscles attached to the skeleton. It is under the direct 
III. MUSCULAR TISSUES 10 
HUMAN PHYSIOLOGY 
control of the will, hence the term "voluntary" which is applied 
to it. 
It consists of fibers collected into bundles, and each individual 
muscle is made up of large numbers of these bundles or fasciculi 
running more or less parallel. 
Structure of Striated Muscle Fiber-Each fiber is cylindrical in 
shape and is enclosed in an envelope called the sarcolemma on the 
inner surface of which many nuclei are found. The fibers rarely 
exceed inches in length and are usually much shorter. 
Examined under the microscope, each fiber shows transverse 
striations caused by alternately dim and clear segments or bands. 
In the middle of the clear segment a line can be made out which is be- 
lieved to represent the edge of a membrane-Krause's membrane-- 
and in the middle of the dim segment another line is seen named 
Hensen's line. 
In preparations hardened in formalin or alcohol each fiber can 
be separated longitudinally into fibrils or sarcostyles, and between 
these sarcostyles a more fluid substance is present called sarco- 
plasm. Each sarcostyle shows the same cross striation as the fiber 
of which it is a constituent with Krause's membrane and Hensen's 
line also visible. 
The portion of the sarcostyle included between two adjacent 
Krause's membranes is called a sarcomere; it includes one dim 
segment with Hensen's line in the middle, and half the clear seg- 
ment on each side. The sarcomere is the unit of muscular struc- 
ture. The sarcostyle consists of a series of sarcomeres placed end 
to end; the muscle fiber is made up of a number of sarcostyles 
lying side by side, and enclosed in an envelope, the sarcolemma; 
the fibers are arranged in bundles or fasciculi which usually run 
parallel from end to end of the muscle, terminating in its tendons; 
the muscle is composed of a larger or smaller number of these 
fasciculi, according to it size. 
Change in Contraction-What is believed to happen when the 
fiber contracts, e. g. under the influence of a nerve impulse, is 
that the hyaline substance of the clear segment is absorbed into 
the sarcous substance of the dim segment which swells up with the 
result that the whole fiber becomes shorter and thicker. When the 
exciting stimulus ceases to act the hyaline substance passes back 
again into the clear segment and the fiber relaxes and elongates. 
The same change taking place in all the fibers (or in many of them), 
at the same time, the whole muscle contracts or relaxes as the case 
may be. 
The fibers are not long enough to reach from one tendinous at- 
tachment of the muscle to the other, but inside each fasciculus 
they cohere to each other, more or less end to end. THE TISSUES 
11 
Motor End Organ-Each muscle fiber is supplied by at least one 
erve fiber which terminates within the sarcolemma in a "motor- 
>nd-organ" or "end-plate." 
Muscle Spindles-Lying amongst the muscle fibers are certain 
organs termed "muscle spindles" which are now believed to be the 
terminations of sensory nerve fibers in the muscle. 
Striated muscle is a very active tissue and consequently is 
abundantly supplied with blood vessels. 
2. Non-striated or Involuntary Muscle is found in the walls of 
the hollow viscera,-lower half of gullet, stomach, small and large 
intestine; in the arteries, veins and larger lymphatics; in the ure- 
ters; urinary bladder and uterus; in the ducts of certain glands; 
in the gall bladder; and in the intrinsic muscles of the eye. 
The fibers are elongated and spindle shaped; they contain a 
single oval or rod-shaped nucleus near the middle and show 
longitudinal fibrillation, but no transverse striation. They prob- 
ably do not possess a true envelope like the sarcolemma of striated 
muscle. The cells (each fiber is a mono-nucleated cell) vary greatly 
in length according to the part or organ in which they are found, 
but do not usually exceed the 1 /600 of an inch. They are joined 
together by a small amount of interstitial cement substance. 
Non-striped muscle receives its nerve supply from the autonomic 
system. 
Its blood supply is not so abundant as in the case of striated 
muscle. 
3. Cardiac Muscle-The muscle fibers of the heart differ from 
those of plain muscle in that they show transverse striation which, 
however, is neither so distinct nor so regular as in voluntary 
muscle. The fibers are short cylinders, joined end to end by ce- 
ment substance which stains dark with silver nitrate. They have 
usually a single nucleus and are frequently branched at the ends. 
They are smaller in diameter than voluntary muscle fibers and 
much shorter. 
Heart muscle receives its nerve supply from two sources-cere- 
bro-spinal and autonomic--which will be considered later. It has 
a rich blood supply. 
The nervous system consists (a) of the brain and spinal cord, 
together with the nerves which come off from them, the whole 
being termed the cerebro-spinal nervous system and (b), a number 
of ganglia connected by nerve fibers lying outside the skull cavity 
and spinal canal, known as the autonomic nervous system. The 
brain and spinal cord together form the central nervous system. 
If the brain or cord be cut across with a knife, and the surface 
of the section examined with the naked eye, it will be seen that 
IV. NERVOUS TISSUE 12 
HUMAN PHYSIOLOGY 
certain areas have a greyish appearance while others are lightei 
color. These consist of grey matter and white matter respective 
the former containing nerve cells, the latter nerve fibers. 
The Neuron-Nervous tissue is usually described as consist' 
of nerve cells and nerve fibers; as a matter of fact, howeve. 
EVERY NERVE FIBER IS THE PROCESS OF A NERVE CELL and the tWO 
together form the Neuron which is the unit of the nervous system. 
A NERVE CELL TOGETHER WITH ALL ITS PROCESSES IS TERMED A 
NEURON. 
The processes which are given off by the cell or cell body are of 
two varieties,--dendrites and axon. Every nerve cell gives off an 
axon which forms the axis-cylinder, the essential conducting part, 
of the nerve fiber, and in addition to this many cells give origin to 
a number of short, branching processes termed dendrites or den- 
drons. 
Although it is convenient to describe the nerve cell and nerve 
fiber separately, it should never be forgotten that the two are parts 
of the same anatomical unit, the neuron, and that the nervous 
system is made up of these units. According to the neuron doctrine, 
each of these units is anatomically independent. The nerve fiber 
of one breaks up into a number of processes termed the terminal 
arborisation, and these surround the cell body of the next neuron 
but do not become continuous with it. There is, however, physio- 
logical continuity, for the nerve impulse is believed to have the 
power of jumping across the interval. This mode of junction is 
termed a synapse or synapsis. 
The Nerve Cell or Cell Body-Nerve cells are found in the grey 
matter of the brain and spinal cord; in enlargments (ganglia) on 
the cranial and spinal nerves; in the ganglia of the autonomic 
system; and in plexuses found in relation to many organs such as 
the heart and intestine. 
They vary greatly in size, shape and structural appearance. 
Some (unipolar nerve cells) have only a single process-the axon; 
others have two processes (bipolar cells), and still others (multi- 
polar cells) have more than two processes-one axon and several 
dendrites. 
All nerve cells have a relatively large and conspicuous nucleus, 
with a very distinct nucleolus and sometimes more than one. 
The protoplasm of the cell contains large granules which stain 
readily with methylene or toluidine blue; these are termed Nissl 
granules. These granules, although present in the dendrites, are 
absent from the axon hillock. 
Chromatolysis.'-When the cells are fatigued by excessive activi- 
ty these Nissl granules gradually disappear. The term chroma- 
tolysis or Nissl degeneration is applied to these changes in the THE TISSUES 
13 
si bodies. A similar change is produced in the cell, in a few 
s, by cutting across the axon or nerve fiber. 
i'ine fibrils-the neurofibrillse-pass through the cell body from 
.cess to process and run out into the axon. 
' The nerve cells found in ganglia are generally enclosed in a 
transparent membranous capsule. 
Nerve Fibers are of two varieties, medullated and non-medul- 
lated. 
A Medullated nerve fiber has the following structure: In the 
center of the fiber lies the axis cylinder; this is the prolongation 
of the axon from the nerve cell and it extends, without interruption, 
throughout the whole length of the fiber. It consists of a number 
of neuro-fibrillas and forms the essential conducting part of the fiber. 
Outside the axis cylinder lies the medullary or myelin sheath. 
It is composed of a semi-fluid, fatty substance termed myelin 
which stains black with osmic acid. 
Next to the medullary sheath, and forming the outermost cover- 
ing of the fiber, is the neurilemma or grey sheath. (The myelin is 
sometimes called the white sheath). This is constricted at inter- 
vals of a millimeter or less, cutting the medullary sheath into 
segments, and these constrictions are termed the nodes of Ranvier. 
Each internodal segment has a single nucleus surrounded by a 
small amount of protoplasm. 
Non-medullated Nerve Fiber-The structure of the non-medul- 
lated nerve fiber is the same as that of the medullated except that 
the myelin sheath is absent. 
The nerve fibers that come off from the brain and spinal cord 
are medullated; those of the autonomic system are non-medullated. 
The function of the medullary sheath is not understood. It 
may be for the purpose of insulating the axis cylinder so as to 
prevent the escape of the nerve impulse, but this is not certain. 
Nerves or Nerve Trunks are made up of bundles of nerve fibers 
bound together by connective tissue. These bundles vary greatly 
in size, and the whole is inclosed in a firm fibrous sheath called the 
epineurium, which contains bloodvessels and lymphatics. 
End Organs--The nerve fibers, when they reach the tissues for 
which they are destined, terminate in end-organs of which there 
is a great variety, some motor and some sensory. 
Neuroglia-The special form of connective tissue which binds 
together the nerve cells and fibers of the brain and spinal cord is 
termed neuroglia. It consists of cells (glia cells) each with a 
great number of short wavy processes. 14 
HUMAN PHYSIOLOGY 
V. BLOOD AND LYMPH 
The blood and lymph may be looked upon as a tissue consisting 
of cells-the red and white blood corpuscles and blood plates- 
with the blood plasma as the intercellular substance. 
The cells of this tissue (blood) retain their primitive structure 
to a large extent and show less differentiation from the original 
mother cells than those of any other tissue. 
It will be studied later in detail. 
Lymph is practically blood without red blood corpuscles. PHYSIOLOGY OF MUSCLE 
PHYSICAL PROPERTIES OF MUSCLE 
Extensibility and Elasticity-Muscle is extensible and also elas- 
tic. The two properties do not necessarily go together; a piece of 
putty or dough is very extensible but scarcely at all elastic; a 
piece of steel or an ivory ball is not extensible but very elastic. 
The extensibility of muscle differs from that of a rubber band or 
steel spring which obeys Hooke's law that equal increments of 
weight produce equal amounts of extension. In muscle equal 
INCREMENTS OF WEIGHT DO NOT PRODUCE EQUAL AMOUNTS OF 
EXTENSION. 
If a muscle be loaded with a weight of 10 grams, say, it will be 
stretched to a certain point; the next weight of 10 gramswill pro- 
duce less effect than the first; the third less than the second, etc. 
This applies to all varieties of muscle. 
If the muscle is not overstretched it will return to its original 
length, i. e., its elastic recoil is perfect. 
In the body the muscles are always in a state of elastic tension, 
i. e., they are always stretched somewhat beyond their normal 
length, apart altogether from muscular tonus which we shall con- 
sider later. This is seen by the fact that when a muscle is cut across 
with a knife, the ends retract, causing the wound to gape. That 
is why a deep wound involving muscle must be stitched by the 
surgeon to bring the faces together. 
Extreme fatigue diminshes the elasticity of muscle while rise of 
temperature, within certain limits, increases it. 
The advantages of this condition of elastic tension in muscle are, 
1st, that when the muscle is stimulated there is no slack to be 
hauled in and so no time is lost. 2d, The shocks and jars which, 
would be communicated to the body, if the muscles were rigid and 
inextensible, are greatly reduced or prevented. It plays the same 
part as the springs on a wagon and adds much to the smoothness 
with which the movements of the skeleton are carried out. 3d, 
Work is economized. 
VITAL PROPERTIES OF MUSCLE 
These are three in number,--excitability, contractility, 
CONDUCTIVITY. 
1. Excitability or Irritability. A muscle responds to stimulation 
by contracting, i. e. changing its shape. Muscle is not alone in 
this respect; all living tissues are excitable or irritable and respond. 
15 16 
HUMAN PHYSIOLOGY 
to stimulation in one way or another,-a muscle fiber by contract- 
ing, a nerve fiber by conducting an impulse, a gland cell by pouring 
out a secretion, etc. 
The natural stimulus for a muscle is a nerve impulse. 
Haller (1739) was the first to prove that muscle can be made 
to contract by artificial stimulation. The ideas prevailing before 
his time regarding the nature of muscular contraction were peculiar. 
It was believed that "animal spirits" (some kind of fluid material), 
generated in the brain, flowed though the connecting nerve into 
the muscle, forcibly causing it to swell up, and thus leading to its 
contraction; the muscle was believed to be passive. The truth of 
this doctrine was easily disproved by showing, as Haller did, that 
a muscle removed entirely from the body, and with its nerve com- 
pletely divided, can still contract when excitied. 
The artificial stimulus for muscle may be mechanical (pinch or 
blow), thermal (application of hot wire), chemical (vapor of am- 
monia, etc.) Or ELECTRICAL. 
Electricity is preferred as a stimulus for both muscle and nerve, 
1, because its strength can be easily regulated, and 2, if not too 
strong, it does not appreciably damage the living tissue. 
The current may be obtained from some form of electrical 
battery,--galvanic or constant current,-or from an induction 
machine,--faradic or induced current. 
The apparatus required for stimulation by galvanic electrcitv 
is as follows: 1. A battery in the form, for example, of a Leclanche 
or dry cell; 2, insulated copper wires attached to electrodes by 
which the current is led into the muscle; 3, a key by which the 
electricity is turned off or on; 4, a switch by which itmay be used 
to stimulate first one tissue and then another, e. g. muscle and 
nerve. The current passes from the positive pole of the battery, 
through the circuit, back to the negative pole, and the key is used 
to close and open this circuit. 
For experimental work the mucles of the frog are very convenient, 
since, in cold blooded animals, the tissues continue to live a long 
time after they are removed from the body. 
A Nerve-muscle preparation consists of the gastrocnemius (calf) 
muscle of the frog together with the sciatic nerve which supplies 
it. If the muscle is left in position in the leg, a straw with a flag 
at the end, may be attached to the toes to indicate to those at a 
distance when the muscles contract, or, a lever may be connected 
with the tendo achilles. 
Stimulation of Muscle and Nerve by Galvanic Electricity- 
The following experiment will show that apparently both muscle 
and nerve may be stimulated by galvanic electricity. A battery 
is connected in circuit with a key and a switch; two pairs of wires 
pass from the switch, the one to the muscle and the other to the PHYSIOLOGY OF MUSCLE 
17 
nerve of a nerve-muscle preparation By closing and opening the 
key, and turning the switch to the one side or the other, the 
muscle and nerve may be excited at will. 
It will be found, from this experiment, that with galvanic elec- 
tricity, a motor nerve is stimulated when the circuit is closed 
(current enters nerve) and again when the circuit is opened 
(current leaves nerve) but not while the current is flowing through 
the nerve, if it does not vary in strength In the case of the muscle, 
however, stimulation takes place at closure, at opening, and also 
while the current is flowing through. 
Is muscle directly excitable? The above experiment, neverthe- 
less, does not prove that the muscle tissue itself is directly excitable. 
Why2 The whole muscle is made up of a great number of muscle 
fibers each of which is provided with a motor end-plate, connected 
with a nerve fiber. Since the wires are stuck into the muscle there 
is no reason why these nerve fibers or end-plates should not be 
stimulated and so lead to an indirect response from the muscle 
fibers. 
John Reid was the first to prove that muscle can be directly 
stimulated by electricity. He divided the sciatic nerve of a 
rabbit (when a nerve is cut off from its cell of origin it undergoes 
degeneration and ceases to conduct an impulse), and at the end of 
two or three weeks, applied electrodes to the muscles supplied by 
the degenerated nerve; these contracted when the current was 
sent through them, and this could not be the indirect result of 
nerve stimulation. 
Claude Bernard's Experiment--This may also be proved by 
eliminating the motor end-plates by means of curara. Curara is a 
drug used by the American Indians as an arrow-poison. It has 
the power of producing voluntary muscular paralysis when in- 
jected under the skin. In the neuro-muscular apparatus there are 
four elements to be considered, the nerve cell in the spinal cord, 
the nerve fiber, the motor end-plate, and the muscle. Claude 
Bernard proved experimentally that the poison curara acts specifi- 
cally on the end-plates, paralyzing them, so that impulses from 
the nerve to the muscle fibers are blocked at this point. This is a 
convenient way to get for experimental work, physiologically pure 
muscular tissue free from nerve elements. 
If now we repeat the experiment on the stimulation of muscle 
and nerve by galvanic electricity, using a curarized muscle, we 
get the same effect from the muscle as before, showing that it is 
capable of being directly excited. 
Faradic Electricity may also be employed to excite muscle and 
nerve. It is obtained by the use of some form of induction machine. 
(For description of induction machine consult some book on 
physics). Du Bois Reymond's inductorium is the form most fre- 18 
HUMAN PHYSIOLOGY 
quently employed in physiology. The primary coil is connected 
with a battery and a simple key, the secondary coil with 
electrodes applied to the tissues, and a short-circuit key. 
When the key is closed in the primary circuit an instantaneous 
shock is received from the seconday coil, and when the key is opened 
in the primary another instantaneous sock is received from the 
secondary, but when the current is flowing through the primary 
circuit no effect is produced in the secondary, if the current does 
not vary in strength. The effect of the primary on the secondary 
is diminished by increasing the distance between the two, and vice- 
versa. 
(Experimental demonstration to show that the break shock is 
stronger than the make). 
Faradic electricity is the more efficient stimulus for nerve and 
galvanic electricity for muscle. 
RESULTS THAT FOLLOW STIMULATION OF MUSCLE 
I. Chemical change. 
II. Mechanical change (contraction). 
III. Production of Heat. 
IV. Production of Electricity. 
I CHEMICAL CHANGE IN MUSCLE FOLLOWING 
STIMULATION 
The potential energy of the chemical compounds in the muscle 
becomes transformed into the kinetic energy of contraction 
(mechanical), heat, and electricity, under the influence of the 
stimulus. 
Chemical change is always going on in muscle, even in the so- 
called resting state, but it is greatly increased when the muscle 
contracts. The main products are carbon dioxide (CO2) and 
sarcolactic acid. 
Production of Sarcolactic Acid-Resting muscle is alkaline, but 
if an excised muscle be made to contract for some time it becomes 
acid. This may be shown by the following experiment: Take 
two test tubes containing a solution of litmus and place a frog's 
gastrocnemius muscle in each. Stimulate the one muscle with 
interrupted shocks but not the other. The contracting muscle will 
soon change the layer of litmus solution in contact with it from 
blue to red, indicating the presence of an acid, while no such effect 
will be produced by the resting muscle. Analysis will show the 
substance produced to be sarcolactic acid. 
Muscles in the living body do not become acid during contraction 
like excised muscle because the sarcolactic acid produced is im- 
mediately neutralized by the alkaline lymph which bathes the PHYSIOLOGY OF MUSCLE 
19 
muscle; the effect is a diminution in the alkalinity of the tissues 
but these never become acid in the living state. 
Production of CO2.-That the production of CO2 is increased 
during contraction of muscle is shown by the following experiment: 
Two tightly stoppered flasks, with an air inlet and outlet, are 
taken, and the hind limb of a frog suspended in each. The outlet is 
connected with a bottle containing a saturated solution of barium 
hydrate, by rubber-tubing, and that again with a water aspirator 
by which a stream of air can be drawn through the two systems. 
(Copy diagram). One muscle is tetanised but the other is not. 
It will be found that the barium solution connected with the con- 
tracting muscle will become milky sooner than the other. This 
proves that CO2 is produced in greater amount in the active 
than in the resting muscle. 
Barium hydrate, Ba(OH)2, which is soluble, when brought into 
contact with CO2, is converted into barium carbonate, BaCO3, 
which is insoluble, and water, according to the following equation: 
Ba(OH)2 + CO2 = BaCO3 + H2O. 
The formation of BaCO3 gives the white precipitate. 
Some degree of cloudiness will appear in the tube connected 
with the resting muscle indicating that a small amount of CO2 is 
produced in it. 
Glycogen is a very important constituent of muscle and this is 
found to diminish as contraction goes on; in exhausted muscle it 
may disappear altogether. The glycogen is derived from sugar, in 
the form of glucose, which is brought to the muscle by the blood. 
This glucose is transformed, by a process of dehydration, into 
glycogen in the muscle and the stored up glycogen is believed to 
be the source of the sarcolactic acid and the CO2. 
The changes in the protein constituents of the muscle during 
contraction have not been determined but they are believed to be 
unimportant. Some of the products of protein metabolism, to- 
gether termed nitrogen extractives, are probably increased 
during contraction; the chief of these is creatin. 
Respiration of Muscle'-If an excised muscle is inclosed in a 
bell-jar containing air of known composition, and left for half an 
hour, say, it will be found on removing the muscle that the air has 
lost oxygen and gained CO2. This shows that even resting muscle, 
removed from the body, respires, i. e. takes in oxygen and gives 
off CO2. If the inclosed muscle be made to contract, the amount 
of CO2 given off will be greatly increased and also the amount of 
oxygen taken up. 
Alizarin Blue Experiment-Muscle in the living body has a 
great affinity for oxygen, so much so, that it will remove the oxygen 
from chemical combination. This can be proved by the following 
experiment: 20 
HUMAN PHYSIOLOGY 
Inject a solution of alizarin blue, one of the aniline dyes, under 
the skin of a live frog; it will be found when the animal is killed 
half an hour afterwards, that the blood is tinted blue but the mus- 
cles have their normal color. On being exposed to the air, however, 
the muscles take on a blue tint. The explanation is that when the 
alizarin blue is carried to the muscles by the blood it is deoxidized 
and rendered colorless by them, while the blood has no power to 
do this. When the dead muscle is exposed to the air oxygen is 
again taken up by this colorless compound and alizarin blue re- 
formed. 
II MECHANICAL CHANGE OR CONTRACTION OF 
MUSCLE 
Contractility appears to be the specific property of muscle, but 
muscle is not the only contractile tissue in the body. This pro- 
perty is also possessed by cilia and by certain of the white blood 
corpuscles which show amoeboid movement. 
When a muscle contracts, each fiber, and consequently the 
muscle as a whole, changes its shape; it becomes shorter and 
thicker but it shows no change in volume. If the gastrocnemius 
muscle of a frog be placed in a flask drawn out to a narrow neck, 
and filled to a definite mark with normal saline, and if the muscle 
be then made to contract by single shocks, the level of the fluid 
does not alter. This shows that there is no change in the volume 
of the muscle. 
Myographic Record of Muscular Contraction-For the proper 
study of muscular contraction the following pieces of apparatus 
are required: A lever to be attached to the muscle; a recording 
surface consisting of a revolving drum covered with smoked 
paper; a Du Bois Reymond inductorium with battery and keys 
for stimulation; a tuning-fork for recording time on the moving 
surface. The term Myograph (muscle writer) is applied to the 
lever, revolving drum and time writer together. 
By the following method the record or graph of a simple muscular 
contraction may be obtained. 
A frog's gastrocnemius nerve-muscle preparation is fixed to a 
strong support and the tendo-Achilles attached to the lever, the 
writing point of which is brought against the revolving drum. 
To the nerve is applied a pair of electrodes connected with the 
secondary coil of the inductorium. The drum is started rotating 
and by means of an automatic key, attached to the drtun, in the 
primary circuit, a single break shock is given to the nerve from 
the secondary. This can be done by moving the secondary coil so 
far away from the primary that the make shock is eliminated. 
The horizontal line drawn around the drum while the muscle is at 
rest is called the abscissa or zero line. Fig- 2 22 
HUMAN PHYSIOLOGY 
When the muscle receives the stimulus it contracts and raises 
the lever, the graph or contraction curve or myogram being written 
off on the revolving drum. The rate at which the drum is moving 
is determined, at the same time, by the vibrating tuning-fork. 
Simple Muscular Curve-On examining the record, which is 
usually known as a simple muscle curve, it will be found to show 
three phases or periods: 1, the latent period, 2, the contraction 
PERIOD, 3, the RELAXATION PERIOD. 
The muscle does not respond immediately on receiving the 
stimulus; the time that intervenes between the application of the 
stimulus and the beginning of the contraction is called the latent 
period. This, in a frog's muscle, measures about 1 /100 of a second, 
at ordinary room temperature. 
If three perpendicular lines be drawn through the curve, the first 
at the point where the curve leaves the abscissa, the second where 
it reaches its maximum height, and the third where it returns to 
the abscissa when the muscle has relaxed again, the contraction 
period is included between the first and second of these lines and 
the relaxation period between the second and third. The contraction 
phase lasts about 4/100 and the relaxation phase about 5/100 of 
a second. The whole contraction, from the application of the 
stimulus to the end of relaxation, including the three phases or 
periods, occupies about 10/100 or 1/10 of a second. 
Influence of Various Conditions on Simple Muscle Contraction. 
(a) Strength of Stimulus-A subminimal stimulus is one not 
sufficiently strong to bring about contraction of the muscle; a 
minimal stimulus is one which is just able to make it contract; 
a maximal stimulus is one which is capable of bringing out the 
maximal or greatest contraction of the muscle; a supermaximal 
is any greater than a maximal, and a submaximal is any between 
minimal and maximal. It is to be kept in mind that a maximal 
stimulus is not the strongest that the induction coil is capable 
of producing, but that which will produce the greatest effect on 
the muscle. It is useless and inadvisable to employ a supermaximal 
stimulus because it may damage the tissue. 
This gradation of effect following progressive increase in strength 
of stimulus is shown by striated and plain muscle, but not by 
heart muscle, as we shall see later. Later investigation, however, 
would appear to show that each muscle fiber, whether in striated, 
plain or cardiac, muscle gives the all-or nothing response when 
stimulated. 
With stimuli between minimal and maximal, the stronger the 
stimulus, the shorter is the latent period and the higher is the curve 
of contraction. 
(b) Resistance to be Overcome, i. e. Load-The heavier the 
load the longer is the latent period and the whole duration of the PHYSIOLOGY OF MUSCLE 
23 
curve, and the shorter is the height of contraction. In this case 
the increase in the length of the curve is mainly due to the pro- 
longation of the relaxation phase. The effect of the extensibility 
of the muscle is to increase the latent period, but apart from this, 
the true, corrected latent period is lengthened. 
(c) The internal condition of the muscle itself influences its 
contraction. Fatigue-As fatigue sets in the latent period and 
the whole duration of the contraction increases, while the 
height diminishes, the chief effect again being on the period of 
relaxation. 
"Treppe"-When a muscle is excited by a series of stimuli of 
equal strength, at intervals of one second, say, the first few con- 
tractions become progressively higher and higher until a maximum 
is reached, and then fatigue begins to manifest itself. This is 
called "treppe" or the "stair-case" phenomenon because the curves 
obtained on a slowly revolving drum resemble somewhat the steps 
of a stair. 
The cause of this treppe is believed to be the development of CO2 
within the muscle, whereby its irritability is increased at first, and 
then diminished. The same result is obtained from all three 
varieties of muscle, and from nerve. It indicates that the effect 
of the activity of the tissue is, in the beginning, beneficial to the 
tissue because it increases its irritability. 
Cause of Fatigue in Muscle-The fatigue of muscle is mainly due 
to the accumulation of the products of contraction-sarcolactic 
acid and CO2. This is proved by the following experiments: 
1. If the extract of fatigued muscle is injected into a fresh 
animal, this animal on being made to exercise, rapidly shows signs 
of fatigue. 
2. If the blood of a fatigued dog be injected into a fresh one, 
fatigue quickly appears in the latter. This proves that the fatigue 
products pass from the muscle into the blood of the general circu- 
lation. 
3. Fatigue can be rapidly induced in a muscle by feeding it 
with a weak solution of lactic acid. 
4. An isolated muscle is much more easily exhausted than one 
through which the blood is circulating. In the one case the poison- 
ous products are allowed to accumulate in the muscle, in the other 
case they are washed away by the blood and lymph. 
It is a well known fact that massage helps to restore fatigued 
muscles; by this treatment the fatigue products are squeezed out 
into the circulation. 
To a certain extent fatigue is due to the using up of the stored 
material within the muscle whose breaking down results in the 
contraction, but this is a less important factor than the other. 
Seat of Fatigue in Muscle-The question that next arises is, 24 
HUMAN PHYSIOLOGY 
when a muscle is stimulated through its nerve, what is the seat of 
fatigue-the nerve fiber, the muscle fiber, or the motor end-plate ? 
Since it will be seen later that a nerve fiber cannot be fatigued, it 
must be either the muscle or the end-plate, and there is reason to 
believe that it first appears in the end-plate. The following ex- 
periment seems to prove this: 
Arrange apparatus to stimulate a muscle through its nerve, once 
every half second, until it is completely fatigued and unable to raise 
the lever with the weight attached; then throw the stimulus, by 
means of a switch, directly into the muscle; it will begin to con- 
tract again. 
Mosso's Ergograph-The onset and effects of fatigue in human 
muscle have been studied by Mosso with an instrument, devised 
by him for the purpose,termed the ergograph. A weight of say 
10 pounds is lifted, by a sudden voluntary flexion of the middle 
finger to which it is attached, once a second, to the beat of a 
metronome. A writing point moving on the surface of a slowly 
revolving drum records, as an ordinate, the height to which the 
weight is lifted each time. A small belt is attached to the mid-finger 
which lifts the weight, the other fingers being strapped down to 
prevent them from assisting, thus one set of muscles alone (the 
flexors of the middle finger) is brought into play. The graphs 
recorded show the rapidity with which fatigue sets in. 
Using this instrument, the following points, among others, were 
made out by Mosso: 
"1. If sufficient time is allowed between contractions little or 
no sign of fatigue is apparent. With a load of 6 kilograms (about 
14 pounds), for instance, the flexor muscles of the finger showed no 
fatigue when a rest of 10 seconds was given between contractions. 
2. After complete fatigue, with a given load, a very long inter- 
val (2 hours) is necessary for the muscle to make a complete recovery 
and give a second record as extensive as the first. 
3. After complete fatigue efforts to still further contract the 
muscle greatly prolongs this period of complete recovery--a fact 
that demonstrates the injurious effect of straining a fatigued muscle. 
4. The power of a muscle to do work is diminished by conditions 
that depress the general nutritive state of the body or the local 
nutrition of the muscle used; for instance,by loss of sleep,hunger, 
mental activity, anaemia of the muscle, etc. 
5. On the contrary, improved circulation in the muscle-pro- 
duced by massage, for example-increases the power to do work. 
Food also has the same effect, and some particularly interesting 
experiments show that sugar, as a soluble and easily absorbed 
foodstuff, quickly increases the amount of muscular work that can 
be performed. PHYSIOLOGY OF MUSCLE 
25 
6. The total amount of work that can be obtained from a muscle 
is greater with small than with large loads, since fatigue sets in more 
rapidly with the larger loads. 
7. Marked activity in one set. of muscles-the use of the leg 
muscles in long walks for instance-will diminish the amount of 
work obtainable from other muscles such as those of the arm. 
It is very evident that the instrument may be used to advantage 
in the investigation of many problems connected with gymnastics, 
dietetics, stimulants, medicines, etc." Howell. 
Sensation of Fatigue-The feeling of discomfort which is ex- 
perienced in fatigued muscle probably results from stimulation of 
the sensory nerve terminations (muscle spindles) within the muscle 
and tendon. This acts as a safeguard against excessive muscular 
activity and overstrain. 
(d) The Temperature of Muscle affects the Character of its 
Contraction--The application of cold to a muscle has an effect 
similar to that of fatigue-the latent period is increased, the height 
of the curve diminished, and the duration of the whole curve 
lengthened, mainly by a prolongation of the relaxation phase. 
Heat, up to a certain limit, has the opposite effect,-the latent 
period is shortened, the height is increased and the length of the 
whole curve shortened. 
At a temperature of about 37° C. for the frog, the muscle loses 
its irritability and dies. At a certain temperature, known as the 
optimum temperature, which differs for different animals, muscle 
transforms energy most rapidly and most economically. 
(e) Drugs-Certain drugs have a pronounced effect on the 
character of the simple muscle curve. The best example is the 
alkaloid veratrin. Under the influence of this drug the contraction 
is very greatly prolonged, the effect again being on the relaxation 
phase. 
Compound Muscular Contraction or Tetanus 
So far we have considered the contraction of a muscle following 
a single stimulus; when a muscle receives a series of stimuli in 
rapid succession, there being no time to relax from one stimulus 
before the next one enters, it gives no longer a simple twitch but 
a compound contraction or tetanus. 
If the rate of stimulation is so rapid that the muscle shows no 
external sign of relaxation between stimuli, the record of this 
compound contraction on a revolving drum will be a continuous 
line; this is known as complete tetanus. When, however, the rate 
of stimulation is not sufficiently rapid, the muscle will relax more 
or less after each stimulus, and the record will show a wavy line, 
each secondary curve representing a muscular contraction; this is 
known as incomplete tetanus. 26 
HUMAN PHYSIOLOGY 
The rate of stimulation necessary to produce complete tetanus 
will vary with the kind of muscle used and with its condition, 
(whether fatigued or not, etc.) and temperature. For insect's 
muscle it will require 300 stimuli per second, for fresh frog's muscle, 
at room temperature, about 25 per second and for non-striped 
muscle one stimulus applied every five seconds may be sufficient; 
it will depend on the rapidity of relaxation shown by the muscles 
in simple contractions. 
There is evidence, however, to show that the rate of compound 
muscular contractions can be more rapid than that necessary to 
produce complete tetanus (See Muscle Sound.) 
Voluntary muscular contractions are tetanic in character; not 
even the most rapidly executed voluntary movement is a simple 
muscle twitch. 
Muscular Work 
Work (W) is defined as the product of the load (1) into the height 
(h) to which it is lifted. 
W = 1 X h. 
When a muscle contracts without lifting a load no work is done, 
forW = o X h = o. Again, when a muscle contracts but is unable to 
raise the load attached to it no work is done, for W = 1 X o = o. 
The English unit of work is the foot-pound; the height is meas- 
ured in feet and the weight in pounds. The French unit is the 
gramme-metre or gramme-millimeter, the height being measured 
in metres or millimeters and the load in grammes. 
The amount of work performed by a frog's muscle may be 
estimated by loading it with known weights and measuring the 
height of the contractions recorded as ordinates on a stationary 
drum moved by hand. The lengths of the ordinates must be re- 
duced for lever-magnification, and then the reduced height in 
millimeters multiplied by the load in grammes. 
If such an experiment be performed it will be found that the work 
done by a muscle increases with the load until the muscle is over- 
loaded. There is one load for each muscle in lifting which it will 
do more work than with either a lighter or a heavier load, and this 
is called the optimum load, i. e. the load in lifting which the muscle 
can do most work. 
The absolute power of a muscle is the load which the muscle is 
just unable to lift. This depends on the cross-section of the 
muscle and on its energizing power. 
The height to which a muscle can lift a weight depends on the 
length of the muscle, the weight to be lifted, and the quality of the 
muscle, the stimulus always being maximal. PHYSIOLOGY OF MUSCLE 
27 
To get the most out of a muscle it must be excited with a maximal 
stimulus, at the optimum temperature, and weighted with the 
optimum load. 
Insects have the most powerful muscles, weight for weight. 
The absolute power of human muscle is more than twice as great 
as that of frog's muscle per unit of the cross sectional area, and 
that of insect muscle is much greater still. 
The power of human muscle is measured by some form of 
dynamometer adapted to the particular group of muscle to be 
tested. The hand dynamometer shown is used for estimating the 
force of contraction of the flexor muscles of the fingers. 
Some heat is always being produced in muscle but the amount 
is greatly increased when the muscle contracts. More than eight- 
tenths of the total heat of the body is produced in muscle. 
Unit of Heat--Just as in measuring mechanical energy we require 
a unit of work, so in dealing with heat energy we must have a unit 
of heat. This is termed the calorie; it is defined as the amount 
of heat necessary to raise one gram of distilled water from 0° C. 
to 1° C. (small calorie) or one kilogram from 0° C. to 1° C. (large 
calorie). 
The quantity of heat produced by a muscle, in a definite time, 
may be calculated by multiplying the increase in temperature by 
the mass (weight) of the muscle by the specific heat of the muscle 
which may be taken as 0.8. For example, if a frog's muscle weigh- 
ing 10 grams had a temperature of 15° C. in the resting state, and 
15.1° C. after a period of contraction, then the amount of heat 
produced by the muscle during this period, assuming that none 
has been lost, will be 0.1 X 10 X 0.8 = 0.8 calorie; that is the 
amount of heat necessary to raise eight-tenths gram of distilled 
water from 0° C. to 1° C. 
In the case of large animals, such as the cat or dog, if the bulb 
of a mercurial thermometer be pushed into the muscles of the 
thigh (animal anassthetised) and then the sciatic nerve stimulated, 
the temperature of the muscle may be raised two or three degrees 
in a short time. For the detection of slight rises in temperature, 
such as occur in the small muscles of a frog, a mercurial thermom- 
eter is not sufficiently sensitive; the thermo-electric method must 
be used. 
Thermopile-Given two junctons of bismuth and antimony (or 
iron and German silver), if one is hotter than the other, electricity 
is developed in the heated junction and passes as a current through 
a galvanometer in circuit. From the amount of deflection of the 
galvanometer needle the difference in temperature between the 
Ill PRODUCTION OF HEAT IN MUSCLE 28 
HUMAN PHYSIOLOGY 
two junctions can be calculated. This is a thermopile. By increas- 
ing the number of junctions connected in series the instrument is 
made more sensitive. 
To determine slight changes of temperature in frog's muscle 
the thermopile is used as follows: Two nerve-muscle preparations 
are made and one is applied to each junction (or set of junctions). 
One muscle is stimulated, through its nerve, to contract while the 
other is allowed to rest. In the contracting muscle heat is de- 
veloped which generates electricity7 and the rise in temperature can 
be calculated from the amount of deflection of the galvanometer 
needle. 
A single contraction will raise the temperature of a frog's 
gastrocnemius about 1 /1000 of a degree centigrade. 
By causing a muscle to lift a weight and at the same time 
measuring the heat produced in it, the ratio of mechanical work 
to heat production can be obtained. While the best mechanical 
engine (gas engine) will not transform more than 25% of the total 
energy of its fuel into mechanical work, the other 75% being wasted 
as heat, a muscle (human) it has been stated will give 40% as me- 
chanical work and the remaining 60% as heat is not wasted for 
it serves to maintain the temperature of the body. 
IV. PRODUCTION OF ELECTRICITY IN MUSCLE 
Galvani's Experiment-Galvani, the professor of physiology in 
the University of Bologna, is credited with the discovery of animal 
electricity. One day when watching some frogs' legs, suspended by 
copper hooks, on an iron railing he observed that whenever the 
wind brought the muscles in contact with one of the rails they 
twitched. He explained the twitching, under these conditions, by 
supposing that the electricity is present in the tissue and that it 
excites the muscle when the circuit is completed. 
Volta, professor of physics at Pavia, maintained, however, that 
the source of the electricity was the junction of the two dissimilar 
metals, copper and iron. The controversy lasted several years and 
it was finally shown that both were right, in part. 
Electric Fishes-Long before the discovery of Galvani it was 
known that certain fishes had electrical properties; these were the 
Torpedo or electric ray, found in the Mediterranean, the Malap- 
terurus electricus, in the Nile, and the Gymnotus or electric eel in 
South America. 
These fishes are provided with a series of plates arranged parallel 
to each other like the elements of a galvanic battery. Each plate is 
supplied by a nerve from the brain through which impulses may 
be sent voluntarily to discharge the organ. In all electric fishes 
the discharges are interrupted; an active fish may give as many as 
200 shocks per second; this is comparable with the compound PHYSIOLOGY OF MUSCLE 
29 
contractions of a muscle. This power is used by the fish to stun 
or kill its prey or enemies. 
In the gymnotus and the torpedo these organs are modified 
muscles, while in malapterurus they are said to be transformed 
skin glands. 
The requirements for the study of the electrical changes in 
muscle are-1, a senstitive galvanometer or electrometer; 
2. NON-POLARIZABLE ELECTRODES; 3. a BATTERY and INDUCTION 
coil for stimulating the muscle. 
If a frog's gastrocnemius muscle be dissected out and one end 
injured, as by cutting it across, and then if one electrode be 
applied to the cut end of the muscle and the other to the longitudi- 
nal surface, a current of electricity will pass through the galvanom- 
eter from the longitudinal surface to the cut end; this is called 
the current of injury, or the demarcation current. The longitudinal 
surface is positive, corresponding to the positive pole of an ordinary 
galvanic battery, and the cut end is negative. 
Contrary to the opinion held by Galvani, it is now known that 
no current is produced by an uninjured and resting muscle. If 
the electrodes be applied to two points on the longitudinal surface 
of an uninjured muscle no effect is seen on the galvanometer, since 
all points on the surface are at the same potential, or iso-electric; 
if, however, the muscle be made to contract by stimulation at one 
end then a current is produced in the muscle. As the contraction 
wave sweeps along, the active (contracting) part of the muscle is 
negative to the inactive parts which are positive,--an area of de- 
pressed potential accompanies the wave of contraction. This is 
known as the current of action or the negative variation. 
This action current is frequently taken as a measure of the ac- 
tivity of the muscle instead of the record produced by a lever 
writing on a drum, from its mechanical contraction. 
Rheoscopic Frog-The action current of one muscle may be used 
to stimulate the nerve of another nerve-muscle preparation and so 
to cause contraction of the second muscle (see diagram). Two such 
preparations are known as the rheoscopic frog. 
A very striking experiment may be performed by laying the 
nerve of a nerve-muscle preparation, longitudinally, on the beating 
heart of a frog; the action current of the heart will stimulate the 
nerve and so the muscle will contract synchronously with the heart. 
When muscle contracts tetanically it gives rise to a musical 
sound which can be heard by applying a stethoscope to the muscle, 
or demonstrated with the microphone. The note that is heard 
corresponds, in pitch, to the number of stimuli sent into the muscle, 
up to a certain point. In the case of frog's muscle when the stimu- 
lation is at the rate of 200 per second, or less, the musical note 30 
HUMAN PHYSIOLOGY 
produced corresponds, but if the rate be increased over 200 then the 
tone fails to rise in pitch. For mammalian muscle the limit is 
about 1000 per second. This shows that, even in apparently com- 
plete tetanus, the character of the contraction is not continuous 
but interrupted, due to what may be supposed to be a series of 
explosive changes in the muscle. The failure to produce tones 
above a certain pitch may be due to the fact that the substance 
exploded requires time to be re-built up, in order to be decomposed 
again, and if this is less than 1 /1000 second in mammalian muscle, 
or 1 /200 second in frog's muscle, then a certain number of stimuli 
must be ineffective. This fraction of a second is known as the 
Refractory Period. 
This interrupted character of muscular contraction, with high 
rates of stimulation, can also be proved by the fact that each 
stimulus is followed by a current of action in the muscle which can 
be demonstrated with the string galvanometer, up to certain 
limits. 
MUSCLE TONUS 
The skeleta1 muscles are always in a state of semi-contraction 
so long as their nervous connection with the spinal cord or brain 
is complete; this is known as muscle tonus. 
Cause of Muscle Tonus--It was at one time believed to be due 
to the automatic discharge of a succession of feeble impulses being 
showered down on the muscle, through its motor nerve, from the 
spinal cord, since, if this nerve is divided, the tonus disappears. 
The same thing happens, however, that is, loss of tonus, if the 
afferent nerve root entering that segment of the cord from which 
the motor root arises, be divided; therefore this is a reflex phe- 
nomenon. 
A living muscle, therefore, with its nerve connections intact, is 
never in a condition of rest, although not in visible contraction; 
it is always the seat of chemical change and of heat production 
even in the so-called resting condition. 
CONDUCTIVITY OF MUSCLE 
This is the last of the three vital properties of muscle (irritability, 
contractility, conductivity) to be considered. Muscular tissue has 
a certain power of transmitting disturbances, produced in it at 
any point, through its substance, although this is the specific 
property of nerve fibers. 
If a curarised muscle (why curarised ?) be stimulated at one end, 
the wave of contraction initiated there will pass to the other end, 
the disturbance being conducted from fiber to fiber. PHYSIOLOGY OF MUSCLE 
31 
Method of Measuring Rate of Muscular Contraction Wave-A 
frog's curarised muscle (preferably one with parallel fibers such as 
the sartorius) is pinned out on a cork plate and two light levers 
laid across it, about an inch apart, their writing points being brought 
against a rapidly revolving drum and on the same vertical line. 
The muscle is simulated at one end by a single induction shock, 
and the contraction wave, which starts at that end, sweeps along 
the muscle raising first the proximal lever and then the distal. 
Two curves will be written off on the drum, the one beginning 
before the other. A tuning fork tracing is taken underneath and 
the time interval between the beginnings of the two curves observed. 
The distance on the muscle between the two levers is then measured. 
This will give all the data necessary for calculating the rate in feet 
or meters per second. 
For example, suppose the time interval between the beginnings 
of the two curves to be 1 /120 second (i. e. the time taken for the 
wave to pass from the proximal to the distal lever), and the dis- 
tance between the two levers one inch; then the wave in one 
second would travel 120 inches or 10 feet. 
The rate in frog's muscle is found to be about 10 feet per second; 
in human muscle about 30 feet per second. 
RIGOR MORTIS 
Muscle when dead loses its irritability, becomes stiff and rigid 
and passes into a condition known as rigor mortis or death stiffen- 
ing. The cause of this is believed to be the coagulation of the 
protein constituents (or some of them) of the muscle. In some 
respects it resembles the clotting of blood. 
A somewhat similar condition is brought about by exposing the 
muscles to too high a temperature; this is known as rigor caloris 
or heat stiffening. Sarcolactic acid is developed in the process in 
both cases. 
In the human subject rigor mortis appears first in the muscles 
of the jaw; then in the neck, arms and trunk, in order, and 
lastly in the muscles of the leg. It passes off in the same order. 
The time of onset varies greatly; it usually supervenes from one 
to seven hours after death, but it may come on a few minutes 
after or it may be delayed for many hours, depending no the 
condition of the muscle. Its appearance is hastened by ex- 
haustion due to starvation, fatigue or wasting disease. Rigor 
usually lasts from 24 to 36 hours but it may continue for as long 
as six days; if it comes on quickly it passes off quickly. The dis- 
appearance of rigor seems to be due to the softening of the coagu- 
lated proteins, by the action of autolytic ferments, in the presence 
of sarcolactic acid. 32 
HUMAN PHYSIOLOGY 
ACTION OF MUSCLES ASSOCIATED WITH THE 
SKELETON 
The skeleton gives support and protection to the soft parts of the 
body and is clothed mainly by muscles whose action is under the 
control of the will, hence they are called skeletal or voluntary 
muscles. They are concerned with the movement of one part in 
relation to another, and also with the locomotion of the body as 
a whole. 
The muscles associated with the limbs cause the movement of 
bones at joints, and according to the nature of these movements, 
the limb muscles are grouped as flexors, extensors, abductors, 
adductors, and in the forearm, supinators and pronators. 
In addition to the limb mucsles other groups are the neck muscles 
which move the head on the trunk; muscles related to other 
regions of the spinal column; muscles forming the walls of the 
thorax (external and internal intercostals) and abdomen; the 
diaphragm, separating the thoracic and abdominal cavities; the 
facial muscles or muscles of expression; muscles of the tongue; 
muscles of the eyeball; and lastly the dermic or skin muscles, 
rudimentary in man, but well developed in certain of the lower 
animals, e. g the horse. 
Each muscle is a separate organ, surrounded by a capsule of 
connective tissue (the deep fascia) which separates it from its 
neighbors. 
In describing a muscle anatomically the following points must 
be stated-its origin, course and insertion; its relations to 
neighboring structures, and its nerve supply. 
The great majority of voluntary muscles are inserted into bones, 
and when they contract, movement of the bones takes place at 
joints, and the action of the muscle will depend on the relation 
between, 1, the point of insertion; 2, the point of application of 
the resistance to be overcome; and, 3, the joint or fulcrum around 
which the movement takes place. 
Examples of each of the three orders of lever are to be found in 
the body. 
First Order--Power (insertion of muscle) at one end, weight or 
resistance at other end, and fulcrum (joint) between. Example- 
action of triceps muscle in extending the forearm. 
Second Order--Power at one end, fulcrum at the other end, and 
the weight or resistance between. Example--action of the 
muscles attached to the lower jaw that open the mouth. 
Third Order--Weight or resistance at one end, fulcrum at other, 
and application of power between. Example--action of biceps 
muscle in flexing the forearm at the elbow joint. PHYSIOLOGY OF MUSCLE 
33 
It must be remembered that even the simplest movement, such 
as bending the finger, for example, requires the co-ordinated action 
of several muscles. This is effected by nerve centers situated in 
the brain and spinal cord. 
NON-STRIATED, PLAIN OR INVOLUNTARY MUSCLE 
The physiological properties of plain muscle differ from those of 
striated mainly in three respects: 1, its nerve supply comes from the 
autonomic system so that it is not under the control of the will 
like striped muscle; 2, it is more sluggish in its response to stimu- 
lation than striped muscle; and 3, it appears to have the power of 
contracting rhythmically, without nervous stimulation, which 
striated muscle has not. 
The contraction of plain muscle is so sluggish, in all its phases, 
that a single contraction may last for minutes instead of the 
fraction of a second as in striated muscle; the latent period may 
be more than half a second. 
The various forms of artificial stimulation used in the case of 
striped muscle are effective for plain, but, as a rule, the latter 
requires a much stronger stimulus. 
Rhythmicity--If a strip of non-striated muscle-e. g. from the 
frog's stomach-be attached to a lever the point of which is 
brought against a slowly revolving drum, the muscle being im- 
mersed in Ringer's solution to keep it alive, a wavy curve will 
be obtained due to the slow and irregular contractions and re- 
laxations of the muscle. If a striated muscle is similarly treated 
the curve obtained will be a straight line unless the muscle be 
stimulated directly or through its nerve. A good example of this 
rhythmical property of plain muscle is to be found in the peris- 
taltic movements of the intestine by which its contents are driven 
onwards. 
Heart Muscle-'The physiological properties of heart muscle will 
be more profitably considered under the circulation. PHYSIOLOGY OF NERVE 
Coming off from the brain and spinal cord, which together form 
the central nervous system, are 43 pairs of nerves-12 pairs from 
the brain (cranial nerves) and 31 pairs from the cord (spinal 
nerves). Each of the spinal nerves arises from the side of the 
spinal cord by two roots--the anterior or ventral root, and the 
posterior or dorsal root, the latter being characterized by a swel- 
ling or ganglion-the posterior root ganglion. A short distance 
from their origin both roots unite to form the spinal nerve. 
The autonomic nervous system lies outside the central 
nervous system but is connected with it; this gives nerve fibers 
to non-striped muscle and various other tissues. 
Anatomical Classification of Nerve Fibers-Nerve fibers (or any 
tissue) may be classified anatomically or physiologically. Ana- 
tomically they are of two kinds-medullated fibers and non- 
medullated fibers, the structure of which has already been 
described. 
Physiological Classification of Nerve Fibers-Classified physio- 
logically, that is, according to their function, nerve fibers may be 
divided into two main groups--efferent (out-going) fibers carrying 
impulses away from the central nervous system, and afferent (in- 
coming) fibers carrying impulses towards the central nervous sys- 
tem. Each of these groups may be further subdivided. 
Efferent fibers are subdivided as follows: 
Motor to voluntary muscles. 
Vaso-motor,-to bloodvessels. 
Cardio-motor,-to heart. 
Viscero-motor,---to viscera such as stomach, in- 
testine, etc. 
Pilo-motor,--to hair muscles. 
1. Motor 
rTo salivary glands. 
To stomach. 
To pancreas, etc. 
/To sweat glands. 
2. Secretory 
3. Inhibitory 
For each variety of motor fiber. 
Foi each variety of secretory fiber. 
Afferent fibers are subdivided into: 
1. Those fibers that carry impulses into the central nervous 
system which affect consciousness (sensory). 
2. Those fibers that carry impulses into the central nervous 
system which do not affect consciousness (reflex). 
34 PHYSIOLOGY OF NERVE 
35 
Motor Nerves--Many nerve fibers carry impulses to muscles 
producing movement; these are termed motor fibers and they are 
subdivided according to the variety of muscle which they supply,-• 
the skeletal muscles (motor fibers); muscular tissue of the blood- 
vessels (vaso-motor fibers), of the heart (cardio-motor fibers), of 
the abdominal viscera (viscero-motor fibers); and the erector pili 
muscles of the hairs (pilo-motor fibers). 
Secretory N erves-Another subdivision of the efferent group carry 
impulses co secreting glands, such as the salivary glands, causing 
them to pour out fluids or secretions. These are termed secretory 
nerve fibers, and they are subdivided according to the variety of 
gland tissue which they supply. 
Inhibitory Nerves-Finally, a large number of nerve fibers are 
not excitatory but inhibitory, that is, impulses coming through 
these nerve fibers check or inhibit some action which was already 
taking place, it may be the contraction of a muscle, or the secretion 
of a gland; these are termed the inhibitory nerve fibers. 
Similarly the afferent group may be subdivided into two classes 
according as the impulses which they convey reach the cerebrum 
(highest division of brain) and so affect consciousness (sensory 
division), or go no farther than the spinal cord and lower regions 
of the brain and bring about reflex action of some kind (reflex 
division). 
When a nerve is cut across the end of the divided nerve next to 
the central nervous system is called the central end and that next 
to the surface of the body or periphery is called the peripheral 
end. 
Methods of Determining Functions of Nerves-If we wish to 
find out the function of a particular nerve two general methods 
may be employed: 1, we may divide the nerve and note what 
happens; 2, we may stimulate either the central or the peripheral 
end and note what happens. For example, if, after cutting a nerve 
across, we find that a certain muscle or group of muscles is para- 
lyzed we know that the nerve is motor in function. This will be 
made more certain still if the muscle or muscles contract on 
stimulating the peripheral end of the nerve. 
Bell-Magendie Law-It v as at one time believed that all nerve 
fibers convey impulses or messages in both directions just like a 
telegraph wire, but in 1811 Sir Charles Bell proved experimentally 
that the anterior roots of the spinal nerves are motor in function 
while the posterior roots are not. He observed, in a recently killed 
animal, that when an anterior root was touched or pinched with 
forceps, certain muscles were thrown into contraction, while this 
did not happen when the posterior roots were excited. Later 
(1822) Magendie showed that in a living animal, when the posterior 36 
HUMAN PHYSIOLOGY 
roots were stimulated, there was evidence of pain. Combining the 
results of both experiments it was stated that the anterior roots 
of the spinal nerves are purely motor, the posterior roots purely 
sensory, and this is known as the Bell-Magendie law. 
Since the time of Bell and Magendie it has been found that in 
addition to purely motor fibers many others,-vaso-motor, secre- 
tory, inhibitory, etc.--emerge in the anterior roots, and that in 
the posterior roots there are fibers stimulation of which causes 
only reflex action, so that now the more general statement may be 
made that all efferent fibers leave the spinal cord by the 
ANTERIOR ROOTS AND ALL AFFERENT FIBERS ENTER BY THE POS- 
TERIOR ROOTS. 
REFLEX ACTION 
Any action which is not directly voluntary is said to be reflex; 
the individual may or may not be conscious of it. For example, 
if the sole of the foot be tickled the leg is involuntarily withdrawn 
by the contraction of its muscles; this is a reflex act, and the sub- 
ject is quite conscious both of the movement and of the sensation, 
but it is not voluntary. During sleep, when consciousness is in 
abeyance, this reflex can be elicited even more readily than in the 
waking state. 
Reflex Arc--The anatomical basis of reflex action is the reflex 
arc. This consists of (a) an end-organ (the receptor) and afferent 
nerve fiber; (b) a nerve center; (c) an efferent fiber ending in a 
muscle, gland cell, etc. (the effector). 
The simplest reflex arc contains two neurons-an afferent and 
an efferent--the synapse being situated within the central nervous 
system (brain or spinal cord). In more complex arcs one or more 
neurons may be interposed between the afferent and efferent. 
Interruption of the arc at any point will abolish the 
REFLEX. 
Inhibition of Reflexes--Reflex actions (motor, secretory, etc.) 
may be inhibited, to a certain extent, voluntarily, or by excitation 
of some other receptive surface, e. g. the reflex movement called 
forth by tickling the sole of the foot may be prevented by a power- 
ful voluntary effort. 
Spinal Reflexes in Decerebrate Animals-Spinal reflexes are 
more easily induced in "decerebrate" (deprived of the cerebrum) 
than in normal animals, the reason assigned being that with the 
brain intact inhibitory impulses are being constantly showered 
down on the lower spinal centers, keeping the reflexes in check; 
when the cerebrum is taken away this controlling or inhibitive PHYSIOLOGY OF NERVE 
37 
influence is removed. This also explains why it is that the spinal 
reflexes are more easily elicited in animals low in the scale (frog) 
than in those higher (cat, dog, man). 
Importance of Reflexes in Medicine-In the human subject the 
character of the spinal reflexes, particularly the plantar reflex and 
the knee-jerk, is of great value to the physician in the practice of 
medicine as diagnostic signs in diseases of the nervous system. 
The Knee-jerk is elicited by giving a sharp blow with the edge of 
the open hand to the patellar tendon in front of the knee, when 
the foot is suddenly and involuntarily jerked or kicked forward. 
This is present in all normal individuals but in the disease known as 
locomotor ataxia it is absent. The lesion in this disease lies in 
the posterior nerve roots, the afferent factor in the reflex arc thus 
being broken. In infantile paralysis (poliomyelitis anterior 
acuta) the part primarily affected is the grey matter of the spinal 
cord (anterior horn). Here again the knee-jerk is absent, the arc 
being interrupted centrally. On the other hand, in some conditions, 
such as spastic paralysis, the knee-jerk is exaggerated. In this 
disease the inhibitory7 influences of the higher brain centers is cut 
off, and all the spinal reflexes are more easily called forth, just as 
they are in a decerebrate animal. 
Classification of Reflex Movements-Reflex movements may be 
SIMPLE, CO-ORDINATED, Or CONVULSIVE. 
1. Simple Reflex-By a simple reflex is meant one in which a 
single muscle is affected. The best example is the winking reflex, 
where the single muscle of the eyelid (orbicularis palpebrarum) 
contracts reflexly as the result of a stimulus received from the 
sensitive conjunctiva. 
2. A Co-ordinated Reflex is one in which a group of muscles 
are called into play so as to bring about a co-ordinated and ap- 
parently purposeful movement. This is well seen in a frog, from 
which the brain has been removed, so as to make voluntary move- 
ments impossible. If such a frog be suspended from a stand and 
a small square of blotting paper, soaked in acetic acid, be applied 
to the skin of the back at one side of the middle line, the limb of 
that side will be drawn up, and the toes directed to the spot which 
is being irritated by the acid so as to remove the blotting paper. 
Here the movement is unconscious and involuntary, nevertheless, 
many muscles are used in perfect co-ordination so as to bring about 
a series of movements with an apparently definite end. 
3. Convulsive Reflexes are those in which a great number of 
muscles (possibly every muscle in the body) are brought into 
action, without any co-ordination, disorderly, purposeless move- 
ments being produced. 38 
HUMAN PHYSIOLOGY 
A co-ordinated reflex may be transformed into a convulsive one, 
1, by strong or persistent sensory stimulation, and 2, by diminishing 
the resistance at the synapses in the reflex arcs. 
1. When the skin of the sole of the foot is tickled, momentarily, 
the foot is withdrawn by the co-ordinated action of the muscles of 
the leg, but if the tickling be persisted in many other muscles are 
brought into play, not associated with the withdrawal of the foot 
at all, notably the respiratory and facial muscles used in the act of 
laughing. 
2. If a small amount of strychnine be injected under the skin 
of a decapitated frog it will soon be absorbed and carried to all the 
tissues including the spinal cord. In such an animal the slightest 
stimulus applied to the skin will cause, not a simple or co-ordinated 
reflex, but powerful convulsive movements of all the muscles in 
the body. The reason given for this action of strychnine is that it 
diminishes the resistance, at the synapses in the reflex arcs, to the 
passage of the impulses that bring about contraction of the muscles. 
PROPERTIES OF NERVE FIBERS 
Nerve fibers have two properties: 1, Irritability; 2, Conductivity. 
Like all living nerve tissues fibers are irritable, i. e. they respond 
to stimulation, but their specific property is conductivity. 
The natural stimulus for a nerve fiber is some change (possibly 
chemical) in its cell of origin, or in its termination in a sensory 
surface. As in the case of muscle, artificial stimulation may be 
mechanical, chemical, thermal or electrical, and the last is preferred 
for the reasons given under muscle, viz.-that its strength is easily 
regulated, and that it does not appreciably injure the tissue. 
When stimulated, naturally or artificially, a nerve impulse is 
initiated at the point excited and travels or is conducted along the 
fiber. No contraction or visible change of shape follows, as we 
saw in the case of muscle; the only known sign of the passage of a 
nerve impulse is an electrical change which passes along the 
fiber accompanying the impulse and which is known as the current 
of action. 
The electrical changes in nerve are studied in the same way 
and with the same apparatus as in muscle. A divided nerve will 
give a current of injury and an active nerve (engaged in conducting 
a nerve impulse) a current of action just as we saw in muscle, and 
th's current of action is taken as the objective sign or index of 
functional activity in investigating the physiological properties 
of nerve fibers. A dead nerve will not conduct an impulse and 
shows no action current. PHYSIOLOGY OF NERVE 
39 
Irritability of Nerve Fiber-The response of a nerve to any 
stimulus depends on three factors-(a) the degree of excitability 
of the nerve; (b) the strength of the stimulus; and (c) the duration 
of the stimulus. 
Summation of Stimuli-The duration of a stimulus, as well as 
its strength, is important. A subminimal stimulus continued for 
some time will ultimately produce an effect. This is known as the 
summation of stimuli. If a decerebrate frog be suspended from a 
stand and the toes of one hind limb dipped into dilute sulphuric 
acid, 1 in 50, the limb will be almost immediately withdrawn 
reflexly; if, however, the irritating acid be more dilute, say 1 in 
500, then it may be many seconds before the limb is withdrawn. 
In the latter case the stimuli are too feeble individually to produce 
any result but may do so when they are summated or added 
together. 
Conductivity of Nerve Fiber 
By this is meant the capacity of a nerve fiber to conduct an 
impulse initiated at any point on its course by natural or artificial 
stimulation. This is the specific property of nerve fiber just as 
contractility is the specific property of muscle fiber. 
Rate of Nerve Impulse-The velocity with which an impulse 
sweeps along a nerve fiber was first successfully measured by 
Helmholtz. He stimulated the sciatic nerve of a frog's nerve- 
muscle preparation first at a point near the muscle, (a), and then 
at a point near the spinal cord, (b), i. e. far away from the muscle, 
and measured the time interval between the beginnings of the two 
curves recorded on a revolving drum. Knowing this interval in 
fractions of a second, and the distance between the two points 
(a) and (b) on the nerve, the rate in feet or meters per second can 
be calculated. For example, suppose the two points on the nerve 
are two inches apart and that the second curve begins 1 /550 
second later than the first, then the distance travelled in one 
second will be 2X550=1100 inches or 91% feet per second. 
The velocity in the frog's motor nerve, at room temperature, 
Helmholtz found to be about 90 feet per second. In human nerve 
it is somewhat greater, viz.: about 110 feet per second. 
Both the velocity and the strength of the impulse are modified 
by temperature, by narcotic drugs, and by mechanical pressure. 
In cooled nerve the velocity is diminished and in warmed nerve 
it is increased, the limits of conductivity lying between 0°C. and 
about 50°C. Cooling a nerve to 0°C. (it is not frozen at this 
temperature because of the saline juices which bathe it) will sus- 40 
HUMAN PHYSIOLOGY 
pend conductivity completely; it will return, however, on warm- 
ing. A temperature of about 50°C. kills the nerve. 
Narcotic drugs such as chloroform, ether, chloral, alcohol, cocaine, 
applied locally, to nerve will block the impulse, i. e. prevent its 
passage; but conductivity will return when the drug is removed. 
This fact is taken advantage of by the surgeon in performing minor 
operations. If cocaine be applied to a nerve trunk supplying a 
certain area of skin, no impression from that area can reach the 
brain and give rise to pain. 
The effect of gases and vapors on the excitability and conductiv- 
ity of nerve is studied by the gas-chamber method. (Copy dia- 
gram). The action of two gases, CO2 and oxygen, is interesting. 
CO2 IN SMALL AMOUNT, i. e. IN GREAT DILUTION, INCREASES THE 
IRRITABILITY AND CONDUCTIVITY OF NERVE FIBERS, BUT IN LARGER 
AMOUNT IT DIMINISHES BOTH AND ULTIMATELY KILLS THE NERVE. 
The relation of oygen to the functional activity of nerve fiber is 
very important when considering the nature of the nerve impulse. 
In an oxygen-free atmosphere conductivity is entirely lost 
BUT IT WILL RETURN IF OXYGEN IS READMITTED. The fact that 
oxygen is essential would seem to point to the conclusion that the 
passage of a nerve impulse implies some chemical change. 
Mechanical pressure on a nerve will also suspend its conductivity 
without permanently injuring the fibers provided the pressure is 
not excessive. 
Fatiguability of Nerve-Can the excitability and conductivity 
of a nerve fiber be suspended by prolonged or excessive stimulation, 
i. e. can a nerve fiber be fatigued? The following experiment ap- 
pears to answer this question: 
Two nerve-muscle preparations from the same frog are arranged 
so that each nerve is stimulated alike with the interrupted current 
and each muscle lifts a weight of 30 grams. In one of the prepara- 
tions (A) an area of the nerve between the point stimulated and 
the muscle is cooled so as to prevent the passage of an impulse, 
thus protecting the muscle; in the other (B) the muscle contracts 
and lifts the weight and in 10 or 15 minutes it will be completely 
fatigued. Now in A remove the block by raising the temperature 
and its muscle will contract. This shows that the nerve of A has 
not been fatigued although it was stimulated equally with that of 
B. It has thus been found that prolonged excitation, for hours, 
will not produce in nerve fibers any signs of fatigue. 
This is not true of nerve cells, however; these quickly show 
signs of fatigue and microscopical changes of the nature of chroma- 
tolysis can be demonstrated in them as the result of stimulation 
and functional activity. PHYSIOLOGY OF NERVE 
41 
Nature of the Nerve Impulse 
Three theories have been advanced to explain the nature of the 
nerve impulse. 1. It may be chemical resembling somewhat the 
firing of a train of gun powder. The stimulus applied to the nerve 
may cause the explosion or breaking down of some unstable sub- 
stance in the axis cylinder, the disturbance being propagated along 
the nerve as a progressive chemical change. It is necessary to as- 
sume here that the substance broken down is immediately rebuilt 
up, because a nerve fiber shows no signs of fatigue or exhaustion 
after stimulation with the interrupted current for hours. Two 
facts give support to this theory. 
1. Oxygen is essential, as we have seen, for nerve conduction, 
and this would seem to indicate chemical change. 2. It has been 
shown by Tashiro that CO2 is produced in nerve fiber, in increased 
amount, when it is stimulated. This again points conclusively to 
chemical change of a disintegrative nature, CO2 being the final 
product of oxidation. 
Certain other facts, however, do not appear to agree with the 
chemical theory. 1. No heat, in detectable quantity is produced 
in nerve fiber, and heat production is always associated with 
oxidative changes. 2. Nerve fiber is unfatiguable. It would ap- 
pear that if the conduction of a nerve impluse depend on the de- 
composition of any substance stored in the fiber, this substance 
would, in the course of time, become exhausted. 
2. It may be something of the nature of a mechanical vibration 
passing along the nerve fiber resembling a sound wave transmitted 
along a steel rod, for example. The fact that no heat is produced, 
and that fatigue cannot be induced seem to support this idea. 
3. It may be something of the nature of an electrical change 
sweeping along the nerve fiber. This is a possibility but it has not 
been proved, and the reasons for and against it cannot be given, 
with advantage, in an elementary text-book. 
A nerve fiber cut off from its cell of origin undergoes degenerative 
changes since the cell acts as the trophic or nutritive center for the 
nerve fiber. This important fact was discovered by Dr. Waller in 
1850 and hence is termed Wallerian or secondary degeneration. 
The degenerative changes in the nerve fiber are evident a few 
days after its section. The myelin of the medullary sheath 
breaks up into fatty globules and ultimately disappears; the 
AXIS CYLINDER also DISAPPEARS, but the NEURILEMMA REMAINS 
and acts as a scaffolding for the new axis cylinder which grows 
down from the central stump when the nerve degenerates. 
Nerve Degeneration 42 
HUMAN PHYSIOLOGY 
When regeneration takes place the function of the nerve is 
restored, that is, motion or sensation returns, as the case may be. 
If a nerve is accidentally torn or cut across, repair may be ex- 
pected in time, but this time is greatly shortened if the surgeon 
unites the two ends of the divided nerve with a few stitches, 
because then the new axis cylinders will find their way more 
readily from the central end into the neurilemma of the peripheral 
part. 
Wallerian degeneration furnishes a very important method of 
tracing the course of nerve fibers, particularly in the brain and 
spinal cord. CIRCULATORY SYSTEM 
The heart and blood-vessels form a closed system filled with 
blood. The mammalian heart is divided by a complete septum 
into four chambers-two auricles and two ventricles, right and left. 
Starting from the left ventricle the blood passes into the aorta, 
then into the smaller arteries and then into the capillaries. From 
the capillaries it is collected into the veins and is finally poured 
into the right auricle. This is called the greater or systemic 
circulation. 
From the right auricle the blood passes into the right ventricle, 
and from there to the lungs through the pulmonary artery which 
soon divides into two branches, one for each lung. In the lungs 
it passes through a second set of capillaries, is collected again by 
the veins and reaches the left auricle through the large pulmonary 
veins, four in number, two for each lung. This is known as the 
lesser or pulmonary circulation. From the left auricle it passes 
to the left ventricle, the point from which we started, thus complet- 
ing the circulation. 
The blood which goes to the stomach, intestine, pancreas and 
spleen, after passing through the capillaries of these organs, is 
collected by their veins which all unite to form a single large vessel 
called the portal vein, and through this it enters the liver where 
it is again driven through a set of capillaries-the hepatic capillaries. 
Finally it leaves the liver by the hepatic veins which join the 
inferior vena cava and so reaches the right auricle. This is termed 
the portal circulation. It will be noted that the blood of the portal 
circulation passes through two sets of capillaries. 
A somewhat accessory circulation is that through the kidneys- 
the renal circulation. A part of the blood takes this course--from 
the aorta, through the renal arteries, capillaries and veins to the 
inferior vena cava. 
The lymphatic vessels really form a part of the circulatory 
system. The lymph, which consists essentially of the fluid part 
of the blood that has passed through the capillary walls into the 
tissues, is collected by the lymphatic vessels, and after passing 
through lymph glands is poured into the blood by two large 
lymphatic vessels--the right lymphatic duct and the thoracic duct 
both of which open into the large veins in the root of the neck 
near the heart. (Copy diagram.) 
As will be explained later, the blood travels much more slowly 
through the capillaries than it does through the arteries and veins, 
Course of Blood in Circulation 
43 Fig. 4 CIRCULATORY SYSTEM 
45 
and this gives time for the interchange of products between the 
blood and the tissues. 
THE HEART 
The heart is a muscular pump whose rhythmical contractions 
force the blood to move in a circle. It is an organ somewhat 
pyramidal in shape, with the apex pointing downwards and to the 
left. It lies behind the sternum and ribs but it is almost entirely 
separated from the anterior chest wall by the lungs, only a small 
area remaining uncovered. In the adult it is about five inches long 
and inches across at its widest part. It weighs 9-10 ounces. 
It gradually increases in weight up to middle life and in old age it 
diminishes. 
Pericardium--The heart is enveloped in a fibrous bag called the 
pericardium, which rests on the diaphragm. It is lined on the 
inside by endothelium which is reflected over the surface of the 
organ, thus two smooth surfaces are apposed-the parietal layer 
and the visceral layer of the pericardium--and this helps to diminish 
friction when the heart contracts. There is no actual space between 
the two layers in health, but if this lining becomes inflamed (peri- 
carditis), serous fluid is poured out and accumulates, forming a 
real space. 
Endocardium-Internally each of the heart cavities is also lined 
by a fibrous membrane covered by a single layer of flattened epi- 
thelial cells or endothelium; this is termed the endocardium. 
Myocardium-Between the endocardium and the visceral layer 
of the pericardium, the muscular tissue is found; this is called the 
myocardium. In addition to heart muscle it contains blood- 
vessels, nerve cells and fibers, and some connective tissue. 
The left ventricle is much thicker in the wall than the right; 
it is connected with the greater or systemic circulation and has 
much more work to do than the right. 
The musculature of the auricles is separated from that of the 
ventricles by a ring of fibrous tissue but a peculiar kind of 
muscular tissue, which is probably not contractile but only con- 
ductive, passes across this ring; it is called the auriculo-ventricular 
or A-V bundle, or the bundle of His, and it is believed to convey 
the stimulus from the auricles to the ventricles. 
Heart Valves 
The heart valves are essentially folds of endocardium strength- 
ened by fibrous tissue; they are smooth and strong. One set, the 
auriculo-ventricular valves, are placed between the auricles and 
ventricles; another set, the semilunar valves, are situated at the 
openings of the aorta and pulmonary artery into the left and right 
ventricles resoectivelv. 46 
HUMAN PHYSIOLOGY 
There are two auriculo-ventricular (A-V) valves, one be- 
tween the right auricle and ventricle, usually called the tricuspid 
valve, and the other between the left auricle and ventricle, common- 
ly known as the mitral valve. 
The tricuspid valve has three cusp shaped flaps, as the name 
indicates, attached at their bases to the fibrous ring surrounding 
the A-V orifice, with their apices projecting into the ventricle. 
Attached to the ventricular aspect of these flaps are numerous fine 
tendinous cords--the chordae tendineae-whose function it is to 
prevent the flaps being shot into the auricle when the ventricle 
contracts. At the other end they are connected with muscular 
elevations on the inner wall of the ventricle known as the papillary 
muscles or musculi papillares. 
The mitral valve on the left side of the heart is built on the same 
plan as the tricuspid, but it is much stronger and has two cusps 
instead of three. The papillary muscles on the left side are also 
better developed and more powerful than those on the right. 
Semilunar Valves-The aortic and pulmonary openings from 
the left and right ventricles, respectively, are each guarded by 
three semilunar valves. The convex margin of each of these 
semilunes is attached to the fibrous ring around the opening 
while the concave free edge projects into the artery. When the 
valves are closed, by pressure on the arterial side, their thin 
edges are applied firmly to each other so that no blood gets through 
between them, the wrong way. Outside each of the flaps there is 
a bulging in the wall of the artery; this is called the Sinus of 
Valsalva; it forms a little pocket in the side of the vessel. There 
are three sinuses to each valve. The function of these sinuses is to 
prevent the valves from sticking to the wall of the vessel. When 
the blood is passing out of the ventricles, they permit of an eddy 
stream getting in behind the valves, and helping to close them the 
moment the axial stream is over. 
The only difference between the aortic and pulmonary valves 
is that the former is the stronger. 
Coronary System of Vessels 
The coronary arteries, right and left, arise from the base of the 
aorta and carry blood to nourish the heart wall. The smaller 
arteries end in capillaries, and these unite into veins which bring 
the blood back to the right auricle, through the coronary sinus. 
The coronary arteries are terminal arteries ; they do not anasto- 
mose, and when one of their branches is blocked, the blood is 
completely cut off from the whole area supplied by that vessel. 
It is on this account that occlusion of one of the coronary arteries, 
by a blood clot or embolus, may be instantly fatal. CIRCULATORY SYSTEM 
47 
In the frog the heart wall is so thin and spongy that no special 
nutrient system of vessels is necessary. 
Openings in the Heart Chambers 
The superior and inferior venae cavae both open into the right 
auricle and also the coronary sinus formed by the union of the 
coronary veins. The right auricle communicates with the right 
ventricle by the right A-V orifice guarded by the tricuspid valve. 
The pulmonary veins---usually four in number in man-open into 
the left auricle and this opens into the left ventricle through the 
left A-V orifice guarded by the mitral valve. There are no valves 
guarding the openings of the great veins at their entrance into 
the auricles. 
Action of Valves in Directing the Flow of Blood 
Through the Heart 
If the heart were without valves the blood would simply flow 
to and fro and not in a circle. 
Before the auricles contract blood is slowly flowing from the 
great veins into the auricles and from these into the ventricles, the 
A-V valves being open. When the auricles contract to expel 
their blood, it is driven into the ventricles which are thus forcibly 
distended. The axial stream may form a kind of eddy behind the 
flaps of the A--V valves and when it ("the axial stream) has come 
to an end that moment the valves close, i. e. before the ven- 
tricles HAVE ACTUALLY BEGUN TO CONTRACT. 
The ventricles then contract and in doing so stretch the A--V 
valves. When the pressure in the ventricles exceeds that in the 
great arteries the semilunar valves open, and the blood is propelled 
into the arteries--pulmonary and aorta. As soon as the blood flow 
from the ventricles is at an end the semilunar valves close, and the 
ventricles begin to relax, taking away the support from the valves 
behind. The blood tends to rush back into the heart, but encoun- 
tering the now closed valves it is unable to do so. There is then a 
complete pause in the heart's action and the whole series of events 
is repeated. 
Both auricles contract synchronously and both ventricles. 
Functions of Chordae Tendineae and Papillary Muscles-When 
the ventricles contract the pressure in them rises suddenly, and the 
flaps of the A--V valves would be everted into the auricles if this 
were not prevented by the chordae tendineae which hang on to 
them. During their contraction, however, the apex of the heart 
approaches the base so that the chordae tendineae might still allow 
the cusps to pass into the auricles if it were not for the fact that 
the papillary muscles, to which they are attached, contract 
and shorten at the same time. This shortening of the papillary 48 
HUMAN PHYSIOLOGY 
muscles compensates exactly for the upward movement of the 
apex and the chordae tendineae are still kept taut. 
Incompetent Valves and Regurgitation 
An incompetent.valve is one which permits the blood to flow 
in the wrong direction; this backward flow is known as regurgita- 
tion. If the ventricles are over distended the flap of the A-V 
valves do not come together and blood escapes backwards or re- 
gurgitates into th e auricles. On account of the comparative th inness 
of its walls this is more liable to occur in the right ventricle than 
in the left. 
It sometimes happens that the valves are roughened as the result 
of disease-endocarditis- and do not meet properly at the edges, 
thus permitting the blood to regurgitate. This condition may be 
found in any of the heart valves. It frequently follows acute 
rheumatism and it is permanent. 
Cardiac Hypertrophy and Compensation-If there are defective 
valves and leakage backwards the tissues will not receive a sufficient 
supply of blood. This is quickly corrected, however, by the heart 
becoming thicker in the wall, i. e. undergoing hypertrophy, so as 
to pump more blood forwards and so make good the leakage. 
Persons may live in good health for many years with defective heart 
valves because of the power which the heart possesses of under- 
going hypertrophy and providing compensation. It is only when 
this compensation fails, with advancing years, that symptoms 
appear. 
The rhythmical movements of the heart may be conveniently 
studied by attaching the apex of a frog's heart to a lever which is 
made to record its movements on a revolving drum. The rhythm 
is sinus-auricles-ventricles-pause. The contraction wave be- 
gins in the sinus, in the frog's heart, then sweeps over the auricles 
and then on to the ventricle there being no intervening connect- 
ive tissue band between the auricles and ventricle in the frog. 
In the mammalian heart there is no sinus but the part that cor- 
responds to it, termed the sino-auricular node or the node of Keith 
and Flack, is situated in the right auricle, near the entrance of the 
superior vena cava. It is at this point that the contraction wave 
begins in the human heart; it sweeps over the auricles, then passes 
on to the ventricles through the A-V bundle before described, 
since there is no continuity of muscular tissue between the auricles 
and ventricles except through this bundle. 
Effect of Temperature on Heart Beat-'The effect of heat and 
cold can be conveniently studied on this frog's heart preparation. 
Heat accelerates the rate and diminishes the force of the beat 
Heart Movements CIRCULATORY SYSTEM 
49 
and cold slows the rate and augments the force. The rapid heart 
of fever is due, in part, to the high temperature of the blood. 
If a tracing be taken from the heart of a turtle, one lever can 
be attached to the auricles and another to the ventricle, each 
recording its movements separately on a revolving drum. If the 
writing points be arranged on the same vertical line, one above the 
other, it will be seen that the auricular contraction is over before 
the ventricular contraction begins, and after that is over there is a 
pause until the next auricular contraction. This sequence of 
EVENTS, FROM THE BEGINNING OF ANY ONE EVENT TO ITS NEXT 
beginning, constitutes a cardiac cycle. Contraction of heart 
muscle is termed the systole and relaxation the diastole, so that 
we have the auricular systole followed by the auricular diastole 
and the ventricular systole by the ventricular diastole. 
The same kind of record would be obtained from the human 
heart, if it were possible to attach levers to the auricles and 
ventricles, but the events would succeed each other much more 
quickly, that is, the cardiac cycle would be shorter. 
The Cardiac Cycle 
Apex Beat 
The apex of the heart rests against the chest wall about the fourth 
or fifth intercostal space; its impact against the chest wall is 
known as the apex beat. This may be recorded by a cardiograph, 
an apparatus consisting of two tambours, one of which is applied 
to the chest wall at the spot where the apex beat can be best felt 
with the finger (usually in the 4th intercostal space about one inch 
to the left side of the sternum), and the other attached to a lever 
which writes on a revolving drum. When the chest wall advances 
(driven forward by the impact), air is driven out of the receiving 
tambour on the chest into the transmitting tambour on the stand, 
so that the lever is elevated; when the chest wall recedes, air 
moves from the transmitting to the receiving tambour and the 
lever falls. 
[The tambour is an instrument, now much used in physiology, 
first devised by Marey of Paris. It consists of a flattened brass 
box closed on one side with a thin rubber membrane, and provided 
with a side tube to which rubber tubing can be attached. To the 
center of the membrane is glued a small metal disc and on this 
rests a lever which rises when air enters the box and causes the 
membrane to bulge, and falls when air escapes from the box. 
Tambours are most frequently usedin pairs, the one receiving 
the movement, the other transmitting it]. 50 
HUMAN PHYSIOLOGY 
Heart Sounds 
"There are two audible sounds produced by the heart during each 
cardiac cycle; they are termed first and second, or long and short, 
or systolic and diastolic. Both are musical notes of low pitch; 
the first is dull, the second is sharp and of higher pitch. The 
sounds may be imitated by pronouncing the words, lubb-dup, at 
the proper pitch. 
The first sound begins the moment the ventricles begin to 
contract but it dies away before the contraction is over; it is suc- 
ceeded by a short period of silence. The Second sound begins the 
moment the ventricle begins to relax, i. e. at the beginning of the 
pause (see diagram); it is shorter than the first and is succeeded 
by a long silence. 
Causes of Heart Sounds-The second sound is due to the 
vibration resulting from the sudden stretching of the semilunar 
valves--not to their closure. It is entirely valvular and this 
can be proved by the following experiments: 
1. If a wire, with a hook on the end of it, be pushed into the 
aorta and another into the pulmonary artery of an anaesthetized 
animal, e. g. a horse, and the flaps of the semilunar valves held aside, 
the second sound disappears and is replaced by a murmur. 
2. If the heart be entirely emptied of blood by clamping the 
superior and inferior venae cavae, it will continue to beat for a 
short time, but the second sound will disappear completely. This 
proves that it is not muscular but purely valvular. 
3. A similar sound can be produced in the excised and dead 
heart, by tying a long glass tube into the aorta and filling it with 
water. If the ventricle be cut away and the valves held open for 
a moment and then allowed to be closed by the pressure of the 
water, a sound is produced by the stretching of the valves similar 
in character to the second heart sound. 
The second sound is produced at both the aortic and pulmonary 
valves, and there are not two second sounds because in health both 
sets of valves are stretched at the same time. The aortic valve 
produces more of the second sound than the pulmonary7 because it 
is three times as strong and is more powerfully stretched by the 
greater blood pressure in the aorta. 
The first sound has two causes. The stretching of the A--V 
valves is the first and most important cause, and the second is the 
contraction of the muscular fibers of the ventricles, but this is not 
so evident as the valvular part. The experimental proofs are not 
so strong as in the case of the second sound; they are as follows: 
1. If the superior and inferior venae cavae be clamped, the heart 
soon empties itself, and the A-V valves can no longer be stretched, CIRCULATORY SYSTEM 
51 
but the first sound does not disappear; it loses much of its musical 
character, however, and becomes a low rumbling noise, such as 
might be produced by contracting muscle. 
2. If the apex of the heart is cut off, allowing the blood to escape, 
so that there can be no presstire in the ventricles, and consequently 
no stretching of the A-V valves, it continues to beat for a few 
minutes, and there is still heard this low rumbling noise represent- 
ing the first sound. 
In the first sound, then, there are two factors-1, stretching of 
the A-V valves; 2, contraction of the ventricular muscle. 
The character of these sounds is an important guide to the 
physician regarding the condition of the heart. For example, a 
loud second sound indicates high blood pressure, and a loud first 
sound points to a powerfully contracting heart, while a feeble first 
sound shows that the heart is weak, etc." 
Heart Murmurs--In valvular disease the heart sounds are re- 
placed by murmurs. For example, if the A-V valves are rough- 
ened so that they can't close properly, when the ventricles contract 
theblood will flow backwards into the auricles, and this rushing of 
fluid over the roughened valves will cause, not a musical sound 
such as is produced by the vibration of the closed valves, but a 
noise which in medicine is called a murmur. This would be a 
systolic murmur because it occurs during the ventricular systole. 
Again, if the semilunar valves are involved, the blood rushing out 
into the arteries during the ventricular contraction will produce a 
systolic murmur and another murmur, diastolic in time, will be 
caused by the blood rushing back again into the ventricles, during 
their relaxation, through the incompetent valves. Here we will 
have a systolic murmur succeeded by a diastolic. 
Points on the Chest Wall where the Heart Sounds are Best 
Heard-The heart sounds may be heard perfectly well by placing 
the ear against the chest, but it is more convenient to use a 
stethoscope. They are not most audible over the valves where 
they are produced, but at the points where the heart or great 
vessels come nearest to the chest wall. The aortic element of the 
second sound is best heard by placing the chest piece of the stetho- 
scope on the second right costal cartilage, at its junction with the 
sternum. This is explained by the fact that the aorta, after it leaves 
the left ventricle, arches upwards and to the right and comes 
closest to the surface at this point. The pulmonary element of the 
second sound is most audible over the second intercostal space, 
close to the left edge of the sternum. The mitral element of the 
first sound is best heard at the apex beat and the tricuspid element 
close to the left border of the sternum, in the 4th intercostal space. 52 
HUMAN PHYSIOLOGY 
It is estimated that each ventricle of the heart ejects about 100 
c.c. of blood at each beat against the resistance of the blood pressure 
in the arteries and that in doing this it performs 285 gram-meter 
units of work; that is, the force exertfed by the heart at each beat 
is capable of raising 285 grams 1 meter high. Knowing that the 
heart in the human subject beats 72 times per minute, the total 
work for the 24 hours is calculated to be 29,558,800 gram-meters. 
Work Done by Heart 
NERVE SUPPLY OF THE HEART 
The heart beats automatically-it will continue to beat for 
hours, under proper conditions, when entirely removed from the 
body'-but its action is controlled by two nerve centers situated 
in the medulla oblongata, viz.--the cardio-motor centre and the 
cardio-inhibitory center. 
[By a nerve center is meant a group of nerve cells situated some- 
where in the central nervous system, i. e. in the brain or spinal 
cord. The medulla oblongata is the lowest division of the brain, 
that next to the spinal cord.] 
Course of Impulses from Cardio-motor Center to Heart-There 
are three superimposed sets of neurons on this path. The first 
arises in the c.--m. center in the medulla oblongata and ends in 
the grey matter of the spinal cord in the upper thoracic region; 
the second leaves the cord through the anterior roots of the 2d, 
3d, and 4th thoracic nerves and ends in the inferior cervical 
ganglion; the third passes from this ganglion to the heart. There 
is in this series one central set of neurons, one preganglionic, and 
one postganglionic. 
[All nerve fibers between the spinal cord or brain and the 
autonomic ganglia are termed preganglionic fibers and all between 
the ganglia and the tissue in which they end are called post- 
ganglionic fibers.] 
On this path there are two cell stations, one in the grey matter 
of the spinal cord and another in the inferior cervical ganglion. 
Stimulation of these fibers, at any point in their course, leads 
to an acceleration of the heart rate and sometimes also to an 
augmentation in the force of the beat. For this reason it is be- 
lieved that there are two kinds of fibers which pass from the 
c--m. center to the heart, viz.:-accelerator fibers and augmentor 
fibers. 
Course of Impulses from Cardio-inhibitory Center to Heart 
On this path there are two superimposed sets of neurons. The 
first joins the vagus or 10th cranial nerve, right and left, and 
passes through that to the heart; the second is situated in the Fig. 4 54 
HUMAN PHYSIOLOGY 
heart wall itself. Groups of nerve cells, forming the intracardiac 
ganglia, are found scattered amongst the muscular fibers; these 
give origin to short nerve fibers which end in the heart muscle. 
The second set of neurons is formed of these intracardiac cells and 
fibers. In this path there is one cell station, that in the heart wall. 
Effect of Dividing Vagus Nerve on Heart Beat-If one vagus 
be divided in the neck, above the point where the heart fibers 
leave it, the heart beats faster; if then the other vagus be cut it 
beats faster still. This proves that, since the heart beats faster 
when it is cut off from the c-i. center, normally this center must 
be in a state of tonic activity, i. e. always in action sending down 
impulses to check the heart rate. 
The cardio-motor center is probably also in tonic activity al- 
though the fact is not so easily demonstrated. 
Effect of Stimulating Peripheral End of Divided Vagus-When 
the peripheral (heart) end of the cut vagus is stimulated the 
heart is slowed and if the stimulus is strong enough it is stopped 
altogether. This proves that the vagus nerve contains inhibitory 
fibers for the heart. 
Action of Atropin and Nicotin on the Heart-The action of two 
drugs much used in physiological investigations, viz.--atropin and 
nicotin, is interesting in this relation. If atropin be administered 
to an animal the heart rate is greatly increased,-just as much, 
in fact, as it would be if both vagi nerves were divided. This 
drug has the property of paralyzing the terminations of the 
second set of neurons in the heart muscle and so the impulses con- 
stantly being sent down from the c--i. center, through the vagi, 
never reach the heart. For the same reason, if the peripheral end 
of the divided vagus be stimulated, after giving the atropin, no 
effect is produced on the heart rate; it is not slowed or stopped. 
Nicotin has the same effect as atropin but it acts in a different 
manner. Nicotin paralyzes the synapses between the first and 
second neurons on the inhibitory path. 
Effect of Stimulating Central End of Divided Vagus-The vagus 
nerve carries afferent or sensory fibers from the heart to the brain 
as well as efferent (inhibitory) fibers from the brain to the heart. 
When the central end is stimulated, therefore, these afferent fibers 
will be excited and this will cause either a slowing of the heart rate 
reflexly or an acceleration of it. It may be impossible to predict 
which action it will have before hand; for example, if the impulses 
which pass up inhibit the activity of the c-i. center of the other 
side, the heart rate will be increased; if they excite the c-i. 
center it will be decreased. Again, if they excite the c-m. center 
the heart will beat faster; if they inhibit its activity it will beat 
slower. By inhibiting the one center and exciting the other it is 
possible to get no alteration at all in the heart rate. CIRCULATORY SYSTEM 
55 
This experiment shows the importance, in studying the function 
of a nerve, of first dividing it before stimulating. For example, if 
the intact vagus nerve had been excited and heart slowing obtained 
it would be impossible to say whether the action were reflex or 
direct. 
Innervation of Heart in Frog-In the frog the arrangement is 
peculiar. The accelerator and augmentor fibers join the vagus in 
the neck and are carried to the heart along with the inhibitory 
fibers, consequently, when the peripheral end of the cut vagus is 
stimulated the effect may be acceleration or inhibition depending 
upon which set of fibers is the more susceptible to the stimulus. 
Influence of Higher Brain Centers on Heart Rate-The cardio- 
inhibitory and cardio-motor centers are affected reflexly by condi- 
tions at the periphery, but they are also under the control of the 
higher brain centers in the cerebrum, particularly the emotional 
centers. For instance, a sudden fright or surprise may cause the 
heart to beat faster (palpitation), or it may cause slowing or 
stoppage of the heart and fainting or even death, according as the 
impulses being showered down from the strongly excited emotional 
centers affect the c--m. center or the c--i. center. We have no 
POWER VOLUNTARILY, HOWEVER, EITHER TO INCREASE OR TO SLOW 
THE HEART RATE. 
NEUROGENIC AND MYOGENIC THEORIES OF 
HEART BEAT 
1. Neurogenic Theory-When nerve 
cells were discovered in the heart itself, 
it was believed that these were respon- 
sible for the heart beat. The impulses 
were supposed to originate in these nerve 
cells and to be conducted, through the 
short nerve fibers, to the heart muscle, 
causing it to contract. This is the neu- 
rogenic THEORY. 
2. Myogenic Theory-Then it was 
shown by Gaskell that strips taken from 
the ventricle of the turtle's heart, con- 
taining neither nerve cells nor nerve fibers, 
if attached to a lever and immersed in 
Ringer's fluid, will beat rhythmically for 
hours. Also in the developing chick, at 
a very early stage, the heart can be seen 
to beat long before any nerve tissue has 
grown into it. For these and other rea- 
sons it is now held by most physiologists 
Fig. 5 Fig. 6 CIRCULATORY SYSTEM 
57 
that the contraction starts in the muscle itself, without any stim- 
ulus received from the nerve cells. This is the myogenic theory. 
The Inner Stimulus-Neither the neurogenic nor the myogenic 
theory, however, explains the rhythmicity of the beat. It is not 
enough to say that this is a property of heart muscle. Something 
must excite the nerve cell or the muscle fiber and that must be 
periodic in its application. The inner stimulus that causes the 
muscle to contract has been looked for in the blood and there is 
some reason to believe that it is furnished by the calcium and 
potassium ions which may be set free in this fluid. Experiments 
on the frog's heart carried out with Schafer's plethysmograph 
appear to throw light on the question. 
Schafer's Frog Heart Plethysmograph-If an opening be made 
in the auricles of a frog's heart and a two way cannula introduced 
into the ventricle, through this opening, and tied in by a ligature 
passed around the A--V junction, and if Ringer's fluid be allowed 
to flow into the ventricle, through one limb of the cannula, from 
a height of 3 or 4 inches and after distending the ventricle flow out 
at the other limb, the preparation will beat rhythmically as the 
frog's heart does in the body. The ventricle is being perfused 
with a fluid (Ringer's) which is practically the same as the frog's 
blood plasma minus its organic constituents,-a solution of the 
inorganic salts of sodium, calcium and potassium in the same 
proportion as they exist in the plasma. 
Ringer's Fluid 
Na Cl0.700% 
K Cl0.030 
Ca Cl20.025 
H Na CO30.003 
Locke's Fluid 
Na Cl0.900% 
K Cl0.042 
Ca Cl20.024 
H Na CO30.020 
Dextrose0.100 
If now the ventricle be introduced into the expanded bulb of a 
U-tube filled completely with Ringer's solution and having a float, 
with a lever attached, in the open limb, then, when the heart 
expands in diastole, the volume will increase, the float will rise 
and the writing point of the lever will descend; when the heart 
contracts in systole, the volume will diminish, the float will fall 
and the lever point ascend. The apparatus is termed a plethysmo- 
graph or volume writer and the record obtained on a revolving 
drum a plethysmogram. In a good preparation the heart will 
continue to beat for hours, on Ringer's fluid alone, which contains 
only inorganic salts. 
If a slight excess of calcium chloride be added to the fluid the 
systole will become more powerful and, by-and-bye, the heart will 
stop with the muscle completely contracted and unable to relax. 
At this stage, if the preparation be fed with Ringer's solution con- 58 
HUMAN PHYSIOLOGY 
taining a slight excess of potassium chloride, it will start to beat 
again, gradually becoming relaxed with the diastole more and more 
prolonged until at last it stops in diastole, i. e. in complete relaxa- 
tion. Therefore, calcium has the power of stimulating heart muscle 
and potassium of inhibiting it. 
Calcium-Potassium Ion Theory of Inner Stimulation-Suppose 
that during diastole calcium ions are set free, when they reach a 
certain concentration the muscle will be excited and the heart will 
pass into systole. Suppose that during systole potassium ions are 
set free, when these reach a certain concentration the contraction 
will be inhibited, and the heart will pass into diastole. It is con- 
ceivable that in this way the heart is kept going between calcium 
stimulation and potassium inhibition, showing the alternate phases 
of systole and diastole. 
By the plethysmographic method records may also be obtained 
from the isolated mammalian heart (even the human heart), but 
here Locke's solution, containing sugar, must be used as the 
perfusing fluid; it must be super-saturated with oxygen, and it 
must be kept warm. 
Physiological Properties of Heart Muscle 
Heart muscle differs from skeletal (voluntary) muscle in three 
respects: 1. A minimal stimulus causes a maximal contraction; 
this is known as the "all or nothing" phenomenon. Recent 
investigation has shown that the same kind of response is given 
by striated muscle. 2. Heart muscle cannot be tetanised be- 
cause of its long refractory period. 3. Heart muscles show 
RHYTHMICAL CONTRACTION. 
These properties of heart muscle may be studied on a frog's 
heart which has been brought to a standstill by a Stannius' ligature. 
If a thread be tied tightly around the sino-auricular junction the 
heart will stop beating for a considerable time. The reason as- 
signed for this is that the contraction wave, which starts in the 
sinus, cannot get across the ligature and so never reaches the 
auricles and ventricle. By-and-bye, however, the auricles them- 
selves will initiate a contraction wave and the heart will start again. 
While the heart is quiescent, under the Stannius' ligature, it 
may be attached to a lever writing on a revolving drum and 
stimulated by electrical shocks just like the gastrocnemius muscle. 
Electrical Changes in the Heart 
The contraction of the heart muscle, like that of skeletal muscle, 
is accompanied by an electrical change. The part in contraction 
becomes negative to the parts at rest and this gives rise to a current 
of action which can be demonstrated and recorded by a string 
galvanometer. The record is known as the electrocardiogram. 
If the heart is diseased the electrocardiogram will be abnormal 
and this method is now extensively used by physicians in diagnos- 
ing heart affections. BLOOD-VESSELS 
These are the arteries, capillaries and veins. 
The arterial system begins at the left ventricle of the heart in 
a large vessel, the aorta. This turns abruptly downwards (arch 
of aorta), and passes through the thorax and abdomen, as a single 
large vessel (the thoracic and abdominal aorta) giving off many 
branches, large and small, in its course. In the abdomen it divides 
into the two common iliac arteries which carry the blood to the 
lower limbs. Just after leaving the heart it gives off three large 
branches--the innominate, the left common carotid and the left 
subclavian arteries-which carry the blood to the head, neck and 
upper limbs. The arteries divide and subdivide into smaller and 
smaller branches, the most m'inute being termed the arterioles, 
and these become continuous with the capillaries. 
The arteries, generally speaking, are placed more deeply than the 
corresponding veins and are protected from pressure and other 
dangers, but there are many exceptions to this rule. In most 
situations their branches anastomose freely with the branches of 
other arteries, so that if one trunk is blocked, a collateral circulation 
is easily established. In certain organs, e. g. the brain and the 
heart, etc., the arteries are terminal, i. e. their branches do not 
anastomose or communicate with other arteries, and the conse- 
quence of obstruction has already been mentioned in the case of 
the heart. 
On account of their elastic walls which recoil on the blood and 
squeeze it out into the veins, in the dead body they are found 
empty, and so before the time of Harvey they were believed to 
contain air; hence the name artery or air pipe. 
Capillaries and Veins--The capillaries, after a short course 
through the tissues, unite to form the smallest venules and these 
to form larger veins, until ultimately two big venous trunks are 
formed, the superior and inferior venae cavae, through which the 
blood is brought back to the heart. In medium and larger sized 
vessels the tributaries of the veins correspond in a general way, 
to the branches of the arteries. 
Veins can easily be distinguished from arteries by the thinness of 
their walls and by the fact that they contain valves-all except 
the smallest and the largest-which the arteries do not. The 
vein is also larger in calibre than the corresponding artery at the 
same distance from the heart. 
59 60 
HUMAN PHYSIOLOGY 
Structure of Arteries and Veins 
There are three coats-outer, middle and inner. 
The outer coat consists of connective tissue containing an ad- 
mixture of white and yellow elastic fibers, the latter arranged in a 
network. 
The middle coat is made up mainly of non-striped muscle 
mixed with some elastic and white fibrous tissue. The muscle 
fibers run transversely to the long axis of the vessel, i. e. they are 
arranged circularly so that when they contract the vessel is 
constricted. 
The inner coat is formed of a layer of elastic tissue termed the 
inner elastic lamina which in many situations is fenestrated. 
The inner lining of the vessel consists of a single layer of delicate 
endothelial cells which form a smooth surface for the blood to 
flow over and thus diminishes fri ction. B etween the internal elastic 
lamina and the endothelial lining is some fine connective tissue. 
In the veins the outer and middle coats are thinner than in the 
corresponding arteries, but otherwise the structure is the same, 
except for the presence of valves in the veins as already mentioned. 
Valves in Veins-These have the same general structure as the 
semilunar valves of the heart. They consist of a fold of endothelium 
strengthened by connective tissue. They are arranged in pairs, 
the one opposite the other, and their free margins are directed 
towards the heart so that the blood is prevented from flowing 
backwards. Any pressure on the veins, such as might be caused 
by the contraction of muscles, will always cause the blood to move 
forwards, towards the heart, because of these valves. The 
position of these valves in the superficial veins of the forearm and 
back of hand can easily be made out by compressing the arm above 
to prevent the venous flow, when they will stand out as little 
swellings or knots through the skin. 
Structure of Capillaries-If we trace the course of the large 
arteries down to the capillaries we find that the outer coat becomes 
thinner and thinner and finally disappears; the middle layer also 
becomes reduced and, in the smallest arteries, consists only of a 
single layer of circular muscle fibers which disappears entirely at 
the capillaries. The elastic membrane of the inner coat also dis- 
appears until, at the capillaries, nothing is left but the single layer 
of endothelial cells. 
All the arteries are elastic (because of the yellow elastic fibers) 
and contractile (because of muscle), but the contractility is most 
marked in the smaller arteries, the elasticity in the larger. The 
capillaries are elastic but not contractile since they contain no 
muscle fibers. BLOOD VESSELS 
61 
INNERVATION OF BLOOD-VESSELS 
The muscular coats of the vessels, particularly the smaller arteries 
or arterioles, are supplied by two sets of nerve fibers-vaso- 
constrictor (commonly called vaso-motor) and vaso-dilator fibers, 
the one excitatory, the other inhibitory, as in the heait. 
The vaso-constrictor (or vaso-motor) center is situated in the 
medulla oblongata, and impulses traveling down from this center 
stimulate the muscle fibers to contract, and so constrict or narrow 
the lumen of the vessels. 
Course of Impulses from Vaso-Constrictor Center to Blood- 
vessels-These impulses travel over three sets of neurons as in 
the case of the cardio-motor fibers,-central, preganglionic and 
postganglionic, adopting the nomenclature of Langley. The two 
cell stations are situated in the grey matter of the spinal cord and 
in the autonomic ganglia respectively. The preganglionic fibers 
leave the cord through the anterior roots of all the spinal nerves 
from the second thoracic to the second lumbar inclusive. There 
are other regions of the central nervous system from which vaso- 
constrictor fibers come off but this is the chief one. 
A vaso-dilator center is believed to exist but it has not been 
located with certainty. 
Both vaso-constrictor and vaso-dilator nerve fibers were discov- 
ered by Claude Bernard. 
Claude Bernard's Experiment which led to the discovery of the 
vaso-constrictor fibers was as follows: On examining the ear of a 
rabbit he observed that there were rhythmical changes in the 
blood vessels; the ear would become pale, owing to the constriction 
of the central artery and its branches, and after one or two minutes, 
it would become flushed again. He then cut the sympathetic 
(autonomic) nerve in the neck and found that after this, the 
vessels remained permanently flushed, and the temperature of the 
ear rose. He next stimulated the peripheral (ear) end of the divided 
nerve and found that the ear became pale again and remained so 
as long as the stimulus was applied. 
The inferences to be drawn from this experiment are, first, that 
there are fibers in the cervical sympathetic nerve stimulation of 
which produces constriction of the blood-vessels of the ear on that 
side, and second, that the nerve center from which the impulses 
come that cause this constriction is in tonic activity. We now call 
this the vaso-constrictor (or vaso-motor) center and know that it 
is located in the medulla oblongata. Prior to this discovery of 
Claude Bernard the existence of such fibers, in any part of the 
body, was unknown. 
Claude Bernard was also the first to demonstrate the existence 
of vaso-dilator nerve fibers. He did this by an experiment on the Fig. 7 BLOOD VESSELS 
63 
submaxillary gland. This gland, which is situated under the lower 
jaw, is supplied by two sets of fibers one of which reaches it through 
the cervical sympathetic nerve and the other through the chorda 
tympani, a branch of the 7th cranial nerve. Bernard cut the chorda 
tympani and observed no change in the blood supply of the gland, 
but when he stimulated the peripheral (gland) end of the divided 
nerve, the gland became flushed at once indicating dilation of the 
bloodvessels. 
The conclusions to be drawn from this experiment are, first, 
that the chorda tympani nerve contains vaso-dilator fibers, and 
second, that the vaso-dilator center is not in tonic activity. 
Stimulation of the cervical sympathetic nerve leads to a blanch- 
ing of the gland, indicating constriction of the bloodvessels, while 
section of this nerve causes a permanent flushing, with the dis- 
appearance of rhythmical changes, showing that the vaso-con- 
strictor center is in tonic activity. 
General Statement based on above experiments. The vaso- 
constrictor center is always in a state of tonic activity main- 
taining vascular tonus. That is to say, impulses proceeding from 
the vaso-constrictor center, in the medulla oblongata, are constantly 
being showered down on the non-striped muscle fibers which form 
the middle coat of the arteries, particularly the arterioles, maintain- 
ing them in a semi-contracted condition and so keeping the vessels 
to some extent constricted. The strength of these impulses varies 
from time to time, and consequently so does the degree of constric- 
tion of the vessels. This variation is the cause of the rhythmical 
changes in the calibre of the vessels, as seen, for example, in the 
rabbit's ear. 
There is no evidence to show that the vaso-dilator center 
is in tonic-activity, Vaso-dilation is the result of an inhibitory 
action on the vaso-constrictor fibers. In the case of the heart we 
saw that both the cardio-motor and cardio-inhibitory centers were 
in tonic activity, the latter more distinctly so than the former, 
however. 
Reflex and Central Stimulation of Vaso-Motor Centers-Both 
the vaso-constrictor and vaso-dilator centers can be excited re- 
flexly by impulses reaching them from the periphery. In this 
respect they resemble other nerve centers; all nerve centers are 
reflex and depend for any change in activity on impulses reaching 
them from the periphery or from the higher brain centers. Cold 
applied to the skin will cause it to turn pale; impulses pass up to 
the vaso-constrictor center from the skin through the nerves that 
convey cold impressions; these excite the cells of the vaso-constrict- 
or center reflexly and lead to a constriction of the bloodvessels of 
the skin and so to blanching. The application of heat has the 
opposite effect, i. e. it leads to vaso-dilatation and flushing of the 64 
HUMAN PHYSIOLOGY 
skin. This may be brought about in two ways, however; it may be 
caused by either stimulation of the vaso-dilator center or inhibition 
of the vaso-constrictor center. 
As in the case of the heart so in the bloodvessels, their centers 
are under the control of the higher brain centers. The emotion of 
shame leads to blushing due to dilatation of the vessels of the skin 
of the face, neck and ears, brought about by inhibition of the vaso- 
constrictor center or excitation of the vaso-dilator center, the im- 
pulses coming, this time, from above and not from the outside. 
Fear, on the other hand, leads most frequently to paling of the face 
due to vaso-constriction. 
Resume-The heart and blood-vessels are connected with the 
medulla oblongata by accelerator (and augmentor) and by in- 
hibitory fibers (heart), and by vaso-constrictor and vaso-dilator 
fibers (bloodvessels). Nerve centers, situated in the medulla 
oblongata, are acted upon by impulses reaching them through 
afferent fibers from the skin and deeper tissues and organs, and 
also by impulses from the higher brain centers. The heart and 
blood vessels are capable of being affected reflexly through these 
centers by excitation of afferent nerves and also by emotional 
states. For instance, any influence which excites the cardio- 
inhibitory center or inhibits the cardio-motor center will produce 
slowing or stoppage of the heart; any influence which excites the 
cardio-motor center or inhibits the cardio-inhibitory. center will 
lead to acceleration of the heart rate. Similarly, excitation of the 
vaso-constrictor center, reflexly or from the higher centers, will 
produce constriction of the arterioles and paling of the skin, etc., 
while inhibition of the vaso-constrictor center or excitation of the 
vaso-dilator center will cause dilatation of the peripheral vessels 
and flushing of the skin, etc. 
The blood pressure may be defined as the force with which 
the blood tends to escape from the vessels. It depends on the 
RELATION BETWEEN THE INFLOW AND OUTFLOW OF BLOOD IN THE 
vascular system, that is, between the force of propulsion repre- 
sented by the heart's action and the force of resistance represented 
by the friction between the blood and the vessel walls. 
The greater the length of the vessel wall the larger is the rubbing 
surface and the more is the resistance, therefore, the blood pressure 
is much greater in the arteries, near the heart, than in the veins. 
When the smaller arteries contract the friction in them increases, 
first because the ratio of the rubbing surface to the mass of blood 
passing through is increased, and second because the velocity is 
BLOOD PRESSURE BLOOD VESSELS 
greater. The normal variations in blood pressure are due mainly 
to changes in the calibre of the arterioles. 
Poiseuille's schema to show fall in pressure from arteries to 
capillaries and veins demonstrated. 
Blood Pressure in Arteries, Capillaries and Veins-The blood 
pressure is highest in the abteries, lower in the capillaries 
and lowest in the veins, and in the great veins near the heart 
it is negative owing to the suction action of the lungs which draws 
the blood out of the veins into the right side of the heart. The 
pressure does not fall uniformly, however, as it does in Poiseuille's 
schema. It is at its maximum in the aorta near the heart, diminishes 
slightly in the larger arteries and then falls suddenly in the ar- 
terioles so that the pressure in the capillaries beyond the arterioles 
is comparatively low. 
Methods of Measuring and Recording Blood Pressure-It 
should be kept in mind that when we talk of blood pressure, with- 
out any qualification, ordinarily, we mean the pressure in the 
larger arteries, that is, arterial blood pressure. 
The blood pressure was first measured by the Rev. Stephen 
Hales (1732). The method he adopted may be described in his 
own words: 
"In December I caused a mare to be tied down alive on her back; 
she was fourteen hands high, and about fourteen years of age, 
had a Fistula on her Withers, was neither very lean, nor yet lusty. 
Having laid open the left crural artery about three inches from 
her belly, I inserted into it a brass pipe, whose bore was one-sixth 
of an inch in diameter; and to that, by means of another brass 
pipe which was fitly adapted to it, I fixed a glass tube, of nearly 
the same diameter, which was nine feet in length. Then untying 
the ligature on the artery, the blood rose in the Tube eight feet 
three inches perpendicular above the level of the left Ventricle 
of the heart. But it did not attain to its full height at once; it 
rushed up about half way in an instant, and afterwards gradually 
at each Pulse twelve, eight, six, four, two and sometimes one 
inch. When it was at its full height, it would rise and fall at and 
after each Pulse two, three, or four inches; and sometimes it 
would fall twelve or fourteen inches, and have there for a time the 
same Vibrations up and down at and after each pulse, as it had, 
when it was at its full height; to which it would rise again, after 
forty or fifty Pulses." 
In this experiment of Hales the pressure in the artery, at the 
point where the tube was inserted, was measured by the height 
to which the blood rose in the tube. 
In Hale's method there are two drawbacks; first the tube is 
too long and unwieldy, and second the blood clots very 
quickly when it escapes into the tube. 
65 66 
HUMAN PHYSIOLOGY 
Two improvements were made by Poiseuille who substituted 
mercury for blood and thus reduced the length of the tube 13.6 
times, and who also used a solution of sodium carbonate between 
the blood and the mercury to delay the blood clotting. 
Kymograph-The method was further perfected by Carl Ludwig 
who placed a float on the top of the mercury, connected with a 
vertical rod and bearing a writing style which was made to record 
its movements on a revolving drum. This is now known as the 
Ludwig Kymograph. 
Method of Recording the Blood Pressure with Ludwig's Kymo- 
graph-To begin with, a positive pressure must be produced in 
the system by means of a pressure bottle (see diagram), and 
this should be a little in excess of what the blood pressure in the 
artery is expected to be,-in the case of a dog, from 5 to 6 inches 
of mercury. 
The animal, dog say, is now anaesthetized with ether, one of the 
common carotid arteries is' exposed in the neck and a cannula 
(glass tube) tied into it. Before the artery is opened a small 
clamp must be placed on it, between the heart and the opening, 
to prevent the escape of blood. The cannula, filled with a solution 
of sodium carbonate, is then attached to a mercurial manom- 
eter (U-tube) by a thick walled rubber tube, and the clamp 
removed from the artery. If the positive pressure in the system 
is equal to or slightly above that in the artery, no blood will escape 
and so will not be liable to clot in the cannula containing the 
sodium carbonate solution. The writing style, attached to the 
open limb of the manometer, is now brought into contact with 
the moving surface and a record of the blood pressure obtained. 
Simultaneously a time tracing can be taken underneath. The 
blood pressure in the carotid artery of the dog lies usually between 
5 and 6 inches of mercury. 
A typical blood pressure tracing or kymogram so obtained from 
an anaesthetized animal will show three sets of waves. The 
smallest represent the heart beats ; next in size are the respira- 
tory waves (pressure rises during inspiration and falls during 
expiration), while the largest, very irregular, are due to changes 
IN THE ACTIVITY OF THE VASO-CONSTRICTOR CENTER whereby the 
blood pressure, in normal conditions, is constantly undergoing 
slight variations. 
This instrument (Ludwig's Kymograph) is not perfect, however. 
There is an objection to mercury as a medium for transmitting 
the pressure because of the great momentum which it acquires in 
the oscillations due to the heart beats so that the finer changes in 
pressure in the artery are lost. 
Hurthle's Manometer-Hurthle has devised a simple apparatus 
which, to a large extent, overcomes this objection. It consists of BLOOD VESSELS 
67 
a small metal box covered with a rubber membrane to which a 
very light lever is attached. This is filled with sodium carbonate 
solution and connected, through a cannula, with the blood-vessel 
in place of the mercurial manometer. When the pressure in the 
artery increases the membrane bulges and the lever rises; when 
the pressure diminishes the membrane collapses and the lever falls. 
The record, so obtained, gives a much more accurate representation 
of the actual changes in blood pressure inside the vessels than the 
Ludwig Kymograph. 
Frank's Manometer-A further improvement on Hurthle's 
manometer has been made by Frank who dispenses with the lever. 
He attaches a small mirror to the edge of the rubber membrane 
and projects a beam of light on it, the reflected ray being photo- 
graphed on a moving sensitive plate. In this way the slightest 
movement of the membrane and mirror is greatly magnified. 
The mass and momentum of the apparatus is here reduced to a 
minimtun. 
With any of these pieces of apparatus we can measure the blood 
pressure in an anaesthetized animal and study the effect of various 
experimental conditions upon it. 
Effect on Blood Pressure of Dividing one or both Vagi Nerves 
in the Neck-When one or both vagi are divided in the neck the 
heart will be freed from the control of the cardio-inhibitory center 
and will beat faster, consequently the blood pressure will rise. 
Effect on Blood Pressure of Stimulating Peripheral End of 
Divided Vagus--Stimulation of the peripheral end of the cut 
vagus will excite the cardio-inhibitory fibers and so slowing or 
stoppage of the heart will take place and a fall in blood pressure 
WILL FOLLOW. 
Effect on Blood Pressure of Stimulating Central End of Divided 
Vagus-The vagus contains fibers (depressor) running up from the 
heart excitation of which causes inhibition of the vaso-constrictor 
center and dilatation of the arterioles with a consequent slow 
fall in blood pressure. Other effects may also be produced. 
In the rabbit these fibers which reflexly cause a fall of blood 
pressure and a slowing of the heart are found in a single nerve-a 
branch of the vagus--called the depressor nerve. In other 
animals, and in man, they are mixed with the other fibers of the 
vagus. 
Effect on Blood Pressure of Stimulating Central End of Divided 
Sciatic Nerve--Most nerves contain afferent fibers (pressor) which 
when stimulated excite the vaso-constrictor center reflexly and so 
raise the blood pressure. This will probably follow stimulation 
of the central end of the sciatic nerve. 
Thus, we see that the blood pressure may be altered by acting 
on the heart or by acting on the arterioles. 68 
HUMAN PHYSIOLOGY 
In man the blood pressure cannot be measured, of course, by 
opening an artery and. tying a glass tube into it, so some other 
device must be resorted to. The apparatus used is called a 
Sphygmomanometer of which there are several forms; the follow- 
ing description refers to the Riva-Rocci pattern. 
The Riva-Rocci Sphygmomanometer consists of a leather 
strap, about 3 inches wide, which is buckled closely around the 
upper arm when it is desired to measure the pressure in the 
brachial artery. Inside this band, between it and the skin, is 
a rubber bag which communicates with a mercurial manometer, 
by means of a rubber tube, and on the same air system is a bicycle 
pump by means of which the bag may be inflated. To use the 
apparatus the physician places his finger on the pulse at the wrist 
of the subject whose blood pressure is to be taken, and then pumps 
air into the bag until the pulse is obliterated. He then, by turning 
a tap, allows air to escape slowly from the system and notes the 
height of the mercury in the manometer at the moment when the 
pulse returns which may be determined either by the finger or 
better by a stethoscope; this will be a measure of the blood pressure 
in the brachial artery. 
The principle is simple. The air pressure in the bag, exerting 
itself through the skin, closes the artery so that the pulse wave 
cannot come through. When air is allowed to escape until the 
pressure inside the artery is just able to open it and let the wave 
through, the pulse is felt again at the wrist or may be heard with 
the stethoscope. At that point the pressure in the artery will be 
practically the same as that in the rubber bag, and the latter is 
indicated by the height of the mercury in the manometer attached. 
This will give the systolic pressure, i. e. the pressure in the vessel 
at the height of the ventricular systole. The pressure in the vessel, 
at the end of diastole, is much lower and is called the diastolic 
pressure which may also be measured, but for a description of the 
method one of the larger text-books must be consulted. 
The mean blood pressure is practically the mean of the systolic 
and diastolic pressures, and with a rapidly beating heart this is 
approximately recorded by the Ludwig mercurial manometer. 
The normal systolic blood pressure in man (20-25 years) is 
about 110 mm. of mercury. It increases with age mainly because 
of loss of elasticity in the arterial walls. 
The dangers of a continuous high blood pressure are: 1, that 
since it is more difficult for the heart to pump the blood against 
a high than a low pressure that organ may fail, and 2, that the ar- 
teries may rupture; this most frequently happens in the brain and 
so leads to apoplexy. 
Blood Pressure in Man BLOOD VESSELS 
69 
Blood Pressure in Veins and Capillaries-Themethod of measur- 
ing the blood pressure in the veins is the same as that employed for 
the arteries, except that water is used in the manometer instead 
of mercury. The pressure in any vein is only from one-tenth to 
one-twentieth of what it is in the corresponding artery. 
In the capillaries it is impossible, on account of their size, to in- 
troduce a cannula so the pressure is measured indirectly by finding 
what weight is necessary to blanch the skin, i. e. squeeze all the 
blood out of the skin capillaries, and from the extent of the area 
so blanched the pressure is calculated. 
PULSE WAVE 
The Pulse Wave is caused by the temporary expansion of a 
blood-vessel due to the sudden injection into it of blood from the 
heart. 
The energy of the heart is expended in two ways-1, in causing 
an onward movement of the blood, and 2, in producing a lateral 
pressure in the arteries. The lateral pressure distends the walls 
of the arteries and this gradually becomes converted into an on- 
ward movement by their elastic recoil. 
The pulse wave is usually confined to the arteries and is rarely 
seen in the capillaries and veins. 
Reason why Pulsation is Confined to Arteries--If two tubes, 
each 10 feet long, say, be connected with a rubber ball pump, the 
one of metal (lead or brass) and rigid, the other of rubber tubing 
and elastic, and if the distal end of each be constricted with a 
narrow glass nozzle, when fluid is pumped into the rigid tube the 
flow will be interrupted or pulsatile, but when pumped through the 
elastic tube it will be almost continuous. In the case of the rigid 
tube just as much fluid flows out as is driven in, because water is 
incompressible, and there is no expansion of the walls; the flow, 
therefore, must be interrupted since the pumping is interrupted. 
With the rubber tube it is different; every time fluid is pumped in, 
the walls expand to accommodate it and then, when the pump ceases 
to act, the recoil of the elastic wall on the enclosed fluid still propels 
it forwards, and so keeps up a continuous stream. This latter is 
the condition in the blood-vessels, and it shows why the pulse is 
obliterated at the arterioles and does not reach the capillaries. If 
the nozzle be removed from the rubber tube--a condition resembling 
dilatation of the arterioles in the body---then the flow is interrupt- 
ed, because the fluid escapes so easily that there is no pressure ex- 
erted on the walls to distend them during the contraction of the 
pump, and consequently no heaping up of fluid in the system. 
The pulse is usually extinguished at the arterioles, 1, because 
normally these are constricted, and 2, because the arterial walls are 70 
HUMAN PHYSIOLOGY 
elastic. If the arterioles were dilated or if the vessel walls were 
rigid then the pulse would not be obliterated at the arterioles; 
it would be present in the capillaries and even in veins. 
The continuous flow in the capillaries permits of the even and 
steady exchange of material between the blood and the tissues, 
and it also prevents the capillaries being ruptured by sudden 
changes in pressure. 
Characters of the Pulse'-The pulse is usually felt in the radial 
artery at the wrist, first because it is superficial and easily reached, 
and second because the artery can be readily compressed against 
the bone which lies behind it. 
Many important facts relating to the condition of the heart and 
circulation can be made out by the trained finger of the physician, 
such as rate, regularity, force etc., but for the determination of 
the secondary waves a tracing must be obtained by an instrument 
known as the sphygmograph. 
Dudgeon's Sphygmograph-This consists essentially of a small 
pad or button which is placed on the radial artery at the wrist and 
which communicates its movements, due to the expansion and 
contraction of the artery, to a series of levers, the last of which is 
provided with a writing point by which its oscillations are recorded 
on a strip of smoked paper moved by clock work. By strapping 
this apparatus to the wrist a pulse tracing or sphygmogram 
can be obtained. 
Character of Pulse Tracing-Such a tracing will show, generally, 
an upstroke or ascending (anacrotic) limb, due to the expansion 
of the artery, and a downstroke or descending (katacrotic) limb, 
due to its retraction. On the descending limb there are two well 
marked secondary elevations called the dicrotic and predicrotic 
waves, D and PD in the diagram, A being termed the primary 
or percussion wave. 
Causes of Waves in Pulse Tracing-The primary wave A is due 
to the sudden expansion of the artery by the blood driven into it 
from the heart, when the aortic semilunar valves open. 
The dicrotic is a reflected wave from the closed semilunar valves. 
When the blood tends to rush back into the heart, as the ventricles 
begin to relax, the aortic semilunar valves give way a little and are 
then suddenly brought up; this gives rise to a secondary wave 
reflected forwards from the closed valves. 
The cause of the predicrotic wave is not so certain. At one time 
it was believed to be purely instrumental, due to the rebound of 
the lever as it falls back on the resisting vessel wall, and not to be 
present at all inside the vessel. It is now believed by many to be 
a reflected wave like the dicrotic, not from the closed semilunar 
valves, however, but from points in the arterial system where the BLOOD VESSELS 
71 
vessels branch. These form a series of promontories projecting 
into the blood bed and parts of the primary waves, striking these 
promontories, are reflected backwards, giving rise to the predicrotic 
wave. The predicrotic wave is best marked when the blood pressure 
is high. 
Velocity of Pulse Wave and How Measured 
The time taken for the pulse wave to travel from the heart to 
the radial artery at the wrist may be measured by placing one 
cardiograph on the chest to record the apex beat, and another on 
the wrist to record the pulse. The two tambour lever points hav- 
ing been arranged on the same vertical line, when the drum re- 
volves the curve produced by the lever connected with the heart 
will appear in advance of that from the lever connected with the 
artery. A time tracing is taken underneath, and a vertical line 
dropped from the point where each curve begins, cutting the time 
tracing. This will give the time interval between the beginnings of 
the two curves, that is, the time taken for the pulse wave to travel 
from the heart to the wrist. Now measure the distance as correctly 
as possible, along the vessel, from the heart to the wrist and 
calculate the velocity in feet or meters per second. 
The pulse wave reaches the wrist about 1 /10th second after it 
leaves the heart; it takes a little longer to reach the foot, viz.: 1/6 
second. Supposing that the distance along the vessel from the heart 
to the wrist is 3 feet, then the velocity will be 30 feet per second. 
See demonstration with long rubber tube to illustrate this 
method. 
THE BLOOD STREAM 
Circulation in Web of Frog's Foot 
If a frog be immobolized with curare and the web of the foot 
spread out under a microscope, the blood may be seen through the 
transparent skin, circulating in the arteries, capillaries and veins. 
The capillaries are so small that the red corpuscles have to go 
through in single file. In the case of the larger vessels the arteries 
can be distinguished from the veins by the fact that the former show 
pulsation while the latter do not. Also, if a part of the field be 
found where a vessel divides, if it is an artery the direction of flow 
will be from the larger stem to the branches, if a vein it will be 
from the branches (tributaries) to the stem. 
The blood everywhere encounters resistance to its movement on 
account of friction. There is internal resistance due to the friction 
between the particles of a viscous fluid, and external resistance 
Velocity of Blood in Arteries, Capillaries and Veins 72 
HUMAN PHYSIOLOGY 
produced by the friction of the blood against the vessel walls; 
this external resistance is much greater than the internal. 
There is a tendency for the blood to adhere to the lining mem- 
brane of the vessels so that the external stratum moves very slowly. 
In the arteries and veins the red blood corpuscles may be seen 
moving in the center of the stream (axial stream) because they 
are heavier than the white corpuscles and the latter are wedged 
aside into the peripheral stream. 
The velocity of the blood stream in arteries and veins may be 
measured by an apparatus termed the hsemodromometer devised 
by Chauveau. It consists of a metal tube, the same size as the 
bloodvessel, let into the side of which is a flange, provided with 
an index, that moves over a graduated scale. 
When this is to be used the artery (or vein) is severed, and the 
metal tube tied in between the two ends, so that the blood must 
pass through it. The impact of the moving fluid against the 
flange, moves the index over the scala, and the number at which 
this stands will give the velocity of the blood in meters per second. 
The instrument must be graduated before use by passing through 
it streams of water of known velocity so that "the angle of de- 
flection may be expressed in terms of absohite velocities." 
The velocity is not constant; it varies with the different phases 
of the heart beat. It is greatest just after the semilunar valves 
open and allow a fresh inflow of blood from the heart, and gradually 
diminishes to the end of diastole. The average for the whole 
cardiac cycle is the mean velocity. 
The mean velocity of the blood in the carotid artery of the horse 
is about 10 inches per second, and in the internal jugular vein 
about half as much, viz. 5 inches per second. The velocity of the 
pulse wave is, as we have seen, 30 feet per second. This is a great 
contrast. The reason is that the one (pulse) is a pressure wave 
while the other is the movement of the whole mass of fluid against 
great resistance. 
The velocity of the blood in the capillaries is estimated by the 
microscope to be from 1 to 1% inches per minute. This is very 
slow, but the capillaries are so short, that the blood quickly passes 
through them, nevertheless. In round numbers it may be said that 
the velocity of the blood in the aorta is one foot per second and in 
the capillaries one inch per minute. 
Relative Velocities in Arteries, Capillaries, and Veins-While the 
absolute velocity will vary with any change in the rate and force 
of the heart or in the calibre of the arterioles, the relative 
velocities in arteries, capillaries and veins will depend entirely 
on the width of the bed through which the blood flows, that is, on 
the united sectional area of the vessels at any level. If the heart BLOOD VESSELS 
73 
beats faster or more powerfully the blood will move faster in arte- 
ries, capillaries and veins, but the relative rate of flow will be the 
same. The blood stream might be compared with a river (arteries) 
expanding into a lake (capillaries) and then narrowing down again 
into an outflow about twice as wide as the inflow river (veins). 
The flow will be swiftest in the narrow part, slowest in the lake 
and intermediate in the emerging stream. 
If an artery divided into two branches, the sum of their sectional 
areas will be greater than the sectional area (area of cross section) 
of the vessel before branching. The arterial system might be 
compared to a tree, where the sum of the cross sections of all the 
branches is greater than that of the single trunk. 
The blood from the heart flows through a bed which grows wider 
and wider as it passes from the aorta to the capillaries and narrower 
and narrower from the capillaries to the large veins. It is esti- 
mated that the united sectional area of all the capillaries is 800 
times as great as that of the aorta, therefore, the velocity will be 
800 times less. The cross section of the large veins (inferior and su- 
perior venae cavae), as they enter the heart, is about twice that of 
the aorta at its origin, so that the velocity will be about half what 
it is in the aorta. 
Circulation Time 
By this is meant the time taken by the blood to pass once from 
the heart, through the vessels, back to the heart again, i. e. to 
make one complete circulation. In man this is something less 
than half-a-minuet (23 seconds calculated). The actual time 
differs in different animals, but in general it appears to take from 
26 to 28 heart beats to effect one complete circulation. In man 
all the blood in the body passes twice through the heart in less 
than one minute. 
It took civilized man several thousand years to discover that 
the blood moves through the heart in a circle. That it actually 
does so was first proved by Harvey about the year 1616. He made 
many experiments on himself and on the lower animals of which 
the following are examples: 
He tied a string around the neck of a common grass snake and 
then beheaded it; the string prevented bleeding. In this cold 
blooded, decapitated animal the heart continues to beat for a long 
time. On this snake preparation (no pain could be felt because the 
brain had been removed) he observed: 
1. That when the large veins entering the heart were ligatured 
the heart and aorta quickly became empty while the veins filled 
up behind the ligature. 
Discovery of the Circulation 74 
HUMAN PHYSIOLOGY 
2. In a similar preparation he tied a ligature around the aorta 
near the heart and then observed that the veins and heart filled 
up and the aorta beyond the ligature became empty. 
3. He opened the aorta and found that the blood spurted out 
in jets from the end next the heart whenever the ventricles con- 
tracted. 
4. He opened the vena cava and observed that the blood flowed 
out steadily, without pulsation, from the end of the vein farthest 
from the heart. 
These experiments showed clearly that the blood flowed from 
the veins into the heart and out of the heart into the aorta. He 
stated that the blood passes from the arteries to the veins through 
pores in the tissues but he was not able to demonstrate the existence 
of capillaries. These were discovered by Malpighi a few years after 
Harvey's death. THE BLOOD 
The blood is an internal medium containing nutrient substances 
on their way to the tissues, and waste products on their way 
from the tissues to be excreted or removed from the body. 
The amount of blood in the body is about 7% of the body weight, 
or about 5 liters (10 pints) in a man of average size. The specific 
gravity varies from 1055 to 1060; the color is red when seen in 
mass but yellow in thin layers; the reaction is slightly alkaline, 
never acid. 
The blood may be regarded as a tissue consisting of cells-the 
red and white blood corpuscles and the blood-plates, and interstitial 
substance-blood plasma or liquor sanguinis-in which the cells 
are suspended. 
Red Blood Corpuscles or Erythrocytes 
The size and shape are constant in the same species but different 
in different species. They are largest in amphibians, such as the 
frog, and in some of these (amphiuma) they can be seen with the 
naked eye. In mammals they are smallest in the deer and largest 
in the elephant. In man they measure about 7 micro-millimeters 
or 1 /3200 inch in diameter. In all mammals, except the camel 
tribe, they are biconcave discs without a nucleus; in the camel 
tribe they are non-nucleated but biconvex and ellipsoidal in shape. 
In all other vertebrates--birds, reptiles, amphibians, fishes-they 
are ellipsoids, flattened but with a central elevation on both sides 
and nucleated. 
Number-In man they number about 5,000,000 per cubic 
millimeter of blood (men 5,000,000 and women 4,500,000) but in 
the goat they reach as high as 20,000,000 per cubic mm. Taking 
the volume of blood in the human body as 5 liters, it is found that 
the total number of red cells in the body reaches the enormous figure 
of 25,000,000,000,000. 
Structure--The red cells form little sacs or vesicles surrounded 
by a delicate transparent colorless cell membrane which is of a 
fatty nature due to the presence of lecithin, and the contents consist 
mostly of a colored substance termed haemoglobin, usually written 
Hb. About 90% of the solids of the red corpuscles and over 14% 
of the whole blood consists of Hb. 
Haemoglobin 
The haemoglobin content of the red corpuscles is of great im- 
portance. It is probably the most complex substance in the body 
75 76 
HUMAN PHYSIOLOGY 
and has a molecular weight of over 16,000. Its chemical formula 
is given as 
C758 H1203 N195 S3 FeO218- 
It consists of a protein element (globin), which contains all the 
sulphur, and a pigment (haematin), which contains all the iron. 
It is easily obtained in the crystalline form by adding water 
or ether to blood; the envelopes of the red cells rupture, the Hb 
escapes and is dissolved in the plasma. On standing it crystallizes 
out and the crystals falling to the bottom may be examined under 
the microscope. In most animals, and in man, they are rhombic 
plates, but in the guinea pig, squirrel and hamster they assume 
peculiar forms. According to recent work by Reichert and Brown 
it would appear that the Hb crystals are specific for each animal 
species, and might thus be used as a character in zoological classifi- 
cation. 
The chief property of Hb is its power of combining with 
oxygen ; it links on oxygen from the air in the lungs and then gives 
it up to the tissues. 
Oxyhaemoglobin (HbO2) is the name applied to the loose chemical 
compound formed between the Hb and oxygen. It is this that gives 
the scarlet color to the blood. This respiratory oxygen can be 
removed from the HbO2 by adding to it certain reducing agents, 
such as ammonium sulphide or Stoke's solution, and then it becomes 
reduced haemoglobin. Both forms, HbO2 and Hb, exist in the 
blood; there is more oxyhaemoglobin in arterial blood and more 
reduced haemoglobin in venous blood. It is only in animals dying 
of suffocation that all the Hb is reduced. 
Experiment to show change in Color of Blood associated with 
HbO2 and Reduced Hb-Place some defibrinated blood in a white 
porcelain dish and pass through it a stream of oxygen or ordinary 
air; it will quickly assume a bright scarlet color due to the pres- 
ence of HbO2. 
Take half a test-tube-full of blood and add to it one-quarter its 
volume of ammonium sulphide solution; in a few minutes it will 
turn a dark brown color. Pour some of the blood treated with oxy- 
gen into another test-tube and compare the two. 
Carbon Monoxide Haemoglobin (HbCO)-Other gases besides 
oxygen form compounds with Hb and of these the most important 
is carbon monoxide (CO). When a stream of ordinary coal gas 
(which contains large quantities of CO) is sent through blood, or 
a solution of haemoglobin, the latter quickly assumes a character- 
istic cherry-red color due to the formation of this compound 
HbCO. This is a much more stable substance than HbO2; the 
oxygen cannot turn out the CO and so the Hb ceases to be a carrier 
of oxygen from the lungs to the tissues. In consequence of this THE BLOOD 
77 
the tissues are deprived of oxygen and quickly die of suffocation. 
This explains the poisonous character of coal or illuminating gas, 
or of CO from any other source. 
Nitric oxide (NO) forms a compound with Hb (HbNO) which is 
more stable even than HbCO and which, if inhaled, will cause 
death in the same way as CO, but this gas is never met with outside 
of the chemist's laboratory. 
In the tissues of the body the protein part of the Hb molecule 
is believed to have the power of linking on CO2 to form HbCO2 
which is dissociated in the lungs. Hb thus acts as a carrier of CO2 
from the tissues to the lungs as well as of O2 from the lungs to the 
tissues. 
Haemolysis 
In various ways the red cells may be destroyed, and then the 
haemoglobin escapes and is dissolved in the plasma, in which 
condition it has not the same power of combining with oxygen. 
This escape of Hb from the corpuscle is known as haemolysis and 
the agents which brings it about are termed haemolysins or haemo- 
lytic agents. 
The following are examples of haemolysins--water, ether, chloro- 
form, blood senrm from an animal of different species, and many 
others. The blood serum of a rabbit, for example, injected into 
the human subject, or into a cat or a dog, will rapidly destroy the 
red corpuscles of the latter. 
These haemolytic agents act in different ways. Water added to 
blood dilutes the plasma and by a process of osmosis the red 
corpuscles swell up and ultimately rupture. The blood is then 
said to be laked. 
Ether and chloroform dissolve the cell membrane, which 
is largely composed of the fat-like substance lecithin, and so the 
contents escape. 
The nature of the action in the case of a foreign serum is not 
understood. 
Ordinarily blood, even in thin layers, is opaque, but when it is 
laked, i. e. when the red corpuscles are destroyed, it becomes 
darker in color by reflected light and more or less transparent by 
transmitted light. The reason for this is as follows: The blood 
corpuscles are solid bodies suspended in a fluid, and reflect a large 
proportion of the rays of light which fall upon them, so that when 
a vessel containing blood is held up between the observer and the 
light little or none gets through; it is opaque. If the corpuscles are 
dissolved and disappear, however, little or no light is reflected and 
much gets through so that it appears darker by reflected light and 
transparent by transmitted light. If the shape of the corpuscles is 
altered, as can be effected by the addition of a strong solution of 78 
HUMAN PHYSIOLOGY 
sodium chloride, so that they reflect more light (they are said to be 
crenated), the color, by reflected light becomes much brighter. 
To demonstrate this take two test tubes and place an inch of blood 
in each, then to the one add an equal quantity of water and to the 
other an equal amount of sodium chloride, saturated solution, 
and compare the colors by reflected and by transmitted light. 
Spectroscope Applied to the Examination of 
Blood Pigments 
When a beam of white light from the sun is passed through a 
glass prism it is spread out into a colored band---red, orange, yellow, 
green, blue, indigo, violet-and this is called a spectrum. The solar 
spectrum is interrupted by a number of dark lines which cross it 
vertically; these are called Frauenhofer's lines and are caused by 
incandescent metallic vapors in the sun's atmosphere. They are 
perfectly constant in position and serve as a natural scale for the 
spectrum. They are named after the letters of the alphabet, 
A, B, C, D, E, etc., the D line in the yellow being especially 
prominent. 
If the beam of light, before falling on the glass prism, be passed 
through a dilute solution of oxyhaemoglobin two dark absorption 
bands will be seen in the spectrum between the D and E lines. 
This is characteristic of HbO2 and is used as a test for the same. 
If now the HbO2 is reduced, by adding to it some ammonium 
sulphide, the two bands will disappear and a single band, much 
broader, but still between the D and E lines, will take their place; 
this is the characteristic spectrum of reduced Hb. 
The spectrum of HbCO also shows two bands and is very similar 
to that of HbO2, but these two substances can be readily dis- 
tinguished, with the spectroscope, by the factthat HbO2 can be 
reduced by ammonium sulphide to show a single band while HbCO 
cannot be so changed. 
Many other pigments, derivatives of Hb, give characteristic 
spectra. 
Origin and Fate of Red Blood Corpuscles 
In the adult the red cells are all believed to be produced in 
the bone marrow normally. In the marrow many nucleated red 
cells are found but the nuclei are lost before the cells are sent out 
into the circulation, ordinarily. After severe haemorrhage, however, 
when the red cells are greatly in demand in the vessels, nucleated 
reds are often found in the circulating blood, having been forced 
out prematurely. They subsequently lose their nuclei in the blood 
stream. 
In the embryo and foetus they are also formed in other tissues, 
particularly the liver and spleen. THE BLOOD 
79 
The red corpuscle, in the blood stream, in mammals, is a cell 
which has lost its nucleus and so probably has only a short exist- 
ence, but just where it breaks down and disappears is a disputed 
question. It has been supposed that the liver, spleen, and lymph 
glands are blood destroying organs'but at present most physiolo- 
gists hold to the belief that they simply go to pieces-die of old 
age so to speak-in the blood stream. It has been estimated that, 
on the average, the red corpuscles do not live more than ten days. 
Large numbers are probably destroyed daily. When they break 
down the Hb which escapes is seized by the liver and there manu- 
factured into bile pigment. 
Effect of Altitude on Number of Red Blood Corpuscles-The 
red cells increase markedly in high altitudes. While the normal 
number at sea level is in the neighborhood of 5,000,000 per 
cmm., a short stay at an elevation of 12,000 feet may raise it 
to 8,000,000 or over. The explanation of this increase given is as 
follows: At high levels the barometric pressure is low and the 
amount of oxygen in the air inhaled at each inspiration, is greatly 
reduced, consequently each red corpuscle that comes to the lungs 
cannot get a full charge and the tissues suffer from lack of oxygen. 
This condition might be remedied in two ways: 1. The heart might 
beat faster or more powerfully so that each carrier (red corpuscle) 
might make the journey between the tissues and the lungs more 
quickly; or 2, the number of carriers might be increased. It is 
the latter method that is adopted by the body. In what way this 
lack of oxygen stimulates the bone marrow to produce more red 
cells is not understood. 
Anaemia 
The blood, like other tissues, is liable to be diseased and here 
the number of red cells may be greatly altered. 
In ordinary or primary anaemia, usually called chlorosis, the red 
corpuscles are only slightly below the normal in number but the 
amount of Hb in each is greatly reduced; it may be only 40% of 
what it ought to be. This condition is not uncommon in girls in 
their "teens," but it is rarely seen in boys or men. The cause 
appears to lie in the bone marrow which is not able to produce a 
sufficient quantity of Hb. The treatment is very simple and con- 
sists in living in the open air, as much as possible, but particularly 
in taking iron, in some form, as a medicine. Iron is an important 
element in the molecule of haemoglobin and this appears to be all 
that is required here. 
Pernicious Anaemia-There is another type of anaemia, much 
more serious than chlorosis, and that is pernicious anemia. 
Here the red cells may be down to 1,000,000 or even 200,000 per 80 
HUMAN PHYSIOLOGY 
cmm. although each cell may contain the normal amount of Hb. 
The cause of this condition is not certain. It is nearly always fatal. 
White Blood Corpuscles 
There are many varieties of the white blood corpuscles. They 
are subdivided according to size and shape of nucleus and the 
character of the granules found in their protoplasm. They fall into 
two main groups, the lymphocytes and the leucocytes. Some are 
small, scarcely bigger than the red cell, and others are very large. 
Most of the leucocytes show amoeboid movement and act as 
phagocytes. 
The normal number of white corpuscles is from 5,000 to 7,000 
per cmm., but this varies from time to time much more than in 
the case of the red cells. 
Functions of White Blood Corpuscles--Without going into de- 
tail, the functions of the white corpuscles are believed to be the 
following: 
1. They are phagocytic, that is, they attempt to destroy all 
foreign bodies found in the blood or tissues, especially pathogenic 
(disease producing) organisms, by absorbing and digesting them. 
They form the main defence of the body against the invasion of 
harmful micro-organisms. All so-called infectious diseases such 
as diphtheria, typhoid fever, tuberculosis, etc., are caused by the 
entrance of disease germs, microbes or micro-organisms into the 
body and one important function of the leucocyte is to protect 
the body from these. 
2. They form antitoxins. When disease germs settle down in 
the tissues they produce toxins or poisons, and to counteract the 
effects of these the leucocytes produce antitoxins, i. e. substances 
which neutralize the toxins. 
3. They take part in the process of blood coagulation in a way 
that will be explained later. 
4. They are said to absorb fats and to some extent proteins 
from the intestinal tract and carry them into the lacteals or 
intestinal lymphatics, whence they reach the blood stream through 
the thoracic duct. 
Blood Plates 
These are circular or ellipsoidal bodies of small size--less than 
the red cells. Little is known regarding their structure because 
they disintegrate very rapidly when the blood is shed and a special 
technique is required for their examination. 
They number about 300,000 per cubic millimeter. Like the 
leucocytes they are believed to be concerned in the clotting of 
blood; in the process of disintegration thrombin or fibrin ferment 
is set free which acts on the fibrinogen of the plasma and trans- THE BLOOD 
81 
forms it into fibrin, the essential part of a blood clot. In the peculiar 
condition of haemophilia, where the blood does not clot at all or 
clots very slowly, the blood plates are found to be greatly reduced 
in number. This occurs in men almost exclusively, very rarely in 
women, yet it is always inherited through the mother. Persons 
suffering from this disease are called "bleeders" or haemophilics. 
The slightest wound will continue to bleed for days and may prove 
fatal. No surgeon will operate on such people. 
BLOOD PLASMA 
The blood plasma is the fluid part of the blood in which the cor- 
puscles are suspended. It contains a great variety of substances 
in solution, since it carries the food material to the tissues and, at 
the same time, conveys the waste products away from them. 
(See composition of plasma on board). 
About 90% is water. The proteins are serum albumin, serum 
globulin, nucleo-protein and fibrinogen. The last is present in 
only small amount but it is a very important constituent. 
Under the heading "extractives" a great variety of substances 
are included. Any organic substance, other than proteins, which 
may be dissolved out of the dried blood residue by water, alcohol 
or ether is called an extractive. Amino acids, although present 
in small quantity at any one time, form one of the most important 
groups. The carbohydrate is present in the form of glucose. 
Less than 1% (0.85%) of the blood plasma consists of inorganic 
salts, the chief of which is sodium chloride. 
In addition to the above, a large number of substances, of un- 
known composition, which cannot be classified, exist in the blood, 
in traces, such as enzymes, hormones, immune bodies, opsonins, 
etc. 
The composition of the blood is very constant. The tissues 
and cells of the body are very sensitive to changes in its com- 
position and strive to keep it constant. 
Coagulation of Blood 
Blood when it escapes from a blood-vessel becomes a solid jelly; 
in other words it clots or coagulates. In man this change takes 
place in from 2 to 4 minutes, in the horse not for about 10 minutes. 
It is very important that the blood should clot, since this serves 
to plug the blood-vessels and spontaneously arrest haemorrhage. 
This blood clot, when examined, is found to consist of fibrin 
threads with the corpuscles entangled. On standing it contracts 
slowly and a clear fluid, called blood serum, is squeezed out. 
When the blood is caught in a beaker and vigorously stirred with 
a stick or a bunch of twigs the fibrin formed collects on the stirrer 82 
HUMAN PHYSIOLOGY 
and can be removed, leaving behind the serum and corpuscles; 
this is termed defibrinated or "whipped" blood. When the fibrin 
is washed free from any corpuscles which may have adhered to it, 
it is found to consist of a mass of fine elastic threads. This is the 
essential part of the clot the corpuscles being merely entangled 
in the fibrin meshwork. How is the fibrin formed? 
Formation of Fibrin-The fibrin is formed from the fibrinogen 
of the plasma. As we have seen, one of the protein constituents 
of the plasma is fibrinogen, which is present in solution, and when 
the blood is shed, this soluble fibrinogen is transformed into 
insoluble fibrin. This change is brought about by an enzyme or 
ferment termed fibrin ferment or thrombin which does not exist 
in the blood-vessels as such but rather as an inactive substance 
termed pro-thrombin or thrombogen. How is the thrombin 
formed ? 
Formation of Thrombin--The formation of thrombin is a com- 
plicated process. The blood plates and, to some extent the leuco- 
cytes, contain prothrombin and when these break down this 
substance is set free. In the red and white corpuscles, and also in 
the other tissues of the body such as muscle, there is another 
ferment termed thrombokinase; this, in the presence of soluble 
calcium salts, acts upon the liberated prothrombin and transforms 
it into thrombin which then, in turn, acts on the fibrinogen of the 
plasma and changesit, in someway, to insoluble fibrin. In tabular 
form the changes which take place may be represented as follows: 
Tissue Cells including Blood Corpuscles - Thrombokinase. 
Thrombokinase + Calcium + Prothrombin = Thrombin. 
Thrombin + Fibrinogen = Fibrin. 
Fibrin + R. and W. Corpuscles caught in Meshes = Clot. 
The above is one of several theories to explain the changes that 
take place in the spontaneous coagulation of the blood. 
Reason why Clotting does not take place in Vessels-Any con- 
dition, such as contact with a foreign body, which will rapidly 
break down the blood plates and white corpuscles will hasten the 
clotting of blood. In the living vessels all the cellular elements 
are constantly undergoing disintegration but not en masse, as is 
the case when the blood comes into contact with a foreign body, 
and not a sufficient quantity of prothrombin and thrombokinase 
is set free at any one time to have an effect. If a needle be thrust 
through the vessel wall, into the blood stream, clotting takes place, 
however, around the needle very rapidly. Injury to the vessel 
walls also induces clotting, because the damaged tissue really 
plays the part of a foreign body which rapidly breaks down the 
blood plates and white cells. 
Calcium in Solution Necessary for Clotting of Blood-The ad- 
dition of a small quantity of potassium oxalate to the blood will THE BLOOD 
83 
prevent clotting. The reason is that the calcium oxalate which is 
formed is insoluble and so all the calcium in the blood is taken out 
of solution. Since soluble calcium is essential, as well as pro- 
thrombin and thrombokinase, to the formation of thrombin, none 
of the latter is produced and clotting does not take place. 
Clotting is RETARDED OR PREVENTED by a LOW TEMPERATURE, 
by leech extract, and by contact with oil, etc. The leech feeds 
on the blood of other animals and the glands of his mouth contain 
a substance termed hirudin which prevents the clotting of blood, 
so that it may be more easily digested. If a trace of this substance 
be injected into the vessels of a living animal the blood will not clot. 
Blood Plates Related to Clotting of Blood-In birds the blood 
plates are few and the blood is long in clotting. 
In "bleeders" or hsemophilics the blood plates are also few in 
number and clotting takes place very slowly or not at all. 
Transfusion of Blood 
By this is meant the passage of blood from one individual into 
another. It is sometimes practised in cases of severe haemorrhage, 
in CO poisoning, and in pernicious anaemia. 
There are two dangers associated with the operation. 1. In- 
travascular clotting may take place. If the peripheral end 
of a vein of the "donor" be connected, by means of a glass tube, 
with the central end of the vein of the "recipient" the blood, in 
flowing through the tube, will come into contact with a foreign 
body (wall of tube) and this may induce clotting in the vessels of 
the recipient. The surgeon, if sufficiently skillful, however, may 
overcome this danger by stitching the two veins together so that 
the blood may flow directly from the one vessel into the other 
without coming in contact with any foreign body. 2. The donor 
and the recipient must belong to the same species because of 
the haemolytic action which the serum of one animal has on the red 
corpuscles of another not of the same species. 3. Aseptic defibrin- 
ated blood may be used. 
In cases of severe haemorrhage it is just as effective to inject 
normal saline (0.9% sodium chloride) into the vein as whole blood, 
since the danger here comes from the fall in blood pressure, not 
from the loss of blood corpuscles, and no risk is connected with the 
procedure. RESPIRATION 
Respiraton consists essentially of an interchange of Oxygen and 
CO2 between the organism and its surrounding medium. All living 
things require oxygen and all give off CO2. In the unicellular 
organism there is no special respiratory apparatus, since this 
exchange takes place through the entire surface of the body. 
In all the higher animals a special apparatus is developed to bring 
the oxygen from the outside environment to the inner tissues and 
the CO2 from the tissues to the outside, the blood acting as the 
medium. 
In fishes the gills fulfill the same functions as the lungs in air 
breathing animals. Essentially a gill or a lung consists of a thin 
membrane one surface of which is exposed to the medium contain- 
ing the oxygen-water or air- and the other to the blood. It is 
through this membrane that the interchange takes place. The 
same exchange occurs in the tissues but in the opposite direction. 
By the term "inner respiration" is meant the exchange of gases 
between the blood and the tissues, and by "outer respiration" the 
exchange between the blood and the air in the lungs (or water in 
the gills). The inner or true respiration takes place in the tissues. 
THE RESPIRATORY APPARATUS 
The respiratory apparatus in man consists of the respiratory 
passages (including the mouth, nose and throat, the larynx, 
trachea, and extrapulmonary bronchi), lungs and muscles of 
respiration (diaphragm and intercostals). 
The Trachea or Windpipe in the human subject is from 4 to 
inches long and from to 1 inch in diameter. It is formed of a 
series of cartilaginous rings, defective behind where they lie against 
the gullet, and is lined by ciliated epithelium. Numerous glands 
pour their secretions, containing a large amount of mucus, into 
the trachea and this helps to entangle any dust particles which 
may be inhaled. The cilia lash upwards and expel the dust laden 
mucus, through the glottis, into the mouth. 
The trachea divides into two extra pulmonary bronchi, right 
and left, the structure of which is practically the same as that of 
the trachea. In the smaller bronchi (intrapulmonary) the cartila- 
ginous rings are entirely absent; they consist essentially of con- 
nective tissue with non-striped muscle fibers, arranged circularly, 
and are lined by ciliated epithelium. 
84 RESPIRATION 
85 
The Lungs, right and left, together with the heart, practically 
fill the chest cavity. Each lung is partially divided into lobes 
of which the right lung has three and the left two. 
Each lung consists essentially of a large number of air saccules 
(alveoli), with a dense network of capillaries in their walls, through 
which an interchange of gases can take place. The bronchi are 
continued down from the trachea and when they enter the lung 
are termed intrapuhnonary bronchi; they divide and subdivide 
just like the branches of a tree, getting smaller and smaller until 
finally each of them terminates in a grape-like cluster of small 
hollow cavities called alveoli or air-cells. The smallest bronchi 
are termed bronchioles just as the smallest arteries, as we have seen, 
are known as arterioles. The wall of the air cell consists of con- 
nective tissue, mostly of the elastic variety; this is lined by endo- 
thelium with a dense network of capillaries underneath. All that 
intervenes between the air and the blood are two layers of endo- 
thelium, one lining the capillaries, the other the alveoli. The total 
extent of the inside of the lung, if it were spread out on a flat sur- 
facers equal to about 90 square meters or nearly 1000 square feet. 
Blood-vessels of Lung-There are two sets of blood-vessels in 
the lung just as there are in the heart-the pulmonary and the 
bronchial. The pulmonary arteries, capillaries and veins carry the 
blood through the lungs, from the right side of the heart to the 
left, for aeration; the bronchial vessels, very much smaller than 
the pulmonary, carry blood to nourish the lung tissue itself, and 
correspond to the coronary or nutrient system of the heart. 
Relation of Lung to Chest Wall-The chest wall is lined by a 
serous membrane, the pleura, which is reflected on to the surface 
of the lungs. There are two layers, therefore, termed the parietal 
(lining chest wall) and visceral (lining lungs) layers of the pleura. 
No actual space exists in health between these two layers, only a 
potential space. The one layer rubs on the other, and to diminish 
friction, the pleura is covered by a single layer of endothelium. 
When the pleura is the seat of inflammation (pleurisy) the surfaces 
may be roughened and then "friction sounds" are heard by the 
physician. 
If in the process of inflammation fluid is effused (poured out) 
between the two layers of the pleura it will cause some collapse of 
the lung; this condition is known as Hydrothorax (water in thorax). 
If an opening be made in the chest wall, or in the lung, air will 
enter the pleural space and the lung will collapse; this is known 
as Pneumothorax (air in thorax). Air may enter through the 
visceral pleura in phthisis pulmonalis and lead to the same result. 86 
HUMAN PHYSIOLOGY 
MECHANISM OF RESPIRATION 
By this is meant the mechanical means by which air is drawn 
into and expelled from the lungs in the alternate phases of inspira- 
tion and expiration. 
Normally the lungs always fill the chest cavity. Any increase 
in the size of the chest cavity causes an increase in the size of the 
lungs therefore air is drawn in through the trachea and this forms 
the act of inspiration. Any diminution in the size of the chest 
cavity is followed by a reduction in the volume of the lungs; air 
is expelled and this forms the act of expiration. 
These changes in the size of the chest cavity are brought about 
as follows: 
In Ordinary Inspiration (quiet breathing) the vertical diameter 
of the chest is increased by the contraction of the diaphragm. 
This sheet of muscle, between the thoracic and abdominal cavities, 
arches upwards into the thorax and when it contracts it tends to 
become flattened, thus increasing the vertical diameter of the cavity. 
The lateral (side to side) and antero-posterior (back to front) 
diameters are increased by the elevation and rotation of the ribs. 
The ribs slope downwards and forwards from the spinal column. 
They articulate with the bodies of the thoracic vertebras behind, 
and in front they are connected with the sternum, through the 
costal cartilages. Between the adjacent costal arches there are two 
thin sheets of muscle-the external and internal intercostal mus- 
cles-and when these contract the ribs are raised. The posterior 
ends are fixed so that each rib describes the arc of a circle, and 
since it slopes downwards, its anterior or sternal end will move 
forwards; this increases the antero-posterior diameter of the chest. 
Not only do the contracting intercostal muscles compel the ribs 
to make this movement, but they (the ribs) are also rotated around 
an axis passing through the head of the rib and its junction with 
the sternum, and in this way the lateral diameter of the chest is 
increased. 
The chest cavity is therefore enlarged in all dimensions, the 
lungs follow and air rushes in = inspiration. (Illustrate with lungs 
in glass jar the bottom of which consists of a thick rubber membrane 
to represent the diaphragm). 
The chief muscles of ordinary inspiration are the diaphragm and 
intercostals, although a few others not mentioned here play a 
minor part. 
Ordinary Expiration is non-muscular. It is brought about by 
four factors, viz.: 
1. The Elasticity of the Lung. It is very important to re- 
member that the lungs are elastic. They are always stretched, 
and tending to collapse, but are unable to do so because they RESPIRATION 
87 
cannot separate the two layers of the pleura. In the so-called 
pleural cavity a vacuum exists, and as soon as air is admitted, 
making the pressure on that aspect of the lung equal to the atmos- 
pheric pressure exerted through the trachea, the lung collapses 
BECAUSE OF ITS ELASTICITY. 
2. Weight of Chest Wall. The ribs, and all the structures 
attached to them, are elevated in inspiration and when the inter- 
costal muscles relax they fall in virtue of gravity. 
3. Untwisting of Costal Cartilages. The costal cartilages, 
attaching the ribs to the sternum, are twisted by the rotatory 
movement of the ribs; they tend to untwist and this helps to bring 
the ribs back to the position of expiration. 
4. Elasticity of Abdominal Walls. In inspiration the de- 
scent of the diaphragm, which is active, exerts pressure against the 
abdominal viscera and pushes out the walls of the abdomen putting 
these under a certain amount of tension. When the diaphragm 
relaxes, at the end of inspiration, the elasticity of the abdominal 
walls exerts itself against the abdominal contents and pushes these 
up against the diaphragm, so that its ascent, by which the vertical 
diameter of the chest is diminished, is passive. The muscular 
part of the diaphragm has no power to arch upwards into the 
thorax if it is not pushed from below. 
Respiratory Movements in Male and Female-Respiration in 
the male is carried on mainly by the diaphragm ; in the European 
and American female mainly by the ribs. In the child the 
movements are chiefly diaphragmatic in both sexes. In Indian 
and Chinese women, who have never worn tight dress, the move- 
ments do not differ from those of the child and of men, and this 
shows that the costal type is due to the vicious habit of wearing 
tight corsets which prevent the normal movements of the abdomi- 
nal walls. 
In dyspnoea or threatened suffocation the breathing is decidedly 
costal. 
A graphic record of the respiratory movements may be obtained 
by the use of an instrument termed the Stethograph or Pneumo- 
graph. The simplest form consists of a wide rubber tube, containing 
a spiral spring, one end of the tube being closed and the other 
attached to a tambour, provided with a lever, writing on a revolv- 
ing drum. The rubber tube is strapped around the chest with an 
inextensible chain. 
During inspiration air is drawn out of the tambour into the tube 
and the lever falls; during expiration the air re-enters the tambour 
and the lever rises. If a time tracing be taken underneath, the 
relative duration of the different phases may be determined. The 
rhythm is, inspiration-expiration-pause, and the relative dura- 88 
HUMAN PHYSIOLOGY 
tion of each phase is 3:4:3. If the pause be taken as a slow ex- 
piration, which in reality it is, then the ratio will be inspiration 3: 
expiration 7. 
The respiratory sounds are caused by the air entering and leav- 
ing the lungs. The character of these is of great importance to 
the physician as a guide to the condition of the lungs. 
Innervation of Diaphragm and Intercostal Muscles-The dia- 
phragm is supplied by the two phrenic nerves, right and left, 
and the intercostal muscles by eleven pairs of intercostal nerves 
from the thoracic region of the spinal cord. The phrenic nerve on 
each side arises from the 4th and 5th cervical spinal segments so 
that section of the spinal cord above this level stops the respiratory 
movements completely and is, of course, fatal. If the cord be 
injured below the 5th cervical segment, although the intercostal 
muscles are cut off, respiration by the diaphragm can still be 
carried on. 
RESPIRATORY CENTER 
The rhythmical movements of respiration require the harmoni- 
ous action of a number of different muscles, and where this is the 
case, these muscles must be presided over by a co-ordinating center 
in the nervous system. 
The respiratory center is situated in the medulla oblongata. 
It is bilateral (half on each side) the two halves being connected 
across the middle line by nerve fibers, so that normally the two 
sides act together. Injury to this part of the brain paralyzes the 
respiratory center and causes instant death. 
The activity of the respiratory center is regulated mainly 
by the amount of co2 in the blood going to it. Every condition 
which leads to an accumulation of CO2 in the blood causes an in- 
crease in the rate and depth of respiration, e. g. muscular exercise 
such as running, hill-climbing, etc. The C02 when it reaches a 
certain level, excites the cells which form the respiratory center; 
these send impulses down the spinal cord and out into the phrenic 
and intercostal nerves, causing the diaphragm and intercostal 
muscles to contract, and so inspiration is brought about. This 
removes CO2 so that its level in the blood falls below the exciting 
point; impulses no longer reaching the muscles in question, they 
relax and expiration follows. 
Diminution of oxygen in the blood acts to some extent as a 
stimulus to the respiratory center, but this is not such an im- 
portant factor as the accumulation of CO2. 
Reflex and Central Stimulation of Respiratory Center-In ad- 
dition to the effect of CO2 in the blood, the respiratory center is 
also acted on by impulses reaching it through the peripheral nerves, 
and by impulses coming down to it from the higher brain centers. RESPIRATION 
89 
Cold water, for example, applied to the skin of the abdomen or chest 
produces deep inspiration, due to the reflex stimulation of the 
respiratory center. Emotion may cause either sobbing or laughing, 
both of which are modified forms of respiration, by excitations 
reaching the respiratory center from above. 
Coughing, sneezing, yawning, sighing, hiccup are all examples 
of modified respiration, and are all produced by stimulation of the 
respiratory center, either reflexly or from the higher brain centers. 
A peculiar form of respiration known as Cheyne-Stokes breath- 
ing is sometimes present in disease. The patient ceases to breathe 
for a time; the respirations then begin; they are feeble at first, 
but gradually increase in depth, until a maximum is reached, when 
they die away again and then cease altogether. After a pause, 
the same succession follows again. In these cases the respiratory 
center is less excitable than usual, and it requires a greater accumu- 
lation of CO2 to start it than is ordinarily sufficient. Once it 
wakes up, however, it continues until the excess of CO2 is swept 
away, and then it goes to sleep again, so to speak. 
Vagus Control of Respiration-The respiratory rate and depth 
are controlled, to a large extent, by impulses from the lungs them- 
selves which reach the respiratory center through the vagi nerves- 
an example of peripheral reflex stimulation. 
If one vagus nerve in the neck is cut, in an .anaesthetized 
animal, the respiratory rate is diminished but the depth is 
increased ; if both are cut the effect is greater still. Two sets of 
afferent nerve fibers are believed to exist in the vagus, carrying 
impulses from the lungs to the respiratory center; stimulation of 
the one excites the center and brings about inspiration, stimulation 
of the other inhibits the center, checking inspiration, and leading 
to expiration. The nerve terminations of these fibers end in the 
walls of the alveoli; one set is excited by the collapse of the walls 
and the other by their stretching. At the end of expiration the 
collapse of the lungs stimulates the inspiratory fibers and starts 
inspiration sooner than would be the case if the CO2 in the blood 
were alone responsible; similarly, at the end of inspiration the 
inhibitory fibers are excited by the expansion of the lungs and this 
checks inspiration. However, when these influences from the lungs 
are entirely cut off, respiration does not come to a standstill; it is 
regulated all the time by the CO2 in the blood but the rhythm 
becomes slower and deeper. 
The respiratory center is under the control of the will, to a con- 
siderable extent. A person can voluntarily accelerate his respira- 
tion or stop it altogether but the latter only for a short time; at 
the end of about one minute, the desire for air becomes so strong, 
that voluntary inhibition is ineffective and respiration is resumed. Fig- 8 RESPIRATION 
91 
Apnoea, Eupncea, Hyperpncea, Dyspnoea, Asphyxia 
By the term Apncea is meant no breathing, or rather no desire to 
breathe. If a person breathe rapidly and deeply for half a minute 
and then stop, for the next twenty seconds or so he has no desire 
to breathe, that is, apnoea. This is because the CO2 of the blood 
has, to a large extent, been swept away and got rid of, and until it 
accumulates again to the exciting point, inspiration does not take 
place. 
The term Eupncea means normal breathing; Hyperpncea ex- 
aggerated breathing; Dyspncea labored or distressful breathing, 
and Asphyxia suffocation from want of air. 
BLOOD GASES 
There are three gases present in the blood-oxygen, nitro- 
gen and CO2; all leave it in a vacuum. All the nitrogen is held in 
solution; the oxygen and CO2 partly in solution and partly in 
chemical combination. 
By means of a complicated piece of apparatus known as a blood 
gas pump a quantity of blood may be drawn into a vacuum and 
the mixture of gases which escapes or boils off from it collected 
and analyzed. If 100 c.c. of blood be treated in the way described 
it will yield about 60 c.c. of gas, and if this be analyzed its composi- 
tion by volume will be about as follows: 
Blood 
0 
CO2 
N 
Arterial 
20 
38 
2 
c.c. 
Venous 
12 
46 
2 
c.c. 
The oxygen comes almost entirely from the red corpuscles 
where it exists in loose combination with haemoglobin as HbO2. 
It is so loosely combined that it dissociates in vacuo. 
The CO2 is contained both in the corpuscles and in the plasma, 
but mostly in the latter, where it exists as sodium bicarbonate 
and in organic combination with the proteins of the plasma. 
Some is present in the red cells as HbCO2. In vacuo these sub- 
stances dissociate and CO2 is given oft, e. g. 2NaHCO3 changes to 
Na2CO3 setting free water and CO2. 
Some oxygen, some CO2 and all the nitrogen exist in the plasma 
in solution just as they would in water. 
Exchange of Gases between Blood and Air in Lungs and between 
Blood and Tissues--Oxygen is at a higher tension in the air cells 
of the lungs than in the blood and it therefore diffuses from the 92 
HUMAN PHYSIOLOGY 
air cells into the blood; CO2 is at a higher tension in the blood than 
in the air cells and it diffuses from the blood into the air cells. 
An exchange the reverse of this takes place in the tissues. This is 
simply a case of the diffusion of gases according to well known phys- 
ical laws. 
Respiratory or Vital Capacity 
This term is applied to the amount of air that can be expelled 
from the lungs by a forcible expiration after they have been fully 
distended by a forcible inspiration. This volume may be deter- 
mined by means of a spirometer. This instrument consists of a 
circular vessel, filled nearly to the top with water, and a graduated 
cylinder inverted over it and counterbalanced by a weight. By 
means of a vertical tube, fixed to the bottom of the reservoir, 
and projecting above the surface of the water, the air expelled 
from the lungs is collected in the cylinder and the amount read off 
on a graduated scale. 
To measure the so-called vital capacity take as deep a breath as 
possible, hold the nostrils and then expel as much air as possible. 
For an individual of average height (5 feet 8 in.) the respiratory 
capacity should be about 3700 c.c. if the lungs are sound. There 
is found to be a relationship between the height and the respiratory 
capacity. According to Hutchinson between 5 and 6 feet every 
inch of stature should add 8cubicinches (130 c.c.) to the respiratory 
capacity. 
The air inspired and expired at each ordinary respiration is 
termed the tidal air; it is only about 500 c.c. At the end of an 
ordinary inspiration 1600 c.c. more can be drawn into the lungs 
by a forced inspiration; this is called the complemental air. Then, 
at the end of an ordinary expiration, 1600 c.c. more can be ex- 
pelled from the lungs; this is called the supplemental air. There 
still remains in the lungs, after the most powerful expiration, 1000 
c.c.; this is termed the residual air. The sum of the complemental, 
tidal and supplemental air constitutes the respiratory capacity. 
The atmospheric air which is inspired is very constant in com- 
position. Every 100 parts by volume of the air inspired contains 
approximately 79 parts of nitrogen, 20.96 of oxygen and 0.04 of 
CO2. The expired air gains about 4% of CO2 and loses about the 
same amount of oxygen. A candle will not burn in air that has 
passed once into the lungs and is then expired. (Experiment to 
prove this). 
Besides the gain in CO2 and loss of oxygen, expired air differs 
from that inspired by having added to it heat and moisture. A 
large quantity of heat is carried away from the body in the expired 
Inspired and Expired Air RESPIRATION 
93 
air, and this is one important factor in the regulation of the body 
temperature. 
Effects of Breathing a Vitiated Atmosphere 
It is difficult to say what are the causes of the evil effects (head- 
ache, general malaise, high temperature, etc.) which follow from 
breathing the vitiated atmosphere of a badly ventilated and over- 
crowded room. These might be due, 1, to excess of CO2, 2, to want 
of oxygen, 3, to the presence of some toxic substance in the 
expired air. 
1. If CO2, prepared in the laboratory, be added to the air of a 
closed chamber until it contains far more than the most badly 
ventilated room, little or no discomfort is felt from breathing it. 
Again, the air of a brewery may contain 10 times as much CO2 as 
the air of a very poorly ventilated room and yet men may work in 
such an atmosphere for years and remain in good health. 
2. There is less oxygen in the air of a mountain health resort 
than in the most overcrowded rooms at sea level, and yet the effect 
is exhilarating rather than otherwise. Thus the oxygen of the air 
can be reduced far below that found in very badly ventilated rooms 
and the CO2 raised far above it, without any ill effects. If, of 
course, the diminution of oxygen or increase of CO2 be very great, 
death may result from asphyxiation or from CO2 poisoning. 
3. Is thecause to be found in some organic poison given off from 
the lungs ? This was answered in the affirmative by Brown-Sequard 
jvho believed that he had proved the presence of a toxin in expired 
air. 
Brown-Sequard's Experiment--He arranged 5 flasks in series 
and connected them to each other and to an aspirator by rubber 
tubing, so that a continuous stream of air might be drawn through. 
Between the 4th and 5th of the series he inserted a tube containing 
sulphuric acid, and then into each he placed a mouse. The sul- 
phuric acid, while it would allow CO2 to pass through it unaffected, 
would decompose and destroy any organic substance. 
When he started the experiment the first mouse to show any 
effect was that in flask No. 4, and in some hours it died. Next 
followed No. 3 and lastly, No. 2, while the mice in flasks 1 and 5 
remained alive and apparently quite well. 
He explained these results as follows: If CO2 had been the poison 
then No. 5 should have died first because it received all the CO2 
given off by the four mice in front of it, and the sulphuric acid did 
not interfere with the passage of this substance, but it did keep 
back any organic toxin which might be produced in the lungs. 
This toxin would be received in larger quantity by No. 4 than by 
any in front of it, hence its early demise, and so on for the others. 94 
HUMAN PHYSIOLOGY 
Nos. 1 and 5 were inhaling air free from this toxin and consequently 
were not affected. 
There is a fallacy in this experiment which was overlooked 
by Brown-Sequard, however, and that is this. He proved that the 
death of Nos. 4, 3 and 2 were due to an organic toxin but he did not 
prove that that toxin came from the lungs. The mice had been 
kept in the flasks for several hours, in a warm room, and during 
this time urine and faeces had accumulated; the toxin or toxins 
were probably some of the decomposition products of these. 
If the moisture present in expired air be condensed, and then 
injected under the skin of an animal, no evil effect results. 
No TOXIN GIVEN OFF BY THE LUNGS HAS EVER BEEN DEMON- 
STRATED. 
It is now generally believed that the physical condition of the 
air has more to do with the bad effects of poor ventilation than its 
chemical constituents. It has a high temperature, it is stagnant 
and saturated with moisture, besides being contaminated with 
offensive exhalations from the teeth, skin and clothes of persons 
in the room. Under these conditions the temperature of the body 
rises until the individual suffers from a mild degree of fever. 
Methods of Applying Artificial Respiration to Persons 
* Apparently Drowned, Electrocuted, etc. 
When respiration is suspended, as in persons apparently drowned, 
or from any other cause such as an overdose of chloroform or ether, 
artificial respiration must be carried on to take the place of the* 
normal movements. Several methods have been employed of 
which the four given below are the most important. 
1. The Marshall Hall Method-This consists in placing the 
subject on his side, rolling him over on to his face, and then back 
to the original side position. These movements are carried out 
at the rate of about 15 per minute. The respiratory exchange 
capable of being effected by this method is not great, and besides, 
the body is liable to be bruised and injured by the manipulation. 
2. The Sylvester Method-The patient is laid on his back 
(supine position), the arms are grasped by the operator below the 
elbows and pulled upwards above the head to expand the chest 
and produce inspiration; they are then brought firmly down on 
the front of the chest so as to expel the air and cause expiration. 
The disadvantages of this method are (a) That the tongue falls 
back and closes the entrance to the lungs; it must therefore be 
held forward by an assistant, (b) No opportunity is given for 
any water, which may have been drawn into the lungs, to escape, 
(c) The liver is liable to be ruptured, (d) It requires a good deal 
of muscular effort on the part of the operator. RESPIRATION 
95 
3. In the Howard Method the subject is laid on his back, and 
pressure is brought to bear upon the lower part of the chest causing 
expiration. When this pressure is withdrawn, the ribs, on account 
of their elasticity, return to their original position leading to 
inspiration. The disadvantages here are the same as in the 
Sylvester method except that it is not so hard on the operator. 
4. The Schafer Method is the latest and the best. "It consists 
in laying the subject in the prone position (face downwards) 
preferably on the ground, with a thick folded garment underneath 
the chest and epigastrium (pit of stomach). The operator puts 
himself athwart or at the side of the subject facing his head (see 
diagram) and places his hands on each side over the lower part of 
the back (lowest 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 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."-Schafer. 
The advantages of the Schafer method over the others are: 
1. It produces a greater exchange of air than any of them--much 
more in fact than takes place in ordinary quiet breathing. 2. The 
tongue falls forward and does not close the entrance to the lungs. 
3. Any water which has been drawn into the lungs has an oppor- 
tunity to flow out by the mouth. 4. It can be carried on for hours 
without fatigue. 5. There is little danger of rupturing the liver. 
The movements above described should be kept up at the rate 
of about 15 per minute for one hour before hope is abandoned, 
unless natural respiration returns earlier, or until rigor mortis has 
set in. 
The most important thing to be remembered is that artificial res- 
piration should be started without an instant's delay. DIGESTION 
All food substances may be divided into five chemical groups, 
viz. :•-proteins, carbohydrates, fats, salts and water and the first 
three of these must be digested in the alimentary canal and pre- 
pared for assimilation by the tissues; none of them are ready for 
use when taken into the body except the inorganic salts and water. 
This digestion or chemical transformation is carried out by se- 
cretions which are poured into the alimentary tract at different 
levels, the active agents being certain enzymes or ferments which 
they contain. 
Divisions of Alimentary Canal 
Mouth 
Pharynx 
Oesophagus or gullet 
Stomach 
Small Intestine 
Duodenum, first 10 or 12 inches. 
Jejunum, first 2/5 of remainder. 
Ileum, last 3 /5 of remainder. 
Coecum and Vermiform Appendix. 
Ascending Colon. 
Hepatic flexure. 
Transverse Colon 
Splenic flexure. 
Descending Colon. 
Sigmoid flexure. 
Large Intestine 
Colon 
Rectum. 
The liver and pancreas are important glands that pour their 
secretions into the small intestine. 
SALIVA 
The first secretion which acts on the food is the saliva. (For 
composition see board). It is poured into the mouth by three 
pairs of salivary glands-the parotid, the submaxillary and the 
sublingual-and by numerous small glands lining the mouth 
cavity termed buccal glands. 
Secreting Glands-At this point a word might be said about 
the structure and functions of secreting glands in general. They 
consist of saccular or tubular depressions on a free surface 
and are lined by a single layer of modified epithelial cells, arranged 
around a central cavity or lumen. The cells have the power of 
96 Fig- 9 98 
HUMAN PHYSIOLOGY 
taking from the blood or lymph which bathes them the materials 
out of which they manufacture the particular substance or sub- 
stances characteristic of the secretion, and this is poured out onto 
the surface, through the communicating channel which is termed 
the duct of the gland. According to their complexity they are 
classified as simple and compound; we have simple saccular and 
compound saccular glands, simple tubular and compound tubular 
glands. 
Salivary Glands 
The three paired glands just mentioned-parotid, submaxillary, 
sublingual-are compound tubulo-saccular glands, each provid- 
ed with a duct. They are contained in a connective tissue capsule 
and are subdivided into lobes and lobules by septa (strands of 
connective tissue) which pass in from the capsule. 
The gland cells, just like muscle fibers, are supplied with nerves 
stimulation of which leads to activity of the cells and a pouring 
out of secretion. The salivary center consists of a group of nerve 
cells situated in the medulla oblongata and from these nerve fibers 
pass to the glands. The submaxillary gland, for example, receives 
fibers from the chorda tympani (a branch of the 7th cranial nerve); 
if this nerve be divided and its peripheral end stimulated, saliva 
will flow from the duct in great abundance. 
All nerve centers are capable of being excited reflexly, and the 
secretion of saliva normally is a reflex act. The stimulus comes 
from the sapid substances in the mouth which excite the afferent 
nerve fibers going to the center. The salivary center in the 
medulla oblongata is also under the influence of the higher brain 
centers, as in the case of the heart and blood-vessels, etc., and so 
the mere thought of food may cause the "mouth to water." 
Emotion of a different kind, such as fear, may have an inhibitory 
influence on the secretion of saliva, and cause the mouth to become 
dry. 
In the resting stage the cells become loaded with granules 
which are discharged during functional activity. 
Physical and Chemical Characters of Saliva 
It is a colorless or opalescent, viscid fluid, slightly alkaline in 
reaction. Its characteristic constituent is the ferment ptyalin 
which has the power of transforming starch into sugar. The 
nature of this transformation is hydrolytic, that is, the molecule 
of starch takes up water and splits up into the simpler molecules 
of dextrin and maltose. Dextrin is an intermediate product; 
the final product is maltose, a reducing sugar. If a warm solution 
of starch be taken into the mouth and held there for a minute, at 
the end of that time all the starch will have disappeared and a 
reducing sugar will be present. DIGESTION 
99 
Ptyalin is destroyed by free mineral acid, such as HC1, even in 
great dilution. It acts most rapidly and most effectively at the 
body temperature; if heated to 60°C. it is destroyed. 
The functions of saliva are partly chemical and partly mechanic- 
al. Its chemical function is to transform starch into sugar; its 
mechanical function is to moisten the mouth and gullet and so to 
assist in speaking, chewing, and swallowing. Its slimy character 
due to the presence of mucin is important in this relation. It also 
serves to dissolve sapid substances in the food so that they can 
be tasted. In the carnivora, where there is no carbohydrate in the 
food, ptyalin is absent from the saliva and it has merely a mechan- 
ical function. 
MASTICATION AND DEGLUTITION 
Mastication or chewing the food is brought about by the biting 
and grinding action of the teeth. This is assisted by the tongue 
pressing the food against the hard palate, and also by the cheek 
muscles which keep pushing the food between the teeth. In the 
process of mastication the salivary glands are very active, and the 
saliva secreted is intimately mixed with the comminuted food. 
Carnivorous animals bolt their food almost un ch ewed, while 
vegetable feeders masticate very thoroughly, especially the rumi- 
nants. In these animals--cud-ch ewers-it is first swallowed 
hurriedly and then brought up again and chewed thoroughly. 
This is important in herbivorous animals where there is much 
starch in the food to be converted into sugar. In man, who is a 
mixed feeder, it is also important that the food be well chewed. 
Deglutition, by which the food, after mastication, is transmitted 
from the mouth to the stomach, is a complicated process. It is 
brought about by the co-ordinated-action of the muscles of the 
tongue, soft palate, pharynx (throat), larynx and gullet. The 
particles of masticated food are collected into a bolus which is 
pushed back into the pharynx by squeezing the tongue against 
the hard palate or roof of mouth. This is voluntary. As it passes 
through the pharynx, the glottis and the openings into the nose 
are closed reflexly to prevent the food entering the lungs or coming 
out through the nose. From the pharynx it passes into the upper 
end of the gullet where it is seized by this muscular tube and 
pushed into the stomach by a peristaltic movement. This is in- 
voluntary. If the muscular coordination is disturbed so that the 
glottis is not closed at the proper time some food particles will 
enter the trachea and give rise to a fit of coughing. 
In man, ordinarily, gravity assists swallowing, but inmost other 
animals, which eat off the ground, swallowing takes place against 100 
HUMAN PHYSIOLOGY 
gravity. Water and other fluids are shot down through the gullet 
very quickly. 
DIGESTION IN THE STOMACH 
The stomach is a saccular dilatation on the alimentary canal for 
receiving and holding the food until it is sufficiently prepared for 
being passed on into the small intestine where digestion is com- 
pleted. It has two openings, the cardiac and the pyloric. The 
cardiac opening communicates with the gullet or oesophagus, a 
muscular tube lined by mucous glands through which the food 
reaches the stomach, and the pyloric opening leads into the 
duodenum. 
Externally it is covered by peritoneum reflected from the 
abdominal wall. The peritoneum is a serous membrane similar 
to the pericardium and pleura, already considered, and is lined 
by a single layer of endothelium. It extends over the abdominal 
wall and is reflected over the abdominal viscera as is the pleura over 
the lungs so that it forms the outer layer of each of them. 
Inside the peritoneum is found the muscular coat which con- 
sists of three layers of non-striped muscle, more or less distinct 
from each other. In the outer layer the fibers are arranged 
longitudinally, i. e. from the cardiac to the pyloric openings, in 
the MIDDLE LAYER CIRCULARLY, and in the INNER LAYER OBLIQUELY. 
The last is incomplete and unimportant. At the pylorus the circu- 
lar layer is greatly thickened and the fibers are so arranged as to 
form a sphincter which closes the opening until the muscle 
relaxes. A similar arrangement is found at the cardiac opening. 
Next to the muscular comes the submucous coat composed of 
loose connective tissue. Here are found the larger blood vessels, 
lymphatics and nerves which supply the muscular coat and the 
mucous membrane. 
The mucous membrane forms the inner layer of the stomach 
wall. It consists of a great number of small glands closely packed 
together side by side which open on to the surface. They are 
supported by a small amount of connective tissue and are sepa- 
rated from the submucous coat by a thin layer of non-striped muscle 
termed the muscularis mucosae which sends a few strands up 
between the glands. 
The stomach is divided more or less artificially into four regions 
(see diagram). The cardia or cardiac region lies immediately 
around the cardiac orifice; the fundus is the blind rounded end 
of the stomach to the left of the cardia; the part between the 
fundus and a transverse band situated towards the pyloric end 
forms the body of the stomach, sometimes termed the prepyloric 
Structure of the Stomach DIGESTION 
101 
region, and the part between this band and the pylorus is known 
as the pyloric region. 
Gastric Glands--Three varieties of glands are found in the 
mucous membrane of the stomach-cardiac, fundic and pyloric 
glands. The cardiac glands, found in the cardia, are simple 
tubular glands formed of columnar epithelial cells. The fundic 
glands, which occupy the mucous membrane of the fundus and 
most of the prepyloric region, are compound tubular glands two 
or three tubules opening into a single duct through which the 
secretion is discharged on the surface. The duct is lined by colum- 
nar epithelial cells which secrete mucin while the deeper lying tubules 
contain two varieties of cells, central and parietal. The central 
cells lie next to the lumen; they are cubical or low columnar 
epithelial cells and granular as distinguished from those lining the 
ducts which are clear like all mucin secreting cells. The parietal 
cells lie between those just mentioned and the walls of the tubule; 
these are oval or spherical and more distinctly granular than the 
central cells. They do not form a continuous layer. The pyloric 
glands, found in the pyloric region, are also of the compound 
tubular variety. They have long ducts lined with clear columnar 
cells and the secreting tubules contain cells corresponding to the 
central cells of the fundic glands and no parietal cells. 
The parietal cells of the fundic glands are believed to secrete 
the HC1 of the gastric juice and the central cells of the fundic 
glands, along with these of the pyloric glands, the pepsin. As in 
all other secreting glands, during the resting stage granules accumu- 
late in the cells and are discharged during the active stage. These 
granules do not consist of the ferment pepsin, however, but of a 
pro-substance which might be called pro-pepsin or pepsinogen 
and which is not active as a digestive agent until, in the process of 
secretion, it is transformed into pepsin. 
Blood-vessels and Nerves of the Stomach-The stomach has a 
very rich blood supply. When it is empty and inactive the mucous 
membrane is pale because the arterioles are constricted, but when 
food enters the stomach these vessels are reflexly dilated, allowing 
more blood to reach the secreting glands, and the surface becomes 
very red. 
The secretory nerve fibers which supply the gastric glands 
reach the stomach through the two vagi nerves, right and left. 
They come from a group of nerve cells situated in the medulla 
oblongata which forms the gastric secretory center. Like those 
nerve centers already studied it is capable of being acted on from 
the periphery reflexly, and from the higher brain centers in the 
cerebrum. 102 
HUMAN PHYSIOLOGY 
Gastric Juice 
The gastric juice or stomach secretion may be obtained free 
from food by establishing, in an animal (dog for example), a 
gastric fistula, that is an opening into the stomach through the 
abdominal wall; this may be temporary or permanent. Occasion- 
ally this procedure is necessary in the human subject, where the 
gullet has been permanently closed from swallowing a corrosive 
poison. In order to prevent the patient from dying of starvation 
an opening (permanent fistula) must be made in the stomach 
through which food can be introduced. 
The fluid so obtained, is clear, colorless and odorless, with an 
acid reaction. (For composition see board). Its characteristic 
CONSTITUENTS ARE THE FERMENT PEPSIN AND FPEE HYDROCHLORIC 
acid. The latter varies from about 0.2% in man to 0.6% in car- 
nivorous animals such as the cat and dog. Afore than 99% is 
water. 
The hydrolytic action of the ptyalin of saliva on starch takes 
place, as we have seen, only in a slightly alkaline or neutral 
medium. This is stopped at once by free HC1 but this does not 
mean that carbohydrate digestion ceases in the stomach. The 
food mixed with the alkaline saliva, is only penetrated slowly by 
the gastric juice and until the center of the mass has been reached 
some transformation of starch into sugar may still go on. Thus, 
for 20 or 30 minutes after the food enters the stomach, the digestion 
of any carbohydrate which may have escaped the action of ptyalin 
in the mouth may be completed in the stomach. Free HC1 
destroys the ptyalin so that it can't resume its action in the 
alkaline secretions of the small intestine. 
Digestive Action of Gastric Juice 
Gastric juice acts mainly on the proteins of the food and scarcely 
at all on the fats and carbohydrates. The carnivora have a more 
powerful gastric secretion than other animals; their diet consists 
mainly of proteins. 
The digestive action of gastric juice can be studied by making 
a mixture of HC1 and pepsin in glass vessels kept at the body 
temperature. This artificial gastric juice transforms coagulable 
proteins, such as egg albumin, meat or fibrin, first into soluble 
globulin, then into acid metaprotein, then into primary pro- 
teoses, then secondary proteoses and finally peptones. The 
process is again one of hydrolysis; water is added, and then the 
complex protein molecule splits up into two simpler molecules, and 
so on until finally the peptones are reached. 
The protein splitting proceeds no farther than the 
PEPTONE STAGE IN THE STOMACH. DIGESTION 
103 
The process of digestion can be studied in the human subject 
by giving a test meal of bread and meat, etc., and withdrawing 
the contents for examination at definite intervals by means of the 
stomach tube. In this way the digestibility of different varieties 
of the protein food substances may be investigated. The results 
are practically the same as those obtained by the method of 
artificial digestion. 
In cases of weakened digestion or prolonged starvation, it is 
advisable to give peptonized food, that is, food that has already 
been artificially digested to the petone stage; or pepsin and HC1 
in the proper quantities may be given along with the food. The 
reason for this procedure in cases of starvation, for example, is 
that the gastric glands are unable to produce any secretion in 
which case the food would remain in the stomach undigested. 
Action of Gastric Juice on Carbohydrates and Fats-The di- 
gestion of carbohydrates takes place almost entirely in the mouth 
and small intestine, and of fats in the small intestine. The action 
of the gastric juice on these two classes of food substances is of 
little importance. 
Cane sugar undergoes inversion into equal molecules of glucose 
and fructose. 
On fats the main action is to dissolve the connective tissue frame 
work of the groups of fat cells, and also the envelopes of these cells, 
so that the fat contained in them is set free in the stomach and is 
ready to be acted on in the small intestine. 
Gastric juice has the power to curdle milk and this is due to the 
presence of a ferment termed rennin which is produced by a 
zymogen which is activated by hydrochloric acid. 
MECHANISM OF GASTRIC SECRETION 
Nervous Factor-The secretion of the gastric juice is under the 
control of a nerve center just as is the salivary secretion, and the 
secretory fibers pass to the stomach in the two vagi nerves. If an 
opening is made into the stomach, through which its interior can 
be inspected, and then if one vagus nerve be divided below the 
point of origin of the cardiac branches, and the peripheral end 
stimulated, gastric juice will be poured out by the fundic and 
pyloric glands, after a latent period of about five minutes. 
The gastric secretory center in the medulla oblongata is capable 
of being excited reflexly from the mouth. This was proved by the 
following experiment first performed by Pawlow: He divided the 
oesophagus of a dog in the neck and brought the two ends to the 
surface, stitching them to the skin. At the same time he made a 
permanent opening (fistula) in the stomach through which he could 
watch the mucous membrane. When the dog masticated and 104 
HUMAN PHYSIOLOGY 
swallowed food it never reached the stomach, but escaped to the 
outside, through the upper oesophageal opening. This Pawlow 
termed "Sham-feeding;" the dog could go on eating for hours and 
still have an appetite. 
It used to be supposed that contact of the food with the gastric 
mucous membrane was necessary to excite secretion, but Pawlow 
observed that a copious flow of gastric juice took place while 
the food was being chewed in the mouth and as long as the sham- 
feeding went on this flow continued. In some cases he was able to 
collect almost 1000 c.c. of pure gastric juice, perfectly free from 
food, in this way, in a few hours. He then divided both vagi 
nerves and found that the secretion stopped although the sham- 
feeding was still continued. 
Here the afferent impulses reach the gastric secretory center 
through the nerves of taste the exciting agent being the sapid 
substance in the mouth. This stimulates the center which in 
turn sends impulses to the gastric glands through the vagi. In- 
terruption of the reflex arc at any point, e. g. by cutting the vagi, 
will stop the secretion. 
Psychical Secretion-Pawlow also observed that if the dog 
were hungry the mere sight of food was enough to cause a flow 
of gastric juice. In this case the stimulus reached the secretory 
center in the medulla oblongata from the higher brain centers, 
and this he called "psychical secretion." 
Pawlow found that in order to produce the reflex secretion the 
food must be agreeable and appetizing; sand or pebbles placed 
in the mouth produced no affect at all on gastric secretion. 
Similar observations have been made on human beings where it 
was necessary to establish a permanent opening in the stomach, 
through the abdominal wall, because of occlusion of the oesophagus, 
and the results agree with those obtained in the dog. When the 
food was swallowed it did not enter the stomach but was regurgi- 
tated back into themouth, nevertheless, an abundant secretion took 
place while mastication was going on. 
Chemical Factor'--The nervous stimulation does not account 
for all the secretion produced during the digestion of a meal, 
however. If a permanent fistula be made in the stomach of a dog, 
and if both vagi be divided so as to cut off completely all nervous 
influence, when meat is introduced into the stomach, through the 
opening, after an interval of some minutes, gastric juice will begin 
to flow. Mechanical stimulation of the mucous membrane, such 
as tickling it with a feather or the introduction of sand, will not 
produce any flow. If bread or starch or egg albumen be used, 
instead of meat, no secretion will result. There must, therefore, 
be some substance in the meat which has the power to excite the 
gastric glands that is not found in bread, etc. If partially digested DIGESTION 
105 
bread be introduced into the stomach secretion does follow, how- 
ever, so it would appear that the exciting substance is produced in 
the early stages of the digestion of bread. 
Secretogogues-Any substance which is capable of stimulating 
the gastric glands is termed a secretogogue, and it is found that 
all foods may be divided into two classes in this connection; 
1, those that contain ready made sceretogogues when they enter 
the stomach, and 2, those that do not contain secretogogues 
ready made but which produce them in the early stages of di- 
gestion. To the first group belong all meats, meat extracts and 
soups; to the second, bread, starchy foods, white of egg, etc. 
Gastrin-These secretogogues do not act directly on the fundic 
and prepyloric glands; they stimulate the glands at the pyloric 
end to produce a hormone termed gastrin or gastric secretin. 
This substance does not escape into the stomach but is absorbed 
into the bloodvessels and carried in the blood stream to the glands 
of the fundus and prepylorus which it excites and which then 
pour out the gastric juice. 
The hormones to which this gastrin belongs form a very im- 
portant group of substances which will be considered later, in con- 
nection with intestinal digestion. 
The normal mechanism of gastric secretion then seems to be 
somewhat as follows: The secretion of gastric juice is first started 
by the presence of food in the mouth, and by the pleasant senations 
and feelings associated with the act of eating (Psychic secretion), 
the gastric secreting center being acted on by the higher cerebral 
centers and reflexly from the mouth. This ensures, at least, the 
beginning of gastric digestion. Then the secretion is further 
carried on by the secretogogues of the food in the stomach; 
meats and soups contain these ready made, as we have seen, and 
in the case of other foods they are produced in the early stages of 
digestion. These act indirectly through the hormone gastrin. 
Once started the chemical secretion is continued as long as the 
stomach contents require it. 
Although meat (proteins) and starchy foods (carbohydrates), 
partially digested, stimulate the secretion of gastric juice, fat ap- 
pears to have a tendency to inhibit it. 
According to the researches of Pawlow it would appear that the 
character of the food determines both the quantity and the quality 
or strength (percentage of HC1 and pepsin present) of the juice, 
although the quantity is also related to the amount of food to be 
digested. The secretion produced by bread, although smaller in 
quantity than that caused by meat, has a stronger digestive power. 
In fact, the juice secreted is that best adapted to digest the par- 
ticular kind of food substance which produced its secretion. 106 
HUMAN PHYSIOLOGY 
The gastric juice has antiseptic properties because of the 
hydrochloric acid present, so that putrefactive changes do not 
take place in the normal stomach. Also, pathogenic micro-organ- 
isms, swallowed in the food, may be destroyed in the stomach on 
this account. 
Movements of the Stomach 
In seed-eating birds, which are not provided with teeth, the 
contractions of the muscular walls of the stomach or gizzard are 
very powerful and serve to grind the food; this takes the place of 
mastication in the mouth. In man the muscular contractions are 
too feeble to produce any effect of this kind, and it is not required, 
if the food has been properly chewed; what these contractions do 
accomplish is to move the food towards the pyloric end. 
The movements of the stomach are best studied with the aid of 
the X rays after a meal containing some salt of bismuth has been 
taken. 
After the food enters the stomach the fundic end becomes 
tonically contracted and presses the contents towards the 
pylorus. When the digestion of part of the food has reached the 
secondary proteose stage, i. e. about twenty minutes after the food 
has entered the stomach, a local constriction of the wall appears 
in the prepyloric region, and a peristaltic wave moves toward 
the pylorus sweeping this portion of the partially digested food or 
chyme in front of it. [Chyme is the term applied to the partially 
digested contents of the stomach which pass into the duodenum.] 
When it reaches the pylorus the sphincter opens and lets it through 
into the duodenum, after which the sphincter closes again. 
As digestion proceeds this is repeated every few minutes until the 
stomach is empty. 
The opening and closing of the pyloric sphincter muscle is 
reflex; acid on the stomach side opens it and acid on the duo- 
denal side closes it. 
The time taken to empty the stomach varies according to the 
size and digestibility of the meal, the activity of the juice, and the 
state of the mind , and body of the individual. Under normal 
conditions it varies from three to five hours. 
Water, which does not require to be digested, is sent into the 
duodenum in jets by the same mechanism very shortly after it 
enters the stomach. 
Nervous Control of Movements-While the movements of the 
stomach are automatic they are controlled from two centers in 
the nervous system, the excitatory fibers being contained in the 
vagus and the inhibitory fibers in the splanchnic nerves. Cannon 
found that in cats the emotions of rage and fear put an end to all 
stomach movements and the same probably occurs in man. DIGESTION 
107 
It is found from experience that digestion proceeds best in the 
absence of severe mental and muscular exercise, the reason being 
that when actively secreting the stomach needs much blood, and 
under the conditions above mentioned, this is taken by the brain 
and the muscles. 
Very little absorption (passage of material into the blood or 
lymph) takes place in the stomach; the intestine is the great organ 
of absorption. 
Hunger Contractions--After the stomach has been empty for 
some time "hunger pains" are felt. These are believed to be 
caused by more powerful contractions of the stomach wall than 
those already described, since they appear simultaneously with the 
hunger sensations. Studying these hunger contractions on a man 
with a permanent gastric fistula, Carlson finds that they are 
arrested by mechanical stimulation of the gastric mucous mem- 
brane, or by the chewing or tasting of palatable food. 
Vomiting-In vomiting the contents of the stomach are forcibly 
expelled through the cardiac orifice into the oesophagus and 
mouth. This is brought about mainly by the action of the dia- 
phragm and abdominal muscles but the stomach itself is not 
passive; an antiperistaltic wave appears and sweeps the contents 
towards the cardiac sphincter, which opens reflexly and allows them 
to escape, the pylorus at the same time being tightly closed. 
The retching movements associated with the act of vomiting 
are produced as follows: A deep inspiration is taken and the glottis 
closed to keep the diaphragm depressed; then the abdominal mus- 
cles are powerfully contracted, to compress the stomach against 
the diaphragm, and so squeeze out its contents. 
Vomiting Center-Vomiting is involuntary and reflex. The 
nerve center controlling it is situated in the medulla oblongata, 
and afferent nerve fibers, from various sources, convey impulses 
to this center which excite it. Efferent impulses are then dis- 
charged from the center to the diaphragm, muscles of the ab- 
dominal wall and of the stomach wall, and the act is brought about. 
Causes of Vomiting-Irritation of the mucous membrane of the 
stomach is the most common cause; the afferent or sensory nerve 
fibers, which run in the vagus, are excited and reflexly stimulate 
the vomiting center. Stimulation of other sensory surfaces, how- 
ever, will bring it about, e. g. tickling the mucous membrane at 
the back of the throat. 
Disturbances in the sense of equilibrium by impressions 
coming in from the semicircular canals frequently result in vomit- 
ing, e. g. in sea-sickness and in air-sickness. The vomiting center 
is also acted on from the cerebrum since the mere recollection of 108 
HUMAN PHYSIOLOGY 
a disagreeable sensation, such as a disgusting odor, will induce the 
act. 
Vomiting is usually, but not always, preceded by a feeling of 
nausea and accompanied by a flow of saliva into the mouth. 
During the retching movements which follow, the openings into 
the trachea and nasal cavities are closed reflexly, as in the case of 
swallowing. 
Emetics are substances which induce vomiting. Some act by 
irritating the gastric mucous membrane, e g. mustard; others, such 
as tartar emetic and apomorphine by exciting the vomiting center 
in the medulla oblongata to which they are carried by the blood 
stream. The latter are more effective when injected hypodermically 
than when taken by the mouth. 
The intestinal tube, in the human subject, is about 30 feet in 
length--small intestine about 23 feet and large intestine from 5 
to 6 feet. The ileo-colic valve, which is a fold of the mucous 
membrane that plays the part of a valve, and allows the intestinal 
contents to pass in only one direction, separates the small from 
the large gut, and the sphincter ani closes tho external opening of 
the latter. 
DIGESTION IN THE INTESTINE 
Small Intestine 
The Small Intestine is divided, more or less artificially, into 
three parts. The first 10.or 12 inches next to the stomach is 
termed the duodenum, the first two-fifths of the remainder the 
jejunum and the last three-fifths the ileum. 
Structure-In general it resembles the structure of the stomach. 
It consists of four coats. Externally is the peritoneum, and next 
to this the muscular coat consisting of an outer longitudinal 
and an inner circular layer, both composed of plain muscle fibers. 
Then comes the submucous coat made up of connective tissue 
which carries the larger blood-vessels, nerves and lymphatics. 
The innermost coat is the mucous membrane which contains the 
secreting glands; between it and the submucous coat is a layer of 
non-striped muscle termed the muscularis mucosa? which sends 
some strands into the villi and between the glands of the mucosa. 
Between the two layers of the muscular coat is found a network of 
nerve fibers and cells termed Auerbach's plexus, and a similar 
network-Meissner's plexus-lies in the submucous coat. 
Valvulae Conniventes-When the two are separated, the tube of 
mucous membrane is longer than the muscular tube in which it is 
placed, and so the former is thrown into a series of transverse 
folds termed valvula? conniventes. They serve to increase the DIGESTION 
109 
surface of the mucous membrane. They begin about two inches 
beyond the pylorus and disapppear about the middle of the ileum 
so that the lower part of the small intestine is free from them. 
Villi-Examined under the microscope a section of the small in- 
testine shows long slender processes projecting from the free surface 
of the mucous membrane into the lumen of the gut. These are 
termed villi and also serve to increase the surface. Each villus is 
lined on the outside by a single layer of columnar epithelial 
cells with a striated hem at the free border; here and there clear 
mucin secreting cells, sometimes termed goblet or chalice cells, 
are scattered amongst the others. In the center of the villus is a 
lymphatic vessel with a blind end termed a lacteal and in close 
relation to it is a network of small blood-vessels. Strands of 
plain muscle fibers pass up into the villus from the muscularis 
mucosie and there is a loose connective tissue frame work with 
many leucocytes scattered throughout. 
Glands of Small Intestine-The mucous membrane contains a 
large number of simple test-tube like glands lying closely packed, 
side by side, in the spaces between the villi and lined by a single 
layer of cubical or low columnar epithelium, along with some chalice 
cells, continuous with that covering the villi; these are termed the 
glands or follicles of Lieberkuhn, and they are found throughout 
the whole extent of the small and large intestine until the rectum 
is reached. In the duodenum in addition to the glands of Lieber- 
kuhn there is a number of ■''mall compound tubulo-saccular glands 
situated in the submucous coat; their ducts pierce the muscularis 
mucosae and open on the surface of the mucous membrane; these 
are termed Brunner's glands. 
Solitary Glands and Peyer's Patches-Throughout the mucous 
membrane there are scattered, at irregular intervals, collections 
of lymph follicular tissue similar in structure to the tonsil; they 
are termed solitary glands. In certain regions, particularly the 
lower part of the ileum, the follicles are aggregated together to 
form larger masses w'hich are known as agminated glands or 
Peyer's patches. They measure half an inch or more in width 
and from one to four inches in length, and number in all about 
thirty. They are very prominent in young people and may be 
absent altogether in the aged. They do not occur in the large 
intestine. 
Large Intestine 
The Large Intestine begins at the ileo-colic (or ileo-coecal) 
valve where the small intestine opens into it, which it does from the 
side, at some distance from its blind end. This blind end is termed 
the coecum and from it there projects a tube-like process, about 110 
HUMAN PHYSIOLOGY 
4 inches in length, and about the thickness of a pencil, the vermi- 
form appendix. 
The large intestine is divided into the coecum with vermiform 
appendix, the colon and the rectum. The colon begins at the right 
side of the abdomen, a little above the inguinal or groin region, 
and is subdivided into the ascending, transverse and descending 
colon with the heptic, splenic and sigmoid flexures. The ascending 
colon runs upwards until it meets the lower surface of the liver; 
here it bends to the left forming the hepatic flexure and crosses 
the abdominal cavity at a slightly lower level than the stomach; 
this division is called the transverse colon. When it reaches the 
spleen, to the left of the stomach, it turns downwards at the 
splenic flexure and is now known as the descending colon; it 
terminates in the left groin, in the sigmoid flexure, which is con- 
tinued into the rectum. 
Structure of Large Intestine-In structure the large intestine 
resembles the small in most respects. It has the same number of 
coats-peritoneal, muscular, submucous and mucous-but the 
lumen is much larger. The outer or longitudinal layer of the 
muscular coat is not continuous all the way around, but is gathered 
into three bands, which are shorter than the walls of the tube 
itself, and this leads to a characteristic puckering of the large 
intestine. These longitudinal muscular bands are not present in 
the rectum, where the outer layer is continuous. At the external 
opening of the rectum (the anus) the muscular coat is increased 
in thickness so as to form a sphincter similar to that at the pvlorus. 
This is termed the sphincter ani; it is normally in a state of tonic 
activity and keeps the orifice firmly closed, unless when it is 
voluntarily relaxed, in the act of defecation. 
In the large intestine there are no valvulge conniventes and no 
villi. The mucous membrane is studded with Lieberkuhn's follicles 
similar to those of the small intestine except that they contain a 
greater number of mucin secreting or goblet cells. 
There are two large and important glands connected with the 
intestine, viz.: the pancreas and the liver. 
Structure of Pancreas 
The pancreas or sweetbread is an elongated, soft organ, of a 
pinkish color, lying along the lower border of the stomach, and 
extending from the spleen to the duodenum. It is a compound 
tubular gland, very similar in structure to the parotid (one of 
the salivary glands). It is surrounded by a connective tissue 
capsule which sends in septa subdividing the gland into lobes 
and lobules. The secretory alveoli open into the lobular 
ducts, these into the lobar ducts and these again into the main 
duct which enters the duodenum from three to four inches below DIGESTION 
111 
the pylorus, along with the common bile duct from the liver. 
In some animals, e. g. the dog, there are two pancreatic ducts. 
Islets of Langerhans-In addition to those lining the secreting 
tubules, other groups of epithelial cells are found, scattered 
throughout the gland, which are known as the islets of Langer- 
hans. They are believed to produce an internal secretion which is 
passed directly into the blood or lymphatic vessels and has no 
relation at all to the process of digestion. They are believed to 
produce the substance insulin which is essential for the proper 
metabolism of sugar and the want of which leads to diabetes 
mellitus. This function of the pancreas will be taken up later. 
The whole organ has a rich blood supply, particularly the islets 
of Langerhans. 
Structure of Liver 
The liver is the largest gland in the body and weighs from 50 to 
60 ounces in the adult. It is of a darkish red color and lies immedi- 
ately under the diaphragm, the greatest mass of it being to the 
right side of the middle line. It is surrounded by a delicate con- 
nective tissue capsule from which septa pass inwards dividing the 
organ into a large number of minute polyhedral lobules. Each 
lobule, which is a miniature liver in itself, has a vein in its center 
and is made up of a great number of hepatic cells arranged in rows 
radiating from the central vein, the adjacent rows being separated 
by blood capillaries. In these cells the bile capillaries take origin. 
Blood Supply of Liver-As we have already seen when dealing 
with the circulation, the blood from the stomach, upper part of 
intestine, pancreas and spleen enters the liver by the portal vein. 
This branches like an artery and ultimately breaks up into the 
intralobular capillaries just mentioned. From these the blood 
passes into the central or intralobular vein, then into the hepatic 
veins which open into the inferior vena cava. Many important 
changes are effected in the portal blood in its passage through the 
liver. 
The hepatic artery is a small vessel, a branch of the aorta, which 
carries blood to the liver to nourish its connective tissue frame work. 
It corresponds in function to the coronary arteries of the heart, and 
the bronchial arteries in the lungs. 
The bile capillaries take origin in the hepatic cells, and in each 
lobule they form a network of very fine vessels. Through these 
capillaries the bile, which is the secretion of the liver, formed by the 
hepatic cells, finds its way into the smaller bile ducts at the 
periphery of the lobules, and from these into larger and larger 
ducts until at last it escapes from the liver, through the hepatic 
duct, and passes either into the gall bladder through the cystic 
duct, or into the duodenum through the common bile duct, the last 
being formed by the union of the hepatic and cystic ducts. 112 
HUMAN PHYSIOLOGY 
Digestive Secretions that Enter the Small Intestine 
The secretions poured into the small intestine are three in number: 
1, the pancreatic juice from the pancreas; 2, the intestinal juice 
(succus entericus) from the glands of the small intestine; 3, bile 
from the liver. 
This may be obtained pure by making a pancreatic fistula, 
i. e. tying a glass tube into the duct, in an anaesthetized animal 
such as a dog, and collecting the juice in a beaker as it flows 
(temporary fistula); or by stitching the orifice of the duct to an 
opening in the abdominal wall and allowing the wound to heal 
(permanent fistula). In the latter case the secretion will be poured 
out on the exterior and may be caught by attaching a clean sponge, 
or a vessel of some kind, to the abdomen. 
It has also been collected in the human subject in cases of tu- 
mour, involving the duodenum, where it was necessary to drain off 
the secretion to the exterior. 
Characters of Pancreatic Juice-It is a clear, colorless, slightly 
viscous fluid, somewhat resembling saliva, with a strongly alkaline 
reaction due to the presence of sodium carbonate. Its important 
constituents are three enzymes or ferments: 1, steapsin, (pancre- 
atic lipase) or fat-splitting ferment; 2, amylopsin, (pancreatic 
amylase) or starch-splitting ferment; 3, trypsin, (pancreatic pro- 
tease) or protein-splitting ferment. The pancreatic juice therefore 
has an action on all three classes of food stuffs. There is 
also present pancreatic rennin, lactase and maltase. 
Action on Fats-The neutral fats (stearin, palmitin and olein) 
are compounds (esters) of glycerin in which the three hydroxyl 
groups are replaced by fatty acid radicles (For formulse see board). 
Under the influence of the ferment lipase the fats are hydrolyzed, 
that is, water is added and the molecule then splits up into glycer- 
in and the corresponding free fatty acid. 
A portion of the fatty acid produced in this way combines with 
the alkali sodium carbonate to form soaps which, acting along with 
the bile, helps to emulsify the fat still undecomposed, that is, to 
break it up into fine globules so that it is more easily reached by the 
ferment. At one time it was believed that this emulsification was 
all that was necessary and that the fine fatty globules were absorbed 
as such by the intestinal epithelium, but now it is held that all the 
fat is decomposed into glycerine and free fatty acid which are 
absorbed, and in the process of absorption, while still within the 
epithelial cells, are recombined to form again the neutral fats 
characteristic of the animal. 
Action on Carbohydrates-The action of the ferment amylopsin 
is believed to be identical with that of the ptyalin of saliva; it 
Pancreatic Secretion DIGESTION 
113 
converts starch into maltose with dextrin as an intermediate 
product. Starchy foods, therefore, which have escaped digestion 
in the mouth and stomach, are completely transformed into the 
reducing sugar maltose in the intestine. The maltose is then acted 
on by the ferment maltase, of the succus entericus, and changed to 
glucose, the form in which all carbohydrates are absorbed into the 
blood. 
Action on Proteins--The ferment trypsin, acting in an alkaline 
medium, transforms coagulable proteins, such as egg albumin, 
into proteoses and peptones like the pepsin of the gastric juice, 
but here the action goes farther than the peptone stage. By a 
process of hydrolysis the proteins are split up into a number of 
substances of low molecular weight belonging to the group of 
amino-acids, examples of which are leucin, tyrosin, asparatic 
acid, and many others. These comparatively simple substances 
(see formulae on board) are absorbed by the epithelium lining the 
intestine, pass into the blood stream directly or through the lacteals, 
and are again built up into the protein molecules of the animal 
which digests them. Where this reconstruction takes place is not 
certain; it may be in the blood or in the tissues themselves, prob- 
ably the latter. 
The action of trypsin is more rapid and more complete following 
peptic digestion. 
Activation of Trypsin-Pure pancreatic juice, as it is obtained 
from a pancreatic fistula, has no digestive action on proteins, but 
when a fragment of mucous membrane from the small intestine is 
added to it, it becomes active. The reason for this is as follows: 
Pancreatic juice when it escapes from the duct contains no trypsin, 
only the zymogen or pro-substance trypsinogen is present which has 
no power to hydrolyze protein. In the small intestine the trypsino- 
gen meets with a ferment enterokinase, contained in the intestinal 
secretion, and this converts the inactive trypsinogen into active 
trypsin which then attacks the protein molecule. 
Changes in Secreting Cells of Pancreas-In the inactive or 
resting stage granules accumulate in the cells lining the tubules; 
these are discharged during secretory activity, just as we have seen 
in the case of the salivary glands. It is probable that these gran- 
ules represent the zymogens or pro-substances which are the pre- 
cursors of the active ferments found in the juice. 
Mechanism of Pancreatic Secretion 
The pancreatic juice does not flow continuously into the small 
intestine. The secretion begins about half an hour after food 
enters the stomach and attains its maximum rate in from two to 
four hours. It flows intermittently until pancreatic digestion is 114 
HUMAN PHYSIOLOGY 
complete, and then it almost ceases. There is little or no flow 
while fasting. 
The stimulus that excites the pancreas to pour out its secretion 
is known to be the presence of acid chyme in the duodenum. 
Dilute acid such as HC1, injected into the duodenum, will produce 
a flow at any time. Until recently this was believed to be simply a 
nervous reflex, through a supposed pancreatic secretory center in 
the medulla oblongata; such, however, is not the case, since 
pancreatic secretion can take place after all the nerves to the 
intestine have been divided, that is, the reflex arc interrupted. 
The stimulation is entirely chemical. What happens is that 
the acid acts on a substance, contained in the mucous membrane 
of the small intestine, termed pro-secretin and converts it into 
secretin which is absorbed into the blood vessels and carried in the 
blood stream to the pancreas, where it excites the cells lining the 
tubules to pour out their secretion. 
This substance secretin can be prepared artificially as follows: 
Scrape the mucous membrane from the duodenum and upper half 
of the jejunum of a recently killed dog and mix it with 100 c.c. 
or 200 c.c. of 0.4% HC1. Allow this to stand for a few hours; 
neutralize, boil and filter; the filtrate will contain secretin. A 
small quantity of this filtrate, injected into a vein, will cause a flow 
of pancreatic juice in about 20 seconds, that is, as soon as it reaches 
the pancreas in the blood stream. 
Hormones 
Secretin, and its action on the pancreas, was discovered by Bayliss 
and Starling and it belongs to the same group as gastrin, which 
we have seen is produced in the pyloric end of the stomach. To 
substances of this class Starling has given the general name of 
hormones or exciters. Many others are now known to exist. 
A hormone may be defined as a chemical substance produced 
IN ONE PART OF THE BODY AND CARRIED IN THE BLOOD STREAM TO 
some other part which it excites to action; for example, 
secretin, produced in the mucous membrane of the small intestine, 
and carried in the blood stream to the pancreas, which it excites 
to action. 
Chemically the hormones are comparatively simple substances 
of low molecular weight and heat stable since they are not de- 
composed by boiling as all ferments are. They are rapidly oxidized 
in the tissues, however, and rendered inert after they have ac- 
complished their purpose. They play a very important r61e in the 
body, taking the place of the nervous system in correlating the 
activity of associated tissues and organs, where there is no great 
necessity for speed. 
It is interesting to observe that in the salivary glands the excit- DIGESTION 
115 
ing stimulus is entirely nervous, in the stomach partly nervous and 
partly chemical, and in the pancreas almost entirely chemical but 
the possiblity of some nervous influence cannot be completely ex- 
cluded. 
Intestinal Secretion 
This is produced by the intestinal glands. To obtain it free 
from the products of digestion a fistula must be made in the 
intestine by one of two methods. By Thiry's method a piece of 
intestine about six inches long is cut out, without damaging its 
bioodor nerve supply; one end is sewed up and the other stitched 
to the abdominal wall. This blind piece of gut is then washed 
out and the secretion collected when it accumulates. In Vella's 
modification of Thiry's method, both ends of the loop are stitched 
to the abdominal wall. 
Pure intestinal juice (Succvs entericus), when thus procured, is 
a turbid fluid, alkaline in reaction, the turbidity being due to leuco- 
cytes and disintegrated epithelium. It contains the following 
important enzymes: 1, Enterokinase which activates trypsinogen; 
2, Erepsin which assists the trypsin of the pancreatic juice in 
splitting up peptones into amino acids; 3, Enzymes, three in 
number, which convert carbohydrates of all varieties into glucose, 
the form in which they are finally absorbed. 
Liver Secretion 
The third secretion which acts on the chyme in the small in- 
testine is bile from the liver. The common bile duct, formed as 
we have seen by the union of the hepatic and cystic ducts, opens 
into the duodenum very obliquely, and this serves as a valve to 
prevent the entrance into the duct of matter from the intestine. 
Unlike the pancreatic juice the bile is being continuously 
secreted by the liver cells, but it does not flow directly into the 
duodenum; it is stored up, during fasting, in the gall bladder, and 
is only discharged when acid chyme comes into the duodenum 
from the stomach; it is not all expelled at once. The cause of this 
expulsion is the reflex stimulation of the muscular walls of the gall 
bladder by the contact of acid with the duodenal mucous mem- 
brane. Some animals, such as the horse and deer, have no gall 
bladder and in these the flow into the intestine is continuous. 
Secretin excites the liver as well as the pancreas. The bile and 
pancreatic juice cooperate in their action on the chyme and they 
are discharged into the duodenum together. 
Bile may be obtained in the usual way by establishing a biliary 
fistula, that is, by stitching the bile duct or the gall bladder to the 
abdominal wall and collecting the secretion as it escapes. It may 
also be secured by emptying the gall bladder of a recently killed 116 
HUMAN PHYSIOLOGY 
animal. In the human subject it is sometimes necessary, in cases 
Qf obstruction of the common bile duct, to establish a biliary 
fistula through the gall bladder. In such cases the amount col- 
lected varies from 500 c.c. to 800 c.c. in the 24 hours. 
Physical Characters of Bile--It is a viscid, yellowish, reddish- 
brown or green fluid, according to the preponderating pigment 
present. It has a peculiar characteristic odor, a bitter taste ("bit- 
ter as gall"), and is alkaline in reaction. The viscidity is due to 
the presence of mucin and this is added to the bile in the gall 
bladder; bile collected from the hepatic duct contains little or no 
mucin. 
Composition of Bile (see board)-The solids total from 2 to 3 
per cent., 97 to 98 percent, being water. The chief constituents 
are the salts of the bile acids (glycocholic and taurocholic acids), 
cholesteroe, bile pigments, lecithin, mucin and mineral salts. The 
composition varies considerably in different samples. The bile 
PIGMENTS ARE DERIVED FROM THE HEMOGLOBIN of broken down red 
blood corpuscles. The usual golden red of human bile is due to 
the preponderance of the red bile pigment or bilirubin, while the 
green color of herbivorous bile is caused by the pigment biliverdin 
which is an oxidation product of bilirubin. Another oxidation 
product is the yellow pigment choletelin. One of the chemical 
tests for bile consists in the production of this play of colors by the 
action of strong oxidizing agents such as fuming nitric acid. 
Functions of Bile'-1. It is alkaline and helps to neutralize 
the acid chyme and so prepare the way for pancreatic digestion, 
which takes place best in an alkaline medium. 2. Another im- 
portant property is its power to emulsify fats, that is, to split 
them up into fine globules so that the ferment steapsin may act 
more readily and effectively on them. If the common bile duct 
is obstructed, preventing the entrance of bile into the intestine, a 
large amount of fat is found free in the excreta because it has not 
been emulsified. The presence of bile in the intestine favors the 
action of all the pancreatic enzymes but more especially that of 
lipase which is due to the emulsifying power of bile salts. The fine 
subdivision of the fat globules enables the lipase to act more 
effectively. 3. It is said to stimulate the peristaltic move- 
ments OF THE LARGE INTESTINE. 
Jaundice-This is most commonly caused by some obstruction 
in the bile duct with the result that the bile, which is normally 
secreted at a low pressure, is dammed back in the liver and ab- 
sorbed by the lymphatics and blood-vessels into the general 
circulation. It is the presence of bile in the blood that gives the 
Seenish yellow tinge to the skin and mucous membrane character- 
;ic of jaundice. DIGESTION 
117 
Movements of the Intestines 
These are of two kinds, peristaltic and segmental. By the former 
the intestinal contents are slowly driven down the gut, from the 
small to the large intestine. The peristaltic movements are pro- 
duced as follows: On the stomach side of a mass of food the 
intestine becomes constricted, by the contraction of its inner 
muscular coat, and on the rectal side of the mass it dilates, an 
example of the "law of the intestine" as shown by Bayliss and 
Starling. In this way the food is moved downwards by these 
waves preceding and following it. The other kind of movement 
(segmental) is due to ring like constriction that take place 
rhythmically at regular distances apart. They do not propel the 
contents downwards, but break them up into segments, so that 
the digestive juices may have more ready access to them. 
It is estimated, from observations made on the human subject 
by the X-rays, that the time required for the passage of the food 
through the small intestine is about 4X hours. This is about the 
time it takes to digest an ordinary meal, so that when the last of it 
is being expelled from the stomach, into the duodenum, the first 
of it has reached the ileo-colic valve. AH the food, completely 
digested and partially digested, which has not been absorbed, will 
be found in the small intestine at one time, viz.: about 4X hours 
after a meal. 
The contents of the small intestine, after being acted on by the 
succus entericus, pancreatic juice, and bile are passed on into the 
large intestine. Here the downward movement is much slower. 
It takes the contents hours to reach the hepatic flexure of the 
colon, 9 hours to reach the splenic flexure, 12 hours to arrive at the 
upper end of the sigmoid flexure, and 18 hours to reach the rectum. 
The products of digestion are absorbed, for the most part, in the 
small intestine, mainly in the lower part of the ileum. The con- 
tents are liquid when passed into the large intestine but there 
much of the water is absorbed, particularly in the ccecum, and they 
soon become semi-solid. 
In the large intestine putrefactive changes take place, due to 
the presence of bacteria, and many substances are formed as 
end-products which are discharged in the excreta. Important 
amongst these are indol and skatol which give the characteristic 
offensive odor to the fasces. 
If the contents remain in the large intestine for any length of 
time, these substances are absorbed into the blood and act in 
a DELETERIOUS MANNER ON THE WHOLE SYSTEM, giving rise to 
general malaise, headache, etc.; hence the necessity of preventing 
constipation. 
In habitual constipation the whole body is being systematically 
poisoned from the large intestine. 118 
HUMAN PHYSIOLOGY 
Mechanism of Absorption from Small Intestine 
The digested materials-glucose, glycerine and fatty acids, and 
the end products of protein digestion, mainly amino acids-pass 
through the columnar epithelitun lining the villi, into two sets of 
vessels,'-lacteals (lymphatic vessels) and blood capillaries. Even 
if the proteoses and peptone are to a slight extent absorbed they are 
so changed by erepsin in the mucous membrane of the intestinal 
wall that the only substances resulting from the digestion of pro- 
teins which reach the blood are the amino-acids. 
The glycerine and fatty acids are probably reconstruct- 
ed INTO NEUTRAL FATS IN THEIR PASSAGE THROUGH THE EPI- 
THELIUM, and these are carried into the lacteals. It would ap- 
pear that the intestinal mucous membrane has the power of modify- 
ing the composition of fat during its absorption. The fat globules 
are seized by leucocytes and conveyed by these into the lacteals, 
the contents of which are ultimately discharged, by the thoracic 
duct, into the great veins at the root of the neck. When chyle is 
collected from a fistula in the thoracic duct only about 60% of the 
fat known to be ingested with the food is recovered. Over 90% 
of the food fat is normally taken up from the intestine. The 
question as to what becomes of the 30% unrecovered has not been 
adequately answered. 
The sugar (glucose) and protein products (mainly amino-acids) 
pass, for the most part, directly into the blood-vessels. 
All the substances absorbed into the blood vessels of the villi pass 
through the liver before they reach the general circulation. 
If the sugar (glucose) content of the blood rises to about 0.3%, 
THE EXCESS IS EXCRETED BY THE KIDNEY, AS A FOREIGN SUBSTANCE, 
and lost to the body. One reason why the liver is placed on the 
portal circulation is to prevent too much sugar getting into the 
blood, at one time, so that it may not be thrown away by the 
kidneys. 
Water is absorbed throughout the whole intestine but mainly in 
the large. The amount which disappears in the small intestine is 
not sufficient to affect greatly the fluidity of its contents because 
the secretions being poured intothegutjustaboutmakeupfor the 
water absorbed. Absorption of water from the large intestine is 
much more noticeable since the contents here are semi-solid or even 
hard feces whereas in the small intestine they are always fluid. 
Glycogenic Function of the Liver 
The liver cells dehydrate the glucose brought to them in 
the blood of the portal vein and convert it into glycogen or animal 
starch. This process is the reverse of what takes place in the 
mouth, stomach and intestine. DIGESTION 
119 
This glycogen is stored up in the liver cells, then gradually 
reconverted into glucose, and given out in small quantities to the 
blood, as it is required. 
These facts were discovered by Claude Bernard. 
The liver is the regulator of the supply of sugar to the blood 
and it strives to keep it constant, and below the point where it 
will be thrown off by the kidneys. Usually it does not rise above 
0.1 per cent. Glycogen is also formed in other situations, chiefly 
in muscle particularly of cold blooded animals. It occurs in traces 
in most animal tissues. 
There is a relationship between the internal secretion of the 
pancreas and the carbohydrate changes in the liver. 
Formation of Urea by the Liver 
In the metabolism of the amino-acids some ammonia com- 
pounds are formed, and these, if allowed to accumulate in the 
circulation (where they probably exist as ammonium carbonate or 
carbamate), will act as poisons. They must therefore be re- 
moved, and their removal is believed to be effected by the liver, 
which converts them into the comparatively harmless substance 
urea. This is then excreted by the kidneys. 
The transformation is brought about by a process of dehydration, 
thus: 
Ammon. Carbamate 
/ ONH4 / NH2 
CO ( -h2o = co ( 
\ nh2 \ nh2 
or 
Urea 
Ammon. Carbonate 
/ ONH4 / NH2 
CO ( -2H2O = CO ( 
\onh4 \nh2 
Proofs that Urea is Formed in the Liver and not in the kidney. 
Urea is the chief end-product in the metabolism of proteins, and is 
excreted in large quantity in the urine, being its principal organic 
constituent. At one time it was supposed that the urea was pro- 
duced in the kidney, but that is no longer believed. 
That urea is formed in the liver has been proved in several ways. 
1. The total extirpation of the liver is such a severe operation, 
in mammals, that they do not usually survive it, but in the frog it 
is otherwise. Ina frog, from which the liver has been removed, 
urea disappears from the urine and ammonium compounds take 
its place. 
2. It is possible in mammals to cut the liver out of the circu- 
laton by uniting the portal vein to the inferior vena cava, so 120 
HUMAN PHYSIOLOGY 
that the blood from the stomach, intestine, etc., does not pass 
through the liver at all, but flows directly into the vena cava. 
This is known as an Eck fistula. Here again, urea is to a large 
extent absent from the urine, and ammonium salts are greatly 
increased. 
3. In extensive disease of the liver, in the human subject, urea 
is diminished in the urine and ammonium salts correspondingly 
increased. 
4. If the liver be removed from a freshly killed animal, and 
perfused through the portal vein with defibrinated blood to which 
ammonium carbonate or carbamate has been added, the blood, as 
it escapes from the hepatic veins, will contain urea,and some of the 
ammonium salts will have disappeared. If the kidney be perfused 
in the same way with normal blood, to which the salts of ammon- 
ium have been added, no urea will be present in the blood as it 
escapes from the organ and there will be no loss of ammonium salts. 
Some urea is formed in the muscles and other tissues, but it 
appears to be the special function of the liver to transform the 
poisonous ammonium compounds produced in the intestines into 
the harmless substance urea. BODY METABOLISM 
All the chemical transformations which the food substances 
undergo, from the time they are absorbed from the alimentary 
canal until thrown off in the excretions, are included under the 
term body metabolism. The oxygen taken in from the lungs must 
also be considered as a food substance, in this relation. In these 
metabolic processes the chemical changes are of two kinds1-kata- 
bolic and anabolic; the tissues are constantly being broken down 
into simpler compounds (katabolism) and at the same time the 
loss is being made good by the building up of new tissue material 
from the nutrient substances brought in the bloodstream (ana- 
bolism) . 
It must be remembered that only from 90 to 95 per cent, of the 
food taken into the stomach is digested and absorbed into the 
blood; the remainder is thrown off in the faeces and simply passes 
through the body taking no part in tissue metabolism. 
Food Substances the Source of Body Heat-The heat of the 
body is produced by the oxidation of the food substances after they 
have been digested and have entered the tissues. The heat pro- 
duced by the complete oxidation of a food-stuff may be determined 
by burning a known quantity of it in a bomb calorimeter. For a 
description of this instrument, see p. 136. 
The amount of heat produced by che complete oxidation of 
one gram of any food-stuff is known as its caloric or heat value. 
If one gram of fat is burnt in a bomb calorimeter it will yield 
9.3 large calories of heat, one gram of carbohydrate, 4.1 calories 
and one gram of protein 5.6 calories. These figures represent the 
physical heat values of fat, carbohydrate and protein. The physio- 
logical heat value is the amount of heat given out by the food 
substance when oxidized in the body. The physical and physio- 
logical heat values of fats and carbohydrates are identical, because 
these are completely oxidized in the body, but the physiological 
heat value of protein is less than its physical heat value, since it 
is not completely oxidized in the body. One of the end-products 
of the protein molecule is urea which is incompletely oxidized and 
still capable of yielding a considerable amount of heat, so that one 
gram of protein oxidized in the body yields only 4.1 calories. 
The physiological heat values of the food substances are as 
follows: 
Fat 9.3 calories 
Carbohydrate 4.1 calories. 
Protein 4.1 calories 
121 122 
HUMAN PHYSIOLOGY 
The energy liberated in the body by the oxidation of the food 
takes the form partly of heat and partly of muscular work. Enough 
food is required to produce daily about 3000 calories of heat. 
In attempting to trace the successive changes through which 
the food substances pass from their entrance into the body to their 
elimination from it, great difficulties are met with, since, although 
we can determine by analysis their chemical composition when 
ingested and that of their end-products when thrown off, we have 
little means of knowing the intermediate stages in the transforma- 
tions. 
The fats, after being digested in the alimentary canal, that is, 
split up into glycerine and free fatty acid, are rebuilt up in the 
process of absorption by the epithelium lining the small intestine, 
and enter the blood stream through the thoracic duct. From the 
blood the fat is stored up, for the most part, in the subcutaneous 
tissue, omentum and other situations, where it serves as a reserve 
stock of fuel, the small proportion needed for immediate use pass- 
ing directly to the tissues. 
When required the stored fat is drawn upon, but it cannot be 
used directly by the muscles and other tissues; it must first under- 
go certain changes in the liver, to which it is carried by the blood 
of the portal vein. These changes consist in desaturating the higher 
neutral fats so that they may be broken up into the lower fatty 
acids (e. g. caproic and butyric) and finally oxidized into carbon 
dioxide and water. 
When an individual neither gains nor loses in weight the fat 
stored, after a meal, is all used up before the next meal; if weight 
is lost the reserve of fat is being depleted and if weight is gained 
more fat is being stored than is used. In starvation the stored fat 
is used by the tissues as their chief source of energy, all of it 
passing first through the liver before it can be utilized by them, 
however. So far as the space which it occupies is concerned fat is 
a very economical form of energy storage; 12 c.c. of adipose tissue, 
weighing about 11 grams, will yield about 100 large calories of 
heat, whereas if this amount of heat were stored in carbohydrate 
(glycogen) in liver tissue it would require more than ten times as 
much or 130 grams at least. 
Formation of Fat from Carbohydrate-In man the fat taken as 
food is the usual source of the fat of the body, but it can be formed 
from the carbohydrates of the food, and in the herbivora most of 
it is so formed. This was proved by the experiment of Lawes and 
Gilbert on the fattening of cattle, sheep and pigs. The food of 
these animals was analyzed and the amount of fat, carbohydrate 
and protein determined. On killing them far more fat was found 
Metabolism of Fat METABOLISM 
123 
in their bodies than could be accounted for by the fat of the food; 
the body fat, therefore, must have been formed from either protein 
or carbohydrate, but it could not be formed from protein since the 
quantity of fat in the body was larger than the amount of protein 
contained in the food; most of it, therefore, must have come from 
the carbohydrate. 
This transformation of carbohydrate into fat cannot be effected 
in the laboratory, and the process by which the change is brought 
about in the living body is not understood. 
Whether fat can be formed from protein is an open question. 
The fact that dogs fed on protein diet, free from fat and carbohy- 
drate, do not put on fat would seem to be against this idea. 
It is a curious fact that fat cannot be completely oxidized unless 
carbohydrate is present in the tissue cells. When the tissues are 
deprived of carbohydrates, as in starvation, or in animals on a 
carbohydrate-free diet, the final products of the oxidation of fat 
are not CO2 and water but (3 oxybutyric acid, diacetic acid and 
acetone which are excreted in the urine as such. This condition 
is known as acidosis; it indicates that the supply of carbohydrates 
to the tissues is insufficient. Under normal conditions these three 
substances are believed to be intermediate products in the oxidation 
of fat, but here the process goes on to the final stage, with CO2 
and water as the end-products. 
As we have already seen, the carbohydrates of the food are all 
transformed into glucose before absorption; this enters the blood 
of the portal circulation and is carried to the liver where most of 
it is converted into glycogen which is stored in the liver cells for 
future use. While the blood of the portal vein during the digestion 
of a meal containing carbohydrates may contain a large quantity 
of sugar, the amount present in the blood of the general circulation 
normally does not rise above 0.1 to 0.2 percent.; if the sugar in 
the blood exceeds this it is thrown off by the kidneys and we have 
the condition known as glycosuria (sugar in the urine). 
The tissues are always removing sugar from the blood to satisfy 
their needs, and the blood would soon be depleted of it were it 
not for the fact that the carbohydrate, stored in the liver as gly- 
cogen, is gradually reconverted into sugar which passes from the 
liver, through the hepatic veins, into the inferior vena cava and 
keeps the supply up to the normal, viz.: 0.1 to 0.2 percent. These 
facts were discovered by Claude Bernard. 
The transformation of sugar into glycogen in the liver is termed 
glycogenesis, the change in the opposite direction from glycogen into 
sugar glycogenolysis, while the taking up and consumption of 
sugar by the tissues is known as glycolysis. All three changes are 
Metabolism of Carbohydrates 124 
HUMAN PHYSIOLOGY 
probably due to the action of a ferment or ferments, or, it may be, 
to hormones. An excessive amount of sugar in the blood (more 
than 0.2 per cent.) is known as hyperglycaemia and this is followed 
by glycosuria. 
There exists in the body a mechanism by which these various 
processes are regulated and adapted to each other, so that the 
demand by the tissues is always met by the liver and the sugar con- 
tent of the blood is kept up to the normal, but little is known of its 
nature. If this regulating mechanism breaks down then, as a rule, 
hyperglycaemia and glycosuria follow. 
Varieties of Glycosuria-Sugar in the urine may be due to 
various causes. 
1. Alimentary Glycosuria is produced by the consumption of 
an excessive amount of sugar in the food in the form of glucose 
for it may be present in such quantity in the blood of the portal 
veins, that the liver is not able to transform the whole of it into 
glycogen, and much may pass through the liver into the general 
circulation, producing hyperglycaemia and glycosuria. This may 
be found in any healthy individual where the liver, pancreas and 
kidneys are prefectly normal. It is temporary and of no signifi- 
cance. This method of producing glycosuria is pometimes used in 
malingering. 
2. Adrenalin Glycosuria-If a small quantity of adrenalin 
(extract of suprarenal capsule) be injected into the circulation 
glycosuria will quickly follow. The explanation is that the adrena- 
lin stimulates the liver to transform glycogen into sugar at a greater 
rate than normal, with the result that an excessive quantity is 
thrown into the blood. If the liver be examined afterwards it will 
be found that the glycogen has almost disappeared from it. 
3. Puncture Diabetes of Claude Bernard-It was discovered 
by Claude Bernard, long ago, that a slight puncture in the floor 
of the fourth ventricle (medulla oblongata), in the rabbit, is 
quickly" followed by the appearance of sugar in the urine. This was 
explained by supposing that in this part of the brain a nerve center 
had been excited, which caused the liver cells to become more active 
in transforming glycogen into sugar. Now it is held that the effect 
on theliver is produced indirectly through the suprarenal capsules; 
the nerve stimulation leads to the secretion of adrenalin by the 
capsules, in excessive amount, and this reaching the liver, through 
the blood, excites the hepatic cells. When the suprarenal capsules 
have been removed the diabetic puncture fails to produce glycosuria. 
4. Phloridzin Glycosuria--A small of the drug phlorid- 
zin, injected into the blood, leads to glycosuria but this is not pre- 
ceded by hyperglcaemia as is usually the case. The phloridzin acts 
on the cells of the kidney tubules and excites them to secrete sugar 
when it is present in even less than the normal amount in the blood. METABOLISM 
125 
The sugar removed from the blood is replaced by the liver, so 
that the store of glycogen is rapidly depleted, and after two or three 
doses, it may disappear altogether. 
5. Pancreatic Glycosuria or Experimental Diabetes. When the 
pancreas is completely removed from animals sugar appears in 
the urine, within a few hours; they lose weight rapidly and die in 
a few weeks. They exhibit the same symptoms as are found in 
the disease known as diabetes mellitus in the human subject. 
The cause of this condition is not the want of pancreatic juice in 
the small intestine, for when the pancreatic duct is tied, or blocked 
with paraffin, no glycosuria results, nor does it occur if a small 
piece of pancreas is left behind. The real cause of this condition is 
the absence of the internal secretion insulin produced by the islets 
of Langerhans which, in the normal animal, enables the tissues, 
particularly muscle, to utilize the sugar brought to them in the 
blood. As a result the sugar accumulates in the blood and is 
thrown off by the kidneys. Certain forms of diabetes mellitus in 
the human subject are attributed to the same cause, since, in many 
cases, the islets of Langerhans have been found to be diseased at 
the post mortem examination. 
The carbohydrates of the food are ultimately oxidized to CO2 
and water but the intermediate steps in the process have not been 
followed; it is believed that lactic acid is one of the intermediate 
products. 
Formation of Carbohydrate from Protein. We have seen that 
fat may be formed from carbohydrate in the animal body, but 
there is no evidence to show that the reverse change takes place, 
viz., the formation of carbohydrate from fat, unless possibly in 
hibernating animals. Carbohydrate may be derived from protein, 
however. In phloridzin glycosuria all the glycogen may disappear 
from the liver, and when the animal is not receiving any carbo- 
dyhrate in its food, sugar is still found in the urine. In this case 
it is believed to be derived from the tissue proteins; the animal 
loses weight rapidly. 
Protein Metabolism 
The final products of protein digestion are various animo-acids 
and these are absorbed by the intestinal epithelium and pass into 
the blood as such. Immediately after or during their absorption, 
part of them are deaminized, that is, lose the animo group NH2, 
the nitrogen being carried to the liver in the form of ammonium 
compounds and there transformed into urea. 
The metabolism of proteins in the body is of two kinds, viz.: 
exogenous and endogenous. The greater part of the animo-acids, 
after deaminization, are carried to the tissues and there oxidized 
into CO2 and water. They never really become a part of the tissue, 126 
HUMAN PHYSIOLOGY 
but are burnt outside it, thus serving as heat and work producers. 
This is known as exogenous protein metabolism. The end products 
excreted in the urine are urea and the inorganic sulphates. A 
certain proportion of the animo-acids, however, becomes a part 
of the living tissues, and the changes through which these pass 
are included in the term endogenous protein metabolism; the 
final products here are creatinine and uric acid. By estimating 
the amount of urea and inorganic sulphates in the urine, on the 
one hand, and the amount of creatinine and uric acid on the other, 
it is possible to tell approximately, the proportion of food protein 
to tissue protein. 
There are certain animo-acids or combinations of animo-acids 
that cannot be formed in the body which are essential in the build- 
ing up of tissue protein, and unless they are present in the food 
tissue protein cannot be formed. Examples of these are trypto- 
phane, tyrosine and phenylalanine. Gelatine is a protein, but, as 
is well known, it will not maintain life and the reason is that it 
does not contain these groupings. Another protein named zein, 
found in maize, will not maintain life because it contains no trypto- 
phane. The food of man and animals, which consists of proteins, carbo- 
hydrates and fats, together with salts and water, is required for 
two purposes, viz.: first, as a source of energy. A constant trans- 
formation of potential energy, stored up in the food substances, 
into the kinetic energy of heat and work is taking place in the body 
and to supply this energy food must be consumed. Second, food 
is necessary to replace the tear and wear of the tissues which are 
continually breaking down and for this purpose proteins are es- 
sential. 
The energy stored up in all food substances is derived ultimately 
from the sun. Under the influence of the sun's rays plants possess 
the power of building up, out of the simple elements carbon, 
nitrogen, hydrogen and oxygen, etc., complex molecules in which 
large quantities of potential energy are stored, and these vegetable 
substances are used either directly or indirectly by animals as food. 
Food as a Source of Energy. It is found, by actual calorimetric 
measurement, that a man of average size, doing light work, gives 
off somewhat less than 3000 large calories of heat in the 24 hours, 
and if the muscular work, performed in that time, transformed into 
heat units be added, it will bring the total energy expended up to 
about this figure, viz.: 3000 kilocalories. In individuals leading 
a sedentary life it will be less and in those doing hard muscular 
work considerably more, over 5000 kilocalories it may be. 
If the body weight is to be maintained enough food must be 
given to supply this energy because it is the oxidation of the food 
substances within the body which is the source of heat and mus- 
cular work. Knowing the physiological heat values of the food 
proximate principles (protein, carbohydrate, fat) we can calculate 
how many grams is necessary to yield this energy. 
Food to Supply Tissue Waste. If the body weight is not to fall 
enough food must be consumed to make good the tissue waste, or, 
in other words, to replace the carbon and nitrogen lost in the ex- 
creta (urine, faeces, expired air). The nutritive value of a food 
depends mainly on the amount of carbon and nitrogen which it 
contains. A man on ordinary diet and doing a moderate amount 
of work will eliminate from 250 to 280 grams of carbon in the 24 
hours and from 15 to 18 grams of nitrogen. This gives a carbon 
to nitrogen ratio of about 16.5:1. In muscular exercise the amount 
of carbon eliminated is greatly increased while the output of ni- 
trogen is scarcely affected, so that in a diet for hard muscular 
work the carbon to nitrogen ratio should be greater than 16.5:1. 
DIETETICS 
127 128 
HUMAN PHYSIOLOGY 
Protein is essential in all dietaries since it alone contains nitrogen 
but a pure protein diet is not suitable because it does not contain 
enough carbon. The carbon to nitrogen ratio in protein is only 
3.5:1, so that when the requisite 15 grams of nitrogen have been 
taken there will be only 53 grams of carbon instead of the necessary 
250 grams. The extra carbon, therefore, must come from fat and 
carbohydrate. 
In arranging a daily diet enough must be given to supply the 
energy given out by the body in the form of heat and muscular 
work, to replace the carbon and nitrogen eliminated from the 
body, and the proportions of the proximate principles must be 
such that the carbon to nitrogen ratio is not far from 16.5:1. In 
making up daily diets on this basis enough protein must be allowed 
first to supply the requisite quantity of nitrogen, and secondly 
fats and carbohydrates to balance the energy lost. In practice it 
is found that fats and carbohydrates may be made to replace each 
other to a considerable extent. 
Voit, from a large number of observations on bodies of workmen, 
body weight 70-75 kilograms, considered the following daily diet 
adequate. 
Grams Calories 
Protein 118 yielding 483 
Fat 56" 520 
Carbohydrate 500 " 2050 
Total3053 
Ranke found that for himself, weighing 74 kilograms and leading 
a sedentary life, the following diet was sufficient: 
Grams Calories 
Protein 100 yielding 410 
Fat 100 " 930 
Carbohydrate 240 " 984 
Total2324 
The daily diet allowed by Atwater in America is more generous: 
Grams Calories 
Protein 125 yielding 512 
Fat 125' " 1172 
Carbohydrate 400 " 1640 
Total3324 
By consulting a table showing the composition of the various 
food-stuffs a diet may be readily constructed containing the proper 
proportions of the three proximate principles, in sufficient quantity, 
but this is not enough. The appetite must be considered, also the 
digestibility of the various food substances as well as their cost. DIETETICS 
129 
For example, vegetable proteins are, as a rule, cheaper than animal 
proteins but they are less digestible, and fats are dearer than 
carbohydrates. The tendency is for well-to-do people to use 
protein in excess while those to whom cost is an important consider- 
ation rely most on carbohydrates. 
The Salts and Water. These in the course of their metabolism 
give out no energy, yet they are essential articles of diet, since 
animals fed on an ash-free diet succumb as rapidly as they would 
have done if deprived of all food, with the exception of water. 
They enter into the composition of all living protoplasm and are 
as important, in this relation, as the proteins. The osmotic pressure 
of the body fluids is due to the presence of salts in solution (mainly 
sodium chloride) and this plays an important part in the control 
of the flow of water to and from the tissues. As we have seen, 
calcium is essential in blood coagulation, and the balanced action 
of calcium on the one hand and sodium and potassium on the other 
may be the essential cause of the heart beat. Iron, again, is nec- 
essary for the production of haemoglobin. 
The function of water is to act as a solvent. 
Vitamines. These are substances, present in minute quantities 
in the husks of certain grains, in vegetables, in milk and other foods, 
which are essential in all diets, if health is to be maintained. Little 
is known, so far, regarding their chemical nature or composition. 
Their absence from the diet for a considerable period leads to such 
diseases as beri-beri, scurry, rickets, and probably pellagra. For 
example, persons living largely on polished rice, that is rice from 
which the outer coat or husk has been removed, suffer from a form 
of neuritis termed beri-beri, and it is now believed that this is due 
to the absence of vitamines. The disease can be cured by using 
rice from which the outer layers have not been removed,, or by 
adding the polishings, or the alcoholic extracts of them, to the diet 
of polished rice. In birds a similar condition can be caused by 
feeding them on polished rice and cured as stated above. ANIMAL HEAT 
Historical-The ancients could give no explanation of the cause 
of animal heat; it was there in the body and existed per se; it was 
different, they believed, from the heat given out by inanimate 
objects and so they called it "animal heat" or "vital heat." 
This vital heat was supposed, by Galen, to be produced in the 
heart and to be distributed through the body by the blood in the 
veins. It was prevented from accumulating in the heart, to too 
great an extent, by the cool air taken into the lungs which were 
placed around the heart for this purpose. The function of respira- 
tion, according to him, was concerned with keeping the body cool. 
In more modern times, after the discovery of the circulation by 
Harvey, in the 17th century, when physiologists began, to some 
extent, to think in terms of physics and chemistry, attempts were 
made to give a more rational explanation of body heat, and since 
only two sources of heat were known at that time, viz.: friction 
and fermentation, they were divided into two schools, the friction- 
ists and the fermentationists. The frictionists taught that the 
heat of the body was produced by the rubbing of the blood against 
the walls of the blood vessels, the fermentationists that it was due 
to fermentative processes going on in the blood. 
Black and Lavoisier, about the end of the 18th century, showed 
that it was caused by oxidation, CO2 being the product of the 
combustion. Lavoisier believed that this oxidation took place in 
the lungs, but if this were so the lungs would be the hottest organs 
in the body, and this is not the case. Later it was shifted to the 
blood stream; but now we know that it takes place neither in the 
lungs nor in the blood stream but in the tissues themselves. 
Oxidation is continually going on in the tissues and this is the 
source of the heat of the body, just as it is in the case of a candle 
flame; the difference is that in the one case (body) it proceeds 
slowly, in the other (candle) more rapidly. 
Body Temperature 
Temperature may be measured by a thermometer; this gives 
the intensity of the heat, but not the quantity of it which is 
measured by a calorimeter. 
Animals are divided into two great groups with regard to body 
temperature, viz.: warm-blooded animals and cold-blooded ani- 
mals, the former including mammals and birds, the latter all other 
animals-reptiles, amphibians, fishes and all invertebrates. An- 
other point of distinction, more important than the actual temper- 
130 ANIMAL HEAT 
131 
ature even, is that the so-called warm-blooded animals have a 
constant body temperature, and are therefore said to be homoio- 
thermal, while the cold-blooded animals have a variable tempera- 
ture, depending on that of the environment in which they live- 
poikilothermal. A man (homoiothermal) has practically the same 
body temperature, no matter whether he reside in the tropics or 
the arctic regions, while a frog or a fish (poikilothermal) takes the 
temperature of the air or water in which it lives. 
Hibernating animals such as the woodchuck, dormouse, bear, 
etc., form a class by themselves, since during one-half of the year 
(summer) they are warm-blooded or homoiothermal and during the 
other half (winter) cold-blooded or poikilothermal. 
Clinically the temperature is taken by placing the bulb of the 
thermometer in the mouth, axilla, or rectum. 
The rectal temperature in the human subject varies between 
97°F. and 99°F.; muscular exercise may raise it to 102°F. or 103°F., 
but it quickly subsides during rest. In high fever it may reach 110° 
F. or over, but 106°F. is serious and 107°F. or 108°F. is dangerous, 
and if it persists for any length of time, death may result from heat 
stroke. 
The normal body temperature is highest in birds; in the hen it 
runs from 107°F. to 109°F., and in the humming bird and swallow 
higher still, viz., 111°F. to 112°F. Most mammals have a higher 
body temperature than man. In all species it is found to be slightly 
higher in females than in males. 
Diurnal Variations in Body Temperature-Man is the most 
perfect homoiothermal animal but even in him the body tempera- 
ture is not absolutely constant. If the rectal temperature be re- 
corded every hour throughout the twenty-four, and the results 
plotted in the form of a curve, it will be found that the temperature 
is lowest in the early morning, when the subject is asleep, from 
2 to 4 a. m., and highest in the late afternoon, when the body is 
most active. It has a greater range in children than in adults. 
Production of Heat in the Body 
The chief seat of heat production is muscle. About one-half 
the total weight of the body is made up of muscle and more than 
four-fifths of the body heat is produced in muscle. When the muscles 
are in active contraction, the heat production is greatly increased. 
Next to muscle, as a source of heat, come the secreting glands, 
and of these the liver is the most important heat producer. It is 
doubtful whether the nervous system adds much to the heat of 
the body, and the metabolism in bone, cartilage and connective 
tissue is so small that little heat is produced in these tissues, for 
the amount of heat produced in any tissue or organ is approximate- 
ly proportionate to the tissue metabolism or oxidation. 132 
HUMAN PHYSIOLOGY 
Loss of Heat from the Body 
The channels of heat loss are the skin, the respiratory passages 
and the excreta. Roughly speaking, 80% is lost by the skin, 17% 
by the air passages and lungs, and 3% by the excreta. 
From the skin heat is lost, 1, by radiation, conduction and con- 
vection, and 2, by evaporation of sweat. If one body is hotter than 
another, heat will radiate from the former to the latter, and the 
greater the difference in temperature between the two the more 
rapidly will radiation take place, and the greater will be the 
amount of heat lost in a given time. In all habitable climates, 
with a few occasional exceptions, the temperature of warm-blooded 
animals is above that of their surroundings, so that heat is con- 
stantly being lost from the body by radiation. 
In temperate and cold climates animals are provided with a 
deposit of fat under the skin and with fur or feathers, all of which 
serve to diminish the loss of heat by conduction; in man clothes 
serve the same purpose. The air held in the meshes of the textures 
of which the clothes are composed, and between the garments, is 
a more effective protection against the loss of heat than the material 
of the garments itself, since air is a very poor conductor of heat. 
When the clothes get wet, the air in the interstices is replaced by 
water, which is a much better conductor, and this explains, in part, 
the cooling effect of wet clothes. 
A large amount of heat is also lost by convection. Even on a 
still day, particularly in those parts of the body unprotected by 
clothes, the layer of air in immediate contact with the surface of 
the body is warmed; it becomes lighter and rises, to be replaced 
by cold air which, in turn, is warmed and ascends, etc. The heat 
lost in this way (by convection) is, of course, greatly increased in 
a draught or on a windy day. 
2. The secretion and evaporation of sweat is a very important 
factor in heat loss from the skin. In the evaporation of sweat a 
large amount of heat is abstracted from the skin and this helps to 
keep the body temperature below that of its surroundings. Sweat 
will evaporate most rapidly if the air is dry; if the air is saturated 
with moisture there will be no evaporation at all. The unclothed 
human body can stand a temperature of 260°F. for several minutes 
if the air is quite dry, whereas if it be saturated with moisture, 
permitting of no loss of heat by evaporation, 89°F. may rapidly 
cause death. Zuntz records the case of a workman without sweat 
glands in his skin; during the summer months, when he could 
lose little heat by radiation, conduction or convection, he was in- 
capacitated for work, since the heat developed from muscular exer- 
cise rapidly raised the body temperature to a dangerous point. 
As long as he kept his shirt wet, however, the temperature remained 
about normal. ANIMAL HEAT 
133 
The relative importance of radiation, etc. and evaporation as a 
means of losing heat depends on the temperature and humidity of 
the outside air. If the humidity is high, near the saturation point, 
little heat can be lost by evaporation, and if the external tem- 
perature is high little is lost by radiation; if both are high the 
temperature of the body must rise, since some heat is always being 
produced in the body, and death may result. Heat stroke is easily 
produced in hot, humid climates and during "heat waves'' in 
summer, in the cities along the Atlantic sea board where the air 
is often near the point of saturation, hundreds of persons die from 
this cause. 
Condition of Cutaneous Bloodvessels and Sweat Glands-The 
loss of heat from the skin varies with the condition of the cutaneous 
bloodvessels and sweat glands. If the vessels are dilated, the 
skin is warmed by the blood from the deeper parts of the body, 
and, consequently, more heat is lost by radiation. The coldest 
blood in the body is found in the cutaneous veins. If the vessels 
are constricted, little warm blood circulates through the skin and 
less heat is lost. Similarly, when the sweat glands are active 
more evaporation takes place and more heat is lost, and vice versa. 
From the Lungs and Air-passages a considerable amount of 
heat is lost in two ways: 1. The air is inhaled cold and, while in 
the lungs, is warmed, and the heat absorbed it carries away when 
exhaled. 2. The evaporation of water from the air passages is 
also an important factor. In animals, such as the dog, which don't 
sweat, a large quantity of heat is lost by evaporation from the 
tongue and air passages. 
The amount of heat lost by the Excreta is comparatively small. 
Most foods, including water, are relatively cold when taken into 
the body and when excreted are warm, but if all the meals be taken 
hot, heat may be added to the body in this way. 
Regulation of Body Temperature 
The temperature of the body will only remain constant so long 
as the production of heat balances the loss; if more heat is lost 
from the body than is produced by it the temperature will fall; 
if more is produced than is lost it will rise. In warm-blooded ani- 
mals, in health, this balance is maintained by a heat-regulating 
mechanism. The essential difference between a warm-blooded 
and a cold-blooded animal is that the latter does not possess such 
a mechanism. 
The Loss of Heat is Regulated both Voluntarily and Involun- 
tarily-1. Voluntary Control is important in man; it is effected 
by the use of clothes, radiators, fires, etc. 
2. Involuntary Control comes into play through centers in the 
nervous system. For example, cold applied to the skin causes the 
cutaneous vessels to become constricted reflexly through the 134 
HUMAN PHYSIOLOGY 
vaso-motor center (not directly), thus sending the warm blood 
from the skin to the deeper parts and diminishing the loss of heat 
from the skin. Warmth has the opposite effect; it reflexly dilates 
the cutaneous vessels and leads to an increased loss of heat. The 
ACTIVITY OF THE SWEAT GLANDS is also Under the CONTROL OF THE 
nervous system. Heat applied to the skin stimulates the glands 
reflexly, through the sweat center, to pour out their secretion 
which on evaporation causes increased loss of heat. That the sweat 
glands are excited reflexly through the sweat center, and not 
directly by the application of heat to the skin, can be proved by 
cutting the nerve which supplies a certain cutaneous field. After 
the nerve is divided no amount of heat applied to that field will 
have any effect on the sweat glands within it; this is because the 
reflex arcs have been interrupted. 
Heat Production is also Controlled by Voluntary and Involuntary 
Means'-1. Voluntary Control is exercised by increasing or dimin- 
ishing muscular activity. When cold, we walk or move about; 
when too warm we rest, thus increasing or diminishing heat pro- 
duction as the case may be. 
2. Involuntary Control is brought about reflexly through the 
nervous system. Cold applied to the skin increases muscular 
tonus; at a certain point the contractions become visible- 
shivering-and more heat is produced. Shivering is involuntary. 
The bracing effect of a cold climate is due to this increase in muscle 
tonus. Warm-blooded animals produce more heat as the external 
temperature falls, while cold-blooded animals produce less. 
Since most of the heat of the body is produced in muscle, this 
would become much warmer than the other tissues if it were not 
for the bloodstream which distributes the heat and so 
equalizes the temperature throughout the body. 
The Effect of Muscular Contraction on Heat Production can be 
shown by giving curara to an animal, such as a rabbit, thus im- 
mobilizing it; here the body temperature will fall rapidly because 
no heat is being produced in the muscles. Anaesthesia with ether 
or chloroform has the same effect, and for this reason the room in 
which a surgical operation is being performed is always kept very 
warm. 
Cold also stimulates the appetite so that more food is con- 
sumed, and it is said to induce a desire for fatty foods especially, 
which have a high heat value. The Eskimo, for example, lives to 
a large extent, on the subcutaneous fat (blubber) of whales, seals 
and walruses. 
Nervous control is less effective in regulating heat production 
and heat loss in young than in old animals, and, in consequence, 
the body temperature is more variable. Newly born animals are 
very nearly poikilothermal. Thus, fever is not such a serious 
symptom in a child as it is in an adult, since in the former a com- ANIMAL HEAT 
135 
paratively slight cause may account for it. Fever, it should be 
remembered, is only a symptom and not a disease. 
Other factors than the influence of the nervous system, which 
govern the relation between heat production and heat loss, are the 
following: 
1. Size-The production of heat is proportional to the mass 
or weight of the body and the heat loss to the surface. The 
former varies with the cube of the linear dimensions and the latter 
with the square, so that in small animals the ratio of surface to 
mass is greater than in large animals, of the same species, hence, 
weight for weight, more heat must be produced, per unit of body 
weight, in small animals than in large, to balance the increased 
heat loss. 
2. The shape of the animal also influences the ratio between 
the surface and the mass and hence between the heat loss and the 
heat production. The sphere offers the smallest surface, per unit of 
mass, and animals, when they wish to diminish heat loss, roll 
themslves into a ball; when, on the other hand, it is desirable to 
increase heat loss they extend the limbs and stretch the body out 
so as to expose a greater surface. 
3. Age -Besides the difference in size and shape young animals 
are growing and are more active than adults and so metabolism 
and heat production are greater weight for weight, in young than 
in old animals of the same species. 
4. The degree of obesity influences the heat loss, to a consider- 
able extent, since the fatty tissue provides a subcutaneous layer 
of bad conducting material. 
Calorimetry 
The temperature of an animal, as indicated by a thermometer, 
is not a measure of the amount or quantity of heat produced by 
the animal. To obtain this some form of calorimeter must be 
used. Animal calorimeters are of two types-the water calorim- 
eter and the air or differential calorimeter. 
The water calorimeter consists of a double walled metal box, 
the space between the walls being filled with water. This is placed 
within a large wooden box and surrounded by such non-conducting 
materials as shavings, sawdust or felt, to prevent loss of heat from 
the outside by radiation. The inner box, in which tlje animal is 
placed, is provided with an air inflow and outflow tube with a 
thermometer in each, so that the difference in temperature between 
the incoming and outgoing air can be determined. Another ther- 
mometer is placed in the water jacket, and during the experiment 
a mixer is kept in motion to ensure that the water is of uniform 
temperature. 
The animal (dog, for example) is placed in the inner box and 136 
HUMAN PHYSIOLOGY 
allowed to remain there for one hour, say. During this time the 
heat given off by the animal is communicated to the water, the 
temperature of which is thus raised. After allowance is made for 
the heat absorbed by the metal of the box, and by the air in its 
passage through, the quantity of heat lost by the animal is found 
by multiplying the weight of water by the increase in temperature. 
Thus, suppose that the water weighs 30 kilograms and that its 
temperature is 1.5°C. higher at the end than at the beginning of the 
experiment, the amount of heat added to the water, that is, given 
off by the animal is 30X1.5 = 45 large calories. 
Corrections must be made, however, for the possible rise or fall 
of the body temperature while the animal is in the calorimeter, 
which is determined by placing a thermometer in the rectum. If 
the rectal temperature does not change the amount of heat given 
off by the animal is equal to that produced; if the rectal tem- 
perature has risen more heat has been produced than lost, and if 
the rectal temperature has fallen, vice versa. For example, if 
the dog weighs 10 kilos and the rectal temperature has risen 0.5°C., 
10X0.5X0.8 (the specific heat of animal tissues) =4 kilocalories, 
which must be added to the amount lost by the animal; if the rec- 
tal temperature has fallen 0.5°C. then the 4 kilocalories must be 
subtracted. What we desire to determine is the amount of heat 
produced by the animal in a given time; what we measure directly 
is the heat lost. 
The Air or Differential Calorimeter consists of two double 
chambers, with the air spaces between the walls of exactly the 
same dimensions, connected to each other by a U-tube containing 
colored fluid as an index. If the air is warmer in one chamber 
than in the other the index will move towards the cooler side, and 
vice versa. 
The animal is placed in a wire cage on one side and a hydrogen 
flame is burnt on the other. The inflow of hydrogen can be 
regulated by a stop-cock so that the heat produced by the animal 
is exactly balanced by that produced by the hydrogen flame; when 
this is so, the index does not move. The amount of heat produced 
by the flame can be determined from the volume of hydrogen 
consumed. 
The Bomb Calorimeter is used for estimating the heat values of 
food substances. In principle it is the same as the water calorim- 
eter described above. A weighed quantity of the food substance 
under investigation-butter, starch, white of egg, etc.--usually 
about 1 gram, is completely burnt in an atmosphere of oxygen 
when the heat of combustion is communicated to a known weight 
of water. One gram of fat, burned in this way, will yield 9.3, one 
gram of carbohydrate (starch) 4.1, and one gram of protein 5.8 
large calories. EXCRETION 
In the general process of nutrition there are three phases to be 
considered: 1, the bringing of new matter to the tissues in the 
form of oxygen from the lungs, and the final products of digestion 
from the alimentary canal; 2, the chemical changes taking place 
in the tissues, included under the term metabolism; 3, the removal 
of waste and useless material, the products of metabolism, from 
the tissues and their elimination from the body. This last is 
known as excretion. 
Water and the waste products of body metabolism are throwp. 
off or excreted through four channels, viz.: the liver, lungs, kidneys 
and skin. It has been shown how the bile constituents are poured 
into the intestines and partly thrown off, along with the undigested 
parts of the food which form the faeces, and also how CO2 is elimi- 
nated by the lungs, but the excretory functions of the kidneys and 
the skin still remain to be studied. 
THE KIDNEYS 
The kidneys are two in number, right and left; they are situated 
in the upper part of the lumbar region, one on either side of the 
middle line, and are surrounded by a large quantity of fat which 
helps to protect them and keep them in position. The main duct 
of the kidney is termed the ureter, and this conveys the urine 
(excretion of kidney) to the urinary bladder, which serves as a 
reservoir in which the urine accumulates until it is expelled through 
the urethra. The entrance of the ureters through the wall of the 
bladder is very oblique and this, as we have already seen in the 
case of the bile and pancreatic ducts, plays the part of a valve in 
preventing the urine from a full bladder passing back into the 
kidneys. 
Structure of Kidney 
Each kidney is about 4 inches in length, 2X inches in width and 
1J4 inches in thickness,and weighs about 4 ounces. The outer 
border is rounded and convex, and the inner border concave showing 
a depression which is termed the hilum; it is here that the blood 
vessels (renal artery and renal vein) enter the organ and the ureter 
emerges from it. The ureter, at its origin, is spread out into a 
funnel shaped expansion termed the pelvis, and this is further 
subdivided into smaller funnels termed calices. The kidney is 
dark red in color, and is enclosed in a tough fibrous capsule which 
is firmly adherent at the pelvis. 
137 138 
HUMAN PHYSIOLOGY 
On making a longitudinal section through the kidney, with a 
sharp knife, the cut surface is seen, by the naked eye to consist of 
two distinct parts, an outer or cortical region and an inner or 
medullary region. The medullary region is made up of a number 
of pyramids, lying side by side, with their bases in the cortex all 
fused together and their apices projecting into the calices of the 
ureter. They are called the Malpighian pyramids and each of them 
is, in a sense, an independent miniature kidney. The ducts of the 
uriniferous tubules open on the surface of these pyramids and 
pour their contents (the urine) into the ureter. 
Bloodvessels of Kidney-The blood enters the kidney by a 
single vessel-the renal artery which is a branch of the aorta- 
and leaves it by a single vessel-the renal vein which joins the 
inferior vena cava. The renal artery, on entering at the hilum, 
divides into branches which run up between the pyramids to which 
they give off some twigs, and terminate in a complicated system of 
capillaries in the cortex and medulla. The renal vein and its 
tributaries take a similar course. 
Uriniferous Tubules-The kidney is a compound tubular gland. 
Each (uriniferous) tubule begins in the cortex as a cup-like ex- 
pansion called the capsule of Bowman which embraces a tuft of 
capillaries called the glomerulus, the two together forming a 
Malpighian corpuscle. This expanded, end narrows into a neck 
as it leaves the glomerulus and then pursues a very devious course, 
bending back upon itself several times, and forming what is called 
the convoluted tubule; ultimately this tubule ends in one of the 
ducts which open into the pelvis of the kidney. The blood enters 
the tuft of capillaries (glomerulus) by a single (afferent) vessel and 
leaves it by a single (efferent) vessel, the latter breaking up into 
a second set of capillaries which surround the convoluted tubules. 
It is the function of the kidney to separate from the blood various 
substances that are formed elsewhere, as the result of tissue meta- 
bolism, and which may be regarded as impurities. This helps to 
keep the composition of the blood constant. Unlike the other 
glands which we have previously considered, none of the con- 
stituents of the secretion (urine) are formed in the kidney; they are 
already present in the blood when it reaches the organ, and are 
simply picked out by the epithelial cells which line the tubules. 
In the case of some of the constituents of the urine they are prac- 
tically entirely removed from the blood, by the kidney, urea, for 
example, almost none being found in the renal vein, whereas in 
other cases it is only the excess, above a certain limit, that is 
removed'--sodium chloride for example. Many constituents of 
the blood again, are not touched at all by the healthy kidney, 
The Function of the Kidneys EXCRETION 
139 
e. g. the proteins, serum albumin, and serum globulin, etc., and 
glucose if it is not present in abnormally large quantities. The 
kidney cells are very senstitive to any change in the composition of 
the blood and foreign substances, such as drugs, which may be 
present in minute quantity, are at once thrown out. 
Diuretics are substances that stimulate the kidney cells to in- 
creased activity and so produce an increase in the quantity of 
urine excreted. Examples are,-caffein, alcohol, water, etc. 
If water is drunk in large quantities, it is quickly absorbed and 
dilutes the blood; in this case the excess of water is picked out by 
the kidney cells and the blood brought back to its normal con- 
centration. One of the hormones found in extracts made from the 
posterior lobe of the pituitary body acts as a powerful diuretic, but 
whether it functions in this way normally is not known. 
Nerves of the Kidney--Both vaso-constrictor and vaso-dilator 
nerves pass to the kidney, but it is difficult to be certain whether 
the organ is supplied with secretory nerve fibers and for the 
following reason: Any circumstance which brings a larger amount 
of blood to the kidney will lead to an increased activity of the 
secreting epithelium, producing an increased flow of urine, and 
similarly, a diminished blood supply will have the opposite effect. 
If one of the nerves going to the kidney be divided and the periphe- 
ral end stimulated, any increase in the secretion of urine which 
might follow would not necessarily prove that this nerve contained 
secretory fibers, because the same effect might be produced by the 
stimulation of vasodilator fibers. 
The secretion of urine by the kidney is continuous; it passes 
through the ureters, into the bladder, where it accumulates until 
the act of micturition takes place. 
Urine 
Human urine is a yellowish, straw or amber-colored fluid, usually 
acid in reaction, the acidity being mainly due to acid sodic phos- 
phate. The specific gravity varies from 1015 to 1025, and the 
quantity passed in 24 hours runs from 1000 c.c. to 2000 c.c. which 
will contain about 50 grams ounces) of solids. 
The Composition of Urine is very complex (see board). It de- 
pends to a considerable extent on the quantity and quality of the 
food. The most abundant constituents are water, urea and sodium 
chloride. The constituents of the urine may be divided into 
inorganic and organic substances. 
The inorganic constituents are chlorides, phosphates and sul- 
phates of sodium, potassium, ammonium, calcium and magnesium, 
the chief of which is sodium chloride. 
The organic constituents are urea, uric acid, hippuric acid, 
creatinin, etc., the chief being urea. Most of the nitrogen of the 140 
HUMAN PHYSIOLOGY 
body is excreted in the form of urea, so that the daily amount of 
this susbstance may be taken as a rough measure of the protein 
metabolism of the body. One hundred grams of protein, oxidized 
in the body, yield 33 grams of urea or 16 grams of nitrogen. As 
we have seen already, much of this is formed in the liver. 
There are chemical tests, both qualitative and quantitative, for 
all the important constituents of the urine. A careful physical 
and chemical examination of the urine is of great value to the 
physician in the diagnosis of disease. 
When the kidneys fail to act the constituents of the urine are 
not removed but accumulate in the blood, and this leads to a 
condition of poisoning known as "uraemia" which is rapidly fatal 
if not relieved. 
The abnormal substances most commonly found in the urine in 
disease, are blood, bile, sugar, albumin and pus. SKIN 
The skin forms the outer covering of the body, becoming con- 
tinuous at the various orifices with the mucous membrane which 
lines the internal cavities. 
Structure of Skin 
It is made up of two layers, the cuticle, epidermis or scarf-skin, 
and the dermis or true skin. The epidermis, which lies on the 
surface, consists of stratified squamous epith elimn, arranged in 
several layers. The most superficial layer consists of keratin, 
a homy substance, which is quite impervious to water. The 
epidermis is pierced by the ducts of the sweat glands and by the 
hairs, where these are present, but it contains no blood vessels 
so that bleeding doesnot take place when the cuticle is cut or 
shaved off, until the true skin is reached. It is thickest on the palms 
of the hands and the soles of the feet. 
The dermis or true skin forms the chief part of the bodily cover- 
ing. It is composed of fibrous tissue, dense in the part next to the 
epidermis, looser deeper down, near the subcutaneous fat; it is 
very vascular and contains the terminal organs of the sensory fibers 
of the skin. It passes insensibly into the areolar, subcutaneous 
tissue which contains numerous groups of fat cells. The superficial 
layer of the dermis is raised up into projections called papillae, 
on which the epidermis is moulded, and this prevents the latter 
from being easily separated or rubbed off. The root of the hair 
in its follicle lies in the true skin, and its stem or shaft penetrates 
the epidermis obliquely. A small muscle consisting of non-striped 
fibers is attached to each hair follicle so that when it contracts the 
hair is erected or set on end. It is called the arrector pili muscle. 
Glands of the Skin--These are of two varieties-the sebaceous 
and the sweat glands. 
The Sebaceous Glands are small, saccular glands the ducts of 
which open into the hair follicles. They are lined by epithelial 
cells, and the sebaceous material secreted by these is poured into 
the hair follicles and serves to lubricate the roots of the hairs and 
the skin. 
The Sweat Glands are found over the whole skin, but they are 
especially numerous on the palms of the hand and the soles of the 
feet. They consist of very long, simple tubules, the deeper or 
secretory portion being coiled up in the dermis, with the duct 
piercing the epidermis and opening on to the surface. The sweat is 
secreted by the epithelial cells that line the deeper part of the 
141 142 
HUMAN PHYSIOLOGY 
tubule and escapes through the duct, spreading over the surface 
of the skin. It has been estimated that there are two million 
sweat glands in the skin, over the whole body. 
The mammary glands are modified sweat glands which secrete 
milk instead of sweat. 
Secretion of Sweat 
The amount of sweat secreted in the 24 hours varies greatly 
from day to day, depending upon outside temperature and mus- 
cular exercise mainly, and no definite figure can be given, but 
probably it is safe to say that, on an average, as much water is 
eliminated by the skin as by the kidneys. When the sweat is 
produced in small quantity it evaporates rapidly, and never be- 
comes visible on the surface; this is known as insensible perspira- 
tion. When it is secreted more rapidly, and there is not sufficient 
time for it to be completely evaporated, it gathers as droplets on 
the skin and this is known as sensible perspiration. If the air is 
warm, dry, and in constant motion, much sweat will be evaporated 
without ever becoming visible. The amount of sweat discharged on 
the skin, other things being equal, will be inversely proportional 
to the quantity of urine secreted by the kidney. 
Animals differ greatly in the amount of sweat which they secrete. 
The horse sweats very freely over the whole body, while the dog 
and cat have no glands except on the pads of the feet, and rabbits, 
rats, mice and goats are said not to sweat at all. In man the skin 
practically all over the body perspires, but is more active in some 
regions, such as the palms of the hands, the soles of the feet and 
the skin of the forehead, than in others. 
Influence of Nervous System on Secretion of Sweat-The 
secretion of sweat is a reflex phenomenon and is under the control 
of the nervous system, as we have seen when discussing heat 
regulation. This can be proved by the following experiment: 
The cat, when exposed to a warm atmosphere, sweats freely from 
the pads of the feet. If the sciatic nerve be divided in a cat and 
the peripheral end stimulated, a copious secretion of sweat will 
appear on the pad of the corresponding foot. When the wound 
heals in this animal, if it be placed in a warm room sweat will be 
formed on the pad of the sound hind limb but none on the side 
where the nerve has been cut. This proves that the application 
of HEAT TO THE SKIN HAS NO DIRECT EFFECT ON THE SWEAT GLANDS. 
The sweat center is situated in the medulla oblongata, and can 
be acted on reflexly from the periphery or from the higher centers 
in the cerebrum. Strong emotion, such as fear, may stimulate the 
sweat center and cause the "cold sweat to break out." 
Sudorifics-Certain drugs which increase the secretion of sweat 
are called sudorifics. Some of these appear to act on the sweat THE SKIN 
143 
center and others on the terminations of the secretory nerves in 
the glands. Of the latter, pilocarpin is the best example, while 
atropin has the opposite effect, paralyzing the secretory fibers. 
Camphor and ammonium acetate stimulate the sweat center. 
Composition of Sweat-It is very difficult to obtain pure sweat 
for chemical analysis because it is mixed with the secretion from 
the sebaceous glands, usually. It is a thin, watery fluid, of low 
specific gravity (1004) and contains only very small quantities of 
solids. The most abundant are sodium chloride and urea, but 
normally these are present in little more than traces. CO2 may 
also be found, but in very minute quantities; sweat in health is 
practically pure water. In disease, where the kidneys are not 
functioning properly, a considerable amount of urea, and also of 
the other constituents of the urine, may be found in the sweat. 
Under these conditions, the skin appears to be able to act vicari- 
ously for the kidney, to a considerable extent. 
Functions of the Skin 
The following functions belong to the skin: 
1. It forms a protective and sensitive covering for the body. 
2. It regulates body tempepature through the blood supply 
and the secretion of sweat. 
3. To only a slight extent in health, but to a considerable extent 
in kidney disease, it serves to eliminate waste products from 
the body. 
4. In mammals it provides nutriment for the off-spring, 
since in the female, the mammae are modified sweat glands. 
5. In thin, moist skinned animals, such as the amphibia, a large 
amount of respiratory exchange takes place through the skin. THE ENDOCRINE ORGANS OR GLANDS OF INTERNAL 
SECRETION 
The glands so far considered, discharge their secretions through 
a duct on to a free surface; these might be called external secre- 
tions. Other glands, still to be studied, manufacture secretions 
which do not escape through a duct but pass directly, or through 
the lymphatics, into the blood vessels. These are termed internal 
secretions and the organs which produce them, glands of internal 
secretion or Endocrine Organs. The study of these glands, their 
secretions and the effects of them on the body form the science of 
Endocrinology. 
Many of these have no ducts and are termed ductless glands; 
others, such as the pancreas and pituitary, produce both external 
and internal secretions and are accordingly provided with ducts. 
These internal secretions belong to the group of substances 
termed hormones which are believed to act in the body as chemical 
co-ordinating agents. This method of correlating the activities 
of organs was originally the only method, before the evolution of 
a nervous system, and still persists, even in the higher animals, 
where rapidity of action is not required. When the activity of one 
organ A, is to excite another organ B, the activity of B may be 
evoked, either by a nerve impulse, passing from A to the central 
nervous system and from this to B (reflex action) or by a special 
chemical substance, produced at A and carried in the bloodstream 
to B (hormone action). 
The following is a list of the glandular structures in the body 
that are believed to produce internal secretions. Some have ducts 
and others are ductless. 
Glands of Internal Secretion 
Suprarenals 
Thyroid 
Parathyroids 
Pineal Body? 
Ovaries 
Ductless Glands, 
Pyloric glands of stomach 
Duodenal glands 
Pancreas 
Pituitary 
Testes 
144 INTERNAL SECRETION 
145 
SUPRARENAL CAPSULES OR ADRENAL GLANDS 
In man, one of these glands lies on the upper end of each kidney, 
forming a sort of capsule, hence the name suprarenal gland or cap- 
sule ; in most other animals they are situated near the kidney, 
and so are termed adrenal glands. 
Structure 
The gland is surrounded by a connective tissue capsule which 
sends in septa between the columns of cells of which the organ is 
composed. When sectioned it is seen to consist of an outer cortex, 
yellowish in color, and an inner medullary portion in the center. 
The cortex is composed of epithelial cells, that are arranged in 
radial columns which are supported by strands of connective tissue, 
in which lie many capillaries. The cells of the medulla form an 
irregular network; they are granular and often pigmented, and 
they contain a substance which is stained brown by the salts of 
chromic acid. This is called chromaffine substance on account of 
its affinity for stains or dyes. 
The cortex and the medulla have a different origin develop- 
mentally, the former coming from the mesoderm, the latter from the 
ectoderm. They are also believed to have distinct and separate 
functions. In the elasmobranch fishes the two exist as separate 
organs, and are not associated anatomically in the way they are 
in mammals, the cortex being represented by the interrenal body 
and the medulla by the paired bodies which lie near the ganglia 
of the sympathetic chain. 
Functions of Suprarenal Glands 
The glands of internal secretion are not infrequently diseased in 
man, and much of our knowledge of their functions has been derived 
from a study of the symptoms arising under these conditions. In 
investigatingthefunctions of these glands generally, we may study 
1, the effects of non-development or disease in man; 2, the effects 
of their removal in animals, and 3, the effects of extracts, made 
from the glands, when injected into other animals or their adminis- 
tration to man as therapeutic agents. We shall follow this order in 
the case of the suprarenal glands. 
In the physiology of the adrenals there are four epoch-making 
discoveries which may be considered.-1, the relation of the de- 
struction of suprarenals to a group of symptoms first observed by 
Addison in 1855; 2, the fact that intravenous injection of an extract 
of the medulla causes a very marked rise in blood pressure discovered 
by Oliver and Schafer in 1894; 3, a third very important addition 
to our knowledge of the subject was made in 1902 when Takamine 
succeeded in isolating this pressor substance and preparing it in the 146 
HUMAN PHYSIOLOGY 
pure crystalline form; 4, and a fourth important generalization 
was made when it was shown that all the various actions of the 
pure secretion, adrenaline, are due to is stimulation of the nerve 
endings of the sympathetic nervous system. 
1. Effects of Disease in Man--The physician Addison of 
London, in 1855, first pointed out that destruction of the supra- 
renal capsules, by tubercular disease, led to a train of symptoms 
of which the following are the chief: (a) General muscular weak- 
ness and prostration, with frequently low blood pressure; (b) pig- 
mentation of the skin; (c) attacks of vomiting. The condition, 
almost invariably, ends fatally. It is now known as Addison's 
disease. It is a rare disease, and usually attacks young adults, 
being more common in men than in women. There is feeble heart 
action, loss of appetite, disturbances of the digestive tract and 
other symptoms which can be accounted for by loss of the normal 
stimulation of the peripheral endings of the sympathetic nervous 
system. 
The most striking feature of the disease is the pigmentation of 
the skin which may represent anything from a slight bronzing to 
the dark hue of a negro. Microscopically the pigment is found in 
the deepest cells of the epidermis. The cause of this pigmentation 
is not known. 
The excessive muscular weakness is always pronounced in Addi- 
son's disease. The patient is always tired without any apparent 
cause. The condition is almost invariably fatal. 
2. Effects of Removal-In animals, when the whole gland is 
removed, on both sides, death follows in from two to four days. 
3. Effects of Injection of Extracts of the Gland-A watery ex- 
tract of the whole gland, when injected into the vein of an anaesthe- 
tized animal, raises the general blood pressure markedly. The 
same effect is obtained from an extract of the medulla alone, and this 
is due to the fact that the medullary portion contains a hormone, 
which excites the nerve endings in the non-striped muscle of the 
arterioles, thereby causing their constriction. With both vagi 
intact the heart rate is slowed, probably a reflex effect through 
the depressor nerve fibers caused by the high blood pressure. 
When the vagi are cut across and all nerve connections destroyed 
the heart beats faster and more powerfully showing that on the 
heart muscle itself the action of adrenaline is favorable. It has 
the specific property of stimulating the nerve terminations of the 
sympathetic system everywhere. 
Cannon has shown that intravenous injection of adrenalin 
causes an increase in blood sugar. Stimulation of the nerves of the 
adrenals have the same effect and this is believed to explain the 
increase of sugar in the blood, which follows emotional excitement. 
The hyperglycemia may lead to glycusoria and this temporary or INTERNAL SECRETION 
147 
emotional glycusoria has been observed in students after examina- 
tion and in the spectators at athletic contests. In this case the 
adrenaline may act on the sympathetic end-organs in the liver so 
as rapidly to transform the glycogen stored in the liver into sugar. 
Whether adrenaline acts on striated muscle has not been proven. 
The beneficial effect on muscular power which appears to follow 
adrenaline administration when studied by means of the ergograph 
may be due to the increased amount of sugar set free in the circu- 
lation. 
It is said that adrenaline shortens the time of blood coagulation. 
If this is so it is the only effect which cannot be explained as the 
result of sympathetic stimulation. 
It is probably the diminished quantity or absence of this hormone 
which leads to the low blood pressure and muscular weakness in 
Addison's disease. 
This hormone was isolated by Takamine and is called adrenalin. 
It is a comparatively simple substance, chemically, and has been 
synthetized in the laboratory and prepared artificially in this way. 
It has the following formula: C6 H3 (OH)2. CH OH. CH2. NH2. CH3 
and is chemically known as dioxy-phenyl-ethanol-methylamin. It 
is used extensively in medicine to stop haemorrhage, because it has 
a powerful constrictor action on blood vessels. This substance is 
being produced continuously in small quantity, by the medulla of 
the suprarenal capsule, passed into the blood, and carried to the 
tissues where it probably helps to maintain the tonus of the 
bloodvessels. 
The Function of the Cortex is not known. It is difficult to in- 
vestigate, in mammals, because it cannot be removed without, at 
the same time, injuring or interfering with the blood supply of the 
medulla. Injection of its extract produces no effect at all, neverthe- 
less, it is believed that the cortex is essential to life, for, when 
this part is removed in fishes, where it exists as a separate organ, 
and can be taken away, without damaging the medullary tissue, 
death follows, whereas, if the medullary portion alone is removed, 
it leads to no bad effects. 
There is some evidence of a relationship between the cortex 
and the generative organs but the real nature of this is unknown. 
Castration is said to cause hypertrophy of the adrenal cortex in 
male animals. The chemical composition of the adrenal cortex 
shows a large quantity of phosphorus-containing lipoids which by 
some are held to be very significant. 
THYROID GLAND 
The thyroid gland lies in the neck and, in the human subject, 
it consists of two lobes, a right and a left, situated one at each side 148 
HUMAN PHYSIOLOGY 
of the upper part of the trachea, and united across the middle line 
by a narrow isthmus. In many animals the isthmus is represented 
by little more than a few strands of connective tissue so that the 
thyroid may be regarded as bilateral. 
Structure 
It is surrounded by a connective tissue capsule and consists of a 
framework of connective tissue, enclosing a large number of small 
saccules or vesicles. Each vesicle is lined by a single layer of 
cubical epithelium and contains a translucent, viscid material termed 
colloid. This colloid substance is believed to be secreted by the 
-epithelial cells lining the vesicles and to contain the active prin- 
ciple of the gland, a substance rich in iodine and named thyroidin 
first isolated by Kendall. The gland is very vascular. 
The function of the Thyroid Gland 
This, as in the case of all the other glands of internal secretion, 
was for a long time an enigma, but these organs were not considered 
to be of much importance to the body. Wharton (1656) states 
that the thyroid gland has four functions: "1, The first and princi- 
pal use of these glands appears to be to take up certain superfluous 
moistures from the recurrent nerve and to bring them back again, 
into the vascular system, by their own lymph channels. 2, To 
cherish the cartilages to which they are fixed which are rather of a 
chilly nature by their own heat. 3, To conduce by their exhalations 
to the lubrication of the larynx and so to render the voice smoother, 
more melodious and sweeter. 4, They contribute much to the 
rounded contour and beauty of the neck; for they fill up the empty 
spaces about the larynx and make its protuberant parts almost to 
subside and become smooth, especially in the female sex, to whom, 
on this account, a larger gland has been assigned,which renders 
their necks more even and beautiful." 
Diseases associated with Abnormal Conditions 
of the Thyroid Gland 
There are three diseases, in the human subject, the cause of 
which is believed to be some abnormal state of the thyroid gland, 
viz.: Myxoedema and cretinism, due to absence or diminished 
secretion of the specific hormone which it produces, and exoph- 
thalmic goitre or Grave's disease, due to over secretion. The first 
two are included under the general term hypothyroidism and the 
last under the term hyperthyroidism. 
Myxoedema (Gull's disease) is a condition that appears in adults, 
most frequently in women. The symptoms are: 1, overgrowth of 
the subcutaneous connective tissue, leading to swelling of the skin 
or oedema, particularly of the hands and face; 2, slow heart beat- INTERNAL SECRETION 
149 
the rate may be down to 40 per minute, 3, mental dulness; the 
patient, who used to be bright and active, becomes slow and clumsy 
in mind and body. There is a general depression of all the func- 
tions accompanied by a lowered bodily metabolism, almost always 
associated with a condition of mental depression. The thyroid 
gland may be enlarged (goitre), but if this is the case, it isnot a true 
overgrowth of the essential gland tissue but is due to a hyper- 
trophy of the connective tissue frame work, leading to an oblitera- 
tion of many of the vesicles and a corresponding diminution in the 
secreting power of the gland. 
The above symptoms, if not remedied, will result in death, which 
may take place only after some years, however. They disappear 
rapidly when the essential substance is supplied by administering 
to the patient the thyroid gland (or its extract) of one of the 
lower animals such as the sheep or pig. When this treatment is 
stopped the symptoms appear again, because the tissues are de- 
prived of the active substance present in the gland. Avery small 
quantity (a few milligrams) of thyroxine administered daily will 
have the same beneficial effect as the gland itself, or its extract. 
Cretinism is the same condition, in the child, as myxoedema in 
the adult, resulting from defective thyroid secretion--a condition 
of hypothyroidism. The gland may be absent at birth, or under 
developed, or diseased. The child, in such a case, remains dwarfed 
and undeveloped in mind and body; at thirty it is a stunted, pot- 
bellied creature with the intelligence of a child of four or five 
years. The treatment is the same as for myxoedema, viz.: the ad- 
ministration of the proper quantity of sheep's or pig's thyroid or 
thyroxine and it must be kept up throughout life, as in the case 
of the adult. 
Iodine is a very important constituent of the thyroid hormone, 
thyroxine, and in regions where it is deficient in the drinking water 
the addition of small quantities of potassium or sodium iodide are 
believed to be useful in counteracting the effects of hypothyroid 
conditions. 
Exophthalmic Goitre or Grave's Disease-This condition is be- 
lieved to be due to excessive thyroid secretion. The cardinal 
symptoms are just the opposite of those of myxoedema, viz.: 
1, goitre, or swelling of the thyroid gland; 2, exophthalmos or 
protrusion of the eyeballs; 3, rapid heart action-the rate may be 
120, or over, per minute; 4, muscular tremors and excessive 
nervousness. The same symptoms may be induced in normal 
individuals by the administration of thyroid extract, leading to 
the presence in the body of an excess of the active substance. 
There is a great increase in oxidation in the body as indicated by 
observations on the basal metabolism, a condition the reverse of 
what is found in hypothyroidism. 150 
HUMAN PHYSIOLOGY 
The treatment here indicated is removal of part of the thyroid, 
in order to lessen the secretion or tying off some of the blood- 
vessels supplying the gland. If all the gland be removed, however, 
myxoedema will follow. Grave's disease is usually fatal if not treat- 
ed in time. 
Extirpation of the Thyroid Gland in animals leads to cretinism 
or myxoedema, according to the age of the animal, while the ad- 
ministration of the gland, or of its extract, by mouth or subcu- 
taneously, produces symptoms of exophthalmic goitre. 
Function of the Thyroid Gland-Judging from the facts related 
above, the function of the thyroid gland would appear to be to 
produce an internal secretion (hormone), in this case thyroxine- 
a substance rich in iodin-which is essential for the proper growth 
and functional activity of the tissues, particularly those of the 
nervous system. 
It has a profound effect on growth and development. The 
metamorphosis of tadpoles and of salamander larvse into adult 
forms are greatly influenced by thyroid feeding. Tadpoles which 
receive small amounts of thyroid in their food are quickly trans- 
formed into very small frogs, while those which have the thyroids 
removed by operation do not metamorphose but grow into large 
tadpoles. 
It has an important effect on the skin and its appendages. In 
thyroidectomized sheep and goats the wool and hair fall out and 
horn growth is greatly retarded. 
PARATHYROID GLANDS 
The parathyroids are small glands, as a rule four in number, 
situated somewhere in relation to the thyroid. There are usually 
two pairs,-an internal pair and an external pair. In the herbivora 
the internal pair are imbedded in the substance of the thyroid 
while the external pair lie outside, usually at some distance from 
the gland. In the carnivora both pairs are either imbedded in 
the substance of the gland or lie within its capsule. 
In Structure the parathyroid is unlike the thyroid. It consists 
of elongated and irregular columns of granular epithelial cells, held 
together by strands of connective tissue extending from the sur- 
roundingcapsule. The cells do not form vesicles as in the thyroid. 
The gland has a very rich blood supply. 
It is not certain whether any specific disease in the human subject 
results from pathological changes in the parathyroids, but many 
believe that the condition known as tetany or "convulsions," 
sometimes seen in children, is caused by a deficiency in parathyroid 
secretion. The connection is not so certain, however, as that be- 
tween cretinism and the thyroid, for example. INTERNAL SECRETION 
151 
Extirpation of all the parathyroids in animals leads to acute 
muscular tetany, with high temperature (the result of severe mus- 
cular contraction) and rapid breathing. Death follows within a 
few days. Similar symptoms have been observed in a few cases, in 
the human subject, where the surgeon has accidentally removed 
the parathyroids along with the thyroids. 
Feeding with parathyroid tissue, or the administration of calcium 
salts, is said to relieve the symptoms both in the human subject 
and in animals, but parathyroid transplantation is most effective. 
Function of the Parathyroids--According to some these glands 
are believed to secrete a substance which is essential for the proper 
metabolism of calcium in the body. The muscular tetany, resulting 
from the removal of the parathyroids, is said to be caused by the 
diminished quantity of calcium in the brain. The substance or 
hormone, secreted by the glands, has not been isolated as it has in 
the case of adrenalin and thyrocine. 
The latest theory of parathyroid function, advanced by Noel 
Paton, is that this gland removes from the circulation certain toxic 
substances produced in normal protein metabolism, viz.: guanidine 
and methylguanidine. If these are allowed to accumulate, as 
happens when the parathyroids are absent or diseased, intoxication 
indicated by mucular spasms and other symptoms follow which 
usually ends fatally. 
The following facts are advanced in favor of this view-1. The 
eating of protein foods aggravates the symptoms in para-thyroidecto- 
mized dogs and hastens death. 2. When salts of guanidine are 
injected into the veins of normal animals tetanic convulsions 
follow. 3. In dogs from which the parathyroids have been removed 
guanidine and methylguanidine are found in excessive quantity 
in the blood. 4. A frog's nerve-muscle preparation placed in 
blood drawn from a dog from which the parathyroids have been 
removed will show tetanic twitchings. 
PINEAL BODY OR GLAND 
This is a small body of a reddish color, found in the upper part 
of the brain, between the two cerebral hemispheres. It has the 
structure of a secreting gland, with cells of epithelial origin, con- 
nective tissue framework, etc., but little is known regarding its 
function. 
In children, disease of this organ, leading to its destruction, is 
held by some to be followed by the early onset of puberty, with 
great sexual and mental precocity, and early death. If this is so, 
the internal secretion produced by the pineal gland, might be re- 
garded as having an inhibitory influence on the development of the 
sexual organs. 152 
HUMAN PHYSIOLOGY 
Whether the pineal gland should be included amongst the endo- 
crine organs may be regarded as an open question. 
GENERATIVE GLANDS OR GONADS 
Besides their generative functions, these organs (ovaries and 
testes) are believed to produce internal secretions which have a 
beneficial effect on the general body metabolism, especially of the 
nervous system. Castration in both sexes is followed by obesity, 
and dulled mentality. When a young male animal (deer or sheep 
for example) is castrated, the secondary sexual characters, such as 
the appearance of horns in the deer or sheep, or the growth of a 
beard in man, do not develop, nor does the sexual appetite appear 
at the age of puberty. If the duct, through which the spermatic 
fluid (external secretion of testes) escapes be tied, on both sides, 
no such results follow however. This would appear to prove that 
an internal secretion is produced in the interstitial cells of the testes 
which normally promotes the growth of the secondary sexual char- 
acters,-horns, beard, etc. 
Steinach tied the spermatic duct on both sides and after some 
months he reported an increase in sexual instinct and in general 
vigor. His explanation is that the cells of the seminiferous tubules 
degenerate while the interstitial tissue undergoes hypertrophy ac- 
companied by an increased production of sex hormone. 
When the ovaries are removed from young animals the growth of 
the uterus is arrested and in the human subject menstruation does 
not begin, or, if the operation is performed after sexualmaturity.it 
ceases. 
PYLORIC AND DUODENAL GLANDS 
The glands at the pyloric end of the stomach and those of the 
duodenum, which produce as internal secretions the hormones 
gastrin and secretin respectively, have already been considered 
under digestion. To the one (gastrin) the term "hormone" was 
first applied, and the other (secretin) was the first of the group of 
hormones to be discovered. 
PANCREAS 
This, like the testes, pyloric and duodenal glands, is a gland with 
both an internal and an external secretion, the latter having been 
taken up in the study of digestion in the small intestine. 
Removal of the Pancreas in Animals is followed by the appear- 
ance of sugar in the urine, which is secreted by the kidney in ex- 
cessive amount; abnormal thirst and hunger, with emaciation and INTERNAL SECRETION 
153 
death in from two to four weeks. Practically the same symptoms 
are present in the disease known as diabetes mellitus in man. 
That these results are not due to the want of the external se- 
cretion (pancreatic juice) is proved as follows: 
1. If, instead of removing the gland, the duct is ligatured or 
blocked with solid paraffin, thus preventing the entrance of the 
external secretion into the duodenum, glycosuria (sugar in the 
urine) and the other symptoms, do not appear. 
2. If a portion of the pancreas, after removal from the abdom- 
inal cavity, is grafted under the skin or in muscle, no symptoms 
follow. 
In these two cases the connection with the duodenum is com- 
pletely cut off, so that none of the pancreatic juice can be available 
for digestion, and yet no symptoms follow. This proves that the 
cause of the symptoms, when the pancreas is removed, is not 
want of the pancreatic juice. 
On the basis of experiments such as the above, it is believed 
that the pancreas forms an internal secretion which plays an im- 
portant part in carbohydrate metabolism. 
Islets of Langerhans-These have been mentioned in the general 
description of the pancreas given under digestion. They consist 
of islands composed of granular epithelial cells, belonging to the 
secretory type, with a rich blood supply, and showing no connection 
with the secreting tubules of the gland which produce the external 
secretion. There is considerable evidence to show that the internal 
secretion of the pancreas comes from these islets of Langerhans; 
they show pathological changes in cases of diabetes mellitus in man. 
Functions of the Internal Secretion of the Pancreas-Several 
theories have been put forward to explain the action of this 
internal secretion. 1. Its presence in the portal circulation may be 
necessary to enable the liver cells (a) to transform sugar into 
glycogen, or (b) to check the too rapid retransformation of glycogen 
into sugar. (See Glycogenic Function of Liver, p. 119). 2. It 
may be required by the tissues, particularly muscle, to enable them 
to utilize the sugar circulating in the blood. On any one of these 
theories, absence of the secretion or hormone, would lead to hyper- 
glycaemia (excess of sugar in the blood) and glycosuria (presence 
of sugar in the urine). 
Since the secretion is effective after boiling, the active substance 
is a hormone and not a ferment. It has not been isolated, so far. 
We have seen that in the case of thyroid deficiency the adminis- 
tration of thyroid substance or of the hormone thyroxine in the 
proper quantity cured the condition for the time being. This would 
seem to indicate that similar treatment, viz.: the administration of 
pancreatic substance containing its active principle, would have 
some beneficial influence on diabetes mellitus. This has been tried 154 
HUMAN PHYSIOLOGY 
but without success. Recently the reason why seems to have been 
given by Banting and Best. These experimenters ligatured the ducts 
of the pancreas in the dog,allowing the animal to live for some weeks. 
On removing the pancreas at the end of that time it was found that 
the acinar cells producing the external secretion had become 
atrophied but not the islets of Langerhans. On injecting an ex- 
tract of this degenerated pancreas into a dog suffering from diabetes 
it was found that the sugar disappeared from the urine at once. 
The apparent cause of the ineffectiveness of an extract of the 
normal pancreas is that the enzymes of the pancreatic juice, 
notably the trypsin, destroy the sugar regulating hormone believed 
to be secreted by the islets of Langerhands and now termed insulin. 
McLeod and his co-workers have shown that active insulin may 
be obtained from the normal pancreas if it be treated with mineral 
acid sufficiently concentrated to destroy the digestive enzymes 
immediately after the animal is killed. 
Insulin is now being used extensively in the treatment of diabetes 
in man. Very small quantities of it quickly lower the blood sugar 
but only for some hours after administration so that it is not a cure. 
In order to keep the blood sugar to the normal limit so that none 
shall appear in the urine insulin must be injected subcutaneously 
once or twice daily. 
By giving too much insulin the blood sugar in normal animals 
may be reduced to such an extent that convulsions follow which 
may end in death unless the effect be counteracted by injecting 
glucose. 
PITUITARY BODY OR HYPOPHYSIS CEREBRI 
The so-called pituitary body is a composite organ consisting of 
two lobes, a glandular lobe and a nervous lobe. It is attached to 
the base of the brain by a slender stalk (stalk of pituitary) and 
lies in a small cavity, in the sphenoid bone, termed the sella turcica 
(Turkish saddle). 
Embryologically the two parts are quite distinct. The large 
anterior or glandular lobe is derived from the epithelium of the 
mouth cavity, while the smaller posterior or nervous lobe is de- 
veloped from the brain. Part of the glandular lobe surrounds, al- 
most completely, the nervous portion and this is termed the pars 
intermedia; it consists of epithelium and is included in the poste- 
rior lobe. 
Acromegaly-Hypertrophy of the anterior lobe in man gives rise 
to the condition known as acromegaly in which there is a great 
overgrowth of bone in particular, and other changes that, after a 
variable time, prove fatal. 
Giantism or Gigantism-If the hypertrophy of the gland takes INTERNAL SECRETION 
155 
place before the epiphyses have united, a great increase in the de- 
velopment of the long bones results and the individual becomes a 
giant in stature the condition being known as gigantism. If the 
disease does not develop until after ossification is complete the 
effects are most evident in the thickening of the long bones, par- 
ticularly at the extremities. There may be some lengthening of the 
spinal column leading to kyphosis (curvature of the spine) but not 
to an increase in stature. The bones of the face enlarge, particu- 
larly the lower jaw. An early symptom is failure of the sexual power. 
This condition is termed acromegaly as distinguished from gi- 
gantism. 
Removal of the whole gland, in animals, was believed to be fatal 
in a few days, as also removal of the anterior lobe alone, while the 
taking away of the posterior lobe alone leads to no ill effects. 
Recent work would seem to show that in dogs complete removal is 
not always fatal. The animals show a general tendency to put on 
fat and the reproductive glands undergo degeneration. 
Removal of the pituitary in young dogs causes dwarfing to a 
marked degree, associated with obesity and a low degree of intelli- 
gence. Basal metabolism is decreased and in many respects the 
condition resembles that due to removal of the thyroids. 
Injection of extracts of the posterior lobe intravenously, in an 
anaesthetized animal, is followed by: 1, a rise in the blood pressure; 
2, increased flow of urine; 3, secretion of milk in lactating animals. 
In this case, however, there does not appear to be a real increase 
in the secretion of milk but rather a more effective emptying 
of the gland by stimulation of plain muscle. There is no increase 
in the quantity of milk yielded in the twenty-four hours. 
While posterior lobe extract stimulates plain muscle generally it 
appears to have a special action on the parturient uterus and on 
this account is extensively used in midwifery. 
It has a very pronounced action on the kidney leading to a 
remarkable increase in the amount of urine excreted. 
Injection of extracts of the anterior lobe produces no evident 
effects at all, and yet, it is the anterior lobe which is essential to 
life, while the physiologically active posterior lobe is not. 
The excretion from the posterior lobe, or a part of it, is external 
and is poured into the third ventricle of the brain, where it mixes 
with the cerebro-spinal fluid; the secretion of the anterior lobe is 
internal and passes directly into the blood vessels. 
Functions of the Posterior Lobe-It produces a hormone (or 
hormones) which, 1, helps to maintain the tonus of non-striped 
muscle; 2, stimulates the kidney to secrete urine; 3, stimulates 
the mammary gland to secrete milk. It may be that a single sub- 
stance brings about all these different effects, or, which is more 156 
HUMAN PHYSIOLOGY 
probable, that there are three distinct hormones furnished by the 
posterior lobe. 
Functions of the Anterior Lobe-This lobe is believed to produce 
one or more hormones which are connected with the processes of 
growth, particularly of the skeleton. A substance tethelin has 
been prepared from the anterior lobe which is said to produce an 
increase in growth in young animals to which it is fed. Its chemical 
constitution has not been determined. 
Much is still unknown regarding the physiology of the pituitary 
gland. 
LIVER AND KIDNEY 
The term "internal secretion'' was first applied by Claude 
Bernard in relation to the liver. He designated bile the "external 
secretion'' and, in contradistinction, the glycogen formed in the 
liver and absorbed as sugar into the bloodvessels the "internal 
secretion.'' This, however, is not a hormone and is not now re- 
garded as a true internal secretion. 
It is reported by some investigators that a true internal secretion 
is produced in the kidney which helps to maintain the tonus of 
the blood vessels, but this statement needs confirmation. 
The Spleen is a ductless gland but there is no evidence that it 
produces an internal secretion. It is not essential to life, since it 
may be entirely removed in animals without any permanent ill 
effects. In foetal life and early infancy it is believed to manufacture 
red blood corpuscles, and probably also in adults, after severe 
haemorrhage. THE SENSES 
The Sensory Mechanism consists of 1, peripheral end organs or 
receptors; 2, a chain of neurons for the transmission of the im- 
pulses arising in the receptors; 3, sensory centers in the brain 
where the impulses arrive and make their impression. 
The receptors are subdivided into three groups, 1, those that 
are situated on the outer surface of the body, termed the extero- 
ceptors; 2, those in the alimentary canal and viscera, the entero- 
ceptors; 3, those found in the muscles, tendons and joints, and in 
the semicircular canals of the ear, the proprioceptors. 
Adequate Stimulus-The end organ of each sense has its own 
specific stimulus; e. g. the retina is excited by light waves but not 
by sound; the hair cells of Corti, in the cochlea of the ear, by sound 
waves and not by light, etc. This is known as the adequate 
stimulus; light is the adequate stimulus for the eye, sound for 
the ear, pressure for certain end organs in the skin, etc. 
Law of Specific Nerve Energy-Stimulation of the end organs 
belonging to any one of the senses gives rise to the sensation peculiar 
to that sense, e. g. ethereal waves, of a certain wave length, falling 
on the retina, give rise to the sensation of sight, but the same 
sensation is aroused when the retina is excited mechanically, as, 
for example, by applying pressure to the eyeball, or by using an 
electrical stimulus. This is known as Muller's law of specific 
NERVE ENERGY" 
Threshold Stimulus--For each sense a certain minimal strength 
of stimulus is necessary to produce the sensation, and this is 
called the liminal or threshold stimulus. This will vary in 
different individuals, and, from time to time, in the same individual, 
depending on the excitability of the end organ. Fatigue, for ex- 
ample, will raise the threshold value and rest will lower it. The 
phenomenon of summation is seen in sensation, as in reflex action, 
where a series of subminimal stimuli, applied in succession, will 
reach the -threshold value and produce an effect. 
Weber's Law-The increase of stimulation necessary to produce 
the smallest perceptible change in a sensation is proportional to 
the strength of the stimulus already acting. For example, if a 
weight of 30 grams be held in the hand, an extra half gram added 
will produce no perceptible difference, but one gram will; if the 
original weight is 60 grams then to make a perceptible increase in 
the sensation 2 grams must be added. Similarly, if a room is 
illuminated with 100 candles one more candle added will make a 
perceptible difference in the illumination, but if there be 1000 
157 158 
HUMAN PHYSIOLOGY 
candles, 9 additional candles will make no difference; 10 will be 
required, that is 1 /100th of the original stimulus. This is known 
as Weber's law. It is only true within certain limits, however. 
One drawback in the study of sensory phenomena is the want 
of a unit by which the end reaction may be measured. In the motor 
nerve we have the muscle which reacts, or the action current, and 
both can be measured quantitatively, but the end reaction for a 
sensory nerve is a state of consciousness for which we have no 
standard of measurement. 
The sensations derived from the skin are those of pressure of 
various kinds, heat, cold, pain. The end organs representing these 
different sensations have not been made out with certainty, but 
it is known that there are localised areas or spots on the skin which, 
when stimulated, give rise to different sensations. 
Warm, Cold, Pressure and Pain Spots-If a blunt pointed wire 
be cooled in ice, and then drawn slowly over the back of the hand, 
every now and again it will come on a spot where it is felt to be 
cold; these spots are termed "cold spots" and can be marked with 
ink; they do not shift their position. If now the wire be heated, 
and the operation repeated, a number of spots will be found which 
give a sensation of warmth. These may also be marked with ink; 
they do not coincide with the cold spots. Similarly, by using 
the prick of a needle as the stimulus, a series of pain spots may be 
located, and lastly spots that respond to pressure alone. 
Each of these spots gives rise to its own specific sensa- 
tion. For example, any stimulus, applied to a warm spot, that 
will excite it, such as electrcity, gives rise to a sensation of warmth, 
and similarly for the others. Menthol applied to the skin appears 
to stimulate the cold spots alone, and so feels cool, while carbon 
dioxide gas, when the arm is immersed in it, excites the warm spots 
only, giving rise to a warm sensation. 
It is found that the pain spots are most numerous; next in order 
come the pressure spots, then the cold spots and lastly the warm 
spots which are fewest in number. 
All nerve fibers conveying impulses from the sense organs of 
the skin enter the spinal cord through the posterior roots of the 
spinal nerves, and the brain through the corresponding parts of the 
cranial nerves, and pass up, over a series of neurons, to reach the 
center for cutaneous sensibility in the cerebrum, which is believed 
to be situated in the region of the Rolandic fissure. 
CUTANEOUS SENSATIONS THE SENSES-TASTE 
159 
SENSE OF TASTE 
The senses of taste and smell are closely associated, and it is not 
always possible to discriminate betwen them. Their end organs 
are excited by chemical substances, either in solution (taste) or in 
the form of vapor (smell). 
The terminations of the nerves of taste are situated, for the most 
part, in the tongue, but the soft palate, the uvula and the upper 
part of the pharynx also possess them to some extent. The 
posterior third of the tongue, as well as the parts just mentioned, 
is supplied by the glosso-pharyngeal nerve and the anterior two- 
thirds by the chorda tympani. These are the nerves of taste. 
The mucous membrane of the posterior part of the tongue 
especially, show epithelial structures termed taste bulbs, which 
are most numerous in the walls of the circumvallate papillee, but 
they are also present in the other papillae and over the general 
surface. Each taste bulb contains two kinds of cells-the gustatory 
cells, the ends of which project into the pore of the taste bulb, and 
surrounding these the supporting cells which form a sort of en- 
velope for the gustatory cells. 
Taste Bulbs 
Taste Sensations 
There are four primary taste sensations-sweet, bitter, acid, 
salt. In order to stimulate the gustatory cells around which the 
terminal fibers of the taste nerves end, the substance to be tasted 
must be in solution; if placed on the tongue dry, no taste sensation 
is felt, until it is dissolved in the saliva. Possibly each of these 
primary taste sensations has its specific sense organ, since they are 
found to be more or less localised in the different regions of the 
tongue surface. Sweet tastes are felt especially at the tip and sides 
of the tongue, while bitter tastes come from the back part and also 
from the soft palate and pharynx. Salts stimulate the tip of the 
tongue and acids the sides. Chewing the leaves of a certain plant- 
Gymnema sylvestre-abolishes the sensations of sweet and bitter 
while those of acid and saline are unaffected and also tactile sense. 
Cocaine, on the other hand, paralyses the tactile sense sooner than 
the sense of taste. This would appear to show that the end organs 
in the tongue are specific for these sensations. 
The peripheral organs of taste and smell are so closely related 
that many of the flavors obtained from substances, introduced into 
the mouth, are really simultaneous sensations of taste and smell. 
Fatigue-If a sapid substance is kept in the mouth for some 
time, or tasted repeatedly, the taste becomes less and less distinct, 
and this would appear to be due to fatigue in some part of the taste 160 
HUMAN PHYSIOLOGY 
mechanism. If a sufficient interval is allowed to elapse, the sensation 
is fully appreciated again. 
It must be remembered that the tongue in addition to taste 
possesses also tactile sensibility. 
SENSE OF SMELL 
End-organs 
The end-organs for the sense of smell are distributed over the 
mucous membrane of the upper part of the nasal cavities. The 
olfactory mucous membrane, as it is called, is covered by epithelial 
cells which are of two kinds'-the olfactory cells and the supporting 
cells. The olfactory cells are long and slender each with a large 
nucleus. Each is a bipolar cell with two processes one of which 
passes to the free surface and ends in a bundle of fibrils resembling 
cilia which show no movement however, while the other penetrates 
the cribriform plate of the ethmoid bone and forms a synapse with 
a mitral cell of the olfactory bulb. The supporting cells are of the 
long columnar type; they are set closely side by side so as to give 
support to the sensory cells. 
The afferent nerve fibers of the sense of smell lie in the first pair 
of cranial nerves-the olfactory--and the impulses from the nasal 
mucous membrane, after passing; over several neurons, leach the 
center for smell in the cerebrum which is believed to be situated 
near the taste center in the uncinate gyrus. 
The smelling mechanism is much more highly developed in 
some animals1-the dog for example, than it is in man and it plays 
a much more important part in the lives of these animals; a dog 
recognizes his friends or his enemies more certainly by the sense of 
smell than by sight. 
Sensations of Smell 
In order to stimulate the olfactory cells the substance must be in 
the form of gas or vapor; when liquids are poured into the nose 
they do not give rise to any odor. No satisfactory classification of 
odors has yet been arrived at such as we have seen in the sense of 
taste. They may be divided roughly into pleasant and unpleasant 
odors. 
The sense of smell is much more delicate than the sense of taste. 
The odor of musk can be detected when the substance has been 
diluted with air eight million times, and mercaptan in a dilution 
of one in twenty-five billions. The smelling mechanism is easily 
fatigued; the sensation obtained from a perfume decreases rapidly 
when inhaled for a considerable time, and people sitting in a badly 
ventilated room very quickly get accustomed to the disagreeable 
odor which is felt at once by a person entering from the fresh air. THE SENSES-HEARING 
161 
Some individuals have no sense of smell at all while in others it 
is partially absent; they perceive certain odors but are insensible 
to others. 
SENSE OF HEARING 
Peripheral End-organs 
The peripheral organs in the sense of hearing are found in the 
cochlea in the inner ear. They consist of modified epithelial cells 
termed the hair cells of Corti; the hairs from their free ends project 
into a fluid, the endolymph, and catch up the sound vibrations 
which are transmitted to that fluid from the tympanic membrane. 
These cells translate the sound vibrations into nerve impulses which 
are carried by the auditory or 8th cranial nerve to the hearing 
center, situated in the temporal lobe of the cerebrum. The outer 
and middle ears are merely mechanical devices for transmitting 
the sound waves to the inner ear. 
The Organ of Hearing is divided into three parts-the outer ear, 
the middle ear or tympanum and the inner ear. 
Outer Ear 
The outer ear consists of the pinna or auricle lying outside the 
skull and the external auditory meatus, a canal leading down to 
the tympanic membrane. The auricle is mobile, being provided 
with two sets of muscles in some of the lower anipaals, the horse 
for example, and serves to collect the sound waves, but in the human 
subject mobility is not essential, since the head can be so easily 
turned round. In man it is of little service as a sound collector, 
because if filled up with wax, the central opening being left patent, 
the hearing is not perceptibly changed. 
The external auditory meatus is about 1% or 1 inch long. It 
follows a curved course, forwards, inwards and slightly upwards 
with the concavity downwards; it is narrowest about the middle 
so that if an object gets pushed in beyond this point it is difficult 
to remove. It is lined by skin, with hairs and ceruminous glands 
in the outer half only. These glands are of the simple tubular 
type, like the sweat glands, and they secrete a bitter, oily substance, 
the cerumen or ear wax, which protects the lining of the canal from 
drying and helps to deter insects from entering. 
Middle Ear or Tympanum 
The middle ear or tympanum consists of an irregular cavity in 
the temporal bone; it is flattened from side to side, and contains 
air, the tympanic ossicles, two muscles, and a nerve-the chorda 
tympani. The cavity communicates with the throat through a 
long duct, the Eustachian tube. The outer wall of the tympanum 
communicates with the external auditory meatus, the opening 162 
HUMAN PHYSIOLOGY 
between the two, in the recent state, being closed by a membrane- 
the tympanic membrane or drum of the ear. On the inner wall 
of the tympanum there are two openings, the fenestra ovalis and 
fenestra rotunda. The former leads into the vestibule of the inner 
ear and is closed by the foot of the stapes, while the latter is closed 
by a membrane which separates the cochlea from the tympanum. 
The higher of the two is the fenestra ovalis, leading in to the vesti- 
bule of the inner ear; it is closed by the foot of the stapes and the or- 
bicular ligament. The other opening is the f enestra rotunda which 
communicates with the cochlea and is also closed by a membrane. 
Sound waves are transmitted by the chain of ossicles and reach the 
inner ear through the fenestra ovalis. 
In birds the transmitting mechanism is very simple; it consists 
of a single rod of bone, called the columella, which extends from 
the tympanic membrane to the fenestra ovalis. By this arrange- 
ment the effect of the sound waves, on the comparatively large 
tympanic membrane, is concentrated on the much smaller fenestra 
ovalis. 
Tympanic Ossicles'-Stretching across the cavity of the middle 
ear is a chain of three small bones'-the tympanic ossicles-named 
the malleus (hammer), the incus (anvil) and the stapes (stirrup). 
These small bones are articulated together in such a way that any 
movement of the tympanic membrane to which the handle of the 
malleus is attached is transmitted to the inner ear by the stapes, 
the incus lying between the malleus and stapes and articulating 
with both. 
These small bones form a series of levers by the action of which 
the movements of the tympanic membrane, caused by the sound 
waves entering through the external auditory meatus, are concen- 
trated on the much smaller fenestra ovalis and transmitted to the 
perilymph of the inner ear with increased force. 
Muscles of Middle Ear-These are two in number, the tensor 
tympani and the stapedius muscles. 
The tensor tympani muscle arises from the cartilage of the 
Eustachian tube and is inserted into the handle of the malleus. 
When this muscle contracts, the handle of the malleus is pulled 
inwards and one idea is that this helps to lock the inco-malleal 
joint more firmly, so that feeble vibrations are transmitted more 
perfectly. It is believed that this muscle is used when we strain 
to hear faint sounds. It is supplied by a small branch of the 5th 
nerve. When sound waves enter one ear the tensor tympani on 
both sides can be observed to contract, the reflex traveling through 
the motor center of the 5th nerve. 
The stapedius is inserted into the neck of the stapes. It is sup- 
plied by a branch of the 7th nerve. The function of this muscle 
is not certain; some say that its contraction by twisting the foot THE SENSES-HEARING 
163 
of the stapes in the fenestra ovalis leads to a tightening of the 
orbicular ligament and so to a damping of the vibrations the muscle 
being used to protect the inner ear from the effects of loud sounds. 
The Eustachian Tube extends from the tympanic cavity to the 
throat. It is a canal ofabout 14 inches long consisting partly of bone 
and partly of cartilage, and its function is to allow of the equalization 
of the air pressure inside and outside the tympanic membrane. 
When this tube is inflamed and occluded, dulness of hearing results. 
The explanation is that the oxygen of the air, in the now closed 
tympanic cavity, is absorbed by the blood and the nitrogen alone 
is left; the pressure inside becomes less and less and the tympanic 
membrane bulges inwards and is stretched thus preventing its 
vibrating so well to the sound waves. Normally it is kept closed 
but is opened periodically by the act of swallowing. 
Inner Ear 
The inner ear is an irregularly shaped cavity in the temporal 
bone. It is divided into the osseous and the membranous laby- 
rinths, which lie one within the other. 
The osseous labyrinth is divided into three parts-the vestibule 
in the middle, into which the sound waves are transmitted, the 
semicircular canals posteriorly and the cochlea anteriorly. The 
whole bony cavity is lined by periosteum, and internal to this is 
a fluid-the perilymph. Inside the osseous labyrinth lies the 
membraneous labyrinth which consists of a series of sacs and ducts 
filled with a fluid termed the endolymph. 
The membranous labyrinth is the essential part of the ear. 
The vestibule contains two membranous sacs called the utricle 
and the saccule; the former communicates with the three mem- 
branous semicircular canals, and the saccule communicates with 
the canal of the cochlea and with the utricle. 
The Cochlea-The human cochlea is a tube, wound spirally two 
and one-half times around a central stem called the modiolus 
through which runs the fibers of the auditory nerve. From the 
modiolus there projects a spiral lamina of bone a little more than 
half across the cochlear canal, the osseous spiral lamina. It re- 
sembles a spiral staircase. From the free margin of the osseous 
spiral lamina a membrane passes across to the periosteum of the 
outer wall; this is named the basilar membrane and it divides the 
bony canal into two divisions, one towards the apex and another 
towards the base. On the apical side of this membrane is placed 
another termed the membrane of Reissner, and in this way the 
whole tube is divided into three canals or scalas. The scala tympani 
lies nearest the base; it is so named because, if followed downwards, 
it opens into the tympanum through the fenestra rotunda which 
is closed by a membrane. The scala vestibuli lies nearest the apex, 164 
HUMAN PHYSIOLOGY 
and at its lower end is the fenestra ovalis, closed by the foot of 
the stapes and the orbicular ligament. The scala media forms the 
middle division; it is also known as the cochlear canal and it forms 
the cochlear division of the membranous labyrinth. 
The scala vestibuli and scala tympani communicate with each 
other at the apex of the cochlea through a round opening called the 
helicotrema. The perilymph in the vestibular canal can flow 
through this opening into the tympanic canal and escape by the 
aqueduct of the cochlea. The cochlear canal contains endolymph 
and each of the others perilymph. 
The basilar membrane increases in breadth from the base to 
the apex of the cochlea. In man it consists of a single layer of con- 
nective tissue fibers running parallel to each other and stretching 
across from the osseous spiral lamina to the periosteum of the other 
side like a suspension bridge. On the apical side of this bridge is 
placed the organ of Corti. 
The Organ of Corti-This is formed of several kinds of epithelial 
cells. The rods of Corti are modified epithelial cells and are arranged 
in two rows-an inner row and an outer row-which meet together 
at the top, like the rafters of a house. They are planted on the 
basilar membrane, to which each is connected by a broad foot, 
and form a tunnel called the canal of Corti. 
A second set of supporting cells are the cells of Deiters. These 
also are modified, epithelial cells, planted by broad bases on the 
basilar membrane. The hair cells of Corti sit on the shoulders of 
Deiter's cells. 
Hair Cells of Corti'-On the inner side of the rods of Corti each 
of these cells is supported on the shoulder of a Deiter's cell and is 
cemented to it by a granular substance; the lower extremity is 
prolonged into a stiff cuticular process which is attached to the 
basilar membrane. They are arranged in rows along the whole 
length of the cochlea; one row lies internal to the pillars of Corti 
and, in man, four or five rows external to them. Each is an 
elongated, epithelial cell with a deeply placed nucleus and granular 
protoplasm around it. From the free extremity there project a 
number of short, stiff, hair-like processes which pass only alittle 
way into the protoplasm; their free ends stick out, through one of 
the apertures in the cuticular lamina, into the endolymph. The 
fibers of the auditory nerve end around the deeper extremities of 
the hair cells as terminal arborisations. There are no bloodvessels 
in the organ of Corti. It is nourished by the endolymph in the 
scala media. It is believed that these hair cells form the apparatus 
by which the sound waves entering the ear give rise to the nerve 
Minute Structure of Cochlea THE SENSES-HEARING 
165 
impulses which are carried by the fibers of the auditory nerve to 
the center for hearing in the brain. 
The membrana tectoria, or membrane of Corti, is placed im- 
mediately above the hair cells. It is a soft, structure which lies 
like a pad over the organ of Corti. This membrane is possibly 
thrown into vibration by the sound waves, thus causing the hairs 
of the hair cells to indent the protoplasm of the cells and so excite 
them. On the other hand, some believe that the membrana tec- 
toria acts as a damper, to check the movements of the hairs after 
the sound vibrations in the endolymph have ceased. 
Cochlear Division of the 8th Nerve-The bipolar cells from 
which the fibers of the cochlear nerve arise are found in the 
spiral ganglion which is situated within the bone in the base of 
the osseous spiral lamina. One branch passes to the hair cells, the 
other into the modiolus and thence to the brain. 
Course of Sound Waves-Sound waves consist of longitudinal 
vibrations of the air molecules giving alternate phases of condensa- 
tion and rarefaction. In the phase of condensation of the sound 
wave the tympanic membrane is pushed inwards and this, through 
the series of ossicles, pushes in the foot of the stapes which com- 
municates its movement to the perilymph, and the condensation 
wave enters the vestibular canal through the fenestra ovalis. 
From the scala vestibuli it passes through Reissner's membrane 
into the scala media, reaching the hairs of the hair cells through 
the membrana tectoria, causing them to indent the protoplasm of 
the cells and so give rise to nerve impulses. Then it passes onwards, 
through the basilar membrane, into the perilymph of the scala 
tympani, bulging out the membrane that closes this canal at the 
fenestra rotunda. In the phase of rarefaction the movements are 
in the opposite direction. For musical sounds these waves may 
vary from seventy feet to a fraction of an inch. 
Sound Sensation 
The auditory sense excels all others in its power of distinguishing 
different qualities of sensation; it can distinguish tones varying in 
frequency from 16 per second to 40,000 per second, that is through 
a range of 11X octaves. It also excels all other senses in its power 
of analysing tones and picking out their component parts. It is 
certain that the cochlea is the part of the ear that enables us to 
perceive musical sounds. How is this brought about ? 
There are several theories of hearing but only two will be 
mentioned here. 1, Helmholtz's resonance theory and 2, Ruther- 
ford's telephone theory. 
Helmholtz's Resonance Hypothesis-This depends on the prin- 
ciple of sympathetic vibration. If one sound a note of a particu- 
lar pitch, in a room containing a piano with the dampers raised, 166 
HUMAN PHYSIOLOGY 
the string of the piano, which is tuned to give that note, will 
immediately respond; if a note of a different pitch is sounded, 
another string will respond, etc. In the cochlea, according to 
Helmholtz, a similar mechanism exists. When a certain note is 
sounded the appropriate fiber of the basilar membrane is thrown 
into vibration, and so causes the stimulation of the hair cells planted 
on it. A note of a different pitch will set another fiber of the 
basilar membrane vibrating which will stimulate another set of hair 
cells and give rise to a different sensation, etc. If two tones are 
sounded together, the two corresponding fibers of the basilar 
membrane will respond, and so an analysis of the compound sound 
wave, which enters the ear, will be effected in the cochlea. ' According 
to this theory there are, within the organ of Corti, vibrating struc- 
tures that respond specifically to all the notes within the range of 
hearing and these structures are claimed to be the fibers of the 
basilar membrane. These fibers range from 0.041 mm. to 0.495 
mm.in length, the shortest ones being found at the base of the cochlea 
and the longest ones at the apex, so that it is probably the width of 
the basilar membrane that determines the tuning to any particular 
note. 
Rutherford's Telephone Hypothesis-According to this theory, 
all the fibers of the basilar membrane and all the hair cells vibrate 
to notes of every pitch, just as the plate of a telephone receiver 
reproduces the pattern of the vibrations falling on the transmitter. 
Each note sets up a different pattern of nerve vibration so to speak, 
and gives rise to a different sensation in the cerebrum. The analy- 
sis is made, not in the cochlea, but in the cerebrum. 
While there are certain facts difficult to explain by either of these 
hypotheses the former appears to be most satisfactory to the 
majority of physiologists. 
The range of hearing for musical tones extends from notes of 
16 vibrations to 40,000 vibrations per second, but when the vi- 
brational frequency is very high the sensation produced is painful. 
Galton's whistle can give out notes from 6,000 to 80,000 per second, 
but any above 40,000 cannot be perceived and the highest notes 
heard are very irritating. 
The acuteness of hearing is measured by the distance from the 
ear at which a watch can be heard ticking. 
The direction from which a sound is coming is judged of by 
turning the head around until the position is found at which the 
sound is heard loudest. 
Sound can be transmitted not only through the outer and middle 
ears but through the bones of the head. If the sound of a 
small tuning fork be allowed to die away until it can no longer be 
heard, and then the handle applied to the teeth of the lower jaw 
or to the top of the head, it can be heard again distinctly. This is THE SENSES-SIGHT 
167 
because the sound is better conducted through the bones of the 
skull than through the air. 
Subjective sound sensations are caused by excitation of the 
auditory center in the brain other than by the entrance of sound 
waves into the ear. The ordinary "singing in the ears" is not a 
subjective sensation; it is probably due to spasm of the tensor 
tympani or stapedius muscle in the middle ear. 
Semicircular Canals 
The Semicircular Canals have no relation to the sense of hearing. 
Stimulation of the cells lining these canals produce rotatory move- 
ments of the head, and their function is to enable us to maintain 
our equilibrium in space. They may be called the peripheral organs 
of the sense of equilibrium. This, along with the muscular sense, 
belongs to the proprioceptive system. 
SENSE OF SIGHT 
The eye is the organ of sight. It is constructed on the principle 
of the camera obscura or photographic camera. It consists of a 
series of lenses for the purpose of focusing the images of objects on 
the retina, which corresponds to the sensitive plate of the camera. 
Structure of Eye 
The eyeball consists of three coats surrounding the transparent 
media which serve as lenses for the formation of images. 
Corneo-sclerotic Coat--The outer covering is formed by the 
sclerotic coat or white of the eye which becomes continuous in 
front with the transparent cornea. It is composed of dense, fibrous 
tissue, mainly, and its function is protective. In front the fibers 
lie parallel to each other, and this arrangement allows of the 
transmission of light, whereas the sclerotic is opaque. This trans- 
parent part is the cornea; it has a somewhat greater convexity 
than the sclera; it is covered in front by stratified squamous epi- 
thelium, while its posterior surface is lined by a single layer of 
squamous epithelium. The radial fibers of the ciliary muscle take 
their origin from the corneo-sclerotic junction. 
The conjunctiva is a connective tissue membrane, with a super- 
ficial layer of stratified squamous epithelium, which covers the front 
of the sclerotic coat and is reflected on to the posterior surface of 
the eyelids. It is represented on the cornea only by the layer of 
stratified squamous epithelium. 
The Middle Coat consists of the choroid prolonged forwards as 
the ciliary processes and the iris. It is very vascular and contains 
a great number of pigment cells. The choroid lines the sclerotic 
coat on the inside to within a short distance of the corneo-sclerotic 168 
HUMAN PHYSIOLOGY 
junction; it is continued forwards, from that point, as the ciliary 
processes, about seventy in number, which consist of connective 
tissue with pigment cells and bloodvessels. They also contain the 
ciliary muscle which is composed of non-striped fibers, and consists 
of a radial portion and a circular portion. The radial fibers arise 
at the junction of the cornea and slcerotic and are inserted into the 
ciliary processes and neighboring part of choroid. The circular 
fibers lie inside the radial fibers. 
The iris is that part of the choroid which extends in front of the 
lens. It consists of fine, fibrous tissue, with many blood capillaries 
and pigment cells. This gives the color to the eye; in dark eyed 
people the pigment is abundant while in blue eyes it is less abun- 
dant. The circular opening in the iris is the pupil of the eye. Be- 
sides the delicate connective tissue, it contains two sets of plain 
muscle fibers. One set is circular and forms the sphincter pupillae 
muscle, because when its fibers contract the pupil becomes smaller 
or is constricted; the other set consists of radiating fibers and forms 
the dilator pupillae muscle, since the contraction of its fibers 
produces a widening or dilatation of the pupil. 
The retina forms the inner coat of the eyeball. It extends for- 
wards to the roots of the ciliary processes. It is a delicate, semi- 
transparent membrane and consists of the following eight layers 
from without inwards. 
1. The layer of pigment cells. 
2. The layer of rods and cones. 
3. The outer nuclear layer. 
4. The outer molecular layer. 
5. The inner nuclear layer. 
6. The inner molecular layer. 
7. The layer of ganglionic nerve cells. 
8. The layer of nerve fibers. 
These various layers are held together by the fibers of Muller. 
Essentially the retina consists of three sets of superimposed neurons, 
extending from the layer of rods and cones inwards to the layer of 
nerve fibers. 
The sensitive elements of the retina are the rods and cones; 
these are the structures that are stimulated by light and give origin 
to the nerve impulses which reach the vision center, in the occipital 
lobe, through the fibers of the optic nerves. The rods are much 
more numerous than the cones over the general surface of the retina 
but at a certain spot-the fovea centralis-only cones are present. 
Each rod and cone has an outer and an inner segment. The outer 
segment of the rod contains a purple or violet pigment while the 
inner segment is clear; both are rod-shaped. In the cone the 
The Retina THE SENSES-SIGHT 
169 
outer segment is colorless and conical in shape while the inner 
segment is bulbous. From the inner segments of both rods and 
cones long slender processes pass inwards, each with an expanded 
portion corresponding to a nucleus. The outer nuclear layer consists 
mainly of the nuclei of the rods and cones. 
The outer layer of the retina really belongs to the choroid. It 
consists of hexagonal pigment cells which send down processes 
between the rods and cones. These processes are believed to act 
as sheaths for the rods and cones and to absorb the light which has 
escaped them. When light falls on the retina the pigment granules 
stream down the processes between the rods and cones and when 
the light stimulation ceases the pigment moves back again. 
Optic Disc or Blind Spot-The retina is deficient at the point 
where the optic nerve enters and this spot is blind; there are only 
nerve fibers here and no visual cells. It is placed about 1 /10th 
inch internal to the optic axis, i. e. the antero-posterior axis of the 
eyeball. When the eye is examined with the ophthalmoscope the 
entrance of the optic nerve is seen as a whitish disc, the whiteness 
being due to the presence of a medullary sheath in the nerve fibers. 
The central artery and vein of the retina radiate from it. 
Marriotte's Experiment--That there is a blind spot in the retina 
can be proved by the following experiment: Make two marks, a 
dot and a cross, on the paper as below, about 3 inches apart; look 
at the cross with the right eye, keeping the left eye closed, and hold 
the book about 10 inches from the eye, when both dot and cross 
will be seen. Now gradually bring the paper towards the eye; 
at a certain distance the dot will disappear, but on bringing the 
book still nearer the dot will appear again. It disappears when its 
image falls on the optic disc or blind spot. (See diagram.) 
The Yellow Spot, so called because yellow pigment is found in 
the retina in this region, is that part of the retina where vision is 
most acute. It is only about 2 mm. in transverse diameter and 
about 1 mm. in vertical diameter. There is a depression in the 
center of this area called the fovea centralis and here the layers of 
the retina, internal to the visual cells, are very thin. At the fovea 
centralis only cones are present, while in the marginal region of 
the yellow spot there are also rods, but the cones are more numer- 
ous. As we pass from the center to the periphery of the retina 
the cones become fewer and the rods more numerous. There are 
no large bloodvessels, only capillaries, at the fovea centralis. 
By Clerk Maxwell's Experiment one may see the yellow spot or 
macula lutea in one's own eye. Place a solution of chrome alum 170 
HUMAN PHYSIOLOGY 
in a glass vessel with parallel walls and look through it at a window; 
a purplish-red, irregularly circular disc will appear in the field of 
vision and this will be a projection of the yellow spot. The chrome 
alum solution transmits red and greenish-blue rays to the retina; 
the yellow pigment of the macula lutea absorbs the greenish-blue 
rays and allows the red rays only to reach the cones, so that a 
purplish-red image, corresponding to the outline of the yellow spot, 
is projected into the field. This experiment proves that the yellow 
pigment lies between the sensitive cells of the retina and the 
source of light. 
The blood vessels of the retina all come from the central artery 
of the retina, which enters the eyeball in the optic nerve, about its 
center. They spread all over the retina, in its inner layers, and 
can be distinctly seen with the ophthalmoscope. That the blood 
vessels of the retina also lie between the visual cells and the 
crystalline lens through which the light enters is proved by 
Purkinjes' Experiment-In a darkened room, with a sheet of 
grey paper on the wTall, illuminate the inside of the eye by holding 
a taper at one side, so that the light enters through the sclerotic 
coat and not through the cornea; a number of branching figures, 
like the roots of a tree, will be seen on the paper. On moving the 
taper backwards and forwards the figures also move. These figures 
are caused by the shadows of the bloodvessels falling on the sensitive 
elements of the retina. These shadows are always being cast on 
the sensitive cells and projected into the field of vision, but it is 
only when the light enters from an unusual direction, and they are 
thrown on parts of the retina unaccustomed to them, that we are 
conscious of them. 
The yellow spot is placed in the visual axis and lies a little 
external to the center of the retina through which the optic axis 
passes. Vision is most acute at the fovea centralis. When we 
look at the page of a book only those letters the images of which 
fall on the fovea centralis are sharply defined. 
Vitreous Humor 
The vitreous humor is a jelly-like mass filling the larger portion 
of the interior of the eyeball, and lies behind the crystalline lens; 
it is enclosed in a transparent envelope called the hyaloid mem- 
brane which separates it from the retina. 
Crystalline Lens 
The crystalline lens is enclosed in a thin, colorless, homogeneous 
capsule which is very tough and difficult to rupture. The lens is 
not attached to the capsule and readily slips out when the latter 
is cut through. It is perfectly transparent and colorless through- 
out the greater part of life. It is biconvex, the posterior surface being more convex than the anterior, and the density increases 
from without inwards. It is composed of fibers, arranged in con- 
centric layers like those of an onion; the fibers are denticulated 
or notched at the edges, the notches interlocking so as to keep 
them accurately in position and allow of no lateral movement. 
The shape of the lens changes with age. In the foetus it is nearly 
spherical and as age advances it grows flatter. About 50 it begins 
to lose its elasticity and becomes denser, these changes being 
progressive. 
Suspensory Ligament-The crystalline lens, inside its capsule, 
is retained in position by the suspensory ligament. At the margin 
of the lens this ligament divides into two layers, comparatively 
short, and continued only a little way over the surface of the cap- 
sule to which they are firmly cemented by a colorless material. 
If the eye of an ox be removed from the head of a recently killed 
animal, and kept for 8 or 10 days, this cement substance decom- 
poses, and the lens within its capsule can easily be shaken free. 
The suspensory ligament is derived from the hyaloid membrane 
which encloses the vitreous humor; at the outer end it is attached 
to the ciliary processes of the choroid coat. When the ciliary 
muscle is relaxed the suspensory ligament is on the stretch, and 
being attached to the capsule of the lens, it pulls on the lens and 
keeps it flattened so that, when the eye is at rest, that is, while 
the ciliary muscle is not contracting, the eye is accommodated for 
distant vision. When the ciliary muscle contracts it pulls forward 
the choroid and ciliary processes; this relaxes the suspensory 
ligament and the crystalline lens, in virtue of its elasticity, bulges 
and becomes more convex; in this way the eye becomes accom- 
modated for near vision. 
The space between the anterior surface of the lens and the cornea 
is filled with a watery fluid termed the aqueous humor. 
THE SENSES-SIGHT 
171 
The Eye considered as an Optical Instrument 
Parallel rays of light are brought to a focus, by a convex lens, 
at a point called the principal focus of the lens, and the distance 
between this point and the lens is the principal focal distance. 
There is, therefore, only one principal focus for each lens, but 
many secondary foci. The rays proceeding from a luminous point, 
placed at the principal focus, are not brought to a focus at all by 
the lens but emerge from it as parallel rays. For the formation 
of an image, the object must be placed outside the principal 
focus (see diagrams). If the object is placed at twice the focal 
distance of a biconvex lens the image is formed at twice the focal 
distance, and these two points are termed conjugate foci. If the 
object be placed farther away than twice the focal distance, the 
image will be smaller than the object, and if it be placed within 172 
HUMAN PHYSIOLOGY 
twice the focal distance, between that and the principal focus, the 
image will be larger than the object. All the images produced on 
the retina are smaller than the objects seen and are inverted. 
The position of the principal focus of a lens, that is, its focal 
distance depends 1, on the curvature of the lens; the greater 
the curvature the shorter is the focal distance; and 2, on the 
refractive index of the substance of which the lens is composed; 
the higher the refactive index the shorter is the focal distance. 
In the normal eye the retina is in the principal focus of its lenses. 
This means that parallel rays of light are brought to a focus on 
the retina without any action of the ciliary muscle; no effort is felt 
when the lens is accommodated for a distant object. 
While the refractive media of the eye are the cornea, the aqueous 
humor, the crystalline lens and the vitreous humor, there are 
virtually only two lenses in the eye; the cornea and the aqueous 
humor together act as a single lens and the crystalline lens is the 
other. The three refracting (and reflecting) surfaces are the 
ANTERIOR SURFACE OF THE CORNEA, the ANTERIOR SURFACE OF 
the lens and the posterior surface of the lens. At the pos- 
terior surface of the cornea there is no refraction because the 
refractive indices of the cornea and of the aqueous humor are 
practically the same, so that the two together form one lens. 
Accommodation of Eye for Near and Distant Objects 
Some adjustment of the eye is necessary in order to see near 
and distant objects with the same system of lenses, since the eye 
cannot at the same time focus both parallel and divergent rays on 
the retina. The adjustment is effected entirely by the crystalline 
lens and this may be brought about in two ways-1, By varying 
THE DISTANCE BETWEEN THE LENS AND THE RETINA, and 2, BY 
altering the curvature of the lens. The former method is 
that adopted in the photographic camera, where the lens is moved 
backwards and forwards, until the image is brought to a focus on 
the ground glass plate. This is also the method used in certain 
fishes, where the lens is moved backwards or forwards by a special 
muscle; but in man, and the higher mammals, the second method 
is the one employed, viz.: that of changing the curvature of the 
lens. The lens is rendered more convex to focus near objects on 
the retina and less convex for distant objects. The means by which 
these changes in the curvature of the lens are brought about is 
termed the 
Mechanism of Accommodation-As mentioned previously, in 
the normal eye, looking at a distant object, the crystalline lens 
is flattened by the tension on the suspensory ligament. When it 
is desired to look at a near object the ciliary muscle contracts, 
pulls forwards the ciliary processes, to which the suspensory liga- THE SENSES-SIGHT 
173 
ment is attached, and so relaxes this ligament. This removes 
the tension from the lens, which because it is elastic bulges or 
becomes more convex, and so the image which, in the resting state 
of the eye fell behind the retina, is now brought forwards until it 
falls on the retina. Accommodation is fatiguing because it requires 
muscular action; when we read a book the ciliary muscle is always 
in action. 
The anterior surface of the lens bulges more in accommodation 
than the posterior surface; in fact the latter hardly changes its 
curvature at all. This is known by a study of 
Sanson's Images-If a candle be held at one side of a person's 
eye an observer, looking in the eye, will see three images of the 
candle flame-one by reflection from the anterior surface of 
the cornea, one from the anterior surface and another from 
the posterior surface of the lens. The first is the brightest; 
the second is larger and dimmer than the first but, like it, erect; 
the third is small and so dim that it is difficult to detect, and it is 
inverted because it is reflected from the posterior surface of the 
lens which acts like a concave mirror. These are termed Sanson's 
images. When the candle is moved images 1 and 2 move in the same 
direction as the candle but image 3 moves in the opposite direction. 
Now ask the subject to look first at a distant and then at a near 
object, and watch the changes that take place in the sizes and 
relative positions of the images. No change occurs in the first 
image but the second image becomes smaller and somewhat 
brighter when the person looks at a near object, and at the same 
time approaches the first. The reduction in size of the second 
image shows that the surface from which it is reflected (anterior 
surface of lens), has increased its curvature, and the increase in 
brightness of the second image, and its approach to the first, 
proves that the anterior surface of the lens comes nearer to the 
cornea. The third image, like the first, shows practically no change, 
either in brightness or position, and this shows that the surfaces 
from which the first and third images are reflected, viz.: anterior 
surface of cornea and posterior surface of lens, do not change dur- 
ing accommodation. 
By means of an instrument termed the phakoscope Sanson's 
images may also be seen, but the simple experiment, just described, 
demonstrates them quite as effectively. 
Changes in Pupil during Accommodation-During accommoda- 
tion the pupil undergoes changes in size; when the eye looks at a 
NEAR OBJECT it DIMINISHES IN SIZE, when it looks at a DISTANT 
object the pupil increases in size. The mechanism by which 
this is brought about will be described under "Functions of the 
Iris," but the object is to diminish spherical aberration. 
Spherical Aberration is a defect of the optical apparatus. Rays 174 
HUMAN PHYSIOLOGY 
of light passing through the circumference of a lens are more bent 
than those passing through the center, therefore they are not 
focused at the same point, and the image is blurred. In an optical 
instrument, such as a telescope or microscope, this defect is cor- 
rected by using a diaphragm to cut out the peripheral rays. In 
the eye this method is also adopted; the iris cuts out the peripheral 
rays. The image of an object will be most distinct and best 
defined when the pupil is smallest, but it must not be too small 
so that enough light is not admitted. When we look at a near object 
it is desirable to have a well defined and distinct image. 
Chromatic Aberration is another defect of the optical apparatus. 
This is due to the fact that when white light is refracted or bent 
it is also dispersed; the red rays are less bent than the violet rays 
and the result is an image surrounded by a colored ring. In optical 
instruments this is corrected by using two lenses of different 
material in combination such as crown glass and flint glass; this 
gives an achromatic system. In the eye this optical defect is 
remedied in a similar way, to some extent, viz.: by admitting the 
light through a combination of lenses. 
Range of Accommodation. Near and Far Points of Vision-In 
every eye there is a limit to the power of accomodation. If a book 
be brought nearer and nearer to the eye, at last a point is reached 
beyond which the type can't be seen distinctly. This is the near 
point of vision. It varies with age; in the adult it is usually about 
6 inches but in the child it is only about 3 inches. 
The near point of vision may be determined as follows: With a 
needle pierce two holes through a piece of paper or cardboard so 
that both shall fall within the diameter of the pupil. 
Hold the paper in front of one eye, closing the other, and look 
at the needle held in front through the holes. Move the needle 
backwards and forwards, nearer and farther from the eye. When 
held at a distance greater than about 6 inches a single needle will 
be seen but on bringing it nearer, it will appear to be doubled and 
slightly blurred. The distance at which the needle ceases to appear 
single is the near point of vision for that eye. 
When the needle is nearer than this the lens cannot be rendered 
sufficiently biconvex to focus the two pencils of light on the retina 
and the result is two blurred images instead of one sharp and clear. 
The far point of vision theoretically is at an infinite distance 
from the eye, if the object is large enough. Practically, however, 
it is found that rays proceeding from a point 20 to 30 feet distant 
are sufficiently parallel to be focused on the retina without accom- 
modation. Any object nearer than 20 to 30 feet in the normal 
eye to be seen clearly requires the use of the ciliary muscle. The 
normal range of vision then is from infinity to the near point 
distant from the eye about six inches. THE SENSES-SIGHT 
175 
Defects of vision occur when the image is imperfectly focused 
on the retina. 
Emmetropia is the term applied to normal vision; parallel rays 
are brought to a focus on the retina without active accommodation. 
Any defect in the refractive surfaces or shape of eyeball renders 
the eye ametropic. 
Presbyopia-At the age of about 50 years for most individuals 
the elasticity of the lens perceptibly diminishes so that it does not 
become so convex as formerly on contraction of the ciliary muscle 
during accommodation. The object to be seen sharply must be 
held at a greater distance from the eye and this is known as 
presbyopia or the long-sightedness of age. It is corrected by 
wearing converging spectacles. 
Hypermetropia is another form of long-sightedness. It is most 
frequently caused by abnormal shortening of the eyeball so that 
parallel rays are focused behind the retina, and it is usually con- 
genital, the eyeball from birth being smaller than the normal. 
When a distant object is looked at a clear image can only be ob- 
tained when the eye is being accommodated so that the ciliary 
muscle is always in action. A hypermetropic eye can never be 
rested. When the muscle is paralyzed by atropine no object can 
be seen clearly. To prevent this constant strain converging spec- 
tacles must always be worn. 
Myopia or Short-Sightedness is exactly the reverse of hyper- 
metropia ; parallel rays are brought to a focus before reaching the 
retina. The cause is usually an elongation of the eyeball, but it 
may be due to an increased curvature of the refracting surfaces. 
This defect may be congenital but it is usually acquired. The 
remedy is diverging glasses to throw the image back to the retina. 
It is usually due to an elongation of the eyeball antero- 
posteriorly, so that parallel rays are brought to a focus in front 
of the retina, but it may be caused by an increased curvature of 
the cornea. The remedy is biconcave or plano-concave lenses so 
as to render the rays of light divergent before they enter the 
eye. 
Astigmatism-This defect, which is present to some extent in 
almost all eyes, is due to irregularity in the curvatures of the 
DIFFERENT REFRACTIVE MEDIA OF THE EYE, USUally the COmea, but 
sometimes the lens. Thus, the eye may be short-sighted in the 
vertical direction and long-sighted for all rays entering horizontally, 
or vice versa, so that all the lines radiating from a center cannot 
be focused sharply at the same time. This condition is corrected 
Common Defects of Vision 176 
.HUMAN PHYSIOLOGY 
by astigmatic lenses of curvature similar to that of the abnormal 
eye but reversed. 
Functions of the Iris 
1. It serves to diminish spherical aberration, as we have al- 
ready seen, but cutting off the peripheral rays, and 2, it regulates 
reflexly the amount of light that enters the eye. 
The iris contains two muscles which are antagonistic in their 
action, viz.: the sphincter pupillae muscle and the dilator pupillae 
muscle. The third cranial nerve supplies the sphincter and the 
sympathetic nerve supplies the dilator pupillae muscle, and both 
nerve centers are in tonic activity so that if the sphincter pupillae 
is paralysed the pupil dilates and if the dilator is paralysed the pupil 
contracts. Stimulation of the cervical sympathetic nerve will 
cause dilatation of the pupil while if the third nerve is excited 
it will contract. 
Action of Atropin on Pupil-When the drug atropin is applied 
to the eye it paralyses the endings of the third nerve, both in the 
sphincter pupillae and in the ciliary muscles, the result being that 
the pupil dilates because the tonic activity of the dilator center 
is unaffected. Also accommodation for near objects is impossible 
in an atropinised eye because the ciliary muscle cannot be used; 
only distant objects can be seen. 
Action of Physostigmin on Pupil-This is the opposite to that 
of atropin since this drug excites the terminations of the third 
nerve and thus produces contraction of the pupil and, at the same 
time, accommodation for near objects by causing contraction of 
the ciliary muscles. An eye treated with physostigmin cannot see 
distant objects clearly. 
Pupillary Light Reflex-When a bright light is thrown into 
the eye the pupil contracts, and not only the pupil of the eye 
into which the light falls but also that of the other eye. Put the 
hand up edgewise, in front of the face, to act as a partition between 
the two eyes, and then bring a candle before one eye; both pupils 
will contract. Again, stand in front of a mirror and note the size 
of the pupils; now close one eye and the pupil of the other eye will 
dilate. The reflex center is situated in the mid-brain; the 
afferent impulse enters from the retina through certain fibers of 
the optic nerve; this center is connected with the nucleus of the 
third nerve, which supplies the sphincter pupillse muscle, and so 
the efferent impulse reaches the muscle by this path, that is, the 
third nerve. THE SENSES-SIGHT 
177 
Visual and Color Sensation 
The sensitive terminations of the retina are found in the rods 
and cones, and the normal stimulus for these are the waves of 
light which enter the eye through the pupil. The radiant energy 
from the sun consists of undulations in the ether and only a limited 
number of these undulations excite the retina. When a beam of 
white light is sent through a glass prism, the undulations or waves 
are spread out to form a spectrum. The undulations at the violet 
end of the spectrum have a frequency of 800 billions per second, 
those at the red end 400 billions per second; the ultraviolet and 
ultrared undulations have no power to excite the retina. Objective 
light is this wave motion; subjective light is the sensation which 
enables us to become aware of the position of the object which 
gives out, transmits or reflects the undulations. 
The only physical difference between the ether waves is fre- 
quency of vibration and wave length. The wave lengths of light 
are very short. The waves at the extreme red end of the visible 
spectrum have a length of 760 millionths of a millimeter and those 
at the extreme violet end 397 millionths of a mm., while the inter- 
mediate colors have wave lengths between these extremes. Waves 
of a certain wave length, falling on the retina, give rise to the sensa- 
tion of redness, those of shorter wave-length to a sensation of 
yellow, etc.; the character of the sensation depends on the wave 
length. When all the waves fall on the retina simultaneously, the 
sensation which we call white is produced. 
Two theories have been advanced to explain color vision, viz.: 
the Young-Helmholtz or tri chromic theory and the Hering theory. 
The Young Helmholtz Theory of Color Vision-According to 
this theory there are three primary color sensations, viz.: red, 
green, and violet, corresponding to three elements or substances 
in the retina. Rays of light at the red end of the spectrum, that 
is, of a certain wave length, excite the red element strongly and the 
other two-green and violet-slightly, the result being a sensation 
of red. Rays from the middle of the spectrum excite the green 
element strongly, the other two only slightly, and a sensation of 
green results, and so for the sensation of violet. Light from the 
yellow part of the spectrum, for example, excites about equally 
the red and green elements, but only slightly the violet, and the 
sensation is yellow, which is therefore a mixed sensation. The 
light from any part of the spectrum excites all three substances or 
elements in unequal degrees, and on the extent to which each 
substance is excited will depend the resulting sensation. If all 
three elements are excited equally the sensation of white is pro- 
duced. 178 
HUMAN PHYSIOLOGY 
The phenomenon of color blindness is difficult to explain on the 
Young-Helmholtz theory. Color blindness may be partial or 
complete. In complete color blindness all color sensations are 
absent; only the forms of objects are seen; in partial color blind- 
ness only certain colors are absent. Partial color blindness is not 
uncommon; it occurs in 4 percent, of all males, and in less than 
1 per cent, of females. Absence of the red and green sensations is 
the most common form,--red-green color blindness. Yellow-blue 
color blindness occurs but it is much less common than the red- 
green variety. On the Young-Helmholtz theory it is difficult to 
understand why red sensations and green sensations disappear 
together, and similarly with regard to blue and yellow. If the red 
substance is absent from the retina it would explain the want of 
red sensations, but there is no reason why the green substance 
should also be absent. Besides, on the Young-Helmholtz theory, 
violet color blindness should also exist but is never found. 
In railway men and seamen, who are guided usually by red and 
green lights, red-green color blindness is a serious defect. It can 
be detected by asking the person under examination to match 
colored wools; if he is color blind he will make many mistakes. 
Hering's Theory of Color Sensation-According to this theory 
there are six luminous sensations arranged in three pairs-white- 
black, a pair; red-green, a pair and yellow-blue, a pair. There 
are three visual substances in the retina, viz.: a white-black sub- 
stance, a red-green substance, and a yellow-blue substance. De- 
composition of the red-green substance, say, gives rise to the sensa- 
tion of red, while its restitution produces a green sensation; and 
similarly for the black-white and yellow-blue substances. This 
theory explains why blindness to red and green sensations go to- 
gether; it is due to the absence in the retina of the red-green 
substance, and yellow-blue blindness is due to the want of the 
yellow-blue substance. 
It also explains the phenomenon of complementary after images. 
Any two colors are said to be complementary which together 
produce white light. Red is complementary to green and yellow 
to blue because, if the spectral colors red and green or yellow and 
blue be mixed in the proper proportions, white is produced. Look 
at a bright red light and then close the eyes; for a few seconds the 
red image will remain but it gradually changes to green which is 
the complementary after image. On the Hering theory this is 
easily explained; the red image is caused by the decomposition of 
the red-green substance and when the eye is closed, and the light 
shut out, the rebuilding up of this substance gives rise to the green 
sensation. THE SENSES-SIGHT 
179 
Field of Vision 
The field of vision is that part of the external world that can be 
seen with one eye, looking straight in front, when the other eye is 
closed. On account of the fact that the rays of light pass through 
the lens, the outer half of the field of vision corresponds to the 
inner half of the retina and inner half of the field to the outer 
half of the retina. The extent of this field, which it is impor- 
tant to know in diagnosing certain diseases of the eye, is ob- 
tained by the use of an instrument called a perimeter. 
Ophthalmoscope-It is also important to be able to obtain a 
view of the retina in the living eye and this is possible by using an 
instrument termed the ophthalmoscope, which consists of a con- 
cave mirror with a hole in the center through which the observer 
looks. THE NERVOUS SYSTEM 
Divisions of the Nervous System 
Brain and Spinal Cord. 
(Central Nervous System). 
Cranial Nerves and Ganglia. 
Spinal Nerves and Ganglia. 
§ 
w 
o 
£ 
£ 
Cerebro-spinal 
Autonomic 
Sympathetic. 
Parasympath etic. 
Enteric 
Auerbach's Plexus. 
Meissner's Plexus. 
The cerebro-spinal nervous system consists of the brain and 
spinal cord (central nervous system) together with the nerves which 
come off from them, viz.: 12 pairs from the brain (cranial nerves) 
and 31 pairs from the spinal cord (spinal nerves). 
CEREBRO-SPINAL NERVOUS SYSTEM 
Cranial Nerves 
The cranial nerves are designated numerically according to the 
order in which they arise from the brain, from before backwards, 
but they are also given alternative names descriptive of their 
functions or of the organs and tissues which they supply. 
The Olfactory or First Cranial Nerve arises from the olfactory 
bulb and passes to the olfactory area of the mucous membrane of 
the nose. It is the nerve of smell and contains only afferent fibers. 
The Optic or Second Cranial Nerve is the nerve of sight and is 
also purely sensory. The nerve fibers when they arise from the 
brain on the two sides form the optic tracts. These decussate, 
partially, in the optic chiasma and then go on to enter the eyeballs 
as the optic nerves which terminate in the retinae. 
The Oculo-motor or Third Cranial Nerve arises from the mid- 
brain and supplies all the muscles of the eyeball except the superior 
oblique and the external rectus. It contains only efferent fibers. 
Paralysis of this nerve gives rise to an external squint. 
The Trochlear or Fourth Cranial Nerve leaves the brain behind 
the third and passes to the superior oblique muscle of the eyeball. 
It is a purely motor nerve. 
The Trigeminal or Fifth Cranial Nerve, which arises from the 
middle of the pons, is a mixed nerve containing both sensory and 
motor fibers. The cells of origin of the sensory fibers are situated 
in the gasserian ganglion which corresponds to the ganglion on 
180 NERVOUS SYSTEM 
181 
the posterior root of one of the spinal nerves. The motor fibers 
supply the muscles of mastication and the sensory fibers are dis- 
tributed to the skin of the face including the eye, and the mucous 
membrane of the mouth and lower part of nose. The sensory 
branches of this nerve are often affected with neuralgia. 
The Abducens or Sixth Cranial Nerve leaves the brain about the 
junction of the medulla and pons, and contains only efferent fibers 
which pass to the external rectus muscle of the eyeball. Paralysis 
of this nerve leads to an internal squint. 
The Facial or Seventh Cranial Nerve is a mixed nerve like the 
fifth. The sensory fibers carry taste impressions from the anterior 
two-thirds of the tongue, while the motor fibers supply the mus- 
cles of the face, otherwise known as the muscles of expression. 
Not infrequently the seventh nerve is damaged and this leads to 
Bell's paralysis; the skin of the face on the affected side is smooth, 
the eye cannot be closed and the mouth is twisted to the unaffected 
side due to the fact that the muscle tonus on the sound side is 
unopposed. In whistling or smiling, the muscles of the affected 
side cannot be moved, while those of the other side are, and this 
gives a peculiar appearance to the face. 
The Auditory or Eighth Cranial Nerve consists of two divisions- 
the auditory nerve proper or nerve of hearing coming from the 
cochlea, and the vestibular division, carrying impulses inwards 
from the semicircular canals to the cerebellum mainly, which en- 
able us to maintain our equilibrium. There are no efferent fibers 
in the eighth nerve. 
The Glossopharyngeal or Ninth Nerve is distributed to the 
tongue and pharynx, as the name indicates. Like the remaining 
three cranial nerves it takes origin from the medulla oblongata. 
It is a mixed nerve containing motor and sensory fibers, the former 
passing to the muscles of the pharynx, the latter coming from the 
posterior third of the tongue and also from the mucous membrane 
of the pharynx and the soft palate. 
The Vagus, Pneumogastric or Tenth Cranial Nerve is a mixed 
nerve and has a very wide distribution, sending fibers, motor, 
sensory, secretory or inhibitory, to the heart, lungs, pharynx, 
larynx, gullet, stomach, intestine, liver, pancreas, spleen, kidneys, 
etc. The functions of the most important of these have already 
been considered. 
The Spinal Accessory or Eleventh Cranial Nerve contains only 
efferent fibers which supply two important muscles of the neck 
and back, viz.: the sterno-mastoid and the trapezius. 
£ The Hypoglossal or Twelfth Cranial Nerve is also a purely 
efferent nerve and supplies the tongue muscles with their motor 
fibers. 182 
HUMAN PHYSIOLOGY 
Spinal Nerves 
There are 31 pairs of nerves that come off from the spinal cord 
and each arises by two roots-the anterior, ventral or efferent 
root and the posterior, dorsal or afferent root, the latter having 
on it a swelling termed the posterior root ganglion. Beyond the 
ganglion the two roots unite to form a mixed nerve carrying efferent 
and afferent fibers to the muscles, bloodvessels, glands, skin and 
other sensory surfaces and structures. 
Structure of Spinal Cord 
The spinal cord is continuous with the medulla oblongata, the 
lowest division of the brain. It lies in the spinal canal but it does 
not occupy the whole width of the canal nor extend throughout 
its whole length; its lower end is on a level with the lower border 
of the first lumbar vertebra. Both the brain and the spinal cord 
are surrounded by three membranes-the dura mater consisting 
of strong connective tissue, the pia mater lying next to the nerve 
substance and carrying many of the blood vessels that nourish 
the nerve tissue, and the delicate arachnoid mater between these 
two. When the membranes (or meninges) of the brain and cord 
are inflamed the condition is termed cerebro-spinal meningitis. 
The spinal cord is cylindrical in outline but it is not of uniform 
thickness throughout its length; it shows two swellings, one in the 
cervical and one in the lumbar region, the cervical and lumbar 
enlargements respectively, and from these the nerve fibers arise 
that supply the upper and lower limbs. The lower end of the cord 
is cone shaped (the conus medullar's) and from this there passes 
downwards a thin filament of grey substance (the filum terminale), 
surrounded by the roots of the lumbar and sacral nerves, which as 
they lie in the spinal canal present the appearance of a horse's 
tail, hence called the cauda equina. 
Internal Structure of Cord 
When a transverse section is made through the spinal cord it is 
seen to consist of white matter externally and grey matter in the 
center. The latter, which consists mainly of nerve cells and their 
processes, presents an appearance somewhat resembling the letter 
H. The projection backwards, on each side, is termed the posterior 
horn and that forwards the anterior horn, while the bridge of grey 
matter connecting the two halves of the cord, across the middle 
line, and through the middle of which runs the central canal, forms 
the posterior and anterior grey commissures. The central canal 
of the cord is continuous with cavities in the brain termed the 
brain ventricles; these, together with the central canal, are filled 
with cerebro-spinal fluid which is practically lymph. NERVOUS SYSTEM 
183 
The spinal cord is almost completely divided into two sym- 
metrical halves by the posterior median septum behind and the 
anterior median fissure in front, while on the lateral aspect of each 
half are two shallow grooves from which the anterior and posterior 
nerve roots emerge. 
Within the grey matter the nerve cells show a more or less 
definite grouping. Those in the anterior horn are large multipolar 
cells and give origin to the fibers that leave the cord through the 
anterior nerve roots and pass to the striated muscles. Another 
group of cells, situated at the base of the posterior horn, on its 
mesial aspect, is known as Clarke's column. The nerve fibers 
which arise from these cells pass into the cerebellar tracts and end 
in the cerebellum. In the lateral part of the grey matter, between 
the anterior and posterior horns, is a collection of nerve cells called 
the intermedio-lateral group; it is most evident in the lower cervical 
and upper thoracic regions. The cells in the posterior horn are 
small and are scattered more or less uniformly throughout the 
grey matter of that region, without any arrangement into distinct 
groups. 
Grey Matter of Spinal Cord 
White Matter of Cord 
The white matter of each half of the cord, which consists of 
medullated fibers, mainly, is divided into three columns by the 
anterior and posterior nerve roots-the posterior column between 
the posterior root and horn and the posterior median septum, the 
lateral column between the anterior and posterior roots, and the 
anterio column between the anterior root and the anterior median 
fissure. 
The nerve cells of the grey matter and the nerve fibers of the 
white matter are held together or supported by a variety of con- 
nective tissue, peculiar to the nervous system, termed neuroglia. 
Tracts of Nerve Fibers in White Columns-The medullated 
nerve fibers which make up the white columns are disposed in 
bundles or tracts, some carrying impusles upwards from the 
periphery to the higher segments of the cord and to the brain, others 
carrying impulses downwards from the brain to the muscles, glands, 
bloodvessels, etc., and others still connecting segments of the cord 
at different levels with each other. The positions which these 
various tracts occupy in the spinal cord can be made out by the 
method of 
Wallerian or Secondary Degeneration-We have already seen 
that when a nerve fiber is cut off from its cell of origin the peripheral 
part of the fiber undergoes degeneration; this is termed Wallerian 
or secondary degeneration. If the spinal cord is cut across about 184 
HUMAN PHYSIOLOGY 
its middle, all the fibers found degenerated in the upper half of the 
cord, i. e. above the section, will have arisen from nerve cells situ- 
ated below the section; these fibers are said to undergo ascending 
degeneration. Similarly, all degenerated fibers in the lower half 
of the cord come off from nerve cells in the brain or cord lying 
above the lesion; these form the descending tracts. With certain 
reagents such as Marchi's fluid, the degenerated fibers stain 
differently from normal fibers, and by this means the two can be 
distinguished. 
Tract of Goll and Tract of Burdach--The axons of the unipolar 
cells of the posterior root ganglia divided into two branches one of 
which runs outwards to the periphery, through the mixed spinal 
nerve, the outer inwards to the spinal cord which it enters in the 
posterior nerve root. This central fiber, on entering the cord, passes 
into the posterior column and soon divides into a short descending 
and a long ascending branch; the former turns into the grey 
matter one or two segments below the point of division; the latter 
may enter the grey matter at a higher level, or it may ascend to 
the medulla oblongata and end in one of two nuclei situated there. 
Those ascending fibers from the lower half of the cord lie nearer 
the posterior medium septum and form the tract or column of Goll; 
they end in relation to cells situated in the nucleus gracilis of the 
medulla. Those from the upper half of the cord occupy the lateral 
part of the posterior column, forming the tract or column of Bur- 
dach, and end in the nucleus cuneatus which lies external to the 
nucleus gracilis. 
From these two nuclei fresh fibers arise, cross the middle line 
in the medulla oblongata as internal arcuate fibers, and run up, 
on the opposite side, as the tract of the mesial fillet which ends 
in the optic thalamus. From the thalamus other fibers carry the 
impulses to the cortex of the cerebrum. 
Cerebellar Tracts---Two important ascending tracts take origin 
from the nerve cells of Clarke's column, viz.: the Dorsal Spino- 
cerebellar or Flechsig's Tract and the Ventral Spino-cerebellar or 
Gowers' Tract. The former runs upwards in the lateral column 
of the cord, at its margin, close to the ventral aspect of the posterior 
horn, and ends in the middle lobe of the cerebellum; the latter 
ascends, also near the surface of the cord, in front of the last 
mentioned tract, and, like that, ends in the vermis of the cerebel- 
lum. Some fibers of Gowers' tract pass to the optic thalamus. 
Ascending Tracts 
Descending Tracts 
Pyramid Tracts-The most important of the descending tracts 
are the cortico-spinal or pyramid tracts. The fibers arise in the NERVOUS SYSTEM 
185 
large pyramidal cells (Betz cells) of the motor cortex of the cere- 
brum and pass downwards, on the same side, through the midbrain, 
pons, and medulla oblongata in the lower part of which they divide 
into three separate bundles. The largest of these crosses over in 
the decussation of the pyramids to the opposite side of the cord 
and forms the crossed lateral pyramid tract, situated in the lateral 
column in front of the posterior horn and internal to the direct 
spino-cerebellar tract. A few fibers which run down without cross- 
ing the middle line occupy a similar position in the lateral column 
of the cord, on the same side, and form the direct lateral pyramid 
tract, while a third uncrossed bundle, lying in the anterior column, 
present only, however, in man and the higher apes, is termed the 
direct anterior pyramid tract. 
The fibers of all three tracts end in relation to the cells of the 
anterior horn which give off the motor fibers that pass out through 
the anterior nerve roots and supply the striated or voluntary 
muscles, those of the direct anterior pyramid tract crossing in the 
anterior white commissure of the cord. Voluntary impulses, arising 
from the motor areas in the cerebrum, reach the skeletal muscles 
through these two superimposed systems of neurones. Owing to 
the crossing of most of the sensory and motor fibers, in the lower 
levels of the medulla oblongata, it will be seen that the one side 
of the brain presides over the opposite side of the body. 
Rubro-spinal Tract-Another descending tract arises in a group 
of nerve cells situated in the midbrain termed the red nucleus 
and runs down, in the lateral column of the cord, either in front 
of or intermingled with the crossed pyramid tract; this is termed 
Monakow's bundle or the rubro-spinal tract. Its fibres end in the 
grey matter of the cord probably forming arborisations around 
the motor cells of the anterior horn. 
Loewenthal's Tract-The descending antero-lateral or Loewen- 
thal's tract lies partly in the lateral and partly in the anterior 
column, the fibers being, to some extent, intermixed with those of 
Gowers' tract, another name for which is the ascending antero- 
lateral tract. It arises in cells situated in the midbrain, pons and 
medulla oblongata and ends in the grey matter of the anterior horn. 
Comma Tract-Another tract which undergoes descending de- 
generation is found in the posterior column and is termed the 
comma tract of Schultze. It consists of the short descending 
branches which are given off by the posterior root fibers soon after 
they enter the cord; these end in the grey matter at lower levels. 
Proprio-spinal Fibers--In addition to the tracts above men- 
tioned, which either run upwards through the cord to the brain 
or downwards from the brain through the cord, there are fibers 
which take origin in the grey matter of the cord in one segment and 186 
HUMAN PHYSIOLOGY 
run upwards or downwards to end in the grey matter of another 
segment. These are known as proprio-spinal fibers; they are con- 
tained for the most part in the anterior, lateral and posterior ground 
bundles which lie next to the grey matter. They belong to the 
system of reflexes and serve to connect together segments of the 
cord at different levels. 
THE BRAIN 
The brain is a very complex organ. It consists of collections of 
grey matter, the nerve cells of which form the brain "centers," 
and numerous strands of fibers (white matter) connecting these 
nerve centers with each other. 
The brain is divided, more or less artificially, into five parts-■ 
the medulla oblongata, the pons, the cerebellum or little brain, 
the midbrain, and the cerebrum or great brain. 
Divisions of Brain 
The Medulla Oblongata 
The medulla oblongata, sometimes known as the bulb, is the 
continuation of the spinal cord upwards into the cranial cavity, 
the edge of the foramen magnum marking the line of division 
between the two. The bulb is shaped like a truncated cone with 
the base upwards. Most of the strands of fibres that connect the 
spinal cord with the parts of the brain above-pons, cerebellum, 
midbrain and cerebrum-pass through the medulla oblongata, and 
besides, new tracts arise from masses of grey matter within it. 
The central canal of the spinal cord is continued upwards through 
the lower half of the medulla, gradually approaching the posterior 
surface, and about its middle this canal comes to the surface, the 
grey matter surrounding it spreading out and forming the floor of 
the fourth ventricle. 
The main masses of grey matter that appear de novo in the 
medulla oblongata, and are not represented in the spinal cord, are 
the inferior olivary nucleus, lying postero-external to the anterior 
pyramid, and the nucleus gracilis and nucleus cuneatus, situated 
on the posterior aspect of the central grey matter. The former is 
connected through a strand of fibers with the lateral lobe of the 
cerebellum of the opposite side, while the gracile and cuneate 
nuclei, as we have seen, give origin to fibers that cross the middle 
line and form the tract of the mesial fillet which ends in the optic 
thalamus, a large mass of grey matter situated in the cerebrum. 
The motor nuclei of the 9th, 10th, 11th, and 12th cranial nerves 
lie in the medulla and also most of the reflex centers already con- 
sidered, viz.: the cardio-motor, cardio-inhibitory, vaso-motor, re- 
spiratory, salivary, swallowing, gastric secretion centers, etc. Be- NERVOUS SYSTEM 
187 
cause of the presence of these reflex centers, some of which are es- 
sential to life (respiratory center), a slight injury to the medulla 
oblongata may prove fatal. 
The anterior pyramids in the upper half of the medulla forming 
rounded elevations at each side of the anterior median fissure in 
front, which is here very shallow, contain the fibers of the pyramid 
tract. These decussate in the lower part, the bundles interlacing 
as they cross the middle line. 
The restiform body on the lateral aspect is made up of many 
groups of fibers, e. g. those of the direct spino-cerebellar tract 
already considered, that pass to the cerebellum. It is also known 
as the inferior cerebellar peduncle. 
Pons Varolii 
The pons varolii is a continuation of the medulla oblongata 
upwards. It forms the bridge in a double sense since it connects 
the bulb with the midbrain and cerebrum above, and the two lateral 
lobes of the cerebellum with each other, through the middle cere- 
bellar peduncles. The pyramid tract at this level is split up into 
separate bundles by the transverse fibers of the pons, and im- 
mediately dorsal to these lies the mesial fillet. The motor nuclei 
of the 5th, 6th, and 7th cranial nerves are found in the grey matter 
of the pons. 
Midbrain or Mesencephalon 
The midbrain lies between the pons and the cerebrum. The 
cerebral peduncles are seen on its anterior aspect and four corpora 
quadrigemina-an anterior pair and a posterior pair-behind. 
When cut transversely, the aqueduct of Sylvius is found near the 
center, a canal similar to that present in the spinal cord. The 
aqueduct is surrounded by grey matter and serves to connect the 
third with the fourth ventricle. The cerebral peduncles, one on 
either side of the middle line, lie in front and contain the fibers of 
the two pyramid tracts at this level. The third and fourth cranial 
nerves have their nuclei of origin in the grey matter in front of 
the aqueduct of Sylvius, and also the superior motor nucleus of 
the fifth. 
The motor nuclei of all the cranial nerves, in the midbrain, pons 
and medulla, correspond to the cells of the anterior roots of the 
spinal cord which give rise to the motor fibers of the efferent roots 
of the spinal nerves. Nerve fibers from the pyramid tract of the 
opposite side cross the median raphe and end in relation to the 
cells of each of these nuclei, so that the voluntary muscles supplied 
by the cranial motor nerves are controlled by the cerebral hemi- 
sphere of the opposite side. 188 
HUMAN PHYSIOLOGY 
Another important collection of nerve cells found in the mid- 
brain is known as the red nucleus which, as we have seen, gives 
origin to the fibers of the rubro-spinal tract. The mesial fillet 
runs upwards on its way to end in the optic thalamus and during 
the passage of this tract through the medulla and pons, its fibers 
are being increased by the addition to it of the secondary neurones, 
related to the afferent fibers of the mixed cranial nerves. 
The Cerebrum 
The cerebrum, in man and the higher mammals, is by far the 
largest and, in many respects, the most important division of the 
brain. It consists of two halves called the cerebral hemispheres, 
connected together across the middle line by a large flattened strand 
of commissural fibers termed the corpus callosum, at the bottom 
of the great longitudinal fissure which incompletely separates the 
hemispheres from each other. 
In the interior of each hemisphere a cavity is found, filled with 
cerebro-spinal fluid; this is termed the lateral ventricle; it com- 
municates with the third ventricle of the brain situated mesially 
between the two optic thalami, and that in turn, through the 
aqueduct of Sylvius, with the fourth ventricle into which, as we 
have seen, the central canal of the spinal cord opens. This whole 
system of canals and ventricles is filled with cerebro-spinal fluid. 
Each cerebral hemisphere, like the other divisions of the central 
nervous system, consists of masses of grey matter and strands or 
tracts of white matter. Most of the grey matter is disposed on the 
surface and forms the cerebral cortex, while deeper down, in the 
base of the hemisphere, near where it joins the midbrain, are three 
additional masses of grey matter termed the basal ganglia; these 
are the optic thalamus, the lenticular nucleus and the caudate 
nucleus, the last two together being known as the corpus striatum. 
Between the lenticular nucleus and a depression on the surface 
termed the island of Reil is another collection of grey matter, 
flattened and elongated, known as the claustrum. 
Internal Capsule--The nerve fibers which form the white matter 
of the hemisphere, in passing down from the cortex to the cerebral 
peduncles in the midbrain, converge like the lines of a fan, and most 
of them at the base of the hemisphere lie between the lenticular 
nucleus on the outside and the head of the caudate nucleus and 
the optic thalamus on the inside; these constitute the internal 
capsule. It is divided into an anterior limb, lying between the 
head of the caudate and the lenticulai nucleus, and a posterior 
limb between the lenticular nucleus and the optic thalamus, the 
point of junction between the two limbs where they form a wide 
angle being called the genu. The layer of white matter between the 
lenticular nucleus and the claustrum is called the external capsule. NERVOUS SYSTEM 
189 
Cerebral Fissures, Lobes and Convolutions-The area of the 
grey matter on the surface of each hemisphere is much increased 
by a great number of depressions called fissures or sulci leading to 
infoldings of the cortex. The largest and deepest of these, termed 
the primary sulci, mark the lines of separation between the lobes 
into which the hemisphere is divided. The lobes are further sub- 
divided into convolutions or gyri by the secondary sulci. 
Inmammalslow in the zoological scale, such as the rabbit, the 
cerebral cortex is smooth, but in animals such as the cat, dog, 
monkey and particularly in man and the higher apes, the brain is 
convoluted, the advantage of this being that it permits of a large 
area of grey matter being accommodated in a comparatively re- 
stricted space, viz.: the cranial cavity. 
On the external aspect of the hemisphere, in man, there are three 
primary fissures which subdivide it into five lobes, viz.: the fissure 
of Sylvius, the fissure of Rolando and the parieto-occipital fissure. 
The fissure of Sylvius, at its origin near the island of Reil, divides 
into a short anterior and a long posterior limb, the former running 
into the frontal lobe and the latter separating the parietal lobe 
above from the tempero-sphenoidal lobe below. 
The fissure of Rolando or central fissure begins below, near the 
Sylvian fissure, and runs upwards and backwards to a point near 
themesial border of the hemisphere; it lies between the frontal 
and parietal lobes and marks the posterior boundary of the motor 
areas. 
The parieto-occipital fissure separates the parietal and occipital 
lobes on the external aspect. It runs parallel to the Rolandic 
sulcus and extends over the border on to the mesial aspect of the 
hemisphere. The part of it that lies on the external surface is 
short in man, and it is usually interrupted by small connecting 
convolutions that are common to the parietal and occipital lobes. 
In the monkey, where the convolution pattern is simpler, it is 
better marked than in man. 
Lobes of the Cerebrum--On the external aspect the lobes of the 
cerebrum are five in number, viz.: the frontal lobe lying in front 
of the Rolandic fissure; the parietal lobe between the Rolandic 
and parieto-occipital fissures and bounded below by the posterior 
limb of the Sylvian fissure; the occipital lobe behind the parieto- 
occipital fissure; the temporo-sphenoidal lobe below the posterior 
limb of the Slyvian fissure; and the island of Reil, an area of grey 
matter sunk beneath the level of the general surface and covered 
over by the convolutions that border the Sylvian fissure at the 
point where it divides. 
Convolutions-Each of these lobes is further subdivided into 
convolutions by secondary fissures. The frontal lobe shows four 
well marked convolutions-ascending, upper, midle and lower 190 
HUMAN PHYSIOLOGY 
frontal convolutions; the parietal lobe contains the ascending 
parietal, the supramarginal, the angular and the superior parietal 
convolutions; the occipital lobe is divided into the upper, middle 
and lower occipital convolutions, and similarly the tempero- 
sphenoidal lobe, by two secondary sulci running parallel with the 
Sylvian fissure, into upper, middle and lower temporal convolu- 
tions. The island of Reil consists of a triangular shaped cluster 
of six more or less distinct convolutions hidden from the surface 
by the overlapping opercula. 
On the mesial aspect of the hemisphere the fissuration is not so 
distinct but the following convolutions may be mentioned-the 
marginal convolution and the callosal convolution, above the corpus 
callosum, both lying in the frontal lobe and partly in the parietal; 
the uncinate gyrus, the cuneus and the precuneus. The calcarine 
fissure in the occipital lobe is well marked on this aspect. 
The orbital surface of the frontal lobe, that is, the surface which 
rests on the orbital plate of the frontal bone, also shows three 
more or less distinct convolutions. 
Microscopical Structure of Cortical Grey Matter 
The grey matter consists of nerve cells and nerve fibers, and of 
these it is possible to make out five layers, although the structure 
of the cortex varies somewhat at different parts. 
Little is know regarding the functions of the nerve cells found 
in these different layers. 
Nerve fibers of the white matter underlying the grey cortex 
may be divided into three groups, viz.: projection fibers, com- 
missural fibers and association fibers. These run in the corona 
radiata, the term applied to the white matter of each cerebral 
hemisphere. 
The projection fibers connect the cerebral cortex with the grey 
matter of the other divisions of the brain at lower levels and with 
that of the spinal cord. They are both efferent and afferent, some 
carrying impulses away from the cortex and others towards it. 
The pyramidal tract is an example of the former, where the fibers 
take origin in the Betz cells of the motor cortex and pass downwards, 
to end in relation to the cells forming the nuclei of the cranial motor 
nerves and in the anterior horns of the spinal cord. 
The commissural fibers, sometimes called bilateral association 
fibers, pass from the convolutions of one hemisphere, across the 
middle line, mainly through the corpus callosum, to those of the 
other hemisphere. 
The association fibers, sometimes called unilateral association 
fibers, connect different convolutions of the hemisphere together. 
Some are short, passing between two adjacent convolutions, while 
others are very long, extending, it may be, from the frontal to the NERVOUS SYSTEM 
191 
occipital poles. The great mass of the white matter of the cerebral 
hemispheres consists of these association fibers, long, short and 
intermediate. 
The cerebellum or little brain lies behind the pons and medulla 
oblongata. It consists of a middle lobe, termed the vermis, and 
two lateral lobes known as the cerebellar hemispheres. 
It is composed of grey and white matter, and like the cerebrum, 
the main part of the grey matter lies on the surface and is known 
as the cerebellar cortex. It is highly convoluted, more so even 
than the cerebrum, the infoldings or fissures being deep and very 
numerous, thereby greatly increasing the area of grey matter. 
Besides this there are several collections of grey matter in the 
interior, the chief being the dentate nucleus, situated deep in each 
lateral lobe, and the roof nuclei in the vermis. 
The white matter consists of nerve fibers leading into the three 
pairs of peduncles that connect this organ with the other divisions 
of the brain. The inferior pair of cerebellar peduncles consist of 
fibers leading up from the spinal cord and bulb to reach the 
cerebellum. The middle peduncles connect the nuclei pontis of 
one side with the lateral lobe of the cerebellum of the opposite 
side; and the superior peduncles carry fibers from the lateral lobe 
of the cerebellum which, through a series of relays, convey impulses 
to the cerebral hemisphere of the opposite side. A very important 
connection exists between the cerebellar cortex and the vestibular 
nerve which has its origin in the semicircular canals of the inner ear. 
Miscroscopical sections through the cerebellum, at right angles 
to the surface, show the cortical grey matter of both vermis and 
hemispheres to consist of two more or less distinct layers, and be- 
tween these are found the antler cells or cells of Purkinje which 
are characteristic of the cerebellum. 
The Cerebellum 
FUNCTIONS OF THE SPINAL CORD 
The functions of the spinal cord are two-fold: 1. It acts as a 
center (or series of centers) for the transformation of afferent im- 
pulses, received through the posterior nerve roots from the peripher- 
al receptors, into efferent impulses transmitted through the anterior 
nerve roots to the effectors such as muscles and glands. This is 
known as reflex action and has already been discussed, in a general 
way. In man an examination of the spinal reflexes is of great im- 
portance and value to the physician in making a diagnosis in cases 
of nervous disease. The reflex actions obtainable from the spinal 
cord may be divided into three classes, from the physicians' point 
of view-1, superficial or skin reflexes; 2, deep or tendon reflexes; 
3, visceral or organic reflexes. 192 
HUMAN PHYSIOLOGY 
The Cord as a Center for Reflexes 
Superficial Reflexes-When the skin of certain parts is stimu- 
lated gently, as, for example, by drawing the finger nail or a pencil 
point across it, certain muscles respond by contracting if the reflex 
arc is complete. A good example is the plantar reflex which con- 
sists in the withdrawal of the foot when the skin of the sole of the foot 
(plantar surface) is tickled. This movement is reflex and conse- 
quently involuntary, but the subject may or may not be conscious 
of it. The spinal center for this reflex lies in the 1 st, 2d, and 3d sacral 
segments of the cord, the afferent path being the nerve fibers from 
the skin of the sole that enter by the posterior roots of the spinal 
nerves belonging to these segments, and the efferent path the motor 
nerves to the muscles of the lower limb used in withdrawing the 
foot. If the physician finds that this reflex cannot be elicited he 
concludes that the reflex arc is interrupted at some point and, if 
by further examination he is able to exclude the afferent and 
efferent factors, this indicates that the lesion exists in the grey 
matter of the cord, in the sacral segments above mentioned. Again, 
it will be remembered that in decerebrate animals, or in animals 
with the spinal cord cut across, the reflexes from the lower half of 
the cord is more easily elicited than in the normal condition, the 
reason being that the inhibitory Impulses which are continually 
being showered down on the spinal cord centers from the higher 
brain centers are absent. If then the physician finds the plantar 
reflex more easily elicited than usual, or the response exaggerated, 
he concludes that a lesion is present in the motor paths (pyramid 
tracts) of the cord at a higher level. 
In addition to the plantar reflex other well known reflexes are 
normally present in man, e. g. the gluteal reflex, the abdominal 
reflex, the epigastric reflex, the interscapular reflex, etc. In each 
case the muscles of those regions contract when the skin over them 
is stimulated in the way stated. The afferent and efferent nerves, 
and the segments of the spinal cord in which the centers for each 
of these reflexes lie, are known, so that a failure to bring out the 
particular reflex helps the physician in many cases to locate the 
lesion. 
A well known superficial reflex, although it is not spinal, is the 
conjunctival reflex which results in the winking of the eye by the 
contraction of the muscle of the eyelid, viz.: the orbicularis palpe- 
brarum, when the conjunctiva is excited. This is often used in 
surgical operations to determine when the patient is sufficiently 
under the influence of the anaesthetic; the disappearance of the 
reflex indicates that this stage has been reached. 
Tendon or Deep Reflexes-The best example of a tendon reflex 
is the "knee-jerk." This is called forth when the extensor tendon, NERVOUS SYSTEM 
193 
just below the patella, is struck a sharp blow with the edge of the 
open hand; the foot is then suddenly and involuntarily kicked or 
jerked forwards. To obtain the jerk the tendon must be slightly 
on the stretch and this is affected by asking the subject to put one 
leg over the other (see diagram). The kick is caused by the sudden 
contraction of the quadriceps extensor group of muscles in front 
of the thigh. The afferent and efferent nerves are known and the 
center lies in the 2d and 3d lumbar segments of the cord. In dis- 
eases such as locomotor ataxia and infantile paralysis, where the 
reflex arc is broken (in the former case in the entering posterior 
root fibers and in the latter case in the grey matter of the cord), 
the knee-jerk is abolished, and in case of damage to the pyramid 
tract fibers, such as might result from a stroke of apoplexy, it is 
exaggerated. 
Another deep reflex which is not present as a rule in the normal 
condition of the nervous system, but only when there is damage 
to the pyramid tract fibers higher up, is the ankle-clonus. This is 
obtained from stretching the tendo achilles by suddenly dorsiflexing 
the foot at the ankle joint (see diagram). Such flexion is followed 
by a series of clonic contractions of the calf muscles leading to 
rhvthmical extension movements of the foot. 
Visceral Spinal Reflexes---Emptying of the urinary7 bladder 
(micturition), of the rectum (defalcation) and of the uterus (par- 
turition) may be brought about reflexly through centers for each 
situated in the lumbo-sacral region of the spinal cord. Each of 
these acts may be carried out, unconsciously of course, when the 
spinal cord is completely severed above the lumbar enlargement 
but not when the grey matter of the lumbo-sacral region is de- 
stroyed. 
Muscular Tonus-All the muscles, whether supplied by the 
spinal or cranial nerves, are maintained normally in a state of 
semicontraction and this condition is known as tonus. If the motor 
nerve to any particular muscle be divided, or if the afferent nerve 
to the same segment of the cord from which the motor fibers 
spring be cut, then this condition of tonus is lost and the muscle 
remains completely and permanently relaxed. Since it is abolished 
by dividing the afferent nerves this tonus is reflex and is maintained 
normally by the continual reception of afferent impulses from the 
skin and other peripheral tissues. 
The spinal cord also serves as a trophic center for the muscles 
and other tissues which are innervated by it. When a motor nerve 
is cut, for example, not only does its peripheral part undergo 
degeneration but also the muscle in which it terminates. The 
muscle fibers atrophy and shrink to less than a quarter of their size, 
The Cord as a Trophic Center 194 
HUMAN PHYSIOLOGY 
it may be, the transverse striation disappears and the whole 
muscle dwindles and decays. For this reason it is believed that 
there is some influence (trophic) normally exerted on the muscle, 
by the nerve cells in the spinal cord, whereby its nutrition is main- 
tained, and when this is cut off the muscle degenerates. Since 
division of the afferent nerve will not produce this effect it is not 
due to the same cause as muscle tonus. 
The Spinal Cord as a Conductor 
According to the Bell-Magendie law all the impulses that reach 
the spinal cord from the periphery enter through the posterior 
nerve roots. The fibers conveying these impulses lie in the tracts 
of Goll and of Burdach, in the posterior columns; some enter the 
grey matter of the cord while others, reach the medulla oblongata 
and terminate in the nucleus gracilis and nucleus cuneatus. The 
mesial fillet fibers arise from cells situated in these two nuclei, 
cross the middle line and end in the optic thalamus from which 
fresh fibers take origin and pass to the grey matter of cerebral 
cortex. Sensory impressions from the skin and other sensitive 
surfaces may reach the sensory areas of the cerebral cortex by 
passing over these three superimposed systems of neurones, viz.: 
periphero-bulbar, bulbo-thalamic, thalamo-cortical, and there give 
rise to sensations. 
Another pathway to the cortex is through the spino-thalamic 
fibers of Gower's tract to the optic thalamus and by a second 
neurone, from there to the sensory areas of the cortex. 
It will be remembered that both dorsal and ventral spino- 
cerebellar tracts arise in the cells of Clarke's column, in the grey 
matter of the cord, and end in the vermis or middle lobe of the 
cerebellum. Many of the fibers of the posterior nerve roots that 
enter the grey matter of the cord end in relation to the cells of 
Clarke's column, so that impulses entering the spinal cord from 
the periphery may reach the cerebellum through the spino-cere- 
bellar tracts, above mentioned, and from there arrive at the 
cerebral cortex. 
A fourth route by which impulses, coming into the cord over 
the posterior nerve roots, may reach the cortex is by the proprio- 
spinal system of fibers found in the ground bundles. These connect 
one segment of the cord with another, the system being continued 
up into the medulla, pons, and midbrain to the optic thalamus as 
the posterior longitudinal bundle. The thalamus it will be seen is 
a great sensory nucleus, or series of nuclei, forming the last cell 
station on the afferent path to the cerebral cortex. 
Most of the fibers of the posterior nerve roots enter the grey 
matter of the spinal cord and connect, either directly or indirectly, NERVOUS SYSTEM 
195 
with cells of the anterior and lateral horns, thus forming reflex 
arcs, the anatomical basis for spinal reflex action. 
From the above description it will be seen that afferent impulses 
coming in through the posterior nerve roots may reach A, the 
cerebral cortex, affecting consciousness and giving rise to sensations; 
B,the cerebellum, leading to the muscular coordination necessary 
to maintain bodily equilibrium; C, the grey matter of the spinal 
cord, giving rise to reflex action. 
Sensory Conduction 
Sensory impressions of various kinds are received from the skin, 
and from the structures underneath it such as the muscles, and 
these, on reaching the spinal cord, travel up one or other of the 
paths described above, to reach the sensory areas of the cerebral 
cortex. Impulses giving rise to sensations of warmth, cold, pain, 
and certain forms of touch are believed to cross early in the cord 
and to travel up the lateral column of the opposite side, possibly 
through the spino-thalamic fibers of Gowers' tract, or by way of 
the cerebellum, to the cerebrum. The sensations arising from 
muscles, tendons and joints,which enable us to tell the position 
of a limb in space, for example, that is the sense of passive position 
and of movement, and the form of touch known as tactile discrim- 
ination, pass up the posterior column (tracts of Goll and Burdach) 
of the same side as that on which they enter and cross to the other 
side in the medulla oblongata, whence they reach the cerebral 
cortex through the mesial fillet and the thalamo-cortical fibers. 
All forms of sensation ultimately cross the middle line before 
reaching the cerebral cortex so that the cerebral hemispheres of 
one side receives its sensory impressions from the opposite side of 
the body; thus, damage to the sensory areas of one side of the 
cerebrum will result in sensory paralysis of the opposite half of 
the body. The crossing is not complete, however, for in all cases 
a few impressions reach the hemisphere of the same side. 
Motor Conduction 
Motor impulses, arising in the motor areas of the cerebral cortex, 
are carried down the cord mainly by the fibers of the pyramid tract. 
Since most of these cross to the opposite side, in the lower region 
of the medulla oblongata, and run down the lateral column of the 
cord as the crossed lateral pyramid tract, to end either directly or 
indirectly, in the anterior horn, from which other fibers emerge 
through the anterior nerve roots, to pass to the voluntary muscles, 
the relationship here is also a crossed one'-one side of the cere- 
brum presides over the muscles of the opposite side of the body. 
Impulses travelling over this system of fibers lead to voluntary 
movements; it is called the primary motor path because it is 196 
HUMAN PHYSIOLOGY 
possible for voluntary impulses to reach the muscles through 
another route known as the secondary motor path. If both pyra- 
mids be divided in a dog, at the point where they decussate in the 
medulla oblongata, the motor paralysis which results is quickly 
recovered from. The reason given for the recovery is that, in the 
absence of the pyramid tract system, the impulses are shunted into 
the rubro-spinal tract or the tract of Loewenthal, and so reach the 
grey matter of the spinal cord by this secondary path, the function 
of which is normally in abeyance. As we ascend the animal scale 
the pyramid tract system (primary motor path) becomes more and 
more important; it is more highly developed in the monkey than 
in the dog and more still in man. In man a lesion in the pyramid 
tract is never recovered from. 
Effects of Lesions in Spinal Cord--Complete transverse section 
of the spinal cord, experimentally in animals, or extensive damage 
by accident or disease in man, will lead to motor and sensory 
paralysis on both sides below the lesion. If the interruption is in 
the thoracic region, so that the lower half of the body is paralysed, 
the condition is known as paraplegia. Hemisection of the cord, 
that is, division of one lateral half, will lead to motor paralysis 
below the lesion, on the same side, and sensory paralysis of the 
opposite side. If the pyramid tract be damaged on one side any- 
where above its decussation in the medulla oblongata, muscular 
paralysis of the opposite side of the body will follow; this is known 
as hemiplegia. This condition is most frequently caused by the 
rupture of a bloodvessel near the pyramidal fibers in the internal 
capsule, the escaping blood tearing the fibers across or the pressure 
of the clot on the fibers preventing the conduction of nerve impulses 
through them. This is one cause of apoplexy. Paralysis of a single 
limb (arm or leg), or one side of the face, is termed monoplegia. 
This may be caused by the disease or destruction of the cells in 
the anterior horns of the cord, or of the motor nuclei in the brain, 
which send fibers to the particular parts involved. 
FUNCTIONS OF MEDULLA OBLONGATA, PONS, 
AND MIDBRAIN 
These divisions of the brain, particularly the medulla, contain 
many important reflex centers such as the cardiac centers, the 
respiratory center, the vasomotor center, thedeglutition center, the 
salivary center and many others. In addition to this they contain 
groups of cells which give origin to the efferent fibers of the cranial 
nerves, termed the cranial motor nuclei, which correspond to the 
cells of the anterior horn of the spinal cord. These nuclei receive 
fibers from the pyramidal tract in its passage downwards, above 
the decussation, which cross the middle line before reaching NERVOUS SYSTEM 
197 
the nuclei. They also contain groups of cells which serve as the 
terminal nuclei for the afferent fibers of the cranial nerves, re- 
sembling in this respect Clarke's column, e. g. in the grey matter 
of the spinal cord in relation to the fibers of the posterior nerve 
roots. The nerve cells of these nuclei give off fibers which cross 
the middle line and join the mesial fillet, the great sensory tract, 
that arises in the medulla oblongata from the nuclei gracilis and 
cuneatus. The motor and sensory conduction paths, pyramid tract 
and mesial fillet respectively, between the cerebrum and spinal 
cord, also pass through these lower divisions of the brain. 
FUNCTIONS OF CEREBRUM 
The grey matter of the cerebral cortex is the seat of intelligence 
and of the will, since, when it is removed in the lower animals, 
both disappear. It is much more highly developed in man than in 
any of the lower animals; it is the organ of psychical activity in 
all animals, where it exists. 
Localization of Functions-Until about the middle of the nine- 
teenth century it was believed that there was no differentiation of 
function in the cerebral cortex, that is, every region of the grey 
matter was supposed to have the same function or functions. 
Speech Center -The first discovery of any localization of func- 
tion in the cerebral cortex was made by Broca, a French physician. 
He observed, that in certain cases of cerebral haemorrhage, where 
the power of speech was lost, on post-mortem examination, the 
lesion was found to be located in the posterior part of the inferior 
frontal convolution. This is now known as Broca's convolution or 
the speech center. Destruction of the grey matter in this region 
leads to the loss of speech, although questions written or oral, are 
understood and can be answered in writing. This speech center 
is only found on one side; in right-handed individuals it lies in the 
left hemisphere and in left-handed individuals in the right 
hemisphere. 
Motor Areas 
These were first postulated by the London physician Hughlings 
Jackson. He observed in certain cases of mild epilepsy (now 
known as Jacksonian epilepsy), where the convulsions were limited 
to one limb, that the movements followed a certain order or march, 
e. g. they would begin in the muscles of the thumb and spread to 
those of the fingers, forearm and upper arm. In some of these 
cases there was the history of injury to the skull in the region of 
the Rolandic fissure and Jackson argued that the irritation to this 
part of the cortex, by a spicule of bone, for example, is the cause 
of the arm movements. This is now known to be the case. 198 
HUMAN PHYSIOLOGY 
The motor areas were first demonstrated by two German 
physiologists, Fritsch and Hitzig. In an anassthetized dog they 
exposed the cerebral cortex in the region of the crucial sulcus and 
stimulated it with galvanic electricity. On applying the electrodes 
to a certain spot in front of this sulcus they obtained movements 
of the head (by contraction of the neck muscles) to the opposite 
side; stimulation of another area, lateral to the first, led to 
movements of the arm on the opposite side; and similarly for the 
leg area lying behind the sulcus. The connection received further 
proof when it was discovered that extirpation of the cortical area 
led to paralysis of the muscles which had previously responded to 
stimulation of that area. 
This work was continued by Ferrier, Horsley, Beevor, Schafer 
and others, on the monkey, the final word, up to the present time, 
being given by Grunbaum and Sherrington on the chimpanzee, 
orang utang and gorilla (see diagram). The motor areas in these 
anthropoid apes (which probably agree closely with those in man) 
occupy on the lateral aspect the whole extent of the ascending 
frontal convolution (often called the precentral convolution) being 
limited behind by the fissure of Rolando or central sulcus. On 
the mesial aspect of the hemisphere it extends only a few milli- 
meters over the border. 
The leg area lies nearest the middle line and extends over the 
mesial border; then, in order, come the body, arm, and neck areas 
from above downwards, and lastly at the lowest extremity of the 
convolution comes the area which on stimulation gives movements 
of the tongue, mouth and face muscles. An area, in front of the 
others, and separated from the precentral convolution, is associ- 
ated with movements of the eyeballs. 
Extirpation of any one of these areas results in paralysis of the 
muscles which respond to its stimulation, and is followed by de- 
generation of the fibers of the pyramid tract which convey impulses 
to the segments of the brain or spinal cord which innervate these 
muscles. Similarly, in man, destruction of the motor areas in 
disease, for example by hsemorrhage or tumour, leads to paralysis 
of the corresponding groups of muscles and to degeneration of the 
fibers that take origin in the nerve cells of these areas. By the 
application of proper staining methods the degenerated fibers may 
be traced to their terminations. 
Sensory Areas 
Ferrier was the first to discover the presence of sensory areas in 
the cerebral cortex. It is in these that the various afferent im- 
pulses from the skin, muscles and special sense organs arrive. 
Destruction of the peripheral organ, of any of the nerve fibers or NERVOUS SYSTEM 
199 
cells on the afferent path, or of the grey matter of the sensory area, 
will result in the loss of that particular sensation. 
Vision Center-This is situated in the occipital lobe. Destruc- 
tion of the occipital lobe, on the right side say, or of that part of 
the grey matter on its mesial aspect in the region of the calcarine 
fissure, will produce blindness in the right halves of the two retinae. 
Objects in the left fields of vision will not be seen. The reason for 
this is that the fibers of the optic nerves which come from the 
inner or nasal halves of the retinae decussate in the optic chiasma 
while those from the outer or temporal halves do not (see diagram). 
Extirpation or destruction of both occipital lobes results in total 
blindness. 
Auditory Center-This is situated in the posterior part of the 
superior temporal convolution. If this area be removed in an 
animal such as a dog or a monkey, or be destroyed in man by 
disease, the opposite ear is deaf. If it be removed or destroyed on 
both sides total deafness follows. The connection here appears to 
be completely crossed. 
The Centers for Taste and Smell have not been located with 
certainty. They are believed to lie in the uncinate gyrus and 
possibly the tip of the temporal lobe. These areas are much more 
highly developed in animals like the dog, where smell is the leading 
sense, than they are in man. 
Tactile Area-This is believed to lie in the ascending parietal or 
postcentral convolution immediately behind the motor areas. 
Those impressions that reach the cortex through the posterior 
columns of the cord and the fillet tract, viz.: the sense of passive 
position and muscular movement, and of tactile discrimination, are 
believed to arrive in this convolution. Whether pain and tempera- 
ture impressions are conducted here also is not certain. 
The Psychical Centers or Silent Areas-When the motor and 
sensory areas have all been mapped out there is a large part of the 
surface left stimulation of which produces no apparent effect; 
these have, on this account, been called the silent areas and are 
believed to be associated with the intellectual or psychical functions 
of the brain. One of these regions lies in the frontal lobe anterior 
to the motor cortex, another in the parietal lobe and a third in the 
island of Reil. These are also known as association centers since 
impressions from the sensory areas are believed to be associated 
or built up into ideas in these regions. It has been shown in 
insane individuals that tnese association centers, in particular, have 
undergone degenerative changes. 
In connection with the functions of the cerebrum the phenomena 
of sleep and narcosis are very interesting. During sleep the 
Sleep and Narcosis 200 
HUMAN PHYSIOLOGY 
activity of the cerebral cortex is in abeyance and consciousness is 
lost. In this respect the brain is no exception for probably all living 
tissues show periods of activity alternating with periods of rest, 
to some extent. The cells of the salivary glands, or pancreas, for 
example, following the phase of active secretion, require a period 
of rest in which to recuperate and to build up again the substances 
to be discharged later. Even the heart which never ceases to beat, 
during each cardiac cycle takes a short rest in the diastole, and the 
respiratory center is quiescent in expiration. In the case of the 
brain cells the period of sleep is the recuperative period in which 
anabolic changes predominate, whereas in the waking state kata- 
bolic changes are most in evidence. 
Functional Changes that Accompany Sleep-The most striking 
accompaniment of sleep is loss of consciousness, but in addition 
to this other changes are observed. The respirations become slower 
and usually deeper and, in many cases, a mild form of Cheyne- 
Stokes breathing is present. Many of the secretory glands become 
less active, e. g. the salivary glands, the lachrymal glands, the 
kidneys and possibly the gastric glands. The heart beats more 
slowly, the bloodvessels of the skin dilate and the blood pressure 
falls; the tonus of the skeletal muscles is greatly diminished, 
and that the bodily metabolism generally is less active is indicated 
by the fact that less CO2 is excreted than in the waking state. 
Cause of Sleep-With regard to the cause of sleep several theories 
have been put forward of which three may be mentioned. 
The Neuron Theory-All afferent impulses that reach the cere- 
bral cortex are conducted over a series of superimposed neurons 
and pass across two or more synapses at which they are supposed 
to meet with a certain amount of resistance depending, it is assumed 
here, on the distance between the dendrites of the one neuron and 
the terminal arborisations of the other. Assuming again that the 
dendrites are contractile, if as a result of functional activity these 
slowly retract, at a certain point the distance will be too great 
for an impulse of ordinary strength to pass across; then the brain 
cortex will be protected from excitation and will go to sleep. 
During the resting (sleeping) period the processes gradually 
elongate, until physiological connection is again established, and 
at this point the sleeper awakes. A sufficiently powerful impulse, 
produced, for example, by a very loud sound, will cross the synapse 
at any time and awake the sleeper. Much of this theory is based 
on assumption, for there is no proof that the dendrites do retract 
under the influence of functional activity. 
Waste Products Theory'-By some the cause of sleep is held to 
be the accumulation of waste products, such as the sarcolactic acid 
produced in muscle, in the blood which leads to loss of excitability NERVOUS SYSTEM 
201 
or fatigue of the brain cells; when a certain stage is reached this 
results in loss of consciousness, that is, sleep. When sarcolactic 
acid or sodium lactate is injected into the blood it is known to 
induce fatigue and ultimately sleep. This would explain the well 
known fact that muscular exercise is one of the best soporifics; 
an outdoor laborer is seldom troubled with insomnia. 
Cerebral Anaemia Theory-Some attribute the loss of cerebral 
activity to a diminished flow of blood to the brain. It is well 
know that fainting or syncope, accompanied by loss of conscious- 
ness, results immediately from a defective supply of blood to the 
brain, caused by stoppage or weakening of the heart's action. In 
animals and in human beings, where the brain has been exposed by 
an opening made in the skull, it is observed that in sleep the 
surface of the cortex becomes paler, indicating a diminished supply 
of blood, and also that the volume of the brain becomes less. The 
diminution in brain volume may be demonstrated by plethysmo- 
graphic records taken on a revolving dram. 
Howell explains the onset of cerebral anaemia which leads to 
sleep as follows: The vasomotor center in the medulla oblongata 
is continually receiving afferent impulses and, as a result, is in 
constant activity or tonus during the waking hours. By and bye 
this center, more particularly that part of it which presides over 
the skin area, begins to exhibit signs of fatigue and loss of tone 
so that the cutaneous bloodvessels dilate and more blood circulates 
through the skin, and it may be also through the abdominal viscera. 
More blood being supplied to the regions mentioned, less is avail- 
able for the brain, and when a certain degree of anaemia is reached 
the blood supply will not be sufficient to maintain the activity of 
the brain cells and loss of consciousness will follow, that is, sleep. 
That the vasomotor center does lose tonus during sleep is proved 
by the general fall in blood pressure. The drowsiness that usually 
follows a heavy meal may be explained as due to the increased supply 
of blood to the digestive organs, leading to a certain degree of 
cerebral anaemia. 
After a sufficient rest the vasomotor center will recover its tone, 
the bloodvessels of the skin and the splanchnic areas will become 
constricted, leading to a diminished supply of blood to these areas, 
and an increased supply to the brain, resulting in spontaneous 
awakening. 
The chief objection that might be raised to this theory is that 
it is almost impossible to say whether the cerebral anaemia is the 
cause of sleep or sleep the cause of the cerebral anaemia. 
No matter what the underlying cause of sleep is, we voluntarily 
assist it by retiring to a darkened and quiet room so as to cut out 
all visual and auditory impressions that might excite the cells of 
the cerebral cortex. 202 
HUMAN PHYSIOLOGY 
Depth or Intensity of Sleep-This is usually measured by the 
loudness of the sound necessary to awaken the sleeper. Using 
this method is it found that sleep is deepest about the end of the 
first hour and that from the second or third hour to the time of 
spontaneous awakening it is very light. It is in this period of light 
sleep that dreams occur, portions of the grey matter being aroused 
by external stimuli; the subjective result is the dream, the im- 
pressions being frequently sufficiently strong to implant themselves 
on the memory. 
Narcosis is the term applied to the loss of consciousness produced 
by drugs such as morphine, alcohol, ether, chloroform, etc. The 
drug probably enters into chemical combination with the proto- 
plasm of the nerve cells, diminishing their excitability, until they 
cease to be affected by outside influences. Ether and chloroform 
are used extensively for producing surgical anaesthesia. The vapor 
of the drug is inhaled and is absorbed by the blood in the lung 
capillaries; it is carried by the blood to the brain and there pro- 
duces its action. The higher brain centers (inhibitory and psychi- 
cal), situated in the cerebral cortex, are the first to succumb; 
consciousness is lost and with it the ability to feel pain, while the 
lower centers in the medulla oblongata (e. g. respiratory center) are 
still active. If the drug be pushed the lower centers are also 
paralysed and respiration stops; the duty of the anaesthetist is to 
keep the patient between the limits of unconsciousness on the one 
hand and of respiratory paralysis on the other. 
FUNCTIONS OF THE CEREBELLUM 
The cerebellum is a large coordinating center for muscular 
movements and its main function is to maintain body equilibrium. 
Removal of the cerebellum, or of a part of it, in animals, or its 
destruction by disease (haemorrhage or tumor) in the human 
subject, results in what is known as cerebellar ataxia, that is a 
staggering, reeling gait due to the want of coordination in the 
muscular movements necessary to maintain equilibrium in the act 
of walking or standing. The reason why a drunken man reels and 
staggers, on attempting to stand orwalk, isthathis cerebellar centers 
have been partially or completely paralysed by the alcohol taken. 
Following removal or disease of the cerebellum there is also some 
loss of muscular strength and tonus but the chief result is the ataxia 
just described. 
Unless the lesion is a very large one the symptoms are soon 
recovered from, other centers in the brain presumably taking on 
the functions of the cerebellum. NERVOUS SYSTEM 
203 
AUTONOMIC NERVOUS SYSTEM 
The autonomic nervous system consists of a chain of ganglia, 
situated outside the spinal canal, extending from the cervical to 
the coccygeal region, and known as the sympathetic system, and 
of other ganglia, mainly in the head and abdominal cavity, termed 
the parasympathetic system. The autonomic system is connected 
with the central nervous system by preganglionic fibers, which 
emerge from the spinal cord through the anterior nerve roots and 
through certain of the cranial nerves, and end in the sympathetic 
and parasympathetic ganglia. From these ganglia postganglionic 
fibers take origin and pass to their terminations in nonstriped 
muscle and secreting glands. There are thus two cell stations on 
the path of impulses passing over the autonomic system, one in 
the grey matter of the spinal cord or brain and the other in the 
sympathetic or parasympathetic ganglia. 
The preganglionic fibers are medullated but considerably smaller 
than the medullated fibers of the cerebro-spinal system, while the 
postganglionic fibers are non-medullated. 
ENTERIC NERVOUS SYSTEM 
This system consists of two plexuses (Auerbach's and Meissner's) 
of nerve fibers and cells situated in the wall of the alimentary 
canal. Auerbach's plexus lies between the longitudinal and circular 
layers of the muscular coat and Meissner's plexus is found in the 
submucous coat. They are believed to be connected with the 
autonomic system but they can act independently of it. Their 
nerve fibers end in the muscles and glands of the alimentary canal. 
THE END