COLUMBIA LIBRARIES OFFSITE

HEALTH SCIENCES STANDARD

HX64088790
QP34.M78 1899 Elementary physiolog

LEJ

RECAP

Physiology

MOORE

Columbia IHnibersitp

in tfte Citp of igeU) |?orfe

COLLEGE OF PHYSICIANS
AND SURGEONS

Reference Library

Given by

^^its^ny^r ^ ^/-QLn^

^^o^ 7^ ^- .^V7t

v^

ELEMENTARY PHYSIOLOGY

ELEMENTARY

PHYSIOLOGY

BY

BENJAMIN MOORE, M.A.

PROFESSOR OF PHYSIOLOGY IN THE IVIEDICAL DEPARTMENT OF YALE UNIVERSITY"

LATE SHAKPEY RESEARCH SCHOLAR, AND ASSISTANT PROFESSOR OF

PHYSIOLOGY, AT UNIVERSITY COLLEGE, LONDON

JVITH ONE HUNDRED AND TWENTY-FIVE
ILL USTRA TIONS

NEW YORK
LONGMANS, GREEN, AND CO.

LONDON AND BOMBAY
1899

All rights reserved

PREFACE

This book is intended to give an idea of the structure of the
body, and of the changes which are constantly taking place in
it during life, to those who have no previous knowledge of the
subject.

It has accordingly been written in as elementary a fashion,
and with the use of as few technical terms, as possible, so as to
be intelligible to the general reader.

The subject is such a vast one that an outline sketch is all
that can be presented in a volume of the dimensions of the
present one, but it is hoped that this sketch will be sufficiently
clear to give a fair idea of the subject to the student of
Physiology before he proceeds to more detailed study with the
aid of larger text-books.

Such a bird's-eye view of the subject as is here shown ought
to prove of advantage to the junior student, who is often
plunged into a mass of detail, and gets so involved in this that
he loses sight of the main outstanding features of the subject.

At the same time, it is hoped that the book may attract a
larger circle of readers and may remove some of that deplor-
able ignorance which is so often met with, even among fairly
well educated people, as to the general structure of their own
bodies and the actions which take place within them during
life.

Physiology is a subject with which a living acquaintance can
only be made by employing experimental methods, and hence

vi Preface.

it is very desirable that the student should as far as possible
perform for himself the experiments described in the text. It
is further to be hoped that doing so will train him to habits of
observation, and awaken his interest so that he will be able in
the end to see for himself things to which his attention is not
specially directed by others.

In conclusion, I desire to thank my friend Mr. D. P.
Rockwood for his kindness in revising the proof-sheets and
preparing the index. Most of the illustrations have been
selected, by kind permission of Prof. Scliafer, from the
"Essentials of Histology," and from Quain's "Anatoniy."

B. MOORE.

University College, London,
October, 1898.

CONTENTS

CHAPTER I'AGE

1. Introduction i

II. The Skeleton and its Articulations 13

III. The Muscular System 56

IV. Position of the Viscera 79

V. The Circulatory System 96

VI. The Blood 121

VII. Diet, Digestion, Absorption, and Metabolism . . 130

VIII. Respiration 173

IX. Animal Heat , . . ..... 192

X. Excretion , 198

XI. The Nervous System . . , , 218

XII. The Senses 245

Appendix , 275

Test Questions • 285

Index - .- 289

ELEMENTARY PHYSIOLOGY

CHAPTER L

INTR OD UCTION.

Physiology is the study of the changes which go on in living
matter. Throughout the entire life of any living creature various
changes are constantly taking place, and it is by means of the
impressions conveyed to our senses by these changes, which
awaken other changes in us, that we are made aware of the life
of the creature.

The forms of life which we can recognize in the world
around us are countless in their variety, but all these forms
have this in common that a store of energy is taken in from the
outer world in various ways and used by the living creature to
carry out the different acts or changes which constitute its life.

In a broad sense of the word, physiology includes the
study of the life of plants as well as that of animals, but plant
life is the special province of the botanist. The physiology of
the lower forms of animal life is also usually left to the care of
the zoologist. We shall hence devote the greater part of the
space at our disposal to giving an outline of the physiology of
man and of those animals which are closely allied to man in
their structure.

It will be well, however, before doing so to very briefly
describe the simplest type of animal life with which we are
acquainted, in order to obtain some general idea of how this
simple form of life is structurally related to the more complex
animals which we have subsequently to consider.

7

/

2 Elementary Physiology.

The simplest form of animal life with which we are
acquainted is a minute speck of material which is only visible
through a microscope. Small as it is, it is however a com-
plete organic whole, capable of showing all the essential
characteristics of life, and of producing, at what corresponds to
the end of its existence as an individual unit, other similar
organisms which repeat again an existence that is a fac-
simile of its own. Examples of organisms of such a simple
type are to be found in the amoeba of pond water, and also in
the leucocytes^ or whitd blood corpuscles^ which carry on a separate
but dependent existence during our life in our blood. These
leucocytes exist in the blood of all the higher animals. They
live in the blood so long as it circulates in the body, but soon
after the blood is shed, or after the animal dies, the leucocytes
perish.

When examined by the aid of a microscope, each of these
simple little organisms is seen to consist of a mass of very
irregular outline. The material of which it is composed is
more or less granular in appearance, and the granules,
although usually minute, vary in size. At one part of the
tiny organism the material is seen to be somewhat denser and
less transparent in structure. This denser portion is usually
rounded or globular in shape, and is also more granular than
the remainder; it is known as the nucleus.^ and when the
organism is in a resting position occupies a central place in
the mass.

If this little mass of living material be carefully and
patiently observed through the microscope, it will be found
that its shape does not remain constant ; there is a slow change
going on in its outline, and in a short time its appearance may
be quite different from that which it had when first observed.
The rate at which the alteration in shape goes on varies with
certain circumstances, such as the temperature of the fluid in
which the organism is immersed and the presence of food
particles in its immediate neighbourhood \ but usually the rate
is not so sudden as to be very obvious to the eye. It is only
when the organism is watched for a time that the change in
shape becomes evident, and a clear idea of how the change

hitroduction. 3

takes place is best obtained by sketching the outUne at intervals
of a minute, or by watching one of the jagged processes in the
irregular outline. It is then seen that the movement which
brings about the change of shape, or it may be a change
in position of the creature as a whole, is a flowing one. At
an indefinite point, a minute process apparently spontaneously
grows out from the main body; this is at first clear in its
interior, later it grows in size and becomes granular inside from
an accession of granular material from the main body. More
than one such process — or, as it might be termed, feeler — is
usually to be seen at any time projecting from the main mass.

Fig. I. — Changes of form of a white corpuscle of newt's blood, sketched at intervals
of a few minutes, showing the inception of two small granules and the changes
of position these underwent within the corpuscle.

The size to which a process can grow depends upon circum-
stances; occasionally after it has reached a certain size it is
again withdrawn into the main mass, as if a determination had
been made at some headquarters that nothing was to be gained
by further motion in that direction. At other times a process
goes on increasing in size until finally the whole of the mass
has flowed into it, and the animal has completely changed its
position. In many cases such a continued growth of a process
is due to the existence of a food particle in that direction. In
such a case the mass of the organism flows round the particle,
envelops it in its mass for a time, and afterwards rejects any
unserviceable debris that may be left by flowing in the same
slow fashion away from it.

The minute organism seems to become aware, in some

4 Elementary Physiology:

way, of the presence of a food particle even before it has
touched it, possibly from minute traces of dissolved substances
surrounding the particle of food which become stronger as it
is approached. Various chemical substances determine move-
ments of the organism towards them, or away from them, or
prevent movement altogether and cause the creature to assume
a spherical shape. Electric shocks passed through the fluid
have a similar effect, the processes are all drawn in, and the
creature becomes a rounded mass. After a time, if the dis-
turbing influence be removed, the processes are gradually
formed again, and the flowing movement recommences. If
stronger chemical reagents are used, or if the fluid in which the
animal lives is heated above a certain temperature, death is
the result. The existence of one of these tiny organisms as an
individual may terminate in one of two ways ; it may die in
some way, or it may resolve itself into two smaller organisms^
which afterwards increase in size and repeat a life history like
that of the single organism from which they originated. When
the organism is well supplied with food it takes up more than
is sufficient to supply energy for the changes in shape above
described. The food is changed within the mass, and is in
part chemically altered and built up into the organism, so that
the bulk goes on increasing until a maximum size is reached.
Soon after this the denser portion, known as the nucleus, begins
to constrict at its middle, becomes hour-glass shaped, and
finally divides into two ; next the general mass also constricts
a portion forming round each of the new nuclei, and finally
there are two smaller masses formed, each with a new nucleus.
Each of these new masses is thus started into life as a new
individual, and so the cycle goes on repeating itself.

When the materials of which the bodies of animals are
made up are examined under the microscope, it is found that
they consist of aggregations of minute units which are related
in their nature to the simple organism which has been described
above. These microscopic units are spoken of as cells. Each
cell typically consists of a minute mass of semi-fluid material
cdiW^di protoplasm. At one part this protoplasm is different from
the remainder ; it is here denser and more complex in its

Introduction. 5

structure, and is known as the nucleus. The nucleus seems to
be the most highly vitalized portion of the mass, and any im-
portant changes in the life of the cell, such as its division into
two new cells, commence first in the nucleus. Granules are
often visible within the cell protoplasm ; these are not part of the
protoplasm in the sense that they are not living, but are usually
composed of material which has been made by the protoplasm
from the food on which it has acted. This material is after-
wards used for carrying on various operations which make up
the life of the cell.

Thus each living cell in the animal's body resembles its
fellows, in that it takes in certain substances in the form of food,
and changes these substances chemically into other substances,
or, as it is called, assimilates them. These new substances
serve various ends. For example, some supply a store of
energy to the cell to carry on its own life ; others are turned
out again to form nutriment to other different cells in the body;
others, again, stream off in solution to act on the food of the
animal, and render it capable of absorption by another class of
cells.

The cells which go to make up the bulk of an animal vary
enormously in their appearance. Only a very small percentage
are capable of freely moving about, and taking in food in the
same fashion as the simple unicellular organism which has been
described above as a type. By far the greater number are fixed
in shape and position ; these must have their food brought to
them in solution, for they cannot absorb solid particles ; they
must be bathed in a nutrient fluid which is carried to them in
some manner, and this nutrient fluid must be prepared in some
fashion from the solid food which the animal, of which they
form a part, eats from time to time.

It is hence easy to see that in animals consisting of large
colonies of cells there must be a division of labour ; one class
of cells performing one class of work, while another class carries
on another. This purpose is best attained by an arrangement
by which those cells which carry on the same kind of labour
are situated at the same place, and such is the arrangement
which is found in the animal body. The cells are not situated

6 Elementary Physiology.

indiscriminately, but in aggregations, each consisting of a very
large number of similar cells carrying out a similar purpose.
To these aggregations of cells different names are given for
purposes of description, such as tissue^ organ^ gland, and
body.

" Tissue" is the most general of these terms, and is applied to
any assemblage of cells designed for a common purpose. Thus
there is a tissue found generally all over the body, the purpose
of which is to bind such aggregations of cells as we have been
describing into bundles and masses, and to form sheaths and
coverings for them ; this is known as connective tissue.
Again, the muscles of the body by which the movements of the
animal are brought about are made up of cells which are
collectively spoken of as muscular tissue.

The term " organ " is more restricted in its application, and
is usually applied when the purpose served by the mass of cells
is more specific in its character. Thus the heart is an organ,
the purpose of which is to send the blood streaming round the
body, carrying nutriment to the cells of which the other tissues
and organs are composed. Since it is chiefly composed of
muscular tissue, it is further called a muscular organ. An organ
may thus be composed of several difterent tissues, just as a
rock may be made up of different minerals. For example, the
heart contains connective tissue, and adipose or fatty tissue, as
well as muscular tissue. Also a tissue may go to form a part
of very diverse organs; thus muscular tissue is found in the
skeletal muscles, in the heart, in the walls of blood-vessels, and
in the coats of the alimentary canal.

A gland consists mainly of aggregations of cells the purpose
of which is to take up certain materials from the blood, and
from these to make new materials, which are either returned to
the blood to be of service to the animal at some other part, or to
be excreted from the body as useless and injurious by some
other gland ; or are sent into a collecting vessel called the duct of
the gland, dissolved in a watery fluid which is known as the
secretion of the gland.

These gland secretions serve many purposes in the body.
Some are lubricants, such as the secretion of the lachrymal

Introduction. 7

gland of the eye, which serves to keep the eyeball moist and
float off any irritant particles which may reach the sensitive
surface of the eyeball. The ^///^_/ function of the saliva secreted
by the salivary glands is similarly to coat the food as it is
swallowed with a slippery envelope, and render its passage to the
stomach an easy one. Other secretions contain substances
dissolved in them which act on the food eaten by the animal,
dissolve it, and render it capable of absorption by those cells
which line the passage called the alimentary canal by which
the food passes through the body. These secretions will be
considered more fully in treating of digestion.

In other cases the secretion of the gland contains only
substances which are either of no service, or directly injurious
to the body, and the purpose of the secretion is to secure the
removal of these from the circulating blood-stream, and so away
from the body; in such a case, the process, though really identical
in nature with that of secretion, is spoken of as excretion. The
best example is that of the kidneys. Here substances which
exist ready formed in the blood, and the accumulation of which
in the blood would lead to the poisoning of the cells and to the
death of the animal, are kept down to a normal level by being
continuously removed by the secreting action of the cells of the
kidney. In the case of the liver the secretion {the bile) has a
mixed character. Some of the substances held in solution are
useless to the body, and are finally carried away from the body,
mixed with the unused and indigestible part of the animal's
foods. Other substances secreted in the bile are useful in the
process of digestion, and are absorbed again by the cells lining
the alimentary canal.

Certain glands in the body do not possess ducts, and have
no obvious secretion. Before their true nature was known,
some of these ductless glands were spoken of as bodies. It is
not yet known how these ductless glands act, but it is certain
that they exert a powerful influence over the welfare of the
animal, and in the case of some of them their removal is
followed by the death of the animal. It is supposed that they
act like glands in removing certain materials from the blood,
and in elaborating from these materials others which are

8 Elementary Physiology.

returned to the blood-stream again, and serve useful and in
some cases indispensable purposes in the preservation of the
health of the animal. A similar kind of work is done by
certain of the other glands which possess ducts. Thus it is
certain that the formation of bile is not the only nor even the
main work of the liver. This organ acts as a storehouse for a
certain class of food, accumulating in its cells a reserve of
material when the supply in the blood which comes to it is too
liberal, and doling this reserve out when the supply slackens
and the other cells of the body require this kind of nutriment.
In addition, the liver has a modifying action on other classes
of food-stuffs, but this subject will be more fully considered
later.

It must not be supposed that in each of these aggregations
of cells that we have been considering, there is to be found
only one kind of cell. It is only meant that one type of cell
preponderates in them. These cells, to do their work, must be
suppUed with blood, and this blood must be carried to them by
blood-vessels, and away from them by other blood-vessels.
The walls of these blood-vessels contain various kinds of cells
specially adapted to form the walls of vessels. There is an
inner lining of flat cells to give a smooth lining to the wall, and
outside this, again, muscle cells encircling the vessel, which by
their state of contraction or relaxation allow less or more blood
to pass to the preponderating cells. ^ Again, the blood itself
contains cells of different kinds. Further, the whole mass of
cells must be supported and bound together by some of that
connective tissue mentioned above. It is hence to be under-
stood that in every tissue and organ of the body there is a
considerable number of different kinds of cell to be found, but
that the nature of these cells corresponds to the kind of work
to be performed by that tissue ; some being fundamentally
concerned with the work in hand, while others are necessary
accessories.

Harking back again to our simple type of a unicellular

^ The blood does not flow into these cells, but flows past close to them.
The manner in which the cells are nourished by the materials carried by the
blood will be shown later.

Introduction. 9

organism, and comparing it with the cells which build up the
body of one of the higher animals, we see that there is
something essentially the same in the two cases. In the case
of the simple organism, one cell carries out all the operations
of existence. It takes in food; it assimilates this food and forms
materials to replenish the waste of the cell ; it rejects what
cannot be assimilated ; it moves about in the fluid in which it
lives, and finally it continues its kind by dividing and producing
new individuals. Similarly, a living cell forming an integral
part of a higher animal carries on to a certain extent an
independent existence. It takes up the food which is prepared
for it and carried to it in solution ; it assimilates this food and
maintains its living condition by its aid, and it is capable of
undergoing cell division and increasing the number of cells
resembling itself. Only the existence of this second kind of
cell is dependent on the life of the animal of which it forms a
part; for on the death of the animal its supply of food stops,
its means of existence are at an end, and in a variable but
always short period of time it inevitably dies.^

In return for the nourishment which each cell receives from
the general stock of circulating food, it performs certain services
which benefit the general community of cells forming the body
of which it is a minute portion.

Thus the most simple type of animal life differs from the
most complex only in this — that in the latter there is minute
division of labour, while in the former there is none. Accom-
panying this division of labour, there is naturally that adaptation
of structure which is necessary to suit the instrument to its
work.

The unicellular organism might be compared to an in-
dividual savage inhabiting an uncivilized region, and a com-
ponent cell of a higher animal to a citizen of a civilized nation ;
in which case the civilized nation might further be taken to
represent the mass of cells which together make up the body.
The savage is able, after a fashion, to supply all his wants for
himself, and carry on a certain low-grade existence without

' The death of the annual as a whole (general death) is hence not
synchronous with that of the tissues (local death).

lo Elementary Physiology.

calling in the aid of others. The civilized man usually does

one type of work only, which is useful to the community at

large, and in return for this mutually receives the service of

his fellows by whom he is virtually fed and clothed and supplied

with comforts in proportion to the value of the work which he

does. In a civilized country it is further found that work can

be best and most expeditiously done when a large number

combine to do it in the same place, and even certain industries

are confined to certain towns and districts to the exclusion of

other places and other industries. Similarly in the body, one

kind of work is done in one part, and a different kind in

another. Again, the centres of industries in a nation must be

kept in communication with one another, and supplies of raw

material and of manufactured articles must be carried from

one to another. Similarly, in the body it is necessary to have

a means of communication and a vehicle of transport between

one part and another, and this is achieved by a stream of

circulating fluid, the blood, which carries suppHes to the

various parts and takes away anything which it is necessary to

remove. Finally, in a civilized community it is essential that

there should be a governing intelligence, and a rapid mode of

intercommunication between this government and the various

parts. Likewise, in the body it is necessary that there should

be a controlling centre or centres which can be rapidly made

aware of any change in the condition of any part. This is

the office of the nervous system^ consisting of a central part,

the brain and spinal cord, which by an immense network of

communicating channels, the nerves., is placed in minute

acquaintance with the state of affairs throughout the body,

and to a certain extent in a position to control any changes.

Between the simplest type of living creature and the most
complex type there are an infinite number of intermediate
stages, so that it is possible to see how the more complex
species are related to the simpler. The same thing is clearly
illustrated in the development of an individual animal of one
of the higher types. Every animal, no matter how compHcated
its final structure, commences life as a single cell of microscopic
dimensions. This primitive cell, or ovum, is formed from the

Introduction. 1 1

body of the female parent, and after being fertilized by being
penetrated by a portion of a cell {spermatozoon) from the body
of a male parent, commences to subdivide, the nucleus first
dividing, as in the case of the simple unicellular organism, and
afterwards the cell protoplasm. The two cells so formed again
subdivide, so giving rise to four cells, and this process is
repeated a great number of times until there is instead of one
cell a spherical mass of many cells. The cells become so
arranged that there are larger cells at one hemisphere than
at the other, and by the growth of one set of cells over the
other there are gradually formed an upper and under layer.
Later there is developed between these two layers a middle
layer, and from these three primitive layers the various organs
take origin — the alimentary canal, digestive glands, and
respiratory system from the inner, the bony skeleton and
muscular system from the middle, and the skin and nervous
system from the outer. As the three layers are formed the
spherical mass elongates, and along one side a groove is
formed by a dipping in of the surface ; this groove deepens
and finally meets and closes at the top so as to form a canal.
Around this canal the spinal cord develops, and it is in this
manner that the centre nervous system comes to be developed
from the external primitive layer.

To give even a complete outline of the manner in which
development goes on to the final production of the perfect
animal would take much more space than is occupied by this
entire volume ; the above is merely intended to indicate roughly
how the operation of development of any animal commences
from a single cell. During the process of development the
growing embryo receives nutriment from the parent, so that it
can grow and increase in bulk. This takes place by means of
a vascular network called the placenta, in which blood-vessels
from the parent ramify alongside blood-vessels from the embryo.
The nutrient material passes from the parent by a process of
diffusion into the blood of the embryo, and the products of
excretion of the embryo pass in the opposite direction. In
this manner the life of the embryonic animal is maintained
until its development is sufficiently advanced for it to carry on

12 Elementary Physiology.

an independent existence, and soon after this stage is reached
it becomes separated from the parent.

Every animal, no matter how comphcated its structure,
then, may be looked upon as an aggregation of physiological
units called cells. These cells, though formed on a com.mon
pattern and produced originally from a single cell, differ from
one another in detail of structure, in shape and in size, and
carry on different functions in the body. The work of all
these cells is designed towards one end, the welfare of the
whole body, and hence the animal though made up of an
immense number of living units may still be regarded as a
single organic whole, and as possessing a life, as a whole, on
which the Hves of the component units depend.

CHAPTER II.

THE SKELETON AND ITS ARTICULATIONS}

The framework on which the body is built is called the
skeleton. The skeleton consists in all of about two hundred
bones, variously joined together so as to afford protection and
support for the soft parts, to give stability to the shape of the
body, and to render possible all those complicated movements
which go on during life. The bones are fitted together at the
joints, or articulations ; those surfaces which come into contact
at the joints being known as the articular surfaces. At some
of the articulations a great freedom of movement is permitted,
in others the amount of movement is but slight, and in others,
again, the opposed surfaces are firmly interlocked, and no
movement is possible.

The bones are living structures provided with a supply of
blood for their nutriment and containing living cells. Between
the organic matter of which the cells are composed, and other
organic matter directly derived from the cells, there is deposited
a large amount of inorganic material, chiefly consisting of the
insoluble phosphate of calcium (tri-calcic phosphate). This
inorganic matter is separated from the blood during growth by
the cells which are engaged in forming the bone. It forms
about two-thirds of the weight of the dried bone, and is that
factor in the materials constituting the bone which gives
hardness and strength to the structure.

^ The student should follow this description with the aid of a set of
dried bones ; or, if these are not at his command, should attempt to gain
access to a museum where he can study the skeleton and see the bones for
himself.

14

Elementary Physiology

A bone is not the same in its structure at all parts ; strength
combined with lightness is obtained by a compact firm structure,
called compact bo?ie, being formed all over the outside, inclosing
a space which, in the case of some bones, such as the ribs and
ends of the long bones of the limbs, is filled with a honey-
combed, lighter structure {cancellous bone), and in the case of
other bones, such as the shafts of the limb bones, is completely
hollow, and filled only with a soft fatty tissue, the mairow of
bone. The purpose is similar to that which causes the engineer

Fig. 2. — Transverse section of compact tissue (of humerus). (Sharpey.)
f Magnified about 150 diameters.)

Three of the Haversian canals are seen, with their concentric rings, or lamellae; also the
lacunae, with the canaliculi extending from them across the direction of the lamellae.

to employ hollow pillars and girders when he designs obtaining
the greatest strength with the least weight of material.

At the ends or surfaces of the bones, where they join or
articiUate together, there is found a layer of tissue known as
cartilage, or gristle. This tissue is closely allied in nature and
mode of origin to bone itself, being indeed the kind of tissue
in which the forms of growing bones are first laid down, and
which is afterwards transformed into bone. In the layer of

The Skeleton and its Articulations.

15

cartilage at the articular surfaces no deposition of calcium
salts takes place, and no after metamorphosis into bone ; this
layer retains throughout life that structure which the whole
bone initially had, and serves an important function in pre-
serving the bones and the whole body from injury by any
sudden jerks or jars. Cartilage is the buffer tissue of the
body, and prevents a sudden knock at any part from being
transmitted through the body. Cartilage is extensible and
compressible in the same sense as a material like indiarubber :

isOc^'

a

J^j c3e> ■siK'.gE? <3®&>

^3 1= .g c-e.

■5 ^ E{» %l

Fig. 3. — Vertical section of articular cartilage covering the lower end of the tibia,
human. (Magnified about 30 diameters.)

a, cells and cell-groups flattened conformably with the surface ; b, cell-groups irregularly
arranged ; c, cell-groups disposed perpendicularly to the surface ; d, layer of calcified
cartilage ; e, bone.

that is to say, it can easily be forced out of shape by pressure
or traction in any direction, and after the deforming force is
removed returns at once to its original shape. Hence it is
used throughout the body to give elasticity to the movements
and to prevent injury by jarring. For this reason there is a
pad of it between each of the separate bones which go to form
the backbone, or vertebral column^ and also on the end of
each of the limb bones. For a similar reason the ribs end in

jg Elementary Physiology.

cartilage so as to give a flexible junction between these and
h breast bone, or sternum, in front, and so drmrn.sh the
fragility of the bony structures which protect the upper cavity
ofihe trunk, known as the chest, or tkora.. The surfaces of
the articular cartilages in those joints where there is consider-
able freedom of motion, such as those of the limbs, are
exceedingly smooth, so that another office of these cartilages is
to prevent friction of bearings at the joints. In this work the

F>0 4 -Sagittal s.cUo„ of the temporo.„,axillary_ articulation of th, right side,
t IG. 4.— sagvi.i.di = (Allen Thomson.) \

. is placed dose .0 the articular eminence and P^o;-^'y^-",SSy°lVo7rLT7aw,
pSstSor portion of the interamcular disc.

smoothness of the cartilaginous surfaces is assisted by a fluid
S fs Lreted at the joint, and is called ^^^^ ^-^■
This synovial fluid is ^^^\^J°^^'ZlTo^^^ ^
Eed"" s;tr.~lous^ =, Ibich completely
rounds the joilt, ^onp^^^jf^ — "a
synovial fluid is secreted. Besides lo™i"g immensely

kind of gear-case for the i^^^X'Tsul^^T^^^
strong and contributes materially to the strengi
^Sre and to keeping the bones in position (.... from getting

TJie Skeleton and its Artienlations. 17

out of joint). In this it is assisted at important joints by
stronger fibrous bands, occurring either as local thickenings of
its wall, or quite distinct from it ; these structures are known as
liganients.

In some important joints, such as the knee joint and that
of the lower jaw with the skull, the junction of the two
articular surfaces is made more perfectly fitting by the inter-
position of a thin disc of cartilage between the two opposed
surfaces, so that there is a kind of double joint, each opposed
surface moving over an opposite surface of the interarticular
disc. The interarticular disc is attached round its margin to
the capsule of the joint, and in this way two cavities are
formed, which may either be completely (jaw) or incompletely
separated (knee).

The foregoing figures show the minute structure of bone
(Fig. 2) and of articular cartilage (Fig. 3) as these are seen
when thin sections of these tissues are prepared and examined
with the microscope. A section through the articulation of the
lower jaw is reproduced as an example of the structure of a
joint (Fig. 4).

The Vertebral Column.

The central structure in the framework of the body is the
backbone, or vertebral column, which lies in the mid line down
the back. It may be looked upon as a central axis of the
skeleton to which the other parts are attached. It supports
the skull upon its upper end, the ribs are attached to it
laterally, and at its lower end it itself is supported between
the hip-bones, through which the weight of the body is trans-
mitted to the legs.

The structure of the vertebral column is shown in the
accompanying figures. It consists of a series of bones, called
vertebrcB, usually twenty-six in number in the adult, which are
articulated together by intervertebral discs of cartilage in such
a way that, although there is very little movement between any
consecutive two, yet the column, as a whole, is flexible, and

c

i8

Elementary Physiology.

admits of a considerable amount of bending, both from before,
backward, and from side to side (see Fig. 7).

^^t^

>^

A A

fi

cov

Fig. 5. — The vertebral
column, viewed from
the left side.

Fig. 6. — The vertebral
column, viewed from
behind.

CI, first cervical vertebra; di, first dorsal vertebra; i.i, first lumbar vertebra ; Si, first
sacral vertebra ; coi, first cocc3'geal vertebra.

The Skeleton and its Articulations.

19

With the exception of the two upper and two lower
members of the series, which will be considered later, the

Fig. 7. — Section through two lumbar vertebrae showing the arrangement of the
intervertebral disc. (R. Quain.) \

I, 2, the fibrous laminae ; 3, the central soft substance ; the capsule of the joint between
the articular processes is also shown.

vertebrae bear a close resemblance, or homology^ to one another,
and hence a description of one typical vertebra will suffice
for all.

The front portion of each vertebra is a short flat cylinder,
or disc, called the body^ a little smaller in its middle than at
the top or bottom. From the posterior part of the body the
neural arch arises by two short stout processes of bone called
the pedicles. These unite at the back in a broad flat plate
called the lamina, which projects backwards as the spinous
process, that part of the vertebra which can be felt through the
skin at the back. A ring of bone is formed in this way
surrounding a cavity which is known as the spinal fora7nen.
It is evident that when the vertebrae are joined together as in
the body, these cavities will form a long canal surrounded and
protected by bone ; this canal is called the spinal canal, and
lodges the spinal cord, a delicate nerve structure which in this
manner is preserved from any chance of injury. The pedicles
are narrower than the body, so that there is a roughly semi-
circular notch at each side above and below each pedicle.
When the vertebrae are in position in the body, it is obvious
that there will thus be at each side between each two vertebrae

20

Elementary Physiology.

a round opening, half of which is contributed by each vertebra ;
these openings are called the intervertebral foramina^ and serve
to transmit the spinal nerves, a pair of which come off the
spinal cord at each vertebra, as well as the blood-vessels,
carrying blood to and from the spinal cord and its
accessories.

From the junction of pedicles and lamina, two processes,
called the transverse processes, arise, as shown in the figure.

SPINOUS PROCESS

RANSVERSE
PROCESS

^SUP AR7IC. PROCESS
PEDICLE

Fig. 8. — Tenth dorsal vertebra, from above. (Drawn by D. Gunn.)

In the case of those vertebrae to which ribs are attached (dorsal
vertebrse), each of these transverse processes bears a small
articular facet, to which a similar facet on the corresponding
rib is applied. On the bodies of these same dorsal vertebrse
above and below on each side there is a half facet, which in
each case unites with its neighbour on the nearest vertebra to
make a whole facet, for articulation with the head of the rib.
So that each rib has two points of attachment, one on the
bodies of the vertebrse, and one on the transverse process of a
vertebra.

The Skeleton mid its Articulaiions,

21

The lamina of the vertebrae imbricate or overlap each
other, so that each vertebra has four other articular surfaces
besides those on the body. This arrangement gives greater
stability to the column, and is a safeguard against dislocation.

The vertebrae are further bound together by strong bands
of ligament passing from one to another, especially between
the spinous processes and between the laminae. The pro-
cesses of the vertebrae serve for the attachment of certain of
the back muscles, which straighten the vertebral column, and
for some of the muscles of the back of the neck, which tilt the
head backwards on the neck.

POSTERIOR ARCH

r 'if ^VERTEBRAL GROOVE

ARTIC. PROC.

TUBERCLE

FOR TRANSV.
LIGAMCNT

ANTERIOR ARCH
ARTIC. SURF. FOR ODONTOID PROC.

Fig. 9. — Atlas, from above. (Drawn by D. Gunn.)
The position of the transverse ligament is indicated by dotted lines.

The seven vertebrae at the upper end of the column are
known as the cervical vertebrae ; these form the bones of the
neck. They are much more slenderly built and lighter than
the lower members of the column, and are capable of moving
on one another to a much greater extent, so as to allow move-
ments of the neck. The two upper members of the cervical
series are much modified in shape, in order to permit movements
of the head on the vertebral column.

The first vertebra which articulates with the occipital bone
of the skull is called the atlas^ and the second vertebra, called
the axis^ articulates above with the atlas, and below with the
third cervical vertebra.

The shape of these tv\^o bones, and the manner in which

22

Elementary Physiology.

they are articulated together, is shown in the accompanying

drawings

The atlas is a ring of bone, the body having disappeared,
and where the body would be if present the odo?itoid process

ODONTOID PROCESS

NF, ARTIC.PROC.

Fig. io. — Axis, from the right side. (Drawn by D. Gunn.)

of the axis projects through, round which, as on a pivot, the
atlas carrying the head can turn. The strong transverse ligmnent
passing behind the odontoid process completes the socket.
On the upper surface of the atlas are two articular facets

ODONTOID PSOCESS

IMF ARTIC PROCESS

SUP ARTIC PROCESr

rRANSV. PHOC.

ITRANSV. PROO.
INF. ARTIC. PROCESS ;

Fig. II. — Atlas and axis, from before. (Drawn by D. Gunn.)

of the form shown in the drawing (Fig. 9). These are
slightly concave on their surface, and receive two similarly
shaped but convex-surfaced facets placed on the occipital bone
of the skull (see Fig. 13), and called the occipital condyles. It

TJie Skeleton and its Articulations. 23

is by a rocking movement of the condyles on tiiese articular
facets of the atlas that nodding of the head up and down is
effected.

The varied movements of the head on the neck are mix-
tures of these two movements simultaneously — namely, of an
up-and-down movement brought about by the rocking of the
occipital condyles on the atlas, and of a rotation movement of
the atlas on the axis around the odontoid process. Changes
in position of the head are further assisted by movements of
the cervical vertebras, i.e. by bending of the neck. If the head
be turned round sharply to one side, and the neck be felt on the
other side, a hard mass will be felt passing from behind the ear
down to the breast-bone and collar-bone on the front of the chest.
This is the mass of the sterno-mastoid muscle. When the
muscle of one side is contracted, and not the opposite one, the
head is rotated towards the side on which the muscle lies, and de-
pressed on that side. When both sterno-mastoids are contracted
the head is depressed in front. The head is elevated by the
muscles of the back of the neck acting in opposition to the
muscle (sterno-mastoid) just referred to, which has been cited
because it lies superficially and can easily be felt. Besides
these there are other muscles which combine in producing the
movements of the head. When a movement takes place, nerve
impulses are sent to these various muscles along the nerves
belonging to them, and a nicely adjusted amount of stimulus
is given to each muscle, so as to cause just the proper amount
of contraction in each in order to produce the desired degree
of motion when combined with the action of the other muscles
involved.

Beneath the seven cervical vertebrae come the twelve dorsal
or ihoradc vertebrae. These are more strongly built than the
cervical, as is fitting, since they have to support a much greater
load. The cervical vertebrae carry only the weight of the head j
but the dorsal have the ribs attached along their sides, and
have communicated to them by these the weight of the upper
part of the trunk {i.e. the thorax) and of the upper limbs. The
dorsal vertebrae may be distinguished by the facets for the ribs
on their bodies and transverse processes.

24 Elermntary Physiology,

The next five vertebrse are the hmibar (or loin) vertebrae.
These are very massive in their structure, because they have to
support the whole weight of the part of the body above them,
and transmit it to the hip-bones. They have no ribs attached
to them, and the soft parts (abdominal viscera) in the part of
the trunk opposite to them are only protected by the strong
sheets of muscle passing from the upper brim of the pelvis to
the lower ribs.

The lowest of the lumbar vertebrae is seated on a strong
bone called the sacrum^ shaped like a curved wedge (see Figs.
5 and 6), and formed by the fusion together of a number of
imperfect vertebrse. The vertebrse which make up the sacrum
are usually five in number, and early in life exist as distinct
bones, but later become completely fused together into one
bone. By two large articular surfaces {atiricular surfaces), one
on each side, the sacrum articulates with the two hip-bones, and
lying between these forms a portion of the pelvis (see Fig. 19).
Strong bands and sheets of ligament pass from the sacrum to
the hip-bones, and a great part of the weight is borne by these
ligaments ; so that the vertebral column and its load are in part
borne by the articulation with the hip-bones, and in part are
borne hammock-fashion by the ligaments passing from hip-
bones to sacrum. The coccyx is attached to the lower end of
the sacrum ; it consists of from three to five (usually four)
rudimentary vertebrae, which are commonly fused together into
one bone (see Figs. 5 and 6).

It will be seen from the above description that the vertebral
column is a strong and somewhat flexible pillar of bones, which
upholds the weight of the part of the body lying above the
hips, and affords an attachment to the ribs ; besides this, it
furnishes a long cavity in which the spinal cord lies. At the
upper end this spinal cord enlarges into the brain, which is
lodged in the large cavity of the cranium, occupying the
greater part of the volume of the skull.

The Skull.

The skull may, for purposes of description, be considered
in two parts — viz. the cranium, or brain-case, and the face.

The Skeleton and its Articulations.

25

The cranium occupies the upper and back part of the skull,
and is formed by eight bones; the face forms the front and
lower part, and is made up of fourteen bones, making twenty-
two bones in the skull in all. The position and names of the
various skull-bones are shown in the accompanying figures.

Fig. 12.— Lateral view of the skull. (Allen Thomson.) \

frontal bone ; 2, parietal bone at the upper temporal line ; X X, coronal suture ; 3, on
the occipital bone ; 3', external occipital protuberance ; 4, sphenoid bone ; 5, squamous
part of temporal ; 6, the same at the root of the zygoma, immediately over the
external auditory meatus ; 7, mastoid portion of temporal, at the front of which is
the mastoid process ; 8, left condyle of occipital bone ; 9, anterior nasal aperture ;
ID, on the lachrymal bone in the inner wall of the orbit ; 11, malar bone, near its
junction with the zygoma ; 12, superior maxilliarybone ; 13, ramus of the lower jaw ;
14, body of the lower jaw, near the mental foramen.

The bones of the skull, with the exception of the lower jaw,
are immovably united together where they come into contact
by the form of articulation known as a sutwe. These sutures

26

Elementary Physiology.

are often dentated or serrated in outline (see Fig. 12); the
serrations strengthen the junction between the two bones.
When it is desired to study the shape of the skull-bones

Fig. 13.— External base of the skull. (Allen Thomson.) \
I palate plate of the superior maxillary bone ; 2, palate plate of the palate bone ;
7 vomer bone ; 12, jugular foramen ; 13, articular emmence of the temporal bone ;
14, external auditory meatus ; 15, glenoid fossa ; 18, basilar process of the occipital
bone ; 19, condyle of the occipital bone ; 20, is placed in the foramen magnum ,
23, external occipital crest running down from the protuberance ; 24, superior curved
line of the occipital bone ; 25, 26, inferior curved line.

separately, the skull must be disarticulated or separated into its
constituent bones; but it is impossible to describe here in
detail all these various bones/

' For such a description, see Quain's "Anatomy," vol. ii. pt. i.

The Skeleton and its Articulations.

27

The cranial bones, by their union, form a hollow case of
bone for the protection of the brain ; the cavity so formed is
rounded or spheroidal above but is flatter at its base. The base
is further divided by ridges of bone into three hollows, or fossae,

21-/-^

Fig. 14.— Section of the adult skull a little to the left of the median plane.
(Allen Thomson.) i.

I, nasal bone ; 2, perpendicular plate of the ethaioid bone with olfactory foramina and
grooves at its upper part ; 3, vomer ; 6, inner surface of the frontal bone ; 7, of the
parietal bone ; 8, squamous part of the temporal bone ; 9, on the occipital bone below
the internal occipital protuberance ; 10, external occipital protuberance ; 17, anterior
nasal spine ; 19, on the inner surface of the ramus of the lower jaw, below the
sigmoid notch, and above t'le inferior dental foramen.

which correspond to divisions of the brain ; the posterior fossa
lodges the cerebellum., or lesser brain ; the middle and anterior
fossae conform to the shape of the base of the greater brain, or
cerebnim. Besides smaller holes, or foramina, for the passage
of nerves and blood-vessels to and from the brain, the base of

28 Elementary Physiology.

the cranium contains a larger opening, the foramen magnum
(see Fig. 13), through which the portion of the central nervous
system called the medulla oblongata passes, uniting brain and
cord. The bones of the face form the greater parts of the
orbits for the accommodation of the eyes, the nose, the hard
palate, the upper and lower jaws. The outline of the nose is
completed by the nasal cartilages. The nasal bones and the
cartilages completing the nose may be felt through the skin
during life ; the flexible front portion is cartilage, the fixed part
behind is formed by the nasal bones. The hard palate, or
roof of the mouth, is formed by the superior maxillary bones,
which also bear the teeth of the upper jaw. It is continued
back, in life, by the fleshy velum., or soft palate., behind which
the posterior 7iares, or inner openings of the nostrils, com-
municate with the pharynx.^ The prominences of the cheek
are formed by the malar bones.

The lower jaw {inferior maxil-a) is the only bone in the
skull which has a movable articulation. A section through
this joint has already been given as an example. We may
now consider somewhat more fully the movements which take
place at this joint. The lower jaw is capable of movement
both upwards and downwards, backwards and forwards, and
from side to side. These movements are necessary to give a
grinding action between the teeth.

The movements of the jaw are brought about by several
muscles ; the strongest set are those which raise it and bring it
into contact with the upper jaw, against the resistance of
anything which may be interposed. The jaw is depressed by its
own weight, but may be forcibly lowered by the action of
special muscles, the chief of which are the digastrics, attached
beneath the jaw in front and running backward parallel to the
line of the jaw on each side (see 12, Fig. 15).

When the jaw opens the condyle and interarticular cartilage
are pulled forward, as you may find by applying your forefinger
to the articulation just in front of the external opening of the

' The pharynx is the upper funnel-shaped part of the alimentary canal
which leads to the gullet, or oesophagus ; into it the mouth and the windpipe,
or trachea, also open.

Fig. 15. — Deep muscles of the left side of the head and neck. (^Allen Thomson,
after Bourgery.) ^]

vertex of head ; /', superior curved line of occipital bone ; c, ramus of lower jaw ; c', its
coronoid process ; d, hyoid bone ; e, sternal end of clavicle ; e', acromial end ; _/j
malar bone divided to show the insertion of the temporal muscle ; y', divided
zygoma, and external ligament of the jaw ; £; thjToid cartilage ; /i, placed on the
lobule of the auricle, points to the styloid process ; i, temporal muscle ; 2, corni-
gator supercilii ; 3, pyramidaiis nasi ; 4, compressor naris ; 5, levator labii superioris ;
6, levator anguli oris ; 7, outer part of the orbicularis oris, the part below the nose
has been removed ; 8, depressor alae nasi ; 9, points to the buccinator muscle,
through which the parotid duct is seen passing ; 10, depressor labii inferioris ;
II, levator menti ; 12, 12, anterior and posterior bellies of the digastric; 13, stylo-
hyoid muscle ; 14, mylo-hyoid ; 15, hyoglossus, between which and 13 is seen a
part of the stylo-giossus ; 16, sterno-hyoid ; 17, on the cla\dcle, indicates the posterior,
and 17', the anterior belly of the omo-hyoid ; 18, sterno-thyroid ; 19, thyro-hyoid ;
20, 21, on the sterno-mastoid muscle, point, the first to the middle, the second to
the lower constrictor of the pharyn.x ; 22, trapezius ; 23, upper part of the complexus ;
24, 25, splenius; 26, levator angulae scapulae; 27, middle scalenus; +, posterior
scalenus ; 28, anterior scalenus.

30 Elementary Physiology.

ear; the same thing happens when the lower jaw is forcibly
moved forward. If one forefinger be placed in this position
on either side, and the jaw be then moved from side to side
laterally, it will be found that one condyle moves forward while
the other remains in the groove. These movements are
brought about by the action of the external pterygoid muscles,
shown ill the drawing (Fig. i6). When these muscles contract
together On the two sides, the jaw is drawn forward ; when they
contract alternately, the jaw is moved from side to side.

Fig. i6. — The pterygoid muscles from outside. (G. D. T.) \

The masseter muscle, the greater portion of the zygomatic arch, the temporal muscle
with the coronoid process, and a large part of the ramus of the jaw have been
removed, i, external pterygoid ; the figure is placed on the lower head ; 2, internal
pterygoid.

The strong set of muscles which raise the lower jaw com-
prise, on each side, the internal pterygoids (2, Fig. 16) on the
inner side of the jaw, the masseters outside, and the temporals
above (see i, Fig. 16). The action of the temporal and
masseter muscles may be felt through the skin. The temporal,
above and in front of the ear (on the temple), may be felt to
contract by its swelling up as the jaw is moved upwards from
an open position; the masseter may be similarly felt at the
angle of the jaw, if the teeth are ground together after the jaw
is closed.

Each jaw is armed with a number of teeth, for the purpose

The Skeleton and its Ai'ticulations.

31

of grinding or masticating the food. The full number of teeth
in the adult is sixteen in each jaw. The teeth are symmetrical
in character in each jaw, and in each side of each jaw. Of
the eight in each half of each jaw, the two nearest the front
(front teeth) are called incisors^ or cutting teeth ; the next is the
canine tooth, which is longer than the others, and is used for
piercing hard food; the remaining five are grinding teeth, and are
more or less flat-topped, but provided with cusps or eminences,
so as to give an uneven surface serviceable for grinding. The
two nearer the front of these five are transitional in character,
and are called bictispids ; the other three are the molar, or " true
molar " teeth. During mastication the food is kept under the
teeth by the action of the
muscles of the cheeks and
tongue, which return it from
either side as it becomes
displaced by the previous
action of the jaw, so replac-
ing it between the teeth.

The Thorax.

The bony framework of
the chest, or thorax, may
be regarded as a kind of
open basketwork of bone,
which gives stability and
elasticity to the outline of
this upper compartment of
the trunk, and at the same i,
time, when acted upon by
appropriate muscles, allows
the volume of the chest
cavity to be rhythmically
altered in the act of breath-
ing, and so occasions air to
which occupy the greater part

The bony thorax, as show

Fig. 17. — Front view of the thorax.

manubrium ; 2, is close to the place of union
of the first costal cartilage ; 3, clavicular
notch ; 4, body of the sternum ; 5, ensiform
process ; 6, groove on the lower border of
the ribs ; 7, the vertebral end of the ribs ;
8, neck ; g, tuberosity ; 10, costal carti-
lage ; 12, first rib ; 13, its tuberosity ; 14,
first dorsal vertebra; 15, eleventh rib; 16,
twelfth rib.

pass into and out of the lungs,

of the space inside.

n in the woodcut, is formed by

32 Elementary Physiology.

the twelve thoracic vertebrae, and by the twelve ribs on each
side (twenty-four in all), and is completed in front by the rib
cartilages, which are attached to the sternum, or breast-bone,
lying in the mid line in front.

The thorax is shaped like a rounded and truncated cone,
and is much longer behind than in front. The ribs at first slope
backwards from their attachments to the vertebrse, and thus
give rise to the hollow in the middle line of the back ; they
then curve round forward in the rest of their length, so com-
pleting the posterior and lateral wall of the thorax. In this
latter part of their course the ribs slope downwards. In front
the ribs are articulated with the costal cartilages, which unite
them to the sternum, in the manner shown in the woodcut.

This bony framework is in the body converted into a closed
box by soft tissues. Two sheets of muscles called the intercostals
(see Fig. i8) lie between the successive ribs, their fibres passing
obliquely from the one rib to the other. The floor of the
cavity of the thorax is formed by a large sheet of muscle and
tendon, called the diaphragm^ which is attached all round the
lower border of the thorax, and has a strong central tendon.
The diaphragm completely shuts off the upper cavity of the
trunk {i.e. the thorax) from the lower cavity or abdomen. It
is pierced at its centre and posterior part by the tubes and
vessels which must pass from one cavity to another. These
are the great blood-vessels carrying the blood to and from the
lower part of the body, and the oesophagtis, or tube which
conveys the food to the stomach.

It must be remembered that the ribs are free to move to a
certain extent up and down, around their attachments to the
vertebrae as an axis. Since the ribs slope downward, anything
which raises them must make them stand out more at right
andes to the vertebral column, and hence must increase the
capacity of the chest by increasing its girth. The increase
takes place both from before backward and laterally. The
raising of the ribs takes place through the contraction of those
layers of intercostal muscles which are known as exte7'nal^
because they lie nearer the skin than the other layer, which are
called the mteimal intercostals. The external intercostals are

The Skeleton and its Artiailations.

33

shown in the accompanying figure in the upper intercostal
space. In the lower space, the external intercostals have been
represented as removed, so as to show the internal intercostals.
A glance at the direction of the muscle fibres of the external
intercostals in the figure will show that when these fibres
shorten, the ribs to which they are attached must together

Fig. rS.— Intercostal muscles of the fifth and sixth spaces. (Allen Thomson, after

Cloquet.) \

A, from the side ; B, from behind.
IV., fourth dorsal vertebra ; V, V, fifth rib and cartilage ; i, i, levatores^ costarum
muscles, short and long ; 2, 2, external intercostal muscle ; 3, 3, internal intercostal
layer, shown in the lower space by the removal of the external laj'er, and seen in A
in the upper space, in front of the external layer ; the deficiency of the internal layer
towards the vertebral column is shown in B.

move upwards.^ The action of the internal intercostals, the
fibres of which slant in the opposite direction in forced expira-
tion, is to lower the ribs. In ordinary breathing the weight of
the chest is, however, sufficient. The internal intercostals are

^ The first rib is held fixed by the muscles above it ; and hence when
the external intercostals contract movement upwards must take place.

D

34 Elementary Physiology.

continued between the rib cartilages in front, and as these slope
tLpwards this portion of the internal intercostals will tend to
raise the cartilages and so aid in inspiration.

The action of the ribs is not the only means by which the
capacity of the thorax is altered ; it has in the diaphragm a
movable muscular floor. The diaphragm is not a sheet lying all
in one plane (see Fig. 52) ; it is somewhat dome-shaped, the top
of the dome being turned towards the thorax, and hence, when
the muscular sheet which forms its peripheral part contracts, the
dome is drawn down towards the abdomen, thus increasing
the capacity of the thorax from top to bottom. Thus, if at the
same time the ribs are raised and the diaphragm contracted,
the dimensions of the thorax are increased in all directions.

Breathing by the action of the ribs is spoken of as costal
respiration, and breathing by the action of the diaphragm as
diaphragmatic or abdo7ninal respiration.

The increase and diminution in the volume of the thorax
causes a quantity of air alternately to enter and leave the lungs,
which fill the greater part of the thoracic cavity ; for the
increase in volume causes a suction on the contents of the
thoracic cavity. Now, the lungs are the only organs within
the thorax which can expand to the necessary extent and
prevent the formation of a vacuous space inside. The lungs
can expand in this manner because they consist essentially of
air-chambers with elastic walls, to which the ultimate sub-
divisions of the windpipe, or trachea, lead. When the thorax
increases in volume, the atmospheric pressure blows the air
down the trachea and distends these air-chambers so as to fill
the increased space, thus increasing the volume of the lungs
and bringing a supply of air to them. On the contrary, when
the ribs fall and the diaphragm rises, the capacity of the chest
diminishes, air is forced out, and the air-spaces in the lungs
become less distended. The purpose of this respiration or
alternate sucking of air in {inspiration) and blowing it out
{expiration) we shall learn subsequently.

The Skeleton and its Artiadations.

r:>

The Pelvis.

The bony pelvis (see Fig. 19) is a strong ring of bone
formed by the union of the two hip-bones with the sacrum j
this receives the weight of the upper part of the body and
transmits it to the thigh-bones. The thigh-bones articulate
with the hip-bones in two deep cups, one on each bone, called
the acetabula (see Figs. 19 and 20).

Fig. 19.

-Adult male pelvis seen from before, in the erect attitude of the body.
(Allen Thomson.) \

I, 2, anterior c.Ktremities of the iliac crests ; 3, 4, acetabula ; 5, 5, thyroid foramina ;
6, subpubic angle or arch.

When the ligaments and muscles attached to the pelvis are
present, it forms a basin-shaped cavity, the floor of which
supports to a certain extent the contents of the abdominal
cavity {abdominal viscera).

Each hip-bone {os innominaticni) is originally formed from
three distinct bones, which persist in youth but become fused
together in the adult (eighteenth to twentieth year). The three
bones are named iliiun, ischium, and p^dns respectively ; the
ilium is the upper and larger part of the bone which articulates
with the sacrum and extends down to form part of the
acetabulum ; the pubis lies in front, and unites with its fellow
to form an arch {tJie pubic arch) ; the ischium is the lower

36 Elementary Physiology.

portion of the bone, that on which one rests in the sitting
posture. These names for the different parts of the bone are

Fig. 20.— Transverse oblique section of the pelvis and hip-joint, cutting the first sacral
vertebra and the symphysis pubis in their middle, from a male subject of about nine-
teen years of age. (Allen Thomson.) \

I, first sacral vertebra ; 2, ilium ; 3, posterior sacro-iliac ligament ; 6, small sacrosciatic
ligament ; 7, great sacro-sciatic ligament ; 8, placed in front of the symphysis pubis,
in the cut surface of which the small median cavity, the adjacent cartilaginous
plates, and the anterior and posterior ligamentous fibres are shown ; 10, cartilaginous
surface of the cotyloid cavity, through the middle of which the incision passes trans-
versely, dividing the interarticular ligament and the fat in the fossa acetabuli ;
II, cotyloid ligament ; 12, interarticular ligament connected with the transverse part
of the cotyloid ligament ; 13, placed on the cut surface of the head of the left femur
near the depression where the interarticular ligament is attached ; 14, 14 , upper and
lower parts of the capsular ligament.

still used even after union into one bone has taken place, in
order to describe the different regions of the bone. The upper
and posterior brim of the pelvis is known as the iliac crest ; it

The Skeleton and its Arliailations. 37

forms in the body the eminence of the haunches. To the
back of the ilium are attached the strong hip muscles {glutei
muscles) which straighten the leg on the body at the hip joint ;
they are used, for example, in straightening the body at the
hips when rising from a stooping posture, as also in walking
and in running. The muscles which flex the leg on the trunk
at the hip joint lie in front.

The sacrum and hip-bones are immovably articulated
together. Where the two hip-bones meet in front a pad of
cartilage is interposed between them, which serves the same
purpose as the intervertebral discs in the case of the vertebrae.
This junction of the two bones is called the pubic symphysis.

The articulation of the thigh-bone with the hip-bone is one
of the best examples in the body of what is known as a ball-
and-socket joint. A similar kind of joint is found in the
shoulder, where the humerus (the bone of the upper arm) is
joined to the scapula (or shoulder-blade) ; but here the socket
is not so deep, and the degree of movement is hence consider-
ably greater. Nevertheless the amount of free movement in
every possible direction at the hip joint is very large.

The nature of the ball-and-socket joint is well seen in the
accompanying drawing (Fig. 20), showing a section through the
hip joint. The round ball-shaped head of the femur (thigh-
bone) is covered with smooth articular cartilage ; as is also the
opposed cavity of the acetabulum, except at its bottom where
it does not touch. The joint is strengthened, in this case, by
a strong ligament uniting the middle of the head of the femur
to the bottom of the socket \ this is called the inferartic^Uar
ligament. There is as usual a capsule surrounding the joint
into which the lubricating synovial fluid is secreted. The
joint is further strengthened by strong ligamentous bands
which pass over the capsule from, hip-bone to thigh-bone.

The Limb Bones.

The bones of the upper and lower limb exhibit a marked
amount of similarity, or homology^ in their arrangement. The
shoulder joint corresponds to the hip joint; the humerus, or

38 Elementary Physiology,

arm-bone, articulating with the shoulder-blade, which although
so different in shape corresponds to the hip-bone. In the
upper arm, as in the thigh, there is but one bone ; while in the
forearm there are two bones, the ulna and radius, correspond-
ing respectively to the tibia and fibula in the leg. Next there
are a number of small bones of the wrist (carpal bones),
corresponding to a number of small bones (tarsal bones) in
the ankle. More distal to these there are a row of bones, five
in each case, called n^etacarpals in the hand and 7netatarsals in
the foot. Next are three rows of bones, in each case, form-
ing the fingers and toes respectively ; these bones are called
phalanges (sing, phalanx) both in the hand and foot. In the
case of the thumb and great toe there are but two phalanges ;
the other digits possess three each.

The Upper Limb.

The upper limb is attached to the body by the shoulder-
girdle, which consists of two bones — the clavicle^ or collar-bone,

and the scapula^ or
^^!^^^^W^ shoulder-blade. The

OE.LTOID yW^^^^^^^^S^m 1-1 1

J"B x<^^^ttiPW"'|^W^a clavicle, as shown m

ACROMIAL TRAPEZOID
FACET

RIAL Pci^R^SIt.^'"'^ / the drawmg, is a bone

STERNAL •.1 111

FACET With a double curve

iROOVE OF SUBCLAVIUS

somewhat like an italic
^ . , . f. It may be felt be-

FiG. 21.— Right clavicle, from below. (Drawn by i -i i •

T. w. P. Lawrence.) ncath the skin at the

junction of neck and

thorax in front. It is articulated to the breast-bone in front,

and passes outward and backward to articulate at its other

outer end with a process of the scapula called the acroinion

process (see Figs. 22 and 23).

The scapula is a triangular-shaped plate of bone which is
placed at the back of the shoulder, the shortest side of the
triangle being directed upwards. Its anterior surface is some-
what concave, following the shape of the underlying parts of
the body, and its posterior surface is correspondingly convex.
Across its dorsal surface there runs a ridge of bone called the

The Skeleton and its Artiadations.

39

spine of the scapula, which becomes more prominent as it
passes outwards and ends in the acromion process above
referred to, where the end of the clavicle is attached.

Beneath the acromion process is the head of the scapula,
which replaces the outer angle of the triangle by a rounded
or elliptical - shaped arti -

ACROMION

cular surface (the glenoid
fossa) covered with cartilage,
against which the head of
the humerus^ or upper arm-
bone, moves. The head of
the humerus has a spherical
surface, also coated with
cartilage, and the joint is
enclosed by a capsule, so as
to make a shallow ball-and-
socket joint. The scapula
and its processes, together
with the clavicle, give at-
tachment to the muscles
which move the arm at the
shoulder-joint.

The attachments of the
sternum, shoulder-girdle, and humerus are represented in
Fig. 23 ; it may be pointed out that there is a disc of cartilage
at the articulation between the clavicle and sternum.

The humeims^ or bone of the upper arm (see Fig. 24),
extends from the shoulder to the elbow ; it articulates at the
shoulder with the scapula, and at the elbow with the ulna and
radi^Ls^ the bones of the forearm. The upper extremity of the
humerus includes the head, a hemispherical articular surface
which, as above described, forms part of the shoulder-joint;
the neck, a narrow groove marking off the head from the rest
of the bone; and the great and small tuberosities, which are
eminences giving attachment to certain of the shoulder muscles.
The shaft or body of the bone bears rough patches on its surface
at places where muscles were attached. It is cylindrical in shape
above, but triangular below. At its lower extremity the bone

Fig. 22. — Dorsal view of right scapula.
(Drawn by T. W. P. Lawrence. )

40

Elementary Physiology,

becomes broader from side to side, and has two articular
surfaces called the trochlea and capitelhim. These articular
surfaces are both convex from before backward ; but the
trochlea, which articulates with the ulna, is concave from side
to side ; while the capitellum, which articulates with the radius,

Fig.

23-

-View from before of the articulations of the shoulder-bones.
Thomson.") \

(Allen

I, acromio-clavicular articulation ; 2, conoid, and 3, trapezoid part of the coraco-clavicular
ligament ; 4, near the suprascapular ligament ; 5, on the coracoid process, points to
the coraco-acromial ligament ; 6, capsular ligament of the shoulder-joint ; 7, coraco-
humeral ligament ; above 6, an aperture in the capsular ligament through which the
synovial membrane is prolonged under the tendon of the subscapularis muscle ;
8, tendon of the long head of the biceps muscle ; 9, right half of the interclavicular
ligament ; 10, interarticular fibro-cartilage of the sterno-clavicular articulation ;
II, costo-clavicular ligament ; 12 and 13, cartilage and small part of the second and
third ribs attached by their anterior chondro-sternal ligaments.

is convex in this direction also. The upper ends of the ulna
and radius bear articular surfaces to match these surfaces on
the humerus (see Fig. 25). The surface on the ulna, which
articulates with the humerus, is convex from side to side, to
suit the concavity in this direction of the trochlea of the

The Skeleton and its Articulations.

41

humerus, and correspondingly concave from above downwards.
The surface by which the head of the radius articulates with

Fig. 24. — RiffVit humerus, from before.
CDrawn by T. W. P. Lawrence.)

Fig. 25. — Anterior view of the right radius
and ulna in supination of the hand.
(Drawn by T. W, P. Lawrence.)

42

Eleinentaiy Physiology.

the himierus is circular and slightly hollowed out, so as to suit

the convexity of the capitellum.

Of the two bones of the forearm (see Fig. 25), the ulna is

large at the elbow and tapers towards the wrist; while the

radius is smaller at the elbow and becomes more massive at

the v/rist.

The radius and ulna articulate with each other both at

elbow and wrist. The articulations are such as to admit of a
rotation of the radius to a certain extent
around the ulna. The head of the radius
at the elbow is held in position by the
orbicular ligament (see Fig. 25). Around
the head is a smooth articular surface which
moves during rotation against a concave
facet on the ulna. At the wrist there is
a convex articular facet on the ulna, and
a concave facet on the radius. The
movement of the radius round the ulna
brings about pronation and supination of
the hand. In supination the bones are
uncrossed; in this position, with the fore-
arm held horizontal, the palm of the hand
is uppermost. In pronation the lower
end of the radius is crossed inwards over
^ . , . the ulna, and the back of the hand is

Fig. 26. — Sagittal section

of the elbow joint uppcrmost. lu the movcment from supi-

through the great sig- . . . ,

moid cavity of the nation to pronation, rotation takes place at

ulna and the trochlear . ... , ,• i j- i i

.surface of the hume- thc radio-umar articuiations, and these are

son.) \ ^^ °"^" hence termed pivot joints. The movement

I, cu.t surface of the hume- between the humcius and ulna is purely

rus ; 2, that of the . in* i i • •

ulna; 3, posterior part, OnC 01 CXtenSlOll and lleXlOn, and this IS

the synovLTcaSfty of brought about by the hinge joint formed
?gaL1nV; V'tendon by thcsc two boncs. The elbow joint is
°^i?at^the^ioweTenci hcncc a compouud joint, including both a
of the oblique liga- hinge joint (humcrus and ulna) and a pivot

joint (radius and ulna).
The movements at the wrist are allowed to take place by
a very complicated series of articulations (see Fig. 27), partially

The Skeleton and its Articulations.

43

between the radius and three of the carpal bones, and partially
between the carpal bones themselves. The ulna takes no
part in the wrist joint, being
shut off, as shown in the figure
(Fig. 27), by a strong liga-
mentous band. The articular
surface of the radius which
touches the carpal bones is
concave both from before back-
ward and from side to side,
and those carpal bones which
articulate with it together fur-
nish a correspondingly convex
surface.

The carpal bones, as shown
in the figure (Fig. 27), are ar-
ranged in two rows ; synovial jj ■ l^
cavities are present between ^'^'^

them, and there is a consider- f^^_ 37.-Section of the inferior radio-
able amount of didinsr motion "^^^^' radio-carpal, intercarpal and
^ ^ carpo-metacarpal articulations. (Allen

possible between the first and Thomson.) ^

QprnnH rowc hnt nnlv n Qlicrhl- i.- triangular fibro-cartilage ; 2, placed on
SeCOna rO^^S, out Oni} a Sngnt the ulna, points to the synovial cavity

amount between bones of the
same row.

Distally^ the carpal bones
01 the second row articulate
with the metacarpal bones,
which are the bones underlying
the palm of the hand. The
amount of movement between
the metacarpal bones of the
four fingers and the carpal
bones is not large, although a
small amount of movement
inward of the fourth and fifth -

of the inferior radio-ulnar articulation ;
3j external lateral, and 4, internal
lateral ligament, and between them
the sjmovial cavity of the wTist ; 5,
scaphoid bone ; 6, lunar ; 7, pyra-
midal ; 8, 8, upper portion, and 8', 8',
lower portion of the general sj-novial
cavity of the intercarpal and carpo-
metacarpal articulations ; between 5
and 6, and 6 and 7, the interosseous
ligaments are seen separating the
carpal articular cavity from the wrist-
joint ; between the four carpal bones
of the lower row, and between the
magnum and scaphoid, the inter-
osseous ligaments are also shown ; the
upper division of the s>-novial cavity
communicates with the lower between
10 and II, and between 11 and 12 ; x,
marks one of the three interosseous
metacarpal ligaments ; 9', separate
synovial cavity of the first carpo-
metacarpal articulation ; 13, first, and
14, fifth metacarpal bone.

' The words " proximal " and "' distal" are used to denote parts re-
spectively nearer and farther from the central part of the body.

- The metacarpal and metatarsal bones are numbered from the thumb

44

Elementary Physiology.

metacarpals takes place in closing the hand. The first meta-
carpal bone (that of the thumb) possesses a separate synovial

cavity (see 9', Fig. 27) between
its proximal end and the bone
of the carpus with which it
articulates (the trapezium). This
joint is what is known as a
saddle joints the two surfaces
concerned being respectively
convex and concave in directions
at right angles to one another,
and fitting against each other
saddle fashion.^ The four meta-
carpal bones of the fingers are
united at their distal extremities
by a strong band of ligament
{tf'ansverse metacai-pal ligament)
which prevents any great move-
ment of these bones with respect
to one another, and strengthens
the hand. This ligament is
shown in Fig. 28, which also
shows a front view of the liga-
ments of the wrist joint.

The joints between the meta-
carpal bones of the four fingers
^' 'TlJTltt^oTi:!';:^.^^. and the first row of phalanges
the lower end of the radius, the Imetacarpo-phalaii^eal joints) are

anterior radio-carpal ligament ; 3. ^ -^ -^ o ^ /

scaphoid bone ; ' 4, pisiform ; 5, tra- tcmied C07ldyloid j ointS. In thcSC

pezium ; 6, unciform ; 7, os magnum, _ . . ^ r 1

with most of the deeper ligaments jointS the dlStal Cnds of the
uniting these bones; I, first meta-

carpo-phalangeal articulation with metacarpal boneS arC COllVCX

its external lateral ligament; II to , ,1 ^ ^ r 1 i j

V, transverse metacarpal ligament ; both frOm betorC backwardS,

luiSL^rXU^rgers^ tCtt^eS and from side to side ; while the
l&Te?naT:r5ytvisiSe'^ '"'"' proximal cnds of the first row

and great toe respectively, so are the phalanges (finger and toe bones) of
the hand and foot.

^ A saddle joint is so called because of its resemblance to that made by
a horseman with his saddle ; it may be represented by holding the thumb

Fig. 28. — General view of the articula-
tions of the wrist and hand from
before. \

The Skeleton and its Articulations. 45

of phalanges are correspondingly concave. There is thus
allowed a certain amount of movement in any direction, so
that each finger is capable of a certain amount of angular
rotation (verify this), as if the joint were a ball and socket.^
The interphalangeal joints in the fingers are simple hinge
joints which allow^ only bending (flexion) and straightening
(extension).

In the thumb the metacarpo-phalangeal joint is a hinge,
and not a condyloid, joint, the function of the latter being
taken on by the saddle joint described above, between the
first metacarpal and trapezium bones. The joint between the
phalanges of the thumb is a hinge joint.

The Lower Limb.

The fe??mr, or thigh-bone, is the longest bone in the
skeleton, and extends from the hip to the knee. Since in
the normal standing position, with the feet close together,
the knees are also close, while the acetabula into which
the heads of the femora are inserted are at opposite sides
of the pelvis, it follows that each femur in this position
inclines somewhat inward as it descends. At the same time it
also inclines slightly backward. The amount of this inclination
inwards is greater in w^oman than in man. The femur (see
Fig. 31), for descriptive purposes, is divided into superior ex-
tremity, shaft, and inferior extremity. The superior extremity
iijcludes the head, neck, and trochanters. The head is spherical
in shape, forming more than half a sphere, is covered with
articular cartilage, and forms the ball of the " ball and socket "
of the hip joint. The neck is a stout rounded mass of bone
connecting the head with the shaft and meeting the shaft at an
angle of 125°. The great 2in6. small trochanters (see Fig. 31)

as far as possible from the fingers of the hand and placing the two forks so
formed in contact at right angles.

^ There is this difference, however, between a ball-and-socket and a
condyloid joint, that in the former one of the bones, viz. that bearing the
ball surface, can rotate in the socket formed by the other ; while in the
condyloid joint there is no such rotation, the angular rotation being due to
a free gliding motion.

46 ' Elementary Physiology.

are two roughened prominences or ridges of bone situated

INT. FIBRO-CART.

SEMILUNAR

PATELLAR

FACET

EXT. TIBIAL I —
SURFACE

INTERNAL
TUBEROSITY

INT. TIBIAL
SURFACE

INTERCONDYLAR FOSSA
POST. CRUC. LIST.

Fig. 29. — Lower extremity of right femur, from below.
(Drawn by T. W. P. Lawrence.) |

where the neck joins the shaft, which serve for the attachment
of muscles to the bone.

The shaft of the femur is not quite straight, but shghtly

ANT. CRUC. LIGT.
INT. FIBRO-CART.

TUBERCLE

EX.T. FIBRO-CART.

ILIO-TIBIAL

BAND

INT. FIBRO-CART. EXT. FIBRO-CART.

POST. CRUC. LIGT.

Fig. 30. — Upper e.xtremity of the right tibia, from above.
(Drawn by T. W. P. Lawrence.) f

curved from above downwards, with the convexity forwards. It
is nearly cylindrical in section in the middle of its length, but

The Skeleton and its Articulations . 47

becomes very much broadened laterally at its lower end, where
it passes into the inferior extremity.

FOSSk

INTCBARTIC.-
LICT

POST. CnuCL'i UuT. ANT. cnuCtAU UCT.

lUTCRCOKOVLAR NOTCH

Fjg. 31. — Right femur, from behind.
(Drawn by T. W. P. Lawrence.)

Fig. 32^Right tibia and fibula, from behind
(Drawn by T. W. P. Lawrence.)

48 Elementary Physiology.

The mferior extremity (see Fig. 29) is the broadened-out
lower end of the shaft; it forms two eminences, called the
external and internal condyles^ which are united in front, but
separated behind by a notch, or fossa {intercondylar fosse). The
surfaces of the condyles are convex both from before backward
and from side to side, while between them in front is a concave
hollow, which extends to some extent up the front of the lower
extremity of the bone. All this surface is covered by smooth
articular cartilage. The grooved surface between the condyles
articulates with the patella (or knee-pan), the convex eminences
of the condyles with the nearly flat upper articular surface of
the tibia (shin-bone), and the semiltmar jibro-cartilages which
are the interarticular cartilages of the knee joint.

The articulation of the head of the femur with the hip-bone
has already been described (see p. 37). The lower extremity
forms part of the knee joint, which is shown, partially in section,
in the accompanying figures.

The bones taking a share in the formation of the knee joint
are the femur, tibia, and patella. The patella is what is known
as a sesamoid bone^ — that is, a bone formed in the tendon of a
muscle, usually where it passes over or bends round a bony
surface. The patella is shaped like a triangle with rounded off
angles, and is placed base upwards just in front of the knee
joint in the combined tendons of the extensor muscles which
straighten (or extend) the leg at the knee. It serves the double
purpose of protecting the knee joint in front and of hardening
and strengthening the extensor tendon where this bends round
the knee.

Between the opposed articular surfaces of the femur and
tibia, which are the two chief bones in the knee joint, are inter-
posed the semihinar fiJ)ro-cartilages.. These are two crescentic-
shaped plates with a free surface both above and below, and
are thick at their outer border (which is attached in each case
to the capsule of the joint), but thin away to an edge at their
inner margin so as to leave part of the upper articular surface
of the tibia free in the centre. This portion of the articular
surface of the tibia comes in contact with that of the femur.
The two ends of each semilunar cartilage are attached to the

TJie Skeleton and its Articulations.

49

non-articular part of the upper extremity of the tibia which is
interposed between the two articular surfaces (see Fig. 30).

Fig. 33. — The superficial parts of the knee joint removed, and the external condyle of
the femur sawn off obliquely-, together with half the patella, so as to expose both
the crucial ligaments together, (Allen Thomson.) ^

In A, the parts are in the position of extension, in B, that of flexion, the figures being
designed to show the different states of tension of the crucial ligaments in these
positions, i, sa^ra surface of femur; 2, sawn surface of patella; 3, ligamentum
patell2e;_4, anterior or external crucial ligament, tense in A, and relaxed in B;
5, posterior or internal crucial ligament, partly relaxed in A, tense in B ; 6, internal^
and 7, external semilunar fibro-cartilage ; 3, transverse ligament ; 9, articular surface
of the tibia, extending behind the external semilunar fibro-cartilage ; 10, on the
head of thefibula, points to the anterior superior tibio-peroneal ligament; 11, upper
part of the interosseous membrane.

The semilunar cartilages' are* thus crescent-shaped wedges
which adapt the articular surfaces of tibia and femur to each
other. In the accompanying figures of the knee joint there are
also shown the two crucial ligaments which connect the femur
and tibia, and, besides strengthening the union, prevent over-
extension.

The movement at the knee is not purely that of a hinge
joint, although the effect is much the same ; but is a compound
of gliding and rolling of the condvles of the femur on the tibia

50 Elementary Physiology.

and semilunar cartilages, accompanied (at the end of extension;
or beginning of flexion) by a small amount of rotation.

The bones of the lower leg are the tibia and fibula (see
Fig. 32) ; of these the tibia is the stronger and more massive.
It transmits all the weight of the body above it to the ankle,
for the fibula takes no part in the knee joint, and the tibia
furnishes the greater part of the surface at ^;he ankle joint for
articulation with the astragalus^ that bone of the foot which
articulates at the ankle.

The tibia and fibula are of almost equal length, the head of
the fibula lying a little lower than that of the tibia at the knee,
while the fibula lies a little lower at the ankle. It here forms
the external malleolus^ a prominence of bone at the outer side
of the ankle. The corresponding prominence on the inner
side is furnished by the tibia, and is known as the internal
malleoliLs.

In position, the tibia lies to the inside and somewhat
anteriorly to the fibula. The two bones articulate both at their
upper and lower extremities ; but there is practically no move-
ment at either articulation, so that there is nothing in the leg,
corresponding to pronation and supination of the forearm.
The tibia and fibula are united by strong interosseous ligaments.
The fibula is a long slender bone which serves to strengthen
the ankle joint. It furnishes to this joint an articular surface
situated on the inner surface of the external malleolus. The
fibula gives attachment to some of the leg muscles. Others of
these muscles are attached to the tibia and to the lower
extremity of the femur ; they cause the movements which take
place at the ankle joint and those of the foot and toes.

The ankle joint is formed by the tibia and fibula above, and
the astragalus beneath. A section across the joint from side to
side is shown in Fig. 34, and another from before backward is
seen in Fig. 35. The movements are those of flexion^ in
which the foot is bent upwards in front towards the leg, and
extension^ in which the foot is depressed and brought into a line
with the leg. The total range of movement is about a right
angle.

There are seven bones in the tarsus (see Fig. 36), named

TJie Skeleton and its Articulations.

51

respectively : i, astragalus ^ which forms the lower articulation
of the ankle joint ; 2, calcanetnn^ or os calcis^ which lies beneath
the astragalus and projects backward to
form the heel, to which the tcndo A chillis is
attached conveying the pull of the calf
muscles to the foot ; 3, navicular^ or
scaphoid^ which lies in front of the astraga-
lus at the inner side of the foot and
articulates with this bone and with the
three cuneiform bones which lie in front of
it; 4, cuboid, which is situated on the
outside of the foot in front of the calcaneum,
with which, and the fourth and fifth meta-
tarsal bones, it articulates; 5, 6, 7, the
internal, middle, and external cimeiforms ^
which lie in front of the navicular on the
inner side of the foot between that bone
and the first, second, and third metatarsal
bones respectively.

The bones of the tarsus, together with
the five metatarsal bones, form an arch
both from before backward, and from side
to side, which is known as the arch of the
foot.

There is very little movement between
the tarsal and metatarsal bones, save a
small amount of gliding motion. A small
amount of rotation can take place between

Fig

astragalus and calcaneum.

34. — Section of the
right ankle joint near
its middle, and of
the posterior astra-
galo-calcaneal articu-
lation, viewed from
before. (Allen Thom-
son.) \

internal ; 2, external
malleolus ; 3, placed
on the astragalus at
the angle between its

• superior and its ex-
ternal surfaces ; 4, in-
lerior interosseous
tibio-fibular ligament ;
5, internal lateral
ligament of the ankle
joint ; 6, sustentacu-
lum tali ; 7, calcaneo-
iibular or middle part
of the external lateral
ligament ; 8, inner
part of the inter-
osseous astragalo-cal-
caneal ligament ; 9,
tuberosity of the cal-
caneum.

movement takes

The chief movement is termed inversion
and eversion. In inversion the outer side
of the foot is depressed and the sole
turned inward ; in eversion the opposite
place.

The movements between metatarsal bones and phalanges
and the interphalangeal movements are similar to those of the
hand ; but the range of movement is somewhat less, especiall}-
in the case of the hallux, or great toe.

52

Elementary Physiology.

The bones of the foot are shown in the
figures (A and B, Fig. 36).

accompanying

Fig. 35. — Sagittal section of the ankle-joint and articulations of the right foot, a little
to the inside of the middle of the great toe. (Allen Thomson.) \

1, synovial cavity of the ankle joint ; 2, posterior astragalo-calcaneal articulation ;
3, 3', astragalo-calcaneo-navicular articulation : the interosseous ligament is seen
separating 2 from 3' ; 4, inferior calcaneo-navicular ligament ; 5, part of the long
plantar ligament ; 6, naviculo-cuneiform articulation ; 7, first cuneo-metatarsal
articulation ; 8, first metatarso-phalangeal articulation ; . 9, section of the inner
sesamoid bone; 10, interphalangeal articulation; 11, placed on the calcaneum,
indicates the bursa between the upper part of the tuberosity of that bone and the
tendo Achillis.

This brief outHne of the structure of the skeleton may be
concluded with the following classification of the articulations : —

Articulations are divided into two classes — viz. synarthroses,
or continuo2Ls articulations ; and diarthroses^ or discontinuous
articulations.

In a synarthrosis the bony surfaces are fixed together either
directly or by some interposed substance, such as a disc of
cartilage. The two most frequent forms of synarthrosis found
in the body are the suture and the symphysis.

The suture is met with only in the case of the skull-bones.
In this form of articulation the opposed surfaces of bone
practically come in contact, being only separated by a thin
layer of fibrous tissue. The edges of the bones where they
come in contact are often deeply indented or serrated^ so as to
firmly interlock with each other. In other cases the edges are
thin and bevelled, to meet each other, forming a squamotis
suture ; or they may be grooved, forming 2i grooved suture.

The Skeleton and its Articulations.

30

In a symphysis the bones are united by a disc of fibro-
cartilage which acts as an elastic pad, as has been described in
the case of the bodies of the vertebra and of the two pubic
bones.

Synchondrosis is a term applied to growing bones separated
by cartilage which aften^-ards become united : as in the case of

U3

Fig. 36.— The bones of the right foot : A, from above ; B, fiom below.
(Alien Thomson.) i

a, navicular bone ; b, astragalus ; c, os calcis ; d, its tuberosity ; ^, internal cuneiform ;
f, middle cuneiform ; g, external cuneiform ; h, cuboid bone ; I to V, the metatarsal
bone ; i, 3, first and last phalanges of the great toe ; i, 2, 3, first, second, and third
phalanges of the second toe-

the three bones which later in life unite to form the hip-bone ;
and as in the case of the occipital bone at the base of the skull
which early in life consists of several distinct bones. As the
component bones grow they approach one another, and finally

54 Elementaiy Physiology.

their edges unite and one bone is formed, the process of union
being described as synostosis.

Syndesmosis is a term applied when the bones are united
by an interosseous Hgament, as is the case in the lower
articulation between tibia and fibula.

In diarthrosis, or discontinuous articulation, the opposed
surfaces of bone remain distinct, they are surrounded by
synovial cavities, and there is more or less freedom of
movement between them. These are the true joints of the
body, and are divided into the following classes : —

1 . Gliding joints in which the surfaces are nearly flat and
capable of gliding to a limited extent over each other ; examples
are found in the carpal and tarsal bones.

2. Hinge joints in which the articular surfaces are arranged
to permit a hinge-like movement by which the joint can be
straightened (extension) or bent (flexion). One surface is
usually pulley-shaped and the other correspondingly ridged.
Examples are the articulation between humerus and ulna at the
elbow joint and the ankle joint.

3. Condyloid joints in which rounded surfaces are present,
one being convex and the other concave. Angular movement
in any direction is allowed by such a joint, and hence cir-
cumduction or angular rotation. The movements of the
fingers at the joint between the metatarsal bone and first
phalanx illustrate the action of such a joint. Similar joints
exist at the corresponding position between the metatarsal
bones and the first row of phalanges in the foot.

4. Saddle joints in which the opposed surfaces are saddle-
shaped {i.e. convex in one direction and concave in another
at right angles to it) and placed so that the convexity of one is
applied to the concavity of the other. Such an articulation is
found between the trapezium and the metacarpal bone of the
thumb.

5. Pivot joints in which one articular surface is either
cylindrical or cone shaped, and the other is correspondingly
concave, as in the case of the articulation between radius and
ulna at the elbow, and that between atlas and axis by means of
which the head is rotated (see pp. 22, 42).

The Skeleton and its Articulations. 55

6. Ball-and-socket joints in which one articular surface is
spherical or nearly so, and the other is hollowed out to form
a cup to receive it. Examples have been described in the
shoulder and hip joints (see pp. 37, 39).

In some cases a joint cannot accurately be described as
belonging rigorously to any one of these six classes, as in the
case of the articulation of the lower jaw with the temporal
bone, where, as already described, there is a hinge movement of
the condyle of the jaw on the lower surface of the interarticular
cartilage, and at the same time a gliding movement of the
upper surface of the interarticular cartilage forward out of the
groove on the temporal bone on to the eminence in front of it.
Again, at the knee joint, while the general movement is a hinge
one, the motion is accomplished by a mixed motion of gliding,
rolling, and rotating.

CHAPTER III.

THE MUSCULAR SYSTEM.

The jointed bony system, or skeleton, of which the structure
has been sketched above, is clothed or covered, except at
certain parts where the bones are subcutaneous, by masses of
flesh, which are made up of muscles, sheathed in coverings of
connective tissue known 2.^ fascia.

The muscles are masses of soft tissue which are capable
of changing their form under the influence of impulses con-
veyed to them along the nerves from the nervous system. We
shall presently see that the muscles are built up of elongated
microscopic elements called muscle fibres. When the nerve
impulse reaches the muscle each fibre cojitracts., or becomes
shortened in length, and at the same time thickened so as
to preserve an unaltered volume. The result of this is that
the muscle as a whole shortens or contracts in the direction
of its constituent fibres, and swells up in a direction at right
angles to this. The effect of such a muscular contraction will
depend upon the attachments of the muscle.

One of the most usual forms of attachment is to two bones,
one on each side of a joint. In such a case the muscle is
often elongated and spindle-shaped, with a thick middle part
forming the belly, and thins out at each end into a fibrous cord
or band, called the tendon of the muscle.^ The tendon is
strongly attached to the bone, and when the muscle contracts
exerts a strong pull upon it. An example of such a muscle
is the biceps ^ of the upper arm, which on contracting bends or

^ A tendon must not be confused with a ligament ; both are fibrous bands,
but the tendon is always connected to a muscle at one end, while the liga-
ment is not.

^ So called because it divides above into two parts.

Elerfientary Physiology. 57

flexes the elbow joint. The thickening of this muscle as it
contracts can be felt as the arm is " put up " or bent at the
elbow.

In other cases the muscles are arranged in broad sheets, which some-
times encircle a cavity. Examples of such muscles are the sheets of muscle
which form the abdominal wall, and the diaphragm or midriff which
separates the thoracic from the abdominal cavity. When these sheets of
muscle contract they compress the cavity towards which their concave
surface is turned. If the diaphragm contracts, for example, it compresses
the contents of the abdominal cavity, unless the pressure is relieved by the
relaxation of the muscles of the abdominal wall in front ; at the same time,
a suction is exercised on the contents of the thoracic cavity, which is re-
lieved by the air entering and filling the lungs. This -is the usual action of
the diaphragm in breathing, but if both diaphragm and abdominal muscles
contract at the same time, the contents of the abdominal cavity are strongly
compressed. This simultaneous contraction takes place in the process of
defcecation, in which, a ring of muscle at the lower end of the intestine being
relaxed, any unused detritus of the food which has not been digested and
taken into the body is ejected at the amis (the lower opening of the
alimentary canal) by means of the pressure so exerted.

The muscles connected with the skeleton {skeletal viuscles)
hence vary greatly in form according to the work which they
are called upon to perform. A complete description of the
skeletal muscles and their actions w^ould fill a greater space
than this entire volume, but some idea of their general arrange-
ment may be gathered from the following diagrams and their
accompanying descriptions.^

All the muscles of the body are not, however, connected
with the skeleton. Some form coats lining various tubes of
the body, and others are found in various organs. Thus, the
heart is a hollow organ containing four chambers, the walls
of which are composed of muscular tissue. By its contractions,
this muscular tissue alters the volumes of these chambers, and
the alteration in volume, aided by an arrangement of valves
to be subsequently described, has the effect of driving the
blood through the heart and the system of tubes connected
with it. The walls of these tubes (the arteries and veins) are
also hned with a muscular coat which alters the bore or calibre
of the vessels. This muscular coat is strongly developed in

^ See also Fig. 15, p. 29,

Fig. 37. — Superficial view of the muscles of the trunk, from before. (Allen Thomson.)

I, sterno-mastoid of the left side ; i', 1", platysma myoides of the right side ; 2, sterno-
hyoid ; 3, anterior, 3', posterior belly of the omo-hyoid ; 4, levator anguliscapulas ;
4', 4", scalene muscles ; 5, trapezius ; 6, deltoid ; 7, upper part of triceps in the left
arm; 8, teres minor; 9, teres major ; 10, latissimus dorsi ; 11, pectoralis major; 11',
on the right side, its clavicular portion ; 12, part of pectoralis minor ; 13, serratus
magnus ; 14, external oblique muscle of the abdomen ; 15, placed on the ensiform
process at the upper end of the linea alba ; 15', umbilicus ; 16, is placed over the
symphysis pubis, at the lower end of the linea alba; above 16, the pyramidal muscles
are seen through the abdominal aponeurosis ; 14 to 17, linea semilunaris at the outer
border of the rectus muscle, the transverse tendinous lines of which are seen through
the abdominal aponeurosis; 18, gluteus mediiis ; 19, tensor vaginae femoris ; 20,
rectus femoris ; 21, sartorius ; 22, femoral part of the iUo-psoas ; 23, pectineus ; 24,
abductor longus ; 25, gracilis.

'iMI/'f

rz--

is

rl6

Fig. 39. — Superficial muscles of the leg, seen from
the inner side. (After Bourgery.)

vastus internus ; 2, sartorius ; 2', its tendon, spreading
on the inner upper part of the tibia ; 3, gracilis ; 4, semi-
tendinosus ; 4', its insertion ; and between 2' and 4', that
of the gracilis ; 5, semi-membranosus ; 6, inner head
of the gastrocnemius ; 7, soleus ; 8, 8', placed upon the
tendon Achillis, point to the tendon of the plantaris
descending on the inner side ; g, small part of the_ tendon
of the tibialis posticus ; 10, flexor longus digitorum ;
II, flexor longus hallucis ; 12, tibialis anticus; 12', its
tendon of insertion ; 13, abductor hallucis.

Fig.' 38.— Superficial muscles of the shoulder and upper limb, from before. (Allen

Thomson.)

I, pectoralis major, its sterno-costal portion ; i', its clavicular portion ; 2, deltoid, its
clavicular part ; 2', its acromial part ; 3, biceps brachii ; 3', its tendon of msertion ;
3", its aponeurotic slip ; 4, brachialis anticus ; 4', its inner and lower port'on ; 5>
long head of the triceps ; 5', inner head of the same, seen arismg from behind the
intermuscular septum ; 6, pronator radii teres ; 7, flexor carpi radialis ; 8, palmans
longus, passing at 8' into the palmar aponeurosis; 9, flexor carpi ulnaris ; 10, 10,
supinator longus ; between 10 and 3', +, supinator brevis ; 11, extensorossis metacarpi
pollicis; 12, extensor brevis pollicis ; 13, lower part of the flexor subliniis digitorum ;
14, flexor longus pollicis ; 15, small part of the flexor profundus digitorum ; 16,
palmaris brevis, lying on the muscles of the little finger; 17, abductor pollicis.

6o Elementary Physiology.

the smaller arteries, where it is chiefly arranged so that its
fibres encircle the blood-vessel. When these fibres contract
the bore of the vessel is diminished, and the blood cannot
flow so rapidly through it; on the other hand, when the
muscle fibres are relaxed the vessel is widened, and the blood
can flow through with greater ease.

To these muscle fibres, nerve fibres pass {vasomotor fibres), and the
muscle fibres are kept in tone ' by impulses sent to them from the nervous
system along these fibres. In this way the blood-supply to any part of the
body is regulated by the nervous system. The nervous centres have nerve
fibres running both to the centre from any part {afferent fibres), and from
the centre to the part {efferent fibres) . When any change takes place in the
part, a nerve impulse passes to the centre by an afferent fibre, and then
a suitable reply is sent back to the part along an efferent fibre. Suppose,
for example, a certain set of muscles are set hard at work contracting ;
then energy is used up rapidly, and an increased supply of blood is required.
The nerve centre being informed of this sends nerve impulses down, which
relax the muscular coats of the small arteries supplying these muscles, and
also stops impulses which had previously been passing down and keeping
them constricted, in this way the supply of blood to the muscles is greatly
increased. Suppose, again, one commences eating something, saliva is at
once secreted, because impulses pass from nerve endings in the mouth up
to a nerve centre, and back from there come impulses to the cells of the
salivary glands, causing these to secrete. Impulses are also sent to the
arterioles (small arteries) which supply the glands relaxing these, and giving
that bigger supply of blood to the cells which is required by them on
account of their increased activity.

Other muscle fibres form coats which line the wall of the
alimentary canal; these cause the onward movement of the
food undergoing digestion by a kind of contraction called
peristalsis. This is a wave of constriction which passes along
the tube and gradually shifts its contents onward, until what
is left unabsorbed during the passage is finally collected in
the rectttm (the last portion of the alimentary canal), from
which it is ejected by the act of defaecation as mentioned
above.

^ The tonicity of muscles means the degree of contraction at which they
are kept by nervous action. A muscle is never completely relaxed ; it is
always kept on the alert, as it were, by the nerves supplying it, and when
a limb, say, is moved the motion takes place by an increase of this tonicity
in one set of muscles, and a diminution in the other opposing set which
move the limb in the opposite way.

The Muscular System. 61

Similar muscular coats are found lining other vessels in
the body, such as the urinary passages. Muscle fibres are
also found in the spleen (see p. 92), which they cause to
contract at intervals of about once a minute.

Muscle is, then, that tissue in the body which is directly
responsible for all movements, but the nature of the movement
varies very greatly. In one case it is a movement of a joint
causing a change in position of the whole animal ; or of part
of it relatively to the rest. In another case it is the mxovement
of a fluid, such as the circulation of the blood in the blood-
vessels by the contraction of the heart muscle. In another
case, the redistribution of the relative supply of blood to
different parts by the contraction or relaxation of the muscle
fibres surrounding the walls of the arterioles. In yet another
case, the passage of food along the alimentary canal by the
peristaltic wave of contraction travelling slowly along the tube.

All these varied acts of muscular contraction are carried
out under the control of the nervous system ; but while some
are also under the control of the individual, or, as it is termed,
are vohuitary\ others are carried out by the nervous system
acting automatically, and are said to be involuntary^ since they
are not under the control of the will.

Muscular tissue is hence divided into two kinds, voluntary
and involuntary, and there are other well-marked differences
in structure between the two kinds of fibre which will presently
be described. Voluntary muscle fibres, for example, when
observed under the microscope, are seen to be marked by
cross striations — that is, they show markings across them,
dividing the fibre into narroiiw bands, alternately light and dark
(see Fig. 41). Such cross striation is absent in involuntary
muscle fibre. Hence the terms cross-striated or striped, and
plain or non-striated muscle, are often synonymously used
instead of the terms voluntary and involuntary respectively.
Heart muscle is intermediate between these two kinds in its
structure, and is accordingly placed in a class by itself as cardiac
muscle. It shows cross striation, but less perfectly marked than
voluntary muscle. Its contractions are not under the control
of the will, so that functionally it is involuntary.

62

Elementary Physiology.

We shall, in the first place, briefly describe the structure of
the different kinds of muscle, and then the general mechanics
of the movements brought about by the contractions of skeletal
muscle.

A skeletal, or voluntary muscle, is enclosed in a sheath of
connective tissue, or muscular fascia, which sends in at places
sheets of like material to form septa, which divide the muscle
into large bundles of fibres. Each of these larger bundles is
divided by thinner sheets of intramuscular connective tissue
into fasciaili, or small bundles, and again, each of these still

»iii;i;uii;aiiiiiiE:;!!H?^

m-

Fig. 40. — Sarcoiemma of mammalian
muscle highly magnified.

The fibre is represented at a place where
the muscular substance has become
ruptured and has shrunk away, leaving
the sarcoiemma (with a nucleus adher-
ing to it) clear. The fibre had been
treated with serum acidulated with
acetic acid.

BiiiiiiiiiiiiHiniiiiHiii

iiillllHliHIllHg

iniiii:ii!:i|iiii ii

illlllilllllllllllBllil

iiiiiiiinii;aiiii|
iiiiiiiniiiiHi;!!^*

iiliiii'i'ffllliiii
[Illlliiljilililili

ii:::in:ii i li

nilliiiiii

Fig. 41. — Muscular fibre of a mammal
examined fresh in serum, highly mag-
nified, the surface of the fibre being
accurately focussed.

The nuclei are seen on the flat at the sur-
face of the fibre, and in profile at the
edges,

contains many hundreds of the fine mt/scle fibres which go to
make up the chief part of the mass of the muscle. So far the
structure can be followed with the naked eye (the fasciculi are
the grain which is seen in boiled flesh), but if it be desired to
follow the structure further, and see the ultimate fibres of which
each fasciculus is made up, the microscope must be employed.

The Musctclav System. 63

In the fasciculus the constituent fibres lie parallel to one another,'
each is about 3555 of an inch in diameter, cylindrical, or nearly so, in
section, and may be an inch or more in length. Each fibre has a sheath,
called the sarcoleinma, which surrounds and incloses the contractile sub-
stance. This sheath is so thin and delicate in structure, and so transparent,
that it is only seen at places where in the process of preparation the con-
tractile substance within it has been broken across (see Fig. 40). At
intervals in the length of the fibre, oval nuclei are to be seen ; these
become more distinct after the use of staining agents.

The contractile substance ^ is marked off into short dark and light discs,
so as to give a striped appearance to the fibre. When the fibre contracts,
the light bands disappear, and the dark ones swell out. It is probable
that the protoplasm forming the light band in this operation passes into the
dark band. The consequence is that the distance from the centre of one^
dark band to the centre of the next becomes very much lessened, and, as
this takes place along the entire length, the fibre as a whole becomes much
shorter and thicker.

The sarcolemma passes over the end of the column of contractile sub-
stance at each end of the fibre, and becomes continuous with a fine thread
of fibrous tissue which passes to form a constituent part of the tendoji by
which the muscle is attached to the bone, and by means of which it pulls
upon the bone when it contracts. This attachment of muscle fibre to tendon
is best seen when the muscle is plunged into hot water, and afterwards a
small shred from the junction between muscle and tendon is taken and
teased and examined under the microscope. The hot water causes the end
of the contractile material to shrink away from the end of the sarcolemma,
as seen in Fig. 42.

The vessels which carry the blood-supply, and the nerve carrying the
nerve-supply, generally enter somewhere near the middle of the muscular
mass. The artery subdivides into branches within the mass, and finally the
small arterial branches (arterioles) break up into capillaries, which run in
oblong meshes, as shown in Fig. 43, with the long branches parallel to the
length of the muscle fibres. No capillaries ever penetrate any of the muscle
fibres ; these are nourished solely by diffusion or passage m sohctiofi, in both
directions, between the blood within the capillary and the muscular sub-
stance within the sarcolemma, and not by any actual contact between blood

^ On account of this arrangement o# the muscle fibres, a shred of muscle
teases with needles more easily in one direction than another, the fibres
being more easily separated than broken across. The same is true of nerve
in which the nerve fibres lie side by side in a similar fashion. To observe
the minute structure of either kind of fibre, a small shred should be teased
out as fine as possible, by means of two needles mounted in M-ooden handles,
and then observed under the microscope.

- Only an outline of the structure of the contractile substance can be
given here ; for more detailed information the student is referred to Schafer's
"Essentials of Histology."

64.

Elenientaj'y Physiology.

and muscle substance. The nutrient materials carried to the muscle fibres
by the blood-stream pass out in solution through the thin wall of the
capillary vessel, which is formed by a single layer of thin flattened cells
tsee p. 96). The fluid so separated from the blood, which holds in solution
the materials necessary for the life and activity of the muscle fibres, is called
the lymph. This lymph bathes the muscle fibres, and the nutrient substances

Fig. 42. — Termination of a muscular FiG. 43. — Capillary vessels of muscle,

fibre in tendon. (Ranvier.)

vt, sarcolemma; s, the same membrane
passing over the end of the fibre ;
/, extremity of muscular substance ;
c, retracted from the lower end of
the sarcolemma-tube ; t, tendon-
bundle passing to be fixed to the
sarcolemma.

which it contains pass through the sarcolemma in solution, and are taken
up and utilized by the contractile substance. Besides this streaming of
nutrient material in towards the muscle substance, there is also a stream in
the opposite direction of waste material away from the muscle substance by
means also of the intermediate lymph back to the capillary vessels.

In addition to this stream of nutrient substance, which has been in the

The Mnsadai' System, 65

first instance formed from the food of the animal, there is also a double
stream of gas in solution : of oxygen, by means of which the muscle sub-
stance is able to decompose or oxidize the nutrient matter and set free the
store of energy which it contains, tozvards the muscle substance ; and of
carho7i dioxide, a chemical product of this oxidation, and hence a waste
product, aioay from the muscle substance and towards the capillaries, Tlie
oxygen so required is taken in at the lungs during respiration in a way
which will subsequently be described, and at the same time the carbon
dioxide is removed from the blood and thrown out with the expired air
into the atmosphere (see p. 173).

This mode of exchange between the blood and the tissues, whereby the
blood supplies nourishment on the one hand, and removes waste materials,
the products of chemical change going on in the tissues, on the other, is
the general method throughout the body. The cells of any tissue are not
bathed in blood, but in lymph which has exuded through the thin walls of
the capillaries ramifying through the tissue. An artery carrying blood
penetrates the part, and subdivides, and again subdivides, many times re-
peated, so giving rise to a large number of very fine branches. As the arterial
branches become more minute at each subdivision, their walls also become
thinner. Finally, they merge into capillaries, which are very fine tubes
with a diavieter oJ\^ to 3— of an inch, and with a wall of extreme thinness,
consisting of a single layer of pavement epithelium — that is, of thin flattened
cells joined edge to edge.^ Through this thin capillary wall the lymph
exudes and bathes the cells which are to be fed by the blood-stream.

The excess of lymph which collects between the capillaries and cells is
gathered up by a different system of capillaries, called lymphatic capillaries,
which by uniting together form larger lymphatic vessels (see Fig. 45).
These larger lymphatics again unite, and finally all the lymph so collected
is carried by two main trunk vessels and poured into two large veins of the
neck, so returning again to the blood-stream. The lymphatic trunk on the
right side of the body is by far the larger of these two, and is known as
the thoracic duct{'s,QQ Fig. 44). There is no propelling agent corresponding
to the heart in the lymphatic system of man, but the lymphatics are closely
beset with valves, which all open in the direction of the thoracic duct,
and hence any compression of the lymphatics by muscular movements
always determines an onward flow of lymph towards the thoracic duct, and
so back to the circulating blood.

The nerve which enters the muscle consists of an immense number of
very fine long fibres, which course p^allel to one another in the nerve.
Within the muscle the nerve subdivides in the same manner as the artery.
At each division the number of nerve fibres in each branch decreases, for
the individual fibres do not divide, and so at length there are within the
muscle a large number of minute nerve twigs, each containing a number of
nerve fibres. Each nerve twig supplies a number of muscle fibres. The

^ See p. 96.

./-.

,^11"!

14

1 g.s'S^^ H

h ^ S ''^ ts ■ c c
^ s g'j; d " ^

C ^ g iw

Jh o tt <u ,,

.S '^
'33 s

K- O

P,.3

> " 1) 4> tj <" 5i

c3 >

O "

% « S c 3 ^^ ft

D S e

lUC'S

i-.a

bo ^

. -^ ^ - o « t«

r^-S ;--5

O *"

c ■?;: o

^ " "
o

'<U O

■■S"^-" "5 bJOiH.^ i;
rt o tv ^.O

S^ O <" e 2

4Ǥga;^g|

P<>j ^ rt ^ 0.3

JJ,rt rtT3 .^c tn__
%,.o iS.S ^^ 33 G

■U "T rj •- U ^, t. >-i

d_^'t! 1) > aJ "^ 4>

■OC-^tSc/i-rt.S
u d o C ot*^^ 7,
.-o . M g M o g ^

>^.S o S'ci s -
V;hoqCrt2'^>.

5 ^o ^ blocks a
rt 0,^ b s-a 3

c +^ <" J', a

o 'Ti S S •* 4»

•g So.s.s.s «-§ ^
o-.a ^i.o.^'S rt

:3

-c g o

The Mtiscular System.

&7

nerve fibres in a twig finally part com-
pany, and each fibre usually branches
into two ; these branches each pierce
the sarcolemma of a muscle fibre near
the middle, and end in a small oval-
shaped plate of coarsely granular proto-
plasm, which rests on the contractile
substance, and is termed an e7id plate.
The nerve impulse arrives at this end
plate, and in some manner acts on the
contractile substance of the fibre, and
causes it to contract. It must not be
assumed, however, that all the fibres in
the nerve which entered the muscle ter-
minate in this way upon the contractile
substance of the muscle fibres. A great
number of them do so ; but some go to
the coats of the arterial branches, and
regulate the blood-supply to the muscle
in the manner above described ; others
are sensory or afferent fibres, and instead
of carrying nerve impulses to the muscle,
carry impulses away from the muscle to
the central nervous system.

Cardiac muscle fibres are also trans-
versely striated, but the striations are
much less distinct. There is no sarco-
lemma inclosing the fibre ; the fibres
branch and the branches unite with those
from neighbouring fibres, and the nuclei
lie within the contractile substance, and
not upon its surface. Each fibre con-
sists of a series of short cylindrical cells
united end to end, and the nuclei of the
fibre are the nuclei of these cells (see
Figs. 46 and 47).

Involuntary or plain muscular tissue
differs structurally from voluntary and
cardiac muscle in that it does not con-^
sist of fibres formed each by the union

Fig. 45. — Superficial lymphatics of the breast, shoulder, and I'pper limb, from before
(after Mascagni and Sappey).
a, placed on the clavicle, points to the external jugular vein ; b, cephalic vein ; c, basilic
vein ; d, radial ; e, median ; /, ulnar vein ; g; great pectoral muscle, cut and turned
outwards ; i, superficial lymphatic vessels and glands above the clavicle ; 2, infra-
clavicular glands ; 3. 3, pectoral glands ; 4, 4, axillary glands ; 5, two small glands
placed near the bend of the arm ; 6, radial lymphatic vessels ; 7, ulnar lymphatic
vessels ; 3, 8, palmar lymphatics.

68

Elementary Physiology.

of a large number of cells, but of single cells. These cells are long
and fusiform in shape (Fig. 48), with an oval or rod-shaped nucleus.^
The cells are longitudinally striated, but show no cross striation (compare
Figs. 41 and 48) ; they are united together by a small amount of connective
tissue, and each is surrounded by a delicate sheath, which is, however,
invisible unless the contractile material has been broken across.

Fig. 46. — Muscular 'fibres from the heart,
magnified, showing their cross-striae,
divisions,! and junctions. (Schweig-
ger-Seidel.)

The nuclei and cell-junctions are only
represented on the right-hand side of
the figure.

Fjg. 47. — Six muscular fibre cells
from the heart. (Magnified 425
diameters. )

a, line of junction between two cells ;
3, c, branching of cells. (From a
drawing by J. E. Neale.)

The different kinds of muscular tissue also vary in the
manner of their response on excitation. Under normal con-
ditions in the body, skeletal muscle never contracts except
when excited to do so by nerve impulses, which travel to it
along the motor (efferent) nerve fibres. The involuntary
muscle fibres which line the intestine, and those lying in the
substance of the spleen, on the other hand, continue to contract
rhythmically even after they have been severed from all their
nerve connections. The same is true of heart muscle, for, not
only will the entire heart of a cold-blooded animal, such as the

^ An oval nucleus is seen in the involuntary muscle cells of the muscular
coats of the small intestine, and a rod-shaped nucleus in the muscular coats
of the larp-er arteries.

The Muscidar System.

69

frog, continue to beat for many hours
after removal from the body, but even
a strip of the muscular tissue, cut out
from the heart, and containing no ner-
vous mechanism, will beat for a long
time.^ The mammalian^ heart ceases
to beat soon after removal from the
body, but this is due to the more rapid
death of the muscle cells, caused by the
stoppage of the blood-circulation through
them; for, when means are taken to
continue the circulation of the blood,
even the mammalian heart continues
to beat.

When a skeletal muscle and the
nerve passing to it are removed from
the body,^ the muscle can be caused to
contract by stimulating its nerve in
various ways, such as (mechanically) by
pinching or hammering it, (chemically)
by applying a crystal of salt to it, or
(electrically) by the action of an electric
current.

Electric stimulation is the most suitable
form for experimental purposes, because the
nerve is not injured thereby, and the excitation
may be many times repeated. By a certain
arrangement of apparatus a graphic record of

^ The heart has, in its substance, small groups
of nerve cells called ganglia, and the purpose
of the strip experiment is to show that the rhyth-
mic beating is not due to these nerve cells, but
is an inherent property of cardiac muscle, the
strip being cut out so as not to include any
nerve cells,

- The mammalia are that class of animals
which suckle their young,

^ These experiments can best be performed
with a muscle and nerve taken from a cold-
blooded animal (frog), for a mammalian muscle
dies very soon after its blood-supply is stopped.

Fig. 48. — Muscular fibre cells
from the muscular coat of
the small intestine, highly
magnified.

A, a complete cell, showing the
nucleus with intra-nuclear
network, and the longitu-
dinal fibrillation of the cell
suhstanC3,with finely vacuo-
lated protoplasm between
the fibrils ; B, a cell broken
in the process of isolation ;
a delicate enveloping mem-
brane projects at the broken
end a little beyond the sub-
Stance of the cell.

70

Elementary Physiology.

the response of a muscle to electrical stimulation may be obtained.^ In-
duced currents from a secondary coil are made use of, and these are
conveyed to the nerve by two wires, called electrodes, placed parallel to
each other and about -j^ of an inch apart. The electrodes are joined up
to the terminals of the secondary coil, and the nerve laid across them.
On making or breaking the electric current in the primary circuit, a tran-
sitory current is induced in the secondary circuit, which excites the nerve
and starts a nerve impulse, and this in turn excites the muscle to con-
tract. • The muscle is fixed at one end, and its other end is attached by a
thread to the short end of a lever. When the muscle contracts the lever

Fig. 49. — Simple muscle curve.

The nerve attached to the muscle was stimulated at a, and at b the muscle began to
contract. The period occupied by the interval between a and b is termed the
" latent period." The lower tracing, taken by a writing-point attached to a vibrating
tuning-fork, shows j^o second intervals ; the small undulations upon it are due to
overtones.

moves, and, by means of a writing-point attached to its long end, traces
a curve on a smoked surface of paper which is caused to move past the
writing-point.

When a single electric stimulu? is applied to the nerve, the tracing
obtained of the muscular contraction is similar to that shown in Fig. 49.
There is first a very short pause before the contraction begins, called the
latent period y then the contraction begins, and is followed by the relaxation.
The relative times occupied by the periods of contraction and relaxation
vary with the load against which the muscle contracts ; but the total period
of contraction and relaxation is fairly constant, and amounts approximately
to -/g second. If now the stimulation be given rhythmically at regular
periods, the effect produced will vary according to the rapidity with which
the stimuli or shocks succeed one another. If a small number of stimuli

^ For details of the methods of obtaining graphic records of muscle con-
tractions, see Brodie, " Essentials of Experimental Physiology."

TJie Muscitlar System. J\

(4 to 6) per second be applied, each conliaction has time to pass otif before
the succeeding one is evoked, and the result is a series of separate con-
tractions or twitches. If the rate of stimulation be more rapid, each
stimulus is applied before the contraction due to the preceding one has
passed, off, and there is a summation of effect ; the muscle never becomes
completely relaxed, but a sinuous line is traced, due to the muscle being
more or less contracted the whole time, the sinuosities being caused by the
individual stimuli. With a still more rapid rate of 30 to 40 stimuli per
second these sinuosities disappear, and the muscle remains permanently
contracted. In this condition each stimulus is thrown in at the height of
the contraction due to the preceding stimulus, and so a maximum con-
traction is maintained. This permanent contraction is known as coiiip 'cle
telanus, the imperfect fusion with a slower rate being termed incomplete
tetaujis. The ordinary" natural contractions of the skeletal muscles in the
body are incomplete tetani ; the rate at which the natural nerve impulses
are sent to the muscles being about 12 to 14 per second. A muscle cannot
be maintained in a tetanized condition for any considerable time, because it
becomes fatigued and gradually relaxes, although the nerve does not become
fatigued and the impulses are still conveyed along it. Even single stimuli
repeated at somewhat longer intervals than ^\j of a second are sufficient to
fatigue a muscle when the stimulation is long continued.

Both cardiac muscle and involuntary muscle have a much longer latent
period than voluntary or skeletal muscle. This may be shown, in the case
of involuntary muscle, by exposing the intestine in an animal which has
just been killed, and directly stimulating it by pricking ^^ith a sharp point.
The intestine contracts at the point touched, but only after an obvious delay
apparent to the eye.^

Cardiac mttscle further differs from voluntary muscle, in that
it cannot be tetanized. The muscle fibres possess naturally the
property of contracting at regular intervals. Just after each
contraction, the fibres pass into a refractory condition, and
cannot by any stimulation be caused immediately to contract.
If this tendency to rhythmic contraction has by any means
been so much weakened that the contractions do not take
place spontaneously, then stimulation may reinforce it, and
cause contractions at intervals ; but by no means can rapidly
repeated rhythmic stimuli be summated, and the heart muscle
sent into tetanus. After a contraction has taken place, the
fibres become inexcitable for a brief period, and relax, in spite
of stimulation. The rate at which the contractions take place

^ The latent period of voluntary muscle is much too short to be appreciated
in such a manner.

J 2 Elementary Physiology.

can, however, be altered by stimulation, either of the heart
muscle directly, or through its nerves.

There are two nerves which carry impulses to the heart,
and alter its rhythm {i.e. rate of beating) ; these nerves are
branches, respectively, from a nerve called the vagus, and from
a chain of nerves called the sympathetic. The vagus branch
slows the heart when stimulated,^ and if sufficiently strongly
excited stops it for a time, but only for a time ; afterwards
the inherent property of contracting which the cardiac muscle
fibres possess asserts itself, and no matter how strong the
stimulus, the accumulated tendency to contract becomes too
strong for the inhibiting ^ stimulus, and the heart recommences
to beat slowly.

The sympathetic branch, on the other hand, increases the
rhythm when it sends impulses to the heart, causing the heart
to beat much more rapidly.'^

The cardiac muscle fibres then possess the property of
rhythmic contractility, and retain this property even when cut
off from all nerve mechanism ; and, further, stimulation
through nerves, or otherwise, can only alter the rate of this
rhythmic contraction, and not altogether remove it, either by
keeping the fibres permanently contracted or permanently
relaxed.

This property of rhythmic contractility is shared to a less
perfect degree by the involuntary muscle fibres of the spleen
and intestine, but is not possessed at all by voluntary muscle,
which only contracts when stimulated to do so. The normal
or natural excitation to contraction of voluntary muscle is
given by nerve impulses, which are sent out from the central
nervous system at a rate of lo to 12 per second, and cause
the muscle to pass into incomplete tetanus.

' This effect may be shown experimentally by cutting these nerves,
and then stimulating, in each case, the end next to the heart.

2 Inhibition means the stoppage of any normal action by nervous
mechanism ; the stoppage of the heart, or its slowing, by the vagus as
described above is an example. In consequence of this action the vagus
is said to be the inhibitory nci've of the heart.

^ This may be shown by stimulating the cardiac branch of the sym-
pathetic.

TJie Muscular System 73

A single skeletal muscle rarely or never contracts alone in the manner
described above, but always in consort with a number of other muscles.
For even the simplest movements require the combined action of several
muscles to carry them out. Also, if the movement is at all complicated,
the different muscles involved in it must contract in a definite order and
time with regard to one another, and with a definite strength of contraction.
This harmonious working of the muscles together is spoken of as co-ordina-
tion. Unless the co-ordination be perfect the movement will be performed
in a clumsy and imperfect way. Complicated movements requiring much
co-ordination, such as walking, talking, and grasping and handling objects,
are learnt by practice by the infant just in the same manner as other skilled
movements, such as writing, piano-playing, cycle-riding, rowing, swimming,
and countless other such accomplishments are learnt later in life. When
once any such complicated movement has been learnt it becomes automatic,
the will is only concerned in starting or stopping the cycle of muscular
contractions, and the various movements are carried out in perfect co-ordina-
tion or rotation by lower nerve centres, without the attention being con-
sciously fixed upon them. If certain parts of the nervous system be injured,
however, this co-ordination is interfered with, some of the paths of nerve
discharge become blocked, and the process has to be learnt again ; if,
indeed, the injury has not been such as to block all possible paths for the
carrying out of the necessary movements. For example, an injury to a
certain part of the brain may affect the speech, so that certain words cannot
be said at will, because the injury has blocked the track of communication
between certain nerve cells in the brain and the muscles of the tongue and
lips, of which the movement is necessary for the production of these words.
Such words may be acquired again by the establishment of communication
through some more roundabout route, as the result of practice or repeated
trial. This is but one example. In nearly all cases where any part of the
brain is injured, "^t paralysis or loss of function which at first appears as a
result gradually vanishes, even though the injury remain permanently,
because other parts of the brain take on the work of the injured part.
There is a free choice of paths along which nerve impulses may pass from
the central nervous system to the muscles, and the usual one is merely the
easiest, most convenient and most used one. If this is shut off, a new one
is soon discovered. Just as with our present intricate meshwork of tele-
graphic communication over the world. When communication is interrupted
between two important places, there is only a temporary hitch, for soon
it is discovered which is the next easiest line of connection between the two
places.

The Mechanics of the Skeletal Movements.

The bones form levers which the muscles attached to them
are capable of moving. A lever is a rigid bar capable of

74

turning about a
CO

tL<-

#^->a:

<

■>

Elementary Physiology.

fixed point called \}i\& fulcrum under the action
g of forces applied along its length at
b different points. Any number of
";; u forces may act on the same lever,
but usually the simplest case only is
considered of two forces acting in
opposition and balanced by their
resultant acting at the fulcrum. One
of these forces is usually the pull of
the earth on a body, and is known
as the weiij^ht (W), while the op-
posing force is called the power (P).

(K<— e

0.^

CO

H§

->a.

•oi

o -Bo
u o g

C (U

4-1 <U

o «

£.i2

It is usual to divide levers into three
orders or classes, according to the dispo-
sition with regard to one another of fulcrum,
power, and weight ; but this division is an
arbitrary one, for the principle is the same
in all three orders. The three orders are
figured in the accompanying sketch ; in the
first order the fulcrum is in the middle,
and the weight and power at opposite sides ;
in the second, the fulcrum is at one end, and
the power at the other, the weight being in
the middle ; while in the third, the fulcrum
is still at one end, but the power is in the
isiddle and the weight at the other end.

When the power and weight balance
each other, there exists a simple relationship
between their magnitude, each being in-
versely proportional to its distance from the
fulcrum. That is to say, if P, for example,
is three times as far from F as W is, it will
only require to be one- third as great to
balance it. If P exceed by a little this
amount, the weight (W) v/ill be raised while
P descends ; but the matter will be equalized
in this way, that P will go down three times
as far as W goes zip.

The ratio of W to P is called the mecha-
nical advantage. This can have any value in
the first order, but must be greater than
unity in the second, and less than unity in

The Muscular System. 75

the third.' The fact that P can be made much less than W is an advantage
ill one respect only, viz. that a small force can be made to lift a large weight,
but in order to do so the small force must move through a correspondingly
greater distance. The force multiplied by the distance is called the ivork
done, and in all cases the work done by P equals that done on W.

In the body it is usually less of an advantage to have a
small force acting through a long distance than to have a
large force acting for a small distance. The muscles and their
tendons are very strong, and the tendons are usually inserted
closer to the fulcrum than the weight or load to be raised ; as,
for example, in raising the forearm. Hence a large range of
movement is obtained with a small amount of contraction of
the muscle, to pay for which advantage the pull of the muscle
through its tendon must be many times greater than the weight
to be raised. If the biceps muscle were attached halfway out
along the radius instead of near the elbow joint, the amount of
contraction of its fibres in order to bring the forearm from the
extended to. the flexed position would be enormous, and, in
fact, impossible for muscle fibres as constituted in the body.
Hence the insertion is near the elbow joint, and a smaller and
correspondingly more powerful pull on the tendon accomplishes
the purpose.

Examples in the body of levers of the first order are the
movements of the head on the atlas, and of the trunk at the
hip joints.

A good example of the second is raising the body on
tiptoe ; here the toes and metatarsal bones lie at the fulcrum,
the weight is at the ankle joint, and the power is applied
through the tendons of the calf muscles at the back.

The third class is by far the commonest in the movements
of the body; it is seen in the movements at the jaw, in flexion
of the elbow and knee joints, and in many movements of the
other joints.

This classification of levers has been given here in
deference to established custom, but it should be remembered
that there is no essential difference between the different

^ The second and third orders interchange when it is equal to unity, for
then P and W are applied at the same point of the lever.

J 6 Elementary Physiology,

orders, and that the classification has no importance save
as a means of description. The essential feature in the lever
is that the force may be diminished if its leverage is in-
creased. But this involves increased movement, and although
for many mechanical purposes this may be advantageous, in
the body it is a decided disadvantage, and hence extent of
contraction is economized by increasing the applied force. It
should also be remembered that in the body the tendons do
not pull parallel to the weight in many cases, and hence the
pull on the tendons is still further increased. It follows that
the tendons of muscles must be exceedingly tough and strong
structures, and this is actually the case, the larger tendons
being capable of standing without breaking a pull many times
greater than the weight of the body.

The erect position of the body can only be maintained
when the vertical line through the centre of gravity falls within
an area drawn to include the soles of the feet ; but this is not
all that is necessary to the maintenance of the erect posture.
The joints must be stiffened by a balancing of forces due to
a definite amount of tonic contraction ^ in the muscles situated
before and behind them. This balancing of forces is learnt
in infancy, and afterwards is always maintained, when we
assume the erect position, without conscious effort or attention.
The activity of the nerve centres is necessary for the main-
tenance of the erect position; hence a shock to the nervous
system, say by a blow on the head, or a sudden fright, causes
the person to fall down in a heap. The nervous impulses
which kept the various sets of muscles tonically contracted
are stopped, the muscles relax, the body is in a position of
unstable equilibrium, bending at the joints takes place, and
the person falls to the ground.

The same thing is seen in nodding of the head as a person
falls asleep in a sitting posture ; the muscles at the front and

^ The term "tonic contraction " means a certain amount of passive or
continuous contraction that a muscle is kept in under given conditions. A
muscle, even when not undergoing active contraction, is never completely
relaxed, but the tendon is kept taut and ready for action, so that there is no
slack in the tendon. The same is true of involuntary fibres surrounding
blood-vessels, etc., which are always in a more or less contracted condition.

The Muscular System.

77

U

back of the neck, the tonic contractions of which had previously
balanced one another, become relaxed, and the head usually
falls forward, because the mass of the head in the normal
sitting position lies more in front of the articulation between
skull and atlas than behind it. Sound sleep
in a standing position, or walking during
sound sleep, are impossible because of this
relaxation of the muscles through inhibition
of the tonic constricting nerve impulses to
them. In cases where people walk or talk
in their sleep the nervous system is not at
rest to the normal amount of sound sleep.
The same is true in the case of dreaming ;
portions of the brain which ought to be in
a resting condition are active. Even in
the soundest sleep, however, there is not
complete quiescence of the entire nervous
system; the respiration must be kept up
throughout by the activity of certain nerve
cells, situated in a part of the central ner-
vous system known as the medulla oblon-
gata (see p. 223), which rhythmically send
out impulses to the respiratory muscles and
cause these to contract. The rhythm of
the heart is also probably regulated during
sleep by the nerves which pass to it ; but,
generally speaking, nervous and muscular
activity are reduced to a minimum, in order
to allow these two tissues, which are con-
tinually in action during the waking hours,
to become refreshed and recuperated by
the nutrient blood-stream, which carries
them fresh supplies and removes their waste
products.

The muscles which by their balanced
tonicity support the body in the standing
position are diagrammatically shown in Fig. 51. The muscles
in certain positions are aided by ligaments which come on

Fig. 51. — Diagram showing
the action of the chief
muscles which keep the
body erect.

The arrows show the direc-
tion in which the muscles
pull. Those in front op-
pose and balance those
at the back.

78 Elementary Physiology.

the stretch in the standing position. Thus, at the knee, the
bones are completely extended by the extensor muscles of the
thigh, and motion forward is prevented by the ligaments of
knee joint.

The reader can easily analyze roughly those muscular
movements which take place in the acts of walking and running.
When a person starts to walk from a standing posture the
weight is first thrown on one foot by a slight movement of the
body to that side, then the other foot is raised from the ground
and carried forward by a flexion in front at the hip joint and
a slight flexion at the knee. At the same time, the other foot
is raised on tiptoe by the contraction of the powerful muscles
of the calf of the leg, and the whole body is swung forward
by a movement at the ankle and hip joints. As this goes on
the foot which was raised and carried forward comes on the
ground, and the weight is gradually received on it as the body
is thrust forward. The other leg is next brought forward by
a flexion at hip and knee, and swung out in front to commence
another step.

In running, the muscular contractions are more vigorous,
and for a brief period during each stride both feet are off the
ground. The body is thrown forward just before each foot
leaves the ground by a sudden extension of the leg at the hip
and knee. In jumping a similar sudden extension, but of both
legs at once, is the means of progression

CHAPTER IV.

POSITION OF THE VISCERA.

The great cavity of the trunk is 'divided into two compartments
by the diaphragm. The upper compartment is called the
thoracic cavity, or thoi'ax^ and the lower is the abdominal
cavity, or abdo7Jien. The organs contained in the thorax are
termed the thoracic viscera^ and those in the abdomen the
abdominal viscera.

The Thoracic Viscera.^

The thorax contains the heart and the great blood-vessels
passing to and from it; the lungs and the branches of the
trachea^ or windpipe, called the hwichi., which convey the air
to them ; the remnants of a gland called the thymus., which is
relatively large in the infant but becomes gradually insignificant
in size towards middle life ; part of the (esophagus., which is a
straight muscular tube serving to convey the food from the
mouth to the stomach ; the thoracic duct., which is a vessel for
conveying the lymph (see p. 65) to the position where it joins
the blood-stream by opening into a vein in the neck ; and
various nerves passing either to these organs or on to the
diaphragm and the abdominal viscera.

The heat't is shaped like a cone with rounded apex and
base, and is about the same size as the closed fist. It lies in
the anterior part of the thorax a little more to the left of the
median line than to the right (see Fig. 53). The apex lies
opposite to a point about an inch and a half below the left

' The student is recommended to accompany this description with the
dissection of an animal.

8o

Elementary Physiology

nipple, and three inches from the middle line. Here the apex of
the heart touches the thoracic wall as it is thrown forward by
each beat, and these beats may be distinctly felt, and in some
cases seen through the skin at this point {the apex beat).
Feeling the heart beat here has given rise to a popular
impression that the heart lies much more to the left side than

Fig. 52. — The lower half of the thorax, with four lumbar vertebrae, showing the
diaphragm from before. (Allen Thomson, after Luschka.) \
a, sixth dorsal vertebra ; b, fourth lumbar vertebra ; c, ensiform process ; d, </, aorta,
passing through its opening in the diaphragm ; e, oesophagus ; y, opening in the
tendon of the diaphragm for the inferior vena cava ; i, central, 2, right, and 3, left
division of the trefoil tendon of the diaphragm ; 4, right, and 5, left costal part,
ascending from the ribs to the margins of the tendon; 6, right, and 7, left crus ;
8, to 8, on the right side, the sixth, seventh, and eighth internal intercostal muscles,
deficient towards the vertebral column, where in the two upper spaces the levatores
costarum and the external intercostal muscles 9, 9, are seen ; 10, 10, on the left side,
subcostal muscles.

is really the case ; for from this point the heart is directed
upwards, and to the right, the base lying above and to the
right, so that a small part of the heart lies to the right side of
the sternum as shown in Fig. 53. The great vessels which

Position of the Viscera. 8i

carry the blood to and from the four chambers of the heart all
enter and leave it in a group at the base (see Fig. 54, p. 83).
k. large vein called the infenor vena cava carries the greater
part of the blood returning to the heart from the lower part of
the body. This vessel penetrates the diaphragm at the back
near the vertebral column (see/, Fig. 52), and passes up parallel

Fjg. 53. — Transverse section through the thorax.
The section is carried above the heart, but below the division of the trachea.
I, sternum; 2, body of dorsal vertebra; 3, spinous Drocess; 4, spinal canal; 5, rib;
6, inner layer of pleura ; 7, outer layer of pleura ; 8, pericardium ; 9, right bronchus ;
10, left bronchus; 11, oesophagus; 12, heart; 17, aorta, ascending; 14, aorta,
descending; 15, left lung; 16, right lung; 17, pulmonary arteries.

and close to the vertebral column in the thorax to enter the
thin-walled right upper chamber of the heart {the right auricle).
The blood coming from the head, neck, and arms is collected
into another great vein (the supenor vena cava), and by this is
also discharged into the right auricle. From the right auricle
the blood flows into the right ventricle, being assisted in its
flow towards the end when the ventricle is nearly full by the
contraction of the auricle, which distends the ventricle.^ The

' For a description of the heart and its valves, see the anatomy of the
circulatory system, p. 1 13; an outline only is given here to enable the
student to understand the arrangement of the great vessels in the thorax.

G

82 Elementayy Physiology.

ventricle next contracts, and the communication between
auricle and ventricle being closed by a valve opening towards
the ventricle, and a communication being opened between the
ventricle and a large artery in connection with it called the
pulmonary artery by the forcing open of a valve directed
towards this artery, the blood is discharged under pressure from
the right ventricle into the pulmonary artery. The pulmonary
artery passes upwards from the heart and soon divides (see
Fig. 52) into two branches, one of which passes to each lung
and enters at the root, which is situated on the mesial
surface of the lung nearer the apex than the base (see Fig. 53).
After circulating through the lung, and undergoing certain
changes there which will subsequently be described (see p. 173),
the blood is collected into the great pulmonary veins which
carry it back to the base of the heart, and discharge it into the
left auricle. By a similar mechanical arrangement to that on
the right side, the blood passes from the left auricle to the left
ventricle, and from the left ventricle to the aorta.

The aorta is the great arterial trunk which carries the blood
from the heart to distribute it by means of its branches to the
entire system. This great vessel leaves the heart near the
middle of the base, and at first passes (see Fig. 54) upwards
and to the right (the ascending aorta) ^ but soon takes a sharp
bend to the left and turns downward (the arch of the aorta).
In its descending course the aorta {descending aorta) lies close
to the vertebral column and a little to the left of it. From
the convexity of the arch of the aorta there arise the large
arteries which carry the blood to the head, neck, and arms
(see Fig. 54), viz. the innonmiate artery (dividing into the right
subclavian and right common carotid) for the supply of the right
side, and the left common carotid and left subclavian arteries
for the left side. From the descending thoracic aorta various
branches are given off, the chief of which are those to the
tissue of the lungs called the bronchial arteries, and the inter-
costal arteries which pass outwards on each side under each pair
of ribs to supply the intercostal muscles. The aorta leaves the
thorax at the back of the diaphragm (see Fig. 52), and passes
down the abdomen as the abdominal aorta, lying immediately in

Fig. 54. — General view of the heart and great blood-vessels of the trunk.

A, right auricle ; £, left auricle ; C, right ventricle ; D, margin of left ventricle ; E, ribs ;
J^, kidneys ; i, arch of the aorta ; 2, descending aorta ; 3 and 4, right and left carotid
arteries ; 5 and 6, right and left subclavian arteries ; 7, arteries supplying the lower
extremities; 8, pulmonary artery ; 9, vena cava superior; 10 and ir, right and left
subclavian veins ; 12 and 13, right and left jugular veins ; 14, vtna ca\a inferior ;
15 and 16, veins which collect blood from the lower extremities.

84 Elementary Physiology.

front of the vertebral column. Here it gives off large branches
to supply blood to the stomach, spleen, liver, intestines, kidneys,
and other abdominal viscera, and finally bifurcates into two
large arteries called the common iliacs, each of which later
divides into two branches, viz. the internal iliac^ for the supply
of the viscera in the lower part of the abdomen {the pelvis); and
the external iliac^ which furnishes the chief blood-supply to the
leg.

The heart, and the roots of the great blood-vessels arising
from it, are surrounded by a strong fibrous bag called the
pericardium^ which is attached below to the diaphragm and
above to the great vessels. The inner surface of this bag is
lined by a smooth membrane {serous membrane). This serous
membrane is reflected above over the roots of the great vessels,
and is continued all over the outer surface of the heart. There
are thus two smooth surfaces opposed to each other, the inner
lining of the pericardium and the outer lining of the heart, and
between these surfaces friction is reduced to a minimum, as
the heart beats within the pericardium. The amount of
friction is still further reduced by a fluid {pericardial fluid) ^
which is secreted by this inner serous coat of the pericardium
and moistens the surface. Besides this function of protecting
the heart from friction as it beats, the pericardium is a strong
envelope capable of protecting the thin-walled auricles from
over distension and injury by any transient engorgement of the
heart with blood.

The mouth, or hiccal cavity., opens posteriorly into a funnel-
shaped tube (the pharynx) lined by muscular walls, and this
tube narrows as it descends in the neck into the oesophagus., or
gullet (see Fig. 91, p. 182). The oesophagus is also a tube with
muscular walls which is collapsed unless when food is passing
along it (see p. 182), and in the neck lies behind the trachea, or
windpipe. Both these tubes enter the thorax at its apex, but
while the trachea soon bifurcates into two branches called the
bronchi, one of which passes to each lung, the oesophagus
passes straight through the thorax, pierces the diaphragm
immediately in front of the aorta, and widens out into the
stomach soon after entering the abdomen.

Position of tJie Viscera. 85

The thymus is a small gland resembling in its structure a
lymphatic gland, which lies at the apex of the thoracic cavity,
surrounding the trachea and in front of it. It is scarcely
recognizable as a gland after middle life.

Practically the whole of the remaining volume of the
thoracic cavity is occupied by the two lungs, and this fraction
of the space greatly exceeds the rest The lungs are moulded
to the shape of the thoracic cavity. The diaphragm beneath,
forming the floor of the thoracic cavity, is convex towards the
thorax, and the bases of the lungs are correspondingly concave.
The diaphragm is placed somewhat lower on the left side, and
the left lung is in consequence longer than the right, but it is
narrower, and more room is notched out of it than out of the
right to accommodate the heart, so that its volume with an equal
amount of distension is slightly less than that of the right lung.
The outer surface of each lung is convex, to fit the concavity of
the thoracic wall. These outer surfaces show in each case a
deep cleft or fissure, which begins near the apex at the posterior
border and passes obliquely forward and downward to end
near the front of the lower margin, thus dividing each lung
into two lobes. The right lung, further, has a cleft running
horizontally forward from the middle of the oblique one, so
that it is divided into three lobes. The inner or apposed
surfaces of the two lungs lie against each other except where
they are separated by the heart, great vessels, and other
structures mentioned above ; each inner surface is indented to
accommodate these structures. The apices of the lungs are
somewhat dome-shaped, and extend up to the neck on each
side, completely fiUing up the apex of the thorax. Each lung
in a healthy condition is unattached save at the root on the
inner surface, where the bronchus, pulmonary arteries and
veins, and bronchial arteries and veins,^ as well as certain
nerves, enter and leave it.

Each lung is further enclosed in a double-walled bag or

' The lung tissue cannot be nourished by the venous blood commg to
it from the heart, but must, like the other tissues of the body, be supplied
with arterial blood ; this is conveyed by special arterial branches given off
from the aorta, which are much smaller than the pulmonary vessels, and
are called the bronchial arteries.

S6 Elementary Physiology.

sac, called the pleura. One layer of this pleura is closely
applied to the inner wall of the thorax, and at the root of the
lung this layer is reflected over the outer surface of the lung,
forming a second layer which is applied to the lung and closely
invests it. These layers of the pleura are exceedingly thin,
and during life there is practically no space between them,
the lungs distending with each inspiration and filling the whole
space. But if an opening of any considerable size be made in
the wall of the thorax, the outer layer adhering closely to the
thoracic wall is cut through in the process, the elasticity of the
distended lung comes into play, the lung shrinks and air is
drawn in between the folds of the pleura. The rhythmical
enlargement of the thorax by the action of the respiratory
muscles is now incompetent to draw air into the lungs.
Instead of the air entering and leaving the lungs, it merely
passes in and out through the artificial opening in the thoracic
wall, and the animal soon suffocates and dies unless some
means be taken to blow air in and out of the lungs. Although
the lungs throughout life, being always less or more distended,
have ever this tendency to shrink in volume and leave the
thoracic wall, they cannot do so, because a vacuum would be
thereby formed^ The thorax is an air-tight cavity, and as
its volume is rhythmically altered in respiration, air must
alternately pass into and out of the lungs. Hence, in a normal
condition of the animal, during life there is a potential space
only between the two layers of each pleura. The pleura is hence
a thin double-walled closed sac, both layers of which lie in
contact with each other surrounding the lung and continuous
round its root. A very small amount of fluid {pleiiritic fltdd) is
normally secreted between the two layers of the pleura and
acts as a lubricant ; but in diseased conditions of the pleura
(such as pleurisy) the amount of this fluid may become
enormously increased, and the two layers of the pleura be
widely separated by it. " Since the volume of the thorax is
limited, any large accumulation of fluid in such a position
becomes dangerous, because the lungs collapse to a corre-
sponding extent. Finally, the animal dies, if the condition be
not relieved, by siffocation or asphyxia ; for it is evident that a

Position of the Viscera. Sy

point can be reached when it is impossible to suck air into the
hmgs by distension of the thorax in presence of such an
accumulation of fluid between the layers of the pleura.

The Abdominal Viscera.

The viscera contained in the abdominal cavity include
the lower and by far the larger portion of the alimentary
canal, the liver, the spleen, the pancreas, the suprarenal
bodies, the kidneys, and the bladder. In addition to these
there are several large lymphatic glands ; certain folds of
connective tissue which suspend or attach the viscera in their
various positions, and carry the blood-vessels which supply
them with blood ; the abdominal aorta and inferior vena
cava, passing along parallel and close to the vertebral column
behind ; certain nerve centres or ganglia and nerves passing to
and from them for the innervation of the viscera ; and, in
the female abdomen, the internal generative organs, consisting
of the uterus or womb, with its appendages, and the ovaries.

These soft viscera are not so thoroughly protected by a complete bony
frame\York as are those of the thoracic cavity, nor is this necessary in the
case of the abdomen. Strong broad sheets of muscle passing from the
margin of the bony pelvis below to the lower ribs above form a sufficient
protection. There is no need to rhythmically change the capacity of the
abdominal capacity as there is in the case of the thorax, for there is here no
respiratory organ, like a lung, into and out of which air must alternately
pass. Further, a complete bony framework surrounding the abdomen
would impede the action of the diaphragm by fixing the position of the
abdominal viscera. When the diaphragm contracts, it forces down the
contents of the abdomen, thus enlarging the volume of the thorax and
sucking air into the lungs ; the abdominal muscles in front relax, and
allow room for the abdominal viscera by an increase in the dimensions of
the abdomen from front to back. Thus the sheets of abdominal muscles
furnish just the needed kind of support to the abdominal viscera, yielding,
yet firm, and suited to the other requirements of the case. Although the
volume of the abdominal cavity does not vary with a quick rhythm, like
that of the thorax, yet it does vary very considerably from time to time,
being distended somewhat after each meal, on account of the increased
volume of stomach and intestine. Here, again, the muscular walls are
suited admirably for plapng their required part ; by relaxing they can yield
the increased volume, and by an increased tonicity they can become more con-
tracted, and give a uniform degree of support when the volume diminishes.

88

Elementary Physiology.

The inner surface of the abdomen, including the under
surface of the diaphragm, is Uned by a thin smooth dehcate

Fig. 55. — The viscera of the thorax and abdomen, viewed from the front.
X, ribs, the front portions of which, together with the sternum, have been removed ;
2, bones of the pelvis ; 3, diaphragm ; 4, thorax ; 5, abdomen ; 6, right lung ; 7, left
lung ; 8, heart; 9, stomach; 10, right lobe of the liver ; 11, left lobe of the liver;
12, spleen; 13, pancreas; 14, small intestine; 15, large intestine; 16, bladder.

Position of the Viscera. CS9

serous membrane called the peritoneum. This smooth mem-
brane is further reflected over both surfaces of the stronger
membranes which suspend the organs (when these do not lie
on the wall of the cavity), and over the organs themselves, thus
forming a very complete smooth investment for all the organs.
A small amount of fluid is secreted, which wets the surface of
the peritoneum, and has that same function as a lubricant
which has been pointed out in the case of several other similar
secretions.

On removing the abdominal wall in front, the lower part of
the liver is seen above and to the right side. But the greater part
of this organ lies beneath the lower ribs, being moulded to the
concavity of the diaphragm, against which its upper dome-
shaped surface fits, and from which, and the abdominal wall
behind, it is suspended by strong folds of connective tissue
coated with peritoneum, termed ligaments. It is divided by a
fissure into a right and left lobe, of which the right is the
larger. On the right side additional fissures further separate
the right lobe into smaller lobes, called the quadrate.^ Spigelian,
and candate lobes. The gall bladder, which temporarily receives
the bile secreted by the liver before it is poured out into the
intestine, is situated in a depression on the under surface of
the right lobe (see Fig. 81, p. 158).

The stomach lies lower down and to the left of the liver. Its
exact position and the extent to which it is visible on opening
the abdominal cavity, depend upon the amount to which it is
distended by food, but the greater part of it is usually con-
cealed by the ribs and the liver. Its upper surface touches the
diaphragm, and it is to some extent suspended from this
structure by the oesophagus. It is further held in position
by ligamentous bands called omenta^ which connect it with
the liver, intestine, and spleen. The stomach is an enlarged
portion of the alimentary canal, which holds the food for a
time after a meal, until it can gradually be passed into and
along the lower part of that canal, called the intestine, or
boiuel. During this stay in the stomach part of the food is
acted on chemically by a fluid secreted by certain glands
imbedded in the inner layer of its walls (see p. 146). After a

90 Elementary Physiology.

time the food is forced on, out of the stomach and into the
intestine, by the contraction of muscular fibres which lie in the
stomach walls external to this glandular layer. The junction
of oesophagus with stomach, called the cardia, is kept closed
unless when food is passing in, by a ring of muscle called a
sphincte7'. A similar sphincter guards the passage from stomach
to intestine {pylorus)^ and the food does not pass continually
from stomach to intestine, but only at intervals, when the
pyloric sphincter is relaxed and the muscular coats of the
stomach contract on the contained food.

A double fold of connective tissue coated with peritoneum, called the
great oment7im, hangs down like an apron from the lower surface of the
stomach, and usually conceals the greater part of the intestine. The front
fold of this loop turns backwards underneath, and is continued as a back
fold, which passes up and is attached to the transverse colon [vide infra).
On removal of the great omentum, the arrangement of the parts of the
intestine can be more clearly seen.

The intestine, by differences in structure and arrangement,
is divided into two distinct parts, the small and the large
intestine; but for descriptive purposes the small intestine is
further divided into three parts, termed the duodenum, jepmum,
and ileum respectively, which are structurally much alike ; and
the large intestine is similarly divided into ascending colon,
transverse colon, descending colon, sigmoid flexure, and rectum.

The small intestine is much the longer portion of the
alimentary canal, and measures in man, on the average, about
twenty feet. The first ten to twelve inches of its length form
a (J-sbaped loop with the stomach, in the bend of which an
important gland called the pancreas lies. This portion is
called the duodenu,m. The upper two-fifths of the small intes-
tine is called the jejunum, and the lower three-fifths ileum ;
but there is no demarcation between them, and the names are
merely used to specify different parts of the length of the small
intestine.

The small intestine is attached to the abdominal wall at
the back by a strong sheet of ligament, covered on both
its surfaces by peritoneal membrane, and called the mesentery.
This mesentery is a strong, but thin and transparent, membrane,

Position of the Viscera. 91

which is attached by one border to the abdominal wall, down
the mid line behind, and broadens out in a fan-shaped fashion
to be attached to the whole length of the small intestine along
its opposite border in front. It carries, between its folds, the
vessels conveying the blood to and from the intestine, as well
as the lymphatic vessels which arise in the walls of the intes-
tine by minute lymphatic capillaries, and unite to form large
lymphatic vessels in the mesentery.^ The arteries run out like
rays in a divergent fashion along the mesentery towards the
intestine, and on nearing it join together and form arches,
from which smaller branches pass inwards to feed the intes-
tinal wall with blood. The returning veins follow the course
of the arteries, and unite to form large veins which lie alongside
the arteries. At its lower end the small intestine opens into
the large intestine by an orifice which is closed by a valve
{ileo-ccecal valve)., formed by two folds arising from the inner part
of the wall. This valve permits the intestinal contents to pass
readily from the small to the large intestine, but bars all
motion in the opposite direction. The entrance of the small
into the large intestine is not placed quite at the beginning of
the large intestine, but at one side near the beginning. That
is to say, the small and large intestine are not joined up end to
end, but the small opens into the large intestine laterally at a
small distance from the end of the large intestine. The large
intestine has hence a blind end or sac, which is termed the
c(Bcic7n.^ and this is further prolonged into a small finger-like
projection of much smaller bore, called the vermiform appendix.
The length and capacity of the caecum vary greatly in different
animals ; in the herbivora (for example, in the rabbit) it is
very capacious, while in the carnivora (such as the cat and dog)
it is rudimentary. It is also very small in man.

The small intestine lies in coils in the abdominal cavity
below the liver and stomach. It occupies the greater part of
the space, and is surrounded by the large intestine, which it
joins in the lower part of the abdomen on the right side (see

^ These mesenteric lymphatics are often termed lacteals, because they
contain a milky white emulsion of fat when fat absorption is going on
(seep. 156).

92 Elementary Physiology.

Fig. 55). The large intestifte, or coloji, has in man an average
length of five to six feet. It passes upwards on the right side
of the abdomen as the ascendmg colon. It then arches over to
the left in a horizontal portion known as the transverse colo7i,
which is seen in front (see Fig. 55) after removal of the great
omentum, lying between the stomach and the folds of the small
intestine. It next bends backward and downward, and passes
down posteriorly to the small intestine as the descending colon
on the left side of the abdominal cavity. The descending
colon bends towards the middle line in the lower and posterior
part of ths abdominal cavity, and this bent portion is termed
the sig7noid flexitre. Finally, the sigmoid flexure leads to a
straight tube with strongly developed muscular walls called the
rectum, which ends in the external opening of the intestine
known as the amis.

The large intestine is fixed in position by folds of
peritoneum which surround it. The transverse colon is less
closely fixed in this fashion than the other parts, being loosely
attached to the abdominal wall at the back by a long fold of
peritoneum {the transverse vieso-colon). On the other hand, the
peritoneum does not usually completely encircle the ascending
or descending colon, but passes over them in a single sheet,
which touches the intestine only for about two-thirds of its
circumference, and is then reflected on to the abdominal wall,
so firmly fixing the intestine in position.

The spleen cannot be seen from the front unless by dis-
placing the stomach, as it is deeply placed behind and to the
leftside of that organ in the left iLpper part of the abdominal cavity.

The spleen is an elongated dark red coloured body filled with blood,
which enters it along one side (which is concave and lies against the
stomach) by a number of fairly large-sized arteries. These arteries sub-
divide within the substance of the spleen and finally give rise to a mesh-
work of capillaries which open out into spaces or sinuses in the tissue of
the organ without any definite lining or wall. The blood is collected up
again from these sinuses by capillaries leading to small veins which unite to
form the splenic veins, and these issue from the spleen alongside the
arteries. The function of the spleen is not clearly known. It is not
essential to life, for animals continue to live in good health after it has been
completely removed. It has the property of rhythmically contracting at
intervals of about once a minute. These contractions are brought about

Position of tJie Viscera. 93

by strands of involuntary muscle fibre which pass inward along septa of
connective tissue called trabeada:. The trabeculoe arise from the outer
sheath of the organ and pass inward, forming a framework from which
lesser strands of connective tissue arise so as to make a network. The
cells of the spleen are contained in this network as well as the capillaries
and sinuses above mentioned. It is supposed that the spleen possesses the
power of destroying the effete or ivorn-oiit red corpuscles of the Mood (see
p. 121), and some observers claim to have observed microscopically such a
process of destruction going on in the spleen cells ; still, the evidence on
this point is not very convincing. The anterior surface of the spleen at
which the blood-vessels enter and leave is concave, and attached to the
stonidich. by the gas tro-sJ)/emc omeutum, a strong ligamentous band, coated
over like everything else in the abdominal cavity by a reflexion of the
peritoneal lining. The outer or posterior surface of the spleen is convex
and unattached, it touches the diaphragm by its upper border, and lies
obliquely against the posterior abdominal wall. The upper end is placed
close to the left suprarenal body {vide infra) near the spine, and from this
point the organ lies outward and downwards, following the course of the
diaphragm for about halfway round the side.

The kidneys are two oval or bean-shaped bodies of a deep
red colour, each about four inches long, two and a half inches
in breadth, and rather more than an inch thick, which lie at
the back of the abdomen on each side of the mid-line, opposite
the last dorsal and upper two or three lumbar vertebra. Their
position is slightly oblique, the upper end being nearer to the
spine than the lower, and the left is placed slightly higher than
the right kidney. The external position opposite to which the
kidneys lie is immediately beneath the last rib at the back,
just on each side of the vertebral column. The kidneys are
embedded in a mass of fat, and lie behind the peritoneal lining
of the abdominal cavity, which is reflected over them in front
as they bulge out into the abdominal cavity. The blood-
vessels enter at the inner border where there is a depression
in the surface called the hilnni ; here also the duct, or ureter^
which conveys away the excreted urine, leaves the kidney.
The ureters are two long slender tubes, one on each side,
which leave the kidneys and pass to the bladder, into which
they continuously pour the urine as it is secreted by the
kidneys. They pass through the bladder wall very obliquely,
and this produces a valve-like effect. For when the pressure
is from the ureter to the bladder, the ureters remain open ; but

94 Elementary Physiology.

when the pressure inside the bladder is increased, as is the
case when it is being emptied, the increased pressure forces
against each other the layers of the bladder wall, and so the
mouths of the ureters, where these pass through obliquely, are
closed, and no passage of urine from the bladder up the ureters
can take place.

The urinary bladder is a distensible bag which serves to
collect the urine between the periods of its discharge from the
body. It lies in the mid-line at the lower or pelvic part of
the abdomen, beneath the arch formed by the pubic bones, but
may when distended appear above them. When distended it
has a rounded or egg-shaped form, the broader end being
towards the base, or fundus, which rests against the rectum
behind. There are three openings to the bladder, of which
two, those of the ureters, have already been described ; these
enter posteriorly at the upper part of the base on each side.
The other opening is the urethra^ by which the urine leaves the
bladder.

The urethra is a tube with muscular walls, which leaves the
bladder at its lower part or neck. It is kept closed, except
when urine is being discharged from the bladder by a muscular
ring or sphincter placed where it leaves the bladder.

The suprarenal bodies are two small yellow coloured glands
situated one immediately above each kidney and close to the
middle line. These bodies, though inconspicuous in size, are
of vital importance in the body, for it is impossible to remove
them without causing death. They are glandular in structure,
and although they possess no ducts, it is probable that the
cells secrete material which is poured directly into the blood-
stream. A diseased condition of these bodies is associated
with a peculiar and fatal malady (Addison's disease), which
is, however, comparatively rare. A characteristic sign of this
disease is a remarkable bronzing of the skin in patches ; this is
accompanied by muscular weakness, and a fall of the pressure
of the blood in the arteries. It has recently been discovered
that extracts of these glands in water, when injected into the
veins of other animals, possess the property of greatly raising
the pressure of the blood in the arteries, by constricting the

Position of the Viscera. 95

small arterioles all over the body, and hence preventing so
rapid an escape of the blood through these, as it is pumped
into the arteries from the heart. It has hence been suggested
that the suprarenal bodies furnish to the blood an iiiter7ial
secretion which is essential for the maintenance of the tonicity
of the muscle fibres surrounding the arterioles.

Another important ductless gland which may be con-
veniently mentioned here, although it is not contained in the
abdomen, is the thyroid gland, which is situated in the neck, on
each side of the trachea, near the thyroid cartilage. Complete
removal of this gland in man, in cases where it has been
diseased, has been found to cause death, and a similar result
has been obtained in the case of certain classes of animals.
The symptoms most closely resemble those seen in man, if
monkeys are used for the experiment. In other animals, such
as the dog, death occurs before certain of the symptoms have
time to become manifest. The most remarkable symptom is
a swollen condition of the connective tissue beneath the skin,
producing a kind of artificial cretinism, which is known as
operative myxoidema, this is accompanied by symptoms of a
disturbance of the central nervous system in the form of
tremors, spasms, and convulsions. These disturbances increase
with the lapse of time, and finally lead to death. The condition
is prevented or palliated by grafting of fresh thyroid tissue
under the skin, or by feeding with fresh thyroid glands obtained
from other animals. Much service has been done, by the
knowledge thus acquired, in practical medicine by feeding
patients similarly affected on the fresh glands of the sheep
and other animals.

The good effects obtained by grafting the thyroid and by
thyroid feeding, show that the thyroid secretes some substance
which has a beneficial action in the body, and negative the
theory^ which has been proposed that the purpose of the
thyroid and similar glands is to remove from the blood certain
noxious substances which tend to accumulate therein.

^ This theory is termed the aiito-intoxication theory, in contra-
distinction to the other, which is known as the theory of internal
secretion.

CHAPTER V.

THE CIRCULATORY SYSTEM.

The circulation of the blood, carrying nutriment to all the
tissues of the body and removing their waste products, is
maintained by means of the heart and the system of tubes
connected with it known as the blood-vessels.

Those vessels which carry the blood away from the heart
are called arteries.^ and those which carry it back to the heart
are called veins. Between these two systems of vessels there
is communication in the tissues by an immense number of
minute vessels, called capillaries., which have exceedingly thin
walls. It is here, in the capillaries, that the real work of the
blood is done ; through the delicate walls of the capillaries,
free interchange of nutrient materials and waste products takes
place ; while the larger vessels (arteries and veins) have thick
impermeable walls, and merely serve the purpose of conveying
and distributing the blood to the capillaries. The entire
blood-vascular system, consisting of heart, arteries, capillaries,
and veins, is completely lined internally by an exceedingly
thin layer of flat pavement cells, which touch each other, and
dovetail into each other by their thin edges. These flat cells
are elongated in form, and each possesses a nucleus (see Figs.
56 and 59). In the case of the capillaries, they form the entire
thickness of the wall, and hence there is free diffusion of sub-
stances in solution between the lymph (bathing the tissue and
its cells) outside the capillary, and the blood inside the
capillary. Also, at the cell junctions there are minute apertures
through which the white cells (leucocytes) present in the blood
can pass from the capillary into the lymph spaces without.
The capillaries branch and anastomose freely with one another,

TJie Circulatory System.

97

and unite at their ends to form arterioles and venules, the
arterioles being next the arteries, and the venules next the

Fig s6.-A small artery, A, and vein, J", from the subcutaneous connective tissue of the

rat. treated with nitrate of silver. (,175 diameters.;
^ n endothelial cells with b, b, their nuclei ; m, m, transverse markings due to staining
' orsubstance' between the muscular fibre cells ; c, c, nuclei of connective-tissue
corpuscles attached to exterior of vessel.

veins. In these small arteries and veins there are disposed
outside the epitheloid coat (endothelium) layers of involuntary
muscle fibres, which are arranged in a circular and slightly

H

98

Elementary Physiology.

spiral fashion around the tiny vessel. These are found at the
places nearer the capillary as isolated cells and incomplete
layers ; but as the distance from the capillary increases, there
is first formed a complete layer of the involuntary muscle
fibres, and at points still more remote the muscle cells are
several layers thick. ^ At the same time, a fine layer of elastic
fibres forming an elastic membrane is found between the inner
lining of pavement cells and the layers of muscle fibres. The
muscular coat is much thicker in the small arteries than in the
small veins, and the elastic coat outside the endothelium is
also much more strongly developed. As the arteries and veins
increase in bore their walls also increase in thickness, and

Fig. 57. —Transverse section of part of the wall of the posterior tibial artery.

(75 diameters.)

a, epithelial and subepithelial layers of inner coat ; b, elastic layer (fenestrated mem-
brane) of inner coat, appearing as a bright line in section ; c, muscular layer (middle
coat) ; d, outer coat, consisting of connective-tissue bundles. In the interstices of
the bundles are some connective-tissue nuclei, and, especially near the muscular coat,
a number of elastic fibres cut across.

there appears in addition an external coat {areolar coat or
timica adventitia) composed of connective-tissue fibres. Many
of these fibres are elastic ; especially in the arteries which require
more elastic distensibility than the veins. It is this external
coat which gives their great strength to the large vessels, and
especially to the arteries in which it is well developed. In the
very largest arteries, such as the aorta and its pfimary branches,
the middle or muscular coat and the outer coat become to a
certain extent blended so that there is an admixture of elastic
and muscular fibres.

In a typical medium-sized artery (see Fig. 57) the wall is

^ The purpose of this muscular coat in the arterioles, and the manner in
which it regulates the supply of blood to a given part, have already been
described (see pp. 57, 60).

TJie Ciradatory System.

99

usually described as consisting of three coats, namely, an imier
or elastic coat {tunica intima), consisting of the innermost pave-
ment layer or endothelium and the elastic membrane \ a middle

Fig. 58.— Transverse section of part of the wall of one of the posterior tibial

veins (man),
a, epithelial and subepithelial layers of inner coat ; b, elastic layers of inner coat :
6-, middle coat consisting of irregular layers of muscular tissue, alternating with con-
nective tissue, and passing somewhat gradually into the outer connective tissue and
elastic coat, d.

or muscular coat (tunica media), consisting of concentric non-
striated muscle cells ; and an external or areolar coat {tunica
cxtima or tunica adventitia). The larger veins
very closely resemble the arteries in structure,
but their walls are much thinner, as they con-
tain both less muscular and less elastic tissue
(compare Figs. 57, 58).

On account of the large amount of elastic
tissue which the arterial walls contain, they are
capable of being distended when the pressure
within them is increased, and of regaining
their original bore when this pressure de-
creases again. The veins, on the other hand,
are not so distensible by pressure. The united
cross section of the large veins is about double
that of the correspondingly large arteries, and f
they are never quite distended by blood under [ fj

normal conditions.

The heart is a double force-pump contain- fig. 59. — Epithelial
ing four chambers, two of which are placed on tSklr tib"S'a?te°y'
the right side, and together constitute what is ^'^^° diameters.)
often termed the right heart, and two on the left side, forming
the left heart. This duplicate arrangement is necessary because

100

Elementary Physiology.

Fig. 6o.'

the entire course of the circulation forms two nearly complete
circuits, as shown in the accompanying diagram (Fig. 6i),

and a given quantity of blood
in a complete round comes
twice to the heart.

The upper chamber on
each side is thin walled, and
incapable of exerting or with-
standing any great amount of
pressure. It is called the
auricle^ and its purpose is, by
a preliminary contraction,^ to
completely fill the lower cham-
ber or ventricle before this
contracts and drives the blood

'.°hyToYd"i^"e?;"t4oikme."rT"°' '"^ ^^e large artery leading

away from it.

The ventricle on each side is a chamber with very thick
muscular walls capable of exerting considerable pressure, when
it contracts, upon the blood contained within it. There is a
valve arranged between each ventricle and its corresponding
auricle which opens towards the ventricle, and another between
each ventricle and the large artery which issues from it, open-
ing towards the artery (see Figs. 65, (^6). On account of these
valves the blood can only move in one direction when the ventri-
cular wall contracts upon it, namely, from the ventricle into the
artery.

There is no valve placed between each auricle and the
great veins which enter it, in order to prevent the blood from
flowing, when the auricle contracts, from the auricle into these
great veins, instead of into the ventricle, because there is less
resistance in the direction of the ventricle than in the direction
of the veins, and the auricle never gets up sufficient pressure
under normal conditions to force the blood back into the veins
instead of into the ventricle. When the auricle contracts, the
blood is not under pressure in the ventricle, and the effect of

^ The contraction of either auricle or ventricle is known as its systole,
and its relaxed or uncontracted condition as its diastole.

The Circulatory System.

lOI

the auricular contraction is merely to slightly distend the
ventricle and to float the anriado-vcntricular valve ^ {i.e. valve

Fig. 6i. — Schematic diagram to illustrate the course of the circulation.
I, right auricle; 2, left auricle; 3, right ventricle; 4, left ventricle; 5, vena cava
superior ; 6, vena cava inferior ; 7, pulmonary' arteries ; 8, lungs ; g, pulmonary
veins; 10, aorta; 11, vessels of alimentarj' canal; 12, vessels of liver; 13, hepatic
arterj-; 14, portal vein ; 15, hepatic vein.

^ The auriculo -ventricular valve on the left side is called the viitral
valve, that on the right the tircnspid valve. The valves placed on the aorta
and pulmonary arteries, as they respectively issue from the left and right
ventricles, are called the aortic and pulmonary seniilimar valves.

102 Elementary Physiology.

between auricle and ventricle) up into a closed position. The
main work at each heart-beat thus comes on the thick-walled
ventricles which have to force the blood into the arteries, where
there exists a considerable pressure {ai'terial pressttre)^ the
cause and need of which will be presently considered.

Beginning at the point where the blood is poured into the
right auricle by the superior and inferior vence, cavce, the course
of a complete circulation may be indicated as follows (see Fig.
6i). The blood flows into the right auricle, and from this,
during the pause before a heart-beat, freely into the right
ventricle; further, an additional quantity is helped in before
the contraction (systole) of the right ventricle by the preced-
ing contraction (systole) of the right auricle. From the right
ventricle the blood is pumped by the systole into the pulmonary
artery, which branches and carries it to the lungs. Here the
blood is spread out in a thin layer by a vast meshwork of
capillaries, and is only separated from the air filling the air
spaces {air cells) of the lung by the thin walls of these capillaries.
While passing through these capillaries gaseous interchange
takes place between blood and air, in consequence of which
the blood loses carbon dioxide formed in the various tissues
by oxidation processes going on there, and takes up a
charge of oxygen for use in the tissues in the next round.
In consequence of these changes in the gases it holds, the
blood changes in colour from dark purple to bright scarlet, or,
as it is termed, is changed from venous to arterial blood.^
The blood is gathered up from the lung capillaries by veins,
which unite with one another to form larger venous trunks,
and leave the lungs as the pulmonary veins, which carry the
arterialized blood back to the heart. Here it enters the left
auricle, and passes, in a similar manner to that on the right
side, into the left ventricle. By the systole of the left auricle,
the left ventricle is completely filled; then this chamber
passes into systole, and the blood is driven under pressure into
the aorta. The aorta and its various branches distribute the
blood to all parts of the body, some passes to this organ and

^ The nature and mode of these changes in the blood will be more fully
discussed elsewhere (see pp. 173, ef seq.).

The Circulatory System, 103

some to that, and after circulating through the capillaries of
the part, each portion is collected up by veins, which gradually
unite and increase in size. The larger veins pour their contents
into the great venous trunks, the superior and inferior ve?ice
cavcE, and these empty the blood into the right auricle, so com-
pletmg the circuit.

The circuit between right auricle and left ventricle via the
lungs is termed the lesser or pulmonary circulation, and that
from left ventricle back to right auricle is known as the greater
or systemic circulation. These pumping operations by the two
force-pumps on the right and left side of the heart go on
simulta?ieo2isly . First, both auricles contract at the same
instant {auricular systole), then, immediately after, both ven-
tricles contract together {I'eutricular systole), and there follows
a pause {diastolic pause), during which all four chambers are
relaxed. This whole cycle of events occupies about y^ to ^
of a second, of which about y^ of a second is taken by the
auricular systole, y^ of a second by the ventricular systole, and
the remainder of yo to 3^ of a second by the diastolic pause.^

Certain sounds, known as the heart sounds, are heard when
the ear is applied to the chest of another person over the region
of the heart, or may be heard in one's self when lying quietly on
one side in bed. Two sounds are heard, one immediately after
the other, followed by a pause. The first sound is lower in
pitch and more prolonged than the second, which is short and
sharp like that produced by softly plucking a piece of linen
cloth, such as a pocket-handkerchief. The sounds have been
compared to those made in pronouncing the syllables, lubb,
dupp — lubb, dupp.

The first sound is produced partially by the contraction
of the muscular substance of the ventricular walls, and partially
by the rush of the blood from the ventricles into the arteries.
The second sound is made by the closure of the valves

^ The rate of the heart-beat varies under different conditions from time
to time, being increased by exercise, excitement, or fever, and lessened by
repose, assuming the recumbent position, or sleeping. It also varies with
age, being about 120 to 150 per minute in the new-born infant, 80 to 90 in
the child, 70 to 80 in the adult, and 50 to 60 in old age. It must be
understood, however, that the individual variations are very considerable.

I04 Elementary Physiology.

guarding the entrances of the aorta and pulmonary arteries.
For the pressure in the ventricles when their contraction is
over falls below that in the arteries, and these valves then
fill out and close together to keep the blood from rushing
back from the arteries into the ventricles again. In conse-
quence, the first heart sound occurs during the ventricular
systole, and the second at the commencement of the diastolic
pause, just as the ventricles relax.

These sounds are of great importance in practical medicine,
because they become altered in many diseased conditions of
the heart, and the character of the alteration gives a clue to the
nature of the disease. The physician listens to them by means
of an instrument called a stethoscope, which is in principle a
hollow tube, or double tube, serving to convey the sounds from
a small area of the chest wall to the observers ear.

Since the right and left ventricle beat at exactly the same
rate, and all the fluid sent from the right ventricle to the
lungs must be afterwards sent from the left ventricle round
the system,^ it follows that the volume of blood discharged
at each systole from the right ventricle must be equal to that
discharged from the left, and as the volume of the ventricle
must become adapted to the volume which it discharges con-
tinually during life, it also follows that the internal volume of
the two ventricles must be the same. It is difficult to deter-
mine this volume accurately, but it has been estimated at 4 to
5 ounces, or 100 to 120 cubic centimetres.^

Although the entire vascular system possesses the exceedingly
smooth epitheloid lining above described, which has the effect
of diminishing the resistance to the flow of the blood through
it to a minimum, yet there is a considerable resistance to the
flow, which is practically all due to the fine bore of the

^ With the exception of a comparatively negligible quantity lost by
evaporation in the kings.

'^ That is, a little more than a medium-sized hen's egg ; hence when the
rate at which the heart beats is taken into account, the vast volume of blood
sent through the heart per day is realized. To obtain the day's work of
the heart this volume must be multiplied by the sum of the pressures at
which it is driven into the aorta and pulmonary artery ; this is equal to a
pressure of about 8 feet of water, so that the amount of work done daily
by the heart is very considerable.

The Circulatory System. 105

capillaries and the smaller arteries and veins adjoining them.
This form of resistance is what is known as fluid resistance.
When a fluid flows along in a channel or tube, there is a
certain amount of resistance to the flow due to the roughness
of the inner wall in impeding the flow of the layers of fluid
nearest to it, called the skin resistance ; this is reduced to a
minimum in a normal condition of the blood-vessels of the
body by the smoothness of their internal coat. But, besides
this, there is a friction, due to the movement of the layers of
fluid on each other. Those portions of fluid in the central
part of the bore of the tube move more rapidly than portions
nearer the wall of the tube, thus there is a brushing of the
fluid against itself, which impedes and delays its motion as a
whole. The amount of this resistance varies with the velocity
of the fluid, with its viscosity^ or internal friction, and with the
hore of the tube. As the bore decreases, the resistance becomes
enormously increased, so that it requires a great pressure to
drive fluid through very fine tubes with any considerable
velocity. Now, the blood is a fluid with considerable viscosity,
and the capillaries through which it has to be driven are
exceedingly narrow, so that to accomplish the purpose the
pressure in the arteries {arterial blood-pressure) must be main-
tained high.

This shows the necessity for the thick-walled muscular
ventricles for the strong auriculo-ventricular, aortic, and pulmo-
nary valves, and for the strong-walled elastic arteries.

The resistance in the pulmonary circuit is not nearly so
great as that in the systemic circuit, and Jience the hlood-pressure
in the pnlmonary artery is only about one-third of that ifi the
aorta. The walls of the pulmonary arteries are hence not nearly
so thick as those of the aorta and its principal branches, and in
the case of the ventricles themselves the left has very much
thicker walls than the right, so that although the internal
volume of the two ventricles is the same as pointed out above,
the external volume of the left ventricle greatly exceeds that
of the right.

The blood pressure in the arteries increases somewhat at
each stroke of the ventricles, and falls back between the strokes ;

io6 Elementary Physiology.

it probably equals on the average in the aorta in man the
pressure of a column of mercury 120-140 millimetres high^ which
is nearly equivalent to a cohmm of water about 6 feet high.

The pressure falls in each part of the circulatory system
proportionately to the resistance passed. Since there is
practically no resistance in the larger arteries, there is very
little fall below that of the aorta (or pulmonary artery respec-
tively) until the arterioles are reached. The amount of
pressure (or head^ as it is called) lost in the arterioles is very
variable, according to whether the muscular walls of these are
constricted or relaxed, but is always much greater than that
lost in the larger arteries. In the capillaries a still greater
resistance is encountered, and a great fall in pressure results;
so that when the veins are reached nearly all the arterial
pressure has been dissipated in overcoming resistance. The
veins are wide tubes, like the arteries, and very little pressure
is required to send the blood along them back to the heart ; so
that, although there is a slow fall, the gradient is very slight.
Finally, when the auricles are again reached by the returning
blood, all that pressure under which it was sent forth on its
round by the ventricles has become lost. In fact, each time
we draw our breath the lungs are filled, as has been previously
said, by the suction on them of the enlarged thorax, and this
suction comes to bear, not only on the lungs, but on all
distensible structures within the thorax ; so that there is a
suction on the large veins within the thorax as also upon the
auricles themselves. ^ Hence, in inspiration, the pressure in
the large veins may not only fall to zero, but become negative,
and as this suction is transmitted to the large veins immediately
outside the thorax, such as the jugular in the neck, if such a
vein be inadvertently cut, air may enter during inspiration, on
account of this negative pressure, and produce fatal results
when it reaches the heart.

^ For methods of measuring blood pressure, the student must consult
larger text-books.

^ Another effect of this suction is to cause a greater flow of blood
towards the thorax, and hence towards the heart, during inspiration. This
increased amount of blood is pumped round by the heart ; so that there is a
small rise of blood pressure during inspiration, and a small fall during
expiration.

The Circulatory System. loy

The arteries, as has been pointed out above, are tubes with
elastic ^ walls capable of being distended under pressure as the
blood is pumped into them from the ventricles, and of recover-
ing their former dimensions as it escapes into the capillaries.
The use of this property is obvious, for if the arteries were rigid
tubes under pressure, each quantity of fluid shot into them
would produce a spurt passing all through the vascular system.
There would be no steady stream passing through the capillaries
to feed the tissues, but instead a useless onward spurt at each
stroke, and between the strokes no forward flow. In fact,
perfectly rigid tubes to replace the arteries it is almost im-
possible to conceive in action ; for the capillaries are so narrow
that they would present an enormous resistance to the flow of
blood through them with such a sudden large velocity, and so
the arteries would burst, or the heart stop dead, incapable of
expelling its contents in face of such a resistance. The dis-
tensibility of the arteries plays in the vascular mechanism
exactly the same part as the air-chamber of a pumping or fire-
engine, or as the indiarubber cover of a chemical bellows.
At each heart-beat the quantity of blood discharged from the
ventricle somewhat further distends the large arteries and
increases the pressure slightly within them. Then, after the
blood ceases to flow in (during the diastolic pause), the elastic
walls of the artery continue to press on the blood and drive it
in a steady stream onward into the capillaries. It is this action
of the arterial walls which is responsible for the difference in
character of the flow from a cut artery and that from a cut
vein respectively. When an artery is cut there is a rapid flow
in jerks /;-^;;^ the e?id nearer the heart ; when a vein is cut there
is a slower, oozing, and constant ^o^ fro7?i the end farther fro f?i
the heart and nearer to the capillaries. The difference in the
ends from which the blood flows is caused by the direction of
the blood-stream ; the more rapid flow from the artery is due
to the higher pressure under which the blood is within it ; and
the spurting flow from the artery is due to the heart-beats

^ The word "elastic" is here used in the popular sense to mean dis-
tensible by pressure and capable of recovery afterwards, and not in the
technical sense used by physicists.

io8 Elementary Physiology.

rhythmically increasing the pressure; while the constant flow
from the vein is the result of the uniform pressure established
by the interposed resistance of the capillaries by means of
which, combined with the elastic pressure of the distended
arteries, the stream is made constant.

It may be gathered from the foregoing account that two
factors are necessary to convert the pulsatile flow of the arteries
into the steady flow of the veins — viz. first, the elastic arterial
wall distended under pressure ; and, secondly, the peripheral
resistance of the small channels (arterioles and capillaries)
interposed between the arteries and veins. When this peri-
pheral resistance is much diminished, as, for example, when
the muscular walls of the arterioles are much relaxed, the
blood can pass too easily through the arterioles and capillaries,
and appreciably more passes when the pressure is increased at
a ventricular systole, than during ventricular diastole : hence the
flow in the veins beyond these distended arterioles becomes
faintly pulsatile or jerky.

Another important outward physical sign of the circulation,
namely the pulse^ is due also to the elasticity of the arterial
walls. When fluid is forced at any point into an elastic tube
distended by internal pressure, a wave is set up which travels
away from this point with a definite velocity depending upon
the elasticity of the wall and the pressure upon it. In this
way an elastic wave is set up in the aorta and its branches
as each quantity of blood is discharged from the left ventricle
into its upper end. This wave is known as the p^Use wave^
and occasions the pulse felt when an artery at any part
of the body is compressed by the finger. The pulse has the
same frequency as the heart-beat, and so is a signal of the
rate and regularity with which the heart is working. By its
character it also gives an indication of the amount of pressure
within the artery. It must not be supposed that the pulse
wave is directly due to the motion of the blood along the
artery, any more than the waves of the sea mean a motion
of the water in the direction of the waves. When a fresh
quantity of blood is thrown into the beginning of the aorta
there is a distension of this portion of the aorta, and afterwards

The Circulatory System. 109

a back-swing, the distension is propagated along the aorta and
its branches, and it is this propagated wave which is the pulse.
If the pulse movements be magnified, as can be done by an
instrument called a sphygmograph (consisting essentially of a pad
which presses on the artery and moves a lever, the longer end
of which writes on a smoked paper surface moved past it by
clockwork), it is seen that the pulse is not a simple wave, but
has several secondary waves upon it (see Fig. 62). These

^■^

Fig. 62. — Sphygmographic tracing.
The tracing, which is to be read from left to right, is a record of the pulsations of the
radial arterj' in man. The first strong upstroke shows the primary or percussion
wave. The notch is the "dicrotic notch," and the wave following it is the "dicrotic
wave."

secondary waves are due to elastic after-vibrations of the artery,
and one, called the dicrotic wave., is accentuated by a smaller
wave caused by the closure of the semilunar valves as the
pressure of the blood in the aorta shuts them after the ventricle
has commenced to relax. This dicrotic wave becomes mag-
nified when the pressure in the arteries is low.

The rate at which the pulse wave travels is variable, being
increased either by greater rigidity of the arterial walls or by
increased arterial pressure ; it is approximately 3 to 4 metres
{i.e. 10 to 13 feet) per second.

The real velocity of the hlood-stream along the arteries is
many times less than this, amounting to only 30 centimetres
(about I foot) per second in the aorta.

There is a definite relationship between the velocities of the
blood in the vai'ious parts (arteries, capillaries, and veins) of the
vasctilar system^ which is determined by one thing ojily, viz.
the relative total cross-section ^ of the vascular system at the

^ i.\ cross-section is the area made by a cut at right angles to the bore
of the tube (artery, capillary, or vein).

1 10 Elementary Physiology.

given point. Each time that an artery branches on its way to
the supply of a tissue, although the branches become smaller
their united cross-section becomes greater. The united cross-
section of the capillaries is many times greater than that of the
great arteries or veins. If the cross-section of the aorta be
taken as unity, then the united cross-section of the capillaries
is approximately 500, and the united cross-section of the ve7i(z
cavcB is about 2 ; it follows that the average velocity of the
blood-flow in the capillaries is 500 times slower than in the
aorta, and in the vence cavce is about half as fast. For the blood
must pass in turn, in making the complete circuit, through the
entire vascular system. A quantity of blood flowing in a given
time along the aorta must flow in the same time exactly
through the united capillaries, and afterwards through the
vence cava;. Now, the velocity of flow is evidently obtained
by dividing the quantity of blood flowing in the unit of time
by the cross-section of the channel or channels through which
it flows. If the channel be of twice the area the blood must
only flow at half the rate for the same quantity to flow in the
same time. Hence, as the same blood flows all round the circuit,
the velocity at any part is inversely proportional to the total
cross-section at that part.

The velocity in the aorta being about 30 centimetres (about
I foot) per second, that in the capillaries will roughly average
on the above basis something over half a millimetre (~ of an
inch) per second, and that in the great systemic veins about
15 centimetres (6 inches) per second.

It is often erroneously stated that the resistance in the
minute capillaries is a cause of the slow flow of the blood in
these vessels as compared with that in the arteries, but this is
a fallacy; the comparative cross-section is the only factor in
determining the relative velocities. It is indeed true that a
diminution of the resistance in the capillaries, or rather in the
small arterioles which lead to them, will increase the flow
through these capillaries, but it will proportionately increase the
velocity of flow in the arteries and veins, if the diminution in
capillary resistance be general all over the body, and hence the
relative velocity will remain unchanged. In this manner, any

Tlie Circulatory System. 1 1 1

general change in velocity at any part of the circuit must tell
backward and forward on the velocity in all other parts of
the circuit, and the average velocity in arteries, veins, and
capillaries must remain purely determined by the relative total
cross-section at these various parts.

The local velocity through the capillaries of a given area —
say of the capillaries of any particular organ in the body — is,
however, much more important than the average capillary
velocity, and this can be altered very widely, without much
altering the velocity in the arteries or veins, by means of
variations in the calibre of the arterioles supplying the part.
These variations are brought about by nerve action through
nerve fibres supplied to the involuntary muscle coat of these
arterioles — the vaso-motor fibres. By this means the distribution
of the blood-stream to the different organs is regulated ; almost
shut off w^hen the demand is small, through the organ becoming-
dormant, and turned on in full when the organ again passes
into a state of activity. The vaso-motor nerve fibres, and the
involuntary muscle coat of the arterioles on which they act,
are thus to the circulation of the blood what the distributing
taps are to a water-supply, allowing the stream to flow where
there is work for it to do, and shutting it off where it is not
required. The only difference is that, in the body, the taps
are never completely shut down, they are only more or less
widely opened. The reason for this is, that living tissue cannot
go on for a long period without a supply of oxygen (which is
carried to it by the blood-stream). Even in a resting condition
a certain amount of activity goes on, accompanied by oxidation
and need for respiration. But when the cells become active,
the amount of change becomes largely increased, and the blood-
supply must also be increased to cope wdth the larger demand.

In certain veins, particularly in those of the limbs, the
action of the heart is assisted by valves placed in the walls at
intervals. These ve?io7is valves are very simple structures, con-
sisting of two pouch-like invaginations of the inner coat of the
vein placed opposite each other (see Fig. 6t,). They open
towards the heart, and allow of the passage of the blood in
that direction, but close and prevent any flow in the opposite

112

Elementary Physiology.

Fig. 63. — Diagram showing the
valves of veins.

direction. The use of these valves is in determining the direc-
tion of flow when a vein is compressed by muscular action in
moving the part. Were there no valves, the contents of the
vein on compression would move in both directions to and

from the heart, so as to empty the
compressed portion, for the venous
pressure (5-15 millimetres of mer-
cury) is too small to prevent this
action. But the valves have the
effect of causing the vein to empty
its contents towards the heart only,
and so assist the heart in its work.
This action is best seen in the long
veins of the leg, which are plentifully
supplied with valves. If a person
remain perfectly at rest for some

A, part of a vein laid open, with . . ...

two pairs of valves ; B, longi- time in an Upright position, the veins

tudinal section of a vein, show- ..11 - r ^-l i i. i. i.t_„

in- the valves closed ; c, por- at the lowcr part of the leg, about the
:XibiSng\swSgltatir anklc, bccoiTie Considerably swollcn,
of valves. bccause the venous pressure is thus

greatly increased, and the veins become rounded and distended
by the increased pressure. The pressure in the veins must,
under such conditions, be equal to the hydrostatic pressure of
a column of blood reaching up to the heart before the blood
can move upward to the heart along these veins, and this is a
considerable pressure.^ But let the person now make a few
muscular movements of the legs, such as by walking about
vigorously, and the distension becomes greatly diminished,
because the valves now come into action, and the blood is
pumped upwards by their action, as the skin compresses them
on account of the movement of the underlying muscles.

The action of the venous valves may be tested by com-
pressing with the finger a long superficial vein of the arm or

^ It is probably impossible to stand so still that there is no action at all
of these valves. It may also be vvxll to point out that the heart has not to
do work against the hydrostatic venous pressure above mentioned, since it
is balanced by an equal column in the arteries, and that the chief evil effect
is the distension of the veins, which is relieved by the action of the valves
when there is sufficient muscular action.

The Circulatory System. 1 1 3

leg.'^ If the vein be stroked towards the heart, the blood runs
easily in front of the finger and leavts the vein almost empty
behind it ; but, if it be stroked in the opposite direction, the
vein swells up, and becomes knotted at points W'here the valves
are situated.^

Structure of the Heart.

We may now consider more in detail the mechanism of
the heart-pump, and the arrangement of its valves. The
description can only be followed profitably if accompanied by
a dissection of a heart. A sheep's heart can easily be pro-
cured from a butcher, and should be obtained with the attached
vessels as long as possible, and uninjured by knife-cuts.

In the dead heart by far the greater part of the bulk is
made up of the two ventricles. The auricles are two com-
paratively inconspicuous appendages of a deep purple colour
placed above the ventricles. This is, in part, due to the fact
that even the internal capacity of the auricles when distended
with blood is somewhat less than that of the ventricles, and in
part to the fact that the auricles have thin and the ventricles
thick walls, so that the external volume of the ventricles is
much greater than that of the auricles.

There is a deep transverse groove round the heart, the
auriculo-ventncular groove^ separating the auricles from the
ventricles. Another somew^hat U-shaped groove, called the
inter-ventriaUar groove, separates the two ventricles ; it con-
tains blood-vessels and some fat, which serve to more clearly
mark its position. This groove passes near, but not over, the
apex of the heart, for the entire apex belongs to the left
ventricle, which serves as a sign to identify it in the excised
heart. Even before making any incision into the heart, the

* Some veins have no valves, for example, the veiut: cava, the pul-
monary veins, and the veins of the liver (portal and hepatic veins).

- This experiment can be best carried out on another person with pro-
minent veins. Compress a long vein of the arm with one forefinger, and
then with the other forefinger stroke it upwards towards the heart ; the vein
empties, and remains empty for a certain distance up, where a prominent
valve appears. On releasing the vein, it fills from the peripheral end. If
the experiment does not succeed with one vein try another,

I

114 Elementary Physiology.

thinner wall of the right ventricle, as compared with the left,
may be appreciated by pressure with the fingers.

When the heart is in position in the body, the right side
lies more anteriorly than the left ; so that when seen from the
front, two-thirds is formed by the right ventricle, and only the
third on the left is left ventricle.

Each auricle is prolonged somewhat at one side into a
process resembling the lobe of the ear, and hence termed the
auriciUar ^ appendage. The main part of each auricle is termed
the atrium^ or sinus venosus^ because the veins open into this part.

The great vessels entering the base of the heart should next
be examined.

The aorta is distinguished by having the thickest wall,
and by the fact that the little finger when inserted into, it,
can be passed into the left ventricle without first passing
through the left auricle. The pulmonary artery has the next
thickest wall, and the finger when passed through it passes
directly into the right ventricle. The remaining vessels are
veins \ two enter the right auricle — these are the s2penor and
inferior vencB cavoR. The veins entering the left auricle come
from the lungs, and are termed the pulmonary veins. Their
number is variable in different animals. In the sheep there are
usually two j in man there are four, two from each lung.

The interior of the heart-chambers and the arrangement of
the valves should next be studied, and it will be well in doing
so to follow the course of the blood through the heart. ^

To expose the inner surface of the right auricle, make a
cut from the superior to the inferior vejia cava, and another
running nearly at right angles from the middle of this into the
auricular appendix. The inner wall shows muscular ridges
over the appendix and upon the right side of the atrium, while
the rest of the wall is much smoother. On the inter-auricular
septum, separating this chamber from the left auricle, an oval
depression may be seen, called the fossa ovalis (see Fig. 64) ;

^ The whole auricle gets its name from a fancied resemblance to the
external ear.

- Before making the incisions above described, the student should, if
provided with the necessary apparatus, perform the experiment described
in practical exercises, to demonstrate the action of the valves.

TJic Circulatory System. 1 1 5

this corresponds to a former opening between the two auricles
which existed in the fcetus, and served an important purpose
before the lungs came into use, in directing the blood into
the left auricle directly, instead of via the lungs ; it became
closed later, as the lungs came into use after birth, by a fold of
tissue, which originally served as a valve for it.

At this stage in the dissection, if the auricle be held open
and the water be poured into it from a slight height, so as to
pass into the ventricle, the action of the auriculo-ventricular
valve may be demonstrated. As the water fills the ventricle,
the flaps of the valve float up and close the opening. This
resembles what happens during life, when the auricle contracts
and forces its contents into the already ahnost full ventricle.
The valve-flaps then float up, and their edges become opposed,
so that all is ready for the ventricular systole, and there is
no back-flow into the auricle at the beginning of the stroke.

Observe, further, that there are no valves at the openings of
the verm cavce into the auricle. Such valves, as has already
been stated, are unnecessary, because there is no great force
exerted by the auricular contraction, which simply serves to
completely fill the ventricle.^

Next open the right ventricle by making an incision with
a knife into it parallel to the inter-ventricular groove, and
following this round at some distance from the groove so as
not to injure the inter-ventricular septum.'^ By means of this
U-shaped incision a flap is made which can be turned outward
from the apex, and so the interior of the ventricle be examined.
Feel with your finger, from the inside of the ventricle, for the
opening of the pulmonary artery into the ventricle, and make
certain that you have found it by seeing that you come out at
the external opening of the pulmonary artery which you have

• Back-flow into these veins is also prevented by the facts, that there is
Httle resistance to the discharge into tlie ventricle, that there is already a
current of blood towards the auricle in the veins, and that the veins assist
the process by a slight contraction of their walls immediately preceding
that of the auricle.

* This is the muscular septum separating the two ventricles. It is
somewhat convex towards the right ventricle ; so that the right ventricle
is crescentic in cross section, while the left ventricle in cross section is oval
or rounded.

Ii6 Elementary Physiology.

previously identified as above described. Now, using your
finger as a guide, continue the previous cut in the ventricle up
towards the pulmonary artery, and cut through the lower
portion of the wall of this, if possible between two of the three
flaps of the semilunar valve which guards its orifice. Finally,
cut away the lower part of the flap of ventricular wall already
partially detached, so as to expose the interior of the ventricle
more completely.

The inner surface of the ventricle is ridged by strong
muscular bundles (the columncE carnece)^ some of which are
attached to the wall of the ventricle all the way, others are free
in the middle, but attached at both ends {trabeailcd)^ while
others, again, are attached to the ventricular wall only at their
base {imisadi papillares). This last set (musculi papillares),
which form two chief bundles, anterior and posterior, serve
as contractile pillars for the attachment of strong fibrous cords
(the chordcs tendinece)^ which are fixed by their other ends to
the edges and under-surfaces of the flaps of the tricuspid valve.
The tricuspid valve (see Fig. 64) guards the passage from the
.right auricle to the right ventricle, and makes this opening
impermeable when the ventricle contracts. It consists of three
thin but strong fibrous flaps, roughly triangular in form, with
their bases attached in a complete ring round the auriculo-
ventricular orifice, and their apices towards the ventricle. To
the edges of the flaps, and to their under-surfaces, the strong
thin ckordcB tefidinece above mentioned are attached, and hold
the flaps in position so as to prevent them bulging into the
auricle when the ventricle contracts and the blood presses upon
their under surfaces.

The musculi papillares are a compensating arrangement. If
the cords by which the flaps are held down were non-contractile,
and ran straight from the valve to the ventricular wall, then
the wall would move towards the valve as the ventricle con-
tracted, the ends of the restraining cords would move with it,
the flaps would be allowed to move too far up, and the valve
become incompetent. This is prevented by the contractile
musculi papillares, which shorten as the ventricle contracts,
and so hold the valves in the proper position.

TJie Circidatory System.

117

The wall of the ventricle near the mouth of the pulmonary
artery is smooth, and conical in shape {conns arteriosus)^ narrow-

FiG. 64. — Interior of the right auricle and ventricle, exposed by removal of the greater
part of their right and anterior walls. (Allen Thomson.) \

I, superior vena cava ; 2. inferior vena cava ; 2', hepatic veins ; 3, septum of the auricles ;
3', fossa ovalis ; the Eustachian valve is just below ; 3", aperture of the coronary sinus
with its valve; +, +, right auriculo-ventricular groove, a narrow portion of the
adjacent walls of the auricle and ventricle having been preserved ; 4, 4, on the septum,
the cavity of the right ventricle ; 4', large anterior papillary muscle ; 5, infundibular,
5', right, and 5", posterior or septal segment of the tricuspid valve ; 6, pulmonaiy
artery, a part of the anterior wall of that vessel having been removed, and a narrow
portion of it preserved at its commencement where the pulmonary valve is attached ;
7, the aortic arch close to the cord of the ductus arteriosus ; 8, ascending aorta
covered at its commencement by the auricular appendix and pulmonary artery ;
9, placed between the innominate and left common carotid arteries ; 10, appendix of
the left auricle ; 11,11, left ventricle.

ing towards the artery. The mouth of the artery is circular,
and about an inch in diameter ; it occupies the summit of the

ii8

Elementary Physiology.

Fig. 65.

-The left auricle and ventricle opened and a part of the wall removed so as to
show their interior. (Allen Thomson.) ■?

The commencement of the pulmonary artery has been cut away, so as to show the aorta ;
the opening into the left ventricle has been carried a short distance into the aorta
between two of the semilunar flaps ; and part of the auricle with its appendix has
been removed, i, right pulmonary veins cut short ; i', placed within the cavity of
the auricle on the left side of the septum, on the part formed by the valve of the
foramen ovale, of which the crescentric border is seen ; 2', a narrow portion of the
wall of the auricle and ventricle preserved around the auriculo-ventricular orifice ;
3, 3', cut surface of the wall of the ventricle, seen to become very much thinner
towards 3", at the apex ; 4, a small part of the wall of the left ventricle which has
been preserved with the left papillary muscle attached to it ; 5, 5, right papillary
muscles ; 5', the left side of the septum ventriculorum ; 6, the anterior or aortic
segment, and 6', the posterior or parietal segment of the mitral valve ; 7, placed in
the interior of the aorta near its commencement and above its valve ; 7', the exterior
of the great aortic sinus ; 8, the upper part of the conus arteriosus with the root of
pulmonary artery and its valve ; 8', the separated portion of the pulmonary trunk
remaining attached to the aorta by 9, the cord of the ductss arteriosus ; 10, the
arteries arising from the aortic arch.

The Circulatory System. 1 1 9

ventricle, being situated slightly higher than the auriculo-
ventricular opening, and is guarded by a valve with three
pouch-like (semilunar) flaps (see Fig. 66), called \kv^ pulmonary
scmiliuiar valve ^ which opens towards the artery.

The construction on the left side of the heart is very similar
to that on the right, except that everything is more strongly

Fig. (>(>. — View of the base of the ventricular part of the heart, showing the relative
position of the arterial and auriculo-ventricular orifices. (Allen Thomson.; \

The muscular fibres of the ventricles are exposed by the removal of the pericardium, fat,
blood-vessels, etc ; the pulmonary artery and aorta and the auricles have been
removed ; the valves are in the closed condition, i, i, right ventricle ; i', conus
arteriosus ; 2, 2, left venticle ; 3, 3, the divided wall of the right auricle ; 4, that of
the left ; 5, the inlundibular ; 5', the right, and 5", the septal segment ot the tricuspid
valve ; 6, the anterior or aortic, and 6', the posterior or parietal segment of the
mitral valve (in the angles between these segments are seen smaller lobes) ; 7, the
pulmonary artery ; 8, placed upon the root of the aorta; 9, the right; 9', the left
coronary artery.

fashioned in the ventricle on account of the greater pressure to
be overcome in the aorta.

Four pulmonary veins enter the left auricle in man {I'ide
supra), two from each lung opening close together into opposite
sides of the cavity. To open the left auricle a cut should be
made across the posterior surface from the right to the left pul-
monary veins, and another shorter cut towards the front at right
angles to this one. The mitral valve guarding the left auriculo-
ventricular opening may next be floated up by pouring in water
in a similar fashion to that directed above in the case of the
tricuspid valve. // has only two flaps instead of three, but is

I20 Elementary Physiology.

much stronger than the tricuspid valve in its structure ; other-
wise the construction of the two valves is much the same.

The left ventricle may be opened by an incision passing
along both anterior and posterior surfaces parallel and close to
the inter-ventricular septum. When the flap so formed is
turned out, the interior of the ventricle is seen. The cavity is
longer and more conical in shape than that of the right ven-
tricle. Similarly to the right ventricle it has two orifices, one
leadingy9v;;^ the auricle, the other mto the aorta. The walls are
also roughened by muscular projections, except near the mouth
of the aorta where they are smoother. The musculi papillares
of the left ventricle, giving attachment to the chordcz tendinece
at one end, are strongly developed, and arranged in two large
bundles which spring respectively from the right and left sides
of the cavity. The aortic opening is situated somewhat higher
up and more to the front than the auriculo- ventricular, and
both these orifices are slightly narrower than those on the right
side. The entrance to the aorta is guarded by the aoi^tic semi-
lunar valve, which like that at the pulmonary artery has three
pouch-like flaps, but is of stronger construction. The pouches
in each case are folds of the inner coat of the artery strengthened
by strong fibrous tissue. Where the three flaps meet in the
closed position of the valve there are small nodules of cartilage
called the corpora Arantii. Opposite each pouch of the valve
there is a swelling outwards of the wall of the vessel known
as the siims of Valsalva, and at the upper margins of two of
these sinuses, two openings are situated, which are the orifices
of the coronary arteries. These arteries, the first branches of
the aorta, supply the heart itself with blood. The veins of the
heart open into the right auricle by various openings, but chiefly
by the coronary sinus, situated between the inferior cava and
the auriculo-ventricular opening. This course via the coronary
artery, heart capillaries, and coronary sinus is the shortest
circuit that the blood can take in the systemic circulation ; while
that to the capillaries of the intestine, from these to the capil-
laries of the liver by the portal vein, and back to the inferior
cava by the hepatic vein, is the longest route which can be taken.

CHAPTER VI.

THE BLOOD.

The nutrient fluid which circulates in the blood-vessels varies
in colour from bright red to dark purple, according to the
amount of oxygen which it contains.-^ It is very opaque, even
in thin layers, and its opacity is due to the same cause as makes
clouds or sea foam opaque though made up of transparent
material, namely, that it contains floating in it myriads of
minute particles (called corpiLscles) which reflect and refract
the light in all directions and refuse to allow it any passage in
unbroken lines. A small drop of blood drawn from the finger
ought to be examined by the student under the high power of
a good microscope, preferably after diluting it with about its
own volume of physiologically normal saline,"-^ for the corpuscles
in the blood are so numerous that they cannot be quite so
clearly seen in undiluted blood.

A large number of small round discs are seen, which are
occasionally discovered rolled over on their edge, w^hen they
are seen to be bi-concave in outline (see r. Fig. 67). They have
a tendency to adhere by their concave surfaces, when these come
in contact, so as to form long rouleaux. In colour they are a

* The bright red blood contains more oxygen, and is seen when an
artery is cut through ; since it is contained in the arteries it is called arterial
blood. The dark purple colour is acquired as the oxygen disappears from the
blood, which it does in the passage through the tissues, and hence the veins
contain such blood, which is accordingly termed venous blood.

- This solution is easily made by dissolving 6 "5 grams of common salt in
a litre (176 pint) of tap water ; for frog's blood, which is also well worth
examining, it must be stronger (8 grams per litre). This solution has
approximately the same strength in salts as the blood, and prevents altera-
tions in shape of the corpuscles ; water makes them swell out, and stronger
salt solutions draw the water out of them and make them become shrunken
and crenated.

122

Elementary Physiology.

pale yellow, for it is only when seen e7i masse that the colouring
matter {hamoglobiii) with which they are charged has a red hue.
There are immense numbers of these red blood corpuscles, or
blood discs, in the blood, the average number being five to
six millions per cubic millimetre,^ but in anaemia the number
may be much reduced.

There are, in addition to the red corpuscles, a much smaller

/

Fig. 6S. — Human red cor-
puscles Ij'ing singly and
collected into rolls. (As
seen under an ordinary-
high power of the micro-
scope.)
I, on the flat ; 2, in profile.

Fig. 67.— Human blood as seen on the warm
stage. (Magnified about 1200 diameters.)

r, r, single red corpuscles seen lying flat ; r', r',
red corpuscles on their edge and viewed in
profile ; r", red corpuscles arranged in rou-
leaux ; c, c, crenate red corpuscles ; /, a
finely granular pale corpuscle ; g, a coarsely
granular pale corpuscle. Both have two_ or
three distinct vacuoles, and were undergoing
changes of shape at the moment of observa-
tion ; in^, a nucleus also is visible.

number of luhite corpuscles^ or leucocytes. There is, on an
average, one of these white corpuscles to every four hundred
of the red corpuscles ; they are usually considerably larger, in
human blood, than the red corpuscles, although they vary
greatly both in size and appearance. Each possesses one or more
nuclei ; some are vacuolated, and others granular, while others

^ A cubic millimetre has about the volume of a large pin's head.

TJie Blood. 123

again are clear or nearly hyaline. They exhibit those amoeboid
movements which have been described in the introduction to
this book, especially when wanned, and if the blood be mixed
with fine particles, such as yeast cells, the white corpuscles may
be observed "with the microscope taking these into their mass.
This inception of foreign particles indicates an important function
of the white blood corpuscles. In a similar manner they
absorb and render innocuous any deleterious particles which
may have found their way in any manner into the blood. They
congregate round a wound or other infected part, and prevent
injurious products from entering the blood. In the combat
many of the leucocytes themselves become poisoned and die,
so that they are found in large number in \k\ftpus flowing from
a suppurating wound.

The leucocytes are probably formed in the lymphatic glands,
and enter the blood-stream with the lymph, for the number of
leucocytes in the lymph is much increased after it has flowed
through a lymphatic gland ; also, the leucocytes of lymph have
a preponderance of smaller and clearer cells, which are termed
lymphocytes, and have the appearance of young leucocytes.

The purpose of the red blood coipuscles is to absorb oxygen
in passing through the lungs and give it up again in passing
through the tissues. About ninety per cent, of their dried
weight consists of a complex substance of a proteid nature,^
called hcemoglobin^ which is capable of forming an unstable
chemical compound with oxygen. This compound is formed
when the pressure of oxygen in the fluid containing the
haemoglobin reaches a certain value, and is decomposed again
when the oxygen pressure falls. Now, in the tissues, oxygen
is being constantly used up by the oxidations going on there,
and consequently there is a low oxygen pressure ; while in the
lungs, by the process of respiration, the oxygen pressure is kept
fairly high. Accordingly, the haemoglobin takes up oxygen in

^ The student is advised to read the part of chapter vii. on the chemistry
of proteid, before reading this chapter.

- The name haemoglobin is used in a general sense ; when combined
with oxygen it is often termed oxy-hKmoglobin, and when free of oxygen
so combined it is termed reduced haemoglobin. In addition to the usual
proteid constituents it contains iron.

124 Elementary Physiology.

the lungs, and gives it up again in the tissues. Thus, a supply
of oxygen is carried by the blood-stream to all parts of the
body to serve in oxidizing, in the life processes of the cells,
those nutrient materials which the blood also carries to the
tissues.

The haemoglobin can be set free from the corpuscles in
various ways, such as, often alternately freezing and thawing,
adding excess of water, or a trace of ether or chloroform. It
then dissolves in the water of the blood and forms a transparent
fluid of a deep lake colour known as laked blood.

The corpuscles of man and of the mammalia generally are,
as stated above, non-nucleated bi-concave discs ; but those of
lower vertebrae (such as amphibia, fishes, and birds) are oval
bodies with prominent nuclei. Nucleated fed blood corpuscles
are, however^ foiDid before birth, even in the mainmaUa. Red
blood corpuscles are probably formed in large numbers, during
life, in the red marrow of the ribs and of the heads of the long
bones, and, as before stated, it is probable that they are, to
some extent, destroyed in the spleen. The liver also probably
takes a share in the disintegration of effete red blood cor-
puscles, for the bile pigments excreted by it are closely allied
to haemoglobin in their chemical constitution (see p. 164).

The corpuscles float in a clear fluid, which is of a pale straw
colour when it is obtained uncontaminated by haemoglobin
shed from the red corpuscles, and is called the plasma, or blood
plasma.

Special precautions must be taken in order to obtain plasma,
because it possesses the property of coagulating, or setting into a
solid mass. If this coagulation takes place in shed blood before
the corpuscles have had time to subside and separate, a thick
solid mass of blood, known as a blood-clot, is the result. The
power of the plasma to undergo spontaneous coagulation after
the blood is shed, and so to cause the blood to set solid, is a
most invaluable quality ; for, without it, an animal would slowly
bleed to death even from an insignificant wound. By its aid,
however, the apertures of the wounded vessels are stopped,
and the leakage of blood ceases.

As will be presently pointed out, the coagulation of the

The Blood. 125

blood is due to a proteid dissolved in the plasma, called
fibrinogen.^ which, under certain conditions, gives rise to an
insoluble substance (also a proteid) called fibrin. The fibrin
so formed when blood clots is but an insignificant part of the
blood, amounting to only, about two parts in a thousand ; but,
as it appears as a meshwork of long filaments, it converts the
whole into a general jelly-like mass, which has a still more solid
appearance if it is formed of the whole blood, so that the
corpuscles are included in the meshes.

The clotting of blood — that is, the formation of the in-
soluble fibrin from the soluble fibrinogen — is affected by various
circumstances, some of which retard or entirely prevent it,
while others accelerate it. A study of these has thrown some
light on the nature of the changes that occur; they may be
summarized as follows : —

1. A certain optimwn temperature^ correspondifig with that of
the body or a little over it, is most favourable for coagulation.
As the temperature sinks below this point, the speed of coagu-
lation diminishes until at the temperature of melting ice it
becomes infinitely slow, so that blood may be kept at 0° C. for
days without undergoing coagulation. During this period, the
corpuscles, which are heavier, sink to the bottom, and the clear
yellow plasma appears above them. "WTien they have com-
pletely subsided, the moist corpuscles occupy about one-third
of the volume. If this iced plasma be decanted off and allowed
to gain the ordinary atmospheric temperature, it slowly coagu-
lates ; if it be heated in a water bath to the temperature of the
body, it coagulates much more rapidly. It hence contains all
the necessary factors for coagulation, but their interaction is
prevented by the low temperature.

2. The presence of an excess of neutral salts ^ such as sodium
or jnagnesium sulphate, delays, and in sufficient q^iantity preve?ifs,
the coagulation of blood. Hence plasma may be obtained by
drawing off the blood from an artery into a sufficient amount
of a saturated solution of such a neutral salt. To prepare such
salted plasma, about one-third as much of a saturated solution
of sodium or magnesium sulphate is taken as there is blood
expected, and the blood is mixed, by stirring, with this fluid

126 Elementary Physiology.

as it flows out of the blood-vessel. ^ This plasma coagulates
when diluted, and still more rapidly if it be heated and have a
portion of blood-clot from another source, or of blood serum,^
added to it. It is of service in investigating the problems of
coagulation, because the fibrinogen which causes coagulation
may, by certain means, be separated from it.

3. Blood may be prevejited from coagulating by removing
its calciiLm salts ^ or converting these into an insoluble form.
This may be done by mixing the blood with a solution of a
soluble oxalate (about 0*2 grammes of potassium or ammonium
oxalate to each 100 cubic centimetres of blood) when oxalate
plasma is obtained. This clots when excess of a soluble
calcium salt is added to it, especially on warming.

4. Certain substances, such as " Wittes^ peptone^'' ^ leech
extract^ and nmssel extract, if previously injected into a vein
of a living animal, prevent the clotting of its blood when this
is drawn soon afterwards. Plasma prepared by this method
is called peptone plasma.

5. Certain other substances when injected into the veins,
on the other hand, cause coagulation to set in within the living
blood-vessels {intra-vasciilar coagtUation). These substances
can be extracted from the testis or thymus, or other glands
rich in cell-nuclei; they were named tissiLe fibrinogens by
their first discoverer (Wooldridge), but are probably derived
from the nuclei of the cells of the gland extracted, and belong
to a class of compounds known as nucleo-albumins.

6. Contact, with foreign bodies, especially if these have
roughened surfaces, hastens clotting, and remaining in contact

' The preparation of the plasma in this, as in some other methods given
in the text, may be hastened by separating the corpuscles by means of the
centrifuge.

^ Blood serum is what is left of the plasma after the clot has separated ;
it is the clear fluid which separates from a blood-clot some time after it has
formed.

^ Blood-plasma, like the other fluids of the body, always contains a
trace of soluble calcium salts. In the preparation of "oxalate plasma,"
these are not removed by filtration, but only thrown out of solution by the
addition of excess of a soluble oxalate,

■* A mixture of albumoses and peptone, obtained as a result of the
peptic digestion of proteid (see p. 148) ; the anti-coagulative action is due
to the albumoses present, and not to the peptone.

The Blood. 127

with the blood-vessels, delays it. Blood drawn off into an
oiled vessel is much longer in clotting than when drawn into a
dry vessel ; it remains still longer fluid if drawn off in drops into
a quantity of oil. Again, if the large jugular vein of the horse
be ligatured at two places some distance apart, so as to be full
of blood, ^ and removed, the blood will remain fluid within the
vein usually for some days, and plasma can be obtained from
it,- which soon clots when drawn oft' into another vessel. The
blood does, however, occasionally coagulate even within the ves-
sels, and is usually found clotted in the heart soon after death.

7. Addition of old blood-clot to a sample of blood or
plasma which only clots slowly, hastens the process.

Fibrinogen^ that proteid of the plasma which causes coagu-
lation of the blood, can easily be obtained from any of the
forms of plasma of which the mode of preparation has been
indicated above by adding to them an equal volume of saturated
sodium chloride solution, that is, by half saturation with sodium
chloride. The fibrinogen appears as a flocculent precipitate,
which may be washed with some half-saturated sodium chloride
solution, and then re-dissolved by adding distilled water,^ which
forms a dilute saline with the adhering sodium chloride. After
being reprecipitated, washed, and redissolved several times,
the fibrinogen is finally obtained in fairly pure solution. It
does not clot spontaneously, in this condition, however long it
be kept at a favourable temperature, but only after two things
have been added to it. One of these is a soluble calcium salt,"
and the other is the material known as " fibrin ferment."

Fibrin fe7'menf is formed after the blood has been shed, and
is believed to be derived from the nuclei of the white blood
corpuscles as these disintegrate. Like those substances which
cause intra- vascular clotting when injected into the veins of a
living animal, it belongs to the class of proteids called nucleo-
proteids, which are found in cell nuclei. This ferment, or
enzyme, in the presence of a soluble calcium salt, converts the

^ To attain this object, the ligature nearer the heart must first be tied,
then the other one.

- This experiment was first performed by Hewson, and is known as the
Ihivg test-Uibe experiment.

^ "Distilled " in order to avoid adding calcium salts.

128 Elementary Physiology.

fibrinogen of the plasma into an insoluble substance called
fibrin. When the fibrin includes the blood corpuscles in its
meshes, a blood-clot is the result. If the blood be whipped
with a feather, or a bundle of twigs or wires, the fibrin of the
blood separates on this, and may be washed and preserved.
After its removal the blood does not clot, and is known as
whipped blood} After blood has clotted, a clear yellow fluid,
which does not clot again, is gradually forced out from the
mass of the clot ; this fluid is blood senmi,

. Serum differs in composition from plasma only in that it
^contains no fibrinogen ; for this has all been converted into
fibrin, and remains in the clot. It is more easily obtained
than plasma, and since it differs so little in composition, may
be used to study the properties of the nutrient fluid in which
the blood corpuscles float. It contains about ten per cent, of
proteid, of which about half is a proteid called serum-glohdin^
which is precipitated by completely saturating the serum with
magnesium sulphate, by adding crystals of that salt, or else by
half saturating with ammonium sulphate by adding an equal
volume, to the serum, of the saturated solution of that salt.
The remainder of the proteid is a mixture of substances closely
allied to one another, and termed serum-albumins ; these are
not precipitated on saturation with magnesium sulphate, or
half saturation with ammonium sulphate, but are precipitated
on complete saturation with ammonium sulphate.

Another method of separating these two forms of proteid
is to place the serum in a dialyzing tube, made of parchment
paper, round the outside of which a current of water is made
to flow. The inorganic salts of the serum pass through the
parchment paper, but the proteids cannot pass, and so remain
within the dialyzer. As soon as the salts have been removed,
the serum-globulin is precipitated; for the globulins are insoluble
in water alone, and only remain soluble in blood plasma because
of the inorganic salts which it also has in solution. The serum-
albumins remain in solution, and can be separated by filtration.
Afterwards, the globulin precipitate can be redissolved in dilute
saline solution.

^ Whipped blood is the serum with the corpuscles suspended in it.

TJie Blood.

129

The serum also contains about two parts per thousand of
dextrose, or grape sugar, which serves as carbohydrate food for
the tissues. It does not contain fat globules, unless it be
obtained immediately after a fatty meal, when it may be quite
milky from suspended fat.

The chief inorganic salts are sodium chloride, which is
present in largest amount (six parts per thousand) sodium
carbonate and sodium phosphate, and traces of calcium salts.
It is to the mixture of sodium carbonate and phosphate that
serum owes its alkaline reaction. Potassium salts are present
only in traces, but are present in greater amount in the
corpuscles and in the cells of the tissues.

Besides these substances, the serum contains very small
amounts of other organic substances, the products of tissue
activity, which are kept down to a minimum by being either
excreted by the kidneys in the urine, or converted into other
substances as the blood passes through the liver.

K

CHAPTER VIL

DIET, DIGESTION, ABSORPTION, AND METABOIISM.

The supplies of nutrient materials which the blood carries
round to the various tissues and organs of the body are
prepared from the food of the animal by a process called
digestion. While undergoing digestion, the food slowly passes
along a tube called the alimentary canal, which is really an
invagination or folding in of the outer surface of the body;
straight in its upper and convoluted in its lower part. Hence,
during the process of digestion, the food is not really within
the body, but only within a hollow tube which passes through
it, and it is only after being digested, and having passed
through the walls of this alimentary canal that it really enters
the body. The indigestible portion which cannot be absorbed,
as well as a small amount of material excreted into the
alimentary canal by certain glands (chiefly the liver), is finally
ejected from the canal at its lower end.

A number of glands pour their secretions into the alimentary
canal at various points along its length, and these secretions
contain substances which act on the various constituents of the
food, and form from these soluble products which are easily
absorbed or taken up by the cells lining the walls of the
alimentary canal, and .after undergoing certain modifications
in their passage through these cells are passed onwards to
finally reach the blood-stream. This process of absorption
takes place chiefly from the lower part of the alimentary canal
— that is, from the intestine. It may be well, before treating of
the process of digestion, to briefly summarize the steps by which
the animal's food is prepared for it by other natural processes.

The sun is the fundamental source of all that energy in the

Diet, Digestion, Absorption^ and Metabolism. 131

form of organic life which we see exhibited by the teeming
myriads of plants and animals inhabiting our planet. Not
only is this true in the sense that without the warmth of the
sun's rays all life on the earth would be impossible, but in the
narrower sense that every act of a living creature requiring
the expenditure of energy is carried out by energy which has
been stored up from the solar rays. The solar energy is
converted first into chemical energy by the agency of living
plants. Every green plant is a laboratory in which the energy
of the sunlight is transmuted into the energy of chemical
substances, which can afterwards, it may be, serve in the form
of food as a source of energy for the supply of an animal, or
after elaboration in the body of one animal may supply food
{i.e. chemical energy) to another animal.

Plants form their substance from inorganic materials.
They build up from these simple bodies others much more
complex in their nature, which are capable in passing back
again to the simple inorganic forms of giving out a supply of
energy which may exhibit itself in other forms, such as heat
and muscular work.

The process of formation of the more complex substances
is spoken of as reduction by the chemist. In it energy is
required, and it can only take place when some source of
energy is available (sunlight in the case of plant life). The
opposite process in which the simpler substances are again
formed is spoken of as comhustioji or oxidation.^ because usually
oxygen ^ is used up in the process. Here energy is set free,
and is perceptible, if the oxidation takes place in the body of
an animal, in the heat which maintains the animal's temperature
above its surroundings, and in the muscular movements which
are continually taking place.

In plants, then, processes of reduction go on, and from
very simple bodies others of complex chemical nature are

^ A gas forming about one-fifth of the atmosphere, which combines with
bodies when these burn (or are oxidized), so giving rise to bodies (oxides)
with a less store of chemical energy than the original bodies. The chemical
energ}' so dissipated takes the form of heat, light, electricity, sound,
muscular movement, or one other of the forms of energ}- ; for energy can
never be destroyed, but merely transmuted from one form to another.

T32 Elementary Physiology.

formed by the aid of energy derived from the solar rays. In
animals this store of energy is taken possession of ; the animal,
after various preliminary processes, absorbs into its body the
complex materials forming the substances of the plants, and
these are gradually oxidized (by means of oxygen taken in by
the lungs) in the body back to simpler bodies. In the process
of oxidation the solar energy, which had been stored up by
the plant as chemical energy, is again set free in the form of
heat and of muscular work, by means of which the animal is
enabled to carry on its existence. It must not, however, be
rashly supposed that in plants the life processes are purely
reductions and accumulations of energy, and in animals the
reverse. It is only true that the total effect in a plant is a
preponderance of reduction and an accumulation of chemical
energy, and in an animal the opposite is true ; but at the same
time the reversed processes, only in less degree, go on in the
two kingdoms of life.

To enter a little more into detail, plants take up carbon
dioxide from the atmosphere, which contains that gas to the
extent of three to four parts per ten thousand, and by the aid of
sunlight assimilate the carbon of the carbon dioxide, and set free
its oxygen. From the supply of carbon so obtained, and the
water of their tissues, plants build up more and more complex
organic substances ; also with the addition of nitrogen salts
obtained from the soil even more complex organic substances
containing nitrogen are formed. These nitrogenous organic
bodies form an indispensable item of animal food, and are
known as vegetable proteids. Although the organic bodies
found in the dried substance of plants are very numerous, by
far the greater weight of their organic material belongs to one
of three great classes of organic bodies, which are termed
carhohydrates., fats^ and proteids.

In carbohydrates the elements present are carbon^ hydrogen^
and oxygen. Of these elements, hydrogen and oxygen are
present in the exact proportions required to form water,^ and

^ It must not be supposed, however, that water is present in the carbo-
hydrate molecule. The hydrogen and oxygen happen to be present in the
same proportion as they are present in water ; but they are combined with
the carbon in a comj^lex manner, and not so as to form water.

Diet, Digestion, Absorption, and Metabolism. 133

the ratio of the carbon to the hydrogen and oxygen is variable
in the different groups of the class. AVhen carbohydrates are
burnt, since there is sufficient oxygen in the molecule to com-
bine with all the hydrogen, oxygen is required only for the
carbon, and as much carbon dioxide is formed by volume as
there is oxygen used up. Hence, if an animal could be fed
purely on carbohydrates, it would give out as much carbon
dioxide by its lungs as it took in oxygen. In the case of the
other two classes of food-stufifs (fats and proteids), there is not
enough oxygen in the molecule to combine completely with all
the hydrogen present to form water during combustion. Hence
some of the oxygen taken in by the lungs combines in the
tissues with this excess of hydrogen to form water, and only
the remainder which combines with the carbon reappears, in
the carbon dioxide exhaled by the lungs, in gaseous form.
When fats and proteids are present in the food, therefore, as
they always are, there is less carbon dioxide given out by
volume through the lungs than there is oxygen taken in. The
ratio of carbon dioxide given out to oxygen taken in is spoken
of as the, respiratory qtwtient ; it is increased by carbohydrate,
diminished by proteid, and still more by fatty food.^

The chief groups of carbohydrates slxq glucoses, saccharoses,
and amy loses, or starches P-

The glucoses (CgHiaOe) are the simplest members ; examples
of them are dextrose, or grape sugar, and IcEvulose, which is
formed in the inversion of cane sugar.

The saccharoses (C12H22O11) are somewhat more complex
in constitution ; when treated wdth dilute mineral acid they are
decomposed into glucoses (inversion). Examples are cane
sugar, maltose, and lactose, or viilk sugar.

The starches [(CeHioOj),,] are much more complex than
either the glucoses or saccharoses, into which they become
converted when they are either boiled with dilute mineral acids,
or are treated with certain ferments contained in the digestive
juices. Examples of starches are the ordinary potato and rice

' It is obvious from the above that the respiratory quotient never can
exceed unity.

2 A more recent terminology is mono-saccharides, di-saccharides, and
poly-saccharides.

134 Elementary Physiology.

starch of commerce. The only starch occurring in the animal
body is a substance called glycogen^ or atiiinal starchy which is
found in the liver, and to a less extent in the muscles. Glycogen
accumulates temporarily in the liver after carbohydrate food,
being gradually converted into sugar, and used up in the tissues
afterwards.

The fats contain the same three elements as the carbo-
hydrates, but, as stated above, in quite different proportions.
Chemically, the fats are glycerides — that is, they are formed by
a combination of glycerine with a fatty acid or acids. The fatty
acid present has a large number of carbon and hydrogen atoms
in its molecule, and is hence a very weak acid. To this weak
fatty acid the glycerine behaves as a base, so that the fats are
neutral bodies. There are three chief fats present in the animal
body, viz. olein, palmitin, and stearin. These three bodies have
different melting-points, and, according to the proportion in
which they are mixed, give rise to the different physical and
other characters which distinguish the fat of different species
of animals.

The proteids are, in their chemical composition, the most
complex class of bodies found in the animal body. They
consist of carbon, hydrogen, oxygen, nitrogen, sulphur, and
sometimes phosphorus, united in proportions which vary some-
what for different members of the class. Their chemical con-
stitution is at present unknown, but it is certain, from various
properties which they possess — such as inability to diffuse
through membranes, small percentage of sulphur or phosphorus,
and large number of decomposition products — that they possess
high molecular complexity. Proteids form the chemical basis
of protoplasm, and are hence present in every living cell of
the body. For the same reason they are an indispensable
form of food for all animals, since they are necessary for the
repair of protoplasmic waste. Thus, while an animal can be
kept alive when fed on proteid alone, and denied all fat or
carbohydrate, its life cannot be sustained on a diet of pure fat
or pure carbohydrate, or a mixture of these two food-stuffs.

Proteid might, therefore, be designated as an essential food,
and carbohydrate and fat as accessory foods, but that this would

Diet, Digestion, Absorption, and Metabolism. 135

minimize too much the importance of fats and carbohydrates
as food-stuffs. For, although an animal can be kept alive on
proteid alone, this forms a very inefficient and imperfect diet. A
part of the proteid so eaten goes to do ^vork which can be
much better done by carbohydrate or fat. Just as the chief
purpose of the proteid of the food is to renovate the cell
protoplasm, to repair the waste of cell substance, the chief use
of carbohydrate and fat is to supply chemical energy for cell
activity. When, for example, a muscle contracts, it does so
mainly at the expense of chemical energy supplied by the com-
bustion in the muscle of carbohydrate material. In the absence
of carbohydrate, proteid may be used as a source of energy,
but it is less effectual, and there is a waste in its application.
When the muscles are set hard at work contracting, as when
prolonged exercise is taken, if a sufficient supply of carbo-
hydrate and fatty food be given, there is found to be little
increase in the amount of proteid used up in the body, but a
considerable increase in the amount of fat and carbohydrate
used.^

The only difference in importance, then, between proteid
on the one hand, and carbohydrates and fats on the other, is
that proteids alone can repair protoplasmic waste.

A proper diet hence consists of a judicious mixture of the
three great classes of foodstuffs, and this is the diet which
we naturally select in the mixture of foods which we eat from
day to day.

The digestive apparatus of man is intermediate in type
between that of herbivora and that of carnivora, as is shown
by the form of the teeth and the length of the alimentary
canal. Such a natural condition of affairs indicates a mixed
diet of flesh and vegetables as the best suited for our
consumption.

^ This is shown by determining the amount of urea (the end product
of proteid change in the body) excreted while the amount of muscular work
is varied. The excretion is then found to be little changed, the increase
being quite insufficient to account for the work done, and merely repre-
senting the increased wear and tear on the protoplasm. On the other
hand, the amount of carbon dioxide given off from the lungs is found to be
largely increased, thus pointing to an increased consumption of carbo-
hvdrate or fat.

136 Elementary Physiology.

The objections urged by vegetarians on ethical grounds
against the slaughter of animals for food are utterly opposed
to natural law. For there are whole classes of animals which
can only exist on animal food, and throughout nearly the
whole of the animal world one species preys upon another;
the stronger attacking, killing, and devouring the weaker.
Indeed, animal life cannot be maintained except by preying
on other forms of life ; the animal organism, of whatever type it
he, cannot prepare its food from inorganic sources, but must
ultimately, directly or indirectly, sustain itself on plants, which
are also living, and must die to supply its food. The diet
used by an animal varies with many circumstances. It varies
first of all with the class of animal. A herbivorous animal
has a capacious stomach and intestine, and fares best with
coarse, bulky food, such as vegetables, grass, and hay. A
carnivorous animal has a much less capacious alimentary canal,
the intestine is very short, and hence some food is necessary
which contains a large amount of nutriment in an easily avail-
able form, and in a small bulk, so that it can be readily
digested and absorbed. Such an animal, therefore, is best fed
on flesh meat. The food varies again with the climate and
time of year. Fats are the form of foodstuff, which produce
in combustion the greatest amount of heat from a given weight,
for they evolve nearly twice as much heat as an equal weight
of either proteid or carbohydrate.^ Hence in cold climates or
in winter time much more fat is naturally taken in the food
than in warm climates or in the summer.

The amount of food required by an individual varies with
the extent of his exercise or labour. Hard work requires a
liberal allowance of food, and with a sedentary occupation
the amount of food must be diminished, or troubles of diges-
tion and nutrition arise. About a quarter of the food daily
taken is solid, and the remainder consists of water. Various
normal diets have been given by different observers, but the
conditions are so variable as to make these of little value.
The two most often quoted are the following, which are given

^ The heats of combustion of proteid and carbohydrate are very nearly
equal.

Diet, Digestion, Absorptio7i, and Metabolism. 137

in round numbers, in weights of dry solids, for an average man
weighing 70 kilograms, or about 150 pounds: — Proteid, 120
grammes ; ^ fat, 50-100 grammes ; carbohydrate, 350-500
grammes (Voit) : — Proteid, 100 grammes ; fat, 100 grammes;
carbohydrate, 240 grammes (Ranke).

Besides the three classes of organic foodstuffs mentioned
above, it is necessary to hfe that a supply of inorganic salts
should be taken in with the food, for each day a considerable
amount of these is excreted in the urine. These are in part
contained in the food itself, and in part dissolved in the water
which is drunk with it, while in addition we daily consume a
certain amount of common salt with our food.

Movements of the Alimentary Canal.

The alimentary canal is lined, almost throughout its entire
length, by muscular fibres, which are arranged in two coats. In
the inner coat (that next the lumen of the tube) the fibres are
disposed circularly round the tube to form the circular coat ;
while in the outer coat the fibres are arranged parallel to the
length of the tube, and constitute the longitudinal coat. It is
by the contraction of these muscle fibres in turn that a wave
of contraction is caused to pass along the tube, so gradually
shifting onward the food which is undergoing digestion. The
muscles of the cheek and pharynx, and of the upper part of
the oesophagus, are striped ; but the muscle fibres lining the
remainder of the alimentary track are involuntary, except at
the anus, where the fibres of the external sphincter are striped.

Mastication, or chewing, consists in the comminution of the
food by grinding it between the teeth, under which it is repeatedly
placed by the action of the muscles of the cheeks and tongue.
When the food has been sufficiently broken up by the teeth it
is swallowed, and passes down the oesophagus into the stomach.
In the process of deghttition^ or swallowing, the food, which has
been rolled into a rounded mass or bolus by the action of the
cheek and tongue muscles, is slid back over the tongue, and
carried back on the base of the tongue into the pharynx. So

' There are about 440 grammes in one pound avoirdupois.

138 Elementary Physiology.

far the process is voluntary, but the remainder is involuntary.
While in the pharynx, and before it enters the oesophagus, the
food is in the way of the air passing to and fro between the nose
or mouth and the trachea, and hence during this second stage
of its journey the muscular movements are very rapid, and respi-
ration is suspended while they take place. The soft palate at
the back of the roof of the mouth is raised, shutting off the
nasal passage from the pharynx, and making a wide passage
for the bolus of food ; the trachea and its upper opening, the
glottis, are pulled up in front beneath the base of the tongue,
so as to prevent any possibility of food entering the trachea ;
the bolus of food is grasped in turn by each of three pairs of
muscles, the constrictors of the pharynx, which contract upon
it, andy^;r^ it downwards into the upper end of the oesophagus.
The third act in the process of deglutition consists in the
passage of the food along the oesophagus to the stomach ; this,
in man, is assisted by the action of gravity, but it is easy to
swallow upwards, and many animals habitually do so. The
passage is caused by a peristaltic wave, which consists of an
annular constriction passing along the oesophagus from its
upper to its lower end, and forcing the food in front of it.

The peristalsis of the oesophagus differs from that of the intestine to be
presently mentioned in that it takes place under the direct action of nerve
impulse. If the oesophagus be ligatured or cut across, the wave passes the
point, and is continued on uninterrupted on the other side. On the other
hand, if the nerves to the oesophagus be cut, the peristalsis does not take
place. In the case of the intestine the reverse is the case in both instances ;
section of nerves does not stop the peristalsis, but section of the muscle
fibres does. Hence it is probable that the peristalsis of the oesophagus is
a succession of reflex discharges of nerve impulses along the tube, while
that of the intestine is a muscular contraction propagated from muscle fibre
to muscle fibre.

In the stomach there is an oblique layer of muscular fibres
interposed between the inner circular and the outer longi-
tudinal coats. By slow peristaltic contractions of these various
layers, which become more energetic one or two hours after
a meal, the food is churned about in the stomach, and at a
later period, by more forcible contractions, it is forced out at

Diet, Digestion, Absorptio7t, and Metabolism. 139

intervals through the pylorus ^ into the duodenum, or upper
part of the small intestine.

Slow peristaltic waves pass along the intestine in the form
of annular constrictions, which move the food slowly down the
intestine. These peristaltic waves may be well seen when the
abdomen of a freshly killed animal is opened, for the cold of
exposure increases them. They may be artificially started
at any point by touching with the point of a knife. After a
considerable pause, for the latent period is very long, an annular
constriction begins at the point touched, and usually a con-
stricted wave, which travels very slowly, passes from this point
both up and down the intestine. These intestinal movements
are stimulated and increased by the presence of food in the
intestine, and are diminished, and gradually subside when the
intestine has been empty of food for some time.

The Digestive Glands.

The food while passing along the alimentary canal is acted
upon by secretions, which are poured in at various points along
the length of the canal. In some instances these secretions
are poured in at the openings of comparatively large ducts,
which carry the secretion that has been collected from large
glands lying at some distance from the intestine. In other
cases the glands are minute, and lie in great numbers in the
inner or mucous coat of the canal, and their secretion is poured
out by minute ducts upon the mucous membrane ^ which forms
the inner surface. Examples of the former type of gland are the
salivary glands, the pancreas, and the liver ; and of the latter
there are the gastric glands lying in the mucous coat of the
stomach, and the glands of Lieberkiihn, which are small tube-
like glands embedded in the mucous coat of the intestine.

* The stomach is closed at its two openings (the cardiac orifice and
pyloric orifice) by muscular rings called sphincters ; that separating it from
the oesophagus is termed the cardiac sphincter, and that separating it from
the duodenum the pyloric sphincter. These rings are only relaxed in order
to allow food to pass in or out of the stomach, and the food does not pass
out continuously, but only at intervals when the pylorus is opened.

^ The term mucous viembrauc is applied to a surface moistened by mucus
which is secreted by some of the cells forming the surface. Such a mucous
membrane lines the alimentary tract, as well as the respiratory and nasal
passages, which may be regarded as prolongations or diverticula of it.

I40

Elementary Physiology.

Besides these glands there are single cells in large numbers
to be found in the inner lining layer of the intestine/ which,
from their shape, are termed goblet cells. These goblet cells
secrete mucus, which moistens the surface of the intestine, and
may be regarded as the simplest type of gland to be found in
the body. In the arrangement of their cells, the salivary glands
and the pancreas are what is known as racemose glands. The
cells which furnish the secretion are aggregated in little lobules

"^^"

Fig. 69.— Section of the submaxillary gland of the dog, showing the commencement of
a duct in the alveoli. (Magnified 425 diameters.)

a, one of the alveoli, several of which are in the section shown grouped around the com-
mencement of the duct it' ; a' , an alveolus, not opened by the section ; b, basement-
membrane in section ; c, interstitial connective tissue of the gland ; d, section of a
duct which has passed away from the alveoli, and is now lined with characteristically
striated columnar cells ; s, semilunar group of darkly stained cells at the periphery
of an alveolus.

or bunches, which are composed of smaller groups of cells
called acini or alveoli. In each acinus the cells are arranged
round a central minute duct, or hcmen^ which serves to carry
off the small quantity of secretion furnished by the acinus.
The minute ducts leading from the acini unite with one another
to form larger ducts, which again unite, until finally there is

1 Such cells are also found amongst the ciliated cells of the trachea and
bronchi, and in other similar situations.

Diet, Digestion, Absorption, and Metabolism. 141

formed one chief duct, which carries the entire secretion of
the whole gland. The whole structure is thus somewhat like
a tree branch or a bunch of grapes in its arrangement, and
it is for this reason that such glands are called racemose.

An idea of the arrangement of the cells in such a gland
may be gathered from the accompanying drawings, which show
the appearance presented by thin sections of the submaxillary

Fig. 70.— Section of the pancreas of the dog. (Klein.)
d, termination of a duct in the tubular alveolus, a.

salivary gland and of the pancreas of the dog. In the
salivary glands the ducts are more numerous than in the
pancreas. The alveoli, or acini, are also much shorter and
more rounded in the salivary glands than in the pancreas,
where they form long columns of cells. The bile duct, which
carries the bile from the liver to the duodenum, arises in a
similar branching fashion from the lobules of the liver. But
the secretion of bile is not the main function of the liver, as
that of the saliva is of the salivary glands ; it has other impor-
tant work, which will be indicated later.

There are three pairs of salivary glands, known as the
parotid, submaxillary, and stiblingiial glands respectively, and
besides these there are a large number of much smaller glands
lying beneath the mucous membrane of the mouth, and opening
upon it by minute ducts.

142

Elementary Physiology.

^ The parotid gland (see Fig. 71) is the largest of the three
salivary glands and weighs nearly one ounce (20 to 30
grammes). It lies in front of and below the ear, and extends
deeply into the cleft behind the ramus of the lower jaw.

Fig. 71. — Sketch of a superficial dissection of the face, showing the position of the
parotid and submaxillary glands. (Allen Thomson.) |

/, parotid gland;/', socia parotidis ; d, the duct of Stensen before it perforates the
buccinator muscle ; a, transverse facial artery ; n, n, branches of the facial nerve
emerging from below the gland ; /, the facial artery passing out of a groove in the
submaxillary gland and ascending on the face; S7n, superficial portion of the
submaxillary gland.

Its duct (Stensen' s duct) leaves the gland at its anterior border,
and runs forward in the cheek external to the masseter muscle,
round the anterior border of which it turns and passes inward
to open into the mouth on the inner surface of the cheek
opposite to the second molar tooth of the upper jaw, where
there is a small papilla.

The submaxillary gland is next in size, weighing about a

Diet, Digestion, Absorption, and Metabolism. 143

quarter of an ounce (8 to 10 grammes). It is ovoidal in form,
and is situated below and to the inner side of the base of the
lower jaw. The duct (Wharton's duct) leaves the gland
posteriorly, and then turning forward and inward beneath the
subhngual gland (see Fig. 72), it runs forward and opens into
the mouth, close to its fellow of the opposite side, at the
frcBmim lijigiirce^ that band which binds down the tongue in front.
The sublingual gldiud (see Fig. 72) is much smaller than the
other two. It lies in the floor of the mouth, covered only by

Fig. 72. — View of the right submaxillary and sublingual glands from the inside. (Allen

Thomson.)

Part of the right side of the jaw, divided from the left at the symphysis, remains ; the
tongue and its muscles have been removed ; and the mucous membrane of the right
side has been dissected off and hooked upwards so as to expose the sublingual
glands ; j w, the larger superficial part of the submaxillar^' gland ; /, the facial
artery passing through it ; j- m', deep portion prolonged on the inner side of the m3-lo-
hyoid muscle in h ; s I, is placed below the anterior large part of the sublingual
gland, with the duct of Bartholin partly shown ; 5 /', placed above the hinder small
end of the gland,, indicates one or two of the ducts perforating the mucous mem-
brane ; d, the papilla, at which the duct of Wharton opens in front behind the incisor
teeth ; a! , the commencement of the duct ; h, the hyoid bone ; n, the lingual nerve.

mucous membrane, between the tongue and the gums of the
lower jaw, where it forms a slight swelling. The sublingual
gland has several ducts (ducts of Rivinus), which either open
separately into the mouth or join Wharton's duct. One of these
is often larger than the others and is termed the duct of
Bartholin ; it usually opens into the submaxillary duct.

The saliva is the mixed secretion of these various glands.
It is a thick stringy or mucous fluid, which serves the double
purpose of moistening the mouth and the food, and of coating
each bolus over with a slippery envelope which facilitates its
passage to the stomach. Its stringiness is due to the mucin
which it contains ; this may be precipitated from it by the

144 Elementary Physiology.

addition of a few drops of acetic acid.^ Besides this physical
action on the food, saliva has in herbivora and in man 2 a
chemical action upon starch, which it converts into a sugar
{maltose) and a mixture of dextrins.^ This conversion is brought
about by traces of a substance <Z2^&^ ptyalin which it contains.
Ptyalin is the first example which we have met of those
substances called unorganized ferments, or enzymes^ to which the
chemical changes in the food brought about by the digestive
juices are due. These peculiar substances have not yet been
isolated by the physiological chemist in a pure condition, so that
their chemical nature is unknown. They exist only in traces
in the several digestive fluids in which they occur, but are so
powerful in their action that this is no disadvantage. The follow-
ing are the chief general characteristics of their mode of action : —

1. They all act best at a certain temperature which is known
as the optimum temperature ; as the temperature falls below
this their action becomes less energetic and finally ceases. If
the fluid, however, be warmed again they become active once
more ; as the temperature rises above the optimum point the
action also slackens, and at a certain temperature (60° to 70° C.)
the enzyme becomes permanently destroyed and does not
work again when the temperature is lowered.

2. They act best with a given reaction of the fluid and a
definite degree of acidity or alkalinity. Most of them act in
an alkaline medium, hut pepsi?i, an enzyme of the gastric juice,
acts only in an acid medium, and is rapidly destroyed by
alkalies ; those which act in an alkaline medium are, on the
other hand, destroyed by acids.

3. The action of all enzymes is catalytic — that is to say, they
are not changed by the reaction which they induce, and an
indefinitely small amount of enzyme will change an indefinitely
large amount of substance, except in so far as it is lost by
dilution. The action is probably, in most cases, one of hydrolysis,
or the taking up of the elements of water.

^ It remains dissolved in the saliva because of the alkaline reaction of
that fluid.

2 This action is wanting in typical carnivora.

2 Dextrins are carbohydrates intermediate between saccharoses and
starches in their properties.

Diet. Digestion, Absorption, and Metabolism. 145

4. The action of each enzyme is specific — that is, each
particular enzyme acts only on one particular class of foodstuff,
and not upon all three. The enzymes are classified according
to the class of food-stuff they act upon, and the manner of their
action upon it. Amylolyfic enzymes act upon starches, and
convert them into maltose and dextrins. Iiiverfing enzymes
act upon the compound sugars, and convert them into simple
sugars. Proteolytic enzymes act upon proteids, and convert
them into albumoses and peptones. Steatolytic enzymes act on
fats, and convert them into fatty acids and glycerine. Besides
these there are certain enzymes known which cause coagula-
tion of fluids, such as re/inin, which causes milk to clot, and
filmn-fermenf, which produces blood coagulation.

The following table gives an enumeration of the chief
enzymes of digestion, the fluids in which they are found, the
reaction with which they work, and a summary of their action : —

Name.

Digestive

fluid in which
found.

Reaction
of fluid.

Action on food-stuff.

I. Amy lo lytic enzymes:

[a) Ftyali7i

Saliva

Alkaline

Converts starch into malt-
ose and dextrins.

{b) A my lop sin

Pancreatic
juice

Do.

Do.

2. Inverting enzymes :

[a) Invertin

Intestinal

Do.

Converts compound sugars

fluid 1

such as cane sugar and
maltose into simple
sugars such as dextrose
and Icevulose.

3. Proteolytic enzymes :

{a) Pepsin

Gastric

Acid

Converts proteids into al-

juice

bumoses and peptones.

[b] Trypsin

Pancreatic
juice

Alkaline

Do.

4. Steatolytic enzymes :

(a) Steapsin

Pancreatic

Alkaline

Converts fats into fatty

juice

acids and glycerine.

5. C oagulating enzymes:-

{a) Ren n in

Gastric

Alkaline,

Coagulates milk.

juice

neutral,
or faint-
ly acid

' Called " Succus cntcricits.''''

^ There is a ferment as yet not named found in pancreatic juice which also
coagulates milk; similar ferments are also found in the juices of many plants.

L

146 Elementary Physiology.

The action of ptyalin on starch takes place in successive
stages. At first, the starch is converted into a more soluble
form, known as soluble starch; next, a body giving a red
coloration with iodine, and called erythro-dextrin^ is formed
simultaneously with a certain amount of a sugar, called maltose ;
finally, there are formed gum-like bodies, called dextri7is^ which,
since they give no colour with iodine, are called achroo-dextrins,
and a larger percentage of maltose. Complete conversion into
maltose never takes place, even after prolonged action of
ptyalin. This action on starch is stopped about half an hour
after the food has entered the stomach — as soon, that is, as the
acid reaction of the first portions of gastric juice secreted has
destroyed the ptyalin. The saliva contains, besides mucin and
ptyalin, only a small amount of inorganic salts. The most
remarkable of these inorganic salts is a trace of potassmm
sulphocyaiiide^ which can be detected by allowing a drop of
saliva to mix on filter-paper with a drop of very dilute ferric
chloride, when a blood-red colour is usually produced.

The digestive secretion of the stomach is termed the gastric
juice. This secretion is furnished by an immense number of
minute glands lying in the thick mucous coat of the stomach,
which pour their secretion out directly upon the mucous
membrane. There are two types of gland found in the
stomach. In one of these types there are two kinds of cell
(see Figs. 74, 75). One kind of cell, which is by far the more
numerous, lines the lumen of the gland, and lies centrally to
the other kind ; these cells are called chiefs or central cells. The
other kind of cell occurs at intervals, and is removed from the
central lumen by the thickness of the row of chief cells ; these
cells are called parietal, or oxyntic cells. This type of gland occurs
at the cardiac end, and also over the greater part of the stomach ;
it is known as the cardiac gland. The other type of gastric
gland {pyloric glajid) contains only one kind of cell, resembling
the chief cell of the cardiac gland. It is also distinguished by
having a much longer duct and shorter alveoli. These glands
are confined to the pyloric end of the stomach. In connection
with this difference in structure there is a corresponding dif-
ference in the character of the secretion of the two kinds of

Fig. T3.. — A pj-loric gland, from a section of the dog's stomach. (Ebstein.)
w, mouth ; «, neck; tr, a deep portion of a tubule cut transversely.

Fig. 74.— Section of the gastric mucous membrane taken across the direction of the

glands (cardiac part).
b, basement membrane ; c, central cells ; o, parietal cells ; r, retiform tissue (with

sections of blood-capillaries) between the glands.
Fig. 75.— a cardiac gland from the dog's stomach. (Highly magnified.) (Klein.)
d, duct or mouth of the gland ; b, base or fundus of one of its tubules. On the rio^ht the
base of a tubule more highly magnified ; c, central cell ; /, parietal cell. °

148 Elementary Physiology.

gland. The two most important constituents of the gastric
juice are pepsin and hydrochloric acid, and it has been shown
that both these constituents are secreted by the cardiac glands,
and only pepsin by the pyloric glands. Since the only kind of
cell of the pyloric gland is very similar to the chief cell of the
cardiac gland, it has been supposed that the chief cells secrete
pepsin, and the parietal or oxyntic cells, the hydrochloric acid.

The gastric juice has little or no action on carbohydrates
or fats, but acts energetically on proteids.

The chief enzyme is pepsin, which dissolves proteid, and
converts it into substances which are afterwards readily
absorbed by the cells lining the intestine. There are several
stages in the peptic digestion of proteids. First the proteid is
dissolved and rendered non-coagulable by heat, by conversion
into acid albumin ; at this stage it is precipitated, if the solution
be neutraHzed by dilute alkali. Next it passes into more soluble
substances, called albumoses, which are not precipitated on
neutralizing, and differs from the original proteids in several
chemical tests. Finally, a portion of the albumose is converted
into peptone, which is still more soluble and more easily
absorbed by the intestinal cells, and here the process stops.
The whole drift of the chemical changes is thus the prepara-
tion of a more soluble material which can be more readily
absorbed.

The gastric juice contains a second ferment, called rejinin,
which has the property of curdling milk. The process bears a
close resemblance to the clotting of blood (see p. 127); in
both cases an unorganized ferment produces the clotting, and
in both cases the presence of a calcium salt is necessary. The
curdling of milk is probably not the only purpose of rennin
in the stomach, for it is found in the gastric juice of fishes.

By the movements of the stomach and the digestive action
of the gastric juice combined, the food is reduced to a soup-
like mass, of varying consistency according to the nature of the
food, which is called chyme. The chyme has a strongly acid
reaction, due to the hydrochloric acid of the gastric juice. At
intervals, portions of it pass through the pylorus into the
duodenum, where they soon become mixed with those alkaline

Diet, Digestion, Absorption, and Metabolism. 149

secretions — the bile and pancreatic y?«V^— which are poured
into the intestine by two ducts/ opening close to each other
about 3 or 4 inches below the pylorus. By admixture with
these secretions, and with the alkaline mucus furnished by the
goblet cells, the chyme soon acquires an alkaline reaction, and
undergoes further attack by the unorganized ferments or
enzymes of these secretions, which are all most active in an.
alkaline medium.

'Y\\Q pancreatic juice is the most important of the digestive
secretions, containing as it does three distinct enzymes, each
acting on a different one of the three great classes of food-stuffs.

Amylopsin completes that conversion of starches into
maltose and dextrins which was temporarily arrested during
gastric digestion. Its action closely resembles that of ptyalin
(see p. 146), but is more rapid.

Trypsin finishes the digestion of proteids which was
commenced by pepsin in the stomach. It differs from pepsin
in that it acts in an alkaline medium, and in that its action is
much quicker and more profound. The albumose stage is
rapidly rushed through, so that it is impossible to isolate
albumoses from an artificial tryptic digestion,^ in the same way
as from a peptic digestion. Further, if the action of trypsin be
allowed to proceed the peptone formed is broken up into a
number of simpler organic bodies.

Steapsin acts upon the fats of the food which have been
previously freed of their enveloping connective tissue, through
the action of the pepsin of the gastric juice upon it. It splits
a portion of the fat up into fatty acid and glycerine, and the
fatty acids so formed combine with the alkali present in the
intestine to form soaps. In the soapy solution, the remainder
of the fat usually becomes suspended in fine drops forming an

' In man the two ducts usually join before opening into the intestine,
and then open by a common orifice, while in many animals there are two
or even more pancreatic ducts.

^ It is by means of artificial digestion in glass vessels that the properties
of the several digestive fluids described in the text have been discovered.
Either a quantity of the digestive secretion itself or an extract of the gland
yielding it, is allowed to act on a portion of the food-stuff for a given time
under favourable conditions of temperature and reaction, then the fluid is
analyzed and the changes in it observed.

ISO

Elementary Physiology.

emiilsion just as the fat of milk does. In this finely divided
form the fat readily undergoes further attack by the steapsin,
and is probably all converted in the end into free fatty acids
and glycerine. In some animals all the free fatty acid com-
bines with alkalies to form soaps, while in others the alkali
present in the intestine is insufficient for the purpose, and the
fatty acids which are insoluble in water are dissolved by

the agency of the bile. In

addition to the bile and pan-
creatic juice there is an alka-
line fluid secreted over all
parts of the mucous mem-
brane of the intestine by
small glands imbedded in it,
which is called the succus
enter icus. The succus enter iciLs
is probably in great measure
secreted by an immense num-
ber of minute glands called
the crypts or glands of Lieher-

FiG. 76. — Cross-section of a small fragment
of the mucovis membrane of the intestine,

including one entire crypt of Lieberkiihn z,/;/„. Thp«;p nrf^ Qinrnlp hihn
and parts of three others. (Magnified ^"/^^^' -»- ilGSe are SimpiC lUDU-

400 diameters.) (Frey.) lar invaginations of the surfacc

a, cavity of the tubular glands or crypts ; 3, _ ,

one of the lining epithelial cells ; c, the OI the mUCOUS membrane
interglandular tissue ; rf, lymph-cells. t j -,1 ,• n /

Imed with secretmg cells (see
Fig. 76); similar crypts are found in the large intestine, but
in these mucin (goblet) cells are more common (see Fig. 77).
Succus entericus is strongly alkaline in reaction, and contains
an inverting ferment {invertin) which acts on the double sugars
(saccharoses) and changes them into simple sugars (glucoses).
In this way it attacks the maltose formed by the action of
the saliva and pancreatic juice on the starch of the food, and
changes each molecule of it into two molecules of dextrose.
In a similar fashion it converts cane sugar into a mixture of
equal parts of dextrose and Isevulose.^

The chemical changes in the food brought about by the

^ The action is a hydrolytic one — that is to say, the elements of a
molecule of water are taken up, as might be represented by the equation—

Diet, Digestion, A bsorption, and Metabolism. 1 5 1

agency of these various digestive secretions are intended to
adapt it for more easy absorption by the cells lining the
intestine, by rendering it more soluble and diffusible so that
it can more readily enter these cells.

We may next turn out* attention to this process of absorption,

Fig. 77. — A gland of the large intestine of the dog. (From Heidenhain and Klose.)
a, in longitudinal ; b, in transverse section.

and it will be well in the first place to consider the arrange-
ment of the absorbing structures. The digested food is in the
first instance absorbed by the 'cells lining the alimentary canal,
and is afterwards passed on, when it has undergone some
modification in these cells, to the tissue underlying them. By
far the greater portion of the food is absorbed in the small

152

Elementary Physiology.

intestine. The mouth and oesophagus are lined by stratified
epithelium ^ resembling in structure that covering the body ex-
ternally (the epidermis),
only softer in its texture ;
such an epithelium is un-
suitable for absorption,
and practically none takes
place. In the stomach
this stratified epithelium
is replaced by columnar
epithelium, which in a
single layer occupies the
interspaces between the
gastric glands and pene-
trates into them as a
lining epithelium for their
ducts. This epithelium
seems more suitable for
absorption, but the sur-
face of the stomach is
smooth, and hence has
a much smaller area than
that of the intestine which
vastly increased by

is

finger - like projections
upon it called villi (see
Fig. 78). Further, the
gastric columnar cells are
found not to absorb most
substances in solution in
water, or only to absorb
them very feebly j so that
only a small share in the
process of absorption is

Fig. 78. — Small intestine, vertical transverse

section with the blood-vessels injected. (Heitz-

mann.)
F, avillus; G, g'andsofLieberkuhn; Af, musculans j-^'h-pi^ "Ky t]->g stomacll

mucosce ; A, areolar coat; R, nng-muscle "-^-^^^^ ^ :i _

(circular layer of muscular coat); L, longitudi- 'Y\\Q, intCStinC is alsO Imcd

nal layer of muscular coat ; P, peritoneal coat.

^ Stratified epithelium means a lining or coating tissue in which the
cells are many layers thick (see Fig. 92, p. 201).

Diet^ Digestion^ Absorption, and Metabolism. 153

throughout with columnar epitheUum in a single layer, except at
the rectum, where it again becomes stratified. In the small
intestine the surface is not smooth, but is covered all over with
minute projections like the piles on a piece of velvet. In this

Fig. 79.— Cross section of a villus of the cat's intestine. (Hiehly maenified.)

(E.A. S.)

e, columnar epithelium ; g, goblet cell ; its mucus is seen partly exuded ; /, lymph-
corpuscles between the epithelium cells; b, basement membrane; c, blood-capillaries ;
in, section of plain muscular fibres ; c/, central lacteal.

way the surface through which absorption can take place is enor-
mously increased, and the rate at which this process can go on
is made correspondingly more rapid. Each of these little pro-

FiG. 80. — Portion of small intestine distended with alcohol and laid open to show the
valvulse conniventes. (Brinton.)

jections is called a villus. The structure of the wall of the in-
testine, the arrangement of the villi upon it, and the structure
of a villus are shown in the accompanying illustrations. The
intestinal wall, like that of the stomach, has four coats which,

154 Elementary Physiology.

enumerated from the outside to the inside of the tube, are
called the serous or peritoneal, the muscular, the areolar or
submucous, and the mucous coats.

The serous coat surrounds the intestine in the jejunum
and ileum, except at a narrow interval along the attached or
mesenteric border, where it passes off and becomes continuous
with the two layers of the mesentery. The duodenum, as well
as portions of the large intestine (ascending and descending
colon), are only partially covered by peritoneum.

The muscular coat is arranged in two portions, each con-
sisting of many layers of muscle fibres ; the fibres of the inner
portion are arranged circularly round the tube {circular nmsctdar
coat), while those of the outer portion run longitudinally along
the intestine {longitudinal muscular coat). The inner or circular
portion is much thicker than the outer or longitudinal portion.
Between the two muscular coats there is a plexus ^ of nerve
fibres, known as Auerbach's plexus. A similar plexus, but
with a closer meshwork of fibres and fewer nerve cells, lies in
the submucous coat (Meissner's plexus).

The submucous coat consists of a layer of connective tissue
underlying the mucous coat, and closely connected with it.
The blood-vessels ramify in this connective tissue before pass-
ing into the mucous membrane.

The mucou^s coat is bounded on the surface nearer the sub-
mucous coat by a layer of plain muscular tissue called the
muscula7-is mucosce, and its intestinal surface is lined with
columnar epithelium, which covers the villi and the interspaces
between them. The space between these two surfaces is
occupied by fine connective tissue {retiform tissue), which
supports the blood-vessels, nerves, lacteals,^ and crypts of
Lieberkiihn, as well as the villi, into the interior of which it
penetrates. In the meshes of this tissue great number of lymph
corpuscles (that is, amoeboid leucocytic cells) are found.

* A nerve plexus is formed by a number of nerve branches uniting with
one another, interchanging fibres, and then separating again. In the
plexuses described above, which can only be made out with the microscope,
there is formed a network of fibres, in the crossings of which ganglia of
nerve cells are situated.

^ Vide infra, p. 155.

Diet, Digestion, Absorption, and Metabolism. 155

A villus is a projection of the mucous membrane into the
cavity of the intestine. It is visible to the naked eye, and the
neighbouring villi are so closely set together that the inner
surface has a velvety appearance. The villi are an arrange-
ment intended both to greatly increase the absorbing area of
the intestine, and also to bring the absorbing channels nearer
to the absorbing cells lining the intestine. The first of these
two purposes is further aided by transverse foldings of the
mucous membrane as a whole ; these larger folds, which are
known as vahmlcs co?mivejtfes, are shown in Fig. 80.

Each villus is covered by columnar epithelium, and within
this lies fine connective tissue, of which the meshes are filled
with lymph cells (lymphoid tissue). This tissue serves to
support a network of capillary vessels lying just beneath the
columnar cells ; one or more lymphatic vessels, here called
lacteals,-"- which occupy a central position in the villus (see
Fig. 79) ; and a few strands of plain muscular fibres which run
longitudmally surrounding the lacteals, and when they contract
shorten the villus and expel the contents of the lacteal.^ The
blood-supply of each villus is usually obtained from one arteriole,
which passes from the submucous coat through the umsailaris
mucoscE. to the base of the villus. It runs up the centre of the
villus parallel to the lacteal for about halfway, and then breaks
up into a number of capillaries, which form a plexus or net-
work all over the villus, lying just beneath the columnar cells.
The blood is collected from this network by one or two venules,
which originate near the tip of the villus, and passing down to
its base join a venous plexus situated in the mucous coat.
From this plexus the blood is conveyed by veins to the larger
venous branches in the submucous coat.

The capillary blood-vessels, which receive a great part of
the absorbed food after it has been acted upon by the columnar
cells, thus lie in the most advantageous position for favouring

^ Because they are filled during fat absorption with a fine emulsion of fat,
which has a milky appearance ; this stream of emulsified fat can then also
be seen filling the lymphatics in the mesentery, A^hich are on this account
termed lacteals.

- These fibres are derived from and connected with the muscularis
viucoscc, mentioned above as underlying the mucous coat.

156 Elementary Physiology.

speedy absorption. These capillaries take up the soluble
materials which have been formed by the digestion of the
proteids and carbohydrates of the food ; but the absorbed fat
passes them, and finally enters the lymphatic capillaries or
lacteals at the centre of the villus.

The proteids and carbohydrates absorbed by the blood
capillaries are carried by a large vein, called the portal vein, to
the liver, and are there modified in certain ways ^ before being
passed on by the hepatic veins (which leave the liver) into the
inferior vena cava, and so into the general circulation.

The fats pass into the central lacteal in a very finely
emulsified condition, ^ forming a milky fluid called chyle, which
is gathered up by larger lacteals lying in the mesentery, and by
these carried to a number of lymphatic glands (abdominal
lymphatics) lying at the attachment of the mesentery to the
abdominal wall.

After passing through these glands, the chyle is carried to
the thoracic duct, the main trunk of the lymphatic system (see
Figs. 44, 45, pp. 66, 67), which passes up through the thorax,
close to the vertebral column on the left side, to enter the
venous system in the neck at the junction of two large veins of
the neck (jugular vein) and shoulder (subclavian vein).

All three classes of food-stuifs after absorption thus eventually
reach the general blood-stream : the proteids and carbohydrates
via the portal vein, liver, and hepatic vein ; the fats via the
lacteals, abdominal lymphatic glands, and thoracic duct.

The absorbed food-stuffs are modified in various ways by
the columnar cells after their absorption, and also in the liver
on their way to join the general circulation.

The absorption by the columnar cells of substances in
solution from the intestine is not by any means purely a
physical process of diffusion. Many substances which are
very soluble, and diffuse readily through dead or through
inorganic membranes, such as the iron salts, are refused
admission altogether by the living columnar intestinal cells;
and other substances which diffuse very slowly, such as albu-

^ Vide infra, p. 165.

2 The so-called "molecular basis of chyle."

Diet, Digestion, Absoi^ption, and Metabolism. 157

moses, peptones, and dextrins, are taken up greedily by these
lining cells. The process of absorption is hence a selective
one, and depends on the vital activity of the columnar cell.
Nor does the columnar cell act like an inert membrane by
allowing those substances which it does absorb from the
intestine to pass through it unaltered into the retiform tissue
underlying it, and so reach the portal circulation unchanged in
nature. For each cell is a minute laboratory in w^hich raw
products are taken in at the end next the intestinal cavity, in
the shape of digestion products, and finished materials, very
different in character, are turned out at the fixed end to pene-
trate into the retiform tissue, and reach the intestinal capillaries
and the lacteal. In this way all the albumose and peptone
with which the columnar cell is fed from the intestine is con-
verted into coagulable proteid again, such as is found in blood
plasma ; for during digestion no albumose or peptone is found
in the blood of the portal vein, nor any proteid dissimilar to
the ordinary proteids of blood. The carbohydrate of the
intestine, in whatever form it may be absorbed by the columnar
cell, is also acted upon by this cell, and changed into dextrose
or grape sugar j for this is the only form of carbohydrate found
during carbohydrate digestion in the portal blood. Similarly,
fats are synthesized again from the fatty acids and glycerine,
or from the soaps and glycerine, as which they w^ere mainly
absorbed by the lining cells, back to neutral fat. For even
when fatty acids are given as food, it is as neutral fats that
they are afterwards found in the thoracic duct.

The columnar cell of the intestine hence plays an important
part in the process of assimilation — that is, in converting the
products of digestion into other products, identical with those
contained in the blood.

The portal vein carries the blood which has been collected
from the intestinal capillaries to the liver, and in this important
organ the blood undergoes various changes which affect not
only the new constituents derived from the food and absorbed
during the passage through the intestinal capillaries, but also
those constituents which were already present in the blood as
it entered the mesenteric arteries to supply these capillaries.

158

Elementaiy Physiology.

The Liver.
The liver is by far the largest gland in the body, and weighs
on an average 50 to 60 ounces in the adult, being about -^ part
of the weight of the body ; but it is proportionately heavier in
early life, forming about y^ part of the body weight at birth.^
It is divided by a fissure into a right and a left lobe ; of these
the right is much the larger, being about four times as great,
and further divided on its under and posterior surface into three
secondary lobes by smaller fissures (see Fig. 81).

iiliiiiiiiliiil^v^

Fig. 81. — The liver of a young subject, sketched from below and behind.

R.L., right lobe ; L.L., left lobe ; L.S., lobe of Spigelius ; L.C., caudate lobe ; L.Q.,
quadrate lobe;/, portal fissure; ii./., umbilical fissure; g:.hL, gall-bladder; v.c.i-,
vena cava inferior ; i-g., impressions on the under surface of the left lobe corresponding
to the stomach ; C, position of the cardia of the stomach ; X, surface of the liver
uncovered by peritoneum.

Posteriorly there is a transverse fissure at right angles to the
longitudinal fissure at which the vessels supplying the liver with
blood enter. The liver differs from all other glands in the
body in that its chief blood-supply is veno2is^ being carried to it
by the portal vein from the capillaries of the stomach, intestines,

^ The student Mall find it advantageous to accompany this description
with a practical examination and dissection of the liver of a sheep or pig.

Diet^ Digestion, Absorption^ and Metabolism. 159

pancreas, and spleen. Since the liver cells cannot be sustained
by venous blood alo7ie^ any more than the lungs can be by the
venous blood carried to them for aeration by the pulmonary
arteries, a supply of arterial blood is carried to the liver by an
artery (which is small compared to the size of the liver) called
the Iiepatic artery^ just as in the case of the lungs a supply of
arterialized blood is carried by the bronchial arteries. In this
way a sufficient supply of oxygen is brought to the liver cells to
carry on the oxidative changes going on in them. This peculiar
blood-supply of the liver, and its position as it lies interposed
between the blood coming from the alimentary canal, charged
with absorbed products, and the general circulation, show that
an important office of the liver is to produce chemical changes
in the portal blood before it passes on to the general circulation
again. Some materials are taken up by the liver cells from the
blood ; from these substances others are formed, and these new
substances are restored to the blood; it may be to perform
service in another part of the body ; it may be in suitable form
for excretion from the body by other glands. This regulation
of chemical changes by the liver in building up some sub-
stances and breaking down others is spoken of as the metabolic ^
function of the liver.

The blood derived from both the portal vein and hepatic
artery is gathered up by a common system of veins after it has
passed through the liver capillaries, and these veins unite into
larger veins called the hepatic veins, which pour their stream
into the inferior vena cava. The portal vein and hepatic artery
enter the liver together at the transverse fissure (see Fig. 81) and
are accompanied by the bile duct.''-' The three vessels branch

^ Metabolism means the chemical alterations of the ingested food which
go on in the body. Further, when substances are built up or synthesized in
the body, the term anabolisni is used to designate the process ; and when
substances are broken up into simpler ones, and energ}- set free in the process.
katabolism is tlie term used.

- The bile duct has different names in different parts of its course : that part
leading from the liver is called the hepatic duct ; lower down this branches,
and the branch going to the gall bladder is termed the cystic duct ; while the
portion of bile duct below the junction leading to the duodenum is called
the common ML' duct. The gall bladder is a distensible bag attached to the
lower surface of the liver into which the bile flows at intervals when it is
not required in the intestine, and from which it is discharged when an in-
creased supply is required in the intestine for digestive purposes.

i6o

Elementary Physiology.

together as they subdivide within the liver and are surrounded
by a common sheath of loose connective tissue known as the

Fig. 82.— Section of a portal canal.

a, branch of hepatic artery ; v, branch of portal vein ; d, bile-duct ; /, /, lymphatics in

the areolar tissue of Glisson's capsule which incloses the vessels.

capsule of Glisson, the whole being termed a portal canal (see
Fig. 82). The hepatic vein does not accompany these vessels,
but takes a separate course through the organ.

Fig. 83. — Diagrammatic representation of two hepatic lobules.
The left-hand lobule is represented with the "intralobular vein cut across ; in the right-
hand one the section takes the course of the intralobular vein. /, znUr\ohu\sir
branches of the portal vein; k, i7ttra\oh\i\ar branches of the hepatic veins; s, sub-
lobular vein ; c, capillaries of the lobules. The arrows indicate the direction of the
course of the blood. The liver-cells are only represented in one part of each lobule.

Diet, Digestion, Absorption, and Metabolism.

i6i

The liver substance is made up of colonies of cells, forming
small polyhedral masses called lobules (see Fig. 84). The liver
lobules are visible to the naked eye, and they may be distmctly
seen on the fresh liver (especially in the case of pig's hver,
where the lobules are more separated) as hexagonal spots, each

Fir 8a —Section of a portion of liver passing longitudinally through a considerable
hepatic vein, from the pig (after Kiernan). (About 5 diameters.)
H heoatic venous trunk, against which the sides of the lobules are applied ; h, h, h
' three sublobular hepatic veins, on which the bases of the lobules rest and through
coats of which they are seen as polygonal figures ; /, mouth of the nitralobular veins,
opening into the sublobular veins ; z', intralobular veins shown passing up the centre
Sf some divided lobules ; c, c, walls of the hepatic venous canal, with the polygonal
bases of the lobules.

about the size of a pin's head. Each lobule is made up of a
large number of liver cells, which, like the lobules themselves,
and from a similar cause, viz. mutual pressure, are polyhedral
in shape. The liver capillaries mn among these cells in a
manner which will be understood by referring to Fig. 83. The
capillaries arise from the ultimate branches of the portal vein,^

1 The hepatic artery supplies the capsule of the liver, the portal canal,
and the walls of the vessels lying therein, and also has interlobular branches,
which are much smaller than those of the portal vein.

M

1 62 Elementary Physiology.

which run round between the lobules, and are hence termed inter-
lobular veins. The capillaries course inward to the centre of
each lobule, where they unite to form an intralobular vein, and
the intralobular veins, after leaving the lobules, open into the
siLblobular veins (see Fig. 84), which are branches of the
hepatic vein.

The liver cells are large, and distinct in their outline ; they
vary in appearance according to the condition of the animal,
being clear during a period of hunger, but full of granules in a
well-fed animal, especially a few hours after a meal. Some of
the granules consist of a reserve form of carbohydrate termed
glycogen, which the liver cells form from any excess of sugar
which may be present in the blood as it comes to the liver.
These granules stain dark brown with iodine. Others of the
granules are fat globules ; for the liver cells also act as a
temporary storehouse for fats.

The bile ducts, which collect the bile and carry it away from
the liver, commence between the hepatic cells as fine passages,
or canaliculi. The bile canaliculi open into the minute bile
ducts at the circumference of the lobule, and these unite with
one another to form larger ducts

The Bile.

Bile is a thick mucous fluid varying in colour from brown
or orange yellow to olive green. The viscidity of bile is due
to mucin, which is added to it in the gall-bladder. The mucin
may be removed as a stringy precipitate by the addition of a
few drops of acetic acid, and the filtrate then forms a mobile
fluid. Bile contains, in addition to mucin, the following con-
stituents in solution in water : viz. bile salts, bile pigments,
small quantities of fats, lecithin, cholestearin, and inorganic
salts.

The bile salts are sodium salts of two complex organic acids
called glycocholic and taurocholic acid. These are compounds
of an acid called cholalic acid with an amido-acid. In the case
of glycocholic acid the amido-acid is glycocoll (amido-acetic
acid), and in the case of taurocholic acid it is taurine (amido-
oxyethyl sulphonic acid). An amido-acid is one in which an

Diet, Digestion, Absorption, and Metabolism. 163

atom of hydrogen has been replaced by a molecule of ammonia,
and hence both glycocoll and taurine contain nitrogen ; taurine
further contains sulphur. The presence of nitrogen and
sulphur in these bodies points out that they are products of the
decomposition (katabolism) of proteids, and hence that the
liver in which they are formed has an important influence on
proteid metabolism.

The bile salts have an important function in rendering
cholestearin and lecithin soluble in the bile, and so providing a
means for the removal of these substances from the body.
Cholestearin and lecithin are decomposition products of the
nervous tissues ; they are insoluble in water, but by the agency
of the bile salts are rendered soluble in the bile, and so can
easily be conveyed to the intestine in solution and removed
from the body. As further evidence of this use of the bile salts
there is the fact that they are re-absorbed in great part from
the intestine and are again excreted by the liver, thus completing
what is known as the circidatioii of the bile. A similar purpose
is served by the bile salts in the absorption of fatty acids and
soaps from the intestine, for the solubility of these is much
increased by the presence of bile salts.

The bile salts have an intensely bitter taste, and may further
be recognized in solution by the intense violet they give when
warmed in a thin film with a drop of strong sulphuric acid and
a crystal of cane sugar [Fettenkofer s test).

The bile owes its colour to the bile pigments. Two pig-
ments are usually present in bile, in varying quantity, called
bilirubin and biliverdin. Bilirubin has a golden yellow colour,
and biliverdin a deep olive green, so that the varying colour of
bile is due to the different proportions in which different
samples contain the two pigments. The two pigments are
closely related in chemical composition, biliverdin being an
oxidized derivative of bilirubin. When yellow-coloured bile is
electrolyzed it turns green round the positive pole on account
of oxidation taking place there ; the same effect is obtained if
it be made more strongly alkaline and exposed to the oxygen
of the air in thin layers; or if it be exposed to a gentle
oxidizing agent such as iodine solution. When a stronger

164 Elementajy Physiology.

oxidizing agent is employed, such as strong nitric acid, the oxida-
tion proceeds further, and after biHverdin a blue coloured body
is formed {bilkyanin), which is finally replaced by a dark brown
substance called choletelin.

On account of these successive oxidations, when a thin film
of bile is spread out on a porcelain vessel or on a piece of
filter-paper, and a drop of strong fuming nitric acid added in
the centre, a change of colours is observed — first green, then
blue, and finally dark brown. These colours form rings round
the central spot, the green being farthest removed from the
acid. These colour changes constitute GineliiUs test for the bile
pigments.

Bilirubin is the most reduced of the naturally occurring bile
pigments, but when it is acted upon by a reducing agent (such
as sodium amalgam) it yields an artificial compound called
hydrobilirubin. The importance of this hydrobilirubin lies in
the fact that it has also been obtained by acting with reducing
agents on hsematin, a decomposition product of haemoglobin,
and hence shows that the bile pigments are products of a down-
ward metabolism of haemoglobin taking place in the liver.
This relationship of the bile pigments to haemoglobin is further
shown by the fact that in old blood-clots formed abnormally in
the blood, the haemoglobin of the red blood corpuscles is con-
verted into a body called hcsjnatoidin, which has been shown to
be identical with bilirubin. The bile pigments are not present
in large amount by weight in the bile, and but for this connec-
tion with haemoglobin would not have any great physiological
importance ; they are not absorbed again like the bile salts, but
pass out with the faeces. In the passage along the intestine
they become completely reduced, and are present in the faeces
as hydrobilirubin, which when it was first discovered here was
termed stercobilin.

Bile has a strongly alkaline reaction, due to the sodium
carbonate and phosphate which it contains ; sodium chloride
is the only other inorganic salt which it contains in appreciable
quantity.

Diet^ Digestion, Absorption, and Metabolism. 165

Metabolism in the Liver.

We have next to consider the chemical action of the Uver
on the blood flowing through it. The function of the liver is
here twofold in character ; in the first place its cells act as a
temporary storehouse for certain classes of food-stuff, and in
the second, the final stages in the degradation or katabolism of
other food-stuffs take place here, yielding products for excretion
from the blood-stream, either by the liver itself or by the
kidneys.

There is no evidence that the liver acts as a temporary
storehouse for proteids ; such evidence would be most difficult
to obtain, because the protoplasm of the liver cells is largely
made up of proteid, and hence a variation in the amount of
proteid would be hard to estimate.^ On the other hand, there
is clear evidence that the liver cells are concerned in the
katabolism of proteid. This is shown, not only by the identity
of the bile pigments, as stated above, with the degradation
products of haemoglobin, but by the fact that urea., as which
practically all the nitrogen of the proteid of the food is removed
from the body, is formed in the liver.

Urea is excreted by the kidneys in the urine, and it might
be supposed at first sight that urea was formed by the kidney
cells, but this has been shown not to be the case. When the
kidneys of an animal are removed, the production of urea in
its body does not cease; on the other hand, the percentage
of urea in the blood increases, and the animal dies from poison-
ing of its blood with urea. This shows that the kidneys do
not form urea, but merely remove it from the blood passing
through them. If the liver, instead of the kidneys, be removed,
there is no accumulation of urea in the blood, but instead
certain ammonium salts make their appearance. Further, if
the blood from the portal vein of a recently fed animal be

^ There never takes place in the body in general so much storage of
proteid as there does of carbohydrate or fat ; this is shown by the fact that
when the proteid in the food is increased, the nitrogen excreted as i;rea is
correspondingly increased. Under such conditions, the excess of proteid is
not used to form tissue, but as a source of energy, and a saver of carbo-
hydrate and fat.

1 66 Elementary Physiology.

passed through the excised Hver of an animal which has not
been recently fed before death, it is found that the percentage
of urea in the blood increases. Again, if an ammonium salt
be added to whipped blood, which is then perfused through a
liver just excised from an animal, it is found that a great deal
of the ammonia disappears and is replaced by urea. It is clear
from these experiments that the liver cells produce urea, and
that they probably manufacture it from ammonium salts cir-
culating in the blood. ^

It was at one time believed that the amount of urea so formed represented
the tissue waste, and hence that the amount of urea formed and excreted
must vary directly as the amount of work done by the tissues. This is not,
however, the case, the amount of urea excreted is directly proportional to
the amount of proteid food consumed. Of course, increased work by the
tissues increases the amount of wear and tear on the protoplasm of their
cells, and hence increases the amount of tissue repair required, but this
amount is insignificant compared with the amount of proteid or other food-
stuff broken down to supply energy for muscular contraction. Carbohydrate
and fat can replace proteid as sources of muscular energy at the expense of
chemical energy. Hence if the amount of carbohydrate food given be
sufficient, and there be no increase of proteid food, increased muscular work
can be done with little or no increase in the formation of urea, i.e. without
increased proteid consumption. There is, however, increased production
of carbon-dioxide and water, which indicates combustion of carbohydrate
to supply the necessary energy for the muscular work done. It follows that
the relative amounts of urea, of carbon-dioxide, and of water formed,
depend on the relative amounts of carbohydrate, of fat, and of proteid
taken as food, and upon these only.

While it is improbable that the liver acts to any appreciable
extent as a storehouse for proteids, there is no doubt whatever
that it has an important function in so acting with regard to
carbohydrates.

When an animal is liberally fed on a carbohydrate food,^
the cells of its liver soon become charged with granules of a
carbohydrate belonging to the amylose (polysaccharide) group,

^ The most probable salt is ammonium lactate, for lactic acid is formed
during muscular contraction, and it is also in the muscles that most oxida-
tion of proteid takes place when proteid food is liberally given ; the ammo-
nium lactate so formed passes to the liver, and, in some unknown way, the
liver cells prepare urea from it, which is restored to the blood, to be again
removed in the kidneys and passed out in the urine.

- Such as rice, potatoes, or carrots.

Diet, Digestion, Absorption, and Metabolism. i6y

and known a.s g/ycogen, or animal starch. The glycogen granules
can be stained a deep brown when sections of the liver are
treated with tincture of iodine ; also the glycogen itself can be
prepared in quantity from such a liver, and its chemical
properties tested.^ After death it quickly changes into grape
sugar; a similar change also takes place during the life of the
animal, if the carbohydrate supply be stopped or greatly
diminished. Glycogen, besides being found in the liver, is
also found in lesser quantity in the muscles, but disappears
after the muscles have been fatigued by over-work, being then
used up to furnish a supply of energy. During a period of
rest, there is a new formation of glycogen, which again becomes
expended in the next period of activity.

Storage of glycogen in the liver cells takes place when the
percentage of dextrose in the portal blood coming to them
exceeds a certain limit ; as, for example, during the digestion
of a meal containing carbohydrate. On the other hand, when
the amount of dextrose in the portal vein falls below a certain
limit (about 2 parts per 1000), there is an insufficient amount
of circulating carbohydrate to supply the necessary energy to
the tissues (especially to the muscles), and accordingly a certain
amount of the carbohydrate previously stored in the liver cells
in the form of glycogen is reconverted into dextrose, and
discharged into the b lood- stream ^ to keep the percentage of
circulating carbohydrate up to the normal mark. These state-
ments are supported by the experimental observations that,
during digestion of carbohydrate the percentage of sugar (dex-
trose) in the portal vein (going to the liver) is greater than that
in the hepatic vein (leaving the liver) ; while, 'during a period
when no digestion is taking place, the situation is reversed,
and there is more sugar in the hepatic blood than in the portal
blood. Thus the liver cells act as governors on the amount of
soluble carbohydrate in the form of dextrose circulating in the
blood. This is an important function, for, on the one hand,
excess of dextrose in the circulation acts injuriously on the
tissues, and the excess is treated as a foreign substance, and

* See Appendix.

^ Not directly, of course, but by the medium of the intervening lymph.

1 68 Elementary Physiology.

excreted by the kidneys, giving rise to a great waste of the
carbohydrate food ; while, on the other hand, an insufficient
amount of circulating carbohydrate leads to debility of the
muscular tissues, and to morbid changes in the cells of the
tissues generally.

This storing of carbohydrate is spoken of as the glycogenic
function of the liver.

This function may be disturbed by certain artificial means, such as removal
of the entire pancreas ; puncture of a portion of the brain, known as the fourth
ventricle ; administration of certain drugs, such as phloridzin, or curare.
When so disturbed, the liver cells do not behave any longer in a normal
fashion ; there appears an excess of sugar in the blood, which is passed out
of the body in the urine [glycosuria^ diabetes)^ and wasting of the tissues is
the result. Under such circumstances, complete stoppage of carbohydrate
in the food, although it diminishes the amount of sugar excreted, does not
entirely stop it ; for the liver cells continue to form an excessive amount of
carbohydrate from the proteid of the food, which they are able to act upon
and convert in part into carbohydrate.

The liver cells are also capable of acting as a temporary
storehouse for fats ; for, after a meal rich in fat, the cells of the
liver are found to contain fat granules.^ But such a storage
of fat in the liver is, under normal conditions, very transitory,
and prolonged storage takes place chiefly in the connective
tissue of certain regions of the body, such as, in the sub-
cutaneous connective tissue generally, and more especially in
that of the abdomen ; in the connective tissue lying under the
peritoneum in which the kidneys are embedded ; in the great
omentum ; and upon the muscular tissue of the heart under-
neath the pericardium. In these parts the cells of the con-
nective tissue become loaded with fat, which first appears in
the cells as minute granules. These granules become larger
and coalesce until the cell becomes mainly a globule of fat
surrounded by a membrane with only the nucleus and a trace
of the cell protoplasm remaining. Connective tissue so altered
to store up fat is termed adipose tissue ; it becomes arranged in
lobules, each of which is copiously supplied with blood by a
small arteriole, from which capillaries are formed surrounding

' The granules are shown to be fat by their staining black with osmic
acid, and by their solubility in ether or xylol.

Diet, Digestion, Absorption^ and Metabolism. 169

the fat cells. The fat cells probably synthesize a great deal of
their fat from carbohydrate instead of from fat, and it is also
probable that when there is a scarcity of fatty or carbohydrate
food, and the storage of fat comes into use, that part of it is
returned, by the aid of the cells, to the blood as carbohydrate.

In the animal organism it is certain that proteid cannot be
formed from carbohydrate, or fat, and nitrogenous inorganic
salts.-^ Such a synthesis can only be carried out by plant cells,
and hence upon plant proteid directly (herbivora) or indirectly
(carnivora) animal life is dependant for its indispensable proteid
supply. With this one reservation, however, it may be stated
that the three classes of food-stuff can replace one another, and
can be converted to a certain extent into one another in the
course of the complex chemical changes which go on in the
cells of an animal's body.

Thus, proteid can be used up by the cells, and either carbo-
hydrate or fat produced in its stead. If the liver of an animal
be freed of glycogen by administration of a drug (such as
phloridzin) which causes the glycogen to be discharged as
sugar in the urine, and then, the administration of the drug
being stopped, is kept on a purely proteid diet for some time
and finally killed, it is found, post-mortem, that the liver cells
contain a fair amount of glycogen. Again, in cases of diabetes,
as stated above, the excretion of sugar in the urine cannot be
completely stopped by placing the patient on a diet from which
carbohydrates are carefully excluded, or even by feeding on
proteid alone.

Similarly, if fat be excluded from the food, and a purely
proteid diet be given to an animal, in a short time the liver
cells become free from fat. If, at this stage, phosphorus be
administered to the animal, and it be killed after some time, it
will be found that the liver cells are loaded with fat. This
production of fat is due to " fatty degeneration " of the proteid
constituents of the protoplasm of the cells under the influence
of the drug. Such an over-production of fat from proteid is
merely an exaggeration of a normal process, and it is probable
that when an excess of proteid food is taken, above that

' This is shown by the fact that animals cannot live without proteid food.

I/O Elementary Physiology,

required for the immediate wants of the animal, that a portion
of the excess is in part converted by chemical changes brought
about by cell protoplasm into carbohydrate or fat.

There is also abundant evidence that carbohydrate can be
converted into fat and stored as such in the adipose tissue of
the body. In fact, carbohydrate food is a more efficient fat
producer than are the fats themselves. This has been shown
by experiments on the fattening of swine. In one case carbo-
hydrate food was given, and in the other a corresponding
weight of fat, and it was found that the animals fed on carbo-
hydrate accumulated much more fat than those fed on fat.
Further, on feeding young animals on a food rich in carbo-
hydrates, and containing as little fat as possible, it is found that
the amount of fat stored up in the body is much greater than
the total amount of fat found by analysis as having been given
in the food.

So marked is this formation of fat from carbohydrate that it
was at one time thought that none of the fat taken in the food
could be directly stored in the body without alteration. It was
supposed that all the fat of the food served as an immediate
source of energy, for muscular work and for heat production
(especially the latter) ; that all the fat found in the adipose
tissue was synthetically formed from other material, chiefly
carbohydrate, by the activity of the cell protoplasm ; and that
none could be directly laid down without change from the fat
of the food. This view was supported by the fact that the fat
of each species of animal has a fairly definite chemical com-
position and melting-point, due to the admixture of the several
fats composing it in definite proportions. Thus, pig's lard has
a lower melting-point than the fat of beef suet, and this again
melts more easily than the fat of mutton suet.^ But it has been
shown that, tmder certai7i circumstances^ the fat of the food may
become directly incorporated as tissue fat. This has been

^ These fats are mixtures of three fats, called olein, palmitin, and
stearin j of these three, olein is fluid at ordinary atmospheric temperatures,
and stearin is solid at even several degrees above body temperature. The
melting-point of the mixture varies with the proportion of each present ; m
lard there is a large amount of olein, in mutton fat very little, and hence
lard is fluid at body temperature, while mutton fat is solid.

Diet, Digestion, Absorption, and Metabolism. 171

shown in two ways. One consists in feeding an animal for a
long time on a form of fat containing a peculiar constituent
which can be easily recognized again by chemical methods, and
which is not contained in the ordinary fats of the animal's
body.^ Afterwards, when the animal has been killed, this
peculiar constituent has been found in the fat of its body. The
other method consists in keeping the animal on a low diet for
some time, until the storage of fat in its body has become
greatly reduced, and then feeding for a considerable time with
fat from another species of animal. It is found that when a
dog is so treated, and fed on mutton suet, that the fat of its
body has a much higher melting-point than is normal for the
fat of the dog — approximating, in fact, to that of mutton fat.

In order to obtain a successful result by either of these
methods, it is necessary, however, that the stock of the animal's
own fat should be low, that it should be allowed to eat no
other kind of fat, and that carbohydrates should also be
excluded from the food. These are somewhat abnormal con-
ditions, and hence, although the experiments demonstrate that
the fat of the food may be directly built up into tissue fat
under certain conditions, they do not show that such direct
deposition takes place to any marked extent under normal con-
ditions. In fact, it is probable that under usual conditions the
greater part of the fat of the body is synthesized by the cells of
the adipose tissue from carbohydrate.

To sum up, then, all three classes of food-stuffs are used
in the body as sources of energy. They supply the cells of
the various tissues with nutriment, and undergo chemical
changes in these cells in the course of which their chemical
energy becomes diminished, being converted into cell activity
and in the end into heat, which serves to keep up the temperature
of the body. Finally, after passing through intermediate stages,
they are resolved into bodies of much simpler chemical

^ Fats which have been used for this purpose are linseed oil, which con-
tains erucic acid ; and spermaceti, which contain cetyl alcohol ; these
substances are not present in the ordinary fats of the food and of the body,
and their presence in the body-fat, after ingestion, serves as a signal that
the fat of which they formed a constituent has not been broken up to any
great extent in the body.

172 Elementary Physiology.

composition,^ and possessed of but little chemical energy, which
are excreted from the body chiefly by the lungs and kidneys.
In yielding to the body a store of energy the different food-
stuffs can replace one another, but a certain amount of proteid
is i7i animal life indispensable, because this form of food alone
can repair the waste of protoplasm going on in the tissues.

^ The chief of these are water and carbon dioxide, in the case of the fats
and carbohydrates ; and in addition to these urea, with traces of other
nitrogenous bodies, in the case of the proteids.

CHAPTER VIII.
RESPIRA TION.

In the processes of oxidation which are continuously going on

in the tissues, oxygen is used up and carbon dioxide formed. The

oxygen is carried to the tissues, and the carbon dioxide from

the tissues by the blood. Hence the blood, in passing through

the systemic capillaries, becomes poorer in oxygen and richer

in carbon dioxide. In order that its composition may remain

unchanged it is obvious that in some other part of the circuit

it must take up oxygen and give off carbon dioxide. This

change takes place in the capillaries of the lung, in its passage

through which the blood takes up oxygen from the air of the

lungs and gives up to it a certain amount of carbon dioxide.

In passing through the systemic capillaries the arterial blood

becomes venous; in the passage through the pulmonary

capillaries the venous blood becomes arterial.^

That a rapid exchange of gases may take place in the lungs

two conditions are necessary : first, that there should be a

large surface of blood exposed to the action of the air in the

lungs, and that this large surface of blood should be as little

as possible separated by tissue from the air, so that rapid

diffusion may take place ; and, secondly, that there should be

some means of quickly changing the air to which the blood

is exposed, so that it may not become charged with that

gaseous product (carbon dioxide) which it is essential should

be removed from the blood, nor poor in that constituent

(oxygen) which it is necessary that it should furnish to the

blood.

^ Hence the pulmonary artery contains venous blood, and the pulmonary
vem arterial blood.

174

Elementary Physiology.

Both these conditions are fulfilled in the respiratory
apparatus, in which a large surface of capillaries is constantly

Fig. 85. — The trachea. Front.
h, hyoid bone ; ti , thyroid cartilage ; c,
cricoid ; e, epiglottis ; tr, trachea ; b
and 3', bronchi.

Fig. 86.— The trachea. Back.
a, arytenoid cartilages ; h, hyoid bone ; tt' ,
thyroid cartilage ; c, cricoid ; c, epi-
glottis ; tr, trachea ; b and b' , bronchi.

exposed to air, which is continually renewed by the respiratory
movements and gaseous diffusion.

The manner in which the air is alternately pumped into and

Respiration. I75

out of the lungs, on account of the lungs being compelled to
follow passively the alterations in volume of the air-tight thorax,
has already been explained (see p. 32) ; it remains to describe
the structure of the respiratory system, and the manner in
which this structure facilitates an exchange of gases between
blood and air.

The respiratory system communicates with the alimentary
canal at the front and lower part of the pharynx, where the
windpipe or air-passage begins in a cartilaginous box called
the larynx, which is the organ of voice. To the lower

Fig. 87. — Diagrammatic representation of the ending of a bronchial tube in sacculated

infundibula.
B, terminal bronchus; LB, lobular bronchiole ; A, atrium ; I, infundibulum ; C, air-
cells, or alveoli.

end of the larynx the trachea is attached (see Figs. 85 and 86) j
this lies in front of the oesophagus in the neck, and enters the
thorax, where it soon bifurcates into two branches termed the
hronchi, which pass one to each lung. The bronchus enters
the lung at the root, and within the lung branches and branches
again, in a tree-like fashion, giving rise to the bronchioles, which
become smaller in section at each successive branching. The
ultimate branches are known as the bronchial tubes, each of
which ends in an expanded sac-like structure known as the
infundibulum. The walls of each infundibulum are lined by
honeycombed recesses known as the air cells, or alveoli. It
is in these minute air-cells, or alveoli, lining the walls of the

176 Elementary Physiology,

infundibula that the real work of gaseous exchange between
the air filhng the infundibula {alveolar aij-) and the blood in the
pulmonary capillaries takes place. It will be readily under-
stood that by this arrangement a very large extent of surface
is obtained. By far the greater part of the volume of a
moderately distended lung is made up of these infundibula
lined with alveoli, so that the total surface so exposed for the
aeration of the blood amounts to many square yards. ^

The trachea, bronchi, and bronchioles are held open, so
as to always allow free passage to the air in and out, by
incomplete hoops or rings of cartilage situated circularly in
the substance of their walls. The cartilages are connected
by fibres of elastic tissue running lengthwise along the air-
passages, internal to the cartilages, and between them. Ex-
ternally there is a coat of connective tissue, and internally
there is a thick mucous membrane. The mucous membrane is
lined throughout by ciliated epithelium ^ from the upper part
of the trachea to the termination of the bronchioles at the
commencement of the infundibula (see Fig. 88). Amongst
the ciliated cells there are situated a large number of goblet
cells which secrete mucus to moisten the ciliated mucous
membrane. The cilia move in such a way as to move any
foreign particles which may have been drawn in with the air
(as well as the mucus moistening them) upwards towards the
pharynx and so away from the lungs.

In this manner, the lungs are greatly protected from being choked with
dust and floating particles of all kinds drawn in with the air. Still, the
protection is not quite complete, and some of the finest particles float in
the air within the air-passages, without touching the walls and so adhering
to them, quite down to the alveoli, where there is no ciliated lining. Such
particles become absorbed, and in the case of old persons who have lived
many years in large cities, the lungs after death are found to be quite black
from having become in this manner impregnated with fine coal dust.^

^ The alveolar area has been estimated at 200 square metres (over 240
square yards).

2 The greater part of the larynx (except over the true vocal cords and
over the epiglottis) is also lined with ciliated epithelium.

^ A portion of this foreign matter is taken up by the numerous lym-
phatics of the lungs and carried to a group of lymphatic glands situated
at the root of each lung where it becomes deposited, so that after some

Respiration.

177

The epithelial cells rest on a membrane {basement jjiemhrane)
which separates them from the rest of the mucous coat under-

•o S I-

c >< c

« C5 S

iJ ° O

.s

part, a
inular la;
een cut

B

VO

u H fi

c

rt.

-7-

c rt

^

1)

iT

0)

-Q

3

.s^ ^

-"■-o

^-^s

[rt

0

§::^

C

-d

« ° 3

0

c u _

0

s

bo

« ^ Si

2 c.y

0

0

1)

^ c H
1)

>

U5

rt ^ <u

a!

£-« y

0

XI C *J

r-

<^J2 J§

.2

-0 M-3

a

S rt -

^

rt " (J

3

0

.. "^-i

1

rt 4J oj

J2

1

.— <->>-

■^

CO

r =^-°

0.

00

rt-^«

«

lying them ; this consists of lymphoid tissue (that is, connective
tissue in which the meshes are filled by lymph corpuscles)

years these dands become quite oritty ; in fact, in old persons who have
lived in cities, more like pieces of coke than glands. This deposition of
carbon is, however, quite innocuous, and is not sufficient to interfere materi-
ally with the functions of the lungs.

N

178 Elementary Physiology.

and supports numerous small blood and lymphatic vessels which
supply the surrounding tissue. In this tissue, as well as deeper
down near the cartilaginous rings, in what is termed the sub-
mucous layer ^ lie numerous mucous glands (see Fig. 88) which
open on the ciliated surface and assist the goblet cells in
providing the moistening mucous fluid. An annular ring of
muscle fibres of the plain or involuntary variety is also present,
underlying the mucous coat, and most strongly developed
opposite that part where the cartilage is incomplete. In the
trachea and bronchi, the cartilages are all incomplete at the
same part, viz. at the back, and here there is a strong band
of involuntary muscle fibres arranged horizontally; while in
the bronchioles the incomplete portions of the rings do not
correspond in situation. The advantage of the incomplete-
ness of the cartilaginous rings at the back of the trachea is
obvious ; here the trachea lies against the oesophagus which
is usually collapsed ; but when a bolus of food passes along,
the oesophagus becomes distended, and the absence of the
cartilages in front where the trachea lies in contact with it
allows the necessary distension to take place more freely.

In the smaller branches of the bronchioles as they near
the infundibula the cartilage disappears, and a complete layer
of plain muscular fibres surrounds the tube, the whole being
enclosed by a layer of loose fibrous tissue. The mucous
membrane lying internal to the muscular layer is constituted
much as in the larger tubes, but there is less connective tissue.
In the stratum of the mucous membrane underlying the
epithelium much elastic tissue is present, of which the fibres
are chiefly arranged parallel to the length of the tube. As
the bronchial tube finally expands into an infundibulum, the
wall thins out, and the several layers above described dis-
appear; at the same time the character of the lining epithelium
alters, and the ciliated cells are chiefly replaced by large
irregularly shaped flattened cells, which form an exceedingly
thin delicate membrane covering the alveolus (see Fig. 89).
At some places this flattened epithelial layer is replaced by
cubical epithelial cells (see Fig. 89), but by far the greater
part of the alveolar surface is covered by the flattened scales,

Respiration.

179

which form the only covering (with the exception of the
equally thin walls of the capillaries themselves) separating the
blood in the pulmonary capillaries from the air in the alveoli.

These pulmonary capillaries are arranged in a close mesh-
work over the concave surface of each alveolus, just beneath
the thin pavement epithelium. The pulmonary artery and
its larger subdivisions follow the branchings of the bronchus

Fig. 89.— Section of part of cat's lung, stained with nitrate of silver. (Klein.)
(Highly magnified.)
The small granular and the large flattened cells of the alveoli are shown. In the middle
is a section of a lobular bronchial tube, with a patch of the granular pavement-
epithelium cells on one side.

at first, but the finer branches, in the end, leaving the smaller
bronchioles, branch independently, and finally small arterioles
are given off from these which run round the margins of the
alveoli and give off capillaries all the way round. The
capillaries unite to form minute veins which collect the blood
into larger venous radicles lying in the connective tissue
between the infundibula, and these unite again to form still
larger vessels, which after pursuing an independent course for

i8o

Elementary Physiology.

some time finally run along the course of the bronchioles, and
eventually form the two pulmonary veins which leave the root
of each lung and carry the oxygenated blood back to the left
auricle.

Fig. 90.— Section of injected lung, including several contiguous alveoli. (F.
Schultze.) (Highly magnified.)

E.

a, a, free edges of alveoli ; c, c, partitions between neighbouring alveoli, seen in section ;
b, small arterial branch giving off capillaries to the alveoli. The looping of the
vessels to either side of the partitions is well exhibited. Between the capillaries is
seen the homogeneous alveolar wall with nuclei of connective-tissue corpuscles and
elastic fibres.

The lungs are never completely emptied of air, even when
the greatest effort to breathe out^ is made, for after the greatest
possible expiratory effort the lungs still contain, on the average,
in the adult, 1000 cubic centimetres (about 60 cubic inches)
of air ; this air is called the residual air. The amount of air
which can be breathed out after an ordinary expiration down

' Breathing out is termed expit'ation, and breathing in is termed in-
spiration.

Respiration. 1 8 1

to the greatest possible forced expiration is termed the suppk-
menfal ov reserve ah; and it measures about 1500 cubic centi-
metres. These two fractions, viz. residual and supplemental air,
together make up what is often called the stationary air, because,
in quiet breathing, it is the amount retained all the time in the
lungs. The amount of air normally passing in and out of the
lungs in quiet breathing is called the tidal air, and measures on
the average about 300 cubic centimetres, although it varies so
in different individuals that the average amount possesses little
importance. Between the amount of air in the lungs at the
end of an ordinary inspiration and the amount at the end of
the greatest possible inspiration there is a difference of about
1700 cubic centimetres, and this frac-tion is termed the comple-
mental air.

The maximum amount of air which can be taken into or
breathed out from the lungs by a single effort, obviously includes
the fractions of reserve^ tidal, and complemental air ; this amount
is known as the vital capacity, and measures 3000 to 4000 cubic
centimetres.

In quiet breathing, the air enters the nostrils and passes
along the nasal passages to enter the pharynx at the posterior
openings of these passages — the posterior nares. It passes
through the pharynx, enters the larynx at the opening of this
from the pharynx, called the glottis, and passes down the
trachea into the bronchi. The amount of air taken in at each
ordinary inspiration is quite insufficient to reach the alveoH,
and is merely enough to fill the nasal passages, phar^^nx,
larynx, trachea, bronchi, and larger bronchioles. The rest of
the work, whereby the alveolar air becomes changed in com-
position, is effected by gaseous diffusion, and so perfect and
rapid is this that the alveolar air differs but Little in composition
from expired air.

The air in its passage through the air-passages is brought
in contact with the warm and moist surface of the mucous
membrane lining the nasal cavities, and is here both warmed
almost to the tem_perature of the body, and nearly saturated
with water vapour. These processes are completed before the
air leaves the body, so that the expired air has the temperatiire

l82

Elementary Physiology.

of the body, and is completely saturated with aqueous vapour
at that temperature.^

The chemical composition of the air is also altered in the
process of respiration. Atmospheric air contains in round

-,5

Fig. 91. — Medial section of the face and neck,
sphenoid bone; 2, nasal cavity ; 3, brain cavity; 4, ethmoid bone ; 5, frontal bone;
6, nasal bone ; 7, superior maxillary bone ; 8, palatal bone ; 9, superior turbinated
bone ; 10, middle turbinated bone ; 11, inferior turbinated bone ; 12, soft palate ; 13,
upper part of pharynx ; 14, lower part of pharynx ; 15, oesophagus ; 16, larynx ; 17,
glottis ; 18, epiglottis ; 19, opening of Eustachian tube ; 20, inferior maxillary bone ;
21, tongue ; 22, tonsil ; a to f, bodies of cervical vertebrae ; s, spinal cord ; /, pro-
cesses of cervical vertebrae ; o, portion of occipital bone.

numbers, in loo parts by volume, about 79 parts of nitrogen,
21 of oxygen, and but 3 parts in 10,000 (in good air) of carbon
dioxide, while expired air contains about 4 per cent, of carbon
dioxide, only 16 per cent, of oxygen, and the balance of
nitrogen. The presence of carbon dioxide in expired air can

^ See p. 192.

Respiration. 183

easily be shown by breathing out through lime water, when a
heavy white precipitate of calcium carbonate is obtained. By
somewhat more delicate analysis, the diminution in the per-
centage of oxygen may be detected; this diminution in the
oxygen is always in excess of the carbon dioxide formed, and
the volume of the expired air is correspondingly less than that
of the inspired air, showing that the oxygen which is missing
has not gone to form a gaseous compound. As already
explained, this oxygen has been utilized in combining with
the excess of hydrogen in the fats and proteids of the food
during the combustion of these bodies in the tissues.

The changes in the air in the process of respiration, then,
are these —

{a) The air is warmed (or cooled) to the temperature of
the body.

{b) The air is saturated with aqueous vapour at that
temperature.

(c) The air is changed in chemical cojnposition^ about 5 per
cent. , roughly^ of oxyge?i disappearing and 4 per cent, of carbon
dioxide appearing.

The chemicnl changes which give rise to the alteration in
chemical composition of respired air do not take place, as was
originally thought, in the lungs, but /;/ the tissues. The process
is really one of slow combustion, or chemical oxidation, going
on in the cells of the tissues, and chiefly in the muscular
tissues, and the lungs are a ventilating agency, taking in stores
of oxygen and removing the carbon dioxide formed.

That the oxidation does not take place in the lungs is shown
by the fact that the arterialized blood leaving the lungs contains
a much higher percentage of oxygen than the venous blood
coming to the lungs, and at the same time a much less per-
centage of carbon dioxide,^ thus showing that the blood has

* Although the above statement is true, still both venous and arterial
blood contain more carbon dioxide than oxygen. Blood contains in round
numbers 60 per cent, of gas, of M'hich, in arterial blood, about 40 volumes
are carbon dioxide, and 20 volumes oxygen, while in venous blood about
46 volumes are carbon dioxide, and 8 to 12 volumes are oxygen. Both
arterial and venous blood contain about I per cent, of nitrogen, which is
simply dissolved in the plasma.

184 Elementary Physiology.

given up carbon dioxide in the lungs and taken up a supply
of oxygen for use somewhere else in the circuit. That the
oxidation does not take place in the blood is shown by the
fact that the composition of the contained gases does not
change until the capillaries have been reached, and that it
does take place in the tissues is shown by the fact that, in
passing through the capillaries of the tissues, all the decrease
in oxygen and increase in carbon dioxide occurs.

We have next to consider in what way oxygen is taken up
in the passage through the lungs, and carbon dioxide given off;
why the reverse change takes place in the tissues, and how the
oxygen and carbon dioxide are held in the blood.

The oxygen and carbon dioxide are held dissolved in the
blood partially by physical and partially by chemical means —
that is to say, partially in solution and partially in chemical
combinations.

The amount of oxygen which the blood plasma is capable
of holding in simple solution is very small, and is only a small
fraction of that which is taken up by the blood ; by far the
greater part is held in a loose state of chemical combination
with the hmmoglohin of the red corpuscles, forming an unstable
compound, called (?.r>'-haemoglobin. When the oxygen pressure
in the plasma is high, as is the case when the blood is in
contact with air containing a fair percentage of oxygen (as, for
example, in the lungs), then the haemoglobin takes up oxygen
from the plasma until it becomes saturated with it. On the
other hand, when the oxygen pressure in the plasma is low,
the compound of haemoglobin becomes broken up,^ and the
oxygen is given out to the plasma.

This taking up or giving out of oxygen by the haemoglobin
is not a very gradual process, the amotmt of oxygen taken up
does not mcrease proportionately to the pressure of oxygen, hut
at a certain pressuj'e of oxygen the gas is taken up rapidly, and
when the press2L7t is only slightly higJur, the h(Enioglohin becomes
almost completely saturated, a7id takes up very little more even
if the oxygen pressure be greatly increased. The pressure at

^ The oxy-haemoglobin is then reduced or converted into reduced*
haemoglobin.

Respiration. 185

which the haemoglobin becomes practically saturated is low,
being considerably less than half that of the oxygen in the
alveolar air. The oxygen pressure in the lymph bathing the
tissues is very low, because the cells of these tissues are con-
tinually using up oxygen ; and, at this low pressure, the
haemoglobin rapidly loses oxygen and becomes partially
reduced.

There is an evident advantage in the oxy^gen being thus
held in loose chemical combination, for when a gas is held in
physical solution in a fluid, the amount (weight) dissolved is
directly proportional to the pressure, and in order to hold the
same amount of oxygen in solution in the plasma as can be
held in chemical combination in the haemoglobin, the pressure
would require to be enormous, so that it would be impossible,
under existing atmospheric conditions, to keep the cells supplied
with that amount of oxygen which they require.

Both in the lungs and in the tissues the plasma in which
the red blood corpuscles float plays the part of an intermediary
between the oxygen and haemoglobin. The venous blood
arriving at the lungs is poor in oxygen, and this poverty is
shared by the plasma and the corpuscles ; part of the haemo-
globin is in a reduced condition, the amount depending upon
the pressure of oxygen in the plasma.^ In the passage through
the pulmonary capillaries, on account of the low pressure of
oxygen in the plasma, a small amount of oxygen is first dis-
solved from the alveolar air, and if there were no hjemoglobin,
this small amount would rapidly raise the pressure of oxygen in
the plasma to that of the oxygen of alveolar air, and absorption

^ The pressure of a gas in a fluid is often spoken of as its tension ; thus,
for example, one speaks of the oxygen tension of arterial or venous blood ;
but the term is of no great use, for this tension is measured by the pressure
of the gas over the fluid when fluid and gas are in equilibrium, and gas
is neither absorbed nor liberated, and hence it is quite correct to speak of the
oxygen pressure in this sense. When the pressure on the surface of the
fluid of the dissolved gas is diminished, as, for example, when a bottle of
soda water is opened, then the pressure of the gas within the fluid is greater
than its pressure outside the fluid, and gas is evolved from the fluid ; and on
the other hand, if the pressure of the gas on the surface be increased, more
gas is dissolved, ^Yhen a mixture of gases are exposed to the fluid, the
amount of each taken up depends on its own pressure in the mixture, or, as
it is called, iis partial pressi^re, and not on the total pressure of the mixture.

1 86 Elementary Physiology,

would stop ; but as the pressure of oxygen in the plasma
increases,' the haemoglobin begins to combine with some of the
oxygen, thus causing the pressure in the plasma to rise more
slowly. So with a much smaller change in pressure, a much
larger quantity of oxygen is taken up. Exactly the reverse
change takes place in the tissues, for here oxygen is always
being used up, and consequently the lymph bathing the cells
is poor in oxygen, and the oxygen pressure in it is very low.
There is accordingly a diffusion of oxygen from the plasma
where the oxygen pressure is high into the lymph where the
oxygen pressure is low, and the pressure in the plasma would
soon fall, and so also the supply of oxygen to the cells, were it
not that the lowered pressure cause the oxy-haemoglobin to split
up and yield a fresh supply of oxygen to the plasma. All the
oxygen is never used up in any one circuit through an organ ;
there is always a large stock of reserve oxygen in venous blood,
otherwise respiration could not be stopped for an instant
without causing suffocation.

The passage of the carbon dioxide takes place in an inverse
direction. The carbon dioxide is not certainly known to form
any definite compound, such as oxygen does with the haemo-
globin. It is more soluble in plasma than oxygen is, and a
certain amount is held in solution ; another fraction is held in
chemical combination, in part as sodium bicarbonate, in part
with the proteids of the plasma, and also probably in part with
the corpuscles. The pressure of the carbon dioxide in the
plasma of the venous blood coming to the lungs is higher than
its pressure in the alveolar air, and hence carbon dioxide is
evolved, lowering the amount in the blood, and increasing the
amount in the alveolar air. In the tissues, the lymph is highly
charged with carbon dioxide, and hence there is a diffusion
stream of carbon dioxide into the plasma, raising the pressure
of carbon dioxide in it. In this way there is a continuous
cycle of change, oxygen is taken in and carbon dioxide thrown
out, in the lungs, and the supply of oxygen so obtained is
borne by the circulating blood to the tissues, where it combines
in part with hydrogen and in part with carbon. That part
which combines with carbon gives rise to carbon dioxide, which

Respiration. i ^J

diffuses out from the cells to the lymph, from the lymph to the
plasma, and in the plasma is carried to the lungs, where it is
removed.

These changes may also be observed in blood outside the
body, if the pressure of oxygen and carbon dioxide upon its
surface be varied.

If blood which has been whipped, to prevent it becoming
solid in clotting, be placed in a bulb which is connected with
a mercurial air-pump, and the air within the bulb removed,
then the gases contained in the blood are given off into the
vacuum. At the same time, as the blood loses its oxygen it
changes its colour from bright red to purple. If air be again
admitted to the bulb, and especially if the blood be shaken up
with it, the reverse change takes place, for the blood absorbs
oxygen and turns back to red in colour.

The same change takes place when blood is left in contact
with reducing agents, for these seize the oxygen from the
oxy-haemoglobin, and reduced hsemoglobin is formed. Thus
if blood be shaken up with ammonium sulphide, and then
allowed to stand for a few minutes, the ammonium sulphide is
oxidized at the expense of the oxygen, and reduced haemo-
globin is formed. A certain amount of reducing power is
possessed by the blood itself, and the amount of reducing sub-
stances in it increases on standing. Hence, when a large clot
of blood, which has stood for some time, is cut into, although
it is red on the surface where it has been in contact with the
oxygen of the air, inside it is dark purple, almost black, in
colour. Because the oxygen of the air has not been able to
penetrate, and the reducing substances formed in the mass
(or the oxidation processes going on there) have reduced the
oxy-haemoglobin.

The changes from the arterial to the venous condition, aiid
back again from venous to arterial, may also be shown by
bubbling alternately a stream of carbon dioxide, and one of
oxygen, or atmospheric air, through some whipped blood.
When the carbon dioxide is passed through, it carries away all
the oxygen, and the blood becomes venous ; when the air is
passed through, it carries off the carbon dioxide, and leaves a

1 88 Elementary Physiology,

supply of oxygen to combine with the haemoglobin, so that the
blood becomes arterial.

Anything which prevents the combination of haemoglobin
with oxygen in the lungs, and so stops the supply of oxygen to
the tissues, endangers the life of the animal. And if the supply
be stopped completely for a short time (two to five minutes),
or be insufficient for a longer period, the animal dies from
suffocation, or asphyxia.

Asphyxia may be produced in several ways. It may be
produced by stopping the windpipe, either from within by a
bolus of food or a growth within the larynx or trachea, or from
without, as in strangulation, thus preventing ingress of oxygen to
the lungs. It may be occasioned by the lungs being filled with
an inert gas or an inert mixture of gases containing too little
oxygen, as in the after-damp of colliery explosions, or in the
air of unventilated cellars or sewers ; or by the lungs being
filled with water, or some other fluid, as in drowning, so that
the oxygen carmot enter. It may be also brought about by
the air containing a much smaller quantity of a poisonous gas,
such as carbon monoxide, which forms a more stable compound
with haemoglobin than does oxygen, and gradually combines
with the haemoglobin permanently, so that there is soon not
enough left to act as an efficient oxygen carrier to the tissues.^
Finally, it may be caused by paralysis of the nervous mechanism,
and this may either be peripheral, as in the case of poisoning
of the nerve endings of the respiratory muscles by curare, or
central, as in poisoning by morphia, and in some cases of
chloroform administration in excess.

When the supply of oxygen is insufficient, there is an ex-
citation of the important nerve-centres lying in the medulla
oblongata by means of the chemical stimulus of the too venous
blood supplied to them.

Thus, the cardio-accelerator -centre is stimulated and the
heart beats faster. At a later stage the cardio-inhibitory centre
is excited, and the heart beats slowly.

^ Such suffocation occurs in poisoning from charcoal fumes, or from
coal gas. In these cases the carbon monoxide compound has a cherry-
red colour, which gives a characteristic hue to the lips and complexion.

Respiration, 1 89

The vaso-motor centre is irritated, and nerve impulses are
despatched along the vaso-constrictor fibres, narrowing the
small arterioles generally over the body, and thus raising the
arterial pressure.

The respiratory centre itself is affected, and there is at first
an increase in both the number and depth of the respirations,
causing laboured breathing or dyspnoea.

But if the dearth of oxygen continue, the venous blood
coming to these centres, which at first stimulated them, later
has a sedative effect upon them ; they become less active, and
finally they are paralyzed and cease to act.

In consequence, the efforts at respiration become slow and
gasping, there are, later on, exaggerated efforts at expiration
only, and finally all respiratory attempts cease. During this
period the vaso-motor centre also passes into slumber, the
arterioles relax, and the arterial pressure falls. The heart-
beats, which had become slow and irregular, in part from
central stimulation, and in part from oxygen starvation of the
cardiac muscle fibres, at length stop altogether from the latter
cause. The arterial blood pressure falls to zero, the circulation
ceases, and the animal dies.

Temporary dearth of oxygen, of much slighter extent, often
occurs during the life of an animal. We see this evidenced in
the laboured breathing which follows severe muscular exercise,
and in the freer breathing which ensues after we have held our
breath from any cause. On the other hand, a condition can
easily be induced, known as apnoea, in which the animal has no
desire for a brief time to breathe, on account of too great re-
spiratory effort immediately preceding it.^ Similarly, during
muscular rest, and more especially during sleep, the respiration
is quieter, because so much tissue oxidation is not going on,
and hence less oxygen is required.

An apnoeic condition can readily be produced bv taking rapidiv about
a dozen deep full breaths, when a short pause occurs, 'during which 'there is
no desire to breathe. The condition is chiefly nervous in character, for it
can be produced by distending an animal's lungs a few times with an
inert gas, such as hydrogen. It is said, however, that apnoea produced by
over-ventilation with oxygen lasts longer than that similarly produced by
an inert gas.

IQO Elementary Physiology.

The increase in respiration and heart action following
violent muscular action is one of the best-known phenomena of
our lives. The demand of the body for oxygen is suddenly
very much increased, as well as the necessity for increased
excretion of carbon dioxide, and there are obviously two means
of meeting the occasion. First, a more rapid circulation,
caused by increased action of the heart, carrying round to the
tissues a larger volume of oxygen, and removing more carbon
dioxide. At the same time, the circulation through the lungs
is also made more rapid, so causing an increase in the intake
of oxygen and in the output of carbon dioxide. Secondly,
there is need for increased ventilation of the lungs, to throw
out the excess of carbon dioxide discharged there, and to replace
it by increased supplies of oxygen from the atmosphere ; this
is provided for by increased respiration.

These desired changes in the rate of action of heart and
lungs are brought about by stimulation of the medullary centres,
by the character of the blood sent to them by the heart ; for
this soon becomes more venous as the muscles go on working.
There is, however, a maximum, and if the muscular activity be
severe and prolonged, the changes in the blood begin to sub-
stitute another and reverse effect instead of exaggerated nervous
activity ; the person becomes qidtc out of breathy or completely
pwnped out^ and has to slacken off or desist altogether.

The rate of respiratioji is slower in large than in small
animals. One reason for this is that the surface of the skin is
larger in proportion to their bulk in small than it is in large
animals, and hence there is a greater comparative loss of heat
which must be made good by a comparatively greater amount
of combustion, and hence of respiratory exchange. For the
same reason the rate of the heart-beat is quicker in small than
in large animals, and there is usually a fairly constant corre-
spondence between the cardiac and respiratory rhythms, one
respiration, as a rule, taking place for about four heart-beats.
For the same reason, heavy persons breathe more slowly than
light persons, and the adult more slowly than the child. At
birth, the rate of respiration is usually over 40 per minute ;
at five years of age it lies between 20 and 30 ; in middle age

Respiration. 191

at 16 to 17;^ and in old age it somewhat increases again,
averaging 17 to 19 per minute. In the same individual con-
siderable variations in the rhythm are found, according to the
circumstances under which observations are made. The effect
of muscular exercise has already been alluded to ; posture has
also a great effect, the rate being fastest while standing, inter-
mediate while sitting, and slowest while lying. The rate is
also affected by the emotions and by sensory stimulation, such
as application of cold water to the skin, or sudden pain. It
can only temporarily be affected by direct application of the
power of the will to that object. We can hold our breath
voluntarily for a short time, but soon the desire to breathe
becomes imperative, and in spite of the utmost voluntary effort
to the contrary, respiration recommences. Also, we can breathe
faster and deeper, or, on the other hand, more slowly than
normal to us, for a short interval of two or three minutes ; but
it is impossible to keep up the attempt for any length of time,
and, in spite of voluntary effort, we soon lapse back to the
normal rate and strength of breathing.

^ There are very wide variations from this average, any number
between lo and 24 per minute being found in different cases.

CHAPTER IX.

ANIMAL HEAT.

All animals may be divided into two great classes, according
to the manner in which they react to changes in temperature of
their surroundings. In one class, the temperature of the
animal's body does not vary in summer or winter, but remains
at a constant level, provided the animal is in a healthy con-
dition ; in the other class, the temperature of the body does
vary with that of the surroundings,^ rising when the air or water
surrounding the animal grows warmer, and falling when the
temperature of these environments sinks. As the temperature
of the first class of animals is as a rule both higher than their
surroundings and than that of the second class of animals, they
are termed warm-blooded animals ; on the other hand, the
animals with variable body temperature are termed cold-
blooded animals.^ To the former class belong the mammalia
(including man), the birds, and, to a certain extent, reptiles ;
to the latter, amphibia, fishes, and the invertebrates.

The temperature is not the same in all species of warm-
blooded animals, but in the same species it is very constant, so
much so tha.t the " clinical thermometer " becomes an invaluable
test for a feverish condition of the body, because the tem-
perature then rises above normal.^

^ Although it is not identical with that temperature, but somewhat
higher.

2 These terms, although they do not express the real difference between
the two classes, are better than the uncouth terms, homoiothermal and
poikilothermal, which have been proposed instead of them.

^ The normal temperature in man, taken in the axilla or arm-pit, is
37° C. (equal to 98-6° Fah.) ; in the mouth it is slightly higher, 37*2° C,
and in the rectum 37 '6° C. The blood in the internal parts is somewhat
warmer than this, and is hottest in the hepatic vein, where its temperature is
about 39'5° C.

Animal Heat, 193

In a healthy condition of a warm-blooded animal the means
provided in the body for regulating the temperature are so
perfect that it does not rise or fall appreciably, no matter how
great be the variation in the temperature of the surroundings,
unless the exposure be very great and prolonged. "When the
temperature does vary considerably in either direction the con-
dition of the animal becomes critical and life soon impossible.

A man may be placed in the rigour of a polar winter, or
beneath the burning sun of the tropics in summer, but, provided
he remain in a healthy condition, the temperature of his body
will remain the same. As soon as the adjusting mechanism
goes out of order this constant temperature is, how^ever, no
longer retained, and the temperature of the body may go above
normal and remain above normal, although the patient be sur-
rounded with ice-bags.^ A person in a healthy condition can go
into a very hot atmosphere, such as that of a Turkish bath, and
remain there for some time, although the temperature may be
sufficient to cook a beef-steak, but such a person must be
supplied with some means of keeping cool ; a large amount of
water must be drunk to supply a large amount which is
evaporated from the skin by the action of the heat-regulating
mechanism of the body, and it is this constant evaporation of
water w^hich keeps down the temperature to a normal level.
Again, a person who is exposed to a low temperature must be
provided with means of producing heat to replace that lost
from the body to surrounding objects, otherwise the temperature
of the body would fall. X liberal supply of heat-producing food
must therefore be eaten, which on combustion in the tissues
yields heat.

Our clothing (and similarly the wool, feathers, and hair of
animals) is another attempt, apart from ornament, to aid the
heat-regulating mechanism of the body. Clothes and other
coverings do not yield heat to the body, they merely, when, as
is usually the case, the temperature of surrounding bodies is
lower than that of the body, prevent loss of heat. They are

^ In such a condition of fever the combustion in the tissues becomes
excessive, and the regulating mechanism of the skin is unable to keep pace
with it.

194 Elementary Physiology.

bad conductors which keep in the heat of the body, and so
diminish the loss by conduction and radiation. White clothing
reflects a good deal of heat, especially direct solar heat, and
hence is cool because it prevents external heat from entering ;
on the other hand, black clothing absorbs solar heat readily,
and hence is bad clothing for hot weather. In winter we
require thick clothing of material which does not conduct heat
well, such as wool, and in summer light clothing, both in
texture and colour, which allows the heat produced in the
body readily to escape, and also reflects as much as possible
the external heat, and does not allow it to reach the body.

It is by being able in this way to modify his clothing at will,
so materially aiding the natural heat-regulating agencies of his
body, that man is enabled to inhabit all climates of the globe,
from the tropics to the polar regions.

We have now to consider the ways in which heat is pro-
duced in and lost from the body, and in what manner the
production and loss are so balanced as to produce an unvarying
temperature.

Heat is produced in the body by chemical change (by
oxidation of the food), and the heat so produced is distributed
to the different parts, so as to maintain these nearly at the
same temperature, by the blood-stream, which in this respect
acts somewhat like a hot- water heating apparatus. The blood
is heated by the chemical changes going on in the glands
(especially in the liver) and in the muscles, so that the venous
blood passing away from a muscle or gland is warmer than the'
arterial blood flowing to these parts. On the other hand, the
blood in the capillaries of the skin is cooled, to some extent
by radiation, but chiefly by evaporation, on the surface of the
skin, of the water separated by the sweat glands. The blood is
cooled by contact with the cooler skin, and therefore the
venous blood passing away from the skin is cooler than the
arterial blood passing to the skin.

Another way in which the body loses heat is by evaporation
in the air-passages leading to the lungs. The air is taken into
these passages in a more or less dry condition, and usually at a
lower temperature than that of the body. Some heat is here

Animal Heat. 195

lost in raising the air to the temperature of the body, but this is
inconsiderable when compared with the larger amount which is
usually lost in saturating the air with aqueous vapour.

It takes a certain definite amount of heat to convert a
definite weight of water into steam. The amount of heat
which so becomes latent in the conversion is very large, for it
takes nearly six times as much heat to convert boiHng water
into steam as it would have required to boil the same quantity
supposing it ice-cold to commence with.

An amount of heat practically equal to this becomes latent
whenever water becomes changed into vapour, no matter whether
the change takes place at the temperature of boiling water or not.
Hence the amount of heat lost in saturating the air, which is
respired, with water vapour, and in evaporating from the skin,
at the temperature of the air, the water furnished by the sweat
glands, is very large.

The production of heat in the body is regulated by the heat
value of the food consumed, and by the amount of exercise
taken by the individual, so as to ensure the conversion of the
whole of the chemical energy of the food into heat energy.
The .rates at which exercise is taken at different times also
regulate the rates of combustion at these times. \Vhen an
animal is exposed to cold surroundings, it runs or walks about,
and uses its muscles so as to keep warm by the heat set free.
Also, in order that the muscles may be supplied with chemical
energy to convert into heat energy, it is necessary, if the
exercise be long maintained, that the food-supply should be pro-
portionately greater. The food is the ultimate source of the heat ;
the muscular energy the means whereby the animal is enabled
to change chemical energy into heat energy ; and the blood-
stream the means of distribution of heat energy so provided.
On the other hand, when an animal is placed in warm sur-
roundings it becomes torpid, there is no need for great heat
production, and consequently there is indisposition to muscular
exercise, so as to produce as little extra heat as possible, while
at the same time the loss of heat is increased by increasing the
blood-supply to the skin. The appetite also becomes less keen,
and a smaller amount of food suffices for the wants of the animal.

196 Elementary Physiology.

The amount of heat lost is regulated chiefly by the blood-
supply to the vessels of the skin. When we are exposed to
cold surroundings ^ the skin becomes pale and bloodless ; when
we are exposed to a warm atmosphere the skin becomes flushed
from a rich supply of blood, and at a certain limit it becomes
wet with perspiration. Before this limit is reached, however,
there is a considerable amount of water being evaporated from
the skin, only the evaporation rate exceeds the rate at which
the sweat glands pour the sweat out, so that there is no accuma-
lation of sweat on the surface. Even when the air in contact
with the skin is hotter than the blood, there is no inconvenience
felt so long as the air is not saturated with water vapour, and
the person is liberally supplied with water. But if the hot air
is also saturated with vapour it soon becomes oppressive, for
then the sweat is not evaporated from the skin.

To sum up, then, the food is oxidized in the tissues by the
agency of the oxygen carried by the haemoglobin of the blood,
and (with the exception of a small fraction which is turned
into external work) all the chemical energy so set free is changed
into heat. This supply of heat keeps the animal's body at a
certain uniform temperature, which is usually above (but
exceptionally may be below) that of its surroundings. To
maintain this constant temperature, heat must be lost at
variable rates, according to the changing temperatures of the
surroundings, and this variation is accomplished mainly by
varying the blood-supply to the more superficial and cooler
part of the body, i.e. the skin, and by varying the amount of
sweat secretion by nervous influence.^ Besides losing heat

* Such an exposure produces a feeling which we refer to as cold, and
say that we are cold, but in reality the body does not become any colder
unless the exposure is very great, and then numbness and unconsciousness
supervene. The feeling of cold is a warning to preserve the temperature of
the body, and is not caused by any appreciable fall in temperature of the
body generally, but merely of the skin. The same is true of the feelings
produced by warmth.

^ It is supposed that there are specific nerve centres for heat regulation,
but the existence of these can scarcely be said to be experimentally proven.
It is certain, however, that vaso-motor action on the skin, and sweat
secretion, are invoked by nervous impulses, which may originate in part
from the change in temperature of the skin, affecting peripheral sensory nerve-
endings, and in part from minute changes in the temperature of the blood
flowing through the nerve-centres.

Animal Heat, 197

through the skin, the body loses a considerable amount by the
lungs, which is chiefly consumed in saturating the respired air
with water vapour.

There are, besides, other minor sources of loss of heat to the
body, which are, however, of no great importance compared to
those considered above ; such, for example, as the food being
occasionally taken into the body at a lower temperature than
that of the body, while the excreta are voided at body
temperature.

CHAPTER X.

EXCRETION.

The waste products of the body are removed by four channels :
viz. by the kings, in the expired air ; by the kidneys, in the
urine ; by the skin, in the sweat ; and by the alimentary canal,
in the faeces.

There is daily taken in along with the food a certain
amount of water, and an equivalent amount of water to this,
together with a much smaller amount arising from the oxidation
of the hydrogen of the food, must daily be removed with the
proper waste products of the body. This supply of water is as
indispensable to the animal as is its food; it may be taken
mixed with the food as water of the food, or it may be drunk
alone, but in some form it must be taken into the body. We
have seen that the food is absorbed from the alimentary canal
in solution, and water is essential for this purpose. Certain of
the waste products are removed from the body in solution, and
here again water is necessary as a vehicle of removal. Further,
a considerable amount of water is daily evaporated from the
skin and lungs, and it is chiefly by variations in the amount of
water so removed that the temperature of the body is regulated
and kept at a constant level in spite of all changes in the
temperature of its surroundings. There is thus a constant
stream of water passed through the body which carries nutrient
material to the blood, carries waste and impurity away from it,
and aids in regulating the body temperature. Although this
water cannot, strictly speaking, be regarded as a waste product,
yet it is intimately connected with the removal of the waste pro-
ducts, and is, moreover, itself removed by the same channels, so
that it can be conveniently considered along with them.

A very little of the waste material of the body, and a very

Excretion. 199

small proportion of the water are removed at the lo\Yer end of
the alimentary canal in the fseces. Most of the solids of the faeces
consist of waste shreds and de'bris of the food which have never
formed part of the body and are merely an indigestible residue
of the food. The only portion which can be regarded as a true
excretion of the body consists of a small amount of bile pig-
ments, cholesterin and lecithin derived from the bile, and a
small quantity of mucus secreted by the intestine epithelial cells
and serving to coat the mass of faeces wdth a slimy surface to
render its passage easy. The w^ater of the faeces when these
are in a normal condition forms but a small fraction of the
quantity of water daily excreted ; for the water taken in with
the food, together with that added by the secretions which are
poured in at the upper part of the alimentary canal, is rapidly
removed by absorption in the low^er part of the small intestine
and in the large intestine, leaving the faeces at length semi-solid
in consistency.

Practically, all the carbon dioxide excreted from the body,
as well as a fair proportion of the water, is removed by the
lungs in the expired air. The manner in which this is done
has already been considered in connection with respiration j it
remains to describe here the other channels of removal — viz.
the skin and kidneys — and the constituents which they remove.

The skin chiefly removes w^ater, accompanied by a small
amount of inorganic salts and carbon dioxide, and traces of
urea and other nitrogenous bodies.

The kidneys also remove water, but their chief function is
the removal of practically the "whole of the nitrogen formed in
the degradation of proteid in the body. In addition, the kidneys
remove the chief part of the inorganic salts excreted from the
body, and certain salts of acids of the aromatic series, which
are chemically vei'y stable^ and being formed in small quantities
(either by changes going on in the body, or by bacterial action
in the large intestine and absorption afterwards), cannot be
broken up or oxidized in the body subsequently, and hence are
excreted unchanged in the urine. ^

^ A considerable fraction of these aromatic compounds is excreted as
sulphates, and in this way a portion of the sulphur formed in proteid

200 Elementary Physiology.

The relative amounts of water excreted respectively by
lungs, skin, and kidneys are so variable with varying circum-
stances that no average figures of any worth can be given. In
cold weather the relative amount excreted by the kidneys is
increased, for then the blood-supply to the skin is diminished
and the production of sweat is decreased because loss of heat
by evaporation is not so much required. In a moist condition
of the atmosphere the amount excreted by the lungs is
diminished, and more especially if the air be both warm and
moist; for the amount excreted by the lungs depends on
saturation of the respired air with water vapour.

The Skin.

The skin forms a protective covering for the surface of
the body. It is composed of two parts, termed the cutis vera,
corium or dermis, and the epidermis Or scarf skin respectively.
Of these two the cutis vera is seated more deeply, and contains
blood-vessels ; while the epidermis is situated superficially,
forming the surface, and has no blood-vessels. The epidermis
is a thick stratified epithelium composed of a large number of
layers of cells, which get different names at different depths, as
their shape and consistency alters (see Fig. 92).

The superficial layers are flattened and horny, those nearest
the surface being quite squamous, while the deeper layers are
somewhat swollen. In these cells the nuclei have degenerated
or at least become invisible. The thin pavement cells of the
outer layers are gradually worn ofl'by friction and abrasion, and
are replaced by the swollen cells, which gradually become flat-
tened as they near the surface, while in turn these cells are re-
placed by others from deeper layers. The deepest stratum of the
horny layer, lying beneath the swollen layer above mentioned,
is composed of clear compressed cells and is known as the
stratum lucidum. These three strata of the horny layer merge
by easy transition into each other. Beneath the horny por-
tion lies the softer portion of the epidermis, which is known as

disintegration is got rid of. The balance is excreted as inorganic sulphates.
The phosphorus formed in the breaking down of proteid appears in the
urine as phosphates of the alkalies and alkaline earths.

Excretion.

20 1

the rctc inucosum (of Malpighi) ; in this part the nuclei of the
cells are still visible, and become more obvious as the cells are
situated more deeply. The most superficial stratum is formed
of a few layers of granular cells, and is o-dW^^strattim gratiulosum ;

Fig. 92. — Section of epidermis. (Ranvier.)
H, horny layer, consisting of 5, superficial horny scales : sw, swollen-out horny cells ;
S.I., stratum lucidum ; IM, rete mucosum or !Malpighian layer, consisting oi p,
prickle-cells, several rows deep ; c, elongated cells forming a single stratum near the
cerium ; and s.gr., stratum granulosum of Langerhans, just below the stratum luci-
dum ; n, part of a plexus of ner\-e fibres in the superficial laj-er of the cutis vera.
From this plexus fine varicose ner\'e fibres may be traced passing up between the
epithelium cells of the Malpighian layer.

beneath- this lies a thick stratum of polygonal-shaped cells
known as prickle cells (of Schulze), which have small inter-
cellular channels between them for the passage of lymph for
the nutriment of the cells. The channels are bridged over at
intervals by processes from the cells, and when the cells are
isolated for examination under the microscope these processes

202

Elementary Physiology.

give them a prickly appearance, from whence their name is
derived. In the deeper layers the prickle cells tend to
become columnar in shape, with the longer dimension per-
pendicular to the surface, and finally there is a layer of long
columnar cells which rests upon the aUis vera lying immediately
beneath it.^ The under surface of the epidermis does not form
a plane surface, but is thrown into ridges and hollows by
papillce which project into it (see Fig. 93) ; these papillae bear

Fig. 93. — Duct of a sweat-gland passing through the epidermis. (Magnified 200
diameters.) (Heitzmann.)
BP, papillae with blood-vessels injected ; V, rete mucosum between the papillae ; E,
stratum corneum ; PL, stratum granulosum ; D, sweat-duct, opening on the surface
at P.

the blood-vessels which form capillary networks in the super-
ficial part of the ctUis vera projecting into the papillae.^ The
outer free surface of the epidermis is also thrown into ridges
corresponding to these, but not so deeply marked, which form
the fine markings seen on the finger tips and elsewhere. Thus
no blood-vessels enter the epidermis, but its deeper layers, where

^ It is in this layer that the pigment is developed which gives colour to
the skin in coloured people,

^ The papillae also lodge the terminations of the sensory nerve fibres
for tactile sensation.

Excretion. 203

the cells are growing, are fed by lymph which exudes from the
capillary networks of the cutis vera and can pass through the
intercellular channels above mentioned.^

The cutis vera is composed of dense connective tissue,
which gradually becomes more open in texture as it passes
into the subcutaneous connective tissue which underlies it
over most parts of the body. In this subcutaneous connec-
tive tissue fat may be largely developed, particularly over the
abdomen, where it forms the pamiiculiis adiposus, and over
the buttocks. The cutis vera is richly supplied with blood-
vessels, which are largely distributed to the surface, forming
the capillary networks in the papillae above mentioned.

The skin becomes modified at various parts, as at the lips,
where it becomes thin and transparent, and passes gradually
into the mucous membrane of the mouth; and on the inner
surface of the eyelids and over the eyeball, where it is changed
into a thin membrane, called the conjunctiva., which contains
many sensory nerve-endings, and is hence extremely sensitive.
Besides such modified parts the skin has modified structures
all over it, which are known collectively as the appendages of
the skiji ; these are the nails., the haiis and their glands
(sebaceous glands), and the siueat glands.

The nails are formed by thickening and alterations of the
stratum lucidum in certain well-known situations. The layers
superficial to the stratum lucidum disappear in the course of
development of the nail, and only a portion remains covered
by these layers, forming the narrow band at the root. The
nail lies on the nail-bed, or matrix, which is formed by a
modified Malpighian layer with longitudinal ridges and grooves.
The nail grows forward both from the end (nail groove) and
from the posterior portion of the bed, and hence the free
border is the thickest part of it. The substance of the nail is
made up of clear compressed horny cells, each of which con-
tains the remains of a nucleus.

The hairs are developed in the hair follicles (see Fig.
95), which are down-growths of the epidermis into the cutis

' On the other hand, fine nerve fibrils do pass between the cells of the
deeper layers of the mucosum (see Fig. 92).

204

Elementary Physiology.

vera, or even into the subcutaneous tissue underlying this.
The hair grows from the bottom of the follicle, where it is
supplied with blood by a small vascular papilla which projects
up into the somewhat expanded knob-like end of the hair.
The hair substance is composed of a pigmented horny fibrous
substance made up of long tapering cells, which can be
separated by the use of acids. Externally the hair is covered

Fig. 94. —Section across the nail and nail-bed. (100 diameters.) (Heitzmann.)
P, ridges with blood-vessels ; B, rete mucosum ; N, nail.

by a cuticle of imbricating scales, which fit against similar
scales sloping in the opposite direction on the inner surface
of the hair follicle.^ The central part of the hair is occupied
by a dark-looking material, and is known as the medulla. When
minute air-bubbles are present in the medulla, or between the

^ On account of these imbricating scales a coarse hair, when rubbed
between finger and thumb, always moves towards the free end, just as does
a blade of coarse grass when similarly treated. The object of the scales is
to firmly fix the hair in its follicle.

Excretion,

20:

fibrous cells of the hair, the hair acquires a white appearance
seen by reflected light.
Each hair follicle has
one or more small
glands in connection
with it which are known
as the sebaceous gla?ids
(see Fig. 95). The
ducts of these glands
open into the hair fol-
licle near its mouth.
The glands secrete a
fatty material called
sebum, which is pro-
bably formed by the
disintegration of the
gland cells. The sebum
imparts oiliness and
softness to the hair.
The hair follicle has
also a tiny muscle (ar-
rector pili) attached to
it, composed of in volun-
tary muscle fibres. This
little muscle is attached
as shown in the figure,
and when it contracts
it erects the hair or
raises it at right angles
to the surface of the
skin.

Sweat glands are
seen in a section of the
skin from any part of
the body, but more
abundantly in the skin
of the palm or sole,
where they lie very

when

a o

^ S

Cm

•5 ^

o .. t:

'"S3

Z. 'Z o

4, C <y
-5 . "

^ "^ s~-r

"=2 "?

^ :- ^
— r: i-

206

Elementary Physiology

close together. They are coiled tubes lined with cubical or
columnar epithelium, which lie deep in the cutis vera, and
send their ducts, which are also coiled corkscrew fashion, up
through the epidermis to open on the surface. The glandular
part of the convoluted tube, in which the sweat is secreted, is
lined outside by a basement membrane (see Fig. 96), inside
which is a single layer of longitudinally placed fibres which

Fig. q5. — Section of a sweat gland in the skin of man.
J!, a, secreting tube in section ; b, a coil seen from above ; c, c, efferent tube ; d, inter-
tubular connective tissue with blood-vessels, i, basement membrane ; 2, muscular
fibres cut across ; 3, secreting epithelium of tubule.

resemble involuntary muscle fibres, and more internally still
surrounding the lumen ^ of the tubule there is a single layer
of columnar cells which yield the secretion. The duct leading
from the secreting portion is fined by two or three layers of
cells, and the lumen is much narrower (see Fig. 96). The
duct where it passes through the epidermis has no proper
wall, and is merely an excavated channel between the epithelial
cells.

' The lumen is the central opening, or bore, of a gland, alveolus, duct,
or canal.

Excretion. 207

The Sweat. — Sweat is continually being produced by the
sweat glands, even when it does not become obvious to the eye
and wet the skin. It only accumulates on the skin when it
is poured out by the glands faster than it can be evaporated
off by the air in contact with the skin. When the sweat
accumulates on the skin the condition is spoken of as sensible
perspiration ; while the term inse7isible perspiration is applied to
the more normal case where it is produced at a less rapid rate
than the air can take it up, so that it does not appear on the
skin.i The more muscular work that is done the greater will
be the amount of heat produced in a given time, and as this
heat must be dissipated from the body, so that its temperature
may remain constant, the greater will be the amount of sweat
produced in a given time (see p. 192); and hence increased
muscular work leads to sensible perspiration. Again, the
hotter the air in contact with the skin, the more water must
be evaporated from the skin surface to keep it cool, and so
cool the blood circulating underneath, and thus keep the body
temperature normal ; so increased temperature of surroundings
leads to increased sweat production, and tends to sensible
perspiration. It is easily seen from these considerations that
the rate of production of sweat is very variable, and hence
that no accurate average estimate for the quantity per diem
can be given, but it probably lies between one and three litres.'^
Each sweat gland is supplied by a small arteriole with a tuft
of capillaries, and the blood-supply to these is controlled by
vaso-motor nerve fibres ; there are also secretory nerve fibres,
through w^hich the gland cells are directly stimulated to secrete.
The sweat is a very watery secretion, and undoubtedly its chief
function is to regulate the body temperature by water evapora-
tion from the surface, and not to purify the blood by excretion
of either organic or inorganic constituents. It contains only
from o'5 to 2 per thousand of total solids, of which about one-
third is inorganic (chiefly sodium chloride), and the remainder

* The drier the air the more rapidly is the sweat evaporated, and hence,
the temperatures being alike, "sensible perspiration" is more easily in-
duced on a moist day than on a dry one.

- A litre is 1*76 pints.

2o8

Elementary Physiology.

organic. It has an acid reaction, probably due to volatile fatty
acids; but after profuse sweating the reaction may become
faintly alkaline. It also contains traces of urea and of carbon
dioxide.

The Kidneys.

The urinary system consists of the kidneys, ureters, bladder,
and urethra. The arrangement of these parts is shown in the

Fig. 97. — The kidneys, bladder, and their vessels. Viewed from behind.

R, right kidney ; U, ureter ; A, aorta ; Ar, right renal artery ; Ve, vena cava inferior ;

Vr, right renal vein ; Vu, bladder ; Ua, commencement of urethra.

accompanying figure as they appear from the back, and their
situation in the abdomen has already been described. The
urine is continuously secreted by the kidneys and trickles
down the ureters into the bladder, where it accumulates, until

Excretion.

209

the distension of the bladder gives rise to a feeling of uneasi-
ness, when it is voluntarily discharged through the urethra by
the relaxation of a sphincter muscle placed at the commence-
ment of that tube. Thus the kidneys secrete the urine, and
the rest of the system is an accessory part for its temporary
storage and convenient expulsion. Hence the kidneys are the
most important portion, and
we have to consider here their
structure and the nature of
their secretion or excretion.^

When a kidney is split
open longitudinally and ex-
amined, it is seen to present
an appearance resembling
that shown in Fig. 98. The
ureter enters at the concave
part, called the hilum, and
expands into a funnel-shaped
dilatation, which is termed the
pelvis. The pelvis divides
into two or three primary
divisions, and these again sub-
divide into a number of short
wide tubes, which are named
calices, or infundibula. These
receive into their mouths the
ends or papillae of the pyra-
mids of the kidney substance,
and are attached all round the
bases of these projections (see
Fig. 98), so as to receive

and carry away the urine which issues at the apices of the
papillae.

On turning the attention to the kidney substance it can be

^ The words ''secrete " and " excrete " are often used indiscriminately.
Any material separated by a glandular structure from the blood may be
termed a secretion ; but rigorously a secretion means a fluid material service-
able to the animal, and an excretion a waste product separated for removal
from the body.

P

Fig. 98.— Plan of a longitudinal section
through the pehds and substance of the
right kidney. One-half the natural
size.

a, the cortical substance ; b. b, broad part of
two of the pyramids of Malpighi ; c, c,
the divisions of the pelvis named calices,
or infundibula. laid open ; c', one of these
unopened ; d, d, summit of the pyramids
or papillae projecting into calices ; e, e,
section of the narrow part of two pyra-
mids near the calices ; />, pelvis or en-
larged portion of the ureter within the
kidney ; j(, the ureter ; s, the sinus ; h,
the hilum.

2IO

Elementary Physiology.

seen to be made up of two portions, differing in appearance
and colour. The outer portion, lying immediately beneath the

Fig. 99. — Diagram of the course of two uriniferous tubules. (Klein.)
A, cortex ; B, boundary zone ; C, papillary zone of the medulla ; a, a', superficial and
deep layers of cortex, free from glomeruli- For the explanation of the numerals,
see the text.

Excretion. 211

fibrous capsule which surrounds the kidney, is termed the
cortex. It is nearly uniform in its appearance, and is of a
reddish brown colour. It extends inwards to the bases of the
conical masses known as the pyramids of Malpighi^ and also
lies between contiguous pyramids. The pyramids constitute
the medulla of the kidney; they vary in number in different
animals ; there are usually over twelve in the human kidney.
The substance of the pyramids is distinctly striated, the stri^
running from base to apex, and marking the course of the
blood-vessels and uriniferous tubules {vide infra), which here
run parallel to one another from the cortex towards the papilla
or apex of the pyramid. The urine is secreted and carried to
the papilla by minute tubules, which can be made out in the
kidney substance with the aid of a dissecting lens. These
tubules are termed uriniferous tubules, and by very patient
dissection under a lens have been shown to have a very
tortuous course in the kidney substance, which is represented
diagrammatically in the accompanying figure.

Each tubule begins in the cortex in a dil ited part which is known as a
Alalpighian corpuscle.^ or glomertcbis (Fig. 99); this contains a much con-
voluted tuft of capillaries over which the tubule commences as the capsule.
This capsule is composed of a double layer of thin pavement cells, one of
which is reflected over the enclosed capillaries, while the other forms the
outer wall of the glomerulus.^ Thus the blood in the capillaries is separated
from the uriniferous tubule only by the thin walls of the capillaries them-
selves and a single layer of flat cells forming the reflected layer of the
capsule. The outer layer of the capsule narrows to a neck (2 in Fig.) at
the opposite pole of the glomerulus to that at which the blood-vessel enters,
and the cells change in shape from pavement to cubical. A tube is thus
formed, which in the first part of its course is convoluted {first convohited
tiibiUe, 3 in Fig.), next spiral {spiral tubide, 4 in Fig.), and then straight,
running down the medulla in one of the pyramids. The tubule next
turns back towards the cortex, forming the loop of Henle (5, 6, 7, 8,
and 9 in Fig.), and in the cortex again becomes zigzag, and then
convoluted {second convohited tuhile, ii in Fig.), finally opening by a
pcnctional tubide (i2 in Fig.) into a collecting ticbide (13 in Fig.). The
collecting tubule, after receiving several junctional tubules, passes straight

^ The arrangement is as if the capsule forming the end of the tubule had
been a ball, against which the tuft of capillaries had been pushed so as to
force in the ball and become enclosed in such a way that one layer formed
a coat for the capillary tuft, while the other surrounded the whole.

212

Elementaiy Physiology.

down the medulla, and opens at the apex
Bellini (15 in Fig.)- The epithelium lining

Fig. 100.— Vascular supply of kidney. (Cadiat.)
Diagrammatic.

«, part of arterial arch ; b, interlobular_ artery ; c,
glomerulus ; d, efferent vessel passing to me-
dulla as false arteria recta ; e, capillaries of
cortex ; /, capillaries of medulla ; g, venous
arch ; h, straight veins of medulla ; j, vena
stellula ; i, interlobular vein.

of a papilla as a duct of
the tubule is set throughout
on a basement membrane,
and it differs in character
in different portions of the
tubule. In the first convo-
luted tubule and the spiral
tubule, the cells are cubical
and fibrillated, and the
lumen is narrow. In the
descending limb of Henle's
loop and the loop itself,
the cells are small and
flattened, and leave a larger
lumen ; in the ascending
limb they become cubical
again ; in the second con-
voluted tubule, they are
cubical and fibrillated ; in
the junctional tubule, flat-
tened ; in the collecting
tubule, clear and cubical ;
and in the duct of Bellini,
clear and columnar.

The arrangement
of the capillary blood-
vessels in the kidney is
somewhat peculiar, for
after being gathered up
from the tuft of capil-
laries in the glomerulus
by a small efferent
vessel which leaves the
glomerulus, the blood
is again distributed to
capillaries by the sub-
division of this efferent
vessel, and the second
capillary system so
formed surrounds the
tubules and runs along-
side them.

Excretion. 2 1 3

Each kidney is supplied with blood by a single large artery (renal artery)
arising from the abdominal aorta, and the blood after circulating through
the kidney returns to the inferior vena cava by a large vein (renal vein).
These blood-vessels enter at the hilum, and passing into the kidney substance
form a system of large branches lying between cortex and medulla. Smaller
branches are given off from these (see Fig. lOO), which pass to cortex and
medulla ; those which go to the cortex supply the glomeruli, and, as stated
above, again form capillaries round the tubules ; on the other hand, those
which go directly to the medulla, form capillaries only once, viz. around
the medullary portions of the tubules.

Urine Secretion.

The glomerulus looks, by reason of its flattened cells, like
a simple filtering arrangement, for its cells do not appear of the
proper type for selective secretion, such as are the cells lining
other parts of the tubule. Still, the pores of the filtering apparatus
must be very small, and unlike those of porous paper, for not
a trace of the proteids of the blood-plasma comes through the
glomeruli. It is probable that certain inorganic salts of the urine
and a portion of the water pass through by filtration under
pressure at the glomeruli, for the concentration of inorganic salts
in urine approximates to that which they have in the blood, at
any rate in the case of the netifral S2i\\.s, such as sodium chloride.
But, in the case of other constituents, it is obvious that the
process of their separation is not one of simple filtration through
the glomeruli. This is so in the case of the acid inorganic
salts to which urine owes its acidity {vide infra) ^ for the blood
is alkaline, while the urine is normally acid. Still more so is
this true for urea^ that organic constituent which is present in
urine in largest quantity, and is of most importance.

Urea is normally present in urine to the amount of 2 per
cent., while in blood-plasma it is never in health present in
more than y^o of this concentration. Hence it cannot be
present in the urine by filtration only, but must be actively
secreted by some of the cells of the kidney. The kidneys
also rapidly excrete any foreign material which may have got
into the blood, such as drugs and medicaments, and these are
found in the urine in much stronger solution than they could
ever have been present in the blood, sure evidence that they
are not removed by filtration.

214 Elementary Physiology.

Now the cubical cells lining certain parts of the uriniferous
tubules possess all the characteristics of secreting cells, and it
is obvious from analogy that these cells have some secreting
function, as well from the appearance of the cells as from the
convoluted course of the tubules, and the abundant blood-
supply to their cells. Further, there is a certain amount of
evidence that dissolved foreign substances are removed from
the blood, not by the glomeruli but by the secreting cells of
the tubules. Hence it is probable that a certain portion of
the water and some of the inorganic salts are separated by
filtration in the glomeruli ; while other inorganic salts, notably
those to which the reaction is due, the urea and other organic
constituents, as well as any foreign substances dissolved by
chance in the blood, are removed by the cells of the tubules.
The secretion of the cells lining the tubule is thus washed
down towards the collecting tubules by the watery secretion,
or rather filtration, of the glomeruli.

The Urine.

The work of the kidneys is to regulate the condition of
the blood, and to keep it pure by removing certain waste
products formed in it as a result of the degradation of the food.
The greater part of the carbon dioxide formed in the body, as
already stated, is removed by the lungs, and a certain amount
of water by the lungs and skin ; all the balance of the work of
maintaining the blood-stream pure is done by the kidneys.

The balance between kidney activity and condition of the
blood is a very delicate one. If too much water has been
absorbed, so that the blood has become slightly too dilute, the
kidneys immediately commence to secrete a urine, which is
much poorer in solid constituents than normal, and the amount
of water in the blood is rapidly reduced. If the blood tends
to become too alkaline, by formation from it of an acid
secretion, as is the case in the first hours after a meal, then
the kidneys at once commence to secrete a less acid, or it may
be even an alkaline, urine and the alkalinity of the blood is
kept unchanged in degree. Suppose some substance reaches
the blood-stream which is an abnormal constituent there, then it

Excretion. 215

is at once treated by the kidney cells as an enemy and promptly
expelled. Similarly, an excess of any normal constituent of
the blood is treated as an abnormality and excreted. Blood
normally contains about two parts per thousand of grape sugar,
and so long as the concentration does not rise above this
the kidney cells take no action with regard to it : but let the
quantity increase, and at once the cells proceed to eliminate
the excess in the urine. In diabetes, the sugar found in the
urine is not formed in the kidneys, it is merely thrown out by
them. Something has gone wrong elsewhere, and as a result
there is an excess of sugar in the urine. Thus, the presence of
sugar in the urine simply shows a normal attempt on the part
of the kidney cells to remove this excess.

Similarly, the presence of the normal constituents of urine
in that excretion is due to the preservation of a balance;
these normal constituents are being continually produced in or
poured into the blood-stream, and as continually are they
removed by the kidneys, so that each is kept down to a certain
normal percentage, which is in some cases very low. For
example, if the amount of proteid in the food be increased, so
also will be the amount of the products of proteid waste in the
blood ; and hence the amount of these waste products excreted
in the urine will be correspondingly increased.

Chemical Composition of Urine.

Urine is a clear yellow or amber-coloured fluid which
usually has an acid reaction. The acid reaction is not due to
free acid, but to acid salts, and chiefly to the acid phosphate
of sodium (NaH.POJ. Th^ average daily amount of urine
excreted by a man of average weight {d^ kilograms, or 145
pounds) is 1500 cubic centimetres, but, as already mentioned,
the daily amount varies very considerably with the amount of
fluid drunk, and with the temperature of the surroundings.
The total amount of solids in this quantity of urine averages
slightly over 70 grammes, of which about 40 grammes is organic
and 30 grammes inorganic matter.

The chief organic constituents of the urine are urea,

2i6 Elementary Physiology,

creatinine, uric acid, hippuric acid, pigment, and aromatic
compounds in traces usually as sulphates.^

The inorganic salts are chiefly chlorides, sulphates and
phosphates of sodmm^ potassium, calcium, magnesium, and
ammonium.^ Urea is the most important of the organic
constituents, as it is in the form of urea that nearly all the
nitrogen, of the proteid of the food, leaves the body. The

ATTT

chemical formula of urea is COCTxttt^; and it hence contains

^ IN J:l2

nearly half its weight of nitrogen.

Urea is not a very stable body, and is easily oxidized by suitable reagents
to carbon dioxide, nitrogen, and water. Such a change can be induced by
mixing with nitrous acid or sodium hypobromite, and may be represented
by the following equation : —

CO(NH2)2 + sNaBrO = COg + No + 2H2O + sNaBr
or, more simply, thus : —

C0(NH,)2 + 30 = CO2 + N2 + 2H2O

This reaction is taken advantage of in order to estimate the amount of
urea in any given sample of urine. A strongly alkaline solution of sodium
hypobromite is used, which absorbs the carbon dioxide given off, and only
allows the nitrogen to escape. From the volume of nitrogen given off, the
amount of urea present can be determined.

After urea is voided in the urine it is attacked by bacteria, and ammonium
carbonate is formed. The chemical action is one of hydrolysis ; thus —

CO{NH2)2 + 2H2O = C03(NH,)2

Creatinine (C4H7N3O) is a body of complex chemical constitution which
is present in urine in small quantity. It is closely related to creatine, a
disintegration product which occurs in muscle, especially on fatiguing the
muscle.

Uric acid (C5H4N4O3) is a bi-basic acid which is not found in a free
condition in the urine, but as acid sodium and potassium urates. It is not
present in large amount in mammalian urine, but forms the greater part of
the solid urine of birds and serpents. The free acid is much less soluble
than its salts, and is hence thrown out of solution on standing, after adding
a strong acid, such as hydrochloric acid, to urine. Still, the urates are not
very soluble in water, and when they are present in excessive amount they

^ The daily amount of each of these constituents in grammes, in round
numbers, is as follows : Urea, 33 '2 ; creatinine, 9 ; uric acid, 5 ; hippuric
acid, 4 ; pigment and other organic substances, 10 ; chlorine, 7 '5 ; sulphuric
acid (SO3), 2; phosphoric acid (P2O5), 3; sodium, 1 1 ; potassium, 2*5 ;
calcium, magnesium, and ammonium, i '2.

Excretion. 2 1 7

pass out of solution ^vhen the urine cools as a brick-red coloured deposit,
which may be distinguished from other urinary deposits by the fact that it
disappears on warming and reappears on cooling. ' The amount of urates
in the urine is increased by excess of proteid food, and by sudden excessive
exercise.

Hippuric acid (C9H9XO3) is a compound of amido-acetic acid with
benzoic acid ; - it is formed chiefly from the aromatic compounds present in
vegetable food, and is hence present in much greater amovmt in the urine
of herbivora. Its amount in the urine is increased by administration of
benzoic acid.

The pigments of the urine, like those of the bile, are probably chiefly
degradation products of haemoglobin, but their chemical relationships to
that substance have not yet been definitely made out.

* Uric acid or urates may be further recognized by the red colour which
they give when evaporated with strong nitric acid, turning purple on making
alkaline with ammonia (murexide test). Uric acid is insoluble in water,
but soluble in alkalies, urates being formed thereby.

2 C6H5.COqH + CH2(XH2)COOH=:COOH.CH2(NH)COC6H5 + H20
Benzoic acid. Amido-acetic acid. Hippuric acid.

CHAPTER XI.

THE NERVOUS SYSTEM.

The nervous system controls all the acts of the life of the
higher animal. It determines all the movements of the body
by initiating or preventing the contraction of the various
muscles ; it determines whether or not the cells of a gland
shall be passive or active, and so whether the gland shall or shall
not yield a secretion ; it regulates by its action on the arterioles
the supply of blood to the various parts, according to their
needs ; it gives the animal information as to its surroundings
by conveying impulses from the outer world to certain struc-
tures, themselves belonging to the nervous system, which
are capable of being acted upon so as to give rise to what are
termed sensations ; it further carries to these excitable structures
forming part of itself impulses from the different parts of the
body of the animal which give information as to the situation,
condition, and well being of these parts ; finally, it is an organ
capable both of judging and estimating present sensations, and
of retaining impressions of past sensations, as well as resolving
its judgment on this complex into definite acts which constitute
the life of the animal as a whole. The nervous system is hence
the seat of intelligence, and the more complicated and highly
developed the nervous organization is, the higher is the grade
of intelligence of the animal. In the lowest forms of animal
life no definite nervous system can be found. It first appears
in the form of isolated knots of cells, called ganglia^ to and
from which long processes or fibres pass, which are outgrowths
of the nerve cells. Higher in the scale of development these
ganglia become arranged in series, or chains, along the axis of
the body, and the ganglia are connected by fibres (nerve fibres)

The Nervoits System. 219

passing between them. In vertebrate animals, the series or
chain of ganglia becomes a connected whole, in which nerve cells
are present along the entire length, and bundles of fibres, called
nerves, are given off laterally in pairs on each side. The nerve
cells and the fibres connecting them together become enclosed
in a bony canal running along the vertebral column, from which
the nerves issue between the successive vertebrae ; further, at
its upper end, the nervous system becomes very much enlarged,
forming the brain, which is enclosed in the bony cavity of the
skull called the cranium. The extent of development of the
brain is an index of the intelligence of the animal and of
its position amongst the vertebrates. It is most developed
in mammals, most of all in man ; and amongst various races
of men, the most highly civilized and intellectual have also the
most highly developed brains.

The nervous system is usually described as divided into
two parts, of which one is much greater than the other. The
part enclosed in the cranium and in the neural canal of the
vertebral column, with the nerves attached to it, is spoken of
as the central nervous system, or cerebrospinal system, and a
double chain of ganglia situated on either side of the vertebral
column in the neck, thorax, and upper part of the abdomen,
and connected to one another and to the central nervous
system by nerve fibres, is known as the sympathetic system.
This sympathetic system is really an outgrowth of the cerebro-
spinal system. The part of the nervous system lodged in the
hollow of the cranium, and in the neural canal, forms the
cerebro-spinal axis, consisting of the brain and spinal cord.

By far the greater part of the volume of the brain in man
and in the higher mammals is taken up by the cerebral hemi-
spheres which occupy the upper and front part of the cranium
(see Figs. loi and 102). The two hemispheres are separated
from each other by a deep cleft (the longitudinal fissure), but
are united to each other at the bottom of this fissure by a thick
band of nerve fibres (the corpns callosiim), passing from the one
hemisphere to the other in order to carry nerve impulses across
from one to the other. The surface of each hemisphere is
marked by deep groves termed stilci, between which are the

220

Elementary Physiology,

Fig. ioi. — View of the cerebro-spinal axis. (After Eourgery.")

The right half of the cranium and trunk of the body has been removed by a vertical
section, and the roots of all the spinal nerves of the right side have been dissected
out and laid separately on the several vertebrse opposite to the place of their natural
exit from the cranio-spinal cavity.

F, T, O, frontal, temporal, and occipital lobes of cerebrum ; C, cerebellum ; P, pons
Varolii ; m o, medulla oblongata ; ju s, in s, point to the upper and lower extremi-
ties of the spinal cord ; c e, on the last lumbar vertebral spine, marks the cauda
equina ; C i, the sub-occipital or first cervical nerve ; C viii, the eighth or lowest
cervical nerve ; D i, the first dorsal nerve ; D xii, the last dorsal ; L i, the first
lumbar nerve ; L v, the last lumbar ; S i, the first sacral nerve ; S v, the fifth ; Co i,
the coccygeal nerve ; s, the left sacral plexus.

The Nei'vous System.

221

eminences known as the convolutions. The purpose of this is
to mcrease the surface and so make room for a greater number
of nerve cells, which are all situated in a thin greyish coloured
layer over the surface, forming what is called the cerebral cortex.
It is in the cells of this cortical layer that all the nerve impulses
which start from the cerebrum originate. The deeper part of
the cerebral substance is white in colour and contains no nerve
cells, but only nerve fibres passing away from or leading to the

Fig. I02. — Plan in outline of the encephalon, as seen from the right side. \

The parts are represented as separated from one another somewhat more than natural so
as to show their connections. A, cerebrum ; c, fissure of Sylvius ; B, cerebellum ;
C, pons Varolii ; D, medulla oblongata ; a, peduncles of the cerebrum ; /', t, d,
superior, middle, and inferior peduncles of the cerebellum ; the parts marked a, b,
form the isthmus encephali.

cells of the cortex and serving to carry impulses to or from these
cells. By these nerve fibres, the cells of the cerebral cortex
are placed in communication with the other parts of the central
nervous system where other cells are situated. Each cerebral
hemisphere is hollow, and the small cavity inside is known as
the ventricle. On the floor of this ventricle are situated certain
other nerve centres containing nerve cells to which some of the
fibres of the cerebral cortex run, and other fibres pass from these
intermediate centres to different parts of the brain. By far the

222

Elementary Physiology.

greater number of the fibres belonging to the cerebral cortex,
however, after being gathered up into a fan-shaped bundle
(known as the corona radiata)^ from all over the surface of
the hemisphere, unite to form a thick stalk or bundle of fibres,
which passes from the hemisphere backwards and downwards
towards the spinal cord and is known as the crtis cerebri (see
Fig. 102). The two crura cerebri form a narrow part of the brain

Fig. 103. — Right half of the brain divided by a vertical antero-posterior section (from
various sources and from nature. (Allen Thomson.)
I, 2, 3, 3«, 3,5, are placed on convolutions of the cerebrum ; 4, the fifth ventricle, and
above It the divided corpus callosum ; 5, the third ventricle ; 5', pituitary body ; 6,
corpora quadrigemlna and pineal gland ; +, the fourth ventricle ; 7, pons Varolii;
8, medulla oblongata ; g, cerebellum ; I, the olfactory bulb ; 11, the right optic
nerve : in, right third nerve.

which is known as the isthmus or mid brain. In the mid brain

there are certain other intermediate cell stations, or nuclei, of

grey matter, containing nerve cells, and to these certain of the

fibres of the crura cerebri pass, but by far the greater number

pass downward towards the spinal cord. Some of these fibres

pass into ^ the cerebellum and form the superior cerebellar

peduncle.

^ The expression "pass into " is here used in an anatomical sense ; in a
physiological sense some of these fibres pass into the cerebellum and carry
impulses to it, while others pass out and carry impulses from it.

The Nervous System. 223

The cerebellum^ like the cerebrum, consists of two exactly
similar hemispheres, which are also deeply indented on their
smface, giving rise to convolutions, but not in an exactly similar
manner to the cerebrum (see Fig. 103).

The two cerebellar hemispheres are further united to each
other by a thick band of fibres called the middle cerebellar
peduncle, which forms part of the brain known as the pons
Varolii} A third peduncle, called the inferior peduncle,
connects each cerebellar hemisphere with the spinal cord below.
The cerebellum is thus connected both with the parts of the
brain above and below and with its fellow of the opposite side.
It has, like the cerebrum, a cortex which contains nerve cells,
and the white part underlying this cortex contains only nerve
fibres.

A great number of the nerve fibres of the crura cerebri
do not pass off laterally to the cerebellum, but pass down
underneath the crossing fibres of the middle cerebellar
peduncle to reach the cord (see Fig. 106) ; these are joined in
this course by the fibres of the inferior cerebellar peduncle, and
together these bundles of fibres make up the greater part of the
medulla oblongata^ which is the name given to that part of the
brain intervening between the pons Varolii and the spinal cord.
In the medulla there are imbedded certain masses of grey
matter containing nerve cells, and here several pairs of nerves
take origin ; also at this part, the white and grey matter begin
to change their relative situation and tend to assume that which
they occupy throughout the spinal cord. In fact, the medulla
may be regarded as the part where transition from brain to cord
takes place. We have seen that in the hemispheres of the brain
the nerve cells lying in the grey matter occupy the cortex or
external part ; while the nerve fibres conducting the impulses
from these nerve cells lie more internally, forming the white
matter. In the spinal cord the position is reversed, for the
cells in the grey matter occupy the central part of the cord and
are everywhere surrounded by nerve fibres forming the white

^ A band of fibres so uniting two bilaterally similar parts of the central
nervous system is known as dL commissure ; thus the corpus callosiim is a great
commissure, so is the middle cerebellar peduncle, and there are similar
commissures uniting the grey masses in the spinal cord.

224 Elementary Physiology,

part of the cord. These nerve fibres run parallel to each other
down the length of the cord, in long strands. As they pass
down the cord their number, and hence the volume of the white
matter, decreases ; for they gradually pass in as they go to com-
municate with the nerve cells at different levels in the cord.
The purpose of these fibres is to give communication between
the cerebrum and other parts of the brain and the nerve centres
of the spinal cord, as well as intercommunication between the
different parts of the cord itself.

At the lower and anterior part of the medulla oblongata a
large number of fibres, coming from the cerebral cortex, cross
over to the opposite side and decussate in bundles as they cross
from each side, thus forming what is called the decussation of the
pyramids (see Fig. to6) ; for these fibres form part of what is
known as the pyramidal tract. The pyramidal tract commences
in the nerve cells of a portion of the cerebral cortex surrounding
a transverse fissure in the brain called the fissure of Rolando
(near A, Fig. 102). This region of the cerebral cortex is known
as the motor area, because the nerve cells of this area regulate
voluntary motion, and when certain portions of the area are
injured, voluntary motion, in definitely corresponding parts of
the body, is paralyzed.^ From the motor area of the cortex the
pyramidal fibres pass down in the corona radiata to the crus
cerebri of the same side, and so reach the medulla, where at the
decussation about four-fifths of them cross, but the proportion is '
very variable. In the cord there are thus two pyramidal tracts
on each side — that which has come from the opposite side of
the brain occupies the postero-lateral part of the white matter
of the cord, and is known as the crossed pyramidal tract; while
that which has not crossed at the decussation lies in the anterior
part of the white matter, and is termed the dii-ect pyramidal
tract. These fibres pass eventually to motor nerve cells ^ lying
at different levels in the cord, and hence the tracts decrease in

^ Also, on stimulating different parts of this area electrically, move-
ments of different parts take place ; and so well localized are the areas for
different movements, that it can be predicted with precision what movement
will follow stimulation of a definite point of the motor cortex (see Fig. 112),

^ These cells lie in the anterior horn of the grey matter of the cord
{pide infra).

The Nervous System.

225

volume as the cord is descended. Although the fibres of the
direct pyramidal tract do not cross at the decussation, they do
all cross at various levels lower down in the cord, and are all
eventually connected with nerve cells of the opposite side of

Fig. 104.— Different views of a portion of the spinal cord from the cervical region with

the roots of the nerves. Slightly enlarged. (Allen Thomson.)
In A, the anterior or ventral surface of the specimen is shown, the anterior nerve root of
the right side having been divided ; in B, a view of the right side is given ; in C,
the upper surface is shown ; in D, the nerve roots and ganglion are shown from
below. I, the anterior median fissure ; 2, posterior median fissure ; 3, antero-lateral
impression, over which the bundles of the anterior nerve root are seento spread
(this impression is too distinct in the figure) ; 4, postero-lateral groove into which
the bundles of the posterior root are seen to sink ; 5, anterior root ; 5', in A, the
anterior root divided and turned upwards ; 6, the posterior root, the fibres of which
pass into the ganglion, 6'; 7, the united or compound nerve; 7',^ the posterior
primary branch, seen in A and D to be derived in part from the anterior and in part
from the posterior root.

the cord. It follows from this complete crossing of the motor
fibres that an injury to the motor area of the brain will cause
motor paralysis, not on the same side of the body as the
injury, but on the opposite side.

When the spinal cord is cut across, or when thin sections of
it are made and examined with the microscope, the situation of
the grey and white matter, and the structure of each, can be

Q

226

Elementary Physiology.

made out. The grey matter is arranged in two somewhat
comma-shaped masses, connected by commissures passing in
front of and behind a small central foramen, which is the central
canal of the spinal cord (see Fig. 105).

The thicker end of each comma is called the anterior horn,
or cornu, and contains many large conspicuous nei*ve cells.

Fig. 105. — Section of spinal cord in the lower cervical region.

which, are connected with motor nerve fibres passing to the
peripheral parts of the body. The more pointed posterior
part is called the posterior horn or cornu, and to this the
sensory fibres pass which convey sensory impulses from the
periphery (skin, etc.) towards the central nervous system.

The spinal nerves arise in pairs from the cord at intervals,
which correspond roughly to the vertebrae, and the nerves issue
between the vertebrae. Each nerve arises by two roots (see
Fig. 104) from the cord, which are termed the anterior and
posterior roots. On the posterior root a ganglion is situated,
containing the nerve cells belonging to the fibres of that root,
and all these fibres are sensory or affe^'ent^ that is, they carry nerve

^ " Afferent " means leading impulses to the central nervous system, and
"efferent " leading impulses from the central nervous system : these are more
general terms than " sensory " and "motor ; " for all fibres leading nerve
impulses to the cord are not sensory, and all leading from are not motor.

The Nervons System. 227

impulses inwards from the periphery to the posterior horn of the
grey matter. The anterior root has no gangHon, and the nerve
cells belonging to the fibres contained in this root are situated
in the anterior horn of the grey matter. All the fibres of the
anterior root are motor or efferent^ and convey nerve impulses
from the central nervous system (anterior cornu) to the periphery.

The union of the two roots gives rise to the mixed nerve
trunk, in which some of the fibres are afterent and some
efferent. These facts as to the nature of the fibres in the two
spinal nerve roots have been made out from the effects of
section or of stimulation of each. When all the posterior roots
of the nerves going to a limb are cut, sensation is lost in the
limb, but it can still be moved. When, on the other hand, all
the anterior roots are cut, sensation is still present in the limb,
but it cannot be moved. Again, when the central end of the
divided anterior root is stimulated electrically no effect is
obtained ; but when the peripheral end is so stimulated there is
a movement of the limb. Further, when a similar experiment
is made with the posterior root no effect is obtained now on
stimulating the peripheral end ; but with the central end there is
evidence that an impulse has reached the cord in the move-
ments of the opposite limb, or of all the other limbs of the
body, according to the strength of the stimulus, or of the same
limb if the anterior roots passing to it have not been divided.

The spinal cord does not extend quite to the end of the
neural canal, but passes at the lower end of the body of the first
lumbar vertebra into a slender filament called thejihwi termi7iale.
The lower lumbar nerve roots pass downward, to their exit,
parallel to the filum terminale, forming together what is known
as the Cauda eqtnnce.

Besides the spinal nerves there are twelve other pairs of
nerves which arise from the cerebro-spinal axis. These nerves
leave the brain within the cranium, and are hence called the
cranial nerves. Enumerated from before backward the cranial
nerves are as follows : —

I. The olfactory nerves arise from the olfactory tract, which
is an outgrowth from the frontal lobe of the cerebrum. About

^ See note on p. 226.

228 Elementary Physiology.

a dozen filaments pass from the olfactory tract to the mucous
membrane (olfactory mucous membrane) lining the upper part
of the nasal passages.

II. The optic nerves passing backward, one from each eye,
meet in the middle line to form the optic commissure, from
which the two optic tracts pass outward and backward (see
Fig. 1 06) to be distributed to those intermediate cell stations,
already mentioned (see p. 221), in the floor of the ventricle
of the cerebrum and in the mid brain. These intermediate
stations are connected by other fibres with the posterior
part of the cortex of the cerebrum (occipital lobe), which is
chiefly concerned with visual sensations (see Fig. 112, p. 242).

III. The oculo-motor nerves supply some of the muscles of
the eyeball and of the interior of the eye.

IV. The trochlear nerves supply one of the muscles of the
eyeball (superior oblique).

V. The t?'igei?imal nerves are so called because they divide
into three chief branches (see V, i, 2, and 3, Fig. 106) : viz. {a)
the ophthalmic division, which chiefly supplies sensory fibres to
the eyeball, to the interior of the eye, and to neighbouring parts ;
ip) the superior maxillary division, which supplies sensory fibres
to the skin of the temple and cheek, to the teeth of the upper
jaw and surrounding parts; and (^) the inferior maxillary division,
which supplies sensory fibres to the mucous membrane on the
inner surface of the cheek, to the anterior two-thirds of the
tongue, and the floor of the mouth, to the teeth of the lower
jaw, to the chin, lower lip and margin of the jaw, and to the
muscles of mastication. This division also supplies motor fibres
to the muscles of mastication.

VI. The abdncejit nerves are the smallest of the cranial
nerves, and are merely motor nerves in each case to that muscle
which causes outward movement of the eyeball (external rectus).

VII. The /^i;(r/<^/ nerves supply motor fibres to the muscles
of the face, and secretory fibres to the salivary glands.

VIII. The auditory nerves supply the organs of hearing ;
they also send fibres to peculiar structures in the internal ear,
called the semi-circular canals, which are concerned with the
sensation of equilibrium.

The Nervotis System,

229

Fig. 106. — View from before of the medulla oblongata, pons Varolii, crura cerebri, and
other central portions of the encephalon. (Allen Thomson.) Natural size.

t', the olfactory tract cut short and lying in its groove ; II, the left optic nerve in front
of the commissure ; II', the right optic tract : Th, the cut surface of the left thala-
mus opticus ; G, the central lobe or island of Reil ; Sy, fissure of Sylvius ; X X,
anterior perforated space ; e, the external, and i, the internal corpus geniculatum ;
h, the hypophysis cerebri or pituitary body ; tc, tuber cinereum with the infundibu-
lum ; a, one of the corpora albicantia; P, the cerebral peduncle or crus ; III, close
to the left oculo-motor nerve ; X, the posterior perforated space. PV, pons Varolii ;
V^, the greater root of the fifth nerve ; +, the lesser or motor root ; \'l, the sixth
nerve ; VII, the facial ; VIII, the auditory nerve ; IX, the glossopharyngeal ; X,
the pneumogastric nerve ; XI, the spinal accessory nerve ; XII, the hypo-glossal
nerve ; C I, the suboccipital or first cerA'ical nerve ; p a, p3"ramid ; o, olive ; d, an-
terior median fissure of the spinal cord, above which the decussation of the pjTamids
is represented ; c a, anterior column of cord ; r, lateral tract of bulb continuous
with c I, the lateral column of the spinal cord.

230 Elementary Physiology,

IX. The glossopharyngeal nerves are the nerves of taste to
the posterior third of the tongue ; they are also sensory nerves
for this region and the neighbouring parts, as well as the upper
part of the pharynx.

X. The pneiimogastric'^ nerves have a very important and
wide distribution. From the wandering course of the nerve it
is also called the vagus. It contains both afferent and efferent
fibres, and sends branches to the pharynx, larynx, oesophagus,
heart, lungs, stomach, intestine, pancreas, and liver. By
means of these branches it exercises an important control
on the operations of swallowing, digestion, and secretion, and
also plays a part in regulating the rhythm of respiration and of
the heart-beat.

It contains both afferent and efferent fibres ; among the afferent fibres
are those to the respiratory centre, which differ in their action according to
the part of the distribution of the nerve from which they come. When both
vagal trunks are cut in the neck, the respiration becomes very slow, and if
now the central end of one vagus be stimulated the respiratory rhythm is
greatly quickened, showing that the vagus trunk contains afferent fibres,^
which normally have an accelerating effect on the rhythm of respiration.
On the other hand, if those fibres of the vagus contained in its superior
laryngeal branch, which supply the mucous membrane of the larynx, be
stimulated, respiration is inhibited, inspiration is stopped, and a violent
expiratory effort takes place. Such a stimulation naturally takes place
when a particle of food or other foreign matter comes in contact with the
mucous membrane of the larynx, and the purpose of the expiratory effort so
produced is to expel the particle and prevent it from dropping into the
trachea. Examples of the distribution of the efferent fibres of the vagus
are : the motor fibres to the muscles of the larynx in the inferior laryngeal
branch; the secretory fibres to the glands of the mucous membrane of
stomach and intestine, and to the pancreas which stimulate the cells of these
glands and cause them to secrete during digestion ; the cardio-inhibitory
fibres to the heart, which slow the rhythm of that organ or stop it temporarily
if stimulated sufficiently strongly.^

^ So called because it gives off branches to both lungs and stomach.

^ These afferent fibres produce their effect reflexly (see p. 237) : when
stimuli passing along them reach the respiratory centre situated in the
medulla oblongata, reflex motor stimuli are sent out from this centre, which
travel along the motor nerves to the respiratory muscles and cause these
muscles to contract.

^ Such stimuli constantly reach the heart along the vagi during life, as
is shown by the great quickening in the heart-beat which always follows the
cutting of both vagi. That the vagi here act as efferent nerves carrying
impulses to the heart is shown by the fact that electrical stimulation of the
peripheral end of the cut nerve {i.e. the end next the heart) causes slowing,
or if strong enough temporary stoppage of the heart.

The Nervous System. 231

XI. The spinal accessoi'y nerves arise by two different roots,
one in the medulla and the other in the lower cervical part of
the spinal cord. This nerve gives branches to the vagus, and
it has been shown that it is to fibres derived from the medullary
origin of the spinal accessory, and which join the vagus, that
the cardio-inhibitory action of the vagus is due. The fibres
which arise from the cervical spinal cord chiefly supply motor
fibres to certain of the muscles of the neck.

XII. The hypoglossal nerves take origin from the medulla
oblongata, and are motor nerves which supply the muscles of
the tongue.

The sympathetic nervous system consisting, as has been
already mentioned, of a double chain of ganglia {i.e. knots of
nerve cells) connected to each other by nerve fibres, is an
outgrowth of the central nervous system, and is connected to
it at intervals along its length by strands of nerve fibres {i-ami
coinimmicantes) . The fibres of the sympathetic system act
chiefly on the vascular and visceral system, sending branches
to the secreting glands, which control the character of their
secretion j to the walls of the small arteries {v as o-motor fibres)^
controlling their calibre ; to the heart, quickening its rhythm,
and so antagonizing the vagus ; to the intestinal muscular walls,
controlling their peristalsis ; to the iris of the eye, increasing
the diameter of the pupil (see p. 269); and, generally, the
sympathetic fibres may be said to carry out operations necessary
to the well-being of the animal, but outside the control of its
will. On the other hand, the nerve impulses of the central
nervous system are partially voluntary and partially involuntary.

We may next briefly consider the minute structure of the
nerve cells, nerve fibres, and nerve endings which form the
essential parts of the nervous mechanism.

If one of the peripheral nerve trunks be exposed, such as
the sciatic nerve in the thigh, and a small length be cut out
and teased with needles in normal saline solution, it will be
found that the nerve is not easily broken across, but that it
divides rather easily into strands along its length. If now a
minute strand be taken and teased out in saline solution
and then examined under the microscope, it will be found

Elementary Physiology.

c^ .a

<U dj

-13

•^ o

D <U

/'i J;/ 1

The Nervous System, 233

that the nerve is made up of bundles of long fibres which run
parallel to one another in the length of the nerve without
branching. The nerve fibres are bound compactly together by
connective tissue which envelopes the whole nerve trunk in a
sheath from which septa are sent in dividing the nerve trunk
into bundles. The nerve fibres do not branch or communicate
with one another throughout the whole length of their course.
By appropriate straining and examination they may be shown
to have the structure represented in the accompanying illus-
trations (Figs. 107, 108).

At intervals there are constrictions of the fibre, which are
termed the nodes of Ranvier. At the nodes the outer thin
sheath of the fibre, known as the neurilemma^ is constricted,
and the inner coat {medidiary s/ieath), which is thick and
consists of soft fatty material, is wanting. Here, where the
medullary sheath is discontinuous, there may be made out a
thin central fibre passing without interruption through the node.
This thin fibre is termed the axis cylinder ; when the medullary
sheath is dissolved away by treatment with ether, it may be
made out passing continuously from node to node throughout
the length of the fibre.

The axis cylinder is the essential part of the nerve fibre,
and the other coats are protecting sheaths for it. It begins as
a process of a nerve cell, and pursues an uninterrupted course
from the nerve cell to the nerve termination throughout the
entire length of the nerve fibre. That the neurilemma or
primitive sheath and the medullary sheath are accessory parts
only, is shown by the fact that each is absent in certain situa-
tions, while the axis cylinder is never wanting. Thus, in the
spinal cord and brain, where the nerve fibre is protected by
other means, and where individual nerve fibres never run alone
as they do in the terminal branchings of the peripheral ner^^es,
the outer sheath is not required, and hence is absent. Again,
in the sympathetic system, the medullary sheath is but feebly
developed, and is often absent, giving rise to what are termed

^ Also called the primitive sheath or sheath of Schiuatiii ; beneath this
sheath cell nuclei are placed at intervals. These nuclei are best shown by
straining with hasmatoxylin after treating with acetic acid.

234

Elementary Physiology.

noii-medullated fibres (see Fig. io8), the usual variety of nerve
fibre being called medulla fed fih-e.

The nerve cells from which the nerve fibres arise vary-
greatly in appearance in different parts of the nervous system.

Fig. log. — Multipolar nerve cell from anterior horn of spinal cord, human. (Gerlach.)'
a, axis-cylinder or nerve-fibre process ; b, pigment.

Some are very large with a large clear spheroidal nucleus con-
taining a nucleolus, while others are very small. They are
classified, according to the number of processes which they

The Nervous System. 235

give off, into unipolar, bipolar, and multipolar. By far the
larger number are multipolar, as this is the chief type found
in the brain and spinal cord ; ^ there are in this type a large
number of processes, one of which becomes an axis cylinder of
a nerve fibre, while the others branch into fine fibrils termed
dendrons (see Fig. 109). Bipolar and unipolar nerve cells are
chiefly found in the ganglia of the posterior roots of the spinal
nerves. The nerve fibre of one nerve cell never communicates
directly with another nerve cell, but breaks up into branches
which interlace with the dendrons of that cell. The fibres of
peripheral nerves communicate with their nerve cell at the
central end ; in the case of the afferent fibres this nerve cell
is situated in the ganglion of the posterior root; in the case
of the efferent fibres it is placed in the anterior cornu of the
grey matter of the spinal cord. At the periphery these fibres
terminate, either in ramifications formed by subdivision called
plexuses, or in special end organs. Plexiform terminations are
found in the cornea, in involuntary muscle and in certain glands.
Special end organs are found in voluntary muscle, tendon, and
skin ; some of these are shown in the diagrams (Figs, no, in).
In certain cases, such as the tactile corpuscles in the skin,
a nerve impulse originates in these end organs, and travelling
towards the central nervous system gives rise to a sensation
when it arrives there ; ^ and in other cases a nerve impulse
arriving at the end organ from the central nervous system
starts cell activity there. For example, on arriving at a
muscular end plate (see Fig. in) a nerve impulse gives
rise to contraction of the muscle fibre. The life of a nerve
fibre depends upon its connection with the nerve cell to which
it belongs ; when severed from this the fibre undergoes certain
changes which are spoken of as nerve degeneration. Only the
part of the fibre which is cut off from the nerve cell undergoes
these changes, the part still in connection remains unchanged

' The cells here are further classified according to their shape, e.g. the
pyramidal cells of the cerebral cortex, and the pear-shaped cells of Purkinje
in the cerebellum.

^ The specialized nerve terminations in the organs of special sense, such
as the eye and ear (see p. 245), may be regarded as such end organs in which
nerve impulses arise giving origin in the brain to special sensations.

236

Elementary Physiology.

in its structure. This gives an important method of investi-
gating the connections and course of nerve fibres, especially in
the brain and cord. By its means tracts have been discovered
in the spinal cord, for the fibres of the white matter do not run
indiscriminately, but those with nerve cells situated above lie
in one part, and those with nerve cells below lie in a different
part. Hence some tracts of the white matter degenerate below

Fig. iio. — Tactile corpuscle within a
papilla of the skin of the hand,
stained with chloride of gold.
(Ranvier.^

Hi two nerve fibres passing to the cor-
puscle ; a, a, varicose ramifications
of the axis cylinders within the
corpuscle.

FiG._ III. — Nerve-ending
in muscular fibre of a
lizard (Lacerta viri-
dis). (Kiihne.)

{descending tracts)^ after section at any level, while other tracts
degenerate above {ascending tracts). In this manner the
ascending and descending tracts shown in Fig. 105 have been
discovered.

The nerve fibres of the sympathetic system have their
nerve cells in the ganglia of the sympathetic chain, and with
these nerve cells fibres from the spinal cord communicate.

In general terms, then, the nervous system may be spoken
of as an exceedingly complex meshwork of nerve cells, nerve

The New 07 IS System. 2'^'j

fibres, and nerve endings, in close communication with one
another, and capable of affecting one another, and calling one
another into activity. By communicating fibres all the different
parts of the brain and spinal cord are, as it were, put in touch
with one another; and there are usually several paths of
nervous communication between any two given points, so that
when one is broken down from any cause another may be
used. At different points, nerves containing immense numbers
or fibres pass off to all the peripheral parts to supply
communication between the central nervous system and all
the other parts of the body ; some of these carry messages
from the centre (efferent fibres), while others carry messages
to the centre (afferent fibres). In addition, there are special
nerves, capable of being rendered active by light, sound, etc.,
which convey impressions to the brain of what is going on
in the outside world, and give rise to actions in the nervous
system, which in turn are resolved into actions upon the other
tissues.

The nerve cells are not arranged indiscriminately in the
brain and cord, but those cells which carry out a common
purpose are placed close together. A collection of nerve cells
which possess a common purpose is termed a nerve centre.
Such nerve centres are found in the different parts of the cord,
medulla oblongata, and brain. The centres in the cord preside
over the muscular movements, and are capable of carrying
out quite complicated movements without assistance from the
brain. This is especially the case in lower types of vertebrates,
such as the frog; in the mammalia generally, and in man,
these spinal centres are much less independent, and are
completely under the control of the more highly developed
brain.

The simplest form of a complete nervous action is what
is termed a reflex act. In this an afferent impulse starting at
the termination of an afferent nerve fibre passes up to the
nerve centre and affects a nerve cell there ; this in turn aftects
another nerve cell connected with an efferent fibre, and a
nerve impulse travels down the efferent fibre, and shows
itself by some action in the tissue to which this efferent fibre

238 Elementary Physiology,

passes. For example, irritation of the skin, by pricking with
a sharp instrument, or by contact with a drop of acid, may
stimulate sensory nerve endings in the skin, thus starting nerve
impulses, in the afferent fibres attached to these nerve endings,
which reach the nerve centre in the spinal cord ; these next
stimulate motor nerve cells and start efferent impulses down the
fibres belonging to these cells which reach motor end plates
in voluntary muscles and cause contraction and muscular
movements.

Such reflex movements may be carried out by the nerve
centres in the spinal cord without at all affecting the brain,
and indeed can take place after the brain has been removed,
or after the cord has been cut across and thus removed from
the influence of the brain. Injuries to the spinal cord which
practically amount to separation from the brain are often
observed in man as the result of accident or disease, and it
is then seen that although the muscles of the part of the body
below the injury are no longer under the control of the will,
they can be moved when the skin of the part is irritated.
Such movements are reflex in character, and the centre for
the reflex lies in the spinal cord ; the brain is not affected by
them, and the patient is not conscious either of the irritation
of the skin or of the muscular movements, except by seeing
them. Suppose, for example, the spinal cord is injured in
the dorsal region so as to be no longer capable of transmitting
nerve impulses up or down ; then the legs become paralyzed,
the patient can neither move them voluntarily, nor can he feel
anything in them. If now the sole of the foot be tickled the
leg will be drawn up violently, but the patient declares that
the tickling was not felt, and that he made no effort to draw
up his leg — in fact, he is only conscious that the leg moves by
seeing it, or by the movement disturbing other parts of his
body from which his brain can still receive sensory impulses.

There is not much co-ordination ^ in these purely spinal
reflexes in man; they are quite disorderly, and in the case
of other mammals, although the co-ordination, on recovery
after complete division of the cord, is greater, it is still by no

^ See p. 73.

The Nervous System. 239

means perfect. It is in cold-blooded animals, such as the
frog, that the cord is most perfect as a reflex centre. When
the head of a frog is cut off, or its brain destroyed by pithing,
the remainder of the animal is still capable of carrying out
most involved and complicated co-ordinated movements. If
it be hung from a support, and a small piece of paper moistened
with acid be applied to one flank, or to the abdomen, there
is, after a pause, a movement of the leg obviously intended to
stroke the irritated spot. If the leg of the same side be held
and prevented from carrying out this purpose, then after a
longer pause, there is a movement of the opposite leg, likewise
designed to remove the source of irritation. If the piece
of paper be applied to the back of the thigh, the muscular
movement is of quite a different type, but designed also to
brush the irritated spot. The movements are so purposeful
as to suggest that they are directed by an intelligent reasoning
cause, and there is no doubt that the nerve centres in the cord
of such an animal act as subsidiary brains, and that although
the frog as an individual is dead, yet there remains a nervous
mechanism to all tests as intelligent as the entire nervous
system of an animal of a somewhat lower type. Such an
amount of independent action is not found in the spinal cord
in warm-blooded animals, where these centres become more
subsidiary to the chief centres in the brain. In warm-blooded
animals the chief centre for voluntary co-ordination of com-
plicated movements lies in the cerebellum, and when this is
removed or injured by disease it is found that co-ordination
becomes exceedingly faulty.

The functions of the different parts of the brain, in so far as
they are yet known to us, have been made out in part from the
study of disease in man, and in part from experimental removal
in animals.

The office of the cerebral hemispheres as a whole is to
receive nerve impulses from sensory fibres, which awaken what
is termed perception, and give rise to varied sensations. As a
result of these sensations, efferent voluntary impulses may be
sent out from the cells of the cerebrum, and give rise to move-
ments ; or contrariwise nerve impulses may be sent out to stop

240 Elementary Physiology,

or inhibit movements which would naturally take place in a
purely reflex manner but for the restraining and guiding
influence of the cerebral centres. For this restraining and
guiding activity of the nerve cells of the cerebrum, many terms
are used in popular language, such as the intelligence, will, and
judgment. When the cerebral hemispheres are removed com-
pletely, the animal loses all this intelligence and guiding power.
It is, like a ship without a rudder, completely at the mercy of
all those influences which play upon it from the outer world.
Each stimulus that reaches it gives rise to a certain response,
just as if the animal were a piece of complicated but unin-
telligent mechanism. In the case of most warm-blooded
animals, death soon follows complete removal of the cerebral
hemispheres ; ^ but, in the frog, removal is not followed by
death for a long time, if only the animal be kept moist.
Such an animal, to casual observation, appears to differ little
from a normal frog ; it sits in a normal manner, and moves
when irritated in any way. If placed on its back, it at once
turns over into the natural position. If thrown into a vessel
containing water, it swims in a regular fashion till it reaches
the side of the vessel, and then, if possible, will crawl up this
and perch passively on the top until it is again disturbed. If
placed on a board which is slowly inclined, it does not slide off
as the inclination is increased, but balances itself and crawls up
the board, to perch in a more comfortable position on the top
as the inclination is increased. If placed in front of an opaque
obstacle and stimulated to jump, it springs to one side so as to
avoid the obstacle. If stroked along the flanks, it croaks
regularly. In fact, the animal is capable of carrying out the
most complicated acts in a perfectly natural manner. But it is
7nerely an automaton^ all these things are performed mechanically,
and with a stimulus definite in amount and kind, it is perfectly

* That the case is much the same in mammals is shown by the fact that
serious injury to large areas of the cerebral cortex by operation leads to an
idiotic condition of the animal, as is also the case when the blood-supply to
the hemispheres, and hence their functional activity is interfered with.
Again, in the case of man, want of development or insufficient development
of the cerebral hemispheres causes idiotcy, and among different races of
men, the grade of intelligence varies directly with the development of the
cerebral hemispheres.

The Nervous System. 241

settled what the frog will do ; there is but one answer to each
stimulus. It never does anything vohmtarily. It never moves
of its own accord, but sits continually in the same spot and
attitude unless stimulated from without. It takes no food,
seems to have no sensations, and if undisturbed will die and
dry up in the same exact position. All its responses to stimuli
are definitely measured out so that a certain strength of stimulus
gives a certain effect, there is no inhibition of movement or
origination of movement by a reasoning centre. Swimming is
caused by the afferent nervous impulses started by contact with
the water, and will go on automatically till the animal reaches
the edge or until it sinks exhausted to the bottom ; the move-
ments are uncontrolled. Again, when the flanks are 5troked^
the animal cannot decide to croak or not croak like a complete
frog, but must croak automatically and in measured degree
each time it is stimulated.

Similar experiments have been performed on the pigeon
with like results ; the animal, when undisturbed, sits as if asleep ;
but, when disturbed, can walk, fly, perch, and balance itself in
a normal manner. It never takes food voluntarily, but swallows
mechanically when food is placed at the back of the throat,
and may be kept alive in this fashion for a long time.

It has already been mentioned that there is localization of
function in different parts of the cortex of the cerebral hemi-
spheres, and that the cortex around the fissure of Rolando
governs the voluntary movements of the muscles of the trunk
and limbs. It has further been discovered that certain other
portions of the cortex are concerned in special sensations and
special movements. These areas are shown in the accompany-
ing diagram (Fig. 112), and have been localized by obser\ing
the effects of disease or injury upon these parts, and by experi-
mental stimulation or removal of these parts of the cortex in
animals. The part marked " lips and tongue " is concerned in
controlling the movements of the muscles of the lips and tongue,
and of regulating speech. Injury to this part causes inco-ordi-
nation of speech and inability either to recollect^ or it may be to
say certain words, a condition which is termed aphasia. This
part of the cerebral cortex is hence termed occasionally the

R

242 Elementary Physiology.

" speech centre." It is usually the left side of the brain which
normally exercises this control ; but in the event of permanent
injury to this portion of the cortex on the left side, the right
side after a time takes up the work, and the aphasia disappears.
Certain important nerve centres lie in the spinal hUb or medulla
oblojigata, and regulate the discharge of nerve impulses which
are essential to the life of the animal. For example, the

Fig. 112. — Diagram of the external surface of th2 brain seen from the left side ; to
indicate the position of the chief centres of localization.

respiratory centre is situated here which regulates respiration.
If the medulla be destroyed in the region of this centre, re-
spiration ceases, and the animal promptly dies of suffocation
or asphyxia. The afferent impulses arrive chiefly by the
vagus, and the efferent impulses are sent out along the phrenic
nerves which supply the diaphragm, and the intercostal nerves
which supply the intercostal muscles. The centre is acted
upon in two chief ways : first, chemically ^ by the nature of the

The New Otis System. 243

blood circulating through it, for when this is poor in oxygen
the cells of the respiratory centre are stimulated to greater
activity, and the rhythm of respiration is quickened ; and
secondly, the centre is stimulated by the number and strength
of the afferent nerve impulses reaching it. When the respiratory
efforts are artificially increased the nerve impulses reaching the
centre become feebler, and the centre acts less energetically ;
on the other hand, diminished respiration, or suspended re-
spiration, for a short time strengthens the afferent impulses
sent to the centre, and the desire to breathe, and to breathe
strongly, "becomes imperative. The respiratory centre is also
affected, and the rhythm changed by nervous impulses reaching
the centre along various other channels. Thus, violent emotions
alter the respiration, and again a plunge into cold water will
awaken sensory impulses from the stimulation of the skin,
which affect the respiratory centre and change the rhythm.
Quiet normal respiration is, however, brought about by an
automatically repeated reflex, which is regularly discharged
from the respiratory centre, and the variations in rhythm are
due to extra afferent impulses arising from temporary causes.

Near the respiratory centre lies also the important vaso-motor
centre which controls the tonicity of the small blood-vessels
and so the distribution of the blood-stream. Here, also, are
situated the cardiac centres, which control the rate of the heart-
beat. Other centres are situated in the medulla, which control
the reflexes for the winking or closing of the eyelids, which pre-
serves the surface of the eyeball clean ; which control reflexly
the diameter of the pupil of the eye, and so regulate the amount
of light entering ; which control the reflex acts of swallowing,
coughing, sneezing, secretion of saliva, vomiting, etc. It must
not be understood, however, that groups of nerve cells corre-
sponding to these centres have been demonstrated, it is merely
known that the medulla is the part of the central nervous
system which regulates these reflex acts.

To sum up, then, the nervous system is an exceedingly
complex network of nerve cells, nerve fibres, and nerve endings.
The nerve cells are situated centrally, and the nerve endings
peripherally, and the two are connected by nerve fibres passing

244 Elementary Physiology.

between centre and periphery. Further, the nerve cells in the
different parts of the central system are connected together in a
most complex fashion, by nerve fibres running from one part to
another, and setting the whole in close communication.

In the spinal cord are nerve centres, which are capable of
controlling complicated muscular movements, and these are
under the control of the nerve cells in the cerebral cortex, by
which they may either be set in motion or inhibited, i.e. put
out of action. In the medulla (that part joining brain and
cord) lie other centres which guide the rhythm of heart-beat
and respiration as well as centres for carrying out other im-
portant work. In the cerebellar cortex lie intermediate cell
stations which govern the co-ordination of voluntary move-
ments, and in the cerebral cortex lie the master cells, which
are so affected by the sensory impulses coming to them as to
give rise, in some way unknown to us, to consciousness, will,
judgment, intelligence, and memory, and have the power of
sending out in reply different impulses which originate the acts
of the animal.

CHAPTER XII.

THE SENSES.

Our sensations are awakened by the stimulation of nerve
centres in the brain by nerve impulses which start by the
excitation of sensory nerve endings in the periphery and
travel to the brain along sensory or afferent nerve fibres.

It is usual to divide sensations into two classes, although
there is no very essential difference in kind corresponding to
the classification. These two classes are common sensations
and special sensations.

The first class includes those general sensations which
cannot be localized accurately, such as fatigue, hunger, and
thirst, as well as the sensations which give rise to coughing,
vomiting, tingling, itching, and such like. All these sensations
are referred to changes going on within the body.

The muscular sense is also usually classed as a common
sensation. This sense gives an impression of the state of
contraction of the skeletal muscles, and so enables the nervous
system to regulate the degree of contraction which is necessary
in the various groups of opposed muscles in carrying out
compUcated movements, such as walking, grasping, writing,
etc. When the muscles contract against resistance the muscular
sense also gives an idea of the amount of effort required, and
it is this sense which we employ when we weigh bodies in the
hand by movements of the forearm.^ It is probable that the
sensory nerve endings for the muscular sense lie in the muscles

* The muscular sense is much more delicate for this purpose than is the
sense of pressure alone. Thus, while a difference of weight of one in
thirty is easily appreciated by most people when movement of the forearm
is allowed, it is scarcely possible to detect a difierence of weight of less
than one in eight when pressure on the palm is alone employed.

246 Elementary Physiology.

themselves, although it has been held by some that sensory
nerve endings in the articular surfaces of the joints have a
ofreat deal to do with the sense of muscular effort, and with
our appreciation of the weights of bodies.

Certain sensory nerves are, however, connected with specific
kinds of sensations, which are produced by influences outside
the body, and these sensations are termed special sensations.
Usually five special senses are recognized, which are popularly
known as the ^'' five senses;'' these are totich, taste^ smelly hearings
and sight.

The sense of touch lies on the borderland between common
and special sensations. It differs from the other four of the
five senses in that it is not conveyed to the brain by any
special nerve, but may be awakened by stimulation of any of
the nerve endings of touch in connection with the cutaneous
nerves. But it resembles the other four special sensations in
that the cause producing it is referred, to somewhere and some-
thing outside the body, to an external cause. When we touch
an object belonging to our surroundings, the sensation pro-
duced is known to be caused by something outside the body;
just as when we see an object, we are aware that the visual
sensation is awakened by something in the external world.
But when our tooth aches, or when we are wearied, we feel that
it is something connected with our body which is giving rise to
the sensation.

We have to deal here more particularly with the special
sensations and the apparatus by which they are evoked; we
have to describe in outline the nature of the peripheral organs
by means of which the various forms of energy reaching the
body from the outer world are enabled to awaken sensory
nerve impulses, which, travelling along definite paths to definite
parts of the cerebrum, awaken in our consciousness each its
own specific sensation.

There are one or two common properties of the special
sensations which it may be well to state before passing to the
description of each of the peripheral organs.

The nerve fibres which pass from a peripheral special sense
organ are only capable of conveymg to the consciousness one

TJie Senses. 247

specific kind of sensation. This is known as the law of
specific sensation. Thus the optic nerves when stimulated in
any manner only give rise in the consciousness to visual
sensations, although the stimulation may be affected in various
ways, such as ordinarily by the stimulation of the proper nerve
endings of the optic nerves by light ; pressure on the eyeball
when the eyelids are closed ; ^ application of the electric current,
or severing the optic nerve. The same is true of each of the
other specific sensations. For example, irritation of the mucous
membrane of the nasal passages where the olfactory nerve
endings lie, as by a catarrh or cold, may produce subjective
sensations ^ of various smells.

Whatever part of a sensory nerve fibre be irritated, the
sensation is always referred by the consciousness to the nerve
ending in the periphery. Thus, after an amputation, when the
cut ends of the nerve by reason of the irritation send impulses
to the brain, these are felt as coming from the cut-off part, and
the patient believes he feels pain in his fingers or toes as if
these still formed part of his body. Again, if the elbow be
immersed in a freezing mixture, the chill irritates the nerve
trunks of the arm at the elbow ; but the sensation experienced
is not felt as coming from the elbow, a sensation of pain instead
is felt which is referred to the finger-tips. The same kind of
thing is felt when the ulnar nerve, w^hich is almost subcutaneous
at the elbow, is irritated by pressure or by a chance blow;
the pricking sensation of " pins and needles " is then felt, not
at the seat of irritation, but all the way down the forearm, where-
ever the ulnar nerve has sensory endings, down to the finger-tips.
Another example of the peripheral reference of sensation by
the sensor i^ivi is the peculiar feeling experienced when a limb,
as it is popularly termed, " goes asleep!' This is due to a slight
continued pressure at any point on the nerve trunk, and not to
anything at the periphery, although the tingling sensation is felt
all over the distribution to the limb of the nerve pressed upon.

^ The coloured images so produced are termed phosphoies.

- A subjective sensation is one produced in an abnormal fashion, giving
rise to an impression of something in the outer world which has no exist-
ence in fact.

248 Elementary Physiology.

It has therefore to be borne constantly in mind that the seat of
injury giving rise to a pain or other sensation may be either
central^ or in the 7ierve trunks or peripheral ; and still, in any of
these cases, the nerve centres can only act and give rise to
consciousness by sending in nerve impulses by the usual
channels. Hence these are estimated as rising in the accus-
tomed fashion, and so give rise in the consciousness, both to
the specific sensation which is normally derived from impulses
carried by that particular nerve route, and to the impression
that the sensation has been awakened in a normal fashion by
stimulation at the peripheral nerve endings.

The strength of sensations is not proportional directly to the
strength of the physical stimulus, but varies as the logarithms ^
of the physical strengths ; this is known as the Weber-Feehner law.
It would never do, for example, if a light equivalent to 1000
candles in the physical intensity of its illumination produced one
thousandfold the effect on the eye. In spite of the arrangements
in the eye, which will subsequently be described, for shutting out
excessive light, if such a condition of things existed, the bright
light would only produce pain by excessive stimulus, and an
illumination much short of the electric light, and far short of
that of a bright sunny day, would absolutely blind us ; either
that or we could not see by the rushlight or starlight. But the
relationship stated above of stimulation to sensation, tends to
equalize sensations so that we can bear and appreciate both
strong and weak stimuli. To a certain extent this toning down
has to be paid for in inability to distinguish small variations in
intensity, but the acuteness in this direction is sufficient for the
ordinary wants of our life.

Although subjective sensations, as stated above, may be
felt when the sensorium is divided from the peripheral nerve

^ For those unacquainted with the nature of logarithms, a simpler
statement of the law is that the change in magnitude of the sensation is
proportional to percentage increase of the physical stimulus. Thus, a
minute trace of difference in intensity of illumination can be made out by
most persons between two lights of 100 candles and loi candles respec-
tively ; now, to see such a difference with an illumination equal to 1000
candles, alight not of 100 1 candles, but of 10 10 candles, must be employed
in comparison.

The Senses. 249

ending, or when a different part of the system than the nerve
ending is irritated, still no objective ^ sensation can arise
except by action in a normal manner of the proper kind of
stimulus on the nerve endings, when these are in connection
physiologically with the sensorium, and when all the entire
physiological system (of nerve endings, connecting machinery
of nerve fibres, and, it may be, intermediate nerve cells, and
central nerve cells) is in working order.

For example, we can only see an object, which has a real
existence apart from our fancy, and the existence of which can
be corroborated by our other senses, when it is illuminated and
casts an image on the retina at the back of the eyeball, thus
awakening, in some unknown manner, nerve impulses in the
fibres of the optic nerve, which travel finally to nerve cells in
the occipital cortex of the cerebrum and set these in activity.
Injury to any part of the physiological chain between retina
and cerebral nerve cell may cause blindness. For example,
the retina may be insensitive from some cause, and then no
object can be seen, although the rest of the visual machinery
may be quite perfect. Under such conditions, subjective sen-
sations may indeed be produced by stimulation of the trunk of
the optic nerve, but it cannot be stimulated by light, and no
object in the external world can be discerned. Again, the eye
may be uninjured, but the optic nerve or optic tract either cut
across, or for some reason inoperative as a conductor, and the
result is blindness. Finally, the cerebral area for vision (see
Fig. 112) may be removed or diseased, and again the effect is
the same.

Cutaneous Sensations.

The skin over the entire surface of the body is supplied
with special sensory nerve endings. These nerve endings have
many different forms in different regions of the skin, and also in
different animals. One of the commonest forms is the tactile
corpuscle (see Fig. no, p. 236) ; these occur in immense numbers
lying in rows immediately beneath the epidermis in the papillae

^ That is, a sensation caused by something with a real existence in the
external world.

250 Elementary Physiology.

of the skin of the hand and foot. The tactile corpuscles are so
called because they are supposed to be specially connected
with the sense of touch, and certainly they are found in greatest
abundance where the sense of touch is most acute. Still, there
are other cutaneous sensations besides tactile sensation, and it
is by no means clearly proved that the tactile corpuscles are
not connected with the appreciation of these other sensations
as much as with that of touch. Stimulation of the skin of any
area by appropriate means may give rise to sensations, either
of heat, of cold, of touch, or of pain, and there is a certain
amount of evidence that these different sensations are produced
by stimulation of different and specific nerve endings. For
example, if a pointed rod of iron, which is either considerably
hotter or considerably colder than the temperature of the body,
be made to touch successively various points on the skin, it will
be found that certain points called " heat points " exist, at
which the hot point is much more acutely appreciated than at
others; similarly there are other points at which the chilled
point gives rise to a much more acute sensation of cold, and
these " cold spots " do not coincide with the " heat spots."
Hence the more acute perception does not depend solely on
closeness to nerve endings, and there must be different nerve
endings, some sensitive to a low and some to a high tempera-
ture. The perception of difference in temperature by the
skin is often spoken of as the temperature sense. This by no
means corresponds in its delicacy at different parts with that
of the tactile sense, for tactile sensation is most acute at the
tip of the tongue or the tips of the fingers ; while the tempera-
ture sense is most acute upon the skin of the cheek, where the
tactile sensation is not nearly so delicately developed. It must
hence be admitted that the tactile sense and the temperature
sense are distinct from each other, and are probably furnished
by quite different sets of nerve endings and nerve fibres.

The delicacy of the sense of touch in different regions of
the skin is estimated by the smallest distance apart at which
two points are still felt as distinct. The testing is carried out
by a pair of compasses so blunted as not to prick the skin or
give rise to sensations of pain. It is found by this method that

The Senses. 251

the two points can still be appreciated as distinct when applied
to the tip of the tongue at a distance apart of about i milli-
metre ; at a less distance than this the points are no longer
felt as two, and the person experimented upon is unable to say
whether both points are applied to the skin or only one by
the person carrying out the test. The tips of the fingers are
next in order of delicacy j here the points can still be felt as
discrete when only 2 millimetres apart. Other parts are less
sensitive ; for example — lip, 9 millimetres ; front of forearm,
15 millimetres; forehead, 23 millimetres; back of hand, 30
millimetres; neck, back, arm, and thigh, 50-70 millimetres.

It is often erroneously stated that the sensation of touch is due to
pressure on the sensory nerve endings in the skin ; it would be much more
correct to say that tactile sensation is due to variation in pressure upon
these nerve endings. Constant localized pressure on the skin if excessive
may give rise to a sensation of pain, but when not excessive does not give
rise to tactile sensation if constantly applied. It is the sudden application
of pressure or removal of pressure which affects the tactile nerve endings,
and gives rise to the consciousness of something touching the part. This
may be shown by laying a light object over the finger, which is kept as still
as possible by resting it upon a table. After a short time the light
object is scarcely felt to touch the finger, being only appreciated by the
excessively slight involuntary movements and by the pulsations of the blood
in the finger, which cause slight variations in pressure. If now the table
be tapped, or the finger slightly moved, the object is felt much more dis-
tinctly. The same thing is experienced when the finger is dipped into a
vessel of mercury. Here a very distinct sensation of touch is experienced
where the surface of mercury touches in a ring around the finger, but little
tactile sensation is awakened at the tip of the finger which is deepest in the
mercury and has the most delicate development of the tactile sensation.
This shows that it is not the pressure which arouses tactile sensation, for
this is greatest at the finger-tip, but the rapid variation or oscillation in
pressure which is greatest at the surface of the mercury where slight move-
ments of the finger are continually alternately increasing and decreasing
the pressure.

The mtaneous sensation of pain is probably specially
localized in certain nerve endings and fibres like the sensa-
tions of heat and cold ; for by a somewhat similar method to
that which was employed for demonstrating " heat spots " and
" cold spots " (namely, gently pricking the skin with a sharp
point), it may be shown that certain localized points are much

252 Elementary Physiology.

more sensitive than others. At the same time, sensations of
pain can be elicited from any point on the skin if the prick be
severe enough, and also sensations of pain can be evoked by
stimulation of nerves which do not supply cutaneous areas, and
generally it seems to be the case that excessive stimulation of
sensory nerve fibres anywhere is capable of awakening unpleasant
sensation which is indefinitely and vaguely spoken of as pain.

There is no doubt that under the general title of pain
many sensations are included together which are really distinct
in character, and nearly any sensation which becomes suffi-
ciently unpleasant is termed pain. For example, the pain of
a headache is different in character from that of a toothache,
and both are different from that of a burn. So that pain is
not a specific sensation, but rather a term used to include
certain somewhat closely allied forms of unpleasant irritation
of the nervous system which usually arise from excessive stimu-
lation of sensory nerves.

Taste and Smell.
The end organs of taste are situated chiefly on the tongue
and soft palate. The terminal ramifications of the gustatory
nerves pass to small ovoid clumps of cells which are known as
'* taste buds." The taste buds (see Fig. 113) are best seen in
the depressions of the cirmmvallate papillce, which are elevations
arranged in a somewhat V-shaped manner at the base of the
tongue, but they are also to be found all over the tongue, and
upon the under surface of the soft palate, lying imbedded in the
stratified epithelium. There are two kinds of cells found in each
taste bud, viz. the gustafo7y cells, which are slender and fusiform,
with a prominent nucleus and a long process at each end ; and
the siLstenfaadar cells, which are elongated, flattened cells,
pointed at their ends. The sustentacular cells lie between,
and appear to support, the more delicate gustatory cells, and
also form a cortical envelope round the outer part of the taste
bulb. It is probable that the gustatory cells are those which
are affected by the dissolved ^ sapid substance, and that this

' Only substances in solution affect the taste organs ; thus, for example,
quinine in powder is insoluble on the tongue and has scarcely any taste,
although quinine in solution is intensely bitter.

TJie Senses.

253

alteration starts nerve impulses in the terminations of the
gustatory nerves which ramify round the gustatory cells. The
nerves of taste are the glosso-pharyngeal, which supplies the
posterior part of the tongue and the soft palate, and the Ungual
branch of the fifth cranial nerve and the chorda tympani,
which supply the anterior part of the tongue/ In some persons

'f^y^'7.' \W^^S'^'^^

Fig. 113. — Section through the middle of a taste bud. CRanvier.)

/, gustatory pore ; 5, gustatory cell ; r, sustentacular cell ; m, lymph cell, containing

fatty granules ; e, superficial ceils of the stratified epithelium ; «, nerve fibres.

no taste sensations whatever can be appreciated on the tip of
the tongue ; while other individuals can only taste sweet sub-
stances in this region. The distribution of the different kinds
of taste sensation is not uniform, and varies in different indi-
viduals. Most usually, sweet substances are tasted at the tip,
and bitter substances at the back of the tongue ; but some
persons have both sweet and bitter tastes at the tip as well as
at the base.

It is usual to state that there are four primitive types of taste sensation,
viz. sweet, bitter, salt, and sour, but it is doubtful whether all possible

^ It is probable that all gustatory fibres arise from the root of the fifth
nerve, and join the glosso-pharyngeal and chorda tympani afterwards.

254

Elementajy Physiology.

tastes can be referred to one of these four classes. Still, it is certain that
the many different fiavotcrs which we experience in the different foods we
eat are to a great extent due to a combination of olfactory sensations with
gustatory sensations. It is a matter of common experience that when the
olfactory mucous membrane is temporarily thrown out of working order by
a severe cold in the head or nasal catarrh that we not only lose the sense

of smell, but that the sense of taste also suffers,
and we can no longer properly taste our food.
This is due to the absence of the olfactory
stimuli which previously formed a complex
with the simpler sensations of taste, and gave
rise to the flavour ; in other words, we are
unable to enjoy the flavour of our food in such
a condition, because we cannot smell it as
well as taste it.

Sensations of smell arise from the
stimulation of certain cells lying in
the mucous membrane lining the upper
portion of the nasal passages. There
are two distinct kinds of epithelium
found in the mucous membrane of
the nasal passages ; that lining the
lower part {ScJmeiderian membrane) is
ciliated like the epithelium of the
trachea, while that of the upper part
{olfactory membrane) is chiefly colum-
nar and devoid of cilia. The air
passing in and out to the lungs through
resrion (M. Schuitze.) ^^ nasal passasfcs passes over the

(Highly magnified.) ^ ...

part lined with ciliated epithelium,
and this portion is not sensitive to
smell. It is only when the molecules
of the odorous substance are carried
upwards by diffusion or air currents to
the olfactory mucous membrane that
the sense of smell is awakened. This takes place much more
rapidly when inspiration is forced, and hence it is that we
sniff or take in the air in little rapid inspiratory gusts when
we attempt to detect a smell. The ultimate end organs of
smell are probably spindle-shaped or bi-polar ceUs which lie

Fig. 114

Cells and terminal
nerve fibres of the olfactory'-

from the frog ; 2, from man ;
a, epithelial cell, extending
deeply into a ramified pro-
cess ; b, olfactory cells ; c,
their peripheral rods ; e,
their extremities, seen in i
to be prolonged into fine
hairs ; d, their central fila-
ments.

The Senses. 255

interposed between the columnar cells of the olfactory mucous
membrane (see Fig. 114). These cells are termed olfadoiy
cells, while the columnar cells which appear to serve to support
them are called siistefitamlar cells. One process of the olfac-
tory cell projects towards the free surface, and in some classes of
animals ends in a number of minute hairs. The other process
is long and delicate, somewhat resembling a non-medullated
nerve fibre and becomes lost to view in a section of the
membrane among the plexus of olfactory nerve fibres, lying
immediately beneath the epithelium. The olfactory nerve fibres
are non-medullated, they unite to form from twenty to thirty
small nerves which pierce the cribriform plate of the ethmoid
bone, and so enter the cranium and pass to the olfactory
lobes.

Hearing.

The organ of hearing is usually described as consisting
of three parts, viz. the external ear, the middle ear or tym-
panum, and the internal ear or labyrinth. The external
and middle ear are accessory parts necessary for conveying
the vibrations of the air, or sound waves, in a modified form
to the internal ear in which are situated the terminations of the
auditory nerve in a complicated structure known as the 07'gan
of Corti.

The external eaj- includes the pinna, which projects from the head and
forms what is termed "the ear" in popular phraseology, and a passage (see
Fig. 115) which leads from this towards the middle ear called the external
auditory meatus. The pinna is composed of a shell of elastic cartilage
covered with skin, which becomes pendulous at the lower part in the lobnle,
and contains there a certain amount of fat. In certain of the lower animals,
the pinnae can be moved about by attached muscles, and besides collecting
the sound waves like an ear-trumpet, and thus making the hearing more
acute, serve to give information of the direction from which a sound is
coming by the direction of pointing in which it is most easily heard. But
in man the pinna is rudimentary, and is more ornamental than useful ; for
persons who have been deprived of their ears have almost normal hearing ;
and also information is obtained as to the directions of sounds not by
moving the ears, although vestiges of muscles still persist, but by moving
the head .

The external auditory meatus is closed at its internal end (see Figs. 115,

256

Elementary Physiology.

117) by the tympanic membrane, or drum of the ear, which is placed
obliquely across the meatus and forms part of the external wall of the cavity
of the tympanum.

The middle chamber of the ear, called the tympanum, is a small
irregular cavity in the substance of the temporal bone. It contains a chain
of three small bones called the auditory ossicles (see Figs. 116, 117), which

Fig. 115. — Diagrammatic view from before of the parts composing the organ of hearing of
the left side. (After Arnold.)

The temporal bone of the left side, with the accompanying soft parts, has been detached
from the head, and a section has "been carried obliquely through it so as to remove
the front of the meatus externus, half the tympanic membrane, and the upper and
anterior wall of the tympanum and Eustachian tube. The meatus internus has also
been opened, and the bony labyrinth exposed by the removal of the surrounding
parts of the petrous bone, i, the pinna and lobule; 2 to 2', meatus externus;
2', membrana tympani ; 3, cavity of^ the tympanum; above 3, the chain of small
bones ; 3', opening into the mastoid cells ; 4, Eustachian tube ; 5, meatus internus,
containing the facial (uppermost) and auditory nerves ; 6, placed on the vestibule of
the labyrinth above the fenestra ovalis ; a, apex of the petrous bone ; b, internal
carotid artery ; c, styloid process ; d, facial nerve issuing from the stylo-mastoid
foramen ; e, mastoid process ; /, squamous part of the bone.

serve to convey with diminished amplitude and somewhat modified the
vibrations of the tympanic membrane to the internal ear, where they
produce an effect on the endings of the auditory nerve. The auditory
ossicles are kept in position by certain slender ligaments attached to the
bony walls of the tympanum, and their tension and that of the tympanic
membrane is adjusted by certain tiny muscles. The most external of the
three ossicles, which is termed the malleus, from a supposed resemblance to

TJie Se?tses.

257

a hammer, is attached by its longer limb to the tympanic membrane in an
eccentric fashion so as to damp the vibrations which that membrane would
tend to make most easily, and so renders it equally responsive to notes of
all pitches. The intermediate ossicle, the incus, articulates by its shorter
limb with the shorter limb of the malleus, and the extremity of its longer limb
is attached by a ligament (in which a tiny ossicle is developed) to the head
of the stapes (or stirrup ossicle). The base of the stapes, which is oval and
surrounded by cartilage, is attached to a membrane closing an opening of
an oval shape situated on the internal wall of the tympanum, called the

Fig. 116. — View of the left membrana tympani and auditory ossicles from the inner side,
and somewhat from above (E. A. S ). f

7n, malleus ; i, incus ; st, stapes ; ^y, pyramid from which the tendon of the stapedius
muscle is seen emerging ; 1 1, tendon of the tensor tympani cut short near its
insertion ; I. a, anterior ligament of the malleus: the processus gracilis is concealed
by the lower fibres of this ligament ; l.s, superior ligament of the malleus ; /. i, liga-
ment of the incus ; ch, chorda tympani nerve passing across the outer wall of the
tympanum.

fenestra ovalis. The fenestra ovalis, as well as another opening on the
internal wall of the tympanum, also covered by membrane and called
the fenestra rotundis, communicates with the internal ear. The purpose
of these two communications will be pointed out later.

The cavity of the middle ear is filled with air, and is in communication
with the atmospheric air by means of a passage or tube called the
Eustachian tube, which opens into the pharynx (see 4, Fig. 115). The purpose
of the Eustachian tube is to keep the air at equal pressure on both sides of
the tympanic membrane, so that this may be free to vibrate, and not be
forced in or out by difference in air-pressure on its two surfaces. The tube

S

258

Elementary Physiology,

does not remain open always, but opens to adjust the pressure when
any variation takes place ; it opens for this reason during the act of
swallowing.

The internal ear or labyrinth lies in a cavity of complicated shape
hollowed out in the substance of the temporal bone, called the osseous

i.aru.m

au.m

Fig. 117. — Profile view of the left membrana tympani and auditory ossicles from before
and somewhat from above. Magnified four times- (E. A. S.)

The anterior half of the membrane has been cut away by an oblique slice, m, head of
the malleus ; sp, spur-like projection of the lower border of its articular surface ;
pr. br, its short process ; pr. gr, root of processus gracilis, cut ; s.l.fu, suspensory
ligament of the malleus ; l.e.tn, its external ligament ; t.t, tendon of the tensor
tympani, cut; i, incus, its long process; st, stapes in fenestra ovalis ; e.au.m,
external auditory meatus ; p-R. notch of Rivinus ; m.t, membrana tympani ; u, its
most depressed point or umbo ; d, declivity at the extremity of the external meatus ;
i.au.vt, internal auditory meatus ; a and b^ its upper and lower divisions for the
corresponding parts of the auditory nerve ; 7t.i>, canal for the nerve to the ampulla
of the posterior semicircular canal; s.s.c, ampullary end of the superior canal ;
/, ampullary opening of the posterior cannl ; c, common aperture of the superior and
posterior canals ; e.s.c, ampullary, and e'.s.c, non-ampullary end of the external
canal ; s.t.c, scala tympani cochleae ; y.r, fenestra rotunda, closed by its membrane ;
a. F, aqueduct of Fallopius.

labyrinth (see Fig. 1 1 9). Within the bony labyrinth lies a membranous
tube of corresponding shape called the membranous labyrinth (see Fig. 118),
which does not entirely fill the cavity in the bone, but leaves a space which
is filled with a fluid called the perilymph.

The mernbranous labyrinth is likewise filled with fluid, which is termed
the endolymph. The parts of the membranous labyrinth (see Fig. 118) are

The Senses.

259

the utricle, the three semicircular canals^ the saccule, and the cochlea ; and
those of the osseous labyrinth are termed the vesiibule, the semicircular
canals, and the cochlea, of which the vestibule accommodates the saccule
and utricle, and the semicircular canals and cochlea the correspondingly
named membranous parts.

The vestibule is the central chamber of the labyrinth, and communicates
in front with the cochlea and behind with the semicircular canals. Its
outer wall, that next the tympanum, contains the fenestra ovalis and
rotundis mentioned above.

The osseous cochlea is a gradually tapering tube wound in a spiral of

\jp.s.o.

Fig. 118. — Plan of the right membranous
labyrinth viewed from the mesial aspect.

utricle, with its macula and the three
semicircular canals with their ampullae ;
J., saccule ; ag.v., aquseductus vestibuli ;
s.e-., saccus endolj'mphaticus ; c.r.,
canalis reuniens ; c.c, canal of the
cochlea.

Fig. 119. — View of the interior of the left
osseous labyrinth.
The bony wall of the labj-rinth is removed
superiorly and externally. i, fovea
hemi-elliptica ; 2, fovea hemlsphasrica ;
3, common opening of the superior and
posterior semicircular canals ; 4, open-
ing of the aqueduct of the vestibule ;
5, the superior, 6, the posterior, and,
7, the external semicircular canals ; 8,
spiral tube of the cochlea ; 9, scala
tympani ; 10, scala vestibuli.

two turns and three-quarters rovmd a slender central pillar of bone, called
the modiolus, from which a spirally wound lamina projects inwards, dividing
the tube partially into two compartments. The membranous labyrinth lies
within this spiral cavity, and between the membrane and the bony wall a
space exists filled with perilymph. The membranous cochlea is divided into
three distinct tubes (see Fig. 120) by two membranes. One of these mem-
branes (the basilar membrane) stretches from the spiral lamina of bone
mentioned above to the opposite wall of the bony cochlea, thus forming
two divisions in the tube, called the scala tympani (beneath) and scala
vestibuli (above). The second membrane {Reissner's mcfnbranc) meets the
basilar membrane at an angle, and shuts off a small spiral chamber from the
scala vestibuli, which is called the canal of the cochlea (see Figs. 120, 121).
It is within this canal of the cochlea that the organ of Corti is placed,
resting upon the basilar membrane (see Fig. 121). The terminations of the

26o

Elementary Physiology.

Fig. I20.— Vertical section of the cochlea of a calf. CKolliker.")

Fig. 121.— Vertical section of the first turn of the human cochlea. (G. Retzius.)
s.v. scala vestibuli ; s.t., scala tympani ; D.C, canal of the cochlea; sp.l, spiral
'lamina ; n, nerve fibres ; l.sp., spiral ligament ; .y^r.t;. stria vascularis ; ssp spiral
eroove • R. section of Reissner's membrane ; I, limbus laminae spiralis M.t., mem-
brana tectoria ; t.C, tunnel of Corti ; b.m., basilar membrane ; h.i., h.e., mternal
and external hair cells.

The Senses. 261

nerve fibres of the auditory nerve lie within this organ of Corti. The fibres
course out in a radiating manner from the central modiolus to be dis-
tributed to the organ of Corti all along the length of the cochlea.

A complete description of the organ of Corti, which is very complex in
its structure, cannot here be given ; sufiice it to say, that it consists of modified
epithelial cells arraiiged in rows (see Fig. 121), and having an appearance
of the general character of those found at the peripheral distribution of the
nerves of special sense. ^ Certain of these cells are furnished with hair-like
processes (auditory hairs), which project into the endolymph bathing the
cells, and hence are brushed upon and easily affected by any movement of
that fluid. The hair cells are placed in two columns, respectively internal
and external, to two rows of cells called the rods of Corti, which incline
towards each other and form the structure known as the tunnel of Corti.
There is only one row of internal hair cells, but three or four rows are
present in the external column. Over the organ of Corti there lies a pad of
soft fibrillar tissue, which is called the me?Jibrana tectoria ; during life this
probably lies upon the hair cells and forms a kind of damper to prevent
excessive disturbance of the auditory hairs.

We are now in a position to follow out the chain of changes
which take place between the arrival of a sound wave at the
membrana tympani and the arrival of the modified disturbance
which it gives rise to at the organ of Corti.

A sound is due to disturbance propagated through the air in the form of
waves of rarefaction and compression. As the waves of rarefaction and
compression reach the membrana tympani this is alternately moved out
and in by them at the same rate. Thus the membrane is set vibrating at
the same rate as that at which the sound waves are produced. For a low-
pitched note the sound waves are produced more slowly, there is a smaller
number of vibrations per second, and for a high-pitched note the vibra-
tions are more rapid. These variations in rate are faithfully copied by
the movements of the membrana tympani. Also, the louder the note, the
more excessive are the variations of rarefaction and compression, and so
correspondingly more extensive are the movements of the membrane. The
movements of the membrana tympani are communicated to the chain of
ossicles in the middle ear, which move in unison with it. This movement
of the chain of ossicles is not a vibration propagated through the substance
of the ossicles, but a movement of each ossicle as a whole, the three bones
forming a system of levers. The length of the arms of the levers is so
adjusted that the extent of movement is diminished about one-third, so that
the excursion of the stapes is only about two-thirds of that of the membrana
tympani. There is also an arrangement at the articulation between malleus

^ Compare the olfactory and gustatory cells already described, and also
the rods and cones of the retina, with the hair cells of Corti's organ.

262 Elementary Physiology.

and incus, which prevents any excessive; movement of the membrane, such
as would be caused by the sound of an explosion, being communicated to
the inner ear and damaging the delicate structures there. ^

The movements of the stapes are communicated to the membrane
covering the fenestra ovalis at its base. It will be remembered that the
internal ear is completely filled with fluid, which is enclosed in a bony wall,
except at two places, viz. the fenestra ovalis and the fenestra rotundis, where
the bone is absent and the perilymph is separated from the tympanic cavity
only by membrane. Now, when the fenestra ovalis membrane is pushed in
by an inward movement of the stapes, the fenestra rotundis must be bulged
out towards the tympanum, because the fluid in the internal ear is incom-
pressible, and, Tjice versa, when the membrane covering the fenestra ovalis is
pulled outward by the stapes the membrane covering the fenestra rotundis
must move inwards. It is thus easy to see why there must be two movable
partitions between tympanum and labyrinth, for otherwise the vibration of
membrana tympani and ossicles could not be communicated to the incom-
pressible fluids of the labyrinth.

As the membranes covering the fenestra ovalis and fenestra rotundis thus
move inward and outward in time to the swingings of the membrana
tympani and ossicles, there is a surging in equal rhythm caused of the
perilymph. This vibratory swinging of the perilymph is in turn com-
municated to the endolymph, and passes round the chambers of the cochlea,
up the scala vestibuli, which lies nearer the fenestra ovalis, to the scala
tampani which communicates at the head of the spiral with the scala
vestibuli, and down this towards the fenestra rotundis. In its passage,
along the cochlear chambers, this fluid vibration affects the fluid in the canal
of the cochlea, disturbs the auditory hairs, and institutes changes in the
cells of the organ of Corti, which are translated somehow into nerve
impulses, and these, finally, travel up to the auditory centres in the brain and
there awaken auditory sensations.

Deafness may arise from an incompetence at any part of this
complicated system. It may be due to blocking of the external
auditory meatus (for example, by impacted wax), preventing
vibration of the tympanic membrane ; it may be due to serious
injury of the membrane itself, although a slight perforation is
often present in persons with normal hearing ; it may be due to
permanent closure of the Eustachian tube, in which case there
is no longer equal pressure on the two sides of the membrane,
the middle ear becomes charged with exuded fluid, and vibra-
tion of the membrane and ossicles becomes impossible ; it may

\ For a complete account of the articulations and movements of the
auditory ossicles, see Quain's "Anatomy," vol. iii. pt. iii. p, 95.

The Senses. 263

be due, though this form is rare, to disease in the auditory
ossicles; it may be due to defects in the internal ear; and,
finally, it may arise from disease, injury, or congenital defect in
the cerebral cortex (see Fig. 112).

The semicircular amals (see Figs. 118, 119), although they
are anatomically so closely associated in the internal ear with the
auditory apparatus, have probably no connection with hearing,
but are concerned with the sense of equilibrium and with the
position (or rather changes in position) of the head in space.
The canals are elliptical in shape, each forming about two-
thirds of an ellipse, and the osseous canals open into the
vestibule hy Jive openings, for two join together at one end.
The membranous canals are much more slender than the bony
canals within which they lie, and the space between is filled
with perilymph. Each membranous semicircular canal has a
swelling near one end termed an ampulla, and on the inner
surface of this there is a ridge called the crista acustica, upon
which an epithelium containing cells with hair-like processes is
placed. This epithelium is provided with nerve fibres derived
from a portion of the auditory nerve, termed the vestibular
nerve. Any motion of the fluid (endolymph) within the canal
affects these hair-like processes, and gives rise to nerve impulses.
The three canals are set in three planes at right angles to one
another, and hence motion of the head cannot take place in any
direction whatever without causing a movement of the endo-
lymph certainly in one, and usually in two, of the three canals.
The movements of the head thus give rise to sensations which
supply a means of judging of the movements and aid in main-
taining equilibrium. Disease of the canals, or experimental
injury to them, causes injury to the sense of equilibrium, failure
in balancing the body, and movements of rotation of the head
corresponding to that canal which is injured.

Sight. ^
In treating of vision it will be well in the first place to
consider the structure of the eye ; next, its action as an optical

^ The student is strongly recommended to accompany this description
by a dissection of an ox or pig's eye.

264

Elementary Physiology.

instniment ; and, lastly, the nature of the peripheral nervous
structures which are affected by the light. The eyeball lies in
the forepart of the orbital cavity, where it is supported upon
a soft circular cushion or pad of fat, and is surrounded by six
muscles, by means of which it can be turned to a certain
extent in various directions.

Fig. 122. — Section through the orbit and its contents.
a, frontal bone ; b, superior maxillary bone ; r, eyebrow ; d, eyelids ; e, conjunctiva ;
f, the muscle which raises the upper lid ; ^and g , recti muscles ; h, inferior oblique
muscle cut across ; i, optic nerve; 2, cornea ; 2', sclerotic ; 3, aqueous chamber;
4, crystalline lens ; 5, vitreous chamber.

Four of these muscles run straight forward from the posterior part of
the orbit to be inserted to the eyeball in front. They are hence called
the recti muscles, and according to position are known as the superior,
inferior, external, and internal rectus, respectively. Their action corre-
sponds to these names ; thus, the superior rectus raises, while the inferior
depresses, the front part of the eyeball ; the external rectus turns it outwards
in front, and the internal turns it inwards.

The other two muscles are called the superior and inferior oblique, from
their mode of attachment. The superior oblique muscle is attached to the
posterior part of the orbit on the inner side, and passing forward ends in
a tendon, which loops round a kind of pulley of fibres attached to the
frontal bone, and then passes backward and outward to become fixed to
the eyeball at its outer and back portion. When this muscle contracts, it
turns the front of the eyeball obliquely outward and downward. The

The Senses. 265

inferior oblique muscle arises from the lower and front part of the orbit
on the nasal side, and passes obliquely backwards and outwards to be
inserted on the posterior and outer part of the eyeball ; by its contraction,
the front of the eyeball is moved upward and outward. The muscles of
the two eyeballs work together in most perfect co-ordination, so that both
eyes are kept trained upon the same object at the same time, and by no
effort of the will can one eyeball be moved without a corresponding move-
ment of the other. This is necessary in order that images of objects may
be cast upon corresponding points of the sensitive screens at the backs of
the two eyeballs ; otherwise, double vision is the result. This may be
shown by artificially changing the direction of one eyeball by pressure upon
it with the forefinger, when objects are at once seen double. In this
complex co-ordination of the oculi-motor muscles, anatomically correspond-
ing muscles in the two eyes do not always work together ; for example,
when the eyes are moved from side to side the external rectus of one eye
contracts at the same time as the internal rectus of the other. In this
manner the internal rectus of one eye and the external of the other are, so
to speak, yoked together ; on the other hand, the two superior recti muscles
work together, and so do the two inferior recti muscles.

^Vhen the external surface of an eyeball is. examined it is
seen to be composed of segments of two spheres.^ One of
these spherical segments forms the middle portion of the front
of the eyeball. It is by far the smaller of the two segments,
forming only about one-fifth of the antero-posterior circum-
ference of the eyeball. The smaller segment is called the
cornea^ and the larger segment the sclerotic. The cornea is
transparent, and allows light to enter the eye ; while the
sclerotic is opaque, and allows no passage to the light.

The external surface of the cornea and the anterior portion of the
selerotic are covered by a delicate mucous membrane called the conjunctiva,
which is also reflected all round over the internal surfaces of the eyelids.
The conjunctiva is plentifully supplied with sensory nerve endings which
make it very sensitive to any irritant, and is kept moist by the secretion of
a special gland lying in the orbit called the lacrymal gland. When for any
reason the secretion of this gland becomes too copious, the space between
the eyeball and eyelids fills up and overflows, so giving rise to tears.

The cornea, to superficial examination, appears to be of
different colours in different persons, but it is really quite
transparent and colourless, as may be seen in dissecting an eye

' This may also be appreciated by closing the eye, placing a forefinger
over the upper eyelid and moving it about, when the prominence of the
anterior segment can be distinctly felt.

266 Elementary Physiology.

or by looking at the cornea of another person's eye from the
the side. The colour is due to a pigmented screen, lying
inside the eyeball behind the cornea, called the iris. In the
middle of the iris there is a circular opening, through which
the light enters the eye, called the pupil. No matter what be
the colour of the eye, this central opening is always black
(except in albinos),^ because all the posterior part of the
internal surface of the eyeball is lined by a black pigmented
coat called the choroid coat, which reflects back no light, and
so gives a black colour to the pupil.

At the back of the eyeball, a little to the nasal side of the
pole of the posterior hemisphere, a thick nerve trunk, that of
the optic nerve, pierces the sclerotic coat and enters the eyeball.
Its branches and endings are spread out over the posterior inner
surface of the eyeball underlying the innermost of the coats of
the eyeball, which is known as the retina and contains the
elements which are sensitive to the light. The blood-vessels
for the supply of the eyeball enter it along with the optic, nerve,
and ramify in the choroid coat. The structures lying within
the eyeball may be seen by making a section from front to
back with a sharp razor, and by dividing the sclerotic with sharp
scissors circularly, parallel to, and somewhat behind the margin
of the cornea.^ The structures displayed in an antero-posterior
section are figured in the accompanying illustration (Fig. 123).
Behind the cornea lies the anterior cha??iber of the eye, which is
filled with a thin watery fluid called the aqueous humour.

The anterior chamber is bounded posteriorly by the
crystalliiie lens, and by the suspensory ligament which attaches
the lens all round to processes (ciliary processes) which arise
from the anterior margin of the choroid coat.

The crystalline lens (see Fig. 123) is a solid body composed
of perfectly transparent fibres. It is a convex lens of which
the convexity, and hence the focal length, can be altered, and
its purpose is to focus images, on the posterior inner surface
of the eyeball, of objects situated at variable distances in the

^ In albinos this black pigment is absent from the choroid coat, and
hence the pupil looks pink, because of the pink colour of the light reflected
back through the pupil from the blood-vessels lying in the choroid,

^ See Appendix.

The Senses.

267

field of view according as the attention is directed to these
various objects.

Behind the lens lies the posterior chamber of the eye, which

Fig. 123. — View of the human ej'e, divided horizontally through the middle.
I, conjunctiva; 2, cornea; 3, sclerotic; 4, sheath of the optic nerve; 5, choroid;
6, ciliary processes ; 7, iris ; 8, pupil ; 9, retina ; 10, anterior limit of the retina ;
II, crj'stalline lens ; 12, suspensory ligament ; 13, ciliary muscle ; 14, anterior
chamber; 15, posterior chamber ; i6, yellow spot ; 17, blind spot.

is completely filled by a clear jelly-like mass called the vitreous
humour.

The iris lies in the anterior chamber in front of the lens,
and its variable central apertm-e (the pupil) lies opposite the
central portion of the lens.

There is a circular muscle within the eyeball called the

268 Elementary Physiology.

ciliary muscle lying at the junction between cornea and
sclerotic. Some of the fibres of this muscle are disposed
circularly, running parallel to the junction of cornea and
sclerotic ; others run longitudinally, and are attached in front
to the corneo-sclerotic junction and behind to the anterior
margin of the choroid coat. When the longitudinal fibres
contract they pull forward the anterior margin of the choroid
coat, and so slacken the suspensory ligament of the lens
which then, by virtue of its elasticity, takes a more convex
form. Since the posterior surface of the lens must conform
to the shape of the vitreous humour behind it, the alteration
in shape takes place chiefly at the anterior surface. The
purpose of this alteration in shape of the lens brought about
by the action of the ciliary muscle is to focus objects upon
the retina. When a near object is examined the lens is made
more convex; when a distant object is looked at it becomes
flatter.^ The circular fibres of the ciliary muscle aid the
longitudinal in pulling the choroid forward.

In the region of the eyeball behind the ciliary muscle and
external to the viteous humour there are three coats in the
wall of the eyeball. Of these, the inner is termed the retina,
it contains the elements which are sensitive to the light; the
middle coat is called the choroid, it contains cells which form
black pigment, and in it run the blood-vessels of the eyeball ;
the external coat is the sclerotic, which is composed of tough
white fibrous tissue.

The eye as an optical instrument bears a certain resem-
blance to a photographic camera. In both there is an inverted
image formed of objects in the external world upon a sensitive
screen placed at the back ; but in the eye the image gives rise
to nerve impulses which affect the consciousness, while in the
photographic camera the effect produced by the light rays is
made obvious by future chemical manipulation. In both

* In the case of an ordinary convex lens of glass, when the object is
brought nearer, the image is formed farther off on the opposite side of the
lens. But this would not be suitable in the eye, in which the screen or retina
is placed at a constant distance ; hence the lens is made accommodati?ig,
becoming more convex when the object is nearer, and so keeping it focussed
upon the retina by bringing the rays to a focus in a shorter distance.

The Senses, 269

there is an internal black lining, the purpose of which is to
prevent blurring of the image by internal reflection of the light.
In both the images are focussed upon the screen at the
back; but in the eye this focussing or accominodatioji is effected
by a change in the focal length of the lens, while in the camera
it is effected by changing the distance of the lens from the
screen. Further, in the eye the whole chamber, by virtue of
its shape, acts as a converging lens in focussing objects upon
the retina, and the lens itself is chiefly of use as an adjusting
mechanism — a kind of fine adjustment. Thus, there is con-
vergence caused by the convex anterior surface of the cornea,
and convergence by the refracting convex mass of the vitreous
humour. In the photographer's camera, on the other hand,
the chamber contains only air, and all the focussing is done
by means of the lens. In both the amount of light which
is allowed to enter is regulated by stops or diaphragms^
according to the intensity of illumination of the objects of
which images are to be cast upon the sensitive screens.

In the eye the adaptation of the size of the pupil, which forms the
diaphragm of the eye, does not take place instantaneously, but follows some-
what slowly by reflex nervous action when the intensity of illumination of
the retina is varied. It is for this reason that we are unable to see for a
few moments when we suddenly leave a brilliantly illuminated room and
pass out into a darkly lit space, until the pupil becomes widened to suit the
dimmer light. Conversely, when we enter from comparative darkness to a
place where there is a bright light we are at first dazzled by the excessive
amount of light entering the eye, until the pupil contracts and relieves the
sensitive retina. These changes are occasioned by two sets of muscle
fibres in the iris ; one set being arranged circularly around the pupil and
the other disposed radially.

The retina or sensitive screen upon which inverted images
of objects are focussed by the optical action of the refractive
substances of the eyeball consists of eight different layers
which are diagrammatically shown in the accompanying figure
(Fig. 124).

Of these layers that called the layer of rods and cones is
the one which is primarily affected by the light. ^ It lies

^ This is shown by the fact that this is the only layer present in the
fovea centralis {vide infra), which is the most sensitive part of the retina.

2/0

Elementary Physiology.

farthest removed from the entering Hght next the choroid coat,
and is bound by certain cells containing pigment called the
pigment cells.

The retina is not equally sensitive in all parts. Where the optic

Fig. 124. — Diagrammatic section of the human retina. (Schuhze.)

nerve enters, a little to the nasal side of the pole of the posterior
hemisphere, the retinal elements are absent, and the branches
of the optic nerve spread out in all directions to form a net-
work. This region is called the " blind spot^' and is quite

TJie Senses.

2/1

insensitive ; so that when the image of an object falls upon it
no visual sensation is produced. About a tenth of an inch to
the temporal side of this there is a small yellow spot called the
macula lutea^ of slightly elliptical shape (see Fig. 125), which

Fig. 125. — View of the posterior part of the retina.

A. — The posterior half of the retina of the left ej-e, viewed from before. (Henle.)

Twice the natural size.

s, cut edge of the sclerotic ; ch, choroid ; r, retina : in the interior at the middle the
macula luteawith the depression of the fovea centralis is represented by a slight oval
shade ; towards the left side the light spot indicates the colliculus or eminence at
the entrance of the optic nerve, from the centre of which the arteria centralis is seen
sending its branches into the retina, leaving the pait occupied by the macula com-
paratively free.

has in its central part a slight depression called the fovea
centralis. The fovea centralis lies at the opposite pole of the
eyeball from the pupil, and is the part of the retina upon
which vision is most acute. When the attention is directed
to an object, the eyeballs are so turned that the image of it is
focussed upon the fovea centralis and the parts of the macula
lutea adjoining. Objects, of which the images are cast upon
other parts of the retina, are only seen very indistinctly, and
are more indistinct as their images fall more distant from the
fovea centralis.

The existence of the blind spot may be easily demon-
strated by making two round dots on a piece of white paper,
each about one-tenth of an inch in diameter, and about two

2/2 Elementary Physiology.

inches apart, and then observing these two spots with each eye
in succession, the other eye being closed in each case.

While observing with the right eye look steadily at the left-
hand dot and move the sheet of paper nearer and farther
alternately from the open eye. When the paper is near the
eye both dots are easily visible, but at a certain distance the
right-hand dot completely disappears and does not re-appear
until the paper has been moved considerably farther from the
eye. A similar result cannot be obtained in the case of the
left-hand dot by looking at the right-hand dot with the right
eye. A disappearance of the left dot can similarly be obtained
in the left eye when the right dot is focussed upon the fovea,
but not of the right dot when the left dot is so focussed.
Remembering that the light rays cross in the eye before
reaching the retina, and hence that the position of images is
inverted, a little consideration will show that these experi-
ments demonstrate that the blind spot lies to the nasal side of
the fovea centralis.

The visual sensation outlasts in time the stimulation of the
retina by the light. The sensation does not quite disappear
until the lapse of about one-tenth of a second after the stimu-
lation. Hence, when a light flashes more than ten times in a
second, the flashes fuse into one another, and the sensation pro-
duced is a continuous light, but unsteady and flickering. If the
flashes succeed one another at a rate of fifty to sixty per second
the flickering sensation disappears and the flashes are fused
into a steady light. This continuance of sensation after stimu-
lation is the explanation of such famihar phenomena as the
circle of fire caused by rotating a stick with a glowing end,
the fusion of colours in the rotating colour top, and the fusion
of the spokes of a rapidly rotating wheel.

We can distinguish many hues and shades of colour in the
objects which we see, but all these varied colours only awaken
three kinds of sensation.^ Coloured light of any given wave-

* Rival theories have been advanced to explain the phenomena of colour
vision ; these theories are both exceedingly artificial and insufficient, and so
an attempt has been made below to give an outline of a few of the prmcipal
facts.

The Senses. 273

length arouses all of these three sensations, but in very varying
degree, and the sensation produced as a resultant varies with
the comparative intensity of the three component sensations.
The three primary sensations are each most strongly produced
by red, green, and violet light respectively, and these are
hence spoken of as primary colours. However, red light only
awakens the red sensation in a preponderating degree ; the
other sensations are also produced, only much more feebly.
So any tint is produced by certain fixed degrees of intensity of
simultaneous stimulation of the three kinds, and any variation
in the relative strengths of the three component stimuli will
cause a corresponding variation in the tint. This can be
experimentally shown by fusing together, so that they affect the
same portion of the retina, coloured lights in varying intensity,
as, for example, by casting them on the same surface, or by
spinning them on a colour top.

The sensation produced by light of any given physical
composition varies greatly with the condition at the time being
of the visual apparatus ; not only so, but the condition of the
part of the retina immediately around the area stimulated has a
profound effect on the sensation evoked.

The visual apparatus is easily fatigued, and while in a
fatigued condition it reacts less easily than when in a fresh
condition; again, it can be fatigued for one colom', and yet
react normally to a colour for which it has not been fatigued.

If a bright white spot be looked upon for 10 to 20 seconds,
and then the eyes be turned to a dull uniform grey background,
or be closed, an image of the spot in black or duller colour,
which is called a negative after-image^ is seen on a brighter back-
ground. If a coloured spot be employed in the first case, an after-
image of the complementary colour is seen — that is, an image
which if used with the original would produce white as a result.
Here the colour of light is changed by previous actions carried
out upon the part of the visual apparatus causing the sensa-
tion— in other words, the sensation produced is shown by the
experiment to vary altogether with the condition at the moment
of the visual mechanism stimulated. Again, if a colour be
viewed upon a background of its complementary colour, it

2/4 Elementary Physiology.

appears much brighter from what is called simultafieoiLS contrast^
showing that the sensation is influenced by the condition of
neighbouring parts of the visual apparatus.

The result of simultaneous contrast is beautifully shown by
the experiment of Mayer, which consists in covering with tissue-
paper a small piece of grey paper placed upon a coloured paper
surface; the grey paper then appears coloured with a tint
complementary to that of the colour upon which it is placed.

In some persons, who are said to be colour blind ^ the per-
ception of colours is imperfect. Such persons usually have
but two colour sensations, and derive all their sensations of tint
from fusion of these two. In the commonest form of colour
blindness, red and green appear alike, and a red cherry is of
the same colour to such people as the leaves of the tree on
which it grows ; in other persons, the middle of the spectrum
is of a neutral tint, and red and blue gradually fuse into each
other. Unfortunately, such persons are always born colour
blind, and so we cannot compare sensations with them, or
else important facts concerning colour sensation might be
discovered.

^ Successive contrast is when a colour is viewed after its complementary
colour ; here again the colour is brighter, showing in a converse manner the
same truth as after-images, that the effect depends upon the immediately
previous history of the part stimulated : this is a contrast in time, while
" smiultanenus contrast" is a contrast in space.

APPENDIX OF PRACTICAL EXERCISES

I . Make dissections of the body of any small mammal, such as a rabbity
guinea-pig, white rat, or kitten.^

Fasten the animal upon its back, make an incision through the skin in
the mid-line in front all the way down, and draw the skin aside. Cut
through the abdominal wall in the mid-line and make a cross cut above,
parallel to the ribs on each side.

Examine the abdominal viscera, and observe their position. If the
animal has just been killed, note the peristaltic movements of the intestine,
and start a contraction at any point by touching with the point of a knife.
Observe the long pause (latent period) before the contraction commences.
Examine the way in which the intestine is attached to the abdominal wall
by the mesentery, and, if the animal has recently been fed on a fatty meal,
look for the lacteals in it, filled with a milky fluid (chyle). Note the posi-
tion of the various viscera as described in the text (pp. 87-95). Look for
the pancreas lying in the loop of the duodenum. In a herbivorous animal
this gland is diffuse and inconspicuous, but it is easily found in the carni-
vora. Pull the stomach to the right, and observe on the left side the spleen
and its attachments. Look also for the opening of the small into the large
intestine, and observe the caecum and vermiform appendix, which are large
in herbivora, but small in carnivora.

Next tie two ligatures around the oesophagus, a short distance apart,
and cutting between the two ligatures, and then through the mesentery, re-
move the intestine down to the rectum, where two other ligatures are to be
tied and cut between. Cut open the small intestine, and observe the velvety
appearance of the inner surface caused by the projecting villi ; look at it with
a magnifying glass after washing it with water. Similarly examine the
inner surfaces of the stomach and large intestine, and note that there are no
villi on these surfaces. Note, in the case of the stomach, the thick glandular
mucous membrane, which is often thrown into ruga:^ or folds.

After the removal of the stomach and intestine, observe the position of
the kidneys, and the small suprarenal bodies lying above them, and of the

^ The animal may be most painlessly killed by placing it under a glass
bell jar or other cover with a piece of cotton wool soaked in chloroform.
All the dissections described below can scarcely be made at one time, but
can be made at intervals as opportunity offers.

276 Elementary Physiology.

liver. Remove the liver and the kidneys, and examine them. Note the
lobes of the liver and the position of the gall-bladder. Observe the impres-
sions on their surfaces, which correspond to the adjacent viscera. Observe
how the vessels enter the gland, cut into it and examine the substance of
it. Slice open one of the kidneys longitudinally, compare it with Fig. 98,
p. 209, and read the description of its naked-eye appearance there given.
Similarly cut open one of the suprarenal glands, and the spleen.

Examine the abdominal cavity after the removal of these organs j note
the smooth lining, and search for the aorta and inferior vena cava, which
will be found, if they have not been accidentally removed in the dissection,
running down in front of and parallel to the vertebral column.

Next dissect the thorax, commencing by removing the front portion of
the thoracic wall. In a small animal, the ribs can be cut through on each
side with a strong pair of scissors, and the front parts of the ribs and the
sternum completely removed. When the thorax is opened the lungs
collapse and occupy a comparatively small volume ; it must be remembered
thai when the thorax is complete they are distended, and occupy the greater
part of the cavity.

Observe the position of the heart and the great vessels passing to and
from it at its base ; note the pericardium which encloses it, and then cut
through this and expose the heart. In an animal which has just been killed,
the heart may often be made to contract by pricking it with a sharp-pointed
instrument. Look at the roots of the lungs and the vessels and bronchi
constituting each. Feel the lung tissue, which gives a crepitant sensation
to the fingers. Dissect up into the neck so as to expose the trachea and
larynx. Find the oesophagus behind the trachea, and trace it through the
thorax to the point at which it passes through the diaphragm. Examine
the diaphragm lying between the thorax and abdomen, and note the shape
of the central tendon. Try to find the places where the inferior vena cava
and the aorta penetrate it, and trace these vessels up to the heart. Remove
the trachea and lungs, tie a tube into the trachea, blow air into it, and show
that the lungs become distended and greatly increased in volume.^ Re-
move the heart and the roots of the great vessels attached to it. Examine
its structure as far as is possible with a heart of such small size (see pp.
1 13-120),

Next remove the skin from the lower part of the animal and dissect one
leg. Turn the animal back upwards, and examine the structure and arrange-
ment of the muscles at the back of the thigh and leg. Separate the muscles
on the back of the leg and search for the sciatic nerve, which will be found
lying deep between two chief groups of muscles. It is in appearance a
strong, white, rounded, glistening cord. Lay it bare in its whole length ;
observe that it gives off branches to the muscles of the back of the leg,

^ This is the reverse mode of distension to that which takes place during
life, when the lungs are distended by diminution of pressure on their outer*
surkceSi

Appendix of Practical Exercises, 277

and near the knee divides into two chief branches, which go to supply
the parts below the knee. Dissect the nerve upwards towards the spinal
cord, and observe that it is derived from three or four nerves which unite
to form what is known as the sciatic plexus} Follow these nerves to the
spinal cord, and note that a nerve trunk is in each case formed by the
union of an anterior and a posterior root which arise separately from
the side of the spinal cord. Cut a piece out of the sciatic nerve, tease it
with needles, and note that it can be easily split up longitudinally, that is,
in the direction in which the fibres run in it. The same thing may next be
done with a piece of muscle.

Remove the skin from the back of the animal and from the back of the
head by making a long incision in the middle line, and turning the skin
outwards to each side. Clear the muscles away from the vertebral column
in the back and neck, and then with a pair of bone forceps snip away the
laminae of the vertebrae so as to expose the spinal cord throughout its length..^
Remove also the upper part of the cranium so as to expose the brain, and
then study the naked-eye structure of the cerebro-spinal axis. Note the
tough, membrane {dura mater) which covers the brain and spinal cord ; re-
move this and observe the convohitions and sulci ou the surface of the brain.
Note the blood-vessels which are carried by a thinner membrane coating
the brain, termed the pia mater. Remove the brain and cord, commencing
at the fore part of the brain. The cranial nerves must be cut through
in order to do this, and also the spinal nerves. Note the thick optic
nerves which meet to form the optic chiasfjia. Study the brain and
cord with the aid of Chapter XI., and the figures given there. Note the
manner in which the nerve roots arise from the brain and from the spinal
cord. Cut a slice from the cerebral cortex and observe the white medullary
centre underlying the grey cortex ; slice deeper and expose the cavity or
ventricle within the cerebrum ; cut into the basal ganglia seen on the floor
of this ventricle. Slice through the cerebellum at right angles to the
direction of the convolutions, and observe again the cortex outside and
medulla within, giving rise to the appearance known as the arbor vitce.

Observe also the pons Varolii connecting the two cerebellar hemispheres ;
the crura cerebri conveying the fibres in two great masses from the cerebrum
towards the cerebellum and cord, and the medulla oblongata or spinal bulb
connecting brain and cord. Cut through the spinal cord at different levels,
and note the grey matter in the centre surrounded by the white. Observe
also the increased area of the grey matter at the enlarged parts of the cord
opposite those portions where the nerves to the limbs are given off, because
here more nerve cells are required ; also the comparatively small grey area
in the dorsal region, where the nerves given off are much smaller. Notice

' Bone forceps or very strong scissors will be required in doing this to
snip through the bones which conceal these nerve roots.

^ This is a somewhat difficult dissection for a beginner, and requires the
exercise of a good deal of patience.

2/8 Elementary Physiology.

also that the shape of the entire section and the contour of the grey matter
vary in the different regions, thus making it easy to distinguish sections of
cord from cervical, dorsal, and lumbar regions respectively.

Finally, clear away the flesh from about one or two of the joints, examine
these, dislocate them and learn their construction.

2. Make a dissection of a frog. In a cold-blooded animal tissues re-
main alive for a longer time after the death of the animal than is the case
with mammals, and hence many important physiological facts can be ascer-
tained. A small depression may be seen behind the head, almost lying at
the apex of an imaginary equilateral triangle with the line joining the two
eyes as base. Insert a pin at this depression and pith the brain of the
animal and its spinal cord, so killing it by destroying the nervous system.

Pin the animal out on its back upon a cork board, and remove the skin
from the thorax and abdomen. Next remove the front of the thorax, taking
care not to go too deep, so as to avoid injuring the heart which lies under-
neath. The heart is then exposed, and is seen beating within the pericardium.
Carefully snip away the pericardium, and observe the beating of the heart ;
notice that the auricles beat first and then the ventricle, followed by a pause. ^
If the lung should happen to be distended, observe that it is of a much
simpler type than in mammals, being simply an air sac on the wall of which
blood-vessels ramify. Snip into the lung with scissors, and it at once collapses.
Remove the skin from the posterior part of the body and look for the sciatic
nerve at the back of the thigh. Expose the nerve for some distance, then
pinch it with forceps, and note that the muscles at the back of the leg
beneath the knee contract vigorously (mechanical stimulation) ; cut the
nerve through with scissors below the point previously pinched, and note
that the muscles again contract. Dissect out the corresponding nerve on
the other side, tie it tightly near the upper end (the muscles contract) ; now
cut it above the point at which it has been tied — no contraction takes place
because the physiological continuity of the nerve has been interrupted. Cut
the nerve below the point at which the ligature was tied, apply a crystal of
salt to the cut end, and note that the muscles commence contracting and go
on twitching, because the nerve is stimulated by the salt (chemical stimula-
tion). Cut the nerve beneath the place at which the salt has acted upon it, and
these twitchings cease. Hold a zinc and copper wire in contact with your
fingers, and place the two wires across the nerve at a short distance apart ;
each time the nerve is touched by the wires, the muscles which it supplies
contract (electrical stimulation).

3. Observe through the microscope the circulation of the blood in the
thin web of the frog's foot, or in the tail of a tadpole, minnow, or other
small fish. The foot should be tied or pinned so that the web is spread out
over a round hole in a piece of cork board or wood, and then the prepara-
tion should be brought under the low power of the microscope. In the case
of the tadpole, this may be wrapped round with wet filter-paper and then

' There is only one ventricle in the frog, instead of two as in mammals.

Appendix of Practical Exercises. I'jc)

placed upon a microscope slide so that the tail lies under a low-power
objective. Notice that the red blood corpuscles move rapidly down the
central part of the vessel observed, while the white corpuscles roll sluggishly
along the wall of the vessel. Bring a small artery into the field of view and
observe the pulsatile flow, the beats of which correspond to the heart-beats.

4. Listen to the heart-sounds in another person by placing your ear
opposite the proper part of the chest wall. At the same time, feel the
pulse and note that it is synchronous with the beat of the heart. Count the
pulse-beats per minute; ask the person to take a short run, then listen to
the heart, and feel and count the pulse once more. Also count the number
of respirations per minute, before and after a run, on another person who
is not aware of what you are doing, and note the increase in rate.

5. Perform the experiment upon a long vein of the arm described on
p. 112.

6. Place three fingers of one hand upon the radial artery at the wrist ;
the pulsations of the artery can be distinctly felt by all three fingers. Com-
press the artery strongly with the middle finger, and the pulse is now felt by
the finger towards the upper arm, but not by the finger next the wrist, thus
showing clearly that the pulse is propagated from the heart.

7. Obtain an uninjured sheep's heart from a butcher, and make the
following experiment to demonstrate the action of the semilunar valves.
Take two glass tubes, about half an inch in diameter and each about eighteen
inches long,^ and tie one securely into the aorta and the other into one of
the pulmonary veins, and afterwards tie the other pulmonary veins. Or,
one tube may be tied into the pulmonary artery and the other into one of
the vence cavce, the other vein being tied up as before. Now pour water
into the tube attached to the vein, both tubes being held vertically, and
alternately compress and relax the ventricles by squeezing with the hand.
The water falls in the tube which is tied in the vein, and rises in the tube
tied in the artery, being upheld during the intervals of relaxation by the
closed semilunar valves. This illustrates the action of these valves
during life.

8. Carry out with the same heart the dissections described on pp.
1 15-120.

9. Make, and examine, the histological preparations of blood corpuscles
described on p. 121.

10. Instruct a butcher to draw off a quantity of blood into a vessel when
killing an animal, and then later study the character of the contents of this
vessel. The clot is red outside, but black when cut into ; also, when the
cut surface is exposed for some time to the air it turns red (see p. 187).

11. Obtain also some whipped blood, and perform the experiments
indicated on p. 187.

' The tubes should be slightly pulled out in a blow-pipe flame close to
one end, so as to form a shoulder round which the ligature can be firmly
applied.

28o ■ Elementary Physiology.

12. Pour off some of the serum which has exuded from the clot obtained
in experiment lo, and dilute it with about three times its volume of water.
With the fluid so obtained perform the following tests, which are characteristic
tests for prpteids : —

{a) Add. a trace of acetic acid, and heat ; before boiling commences a
white coagulum is thrown down. This is what happens to the white of egg
when an &g^ is boiled, and is termed heat coagulation.

{b) Add a single drop of a dilute solution of copper sulphate, and after-
wards excess of caustic potash, and the solution will turn a violet colour
(Biuret test).

{c) Add strong nitric acid ; the solution turns yellow and gives a white
precipitate which turns yellow on boiling. Allow the solution to cool, and
then add excess of ammonia, when the solution will turn orange-coloured
(xantho-proteic test).

{d) Add excess of alcohol and a white precipitate will be thrown down.

13. If possible, carry out the separations described on p. 128.

14. Make some starch paste by powdering a small quantity of starch ^
and then boiling with water. If a drop of this paste be added to a drop of
a dilute solution of iodine, a deep blue colour is the result. Now add some
saliva from the mouth to a quantity of this starch paste, keep the mixture
warm, but not too hot,^ and test, by means of the iodine solution, drops
taken from it from time to time. It will be found that the blue coloration
after a time no longer appears, and finally, after a transitory stage in which
a red is obtained, no coloration whatever is produced by the iodine. This
experiment shows that the starch has been converted into something else by
the action of the saliva, and if some of the solution be now taken and tested
by adding a few drops of copper sulphate and excess of caustic potash it
will be found that the copper salt is reduced. In fact, as can be shown by
more elaborate experiments, a reducing sugar called maltose, mixed with
certain bodies intermediate in chemical nature between starches and sugars,
and called dextiins, has been produced by the action of the saliva on the
starch (see p. 146).

15. Feed an animal on food containing fat, such as fat meat, and kill it
after an interval of about five hours. Open the abdomen and observe the
milky lacteals in the mesentery. Open the thorax and search for the thoracic
duct, which is charged with milky fluid and can on this account easily be
found ; trace this vessel to its entrance into the junction of the subclavian
and jugular veins in the neck. Note how richly it is supplied with valves,
which are shown by the swellings upon it.

16. Feed a rabbit freely on rice or carrots for one or two days, and then
kill it four or five hours after a meal. Rapidly cut out the liver and throw
it in small pieces into a vessel containing boiling water just acidulated with

^ Half a teaspoonful will make a cupful of paste.

- The mixture should be kept at such a temperature that the finger can
be kept in it without discomfort, that is to say, at about body-temperature.

Appendix of Practical Exercises. 281

acetic acid. This process coagulates the proteid present in the liver. Take
the pieces of coagulated liver out of the boiling water after they have been
immersed for about a minute, and grind them up with some cold water in a
mortar. Filter through muslin, and a very opalescent, almost milky fluid
will be obtained which is rich in glycogen or animal starch. If the liver be
left for some time after death before extracting as described above, all the
glycogen becomes converted into grape sugar. A similar change can be
induced in the glycogen solution by heating it with acids or with saliva or
pancreatic extract. If dilute iodine solution be added to the glycogen solu-
tion a deep brown or port-wine colour is produced, which disappears on
heating and reappears on cooling. The glycogen may be precipitated from
solution by the addition of 60 per cent, of alcohol, and so obtained as a
white amorphous powder.

This experiment shows that the liver stores up excess of carbohydrate
in the form of a variety of starch.

17. Obtain some ox-bile from a butcher and perform the chemical tests
for bile salts and bile pigments described on pp. 163 and 164.

18. Fit a bottle with a cork through which two glass tubes, bent once
at right angles, pass, one tube reaching to the bottom of the bottle, and
the other only just passing through the cork. Fill the bottle nearly full of
lime-water, and place the cork in position, apply your mouth to the shorter
tube, and suck so that atmospheric air bubbles through the lime-water ; no
change takes place in the lime-water unless the process be continued for a
very long time. Now apply your mouth to the longer tube, and blow air
from your lungs through the lime-water. One or two breaths will prove
sufficient to give a white precipitate of calcium carbonate in the lime-water ;
this is produced by the action of the carbon dioxide liberated in the lungs.

19. Breathe against a clear cold mirror ; it becomes clouded by the
moisture deposited from the air coming from the lungs, which is saturated
M'ith water vapour at the temperature of the body, and hence causes a
deposit of water upon any cold surface with which it comes in contact.

20. Tie a glass tube, connected by indiarubber tubing to a U-shaped
tube containing a coloured fluid, into the trachea of a dead animal. Now
cut into the thorax and thus allow the full atmospheric pressure to act upon
the lungs ; these partially collapse, and the coloured fluid is driven up in
the distal limb of the U-tube. This shows that the lungs are held distended
by a partial vacuum existing within the thorax, and at the same time points
to how, when the volume of the thorax is enlarged during life that of the
lungs must also be enlarged on account of the pressure of the atmospheric
air from without acting down the trachea, bronchi, and bronchioles.

21. Perform the experiment on reflex activity indicated on p. 239.^

^ The frog's head should be removed with scissors, leaving the upper
end of the spinal cord exposed, and the remainder of the animal should be
vertically suspended by means of a bent pin used as a hook from some con-
venient support.

282 Elementary Physiology.

Two per cent, solution of acetic acid may be employed for the stimula-
tion.

By diluting the acid, find a strength such that the leg is not drawn up
out of a beaker containing the acid until 20 to 30 seconds have elapsed.
Wash the leg free again from acid by dipping it in water. Now apply a
pinch of common salt to the cut end of the spinal cord, and after a pause
of a few seconds, again test the time that elapses before the leg is with-
drawn from the same acid. It will usually be found that the interval is
enormously increased. This experiment illustrates inhibition ; that is, it
shows that reflexes can be stopped or delayed by controlling impulses
coming from the higher centres. For the salt stimulates nerve fibres in the
cord which in a normal condition of the animal would be connected with
nerve cells in the brain, and originates impulses down these fibres which
for a time stop the reflex act due to the irritation of the lower cells by the
acid.

22. Test with a blunt-pointed pair of compasses, the various distances
apart at which the two points can be appreciated as distinct, over different
regions of the skin (see p. 250).

23. Obtain two or three ox's or pig's eyes from a butcher and make
dissections of them.

Clear away from the eyeball all adhering fat and muscles, and note the
place where the optic nerve enters, and the external appearance of the eye-
ball. Insert one blade of a pair of scissors through the sclerotic coat
about a quarter of an inch behind the corneo-sclerotic junction, and cut
completely round parallel to this junction, and about a quarter of an inch
behind it,^ thus dividing the eyeball into an anterior and a posterior part.
When the two parts are pulled apart the vitreous humour is exposed, and
is seen to be a beautifully clear jelly-like mass. The vitreous humour
usually adheres to the front portion, but can easily be detached and
examined. After the vitreous humour has been removed examine the front
portion from the inner side.- Observe the crystalline lens lying in the
middle opposite to the pupil and its mode of attachment by the ciliary
processes all round the margin to the anterior portion of the choroid coat
which is seen surrounding it and forming a black internal coating to the
eyeball.^ The lens is enclosed in a capsule, which is very delicate and
easily ruptured. Remove the lens by pressing against it with the handle of
a scalpel at one side, or by passing a pin round the margin. Hold the lens
up to the light and notice how clear and transparent it is ; but on looking
through it, especially some time after the death of the animal, three radial
lines may be seen which meet at angles of 120° at the centre. These three

^ It is advantageous to do this in a vessel of water, the parts are then
better seen and less injured in the dissection.

" It may be removed from the water for this purpose.

^ Identify these various structures by the aid of the diagram given on
p. 267.

Appendix of Practical Exercises. 283

lines are due to the structural arrangements of the lens. By breaking up,
in a small quantity of water, with needles it may be seen that the lens is
made up of an immense number of transparent fibres running from front to
back.^ These fibres are arranged in three equal bundles, and the tri-radiate
arrangement described above is due to the junctions of these bundles.
Examine the anterior portion again after the removal of the lens. Observe
the ii'is, with its central aperture, \h(t picpil, for the admission of the light.
The pupil is lenticular in shape in the ox's or pig's eye, and not circular as
in the human eye. The back surface of the iris is always black, from the
continuation of the choroid containing black pigment over it, the front
surface is variously coloured in different individuals. Note how the outer
margin of the iris is attached all round at the junction of the cornea and
sclerotic coat. Rub away with a blunt knife the black pigment of the
choroid lying behind the junction of the cornea with the sclerotic, and you
will expose a circular ridge, lying all round the junction. This ridge is
formed by the fibres of the ciliary muscle. Make a cut forwards at one
point with a sharp knife, so as to pass at right angles through the corneo-
sclerotic junction. Some of the fibres of the ciliary muscle run circularly
parallel to the junction of cornea and sclerotic (circular fibres), another set
run at right angles to the junction, and are attached in front at the junction
and behind to the choroid coat. It is evident that when the fibres of the
ciliary muscle contract, the choroid will be pulled forward, and hence the
processes (ciliary processes), which were previously observed attacking
the lens all round to the choroid, will be slackened. The lens will, in
consequence, not be so much flattened against the vitreous humour by the
pull of these processes, and will become more rounded. This happens
when we look at a near object.

Examine next the posterior half. Observe the retina lying innermost of
the three coats ; ^ this coat is usually detached from the choroid except
where the optic nerve enters, and is seen floated up by the water in which
it is immersed as a yellow-coloured almost transparent film. Outside this
is seen the black choroid coat, and outermost of all there is the sclerotic
coat,

24. If an eye can be obtained in a fresh enough condition,' the follow-
ing experiment may be made. Cut a circular opening out of the back part
of the eye, including the optic nerve, and about one-third of the posterior
surface of the eyeball. Take care that the opening is not large enough to
allow the vitreous humour to escape. Now apply a piece of greased paper

^ These fibres can be better seen in the case of a lens which has been
hardened for some days in two per cent, solution of potassium chromate.

- In the eye of the ox there is a thin glistening coat, of a green colour,
\\hich is termed the tapehim, to the inside of the retina. The purpose of
this coat is unknown. It is not present in the human eye.

' The cornea rapidly loses its transparency after death, and hence the
above experiment is only successful with a fresh eye.

284 Elementary Physiology.

against the opening and turn the eyeball so that light from a window enters
it in front. An image of the window will be formed upon the paper
behind.

25. The shape of the blind spot may be roughly mapped out by the
following method. Make an ink spot upon a piece of white paper, close
the left eye, and place the right eye vertically opposite the ink spot and
about ten inches distant from it. Move the point of a pencil or pen to the
right, away from the ink spot, keeping the eye always direct upon the ink
spot, and not upon the pen point, until the point just ceases to be visible,
and mark this spot ; continue moving the pen point still further to the right
until it again becomes visible, and mark this point also. This gives the
horizontal diameter of the blind spot, and the colour may be marked in by
moving the pen point up and down above and below this line at inter-
mediate points, and marking in each case the spot at which it is just visible.

26. Try Mayer's experiment, described on p. 274.

TEST QUESTIONS^

1. Describe the life-history of the simplest animal organism.

2. State the similarity which exists between the simplest type of living
animal and the more complex types.

3. Define the terms "tissue," "organ," and "gland."

4. Describe a typical vertebra, and state in general terms how this
structure is modified in various parts of the vertebral column.

5. Describe the articulation of the lower jaw with the skull, and the
movements which take place at this joint.

6. Describe the bony framework of the thorax, and the manner in which
the volume of the thorax is altered during respiration.

7. State the bones and joints which correspond to one another in the
upper and lower limbs.

8. Give a classification of joints, stating the nature of the movement in
each, and illustrating by examples.

9. Name the different classes of muscular tissue, state where each kind
is to be found, its microscopic appearance, and in general terms its use in
the body.

10. How is the erect position of the body maintained ^

11. Enumerate the viscera found in the thorax and in the abdomen
respectively, and state their position in these cavities.

12. Describe the course of the aorta, and name the principal branches
which it gives off.

13. Beginning at the aorta, describe the path of a blood corpuscle which
makes a complete circuit. What is the longest, and what the shortest, path
which such a corpuscle could take ?

14. State in general terms the purposes served in the body by the
circulation of the blood.

15. Describe the walls of the blood-vessels in the cases of arteries,
capillaries, and veins, pointing out similarities and differences.

16. "What is meant by the " heart sounds " ? and how are these caused ?

1 7. Why are the auricles thin-walled and the ventricles thick-walled ? and
why is the wall of the left ventricle thicker than that of the right ventricle ?

18. Where in the circuit does the greatest fall in blood-pressure take
place ? and what is the cause of this rapid fall ?

^ These questions are intended to point out important points, which
should be specially studied and understood ; they should not be attempted
until the book has been once carefuUv read over.

286 Element a7'y Physiology.

19. What is the cause and the nature of the pulse in the arteries ? and
why does the blood-flow become uniform in the veins ?

20. What determines the relative velocity of the blood-flow in arteries,
capillaries, and veins respectively, and what the local velocity in any
particular area ?

21. Describe the venous valves, and state what use they serve. Which
veins have no valves ?

22. How would you identify the different chambers in an excised heart ?
What cuts would you make in it in order to expose the interior of each
chamber ? What differences are found in the two sides ?

23. Describe the tri-cuspid valve. What are the uses of the chorda
tendijice and musculi papillares ?

24. Describe the corpuscles found in the blood, and state their functions.

25. What are the names given to the fluid part of the blood before and
after clotting respectively ? What substances are dissolved in this fluid part
of the blood ?

26. Enumerate the conditions which respectively hasten and retard
coagulation of the blood.

27. State the part which plants play in ^^reparing the food of animals.

28. Into what classes or groups can the substances present in food be
divided, and what are the characteristics of each class ? Give examples.

29. Give the position of the salivary glands. What is the action of
saliva on food ?

30. What is meant by the term ^^ enzyme'''' 1 Enumerate the digestive
enzymes, stating in which digestive secretion each is found, and discuss
briefly their action on food.

3 1 . State the general characteristics of enzymic action.

32. Describe the two types of gastric gland, and state what constituents
of the gastric juice are probably secreted by each type,

33. Enumerate and briefly describe the coats of the wall of the small
intestine.

34. Describe a vilhis of the small intestine. What use is served by the
villi ?

35. Describe, first, the naked-eye appearance of the liver ; secondly,
its minute structure ; and thirdly, the nature and arrangement of its blood-
supply.

36. Describe the physical properties of bile ; enumerate its chief
chemical constituents, and state veiy briefly the properties of each of these.

37. How have the bile pigments been shown to be related to haemoglo-
bin? What information does this give as to their source in the body?
What is their ultimate fate ?

38. What uses do the bile salts serve in the body ? W^hat is meant by
the term ^''circulation of the bile " ?

39. What is meant by the "glycogenic function" of the liver? State
the evidence that the liver can also act as a temporary storehouse for other
food-stuffs than carbohydrates.

Test Questions. 287

40. What are the meanings of the terms "metabolism," "katabolism,"
and "anabolism " ?

41. Where is fat chiefly stored in the body ? Describe the modifications
which the cells undergo as they become charged with fat.

42. To what extent can the various kinds of food-stuff replace one
another, and be converted into one another in the metabolic processes ?

43. Describe concisely the structure of the respiratory apparatus.

44. Describe the changes which [a) the air and (/;) the blood undergo
in the lungs in the process of respiration. How may some of these changes
be demonstrated?

45. In what manner is the oxygen held in the blood? What circum-
stances determine the amount of oxygen so held ?

46. Why does the blood lose oxygen and take up carbon dioxide in
passing through the capillaries of the tissues ? Describe the manner of the
exchange, as far as is known, between the tissue cells and the blood.

47. What is asphyxia I Enumerate some of the ways in which it may
be caused.

48. How is the temperature of the body maintained constant ?

49. State the channels by which the waste of the body is removed, and
mention the chief waste products removed by each.

50. Describe the minute structure of the skin. Briefly describe the
structure of a nail, of a hair follicle, and of a sweat gland.

51. Name the parts of the urinary system, and state their relationship to
one another.

52. Describe the naked-eye appearance of a kidney as seen in longitu.
dinal section.

53. Describe the course of a uriniferous tubule, and the structure of the
cells lining it at various parts.

54. Describe the arrangement of the blood-supply of the kidney.

55. State what you know as to the manner in which the urine is secreted
in the kidney.

56. What evidence have we that urea is formed in the liver and not in
the kidney ? From what substances is urea probably formed in the liver ?
(See p. 165.)

57. What is the chemical formula of urea, and what is its nature as a
chemical compound ? What gives this substance its chief importance to the
physiologist ?

58. Enumerate the other important constituents of normal urine.

59. Under what conditions do abnormal constituents appear in the
urine, and what does their presence under such conditions teach us as to
the function of the kidneys ?

60. State in general terms the work performed in the body by the
nervous system.

61. Enumerate the parts of the central nervous system, and very briefly
describe each part.

62. Why is it that when one side of the brain is injured, effects are
produced upon the opposite side cf the body ?

288 Elementary Physiology.

63. Enumerate the cranial nerves, and very briefly state the use of
each pair.

64. Describe the sympathetic nervous system, and state in general terms
w^hat is the nature of its action in the body.

65. Describe a medullated nerve fibre.

66. Describe a simple reflex act.

67. What is meant by the term " co-ordination " ?

68. Describe the condition of an animal, such as a frog, from which the
cerebral hemispheres have been removed.

69. State concisely the functions of the different parts of the brain and cord.

70. What is the chief difference between the so-called common and
special sensations ?

71. What is meant by the " muscular sense " ? By what nerve endings
is it probably called into action ?

72. What is the law of " specific sensation " ?

73. What is the Weber-Fechner law ?

74. State the various injuries to the visual apparatus by which blindness
may be occasioned.

75. What evidence is there that there are nerve endings in the skin
which are connected with different kinds of sensation ?

76. How is the delicacy of touch estimated in different areas of the skin ?
77- Describe the histological appearance of the end organs of taste and

of smell.

78. Name the four taste sensations which are usually referred to as
primitive. How are these modified so as to give rise to the many flavours
which we experience ?

79. Describe the structure of the middle ear, and the movements of the
auditory ossicles.

80. Describe the structure of the cochlea.

81. Enumerate the changes which take place between the arrival of a
sound wave at the membrana tympani and the stimulation of the nerve
endings in the organ of Corti.

82. From what defects in the auditory apparatus may deafness arise?

83. Describe the semicircular canals, and state what is their function.

84. Draw a diagram of a section, from front to back, of the eyeball
and mark in the names of the various parts.

85. What is the iris? Why does the pupil vary in size, and how are
the changes in size produced ?

86. What changes take place in the eye when the attention is directed
from a distant to a near object, and how are these changes brought about ?

87. Which layer of the retina is sensitive to light, and how has this been
shown ?

88. What is the "blind spot," and how is its existence demonstrated?

89. What is an " after-image " ?

90. What is meant by sirmdtaneous^ and what by successive contrast ?
Describe an experiment to show the effect of simultaneous contrast*

INDEX

Abdomex, 32

Absorption, 5, 151, 156, i57

of carbohydrates, 156

of fatty acids, 156

ofproteids, 156

Accommodation, 268, 269
Acetabulum, 35
Acidity of urine, 215, 216
Acromion process, 38
Adipose tissue, 168
Afferent nerves, 226
After-image, 273
Air, alveolar, 176

, changes in respiration, 183

, complemental, 257

, composition of, 182

, residual, 180

, stationary, 181

, supplemental, 181

, tidal, 181

Alimentary canal, 152-154

Amoeba, structure of, 2

Amoeboid movements, 2, 3

Amylolytic ferment, 145

Amylopsin, 149

Anabolism, definition, 9

Animal heat, 192-194

Antero-lateral tract, 236

Anus, 57, 92

Aorta, 82

Aphasia, 241

Apnoea, 189

Aqueous humour, 266

Arterial blood-pressure, 105

Artery, structure of, 98, 99

Articular surface, 13

Articulations of ankle, 50

, astragalus, 50

of atlas and axis, 21

Articulations, carpal, 43

, carpo-metacarpal, 43

, classification of, 52-55

of elbow, 40

of face, 28

of foot, 50, 51

of hand, 44

of hip, 36

of knee, 48

of lower jaw, 30

of pelvis, 35

of ribs, 32

of shoulder, 39

of skull bones, 25

of thorax, 32

of vertebral column, 18

Asphyxia, 85, 188

Assimilation, 5, 157

Astragalus, 50

Atlas, 21

Auditory meatus, 255

ossicles, movement of, 261

Auricle of heart, 81, 100, 117

Auricular appendages of heart, 114
Automatic action, 73

Axis, 22

Basilar membrane, 259
Bicuspid tooth, 31
Bile, 149, 162

circulation, 163

duct, 159, 162

■ pigments, 163, 164

salts, 162, 163

, tests for, 163, 164

BiUrubin, 163
Biliverdin, 164
Bladder, urinary, 94
Bhnd spot, 270

U

290

Index.

Blood, changes in, 102, 184

, circulation of the, 81, 99-103

, coagulation of, 124-127

; composition of, 128, 129

gases, 187

serum, 128

Blood- corpuscles, red, 121-123

, white, 2, 122

Blood-plasma, 124, 185
Blood-pressure, 105, 106
Blood-serum, 128, 129
Bone, cancellous, 14

, compact, 14

, structure of, 13

Bronchi, 79, 175

, structure of, 177, 178

C^CUM, 91

Calcaneum, 51

Calcium, effect on clotting, 126

Canine tooth, 31

Capillary, structure of, 65, 95

Capitellum, 40

Carbohydrates, 132, 133

, absorption of, 156

■, digestion of, 145

Carbon dioxide, excretion of, 185,

186, 199
Carotid artery, 82
Cartilage, structure of, 14, 15
Cauda equina, 227
Caudate lobe of liver, 89
Cell, the, 4
Centre, respiratory, 242

, speech, 242

, vasa motor, 243

Centres in spinal cord, 244
Cerebellar peduncles, 222
Cerebellum, 223
Cerebral convolutions, 220

cortex, 221

hemispheres, 219

■ localization, 241

Cerebrum, function of, 239-241

, removal of, 239

Cholalic acid, 162
Cholestearin, 163
Chordae tendinae, 116, 120
Choroid coat, 264
Chyle, 148

Ciliary muscle, 264

process, 268

Circulation of the blood, 81, 99
Circumvallate papillae, 252
Clavicle, 38

Coagulation of blood, 124
Coccyx, 24
Cochlea, 259

Cold-blooded animals, 193
Colon, 90, 92
Colourblindness, 274

vision, 272

Complemental air, 181
Complementary colour, 273
Conjunctiva, 203
Contraction of heart, 102

of muscle, 70

Contrast phenomena, 274
Co-ordination of movement 238, 273
Cornea, 265, 268
Corona radiata, 222
Corpora Aurantii, 120
Corpus collosum, 219
Corpuscles, red, 121

, tactile, 235, 236, 249

white, 2, 122

Corti, organ of, 255, 259, 261

, tunnel of, 261

Cranial cavity, 27

nerves, 227-231

Cranium, 25
Creatinine, 216
Crystalline lens, 266
Cuneform bones, 51
Cutaneous sensation, 251
Cutis vera, 2co, 203

Deafness, 263
Decussation of pyramids, 224
Defaecation, 57
Degeneration of nerves, 235
Deglutition, 137
Dendrons, 235
Dextrin, 146
Diabetes, 168
Diaphragm, 32

, action of, 34

Diarthrosis, 52
Dicrotic wave, 109
Diet, 135-137

Index.

291

Digestion, 145-149
Duct, bile, 159, 162
Duct of Rivinus, 143

of Stensen, 142

, thoracic, 65, 79

Ductless glands, 7, 8
Duodenum, 90

Ear, lobule of, 255

Efferent nerve fibres, 227

Elbow, articulation of, 40

Electrodes, 70

Endolymph, 258

Endplate of nerve, 6j, 235

Enzymes, 144-149

Epidermis, 200

Erect position, mechanism of, j'j

Eustachian tube, 257

Excretion of carbon dioxide, 199

Expiration, 34

Eye, anterior chamber, 266

, movements of, 264

, muscles of, 264, 265

, posterior chamber, 266

Face, bones of, 28

Fascia, 56

Fats, 134, 168-171

, absorption of, 156

, digestion of, 149

Femur, 37, 45, 47
Fenestra ovalis, 257

rotundis, 257

Ferments, 144-149

Fibrin ferment, 127, 145, 148

Fibrinogen, 125, 127

Fibula, 50

Filum terminale, 227

Fissure of Rolando, 241

Foramen magnum, 28

Fossa ovalis, 114

Fovea centralis, 271

Gall bladder, 89
Gastric juice, 146, 148
Glands, 6

, ductless, 7, 8

, gastric, 146, 147

, lachrymal, 6, 7, 265

Glands of Lieberkiihn, 139, 150

, mucous, 139

, parotid, 142

, racemose, 140

, salivary, 7, 140

, sebaceous, 205

, subungual, 143

, submaxillary, 142

, sweat, 205

, thymus, 79, 85

, thyroid, 96

Glisson's capsule, 160

Glomeruli of kidney, 211

Glottis, 181

Gluteal muscles, 37

Glycocholic acid, 162

Glycogen, 162, 167

Glycosuria, 168

Gmelin's test for bile pigments, 164

Goblet cells, 140

Grooved suture, 52

Gustatory cells, 212.

H^.MAGLOBIX, 123, 124, 184

Haematoidin, 164

Hair, 204

Hallux, 51

Head, movements of, 23

Hearmg, 255

Heart, 79, 80, 11 3-1 20

, auricles of, 81, 100. 117

, beats of, 103

, grooves of, 113

, movements of, 102

, nerves of, 72

, output of, 104

, sounds of, 103, 104

, ventricles of, 81, 100, 11 7- 120

, work of, 104

Heat, latent, 195

, loss of, 194

, production of, 194

Hip-bone, 35
Hippuric acid, 217
Homology of limb bones, 37
Humerus, 39

, Iliem, 90
I Ilium, 35

Incisor tooth, 31

292

Index.

Incus, 257

Innominate artery, 82
Inspiration, 34
Intercostal muscles, 33
Internal secretion, 96
Intestine, 89-92
Invertin, 145, 150
Iris, 267
Ischium, 35

Jaw, 28

, movements of, 28, 30

, muscles of, 30

Jejunum, 90

Joints, ball-and-socket, 37 '

, different kinds of, 52-54

Kidney, blood-vessels of, 212

, calix of, 209

— — , cortex of, 211

, function of, 214, 215

, glomeruli of, 211

, Malpighian corpuscles of, 211

, medulla of, 21 1

, pelvis of, 204

, position and relations of, 93

, structure of, 209-212

Knee-joint, articulation of, 48
, movements of, 49

Labyrinth of ear, 255, 258

Lacteals, 91

Larynx, 175

Lecithine, 163

Leucocytes, 2

Levers, three orders of, 74

Ligaments, 17

■ , orbicular, 42

Limb bones, homology of, 37
Liver, 89, 158

, caudate lobe of, 89

, functions of, 159, 165-172

, quadrate lobe of, 89

, spigelian lobe of, 89

, structure of, 159-162

Lobule of ear, 255
Lungs, 85

, root of, 85

, structure of, 179, 180

Lymph, 64

Macula lutea, 271

Malleus, 256

Mastication, muscles of, 30

Mayer's experiment, 274

Medulla oblongata, 223, 2_j2, 243,

Membrana tectoria, 259

Membrane basilar, 259

of Ressinus, 259

Mesentery, 90, 91
Meso-colon, 92
Metacarpal bones, 43, 44
Metatarsal bones, 51
Modiolus, 259
Molar tooth, 31

Motor area, 224

Movements, co-ordinate, 238, 275

of elbow, 42

of fingers, 45.

of head, 23

of knee, 48

of v^rist, 42.

, reflex, 238

Muscle, 56, 57

, cardiac, 68, 71

, chemical stimulation of, 69

, ciliary, 264

, contraction of, 70

, co-ordination of, 73

, curve, 70

, electrical stimulation of, 69

, excitation of, 68

, involuntary, 61, 67, 69

, latent period of, 70

, mechanical stimulation of,

69

-, nerve ending in, 67

-, nerve of, 65

-, nutrition of, 63-65

-, relaxation of, 70

-, sense, 245

-, structure of, 61-64

-, summation of stimuli, 71

-, tone of, 60

-, voluntary or skeletal, 61, 62

Muscles of eye, 264, 265

, gluteal, 37

, intercostal, 32, 33

of mastication, 30

of respiration, 65

Musculi papillaris, 116, 120

Index.

^93

Xail and nail bed, 204, 208
Navicular bone, 51
Xerve, afferent, 60, 226

■ cells, 234

-, cranial, 227-231

, degeneration of, 235

, efferent, 60, 227

, medullated and non-meduUated,

233
• ■ of heart, 72

roots, 226

, spinal, 226

, structure of, 232-235

-, vasa motor, 60, in

Nerves, 28, 281
Nucleus, 2, 5

Occipital condyles, 23
CEsophagus, 32, 79, 84
Olfactory' cells, 254

membrane, 254

Omentum, 89

, great, 90

, small, 90

Optic ner\'e, 266

Os calcis, 51

Os innominatum, 35

Ovum, segmentation of, 10

Oxy-haemoglobin, 184

Palate, hard, 28

, soft, 28

Pancreas, 90
Pancreatic juice, 149

— , action of, 150

, composition of, 150

Patella, 48
Pelvis, 35

Pepsin, 148

Peptone, 126

Pericardium, 84

Perilymph, 258

Peripheral resistance to blood, 10:
T07

Peristalsis, 60, 138

Peritoneum, 88, 89

Perspiration, insensible, 207

, sensible, 207

Pettenkofer's test for bile salts, 163

Pharvnx, 28, 84

Pinna, 255
Pleura, 85, 86
Pons varolii, 223
Portal canal, 160

vein, 157

Pronation of hand, 42
Proteids, 132, 134

, metabolism of, 169

Proteolytic ferment, 145
Protoplasm, 4, 34

Ptyahn, 144-146

Pubic symphasis, 37

Pubis, 35

Pulmonary circulation, 102

veins, 114, 119

Pulse, 1C7-109

wave, 108

Pupil, 266, 269

Pyramidal decussation, 224

tracts, 224

Quadrate lobe of liver, 89

Radius, 39
I Rectum, 60, 92
I Reflex act, 237-239, 243

Rennin ferment, 145, 148

Reserve air, 181

Residual air, 180

Respiration, abdominal, 34

, costal, 34

. , rate of, 190, 191

Respiratory centre, 242

Retina, 266, 269

Ribs, 32

Rolando, fissure of, 241

Sacrum, 24
Saliva, 7, 143, 144

, action of, 146

, composition of, 146

Scala tympani, 259
Scaphoid bone, 51
Scapula, 37, 38
Schneiderian membrane, 254
Sclerotic coat, 265, 268
Secretion, internal, 96

of urine, 213

Semicircular canals, 259, 263
Semilunar fibro-cartilages, 48

294

Index.

Sensation, common, 245;

, cutaneous, 245

— — , special, 245
Sanse, muscular, 245

of hearing, 255

• of smell, 252, 254

of taste, 252 -254

of touch, 246

Serum, 128, 129
Sesamoid bone, 48
Shoulder, articulation of, 39, 40
Simultaneous contrast, 274
Skeletal movements, t^, 75, 76
Skin, appendages of, 203

, structure of, 200-202

Skull, 24-26

Special sensation, law of, 247
Spermatozoon, 11
Sphygmograph, 109
Spigelian lobe of liver, 89
Spinal cord, 225

, central canal, 226

centres, 244

tracts, 236

nerves, 226

Spleen, 92, 93
Stapes, 257

Steatolytic ferment, 145
Stercobilin, 164
Stomach, 89, 90, 139

, movements of, 138, 148

, mucous membrane of, 146, 147

, structure of, 138

Subjective sensation, 247, 250, 251
Successive contrast, 274
Succus entericus, T50
Sulci of brain, 219
Supination of hand, 42
Supplementary air, 181
Suprarenal bodies, 94, 96
Suspensory ligament of lens, 266
Sutures, 25, 52
Sweat, composition of, 207

glands, 205, 206

Sympathetic system, 219, 231, 236-

237
Symphasis pubis, 52
Synarthrosis, 52
Synchondrosis, 53
Synovial fluid, 16

Tarsus, 50, 51
Taste, 252
Taurine, 162
Taurocholic acid, 162
Temperature of animals, 192

sense, 250

Tendo Achillis, 51
Tendon, 56, 63
Tetanus, 71
Thoracic duct, 65, 79
Thorax, movements of, 31-34

■, structure of, 31, 32

Thymus gland, 79, 85

Thyroid gland, 96

Tibia, 50

Tidal air, 181

" Tissue," definition of, 6

Tone of muscle, 60

Tooth, bicuspid, 31

, canine, 31

, incisor, 31

, molar, 31

Trachea, 174, 175

, structure of, 176

Trochanter, 45, 46
Trochlea, 40
Trypsin, 149

Tympanic membrane, 256
Tympanum, 255, 256

Ulna, 39, 213, 216

Urea, 165, 216

Ureter, 93

Urethra, 94

Uric acid, 216

Urine, composition of, 215, 217

, secretion of, 213, 214

Uriniferous tubules, 211

Valve, aortic, 120

, auriculo-ventricular, 115

, ileo-cascal, 91

, mitral, 119

, pulmonary, 119

, semi-lunar, 119

, tricuspid, 120

of veins, iii, 112

Valvulas conniventes, 155
Vasa motor centre, 243

Index.

295

Vasa motor nerves, 60, iii
Vein, portal, 157

, pulmonary, 114, 119

, structure of, 99

Velocity of blood, 109, no

of pulse, 109

Velum, 28

Vena cava, inferior, 81, 114, 159

, superior, 81

Ventricles of brain, 221

of heart, 100, 117-120.

Vermiform appendix, 91
Vertebra, cervical, 21

Vertebra, cocygeal, 24

, dorsal or thoracic, 23

, lumVjar, 24

, sacral, 24

Vertebral column, 17, 18
Vestibule of ear, 259
Villi of intestine, 152-155
Vital capacity, 181
Vitreous humour, 267

Warm-blooded animals, 192, 193
Water, excretion of, 198
Weber-Feehner law, 248

PRINTED BY WILLIAM CLOWES AND SONS, LIMllfci), LONUOX ANU BECCLES.

COLUMBIA UNIVERSITY LIBRARIES

This book is due on the date indicated below, or at the
expiration of a definite period after the date of borrowing, as
provided by the library rules or by special arrangement with
the Librarian in charge.

DATE BORROWED

DATE DUE

DATE BORROWED

DATE DUE

4m 1 0* 194

7

/

/

/

/

/

/

/

C28(546)M25

■ /

^A^^

JAN lO^M^ 0

\ff-m^^!fm-- ..^