eiouxnr

LIBRARY
G

ESSENTIALS

OF

HUMAN PHYSIOLOGY

BY

D. NOEL P^TON, M.D., B.Sc., F.R.C.P. ED.

SUPERINTENDENT OP THE RESEARCH LABORATORY OF THE ROYAL COLLEGE OF PHYSICIANS

OF EDINBURGH; LECTURER ON PHYSIOLOGY, SCHOOL OF MEDICINE OF THE ROYAL

COLLEGES, EDINBURGH ; EXAMINER IN PHYSIOLOGY IN THE UNIVERSITY

OF GLASGOW AND FOR THE ROYAL COLLEGE OF PHYSICIANS,

EDINBURGH ; AND LATE EXAMINER IN THE

UNIVERSITY OF EDINBURGH

SECOND EDITION

REVISED AND ENLARGED

or THE

SSITY

WILLIAM GREEN <3£ SONS
EDINBURGH AND LONDON

W. T. KEER & CO.,
CHICAGO.

BIOLOGY
LIBRARY

PREFACE TO FIRST EDITION

THE object of this volume is to put before medical students
as succinctly as possible the essential facts of human
physiology, and to emphasise specially those parts of the
science which are of cardinal importance in medicine and
surgery.

Physiology has now become so wide in scope that the
ordinary student must limit his attention to matters which
have a direct bearing upon his professional work. Fortunately
the study of this limited field affords ample opportunity for
cultivating the scientific methods of observation and of
thought, which should be gained by the student before
approaching his clinical studies.

In writing this book, I have endeavoured to recognise
and to adhere to these limitations, and hence many parts
of physiology which occupy considerable space in the
ordinary text-books have been relegated to minor positions,
while parts which have a direct bearing upon the study
of medicine have been purposely given a prominence which
their importance, when viewed from the purely scientific
standpoint, would hardly warrant.

Physiology, like anatomy, must be studied practically.
But it is impossible for students to perform more than a
small number of the experiments which are the ground-
work of the science, and it is the duty of the teacher to in-
dicate such a course of practical exercises as it is possible for
them to accomplish, and which will form a basis upon which

144J48

vi PREFACE TO FIRST EDITION

to build a really sound knowledge. Such practical courses
are apt in the student's mind to be entirely dissociated from
the systematic study of physiology. Microscopic prepara-
tions are made too often with more consideration of the
methods employed, than of the structures to be studied,
and experiments are performed as feats of manipulative
skill, rather than in relationship to what they teach.

I have attempted to bring the practical and systematic
study into closer relationship, by constant references to the
practical work which the student must undertake. Since
no amount of reading will give the real knowledge of the
histological methods which are employed, and the structures
revealed by these methods, or of the chemical tests which
are daily more and more largely used by physicians, or of
the experimental procedure by which physiology has been
built up, lengthy descriptions of these methods seem un-
called for ; and hence those details which the student must
master practically are here merely touched upon, and he
is referred to his practical exercises.

Every student in a recognised school now receives such
a practical training, and the knowledge thus acquired may
be refreshed by reference to those books which are used
as guides during the prosecution of the practical work
— such books as Schafer's " Experimental Physiology,"
Schafer's "Essentials of Histology," Halliburton's "Essen-
tials of Chemical Physiology," and "The Practical Physi-
ology" of Beddard, Edkin, Hill, M'Leod, and Pembrey.
Descriptions of apparatus have been purposely excluded.
The student must make himself practically familiar with
such pieces of apparatus as he has to use himself, or to
see used in demonstrations. With other apparatus he is not
concerned. It is as absurd that the student of physiology
should burden his memory with the construction of machines
he will never see in use, as that the surgeon should attempt

PREFACE TO FIRST EDITION vii

to learn all the various instruments which he will never
require in his practice.

I have pleasure in acknowledging the kindly help which
I have received from many of my friends. Drs. Gibson,
Bruce, Gulland, Dunlop, Boyd, and Ballantyne have given
me the benefit of their criticism on those sections of the
book which bear upon the parts of medical science with
which each of them is specially conversant. I desire also
to record my thanks to my class assistants, Dr. Goodall,
Messrs. Barcroft, Kidston, and Mackenzie for help in revising
the proofs, and to Mr. Graham Brown for some of the

histological diagrams.

D. N. P.

PREFACE TO SECOND EDITION

IN preparing a second edition of my Essentials of Physi-
ology, I have endeavoured to bring the information up to
date, and to increase the usefulness of the book by rearrang-
ing and extending certain sections and by the introduction
of many additional figures.

The purpose and character of the book remain unchanged.
It pretends to give no more than the Essentials of Human
Physiology, and it is not intended to replace, but only to
supplement the practical work and demonstrations from
which alone a real knowledge of the subject can be gained.
To facilitate the linking of systematic and practical study,
references are given to the practical work which the student
should do as described in Schafer's Class Work in Prac-
tical Physiology, and in the author's Practical Course of
Elementary Chemical Physiology.

My thanks are due to Dr. Goodall and to Mr. Graham
Brown, B.Sc., for the new illustrations, and to Dr. I. Cameron
for reading the proof-sheets and revising and extending the
Index.

D. N. P.

CONTENTS

PAGE

INTRODUCTION 1

PART I
SECTION I

PROTOPLASM

SECTION II

THE CELL 14

SECTION III

THE TISSUES 22

A. THE VEGETATIVE TISSUES .22

Epithelium —

1. Squamous Epithelium 22

2. Columnar Epithelium 23

3. Glandular Epithelium 25

4. Ciliated Epithelium 27

Connective Tissues —

1. Mucoid Tissue 27

2. Fibrous Tissue 28

3. Cartilage 33

4. Bone 34

B. THE MASTER TISSUES— MUSCLE AND NERVE ... 38

Muscle —

1. Physical and Chemical Characters of Muscle at rest . 39

2. The Methods of making Muscle Contract ... 45

3. The Changes in Muscle during Contraction ... 51

4. The Chemical Changes in Muscle and the Source of the

Energy evolved 67

5. Death of Muscle 74

Nerve 75

The Neuro- Muscular Mechanism 88

xi

xii CONTENTS

SECTION IV

PAGE

THE SENSES 95

A. COMMON SENSIBILITY 95

B. MUSCLE AND JOINT SENSE 97

(7. SPECIAL SENSES 98

Tactile Sense 98

Temperature Sense 101

Vision 102

Hearing .......... 129

Taste 137

Smell 139

SECTION V

THE CENTRAL NERVOUS SYSTEM—

Spinal Nerves 141

Spinal Cord 148

Medulla and Cranial Nerves 154

Pons Varolii 160

Cerebellum 160

Crura Cerebri and Corpora Quadrigeinina .... 167

Cerebrum . 168

PART II
THE NUTRITION OF THE TISSUES

SECTION VI

MANNER IN WHICH NOURISHMENT is SENT TO THE TISSUES
BLOOD AND LYMPH 187

SECTION VII

THE CIRCULATORY SYSTEM—

General Arrangement . 209

Circulation through the Heart 211

Circulation in the Blood and Lymph Vessels . . . 244

CONTENTS xiii

SECTION VIII

SUPPLY OF NOURISHING MATERIAL TO BLOOD AND LYMPH, AND
ELIMINATION OF WASTE MATTER FROM THEM

PAGE

KESPIRATION 277

SECTION IX

FOOD AND ITS DIGESTION AND UTILISATION 312

Food 312

Digestion 324

Absorption 357

Heat Production and Temperature Regulation . . . 360

Hepatic Metabolism 366

General Metabolism . . . . . . 371

Dietetics . 376

SECTION X

INTERNAL SECRETIONS— Toxic ACTION AND IMMUNITY . . 384

SECTION XI

EXCRETION 394

RENAL EXCRETION—

Urine . . . . . . . .'"..-. . 394

Kidneys .......... 403

Renal Secretion 403

CUTANEOUS EXCRETION—

Sweat Secretion 409

Sebaceous Secretion 410

Milk Secretion 410

PART III

SECTION XII
REPRODUCTION 413

APPENDIX 421

INDEX 429

REFERENCES

Practical Physiology refers to " Directions for Class Work
in Practical Physiology, Elementary Physiology of
Muscle and Nerve, and of the Vascular and Nervous
Systems," by E. A. SCHAFER, LL.D., F.R.S., Pro-
fessor of Physiology in the University of Edinburgh.
London: Longmans, Green, & Co. 1901.

Chemical Physiology refers to " Practical Course of Ele-
mentary Chemical Physiology for Medical Students,"
by D. NOEL PATON. (Second Edition.) Edinburgh :
William Green & Son. 1905.

xiv

Of THE

DIVERSITY
or

INTRODUCTION

PHYSIOLOGY is really an older science than anatomy, for
even before any idea of pulling to pieces, of dissecting the
animal machine had suggested itself to our forefathers,
crude speculations in regard to the causes and nature of the
various vital phenomena must have been indulged in —
speculations based upon the vivid belief in the action of
spiritual agencies, and perhaps unworthy of the name of
science. Still the physiology of to-day is the offspring of
such speculations.

Organs and Function. — The first great and true advance
was through anatomy. As that science showed how the
body is composed of distinct and different parts, it became
evident that these parts or organs had separate actions or
functions ; and hence arose the important conception of the
co-relation of organ and function.

From the early metaphysical speculations to such true
inductions was a great stride, for a scientific method of
advance had been established.

Ever since this, until quite recent times, physiology has
followed in the footsteps of anatomy, or, to use a more com-
prehensive term, of morphology. The connection between
organ and function having been demonstrated, the ques-
tions, why are these various functions connected with the
respective organs ? why should the liver secrete bile, and the
biceps muscle contract ? next forced themselves upon the
attention.

Tissues and Function. — Again anatomy paved the way
for the explanation. The dissecting knife and the early and
defective microscope showed that the organs are composed
of certain definite structures or tissues, differing widely from
one another in their physical characters and appearance, and,
as physiologists soon showed, in their functions. It now

1

2 INTRODUCTION

became evident why the liver secreted and the biceps con-
tracted : the one is composed of secreting tissue, and the
other of contracting tissue.

Cells and Function. — Physiologists and anatomists alike
devoted their energies to the study of these various tissues,
and, as the structure of the microscope improved, greater and
greater advances were made in their analysis, till at length
Schwann was enabled to make his world-famous generalisa-
tion, that all the tissues are composed of certain similar
elements more or less modified, which he termed cells, and
it became manifest that the functions of the different tissues
are due to the activities of their cells.

The original conception of the cell was very different from
that which we at present hold. By early observers it was
described as composed of a central body or nucleus, sur-
rounded by a granular cell substance with, outside all, a cell
membrane. As observations in the structure of the cell were
extended, it soon became obvious that the cell membrane
was not an essential part, and later, the discovery of cells
without any distinct nucleus rendered it clear that the
essential part is the cell substance, and this substance Von
Mohl named protoplasm, by which name it is since generally
known.

Protoplasm and Function. — So far physiology had followed
in the tracks of anatomy, but now another science became
her guide. Chemistry, which during the last century has
advanced with enormous strides, and has thrown such im-
portant light upon the nature of organic substances, now lent
her aid to physiology ; and morphologists having shown that
the vital unit is essentially simply a mass of protoplasm, the
science of life bids fair to become the science of the chemistry
of protoplasm.

The prosecution of physiology on these lines is still in its
infancy, but already it has changed the whole face of the
science. Physiology is no longer the follower of anatomy.
It is become its leader, and at the present time, as we shall
afterwards see, not only the various activities, but also the
various structural differences of the different tissues are to be
explained in terms of variations in the chemical changes in
protoplasm.

INTRODUCTION 3

In the study of physiology this order of evolution must be
reversed, and from the study of protoplasm the advance
must be made along the following lines : —

1. Protoplasm — the physical basis of life; its activities

and nature.

2. Cells. — Manner in which protoplasm forms the vital

units of the body.

3. Tissues. — Manner in which these are formed by cells.

Their structure, physical and chemical properties,
and vital manifestations.

4. Nutrition of Tissues.

a. Fluids bathing the tissues —

Blood and Lymph.

b. Manner in which fluids are brought into relation-

ship with tissues —
Circulatory System.

c. Manner in which substances necessary for the

tissues are supplied to these fluids —
Respiratory System.
Digestive System.
Food, its nature and quantity.

d. Chemical changes in the tissues generally —

Metabolism and Heat Production.

e. Manner in which the waste products of tissues

are eliminated — Excretion, Hepatic, Renal, Pul-
monary, Cutaneous.

5. Reproduction and Development.

PART I

SECTION I

PROTOPLASM

THE first step in the study of physiology must be to acquire
as clear and definite a conception as possible of the nature
of protoplasmic activity in its most simple and uncomplicated
form, for in this way an idea of the essential and non-essen-
tial characteristics of life may best be gained.

I. Structure. — Protoplasm is a semi-fluid transparent
viscous substance. It usually occurs in small individual
particles — CELLS — more or less associated, but it may occur
as larger confluent masses — PLASMODIA.

Sometimes protoplasm seems perfectly homogeneous, but
generally a reticulated appearance can be made out even in
the living condition (Fig. 1), and from this it has been con-
cluded that there is a more solid part arranged like the
fibres of a sponge, or like the films of a mass of soap-
bubbles, with a more fluid interstitial part. In all proto-
plasm, therefore, there seems to be a certain amount of
organisation, and in certain cells this organisation becomes
very marked indeed.

II. Physiology. — A knowledge of the essentials of the
physiology of protoplasm may be gained by studying the
vital manifestations of one of the simplest of living things,
the yeast plant (Saccharomyces Cerevisae).

This plant consists of very minute oval or spherical bodies
frequently connected to form chains, each composed of a
harder outer covering or capsule and of a softer inner sub-
stance which has all the characters of protoplasm.

PROTOPLASM

5

Its physiology may be studied by placing a few torulse in
a solution, containing glucose, C6H1206, and urea, CON2H4,
with traces of phosphate of soda, Na2HP04, and sulphate of
potash, K2SO4.

If the vessel is kept all night in a warm place the clear
solution will in the morning be seen to be turbid. An
examination of a drop of the fluid shows that the turbidity

FIG. 1. — (a) Foam structure of a mixture of olive oil and cane sugar ; (b) Reticu-
lated structure of Protoplasm ; (c) Reticulated structure of Protoplasm in the
cell of an earth-worm (after BUTSCHLI).

is due to the presence of myriads of torulse. In a few hours
the few torulse placed in the fluid have increased many
hundredfold. The whole mass of yeast has grown in amount
by the growth and multiplication of the individual units.

This power of growth and reproduction under suitable
conditions is the essential characteristic of living matter.

What are the conditions necessary for the manifestation of
these phenomena of life ?

6 HUMAN PHYSIOLOGY

1. If the yeast be mixed with the solid constituents of the
solution in a dry state no growth or reproduction occurs.
Water is essential.

2. If the yeast, mixed with the solution, be kept at the
freezing point no growth takes place, but this proceeds
actively at about 36° C. A certain temperature is necessary
for the vitality of protoplasm. In the absence of these
conditions, protoplasm is only potentially alive, and in
this state it may remain for long periods without under-
going any change, as in the seeds of plants and in dried
bacteria.

These conditions being present, in order that the growth
of the yeast may take place, there must be : —

(a) A SUPPLY OF MATERIAL from which it can be formed.
(6) A SUPPLY OF ENERGY to bring about the construction.

The chemical elements in protoplasm are carbon, hydrogen,
oxygen, nitrogen, sulphur, and phosphorus. These elements
are contained in the ingredients of the solution used. If
yeast be sown in distilled water, even if it be kept at a
temperature of 36° C., it does not grow.

The energy is got by the breaking down of the sugar,
C6H1206, into alcohol, C2H6O, and carbon dioxide, CO2. Such
a breaking down of a complex into simpler molecules liberates
energy, as is well seen when nitro-glycerine explodes, breaking
into carbon dioxide, water, oxygen, and nitrogen —

2C3H6(N03)3 = 6C02 + 5H20 + 0 + ON.

The energy can be used for the performance of work of
any kind, as, for example, the work of building up a fresh
quantity of the yeast plant out of the substances contained
in the solution. The history of the yeast plant shows that
protoplasm, when placed in suitable conditions, has the
power of breaking down certain complex substances, and
of utilising the energy liberated for building itself up. It
is this power which has enabled living matter to exist and
to extend over the earth.

How does protoplasm liberate the potential energy of such
substances ? At present the answer to this question cannot

PROTOPLASM 7

be absolutely definite, but in all probability the yeast forms
something which acts upon glucose, since, by applying
pressure to it, a fluid can be extracted which contains a
substance having the same action as the living yeast. Such
a substance is called an Enzyme or Zymin. These Zymins
act only in the presence of water, and at suitable tempera-
tures, and their action is to accelerate changes which occur
more slowly without their presence. Very minute quantities
can bring about changes in large quantities of the substance
on which they act, and in acting they undergo no marked
change.

Many complex substances besides sugar very readily break
down and liberate energy. For instance, formate of lime
breaks into carbonate of lime, carbonic acid, and hydrogen,
under different conditions : first, under the influence of certain
bacteria, minute living organisms resembling yeast in many
particulars; second, under the influence of some substance
contained in the bacteria after they are killed; and lastly,
in the presence of finely divided iridium, rhodium, and
ruthenium — i.e. the same change is brought about by a
living organism, by a substance contained in it, and by a
metal.

Hence mere vibrations of molecules, occurring in different
ways and in different substances, may be sufficient to bring
about these changes, and however the change is brought
about the result is to set free energy. Such a process has
been termed Catalysis.

Living yeast differs from these dead substances simply in
the fact that it uses the energy liberated from the glucose.
In virtue of this, the yeast has the power of repair and of
growth.

But protoplasm is also constantly breaking down, and if

yeast be kept at a suitable temperature in water without any
supply of material for construction, it gives off carbon dioxide
and decreases in bulk on account of these disintegrative
changes. These are as essential a part of the life of living
matter as the building-up changes, and it is only when they
are in progress that the latter are possible.

Protoplasm (living matter) is living only in virtue of its

8 HUMAN PHYSIOLOGY

constant chemical changes, metabolism, and these changes
are on the one hand destructive (katabolic), on the other
constructive (anabolic). Living matter thus differs from
dead matter simply in this respect, that side by side with
destructive changes, constructive changes are always going
on, whereby its amount is maintained or increased.

Hence our conception of living matter is not of a definite
chemical substance, but of a substance constantly undergoing
internal changes. It might be compared to a whirlpool con-
stantly dragging things into its vortex, and constantly throw-
ing them out more or less changed, but itself continuing
apparently unchanged throughout. Hoppe-Seyler expresses
this by saying : " The life of all organisms depends upon, or,
one can almost say, is identical with a chain of chemical
changes." Foster puts the same idea in more fanciful
language : " We may speak of protoplasm as a complex sub-
stance, but we must strive to realise that what we mean by
that is a complex whirl, an intricate dance, of which, what
we call chemical composition, histological structure, and gross
configuration are, so to speak, the figures."

The rate of these changes may be quickened or slowed by
changes in the surroundings, and such changes are called
stimuli. If the stimulus increases the rate of change, it is
said to excite ; if it diminishes the rate of change, it is said
to depress. Thus the activity of the changes in yeast may
be accelerated by a slight increase of the temperature of the
surrounding medium, or it may be depressed by the addition
of such a substance as chloroform water.

While the continuance of these chemical changes in proto-
plasm is life, their stoppage is death. For the continuance
of life the building-up changes must be in excess of or equal
to the breaking-down, and when failure in the supply or in
the utilisation of the material used in construction occurs,
the protoplasm dwindles and disintegrates. Death is sudden
when the chemical changes are abruptly stopped, slow when
the anabolic changes are interfered with. The series of
changes which occur between the infliction of an incurable
injury and complete disintegration of the tissue constitute
the processes of Necrobiosis, and their study is of importance
in pathology.

PROTOPLASM 9

III. Chemistry. — It is impossible to analyse such an ever-
changing substance as protoplasm, and although what is left
when these chemical changes are stopped can be examined,
such analyses give little insight into the essential nature of
the living matter.

That substances of great complexity take part in the con-
stant whirl is shown by the analyses of what is left after
death. Five or six elements — carbon, hydrogen, oxygen,
nitrogen, sulphur, and phosphorus are present, and these
are linked together to form molecules of enormous size.

Water is the most abundant constituent of protoplasm,
amounting, as it does, to about 75 per cent.

The Solids, constituting the remaining 25 per cent., con-
sist chiefly of a series of bodies closely allied to one another
and called " chief substances " or Proteids. In addition to
these, certain inorganic salts are found in the ash when
protoplasm is burned, indicating the presence of POTASSIUM
and CALCIUM along with PHOSPHORUS and SULPHUR. Small
and varying quantities of FATTY SUBSTANCES, and of CARBO-
HYDRATES, with traces of a number of other organic sub-
stances which need not here be enumerated, are also usually
present.

Of these substances the Proteids alone have to be con-
sidered here, since they constitute the really important part
of the material.

PROTEIDS

White of egg may be taken as an example of such pro-
teids dissolved in water with some salts. If the salts be
separated, and the water carefully driven off at a low
temperature, a pure proteid is left.

(A) Physical Characters. — Proteids have a white, yellow,
or brownish colour. In structure they are usually amorphous,
but many have been prepared in a crystalline condition,
and it is probable that all may take a crystalline form.
The crystals vary in shape, being usually small and needle-
like, but sometimes forming larger rhombic plates. Some
proteids are soluble in water, others require the presence
of neutral inorganic salts, others of an acid or alkali,
while some are completely insoluble without a change

io HUMAN PHYSIOLOGY

in their constitution. All are insoluble in alcohol and
ether.

When in solution, or apparent solution, many of the pro-
teids do not dialyse through an animal membrane, and they
are hence called Colloids. Other colloidal bodies reacting
much like the proteids have been prepared synthetically by
chemists — e.g. by heating together amido-benzoic acid and
phosphoric anhydride. Like other colloids they tend to
coagulate, forming a clot just as, for instance, silicic acid
may clot when carbon dioxide is passed through its solution.

All proteids rotate the plane of polarised light to the left.

(B) Chemistry. — Proteids contain the following chemical
elements : carbon, hydrogen, oxygen, nitrogen, and sulphur,
in about the following percentage amounts : —

c. H. N. s. o.

52 7 16 1 24

It is important to remember the amounts of nitrogen and
carbon, since proteids are the sole source of the former
element in the food and an important source of the latter.

As regards the number of atoms of these elements which
go to form a single molecule, information has been obtained
by studying compounds with various metals. The following
probable formula of the molecule of the chief proteid of the
white of egg is given simply to show how complex these
substances are : C^Hg^N^OgA.

Our knowledge of the constitution of the molecule is still
very imperfect.

The simplest bodies having the characters of proteids are
the Protamines, basic substances which are found in the heads
of spermatozoa, combined with nucleic acid (p. 12). When
they are broken down they yield chiefly Hexone Bases, so
called from having six atoms of carbon in their molecule.
These are —

DIAMIDO ACIDS — acids having two ainidogens — NH2 —
in their molecule. The most important is : —

(1) Arginin, diamido-valerianic acid linked to guani-

din, and therefore allied to creatin. (See p. 43.)

(2) Lysin, diamido-caproic acid, and

(3) Histidin, a substance of unknown constitution.

PROTOPLASM 1 1

The ordinary proteids are built on the same plan as the
protamines, but have complex side chains of MONAMIDO
ACIDS linked to the hexone bases.1

Of these monamido acids perhaps the best known are —

(1) Leucin (amido-caproic acid)— C5H10NH9CO.OH.

(2) Tyrosin, in which amido-proprionic acid is linked

to an aromatic nucleus.2

H NH20

c« H4 ] _c_ C— C— 0— H

H H

CLASSIFICATION OF THE PROTEIDS

(A) Simple Proteids. — 1. Native Proteids. — These pro-
teids, either alone, or combined with certain other sub-
stances, are constant ingredients of dead protoplasm, and
of the fluid constituents of the body. They are distinguished
from all other proteids by being coagulated on heating.

There are two groups — Globulins and Albumins — the
former characterised by being insoluble in distilled water,
by requiring the presence of a small quantity of a neutral
salt to form a solution, and by being precipitated from
solution by half saturating with sulphate of ammonia.

2. Proteoses (Proteids with a less complex molecule than
albumins and globulins). — They may be formed from albu-
mins (albumoses) and globulins (globuloses), by the action
of superheated steam and during digestion. Under the
influence of these agents, the complex molecule splits into
simpler molecules and takes up water.

These proteoses form a series between the original proteids
on the one hand, and the peptones or simplest proteids on
the other. They may be divided into two classes : —

(a) Those nearly allied to the original proteids — Proto-

1 For the tests for proteids and the methods of distinguishing the individual
proteids, see Chemical Physiology, p. 3.

2 For some elementary facts of organic chemistry necessary for the com-
prehension of these details, see Appendix, p. 421 et seq.

12 HUMAN PHYSIOLOGY

proteoses, which are precipitated in a saturated solution of
common salt, NaCl.

(6) Those more nearly allied to the peptones — Deutero-
proteoses, which are not precipitated in a saturated solution
of NaCl, but are precipitated by a saturated solution of
sulphate of ammonia.

3. Peptones. — These are the ultimate products of the
action of gastric juice on proteids. Their characteristic
reaction is their solubility in hot saturated sulphate of
ammonium solution. They diffuse very readily through
an animal membrane.

(B) Conjugated Proteids.— Proteids have a great tendency
to link with other substances —

(1) Proteates are formed by linking acids or alkalies to the
native proteids.

(2) Nucleins, so called because their existence was first
demonstrated in the nuclei or central parts of the cells of
the body, may readily be split into a proteid part and
into nucleic acid, a phosphorus - containing material of
definite composition, having an acid reaction, and containing
about 10 per cent, of phosphorus. In certain places the
amount of nucleic acid is large in proportion to the proteid,
in others it is small. The term nuclein is usually confined
to the former, nucleo-albumin to the latter of these. From
the pure nucleic acid, which occurs along with protamine
in the heads of spermatozoa, to the proteids almost free of
phosphorus there is a continuous series.

Nucleic acid when decomposed yields phosphoric acid and
a series of bodies called the Purin bodies which belong to the
class of diureides, and consist of two more or less modified
urea molecules linked together by the radicle usually of
acrylic acid (see p. 397).

(3) Pseudo-Nucleins. — Other compounds of proteids with
phosphorus-containing molecules occur which do not yield
purin bodies when decomposed. Of these vitellin, the pro-
teid of the yolk of egg, is an example.

(4) Histones are proteids linked to protamine. They occur
in the globin which may be separated from blood pigment.
They have a basic reaction.

(5) Glyco-proteids. — Proteids are linked with sugar-like

PROTOPLASM 13

substances to form compounds. The best-known example is
Mucin (see p. 26).

(6) Ferro-proteids. — In the pigment of the blood (Haemo-
globin) proteids occur linked to an iron-containing molecule
(p. 199).

(7) In horn (Keratin) a sulphur- containing molecule joined
to the proteid gives the special characters to the substance
(p. 23).

SECTION II
THE CELL

PROTOPLASM occurs in the animal body as small separate
masses or CELLS. These vary considerably in size, but, on
an average, they are from 7 to 20 micro-millimetres in
diameter. The advantage of this subdivision is obvious.
It allows nutrient matter to reach every particle of the proto-
plasm. In all higher animals each CELL has a perfectly
definite structure. It consists of a mass of protoplasm, in
which is situated a more or less defined body, the nucleus.

(A) Cell Protoplasm.— This has the structure .already
described under protoplasm, and in different cells the reti-
culum or cytomitoma is differently arranged. In some cells
there is a condensation of the reticulum, round the periphery,
to form a sort of cell membrane.

At some point, in the protoplasm of many cells, one or two
small spherical bodies, the centrosomes (Fig. 2), are found,
from which rays pass out in different directions. For the
detection of these bodies special methods of staining and the
use of very high magnifying powers are required. They will
be again considered when dealing with the reproduction of
cells.

The cell protoplasm frequently contains granules, either
formed in the protoplasm (p. 25), or consisting of material
ingested by the cell.

In the protoplasm, vacuoles are sometimes found, and
from a study of these vacuoles in protozoa, it appears that
they are often formed round material which has been taken
into the protoplasm, and that they are filled with a fluid
which can digest the nutritious part of the ingested par-
ticles. In some cells vacuoles may appear in the process
of disintegration.

14

THE CELL 15

In certain cells protoplasm undergoes changes in shape.
This may well be studied in the white cells in the blood of
the frog or newt. Processes are pushed out, and these are
again withdrawn, or the whole cell may gradually follow the
process, and thus change its position. The processes are
called pseudopodia (false feet), and the mode of movement,

FIG. 2. — (a) White Cell from blood to show the centrosome and nucleus ; (6) Egg
Cell, dividing, shows reticulated structure of Protoplasm, two centrosomes
and nuclear fibres in mitosis (division of nucleus).

from its resemblance to that seen in the amoeba, is called
amoeboid.

The part played by reticulum and hyaloplasm in these
movements is not clearly understood. The pseudopodia are
at first free of reticulum ; but \vhether the hyaloplasm is
pressed out by contraction of the reticulum, or whether it
actively flows out, is not known. In some cells among the

1 6 HUMAN PHYSIOLOGY

protozoa movements take place along some definite line,
and the reticulum is arranged more or less parallel to the
line of movement. Such contractile processes, from their
resemblance to muscles, have been termed myoids. In
other protozoa the pseudopodia may manifest a to-and-fro
rhythmic waving movement, which may cause the cell to
be moved along, or may cause the adjacent fluid to move
over the cell. Such mobile processes when permanent have
been called cilia.

These movements are modified by the various STIMULI
which modify the activity of the chemical changes in the
protoplasm (p. 8). Thus cooling diminishes, and finally
stops them. Gentle heat increases them, but when a certain
temperature is reached, they are stopped. Drying and
various drugs, such as chloroform, quinine, &c., also arrest
the movements.

Changes in the surroundings may cause either contraction
or expansion, may repel or attract. When a repelling or
attracting influence, a positive or negative stimulus, acts at
one side of the cell — unilateral stimulation — it may lead to
movement of the cell away from it or towards it. Movements
are produced by various chemical substances (chemiotaxis),
or by light (photo taxis) or by electricity (galvanotaxis). If
the action is towards the stimulus, it is said to be positive,
if away from it negative.

Chemiotaxis is the attraction or repulsion produced by one-
sided application of chemical stimuli. This is well seen in
the plasmodial masses of aethalium septicum which grow on
tan. Oxygen and water both attract it towards them, and
exercise a positive chemiotaxis. It is also seen in the stream-
ing of the white cells of the blood to disintegrating tissues,
or to various micro-organisms which have to be destroyed to
prevent their poisoning the organism, and in the attraction
exercised by the ovum upon the male element in repro-
duction.

Barotaxis is the effect of unilateral pressure or mechanical
stimulation. Many protozoa appear quite unable to leave
the solid substance — e.g. the microscope slide — with which
they are in contact, the unilateral pressure seeming to cause
a positive attraction in that direction.

THE CELL 17

Phototaxis. — Light, which plays so important a part in
directing the movements of the higher plants, also acts
positively or negatively on many unicellular organisms.
Thus the swarm spores of certain algse are positively at-
tracted by moderate illumination, streaming to the source
of light, while they are negatively stimulated by strong
light, and stream away from it. Light also plays an im-
portant part in directing the movements of certain bacteria.

Thermotaxis. — The unilateral influence of temperature is
well seen in the plasmodium of sethaliuin septicum which
streams from cold water towards water at a temperature of
about 30° C.

Gfalvanotaxis. — As would naturally be expected from its
stimulating action, a current of electricity has a most power-
ful effect in directing the movements of many cells. Certain
infusoria when brought between the poles of a galvanic
battery may be observed to stream towards the negative pole.

The effects of this unilateral stimulation are of great
importance in physiology and pathology, since they explain
the streaming of leucocytes to attack micro-organisms and
other poisons to the animal body, and since they seem to
explain many of the apparently volitional acts of unicellular
organisms. Many of these organisms appear to definitely
select certain foods, but in reality they are simply compelled
towards them by this unilateral stimulation.

(B) Nucleus.

(1) Structure. — The nucleus, seen with a moderate mag-
nifying power, appears in most cells as a well-defined circular
or oval body situated towards the centre of the cell. (Figs. 1
and 2.) Sometimes it is obscured by the surrounding proto-
plasm. It has a granular appearance, and usually one or
more clear refractile bodies — the nucleoli — are seen within
it. It stains deeply with many reagents of a basic reaction,
such as hsematoxylin, carmine, methylene blue, &c. In
some cells the nucleus is irregular in shape (Fig. 2), and in
some it is broken up into a number of pieces, giving the cell
a multi-nucleated character.

It is usually composed of a mass of fibres arranged in a

2

1 8 HUMAN PHYSIOLOGY

complicated network (Fig. 1), and it is these fibres which
have a special affinity for basic stains. Between these fibres
is a more fluid material which may be called the nuclear
plasma. Digestion in the stomach removes the nuclear
plasma, but leaves the network unacted upon.

The nuclear fibres are made up of two substances, a ground
substance or matrix, which has no affinity for colouring matter,
and which is hence called achromatin substance ; and a sub-
stance with a marked affinity for various dyes, set in the
former in a series of particles variously arranged, and dis-
tinguished as chromatin. It is the staining of this material
which colours the nucleus so deeply.

The chromatin substance contains a large amount of
nucleic acid, and its richness in phosphorus has been demon-
strated by treating the cells with ammonium molybdate and
pyrogallol, which colours parts rich in phosphorus of a brown
or black tint.

The nuclear fibres vary in their arrangement in different
cells. Usually they form a network, but occasionally they
are disposed as a continuous skein. In nuclei, with the
former arrangement of fibres, swellings may be observed
where the fibres unite with one another — the nodal swellings.
The nucleoli are probably not all of the same nature, and
some may be simply specially well-marked nodal swellings.
The resting nucleus appears to be surrounded by a distinct
nuclear membrane, which is, however, probably really a
basket-like interlacement of the fibres at the periphery.

(2) Functions. — The part taken by the nucleus in the
general life of the cell is not yet fully understood. 1st, It
exercises an influence on the nutritive processes, since it
has been observed in certain of the large cells in lower
organisms that a piece of the protoplasm detached from the
nucleus ceases to grow, and, after a time, dies. Important
interchanges of material go on between the nucleus and the
protoplasm. 2nd, It is the great reproductive organ of the
cell, probably playing an important part in transmitting in-
herited characters.

Reproduction of Cells. — Cells do not go on growing in-
definitely. When they reach a certain size they generally

THE CELL

either divide, to form two new cells, or they die and undergo
degenerative changes. The reason of this is possibly to be
found in the well-known physical fact that, as a sphere
increases in size, the mass increases more rapidly than the
periphery. Hence, as a cell becomes larger and larger, the
surface for nourishment becomes smaller and smaller in
relationship to the mass of material to be nourished. Pro-
bably the altered metabolism so produced sets up the
changes which lead to the division of the cell. These
changes have now been very carefully studied in a large
number of cells, and it has been shown that the nucleus
generally takes a most important part in division.

(3)

FIG. 3. — Nucleus in Mitosis : (1) Convoluted stage ; (2) Monaster stage ;
(3) Dyaster stage ; (4) Complete division.

Mitosis. — In a cell about to divide, the first change is a
general enlargement of the nucleus. At the same time the
centrosome becomes double, and the two portions travel from
one another, but remain united by delicate lines to form a
spindle-shaped structure (Fig. 3, 1). These lines may be

20 HUMAN PHYSIOLOGY

actual fibres or they may be lines of movement in the proto-
plasm. The spindle passes into the centre of the nucleus, and
seems to direct the changes in the reticulum. The nuclear
membrane disappears, and the nucleus is thus not so sharply
marked off from the cell protoplasm. The nucleoli and
nodal points also disappear, and with them all the finer
fibrils of the network, leaving only the stouter fibres, which
are now arranged either in a skein or as loops with their
closed extremity to one pole of the nucleus and their open
extremity to the other. The nucleus no longer seems to
contain a network, but appears to be filled with a con-
voluted mass of coarse fibres, and hence this stage of nuclear
division is called the convoluted stage.

The spindle continues to grow until it occupies the whole
length of the nucleus. The two centrosomes are now very
distinct, and from them a series of radiating striae extends
out into the protoplasm of the cell.

The nuclear loops of fibres break up into short, thick
pieces; and these become arranged around the equator of
the spindle in a radiating manner, so that when the nucleus
is viewed from one end it has the appearance of a rosette or
a conventional star. This stage of the process is hence often
called the single star or monaster stage (Fig. 3, 2).

Each loop now splits longitudinally into two, the divisions
lying side by side (Fig. 3, 2).

The next change consists in the separation from one
another of the two halves of the split loops — one half of each
passing up towards the one polar body, the other half passing
towards the other. It is the looped parts which first separate
and which lead the way — the open ends of the loops remain-
ing in contact for a longer period, but, finally, also separating.
In this way, around each polar body, a series of looped fibres
gets arranged in a radiating manner, so that the nucleus
now contains two rosettes or stars, and this stage of division
is hence called the dy aster stage (Fig. 3, 3).

The single nucleus is now practically double. Gradually
in each half finer fibres develop and produce the reticular
appearance. Nuclear nodes, nucleoli, and the nuclear mem-
brane appear, and thus two resting nuclei are formed from a
single nucleus. Between these two nuclei a delicate line

THE CELL 21

appears, dividing the cell in two, and the division is accom-
plished (Fig. 3, 4).

The network of the nucleus of actively dividing cells is
rich in nucleic acid, but in cells which have ceased to divide,
in which the nucleus has ceased to exercise its great repro-
ductive function, the amount of phosphorus — i.e. of nucleic
acid — diminishes, and may be actually less than the amount
in the cell protoplasm.

Amitotic Division. — In some cells the nucleus does not
appear to take an active part, the cell dividing without the
characteristic changes above discussed.

SECTION III

THE TISSUES

FROM the protoplasm of the cells the various tissues of the
body — bone, cartilage, muscle, £c., are formed. The structure
of these must be studied practically; all that will be at-
tempted here is to indicate how they are formed from the
primitive cell.

The human body is originally a single cell, and from this,
by division, a mass of simple cells is produced. In the
embryo, these cells get arranged in three layers — an outer,
a middle, and an inner — the epiblast, mesoblast, and hypo-
blast.

(A) THE YEGETATIYE TISSUES

The Vegetative Tissues are those which support, bind to-
gether, protect, and nourish the body. They may be divided
into the Epithelial Tissues, formed from the epiblast or hypo-
blast, and consisting of cells placed upon surfaces, and the
Connective Tissues developed from the mesoblast, and con-
sisting chiefly of formed material between cells.

I. EPITHELIUM

1. Squamous Epithelium—

(a) Simple Squamous Epithelium. — This is seen lining the
air vesicles of the lungs. It consists of a single layer of flat,
scale-like cells, each with a central nucleus. The outlines
of these cells are made manifest by staining with nitrate
of silver, which blackens the cement substance between the
cells.

(6) Stratified Squamous Epithelium (Fig. 4). — The skin
and the lining membrane of the mouth and gullet are covered
by several layers of cells. The deeper cells divide, and as
the young ones get pushed upwards towards the surface, and
away from the nourishing fluids of the body, the chemical

THE TISSUES 23

changes are interfered with, and the protoplasm undergoes
a change into a body closely allied in composition to the
proteids, keratin. This substance is a hard, horny material,
which is well seen in the nails and hair, and in the horns and
hoofs of certain animals. It first makes its appearance as a
number of little masses or granules in the cells, and these
run together to fill the cells, which from pressure become
flattened out into thin scales.

FIG. 4. — Stratified Squamous Epithelium from the cornea.

Keratin forms an admirable protective covering to the
body, not only on account of its hardness and toughness, but
because poisons cannot readily pass through it, and also
because it is not easily acted on by chemicals. Like the
proteids, it contains carbon, hydrogen, oxygen, nitrogen, and
sulphur; and the first four of these elements are in about
the same proportion as in the proteids. But the sulphur is
in greater proportions (3 to 5 per cent.), and readily enters
into combination with various substances. Hence, lead solu-
tions, which give the black sulphide of lead, colour keratin
black, and are largely used to dye the hair (see Chemical
Physiology, p. 6). A slightly modified stratified squamous
epithelium lines the urinary passages.

2. Columnar Epithelium (Fig. 5, a). — The inmost set of
cells in the embryo, lining the stomach and intestine, elongate

24 HUMAN PHYSIOLOGY

at right angles to their plane of attachment, and become
columnar in shape. The free border of the cells looks like
a hem, an appearance which is due to a series of short rods

Fio. 5.— (a) Columnar Epithelium from the small intestine; (b) Ciliated
Epithelium from the trachea.

placed side by side. Probably this is a special development
of the reticulum of the protoplasm. The great function of
this form of epithelium is to take up the digested matter
from the stomach and intestine, and to pass it on to the
blood.

Among these columnar cells a certain number of peculiarly

Fio. 6. — A Zymin-secreting Gland, to show duct and acinus lined with secreting
cells containing zymogen granules.

modified cells, chalice cells, are always found. They are
larger than the columnar cells, and somewhat pear-shaped,
being attached by their small extremity. Their protoplasm

THE TISSUES 25

is collected at their point of attachment, while the body of
the cell is filled with mucin, a clear, transparent material.

3. Glandular Epithelium. — A number of cells, having for
their function the production of some material which is to be
excreted from the cell, are arranged as the lining of depres-
sions, the glands.

The simplest form of gland is the simple tubular — a test-
tube-like depression, lined by secreting cells. Instead of
being simple, the tube may be branched, when the gland is
said to be racemose. In many glands the secreting epi-
thelium is confined to the deeper part of the tube, the
alveolus or acinus (Fig. 6), while the more superficial part
is lined by cells which do not secrete, forming the duct.

In many situations several simple glands are grouped
together, their ducts opening into one common duct, and
a compound gland results.

Secreting epithelium varies according to the material it
produces.

(A) Mucin-secreting Epithelium. — Many glands have for
their function the production of mucin, a slimy substance
of use in lubricating the mouth, stomach, intestine, &c.
The acini containing such cells are usually large. The cells
themselves are large, and are placed on a delicate basement
membrane, a condensation of the subjacent fibrous tissue,
which bounds the acinus. The nuclei are situated near
to the attached margin of the cells, which are somewhat
irregular in shape, and are packed close together. Their
appearance varies according to whether the gland has been
at rest or has been actively secreting.

Resting State. — In the former case, in the fresh condi-
tion, the cells are large, and pressed closely together. Their
protoplasm is filled with large shining granules. After treat-
ment with reagents, each cell becomes distended with clear,
transparent mucin, formed by the swelling and coalescence
of the granules.

After Activity. — After the gland has been actively secret-
ing, the cells are smaller and the granules are much less
numerous, being chiefly situated at the free extremity of the
cell, and leaving the nucleus much more apparent.

This form of epithelium, during the resting condition of

26 HUMAN PHYSIOLOGY

the gland, takes up nourishing matter and forms this mucin-
yielding substance. During the active state of the gland,
the mucin-yielder is changed to mucin, and is extruded from
the cells into the lumen of the gland.

Mucin is a substance of great importance in the animal
economy. When precipitated and freed from water it is
white and amorphous. On the addition of water it swells
up and forms a glairy mass. In the presence of alkalies, it
forms a more or less viscous solution, and from this solution
it is precipitated by acetic acid. In composition it is a pro-
teid linked to a molecule allied to the sugars, glucosamine
C6H11NH205, and is therefore called a glyco-proteid. When
boiled with an acid it yields sugar. (See Chemical Physiology,
p. 6.)

In adult life the great function of mucin is to give to
certain secretions a slimy character which renders them of
value as lubricants.

(B) Zymin-secreting Epithelium. — Another form of secret-
ing epithelium of great importance is that which forms the
various juices which act upon the food to digest it. These
juices owe their activity to the presence of enzymes or zymins.

A zymin-forming gland after a prolonged period of rest
shows cells closely packed together, so that it is difficult to
make out their borders. The protoplasm is loaded with
granules which are much smaller than those seen in the
mucin-forming cells, and which do not swell up in the same
way, under the action of reagents. The nucleus is often
obscured by the presence of these granules.

When the gland has been actively secreting, the granules
become fewer in number, and are confined to the free ex-
tremity of the cell; they are obviously passing out. The
cell becomes smaller, and its outlines are more distinct and
the nucleus more apparent.

The granules which fill the cells are not composed of the
active enzyme. If extracts of the living cells be made, they
are inert, and it is only after the granules have left the cell,
or are in the process of leaving, that they become active.
Hence, the granules are said to be composed of zymin-
forming substance or zymogen.

The series of changes are parallel to those described in the

THE TISSUES 27

mucin-forming cells. During the so-called resting state of
the gland, the cells are building up zymogen. When the
gland is active, the cells throw off the material they have
accumulated, and it. undergoes a change to zymin.

(C) Excreting Epithelium does not manufacture materials
of use in the animal economy, but passes substances out of
the body. Such epithelium is seen in the kidneys, sweat
glands, sebaceous glands, mammary glands, and perhaps in
the liver. The cells are composed of a granular protoplasm,
in which the presence of the material to be excreted either
in its fully elaborated condition, or in process of preparation,
may frequently be demonstrated — e.g. fat globules, iron con-
taining particles, &c. These cells do not merely take up
material from the blood and pass it out, but they may pro-
foundly alter it before getting rid of it.

4. Ciliated Epithelium (Fig. 5, p. 24). — The cells are
usually more or less columnar, and the free border is pro-
vided with a series of hair-like processes, the cilia, which
vary in size in different situations.

In the living state the cilia are in constant rhythmic
motion, each cilium being suddenly whipped or bent down
in one direction, and then again assuming the erect position.

All the cilia on a surface work harmoniously in the same
direction, and the movement passes from the cilia of one cell
to those of the next in regular order, beginning at one end
of the surface and passing to the other.

As a result of this constant harmonious rhythmic move-
ment, any matter lying upon the surface is steadily whipped
along it; and since the cilia usually work from the inner
parts of the body to the outside, this matter is finally ex-
pelled from the body.

The movements of the cilia are dependent on the changes
in the protoplasm, and everything which influences the rate
of chemical change modifies the rate of ciliary movement,
which may thus be taken as an index of the protoplasmic
activity.

II. CONNECTIVE TISSUES

1. Mucoid Tissue. — The cells of the mesoblast of the
embryo, which at first lie in close apposition with one

28

HUMAN PHYSIOLOGY

another, become separated, remaining attached by elongated
processes. Between the cells, a clear, transparent, viscous
mucin-like substance makes its appearance, forming a soft
jelly-like tissue. This tissue is widely distributed in the
embryo as a precursor of the connective tissues, and after
birth it is still to be seen in the pulp of a developing tooth
and in the vitreous humour of the eye (Fig. 7).

FlO. 7.— Mucoid Tissue from an embryo rabbit.

2. Fibrous Tissue. — As development advances, the cells of

mucoid tissue elongate
and become spindle-
shaped, and are continued
at their ends into fibres
(Fig. 8). These cells are
often called fibroblaats.
The connective tissues are
thus clearly distinguished
from the epithelia byhav-

Fio.S.-Fibroblasts from young fibrous - the forme(i material

tissue. ,

between and not in the

cells. They are composed of the following parts : —
I. Formed material.
(a) Fibres.
(6) Matrix.

II. Spaces (Connective Tissue Spaces).
III. Cells.

THE TISSUES

29

I. Formed Material. — (A) Fibres (Fig. 9)— 1st, Non-
elastic (white fibres). These are delicate, transparent fibrils
arranged in bundles, which do not branch, and which have
a mucin-like matrix between them. They are composed of
a non-elastic substance, collagen. This is closely allied to
the proteids, and gives the proteid reactions faintly, but it
does not yield tyrosin when decomposed, while it does yield
amido-acetic acid. It is insoluble in cold water, but swells
up and becomes transparent in acetic acid. It has a great
affinity for carmine, and stains a pink colour with it. When
boiled, it takes up water to form a hydrate, Gelatin ; a sub-

FIG. 9.— Bundles of White Fibres, with Fibroblasts (a) and Elastic Fibres (6)
anastomosing with one another.

stance soluble in hot water, and forming a jelly on cooling.
(See Chemical Physiology, p. 7.)

2nd, Elastic Fibres. These are highly refractile elastic
fibres, which branch and anastomose with one another.
They are composed of Elastin, a near ally of the proteids,
which is insoluble both in cold and in hot water, and is not
acted on by acetic acid. It stains yellow with picric acid,
and has no affinity for carmine.

(B) Interstitial Substance. — This is composed of mucus-
like material.

Types of Fibrous Tissue. — According to the arrangement
of these fibres, and to the preponderance of one or other
variety, various types of fibrous tissue are produced.

3o HUMAN PHYSIOLOGY

When a padding is required, as under the skin and under
mucous membranes, the fibres are arranged in a loose felt
work to constitute Areolar Tissue.

In fascia, in tendon sheaths, and in flat tendons, the fibres
are closely packed together to form more or less definite
layers. In tendons and ligaments the fibres run parallel
and close together. In ordinary tendons, where no elasticity
is required, the fibres are of the white or non-elastic variety.
In ligaments, where elasticity is desirable, the elastic fibres
preponderate.

Lymph Tissue. — One peculiar modification of fibrous tissue
is often described as a special tissue under the name of Lymph
Tissue. It is composed of a delicate network of white fibres,
the interstices of which communicate with lymphatic vessels,
and contain masses of simple protoplasmic cells, leucocytes,
often in a state of active division. So numerous are these
that it is impossible to make out the network under the
microscope, until they have been removed by washing.

Lymph tissue is very widely distributed throughout the
body, and is of great importance in connection with nutrition.

II. The spaces of fibrous tissue vary with the arrangement
of the fibres. In the loose areolar tissue under the skin
they are very large and irregular, in fascia they are flattened,
while in tendon, where the fibres are in parallel bundles, they
are long channels.

III. The cells of fibrous tissue (Fibroblasts) vary greatly
in shape. In the young tissue they are elongated spindles,
from the ends of which the fibres extend. In some of the
loose fibrous tissues they retain this shape, but in the denser
tissues they get squeezed upon, and are apt to be flattened
and to develop processes thrust out into the spaces.

In certain situations, peculiar modifications of Connective
Tissue cells are to be found —

(A) Endothelium. — When cells line the larger connective
tissue spaces they become flattened, and form a covering
membrane, called an endothelium. Such a layer lines all
the serous cavities of the body, and the lymphatics, blood
vessels, and heart, which are all primarily large connective
tissue spaces. To demonstrate the outlines of these cells it
is necessary to stain with nitrate of silver, which has a special

THE TISSUES 31

affinity for the interstitial substance, and which thus forms
a series of black lines between the cells.

(B) Fat Cells. — In the areolar tissue of many parts of the
body fat makes its appearance in the cells round the smaller
blood vessels, and when these cells occur in masses Adipose
Tissue is produced.

Little droplets of oil first appear, and these become larger,
run together, and finally form a large single globule, distend-
ing the cell, and pushing to the sides the protoplasm and
nucleus as a sort of capsule (Fig. 10).

FIG. 10. — Fat Cells stained with osmic acid, and lying alongside
a small blood vessel.

If the animal be starved, the fat gradually disappears out

of the cell, and in its place is left a clear albuminous fluid

which also disappears, and the cell resumes its former shape.

The fats are ether derivatives usually of the triatomic

alcohol glycerin —

(OH

C3H5 OH
(OH

by the replacement of the hydrogen of the hydroxyl molecules
by the radicles of the fatty acids.

The most abundant fatty acids of the body are : —

Palmitic Acid, C16H31O,OH

Stearic Acid, C18H350,OH

Oleic Acid, C18H330,OH

32 HUMAN PHYSIOLOGY

and from these the three fats —

Palmitin C3H5(0,C18H3I0)3 - C51H9806

Stearin

Olein

are produced.

It will be observed that the molecules of these fats are
very rich in carbon and hydrogen, and very poor in oxygen
— i.e. they contain a large amount of material capable of
being oxidised, and thus capable of affording energy in the
process of combustion.

The fats resemble one another in being insoluble in water,
but soluble in ether and in hot alcohol. As the alcohol
cools, they separate out as crystals. They differ from one
another in their melting point, palmitin melting at the
highest and olein at the lowest temperature. Fat which is
rich in palmitin and stearin, like ox fat, is thus hard and
solid at the ordinary temperature of the air, while fats rich
in olein, like dogs' fats, are semi-fluid at the same tempera-
ture. The olein acts as a solvent for the fats of a higher
melting point. (For tests, see Chemical Physiology, p. 12.)

The functions of adipose tissue are twofold : —

1st, Mechanical. — The mass of adipose tissue under the
skin is of importance in protecting the deeper structures
from injury. It is a cushion on which external violence
expends itself. Further, this layer of subcutaneous fat pre-
vents the loss of heat from the body, being, in fact, an extra
garment.

2nd, Chemical. — Fat, on account of its great quantity of
unoxidised carbon and hydrogen, is the great storehouse of
energy in the body (p. 379).

(C) Pigment Cells. — In various parts of the eye the
connective tissue and other cells contain a black pigment —
Melanin. The precise chemistry and mode of origin of this
pigment is not known. It contains carbon, hydrogen, nitro-
gen, oxygen, and it may also contain iron. It may be formed
directly in the cells, or it may be produced by the cells from
the pigment of the blood. Its function in the eye is to
prevent the passage of light through the tissues in which it
is contained.

THE TISSUES

33

The cells containing the pigment are branched, and in
many cases they possess the power of movement. This is
specially well seen in such cells in the skin of the frog, where
contraction and expansion may be easily studied under the
microscope. By these movements the skin, as a whole, is
made lighter or darker in colour. The movements of these
cells are under the control of the central nervous system.

3. Cartilage. — While fibrous tissue is the great binding
medium of the body, support is afforded in foetal life and in
certain situations in adult life by cartilage.

When cartilage is to be formed, the embryonic cells be-
come more or less oval, and secrete around them a clear
pellucid capsule. This may become hard, and persist
through life as in the so-called parenchymatous cartilage
of the mouse's ear.

(1) Hyaline Cartilage. — Development, however, usually
goes further, and before the capsule has hardened, the carti-
lage cells again divide, and each
half forms a new capsule which
expands the original capsule of
the mother cell, and thus in-
creases the amount of the
formed material. This formed
material has a homogeneous,
translucent appearance, and a
tough and elastic consistence,
and cuts like cheese with the
knife (Fig. 11).

The formed material of car-
tilage is not a special substance,
but a mixture of chondroitin-
sulphuric acid with collagen in
combination with proteids. Chondroitin when decomposed
yields glucosamine, a sugar-like substance containing nitrogen,
and glycuronic acid, another substance closely related to the
sugars.

Cartilage is surrounded by a fibrous membrane, the peri-
chondrium, and no hard and fast line of demarcation can
be made out between them. The fibrous tissue gradually
becomes less fibrillated — the cells become less elongated and

3

FIG. 11. — Hyaline Cartilage covered
by perichondrium.

34 HUMAN PHYSIOLOGY

more oval, as if the interfibrillar substance increased in
amount and became of the same refractive index as the
fibres. During old age, a fibrillation of the homogeneous-
looking cartilage is brought out, especially in costal cartilage,
by the deposition of lime salts in the matrix, between the
fibres. It was long ago shown that in inflammation of
cartilage this fibrillation appears ; and by digesting in baryta
water, a similar structure may be brought out. The close
connection of cartilage with fibrous tissue is thus clearly
demonstrated.

Such homogeneous or hyaline cartilage precedes most of
the bones in the embryo, and covers the ends of the long
bones in the adult (articular cartilage), forms the frame-
work of the larynx and trachea, and constitutes the costal
cartilages.

(2) Elastic Fibro-Cartilage. — In certain situations, a
specially elastic form of cartilage is developed — e.g. in the
external ear, elastic fibres appearing in the cartilaginous
matrix, and forming a network through it.

(3) White Fibro-Cartilage. — In other situations — e.g. the
intervertebral discs — a combination of the binding action of
fibrous tissue with the padding action of cartilage is required ;
and here strands of white fibrous tissue with little islands of
hyaline cartilage are found. It is also found when white
fibrous tissue, as tendon, is inserted into hyaline cartilage,
and is really a mixture of two tissues — white fibrous tissue
and cartilage.

4. Bone. — The great supporting tissue of the adult is BONE.

(1) DEVELOPMENT AND STRUCTURE. — Bone is formed by a
deposition of lime salts in layers or lamellae of white fibrous
tissue ; but while some bones, as those of the cranial vault,
face, and clavicle, are produced entirely in fibrous tissue,
others are performed in cartilage, which acts as a scaffolding
upon which the formation of bone goes on.

Intra-membranous Bone Development. — This may be well
studied in any of the bones of the cranial vault where
cartilage is absent (Fig. 12).

At the centre of ossification the matrix between the fibres
becomes impregnated with lime salts, chiefly the phosphate
and carbonate. How this deposition takes place is not

THE TISSUES

35

known ; and how far it is dependent on the action of cells
has not been clearly determined. As a result of this, the
connective tissue cells get enclosed in definite spaces,
lacunae, and become bone corpuscles. Narrow branching
channels of communication
are left between theselacunae,
the canaliculi. This deposi-
tion of lime salts spreads out
irregularly from the centre

into the adjacent fibrous **%*

tissue. The fully formed
adult bone, however, is not
a solid block, but is com-
posed of a compact tissue
outside, and of a spongy bony
tissue, cancellous tissue, in-
side. This cancellous tissue
is formed as a secondary
process. Into the block of ^

calcareous matter, formed HHHI
as above described, processes
of the fibrous tissue, with
blood vessels, lymphatics,
and numerous cells, burrow.
This burrowing process
seems to be carried on by
the connective tissue cells

which eat up the bony matter formed. In doing this they
frequently change their appearance, becoming large and
multi-nucleated. Thus the centre of the bone is eaten out
into a series of channels, in which the marrow of the bone is
lodged, and between which narrow bridges of bone remain.

It is by the extension of the calcifying process outwards,
and the burrowing out of the central part of the bone, that
the diploe and cancellous tissue are produced.

Intra-cartilaginous Bone Development. — In the bones
preformed in cartilage, the process is somewhat more com-
plex, although all the bone is formed in connection with
fibrous tissue, the cartilage merely playing the part of a
scaffolding and being all removed. Where the adult bone

FIG. 12. — Intra - membranous Bone
Development in the lower jaw of a
foetal cat. Above, the process of
ossification is seen shooting out
along the fibres, and on the lower
surface the process of absorption is
going on. Three osteoclasts — large
multi-nucleated cells — are shown.

36 HUMAN PHYSIOLOGY

is to be produced, a minute model is formed in hyaline car-
tilage in the embryo, and this is surrounded by a fibrous
covering, the perichondrium. In the deepest layers of this
perichondrium the process of calcification takes place as
described above, and spreads outwards, thus encasing the
cartilage in an ever thickening layer of bone (Fig. 13). This
was demonstrated by inserting a silver plate under the peri-
osteum, and showing that bone was deposited outside of it.

Flo. 13. — Intra-cartilaginous Bone Development. A phalanx of a foetal finger
showing the formation of periosteal bone round the shaft ; the opening up
of the cartilage at the centre of ossification ; the vascularisation of the
cartilage by the invasion of periosteum ; and the calcification of the carti-
lage round the .spaces.

At the same time, in the centre of the cartilage, at what
is called the centre of ossification, the cells begin to divide
actively, and, instead of forming new cartilage, eat away
their capsules, and thus open out the cartilage spaces
(Fig. 13). Into these spaces processes of the perichondrium
bore their way, carrying with them blood vessels, and thus
rendering the cartilage vascular (Fig. 13). The vas-
cularisation of the centre of the cartilage having been
effected, the process of absorption extends towards the
two ends of the shaft of cartilage, which continues to
elongate. The cartilage cells divide and again divide, and,
by absorbing the material between them, form long irregular
canals running in the long axis of the bone, with trabeculse

THE TISSUES

37

of cartilage between them. Into these canals the processes
of the periosteum extend, and fill them with its fibrous
tissue. In this fibrous tissue, the deposition of lime salts
takes place upon the trabeculae, enclosing cells, and thus
forming a crust of bone, while the cartilage also becomes
calcified. If this calcification of the cartilage and deposition
of bone were to go on unchecked, the block of cartilage
would soon be reduced to a solid mass of calcified tissue.
But this does not occur. For, as rapidly as the trabeculse
become calcified, they are
absorbed, while the active
changes extend farther and
farther from the centre, which
is thus reduced to a space
filled by fibrous tissue which
afterwards becomes the bone
marrow.

The process of absorption
does not stop at the original
block of cartilage, which is
all removed. After all of this
has been absorbed, the bone
formed round the cartilage (the
periosteal bone) is attacked by
burrowing processes from in-
side and outside, which hollow
out long channels running in
the long axis of the bone.
These are -the Haversian
spaces (Fig. 14). Round the
inside of these, calcification

occurs, spreading inwards in layers, and enclosing connective
tissue cells, until, at length, only a small canal is left, an
Haversian canal, containing some connective tissue, blood
vessels, lymphatics, and nerves, with layer upon layer of
bone concentrically arranged around it. This constitutes an
Haversian system. In this way the characteristic appearance
of the shaft of a long bone is produced (Fig. 14), with layers
of calcified fibrous tissue, the bone lamellae arranged as shown
as Haversian, interstitial, peripheral, and medullary lamellae.

FIG. 14. — Cross Section through part
of the shaft of an adult long bone to
show the arrangement in lamellae
distributed as Haversian (1), inter-
stitial (2), peripheral (3), and medul-
lary (4).

38 HUMAN PHYSIOLOGY

One important function performed by the cartilage is in
bringing about the increase in length of the bones. In
addition to the centre of ossification in the shaft, at each
end of the bone one or more similar centres of ossification
form. These are the epiphyses. Between these and the
central rod of bone — the diaphysis — a zone of cartilage
exists until near adult life, when the bones stop growing.
In this zone, the cells arrange themselves in vertical rows,
divide at right angle to the long axis of the bone, and form
cartilage. This cartilage as it is formed is attacked by the
bone-forming changes at the diaphysis and epiphyses, but
the amount of new cartilage formed is proportionate to this,
and thus a zone of growing cartilage continues to exist until
early adult life, when epiphyses and diaphysis join and
growth in length is stopped. The rate and extent of this
growth of the cartilage has an important influence on the
growth of the individual.

(2) CHEMISTRY. — The composition of adult bone is roughly
as follows : —

Water, 10 per cent.
Solids, 90 per cent.

Organic, 35 per cent. — chiefly collagen.
Inorganic, 65 per cent.
Calcium phosphate, 51.
„ carbonate, 11.

fluoride, 0*2.

Magnesium phosphate, 1.
Sodium salts, 1.

The points to be remembered are the small amount of
water, the large amount of inorganic matter, chiefly calcic
phosphate, and the nature of the organic matter — collagen.

(B) THE MASTER TISSUES OF THE BODY,
MUSCLE AND NERYE.

By means of the epithelial and connective tissues the
body is protected, supported, and nourished. It performs
purely vegetative functions, but it is not brought into
relationship with its environments. By the development

THE TISSUES 39

of nerve and muscle the surroundings are able to act upon
the body, and the body can react upon its surroundings.

These tissues may thus be called the Master Tissues,
and it is as the servants of these tissues that all the others
functionate.

So far as the chemical changes in the body are concerned,
muscle is more important than nerve, for three reasons —
First, it is far more bulky, making up something like 42
per cent, of the total weight of the body in man ; second,
it is constantly active, for even in sleep the muscles of
respiration, circulation, and digestion do not rest ; and third,
the changes going on in it are very extensive, since its great
function is to set free energy from the food. So far as the
metabolism of the body is concerned, muscle is the master
tissue. For muscle we take food and breath, and to get
rid of the waste of muscle the organs of excretion act.
Hence it is in connection with muscle that all the problems
of nutrition — digestion, respiration, circulation, and excretion
— -have to be studied.

I. MUSCLE.

The two great functions of muscle are —

To perform mechanical work.

To liberate heat.

The study of the physiology of muscle may be divided
into —

1. The structure, chemical and physical characters of
muscle at rest.

2. The methods of making muscle contract.

3. The changes which take place in muscle during con-
traction.

4. The chemical change in muscle and the source of the
energy evolved.

5. Death of muscle.

1. STRUCTURE, CHEMICAL AND PHYSICAL CHARACTERS OF
MUSCLE AT REST.

The first trace of the evolution of muscle is found among
the infusoria, where, in certain cells, in parts of the proto-
plasm, the network or cytomitoma is arranged in long parallel

40 HUMAN PHYSIOLOGY

threads in the direction of which the cell contracts and
expands. Such a development has been termed a myoid.

i. Structure of Muscle.

Even a cursory examination of mammalian muscles shows
that those of the trunk and limbs, skeletal muscles, are
different from those of such internal organs as the bladder,
uterus and alimentary canal, visceral muscles.

The visceral muscles appear to be formed from cells
similar to ordinary connective tissue cells. These elongate,
acquire a covering, and their protoplasm becomes definitely
longitudinally fibrillated by the arrangement of the cytomi-
torna. They thus become spindle-shaped cells, varying

FIG. 15.— (a) Fibres of Visceral Muscle ; (b) Fibres of Skeletal Muscle
to show sarcolemma, muscle corpuscles, and sarcous substance
composed of fibrils showing transverse markings.

in length from about 50 to 200 micro-millimetres. The
covering membrane, sarcolemma, is thin, but tough and
elastic, and adapts itself to the surface of the cell, unless
when this is excessively shortened, in which case the
sarcolemma may be thrown into folds, which give the cell
the appearance of cross-striping. The nucleus is usually
long, almost rod-shaped, and is independent of the cytomi-
toma (Fig. 15, a).

The skeletal muscles develop from a special set of cells,
early differentiated as the muscle plates in the mesoblast
down each side of the vertebral column of the embryo. Each
cell elongates. The nucleus divides across, but the cell,
instead of also dividing, lengthens and continues to elongate
as the two daughter nuclei again divide. The cytomitoma
becomes arranged longitudinally, and a series of transverse

THE TISSUES 41

markings appear across the cell. Lastly, a covering develops,
and the fully-formed fibre is produced (Fig. 15, 6).
This consists of three parts —

1. The Sarcolemma is a delicate, tough, elastic membrane
closely investing the fibre, and attached to it at Dobie's
lines.

2. The Muscle corpuscles consist of little masses of
protoplasm with a nucleus, which lie just under the sarco-
lemma.

3. The Sarcous substance is made up of a series of
longitudinal fibrils consisting of alternate dim and clear bands
— the former staining deeply with eosin. In the middle
of the clear band is a narrow dim line, Dobie's line. The
fibres and fibrils tend to break across in the region of the
clear band, showing that they are weakest at that part. The
clear band differs from the dim band, not only in not taking
up eosin, but also in the fact that it entirely prevents the
passage of polarised light except in one position of the
analysing prism, while the dim band allows polarised light to
pass, whatever be the position of the prisms.

Two explanations of these facts have been suggested:
(1) That the sarcous substance is made up, like other proto-
plasm, of a mitoma and plasma ; that the mitoma is arranged
in a series of longitudinal fibres, which are broader and
stronger in the dim band, and lie closely applied to one
another, side by side, while in the clear band they are
thinner, and are separated from one another by plasma ; and
that at Dobie's line there is a swelling on each fibril ; (2)
thai each fibril is a hollow tube consisting of a sponge-work
in the dim band with a fluid part in the clear band inter-
sected by a septum at Dobie's line.

2. Chemistry of Muscle.

Like all other living tissues, muscle is largely composed of
water. It contains about 75 per cent. The 25 per cent,
of solid constituents is made up of a small quantity, about 3
per cent., of ash, and 21 per cent, of organic substances.
The ash consists chiefly of potassium and phosphoric acid,
with small amount of hydrochloric acid and sodium, mag-

42 HUMAN PHYSIOLOGY

nesium, calcium, and iron. But a part of the phosphoric
acid is derived from the phosphorus of the nuclei of
muscle, and probably from phosphocarnic acid.

1. Proteids. — Of the organic constituents, by far the
greater part is made up of Proteids. These may be divided
into —

(a) Those soluble in neutral salt solutions.
(6) Those insoluble in them.

(a) The first class of bodies consist entirely of three
globulins. Two of these — Myosinogen and Paramyosinogen
—have the peculiar property of clotting under certain condi-
tions, of forming what is called Myosin, and this process,
which occurs after death, is the cause of death stiffen-
ing. The post-mortem change is supposed to be brought
about by the development of an enzyme, since a glycerine
extract of dried muscle rapidly causes the formation of
myosin. The third globulin, Myoglobulin, does not undergo
this change. These three proteids are contained in the
plasma — the juice which can be expressed from muscles kept
near the freezing point. If the plasma is warmed it rapidly
forms a jelly, clots just as it does post mortem.

(b) The insoluble proteid of muscle, Myostromin, seems
to be in the nature of a nuclein, and probably forms the
framework of the fibres. It is always mixed with the
collagen of the fibrous tissue of muscle, and it may be
separated by dissolving in carbonate of soda solution. (See
Chemical Physiology, p. 6.)

Collagen of the fibrous tissue holding the muscle fibres
together is also present, and yields gelatin.

In addition to the proteids, small quantities of other
organic substances are found in muscle.

2. Carbohydrates. — Glucose (C6H1206) is present in muscle,
as in all other tissues.

Glycogen #(C5H10O5) — a substance closely allied to ordi-
nary starch, but giving a brown reaction with iodine — is
always present in muscle at rest. If the muscle has been
active, the amount of glycogen diminishes, being probably
converted to glucose, and used for the nourishment of the
tissue. (For the chemistry of these, see p. 316.)

THE TISSUES 43

3. Fat is present in small quantities.

4. Inosite, formerly called muscle sugar, is present in small
amounts. It is not a sugar, but a benzene compound.

5. Sarcolactic Acid. — Hydroxy-propionic acid —

H OH 0
H— C— C— C— 0— H

This isomer of ordinary lactic acid is increased in muscle
during activity and during death stiffening.

6. Extractives. — If dried muscle is treated with alcohol a
series of bodies containing nitrogen may be extracted. The
chief of these is Creatin, or methyl-guanidin-acetic acid.
Guanidin C.NH(NH2)2 is a near ally of urea CO(NH2)2
being formed by replacing the 0 with NH.

Methyl-guanidin is produced by replacing an H in gua-
nidin by CH3 —

NH CH,

I! I '
H2N— C — N— H

If this is linked to acetic acid —

H— N

H2N— C— N-
Methyl-guanidin

H 0

-LL

OH

acetic acid

is produced.

7. The Colour of Muscle varies considerably, some muscles
being very pale, almost white in colour — e.g. the breast
muscles of the fowl ; others again being distinctly red, even
after all the blood has been removed. This red colour is, in
some cases, due to the presence of the pigment of blood,
hcemoglobin, but in certain muscles it is due to a peculiar
set of pigments, Myohaematins, giving different reactions from
the blood pigment.

44 HUMAN PHYSIOLOGY

3. Physical Characters of Muscle.

1. Muscle is translucent during life, but, as death stiffen-
ing sets in, it becomes more opaque.

2. Muscle is markedly extensile and elastic. A small force
is sufficient to change its shape, but when the distorting force
is removed it returns completely to its original shape, pro-
vided always that the distortion has not overstepped the
limits of elasticity.

When a distorting force is suddenly applied to muscle —
e.g. if a weight is suddenly attached — the distortion takes
place at first rapidly, and then more slowly, till the full effect
is produced. If now the distorting force is removed the
elasticity of the muscle brings it back to its original form,
at first rapidly, and then more slowly. (Practical Physi-
ology, Chap. VI.)

The advantages of these properties of muscle are, that
every muscle in almost all positions of the parts of the body
is stretched between its point of origin and insertion. When
it contracts it can therefore at once act to bring about the
desired movement, and no time is lost in preliminary tighten-
ing. Again, the force of contraction, acting through such an
elastic medium, causes the movement to take place more
smoothly, and without jerks. Experimentally, too, it has
been ascertained that a force acting through such an elastic
medium produces more work than when it acts through a
rigid medium.

The extensibility of muscle is of value in allowing a group
of muscles to act without being strongly opposed by their
antagonistic group. For instance, suppose the extensor
muscles of the arm were not readily extensile, when the
flexors acted, a large amount of their energy would have to
be employed in elongating the extensors. Similarly the
elasticity of the muscles tends to bring the parts back to
their normal position when the muscles have ceased to
contract. It must not, however, be imagined that, in all
movements of one set of muscles, the antagonistic muscles
are relaxed, although they may be elongated. Often they
are in a state of activity so as to guide the movements being
produced (see p. 60).

THE TISSUES 45

Tonus of Muscle. — The tense condition of resting
between its points of origin and insertion is not merely due
to passive elasticity, but is in part caused by a continuous
contraction, kepj^up by the action of the nervous system. Jf^
the nerve to a group of muscles be cut, the muscles become
soft aridHabby, and lose their tense feeling.

3. Heat Production. — Muscles, like all other living proto-
plasm, is in a state of continued chemical change, constantly
undergoing decomposition and reconstruction. • As a result
of this chemical change, heat is evolved. This is a matter
of considerable importance, as it explains how, even when the
muscles are at rest, the body is still kept at a comparatively
high temperature.

4. Electrical Conditions. — Muscle when at rest is iso-
electric, but if one part is injured, it acts to the rest like the
zinc plate in a galvanic battery — becomes electro-positive ; and
hence if a wire passes from the injured to the uninjured part
round a galvanometer, a current is found to flow along the
wire from the uninjured to the injured part just as, when the
zinc and copper plates in a galvanic cell are connected, a cur-
rent flows through the wire from copper to zinc. This is the
Current of Injury (p. 65). (Practical Physiology, Chap.
VIII.)

2. METHODS OF MAKING MUSCLE CONTRACT.

Skeletal muscle remains at rest indefinitely until stimu-
lated to contract, usually by changes in the nerves. We
desire to contract our biceps : certain changes occur in our
brain, these set up changes in the nerves passing to the
biceps and the muscle contracts.

Can skeletal muscle be made to contract without the
intervention of nerves — can it be directly stimulated ?

To answer this, some means of throwing the nerves out of
action must be had recourse to. If curare, a South American
arrow poison, be injected into an animal — e.g. into a frog, the
brain of which has been destroyed — it soon loses the power
of moving. When the nerve to a muscle is stimulated, the
muscle no longer' contracts. But if th^ TmiPfi1ft ^Q ^ir^tly
stimulated by any of the various agents to be__afterwards
m entidneH, it at once, contracts. ~~

46 HUMAN PHYSIOLOGY

It might be urged that the curare leaves unpoisoned the
endings of the nerve in the muscle, and that it is by the
stimulation of these that the muscle is made to contract.
But that these are poisoned is shown by the fact that if the
artery to the leg be tied just as it enters the muscle, so that
the poison acts upon the whole length of the nerve except
the nerve endings in the muscle, stimulation of the nerve
still causes muscular contraction. Only when the curare is
allowed to act upon the muscle and the nerve endings in the
muscle^ does stimulation of tEe~nerve fail to produce any re-
action in the muscle, while direct stimulation of the muscle

causes it to contract. This
clearly shows that it is the
nerve endings which are poi-
soned by _jzygdre, and that
therefore the application of
stimuli to the muscle must

FIG. 16. — Curare Experiment to show , j. ,1 x, ,

sciatic nerves exposed to curare, ^Ct directly Upon the muscular

but nerve endings protected on the fibres (Fig. 16). (Practical

left side; while on the right side p^flfc™ Chap< IV<)
the curare is allowed to reach the y &*'

nerve endings in the muscle. Muscle, however, is more

readily stimulated through its

nerves, and a knowledge of the points of entrance of the
nerves into muscles, the motor points, is of importance in
medicine in indicating the best points at which to apply
electrical stimulation.

Various means may be used to make the muscle contract.

1st. Various chemical substances when applied to a muscle
make it contract before killing it, while others kill it at once.
Among the former may be mentioned dilute mineral acids
and metallic salts. (Practical Physiology, Chap. II.)

2nd. A sudden mechanical change such as may be pro-
duced by pinching, tearing, or striking the muscle will cause
it to contract. (Practical Physiology, Chap. II.)

3rd. Any sudden change of temperature, either heating or
cooling, stimulates muscle. A slow change of temperature
has little or no effect. Every muscle, however, passes into a
state of contraction — heat stiffening when a sufficiently high
temperature to coagulate its proteid constituents is reached.

THE TISSUES

47

This, however, is not a true living contraction. (Practical
Physiology, Chap. III.)

4th. Muscle may also be made to contract by any sudden
change in an electric current passed through it, whether the
current be suddenly allowed to pass into it or suddenly cut
out of it, or whether it is suddenly made stronger or weaker.
(Practical Physiology, Chap. II.)

This method of stimulating muscle is constantly used in
medicine. It is a matter of no importance how the elec-
tricity is procured, but most usually it is obtained either —

1st. Directly from a galvanic battery ; or

2nd. From an induction coil.

FIG. 17. — Rheonome, consisting of a circular trough filled with sulphate of
zinc solution, into which dip the arras of a bridge which can be brought
rapidly or slowly into the proximity of the wires of a galvanic circuit.

If a galvanic battery is used — (1) On making (closing) the
current, and upon breaking (opening) the current, a contrac-
tion results. While the current is flowing through the muscle,
the muscle usually remains at rest ; but if the current is
suddenly increased in strength or suddenly diminished in
strength, the muscle at once contracts. With strong cur-
rents a sustained contraction — galvanotonus — may persist
while the current flows. (Practical Physiology, Chap. II.)

It is the suddenness in the variation of the strength of the
current rather than its absolute strength which is the factor
in stimulating, as may be shown by inserting some form of
rheonome into the circuit by which the current may be either
slowly or rapidly varied (Fig. 17).

48 HUMAN PHYSIOLOGY

(2) If a current be made weaker and weaker, breaking
ceases to cause a contraction, while making still produces it.
That is, it requires a stronger current to cause a contraction
on breaking than on making.

(3) The two poles do not produce the same effect. The
negative pole or cathode — that coming from the zinc plate
of the battery — causes contraction of the muscle on closing ;
while the positive pole or anode causes contraction at opening.
This may be summarised as follows : —

1. Contraction on closing ; contraction on opening.

2. Closing contraction stronger than opening contraction.

3. Contraction at cathode on closing, at anode on opening.
(Practical Physiology, Chap. VII.)

1. C.C • C.O

2. CC > CO

3. CCC CAO
How can these facts be explained ?

Electrotonus.

A study of the influence of the current on the muscle
while it is passing through it throws important light on this
point. (Practical Physiology, Chap. VII.)

While the current simply flows through the muscle no
contraction is produced, but the excitability is profoundly
modified.

Round the cathode it becomes more easily stimulated,
while round the anode or positive pole it becomes less
easily stimulated. This may be expressed by saying that
the part of the muscle under the influence of the cathode is
in a state of cathelectrotonus, of increased excitability or of
more unstable equilibrium, while the part of the muscle
under the influence of the anode is in a state of anelectro-
tonus, of decreased excitability or of more stable equilibrium.
Now it is well known that any sudden disturbance of the
equilibrium or balance of a series of bodies is apt to cause
them to fall asunder. If, for instance, from a house of cards,
one card is suddenly drawn out, the whole structure passes
into a condition of unstable equilibrium, and is apt to fall
to pieces. So with a muscle, if it is suddenly made unstable
as at the cathode on closing, disintegration or katabolic

THE TISSUES

49

changes are apt to be set up and a contraction results. If,
on the other hand, a house of cards is built and made extra
stable by introducing some additional cards at the founda-
tion, if these cards are suddenly withdrawn the chances
are that the house falls to bits. So with a muscle. When
the current is opened the removal of the state of increased
stability at the positive pole may cause disintegration and
produce the anodal opening contraction.

The study of electrotonus thus explains why any sudden
change in the flow of electricity through a muscle stimulates
it. It further explains why the stimulation and contraction
start from the cathode on closing and from the anode on
opening; and why the closing contraction is stronger than
the opening, since the sudden pro-
duction of a condition of actual in-
stability must act more powerfully
than the simple sudden removal of
a condition of increased stability.

This law of Polar Excitation, while
it applies to muscle and nerve, does
not apply to all protoplasm. Thus
amoeba shows contraction at the
anode and expansion at the cathode
when a galvanic current is passed
through it.

When muscle is stimulated by
induced electricity (Fig. 21) the ques-
tion is much easier, for, with each make
and break or each sudden alteration
in the strength of the primary circuit,
there is a sudden appearance and
equally sudden disappearance of a
flow of electricity in the secondary
coil. If, therefore, wires from the
secondary coil are led off to a muscle, each change in the
primary circuit causes the sudden and practically simul-
taneous appearance and disappearance of an electric current
in the muscle, and this of course causes a contraction.
But here the effects of closing and opening the current are

4

FIG. 18.— Course of Electric
Current in primary circuit
(lower line), and in secon-
dary circuit (upper line) of
an induction coil. Observe
that in the secondary the
make (upstroke) and break
(downstroke) are com-
bined, and that a stronger
current is developed in
the secondary circuit upon
breaking than upon mak-
ing the primary circuit.

50 HUMAN PHYSIOLOGY

practically fused, and hence the influence of the anode and
cathode, or of closing and opening, need not be considered.
(Practical Physiology, Chap. II.)

It must, of course, be remembered, that the opening of
the primary circuit produces a more powerful current in the
secondary coil, and therefore a more powerful stimulation of
the muscle than the closure of the primary circuit (Fig. 18).

When the galvanic current is used to stimulate muscles
through the skin in man or in other living animals, the
different action of the two poles is not so marked as in the

Cathode Anode

Skin

Muscle

FlO. 19. — Electrical Stimulation of human muscle or nerve to show the passage of
the current across the structure, and the consequent combination of effects
under each pole.

excised muscle of the frog, because the current, passing
through the skin above the muscle to enter the body, flows
not along but rather across the muscle, and thus, under each
pole applied to the skin there is on one side of the muscle
the effect of an entering current — anode — and on the other,
of a leaving current— -cathode (Fig. 19). Thus the same bit
of muscle or nerve is subjected to anelectrotonus on one
side and cathelectrotonus on the other, and the effects of
closing and of opening therefore tend to be combined.
Hence with a strong current contraction occurs both on
closing and on opening at both poles, but as the current
is weakened first the contraction at the cathode on opening,
next at the anode on opening, then at the anode on closing,

THE TISSUES 51

and, finally, at the cathode on closing, disappear. When
the muscle is in one stage of the degeneration which follows
separation from its nerve, the anodal closing contraction
(Fig. 19, Anode M) becomes much exaggerated. This is
called the reaction of degeneration.

3. THE CHANGES IN MUSCLE DURING CONTRACTION.
I. Change in Shape.

The most manifest change is an alteration in the shape
of the muscle. It becomes shorter and thicker. This
any one can see by studying their own biceps muscle.
Contraction of muscle, however, is not a necessary result
of excitation. Thus a part of a
muscle when dipped in water may
fail to contract when stimulated, but
may manifest its excitation by caus-
ing the part of the muscle not in the
water to contract — by conducting.

In skeletal muscle the shortening
and thickening of the muscle as a
whole is due to the shortening and
thickening of the individual fibres
and their fibrils.

In these fibrils the shortening and
thickening is most marked in the
dim band. The clear band also
shortens, and at the same time it
becomes darker till, in the fully con-
tracted muscle, it may be as dark as
the dim band.

These appearances may best be explained on the assump-
tion that the fibrils are the part of the fibre which shorten and
thicken, possibly by a flow of fluid into them, and that these

fibrils chiefly shorten where they are thickest in the dim

band. At the same time by the contraction of the fibrils in
the clear band, adjacent dim bands may be supposed to be
pulled nearer to one another, and to cast a shadow over the
clear band. That no actual chemical change takes place in

FIG. 20. — Contraction of
Skeletal Muscle — relaxed
above, contracted below.
A is a diagram of the
change in a fibril ; B shows
the shading of the clear
band ; and C shows the
absence of any alteration
in the influence of the two
bands on polarised light.

52 HUMAN PHYSIOLOGY

the clear band seems to be indicated by the facts that it
retains its reactions to polarised light and staining reagents.

"Usually the contraction of a muscle occurs simultaneously
in all the fibres. This is because a nerve fibre passes to every
muscular fibre, and sets them all in action together. When,
as sometimes occurs in disease, the nerve fibres become
implicated, the muscular fibres may not all act at once, and
a peculiar fibrillar twitching of the muscle may be produced.
] If the muscle be directly stimulated at any point, the
contraction starts from that point and passes as a wave of
'contraction outwards along the fibres. This may be seen by
sharply percussing the fibres of the pectoralis major in the
chest of an emaciated individual. The rate at which the
wave of contraction travels is ascertained by finding how
long it takes to pass between any two points at a known
distance from one another. Its velocity is found to vary
much according to the kind of muscle and the condition of
the muscle. In the striped muscular fibres of a frog in good
condition it travels at something over three metres per second.
When the muscle is in bad condition the wave passes more
slowly, and in an exhausted muscle it may remain at the
point of stimulation. (Practical Physiology, Chap. IV.)

The cause of the propagation of this wave is simply the
continuity of the muscle fibres. The fibres stimulated are
set in action, and the evolution of energy in these stimulates
the adjacent fibres, and so the contraction passes along the
muscle as a flame passes down a trail of gunpowder.

Contraction of Muscle as a whole may best be studied
under the following heads : —

1st. The course of contraction.
2nd. The extent of contraction.
3rd. The force of contraction.

1st. Course of Contraction (Fig. 21).

By attaching the muscle (M) to a lever (L\ and allowing
the point of the lever to mark upon some moving surface,
a magnified record of the shortening of the muscle when
stimulated may be obtained.

A revolving cylinder covered with a smoke-blackened,
glazed paper is frequently used for this purpose, and to

THE TISSUES

or

53

stimulate and mark the moment of stimulation an induction
coil (p.c., s.c.), with an electro-magnetic marker (T.M.), intro-
duced in the primary circuit, may be employed.

B

s.c,

FIG. 21.— A, Method of Recording Muscular Contraction. B, Key to Parts of
Apparatus. M, Muscle attached to crank lever L. p.c., Primary circuit,
and, s.c., secondary circuit of an induction coil with short circuiting key, k',
in secondary circuit. B, Galvanic cell, and, k, a mercury key for closing and
opening the primary circuit. T.M., A lever moved by an electro-magnet
placed in the primary circuit and marking the moment of stimulation. In As
a tuning fork beating 100 times per second is shown recording its vibration
on the drum.

54

HUMAN PHYSIOLOGY

To find the duration of the contraction, a tuning fork,
vibrating 100 times per second, may be made to record its
vibrations on the surface. (Practical Physiology, Chap.

in.)

In this way such a tracing as is shown in Fig. 22 is pro-
duced.

From this it is evident that the muscle] does not contract
the very moment it is stimulated, but that a short latent
period supervenes between the stimulation and the contrac-

tion. In the muscle of the
frog attached to a lever this
usually occupies about T^th
second ; but if the change in
the muscle is directly photo-
graphed without any lever

FIG. 22. — Trace of Simple Muscle
Twitch (1) showing periods of
latency, contraction, and relaxa-
tion ; record of moment of stimu-
lation (2) ; and a time record made
with a tuning fork vibrating 100
times per second (3).

being attached to it, this
period is found to be very
much shorter.

The latent period is fol-
lowed by the period of con-
traction. At first it is sudden,
but it becomes slower, and finally stops. Its average dura-
tion in the frog's muscle is about -j-J^-th second.

The period of relaxation follows that of contraction, and
it depends essentially on the elasticity of the muscle, whereby
it tends to recover its shape when the distorting force is re-
moved. The recovery is therefore at first fast and then slow,
and it lasts in the frog's muscle about To4rth second.

The whole contraction thus lasts only about TVth second
in the frog's muscle. In mammalian muscle it is much
shorter, and in the muscle of insects shorter still.
2nd. Extent of Contraction.

While, as will be afterwards considered, the extent of con-
traction is modified by the strength of stimulus and the state
of the muscle, the total amount of contraction is primarily
determined by the length of the muscle. If a muscle of two
inches contracts to one-half its length, the amount of con-
traction is one inch, but if a muscle of four inches contracts
to the same amount, it shortens by two inches.

3rd. Force of Contraction is measured by finding what

THE TISSUES 55

weight the muscle can lift, and the absolute force of a muscle
may be expressed by the weight which is just too great to be
lifted. The lifting power of a muscle depends primarily upon
its thickness or sectional area. The absolute force of a muscle
may therefore be expressed per unit of sectional area. In
man the absolute force per 1 sq. cm. is from 5000 to 10,000
grams. The force of contraction is, however, modified by so
many other conditions that no definite figure can be given.

The force of contraction during different parts of the con-
traction period may be recorded by making the muscle pull
upon a strong spring, so that it can barely shorten. The
slight bending of the spring may be magnified and recorded
by a long lever, and in this way it is found that the ordinary
curve of contraction gives a fair representation of the varia-
tions in the force.

This method of recording the force of contraction is some-
times called the isometric method, in distinction to the
isotonic method of letting the muscle act on a light lever.
In clinical medicine the DYNAMOMETER is used for measuring
the force of muscular contraction. (Practical Physiology,
Chap. V.)

II. The Factors modifying the Contraction.

1. Kind of Fibre. — In skeletal muscles the pale fibres con-
tract more rapidly and completely than the red fibres. The
peculiarities of the contraction of visceral muscles will be
considered later (p. 66).

2. Species of Animal. — In vertebrates the contraction of
the muscles of warm-blooded animals is more rapid than the
contraction in cold-blooded animals. The most rapidly con-
tracting muscles are met with in insects.

3. State of the Muscle. — (1) Continued Exercise. — If a
muscle is made to contract repeatedly, the contractions take
place more and more slowly. At first each contraction is greater
in extent, but, as the contractions go on, the extent diminishes
as fatigue becomes manifest, and finally stimulation fails to
call forth any response. This condition is probably caused
by the accumulation of the products of activity \\\ t.TiP TY^^IA
acting as poisons upon its protoplasm, for the same pheno-

HUMAN PHYSIOLOGY

mena may be induced by the application of dilute acids and
certain other drugs, and may be removed for a time_by.jyash-

out the muscle with salt

FIG. 23. — Influence of continued
Exercise on Skeletal Muscle— (1)
the first trace ; (2) a trace after
moderate exercise ; (3) a trace
when fatigue has been induced.

solutions^ (Fig. 23). (Practical
Physiology, Chap. III.)

(2) Temperature. — If a muscle
be warmed above the normal
temperature of the animal from
which it is taken, all the phases
ofcontraction become more rapid,
and the~ contraction is jit first
increased in extent, but subse-
quently decreased in force. If,

on the other hand, a jmu_sdfi_hfi_ cooleil, the various periods
are prolonged. At first the contraction becomes greater
and more powerful, but as the cooling process goes on
it becomes less and less, until finally the most powerful
stimuli produce no effect. Cooling has thus practically the
same effect as fatigue (Fig. 23). (Practical Physiology, Chap.
III.)

4. Drugs. — Many drugs modify muscular contraction, e.g.
veratrin enormously prolongs the relaxation period. (Prac-
tical Physiology, Chap. IV.)

5. Strength of Stimulus. — A stimulus must have a certain
intensity to cause a

contraction. The pre-
cise strength of this
minimum stimulus
depends upon the con-
dition of the muscle.
The application of
stronger and stronger
stimuli causes the
muscular contraction
to become more and
more rapid, more and
more complete, and

Con.

Flo. 24. — Influence of increasing the Strength of
the Stimulus upon the contraction of Skeletal
Muscle. St., the stimulus; Con., the result-
ing contraction. A , a subminimal stimulus ;
B, the minimum adequate stimulus ; C, tbe
optimum stimulus.

more and more powerful. But increase in

the contraction
is notjproportionate to the increase in the stimulus. If the
stimulus is steadily increased, the increase in contraction

THE TISSUES 57

becomes less and less. This may be represented diagram-
matically in the accompanying figure, where the continuous
lines represent the strength of the stimuli, and the dotted
lines the extent of the contractions (Fig. 24).

After a certain strength of stimulus has been reached,
further increase of the stimulus does not_cause any increase
in the muscular contraction. Tljis smallest stimulus which
causes the maximum muscular contraction is called the
optimum stimulus.

Increasing the str^ngth^nf t.Vm pt.imnbia shortens the
latent periocl/but lengthens the periods of contraction and
relaxation.

6. Resistance to Contraction — Weight to be Lifted.—
Starting from the extent of muscular contraction without

IG. 25. — Influence of Load on a Muscular Contraction, (a) The effect of increas-
ing the load on the extent of contraction ; (b) the effect of load on the course
of contraction.

any load, it is found that small weights attached to the
muscle actually increase the extent of contraction, but
greater weights diminish it, until finally, when a sufficient
weight is^tpplied, the muscle no longer contracts at all, but
may actually slightly lengthen, because its extensibilities
increased during contraction (Fig. 25, a).

The application of weights to a muscle causes the latent
period and period of contraction to be delayed, while it
renders the period of relaxation more rapid, and an over-
extension may be produced followed by a recovery resem-
bling a small after-contraction (Fig. 25, b). (Practical
Physiology, Chap. VI.)

7. Successive Stimuli. — So far we have considered the
influence of a single stimulus on the shape of muscle. But
in nearly every muscular action the contraction of the
muscles must last much longer than y^-th of a second.

58 HUMAN PHYSIOLOGY

How is this continued contraction of muscles produced ?
To understand this it is necessary to study the influence of
a series of stimuli on muscle.

If, to a frog's muscle which takes TVth of a second to
contract and relax, stimuli at the rate of five per second
are applied, it is found that a series of simple contractions,
each with an interval of TVth of a second between them
are produced (Fig. 26, 1). If the stimuli follow one another
a b c at the rate of ten per second, a

series of simple contractions is still
produced, but now with no interval
between them.

r If stimuli be sent more rapidly
to the muscle, say at the rate
of twelve per second, the second
stimulus will cause a contraction
before the contraction due to the
irst stimulus has entirely passed
>ff (Fig. 26, 2). The second con-
raction will thus be superimposed
>n the first, and it is found that
he second contraction is more
jomplete than the first, and the
hird than the second. But while

\

A

Fio. 26.— Effect of a series of
Stimuli on Skeletal Muscle.
(See text.)

the second contraction is

markedly greater than the first, the

third is not so markedly greater than the second, and each
succeeding stimulus causes a less and less increase in the
degree of contraction until, after a certain number, no
further increase takes place, and the degree of contraction
I is simply maintained.

When the contractions follow one another at such a rate
that the relaxation period of the first contraction has begun,
but is not completed, before the second contraction takes
place, a lever attached to the muscle, and made to write on
a moving surface, produces a toothed line. The contraction
is not uniform, but is made up of alternate shortenings and
lengthenings of the muscle. This constitutes "incomplete

If the second stimulus follows the first so rapidly that the
contraction period has not given place to relaxation, then

THE TISSUES 59

the second contraction will be superimposed on the first,
the third on the second, and so on continuously and
smoothly without any slight relaxations, and thus the lever
will describe a smooth line, rising at first rapidly, then more
slowly, till a maximum is reached, and being maintained
at this till the series of stimuli causing the contraction is
removed, or until fatigue causes relaxation of the muscle.
This is the condition Q{ " complete tetanus" (Fig. 26, 3).
(Practical Physiology, Chap. V.)

The rate at which stimuli must follow one another
in order to produce a tetanus depends on a large number
of factors. Anything which increases the duration of a
single contraction renders a smaller number of stimuli per
second sufficient to produce a tetanus, and thus all the
various factors modifying a single muscular contraction,
modify the number of stimuli necessary to produce a tetanus
(see p. 55). D'Arsonval has shown that an alternating
current with very frequent interruptions of about 1,000,000
per second causes no contraction.

Every voluntary contraction of any group of the muscles
is probably of the nature of a tetanus ; and the question thus
arises, at what rate do the stimuli which cause such a tetanus
pass from the spinal cord to the muscles ?

In a tracing of a continued voluntary contraction, indica-
tions of about ten variations per second are to be seen,
while the rate of the clonic tremor of the leg which may
be produced during fatigue by supporting the weight of
the leg on the toes is about ten, backward and forward
movement, per second, and in various morbid muscular
spasms the rate is about the same. (Practical Physiology,
Chap. V.)

All this would seem to indicate that the number of
stimuli which pass to human muscle from the central
nervous system is probably about ten per second.

It has, however, been found that passing a strong
galvanic current into a muscle may lead to rhythmic
contraction, and hence it is possible that the contractions
of muscle induced by the central nervous system may
be caused by a continued discharge from the nerve
centres.

60 HUMAN PHYSIOLOGY

III. Mode of Action of Muscles.

The skeletal muscles act to produce movements of th
body from place to place, or movements of one part of th
body on another. This they do by pulling on the bony frame
work to cause definite movements of the various joints.

In relationship to each joint the muscles are arrangec
in opposing sets — one causing movement in one direction
another in the opposite direction — and named according t<
their mode of action, flexors, extensors, adductors, abductors,
&c. But in the production of any particular movement —
say flexion of the forearm at the elbow — not only are the
muscles manifestly causing the movement in contraction,
but the opposing group, the extensors, are also in action to
guide and direct the force and extent of the movement.
This Co-operative Antagonism of groups of muscles is of
very great importance, since it explains many of the results
observed in paralysis. Thus, if the extensors of the hand
be paralysed, as in lead palsy, it is found impossible to
clench the hand, although the flexors are intact. Again,
if part of the brain which causes flexion of the hand of
the monkey be stimulated, and the nerve to the flexors
divided, the co-operative action of the extensors brings about
an extension of the hand.

The muscles round the various joints act on the bones,
arranged as a series of levers, of the three classes (Fig. 27).

FlQ. 27. — The three types of lever illustrated by the movements
at the ankle joint.

1st Class. — Fulcrum between power and weight. In the
ankle this is seen when, by a contraction of the gastro-
cnemius, we push upon some object with the toes.

2nd Class. — Weight between fulcrum and power. In
rising on the toes the base of the metatarsals is the fulcrum,

THE TISSUES 61

the weight comes at the ankle, and the power on the
os calcis.

3rd Class. — Power between fulcrum and weight. In
raising a weight placed on the dorsal aspect of the toes by
the contraction of the extensors of the foot, we have the
weight at the toes, the power at the tarsus, and the fulcrum
at the ankle.

In the other joints, actions involving the principle of each
of these levers may be found.

IY. Muscle Work.

As a result of the changes in shape, muscle performs its
great function of doing mechanical work; and the most
important question which has to be considered in regard
to muscle, as in regard to other machines, is the amount
of work it can do.

Since the work done depends upon the weight moved and
the distance through which it is moved, the work-doing
power of muscle is governed by the
force of contraction, which determines
the weight which can be lifted, and
by the amount to which the muscle
can shorten, for this will govern the
distance through which the weight
may be moved.

It has been already shown that the
force of contraction depends chiefly FlQ- 28.— influence of the
upon the sectional area of a muscle. ^/^ daon^uscle upon
A thick muscle is stronger than a

thinner one. But on the other hand, the amount of con-
traction depends upon the length of the muscle, since each
muscle can contract only to a fixed proportion of its original
length. A glance at the diagram will at once make this
plain (Fig. 28).

The size of the muscle is thus the first great factor which
governs its work-doing power. But the many factors in-
fluencing the force of muscular contraction also influence
the work-doing power of the muscle (see p. 55). (Practical
Physiology, Chap. VI.)

Load in
Grams.

Space through which
lifted in mm.

0

X

4-5

20

X

3-0

40

X

2-37

60

X

2-00

80

X

1-75

100

X

1-2

62 HUMAN PHYSIOLOGY

One factor requires special consideration, namely, the
Load.

We have already seen that as the load is increased the
extent of contraction is diminished.

The following experiment will illustrate the influence of
increasing the load on the work-doing power of a muscle : —

Work in Gram,
mm.

O'O

60-0
94-8
120-0
140-0
120-0

It will be seen that increasing the load at first increases
the amount of work done, jut after a certain weight is
reacrjecr. diminishes it. There~is, therefore, for^very muscle,
so far as its working power is concerned, an " optimum "
load.

In studying the amount of work a muscle or set of
muscles can do, the element of time must always be con-
sidered. Obviously contracting muscles will do more work
in an hour than in a minute. Hence, in trying to form
any idea of the amount of work a muscle can do, this must
be expressed in work units, per unit of bulk and per unit
of time.

The average working capacity of skeletal muscle may be
estimated as follows : — A labourer who raises 130,000 kilos,
through one metre during his eight hours of work does a
good average day's work. His muscles weigh about 25
kilos., and thus each gram, of his muscle will do 5 kilogram-
metres per diem, or 0*06 gram, metres per second.

When required, much larger amounts of work can be
done for short periods. It has been calculated that in
the sprint of a 100 yards race, work is done at something
like 2 gram, metres per second, about thirty times the rate
at which a labourer's muscles work. But to increase the
rate at which work is done requires an increase in the
expenditure of the energy-yielding materials in greater pro-

THE TISSUES 63

portion than the increased work — just as to increase the
speed of a ship or an engine requires an increase of coal
consumption in a proportion roughly corresponding to the
square of the increased speed.

Y. Heat Production.

In muscle, as in other machines, by no means the whole
of the energy rendered kinetic is used for the production
of mechanical work. In a steam-engine much of the
energy is dissipated as heat, and the same loss occurs in
muscle.

If heat is given off when a muscle contracts, either the
muscle itself, or the blood coming from it, will become
warmer. Hence to detect such a change some delicate
method of measuring changes of temperature must be
employed. The mercurial thermometer is hardly sufficiently
sensitive, and, therefore, the thermo-electrical method is most
generally employed. Various forms of thermopile may be
used.

The rise of temperature in a muscle after a single con-
traction is extremely small, but after a tetanic contraction,
lasting for two or three minutes, it is very much greater.

The amount of heat produced may be calculated if
(a) the weight of the muscle; (6) its temperature before
and after contraction; and (c) the specific heat of muscle,
are known.

The specific heat of muscle is slightly greater than that
of water, but the difference is so slight that it may
be disregarded. If, then, ten grammes of muscle had a
temperature of 15° C. before it was made to contract, and
a temperature of 15*05° C. after a period of contraction, then
0*5 gramme-degrees of heat have been produced ; i.e. heat
sufficient to raise the temperature of 0*5 gramme of water
through 1° C.

The amount of heat produced by muscle in different
conditions varies so greatly that it is unnecessary to consider
it further.

Relationship of Heat Production to Work Production. —
Since it is possible to measure both the mechanical work

64 HUMAN PHYSIOLOGY

done by a muscle and the amount of energy dissipated as
heat, it is possible to determine the relationship of these
to one another, and thus to compare muscle with other
machines as to proportion of energy which is utilised to
produce work. To do this it is necessary to be able to
convert "work units" into "heat units," or vice versa. It
has been found that 0-45 gramme-degrees is equivalent to
1 kilogram-metre.

I The proportion of work to heat is not constant. Gradually
increasing the stimulus increases both work production and
heat production, but the latter is increased more rapidly,
and reaches its maximum sooner. Again, as muscle becomes
f exhausted its heat production declines more rapidly than its
\work production. Exhausted muscle, therefore, works more
economically. If an unloaded muscle is made to contract no
/work is done and all the energy is given off as heat, and the
J same thing happens where a muscle is so loaded that it
cannot contract when stimulated.

But the point of practical importance to determine is —
How much of the energy liberated by muscle in normal
conditions is usually used for mechanical work, and how
much is lost as heat? It will afterwards (p. 312) be shown
that all the energy of the body comes from the food, and
the amount of energy yielded by any food may be deter-
mined by burning it in a calorimeter. To determine the
energy used in mechanical work some form of work measurer
or ergograph may be used — e.g. a wheel turned against a
measured resistance. By converting the work units of the
work thus done into heat units, and subtracting this from
the total energy of the food, the energy lost as heat may
be determined, and thus the relationship between work pro-
duction and heat production may be found. By experiments
on men, horses, and dogs, Zuntz has found that about Jrd
of the energy liberated may, under favourable conditions, be
available for mechanical work, while §rds is lost as heat.
Compared with other machines, such as steam-engines,
muscle must be regarded as an economical worker, and it
has the advantage that the heat liberated is necessary to
maintain the temperature at which the chemical changes
which are the basis of life can go on.

THE TISSUES 65

YI. Electrical Changes.

When a muscle contracts certain electrical changes occur.
These may be best studied in the heart, which is a muscle
which can be exposed without injury. With other muscles
the injury inflicted
in isolating them sets
up electrical currents
of injury (p. 45).

If one end of a wire (f •/ \\

be brought in con-
tact with the base of
the ventricle by means
of a non-polarisable

electrode (in which FIG. 29.— To show electric current of action in a
SOme material which muscle (a) compared with that in a galvanic

J *-"Urt cel1 (b)' Tne contracting part of the muscle

does not act upon the is sh^ded (g] Galvanometer.
muscle and is not

acted upon by the muscle is in contact with it), and another
wire be similarly connected with the apex, and if these
wires are led off round a galvanometer, it will be found
that with each contraction of the heart an electric current
is set up, the one part of the heart becoming first positive
and then negative to the other part.

This means that, when the contraction occurs, the part
which first contracts becomes of a higher electric potential
than the rest of the muscle, so that electricity flows from
it to the uncontracted part in the organ, and from the
uncontracting part to the contracting part in the wire round
the galvanometer. The contracting part is thus similar to
the positive element of a battery, the zinc, the uncontracting
part to the negative element, and the wire coming from the
contracting part will, therefore, correspond to the negative
pole — that from the uncontracting part to the positive pole.

It has now been shown that this current of action occurs
along with, and does not precede, the period of contraction.
The electric change in contracting muscle may be demon-
strated by laying the nerve of a muscle nerve preparation
over the muscle of another muscle nerve preparation or over

5

66 HUMAN PHYSIOLOGY

the beating heart. The current of action in the second
muscle in each case may stimulate the first. (Practical
Physiology, Chap. VIII.)

YII. Extensibility.

The extensibility of muscle is increased during contraction
so that the application of a weight causes a greater lengthen-
ing than when the muscle is at test.

Visceral Muscles.

In several important respects the visceral muscles differ in
their mode of action from the skeletal muscles.

1. Their connection with nerves is by no means so definite
and precise, for, instead of each nerve-fibre ending in a
muscle- fibre, the nerves to non- striped muscle form an
irregular network upon them, and the muscle-fibre appears
to be capable of action, both before these nerves have
developed in the embryo and when the influence of these
nerves has been cut off in the adult. In the intestine the
mode of action of the muscles is largely dominated by the
plexus of nerves (see p. 328).

2. The great features of the action of visceral muscle are
— 1st, its tendency to sustained tonic contraction; and 2nd,
its spontaneous regular rhythmic contraction and relaxation.

1st. The continuous slight tonic contraction is seen in all
the visceral muscles, and, while it may be increased or
diminished by the intervention of nerves, it appears to be
largely independent and an expression of the continuous
metabolism of the muscle protoplasm.

2raZ. The rhythmic contractions and relaxations are not
equally manifest in all situations, nor are they so continuous,
but they are well marked in the muscles around such hollow
viscera as the intestines, bladder, and uterus. Like the tonic
contractions, they are to a certain extent independent of
nerve action, but are influenced by it.

These contractions recur at regular intervals of varying
duration. Each contraction lasts for a considerable period—
sometimes over a minute — and the relaxation is correspond-
ingly long. Everything which increases the rate of chemical

THE TISSUES 67

change increases the rapidity of the rhythm. Thus warming
the muscle and the action of a galvanic current have this
action.

3. When the muscle is at rest, a contraction may be pro-
duced by any of the modes of stimulation which will cause
the skeletal muscles to contract ; and it may thus be demon-
strated that the latent period is very long.

4. Unlike skeletal muscles, the extent of contraction is
not increased by increasing the strength of the stimulus.
The smallest available stimulus causes the maximum con-
traction; but if the same stimulus is repeated at regular
intervals the resulting contractions become greater and
greater during the application of the first four or five stimuli,
so that the record of a series of contractions has a somewhat
stair-like appearance.

5. A series of stimuli do not cause a tetanus, but simply
increase the rapidity and force of the individual contractions.

When the muscles are arranged round hollow viscera, the
rhythmic contraction, starting at one end, travels along the
group of muscle-fibres as a regular wave — the peristaltic
wave — and thus drives onwards the contents of the tube.
The rate at which this wave travels varies very considerably.
(Practical Physiology, Chap. IX.)

Cardiac Muscle physiologically resembles other visceral
muscles, but its period of contraction is shorter and its
rhythm generally more rapid.

4. THE CHEMICAL CHANGES IN MUSCLE AND THE

SOUKCE OF THE ENERGY EVOLVED.

Chemical changes are constantly going on in muscle, and
the study of these chemical changes in resting muscle and
in contracting muscle explains the source of the energy of
muscle, disintegration leading to the liberation of energy and
construction leading to the repair of the muscles and the
storage of energy.

No part of physiology is of more importance ; for it is the
chemical changes in muscle which give rise to the great
waste products of the body, and it is to make good these
losses that fresh nourishment has to be supplied. The

68 HUMAN PHYSIOLOGY

chemical changes in muscle therefore govern both the intake
and output of matter from the body.

By studying the question from a number of different
standpoints, and by comparing the results so obtained, a
fairly clear conception of the chemical changes and the
source of muscular energy has been obtained.

1. Composition of Muscle before and after Contraction. —
The method which most naturally presents itself is to take
two muscles or groups of muscles corresponding to one
another, and to examine the chemistry of one before it had
been made to contract, and of the other after it had been
contracting for some time.

Resting muscle is alkaline ; but if an excised muscle, out-
side the body, be kept contracting for some time, it becomes
acid, and this acidity is due to the appearance of sarcolactic
acid. Muscle in the body does not become acid, because
the alkaline lymph at once neutralises the acid which is
produced.

Again, after contraction, the glycogen of the muscle is
found to be diminished. But the most important change is
that the amount of carbon dioxide, CO2, which can be ex-
tracted from, muscle is very greatly increased.

As yet the changes, if any, in the proteids of muscle
during contraction have not been fully investigated, while
the results of the work accomplished on the nitrogenous
extractives, which are formed by the decomposition of the
proteids, are not trustworthy. They seem to indicate that
these bodies are increased during muscular contraction in
the excised muscle. These changes in a piece of muscle
may be diagrammatically represented as follows :—

-f Carbon dioxide.

+ Sarcolactic acid.

+ Nitrogenous extractives ?

— Glycogen.

The results obtained by this method of investigation are
thus of considerable value, but alone they give us no clear
idea of the nature of the chemical changes.

THE TISSUES

69

2. Respiration of Excised Muscle. — By enclosing the ex-
cised muscle in a closed space containing air of known com-
position, and by investigating the changes in the components
of the air after the muscle has either been kept at rest for
some time or made to contract, important light has also been
thrown on these chemical changes.

It has been found that the resting muscle constantly takes
up oxygen from the air round about it, and constantly gives
off carbon dioxide. In contracting, more carbon dioxide is
given off, and usually the amount of oxygen taken up is also
increased. (Fig. 30.)

Here we have at once evidence that muscle breathes, and
that this process of respiration is increased during muscular

FIG. 30. — Respiration of muscle in a closed chamber.

activity. The affinity of muscle for oxygen is very great, so
great that jt can actually talrp nvrrpn n^t ^f plifiinjjfiT com-

binaiions. If methyl ene blue or alizarin blue are injected
into the vein of an animal, the blood becomes blue, but the
muscles remain colourless, having deoxidised the pigment
and reduced it to a colourless condition. When freely ex-
posed to air after death, the blue colour returns.

3. Changes in Blood passing through Muscle. — Observa-
tions on excised muscle carry us no further, but by ascertain-
ing the changes in composition which the blood undergoes
in passing through a muscle, further information may be
gained.

For this purpose the two hind limbs of a dog have been
used. The blood going to one leg, and the blood coming
from the other, are collected at the same time. It is found

70 HUMAN PHYSIOLOGY

that the blood in passing through the muscles has gained
carbon dioxide and lost oxygen. If the muscles be kept
contracted, it is further ascertained that the amount of
carbon dioxide gained is increased, while usually the amount
of oxygen taken up is also increased. This observation thus
confirms the investigations on the changes in the air sur-
rounding a muscle.

But the solid constituents of the blood are also changed.
If the muscles have been contracting, the blood is found to
contain sarcolactic acid probably combined with ammonia.

We shall afterwards find that blood contains small
quantities of glucose, C6H1206. As it passes through
muscle it loses some of this, even when the muscle is at

,GLUC05 E
FAT
OXYGEN

SARCOLACTIC ACID
CARBON DIOXIDE

FIG. 31. — Exchanges between muscle and blood.

rest, and a much larger amount when the muscle has been
active.

The changes in the proteids of the blood going to and
coming from muscle have not been properly investigated.

Some observers have obtained results which seem to
indicate that the amount of fat which is found in the blood
is diminished as the blood passes through the muscles, but
whether this diminution is greater during muscular activity
has not been studied.

Such direct observations on muscle and the blood nourish-
ing it indicate that constant chemical changes are going on
when the muscle is at rest. It is constantly giving off carbon
dioxide and constantly consuming oxygen, glucose, and pos-
sibly fats and proteids. When doing work these chemical
changes become more active. We may compare resting

THE TISSUES 71

inuscle as to its chemical changes to an engine with its fires
banked down. Active muscle is comparable to the engine
with its fires in full blast.

4. Effects of Muscular Work upon the Excreta. — Another
method of study has yielded results of very great value —
the investigation of the effects of muscular work upon the
excreta.

Not only is muscle the most bulky and most constantly
active tissue, but it is the tissue in which very extensive
chemical changes occur, in the liberation of the energy for
work and heat production, and hence the waste products of
the body are chiefly derived from muscle, and their amount
and character must afford an indication of the changes in
this tissue.

This was long ago recognised, but the older experimenters
did not sufficiently realise that the excretions are modified
by the amount and character of the food taken, and hence
their results are of little value. In studying the influence
of muscular work on the excreta, food must be withheld or
must be unvarying during the experiment.

If this precaution is taken, it is found that the excretion
of the various elements composing muscle is modified by
muscular work.

Attention has chiefly been directed to the variations in the
carbon and nitrogen thrown off, the former mainly as carbon
dioxide in the expired air, the latter as urea in the urine.
It has been found that if a fasting or underfed animal is
made to do work, the excretion of both these elements is
increased, the carbon proportionately to the work done,
the nitrogen in quantities not strictly proportionate to
the work, being greater the more underfed the animal
is and the harder the work done, and being less the better
nourished the animal is or the less the work that is done.
(Fig. 32, 1).

If a lean animal be fed on an exclusively proteid diet, the
excretion of carbon and nitrogen is increased, practically
proportionately to the work done (Fig. 32, 2).

But if the animal be well fed on an ordinary diet, contain-
ing proteids, carbohydrates, and fats, the performance of
muscular work increases the excretion of carbon proportion-

72 HUMAN PHYSIOLOGY

ately to the work done, but may cause only a very slight
increase in the excretion of nitrogen (Fig. 32, 3).

From the increased excretion of nitrogen and carbon the
consumption of proteids may be calculated, since proteids
contain 16 per cent, of nitrogen and 52 per cent, of carbon —
i.e. 3*4 times more carbon than nitrogen. Each gram, of
nitrogen excreted thus represents the breaking down of 6-25
grams, of proteid, and it is accompanied by 3*4 grams, of
carbon. If more carbon is excreted, it must come from
carbohydrates or fat.

Proceeding in this way, it is found that in the fasting
animal and in the animal fed on proteids, the muscles get
their energy from proteids, but that in an animal on an

.-

/

//

FIG. 32.— To illustrate the influence of Muscular Work upon the
Excretion of Carbon and of Nitrogen — (1) in a fasting or
underfed animal ; (2) in an animal fed on proteids ; (3) in an
animal on a normal diet.

ordinary diet the muscles get it chiefly from the carbo-
hydrates and fats of the food.

An example of such an investigation may be given.
Suppose that a man during a period of rest excretes daily
10 grams, of nitrogen, and that he then does 100,000 Kgms.
of work, and during the next three days the excretion of
nitrogen is raised 2 grams, above the 10 per diem. This
means that 2 x 6*25 = 12-5 grams, of proteid has been decom-
posed. Now the amount of energy which can be liberated
from 1 gram, of proteid has been found to be equivalent to
1738 Kgms. (kilogrammetres), and therefore the 12*5 grams,
decomposed in the experiment is sufficient to yield 21,635
Kgms. of energy, about 20 per cent, of the total energy
expended in the work. The rest of the energy must be
derived from the fats and carbohydrates.

5. A study of the ordinary diet of men doing muscular

THE TISSUES 73

work corroborates the conclusions arrived at by an examina-
tion of the excreta. In this country the diet of a labourer
consists of something like the following proportions of food
constituent s : —

Amount. Yielding Calories.

Proteids ... 130 533

Fats . . . . 100 930

Carbohydrates . . 500 2050

The energy is here expressed in heat units, Calories — the
amount of heat required to raise 1 kilogramme of water
through 1 degree Centigrade. Of the total 3513 Calories of
energy daily taken in the food, only 15 per cent, is derived
from proteids, the rest comes from the carbohydrates and fats.
The same is found to be the case in the diet of many other
animals, such as the horse.

Thus during muscular work the three great constituents
of the body and of the food — proteids, fats, and carbo-
hydrates— are broken down to liberate their energy, and
apparently the muscle tends to use the non-nitrogenous fats
and carbohydrates in preference to the proteids. Only when
forced to do so does it take a large proportion of its energy
from these substances.

It may be urged that in athletic training proteids must
be a source of energy, since experience has taught that
they are of such value. But their great value is as material
from which the energy-liberating machine, the muscles, can
be built up and increased, so that it can dispose of larger and
larger quantities of food.

Muscle then is a machine which has the power of libe-
rating energy from proteids, fats, and carbohydrates, but it
uses proteids more especially in construction and repair.

The muscles liberate energy from these substances by
breaking them down into simpler molecules just as a blow
causes the disintegration of nitre-glycerine and liberates its
stored energy. There is not such a direct oxidation as occurs
in the coals in the furnace of an engine, for if this were so,
the consumption of oxygen would always be equivalent to
the elimination of carbon dioxide and the other products of
disintegration. It has, however, been shown that a frog,

74 HUMAN PHYSIOLOGY

deprived of all free oxygen by placing it in the receiver of
an air pump, and then transferred through mercury to an
atmosphere of nitrogen, still continues to produce carbon
dioxide. This means that its oxygen must be intramolecular,
must be in the muscle molecule, like the oxygen of nitro-
glycerine. Probably the presence of this oxygen is one of
the causes of the instability of the molecule.

The muscle then takes these substances into itself — makes
them part of its molecule — assimilates them before breaking
them down. It is not necessary to suppose that all the
substances are equally intimately associated with the muscle
protoplasm. In all probability the proteid becomes much
more truly a part of the muscle than the carbohydrates and
fats, but with each one of them it is essential that it should
come into the domain of the muscle and not simply remain
in the blood and lymph, in which it cannot be used.

5. DEATH OF MUSCLE.

The death of the muscle is not simultaneous with the
death of the individual. For some time after somatic death
the muscles remain alive and are capable of contraction
under stimulation. Gradually, however, their irritability
diminishes and finally disappears. They are then dead,
and necrobiotic changes begin. The first of these — Rigor
Mortis — is a disintegrate chemical change whereby carbon
dioxide and sarcolactic acid are set free, and at the same
time the soluble myosinogen changes to the insoluble myosin
and the muscle becomes contracted, less extensile, less elastic,
and more opaque. The contraction is a feeble one, and since
it affects flexors and extensors equally, it is not generally
able to alter the position of the limbs, although it may some-
times do so. As these changes occur, heat is evolved and the
muscles become warmer.

The time of onset of rigor varies with the condition of the
muscles. If they have been very active just before death,
stiffening tends to appear rapidly.

It lasts for a period which varies with the species of
animal and with the condition of the muscles, and as it
disappears the muscles again become soft, and the body

THE TISSUES 75

becomes limp. In all probability this latter change is due
to a solution of the coagulated proteid by the enzyme
of the stomach — pepsin — which seems to exist in all the
tissues. This can act only in the presence of an acid, and
the appearance of sarcolactic acid, therefore, allows it to come
into play.

II. NERVE.

It is through the nerves that our surroundings act upon
us, and through nerves that our muscles are made to respond
appropriately to the surrounding conditions.

1. Structure and Development.

In unicellular organisms changes in the surroundings
act directly on the cell protoplasm, e.g. an amoeba, when
touched, draws itself together. But,
even in these simplest organisms,
certain kinds of external conditions will
produce one kind of change, while others
will produce a different one, as has
been shown in considering unilateral
stimulation (p. 16). Among unicellular
organisms somewhat higher than amoeba
— the infusoria — animals are found in
which the cell is differentiated into a
receiving and reacting part. Poterio-
dendron, a little infusorion sitting in
a cup-like frame, consists of a long pro- FlG- 33.— Poteriodendron

.,. ,. f r, to illustrate first stage

cess or cilmm extending up ironi the in the evo]ution of a
cell body, while a contractile myoid neuro-muscuiar system,
attaches the cell body to the floor of
the cup. When the cilium is touched the myoid con-
tracts, and draws the creature into the protection of its
covering.

But in more highly organised animals, where the reaction
has to be more definitely appropriate to the surrounding
conditions, and where the complexity of the mechanism
involved is greater, there is a development by which special
conditions at special parts of the surface each lead to a

76 HUMAN PHYSIOLOGY

special reaction. This is brought about by the establishment
of what may be compared to a series of shunting stations
between the receptive mechanism on the surface and the
reacting mechanism, the muscles, glands, &c. To form this
a part of the original covering of the embryo sinks inwards as
a canal composed of the surface cells, and these cells remain
in functional connection with the surface on the one hand
and with the reacting structures on the other. At first the
cells forming this tube are undifferentiated and alike, but
some throw out processes towards the surface and others
towards the reacting structures, and they are connected,
not by actual continuity, but by coming in close relationship
to one another in a series of branching processes, forming a
synapsis (Fig. 34).

Each of the units so formed has been called a neuron ;
and a neuron may be defined as one of the cells, with all

.

Fia. 34.— To show a receiving (c) and a reacting Neuron (a), each with
dendrites at its extremities, and their connection to one another
through a Synapsis (6). , »

its processes, which build up the narvous $ystem. These
neurons may be divided into the deceiving and reacting
series, but in structure they are alike. The shape and
characters of the cells, and their position upon the processes
of the neuron — the fibre — varies greatly, but they have all
the following characters in common : — They are nucleated
protoplasts, the protoplasm of which shows a well-marked
network, in the meshes of which a material which stains
deeply with basic stains, and which seems to be used up
during the activity of the neuron, may accumulate. The
granules formed of this material are generally known as
Nissl's granules (Fig. 35).

All these cells give off at least one process, which continues
for some distance, as the axon or axis cylinder of a nerve
fibre. Frequently other processes are given off, which may
either pass away as fibres, or may, while still in close
proximity to the cell, form a branching system of dendrites.

THE TISSUES

77

The axons end in much the same manner, so that all
the processes are essentially the same. These processes are

FIG. 35. — (a) A Nerve Cell with Nissl's granules ; (b) a similar cell
showing changes on section of its axon.

fibrillated, and the fibrillse may be traced through the proto-
plasm of the cells (Fig. 36). In many cases the dendrites

FIG. 36. — A. Nerve Cell highly magnified to show passage of processes
through the protoplasm.

show little buds or gemmules upon their course, and,
according to some observers, it is through these that one
neuron is brought into definite relationship at one time with

78 HUMAN PHYSIOLOGY

one set of neurons, and at another with other adjacent
neurons. There is also some evidence that the dendrites
as a whole may expand and contract, and thus become
connected with those of adjacent neurons.

Axon. — The axis cylinder process, as it passes away from
the cell, becomes a Nerve Fibre, and acquires one or two
coverings.

1. A thin transparent membrane is present in all peripheral
nerves, and has been called the neurilemma. Between it and
the axis cylinder a series of nuclei surrounded by a small
quantity of protoplasm, forming the so-called nerve cor-
puscles, is found at intervals. The mode of origin of these
is unknown. Fibres with only this sheath have a grey
colour, and may be called non-medullated fibres. They are
abundant in the visceral nerves.

2. A thick white sheath — the medullary sheath or white
sheath of Schwann — which gives the white colour to most of

Fio. 37.— Pieces of two white Nerve Fibres.

the nerves of the body, appears somewhat late in the develop-
ment of many nerve fibres. It lies between the primitive
sheath with the nerve corpuscles and the axis cylinder. It
is not continuous, but is interrupted at regular intervals at
the nodes of Ranvier (Fig. 37). It is composed of a sponge-
work or felt-work of a horn-like material — neuro-keratin —
the meshes of which are filled with a peculiar fatty material,
from which certain chemical substances have been isolated,
the relationships of which to one another are little under-
stood. The most abundant of these has been called pro-
tagon. It yields stearic acid ; hence it is allied to the fats,
and it contains nitrogen and phosphorus. Its constitution
is not known. Along with protagon, or as a result of its de-
composition, lecithin occurs. This is a fat in which one of
the acid radicles is replaced by phosphoric acid linked to
cholin — Hydroxyethyl-trirnethyl-ainmonium-hydroxide.

THE TISSUES 79

r Fatty acid.
Glycerol J Fatty acid.

{ Phosphoric acid.

Cholin.

H H

HO— C— C-

OH

- N

Hydro xyethyl.

trimethyl ammonium

hydroxide.

The chief interest of cholin is that it is toxic, and some
of the symptoms occurring in degenerative changes of the
nervous system may be due to its presence. It is closely
allied to inuscarin, a very powerful vegetable poison.

Cholesterin may also be obtained in considerable amounts
from the fatty substance of the white sheath. Like the
glycerine of ordinary fats, it is an alcohol — C26H43,OH — and
it is capable of linking with fatty acids. It is very soluble in
hot alcohol, and crystallises out on cooling in characteristic
square plates, with a notch out of one corner.

The nerve fibres run together in bundles to constitute the
nerves of the body, and each bundle is surrounded by a dense
fibrous sheath, the perineurium. When a bundle divides,
each branch has a sheath of perineurium, and in many nerves
this sheath is continued, as the sheath of Henle, on to the
single fibres which are ultimately branched off from the
nerve.

2. Physiology.

1. Inter-relationship of Neurons. — The neurons form a
most intricate labyrinth throughout all parts of the body,
and more especially throughout the central nervous system.

By their dendritic terminations each is brought into rela-
tionship with many others, and hence there is a continued
interaction between them, the activity of any one influencing
the activity of many others. In this way the constant
activity of the nervous system, which goes on from birth to
death, during consciousness and in the absence of conscious-
ness, is kept up.

It is unnecessary and gratuitous to invoke the conception

8o HUMAN PHYSIOLOGY

of automatic action on the part of any portion of the nervous
system. Throughout life these neurons are constantly being
acted upon from without, and activity, once started by any
stimulus, must necessarily set up a stream of action which
will be co-existent with life.

2. Stimulation of Neurons. — This implies that a neuron is
capable of stimulation, that, like all other protoplasm, it
reacts to changes in external conditions. A neuron may be
stimulated at any part, but it is usually stimulated from one
or other of its terminal dendritic endings, either by changes in
the tissues round about or by changes in other neurons. Thus
(Fig. 34) a neuron may be thrown into action by changes in
the tissue at its extremity, while another may be stimulated
by the activity of the former. They may also be stimulated
in their course, as is demonstrated by pinching the ulnar
nerve behind the internal condyle of the humerus.

Means of Stimulation. — Just as with muscle, so with nerve ;
any sudden change excites it to activity — be this change a
mechanical one, as in pinching a nerve, or a change in the
temperature, or in the electric conditions in its neighbour-
hood, or in the chemical surroundings of the neuron —
agents which withdraw water, like glycerine, stimulating most
strongly. All that has been said of the stimulation of muscle
applies to the stimulation of nerve (see p. 46 et seq.).

The condition of the neuron modifies the effect of the
stimulus, and the condition of other neurons modifies the
ultimate result of the stimulus on the body.

The excitability of a neuron is modified by the many fac-
tors. It may be increased by a slight cooling, but is de-
creased at lower temperatures. It is increased by a warming
up to a certain point. Drying at first increases excitability,
then abolishes it. During the flow of an electric current it
is increased in the neighbourhood of the negative pole, de-
creased around the positive pole, in the same way as in muscle
(see p. 48). It is influenced by many chemical substances,
some of which increase its excitability in small doses, and
diminish it in larger doses ; some again even in the smallest
dose depress its activity, e.g. potassium salts. Continued
activity has no effect on the excitability of nerve, and the
phenomena of fatigue are not manifested as in muscle.

THE TISSUES 81

3. Manifestations of the Activity of Neurons. — So far as

is at present known, the activity of neurons is not accom-
panied by any obvious change, although it is possible that
movements of the dendrites or of the gemmules upon them
may occur. The activity of neurons is made evident —

1st By their action upon other structures, e.g. muscles,
glands, &c., either (a) directly or (6) indirectly, through other
neurons.

2nd. By electric changes in the neuron ; and

3rd. By changes in the consciousness.

The activity of the outgoing neurons — neurons conducting
impulses from the central nervous system to muscles, glands,
&c. — is manifested by changes in the muscles or other
structures to which they go: while the activity of ingoing
neurons is made evident by their action on outgoing neurons
to muscles, &c., and sometimes by modifications in the state
of consciousness, which may be of the nature of a simple
brief sensation, or, by the implication of a number of other
neurons, may develop into a series of changes accompanied
by sensations.

Very interesting results follow from this fact that the
activity of neurons is made manifest by changes in the
structures to which they pass. Kennedy has shown that, if
the nerves to the flexors and the nerves to the extensors
of a dog's thigh be cut, and the central end of the first
united to the peripheral end of the second, and vice versa,
co-ordinate movements occur, and that if that part of the
brain which naturally causes extension be stimulated, flexion
occurs. Langley has demonstrated that if the vagus which
conducts downwards to the abdominal viscera be cut, and
the cervical sympathetic which conducts up to the head be
also cut, and the central end of the vagus united to the
peripheral end of the sympathetic, vagus fibres grow out-
wards, and when the vagus is stimulated, the results which
naturally follow stimulation of the sympathetic occur.

4. Conduction. — It is found that if a neuron is stimulated
at any one point, some time elapses before the result of the
stimulation is made manifest, and that the farther the point
stimulated is from the structure acted upon, the longer will
be this latent period. This of course indicates that the

6

82

HUMAN PHYSIOLOGY

change, whatever it is, does not develop simultaneously
throughout the neuron, but, starting from one point, be it
one end or the middle, travels or is conducted along. The
rate of conduction may be determined —

~Lst. By stimulating a nerve going to a muscle at two
points at known distances from one another, and measuring
the difference of time which elapses between the contraction

Fio. 38. — M, Muscle attached to crank lever marking on revolving drum. The
secondary circuit of an induction coil is connected with a commutator, with
the crossed wires removed so that the current may be sent either through
the wires going to the nerve at A far from the muscle, or at B, a point at a
measured distance nearer the muscle. On the drum, A represents the onset
of contraction on stimulating at A, and B the onset on stimulating at B.
To secure stimulation in each case with the drum in the same position, the
make and break of the primary circuit is caused by the point K touching
and quitting the point P.

resulting from stimulation at each. (Practical Physiology,
Chap. VII.) (Fig. 38.)

2nd. By taking advantage of the fact that the active
part of a neuron — like the active part of a muscle — is
electro-positive or zincy to the rest, and by finding how long
after stimulation at one point this electric change reaches
another point at a measured distance oft'. (Fig. 39.)

The rate of conduction varies considerably, everything
stimulating protoplasmic activity accelerating, and every-

THE TISSUES 83

thing depressing protoplasmic activity diminishing it. Under
normal conditions in the fresh nerve of the frog, the nerve
change travels about 33 metres per second.

Factors modifying conduction. — Conduction is modified
by the temperature. Cooling a nerve lowers its powers of
conduction, gently heating it increases it. Various drugs
which diminish protoplasmic activity — e.g. chloroform-
diminish conduction. The electric current acts differently
on conduction and on excitability. While a weak current
has little or no effect, a strong current markedly decreases
conductivity round the positive pole, and to a less extent

FIG. 39. — N, a piece of a Nerve connected by non-polarisable electrodes
to the galvanometer, G. By an induction coil, it may be stimu-
lated at A. And when the nerve impulse reaches a, a deflection
of the galvanometer needle takes place.

decreases it at the negative pole, so that the general effect
of a strong current is to decrease conductivity.

From this influence of the electric current upon excita-
bility and conductivity certain differences are to be observed
in the effects of stimulating an exposed nerve with cur-
rents of various strengths and directions. These have
been formulated as Pfltiger's Law, but since they have
no bearing upon the stimulation of nerve in the living
body they need not here be considered. (Practical Physi-
ology, Chap. VII.)

By using the electric changes as an index of nerve
action, it has been found that when a neuron is stimulated

84 HUMAN PHYSIOLOGY

in the middle, the change travels in both directions,

although its result is made manifest by the action of the
structure on which it normally acts. This two-way con-
duction may also be demonstrated by the experiment of
paradoxical contraction, in which by stimulating the branch
of the sciatic nerve of the frog going to the muscles of
the thigh, the nerve fibres to the gastrocnemius lying
alongside of them are also stimulated, and cause that
muscle to contract. (Practical Physiology, Chap. VIII.)

5. Classification of Neurons. — Since a nerve is normally
stimulated from one or other end, and hence conducts in one
direction, and since the passage of impulses along it are made
manifest by changes in the structure to which it goes, it is
possible to classify nerve fibres according to whether they
conduct to or from the central nervous system, and accord-
ing to the structure upon which they act.

To find out the direction of conduction and the special
mode of action of any nerve, two methods of investigation
are employed : —

1st. The nerve may be cut, and the results of section
studied.

2nd. The nerve may be stimulated, and the result of
stimulation noted.

Usually these methods are used in conjunction ; first, the
nerve is cut, and when the changes thus produced have been
noted, the upper end and the lower end of the cut nerve are
stimulated.

It is, of course, only if a nerve is constantly transmitting
impulses that section reveals any change. If the nerve is
not constantly active, stimulation alone will teach anything
of its functions.

Outgoing or Efferent Nerves. — Section of certain nerves
produce a change of action in muscles, glands, &c., or, if
the nerve is not constantly acting, stimulation of the peri-
pheral end of the cut nerve causes some change in the
activity of these structures. Stimulation of the central end
of such nerves produces no effect. These nerves therefore
conduct impulses from the central nervous system outward.

Many of these nerves produce an increase on the activity
of the parts to which they go, but others diminish or inhibit

THE TISSUES 85

activity. The former class may be called augmentor nerves,
the latter inhibitory nerves.

The augmentor nerves may further be divided into groups
according to the structures upon which they act. Those act-
ing on muscle may be called motor nerves ; those acting to
cause secretion from a gland, secretory nerves ; those acting
to constrict blood vessels, vaso-constrictor nerves.

The inhibitory nerves may be similarly subdivided into
musculo-inhibitory, secreto-inhibitory , and vaso-inhibitory
nerves.

Ingoing or Afferent Nerves. — Section of another set of
nerves may produce loss of sensation in some part of the
body. When the peripheral end of the cut nerve is stimu-
lated no result is obtained. When the central end is stimu-
lated, sensations or some kind of action results. Such nerves
obviously conduct to the central nervous system. Those
which, when stimulated, give rise to sensations may be called
sensory; those which give rise to some action are called
excito-reflex, because the action which results is produced by
what is called reflex action. As an example of such a nerve
we may take the branches of the fifth cranial nerve which
pass to the conjunctiva of the eye. When the conjunctiva is
touched — i.e. when this nerve is stimulated — the orbicularis
palpebrarum is brought into action through the seventh
cranial nerve, and the eye is closed. The conjunctival branch
of the fifth cranial nerve is thus an excito-motor nerve.

When the terminations of the lingual nerve in the tongue
are stimulated the result is a free flow of saliva, through the
action of the seventh nerve and the secretory branches of the
glosso-pharyngeal. The lingual nerve is thus excito-secretory.
Further stimulation of the nerves from any part — e.g. by a
mustard blister — causes relaxation of the vessels, and such
afferent nerves may be called excito-vaso-inhibitory.

A consideration of these various examples shows that there
is no hard and fast distinction between sensory and excito-
reflex nerves. Many produce both sensation and reflex action.
Some may at one time produce sensation alone, at another
reflex action alone.

Many nerves of the body contain both afferent and efferent
nerve fibres, and are called mixed nerves.

86 HUMAN PHYSIOLOGY

6. The nature of the " impulse " which passes along a nerve
is due to changes in the axis cylinder, since this without
its sheath can conduct impulses. Secondly, it is dependent
on the vitality of the nerve. Death of the nerve at once stops
the transmission of an impulse.

We may at once dismiss the idea that the impulse is due
to a flow of electricity. Electricity travels along a nerve
with a much higher velocity than the nerve impulse.

Two possibilities remain. The impulse may be of the
nature of a molecular vibration, such as occurs in a stetho-
scope which conducts sound vibration, or it may consist of
a series of chemical changes such as cause the activity of
protoplasm generally.

In considering this matter it must be remembered that
the amount of energy evolved in a nerve impulse need not
be great. All it has to do is to start the activity of the part
to which it goes. Hence if chemical changes are the basis
of the impulse, these may be extremely small in amount and
difficult to detect, while at the same time recuperation may
be extremely active.

As a matter of fact, the evidence of chemical changes in
nerve fibres is entirely wanting. No change in reaction,
no heat production, and no phenomena of fatigue can be
demonstrated.

7. The great function of the cell is to preside over the
nutrition of the neuron. If any part of the neuron is cut off
from its connection with the cell, it dies and degenerates.
In the white nerve fibres this degeneration begins in the
white sheath, which breaks down into globules, and under-
goes chemical changes, so that it is more readily stained with
osmic acid. This is taken advantage of in Marchi's method
for tracing degenerated fibres. The change extends through-
out the whole extent of the nerve at the same time. The axis
cylinder next breaks up, and the nerve corpuscles proliferate
and increase, and absorb the remains of the white sheath and
of the axis cylinder, so that after about twenty days nothing
is left of the nerve but the primitive sheath, filled with the
nucleated protoplasts. According to some observers, after a
time these nucleated structures begin to throw out processes
along the course of the degenerated fibre, and these processes,

THE TISSUES 87

joining together, regenerate the nerve, so that frequently when
the detached end is united to the central part, the nerve is
very rapidly capable of performing its functions again. Other
investigators maintain that the regeneration is always due to
outgrowths from the axis cylinder, either of the central end
of the cut nerve or from adjacent nerves, and this is sup-
ported by the fact that a cut nerve will grow down into any
other nerve with which it is connected (see p. 81).

The cell of the neuron appears to have the power of accu-
mulating a reserve of material as Nissl's granules, for it
has been found that after continued action these granules
diminish in amount. The nucleus, too, would seem to have
the power of giving off material for the nourishment of
the neuron, since in conditions of excessive activity it has
been found shrunken and distorted. Whether the cells play
any other part in the physiology of the neuron is not known.

But the cell is also dependent for its proper nutrition upon
the condition of the rest of the neuron. When the axon is
cut, the chromatin of the cell nucleus decreases, and the
nucleus becomes displaced to one side, and ultimately the
whole cell degenerates. This is sometimes called Nissl's
Degeneration (see Fig. 35).

The passage of excitation from one neuron to others

occupies a very appreciable time. This may be readily
demonstrated in what is called reflex action, which may be
denned as the response through outgoing neurons which
follows the stimulation of ingoing neurons. As examples of
this the drawing up of the leg when the sole is tickled, or
the winking of the eye when the eyeball is touched, may be
taken. In each of these the end of an ingoing neuron is
excited ; the change passes in and sets up changes in out-
going neurons, which act upon muscles. In the case of the
eye, about '06 second elapses between the touching of the
eye and the resulting " wink."

Knowing the rate at which nerve changes pass along
nerves, and knowing the length of the ingoing and of the
outgoing neurons, the time taken in the passage of the
change along these is readily calculated. In a reflex wink
it is about '01 second.

88 HUMAN PHYSIOLOGY

Hence only £th of the total " latent time " of the reflex
action is occupied in the passage of the change along the
neurons, and '05 second, or £th of the whole is taken up in
the passage of the change from one neuron to another.
Obviously the synapsis between the dendrites of the neurons
offers a resistance, and this resistance varies with the con-
dition of the neurons involved, possibly with the condition
of the dendrites which form the synapses.

Not only does the time of the resulting action vary with
the state of the neurons, but the extent also varies. If the
toe of a frog deprived of its brain is pinched, it draws up the
leg; but if a dose of strychnine is first administered, even
touching the toes causes a violent spasm of every muscle in
the body. If, on the other hand, a dose of bromide of
potassium has been administered, or if ice be put on the
back of the animal, much more powerful stimulation is
required to produce any reaction. The activity of the
central synapses may be increased or diminished in various
ways, and hence it is never easy to predicate the ultimate
result of any stimulation of the nervous system. But, other
things being equal, the strength of stimulus applied to the
first neuron — that is the extent of excitation— directly affects
the extent of the resulting action. (Practical Physiology,
Chap. XII.)

THE NEURO-MUSCULAR MECHANISM.

The study of the physiology of muscle and nerve leads to
the consideration of how the neuro-muscular mechanism
acts, so that (1st), the various visceral muscles perform
their functions, and (2nd), so that the co-relationship of the
animal with its surroundings may be maintained. With the
second of these we shall at present deal, leaving the former
for consideration when studying the physiology of the viscera.
This interaction of the body on the surroundings may
be considered first as regards the simpler reactions — the
so-called reflex actions; and secondly as regards the more
complex reactions cominonty classified as voluntary actions.
This neuro-muscular mechanism is controlled by three chains

THE TISSUES

89

or arcs of neurons, consisting of ingoing neurons on the one
side, and outgoing neurons on the other.

1. Spinal or Peripheral Arc— A. Ingoing (Fig. 40, A).—
These neurons start in dendritic expansions at the peri-
phery, and enter the cord by the posterior roots of the spinal
nerves. In these roots they are connected with cells by lateral
branches (see p. 142). When they enter the cord they pass
to the posterior portion, and divide into (a) branches running
for a short distance down the cord; (6) branches run-
ning right up to the top of the spinal cord to end in

I'iC-. 40. — To show the three Arcs in the Central Nervous System. A, Peripheral
ingoing neuron giving off collaterals in the cord and terminating above in the
nuciM of posterior columns ; B, peripheral outgoing neurons ; C, ingoing
cerebra1 neurons; D, outgoing cerebral neurons, decussating at// above the
cord ; E, r'ngoing cerebellar neurons ; F, outgoing cerebellar neurons.

synapses round masses of cells — the nuclei of the posterior
columns.

From these, collateral branches are given off which pass
forward, and, for the most part, end in synapses adjacent to
cells placed in the front part of each side of the cord.

B. Outgoing (Fig. 40, B). — From these cells, fibres are given
off which pass out in the anterior roots of the spinal nerves
to muscles, glands, and other reacting structures. The action
of these neurons is controlled and modified by the two other
series of central neurons.

2. Cerebral Arc— A. Ingoing (Fig. 40, O)— (a) Lower
Neurons. — These are (i.) ingoing neurons of the spinal arc
which lead up to the top of the spinal cord and end in
synapses in the nuclei of the posterior column ; (ii.) inter-
mediate neurons. These start from the cells in the nuclei of

90 HUMAN PHYSIOLOGY

the posterior columns, and, crossing the middle line, run up
to the base of the great brain, where they end in synapses
round other cells in the thalamus opticus.

(6) Upper Neurons. — From these cells, processes pass up
to the surface of the great brain, to end in synapses with the
cells situated there.

B. Outgoing (D). — These start in the cells of the cortex
cerebri, and pass down to the upper part of the spinal cord,
where most of them cross and run down the lateral column
of the spinal cord, giving off collaterals which end in
synapses round the cells in the anterior horn of grey matter,
from which the spinal outgoing neurons pass to the muscles,
&c. Those which do not cross run down the anterior column
of the cord for some distance, and end by crossing and
becoming associated with the cells in the anterior horn.

3. Cerebellar Arc— A. Ingoing (E).— Some of the collaterals
of the spinal ingoing neuron end in synapses round a mass
of nerve cells at the side of the grey matter of the spinal
cord — the cells of Lockhart Clarke. From these cells, fibres
extend up at the side of the cord to the lesser brain or
cerebellum, to form, directly or indirectly, synapses round
the cells in this organ.

B. Outgoing (F). — From cells near the surface of the cere-
bellum and in the roof nucleus (a) axons extend to the
medulla oblongata, where they end in synapses round a mass
of cells — the nucleus of Deiters. From these cells, fibres
extend down the lateral columns of the spinal cord, and
give off collaterals to the cells of the anterior horn of grey
matter. (6) Other neurons pass to the cerebrum.

The nervous system may thus be considered as built
up of these three sets of arcs.

1st. The Spinal arcs, consisting of the peripheral ingoing
neurons and the peripheral outgoing neurons. These arcs
are not only at the level of the cord at which the ingoing
neuron enters, but at various levels above and below this
point.

2nd. The Cerebral arcs, consisting of the peripheral, the
intermediate and the upper ingoing neurons, and the central
outgoing neurons, and the peripheral outgoing neurons.

3rd. The Cerebellar arcs, consisting of peripheral ingoing

THE TISSUES 91

neurons, the cerebellar ingoing neurons, the outgoing cere-
bellar neurons, either direct to the cord or through the
cerebrum, and the peripheral outgoing neurons.

A. Simple Reactions.

Reflex Action. — This has already been considered shortly
in dealing with the stimulation of outgoing neurons through
their synapses with ingoing neurons. It has been shown
that considerable time is occupied in the passage of the
stimulus, and that the resulting action depends upon the
condition of the neurons. In the ordinary reflex action the
synapses involved are situated in the spinal cord.

Spinal Reflex Action. — The simplest manifestation of
reflex action is to be seen in the effect of pinching the toe
of a frog in which the brain has been destroyed so that the
spinal arcs can act without interference. If the pinch is
gentle the foot is simply drawn up. If the pinch is stronger
a more extensive movement occurs, and if the leg is held
firmly the opposite limb is drawn up. If a piece of paper
dipped in acetic acid is laid on the animal's flank, extensive
and well co-ordinated movements are made with one or both
limbs to remove the irritant. In all cases the act is a definite
and co-ordinated one, bringing about an appropriate, but
inevitable reaction of the animal on its surroundings. This
is due to the passage of an impulse inwards, which sets
up changes in the outgoing neurons. With a very gentle
stimulus the effect manifests itself at the same level of the
cord, but with more powerful stimuli other levels of the cord
are acted upon and a more extensive movement results.
Generally speaking, stimulating one hind limb causes a
movement first in the limb stimulated, then in the fore limb
of the same side, then in the opposite fore limb, and lastly in
the opposite hind limb. (Practical Physiology, Chap. XII.)
In the " spinal dog " — a dog with the spinal cord separated
from the brain by section in the dorsal region — Sherrington
finds that different kinds of stimuli produce different kinds
of result.

In reflex action the shortest arc is usually the line of least
resistance along which the nerve-change travels, and more

92 HUMAN PHYSIOLOGY

distant arcs are less readily brought into play. But in various
more complex reflex actions certain special arcs are associated,
apparently as the result of their having been educated to
act together, and whether this association is an inherited one,
or whether it has been acquired, there seems to be a ten-
dency for nerve action, which has once travelled by a definite
route, to take the same route again. Definite channels of
communication connecting ingoing impulses with definite
outgoing reaction are thus established.

The spinal reflexes are modified — 1st, By the condition of
the neurons (p. 81). 2nd, By stimuli from adjacent areas.
The reflex act of sneezing, set up by stimulation of the
nasal mucous membrane, may be checked by firmly pressing
over the bridge of the nose. 3rd, By the upper arcs.
Spinal reflexes are increased when the cord is separated from
the brain.

Cerebral and Cerebellar Reflex Actions. — The upper arcs
may also act reflexly — i.e. inevitably. Thus stimulating
certain areas of skin in the dog leads to a reflex scratching
in which the body is bent and balanced, while the hind
leg performs complex movements. Here the balancing
action of the cerebellar arc, as well as the directive action of
the cerebral arc, are both involved. But since the action is
obviously inevitable, it is classed as a reflex.

B. More Complex Reactions.

It has been seen that the spinal reflexes are noc absolutely
inevitable since they are modified by the condition of the
nervous structures involved.

When the more complex reactions, in which the cerebral
arc is involved, are studied, the resulting action appears
to be less inevitable, and to be influenced by the sensations
and other changes in consciousness which accompany it.
It has, in fact, been assumed that the state of consciousness
is the determining factor in the result, and hence such
actions have been called voluntary. But since in such
conditions as sleep-walking and hypnosis the most complex
and selective actions are performed without the intervention

THE TISSUES 93

of consciousness, it must be admitted that this metaphysical
phenomenon is not an integral part of the response of the
nervous system.

On the other hand, we know that the character of the
reaction to any stimulus is largely dependent upon the state
of the nervous centres. Just as a touch produces a different
effect in a frog poisoned with strychnine, and in one under
the influence of bromide of potassium, so a sudden noise may
produce a totally different reaction upon a person with a
fatigued brain or a brain poisoned by alcohol, and upon one
with the brain in a good state of nutrition.

Not only does the temporary state of nutrition thus
modify the result of a stimulus, but the paths of action
previously opened and defined through the centres also have
a marked influence. These paths may have been formed in
past generations and inherited from the parents. In young
fowls, as soon as they are hatched, the acts of walking and
of pecking are at once performed, and in many families
particular gestures or expressions follow certain modes of
stimulation in many different individuals without the con-
sciousness of the person being involved. They are inherited
cerebral reflexes. Paths may also have been developed
in the individual as the result of previous activities of the
nervous mechanism. For, if a given action has once followed
a given stimulus, it always tends to follow it again. This,
in fact, is the basis of all rational education — to open up
paths in the nervous system by which the most suitable
response may be made to any given stimulus; and to
prevent the formation of paths by which inappropriate
reaction may be produced.

It is very important to recognise clearly the influence
of these factors upon the conduct of the individual — the
nutrition of the brain at any moment, and the inherited and
acquired tendencies in particular directions — since various
abnormalities in moral and social conduct may be explained
by reference to them, and since cases of so-called insanity
are frequently dependent upon them.

With the relationship of consciousness to these reactions
of the nervous system we shall not deal at present. Con-
sciousness is a purely metaphysical conception, and we do not

94 HUMAN PHYSIOLOGY

know its relationship to cerebral action further than that we
have no evidence that consciousness can manifest itself apart
from such cerebral activity.

Fatigue of the Neuro-Muscular Mechanism.

Continued action of this mechanism leads to fatigue, and
this may best be studied by means of some form of ergograph,
an instrument which enables the response of a muscle to
stimuli to be recorded. If a muscle be reflexly stimulated
again and again it finally ceases to react, but if now the out-
going nerve is stimulated the muscle contracts at once. This
shows that fatigue first manifests itself in the central synapses.
If the outgoing nerve be repeatedly stimulated, after a time
the muscle no longer responds, but if the muscle be directly
stimulated it contracts; but since the electrical changes
which accompany conduction in a nerve still go on it is
obvious that the nerve still acts. It is therefore the nerve
ending in the muscle which is fatigued. Fatigue is due
to the accumulation of the products of the activity of muscle,
and it may be induced in a normal dog by injecting the
blood from a dog which has been fatigued.

It is most important to keep clearly in mind the meaning
of stimulus, reaction, and sensation, (a) Stimulus is the
change in the surroundings which produces (6) the Reaction,
the modification in the action of some part of the body.
(c) Sensation is the change in the consciousness which may
accompany the application of a stimulus and the reaction.

SECTION IV

THE SENSES

IN order that each particular kind of change in our sur-
roundings should produce its appropriate reaction, it is
essential that the different kinds of changes should act in
different ways — that the contact of gross matter, changes of
temperature, vibrations of the air, vibrations of the ether,
and various chemical changes should each produce a special
effect.

To secure this, special peripheral developments of neurons
have been evolved which react more particularly to each of
these special kinds of change, and with these peripheral
neurons particular parts of the brain are connected and
associated, so that a reaction to the various stimuli may
occur. These reactions may be accompanied by changes in
the consciousness — by sensations ; and since our consciousness
is our instrument of knowledge — our Ego — these sensations
appear to us the chief and most important part of the action
of the mechanism. That, in many reactions, it is not an
essential part, we have already indicated.

A. COMMON SENSIBILITY.

Before dealing with the special senses the phenomena of
common sensibility may be considered. By this is meant a
series of somewhat indescribable but quite definite sensations
by which our attention is directed to the state of the body
as a whole or of various parts of the body. The ordinary
sensations of thirst and hunger are examples of these, sensa-
tions which, although due to changes in the mouth, throat,
or stomach, give us information as to the general needs of
the body. Such sensations may be considered as normal
and physiological. But when abnormal conditions exist in

95

96 HUMAN PHYSIOLOGY

certain localities they produce sensations such as tickling,
tingling, &c., and generally lead to an endeavour to remove
the abnormal stimulus.

Those modifications of consciousness produced by this
mechanism of common sensibility may be small or great
according to the strength of the stimulus, and according to
the state of the central nervous system, and when excessive
the sensation produced is called pain. All pain, since it
means a change in our consciousness, is metaphysical. There
is not such a thing as " physical pain." The fatigue and
other sequences to any kind of pain are frequently cited as
proofs of the influence of mind on the body. But we have
no right to assume that they are caused by the pain rather
than by the physical disturbances in the nervous system of
which the pain is an accompaniment. It must be recognised
that pain is a purely relative term, and that conditions which
in one individual will cause pain will not cause it in another,
while stimuli which will produce what are called painful
sensations when the nervous system is debilitated may give
rise to sensations not considered as painful when the nervous
system is normal.

The mechanism of common sensibility and pain is not
acted on by the same stimuli in all parts of the body. The
mouth and throat are the parts to which the sensations are
referred in abstinence from fluids, the stomach in absence of
food. The intestine appears to give rise to sensation only
when abnormally stimulated. In the skin the mechanism of
common sensibility is so closely associated with the mechanism
of the tactile and temperature senses that it is difficult to
differentiate them. Abnormal stimulation of the skin pro-
duces painful sensations very readily, while similar changes
in other tissues — e.g. muscles — cause no modification of
consciousness.

The nerve channels by which the changes producing
common sensibility and pain are transmitted to the central
nervous system appear to be distinct from those connected
with the tactile and other senses, since common sensibility
may persist while the tactile sense is lost.

THE SENSES

97

B. MUSCLE AND JOINT SENSE.

Closely allied to the mechanism of common sensibility is
the arrangement by which we are made aware of the
movements and thus of the position of various parts of
the body. A double mechanism is here involved — 1st,
A mechanism stimulated by the contraction of the muscles ;
and 2nd, a mechanism acted on by movements at the
joints.

(1st) Muscle Spindles. — Among the fibres of the muscles
are found long fusiform structures containing modified parts
of the muscle fibres. Into each spindle a medullated nerve
passes and breaks up into a non-medullated plexus round
the fibres. (Fig. 41.)

(2nd) Organs of Golgi are swellings
in the tendons near the muscle fibres
into which a medullated fibre enters, and
losing its white sheath forms a plexus
of fibrils with varicosities upon them.

(3rd) Varicose terminations of axons
surrounded by fibrous tissues are found
in the synovial membranes and round
joints.

Through these mechanisms informa- Fia 4L —"structure
tion is transmitted to the central nervous
system as to the position and move-
ments of the various parts, and this,
although not necessarily modifying the
consciousness, is of service in guiding
the movements. When the conscious-
ness is affected, valuable information as to the conditions of
the surroundings may be gained. In estimating the weight
of bodies, these sensations are much used. The body is taken
in the hand, and by determining the amount of muscular
contraction required to support or raise it, the weight is
estimated. The shape and size of objects are also determined
by this sense in conjunction with the sense of touch. If
we touch a book on the table we can form an idea of its
shape and size by estimating the distance through which

' 7

of

muscle spindle— only
one fibre represented.
6, capsular space ; c,
capsule; p, motor ter-
mination ; T.S., sen-
sory termination on the
spindle fibre. (From
EEGAUD and FAVRB. )

98

HUMAN PHYSIOLOGY

the hand may be moved in different directions. In the dark
the distance of objects is also judged by estimating the extent
of movement of the hand and arm necessary to touch them.
The sensations derived from the joint and muscle senses are
often lost or impaired, and the condition of the mechanism
may be tested by ascertaining the smallest difference of
weight which can be appreciated. With moderate weights of
about one pound, a difference of about 5 per cent, can usually
be detected in the normal condition. (Experiment.)

C. SPECIAL SENSES.

I. Tactile Sense.

The Tactile Corpuscles, which consist of a naked branch-
ing varicose termination of an axon surrounded by sheaths

of fibrous tissue, are situated in
the papillae of the true skin
(Fig. 42).

The study of the sense of
touch may be approached by
touching the table and analys-
ing the manner in which the
conclusion is arrived at that a
table is touched. We conclude
it is a table because the surface
is hard and smooth. This judg-
ment may or may not be correct.
But even in saying that the

Fio. 42.— Simple form of sensory body W6 touch IS hard and
nerve termination. In the tac- ., i r •

tile corpuscle the nerve fibre Smooth, W6 are also forming

coils round the capsule before judgments from the sensations
entering. (DooiEL.) experienced. When we say the

surface is hard we mean it resists pressure, and when we
say it is smooth we mean that the skin of our finger is
uniformly touched and not pressed upon at certain points
as it would be if the surface were rough. Or it may be
that we draw our finger over the table and feel a con-
tinuous contact, and not the series of contact which we
should experience were the surface rough. The determina-

THE SENSES 99

tion of the resistance to pressure implies the power of dis-
tinguishing differences of pressure. The determination of a
continuous contact instead of a series of local contacts im-
plies the power of determining or localising the part or parts
touched, and the ability to distinguish between a continuation
of contact and a succession of contacts implies the power of
differentiating contacts in time.

The tactile sense may thus be best studied under three
heads : —

1. The Power of distinguishing Differences of Pressure.
— Variations of pressure in time and space are alone dis-
tinguished. We live under a pressure of 760 mm. of mer-
cury, but this gives rise to no sensation. Any sudden in-
crease or diminution of pressure, however, leads to a marked
change of sensation, but a slow change causes a lesser modi-
fication of consciousness. If a part of a body is uniformly
pressed on, as when a finger is immersed in mercury, the
sensation of pressure is felt as a ring at the surface of the
mercury, where the greater pressure of the mercury joins the
lesser pressure of the air. (Experiment.)

For these reasons in testing the acuteness of the pressure
sense over the surface of the body, these two factors must
be kept in mind. The rate at which the pressure is
varied and the pressure on adjacent parts of skin must be
kept uniform. The part of the body being supported so that
the muscular sense cannot come into play, different weights
may be applied to ascertain the smallest difference of weight
which can be distinguished. (Experiment.)

The pressure sense varies in different parts of the body,
being most acute where the nerve terminations are most
abundant, and less acute where they are fewer. Over the
points of the finger, with a weight of about 1 gram., a differ-
ence of about 10 per cent, can be distinguished, but over the
leg the difference must be much greater. Everything which
diminishes the activity of protoplasm diminishes the acute-
ness of the pressure sense, and therefore when the skin is
cold the sense is much less acute than when it is warm.

Again, since the sensation is a modification of conscious-
ness, it is modified by the state of the central nervous
system. This is readily fatigued, and hence, if the tests are

ioo HUMAN PHYSIOLOGY

applied for too long a period at one time, the acuteness of
perception diminishes.

2. The Power of Localising the Place of Contact.—
Where the tactile organs are abundant, the power of dis-
tinguishing accurately the point touched is more acute than
in places where these are more scattered. For this reason, if
two contacts are made at the same time, these may be very
close together in the former situation, and each of them may
be localised and felt as distinct from the other, whereas in
the latter situation, they would be felt as a single contact.
The power is therefore tested by determining how near two
points of contact may be brought to one another, and still
cause a double sensation. This may be done by means of
some form of aesthesiometer — e.g. a pair of compasses ; and, in
using it, it is necessary to observe certain precautions. First,
the two points must touch the skin simultaneously. Second,
they must touch it lightly and with equal pressure each time.
Third, they must not be worked steadily from close together
to further apart, or vice versa, but must be used now in the
one way now in the other. (Experiment.)

In this way it is easy to demonstrate that over the tips of
the fingers where tactile organs are very abundant the two
points of contact will give rise to a double sensation when
no more than 2 mm. apart, while over the back of the
hand they do not give a double sensation till they are about
30 mm. apart. Over the thigh and back they must be no
less than 50 to 70 mm. apart.

Both the peripheral mechanism and the central nervous
system are involved in this localisation, and hence the power
varies with the condition of the skin as regards tempera-
ture, &c., and with the state of the nervous system.

3. The Power of distinguishing Contacts in Time. — If
the finger be brought against a toothed wheel rotated slowly,
the contacts of the individual teeth will be separately felt.
But if the wheel is made to rotate more and more rapidly,
the separate sensations are no longer felt, but a continuous
sense of contact is experienced. This indicates that, if stimuli
follow one another sufficiently rapidly, the sensations pro-
duced are fused. From this it is obvious that the sensation
lasts longer than the stimulus — the contact.

I I I I I

L^ lX^~b<r~P^\i\
L - M i \ l i I i

THE SENSES 101

The duration of the sensation depends upon the degree
of stimulation of the peripheral tactile organs. When
the stimuli are strong, as in
the contact of the finger with
the rough edge of a toothed
wheel, the sensations last a
considerable time, and thus
stimuli, following one another

at about 500 times per Second, FlG- 43.— Relationship of Sensation
r r , . n to Stimulus, with weak and strong

cause a fusion of sensation. On stimuli> stirauli represented by

the Other hand, if a Violin String vertical lines— the strength being

is made to vibrate against the ^^ed by their height. Sensa-

, ... tions represented by the curves.

nnger, the stimuli are weak, and

the resulting sensation of short duration, and hence stimuli
may follow one another with greater rapidity, say to 1200
per second, and still be distinguished as separate sensations.

II. Temperature Sense.

Heat, like light, is physically a form of vibration of the
ether. The temperature sense depends upon the fact that
when heat is withdrawn from the body we have one kind
of sensation which we call cold, and when heat is added to
our body another sensation which we call hot. This depends
upon the temperature of our body in relationship to the
surroundings, and not merely on the temperature of sur-
rounding bodies. If three basins of water are taken, one
very hot, one very cold, and one of medium temperature, and
if a hand be placed, one in the very hot and one in the very
cold water for a short time, and then transferred to the basin
with water at a medium temperature, the water will feel hot
upon the hand that has been in the cold water, and cold to
the hand that has been in the hot water. (Experiment.)

The rate at which heat is abstracted or added is the
governing factor in causing the sensation ; a sudden change
of temperature stimulates far more powerfully than a slow
change. For this reason the thermal conductivity of sub-
stances in contact with the skin has an influence upon the
sensation. If a piece of iron and a piece of flannel side by
side be touched, the first will feel cold, the second will not,

102 HUMAN PHYSIOLOGY

because the first has high thermal conductivity, the second
has not, and thus the former abstracts heat more rapidly
than the latter.

Certain parts of the skin are stimulated by the withdrawal
of heat to give rise to sensations of cold, while others are
stimulated by the addition of heat. This may be demon-
strated by taking the cold point of a pencil and passing it
over the back of the hand when it will be felt as cold only at
certain points ; such points have been called cold spots, while
similar spots stimulated by the addition of heat are called
hot spots. (Experiment.)

The acuteness of the temperature sense may be tested by
finding the smallest difference of temperature which can be
distinguished. This may be done by taking two test tubes
in which thermometers have been placed, and filling them
with water at slightly different temperatures, and then apply-
ing them to different parts of the skin. If the temperature
is very low or very high, differences of temperature are not
readily distinguished, and in fact painful sensations may
take the place of temperature sensations. But between 15°
and 50° C. the power of distinguishing differences of tempera-
ture is fairly constant, but it varies in different parts of the
body. Over the cheek as small a difference as '2° C. can be
appreciated, while on the back the difference must be as great
as -9° C. (Experiment.)

The temperature sense is independent of the tactile sense.
The one may be lost and the other retained. It is probable
that the nerve endings in the deeper layers of epithelium
are connected with the temperature sense. But although
independent, the tactile and thermal senses influence one
another. A cold body placed on the skin feels heavier than
a warm body, as may be shown by placing first a cold penny
and then a warm penny on the skin of the forehead.

III. Vision.

While the addition to and withdrawal from the surface of
the body of the slower waves of ether which are the basis of
heat act upon the special nerve terminations in the skin to
give rise to sensations of heat and cold, a certain range of

THE SENSES 103

more rapid vibrations act specially upon the nerve endings
in the eye to produce molecular changes which in turn affect
the centres in the brain and give rise to changes in con-
sciousness which we call light. The range of vibrations
which can act in this way is comparatively limited, the
slowest being about 435 billions per second, the most rapid
about 764 billions. Vibrations more rapid than this, which
are capable of setting up chemical changes, as in photo-
graphy, do not act upon the eye.

The action of light upon the protoplasm of lower organ-
isms has been already considered (p. 16), and it has been
seen that it may be either general or unilateral, producing
the phenomenon of positive or negative phototaxis. In more
complex animals special sets of cells are specially set aside to
be acted on by light, and these are generally imbedded in
pigmented cells to prevent the passage of light through the
protoplasm. Such an accumulation of cells constitutes an
eye, and in the simpler organisms such an eye can have no
further function than to enable the presence or absence of light
or various degrees of illumination to produce their effects.

But in the higher animals these cells are so arranged that
certain of them are stimulated by light coming in one direc-
tion, others are stimulated by light coming in another, and
while the former are connected with one set of synapses in
the brain, the latter are connected with another. Thus light
coming from one point will stimulate one set of cells which
will excite one part of the brain, and light from another will
act upon other cells which will excite another part of the
brain, and thus not merely the degree of illumination but the
source of illumination becomes distinguishable.

It is by this arrangement that it becomes possible to form
ideas of the shape of external objects. One directs the eye
to the corner of the ceiling, and the idea that it is a corner
is due to the fact that three different degrees of illumination
are appreciated, and that these can be localised — one above,
one to the right, and one to the left. One set of cells is
stimulated to one degree, another set of cells to another
degree, and a third set of cells to a third degree ; and the
different stimulation of these different sets of cells leads to a
different excitation of separate sets of cells in the brain.

104 HUMAN PHYSIOLOGY

This is associated with the perceptions of the three parts
differently illuminated. From the previous training of the
nervous system we are taught to interpret this as due to a
corner. But this interpretation is simply a judgment based
upon the sensations, and it may or may not be right. Thus,
instead of actually looking at a corner we may be looking at
the picture of one.

From the very first it must be remembered that the modi-
fication of our consciousness which we call vision is not
directly due to external conditions, but is due to changes set
up in our eye by these external states. We do not perceive
the object we are looking at, but simply the changes in our
brain produced by changes in the eye set up by rays of light
coming from the object.

Usually such changes are set up by a certain range of
vibrations of the ether, but they may be set up in other
ways — e.g. by the mechanical stimulation of a blow on the
eye ; but, however set up, they give rise to the same kind of
changes in consciousness — visual sensations. This fact has
been formulated in the doctrine of specific nerve energy,
that different varieties of stimuli, applied to the same organ
of sense, always produce the same kind of sensation. And the
converse that the same stimulus applied to different organs
of sense produces a different kind of sensation for each also
holds good.

The visual mechanism not only gives the power of appre-
ciating the degree and source of illumination, but also of
appreciating colour. Physically the different colours are
simply different rates of vibration of the ether, physiologi-
cally they are different sensations produced by different
modes of stimulation of the eye. The slowest visible
vibrations produce changes accompanied by a sensation
which we call red, the most rapid vibrations produce
different changes which we call violet. But, as will be
afterwards shown, these sensations may be produced by
other modes of stimulating the eye.

The visual mechanism in this way gives a flat picture of
the outer world, and from this flat picture we have to form
judgments of the size, distance, and thickness of the bodies
looked at.

THE SENSES 105

The idea of size depends upon the extent of the eye-cells
stimulated by the light coming from a body. If a large
surface is acted upon, the body seems large ; if a small
surface, the body seems small. But the extent of eye-cells
acted on depends not merely upon the size of the object,
but also upon its distance from the eye. Hence, our ideas of
size are judgments based upon the size of the picture in the
eye, and the appreciation of the distance of the object. The
distance of an object, when over fifty or sixty metres from the
eye, and very probably even when over as little as six metres,
is judged by the modifications in its shading and colour due
to the condition of the atmosphere. A range of hills will at
one time be judged to be quite near, at another time to
be distant. Since the estimation of the size of an object
depends upon the judgment of its distance, the estimation
we make of the size of such objects as a range of hills is often
most erroneous. When objects are nearer to the eye, a
special mechanism comes into play to enable us to determine
their distance (see p. 114).

The idea of thickness or contour of an object is also largely
a judgment based upon colour and shading. When a cube is
looked at, we judge that it is a cube because of the degrees of
illumination of the different sides— degrees of illumination
which may be reproduced in a flat picture of such a cube.
When the object is near the eyes, by using the two eyes
together a means of determining solidity comes into action
(see p. 124).

When the manner in which we gain knowledge of our
surroundings by vision is analysed, it must be admitted that
the dictum " seeing is believing" has at best an unsubstantial
physiological basis, and that most of the points about any-
thing which we say we see — e.g. its size, distance, and contour —
are largely judgments formed by us upon a flat picture pro-
duced in the cells of the eye, which flat picture has in turn
led to these changes in our brain which are accompanied by
the changes in our consciousness upon which our judgment
has to act.

Any defect in the visual mechanism must, and does, lead
to defects in the mental picture formed, and the accuracy of

106 HUMAN PHYSIOLOGY

the judgment will depend upon the accuracy of the picture,
and upon the previous experience and training of the nerve
structures involved.

But, further, while the parts of the brain connected with
the visual sense are usually stimulated by changes in the
cells of the eye, they may be directly stimulated ; and when
this is the case, a sensation of light, apparently in front of
the eye, is experienced, because the centres are naturally
always stimulated by such illumination. Sensations thus
produced are called illusions, and they are well illustrated by
the flashes of light before the eyes which sometimes precede
tin epileptic attack and which are caused by direct irritation
of the surface of the brain.

The study of vision may be taken up in the following
order : —

1. The mode of formation of pictures on the nerve struc-
tures (retina) of the eye.

(1) One eye (monocular vision).

A. The method in which rays of light are focussed (diop-
tric mechanism).

B. The method in which the retina is stimulated.

(2) Two eyes (binocular vision).

2. The conduction of the nerve impulses from the retina
to the brain.

3. The position and mode of action of the parts of the
brain in which the changes are set up which accompany
visual sensations (the visual centre).

1. The Mode of Formation of Pictures upon the Retina.

(1) MONOCULAR VISION.
A. The Dioptric Mechanism.

Anatomy. — Before attempting to study the physiology
of the eye, the student must dissect an ox's or a pig's eye,
and then make himself familiar with the microscopic struc-
ture of the various parts.

THE SENSES

107

C,IM

The eye may be described as a hollow sphere of fibrous
tissue (Fig. 44), the posterior part, the sclerotic (Scl.), being
opaque ; the anterior part, the cornea (Cor.), being transparent
and forming part of a sphere of smaller diameter than the
sclerotic. Inside the sclerotic coat is a loose fibrous layer,
the choroid (Chor.), the connective tissue cells of which are
loaded with melanin, a black pigment. This is the vascular
coat of the eye — the larger
vessels running in its outer
part, and the capillaries in its
inner layer. Anteriorly, just
behind the junction of the
cornea and sclerotic, it is
thickened and raised in a
number of ridges, the ciliary
processes (Oil. J/.), running from
behind forward and termi-
nating abruptly in front. In
these the ciliary muscle is
situated. It consists of two

Sets of non-Striped muscular FIG. 44. —Horizontal section through the

fibres — first, radiating fibres,
which take origin from the
sclerotic just behind the corneo-
sclerotic junction, and run
backwards and outwards to be
inserted with the bases of the
ciliary processes; second, circu-
lar fibres which run round the processes just inside the
radiating fibres. The choroid is continued forward in front
of the ciliary processes to the pupil as the iris, and in it are
also two sets of non-striped muscular fibres — first, the circular
fibres, a well-marked band running round the pupil, and
called the sphincter pupilLv (Sph. P.) muscle ; second, a less
well-marked set of radiating fibres, which are absent in
some animals, and which constitute the dilator pupillte
muscle (D.P.).

That part of the eye in front of the iris is filled by a
lymph-like fluid, the aqueous humour, while the part behind
is occupied by a fine jelly-like mucoid tissue, the vitreous

Left Eye. Cor. .cornea ; Scl. , sclerotic ;
0[>t. N., optic nerve ; Chor. , choroid ;
CiL M., ciliary processes with ciliary
muscle ; D. P. .dilator pupillse muscle ;
Sph. P. , sphincter pupillse muscle ;
L. , crystalline lens; S.L., hyaloid
membrane forming suspensory liga-
ment and capsule of lens. Ret.,
retina.

io8

HUMAN PHYSIOLOGY

humour. The vitreous humour is enclosed in a delicate
fibrous capsule, the hyaloid membrane, and just behind the
ciliary processes this membrane becomes tougher, and is
so firmly adherent to the processes that it is difficult to strip
it off. It passes forward from the processes as the suspensory
ligament (S.L.), and then splits to form the lens capsule.
In this is held the crystalline lens (L.), a biconvex lens, with

its greater curvature on its pos-
terior aspect, and characterised by
its great elasticity. Normally it
is kept somewhat pressed out and
flattened between the layers ot'
the capsule, but if the suspensory
ligament is relaxed its natural
elasticity causes it to bulge for-
ward. This happens when the
ciliary muscle contracts and pulls
forward the ciliary processes with
the hyaloid membrane.

Between the hyaloid membrane
and the choroid is the retina (Ret.).
This is an expansion of the optic
nerve, which enters the eye at 3 to
4 mm. to the inner side of the
posterior optic axis (Fig. 45). The
white nerve fibres pass through
the sclerotic, through the choroid,
and through the retina, to form the
white optic disc, and then losing
their white sheath, they spread
out in all directions over the front
of the retina, to form its first layer

—the layer of nerve fibres (1). These nerve fibres take
origin from a layer of nerve cells (2) behind them, forming
the second layer. The dendrites of these cells arborise with
the dendrites for the next set of neurons in the third layer,
the internal molecular layer (3). The cells of these neurons
are placed in the next or fourth layer, the inner nuclear
layer (4), and from these cells, processes pass backwards to
form synapses in the fifth, or outer molecular layer (5), with

FIG. 45. — Diagram of a Sec-
tion through the Retina
stained by Golgi's me-
thod. For description,
see text. (From VAN
GBHUCHTEN. )

THE SENSES

109

the dendrites of the terminal neurons. These terminal
neurons have their cells in the sixth or outer nuclear layer
(6) of the retina, and they pass backwards and end in two
special kinds of terminations in the seventh layer of the
retina — the rods and cones (7). These structures are com-
posed of two segments — a somewhat barrel-shaped basal piece,
and a transparent terminal part which in the rods is cylindri-
cal and in the cones is pointed. Over the central spot of the
eye there are no rods, but the cones lie side by side, and the
other layers of the retina are thinned out. The rods and
cones are imbedded in the last or eighth layer of the retina

FIG. 46. — To show how parallel rays are brought to a focus on the
retina by refraction at the three surfaces (a), anterior sur-
face of the cornea ; (6), anterior surface of the lens ; and
(c), posterior surface of the lens.

— the layer of pigment cells, or tapetum nigrum. The
retina stops abruptly in front at the ora serrata, but the
tapetum nigrum, along with another layer of epithelial cells
representing the rest of the retinal structures, is continued
forwards over the ciliary processes and over the back of the
iris.

The blood vessels of the retina enter in the middle of the
optic nerve, and run out and branch in the anterior layer of
the retina.

The interior of the eye may be examined by the Ophthal-
moscope, which consists essentially of a small mirror from
which light can be reflected into the back of the eye,

i io HUMAN PHYSIOLOGY

with a small hole in the centre through which the observer
can study the illuminated part of the chamber. (Experi-
ment.)

Physiology. — The eye may be compared to a photographic
camera, having in front a lens, or lenses, to focus the light
upon the sensitive screen behind (Fig. 46). The picture
is formed on the screen by the luminous rays from each
point outside being concentrated to a point upon the screen.
This is brought about by refraction of light as it passes
through the various media of the eye — the cornea, aqueous,
crystalline lens, and vitreous. The refractive indices of
these, compared with air as unity, may be expressed as
follows : —

Cornea . . 1'33 Lens . . 1'4~>
Aqueous . 1'33 Vitreous . 1'33

Thus light passes from a medium of one refractive index
into a medium of another refractive index —

1. At the anterior surface of the corn* -a :

2. At the anterior surface of the lens ;

3. At the posterior surface of the lens ;

and at these surfaces it is bent. The degree of bending
depends upon — 1st, The difference of refractive index. 2nd,
The obliquity with which the light hits the surface. This
will vary with the convexity of the lens — being greater the
greater the convexity.

The posterior surface of the lens has the greatest con-
vexity, with a radius of 6 mm. The anterior surface of
the cornea has the next greatest, with a radius of 8 mm.
The anterior surface of the lens has the least, with a radius
of 10 mm. A ray of light passing obliquely through these
media will be bent at the three surfaces.

xThese media in fact form a compound lens composed of
a convexo-concave part in front, the cornea and aqueous,
and a biconvex part behind, the crystalline lens. In the rest-
ing normal eye (the emmetropic eye) the principal focus is

THE SENSES in

exactly the distance behind the lens at which the layer of
rods and cones in the retina is situated, and thus it is upon
these that light coming from luminous points at a distance
is focussed.

Positive Accommodation. — If an object is brought nearer
and nearer to the eye, the rays of light entering the eye
become more and more ' divergent,
and if the eye be set so that rays
from a distance — i.e. parallel rays
— are focussed, then rays from a ~^^_^^ J

nearer object will be focussed be- FlG> 47._T0 show that rays
hind the retina, and a clear image from distant and near objects
will not be formed (Fig. 4V). This ^l™ °° the reti™
means that near and far objects

cannot be distinctly seen at the same time, a fact which
can be readily demonstrated by Scheiners Experiment.
(Experiment.)

Make two pin holes in a card so near that they fall witnin
the diameter of the pupil. Close one eye and hold the holes
in front of the other.
Get some one to
hold a needle against
a white sheet of
paper at about three

x L .. ,, FIG. 48. — Schemer s Experiment.

yards from the eye,

and hold another needle in the same line at about a foot
from the eye. When the far needle is looked at the near
needle becomes double.

It is found practically that objects at a greater distance
than 6 metres may be considered as " distant," and that they
are focussed on the retina.

Objects may be brought nearer and nearer to the eye, and
yet be seen distinctly up to a certain point, the near point
of vision within which they cannot be sharply focussed upon
the retina. This, however, requires a change in the lens
arrangement of the eye, and this change, beginning 'when
the object comes within about 6 metres (the far point of
vision), becomes greater and greater till it can increase no
further when the near point is reached. The change is
called positive accommodation, and it consists in an in-

H2 HUMAN PHYSIOLOGY

creased curvature of the anterior surface of the lens. This
may be proved by examining the images formed from the
three refracting surfaces when it will be found that the
image from the anterior surface of the lens becomes smaller
and brighter (Sanson's images). The examination of these
images is facilitated by the use of the Phacoscope. (Experi-
ment.)

Positive accommodation is brought about by contraction
of the ciliary muscle, which pulls forward the ciliary pro-
cesses to which the hyaloid membrane is attached, and thus

relaxes the suspensory
ligament of the lens and
the front of the lens
capsule, and allows the
natural elasticity of the
lens to bulge it forward
(Fig. 49).

FIG. 49.— Mechanism of Positive Accommodation. rpu: rV,imr« of nosj
The continuous lines show the parts in . P°

negative accommodation, the dotted lines tlV6 accommodation is

the positive accommodation. accompanied by a con-

traction of the pupil

due to contraction of the sphincter pupilhe muscle. By this
means the more divergent peripheral rays which would
have been focussed behind the central ones are cut off, and
spherical aberration is prevented.

The muscles acting in positive accommodation — the ciliary
and sphincter pupilLe (Fig. 50, C.M. and S.P.) — are supplied
by the third cranial nerve (///.), while the dilator pupilLr is
supplied by fibres passing up the sympathetic of the neck.
The centre for the third nerve is situated under the aqueduct
of Sylvius, and separate parts preside over the ciliary muscle
and the sphincter pupillse (see p. 107).

The sphincter centre is refiexly called into action, and
the pupil contracted. 1st, When strong light falls on the
retina and stimulates the optic nerve. In this way the
retina is protected against over stimulation. 2nd, When
the image upon the retina becomes blurred as the object
approaches the eye. At the same time the centre for the
ciliary muscle is also called into play.

The centre for dilatation of the pupil is situated in the

THE SENSES

D.f-

S.CG

medulla oblongata. Like the centre of the sphincter it
may be reflexly excited, stimulation of ingoing nerves
causing a dilatation of the pupil when the medulla is
intact (Fig. 50).

The dilator fibres pass down the lateral columns of the
spinal cord to the lower cervical and upper dorsal region
where they arborise round cells in the anterior horn.
From these, fibres pass by the anterior root of the second
(2 D.N.), possibly also of the first and third dorsal nerves,
and, passing up through the inferior cervical ganglion, run
on to the superior ganglion, where they arborise round
cells which
send axons to
the Gasserian
ganglion of
the fifth cran-
ial nerve (F.),
and from there

the fibres pass along the ophthalmic
division and its long ciliary branches to
the dilator fibres (D.P.).

The importance of the course taken
by these dilator fibres is considerable,
because diseases of the spinal cord in - ^
the lower cervical and upper dorsal
region (the cilio-spinal region), and
tumours in the upper mediastinum, may
interfere with their acjj^, and by stimu-
lating cause
pupil, or by
of the pupi
fibres of th
animals, it h)

FIG. 50.— Nerve Supply
of the Intrinsic Muscles

^^^^^^ ^^^ of the Eye (see text).

e dilaB muscle

fff been demonstrated in all
^gested that the nerve may act by
inhibiting the sphincter pupilloe, but the evidence on this
point is not conclusive.

The non-striped muscle of the iris, like non-striped muscle
elsewhere, may act independently of nerve fibres as may be
seen in the eye of a decapitated cat. Further, various drugs
seem to act directly upon them — e.g., physostigmin causes a
contraction, while atropin causes a dilatation.

8

ID.N.
2D.N

Bptation of the
wejBydilata\ion

ii4 HUMAN PHYSIOLOGY

The power of positive accommodation varies at different
ages, being greatest in young children, since in early life the
lens is most convex.

The distance of the near point in cms. is represented in
the accompanying figure. The "range of accommodation,"

Years. Near point in cms.

10

7

20

10

30

14

40

22

50

40

60

100

FIG. 51. — To show Variations in the power of Positive Accommodation
throughout life.

i.e. the difference between the "near point" and the "far
point," steadily decreases as age advances. After about
sixty years of age, on account of the flattening of the lens,
not even parallel rays can be focussed except by using

FIG. 52. — To illustrate Presbyopia, Myopia, and Hypermetropia. A, emmetropic
eye; B, presbyopic eye; C, hypermetropic eye; D, myopic eye; N.P.o.
the near point, and F.P.'S., the far point of accommodation.

positive accommodation. This is the fully developed con-
dition of Presbyopia — old-sightedness (Fig. 52, B).

Imperfections of the Dioptric Mechanism — (1) Myopia.—
In certain individuals the antero-posterior diameter of the eye

THE SENSES

is too long, and as a result parallel rays — rays from distant
objects — are focussed in front of the retina, and it is only
when the object is brought near to the eye that a perfect
image can be formed. In such an eye, no positive accom-
modation is needed till the object is well within the normal
far point; and the near point is approximated to the eye.
To enable distant objects to be seen it is necessary to provide
concave glasses by which the parallel rays are rendered
divergent (Fig. 52, D).

(2) Hypermetropia. — The eye of a considerable number of
people is too short from before backwards, and thus, in the
resting state, parallel rays are focussed behind the retina,
and to see even a distant object the individual has to use
his positive accommodation. As the object is approached
to the eye it is focussed with greater and greater difficulty
and the near point is further off than in the emmetropic
eye (Fig. 52, C).

The long-sighted eye differs from the slightly presbyopic
in the fact that not merely divergent, but also parallel rays,
are unfocussed in the resting state.

The condition is corrected by using convex glasses which
render the rays convergent, and, therefore, capable of being
focussed upon the retina of the
shortened eye.

(3) Astigmatism is a defect
due to unequal curvature of
the one or more of the refract-
ing surfaces in different planes.
If the vertical curvature of the
cornea is greater than the
horizontal, when a vertical line
is looked at, horizontal lines
will not be sharply focussed at
the same time. To correct
this condition, the lesser cur-
vature in any particular plane
must be made as great as the
greater curvature in the other

O

plane, and this is done by placing a cylindrical or part
cylindrical glass in front of the eye so that its curvature is

FIG. 53.— To show the cause of
Astigmatism. A, a slight curva-
ture of the cornea in the vertical
plane ; B, more marked curvature
in the horizontal plane, leading to
rays from b — a horizontal line being
focussed in front of the retina when
a — a vertical line — is looked at.

n6 HUMAN PHYSIOLOGY

in front of the lesser curvature of the eye and thus equalises
it with the other curvature (Fig. 53).

B. Stimulation of the Retina.

1. Reaction to Varying Illuminations. — (1) The Blind
Spot. — At the entrance of the optic nerve the retina can-
not be stimulated because there are no end organs in that
situation. The existence of such a blind spot may be

FIG. 54.— Methods of demonstrating the Blind Spot, a, by Mariotte's
Experiment ; 6, by moving a pencil along a sheet of paper.

demonstrated — 1st, By Mariotte's experiment, which consists
in making two marks in a horizontal line on a piece of paper,
closing the left eye, fixing the right eye on the left-hand
mark with the paper held at a distance from the eye, when
both marks are visible, then bringing the paper nearer to the
eye, when the right-hand mark will first disappear, and when
the paper is brought still nearer will reappear (Fig. 54, a).
(Experiment) 2nd, By making a mark on a sheet of paper,
and with the head close to the paper moving the point of a
pencil to the right for the right eye, or to the left for the
left eye, when the point will disappear and again reappear
(Fig. 54, b). (Experiment.)

THE SENSES

117

The eye is blind for all objects in the shaded region. By
resolving the various triangles the distance of the blind spot
from the central spot of the eye may be determined (3 to
4 mm.), and the diameter of the blind spot (1-5 mm.) may
also be ascertained.

The shape of the blind spot may be mapped out by fixing
the head close to the paper, moving the point of the pencil
out till it disappears, and then moving it in different direc-
tions and marking when it re-appears. It is never quite
circular, and often shows rays extending from its edge which
are due to the blood-vessels.
(Experiment.)

(2) The Field of Yision.-
The rest of the retina forward
to the ora serrata is capable
of stimulation, but when the
eye is directed forwards the
extent of retina stimulated
is influenced by the eyebrow
cutting off rays from above
and thus preventing the lower
part of the retina being
stimulated to its margin,
and by the nose intercepting
rays from the nasal side, thus
protecting the outer part of
the retina.

The whole range of objects
which can be seen at one time

constitutes the field of vision, and it, may be indicated by
the optical angle subtended by that range of objects. As
the distancse from the eye increases the field of vision expands.
It may be investigated by the perimeter, an instrument
which can readily be made by describing the arc of a circle
upon a blackboard or sheet of paper, placing the eye at
the centre and directing it to a mark in the middle of the
circumference, and then bringing a piece of chalk inwards
along the line until it is seen (Fig. 55). (Experiment) On
bringing an object from above it is not seen till A is reached,
while on bringing it from below it is seen at D. The angle

FIG. 55.— The Field of Vision, and the
method of investigating it by the
Perimeter. (7, the point in the arc
of a circle to which the eye B is
directed.

HUMAN PHYSIOLOGY

DBG is larger than CBA. The angle ABC is the measure of
the vertical field of vision, and it will be observed how it is
constricted by the eyebrow. The vertical angle amounts to
about 130° ; 60° in the upper field and 70° in the lower field.

The horizontal field worked out in the same way gives
about 150°, of which no less than 90° are on the outer side
and only 60° on the inner.

Of all parts of the retina the central spot is the most
sensitive to differences of illumination.

(3) The layer of the retina capable
of stimulation is the layer of rods and
cones. This is proved by the experi-
ment of Purkinje's images. It depends
upon the fact that if a ray of light is
thrown through the sclerotic coat of
the eye the shadow of the blood-vessels
stimulates a subjacent layer (Fig. 56, c),
and these vessels appear as a series of
wriggling lines on the surface looked
at. If the light is moved the lines
seem to move, and, by resolving the
triangles, it is possible to calculate the

FIG. 56.-To show that the distance behind the vessels of the part
stimulated, and this distance is found
to correspond to the thickness of the
retina. (Experiment.)

(4) Modes of Stimulation. — The
rods and cones are generally stimu-
lated by the ethereal light vibration,
but they may be stimulated by

mechanical violence or by sudden changes in an electric
current. But, however stimulated, the kind of sensation is
always of the same kind — a visual sensation. (See p. 104.)

(5) Of the nature of the changes in the retina when
stimulated we know little. But we know—

1st. That under the influence of light the cells of the
tapetum nigrum expand forward between the rods and cones.

2nd. That a purple pigment which exists in the outer
segment of the rods is bleached. Even although there is
no purple in the cones, which alone occupy the sensitive

hindmost layer of the re-
tina is stimulated. (Pur-
kinje's Images.} a, source
of light ; b, blood - vessel
of retina; c, shadow of
vessel on rods and cones ;
d, image of shadow men-
tally projected on to the
wall.

THE SENSES

119

central spot of the eye, this change in colour suggests that a
chemical decomposition accompanies stimulation.
3rd. Electrical changes. (See p. 83.)

2. The power of localising the source or direction of
illumination has now to be considered. Its acuteness
may be determined in the same way as in studying the
sense of touch — by finding how near two points may be
stimulated and still give rise to a double sensation. Over
the central spot two points of illumination may be as
near to one an-
other as about
four micro-milli-
metres, and still
two sensations be
experienced. This
is determined by
finding the small-
est optical angle
which can be sub-
tended by, say
two stars, with-
out their images
being fused. This
angle varies from
seventy-three to

FIG. 57. — The Power of localising the Source of Illu-
mination on different parts of the retina. The two
points, a-b, subtended by the small angle, fall close
together at a-b near the centre of the retina, and
still give rise to a double sensation ; but if two
points, c-d, have their images formed on the peri-
phery of the retina, c-d, these images must be far
apart to cause a double sensation.

fifty seconds in

different individuals, and this corresponds to from 5 -31 to
3*65 micros on the retina (Fig. 57). Over the central spot
the centres of the cones are about this distance from one
another, and it would seem that, to get a double sensation,
two cones must be stimulated. On passing to the more
peripheral part of the retina, where the cones are more scat-
tered, the power of localising decreases, and larger and larger
optical angles must be subtended by the two objects — e.g.
the points of a pair of compasses — in order that both may
be seen. (Experiment.)

3. Colour Sensation — Physics of Light Vibration. — Phy-
sically the various colours are essentially different rates of

120

HUMAN PHYSIOLOGY

vibration of the ether, and only a comparatively small range
of these vibrations stimulate the retina. The slowest acting
vibrations are at the rate of about 435 billions per second,
while the fastest are not more than 764 billions — the relation-
ship of the slowest to the fastest is something like four to
seven. The apparent colour of objects is due to the fact
that they absorb certain parts of the spectrum, and either

transmit onwards other parts,
or reflect other parts. The
vast variety of colours which
are perceived in nature is
due to the fact that the pure
spectral colours are modified
by the brightness of illumina-
tion, and by admixture with
other parts of the spectrum.
Thus a surface which in bright
sunlight appears of a brilliant
red, becomes maroon, and
finally, brown and black, as the

4

Agam> a PUre

of the retina (Colour Perimeter). re when diluted with all
A indicates the extent of retina fch spectrum— i.e. With white
stimulated by white and black ; . r .

J5, the part also capable of stimula- light — becomes pink as it be-
tion by blue and yellow; and C, comes less and less saturated.

of n>y"i°i<*y of Colour Sensa-

tion. — 1. The peripheral part
of the retina is colour blind — is incapable of acting so as to
produce colour sensations. This may be shown by means
of the perimeter and coloured chalks. Until the chalk is
brought well within the field of vision its colour cannot
be made out. As the image of the chalk travels in along
the retina it is found that yellow and blue can be dis-
tinguished before red and green — that is, that there is a
zone of retina which is blind to red and green, but which
can distinguish blue and yellow. Only the central part
of the retina is capable of being stimulated by all colours.
These zones are not sharply defined, and vary in extent
with the size and brightness of the coloured image. (Ex-
periment.) (Fig. 58.)

Fio. 58. — Distribution of Colour Sensa- •,• -, . r ,
tion in relationship to the surface hght fades-

THE SENSES 121

2. While the various sensations which we call colour are
generally produced by vibrations of different lengths falling
on the retina, colour sensations are also produced in
various other ways.

(a) By mechanical stimulation of the retina. By pressing
on the eyeball as far back as possible a yellow ring, or part
of a ring, may often be seen. (Experiment.)

(b) Simple alternation of white and black upon the
retina may produce colour sensation, as when a disc of
paper marked with lines is rotated rapidly before the eye.
(Experiment.) (Fig. 59.)

3. By mixing different parts of the spectrum, some inter-
mediate part or white may be produced. This may be done
by colouring the surface of a top with different colours, or by
means of a sheet of glass allowing the image of one wafer to
fall on another of a different colour. (Experiment.)

This means that by different modes of stimulation of the
retina the same sensation may be produced. The sensation
of orange may be produced either
when vibrations at about 580
billions per second fall on the
eye, or when two sets of vibrations,
one about 640 and one about
560 billions, reach it. By no pos-
sible physical combination of the
two is it possible to produce the
intermediate rate of vibration.

The sensation of colour, there-
fore, depends upon the nature of

., , .. FIG. 59.— Disc which, when ro-

the change set up in the retina, tated in a bright light, gives
and not upon the condition pro- impression of colours.
ducing that change.

4. After looking for some time at any one colour, on
removing the colour another appears in its place — the
complemental colour. If the first colour is —

Red, the second will be green blue ;
Orange, ,, „ blue ;

Green, „ „ pink;

Yellow, „ „ indigo blue ;

and vice versa. (Experiment.)

122 HUMAN PHYSIOLOGY

Theories of Colour Vision. — 1. From consideration of the
peripheral colour blind zone of the retina and of the more
limited area giving sensations only of blue and yellow when
stimulated, and of the most limited central part giving also
sensations of red and green, it would seem that some special
substance or substances must exist in each of these areas
which by its or their stimulation give rise to the various
sensations.

2. The phenomenon of complemental colours suggests the
possibility of there being one substance which when under-
going one change, say breaking down, produces blue, and
when undergoing another change, say building up, produces
yellow, and another substance which when undergoing one
change produces red, and another change produces green;
or that there are four different substances, one when changed
giving rise to yellow, another to blue, another to red, and
another to green. When the substance giving the sensation
of yellow is used up, then the parts stimulated by the rest of
the spectrum would react to white light and give a com-
plemental colour, and so on through the other substances.
If such a view be correct, it becomes almost necessary to
postulate the existence of another substance which when
stimulated gives rise to sensations which we call white.

It has also been suggested that the facts may be explained
on the assumption that there are three substances in the
retina, one more especially stimulated by the red rays but
also acted on by the others, one chiefly stimulated by the
green rays, and one chiefly acted on by the blue rays. Such
theories, however, do not call for consideration from the
ordinary student.

Colour-blindness. — While every one is colour blind at the
periphery of the retina, a certain proportion of people — about
5 per cent. — are unable to distinguish reds and green, even
at the centre of the retina. Individuals who manifest this
condition are often able to name colours fairly accurately,
but when asked to match a piece of red wool from a number
of others, they tend to put beside it green wools. The con-
dition is of great importance to engine drivers and seamen.
(Experiment.)

Colour blindness for yellow and blue is very rare.

THE SENSES 123

(2) BINOCULAR VISION.

By the fact that there are two eyes instead of only one,
the following advantages are attained : —

1. The Field of Vision is increased, but is not doubled.
This is shown in Fig. 60, where the two eyes are directed to
a spot A, and where the field of vision of the right eye is
indicated by continuous lines, that of the left eye by dotted
lines. The two fields greatly overlap, and the central part
is common to the two eyes. (Experiment.) In animals

\

; L.FV

FIG. 60.— The Field of Vision in Binocular Vision. The two eyes are directed to a
point, A. The field of vision of the right eye subtends the angle formed by
the continuous lines, and that of the left that subtended by the dotted lines.
The overlap of the fields is shown on the surface looked at and in the figure
below, R.F.V. andL.F.V.

where the eyes are placed laterally, the two fields are
independent.

2. A mechanism is afforded for the determination of the
distance of near objects, because as an object is approached,
the two eyes have to be turned inwards by the internal recti
muscles, and by the degree of contraction of these, an
estimation of the distance is made. The importance of
this may be demonstrated by fixing a stick vertically,
rapidly walking up to it with one eye shut, and endeavour-
ing to touch it with the finger. (Experiment.)

3. A means of determining the solidity of an object
is afforded, because if the object is near, a slightly dif-
ferent picture is given on each retina, and experience has

124

HUMAN PHYSIOLOGY

taught us that this stereoscopic vision indicates solidity
(Fig. 61).

Corresponding Areas of the Two Retinae.— In order that

Fio. 61.— Stereoscopic
Vision.

Fio. 62. — Corresponding Areas of the
two Retinae in Binocular Vision.
The upper and outer area of the
right retina corresponds to the
upper and inner area of the left
retina, and the other areas corre-
spond as shown by the shading.
In each pair of areas definite points
correspond with one another,

with the two eyes single

vision may occur, the eyes

must be directed to the same

place, so that the image

of that place falls on each

central spot. If this does

not occur, double vision results. If the central spot of one

eye corresponds to the central spot of the other, certain

points in each retina will have corresponding points in the

other which will be stimulated by the same part of the

picture when the eyes are working together (Fig. 62).

Movements of Eyeballs. — To

£ £ secure this harmonious action of

1 the two retinae, it is necessary

that the eyes should be freely
movable. Each eye in its orbit
is a ball and socket joint in
which the eyeball moves round
every axis (Fig. 63). The axis
of the eye (a) is set obliquely to
the axis of the orbit (6), and the
centre of rotation is behind the
Fm. 63.— The left Eyeball in the centre of the ball. The move-
Orbit, with the Muscles acting ments are produced by three

upon it. i*

pairs of muscles.

1. The internal and external recti (LR. and Ex. R.).

2. The superior and inferior recti acting along the lines
indicated (S.R.).

3. The superior and inferior obliques acting in the line (S.ob.).
The internal rectus rotates the pupil inwards.

external outwards.

S.ob.

THE SENSES 125

The superior rectus rotates the pupil upwards and inwards,
inferior (downwards and in-

\ wards.

superior oblique (downwards and out-

l wards.
„ inferior „ „ „ upwards and outwards.

In directing the eyes to the right, the external rectus of
the right eye acts along with the internal rectus of the left.
In directing the eyes straight upwards, the superior rectus
and inferior oblique of each eye act together ; and in looking
downwards, the inferior rectus and superior oblique come
into play (Fig. 64).

When a distant object is looked at, the axes of the two
eyes may be considered as parallel; but as an object is
approached to the eyes, the axes converge. It is not possible
by voluntary effort to diverge the optic axis or to rotate
the eyes round antero-posterior axes.

When the eyes are Nose

allowed to sweep over a In 01.

landscape or any series of ^

objects, or when these

move rapidly past the £*/?.< — [ ( ^ J— > InR.

eyes, or the eyes rapidly
past them, as in travelling
by train, the axes are -s-di,.

directed in a Series Of FIG. 64.— The Movements of the Pupil

glances to different points caused by the various Muscles of the

r ' Eye. (Eight Eye.)

and the succession of pic-
tures thus got gives the idea of the continuous series of
objects. This jerking movement of the eyes may be well
seen in a passenger looking out of a railway carriage in
motion.

A somewhat complex nervous mechanism presides over
these various movements of the eyes. All the muscles are
supplied by the third cranial nerve, except the superior
oblique, which is supplied by the fourth nerve, and the
external rectus, which is supplied by the sixth nerve (Fig.
65 ; see also Fig. 85, p. 158).

The centres for the third and fourth nerves are situated
in the floor of the aqueduct of Sylvius under the corpora

126

HUMAN PHYSIOLOGY

quadrigemina, while the centre for the sixth is in the pons
Varolii (Fig. 86, p. 159, and Fig. 85, p. 158). The various
centres are joined by bands of nerve fibres which pass
between the sixth and fourth and third centres, and in part
at least cross the middle line.

A combined mechanism, each
part of which acts harmoni-
ously with the other parts,
thus presides over the ocular
movements, and this mechanism
is controlled by impulses con-
stantly received from the two
retinae, from the ear and from
the brain.

2. Connections of the Eyes
with the Central Nervous
System.

From each eye the optic nerve
extends backwards and inwards
to join the other optic nerve
at the chiasma. From the
chiasma the two optic tracts
pass upwards round the crura
cerebri to end in two divisions —
1. A posterior division passing

internal rectus of the opposite to the anterior corpora quadri-
side through the nucleus of the gemma on the same side (Fig. 66
*"'*-" A.C.Q.).

2. An anterior running to the geniculate body on the
posterior aspect of the thalamus opticus (Fig. 66, Op. Th.).

A partial crossing of the fibres takes place in the chiasma
— fibres from the middle and internal part of the retina
decussating, those from the outer part remaining on the same
side. For this refs%n, when the right optic tract is cut, it
leads to partial blindness of both retinaa — on the outer part of
the right eye and on the inner and middle part of the left
eye. Thus objects on the left side of the field of vision are
not seen.

The fibres of the posterior termination of the optic tract

FIG. 65. — The Nervous Mechanism
presiding over the combined
movements of the two Eyes.
IR. , Internal rectus ; ER. , ex-
ternal rectus; CO., convergent
centre acting on the internal
recti through the nuclei of the
third nerve; S.O., superior olive
(centre for lateral divergence)
acting on the external rectus
of the same side through the
nucleus of the sixth, and on the

THE SENSES

127

end in synapses with neurons in the corpora quadrigemina,
and the fibres of these neurons pass downwards and control
the oculo-inotor mechanism already described (Fig. 65,
p. 126).

The fibres of the anterior division make synapses with
other neurons in the posterior part of the thalamus, and

Occ. L.

FlG. 66.— The Connections of the Retinae with the Central Nervous System.
R, retinae; Ch., chiasma leading to optic tract; Op. Th., optic thalamus ;
A.C.Q., anterior corpora quadrigemina; Oc. M., oculo-motor mechanism
(Fig. 65) ; Occ. L., occipital lobe of the cerebrum ; Teg.) tegmentum.

these neurons send their fibres backwards to the occipital
lobe of the brain where they connect with the cortical
neurons (Fig. 66, Occ.) (see p. 181). ^

When the right occipital lobe or the strand of fibres leading
to it is destroyed, blindness on the outer part of the right
retina and on the inner and middle part of the left retina
results — the individual is blind for all objects in the left half
of the field of vision.

128 HUMAN PHYSIOLOGY

3. The Visual Centre.

A response to stimulation on the part of the neurons in the
occipital lobe of the brain (p. 181) is the physical basis of our
visual sensations, and hence this part of the brain is called
the visual centre. Usually the visual centre is stimulated
by changes in the chain of neurons passing from the retina
and set in action by retinal changes ; but direct stimulation
of the occipital lobe may induce visual sensations, as is some-
times seen in the early part of an epileptic fit.

The strength of the sensation depends upon the strength
of the stimulus, and the smallest difference of sensation
which can be appreciated is a constant factor of the degree
of stimulation. Thus, to produce a change in visual sen-
sation, the strength of the stimulus must vary by about
Tc75-th of the stimulus.

The sensation lasts longer tlian the stimulus, and thus,
if a series of stimuli follow one another at sufficiently rapid
intervals, a fusion of sensation is produced. If a wheel
rotating slowly is looked at, the individual spokes are seen,
but when it is going more rapidly, the appearance of a con-
tinuous surface is presented. If the light is dim, this fusion
takes place more readily than when the light is bright. From
this it is concluded that a strong stimulus causes a more
sudden and acute sensation than a weak one, and, therefore,
the individual sensations are distinguished.

The visual centre of each side must be regarded as a chart
of the opposite field of vision, each part corresponding to
a particular part of the field. The two centres acting
together give the whole field of vision. Since the blind
spot is not represented in the centre, it is not perceived
in the field of vision. The centre is said to rectify the
inverted image formed on the retina, but this simply means
that as a result of experience, we have learned that changes
in, say the lower part of the retinae and in the corresponding
parts of the visual centres, are produced by light from above
the head.

Since the retinal changes differ simply according to the
degree of illumination and the rate of the ethereal waves,
and since the part of the retina acted on is determined by

THE SENSES

129

the direction of the rays, we can be conscious only of
different changes in different parts of the visual centre depen-
dent on the changes set up in the retinae. We have there-
fore only the means of getting a flat picture of what we look
at, but no special arrangement for having different sensations
according to the distance of an object or according to whether
it is flat or in relief. Thus the means of determining the
form and size of objects by the retinas and visual centres is
very limited.

To gain knowledge of the size, distance, or contour of an
object we have to combine certain other sensations, or certain
past experiences with the visual sensation, and to form a
judgment or concept of what we are looking at.

It is not then wonderful that erroneous judgments are
frequently made. The possibility of such may be indicated
by one or two examples of what are called modified per-
ceptions.

(1) If an imaginary line be divided into two equal parts,
and if a series of dots are put

along the one half, that part will

appear longer than the other.

(2) If a black wafer on a white ground, and a white wafer
on a black ground be looked at, the latter will appear larger
than the former.

(3) If a square be ruled with parallel diagonal lines, and
if short vertical and horizontal lines be placed alternately
upon them, they will no longer appear

parallel.

IY. Hearing.

While through the sense of touch
we are made aware of differences of
pressure, through the sense of hearing
certain vibratory changes of pressure
specially affect the consciousness. Even
simple organisms, devoid of any special
organ of hearing, may be affected by vibratory changes, and
in fish it is difficult to be certain how far such vibrations
produce their effect through the ear or through the body
generally; but in higher vertebrates it is chiefly through

9

FIG. 67.— To illustrate
Optical Illusions.

130 HUMAN PHYSIOLOGY

the ears that they act. In these there is a special arrange-
ment by which the vibrations of the air are converted into
vibrations of a fluid in a sac situated in the side of the
head into which the free ends of neurons project.

In mammals the organ of hearing consists of an external,
a middle, and an internal ear.

A. External Ear. — The structure of this presents no point
of special physiological interest. In lower animals the pinna

LxM

Fio. 68. — Diagram of the Ear. ExM., external meatus ; Ty. , tympanic mem-
brane; m. , malleus; t., incus; «., stapes; /.o., fenestra ovalis; f.r., fenestni
rotunda; EnT., Eustachian tube; t>., vestibule; «.c., semicircular canal;
Coch., cochlea.

is under the control of muscles, and is of use in determining
the direction from which sound comes.

B. Middle Ear. — The object of the middle ear is to over-
come the mechanical difficulty of changing vibrations of air
into vibrations of a fluid. It consists of a chamber, the
tympanic cavity, placed outside of the petrous part of the
temporal bone (Fig. 68). Its outer wall is formed by a
membrane, the membrana tympani (Ty.), which is attached
to a ring of bone. Its inner wall presents two openings into
the internal ear — the fenestra ovalis (/.o.), an oval opening,
situated anteriorly and above, and the fenestra rotunda (/.?*.),
a round opening placed below and behind. Throughout life
these are closed, the former by the foot of the stapes, which

THE SENSES 131

is attached to the margin of the hole by a membrane, the
latter by a membrane. The posterior wall shows openings
into the mastoid cells and presents a small bony projection
which transmits the stapedius muscle. The anterior wall
has above a bony canal carrying the tensor tympani muscle,
and below this the canal of the Eustachian tube which com-
municates with the posterior nares (Fig. 68, EnT.).

In the tympanic cavity are three ossicles — the malleus (m.),
incus (i.), and stapes (s.), forming a chain between the mem-
brana tympani and the fenestra ovalis. The handle of the
malleus is attached to the membrana tympani, and each time
a wave of condensation hits the membrane, it drives in the
handle of the malleus. This, by a small process, pushes
inwards the long process of the incus which thrusts the stapes
into the fenestra ovalis, and thus increases the pressure in the
enclosed fluid of the internal ear. The fenestra rotunda with
its membrane acts as a safety valve. The bones rotate
round an antero-posterior axis passing through the heads of
the malleus and incus. They thus form a lever with the
arm to which the power is applied — the handle of the malleus
— longer than the other arm. The advantage of this is that,
while the range of movement of the stapes in the fenestra
ovalis is reduced, its force is proportionately increased.

The range of movement is still further controlled by the
stapedius muscle which twists the stapes in the fenestra.
This muscle seems to act when loud sounds fall on the ear,
and when its nerve supply, derived from the facial nerve, is
paralysed, such sounds are heard with painful intensity.

If the membrana tympani is violently forced outwards by
closing the nose and mouth and forcing air up the Eustachian
tube, the incus and stapes do not accompany the malleus and
membrane, since the malleo-incal articulation becomes un-
locked.

The membrana tympani is so loosely slung that it has no
proper note of its own, and responds to a very large range
of vibrations. By the attachment to it of the handle of the
malleus it is well damped, and stops vibrating as soon as
waves of condensation and rarefaction have ceased to fall
upon it. The tensor tympani muscle, supplied by the
fifth cranial nerve, has some action in favouring the vibration

132 HUMAN PHYSIOLOGY

of the membrane, and its paralysis diminishes the acuteness
of hearing.

The Eustachian tube has a double function. It allows the
escape of mucus from the middle ear, and it allows the en-
trance of air, so that the pressure is kept equal on both sides
of the membrana tympani. Its lower part is generally closed,
but opens in the act of swallowing. It is surrounded by an
arch of cartilage to one side of which fibres of the tensor
palati are attached, so that when this muscle acts in swallow-
ing, the arch of cartilage is drawn down and flattened, and
the tube opened up (Fig. 69).

When the Eustachian tube gets occluded, as a result of
catarrh of the pharynx, the oxygen in the middle ear is ab-
sorbed by the tissues, and the pressure falls. As a result, the
membrane is driven inwards by the atmospheric pressure, and
does not readily vibrate, and hearing is impaired.

C. Internal Ear. — The internal ear is a somewhat complex
cavity in the petrous part of the temporal bone, the osseous
labyrinth. It consists of a central space,
the vestibule ( V), into which the fenestra
ovalis opens. From the anterior part of
this, a canal makes two and a half turns
round a central pillar, and then, turning
sharply on itself, makes the same number
of turns down again, and ends at the
fenestra rotunda. This is the osseous

FIG. 69.- Transverse cochlea (Fig. 68, Cock.) The ascending
Section through Car- p.

tiiaginous lower part and descending canals are separated from

of Eustachian Tube One another, partly by a bony plate,

£±L"± ?<«% by a membranous partition-the

and the way in which basilar membrane. At the base, the

it is pulled down and hony iainena js broad, but at the apex

the tube opened in . , . . n

swallowing (shaded). lts place is chiefly taken by the mem-
brane, which measures at the apex more
than ten times its width at the base.

From the posterior and superior aspect of the vestibule
three semicircular canals (Fig. 70) open, each with a
swelling at one end. One runs in the horizontal plane,
and has the swelling or ampulla anteriorly (Fig. 70, h.c.).

THE SENSES

133

h.c.

The other two run in the vertical planes indicated in the
diagram, the anterior being called the superior canal (s.c.),
and having its ampulla in front, the posterior (p.c.) having
its ampulla behind. They join
together, and enter the vestibule
by a common orifice.

The bony labyrinth is filled
with a lymph-like fluid, the peri-
lymph, and in it lies a complex
membranous bag, the membranous
labyrinth. FIG> 7a_The Keiationship of the

In the vestibule this is divided Semicircular Canals to one

into two little sacs, the utricle

and the saccule, joined together

by a slender canal. From the

saccule comes off a canal which runs into the cochlea

upon the basilar membrane, forming a middle channel

between the other two, the scala media or membranous

p.C.

another. h.c., horizontal
canal ; s. c. , superior canal ;
p.c, , posterior canal.

FIG. 71. — Transverse Section through one turn of the Cochlea to show the Organ of
Corti on the Basilar Membrane. S. M. , scala media ; S.V., scala vestibuli ;
S. T. , scala tympani.

cochlea. This terminates blindly at the apex. From the
utricle a membranous canal extends into each of the bony
semicircular canals, being provided with an ampulla, which

134 HUMAN PHYSIOLOGY

nearly fills up the bony ampulla, while the canal portion
is small, and occupies only a small part of the bony canal.
(Fig. 89, p. 165.)

In the membranous cochlea the lining cells form the organ
of Gorti (Fig. 71). This is set upon the basilar membrane, and
consists from within, outwards, of — 1st, A set of elongated sup-
porting cells. 2nd, A row of columnar cells, with short, stiff,
hair-like processes projecting from their free border. 3rd, The
inner rods of Corti, each of which may be compared to an ulnar
bone attached by its terminal end, and fitting on to the heads
of the outer rods. 4th, The outer rods of Corti, each resem-
bling a swan's head and neck — the neck attached to the basi-
lar membrane, and the back of the head fitting into the hollow
surface of the inner rods. 5th, Several rows of outer hair
cells, with some spindle-shaped cells among them. 6th, The
outer supporting cells. 7th, Lying over the inner and outer
hair cells is the membrana reticularis, resembling a net,
through the meshes of which the hairs project. 8th, Arch-
ing over this organ is a homogeneous membrane — the
membrana tectoria.

The membranous labyrinth is attached to the inner wall
of the bony labyrinth at certain points through which fibres
of the auditory nerve enter it. A set of fibres goes to the
utricle, a set to each of the ampullae, and a set to the
saccule and the cochlea.

The membranous labyrinth has an outer fibrous coat, and
inside this a homogeneous layer which is markedly thick-
ened where the nerves enter it. It is lined by flattened
epithelium, which become columnar, and is covered with
stiff hair-like processes over the thickenings at the entrance
of the nerves.

The terminal neurons of both the vestibule and the
cochlea end in dendrites between the hair cells, and the
cell of each is upon its course to the medulla.

The auditory nerve is essentially double, consisting of a
dorsal or cochlear, and a ventral or vestibular part.

Cochlear Root (Fig. 72). — This is the true nerve of hear-
ing. Its fibres (Cock. R.) begin in dendrites between the cells
of the organ of Corti, have a cell upon their course, and when

THE SENSES

135

they enter '.the medulla they branch into two divisions, which
end either in the tuberculum acusticum or the nucleus
accessorius (N. Ace.), where they form synapses. From
the cells, axons pass (a) to the oculo-motor mechanism
of the 'Same side and the opposite side (N.vi.\ and (6) up

EYE,

FIG. 72. — Connections of Cochlea with Central Nervous System. Cock.
R., cochlear root of eighth nerve ; N. Ace., tuberculum acusticum
and nucleus accessorius sending fibres to the cerebrum (C.B.) and
to the oculo-motor mechanism (N.vi.).

to the cerebrum (CB.) of the same and of the opposite
side.

Yestibular Root (Fig. 73). — The fibres of this root take
origin in dendrites between the cells of the maculse, and have
their nerve cell upon their course (Ves. R.). As they enter
the medulla they divide into two, forming an ascending

1 36 HUMAN PHYSIOLOGY

and a descending branch. (1) The ascending branch sends
fibres on to the cerebrum (CB.\ and to the superior
vermis of the cerebellum (CBL.). These fibres give off
collaterals to the nucleus of Deiters (N. Deit.), from
the cells of which fibres pass, which divide, some run-
ning on the same side, some on the opposite side ; one
branch passing up to the oculo-motor mechanism (N.vi.),
the other passing down the spinal cord to send collaterals
to the cells in the grey matter. (2) The descending
branch forms connections with the medullary nuclei as it
passes down.

Sound Perception. — The qualities of sound which can
be distinguished by the sense of hearing, are loudness—
amplitude of vibration ; pitch — rate of vibration ; and
quality — the character of the sound given by the over-
tones. The perception of this last is essentially a perception
of pitch.

It is easy to understand how the peripheral neurons
in the internal ear are more powerfully stimulated by
the greater variations in the degree of pressure which
are produced by more powerful aerial waves, and how
the greater stimulation of the receptive centre in the
brain will be accompanied by a sensation of greater
loudness.

A study of the structure of the cochlea seems to
show a mechanism well suited to afford a means of
estimating the pitch of a note and the existence of over-
tones. The fibres of the basilar membrane may be com-
pared to the strings of a piano, each one of which, or
each set of which, will be set in vibration by a particular
note.

The power of distinguishing differences of pitch varies
in different individuals. It is more acute for notes of a
moderate rate of vibration, from 46 to 4000, than for very
slow or very fast vibrations.

The range of perception of pitch also varies greatly, some
people hearing notes as low as 20 vibrations per second,
and others hearing them up to 40,000 per second.

THE SENSES

137

Y. Taste.

Mechanism. — The nerve endings connected with the
sense of taste are disposed in barrel-shaped structures

CELLS OP

ANT. HORN

FIG. 73. — Connections of Semicircular Canals with Central Nervous
System. Fes. R. , Vestibular root of eighth nerve sending fibres
upwards to CB. (cerebrum) and CBL. (cerebellum), downwards to
the centre in medulla oblongata (Med.)t and to Deiters' nucleus
(N. Deit.}, from which fibres pass to the oculo-motor mechanism
(N.vi.) and to the centres in the anterior horn of the spinal
cord.

in the epithelial covering of the mouth. These taste bulbs
are most abundant at the back of the tongue, on the sides

138 HUMAN PHYSIOLOGY

of the large circumvallate papillae which form the prominent
V-shaped line on the posterior part of the dorsum.

Each is composed of a covering of elongated cells like the
staves of a barrel, enclosing a set of spindle-shaped cells with
which the dendrites on the end of the nerve fibres are closely
associated.

These nerve fibres pass to the brain in the fifth nerve.
Several cases of complete loss of taste have been recorded
by Gowers in which the root of the fifth nerve alone was
destroyed, but it is usually thought that the glosso-pharyngeal
also carries nerve fibres connected with taste, and the recent
observations favour this view.

Physiology. — As to the way in which this mechanism
is stimulated our knowledge is very imperfect. In order
to act the substance must be in solution. The strength of
the sensation depends on the concentration of the solution,
upon the extent of the surface of the tongue acted upon,
upon the duration of the action, and upon the tempera-
ture of the solution. If the temperature is very high or
very low the taste sensation is impaired by the feelings
of cold or heat.

It is most difficult to classify the many various taste sensa-
tions which may be experienced, but they may roughly be
divided into four main groups :—

1. Sweet. 3. Acid.

2. Bitter. 4. Saline.

Whether different sets of terminations react specially to
each of these is not known, but it has been found that sub-
stances giving rise to the sensation which we call bitter act
best on the back of the tongue, while substances producing
sweet or acid sensations act on the sides and front. Again,
chewing the leaves of gymnema sylvestre abolishes sensations
of sweet and bitter, but does not interfere with those of acid
and saline, and leaves the tactile sense unimpaired. On the
other hand, cocaine paralyses the tactile sense before it inter-
feres with the sense of taste.

The sense of taste is very closely connected with the sense
of smell, and, when the latter is interfered with, many sub-

THE SENSES 139

stances seem tasteless which under normal conditions have a
marked flavour.

YI. Smell.

Mechanism. — Over the upper part of the nasal cavity the
columnar epithelial cells are devoid of cilia, and between them
are placed spindle-shaped cells (Fig. 74, 01. c.\ which send
processes through the mucous membrane, and through the
cribriform plate of the ethmoid into the olfactory bulb. Here

C.T*.

FIG. 74.— The Connections of the Olfactory Fibres. 01. c., olfactory cells ; 01. Gl.t
glomeruli; M.G., mitral cells; 01. T., olfactory tract; C.M., corpus
mammilarium ; Hipp. , hippocampus ; Op. Th. , optic thalamus ; F. , fornix
(modified from SCHAFER).

they form synapses with other neurons (01. GL), the axons of
which pass to the base of the olfactory tracts(6). Some fibres (a)
pass straight on to the optic thalamus, while others (6) form
synapses with other neurons, the fibres of which, passing
round the fornix, end by arborising round cells in the corpora
mammilaria (C.M.), which send processes on to the thalamus.
There is some evidence that the fibres to the cortex end in
the hippocampus.

Physiology. — To act upon the olfactory mechanism the
substance must be volatile, and must be suspended in the air.
In this condition infinitesimal quantities of such substances
as musk are capable of producing powerful sensations. The

140 HUMAN PHYSIOLOGY

mucous membrane must be moist, and this is secured by the
activity of Bowman's glands, situated in the mucous mem-
brane. These are under the control of the fifth cranial nerve,
and section of this leads indirectly to loss of the sense of
smell.

SECTION V
THE NERVOUS SYSTEM

SPINAL NERVES

HAVING studied the various organs of special sense and the
muscles by which we can react to the impressions received
through them, the general characters of the connecting
nerves between the central nervous system, and the various
structures involved, may now be considered. These nerves
may be classified as ingoing and outgoing, and they may

FIG. 75. — Structure of a Typical Spinal Nerve. P.R. , posterior root with ganglion ;
A.R., anterior root; S.L, ganglion of sympathetic chain; W.R., its white
ramus ; G.R., its grey ramus ; V.N., visceral nerve with collateral ganglion ;
S.N., somatic nerve.

roughly be divided into those connected with the body wall
and its appendages and those connected with the viscera.

General Structure. — The arrangement of fibres may best
be understood by studying the constitution of one of the
typical spinal nerves coming off from, say the dorsal region,
of the spinal cord (Fig. 75).

A posterior root (P.R.) comes off from the postero-lateral
aspect of the cord and has a swelling upon it, the ganglion

142 HUMAN PHYSIOLOGY

of the posterior root. It joins an anterior root (A.R.)
coming from the antero-lateral margin. These form the
spinal nerve which is distributed to the body wall. Lying in
front of this is a swelling or ganglion (S.I.) joined to the
nerve by two roots, a white ramus ( W.R.) and a grey ramus
(G.R.)', and from this a nerve extends towards the viscera
(V.N.). Before this nerve reaches its final distribution it
passes through another ganglion.

Roots of the Spinal Nerves. — The posterior root is the
great ingoing channel to the spinal cord, and the anterior
root is the great outgoing channel. Section of a series
of posterior roots leads to (a) loss of sensation in the
structures from which the fibres come, and (b) to a loss
of muscular co-ordination, as a result of cutting off the
afferent impressions connected with the muscle sense
(p. 97).

As a result of this section, the parts of the fibres cut off
from the cells of the ganglia on the posterior root die and
degenerate. Therefore if the root is cut inside the ganglion,
the degeneration extends inwards and up the posterior
columns of the cord, and if it is cut outside, the degeneration
passes outwards to the periphery.

Section of the anterior root causes paralysis of the muscles
and other structures supplied by the outgoing fibres, and
these fibres die and degenerate.

The nerve to the somatopleur or body wall (S.N.) is
composed of incoming and outgoing fibres. 1st, Incoming
fibres are medullated and take origin in the various peri-
pheral sense organs. As they pass through the ganglion on
the posterior root each fibre is connected by a side branch
with a nerve cell — the trophic centre of the neuron — and it
then enters the spinal cord, and, passing to the postero-
lateral column, it breaks up into an ascending branch and a
descending branch (Fig. 40, p. 89). 2nd, Outgoing fibres,
which are medullated, take origin from the large cells in the
anterior horn of the grey matter of the cord and pass on to
be connected with muscle fibres by end plates, or to gland
cells by less definite synapses.

The nerve to the viscera or splanchnopleur (V.N.), and to
the involuntary structures in the somatopleur, contains — 1st,

THE NERVOUS SYSTEM 143

Incoming Fibres. — These take origin either in definite peri-
pheral structures, such as Pacinian corpuscles, or in some
less defined endings, and as medullated fibres pass throu^n
the various ganglia, and, so far as is at present known, have
their cell stations in the ganglion on the posterior root.
2nd, The Outgoing Fibres, characterised by their small size,
take origin chiefly in a lateral column of cells, which is well
developed in the dorsal region of the cord, and pass out as
medullated fibres by the anterior root. From this they pass
by the white root to a sympathetic ganglion, whence they
may proceed in one of two different ways.

(a) They may form synapses with cells, and from these
cells fibres may pass —

1. Outwards with the splanchnic nerves ; or,

2. Back into the spinal nerve by the grey root and so
down the somatic nerve to blood-vessels, hairs, sweat glands,
&c. The ganglia from which fibres pass back into spinal
nerve are known as lateral ganglia.

(b) They may pass through these ganglia on to one more
peripherally situated in which they form synapses and are
continued onwards. These ganglia from which fibres do not
pass back are called collateral ganglia. Before their first
interruption they are termed pre-ganglionic fibres, after their
interruption post-ganglionic.

The various fibres after their interruption proceed as non-
medullated or grey fibres to their termination, where they
break up into a network of anastomosing fibres with cells — a
sort of terminal ganglion. Many drugs have a special action
on the terminal ganglia, e.g. apocodein paralyses them, while
adrenalin — the extract of the medullary part of the supra-
renals — stimulates them.

The interruption of fibres in ganglia, or their passage
through these structures, has been determined by taking
advantage of the fact that nicotine in one per cent, solution
when painted on a ganglion poisons the synapses but does
not influence the fibre. Hence, when a ganglion is painted
with nicotine, if stimulation of the fibres on its proximal side
produces an effect, it is proved that the break is not in that
ganglion.

144 HUMAN PHYSIOLOGY

General Distribution.

A. SOMATIC FIBRES.

(a) Outgoing Fibres. — The course of these must be studied
in the dissecting-room.

(6) Ingoing Fibres — Cutaneous Fibres. — The fibres pass-
ing in by each pair of nerves come from zones of skin
encircling the body. These are, however, interrupted by the
limbs. Each limb may be considered to be an outgrowth at
right angles to the trunk, composed of a pre-axial and post-
axial part.

B. SPLANCHNIC FIBRES.

These are small medullated fibres.

(a) The Outgoing Fibres may be subdivided as follows
(Fig. 76):-

1. Head and Neck. — These leave the spinal cord by the
upper five dorsal nerves and pass upwards in the sympathetic
cord of the neck to the superior cervical ganglion where they
have their cell stations. From these, fibres are distributed
to the parts supplied. The chief functions of these fibres
are — 1st, Vaso-constrictor to the vessels of the face and head ;
2nd, Pupilo-dilator (see p. 103) ; 3rd, Motor to the muscle of
Miiller; 4th, Secretory to the salivary glands, lachrymal
gland, and sweat glands. The course of these fibres is of
importance in medicine, since tumours in the upper part of
the thorax may press upon them.

2. Thorax. — The fibres to the thoracic organs also come
off in the five upper dorsal nerves, have their cell stations in
the stellate ganglion, and pass to the heart and lungs.

3. Abdomen. — These fibres come off by the lower six dorsal
and upper three lumbar nerves. They course through the
lateral ganglia and form synapses in the collateral ganglia of
the abdomen — the solar plexus and the superior and inferior
mesenteric ganglia. From these they are distributed to the
abdominal organs, being vaso-constrictor to the vessels, in-
hibitory to the muscles of the stomach and intestine, and
possibly secretory to the pancreas.

4. Pelvis. — The fibres for the pelvis leave the cord by the

THE NERVOUS SYSTEM

145

lower dorsal and upper four lumbar nerves, and have their
cell stations in the inferior mesenteric ganglia, from which
they run in the hypogastric nerves to the pelvic ganglia.

Head.

Thorax.

Abdomen.

D.I

2.

5
4
S

6

1

Pelvis.

FIG. 76. — Scheme of distribution of Splanchnic Nerves.

They are vaso-constrictor, inhibitory to the colon, and motor
to the bladder, uterus and vagina and the retractor
penis.

5. Arm. — These fibres, coming out by the fourth to the
tenth dorsal nerves, have their synapses in the sympathetic
ganglia of the sympathetic chain, and passing back into the

10

146 HUMAN PHYSIOLOGY

spinal nerves by the grey rami, course to the blood vessels,
veins, and sweat glands of the limb.

6. Leg. — The fibres take origin from the eleventh dorsal
to the third lumbar nerves, have their cell stations in the
lateral ganglia, and pass to the leg in the same way as do the
fibres to the arm.

Outgoing Visceral Fibres not connected with Sympathetic
Ganglia. — Certain outgoing fibres to the viscera and involun-
tary structures pass directly without going through the
sympathetic ganglia.

1. The third cranial nerve carries fibres which have their
synapses in the ciliary ganglion, and pass on to the sphincter
pupilke and ciliary muscle.

2. The seventh nerve carries fibres through the chorda
tympani to cell stations in the submaxillary and sublingual
ganglia. These are secretory to the submaxillary and sub-
lingual glands.

3. The ninth nerve sends fibres to the parotid gland, which
have their cell station in the otic ganglion.

4. The vagus sends inhibitory fibres to the heart, which
form synapses in the cardiac plexus. It also sends motor
fibres to the oesophagus and stomach, which, in some animals
at least, have the cell stations in the ganglion of the
trunk.

5. The nervi erigentes come off from the second and third
sacral nerves, and pass to the hypogastric plexus near the
bladder where the fibres have their cell stations. They are
the vaso-dilator nerves to the pelvic organs, inhibit the
retractor penis, and are motor to the bladder, colon, and
rectum.

(6) Ingoing Fibres. — The course of these from the viscera
is not so clearly known ; but they appear to enter the main
nerve largely by the white rami. In the normal condition
stimulation of their peripheral endings does not lead to
modifications of consciousness, and is therefore not accom-
panied by pain (see p. 96). But in abnormal conditions
painful sensations are produced. In some cases, abnormal
stimulation of visceral nerves leads to painful sensations
referred to the cutaneous distribution of the spinal nerve
with which they are connected. Thus disease of the heart

THE NERVOUS SYSTEM

147

is often accompanied by pain in the distribution of the upper
dorsal nerves in the left arm.

SPINAL CORD AND BRAIN.

The anatomy and histology of each part of the central
nervous system should be mastered before its physiology is

PONS.

CORD.

FIG. 77. — Mesial Section through the Brain and upper part of the Spinal Cord
to show the positions at which the sections figured in later diagrams have
been made.

studied. An outline sufficient to make the description of
the physiology intelligible is all that is given here.

148

HUMAN PHYSIOLOGY

A. SPINAL CORD.
Structure.

The spinal cord is a more or less cylindrical mass of nerve
tissues which passes from the base of the brain down the
vertebral canal. It terminates in a pointed extremity
at the level of the 1st lumbar vertebra. There are two
enlargements upon it, one in the cervical region, one in the
lumbar region, and from these the nerves to the arms and
legs come off. A fine central canal runs down the middle,

FIG. 78. — Cross Section of the Spinal Cord through the second Dorsal Segment,
to show disposition of grey and white matter. P., Posterior Horn ; A., An-
terior Horn with large cells; I.L., Intermedio-lateral Horn with small cells ;
L.C., Lockart Clarke's Column of Cells; P.M. and P. L. , Postero-median and
Postero-lateral Columns; D.C., Direct Cerebellar Tract; Asc. and Desc.
Ant. Lat., Ascending and Descending Antero-lateral Tracts; B.B., Basis
Bundles; C.Pt/., Crossed Pyramidal Tract; O.Py., Direct Pyramidal Tract.
On opposite side tracts which degenerate upwards are marked with horizontal
lines ; tracts degenerating downwards with vertical lines. (After BRUCE.)

and the two sides are almost completely separated from one
another by an anterior and a posterior mesial fissure (Fig. 78).
Each half is composed of a core of grey matter arranged in
two processes or horns — the anterior and posterior horns
(A. and P.) — which divide the white matter surrounding the
grey into a posterior, a lateral, and an anterior column. In
the dorsal region a lateral horn of grey matter projects into
the lateral column -^(I.L.). The white matter is composed of
white nerve fibres, the grey matter very largely of cells and

THE NERVOUS SYSTEM 149

synapses of neurons supported by branching neuroglia cells.
The cells of the grey matter are largest and most numerous
in the anterior horn, where they constitute the cells from
which the majority of nerve fibres come off. In the dorsal
region a group of cells in the lateral horn give off visceral
fibres (I.L.). In the dorsal region also a set of cells lie on
the mesial aspect of the posterior horn constituting the cells
of Lockhart Clarke (L.C.).

Functions.

The spinal cord is the great mechanism of reflex action,
and the great channel of conduction between the brain and
the peripheral structures.

A. REFLEX FUNCTIONS.

If the brain of such an animal as a frog is destroyed, the
animal lies for any length of time prone on its belly and
immovable. .If the skin of the leg is pinched the limb is
withdrawn, and if a piece of paper dipped in acetic acid is
placed on the flank, definite co-ordinated movements are
made to remove it. The animal has the power of reflex
movements with definite co-ordination of its muscles, but it
has no power of balancing itself, and manifests no spon-
taneous movements.

The reflexes conriected with various groups of skeletal
muscles are definitely associated with different levels of the
cord. In man the reflex movement of the foot on tickling
the sole is connected with the part of the cord from which
the 1st, 2nd, and 3rd sacral nerves come off. The reflex of
the cremaster muscle with the 1st, 2nd, and 3rd lumbar;
that of the abdominal muscles with the 8th to the 12th
dorsal; of the epigastric muscles with the 4th to the 7th
dorsal ; and of the scapular muscles with the 5th cervical to
the 12th dorsal. By taking advantage of these reflexes the
condition of the cord at different levels may be studied.

Reflex action in connection with various visceral muscles
are also connected with the spinal cord. Many of these are
complex reflexes involving inhibition of certain muscles and
increased action of others, some visceral, some skeletal. The

HUMAN PHYSIOLOGY
best marked of these are the reflex acts of micturition

External rotators of hij>.
Hamstrings.

Calf muscles and extrinsic muscles
of foot.

Intrinsic muscles of foot.

Muscles of bladder and urethra.
Extrinsic muscles of foot.
Intrinsic muscles of foot.

Levator and sphincter ani.

FIG. 79.— The Groups of Cells in the Anterior Horn of grey matter at the level
of the second, third, and fourth Sacral Nerves. (From BRUCE.)

(p. 407), defaecation (p. 355), erection, and ejaculation

./VVVVVA/SAAAAAAAAA/* T.

FIG. 80. — The neuro-iuuscular mechanism concerned in the knee jerk.
and the time of the knee jerk (A.J.) compared with the time
of a reflex action (4.72.).

(p. 415). The lumbar enlargement is the part of the cord
involved.

THE NERVOUS SYSTEM 151

The synapses in the cord are not only capable of acting
reflexly to set up definite contractions in muscles, but they
seem to exercise a constant tonic action upon them, and
when this tonic action is interfered with, the effect of directly
stimulating a muscle is diminished. This is very well seen
in the contraction of the quadriceps extensor femoris which
occurs when the ligamentum patellae is struck sharply causing
a kick at the knee joint — the knee jerk (Fig. 80). When
the reflex arc in the cord in the lower lumbar region is
interfered with, the knee jerk is diminished or is absent, and
when the activity of the arc is increased by the removal of
the influence of the brain, the jerk is increased. That the
jerk is not a reflex is shown by the fact that the latent
period is very much shorter than that of a reflex action
(Fig. 80). The reflex arc, however, is necessary for the tonus.
This tonus is increased by tension of the muscle and also by
fatigue of the nervous system which may lead to cramp.

The degeneration of the cells which follow amputation of
the leg at different levels seems to indicate that the various
groups of cells in the anterior horn of grey matter have
definite connections with individual muscles (see Fig. 79).

B. CONDUCTING PATHS.

Outgoing and ingoing fibres chiefly pass down the side of
the cord upon which they act or from which they come.
Section of one side of the cord leads to loss of the so-called

„

/

.-_„_-„- -/,

77

/,!—

t

///

Spinal Nerve

FIG. 81.— To show the course of upgoing (dotted line) and downgoing
fibres (continuous line) in the spinal cord.

voluntary movements and loss of sensation below the point
of section on the same side. Since there is, at least some-
times, a slight loss of voluntary power and of sensation on

152 HUMAN PHYSIOLOGY

the opposite side, it has been concluded that a few of both
sets of fibres decussate in the cord. It will afterwards be
shown that the main set of fibres cross the middle line in
the medulla (Fig. 83).

The two kinds of fibres run in different strands or tracts
of the cord, and these tracts have been defined by different
methods.

1. Degeneration or Wallerian Method. — This depends
upon the fact that nerve fibres degenerate when cut off
from their cells (p. 86), and that they, generally speaking,
conduct in the direction in which they degenerate, although,
as has been seen in the fibres of the posterior roots of spinal
nerves, this is not always the case. If the spinal cord be cut
across, certain tracts of fibres degenerate upwards, others
degenerate downwards, while some do not degenerate to any
great distance from the point of section.

Degenerations which reveal these tracts are often produced
by diseases and injuries of the cord, and thus the results
experimentally produced on animals have been confirmed on
the human subject. These degenerations may be demon-
strated when recent by Marchi's method of staining, which
depends upon the fact that while the white sheath of normal
fibres is not stained black when the tissue is placed in a
solution of chrome salt with osmic acid, it is so stained when
it begins to degenerate (p. 86). When the white sheaths
have entirely disappeared the degeneration is best demon-
strated by Weigert's method of staining the white sheaths of
normal fibres with hsematoxylin, which leaves the degenerated
fibres unstained.

A. Fibres degenerating upwards (see Fig. 78, p. 148).—
1. The fibres making up the posterior columns which are
derived from the posterior roots of the spinal nerves (P.L.
and P.M.). A few of these fibres degenerate downwards,
because the fibres of the posterior roots bifurcate when they
enter the cord (Fig. 40, p. 89), the one division passing right
up to the top of the cord, the other passing down for a short
distance. The ascending fibres of the posterior columns for
the most part end in synapses in the upper end of the cord,
in the nucleus gracilis and nucleus cuneatus (Fig. 40, p. 89).
From these fresh fibres pass upwards to the cerebrum

THE NERVOUS SYSTEM 153

(Fig. 40, (7.). 2. A thin layer of fibres round the margin of the
lateral column degenerates upwards, and has been traced as
far as the cerebellum (Fig. 78, D.C. ; Fig. 40, E.). It
consists of two sets of fibres. Those behind constitute the
direct or dorsal cerebellar tract ; those in front which take
a somewhat different course forming the ascending antero-
lateral or ventral cerebellar tract (Fig. 78, Asc. Ant. Lat.).
They both take origin in Lockhart Clarke's column of cells,
which is specially well developed in the dorsal and upper
lumbar region of the cord.

B. Fibres degenerating downwards. — 1. A very strong
band of fibres lying in the posterior part of the lateral
column, just inside the direct cerebellar tract, and becoming
smaller as the lower part of the cord is reached. This is
the crossed pyramidal tract (Fig. 78, C.Py.), which comes
from the cells of the cortex cerebri of the opposite side, and
gives off collateral branches to the cells in the anterior horn
of the spinal cord (Fig. 40, D.).

2. Certain fibres from the cortex cerebri do not cross, but
run down, some in the crossed pyramidal tract, some in the
direct pyramidal tract (Fig. 78, O.Py.), which runs along the
margin of the anterior fissure, and extends downwards only
into the dorsal region. These fibres decussate in the cord.

3. A set of fibres just inside the antero-lateral ascending
tract, which may be called the antero-lateral descending
tract (Fig. 78, Desc. Ant. Lat.). This comes from Deiters'
nucleus (see p. 89), and, as it passes down, gives off fibres to
the cells in the anterior horn of the grey matter of the cord.
Deiters' nucleus receives fibres from the cerebellum, and
these fibres probably carry down impulses from that organ.

C. Fibres not degenerating for any distance. — Round the
grey matter, a band of fibres — the basis bundles (Fig. 78,
B.B.) — do not degenerate far, and seem to be commissural
between adjacent parts of the grey matter.

Other tracts of fibres have been described, such as Lis-
sauer's tract and the septo-marginal tract, but their relations
have not been satisfactorily investigated.

2. Developmental Method. — The development of the cord
also helps to demonstrate the various tracts, since it has been
found that the fibres of outgoing tracts become functionally

154

HUMAN PHYSIOLOGY

active, and get their medullary sheath later than the fibres
of ingoing tracts. The crossed and direct pyramidal tracts
are non-medullated at birth, while the ingoing tracts are
medullated.

nc-

cyen

-pern.

Fio. 82. — View from above of the medulla oblongata, corpora quadri-
gemina, and optic thalami. c.l.a., posterior columns of cord ;
VIII., XII., JT., indicate the nuclei of these cranial nerves
in the floor of the fourth ventricle ; p.c.i. , the restiform body ;
p.c.m., the middle peduncle of the cerebellum; p.c.s., the
superior peduncle of the cerebellum; t.q., the anterior and
posterior corpora quadrigemina ; c.o. , the optic thalamus with
pulvinar (pulv.) and external and internal geniculate bodies
behind it ; e.p., the pineal body. (VAN GEHUCHTEN.)

B. THE MEDULLA OBLONGATA.
1. Structure.

The medulla oblongata may be regarded as the upper end
of the spinal cord, and it connects that structure with the
brain (Fig. 82). The cord expands and the posterior mesial

THE NERVOUS SYSTEM 155

fissure is opened out, so that the central canal comes to the
surface, and expands into a lozenge-shaped area — the floor
of the fourth ventricle. The lateral columns of the cord
pass outwards to the cerebellum to form part of its inferior
peduncles — the restiform bodies. Between the lateral and
the anterior columns an almond-shaped swelling, the olive,
appears (Fig. 84, 0.). Above this the medulla is encircled
by a mass of transverse fibres — the middle peduncles of
the cerebellum, or the pons Varolii (Fig. 86, P.). The
floor of the fourth ventricle is constricted above by the
approximation of the superior peduncles of the cerebellum
to again become a canal.

The grey matter of the cord gets broken up into separate
masses, of which the most important are : —

1. The nuclei of the posterior columns — the nucleus gracilis
and nucleus cuneatus (Fig. 83, N.C. and N.G.) — masses of
cells and synapses in which the fibres of the posterior
columns end, and from which the upgoing fibres of the fillet
start.

2. The inferior olivary nucleus (Fig. 84, 0.), which lies in
the olive, and which is connected by bands of fibres with the
dentate nucleus of the cerebellum (Fig. 86, Deit.).

3. The nucleus of Deiters (Fig. 86, Deit.), lying higher
up in the pons Varolii, and connected with fibres from
the cerebellum and from the semicircular canals (see
Fig. 73).

4. The masses of cells from which the cranial nerves take
origin (Fig. 85).

2. Conducting Paths.

A. Ingoing. — 1. The posterior columns of the spinal cord
terminate in two masses of grey matter on each side, the
nucleus gracilis and nucleus cuneatus. From these, fibres
pass downwards and across the middle line forming the
decussation of the fillet (Fig. 83, F.). The crossed fibres
(Fig. 84, F.) then pass up in a vertical series on each side
of the middle line until the pons Varolii is reached, when
they spread out horizontally like a fan (Fig. 86, F.) above
the deep transverse fibres. Above the pons they divide
into two sets (Fig. 90, F.) — a lateral fillet, which ends in

156

HUMAN PHYSIOLOGY

the anterior corpora quadrigemina, and a mesial fillet, which
passes on to the optic thalamus, and there ends by forming
synapses.

2. The direct cerebellar tract passes up into the restiform
body, and so on to the superior vermis of the cerebellum. Its
fibres form synapses round cells chiefly on the opposite side.

3. The ascending antero-lateral tract passes up beside
the last, but it leaves it in the restiform, and courses forward,
to arch back into the cerebellum round the superior cere-

FIG. 83. — Cross Section through Medulla Oblongata above the decussation of the
Pyramids. P.M. and, P.L., Postero-median and Postero-lateral tracts of the
Cord ; N.G. and N.C., Nucleus Gracilis and Cuneatus. giving off the Fillet
Fibres crossing at F. ; V., Ascending Root of Fifth Nerve; G.t Nucleus of
Glossopharyngeal Nerve ; A.H., Anterior Horn of Spinal Cord ; P., The
Anterior Pyramids; Z>.(7., Direct Cerebellar Tract; A. and D. Ant. L.,
Ascending and Descending Antero-lateral Tracts. (After BRUCE.)

bellar peduncle and to form synapses with the cells of the
superior vermis.

B. Outgoing. — 1. The fibres from the cerebral cortex,
which form in the cord the crossed and direct pyramidal
tracts, pass down in the middle part of the crusta (Fig.
90, P.) of the crura cerebri, and coursing between the super-
ficial and deep transverse fibres of the pons (Fig. 86, P.),
come to lie in the anterior pyramids of the medulla (Fig.
83, P.). At the lower end of the medulla most of these
fibres cross over to the lateral column of the cord; some,

THE NERVOUS SYSTEM

157

however, run down the direct and crossed pyramidal tracts
of the same side.

2. The fibres of the descending antero -lateral tract take
origin in a mass of nerve cells, which lies in the dorsal and
lateral part of the pons Varolii (Deiters' nucleus) (Fig. 86,
Deit.).

C. Commissural Fibres.

(1) The antero-lateral basis bundles of the cord form in
the medulla a strong band of fibres connecting the grey

V7/7 AccN

FIG. 84. — Cross Section of Medulla through the Olive. The Central Canal has
opened out to form the Floor of the Fourth Ventrical, 4th V. ; the Lateral
Columns are passing out to form the Inferior Peduncles of the Cerebellum ;
F. , Fillet; O. , Inferior Olivary Nucleus; />., Anterior Pyramids; Rest.,
Fibres of Kestiform Body; F., Ascending Root of Fifth Nerve; VIII.
Ace. N., Accessory Nucleus of the Eighth Nerve. (After BRUCE.)

matter at different levels, and known as the posterior longi-
tudinal fasciculus.

(2) A set of fibres run from each olivary body across the
middle line to the dentate nucleus of the cerebellum of the
opposite side.

3. Cranial Nerves.

(The physiology of these should be studied while dissecting
them.) The nerves springing from and entering the medulla,
do not come off in the same regular fashion as the spinal
nerves. The outgoing fibres of each spring from a more or

158 HUMAN PHYSIOLOGY

less definite mass of cells. The ingoing fibres generally form
synapses with cells arranged in definite groups. In this way
the so-called nuclei of the cranial nerves are formed. The
position of these is indicated in Fig. 85. In the cranial
nerves no sharp differentiation into anterior and posterior
roots can be made out, but they contain the same component
elements as the spinal nerves, the fibres running either
together or separately.

Ingoing Fibres. — Somatic and splanchnic enter the medulla
and have their cell stations in ganglia upon the nerves.

Outgoing Fibres. — Somatic and splanchnic pass out, the

FIG. 85.— The Nuclei and Roots of the Cranial Nerves. (After EDINGER.)

latter being characterised by their small size, and by forming
synapses before their final distribution.

The XII. (Hypoglossus) is purely an anterior root nerve,
and is motor to the muscles of the tongue.

The X. (Vagus) is essentially the posterior root of the XI.
(Spinal Accessory), but it transmits some outgoing fibres.
It is the great ingoing nerve from the abdomen, thorax,
larynx, and gullet, while, by outgoing fibres passing through
the vagus or accessorius, it is augmentor for the muscles of
the bronchi and alimentary canal, inhibitory to the heart,
dilator to blood-vessels of the thorax and abdomen, and
motor to the muscles of the larynx and to the levator palati.
The accessorius is also motor to the sterno-cleido-mastoid
and trapezius.

The IX. (Glossopharyngeal) is essentially a posterior root,
and is the ingoing nerve for the back of the mouth, the
Eustachian tube, and tympanic cavity. It transmits out-

THE NERVOUS SYSTEM

159

going fibres which are motor to the stylo-pharyngeus and
middle constrictor of the pharynx.

The III. (Oculomotorius), IV. (Trochlearis), VI. (Ab-
ducens), and VII. (Facial), along with the V. (Trigeminal),
form what may be regarded as a pair.

The Trigeminal is chiefly a posterior root, but it has a

Rest

FlG. 86. — Cross Section through Region of Pons, Cerebellum, and Fourth Ventricle.
S. V. , Superior Vermis ; M.N., Roof Nucleus; Dent., Dentate Nucleus;
Rest., Restiform Body; S.P., Superior Peduncle of Cerebellum; Deit.,
Deiters' Nucleus; VI., Nucleus of the Sixth Nerve; F., Fillet; S.O.,
Superior Olive ; D. T. and S.T., Deep and Superficial Transverse Fibres ; P.,
Pyramidal Fibres. (After BRUCE. )

distinct anterior or motor root which joins it, and carries the
motor fibres to the muscles of mastication.

It is the great ingoing nerve for all the face.

The VII. is almost purely an anterior root, transmitting
the motor fibres to the muscles of expression, and secretory
fibres to the submaxillary and sublingual glands and the

i6o HUMAN PHYSIOLOGY

glands of the mouth. It, however, carries ingoing fibres from
the anterior two- thirds of the tongue.

The VI. supplies the external rectus of the eye.

The IV. supplies the superior oblique. The III. supplies
all the muscles of the eyes except those supplied by the VI.
and IV.

The fibres coming from the nuclei of these cranial nerves
do not always pass out in the nerve itself. Thus, fibres from
the nucleus of the III. to the orbicularis oculi pass out in
the VIL, while fibres for the posterior belly of the digastric
which pass out in the VII. probably come from the nucleus
of the XII.

4. Reflexes of the Medulla.

The extensive series of synapses in the medulla form
arrangements by which various combined and co-ordinated
movements are controlled. Thus, part of the nucleus of the
vagus governs the movements of respiration, while other
parts preside over the slowing mechanism of the heart. To
these various reflex arrangements the name of centres has
been given.

C. REGION OF PONS VAROLII.

Outgoing Fibres. — 1. The fibres to the face muscles cross
the middle line to become associated with the various nuclei
of the cranial nerves. For this reason a tumour in one side
of the pons may cause paralysis of the face muscles on one
side and of the muscles of the rest of the body on the
opposite side. 2. Fibres to the limbs and trunk run down
between the deep and superficial transverse fibres (Fig.
86, P.).

Ingoing Fibres. — The fillet fibres in the pons, instead of
running up on each side of the middle line, spread out into a
horizontal arrangement above the crossed fibres.

D. CEREBELLUM.

1. Structure. — The cerebellum or lesser brain lies above
the fourth ventricle, and is joined to the cerebro-spinal axis
by three peduncles on each side. It consists of a central

THE NERVOUS SYSTEM

161

lobe, the upper part of which is the superior vermis, and two
lateral lobes, each with a secondary small lobe, the flocculus.
Its surface is raised into long ridge-like folds running in the
horizontal plane, and is covered over with grey matter, the
cortex. In the substance of the white matter forming the
centre of the organ are two masses of grey matter on each
side — 1, the roof nucleus; and 2, the dentate nucleus
(Fig. 86, R.N. and Dent.).

The cortex may be divided into an outer somewhat homo-
geneous layer (the molecular layer, Fig. 87, G.L.) and an inner

FIG. 87.— Diagram of the Arrangement of Fibres and Cells in the Cortex of the
Cerebellum. G.L., Molecular Layer; N.L., Nuclear Layer; P., Purkinje's
Cells. (After RAMON Y CAJAL. )

layer studded with cells (the nuclear layer, N.L.). Between
these is a layer of large cells — the cells of Purkinje (P.).

By Golgi's method the arrangement of fibres and cells in
the cerebellum has been shown to be as follows : —

Fibres coming into the cortex from the white matter end
either in synapses round cells in the nuclear layer, or proceed
at once to the outer layer (Fig. 87). From the cells in the
nuclear layer processes pass to the outer layer and there form
synapses with other cells. From these, processes pass to
the cells of Purkinje, round which they arborise, and from

11

162 HUMAN PHYSIOLOGY

Purkinje's cells the outgoing fibres of the cerebellum pass
into the white matter, to the roof nuclei, and hence to
Deiters' nuclei (Fig. 86).

2. Connections. — The cerebellum is connected (Fig.
88):—

i. With the Spinal Cord.

a. Incoming Fibres. — 1. The direct cerebellar tract (p. 156)
passes up in the restiforin body to end chiefly in the superior
vermis. 2. The ascending antero-lateral tract (p. 156) passes
to the cerebellum in the superior peduncle and ends in
the superior vermis. 3. Fibres from the nuclei of the
posterior columns of the same side (Fig. 83, p. 156) pass
in the restiform body to the cerebellum. 4. Fibres from
the vestibular root of the eighth nerve also pass to the
cerebellum (p. 137).

6. Commissural Fibres. — Strong bands of fibres connect
the inferior olive of one side with the dentate nucleus of
the other.

c. Outgoing Fibres. — Fibres pass from the superior vermis
to the roof nuclei, and, from these, fibres pass on to Deiters'
nuclei (Fig. 86, p. 159), from which fibres pass down in the
descending antero-lateral tract of the cord.

ii. With the Cerebrum. — 1. The fibres of the middle
peduncles cross in the middle line embracing the medulla,
and become associated with cells from which fibres pass up
in the lateral parts of the crura cerebri to the cerebral
cortex (Fig. 90, CO. CO., p. 167). 2. The fibres of the superior
peduncle cross and end in the red nuclei (Fig. 90, S.C.P.),
from which fibres seem to pass upwards to the cerebrum.
How far these are upward conducting and how far down-
ward is not definitely known.

3. Functions. — Removal of the cerebellum deprives the
animal, for a time at least, of the power of balancing itself.
This may be easily demonstrated in the pigeon (Fig. 95,
p. 171). But in some cases, when slowly progressing disease
has destroyed the organ, no loss of equilibration has appeared,
and in other cases the cerebellum has been congenitally
almost absent, and yet the individual has not shown any
sign of want of power of maintaining his balance. Evi-

THE NERVOUS SYSTEM

163

dently, therefore, some other part of the brain can com-
pensate for its absence.

The manner in which the cerebellum acts has been chiefly
elucidated by removing parts of the organ and keeping the

FIG. 88. — Connections of the Cerebellum with the Cerebro-spinal Axis (for
explanation, see text).

animals under observation for prolonged periods. If one
side of the cerebellum is removed the first symptoms are
(1) a tonic contraction of the muscles of the limbs of the
same side by which the fore limbs may be powerfully ex-

1 64 HUMAN PHYSIOLOGY

tended, and an arching of the body with the convexity
towards the side of the lesion, while the animal may be
driven round its long axis to the opposite side. (2) These
irritative symptoms soon pass off, and the animal then mani-
fests inadequacy or weakness in the limbs of the affected
side, so that it droops to that side, and if a quadruped may
circle to that side. (3) After some weeks these symptoms
disappear, and the loss of one side of the cerebellum is
apparently completely compensated for.

From these experiments it would appear that the cere-
bellum is to be regarded as a mechanism supplementary to
the great cerebro-spinal mechanism, and that it has for its
purpose more especially the muscular adjustment required
in maintaining the balance. This it may do in one or both
of two ways.

1. By receiving impulses from without, and sending
impulses downwards to act upon the spinal mechanism.

2. By receiving impulses, and sending impulses upwards
to the cerebrum to modify its action. A channel for such
impulses exists in the fibres of the pons which cross the
middle line to connect with cells from which fibres pass
upwards to the occipital and frontal lobes of the cerebrum
(Fig. 40, p. 89).

To maintain the constant muscular adjustments involved
in balancing the body requires an arrangement whereby any
disturbance of the equilibrium can produce an appropriate
reaction.

The ingoing impulses which are more especially of service
in this way are (1) the muscular sense (see p. 97); (2) the
tactile sense from the soles of the feet ; (3) vision ; and (4)
the sense of acceleration or retardation of motion in any
plane or planes of the body derived through the semicircular
canals. The importance of the muscular, tactile, and visual
senses in maintaining the balance is so obvious that it need
not be further considered.

4. Physiology of the Semicircular Canals. — That there is
no special mechanism making us aware of uniform movement
is proved by the fact that we are not conscious of whirling
through space with the earth's surface, and that in a smoothly

THE NERVOUS SYSTEM 165

running train we lose all sense of forward movement. It is
only as the train starts or stops that we have a sensation of
movement or retardation. The same thing has been demon-
strated by strapping a man to a table rotating smoothly
round a vertical axis and setting the table spinning. A
sense of rotation is experienced as the table starts but is lost
when the movement becomes uniform, while stopping the
table gives rise to a sensation of being rotated in the opposite
direction.

The semicircular canals form a mechanism which is
capable of acting in this way. They are arranged in pairs
in the two ears thus — The two horizontal canals are in a
horizontal plane, the superior canal of one side and the
posterior canal of the other are in parallel planes oblique
to the mesial plane of the body (Fig. 89, a).

FIG. 89.— (a) Arrangement of the Semicircular Canals on the two sides ; (6) Bony
and Membranous Canal and Ampulla to illustrate their mode of action.

The horizontal canals may be considered as forming the
arc of a circle with an ampulla at each end. The superior
canal of one side has its ampulla in front, while its twin — the
posterior of the opposite side — has its ampulla behind, and
they together form the arc of a circle with an ampulla at
each end (Fig. 89, a).

The membranous canals are very narrow, and occupy but
a small part of the osseous canals. The membranous
ampullae are large and almost fill the osseous ampullae
(Fig. 89, 6).

If the head is moved in any plane, certain changes will be
set up in the arnpullse towards which the head is moving,
and converse changes in the ampullae at the other end of the
arc of the circle.

1 66 HUMAN PHYSIOLOGY

If, for example, the head is suddenly turned to the right,
the inertia of the endolymph and perilymph tend to make
them lag behind. Thus the endolymph in the ampulla of
the right horizontal canal will tend to flow into the canal,
but the canal is so small that it will merely accumulate in
the ampulla, and thus a high pressure will be produced (Fig.
89, b + +). The perilymph will tend to lag behind, and a
low pressure will result outside (Fig. 89, b — ). The converse
will take place in the opposite horizontal canal.

When the movement is continued the pressures will be
readjusted, and, on stopping the movement, the opposite
conditions will be induced, and a sensation of moving in an
opposite direction will be experienced.

In forward movement, the two superior canals have the
pressure of endolymph increased in their ampullae — in back-
ward movement this occurs in the two posterior canals. In
nodding to the right the superior and posterior canals of the
right ear undergo this change.

There is also evidence that the semicircular canals assist
in maintaining the tone of the skeletal muscles and that
destruction of the canals is followed by a loss of tone. This
might be expected from their intimate connection with the
cerebellum.

When the information as to our relationship with our
surroundings derived from these various sources is not
concordant — e.g. when through the semicircular canals we
have a sensation of movement, and through the eyes an
apparent absence of movement — balancing becomes difficult,
and a feeling of giddiness results. This may be readily
demonstrated by setting a poker vertically on the floor,
holding it in the hand, placing the forehead on the top,
walking rapidly three times round it, then standing up and
trying to walk out of the room. The sudden stoppage of
the rotatory movement causes a disturbance in the semi-
circular canals giving a sense of rotation in the opposite
direction, while the eyes tell us that no rotation is taking
place. The feeling of giddiness is, however, not the cause
of the loss of balancing, but a mere accompaniment.
(Experiment.)

THE NERVOUS SYSTEM

167

E. THE CRURA CEREBRI AND CORPORA
QUADRIGEMINA.

Above the pons Varolii, the two halves of the medulla
diverge from one another and form the peduncles of the
cerebrum (Fig. 90, CC., P.), while posteriorly the two superior
peduncles of the cerebellum having crossed join together
(8.C.P.). Above these, two swellings develop on each side —
the anterior and posterior corpora quadrigemina (Fig. 82,
p. 154).

The crusta, or anterior parts of each peduncle of the

FIG. 90. — Cross Section through Anterior Corpora Quadrigemina and Cerebral
Peduncles, A.S., Aqueduct of Sylvius; III., Nucleus of Third Nerve;
S.C.P., Superior Cerebellar Peduncles; F., Fillet; P., Pyramidal Tract;
CC., Cerebello-cerebral Fibres. (After BRUCE.)

cerebrum, is composed, in its central part, of the pyramidal
fibres passing down from the cerebrum to the spinal cord
(P.), and, on each side, of the cerebello-cerebral fibres pass-
ing upwards from the pons (CC.). The posterior part, or
tegmentum, contains — 1st, the fillet fibres going partly to
the corpora quadrigemina, partly onwards to the thalamus
opticus (P.); 2nd, the nuclei of the 3rd and 4th cranial
nerves; 3rd, the fibres of the superior peduncles of the
cerebellum which cross the middle line (S.C.P.); and 4th,
the red nuclei in which most of these fibres end.

168

HUMAN PHYSIOLOGY

The functions of this segment of the brain are chiefly
conducting, but the anterior corpora quadrigemina forms the
shunting station between the incoming fibres of the optic
tract and the oculo-motor mechanism (see p. 126).

FlG. 91. — Diagrammatic Horizontal Section through Base of Cerebral Hemisphere,
showing (1) the Outgoing Fibres for the Leg, Arm, and Face springing from
the Cortex of the Rolandic Areas, passing through the Internal Capsule
between the Thalamus and the Lenticular Nucleus. The Face Fibres cross in
the Pons, the Leg and Arm Fibres in the Medulla. (2) The Incoming Fibres
(Fillet, Eye) form their stations in the Thalamus, and then pass on to the
Cortex.

F. THE CEREBRUM.

Each peduncle terminates in its half of the cerebrum.
As the fibres pass from peduncle to cerebrum and vice versa
they come into relationship with three masses of grey matter
lying in the midst of the cerebrum. These are the thalamus
options into which the ingoing fibres enter; the lenticular
nucleus, between which and the thalamus the outgoing

THE NERVOUS SYSTEM

169

fibres run ; and the caudate nucleus, the main part of which
lies in front of the other two (Fig. 91).

FIG. 92. — Diagram of the Arrangement of Cells in a typical part of the Cerebral
Cortex (see p. 168). (After RAMON Y CAJAL.)

The fibres, above these nuclei, spread out to form the

FIG. 93. — Diagram of Collateral Connections of different parts of the Cerebral
Cortex, a, &, c, Pyramidal Cells of the Cortex, all connected by Collateral
Branches with other parts of the Cortex in the same and in the opposite
hemisphere, a gives off the Pyramidal Fibres to the Cord. (After RAMON Y
CAJAL.)

corona radiata and enter a crust of grey matter, the cortex
cerebri, which covers over the cerebrum, and which in the

HUMAN PHYSIOLOGY

higher animals is raised into a number of folds or con-
volutions marked off from one another by fissures and sulci
(Fig. 94).

The method of Golgi shows that the neurons in a typical
part of the cortex are arranged in the following manner
(Fig. 92) :—

FiQ. 94. — A shows the chief convolutions on the outer aspect (front to right), and
B on the inner aspect (front to left) of a cerebral hemisphere.

1. The incoming fibres pass right up to the surface and
end in dendrites (1).

2. In this region are a set of small horizontal cells with
numerous processes (2).

3. Underneath these are several rows of small pyramidal
cells sending dendritic processes to the surface and their
axons downwards (3).

4. Below these are rows of large pyramidal cells similarly
arranged (4).

THE NERVOUS SYSTEM

171

5. Lowest of all are some irregular cells with a similar
disposition of processes. (These are not shown in Fig.
92.)

This arrangement is considerably modified in certain
regions of the cortex, the large pyramidal cells being best
developed in those parts from which the great mass of fibres
pass down to the spinal cord.

From the various fibres collaterals come off which connect
different parts of the cortex of the same side, and which also
connect the cortex of one side with that of the other, and
with the basal ganglia (Fig. 93).

The diagram opposite shows the more important con-
volutions of the cortex cerebri (Fig. 94).

FIG. 95. — A, Pigeon with the Cerebellum destroyed to show struggle to maintain
the balance ; JB, Pigeon with Cerebrum removed to show balance maintained,
but the animal reduced to a somnolent condition.

Functions of the Cerebrum — 1. General Consideration.—

The functions of the cerebrum may be best understood by
first contrasting the condition of animals with, and of animals
without, this part of the brain.

(1) In the frog the cerebral lobes may easily be removed.
The animal sits in its characteristic attitude. When touched
it jumps, when thrown into water it swims. It is a perfect
reflex machine, with the power of balancing itself unimpaired.
But it differs from a normal frog in moving only when
directly stimulated, and in showing no signs of hunger or of
thirst. A worm crawling in front of it does not cause the
characteristic series of movements for its capture which are
seen in a normal frog.

(2) In the pigeon (Fig. 95, B), removal of the cerebral
hemispheres reduces the animal to the condition of a

i/2 HUMAN PHYSIOLOGY

somnolent reflex machine. The bird sits on its perch,
generally with its head turned back, as if sleeping. If a
sudden noise is made, if light is flashed in its eye, or if it is
touched, it flies off its perch and lights somewhere else.
But every stimulus produces the same result. Clapping
the hands and letting peas fall on the floor are both
obviously heard and both produce a start, but the bird
makes no endeavour to secure the peas as it would do in the
normal state.

(3) In the dog, by a succession of operations, Goltz has
removed the greater part of the cerebral cortex without
causing paralysis of the muscles. The animal became dull
and listless, and did not take food unless it was given to it.
It showed no sign of recognising persons or other dogs, and
did not respond in the usual way when patted or spoken to.
But it snapped when pinched, shut its eyes and turned its
head away from a bright light, and shook its ears at a loud
sound. It did not sit still, but walked constantly to and fro
when awake. It slept very heavily. In fact all the responses
of the animal might be classed as reflex responses to imme-
diate excitation.

(4) In monkeys, removal of the cerebral cortex leads to
such loss of the so-called voluntary movements that all
other symptoms are masked.

In the decerebrated animal different stimuli do not produce
distinctive reactions, but, with the cerebrum intact, there is
at least the possibility of small differences in the modes of
stimulation producing marked differences in the resulting
action.

These resulting actions are in part at least determined by
(1) the previous training and education of the brain; for,
just as in the spinal cord channels of action are formed, so
in the cerebrum, if a given reaction once follows a given
stimulus, it will tend to follow it again, (a) This training is
in part hereditary. Each individual of a race is born with
well-established lines of action in the process of development,
and throughout life these inherited channels play an im-
portant part in determining the results of stimulation.
(b) But it is also largely acquired by the individual, since
the reception of each stimulus and the performance of a

THE NERVOUS SYSTEM

173

resulting action, however this be determined, tends to lay
down a path which will again be followed.

(2) Not only will the previous training of the brain thus
act as the directive force in the response to stimuli, but the
nutrition of the brain also plays an important part. The

S TOMNG.

RECEIVING.

\

p I (VA L AR.C

FIG. 96. — Diagram to illustrate different possible channels of cerebral
response to stimulation.

action of a brain when well nourished and freely supplied
with pure blood is often very different to that of the same
brain when badly nourished or imperfectly supplied with
healthy blood. Since the education of the brain really con-
sists in developing proper responses to various stimuli, the

174 HUMAN PHYSIOLOGY

importance of the brain being in a healthy and well-nourished
condition during the training is manifest.

The power of differentiating various stimuli is dependent
on the development of the brain, and becomes more perfect
as the animal scale is ascended ; and the complexity of the
cerebral action in the higher animals has its basis in the
greater number of distinct impressions which have been
received and reacted to. Each separate stimulus leaves its
mark upon the brain, or as we may say, is sto?*ed in the brain,
and each subsequent similar stimulus is sent into these
channels, or is associated with the past reactions, and thus
the present response is determined, what may be described
as an unconsciousness judgment being made. For appro-
priate reaction the whole mechanism must be normal — a
very small injury to any part may completely alter the
character of the response to any given stimulus — as may be
well seen in the insane.

So far, cerebral action may be considered in a purely
material manner as consisting of a series of reflex acts, higher
and more complex than those in the spinal cord, in which
the result of the stimulus varies in the same way, but to a
much greater degree, from its association with more complex
past impressions.

But this cerebral action is generally accompanied by
changes in the consciousness of the individual, some of
which are termed simply sensations, while others may be
described as trains of thought. Essentially, however, a train
of thought is nothing more than a train of sensations, each
evoked by a stimulus or by a preceding sensation, and, if
this be admitted, we may say that it is by sensation alone
that we are aware of consciousness, and that, therefore, the
two are coterminous. It is impossible to conceive conscious-
ness without sensation, or sensation without consciousness.

Cerebral action frequently goes on without consciousness
being implicated ; but so far as we know, consciousness
without accompanying cerebral action is unknown, and there
is evidence that it is only when the action of the various
parts of the cerebrum is co-ordinated that consciousness
is possible. In cases of Jacksonian epilepsy, as a result
of a small centre of irritation on the surface of the brain,

THE NERVOUS SYSTEM 175

a violently excessive action of the cerebral neurons starts
at the part irritated and passes to involve more and more
of the brain. In such fits it is found that at first the
patient's consciousness is not lost, but that, when a suffi-
cient area of brain is involved in this excessive and unco-
ordinated action, consciousness disappears.

The study of the action of drugs which abolish, conscious-
ness— e.g. chloroform and morphine — on the dendrites of
brain cells, suggests a physical explanation of the loss of
consciousness. It is found that these drugs cause a general
extension of the gemmules of all the dendrites ; and, if we
imagine that the co-ordinated action of any part of the brain
is secured by definite dendrites of one set of neurons coming
into relationship with definite dendrites of another set of
neurons by their gemmules, the want of co-ordinate relation-
ship established by the general expansion might obviously
explain the disappearance of the definite sensations which
constitute consciousness.

It is manifest that the range of consciousness must neces-
sarily be wider where the stored impressions are most
abundant, and where the present stimulus most readily
calls into action these previous lines of cerebral activity. The
storage of impressions is the basis of MEMOKY, the power of
associating these stored impressions with the present stimu-
lus is the basis of RECOLLECTION. It is the implication
of consciousness in this part of brain action which is the
basis of mental activity. How far the mental action is a
mere accompaniment of the physical changes, and how
far it can react upon them, is a question which cannot be
discussed here. But the study of the insane seems to point
to the conclusion that the individual does certain things and
has certain ideas as concomitant results of faulty brain
action, rather than that his actions are a result of the
modified ideas.

2. Time of Cerebral Action — The cerebral mechanism
takes a very appreciable time to act, and the time varies
with the complexity of the action and with the condition
of the nervous apparatus.

Of the time between the presentation of a flash of light to

i/6 HUMAN PHYSIOLOGY

the eye or a touch to the skin and a signal made by the
person acted upon when it is perceived, part is occupied
by the passage of the nerve impulses up and down the
nerves and in the latent period of muscle action, but some-
thing over TVth of a second remains, representing the time
occupied in the cerebral action. (Practical Physiology,
Chap. XII.)

If the observation be complicated by requiring a dis-
crimination to be made between different stimuli, the reaction
time is longer, and, the more unaccustomed the differentia-
tion, the longer will the reaction time be. Thus in one
accustomed to deal with figures, the discrimination of a
series of these is more rapidly carried out than in one
unaccustomed to do so. Prolonged action of the nerve
centres soon leads to a prolongation of the reaction time,
and the same thing is produced by the action of alcohol,
chloroform, and other poisons. This fatigue of the nerve
mechanism is the physical basis of that state of the con-
sciousness which is called loss of attention.

3. Fatigue of Cerebral Mechanism. — This naturally leads
to the consideration of fatigue of the cerebral mechanism.
The way in which, as a result of poisons, the definite
co-relationship of certain sets of neurons with certain other
sets is abolished by the generalised expansion of the
gemmules of the dendrites has been already dealt with.
In all probability the same thing occurs in fatigue, and by
interrupting the definite chain of action allows rest and
recovery to supervene. But continued action leads to well-
marked changes in the cell protoplasm of the neurons.
The Nissl's granules diminish and the nucleus shrivels and
becomes poorer in chroinatin.

In all reflex action, whether spinal or cerebral, it is the
central part of the mechanism which first becomes fatigued.
If, by direct excitation of the central nervous system of
a frog, muscular movements are caused for some time, the
stimulation ultimately fails to act ; but, if the nerves going
to the muscles are stimulated, the muscles at once respond,
showing that the central mechanism has given out before
the peripheral structures.

THE NERVOUS SYSTEM 177

Fatigue of the central nervous system is manifest both
upon the receiving and reacting mechanism ; upon the
receiving, on the physical side by prolongation of the
reaction period, and on the metaphysical side by diminished
power of attention. Upon the reacting side it is shown by
lessened power of muscular contraction. (See also p. 94.)

4. Sleep. — Fatigue of the cerebral mechanism is closely
connected with sleep. As the result of fatigue, external
stimuli produce less and less effect, and thus the changes
which are the physical basis of consciousness become less
and less marked. At the same time, by artificial means
stimuli are usually excluded as far as possible. Absence of
light, of noise, and of tactile and thermal stimuli all conduce
to sleep. Consciousness fades away, and, as the cerebral
activity diminishes, the arterioles throughout the body
dilate, and the arterial blood pressure falls, and thus less
blood is sent to the brain, and the organ becomes more
bloodless. The eyelids close, the eyeballs turn upwards,
the pupils contract, and the voluntary muscles relax.

The depth of sleep may be measured by the strength of
the stimuli required to overcome it, and it has been found
that usually it is deepest at about the end of an hour,
and that it then rapidly becomes more and more shallow
until, as the result of some stimulus, or when the brain has
regained its normal condition, it terminates. In the later
hours of sleep the consciousness may be temporarily aroused
without the other conditions of sleep disappearing, and as a
result of this dreams may ensue. Or, on the other hand,
without consciousness being necessarily restored, stimuli may
lead to muscular response of a perfectly definite and pur-
posive character, and sleep-walking may occur.

Hypnosis is a condition in some respects allied to sleep.
It may be produced in many individuals by powerfully
arresting the attention, and is probably due to a removal
of the influence of the higher centres over the lower. When
the condition is produced the respiration and pulse become
quickened, the pupil expands, the sensitiveness of the neuro-
muscular mechanism is so increased that merely stroking
a group of muscles may throw them into firm contraction.

12

1 78 HUMAN PHYSIOLOGY

The individual becomes a pure reflex machine even as
regards the cerebral arc, and each stimulus is followed
by an immediate reaction. The power of suggestion is
exaggerated. If a hypnotised person is told that he sees
anything he acts at once as if he did actually see it.

5. Localisation of Functions. — The question must n« xt,
be considered whether special parts of the brain are more
especially connected with its three great functions—

1. The reception of stimuli.

2. The storing of effects, and the associating of present
stimuli with these stored impressions.

3. The production of the resulting actions.

1. Reception of Stimuli. — In investigating the existence of
special mechanisms, for this purpose, two methods of inquiry
are available.

1st. By removing or stimulating parts of the brain in the
lower animals and studying the results.

2nd. By observations during life on the sensations or
absence of sensation in patients suffering from disease of
the brain, and the determination of the seat of the lesion
after death.

1st. Sensations are the usual accompaniment of the activity
of the receiving mechanism. But, in the lower animals, it is
not possible to have a direct expression of whether sensations
are experienced or not, and, therefore, in determining whether
removal of any part of the brain has taken away the power
of receiving impressions, we have to depend on the absence
of the usual modes of response to any given stimulus. But
the absence of the usual response may mean, not that the
receiving mechanism is destroyed, but either that the react-
ing mechanism is out of action or that the channels of
conduction have been interfered with. (See Fig. 97.)

Thus, if light be flashed in the eye of a monkey, it re-
sponds by glancing towards the source of illumination : and
if these movements are absent this may be due to loss of the
receiving mechanism, to loss of the mechanism causing the
movements, or to interruption of the channels between these.

THE NERVOUS SYSTEM

179

Again, it is quite possible that, after removing the receiving
mechanism in the cerebrum, external stimuli may lead to
the usual response by acting through lower reflex arcs
(Fig. 97). Thus, if we suppose the receiving part of
the cerebrum connected with the reception of tactile im-

RECCIVING.

»

D14CHAP.C.ING.

FlG. 97. — Diagram to show how through reflex action of the lower arcs the action
of the higher arcs may be stimulated.

pressions entirely destroyed, scratching the sole of the foot
may still cause the leg to be drawn up, just as if a sensation
had been experienced. Here although the upper arc is out
of action the lower arc still acts.

In the lower animals, stimulation of a part of the brain,
if it be connected with the reception of impressions, may
cause the series of movements which naturally follow such
an impression. But these movements may also be caused

B

Of CM,

i8o

HUMAN PHYSIOLOGY

by directly stimulating the reacting mechanism. When,
however, removal of a part of the brain causes no loss of
power of movement, and }^et prevents a stimulus from
causing its natural response, it is justifiable to conclude that
that part of the brain is connected with reception.

Km. 98.— (a) Surface of the left Cerebral Hemisphere to show the situations of
some of the Receiving and Discharging Mechanisms (front to left) ; (b) Mesial
Surface of the same Hemisphere (front to right).

2nd. In man, the chief difficulty of obtaining information
is in finding cases where only a limited part of the brain is
affected. But such cases have, in many instances, been
observed. Tumours of an occipital lobe, for instance, have
been found to be associated with loss of visual sensations

THE NERVOUS SYSTEM 181

without loss of muscular power, and thus the conclusion has
been drawn that the occipital lobe is the receiving mechanism
for stimuli from the eyes.

Visual Centre. — The way in which the fibres, coming from
the two retinae, are connected with each thalamus opticus and
occipital lobe has been already considered, and it has been
shown that the optic tract passes into the geniculate bodies
on the posterior aspect of the thalamus, and that a strong band
of fibres, called the optic radiation, extends from these back-
wards to the occipital lobes (Fig. 66, p. 127). An extensive
lesion of one — say the right — occipital lobe, especially if on
the inner aspect in the region of the cuneate lobe, is accom-
panied by no loss of muscular power but by blindness for all
objects in the opposite side of the field of vision — i.e. the right
side of each retina is blind. The central spot of neither eye
is completely blinded because the fibres from the macula
lutea only partially decussate at the chiasma. Probably each
part of the occipital lobe is connected with definite parts of
the two retinse. Certain it is that there is no part of this
cortical mechanism connected with the blind spot, and hence
this is not perceived in ordinary vision.

Colour perception seems to be a less fundamental function
of the visual apparatus than perception of the degree and
direction of illumination, and hence colour perception may be
lost in less extensive lesions of the occipital lobes without the
other functions being impaired. One of the most recently
acquired functions of the mechanism is the power of appre-
ciating the significance of the signs used in written language,
and it is found that in small and superficial lesions this
function may alone be lost.

The cortex of the occipital lobe is rich in the smaller variety
of cells and poor in the larger pyramidal cells, which are found
in areas connected with the production of movements.

While there is good evidence that a special localised area
of the cortex is connected with the reception of stimuli from
the eyes, the evidence of the existence of similar areas or
centres connected with the other organs of special sense is
by no means satisfactory.

Auditory Centre. — Ferrier, by removing the superior

1 82 HUMAN PHYSIOLOGY

temporo-sphenoidal lobe in the monkey, produced no motor
disturbance, but found evidence of loss of hearing in the
opposite ear. He described the monkey as pricking its ears,
and looking to the opposite side when the region is stimu-
lated, and he considered that these observations prove the
existence of a special localised mechanism for the reception
of stimuli from the ear. But, more recently, Schafer has
removed these convolutions from both sides in the monkey,
with aseptic precautions, kept the animal alive, and found
no evidence of loss of auditory sensations. This conclusion
depends upon the observation that stimulation of the auditory
mechanism in such animals still leads to the usual muscular
response. But since lower connections probably exist between
the auditory nuclei in the medulla and the centres for mus-
cular movement in the spinal cord (Figs. 72 and 73), this
observation cannot be accepted as excluding the relationship
of the superior temporo-sphenoidal convolution with hearing.
This relationship is strongly supported by pathological evi-
dence. Cases of " Jacksonian Epilepsy" have been recorded,
in which the first symptom of the fit was the hearing of
sounds, and in which the lesion was found on this lobe.

Taste and Smell. — So far satisfactory evidence of the exist-
ence of a special localised mechanism connected with these
senses is wanting. Ferrier states that removal of the hippo-
campal convolution in monkeys leads to loss of taste and
smell, and that stimulation causes torsion of the nostrils and
lips, as if sensations of smell and taste were being experienced.
But Schafer was unable to observe any indication of loss
of taste and smell in monkeys from which the temporo-
sphenoidal lobe had been to a large extent removed.

Other experimenters have observed interference with the
sense of smell in destructive lesions of the hippocampal lobe,
and one case at least has been described in which a tumour
of the right gyrus hippocampus was associated with sensa-
tions of smell. In monotremes the fibres from the olfactory
lobe have been traced to the neighbourhood of the hippo-
campus.

Touch. — Here, again, the difficulty of drawing conclusions
from observations on lower animals is encountered. Ferrier
thought that removal of the hippocampal convolution caused

THE NERVOUS SYSTEM 183

loss of tactile sense, while Schafer believes that the gyrus for-
nicatus is the centre for this sense. It has been objected
that in removing this lobe the fibres going to the areas on
the outside of the cortex are apt to be injured. According
to the observations of Mott, when the cortex round the fissure
of Rolando — in which the mechanism for causing the various
combinations of muscular movements is situated — is removed
in the monkey, clips may be attached to the skin on the
opposite side of the body without attracting attention,
while if they are placed on the same side they are at once
removed. He therefore regards the Rolandic area of the
brain as connected with the reception of tactile impressions.

As already indicated, this centre must act as a chart of the
surface of the body, stimulation of any definite part of the
body leading to changes in a definite part of the centre, and
these changes are accompanied by sensations referred to the
part stimulated.

We know nothing of the existence of centres connected
with the thermal and muscular senses.

2. Storing and Association Mechanism. — The existence of
a special part or parts of the brain connected with the storing
of impressions, so that they may be associated with present
sensations, is indicated by the following considerations : —
It is this association of present stimuli with past sensations
which is the basis of intellectual life, and in man, where
apparently the intellectual functions are most highly deve-
loped, the frontal lobes of the brain are much larger than in
the lower animals. So far stimulation of these frontal lobes
has failed to give indication of resulting sensations or to
produce muscular movements. They may be extensively
injured without loss of sensation and without paralysis, and
hence it has been concluded that in them the storing and
associating functions must be chiefly located.

3. Discharging Mechanism. — The position of the dis-
charging mechanism for cerebral action has been definitely
localised, by pathological and experimental observation, in
the cerebral convolutions round, or probably chiefly in front of,
the fissure of Rolando or sulcus centralis. Destructive lesions

1 84

HUMAN PHYSIOLOGY

of this area on one side cause a loss of the so-called voluntary
action of groups of muscles on the opposite side of the body.
The cerebral arc is stimulated and acts along certain lines —
possibly with the accompaniment of consciousness in a sen-
sation of decision as to the line of action to be taken and a
desire to accomplish it — but this so-called volition is not
accompanied by the appropriate muscular action. From
the frequent involvement of the so-called volition in these
actions, and from the fact that these metaphysical changes

Fio. 99. — Left Hemisphere of Brain of Chimpanzee to show the results of stimu-
lating different parts. The Sulcus Centralis is the fissure of Rolando. (Front
GRCNBAUM AND SHERRINGTON.)

figure more largely in our consciousness than the physical
changes which are their basis, we are accustomed to assume
that the movements produced are the result of volition, and
to speak of them as voluntary movements, and of the brain
mechanism producing them as voluntary centres. There is
no harm in doing so if we remember that these centres can and
do act without the involvement of consciousness, and, there-
fore, without volition ; but that their action generally implies
the previous action of parts of the receiving and associating
mechanism of the cerebrum.

But certain lesions may directly stimulate these centres,

THE NERVOUS SYSTEM 185

causing them to act without the previous action of the other
cerebral mechanisms. This is seen in Jacksonian epilepsy,
where, as the result of it may be a spicule of bone or a
thickened bit of membrane, one part of the cortex is from
time to time excited, and by its action produces movement
of certain groups of muscles.

Experimental observations have fully confirmed and
extended the conclusions arrived at from such pathological
evidence.

If parts of these convolutions be excised in the monkey,
the animal loses the power of voluntary movement of certain
groups of muscles, while if they are stimulated by electricity
these groups of muscles respond.

These Rolandic convolutions, in front of the fissure, may be
considered as a map of the various muscular combinations
throughout the body, the map being mounted so that the
lower part represents the face, the middle part the arm, and
the upper part the leg. Each large division is filled in so
that all the various combinations of muscular movement are
represented. (Figs. 98 and 99.) It must be remembered
that these centres do not send nerves to single muscles, but
act upon groups to produce definite movements, through the
lower spinal centres, and their action may involve not merely
stimulation of some muscles but inhibition of others. (See
p. 60.)

In these areas the lesion must be extensive to cause com-
plete paralysis of any group of muscles. A limited lesion
may simply cause a loss of the finer movements. Thus, a
monkey with part of the middle portion of the Rolandic areas
removed may be able to move its arm and hand about, but
may be quite unable to pick up objects from the floor of
its cage.

As in the case of the receiving, so in that of the discharg-
ing mechanism, it is the most recently acquired functions
which are most easily lost. This is well illustrated by the
results of lesions of the left inferior frontal convolution —
the area which presides over movements of the lips and
tongue. This is the centre which has to be specially educated
in the use of spoken language ; and, when this region is only
slightly injured, the power of using language may be lost

1 86 HUMAN PHYSIOLOGY

without there being any impairment in the power of moving
the muscles. This was long ago observed by Broca, and hence
this part of the brain is often called Broca's convolution.
Similarly, in the hand area, while crude lesions are required
to cause loss of power of moving the hand, very slight inter-
ferences with nutrition may cause loss of power of expressing
language in writing.

PART II
NUTRITION OF THE TISSUES

SECTION VI

FLUIDS BATHING THE TISSUES

BLOOD AND LYMPH

THE blood carries the necessary nourishment to the tissues,
and receives their waste products. But it is enclosed in a
closed system of vessels, and does not come into direct rela-
tionship with the cells. Outside the blood vessels, and bath-
ing the cells, is the lymph which plays the part of middleman
between the blood and the tissues, receiving nourishment
from the former for the latter, and passing the waste from
the latter into the former.

A. BLOOD.

The various physical, chemical, and histological characters
of blood must be practically investigated.

I. General Characters.

Colour. — Blood changes its colour from purple to cinnabar
red on shaking with air, showing that the pigment of the
blood may exist in two conditions. Elements of Blood. —
Microscopic examination shows that blood is composed of a
clear fluid (Liquor Sanguinis or Plasma) in which float
myriads of small disc-like yellowish-red cells (Erythrocytes),
and a smaller number of greyish cells (Leucocytes), and
certain more minute grey particles (Blood Platelets). The
Opacity of Blood is due to the erythrocytes. When the
pigment is dissolved out of them by water, and they are

1 88 HUMAN PHYSIOLOGY

rendered transparent, the blood as a whole becomes trans-
parent and " laked." The Specific Gravity is about 1055. It
may be estimated by finding the specific gravity of a sodium
sulphate solution in which a drop of blood neither sinks nor
floats.

Taste and Smell are characteristic, and must be experi-
enced. Reaction. — Blood is alkaline, arid the degree of
alkalinity is very constant in health. It is equivalent to
about 0*3 per cent, of Na2C03. It is increased during
digestion, and diminished after muscular exercise.

Clotting or Coagulation. — In the course of about three
minutes the blood when shed becomes a solid jelly. The
process starts from the sides of the vessel, and spreads
throughout the blood until, when clotting is complete, the
vessel may be inverted without the blood falling out. In a
short time drops of clear fluid appear upon the surface of the
clot^and in a few hours these have accumulated to a con-
siderable extent, while the clot has contracted and drawn
away from the sides of the vessel, until it finally floats in the
clear fluid — the Serum. Clotting is due to changes in the
plasma, since this fluid will coagulate in the absence of
corpuscles.

The change may be represented thus :—

Blood

i • i

Plasma Corpuscles

Serum Clot

The change consists in the formation of a series of fine
elastic threads of fibrin throughout the plasma, and if red
corpuscles are present they are entangled in the meshes of
the network and give the clot its red colour.

These threads may be readily collected in mass upon a
stick with which the blood is whipped as it is shed. The
red fluid blood which is left, consisting of blood cells and
serum, is said to be defibrinated.

THE BLOOD 189

Fibrin is a proteid substance. It is slowly dissolved in
solutions of neutral salts. It is coagulated by heat, and is
precipitated when an excess of a neutral salt is added. It
is therefore a globulin.

The plasma before clotting and the serum squeezed out
from the clot agree in containing an albumin (serum albumin)
and a globulin, or series of globulins which may be classed
together as serum globulin, in the same proportion in each.
But the plasma contains a small quantity — about 0*2 per
cent. — of another globulin (fibrinogen) which coagulates at a
low temperature, and which is absent from serum. It is this
which undergoes the change from the soluble form to the
insoluble form in coagulation. When separated from the
other proteids it can still be made to clot.

The substance which usually causes clotting appears to be
an enzyme, which is formed by the union of a nucleo-proteid
or a derivative of a nucleo-proteid with a lime salt. The
enzyme may be called thrombin,and its precursor prothrom-
bin. Oxalates, when added to blood, precipitate the soluble
lime salts, and prevent the formation of thrombin, and thus
prevent coagulation.

There is some evidence that the prothrombin exists in
solution in the plasma, but it certainly is derived from the
breaking down of the cells of the blood. It is also formed
from the breaking down of the cells of such tissues, as
lymphatic glands and thymus.

Many circumstances influence the rapidity of clotting.
Temperature has a marked effect ; a low temperature re-
tarding it, a slight rise of temperature above the normal of
the particular animal accelerating it. If a trace of a neutral
salt be added to blood, coagulation is accelerated ; but if blood
be mixed with strong solutions of salt, coagulation is pre-
vented. Salts of lime have a marked and important action,
and if they are precipitated by the addition of oxalate of
soda, blood will not clot, apparently because thrombin cannot
be formed.

The injection into the blood vessels of a living animal of
commercial peptones, which chiefly consist of proteoses, or
of an extract of the head of the medicinal leech, retards
coagulation after the blood is shed. They appear to cause

1 90 HUMAN PHYSIOLOGY

the development in the liver of some body which retards
coagulation, and if the liver be excluded from the circulation
they are incapable of acting.

Why is it that blood does not coagulate in the vessels and
does coagulate when shed ? Such a general statement is
not absolutely correct, for blood may be made to coagulate
in the vessels of a living animal in various ways. If inflam-
mation is induced in the course of a vessel, coagulation at
once occurs. If the inner coat of a vessel be torn, as by a
ligature, or if any roughness occurs on the inner wall of a
vessel, coagulation is apt to be set up. Again, various sub-
stances injected into the blood stream may cause the blood
to coagulate, and thus rapidly kill the animal. Among such
substances are extracts of various organs — thymus, testis,
and lymph glands — which yield prothrombin, although
curiously enough the injection of pure thrombin does not
usually act in this way. Nor does blood necessarily coagulate
when shed. If it is received into castor oil, or into a vessel
anointed with vaseline and filled with paraffin oil, it will
remain fluid for a considerable time.

Apparently some roughness in the wall of the blood vessel
or of the vessel in which the blood is received is required to
start the process, acting as a focus from which it can spread
outwards.

The advantages of coagulation of blood are manifest.
By means of it wounds in blood vessels are sealed and
haemorrhage stopped.

Although an important and very prominent change in
the blood, clotting is really produced by change in one
constituent of the plasma, which is present in very small
quantities.

II. Plasma and Serum.

These may be considered together, since serum is merely
plasma minus fibrinogen. As serum is so much easier to
procure, it is generally employed for analysis.

Both are straw-coloured fluids, the colour being due to a
yellow lipochrome. Sometimes they are clear and trans-
parent, but after a fatty diet they become milky. They are
alkaline in reaction, and have a specific gravity of about

THE BLOOD 191

1025. They contain about 90 per cent, of water and 10 per
cent, of solids. The chief solids are the proteids — serum
albumin and serum globulin (with, in the plasma, the addition
of fibrinogen). The proportion of the two former proteids
to one another varies considerably in different animals, but
in the same animal at different times the variations are
small. The globulin probably consists of at least two
bodies — englobulin precipitated by weak acid, and pseudo-
globulin not so precipitated. The amount of albumin is
generally greater when the body is well nourished. In man,
they together form about 7 per cent, of the serum.

The other organic constituents of the serum are in much
smaller amounts and may be divided into —

1. Substances to be used by the tissues.

Glucose is the most important of these. It occurs only in
small amounts — about 1 to 2 per mille. Part of it is free,
but part is probably combined in organic combinations such
as jecorin. It is probably in larger amount in blood going
to muscles than in blood coming from muscles, and this
difference seems to be specially well marked when the
muscles are active.

Fats occur in very varying amounts, depending upon the
amount taken in the food.

2. Substances given off by the tissues.

The chief of these is urea, which occurs constantly in
very small amounts in the serum — about -05 per cent.
We shall afterwards see that it is derived from the liver,
and that it is excreted in the urine by the kidneys.

Creatin (p. 43), uric acid (p. 397), and some allied bodies
appear to be normally present in traces, and their amount
may be increased in diseased conditions.

Of the inorganic constituents of the serum the most
abundant is chloride of sodium, but in addition sodium
carbonate, and alkaline sodium phosphate, are also present.
Calcium, potassium and magnesium occur in very small
amounts.

192

HUMAN PHYSIOLOGY

III. Cells of Blood.

1. Leucocytes — White Cells. — These are much less
numerous than the red cells, and their number varies
enormously in normal conditions. On an average there
are about 7500 per cubic millimetre.

They are soft, extensile, elastic, and sticky, and each
contains a nucleus and a well-developed double centrosome.
In size they vary considerably, some being much larger than
the red cells, some slightly smaller. The character of the

FIG. 100.— Cells of the Blood, (a) Erythrocytes ; (b) Large, and (c) Small Lym-
phocyte ; (d) Polymorpho-uuclear Leucocyte ; (e) Eosinophil Leucocyte.

nucleus varies greatly, and from this and variations in the
protoplasm, they may be divided into four classes.

1st. Lymphocytes. — Cells with a clear protoplasm and a
more or less circular nucleus. Some are very small, while
others are larger. They constitute about 20 to 25 per cent,
of the leucocytes (Fig. 100, b and c).

2nd. Polymorpho-nuclear leucocytes, with a much distorted
and lobated irregular nucleus and a finely granular proto-
plasm, whose granules stain with acid and neutral stains.
These constitute about 70 to 75 per cent, of the leucocytes.

3rd. Eosinophil or oxyphil leucocytes, with a lobated
nucleus like the last, but with large granules in the proto-
plasm which stain deeply with acid stains. From 1 to 4
per cent, of the leucocytes are of this variety.

THE BLOOD 193

4th. Basophil leucocytes, practically absent from normal
blood, with a lobated nucleus and granules in the protoplasm,
staining with basic stains.

Myelocytes are large , leucocytes with a large circular or
oval nucleus and a finely granular protoplasm. They are
not normal constituents of the blood, but appear when the
activity of the bone marrow is increased in certain patho-
logical conditions.

These various forms have certain properties — (a) Amoeboid
movement. They can, under suitable conditions, undergo
changes in shape, as may be readily seen in the blood of the
frog or other cold-blooded animal. The motion may consist
simply of the pushing out and withdrawal of one or more
processes (pseudopodia), or, after a process is extended, the
whole corpuscle may follow it and thus change its place, or
the corpuscle may simply retract itself into a spherical mass.
As a result of these movements the corpuscles, in certain con-
ditions, creep out of the blood vessels and wander into the
tissues (Diapedesis).

(6) Phagocyte action. — The finely granular leucocytes and
the lymphocytes have further the power of taking foreign
matter into their interior, and of thus digesting it. By this
devouring action, useless and effete tissues are removed and
dead micro-organisms in the body are taken up and got rid of.
In disposing of these micro-organisms the large granular cor-
puscles seem also to play an important part. This scavenger
action of the leucocytes is of vast importance in pathology.

Chemistry of Leucocytes. — The nucleus is chiefly made up
of nuclein, and in the protoplasm a nucleo-albumin, along
with two globulins and a small amount of an albumin, are
found. Along with these proteid substances glycogen and a
small amount of fat are present, while the chief inorganic
constituents are potassium salts.

2. Blood Platelets. — These are small circular or oval dis-
coid bodies, about one-third the diameter of a red blood cor-
puscle. Some observers have stated that they contain a
central nucleus. They are very sticky, and mass together
when blood is shed and adhere to a thread passed through
blood or to any rough point in the lining of the heart or

13

194 HUMAN PHYSIOLOGY

vessels. They there form clumps, and from these clumps
fibrin threads are seen to shoot out. They thus appear to
play an active part in clotting, possibly by yielding thrombin.
They are present in the blood of mammals only. Their source
is not definitely known, but it has been suggested that they
are the extruded nuclei of developing erythrocytes.

3. Erythrocytes — Red Cells. — Shape. All mammals ex-
cept the camels have circular, biconcave, discoid erythrocytes,
which, when the blood is shed, tend to run together like piles
of coins. The camels have elliptical biconvex corpuscles. A
nucleus is not present in the fully developed mammalian
erythrocyte. In birds, reptiles, amphibia and fishes, the
corpuscles are elliptical biconvex bodies, with a well-marked
central nucleus. The size of the human erythrocytes is fairly
constant — on an average 7 '6 micro-millimetres in diameter.
The number of red cells in health is about 5,000,000 in
men and 4,500,000 in women, per cubic millimetre ; but in
disease it is often decreased.

The number of corpuscles per cubic millimetre is estimated
by the Haeinocytometer. This consists of (1) a pipette by
which the blood may be diluted to a definite extent with
normal salt solution, and (2) a cell ruled in squares each con-
taining above it a definite small volume of blood so that the
number of corpuscles in that volume may be counted under
the microscope. (Experiment.)

The pale yellow colour of the individual corpuscles is due
to the pigment they contain being spread out in so thin a
layer. This pigment may be dissolved out by various agents
— e.g. salts of the bile acids, distilled water, dilute acids, or
alkalies, &c., leaving a colourless shadow-like corpuscle, which
seems to be composed of a sponge-like stroma in the inter-
stices of which the pigment is held.

Chemistry. — The stroma of the erythrocytes is made up of
a globulin-like substance, in connection with which lecithin
and cholesterin occur in considerable quantities. Potassium
is the base most abundantly present. This stroma seems to
form a capsule round the cell, and through this capsule water
can pass under the influence of osmotic pressure. It is well
known that, if two solutions of different molecular concen-

THE BLOOD 195

tration are separated by an animal membrane, water will
pass from that of lower concentration into that of higher
concentration. Hence if blood corpuscles are placed in a
series of saline solutions, it will be found that, if the solution
is of lower molecular concentration than the corpuscle, fluid
will pass in, swell the corpuscle, and perhaps burst it and set
free the pigment, while, if the fluid is of higher concentra-
tion, the fluid will pass from the corpuscle which will shrivel.
Hence the change in the corpuscle is used as a means of
determining the osmotic pressure and molecular concentra-
tion of various solutions.

The pigment is Haemoglobin. It constitutes no less than
90 per cent, of the solids of the corpuscles. In many animals
when dissolved from the corpuscles this substance tends to
crystallise very readily. The crystals prepared from the
human blood are rhombic plates. When exposed to air they
are of a bright red colour, but if placed in the receiver of an
air-pump at the ordinary temperature they become of a pur-
plish tint. The same thing occurs if the haemoglobin is in
solution, or if it is still in the corpuscles. The addition of
any reducing agent such as sulphide of ammonia or a ferrous
salt also causes a similar change. This is due to the fact that
haemoglobin has an affinity for oxygen, which it takes up from
the air, forming a definite compound of a bright red colour in
which one molecule of haemoglobin links with a molecule of
oxygen, Hb02, and is known as oxyhaemoglobin.

Haemoglobin is closely allied to the proteids, but differs
from them in containing 0*42 per cent, of iron.

When light from the sun is allowed to pass through these
solutions certain parts of the spectrum are absorbed, and
when the spectrum is examined dark bands — the absorption
spectra — are seen. In a weak solution of oxyhaemoglobin- a
dark band is seen in the green and another in the yellow part
of the spectrum between Frauenhofer's lines D and E, while
the violet end of the spectrum is absorbed (Fig. 101). These
bands may be broadened or narrowed by strengthening or
weakening the solution. When the oxygen is taken away and
the purple reduced haemoglobin is formed, a single broad band
between D and E takes the place of the two bands (Fig. 101),

The property of taking oxygen from the air and of again

196 HUMAN PHYSIOLOGY

giving it up at a moderate temperature and under a low
pressure of oxygen is the great function of the blood pigment
in the body. The haemoglobin plays the part of a middle-
man between the air and the tissues, taking oxygen from the
one and handing it on to the others, just as it may be made
to act when in solution in a test tube containing sulphide of
ammonia or a ferrous salt. (Chemical Physiology, p. 14.)

Haemoglobin constitutes about 13 or 14 per cent, of the
blood, but in various diseases its amount is decreased. The
best method of estimating its amount is by Haldane's Hsemo-
globino meter. This consists of two tubes of uniform calibre,
one tilled with a 1 per cent, solution of normal blood
saturated with CO, and another in which 20 cmm. of blood
to be examined, measured in a pipette, is placed in water,
mixed with coal gas to saturate with CO, and then diluted
till it has the same tint as the standard tube. The per-
centage of haemoglobin in terms of the normal is indicated by
the mark on the tube at which the fluid stands. (Chemical
Physiology, p. 15.)

Methaemoglobin. — Haemoglobin forms another compound
with oxygen — methaemoglobin. The amount of oxygen is
the same, but methaemoglobin must be acted on by the
strongest reducing agents before it will part with its oxygen.
When therefore this pigment is formed in the body, the
tissues die for want of oxygen. It may be produced by the
action of various substances on oxyhsemoglobin. Among
these are ferricyanides, nitrites, and permanganates. It
crystallises in the same form as oxyhaemoglobin, but has a
chocolate brown colour. Its spectrum is also different from
haemoglobin or oxyhaemoglobin, showing a narrow sharp band
in the red part of the spectrum, with two or more bands in
other parts according to the reaction of the solution in which
it is dissolved (Fig. 101). It is of importance since it occurs in
the urine in some pathological conditions. In all probability

the molecule of oxyhaemoglobin has the formula — Hb/

X)

while in methaemoglobin the atoms are arranged

THE BLOOD

197

Haemoglobin also combines with certain other gases.
Among these is Carbon monoxide. For this gas haemoglobin
has a greater affinity than for oxygen, so that when carbon
monoxide haemoglobin is once formed in the body, the
blood has little power of taking up oxygen, and the animal
dies. This gas is evolved freely in the fumes from burn-
ing charcoal, and the production of the compound in the
blood is the cause of death from inhaling such fumes in

RED

YELLOW. GREEN.

BLUE.

Carbon-monoxide ^ f
Hemoglobin . > f

I

j

,

- •
• ' i

Haemoglobin . |

t

Methfemoglobin . 1 :
Acid Hjematin . / j

1

,?.±j

.

Reduced Alkaline \
Hajmatin . . / i

i IL

FIG. 101. — Spectra of the more important Blood Pigments and their more important
derivatives. (The Spectrum of Acid Haematin is not identical with that of
Methsemoglobin).

closed spaces. It is also found in the air of coal mines
after explosions. The carbon monoxide haemoglobin forms
crystals like oxy haemoglobin, and has a bright pinkish
red colour, without the yellow tinge of oxy haemoglobin.
Since after death it does not give up its carbon monoxide
and become changed to purple haemoglobin, the bodies of
those poisoned with the gas maintain the florid colour of
life. Its spectrum is very like that of oxyhaemoglobin, the
bands being slightly more to the blue end of the spectrum
(Fig. 101).

198 HUMAN PHYSIOLOGY

It may be at once distinguished by the fact that when
gently warmed with sulphide of ammonia it does not yield
reduced haemoglobin. (Chemical Physiology, p. 14.)

Decomposition of Haemoglobin. — Haemoglobin is a some-
what unstable body, and, in the presence of acids and alkalies,
splits up into about 96 per cent, of a colourless proteid sub-
stance belonging to the globulin group, and about 4 per cent,
of a substance of a brownish colour called haematin. The
spectrum and properties of this substance are different in
acid and alkaline media. In acid media it has a spectrum
closely resembling methaemoglobin, but it can at once be
distinguished by the fact that it is not changed by reducing
agents. In medicine it is sometimes important to distinguish
between these pigments. Haematin in alkaline solution
can take up and give off oxygen in the same way as
haemoglobin does. Reduced alkaline haematin or hsemo-
chromogen has a very definite spectrum (Fig. 101), and its
preparation affords a ready means of detecting old blood
stains. Haematin contains the iron of the haemoglobin, and
it is this pigmented iron-containing part of the molecule
which has the affinity for oxygen. Apparently it is the pre-
sence of iron which gives it this property, because, if the iron
be removed by means of sulphuric acid, a purple-coloured
substance, iron-free hcematin, haematoporphyrin, is formed,
which has no affinity for oxygen. This pigment occurs in
the urine in some pathological conditions. (Chemical Physi-
ology, p. 15.)

In the liver haemoglobin is broken down to form bilirubin
and the other bile pigments. These are iron-free, and, like
iron-free haematin, do not take up and give off' oxygen. But
not only is this iron-free pigment formed from haemoglobin
in the liver, but the cells of any part of the body have the
faculty of changing haemoglobin in blood extravasations into
a pigment known as haematoidin, which is really the same as
bilirubin.

Haemin — the hydrochloride of haematin — is formed when
blood is heated with chloride of sodium and glacial acetic
acid. It crystallises in small steel black rhombic crystals,
and its formation is sometimes used as a test of blood stains.
(Chemical Physiology, p. 16.)

THE BLOOD

199

The following table shows the relationship of these pig-
ments to one another : —

RELATIONSHIP OF HB AND ITS DERIVATIVES.
Methsemoglobin Hb02 HbCO

Hb

Haematin

Globin

Acid Hsematin

Alkaline Hsematin

Oxidised Reduced

(Hsemochromogen)

Contain Iron.

Iron-free Haematin
(Hsematoporphyrin)

Haamatoidin
Bilirubin

j- Iron-free.

IY. Gases of the Blood.

The muscles and other active tissues are constantly con-
suming oxygen and constantly giving off carbon dioxide.
The oxygen must be brought to the tissues by the blood,
and the carbon dioxide carried away by the same medium.

Various methods of carrying out the examination of the
gases of the blood have been devised, and many different gas
pumps have been invented in which the gases may be col-
lected in the Torricellian vacuum over mercury. Haldane and
Barcroft have devised a convenient method, which depends
upon the fact that the oxygen can be driven off from blood
treated with dilute ammonia by the addition of ferricyanide
of potassium, and that the carbon dioxide is liberated by
adding an acid, and the amount of gas estimated by measur-
ing the increased pressure in the tube in which the gas has
been given off.

About 60 c.c. of gas measured at 0° C. and 760 mm. pres-
sure can be extracted from 100 c.c. of blood. The proportion
of the gases varies in arterial and in venous blood.

200 HUMAN PHYSIOLOGY

AMOUNT OF GASES PER HUNDRED VOLUMES OF BLOOD.

Arterial Blood. Venous Blood.

Oxygen 20 12

Carbon dioxide . 40 46

There are two ways in which gases may be held in such a
fluid as the blood—

1st. In simple solution.

2nd. In chemical combination.

Oxygen. — At the temperature of the body the blood can
hold in solution less than 1 per cent, of oxygen. Now the
amount of oxygen actually present is about 20 per cent. So
that by far the greater quantity of the gas is not in solution.
We have already seen that it is in loose chemical union with
hsemoglobin.

Carbon dioxide. — In the animal body the blood can dis-
solve about 2J per cent, of carbon dioxide. But it may con-
tain as much as 46 per cent. Hence the greater part of the
gas must be in chemical combination. Since the proportion
of carbon dioxide is greater in the plasma than in entire
blood, this gas must be held in the plasma. Analysis of the
ash of the plasma shows that the sodium is more than
sufficient to combine with the chlorine and phosphoric acid,
and is thus available to take up carbon dioxide. Carbonate
of soda and basic phosphate of soda are therefore present
together in the plasma, and if a stream of carbon dioxide be
passed through a solution of basic phosphate of soda, it
appropriates a certain amount of soda and leaves the neutral
phosphate. On the other hand, if the amount of carbon
dioxide is small, the phosphoric acid again seizes on the soda,
and turns out the carbon dioxide. Thus the soda is the
carrier of carbon dioxide in the blood plasma. The blood,
in passing through the tissues which are loaded with this
gas, gains carbon dioxide, and some of the basic phosphate
of s*oda is changed to the less alkaline neutral phosphate.
When the lungs are reached the blood is exposed to air poor
in carbon dioxide, and the phosphoric acid is able to turn
out the carbon dioxide. In fact, the carriage of carbon

THE BLOOD 201

dioxide and its excretion are the result of a struggle between
that gas and phosphoric acid for the soda of the plasma.

Nitrogen. — The amount of nitrogen in the blood is not in
excess of what can be held in solution ; and we may there-
fore infer that it is simply dissolved in the blood plasma.

Y. Source of the Blood Constituents.

A. Of the Plasma. — The water of the blood is derived
almost entirely from the water ingested.

The source of the proteids has not been fully investigated.
Undoubtedly they are partly derived, somewhat indirectly as
we shall afterwards see, from the proteids of the food. Very
probably, too, they are in part derived from the tissues. But
the significance of the two proteids, albumin and globulin,
and their variations has not yet been elucidated.

The glucose is derived from the carbohydrates and possibly
from the proteids of the food, and during starvation it is
constantly produced in the liver and poured into the blood.

The fats are derived from the fats and carbohydrates and
possibly from the proteids of the food.

The urea and other waste constituents are derived from
the various tissues.

B. Of the Cells — I. Leucocytes. — These are formed in the
lymph tissue and in the red marrow of bone.

1. Lymph Tissue (see p. 30) is very widely distributed in
the body, occurring either in patches of varying shape and size,
or as regular organs, the lymphatic glands (Fig. 102). These
are placed on the course of a lymphatic vessel, and consist of
a sponge-work of fibrous tissue, in the interstices of which are
set the patches of lymph tissue or germ centres, each sur-
rounded by a more open network, the sinus, through which
the lymph flows, carrying away the lymphocytes from the
germ centres. Round some of the lymphatic glands of
certain animals large blood spaces or sinuses are seen, and
these glands are called hsemolymph glands. They are inter-
mediate between lymphatic glands and the spleen. While
these glands produce lymphocytes, they also play an im-
portant part in disposing of disintegrating erythrocytes and
in storing iron. *

202 HUMAN PHYSIOLOGY

2. Bone Marrow. — The structure of bone marrow is con-
sidered below, but it may be stated here that young leuco-
cytes or leucoblasts, in the condition of mitosis, are abundant,
and that they seem to pass away in the blood stream. They
are of all varieties. In certain pathological conditions the
formation of these cells is increased and a leucocytosis
results (Fig. 103).

II. Erythrocytes.— These are formed after birth in the
red marrow of bone. Marrow consists of a fine fibrous
tissue with large blood capillaries or sinuses running in it.
In the fibrous tissues are numerous fat cells, and generally

FIG. 102. — Section of a Lymph Gland, a., capsule ; 6., germ centres of cortex ;
c., sinuses ; d. , trabecula ; e. , germ centres of medulla.

a considerable number of multi-nucleated giant cells. In
addition to these are the young leucocytes, leucoblasts, which
used to be called the proper cells of the bone marrow, and
lastly young nucleated red cells, the erythroblasts. After
haemorrhage, the formation of these becomes unusually
active, and may implicate parts of the marrow not generally
concerned in the process, and hence the red marrow may
spread from the ends of the long bones, where it is usually
situated, towards the middle of the shaft. The nuclei of the
erythroblasts atrophy or are shed and the cells escape into
the blood stream. The red marrow has the power of re-
taining the iron of disintegrate'd erythrocytes, which are

THE BLOOD 203

often found enclosed in large phagocytes. The iron is often
very abundant after a marked destruction of erythrocytes.

YI. Total Amount of Blood in the Body.

This was formerly determined by bleeding an animal,
measuring the amount of blood shed, and determining the
amount of haemoglobin contained in it; then washing out
the blood vessels, and after measuring the amount of fluid

FlG. 103. — Section of Red Marrow of Bone. a. , lymphocyte ; b. , fat cell ;
c. , erythroblast ; d., giant cell; e. , erythrocyte ; /.,' erythroblast in
mitosis; g. , neutrophil myelocyte ; A., eosinophil myelocyte ; k.,
eosinophil leucocyte ; I. , polymorpho-nuclear leucocyte.

used, determining the amount of haemoglobin in it to ascer-
tain the amount of blood it represents. By this method
the amount of blood was found to be about TV of the body
weight.

Haldane and Lorrain Smith have devised a method which
can be applied to the living animal. It depends upon the
fact that after a person has inhaled carbon monoxide it is
possible to determine to what proportion the gas has replaced
oxygen in the oxy haemoglobin. If then an individual breathes
a given volume of carbon monoxide, and if a measured speci-
men of blood contains a definite percentage of the gas, the

204 HUMAN PHYSIOLOGY

rest of the gas must be equally distributed through the
blood, and thus the amount of blood may be deduced.

By this method they conclude that the volume of the
blood is about ?\ih of the weight of the body.

VII. Distribution of the Blood.

Roughly speaking, the blood is distributed somewhat as
follows : —

Heart, lungs, large vessels J

Muscles ....... J

Liver

Other organs J

YIII. Fate of the Blood Constituents.

The water of the blood, constantly renewed from outside,
is constantly got rid of by the kidneys, skin, lungs, and
bowels.

About the fate of the proteids we know nothing. They
must be used up in the construction of the tissues, but
experimental evidence of this is wanting.

The glucose and fat are undoubtedly used up in the
tissues.

The urea and waste products are excreted by the kidneys.

The fate of the salts is not fully worked out. The chlorides
are partly excreted by the kidneys and are partly split up
to form the hydrochloric acid required for stomach diges-
tion. The phosphates and sulphates are excreted in the
urine, but whether they are also used in the tissues is not
known.

The leucocytes break down in the body — but when and
how we do not know. We shall afterwards find that they
are greatly increased in number after a meal of proteids, and,
since the increase is transitory, lasting only for a few hours,
they are probably rapidly broken down, possibly to feed the
tissues. It would thus seem that a leucocyte lives in the
blood only for a short time.

The erythrocytes also break down. How long they live
is not known. It is found, after injecting blood, that the

THE BLOOD 205

original number of corpuscles is not reached for about a fort-
night, and hence it has been concluded that the corpuscles
live for that period. The experiment, however, is far from
conclusive, and must be accepted with reservation.

Organs Connected with Haemolysis. — The process of
breaking down old erythrocytes and eliminating their pig-
ment is often called the process of haemolysis. Certain
organs appear to be specially connected with this, but the
precise part played by each of them is not very clearly
understood.

That the liver acts in this way is indicated, first, by the

FIG. 104.— To show the relationship of the Spleen to Lymph Glands and
Hsemolymph Glands. The black indicates Lymphoid Tissue ; the
coarsely spotted part Lymph Sinuses, and the finely dotted part
Blood Sinuses. (LEWIS.)

fact that the blood passing from the organ during digestion
contains fewer erythrocytes than the blood going to it;
second, by the formation of bile pigment, a derivative of
haemoglobin, in the liver cells; third, by the presence of
pigment and of iron in simple combinations in the liver cells
under certain conditions. It is possible that the reabsorbed
salts of the bile acids in the portal blood may dissolve the
pigment out of the old erythrocytes, and that the liver cells
may then act upon the liberated pigment. Under ordinary
conditions the liver does not store much iron.

The spleen is generally said to have a similar action. This
organ is composed of a fibrous capsule containing non-striped

206 HUMAN PHYSIOLOGY

muscle and a sponge-work of fibrous and muscular trabeculae,
in the interstices of which is the spleen pulp. The branches
of the splenic artery run in the trabeculae, and twigs, leaving
these trabeculse, pass out and are covered with masses of
lymph tissue forming the Malpighian corpuscles. Beyond
these, the vessels open into a series of complex sinuses from
which the blood is collected into channels, the venous
sinuses, which carry it back to branches of the splenic vein
in the trabeculse. The pulp is thus made up of processes
of fibrous tissue and of a set of branching endothelial-like
cells, the spaces between which are filled with blood. It is
comparable with the blood sinuses of the hsernolymph glands.

So far no decrease in the number of erythrocytes in the
blood leaving the organ has been recorded. In the cells of
the spleen pulp, yellow pigment and simple iron compounds
are frequently seen, indicating that haemoglobin is being
broken down. But the idea that the spleen plays an im-
portant part in the actual destruction of erythrocytes seems
to be negatived by the fact that, when blood is injected, the
cells are broken down no faster in an animal with the spleen
than in an animal from which the spleen has been removed.
While the spleen appears to have no action in killing and
destroying erythrocytes, its cells have the power of taking
up dead and disintegrating erythrocytes and storing the iron
for future use in the body.

The non-striped muscle in the framework of the spleen
undergoes rhythmic contraction and relaxation, and the
organ thus contracts and expands at regular intervals of
about a minute.

Lymph Glands, Hsemolymph Glands, and Red Marrow of
Bone. — When erythrocytes break down or haemoglobin is
injected, the pigment accumulates in these, and they there-
fore probably act as the graves of old erythrocytes in the
same way as the spleen.

B. LYMPH.

1. Characters of Lymph.— The lymph is the fluid which
plays the part of middleman between the blood and the
tissues. It fills all the spaces in the tissues and bathes the

THE BLOOD 207

individual cell elements. These spaces in the tissues open
into vessels — the lymph vessels — in which the lymph flows
and is conducted through lymph glands and back to the
blood through the thoracic duct. (See Fig. 105, p. 209.)

Lymph varies in character according to the situation from
which it is taken and according to the condition of the
animal.

Lymph taken from the lymph spaces — e.g. the pericar-
dium, pleura or peritoneum — is a clear, straw-coloured fluid.
It has little or no tendency to coagulate. Microscopic
examination shows that it contains few or no cells — any
cells which may exist being lymphocytes. In reaction it
is alkaline. The specific gravity varies according to its
source.

Apparently the cause of the non-coagulation of such lymph
is the absence of cells from which thrombin may be set free.
If blood or white corpuscles be added to it, a loose coagulum
forms.

If the lymph be taken from lymphatic vessels after these
have passed through lymphatic glands, it is found to contain
a number of lymphocytes, and it coagulates readily.

Chemically, lymph resembles blood plasma in which the
proteids are in smaller amount, but the inorganic salts in
the same proportion as in the blood. The amount of solids
varies in lymph from different organs.

Lymph of Proteids.

Limbs 2-3 per cent.

Intestines .... 4-6 „
Liver 6-8 „

In the lymphatics coming from the alimentary canal during
starvation, the lymph has the characters above described.
But after a meal it has a milky appearance and is called
chyle. This milky appearance is due to the presence of fats
in a very fine state of division, forming what is called the
molecular basis of the chyle.

Lymph in various diseases tends to accumulate as serous
effusions in the large lymph spaces — e.g. the pleura, peri-
toneum, pericardium — and these effusions behave differently

208

HUMAN PHYSIOLOGY

as regards coagulation. The following table helps to explain
this (S. A. is Serum Albumin, S. G. Serum Globulin) :—

COAGULABILITY OF LYMPH, SERUM, AND EFFUSIONS.

Plasma and
Lymph.

Serous Effusion.

Serum.

Coag.

Coag. with
Thrombi ii.

Uncoag.

Uncoag.

S. A.
S. G.
Fibrinogen.
Fib. Zym.

S. A.
S. G.
Fibrinogen.
Fib. Zym.

S. A.
S. G.
Fibrinogen.

S. A.
S. G.

S. A.
S. G.

2. Formation of Lymph. — Lymph is derived partly from
the blood and partly from the tissues. The formation of
lymph from the blood depends upon the permeability of the
walls of the capillaries and the pressure of blood in the blood
vessels. Thus, although the pressure in the blood vessels of
the limbs is much higher than the pressure in the vessels of
the liver, hardly any lymph is usually produced in the former,
while very large quantities are produced in the latter —
apparently because of the small permeability of the limb
capillaries and the great permeability of the hepatic capil-
laries. The permeability may be increased by anything
which injures the capillary wall. Thus the injection of hot
water or of proteoses at once leads to an increased flow of
lymph.

But, while the permeability of the vessel wall is the most
important factor controlling lymph formation, any increase
of the intra-vascular pressure of a region increases the flow
of lymph, and for this reason any obstruction to the free flow
of blood from a part leads to increased lymph production.

That lymph is also formed from the tissues is indicated
by the fact that the injection of substances of high osmotic
equivalent into the blood — such as sugar or sulphate of soda
— leads to a flow of fluid into the blood, so that it becomes
diluted, and also to an increased flow of lymph, and this in-
crease of water in both can be explained only by its withdrawal
from the tissues.

SECTION VII

THE CIRCULATION

I. General Arrangement

THE arrangement by which the blood and lymph are dis-
tributed to the tissues may be compared to a great irrigation
system.

It consists of a central force pump — the systemic heart
(Fig. 105, S.H.) — from which pass a series of conducting tubes

LUNG

CAP

FIG. 105.— Scheme of the Circulation. S.H., Systemic Heart sending Blood to the
Capillaries in the Tissues, Cap. The Blood brought back by Veins and
the exuded Lymph by Lymphatics, Ly.t passing through glands ; Blood sent
to the Alimentary Canal, Al.C., and from that to the Liver, Liv. ; Blood also
sent to the Kidneys, Kid. ; the Blood before again being sent to the body is
passed through the Lungs by the Pulmonic Heart, P.H.

—the arteries — leading off to every part of the body, and
ending in innumerable fine irrigation channels — the capil-
laries (Cap) — in the substance of the tissues. From these, a
considerable proportion of the blood constituents is passed
into the spaces between the cells as lymph. From these
spaces the fluid is either passed back into the capillaries, or
is conducted away in a series of lymph vessels, which carry
it through lymph glands (Ly.\ from which it gains certain

-9

210 HUMAN PHYSIOLOGY

necessary constituents, and finally bring it back to the central
pump.

The fluid, which has not passed out of the capillaries into
the tissues, has been deprived of many of its constituents,
and this withdrawal of nutrient material by the tissues is
made good by a certain quantity of the blood being sent
through the walls of the stomach and intestine (Al.C.), in
which the nutrient material of the food is taken up and added
to the blood returning to the heart. At the same time, the
waste materials added to the blood by the tissues are partly
got rid of by a certain quantity of the blood being sent
through the liver and kidneys (Liv. and Kid.).

The blood is then poured back, not at once into the great
pump which sends it through the body, but into a subsidiary
pump — the pulmonic heart (P.H.) — by which it is pumped
through the lungs, there to obtain a fresh supply of oxygen,
and to get rid of the carbon dioxide excreted into it by the
tissues. Finally the blood, with its fresh supply of oxygen
from the lungs, and of nourishing substances from the ali-
mentary canal, is poured into the great systemic pump — the
left side of the heart — again to be distributed to the tissues.

Thus the circulation is arranged so that the blood, ex-
hausted of its nourishing material by the tissues, is replenished
in the body before being again supplied to the tissues.

The sectional area of this irrigation system varies enor-
mously. The aorta leaving the heart has a comparatively
small channel. If all the arteries of the size of the radial
were cut across and put together, their sectional area would
be many times the sectional area of the aorta. And, if all
the capillary vessels were cut across and placed together, the
sectional area. would be about 700 times that of the aorta.

From the capillaries, the sectional area of the veins and
lymphatics steadily diminishes as the smaller branches join
with one another to form the larger veins and lymphatics :
but, even at the entrance to the heart, the sectional area of
the returning tubes, the veins, is about twice as great as that
of the aorta (Fig. 128, p. 271).

The circulatory system may thus be compared to a stream
which flows from a narrow deep channel, the aorta, into a
gradually broadening bed, the greatest breadth of the channel

THE CIRCULATION 211

being reached in the capillaries. From this point the channel
gradually narrows until the heart is reached.

Hence the blood stream is very rapid in the arteries where
the channel is narrow, and very sluggish in the capillaries
where the channel is wide, so that in them plenty of time is
allowed for exchanges between the blood and the tissues.

II. The Central Pump— The Heart.

A. Structure. — A very simple form of heart exists in the
ascidians. At one point on a large vessel there is a thicken-
ing in the wall composed of non-striped muscular fibres. A
contraction is seen to pass from one end of this to the other
at frequent regular intervals, thus forcing the fluid through
the vessels. The embryonic heart in man has a similar
structure.

In the snail and cuttle-fish, in addition to the contracting
muscular thickening, there is also a thin- walled receiving
chamber into which the blood flows before it is expelled
onwards. The heart is thus composed of two chambers.

1st. A receiving chamber — the auricle.

2nd. An expelling chamber — the ventricle.

In fish the heart has a similar structure. But in lung-
bearing animals a more complex arrangement is required,
and a double heart is found, one concerned with the pro-
pulsion of blood to the system generally, and hence called
the systemic heart ; one propelling blood to the lungs, and
hence called the pulmonic heart. In mammals, the former
chamber is on the left side, the latter on the right. Each
consists of a receiving and expelling chamber — an auricle
and a ventricle.

The walls of these chambers are essentially muscular ; but
this muscular layer, or myocardium, lies between two fibrous
layers, the pericardium and the endocardium.

The musculature of the auricles is separate from that of
the ventricles, but some fibres more like ordinary visceral
fibres than cardiac muscles extend from one to the other.
If the heart is boiled, the auricles, the aorta and the pul-
monary artery may be separated from the ventricles. This
is because boiling converts fibrous tissue to gelatine and

212 HUMAN PHYSIOLOGY

dissolves it, and it is by white fibrous tissue that the auricles
and great arteries are attached to the ventricles. This tissue
is arranged in three rings, one encircling the opening between
the right auricle and ventricle and crescentic in shape ; one,
more circular in shape, encircling in common the left auri-
culo-ventricular arid the aortic orifice, and one encircling
the pulmonary opening. The auricles are attached to the
auriculo-ventricular rings above, the ventricles are attached
below, while the valves of the heart are also connected with
them. I v

The muscular fibres of the auricles are arranged in two
badly defined layers — .

1st. An outer layer runnings, horizontally round both
auricles.

2nd. An inner layer arching over each auricle, and con-
nected with the auriculo-ventricular rings.

Contraction of the first layer diminishes the capacity of
the auricles from side to side. Contraction of the second
pulls the auricles downwards towards the ventricles, and
thus diminishes their eapacity from above downwards.

The peculiar striped muscle fibres of the auricular walL
extend for some distance, along the great veins which open
into these chambers.

The left ventricle forms the cylindrical core to the heart,
and the right ventricle is attached along one side of it. The
septum between the ventricles is essentially the right wall of
the left ventricle, and it bulges into the right ventricle with
a double convexity from above downwards and from before
backwards (Fig. 106).

The muscle fibres of the ventricle are arranged essentially
in three layers —

1st. The outmost layer takes origin from the auriculo-
ventricular rings, and passes downwards and to the left till
it reaches the apex of the heart. Here it turns inwards,
forming a sort of vortex, and becomes continuous with the
inmost layer.

2nd. The middle layer is composed of fibres running hori-
zontally round each ventricle. It is the thickest layer of
the heart, and in contracting it pulls the walls of the ven-
tricles towards the septum ventriculi.

THE CIRCULATION

213

3rd. The inmost layer is continuous with the outmost layer,
as it turns in at the apex. It may be considered as composed
of two parts —

(a) A layer of fibres running longitudinally along the in-
side of each ventricle from the apex upwards to the auriculo-
ventricular ring. These fibres are raised into fleshy columns,
the columnse carneae.

(6) A set of fibres, constituting the papillary muscles
(Fig. 107, P.M.), which, taking origin from the apical part of
the ventricles, extend freely upwards to terminate in a series

\FlG. ],OG. — Cross Section thrjaugfr the Ventricles of thfi__He.act looking towards

Auricles, to show the right Ventricle placed on the Central Core of the left
Ventricle. The cusps of the Auriculo-ventricular Valves are also shown.

of tendinous cords (the chordae tendineae), which are inserted
partly into the auriculo-ventricular valves, presently to be
described, and partly into the auriculo-ventricular rings.
The papillary muscles are merely specially modified columme
carnese. In many cases, actual muscular processes extend
from the apex of the papillary muscles to the auriculo-
ventricular ring.

In the left ventricle there are two papillary muscles, or
groups of papillary muscles, one in connection with the
anterior wall of the ventricle, called the anterior muscle;
and one in connection with the posterior wall, called the
posterior muscle.

In the right ventricle there are — 1st. One or more small

2i4 HUMAN PHYSIOLOGY

papillary muscles just under the pulmonary orifice, and
having a horizontal direction, their apices pointing back-
wards— the superior papillary muscle (Fig. 107, S.P.M.).

2nd. A large papillary muscle taking origin from the mass
of fleshy columns at the apex of the ventricle. It is directed
upwards. It may be called the anterior papillary muscle
(AJP.M.).

3rd. One or more papillary muscles of varying size arising
from the posterior part of the apical portion of the ventricle
and constituting the posterior papillary muscle (P. P.M.).

4th. A number of small septal papillary muscles arising
from the septum.

The distribution of the chorda? from these muscles will be
considered in connection with the auriculo-ventricular valves.

In contraction, the outmost and inmost layers of the
ventricles tend to approximate the apex to the base of the
ventricles, but this is resisted by the contracting middle layer.
The apex tends to be tilted towards the right, the papillary
muscles shorten, the colummu cameo; by their shortening and
thickening encroach upon the ventricular cavity, and help
to abolish it, while the auriculo-ventricular rings are drawn
downwards and inwards towards the septum.

The endocardium forms a continuous fibrous foyer, lined
by endothelium, extending from the vessels over the inner
aspect of auricles and ventricles. At certain points flaps
of this endocardium are developed to form the valves of
the heart.

In the heart, valves are situated at the entrance to and at
the exit from the expelling cavities. There is thus on each
' side of the heart a valve between the auricles and ventricles,
\ and a valve between the ventricles and the great arteries.

Auriculo-ventricular Valves, — On each side of the heart
the auriculo-ventricular valve is formed by flaps of endocar-
dium, which hang downwards from the auriculo-ventricular
ring like a funnel into the ventricular cavity, and are attached
to the apices of the papillary muscles by the chordae tendinese
(Figs. 107 and 108).

On the left side, of the heart there are two main cusps,
forming the mitral valves (Fig. 108)—

1st. An anterior or right cusp, which takes origin from,

THE CIRCULATION 215

and is continuous with, the right posterior wall of the aorta,
and hangs down into the ventricle between the aortic and
auriculo-ventricular orifices, thus dividing the ventricle into
two parts, an aortic and an auricular part. This cusp is
very strong, and in many animals bone is developed in it
towards its base. It is composed of dense fibrous tissue, is
smooth on both sides, and the chordae are inserted chiefly
along its edges.

2nd. The posterior or left cusp takes origin from the
back part of the auriculo-ventricular ring, and hangs in the
ventricle in its relaxed state against the posterior and left
wall. It is smaller and less strongly made than the anterior
cusp. The chordae tendineae are not only inserted into its
edge, but run up along its posterior aspect to be inserted
into the auriculo-ventricular ring, and they thus give the
posterior aspect of the cusp a rough ridged appearance.

The chordae from the anterior papillary muscle are inserted
into the anterior or left edge of both cusps, those from the
posterior papillary muscle into the posterior or right edges of
both cusps.- When the papillary muscles contract, the cusps
are thus drawn together. The edge of each cusp thins out
to form a delicate border, which, when the cusps are approxi-
mated, completely seals the aperture.

On the right side of the heart the auriculo-ventricular
orifice is separate from the pulmonary opening, and the three
cusps of the tricuspid valve are developed in connection
with the crescentic opening from the auricle (Fig. 106). One
rises from the ring above the septum, and hangs down into
the ventricle upon the surface of the septum. This cusp is
small, thin, and delicate. It is attached by its lower border
to the septal papillary muscles. The chief or infundibular
cusp (Fig. 107, I.C.} rises from the front part of the ring
between the pulmonary infundibulum and the auriculo-
ventricular opening. It is connected by its anterior border
with the horizontal fibres from the superior papillary muscles,
and by its lower and inferior border by the chordae from the
anterior papillary muscle. When these two sets of papillary
muscles contract, this cusp is drawn flat against the bulging
septum.

The posterior cusp (P. C.) takes origin from the posterior

2l6

HUMAN PHYSIOLOGY

and outer part of the ring, and hangs down into the posterior
part of the ventricle. It is connected by its anterior margin
with the anterior papillary muscle and by its posterior margin
with the posterior papillary muscle. Contraction of these
muscles will therefore approximate its anterior edge to the
infundibular cusp, its posterior edge to the septal cusp.

In both the infundibular and posterior cusps many of the
chordae pass up to be inserted into the auriculo-ventricular
ring.

Semilunar Valves. — The valves, situated at the opening

PA

PPM

FlO. 107. — The Right Ventricle and Tricuspid Valve to show relationship of the
Papillary Muscles and Chorda Tendineae to the Cusps of the Valve. (See text.)

of the ventricles into the great arteries, are also formed as
special developments of the endocardium.

Each is composed of three half-moon-shaped membranous
pouches attached along their curved margin to the walls of
the artery and upper part of the ventricle, and with their
concavities directed away from the ventricle. In the centre
of the free margin is a fibrous thickened nodule, the corpus
Arantii, from which a very thin piece of membrane, the
lunule, extends to the attached margin of the edges. A
pouch, the sinus of Valsalva, lies behind each cusp.

The arrangements of these various cusps is of importance
in connection with their action (Fig. 108).

THE CIRCULATION 217

Aortic Valve. — The anterior cusp is largest, and lies some-
what deeper in the heart than the others. At each side it
is attached to the aortic wall, but below it is attached to the
upper part of the septum ventriculi, so that the base of the
sinus of Valsalva is formed by the upper part of the
septum. At a somewhat higher level is a cusp which
is partly attached to the upper part of the septum, partly

FIG. 108. — Vertical Mesial Section through Heart to show Aortic and Mitral Valves.
R.V., Right Ventricle; L.V., Left < Ventricle with Papillary Muscle; L.A.,
Left Auricle ; Ao. , Aorta with Anterior Cusp.

to the posterior wall of the aorta, where this becomes con-
tinuous with the anterior cusp of the mitral. The third
cusp is still higher, and is attached to the aortic wall
where it becomes continuous with the anterior cusp of
the mitral.

Pulmonary Yalve. — The posterior cusp is mounted on the
top of the septum ventriculi, and is at a somewhat lower
level than the other two.

Thus in each valve the cusp placed lowest is mounted on

218

HUMAN PHYSIOLOGY

a muscular cushion, the use of which will afterwards be
considered.

Attachments and Relations of the Heart (Fig. 109). —
The heart is attached, by the great vessels coming from it,
to the posterior wall of the chest at the level of the 5th
to the 8th dorsal vertebrae. Its plane of attachment faces
forwards and downwards. From this
the heart projects into the chest as a
conical mass downwards, forwards, and
to the left. It does not lie at right
angles to its plane of attachment, but,
when not contracting, it is limp and
hangs down, as shown in the diagram
(continuous line). Nor can it assume
a position at right angles to its plane
of attachment (dotted line), because the
front part of the heart lies against the
anterior chest wall over an area (the
prsecordium) bounded to the right by
the midsternal line, above by the fourth

FIG. 109. — Mesial Section i rv *u i i i_ *.i ^i i A. -i

through the Thorax to left rib> below by the seventh left rib,
show the attachment and to the left by a vertical line inside
the nipple line.

From the oblique position of the
heart, it is the right side — auricle and ventricle — which is
directed forwards, and it is a portion of the right ventricle
which lies in relationship with the chest wall. The rest of
the organ is covered by the lungs. The only parts of the
left side which can be seen from the front are the tip of
the left auricular appendix, and a narrow strip of the left
ventricle.

Below and behind the heart lies upon the central tendon
of the diaphragm to which the pericardium is attached.

All round it are the lungs completely filling up the rest of
the thorax.

The heart is enclosed in a strong fibrous bag, the Peri-
cardium, which supports it and prevents over-distension.
When fluid accumulates in this bag the auricles are pressed
upon and the flow of blood into them is impeded.

and relations of the
Heart.

THE CIRCULATION

219

s/nus

B. Physiology of the Heart.

Cardiac Cycle. — If any one part of the heart of a frog is
watched it is seen to undergo contractions and relaxations at
regular rhythmical intervals.

1. General Description. — In the frog a contraction, starting
from the openings of the veins, suddenly involves the sinus
venosus, causing it to become smaller and paler. This con-
traction is rapid and of short duration, and is followed by a
relaxation, the cavity again regaining its former size and
colour. As this relaxation
begins, the two auricles
are suddenly contracted
and pulled downwards
towards the ventricle at
the same time becoming
paler, while the ven-
tricle becomes more dis-
tended and of a deeper
red. The rapid brief auri-
cular contraction now gives
place to relaxation, and,
just as this begins, the
ventricle is seen to be-
come smaller and paler,
and if held in the finger
is felt to become firmer. This event takes place more slowly
than the contraction of either sinus or auricles. The chief
change in the ventricle is a diminution in its lateral diameter,
though it is also decreased in the antero- posterior and
vertical directions. During ventricular contraction the
bulbus is seen to be distended and to become of a darker
colour. The ventricular contraction passes off suddenly, the
ventricle again becoming larger and of a deep red colour.
At this moment the bulbus aortse contracts and becomes pale
and then relaxes before the next ventricular contraction.

Each chamber of the heart thus passes through two
phases — a contraction phase, a systole of short duration,
and a longer relaxation phase, the diastole. And the

FIG. 110.— Scheme of the Cardiac Cycle in
the Frog. S.S., Sinus Systole; A.S.,
Auricular Systole; V.S., Ventricular
Systole; B.S., Bulbus Systole; P., Rest
of all chambers.

220 HUMAN PHYSIOLOGY

sequence of events in the frog's heart might be schematically
represented us in Fig. 110.

The events from the beginning of the contraction of any
cavity to the beginning of the next contraction of that cavity
constitute a cardiac cycle. Usually the cardiac cycle is
reckoned from the beginning of the systole of the sinus
(Practical Physiology, Chap. IX.).

In the mammalian heart the rate of recurrence of the
cardiac contraction varies with the animal examined. In
man it is in adult life about 72 per minute. Many factors
modify the rate of the heart.

1. Period of Life. — The following table shows the average
rate of the heart at different ages : —

Foetus . . . 140 per minute.

Under 1 year . . 120

1 to 3 years . . 100

7 to 14 years . . 85

21 to 61 years . . 70 to 75 „

Old age ... 75 to 80 „

2. Period of the Day. — The pulse is generally slowest in
the early morning, and quickest in the evening.

3. Temperature of the Body. — The pulse varies with the
body temperature, generally being increased about 10 beats
with each degree Fahr. of elevation of temperature.

4. Muscular Exercise increases the rate of the heart, first
by driving the blood from the muscles into the great veins
(p. 274), and second, by developing toxic substances which
act directly upon the heart.

5. Posture has also an important influence. Suddenly
assuming the erect position accelerates the heart by caus-
ing the blood to accumulate in the abdominal veins,
and thus checking its transference on into the arteries
(p. 276).

6. The condition of the central nervous system may
modify the rate of the heart, any disturbance accompanied
by emotional changes either accelerating or retarding the
rate.

7. Stimulation of certain nerves — especially those of the

THE CIRCULATION

221

abdomen — tend to cause a retardation in the rate of the
heart.

The sequence of events making up the cardiac cycle is
simpler than in the frog.

The contraction starts in the great veins which enter the
auricles, and spreads down along them to these chambers.
This corresponds to the contraction of the sinus in the frog's
heart. It is followed by a short sharp contraction of the
auricles, which become smaller in all directions and seem to
be pulled down towards the ventricles. The contraction of
the auricles in mammals is not accompanied by so marked a
dilatation of the ventricles as in the frog.

After the auricles have fully contracted, the contraction of
the ventricles begins, and immediately the auricles relax and
resume their original size.

The ventricular contraction develops suddenly, lasts for
some time, and then suddenly passes of.

The contraction of the ventricle is followed by a period
during which both auricles and ventricles remain relaxed.
This is called the pause of the cardiac cycle.

The cardiac cycle in mammals may be represented as
in Fig. Ill :—

2. Duration of the Phases. — Ventricular systole lasts three
times as long as auricular systole ; the auricles contract for
about O'l of a second,
the ventricles for 0*3
of a second.

The duration of
these two phases in
relationship to the
pause varies very

greatly. Whatever FIG. 111. — Scheme of the Cardiac Cycle in the

<-"U^ M«4^ ^f Human Heart. A.S. , Auricular Systole ; V.S.,

may be the rate Of Ventricular Systole; P., Pause.

the heart, the auri-
cular and ventricular systoles do not vary, but in a rapidly
acting heart the pause is short, in a slowly acting heart it
is long. Taking the ordinary heart rate of 72 per minute,
the auricular systole lasts for |th of the whole cardiac
cycle, the ventricular for f ths, and the pause for f ths.

AWICLES

VENTRICLES

222 HUMAN PHYSIOLOGY

3. Changes in Shape of the Chambers.

1. Auricles. — These simply become smaller in all direc-
tions during systole.

2. Ventricles. — The changes in the diameters of the
ventricles may be studied by fixing them in the various
phases of contraction and measuring the alterations in the
various diameters,

The shape in diastole may be investigated after death-
stiffening has passed off and has left the walls relaxed. The
condition at the end of systole may be studied by rapidly
excising it while it is still beating and plunging it in some
hot solution to fix its contraction.

The condition at the beginning of systole, before the blood
has left the ventricles, may be studied by applying a ligature
round the great vessels and then plunging the heart in a hot
solution to cause it to contract round the contained blood
which cannot escape.

Measurements of hearts so fixed show that at the begin-
ning of contraction the antero-posterior diameter is increased,
while the lateral diameter is diminished. In contracting,
the lateral walls appear to be pulled towards the septum —
the increase in the antero-posterior diameter being largely due
to the blood in the right ventricle pressing on and pushing
forward the thin wall of the conus.

As the ventricles drive out their blood, both antero-
posterior and lateral diameters are diminished — but the
diminution in the lateral direction is the more marked.

There is no great shortening in the long axis of the heart.
Although the contraction of the longitudinal fibres tends to
approximate base and apex, this is in part prevented by the
contraction of the circular fibres.

4. Change in Position of the Heart. — During contraction
the heart undergoes, or attempts to undergo, a change in
position. In the relaxed condition it hangs downwards and
to the left from its plane of attachment, but when it becomes
rigid in ventricular contraction it tends to take a position at
right angles to its base — Cor sese erigere, as Harvey describes
the movement. Since the apex and front wall are in contact
with the chest, the result of this movement is to press the

THE CIRCULATION 223

heart more forcibly against the chest wall. This gives rise
to the cardiac impulse which is felt with each ventricular
systole over the prsecordium (Fig. 109).

If the chest is opened and the animal placed on its back
this elevation of the apex is readily seen. If the animal is
placed on its belly, so that the heart when relaxed hangs
forwards, the apex is tilted back during contraction.

Since the apex is twisted to the left, the movement of the
ventricle is not simply directly forward, but also from left
to right. This tilting of the apex from left to right is
further favoured by the direction of the muscular fibres of
the ventricles which pass from the auriculo- ventricular rings
downwards and to the left.

The increased thickness of the heart from before back-
wards also assists, to some extent, in the production of the
impulse.

The study of the position and characters of the cardiac
impulse is of great importance in medicine.

The position is determined by the relationship of the heart
to the anterior chest wall and to the lungs. The boundaries
of the part of the heart lying in relationship to the chest
wall have been already defined (p. 218), and it is at the outer
and lower part of this area, a region bounded above by the
fifth rib, below by the sixth rib, outside by a line drawn verti-
cally through the nipple, and inside by a line drawn vertically
midway between the nipple and the left edge of the sternum,
that the cardiac impulse is felt. Normally it does not extend
outwards beyond the nipple line, but frequently when the
left lung is voluminous the impulse only extends out to an
inch or so inside of the nipple line. In children, on account
of the size of the liver, it is often felt between the fourth and
fifth ribs. This impulse is often called the apex beat — but it
is not the apex which presses on the chest wall but a part of
the front of the right ventricle.

In character it is felt as a forward impulse of the tissue
occupying the fifth interspace, which develops suddenly,
persists for a short period, and then suddenly disappears.
In many forms of heart disease its character is markedly
altered.

The cardiac impulse may be recorded graphically by

224

HUMAN PHYSIOLOGY

means of any of the various forms of cardiograph, one of
the simplest consisting of a receiving and recording tam-
bour connected by means of a tube (Fig. 112). (Practical
Physiology, Chap. XL)

FlO. 112. — Cardiograph consisting of a Receiving Tambour, with a button on the
Membrane which is placed upon the Cardiac Impulse, and a Recording
Tambour connected with a Lever.

The form of the trace varies according to the part of the
heart upon which the button is placed, but it has the char-
acter shown in Fig. 113
if the button is upon
the cardiac impulse.

At the moment of
ventricular systole the
lever is suddenly
thrown up to a certain
level (a to b). From
this point it suddenly falls slightly (6 to c), but is maintained
during the ventricular systole above the abscissa (c to d).
At the end of the ventricular systole, as the heart falls
away from the chest wall, the lever falls to its original level
(d to e). In many tracings a small rise of the lever may

FIG. 113.— Cardiographic Trace, a to d,
Ventricular Contraction.

THE CIRCULATION 225

be seen just before the great upstroke. This corresponds
to the contraction of the auricles.

In various diseases of the heart the cardiogram is mate-
rially modified. Hence it is important to have a clear
conception of the various parts of the trace.

The elucidation of the various parts of the cardiogram is
only possible after careful study of the other changes in the
heart during the cycle.

5. Changes in the Intracardiac Pressure.

These can be studied only in the lower animals.

The most common way of determining the pressure in a
cavity is to connect it to a vertical tube and to see to what
height the fluid in the cavity is raised. If such a method is
applied to the heart, the blood in the tube undergoes such
sudden and enormous changes in level that it is impossible
to get accurate results.

The same objection applies to the method of connecting
the heart with a U tube filled with mercury. When this is
done the changes in pressure are so sudden and so extensive
that the mercury cannot respond to them on account of its
inertia.

Various means of obviating these difficulties have been
devised. One of the best is to allow the changes of pressure
to act upon a small elastic membrane tested against known
pressures. A tube is thrust through the wall of the heart
and connected with a tambour covered by a membrane to
which a lever is attached.

A. Pressure in the Great Veins (small dotted line in
Fig. 114). — When the auricles contract the flow of blood from
the great veins into these chambers is arrested, and, as a
result, the pressure in the veins rises. As the auricles relax
the blood is sucked from the veins and the pressure falls, but,
as the auricles fill up, it again rises. When the ventricles
relax and suck blood from the auricles, blood again flows in
from the great veins and the pressure falls, again to rise as
the auricles and veins are both filled up, towards the end of
the pause.

B. Pressure in the Auricles (dash line in Fig. 114).

At the moment of auricular contraction there is a marked

15

226

HUMAN PHYSIOLOGY

rise in the intra-auricular pressure. When the auricular
systole stops, the pressure falls rapidly, reaching its lowest

A S. V. S.

AS.

PRESSUHE IN

Artery.

t

Auricle.
Great Veins
Ventricle. *

FLOW of BLOOD
from

1. Great Veins to Auricles.

2. Auricles to Ventricles-

S. Ventricles to Arteries.

CLOSURE o^

1. Aunculo-Ventricular Valve.

2. Semilunar Valves.
SOUNDS of HEART.

CARDIAC IMPULSE.

FIG. 114. — Diagram to show the relationship of the events in the Cardiac Cycle to
one another. A.S., Auricular Systole ; V.S., Ventricular Systole ; P., Pause.

level when the ventricles are throwing their blood into the
arteries. Sometimes there is an interruption to this descent,

THE CIRCULATION 227

apparently synchronous with the closure of the auriculo-
ventricular valves. From this point the pressure in the
auricles rises until the moment when the ventricles relax,
when another fall in the pressure is observed. The pressure
remains about constant from this point until the next
auricular contraction.

C. Pressure in the Ventricles (continuous line in Fig. 114). —
The intra-ventricular pressure suddenly rises at the moment
of ventricular systole to reach its maximum. From this it
falls, but 4, he fall is gradual, and is interrupted by a more or
less well-marked period during which the pressure remains
constant. As the ventricles relax the pressure suddenly falls
to below zero, and then rises to a little above zero, at which it
is maintained until the next ventricular systole. The dia-
stolic expansion of the ventricle is in part due to the elasticity
of the muscular wall, and in part to the filling of the coronary
arteries which takes place only as the muscular fibres relax.

D. Pressure in the Arteries (dot-dash line in Fig. 114). —
There is a sudden rise in the aortic pressure as the blood
rushes out of the ventricles. The pressure then falls, but the
fall is not steady. Often it is interrupted by a more or less
marked increase corresponding to the later part of the
ventricular contraction. At the moment of ventricular
diastole, the fall is very sharp and is interrupted by a well-
marked and sharp rise. Following this the fall is continuous
till the next systolic elevation.

In the dog the extent of variation of the pressure in
auricles and ventricles is roughly as follows — measured in
millimetres of Hg —

Left Right Eight

Ventricle. Ventricle. Auricle.

Maximum . +140 +60 +30

Minimum . — 30 —15 — 7

These changes in the pressure in the different chambers
are due —

1st To the alternate systole and diastole of the chambers,
the first raising, the second lowering the pressure in the
chambers.

2nd. To the action of the valves.

228

HUMAN PHYSIOLOGY

6. Action of the Valves of the Heart.

A. Auriculo-ventricular (Fig. 115). — These valves have
already been described as funnel-like prolongations of the
auricles into the ventricles. They are firmly held down in
the ventricular cavity by the chordae tendineae. When the
ventricle contracts the papillary muscles pull the cusps of the
valves together and thus occlude the opening between auricles
and ventricles. The cusps are further pressed face to face by
the increasing pressure in the ventricles, and they may become
convex towards the auricles. They thus form a central core
around and upon which the ventricles contract.

On the -left side of the heart the strong anterior cusp of
the mitral valve does not materially shift its position. It

FlG. 115. — State of the various parts of the Heart throughout the Cardiac Cycle.
1, Auricular Systole ; 2, Beginning of Ventricular Systole (latent period) ;
3, Period of Outflow from the Ventricle ; 4, Period of Residual Contraction ;
5, Beginning of Ventricular Diastole.

may be somewhat pulled backwards and to the left. The
posterior cusp is pulled forwards against the anterior.

On the right side the infundibular cusp of the tricuspid
valve is stretched between the superior and inferior papillary
muscles, and is thus pulled towards the bulging septum,
against which it is pressed by the increasing pressure inside
the ventricles. The posterior cusp has its anterior margin
pulled forward and its posterior margin backwards, and is
thus also pulled toward the septum. The septal cusp
remains against the septum. The greater the pressure in
the ventricle the more firmly are these cusps pressed against
one another or against the septum, and the more completely
is the orifice between the auricle and the ventricle closed.
On the right side of the heart other factors play an impor-
tant part in occluding the orifice ; the muscular fibres which

THE CIRCULATION 229

surround the auriculo-ventricular opening contract, while the
papillary muscles pull the auriculo-ventricular ring down-
wards and inwards through the chordae which are inserted
into it.

Nevertheless the occlusion of this orifice is apt to be in-
complete when the right side of the heart becomes in the
least over-distended, giving rise to a safety valve action for
the right ventricle.

The auriculo-ventricular valves are open during the whole
of the cardiac cycle, except during the ventricular systole
(Fig. 114).

B. Semilunar Valves. — Before the ventricles contract
these valves are closed and the various segments pressed
together by the high pressure of blood in the aorta.

As the ventricles contract the pressure in them rises, until
the intra-ventricular pressure becomes greater than the pres-
sure in the arteries. Instantly the cusps of the valves are
thrown back and remain thus until the blood gushes out.
When the outflow of blood is completed, the cusps are again
approximated by the pressure of blood in the arteries. As
relaxation of the ventricles occurs, the intra-ventricular
pressure becomes suddenly very low, and the high pressure
of the blood in the arteries at once falls upon the upper
surfaces of the cusps which are thus forced downwards and
completely prevent any backflow of blood.

The prejudicial effect of too great pressure upon these
cusps is obviated by the lower cusp being mounted on
the top of the muscular septum upon which the pressure
comes — the other cusps shutting down upon this one
(Fig. 108).

7. Flow of Blood through the Heart. — The circulation of
blood through the heart depends upon these differences of
pressure in the different chambers.

A fluid always flows from a point of high pressure to a
point of lower pressure. We may then consider the flow

A. From Great Veins into Auricles. — This occurs when
the pressure in the great veins is greater than the pressure in
the auricles (Fig. 114).

The pressure in the auricles is lowest at the moment of

230 HUMAN PHYSIOLOGY

their diastole. At this time there is therefore a great fl«>\v
of blood into them, but gradually this becomes less and less,
until, when the ventricles dilate, another fall in the auricular
pressure takes place and another rush of blood from the gr» at
veins occurs. Gradually this diminishes, and by the time
that the auricles contract the flow from the great veins has
stopped.

The contraction of the mouths of the great veins, which
precedes the auricular systole, drives blood from the veins
into the auricles, and, as these enter into contraction, no flow
from the veins can occur, and no back flow from the auricles
is possible (Fig. 114).

B. From Auricles to Ventricles. — As the ventricles dilate,
a very low pressure develops in them, and hence a great rush
of blood occurs from the auricles. During the passive stage
of ventricular diastole, the intra- ventricular pressure becomes
nearly the same as the auricular, and the flow diminishes or
may stop. When the auricles contract a higher pressure is
developed, and a fresh flow of blood occurs into the ventricles.
When the ventricles contract the auriculo-ventricular valves
are closed, and all flow of blood from the auricles is stopped
(Fig. 114).

C. From Ventricles to Arteries. — When the ventricles
begin to contract the intra-ventricular pressure is low, while
the pressure in the arteries is high and keeps the semilunar
valves shut. As ventricular systole goes on the intra-ven-
tricular pressure rises until after about 0-03 of a second it
becomes higher than the arterial pressure (Latent Period).
Immediately the semilunar valves are forced open and a rush
of blood occurs from the ventricles (Period of Overflow).
This usually lasts less than O2 second. If the ventricles are
acting powerfully, and if the pressure in the arteries does not
offer a great resistance to the entrance of blood, then the
ventricles rapidly empty themselves into the arteries. If the
heart, however, is not acting forcibly, or if the arterial pressure
offers a great resistance to the entrance of blood, then the
outflow is slow and more continued. In the former case we
get a trace of the intra-ventricular pressure like Fig. 121, a,
p. 252, with a well-marked Period of Residual Contraction,
and in the latter case the trace is like Fig. 121, b. It is not

THE CIRCULATION 231

so much the absolute force of the cardiac contraction or the
absolute intra-arterial pressure which governs this, as the
relationship of the one to the other. The heart may not be
acting very forcibly, but still if the pressure in the arteries is
low its action may be 'relatively strong.

The Coronary Arteries, unlike all the other arteries, are
filled during ventricular diastole. During systole they are
compressed by the contracting muscle of the heart, and it
is only when that compression is removed in diastole that
blood rushes into them and helps to dilate the ventricles.

The interpretation of the various details of the Cardiogram
is now rendered more easy. The ventricles, still full of
blood, are suddenly pressed against the chest wall. As the
blood escapes into the arteries they press with less force, and
hence the sudden slight downstroke (Fig. 113, b to c). But so
long as the ventricles are contracted the apex is kept tilted
forward, and hence the horizontal plateau is maintained
(c to d) and disappears as the ventricles relax (e).

8. Sounds of the Heart. — On listening in the region of the
heart, a pair of sounds may be heard with each cardiac cycle,
followed by a somewhat prolonged silence. These are known
respectively as the First and Second Sounds of the Heart
(Fig. 114). (Practical Physiology, Chap. XL)

By placing a finger on the cardiac impulse while listening
to these sounds it is easy to determine that the first sound
occurs synchronously with the cardiac impulse — i.e. syn-
chronously with the ventricular contraction.

It develops suddenly, and more slowly dies away. In
character it is dull and rumbling, and may be imitated by
pronouncing the syllable lub. In pitch it is lower than the
second sound.

The second sound is heard at the moment of ventricular
diastole. Its exact time in the cardiac cycle has been deter-
mined by recording it on the cardiac tracing by means of a
microphone. It develops suddenly and dies away suddenly.
It is a clearer, sharper, and higher pitched sound than the
first. It may be imitated by pronouncing the syllable dupp.

According to the part of the chest upon which the ear is

232 HUMAN PHYSIOLOGY

placed, these sounds vary in intensity. Over the apical
region the first sound is louder and more accentuated ; over
the base the second sound is more distinctly heard.

The Cause of the Second Sound is simple. At the moment
of ventricular diastole, when this sound develops, fehe only
occurrence which is capable of producing a sound is the
sudden stretching of the semilunar valves by the high
arterial pressure above them and the low intra-ventricular
pressure below them. The high arterial pressure comes on
them suddenly like the blow of a drum-stick on a drum-
head, and, by setting the valves in vibration, produces the
sound.

Aortic and Pulmonary Areas. — The second sound has thus
a dual origin — from the aortic valve and from the pulmonary
valve ; and it is possible by listening in suitable positions to
distinguish the nature of each of these.

The aortic valve is placed behind the sternum at the level
of the lower border of the third costal cartilage. But it is
deeply situated. The aorta, passing upwards and forwards,
lies in close relationship to the chest wall at the junction of
the right side of the sternum and the right second costal
cartilage. The sound produced by the valve is conducted
up the aorta, and may best be heard in this " aortic
area."

On the other hand, the pulmonary valve lies in close re-
lationship to the anterior chest wall — being covered only by
the anterior border of the left lung — close to the edge of the
sternum in the second left interspace. The pulmonary
element of the second sound may best be heard here.

The Cause of the First Sound is by no means so simple.
When it is heard, two changes are taking place in the heart,
either of which would produce a sound.

1st. The muscular wall of the ventricles is contracting.

2nd. The auriculo-ventricular valves are being closed and
subjected on the one side to the high ventricular pressure,
and on the other to the low auricular pressure.

That the first factor plays an important part hi the pro-
duction of the first sound is proved by rapidly cutting out
the heart of an animal, and while it is still beating — but
without any blood passing through it to stretch the valves —

THE CIRCULATION 233

listening to the organ with a stethoscope. With each beat
the lub sound is distinctly heard.

Apparently the wave of contraction, passing along the
muscular fibres of the heart, sets up vibrations, and when
these are conducted to the ear the external meatus picks out
the vibration corresponding to its fundamental note, and thus
produces the characters of the sound.

2nd. The stretching of the auriculo-ventricular valves also
plays a part. If the valves be destroyed or diseased the
characters of the first sound are materially altered, or the
sound may be entirely masked by a continuous musical
sound — a murmur. Again, it has been maintained that a
trained ear can pick out in the first sound the note corre-
sponding to the valvular vibration.

The idea that the impulse of the heart against the chest
wall plays a part in the production of this sound is based upon
the fallacious idea that the heart " hits " the chest wall. All
that it does is to press more firmly against it.

Mitral and Tricuspid Areas. — On account of the part
played by the valves in the production of the first sound it
may be considered to be double in nature — partly due to
the mitral valve, partly to the tricuspid. The mitral valve
element may best be heard not over the area of the mitral
valve — which lies very deep in the thorax — but over the
apex of the heart, as at this situation the left ventricle, in
wnich the valve lies, comes nearest to the thoracic wall and
conducts the sound thither. The tricuspid element may be
best heard over the area of the valve, and in listening to it
it is usual to go to the right extremity of the area in order
as far as possible to eliminate the mitral sound. The best
situation to select is at the junction of the fifth right costal
cartilage with the sternum.

Cardiac Murmurs. — When these valves are diseased and
fail to act properly, certain continuous sounds called cardiac
murmurs are heard.

These owe their origin to the fact that, while a current of
fluid passing along a tube of fairly uniform calibre is not
thrown into vibrations and therefore produces no sound,
when any marked alterations in the lumen of the tube
occurs — either a sudden narrowing or a sudden expansion —

234 HUMAN PHYSIOLOGY

the flow of fluid becomes vibratory, and, setting up vibrations
in the solid tissues, produces a musical sound.

Such changes in the calibre of the heart are produced in
two ways : —

1st. By a narrowing, either absolute or relative, of the
orifices between the cavities — stenosis.

2nd. By a non-closure of the valves — incompetence.

Stenosis. — If one of the auriculo-ventricnlar orifices is
narrowed, we then hear the murmur during the period at
which blood normally flows through this opening. A refer-
ence to Fig. 114 at once shows that this occurs during the
whole of ventricular diastole, and that the flow is most
powerful during the first period of ventricular diastole and
during auricular systole.

If the aortic or pulmonary valve is narrowed the murmur
will be heard (Fig. 114) during ventricular systole.

The narrowing need not be absolute. A dilatation of the
artery will make the orifice relatively narrow, and will produce
the same result.

Incompetence. — If the auriculo-ventricular valves fail to
close properly, then during ventricular systole blood will be
driven back into the auricles, and a murmur will be heard
during this period.

If the aortic or pulmonary valves fail to close, the blood
will regurgitate into the ventricles from the arteries during
ventricular diastole, and a murmur will be heard during this
period.

By the position of these murmurs the pathological con-
dition producing them may be determined.

9. Work of the Heart. — The heart in pumping blood
through it is doing work, and the amount of work may be
expressed in work units — e.g. kilogram metres. With each
beat something under 80 grms. of blood are thrown from
each ventricle into the aorta and pulmonary artery. Thus
the weight lifted may be 0*08 kilos. The output of blood
at each beat of the heart of the dog is measured by Roy's
cardiometer, a rigid walled air-tight case, which is placed
round the heart and connected with a piston-recorder, so
that the decrease in the volume of the enclosed heart due

THE CIRCULATION

235

to the blood leaving it may be directly recorded by means
of a lever attached to the piston.

The average resistance in the aorta may be taken at about
1-5 metres of blood. Hence, with each beat, the left ventricle
may perform 0-08 x 1'5 = 0-12 Kgms. of work. The right
ventricle is only one-third as
strong as the left, and hence
the work done by each beat
is only 0*04 Kgms.

If the heart is beating 72
times per minute, the amount
of work per minute will be
something under 11 '5 Kgms.,
or 16,560 Kgms. in 24 hours.
Some investigators estimate
it as low as 10,000 Kgms.

In cardiac muscle the
greater the resistance to con-
traction the stronger the force
of contraction. Hence, when
extra blood is poured into the
heart from the veins, or when
the outflow from the ventricles
into the arteries is impeded,
the increased strain put upon
the heart muscle is met by
increased contraction, and the
additional work thrown upon

the organ is effectually performed. Not only is this the
case when temporary disturbances of the circulation occur,
but when these disturbances are permanent, the heart adapts
itself to them, and, if it has continuously to perform extra
work, its muscular wall hypertrophies, just as the skeletal
muscles grow by continual use. Of course, to allow such
compensation to be established, the blood supply to the
heart muscle must be sufficient, and hence, when the
coronary arteries are diseased, heart failure rapidly ensues.
If the coronary arteries are clamped and then relaxed,
a peculiar fibrillar contraction of the heart muscle
occurs.

FIG. 116. — Eoy's Cardiometer to
measure the output of blood from
the heart. b, heart in cardio-
meter chamber ; c, piston re
corder working on lever against
rubber band, d.

236 HUMAN PHYSIOLOGY

10. Nature of Cardiac Contraction. —The contraction of the
ventricle lasts for a considerable period — 0*3 seconds. Is it
of the nature of a single contraction, or of a tetanus ?

It is impossible to tetanise heart muscle, even by rapidly
repeated induction shocks. A single stimulus applied to heart
muscle produces a single prolonged contraction. Again, the
mode of development of the currents of action does not indi-
cate anything of the nature of a tetanus. With each beat of
the ventricles the variation in the electric potential begins
at the base and travels rapidly to the apex. This passage
of the contraction wave along the fibres explains the great
length of the ventricular systole as a whole. There can be
no doubt that each contraction of heart muscle is of the
nature of a muscle twitch. In this respect heart muscle
resembles non-striped muscle.

It further resembles it in that the minimum stimulus is
also a maximum stimulus — i.e. the smallest stimulus which
will make the muscle contract makes it contract to the
utmost. But while this is the case the strength of stimulus
necessary to call forth a contraction varies at different
periods. To produce another contraction while the muscle is
alreadyjin the period of contraction is difficult, but as it relaxes
it reacts more and more readily to stimuli In cardiac muscle,
perhaps more than in any other, the staircase increase in the
extent of contraction with a series of stimuli is manifested.

11. How is the Rhythmic Contraction of the Heart
maintained? — The mechanism is in the heart itself, for
the excised heart continues to beat.

In considering what this mechanism is, it must be borne
in mind^that two distinct questions have to be investigated.

1st. How does the contraction, once started, pass in regular
sequence from one part of the heart to the other ?
2nd. What starts each rhythmic contraction ?

1st. Propagation of the Wave of Contraction. — In the

heart of many of the lower animals, and in the embryo of
mammals, no nervous structures are to be found, and the
rhythmic contraction is manifestly simply a function of the
muscular fibres.

THE CIRCULATION 237

Even in the heart of animals with well-marked nerve cells
in the walls of the heart, and with nerve fibres coursing
among the muscular fibres, the conduction of the contraction
is purely a function of the muscles. For if the heart of a
frog be cut across and across, so that all nerve fibres are
severed, the contraction passes along it. The rate at which
the contraction travels is slow, only about 10 to 15 centi-
metres per second.

Since in the mammalian heart muscular continuity be-
tween auricles and ventricles is partially interrupted, the
wave of contraction is delayed at this point, and in the dying
heart, and in various pathological conditions, the contraction
frequently fails altogether to pass this block, and thus the
ventricles either stop beating before the auricles, or respond
to every second or third auricular contraction.

2nd. Starting Mechanism of Contraction. — In the ascidian
heart no nerve structures have been found, yet it beats
regularly and rhythmically. In the apex of the ventricle
of the frog there are no nerve structures, yet, if the apex be
cut off and repeatedly stimulated at regular intervals with
galvanic making and breaking stimuli, it will, after a time,
begin to contract spontaneously, regularly, and rhythmically.
Not only so, but if the apex be tied on to a tube, and a
stream of blood passed through it, it will again start con-
tracting regularly and rhythmically.

These experiments clearly show that regular rhythmic
contraction is a function of cardiac muscle.

In the cardiac cycle in the frog each contraction starts in
the sinus. What part does the sinus take in initiating
contraction ?

If a ligature be tightly applied between the sinus and
auricles (Stannius' Experiment), the sinus continues to
beat, and the auricles and ventricles usually stop beating
for a longer or shorter period. But ultimately they begin
to beat again. Hence it would seem that it is not any
special mechanism in the sinus which is essential in starting
cardiac contraction. A ligature subsequently applied between
auricles and ventricles sometimes starts the one, sometimes
the other, sometimes neither. Hence we see that any
part of the heart has the power of originating rhythmical

HUMAN PHYSIOLOGY

contractions, although usually the sinus initiates it. The
sinus more than any other part of the heart has the
property of rhythmic contraction (Practical Physiology,
Chap. IX.).

We have no evidence that the nerve cells in the sinus or
elsewhere have anything to do with this ; and so far as we
at present know, the initiation as well as the propagation of
the cardiac contraction is a function of the muscular fibres.

3rd. Intra-cardiac Nervous Mechanism. — In the frog's
heart nervous structures exist, and are distributed as
follows : —

1st. In the wall of the sinus venosus there are a number of

AOFTTA

V.C.I.

FlQ. 117. — Scheme of the various chambers of the Frog's Heart and of the
distribution of the intracardiac nervous mechanism.

nerve cells constituting the ganglion of the sinus (Remak's
ganglion).

2nd. In the inter-auricular septum a number of nerve cells
constitute the ganglion of the auricular septum.

3rd. In the auriculo-ventricular groove a number of
nerve cells are also found forming the auriculo-ventricular
ganglion (Bidder's ganglion). With these intra-cardiac
ganglia the terminations of the vagi nerves form definite
synapses.

In the mammalian heart nerve cells exist, but there is not

THE CIRCULATION 239

the same differentiation into distinct groups. Nevertheless
they are abundant round the mouths of the great veins,
round the edges of the inter- auricular septum, and round
the auriculo-ventricular groove.

While there is no evidence that the nervous structures
play an important part in starting or keeping up the con-
tractions, there is evidence that they exercise a checking or
controlling action.

If the region between the sinus and auricles in the frog's
heart is stimulated by the interrupted current from an
induction coil, the heart is slowed or stopped. (Practical
Physiology, Chap. IX.)

If atropine is first applied electric stimulation is without
result. (Practical Physiology, Chap. IX.)

These experiments seem to indicate that there is in the
heart a checking mechanism which may be stimulated by
electricity, and which is paralysed by atropine.

4:th. Connections of the Heart with the Central Nervous
System. — In the frog a branch from the vagus connects the
central nervous system with the heart. When the branch is
cut no effect is produced, showing that it is not constantly
in action ; but when the lower end is stimulated, the heart is
generally slowed or brought to a standstill. Sometimes the
effect is not marked. The reason for this is that the cardiac
branch of the vagus in the frog is really a double nerve
derived in part from the spinal accessory and in part from
fibres Avhich reach the vagus from the superior thoracic sym-
pathetic ganglion. If the spinal accessory is stimulated, the
heart is always slowed ; and if the sympathetic fibres are
stimulated, it is quickened. Generally stimulation of the
cardiac branch containing these two sets of fibres simply
gives the result of stimulating the former, but sometimes
the stimulation of the latter masks this effect. (Practical
Physiology, Chap. IX.)

In the mammal three sets of nerve fibres pass to the
heart : —

1st. Superior cardiac branch of the vagus starts from near
the origin of the superior laryngeal nerve, and passes to the
heart to end in the endocardium (Fig. 118, S.C.).

240

HUMAN PHYSIOLOGY

s.c

2nd. Inferior cardiac branch of the vagus leaves the main

nerve near the re-
current laryngeal,
and passes to join
the superficial car-
diac plexus in the
heart (Fig. 118,/.(7.).
3rd. Sympathetic
nerve fibres come
from the superior
thoracic and in-
ferior cervical gan-
glia, and also end
in the superficial
cardiac plexus (Fig.
118, 8.).

Functions of the
Cardiac Nerves. —
A. The Superior
Cardiac Branch of
the Yagus is an in-
going nerve. Sec-
tion produces no
effect ; stimulation
of the lower end
causes no effect ;
stimulation of the
upper end causes
slowing of the heart
and a marked fall
in the pressure
of blood in the
arteries, and it
may cause pain.

Flo. 118.— Connections of the Heart with the Central
Nervous System. Au., Auricle; V., Ventricle;
V.D.C.y Abdominal Vaso-dilator Centre; C.I.C.,
Cardiac Inhibitory Centre; C.A.C., Cardio-aug-
mentor Centre; S.C., Superior Cardiac Branch of
the Vagus; 7.C7., Inferior Cardiac Branch of the

Vagus with Cell Station in the Heart; S., Cardio- The slowing of the
sympathetic Fibres with Cell Station in the Len- heart is a reflex
ticular Ganglion; V. D. A b. , Vaso-dilator Fibres
to Abdominal Vessels. The continuous lines are
outgoing ; the broken lines are ingoing Nerves.

effect through the
inferior cardiac
branch ; and the
fall of blood pressure, which is the most manifest effect, is

THE <2!ROTLATMT ., 241

\ O V

due to a reflex dilatation of the vessels of the abdomen,
causing the blood to accumulate there, and thus to lessen
the pressure in the arteries generally. On account of the
action in the blood pressure, it is called the depressor
nerve.

B. Inferior Cardiac Branch of Yagus. — Section of the
vagus or of this branch causes acceleration of the action
of the heart. The nerve is therefore constantly in action.
Stimulation of its central end has no effect; stimulation
of its peripheral end causes a slowing or stoppage of the
heart. It is therefore the checking or inhibitory nerve of
the heart.

1. dwtse.tf the Fibres. — These fibres leave the central ner-
vous system by the spinal accessory, and pass to the heart
to form synapses with the cells of the cardiac plexuses.

2. Centre. — The fibres arise from a centre in the medulla
oblongata, which can be stimulated to increased activity
either directly or reflexly. (1) Direct stimulation is brought
about by (a) sudden anaemia of the brain, as when the arteries
to the head are clamped or occluded ; (b) increased venosity
of the blood, as when respiration is interfered with ; (c) the
concurrent action of the respiratory centre (see p. 295). (2)
Reflex stimulation is produced through many nerves. In
the rabbit stimulation of the 5th cranial nerve by the inhala-
tion of ammonia vapours has this action, and in all animals
stimulation of the abdominal nerves produces the same effect.
This reflex stimulation of the centre is used to determine its
position in the medulla. It can be induced after removal of
the brain above the medulla, but destruction of the, medulla
entirely prevents it.

3. Mode of Action. — These inhibitory fibres appear to act
by stimulating the local inhibitory mechanism in the heart ;
and when this has been poisoned by atropine, they cannot
act. According to the observation of Gaskell, they excite
in the heart anabolic changes, since the electric current of
injury is increased when they are stimulated, indicating that
the difference between the living part of the heart and the
injured part is increased (see p. 65).

4. Result of Action.

(a) The output of blood- from the heart is diminished, and

16

242

HUMAN PHYSIOLOGY

thus less blood is forced into the arteries, and the blood
pressure falls.

(6) The rhythm of both auricles and ventricles is slowed,
but the effect on the auricles is more marked than upon the
ventricles, and the ventricles may show a contraction rhythm
independent of that of the auricles (Fig. 119, A.).

(c) The force of contraction of the auricles is decreased.

FIG. 119. — Simultaneous Tracing from Auricles and Ventricles. A., During Stimu-
lation of the Vagus; B.t During Stimulation of the Sympathetic. Each
downstroke marks a systole, each upstroke a diastole. (From ROY and
ADAHI. )

In the ventricles the systole becomes less complete and the
cavities become more and more distended. In the heart
of the tortoise excitability and conductivity are decreased,
and the auricular contraction may fail to pass to the ven-
tricles.

THE CIRCULATION 243

C. Sympathetic Fibres. — The outgoing fibres are the
augmentors and accelerators of the heart's action. When
they are cut, no effect is produced, therefore the centre
is not constantly in action; but when the peripheral
end is stimulated, the rate and force of the heart are
increased.

1. Course of the Fibres. — These are small medullated
fibres. They leave the spinal cord by the anterior roots of the
2nd, 3rd, and 4th dorsal nerves passing to the stellate gang-
lion where they have their cell stations (Fig. 118). From the
cells in this ganglion non-medullated fibres run on in the
annulus of Vieussens, and from this and from the inferior
cervical ganglion they pass out to the muscular fibres of
the heart.

2. The Centre is in the medulla, and it may be stimulated
by stimulating various ingoing nerves, such as the sciatic ; or
it may be set in action from the higher nerve centres in
various emotional conditions.

3. Mode of Action. — These fibres seern to act directly
upon the muscular fibres, increasing their excitability and
conductivity.

4. Result of Action —

(a) The output of blood from the heart is increased, and
the pressure of £>lood in the arteries is raised.

(6) The rate of the rhythmic movements of auricles and
ventricles is increased.

(c) The force of contraction of auricles and ventricles is
increased.

It is probable that the cardiac sympathetic also carries
ingoing fibres which enter the cord in the lower cervical
region. The pain experienced in the arm in heart disease
is generally thought to be due to the implication of these
fibres leading to the sensation which is referred to the cor-
responding somatic nerves (p. 146).

The vagus is thus the protecting nerve of the heart,
reducing its work and diminishing the pressure in the
arteries.

The sympathetic is the whip which forces the heart to
increased action in order to keep up the pressure in the
arteries.

244 HUMA PHYSIOLOGY

III. Circulation in the Blood Vessels.

The general distribution of the various vessels — arteries,
capillaries, veins, and lymphatics — has been already con-
sidered (Fig. 105, p. 209).

(The structure of the walls of each must be studied
practically.)

The capillaries are minute tubes of about 12 micro-
millimetres in diameter, forming an anastomising network
throughout the tissues. Their wall consists of a single
layer of endothelium. On passing from the capillaries to
arteries on the one side, and to veins and lymphatics on
the other, non-striped muscle fibres make their appearance
encircling the tube. Between these fibres and the endo-
thelium a fine elastic membrane next appears, while out-
side the muscles a sheath of fibrous tissue develops. Thus
the three essential layers of the coats of these vessels are
produced :—

Tunica intima, consisting of endothelium set on the in-
ternal elastic membrane.

Tunica media, consisting chiefly of the visceral muscular
fibres.

Tunica adventitia, consisting of loose fibrous tissue.

The coats of the arteries are thick ; those of the veins are
thin. In the large arteries the muscular fibres of the media
are largely replaced by elastic fibres so that the vessels may
better stand the strain of the charge of blood which is shot
from the heart at each contraction. In the veins double
flaps of the tunica intima form valves which prevent any
regurgitation of blood.

The great characteristic of the walls of the large arteries
is the toughness and elasticity given by the abundance of
elastic fibrous tissue, of the small arteries, the contractility
due to the preponderance of muscular fibres.

The circulation of blood in the vessels is that of a fluid in
a closed system of elastic-walled tubes, at one end of which
(the great arteries) a high pressure, and at the other (the
great veins) a low pressure is kept up. As a result of this

m

distribution of pressure there is a constant flow of blood
from arteries to veins.

Many points in connection with the circulation may be
conveniently studied on a model made of indiarubber tubes
and a Higginson's syringe. (Practical Exercise.)

A. — Blood Pressure.

The distribution of pressure is the cause of the flow of
blood, and must first be considered.

1. General Distribution of Pressure.

(See Fig. 125, p. 258.)

The pressure in any part of a system of tubes depends
upon two factors —

1st. The force propelling fluid into that part of the
tubes.

2nd. The resistance to the outflow of fluid from that part
of the tubes.

The pressure in the arteries is high, because with each
beat of the heart about 80 grms. of blood are thrown with
the whole contractile force of each ventricle into the cor-
responding artery; and because the resistance offered to
the outflow of blood from the arteries into the capillaries
and veins is enormous. For, as the blood passes into
innumerable small vessels, it is subjected to greater and
greater friction — just as a river in flowing from a deep
narrow channel on to a broad shallow bed is subjected to
greater friction.

Thus in the arteries the powerful propulsive force of the
heart and the great resistance to outflow keep the pressure
high.

When the capillaries are reached much of the force of
the heart has been lost in dilating the elastic coats of the
arteries, and thus the inflow into the capillaries is much
weaker than the inflow into the arteries. At the same
time the resistance to outflow is small, for in passing from
capillaries to veins the channel of the blood is becoming

246 HUMAN PHYSIOLOGY

less broken up and thus opposes less friction to the inflow
of the blood.

When the veins are reached the propelling force of the
heart is still further weakened, and hence the force of inflow
is very small. But, instead of there being a resistance to
outflow from the veins into the heart, this is favoured by
the suction action of the heart during diastole, and also by
the fact that the great veins, in entering the heart, pass into
the thorax, an air-tight box in which during each inspiration
a very low pressure is developed.

What has been said of the veins applies equally to the
lymphatics.

2. Variations in Blood Pressure.

Before considering the exact measurements of pressure in
these different vessels, the rhythmic variations in pressure
may be considered.

I. Synchronous with the Heart Beats.

A. Arterial Pulse.

If the finger be placed on any artery, a distinct expansion
will be felt following each ventricular systole.

This expansion develops suddenly and disappears more
slowly. In some cases it may be felt by simply laying the
finger on the surface of the artery without exerting pressure,
in other cases it may be necessary to compress the artery
before the pulsation is distinctly felt.

If any vein be investigated in the same way it will be
found that no pulse can be detected. In the capillaries too
this pulse does not exist.

It is best marked in the great arteries, and becomes less
and less distinct as the small terminal arteries are reached.

Cause of Pulse. — The arterial pulse is due to —

1st. The intermittent inflow of blood. The arteries expand
with each sudden inflow of 80 grms. of blood from the heart
into the arterial system.

2nd. The resistance to outflow from the arteries into the
capillaries.

If blood could flow freely from the arteries into the capil-

THE CIRCULATION 247

laries, then the inrush of blood from the heart would simply
displace the same amount of blood into the capillaries and
the arteries would not be expanded. As already indicated,
the friction between the walls of the innumerable small
arterioles and the blood is so great that the flow out of the
arteries is not so free as to allow the blood to pass into the
capillaries so rapidly as it is shot into the arteries. Hence
with each beat of the heart an excess of blood must
accumulate in the arteries.

3rd. To allow of their expanding to accommodate this
excess of blood their walls must be elastic.

It is upon these three factors that the arterial pulse depends.
Do away with either, and the pulse at once disappears.

Why is there no Pulse in the Veins? — Their walls are
elastic, but, in the first place, instead of there being an
obstruction to the outflow of blood from the veins into the
heart, this is favoured by the suction action of the heart and
thorax. Hence, even if an intermittent inflow were well
marked, the absence of resistance to outflow would in itself
prevent the development of a venous pulse. But the inflow
is not intermittent. With each beat of the heart the blood
does not pass freely from the arteries into the capillaries and
veins, but it only slowly escapes, just as much passing out
between the beats as during the beats. Hence the most
important factor in causing a pulse, an intermittent inflow,
is absent.

With no sudden intermittent inflow, and with no resist-
ance to outflow, the development of a pulse is impossible.

In certain abnormal conditions, where, from the extreme
dilatation of the arterioles, the inflow into the veins is very
free, and where the outflow from the part of the body is not
so free, a local venous pulse may develop.

Characters of the Pulse Wave. — If a finger be placed on
the carotid artery and another upon the radial artery it will
be felt that the artery near the heart expands (pulses) before
that further from the heart.

The pulse develops first in the arteries near the heart
and passes outwards towards the periphery. The reason for
this is obvious. The arteries are always overfilled with blood.
The ventricle drives its contents into this overfilled aorta,

248 HUMAN PHYSIOLOGY

and to accommodate this the aortic wall expands. But since
the aorta communicates with the other arteries this increased
pressure passes outwards along them expanding their wall as
it goes.

The pulse wave may thus be compared to a wave at sea,
which is also a wave of increased, pressure, the only differ-
ence being that, while the waves at sea travel freely over the
surface, the pulse wave is confined in the column of blood,
and manifests itself by expanding the walls of the arteries.

It greatly simplifies the study of the pulse to regard it in
this light, and to study it just as we would study a wave
at sea.

1. Velocity. — To determine how fast a wave is travelling
we might select two points at a known distance from one
another, and with a watch note how long the wave took to
pass from one to the other. So with the pulse wave, two
points on an artery at a known distance from one another
may be taken and the time which the wave takes to pass
between them may be measured.

It is thus found that the pulse wave travels at about 9 or
10 metres per second — about thirty times as fast as the blood
flows in the arteries.

2. Length of the Wave. — To determine this in a wave at
sea is easy if we know its velocity and know how long it
takes to pass any one point. Suppose it is travelling at
50 feet per second, and that it takes 1 second to pass a par-
ticular point, obviously it is 50 feet in length. The same
method may be applied to the pulse wave. We know its
velocity, and by placing the finger on an artery we may
determine that one wave follows another in rapid succession,
so that there is no pause between them. Each wave corre-
sponds to a ventricular systole, and therefore each wave must
last, at any point, just the time between two ventricular
systoles — just the time of a cardiac cycle. There are about
70 cycles per minute — i.e. per 60 seconds ; hence each must
last 0-88 second. The pulse wave takes 0'8S second to pass
any place, and it travels at 30 feet per second ; its length then
is 26*4 feet, or about Jive times the length of the body. It is
then an enormously long wave, and it has disappeared at
the periphery long before it has finished leaving the aorta.

THE CIRCULATION 249

3. The Height of the Wave. — The height of the pulse wave,
as of the wave at sea, depends primarily on the pressure caus-
ing it, but the character of the arterial wall modifies it very
largely. Thus the true height of the pulse wave in the great
arteries near the heart is masked by the thickness of the
arterial wall.

Speaking generally, however, we may say that the pulse
wave is highest near the heart, and becomes lower and lower
as we pass out to the periphery, where, as already seen, it
finally disappears altogether. This disappearance is due to
its force becoming expended in expanding the arterial wall.

4. The Form of the Wave. — Waves at sea vary greatly in
form, and the form of the wave might be graphically recorded
on some moving surface like the side of a ship by some float-
ing body. If the ship were stationary a simple vertical line
would be produced, but if she were moving a curve would be
recorded, more or less abrupt according to her speed. From
this curve the shape of the wave might be deduced, if we
knew the speed of the vessel.

The same method may be applied to the arterial pulse.
By recording the changes produced by the pulse wave as it
passes any point in an artery the shape of the wave may
be deduced.

This may be done by any of the various forms of sphygmo-
graph. (Practical Physiology, Chap. XI.)

Such a tracing is not a true picture of the wave, but
simply of the effect of the wave on one point of the arterial
wall. Its apparent length depends upon the rate at which
the recording surface is travelling and not on the length of
the wave.

Its height depends in part upon the length of the recording
lever, in part upon the resistance offered by the instrument,
in part upon the degree of pressure with which the instru-
ment is applied to the artery, and in part on the thickness of
the arterial wall.

Such a trace shows (Fig. 120) —

1st. That the pulse waves follow one another without any
interval.

2nd. That the rise of the wave is much more abrupt than
the fall.

250 HUMAN PHYSIOLOGY

3rd. That upon the descent of the primary wave there are
one or more secondary waves.

One of these is constant and is often very well marked.
It forms a second crest, and is hence called the dicrotic wave.

Between the chief crest and this secondary crest, a smaller
crest is often manifest (Fig. 120, 3), and, from its position, it is
called the predicrotic wave. Sometimes other crests appear.

Medium Pressure.

Medium Pressure.

Low Pressure.

FlG. 120. — Three Sphygmographic Tracings made from the radial artery of a
healthy man in the course of one hour without removing the Sphymograph.
1 was made immediately after muscular exercise ; 2 was made after sitting
still for half-an-hour ; and 3, after an hour.

If the wave has only one crest it is called a one-crested or
monocrotic wave. If only the dicrotic crest is well marked
it is called dicrotic. If three crests are present, tricrotic ;
if several crests, polycrotic.

To understand the various parts of the pulse wave it is
necessary to compare it with the intra-ventricular pres-
sure changes. This may be done by taking synchronously
tracings of the intra-ventricular pressure, and of the aortic
pressure (Fig. 121).

THE CIRCULATION 251

Such a tracing shows that at the moment of ventricular
systole the pressure in the aorta is higher than that in the
left ventricle.

As ventricular systole advances the intra-ventricular pres-
sure rises and becomes higher than the aortic. At that
moment the aortic valves are thrown open and a rush of
blood takes place into the aorta, raising the pressure and
expanding the artery, and causing the upstroke, and crest of
the pulse curve. As the ventricle empties itself the intra-
ventricular pressure tends somewhat to fall, and, at the same
time, a fall in the intra-aortic pressure also takes place. If
all the blood does not leave the ventricle in the first gush,
e.g. when the intra-aortic pressure is high as compared with
the force of the heart (Fig. 121, continuous line), there is a
residual outflow which arrests the diminution in the aortic
pressure, or may actually raise it, causing the predicrotic
wave. As this residual outflow diminishes, the aortic pres-
sure again falls and continues to fall until the moment of
ventricular diastole. At this instant the intra-ventricular
pressure suddenly becomes less than the intra-aortic and the
semilunar valves are forced downwards towards the ventricles,
and thus the capacity of the aorta is slightly increased and
the pressure falls. This fall in pressure is indicated by the
dicrotic notch. But the elasticity of the semilunar valves at
once makes them again spring up, thus increasing the pres-
sure in the aorta and causing the second crest, the dicrotic
wave. After this the pressure in the arteries steadily
diminishes till the mean is reached, to be again increased
by the next ventricular systole.

The form of the pulse wave varies according to the
relationship between the arterial pressure, and the activity
of the heart.

If the heart is active and strong in relation to the arterial
pressure, the main mass of the blood is expelled in the first
sudden outflow, and the residual flow is absent or slight
(Fig. 121, dotted line). In this case there is a sudden and
marked rise of the arterial pressure, followed by a steady fall
till the moment of ventricular diastole. The rebound of the
semilunar valves is here marked and causes a very prominent
dicrotic wave, while the predicrotic wave is absent (Fig. 120, 1).

252

HUMAN PHYSIOLOGY

Such a condition is well seen after violent muscular exertion,
and in certain fevers. In these conditions the dicrotic wave
is so well marked that it can be readily perceived with the
finger. It is to this form of pulse that the term dicrotic is
applied in medicine.

On the other hand, if the ventricles are acting slowly and
feebly in relationship to the arterial pressure, the initial
outflow of blood does not take place so rapidly and com-
pletely (Fig. 121,
continuous line),
and the initial
rise in the pulse
is thus not so
rapid. The re-
sidual outflow of
blood is more
marked and
causes a well-
marked secon-
A5 vs. V-<>- ~'~p dary rise in the

FIG. 121.— Diagram to show the effect of altering the pulse Curve — the

relationship between the activity of the Heart and predicrotic Wave.

the arterial blood pressure. — - — - — b is the y pprf Jn r»flqfts

curve of intra-ventricular pressure, and bl is

a pulse curve with an active heart and a relatively this may be

low arterial pressure. — a and a1 are the same hjcrher than the
with a sluggish heart and a relatively high arterial

pressure. primary crest,

producing the

condition known as the anacrotic pulse. The relatively
high intra-arterial pressure here prevents the development
of a well-marked dicrotic wave.

In extreme cases of this kind, when the arterial walls are
very tense, they may recover after their expansion in an
irregular jerky manner, and may give rise to a series of kata-
crotic crests producing a polycrotic pulse (Fig. 120, 3).

From what has been said it will be seen that a study of
the pulse wave gives most valuable information as regards
the state of the circulation, and the physician constantly
makes use of the pulse in diagnosis.

Palpation of Pulse. — On placing the finger on the radial
artery the points to determine are —

THE CIRCULATION 253

1st. The rate of the pulse — i.e. the rate of the heart's
action.

2nd. The rhythm of the pulse — i.e. of the heart's action as
regards — (1) Strength of the various beats. Normally the
beats differ little from one another in force — since the
various heart beats have much the same strength. Respira-
tion has a slight effect which will afterwards be considered
(see p. 295). In pathological conditions great differences in
the force of succeeding pulse waves occur. (2) Time relation-
ship of beats. Normally the beats follow one another at
regular intervals — somewhat shorter during inspiration —
somewhat longer during expiration. In pathological con-
ditions great irregularities in this respect may occur.

3rd. The volume of the pulse wave. Sometimes the wave
is high and greatly expands the artery — sometimes less high
and expanding the artery less. The former condition is called
a full pulse (pulsus plenus), the latter a small pulse (pulsus
parvus). The fulness of the pulse depends upon two factors.
1st. The average tension in the arteries between the pulse beats.
If this is high, the walls of the artery are already somewhat
stretched, and therefore the pulse wave expands them further
only slightly. On the other hand, if the average pressure is
low, the arterial wall is lax, and is readily stretched to a greater
extent. 2nd. The force of the heart. To stretch the arterial
wall to a large extent requires an actively acting heart throw-
ing a sudden large wave of blood into the arterial system at
each systole. The full pulse is well seen after violent exer-
tion, when the heart is active and the peripheral vessels fully
dilated, thus allowing a free flow of blood from the arteries
and thus keeping the mean arterial pressure low.

4:th. Tension of the pulse. Sometimes the pulse wave is
easily obliterated by pressing on the artery — sometimes con-
siderable force is required to prevent it from passing. To
test this two fingers must be placed on the artery. That
placed nearest the heart must be pressed more and more
firmly on the vessel until the pulse wave is no longer felt by
the second finger. In this way the tension or force of the
pulse may be roughly determined. So important, however,
is this point that various instrumental methods for deter-
mining it have been devised.

254 HUMAN PHYSIOLOGY

The tension of the pulse varies directly with the force of
the heart and with the peripheral resistance. The first
statement is so obvious as to require no amplification. It is
also clear that if the peripheral resistance is low so that blood
can easily be forced out of the arteries into the capillaries,
the arterial wall will not be so forcibly expanded as when
the resistance to outflow is great. Hence a high tension
pulse is indicative of a strongly acting heart with constriction
of the peripheral vessels. It is well seen during the shivering
fit which so frequently precedes a febrile attack, since at that
time the peripheral vessels are constricted and the heart's
action excited. The tension of the pulse wave must not be
confused with the mean arterial pressure.

oth. The form of the pulse wave may be investigated by
means of the finger alone or by means of the sphygmograph.
The points to be observed are —

(1) Does the wave come up suddenly under the finger ?
In the pulsus celer (or active pulse) it does so ; in the
pulsus tardus, on the other hand, it comes up slowly. The
former condition is indicative of an actively acting heart
with no great peripheral resistance — the latter indicates that
the heart's action is weak in relationship to the arterial blood
pressure.

(2) Does the wave fall slowly or rapidly ? Normally the
fall should not be so sudden as the ascent. When the
aortic valves are not closed properly the descent becomes
very rapid.

(3) Are there any secondary waves to be observed ? The
only one of these which can be detected by the finger is the
dicrotic wave, and this only when it is well marked. When
it can be felt, the pulse is said to be dicrotic, and, as before
stated, this indicates an actively acting heart with an arterial
pressure low relatively to the strength of the ventricles.

B. Capillary Pulse.

Normally there is no pulse in the capillaries. Their thin
endothelial wall is not well adapted to bear such an inter-
mittent strain. If, however, the arterioles to a district are
freely dilated so that little resistance is offered to the escape

THE CIRCULATION

255

of blood from the arteries, and if at the same time the out-
flow from the capillaries is not proportionately increased,
intermittent inflow and resistance to outflow are developed,
and a pulse is produced. Such a condition is seen in certain
glands during activity.

C. Yenous Pulse.

1. The absence of a general venous pulse has been already
explained. But just as in the capillaries so in the veins, a
local pulse may develop.

2. In the veins entering the auricles a pulse occurs,
but a pulse having no resemblance to the arterial pulse,
although depend-
ing on the same

three factors.

Its form is indi-
cated in Fig. 122.

Its features are
to be explained as
follows : —

Blood is con-
stantly flowing
into the great
veins, pressed on
by the vis a tergo.
When the auricles
contract the out-
flow from these
veins into the heart is suddenly checked, and consequently
the veins distend. At the moment of auricular diastole
the outflow is again free, a rush of blood takes place
into the distending auricles, and thus the pressure in
the veins falls. But as the ventricular systole prevents
blood from passing through the auricles, a second obstruction
to outflow occurs, and thus a second increase in pressure is
developed in the veins. At the moment when the ventricles
dilate a sudden rush of blood takes place from the veins and
auricles into the ventricles, and thus a sudden fall in the
pressure is produced. Gradually, as the ventricles fill, the

FIG. 122. — Tracings of the Pulse in the great Veins
in relationship to the Cardiac Cycle.

Normal Venous Pulse.

Venous Pulse in Tricuspid Incompetence.

256 PHYSIOLOGY

pressure in the auricles and veins increases and they are
again expanded.

This is the normal venous pulse. But, if the auriculo-ven-
tricular valves are incompetent, when the ventricles contract,
blood is forced back into the auricles and veins and a crest
develops between the two normal crests. The height of this
third crest is a good index of the amount of regurgitation.

II. Respiratory Variations in Blood Pressure.

Not only do rhythmic changes in the arterial pressure occur
with each beat of the heart, but larger changes are caused
by the respirations — the rise in pressure in great measure
corresponding to the phase of inspiration, the fall in pressure
to the phase of expiration. This statement is not quite
accurate, as will be seen when considering the influence of
respiration on circulation (see page 295). These variations

I I I

FlO. 123. — Tracing of Arterial Blood Pressure to show large Respiratory Varia-
tions, and small Variations due to Heart Beats, a to 6, Inspiration ; 6 to a1,
Expiration.

are easily seen in a tracing of the arterial pressure taken with
the mercurial manometer (Fig. 123).

A pulse synchronous with the respirations may also be
observed in the great veins at the root of the neck and in
the cranium when opened. With each inspiration they tend
to collapse, with each expiration they again expand. The
reason for this is that during inspiration the pressure inside
the thorax becomes low and hence blood is sucked from the
veins into the heart, while during expiration the intra-thoracic
pressure becomes higher and thus the entrance of blood into
the heart is opposed.

THE CIRCULATION

257

3. Mean Blood Pressure.

I. GENERAL DISTRIBUTION.

That the pressure is positive — greater than the pressure of
the atmosphere — throughout the greater part of the blood
vessels is indicated by the fact that if a vessel is opened, the
blood flows out of it. The force with which blood escapes is
a measure of the pressure in that particular vessel. If an

B

FIG. 124. — A, The Mercurial Manometer with recording float, used in taking
records of the arterial blood pressure of lower animals. B, The Hill- Barnard
Spbygmometer, for measuring the arterial pressure in man.

artery be cut, the blood escapes with great force ; if a vein be
cut, with much less force.

Actually to measure the pressure in arteries and veins in
the lower animals is easy. It is only necessary to let the
escaping blood act against some measured force — e.g. the
force of gravity, or a column of mercury. The instru-
ment most frequently employed is a U tube containing
mercury, one limb of which is connected with an artery by

17

258

HUMAN PHYSIOLOGY

means of a rigid-walled tube filled with some fluid to pre-
vent coagulation. Before starting an observation the pres-
sure in this tube is raised to something like that expected
in the artery, and thus a rush of blood into the tube is
prevented. (Fig. 124, A.)

In man it may be done by taking advantage of the fact
that, when the pressures inside and outside an artery are
equal, the pulse wave causes the greatest variation in the
size of the artery. This may be determined by Barnard and
Hill's Sphygmometer, which is made on the principle of an
anseroid barometer attached to an elastic sac placed over an
artery. As the pressure is increased in this system, the
pulse of the artery, as shown by the hand of the android,
becomes more and more marked, until it reaches a maximum,
when the pressure in the sac is equal to that in the artery.
(Fig 124, A)

Arteries. — If the pressure in the aorta, in the radial, in
the dorsalis pedis, and in one of the smallest arteries is

measured, it is
V. found that

while it is great
in the great
arteries — about
160 mm. Hg in
the aorta — it is
much less in the
small arteries.
This distribu-
tion of arterial
pressure might
. be plotted out as

160

Ar

20

FlG. 125.— Diagram of the Distribution of Mean Blood Pres- in Fig. 125, Ar.

sure throughout the Blood Vessels. Ar., the Arteries ; Vftins If

C., the Capillaries ; V., the Veins.

the pressure in

any of the small veins, in a medium vein, and in a large
vein near the heart be measured, it will be found—

1st. That the venous pressure is less than the lowest
arterial pressure.

2nd. That it is highest in the small veins, and becomes
lower in the larger veins. In the great veins entering the

THE CIRCULATION 259

heart it is lower than the atmospheric pressure during the
first part of each ventricular diastole. (Fig. 125, V.)

Capillaries. — The pressure in the capillaries must obviously
be intermediate between that in the arteries and in the veins.
It is not so easily measured, but it may be approximately
arrived at by finding the pressure which is required to empty
the capillaries — e.g. to blanch a piece of skin.

II. ARTERIAL PRESSURE.

The force of the heart and the degree of peripheral resist-
ance both modify the arterial pressure, and normally these so
act together that disturbance of one is compensated for by
changes in the other. Thus, if the heart's action becomes
increased and tends to raise the arterial pressure, the peri-
pheral resistance falls and prevents any marked rise.
Similarly, if the peripheral resistance is increased, the
heart's actidn is diminished, and no rise in the pressure
occurs. Under certain conditions, however, this compen-
satory action is not complete, and changes in the arterial
pressure are thus brought about.

The volume of blood has a comparatively small influence
on the arterial pressure because the veins are so large that
they accommodate very varying amounts of fluid.

Factors controlling Arterial Pressure.

(a) Heart's action. — The influence of this may be readily
demonstrated by stimulating the vagus nerve while taking a
tracing of the arterial pressure. The heart is inhibited,
less blood is forced into the arteries, and the pressure
falls.

If, on the other hand, the accelerator nerve is stimulated,
the increased heart's action drives more blood into the
arteries, and the pressure rises.

(6) Peripheral resistance. — The resistance to outflow
from the arteries depends upon the resistance offered in the
small arteries, the walls of which are chiefly composed of
muscular tissue. When these fibres are contracted, the
lumen of the vessels is small and the resistance is great.

260 HUMAN PHYSIOLOGY

When it is relaxed, the lumen of the vessels dilates, and the
resistance to outflow is diminished. This muscular tissue
of the arterioles acts as a stop-cock to the flow of blood
from the arteries to the capillaries. It is of great import-
ance—

1st. In maintaining the uniform pressure in the arteries.

2nd. In regulating the flow of blood into the capillaries.

During the functional activity of a part, a free supply of
blood in its capillaries is required. This is brought about
by a relaxation of the muscular coats of the arterioles leading
to the part. When the part returns to rest, the free flow of
blood is checked by the contraction of the muscular walls of
the arterioles.

The action of the arterioles is well seen under the influence
of certain drugs (vaso-dilators and vaso-constrictors). If,
while a tracing of the arterial pressure is being taken, nitrite
of amyl is administered to the animal, it will be seen that the
skin and mucous membranes become red and engorged with
blood, while at the same time the arterial pressure falls.
Nitrites cause the muscular coat of the arterioles to relax,
and thus, by diminishing peripheral resistance, permit blood
to flow freely from the arteries into the capillaries.

Salts of barium have precisely the opposite effect, causing
the skin to become pale from imperfect filling of the capil-
laries, and producing a marked rise in the arterial pressure.
Contraction of the muscles of the arterioles is produced, and
the flow of blood from arteries to capillaries is retarded.

Not only is the state of the arterioles influenced thus by
drugs, but it is also affected by the internal secretions (see
p. 284) from certain organs. In all probability a vaso-dilator
substance is formed in the thyroid, while a powerful vaso-
constrictor is certainly produced in the medulla of the supra-
renals.

The condition of the arterioles may be studied in many
different ways —

1st. By direct observation. 1. With the naked eye. A red
engorged appearance of any part of the body may be due to
dilatation of the arteriole leading to it. It may, however, be
due to some obstruction to the outflow of blood from the part.
2. With the microscope. In certain transparent structures,

THE CIRCULATION 261

such as the web of the frog's foot, or the wing of the bat, or
the mesentery, it is possible to measure the diameter of the
arterioles by means of an eye-piece micrometer, and to study
their dilatation and contraction.

2nd. The engorgement of the capillaries brought about by
dilatation of the arterioles manifests itself also in an increased
size of the part. Every one knows how on a hot day, when
the arterioles of the skin are dilated, it is difficult to pull on
a glove, which, on a cold day, when the cutaneous arterioles
are contracted, feels loose. By enclosing a part of the body
in a case with rigid walls filled with fluid or with air which is
connected with some form of recording tambour, an increase
or decrease in the size of the part due to the state of its
vessels may be registered. Such an instrument is called a
plethysmograph.

3rd. When the arterioles to a part are dilated and the
blood is flowing freely into the capillaries, the part becomes
warmer, and by fixing a thermometer to the surface conclu-
sions as to the condition of the arterioles may be drawn.

4tth. By streaming blood through the vessels and observing
the rate at which it escapes the changes in the state of the
arterioles may be made out. This perfusion method is much
used in studying the action of drugs. (Practical Physiology,
Chap. X.)

5th. Since the state of the arterioles influences the arterial
pressure, if the heart's action is kept uniform, changes in the
arterial blood pressure indicate changes in the arterioles,
a fall of pressure indicating dilatation, a rise of pressure,
constriction.

Normal State of Arterioles. — If an arteriole in some
transparent tissue be examined, it will be found to main-
tain a fairly uniform size, but to undergo periodic slow
changes in calibre. If the ear of a white rabbit be studied,
it will be seen to undergo slow changes, at one time appear-
ing pale and bloodless, at another time red and engorged.
During this latter phase numerous vessels appear, which in
the former condition were invisible. These slow changes
are independent of the heart's action and of the rate of
respiration. They appear to be due to the periodic rhythmic
contraction which is a characteristic property of non-striped

262 HUMAN PHYSIOLOGY

muscle fibres. This rhythmic action is better marked in
some vessels than in others.

Yaso-motor Mechanism. — If the sciatic nerve of a frog be
cut, the arterioles in the foot at once dilate. If the sciatic is
stimulated, the arterioles become smaller. The same results
follow if the anterior roots of the lower spinal nerves, from
which the sciatic takes origin, be cut or stimulated.

We must, therefore, conclude (1) that the central nervous
system exerts a constant tonic influence upon the arterioles,
keeping them in a state of semi-contraction ; and (2) that this
influence may be increased, and thus a constriction of the
arterioles caused, and in this way the flow of blood from
arteries to capillaries obstructed and the arterial pressure
raised ; and (3) that this influence may be diminished, so
that the arterioles dilate and allow an increased flow into the
capillaries from the arteries, and thus lower the arterial
pressure.

These mobile arterioles, under the control of the central
nervous system, constitute a vaso-motor mechanism, which
plays a most important part in connection with nearly every
vital process in the body. By it the pressure in the arteries
is governed, by it the supply of blood to the capillaries and
tissues is controlled, and by it the loss of heat from the body
is largely regulated.

This vaso-motor mechanism consists of the three parts : —

1st. The contractile muscular walls of the arterioles with
the nerve terminations in them.

2nd. The nerves which pass to them.

3rd. The portions of the central nervous system presiding
over these.

1. Muscular Walls of the Arterioles. — The muscular fibres
are maintained in a state of tonic semi-contraction by nerves
passing to them, and when these nerves are divided, the
muscular fibres relax. But if, after these nerves have been
cut, the animal be allowed to live, in a few days the arterioles
again pass into a state of tonic semi-contraction, although
no union of the divided nerve has taken place.

Certain drugs, e.g. digitalis and the salts of barium, act as
direct stimulants to these muscle fibres.

THE CIRCULATION

263

It appears that the muscular fibres in the arterioles,
as elsewhere, tend to maintain themselves in a state
of partial contraction, which increases and diminishes
in a regular rhythmic
manner.

The precise part played
by the nerve terminations
has not been definitely estab-
lished, but certain drugs
appear to act specially upon
them. Thus apocodeine,
while it does not prevent
barium salts from constrict-
ing the vessels, prevents the
constricting action of ex-
tracts of the medulla of
the suprarenals, even when
the nerves are cut ; and
hence it must be concluded
that it paralyses a nervous
mechanism in the arteriole
wall, which is stimulated
by the suprarenal extract.

Normally this muscular
mechanism is controlled by
the nervous system.

2. Yaso-motor Nerves.—

When a nerve going to any FlG. i26.-Diagram of the Distribution of
part of the body is cut the
arterioles of the part gene-
rally dilate, when it is stimu-
lated the arterioles are
usually contracted ; some-
times, however, they are
dilated. In no case does section of a nerve cause constric-
tion of the arterioles.

These facts prove that the vaso-motor nerves may be
divided into two classes: —

1st. Vaso-constrictor.

Vaso-motor Nerves. The continuous
line shows the Vaso-constrictors ; the
dotted line the Vaso-dilators. C.N.,
Cranial Nerves; Vag., Vagus; T.S.,
Thoracic Sympathetic ; A. St., Ab-
dominal Sympathetic; N.L., Nerves
to the Leg.

264 HUMAN PHYSIOLOGY

2nd. Vaso-dilator.

A. Yaso-constrictor Nerves. — The fact that section of
these at once causes a dilatation of the arterioles proves
that they are constantly transmitting impulses from the
central nervous system.

Course. — The course of these fibres has been investigated
by section and by stimulation (Fig. 126).

They leave the spinal cord chiefly in the dorsal region
by the anterior roots of the spinal nerves, pass into the
sympathetic ganglia, where they have their cell stations,
and then as non-medullated fibres pass, either along, the
various sympathetic nerves to the viscera, or back through
the grey ramus (see Fig. 75, p. 145) into the spinal nerve,
and run in it to their terminations.

B. Yaso-dilator Nerves. — A good example of such a nerve
is to be found in the chorda tympani branch of the facial
nerve, which sends fibres to the submaxillary and sublingual
salivary glands. If this nerve is cut, no change takes place
in the vessels of the gland, but, when it is stimulated, the
arterioles dilate and allow an increased flow of blood through
the capillaries. These fibres, therefore, instead of increasing
the activity of muscular contraction, diminish or inhibit it.
They play the same part in regard to the muscular fibres of
the arterioles as the inferior cardiac branch of the vagus
does in regard to the cardiac muscular fibres. As examples
of vaso-dilator nerves we may take the gastric branches of
the vagus carrying vaso-dilator fibres to the mucous mem-
brane of the stomach, and the nervi erigentes carrying vaso-
dilator fibres to the external genitals.

The vaso-dilator nerves of most parts of the body run side
by side with the vaso-constrictor nerves ; and, hence, curious
results are often obtained. If the sciatic nerve of a dog is
cut, the arterioles of the foot dilate. If the peripheral end
of the cut nerve is stimulated, the vessels contract. But,
after a few days, if the nerve be prevented from uniting, the
arterioles of the foot recover their tonic contraction, and if
the sciatic nerve is then stimulated, a dilatation, and not a
constriction, is brought about. The vaso-constrictor fibres
seem to die more rapidly than the vaso-dilator fibres which
run alongside of them. Under certain conditions the activity

THE CIRCULATION 265

of the vasodilator fibres seems to be increased. Thus, if,
when the lirnb is warm, the sciatic nerve is stimulated,
dilatation rather than constriction may occur. Again, while
rapidly repeated and strong induction shocks are apt to
cause constriction, slower and weaker stimuli tend to pro-
duce dilatation.

Course. — The vaso-dilator nerves pass out by the anterior
roots of the various spinal nerves, and do not pass through
the sympathetic ganglia, but run as medullated fibres to
their terminal ganglia (Fig. 126).

3. Portions of Nervous System Presiding over the Yaso-
motor Mechanism. — Since a set of nerves causing constriction
of the arterioles, and another set causing dilatation exist,
we must conclude that there are two mechanisms in the
central nervous system, one a vaso-constrictor, the other a
vaso-dilator.

A. Yaso- constrict or Centre. — (a) Mode of Action. — This
mechanism is constantly in action, maintaining the tonic
contraction of the arterioles.

If any afferent nerve be stimulated, the effect is to increase
the activity of the mechanism, to cause a general constriction
of arterioles, and thus to raise the general arterial pressure.
It is, therefore, capable of reflex excitation. In ordinary
conditions so many afferent nerves are constantly being
stimulated, that it is not easy to say how far the tonic action
of this centre is reflex and dependent on the stream of
afferent impulses.

But this centre may also be directly acted upon by the
condition of the blood and lymph circulating through it.
When the blood is not properly oxygenated, as in asphyxia
or suffocation, this centre is stimulated and a general con-
striction of arterioles with high blood pressure results.

(6) Position. — In investigating the position of the centre
we may take advantage of—

1st. Its constant tonic influence. Removal of the centre
at once causes dilatation of arterioles.

2nd. The fact that it may be reflexly stimulated. If the
vaso-constrictor centre be removed, stimulation of an afferent
nerve no longer causes constriction of the arterioles.

266 HUMAN PHYSIOLOGY

Removal of the whole brain above the pons Varolii leaves
the centre intact.

Separation of the pons Varolii and medulla oblongata
from the spinal cord at once causes a dilatation of the
arterioles of the body, and at once prevents the produc-
tion of reflex constriction by stimulation of an afferent
nerve.

The main part, at least, of the vaso-constrictor mechanism
is therefore situated in the pons Varolii and medulla
oblongata.

The extent of this centre has been determined by slicing
away this part of the brain from above downwards, and
studying the influence of reflex stimulation after the removal
of each slice.

It is found that at a short distance below the corpora quad-
rigemina, the removal of each succeeding part is followed by
a diminution in the reflex constriction, until, at a point close
to and just above the calamus scriptorius, all reflex response
to stimulation stops.

The centre is therefore one of very considerable longi-
tudinal extent.

But it has been found that, if, after section of the spinal
cord high up, the animal be kept alive for some days, the
dilated arterioles again contract, and stimulation of afferent
nerves entering the cord below the point of section causes
a further constriction. If now another section be made
further down the cord, the arterioles supplied by nerves
coming from below the point of section will again dilate.
This shows that secondary vaso-constrictor centres, tonic
in action and capable of having their activity reflexly
increased, exist all down the grey matter of the spinal cord.
Normally these are under the domain of the dominant
centre, but when this is out of action they then come into
play.

B. Yaso-dilator Centre. — (a) Mode of Action.— This
mechanism is not constantly in action. Section of a vaso-
dilator nerve does not cause vascular dilatation. It may be
excited reflexly, but in a different manner from the vaso-
constrictor mechanism.

Stimulation of an afferent nerve causes a dilatation of the

THE CIRCULATION 267

arterioles in the part from which it comes, and a constric-
tion of the arterioles throughout the rest of the body. If
a sapid substance, such as pepper, be put in the mouth,
the buccal mucous membrane and the salivary glands
become engorged, while there is a constriction of the
arterioles throughout the body. The vaso-dilator central
mechanism is not general in its action like the vaso-
constrictor, but is specially related to the different parts of
the body.

This is a matter of the greatest importance in physiology
and pathology. It explains the increased vascularity of a
part when active growth is going on. The changes in the part,
or the products of these, stimulate the afferent nerve. This
reflexly stimulates the vaso-dilator mechanism of the part,
and thus causes a free flow of blood into the capillaries, and
at the same time, by causing a general constriction of the
arterioles, maintains or actually raises the arterial pressure,
and thus forces more blood to the situation in which it is
required.

The same process occurs in the case of the stomach during
digestion, in the case of the kidney during secretion, and
in the process of inflammation.

Not only does peripheral stimulation act in this way, but
various states of the brain, accompanied by emotions, may
stimulate part of the vaso-dilator mechanism, as in the act
of blushing.

Again, it has been shown that stimulation of the central
end of the depressor nerve (superior cardiac branch of the
vagus) causes a dilatation of the arterioles chiefly in the
abdominal cavity, but also throughout the body generally.
This is the most generalised vaso-dilator reflex known
(see p. 240).

(b) Position. — While the dominant vaso-constrictor centre
is in the medulla, the vaso-dilator centres seem to be dis-
tributed throughout the medulla and spinal cord.

III. CAPILLARY PRESSURE.

It has already been shown that this is less than in the
arteries and greater than in the veins.

268 HUMAN PHYSIOLOGY

Like the pressure in the arteries it depends upon the two
factors —

1st. Force of inflow.

2nd. Resistance to outflow.

(a) Variations in the Force of Inflow. — The capillary
pressure may undergo marked local changes through the
vaso-motor mechanism. Wherever the function of a part is
active, dilatation of the arterioles and an increased capillary
pressure exists, and, when the influence of vaso-dilator nerves
is withdrawn, the capillary pressure falls.

But the capillary pressure may also be modified by
the heart's action, inasmuch as the arterial pressure, by
which blood is driven into the capillaries, depends upon
this. In cardiac inhibition not only is arterial pressure
lowered, but capillary pressure may also fall. In aug-
mented heart action both arterial and capillary pressure
are raised.

(6) Variations in Resistance to Outflow. — Normally the
flow from capillaries to veins is free and unobstructed ; but,
if the veins get blocked, or if the flow in them is retarded by
gravity, the capillaries get engorged with the blood which
cannot escape from them. This increased pressure in the
capillaries is very different from that caused by increased
inflow. The flow through the vessels is slowed or may
be stopped instead of being accelerated, and the blood gets
deprived of its nourishing constituents, loaded with waste
products, and tends to exude into the lymph spaces, causing
dropsy.

It is, therefore, most important to distinguish between
high capillary pressure from dilated arterioles or an
active heart, and high pressure due to venous obstruc-
tion.

A condition very similar to that described, but producing
a capillary pressure high relatively to the pressure in the
arteries — though not absolutely high — is seen in cases of
failure of the heart, when that organ is not acting sufficiently
strongly to pass the blood on from the venous into the
arterial system. Here the arterial pressure becomes lower
and lower, the venous pressure higher and higher, and along
with this the capillary pressure becomes high in relationship

THE CIRCULATION

269

to the arterial pressure ; the blood is not forced through these
channels, and congestion of the capillaries and dropsy may
result.

The influence of gravity plays a very important part on
the capillary pressure, since it has so marked an influence
on the flow of blood in
the veins. When, through
heart failure, the blood is
not properly sucked up
from the inferior extremi-
ties, this increased pressure
becomes very marked in-
deed.

But the pressure in the
capillaries may also to a
certain extent be varied by
the withdrawal of water

from the body, as in pur- Fia 127.— The Changes in Blood Pressure
,. -,. . in the Capillaries produced by increas-

gation or diuresis, or by ing the ^rterial ;ressure .5 ,

the addition of large quan- and by obstructing the venous flow

tities of fluid to the blood. • 7f • Arteries; &• Cnpii-

.,,, . , lanes ; V. , Veins.

I he venous system is, how-
ever, so capacious that very great changes in the amount
of blood in the vessels may take place without materially
modifying the arterial or capillary pressure while affecting
temporarily the venous pressure.

IV. VENOUS PEESSURE.

In the veins the force of inflow is small; the resistance
to outflow is nil. Hence the pressure is small, and steadily
diminishes from the small veins to the large veins entering
the heart (Fig. 127).

The venous pressure may be modified by variations in
these two factors. Constriction of the arterioles tends to
lower the venous pressure, dilatation to raise it. On the
other hand, increased heart's action, which so markedly
tends to raise arterial pressure, diminishes the pressure in
the larger veins, because the blood is thus more rapidly
driven from veins into arteries, and because the heart, which

270 HUMAN PHYSIOLOGY

in its powerful systole drives out more blood, in its diastole
sucks in more.

Compression of the thorax has a very marked effect in
retarding the flow of blood from the great veins into the
heart, and thus tends to raise the venous pressure and to
lower the arterial pressure. Venous pressure may be tem-
porarily modified by the loss or gain of water.

V. LYMPHATIC PRESSURE.

No exact determination of the lymph pressure in the
tissue spaces has been made, but since there is a constant
flow from these spaces through the lymphatic vessels and
through the thoracic duct into the veins at the root of the
neck, the pressure in the tissue spaces must be higher than
the pressure in the great veins.

This pressure is kept up by the formation of lymph from
the blood, and from the cells of the tissues (see p. 207).

B.— Flow of Blood.

The flow of blood, as already indicated, depends upon the
distribution of pressure, a fluid always tending to flow from
the point of higher pressure to the point of lower pressure.
Since a high pressure is maintained in the aorta and a low
pressure in the veins entering the heart and in the cavities
of the heart during its diastole, the blood must flow through
the circulation from arteries to veins.

The velocity of the flow of a fluid depends upon the
width of the channel. Since in unit of time unit of
volume must pass each point in a stream if the fluid is
not to accumulate at one point, the velocity must vary
with the sectional area of the channel. In the case of a
river, in each second the same amount of water must pass
through the narrowest and through the widest part of its
channel. Now for a ton of water to get through any point
in a channel one square yard in sectional area in the same
time as it takes to pass a point in a channel ten square yards
in area, it must obviously flow with greater velocity. This
may be stated in the proposition that the velocity (V)

THE CIRCULATION

271

of the stream is equal to the amount of blood passing any
point per second (v) divided by the sectional area of the
stream (S) —

where S is the radius squared multiplied by the con-
stant 314.

In the vascular system the sectional area of the aorta
is small when compared with the sectional area of the

AR

FlG. 128. — Diagram of the Sectional Area of the Vascular System, upon
which the Velocity of the Flow depends. AR., Arteries; (7.,
Capillaries ; V., Veins.

smaller arteries; while the sectional area of the capillary
system is no less than 700 times greater than that of the aorta.
In the venous system the sectional area steadily diminishes,
although it never becomes so small as in the corresponding
arteries, and where the great veins enter the heart it is about
twice the sectional area of the aorta (Fig. 128).

This arrangement of the sectional area of the stream gives
rise to a rapid flow in the arteries, a somewhat slower flow in
the veins, and to a very slow flow in the capillaries.

The suddenness of the change of pressure has a certain
influence on the rapidity of flow, as is well seen in a river.
If the water descends over a sudden declivity to a lower level
it attains a much greater velocity than if the declivity is

272 HUMAN PHYSIOLOGY

gentle. In the first case the change of pressure is sudden,
in the second case it is slow.

Hence, if from any cause the pressure is raised at any
point, the flow will tend to be more rapid from that point
onwards till the normal distribution of pressure is re-
established.

Friction has also a certain effect. A river runs much
faster in mid-stream than along the margins, because near
the banks the flow is delayed by friction, and the more
broken up and subdivided is the channel, the greater is
the friction and the more is the stream slowed.

When, therefore, in the capillary system, the blood stream
is distributed through innumerable small channels, the
friction is very great, and this tends to dam back the
blood.

The velocity of flow in the arteries and veins may be
measured by various methods, of which one of the best is
that by means of the stromuhr, an instrument by which
the volume of blood passing a given point in an artery or
vein in a given time may be determined. The velocity of
the flow in the capillaries may be measured in transparent
structures by means of a microscope with an eye-piece
micrometer. (Practical Exercise.} The velocity of the
blood is—

Carotid of the dog about . . 300 mm. per sec.
Capillaries about . . . 0-5 to 1 m. „
Vein (jugular) about . .150 mm. „

It is not so easy to give definite figures for the velocity of
the lymph stream.

Disturbance of any of the factors which govern the rate
of flow will bring about alterations in the velocity of the
blood in arteries, capillaries, and veins. Thus, an increased
venous pressure, by leading to a diminution in the difference
of pressure between arteries and veins, will materially slow the
blood stream. Great dilatation of the arterioles will slow the
blood stream in them ; and increased viscosity of the blood
by increasing friction with the vessel wall will also slow the
stream.

THE CIRCULATION 273

Special Characters of Blood Flow.

(a) Arteries. — The flow of blood in an artery is rhythmi-
cally accelerated with each ventricular systole. This is due
to the pulse wave. As the wave of high pressure passes
along the vessels, the blood tends to flow forwards and
backwards from it — so that in front of the wave there is
an acceleration of the stream and behind it a retardation.
In a wave at sea the same thing happens, and a cork
floating on the surface is moved forward in front of the
wave and again backwards after the wave has passed.

(b) Capillaries. — In the capillaries the flow is uniform.

(c) Veins. — In most veins, too, it is uniform, but in the
great veins near the heart it undergoes periodic accelera-
tions—

1st. With each diastole of auricle and ventricle.
2nd. With each inspiration.

3rd. By muscular action squeezing the blood out of the
small veins.

In all vessels the blood in the centre of the stream
moves more rapidly than that at the periphery on account
of the friction between the blood and the vessels. An
"axial" rapid and "peripheral" slow stream are, there-
fore, described. This is well seen in any small vessel placed
under the microscope, and in such situations it will be found
that, while the erythrocytes are chiefly carried in the axial
stream, the leucocytes are more confined to the peripheral
stream, where they may be observed to roll along the vessel
wall with a tendency to adhere to it.

When from any cause the flow through the capillaries is
brought to a standstill, these leucocytes creep out through
the vessel wall and invade the tissue spaces. This is the
process of diapedesis, which plays an important part in
inflammation.

In at least two situations the circulation presents special
characters.

1. Circulation Inside the Cranium. — Here the blood circu-
lates in a closed cavity with rigid walls, and therefore its

18

274 HUMAN PHYSIOLOGY

amount can vary only at the expense of the cerebro-spinal
fluid. This is small in amount, and permits of very small
variations in the volume of blood. Increased arterial pres-
sure in the body does not therefore increase the amount of
blood in the brain, but simply drives the blood more rapidly
through the organ. There seems to be no regulating nervous
mechanism connected with the arteriolcs of the brain, and
the cerebral pressure simply follows the changes in the
general arterial pressure. The splanchnic area is the great
regulator of the supply of blood to the brain. Since the
cerebral arteries are supported and prevented from dis-
tending by the solid wall of the skull, the arterial pulse
tends to be propagated into the veins, and in these veins
the respiratory pulse is very well marked (Fig. 129).

2. Circulation in the Lungs. — Vaso-motor nerves seem
to be absent, and hence drugs like adrenalin fail to cause a
constriction of the arterioles. The amount of blood in the
lungs is regulated by the blood pressure in the systemic
vessels.

3. Circulation in Heart Wall. — A peripheral vaso-motor
mechanism is not present in the arterioles of the coronary
vessels (see also p. 231).

Extra-Cardiac Factors Maintaining Circulation.

In considering the flow of blood through the vessels, due
to the distribution of pressure in arteries and veins, it must
be remembered that the central pump or heart is not the
only factor maintaining it (Fig. 129).

The thorax is to be looked upon as a suction pump of
considerable power, which draws blood into the heart during
inspiration. Again, the abdominal blood vessels are to be
regarded as the great blood reservoir, and when the abdo-
minal muscles are tightened and the respiratory movements
of the thorax are increased, as in the panting which accom-
panies intermittent muscular exercise, the blood is partly
pressed, and partly sucked from the abdomen into the heart,
and so forced on into the arteries. Even expiration helps
in this, for the blood which has filled the vessels of the lungs
in inspiration is driven on into the left side of the heart in

THE CIRCULATION

275

expiration. The blood is thus forced on into the arteries and
so to the muscles, and they, by their alternate contraction
and relaxation, still further help to drive it on and to accele-
rate the* circulation. The high arterial tension tends to
drive the blood through the cranial vessels. The benefit

INTRACRANIAL CIRCULATION

PULMONARY CIRCULATION

[ABDOMINAL CIRCULATION

CIRCULATION IN LIMBS

FIG. 129. — Scheme of the Circulation, modified from Hill, to illustrate the influence
of the various extra-Cardiac Factors which maintain the Flow of Blood.

of intermittent muscular exercise on the circulation is thus
manifest.

When, on the other hand, some sustained muscular strain
has to be undergone, the thorax, is fixed and hence the
pressure in the heart and thoracic organs is raised, and the
increased pressure in the thorax helps to support the heart
and to prevent over-distension. The abdominal vessels are

276 HUMAN PHYSIOLOGY

also pressed upon, and the sustained contraction of the limb
muscles tends to prevent the blood flowing through them.
It is thus forced to the central nervous system in which the
pressure rises, and, if a weak spot in the vessels is -present,
rupture is apt to occur. But, if the effort is still further
sustained, the high intra-thoracic pressure tends to prevent
proper diastolic filling of the heart ; blood is therefore not
sent on from the veins into the arteries, the veins become
congested and the arterial pressure falls, less blood goes to
the brain, and thus fainting may result.

In man the position of the abdominal reservoir of blood
at a lower level than the heart increases the work of that
, organ. For this reason, in people with a weak heart, the
sudden assumption of the erect position may lead to a failure
of the heart and to fainting. Especially is this the case when
the abdominal wall is lax, so that accumulation of blood in
the abdominal vessels is not prevented. In the recumbent
position, when the reservoir is on the same level as the pump,
the work is much easier.

On the other hand, in the " head down position," the accu-
mulation of blood in the dependent parts is prevented in
the head by the vessels being packed inside the skull and in
the right side of the heart by the supporting pericardium.

The Time taken by the Circulation.

This was first determined by injecting ferrocyanide of
potassium into the proximal end of a cut vein, and finding
how long it took to appear in the blood flowing from the
distal end. From observation in the horse, dog, and rabbit,
it appears that the time corresponds to about twenty-seven
beats of the heart, so that in man it should amount to about
twenty-three seconds.

Stewart has investigated the rate of flow through different
organs by injecting salt solution into the artery, and by
detecting its appearance in the vein by the change in the
electric conductivity of the contents of the vessel.

SECTION VIII

SUPPLY OF NOURISHING MATERIAL TO BLOOD AND LYMPH,
AND ELIMINATION OF WASTE MATTER FROM THEM

KESPIRATION
I. EXTERNAL RESPIRATION

IF an animal be placed in a closed chamber filled with
ordinary atmospheric air which contains by volume 79 parts
of nitrogen and 21 parts of oxygen, and if the air is examined
after a time, it will be found that the oxygen has diminished
in amount, and that a nearly corresponding amount of carbon
dioxide has been added.

The same thing occurs in aquatic animals — the water
round them loses oxygen and gains carbon dioxide. An
animal takes up oxygen and gives off carbon dioxide. This
is the process of external respiration.

I. Respiratory Mechanism.

In aquatic animals the mechanism by which this process
is carried on is a gill or gills. Each consists of a process
covered by a very thin layer of integument, just below which
is a tuft of capillary blood vessels. The oxygen passes from
the water to the blood ; the carbon dioxide from the blood
to the water.

A lung is simply a gill or mass of gills, turned outside in
with air instead of water outside the integument. While in
aquatic gill-bearing animals there is constantly a fresh supply
of water passing over the gills, in lung-bearing animals the
air in the lung sacs must be exchanged by some mechanical
contrivance.

The lungs consist of myriads of small thin-walled sacs

attached round the funnel-like expansions in which the air

277

278 HUMAN PHYSIOLOGY

passages (infundibular passages) terminate. (The structure
of the various parts of the respiratory tract must be studied
practically.)

Each sac is lined by a layer of simple squamous epithelium,
supported by a framework of elastic fibrous tissue richly
supplied with blood vessels. It has been calculated that, if
all the air vesicles in the lungs of a man were spread out in

one continuous sheet, a surface
of about 100 square metres
would be produced and that
the blood capillaries would
occupy about 75 square metres
of this. Through these vessels
about 5000 litres of blood would
pass in twenty-four hours.
FIG. 130.— Scheme of the Distribution The larger air passages are

sufported°by pieces of hyaline
cartilage in their walls, but the
smaller terminal passages, the bronchioles, are without this
support, and are surrounded by a specially well-developed
circular band of non-striped muscle — the bronchial muscle
—which governs the admission of air to the infundibula and
air sacs.

The lungs are packed in the thorax round the heart, com-
pletely filling the cavity.

They may be regarded as two compound elastic-walled
sacs, which completely fill an air-tight box with movable
walls — the thorax — and communicate with the exterior by
the windpipe or trachea.

No air exists between the lungs and the sides and base of
the thorax, so that the so-called pleural cavity is simply a
potential space. If the thoracic wall be punctured so that
this potential pleural cavity is brought into connection with
the air, the lungs immediately collapse and occupy a small
space posteriorly round the large bronchi. This is due to
their elasticity (Fig. 131).

The lungs are kept in the distended condition in the
thoracic cavity by the atmospheric pressure within them.

Their elasticity varies according to whether the organs are
stretched or not. As they collapse, their elastic force natu-

THE RESPIRATION

279

760

rally becomes less and less ; as they are expanded, greater and
greater. Taken in the average condition of expansion in
which they exist in the chest, the elasticity of the excised
lungs of a man is capable of supporting a column of mercury
of about 30 mm. in height, so that they are constantly
tending to collapse with this force.

But the inside of the lungs freely communicates with the
atmosphere, and this
at the sea level has
a pressure of about
760 mm. Hg. Dur-
ing one part of
respiration this pres-
sure becomes, a few
mm. smaller, during
another part a few
mm. greater ; but
the mean pressure
of 760 mm. of mer-
cury is constantly
expanding the lung,

and acting against a pressure of only 30 mm. of mercury,
tending to collapse the lung.

Obviously, therefore, the lungs must be kept expanded
and in contact with the chest wall.

When a pleural cavity is opened, the distribution of forces
is altered, for now the atmospheric pressure tells also on the
outside as well as on the inside of the lung, and acts along
with the elasticity of the organ. So that now a force of 760
mm. + 30 mm. = 790 mm. act against 760 mm., causing a
collapse of the lungs.

In the surgery of the thorax, as well as in the physiology
of respiration, these points are of great importance.

FIG. 131. — Shows the Distribution of Pressure in the
Thorax with the chest wall intact, and with an
opening into the Pleural Cavity, (j) Indicates
the Atmospheric Pressure of 760 mm. of Mercury :
30 is the elasticity of the Lungs also in -mm. Hg.

II. Physiology.

The process of respiration consists of two parts —
1st. The passage of air into and out of the air sacs.
2nd. The interchange of gases between the air in the air
vesicles and the blood in the capillaries.

280 HUMAN PHYSIOLOGY

A. Passage of air into and out of the Lungs. — This is
brought about —

1st. By the movements of respiration — breathing.

2nd. By diffusion of gases.

The air is made to pass into and out of the lungs by
alternate inspiration and expiration.

I. Movements of Respiration — A. Inspiration. — During this
act the thoracic cavity is increased in all directions — lateral,
antero-posterior, and vertical. As the thorax expands, the
air pressure inside the lungs keeps them pressed against the
chest wall, and the lungs expand with the chest. As_ a
.result of this expansion of the lungs the pressure inside
becomes less than the atmospheric pressure, and uir
rushes in until the pressure inside and outside airain
become e<mal. This can be shown by placing u lulu,- in
the mouth or in a nostril and connecting it with a water
manometer.

This expansion of the lungs can readily be deter-
mined in the vertical direction by percussion, and in
the horizontal planes by measurement. By tapping
the chest with the finger over the lung in the right
intercostal spaces, a resonant note is produced, while if
the percussion is performed below the level of the
lung, a dull note is heard. If the lower edge of
this resonance be determined before an inspiration,
and again during it, it will be found to have
descended.

As a result of inspiration, the form of the chest is
markedly modified, the change being best seen in trans-
verse sections. In expiration the chest in transverse section
is an elongated ellipse from side to side, in inspiration it
becomes more circular (Fig. 132). The change from side
to side and from behind forwards is best marked towards
the lower part of the chest, less marked in the upper part.
These changes may be recorded by means of a Cyrto-
meter, a piece of flexible gas tubing hinged behind, so that
it can be modelled to the chest.

The change from above downwards cannot be directly
seen, but it is indicated by a forward movement of the wall

THE RESPIRATION 281

of the abdomen. It will be described when considering the
mechanism by which it is brought about.

The expansion of the chest in inspiration is a muscular
act and is carried out against the following forces —

1st. The elasticityjif the Lungs.— To expand the lungs
their elastic force has to be overcome, and the more they are
expanded the ^eater^^s^their elasticity. This factor there-
fore plays a smaller part at the~Beginmng than towards the
end of inspiration.

2nd. The elasticity of the Chest Wall— The resting posi-

FIG. 132. — Vertical-tangential, Transverse, and Vertical Mesial Sections of the
Thorax in Inspiration and Expiration.

tion of the chest is that of expiration. To expand the chest
the costal cartilages have to be twisted.

3rd. The elasticity of the Abdominal Wall. — As the cavity
of the thorax increases downwards, the abdominal viscera are
pushed against the muscular abdominal wall, which in virtue
of its elasticity resists the stretching force.

In studying how these changes are brought about we may
consider —

1st. Increase in the thorax frnnm. Yy-foflf^n/y/yp^yy/ivfo.

This is due to the contraction ollhe diaphragm (Fig. 132).

In expiration this dome-like muscle, rising from the
vertebral column and from the lower costal margin,, arches
upwards, lying for some distance along the inner surface of

282 HUMAN PHYSIOLOGY

the ribs and then curving inwards to be inserted into the
flattened central tendon to which is attached the pericardium
and on which rests the heart.

In inspiration the muscular fibres contract. But the
central tendon bein^ lix»id l>y the pericardium does not
undergo extensive movement. The result of the muscular
contraction is thus to flatten out the more marginal part of
the muscle and to withdraw it more or less from the chest
wall — thus opening up a space, the complemental pleura,
into which the lungs expand.

The existence of this complemental pleura is of great
importance in cases of accumulation of fluid in the chest,
for the fluid may collect and push down the diaphragm
without materially altering the lower margin of the lungs.

It might be expected that this contraction of the
diaphragm would pull inwards the chest wall — but this
is prevented by the expansion of the thorax in the lateral
and antero-posterior diameters as a result of the mechanism
which has next to be considered. Nevertheless, in young
children, in whom the chest wall is soft, an indrawing of the
chest with each contraction of the diaphragm may occur,
and may lead to permanent distortion of the thorax.

2nd. T^wpfiAP j^L-Jh^^JiffSt JAL in^Afi g/n,tfi,r'o-fpo8t(>Tior d'nd
lateral diameters.

This is brought aboqt by the elevation of the ribs which
rotate round the axis of their attachment > in the vertebral
column.

To understand this, the mode of the connection of the ribs
to the vertebral column must be borne in mind. The head
of the rib is attached to the bodies of two adjacent vertebrae.
The tubercle of the rib is attached to the transverse process
of the lower of these vertebrae. From this the shaft of the
rib projects outwards, downwards and forwards, to be attached
in front to the sternum by the costal cartilage. If the rib
is made to rotate round its two points of attachment, its
lateral margin is elevated and carried outwards, while its
anterior end is carried forwards and upwards (see Fig. 133).

Further, as we pass from above downwards, each pair of
ribs forms the arc of a larger and larger circle, and as each
pair rises it takes the place of a smaller pair above. In these

THE RESPIRATION 283

ways the chest is increased from before backwards and from
side to side.

The first pair of ribs does not undergo this movement;
the motion of the second pair of ribs is slight, but the
range of movement becomes greater and greater as we pass
downwards until the floating ribs are reached, and these are
fixed by the abdominal muscles. This greater movement is
simply due to the greater length of the muscles moving them.
The muscles moving the ribs are chiefly the external inter-
costal muscles, and these may be considered as acting from
the fixed first rib. Now if the
fibres of the first intercostal
muscle are one inch in length,
the second rib can be raised,
say, half an inch. The first and
second intercostals acting on the
third rib will together be two
inches in length, and in con-
tracting they can raise the third
rib through, say, half of two
inches — i.e. one inch. The first,
second, and third intercostals, FIG. 133.— Shows the Movements of
acting on the fourth rib, are tho Kibs from their Position in

, i i • i .-, i Expiration to their Position in

three inches m length, and can Ration.

therefore raise this rib half of

three or one and a half inches, and so through the other

ribs, until the floating ribs fixed by the abdominal muscles

are reached.

When tho diaphragm takes the chief part in inspiration
the breathing is said to be abdominal in type — when the
intercostals chiefly act in raising the ribs it is said to be
thoracic. Abdominal breathing is best marked in males —
thoracic in females.

Along with the intercostal muscles, the levatores costarum
also act in raising the ribs and in increasing the thorax in
the transverse and antero-posterior diameters.

These are the essential muscles of inspiration, but other
muscles also participate in the act. In many individuals,
even when breathing quietly, it will be seen that the nostrils

284 HUMAN PHYSIOLOGY

dilate with each inspiration. This is due to the action of
the dilatores narium which contract synchronously with
the other muscles of inspiration. Again, if the larynx be
examined, it will be found that the vocal cords slightly
diverge from one another during inspiration. This is
brought about by the action of the posterior crico-arytenoid
muscles (p. 308).

Forced Inspiration. — This comparatively small group of
muscles is sufficient to carry out the ordinary act of inspira-
tion. But, in certain conditions, inspiration becomes forced.
A forced inspiration may be made voluntarily, often it is pro-
duced involuntarily. Such forced inspiration is well seen in
patients suffering from heart disease, in whom the blood is
not properly oxygenated, and by whom powerful efforts are
made to get as much air into the lungs as possible. In this
condition every muscle which can act upon the thorax to
expand it is brought into play. The body and spinal column
are fixed in the erect position. The head is thrown back and
fixed by the posterior spinal muscles. The arms and shoulders
are fixed — usually by holding on to the sides or arms of the
chair — and every muscle which can act from the fixed spine,
head and shoulder girdle upon the thorax is brought into
play. Normally, these act from the thorax upon the parts
into which they are inserted ; now they act from their in-
sertion upon their point of origin. The sterno-mastoids,
sterno-thyroids, and sterno-hyoids assist in elevating the
thorax. The serratus magnus, pectoralis minor, and upper
fibres of the pectoralis major, and the part of the latissimus
dorsi which passes from the humerus to the three last ribs,
also pull these structures upwards. The facial and laryngeal
movements also become exaggerated.

B. In Expiration the various muscles of inspiration cease
to act, and the forces against which they contended again
contract the thorax in its three diameters.

The elasticity of the lungs is no longer overcome by the
muscles of inspiration, and the external atmospheric pressure
acting along with it drives the chest wall inwards (see p. 279).

The elasticity of the costal cartilages and — in the erect
position — the iveight of the chest wall cause the ribs again
to be depressed, and finally the elasticity of the abdominal

THE RESPIRATION 285

wall drives the abdominal viscera against the relaxed dia-
phragm and again arches it towards the thorax, squeezing its
marginal portion against the ribs and occluding the comple-
mental pleura. Experimental evidence shows that the inter-
nal intercostals contract with each expiration, and help to
draw the ribs downwards.

Ordinary expiration is thus normally mainly a passive
act, being simply a return of the thorax to the position of
rest. But voluntarily, and in certain conditions involuntarily,
expiration may be forced. Forced expiration is then partly
due to the above factors, and partly due to the action of
muscles. Every muscle which can in any way diminish the
size of the thorax comes into play.

Chief of these are the abdominal muscles, which by
compressing the viscera push them upwards and press
the diaphragm further up into the thorax. At the same
time by acting from the pelvis to pull down the ribs
they decrease the thorax from side to side, and from before
backwards.

The serratus posticus inferior and part of the sacro-lumbalis
pull downwards the lower ribs, and the triangularis sterni also
assists in this.

By this constriction of the thorax, brought about by
ordinary or by forced expiration, the air inside is compressed
and the pressure raised. During ordinary expiration the
highest pressure reached is about 2 to 3 mm. Hg, in forced
expiration about 80 mm.

The pressure of the air outside is less than this, and the air
inside the chest is driven out.

Special Respiratory Movements. -- There are several
peculiar and special reflex actions of the respiratory muscles,
each caused by the stimulation of a special district, and
each having a special purpose.

Coughing. — This consists of an inspiration followed by a
strong expiratory effort during which the glottis is constricted
but is forced open repeatedly by the current of expired air.
It is generally due to irritation of the respiratory tract, and
its object is to expel foreign matters.

Sneezing. — This is generally produced by irritation of the
nasal mucous membrane. It consists in an inspiratory act

286

HUMAN PHYSIOLOGY

followed by a forced expiration during which, by contraction
oflhe pillars of the fauces and descent of the soft palate, the
air is forced through the nose.

Hiccough consists in a sudden reflex contraction of the
diaphragm causing a sudden inspiration which is interrupted
by a spasmodic contraction of the glottis. Abdominal irrita-
tion is its chief cause.

Sighing and Yawning are deep involuntary inspirations
which serve to accelerate the circulation of the blood when
from any cause this becomes less active (see p. 294). They are
probably due to cerebral anaemia, which they help to correct
by increasing the general arterial pressure.

II. Amount of air respired. — The amount of air respired
varies according to whether the respirations are ordinary or
forced.

In ordinary respiration about 300 ccms. of air enter and
leave the chest. This is called the tidal air. Its amount
varies with the size and muscular develop-
ment of the chest.

By a forced inspiration a much larger
Co'^c.f- (luant^ty °f ftir mav be made to pass into

/5OO CC

the lungs — a quantity varying with the
size and strength of the individual — but
on an average about 1500 ccms.
This is called the complemental air.
By forced expiration an amount of air
FIG. 134.— The amount much larger than the tidal can be ex-
of air respired in pelled, an amount usually about the
same as the complemental air, and called

the reserve air.

The total amount of air which an in-

dividual can draw into and drive out of his lungs is a fair
measure of the size and muscular development of the thorax,
and it has been called the vital capacity of the thorax.
This vital capacity may be measured by means of a spiro-
meter. Its amount, as thus indicated, depends a good deal
upon practice ; the instrument, therefore, cannot be con-
sidered as of much practical value, and a more reliable
conclusion as to the vital capacity may be arrived at by

tion and expiration.

THE RESPIRATION 287

measuring the circumference of the chest in expiration and
in inspiration.

Even after the whole of the reserve air has been driven
out of the chest, a considerable quantity still remains in
the air vesicles, its amount depending upon the size of the
chest, but averaging about 2000 ccms. This is called the
residual air.

This very important point must always be remembered,
that the air taken into the chest never fills the air vesicles,
and that air is never driven completely out of them. The
air in them is thus not changed by the movements of respira-
tion but by the process of diffusion.

III. Interchange of air in the lungs by diffusion of gases.—

It has been shown that only the air in the trachea and bronchi
undergoes exchange in mass, but that the air of the vesicles is
not driven out of the chest. The renewal of co^ o
this air depends upon the diffusion of gases.

If two gases are brought into relationship
with one another they diffuse and tend to
form a mixture uniform throughout. But,
if at one point of a system, one gas is con-
stantly being taken away and another con-
stantly added, there will be a constant
diffusion of the former towards the part
where it is being taken up, and a constant
diffusion of the latter away from the point
from which it is being given off. Suppose a FIG. 135. — Shows
tube (Fig. 135) containing oxygen and carbon *he diffusion of

j- -j j ^tA / J £ ^ Oxygen into, and

dioxide, and suppose that at one end of the of carbon Dioxide
tube oxygen is constantly being taken up and out of, the Air
carbon dioxide constantly given off, a diffu- Ves
sion of the gases in the direction indicated by the arrows
will continually go on, and thus a constant supply of oxygen
will be conveyed to the bottom of the tube, while carbonic
acid will constantly be cleared out. This is exactly the con-
dition in the lungs.

The lower part of the tube corresponds to the air vesicles
—the upper part to the air passage in which the air is con-
stantly being exchanged by the movement of respiration.

t

288 HUMAN PHYSIOLOGY

The mechanism by which the gaseous exchange in the
alveoli is carried out is thus a double one.

IY. Breath Sounds. — The air as it passes into and out of
the lungs produces sounds, which may be heard on listening
over the thorax. The character of the breath sounds are of
the utmost importance in the diagnosis of diseases of the
lungs, and must be studied practically.

On listening over the trachea or over the bifurcation of
the bronchi behind (between the 4th and 5th dorsal vertebra?),
a harsh sound, something like the guttural ch (German ich),
may be heard with inspiration and expiration. This is called
the bronchial sound.

If the ear be applied over a spot under which a mass of
air vesicles lies, a soft sound, somewhat resembling the sound
of gentle wind among leaves, may be heard throughout
inspiration and for a third or less of expiration. This is
called the vesicular sound.

When the air vesicles become consolidated by disease the
vesicular sound is lost and the bronchial sound takes its
place. The cause of the vesicular character is therefore to
be sought in the vesicles, infundibula, or small bronchi.

The cause of the bronchial sound has been determined by
experiments on horses. It has already been seen that a
column of fluid — and the same is true of a column of air —
moving along a tube of uniform calibre, or with the calibre
only slowly changing, produces no sound. Any sudden
alteration in calibre produces vibration and a musical sound,
as explained on p. 233. The first sharp constriction of the
respiratory tract is at the glottis, and it is here that the
bronchial sound is produced. If the trachea is cut below
the larynx and drawn freely outwards, the bronchial sound
at once stops, and the vesicular sound becomes lower and less
distinct.

The cause of the vesicular sound is not so satisfactorily
explained. It is in part due to propagation of the bronchial
sound, altered by passing through vesicular tissue ; but it is
also probably due to the expansion and contraction of the
air vesicles drawing in and expelling air, either through their
somewhat narrow openings into the infundibula, or through

THE RESPIRATION 289

the narrow opening of the infundibula into the bronchioles.
The reason why the sound is best heard during inspiration
may be that the sound is best conducted in the direction of
the air stream.

Y. Rhythm of Respiration. — These movements of respira-
tion are carried on in a regular rhythmic manner. Their
rate varies with many factors ; but the average number of
respirations per minute in the adult male is about sixteen,
or about one to every four or five beats of the heart. The
rate of respirations may be modified by the will, and,
therefore, in counting the respirations in a patient, it is well
to prevent his being aware of what is being done. Similarly,
on account of this influence of the upper part of the brain,
the respirations should not be counted while the patient is
excited or nervous.

The most important factor modifying the rate of respira-
tion is the age of the individual. The following table gives
the average rate at different ages : —

Under 1 year . . 44

„ 5 years . . 26

„ 20 „ . 19

. Adult .... 16

The other modifications in the rate of breathing will be
better understood after studying the nervous mechanism of
respiration.

In these respiratory movements the phase of inspiration
bears a certain proportion to that of expiration. Inspiration
is much more rapid than expiration (see Fig. 134). As soon
as inspiratian is completed, a reverse movement occurs, which
is at first rapid, but gradually becomes slower, and may be
followed by a pause, during which the chest remains in the
collapsed condition. The existence and duration of this
pause varies much, and it may really be considered as the
terminal period of expiration. Considering it in this light,
we may say that inspiration is to expiration as 6 is to 7.

YI. Nervous Mechanism of Respiration. — The rhythmic
movements of respiration require the harmonious action of

19

290

HUMAN PHYSIOLOGY

a number of muscles, and this is directed by the nervous
system.

The diaphragm is supplied by the phrenic nerves rising
from the third and fourth, and partly from the fifth cervical
nerves. The intercostals are supplied by branches from their
corresponding dorsal nerves.

If the spinal cord be cut below the fifth cervical nerve the
intercostal muscles cease to act. If the section is made above
the third cervical nerve, the diaphragm, too, is paralysed,
and the animal dies of suffocation.

Respiratory Centre. — Obviously, then, there is some ner-

R.C.

Diaph

FIG. 130.— Nervous Mechanism of Respiration. R.C., Respiratory Centre; Cut.,
Cutaneous Nerves; Ph., Phrenics; In. C., Intercostal Nerves; P., Pul-
monary Branches of Vagus; S.L., Superior Laryngeal Branch of Vagus;
I/a., the Larynx; O. Ph., Glossopharyngeal Nerve; Diaph., Diaphragm.

vous mechanism above the spinal cord presiding over these
muscles.

Removal of the brain above the medulla oblongata does
not stop the respiratory rhythm.

The mechanism must, therefore, be situated in the medulla
oblongata.

If the medulla is split into two by an incision down the
middle line, respiration continues, but the two sides do not
always act at the same rate. The mechanism, then, is bi-
lateral, but normally the two parts are connected, and thus
act together.

Destruction of the part of the medulla lying near the root
of the vagus arrests, respiration, and it may, therefore, be

THE RESPIRATION 291

concluded that the nervous mechanism presiding over this
act is situated there.

It must not be imagined that this centre sends fibres
directly to the muscles concerned in respiration. The
nerves passing to these come from the cells in the grey
matter of the spinal cord, and it is by influencing the
activity of these cells that the respiratory centre controls
the act of respiration.

Since expiration, when forced, is a complex muscular act,
it is reasonable to suppose that the respiratory centre con-
tains two parts — one presiding over inspiration, one presiding
over expiration. While the inspiratory centre is constantly
in rhythmic action, the expiratory centre is only occasionally
at work.

Mode of Action of the Respiratory Centre. — Both parts of
the respiratory centre are under the control of higher nerve
centres, and through these they may be thrown into action
at any time, or even prevented from acting for the space of
a minute or so. But, after the lapse of this period, the
respiratory mechanism proceeds to act in spite of the most
powerful attempts to prevent it.

To determine its mode of action the influence of afferent
nerves upon the centre must be considered.

Yagus. — Since the vagus is the nerve of the respiratory
tract we should expect it to have an important influence on
the centre (Fig. 136).

Section of one vagus causes the respiration to become
slower and deeper ; but, after a time, the effect wears
off, and the previous rate and depth of respiration is
regained (Fig. 137).

Section of both vagi causes a very marked slowing and
deepening of the respiration, which persists for some time,
and passes oft slowly and incompletely. Now, if after the
vagi have been cut, the connection of the centre with the
upper brain tracts is severed, the mode of action of the
centre totally changes. Instead of discharging rhythmically
it remains for a long period at rest, then the inspiratory
centre discharges violently, causing a strong and prolonged
contraction of the muscles of inspiration. This passes off,
and again a period of rest of variable duration sets in, to be

292 HUMAN PHYSIOLOGY

again interrupted by another more or less long and strong
discharge.

Separation of the respiratory centre from the vagi and
upper brain tracts brings about a loss of its rhythmic
action, but does not stop its activity. The centre owes the
rhythmic nature of its action to afferent impulses. These
afferent impulses reach it normally through the vagi, but
when these are cut the upper brain takes upon itself the
function of maintaining the rhythm.

To investigate further this influence of the vagus it is
necessary to study the effect of stimulating the nerve.

Strong stimulation of the pulmonary branches of one

d'

FlO. 137. — Tracings of the Respirations — Downstrokes, Inspiration; Upstrokes,
Expiration. At a one Vagus Nerve was cut ; at 6 the second was divided ;
at c the Upper Brain Tracts also were cut off ; d and d' show the effect of
Stimulating the Glossopharyngeal Nerve.

vagus (vagus below the origin of the superior laryngeal)
causes the respiration to become more and more rapid, the
inspiratory phase being chiefly accentuated. If the stimulus
is very strong respirations are stopped in the phase of in-
spiration. Weak stimuli, on the other hand, may cause
inhibition of inspiration.

Such experiments prove that impulses are constantly
travelling from the lungs to the centre whereby the rhythmic
activity of the centre is maintained.

How do these impulses originate in the lungs ? Appa-
rently from their alternate expansion and contraction.

If the lungs be forcibly inflated — e.g. with a bellows — the
inspiration becomes feebler and feebler and finally stops.

THE RESPIRATION 293

The nature of the gas, if non-irritant, with which this in-
flation is carried out is of no consequence. If on the other
hand the lungs be collapsed by sucking air out of them, the
inspiration becomes more and more powerful, and may end
in a spasm of the inspiratory muscles.

This shows that with each expiration a stimulus passes up
the vagus which acts upon the inspiratory centre to make it
discharge. The vagus is thus a true excito-motor nerve,
making the centre act in a reflex manner. With each col-
lapse of the lung the vagus is thrown into action, as the
lungs expand it ceases to act, and, as a result, the inspiratory
centre stops acting, the muscles of inspiration cease to con-
tract, and expiration occurs.

While ordinary respiration may thus be considered as a
rhythmic reflex act, it must not be forgotten that the respira-
tory centre can and does act rhythmically under the influence
of the higher centre, or a-rhythmically and spasmodically
when these as well as the vagi are severed from it.

So far the pulmonary branches of the vagus alone have
been considered.

But the upper part of the respiratory tract, the larynx,
receives its sensory fibres from this nerve. Section of the
superior laryngeal branch of the vagus does not alter the
rhythm of respiration. Stimulation of the upper end of the
cut nerve causes first an inhibition of inspiration and, if
stronger, produces forced expiratory acts. This is well illus-
trated by the very common experience of the effect of a
foreign body, such as a crumb, in the larynx. The fit of
coughing is a series of expiratory acts produced through
this nerve.

Another set of visceral nerves having an important influ-
ence on the respiratory rhythm are the splanchnics.

When these are stimulated inspiration is inhibited. Every
one has experienced the "loss of wind" as the result of a
blow on the abdomen.

The glossopharyngeal, which supplies the back of the
tongue, when stimulated, as by the passage of food in the
act of swallowing, causes an instant arrest of the respiratory
movements either in inspiration or expiration. The advan-
tage of this in preventing the food as it is swallowed from

294 HUMAN PHYSIOLOGY

passing into the trachea must be obvious (Fig. 137, d
and d').

Stimulation of the Cutaneous Nerves stimulates the in-
spiratory centre and causes a deep inspiration. This is seen
when cold water is dashed upon the skin, and is more clearly
demonstrated in animals with the vagi and upper nerve
tracts cut across. In such animals if the skin be touched an
inspiratory movement is made.

The action of the respiratory centre is thus regulated by
many afferent nerves. Its activity is also modified by the
condition of the blood and lymph going to it.

If the supply of blood to the medulla be interfered with —
e.g. by ligaturing the arteries to the head — breathing becomes
deeper and more laboured. The activity of the respiratory
centre is increased in the same way if the proper exchange
of gases between the blood and the air in the lungs is inter-
fered with. This may be due either to the withdrawal of
something necessary for the nutrition of the nerve cells of
the centre, or to the accumulation of products of activity.

One of the most important substances yielded by the
blood for the nutrition of cells is oxygen. That the absence
of oxygen is alone sufficient to stimulate the activity of the
respiratory centre is shown by the fact that it' an animal is
made to breathe an atmosphere of nitrogen which prevents
fHel)Iood from obtaining oxygen, but docs not interfere with
its getting rid of its carbon dioxide, increase in the activity of
the respiratory centres is produced.

This experiment does not, however, exclude the possibility
that the accumulation of carbon dioxide may also stimulate
the centre. If an animal be supplied with air in which there
is an abundance of oxygen, but which is loaded with carbonic
acid, the result is that the carbonic acid of the blood is not
got rid of. In such an animal the breathing becomes quicker,
deeper and more laboured ; but ultimately there is a tendency
for the breathing to diminish and for the animal to sink into
a state of deep coma. Since, under ordinary conditions, the
amount of oxygen in the blood does not materially vary,
while the amount of C02 may be considerably altered, it is
probable that the increase of C02 under certain conditions is
the more frequent stimulus to the respiratory centre.

THE RESPIRATION 295

The accumulation of other Waste Products in the blood

undoubtedly stimulates these nerve cells. Muscular exertion
increases the activity of respiration. That this is really due
to the accumulation of products of muscle waste in the blood
and not to any reflex influence through the nerves is shown
by the fact that it occurs in a dog when the spinal cord is
cut across, and the muscles of the hind limb are made to
contract by stimulating the nerves with electricity. The fact
that the injection of acid into the blood of animals, which
are unable rapidly to neutralise it, increases the respirations,
suggests the possibility that sarcolactic acid may be among
the substances which can stimulate these nerve cells.

Yet another factor of importance in modifying the activity
of this centre is the temperature of the animal. Increase in
temperature accelerates the rate of the heart — so, too, it
accelerates the rate of the respirations, and in about the
same proportion, as is seen in feverish attacks, where pulse
and respiration are proportionately quickened so that their
ratio remains unaltered. When the respiratory rate rises
out of proportion to the rate of the pulse, it is usually an
indication that some pulmonary irritation is present.

The lungs and heart being packed tightly together in the
air-tight thorax, and both undergoing periodic changes,
necessarily influence one another. At the same time, the
close proximity of the respiratory and cardio-motor centres
in the medulla seems to lead to the activity of one influencing
the other.

Influence of Respiration on Circulation. — The circulation
is modified in two ways by respiration. First, the rate of
the heart ; and second, the arterial blood pressure undergo
alterations.

1st. Rate of Heart. — If a sphygmographic trace giving the
pulse waves during the course of two or three respirations be
examined, it will be found that during inspiration the heart
is acting more rapidly, while during expiration its action is
slowed.

If the vagus is cut these changes are not seen, showing
that the inspiratory acceleration is not the result simply of
the larger amount of blood which enters the heart during

296 HUMAN PHYSIOLOGY

inspiration, but is really due to changes in the cardio-motor
centre — the accelerating part of which has its activity in-
creased during inspiration, while the inhibitory part is more
active during expiration. This is thus a reflex effect from
the lung through the vagus, and it may be in part due to the
proximity of the centres in the medulla.

But not only is the pulse more rapid during inspiration
and slower during expiration, but the waves are smaller
during inspiration and larger during expiration. The size
of the wave depends much upon the pressure of blood in the
arteries, and this change in the pulse thus leads to the fuller
consideration of the changes in the arterial pressure due to
respiration.

2nd. Changes in Blood Pressure. — If a tracing of the
arterial pressure and of the respiratory movements are taken
at the same time, it is found that there is a general rise of
pressure during inspiration and a general fall during expira-
tion, but that at the beginning of inspiration the pressure is
still falling, and at the beginning of expiration it is still
rising (Fig. 120, p. 256). This influence of respiration on
arterial pressure is chiefly a mechanical one, depending on
the variations in the pressure in the thorax during inspiration
and expiration.

During inspiration the pressure in the thorax falls to
below the atmospheric pressure, and thus during this period
the heart and great vessels are under a diminished pressure.
This diminution in pressure has little influence on the thick-
walled ventricles and arteries, but tells markedly on the thin-
walled auricles and veins. In these there occurs a diminution
in pressure, which, in the case of the vena cava, may fall
below an atmospheric pressure, and as a result an increased
flow of blood into these vessels from the veins outside the
thorax takes place (Fig. 129, p. 275).

But when more blood enters the heart the activity of the
organ is increased, and more blood is pumped through it into
the arteries, and the pressure in these rises. This explains
the great rise in arterial pressure during inspiration.

During expiration the pressure in the thorax rises to above
the atmospheric pressure, and thus the pressure on the vessels
in the thorax is increased. This tells on the thin-walled

THE RESPIRATION 297

veins and auricles, and thus the flow of blood into them is
retarded (Fig. 129), and, less blood passing into the heart, less
is pumped into the arteries, and the arterial pressure falls.

This, however, does not explain the slight fall of pressure
at the beginning of inspiration, or the slight rise at the
beginning of expiration. To understand these, the action of
the pulmonary circulation has to be taken into account.

As inspiration develops the lungs are dilated, and the
capillaries in them are also expanded. These expanding
capillaries require more blood to fill them. They are situated
on the course of the blood from the right side to the left side
of the heart, and thus blood is retained from this stream to
fill them, and less blood passes on into the left side of the
heart and out into the arteries, and thus at the beginning
of inspiration a small fall in the arterial pressure occurs
(Fig. 129).

Similarly, at the beginning of expiration the lungs are
compressed and their blood vessels squeezed, and thus the
blood is driven out from them. Now, this blood cannot pass
back into the right side of the heart, so it must pass on into
the left side — more blood is driven into the arteries, and thus
the pressure rises. As soon, however, as the excess of blood
has been squeezed out of the lungs, the contracted state of
the vessels further retards the passage of blood to the left
side of the heart, and assists in diminishing the arterial
pressure.

Influence of the Action of the Heart on Respiration.

—The heart lies in the thorax surrounded by the elastic
lungs. As it contracts and dilates it must alternately pull
upon and compress the lungs, and thus tend to cause an inrush
and an outrush of air — the cardio-pneumatic movement.

If a simultaneous tracing of the heart-beat and of the
movements of the air column be taken, it will be seen that
at the beginning of ventricular systole there is a slight out-
rush of air from the lungs, probably caused by the blow given
to the lungs by the suddenness of the systolic movement.
This is followed by a marked inrush of air corresponding to
the outflow of blood from the ventricles, and caused by the
fact that the contracting ventricles draw on and expand the

298

HUMAN PHYSIOLOGY

lungs. This is followed by a slower outrush of air corre-
sponding to the active filling of the ventricles during the
beginning of ventricular diastole. Lastly, during the period

of passive diastole, the cardio-
pneumatic movements of air are
in abeyance.

These cardio-pneumatic move-
ments are of great importance in
animals which hibernate. Dur-
ing their winter sleep the ordi-
nary respirations almost stop,
but a sufficient gaseous inter-
change is kept up by these
cardio-pneumatic movements.

FIG. 138,-To show relations of In the examination of the
Cardio-pneumatic Movements, heart sounds they must always

A to Cardiac Cycle, B. In A be bome ^ mind> because; if

there is any constriction in a
small bronchus near the heart,
the rush of air through this may give rise to a murmuring
sound, in character very like a cardiac murmur and syn-
chronous with the heart's action. On making the patient
cough, such a murmur at once disappears.

the upstrokes are Expiratory,
the downstrokes Inspiratory.

B. Interchange between the Air breathed and the Blood
in the Lung Capillaries.

I. Effect of Respiration upon the Air breathed. — To

determine this, some method of analysing the air exhaled
must be employed. In the case of larger animals a mask
with a valve to allow the collection of the expired air may be
used, while small animals may be placed in a closed space to
which measured quantities of air are admitted, and samples
of the air drawn off, or the whole air drawn off, may be
collected and analysed.

The following table shows the average percentage coin-
pos^tion of the air inspired and the air expired : —

Per Cent, of N. O. CO2.

Inspired air . . .79 21 0

Expired air . . . 79 17 4

THE KESPIRATION

299

i.e. about 4 per cent, of oxygen is taken from the air, and
about 4 per cent, of carbon dioxide is added to it. In man
the amount of carbon dioxide given off is smaller than the
amount of oxygen taken up. The proportion between the

"Q + k is called the Respiratory Quotient, and it is

thus less than unity — usually about '8 to *9 — that is, for

every five volumes of oxygen taken up only four volumes are

given off in carbon dioxide, the

remainder being combined with

hydrogen to form water. The

various factors modifying this

quotient will be considered while

dealing with the extent of the

respiratory changes.

FIG. 139.— Shows the Composition
of Inspired and of Expired Air.

Expired air usually contains more water than inspired air.
It is always saturated with watery vapour.

Expired air also contains small amounts of organic matter,
which give it its offensive odour. These may possibly be to
a small extent formed in the lungs, but are to a greater
extent produced by putrefactive change in the mouth and
nose. It is probable that the accumulation of these products
in the air is one of the great causes of the injurious effect of
the " foul air " in overcrowded spaces.

Expired air is usually warmer than inspired air, because
usually the body is warmer than the surrounding atmosphere.
When, however, the temperature of the air is higher than
that of the body, the expired air is cooler than the inspired.

This is illustrated by the figures of an experiment —

Temperature of Inspired Air.

6-3° C.

17-19° C.

41° C.

44° C.

Temperature of Expired Air.

30° C.
37° C.

38° C.
38-5° C.

II. Effect of Respiration on the Blood.— To understand
these changes in the air we must refer to the changes in the
gases of the blood in passing through the lungs (p. 199).
Analyses show that the blood going to the lungs is poorer in
oxygen and richer in carbon dioxide than the blood coming

300

HUMAN PHYSIOLOGY

FIG. 140.— Shows the Difference
in the Gases of Arterial and
Venous Blood.

from the lungs (Fig. 137). Oxygen is taken by the blood
from the air, carbon dioxide is given by the blood to the air.
How is this effected ? The extensive capillary network in
the walls of the air vesicles, if spread out in a continuous

sheet, would present a surface of
about 75 square metres. Between
the blood in the capillaries and
the air in the air vesicles are two
layers of living cells —

1st. The endothelium lining the
capillaries.

2nd. The flattened cells lining
the air vesicles. Through these cells the interchange of
gases must take place.

This interchange might take place in two different ways —
1st. By simple mechanical diffusion.
2nd. By some special action of the cells.
If the process follows strictly the laws of simple diffusion,
it is then unnecessary to invoke the activity of the cells as
playing a part. But if the gaseous interchange does not
strictly follow these laws, we must conclude that the cells
do play a part.

Whether a gas is simply dissolved or whether it be held
in loose chemical combination, the amount held will depend
upon the temperature of the fluid and upon the pressure of
the gas over the fluid. If the temperature is raised the
fluid will hold less of the gas in solution, and any chemical
combination will tend to split, as is seen when carbonate of
lime is heated and the carbon dioxide is driven off.

If the pressure of any gas over a fluid be decreased the gas
will tend to come off from the fluid, if it is increased it will
be taken up by the fluid. This is well seen in the case of
soda water. But the same law applies to such chemical
compounds as carbonate of lime. If it is heated in ordinary
air — i.e. under a low pressure of carbon dioxide — the gas is
given off; but if lime is in an atmosphere of C02 the gas does
not come off, but is taken up and the carbonate is formed.

It will thus be seen that for every temperature there is a
certain pressure of the gas at which the solution or chemical
combination will neither give off nor take up more of the gas.

THE RESPIRATION 301

This may be determined by exposing the material in a series
of chambers to air containing different proportions of the gas,
and ascertaining by analyses of the air whether the gas has
been given off or taken up or has remained unaltered.

We know that the pressure of gas in an atmosphere
depends upon the proportion present. Suppose an atmos-
phere contains 20 per cent, of oxygen, then the pressure, or
partial pressure, of the oxygen is got by multiplying the
percentage amount of the gas by 760 — i.e. a whole atmos-
phere's pressure — and dividing by 100 —

20x760

100

= 152 mm.

By ascertaining at what partial pressures of a gas, that gas
is neither given off nor taken up by a fluid, we may determine
the " tension " of the gas in the fluid whether it be in solution
or in loose chemical combination.

The presence of a moist membrane between the fluid and
the air makes no difference to these interchanges.

These facts may now be applied to the question as to
whether the gaseous interchange between the air in the
air vesicles and the blood is due simply to diffusion.

The questions to be decided are —

1st. What is the partial pressure of these gases in the blood
going to and coming from the lungs ?

2nd. What is their partial pressure in the air vesicles ?

Partial pressure of Oxygen and Carbon Dioxide in the
Blood. — The percentage amount of these gases tells us nothing
of their partial pressure since they are in chemical combina-
tion and not in solution. Experiments made upon the
subject have given very varying results, but the most recent
and perfect series of observations go to show that the partial
pressure of oxygen in blood coming from the lungs — i.e. in
arterial blood — is about 100 mm., while the C02 pressure in
the same blood varies enormously, but on an average is about
20 mm. of Hg.

In venous blood the oxygen pressure must be much lower —
some have put it as low as 21 mm. Hg ; while the pressure
of C02 is higher, something over 40 mm. Hg.

Partial pressure of Gases in the Air Vesicle. — The air in

302 HUMAN PHYSIOLOGY

the alveoli is not renewed by direct ventilation from without,
but by a process of diffusion (p. 287). For this reason the
amount of oxygen in the alveoli must be much smaller, the
amount of carbonic acid much larger, than in the respired air.

By catheterisation, samples of air have been withdrawn
from the deeper part of the lungs and have been analysed.

Such analysis tends to show that in the alveolar air there is —

Oxygen about 10 per cent, at a partial pressure of 76
mm. Hg.

Carbon dioxide about 4 per cent, at a partial pressure of
about 30 mm. Hg.

The difference of the partial pressure of these gases on the
two sides of the membrane — in the alveolar air and in the
blood — may be represented as follows : —

Oxygen,
ir Air, 76

/

Carbon Dioxide.
30

*

^

20?
Venous

100

Arterial

/
46
Venous

0—40
Arterial

Blood

This shows that when the blood reaches the lungs the
distribution of pressure in the gases is such that, by the laws
of diffusion, oxygen will pass from the alveolar air into the
blood and carbon dioxide from blood to air ; but, before the
blood has left the lungs, the distribution is such that oxygen
should, by the laws of diffusion, pass from blood to lungs
and carbon dioxide from lungs to blood, which is exactly the
reverse of what occurs. The passage of oxygen to the blood
and the passage of carbon dioxide from the blood is much
greater than could be accounted for by diffusion.

We must, therefore, conclude that the exchange of gases
between the alveolar air and the blood is not due entirely
to diffusion, but is in part, at least, brought about by the
activity of the cells lining the vessels and the alveoli. These
conclusions are confirmed by the observations of Haldane,
that when air containing 0*045 per cent, of CO is shaken
with blood, 31 per cent, of the haemoglobin is combined with
that gas, while, if an individual breathes such air, only 25 per
cent, of the haemoglobin of the blood is combined. In some

THE RESPIRATION 303

way the oxygen of the air is passed to the blood more rapidly
than the CO.

It should, however, be stated that, in spite of these figures,
some physiologists maintain that simple diffusion will explain
these interchanges, 1st, because, when the amount of oxygen
in the atmosphere — i.e. when the partial pressure of O falls
below a certain point, the gas is no longer taken up by the
blood, and 2nd, because, when the amount and pressure of
C02 rises, the C02 is not given off from the blood.

It has been ascertained by experiment that the pressure
of oxygen in the air may fall to about half its usual pressure
without interfering with the oxygenation of the blood. If it
falls below this oxygen is not taken up. Hence it is possible
for men to live at high altitudes where the oxygen tension
is much reduced. But under these conditions the rate of
respirations and the rate of blood flow through the lungs
have to be accelerated, and hence hyperpnoea is apt to be
induced, especially on exertion.

II. INTERNAL RESPIRATION.

Having seen how the blood gets its oxygen and gets rid of
its carbon dioxide in the lungs, it is necessary to consider
the exchanges of these gases between the blood and the
tissues.

1st. Passage of Oxygen from Blood to Tissues.

In studying the physiology of muscle, which may be taken
as a type of all the active tissues, it was seen that oxygen
is constantly being built up into the muscle molecule, and
that the living tissues have such an affinity for oxygen that
they can split it off from such pigments as methylene blue.
The tension of oxygen in muscle is therefore always very
low. We have seen that the tension of oxygen in arterial
blood is nearly 100 mm. Hg, therefore, when the blood is
exposed to a low tension of oxygen in the tissues, the oxygen
comes off from the blood and passes into the tissues by the
ordinary laws of diffusion.

But it must be remembered that this takes place in three
stages.

(1) The tissue elements are always taking up oxygen

304 HUMAN PHYSIOLOGY

from the lymph, because of the very low pressure of oxygen
in the protoplasm and because the protoplasm has an affinity
for oxygen as for other nourishing substances.

(2) As a result of this the oxygen pressure in the lymph
falls and becomes lower than the oxygen pressure of the
blood plasma, and thus the gas passes from the blood
through the capillary walls to the lymph. How far this
is simply the result of mechanical diffusion, and how far it
is carried on by the vital action of the endothelium of the
capillaries, we do not know.

(3) As a result of the withdrawal of oxygen from the
plasma, the partial pressure round the erythrocytes is
diminished, and the blood being at a high temperature, a
dissociation of oxyhsemoglobin takes place, and the oxygen
passes out into the plasma, leaving reduced haemoglobin in
the erythrocytes.

2nd. Passage of Carbon Dioxide from Tissues to Blood.

The tissues are constantly producing carbon dioxide. In
the blood the carbon dioxide is combined with the soda of
the plasma and is thus at a low tension. Hence there is a
constant passage of carbonic acid from the tissues to the
blood.

III. Extent of Respiratory Interchange.

The extent of the respiratory interchange in the lungs is
governed by the extent of the internal respiratory changes
— i.e. by the activity of the tissues. Every factor which
increases the activity of the metabolic changes in the tissues
increases the intake of oxygen and output of carbon dioxide
by the lungs.

Under average conditions a normal man of 65 kgs.
excretes, in 24 hours, about 432 litres (about 850 grms.) of
C02, or 230 grms. of C, and takes up 540 litres of 0 (about
770 grms.). That is, in one hour, on an average, he excretes
20 litres of C02 and absorbs 22 litres of 0. The rate of
exchange per unit of weight is more active in the child than
in the adult.

1. Muscular Work. — Since muscle is the most abundant
and active tissue of the body, muscular work more than
anything else increases the respiratory changes (see p. 71).

THE RESPIRATION 305

2. Food. — The taking of food at once sets up active changes
in the digestive apparatus. The muscular mechanism is
set in action, and the various glands secrete. As a result
of these processes the respiratory interchange is at once
increased. That the increase is dependent upon the in-
creased activity is shown by the fact that it is produced
by the taking of substances which cannot be absorbed and
used in the metabolic processes of the body.

But while the immediate increase in the respiratory inter-
change following the taking of food is due to the increased
activity of the digestive structures, there is also an increase
due to the utilisation in the body of the food taken.
Whether proteids, fats, or carbohydrates, or all of these form
the diet, more oxygen is consumed and more carbon dioxide
given off than during starvation. The proportion between
the oxygen taken and the carbon dioxide excreted is not,
however, the same with all these food-stuffs. If the food
is rich in carbon and poor in oxygen, a greater quantity of
oxygen must be taken to oxidise it than if it is rich in
oxygen. Thus while the carbohydrates contain about 40 per
cent, of C and 53 per cent, of O, the fats contain about 76
per cent, of C and only 12 per cent, of 0, and the proteids
contain 52 per cent, of C and 22 per cent, of 0.

Hence on a carbohydrate diet the respiratory quotient

(p. 299) £§* is high> about °'9 to 1* wnile on a fatty or
proteid diet it is low, 0-7 to 0-8.

3. Temperature. — If the temperature of the body is in-
creased, the metabolic processes become more active and
the respiratory interchanges are increased. But if the tem-
perature of the air round the body is elevated, the metabolic
processes may be diminished in activity, and the respiratory
exchange decreased.

4. Light. — It has been shown that light increases the
metabolic changes and therefore the respiratory activity.

5. Sleep. — Since in sleep the animal is in a condition of
muscular rest, since light is excluded from the eyes, and
since food is not taken, the respiratory exchanges are less
active during sleep than during the waking hours. Simi-
larly, in the long winter sleep of certain animals (hiber-
nation), these factors as well as the diminished temperature

20

3o6 HUMAN PHYSIOLOGY

of the body cause a great reduction in the intake of oxygen
and output of carbon dioxide.

It is thus the internal which governs the external respira-
tion. Merely increasing the number or depth of the
respirations has no influence on the amount of the
respiratory interchanges.

Ventilation.

The rate of gaseous exchange governs the necessary
supply of fresh air. It has been found that if the supply
is insufficient, headache and sleepiness are apt to supervene,
and experience has taught that each adult should have
2000 cubic feet of fresh air per hour. If 1000 cubic feet
of space is allowed to each individual, the ordinary methods
of exchange of air — through the chimneys and through
chinks in windows and doors — should supply this quantity
of air.

Asphyxia.

This is the condition caused by any interference with the
supply of oxygen to the blood and tissues. It may be
induced rapidly and in an acute form by preventing the
entrance of air to the lungs, as in drowning or suffocation, or
by causing the animal to breathe air deprived of oxygen, or
by interfering with the flow of blood through the lungs, or
with the oxygen-carrying capacity of the blood. It is slowly
induced in a less acute form as the muscles of respiration
fail as death from almost any disease approaches.

In acute asphyxia there is an initial stage of increased
respiratory effort, the breathing becoming panting, and the
expirations more and more forced. The pupils are small,
and the heart beats more slowly and more forcibly, while
the arterioles are strongly contracted, and a marked rise
in the arterial pressure is produced. Within a couple of
minutes a general convulsion, involving chiefly the muscles
of expiration, occurs. The intestinal muscles and the
muscles of the bladder may be stimulated, and the faeces and
urine may be passed involuntarily. Then the respirations
stop, deep gasping inspirations occurring at longer and longer
intervals. The pupils are dilated, and consciousness is

THE RESPIRATION

307

abolished. The heart fails, and thus, although the arterioles
are still contracted, the pressure in the arteries falls. Finally
the movements of the heart cease and death supervenes.

VOICE.

In connection with the respiratory mechanism of many
animals, an arrangement for the production of sound or voice
is developed. This is constructed on the principle of a
wind instrument, and it consists of a bellows, a windpipe, a
vibrating reed, and resonating
chambers. In man and other
mammals the bellows are formed
by the lungs and the thorax.
The trachea is the windpipe.
The vocal cords in the larynx
are the vibrating reeds, and the
resonating chambers are the
pharynx, nose, and mouth.

A. Structure. — The points of
physiological importance about
the structure of the larynx are
the following : —

1. Cartilages (FigS. 141, 142). FIG. 141.— Side View of the Carti-

— The ring-like cricoid (Cr.) at
the top of the trachea is thickened
from below upwards at its pos-
terior part and carries on its
upper border two pyramidal car-
tilages triangular in section — the arytenoids (Ar.). These
articulate with the cricoid by their inner angle. At the
outer angle the posterior and lateral crico-arytenoid
muscles are attached. From their anterior angles the
vocal cords arise and run forward to the thyroid. The
thyroid cartilage (Th.) forms a large shield which articu-
lates by its posterior and inferior process with the sides of
the cricoid so that it moves round a horizontal axis. To
the upper and anterior part, the epiglottis or cartilaginous
lid of the larynx is fixed.

2. Ligaments. — The articular ligaments require no special

Cr.

lages of the Larynx. Cr., Cri-
coid Cartilage; Ar., Right
Arytenoid Cartilage ; Th. , Thy-
roid Cartilage. The dotted line
shows the change in the position
of the Thyroid by the action of
the Crico-thyroid Muscle.

308 HUMAN PHYSIOLOGY

attention. The true vocal cords are fibrous ligamentous
structures which run from the arytenoids forward to the
posterior aspect of the middle of the thyroid. They contain
many elastic fibres and are covered by a stratified squamous
epithelium and appear white and shining.

The vocal cords increase in length as the larynx grows,
and in adult life they are generally longer in the male than
in the female, and the whole larynx is larger.

3. Muscles. — The crico-thyroids take origin from the
antero-lateral aspects of the cricoid, and are inserted into
the inferior part of the lateral aspect of the thyroid. In
contracting they approximate the two cartilages anteriorly,
and render tense the vocal cords (Fig. 141).

The crico-arytenoidei postici arise from the back of the
cricoid and pass outwards to be inserted into the external
or muscular process of the arytenoids. In contracting
they pull these processes inwards, and thus diverge the
anterior processes and open the glottis (Fig. 142).

The crico-arytenoidei laterales take origin from the lateral
aspects of the cricoid, and pass backwards to be inserted into
the muscular processes of the arytenoids. They pull these
forward and so swing inwards their anterior processes, and
approximate the vocal cords (Fig. 142).

A set of muscular fibres run between the arytenoids — the
arytenoidei — while other fibres run from the arytenoids up
to the side of the epiglottis. These help to close the upper
orifice of the larynx.

The thyro-arytenoid is a band of muscular fibres lying
in the vocal cords and running from the thyroid to the
arytenoids. Its mode of action is not fully understood.

5. Mucous Membrane. — The mucous membrane of the
larynx is raised on each side into a well-marked fold
above each true vocal cord — the false vocal cord. Be-
tween this and the true cord is a cavity — the ventricle
of the larynx. The other folds of mucous membrane,
although of importance in medicine, have no special physio-
logical significance.

The interior of the larynx may be examined during life by
the laryngoscope. (Practical Exercise.)

5. Nerves. — The muscles of the larynx are supplied chiefly

THE RESPIRATION

309

by the recurrent laryngeal branch of the vagus which comes
off in the thorax and arches upwards to the larynx. On the
left side, where it curves round the aorta, it is apt to be
pressed upon in aneurismal swellings. Paralysis of this
nerve causes the vocal cord on that side to assume the
cadaveric position, midway between adduction and abduc-
tion, and makes the voice hoarse or abolishes it altogether.
The superior laryngeal is the great ingoing nerve, but it
also supplies motor fibres
to the crico-thyroid muscle.
Paralysis prevents the
stretching of the vocal cords,
makes the voice hoarse and
renders it impossible to
produce a high note.

Centre. — These nerves are
presided over by (a) a centre
in the medulla. When this
is stimulated abduction of the
vocal cords is brought about.
(6) This centre is controlled
by a cortical centre situated
in the inferior frontal con-
volution. Stimulation of
this causes adduction of the
cords as in phonation, while
destruction leads to no

Cr.

Ar.

marked change.

FIG. 142. — Cross Section of the Larynx,
to show the Cricoid, Cr. ; Thyroid,
Th. ; Arytenoid Cartilages, Ar. The
continuous line shows the parts at
rest. The dotted line under the action
of the Lateral Crico-arytenoid Muscle,
and the dot-dash line under the action
of the Posterior Crico-arytenoid.

B. Physiology. — When a

blast of air is forced between the vocal cords they are set
in vibration both wholly and in segments like other
vibrating reeds, and sounds are thus produced. These
sounds may be varied in loudness, pitch, and quality.

The loudness, or amplitude of vibration, depends upon the
size of the larynx and upon the force of the blast of air
acting on the cords.

The pitch or number of vibrations per second depends
upon the length and tension of the vocal cords. The greater
length of the vocal cords in the male, as compared with the

3io

HUMAN PHYSIOLOGY

female, makes the voice deeper. The tension of the cords
may be varied by the action of the crico- thyroid muscle.

The power of varying the pitch of the voice differs greatly
in different people. The average difference between the
lowest and the highest note which the ordinary individual
can produce is about two octaves.

The quality of the sound depends upon the overtones
which are made prominent by resonance in the pharynx,
nose, and mouth. By varying the shape and size of these
cavities and more especially of the mouth, the quality of
sound may be considerably varied.

Singing Voice. — This is often classified by its average pitch

A i u

FIG. 143.— Changes in the Shape of the Mouth in Sounding the Vowels A, 7, U.

and by its quality into bass and tenor in males, and contralto
and soprano in females.

The true or chest voice is produced by a blast of air from
the chest setting the cords into vibration, but the falsetto
voice, which is generally higher in pitch than the true
voice, is probably produced by vibrations of the edges of
the cords.

Articulate Speech. — Spoken language is produced by
varying the resonating chambers of the pharynx, nose, and
mouth, and thus modifying the sounds produced in the
larynx. Whispered speech is produced by setting in vibra-
tion in the resonating chambers a stream of air which has
not been allowed to act upon the vocal cords.

In articulate speech two classes of sounds may be dis-

THE RESPIRATION

tinguished — (1) Yowel Sounds. These are musical sounds
produced by a blast of air which is so modified by changing
the shape of the mouth as to produce the well-known a, e, i,
o, u (Fig. 143).

In sounding a, the mouth resembles a funnel with the
wide part forward.

In i, it may be compared to a flask with the belly behind
and the neck forward.

In u, the flask is reversed, the belly being forward.

In o, the mouth is intermediate between its position in
u and a, and in e between a and i.

(2) Consonant Sounds. — These are more of the nature of
noises — irregular vibration. They are produced either — (1)
at the lips, (2) between the teeth and hard palate and
the tongue, or (3) at the soft palate and back of the tongue.
At these situations sounds may be produced — (1) by closing
the orifice and then suddenly forcing it open, (2) by sending
a current of air over a narrowing produced at one of these
places, (3) by setting the edges of the narrowing in vibration.
When the mouth is closed at one of these situations and air
is forced through the nose the nasal consonant sounds result.

Thus, according to their mode of production, the consonant
sounds may be classified into explosives, aspirates, vibratories,
or nasals.

Labials.

Dentals.

Gutturals.

Explosives .
Aspirates .
Vibratory .
Nasals

P. B.

F. V.

M.

T. D.
S. L. Th.
R. (English).

•j/- pi

Ch. (German).
R. (French).

Ng.

SECTION IX
THE FOOD AND DIGESTION

I. FOOD

The great use of food is to supply energy to the body. The
muscles of the body are constantly active; they are con-
stantly liberating energy by breaking down the complex
molecules of proteids, fats, and carbohydrates, and hence a
constant fresh supply of such material is necessary to pre-
vent the body living on its own material and wasting away
(see p. 73). During the growth of the body, too, the material
from which it is to be built up, and the energy used in its
construction, must be supplied by the food.

Hence a suitable food is one which will yield the necessary
amount of energy, and will supply the materials necessary
for repair and for growth.

But food must also supply the water and salts required to
keep the various constituents of the body in solution, so that
the essential chemical changes may go on.

Nature of Food.

Food may be divided into foods not yielding energy and
foods yielding energy — the former acting chiefly as solvents.

A. Food-stuffs not yielding Energy. — 1. Water is the
chief constituent. Since it is daily given off in large
quantities by the kidneys, lungs, skin, and bowels, it must
daily be supplied in sufficient amounts or the chemical
changes cannot go on and death supervenes.

2. Inorganic salts. — The most important of these is
chloride of sodium, which is essential for the maintenance of
the chemical changes in the body. When it is not freely

THE FOOD AND DIGESTION 313

supplied in the food, it is retained in the tissues, and hence
animals can, when necessary, live with a comparatively small
supply.

3. Salts of organic acids. — The sodium and potassium
salts of citric, malic, and tartaric acid, which are found
abundantly in various vegetables, are, when taken into the
tissues, oxidised into carbonates which are strongly alkaline
salts. The proteids which are decomposed in the body
contain sulphur and phosphorus, and these are oxidised into
sulphuric and phosphoric acids. In herbivorous animals
the prejudicial effect of such acids is counteracted by the for-
mation of these alkaline carbonates, which neutralise the acids.
In carnivorous animals these salts are not so necessary, since
ammonium is formed from the nitrogen of the proteids in
sufficient quantity to neutralise the acids produced. Man
occupies a position midway between the herbivora and the
carnivora. The amount of energy yielded by the breaking
down of these salts into carbonates is so small that it is of no
importance.

B. Food-stuffs yielding Energy. — These are complex
combinations of carbon, hydrogen, and oxygen, with or
without nitrogen, sulphur, phosphorus, and iron. They are
of the same nature as the materials which are found on
analysis of dead protoplasm. They are commonly spoken of
as the Proximate Principles of the food, and they may be
classified as follows : —

1. Nitrogen-containing — Proteids and Albuminoids or
Modified Proteids.

2. Non-nitrogenous — Fats and Carbohydrates.

In studying the value of these food-stuffs it is necessary to
consider their Energy Yalue — that is, the amount of energy
which can be yielded by the decomposition of a definite
quantity of each in the body.

The fats and carbohydrates leave the body as carbon
dioxide and water, the proteids leave it partly as carbon
dioxide and water, partly as urea.

Such a body as glucose, C6H1206, by being oxidised to C02
and H20, gives off' a certain amount of kinetic energy, and
the amount of energy liberated is the same whether the

3i4 HUMAN PHYSIOLOGY

oxidation is direct or takes place through any of many
possible lines of chemical change.

In whatever ways a chemical substance breaks down into
certain final products the energy set free is always the same,
and this principle is taken advantage of in determining the
energy value of the food-stuffs. If fats and carbohydrates
are changed to carbon dioxide and water in the body, and if
the energy given off as they undergo this change can be
measured outside the body, their energy value as foods can
be ascertained.

Determination of Energy Value. — This is done by burning
a definite quantity of the material and finding how many
degrees a definite quantity of water is heated. This gives
the energy in heat units, and, by Joule's law, it can be con-
verted into the equivalent for any other kind of energy, such
as mechanical work.

It is known that the energy required to heat one kilo-
gramme of water through one degree Centigrade is sufficient
to raise 423 kilogrammes of matter to the height of one
metre against the force of gravity, and thus if the energy
value of any material as a producer of heat is known in
heat units — kilogramme degrees or Calories — by multiplying
by 423 we get the value in work units — kilogramme metres
(Kgms.).

The apparatus used for ascertaining the heat produced by
the combustion of material is called a calorimeter. Many
different forms are in use, but the object in the water calori-
meter is to secure that all the heat is used in raising the
temperature of a known volume of water.

The value of the three great proximate principles of the
food must be considered in detail.

1. Proteids. — The chemistry of these bodies has been
already considered (pp. 9 and 10). They are the " chief sub-
stances" of living matter, forming about 80 or 90 per cent,
of its dry residue. The molecule is one of great complexity,
and contains C.H.O.N.S., and sometimes P. and Fe.

It is from the proteids of the food alone that the nitrogen
and sulphur required in the construction and repair of the
living tissues are obtained. The carbon and hydrogen

THE FOOD AND DIGESTION 315

required are also contained in these substances; and, as
will be presently shown, they have a considerable energy
value. Hence Proteids form THE essential organic con-
stituent of the food. Theoretically it should be perfectly
possible for an animal to live on proteids alone, with a
suitable addition of water and salts.

In estimating the actual energy value of proteids in the
body a difficulty arises in the fact that, instead of being
decomposed to C02, H90, NH3, S03, as they are during com-
bustion, in the body the nitrogenous part is not broken down
further than urea — CON2H4, 3 grms. of proteid yielding 1
grm. of urea. If the energy value of the complete combus-
tion of a definite amount of proteids is first ascertained, and
then the energy value of the amount of urea derived from
the same amount of proteid is determined, by subtract-
ing the latter from the former, the energy value of proteids
in their decomposition in the body is found (see p. 318).
The combustion of 1 grm. of proteid to urea yields 4*1
Calories of Energy.

2. Modified Proteids (Albuminoids). — In studying the
chemistry of the formed material of the various protecting,
supporting, and connecting tissues, these substances have
been considered (p. 29).

Keratin, elastin, and mucin seem to be of no importance
as articles of food. If taken in the food they pass through
the alimentary canal practically unchanged.

While raw collagen seems also to be of little use, gelatin,
formed by boiling collagen, has a certain value. Although
it cannot take the place of proteids, because it cannot be
used for building up the living tissues of the body, it is
nevertheless decomposed into urea, and in decomposing it
yields the same amount of energy as the proteids. It has,
therefore, a definite though restricted value as a food
stuff.

3. Fats. — The chemistry of the fats has already been con-
sidered (p. 31). From the fact that they contain so little
oxygen in proportion to their carbon and hydrogen, a large
amount of energy is liberated in their combustion, and
therefore they have a high energy value as food-stuffs.

One gramme of fat yields twice as much energy as the

3i6 HUMAN PHYSIOLOGY

same amount of proteid or carbohydrate. The combustion
of 1 gramme of fat yields 9-3 Calories of Energy.

4. Carbohydrates (for tests for different carbohydrates,
see " Chemical Physiology," p. 8, et seq.). — The carbohydrates
— starches and sugars — form a group of bodies which do not
occur largely in animals, but are abundant constituents of
plants.

They contain carbon, hydrogen, and oxygen, the carbon
atoms of the molecule usually numbering six or some mul-
tiple of six, and the hydrogen and oxygen being in the same
proportions in which they occur in water. They are
aldehydes or ketones, and derivatives from these, of the
hexatomic alcohol, CCHUO6 (p. 422). A group of carbo-
hydrates having five carbon atoms, and hence called Pentoses,
have been found in the animal body, but they are of minor
importance.

The simplest carbohydrates are the monosaccharids, of
which dextrose, the aldehyde of mannite, is the most im-
portant. Dextrose is the sugar of the animal body. It
has been called glucose, grape sugar, and blood sugar.

Closely allied to dextrose in chemical composition is
kevulose, a sugar which, instead of rotating the plane of
polarised light to the right, rotates it to the left, but which
in other respects behaves like dextrose. It occurs in certain
plants.

The other monosaccharid of importance is galactose, a
sugar produced by the splitting of milk sugar.

These monosaccharids, when boiled with a solution of
cupric acetate in acetic acid (Barfoed's solution), are oxidised,
taking oxygen from the cupric salt and reducing it to the
cuprous state. When boiled with caustic potash, they, along
with certain of the double sugars, are oxidised, and if a
metallic salt be present which can readily give up its oxygen
it becomes reduced, the sugar appropriating the oxygen.
On this depends Fehling's and many other tests for glucose.

Under the influence of yeast they split into ethyl alcohol
and carbon dioxide.

They also form crystalline compounds, osazones, with
phenylhydrazin. These have proved most useful in dis-
tinguishing different sugars.

THE FOOD AND DIGESTION

By the polymerisation of two monosaccharid molecules
with the loss of water, disaccharids, or double sugars, are
formed. Thus, two glucose molecules polymerise to form
one maltose molecule.

1206 = 2 (c6H1206)-H20

Maltose is the sugar formed by the action of malt and other
vegetable and animal zymins upon starch. By the action of
dilute acids and other agents it can be split into two dextrose
molecules. Like the monosaccharids, it ferments with yeast.

Lactose, the sugar of milk, is a disaccharid composed of
a molecule of dextrose united to a molecule of galactose
with dehydration. It readily splits into these two mono-
saccharids, but does not ferment with yeast.

Dextrose, polymerising with laevulose, yields cane sugar,
and this sugar, so largely used as an article of food, can be
split into dextrose and lasvulose. It does not reduce Fehling's
solution, and does not ferment with yeast.

By further polymerisation of monosaccharids with the loss
of water, molecules of greater size are produced and form the
set of substances known as the polysaccharids. Among the
simplest of these is dextrin, produced by the polymerisation
of twelve molecules of glucose with the loss of twelve
molecules of water.

12(C6H1206-H20)
= 12(C6H1005)

Closely allied to dextrin is inulin. But while dextrin is
formed of dextrose molecules, inulin contains Isevulose mole-
cules. Both are formed from the splitting of the more
complex starches. The molecule of soluble starch is built up
of no less than thirty dehydrated monosaccharid molecules,
and has a molecular weight of 9000. Ordinary starch seems
to have a molecular weight of 20,000 or 30,000, .and hence
must be of still greater complexity.

These polysaccharids are distinguished from the sugars by
being precipitated from their solutions by the addition of
alcohol. They are not oxidised when boiled with caustic
potash, nor do they change to alcohol and carbon dioxide
under the influence of yeast. In cold neutral or acid solu-

3i8 HUMAN PHYSIOLOGY

tions most of them strike a blue or brown colour with iodine.
By boiling with a mineral acid and by the influence of various
ferments they break down, take up water, and become mono-
saccharids — the starches yielding the dextroses, and inulins
yielding Isevulose.

Glycogen is animal starch. It gives an opalescent solution
and strikes a brown with iodine.

The energy value of the carbohydrates is about the same
as that of the proteids.

Energy Yalue of the Proximate Principles of the Food.—
1 gramme of —

Proteid yields . . . .4-1 Calories.
Carbohydrate yields . . .4-1
Fat yields .... 9-3 „

The Sources of the Various Proximate Principles of
the Food.

The proximate principles are in part derived from the
animal, in part from the vegetable kingdoms. While some
races procure their food entirely, or almost entirely, from the
former, others depend almost entirely on the latter. The
vast majority of mankind, however, use a mixture of animal
and vegetable foods.

Animal foods may be classified as—

1. Milk and its Products, Cream, Butter, and Cheese.

2. Flesh.

3. Eggs.

1. Milk. — The characters of human milk are considered at
p. 411. Cow's milk is an important constituent of the diet.
Its average composition compared with human milk is as
follows : —

Cow's.

Human.

Water ....

88-3

88-8

Proteids ...

3-0

1-0

Fats

3'5

3'5

Carbohydrates .

4-5 6'5

Salts

0-7

0-2

THE FOOD AND DIGESTION 319

The chief proteid of milk is caseinogen, a nucleo-proteid
with a very small amount of phosphorus, which exists as a
soluble calcic compound. It is held in solution in milk, but
under the influence of various agents it clots or curds. It is
split by the action of acids, and the casein is precipitated.
Under the influence of rennet it is also split into whey
albumin which remains in solution, and calcic paracasein
which is insoluble. In cow's milk a small amount of an
albumin is also present.

The fats of milk occur as small globules of varying size
floating in the fluid, each surrounded by a proteid envelope
which must be removed by means of an alkali or an acid
before the fat can be extracted with ether. The fats are
chiefly olein with smaller quantities of palmatin and stearin,
and still smaller amounts of such lower fats as butyrin,
capronin, and caprylin.

The carbohydrate of milk is lactose, a disaccharid, which
splits into dextrose and galactose.

The ash of milk is rich in phosphoric acid, calcium, and
potassium — poor in sodium and iron.

Butter and Cream are simply the fats of the milk more or
less completely separated from the other constituents.

Cheese is produced by causing the coagulation of the
casein, which carries with it a large amount of the fats. If
cheese is made before the removal of the cream it is rich in
fats, if after the removal of the cream it is poor in fats.
Cheese contains between 25 and 30 per cent, of proteid, and
between 10 and 30 per cent, of fat. It is as a source of
proteid that it is of chief value.

Cheese, when allowed to stand, affords a suitable nidus
for the growth of micro-organisms by the action of which
the proteids are digested into peptones and simpler bodies,
and the fats split up into glycerine and the lower fatty
acids. These free fatty acids give the peculiar flavour to
ripe cheese. The lactose is in part converted into lactic
acid.

2. Flesh. — Under this head may be included not only the
muscles of various animals, but also such cellular organs as
the liver and kidneys.

When free of fat they contain about 20 per cent, of pro-

320 HUMAN PHYSIOLOGY

teids. The amount of fat may vary from almost nil in white
fish to about 80 per cent, in fat bacon.

Flesh is thus a source of proteids and albuminoids, and to
a smaller extent of fats. In animals specially fed the amount
of fat may be enormously increased, and ordinary butchers'
meat may have more fat than proteid. The extractives
include such bodies as creatin, xanthin, inosit, &c. (see p. 43),
which give the peculiar flavour to the flesh of various animals.
Flesh may be preserved in various ways — e.g. by simply
drying, by salting, or by smoking. The result of each of
these procedures is to dimmish the amount of water, and
thus to increase the solids.

3. Eggs. — The egg of the domestic fowl need alone be
considered. The composition of the white and of the yolk
naturally differs considerably. The white of egg is nothing
more than a solution of proteids.

In the yolk there is a very large amount of lecithin (p. 78)
along with ordinary fats, and a large amount of a phospho-
proteid, and the great value of eggs is thus that they con-
tain both proteids, ordinary fats, and this special fat. The
whole egg contains a little more than 10 per cent, each
of proteids and of fats.

Speaking generally, we may say that the animal food-
stuff's are rich in proteids and fats, but are poor in carbo-
hydrates.

Vegetable Food-stuffs. — In the food of man vegetables
play as important a part as animal products.

The peculiarity of special importance in vegetables is the
existence of a capsule to the cells, composed of cellulose— a
substance allied to starch in its composition, but which is
very resistant to the action of the human digestive juices,
and thus hinders the utilisation of the cell contents. In
order that these may be digested and absorbed from the
stomach and intestine, this capsule must be broken down
either by some preparatory treatment, or by the teeth in the
act of chewing. Although of practically no value as a food-
stuff, it acts as a natural purgative by stimulating the intes-
tines, and is of great value in keeping up the regular action
of the bowels.

THE FOOD AND DIGESTION 321

I. Cereals. — From the seeds of these, meals and flours are
prepared.

Oatmeal contains about 15 per cent, of proteids, about
6 per cent, of fats, and about 65 per cent, of carbohydrates.

Wheaten flour contains about 10 per cent, of proteid, 1 per
cent, of fat, and 75 per cent, of carbohydrates.

Ordinary white bread, prepared from wheaten flour, con-
tains only about 7 per cent, of proteids and 55 per cent, of
carbohydrates.

The proteids of these cereals are mixtures of various
albumins and globulins which do not differ in their char-
acters from the animal proteids. They are most abundant
in the outer part of the grain, just under the capsule.

While oatmeal and maize contain a fair proportion of fat,
the other cereals are poor in this constituent.

The chief constituents of all these seeds are the carbo-
hydrates stored as starch.

II. Legumens. — The seeds of the leguminosse — peas, beans,
and lentils — are valuable constituents of the diet, being speci-
ally rich in proteids. In the dry state they contain some-
thing over 20 per cent, of proteid and about 50 per cent, of
carbohydrates.

The proteids are a mixture of albumin and globulin, which
have been classified together as legumin.

The fats are small in amount, and the carbohydrates,
though abundant, are less so than in the cereals.

III. Bulbous Plants. — The underground stems of certain
plants develop tuberous growths in which material for the
nourishment of the plant is stored. These plants belong to
different natural orders, but they may here be classified to-
gether. The most commonly employed are potatoes, turnips,
and carrots. The amount of proteid is small, something
under 2 per cent., while the carbohydrates in the potato reach
20 per cent., but in other tubers only about '10 per cent.

Such tubers are chiefly valuable as a source of carbo-
hydrates— though they also contain a small proportion of
proteids.

IY. Green Vegetables. — Cabbage, cauliflower, spinach,
lettuce, &c., are useful additions to the diet, but their
value as a source of the proximate principle of the

21

322 HUMAN PHYSIOLOGY

food is not great on account of the amount of water they
contain.

They are rich in the potash salts of the organic acids, the
importance of which has already been discussed (p. 313).
The cellulose forming the walls of the cells in the young
and growing part of the leaves seems to be capable of partial
digestion in the human intestine.

Y. Fungi. — Mushrooms and other such fungi do not con-
stitute a sufficiently important part of the diet to require
attention.

YI. Fruits. — These vary considerably in composition.
Most are rich in water and carbohydrates, poor in proteids,
and contain practically no fat. Their great value lies in the
amount of free and combined organic acids they contain.
Bananas, which contain about 23 per cent, of soluble carbo-
hydrates, may be considered as a food of some import-
ance, while dried fruits, especially dates and figs, which
contain between 4 and 5 per cent, of proteids and about
65 per cent, of carbohydrates, are of considerable nutritive
value.

YII. Nuts. — These, unlike the fruits, are for the most part
poor in water and carbohydrates but rich in proteids and
fats. Chestnuts, however, contain an abundance of carbo-
hydrates, and a smaller proportion of proteids and fats.

Cooking. — Few of these food-stuffs are used by civilised
man in a raw and unprepared condition. With the object
of rendering them more palatable and more easily digested,
and also in order to destroy bacteria, they are usually cooked.
This process of cooking produces important changes in many
of the foods, and its effects must be briefly considered.

Milk and its products are practically unaltered by cooking.

Flesh is cooked either by exposing it directly to heat, or
by treating it with boiling water.

Roasting, grilling, broiling, and frying are modifications of
the former method, and in all of them the heat at once
coagulates the proteids at the outer part of the piece of flesh,
and thus forms a more or less impermeable covering, which
prevents the escape of the juices of the meat. Hence these
methods of cooking, although bringing about a burning of

THE FOOD AND DIGESTION 323

the outer layer of the flesh, leave all the constituents in only
slightly altered proportions.

On the other hand, if a piece of flesh be put into cold
water and boiled, the proteids, the salts, and the various
extractive bodies which give it its flavour are extracted, and,
as the water warms, the fats also are dissolved out, and the
meat becomes poorer in these constituents, while the sur-
rounding water becomes a soup. In this soup the dissolved
proteids precipitate as the temperature rises, and when the
soup is cooled they rise to the top with the fats, and are
generally removed as a scum. Hence soups are poor in the
proximate principles of food, but rich in the extractives and
salts of meat.

If, however, the piece of meat to be cooked is plunged into
a large quantity of boiling water, the proteids at its outer
part at once coagulate and form a covering which prevents
the loss of the nutrient material which occurs when the meat
is slowly boiled.

Stewing is a modification of boiling by which much of the
nutrient material of meat is extracted, but this is served as a
gravy.

It is in vegetables, however, that cooking is of the greatest
importance, since by it the cellulose envelopes which enclose
the digestible portions of the plant are ruptured.

II. DIGESTION
I. Structure of Alimentary Canal

THE anatomy and histology of the alimentary tract must
be studied practically. We shall here merely give such an
outline of the various structures as will assist in the com-
prehension of their physiology.

The Alimentary Canal (Fig. 144) may be divided into the
mouth, the oesophagus or gullet, the stomach, the small and
large intestines, and three sets of supplementary structures —
the salivary glands, the liver, and the pancreas.

The Mouth, provided with its teeth, and surrounded by
its mobile muscular wall, with the muscular tongue lying in
its floor, is the part of the canal in which the food is broken
up and prepared for digestion. Into the mouth three pairs
of compound glands — the Salivary Glands — open. The
parotid, lined entirely by enzyme -secreting epithelium,
opens on the side of the cheek, while the submaxillary
gland, composed partly of acini with enzyme-secreting, and
partly of acini with mucin-secreting epithelium, and the sub-
lingual, composed entirely of mucin-secreting acini, open
under the tongue (S.C.).

The tongue is covered with a fine fur of processes, the fili-
form papilla?, which are of use in passing the food backwards
along its surface in the act of swallowing. (For Organs of
Taste see p. 137.)

Posteriorly, the mouth opens into the pharynx (Ph.) or
upper part of the gullet. On each side, between the mouth
and the pharynx, is the tonsil (T.), an almond-like mass of
lymphoid tissue. The pharynx is a cavity which can be shut
off above from the posterior nares by raising the soft palate,
and by pulling forward the posterior pharyngeal wall. It is
surrounded by three constrictor muscles, which, by contracting

324

THE FOOD AND DIGESTION

325

from above downwards, force the food down the gullet towards
the stomach.

The (Esophagus (Oe.) is a muscular walled tube lined by a
stratified squamous epithelium.
The muscles, below the lowest
constrictor of the pharynx, are
of the visceral type, and are
arranged in two layers, an
outer longitudinal layer, and
an inner circular layer.

The Stomach is a dilatation
of the alimentary canal into
which the gullet opens. To
the left it expands into a sac-
like cardiac end (C.), and to
the right it narrows, forming
the pyloric end (Py.). Like
the gullet, it is surrounded
by visceral muscular fibres,
arranged essentially in two
sets. At the cardiac orifice,
the circular fibres form a not
very marked cardiac sphincter,
and at the pyloric end they
form a very thick and strong
pyloric sphincter.

The mucous membrane,

Which is Covered by a Colum- FIG 144 -Diagram of the Parts of

•/ the Alimentary Canal, from Mouth

nar epithelium, is largely com-
posed of tubular glands, those
at the cardiac end containing
two kinds of cells, the peptic
and the oxyntic cells, those
at the pyloric end containing
peptic cells alone.

The Small Intestine is a tube of about 7 metres in length.
It has a double muscular coat like the stomach. The mucous
membrane, which is covered by a columnar epithelium, is
thickly set with simple test-tube like glands — Lieberkuhn's
follicles — and is projected into the lumen of the tube, as a

to Anus. T., Tonsils; Ph.,
Pharynx; S.C., Salivary Glands;
Oe., (Esophagus; C., Cardiac;
Py-, Pyloric Portion of Stomach ;
D. , Duodenum ; Li. , Liver ; P. ,
Pancreas ; J. , Jejunum; /., Ileum ;
V., Vermiform Appendix; Col.,
Colon ; It., Rectum.

326 HUMAN PHYSIOLOGY

series of delicate finger-like processes, the villi. The tissue
of the villi and that between the Lieberkiihn's follicles is
chiefly lymphoid, and in certain places this lymphoid tissue
is massed in nodules which are either placed singly or grouped
together in the lower part of the small intestine to form
Peyer's patches. In the first part of the small intestine —
the upper part of the duodenum (D.) — the submucous layer
is full of small branching glands lined by an enzyme-secreting
epithelium (Brunner's glands).

The Large Intestine is about 2 metres in length. The
small intestine enters it at one side, and the opening is
guarded by a fold of mucous membrane which forms the
ileo-csecal valve. Above the opening of the small intestine
a csecal pouch exists, and at the top of this is the vermiform
appendix ( F.), a narrow tube with an abundance of lymph
tissue in its wall. Below the opening of the small intestine
is the colon (Col.\ which, after passing up the right side
across the abdomen and down the left side, takes an S-like
bend to end in the rectum (R.), which, passing forward,
suddenly turns down and opens at the anus. The sudden
bend is of importance in retaining the contents of the rec-
tum. The last part of the rectum is surrounded by a strong
band of muscle — the internal sphincter ani — by which it
is compressed. The whole large intestine is covered by
columnar epithelium, and is studded with Lieberkiihn's
follicles, in which the epithelium is chiefly mucus-secreting
in type. There are no villi. The muscular coat of the colon
differs from that of the rest of the alimentary canal, in that
the longitudinal fibres are arranged in three bands.

Into the duodenum, the bile duct and the duct of the
pancreas open. The bile duct is formed by the union of
the ducts from the lobes of the liver. Upon its course is a
diver ticulum, the gall bladder. The Liver (Li.) is a large
solid-looking organ, formed originally as a double outgrowth
from the alimentary canal. These outgrowths branch, and
again branch, and between them the blood coming from the
mother to the foetus flows in a number of capillary channels.
Later, when the alimentary canal has developed, the blood
from it is streamed between the liver tubules. In man and
other mammals, the fibrous tissue supporting the liver cuts

THE FOOD AND DIGESTION

327

it up into a number of small divisions, the lobules, each
lobule being composed of a series of tubules arranged radially
with blood vessels coursing between them.

The portal vein which takes blood from the stomach, intes-
tine, pancreas, and spleen breaks up in the liver (Fig. 105,
p. 209), and carries the blood between the lobules. From the
interlobular branches, capillaries run inwards and enter a
central vein which carries the blood from each lobule, and
pours it into the hepatic veins which join the inferior vena
cava. The supporting tissue of the liver is supplied by the
hepatic artery, -and the ter-
minal branches have a very
free communication with
those of the portal vein.

The Pancreas is essenti-
ally the same in structure as
the parotid gland. But in
the lobules are certain little
masses of epithelium - like
cells closely packed to-
gether, the Islets of Lan-
gerhans (Fig. 145).

The Nerve Supply of the
alimentary canal. The
muscles round the mouth
are supplied by the fifth,
seventh, and twelfth cranial

nerves. The nerve Supply FiG.145. -Section of Pancreas to show Acini

-ri of Secreting Cells ; a large duct ; and m
Ot the Salivary glands Will the centre an Island of Langerhans.

be considered later. The

pharyngeal muscles are supplied by the ninth and tenth

cranial nerves, and the oesophagus is supplied by the tenth.

The stomach and intestine get their nerve fibres from two
sources (Fig. 126, p. 263) — above the descending colon from the
vagus and the abdominal sympathetic, and below this from
the nervi erigentes and abdominal sympathetic — the various
fibres passing through the abdominal sympathetic ganglia.
In the wall of the stomach and intestine, these nerves end by
forming an interlacing set of fibres, with nerve cells upon
them, from which fibres pass to the muscles and glands.

328 , HUMAN PHYSIOLOGY

One of these plexuses (Auerbach's) is placed between the
muscular coats — the other (Meissner's) is placed in the
submucosa.

II. Physiology.
I. Digestion in the Mouth.

A. Mastication. — In the inouth, by the act of chewing,
the food is thoroughly broken up and mixed with saliva.

The muscular mechanism of mastication may be here
briefly indicated.

MOVEMENTS OF MASTICATION. — Movements of Lower Jaw
in — 1. Vertical Plane, a. Elevation, a. Temporal. @.
Masseter. 7. Internal Pterygoid. b. Depression, a. Weight
of Jaw. $. Anterior Belly of Digastric. 7. Mylo- and
Genio-Hyoid. B. Platysma. Hyoid fixed by — Omo- and
Sternohyoid, Sternothyroid, and Thyrohyoid. 2. Horizontal
Plane — Forwards: External Pterygoids. Backwards: Pos-
terior Fibres of Temporal. To Right: Left External Ptery-
goid. Right Temporal (Posterior Fibres). Other Muscles of
Mastication — Buccinator and Orbicularis oris.

B. Saliva. — The saliva is formed by the salivary glands
(viz., the parotid, submaxillary, sublingual, and various
small glands in the mucous membrane of the mouth).

Characters. — It is a somewhat turbid frothy fluid which,
when allowed to stand, throws down a white deposit consist-
ing of shed epithelial scales from the mouth, leucocytes,
amorphous phosphates of lime and magnesia, and generally
numerous bacteria. Its specific gravity is low — generally
about 1003. In reaction it is neutral or faintly alkaline.

Chemically it is found to contain a very small proportion
of solids. The saliva from the parotid gland contains only
about 0'4 per cent., while that from the sublingual may
contain from 2 to 3 per cent. The sublingual and sub-
maxillary saliva in man is viscous, from the presence of mucin
formed in these glands, while the parotid saliva is free from
mucin. In addition to mucin, traces of proteids are readily
demonstrated, and with these proteids is associated the active
constituent or enzyme of the saliva — ptyalin.

Saliva generally contains traces of sulphocyanide of potas-

THE FOOD AND DIGESTION 329

slum, and in some pathological conditions this may be
markedly increased.

The functions of the saliva are twofold : —

1. Mechanical, to moisten the mouth and gullet, and thus
to assist in speaking, chewing and swallowing. Since the
salivary glands are absent from aquatic mammals, it would
appear that this is the more important function.

2. Chemical — Under the action of the ptyalin of the
saliva, polysaccharids, like the starches, are broken down into
sugars. Like other enzyme actions the process requires the
presence of water and a suitable temperature, and it is
stopped by the presence of strong acids or alkalies, by various
chemical substances, and by a temperature of over 60° C.,
while it is temporarily inhibited by reducing the temperature
to near the freezing point. During the short time the saliva
acts on the food the conversion is by no means complete.
The starch is first changed into the dextrins, giving a brown
colour with iodine, and hence called erythrodextrins, then into
dextrins which give no colour with iodine, achroodextrins,
and lastly into the disaccharid maltose (see p. 317). (Chemical
Physiology, p. 18.)

Physiology of Salivary Secretion. — In order to study
the physiology of salivary secretion, a canula may be inserted
into the duct of any of the salivary glands and the flow of
saliva or pressure of secretion may be thus measured. In this
way it may be shown that the taking of food, or simply the
act of chewing, and in some cases the mere sight of food,
causes a flow of saliva. This shows that the process of
secretion is presided over by the central nervous system.

The submaxillary and sublingual are supplied — (1) By
branches from the lingual division of the fifth cranial nerve ;
and (2) by branches of the perivascular sympathetic fibres
coming from the superior cervical ganglion. The parotid
gland is supplied by the auriculo-temporal division of the
fifth and also by sympathetic fibres (Fig. 146).

The influence of these nerves has been chiefly studied on
the submaxillary and sublingual glands.

It has been found that when the lingual nerve is cut the
reflex secretion of saliva still takes place, but that, when the
chorda tympani (Ch. T.), a branch from the seventh nerve

330 HUMAN PHYSIOLOGY

which joins the lingual, is cut, the reflex secretion does not
occur. Stimulation of the chorda tympani causes a copious
flow of watery saliva, and a dilatation of the blood vessels of
the glands. If atropine has been first administered the
dilatation of the vessels occurs without the flow of saliva,
indicating that the two processes are independent of one
another. The secreting fibres all undergo interruption before

FIG. 146.— Nervous Supply of the Salivary Glands. Par., Parotid, and S.M. and
S.L., the Submaxillary and Sublingual Glands; VII. , the Seventh Cranial
Nerve, with Ch. T., the Chorda Tympani Nerve, passing to L.t the Lingual
Branch of V., the Fifth Nerve, to supply the Glands below the Tongue, T. ;
IX., the Glossopharnygeal giving off J.N., Jacobson's Nerve, to the O.,
Otic Ganglion, to supply the Parotid Gland through Aur. T., the Auriculo-
temporal Nerve.

the glands are reached; the fibres to the sublingual gland
having their cell station in the submaxillary ganglion (S.M.G.),
the fibres to the submaxillary gland having theirs in a little
ganglion at the hilus of the gland (S.M.). This was demon-
strated by painting the two ganglia with nicotine. When
applied to the submaxillary ganglion it does not interfere
with the passage of impulses to the submaxillary gland, but
stops those going to the sublingual.

If, when the chorda tympani is stimulated, the duct of the

THE FOOD AND DIGESTION 331

gland is connected with a mercurial manometer, it is found
that the pressure of secretion may exceed the blood pressure
in the carotid.

When the perivascular sympathetics, or when the sym-
pathetic cord of the neck is stimulated, the blood vessels of
the gland constrict, and a flow of very viscous saliva takes
place.

On the parotid gland the auriculo-temporal nerve (Aur. T.)
acts in the same way as the chorda tympani acts on the
other salivary glands. But stimulation of the fifth nerve
above the otic ganglion, from which the auriculo-temporal
takes origin, fails to produce any effect. On the other hand,
stimulation of the glossopharyngeal nerve (IX.) as it comes
off from the brain, acts upon the parotid gland, and since the
glossopharyngeal is united to the small superficial petrosal
which passes to the otic ganglion, by Jacobson's nerve (J.N.),
it is obvious that these parotid fibres take this somewhat
roundabout course. When the sympathetic fibres to the
gland alone are stimulated, constriction of the blood vessels
but no flow of saliva occurs, but if, when the flow of watery
saliva is being produced by stimulating the glossopharyngeal
or Jacobson's nerve, the sympathetic fibres are stimulated,
the amount of organic solids in the parotid saliva is very
markedly increased.

The nerve fibres passing to the salivary glands are pre-
sided over by a centre in the medulla oblongata which acts
reflexly. So long as this is intact, stimulation of the lingual
or glossopharyngeal leads to a reflex flow of saliva. Other
nerves may also act on this centre. Thus, gastric irritation,
when it produces vomiting, causes a reflex stimulation of
salivary secretion.

II. Swallowing.

The food after being masticated is collected on the surface
of the tongue by the voluntary action of the buccinators and
other muscles, and then, the point of the tongue being
pressed against the hard palate behind the teeth, by a
contraction passing from before backwards, the bolus of
food is driven to the back of the tongue. When this is

332

HUMAN PHYSIOLOGY

. TIE7H

reached the act becomes involuntary and reflex, and the
food is forced through the pillars of the fauces into the
pharynx. It is prevented from passing up into the posterior
nares by the contraction of the palato-pharyngeus muscle,
and of the levator palati. The larynx as a whole is pulled
upwards by the stylo-hyoid and stylo-thyroid and the thyro-

hyoid, and its entrance
into the larynx is pre-
vented by the closure of
upper part of aperture.
The arytenoid carti-
lages are pulled forward
by the thyro-arytenoid
muscles, and approxi-
mated by the aryte-
noidei, while a cushion
on the posterior surface
of the epiglottis be-
comes applied to their
tips, forming a tri-
radiate fissure or chink
through which food
cannot pass. The lateral
crico - arytenoids also
approximate the vocal
cords, and close the
glottis (Fig. 147).

The constrictors of
the pharynx contract
from above downwards,
and force the food into the grasp of the oesophagus, and this
by a slow peristaltic contraction sends the food onwards to
the stomach. Under normal conditions this peristalsis is not
essential. It is abolished by section of the vagi. In swallow-
ing liquids, the peristalsis of the oesophagus is not brought
into play, but the fluid is forced by the tongue down the
relaxed oesophagus into the stomach. The passage of the
food down the gullet may be heard by applying a stetho-
scope to the right side of the spinal column, and any delay
caused by a stricture may thus be determined.

FIG. 147.— Vertical Mesial Section of Mouth and
Pharynx to show how, in swallowing, the food
slips along the back of the Epiglottis.

THE FOOD AND DIGESTION 333

In swallowing fluids two sounds are heard, one immediately,
and one after about six seconds.

III. Digestion in the Stomach.

Various opportunities have occurred and have been taken
advantage of to study the interior of the human stomach
during life. The best known investigation of the kind was
undertaken by Dr. Beaumont, a Canadian physician, on the
person of St. Martin, a backwoodsman, who had received a
gunshot wound in the abdomen, which had left him with
an opening through the front wall of his stomach. Dr.
Beaumont engaged St. Martin as his servant, and made a
prolonged and valuable study of the changes which take
place in the viscus.

He found that the condition varies greatly in fasting and
after feeding.

A. Stomach during Fasting.

The organ is collapsed, and the mucous membrane is
thrown into large ridges. It is pale in colour because the
blood vessels are not dilated. Movements are not marked
and the secretion is scanty, only a little mucus being formed
on the surface of the lining membrane.

B. Stomach after Feeding.

When food is taken, the blood vessels dilate, a secretion is
poured out, and movements of the organ become more
marked.

1. Vascular Changes. — The arterioles dilate, and the
mucous membrane becomes bright red in colour. This is
a reflex vaso-dilator effect, impulses, passing up the vagus
to the vaso-dilator centre in the medulla, and coming down
the vagus from that centre. Section of the vagi prevents
its onset.

2. Secretion. — (a) Characters of Gastric Juice. — Very
rapidly a free flow of gastric juice occurs from all the glands

334 HUMAN PHYSIOLOGY

in the mucous membrane. The gastric juice is a clear
watery fluid, which is markedly acid from the presence of
free hydrochloric acid. In the dog the free acid may
amount to O2 per cent., but in man it is less abundant,
and when the gastric juice is mixed with food the acid
rapidly combines with alkalies and with proteids, and is no
longer free. In addition to the HC1 small quantities of
organic salts are present. Traces of proteids may also be
demonstrated, and with these two enzymes are associated-
one a proteolytic or proteid-digesting enzyme, pepsin, the
other a milk-curdling enzyme, rennin.

(b) Course of Gastric Digestion — (1) Amylolytic Period.—
The action of the gastric juice does not at once become
manifest. For half-an-hour after the food is swallowed the
ptyalin of the saliva goes on acting, and the various micro-
organisms swallowed with the food grow and multiply, and
thus there is a continuance of the conversion of starch to
sugar which was started in the mouth, and at the same time
the micro-organisms go on splitting the sugar to form lactic
acid, which may thus be regarded as a normal constituent
of the stomach during the first half-hour after a mixed
meal.

(2) Proteolytic Period. — Before the amylolytic period is
completed, the gastric juice has begun its special action on
proteids. This may be readily studied by placing some
coagulated proteid in gastric juice, or in an extract of the
mucous membrane of the stomach made with dilute hydro-
chloric acid, and keeping it at the temperature of the body.
The proteid swells, becomes transparent, and dissolves. The
solution is coagulated on boiling — a soluble native proteid
has been formed. Very soon it is found that, if the soluble
native proteid is filtered off, the filtrate gives a precipitate
on neutralising, showing that an acid proteate has been
produced. If the action is allowed to continue and the acid
proteate precipitated and filtered off, it will be found that
the filtrate gives a precipitate on saturating with common
salt, showing that a proto-proteose has been formed. Along
with this a certain amount of hetero-proteose is also formed.

THE FOOD AND DIGESTION 335

It is characterised by being precipitated on neutralisation
and by being insoluble in distilled water. On filtering off
these, the filtrate yields a precipitate on saturating with
sulphate of ammonia, indicating the formation of a deutero-
proteose, and, if the filtrate from this be tested, the presence
of a proteid may be demonstrated. Peptone has been
produced. (Chemical Physiology, p. 18.)

These changes may be represented in the following
table :—

Coagulated Proteid.

Soluble Native Proteid.
Acid Proteate.

| I

Proto-proteose. Hetero-proteose.

Deutero-proteose. Deutero-proteose.

Peptone. Peptone.

The process is one of breaking down a complex molecule
into simpler molecules, probably with hydration.

The object of this was formerly supposed to be to allow of
the diffusion of the proteid in the form of peptone through
the wall of the intestine. It is now known that absorption
is not due to diffusion, and it is more probable that the
change to the simplest proteid molecule is a necessary step
in the building up of the proteid into the special protoplasm
of the body of the particular animal.

On certain proteids and their derivatives the gastric juice
has a special action. On collagen the HC1 acts slightly in
converting it to gelatin. On gelatin the gastric juice acts,
converting it to a gelatin peptone.

On nucleo-proteids it acts by digesting the proteid part
and leaving the muclein undissolved.

Haemoglobin is broken down into hsematin and globin, and
the latter is changed into peptone.

The casein of milk is first coagulated and then changed to
peptone. The coagulation is brought about by the presence
of the second enzyme of the gastric juice — rennin.

336 HUMAN PHYSIOLOGY

This may be separated from pepsin in various ways, and
unlike pepsin it acts in a neutral medium.

The change set up by it seems to be due to a splitting
of the soluble lime salt of casein which exists in milk
into calcic paracasein, which is insoluble and is thrown
down, while a small quantity of whey albumin remains
in solution. The nuclein part of the paracasein remains
undigested.

On Fats the gastric juice has no action, but, when these
are contained in the protoplasm of cells, it sets them free by
digesting the proteid covering.

On Carbohydrates the free mineral acid of the gastric
juice has a slight action at the body temperature, splitting
the polysaccharids and disaccharids into monosaccharids.

(c) Digestion of the Stomach Wall.— When the wall of
the stomach dies either in whole, as after the death of the
animal, or in part, as when an artery is occluded or ligatured,
the dead part is digested by the gastric juice and the wall of
the stomach may be perforated.

(d) Antiseptic Action of the Gastric Juice. — In virtue of
the presence of free HC1 the gastric juice has a marked action
in inhibiting the growth of or in killing bacteria. The bacillus
of cholera is peculiarly susceptible, and a healthy condition of
the stomach is thus a great safeguard against the disease.
Other organisms, while they do not multiply in the stomach,
pass on alive to the intestine where they may again become
active. When HC1 is not formed in sufficient quantities to
exist free in the stomach, the activity of these bacteria
may lead to various decompositions and to many of the
symptoms of dyspepsia.

(e) Source of the Constituents of the Gastric Juice.—

The hydrochloric acid is formed at the cardiac end of the
stomach. This may be shown by isolating a part of the
stomach so that it opens on the surface. Since the parietal
or oxyntic cells are confined to this portion of the stomach, it
may be concluded that they are the producers of the acid.
They manufacture it from the NaCl of the blood plasma.

THE FOOD AND DIGESTION

337

Probably the C02 liberated in the cells seizes on some of the
Na and turns out HC1.

The Pepsin and Rennin are produced in the chief or peptic
cells which line the glands both of the cardiac and pyloric
parts of the stomach. During fasting granules, are seen to
accumulate in these cells, and when the stomach is active
they are discharged. These granules are not pepsin but the
forerunner of pepsin — pepsinogen.

(/) Influence of Various Diets upon the Gastric Juice. —
This has been chiefly worked out by Pawlow on dogs. By a

FIG. 148. — Diagram of Pawlow's Pouch made on the Stomach of a Dog.

longitudinal incision along the great curvature of the stomach
he separated a V-shaped piece (Fig. 148). The cut edges of
the stomach are then stitched up and the sides of the
V-shaped portion are also united, except that at the apex they
are attached to the skin surface. The mucous membrane
between the stomach cavity and the " pouch " formed is then
united, and a small stomach is thus produced, still connected
with the nerves and vessels, but separated from the main
cavity and pouring its contents on to the surface. Any
modification in the secretion of the stomach is indicated by
a modification in the secretion from this pouch.

22

338 HUMAN PHYSIOLOGY

It has thus been found that — (1) The amount of secretion
depends upon the amount of food taken. (2) The amount
and course of secretion varies with the kind of food taken.
Thus, with flesh the secretion reaches its maximum at the end
of one hour, persists for an hour and then rapidly falls, while
with bread it reaches its maximum at the end of one hour,
rapidly falls but persists for a much longer period than in
the case of flesh. (3) The digestive activity of the juices
varies with the kind of food and with the course of diges-
tion. It is higher and persists longer after a diet of bread,
which is difficult to digest, than after a diet of flesh, which is
more easily digested. (4) The percentage of acid does not
vary markedly. When more acid is required more gastric
juice is secreted. (5) The work done by the gastric glands is
greater in the digestion of bread than in the digestion of
flesh.

(g) Nervous Mechanism of Gastric Secretion. — It has
been proved that in the dog the secretion of gastric juice can
go on after the nerves to the stomach have been divided, and
this has been ascribed to a reflex stimulation of the nerve
plexus in the submucosa. But while this is the case, the
vagus also exercises a direct influence. This was proved by
experiments on dogs in which Pawlow's pouch had been
made, and in which the oesophagus was opened in the
neck, so that when food was taken it did not pass into the
stomach. It was found that letting the dog swallow food,
which of course escaped by the opening in the oesophagus,
or merely showing food to the dog, caused a secretion of
gastric juice if the vagi were intact, but not after they were
divided.

3. Movements of the Stomach. — These have been studied
by feeding an animal with food containing bismuth, and then
applying X rays, which are intercepted by the coating of
bismuth, so that a shadow picture of the shape of the
stomach is given (Fig. 149).

It is found that, soon after food is taken, a constriction
forms about the middle of the stomach and slowly passes on
towards the pylorus. Another constriction forms and follows
the first, and thus the pyloric part of the stomach is set into

THE FOOD AND DIGESTION 339

active peristalsis. The cardiac fundus acts as a reservoir, and,
by a steady contraction, presses the gastric contents into the
more active pylorus, so that at the end of gastric digestion it
is completely emptied.

The pylorus is closed by the strong sphincter muscle,
which, however, relaxes from time to time during gastric
digestion to allow the escape of the more fluid contents of
the stomach into the intestine. These openings are at first
slight and transitory, but as time goes on they become more
marked and more frequent, and when gastric digestion is
complete — usually at the end of five or six hours — the

FIG. 149. — Tracings of the shadows of the contents of the stomach and intestine of a
cat two hours after feeding (A) with boiled lean beef, and (B) with boiled rice.
The small divisions of the food in some of the intestinal loops represent the
process of rhythmic segmentation (see p. 354). (CANNON.)

sphincter is completely relaxed and allows the stomach
to be emptied. The rate of passage from the stomach of
various kinds of food — proteid, carbohydrate, and fat — has
been studied by feeding cats with equal amounts of each
special food mixed with bismuth, and then, by X rays,
getting the outline of the contents of the small intestine
at different periods. Carbohydrates were found toipass on
most rapidly and fats most slowly.

Nervous Mechanism of the Movements. — Even after the
section of all the gastric nerves, movements of the stomach
may be observed, but the mechanism of these has not been
fully studied. The action of the vagus and sympathetic
fibres is complicated, and their influence on the wall of the
stomach and the sphincters requires further investigation.

340 HUMAN PHYSIOLOGY

Speaking generally, the vagus seems to increase the move-
ments, while the sympathetic fibres check them ; but the
vagus seems to inhibit the cardiac sphincter.

Absorption from the Stomach.

The stomach appears to play a small part in the absorp-
tion of food. Very little water is absorbed, fats are not
absorbed, but sugar and peptones seem to be absorbed to a
considerable extent.

Importance of the Stomach in Digestion.

The chief function of the stomach is as a reservoir for the
food. While it plays a certain part in digestion its action
is by no means indispensable, for it has been removed in
animals and in men without disturbance of the health. Pro-
bably the antiseptic action of its secretion is of very consider-
able practical importance.

Vomiting.

Sometimes the stomach is emptied upwards through the
gullet instead of downwards through the pylorus. This act
of vomiting is generally a reflex one, resulting from irritation
of the gastric mucous membrane and more rarely from
stimulation of other nerves. Usually the act is preceded by
a feeling of nausea and by a free secretion of saliva. In
vomiting, the glottis is closed, and, after a forced inspiratory
effort by which air is drawn down into the gullet, a forced
and spasmodic expiration presses on the stomach, while at
the same time the cardiac sphincter is relaxed through the
action of the vagus, and the contents of the stomach are
sent upwards. They are at first prevented from passing into
the nares by the contraction of the soft palate ; but, as the
act continues, these muscles are overcome, and the vomited
matter escapes through mouth and nose. The wall of the
stomach also seems to act, but its action is non-essential,
since vomiting may be produced in an animal in which a
bladder has been made to replace the stomach.

THE FOOD AND DIGESTION 341

The centre which presides over the act is in the medulla
oblongata, and while it is usually reflexly called into action,
it may be stimulated directly by such drugs as apomorphine.

IY. Intestinal Digestion

After being subjected to gastric digestion the food is gene-
rally reduced to a semi-fluid grey pultaceous condition of
strongly acid reaction known as chyme, and in this condition
it enters the duodenum.

Here it meets three different secretions : —
Bile.

Pancreatic secretion.
Intestinal secretion.

A. Bile.

i. Characters and Composition. — The bile is the secretion
of the liver, and it may be procured for examination — (a)
From the gall bladder, or (6) from the bile passages by
making a fistula into them. Bile which has been in the
gall bladder is richer in solids than bile taken directly from
the ducts, because water is absorbed by the walls of the
bladder and the bile thus becomes concentrated.

Analyses of gall bladder bile thus give no information as
to the composition of the bile when formed. In several cases,
where surgeons have produced biliary fistulse, opportunities
have occurred of procuring the bile directly from the ducts
during life in man.

Such bile has a somewhat orange-brown colour, and is
more or less viscous, but not nearly so viscous as bile taken
from the gall bladder. It has a specific gravity of almost
1005, while gall bladder bile has a specific gravity of about
1030. Its reaction is slightly alkaline, and it has a charac-
teristic smell.

It contains about 2 per cent, of solids, of which more than
half are organic.

The most abundant solids are the salts of the bile acids.
In man the most important is glycocholate of soda. Tauro-
cholate of soda occurs in small amounts. These salts are
readily separated from an alcoholic solution of dried bile by

342 HUMAN PHYSIOLOGY

the addition of water-free ether, which makes them separate
out as crystals. (Chemical Physiology, p. 21.)

Glycocholic acid splits into glycin, amido-acetic acid —

H 0

H\ 1 »

>N— C— C— O— H

i

and a body of unknown constitution, cholalic acid, C^H^O^
Taurocholic acid yields amido-ethane-sulphuric acid or
taurin. This is a molecule closely resembling amido-acetic
acid linked to sulphuric and cholalic acids.

Since both acids contain nitrogen they must be derived
from proteids. That they are formed in the liver and not
merely excreted by it, is shown by the fact that, while
they accumulate in the blood if the bile-duct is ligatured,
they do not appear if the liver is excluded from the
circulation.

Action of Bile Salts. — 1. The bile salts are solvents of fats
and fatty acids, and they thus assist in the digestion and
absorption of fats. When bile is excluded from the intes-
tines no less than 30 per cent, of the fats of the food may
escape absorption and appear in the faeces. When this is
the case the feces have a characteristic white or grey appear-
ance from the abundance of fat.

2. These salts keep cholesterin in solution.

3. While the salts have no action on proteids, free tauro-
cholic acid precipitates native proteids and acid proteate.
In the human intestine this is an action of no importance.

4. These salts are powerful ha^molytic agents, and rapidly
dissolve haemoglobin out of the erythrocytes.

Bile Pigments. — These amount to only about 0*2 per cent.
In human bile the chief pigment is an orange-brown sub-
stance, bilirubin, C32H36N4O6, while in the bile of herbivora,
biliverdin, a green pigment somewhat more oxidised than
bilirubin, C^H^N^Og, is more abundant. By further oxi-
dation with nitrous acid other pigments — blue, red, and
yellow — are produced, and this is used as a test for the pre-
sence of bile pigments (Gmelin's test). (Chemical Physiology,
p. 21.)

THE FOOD AND DIGESTION 343

The pigments are closely allied to hsematoporphyrin
and hsematoidin, and they are derived from haemoglobin.
Their amount is greatly increased when haemoglobin is
set free or injected into the blood. They are formed
in the liver, since, when the liver is excluded from the
circulation, the injection of haemoglobin does not cause
their formation.

The liver has the property of excreting not only these
pigments formed by itself, but also other pigments. Thus
the liver of the dog can secrete the characteristic pigment of
sheep's bile.

Cholesterin is a monatomic alcohol — C26H43OH — which
occurs free in small amounts in the bile. It is very insoluble
and is kept in solution by the salts of the bile acids. It
readily crystallises in rhombic plates, generally with a notch
out of the corner. On account of its insolubility, when it is
in excess in the bile or when the bile salts are decreased,
it may form concretions or biliary calculi — gall stones —
which may accumulate in the gall bladder and may get
caught in the bile passages, obstructing the flow of bile and
leading to its absorption throughout the system. Jaundice
is thus produced. When these stones are forced along the
bile passages as a result of muscular contraction, intense
agony — biliary colic — may be produced. When they are
passed by the rectum, their nature is readily demonstrated
by breaking them up in a mortar, dissolving in hot alcohol,
and allowing the solution to cool, when the characteristic
crystals separate out. (Chemical Physiology, p. 12.) The
source of the cholesterin of the bile is not definitely known.
It is not an excretion of cholesterin formed elsewhere, because
the injection of cholesterin does not lead to an increase in the
amount in the bile. According to Naunyn's observations
it is most abundant in cases of inflammation of the bile
passages, and he therefore thinks it is formed by the
breaking down of the epithelium lining these ducts.

Fats and Lecithin. — The true fats and the phosphorus con-
taining lecithin are present in small amounts in the bile, and
apparently they are derived from the fats of the liver cells;
and they may be increased in amount by the administration
of fatty food.

344 HUMAN PHYSIOLOGY

Nucleo-proteid and Mucin. — The bile owes its viscosity to
the presence of a mucin-like body, which, however, does not
yield sugar on boiling with an acid and which contains phos-
phorus. It is precipitated by acetic acid, but is soluble in
excess. It is therefore a nucleo-proteid. In some animals
a certain amount of mucin is also present. (Chemical
Physiology, p. 21.)

Inorganic Constituents. — The most abundant salt is phos-
phate of calcium. Phosphate of iron is present in traces.
Carbonate of soda and of calcium and chloride of sodium are
the other chief salts.

2. Flow of Bile. — The bile, when secreted by the liver
cells, may accumulate in the bile passages and gall bladder
to be expelled under the influence of the contraction of the
muscles of the ducts or of the pressure of the abdominal
muscles upon the liver. The flow of bile into the intestines
thus depends upon — 1st, The secretion of bile; 2nd, the
expulsion of bile from the bile passages. It is exceedingly
difficult to separate the action of these two factors. The flow
of bile in the human subject has now been studied in several
cases in which the surgeon has had to make a fistula into
the gall bladder through which all the bile secreted escaped
and could be collected.

The flow of bile begins in intra-uterine life before the
twelfth week, and it continues without intermission through-
out the whole of life, even during very prolonged fasts. The
taking of food increases the flow of bile, and the extent to
which it is increased depends largely on the kind of food
taken. In the dog a proteid meal has the most marked
effect, a fatty meal a less marked effect, and a carbohydrate
meal hardly any effect. The increased flow of bile following
the taking of food does not reach its maximum till six or
nine hours after the food is taken, and some observers have
found that the period of maximum flow is even further pro-
longed. Very often, immediately after food is taken, there
is a markedly increased flow due to the stimulation of the
muscles of the bile passages, but the later increase seems to
be due to a true increased formation of bile. When the indi-
vidual is taking a liberal diet the secretion of bile appears to

THE FOOD AND DIGESTION 345

be greater than when the diet is low. In fever there is a very
marked fall in the secretion, the fluid flowing from a fistula
becoming colourless and almost devoid of bile salts and pig-
ments. Certain drugs markedly modify the formation of
bile — the salts of the bile acids stimulating the liver to form
more solids and to secrete more water, the salicylates acting
in much the same way, and all drugs which cause haemolysis
— i.e. the solution of the pigment of the erythrocytes — pro-
ducing an increased formation of bile pigments.

Influence of Nerves upon the Flow of Bile — (a) Expulsion
of Bile. — There is good evidence that fibres pass to the muscles
of the bile passages and may cause an expulsion of bile by
stimulating them to contract.

(b) Secretion of Bile. — There is no evidence that nerve
fibres act directly upon the secretion of bile. This
appears to be governed by the nature of the material
brought to the liver by the blood and by the activity of
the liver cells.

3. Mode of Formation of Bile. — It has been seen that the
bile salts and pigments are actually formed in the liver cells,
and there is good evidence that the water of the bile is not
a mere transudation but is the product of the living activity
of these cells. The pressure under which bile is secreted
may be determined by fixing a canula in the bile duct or in
a biliary fistula. In man the pressure is as much as 20 to
30 mm. Hg, while the pressure in the portal vein of the dog
is only 7 to 16 mrn. Hg.

4. Nature of Bile. — Bile is not a secretion of any import-
ance in digestion. It has no action on proteids or carbo-
hydrates, and its actions on fats is merely that of a solvent.
Its secretion in relationship to food does not indicate that it
plays an active part in digestion. It is formed during intra-
uterine life and during fasting, and it is produced many
hours after food is taken, when digestive secretions are no
longer of use in the alimentary canal. Digestion can go on
quite well without the presence of bile in the intestine,
except that the fats are not so well absorbed. The composi-
tion of bile strongly suggests that it is a waste product. The

346 HUMAN PHYSIOLOGY

pigment is the result of the decomposition of haemoglobin
and the acids are the result of proteid disintegration.

All these facts seem to indicate that bile is the medium
by which the waste products of hepatic metabolism are
eliminated, just as the waste products of the body generally
are eliminated by the kidneys.

B. Pancreatic Secretion.

The secretion of the pancreas may, in the dog, be procured
by making either a temporary or a permanent fistula. In
the former case the duct is exposed, and a canula fastened
in it ; in the second the duct is made to open on the surface
of the abdomen.

1. Characters and Composition. — When obtained from a
temporary fistula, immediately after the operation, the pan-
creatic juice is a clear, slimy fluid, with a specific gravity
often of 1030 and an alkaline reaction. It contains an
abundance of a native proteid having the characters of a
globulin, and the alkalinity is probably due to carbonate
and alkaline phosphate of soda. From a permanent fistula
a more abundant flow of more watery secretion may be
collected.

2. Action. — Closely associated with the proteids, and pre-
cipitated by alcohol along with them, are the enzymes upon
which the action of the pancreatic juice depends. They are
three in number. (Chemical Physiolgy, p. 20.)

1st. A Proteolytic Enzyme — Trypsin. This, in a weakly
alkaline or neutral fluid, converts native proteids into
peptones, and is capable of still further breaking up these
peptones into simpler non-proteid bodies. It does not cause
solid proteids to swell up but simply erodes them away.

The pancreatic juice brings about this breaking down in
stages. Fibrin and similar bodies first pass into the condi-
tion of soluble native proteids and then into deutero-proteose,
while boiled egg white appears at once to yield deutero-pro-
teose. The deutero-proteose is then changed into peptone,
and part of that peptone may split up still further into a

THE FOOD AND DIGESTION 347

series of bodies which no longer give the biuret test. These
consist chiefly of the component amido - acids, of which
the most important are leucin and tyrosin, and ammonia
compounds (see p. 11).

Tryptophan. — If chlorine water is added to a pancreatic
digestion, which has proceeded for a long time, a rose-red
colour is struck, and the substance yielding this, to which
the name of tryptophan has been given, appears to be amido-
acetic acid linked to skatol (see p. 399).

While trypsin has the power of splitting some of the
peptone in the manner indicated, it has probably little
opportunity of doing so in the intestine, because the proteid
is rapidly absorbed as it is changed to peptone.

On nucleo - proteids the trypsin acts by digesting the
proteid and dissolving the nucleic acid so that it can be
absorbed.

On collagen and elastin the trypsin has little action ; but
on gelatin it acts as upon proteids.

2nd. An Amylolytic Enzyme — Amylopsin. This acts in
the same way as ptyalin, but more powerfully, converting a
certain part of the maltose into dextrose. It acts best in a
faintly acid medium.

3rd. A Fat Splitting Enzyme — Pialyn. This is the most
easily destroyed and the most difficult to separate of the
zymins. It breaks the fats into their component glycerin and
fatty acids. The fatty acid links with the alkalies which are
present to form soaps, and in this form, or dissolved as free
fatty acids in the bile, they are absorbed. But the formation
of soaps also assists the digestion of fats by reducing them
to a state of finely divided particles, an emulsion upon which
the pialyn can act more freely. This process of emulsifica-
tion is assisted by the presence of proteid in the pancreatic
juice and also by the presence of bile.

That these enzymes are independent of one another is
shown by many facts.

1. Amylopsin does not appear till a month after birth.

2. Amylopsin is taken up by dry glycerin while trypsin
is not.

3. Trypsin may be precipitated and separated by shaking
with collodion.

348 HUMAN PHYSIOLOGY

4. Trypsin acts in O'Ol percent, ammonia while amylopsin
does not.

5. The proportion of the zymins varies with the character
of the diet.

This is well shown by experiments carried out in Paw low's
laboratory upon dogs with pancreatic fistuke. The effects of
diets of milk, bread, and flesh were compared, in each case
the amount of the food given containing the same amount
of nitrogen (proteid). The total quantity of ferment unit
is got by multiplying the quantity of the juice in ccrn. by
the strength of the juice. The following table indicates the
results obtained : —

Quantity of Enzyme.
Diet.

Proteolytic.

BOO

Bread, 250 grin. . . ! 1978 1601

Milk, 600 cc 1085 432

Flesh, 100 grm. . . | 1502 648 8000

Amylolytic. Fat Splitting.

Bread contains a proteid difficult of digestion, plenty of starch,
and little fat. Milk contains an easily digested proteid, and
plenty of fat, but no starch; while flesh contains a com-
paratively easily digested proteid, no starch, and little fat.
The first food causes a copious production of trypsin and
amylopsin, and little pialyn. The second causes the produc-
tion of less trypsin, little amylopsin, but most pialyn. The
last causes a moderate production of trypsin, little amy-
lopsin, and a comparatively large amount of pialyn.

As to the mode of production of these enzymes, it is
known that trypsin is not formed as such in the cells, but
that a forerunner of trypsin — trypsinogen — is produced, and
that this changes into trypsin after it is secreted. The in-
testinal secretion contains something which has been termed
enterokinase, which has the power of bringing about this
change, and in all probability the cells lining the ducts of
the pancreas also produce this or a similar substance.

It is doubtful whether the pancreatic secretion contains

THE FOOD AND DIGESTION 349

any true rennin, although it produces a modified clotting of
milk, under certain conditions.

The influence of the pancreas in the general metabolism
will be considered later (p. 390).

3. Physiology of Pancreatic Secretion. — The secretion
of pancreatic juice is not constant, but is induced when
the acid chyme passes into the duodenum. This occurs
even when all the nerves to the intestine have been
cut, and it appears to be due to the formation in the
epithelium lining the intestine, under the influence of an
acid, of a material which has been called secretin. This
is absorbed and, on being carried to the pancreas, stimulates
it to secrete. It has been shown that the injection into the
blood of an extract of the lining membrane of the upper part
of the small intestine, made with dilute hydrochloric acid,
leads to a flow of pancreatic juice. This secretin is not
destroyed by boiling, and is soluble in strong alcohol. It is
therefore not of the nature of an enzyme.

But while secretin seems to play so important a role,
it has been found that stimulation of the vagus nerve,
after a latent period of two minutes, increases pancreatic
secretion, so that it must be concluded that the secretion
of the fluid is, to a certain extent, under the control of the
nervous system.

C. Secretion of the Intestinal Wall (Succus Entericus).

This is formed in the Lieberkiihn's follicles of the intes-
tine, and it may be procured by cutting the intestine across
at two points, bringing each end of the intermediate piece to
the surface, and connecting the ends from which this piece
has been taken away. On mechanically irritating the
mucous membrane, a pale yellow clear fluid is secreted,
which contains native proteids and mucin, and is alkaline
in reaction from the presence of carbonate of soda.

Action. — The succus entericus contains: (1) An enzyme
which splits some disaccharids, as maltose and cane sugar,
into monosaccharids, but does not seem to act on lactose.
(2) In the intestine of animals taking milk a special zymin,

350 HUMAN PHYSIOLOGY

lactase, which splits inilk sugar. (3) Erepsin, an enzyme
which seems to act more powerfully than trypsin in splitting
peptone and many other proteids into their component
non-proteid crystalline constituents such as the di-amido
acids and non-amido acids, e.g. leucin and tyrosin. The
object of this is not easy to understand, but it may be that
the nitrogen of the proteid is largely treated as a waste
product. Vernon has shown that a similar enzyme is widely
distributed in the tissues, being specially abundant in the
kidney. (4) Enterokinase — a zymin which, acting on tryp-
sinogen, converts it into active trypsin (p. 348).

Nervous Mechanism of Secretion. — So far very little is
known on this point. It has been found that, when the in-
testine is ligatured in three places so as to form two closed
sacs, if the nerves to one of these be divided, it becomes filled
with a clear fluid closely resembling lymph. The dilatation
of the blood-vessels may account for this without secretion
being implicated.

Bacterial Action in the Alimentary Canal.

With the food, water, and saliva, numerous micro-
organisms of very diverse character are swallowed. It has
been suggested that the leucocytes formed in the lymphoid
tissue at the back of the mouth and pharynx, attack and
destroy such organisms, but so far definite proof of this is
not forthcoming. When the food is swallowed, the micro-
organisms multiply for some time in the warm moist stomach,
and certain of these, by splitting sugars, form lactic and
sometimes acetic acid. But when sufficient gastric juice is
poured out for the hydrochloric acid to exist free, the growth
of micro-organisms is inhibited, and some, at least, are
killed. Others pass on into the intestine, and, as the acid in
the chyme becomes neutralised, the acid-forming organisms
begin to grow, and, by splitting the sugars, form lactic or acetic
acid, and render the contents of the small intestine slightly
acid. Towards the end of the small intestine, and more
especially in the large intestine, the alkaline secretions have
neutralised these acids, and in the alkaline material so pro-
duced the putrefactive organisms begin to flourish and to

THE FOOD AND DIGESTION

attack any proteid which is not absorbed — splitting it up and
forming among other substances a series of aromatic bodies,
of which the chief are indol, skatol, and phenol.
Indol is a derivative of ethyl-amido-benzene.

Amido-benzene.

H H H

\ I I I
TT x — N— C— C— H Ethyl-amido-benzene.

A A

In Skatol a hydrogen of the ethyl of indol is replaced
by CH3.
Phenol is

— O— H

By taking embryo guinea-pigs at full time from the uterus
and keeping them with aseptic precautions, it has been shown
that the absence of micro-organisms from the intestine does
not interfere with digestion.

The most important and abundant organism present in
the intestinal tract is the bacillus coli communis, which has
a certain power of splitting proteids and a marked action
in producing acids from sugars. Its presence in water is
generally considered indicative of sewage contamination.

352 HUMAN PHYSIOLOGY

Fate of the Digestive Secretions.

1. Water. — Although it is impossible to state accurately
the average amount of the various digestive secretions poured
into the alimentary canal each day, it must be very consider-
able, probably not far short of 3000 ccms., or something
considerably more than one-half of the whole volume of the
blood. Only a small amount of this is given off in the faeces,
and hence the greater part must be re-absorbed. There is
thus a constant circulation between the blood and the
alimentary canal, or what may be called an entero-hcemal
circulation. One portion of this is particularly important.
The blood vessels of the intestine pass to the liver, and many
substances, when absorbed into the blood stream, are again
excreted in the bile and thus are prevented from reaching
the general circulation. Among these substances are the
salts of the bile acids and their derivatives, many alkaloids
such as curarine, and in all probability a set of animal alka-
loids called ptomaines formed by putrefactive decomposition
of proteids in the gut. If, from disturbances in the functions
of the liver, these are allowed to pass through that organ, the
feelings of lassitude and discomfort which are associated with
intestinal dyspepsia are produced. The liver thus forms a
protective barrier to the ingress of certain poisons.

2. Enzymes. — Ptyalin appears to be destroyed in the
stomach by the hydrochloric acid. Pepsin is probably
partly destroyed in the intestine, but it seems also to be
absorbed and excreted in the urine ; for, on the addition of
hydrochloric acid, the urine has a peptic action on proteids.
Trypsin appears to be destroyed in the alimentary canal ;
but the fate of the other pancreatic enzymes and of the
enzymes of the succus entericus is unknown.

3. Bile Constituents. — 1. The bile salts are partly re-
absorbed from special parts of the small intestine —
glycocholate of soda being taken up in the jejunum and
taurocholate in the ileum. The acids of these salts are also
partly broken up. The glycocholic acid yields amido-acetic
acid, which is absorbed and passes to the liver to be excreted
as urea; while the taurocholic acid yields amido-isethionic
acid which goes to the liver and yields urea and probably

THE FOOD AND DIGESTION 353

sulphuric acid. The fate of the cholalic acid is not known,
but it is supposed to be excreted in the faeces. 2. The
pigments undergo a change and lose their power of giving
Gmelin's reaction. They appear in the faeces as what may
be called stercobilin. It is probably formed by reduction of
bilirubin in the intestines as the result of the action of
micro-organisms. 3. The cholesterin is passed out in the
faeces.

Faeces.

The materials not absorbed from the intestine, whether
these are derived from the food or from the alimentary
canal, are thrown off from the rectum as the faeces. In
fasting animals these are passed at long intervals, and
consist of mucin, shed epithelium, the various products
of the bile constituents, and inorganic salts. In feed-
ing animals the amount and character of the faeces de-
pends largely upon the amount and character of the food,
and upon the bacteria which are growing in the large in-
testine. The unabsorbed material, as it passes down the
large intestine, becomes inspissated from the absorption of
water, but, if much undigested matter is present, water
may also be added, and the consistence of the faeces may
thus be varied. In the average condition they contain
about 70 or 80 per cent, of water. The colour is normally
brown, from the haematin of the flesh eaten, while the
sulphide of iron formed by the splitting of the haematin
compounds in the intestine may make them darker in
colour. On a milk diet they are light yellow in colour,
and if a large excess of fatty food is taken, or if fat is not
absorbed, as in jaundice, they become clay coloured.

The derivatives of the bile pigments play but a small part
in colouring the fasces. In infants, before bacteria are
introduced and begin to exert their reducing action, the
faeces may be green from the presence of unaltered biliverdin.
The reaction of the faeces varies. Usually the outside of the
mass is alkaline from the alkaline secretion of the intestine,
while the inside is acid from the free fatty acids and other
acids formed by the action of such acid-forming bacteria as

23

354 HUMAN PHYSIOLOGY

the bacillus coli cominunis. The amount of solid faeces
depends on the amount of food — a fairly average amount
per diem is 150 grms. of dried solids. On a vegetable diet,
from the presence of undigested cellulose, the amount is
very much greater. The solids of the faeces of a feeding
animal consist of the same constituents as the faeces in a
fasting animal, with the addition of all the undigested con-
stituents of the food — elastic and white fibrous tissue, remains
of muscle fibres, often fat and the earthy soaps of the fatty
acids ; and, when a vegetable diet is taken, the cellulose of
the vegetable cells, and frequently starch. The cellulose, by
stimulating the intestine, is a valuable natural purgative.

The odour is due to the presence of aromatic bodies such
as indol and skatol.

Meconium is the name given to the first fyeces passed by
the child after birth. They are greenish-black in colour,
and consist of inspissated bile and shed epithelium from
the intestine.

Movements of the Intestine.

These are of two kinds — myogenic and peristaltic. The
myogenic movements are slight rhythmic contractions which
pass rapidly along the intestine, and are insufficient to
drive on the contents, but are probably of use in churning
and mixing them. By feeding with food mixed with bismuth,
and employing X rays, Cannon finds that the contents of the
small intestine get broken up into small segments. This is
possibly due to these myogenic movements (Fig. 149, p. 339).
They occur when all the nerves have been divided, and when
the ganglia in the intestinal walls have been poisoned with
nicotine, and they are therefore due to the muscle fibres
alone.

The peristaltic movements are much more complex and
powerful. They consist of a constriction of the muscles,
which seems to be excited by the passage of the food, and
may be caused by inserting a bolus of cotton-wool covered
with vaseline. Starting at the upper end of the intestine,
they pass slowly downwards. In front of this contraction
the muscular fibres are relaxed, and thus the contracting

THE FOOD AND DIGESTION 355

part drives its contents into the relaxed part below. These
peristaltic movements go on after the nerves to the gut are
cut, but they are stopped when the ganglia in the wall of the
intestine are poisoned with nicotine. It has therefore been
concluded that the nerve ganglia in the intestinal wall form
a local reflex mechanism, which is stimulated by the presence
of foreign matter in the intestine, and which brings about
the co-ordinated contraction and relaxation, which together
constitute a true peristalsis.

But while peristalsis is thus independent of the central
nervous system, it is nevertheless controlled by it. The
splanchnic nerves inhibit, while the vagus to the small intes-
tine and upper part of the large gut, and the nervi erigentes
to the lower part of the large gut are augmentor nerves,
increasing the peristalsis.

As the contents of the small intestine are forced through
the ileo-csecal valve, the large intestine relaxes to receive
them, and then, a series of contractions passing from below
upwards — an anti-peristalsis — sets in by which the contents
are very thoroughly churned. Afterwards they are forced
downwards by tonic peristaltic waves.

The intestinal movements are inhibited by emotions.

Defsecation.

By the peristalsis of the intestine, the matter not absorbed
from the wall of the gut is forced down and accumulates in
the part of the rectum which passes horizontally forward
to end in the anal canal, into which it is prevented from
escaping by the sharp fold which the last part of the bowel
makes, and by the contraction of the strong sphincter ani.

Defsecation depends primarily on the intestinal peristalsis,
without which it cannot be performed. When faeces accumu-
late in the rectum, the mucous membrane is stimulated, and
impulses are sent up to inhibit a centre in the lumbar region
of the cord which keeps the sphincter ani contracted, and
the sphincter is relaxed, and the escape of faeces made pos-
sible. In some diseases of the cord this centre is stimulated
and cannot be inhibited, and thus defaecation is interfered

356 HUMAN PHYSIOLOGY

with, while in other diseases, when this centre has been
destroyed, the sphincter does not contract, and faeces may
escape continuously. Normally the act of defecation is
partly voluntary and partly involuntary. The voluntary part
of the act consists in closing the glottis, and making a forced
expiration so as to press upon the contents of the abdomen,
while at the same time the perineal muscles are relaxed, and
the rectum straightened, and thus the contents are allowed
to pass into the anal canal. The act is completed by the
emptying of the rectum by the contraction of the levatores
ani muscles.

III. ABSORPTION OF FOOD

1. State in which Food leaves the Alimentary Canal.— The

carbohydrates generally leave the alimentary canal as
monosaccharids ; but some resist the action of digestion
more than others. Lactose seems to be broken down in the
intestine only when the special lactase is present in the succus
entericus, but in all cases it must be broken down before it
reaches the liver. Cane sugar when taken in large excess
may also be absorbed and it is then excreted by the kidneys.

The proteids are absorbed as peptones, possibly as proteoses,
and as the diamido acids and other crystalline compounds
formed by the action of erepsin (p. 350). Native proteids
may be absorbed unchanged from the lower bowel, since
it has been found that when egg white is injected into an
isolated part of the rectum it disappears to a very consider-
able extent.

The fats are chiefly absorbed as soaps and as fatty acids,
and it is very doubtful if they leave the gut as fats.

2. Mode of Absorption of Food. — Absorption does not
occur uniformly throughout the alimentary canal. Thus,
while sugar and peptones are absorbed from the stomach,
water is absorbed only to a small extent.

That absorption is not due merely to a process of ordinary
diffusion or osmosis is clearly indicated by many facts.

1. Heidenhain has shown that absorption of water from
the intestine takes place much more rapidly than diffusion
through a dead membrane.

2. The relative rate of absorption of different substances
does not follow the laws of diffusion. Griibler's peptone passes
more easily through the intestine than glucose, but glucose
passes more readily through parchment paper, while sodium

357

358 HUMAN PHYSIOLOGY

sulphate, which is more diffusible than glucose, is absorbed
much less readily. Again, as shown by Reid, an animal can
absorb its own serum under conditions in which filtration
into blood capillaries or lacteals is excluded. In such a
case osmosis cannot play a part. Absorption is stopped
or diminished when the epithelium is removed, injured,
or poisoned with fluoride of sodium, in spite of the fact
that this must increase the facilities for osmosis and
filtration.

3. Channels of Absorption. — There are two channels of
absorption from the alimentary canal (see Fig. 105, p. 209)—
the veins which run together to form the portal vein of the
liver, and the lymphatics which run in the mesentery and,
after passing through some lymph glands, enter the recep-
taculum chyli in front of the vertebral column. From this,
the great lymph vessel, the thoracic duct, leads up to the
junction of the subclavian and innominate veins, and pours
its contents into the blood-stream. The lymph formed in
the liver also passes into the thoracic duct.

1. Proteids. — Peptones and the further products of their
digestion under the influence of erepsin are formed from
proteids in digestion, but the peptones undergo a change in
the intestinal wall before passing to the tissues, since they
are not found in the blood. That in some altered condition
they leave the intestine by the blood and not by the lymph
is shown by the fact that their absorption is not interfered
with by ligature of the thoracic duct.

During the digestion of proteids the number of leucocytes is
enormously increased, sometimes to more than double their
previous number, and in all probability it is they which carry
the products of digestion from the intestine. According to
the observations of Pohl, the leucocytes are derived from the
lymph tissue in the intestinal wall, but more recent experi-
ments tend to show that they come from the bone marrow,
being probably attracted to the intestine by a positive
chemiotaxis. By breaking down in the blood-stream they
probably set free the proteids for use in the tissues.

When an excess of proteids is taken in the food, it is
broken down in the lining membrane of the gut, its nitro-

THE FOOD AND DIGESTION 359

genous part forming ammonia compounds, probably carba-
mate of ammonia —

o

H ,!

\N C— 0-

H

H
H

Its non-nitrogenous part -probably yields carbohydrates,
since a proteid diet may lead to the accumulation of these
substances in the liver. The ammonia compounds are car-
ried to the liver and there changed to urea, and excreted
as such. Thus the entrance of an excess of nitrogen into the
tissues is prevented.

It has been pointed out that gastric juice does not dissolve
the nucleo-proteids, but that they are dissolved by the
pancreatic juice. Phosphorus is undoubtedly absorbed in
organic combination, but the mode of absorption and the
channels by which it passes from the intestine have not been
investigated.

2. Carbohydrates. — Although the chief monosaccharid
formed in digestion is dextrose, others are also produced —
laevulose from cane sugar, and galactose from milk sugar.
All these are absorbed in solution, and are carried away in
the blood by the portal vein.

3. Fats. — It was for long thought that the fats are absorbed
as a fine emulsion ; but the most recent investigations seem
to indicate that after being split up into the component acids
and glycerin, they pass, as soluble soaps or as fatty acids
soluble in the bile, through the borders of the intestinal
epithelium. Here they appear to be again converted into
fats by a synthesis of the acid with glycerin. Fine fatty
particles are found to make their appearance in the cells
at some distance from the free margin and to increase in
size. They are passed on from the cells, through the lymph
tissue of the villi, into the central lymph vessels, and thus
on through the thoracic duct to the blood-stream. Unlike
the proteids and carbohydrates, they are not carried directly
to the liver.

IV. FATE OF THE FOOD ABSORBED

THE food absorbed may be (A) used immediately as a source
of energy, for (1) the Construction or Reconstruction of
Tissues ; (2) the Production of Mechanical Work ; (3) the
Production of Heat ;

Or (B) it may be stored for future use in the body.

The processes of construction and repair of the tissues
and the production of mechanical work have already been
considered (p. 67 et seq.\ and the production of heat and the
regulation of temperature may now be dealt with.

I. PRODUCTION OF HEAT AND REGULATION OF
TEMPERATURE.

1. Production of Heat.

A. Muscle. — The production of heat in muscle has been
already studied (p. G3). It has been shown that muscle,
from its great bulk and constant activity, is the main source
of heat in the body. Not only may it be demonstrated
that the temperature of contracting muscle rises, but it
has been found that the temperature of blood coming from
the muscles is slightly higher than that of blood going
to them. Muscular exercise raises the temperature of the
body, and the shivering fit which is induced by exposure to
cold is really a reflex reaction by which heat production is
increased. Drugs which interfere with muscular contraction,
such as curare, diminish the temperature, and young animals,
before their muscular tissues become active, have a low
temperature unless kept in a warm atmosphere.

B. Glands. — Wherever chemical change goes on in proto-
plasm, heat is liberated. Therefore in glands during activity
a certain amount of heat is produced, but the production in
them is trivial when compared with the production in muscle.

THE FOOD AND DIGESTION 361

During the period of active digestion the temperature of the
blood coming from the liver may be nearly 1° C. higher than
that of the blood going to the organ. The liver alone among
glandular structures contributes an appreciable amount of
heat to the body, since the amount of blood passing through
the organ is large, and thus a considerable amount of heat is
derived from it.

C. Brain. — Some physiologists have maintained that the
fact that the temperature of the brain rises during cerebral
activity indicates that the chemical changes going on are
sufficient to yield a certain amount of heat. But it is more
probable that the rise of temperature is due to the increased
flow of blood through the organ, since a study of the blood
gases in the brain gives no indication of any marked increase
of chemical change during periods of increased cerebral
action.

2. Regulation of Temperature.

Since heat is constantly being produced, the temperature
of the body would tend to rise higher and higher, were
there not some arrangement by which just as much heat is
eliminated as is produced, and by which the temperature is
thus kept constant.

Elimination of Heat. — Heat is got rid of by three
channels. A. Skin. — Since the body is generally warmer
than the surrounding air, heat is constantly lost by conduc-
tion, convection, and radiation, and the extent of this loss
depends mainly upon the difference between the temperature
of the body and that of the air. Radiation plays the most
important part when a person is sitting quiet in still air ;
conduction and convection when the exchange of air over
the surface is rapid. The temperature of the skin is increased
when, from dilatation of the cutaneous vessels, more blood is
brought to the surface, and conversely it is lowered by con-
striction of these vessels. The influence of variations in the
temperature of the air is generally minimised in man by the
covering of clothes, and in animals by the covering of fur or
feathers, which retains a stationary layer of air at about 25°
to 30° C. over the skin. It has been calculated that over

362 HUMAN PHYSIOLOGY

70 per cent, of all the heat is lost by conduction and
radiation.

By the evaporation of sweat, heat is rendered latent, and
is taken from the body, which is thus cooled just as the hand
may be cooled by allowing ether to evaporate upon it. If
the amount of sweat vaporised is known, it is possible to
calculate the amount of heat removed from the body hi
this way. The loss is comparatively small — only about
14 per cent, of the whole. The extent depends upon the
rapidity with which evaporation goes on, and this is governed
by the amount of sweat secreted, and by the dryness and
temperature of the atmosphere. Thus a warm dry climate
is better borne than one which is warm and moist, since in
the former the loss of heat by evaporation is so much freer.
Of the various factors increasing sweat secretion, heat is
probably the most important.

Since the temperature of the skin is governed by the state
of the cutaneous vessels, and the amount of sweat produced
by the state of the sweat glands, and since both of these
are under the control of the nervous system, the elimina-
tion of heat from the skin is presided over by a nervous
mechanism.

B. Respiratory Passages. — By conduction and radiation,
and by evaporation from the respiratory passages, about 10
per cent, of the heat is got rid of in man. In the dog and
some other animals, the proportion of heat eliminated in
this way is considerably greater.

C. Urine and Faeces. — Since these are warmer than the
surrounding air, a certain amount of heat is lost through
them. The amount is small— something less than 2 per
cent, of the whole.

Temperature. — In all higher animals, the loss of heat and
the production of heat are so nicely balanced that the tem-
perature of the body remains fairly constant under all con-
ditions. If an extra amount of heat is produced, say in
muscular exercise, it is at once eliminated by the skin, and,
if the body is exposed to a low temperature, loss of heat is
rapidly checked by contraction of the cutaneous vessels and
diminished activity of the sweat glands.

THE FOOD AND DIGESTION 363

Since heat is constantly being given off, the temperature at
the surface of the body is always lower than the temperature
in the interior. The temperature of the rectum may be taken
as a measure of the internal temperature.

The mean daily temperature of a healthy man is : —

0 c. ° F.

Kectum . . . 37*2 98-96

Axilla .... 36-9 9845

Mouth .... 36-87 98-36

But the temperature varies throughout the course of life.

°c.

Infant . . . . 37 '5
Under 25 years . . 37*2
About 40 . . . 37-1
Old Age . . . 37-2 to 37*5

It also varies throughout the course of the twenty-four
hours, and since this is a matter of great importance in

FIG. 150. — Chart of the daily variation of temperature in the normal human subject
in degrees Centigrade. (RiCHET. )

medicine, it has been very carefully studied by many
observers. The difference is not more than 1° C. It is
lowest in the early morning and highest in the afternoon.
Under all normal conditions the temperature of man

364 , HUMAN PHYSIOLOGY

undergoes only small variations, because the balance between
production of heat and elimination of heat is so well main-
tained. But under abnormal conditions the balance is
frequently upset. Thus severe muscular work causes a
temporary rise of temperature, because heat elimination does
not quite keep pace with heat production. Exposure to very
high temperatures may cause a slight rise of temperature,
while exposure to excessive cold may cause a slight fall;
but unless in the case of those unable to use their muscles —
e.g. in those suffering from alcoholic poisoning — the change
is small.

Although the normal variation of temperature is so small,
life may be sustained when the temperature falls for a time
to about 25° C., or rises to nearly 43° C. Cases are recorded in
which it has even risen to 46'6° C., without death supervening.

While the higher "warm-blooded animals," mammals and
birds, maintain a constant temperature, the lower vertebrates,
" cold-blooded animals," reptiles, amphibia and fishes, do not
do so, and their temperature varies with that of the surround-
ing medium.

But even in mammals the mechanism for the regulation of
temperature is not absolutely perfect, and in every species
of animal there is a limit to the power of adjustment.

Mammals which hibernate become for the time " cold-
blooded animals," and lose their power of regulating their
temperature.

The regulation of temperature may be effected either
by modifying heat production, or by altering the rate of
elimination.

Heat production is voluntarily modified when muscular
exercise is taken during exposure to cold, and involuntarily
when muscles are set in action by a shivering fit. There is,
however, no evidence of the existence of a special nervous
mechanism presiding over heat production in muscle.

But it is not so much by changes in the rate of heat pro-
duction, as by alteration in heat elimination through the
skin, that the temperature is kept uniform. The nerves to
the cutaneous vessels, and to the sweat glands, are the great
controllers of temperature. It is through failure of this
mechanism under the action of the toxins of micro-organisms

THE FOOD AND DIGESTION 365

that heat elimination is diminished, and the temperature is
raised in fevers.

It is not necessary to assume that there is a special heat
regulating nervous mechanism, since the nervous arrange-
ments presiding over the vessels and glands of the skin are
capable of immediately responding to change of condition
calling for their intervention.

II. STOKAGE OF SURPLUS FOOD.

This storage takes place chiefly in three situations : (1)
Fatty tissue ; (2) muscle ; (3) liver.

1. In Fatty Tissues. — In most mammals the chief storage
of surplus food is in the fatty tissues.

(1) That the fat of the food can be stored in them is
shown by the fact that the administration of large amounts
of fats different from those of the body leads to their appear-
ance in those tissues.

(2) Fats are also formed from the carbohydrates of the
food. Feeding experiments upon pigs and other animals
have definitely proved that sugary foods are changed to fat
in the body and stored in that form. The following may be
given as an example of such experiments. Two young pigs
of a litter were taken, and one was killed and analysed. The
other was fed for weeks on maize, the amount eaten being
weighed and the excretion of nitrogen by the pig being
determined. The animal was then killed and analysed, and
it was found that the fat gained was more than could be
produced from the fat and proteid of the food eaten. It must
therefore have been formed from the carbohydrates.

(3) There is good evidence that in excessive proteid feed-
ing the non-nitrogenous part of the proteid molecule may be
stored as fat, at least in carnivorous animals. Thus, a dog
fed on lean meat does not lose all his fat. It has recently
been maintained that the evidence in favour of the formation
of fats directly from proteids is unsatisfactory ; but since
proteids yield carbohydrates, and since carbohydrates form
fats, it must be admitted that proteids may be a source
of fats.

2. In Muscle. — Some animals, as the salmon, store fats

366 HUMAN PHYSIOLOGY

within their muscle fibres ; but in mammals such a storage
is limited in amount. The salmon also stores surplus proteid
material in the muscles, and mammals too appear to do the
same. How far a passive storage may occur is not known,
but feeding experiments on mammals indicate that only a
small amount of proteid can be accumulated. On the other
hand, the experience of athletic training shows that the
muscles may be enormously increased by the building up of
the proteid of the food into their protoplasm. Glycogen also
is stored in the muscles.

3. In the Liver. — The liver is a storehouse of carbohydrates
and fats (p. 368). Lecithin is always present in the liver,
even in prolonged fasting.

III. THE LIVER IN RELATIONSHIP TO ABSORBED
FOOD AND TO THE GENERAL METABOLISM.

The liver develops as a couple of diverticula from the
embryonic gut, and is thus primarily a digestive gland. But
in mammals, early in fcetal life, it comes to have important
relationships with the blood going to nourish the body from
the placenta. In invertebrates it remains as a part of the
intestine both structurally and functionally. The vein bring-
ing the blood from the mother breaks up into a series of
capillaries in the young liver, and in these capillaries, for a
considerable time, the development of the cells of the blood
goes on. Soon the liver begins to secrete bile, while animal
starch and fat begin to accumulate in its cells. Gradually
the formation of blood cells stops, and the mass of liver cells
become larger in proportion to the capillaries. As the foetal
intestine develops, the vein bringing blood from it — the
portal vein — opens into the capillary network of the liver, so
that, when at birth the supply of nourishment from the
placenta is stopped, the liver is still associated with the blood
bringing nutrient material to the tissues.

1. Relation to Carbohydrates — Glycogenic Function.—
Claude Bernard discovered that there is a constant forma-
tion of sugar in the liver. On account of this constant
supply, even when an animal undergoes a prolonged fast,
the amount of sugar in the blood does not diminish. In

THE FOOD AND DIGESTION 367

starvation there are only two possible sources of this glucose
—the fats and the proteids of the tissues. There is no con-
clusive evidence that fats can be changed to sugar in the
liver, although it is difficult to explain the large amount of
sugar which is excreted in phloridzin poisoning, unless it is
formed from fats. That proteids are a source of sugary
substances is shown by the accumulation of glycogen in the
liver in animals fed upon proteids, and by continued excre-
tion of glucose in the urine of a dog without its pancreas,
and fed exclusively on proteids. It is therefore probable
that in starvation the proteids of the body are broken down
and their non-nitrogenous part changed to sugar. But, not
only does the liver manufacture sugar for the tissues in star-
vation, but, when the supply of sugar is in excess of the
demands of the tissues, it stores it as a form of starch —
glycogen (see p. 318) — and gives it out as sugar as that
substance is required. On a carbohydrate diet the accumu-
lation of glycogen in the liver is very great ; but even on a
proteid diet, in dogs at least, a smaller accumulation takes
place. The observation that various monosaccharids are
stored as the same form of glycogen shows that they must
first be assimilated by the liver protoplasm and then con-
verted to glycogen, the process being one of synthesis.

The way in which glycogen is again changed to sugar is
more doubtful. The fact that the liver after treatment with
alcohol can change glycogen to glucose, has induced some
physiologists to believe that it is by an enzyme that this
conversion goes on during life. But it has been shown (1)
that the injection of methylene blue, which poisons proto-
plasm but does not interfere with the action of enzymes
checks the conversion, and (2) that stimulating the splanchnic
nerves going to the liver increases the conversion without
increasing the amylolytic enzyme in the liver or blood.
It is therefore probable that the conversion results from
chemical changes in the protoplasm which are controlled by
the nerves of the liver.

If more sugar is taken than the liver can deal with, it then
passes on into the general circulation, and is excreted in the
urine. Every individual has a certain power of oxidising
and using sugar, and most persons can dispose of about 200

368 HUMAN PHYSIOLOGY

grms. at a time. But the carbohydrate capacity varies
greatly, and even in the same individual it is different under
different conditions. When the glycogen stored in the liver
is changed to glucose more quickly than is required by the
tissues, the glucose accumulates in the blood and is excreted
in the urine -(glycosuria). This is seen in Bernard's experi-
ment of puncturing the floor of the fourth ventricle at its
posterior part in a rabbit. If glycogen is abundant in the
liver, glycosuria results.

Another way in which sugar may be made to appear in
the urine is by injecting phloridzin. But since under the
influence of this drug the sugar in the blood is decreased, it
must be concluded that it acts by causing the kidneys to
excrete glucose too rapidly, so that it is not available for the
tissues.

The injection of large doses of extract of the suprarenal
bodies causes a glycosuria with an increase of sugar in the
blood ; but so far it is not known whether the condition is
one of increased production of sugar or of diminished
utilisation (p. 385).

Removal of the pancreas also causes glycosuria (p. 390).

2. Relation to Fats. — Although the fats are not carried
directly to the liver, as are proteids and carbohydrates, they
are stored in large amounts in the liver of some animals —
e.g. the cod among fishes and the cat among mammals.
Animals which have little power of storing fat generally
throughout the muscles and other tissues, seem to have a
marked capacity for accumulating it in the liver. Even in
starvation the fats do not disappear from the liver, and
throughout all conditions of life a fairly constant amount of
lecithin — a phosphorus and nitrogen containing fat (see
p. 78) — is present in the liver cells. Lecithin is an inter-
mediate stage in the formation of the more complex nucleins
of living cells, and the formation of lecithin in the liver by
the synthesis of glycerin, fatty acids, phosphoric acid, and
cholin is probably a first step in the construction of these
nucleins. If this is so, the fat of the liver must play an
important part in retaining and fixing phosphorus in the
body.

3. Relation to Proteids. — Along with the intestinal wall,

THE FOOD AND DIGESTION 369

the liver regulates the supply of proteids to the body. A
study of the chemical changes in muscle has shown that the
waste of proteid is normally small in amount, and that a
great part of the nitrogen is capable of being used again if a
supply of oxygen and carbonaceous material is forthcoming
(see p. 72). Hence the demand for nitrogen in the muscles
is small, and for this reason, apparently, any excess of proteid
in the food is decomposed either by erepsin or by the intes-
tinal wall into ammonia compounds, which are changed into
urea in the liver.

Urea, the chief waste substance excreted in the urine, is
the bi-amide of carbonic acid.

O H, ° H

VNT f1 N"/
H-O-C-O-H H/ \H

It contains 46-6 per cent, of nitrogen. It is a white
substance crystallising in long prisms* It is very soluble
in water and alcohol — insoluble in ether. With nitric and
oxalic acids it forms insoluble crystalline salts. It is readily
decomposed into nitrogen, carbon dioxide and water by
nitrous acid and by hypobromite of soda in excess of soda.

Urea is chiefly formed in the liver. — That it is not
produced in the kidneys is shown by the following facts :
(1) When these organs are excised, urea accumulates in the
blood. (2) When carbonate of ammonia is added to blood
artificially circulated through the kidney of an animal just
killed, no urea is formed.

That it is not formed in the muscles is shown — (1) By the
absence of a definite increase in urea formation during mus-
cular activity ; (2) by the fact that when blood containing
carbonate of ammonia is streamed through muscles, urea is
not produced.

That it is formed in the liver is indicated — (1) By the fact
that when an ammonia salt such as the carbonate dissolved
in blood is streamed through the organ, it is changed to
urea ; (2) by the observation that, when the liver is cut out
of the circulation, the urea in the urine rapidly diminishes,
and ammonia and lactic acid take its place.

The exclusion of the liver from the circulation in

24

370 HUMAN PHYSIOLOGY

mammals is difficult, because, when the portal vein is liga-
tured, the blood returning to the heart tends to accumulate
in the great veins of the abdomen. But this difficulty has
been overcome by Eck, who devised a method of uniting the
peripheral end of the divided portal vein with the inferior
vena cava, and of thus allowing the blood to return from the
abdomen to the heart.

Source of Urea. — Urea is produced from the proteids of
the food and tissues. The manner in which excess of proteid
in the food is broken down into ammonia compounds and
sent to the liver has been already described (p. 369). But
the fact that even in starvation urea is produced, seems to
indicate that the initial stages of decomposition of proteids
may go on elsewhere than in the -intestinal wall. The fate of
haemoglobin tends to show that the whole process may be
conducted in the liver cells. When haemoglobin is set free
from the corpuscles, the nitrogen of its proteid part is
changed to urea, while the pigment part is deprived of its
iron and excreted as bilirubin. Whether the proteids of
muscle and other tissues are thus directly dealt with, or
whether the initial stages of decomposition go on outside the
liver is not known. It is probable that lactate of ammonia
is produced in muscle, and that this is converted into urea
in the liver.

Speculations as to the way in which the proteid molecule
is broken down are of little value. It is one thing to show
that urea may be formed in a particular way outside the
body, but quite another to prove that it is formed in that
particular way in living protoplasm.

The nitrogen excreted is not all in the form of urea, but
some is combined in ammonia salts, in uric acid and other
purin bodies (see p. 397), and in creatinin. In the mam-
malian body ammonia and the purin bodies can be changed
into urea, and it is probable that the small amounts of these
substances which appear in the urine have simply escaped
this conversion. Certain drugs (alcohol, sulphonal, &c.) and
toxins (diphtheria) markedly decrease their conversion into
urea and so increase their quantity in the urine. Although
urea may be prepared from creatin, there is no evidence that
such a process goes on in the body.

THE FOOD AND DIGESTION 371

After the nitrogenous portion of the proteid molecule is
split up, the liver has the further power of turning the non-
nitrogenous part into sugar, and either sending it to the
tissues or storing it as glycogen.

Summary of the Functions of Liver. — The functions of the
liver may be briefly summarised as follows : (1) It regulates
the supply of glucose to the body (a) by manufacturing it
from proteids when the supply of carbohydrates is insufficient,
and (b) by storing it as glycogen when the supply of carbo-
hydrates is in excess, giving it off afterwards as required.
(2) Along with the intestinal wall it regulates the supply of
proteids to the body, by decomposing any excess, and giving
off the nitrogen as urea, &c. (3) It regulates, in many animals
at least, the supply of fat to the body by storing any excess.
(4) It regulates the number of erythrocytes by getting rid of
waste haemoglobin and retaining the iron for further use.
(-5) From the part it plays in the entero-hepatic circulation,
it protects the body against certain poisons by excreting
them in the bile.

V. GENERAL METABOLISM.

Having considered how the food is digested and absorbed,
and how it is then either stored or at once used (a) for
building up and repairing the tissues, or (6) as a source of
energy, the rate at which the various chemical changes go
on and the factors modifying them may be dealt with.

The changes in the two great constituents of the body —
proteids and fats — have to be separately studied.

1. Method of Investigating.

A. Proteid Metabolism. — The amount of proteid used in
the body is readily calculated from the amount of nitrogen
excreted, since, under normal conditions, unless nitrogen
in some unusual combination is being taken, it is derived
entirely from the proteids in the body. Proteids contain
16 per cent, of nitrogen, and hence each grm. of nitrogen
excreted is derived from 6'25 grms. of proteid.

The nitrogen is almost entirely excreted in the urine.

372

HUMAN PHYSIOLOGY

Only a small amount escapes by the bowels and skin, and
hence only when very accurate observations are desired is it
necessary to analyse the faeces and sweat.

Since nucleo-proteids form so important a constituent of
living matter, it is sometimes desirable to study the chemical
changes which they are undergoing. To do this the excre-
tion of phosphorus and the purin bases must be investigated.
But it is difficult to arrive at reliable conclusions, because
there are other phosphorus- containing substances besides
nucleins in the body — e.g. the bones ; and secondly, the
purin bodies all tend to be converted into urea before being
excreted.

B. Metabolism of Fats. — Proteids contain nearly three
and a half times as much carbon as nitrogen, and hence,
when broken down, for each grm. of nitrogen excreted,
3 '4 grms. of carbon are given off.

The carbon is chiefly excreted from the lungs as carbon
dioxide, and in this form it may be collected and estimated.

Any excess of carbon excreted over 3-4 times the amount
of nitrogen given oft', must be derived from the fats of the
body or from the fats and carbohydrates taken in the food.
Any carbon retained in the body, apart from that in proteids,
is stored ultimately as fat, and since carbon constitutes
76*5 per cent, of fats, the amount of fat is calculated by
multiplying the carbon by 1/3.

The following tabular example of an investigation of the
metabolism may be given :—

Intake in Grammes.

Output.

c.

X.

C.

N.

Proteids .

100

54

16

Fats ....

100

76

Carbohydrates .

400

200

...

...

330

16

300

14

Two grms. of nitrogen are retained as proteid ; that is, 2 x
6*25 = 12-5 grms. of proteid — are being daily laid on. Thirty
grms. of carbon are also retained in the body, and of this

THE FOOD AND DIGESTION 373

3'4x2 = 6'8 grms. are combined with the nitrogen in the
proteid. The remainder, 26'6 grms., go to form fats, the
amount of which is 26'6 x 1*3 = 34'6 grms. of fat.

2. Metabolism during Fasting.

When the usual supply of energy in the food is cut off,
the animal liberates the energy required by oxidising its own
stored material and tissues. This is shown by the fact that
the animal loses weight and goes on excreting carbon dioxide,
urea, and the other waste products of the activity of the
tissues.

Several prolonged fasts have been undertaken by men, and,
in one or two of these, careful observations have been made
by physiologists. It has been found that during the first day
or two of a fast, the individual goes on using proteids and
fats at something like the same rate as he did while taking
food, but that gradually he uses less and less proteid each
day. This is well shown in the case of Succi, who underwent
a fast of thirty days.

Day of Fast. Proteid used in Grms. Fat used.

1st 104 Not estimated.

10th 51 170

20th 33 170

29th 31 163

It is from the stored fats that the energy is chiefly derived,
and the result of this is that before death the fats of the body
are largely used up. The proteid-containing tissues waste
more slowly and waste at different rates, the less essential
being used up more rapidly than the more essential, which,
in fact, live upon the former. In cats deprived of food till
death supervened the heart and central nervous system had
hardly lost weight ; the bones, pancreas, lungs, intestines, and
skin each had lost between 10 to 20 per cent, of their weight,
the kidneys, blood, and muscles between 20 to 30, and the
liver and spleen between 50 to 70.

The rate of waste during a fast depends upon the amount
of energy required, and it is therefore increased by muscular
work and by exposure to cold. When a man is kept quiet

374 HUMAN PHYSIOLOGY

and warm and supplied with water, a fast of thirty days may
in some cases be borne without injury.

3. Effect of Feeding.

When food is given to a fasting animal or man, the first
effect is to increase the rate of wasting by calling into action
the muscles and glands concerned in digestion. The result
is an immediate increase in the excretion of nitrogen and
carbon, indicating an increased breaking down of proteids
and fats. Zuntz and Magnus Levy found that a diet of white
bread and butter increased the metabolic processes by an
amount equivalent to about 10 per cent, of the energy value
of the diet. For this reason, to give an animal which is fast-
ing a diet containing just the amount of nitrogen and of
carbon which the animal is excreting, will not at once stop
the loss of weight. Suppose, for instance, that to a fasting
animal using daily 30 grms. of the proteids and 160 grms. of
the fats of his body, a diet containing these amounts is given,
the disintegration of proteids and of fats will at once rise, say,
to 50 grms. of proteid and 280 of fat. Thus the result will
be that, instead of his losing 30 grms. of proteid, he will lose
only 20 grms. per diem, and instead of 160 grms. of fat, only
120 grms. But, if the diet is sufficient to supply the energy
required, in a few days the intake and output will balance,
and the individual is then said to be in metabolic equili-
brium, and he neither gains nor loses weight. The following
table gives an idea of how this adjustment of the metabolism
is reached : —

Day.

Intake.

Disintegrated.

Waste diminished to

Proteid.

Fat.

Proteid.

Fat.

Proteid.

Fat.

1

0

0

30

160

2

30

160

50

280

20

120

3 30

160

40

240

10

60

4

30

160

30

200

0

40

5

30

160

30

160

0

0

6 30

160

30

160

0

0

If the amount of food be further increased, a small pro-
portion of the proteids and a larger proportion of the fats are

THE FOOD AND DIGESTION 375

retained, and weight is gained. As already indicated, the
power of storing proteids is generally small.

Proteid Diet. — Proteids contain all the chemical elements
required for the building and repair of the tissues, and from
the complexity of their molecules they also supply latent
energy. It is therefore theoretically possible for an animal
to sustain life on proteids, and certain animals can be fed
exclusively upon them. Thus Pfliiger kept a dog for many
months upon a purely proteid diet without injury to its
health. But to supply the necessary energy in proteids
alone requires the consumption of excessively large quanti-
ties. For a man to get the energy equivalent to 3000 Calories
— a very moderate expenditure per diem — he would have to
eat more than seven times the usual amount of proteid.
Further, it has been shown that, when large quantities are
taken, a portion is broken up in the intestinal wall and
formed into urea by the liver and excreted by the kidney,
and thus excessive work is thrown upon these excretory
organs. While these organs usually form and excrete
about 33 grms. of urea per diem, on such a diet they would
have to deal with no less than 231 grms.

It is therefore not advantageous to adopt a too purely5
proteid diet. The great use of proteids is as muscle-builders.
When the muscles are in a state of constant activity they
have a certain power of laying on proteid as they grow.
Hence the value of proteids in muscular training.

Gelatin, although undergoing digestion and absorption like
the proteids, is not available as a muscle-builder. Its sole
use is as an energy yielder, and in this respect it has a value
equal to the proteids.

Carbohydrate Diet. — Carbohydrates are of equal value
with proteids as a source of energy, but they contain no
nitrogen, and they are not available for building up and
repairing the protoplasm of muscles and other tissues. Car-
bohydrates alone will not support life, but when added to
proteids they enable the animal to do with smaller quantities
of the latter. They are thus sometimes termed proteid

376 HUMAN PHYSIOLOGY

sparers. Their use in diminishing the consumption of pro-
teids is, however, strictly limited.

Fat Diet. — Fats, like carbohydrates, will not support life,
because they cannot be used for building up protoplasm, but,
like carbohydrates, they are a source of energy, and they have
more than twice the energy value of proteids or of carbo-
hydrates (p. 315). They are thus proteid sparers. But
experiment has shown that, in spite of their higher energy
value, they have not the same power as carbohydrates of
sparing proteids, since greater work is required in their
digestion and absorption.

A knowledge of the part played by proteids, carbohydrates,
and fats in the animal body is the groundwork of the study
of Dietetics.

DIETETICS.

The great essentials of a diet capable of maintaining health
are : —

1st. That it should supply the energy required.

2nd. That it should contain sufficient proteids to make
good the waste of these substances.

3rd. That it should be capable of digestion, absorption,
and assimilation.

I. The energy requirements vary with the mode of life and
with the age and size of the individual.

Size. — Other things being equal, a large man requires more
energy and more proteid than a small man. For this reason
the energy requirements are sometimes stated as per unit of
weight, but it is more convenient to take as the standard an
adult man of average weight, say 65 kgs. It must further
be remembered that the smaller the animal the greater the
surface in proportion to its weight ; and hence the greater
the loss of heat per unit of weight. For this reason alone
small animals and children require more energy per unit of
weight than larger animals or older people.

Age. — In children the metabolism is more active than in
adults. They are more constantly in motion, and they re-
quire energy and material to build up their tissues, and, as

THE FOOD AND DIGESTION

377

just stated, the loss of heat per unit of weight is greater than
in their elders. Weight for weight, a child thus requires
a greater supply of energy than a man. The following
results, based upon observations on diets recorded by
Camerer, illustrate this : —

Age.

Weight in Kilos.

Energy used per Kilo
in Calories.

Total Energy
in Calories.

4
12
30

14
30
66

91-3

57-7
42-4

1280
1730
2800

It will thus be seen that the energy requirements of chil-
dren at different ages may be stated in terms of the energy
requirements of an adult man doing average work. Atwater
formulates this as follows : —

Taking a man at
A woman
A boy of 14 to 16
A girl

A child 10 to 13

6 to 9

2 to 5

under 2

. 1-0

is equivalent to 0-8 of a man.
0-8 „
0-7

0-6 „
0-5 „
0-4
0-3

Mode of Life. — An individual kept warm and at rest, re-
quires much less energy than if he is required to do muscular
work and is exposed to cold. The variations in the amount
of material used in different conditions is well illustrated by
the experiments of Zuntz on the excretion of carbon dioxide
in the dog.

Resting lying

„ standing ....
Forward movement (unloaded)

(drawing weight)

CO2 excreted per
minute in corns.

124-7
170-2
525-0
798-9

These results are corroborated by a study of the diet re-
quired to maintain the weight and health of men doing
different amounts of work. From a large series of such

378 HUMAN PHYSIOLOGY

observations Atwater concludes that the energy requirements
of the diet varies as follows : —

Calories.

Man without muscular work . 2700

„ with light muscular work . 3000

„ „ moderate muscular work 3500

„ „ severe muscular work . 4500

It must, of course, be remembered that all the energy of
the constituents of the diet is not available, since a consider-
able and varying proportion of the food is not digested or
absorbed (p. 381). The gross energy of the diet must there-
fore be well above the nett energy requirements of the body.

A rough idea of these nett requirements may be arrived at
by considering the average expenditure of energy in different
forms. Taking the case of a man doing a moderate day's
muscular work equivalent to 150,000 kgms. or 350 Calories
(see p. 73), he gives off, according to N. Stewart's calcula-
tions (Manual of Physiology, p. 425), 2590 Calories of heat.
The loss of energy is thus —

Calories.

In external muscular work, equivalent to 350
As heat 2600

Total . . . 2950

To make good this loss of energy the diet should contain
more than 3000 gross Calories, and the results of Zuntz
clearly show that, when muscular work is large in amount,
a greater supply of energy must be forthcoming, while in
individuals leading a sedentary life the energy requirement
will be smaller. In the case of a labourer it is safe to allow
at least 3500 Calories.

II. Proteid Requirements. — Many experiments have been
made to determine the smallest amount of proteids upon
which life can be maintained, and conflicting results have
been arrived at. Various investigators have succeeded in
maintaining a nitrogenous equilibrium for short periods on
an intake of proteid no greater than that metabolised in
fasting.

THE FOOD AND DIGESTION 379

Recently Chittenden has recorded a prolonged series of
investigations upon this question by which he shows that in
five professional men for periods of from seven to nine
months, in eight student athletes for from four to seven
months, and in thirteen soldiers for five months, a nitro-
genous equilibrium and perfect physical and mental health
could be maintained with an intake of from 35 to 56 grms.
of proteid per diern.

The work done by these individuals is not recorded, but
from the daily routine of the soldiers, whose chief work
appears to have been two hours' exercise in a gymnasium, it
must be classed as very moderate. The fact that the energy
value of the diets in the case of the soldiers was only 2500 to
2800 Calories seems either to confirm the opinion that they
were not called upon to do severe work or to suggest that our
present estimates of the loss of energy as heat from the body
(p. 378) are erroneous.

While these results certainly prove that men can maintain
health and muscular efficiency for long periods on about half
the amount of proteid which is usually consumed, they do
not demonstrate that, in the case of those subjected to
strenuous and sustained muscular work, such a reduction is
desirable, nor do they indicate that in growing children a
reduction in the amount of proteids usually consumed may
be safely allowed. A study of a very large series of dietaries
of different races shows that, unless absolutely prevented by
poverty, the average man consumes over 100 grms. of pro-
teid per diem, and that those in muscular training tend to
consume very much larger amounts. Bearing in mind the
importance of proteids as muscle builders, it is safe to con-
clude that about 120 grms. of proteids should be allowed per
man per diem, at least in the labouring classes.

Ill; Part played by Carbohydrates and Fats. — The carbo-
hydrates and fats have to supply the energy not supplied by
the proteids. The amounts which must be consumed will
thus depend, first, on the amount of proteid taken, and,
second, on the energy requirement of the individual. If 100
grms. of proteid are taken, this will yield 410 Calories of
energy, while if only 50 are consumed, 205 Calories will be

38o HUMAN PHYSIOLOGY

yielded. The difference between these amounts and the total
energy requirements of the person must be made up from
carbohydrates and fats. In the case of a working-man
requiring 3500 Calories, and consuming 100 grms. of proteid
per diem, this leaves about 3000 Calories to be supplied by
fats and carbohydrates.

In determining the proportionate amounts of these, two
factors have to be considered — first, the relative cost, and,
second, the limitation of the power of digestion of each.

Carbohydrates are enormously cheaper than fats as a
source of energy. Margarine at 8d. per Ib. will yield 435
Calories for a penny, while sugar at 2|d. per Ib. will yield
1860 Calories for the same sum. But the use of carbo-
hydrates as a source of energy is limited by the fact that in
most individuals digestive disturbances are apt to supervene
if more than from 500 to 600 grms. are consumed.

A diet should therefore not contain much more than 500
grms. of carbohydrates, and these will yield 2050 Calories of
energy.

This leaves about 950 Calories to be supplied by fats, an
amount which is nearly met by 100 grms. of fat yielding 930
Calories of energy. This quantity of fat most people can
easily digest.

A typical diet for an average man doing moderate muscular
work would thus be —

Amount. Energy Value in

Calories.

Proteids ... 120 410

Carbohydrates . . 500 2050

Fats 100 930

3390

or very nearly the 3500 Calories.

A reference to Atwater's table gives the dietary require-
ments of a woman or of a child of any age, and thus renders
it easy to make out the diet of a family or public institution.

IY. The Diet must be capable of Digestion, Absorption,
and Assimilation. — While proteids, carbohydrates, and fats
in the alimentary canal undergo the changes already de-

THE FOOD AND DIGESTION 381

scribed, they are frequently, when taken in the food, in an
unfavourable state for the action of digestive juices. Thus,
while the proteids of flesh are exposed to the gastric and
pancreatic secretion, the proteids of many vegetables, unless
carefully prepared, cooked, and masticated, are protected and
escape digestion and absorption. The same may be said of
the crude starch of vegetables. The digestibility and value
as an article of diet of the cellulose and allied substances in
plants is an important question at present requiring further
investigation.

The fats of vegetables are also generally in a less favourable
state for digestion than the fats of animals.

When once digested there seems to be no difference in the
absorbability and assimilability of the proximate principles
of vegetables and animals ; a given weight of vegetable pro-
teid may be substituted for the same quantity of animal
proteid.

As a result of the difference in digestibility the availa-
bility of food-stuffs varies. The following table serves to
show some of these variations.

Absorption of Food-stuffs.

Per Cent. Absorbed.
Proteid. Fat. Carbohydrate.

Flesh ... 97 95

Egg .... 97 95

Milk .... 89-99 96 100

Bread ... 78 ... 99

Potatoes (boiled) . 68 ... 92

Carrots (raw) . .61 ... 82

The availability of almost any article of food varies with
the state of the teeth and the digestive organs of the
individual, with the manner in which it is eaten — whether
leisurely or too rapidly and without proper mastication, and
with the manner in which it is prepared. For example, the
mode of manufacture of the flour used in bread-making
has a very marked influence upon the amount digested and
absorbed. In vegetable foods especially the thoroughness
of the cooking has a most important influence on the availa-
bility of their constituents.

382 HUMAN PHYSIOLOGY

By assimilation is meant the taking of the food con-
stituent from the blood by the muscle and other tissues so
that it may be used, for until the constituents of the food
have become part of the protoplasm of the tissues they
are not oxidised. Apparently the only substances which
can be freely assimilated and used by the tissues are the
three proximate principles of the food, although alcohol and
possibly some other similar substances may be utilised to a
small extent.

Y. The subject of Dietetics cannot be left without allud-
ing to the influence of two very universally used materials —
alcohol and tea.

Alcohol. — The influence of alcohol may be considered
under two heads.

1st. Before absorption. Moderate doses of alcohol taken
along with food do not appear to interfere with the digestion
and absorption of proteids, of carbohydrates, or of fats.
When taken apart from food and in such excessive doses
as to act as an irritant and to cause catarrh of the
alimentary canal, alcohol of course has a prejudicial
influence.

2nd. After absorption. The tissues have the power of
oxidising a small quantity of alcohol, and since the com-
bustion of 1 grm. of alcohol liberates 7'06 Calories of energy,
alcohol must be regarded as a food just in the same way as
sugar is a food. The amount of alcohol which can be thus
oxidised varies in different individuals, but the average
man cannot use more than 50 grms. per diem. Any
alcohol taken above the amount which can be oxidised is
excreted in the urine and breath, and in its passage through
the body it acts as a poison to the protoplasm, first diminish-
ing its activity and so diminishing the rate of waste, and
secondly causing the death of the protoplasm and the
removal of the broken-down nitrogenous constituents in the
urine. Its poisonous action on the liver is demonstrated
by the diminished building up of urea, so that a greater
quantity of waste nitrogen is excreted in other forms. In
moderate doses it stimulates the heart and dilates the
arterioles. Its first action in checking the activity of meta-

THE FOOD AND DIGESTION 383

holism, along with its influence in producing dilatation of
the cutaneous vessels, is taken advantage of in its use in
the treatment of high temperature in some fevers.

Tea, Coffee, and Cocoa have as their essential constituents
the substances theine and caffeine, which are methyl deriva-
tives of xanthin, a diureide which is excreted in the urine
(see p. 397). They tend to paralyse the sensory mechanism
of the brain, and hence abolish the sense of fatigue. They
stimulate the heart and the secreting action of the kidneys,
but they have no effect upon the metabolism. Cocoa nibs
contain about 8 per cent, of proteid and about 50 per cent,
of fat. Cocoa is therefore an energy -yielding food.

SECTION X

INTERNAL SECRETIONS— THEIR PRODUCTION
AND ACTION

THE products of the metabolism of the various organs
are carried away in the lymph and blood, and certain of
these products exercise an important influence upon other
structures in the body. These products have been termed
Internal Secretions, and the mode of action of some of
them has been more or less investigated. Much, however,
remains to be done before our knowledge can be deemed
anything like satisfactory.

Among the structures which are known to yield such
active internal secretions are the following : —

1. Suprarenal Bodies. — These structures lie just above
the kidneys. Each consists of a tough cortex composed
of epithelial-like cells arranged in columns, and a soft
medulla consisting of cells derived from neuron cells
which stain of a peculiar brown colour with chromic acid.
The medulla is developed from the sympathetic chain of
ganglia. The cortex is a perfectly independent structure
derived from the surrounding mesoblast, and in teleostean
fishes it is quite apart from the representative of the medul-
lary part. A suprarenal body is thus two distinct and
independent organs combined with one another (Fig. 151).

Long ago Brown-Sequard stated that removal of these
bodies causes great muscular weakness, loss of tone of the
vascular system, loss of appetite, and finally death in a short
time. Addison had already pointed out that a similar set
of symptoms, accompanied by pigmentation of the skin, is
associated with diseased conditions of these organs in man.
Within the last few years it has been demonstrated that in-
jections of small quantities of the medullary portion of the

bodies have a powerful effect on the muscular system gene-

384

INTERNAL SECRETIONS

385

rally, and especially on the muscles of the vascular system,
causing contraction of the arterioles, and an enormous rise
in the blood-pressure. It seems to act not directly on the
muscle substance, but rather through the sympathetic nerve
endings. Thus, McFie got no action on the heart of the chick
in which the nerves are not yet developed, while Brodie has
failed to get any effect on the vessels of the lungs in which
the sympathetic nerve terminations are said to be absent.

FIG. 151. — Section through Cortex and Medulla of the Suprarenal Body of
a mammal ; a, 6, c, d, Cortex ; /, Medulla.

Apocodeine, which poisons the nerve endings, abolishes the
effect of suprarenal extracts on the blood vessels.

On the heart it has an inhibitory action through the
vagi, and this may be removed by section of the nerves or
by the administration of atropine, which poisons their termi-
nations.

As already indicated (p. 368), injections of extracts of the
suprarenal bodies profoundly modify the metabolism, lead-
ing to an increase of sugar in the blood and to its excretion
in the urine. This is best marked when the animal is well
fed and has a store of glycogen in its liver, but since it
occurs in fasting animals after the stored carbohydrates

25

386 HUMAN PHYSIOLOGY

have been cleared out by the administration of phloridzin
(p. 368), it would appear to be due, in part at least, either
to a non-utilisation of sugar by the tissues or to an increased
production of sugar from proteids. A decrease in the nitrogen
in the form of urea and an increase of that in ammonia,
similar to that found in cases of true diabetes, have also
been observed.

It has been suggested that the suprarenal secretion acts
through the pancreas by preventing the formation of the
internal secretion which has been supposed to act on the
liver (see p. 390). But in birds it acts even after removal
of the pancreas.

The essential principle of the suprarenals is a substance,
adrenalin, the constitution of which is now known. Various
more or less successful attempts have been made to prepare
it synthetically.

2. Pituitary Body. — This lies at the base of the mid-
brain, and consists of a posterior part of nervous tissue,
somewhat resembling the medulla of the suprarenals, and
an anterior part derived from the alimentary canal, and
consisting of masses of epithelial-like cells.

Removal of this body causes in cats and dogs a fall of
temperature, lassitude, muscular twitchings, dyspmjea, and
ultimately death. Injection of extracts of the substance is
said to diminish these symptoms. In the healthy animal
the injection of extracts of the posterior or nerve parts of
the pituitary causes an augmentation of the force of cardiac
contraction, and a contraction of the arterioles, and thus
raises the arterial pressure.

3. Thyroid Gland (Fig. 152). — This structure is formed as a
hollow outgrowth for the anterior part of the alimentary canal,
which branches and again branches. It early loses its con-
nection with the alimentary canal, and becomes cut up by
fibrous tissue into a number of small more or less rounded
cysts or follicles, each lined with epithelium, and filled with
a mucus-like substance, which contains a nucleo-proteid, and
a substance with a marked power of combining with iodine.
This has been called lodothyrin. It contains about 3'6 per
cent, of iodine, and it seems to be the active constituents of
the internal secretion of the gland.

INTERNAL SECRETIONS

387

The removal of the thyroid usually leads to a train of
symptoms which varies somewhat in different animals, but is
essentially the same in nearly all. The connective tissues
tend to revert to the embryonic conditions, and the amount
of mucin increases. The temperature falls, muscular tremors
appear, and in dogs these may go on to convulsions. They
do not disappear on removing the cortex cerebri, but are
stopped by section of the nerves to the muscles, and
thus appear to be spinal in origin. The function of the

TV,

FIG. 152.— Section through part of the Thyroid (Th.) and a Parathyroid (P.)
of a mammal.

higher nervous system becomes sluggish, and the animal
usually dies. By administering the substance of the
thyroid, or by giving extracts of the thyroid, most of these
symptoms may be delayed or prevented. When thyroid
gland or extract is given to healthy animals in moderate
doses it causes an increased metabolism of both fats and
proteids, and may thus induce emaciation. It would appear
as if one function of the organ is to produce an internal
secretion, which regulates the rate of the metabolic processes
in the body by increasing them when such an increase is
desirable. It seems also to act slightly on the arterioles as a

388 HUMAN PHYSIOLOGY

vaso-dilator. When the thyroid is not developed, the growth
and development of the individual are partially arrested,
and the condition of cretinism is produced. Atrophy of the
structure in adult life causes a train of symptoms somewhat
resembling those produced by its removal, and constituting the
disease, myxcedema. It has been suggested that a condition
of increased activity of the action of the heart, usually accom-
panied by prominence of the eyes and swelling in the region
of the thyroid — exophthalmic goitre — may be due either to
increased activity of the structure, or to deficient action of
the parathyroids.

4. Parathyroids. — Two to four small nodules are found in
close relationship to each lobe of the thyroid often lying in
its substance, and these are formed of columns of cells with
capillary blood vessels between them (Fig. 152). More or less
successful attempts have been made in different animals to
remove them without the thyroid, or the thyroid without
them, and the general result of these experiments is that the
nervous symptoms which follow ordinary thyroidectomy — the
tremors, &c. — seem to be due to the loss of the parathyroids,
while the metabolic changes are probably due to want of the
internal secretion of the thyroid.

5. Ovaries and Testes. — It is well known that removal of
these organs causes characteristic changes in the animal ; a
tendency to the deposition of fat being produced, the activity
of the central nervous system being somewhat modified, the
voice in the male losing its masculine character, and the
thymus persisting, in the male at least, for a considerably
longer period than in the normal. Several years ago Brown-
Sequard maintained that by the administration of testicular
substance the general effects of atrophy of these organs might
be obviated ; and more recently, as a result of clinical experi-
ence, the administration of extracts of the ovaries has been
described as relieving certain of the nervous symptoms
which supervene on their removal or atrophy. It has further
been found that ovarian substance when given to dogs,
whether male or female, causes an increase in the rate of
proteid metabolism, although no similar action is found
with testicular substance. There is thus evidence that the
ovaries, like the thyroid, form an internal secretion having

INTERNAL SECRETIONS

389

an important action in accelerating the metabolism, and
it is at least probable that the testes produce a similar
substance.

6. Thymus. — This structure develops as an epithelial
outgrowth from one or more of the branchial arches of the
embryo. Round these outgrowths masses of lymph' tissue

FIG. 153. — Section of the lobules of the Thymus to show the lobules,
with Hassall's corpuscles in the central part.

collect, and thus a much lobulated structure lying in the front
of the neck and upper part of the thorax is formed. As de-
velopment goes on the epithelial core of each lobule breaks
up and forms nests of cells which are often disposed concen-
trically and form the corpuscles of Hassall. These lie in a
loose lymphoid tissue, the medulla of the lobule, and this
is surrounded by a cortex of dense lymphoid tissue. The
thymus is largest in relationship to the body weight about
the time of birth, but it continues to grow, although not in
proportion to the growth of the body, till about the age of
puberty. After about twenty-four years of age it atrophies
and is replaced by a mass of fatty tissue. The Hassall's cor-
puscles seein to atrophy earlier than the lymphoid tissue.
Castration in cattle and guinea-pigs markedly retards the

390 HUMAN PHYSIOLOGY

onset of atrophy, so that the thymus of the ox is much
larger than that of the bull of the same age. Not only so,
but removal of the thymus in young guinea-pigs seems to be
followed by a more rapid growth of the testes. It is therefore
probable that the thymus yields an internal secretion which
controls the growth of the testes.

The only other effect of its removal in young guinea-pigs
is a diminution in the number of leucocytes. This seems to
lead to a diminished power of resisting the invasion of those
micro-organisms — e.g. staphylococci — which are normally
combated by the leucocytes.

7. Pancreas. — That- the pancreas is the most important of
the digestive glands has been for long known, but a further
function has more recently been demonstrated. It has been
found that the excision of the pancreas in dogs and other
mammals produces a condition of diabetes — an increase of
sugar in the blood, its appearance in the urine, an increased
excretion of nitrogen, and a general emaciation. In ducks
and geese this effect is not produced. These symptoms do
not occur when the duct is tied or occluded until degenera-
tion has developed, but they are invariable and immediate
when a sufficient amount of the gland is removed. They
are not prevented by the administration of pancreas, either
fresh or as extracts. In this condition, sugar is formed
from the proteids, since it appears after all the glycogen has
been removed, and its amount is proportionate to the amount
of nitrogen excreted. The pancreas seems therefore to form
something which either controls the production of sugar in
the liver or causes its utilisation by the muscles. Since the
only respect in which the pancreas differs in histological
character from the parotid gland — removal of which has no
effect on the metabolism — is in the presence of the islets of
Langerhans, it has been suggested that they are related to this
function of the organ. Some recent observations, however,
tend to show that these islets are not permanent structures,
but that they are formed from and revert to the ordinary
secreting tissue. The fact that, in ducks and geese in
which removal of the pancreas does not cause diabetes,
the injection of suprarenal extracts causes glycosuria, is
opposed to the view that it acts through the pancreas,

INTERNAL SECRETIONS 391

and suggests that it must act directly on the liver or other
tissues.

8. Duodenum. — As already pointed out (p. 349), the duo-
denum yields an internal secretion (secretin), which acts
directly upon the pancreas to stimulate its secretion.

Toxic Action and Immunity.

It is not definitely known how these internal secretions
each perform its special action, but light seems to be thrown
upon the question by the study of the mode of action of
various toxic substances, and the mode of production of a
condition of immunity against them. As will be presently
shown, a process of the same nature as the production of
internal secretions is involved.

Snake and Diphtheria Toxins. — It may be most simply
explained by considering first the probable mode of action
of the toxin or poison of snake venom, or of that produced
by the diphtheria bacillus, and the way in which protection
against these is established by the development of anti-
toxins.

By injecting under the skin of the horse increasing
doses of such toxins the animal becomes quite resistant
to the poison, and if now some of the horse's serum is
taken it is found that a certain quantity can neutralise a
definite quantity of the toxin, so that if the mixture is in-
jected it does no harm to the animal. Something has been
formed in the horse which seizes on the molecules of the
toxin and makes them harmless, just as when soda is added
to sulphuric acid it forms a neutral salt.

The two molecules have a definite chemical affinity for one
another, so that the toxin is no longer free to seize upon the
protoplasm of the animal's body. To explain this Ehrlich
has suggested that the protoplasm molecule, like the proteid
molecule (p. 10), is to be considered as made up of a central
core with a number of side chains or hands which play an
important part in taking up nourishment of different kinds,
for each variety of which special side chains have a special
affinity (Fig. 154). He supposes that some of these side chains
fit the toxin molecule, and are thus capable of anchoring it

392 HUMAN PHYSIOLOGY

to the cell and allowing it to exercise its toxic action, and he
explains the production of antitoxin by supposing that, as
these side chains get linked to the toxin and are thus as it
were thrown out of action, others are produced to take their
place, since they are necessary for the nourishment of the
protoplasm. If the toxin is continually administered in
small doses this production of side chains may be so in-
creased that they get thrown off into the blood and in it
are capable of linking to the toxin and so preventing it
from fixing itself to the cells. If therefore some of the
blood is injected into an animal which afterwards receives
a dose of the toxin, that toxin will not act, and the animal

will be immune.

Typhoid Toxin. — But immu-
nity may also be established not
against toxins separate from
organisms, but against organisms
which hold their toxin, as in the
case of the bacillus of typhoid
fever. Here repeated injections

FIG 154.-TO illustmte the forma- of increasing doses produce a
tion of Side Cha:ns, SC, by . ° .

which the toxin molecules, T, serum which has the power of

are either anchored to the cell destroying the Organism when

:Lr±tt7"an?- added to it even outside the
toxin is formed. body. But this is not a simple

combination, because if the serum

be heated to 55° C. it loses its power, but if a few drops
of the fresh serum of an unimmunised animal be added,
the power is restored. Obviously the anti-body which
destroys the organism — the bacteriocidal or bacteriolytic
body — requires the co-operation of another body to enable
it to act, and this body has been called the complement.
Ehrlich supposes that the immune body does link to the
protoplasm of the organism, but that it must in its turn be
linked to the complement. The figure may help to explain
this (Fig. 155).

Cytotoxins. — Similar anti-bodies, acting upon the cells
of the animal body, may be produced by injecting
the particular kind of cell into an animal of another
species. Thus, if human blood be repeatedly injected

INTERNAL SECRETIONS

393

into a rabbit the serum of the rabbit's blood becomes
hwnolytic — i.e. acquires the power of dissolving the

com

a

erythrocytes in human blood. In
this case too the immune body re-
quires the presence of a comple-
ment, readily destroyed at a low
temperature, to enable it to act.

Precipitins. — By the injection of
the proteids of the blood of any parti-
cular animal into an animal of another
species a serum is developed which

precipitates the proteids of the blood FIG. 155.— To illustrate the

of the first species and of no others.

In these various cases the active
body is produced by the throwing
off of side chains from protoplasm,
and as these products are carried away in the blood the
process is exactly analogous to the formation of internal
secretions.

anchoring of the anti-
body, a, to the cell by
a side chain, sc, and
the action upon it of
complement, com.

SECTION XI
EXCRETION OF MATTER FROM THE BODY

1. EXCRETION BY THE LUNGS (see Respiration, p. 277)
2. EXCRETION BY THE KIDNEYS

URINE

THE water and waste nitrogen of the body are chiefly elimi-
nated in the urine, which is secreted by the kidneys.

The testa for the various constituents of the urine must be
studied practically. (Chemical Physiology, p. 22 et seq.)

I. Physical Characters.

The characters of the urine depend largely on the
relative proportion of water and of solids which are excreted
in it : at one time it may be very concentrated, while at
another time it may be very dilute indeed. For this reason
its specific gravity, which depends upon the percentage of
solids in solution, varies within wide limits, being often as
high as 1030 and frequently as low as 1005 ; but the average
specific gravity is about 1020. It is possible from the
specific gravity to form a rough idea of the amount of solids
present, for by multiplying the last two figures by 2'22 the
amount of solids per 1000 parts is given.

Since the percentage of pigments in the urine varies like
that of the other constituents, the colour of the urine shows
wide divergence in the normal condition. A concentrated
urine has a dark amber colour, while a dilute urine may be
almost colourless. Under average conditions the urine has
a straw yellow colour.

The reaction of urine is normally acid in man, chiefly
from the presence of acid sodium phosphate, NaH2P04, and

394

EXCRETION OF MATTER FROM THE BODY 395

the degree of acidity varies with the concentration. But the
acidity of the urine may also be varied by different con-
ditions. It is increased when there is an increased oxidation
of proteids, for the sulphuric acid and phosphoric acid thus
formed have to be neutralised by combining with the
alkalies. It is decreased by taking alkalies, or by taking
such vegetable salts as the citrate, malate or tartrate of
soda, because these are oxidised in the body and excreted
as the carbonates. The NaH2P04 may then be changed
to Na2HPO4.

Urine is normally transparent; but when it has stood for
a few hours, a cloud of a mucin-like substance is seen floating
in it. When it is alkaline it is turbid from the separation
of a white deposit of phosphates of lime and magnesia. In
the alkaline urine of herbivora the white deposit is chiefly
composed of carbonate of lime.

A brick-red deposit of urates tends to fall as the urine
cools when it is concentrated and very acid.

The smell of urine is characteristic, and it may be
modified by the ingestion of many different substances.
*

II. Composition.

Since the relative amounts of water and solids vary
within such wide limits, the percentage composition of urine
is of little moment. Under average conditions the water
constitutes about 96 per cent., and the solids about 4 per
cent. Of these solids, rather more than half are organic,
rather less than half are inorganic. Since water and solids
are derived from the water and solids taken by the indi-
vidual, the amounts excreted depend upon the amounts
taken, and must be considered in connection with them.
Thus if a man takes little fluid, he will pass little water in
the urine. If he takes little food, a small quantity of solids
will be excreted by the kidneys. Since excretion and inges-
tion must be studied in relationship to one another, it is
convenient to compare them during a definite period of
time, and the natural division into days of twenty-four
hours is generally adopted.

Under ordinary conditions the amount of solid food taken

396 HUMAN PHYSIOLOGY

per day does not vary very greatly, but the amount of fluids
imbibed varies within much wider limits. For this reason,
while the amount of water excreted in the urine per diem
varies enormously, the amount of solids is more fixed. In a
man on an average diet, it may be stated that something
like 1500 c.cm. of water and 60 to 70 grms. of solids are
daily eliminated.

I. Nitrogenous Substances.

A. Urea. — The chemistry and mode of formation of urea
have been discussed on p. 369. Since it is as urea that,
on an ordinary diet, nearly 90 per cent, of the waste nitrogen
is eliminated in the urine, the amount excreted depends
upon the amount of proteids taken in the food. For this
reason, during fasting, the excretion of urea may fall as low
as 6 grms. per diem, while on a diet containing the ordinary
amount of proteids, about 33 grms. — 15*4 grms. of nitrogen —
are excreted. On a normal diet from 86 to 90 per cent, of
the waste nitrogen is excreted as urea, but, when the nitrogen
intake is decreased, the proportion of urea-nitrogen may fall
to as low as 60 per cent.

When the urine is allowed to stand, certain micro-
organisms are apt to get into it, and to cause a hydration of
the urea, whereby it is changed into ammonium carbonate —

TT T_T

0 H H) [H

H N-O-C-O-N '

H' IH

The urine is thus made alkaline, and the phosphates of
the earths are precipitated. The phosphate of magnesia
combines with the ammonia to form ammonio-magnesium-
phosphate, NH4MgP04-f 6H2O, which crystallises in charac-
teristic prism-like crystals.

B. Non-Urea Nitrogen. — The 10 or 12 per cent, of nitrogen
which on an ordinary diet is not excreted as urea is dis-
tributed in : —

1. Ammonium Salts. — About 4 or 5 per cent, of the total

EXCRETION OF MATTER FROM THE BODY 397

nitrogen is normally excreted as ammonium salts. But
under certain conditions the proportion is greatly increased.
Anything which causes an increased breaking down of
proteid, and an increased formation of acids, leads to an
increased excretion of ammonia — the ammonia being formed
from the proteids to neutralise the acids.

2. Diureides. — The members of this series of bodies
consist of two unmodified or modified urea molecules, linked
together by an acid nucleus. The most important of the
series have as the linking molecule acrylic acid, and they
constitute the purin bodies.

0

II
j H— N— C— N— H i Urea.

C=C — C = 0 Acrylic acid nucleus.

; H— N N— H !

i \/ I

C Urea.

II
0

In birds and reptiles they replace urea as the substances
in which nitrogen is chiefly eliminated. In these animals
they are formed in the liver from lactate of ammonia,
derived from proteids, but in mammals they appear to be
very largely derived from the decomposition of nucleic acid.
Even when all supplies from without of nucleins and
purin bodies are cut off, a certain amount of these purin
bodies is daily eliminated. These have been called the
"endogenous" purins, while those derived from the con-
stituents of the food are termed the " exogenous " purins. A
certain amount of the purins formed are changed to urea
before being excreted, and, therefore, when disturbances
of the chemical processes in the liver occur, the purins
appear to be increased at the expense of the urea.

Uric Acid is the most important member of the series.
Its constitution is shown above.

398 HUMAN PHYSIOLOGY

It is an exceedingly insoluble substance which tends to
crystallise in large irregular crystals, and in the urine these
are generally coloured brown by the urinary pigment.

It occurs as salts of sodium and potassium, and, according to
Roberts, the acid salt, NaHlT, is linked to a molecule of the
acid to form NaHU-H2TT, or, what he calls, a quadriurate ;
but the evidence of this is not conclusive.

Although the salts of uric acid are more soluble than the
free acid, only a small quantity can be dissolved in the
urine. Apparently inorganic salts, such as phosphate of
soda, act as solvents. When the urine cools, especially if it
is unusually acid, the urates tend to separate out and fall as
a brick-red deposit, which generally shows no crystalline
structure under the microscope.

If the urine has become ammoniacal on standing, the
deposit frequently contains characteristic spinous pigmented
crystals of urate of ammonia.

From their insolubility, uric acid and the urates tend to
form calculi or concretions in the urinary passages. The
presence of uric acid in such concretions is recognised by the
murexide test, which depends upon the fact that uric acid
heated with nitric acid is oxidised to alloxantin, which
strikes a purple colour with ammonia, yielding murexid —
the ammonium salt of purpuric acid.

Other members of the series, such as xanthin and
hypoxanthin, occur in the urine in small quantities.

Allantoin, which occurs in the urine of the foetus, and in
the urine of dogs after the administration of nucleic acid, is
a diureide in which gly coxy lie acid with two carbon atoms is
the linking band.

3. Greatinin. — Creatinin is the form in which the creatin
of muscle is excreted.

Creatin is methyl-guanidin-acetic acid (see p. 43). By
dehydration creatinin is produced. The amount excreted is
always small, and depends upon the amount of muscular
tissue broken down in the body. According to the in-
vestigations of Folin the amount of creatinin excreted
per diem on a flesh-free diet is very constant in each in-
dividual, and does not vary with the amount of proteid
food taken.

EXCRETION OF MATTER FROM THE BODY 399
4. Hippuric Acid. — This is benz-amido-acetic acid.

H

H 0

_C— C— 0— H
H

It is formed from benzoic acid taken in the food by link-
ing it to glycocoll — amido-acetic acid. This synthesis appears
to take place in the kidneys, for it has been found that
hippuric acid is not formed when these organs are excised,
and that, when blood containing benzoates is circulated
through them, hippuric acid is produced. Its chief interest
is in the fact that it is one of the first organic compounds
which were demonstrated to be formed synthetically in the
animal body. Normally it is present in human urine in
very small quantities, but in the urine of herbivora the
amount is considerable, from the presence of benzoic acid
in the fodder. The acid itself is insoluble, and it occurs as
the soluble soda salts.

II. Sulphur-containing Bodies.

The sulphur excreted in the urine is derived from the
sulphur of the proteid molecule, and the amount of sulphur
excreted may be taken as a measure of the amount of pro-
teid decomposed. This is sometimes used as a check upon
an estimation from the excretion of nitrogen.

A. Acid Sulphur. — The greater part of the sulphur is
fully oxidised to S03. (a) Preformed Sulphates. About
nine-tenths of this is linked with bases to form ordinary
sulphates.

(b) The other one-tenth is in organic combination, linked
to benzene compounds, Ethereal Sulphates. The indol,
skatol, and phenol (see p. 351), formed by the putrefaction
of proteids in the bowel, being excreted in the urine in an
oxidised form linked with sulphuric acid. Indol, as already
shown, is related to amido-ethyl-benzene.

40O

HUMAN PHYSIOLOGY

It is oxidised into indoxyl thus —

H

_N— C— H

//
C

O'H

This when linked to sulphate of potassium forms indoxyl-
sulphate of potassium or indican.

S02OK

From skatol, which is methyl-indol, skatoxyl-sulphate of
potassium is formed in the same way.

These bodies are colourless, but when oxidised they yield
pigments — indican yielding indigo blue, skatoxyl-sulphate of
potassium yielding a rose colour.

— 0— H

— 0— S02OK

Phenol is also linked with sulphate of potassium, and
excreted in the urine.

Their amount depends upon the activity of putrefaction in
the intestine, and is a good index of its extent.

EXCRETION OF MATTER FROM THE BODY 401

When Dioxybenzene or Pyrocatechin is formed in the body,
it too is linked to sulphate of potas-
sium and excreted. When urine
containing this substance stands, it be-
comes oxidised and yields a greenish
brown or black pigment.

B. Neutral Sulphur. — A small quantity of sulphur is some-
times excreted in a less oxidised state, in the form of neutral
sulphur. The most important compound of this kind is
cystin, the disulphide of amido-propionic acid, two molecules
of amido-propionic acid linked by sulphur —

Amido-propionic acid

i

Sulphur

I
Sulphur

i .

Amido-propionic acid

In some individuals and in certain conditions of the meta-
bolism not yet fully understood, the amount of cystin is
increased, and it then tends to crystallise out in peculiar
hexagonal plates.

III. Phosphorus-containing Bodies.

The phosphorus in the urine is derived partly from phos-
phates taken in the food, and partly from the nucleins of
the food and tissues and from the bones.

(a) Normally the phosphorus is fully oxidised to P205,
which is linked to alkalies and earths, and excreted in the
urine. The most important phosphate is the phosphate of
soda, NaH2P04, which is the chief factor in causing the
acidity of the urine. About one quarter of the phosphoric
acid is linked to lime and magnesia, and it is these earthy
phosphates which precipitate when the urine becomes
alkaline. When the urine becomes ammoniacal, triple
phosphate is formed (p. 396).

(b) It is probable that a small quantity of the phosphorus

26

402 HUMAN PHYSIOLOGY

is excreted in organic compounds, such as glycero-phosphates ;
but so far these have not been fully investigated.

IY. Chlorine-containing Bodies.

Chloride of sodium is the chief salt of the urine. It is
entirely derived from the salt taken in the food, and its
amount varies with the amount ingested. From 10 to 15
grms. are usually excreted per diem in a person on normal
diet.

In starvation, and still more in fever, the tissues of the
body have an extraordinary power of holding on to the
chlorine, and the chlorides may almost disappear from the
urine.

Y. Bases of the Urine.

Sodium, potassium, calcium, and magnesium occur in the
urine in amounts varying with the amounts taken in the
food. Generally speaking sodium is in excess of the others,
but on a flesh diet and in starvation it may fall below the
potassium. Calcium and magnesium are present in much
smaller quantities.

YI. Pigments.

A brown hygroscopic substance, which gives no bands in
the spectrum, may be extracted from urine. This has been
termed urochrome. By reducing this, another pigment,
urobilin, is produced, which gives definite bands, and which
is frequently present in the urine. It is probably identical
with the hydrobilirubin which has been prepared from the
bile pigments, and it contains C. H. O. and N. The pigment
which gives the pink colour to urates has been called uro-
erythrin, and its chemical nature is unknown. Haemato-
porphyrin (see p. 198) is normally present in small traces in
the urine, but in certain pathological states it is increased in
amount, and gives a brown colour to the urine.

YII. Nucleo-proteid.

A mucin-like substance derived from the urinary passages
is always present in small amounts, and forms a cloud when
the urine stands.

EXCRETION OF MATTER FROM THE BODY 403

YIII. Carbonic and Oxalic Acids.

1. Carbonic Acid. — Small amounts of this are present
in urine, and after the administration of citrates, malates,
or tartrates, the amount may be considerably increased,
and the urine may then effervesce strongly when an acid
is added.

2. Oxalic acid is a substance in a
stage of oxidation just above that of

u

TT Q Q Q Q jj carbonic acid. It is frequently pre-
sent in the urine linked with lime,

and the lime salt tends to crystallise out in characteristic
octohedra, looking like small square envelopes under the
microscope. Under certain conditions these crystals assume
other shapes. The oxalic acid of the urine is chiefly derived
from oxalates in vegetable foods, but it has been detected in
the urine of animals on a purely flesh diet.

SECRETION OF URINE.

Structure of the Kidney.

(This must be studied practically.} The kidney (Fig. 156)
is a compound tubular gland, consisting of innumerable
tubules, each made up of—

A closed extremity or Malpighian body (M.B.), consisting
of an expansion at the end of the tubule — Bowman's capsule
—into which a tuft of capillary vessels — the glomerulus —
projects. Extending away from this is —

A proximal convoluted tubule (P.C.T.) lined by pyramidal
and granular epithelial cells. This dives into the medulla
and again ascends to the cortex, forming Henle's Loop
{H.L.), which terminates in the distal convoluted tubule.
This exactly resembles the proximal (D.C.T.). It opens
into a collecting tubule (C.T.) lined by a low transparent
epithelium, which conducts the urine to the pelvis of the
kidney.

The renal artery breaks up and gives off a series of straight
branches — the interlobular arteries (IL.A.) — which, as they
run towards the surface, give off short side branches which
terminate in the glomeruli. The efferent vein passing from

404

HUMAN PHYSIOLOGY

these breaks up again into a series of capillaries between
the convoluted tubules, and these pour their blood into the
interlobular veins (IL.V.). This arrangement must help
to maintain a high pressure in the capillary loops of the
glomerular tuft.

Physiology of Secretion.

I. Secretion in the Malpighian Bodies. — A consideration
of the structure of the Malpighian bodies tends to the conclu-

FlO. 156. — Diagram of the Structure of the Kidney. M.P., Malpighian
Pyramid of the Medulla; M.R., Medullary Ray extending into
Cortex; L., Labyrinth of Cortex; M.B., a Malpighian Body con-
sisting of the Glomerular Tuft and Bowman's Capsule ; P.C.T., a.
Proximal Convoluted Tubule; H.L., Henle's Loop on the Tubule;
D.C.T,, Distal Convoluted Tubule ; C.T., Collecting Tubule ; R.A.,
Branch of Renal Artery, giving off IL.A.. Interlobular Artery, to
supply the Glomeruli and the Convoluted Tubules ; IL. V, , Inter-
lobular Artery bringing Blood back from the Cortex.

sion that in them the water of the blood must filter out into
Bowman's capsule, under the influence of the intravascular
pressure. That they do act in this way is demonstrated by
(a) the influence of the blood pressure in the kidneys on the
flow of urine, and (6) the effect of increasing the pressure in
the ureters on the flow of urine.

EXCRETION OF MATTER FROM THE BODY 405

(a) Blood pressure in the vessels of kidney. The pressure
of blood in the glomerular tufts may be increased in two
ways — (a) By dilating the arteries, and (b) by constricting or
occluding the veins. The former leads to an increased pres-
sure on the arterial side of the capillary loops and to a more
rapid flow of blood, the latter to a rise in pressure on the
venous side and a slower flow of blood (see p. 269). In both
cases the amount of blood in the kidney is increased, and the
organ expands ; but while the former condition is associated
with an increased flow of urine, the latter is accompanied by
a decreased or arrested flow. A high pressure in the glome-
ruli, with a rapid flow of blood, increases the flow of urine.
This may be produced (a) by cutting the renal nerves, which
maintain a tonic constricting action on the arterioles ; (b) by
raising the general arterial pressure and thus forcing more
blood into the kidneys.

The converse condition of decreased pressure in the gloine-
ruli and diminished rate of blood-flow may be produced by
stimulating the renal nerves derived from the llth, 12th,
and 13th dorsal nerves, or by causing a decrease of the
general arterial pressure. A fall of the carotid pressure to
about 50 mm. Hg in the dog is generally sufficient to arrest
the flow of urine. The influence of such a fall of pressure is
often seen in heart disease, where, as a result of it, the excre-
tion of water by the Malpighian bodies is so impeded that
dropsy supervenes.

(b) If the ureter be ligatured the production of urine stops
when the pressure behind the ligature rises to about 50 mm.
Hg. This further supports the view that the formation of
urine in the Malpighian bodies is due to filtration under
pressure.

The urine formed in these bodies is alkaline, as has been
demonstrated by applying an indicator — a substance the
colour of which is changed by alkalies — to the cut surface
of the kidney ; and this further supports the conclusion that
it is simply an exudation from the blood.

That various substances in solution pass out with the
water is demonstrated by the fact that when haemoglobin is
injected into the blood vessels it soon makes its appearance
in Bowman's capsules and in the urine.

406 HUMAN PHYSIOLOGY

It may also be demonstrated by taking advantage of the
fact that in the frog the Malpighian bodies are chiefly, if
not entirely, supplied from the renal artery, and the tubules
from a portal vein. When sugar or commercial peptone is
injected into the circulation, it appears in the urine, but
if the renal arteries are first ligatured, it does not appear,
proving that it is by the gloineruli that it is passed out.

II. Secretion in the Tubules. — The alkaline urine formed in
the Malpighian bodies undergoes changes as it passes along
the tubules. It becomes acid and the various solids must
be increased, for urine contains a higher proportion of these
than does the blood. The blood contains only about 0'03 per
cent, of urea, but the urine usually contains 2 per cent. It
has been suggested that this is due to absorption of water
from the tubules, by which the urine formed in the Mal-
pighian bodies become more concentrated. But there is no
evidence that this takes place. On the other hand, the fol-
lowing facts seem to show that solids are added to the urine
by a process of secretion from the cells of the tubules.

(a) Uric acid crystals are frequently found in the cells of
the convoluted tubules of the kidney of birds. (6) Heiden-
hain, by injecting a blue pigment — sulph-indigotate of soda
— into the circulation of the rabbit, demonstrated that the
cells of the convoluted tubules take it up and pass it into the
urine. In the normal rabbit the whole of the kidney and the
urine become blue. But if the formation of urine in the
Malpighian bodies is stopped by cutting the spinal cord in
the neck so as to lower the blood pressure, then the blue pig-
ment is found in the cells of the convoluted tubules and of
the ascending limb of Henle's tubule, (c) When the Mal-
pighian bodies of the frog have been thrown out of action by
ligaturing the renal arteries, the injection of urea still causes
a flow of urine and the excretion of that substance by the
tubules.

This last experiment seems to show that the cells of the
convoluted tubules are capable of secreting water, and that
they can do so is further demonstrated by the fact that, if
by cutting the spinal cord in the neck the formation of
urine in the Malpighian bodies of a dog is stopped, the ad-
ministration of caffeine and of some other substances causes

EXCRETION OF MATTER FROM THE BODY 407

an increased flow of urine, although the blood pressure in the
kidney is not raised. This is taken advantage of in cases of
heart disease, when the secretion of urine is almost arrested
from low arterial pressure, and when dropsy is rapidly ad-
vancing. Until the heart is toned up, the kidneys may be
stimulated to get rid of water by means of such diuretics as
caffeine.

EXCRETION OF URINE.

1. Passage from Kidney to Bladder. — The pressure under
which the urine is secreted is sufficient to drive it along the
ureters to the bladder. If these are constricted the pressure
behind the constriction rises, and may distend the ureter and
pelvis of the kidney, but when it reaches about 50 mm. Hg
in the dog, the secretion of urine is stopped. The muscular
walls of the ureters show a rhythmic peristaltic contraction,
which must also help the onward passage of the urine to the
bladder.

2. Micturition. — As the urine accumulates in the urinary
bladder the rhythmic contraction of the non-striped muscle
becomes more and more powerful. These contractions are
chiefly excited by the fibres of the nervi erigentes of the
second and third sacral nerves, although fibres passing down
from the inferior mesenteric ganglion also probably act either
in exciting or inhibiting in different animals. Very different
results as regards the action of these nerves have been
observed, and at present no definite conclusion as to their
mode of action is possible. The backward passage of the
urine into the ureters is prevented by the oblique manner
in which these tubes pass through the muscular coat of the
bladder.

The passage of urine into the urethra is at first prevented
either by the oblique manner in which the urethra leaves the
bladder, or more probably by the contraction of a strong
band of non-striped muscle, the sphincter trigonalis. This
muscle or the striped fibres which surround the membranous
part of the urethra are under the control of a centre in the
lumbar enlargement of the spinal cord, and the expulsion of
urine must be preceded by their relaxation. In some cases
of inflammation of the spinal cord the increased activity of

408 HUMAN PHYSIOLOGY

the centre may prevent the expulsion of urine, while later in
the disease, when the nerve structures have been destroyed,
the urine is not retained and dribbles away on account of the
absence of the tonic contraction of the muscles.

The expulsion of the last drops of urine is carried out by
the rhythmic contraction of the bulbo-cavernous muscle,
while the peristaltic contraction of the bladder wall is assisted
by the various muscles which can press upon the contents of
the bladder.

In man, in early life, micturition is a purely reflex act, and
in the dog it is perfectly performed when the spinal cord is
cut in the back. As age advances the reflex mechanism
comes to be more under the control of the higher centres,
and the activity of the sphincters may be increased or
abolished as circumstances indicate.

3. EXCRETION BY THE SKIN.

The skin is really a group of organs, and some of these
have been already studied. (The structure of the skin and
its appendages must be studied practically.)

(1) The Protective functions of the horny layer of epi-
dermis, with its development in hair and nail, and of the
layer of subcutaneous fat, are manifest.

Hair. — In man the hair has largely ceased to have the im-
portant protective function it fulfils in many of the lower
animals, but the muscular mechanism by which the position
of the hairs can be modified still persists. Attached to each
hair follicle is a band of non-striped muscle, the arrector pili,
which by contracting can erect the hair. These muscles are
under the control of the central nervous system, and the
nerve fibres have been demonstrated in the cat to take much
the same course as the vaso-constrictor fibres of somatic
nerves (see p. 264). A hair after a time ceases to grow, and
the lower part in the follicle is absorbed and the hair is
readily detached. From the cells in the upper part of the
follicle a new down-growth occurs, a papilla forms, and the
hair is regenerated. In many of the lower animals this
process occurs twice a year.

EXCRETION OF MATTER FROM THE BODY 409

(2) The Sensory functions have been studied under the
Special Senses (p. 98 et seq.).

(3) The Respiratory action of the skin in mammals is of
little importance.

(4) The Excretory Function of the Skin. — Three sets of
glands develop in the skin — sweat glands and sebaceous
glands, which are common to both sexes and are constantly
active — and mammary glands, which are active in the female
during the period of suckling.

A. Sweat Secretion — 1. Sweating. — The simple tubular
sweat glands are exceedingly numerous. It has been calcu-
lated that a man possesses about two and a half million, and
that if spread out they would present a surface of very great
extent.

From these glands a considerable amount of sweat is
poured out, but to form any estimate of the daily amount
is no easy matter, since it varies so greatly under different
conditions. Probably about 1000 c.cm. is an average amount.
When poured out, sweat usually evaporates, and is then called
insensible perspiration, but when large quantities are formed,
or when, from coldness of the surface, or of the air, or from
the large quantity of watery vapour already in the air, eva-
poration is prevented, it accumulates, and is called sensible
perspiration.

A free secretion of sweat is usually accompanied by a
dilatation of the blood vessels of the skin, but this may be
absent, and it may occur without any sweat secretion — e.g.
under the influence of atropine.

2. Nervous Mechanism of Sweat Secretion — The sweat
glands are under the control of the central nervous system.
This may be very conveniently studied in the cat, in which
animal the sweat glands are chiefly in the pads of the feet.
If a cat be put in a hot chamber it sweats on the pads of all
its feet. But if one sciatic nerve be cut the foot supplied
remains dry. If the cat be placed in a warm place and the
lower end of the cut sciatic stimulated, a secretion of sweat
is produced. These sweat-secreting fibres all pass through
the sympathetic ganglia, and back into the spinal nerves.

4io HUMAN PHYSIOLOGY

Those to the leg and foot come from the upper lumbar region,
those for the hand and arm from the fifth to the eighth cervical
nerve, and those for the head chiefly from the medulla.

The centres presiding over these nerves have not been
definitely located. But they are capable of (a) reflex stimula-
tion, as when pepper is taken into the mouth ; and (6) of
direct stimulation by a venous condition of the blood, as in
the impaired oxygenation of the blood which so frequently
precedes death as the respirations fail.

But even after the nerves to the sweat glands are cut, the
glands can be stimulated by certain drugs — e.g. pilocarpin.
The action of heat seems also to be chiefly peripheral, setting
up an unstable condition of the gland cells so that they re-
spond more readily to stimulation.

3. Chemistry of Sweat. — Sweat is a clear, watery fluid,
which, when pure, has a neutral or faintly alkaline reaction,
but which is generally acid from admixture with the sebaceous
secretion. Its specific gravity is low, about 1004, and it
contains less than 2 per cent, of solids, of which the chief is
NaCl. Of the organic solids, urea is the most important.
About 4 or 5 per cent, of the total nitrogen excreted from
the body is thrown off by the skin in this form, and, when
the action of the kidneys is interfered with, very considerable
quantities may be eliminated. The sweat nearly always con-
tains epithelial squames from the epidermis, and oil globules
derived from the sebum.

B. Sebaceous Secretion. — The sebaceous glands are simple
racemose glands which open into the hair follicles, and their
function is to supply an oily material to lubricate the hairs.
This secretion is produced by the shedding and breaking
down of the cells formed in the follicles of the glands.
Those lining the basement membrane are composed of proto-
plasm and actively divide, but the cells thrown off into the
lumen of the follicle disintegrate and become converted into
a semi-solid oily mass, which consists of free fatty acids and
of neutral fats of glycerine and of cholesterin. These latter
are the lanolins, which differ from ordinary fats in being
partly soluble in water. Free cholesterin is also present in
the sebum.

C. Milk Secretion — 1. Physiology. — JBefore pregnancy

EXCRETION OF MATTER FROM THE BODY 411

occurs the mammary glands are largely composed of fibrous
tissue, with a large amount of fat, in which run the branch-
ing tubules of the glands as small solid blocks of cells
completely filling the lumen.

As pregnancy advances these tubules grow outwards and
increase, and the cells begin to divide, some remaining
attached to the basement membrane, some coming to lie
in the middle of the tubules. These latter undergo a fatty
change and break down, and they are shed in the first milk
which is secreted, which is known as the colostrum. The
cells left upon the basement membrane elaborate the con-
stituent of milk, and the presence of fat globules in their
protoplasm is very manifest.

The milk, after being secreted, collects in the ducts of the
glands and in the sinus below the nipple, and is expelled
from these by the contraction of the muscular fibres in their
walls, and by the suction of the young animal. The excre-
tion of milk from the ducts is directly under the control of
the nervous system, but the evidence as to the way in which
the central nervous system influences the secretion of milk
is by no means satisfactory. Clinical experience shows that
it is profoundly modified by nervous changes, but so far
stimulation of the nerves to the glands has not yielded
definite results.

2. Characters of Milk. — The white colour of milk is due
to its being a fine emulsion of fat globules floating in a clear
plasma. Hence, when the cream is removed, milk becomes
less white and less opaque. Its specific gravity is about
1030, and in man and herbivorous animals its reaction is
alkaline.

Its composition in the human subject and in the cow is
given on p. 318.

Proteids. — The chief proteid is caseinogen, which exists as
a soluble calcic compound. It is of the nature of a phospho-
proteid, but contains very little phosphorus, and does not
yield purin bases. It is not coagulated on boiling, but its
combination with lime is split under the action of acids and
the casein is precipitated. Under the influence of rennet it
splits into whey albumin and paracasein lime, which is in-
soluble, and separates out as a curd which generally holds

4i2 HUMAN PHYSIOLOGY

the fat of the milk, and is therefore white. From the curd
a colourless whey containing albumin and milk sugar exudes.

Fats. — Olein is the chief fat, but fats of the lower fatty
acids are also present. They exist in a fine state of sub-
division, suspended in the milk plasma, and each globule is
apparently surrounded by a thin covering of proteid, which
has to be removed by the action of an acid or an alkali before
the fat can be extracted with ether.

Sugar. — The disaccharid lactose is the sugar of milk
(see p. 317). Under the action of various micro-organisms
it is split up to form lactic acid, thus causing the souring of
milk.

Phosphorus Compounds. — In addition to caseinogen, milk
contains other organic phosphorous compounds. Among
these is lecithin and a compound which has been called
phosphocarnic acid, the constitution of which is not fully
understood. In human milk the greater part of the
phosphorus is in organic combinations, while in cow's milk
the amount in inorganic compounds is much greater.

Milk is specially rich in calcium and potassium, but the
amount of iron in milk is very small, and therefore, when
the child has used up the store of iron which it has in its
body at birth, it is necessary to replace the milk-diet l>y
foods containing more iron.

PART III

SECTION XII
REPRODUCTION

So far the animal has been studied simply as an individual.
But it has also to be regarded as part of a species, as an entity
which has not only to lead its own life, but to transmit that
life to offspring.

The various problems of reproduction have been already
studied by the student in connection with biology, and it is
here sufficient to indicate some of the main points in the
physiology of the process in mammals.

(The structure of the organs of reproduction must be
studied practically.)

While the individual is actively growing, the reproductive
organs are quiescent ; but when puberty is reached, they begin
to perform their functions — the testes to produce sperma-
tozoa, the ovary to produce mature ova.

The removal of the sexual organs in the young animal
leads to arrest in the development of the special sexual
characters, especially in the male, in which these characters
are generally best marked. Simple ligature of the vas
deferens has not this effect.

The genital gland in both sexes is formed from a longi-
tudinal thickening or ridge at the posterior part of the
coelom or peritoneal cavity. Over this ridge the endothelium
thickens and becomes epithelial-like in structure. Groups
of cells grow down into the tissue below.

In the ovary one of these cells in a group takes a central
position and forms the ovum, while the other cells get
arranged around it to form the zona granulosa, the whole

group constituting a Graaiian follicle.

413

4i4 HUMAN PHYSIOLOGY

In the testis the groups of cells form seminiferous tubules,
in which the spermatozoa or male elements are developed.

Ovary. — In the adult the ovaries are oval structures covered
by a columnar germinal epithelium. In the stroma are
seen Graafian follicles in different stages of development.
The central cell, the ovum, enlarges. The nucleus becomes
prominent, and is sometimes called the germinal vesicle.
The nucleolus is also large, forming the germinal spot. The
protoplasm becomes encased in a transparent capsule — the
zona pellucida. The cells of the zona granulosa multiply,
and fluid (the liquor folliculi) appears among them,
dividing them into a set attached to the capsule of the
follicle and a set surrounding the ovum. When the follicle
is ripe it projects on the surface of the ovary, and finally
bursts, setting free the ovum into the peritoneal cavity. It
passes into the trumpet-shaped fimbriated upper end of the
Fallopian tube, through which it reaches the uterus.

Testis. — In the adult this is enclosed in a dense fibrous
capsule — the tunica albuginea. Posteriorly this is
thickened, and forms the corpus Highmori. From it pro-
cesses extend and form a supporting framework. In the
spaces are situated the seminiferous tubules, which open
into irregular spaces in the corpus Highmori — the rete testis,
from which the efferent ducts (vasa efferentia) pass away to
join together to form the vas deferens.

In the seminiferous tubules the spermatozoa are produced.
Some of the lining cells divide into two, forming a sup-
porting cell and a spermatogen. The latter divides and
subdivides till a group of cells lie on the top of the sup-
porting cell. These are the spermatoblasts. In each sper-
matoblast the nucleus elongates and passes to the attached
extremity, the protoplasm decreases in amount, and a long
cilium develops from the free end, and the spermatozoon
is thus produced.

Semen. — When the testes have become active, the glands
of the prostate increase and produce a fluid which, with the
spermatozoa, forms the semen.

Menstruation. — In the case of the female the maturation
of the ova is accompanied by changes in the uterus. Every
four weeks the superficial layers of the mucous membrane of

REPRODUCTION 415

the uterus break down, and are shed during the menstrual
period. After the menstrual period, the membrane again
regenerates.

Impregnation is effected by the transmission of sperma-
tozoa into the passages of the female. For this purpose
erection of the penis is brought about reflexly through a
centre in the lumbar enlargement of the cord, the outgoing
nerves being the nervi erigentes, which dilate the arterioles,
and the internal pudics supplying the transversus perinei
and bulbo-cavernous muscles by which the veins are
constricted.

The semen is ejected by a rhythmic contraction of the
bulbo-cavernous and other perineal muscles, an action which
is also presided over by a centre in the lumbar region of the
cord (p. 150).

The spermatozoon meets the ovum in the Fallopian tube,
or upper part of the uterus.

DEVELOPMENT.
1. Early Stage.

It is unnecessary here to describe the changes in the
ovum before or immediately after its conjugation with
the spermatozoon, since
they are so fully dealt
with in all works on

The mammalian ovum
is holoblastic, that is
undergoes complete seg-
mentation, and forms a

mulberry -like mass of JSK^g™ B.

cells (Fig. 157, A). The

Cells then get disposed FIG. 157.—^1. Ovum with central cells forming

in tWO SetS, a layer of Blastoderm. B. Blastoderm now made up

,. •,, of two layers of cells — Epiblast and

small surrounding cells Hypobiast.

and a set of large central

cells. These latter spread out at one pole to form the

4i6

HUMAN PHYSIOLOGY

blastoderm, and dispose themselves in three layers — the
epiblast, mesoblast, and hypoblast (Fig. 158). From these
layers the various parts of the body are derived as follows : —
I. Epiblast. — Nervous system, epidermis, and appendages.
Epithelium of mouth, nose, naso-pharynx, and all cavities

and glands opening into them,
and the enamel of teeth.

II. Hypoblast. — Epithelia
of (a) alimentary canal from
back of mouth to anus and of
all its glands ; (b) of Eustachian
tube and tympanum ; (c) of
trachea and lungs ; (d) of
thyroid and thy m us ; and
(e) of urinary bladder and urethra.
III. Mesoblast. — All other structures.

By the formation of a vertical groove down the back of
the blastoderm, a tube of epiblast cells (the neural canal) is
enclosed, from which the nervous system develops by the

FIG. 158. — Transverse section of more
advanced Blastoderm, to show Epi-
blast, Mesoblast, and Hypoblast,
formation of Neural Groove and
splitting of the Mesoblast.

FlQ. 159. — Longitudinal Section through Embryo to it sinking down into ovum and
the formation of the Amnion, am. In the Mesoblast round, all., the Allantois,
the blood vessels grow out to form the placenta.

conversion of some of the cells into neurons, and others
into neuroglia cells (Fig. 158).

The mesoblast on each side of this splits, and the outer part,
with the epiblast, goes to form the body wall (Somatopleur),
while the inner part with the hypoblast gets tucked in to pro-
duce the alimentary canal (Splanchnopleur) (Fig. 158).

The developing embryo sinks into the ovum, and, as a
result of this, the somatopleur folds over it and, uniting
above, encloses it in a sac — the amniotic sac (Fig. 159, am.),
which becomes distended with fluid — the amniotic fluid, in
which the embryo floats during the later stages of its develop-

REPRODUCTION

ment, and which acts as a most efficient protection against
external violence.

2. Attachment to the Mother.

The ovuin gets enclosed in the uterine mucous membrane,
which regenerates round it as the decidua reflexa after men-
struation (Fig. 160, D.K).

Almost as soon as the ovum is embedded in the maternal
mucous membrane, it
becomes surrounded
by a nucleated mass
of protoplasm — the
trophoblast, formed of
the cells of the ecto-
derm, and this probably
transfers nourishment
from the mother to
the ovum. At the
end of about a fort-
night, the mesoblast
of the embryo extends
out in a number of
finger - like processes
into the trophoblast,
and soon afterwards
blood vessels shoot
into these, and the
chorionic villi are
formed (Fig. 161).
The precise origin of the first blood vessels in these is
not known, but ultimately they are derived from the
allantoic arteries which pass out from near . the posterior
end of the hind gut. As the villi grow, the blood vessels
in the maternal mucosa are dilating, and the capillaries
form large sinuses or blood spaces. Into these the
chorionic villi pass, and thus the loops of foetal vessels
hang free in the maternal blood, and an exchange of material
is possible between the mother and foetus. The placenta
thus formed acts as the fcetal lung, giving the embryo the
necessary oxygen and getting rid of the waste carbon dioxide.

27

FIG. 160. — Longitudinal Section through the
human uterus and ovum at the fifth week of
pregnancy. D.S., Decidua serotina, which
will become the placenta; D.R., Decidua
reflexa ; D. V. , The uterine mucous membrane
called the Decidua vera.

4i8

HUMAN PHYSIOLOGY

JV.S.P

FlQ. 161. — Longitudinal Section
through the tip of a villus of
the human placenta, covered by
its trophoblast layer, and con-
taining a loop of blood vessels,
and projecting into a large blood
sinus, I.V.S., in the maternal
mucosa.

It is the foetal alimentary canal supplying the necessary
material for growth and development; and it is the foetal
kidney through which the waste nitrogenous constituents are

thrown otf. In the mesoblast,
through which the allantoic
arteries pass out, a large vesicle
filled with fluid, and at first
communicating with the pos-
terior gut, is developed. This
is the allantois (Fig. 159, ////.).

3. Foetal Circulation.

Perf°rmance °f these
functions by the placenta is
associated with a course of
circulation of the blood some-
what different to that in the
post-natal state (Fig. 162).

The blood coming from the
placenta to the foetus is collected
into a single umbilical vein which passes to the liver. This
divides into the ductus venosus, passing straight through
the organ, and into a series of capillaries among the cells.
From these the blood flows away in the hepatic vein to the
inferior vena cava, which carries it to the right auricle. In
this it is directed by a fold of endocardium, through the
foramen ovale, a hole in the septum between the auricles,
and it thus passes to the left auricle, and thence to the left
ventricle, which drives it into the aorta, and chiefly up to
the head. From the head the blood returns to the superior
vena cava, and, passing through the right auricle, enters the
right ventricle, which drives it into the pulmonary artery.
Before birth this artery opens into the aorta by the ductus
arteriosus, while the branches to the lungs are still very
small and unexpanded. In the aorta, this impure blood
from the head mixes with the purer blood from the left ven-
tricle, and the mixture is sent to the lower part of the body
through the descending aorta. From each iliac artery an
umbilical artery passes off, and these two vessels carry the
blood to the placenta.

REPRODUCTION

419

When the child is born, the flow of blood between it and
the mother is arrested, the umbilical cord being tied. As a
result of this the respiratory centre is no longer supplied
with pure blood, and is
stimulated to action. The
lungs expand and the
blood flows through them.

In the ductus venosus a a -KC-

clot forms and the vessel
becomes obliterated. The
ductus arteriosus also
closes up, and the fora-
men ovale is occluded.
The circulation now takes
the normal course in
post-natal life.

Our knowledge of the
differences between the
physiological processes
in embryonic and in
extra- uterine life is still
very imperfect, and the
subject cannot be further
discussed here.

$. Gestation and
Delivery.

The child remains in
the uterus for nine
months, and at the end
of that period it is
expelled during labour.
Labour may be

FIG. 162.— Scheme of Circulation in the
Foetus, u.v., umbilical vein ; d.v., ductus
venosus ; p.v.c., inferior vena cava pour-
ing blood through the right auricle and
through the foramen ovale, jjf.o. , into the
left heart; a.v.c., superior venfa cava bring-
ing blood from head to pass through the
right side of the heart, and through the
ductus arteriosus, d.a.

divided
into three stages. In the

first stage the uterus passes into contractions at intervals,
and the lower part or cervix is dilated. In the second stage
the contractions become stronger, and with the help of the
contractions of the abdominal muscles, the child is expelled
through the vagina. After this the uterus is usually

420 HUMAN PHYSIOLOGY

quiescent for a short time, and then contractions supervene,
and the placenta and lining of the uterus are expelled as
the after-birth. These uterine contractions are presided over
by a nerve-centre in the lumbar enlargement of the cord, and
in all probability the nervi erigentes play an important part
in their production.

APPENDIX

SOME ELEMENTARY FACTS OF ORGANIC CHEMISTRY

THE following elementary facts may help the student who has
neglected the study of the outlines of Organic Chemistry.

Organic compounds are built round the four-handed carbon atom

— C—

I

When each hand links to the one-handed hydrogen atom,
METHANE — H

H— C— H

is formed.

H

By taking away a hydrogen atom from two Methane mole-
cules and linking these molecules together
ETHANE — H H

i i

H — C — C — H is produced.

H H

By further linking more and more of these molecules together,
similar molecules containing three, four, five or more carbon atoms
are produced.

When each carbon has its due proportion of hydrogen atoms it
is saturated, but if two hydrogen atoms are let go, the unoccupied
hands of the carbon may join and form an unsaturated molecule,
thus : —

Ethane becomes Ethylene H H

H— C=C— H

When one hydrogen atom is taken away and the molecule has a
hand ready to link with some other substance a radical is consti-
tuted, and these are known as METHYL, ETHYL, &c.

422 APPENDIX

Alcohols. — When the two-handed oxygen atom, — O — linked to
hydrogen H — and thus forming the hydroxyl molecule — OH is
linked to the vacant hand of the radical, an alcohol is formed, e.g. —

— C

H— C— C— O— (H) Ethyl Alcohol.

When the terminal carbon is thus oxidised a Primary Alcohol
is formed — but if a middle carbon atom is oxidised, a Secondary
Alcohol is produced—
H OH H

I I I
H— C — C — C— H Secondary Propyl Alcohol.

Aii

But the oxidation may involve more than one carbon atom and
thus the atomic Polyvalent Alcohols are produced—
OH OH OH

I I I
H — C — C — C — H Glycerin.

I I I
H H H

Aldehydes. — When from a Primary Alcohol the two hydrogens
in brackets are removed, the vacant hand of the oxygen links to
the vacant hand of the carbon to form an Aldehyde —
H

H— C— C=0 Ethyl Aldehyde.

H (H)

Ketones. — These are formed in the same way from the Secondary
Alcohols, thus : —
H 0 H

r< Ji n XT Acetone, the Ketone of Secondary

I I Pr°Pyl AlcohoL

H H

Acids. — If the hydrogen of the Aldehyde in brackets is replaced
by hydroxyl — OH an acid is produced—

H : 0

i i n

H— C-;-C— 0— H Acetic acid

I i
H :

APPENDIX 423

The carboxyl group to the right of the dotted line is characteristic
of the acids.

The oxidation may be carried on at each end of the line and the
divalent acids are thus produced

O 0

II II
H— 0— C— C— 0— H Oxalic acid.

If in the radical of one of these acids a hydrogen is replaced by
hydroxyl — OH an oxi-acid is formed, thus : —

H H 0

I I II
H— C— C— C— 0— H Propionic acid

may be converted to the two Lactic acids called (a) and (b) oxy-
propionic acid, according to the carbon which is oxidised

H OH O

I I I
H— C— C— C— O— H

I I
H H

and

OH H 0

I II

H— C— C— C— 0— H

H H

Similar oxy-acids are formed from the divalent acids.

BENZENE COMPOUNDS.

An important series of carbon compounds contain a ring of six
carbons, each with an unsatisfied affinity, thus : —

_/ V
J L

V

When each hand holds a hydrogen, benzene is formedl

424 APPENDIX

These hydrogens may be replaced by various molecules giving
rise to a large series of different compounds.

NITROGEN-CONTAINING COMPOUNDS.

Ammonia. — The three-handed Nitrogen by linking with three
hydrogens forms Ammonia,

H

H— N— H

If one of these hydrogens is removed, Amidogen, which can link
with other molecules, is produced.

Amido Acids. — If one of the hydrogen atoms in the radical of
an acid is replaced by amidogen a mon-amido acid is formed, thus : —

H 0
Hx | ||

\N_C-C— 0—H Amido acetic acid.
H/ |
H

When two hydrogen atoms are thus replaced, a di-amido acvl is
produced —

NH0 NH, 0

i r ii

H — C - - C — C — 0 — H Di-amido propionic acid.

H H

Amides. — If the amidogen molecule takes the place of the
hydroxyl in the carboxyl of an acid an amide results, thus : —
0

II
H— C— 0— H Formic acid.

0

/
H— C— N<

\H

H

Formamide.

From the dibasic carbonic acid,

0

H— O— C— 0— H
is formed

0

Hv || /H

\N— C— N<

H/ XH

the important substance urea.

APPENDIX 425

Urea molecules may link together —

(a) By dropping hydrogens when Biuret is produced,

0 H H 0

Hv || | || H

\N— C— N— N— C— N<

is/ XH

(b) By holding on to an intermediate radical of an acid, e.g.
the unsaturated three carbon acrylic acid. These are Diureides of
which the most important is Uric Acid.

0

H— N— C— N— H;

0=0— C=0

I I
H— N N— H

Y
4

INDEX

INDEX

ABDOMINAL breathing, 283
Abducens nerve, 159
Absorption of carbohydrates, 359
channels of, 358
of fats, 359
of food, 357, 381
of food proteids, 358
from the stomach, 340
Accessorius, 158
Accommodation, positive, 111 et seq.

range of, 114
Acetone, 422
Achromatin, 18
Achroo-dextrin, 329
Acid proteate, 334

sulphur in the urine, 399
Acids, divalent, 423

in gastric juice (see Gastric juice)
organic, 422
Acinus, 25

Acrylic acid, 397, 425
Action, reflex, 87, 88, 91
latent time of, 88
voluntary, 88, 92
Addison's disease, 384
Adipose tissue, 31
Adrenalin, 386
JEthalium septicum, 16
After-birth, 420
Air (see Complemental, &c.)
Air vesicles in the lungs, 277
Albumin, 11

Albuminoids, energy value of, 315
Albumose, 11

Alcohol as a food accessory, 382
Alcohols, chemistry of, 422
polyvalent, 422
primary, 422
secondary, 422
Aldehydes, 422
Alimentary canal, 324 et seq.
bacterial action in, 350
development of, 416
nerve supply of, 327
physiology of, 328
structure, 324
Allantoic arteries, 417
Allantoin, 398
Allantois, 418
Alloxantin, 398

Amides, 424
Amido-benzene, 351
Amido-ethane-sulphuric acid (tauro-

cholic acid), 342
Amido-isethionic acid, 352
Amido-propionic acid, 401
Amitotic division, 21
Ammonia, 11, 424

Ammonio-magnesium phosphate, 396
Ammonium salts in urine, 396
Amniotic fluid, 416

sac, 416

Amoaboid movement, 193
Amount of respired air, 286
Ampulla, 165
Amyl nitrite and influence on blood

pressure, 260

Amylolytic period of gastric diges-
tion, 334
Amylopsin, 347
Anabolic phenomena, 8
Anacrotic pulse, 252
Analysis of alveolar air, 302
Anelectrotonus, 48
Annulus of Vieussens, 243
Anterior corpora quadrigemina, 156,

167

pyramids of the medulla, 156
Antero-lateral ascending tract, 153,

156, 162

descending tract, 153, 157
Antitoxin, 392
Anus, 326
Apex beat, 223
Appendix, auricular, left, 218
Appendix, vermiform, 326
Apocodeine, 143, 263, 385
Aqueous humour, 107
Arc, cerebellar, 90
cerebral, 89
spinal, 89
Areolar tissue, 30
Arginin, 10
Arteries, 209

pressure in, 227, 245, 258, 259

et seq.
Arterioles, methods of studying their

condition, 260
normal state of, 261
walls (muscular) of, 262

429

430

INDEX

Arytenoid cartilages, 307, 332

Ash of milk, 319

Asphyxia, 306

Aspirates (see Consonants)

Assimilation, 382

Association, mechanism cerebral, 183

Astigmatism, 115

Atropin, 385

effect on heart, 239

effect on salivary secretion, 330

effect on sweat secretion, 409
Atwater on energy requirements, 378
Auditory centre, 181

nerve, 134 et seq.

cochlear root of, 134

vestibular root of, 135
Auerbach's plexus, 328
Auricle, 211

musculature of, 211

pressure in, 225

Auriculo-ventricular valves, 228
Availability of food-stuffs, 381
Axon, 76

BACILLUS coli communis, 351
Bacterial action in the alimentary

canal, 350

Bananas as food, 322
Barcroft and Haldane's method for
estimating gases in the blood,
199

Barfoed's solution, 316
Barium salts, influence on blood pres-
sure, 260

Barnard Hill's sphygmometer, 258
Barotaxis, 16
Bases, hexone, 10

of the urine, 402
Basilar membrane, 132
Basis bundles, 153
Basophil leucocytes (see Leucocytes)
Beans as food, 321
Beaumont's work on gastric digestion,

333

Benzene compounds, 423
Benzoates in the circulation, 399
Bi-amide of carbonic acid (see Urea)
Bidder's ganglion, 238
Bile, 341 et seq.
Bile, action of salts of, 342

as a haemolytic agent, 342

crystalline, 342

duct, 326

effect of drugs on, 345

flow of, 344

influence of nerves on flow of,
345

mode of formation of, 345

nature of, 345

pigments, 342

pigments, fate of, 353

Bile, pressure under which secreted,

345
Biliary calculi, 343

colic (see Colic)
Bilirubin, 198, 342, 370
Biliverdin, 342, 353
Binocular vision, 123
Biuret test, 347
Bladder, urinary, 407
Blastoderm, 416
Blind spot, 116, 181
Blood, 187 et seq.

coagulation of, 188

constituents, fate of, 204

constituents, source of, 201

defibrinated, 188

distribution of, 204

flow, 270

flow in arteries, 273

flow in capillaries, 273

flow in veins, 273

gases, 199

plasma, 187

platelets, 187, 193

serum, 188

specific gravity, 188

sugar, 316

total amount in body, 203
Blood corpuscles (see Leucocytes, Ac.),

192 et seq.
Blood-pressure, 245 et seq.

general distribution, 245

in kidney, 405

mean, 257

method of measuring, 257

method of measuring in man, 2;18

respiratory changes in, 256, 29(1

tracing, 256

variations in, 246
Blood vessels, circulation in, IM:;

coats of, 244
Bolus of food, 331
Bone, 34 et seq.

chemistry of, 38

development, intra-cartilaginous,
35

development, intra-membranous,
34

marrow, 202

structure of, 34 et seq.
Bowman's capsule, 403

glands, 140

Brain (see Cerebrum, &c.)
Breath sounds, 288
Broca's convolution, 186
Brodie, on extract of suprarenals, 385
Bronchial sound, 288
Brown-Sequard, on administration of

testicular substance, 388
Brown-Sequard, removal of the supra-
renals, 384

INDEX

431

Brunner's glands, 326
Buccinator muscle, 331
Bulbous plants, 321
Butter as food, 319
Butyrin, 319

CAFFEINE, 383
Calamus scriptorius, 266
Calcification of cartilage, 37
Calculi, urinary, 398
Calories, 314

definition of, 73
Calorimeter, 314

Camerer, observations on diet, 377
Canal, Haversian, 37
Canaliculi, 35
Canals, semicircular, 133

physiology of, 164
Cancellous tissue of bone, 35
Cane sugar, 317
Capillaries, 209, 244

pulse in, 254

pressure in, 245, 259, 267 et seq.
Capronin, 319
Capsule, internal, 168
Carpylin, 319

Carbohydrates, action of gastric juice
on, 336

as diet, 375

as they leave the alimentary
canal, 357

definition of, 316

energy value of, 318

requirement of, 379

tests for, 316 et seq.

their conversion to fat, 365
Carbon dioxide in blood, 200

in the urine, 403

Carbon monoxide haemoglobin, 197
Carbonate of ammonia, 359
Cardiac contraction, nature of, 236
Cardiac cycle, 219

duration of phases of, 221
Cardiac end of stomach, 325

failure, 268

glands of stomach, 325

impulse, 223, 233

muscle, 67

plexus, superficial, 240

branches of the vagus, 239 et

seq., 267

Cardiogram, 225, 231
Cardiograph, 224
Cardiometer (Roy's), 234
Cardio-pneumatic movements, 297

during hibernation, 298
Carrots as food, 321
Cartilage, 33

hyaline, 33

parenchymatous, 33
Cartilages of the larynx, 307

Casein (see Milk)
Caseinogen, 319, 411
Catalysis, 7
Cathelectrotonus, 48
Catheterisation of lungs, 302
Caudate nucleus, 169
Cells, chalice, 24

fat, 31

nerve, 76

pigment, 32

reproduction of, 18
Cellulose, 320
Central spot of the eye, 181
Centre, auditory, 181

for dilator pupillae, 112

for smell, 140, 182

for speech, 185

for sphincter pupillae, 112

for vomiting, 341

respiratory, 290

taste, 182

touch, 182, 183

vaso-constrictor, 265

visual, 128

voluntary, 184
Centrosome, 14
Cereals, 321
Cerebellar tracts, 152
Cerebellum, 160 et seq.

connections of, 162

cortex of, 161

functions of, 162

grey matter of, 161

peduncles of, 155, 162

removal of, 162

superior vermis of, 156
Cerebral cortex, 156, 169

action of, 174

action of drugs on, 175

action, time of, 175
Cerebrum, 168 et seq.

differentiation of stimuli of, 174

functions of, 171

grey matter of, 168

localisation of functions, 178

peduncles of, 167

removal of, 171
Cheese as food, 319
Chemiotaxis, 16
Chest voice (see Voice)
Chittenden on proteid requirements,

379
Chlorine-containing bodies in urine.

402

Cholalic acid, 342
Cholera bacillus, action of gastric

juice on, 336

Cholesterin, 79, 194, 342 et seq., 353, 410
Cholin, 78, 79
Chondroitin, 33
Chorda tympani, 264, 330

432

INDEX

Chordae tendinse, 213, 215
Chorionic villi, 417
Choroid, 107
Chromatin, 18
Chyle, 207
Chyme, 341
Cilia, 16, 27
Ciliary muscle, 107

processes, 107
Cilio-spinal region, 113
Circulation, the, 209 et seq.

factors, extra-cardiac, in, 274etseq.

foetal, 418

in blood vessels, 244

in kidney, 403

in the heart wall, 274

inside the cranium, 273
Circulation in the lungs, 274

respiration, influence on, 295
Claude Bernard — formation of sugar

in the liver, 366
Coagulation of the blood, 188 et seq

advantages of, 190

factors, influencing, 189

of milk, 319 (see also Souring of

milk)

Cochlea, 132
Cocoa as food, 383
Coffee as food, 383
Cold spots, 102
Colic, biliary, 343
Collagen, 29

action of hydrochloric acid on,
335

action of trypsin on, 347
Collodion as a means of separating

trypsin, 347
Colloids, 10
Colon, 326
Colostrum, 411
Colour, 104

blindness, 122

sensation of, 119
Colours, compleruental, 121
Columnae carneae, 213
Complement, 392
Complemental air, 286
Conducting paths (sec Spinal cord, &c.)
Conduction of heat from skin, 361
Conduction of nerve impulse, 81 et seq.
Conductivity, thermal, 101
Connective tissues (see Tissues)
Consciousness, 93, 174
Consonant sounds. 311
Contraction of the pupil, 112
Convection of heat from skin, 361
Convoluted stage of mitosis, 20
Convoluted tubules of the kidney, 403
Convolutions, cerebral, 171

left inferior frontal, 185
Cooking, effects of, on food, 322

Cooking, methods of, 322

of vegetables, 323
Co-operative antagonism of groups of

muscles, 60

Cord, spinal (see Spinal cord)
Cornea, 107
Corona radiata, 169
Coronary arteries, 231
Corpora quadrigemina, 167
Corpus Arantii, 216

Highmori, 414
Corpuscles, blood, 192 et sf<i.

bone, 35

muscle, 41

nerve, 78

tactile, 98
Coiti, organ of, 134
Coughing, 285
Cranial nerves, 157 et seq.
Cream as food, 319
Creatin, 43, 191, 320, 370, 398
Creatinin, 370, 398
Cretinism, 388
Cricoid cartilage, 307
Crossed pyramidal tract, 153
Crura cerebri, 162, 167 et seq.

< rusta of, 156, 167
Cuneate lobe, 181
Curare, experiment, 46
Current of action, 66

injury, 65

Cutaneous nerves, influence on res-
piration, 294
Cyrtometer, 280
Cystin, 401
Cytomitoma, 14
Cytotoxins, 392

DECUSSATION of the fillet, 155

of the pyramids, 156

of the optic nerves, 128
Defalcation, 355
Degeneration, Nissl's, 87

reaction of, 51

Deiters' nucleus, 153, 155, 157, 162
Delivery, 419
Dendrites, 76
Dentals (see Consonants)
Dentatenucleusof the cerebellum, 161
Depressor nerve (see Cardiac branches

of vagus)

Depth of sleep, 177
Descending antero-lateral tract. 157
Deutero-proteose, 12, 335, 346
Development, 415 ft ser/.
Developmental method of staining

nerve tracts, 153
Deviation of eyes, 126
Dextrin, 317, 329
Dextrose, 316
Diabetes, 386, 390

INDEX

433

Diamido acids, 10, 350, 424
Diapedesis, 193
Diaphragm, 281
Diaphysis, 38

Diastole of heart, 219, 221
Dicrotic notch, 251

wave, 250, 251
Diet, 375 et seq.

during muscular work, 72

typical, for an average man,

380

Dietetics, 376
Diffusion of gases in lungs, 287, 300,

303
Digestion, 312 et seq.

fate of the secretions of, 352

intestinal, 341

in mouth, 328

in stomach, 333

of stomach wall, 336
Dilatores narium, 284

pupillse, 107

pupillse, course of fibres, 113
Dioptric mechanism, 106
Dioxybenzene, 401
Diphtheria bacillus, 391

toxin, 391

Direct or dorsal cerebellar tract, 153,
156, 162

pyramidal tract, 153
Disaccharids, 317
Discharging mechanism (cerebral),

183
Distinction of differences of pressure,

99
Distinction of place of contact, 100

of time contacts, 100
Diureides, 397, 425
Divalent acids, 423
Dobie's line, 41
Ductless glands, 384 et seq.
Ductus arteriosus, 418

arteriosus, closure of, 419

venosus, 418

venosus, obliteration of, 419
Duodenum, 326
Dyaster stage of mitosis, 20
Dynamometer, 55

EAR, anatomy of, 130 et seq.

external, 130

internal, 132 et seq.

middle, 130 et seq.
Eck's fistula, 370
Eggs as food, 320
Ehrlich's side chain theory, 391
Elastin, 29, 315

action of trypsin on, 347
Electrode, non-polarisable, 65
Electrotonus, 48
Eleventh nerve (see Spinal accessory)

Emmetropia, 110

Emulsification, 347

Endocardium, 214

Endolymph, 166

Endothelium, 30

Energy requirements of individuals,

377

factors modifying, 376
Energy value of food, 313
determination of, 314
Entero-hsemal circulation, 352
Enterokinase, 348, 350
Enzymes, 7

Eosinophiles (see Leucocytes)
Epiblast, 416

parts developed from, 416
Epiglottis, 307
Epiphysis, 38
Epithelium, ciliated, 27
columnar, 23
excreting, 27
glandular, 25
mucin-secreting, 25
simple squamous, 22
stratified squamous, 22
zymin-secreting, 26
Erection, 415

mechanism of, 415
Erepsin, 350, 369
Ergograph, 64

Erythrocytes, 187, 194 et seq., 202
chemistry of, 194
fate of, 204
Erythro-dextrin, 329
Estimation of weight of objects,

97

Ethane, 421

Ethereal sulphates in urine, 399
Ethylene, 421
Euglobulin, 191
Eustachian tube, 131, 132
Excretion, effect of muscular work

on, 71

by the kidneys, 394 et seq.
by the lungs (see Respiration)
by the skin, 408
Exophthalmic goitre, 388
Expiration, 284
forced, 285
Expired air, 299
Eye, 103 et seq.

anatomy of, 106 et seq.
connection with central nervous

system, 126

distance of central spot, 117
emmetropic, 110
hyaloid membrane of, 108
nervous mechanism of movement,

125

physiology of, 109
Eyeballs, movement of, 124

28

434

INDEX

FACIAL nerve, 159
F«3ces, 353

in fasting animals, 353
in feeding animals, 353
in infants, 353
Fallopian tube, 414, 415
Falsetto voice (see Voice)
Far point of vision, 112
Fasting, metabolism during, 373
Fat cells, 31
Fatigue of cerebral mechanism, 176

of muscle, 55
Fats, 31, 191

in bacon, 320

in diet, 376

energy value of, 315

gastric juice on, 336

metabolism of, 372

as they leave the alimentary

canal, 357
Fatty acids, 31

tissue, storage in, 365
Feeding, effect of, 374
Fehling's test, 316
Fenestra ovalis, 130

rotunda, 130
Ferrier, experiments on brain of

monkey, 181, 182
Ferro-proteids, 13
Fibres, elastic, 29
nerve, 78
non-elastic, 29
non-medullated nerve, 78
pre-ganglionic and post-gangli-

onic fibres, 143
splanchnic, 144 et seq.
somatic, 144
Fibrin, 189
Fibrinogen, 189
Fibroblasts, 28
Fibro-cartilage, elastic, 34

white, 34
Fibrous tissue, 28
Field of vision, 117
Fifth cranial nerve (see Trigeminal)
Filiform papillae, 324
Fillet, 155 et seq.
Fissure of Rolando, 183
Flesh, 319

effect of cooking on, 322
preserved, 320
Flocculus, 161

Floor of the fourth ventricle, 155
Fluoride of sodium, action on epithe-
lium, 358

Foetal circulation, 418
Food, 312 et scq.

absorption of, 340 et seq.
animal, 318

effect of, on respiratory inter-
change, 305

Food, fate of absorbed, 360

nature of, 312

vegetable, 320
Foramen ovale, 418

occlusion of, 419
Formamide. \'l 1
Formic acid, 424
Foster, changes in protoplasm, 8
Fourth cranial nerve (»cc Trochlearis)
Frauenhofer's lines, I'.t.'j
Frog's heart, 219
Functions of the spinal cord, 149

GALACTOSE, 316

Gall-bladder, 326

Gall-stones, 343

Galvanic current, stimulation of

muscle by, 50
Galvanotaxis, 16
Galvanotonus, 47
Ganglia, collateral, 143

inferior mesenteric, 144

lateral, 144

superior cervical, 144

superior meseuteric, 144
Gases of the blood, 199
Gaskell, nervous mechanism of the

heart, 241

Gastric digestion, amylolytic period,
333

proteolytic period, 334
Gastric glands, 325
Gastric juice, 333

action on gelatin, 335

antiseptic action, 336

influence of diet on, 337

source of constituents, 336
Gelatin, 29, 315

action of trypsin on, 347
Gemmules, 77, 175
Generative organs, 413 et seq.
Geniculate bodies, 1M
Germ centres, 201
Germinal epithelium, 414

spot, 414

vesicle, 414
Gestation, 419
Giant cells, 202
Glands, compound, 25

racemose, 25

simple tubular, 25
Globin. 11)9
Globulin, 11
Globulose, 11

Glomerulus of kidney. 403
Glossopharyngeal nerve, 158, 331

influence on respiration, 293
Glucosamine, 26, 33
Glucose, 42, 191, 313

of blood, 201
Glycerin, 422

INDEX

435

Glycero-phosphates in urine, 402

Glycocholic acid, 341

Glycocoll, 399

Glycogen, 42, 318

Glycogenic function of the liver (see
Liver)

Glyco-proteids, 13, 26

Glycosuria, 368

Glycuronic acid, 33

Gmelin's test, 342, 353

Golgi, organs of, 97

Golgi's method of nerve staining, 170

Goltz, experiments on cerebral cortex,
172

Graafian follicle, 413

Granules, Nissl's, 76

Gravity, influence on capillary pres-
sure, 269

Grey matter (see Cerebrum, &c.)

Growth in length of bone, 38

Grubler's peptone, 357

Guanidin, 43

Gutturals (see Consonants)

H^MATIN, 198
Hsematoidin, 198, 343
Hsematoporphyrin, 198, 343, 402
Haemin, 198
Hasmochromogen, 198
Haemocytometer, 194
Haemoglobin, 43, 195

action of hydrochloric acid on,
335

decomposition of, 198

reduced, 195

Hsemoglobinometer, Haldane's, 196
Haemolymph glands, 201, 205
Haemolysis, 205, 393
Hair, 408

Haldane, experiments on air contain-
ing carbon monoxide, 302

and Barcroft, method for gases
in blood, 199

and Lorrain Smith, method for

total blood, 203
Hassall, corpuscles of, 389
Haversian canals, spaces, &c., 37
Hearing, 129 et seq.
Heart, 211 et seq.

changes in shape, 222

changes in position, 222

connection with central nervous
system, 239 et seq.

connection in frog, 239

connection in mammal, 239

effect of atropin on, 239

electrical stimulation of, 239

fibrous rings of, 212

of frog, 219

physiology of, 219

pulmonic, 210, 211

Heart, relations of, 218

sounds of, 231 et seq.

structure, 211 et seq.

systemic, 209, 211

valves (see Valves)

work of, 234
Heat elimination, 361, 364

elimination, nervous mechanism
in, 365

elimination from skin, 361

elimination from respiratory pas-
sages, 362

elimination in urine and faeces, 362

production, 360, 364

production in the brain, 361

production in glands, 360

production in muscle, 360

units, 314.
Henle's loop, 403
Hemisection of spinal cord, 151
Hemispheres, cerebral (see Cerebrum)
Herbivorous animals, 313
Hetero-proteose, 334
Hexone bases, 10
Hiccough, 286
High tension pulse, 254
Highmori, corpus (see Corpus)
Hippocampal convolution, 182
Hippuric acid, 399
Histidin, 10
Histones, 12

Holoblastic segmentation, 415
Hoppe-Seyler, changes in proto-
plasm, 8
Hot spots, 102
Hyaline cartilage, 33
Hyaloid membrane, 108
Hyaloplasm, 15
Hydrocarbons, unsaturated, 421

saturated, 421
Hydrochloric acid, 334
Hydroxyl molecule, 422
Hypermetropia, 115
Hypnosis, 177
Hypoblast, 416

parts developed from, 416
Hypoglossal nerve, 158
Hypoxanthin, 398

ILEO-C^ECAL valve, 326
Illumination, degree of, 103

source of, 103
Illusions, optical, 129
Immune body, 392
Immunity, 391
Impregnation, 415
Incompetence (cardiac), 234
Incus, 131
Indol, 351, 354, 399
Indoxyl, 400
Indican, 400

436

INDEX

Induced electricity, stimulation of

muscle by, 47
Inferior oblique muscle, 125

olivary nucleus, 155
Inflation of lungs, artificial, 292
Infundibula, pulmonary, 215, 278
Inhibition, intra-cardiac mechanism.

238

Inhibitory action of vagus, 241
Inhibitory nerves, 85
Inorganic salts in food, 312
Inosite, 43, 320
Inspiration, 280
forced, 284
forces opposing, 281
Intercostal muscles, external, 283
Internal capsule. 168
Internal secretion, 384

secretion, effect on blood pres-
sure, 260
Intestines, 325 et set].

absorption from, 358 et seq.
digestion in, 341 et seq.
movements of, 354
nerve supply of, 327
secretion of wall (tee Succus

entericus)
Intra-cardiac pressure, changes in,

225
pressure, method of determining,

225

Inulin, 317
lodothyrin, 386
Iris, 107

Islets of Langerhans, 327
Isometric method of measuring mus-
cular contraction, 55
Isotonic method of measuring mus-
cular contraction, 55

JACKSONIAN epilepsy, 174, 182, 185
Jacobsen's nerve, 33i
Jecorin, 191
Joule's law, 314
Juice, gastric, 333 et seq.
pancreatic, 346

KATABOLIC changes, 8

Katacrotic crests, 252

Kennedy's experiments on nerve

crossing, 81
Keratin, 13, 23, 315
Ketones, 422
Knee jerk, 151
Kidney, 403 et seq.

circulation in, 403

physiology, 404

structure, 403

LABIALS (see Consonants)
Labour, 419

Labyrinth of ear, 132
Lactase, 350
Lactate of ammonia, 370
Lacteals, 358
Lactic acid, 412, 423
Lactose, 317, 319
Lacunae, 35
Laevulose, 316
Lamellae, Haversian, 37

interstitial, 37

medullary, 37

peripheral, 37

Langerhans, islets of, 327, 390
Langley, experiment on vagus and

sympathetic nerves, 81
Language, spoken, 310
Lanolins, 410
Large intestine, 326
Laryngoscope, 308
Larynx, cartilages of, 307

mucous membrane of, 308

muscles of, 308

nervous mechanism of, 309

physiology of, 309

position in deglutition, 332

structure of, 307
Lateral fillet. I",
Law, Pfliiger's, 83

of polar excitation, 49
Lecithin, 78,194, 320, 343, 366, 368, 412
Left inferior frontal convolution, 185
Legumens, 321
Lens, crystalline, 108
Lenticular nucleus, 168
Lentils, as food, 321
Lesions of the cerebral cortex, 185
Leucin, 11, 347, 350
Leucoblasts, 202
Leucocytes, 187, 192 et seq., 201

chemistry of, 193

fate of, 204
Leucocytosis, 30, 202

digestion, 358
Levatores costarum, 283
Levy and Zuntz's experiments or

diet, 374

Lieberkiihn's follicles, 325, 326, 349
Light, 103
Liquor folliculi, 414

sanguinis, 187
Lipochrome, 190
Lissauer's tract, 153
Liver, 205, 326 ct seq.

development of, 366

formation of urea in, 369

glycogenic function, 366

relation to fats.

relation to general metabolism,
366

relation to proteids, 368

storage of carbohydrates, 366

INDEX

437

Liver, summary of functions, 373
Localisation of functions, 178
Lockhart Clarke's column, 90, 153
Lorrain Smith and Haldane's method

for total blood, 203
Loudness of sounds, 309
Lungs, 278, 279

air sacs, 278

capillaries, 278

elasticity of, 278

interchange between air and
blood in, 298

physiology of, 279 et seq.
Lymph, 187 et scq., 206 et seq.

chemistry of, 207

in disease, 207

formation of, 208

specific gravity of, 207

vessels, 209

Lymphatic glands, 201, 209, 258
Lymphatics, pressure in, 246, 270
Lymphocytes, 192
Lymphoid tissue, 30, 201
Lysin, 10

MACULA lutea, 181
Maize, 321
Malic acid, 313
Malleus, 131
Malpighian bodies, 403

bodies, secretion in, 404

corpuscles, 206
Maltose, 317, 329
Mammary glands, 409, 411
Marchi's method of nerve-staining,

86, 152

Mariotte's experiment, 116
Mastication, 328

Maternal attachment of ovum, 417
M'Fie, experiments with extract of

suprarenal, 385
Meconium, 354
Medulla of kidney, 403
Medulla oblongata, 154 et seq.

commissural fibres of, 157

conducting paths in, 155

grey matter of, 155

pyramids of, 156

reflexes of, 160

structure of, 154
Medullary sheath, 78
Meissners plexus, 328
Melanin, 32

Membrana tympani, 130
Membranous labyrinth, 133
Memory, 175
Menstrual period, 415
Menstruation, 414
Mesial fillet, 156
Mesoblast, 416

parts developed from, 416

Metabolic equilibrium, 374
Metabolism, general, 8, 371

during fasting, 373

of fats, 372

proteid, 371

method of investigating, 371
Methsemoglobin, 196
Methane, 421
Methylene blue, use in muscle ex-

" periments, 303
Micro-organisms, 193

in cheese, 319
Micturition, 407
Milk, 318

composition of cow's, 318

composition of human, 318

effect of cooking on, 322

fats, 412

phosphorus compounds of, 412

proteids of, 411

secretion of, 410

souring of, 412

sugar of, 412
Mitosis, 19
Mitral area, 233

valve, 214

Molecular layers of retina, 108
Monamido acids, 11, 424
Monaster stage of cell division, 20
Monosaccharids, 316
Monotremes, 182
Motor nerves (see Nerves)

points, 46

Mott, views on the Kolandic area, 183
Mouth, 324

nerve supply of, 327
Mucin, 13, 25, 26, 315, 344, 387

secreting epithelium, 25
Mucoid tissue, 27

Mucous membrane of large intestine,
326

of small intestine, 325

of stomach, 325

of uterus, 414
Murexide test, 398
Murmurs, cardiac, 233

simulation of, by breath sounds,

298
Muscle, 40 et seq.

absolute force of, 55

application of weights to, 57

carbohydrates of, 42

cardiac, 67

changes in blood passingthrough,
69

changes in shape, 51

chemical changes in, 67

chemistry of, 41

death of, 74

duration of contraction, 54

electrical changes in, 65

438

INDEX

Muscle, electrical condition of, 45

extensibility of, 44, 66

fatigue, 55

heat production in, 45, 63

latent period of contraction, 54

minimum stimulus, ;"f.

optimum load, 62

optimum stimulus, 57

physical characters of, 44

proteids of, 42

respiration of excised muscle, 69

skeletal, 40

spindles, 97

stimulation by galvanic current,
50

stimulation by induced elec-
tricity, 49

storage of food in, 366

structure of, 40

successive stimuli, 57

temperature effects, 56

visceral, 40, 66

visceral, staircase contraction of,
67

voluntary contraction, 59

wave of contraction, 52

work, 61
Muscles, co-operative antagonism of,

60
Muscular sense (see Sense)

sense, centre for, 183
Muscular work, effect on excreta, 71

work, man's, for a day, 378
Myelocytes, 193
Myogenic movements of intestine,

354

Myoglobulin, 42
Myohacruatin, 43
Myoids, 16, 40
Myopia, 114
Myosin, 42
Myosinogen, 42
Myostromiii, l_
Myxoedema, 388

NASAL cavity, 139

Nasals (see Consonants)

Naunyn's observations on cholesterin,

343

Near point of vision, 112
Necrobiosis, 8
Nerve cells, 76

cells, function of, 86

centres (see Centres)

corpuscles, 78

fibres, 78

fibres, medullary sheaths of, 78

fibres, non-medullated, 78

specific energy of, 104
Nerves, 75 et seq.

afferent, 85

Nerves, auditory, 135 ; cochlear root
of, 136 ; vestibular root of, 137

augmentor, 85

efferent, 84

excito-motor, 85

excito-reflex, 85

excito-secretory, 85

factors modifying conduction in,
83

inhibitory, 85

mixed, 85

motor, 85

nature of impulse in, 86

physiology of, 86

rate of conduction in, 82

secretory, 85

sensory, 85

spinal (see Spinal nerves)

vaso-constrictor, 85, 263, 264

vaso-dilator, 264

Nervi erigentes, 146, 264, 355, 407
Nervous mechanism, intra-cardiac, 238

system, 141 et seq.
Neural canal, 41»>
Neurilerama, 78
Neuroglia, 148, 416
Neuro-keratin, 78
Neuro-muscular mechanism, 88 et seq

fatigue of, 94

Neutral sulphur in urine, 401
Neurons, 76, 416

classification of, 84

excitability of, 80

extent of excitability of, 88

ingoing, 81

manifestation of activity of, 81

outgoing, 81

stimulation of, 80
Nicotine, effect of painting ganglia

with, 143, 330
Ninth nerve, 146, 158
Nissl's degeneration, 87

granules, 76, 87, 176
Nitrites, effect on blood pressure, 260
Nitrogen in the blood, 201

in the urine, 396 et m>/.
Nodal swellings, 18
Nodes of Ranvier, 78
Nuclei of the cranial nerves, 158

of the posterior columns, 89
Nucleins, 12

Nucleo-albumin in the urine, 402
Nucleolus, 17
Nucleo-proteids, 344

action of gastric juice on, 335

action of trypsin on, 347
Nucleus, 17 et seq.

cuneatus, 152, 155

gracilis, 152, 166
Nutrition of the brain, 173
Nuts, as food, 322

INDEX

439

OATMEAL, as a food, 321
Occipital lobe, 181
Oculomotor nerve, 159
(Esophagus, 325

application of a stethoscope to,
332

peristaltic action of, 332
Oleic acid, 31
Olein, 32, 319
Olfactory bulb, 139

tracts, 139
Olive, the, 155
Ophthalmoscope, 109
Optic chiasma, 126

disc, 108

nerve, 108

radiation, 181

thalamus (see Thalamus)
Ora serrata, 109
Organ of Corti, 134
Organic acids as salts in food,

313

Organs of Golgi, 97
Osazones, 316
Osmosis, 358
Osmotic pressure, 194
Ossification, process of, 34 ct seq.

centre of, 36
Otic ganglion, 331
Ovary, 388, 414
Ovum, 413
Oxalates, 189
Oxalic acid, 403, 423
Oxy-acids, 423
Oxygen in blood, 200
Oxyhasmoglobin, 195
Oxyntic cells, 325
Oxyphil leucocytes, 192

PAIN, sensation of, 96
Palmitic acid, 31
Palmitin, 32, 319
Pancreas, 327, 390

removal of, 368
Pancreatic secretion, 346

secretion, action of, 346

secretion, character of, 346

physiology of, 349
Papillary muscles, 213
Paracasein calcic. 319, 336, 411
Paradoxical contraction, 84
Paralysis of laryngeal nerves, 309
Paramyosinogen, 42
Parathyroid, 388
Parotid gland, 324

nerve supply, 329

reflex stimulation of, 331
Partial pressure of oxygen and carbon
dioxide in blood, 301

pressure of gases in air vesicle,
301

Passage of carbon dioxide from tis-
sues to blood, 304

of oxygen from blood to tissues,

303

Pawlow, effect of diet on gastric juice,
337

pancreatic fistulae, 348
Peas as food, 321
Penis, 415
Pepsin, 75, 337

fate of, 352

source of, 336
Pepsin ogen, 337
Peptic glands, 325
Peptone, 12, 189, 335, 346
Perceptions, modified, 129
Perfusion method of studying action

of drugs, 261
Pericardium, 218
Perichondrium, 36
Perilymph, 133, 166
Perimeter, 117
Perineurium, 79
Peripheral resistance, 259
Peristalsis of bladder wall, 408

of intestine, 354

nervous mechanism of, 355

of wall of ureter, 407

wave of, 67

Petrosal nerve, small superficial, 331
Peyer's patches, 326
Pniiger's experiments on proteid diet,
375

law of contraction, 83
Phacoscope, 112
Phagocyte action, 193
Pharynx, 324

nerve supply of, 327
Phenol, 351, 399
Phenylhydrazin, 316
Phloridzin, injection of, 368

poisoning, 367
Phosphocarnic acid, 42, 412
Phospho-proteid, 320
Phosphorus-containing bodies in the

urine, 401 et seq.
Phototaxis, 16
Phrenic nerves, 290
Pialyn, 347
Pigment cells, 32
Pigments of the urine, 402
Pillars of the fauces, 332
Pilocarpine, effect on sweat glands,

410

Pitch of sounds, 136, 309
Pituitary body, 386

injection of extracts of, 386
Placenta, 417
Plasma, 190 et seq.
Plasmodia, 4
Plethysmograph, 261

440

INDEX

Pleura, complemental, 282
Pleural cavity, 278
Plexus, hypogastric, 146

Auerbach's, 328

Meissner's, 328
Pohl's observations on Leucocytosis,

358

Polar excitation, law of, 49
Polycrotic waves, 250

pulse, 252
Polymerisation of raonosaccharids,

317
Polymorpho-nuclear leucocytes (see

Leucocytes)
Polysaccharids, 317
Pons Varolii, 155, 160
Portal circulation, 327

vein, 327
Positive accommodation, 111 et seq.

accommodation, varying power

of, 114
Posterior columns of spinal cord, 152

columns, nuclei of, 89

corpora quadrigernina, 166
Potatoes, as food, 321
Poteriodendron, 75
Prsecordium, 218
Precipitins, 393
Predicrotic wave, 250, 251
Preformed sulphate in urine, 399
Preganglionic fibres, 143
Presbyopia, 114
Pressure, arterial, 259

arterial, factors controlling, 259
Primitive nerve sheath (see Schwann)
Propionic acid, 423
Prostate gland, 414
Protagon, 78
Protamines, 10
Proteid metabolism, 371
Proteids, 9 et seq.

as they leave the alimentary
canal, 357

of blood, 201

of cereals, 321

conjugated, 12

conversion into fat, 365

diet, 375

energy value of, 314

native, 11

requirements, 378

sparers, 375

Proteolytic period (see Gastric secre-
tion)

Proteoses, 11
Prothrombin, 189
Protoplasm, 4 et seq.

cell, 14

Proto-proteoses, 12, 334
Proximate principles, 313

sources of, 318

Pseudoglobulin, 191
Pseudopodia, 193
Pseudo-nucleins, 12
Ptomaines, 352
Ptyalin, 328

fate of, 352
Puberty, 413
Pulmonary area, 232

valve, 217
Pulse, anacrotic, 252

arterial, 246

capillary, 254

cause of, 246

characters of wave, 247

form of wave, 249, 254

height of wave, 249

high tension, 254

length of wave, 248

palpation of, 252

rate of, 253

rate, variations in, 220

rhythm, 253

tension, 253

velocity of wave, 248

venous (see Venous pulse)

volume of, 253
Pulsus celer, 254

parvus, 253

plenus. 253

tardus, 254
Pupil, 107

contraction of, 112
Purin bodies, 370, 372, 397

bodies, endogenous and exo-
genous, 397
Purkinje's cells, 161

images, 118

Pyloric end of the stomach, 325
Pyramidal tracts, 153
Pyramids of the medulla, 156
Pyrocatechin, 401

QUADRIURATES, 398

Quality of sounds, 310

RADIATION of heat from skin, 361
Radicals, organic, 421
Ranvier, nodes of, 78
Reaction, 94

of degeneration, 51
Receptaculum chyli, 358
Reception of stimuli, 178
Recollection, 175
Recti muscles (eye), 124, 125
Rectum, 326

Recurrent laryngeal nerve, 240
Red marrow of bone, 202, 206
Red nuclei, 162, 167
Reduced alkaline hzematin, 198
Reflex action, 87, 91, 176

function of the spinal cord, 149

INDEX

441

Refraction of light, 110
Reid on absorption, 358
Relation of haemoglobin and its

derivations, 199
Remak's ganglion, 238
Rennet, 319, 411
Rennin, 337

source of, 33G
Reproduction, 413
Reserve air, 286
Residual air, 287

contraction, period of (see Systole)
Respiration, 277 ct seq.

effect on air breathed of, 298

effect on the blood, 299

at high altitudes, 303

influence of heart's action on,
297

internal, 303

mechanism of, 277 et seq.

movements in (special), 285

nervous mechanism of, 289

physiology of, 279

rhythm of, 289
Respiratory centre, 290

action of, 291

effect of absence of oxygen on,
294

effect of accumulation of waste
products in the blood, 295

effect of carbon dioxide on, 294

effect of temperature of animal
on, 295

modifications in activity of,

294

Respiratory interchange, extent of,
304

interchange, factors modifying,
304, 305

quotient, 299

quotient, effect of diet on, 305
Restiform bodies, 155
Rete testis, 414
Retina, 108 et seq.

blood vessels of, 109

corresponding areas on the two
sides, 124

nature of changes in, 118

stimulation of, 116, 118

rods and cones of, 109
Rhythmic contraction of the heart,
236

contraction, propagation of, 236

contraction, starting mechanism

of, 237

Rigor mortis, 74
Rolandic area, 183, 185
Rolando (see Fissure)
Roof nucleus of the cerebellum, 161
Roots of the spinal nerves, 142
Roy's cardiometer, 234

SACCHAKOMYCES cerevisae, 4
Saccule, 133

St. Martin (see Beaumont)
Saliva, 328

functions of, 329

influence of chorda tympani in
secretion of, 330

physiology of, 329

reflex stimulation of secretion,
331

stimulation of sympathetic, effect

on, 331
Salivary glands, 324 et seq.

glands, nerve supply, 327, 329

et seq.

Salol, rate of passage through intes-
tine, 355
Salts of the bile acids, 341, 352

fate of, 352
Sanson's images, 112
Sarcolactic acid, 43
Sarcolemma, 40, 41
Sarcous substance, 41
Scala media, 133
Schafer's investigations of brain

functions, 182, 183
Schemer's experiment, 111
Schwann, white sheath of, 78
Sciatic nerve, 264
Sclerotic, 107
Sebaceous glands, 409-410
Secretion, 391

internal, 384 et seq.
Sectional area of circulatory system,

210

Semen, 414, 415
Semicircular canals, 132 et seq.

canals, physiology of, 164
Semilunar valves, 216

valves, action of, 229
Seminiferous tubules, 414
Sensation, 94

colour, 119

physiology of colour, 120

production of colour, 121

of hunger, 95

of pain, 96

of smell, 139

of taste, 138

of thirst, 95

visual, 129
Sense (The Senses), 95 et seq.

of acceleration and retardation
of motion, 164

joint and muscle, 97 et seq., 164

special, 98 et seq.

of smell, 139

tactile, 98

of taste, 138

temperature, 101

visual, 102 et seq.

29

442

INDEX

Sensibility, common, 95
Septo-marginal tract, 153
Serum, 190 et seq.

albumin, 189, 191

globulin, 189, 191

Seventh cranial nerve, 146 (see Facial)
Sexual organs, development of, 413

organs, removal of, 413
Side-chain theory (see Ehrlich)
Sighing, 286
Singing voice, 310
Sinus of Valsalva, 216
Sixth nerve (see Abdncens)
Skatol, 347, 354, 399
Skatoxyl-sulphate of potassium, 400
Skin, excretion by, 408
Sleep, 177

respiratory changes during, 305

walking, 92, 177
Small intestine, 325
Smell, 139

centre for, 182

mechanism of, 139

physiology of, 139
Snake toxin, 391
Sneezing, 285
Soluble native proteids, 346
Somatopleur, 142, 416
Sound perception, 136
Sounds of the heart (tee Heart)
Soup, as food, 323
Space, Haversian, 37
Specific nerve energy, 104
Spectrum analysis, 195
Speech (spoken language), centre for,

185

Spermatoblasts, 414
Spermatogen, 414
Spermatozoa, 413, 414

development of, 414
Sphincter an'., 326, 335

pupillse, 107

trigonalis, 407
Sphygmograph, 249

tracings, 250
Sphygmometer, 258
Spinal accessory nerve, 15s

cord, 148 et seq.

cord, conducting paths in, l.'l

cord, grey matter of, 148

cord, posterior columns of, 152

cord, reflex function of, 149

cord, section of one side of, 151
" Spinal dog," 91
Spinal nerves, 141

nerves, roots of, 142
Spirometer, 286
Splanchnic area, 274

influence on respiration, 293
Splanchnopleur, 142, 416
Spleen, 205

Spot, blind, 116

Staircase contraction, 236

Stannius' experiment, L'.'iT

Stapedius muscle, 131

Stapes, 131

Starches, 317

Stearic acid, 31

Stearin, 32, 319

Stenosis, 234

Stercobilin, 353

Stethoscope, 233

Stewart's method of estimating time

taken in circulation, 276
Stewart on loss of heat, 378
Stimulus, 94
Stomach, 325

during fasting, 333

during feeding, 333

importance in digestion, 340

movements of, 338

nervous mechanism of, 339

rate of passage of food from,
339

vascular changes, 333
Storage of surplus food, 365
Storing mechanism (cerebral), 183
Stromuhr, 272
Sublingual gland, 324

gland, nerve supply, 329
Submaxillary gland, 324

gland, nerve supply, 329
Succi's fast, 373
Succus entericus, 349

entericus, action of, 349

entericus, nervous mechanism of

secretion, 350
Sulcus centralis, 183
Sulphur-containing bodies in the

urine, 399 et seq.
Superior oblique muscle, 125
Suprarenal bodies, 384

extract of, 263, 368

medulla, 384

metabolism after injection of,

385

Suspensory ligament of eye, 108
Swallowing, 331
Sweat, chemistry of, 410

evaporation, 362

glands, 409

nervous mechanism of secretion

of, 409
Sympathetic fibres to the heart, 240-

243

Synapsis, 76
System, Haversian, 37
Systole of heart, 219, 221

latent period, 230

period of overflow, 230

period of residual contraction,
230

INDEX

443

TACTILE sense (see Sense)
Tapetum nigrum, 109
Tartaric acid, 313
Taste, 137 et seq.

bulbs, 137

centre for, 182

mechanism of, 137

physiology of, 138
Taurocholic acid, 342
Tea, 383
Tegmentum, 167
Temperature, 362 et seq.

in cold-blooded animals, 364

in hibernating animals, 364

in warm-blooded animals, 364

mean daily, of man, 363

regulation, 361 et seq.

variations in, 363
Temporo-sphenoidal lobe, 182
Tension of gas in a fluid, 301

of the vocal cords, 310
Tensor tympani muscle, 131
Tenth nerve (see Vagus)
Testis, 388, 414
Tetanus, complete, 59

incomplete, 58

Thalamus opticus, 167, 168, 181
Theine, 383

Thermal sense, centre for, 183
Thermotaxis, 17
Third nerve, 146, 159
Thoracic breathing, 283
Thrombin, 189
Thymus gland, 189, 389

effect of castration on, 390

removal of, 390
Thyroid cartilage, 307

gland, 386

administration of extracts of, 387

removal of (thyroidectomy), 387,

388

Tidal air, 286
Tissue, adipose, 31

areolar, 30

cancellous, 35

connective, 27 et seq.

fibrous, 28

lymph, 30

mucoid, 27

vegetative, 23 et seq.
Tongue, 324
Tonsil, 324

Torricellian vacuum, 199
Touch, centre for, 182
Toxic action, 391 et seq.
Trabecula3, 36
Tricrotic pulse, 250
Tricuspid valve, 215

area, 233

Trigeminal nerve, 159
Trochlearis nerve, 159

Trophoblast, 417
Trypsin, 346

fate of, 352
Trypsinogen, 348
Tryptophane, 347
Tubular glands of stomach, 325
Tubules, secretion in kidney, 406
Tunica albuginea, 414
Turnips as food, 321
Twelfth nerve, 158
Tympanic cavity, 130
Typhoid toxin, 392
Tyrosin, 11, 347, 350

UMBILICAL arteries, 420

veins, 418

Unilateral stimulation, 16
Uiates, 395
Urea, 315, 369, 372, 396, 424

of blood, 201

effect of drugs on formation of,
370

source of, 370
Ureter, ligature of, 405
Urethra, 407

Uric acid, 191, 397, 406, 425
Urinary bladder, 4.07

nerves of, 407
Urine, 394 et seq.

excretion of, 407

method of estimating solids in,
394

micro-organisms in, 396

non-urea nitrogen in, 396

of herbivora, 395

pigments of, 402

secretion of, 403
Urabilin, 402
Urochrome, 402
Uroerythrin, 402
Uterus, 414, 415

centre for contractions of, 420
Utricle, 133

VACUOLES, 14
Vagus, 146, 158

cardiac branches, 239 et seq.

frog's, 239

gastric branches, 264

effect on respiration of section,
291 et seq.

effect on respiration of stimula-
tion, 292 et seq.
Valsalva, sinus of, 216
Valves of the heart, 214 et seq.

action of, 228

aortic, 217

auriculo - ventricular, 228 (see
Mitral and Tricuspid)

mitral, 214

pulmonary, 217

444

INDEX

Valves of the heart, semilunur, 216

(see Aortic and Pulmonary)
Valve, ileo-caecal, 326
Vas deferens, 414

deferens, ligature of, 413
Vasa efferentia, 414
Vaso-constrictor centre, 265

centres, secondary, 266

nerves, 264
Vaso-dilator centre, l>t'>r,

nerves, 264
Vaso-motor centre (see Centre)

mechanism, 262

nerves, 263

nerves, section of, 263
Vegetables, as food (see Food)

effect of cooking, 323

green, 321

Veins, pressure in, 225, 246, 258, 269
et seq.

umbilical (see Umbilical)
Vena cava, inferior (foetal), 418

superior (foetal), 418
Venous pulse, 2.V.

pulse, normal. 2.">»;
Ventilation, 306
Ventricles of the heart, 211

contraction of, 214

left, 212

muscular structure of, 212. 214

pressure in. 'I'll

right, 214

Veratrin, effect on muscular contrac-
tion, ."it;

Vermiform appendix, 326
Vernon on erepsin in the tissues.

350

Vesicular sound, 288
Vestibule of the ear, 132
Vestibular root of the eighth nerve,

135, 162

Vibrations, light, 119
Vibratories (see Consonants)
Vieussens, annulus of (sec Annulus)
Villi. chorionic, 417

of intestine, 326
Vision, 102 et seq., 164

binocular, 123

far point of, 111

field of, 118

Vision, monocular, 106 et seq.

near point of, 111

theories of colour, 122
Visual centre, 128, 181

sensation, duration of, 128

sensation, strength of, 128
Vital action of endothelium of capil-
laries, 304

Vital capacity of the thorax, 286
Vitreous humour, 107
Vocal cords (false and true), 308
Voice, 307 et seq.

chest, 310

falsetto, 310
Volition, 184
Voluntary actions, 88, 92

centres, 184
Vomiting, 340

centre, 341
Vowel sounds, 311

WALLERIAN degeneration method,

152

Waste, rate of (during fasting), 373
Water in food-stuffs, 312
Weigert's method of nerve-staining,

152

Wheaten flour as food, 321
Whey albumin, 319, 336
Whispered speech, 310
White bread as food, 321

XANTHIN, 320, 383, 398

X rays in the examination of the

stomach, 338
of the intestine, 354

YAWNING, 286
Yeast, 4, 316

ZONA granulosa, 413, 414

pellucida, 414

Zuntz on excretion of CO-, in the dog,
377

experiments on muscle work. (J4
378

and Levy, on diet, 374
Zymin, 7, 317

secreting epithelium, 26
Zymogen, 26

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