GIFT OF
MICHAEL REESE

AMERICAN SCIENCE SERIES— ADVANCED COURSE

THE HUMAN BODY

AN .ACCOUNT OF

ITS STRUCTURE AND ACTIVITIES

AND THE CONDITIONS OF ITS

HEALTHY WORKING

BY

II. NEWELJj MARTIN, D.Sc., M.A., M.D., F.R.

Late Professor of Biology in the Johns Hopkins University

and of Physiology in the Medical Faculty

of the same

EIGHTH EDITION, REVISED

NEW YORK
Mr/ HOLT AND COMPANY

1898

BIOLOGY

LIBRARY

G

Copyright, 1881, 1836,

BY
HENRY HOLT & CO.

x

PUBLISHEBS' NOTE.

THE Appendix on Reproduction of the earlier editions
appears in this as Chapter XXXIX. Copies of the book

without the chapter can be had when specially ordered.

iii

PKEFACE TO THE SEVENTH EDITION.

THIS edition has been very thoroughly worked over and,
I trust, improved. A considerable amount of new matter
has been added, especially in connection with the cardiac
and vascular nerves, and the physiology of the brain; but
throughout the whole book many paragraphs have been re-
written; and many corrections, rendered necessary by the
discoveries of the last three or four years, have been made.
I hope therefore that the edition will be found as well up to
date as it is possible for a text-book to be : for a text-book
must always incline to the conservative side, and deal with
well-established facts rather than with even the most fasci-
nating novelties. Still, as in previous editions, I have tried
to show where the outposts and the outlooks of Physiology
are.

H. N. M.
May 1, 1896.

PKEFACE TO THE FIEST EDITION.

IN the following pages I have endeavored to give an
account of the structure and activities of the Human Body,
which, while intelligible to the general reader, shall be accu-
rate, and sufficiently minute in details to meet the require-
ments of students who are not making Human Anatomy and
Physiology subjects of special advanced study. Wherever it
seemed to me really profitable, hygienic topics have also been
discussed, though at first glance they may seem less fully
treated of than in many School or College Text-books of
Physiology. Whoever will take the trouble, however, to
examine critically what passes for Hygiene in the majority of
such cases will, I think, find that, when correct, much of it is
platitude or truism : since there is so much that ic of impor-
tance and interest to be said it seems hardly worth while to
occupy space with insisting on the commonplace or obvious.

It is hard to write a book, not designed for specialists,
without running the risk of being accused of dogmatism, and
some readers will, no doubt, be inclined to think that, in
several instances, I have treated as established facts matters
which are still open to discussion. General readers and
students are, however, only bewildered by the production of
an array of observations and arguments on each side of every
question, and, in the majority of cases, the chief responsi-
bility under which the author of a text-book lies is to select
what seem to him the best supported views, and then to state
them simply and concisely: how wise the choice of a side has
been in each case can only t>e determined by the discoveries
of the future.

Others will, I am inclined to think, raise the contrary
objection that too many disputed matters have been dis-

VI PREFACE TO TEE FIRST EDITION.

cussed : this was deliberately done as the result of an experi-
ence in teaching Physiology which now extends over more
than ten years. It would have been comparatively easy to
slip over things still uncertain and subjects as yet unin-
vestigated, and to represent our knowledge of the workings
of the animal body as neatly rounded off at all its contours
and complete in all its details — totus, teres, et rotimdus.
But by so doing no adequate idea of the present state of
physiological science would have been conveyed; in many
directions it is much farther travelled and more completely
known than in others; and, as ever, exactly the most inter-
esting points are those which lie on the boundary between
what we know and what we hope to know. In gross Anatomy
there are now but few points calling for a suspension of judg-
ment; with respect to Microscopic Anatomy there are more;
but a treatise on Physiology which would pass by, unmen-
tioned, all things not known but sought, would convey an
utterly unfaithful and untrue idea. Physiology has not fin-
ished its course. It is not cut and dried, and ready to be
laid aside for reference like a specimen in an Herbarium, but
is comparable rather to a living, growing plant, with some
stout and useful branches well raised into the light, others
but part grown, and many still represented by unfolded buds.
To the teacher, moreover, no pupil is more discouraging than
the one who thinks there is nothing to learn; and the boy
who has " finished " Latin and " done " Geometry finds some-
times his counterpart in the lad who has " gone through "
Physiology. For this unfortunate state of mind many Text-
books are, I believe, much to blame : difficulties are too often
ignored, or opening vistas of knowledge resolutely kept out of
view: the forbidden regions may be, it is true, too rough for
the young student to be guided through, or as yet pathless
for the pioneers of thought; but the opportunity to arouse
the receptive mental attitude apt to be produced by the rec-
ognition of the fact that much more still remains to be learned
— to excite the exercise of the reasoning faculties upon dis-
puted matters — and, in some of the better minds, to arouse
the longing to assist in adding to knowledge, is an inesti-
mable advantage, not to be lightly thrown aside through the
desire to make an elegantly symmetrical book. While I
trust, therefore, that this volume contains all the more impor-
tant facts at present known about the working of our Bodies,

PREFACE TO THE FIRST EDITION. vn

I as earnestly hope that it makes plain that very much is yet
to be discovered.

A work of the scope of the present volume is, of course,
not the proper medium for the publication of novel facts;
but, while the " Human Body," accordingly, professes to be
merely a compilation, the introduction of constant references
to authorities would have been out of place. I trust, how-
ever, that it will be found throughout imbued with the influ-
ence of my beloved master, Michael Foster; and on various
hygienic topics I have to acknowledge a special indebtedness
to the excellent series entitled Health Primers.

The majority of the anatomical illustrations are from
Henle's Anatomie des Menschen, and a few from Arendt's
Schulatlas, the publishers of each furnishing electrotypes.
A considerable number, mainly histological, are from Quain's
Anatomy, and a few figures are after Bernstein, Carpenter,
Frey, Haeckel, Helmholtz, Huxley? McKendrick, and Wundt.
About thirty, chiefly diagrammatic, were drawn specially for
the work.

Quantities are throughout expressed first on the metric sys-
tem, their approximate equivalents in American weights and
measures being added in brackets,

H. NEWELL MAKTIN.

BALTIMORE, October, 1880.

CONTENTS.

CHAPTER I.

THE GENERAL STRUCTURE AND COMPOSITION OF THE
HUMAN BODY.

PAGE

Definitions. Tissues and organs. Histology. ZoSlogical position of
man. The vertebrate plan of structure. The mammalia.
Chemical composition of the Body 1

CHAPTER II.
THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS,

The properties of the living Body. Physiological properties. Cells.
Cell division. Indirect, karyokinetic or mitotic cell division.
Assimilation ; reproduction. Contractility. Irritability. Con-
ductivity. Spontaneity. Protoplasm. The fundamental physi-
ological properties 15

C CHAPTER III.

THE DIFFERENTIATION OF THE TISSUES AND THE
PHYSIOLOGICAL DIVISION OF EMPLOYMENTS.

Development. The physiological division of labor. Classification
of the tissues. Undifferentiated tissues. Supporting tissues.
Nutritive tissues. Storage tissues. Irritable tissues. Coordi-
nating and automatic tissues. Motor tissues. Conductive
tissues. Protective tissues. Reproductive tissues. Organs.
Physiological mechanisms. Anatomical systems. The Body
as a working whole „ . 29

CHAPTER IV.
THE INTERNAL MEDIUM.

The external medium. The internal medium. The blood. The
lymph. Histology of blood. Blood crystals. Histology of

lymph 40

ix

X CONTENTS.

CHAPTER V.

THE CLOTTING OF BLOOD.

PAGE
Coagulation of the blood. Cause of coagulation. Whipped blood.

The buffy coat. The source of blood fibrin. Artificial clot.
Fibrin ferment. Proximate causes of normal blood coagulation.
Relation of the blood-vessels to coagulation. Chemical com-
position of the blood. Quantity of blood. The life-history of
the blood-corpuscles. Chemical composition of lymph ........ 51

* CHAPTER VI.
THE SKELETON.

Exoskeleton and endoskeleton. The bony skeleton. Segmentation
of the skeleton. Homologies of the bones of the anterior and
posterior limbs. Peculiarities of the human skeleton ...... , . . 63

CHAPTER VII.
THE STRUCTURE AND COMPOSITION OF BONE. JOINTS.

Gross structure of the bones. Microscopic structure of bone. Chem-
ical composition of bone. Articulations. Joints. Hygiene of
the joints .................................................. 85

CHAPTER VIII.
CARTILAGE AND CONNECTIVE TISSUE.

Temporary and permanent cartilages. Varieties of cartilage. The
connective tissues. Elastic cartilage and fibre-cartilage. Ho-
mologies of the supporting tissues. Hygiene of the developing
skeleton. Adipose tissue .................................. 98

CHAPTER IX.
THE STRUCTURE OF THE MOTOR ORGANS.

Motion in animals and plants. Amoeboid cells. Ciliated cells. The

muscles. Histology of striated muscle. Structure of un-
striated muscular tissue. Cardiac muscular tissue. The chem-

istry of muscular tissue. Beef -tea and Liebig's extract ........ 109

CHAPTER X.
THE PROPERTIES OF MUSCULAR TISSUE.

Contractility. Irritability. A simple muscular contraction. Phys-
iological tetanus. Causes affecting degree of contraction.
Measure of muscular work. Muscular elasticity. Electrical
currents of muscle. Secondary contraction. Secondary tetanus.
Source of muscular energy. Physiology of plain muscular
tissue . . .127

CONTENTS. xi

CHAPTER XI.
MOTION AND LOCOMOTION. HYGIENE OF MUSCLES.

PAGE

Special physiology of the muscles. Levers in the Body. Postures.
Walking. Running. Hygiene of muscles. Exercise. Train-
ing 144

CHAPTER XII.
ANATOMY OF THE NERVOUS SYSTEM. **

Nerve-trunks. Nerve-centres. Cerebro-spinal centre and its mem-
branes. Spinal cord. Spinal nerves. Brain. Cranial nerves.
Sympathetic system. Sporadic ganglia. Histelogy of nerve-
fibres. Histology of nerve-cells. Neuroglia. Histology of
spinal cord. Structure of a spinal ganglion 158

CHAPTER XIII.
THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM.

The properties of the nervous system. Functions of nerve-centres
and nerve-trunks. Excitant and inhibitory nerves. Classifica-
tion of nerve-fibres. Electrical phenomena of nerves. Stimuli
of nerve-fibres. General nerve stimuli. Special nerve stimuli.
Specific nerve energies. Proof that all nerve-fibres are physio-
logically alike. The nature of a nervous impulse. Rate of trans-
mission of a nervous impulse. Functions of special nerve roots.
Cranial nerves. Intercommunication of nerve-centres. Degen-
eration of nerve-fibres when separated from their centre 186

CHAPTER XIV.

THE ANATOMY OF THE HEART AND BLOOD-VESSELS.
General statement. Position of heart. Membranes of heart. Anat-
omy of heart. Valves of heart. The arterial system. Aorta
and its branches. The capillaries. The veins. The pulmo-
nary circulation. The portal circulation. Arterial and venous
blood. Structure of the arteries, of the capillaries, and of the
veins 211

CHAPTER XV.

THE WORKING OF THE HEART AND BLOOD-VESSELS.
The beat of the heart. Cardiac impulse. Use of papillary muscles.
Sounds of the heart. A cardiac cycle. Work done by the heart.
The circulation in the blood-vessels. Conversion of intermit-
tent into a continuous flow 227

CHAPTER XVI.

ARTERIAL PRESSURE. THE JP&LSE.

Weber's schema. Arterial pressure. The pulse. The rate of the
blood flow. Secondary causes of the circulation. Aspiration
of the thorax. Proofs of the circulation of the blood . . .240

Xli CONTENTS.

CHAPTER XVII.

THE NERVES OF THE HEART AND SOME PHYSIOLOGICAL
PECULIARITIES OF CARDIAC MUSCLE.

The coordination of heart and blood-vessels. Physiological peculi-
arities of cardiac muscle. The beat of the frog's heart. Heart-
beat not tetanic. Ventricular contraction always maximal.
Extrinsic nerves of mammalian heart. Cardio-inhibitory fibres.
The arterial manometer. The cardio- inhibitory centre. The
cardio-accelerator nerves. The iniluence of temperature
changes and of calcium salts on the heart-beat 253

CHAPTER XVIII.
THE VASO-MOTOR NERVES AND NERVE-CENTRES.

The nerves of the blood-vessels. Vaso-constrictor nerves. Vaso-
constrictor centre. Blushing. Taking cold. Vaso-dilator
nerves and centre. Vascular phenomena of inflammation . „ . . . 273

CHAPTER XIX.
THE SECRETORY TISSUES AND ORGANS.

Organs of secretion. Glands. Physical processes in secretion.
Chemical processes of secretion. Mode of activity of secretory
cells. Influence of nervous system on secretion. Secretion by
the subrnaxillary and parotid glands. ..„...., 282

CHAPTER XX.
THE INCOME AND EXPENDITURE OF THE BODY.

The material losses of the Body. The losses of the Body in energy.
The conservation of energy. Potential and kinetic energy. The
energy of chemical affinity. Relation between matters removed
from the Body and energy spent by it. Conditions of oxidation
in the living Body. The fuel of the Body. Utilization of en-
ergy in the living body 399

CHAPTER XXI.
FOODS.

Foods as tissue-formers. The food of plants. Non-oxidizable foods.
Definition of foods. Conditions which a food must fulfil. Pro-
teid or albuminous alimentary principles. Albuminoid or gela-
tinoid alimentary principles. Hydrocarbons. Carbohydrates.
Inorganic foods. Mixed foods. Flesh. Eggs. Milk. Veg-
etable foods. Alcohol. Advantage of a mixed diet 313

CHAPTER XXII.
THE ALIMENTARY CANAL AND ITS APPENDAGES.

General arrangement. Subdivisions of alimentary canal. Mouth.
Teeth. Tongue. Salivary glands. Fauces. Pharynx. CEsoph-

THE HUMAN BODY.

CHAPTER I.

THE GENERAL STRUCTURE AND COMPOSITION OF THE
HUMAN BODY. ' ,

Definitions. The living Human Body may be considered
from either of two aspects. Its structure may be especially
examined, and the forms, connections and mode of growth of
its parts be studied, as also the resemblances or differences in
such respects which appear when it is compared with other
animal bodies. Or the living Body may be more especially
studied as an organism presenting definite properties and
performing certain actions; and then its parts will be investi-
gated with a view to discovering what duty, if any, each ful-
fils. The former group of studies constitutes the science of
Anatomy, and in so far as it deals with the Human Body
alone, of Human Anatomy; while the latter, the science con-
cerned with the uses — or in technical language the functions
—of each part is known as Physiology. Closely connected
with physiology is the science of Hygiene, which is concerned
with the conditions which are favorable to the healthy action
of the various parts of the Body; while the activities and
structure of the diseased body form the subject-matters of
the sciences of Pathology and Pathological Anatomy.

Tissues and Organs. Histology. Examined merely from
the outside our Bodies present a considerable complexity of
structure. We easily recognize distinct parts as head, neck,
trunk and limbs; and in these again smaller constituent
parts, as eyes, nose, ears, mouth; arm, forearm, hand; thigh,
leg and foot. We can, with such an external examination,
go even farther and recognize different materials as entering
into the formation of the larger parts. Skin, hair, nails and
teeth are obviously different substances; simple examination

2 THE HUMAN BODY.

by pressure proves that internally there are harder and softer
solid parts; while the blood that flows from a cut finger shows
that liquid constituents also exist in the Body. The concep-
tion of complexity which may be thus arrived at from exter-
nal observation of the living, is greatly extended by'dissection
of the dead Body, which makes manifest that it consists of a
great number of diverse parts or organs, which in turn are
built up of a limited number of materials; the same material
often entering into the composition of many different organs.
These primary building materials are known as the tissues^
,and that branch" of anatomy which deals with the characters
tof the. tissues -.amd their arrangement in various organs is
known as. Histology j or, since it is mainly carried on with the
aid of the itik-rc^oope, as Microscopic Anatomy. If, with the
poet, we compare the Body to a house, we may go on to liken
the tissues to the bricks, stone, mortar, wood, iron, glass and
so on, used in building ; and then walls and floors, stairs and
windows, formed by the combination of these, would answer
to anatomical organs.

Zoological Position of Man. External examination of the
human Body shows also that it presents certain resemblances
to the bodies of many other animals: head and neck, trunk
and limbs, and various minor parts entering into them, are
not at all peculiar to it. Closer study and the investigation
of internal structure demonstrates further that these resem-
blances are in many cases not superficial only, but that our
Bodies may be regarded as built upon a plan common to them
and the bodies of many other creatures: and it soon becomes
further apparent that this resemblance is greater between the
Human Body and the bodies of ordinary four-footed beasts,
than between it and the bodies of birds, reptiles or fishes.
Hence, from a zoological point of view, man's Body marks
him out as belonging to the group of Mammalia (see Zoology),
which includes all animals in which the female suckles the
young ; and among mammals the anatomical resemblances
are closer and the differences less between man and certain
apes than between man and the other mammals; so that
zoologists still, with Linnaeus, include man with the monkeys
and apes in one subdivision of the Mammalia, known as the
Primates. That civilized man is mentally far superior to
any other animal is 010 valid objection to such a classification,
for zoological groups are defined by anatomical and not by

GENERAL STRUCTURE AND COMPOSITION. 3

physiological characters; and mental traits, since we know
that their manifestation depends upon the structural integ-
rity of certain organs, are especially phenomena of function
and therefore not available for purposes of zoological ar-
rangement.

As man walks erect with the head upward, while the great
majority of Mammals go on all lours with the head forward
and the back upward, and various apes adopt intermediate
positions, confusion is apt to arise in considering correspond-
ing parts in man and other animals unless a precise mean-
ing be given to such terms as "anterior" and "posterior."
Anatomists therefore give those words definite arbitrary sig-
nifications. The head end is always an_tej'ior whatever the
natural position of the animal, and the opposite end posterior;
the belly side is spoken of as ventral, and the opposite side as
dorsal ; rigid and left of course present no difficulty: the
terms cephalic and caudal as equivalent, respectively, to ante-
rior and posterior, are sometimes used. Moreover, that end
of a limb nearer the trunk is spoken of as proximal with refer-
ence to the other or distal end. The words upper and loiver
may be conveniently used for the relative position of parts in
the natural standing position o.f the animal.

The Vertebrate Plan of Structure. Neglecting such
merely apparent differences as arise from the differences of
normal posture above pointed out, we find that man's own
zoological class, the Mammals, differs very widely in its broad
structural plan from the groups including sea-anemones, in-
sects or oysters, but agrees in many points with the groups of
fishes, amphibians, reptiles and birds. These four are there-
fore placed with man and all other Mammals in one great
division of the animal kingdom known as the Vertebrata.
The main anatomical character of all vertebrate animals is
the presence in the trunk of the body of two cavities, a dorsal
and a ventral, separated by a solid partition: in the adults of
nearly all vertebrate animals a hard axis, the vertebral column
(backbone or spine), develops in this partition and forms a
central support for the rest of the body (Fig. 2, ee). The
dorsal cavity is continued through the neck, when there is
one, into the head, and there widens out. The bony axis is
also continued through the neck and extends into the head
in a modified form. The ventral cavitf, on the other hand,
is confined to the trunk. It contains the main organs con-

4 THE HUMAN BODY.

nected with the blood-flow and is often called the hcemal
cavity.

Upon the ventral side of the head is the mouth-opening
leading' into a tube, the alimentary canal, /', which passes
back through the neck and trunk and opens again on the
outside at the posterior part of the latter. In its passage
through the trunk-region -this canal lies in the ventral cavity.

The Mammalia. In many vertebrate animals the ven-
tral cavity is not subdivided, but in the Mammalia it is; a

FIG. 1.— The Body opened from the front to show the contents off its ventral
cavity, lu, lungs; h, heart, partly covered by other things; le, le', right and left
liver-lobes respectively ; ma, stomach ; we, the great omentum, a membrane con-
taining fat which hangs down from the posterior border of the stomach and covers
the intestines.

membranous transverse partition, the midriff or diaphragm
(Fig. ], z), separating it into an anterior chest or thoracic

GENERAL STRUCTURE AND COMPOSITION.

O

cavity, and a posterior or abdominal cavity. The alimentary
canal and whatever else passes from one of these cavities to
the other must therefore perforate
the diaphragm.

In the chest, besides part of the
alimentary canal, lie important or-
gans, the hearty h, and lungs, lu;
the heart being on the ventral side
of the alimentary canal. The ab-
dominal cavity is mainly occupied by
the alimentary canal and organs con-
nected with it and concerned in the
digestion of food, as the stomach,
ma, the liver, le, the pancreas, and
the intestines. Among the other
more prominent organs in it are the
kidneys and the spleen.

In the dorsal or neural cavity lie
the brain and spinal cord, the former
occupying its anterior enlargement
in the head. Brain and spinal cord
together form the cerebro-spinal
nervous centre; in addition to this
there are found in the ventral cavity
a number of small nerve-centres
united together by connecting cords,
and with their offshoots forming the
sympathetic nervous system.

The walls of the three main cavi-

ties are lined by Smooth, moist the brain; ee, vertebrae form-

. ing the solid partition between

SerOUS membranes. That lining the the dorsal and ventral cavities;

. ,, , . , ,, . &, the pleural, and c, the abdom-

QOrsal Cavity IS the arachnoid; that inal division of the ventral cav-

-, . . i J.T 7 i. ily» separated from one another

lining the chest the pleura; that by the diaphragm, d\ i, the

Fm. 2._Diagrammatic
the'neuSfSe?^

lining the abdomen the peritoneum;
the abdominal cavity is in conse-
quence often called the peritoneal

pavitv T^Yfprrmllv flip walls nf fTioco nervous chain. From the stom-
.avity. Externally t nese ach, /, the intestinal tube leads

cavities are covered by the skin, through the abdominal cavity

•* ' to the posterior opening of the

Which Consists Of two layers : 'an Outer alimentary canal.

horny layer called the epidermis, which is constantly being
shed on the surface and renewed from below; and a deeper
layer, called the dermis and containing blood, which the

6

HUMAN BODY.

epidermis does not. Between the skin and the lining serous
membranes are bones, muscles (the lean of meat), and a great
number of other structures which we shall have to consider
hereafter. All cavities inside the body, as the alimentary
canal and the air-passages, which open directly or indirectly
on the surface are lined by soft and moist prolongations of
the skin known as mucous membranes. In these two layers
are found as in the skin, but the superficial bloodless one is
called epithelium and the deeper vascular one corium.

Diagrammatically we may represent the Human Body
in longitudinal section as in Fig. 2, where aa' is the dorsal
or neural cavity, and b and c, respectively, the thoracic and
abdominal subdivisions of the ventral cavity; d represents
the diaphragm separating them ; ee is the vertebral column
with its modified prolongation into the head beneath the
anterior, enlargement of the dorsal cavity; / is the ali-
mentary canal opening in front through the nose, ?', and
mouth, 0; li is the heart, I a lung, s the sympathetic nervous
system, and k a kidney.

A transverse section through the chest is represented by the
diagram Fig. 3, where x is the neural canal containing the
spinal cord. In the thoracic cavity are seen the heart, h,

FIG. 3.— A diagrammatic section across the Body in the chest region, x, the
dorsal tube, which contains the spinal cord; the black mass surrounding it is a
vertebra; a, the gullet, a part of the alimentary canal; h, the heart; sy, sympa-
thetic nervous system; M, luags; the dotted lines around them are the pleurae; IT,
ribs; st, the breast-bone.

the lungs, II, part of the alimentary canal, a, and the sympa-
thetic nerve-centres, sy ; the dotted line on. each side covering
the inside of the chest- wall and the outside of the lung
represents the pleura.

Sections through corresponding parts of any other Mam-
mal would agree in all essential points with those represented
in Figs. 2 and 3.

GENERAL STRUCTURE AND COMPOSITION. 1

The Limbs. The limbs present no such arrangement of
cavities on each side of a bony axis as is seen in the trunk.
They have an axis formed at different parts of one or more
bones (as seen at U and R in Fig. 4, which represents a cross-
section of the forearm near the elbow-joint), but around this
are closely-packed soft parts, chiefly muscles, and the whole
is enveloped in skin. The only cavities in the limbs are
branching tubes which are filled with liquids during life,
either blood or a watery-looking fluid known as lymph. These
tubes, the blood and lymph vessels respectively, are not, how-

u

FIG. 4.— A section across the forearm a short distance below the elbow-joint. R
and £7, its two supporting bones, the radius and ulna; e, the epidermis, and d, the
dermis of the skin; the latter is continuous below with bands of connective tissue,
s, which penetrate between and invest the, muscles, which are indicated by nuin
bers ; n n. nerves and vessels.

ever, characteristic of the limbs, for they are present in
abundance in the dorsal and ventral cavities and in their
walls.

Chemical Composition of the Body. In addition to the
study of the Body as composed of tissues and organs which
are optically recognizable, we may consider it as composed of
a number of different chemical substances. This branch of
knowledge, which is still very incomplete, really presents two
classes of problems. On the one hand we may limit ourselves
to the examination of the chemical substances which exist in
or may be derived from the dead Body, or, if such a thing
were possible, from the living Body entirely at rest; such a
study is essentially one of structure and may be called Chem-
ical Anatomy. But as long as the Body is alive it is the seat
of constant chemical transformations in its material, and
these arc inseparably connected with its functions, the great
majority of which are in the long-run dependent upon chem-
ical changes. From this point of view, then, the chemical
study of the Body presents physiological problems, and it is
usual to include all the facts known as to the chemical com-
position and metamorphoses of living matter under the name

8 THE HUMAN BODY.

of Physiological Chemistry. For the present we may confine
ourselves to the more important substances derived from or
known to exist in the Body, leaving questions concerning the
chemical changes taking place within it for consideration
along with those functions which are performed in connection
with them.

Elements Composing the Body. Of the elements known
to chemists only sixteen have been found to take part in the
formation of the human Body. These are carbon, hydrogen,
nitrogen, oxygen, sulphur, phosphorus, chlorine, fluorine,
silicon, sodium, potassium, lithium, calcium, magnesium, iron
and manganese. Copper and lead have sometimes been found
in small quantities, but are probably accidental and occa-
sional.

Uncombined Elements. Only a very small number of the
above elements exist in the Body uncombined. Oxygen is
found in small quantity dissolved in the blood; but even there
most of it is in a state of loose chemical combination. It is
also found in the cavities of the lungs and alimentary canal,
being derived from the inspired air or swallowed with food
and saliva; but while contained in these spaces it can hardly
be said to form a part of the Body. Nitrogen also exists un-
combined in the lungs and alimentary canal, and in small
quantity in solution in the blood. Free hydrogen has also
been found in the alimentary canal, being there evolved by
the fermentation of certain foods.

Chemical Compounds. The number of these which may
be obtained from the Body is very great; but with regard to
very many of them we do not know that the form in which
we extract them is really that in which the elements they
contain were united while in the living Body; since the
methods of chemical analysis are such as always break down the
more complex forms of living matter and leave us only its de-
bris for examination. We know in fact, tolerably accurately,
what compounds enter the Body as food and what finally
leave it as waste; but the intermediate conditions of the ele-
ments contained in these compounds during their sojourn
inside the Body we know very little about; more especially
their state of combination during that part of their stay when
they do not exist dissolved in the bodily liquids, but form
part of a solid living tissue.

For present purposes the chemical compounds existing in

GENERAL STRUCTURE AND COMPOSITION. (9

or derived from the Body may be classified as organic and in-
organic, and the former be subdivided into those which con-
tain nitrogen and those which do not.

Nitrogenous or Azotized Organic Compounds. These
fall into several main groups: proteids, peptones, albuminoids,
enzymes, crystalline substances, and coloring matters.

Proteids are by far the most characteristic substances
obtained from the Body, since they are only known as exist-
ing in or derived from living things, either animals or plants.
The type of this class of bodies may be found in the white of
an egg, where it is stored up as food for the developing chick-;
from this typical form, which is called egg-albumin, the pro-
teids in general are often called albuminous- bodies. Each
of them contains carbon, hydrogen, oxygen, sulphur and
nitrogen united to form a very complex molecule, and although
different members of the family differ from one another in
minor points they all agree in their broad features and have
a similar percentage composition. The latter in different
examples varies within the following limits:

Carbon 50 to 55 per cent.

Hydrogen 6.8 to 7.3

Oxygen 22.8 to 24.1 "

Nitrogen.. 15.4 to 18.2 "

Sulphur ." 0.4 to 5.0 "

In addition a small quantity of ash is usually left when a
proteid is burnt.

Proteids are recognized by the following characters:

1. Boiled, either in the solid state or in solution, with strong
nitric acid they give a yellow liquid which becomes orange on
neutralization with ammonia. This is the xantho-proteic test.

2. Boiled with a solution containing subnitrate and per-
nitrate of mercury they give a pink precipitate, or, if in very
small quantity, a pink-colored solution. This is known as
Millon's testf

3. If a solution containing a proteid be strongly acidulated
with acetic acid and be boiled after the addition of an equal
bulk of a saturated watery solution of sodium sulphate, the
proteid will be precipitated.

Among the more important pfoteids obtained from the
Human Body are the following:

Serum-albumin. 0 This exists in solution in the blood and

10 THE HUMAN BODY.

is very like egg-albumin in its properties. It is coagulated
(like the white of an egg) when boiled, arid then passes into
the state of coagulated proteid which is, unlike the original
serum-albumin, insoluble in dilute acids or alkalies or in
water containing neutral salts in solution. All other proteids
can by appropriate treatment be turned into coagulated
proteid.

Fill-in. This forms in blood when it " clots," either in-
side or outside of the Body; it is insoluble in water and dilute
acids or alkalies; soluble in strong acids and alkalies and,
though slowly, in ten per cent neutral saline solutions.

Myosin. This is derived from the muscles, in which it
develops and solidifies after death, causing the "death-stiffen-
ing."

Globulin exists in the red globules of the blood and dis-
solved in some other liquids of the body. In the blood-cor-
puscles it is combined with a colored non-proteid substance
to form licemoglolin, which is crystallizable. Allied sub-
stances, paraglolulin and fibrinogen, are found dissolved in
the blood-liquid. When blood clots the fibrinogen gives rise
to fibrin.

Casein or, as it is better named, caseinogen exists in
milk. Its solutions do not coagulate spontaneously or, like
that of serum-albumin, on boiling. When milk turns sour on
keeping, or when it is very slightly acidulated with dilute
acetic acid, the casein is precipitated. The clot or curd which
forms when milk is gently warmed with gastric juice or with
rennet, is also derived from caseinogen; it differs from true
casein and is named tyrein : it is the chief constituent of
cheese.

Peptones. These are formed in the alimentary canal by
the action of some of the digestive liquids upon the proteids
swallowed as food. They contain the same elements as the
proteids and give the xantho-proteic and Millon's reactions,
but are not precipitated by boiling with acetic acid and
sodium sulphate. Their great distinctive character is, how-
ever, their diffusibility. The proteids proper will not dialyze
(see Physics), but the peptones in solution pass readily
through moist animal membranes.

Albuminoids or Gelatinoids. These contain carbon,
hydrogen, oxygen and nitrogen, but rarely any sulphur. Like
the proteids, the nearest chemical allies of which they seem

GENERAL STRUCTURE AND COMPOSITION. 11

to be, they are only known in or derived from living beings.
Gelatin, obtained from bones and ligaments by boiling, is a
typical albuminoid; as is chondrin, which is obtained similarly
from gristle. Mucin, which gives their glairy tenacious char-
acter to the secretions of the- mouth and nose, is anothei
albuminoid.

Enzymes or Soluble Ferments are a group of substances
which seem to be allied in chemical composition to the true
proteids, but it is so difficult to be sure of the purity of any
specimen that their composition is still in doubt. The
enzymes have the power, even when present in very small
quantity, of bringing about extensive changes in other sub-
stances, and they are not themselves necessarily used up or
destroyed in the process. Many enzymes of great physiologi-
cal importance exist in the digestive fluids and play a part in
fitting food for absorption from the alimentary canal. For
example, pepsin found in the gastric juice and trypsin found
in the pancreatic secretion convert, under suitable conditions,
albuminous substances into peptones; and ptyalin, found in
the saliva, converts starch into sugar. Other ferments
cause the clotting of various animal liquids: rennin from the
gastric juice clots the caseinogen of milk preparatory to its
digestion; and a ferment which forms in drawn blood con-
verts fibrinogen into fibrin. We shall have occasion later to
study several enzymes more in detail in connection with their
physiological uses.

Crystalline Nitrogenous Substances. These are a heter-
ogeneous group, the great majority of them being materials
which have done their work in the Body and are about to be
got rid of. Nitrogen enters the Body in foods for the most
part in the chemically complex form of some proteid. In the
vital processes these proteids are broken down into simpler
substances, their carbon being partly combined with oxygen
and passed out through the lungs as carbon dioxide; their
hydrogen is similarly in large part combined with oxygen and
passed out as water; while their nitrogen, with some carbon
and hydrogen and oxygen, is usually passed out in the form
of a crystalline compound, containing what chemists call an
"ammonium residue/' Of these the most important is urea
(Carbamide, 2NH2, CO), which is eliminated through the kid-
neys. Uric acid is another nitrogenous waste product, and
many others, such as kreatin and leucin, seem to be inter-

12 THE HUMAN BODY.

mediate stages between the proteids which enter the body and
the urea and uric acid which leave it.

In the bile or gall, two crystallizable nitrogen-containing
bodies, glycocholic and taurocholic acid, are found combined
with soda.

Nitrogenous Coloring Matters. These form an artificial
group whose constitution and origin are ill known. Among
the most important are the following:

Hcematin, derived from the red corpuscles of the blood in
which a residue of it is combined with a proteid residue to
form haemoglobin.

Bilirubin and liliverdin, which exist in the bile; the
former predominating in the bile of man and of carnivorous
. animals and giving it a reddish-yellow color, while biliverdin
predominates in the bile of Herbivora, which is green.

Non-Nitrogenous Organic Compounds. These may be
conveniently grouped as hydrocarbons or fatty bodies; carbo-
hydrates or amyloids / and certain non-azotized acids.

Pats. The /ate all contain carbon, hydrogen and oxygen,
the oxygen being present in small proportion as compared
with the hydrogen. TTlT1fifl , fqfg npf>ip- *'" ^^ ^oriy in large
qnantities1 vig^ palmatin (C^H^O,.'). stearin (CB,H1I006),
fl,i)rl nlp;ip_^r^ H |u( }j. The two fanner when pure are solid
at the temperature of the Body, but in it are mixed with
olein (which is liquid) in such proportions as to be kept fluid.
The total quantity of fat in the Body is subject to great vari-
ations, but its average quantity in a man weighing 75 kilo-
grams (165 pounds) is about 2.75 kilograms (6 pounds).

Each of these fats when heated with a caustic alkali, in
the presence of water, breaks up into a fatty acid (stearic,
palmitic or oleic as the case may be), and glycerin. The
fatty acid unites with the alkali present to form a soap.

Carbohydrates. These also contain carbon, hydrogen
and oxygen, but there is one atom of oxygen present for
every two of hydrogen in the molecule of each of them.
Chemically they are related to starch. The more important
of them found in the Body are the following:

Glycogen (G^H^OJ, found in large quantities in the liver,
where it seems to be a reserve of material answering to the
starch stored up by many plants. It exists in smaller quanti-
ties in the muscles.

Glucose, or grape-sugar (C6H]206), which exists in the

I STRUCTURE AND COMPOSITION. 13

liver in ,'rnall quantities; also in the blood and lymph. It is
largely derived from glycogen, which is very readily Converted
into it.

Lactose, or sugar of ntilJc (CiaH3aOn -f H20), found in
considerable quantity in milk.

Inosit (C6H1206 -f 2H20), also called muscle -sn gar aiv"
formerly classed in this group, is now known to be chf
cally not a real sugar or true carbohydrate. It exi^
muscles, liver, spleen, kidneys, etc.

Organic Non-Nitrogenous Acids. Of these the most im-
portant is carbon dioxide (C02), which is the form in which
by far the greater part of the carbon taken into the Body
ultimately leaves it. United with calcium it is found in the
bones and teeth in large proportion.

Formic, acetic and butyric acids are also found in the
Body; stearic, palmitic, and oleic have been above mentioned
as obtainable from fats. Lactic acid is sometimes found in
the stomach, and when milk turns sour is formed from lactose.-
A body of the same percentage composition, C3H603 (sarco-
lactic acid), is formed in muscles when they work or die.

Glycerin phosphoric acid (C3H9P06) is obtained on the de-
composition of lecithin, a complex nitrogenous fat found in
nervous tissue.

Inorganic Constituents. Of the simpler substances en-
tering into the structure of the Body the following are the
most important :

Water; in all the tissues in greater or less proportion and
forming about two thirds of the weight of the whole Body.
A man weighing 75 kilos (1G5 Ibs.), if completely dried
would therefore lose about 50 kilos (110 Ibs.) from the evapo-
ration of water. Of the constituents of the Body the enamel
of the teeth contains least water (about 2 per cent), and the
saliva most (about 99.5 per cent) ; between these extremes
are all intermediate steps — bones containing about 22 per cent,
muscles 75, blood 79.

Common salt — Sodium chloride — (NaCl); found in all the
tissues and liquids, and in many cases playing an important
part in keeping other substances in solution in water.

Potassium chloride (KC1) ; in the blood, muscles, nerves
and most liquids.

Calcium phosphate (Ca32P04) ; in the bones and teeth in
large quantity. In less proportion in all the other tissues.

14 THE HUMAN BODY.

Besides the above, ammonium chloride, sodium a id potas-
sium phosphates, magnesium phosphate, sodium sulphate,
potassium sulphate and calcium fluoride have been obtained
?rom the body.

* Uncombined hydrochloric acid (HC1) is found in the
ga?*^ juice.

V

CHAPTER II.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS.

The Properties of the Living Body. When we turn
from the structure and composition of the living Body to
consider its powers and properties we meet again with great
variety and complexity, the most superficial examination be-
ing sufficient to show that its parts are endowed with very
different faculties. Light falling on the eye arouses in us a
sensation of sight, but falling on the skin has no such effect;
pinching the skin causes pain, but pinching a hair or a nail
does not; when the ears are stopped, sounds arouse in us no
sensation; we readily recognize, too, hard parts formed for
support, joints to admit of movements, apertures to receive
food and others to get rid of waste. We thus perceive that
different organs of our Bodies have very different endow-
ments and serve for very distinct purposes; and here also
the study of internal organs shows us that the varieties of
quality observed on the exterior are but slight indications of
differences of property which pervade the whole, being some-
times dependent on the specific characters of the tissues con-
cerned and sometimes upon the manner in which these are
combined to form various organs. Some tissues are solid,
rigid and of constant shape, as those composing the bones
and teeth; others, as the muscles, are soft and capable of
changing their forms; and still others are capable of working
chemical changes by which such peculiar fluids as the bile
and the saliva are produced. We find elsewhere a number of
tissues combined to form a tube adapted to receive food and
carry it through the Body for digestion, and again similar
tissues differently arranged to receive the air which we breathe-
in, and expel it after abstracting part of its oxygen and
adding to it certain other things; and in the heart and blood-
vessels we find almost the same tissues arranged to propel
and carry the blood over the whole Body. The working of

15

16 THE HUMAN BODY.

the Body offers clearly even a more complex subject of study
than its structure.

Physiological Properties. In common with inanimate
objects the Body possesses many merely physical properties,
as weight, rigidity, elasticity, color, and so on; but in addi-
tion to these we find in it while alive many others which it
ceases to manifest at death. Of these perhaps the power of
executing spontaneous movements aiid of maintaining a high
bodily temperature are the most marked. As long as the
Body is alive it is warm and, since the surrounding air is
nearly always cooler, must be losing heat all day long to
neighboring objects; nevertheless we are at the end of the
day as warm as at the beginning, the temperature of the
Body in health not varying much from 37° 0. (98.4° F.), so
that clearly our Bodies must be making heat somehow all
the time. After death this production of heat ceases and the
Body cools down to the temperature in its neighborhood; but
so closely do we associate with it the idea of warmth that
the sensation experienced on touching a corpse produces so
powerful an impression as commonly to be described as icy
cold. The other great characteristic of the living Body is its
power of executing movements; so long as life lasts it is
never at rest; even in the deepest slumber the regular breath-
ing, the tap of the heart against the chest-wall, and the beat
of the pulse tell us that we are watching sleep and not death.
If to this we add the possession of consciousness by the living
Body, whether aroused or not by forces immediately acting
upon sense-organs, we might describe it as a heat-producing,
moving, conscious organism.

The production of heat in the Body needs fuel of some
kind as much as its production in a fire ; and every time we
move ourselves or external objects some of the Body is used
up to supply the necessary working powerp just as some coals
are burnt in the furnace of an engine for every bit of work it
does; in the same way every thought that arises in us is ac-
companied with the destruction of some part of the Body.
Hence these primary actions of keeping warm, moving, and
being conscious, necessitate many others for the supply of
new materials to the tissues concerned and for the removal of
their wastes; still others are necessary to regulate the pro-
duction and loss of heat in accordance with changes in the
exterior temperature, to bring the moving tissues into rela-

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 17

tion with the thinking, and so on. By such subsidiary ar-
rangements the working of the whole Body becomes so com-
plex that it would fill many pages merely to enumerate what
is known of the duties of its various parts. However, all the
proper physiological properties depend in ultimate analysis
on a small number of faculties which are possessed by all
living things, their great variety in the human Body depend-
ing upon special development and combination in different
tissues and organs; and before attempting to study them in,
their most complex forms it is advantageous to
examine them in their simplest and most gen-
eralized manifestations, as exhibited by some of
the lowest living things or by the simplest con-
stituents of our own Bodies.

Cells. Among the anatomical elements
which the histologist meets with as entering into
the composition of the human Body are minute
granular masses of a soft consistence, about
0.012 millimeter (-g^Vcr °^ an incn) in diameter
(Fig. 5, b). Imbedded in each lies a central
portion, not so granular and therefore different
in appearance from the rest. These anatomical
units are known as cells, the granular substance Fio.5.— Forms
being the cell-body and the imbedded clearer por- gf0|jel's from the
tion the cell-nucleus,. Inside the nucleus may
often be distinguished a still smaller body — the nucleolus.
Cells of this kind exist in abundance in the blood, where they
are known as the white blood-corpuscles, and each exhibits of
itself certain properties which are distinctive of all living
things as compared with inanimate objects.

Cell Growth. In the first place, each such cell can take
up materials from its outside and build them up into its own
peculiar substance; and this does not occur by the deposit of
new layers of material like its own on the surface of the cell
(as a crystal might increase in an evaporating solution of the
same salt), but in an entirely different way. The cell takes
up chemical elements, either free or combined in a manner
different from that in which they exist in its own living sub-
stance, and works chemical changes in them by which they
are made into part and parcel of itself. Moreover, the new
material thus formed is not deposited, at any rate necessarily
or always, on the surface of the old, but is laid down in the

18 THE HUMAN BODY.

substance of the already existing cell among its constituent
molecules. The new-formed molecules therefore contribute
to the growth of the cell not by superficial accretion, but by
interstitial deposit or intussusception.^

Cell Division. The increase of size, which may be brought
about in the above manner, is not indefinite, but is limited in
two ways. Alongside of the formation and deposit of new
material there occurs always in the living cell a breaking
down and elimination of the old; and when this process

FIG. 6.— Diagrams illustrating direct cell-division, a, cell, body; 6, nucleus;
c, uucleoJus.

equals the accumulation of new material, as it does in all the
cells of the Body when they attain a certain size, growth
of course ceases. In fact the work of the cell increases
as its mass, and therefore as the cube of its diameter;
while the receptive powers, dependent primarily upon the
superficial area, only increase as the square of the diameter.
The breaking down in the cell increases when its work
does, and so comes at last to equal the reception and con-
struction. The second limitation to indefinite growth is
connected with the power of the cell to give rise to new cells
by division.

Until recently it was believed that cell division was in all
^ases a comparatively simple process (Fig. 6). It was thought
that the nucleus, without any important structural change,
enlarged somewhat, became elongated, and then divided by
simple constriction into two equal parts, forming two smaller
daughter nuclei; and that the rest of the cell then divided,
its halves arranging themselves around the new nuclei. The
nucleolus when present was supposed to divide before the
nucleus. Such a mode of cell multiplication is known as
direct division : it possibly occurs in some cases, but in the
great majority of cells division is preceded by marked changes
in the structure of the nucleus and by a rearrangement of its
material: such cell division is named indirect, and the attend-
ant nuclear changes are known as the phenomena of karyoki-
nesis or mitosi*

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 19

Indirect, Karyokinetic or Mitotic Cell Division. Before
attempting to describe the phenomena of indirect cell divi-
sions it is necessary to give some
account of the structure of a typical
primitive cell as made out in speci-
mens carefully prepared and studied
with the highest powers of the
microscope. The main bulk of the
cell, surrounding the nucleus, is the
cell-body, and in some cases is en-
closed in an envelope or sac, which,
however, when present, plays but a
secondary or passive part in cell divi- ium; e, nucleus." aaudt. together

__, r,. . form the cell-body.

sion. The cell-body, known also as

the cell-protoplasm (Fig. 7), consists of a network of extremely
fine threads,"the reticulum or spongioplasm, the meshes of which
are occupied by a different substance, the hyaloplasm : the
proportions of hyaloplasm and spongioplasm vary in different
cells and often in different parts of the same cell; in fact a
layer of hyaloplasm unmixed with spongioplasm frequently
exists on the exterior of the cell, and the hyaloplasm appears
to be the more immediately concerned in the activities of the
living cell. In addition there is to be found, imbedded in the
cell-body and near the nucleus or attached to it, an extremely
minute particle, the attraction-particle or cent r.o some, near

which a radial arrangement of the
cell-substance may often be ob-
served.

The nucleus (Fig. 8) of a
resting cell, that is of a cell not
in process of division, consists of
an amorphous material (nucleo-
plasm] which is perhaps similar
in composition to the hyaloplasm,
and a filamentous material, dif-
ferent from spongioplasm, and
named chromoplnsm or karyo-

SS^pl^r^f'SrS?"^ ?*««»• ^ P«™*1 ^ "S behavior

h staining fluids and other
reagents karyoplasm is quite different chemically from the
spongioplasm of the cell-body. One or more granules (nit-
cleoli) which may be found within most nuclei are probably

FIG. 8. — Diagram of a resting
nucleus, a, nuclear membrane; fo,
nucleoplasm; c, nucleolus; d, chro-
ome of the surround-
m of the cell, the
structure of which is not indicated.

20 THE HUMAN BODY.

local accumulations of chromoplasm; a membrane (nuclear
membrane) which surrounds the nucleus of cells not in process
of division is also probably composed of chromoplasm.

The first observed step in cell division is binary division of
the attraction-particle: its halves evolve a set of very fine
achromat in filaments uniting them, so that each half is one of
the poles of a spindle-shaped collection of fibres, the nuclear
spindle. Meanwhile the nucleolus and nuclear membrane
disappear, being probably taken up into the rest of the chro-
moplasm, which now, instead of its original reticular arrange-

- d. b

FIG. 0.— Diagrams ot a nucleus in an early stage of karypkinesis, A showing the
polar, B the anripolar region; o, nuclear or achromatin spindle; />, part of general
cell-protoplasm around the nucleus; c, looped chromatic filament; d, nucleoplasm.

ment, takes the form of a single long chromatic filament
coiled in the nucleoplasm. At one portion of the nucleus
(pole) the loops of the chromatic filament leave a space free
from them (Fig. 9, ^4), and in the neighborhood of this space
the nuclear spindle is first seen within the nucleus. At the
opposite side of the nucleus or antipole (Fig. 9, B) the loops
of the chromatic filament leave no clear space, but cross ir-
regularly. In the next stage the loops at the antipolar end
break through, and in this way the filament is divided into a
number of irregular elongated Vs, each with its closed angle
near the pole and its open end near the antipole. The spindle
meanwhile passes to the centre of the nucleus and takes a posi-
tion in which its long axis coincides with that joining pole and
antipole, and then the Vs of chromoplasm become shorter and
their limbs thicker, and they also shift position so as to group
themselves radially around the equator of the spindle (A, Fig.
10) with their angles directed centrally. Each V then di-
vides along its whole length, and one half passes towards the

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 21

pole, the other towards the antipole. The whole nucleus
elongates in the direction of the long axis of the spindle; the
achromatin filaments disappear, and the nucleus dividing in

B

FIG. 10. — Diagrams representing more advanced stages of karyokinesis than those
illustrated in Fig. 9. a, polar, and e, antipolar end of nuclear spindle; b and c, por-
tions of the chromatic filament; rf, nucleoplasm; /, cell protoplasm with indications
of a radial arrangement in the neighborhood of the pole and antipole.

The nuclear spiudle is seen to have lengthened and become placed in the centre
of the nucleus, the pole and antipole of which its ends reach. In A the Vs which
resulted from divisions of the chromatic filament at its antipolar loops are seen to
have become much shorter and thicker and to have changed position, so that in-
stead of lying lengthwise in the nucleus, with their points towards the pole, they lie
equatorially, with their points towards the spindle and their open ends towards the
periphery of the nucleus. For the sake of clearness only two are represented out
of the set of them which surrounds the spindle; 6 is still uncleft; c has nearly com-
pleted its longitudinal division into two Vs, the angle of one of which is commencing
to travel towards the pole and of the other towards the antipole. In B the splitting
of the Vs and the progress of their halves towards the ends of the nucleus is more
advanced.

the equatorial plane, two nuclei are formed, each with nucleo-
plasm and chromoplasm : the chromoplasm of each is derived,
as follows from the preceding description, from both polar and
antipolar regions of the parent nucleus. The chromoplasm in
each daughter nucleus unites into a single convoluted chro-
matic filament like that represented for the parent nucleus in
Fig. 9, and this filament breaks up and becomes arranged
into reticulum, nucleolus and nuclear membrane as in the
resting cell (Figs. 7 and 8). Around the new nuclei the cell-
protoplasm rearranges itself and divides to form a new cell-body
enveloping each; during its rearrangement its material fre-
quently presents a radial structure, the radii converging to-
wards the ends of the nuclear spindle. The poles of the nuclear
spindle, which it will be remembered represent the halves of

22 THE HUMAN BODY.

the original centrosome, probably pass out of the new nuclei
and become the attraction particles of the new cells.

The phenomena of karyokinesis show clearly that in spite of
its small size the animal cell is a complicated structure, made
up of very distinct parts possessing very distinct properties
and no doubt very different functions.

Assimilation: Reproduction. The two powers, that of
working up into their own substance materials derived from
outside, known as assimilation, and that of, in one way or an-
other, giving rise to new beings like themselves, known as re-
production, are possessed by all kinds of living beings, whether
animals or plants. There is, however, this important differ-
ence between the two: the power of assimilation is necessary
for the maintenance of each individual cell, plant or animal,.
since the already existing living material is constantly break-
ing down and being removed as long as life lasts, and the loss
must be made good if any of them is to continue its existence.
, *Fhe power of reproduction, on the other hand, is necessary
only for the continuance of the kind or race, and need be, and
often is, possessed only by some of the individuals composing it.
Working bees, for example, cannot reproduce their kind, that
duty being left to the queen-bee and the drones of each hive.

The breaking down of already existing chemical compounds
into simpler ones, sometimes called dissimilation, is as inva-
riable in living beings as the building up of new complex mole-
cules referred to above. It is associated with the assumption
of uncombined oxygen from the exterior, which is then com-
bined directly or indirectly with other elements in the cell, as,
for example, carbon, giving rise to carbon dioxide, or hydro-
gen, producing water. In this way the molecule in which the
carbon and hydrogen previously existed is broken down and
at the same time energy is liberated, which in all cases seems
to take in part the form of heat just as when coal is burnt in
a fire, but may be used in part for other purposes, such as pro-
ducing movements. The carbon dioxide is usually got rid
of by the same mechanism as that which serves to take up the
oxygen, and these two processes constitute the function of
respiration which occurs in all living things. Assimilation
and dissimilation, going -on side by side and being to a certain
extent correlative, are often spoken of together as the process
of nutrition : the assimilative or chemically constructive pro-
cesses are also named anabolic, and the dissimilative katabolic.

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 23

Contractility. Nutrition and (with the above-mentioned
partial exception) reproduction characterize all living creat-
ures; and both faculties are possessed by the simple nucleated
cells already referred to as found in our blood. But these
cells possess also certain other properties which, although not
so absolutely diagnostic, are yet very characteristic of living
things. Examined carefully with a -microscope in a fresh-
drawn drop of blood, they exhibit changes of form independent
of any pressure which might distort them or otherwise mechani-
cally alter their shape. These changes may sometimes show
themselves as constrictions ultimately leading to the division
of the cell ; but more commonly (Fig. 15*) they have no such
result, the cell simply altering its form by drawing in its sub-
stance at one point and thrusting it out at another. The
portion thus protruded may in turn be drawn in and a pro-
cess be thrown out elsewhere ; or the rest of the cell may col-
lect around it, and a fresh protrusion be then made on the
same side ; and by repeating this manoeuvre these cells ma
change their place and creep across the field of the micro-
scope. Such changes of form from their close resemblance to
those exhibited by the microscopic animal known as the
Amceba (see Zoology) are called amoeboid, and the faculty in
the living cell upon which they depend is known in physiol-
ogy as contractility. It must be borne in mind that physiol-
ogical contractility in this sense is quite different from the
so-called contractility of a stretched india-rubber band,
which merely tends tp reassume a form from which it has
previously freen forcibly removed.

Irritability. Another property exhibited by these blood-
cells is known as irritability. An Amoeba coming into con-
tact with a solid particle calculated to serve it as food will
throw around it processes of its substance, and gradually
carry the foreign mass into its own body. The amount of
energy expended by the animal under these circumstances is
altogether disproportionate to the force of the external contact.
It is not that the swallowed muss pushes-in mechanically the
surface of the Amoeba, or burrows into it, but the mere touch
arouses in the animal an activity quite disproportionate to the
exciting force, and comparable to that set free by a spark
falling into gunpowder or by a slight tap on a piece of gun-
cotton. It is this disproportion between the excitant (known

* P. 48.

24 THE HUMAN BODY.

in Physiology as a stimulus) and the result, which is the es-
sential characteristic of irritability when the term is used in
a physiological connection. The granular cells of the blood
can take foreign matters into themselves in exactly the same
manner as an Amoeba does; and in this and in other ways, as
by contracting into rigid spheres under the influence of elec-
trical shocks, they show that they also are endowed with irri-
tability.

Conductivity. Further, when an Amoeba or one of these
blood-cells comes into contact with a foreign body and pro-
ceeds to draw it into its own substance, the activity excited
is not merely displayed by the parts actually touched. Dis-
tant parts of the cell also co-operate, so that the influence of
the stimulus is not local only, but in consequence of it a change
is brought about in other parts, arousing them. This prop-
erty of transmitting disturbances is known as conductivity.

Finally, the movements excited are not, as a rule, random.
They are not irregular convulsions, but are adapted to attain
a certain end, being so combined as to bring the external par-
ticle into the interior of the cell. This capacity of all the
parts to work together in definite strength and sequence to
fulfil some purpose, is known as co-ordinations

These Properties Characteristic but not Diagnostic.
These four faculties, irritability, conductivity, contractility
and co-ordination, are possessed in a high degree by our
Bodies as a whole. If the inside of the nose be tickled with
a feather, a sneeze will result. Here the feather-touch (stim-
ulus) has called forth movements which are mechanically
altogether disproportionate to the energy of the contact, so
that the living Body is clearly irritable. The movements,
which are themselves a manifestation of contractility, are not
exhibited at the point touched, but at more or less distant
parts, among which those of abdomen, chest and face are
visible from the exterior ; our Bodies therefore possess physio-
logical conductivity. And finally these movements are not
random, but combined so as to produce a violent current of
air through the nose tending to remove the irritating object;
and in this we have a manifestation of co-ordination. Speak-
ing broadly, these properties are more manifest in animals
than in plants, though they are by no means absolutely con-
fined to the former. In the sensitive plant touching one leaflet
will excite regular movements of the whole leaf, and many of

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 25

the lower aquatic plants exhibit movements as active as those
of animals. On the other hand, no one of these four faculties
is absolutely distinctive of living things in the way that growth
by intussusception and reproduction are. Irritability is but
a name for unstable molecular equilibrium, and is as marked
in nitroglycerin as in any living cells; in the telephone the
influence of the voice is conducted as a molecular change
along a wire, and produces results at a distance; and many
inanimate machines afford examples of the co-ordination of
movements for the attainment of definite ends.

Spontaneity. There is, however, one character belonging
to many of the movements exhibited by amoeboid cells, in
which they appear at first sight to differ fundamentally from
the movements of inanimate objects. This character is their
apparent spontaneity or automat icity. The cells frequently
change their form independently of any recognizable external
cause, while a dead mass at rest and unacted on from outside
remains at rest. This difference is, however, only apparent
and depends not upon any faculty of spontaneous action pe-
culiar to the living cell, but upon its nutritive powers. It
can be proved that any system of material particles in equi-
librium and at rest will forever remain so if not acted upon
by an external force. Such a system can carry on, usder cer-
tain conditions, a series of changes when once a start has
been given; but it cannot initiate them. Each living cell
in the long-run is but a complex aggregate of molecules,
composed in their turn of chemical elements, and if we sup-
pose this whole set of atoms at rest in equilibrium at any
moment, no change can be started in the cell from inside; in
other words, it will possess no real spontaneity. When, how-
ever, we consider the irritability of amoeboid cells, or, ex-
pressed in mechanical terms, the unstable equilibrium of their
particles, it becomes obvious that a very slight external cause,
such as may entirely elude our observation, may serve to set
going in them a very marked series of changes, just as pressing
the trigger will fire off a gun. Once the equilibrium of the cell
has been disturbed, movements either of some of its constitu-
ent molecules or of its whole /mass will continue until all the
molecules have again settled down into a stable state. But
in living cells the reattainment of this state is commonly in-
definitely postponed by the reception of new particles, food
in one form or another, from the exterior. The nearest ap-

26 THE HUMAN BODY.

proach to it is probably exhibited by the resting state into
which some of the lower animals, as the wheel-animalcules,
pass when dried slowly at a low temperature; the drying act-
ing by checking the nutritive processes, which would other-
wise have prevented the reattainment of molecular equilib-
rium. All signs of movement or other change disappear
under these circumstances, but as soon as water again soaks
into their substance and disturbs the existing condition, then
the so-called "spontaneous" movements recommence. If,
therefore, we use the term spontaneity to express a power in
a resting system of particles of initiating changes in itself, it
is possessed neither by living nor not-living things. But if
we simply employ it to designate changes whose primary
cause we do not recognize, and whose cause was in many
cases long antecedent to the changes which we see, then the
term is unobjectionable and convenient, as it serves to ex-
press briefly a phenomenon presented by many living things
and finding its highest manifestation in many human actions.
It then, however, no longer designates a property peculiar to
them. A steam-engine with its furnace lighted and water in
its boiler may be set in motion by opening a valve, and the
movements thus started will continue spontaneously, in the
above sense, until the coals or water are used up. The differ-
ence between it and the living cell lies not in any spontaneity
of the latter, but in its nutritive powers, which enable it to
replace continually what answers to the coals and water of
the engine.

Protoplasm. The cell-body was formerly regarded as es-
sentially made up of a single substance, which was named
protoplasm : and now that its structure is known to be com-
plex the term is retained as a convenient one for that mixture
of spongioplasm and hyaloplasm which constitutes the main
bulk of the bodies of most cells. With the protoplasm other .
things are frequently presen^the most important ofjyjikili
are either materials undergoing anabolic changes butno-t ^vet
completely built up into protoplasm, or katabolic materials
rpsnmnfr from the chemical degradation of protoplasm":
these secondary matters, mingled with the completed proto-
plasm, are conveniently spoken of as the cell deutoplasm or
par a plasm. As between the spongioplasm and hyaloplasm
there are still some differences of opinion as to which is the
more immediate agent in the manifestation of the vital activ-

THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 27

ities of the cell. So far as the manifestation of the power of
movement is concerned the evidence seems in favor of the
hyaloplasm : the outermost parts of a white blood-corpuscle,
for example, exhibit active contractile power, yet they con*
tain no spongioplastic filaments; and many unicellular living
things are known in which no reticular structure can be dis-
covered and which nevertheless nourish themselves and are
reproductive, irritable, contractile, conductive, co ordinative
and automatic. It is therefore possible that the filaments
when present are to be regarded as secondary in importance
to the hyaloplasm, partly serving as a mechanical support;
but in addition they may play an important part in the inter-
nal economy of the cell. The study of the physiology of in-
dividual cells presents very great difficulties and is yet in its
beginnings, so that we can do little more than speak of the
properties of the cell as a whole, though from the frequent
radial arrangement of the cell-protoplasm, in its neighborhood
and from the part it plays in the initiation of cell division,
the attraction-particle appears to have a very important role.

Of the actual chemical composition of living matter we
know only that its molecule is one of great complexity: all
methods of chemical analysis break it up and alter it funda-
mentally, so that what is really analyzed is not living matter
but a mixture of the products of its decomposition, among
which proteid substances are always prominent.

Cell-protoplasm no doubt varies a little in different cells,
so that the name is to be regarded as a general term designat-
ing a number of closely-allied substances agreeing with one
another chemically in main points, as the proteids do, but
differing in minor details, in consequence of which one cell
differs from another in faculty. On proximate analysis every
mass of protoplasm is found to contain much water and a
certain amount of mineral salts; the water being in part con-
stituent or entering into the structure of the particles of pro-
toplasm, and in part probably deposited in layers between
them. Of organic constituents protoplasm always yields one
or more proteids, some fats, and some starchy or saccharine /
body/ So that the original protoplasm is probably to be re-
garded as containing chemical "residues" of proteids, fats
and carbohydrates, combined with salts and water.

The name nuclein has been given to a substance or mix-
ture of substances which are left behind when the cell-proto-

28 THE HUMAN BODY.

plasm has been dissolved away by various reagents : it con-
tains a considerable quantity of phosphorus. In the living
nucleus nuclein seems to be combined with various proteids
to form nudeo-albumins.

The Fundamental Physiological Properties. All living
animals possess in greater or less degree the properties con-
sidered in this chapter; and since the science of physiology
is virtually concerned with considering how these properties
are acquired, maintained and manifested,, and for what ends
they are employed, we may call them the fundamental physi-
ological properties.

CHAPTER III.

THE DIFFERENTIATION OF THE TISSUES AND THE
PHYSIOLOGICAL DIVISION OF EMPLOYMENTS.

Development. Every Human Body commences its indi-
idual existence as a single nucleated cell. This cell, known
s the ovum, divides or segments and gives rise to a mass con-

FtG. llA.-^l, an ovum; B to E, successive stages in its segmentation until the
orula, F, is produced; a, cell-sac; 6, cell contents; c, nucleus.

sting of a number of similar units and called the mulberry
mss or the morula. At this stage, long before birth, there
re no distinguishable tissues entering into the structure of

Body, nor are any organs recognizable.

For a short time the morula increases in size by the
rowth and division of its cells, but very soon new processes
ccur which ultimately give rise to the complex-adult body
ith its many tissues and organs. Groups of cells ceasing to
row and multiply like their parents begin to grow in ways
eculiar to themselves, and so come to differ both from the
riginal cells of the morula and from the cells of other groups,
ad this unlikeness becoming more and more marked, a
iried whole is finally built up from one originally alike in

29

30 THE HUMAN BODY.

ail its parts. Peculiar growth of this kind, forming a com
plex from a simple whole, is called development; and the pro
cess itself in this case is known as the differentiation of tli
tissues, since by it they are, so to speak, separated or special
izcd from the general mass of mother-cells forming th
morula.

As the differences in the form and structure of the con
ells of the morula become marked, differences ii
property arise, and it becomes obvious that the whole cell
gate is not destined to give rise to a collection of hide
pendent living things, but to form a single human being, ii
whom each part, while maintaining its own life, shall hav
duties to perform for the good of the whole. In other words
a single compound individual is to be built up by the unio]
and co-operation of a number of simple ones represented b
the various cells, each of which thenceforth, while primaril
looking after its own interests and having its own peculia
faculties, has at the same time its activities subordinated t
the good of the entire community.

The Physiological Division of Labor. The fundaments
physiological properties, originally exhibited by all the cells
become ultimately distributed between the different modifie
cells which form the tissues of the fully developed Bod
much in the same way as different employments are dis
tributed in a civilized state; for the difference between th
full} developed Human Body and the collection of amosboi
cells from which it started is essentially the same as tha
between a number of wandering savages arid a civilized natioi
In the former, apart from differences dependent on sex, eac
individual has no one special occupation different from tha
of the rest, but has all his own needs to look after: he mug
collect his own food and prepare it for eating, make his ow
clothes, if he wear any, provide his own shelter, and defen
himself from wild beasts or his fellow men. In the civilize
country, on the other hand, we find agriculturists to rais
food and cooks to prepare it, tailors to make clothes, an
policemen and soldiers to provide protection. And just £
we find that when distribution of employments in it is moi
minute a nation is more advanced in civilization, so is a
annual higher or lower in the scale according to the degre
in which it exhibits a division of physiological duties betwee
its different tissues.

THE DIFFERENTIATION OF TISSUES.

From the subdivision of labor in advanced communities
several important consequences arise. In the first p!
man devoting himself to one kind of work mainly and relying
upon others for the supply of his other needs, every sort of
work is better done. The man who is constantly making
boots becomes more expert than one whose attention is con-
stantly distracted by other duties, and he can not only make
more boots in a given time, but better ones; and so with the
performance of all other kinds of work. In the second place,
a necessity arises for a new sort of industry, in order to con-
vey the produce of one individual in excess of the needs of
limself and his family to those at a distance who may want
it, and to convey back in return the excess of their produce
which he needs. The carriage of food from the country to
cities, and of city produce to country districts, and the occu-
pation of shopkeeping, are instances of these new kinds of
labor which arise in civilized communities. In addition there
is developed a need for arrangements by which the work of
.ndividuals shall be regulated in proportion to the wants of
;he whole community, such as is in part effected by the
agency of large employers of labor who regulate the activities
of a number of individuals for the production of various
articles in the different quantities required at different times.

Exactly similar phenomena result from the subdivision of
iabor in the Human Body. By the distribution of employ-
ments between its different tissues, each one specially doing
one work for the general community and relying on the
others for their aid in turn, every necessary work is better
performed. And a need arises for a distributive mechanism
by which the excess products, if any, of various tissues shall
be carried to others which require them, and for a regulative
mechanism by which the activities of the various tissues shall
be rendered proportionate to the needs of the whole Body at
different times and under different circumstances.

Classification of the Tissues. — As we might separate the
inhabitants of the United States into groups, such as lawyers,
doctors, clergymen, merchants, farmers, and so forth, so we
may classify the tissues by selecting the most distinctive
properties of each of those entering into the construction of
the adult Body and arranging them into physiological groups;
those of each group being characterized by some one promi-
nent employment. No such classification, however, can be

THE HUMAN BODY.

nore than approximately accurate, since the same tissue has
often more than one well-marked physiological property.
The following arrangement, however, is practically convenient

1. UNDIFFERENTIATED TISSUES. These are composed oil
cells which have developed along no one special line, but
retain very much the form and properties of the cells forming
the very young Body before different tissues were recognizable!
in it. The lymph-corpuscles and the colorless corpuscles oJ
the blood belong to this class.

2. SUPPUKIJNG TISSUES. Including cartilage (gristle),
and connective tissue. Of the latter there are seven

subsidiary varieties, the two more important being -whiti
fibrous connective tissue, composed mainly of colorless inex-
tensible fibres, and yelloio fibrous tissue, composed mainly of
yellow elastic fibres. All the supporting tissues are used in
the Body for mechanical purposes : the bones and cartilage
form the hard framework by which softer tissues are supporte
and protected; and the connective tissues unite the various
bones and cartilages, form investing membranes around dif-
ferent organs, and in the form of fine networks penetrate their
substance and support their constituent cells. The functions
of these tissues being for the most part to passively resist
strain or pressure, none of them has any very marked phy-
siological property; they are not, for exai( ^ .\ fri-i table or
contractile, ;iiid their mass is chiefly made up o'i an intercell-
ular substance which has been formed by the actively living
cells sparsely scattered through them, as for instance in
cartilage, Fig. 45, where the cells are seen imbedded in cavi-
ties in a matrix which they have formed around them; and
this matrix by its firmness and elasticity forms the func-
tionally important part of the tissue. *

3. NUTRITIVE TISSUES. These form a large group, the
members of which fall into three main divisions, viz. :

[ssimilative tissues, concerned in receiving and preparing
food materials, and including — (a) Secretory tissues, com-
posed of cells which make the digestive liquids poured into the
alimentary canal and used to bring about chemical or other
changes in the food, (b] Receptive tissues, represented by
cells which line parts of the alimentary canal and take up the
digested food.

Eliminative or feretory tissues, represented by cells in the

THE DIFFERENTIATION OF TISSUES, 33

kidneys, skin, and elsewhere, whose main business it is to get
rid of the waste products of the various parts of the Body.

Respiratory tissues. These are concerned in the gaseous
interchanges between the Body and the surrounding air.
They are constituted by the cells lining the lungs and by the
colored corpuscles of the blood.

As regards the nutritive tissues it requires especially to be
borne in mind that although such a classification as is here
given is useful, as helping to show the method pursued in the
domestic economy of the Body, it is only imperfect and
largely artificial. Every cell of the Body is in itself assimi-
lative, respiratory, and excretory, and the tissues in this class
are only those concerned in the first and .ast interchanges
of material between it and the external world. They provide
or get rid of substances for the whole Body, leaving the feed-
ing and breathing and excretion of its individual tissues to be
ultimately looked after by themselves, just as even the mandarin
described by Robinson Crusoe who found his dignity promoted
by having servants to put the food into his mouth, had finally
to swallow and digest it for himself. Moreover, there is no
logical distinction between a secretory and an excretory cell:
each of them is characterized by the separation of certain sub-
stances which are poured out on a free surface on the exterior
or interior of tl^ * ^ . Many secretory cells too have no
concern with the digestion of food, as for example those
which form the tears and sweat.

4. STORAGE TISSUES. The Body does not live from hand
to mouth: it has always in health a supply of food-materials
accumulated in it beyond its immediate needs. This lies in
part in the individual cells themselves, just as in a prosperous
community nearly every one will have some little pocket-
money. But apart from this reserve there are certain cells,
a sort of capitalists., which store up considerable quantities of
material and constitute what we will call the storage tissues.
These are especially represented by the liver-cells and fat-
cells, which contain in health a reserve fund for the rest of
the Body. Since both of these, together with secretorv and
excretory cells, are the seats of great chemical changes they
are all often called metabolic tissues.

5. IRRITABLE TISSUES. The maintenance, or at any rate
the best prosperity, of a nation is not fully secured wheu a
division of labor has taken place in food-supply and food-dis-

34 THE HUMAN BODY.

tribution employments. It is extremely desirable that means
shall be provided by which it may receive information of ex
ternal changes which may affect it as a whole, such as the
policy of foreign countries; or which shall enable the inhabi-
tants of one part to know the needs of another, and direct
their activity accordingly. Foreign ministers and consuls and
newspaper correspondents are employed to place it in com-
munication with other states and keep it informed as to its
interests; and we find also organizations, such as the meteor-
ological department, to warn distant parts of approaching
storms or other climatic changes which may seriously affect
the pursuits carried on in them. In the Human Body we
have a comparable class of intelligence-gaining tissues lying
in the sense-organs, whose business it is to obtain and com-
municate to the whole information of external changes which
occur around it. Since the usefulness of these tissues
depends upon the readiness with which slight causes excite
them to activity, we may call them the irritable tissues.

6. CO-ORDINATING AND AUTOMATIC TISSUES. Such in-
formation as that collected by ministers in foreign parts or by
meteorological observers is usually sent direct to some central
office from which it is redistributed; this mere redistribution
is, however, in many cases but a small part of the work carried
on in such offices. Let us suppose information to be obtained
that an Indian chief is collecting his men for an attack on
some point. The news is probably first transmitted to Wash-
ington, and it becomes the duty of the executive officers there
to employ certain of the constituent units of the nation in
such definite work as is needed for its protection. Troops,
have to be sent to the place threatened perhaps; recruits en-
listed ; food and clothes, weapons and ammunition, must be
provided for the army; and so on. In other words, the work
of the various classes composing the society has to be organ-
ized for the common good ; the mere spreading the news of
the danger would be of little avail. So in the Body: the
information forwarded to certain centres from the irritable
tissues is used in such a way as to arouse to orderly activ-
ity other tissues whose services are required ; we find
thus in these centres a group of co-ordinating tissues,
represented by nerve-cells and possibly by certain other
constituents of the nerve-centres. Certain nerve-cells are
also automatic in the physiological sense already pointed

THE DIFFERENTIATION OF TISSUES. 35

out. The highest manifestation of this latter faculty, shown
objectively by muscular movements, is subjectively known as
the " will," a state of consciousness; and other mental phe-
nomena, as sensations and emotions, are also associated with
the activity of nerve-cells lying in the brain. ^JHow it is that
any one state of a material cell should give rise to a particular
state of consciousness is a matter quite beyond our powers
of conception Abut not really more so than how it is that
every portion 01 matter attracts every other portion according
to the law of gravitation. In the living Body, as elsewhere
in the universe, we can study phenomena and make out their
relations of sequence or coexistence; but why one phenom-
enon is accompanied by another, why in fact any cause pro-
duces an effect, is a matter quite beyond our reach in every
case; whether it be a sensation accompanying a molecular
change in a nerve-cell, or the fall of a stone to the ground in
obedience to the force of gravity.

7. MOTOR TISSUES. These have the contractility of the
original protoplasmic masses highly developed. The more
important are ciliated cells and muscular tissue. The former
line certain surfaces of the body, and possess on their free
surfaces fine threads which are in constant movement. One
finds such cells, for example (Fig. 50), lining the inside of
the windpipe, where their threads or cilia serve, by their
motion, to sweep any fluid formed there towards the throat,
where it can be coughed up and got rid of. Muscular tissue
occurs in two main varieties. One kind is found in the mus-
cles attached to the bones, and is that used in the ordinary vol-
untary movements of the Body. It is composed of fibres which
present cross-stripes when viewed under the microscope (Fig.
56), and is hence known as striped or striated muscular tis-
sue. The other kind of muscular tissue is found in the walls
of the alimentary canal and some other hollow organs, and con-
sists of elongated cells (Fig. 60) which present no cross-stria-
tion. It is known as plain or unstriated muscular tissue.

The cells enumerated under the heading of "undiffer-
entiated tissues" might also be included among the motor
tissues, since they are capable of changing their form.

8. THE CONDUCTIVE TISSUES. These are represented by
the nerve-fibres, slender threads, each of which has as its essen-
tial part a branch of a nerve-cell having the property of physio-
logical conductivity highly developed; the fibres therefore

36 THE HUMAN BODY.

readily transmit molecular disturbances. When its equilib-
rium is upset at one end, a nerve-fibre transmits to its other
end a molecular movement known as a "nervous impulse"
and so can excite parts distant from the original exciting
force. Nerve-fibres place, on the one hand, the irritable
tissues in connection with the automatic, co-ordinating, and
sensory; and on the other put the three latter in communica-
tion with the muscular, secretory and other tissues.

9. PROTECTIVE TISSUES. These consist of certain cells lin-
ing cavities inside the body and called epithelial cells, and cells
covering the whole exterior of the Body and forming epider-
mis, hairs and nails. The enamel which
covers the teeth belongs also to this
group.

The class of protective tissues is, how-
ever, even, more artificial than that of the
nutritive tissues, and cannot be defined by
positive characters. Many epithelial cells
are secretory, excretory or receptive; and
ciliated cells have already been included
\r^ FI -th amorig the motor tissues, although from
lium-ceiis from the sur- the fact that the movements of their cilia

face of the peritoneum. , , _, _ . _,

«, cell-body ; c, nucleus; so on in separated cells and independently

6, nucleoli. &. • ui i *.- v ^

of recognizable external stimuli, they
might well have been put among the automatic. The protec-
tive tissues may be best defined as including cells which cover
free surfaces, and whose functions are mainly mechanical or
physical. In their simplest form epithelial cells are flat
scales, as, for example, those represented in Fig. HB, from
the lining membrane of the abdominal cavity.

10. THE EEPRODUCTIVE TISSUES. These are concerned in
the production of new individuals, and in the Human Body
are of two kinds, located in different sexes. The conjunction
of the products of each sex is necessary for the origination
of offspring, since the female product, egg-cell or ovum,
which directly develops into the new human being, remains
dormant until it has \>QZ\\ fertilized, and fertilization consists
essentially in the fusion of its nucleus with the nucleus of a
cell produced by the male.

The Combination of Tissues to Form Organs. The va-
rious tissues above enumerated forming the building materials
of the Body, anatomy is primarily concerned with their struc-

THE DIFFERENTIATION OF TISSUES. 37

ture, and physiology with their properties, If this, however,
were the whole matter, the problems of anatomy and physi-
ology would be much simpler than they actually are. The
knowledge about the living Body obtained by studying only
the forms and functions of the individual tissues would be com-
parable to that attained about a great factory by studying
separately the boilers, pistons, levers, wheels, etc., found in
it, and leaving out of account altogether the way in which
these are combined to form various machines; for in the
Body the various tissues are for the most part associated to
form organs, each organ answering to a complex machine
like a steam-engine with its numerous constituent parts.
And just as in different machines a cogged wheel may per-
form very different duties, dependent upon the way in which
it is connected with other parts, so in the Body any one tissue,
although its essential properties are everywhere the same,
may by its activity subserve very various uses according to
the manner in which it is combined with others. For ex-
ample: A nerve-fibre uniting the eye with one part of the
brain will, by means of its conductivity, when its end in the
eye is excited by the irritable tissue attached to it on which
light acts, cause changes in the sensory nerve-cells connected
with its other end and so arouse a sight sensation; but an ex-
actly similar nerve-fibre running from the brain to the mus-
cles will, also by virtue of its conductivity, when its ending
in the brain is excited by a change in a nerve-cell connected
with it, stir up the muscle to contract under the control of
the will. The different results depend on the different parts
connected with the ends of the nerve-fibres in each case, and
not on differences in the properties of the nerve-fibres them-
selves.

It becomes necessary then to study the arrangement and
uses of the tissues as combined to form various organs, and
this is frequently far more difficult than to make out the
structure and properties of the individual tissues. An ordi-
nary muscle, such as one sees in the lean of meat, is a very
complex organ, containing not only contractile muscular tis-
sue, but supporting and uniting connective tissue and con-
ductive nerve-fibres, and in addition a complex commissariat
arrangement, composed in its turn of several tissues, con-
cerned in the food-supply and waste removal of the whole
muscle. The anatomical study of a muscle has to take into

38 THE HUMAN BODY.

account the arrangement of all these parts within it, and also
its connections with other organs of the Body. The physi-
ology of any muscle must take into account the actions of all
these parts working together and not merely the functions
of the muscular fibres themselves, and has also to make out
under what conditions the muscle is excited to activity by
changes in other organs, and what changes in these it brings
about when it works.

Physiological Mechanisms. Even the study of organs
added to that of the separate tissues does not exhaust the
matter. In a factory we frequently find machines arranged
so that two or more shall work together for the perform-
ance of some one work: a steam-engine and a loom may,
for example, be connected and used together to weave carpets.
Similarly in the Body several organs are often arranged to
work together so as to attain some one end by their united
actions. Such combinations are known as physiological ap-
paratuses. The circulatory apparatus, for example, consists
of various organs (each in turn composed of several tissues)
known as heart, arteries, capillaries and veins. The heart
forms a force-pump by which the blood is kept flowing
through the whole mechanism, and the rest, known together
as the blood-vessels , distribute the blood to the various organs
and regulate the supply according to their needs. Again, in
the visual apparatus we find the co-operation of (a) a set of
optical instruments which bring the light proceeding from
external objects to a focus upon (b) the retina, which con-
tains highly irritable parts; these, changed by the light,
stimulate (c) the optic nerve, which is conductive and trans-
mits a disturbance which arouses in turn (d) sensory parts in
the brain. In the production of ordinary sight sensations all
these parts are concerned and work together as a visual appa-
ratus. So, too, we find a respiratory apparatus, consisting
primarily of two hollow organs, the lungs, which lie in the
chest and communicate by the windpipe with the back of the
throat, from which air enters them. But to complete the
respiratory apparatus are many other organs, bones, muscles,
nerves and nerve-centres, which work together to renew the
air in the lungs from time to time; and the act of breathing
is the final result of the activity of the whole apparatus.

Many similar instances, as the alimentary apparatus, the

THE DIFFERENTIATION OF TISSUES. 39

auditory apparatus, and so on, will readily be thought of.
The study of the working of such complicated mechanisms
forms a very important part of physiology.

Anatomical Systems. From the anatomical side a whole
collection of bodily organs agreeing in structure with on,e
another is often spoken of as a system ; all the muscles, for
example, are grouped together as the muscular system, and
all the bones as the osseous system, and so on, without any
reference to the different uses of different muscles or bones.
The term system is, however, often used as equivalent to
"apparatus": one reads indifferently of the "circulatory sys-
tem " or the " circulatory apparatus." It is better, however,
to reserve the term system for a collection of organs classed
together on account of similarity of structure; and "appa-
ratus " for a collection of organs considered together on ac-
count of their co-operation to execute one function. The
former term will then have an anatomical, the latter a phy-
siological, significance.

The Body as a Working Whole. Finally it must all
through be borne in mind that not even the most complex
system or apparatus can be considered altogether alone as an
independently living part. All are united to make one living
Body, in which there is throughout a mutual interdepend-
ence, so that the whole forms one human being, in whom the
circulatory, respiratory, digestive, sensory and other appara-
tuses are constantly influencing one another, each modifying
the activities of the rest. This interaction is mainly brought
about through the conductive and co-ordinating tissues of
the nervous system, wrhich place all parts of the Body in com-
munication. But in addition to this another bond of union
is formed by the blood, which by the circulatory apparatus is
carried from tissue to tissue and organ to organ and so, bring-
ing materials derived in one region to distant parts, enables
each organ to influence all the rest for good or ill.

Besides the blood another liquid, called lymph, exists in
the Body. It is contained in vessels distinct from those
which carry the blood, but emptying into the blood-vessels at
certain points. This liquid being also in constant movement
forms another agency by which products are carried from
part to part, and the welfare or ill-fare of one member en-
abled to influence all.

V CHAPTER IV.

«

THE INTEKNAL MEDIUM.

• »

The External Medium. During the whole of life inter-
changes of material go on between every living being and the
external world; by these exchanges material particles that
one time constitute parts of inanimate objects come at an-
other to form part of a living being; and later on these
same atoms, after having been a part of a living thing, are
passed out from it in the form of lifeless compounds. As
the foods and wastes of various organisms differ more or
less, so are more or less different environments, suited for
their existence; and there is accordingly a relationship be-
tween the plants and animals living in any one place and the
conditions of air, earth and water prevailing there. Even
such simple unicellular animals as the amoebae live only in
water or mud containing in solution certain gases, and in sus-
pension solid food-particles; and they soon die if the water
be changed either by essentially altering its gases or by taking
out of it the solid food. So in yeast we find a unicellular
plant which thrives and multiplies only in liquids of certain
composition, and which in the absence of organic compounds
of carbon in solution will not grow at all. Each of these
simple living things, which corresponds to one only of the in-
numerable cells composing the full-grown Human Body, thus
requires for the manifestation of its vital properties the pres-
ence of a surrounding medium suited to itself: the yeast
would die, or at the best lie dormant, in a liquid containing
only the solid, organic particles on which the amoeba lives ;
and the amoeba would die in such solutions as those in which
yeast thrives best.

The Internal Medium. A similar close relationship be-
tween the living being and its environment, and an inter-
change between the two like that which we find in the amoeba
and the yeast-cell, we find also in even the most complex
living beings. When, however, an animal comes to be com-

TEE INTERNAL MEDIUM. 41

p03ed of many cells, some of which are placed far away from
the surface of its body and from immediate contact with
the environment, there arises a new need — a necessity for an
internal medium o\* plasma which shall play the same part
toward the individual cells as the surrounding air, water and
food to the whole animal. This internal medium kept in
movement and receiving at some regions of the bodily sur-
faces materials from the exterior, while losing substances to
the exterior at the same or other surfaces, forms a sort of
middleman between the individual tissues and- the surround-
ing world, and stands in the same relationship to each of the
cells of the Body as the water in which an amoeba lives does
to that animal, or beer-wort does to a yeast-cell. We find
accordingly the Human Body pervaded by a liquid plasma,
containing gases and food-material in solution, the presence
of which is necessary for the maintenance of the life of the
tissues. Any great change in this medium will affect in-
juriously few or many of the groups of cells in the Body, or
may even cause their death; just as altering the media in
which they live will kill an amoeba or a yeast-cell.

The Blood. In the Human Body the internal medium is
primarily furnished by the Uood, which, as every one knows,
is a red liquid very widely distributed over the frame, since
it flows from any part when the skin is cut through. There
are in fact very few portions of the Body into which the
blood is not carried. One of the exceptions is the epidermis
or outer layer of the skin : if a cut be made through it only,
leaving the deeper skin-layers intact, no blood will flow from
the wound. Hairs and nails also contain no blood. In the
interior of the Body the epithelial layers lining free surfaces,
such as the inside of the alimentary canal, contain no blood,
nor do the hard parts of the teeth, the cartilages, and the
refracting media of the eye (see Chap. XXXII), but these
interior parts are moistened with liquid of some kind, and
unlike the epidermis are protected from rapid evaporation.
All these bloodless parts together form a group of non-vas-
cular tissues; they alone excepted, a wound of any part of
the Body will cause bleeding.

In many of the lower animals there is no need that the
liquid representing their blood should be renewed very rapidly
in different parts. Their cells live slowly, and so require but
little food and produce but little waste. In a sea-anemone,

42 THE HUMAN BODY.

for example, there is no special arrangement to keep the
blood moving; it is just pushed about from part to part by
the general movements of the body of the animal. But in
higher animals, especially those with an elevated temperature,
such an arrangement, or rather absence of arrangement, as
this would not suffice. In them the constituent cells live
very fast, making much waste and using much food, and
altering the blood in their neighborhood very rapidly. Be-
sides, we have seen that in complex animals certain cells are
set apart to get food for the whole organism and certain
others to finally remove its wastes, and there must be a sure
and rapid interchange of material between the feeding and
excreting tissues and all the others. This can only be brought
about by a rapid movement of the blood in a definite course,
and that is accomplished by shutting it up in a closed set of
tubes, and placing somewhere a pump, which constantly takes
in blood from one end of the system of tubes and forces it
out again into the other. Sent by this pump, the hearf,
through all parts of the Body and back to the heart again,
the blood gets food from the receptive cells, takes it to the
working cells, carries off the waste of these latter to the ex-
creting cells; and so the round goes on.

The L'ymph. The blood, however, lies everywhere in
closed tubes formed by the vascular system, and does not
come into direct contact with any cells of the Body except
those which float in it and those which line the interior of the
blood-vessels. At one part of its course, how-
ever, the vessels through which it passes have
extremely thin coats, and through the walls of
these capillaries liquid transudes from the blood
and bathes the various tissues. The transuded
liquid is the lymph, and it is this which forms
Fro. 12.— Adia- the immediate nutrient plasma -of the tissues
fug" Apparatus" except the few which the blood moistens di-

containing two vppHv
liquids, 6 and c, r 'UJ;

separated by a Dialysis. When two liquids containing dif-

moist animal . °

membrane. ferent matters in solution are separated from
one another by a moist animal membrane, an interchange of
material will take place under certain conditions. If A be a
vessel (Fig. 12) completely divided vertically by such a mem-
brane, and a solution of common salt in water be placed on
the side Z>, and a solution of sugar in water on the side c, it

THE INTERNAL MEDIUM. 43

will be found after a time that some salt has got into c and
some sugar into b, although there are no visible pores in the
partition. Such an interchange is said to be due to dialysis
or osmosis, and if the process were allowed to go on for some
hours the same proportions of salt and sugar would be found
in the solution on either side of the dividing membrane.

The Renewal of the Lymph. Osmotic phenomena play
a great part in the nutritive processes of the Body. The
lymph present in any organ gives up things to the cells there
and gets things from them ; and thus, although it may have
originally been tolerably like the liquid part of the blood, it
soon acquires a different chemical composition. Diffusion
or dialysis then commences between the lymph outside and
the blood inside the capillaries, and the latter gives up to the
lymph new materials in place of those which it has lost and
takes from it the waste products it has received from the tis-
sues. When this blood, altered by exchanges with the lymph,
gets again to the neighborhood of the receptive cells, having
lost some food-materials it is poorer in these than the richly
supplied lymph around those cells, and takes up a supply by
dialysis from it. When it reaches the excretory organs it has
previously picked up a quantity of waste matters and loses
these by dialysis to the lymph there present, which is special-
ly poor in such matters, since the excretory cells constantly
deprive it of them. In consequence of the different wants
and wastes of variou-s cells, and of the same cells at different
times, the lymph must vary considerably in composition in
various organs of the Body, and the blood flowing through
them will gain or lose different things in different places.
But renewing during its circuit in one what it loses in
another, its average composition is kept pretty constant, and,
through interchange with it, the average composition of the
lymph also.

The Lymphatic Vessels. The blood, on the whole, loses
more liquid to the lymph through the capillary walls than it
receives back the same way. This depends mainly on the
fact that the pressure on the blood inside the vessels is greater
than that on the lymph outside, and so a certain amount of
nitration of liquid- from within out occurs through the vas-
cular wall in addition to the dialysis proper. The excess is
collected from the various organs of the Body into a set of
lymphatic vessels which carry it directly back into some of

44 THE HUMAN BODY.

the larger blood-vessels near where these empty into the
heart; by this flow of the lymph, under pressure from behind,
it is renewed in various organs, fresh liquid filtering through
the capillaries to take its place as fast as the old is carried off.

The Lacteals. Jn the walls of the alimentary canal cer-
tain food-materials after passing through the receptive cells
into the lymph are not transferred locally, like the rest, by
dialysis into the blood, but are carried off bodily in the lymph-
vessels and poured into the veins of a distant part of the
Body. The lymphatic vessels concerned in this work, being
frequently filled with a white liquid during digestion, are
called the milky or lacteal vessels.

Summary. To sum up: the blood and lymph form the
internal medium in which the tissues of the Body live; the
lymph is primarily derived from the blood and forms the im-
mediate plasma for the great majority of the living cells of
the Body; and the excess of it is finally returned to the
blood. The lymph moves but slowly, but is constantly reno-
vated by the blood, which is kept in rapid movement, and
which, besides containing a store of new food-matters for the
lymph, carries off the wastes which the various cells have
poured into the latter, and thus is also a sort of sewage stream
into which the wastes of the whole Body are primarily col-
lected.

Microscopic Characters of Blood. If a finger be pricked,
and the drop of blood flowing out be spread on a glass slide,
covered, protected from evaporation, and examined with a
microscope magnifying about 400 diameters, it will be seen
to consist of innumerable solid bodies floating in a liquid.
The solid bodies are the blood-corpuscles, and the liquid is
the blood-plasma or liquor sanguinis.

The corpuscles are not all alike. While currents still exist
in the freshly-spread drop of blood, the great majority of
them are readily carried to and fro; but a certain number
more commonly stick to the glass and remain in one place.
The former are the red, the latter the pale or colorless blood-
corpuscles.

Red Corpuscles. Form and Size. The red corpuscles
as they float about frequently seem to vary in form, but by ^
little attention it can be made out that this appearance is due
to their turning round as they float, and so presenting differ-
ent aspects to view; just as a silver dollar presents a different

THE INTERNAL MEDIUM.

45

outline according as it is looked at from the front or edge-
wise or in three-quarter profile.

Sometimes the corpuscle (Fig. 13, B] appears circular;
then it is seen in full face; sometimes linear ((?), and slightly
narrowed in the middle; sometimes oval, as the dollar when
half-way between a full and a side view. These appearances
show that each red corpuscle is a circular disk, slightly hol-
lowed in the middle (or biconcave) and about four times as
wide as it is thick. The average transverse diameter is 0.008
milimeter (^Vo inch). Shortly after blood is drawn the

FIG. 13.— Blood-corpuscles. A, magnified about 400 diameters. The red corpus-
cles have arranged themselves in rouleaux ; a, a, colorless corpuscles ; B, red cor-
puscles more magnified and seen in focus : E. a red corpuscle slightly out of focus.
Near the right-hand top corner is a red corpuscle seen in three-quarter face, and at
Cone seen edgewise.' F, (?, H, I, white corpuscles highly magnified.

corpuscles arrange themselves in rows, or rouleaux, adhering
to one another by their broader surfaces.

Color. — Seen singly each red corpuscle is of a pale yellow
color; it is only when collected in masses that they appear
red. The blood owes its red color to the great numbers of
these bodies in it; if it is spread out in a very thin layer it,
too, is yellow. In a cubic millimeter (^ inch) of blood there
are about five million red corpuscles.

Structure.— Seen from the front the central part of
each red corpuscle in a certain focus of the microscope
appears dimmer or darker than the rest (Fig. 13, B), ex-

46 THE HUMAN BODY.

cept a narrow band near the outer rim. If the lens of the
microscope be raised, however, this previously dimmer central
part becomes brighter, and the previously brighter part ob-
scure (E). This difference in appearance does not indicate
the presence of a central part or nucleus different from the
rest, but is an optical phenomenon due to the shape of the
corpuscle, in consequence of which it acts like a little bicon-
cave lens. Rays of light passing through near the centre of
the corpuscles are refracted differently from those passing
through elsewhere; and when the microscope is so focussed
that the latter reach the eye, the former do not, and vice
versa ; thus when the central parts look bright, those around
them look obscure, and the contrary.

There is no satisfactory evidence that these corpuscles
have any enveloping sac or cell-wall. All the methods used
to bring one into view under the microscope are such as
would coagulate the outer layers of the substance composing
the corpuscle and so make an artificial envelope. So far as
optical analysis goes, then, each corpuscle is homogeneous
throughout. By other means we can, however, show that at
least two materials enter into the structure of each red cor-
puscle. If the blood be diluted with several times its own
bulk of water and examined with the microscope, it will be
found that the formerly red corpuscles are now colorless and
the plasma colored. The dilution has caused the coloring
matter to pass out of the corpuscles and dissolve in the liquid.
This coloring constituent of the corpuscle is h&moglolin, and
the colorless residue which it leaves behind and which swells
up into a sphere in the diluted plasma is the stroina. In the
living corpuscle the two are intimately mingled throughout
it, and so long as this is the case the blood is opaque; but
when the coloring matter dissolves in the plasma, then the
blood becomes transparent, or, as it is called, laky. The
difference may be very well seen by comparing a thin layer of
fresh blood diluted with ten times its volume of ten-per-cent
salt solution with a similar layer of blood diluted with ten
volumes of water. The watery mixture is a dark transparent
red; the other, in which the coloring matter still lies in the
corpuscles, is a brighter opaque red.

Consistency. — Each red corpuscle is a soft jelly-like mass
which can be readily crushed out of shape. Unless the pres-
sure be such as to rupture it, the corpuscle immediately reas-

&

THE INTERNAL MEDIUM. 47

sumes its proper form when the external force is removed.
The corpuscles are, then, highly elastic; they frequently can
be seen much dragged out of shape inside the vessels when
the circulation of the blood is watched in a living animal
(Chap. XV), but immediately springing back to their normal
form when they get a chance.

Blood-crystals. Haemoglobin is, as above shown, readily
soluble in water. In this it soon decomposes if kept in a
warm room, breaking up into a colorless proteid substance
called globulin and a red body, Jicematin. By keeping the
haemoglobin solution very cold, however, this decomposition,
can be greatly retarded, and at the same time the solubility
of the haemoglobin in the water much diminished. In dilute
alcohol haemoglobin is still less soluble, and so if its ice-cold

watery solution have one

j ^~ z^a . fourth of its volume of

% ^r f ^ -— cold alcohol added to it
iP^ rd^ ^H and the mixture be put in,
^0^ <|j a refrigerator for twenty-

S^"~ 3 ^ittK f°U1' hours, a part of the
haemoglobin will often
crystallize out and sink to
the bottom of the vessel,

FIG. 14,-Biood.crystais, or haemoglobin where it can be collected for
crystals, examination. The haemo-

globin of the rat is less soluble than that of man, and there-
fore crystallizes out especially easily; but these haemoglobin
crystals, or, as they are often called, blood-crystals, can also
be obtained from human blood. In 100 parts of dry human
red blood-corpuscles there aro of 90 haemoglobin. The haemo-
globin is the essential constituent of the red blood-corpuscles,
enabling them to pick up Targe quantities of oxygen in the
lungs and carry it to other parts. (See Respiration.)

Haemoglobin contains a considerable quantity of iron, much,
more than any other proximate constituent of the Body.

The Colorless Blood -corpuscles (Fig. 13, F, H, G). The
colorless, pale or white corpuscles of the blood are far less
numerous thai: the red; in health there is on the average
about one white to three hundred red, but the proportion
may vary considerably Each is finely granular and consists
of a soft mass of protoplasm enveloped in no definite cell-wall,
but containing a nucleus. The granules in the protoplasm.

48 THE HUMAN BODY.

commonty hide the nucleus in a fresh corpuscle, but dilute
acetic acid dissolves most of them and brings the nucleus into
view. These pale corpuscles belong to the group of undiffer-
entiated tissues, and differ in no important recognizable
character from the cells which make up the whole very young
Human Body, nor indeed from such a unicellular animal as
an Amoeba. They have the power of slowly changing their
form spontaneously. At one moment a pale corpuscle will
be seen as a spheroidal mass; a few seconds later (Fig. 15)
processes will be seen radiating from this, and soon after
these processes may be retracted and
ot hers thrust out; and so the corpuscle
goes on changing its shape. These slow
amoeboid movements are greatly promoted
by keeping the specimen of blood at the
temperature of the Body. By thrusting
out a process on one side, then drawing
FIG. 15. -A white blood- the rest of its body up to it, and then
\enterfaiseof aa sfew sending out a process again on the same
fW^dS VIS side, the corpuscle can slowly change its
movements. I^QQ and creep across the field of the
microscope. Inside the blood-vessels these corpuscles often
execute similar movements; and they sometimes bore right
through the capillary walls and, getting out into the lymph-
spaces, creep about among the other tissues. This migration
is especially frequent in inflamed parts, and the pus or
"matter" which collects in abscesses is largely made up of
whiue blood-corpuscles which have in this way got out of the
blood-vessels. The average diameter of the white corpuscles
is one third greater than that of the red.

The colorless corpuscles, or some of them, are capable of
taking into themselves foreign particles present in the blood;
this they do in a manner similar to that in which an amoeba
feeds: the process is known as phagocytosis and the cells ex-
hibiting it as phagocytes. Among the substances observed to
be taken up by white corpuscles are the minute organisms
known as Bacteria, certain species of which have been proved
to be the causes of some diseases (zymotic diseases). The
white corpuscles may in this way play an important part in
the cure of such diseases, or in their prevention in persons
exposed to infection. The accumulation of white corpus-
cles in inflamed or injured parts is probably primarily as-

THE INTERNAL MEDIUM. 49

sociated with the removal of dead and broken-down tissues,
though it may be carried to excess as in the case of purulent
accumulations.

The Blood Platelets or Plaques are a third kind of blood-
corpuscle, considerably smaller than the red, but somewhat
resembling them in form. They adhere together, break down
and form sticky clumps with great rapidity in drawn blood
unless special precautions are taken.

Blood of Other Animals. In all animals with blood the
pale corpuscles are pretty much alike, but the red corpuscles,
which with rare exceptions are found only in Vertebrates,
vary considerably. In all the classes of the mammalia they
are circular biconcave disks, with the exception of the camel
tribe, in which they are oval. They vary in diameter from 0.02
mm. (ygVo inch) (musk deer) to .011 mm. (^^Vo inch) (ele-
phant). In the dog they are nearly the same size as those of
man. In no mammals do the fully-developed red corpuscles
possess a nucleus. In all other vertebrate classes the red cor-
puscles possess a central nucleus, and are oval slightly bi-
convex disks, except in a few fishes in which they are cir-
cular. They are largest of all in the amphibia. Those of
the frog are 0.02 mm. (r/ffQ inch) long and .007 mm. (^¥
inch) broad.

Histology of Lymph. Pure lymph is a colorless watery-
looking liquid; examined with a microscope it is seen to con-
tain numerous pale corpuscles closely resembling those Of the
blood, and no doubt many are pale blood-corpuscles which
have migrated. These lymph-corpuscles or leucocytes have,
however, another more important origin. In many parts of the
Body there are collections of a peculiar lymphoid or adenoid
tissue, sometimes in nodular masses (lymphatic glands).
This tissue consists essentially of a fine network, the meshes
of wnich are occupied with leucocytes which frequently show
signs of division. The meshes of the network communicate
with lymphatic vessels and the lymph flowing through picks
up and-cairies off the new-formed leucocytes. The lymph
being ultimately poured into the blood, the leucocytes be-
come the colorless corpuscles of the latter; and the migrating
cells of the blood are therefore but lymph-corpuscles restored
to the lymph, perhaps somewhat changed during their life in
the blood-plasma.

The lymph flowing from the intestines during digestion

50 THE HUMAN BODY.

is, as already mentioned, not colorless, but white and milky.
It is known as chyle, and will be considered with the process
of digestion. During fasting the lymph from the intestines
is colorless, like that from other parts of the Body.

CHAPTEE V.
THE CLOTTING OF BLOOD.

The Coagulation of the Blood. When blood is first
drawn from the living Body it is perfectly liquid, flowing in
any direction as readily as water. This condition is, however,
only temporary; in a few minutes the blood becomes viscid
and sticky, and the viscidity becomes more and more marked
until, after the lapse of five or six minutes, the whole mass
sets into a jelly which adheres to the vessel containing it, so
that this may be inverted without any blood whatever being
spilled. This stage is known as that of gelatinization and is
also not permanent. In a few minutes the top of the jelly-
like mass will be seen to be hollowed or " cupped " and in the
concavity will be seen a small quantity of nearly colorless
liquid, the blood-serum. The jelly next shrinks so as to pull
itself loose from the sides and bottom of the vessel containing
it, and as it shrinks squeezes out more and more serum. Ulti-
mately we get a solid clot, colored red and smaller in size
than the vessel in which the blood coagulated though retain-
ing its form, floating in a quantity of pale yellow serum. If,
however, the blood be not allowed to coagulate in perfect rest,
a certain number of red corpuscles will be rubbed out of the
clot into the serum and the latter will be more or less reddish.
The longer the clot is kept the more serum will be obtained:
if the first quantity exuded be decanted off and the clot put
aside and protected from evaporation, it will in a short time
be found to have shrunk to a smaller size and to have pressed
out more serum; and this goes on until putrefactive changes
commence.

Cause of Coagulation. If a drop of fresh-drawn blood
be spread out- very thin and watched for a few minutes with a
microscope magnifying 600 or 700 diameters, it will be seen
that the coagulation is due to the separation of very fine solid
threads which run in every direction through the plasma and
form a close network entangling all the corpuscles. These

51

THE HUMAN BODY.

threads are composed of the proteid substance fibrin. When
they first form, the whole drop is much like a sponge soaked
full of water (represented by the serum) and having solid
bodies (the corpuscles) in its cavities. After the fibrin threads
have been formed they tend to shorten; hence when blood
clots in mass in a vessel, the fibrinous network tends to shrink
in every direction just as a network formed of stretched
india-rubber bands would, and this shrinkage is greater the
longer the clotted blood is kept. At first the threads stick
too fkmly to the bottom and sides of the vessel to be pulled
away, and thus the first sign of the contraction of the fibrin
is seen in the cupping of the surface of the gelatinized blood
where the threads have no solid attachment, and there the
contracting mass presses out from its meshes the first drops of
serum. Finally the contraction of the fibrin overcomes its ad-
hesion to the vessel and the clot pulls itself loose on all sides,
pressing out more and more serum, in which it ultimately
floats. The great majority of the red corpuscles are held back
in the meshes of the fibrin, but a good many pale corpuscles,
by their amoeboid movements, work their way out and get
into the serum.

Whipped Blood. The essential point in coagulation
being the formation of fibrin in the plasma, and blood only
forming a certain amount of fibrin, if this be removed as fast
as it forms the remaining blood will not clot. The fibrin
may be separated by what is known as" whipping" the blood.
For this purpose fresh-drawn blood is stirred up vigorously
with a bunch of twigs, and to these the sticky fibrin threads
as they form, adhere. If the twigs be withdrawn after a few
minutes a quantity of stringy material will be found attached
to them. This is at first colored red by adhering blood-cor-
puscles: but by washing in water they may be removed, and
the pure fibrin thus obtained is perfectly white and in the
form of highly "elastic threads. It is insoluble in water and
in dilute acids, but swells up to a transparent jelly in the
latter. The " whipped " or " defibrinated blood " from which
the fibrin has been in this way removed, looks just like ordinary
blood, but has lost the power of coagulating spontaneously,

The Buflfy Coat. That the red corpuscles are not an
essential part of the clot, but are merely mechanically caught
up in it, seems clear from the microscopic observation of
the process of coagulation; and from the fact that perfectly

THE CLOTTING OF BLOOD. 53

formed fibrin can be obtained free from corpuscles by whip-
ping the blood and washing the threads which adhere to the
twigs. Under certain conditions, moreover, one gets a natu-
rally formed clot containing no red corpuscles in one part of
it. The corpuscles of human blood are a little heavier, bulk
for bulk, than the plasma in which they float; hence, when
the blood Is drawn and left at rest they sink slowly in it;
and if for any reason clotting take place more slowly or the
corpuscles sink more rapidly than usual, a colorless top
stratum of plasma, with no red corpuscles in it, is left
before gelatinization occurs and stops the further sinking of
the corpuscles. The uppermost part of the clot formed
under such circumstances is colorless or pale yellow, and is
known as the buffy coat; it is especially apt to be formed in
the blood drawn from febrile patients, and was therefore a
point to which physicians paid much attention in the olden
times when bloodletting was thought to be almost a panacea.
In horse's blood the difference between the specific gravity of
the corpuscles and that of the plasma is greater than in
human blood, and horse's blood also coagulates more slowly,
so that its clot has nearly always a buffy coat. The colorless
buffy coat seen sometimes on the top of the clot must, how-
ever, not be confounded with another phenomenon. When
a blood-clot is left floating exposed to the air its top becomea
bright scarlet, while the part immersed in the serum assumes
a dark purple-red color. The brightness of the top layer is
due to the action of the oxygen of the air, which forms a
scarlet compound with the coloring matter of the red cor-
puscles. If the clot be turned upside down and left for a
short time, the previously dark red bottom layer, now exposed
to the air, becomes bright; and the previously bright top
layer, now immersed in the serum, loses iis oxygen and be-
comes dark.

Uses of Coagulation. The clotting of the blood is so im-
portant a process that its cause has been frequently investi-
gated; but it is not yet completely understood. The living
circulating blood in the healthy blood-vessels does not clot;
it contains no solid fibrin, but this forms in it, sooner or later,
when the blood gets by any means out of the vessels or when
the lining of these is injured. In this way the mouths of the
small vessels opened in a cut are clogged up, and the bleed-
ing, which would otherwise go on indefinitely, is stopped.

64 THE HUMAN BODY.

So, too, when a surgeon ties up an artery before dividing it,
the tight ligature crushes or tears its delicate inner surface,
and the blood clots where that is injured, and from there a
•coagulu'm is formed reaching up to the next highest branch of
the vessel. This becomes more and more solid, and by the time
the ligature is removed has formed a firm plug in the cut end
of the artery, which greatly diminishes the risk of bleeding.

The Source of Blood-fibrin. Since fresh blood-plasma
contains no fibrin but does contain considerable quantities of
other proteids, we look first to these as a possible source of
the fibrin formed during coagulation. Blood drawn from a
living animal into one third of its bulk of a cold saturated
solution of magnesium sulphate and kept cold will not clot
for a long time. The corpuscles slowly sink in the mixture,
and after a time considerable quantities of colorless " salted "
plasma can be drawn off from its upper part. The salted
plasma still contains something which can form fibrin, for if
diluted with six or seven times its volume of water it clots in
a manner quite similar to pure blood-plasma (though the clot
is a little less firm); and also, fibrin can be obtained by
whipping it.

If salted plasma be saturated with sodium chloride it
yields a whitish rather sticky precipitate, called plasmim.
The remaining liquid is then found to have lost the power of
clotting, but if the plasmine be treated with a little dilute
saline solution it dissolves, and the solution soon clots, with
the formation of fibrin.

The plasmine is not a single body. If its solution before
it clots have sodium chloride added to it in the proportion
of about 15#, a white sticky precipitate is formed, and may
be collected on a filter; it is a substance named fibrinogen.
If more sodium chloride or some magnesium sulphate be
added to the filtrate a second white precipitate is obtained :
this is par a globulin.

Paraglobulin dissolves in dilute solutions of common salt:
such solutions cannot be made to yield fibrin, though they
are coagulated with the formation of coagulated proteid
(p. 10) at the temperature 75° C. (167° F). Purified fibrin-
ogen also- dissolves in dilute solution of common salt, and
such solution is coagulated by heat (56° C. or 133° F.) : but
under certain conditions it clots with the formation of true
fibrin. During the clotting the fibrinogen disappears, but

THE CLOTTING OF BLOOD. 55

the quantity of fibrin formed never is quite equal in weight
to the fibrinogen which, disappears, so the process is not a
mere direct transformation of one substance into the other.

We are thus led to the conclusion that the natural clot-
ting of fresh blood is due to the formation of fibrin from
fibrinogen which existed in solution in the plasma of the
circulating blood and has been altered in the clotted, giving
origin to fibrin. But as normal blood circulating in healthy
uninjured blood-vessels does not clot nor do pure solutions
of fibrinogen, we have still to seek the exciting cause of the
change.

If to a solution of fibrinogen there be added a few drops
of blood or of blood-serum, or of the washings of a blood-clot,
fibrin will be forme^; therefore drawn blood and serum and
natural clot each contain something which can effect the con-
version of fibrinogen into fibrin. This substance is the
enzyme named fibrin -ferment.

The Fibrin-ferment. When blood-serum is treated with
several times its volume of strong alcohol its various proteids
and most of its salts are precipitated : if the precipitate be
left standing in alcohol for some months the proteids become
almost entirely insoluble in water, but a few drops of the
watery extract cause clotting in a saline solution of fibrin-
ogen, and clearly contain some of tho ferment. A very
minute quantity of the ferment will cause the conversion of
an indefinite quantity of fibrinogen and does not appear to be
itself used up in the process: it acts somehow by its mere
presence, and the clotting of blood is to be relegated to that
obscure group of physico-chemical processes known as cata-
lytic. Solutions containing the ferment always give some
proteid reactions and it may be a proteid, but this is doubt-
ful; for the proteid present may be only an impurity. Watery
solutions of ferment completely lose their activity when
boiled.

If fibrinogen be dissolved in the least possible amount of
dilute caustic potash and a few drops of as pure as possible a
solution of fibrin ferment, freed from its salts by dialysis,
be added, clotting does not occur: but it may be brought
about by the addition of a very small quantity of a calcium
salt. The presence of some calcium seems to be an essential,
but the part it plays is unknown. Of the four substances
which take part in the coagulation of blood, the fibrinogen

56 THE HUMAN BODY.

primarily determines the quantity of fibrin formed : the more
fibrinogen the more fibrin, though never quite so much as the
fibrinogen which disappears. The ferment acting on fibrin-
ogen in the presence of a salt of calcium, in some way causes
it to become fibrin, but does not itself enter into the fibrin ; it
is not used up in the process, and the amount of fibrin ulti-
mately formed is the same whether much or little ferment
be present; but the more ferment the quicker the clotting.
The presence in small quantity of many neutral salts seems to
favor coagulation, but none except the lime-salts are essential.
The part they play is obscure; and when present in large pro-
portions they prevent coagulation of blood or plasma, prob-
ably by hindering the formation of ferment. If fresh blood
be mixed with an equal bulk of a saturated solution of mag-
nesium sulphate (Epsom salts) 'or of common salt, it will not
clot; but if this mixture be largely diluted with water, then
some ferment is formed and clotting takes place.

The Proximate Causes of Normal Blood Coagulation.
As all the phenomena of clotting, with the formation of fibrin
agreeing in all respects with that formed during the natural
coagulation of drawn blood, can be obtained in artificial solu-
tions of fibrinogen, it is obvious that the process is not, as was
once supposed, a so-called vital but a purely chemical one :
but we still are far from a satisfactory explanation why the
fibrinogen of the plasma does not clot in normal circulating
blood contained in healthy blood-vessels. It is, in fact, much
easier to point out what are not the proximate causes of the
coagulation of drawn blood than what are.

Blood when removed from the Body and received in a
vessel comes to rest, cools, and is exposed to the air, from
which it may receive or to which it may give off gaseous
bodies. But it is easy to prove that none of these three
things is the cause of coagulation. Stirring the drawn blood
and so keeping it in movement does not prevent but hastens
its coagulation: and blood carefully imprisoned in a living
blood-vessel, and so kept at rest, will not clot for a long time;
not until the inner coat of the vessel begins to change from
the want of fresh blood. Secondly, keeping the blood at the
temperature of the Body hastens coagulation, and cooling re-
tards it; blood received into an ice-cold vessel and kept sur-
rounded with ice will clot more slowly than blood drawn and
left exposed to ordinary temperatures. Finally, if the blood

THE CLOTTING OF BLOOD. 57

be collected over mercury from a blood-vessel, without having
been exposed to the air even for an instant, it will clot per-
fectly.

The formation of fibrin is then due to changes taking
place in the blood itself when it is removed from the blood-
vessels; the clotting depends solely upon some rearrangement
of the blood-constituents, and the primary change seems to
be the formation of fibrin-ferment. That healthy circulating
blood contains no ferment but that this forms in drawn blood
may be shown as follows: Blood is drawn from an artery
into four separate vessels. To one specimen a large quantity
of alcohol is added at once; to a second after five minutes, to
a third after ten, to the fourth after fifteen. The precipitate
in each is collected and dried, and then treated with water
which will dissolve any ferment present. The watery extract
from the first specimen will not cause clotting when added to
a fibrinogen solution: from the second only slowly; the third
more quickly, and the fourth quickest of all. It is hence con-
cluded that there is no ferment in perfectly fresh blood, but
that this begins to form as soon as blood is drawn and for
some time goes on increasing, so that there is more in blood
drawn ten minutes than in blood drawn only five. The
alcohol in each sample precipitates all the ferment already
present and prevents the formation of more. There is some
evidence that a good many pale corpuscles disintegrate when
blood is drawn, and it has been maintained that they then
give origin to the fibrin-ferment along with other things : but
of late evidence seems rather to point to the platelets as
the main source of the ferment. As already stated they
rapidly break down when blood is removed from the body,
part of their substance going into solution in the plasma and
part remaining as a sticky mass which tends to adhere to its
fellows to form little clumps. If the formation of fibrin in
clotting blood be watched with the aid of a microscope the
fibrin threads are seen to appear first in the neighborhood of
these clumps, and in many cases to radiate from them. More-
over those substances which check or retard the clotting of
blood also hinder the disintegration of the platelets: 'and if a
fine thread be passed through the blood-vessel of a living
animal fibrin forms around it after a time, and this formation
is preceded by adhesion to the thread and disintegration of
platelets. But be the source of the ferment platelets or pale

58 THE HUMAN BODY.

corpuscles or both, we have still the problem why, under
normal conditions, do not these break down in the circulating
blood : have perchance the blood-vessels some part in the
matter ?

Relation of the Blood-vessels to Coagulation. As to
the role of the blood-vessels with respect to coagulation, two
views are held, between which the facts at present known do
not permit a decisive judgment to be made; and there may
be some truth in both. One theory is that the vessels actively
prevent coagulation by constantly absorbing from the blood
some substance, as the fibrin-ferment, the presence of which
is a necessary condition for the formation of fibrin and which
is supposed to be constantly forming in the blood, but to be
as steadily removed from it or destroyed by the lining cells of
the blood-vessels. In support of this opinion is brought for-
ward the fact that it is possible to inject considerable quanti-
ties of a solution of fibrin-ferment into the blood of a living
animal without causing intravascular coagulation.

The other view is that the blood-vessels are passive. They
simply do not excite those changes in the blood constituents
which give rise to the formation of fibrin-ferment, while
foreign bodies in contact with the blood do excite these
changes and so lead to coagulation. In support of this view
are brought forward the facts that drawn blood clots faster in
vessels of such shapes that a large surface of blood is exposed
to foreign contact; and that coagulation takes place rapidly
in a vessel with a rough interior, while in a chemically clean
glass vessel it occurs slowly. The experiment already men-
tioned of getting a clot around a thread passed through a blood-
vessel, and also that of getting extensive clotting within the
blood-vessels by the injection into a vein of extract of the
thymus body, may be cited as tending to show that the linings
of the blood-vessels cannot actively prevent coagulation; but
it may be objected that in the one case locallys and in the other
generally, the ferment is set free in the blood so fast that the
vessels cannot remove it in time to prevent the formation of
fibrin. Blood poured out from a torn vessel among other
tissues of the body often clots very slowly; this may be due
either to the tissues in general possessing the power of de-
stroying fibrin-ferment or to their being merely indifferent
substances not exciting the changes which lead to fibrin
formation.

THE CLOTTING OF BLOOD. 59

Whatever the part played by the blood-vessels in reference
to coagulation it is only exhibited when their inner surfaces
are healthy and uninjured. If their lining be ruptured or
diseased the blood clots. Accordingly, after death, when
post-mortem changes have affected the blood-vessels, the
blood clots in them; but often very slowly, since the vessels
only gradually alter. If the Body be left in one position
after death the clots formed in the heart have often a marked
buffy coat, because the corpuscles have had a long time to
sink in the plasma before coagulation occurred. In medico-
legal cases it is thus sometimes possible to say what was the
position of a corpse for some hours after death, although it
has been subsequently moved.

Lymph clots like the blood, but not so firmly. The clot
formed is colorless.

Composition of the Blood. The average specific gravity
of human blood is 1055. It has an alkaline reaction, which
becomes less marked as coagulation occurs. About one half
of its mass consists of moist corpuscles and the remainder of
plasma. Exposed in a vacuum, 100 volumes of blood yield
about 60 of gas consisting of a mixture of oxygen, carbon
dioxide and nitrogen.

Chemistry of Serum. Blood -serum is plasma which has
lost its fibrinogen and gained fibrin-ferment and probably
some additional paraglobulin; from an analysis of it we can
draw conclusions as to the plasma. In 100 parts of serum
there are about 90 parts of water, 8.5 of proteids, and 1.5 of
fats, salts and other less-known solid bodies. Of the proteids
present the most abundant are serum-albumin and para-
globulin. Serum-albumin agrees with egg-albumin in coagu-
lating when heated : for this reason serum when boiled sets
into an opaque white mass, just as the white of an egg does.
Serum-albumin differs from egg-albumin in not being coagu-
lated by ether; and in the fact that although present in such
large quantities in the blood, it is not excreted by the kid-
neys, as egg-albumin is, if injected into a blood-vessel. The
paraglobulin is also precipitated by heat, but may be pre-
cipitated alone by saturation of the serum with magnesium
sulphate. Fats are present in the serum in small quantity
except after a meal at which fatty substances have been
eaten; serum obtained from the blood of an animal soon
after such a meal is often milky in appearance from the large

60 THE HUMAN BODY.

amount of fats present, instead of being colorless or pale yel-
low and transparent as it is after fasting. The salts dissolved
in the serum are mainly sodium chloride and carbonate;
small quantities of sodium, calcium, and magnesium phos-
phates are also present.

Chemistry of the Red Corpuscles. In these in the fresh
moist state there are, in 100 parts, 56 of water and 44 of
solids. Of the solids about one per cent is salts, chiefly potas-
sium phosphate and chloride. The remaining solids contain,
in 100 parts, 90 of hsemoglobin and about 8 of other proteids;
the residue consists of less well-known bodies.

Chemistry of the White Corpuscles. Besides much water,
these yield several proteids, some fats, glycogen (see Chap.
XXIX) and salts; and smaller quantities of other bodies.
The predominant salts, like those of the red corpuscles, are
potassium phosphates.

Variations in the Composition of the Blood. The above
statements refer only to the average composition of the
healthy blood and to its better known constituents. From
what was said in the last chapter it is clear that the blood
flowing from any organ will have lost or gained, or gained
some things and lost others, when compared with the blood
which entered it. But the losses and gains in particular parts
of the Body are in such small amount as, with the exception
of the blood-gases, to elude analysis for the most part: and
the blood from all parts being mixed in the heart, they
balance one another and produce a tolerably constant average.
In health, however, the specific gravity of the blood may vary
from 1045 to 1075; the red corpuscles also are present in
greater proportion to the plasma after a meal than before it.
Healthy sleep in proper amount leads to increase in the pro-
portion of red corpuscles, and want of it tends to diminution
of their number, as may be recognized in the pallid aspect of
a person who has lost several nights' rest.

The proportion of the red corpuscles has a great impor-
tance since, as we shall subsequently see, they serve to carry
oxygen, which is necessary for the performance of its func-
tions, all over the Body. Ancemia is a diseased condition
characterized by pallor due to deficiency of red blood-corpus-
cles, and accompanied by languor and listlessness. It is not
unfrequent in girls on the verge of womanhood, and in per

THE CLOTTING OF BLOOD. 61

sons overworked and confined within doors. In such cases
the best remedies are open-air exercise and good food.

Summary. Practically the composition of the blood may
be thus stated: It consists of (1) plastna, consisting of watery
solutions of serum -albumin, paraglobulin, fibrinogen, sodi-
um and other salts, and extractives of which the most con-
stant are urea, kreatin, and grape-sugar; (2) red corpuscles,
containing rather more than half their weight of water, the
remainder being mainly haemoglobin, other proteids, and pot-
ash salts; (3) white corpuscles, consisting of water, various
proteids, glycogen, and potash salts; (4) the platelets ; (5)
gases, partly dissolved in the plasma or combined with its
sodium salts, and partly combined (oxygen) with the haemo-
globin of the red corpuscles.

Quantity of Blood. The total amount of blood in the
Body is difficult of accurate determination. It is about -fa
of the whole weight of the Body, so the quantity in a man
weighing 75 kilos (165 Ibs.) is about 5.8 kilos (12.7 Ibs.). Of
this at any given moment about one fourth would be found in
the heart, lungs and larger blood-vessels; and equal quantities
in the vessels of the liver, and in those of the muscles which
move the skeleton; while the remaining fourth is distributed
among the remaining parts of the Body.

The Origin and Fate of the Blood-corpuscles. The white
blood-corpuscles vary so rapidly and frequently in number in
the blood that they must be constantly in process of altera-
tion or removal, and formation ; their number is largely in-
creased after taking food, even more than that of the red, so
that their proportion to the red rises, from 1 to 1000 during
fasting, to 1 to 250 or 300 after a meal. This increase is
mainly due to increased flow of lymph at this time through
the lymphatics of the alimentary canal which have much
lymphoid tissue on their course; and, as already pointed out,
lymph-corpuscles are constantly multiplying in this tissue
and are gathered from it by the lymph, to be poured into the
blood (see also Chap. XXIII). Migrated pale corpuscles of
the blood and the leucocytes of the lymph retain many of the
characters of undifferentiated and unspecialized embryonic
cells; and there is some evidence that they may develop new
tissues in the repair of injured parts.

Amphioxus, the lowest undoubted vertebrate animal (see
Zoology), possesses only colorless corpuscles in its blood.

62 THE HUMAN BODY.

Higher and more complex animals need more oxygen and, as
blood-plasma dissolves very little of that gas, they develop in
addition the haemoglobin-containing corpuscles which pick
it up in the gills or lungs and carry it to all parts of the
Body, leaving it where wanted (see Chap. XXVI). In cold-
blooded vertebrates the red corpuscles are not nearly so many
in proportion as in the warm-blooded, which use far more
oxygen. The older view, was that the mammalian red cor-
puscle represented the nucleus of one of the white, in which
haemoglobin had been formed and from about which the rest
of the corpuscle had disappeared. This, however, does not
seem to be the case. In adults new red blood -corpuscles are
formed by the segregation of portions of the protoplasm of
peculiar cells (hcematoblasts) found in various parts of the
Body, but especially in the red marrow of certain bones (p.
95). In the embryo some cells of the liver, and in new-born
animals (possibly also in adult) some connective-tissue cor-
puscles (p. 112) form new red blood-corpuscles.

How long an individual red corpuscle lasts is not known,
nor with certainty how or where it disappears : there is, how-
ever, some reason to believe that many are finally destroyed
in the spleen (see Chap. XXIII). Their average rate of dis-
appearance and new formation is unknown, but in emergen-
cies (as after severe haemorrhages) they can be reproduced
with great rapidity.

Chemistry of Lymph. Lymph is a colorless fluid when
pure, feebly alkaline, and with a specific gravity of about
1045. It may be described as blood minus its red corpuscles
and much diluted, but of course in various parts of the Body
it will contain minute quantities of substances derived from
neighboring tissues. It contains a considerable quantity of
carbon dioxide gas which it gives up in a vacuum, but no un-
combined oxygen, since any of that gas which passes into it
by diffusion from the blood is immediately picked up by the
living tissues among which the lymph flows.

CHAPTER VL
THE SKELETON.

Exoskeleton and Endoskeleton. The skeleton ol an
animal includes all its hard protecting or supporting parts,
and is met with in two main forms. One is an exoskeleton
developed in connection with either the superficial or deeper
layer of the skin, and represented by the shell of a clam,
the scales of fishes, the horny plates of a turtle, the
bony plates of an armadillo, and the feathers of birds.
In man the exoskeleton is but slightly developed, but it
is represented by the hairs, nails and teeth; for although
the latter lie within the mouth, the study of development
shows that they are developed from an offshoot of the skin
which grows in and lines the mouth long before birth. Hard
parts formed from structures deeper than the skin constitute
the endoskeleton, which in man is highly developed and con-
sists of a great many bones and cartilages or gristles, the
bones forming the mass of the hard framework of the Body,
while the cartilages finish it off at various parts. This frame-
work is what is commonly meant" by the skeleton; it pri-
marily supports all the softer parts and is also arranged so as
to surround cavities in which delicate organs, as the brain,
.heart or spinal cord, may lie with safety. The gross skeleton
thus formed is completed and supplemented by another made
of the connective tissues, which not only, in the shape of
tough bands or ligaments, tie the bones and cartilages to-
gether, but also in various forms pervade the whole Body as
a sort of subsidiary skeleton running through all the soft
organs and forming networks of fibres around their other
constituents; they make, as it were, a microscopic skeleton
for the individual modified cells of which the Body is so
largely composed, and also form partitions between the mus-
cles, cases for such organs as the liver and kidneys, and
sheaths around the blood-vessels. The bony and cartilagin-
ous framework with its ligaments might be called the skele-

63

64 THE HUMAN BODY.

ton of the organs of the Body, and this finer supporting
meshwork the skeleton of the tissues. Besides forming a
support in the substance of various organs, the connective
tissues are often laid down as a sort of packing material in the
crevices between them ; and so widely are they distributed
everywhere from the skin outside to the lining of the alimen-
tary canal inside, that if some solvent could be employed
which would corrode away all the rest and leave only these
tissues, a very perfect model of the whole Body would be left;
something like a " skeleton leaf," but far more minute in its
tracery.

The Bony Skeleton (Fig. 16). If the hard framework
of the Body were joined together like the joists and beams of
a house, the whole mass would be rigid; its parts could not
move with relation to one another, and we should be unable
to raise a hand to the mouth or put one foot before another.
To allow of mobility the bony skeleton is made of many sepa-
rate pieces which are joined together, the points of union be-
ing called articulations, and at many places the bones enter-
ing into an articulation are movably hinged together, forming
what are known &s joints. The total number of bones in the
Body is more than two hundred in the adult; and the number
in children is still greater, for various bones which are dis-
tinct in the child (and remain distinct throughout life in
many lower animals) grow together so as to form one bone in
the full-grown man. The adult bony skeleton may be de-
scribed as consisting of an axial skeleton, found in the head,
neck and trunk; and an appendicular skeleton, consisting of
the bones in the limbs and in the arches (u and s, Fig., 16)
by which these are carried and attached to the trunk.

Axial Skeleton. The axial skeleton consists primarily
of the vertebral column or spine, a side view of which is rep-
resented in Fig. 17. The upper part of this column is com-
posed of twenty-four separate bones, each of which is a ver-
tebra. At the posterior part of the trunk, beneath tjie
movable vertebrae, comes the sacrum (8 1), made up of five
vertebrae, which in the adult grow together to form one bone,
and below the sacrum is the coccyx (Co 1-4), consisting of
four very small tail vertebrae, which in advanced life also
unite to form one bone.

On the top of the vertebral column is borne the skull,
made up of two parts, viz., a great box above which incloses

THE SKELETON. 65

the brain and is called the cranium, and a large number of

FIG. 16. — The bony and cartilaginous
skeleton.

FIG. 17.— Side view of the
spinal column.

bones on the ventral side of this which form the skeleton of

66 THE HUMAN BODY.

the face. Attached by ligaments to the under side of the
cranium is the Jiyoid lone, to which the root of the tongue is
fixed. •

Of the twenty-four separate vertebrae of the adult the seven
nearest the skull (Fig. 17, C 1-7) lie in the neck and are
known as the cervical vertebra. These are followed by
twelve others which have ribs attached to them (see Fig. 16)
and lie at the back of the chest; they are the thoracic or dorsal
vertebrae (D 1-12). The ribs (Fig. 28) are slender curved
bones attached by their dorsal ends, called their heads, to the
thoracic vertebrae and running thence round the sides of the
chest. In the ventral median line of the latter is the breast-
bone or sternum (d, Fig. 16). Each rib near its sternal end
ceases to be bony and is composed of cartilage.

These parts — skull, hyoid bone, vertebral column, ribs,
and sternum — constitute the axial skeleton.

The Thoracic or Dorsal Vertebras. If a single vertebra,
say the eleventh from the skull, be examined carefully it will
be found to consist of the following parts (Figs. 18 and 19) :

First a bony mass, C, rounded on the sides and flattened
on each end where it is turned towards the vertebrae above and
below it. This stout bony cylinder is the "body " or centrum
of the vertebra, and the series of vertebral bodies (Fig. 17)
forms in the trunk that bony partition between the dorsal
and ventral cavities of the body spoken of in Chapter I. To
the dorsal side of the body is attached an arch — the neural
arch, A9 which with the back of the body incloses a space,
Fv, the neural ring. In the tube formed by the rings of the
successive vertebrae lies the spinal cord. Projecting from the
dorsal side of the neural arch is a long bony bar, Ps, the
spinous process : and the projections of these processes from
the various vertebrae can be felt through the skin all down
the middle of the back. Hence the name of spinal column
often given to the whole back-bone.

Six other processes arise from the arch of the vertebra:
two project forwards, i.e., towards the head; these, Pas, are
the anterior articular processes and have smooth surfaces,
covered with cartilage, on their dorsal sides. A pair of simi-
lar posterior articular processes, Pai, runs back from the
neural arch, and these have smooth surfaces on their ventral
aspects. In the natural position of the vertebra, the smooth
surfaces of its anterior articular processes fit upon the poste-

THE SKELETON.

67

rior articular processes of the vertebra next in front, forming
a joint, and the two processes are united by ligaments. Sim-

FIG. 18.

FIG. 19.

FIG. 18.— A thoracic vertebra seen from behind, i.e., the end turned from the head.

FIG. 19.— Two thoracic vertebrae viewed from the left side, and in their natural
relative positions. C, the body ; A, neural area ; Fv, the neural ring ; Ps, spinous
process; Pas, anterior articular process; Pai, posterior articular process; Pt,
transverse process ; Ft, facet for articulation with the tubercle of a rib ; Fes, Fci,
articular surfaces on the centrum for articulation with a rib.

ilarly its posterior articular processes form joints (Fig. 19)
with the anterior articular processes of the vertebra next be-
hind.

The remaining processes are the transverse, Pt, which
run outwards and a little dorsally. Each of these has a
smooth articular surface, Ft, near its outer end.

On the " body " are seen two articular surfaces on each
side : one, Fes, at its anterior, the other, Fci, at its posterior
end, and both close to the attachment of the neural arch.
Each of these surfaces forms with corresponding areas on
the vertebrae in front and behind a pit into which the end
of a rib fits and the rib attached in this way to the anterior
part of the " body " is also fitted on, a little way from its
dorsal end, to the articular surface at the end of the transverse
process.

The Segments of the Axial Skeleton. If a thoracic verte-
bra, say the first (Fig. 20), be detached with the pair of ribs,
Cv9 belonging to it and the bit of the sternum, S, to which
these ribs are fixed ventrally, we would find a bony parti-
tion formed by the body of the vertebra, lying between

68 THE HUMAN BODY.

two arches which surround cavities. The dorsal cavity
inclosed by the "body" and "neural arch" contained origi-
nally part of the spinal cord. The other ring, made up by

the body of the vertebra dor-

^^^^11 i^r-^^ sally, the sternum ventrally,

j ^^ and the ribs on the sides, sur-

rounds the chest-cavity with
its contents. All of these parts
together form a typical seg-
ment of the axial skeleton,
which, however, only attains
this completeness in the tho-
racic region of the trunk. In

FIG. 20.— Diagrammatic representa- ,, elrnll if is m-patlv rnnHifiprl •
tiou of a segment of the axial skeleton, tne SKlll I It IS glCdliy n

and in the neck and the lower

Part of the trunk tv~ 'ibs are

is the rib-cartilage. either absent or -V. small,

appearing only as processes of the vertebrae; and the sternal
portion is wanting altogether.

Nevertheless we may regard the whole axial skeleton as
made up of a series of such segments placed one in front of
another, but having different portions of the complete seg-
ment much modified or rudimentary or even altogether
wanting in some regions. Parts which in this way really
correspond to one another though they differ in detail, which
are so to speak different varieties of one thing, are said in
anatomical language to be homologous to one another; and
when they succeed one another in a row, as the trunk seg-
ments do, the homology is spoken of as serial.

The Cervical Vertebrae. In the cervical region of the
vertebral column the bodies of the vertebrae are smaller than
in the dorsal, but the arches

are larger; the spinous pro- BB ' Frt

cesses are short and often bifid

and the transverse processes

appear to be perforated by a

canal, the vertebral foramen.

The bony bar bounding this Jf \) \,

aperture on the ventral side,

FIG. 21.— A cervical vertebra. Frt,
however, IS in reality a Very vertebral foramen; Pat, anterior ar-

small rib which has grown into ticuIarPro

continuity with the body and true transverse process of

THE SKELETON. 69

the vertebra, although separate in very early life: the trans-
verse process proper bounds the vertebral foramen dorsally.
In this latter during life runs an artery, which ultimately
enters the skull-cavity.

The Atlas and Axis. The first and second cervical verte-
brae differ considerably from the rest. The first, or atlas
(Fig. 22), which carries the head, has a very small body, Aa,
and a large neural ring. This ring is subdivided by a cord,
the transverse ligament, L, into a dorsal moiety in which the
spinal cord lies and a ventral into which the bony process D
projects. This is the odontoid process, and arises from the
front of the axis or second cervical vertebra (Fig. 23).
Around this peg the atlas rotates when the head is turned
from side to side, carrying the skull (which articulates with
the large hollow surfaces Fas) with it.

The odontoid process really represents a large piece of the
body of the atlas which in early life separates from its own
vertebra and becomes united to the axis.

The Lumbar Vertebrae (Fig. 24) are the largest of all the
movable vertebra and have no ribs attached to them. Their
spines are short and stout and lie in a more horizontal plane

FIG. 22. FIG

FIG. 22.— The atlas. FIG. 23.— The axis. Aa, body of atlas; D, odontoid process;
Fas, facet on frout of atlas with which the skull articulates; and in Fig. 23, ante-
rior articular surface of axis; L. transverse ligament; Frt, vertebral foramen; Ap,
neural arch; Tp, spinous process.

than those of the vertebrae in front. The articular and trans-
verse processes are also short and stout.

The Sacrum, which is represented along with the last lum-
bar vertebra in Fig. 25, consists in the adult of a single bone;
but cross-ridges on its ventral surface indicate the limits of
the five separate vertebrae of which it is composed in
childhood. It is somewhat triangular in form, its base

70

THE HUMAN BODY.

being directed upwards and articulating with the under
surface of the body of the fifth lumbar vertebra. On its
sides are large surfaces to which the arch bearing the lower

Pni

Pa

*

FIG. 24. — A lumbar vertebra seen from the left side. Ps, spinous process; Pas,
anterior articular process; Pai, posterior articular process; Pt> transverse process.

Fsa

FIG. 25. — The last lumbar vertebra and the sacrum seen from the ,entral side.
Fsa, anterior sacral foramina.

limbs is attached (see Fig. 16). Its ventral surface is con-
cave and smooth and presents four pairs of anterior sacral

THE SKELETON. . 71

foramina, Fsa, which communicate with the neural canal.
Its dorsal surface, convex and roughened, has four similar
pairs of posterior sacral foramina.

The coccyx (Fig. 26) calls for no special description. The
four bones which grow together, or ankylose,
to form it, represent only the bodies of vertebrse,
and even those incompletely. It is in reality
a short tail, although not visible as such from
the exterior.

The Spinal Column as a Whole. The ver-
tebral column is in a man of average height
about twenty- eight inches long. Viewed from
the side (Fig. 17) it presents four curvatures;
one with the convexity forwards in the cervical
region is followed, in the thoracic, by a curve with its concavity
towards the chest. In the lumbar region the curve has again
its convexity turned ventrally, while in the sacral and coccy-
geal regions the reverse is the case. These curvatures give the
whole column a good deal of springiness such as would be
absent were it a straight rod, and this is farther secured by the
presence of compressible elastic pads, the intervertebral disks,
made up of cartilage and connective tissue, which lie between
Ihe bodies of those vertebras which are not ankylosed together,
and fill iip completely the empty spaces left between the
bodies of the vertebrae in Fig. 17. By means of these pads,
moreover, a certain amount of movement is allowed between
each pair of vertebras; and so the spinal column can be bent
to considerable extent in any direction; while the movement
between any two vertebrae is so limited that no sharp bend
can take place at any one point, such as might tear or other-
wise injure the spinal cord contained in the neural canal.
The amount of movement permitted is greatest in the cervical
region.

In the case of the movable vertebrae, the arch is somewhat
narrowed where it joins the body on each side ; this nar-
rowed stalk is the pedicle (li, Fig. 19), while the broader
remaining portion of the arch is its lamina. Between
the pedicles of two contiguous vertebrae there are in this
way left apertures, the intervertebral holes which form a
series on each side of the vertebral column, and one of which,
Fi, is shown between the two dorsal vertebras in Fig. 19.
Through these foramina nerves run out from the spinal cord

THE HUMAN BODY.

to various regions of the Body. The sacral foramina, anterior
and posterior, are the representatives of these apertures, but
modified in arrangement, on account of the fusion of the
arches and bodies of the vertebrae between which they lie.

Sternum. The sternum or breast-bone (Fig. 27 and d,
Fig. 16) is wider from side to side than dorso-ventrally. It
consists in the adult of three pieces, and seen from the ven-
tral side has somewhat the form of a dagger. The piece M
nearest the head is called the handle or manubrium, and pre-
sents anteriorly a notch, Id, on each side, with which the
collar-bone articulates (u, Fig. 16); farther back are two
other notches, Id and Jc2, to which the sternal ends of the
first and second ribs are attached. The middle piece, C, of
the sternum is called the body; it completes the notch for
the second rib and has on its sides others,
Ic 3-7, for the third, fourth, fifth, sixth,
and seventh ribs. The last piece of the
sternum, P9 is the ensiform or xiphoid
process; it is composed of cartilage, and
has no ribs attached to it.

The Ribs. (Fig. 28). There are twelve
pairs of ribs, each being a slender curved
bone attached dorsally to the body and
transverse process of a vertebra in the
manner already mentioned, and continued
ventrally by a costal cartilage. In the
case of the anterior seven pairs, the costal
cartilages are attached directly to the sides
of the breast-bone; the next three carti-
lages are each attached to the cartilage of
the preceding rib, while the cartilages of
the eleventh and twelfth ribs are quite

FIG. 27.— The sternum ,, .-, TI j

seen on its ventral aspect, unattached ventrally, so these are called

the free or floating ribs. The convexity
of each curved rib is turned outwards so
rib-cartiiages. " " as io ^ rouil(jnes8 to the sides of the

chest and increase its cavity, and each slopes downwards from
its vertebral attachment, so that its sternal end is consider-
ably lower than its dorsal.

The Skull. (Fig. 29) consists of twenty-two bones in the
adult, of which eight, forming the cranium, are arranged so
as to inclose the brain-case and protect the auditory organ,

THE SKELETON.

while the remaining fourteen support the face
round the mouth, the nose, and the eye-sockets.

and

73

sur-

Fio. 28.— The ribs of the left side, with the dorsal and two lumbar vertebrae, the
rib-cartilages and the sternum: a, first and, 6, twelfth thoracic vertebra; c, first
lumbar vertebra.

Cranium. The cranium is a box with a thick floor and
thinner walls and roof. Its floor or base represents in the
•head (as is depicted in the diagram Fig. 2) that partition be-
tween the dorsal and ventral cavities which in the trunk is
made up of the bodies of the vertebrae. In very early life it

74

THE HUMAN BODY.

presents in the middle line a series of four bones, the basi-
occipital, basi-sphenoid, presphenoid, and basi-ethmoid, which
answer pretty much to the bodies of four vertebra?, and have
attached to them the thin bones which inclose the skull-cavity
(which may be likened to an enlarged neural canal) on the
sides and top. In the Human Body, however, these bones

o

FIG. 29.— A side/viexv of the skull. O, occipital bone : T, temporal ; Pr, parie-
tal; F, frontal; &', sphenoid ; Z, malar ; MX, maxilla; N, nasal; E, ethmoid; £,,
lachrymal; Md, inferior maxilla.

very soon ankylose with others or with one another; although
they remain distinct throughout life in the skulls of very
many lower animals. On the base of the skull, besides many
small apertures by which nerves and blood-vessels pass in or
out, is a large aperture, the foramen magnum, through which
the spinal cord passes in to join the brain.

THE SKELETON. 75

The cranial bones are the following:

1. The occipital bone (Fig. 29, 0), unpaired and having in
it the foramen magnum. It is made up by the fusion of the
basi-occipital with other flatter bones.
2. The frontal lone (Fig. 29, F), also
unpaired in the adult, but in the
child each half is a separate bone. 3.
A pair of thin platelike parietal bones
(Fig. 29, Pr] which meet one another
along the middle line in the top of the
skull, and roof-in a great part of the
cranial cavity. 4. A pair of temporal
bones (Fig. 29, T), one on each side of
the skull below the parietal. On
each temporal bone is a large aperture
leading into the ear-cavity, the essen-

tial parts of the organs of hearing Fra. ^^ ^ of the
being contained in these bones. 5. skull. The lower jaw has been

.,,, -. . -, , , 1,1 removed. At the lower part

The Sphenoid OOne, made Up by the of the figure is the hard palate

» , T 7 • 7 . 7 , forming the roof of the mouth

UniOll OI the OaSl- SplienQld and pre- and surrounded by the upper

sphenoid (lying on the base of skull in '

front of the basi-occipital) with one

another and with flatter bones, is seen

partly (Fig. 29, 8) on the sides of the

cranium in front of tho temporals. 6. at las, on its sides.

The ethmoid, like the sphenoid, single in the adult, is really

made up by the union of a single median basi-ethmoid with

a pair of lateral bones. It closes the skull-cavity in front,

and lies between it and the top of the nasal chambers, being

perforated by many small holes through which the nerves of

smell pass. A little bit of it is seen on the inner side of the

eye-socket at E in Fig. 29.

Facial Skeleton. The majority of the face-bones are in
pairs; two only being single and median. One of these is
the lower jaw-bone or inferior maxilla (Fig. 29, Mrl)', the
other is the vomer, which fcrms part of the partition between
the two nostrils.

The paired face-bones are: 1. The maxilla, or upper jaw-
bones (Mx, Fig. 29), one on each side, carrying the upper
row of teeth and forming a great p^rt of tlie hard palate,
which separates the mouth from the nose. 2. The palatine
'bones, completing the skeleton of the hard palate, and behind

76 THE HUMAN BODY.

which the nose communicates by the posterior nares (Fig. 30)
with the throat-cavity, so that air can pass in or out in breath-
ing. 3. The malar bones, or cheek-bones, (Z, Fig. 29.) lying
beneath and on the outside of the orbit on each side. 4. The
nasal bones (N, Fig. 29), roofing in the nose-. 5. The lach-
rymal bones (L, Fig. 29), very small and thin and lying be-
tween the nose and orbit. 6. The inferior turbinate bones,
lying inside the nose, one in each nostril-chamber.

The Hyoid. Besides the cranial and facial bones there
is, as already pointed out, one other, the hyoid (Fig. 31),
which really belongs to the skull, although it lies in the
neck. It can be felt in the front of the throat, just above
" Adam's apple." The hyoid bone is U-shaped, with its con-
vexity turned ventrally, and consists of a
body and two pairs of processes called cor-
nua. The smaller corima (Fig. 31, 3) are
attached to the base of the skull by long
Fro. 81.— The hyoid ligaments. These ligaments in many ani-
great cornua3; ' I mals are represented by bones, so that the
small corima. ' hyoid, with them, forms a bony arch at-
tached to the base of the skull much as the ribs are attached
to the bodies of the vertebrae. In fishes behind this Jiyoidean
arch come several others which bear the gills; and in the
very young Human Body these also are represented, though
they almost entirely disappear long before birth. The hyoid,
then, with its cornua and ligaments answers pretty much to
a gill-arch, or really to parts of two gill-arches, since the
great and small cornua belong to originally separate arches
present at an early stage of development. It is a remnant of
a structure which has no longer any use in the Human Body;
but in the young frog-tadpole parts answering to^it carry
gills and have clefts between them which extend into the
throat just as in fishes. The gills are lost afterwards and the
clefts closed up when the frog gets its lungs and begins to
breathe by them. In the embryonic human being these gill-
clefts are also present and several more behind them, but the
arches between them do not bear gills, and the clefts them-
selves are closed long before birth. As they have no use their
presence is hard to account for; those who accept the doc-
trine of evolution regard them as developmental reminis-
cences of an extremely remote ancestor in which they were
of functional importance somewhat as in the tadpole: of

THE SKELETON. 77

course this does not mean that men were developed from
tadpoles.

The Appendicular Skeleton. This consists of the
shoulder-girdle and the bones of the fore limbs, and the
pelvic girdle and -the bones of the posterior limbs. The two
supporting girdles in their natural position with reference to
the trunk skeleton are represented in Fig. 32.

The Shoulder-girdle, or Pectoral Arch. This is made
up, on each side, of the scapula or shoulder-blade, and the
clavicle or collar-bone.

The scapula (S, Fig. 32) is a flattish triangular bone
which can readily be felt on the back of the thorax. It is
not directly articulated to the axial skeleton, but lies im-
bedded in the muscles and other parts outside the ribs on each
side of the vertebral column. From its dorsal side arises a
crest to which the outer end of the collar-bone is fixed, and
on its outer edge is a shallow cup into which the top of the
arm-bone fits: this hollow is known as the glenoid fossa.

The collar-bone (C, Fig. 32) is cylindrical and attached at
its inner end to the sternum as shown in the figure, fitting
into the notch represented at Id in Fig. 27.

The Fore Limb. In the limb itself (Fig. 33) are thirty
bones. The largest, a, lies in the upper arm, and is called the
humerus. At the elbow the humerus is succeeded by two
bones, the radius and ulna, c and b, which lie side by side,
the radius being on the thumb side. At the distal ends of
these bones come eight small ones, closely packed and forming
the wrist, or carpus. Then corno five cylindrical bones
which can be felt through the soft parts in the palm of the
hand; one for the thumb, and one for each of the fingers.
These are the metacarpal bones, and are distinguished as first,
second, third, and so on, the first being that of the thumb.
In the thumb itself are two bones, and in each finger three,
arranged in rows one after the other; these bones are all called
phalanges .

The Pelvic Girdle (Fig. 32). This consists of a large
bone, the os innominatum, Oc, on each side, which is firmly
fixed dorsally to the sacrum and meets its fellow in the mid-
dle ventral line. In the child each os innominatum consists
of three bones, viz., the ilium, the ischium, and pubis.
Where these three bones meet and finally ankylose there is a
deep socket, the acrtabulum, into which the head of the thigh-

78

THE HUMAN BODY.

bone fits (see Fig. 16). Between the pubic and ischial bones ,
is the largest foramen in the whole skeleton, known as the
doorlike or thyroid foramen. The pubic bone lies above

FIG. 32.— The skeleton of the trunk and the limb arches seen from the front. C,
clavicle; S, scapula; Oc, innominate bone attached to the side of the sacrum dor-
sally and meeting its fellow at t.hepufoic symphysis in the ventral median line.

and the ischial below it. The ilium forms the upper expanded
portion of the os innominatum to which the line drawn from
Oc in Fig. 32 points.

The Hind Limb. In this there are thirty bones, as in the
fore limb, but not quite similarly arranged; there being one
less at the ankle than in the wrist, and one at the knee not
present at the elbow-joint. The thigh-bone or femur (a,
Fig. 34) is the largest bone in the body and extends from the
hip to the knee-joint. It presents above a large rounded
head which fits into the acetabulum and, below, it is also

THE SKELETON.

79

enlarged and presents smooth surfaces which meet the bones
of the leg. These latter are two in number, known as the
tibia, c, or shin-bone, and fibula, d; the tibia being on the
great-toe side. In front of the knee-joint is the knee-cap, or
patella, b.

FIG. 33. FIG. 34.

FIG. 33. — The hones of the arm. a, hunierus; 6. ulna; c, radius; d, the carpus;
e, the fifth metacarpal; f, the three phalanges of the fifth digit (little finger); g,
the phalanges of the poliex (thumb).

FIG. 34. -Bones of the leg. a, femur; 6, patg.Ha; c, tibia: d, fibula; h, calca-
neum; e, remaining tarsal bones; /, metatarsal bones; g, phalanges.

At the distal end of the leg-bones comes the foot, consist-
ing of tarsus, metatarsus, and phalanges. The tarsus, which
answers to the carpus of the fore limb, is made up of seven
irregular bones, the largest being the heel-bone, or calcaneum,

80

THE HUMAN BODY.

h. The metatarsus consists of five bones lying side by side,
and each carries a toe at its distal end. In the great toe (or
hallux) there are two phalanges, in each of the others three,
arranged as in the fingers, but smaller.

Comparison of the Anterior and Posterior Limbs. It is
clear that the skeletons of the arm and leg correspond pretty

0 P

FIG. 35.— The skeleton of the arm and leg:. H, the humerus; Cd, its articular
head which fits into the glenoid fossa of the scapula; U. the ulna: R, the radius;
O, the olecranon; Fe, the fernur; P, the patella; Ft, the fibula; T, the tibia.

closely to one another. They are in fact quite alike in very
early life, and their differences at birth depend upon their
taking different ways as they develop from their primitive

THE SKELETON. 81

simplicity; as both may be regarded as modifications of the
same original structure, they are homologous. The pelvic
girdle clearly corresponds generally to the pectoral arch, the
tibia and fibula to the radius and ulna; the five metatarsal
bones to the five metacarpal, and the phalanges of the toes to
those of the thumb and fingers. On the other hand, there is
in the arm no separate bone at the elbow-joint corresponding
to the patella at the knee, but the ulna bears above a bony
process, the olecranon (0, Fig. 35), which at first is a separate
bone and is the representative of the pafcella. There are in
the carpus eight bones and in the tarsus but seven. The

FIG. 36.— Diagram showing the relation of the pectoral arch to the axial
skeleton.

astragalus of the tarsus (Ta, Fig. 38) represents, however, two
bones which have grown together. The elbow-joint bends
ventrally and the knee-joint dorsally.

Comparing the limbs as a whole, greater differences come
to light, differences which are
mainly correlated with the dif-
ferent uses of the two limbs.
The arms, serving as prehensile ,A
organs, have all their parts as
movable as is consistent with
the requisite strength, while

,1 , TIT- i FIG. 37.— Diagram showing • the at-

tne lower limbs, having to bear tachment of the pelvic arch to the axial

the whole weight of the Body, s

require to have their parts much more firmly knit together.
Accordingly we find the shoulder-girdle, represented red in
the diagram (Fig. 36), only directly attached to the axial
skeleton by the union of the inner ends of the clavicles with
the sternum, and capable of considerable independent move-
ment, as seen, for instance, in " shrugging the shoulders."

82 THE HUMAN BODY.

The pelvic arch, on the contrary, is firmly and immovably
fixed to the sides of the sacrum. The socket of the scapula,
into which the head of the humerus fits, is very shallow and
allows a far greater range of movement than is permitted by
the deeper socket on the pelvis, into which the head of the
femur fits. Further, if we hold the right humerus tightly
in the left hand and do not allow it to move, we can still
move the forearm bones so as to turn the palm of the hand
either up or down: no such movement is possible between
the tibia and fibula. Finally, in the foot the bones are much
less movable than in the hand, and are arranged so as to make
a springy arch (Fig. 38) which bears behind on the calcaneum,
Ca, and in front on the distal ends of the metatarsal bones,
Os ; over the crown of the arch, at Ta, is the surface with

Sfhl

FIG. 38.— The bones of the foot. Ca, calcaneum, or os calcis ; Ta. articular sur-
face for tibia on the astragalus ; N, scaphoid bone ; CL CII, first and second
cuneiform bones ; Cb, cuboid bone ; Ml, metatarsal bone of great toe.

which the leg-bones articulate and on which the weight of
the Body bears in standing.

The toes, too, are far less movable than the fingers, and
this difference is especially well marked between the great
toe and the thumb. The latter can be made to meet each of
the finger-tips and so the hand can seize and manipulate very
small objects, while this power of opposing the first digit to
the rest is nearly absent in the foot of civilized man. In
children, however, who have never worn boots, and in savages,
the great toe is far more movable, though it never forms as
complete a thumb as in many apes, which use their feet, as
well as their hands, for prehension. By practice, however,
our own toes can be made much more mobile than they
usually are, so that the foot can to a certain extent replace
the hand; as has been illustrated in the case of persons born

THE SKELETON.

without hands who have learned to write and paint with
their toes.

Peculiarities of the Human Skeleton. These are largely
connected with the division of labor between the fore and
hind limbs referred to above, which is carried farther in man
than in any other creature. Even the highest apes frequently
use their fore limbs in locomotion and their hind limbs in
prehension, and we find accordingly that anatomically they
present less differentiation of hand and foot. The other more
important characteristics of the human skeleton are correlated
for the most part with the maintenance of the erect posture,
which is more complete and habitual in man than in the
animals most closely allied to him anatomically. These
peculiarities, however, only appear fully in the adult. In the
infant the head is proportionately larger, which gives the
centre of gravity of the Body a comparatively very high posi-
tion and renders the maintenance of the erect posture difficult
and insecure. The curves of the vertebral column are nearly
absent, and the posterior limbs are relatively very short. In
all these points the infant approaches more closely than the
iidult to the ape. The subsequent great relative length of the
posterior limbs, which grow disproportionately fast in child-
hood as compared with the anterior, makes progression on
them more rapid by giving a longer stride and at the same
time makes it almost impossible to go on "all fours" except
by crawling on the hands and knees. In other Primates this
disproportion between the anterior and posterior limbs does
not occur to nearly the same extent.

In man the skull is nearly balanced on the top of the
vertebral column, the occipital condyles which articulate with
the atlas being about its middle (Fig. 30), so that but little
effort is needed to keep the head erect. In four-footed beasts,
on the contrary, the skull is carried on the front end of the
horizontal vertebral column and needs special ligaments to
sustain it. For instance, in the ox and sheep there is a great
elastic cord running from the cervical vertebrae to the back
of the skull and helping to hold up the head. Even in the
highest apes the skull does not balance on the top of the
spinal column; the face part is much heavier than the back?
while in man the face parts are relatively smaller and the cra-
nium larger, so that the two nearly equipoise. To keep th,e
head erect and look things straight in the face, "like a man,"

84 THE HUMAN BODY.

is for the apes far more fatiguing, and so they cannot long
maintain that position.

The human spinal column, gradually widening from the
neck to t-he sacrum, is well fitted to sustain the weight of the
head, upper limbs, etc., carried by it; and its curvatures,
which are peculiarly human, give it considerable elasticity
combined with strength. The pelvis, to the sides of which
the lower limbs are attached, is proportionately very broad in
man, so that the balance can be more readily maintained
during lateral bending of the trunk. The arched instep and
broad sole of the human foot are also very characteristic.
The majority of four-footed beasts, as horses, walk on the
tips of their toes and fingers; and those animals, as bears and
apes, which like man place the tarsus also on the ground, or in
technical language are plantigrade, have a much less marked
arch there. The vaulted human tarsus, composed of a num-
ber of small bones, each of which can glide a little over its
neighbors, but none of which can move much, is admirably
calculated to break any jar which might be transmitted to
the spinal column by the contact of the sole with the ground
at each step. A well-arched instep is therefore rightly con-
sidered a beauty; it makes progression easier, and by its
springiness gives elasticity to the step. In London flat-footed
candidates for appointment as policemen are rejected, as they
cannot stand the fatigue of walking the daily " beat/'

CHAPTER VII.
THE STRUCTURE AND COMPOSITION OF BONE. JOINTS.

Gross Structure of the Bones. The bones of the Body
have all a similar structure and composition, but on account
of differences in shape they are divided by anatomists into
the following groups: (1) Long bones, more or less cylindrical
in form, like the bones of the thigh and arm, leg and forearm,
metacarpus, metatarsus, fingers and toes. (2) Tabular bones,
in the form of expanded plates, like the bones on the roof and
sides of the skull, and the shoulder-blades. (3) Short bones,
rounded or angular in form and not much greater in one
diameter than in another, like the bones of the tarsus and
carpus. (4) Irregular bones, including all which do not fit
well into any of the preceding groups, and commonly lying
in the middle line of the Body and divisible into similar
halves, as the vertebrae. Living bones have a bluish-white
color and possess considerable elasticity, which is best seen in
long slender bones such as the ribs.

To get a general idea of the structure of a bone, we may
select the humerus for examination. Externally in the fresh
state it is covered by a dense white fibrous membrane very
closely adherent to it and containing many small blood-vessels.
This membrane is the periosteum ; on its under side new
osseous tissue is formed while the bone is still growing, and
all through life it is concerned in maintaining the nutrition
of the bone, which dies if it be stripped off. The periosteum
covers the whole surface of the bone except its ends in the
elbow and shoulder joints; the surfaces there which come into
contact with other bones and glide over them in the move-
ments of the joint have no periosteum, but are covered by
a thin layer of gristle, known as the articular cartilage. Very
early in the development of the Body the bone in fact -was
represented entirely by cartilage; but afterwards nearly all
this was replaced by osseous tissue, leaving only a thin car-
tilaginous layer at the ends.

85

86

THE HUMAN BODY.

The bone itself, Fig. 39, consists of a central nearly cylin-
drical portion or shaft, extending between the dotted lines x
and z in the figure, and two enlarged articular extremities.

Cpi

FIG. 39.— The right humenis, seen
from the front. For description, see
text.

FIG. 40.— The humerus
bisected lengthwise, a,
marrow-cavity; 6, hard
bone; c, spongy bone;
d, articular cartilage.

On the upper articular extremity is the rounded surface,
Cp, which enters into the shoulder- joint, fitting against the

STRUCTURE AND COMPOSITION OF BONE. JOINTS. 87

glenoid cavity of the scapula; and on the lower are the simi-
lar surfaces, Cpl and Tr, which articulate with the radius and
ulna respectively. Besides carrying the articular surfaces,
each extremity presents several prominences. On the upper
are those marked Tmj and Tm (the greater and smaller tro~
chanters), which give attachment to muscles; and similar
eminences, the external and internal condoles, El and Em,
are seen on the lower end. Besides these, several bony ridges
and rough patches on the shaft indicate places to which mus-
cles of the arm were fixed.

Internal Structure. If the bone be divided longitudinally,
it will be seen that its shaft is hollow, the space being known
as the medullary cavity, and in the fresh bone filled with
marrow. Fig. 40 represents such a longitudinal section. It
will be seen that the marrow-cavity does not reach into the ar-
ticular extremities, but that there the bone has a loose spongy
texture, except a thin layer on the surface. In the shaft, on
the other hand, the outer compact layer is much the thicker,
the spongy or cancellated bone forming only a thin stratum
immediately around the medullary cavity. To the naked
eye the cancellated bone appears made up of a trellis-work of
thin bony plates which intersect in all directions and sur-
round cavities rather larger than the head of an ordinary
pin; the compact bone, on the contrary, appears to have no
cavities in it until it is examined with a magnifying-glass.
In the spaces of the spongy portion lies, during life, a sub-
stance known as the red marroiv, which is quite different from
the yellow fatty marrow lying in the central cavity of the
shaft.

Microscopic Structure of Bone. The microscope shows
that the compact bone contains cavities and only differs from
the spongy portion in the fact that these are much smaller,
and the hard true bony plates surrounding them much more
numerous in proportion than in the spongy parts. If a
thin transverse section of the shaft of the humerus be
examined (Fig. 41) with a microscope magnifying twenty
diameters, it will be seen that numerous openings exist all
over the compact parts of the section and gradually become
larger as this passes into the cancellated part, next the medul-
lary cavity. These openings are the cross-sections of tubes
known as the Haversian canals, which ramify all through the
bone, running mainly in the direction of its long axis, but

88

THE HUMAN BODY.

united by numerous cross or oblique branches as seen in the
longitudinal section (Fig. 42). The outermost ones open on
the surface of the bone beneath the periosteum, and in the
living bone blood-vessels run from this through the Haversian
canals and convey materials for its growth and nourishment.

B

FIG. 41.— A, a transverse section of the ulna, natural size; showing the medullary
cavity. B, the more deeply shaded part of A magnified twenty diameters.

The average diameter of the Haversian canals is 0.05 mm.
(^ of an inch).

Around each Haversian canal lies a set of plates, or lamellae,
of hard bony substance (see the transverse section Fig. 41),
each canal with its lamellae forming an Haversian system :
and the whole bone is made up of a number of such systems,
with the addition of a few lamellae lying in the corners be-
tween them, and a certain number which run around the
whole bone on its outer and inner surfaces. In the spongy

STRUCTURE AND COMPOSITION OF BONE. JOINTS. 89

parts of the bone the Haversian canals are very large and the

intervening lamellae few in number.

Between the lamellae lie small cavities, the lacunce, each of

which is lenticular in form, somewhat like the space which

would be inclosed by two watch-
glasses joined by their edges.
From the lacunae many extremely
fine branching canals, the canali-
culi, radiate and penetrate the
bony lamellae in all directions.
The innermost canaliculi of each
system open into the central Ha-

. FIG. 42.-A thin'ion^tudinai^ec- versian canal; and those of various

tion of bone, magnified about 350 lop1irijYi infprpnmTrmrnnufiTirr fViaao
diameters, oo, Haversian canals. 1RCl1 miCdtlllg, tnese

fine tubes form a set of passages

through which liquid which has transuded from the blood-
vessels in the Haversian canals can ooze all through the
bone. The lacunae and canaliculi are shown in Fig. 42.

In the living bone a granular nucleated cell lies in each
lacuna. These cells, or bone-corpuscles, are the remnants of
those which built up the bone, the hard parts of the latter
being really an intercellular substance or skeleton formed
around and by these cells, much in the same way as a calca-
reous skeleton is formed around a Foraminifer by the activity
of its protoplasm. By the co-operation of all the bone-
corpuscles, and the union of their skeletons, the whole bone
is built up.

In other bones we find the same general arrangement of
the parts, an outer dense layer and an inner spongy portion.
In the flat and irregular bones there is no medullary cavity,
and the whole centre is filled up with cancellated tissue with
red marrow in its spaces. For example, in the thin bones
roofing in the skull we find an outer and an inner hard layer of
compact bone known as the outer and inner table respectively,
the inner especially being very dense. Between the two tables
lies the spongy bone, red in color to the naked eye from the
marrow within it, and called the diploe. The interior of the
vertebrae also is entirely occupied by spongy bone. Every-
where, except where a bone joins some other part of the skel-
eton, it is covered by the periosteum.

Chemical Composition of Bone. Apart from the bone-
corpuscles and the soft contents of the Haversian canals and

90 THE HUMAN BODY.

of the spaces of the cancellated bone, the bony substance
proper, as found in the lamellae, is composed of earthy and
organic portions intimately combined, so that the smallest
distinguishable portion of bone contains both. The earthy
matters form about two thirds of the total weight of a dried
bone, and may be removed by soaking the bone in dilute
hydrochloric acid. The organic portion left after this treat-
ment constitutes a flexible mass, retaining the form of the
original bone; it consists chiefly of an albuminoid, ossein,
which by long boiling, especially under pressure at a higher
temperature than that at which water boils when exposed
freely to the air, is converted into gelatin, which dissolves
in the hot water. Much of the gelatin of commerce is pre-
pared in this manner by boiling the bones of slaughtered
animals, and even well-picked bones may be used to form a
good thick soup if boiled under pressure in a Papin's digester;
much nutritious matter being, in the common modes of do-
mestic cooking, thrown away in the bones.

The earthy salts of bone may be obtained free from organic
matter by calcining a bone in a clear fire, which burns away
the organic matter. The residue forms a white very brittle
mass, retaining perfectly the shape and structural details of
the original bone. It consists mainly of normal calcium
phosphate, or bone-earth (Ca3, 2P04) ; but there is also pres-
ent a considerable proportion of calcium carbonate (CaC03)
and smaller quantities of other salts.

Hygiene of the Bony Skeleton. In early life the bones
are less rigid, from the fact that the earthy matters then pres-
ent in them bear a less proportion to the softer organic parts.
Hence the bones of an aged person are more brittle and easily
broken than those of a child. The bones of a young child
are in fact tolerably flexible and may be distorted by any con-
tinued strain ; therefore children should never be kept sitting
for hours, in school or elsewhere, on a bench which is so high
that the feet are not supported. If this be insisted upon (for
no child will continue it voluntarily) the thigh-bones will al-
most certainly be bent over the edge of the seat by the weight
of the legs and feet, and a permanent distortion may be pro-
duced. 'For the same reason it is important that a child
be made to sit straight while writing, to avoid the risk of
producing a lateral curvature of the spinal column. The
facility with which the bones may be moulded by prolonged

STRUCTURE AND COMPOSITION OF BONE. JOINTS. 91

pressure in early life is well seen in the distortion of the
feet of Chinese ladies, produced by keeping them in tight
shoes; and in the extraordinary forms which some races of
man produce in their skulls, by tying boards on the heads of
the children.

Throughout the whole of life, moreover, the bones remain
amoug the most easily modified parts of the Body ; although
judging from the fact that dead bones are the most permanent
parts of fossil animals we might be inclined to think other-
wise. The living bone, however, is constantly undergoing
changes under the influence of the protoplasmic cells im-
bedded in it, and in the living Body is constantly being ab-
sorbed and reconstructed. The experience of physicians
shows that any continued pressure, such as that of a tumor,
will cause the absorption and disappearance of bone almost
quicker than that of any other tissue; and the same is
true of any other continued pressure. Moreover, during life
the bones are eminently plastic; under abnormal pressures
they are found to quickly, assume abnormal shapes, being
absorbed and disappearing at points where the pressure
is most powerful, and increasing at other points; tight
lacing may in this way produce a permanent distortion of
the ribs.

When a bone is fractured a surgeon should be called in
as soon as possible, for once inflammation has set in and
the parts have become swollen it is much more difficult to
place the broken ends of the bone together in their proper
position than before this has occurred. Once the bones are
replaced they must be held in position by splints or bandages,
or the muscles attached to them will soon displace them
again. With rest, in young and healthy persons complete
union will commonly occur in three or four weeks; but in
old persons the process of healing is slower and is apt to be
imperfect.

Articulations. The bones of the skeleton are joined
together in very various ways; sometimes so as to admit
of no movement at all between them; in other cases so as
to permit only a limited range or variety of movement; and
elsewhere so as to allow of very free movement in many
directions. All kinds of unions between bones are called ar-
ticulations.

Of articulations permitting no movements, those which

92 THE HUMAN BODY.

unite the majority of the cranial bones afford a good example.
Except the lower jaw, and certain tiny bones inside the tem-
poral bone belonging to the organ of hearing, all the skull-
bones are immovably joined together. This union in most
cases occurs by means of toothed edges which fit into one
another and form jagged lines of union known as sutures.
Some of these can be well seen in Fig. 29 between the
frontal and parietal bones (coronal suture) and between the
parietal and occipital bones (lambdoidal suture)', while an-
other lies along the middle line in the top of the crown
between the two parietal bones, and is known as the sagittal
suture. In new-born children where the sagittal meets the
coronal and lambdoidal sutures there are large spaces not yet
covered in by the neighboring bones, which subsequently
extend over them. These openings are known asfontanelles,
At them a pulsation can often be felt synchronous with each
beat of the heart, which, driving more blood into the brain,
distends it and causes it to push out the skin where bone is
absent. Another good example of an articulation admitting
of no movement is that between the rough surfaces on the
sides of the sacrum and the innominate bones.

We find good examples of the second class of articulations
—those admitting of a slight amount of movement — in the
vertebral column. Between every pair of vertebrae from the
second cervical to the sacrum is an elastic pad, the interqer-
tebral disk, which adheres by its surfaces to the bodies of the
vertebras between which it lies, and only permits so much
movement between them as can be brought about by its own
compression or stretching. When the back-bone is curved to
the right, for instance, each of the intervertebral disks is
compressed on its right side and stretched a little on its left,
and this combination of movements, each individually but
slight, gives considerable flexibility to the spinal column as a
whole.

Joints. Articulations permitting of movement by the glid-
ing of one bone over another are known as joints, and all
have the same fundamental structure, although the amount
of movement permitted in different joints is very different.

Hip-joint. We may take this as a good example of a true
joint permitting a great amount and variety of movement.
On the os innominatum is the cavity of the acetabulum (Fig.
43), which is lined inside by a thin layer of articular carti-

STRUCTURE AND COMPOSITION OF BONE. JOINTS. 93

lage which has an extremely smooth surface. The bony cup
is also deepened a little by a cartilaginous rim. The proximal
end of the femur consists of a nearly spherical smooth head,
borne on a somewhat narrower neck, and fitting into the ace-
tabulum. This head also is covered with articular cartilage;
and it rolls in the acetabulum like a ball in a socket. To
keep the bones together and limit the amount of movement,
ligaments pass from one to the other. These are composed
of white fibrous connective tissue (Chap. VIII) and are ex-
tremely pliable, but quite inextensible and very strong and

FIG. 43.— Section through the hip-joint.

tough. One is the capsular ligament, which forms a sort of
loose bag all round the joint, and another is the round liga-
ment, which passes from the acetabulum to the head of the
femur. Should the latter rotate above a certain extent in
i'ts socket, the round ligament and one side of the capsular
ligament are put on the stretch, and any further movement
which might dislocate the femur (that is, remove the head
from its socket) is checked. Covering the inside of the cap-
sular ligament and the outside of the round ligament is a
layer of flat cells, which are continued in a modified form
over the articular cartilages and form the synovial membrane.
This, which thus forms the lining of the joint, is always

94 THE HUMAN BODY.

moistened in health by a small quantity of glairy synovial
fluid, something like the white of a raw egg in consistency,
and playing the part of the oil with which the contiguous
moving surfaces of a machine are moistened; it makes all
run smoothly with very little friction.

In the natural state of the parts, the head of the femur and
the bottom and sides of the acetabulum lie in close contact,
the two synovial membranes rubbing together. This contact
is not maintained by the ligaments, which are too loose and
serve only to check excessive movement, but by the numerous
stout muscles which pass from the thigh to the trunk and
bind the two firmly together. Moreover, the atmospheric
pressure exerted on the surface of the Body and transmitted
through the soft parts to the outside of the air-tight joint
helps also to keep the parts in contact. If all the muscles
and ligaments around the joint be cut away, it is still found in
the dead Body that the head of the femur will be kept in its
socket by this pressure, and so firmly as to bear the weight of
the whole limb without dislocation, just as the pressure of
the air will enable a boy's " sucker " to lift a tolerably heavy
stone.

Ball-and-socket Joints. Such a joint as that at the hip is
called a ball-and-socket joint and allows of more free move-
ment than any other. Through movements occurring in it
the thigh can be flexed, or bent so that the knee approaches
the chest; or extended, that is, moved in the opposite direc-
tion. It can be abducted, so that the knee moves outwards;
and adducted, or moved back towards the other knee again.
The limb can also by movements at the hip-joint be circum-
ducted, that is, made to describe a cone of which the base is
at the foot and the apex at the hip. Finally, rotation can
occur in the joint, so that with knee and foot joints held
rigid the toes can be turned in or out, to a certain extent, by
a rolling around of the femur in its socket.

At the junction of the humerus with the scapula is another
ball-and-socket joint permitting all the above movements to
even a greater extent. This greater range of motion at the
shoulder-joint depends mainly on the shallowness of the
glenoid cavity as compared with the acetabulum, and upon
the absence of any ligament answering to the round ligament
of the hip-joint. Another ball-and-socket joint exists between
the carpus and the metacarpal bone of the thumb; and others

STRUCTURE AND COMPOSITION OF BONE. JOINTS. 95

with the same variety, but a much less range, of movement
between each of the remaining metacarpal bones and the
proximal phalanx of the finger which articulates with it.

Hinge-joints. Another form of synovial joint is known as
a hinge-joint. In it the articulating bony surfaces are of
such shape as to permit of movement, to and fro, in one plane
only, like a door on its hinges. The joints between the pha-
langes of the fingers are good examples of hinge-joints. If
no movement be allowed where the finger joins the palm of
the hand it will be found that each can be bent and straight-
ened at its own two joints, but not moved in any other way.
The knee is also a hinge-joint, as is the articulation between
the lower jaw and the base of the skull whicli allows. us to
open and close our mouths. The latter is, however, not a
perfect hinge- joint, since it permits of a small amount of
lateral movement such as occurs in chewing, and also of a
gliding movement by which the lower jaw can be thrust for-
ward so as to protrude the chin and bring the lower row of
teeth outside the upper.

Pivot-joints. In this form one bone rotates around
another which remains stationary. We have a good example
of it between the first and second cervical vertebrae. The
first cervical vertebra or atlas (Fig. 22) has a very small
body and a very large arch, and its neural canal is subdivided
by a transverse ligament (L, Fig. 22) into a dorsal and a ven-
tral portion ; in the former the spinal cord lies. The second
vertebra or axis (Fig. 23) has arising from its body the stout
bony peg, D, called the odontoid process. This projects into
the ventral portion of the space surrounded by the atlas, and,
kept in place there by the transverse ligament, forms a pivot
around which the atlas, carrying the skull with it, rotates
when we turn the head from side to side. The joints on each
side between the atlas and the skull are hinge-joints and per-
mit only the movements of nodding and raising the head.
When the head is leaned over to one side, the cervical part of
the spinal column is bent.

Another kind of pivot- joint is seen in the forearm. If
the limb be held straight out, with the palm up and the elbow
resting on the table, so that the shoulder-joint be kept steady
while the hand is rotated until its back is turned upwards, it
will be found that the radius has partly rolled round the ulna.
When the palm is upwards and the thumb outwards, the

96

THE HUMAN BODY.

fr-

lower end of the radius can be felt on the outer side of the
forearm just above the wrist, and if this be done while the hand
is turning over, it will be easily discerned that during the
movement this end of the radius, carrying the hand with it,
travels around the lower end of the ulna so as to get to its
inner side. The relative position of the bones when the palm
is upwards is shown at A in Fig. 44, and when the palm is

down at B. The former position
is known as supination ; the latter
as pronation. The elbow end of
the humerus (Fig. 39) bears a
large articular surface: on the
inner two thirds of this, Tr, the
ulna fits, and the ridges and
grooves of both bones interlock-
ing form a hinge-joint, allowing
only of bending or straightening
the forearm on the arm. The
radius fits on the rounded outer
third, Cpl, and forms there a ball-
and-socket joint at which the
movement takes place when the
hand is turned from the supine
to the prone position ; the ulna
forming a fixed bar around which
the lower end. of the radius is
moved.

Gliding Joints. These per-
mit as a rule but little movement:
examples are found between the closely packed bones of the
tarsus (Fig. 38) and carpus, which slide a little over one
another when subjected to pressure.

Hygiene of the Joints. When a bone is displaced or
dislocated the ligaments around the joint are more or less
torn and other soft parts injured. This soon leads to inflam-
mation and swelling which make not only the recognition of
the injury but, after diagnosis, the replacement of the bone,
or the reduction of the dislocation, difficult. Moreover the
muscles attached to it constantly pull on the displaced bone
and drag it still farther out of place; so that it is of great
importance that a dislocation be reduced as soon as possible.
In most cases this can only Ipe attempted with safety by one

FIG. 44.— A, arm in supiuation; B,
arm in pronation. H, humerus; R,
radius; U, ulna.

STRUCTURE AND COMPOSITION OF BONE. JOINTS. 97

who knows the form of the bones, and possesses sufficient ana-
tomical knowledge to recognize the direction of the displace-
ment. No injury to a joint should be neglected. Inflamma-
tion once started there is often difficult to check and runs on,
in a chronic way, until the synovial surfaces are destroyed,
and the two bones perhaps grow together, rendering the joint
permanently stiff. A sprained joint should get immediate
and complete rest, for weeks if necessary, and if there be
much swelling, or continued pain, medical advice should be
obtained. An improperly cared-for sprain is the cause of many
a useless ankle or knee.

CHAPTER VIII.

CAKTILAGE AND CONNECTIVE TISSUE.

Temporary and Permanent Cartilages. In early life a
great many parts of the supporting framework of the Body,
which afterwards become bone, consist of cartilage. Such for
example is the case with all the vertebrae, and with the bones
of the limbs. In these cartilages subsequently the process
known as ossification takes place, by which a great portion of
the original cartilaginous model is removed and replaced by
true osseous tissue. Often, however, some of the primitive
cartilage is left throughout the whole of life at the ends of
the bones in joints where it forms the articular cartilages;
and in various other places still larger masses remain, such as
the costal cartilages, those in the external ears forming their
framework, others finishing the skeleton of the nose which is
only incompletely bony, and many in internal parts of the
Body, as the cartilage of " Adam's apple/7 which can be felt
in the front of the neck, and a number of rings around the
windpipe serving to keep it open. These persistent masses
are known as the permanent, the others as the temporary
cartilages. In old age many so-called permanent cartilages
become calcified— that is, hardened and made unyielding by
deposits of lime-salts in them — without assuming the histo-
logical character of bone, and this calcification of the perma-
nent cartilages is one chief cause of the want of pliability and
suppleness of the frame in advanced life.

Hyaline Cartilage. In its purest form cartilage is flexi-
ble and elastic, of a pale bluish-white color when alive and
seen in large masses, and cuts readily with a knife. In thin
pieces it is quite transparent. Everywhere except in the
joints it is invested by a tough adherent membrane, the peri-
chondrium, which resembles in structure and function the
periosteum of the bones. When boiled for a long time in
water, such cartilages yield a solution of chondrin, which
differs from gelatin in minor points, but agrees with it in the
fact that its solution in hot water " sets " or gelatinizes on cool-

98

CARTILAGE AND CONNECTIVE TISSUE. 99

ing. When a thin slice of hyaline cartilage is examined with
a microscope it is found (Fig. 45) to consist of granular nucle-
ated cells, often collected into groups of two, four, or more,
scattered through a homogeneous or faintly granular ground-
substance or matrix. Essentially, cartilage resembles bone,
being made up of protoplasmic cells and a proportionately
large amount of non-protoplasmic intercellular substance, the

FIG. 45.— A thin slice of cartilage, magnified, to show the cells imbedded in the
homogeneous matrix, a, a cell in which the nucleus has divided; 6, a cell in which
division is just complete; c, e, a group of four cells resulting from further division
of a pair like 6; the new cells have formed some matrix between them, separating
them from another; d. d. cavities in the matrix from which cells have dropped out
during the preparation of the specimen.

cells being the more actively living part and the matrix their
product. Examples of this hyaline variety (so called from
its glassy transparent appearance) are found in all the tempo-
rary cartilages, and in the costal and articular among the
permanent.

Cartilages rarely contain blood-vessels except at points
where a temporary cartilage is being removed and replaced
by bone; then blood-vessels run in from the perichondrium
and form loops in the matrix, around which it is absorbed
and bony tissue deposited. In consequence of the usual
absence of blood-vessels the nutritive processes and exchanges
of material must be small and slow in cartilage, as might in-
deed be expected from the passive and merely mechanical
role which this tissue plays.

Hyaline cartilage is the type, or most characteristically
developed form, of a tissue found with modifications else-
where in the Body. One of its other modifications is the so-
called cellular cartilage, which consists of the cells with
hardly any matrix, only just enough to form a thin capsule
around each. This form is that with which all the carti-

100 THE HUMAN BODY

lages commence, the hyaline variety being built up by the in-
crease of the cell- capsules and their fusion to form the ma-
trix. It persists throughout life in the thin cartilaginous
plate of a mouse's external ear. Other varieties of cartilage
are really mixtures of true cartilage and connective tissues,
and will be considered after the latter.

The Connective Tissues. These complete the skeleton,
marked out in its coarser features by the bones and cartilages,
and constitute the final group of the supporting tissues.
They occur in all forms, from broad membranes and stout
cords to the finest threads forming networks around the other
ultimate histological elements of various organs. In addition
to subsidiary forms, three main varieties of this tissue are
readily distinguishable, viz., areolar, white fibrous, and yellow
elastic. Each consists of fibres and cells, the fibres being of
two kinds, mixed in nearly equal proportions in the areolar
variety, while one kind predominates in one and another in
the second of the remaining chief forms.

Areolar Connective Tissue. This exists abundantly be-
neath the skin, where it forms a loose layer which permits
the skin to be moved, more or less, to and fro over the sub-
jacent parts. Areolar tissue consists of innumerable bands
and cords interlacing in all directions, and can be greatly dis-
tended by blowing air in at any point, from whence it travels
widely through the intercommunicating meshes: if dried
while distended it is somewhat like raw cotton in appearance
but not so white. In dropsy of the legs or feet the cavities
of this tissue are distended with lymph, which in health is
present only in sufficient quantity to moisten them. From
beneath the skin the areolar tissue extends all through the
Body between the muscles and around the blood-vessels and
nerves; and still finer layers of it enter into these and other
organs and -unite their various parts together. It constitutes
in fact a soft packing material which fills up the holes and
corners of the Body, as for instance around the blood-vessels
and between the muscles in Fig. 4.

Microscopic Structure of Areolar Tissue. When exam-
ined with the microscope areolar tissue is seen to consist of
nucleated cells imbedded in a ground-substance which is per-
meated by fibres. The fibres everywhere form the predomi-
nant feature of the tissue (the homogeneous matrix and the
cells being inconspicuous) an'd are of two very different kinds.

CARTILAGE AND CONNECTIVE TISSUE.

101

In a strict sense indeed the areolar tissue ought to be consid-
ered as a mixture of two tissues, one corresponding to each
variety of fibres in it. It is characterized by its loose texture
and by the fact that the two forms of fibres are present in
about equal quantities. In many places a tissue containing
the same histological elements as the areolar tissue is found
in the form of dense membranes, as for example periosteum
and perichondrium.

White Fibrous Tissue. One of the variiet]e> bt fibres per-
vading the matrix of areolar tissue exists "olmSst unmixed'
with the other kind in the cords or tendSns whifch miffce'-r-i US-

FIG. 46. Fro. 46a.

FIG. 46.— White fibrous connective tissue, highly magnified. The nucleated cor-
puscles, seen edgewise and appearing spindle-suaped, are seen here and there on
the surface of the bundles of fibres.

FIG. 46a.— Yellow elastic tissue, magnified after its fibres have been torn apart.

cles to the bones. This form, known as the white fibrous con-
nective tissue, is also found fairly pure in the ligaments around
most joints. Physically it is very flexible but extremely
tough and inextensible, so that it will readily bend in any
direction but is very hard to break; when fresh it has an
opaque white color.

White fibrous tissue (Fig. 46) consists of a matrix, contain-
ing cavities in which cells lie, and pervaded by bundles of
extremely fine fibres. These fibres run in each bundle toler-

102 THE HUMAN BODY.

ably parallel to one another in a wavy course (Fig. 46) and
never branch or unite. Their diameter varies from 0.0005 to
0.001 millimeter (-5-00-00- to ^-jhrTr °f an inch).

Chemically this tissue is characterized by the fact that its
fibres swell up and become indistinguishable when treated
with dilute acetic acid, and by the fact that it yields gelatin
when boiled in water. The substance in it, called ossein in
bones, which is turned into gelatin by such treatment, is here
known us i.oU'.tgen. Glue is impure gelatin obtained from
tendons and ligaments, and calf s-foot jelly, so often recom-
mended, to invalids, Is a purer form of the same substance
oiriained by boiling' the feet of calves, which contain the ten-
dons of many muscles passing from the leg to the foot.

Elastic Tissue. This is almost invariably mixed in some
proportion in all specimens of white fibrous tissue, even the
purest, such as the tendons of muscles; but in certain places
it exists almost alone, as for example in the ligaments (liya-
menta sub/lava) between the arches of the vertebrae, and in
the coats of the larger arteries. In quadrupeds it forms the
great ligament already referred to (p. 83), which helps to sus-
tain the head. This tissue, in mass, is of a dull yellow color
and extremely extensible and elastic; when purest nearly as
much so as a piece of india-rubber. Sometimes it appears
under the microscope to be made up of delicate membranes,
but more often it is in the form of fibres (Fig. 46«) which are
coarser than those of white fibrous tissue and frequently
branch and unite. It is unaffected by acetic acid and does
not yield gelatin when boiled in water.

Connective-tissue Corpuscles. The fibres of white fi-
brous tissue, wherever it is found, are united into bundles by
a structureless ground-material known as the cement-sub-
stance, which also invests each bundle, or skein as we may
call it, with a delicate coating. In this ground-substance are
numerous cavities, branched and flattened in one diameter,
and often intercommunicating by their branches. In these
cavities lie nucleated masses of protoplasm (Fig. 47), fre-
quently also branched, known as the connective-tissue cor-
puscles. These it is which build up the tissue, each
cell in the course of development forming around it a
quantity of intercellular substance, which subsequently be-
comes fibrillated in great part, the remainder forming the
cement. The cells do not quite fill the cavities in which they

CARTILAGE AND CONNECTIVE TISSUE.

103

lie, and these opening into others by their offsets there is
formed a set of minute tubes ramifying through the con-
nective tissues; and (since these in turn permeate nearly all
the Body) pervading all the organs. In these cell-cavities
and their branches the lymph flows before it enters definite
lymphatic vessels, and they are accordingly known as lymph

FIG. 47.— Connective-tissue corpuscles : a. from areolar tissue ; 6, from tendon ;
c, wandering cells.

canaliculi. In addition to the fixed branched connective-
tissue corpuscles there are often found other cells, when livi-ng
connective tissue is examined. These cells much resemble
white blood-corpuscles, and probably are such which have
bored through the walls of the finer blood-vessels. They
creep about along the canaliculi by means of their faculty
of amoeboid movement, and are known as the "wandering
cells. »

Subsidiary Varieties of Connective Tissue — In various
parts of the Body are connective-tissue structures which have
not undergone the typical development, but have departed
from it in one way or another. The cells having formed a
non-fibrillated intercellular substance around them, develop-
ment may go no farther and the mass remain permanently as
the jellylike connective tissue ; or, as in the vitreous humor
of the eye (Chap. XXXI), the cells having formed the soft
matrix, may disappear and leave the latter only. In other
cases the intercellular substance disappears and the cells
branching, and joining by the ends of their branches, form a
network themselves, nucleated or not at the points answering
to the centre of each originally separate cell. This is known
as adenoid connective tissue. In other cases the cells almost
alone constitute the tissue, becoming flattened, closely fitted
at their edges, and united by a very small amount of cement-
substance. Membranes formed in this way lie beneath
epithelium in many places and are known as basement-

104 THE HUMAN BODY.

membranes : the flat cells (Fig. 11, I) which form the
epithelium of the serous cavities are themselves a layer of
modified connective-tissue corpuscles.

In brain and spinal cord, protecting and supporting the
nerve-tissues, are found branched cells forming the neuroglia.
They are not true connective tissue, but correspond to cells
of the horny layer of the epidermis, shut in when the
medullary canal was closed in the embryo.

Elastic Cartilage and Fibro-cartilage. We may now
return to cartilages and consider those forms which are made
up of more or less true cartilage mixed with less or more con-
nective tissue of one kind or another. The cartilages of the
ear and nose and some others have their matrix pervaded by
fine branching fibres of yellow elastic tissue, which form net-
works around the groups of cartilage-cells. Such cartilages
are pliable and tough and possess also considerable extensibil-
ity and elasticity. They are known as elastic or, from their
color, as yellow cartilages. Elsewhere, especially in the carti-
lages which lie between the bones in some joints, we find
forms which have the matrix pervaded by white fibrous tissue
and known as fibro-cartilage. For example the articular
cartilage on the end of the lower jaw does not come into

FIG. 48.— Section through the joint of the lower jaw showing its interarticular
fibre-cartilage, x, with the sy no vial cavity on each side of it.

direct contact with that covering its socket on the skull, but
lying between the two in the joint (Fig. 48) is an interartic-
ular fibro-cartilage : similar cartilages exist in the knee-joint;

CARTILAGE AND CONNECTIVE TISSUE. 105

and the intervertebral disks are also made up of this tissue.
Both elastic cartilage and fibro-cartilage often shade off
insensibly into pure elastic or pure white fibrous connective
tissue.

Homologies of the Supporting Tissues. Bone, cartilage,
and connective tissue all agree in broad structural characters,
and in the uses to which they are applied in the Body. In
each of them the cells which have built up the tissue, with
few exceptions, form an inconspicuous part of it in its fully
developed state, the chief mass of it consisting of intercellular
substance. In hyaline cartilages this latter is not fibrillated;
but these cartilages pass insensibly in various regions of the
Body into elastic or fibro-cartilages, and these latter in
turn into elastic or fibrous connective tissue. The lamellae
of bone, too, when peeled off a bone softened in acid and
examined with a very high magnifying power, are seen to be
pervaded by fine fibres. Structurally, therefore, one can
draw no hard and fast line between these tissues. The same
is true of their chemical composition; bone and white fibrous
tissue contain a substance (collagen) which is converted into
gelatin when boiled in water; and in old people many carti-
lages become hardened by the deposit in their matrix of the
same lime-salts which give its hardness to bone. Further,
the developmental history of all of them is much alike. In
very early life each is represented by cells only : these form
an intercellular substance, and this subsequently may become
fibrillated, or calcified, or both. Finally they all agree in
manifesting in health no great physiological activity, their
use in the Body depending upon the mechanical properties
of their intercellular portions.

The close alliance of all three is further shown by the
frequency with which they replace one another. All the
bones and cartilages of the adult are at first represented only
by collections of connective tissue. Before or after birth this
is in some cases substituted by bone directly (as in the case of
the collar-bone and the bones on the roof of the skull), while
in other cases cartilage supplants the connective tissue, to be
afterwards in many places replaced by bone, while elsewhere
it remains throughout life.

Moreover in different adult animals we often find the
same part bony in one, cartilaginous in a second, and com-
posed of connective tissue in a third: so that these tissues

106 THE HUMAN BODY.

not only represent one another at different stages in the life
of the same animal but permanently throughout the whole
life of different animals. Low in the animal scale we find
them all represented merely by cells with structureless inter-
cellular substance: a little higher in the scale the latter
becomes fibrillated and forms distinct connective tissue.
In the highest Mollusks (as the cuttle-fishes) this is
partly replaced by cartilage, and the same is true of the low-
est fishes; while in some other fishes and the remaining
Vertebrates we find more or less bone appearing in place of
the original connective tissue or cartilage.

From the similarity of their modes of development and
fundamental structure, the transitional forms which exist
between them, and the frequency with which they replace
one another, histologists class the three (bone, cartilage, and
connective tissue) together as homologous tissues and regard
them as differentiations of the same original structure.

Hygienic Remarks. Since in the new-born infant many
parts which will ultimately become bone consist only of car-
tilage, the young child requires food which shall contain a
large proportion of the lime-salts which are used in building
up bone. Nature provides this in the milk, which is rich in
such salts (see Chap. XXI), and no other food can thoroughly
replace it. Long after infancy milk should form a large
part of a child's diet. Many children though given food
abundant in quantity are really starved, since their food does
not contain in sufficient amount the mineral salts requisite
for their healthy development.

At birth even those bones of a child which are most ossi-
fied are often not continuous masses of osseous tissue. In the
humerus, for example, the shaft of the bone is well ossified
and so is each end, but between the shafts and each of the
articular extremities there still remains a cartilaginous layer,
and at those points the bone increases in length, new cartilage
being formed and replaced by bone. The bone increases in
thickness by new osseous tissue formed beneath the perios-
teum. The same thing is true of the bones of the leg. On
account of the largely cartilaginous and imperfectly knit
state of its bones, it is cruel to encourage a young child to
walk beyond its strength, and may lead to "bow-legs" or
other permanent distortions. Nevertheless here as elsewhere
in the animal body, moderate exercise promotes the growth of

CARTILAGE AND CONNECTIVE TISSUE. 107

the tissues concerned, and it is nearly as bad to wheel a child
about forever in a baby-carriage as to force it to over exertion.

The best rule is to let a healthy child use its limbs when
it feels "inclined, but not by praise or blame to incite it to
efforts which are beyond its age, and so sacrifice its healthy
growth to the vanity of parent or nurse.

The final knitting together of the bony articular ends
with the shaft of many bones takes place only comparatively
late in life, and the age at which it occurs varies much in
different bones. Generally speaking, a layer of cartilage re-
mains between the shaft and the ends of the bone, until the
latter has attained its full adult length. To take a few
examples : the lower articular extremity of the humerus
only becomes continuous with the shaft by bony tissue in the
sixteenth or seventeenth year of life. The upper articular
extremity only joins the shaft by bony continuity in the
twentieth year. The upper end of the femur joins the shaft
by bone from the seventeenth to the nineteenth year, and
the lower end during the twentieth. In the tibia the upper
extremity and the shaft unite in the twenty-first year, and
the lower end and the shaft in the eighteenth or nineteenth :
while in the fibula the upper end joins the shaft in the
twenty-fourth year, and the lower end in the twenty-first.
The separate vertebras of the sacrum are only united to form
one bone in the twenty-fifth year of life; and the ilium,
ischium, and pubis unite to form the os innominatum about
the same period. Up to about twenty-five then the skeleton
is not firmly "knit," and is incapable, without risk of injury,
of bearing strains which it might afterwards meet with im-
punity. To let lads of sixteen or seventeen row and take
other exercise in plenty is one thing, and a good one; but to
allow them to undergo the severe and prolonged strain of
training for and rowing a long race is quite another, and not
devoid of risk.

Adipose Tissue. Fatty substances of several kinds exist
in considerable quantity in the Human Body in health, some
as minute droplets floating in the bodily liquids or imbedded
in various cells, but most in special cells, nearly filled with
fat, and collected into masses with supporting and nutritive
parts to form adipose tissue. In fact almost in every spot
where the widely distributed areolar tissue is found, there is
adipose tissue in greater or less proportion mixed with it.

108 THE HUMAN BODY.

Considerable quantities exist for example in the subcuta-
neous areolar tissue, especially in the female sex, giving the
figure of the woman its general more graceful roundness of
contour when compared with that of the male. Large quanti-
ties commonly lie in the abdominal cavity around the kid-
neys; in the eye-sockets, forming a
pad for the eyeballs ; in the mar-
row of bones; around the joints,
and so on.

Examined with the microscope
(Fig. 49) adipose tissue is found to
consist of small vesicles from 0.2
mm. to 0.09 mm. (T-J-Q to ^-J-^ inch)
in diameter, clustered together into
little masses and bound to one an-

FIG. 49. — Fat-cells imbedded ., , ,. ,. -, , -, ^

in areolar tissue, a, nucleus; Other by Connective tlSSlie and blood-
6, protoplasmic, oil-droplet.^ veggels ^^ intertwine around

them; in this way the little angular masses which are seen in
beef-suet are formed, each mass being separated by a some-
what coarser partitioii of areolar tissue from its neighbors.
The individual fat-cells are spherical or ovoid except when
closely packed ; then they become polygonal. Each consists
of a delicate envelope containing oily matter, which in life
is liquid at the temperature of the Body. Besides the oily
matter, a nucleus is commonly present in each fat-cell; and
a thin layer of protoplasm, exaggerated in Fig. 49, forms a
lining to the cell-wall. The oily matter consists of a mixture
of palmatin, olein and stearin, which are compounds of pal-
mitic, stearic and oleic acids with glycerin, three molecules
of the acid being combined with one of glycerin, with the
elimination of water; as for example:

3(C,,H3S0 | Q) +OA J Q _ 3(0,^0) J0s + 3H20

Stearic acid. Glycerin. Stearin. Water.

CHAPTER IX.
THE STRUCTURE OF THE MOTOR ORGANS.

Motion in Animals and Plants. If one were asked to
point out the most distinctive property of living animals, the
answer would probably be, their power of executing spontane-
ous movements. Animals as we commonly know them are
rarely at rest, while trees and stones move only when acted
upon by external forces, which are in most cases readily re-
cognizable. Even at their quietest times some kind of motion
is observable in the higher animals. In our own Bodies
during the deepest sleep the breathing movements and the
beat of the heart continue; their cessation is to an onlooker
the most obvious sign of death. Here, however, as elsewhere
in Biology, we find that precise boundaries do not exist; at
any rate so far as animals and plants are concerned we cannot
draw a hard and fast line between them with reference to the
presence or absence of apparently spontaneous motility. Many
a flower closes in the evening to expand again in the morning
sun; and in many plants comparatively rapid and extensive
movements can be called forth by a slight touch, which in
itself is quite insufficient to produce mechanically that amount
of motion in the mass. The Venus's flytrap (Dioncea musci-
pula) for example has fine hairs on its leaves, and when these
are touched by an insect the leaf closes up so as to imprison
the anirial, which is subsequently digested and absorbed by
the leaf. The higher plants it is true have not the power of
locomotion, they cannot change their place as the higher ani-
mals can; but on the other hand some of the lower animals
are permanently fixed to one spot; and among the lowest
plants many are known which swim about actively through
the water in which they live. The lowest animals and plants
are in fact those which have undergone least differentiation
in their development, and which therefore resemble each
other in possessing, in a more or less manifest degree, all the
fundamental physiological properties of that simple mass of

109

HO THE HUMAN BODY.

protoplasm which formed the starting-point of each individ-
ual. With the physiological division of labor which takes
place in the higher forms we find that, speaking broadly,
plants especially develop nutritive tissues, while animals are
characterized by the high development of tissues with motor
and irritable properties; so that the preponderance of these
latter is very marked when a complex animal, like a dog or a
man, is compared with a complex plant, like a pine or a hick-
ory. The higher animal possesses in addition to greatly de-
veloped nutritive tissues (which differ only in detail from
those of the plant, and constitute what are therefore often
called organs of vegetative life] well-developed spontaneous,
irritable and contractile tissues, found mainly in the nervous
and muscular systems, and forming what have been called the
organs of animal life. Since these place the animal in close
relationship with the surrounding universe, enabling slight
external forces to excite it, and it in turn to act upon external
objects, they are also often spoken of as organs of relation.
In man they have a higher development on the whole than in
any other animal, and give him his leading place in the ani-
mate world, and his power of so largely controlling and direct-
ing natural forces for his own good, while the plant can only
passively strive to endure and make the best of what happens
to it; it has little or no influence in controlling the happening.

Amoeboid Cells. The simplest motor tissues in the adult
Human Body are the amoeboid cells (Fig. 15) already de-
scribed, which may be regarded as the slightly modified
descendants of the undifferentiated cells which at one time
made up the whole Body. In the adult they are not attached
to other parts, so that their changes of form only affect them-
selves and produce no movements in the rest of the Body.
Hence with regard to the whole frame they can hardly be
called motor tissues, and are classed in the group of undiffer-
entiated tissues.

Ciliated Cells. As the growing Body develops from its
primitive simplicity we find that the cells lining some of the
tubes and cavities in its interior undergo a very remark-
able change, by which each cell differentiates itself into a nu-
tritive and a highly motile and spontaneous portion. Such
cells are found for example lining the windpipe, and are
represented in Fig. 50. Each has a conical form, the base of
the cone being turned to the cavity of the air-tube, and con-

THE STRUCTURE OF THE MOTOR ORGANS, 111

tains an oval nucleus with a nucleolus. On the broader free
end are a number (about thirty on the average) of extremely
fine processes called cilia. During life
these are in constant rapid movement,
lashing to and fro in the liquid which
moistens the interior of the passage; and
as the cells are very closely packed, a bit
of the inner surface of the windpipe, ex-
amined with a microscope, looks like a
field of wheat or barley when the wind

_ . .f. FIG. 50.— Ciliated cells.

blows over it. .bach cilmm strikes with
more force in one direction than in the opposite, and as this
direction of more powerful stroke is the same for all the cilia
on any one surface, the resultant effect is that the liquid in
which they move is driven one way. In the case of the wind-
pipe for example it is driven up towards the throat, and the
tenacious liquid or mucus which is thus swept along is finally
coughed or "hawked" up and got rid off, instead of accumu-
lating in the deeper air-passages away down in the chest.

These cells afford an extremely interesting example of the
division of physiological employments. Each proceeds from
a cell which was primitively equally motile, automatic and
nutritive in all its parts. But in the fully developed state
the nutritive duties have been especially assumed by the
conical cell-body, while the automatic arid contractile prop-
erties have been condensed, so to speak, in that modified
portion of the primitive protoplasmic mass which forms the
cilia. These, being supplied with elaborated food by the rest
of the cell, are raised above the vulgar cares of life and have
the opportunity to devote their whole attention to the per-
formance of automatic movements; which are accordingly far
more rapid and precise than those executed by the whole cell
before any division of labor had occurred in it.

That the movements depend upon the structure and com-
position of the cells themselves, and not upon influences
reaching them from the nervous or other tissues, is proved by
the fact that they continue for a long time in isolated cells,
removed and placed in a liquid, as blood-serum, which does
not alter their physical constitution. In cold-blooded animals,
as turtles, whose constituent tissues frequently retain their
individual vitality long after that bond of union has been
destroyed which constitutes the life of the whole animal as

112 THE HUMAN BODY.

distinct from the lives of its different tissues, the ciliated cells
in the windpipe have been found still at work three weeks
after the general death of the animal.

The Muscles. These are the main motor organs ; their
general appearance is well known to every one in the lean of
butcher's meat. While amoeboid cells can only move them-
selves, and (at least in the Human Body) ciliated cells the
layer of liquid with which they may happen to be in contact,
the majority of the muscles, being fixed to the skeleton, can,
by alterations in their form, bring about changes in the form
and position of nearly all parts of the Body. With the skele-
ton and joints, they constitute pre-eminently the organs of
motion and locomotion, and are governed by the nervous
system which regulates their activity. In fact skeleton,
muscles, and nervous system are correlated parts: the degree
of usefulness of any one of them largely depends upon the
more or less complete development of the others. Man's
highly endowed senses and his powers of reflection and
reason would be of little use to him, were his muscles less
fitted to carry out the dictates of his will or his joints less
numerous or mobile. All the muscles are under the control
of the nervous system, but all are not governed by it with the
co-operation of will or consciousness; some move without our
having any direct knowledge of the fact. This is especially the
case with certain muscles which are not fixed to the skeleton
but surround cavities or tubes in the Body, as the blood-vessels
and the alimentary canal, and by their movements control
the passage of substances through them. The former group,
or skeletal muscles, are also from their microscopic characters
known as striped muscles, while the latter, or visceral muscles,
are called unstriped or plain muscles. The skeletal muscles
being generally more or less subject to the control of the will
(as for example those moving the limbs) are frequently spoken
of as voluntary, and the visceral muscles, which change their
form independently of the will, as involuntary. The heart-
muscle forms a sort of intermediate link; it is not directly
attached to the skeleton, but forms a hollow bag which drives
on the blood contained in it and that quite involuntarily; but
in its microscopic structure it resembles somewhat the skeletal
voluntary muscles. The muscles of respiration might perhaps
be cited as another intermediate group. They are striped
skeletal muscles and, as we all know, are to a certain extent

THE STRUCTURE OF THE MOTOR ORGANS. 113

subject to the will; any one can draw a deep breath when he
chooses. But in ordinary quiet breathing we are quite un-
conscious of their working, and even when attention is turned
to them the power of control is limited; no one can voluntar-
ily hold his breath long enough to suffocate himself. As we
shall see hereafter, moreover, any one or all of the striped
muscles of the Body may be thrown into activity independ-
ently of or even against the will, as, to cite no other instances,
is seen in the "fidgets" of nervousness and the irrepressible
trembling of extreme terror; so that the names voluntary and
involuntary are not good ones. The functional differences
between the two groups depend really more on the nervous
connections of each than upon any essential difference in the
properties of the so-called voluntary or involuntary muscular
tissues themselves.

The Skeletal Muscles. In its simplest form a skeletal
muscle consists of a red soft central part, the belly, which
tapers at each end and there passes into one or more dense
white cords which consist almost entirely of white fibrous
connective tissue. These terminal cords are called the tendons
of the muscle and serve to attach it to parts of the bony or
cartilaginous skeleton. In Fig. 51 is shown the biceps muscle
of the arm, which lies in front of the humerus. Its fleshy
belly is seen to divide above and end there in two tendons,
one of which, Bl', is fixed to the scapula, while the other, Bl),
joins the tendon of a neighboring muscle (the coraco-brachial,
Cb), and is also fixed above to the shoulder-blade. Near the
elbow-joint the muscle is continued into a single tendon,
B', which is fixed to the radius, but gives an offshoot, B", to
the connective -tissue membranes lying around the elbow-
joint.

The belly of every muscle possesses the power of shorten-
ing forcibly under certain conditions. In so doing it pulls
upon the tendons, which being composed of inextensible
white fibrous tissue transmit the movement to the hard parts
to which they are attached, just as a pull at one end of a rope
may be made to act upon distant objects to which the other
end is tied. The tendons are merely passive cords and are
sometimes very long, as for instance in the case of the mus-
cles of the fingers, the bellies of many of which lie away in
the forearm.

If the tendons at each end of a muscle were fixed to the

114

THE HUMAN BODY.

I

O ?

^

&

THE STRUCTURE OF THE MOTOR ORGANS. 115

same bone the muscle would clearly be able to produce no
movement, unless by bending or breaking the bone; the
probable result in such a case would be the tearing of the
muscle by its own efforts. In the Body, however, the two
ends of a muscle are always attached to different parts,
usually two bones, between which more or less movement is
permitted, and so when the muscle pulls it alters the relative
positions of the parts to which its tendons are fixed. In the
great majority of cases a true joint lies between the bones on
which the muscle can pull, and when the latter contracts it
produces movement at the joint. Many muscles even pass
over two joints and can produce movement at either, as the
biceps of the arm which, fixed at one end to the scapula and
at the other to the radius, can move the bones at either the
shoulder or elbow joint. Where a muscle passes over an ar-
ticulation it is nearly always reduced to a narrow tendon;
otherwise the bulky bellies lying around the joints would
make them extremely clumsy and limit their mobility.

Origin and Insertion of Muscles. Almost invariably
that part of the skeleton to which one end of a muscle is

• FIG. 52.— The biceps muscle and the arm-bones, to illustrate how, under ordinary
circumstances, the elbow-joint is flexed when the muscle contracts.

fixed is more easily moved than the part on which it pulls by
its other tendon. The less movable attachment of a muscle
is called its origin, the more movable its insertion. Taking
for example the biceps of the arm, we find that when the
belly of the muscle contracts and pulls on its upper and lower
tendons, it commonly moves only the forearm, bending the
elbow-joint as shown in Fig. 52. The shoulder is so much
more firm that it serves as a fixed point, and so that end is

116

THE HUMAN BODY.

the origin of the muscle, and the forearm attachment, P, the
insertion. It is clear, however, that this distinction in the
mobility of the points of fixation of the mnscle is only rela-
tive, for, by changing the conditions, the insertion may become
the stationary and the origin the moved point; as for instance
in going up a rope " hand over hand." In that case the radial
end of the muscle is fixed and the shoulder is moved through
space by its contraction.

Different Forms of Muscles. Many muscles of the Body
have the simple typical form of a belly tapering to a single
tendon at each end as A, Fig. 53; but others divide at one
end and are called two-headed or biceps muscles; while some
are even three- headed or triceps muscles. On the other hand
some muscles have no tendon at all at one end, the belly run-
ning quite up to the point of attachment; and some have no
tendon at either end. In many muscles a tendon runs along
one side and the fibres of the belly are at-
tached obliquely to it: such muscles (B, Fig.
53) are called penniform or featherlike;
or a tendon runs obliquely down the middle
of the muscle and has the fibres of the belly
fixed obliquely on each side of it (C, Fig. 53),
forming a bipenniform muscle : or even two
tendons may run down the belly and so form
a tripenniform muscle. In a few cases a
tendon is found in the middle of the belly

FIG. 53.— Diagrams .

illustrating typical as well as at each end oi it; such muscles

muscle with a central -\-iii- * • * ^ .c j_i • j?

belly and two termi- are called diastric. A muscle of this form

(Fig. 54) isfonnd in connection with the

bipenniform muscle. IQ^ ^ ^ ^^ by ft tend(m attached

to the base of the skull; from there its first belly runs down-

wards and forwards to the neck by the side of the hyoid bone,

where it ends in a tendon which passes through a loop serving

as a pulley. This is succeeded by a second

belly directed upwards towards the ciiin,

where it ends in a tendon inserted into the

lower jaw. Running along the front of the

abdomen from the pelvis to the chest is a lonqr

muscle on each side of the middle line called

the rectus abdominis: it is polygastric, con

sisting of four bellies separated by short tendons. Many

muscles moreover are not rounded but form wide flat masses,

THE STRUCTURE OF THE MOTOR ORGANS. 117

us for example the muscle Ss seen on the ventral side of the
shoulder-blade in Fig. 51.

G-ross Structure of a Muscle. However the form of the
skeletal muscles and the arrangement of their tendons may
vary, the essential structure of all is the same. Each consists
of a proper striped muscular tissue, which is its essential
part, but which is supported by connective tissue, nourished
by blood-vessels and lymphatics, and has its activity governed
by nerves; so that a great variety of things go to form the
complete organ.

A loose sheath of areolar connective tissue, called the peri-
mysium, envelops each muscle, and from this partitions run
in and subdivide the belly into bundles or fasciculi which
run from tendon to tendon, or for the whole length of the
muscle when it has 110 tendons. The coarseness or fineness
of butcher's meat depends upon the size of these primary
fasciculi, which differs in different muscles of the same ani-
rnal. These larger fasci-
culi are subdivided by finer
connective tissue, mem-
branes into smaller ones
(as shown in Fig. 55, which
represents a few primary
fasciculi of a muscle and
the secondary fasciculi into
which these are divided), FlG. 55._A smaTi bit of muscle composed
each of which consists of a of five g^^Bgvdfl^thne dKnwters^to

Certain number Of micro- show the secondary fasciculi of which the

primary are composed.

scopic muscular fibres

bound together by very fine connective tissue and enveloped
in a close network of blood-vessels. Where a muscle tapers
the fibres in the fasciculi become less numerous, and when a
tendon is formed disappear altogether, leaving little but the
connective tissue.

Histology of Muscle. For the present we need only
concern ourselves with the muscular fibres. Each of these is
from eight to thirty-five millimetres (£ to H inches) long, but
only from 0.034 to 0.055 mm. (Tfo to rhr inch) in diameter
in its widest part and tapering to a blunt point at each end.
In cross-section the fibres are irregularly polygonal. In long
muscles with terminal tendons, no fibre runs the whole length
of a fasciculus, which may be a foot or more long, but the

118

THE HUMAN BODY.

fasciculus is made up of many successive fibres, the narrow
end of each fitting in between the ends of those which follow
it. In muscles with short fasciculi, the fibres
may run the whole length of each of the
latter.

The tissue of the skeletal muscles is very
easily recognized under the microscope: even
when magnified only two or three hundred
diameters each fibre is seen to be crossed for
its whole width by regularly alternating dim-
mer and brighter bands (Fig. 56) or stripes.
In a relaxed fibre each band is about jrfo
min. (V^TTTr inch) in width, but the brighter

FIG. 56.-A small V 1 ? 0 ( 0 / &

part of a muscle-fibre, bands are a little broader than the darker.
itl^cross^tV' Yo0rTandIn the contracted fibre both kinds of bands
become narrower, especially the brighter, and
these latter at the same time undergo an optical change and
divert the light so that but little of it reaches the
eye when the fibre is in focus ; in consequence they
then look darker than the original dimmer bands
lying between them and now appearing as the
brighter of the two. A fresh muscle-fibre shows
on close examination a faint longitudinal striation.
This is much more marked in specimens which
have been preserved in alcohol> and these may be
teased out into very fine threads which have been
named fibrillai.

On careful examination each fibre can be made
out to possess an external envelope, the sarco-
lemma, enveloping a softer material which makes
up the main mass of the fibre; but there are in
addition a number of oval nuclei which lie im-
mediately under the sarcolemma and are placed muscuia
lengthwise in the fibre. On account of its extreme fibre hat
thinness and transparency the sarcolemma cannot fwStSf

FIG.

be recognized when lying in its natural position,
closely applied to the striped contents, but being
tougher than these it sometimes remains unbroken where so ciose-

ly applied to

when they are crushed and then (Fiff. 57) comes the rest as to

. . . be invisible,

into view as an apparently structureless mem- remains un-
brane bridging over the gap. The sarcolemma spiSuous. cc
is imperforate except at one point where the central por-

THE STRUCTURE OF THE MOTOR ORGANS. 119

tion (or axis cylinder, see Chap. XII) of a nerve-fibre pene-
trates it.

The striped contents which occupy most of the cavity en-
closed by the sarcolemma are the essential contractile portion
of the fibre and during life are soft or semi-fluid: soon after
death they solidify or clot and thus death-stiffening (rigor
mortis] is produced. At intervals, corresponding to the
middle zone of each bright band of the relaxed fibre, an
extremely delicate membrane (membrane of Krause, K, Fig.
58) crosses the fibre, thus dividing the rest of the contents
into a series of disks, each consisting of a dim centre answering

K

K

FIG. 58.— Diagrams to illustrate the structure of a small piece of a striped
muscle-fibre. A, in the relaxed, B, in the contracted condition; K, K, membranes
of Krause; H, H, bands of Hensen ; aa, bb, parts of sarcostyles, showing their con-
strictions near Krause's membranes, and the tubulated sarcosomes in each ; c,d,e,/,
a sarcous element; o, hyaloplasm; g, sarcoplasm.

to the whole of a dark band, and two brighter ends, each cor-
responding to half of a bright band. Each disk is a sar comer e.
Under certain conditions, in fact, a fibre may be split up
crosswise into a number of such disks. When a fresh muscle-
fibre is artificially stretched and examined with a very high
magnifying power there may further be made out in the
middle of each dim band a transverse line (band of Hensen,
H, Fig. 58) slightly brighter than the rest of the dim band.

The main bulk of each fibre consists of polygonal rods, the
muscle-columns or sarcostyles (aa, bb, Fig. 58), which are inter-

120 THE HUMAN BODY.

rupted in their course by Krause's membranes. That portion
of a sarcostyle, cdef, included between two consecutive mem-
branes is a sarcous element. The terminal portions of each
sarcous element are of softer consistence than most of the mid-
dle part and correspond to the hyaloplasm (Fig. 7) of a
typical primitive cell, and the material composing them may
be designated by the same name. The central portion of each
sarcous element is mainly made up of a firmer material which
stains with hemotoxylin and answers in general to the reticulnm
of a primitive cell : it is named the sarcous substance or, better,
the sarcosome. Each sarcosome is permeated by fine longi-
tudinal tubules which commence at its ends bat do not reach
to its centre and are thus divided into two sets by a median
transverse partition in which the band of Hensen lies. These
tubules are filled with hyaloplasm. The sarcous elements are
constricted where they abut on Krause's membrane and in
consequence each sarcostyle is narrowed at regular intervals
along its course. The spaces between the sarcostyles are
filled by a very soft sarcoplasm, which is of course more
abundant in the regions of Krause's membranes, where the
muscle-columns are constricted. In mammalian muscle the
sarcoplasm is present in relatively much smaller amount than
indicated in Fig. 58. In fresh specimens it can, however,
be made out in the form of fine dark lines with swollen ends,
lying between contiguous sarcous elements. Gold chloride
stains the sarcoplasm deeply but leaves the sarcostyles un-
colored : hence in specimens so prepared the edges or ends of
the sarcoplastic septa appear as very conspicuous lines, which
look, especially in cross-sections, as if due to a network of
fibres, as which they have been described by several observers,
and been regarded as the essential contractile part of the
fibre. In a relaxed muscle-fibre (A, Fig. 58) the sarcosomes
are comparatively long and narrow; but during contraction
(B) they become shorter and thicker and bulged out in the
middle, and more hyaloplasm passes into their tubules, which
become distended, especially near their deeper ends: the band
of Hensen also ceases to be visible. Contraction of the whole
fibre is thus accompanied by or, rather, is due to a transfer-
ence of hyaloplasm from the ends of each sarcomere into the
interior of the sarcosomes of its central portion, in conse-
quence of which the whole fibre becomes shorter and thicker.
The swelling of the sarcosome pushes aside some of the

THE STRUCTURE OF THE MOTOR ORGANS. 121

sarcoplasm lying between them and the displaced portion ac-
cumulates nearer the ends of the sarcous elements, in the
space left by that portion of the hyaloplasm which has entered
the tubules: compare gg in A and B, Fig. 58.

Arguing from the analogy of the amoeboid cell in which,
as we have seen (p. 27), parts consisting only of hyaloplasm
can exhibit movements, it would seem probable that in the
muscle-fibre the hyaloplasm is to be regarded as the active
contractile portion and the sarcosomes as a framework
directing the form which the contracted hyaloplasm shall
assume, and assuring that it shall be a precise and definite
shortening in the direction of the long axis of the fibre with
a widening in the transverse direction, instead of such irreg-
ular changes of form as are exhibited by the amoeboid cell
with its irregularly arranged or, sometimes, entirely absent
reticulum. That the hyaloplasm and not the sarcoplasm
form the contractile part of the fibre is proved by the fact
that in some insect-muscles in which they are unusually large,
it is possible to isolate them while alive and observe them still
contracting.

The nuclei of the fibres lie in the sarcoplasm, which rep-
resents a part of the original protoplasm of the row of cells
from which each muscle-fibre develops, that has remained
but little changed while the rest was differentiated into
sarcous elements.

The blood-vessels and nerve-fibres supplied to the skeletal
muscles are numerous. The larger blood-vessels run in the
coarser partitions of the connective tissue lying between the
fasciculi and give off fine branches which form a network be-
tween the individual fibres but never penetrate the sarcolemma.

Connected with each muscle-fibre is a nerve-fibre of the
white variety (Chap. XII). The central core of the nerve-fibre
ends in an oval expansion (end plate) which contains many
nuclei and lies close under the sarcolemma, its deeper side
being in immediate contact and possibly continuous with
the striated contents. These nerve-fibres are motor or con-
cerned in exciting a contraction of the muscle-fibre. Other
white nerve-fibres are connected with very peculiar bodies
found scattered throughout the muscle, but especially numer-
ous near the tendons. They are usually of a size just visible
to the unaided eye and from their form have been named
muscle-spindles. They appear to be sensory in function.

122

THE HUMAN BODY.

Somewhat similar bodies (Golgi's tendon-organs] are found
in the tendons and are also richly supplied with nerve-fibres.
In histological structure the tendon-organs and the muscle-
spindles appear to be allied to Pacinian bodies (Chap. XXXV).
Structure of the Unstriped Muscles. Of these the
muscular coat of the stomach (Fig. 59) is a good example.

FIG. 59.— The muscular coat of the stomach.

They have no definite tendons, but form expanded membranes
surrounding cavities, so that they have no
definite origin or insertion. Like the skel-
etal muscles they consist of proper contractile
elements, with accessory connective tissue,
blood-vessels and nerves. Their fibres, how-
ever, have a very different microscopic struc-
ture. They present a slightly marked longi-
tudinal but no cross striation and are made
up of elongated cells (Fig. 60), bound to-
gether by a small quantity of cementing
material. The cells vary considerably in
size, but on the average are about ^ mm.
(•{T^g- inch) in length. Each is flattened
in one plane, tapers off at each end, and
possesses a very thin enveloping membrane;
in its interior lies an elongated nucleus with
one or two nucleoli. These cells have the
— Unstriped power of shortening in the direction of their
muscle-ceils. long axes, and so of diminishing the capacity

of the cavities in the walls of which they lie.

THE STRUCTURE OF THE MOTOR GROANS. 123

Cardiac Muscular Tissue. This consists of nucleated
branched cells which unite to form a network, in the inter-
stices of which blood-capillaries and
nerve-fibres run. The cells present
transverse striations, but riot so distinct
as those of the skeletal muscles, and are
said to have no sarcolemma.

The Chemistry of Muscular Tissue.
The chemical structure of the living-
muscular fibre is unknown, but some
idea as to it may be obtained from ex-
amination of the substances it yields on
proximate analysis. Muscle contains 75 Fl«^._Cardiacmuscu-
per cent of water; and, among other Jg; ^^gpT'^S

inorganic Constituents, phosphates and boundaries and cell-nuclei

0 ' r -t are indicated only in the

Chlorides Of potassium, SOdium, and right-hand portion of the

* ' . figure.

magnesium. When at rest a living
muscle is feebly alkaline, but after hard work, or when dying,
this reaction is reversed through the formation of sarcolactic
acid (C3H603). Muscles contain small quantities of grape-
sugar and glycogen, and of organic nitrogenous crystalline
compounds, especially kreatin (C4H9N3Oa). As in the case of
all other physiologically active tissues, however, the main
post-mortem constituents of the muscular fibres are proteid
substances, and it is probable that like protoplasm itself (p.
27) the essential contractile part of the tissue consists of a
complex body containing proteid, carbohydrate and fatty
residues; and that during muscular work this is broken up,
yielding proteids, carbon dioxide, sarcolactic acid, and prob-
ably other things.

During life and for a certain time after general death the
muscles are soft, translucent, extensible and elastic, and
neutral or feebly alkaline in reaction; after a period which in
warm-blooded animals is brief (varying from a few minutes
to three or four hours) they gradually become harder, more
opaque, less extensible and less elastic, and distinctly acid in
reaction. The result of these changes is the well-known
cadaveric rigidity or rigor mortis. The rigid condition lasts
for a day or longer and then it gradually and finally disappears
and more marked decomposition changes commence. Until
a short time before the commencement of rigor the muscles
remain contractile and can be thrown into activity by various

124 THE HUMAN BODY.

excitants, as electric shocks; that is to say, although the body
in general is dead and the beat of the heart and the flow of
blood have ceased, yet the muscles retain their vitality for a
while. This is especially the case with the muscles of cold-
blooded animals, as frogs and turtles, the muscles of wjiich
may, especially if kept cool, retain their living propertied for
several hours after removal from the body of the animal/

If muscles be taken in an early stage of rigor, rapidly^reed
as much as possible from tendons, fats and connective tissue,
and then finely minced and thoroughly washed with water,
most of the salts and crystallizable muscle ingredients eanjpe
dissolved away, along with a small amount of albumens; but by
far the greater part of the albumen is left behind in the form
of myosin, a proteid which is insoluble in water. On treating
the residue with a 10 per cent solution of ammonium chloride
the myosin dissolves and may be obtained as a flocculent
white precipitate by allowing the solution to fall drop by drop
into a large quantity of water, or by adding to it a consider-
able proportion of common salt. Myosin is related chemically
to fibrinogen and globulin, and its solutions in 10 per cent
neutral saline are coagulated by heat at the same temperature
(56° C. or 158° F.) as the former.

Although myosin is apparently the least altered form in
which its chief proteid constituent can be separated from
muscle, it does not appear to exist, or at least exists in small
quantity if at all, in. living muscle; it is an early product of
post-mortem chemical changes. Its precursor in living muscle
has been named myosinogen, and a solution containing that
substance may be obtained as follows: Perfectly fresh and
still contractile muscles are cut out from a frog which has
just been killed by destruction of its brain and spinal cord, a
proceeding which entirely deprives the animal of conscious-
ness and the power of using its muscles, but leaves these lat-
ter unaltered and alive for some time. The excised muscles
are thrown into a vessel cooled below 0° C. by a freezing mix-
ture and are thus frozen hard before any great chemical
change has had time to occur in them. The solidified mus-
cles are then cut up into thin slices, the bits thrown on a
cooled filter and let gradually warm up to the freezing-point
of water, after the addition of some ice-cold 0.5 per cent solu-
tion of common salt. Gradually a small quantity of a tena-
cious alkaline and transparent liquid filters through. This

THE CHEMISTRY OF MUSCLE. 125

liquid, known as the muscle-plasma, contains myosinogen and
like blood-plasma is spontaneously coagulable. It quickly
sets into a transparent jelly and this soon separates into mus-
cle-serum and muscle-clot, the latter consisting of myosin.
Dissolved in the muscle-serum are found small quantities of
several albumens, one much resembling the serum-albumen of
blood. The spontaneous clothing of the plasma, and presum-
ably the natural formation of myosin during rigor mortis, are
due to the action on myosinogen of an enzyme, muscle-fer-
ment, much resembling fibrin-ferment. The clotting is
accompanied by a change of reaction from the alkaline or
neutral of the plasma to a markedly acid one: this appears to
be mainly due to the formation of sarcolactic acid, the quan-
tity of which bears a proportion to that of the myosin formed,
suggesting that both may be products of the breaking-down
of a pre-existent more complex substance. It has further been
shown that when a muscle passes into the state of rigor it
evolves a certain amount of carbon dioxide, and that the
quantity of this varies with the quantity of myosin and of
sarcolactic acid formed. Hence it has been suggested that in
the living muscle there is a substance which after death
breaks up yielding (with possibly other things) myosinogen,
sarcolactic acid and carbon dioxide; and further that this
chemical change is associated with the liberation of energy
(Chap. XX) which in the dead muscle is set free mainly as
the heat which is known to be evolved by muscles passing
into rigor.

The precipitate produced when myosin solutions are
heated is coagulated proteid (p. 10) and insoluble in dilute
acids and alkalies in which myosin itself is very soluble.
When dissolved in dilute acids myosin is converted into syn-
tonin, which was formerly supposed to be the chief form of
proteid present in dead muscles. Syntonin is insoluble in
water and neutral saline solutions, but soluble in dilute acids
and alkalies, and its solutions are not coagulated by boiling.

Beef Tea and Liebig's Extract. From the above-stated
facts it is clear that when a muscle is boiled in water its myo-
sin is coagulated and left behind in the meat : even if cook-
ing be commenced by soaking in cold water the myosin still
remains, as it is as insoluble in cold water as in hot. Beef tea
as ordinarily made, then, contains little but the flavoring
matters and salts of the meat, traces of some albumens and

126 THE HUMAN BODY.

some gelatin, the latter derived from the connective tissues
of the muscle. The flavoring matters and salts make it decep-
tively taste as if it were a strong solution of the whole meat,
and the gelatin causes it to " set " on cooling, so the cook
feels quite sure she has got out " all the strength of the meat/'
whereas the beef tea so prepared contains but little of the
most nutritious proteid portions, which in an insipid shrunken
form are left when the liquid is strained off. Various pro-
posals have been made with the object of avoiding this and
getting a really nutritive beef tea; as for example chopping
the raw meat fine and soaking it in strong brine for some
hours to dissolve out the myosin; or extracting it with dilute
acids which turn the myosin into syntonin and dissolve it and
at the same time render it non-coagulable by heat when subse-
quently boiled. Such methods, however, make unpalatable
compounds which invalids will not take. Beef tea is a slight
stimulant, and often extremely useful in temporarily main-
taining the strength and in preparing the stomach for other
food, but its direct value as a food is slight, and it cannot be
relied upon to keep up a patient's strength for any length of
time. There can be no doubt that thousands of sick persons
have in the past and are being to-day starved to death on it.
Liebig's extract of meat is essentially a very strong beef tea;
containing much of the flavoring substances of the meat,
nearly all its salts and the crystalline nitrogenous bodies, such
as kreatin, which exist in muscle, but hardly any of its really
nutritive parts, as was pointed out by Liebig himself. From
its stimulating effects it is often useful to persons in feeble
health, but other food should be given with it. It may also
be used on account of its flavor to add to the " stock " of soup
and for similar purposes ; but the erroneousness of the com-
mon belief that it is a highly nutritious food cannot be too
strongly insisted upon. Under the name of liqiiid extracts
of meat other substances have been prepared by subjecting
meat to chemical processes in which it undergoes changes
similar to those experienced in digestion: the myosin is thus
rendered soluble in water and uncoagulable by heat, and such
extracts if properly prepared are nutritious and can often be
absorbed when meat in the solid form cannot be digested:
they may thus help the stomach over a crisis, but are not,
even the best of them, to be depended on as anything but
temporary substitutes for other food; or in some cases as use-
ful additions to it.

CHAPTER X.
THE PROPERTIES OF MUSCULAR TISSUE.

Contractility. The characteristic physiological property
of muscular tissue, and that for which it is employed in the
Body, is the faculty possessed by its fibres of shortening
forcibly under certain circumstances. The direction in which
this shortening occurs is always that of the long axis of the
fibre in both plain and striped muscles, and it is accompanied
by an almost equivalent thickening in other diameters, so that
when a muscle contracts it does not shrivel up or diminish
its bulk in any appreciable way; it simply changes its form.
When a muscle contracts it also becomes harder and more
rigid, especially if it has to overcome any resistance. This
and the change of form can be well felt by placing the fingers
of one hand over the biceps muscle lying in front of the bu-
rn erus of the other arm. When the muscle is contracted so
as to bend the elbow it can be felt to swell out and harden as
it shortens. Every schoolboy knows that when he appeals to
another to " feel his muscle " he contracts the latter so as to
make it thicker and apparently more massive as well as
harder. In statues the prominences on the surface indicating
the muscles beneath the skin are made very conspicuous
when violent effort is represented, so as to indicate that the
muscles are in vigorous action. In a muscular fibre we find
no longer the slow, irregular, and indefinite changes of form
seen in amoeboid slightly differentiated cells; they are replaced
by a precise, rapid and definite change of form. Muscular
tissue represents a group of cells in the bodily community
which have taken up the one special duty of executing
changes of form, and in proportion as these cells have fewer
other things to do, they do that one better. This contractility
of the muscular fibres may be briefly described as a passage
from the state of rest, in which the fibres are long and narrow,
into the state of activity, in which they are shorter and thicker:
this change is made with considerable force, and thus the mus-

127

128 THE HUMAN BODY.

cles move parts attached to their tendons. AVhen the state of
activity has passed off the fibres suffer themselves to be ex-
tended again by any force pulling upon them, and so regain
their resting shape; and since in the living Body almost in-
variably other parts are put upon the stretch when any mus-
cle contracts, these by their elasticity serve to pull the latter
back again to its primitive shape.' No muscular fibre is
known to have the power of actively expanding after it has
contracted: in the active state it forcibly resists extension, but
once the contraction is completely over, it suffers itself readily
to be pulled back to its resting form. The contracted state
lasts always longer, however, than the mere time occupied by
the muscle in shortening: as will be seen later, the full state
of contraction is gradually attained and gradually disappears.

Irritability. With that modification of the primitive
protoplasm of an amoeboid embryonic cell which gives rise to
a muscular fibre with its great contractility, there goes a loss
of other properties. Nearly all spontaneity disappears; mus-
cles are not automatic like primitive protoplasm or ciliated
cells; except under certain very special conditions they remain
at rest unless excited from without. The amount of external
change required to excite the living muscular fibre is, how-
ever, very small; muscle tissue is highly irritable, a very
little thing being sufficient to call forth a powerful contrac-
tion. In the living Human Body the exciting force, or stim-
ulus, acting upon a muscle is almost invariably a nervous
impulse, a molecular movement transmitted along the nerve-
fibre attached to it, and upsetting the molecular equilibrium
of the muscle. It is through the nerves that the will acts
upon the muscle-fibre, and accordingly injury to the nerves of
a part, as the face or a limb, causes paralysis of its muscles.
They may still be there, intact and quite ready to work, but
there are no means of sending commands to them, and so
they remain idle.

Although a nervous impulse is the natural physiological
muscular stimulus it is not the only one known. If a muscle
be exposed in a living animal and a slight but sudden tap be
given to it, or a hot bar be suddenly brought near it, or an elec-
tric shock be sent through it, or a drop of glycerin or of solu-
tion of ammonia be placed on it, it will contract; so that in
addition to the natural nervous stimulus, muscles are irritable
under the influence of mechanical, thermal, electrical, and

THE PROPERTIES OF MUSCULAR TISSUE. 129

chemical stimuli. One condition of the efficacy of each. of
them is that it shall act with some suddenness; a very slowly
increased pressure, even if ultimately very great, or a very
slowly raised temperature, or a slowly increased electrical cur-
rent passed through it, will not excite the muscle; although
far less pressure, warmth, or electricity more rapidly applied
would stimulate it powerfully. Once an electric current has
been set up through a muscle, its steady passage does not act
as a stimulus; but a sudden diminution or increase of it does
It may perhaps still be objected that it is not proved that any
of these stimuli excite the muscular fibres, and that in all
these cases it is possible that the muscle is only excited
through its nerves. For the various stimuli named above
also excite nerves (see Chap. XIII), and when we apply them
to the muscle we may really be acting first upon the fine
nerve-endings there, and only indirectly and through the
mediation of these upon the muscular fibres. That the mus-
cular fibres have a proper irritability of their own, independ-
ently of their nerves, is, however, shown by the action of cer-
tain drugs — for example curare, a South American Indian
arrow poison. When this substance is introduced into a
wound all the striped muscles are apparently poisoned, and
the animal dies of suffocation because of the cessation of the
breathing movements. But the poison does not really act on
the muscles themselves: it kills the muscle-nerves, but leaves
the muscle intact; and it has been proved to kill the very
endings of the muscle-nerves right down in the muscle-fibres
themselves. Yet after its administration we still find that
the various non-physiological stimuli referred to alove make
the muscles contract just as powerfully as before the poison-
ing, so we must conclude that the muscles themselves are
irritable in the absence of all nerve stimuli — or, what amounts
to the same thing, when all their nerve-fibres have been poi-
soned. The experiment also shows that the contractility of a
muscle is a property belonging to itself, and that its contract-
ing force is not something derived from the nerves attached
.to it. The nerve stimulus simply acts like the electric shock
or sudden blow and arouses the muscle to manifest a property
which it already possesses The older physiologists observing
that muscular paralysis followed when the nervous connection
between a muscle and the brain was interrupted, concluded
that the nerves gave the muscles the power of contracting,

130 THE HUMAN BODY.

They believed that in the brain there was a great store of a
mysterious thing called vital spirits, and that some of this
was ejected from the brain along the nerve to the muscle,
when the latter was to be set at work, and gave it its working
power. A\ v3 now know that such is not the case, but that a
muscle-fibre is a collection of highly irritable material which
can have its equilibrium upset in a definite way, causing it to
change its shape, under the influence of certain slight disturb-
ing forces, one of which is a nervous impulse; and since in
the Body no other kind of stimulus usually reaches the mus-
cles, they remain at rest when their nervous connections are
severed. But the muscles paralyzed in this way can still, in
the living Body, be made to contract by sending electrical
shocks through them. Physiologically, then, muscle is a con-
tractile and irritable, but not an automatic, tissue.

A Simple Muscular Contraction. Most of the details con-
cerning the physiological properties of muscles have been
studied on those of cold-blooded animals. A frog's muscle
will retain all its living properties for some time after re-
moval from the body of the animal, and so can be experi-
mented on with ease, while the muscles of a rabbit or cat
soon die under those circumstances. Enough has, however,
been observed on the muscles of the higher animals to show
that in all essentials they agree with those of the frog or ter-
rapin.

AVhen a single electric shock is sent through a muscle, the
nerves of which have been thrown out of action by curare, it
rapidly shortens and then, if a weight be hanging on it, rap-
idly lengthens again. The whole series of phenomena from
the moment of stimulation until the muscle regains its rest-
ing form is known as a simple muscular contraction or a
"twitch" : it occupies in frog's muscle about one tenth of a
second. So brief a movement as this cannot be followed in
its details by direct observation, but it is possible to record it
and study its phases at leisure. This may be done by firmly
fixing the upper tendon of an isolated muscle, M, Fig. 62,
and attaching the other end at d to a lever, I, which can move
about the fulcrum/: the end cf the long arm of the lever
bears a point, p, which scratches on a smooth smoked surface,
S. Suppose the surface to be placed so that the writing point
of the lever is at «; if the muscle now contracts it will raise
the point of the lever, and a line ac will be drawn on the

THE PROPERTIES OF MUSCULAR TISSUE. 131

THE HUMAN BODY.

smoked surface, its vertical height, cm, being dependent, first,
on the extent of the shortening of the muscle, and second, on
the proportion between the long and short arms of the lever:
the longer fp is as compared with fd, the more will the actual
shortening of the muscle be magnified. With the lever shown
in the figure this magnification would be about ten times, so
that one tenth of cm would be the extent of the shortening
of the muscle. Suppose, next, the smoked surface to be moved
to such position that the writing point of the lever touches it
at i, and, the muscle being left at rest, the surface to be
moved evenly from left to right; the horizontal line io would
then be traced, its length depending on the distance through
which 8 moved during the time the lever was marking on it :
and it is clear that if S move uniformly, and we know its rate
of movement, we can very readily calculate from the length of
io how long 8 was moving while that line was being traced:
for example, if we know the rate of movement to be ten
inches per second, and on measurement find io to be an inch
long, the time during which the surface was moving must
have been -3^ of a second; and each tenth of io correspond
to YJ-Q- of a second.

If we set the recording surface in motion and while the
lever point is tracing a horizontal line cause the muscle to
contract, the point will be raised as long as the muscle is
contracted, and the line drawn by it will be due to a
combination of two .simultaneous movements — a horizontal,
due to the motion of 8, a nearly vertical, due to the shorten-
ing of the muscle; the resulting line is a curve known
as the curve of a simple muscular contraction. Let the
surface 8 be placed so that the writing point is at q and
then be set in uniform motion from left to right at the same
rate as before (ten inches per second). When the point is
opposite t, stimulate the muscle by an electric shock; the
result, until the muscle has fully lengthened again, will be the
curve tuvwxy, from which many things may be learned. In the
first place we see that the muscle does not commence to con-
tract at the very instant of stimulation, but at an appreciably
later time, and during the interval the lever draws the hori-
zontal line tu\ this period, occupied by preparatory changes
within the muscle, is known as the period of latent excitation.
Then the muscle begins to shorten and the lever to rise, at first
slowly from u to v, then more rapidly, and again more slowly

THE PROPERTIES OF MUSCULAR TISSUE. 133

until the summit of the contraction is reached at w. The
muscle does not now instantly relax, but only gradually passes
back to the resting state : beginning at w, we see the descent
of the curve is for a time slow, then more rapid, and finally
slow again from x to y, when the contraction is completed
and the lever once more traces only the horizontal line yp, due
to the continued movement of the recording surface. The
curve then shows three distinct phases in the contraction : the
period of latent excitation; the period of shortening; the
period of elongating. Knowing the rate of horizontal move-
ment, we can measure off the time occupied by each phase.
The horizontal distance from I to u represents the time taken
by the latent excitation 5 from u to z, the time occupied in
shortening; from z to y, the time taken in elongation: in a
fresh frog's muscle these times are respectively Tf^, T^, Tf¥
of a second. In the muscles of warm-blooded animals they
are all shorter, but the difficulties in the way of accurate ex-
periment are very great. If we know the relative lengths of
the arms of the lever we can of course readily calculate from
the height, wz, of the curve the extent of shortening of
the muscle. With a single electrical stimulation this is never
more than one fourth the total length of the muscle.

In Fig. 62 the accessory apparatus used in practice to in-
dicate on the moving surface the exact instant of stimulation
and to measure the rate at which S moves have been omitted.

Physiological Tetanus. It is obvious that the ordinary
movements of the Body are not brought about by such tran-
sient muscular contractions as those just described. Even a
wink lasts longer than one tenth of a second. Our movements
are, in fact, due to more prolonged contractions which may be
described as consisting of several simple contractions fused
together, and known as " tetanic contractions "; it might be
better to call them "compound contractions," since the word
tetanus has long been used by pathologists to signify a dis-
eased stato, such as occurs in strychnine poisoning and hydro-
phobia, in which most of the muscles of the Bcdy are thrown
into prolonged and powerful involuntary contractions.

If, while a frog's muscle is still shortening under the in-
fluence of one electric shock, another stimulus be given it, it
will contract again and the new contraction will be added on
to that already existing, without any period of elongation
occurring between them. While the muscle is still contract-

134 THE HUMAN BODY.

ing under the influence of the second stimulus a third electric
shock will make it contract more, and so on, until the muscle
is shortened as much as is possible to it for that strength of
stimulus. If now the stimuli be repeated at the proper in-
tervals, each new one will not produce any further shortening,
but, each acting on the muscle before the effect of the last
has begun to pass off, the muscle will be kept in a state of
permanent or "tetanic" contraction; and this can be main-
tained, by continuing the application of the stimuli, until the
organ begins to get exhausted or " fatigued "; elongation then
commences in spite of the stimulation. When our muscles
are stimulated in the Body, from the nerve-centres through
the nerves, they receive from the latter a sufficient number
of stimuli in a second (the exact number is still doubtful) to
throw them into tetanic contractions. In other words, not
even in the most rapid movements of the Body is a muscle
made to execute a simple muscular contraction; it is always
a longer or a shorter tetanus. When very quick movements
are executed, as in performing rapid passages on the piano,
the result, is obtained by contracting two opposing muscles
and alternately strengthening and weakening a little the
tetanus of each.

Causes affecting the Degree of Muscular Contraction.
The extent of shortening which can be called forth in a mus-
cle varies with the stimulus. In the first place, a single stim-N
ulus can never cause a muscle to contract as much as rapidly
repeated stimuli of the same strength — since in the latter
case we get, as already explained, several simple contractions
such as a single stimulus would call forth, piled on the top
of one another. With powerful repeated electrical stimuli
a muscle can be made to shorten to one third of its resting
length, but in the Body the strongest effort of the Will never
produces a contraction of that extent. Apart from the rate
of stimulation, the strength of the stimulus has some influ-
ence, a greater stimulus causing a greater contraction; but
very soon a point is reached beyond which increase of stimu-
lus produces no increased contraction; the muscle has reached
its limit. The amount of load carried by the muscle (or the
resistance opposed to its shortening) has also an influence,
and that in a very remarkable way. Suppose we have a frog's
calf-muscle, carrying no weight, and find that with a stimulus
of a certain strength it shortens two millimeters (T^ inch).

THE PROPERTIES OF MUSCULAR TISSUE. 135

Then if we hang one gram (15.5 grains) on it and give it the
same stimulus, it will be found to contract more, say four or
five millimeters, and so on, up to the point when it carries
eight or ten grams. After that an increased weight will,
with the same stimulus, cause a less contraction. So that up
to a certain limit, resistance to the shortening of the muscle
makes it more able to shorten : the mere greater extension of
the muscle due to the greater resistance opposed to its short-
ening, puts it into a state in which it is able to contract more
powerfully. Fatigue diminishes the working power of a
muscle and rest restores it, especially if the circulation of the
blood be going on in it at the same time. A frog's muscle
cut out of the body will, however, be considerably restored
during a period of rest, even although no blood flow through it.

Cold increases the time occupied by a simple muscular
contraction, and also impairs the contractile power, as we
find in our own limbs when " numbed" with cold, though in
that case the hurtful influence of the cold on the nerves no
doubt also plays a part. Moderate warmth on the other hand,
up to near the point at which death stiffening (often in this
case spoken of as heat rigor] occurs, diminishes the time
foikeii by a contraction, and increases its height. Heat rigor
is produced in excised frog's muscle by heating it to about
40° 0. (104° F.) The required temperature is higher in warm
blooded animals, especially while the circulation through the
muscle is maintained: in fevers temperatures considerably
greater than the above have been observed without the occur-
rence of muscular rigor.

The Measure of Muscular Work. The work done by a
muscle in a given contraction, when it lifts a weight verti-
cally against gravity, is measured by the weight m'oved, mul-
.tiplied by the distance through which it is moved. When a
muscle contracts carrying no load it does very little work,
lifting only its own weight; when loaded with one gram and
lifting it five millimeters it does five gram-millimeters of
work, just as an engineer would say an engine had done so
many kilogrammeters or foot-pounds. If loaded with ten
grams and lifting it six millimeters it would do sixty gram-
millimeters of work. Even after the weight becomes so great
that it is lifted through a less distance, the work done by the
muscle goes on increasing, for the heavier weight lifted more
than compensates for the less distance through which it is

136 THE HUMAN BODY.

raised. For example, if the above muscle were loaded with
fifty grams it would maybe lift that weight only 1.5 millime-
ters, but it would then do seventy-five gram-millimeters of
work, which is more than when it lifted ten grams six milli-
meters. A load is, however, at last reached with which the
muscle does less work, the lift becoming very little indeed,
until at last the weight becomes so great that the muscle can-
not lift it at all and so does no work when stimulated. Starting
then from the time when the muscle carried no load and did
no work, we pass with increasing weights, through phases in
which it does more and more work, until with one particular
load it does the greatest amount possible to it with that stim-
ulus: after that, with increasing loads less work is done, until
finally a load is reached with which the muscle again does no
work. What is true of one muscle is of course true of all,
and what is true of work done against gravity is true of all
muscular work, so that there is one precise load with which
a beast of burden or a man can do the greatest possible
amount of work in a day. With a lighter or heavier load the
distance through which it can be moved will be more or less,
but the actual work done always less. In the living Body,
however, the working of the muscles depends so much on
other things, as the due action of the circulatory and respira-
tory systems and the nervous energy or "grit" (upon which
the stimulation of the muscles depends) of the individual
man or beast, that the greatest amount of work obtainable is
not a simple mechanical problem as it is with the excised
muscle.

From what precedes it is % clear that the molecular changes
which take place in a contracting muscle fibre are eminently
susceptible of modification by slight changes in its environ-
ment. The evidence indicates that the contractility of a
muscle depends, not upon a vital force entirely distinct from
ordinary inanimate forces, but upon an arrangement of its
material elements which is only maintained under certain
conditions and is eminently modifiable by changes in the
surroundings.

Influence of the Form of the Muscle on its Working
Power. The amount of work that any muscle can do de-
pends of course largely upon its physiological state; a healthy
well -nourished muscle can do more than a diseased or starved
one; but allowing for such variations the work which can be

THE PROPERTIES OF MUSCULAR TISSUE. 137

done by a muscle varies with its form. The thicker the mus-
cle, that is the greater the number of fibres present in a sec-
tion made across the long axes of the fasciculi, the greater
the load that can be lifted or the other resistance that can be
overcome. On the other hand, the extent through which a
muscle can move a weight increases with the length of its
fasciculi. A muscle a foot in length can contract more than
a muscle six inches long, and so would move a bone through
a greater distance, provided the resistance were not too great
for its strength. But if the shorter muscle had double the
thickness, then it could lift twice the weight that the longer
muscle could. We find in the Body muscles constructed on
both plans; some to have a great range of movement, others
to overcome great resistance, besides numerous intermediate
forms which cannot be called either long and slender or short
and thick; many short muscles for example are not specially
thick, but are short merely because the parts on which they
act lie near together. It must be borne in mind, too, that
many apparently long muscles are really short stout ones —
those namely in which a tendon runs down the side or middle
of the muscle, and has the fibres inserted obliquely into it.
The muscle (gastrocnemius) in the calf of the leg for instance
(Fig. 53, B) is really a short stout muscle, for its working
length depends on the length of its fasciculi and these are
short and oblique, while its true cross-section is that at right
angles to the fasciculi and is considerable. The force with
which a muscle can shorten is very great. A frog's muscle of
1 square centimeter (0.39 inch) hi section can just lift 2800
grams (98.5 ounces), and a human muscle of the same area
more than twice as much.

Muscular Elasticity. A clear distinction must be made
between elasticity and contractility. Elasticity is a physical
property of matter in virtue of which various bodies tend to
assume or retain a certain shape, and when removed from it,
forcibly to return to it. When a spiral steel spring is stretched
it will, if let go, "contract" in a certain sense, by virtue of its
elasticity, but such a contraction is clearly quite different
from a muscular contraction. The spring will only contract
as a result of previous distortion; it cannot originate a change
of form, while the muscle can actively contract or change its
shape when a stimulus acts upon it, and that without being
previously stretched. It does not merely tend to return to a

138 ^ ^ THE HUMAN BODY.

natural shape from which it has been removed, but it assumes
a quite new natural shape, so that physiological contractility
is a different thing from mere physical elasticity; the essen-
tial difference being that the coiled spring or a 'stretched band
only gives back mechanical work which has already been spent
on it, while the muscle originates work independently of any
previous mechanical stretching. In addition to their contrac-
tility, however, muscles are highly elastic. If a fresh muscle
be hung up and its length measured, and then a weight be
hung upon it, it will stretch just like a piece of india-rubber,
and when the weight is removed, provided it has not been so
great as to injure the muscle, the latter will return passively,
without any stimulus or active contraction, to its original
form. In the Body all the muscles are so attached that they
are usually a little stretched beyond their natural resting
length; so that when a limb is amputated the muscles divided
in the stump shrink away considerably. By this stretched
state of the resting elastic muscles two things are gained. In
the first place when the muscle contracts it is already taut,
there is no "slack" to be hauled in before it pulls on the
parts attached to its tendons: and, secondly, as we have
already seen the working power of a muscle is increased by
the presence of some resistance to its contraction, and this is
always provided for from the first, by having the origin and
insertion of the muscles so far apart as to be pulling on it
when it begins to shorten.

The Electrical Currents of Muscle. When a muscle is
exposed in the body or carefully removed from it and suitable
electrodes connected with a sensitive galvanometer are applied
to different parts of its surface, there is nearly always to be
found evidence of a difference of electric potential between
different parts of the muscle. These differences give rise to
currents which are shown by the galvanometer to travel
through the wires of the circuit from any central portion of
the muscle to any part nearer one end, or from any part of the
belly to a tendon. The less injured the muscle the more
feeble are these currents, and in very fresh and very carefully
exposed muscles they may be absent altogether. They are
probably altogether absent from perfectly uninjured resting
muscles, and when present in a resting muscle are due to the
fact that any more living part of a muscle is electrically posi-
tive to a more injured or dead. When a muscle is exposed

THE PROPERTIES OF MUSCULAR TISSUE. 139

its thinner ends die more quickly than its central parts, or
the ends are directly injured when the muscle is cut across to
remove it from the animal; and in that way the currents so
usually observable arise. When all of a muscle is dead, its
surface is isoelectric; no currents can be led oif from it.

Even a quite uninjured muscle is however, capable, of giv-
ing rise to currents when it contracts, and these currents
pass in such direction as to show that a portion of muscle
in contraction is electronegative to a portion at rest. If a
curarized muscle be stimulated at one point, its contraction
commences at that point and travels from it over the remainder
of the muscle; so that by the time a distant portion is in con-
traction the part which just contracted has come to rest. By
electrodes suitably applied it can be observed that immedi-
ately after the stimulation the region of muscle close to the
point of stimulation is electro-negative to a more distant part;
but that afterwards, when a distant portion is in contraction
and the stimulated region has returned to rest, the reverse is
the case. Electrically, therefore, any contracting part of a
muscle has to any resting part a relation similar to that of a
dying or injured part of a muscle to an uninjured. The cur-
rents which arise in consequence of the changes going on in
contracting muscle are known as the action currents to dis-
tinguish them from the resting currents due to unequal rates
of death usually found between different parts of an exposed'
muscle in rest.

When a muscle is stimulated through its nerve the action
current is less easy to demonstrate, because the nerve fibres
branch all through the muscle and stimulate all parts of it at
once, and throw all simultaneously into contraction. The cur-
rent may, however, be shown indirectly. A muscle is removed
with its nerve attached and electrodes put on it — one, for ex>-
ample, on the middle of the belly and the other on the tendon,
so as to show on the galvanometer a resting current. If the
muscle be now made to contract by stimulating its nerve the
current is diminished, or, as is said, shows a negative varia-
tion. The cause of this is as follows : The amount of resting
current depends on the difference between the less injured
belly of the muscle and the injured end; anything which
makes these two less different electrically must diminish this
current; and as contracted muscle is electrically like dying
muscle, when we throw the whole into activity the previously

140 THE HUMAN BODY.

existing difference is less than it was, and this the galva-
nometer shows as the negative variation.

Secondary Contraction. It is possible to use the action
current of one muscle to stimulate the nerve of a second and
produce a contraction. For this purpose two frogs' muscles,
A and B, are carefully dissected out with their nerves at-
tached. The nerve of B is laid over A so that one part of it
lies on the belly and another on the tendon. If the nerve
of A be stimulated by a single induction shock, for each
contraction of A we get a contraction of B, the negative
variation of the muscle current of A being the stimulus for
the nerve of B.

Secondary Tetanus. If the nerve of A be given rapidly
repeated stimuli so as to throw that muscle into tetanic con-
traction, B is also tetanized. This is of importance, as tend-
ing to show that the tetanus of A is really a compound con-
traction, although to the eye or as recorded by a lever it is one
unbroken shortening. If the electrical condition of A
remained uniform during contraction, there should be no
fetanus of B, but merely a simple contraction due to the set-
ting up of the action current or negative variation when A
commenced to contract, and a second due to the cessation of
this current when A came to rest again. The tetanus of B
must be due to rapidly repeated electrical variations in A, and
these probably correspond to the potentially separate con-
tractile changes going on in A, and fused into its apparently
uniform tetanic contraction.

The Source of Muscular Energy will be more fully dis-
cussed in the chapter on nutrition, but a few of the main
points may be mentioned here. A muscle where it contracts
is able to do work by using energy set free by chemical
changes occurring within it, as a steam-engine does work by
using the energy set free by the chemical changes occurring
in the combustion of its fuel; and as in the steam-engine,
so here, the fundamental change is an oxidative one, though in
the muscle a very indirect oxidation. A fresh frog's muscle
deprived of blood contains no uncombined oxygen; hung up
in an atmosphere of pure nitrogen it can be made to contract
and do a great deal of work before it dies and passes into
rigor mortis. While doing this work it gives off carbon-diox-
ide gas and becomes acid from the formation (probably) of
sarcolactic acid, but there does not appear to occur any ap-

THE PROPERTIES OF MUSCULAR TISSUE. 141

preciable increase of oxygen-containing nitrogen compounds
in it. As, under the conditions of the experiment, no free
oxygen is available, the carbon dioxide must be derived from
the breaking down of something present in the muscle; and
as the formation of sarcolactic acid varies in amount with
that of carbon dioxide, and both increase with the work done
by the muscle, it would seem as if the energy set free were
obtained by the breaking down of some highly unstable
non-nitrogenous energy-yielding matter stored in the muscle.
And such a view gains support from the fact that a man
doing hard muscular work gives off per hour a great deal
more carbon dioxide through his lungs than a man at rest,
and does not give off any or very little more nitrogenous
waste matter.

But a muscle placed as above described and made to work
passes into rigor sooner than a muscle similarly situated and
left at rest : and this shows that work tends to favor the pro-
duction of myosin, or rather of its immediate precursor myo-
sinogen, in the muscle : so here we get some evidence that the
nitrogenous muscle constituents are influenced and altered
though not oxidized during work. Further, when a muscle
passes into rigor it gives off carbon-dioxide gas, whether it
has been worked previously or not; if so situated as to be
deprived of all exterior sources of supply, it gives off less
when becoming rigid after work than when becoming rigid
without having been worked; but the difference is almost
accurately accounted for by the greater quantity of carbon
dioxide the working muscle had previously given out. This
suggests that the chemical phenomena of rigor and of work
are essentially alike, being merely carried to an extreme in
the former.

Most of the facts can be accounted for by the supposition
that there is in living muscle a store of an unstable substance
containing nitrogen, hydrogen, carbon, and oxygen. For this
hypothetic substance the name inogen has been proposed.
During work inogen is used up and broken into a highly
oxidized part, carbon dioxide; an oxidized body containing
carbon and hydrogen, as sarcolactic acid (C3H603); and a
third body allied to myosinogen and containing all the nitro-
gen and some of the oxygen, carbon, and hydrogen of the
original inogen. In the products of this alteration stronger
chemical affinities are satisfied than in the original compound,

142 THE HUMAN BODY.

and thus energy is liberated and used by the muscle. In the
ordinary course of events the carbon dioxide is carried off by
blood and lymph and eliminated from the Body; the sarco-
lactic or other similar substance or substances are also carried
off and oxidized elsewhere to form carbon dioxide and water
and be then eliminated ; but the nitrogen-containing product
remains behind, and with the help of fresh oxygen and of
other food material brought by the blood is reconstructed into
the original inogen. In the excised muscle there is but
scant store of material for repair; carbon dioxide is given
off when the muscle contracts, and the sfrcolactic acid and
nitrogen-containing product accumulate: the latter then
undergoes further changes, and ultimately becomes myosin.
If the excised muscle be thrown into rigor quickly (as by
heat), then the inogen is at once broken up, forming myosin
and carbon dioxide and sarcolactic acid: if it be worked
for a time before being thrown into rigor, then some of its
inogen will have been already broken up, so there will be less
to give rise to carbon dioxide at the moment of rigor, but the
missing amount is found in that given off during work. If
some such view as this, which may be called the " inogen
theory," be the correct one, then the energy liberated by a
resting muscle passing into rigor must take some other form
than muscular work. As a matter of fact a good deal of heat
is liberated during death stiffening, but whether sufficient to
account for all the missing energy is by no means clear. The
whole subject of the immediate source of muscular work is
still in much need of elucidation.

Physiology of Plain Muscular Tissue. What has hither-
to been said applies especially to the skeletal muscles; but
in the main it is true of the unstriped muscles. These also
are irritable and contractile, but their changes of form are
much more slow than those of the striated fibres. Upon
stimulation, a longer period of latent excitement elapses
before the contraction commences and when, finally, this
takes place it is comparatively very slow, gradually attaining
a maximum and gradually passing away.

Unstriped muscular tissue has a remarkable power of
remaining in the contracted state for long periods : the mus-
cular coats of many small arteries, for example, are rarely
relaxed ; sometimes they may be more contracted, sometimes
less, but in health seldom if eyer completely at rest. There

THE PROPERTIES OF MUSCULAR TISSUE. 143

seems to be some connection between that arrangement of
the contractile substance which shows itself under the micro-
scope as striation and the power of rapid contraction, since
we find that the heart, which is not a skeletal or voluntary
muscle but yet one that contracts rapidly, agrees with these
in having its fibres striated. This connection is further illus-
trated by facts of comparative anatomy: insects are, as a
rule, rapidly moving animals, and they are characterized by
very marked striation of nearly all their muscular tissue;
while in the slow-moving molluscs nearly all the muscular
tissue is unstriped except in a few, as Pecten, which make
rapid movements, and in that genus the muscles concerned
in producing these movements are striated..

CHAPTER XL
MOTION AND LOCOMOTION.

The Special Physiology of the Muscles. Having now
considered separately the structure and properties in general
of the skeleton, the joints, and the muscles, we may go on to
consider how they all work together in the Body. Although
the properties of muscular tissue are everywhere the same,
the uses of different muscles are very varied, by reason of the
different parts with which they are connected. Some are
muscles of respiration, others of deglutition; many are known
as flexors because they bend joints, others as extensors because
they straighten them. The exact use of any particular mus-
cle, acting alone or in concert with others, is known as its
special physiology, as distinguished from its general physiol-
ogy, or properties as a muscle without reference to its use as
a muscle in a particular place. The functions of those mus-
cles forming parts of the physiological mechanisms concerned
in breathing and swallowing will be studied hereafter; for
the present we may consider the muscles which co-operate in
maintaining postures of the Body; in producing movements
of its larger parts with reference to one another; and in pro-
ducing locomotion or movement of the whole Body in space.

In nearly all cases the striped muscles carry out their func-
tions with the co-operation of the skeleton, since nearly all
are fixed to bones at each end, and when they contract pri-'
marily move these, and only secondarily the soft parts attached
to them. To this general rule there are, however, exceptions.
The muscle for example which lifts the upper eyelid and
opens the eye arises from bone at the back of the orbit, but
is inserted, not into bone, but into the eyelid directly; and
similarly other muscles arising at the back of the orbit are
directly fixed to the eyeball in front and serve to rotate it
on the pad of fat on which it lies. Many facial muscles again
have no direct attachment whatever to bones, as for example

144

MOTION AND LOCOMOTION. 145

the muscle (orMcularis oris) which surrounds the mouth-
opening, and by its contraction narrows it and purses out the
lips; or the orlicularis palpebrarum which similarly sur-
rounds the eyes and when it contracts closes them.

Levers in the Body. When the muscles serve to move
bones the latter are in nearly all cases to be regarded as levers
whose fulcra lie at the joint where the movement takes place.
Examples of all the three forms of levers recognized in me-
chanics are found in the Human Body.

Levers of the First Order. In this form (Fig. 63) the
fulcrum or fixed point of support lies between the " weight "

I F \

P jj^ W

FIG. 63.— A lever of the first order. .F, fulcrum ; P, power ; W, resistance or

weight.

or resistance to be overcome and the " power " or moving
force, as shown in the diagram. The distance PF, from the
power to the fulcrum, is called the "power-arm;" the dis-
tance FW is the " weight-arm." When power-arm and
weight-arm are equal, as is the case in the beam of an ordi-
nary pair of scales, no mechanical advantage is gained, nor is .
there any loss or gain in the distance through which the weight
is moved. For every inch through which P is depressed, W
will be raised an equal distance. When the power-arm is
longer than the other, then a smaller force at P will raise a
larger weight at W, the gain being proportionate to the dif-
ference in the lengths of the arms. For example if PF is
twice as long as FW, then half a kilogram applied at P will
balance a whole kilogram at Tf, and just more than half a
kilogram would lift it ; but for every centimeter through
which P descended, W would only be lifted half a centimeter.
On the other hand when the weight-arm in a lever is longer
than the power-arm, there is loss in force but a gain in the
distance through which the weight is moved.

Examples of the first form of lever are not numerous in
the Human Body. One is afforded in the nodding move-
ments of the head, the fulcrum being the articulations be-
tween the skull and the atlas. When the chin is elevated
the power is applied to the skull, behind the fulcrum, by

146 THE HUMAN BODY.

small muscles passing from the vertebral column to the occi-
put; the resistance is the excess in the weight of the part of
the head in front of the fulcrum over that behind it, and is
not great. To depress the chin as in nodding does not neces-
sarily call for any muscular effort, as the head will fall for-
ward of itself if the muscles keeping it erect cease to work,
as those of us who have fallen asleep during a dull discourse
on a hot day have learnt. If the chin however be depressed
forcibly, as in the athletic feat of suspending one's self by
the chin, the muscles passing from the chest to the skull in
front of the atlanto-occipital articulation are called into play.
Another example of the employment of the first form of lever
in the Body is afforded by the curtsey with which a lady
salutes another. In curtseying the trunk is bent forward at
the hip-joints, which form the fulcrum; the weight is that of
the trunk acting as if all concentrated at its centre of gravity,
which lies a little above the sacrum and behind the hip-joints;
and the power is afforded by muscles passing from the thighs
to the front of the pelvis.

Levers of the Second Order. In this form the weight or
resistance is between the power and the fulcrum. The
power-arm PFis always longer than the weight-arm WF,
and so a comparatively weak force can overcome a consider-
able resistance. But it is disadvantageous so far as regards
rapidity and extent of movement, for it is obvious that when
P is raised a certain distance W will be moved a less distance
in the same time. As an example of the employment of such
levers (Fig. 64) in the Body, we may take the act of standing
on the toes. Here the foot represents the lever, the fulcrum
is at the contact of its fore part with the ground ; the weight

F

W

FIG. 64.— A lever of the second order. F, fulcrum ; P, power ; TF, weight. The
arrows indicate the direction in which the forces act.

is that of the Body acting down through the ankle-joints at
Ta, Fig. 65; and the power is the great muscle of the calf
acting by its tendon inserted into the heel-bone (Ca, Fig. 65).
Another example is afforded by holding up the thigh when
one foot is kept raised from the ground, as in hopping on the

MOTION AND LOCOMOTION. 147

other. Here the fulcrum is at the hip-joint, the power is ap-
plied at the knee-cap by a great muscle (rectus femoris) which

Spl

FIG. 65. — The skeleton of the foot from the outer side. Ta, surface with which
the leg-bones articulate ; Ca, the calcaueum into which the tendon (tendo Achillis)
of the calf muscle is inserted ; M5. the metatarsal bone of the fifth digit ; N, the
scaphoid boue-, C/, CII, GUI, first, second, and third cuneiform bones ; Cb, the
cuboid bone.\

is inserted there and arises from the pelvis; and the weight
is that of the whole lower limb acting at its centre of gravity,
which lies somewhere in the thigh between the hip and
knee-joints, that is between the fulcrum and the point of ap-
plication of the power.

Levers of the Third Order. In these (Fig. 66) the power-
is between the fulcrum and the weight. In such levers the
weight-arm is always longer than the power-arm, so the power
works at a mechanical disadvantage, but swiftness and range
of movement are gained. It is the lever most commonly used
in the Human Body. For example, when the forearm is
bent up towards the arm, the fulcrum is the elbow-joint, the
power is applied at the insertion of the biceps muscle (Fig.
52) into the radius and of another muscle (not represented
in the figure, the brachialis anticus, into the ulna), and the

W F

FIG. 66. -A lever of the third order. F, fulcrum ; P, power ; W, weight.

weight is that of the forearm and hand, with whatever may
be contained in the latter, acting at the centre of gravity of
the whole somewhere on the distal side of the point of appli.

148 THE HUMAN BODY.

cation of the power. In the Body the power-arm is usually
very short so as to gain speed arid range of movement, the
muscles being powerful enough to still do their work in spite
of the mechanical disadvantage at which they are then placed.
The limbs are thus made much more shapely than would be
the case were the power applied near or beyond the weight.

It is of course only rarely that simple movements as those
described above take place. In the great majority of those
executed several or many muscles co-operate.

The Loss to the Muscles from the Direction of their Pull.
It is worthy of note that, owing to the oblique direction in
which the muscles are commonly inserted into the bones,
much of their force is lost so far as producing movement is
concerned. Suppose the log of wotid in the diagram (Fig.
67) to be raised by pulling on the rope in the direction a\ it
is clear at first that the rope will act at a great disadvantage;
most of the pull transmitted by it will be exerted against the
pivot on which the log hinges, and only a small fraction be
available for elevating the latter. But the more the log is
lifted, as for example into the position indicated by the dotted
lines, the more useful will be the direction of the pull, and the
more of it will be spent on the log and the less lost unavail-
ingly in merely increasing the pressure at the hinge. If we
now consider the action of the biceps (Fig. 52) in flexing the
elbow-joint, we see similarly that the straighter the joint is,
the more of the pull of the muscle is wasted. Beginning

FIG. 67.— Diagram illustrating the disadvantage of an oblique pull.

with the arm straight, it works at a great disadvantage, but
as the forearm is raised the conditions become more and more

MOTION AND LOCOMOTION. 149

*

favorable to the muscle. Those who have practised the gym-
nastic feat of raising one's self by bending the elbows when
hanging by the hands from a horizontal bar know practically
that if the elbow-joints are quite straight it is very hard to
start; and that, on the other hand, if they are kept a little
flexed at the beginning the effort needed is much less; the
reason being of course the more advantageous direction of
traction by the biceps in the latter case.

Experiment proves that the power with which a muscle
can contract is greatest at the commencement of its short-
ening, the very time at which, we have just seen, it works
at most mechanical disadvantage; in proportion as its force
becomes less the conditions become more favorable to it.
There is, however, it is clear, nearly always a considerable
loss of power in the working of the skeletal muscles, strength
being sacrificed for variety, ease, rapidity, extent, and ele-
gance of movement.

Postures. The term posture is applied to those positions
of equilibrium of the Body which can be maintained for some
time, such as standing, sitting, or lying, compared with leap-
ing, running, or falling. In all postures the condition of
stability is that the vertical line drawn through the centre of
gravity of the body shall fall within the basis of support
afforded by objects with which it is in contact ; and the
security of the posture is proportionate to the extent of this
base, for the wider it is the less is the risk of the perpendicu-
lar through the centre of gravity falling outside of it on slight
displacement.

The Erect Posture. This is pre-eminently characteristic
of man, his whole skeleton being modified with reference to
it. Nevertheless the power of maintaining it is only slowly
learnt in the first years after birth, and for a long while it is
unsafe. And though finally we learn to stand erect without
conscious attention, the maintenance of that posture always
requires the co-operation of many muscles, co-ordinated by
the nervous system. The influence of the latter is shown by
the fall which follows a severe blow on the head, which may
nevertheless have fractured no bone nor injured any muscle :
the concussion of the brain, as we say, "stuns" the man,
and until its effects have passed off he cannot stand upright.
In standing with the arms straight by the sides and the feet
together the centre of gravity of the whole adult Body lies

150

THE HUMAN BODY.

(L

in the articulation between the sacrum and the last lumbar
vertebra, and the perpendicular drawn from it will reach the
ground between the two feet, within the basis of support af-
forded by them. With the feet close together, however, the
posture is not very stable, and in standing we commonly
make it more so by slightly separating them so as to increase
the base. The more one foot is in front
of the other the more swaying back and
forward will be compatible with safety ; and
the greater the lateral distance separating
them the greater will be the lateral sway
which is possible without falling. Conse-
quently we see that a man about to make
great movements with the upper part of
his Body, as in fencing or boxing, or a sol-
I J*H dier preparing for the bayonet exercise,

always commences by thrusting one foot
forwards obliquely, so as to increase his
basis of support in both directions.

The ease with which we can stand is
largely dependent upon the way in which
the head is almost balanced on the top of
the vertebra] column, so that but little
muscular effort is needed to keep it up-
right. In the same wray the trunk is almost
balanced on the hip joints, but not quite>
its centre of gravity falling rather behind
them; so that just as some muscular effort
is needed to keep the head from falling
forwards, some is needed to keep the trunk
from toppling backwards at the hips. In
a similar manner other muscles are called

illto PlaJ at otner Joints : as between the
lines) which pass before vertebral column and the pelvis, and at

and behind the joints r

and by their balanced the knees and ankles i and thus a certain

activity keep the joints . _ ,,

rigid and the body erect, rigidity, due to muscular effort, extends all
along the erect Body : which, on account of the flexibility of its
joints, could not otherwise be balanced on its feet, as a
statue can. Beginning (Fig. 68) at the ankle-joint, we find
it kept stiff in standing by the combined and balanced con-
traction of the muscles passing from the heel to the thigh,
and from the dorsum of the foot to the shin-bone (tibia).

il I J

FIG. 68.— Diagram ii-

MOTION AND LOCOMOTION. 151

Others passing before and behind the knee- joint keep it from
yielding; and so at the hip-joints: and others again, lying in
the walls of the abdomen and along the vertebral column,
keep the latter rigid and erect on the pelvis; and finally the
skull is kept in position by muscles passing from the sternum
and vertebral column to it, in front of and behind the occipi-
tal condyles.

Locomotion includes all the motions of the whole Body
in space, dependent on its own muscular efforts: such as
walking, running, leaping, and swimming.

Walking. In walking the Body never entirely quits the
ground, the heel of the advanced foot touching the ground in
each step before the toe of the rear foot leaves it. The ad-
vanced limb supports the Body, and the foot in the rear at
the commencement of each step propels it.

Suppose a man standing with his heels together to com-
mence to walk, stepping out with the left foot; the whole
Body is at first inclined forwards, the movement taking place
mainly at the ankle-joints. By this means the centre of
gravity would be thrown in front of the base formed by the
feet and a fall on the face result, were not simultaneously the
left foot slightly raised by bending the knee and then swung
forwards, the toes just clear of the ground and, in goojd
walking, the sole nearly parallel to it. When the step is
completed the left knee is straightened and the sole placed
on the ground, the heel touching it first, and the base of sup-
port being thus widened from before back, a fall is prevented.
Meanwhile the right leg is kept straight, but inclines for-
wards above with the trunk when the latter advances, arid as
this occurs the sole gradually leaves the ground, commencing
with the heel. When the step of the left leg is completed the
great toe of the right alone is in contact with the support.
With this a push is given which sends the trunk on over the
left leg, which is now kept rigid, except at the ankle-joint;
and the right knes being bent that limb swings forwards,
its foot just clearing the ground as the left did before. The
Body is meanwhile supported on the left foot alone, but when
the right completes its step the knee of that leg is straight-
ened and the foot thus placed, heel first, on the ground.
Meanwhile the left foot has been gradually leaving the
ground, and its toes only are at that moment upon it: from
these a push is given, as before, with the right foot, and the

152 THE HUMAN BODY.

knee being bent so as to raise the foot, the left leg swings for
wards at the hip-joint to make a fresh step.

During each step the whole Body sways up and down
and also from side to side. It is highest at the mo-
ment when the advancing trunk is vertically over the
foot supporting it, and then sinks until the moment
when the advancing foot touches the ground, when it is
lowest. From this moment it rises as it swings forward
on this foot, until it is vertically over it, and then sinks
again until the other touches the ground; and so on. At
the same time, as its weight is alternately transferred from
the right to the left foot and vice versa, there is a slight
lateral sway, commonly more marked in women than in men,
arid which when excessive produces an ugly "waddling"
gait.

The length of each step is primarily dependent on the
length of the legs; but can be controlled within wide limits
by special muscular effort. In easy walking little muscular
work is employed to carry the rear leg forwards after it has
given its push. When its foot is raised from the ground it
swings on, like a pendulum; but in fast walking the muscles,
passing in front of the hip- joint, from the pelvis to the limb,
by their contraction forcibly carry the leg forwards. The
easiest step, that in which there is most economy of labor, is
that in which the limb is let swing freely, and since a short
pendulum swings faster than a longer, the natural step of
short-legged people is quicker than that of long-legged ones.

In fast walking the advanced or supporting leg also aids in
propulsion; the muscles passing in front of the ankle-joint
contracting so as to pull the Body forwards over that foot
and aid the push from the rear foot. Hence the fatigue and
pain in front of the shin which is felt in prolonged, very fast
walking. From the fact that each foot reaches the ground
heel first, but leaves it toe last, the length of each stride is
increased by the length of the foot.

Running. In this mode of progression there is a moment
in each step when both feet are off the ground, the Body
being unsupported in the air. The toes alone come in con-
tact with the ground at each step, and the knee-joint is not
straight when the foot reaches the ground. When the rear
foot is to leave the support, the knee is suddenly straight-
ened, and at the same time the ankle-joint is extended so as

MOTION AND LOCOMOTION. 153

to push the toes forcibly on the ground and give the whole
Body a powerful push forwards and upwards. Immediately
after this the knee is greatly flexed and the foot raised from
the ground, and this occurs before the toes of the forward
foot reach the latter. The swinging leg in each step is vio-
lently pulled forwards and not suffered to swing naturally, as
in walking. By this the rapidity of the succession of steps
is increased, and at the same time the stride is made greater
by the sort of one-legged leap that occurs through the jerk
given by the straightening of the knee of the rear leg just
before it leaves the ground.

Leaping. In this mode of progression the Body is raised
completely from the ground for a considerable period. In a
powerful leap the ankles, knees, and hip-joints are all flexed
as a preparatory measure, so that the Body assumes a crouch-
ing attitude. The heels, next, are raised from the ground and
the Body balanced on the toes. The centre of gravity of the
Body is then thrown forwards, and simultaneously the flexed
joints are straightened, and by the resistance of the ground,
the Body receives a propulsion forwards; much in the same
way as a ball rebounds from a wall. The arms are at the
same time thrown forwards. In leaping backwards, the Body
and arms are inclined in that direction; and in jumping ver-
tically there is no leaning either way and the arms are kept
by the sides.

Hygiene of the Muscles. The healthy working of the
muscles needs of course a healthy state of the Body gener-
ally, so that they shall be supplied with proper materials for
growth and repair, and have their wastes rapidly and effi-
ciently removed. In other words, good food and pure air are
necessary for a vigorous muscular system, a fact which train-
ers recognize in insisting upon a strict dietary, and in super-
vising generally the mode of life of those who are to engage
in athletic contests. The muscles should also not be exposed
to any considerable continued pressure, since this interferes
with the flow of blood and lymph through them.

As far as the muscles themselves are directly concerned,
exercise is the necessary condition of their best development.
A muscle which is permanently unused degenerates and is
absorbed, little finally being left but the connective tissue of
the organ and a few muscle fibres filled with oil-drops. This
is well seen in cases of paralysis dependent on injury to the

154 THE HUMAN BODY.

nerves. In such cases the muscles may themselves be per*
fectly healthy at first, but lying unused for weeks they become
altered, and finally, when the nervous injury has been healed,
the muscles may be found incapable of functional activity.
The physician therefore is often careful to avoid this by exer-
cising the paralyzed muscles daily by means of electrical
shocks sent through the part, while at the same time he tries
to restore the nerves; passive exercise, as by proper massage,
is frequently of great use in such cases. The same fact is
illustrated by the feeble and wasted condition of the muscles
of a limb which has been kept for some time in splints. After
the latter have been removed it is only slowly, by judicious
and persistent exercise, that the long-idle muscles regain
their former size and power. The great muscles of the
" brawny " arm of the blacksmith or wrestler illustrate the
reverse fact, the growth of the muscles by exercise. Exer-
cise, however, must be judicious; repeated frequently to the
point of exhaustion it does harm; the period of repair is' not
sufficient to allow replacement of the parts used in work, and
the muscles thus waste under too violent exercise as with too
little. Rest should alternate with work, and that regularly,
if benefit is to be obtained. Moreover, violent exercise should
never be suddenly undertaken by one unused to it, not
only lest the muscles suffer, but because muscular effort
greatly increases the work of the heart, not merely because
more blood has to be sent to the muscles themselves, but they
produce great quantities of carbon dioxide, which must be
carried off in the blood to the lungs for removal from the
Body, and the heart must work harder to send the blood faster
through the lungs, and at the same time the breathing be
hastened so as to renew the air in those organs faster. The
least evil result of throwing too violent work on the heart
and lungs in this way is represented by being " out of
breath," which is advantageous insomuch as it may lead to a
cessation of the exertion. But much more serious, and
sometimes permanent, injuries of either the circulatory or
respiratory organs may be caused by violent and prolonged
efforts without due previous training. No general rule can
be laid down as to the amount of exercise to be taken; for a
healthy man in business the minimum would perhaps be rep-
resented by a daily walk of five miles.

Varieties of Exercise. In walking and running the

MOTION AND LOCOMOTION. 155

muscles chiefly employed are those of the lower limbs and
trunk. This is in part true of rowing, which when good is
performed much more by the legs than the arms: especially
since the introduction of sliding seats. Hence any of these
exercises alone is apt to leave the muscles of the chest and
arms imperfectly exercised. Indeed, no one exercise employs
equally or proportionately all the muscles: therefore gym-
nasia in which various feats of agility are practised, so as to
call different parts into play, have very great utility. It
should be borne in mind, however, that the legs especially
need strength; while the upper limbs, in which delicacy of
movement, as a rule, is more desirable than power, do not re-
quire so much exercise; and the fact that gymnastic exercises
are commonly carried on indoors is a great drawback to their
value. When the weather permits, out-of-door exercise is far
better than that carried on in even the best ventilated and
lighted gymnasium. For those who are so fortunate as to
possess a garden there is no better exercise, at suitable sea-
sons, than an hour's daily digging in it; since this calls into
play nearly all the muscles of the Body; while of games, the
modern one of lawn tennis is perhaps the best from a hygienic
view that has ever been invented, since it not only demands
great muscular agility in every part of tne Body, but trains
the hand to work with the eye in a way that walking, run-
ning, rowing, and similar pursuits do not. For the same
reasons baseball, cricket, and boxing are excellent.

Exercise in Infancy and Childhood. Young children
have not only to strengthen their muscles by exercise, but
also to learn to use them. Watch an infant trying to con-
vey something to its mouth, and you will see how little
control it has over its muscles. On the other hand, the
healthy infant is never at rest when awake; it constantly
throws its limbs around, grasps at all objects within its
reach, coils itself about, and so gradually learns to exercise its
powers. It is a good plan to leave every healthy child more
than a few months old several times daily on a large bed, or
even on a rug or carpeted floor, with as little covering as is
safe, and that as loose as possible, and let it wriggle about as it
pleases. In this way it will not only enjoy itself thoroughly,
but gain strength and a knowledge of how to use its limbs.
To keep a healthy child swathed all day in tight and heavy
clothes is cruelty.

156 THE HUMAN BODY.

When a little later the infant commences to crawl it is safe
to permit it to as much as it wishes, but unwise to tempt it to
do so when disinclined : the bones and muscles are still feeble
and may be injured by too much work. The same is true of
learning to walk.

From four or five to twelve years of age almost any form
of exercise should be permitted, or even encouraged. During
this time, however, the epiphyses of many bones are not firmly
united to their shafts, and so anything tending to throw too
great a strain on the joints should be avoided. After that up
to commencing manhood or maidenhood any kind of out-
door exercise for healthy persons is good, and girls are all the
better for being allowed to join in their brothers' sports.
Half of the debility and general ill-health of so many of our
women is the consequence of deficient exercise during early
life; and the day, which fortunately seems approaching,
which will see dolls as unknown to or as despised by healthy
girls as by healthy boys will see the beginning of a great im-
provement in the stamina of the female portion of our popu-
lation.

Exercise in Youth should be regulated largely by sex; not
that women are to be shut up and made pale, delicate, and
unfit to share the duties or participate fully in the pleasures
of life; but the other calls on the strength of the young woman
render vigorous muscular work often unadvisable, especially
under conditions where it is apt to be followed by a chill.

A healthy boy or young man may do nearly anything; but
until twenty-two or twenty-three very prolonged effort is un-
advisable. The frame is still not firmly knit or as capable of
endurance as it will subsequently become.

Girls should be allowed to ride or play out-door games in
moderation, and in any case should not be cribbed in tight
stays or tight boots. A flannel dress and proper lawn tennis
shoes are as necessary for the healthy and safe enjoyment of an
afternoon at that game by a girl as they are for her brother in the
baseball field. Eowing is excellent for girls if there be any
one to teach them to do it properly with the legs and back,
and not with the arms only, as women are so apt to row.
Properly practised it strengthens the back and improves the
carriage.

Exercise in Adult Life. Up to forty a man may carry on
safely the exercises of youth, but after that sudden efforts

MOTION AND LOCOMOTION. 157

should be avoided. A lad of twenty-one or so may, if trained,
safely run a quarter-mile race, but to a man of forty-five it
would be dangerous, for with the rigidity of the cartilages
and blood-vessels which begins to show itself about that time
comes a diminished power of meeting a sudden violent de-
mand. On the other hand, the man of thirty would more
safely than the lad of nineteen or twenty undertake one of
the long-distance walking matches which have lately been in
vogue; the prolonged effort would be less dangerous to him,
though a six-days' match, with its attendant loss of sleep,
cannot fail to be more or less dangerous to any one. Prob-
ably for one engaged in active business a walk of two or
three miles to it in the morning and back again in the after-
noon is the best and most available exercise. The habit
which Americans have everywhere acquired, of never walking
when they can take a street car, is certainly detrimental to
the general health; though the extremes of heat and cold to
which we are subject often render it unavoidable.

For women during middle life the same rules apply: there
should be some regular but not violent daily exercise.

In Old Age the needful amount of exercise is less, and it
is still more important to avoid sudden or violent effort.

Exercise for Invalids. This should be regulated under
medical advice. For feeble persons gymnastic exercises are
especially valuable, since from their variety they permit of
selection according to the condition of the individual; and
their amount can be conveniently controlled.

Training. If any person attempt some unusual exercise
he soon finds that he loses breath, gets perhaps a " stitch
in the side," and feels his heart beating with unwonted
violence. If he persevere he will probably faint — or vomit,
as is frequently seen in the case of imperfectly trained men at
the end of a hard boat-race. These phenomena are avoided
by careful gradual preparation known as " training." The
immediate cause of them lies in disturbances of the circula-
tory and respiratory organs, on which excessive work is
thrown.

CHAPTEE XH.
\
ANATOMY OF THE NERVOUS SYSTEM.

Nerve-Trunks. In dissecting the Human Body numerous
white cords are found which at first sight might be taken for
tendons. That they are something else however soon becomes
clear, since a great many of them have no connection with
muscles at all, and those which have usually enter somewhere
into the belly of the muscle, instead of being fixed to its ends
as most tendons are. These cords are nerve-trunks : followed
in one direction each (Fig. 69) will be found 'to break up into
finer and finer branches, until the subdivisions become too
small to be followed without the aid of a microscope. Traced
the other way the trunk will in most cases be found to in-
crease by the union of others with it, and ultimately to join
a much larger mass of different structure, from which other
trunks also spring. This mass is a nerve-centre. That end
of a nerve attached to the centre is naturally its central,
and the other its distal or peripheral end. Nerve-centres,
then, give origin to nerve-trunks; these latter spread all over
the Body, usually branching and becoming smaller and smaller
as they proceed from the centre; they finally become very
small, and how they ultimately end is not in__alLcas_es certain,
but it is known that some have sense-organs at their termina-
tions and others muscular fibres. The general arrangement
of the larger nerve-trunks of the Body is shown in Fig. 69.
Physically a nerve is not so tough or strong as a tendon of
the same size; it may readily be split up into longitudinal
strands, each of which consists of a number of microscopic
threads, the nerve-fibres, bound together by connective tissue.

Plexuses. Very frequently several neighboring nerve-
trunks send off communicating branches to one another, each
branch carrying fibres from one trunk to the other. Such
networks are called plexuses (Fig. 72), and through the inter-
changes taking place in them it often happens that the distal

158

ANATOMY OF THE NERVOUS SYSTEM.

159

brandies of a nerve-trunk contain fibres which it does not
possess as it leaves the centre to which it is connected.

FIG. 69.— Diagram illustrating the general arrangement of the nervous system.

Nerve-Centres. The great majority of the nerves take
their origin from the brain and spinal cord, which together
form the great cerebro-spinal centre. Some, however, com-

160 THE HUMAN BODY.

mence in rounded or oval masses which vary in size from that
of the kernel of an almond down to microscopic dimensions,
and which are widely distributed in the Body. Each of these
smaller scattered centres is called a ganglion, and the whole
of them are arranged in three sets. A considerable num-
ber of the largest are united directly to one another by
nerve-trunks, and also give off nerves to various organs, espe-
'cially to the blood-vessels and the viscera in the thoracic and
abdominal cavities. These ganglia and their branches form
the sympathetic nervous system, as distinguished from the
cerebro-spinal nervous system consisting of the brain and
spinal cord and the nerves springing from them. Of the re-
maining ganglia some are connected with various cerebro-
spinal trunks near their origin, while the rest, for the most
part very small and connected with the peripheral branches
of sympathetic or other nerves, are known as the sporadic
ganglia.

The Cerebro-Spinal Centre and its Membranes. Lying
inside the skull is the brain and in the neural canal of the
vertebral column the xpinal cord or spinal marrow, the cwo
being continuous through the foramen magnum of the oc-
cipital oone and forming the great cerebro-spinal nerve-centre.
This centre is bilaterally symmetrical throughout except for
slight differences on the surfaces of parts of the brain, which
are often found in the higher races of mankind. Both brain
and spinal cord are very soft and easily crushed, the con-
nective tissue and a peculiar supporting tissue (neuroglia)
which pervade them being delicate; accordingly both organs
are placed in nearly completely closed bony cavities and are
also enveloped by membranes which give them support. These
membranes are three in number. Externally is the dura
mater, very tough and strong and composed of white fibrous
and elastic connective tissues. In the cranium the dura
mater adheres by its outer surface to the inside of the skull
chamber, serving as the periosteum of its bones; this is
not the case in the vertebral column, where the dura mater
forms a loose sheath around the spinal cord and is only at-
tached here and there to the surrounding bones, which have
a separate periosteum of their own. The innermost membrane
of the cerebro-spinal centre, lying in immediate contact with
the proper nervous parts, is the pia mater, also made up of
white fibrous tissue interwoven with elastic fibres, but less

ANATOMY OF THE NERVOUS SYSTEM.

161

B

D

F-;

closely than in the dura mater, so as to form a less dense and
tough membrane. The pia mater
contains many blood-vessels which
break up in it into small branches
before entering the nervous mass
beneath. Covering the outside of
the pia mater is alayer of flat closely
fitting cells; a similar layer lines the
inside of the dura mater, and these
two layers are described as the third
membrane of the cerebro-spinal cen-
tre, called the arachnoid. In the
space between the two layers of the
arachnoid is contained a small quan-
tity of watery cerebro-spinal liquid.
The surface of the brain is folded
and the pia mater follows closely these
folds; the arachnoid often stretches
across them : in the spaces thus left
between it and the pia mater is con-
tained some of the cerebro-spinal
liquid.

The Spinal Cord (Fig. 70) is
nearly cylindrical in form, being
however a little wider from side to
side than dorsoventrally, and taper-
ing off at its posterior end. Its
average diameter is about 19 milli-
meters (f inch) and its length 0.43
meter (17 inches). It weighs 42.5
grams (1^ ounces). There is no
marked limit between the spinal cord
and the brain, the one passing grad-
ually into the other (Fig. 77)^ but
the cord is arbitrarily said to com-
mence opposite the outer margin of
the foramen magnum of the occipital
bone: from there it 'extends to the
articulation between the first and
second lumbar verteTira^here it
narrows off to a slender filament, the different levels.
filum terminate (cut off and represented separately at B in Fig

-8 F

G

H

FIG. 70. — The spinal cord and
medulla oblongata. A. from the

162

THE HUMAN BODY.

70), which runs back to the end of the neural canal behind
the sacrum. In its course the cord presents two expansions,
an upper, 10, the cervical enlargement, reaching from the third
cervical to the first dorsal vertebrae, and a lower or lumbar
enlargement, 9, opposite the last dorsal vertebra.

Running along the middle line on both the ventral and the
dorsal aspects of the cord is a groove, and a cross-section shows
that these grooves are the surface indications of fissures which
extend deeply into the cord (C, Fig. 71) and nearly divide it
into right and left halves.

The anterior fissure (1, Fig. 71) is wider and shallower
than the posterior, 2, which indeed is hardly a true fissure,
being completely filled up by an ingrowth of pia mater. The
transverse section, C, shows also that the substance of the

FIG. 71. — The spinal cord and nerve-roots. A, a small portion of the cord seen
from the ventral side; B, the same seen laterally ;•<?, a cross section of the cord;
D, the two roots of a spinal nerve; 1. anterior (ventral) fissure; -', posterior (dorsal)
fissure; 3, surface groove along the line of attachment of The anterior nerve-roots;
4, line of origin of the posterior roots; 5, anterior root filaments of spinal perve;
6. posterior root filaments; 0', ganglion of the posterior root; 7, 7', the first two
divisions of the nerve-trunk after its formation by the union of the two roots. The
grooves are much exaggerated.

cord is not alike throughout, but that its white superficial
layers envelop a central gray substance arranged somewhat in
the form of a capital H. Each half of the gray matter is
crescent-shaped, and the crescents are turned back to back and
united across the middle line by the gray commissure. The

ANATOMY OF THE NERVOUS SYSTEM. 163

tips of each crescent are called its horns or cornua, and the
ventral, or anterior cornu, on each side is thicker and larger
than the posterior. In the cervical and lumbar enlargements
the proportion of white to gray matter is greater than else-
where; and as the cord approaches the medulla oblongata its
central gray mass becomes irregular in form and begins to
break up into smaller portions. If lines be drawn on the
transverse section of the cord from the tip of each horn of the
gray matter to the nearest point of the surface, the white sub-
stance in each half will be divided into three portions: one
between the anterior fissure and the anterior cornu, and
called the anterior white column ; one between the posterior
fissure and the posterior cornu, and called the posterior white
column ; while the remaining one lying in the hollow of the
crescent and between the two horns is the lateral column : the
anterior and lateral columns of the same side are frequently
named the antero -lateral column. A certain amount of white
substance crosses the middle line at the bottom of the ante-
rior fissure; this forms the anterior white commissure. There
is no posterior white commissure, the bottom of the posterior
fissure being the only portion of the cord where the gray sub-
stance is uncovered by white. Running along the middle
of the gray commissure, for the whole length of the cord, is
a tiny channel, just visible to the unaided eye; it is known as
the central canal (canalis centralis).

The Spinal Nerves. Thirty-one pairs of spinal nerve-
trunks enter the neural canal of the vertebral column through
the intervertebral foramina (p. 71). Each divides in the fora-
men into a dorsal and ventral portion known respectively
as the posterior and anterior roots of the nerve (6 and 5, Fig.
71), and these again subdivide into finer branches which are
attached to the sides of the cord, the posterior root at the
point where the posterior and lateral white columns meet,
and the anterior root at the junction of the lateral and anterior
columns. At the lines on which the roots are attached there
are superficial furrows on the surface of the cord. On each
posterior root is a spinal ganglion (6', Fig. 71), placed just be-
fore it joins the anterior root to make up the common nerve-
trunk. Immediately after its formation by the mixture of
fibres from both roots, the trunk divides (D, Fig. 71), into
a posterior primary, an anterior primary, and a communi-
cating branch. The branches of the first set go for the most

164 THE HUMAN BODY.

part to the skin and muscles on the back, the second form
a series of plexuses from which the nerves for the sides and
ventral region of the neck and trunk and for the limbs arise;
the communicating branches go to neighboring sympathetic
ganglia.

The various spinal nerves are named from the portions of
the vertebral column through the intervertebral foramina of
which they pass out; and as a general rule each nerve is named
from the vertebra in front of it. For example the nerve pass-
ing out between the fifth and sixth thoracic vertebrae is the
" fifth thoracic" nerve, and that between the last thoracic and
first lumbar vertebrae, the " twelfth thoracic." In the cervi-
cal region, however, this rule is not adhered to. The nerve
passing out between the occipital bone and the atlas is called
the "first cervical" nerve, that between the atlas and axis the
second, and so on; that between seventh cervical and first
thoracic vertebrae being the "eighth cervical" nerve. The
thirty-one pairs of spinal nerves are then thus distributed: 8
cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal; the
latter passing out between the sacrum and coccyx. Since the
spinal cord ends opposite the upper lumbar vertebrae while
the sacral and coccygeal nerves pass out from the neural canal
much farther back, it is clear that the roots of those nerves,
on their way to unite in the foramina of exit and form nerve-
trunks, must run obliquely backwards in the spinal canal for
a considerable distance. One finds in fact the neural canal
in the lumbar and sacral regions, behind the point where the
spinal cord has tapered off to form the filum terminale, oc-
cupied chiefly by a great bunch of nerve-roots forming the
so-called "horse's tail" or cauda equina.

Distribution of the Spinal Nerves. It would be out of
place here to go into detail as to the exact portions of the
Body supplied by each spinal nerve, but the following general
statements may be made. The anterior primary branches of
the first four cervical nerves form on each side the cervical
plexus (Fig. 72) from which branches are supplied to the
muscles and integument of the neck: also to the outer ear
and the back part of the scalp. The anterior primary
branches of the remaining cervical nerves and the first dorsal
form the Irachial plexus, from which the upper limb is
supplied. The roots of the trunks which form this plexus
arise from the cervical enlargement of the spinal cord.

ANATOMY OF THE NERVOUS SYSTEM.

165

From the fourth and fifth cervical nerves on each side, small
branches arise and unite to make the phrenic nerve (4', Fig.
72) which runs down through the chest and ends in the
diaphragm.

The anterior primary branches of the dorsal nerves, except
part of the first which enters the brachial plexus, form no

FIG. 72.— The cervical and brachial plexuses of the left side of the Body.

plexus, but each runs along the posterior border of a rib and
supplies branches to the chest- walls, and the lower ones to
those of the abdomen also.

The anterior primary branches of the four anterior lumbar
nerves are united by branches to form the lumbar plexus.

166

THE HUMAN BODY.

It supplies the lower part of the trunk, the buttocks, the
front of the thigh, and inner side of the leg.

The sacral plexus is formed by the anterior primary
branches of the fifth lumbar and the first four sacral nerves,
which unite in one great cord and so form the sciatic nerve,
which is the largest in the Body and, running down the
back of the thigh, ends in branches for the lower limb. The
roots of the trunks which form the sacral plexus arise from
the lumber enlargement of the cord.

The Brain (Fig. 73) is far larger than the spinal cord
and more complex in structure. It weighs on the average

FIG 73.— Diagram illustrating the general relationships of the parts of the brain.
A, fore-brain; b. mid-brain ; B, cerebellum; (7, pons Varolii ; D, medulla oblon-
gata ; B, C, and D together constitute the hind-brain.

about 1415 grams (50 ounces) in the adult male, 'and about
155 grams (5.5 ounces) less in the female. In its simpler
forms the vertebrate brain consists of three masses, each with
subsidiary parts, following one another in series from before
back, and known as the fore-brain, mid-brain, and hind-
brain respectively. In man the fore-brain, A, weighing
about 1245 grams (44 ounces), is much larger than all the
rest put together and laps over them behind. It consists
mainly of two large convoluted masses, separated from one
another by a deep median fissure, and known as the cerebral
hemispheres. The immense proportionate size of these is
very characteristic of the human brain. Beneath each cere-

ANATOMY OF THE NERVOUS SYSTEM. 167

bral hemisphere is an olfactory lobe, inconspicuous in man
but in many animals larger than the cerebral hemispheres.
Buried in the fore-brain on each side are two large gray
masses, the corpora striata and optic thalami. The mid-
brain forms a connecting isthmus between the two other
divisions and presents on its dorsal side four hemispherical
eminences, the corpora quadrigemina. On its ventral side
it exhibits two semicylindrical pillars (seen under the nerve
IV. in Fig. 77), known as the crura cerebri. The hind-
brain consists of three main parts : on its dorsal side is the
cerebellum, B (Fig. 73), consisting of a rigid, a left, and a
median lobe; on the ventral side is the pons Varolii, C
(Fig. 73), and behind that the medulla oblonyata, D (Fig. 73),
which is continuous with the spinal cord.

In nature, the main divisions of the brain are not sepa-
rated so much as has been represented in the diagram for

Cbl

"^^^^

Mo

FIG. 74.— The brain from the left side. Cb, the cerebral hemispheres forming
the main bulk of the fore-brain; Cbl, the cerebellum; Mo, the medulla oblon-
gata; />, the pons Varolii ;* the fissure of Sylvius.

the sake of clearness, but lie close together, as represented
in Fig. 74, only some folds of the membranes extending be-
tween them ; and the mid-brain is entirely covered in on its
dorsal aspect. Nearly everywhere the surface of the brain
is folded, the folds, "known as gyri or convolutions being
deeper and more numerous in the brain of man than in that
of the animals nearest allied to him; and in the human
species more marked in the higher than in the lower races.
It should however be added that some species of animals

168

THE HUMAN BODY.

which are not markedly intelligent have much convoluted
cerebral hemispheres.

The brain like the spinal cord consists of gray and white
nervous matter, but somewhat differently arranged, for while
the brain, like the cord, contains gray matter in its interior,
a great part of its surface is also covered with it. By the
external convolutions of the cerebellum and the cerebral
hemispheres the surface over which this gray substance is
spread is very much increased (see Fig. 74).

The Ventricles of the Brain. The minute central canal
of the spinal cord is continued into the brain and expands

FIG. 75.— Diagram of the right half of a vertical median section of the brain.
Hi H, convoluted inner surface of right cerebral hemisphere; 5, the fifth ventricle;
the figure is placed on the thin inner wall of the right lateral ventricle; Cc, cor-
pus catlosum; 3, the third ventricle ; the partition separating it from the fifth ven-
tricle is the fornix, and just behind the anterior thickened end of the fornix is
shown part of the right foramen of Monro m, leading to the right lateral ven-
tricle ; t, the soft commissure cut across; in the fore part of the fornix is the
anterior commissure; the anterior portion of the floor of the third ventricle
shows two downward prolongations, one directed to the optic commissure, z, the
other (iiifundibnluni) to the pituitary body, pt; «, the pineal body; the thickening
immediately beneath its root is the posterior commissure; the mass forming the
exposed wall of the ventricle and on which the figure 3 is placed is the inner side
of the right optic thalamus ; o, d, the anterior and posterior corpora quadri-
ffemina of the right side; 4, the fourth ventricle lying near the dorsal side of the
medulla oblongata, Mo, and connected by the tier with the third ventricle; pos-
teriorly it is continued to join the central canal of the spinal cord; CV, right crus
cerebri; P, pans Varolii; Cb, cerebellum.; where it is divided in the middle line
the radial arrangement of its central white matter forming the so-called arbor
vitce is seen; op. right optic nerve proceeding from the optic commissure ; oc, the
third cranial nerve arising from the crus cerebri; 1, callosal convolution.

there at several points into chambers known as the ventri-
cles. Entering the medulla oblongata it approaches its
upper surface and dilates into- the fourth ventricle, 4, Fig. 75,

ANATOMY OF THE NERVOUS SYSTEM.

169

which has a very thin roof, lapped over by the cerebellum.
From the front of the fourth ventricle rims a narrow pas-
sage (aqueduct of sylvius or iter) which enters another dila-
tation, 3, Fig. 75, lying in the middle line near the under
side of the fore-brain and known as the third ventricle.
From the third ventricle two apertures (the foramens of
Monro), one of which is partly seen at m in the diagram,
lead into the first and second, or lateral ventricles, one of
which lies in each of the cerebral hemispheres. The front
ends of these two ventricles are seen in the vertical trans-
verse section of the brain represented in Fig. 76.

VI

FIG. 76.— A vertical section across the cerebral hemispheres taken in front of the
fifth ventricle. Cci2, anterior part of corpus callopum ; VI, the anterior end of the
right lateral ventricle: the gray mass on its exterior is the front end of the corpus
striatum. On the left side the superficial gray matter covering the convolutions
is shaded.

The ventricles contain a small amount of cerebro-spinal
liquid, and are lined by epithelium which is ciliated in early
life. Part of the posterior wall of the third ventricle is ex-
tremely thin, consisting of little but this epithelium sup-
ported by a thin layer of pia mater: this part is pushed in or
doubled into the cavity of the ventricle in the form of a
triangular membrane, the velum interpositum, which lies
beneath the fornix and sends offshoots into the lateral ven-
tricles. Between the upper and lower layers of the indupli-
cated velum interpositum arteries enter and there break up
into plexuses — the choroid plexuses— covered everywhere by

170 THE HUMAN BODY.

the pushed-in epithelium. These plexuses occupy a consid-
erable part of the third and lateral ventricles: and a pair of
similar vascular tufts drive in before them part of the thin
roof of the fourth ventricle and encroach on its cavity.

Note. A frequent cause of apoplexy is a hemorrhage
into one of the lateral ventricles; the outpoured blood accu-
mulating and pressing upon the cerebral hemispheres, their
functions are suppressed and unconsciousness produced.
When a person is found in an apoplectic fit therefore the
best thing to do is to leave him perfectly quiet until medical
aid is obtained: for any movement may start afresh a bleed-
ing into the ventricle which had been stopped by clots
formed in the mouths of the torn blood-vessels.

Sections of the Brain. Having got a general idea of the
parts composing the brain, the best way to continue the study
of its anatomy is to examine sections taken in various direc-
tions. Two such are given in Figs. 75 and 7G. Fig. 75 rep-
resents the right half of a vertical section of the brain, taken
from before back in the middle line and viewed from the
inner side. Above, the knife has passed between the two
cerebral hemispheres, in the longitudinal fissure, without cut-
ting either, and the convoluted inner surface of the right one
is seen. The sickle-shaped mass lower down, Cc to Cc, rep-
resents the cut surface of a connecting band of white nervous
tissue called the corpus callosum^ which runs across the mid-
dle line from one cerebral hemisphere to the other and puts
them in communication. Beneath the corpus callosurn
the knife has opened, a cavity, the fifth ventricle, 5,
bounded on each side by a very thin wall, which forms part
of the inner wall of the corresponding lateral ventricle; the
median partition formed by these two walls and containing
the\slit-like fifth ventricle is the septum lucidum. The fifth
is quite different in origin from the remaining cerebral ven-
tricles, not being a continuation of the canalis centralis of
the spinal cord.

Forming the floor of the fifth ventricle and separating it
from the third ventricle, 3, is the for nix, mainly made up of
fibres running from before back. The anterior downward-
curved end of the fornix is thickened, and contains the an-
terior commissure, a small cord of transverse nerve-fibres.
The cavity of the third ventricle is narrow from side to side,
and is bounded laterally by the optic tlialami, of which the

ANATOMY OF THE NERVOUS SYSTEM. 171

right, having the figure 3 placed on it, has its median side
exposed in the section. The third ventricle is crossed about
its middle by the middle commissure, t, and from its anterior
end the foramina of Monro, of which the right, m, is partly
exposed in the section, lead to the lateral ventricles. From
the fore%part of the third ventricle two conical extensions pass
downward, one directed to z, the optic commissure, from which
the optic nerves pass, and the other, named the infundibulum,
to the pituitary body, fit. The latter consists of an anterior
and posterior lobe, and in the human brain contains no ner-
vous elements. The anterior lobe, indeed, is an outgrowth
from the pharynx of the embryo, and only secondarily be-
comes attached to the brain. It is not known to have any
function in existing vertebrates. From the posterior part of
the floor of the third ventricle the iter leads as a narrow pas-
sage dorsal to the crura cerebri, Cr, and ventral to the corpora
quadrigemina, o, d, to the fourth ventricle, 4. Projecting
from the posterior wall of the third ventricle is a small coni-
cal non-nervous mass, the pineal body, which, though of no
functional importance, is of interest, in the first place be-
cause the philosopher Descartes considered it the special seat
of the soul, and in the second because embryology and com-
parative anatomy show that it is the remnant of a third
median eye, which primitive vertebrates possessed on the
dorsal side of the head. In some existing reptiles its original
structure is more complete than in man, but in none is it
functional. Just beneath the attachment of the pineal body
is a slight thickening of the posterior wall of the third ven-
tricle containing transverse fibres, and named the posterior
commissure. The third ventricle and the parts immediately
surrounding it constitute the inter-brain or thalamencephalon,
which with the two cerebral hemispheres and the corpus cal-
losum and fornix makes up the fore brain.

The mid-brain, consisting mainly of the crura cerebri, Cr,
and the corpora quadrigemina, o, d,and traversed by the nar-
row iter, is continuous posteriorly with the hind brain, con-
sisting of pons Varfilii, P; cerebellum, Cb; and medulla oblon-
gata, Mo. The 'thin-roofed cavity of the fourth ventricle, 4,
lies near its dorsal side. Where cut in making the section
the cerebellum shows a curious branching core of white nerve
matter, surrounded by gray, named arbor vitce by the old
anatomists. The pons consists mainly of transverse fibres

172 THE HUMAN BODY.

uniting the right and left halves of the cerebellum; the
medulla oblongata and crura contain mainly longitudinal
fibres, but there are many transverse.

Fig. 76 represents a vertical transverse section of the brain
taken through the forepart of the corpus callosum (CcF) and
altogether in front of the third ventricle. It shows the foldings
of the cerebrum and its superficial layer of gray substance; the
anterior ends of the lateral ventricles, VI, with a gray mass, the
corpus striatum lying beneath and on the outer side of each.
If the section had been taken a little farther back the optic
tlialami would have been found reaching the floor of each ven-
tricle. Like the optic thalamus, to the front of and partly to
the outer side of which it lies, the corpus striatum is mainly
composed of gray nerve matter. It is, however, divided in
its posterior region into an inner and outer portion by a well
marked band of white substance, consisting of nerve fibres,
passing through on the way to or from the surface of the
cerebral hemispheres: this band is the internal capsule.

The Base of the Brain and the Cranial Nerves. Twelve
pairs of nerves leave the skull by apertures in its base, and
are known as the cranial nerves. Most of them spring from
the under side of the brain, and so they are best studied in
connection with the base of that organ, which is represented
in Fig. 77. The first pair, or olfactory nerves, spring from
the under sides of the olfactory lobes, 1, and pass out through
the roof of the nose. They are the nerves of smell. The
second pair, or optic nerves, II, spring from the optic thalami
and corpora quadrigemina, and, under the name of optic tracts,
run down to the base of the brain, where they appear passing
around the crura cerebri, as represented in the figure. In the
middle line the two optic tracts unite .to form the optic com-
missure (seen in section at z, in Fig. 75), from which an optic
nerve proceeds to each eyeball. Behind the optic commis-
sure is seen the conical stalk of the pituitary body or hy-
pophysis cerebri (pt in Fig. 75), and still further back a pair of
hemispherical masses, about the size of split peas, known as
the corpora albicantia.

All the remaining cranial nerves arise from the hind-
brain. The third pair (motor es oculi) arise from the front of
the pons Varolii, and are distributed to most of the muscles
which move the eyeball and also to that which lifts the upper
eyelid. The four-sided space bounded by the optic tracu;

ANATOMY OF THE NEUVOUS SYSTEM. 173

and commissure in front and the third pair of nerves behind,
and having on it the pituitary body and the corpora albi-
cantia, lies beneath the third ventricle, so that a probe pushed
in there would enter that cavity (see Fig. 75).

in

FIG. 77.— The base of the brain. The cerebral hemispheres are seen overlap-
ping all the rest. 7, olfactory lobes; II, optic tract passing tothe optic commissure
from which the optic nerves proceed ; HI, the third nerve or motor oculi ; 1 V, the
fourth nerve or patheticus ; V, the fifth nei*ve or trigeminalis; VI. thn sixth nerve
or abducens; VII, the seventh or facial nerve or portio dura; VIII, the auditory
nerve or portio mollis; IX, the ninth or glosso-pharyngeal ; X, the tenth or pneu-
mogastric or vagus; XI, the spinal accessory; XII, the hypoglossal; ncl, the first
cervical spinal nerve.

The fourth pair of nerves, IV (patlietici\ arise from the
front part of the roof of the fourth ventricle. From there,
each curls around a crus cerebri (the cylindrical mass seen
beneath it in the figure, running from the pons Varolii to
enter the under surface of" the cerebral hemispheres) and ap-
pears on the base of the brain. Each goes to one muscle of
the eyeball.

The fifth pair of nerves (trigeminales), V, resemble the

174 THE HUMAN BODY.

spinal nerves in having two roots; one of these is much
larger than the other and possesses a ganglion (the Gasserian
ganglion) like the dorsal root of a spinal nerve. Beyond
the ganglion the two roots form a common trunk which
divides into three main branches. Of these, the ophthalmic
is the smallest and is mainly distributed To~the muscles and
skin over the forehead and upper eyelid; but also gives
branches to the mucous membrane lining the nose,, and to
the integument over it. The second division (superior maxil-
lary nerve) of the trigeminal gives branches to the skin over
the temple, to the cheek between the eyebrow and the angle
of the mouth, and to the upper teeth; as well as to the
mucous membrane of the nose, pharynx, soft palate and roof
of the mouth. The third division (inferior maxillary) is the
largest branch of the trigeminal; it receives some fibres from
the larger root and all of the smaller. It is distributed to
the side of the head and the external ear, the lower lip and
lower part of the face, the mucous membrane of the mouth
and the anterior two thirds of the tongue, the lower teeth,
the salivary glands, and the muscles which move the lower
jaw in mastication.

The sixth pair of cranial nerves (Fig. 77), VI, or abdu-
centes arise from the posterior margin of the pons Varolii,
and each is distributed to one muscle of the eyeball.

The seventh pair (facial nerves), VII, appear also at the
posterior margin of the pons. They are distributed to most
of the muscles of the face and scalp.

The eighth pair (auditory nerves) arise close to the facial.
They are the nerves of hearing and are distributed entirely
to the internal ear.

The ninth pair (glossopharyngeals), IX, arising close to
the auditories, are distributed to the mucous membrane of
the pharynx, the posterior part of the tongue, and the middle
ear.

The tenth pair (pneumo gastric nerves or vagi), X, arise
from the sides of the medulla oblongata. Each gives branches
to the pharynx, gullet and stomach, the larynx, windpipe
and lungs, and to the heart. The vagus runs farther through
the body than any other cranial nerve.

The eleventh pair (spinal accessory nerves), XI, do not
arise mainly from the brain but by a number of roots attached
to the lateral columns of the cervical portion of the spinal

ANATOMY OF THE NERVOUS SYSTEM. 175

cord, between the anterior and posterior roots of the proper
cervical spinal nerves. Each, however, runs into the skull
cavity alongside of the spinal cord and, getting a few fila-
ments from the medulla oblongata, passes out along with the
glossopharyngeal and pneumogastric nerves. Outside the
skull it divides into two branches, one of which joins the
pneumogastric trunk, while the other is distributed to mus-
cles about the shoulder.

The twelfth pair of cranial nerves (liypoglossi), XII, arise
from the sides of the medulla oblongata; they are distributed
mainly to the muscles of the tongue and hyoid bone.

Deep Origins of the Cranial Nerves. The places referred
to above, at which the various cranial nerves appear on the
surface of the brain, are known as their superficial origins.
From them the nerves can be traced for a lesser or greater way
in the substance of the brain until each is followed to one or
more masses of gray matter, which constitute its proper start-
ing-point and are known as its deep origin. The deep origins
of all except the first and second and part of the eleventh lie
in the medulla oblongata, midbrain, and thalamen cephalon.

The Ganglia and Communications of the Cranial Nerves.
Besides the Gasserian ganglion above referred to, many others
are found in connection with the cranial nerves. Thus for
example there is one on each of the main divisions of the
trigeminal, two are found on each pneumogastric and two in
connection with the glossopharyngeal. At these ganglia and
elsewhere, the various nerves often receive branches from
neighboring cranial or spinal nerves, so that very soon after
it leaves the brain hardly any, except the olfactory, optic, and
auditory, remains free from fibres derived from other trunks.
This often makes it difficult to say from where the nervej of
a special part have come; for example, the nerve-fibres going
to the submaxillary salivary gland from the trigeminal leave
the brain first in the facial and only afterwards enter the
fifth; and many of the fibres going apparently from the
pneumogastric to the heart come originally from the spinal
accessory.

The Sympathetic System. The ganglia which form the
main centres of the sympathetic nervous system lie in two
rows (s, Fig. 2, and sy, Fig. 3), one on either side of the
bodies of the vertebrae. Each ganglion is united by a nerve-
.trunk with the one in front of it, and so two great chains are

176 THE HUMAN BODY.

formed reaching from the base of the skull to the coccyx.
In the trunk region these chains lie in the ventral cavity,
their relative position in which is indicated by the dots sy in
the diagrammatic transverse section represented on p. 6 in
Fig. 3. The ganglia on these chains are forty-nine in num-
ber, viz., twenty-four pairs, and a single one in front of the
coccyx in which both chains terminate. They are named
from the regions of the vertebral column near which they lie;
there being three cervical, twelve thoracic, four lumbar, and
five sacral pairs.

Each sympathetic ganglion is united by communicating
brandies with the neighboring spinal nerves, and near the
skull with various cranial nerves also; while from the gan-
glia and their uniting cords arise numerous trunLs, many of
which, in the thoracic and abdominal cavities, form plexuses,
from which in turn nerves are given olf to the viscera.
These plexuses frequently possess numerous ganglia of their
own; two of the most important are the cardiac plexus
which lies on the dorsal side of the heart, and the solar plexus
which lies in the abdominal cavity and supplies nerves to the
stomach, liver, kidneys, and intestines. Many of the sympa-
thetic nerves finally end in the walls of the blood-vessels of
various organs. To the naked eye they are commonly grayer
in color than the cerebro-spinal nerves.

Tho Sporadic Ganglia. These are found scattered in
nearly all parts of the Body except the limbs. They are for
the most part small, even microscopic in size, though several
large ones exist in the abdominal cavity. The}7 are especially
abundant in the neighborhood of secretory tissues and about
blood-vessels, while a very important set is found in the
heart. Nerves unite them with the cerebro-spinal and sym-
pathetic centres, and probably most of them should be classi-
fied as belonging to the sympathetic system.

The Histology of Nerve-Fibres. The microscope shows
that in addition to connective tissue and other accessory
parts, such as blood-vessels, the nervous organs contain tis-
sues peculiar to themselves and known as nerve-fibres and
nerve-cells. The cells are found in the centres only; while
the fibres, of which there are two main varieties known as
the white and the gray, are found in both trunks and cen-
tres: the white variety predominating in most cerebro-spinal
nerves and in the white substance of the centres, and the

ANATOMY OF THE NERVOUS SYSTEM.

177

gray in the sympathetic trunks and the gray portions of the
central organs.

If an ordinary cerebro-spinal nerve-trunk be examined it
will be found to be enveloped in a loose sheath of areolar
connective tissue, which forms a packing for it and unites
it to neighboring parts. From this sheath, or perineurium,
bands of connective tissue penetrate the nerve and divide it
up into a number of smaller cords or funiculi, much as a
muscle is subdivided into fasciculi; each funiculus has a
sheath of its own called the neurilemma, composed of several

FIG. 78. FIG. 79.

FIG. 78. — White nerve-fibres soon after removal from the Body and when they
have acquired their double contour.

FIG. 79.— Diagram illustrating the structure of a white or medullated nerve-fibre.
I, 1, primitive sheath; 2, 2, medullary sheath; 3, axis cylinder.

concentric layers of a delicate membrane, within which the
true nerve-fibres lie. These, which would be nearly all of
the white kind, consist of extremely delicate threads, on the
average, 0.0125 mm. (^oVo- incn) in diameter, though often
considerably smaller, and of a length which is in proportion
very great. The core of each nerve-fibre in fact is continuous
from a nerve-centre to the organ in which it ends, so that the
fibres, e.g., which pass out through the sacral plexus and then
run on through the sciatic nerve and its branches to the skin

178

THE HUMAN BODY.

B

of the toes, are three to four feet long. If a fresh while nerve*
fibre be examined with the microscope it presents the appear-
ance of a perfectly homogeneous glassy thread ; but soon it
acquires a characteristic double contour (Fig.
78) from the coagulation of a portion of its
substance. By proper treatment with re-
agents three layers may be brought into view.
Outside is a fine transparent envelope (1,
Fig. 79) called the primitive sheath ; inside
this is a fatty substance, 2, forming the
medullary sheath (the coagulation of which
gives the fibre its double border), and in the
centre is a core, the axis cylinder, 3, which
is longitudinally fibrillated and is clearly the
essential part of the fibre, since near the end-
ing the primitive and medullary sheaths are
frequently absent. At intervals of about
one millimeter (fa inch) along the fibre are
found nuclei (c, Fig. 80), around each of
which lies a little protoplasm. These are
indications of the primitive cells which have
elongated and formed an envelope for the
axis cylinder, which itself is a branch given
off by a nerve-cell in some centre. The
medullary sheath is interrupted half-way
between each pair of nuclei at a point called
the node of Ranvivr (R, Fig. 80), which is
the boundary between two of the enveloping
cells. In the course of a nerve-trunk its
fibres rarely divide; when a branch is given
off some fibres merely separate from the

°f

four hundred diame- rafced out at one end into smaller bundles

terse they nsivc ot*cn _^ _ . -.

treated with, osmic containing fewer threads. .Near their ends,

acid, which stains the , , -, •,

medullary sheath however, nerve-fibres frequently brancn, ana
8r^ *£*!£& then a division of the axis cylinder goes to

c, c, and nodes of , , ,

Ranvier, R. The axis each branch.

Gray Nerve-Fibres. Some of these are
merely white fibres which near their peri-
pheral ends have lost their medullary sheaths; others have no
medullary sheath throughout their whole course, and consist
merely of an axis cylinder (often striated) and nuclei, with

the nodes.

ANATOMY OF THE NERVOUS SYSTEM.

179

or without a primitive sheath. Such fibres are especially
abundant in the sympathetic trunks; and they alone form
the olfactory nerve. In the communicating branches between
the sympathetic ganglia and the spinal nerves both white
and gray fibres are found; the former are cerebro-spinal
fibres passing into the sympathetic system, while the grav
fibres originate in the sympathetic system and pass to the
membranes and blood-vessels of the spinal cord and spinal
column. Another group of gray nerve-fibres may be called
nerve-fibrils : they are extremely fine,, and result from the
subdivision of axis cylinders, close to their final endings in
many parts of the Body, after they have already lost both
primitive and medullary sheaths. Many fine gray fibres exist
in the nerve-centres.

The Histology of Nerve-Cells. The only structures
known with certainty to be connected with the central ends
of nerve-fibres are nerve-cells, and so many nerve-fibres have

FIG. 81.— Nerve-cell from anterior horn of srey matter of spinal cord; a, axis-
cylinder process. 2, Cell from posterior horn of spinal cord.

been traced into continuity with nerve-cells, that it is fairly
certain all arise in this way. The latter may therefore be re-
garded as the central organs of the nerve-fibres.

180 THE HUMAN BODY.

At 1, Fig. 81, is shown a typical nerve-cell such as may
be found in an anterior horn of the gray matter of the spinal
cord. It consists of the cell body, or cell protoplasm, in
which is a large nucleus containing a nucleolus. From the
body of the cell arise several branches,, the great majority of
which are granular and divide frequently in a forking or
" dichotomous " manner. These are known as the " proto-
plasmic " branches of the cell, and possibly serve merely to
absorb nourishment for it. One branch, however, a, gives oft'
at right angles smaller filaments, but still maintains its in-
dividuality and ultimately becomes the axis cylinder of a
nerve-fibre. Its side branches probably put it in anatomical
continuity with other nerve-fibres and other nerve-cells.
Nerve cells from the posterior horn of the grey matter of
the spinal cord (2, Fig. 81) also possess numerous granular
protoplasmic processes and a nerve-fibre process (#); but the
latter, instead of continuing directly into an axis cylinder,
breaks up into a network of fine branches which frequently
unite with one another and also, no doubt, with fibrils from
neighboring cells. It is almost certain that one or more of
these fibrils or a bunch of them forms the axis cylinder of a
fibre in a dorsal root of a spinal nerve.

As we shall learn later, the dorsal roots are concerned in
carrying impulses from the skin and other sensitive parts to
the spinal cord; the anterior roots in conveying impulses from
the nerve-centres to the organs (muscles, glands, etc.) of the
Body. Therefore, in general terms, we may speak of the type
of nerve-cell 1, Fig. 81, as a motor nerve-cell; and the type
of cell 2, Fig. 81, as a sensory nerve-cell. Both varieties of
cells are found abundantly in the gray matter of the brain
(Fig. 83), along with other forms, of which the pear-shaped
cells of Purkinje existing in the cerebellum may be mentioned
(Fig/82).

In the sympathetic and sporadic ganglia somewhat simpler
forms of nerve-cells, having fewer branches, occur. As a rule
nerve-cells are comparatively large and have conspicuous
nuclei, but in the brain many small ones exist.

Neuroglia. In the brain and spinal cord the true nervous
elements are intertwined with and supported by connective
tissue and minute blood-vessels, but in addition there is found
closely twisted around the cells and fibres a peculiar tissue

ANATOMY OF THE NERVOUS SYSTEM.

181

made of greatly branched cells (Fig. 83), and named the
neuroglia or sus tentacular tissue.

Nerve-Centres consist of white and gray nerve-fibres, of
nerve-cells, of neuroglia, and
of connective tissue and

blood-vessels arranged in -^^W^l^&tiM'hln
different ways in the differ-
ent centres. They are es-
sentially collections of nerve- " v \j/ \&M 7*\/$r^n.

FIG. 82.— A thin section of the cere- FIG. 83.— Cells from the surface gray mat-
bellum showing: pear-shaped cells of ter of a cerebral convolution : p, nerve-cells
Purkinje, and numerous other small with axis cylinder processes, o ; ?i, non-ner-
nerve-cells. vous neuroglia cells. The method of prepa-

ration (Qolgi's) stains the cells an uniform
black.

cells and nerve-fibres, some of the latter being connected with
the cells, while others may merely pass through on their way
to or from other centres. As an illustration of the structure
of a more complex nerve-centre we may study the spinal cord.
Histology of the Spinal Cord. If a thin transverse sec-
tion of the spinal cord be examined with a microscope it will
be found that enveloping the whole is a delicate layer of
connective tissue, the pia mater. Fine bands of it ramify

182

THE HUMAN BODY.

through the cord, supporting the nervous elements; some of
the coarser of these are represented at 6, 7, and elsewhere in
Fig. 84, but from these still finer processes arise, as represented
at d and e in Fig. 85. The ultimate finest tissue directly
supporting the nervous elements directly, is the neuroglia.
In the white columns, the cord (Fig. 85) will be seen to be

/IG. 84.— A thin transverse section of half of the spinal cord magnified about
10 diameters. 1, anterior fissure ; 2, posterior fissure ; 3. canalis central is ; 8, pia
mater enveloping the cord ; 6, 7, bands of pia mater penetrating the cord and sup-

gorting its nerve elements ; 9, a posterior root ; 10, bundles of an anterior root ; a,
, c, d, e, groups of nerve cells in the gray matter.

mainly made up of medullated nerve-fibres which run longi-
tudinally and therefore appear in the transverse section as
circles, with a dot in the centre, which is the axis cylinder.
At b in Fig. 85 these fibres are represented, the intermediate
connective tissue being omitted, while at e this latter alone is
represented in order to show more clearly its arrangement.
At the levels of the nerve-roots horizontal white fibres are
found (9 and 10, Fig. 84, and «,.Fig. 85), running into the
gray matter, and others exist at the bottom of the anterior
fissure, running from one side of the cord to the other. In

ANATOMY OF THE NERVOUS SYSTEM.

183

the gray substance the same supporting network of connec-
tive tissue is found, but in it the majority of the nerve-fibres
are non-medullated, and at certain points nerve-cells, such as
are totally absent in the white substance, are found. One
collection of these nerve-cells is seen at c in Fig. 84, and
others are represented at a, e,f, and elsewhere. The nerve-
fibres in the gray matter are for the most part branches of
the axis cylinder processes of these cells (see Fig. 81), and as
they unite with one another freely they form a structurally
continuous network through the whole gray substance. The
fibres of the anterior roots of the spinal nerves enter the gray
matter and there most of them soon become continuous with
the axis cylinder process of a nerve-cell; the ending of the
posterior root-fibres is not quite certain, but they appear to
break up and join the gray network, to be by it placed indi-
rectly in connection with nerve-cells. In any case the funda-
mental fact remains that every nerve-fibre joining the spinal
cord is directly or indirectly in continuity with the gray net-
work, and so with all the other fibres of all the spinal nerves.,

FIG. 85.— A small bit of the section represented in Fig, 84 more magnified, a, a
bundle of fibres from an anterior root passing tli rough the white substance on its
way to the gray. Towards the right of the figure the nerve-fibres of the anterior
column have been omitted so as to render more conspicuous the supporting con-
nective tissue, d and e. Elsewhere the nerve-fibres alone are represented ; c, envel-
oping pia mater. The neuroglia is not indicated.

From the sides of the gray substance, fibres continually pass
out into the white portion and become medullated ; some of
these enter the gray network again at another level and so
bring parts of the cord into especially close union, while
others pass on into the brain. At the top of the neck, more-
over, the gray matter of the cord is continuous with that of

184

THE HUMAN BODY.

the medulla oblongata and through it with the rest of the

brain, so that nervous disturbances can pass by anatomically

continuous paths from one to the other.

The Structure of a Spinal Ganglion. When one of these

ganglia is cut lengthwise, and the section examined micro-
scopically, it is seen that
connective tissue forms an
envelope for it, and sends
ramifying bundles through
it. The fibres of the poste-
rior root become separated
into bundles when they
enter a ganglion and unite
into a single bunch when
they leave it to join the
mixed spinal nerve trunk.
Between the bundles of
nerve-fibres within the gan-
glion are groups of nerve-
cells, and probably each fibre
on its way through the gan-
glion is connected with a

FIG. 86. — Diagram of a spinal ganglion -M rnr,-

cell: uc, its flbrillated process, which ac- Cell. IhlS Connection OC-

quiring primitive sheath, ps, and medullary • eOrr,pWV,at r»PPnliir
sheath. m«, becomes a fibre which at the c

node of Ranvier nr joins a posterior root w«v TVm polla ^TTio- 8fi\

fibre, part of its axis cylinder, c, running WaJ' ± ne C6llS I'H?' °*V

centrally in this, and part, d, distaiiy. are pear-shaped, granular,
contain a large nucleus and nucleolus, and average -£$• mm.
(^l-Q inch) in long diameter. Near its narrow end the cell
substance is fibrillated, and a bundle of these fine fibres, ac,
passes from it, something like the stalk from a pear. This
stalk is an axis cylinder, and has on it small nuclei. A little
way from the cell the axis cylinder acquires a primitive sheath,
ps, and a little farther on a medullary sheath, ms, so that it
becomes a fully formed white nerve-fibre. At a node of Ran-
vier (usually that one nearest the cell), nr, this divides, its
branches diverging from it at right angles: one branch runs
to the posterior root and enters the spinal cord; the other
continues through the ganglion as a fibre of the mixed
nerve-trunk. The axis cylinders of these branches, c and d,
in some cases at least, contain fibrillae not derived from the
pear-shaped cell in addition to those which are. Each cell as
it lies in the ganglion is gncased in a delicate envelope of

ANATOMY OF THE NERVOUS SYSTEM. 185

flattened nucleated cells (not indicated in the figure).,
probably belonging to the surrounding connective tissue.
Blood-vessels are distributed in the ganglion, the capillaries
being especially numerous about the nerve-cells.

Most of the cells of sympathetic and other peripheral gan-
glia seem to have several branches, no one of which differs
essentially from the rest; probably each branch becomes part
of the axis cylinder of a different fibre, the cell thus placing
several distinct fibres in communication.

186 THE HUMAN BODY.

CHAPTER XIII.
THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM.

The Properties of the Nervous System. General Con-
siderations. If the finger of any one unexpectedly touches
a very hot object, pain is felt and the hand is suddenly
snatched away; that is to say, sensation is aroused and cer-
tain muscles are caused to contract. If, however, the nerves
passing from the arm to the spinal cord have been divided, or
if they have been rendered incapable of activity by disease, no
such results follow. Pain is not then felt on touching the
hot body nor does any movement of the limb occur; even
more, under such circumstances the strongest effort of the
will of the individual is unable to bring about movement
of his hand. If, again, the nerves of the limb have uninjured
connection with the spinal cord, but parts of the latter,
higher up, between the brain and the point of junction of the
nerves of the brachial plexus with the cord, are injured, then
a sudden contact with the hot body will cause the arm to be
snatched away, but no pain or other sensation due to the
contact will be felt, nor can the will act upon the muscles of
the arm. From the comparison of what happens in such
cases (which have been observed over and over again upon
wounded or diseased persons) with what occurs in the natural
condition of things, several important conclusions may be
arrived at:

1. The feeling of pain does not reside in the burnt part it-
self; although that may be perfectly normal, no sensation
will be aroused by any external force acting upon it, if the
nervous cords uniting it with the centres be previously
divided.

2. The hot body has originated some change which, when
propagated along the nerve-trunks, has excited a condition of
the nerve-centres which is accompanied by a sensation, in this
particular case a painful one. This is clear from the fact
that the loss of sensation immediately follows division of the

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 187

nerves of the limb, but not the injury of any of its other
parts; unless of such a character as to cut off the supply of
blood, when of course the nerves soon die, with the rest.
Even, however, some time after tying the vessels which carry
blood to a limb one can observe in experiments upon the
lower animals that sensibility is still retained if the nerves
be not directly injured.

3. When a nerve in the skin is excited by a burn, or other-
wise, it does not directly call forth muscular contractions; for
if so, touching the hot body would cause the limb to be moved
even when the nerve had been divided high up in the arm, and
as a matter of observation and experiment we find that no such
result follows if the nerve-fibres have been cut in any part of
their course from the burned part to the spinal marrow. It
is therefore through the nerve-centres that the change trans-
mitted from the excited part of the skin is reflected or sent
back, to act upon the muscles.

4. The last deduction makes it probable that nerve-fib? es
must pass from the centre to muscles as well as from the skin
to the centre. This is confirmed by the fact that if the nerves
of .the limb be divided the will is unable to act upon its
muscles, showing that these are excited to contract through
the nerves. That the nerve-fibres concerned in arousing
sensation and muscular contractions are different, is shown
also by cases of disease in which the sensibility of the limb is
lost while the power of voluntarily moving it remains, and by
other cases in which the reverse is seen, objects touching the
hand being felt while it cannot be moved by the will. We con-
clude then that certain nerve-fibres when stimulated convey
something (a nervous impulse) to the centres, and that these
when excited may radiate impulses through other nerve-fibres
to distant parts, the centre serving as a connecting link be-
tween the fibres which carry impulses from without in, and
those which convey them from within out.

5. Further we conclude that the spinal cord can act as
an intermediary between the fibres carrying -in nervous im-
pulses and those carrying them out, but that sensations can-
not be aroused by impulses reaching the spinal cord only,
nor has the Will its seat there; volition and consciousness are
dependent upon states of the brain. This follows from the
unconscious movements of the limbs which follow stimula-
tion of its skin after such injury to the spinal cord as pre-

188 THE HUMAN BODY.

vents the farther transmission of nervous impulses (show-
ing that the cord is a reflex centre], and from the absence, in
such cases, of sensation in the part whose nerves have been
injured, and the loss of the power of voluntarily causing its
muscles to contract.

6. Finally we conclude that the spinal cord in addition to
being a centre for rt flex actions serves to transmit nervous im-
pulses to and from the brain; a fact which is confirmed by the
histological observation that in addition to the nerve-cells,
which are the characteristic constituents of nerve-centres, it
contains the simply conductive nerve-fibres, many of which
pass on to the brain. In other words, the spinal cord, besides
containing fibres which enter it from and pass from it to peri-
pheral parts, contains many which unite it to other centres;
and others which connect the various centres in it, as those
for the arms and legs, together. This is true not only of the
spinal cord, but of the brain (which contains many fibres
uniting different centres in it), and probably of all nerve-
centres.

The Functions of Nerve-Centres and Nerve-Trunks.
From what has been stated in the previous paragraphs it is
clear that we may distinctly separate the nerve-trunks from
the nerve-centres. The fibres serve simply to convey impulses
either from without to a centre or in the opposite direction,
while the centres conduct and do much more. Some, as the
spinal cord, are merely reflex centres and have nothing to do
with states of consciousness. A man with }ris spinal cord
cut or diseased in the thoracic region will kick violently if the
soles of his feet be tickled, but will feel nothing of the tick-
ling, and if he did not see his legs would not know that they
were moving. Reflex centres moreover do not act, as a rule,
indifferently and casually, but rearrange the impulses reach-
ing them, so as to produce a protective or in some way advan-
tageous result. In other words, these centres, acting in
health, commonly co-ordinate the incoming impulses and give
rise to outward-going impulses which produce an apparently
purposive result. The burnt hand or the tickled foot, in the
absence of all consciousness, is snatched away from the irri-
tant; and food chewed in the mouth excites nerves there
which act on a centre which causes certain cells in the salivary
glands to make and pour into the mouth more saliva. In
addition to the reflex centres we have others, placed in the

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 189

brain, the excitation of which is accompanied in us by various
states of consciousness, as sensations, emotions, and the will;
concerning these centres of consciousness our physiological
knowledge is still very incomplete; what we know about them
is based rather on psychological than physiological observa-
tion. The brain also contains a great many important reflex
centres, as that for the muscles of swallowing, an act which
goes on perfectly without our consciousness at all. In fact
if we pay attention to our swallowing we fail to perform it as
well as if we let the nervo-muscular apparatus alone, as is
illustrated by the difficulty many persons experience on trying
to swallow a pill. To complete the statement of the functions
of the nerve-centres we must probably add two other groups.
The first of these is that of the automatic centres, which are
centres excited not directly by nerve-fibres conveying impulses
to them, but in other ways. For example the breathing
movements go on independently of our consciousness, being
dependent on stimulation of a nerve-centre in the brain by
the blood which flows through it (see Chap. XXVII); and
the beat of the heart is also much dependent (Chap. XVIII)
upon nerve-centres, the excitant of which is unknown. The
final group of nerve-centres is represented by certain sporadic
sympathetic and cerebro-spinal ganglia which are not known
to be either reflex, automatic, or conscious in function. They
may be called relay and junction centres, since in them prob-
ably an impulse entering by one nerve-fibre excites a cell,
which by its communicating branches arouses many others,
and these then send out impulses by the many nerve-fibres
connected with them. By such means a single nerve-fibre can
act upon an extended region of the Body. In other cases it
seems likely that a feeble nervous impulse reaching an irri-
table nerve-cell excites changes in this comparable to those
produced in a muscle when it is stimulated; the cell is in
fact a store of readily decomposable material which breaks
down when stimulated through one branch, with the liberation
of energy, the discharge of which takes the form of reinforced
nerve impulses sent along other branches or one of them.

That nerve-cells are the seats of considerable metabolic
changes is indicated by the abundant supply of blood always
sent to regions where they are numerous: and that some of
their material is used up, or undergoes katabolism, during
their activity and is replaced by anabolic processes during

190 THE HUMAN BODY.

rest, can be demonstrated histologically. If the sensor}1
nerves of one fore limb of an animal be left at rest and those
of the other simultaneously excited for several hours, it will
be found, at the end of that time, that the nuclei of many
cells of the spinal ganglia of the brachial nerves on the stim-
ulated side are shrunken and distorted when compared with
those of the other side. But if some hours be suffered to
elapse before the animal is killed and the ganglia examined,
the nuclei of the cells on both sides will be found equally
large and well rounded. In carrier-pigeons after a long flight
and in wild sparrows shot at the close of day, the nuclei of
the nerve-cells connected with the origin of motor nerve-fibres
are found to be shrivelled, and the whole cell frequently dimin-
ished in size when compared with specimens taken from birds
after a period of rest. In old age the nuclei of many nerve-
cells are small and distorted, even after prolonged rest.

Nerve-trunks and the white portions of nerve-centres are
sparsely supplied with blood and undergo but small and slow
nutritive changes in health. Their activity appears to consist
in the transmission of some molecular motion not affecting
the nutrition and chemical composition of the fibre, and not
using up its material.

Excitant and Inhibitory Nerves. The great majority
of the nerve-fibres of the Body when they convey^n^rvous
impulses to a part arouse it to activity; they are exciffrn^
fibres. ' There is, however, in the Body another very impor-
tant set which arrest the activity of parts and which are
known as inhibitory nerve-fibres. Some of these check the
action of central nervous organs, and others the work of
peripheral parts. For instance, taking a pinch of snuff will
make most persons sneeze; it excites centrally acting fibres
in the nose, these excite a centre in the brain, and this in
turn sends out impulses by motor fibres which cause various
muscles to contract. But if the skin of the upper lip be
pinched immediately after taking the snuff, in most cases the
reflex act of sneezing, which the Will alone could not pre-
vent, will not take place. The afferent impulses conveyed
from the skin of the lip have " inhibited " what we may call
the "sneezing centre;" and afford us therefore an example
of inhibitory fibres checking a centre. On the other hand,
the heart is a muscular organ which goes on beating steadily
throughout life; but if certain branches of the pneumogastric

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 191

nerve going to it be excited, the beat of the heart will be
stopped; it will cease to work and lie in a relaxed resting
condition : in this we have an instance of an inhibitory nerve
checking the activity of a peripheral organ.

Classification of Nerve-Fibres. Nearly all the nerve-
fibres of the Body fall into one of two great groups corre-
sponding to those which carry impulses to the centres and
those which carry them out from the centres. The former
are called afferent or centripetal fibres, and the latter efferent
or centrifugal. Since the impulses reaching the centres
through the afferent fibres frequently cause sensations they are
often called sensory fibres ; and as many of those which carry
out impulses from the centres excite movements, they are
frequently called motor fibres ; but these last names are bad,
since even excluding inhibitory nerves, many afferent fibres
are not sensory and many efferent are not motor.

"We may distinguish as subdivisions of afferent fibres the
following groups. 1. Sensory fibres proper, the excitement
of which is followed by a sensation when they are connected
with their brain-centre, which sensation may or may not give
rise to a voluntary movement. 2. Reflex fibres, the excitation
of which may be attended with consciousness but gives rise
to involuntary efferent impulses. Thus for example light
falling on the eye causes not only a sensation, but also a nar-
rowing of the pupil, which is entirely independent of the
control of the will. No absolute line can, however, be drawn
between these fibres and those of the last group: any sudden
excitation, as an unexpected noise, will cause an involuntary
movement, while the same sound if expected would cause a
movement or not according as was willed. 3. Excito motor
fibres. The excitation of these when reaching a nerve-centre
causes the stimulation of efferent fibres, but without the par-
ticipation of consciousness. During fasting, for instance, bile
accumulates in the gall-bladder and remains there until some
semi-digested food passes from the stomach into the intestine.
This is acid and stimulates nerves in the mucous membrane
lining the intestine, and these convey an impulse to a centre,
which in consequence sends out impulses to the muscular coat
of the gall-bladder causing it to contract and expel its con-
tents into the intestine: but of all this we are errtirely un-
conscious. 4. Centro-inliibitory fibres. Whether these exist
as a distinct class is at present doubtful. It may be that

192 THE HUMAN BODY.

they are only ordinary sensory or reflex fibres, and that the
inhibition is due only to the interference of two impulses
reaching a central organ at the same time and impeding or
hindering the full production of the normal result of either.

In efferent nerve-fibres physiologists also distinguish sev-
eral groups. 1. Motor fibres, which are distributed to the
muscles and govern their contractions. 2. Vaso-motor fibres.
These are not logically separable from other motor fibres;
but they are distributed to the muscles of the blood-vessels,
and by governing the blood-supply of various parts, indirectly
produce such secondary results as entirely overshadow their
primary effect as merely producing muscular contractions.
3. Secretory fibres. These are distributed to the cells of the
Body which form various liquids used in it, as the saliva and
the gastric juice, and arouse them to activity. The salivary
glands, for instance, may be made to form saliva by stimulat-
ing nerves going to them, and the same is true of the cells
which form the sweat poured out upon the surface of the
Body. 4. Trophio nerve-fibres. Under this head are included
nerve-fibres which hate-^been supposed to govern the nutri-
tion of the various tissues, and so to control their healthy
life. It has been doubted if any such nerve-fibres exist as a
distinct class, and no doubt many of the facts which have been
cited to prove their existence are otherwise explicable. For
instance, shingles is a disease characterized by an eruption on
the skin along the line of certain nerves, oftenest those which
run between the ribs; but it may be dependent upon disease
of the vaso-motor nerves which control the blood-supply of
the part. In other cases diseases ascribed to injury of trophic
nerves have been shown to be due to injury of the sensory
nerves of the part, which having lost its feeling, is exposed to
injuries from which it would otherwise have been protected.
There are, however, cases which seem to indicate a direct niv
tritive influence of the nervous system on the tissues; as for
example the acute bedsores seen in some diseased states of the
spinal cord and leading to extensive destruction of the skin in
a very few hours; and there is direct experimental evidence
to show that stimulation of the branches of the pneumogastric
nerve going to the heart tends to restore that organ when ex-
hausted, Vhile stimulation of the sympathetic branches has a
precisely opposite effect (see Chapter XVIII). There is also
no doubt that each nerve fibre depends for the maintenance

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 193

of its nutrition on a nerve-cell since, if divided in its course,
the part separated from the cell rapidly degenerates. It
might also be urged that secretory nerves are trophic nerves
in the true sense of the word, since when excited they
cause the secretory cells to live in a special way, and produce
substances which when unacted upon by their nerves they
do not form. But if we call secretory nerves trophic we
must include also under that name all other efferent nerves;
the nutritive processes going on in a muscular fibre when at
work are different from those in the same fibre when at rest,
and the same is true of all other cells the activity of which is
governed by nerve-fibres. 5. Peripherally -act ing inhibitory
nerves, such as those which slow or stop the beat of the heart.

Intercentral Nerve-Fibres. These, which do not convey
impulses between peripheral parts and nerve-centres, but
connect one centre with another, form a final group in addi-
tion to efferent and afferent nerve-fibres. Many of them
connect the sporadic and sympathetic ganglia with one an-
other and with the cerebro-spinal centre, while others place
different parts of this latter in direct communication; as for
instance different parts of the spinal cord, the brain and the
spinal cord, and the two halves of the brain. The paths taken
by some of these commissural fibres will be stated in connec-
tion with the physiology of the brain and spinal cord.

General Table. We may physiologically classify nerve-
fibres as in the following tabular form, which is founded upon
the facts above stated.

Nerve-fibres.

Peripheral.

f Suasory.

Afferent.

Efferent.

Excito-motor,
Inhibitory.

[ Motor.
| Vaso-motor.
\ Secretory.
I Trophic.
Inhibitory.

Intercentral.

Inhibitory.

The Electrical Phenomena of Nerves. Under certain
conditions electrical currents can be led off from living nerve-

194 THE HUMAN BODY.

trunks and studied by aid of a galvanometer: in all respects
these currents correspond to those of muscle, except that they
are feebler. A perfectly fresh uninjured nerve at rest is
isolectric, and so is a completely dead nerve. A dying por-
tion of a nerve is negative to a more normal portion, and in
consequence, if electrodes be placed, one on the centre and
the other on the cut end of a freshly-removed portion of nerve,
a current will be found passing through the connecting wire
from the central portion of the piece of nerve towards the
peripheral. A region of nerve in activity, that is transmit-
ting a nervous impulse, is electro-negative to a region at rest,
other things being equal; accordingly, an act ion -cur rent or
negative variation can be demonstrated on nerves as on mus-
cles; the electrical change starting from the point of stimu-
lation and travelling along the trunk, to be found at a distant
part at a time when it has gone from the place of its first ap-
pearance. The account of the, similar phenomena in muscle
(Chap. X) may be consulted for a fuller statement.

The Stimuli of Nerve-Fibres. Nerve-fibres, like mus-
cular fibres, possess no automatic! ty; acted upon by certain
external forces or stinmli\ihey propagate a change, which is
known as ,a nervous impure, from the point acted jupon to
their ends; but they do not generate, nervous impulses when
left entirely to themselves. Formally, in the living Body
the stimulus acts on a nerve-fibre at one of its ends, being
either some change in a nerve-centre with which the fibre is
connected (efferent nerves) or some change in an organ at-
tached to the outer end of the nerve (afferent fibres). Ex-
periment shows, however, that a nerve can be stimulated in
any part ot its course; that it is irritable all through its ex-
tent. If, for example, the sciatic of a frog be exposed in the
thigh and divided, it will be found that electric shocks ap-
plied at the point of division to the outer half of the nerve
stimulate the motor fibres in it, and cause the muscular fibres
of the leg to contract: and similarly such shocks applied to
the cut end of the central half irritate the afferent fibres in
it, as shown by the signs of feeling exhibited by the animal.
In ourselves, too, we often have the opportunity of observing
that the sensory fibres can be stimulated in their course at
some distance from their ends. A blow at the back of the
elbow, at the point commonly known as the "funny bone "or
the " crazy bone," compresses the ulnar nerve there against the

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 195

subjacent bone, and in addition to irritating the nerves of
the skin at the point struck, starts nervous impulses which
make themselves known by severe tingling pain referred to
the little and ring fingers to which the ulnar nerve is dis-
tributed. This shows not only that the nerve-fibres can be
irritated in their course as well as at their ends, but also that
sensations do not directly tell us where a nerve-fibre has been
excited. No matter where in its course an impulse causing
sensation has been started, we irresistibly refer its origin to
the peripheral end of the afferent nerve-fibre affected.

General and Special Nerve-stimuli. Certain external
forces excite all nerve-fibres, and in any part of their course.
These are known as general nerve-stimuli ; others act only
on the end-organs of nerve-fibres, and often only on one kind
of end-organ, and hence cannot be made to excite all nerves:
these latter are commonly known as special nerve-stimuli,
In reality they are not properly nerve-stimuli at all; but
only things which so affect the irritable tissues attached to
the ends of certain nerve-fibres as to make these tissues in
turn excite the nerves. For example, light itself will not
stimulate any nerve, not even the optic: but in the eye it
effects changes (perhaps of a, chemical nature) by which
nerve-stimuli are produced and these stimulate the optic
nerve-fibres. The ends of the nerves in the skin are not
accessible to light nor are the proper end organs on which
the light acts present there, so light does not lead to the pro-
duction of nervous impulses in them: but the optic nerve
without its peculiar end-organs would be just as insensible
to light as these are. Similarly the aerial vibrations which
affect us as sounds do not stimulate directly the fibres of the
auditory nerve. They act on terminal organs in the ear, and
these then stimulate the fibres of the nerve of hearing, just
as they would any other nerve which happened to be con-
nected with them.

General Nerve Stimuli. Those known are : (1) Electric
currents. An electric shock passed through any part of any
nerve-fibre powerfully excites it. A steady current passing
through a nerve does not stimulate it, but any sudden
change in this, whether an increase or a decrease, does. A
very gradual change in the amount of electricity passing
through a nerve in a unit of time does not stimulate it.
(2) Mechanical stimuli. Any sudden pressure or traction, as

196 THE HUMAN BODY.

a blow or a pull, will stimulate a nerve-fibre. On the other
hand steady pressure, or pressure very slowly increased from
a minimum, will not excite the nerve. (3) Thermal stimuli.
Any sudden heating or cooling of a nerve, as for instance
bringing a hot wire close to it, will stimulate; slow changes
of temperature will not. (4) Chemical stimuli. Manv sub-
stances which chemically alter the nerve-fibre stimulate
before killing it; thus dipping the cut end of a nerve into
a strong solution of common salt will excite it; very slow
chemical change in a nerve fails to stimulate.

In the case of all these general stimuli it will be seen that
as one condition of their efficacy they must act with .con-
siderable suddenness. On the other hand too transient in-
fluences have no eifect. An electric shock sent for only
0.0015 of a second through a nerve does not stimulate it: ap-
parently the inertia of the nerve molecules is too great to be
overcome by so brief an action. So, also, strong sulphuric
acid and some other liquids kill nerves immediately, altering
them so rapidly that they die before being stimulated.

Special Nerve-stimuli. These as already explained act
only on particular nerves, not because one nerve is essen-
tially different from another, but because their influence is
excited through special end-organs which are attached to some
nerves. These stimuli are: (1) Changes occurring in central
organs, of whose nature we know next to nothing, but which
excite the efferent nerve-fibres connected with them. The
remaining special stimuli act on afferent fibres through the
sense-organs. They are: (2) Light, which by the interven-
tion of organs in the eye excites the optic nerve. (3) Sound,
which by the intervention of organs in the ear excites the
auditory nerve. (4) Heat, which through end-organs in
the skin is able, by very slight changes, to excite certain
nerve-fibres: such slight changes of temperature being
efficient as would be quite incapable of acting as general
nerve-stimuli without the proper end-organs. (5) Chemical
agencies, which when extremely feeble and incapable of
acting as general stimuli can act as special stimuli through
special end-organs in the mouth and nose (as in taste and
smell) and probably in other parts of the alimentary tract,
where very feeble acids and alkalies seem able to excite cer-
tain nerves, and reflexly through them excite movements or
render active the cells concerned in making the digestive

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 197

liquids. (6) Mechanical stimuli when so feeble as to be ineffi-
cient as general stimuli. Pressure on the skin of the fore-
head or of the back of the hand, equal to .002 gram (.03 grain)
can be felt through the end-organs of the sensory fibres there,
but would be quite incapable of acting as a general stimulus
if applied directly to the nerve-fibre.

It will be noticed as regards the special stimuli of afferent
nerves that many of them are merely less degrees of general
stimuli ; the end-organs in skin, mouth, and nose are in fact
excited by the same things as nerve-fibres, but they are far
more irritable. In the case of the higher senses, seeing and
hearing, however, the end-organs seem to differ entirely in
property from nerve-fibres, being excited by sonorous and
luminous vibrations which, so far as we know, will in no
degree of intensity directly excite nerve-fibres. To construct
an end -organ capable of recognizing very slight pressures we
may imagine that all that would be needed would be to expose
directly a very delicate end-branch of the axis cylinder ; and
tnis seems in fact to be the case in the nerves of the-transpar-
ent exposed part of the eyeball, if not in some other parts of
the external integument of the Body. But as axis cylinders
are quite unirritable by light or sound a mere exposure of
them would be useless in the eye or ear, and in each case we
find accordingly a very complex apparatus developed, whose
function it is to convert modes of motion which do not
excite nerves into others which do. We might compare
this apparatus to a fuse with a detonating cap attached ; the
stimulus of a blow from a hammer which would not itself
ignite the fuse, acting on the detonating material (repre-
senting an " end -organ"), causes it to give off a spark, and
this in turn ignites the fuse which answers to the nerve-fibre.

Specific Nerve-energies. We have already seen that a
nervous impulse propagated along a nerve-fibre will give rise
to different results according as different nerve-fibres are
concerned. Travelling along one fibre it will arouse a sensa-
tion, along another a movement, along a third a secretion. In
addition we may observe readily that these different results
may be produced by the same physical force when it acts
upon different nerves. . Radiant energy, for example, falling
into the eye causes a sensation of sight, but falling upon the
skin one of heat, if any. The different results which follow
the stimulation of different nerves do not then depend upon

198 THE HUMAN BODY.

differences in the physical forces exciting them. This is
still further shown by the fact that different physical forces
acting upon the same nerve arouse the same kind of sensa-
tion. Light reaching the eye causes a sight sensation, but if
the optic nerve be irritated by a blow on the eyeball a sensa-
tion of light is felt just as if actual light had stimulated the
nerve-ends; and a similar result follows if an electric shock be
sent through the eyeball and optic nerve. Different nerves
excited by the same stimulus produce different results, and
the same nerve excited by different stimuli gives the same
result. How are these facts to be explained ?

The first explanation which suggests itself is that the
various nerves differ in their properties : that electricity ap-
plied to a motor nerve causes a muscle to contract, and
to the optic nerve a visual sensation, and to the lingual
nerve a sensation of taste, because nervous impulses in
the motor, optic, and lingual nerves differ from one an-
other. This was the view held by the older physiologists;
and that supposed peculiarity of a muscular nerve by which
its irritation caused a muscular contraction, and that of
of the optic nerve in consequence of which its excitation
caused a sensation of sight, and so on, they called the specific
energy of the nerve. Seeing further that when a pure motor
nerve was cut and its peripheral stump pinched the muscles
connected with it contracted, but that when its central end
was pinched no sensation or other recognizable change fol-
lowed, while exactly the reverse was true of a sensory nerve,
they believed that afferent nerves differed essentially from
efferent nerves, inasmuch as the latter could only propagate
impulses centrifu gaily and the former only centripetally.
Now, however, we have much reason to believe that this view
is wrong, and that all nerve-fibres, though perhaps exhibiting
some minor differences, are essentially alike in their physio-
logical properties, and can carry nervous impulses either way*
The differences observed depend in fact not on any differ-
ences in the nerve-fibres, but on the different parts connected
with their ends; that is to say, on the different terminal
organs excited by the impulses conveyed by the fibre. A
motor fibre is one which has at its peripheral end a muscular
fibre, and a centrifugally travelling impulse reaching this will
cause it to contract: but the cells connected with its central
end are not of such a nature as to give rise to sensations

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 199

when centripetally travelling impulses reach them, or to
transmit these to other efferent fibres and so cause reflex
movements; and therefore when a motor fibre is stimulated
in the middle of its course the out ward -going impulse causes
a movement, while the centrally travelling impulse, starting
at the same time, gives no direct sign of its existence. Simi-
larly for a sensory nerve such as the ulnar, already referred
to: if it be stimulated at the elbow the centrally travelling
impulse will cause a sensation of pain by exciting the brain-
centre connected with it, but the outward travelling impulse
not reaching muscular fibres or other parts which it can
arouse to activity, remains concealed from our notice. In
other words, the so-called specific energy of a nerve-fibre de-
pends upon the terminal organs on which it can act, and not
on any peculiarity of the nerve-fibre itself.

proofs that all Nerve-Fibres are Physiologically Alike.
(1) Thelnost marked difference between nerve-fibres is obvi-
ously that between efferent and afferent, and the microscope
fails entirely to show any differences between the two. Some
motor and some se!nsory fibres may be bigger or less than
others, some may be white and others may be gray; but such
differences are secondary, and have no direct relation to the
function of a fibre as afferent or efferent. We can recognize
no constant difference in structure between the two. (2)
The physical properties and chemical composition of motor
and sensory nerves agree in all known points. (3) When a
nerve, say a motor one, is stimulated half-way between the
centre and a muscle, a nervous impulse, as we call it, is
propagated to the muscle, which impulse starts at the point
stimulated and travels at a definite rate to the muscle, the
contraction of which latter gives proof of its arrival. Now
starting at the same moment from the same point, and
travelling at the same rate, is that change in the elec-
trical condition of the nerve which can be detected by a
galvanometer, the so-called negative variation or action cur-
rent. When a nerve is excited from its end-organ, as for
example the optic nerve by light falling into the eyeball,
or a motor nerve by a stimulus arising in a centre, an action
current exactly like that observed with artificial stimulation
travels along it. Since this negative variation always accom-
panies a nervous impulse, appearing when it appears and dis-
appearing when it disappears, we conclude that it is a change

200 THE HUMAN BODY.

in the electrical properties of the nerve dependent on that
internal movement of its molecules which constitutes a ner-
vous impulse. It is an externally recognizable physical sign,
and the only known one, of the existence of the nervous im-
pulse while it is travelling along the fibre. If the muscle were
cut away from the end of the nerve we could still detect that
a nervous impulse had travelled from the point of stimulation
to that where the fibres were divided, by tracking the nega-
tive variation. If, while stimulating a motor nerve half-way
in its course, we examine galvanometrically the portion lying
central to the stimulated point we find a well-marked centripe-
tally travelling action current; it starts at the same moment as
the efferent negative variation and travels in the same manner,
but the nervous impulse of which it is a sign produces no more
effect than the efferent impulse would after the muscle had been
cut away; for it does not reach any muscular fibre, or sen-
sory or reflex centre, which it can arouse to activity. Hence
it is clear that the motor nerve can conduct impulses equally
well in either direction; and similar experiment proves the
same thing for pure sensory nerves.

While, however, by chemical or electrical stimulation of
a motor or a secretory nerve we can get a muscular con-
traction or a secretion apparently quite identical with that
produced by natural stimulation, we cannot make the same
assertion with regard to afferent nerves. It is possible by
gentle stimulation of a cutaneous afferent nerve through its
end-organs in the skin to excite the centres, so that they in
turn give rise to definitely combined reflex muscular con-
tractions, producing, even in the absence of all consciousness,
an useful movement. But if the skin be removed and the
outer end of its afferent nerve stimulated directly, though
the centres can be thus excited and caused to send out im-
pulses to muscles, the movements which result are random
kicks and jerks, very different from the definite, orderly
movements which follow suitable stimulation through the
skin. And as regards certain nerves of special sense some-
thing similar seems to be true. It has indeed been stated
that mechanical injury of the optic nerve, as by cutting it
during a surgical operation, causes a sensation of light in
patients not anaesthized, but this has been denied; and
though one positive observation counts for more in such a
case than many negative, we must take into account the

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 201

fact that in no other sense-organ has the direct stimulation
of the proper nerve-trunk in any other way than through
the sense-organ at its outer end, been proved to give rise
to true sensations of special sense. Stimulation of the nerves
exposed in a wound does not cause a true touch sensation,
but a feeling of pain; and similarly irritation of the trunks
of the nerves of taste by diseased conditions does not seem to
ever cause true taste sensations unless the end-organs in the
mouth be also excited. Even if it turn out to be true that
a nerve of special sense is only capable of giving rise to
feelings belonging to the sphere of that sense when ex-
cited through its proper end-organs, that does not prove
that its nerve-fibres have any unique faculty distinguishing
them in property from other nerve-fibres. It only means
that the brain organ, the central nerve-cell mechanism, to be
excited by the nerve is highly complex, and only responds
with the proper sensation when stimulated in proper strength
and proper rhythm, and the sense organs accomplish this.
Even the most delicate artificial stimulation that we can
apply to a naked nerve-trunk is undoubtedly a crude and
gross thing compared with the stimuli arising in the retina
when light enters the eye, or in certain skin nerve end-
organs when moderate heat falls on them. If we merely
wish to get a noise out of a piano it does not matter how
we strike it, if we strike hard enough ; and a muscular con-
traction or an irregular set of muscular contractions excited
by direct stimulation of a nerve-trunk may be compared to
such a noise. If we wish for a definite musical chord we
must strike through the keyboard in a definite way; and the
orderly combined muscular movements and the special sensa-
tions which follow stimulation through the proper sense-
organs may be compared to such chords. In our bodies the
keyboards are different in eye, ear, and skin, and adapted to
be set in action by different external physical agencies, and
the strings in connection with each keyboard are different
and give different results; but the connecting apparatus, the
nerve-fibre, lying between the keys in the sense-organs and
the strings respectively responding to them in the centres,
is essentially the same in all cases.

To put the case more definitely: Light outside the eye
exists as ethereal vibrations, sound outside the ear as vibra-
tions of the air (commonly). Each kind of vibration acts on

202 THE HUMAN BODY.

a particular end-organ in eye or ear which is adapted to be
acted upon by it, and in turn these end-organs excite the
optic and auditory nerve-fibres; these in consequence trans-
mit impulses, which reaching different parts of the brain
excite them ; the excitement of one of these brain-centres is
associated with sonorous and of the other with visual sensa-
tions. The nervous impulse in the two cases is quite alike,
at least as to quality (though it may differ in quantity and
rhythm) and the resulting difference in quality of the sensa-
tions cannot depend on it. The quality differences in these
cases must be products of the central nervous system. If we
had a set of copper wires we might by sending precisely
similar electric currents through them produce very different
results if different things were interposed in their course.
In one case the current might be sent through water and
decompose it, doing chemical work; in another, through the
coil of an electro-magnet and raise a weight; in a third,
through a thin platinum wire and develop light and heat;
and so on, the result depending on the terminal organs, as we
may call them, of each wire. Or, on the other hand, we
might generate the current in each wire differently, in one
oy a Darnell's cell, in a second by a thermo-electric machine,
iti a third by the rotation of a magnet inside a coil, but the
currents in the wires would be essentially the same, as the
nervous impulses are in a nerve-fibre. No matter how they
have been started, provided their amount is the same,
whether they shall produce similar or dissimilar results, de-
pends only on whether they are connected with similar or
dissimilar end-organs.

To sum up : Afferent and efferent nerve-fibres differ in no
fundamental physiological property; they are simple trans-
mitters, everywhere alike in faculty. We may extend this
statement to the subdivisions of each kind, and say that
motor, vasomotor and secretory efferent fibres, and tactile,
auditory and visual afferent fibres are in all essentials like one
another; and that a nervous impulse is alike in every nerve,
varying it may be in intensity and in the rate at which
others succeed it, in different cases, but the same in kind.
Just as all muscles are alike in general physiological proper-
ties, and differ in special function according to the parts on
which they act, so are all nerve-fibres alike in general physio-
logical properties, and differ in special function only because

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 203

they are attached to special things. The special physiology
of various nerves will hereafter be considered in connection
with the working of various mechanisms in the Body.

The Nature of a Nervous Impulse. Since between
sense-organs and sensory centres, and these latter and the
muscles, nervous impulses are the only means of communi-
cation, it is through them that we arrive at our opinions con-
cerning the external universe and through them that we are
able to act upon it; their ultimate nature is therefore a
matter of great interest, but one about which we unfortu-
nately know very little. We cannot well imagine it any-
thing but a mode of motion of the molecules of the nerve-
fibres, but beyond this hypothesis we cannot go far. In
many points tl^e phenomena presented by nerve-fibres as
transmitters of disturbances are like the phenomena of wires
as transmitters of electricity, and when the phenomena of cur-
rent electricity were first observed there was a great ten-
dency, explaining one unknown by another, to consider ner-
vous impulses merely as electrical currents. The increase of
our knowledge concerning both nerves and electric currents,
however/has made such an hypothesis almost, if not quite,
untenable. In the first place nerve-fibres are extremely bad
conductors of electricity — so bad that it is impossible to sup-
pose them used in the Body for that purpose; and in the
second place, merely physical continuity of a nerve-fibre,
such as would not interfere with the passage of an electric
current, will not suffice for the transmission of a nervous im-
pulse. For instance if a damp string be tied around a nerve,
or if it be cut and its two moist ends placed in contact, no
nervous impulse will be transmitted across the constricted or
divided point although an electrical current would pass
readily. An electrical shock may be used like many other
stimuli to upset the equilibrium of the nerve-molecules and
start a nervous impulse, which then travels along the fibre,
but is just as different from the stimulus exciting it as a
muscular contraction is from the stimulus which calls it
forth.

Careful study of the action-current give's, perhaps, some
information regarding the nature of nervous impulses. That
local negativity which causes the current begin& at the stimu-
lated point of a nerve at the same time as the nervous impulse
and travels along the nerve at the same rate. Hence we con-

204: THE HUMAN BODY.

elude that the new internal molecular arrangement in a nerve-
fibre which constitutes its active as compared with its resting
state, is one which changes also the electrical properties of the
fibre. Now it is found that the negative variation travels along
the nerve (in the frog) at the rate of 28 metres (92.00 feet) in
a second, and takes .0007 second to pass by a given point :
accordingly at any one moment it extends over about 18 mm.
(0.720 inch) of the nerve-fibre. Moreover, when first reach-
ing a point it is very feeble, then rises to a maximum,, and
gradually fades away again. Taking it as an indication of
what is going on in the nerve, we may assume that the nerv-
ous impulse is a progressive molecular change of a wavelike
character, rising from a minimum to a maximum, then grad-
ually ceasing, and. about 18 millimetres in wave-length.

A nervous impulse does not appear to exhaust a fibre when
transmitted along it. As a ray of light traversing the ether
sets up a transient change in it but does not in any way use it
up or leave it less fit to transmit a succeeding ray, so it is
with the nervous impulse in its transmission. It is true that
when a motor nerve attached to a muscle is continuously
stimulated the muscular contractions cease after a certain
time, though the muscle still responds to electrical stimula-
tion directly applied, and it has been argued that we thus get
evidence of the exhaustion of the nerve; but it must be
borne in mind that an electrical shock directly applied is un-
doubtedly a much more powerful stimulus to the muscle than
any nervous impulse, and the muscle may have been so
fatigued by its previous work as to have become irresponsive
to stimulation through its nerve, though still reacting to the
grosser excitation. And we have direct evidence that stimu-
lation of a nerve may be continued for a very long time with
out causing loss of activity. As an instance, we may take the
nerve already mentioned which stops the beat of the heart :
when it is stimulated continuously for a few seconds the heart
breaks beyond its control and begins to beat again, though the
stimulation of the nerve be kept up. This, however, is due to
fatigue of the endings of the nerve in the heart, and not of
the nerve fibres, as may be proved in this way: the nerve
(pneumo-gastric) being carefully exposed in the neck is arti-
ficially cooled in one region to below the temperature at
which it can conduct a nervous impulse; it is then stimu-
lated at a point nearer the head than the cooled portion: the

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 205

resulting impulses being blocked on their way to the heart it
goes on beating regularly. After stimulation of the nerve
has been continued for several minutes the cooled tract of the
nerve is allowed to warm again until it becomes capable of
transmitting a nervous impulse; then the heart-beat is found
to be promptly stopped or slowed. This shows that if the
cardiac endings of the nerve be protected from fatigue, pro-
longed stimulation of the nerve-trunk does not interfere with
its functional capacity: the stimulation still starts nervous
impulses in it, which as soon as they can pass on produce their
normal effect on the heart. When long-continued sensations
become dulled the explanation is no doubt similar: it is the
end-organs, central or peripheral, or both, which are ex-
hausted, not the nerve-fibres themselves. It has, however,
been observed that when artificial stimulation is long applied
to one point on a nerve-trunk that point sometimes becomes
unexcitable, though the nerve in general is still quite func-
tional and acts perfectly when the point of application of the
stimulus is shifted a little: this is especially the case with
gray nerve-fibres and white fibres having a thin medullary
sheath.

The very sparse blood-supply of nerve-trunks is in great
contrast to the rich supply of those parts of the nervous system
containing nerve-cells and to the abundant supply of muscles,
aiid is an evidence that the chemical changes taking place in
them during both rest and activity are but small. Seeing
that functional activity leads to little or no using up of the
conductive substance of a nerve-fibre any more than the
transmission of a galvanic current uses up a copper wire, the
term irritable is not properly applicable to nerve-fibres. Ir-
ritability in its physiological sense we have defined as a con-
dition of a living tissue such that a very small extraneous
force acting on it may cause it to set free a disproportionately
large amount of energy, and in that sense muscle-fibres and
nerve-cells are truly irritable, and they both use up their ma-
terial when at work and are subject to exhaustion. Nerve-
fibres are excitable and conductive, but not really irritable,
though on account of their great excitability they are very
generally spoken of as irritable.

The Rate of Transmission of a Nervous Impulse.
This can be measured in several ways. One of the simplest
is a modification of the simple nerve-muscle experiment il-

206 THE HUMAN BODY.

lustrated in Fig. 62. The muscle M is dissected out with its
motor nerve attached, and the stimulus applied to the nerve
and not directly to the muscle. First the stimulus is given
to the nerve close to the muscle: it is then found that the
period of latent excitation, as shown by the greater length of
tu, is a very little longer than when the muscle is directly
stimulated. Next the stimulus is applied to the nerve, say two
inches from the muscle, and it is found that tu is consider-
ably longer, the increase in its length being due to the time
taken by the nervous impulse in travelling along two inches
of nerve. As we know the rate of movement of the surface
S, we can readily calculate the amount of the time increase.
The rate of travel of the nervous impulse as thus ascertained
is almost incomparably slower than that of an electric cur-
rent, being 28 metres (92.00 feet) per 1". In the motor nerves
of warm-blooded animals the rate of transmission is somewhat
faster. Considerable difficulties are met with in making cor-
responding measurements on afferent nerves, and the rates
obtained by different observers differ widely: probably the
impulse travels at about the same speed as in the motor nerves
of the same animal.

Functions of the Spinal Nerve-Roots. The great ma-
jority of the larger nerve-trunks of the Body contain both
afferent and efferent nerve-fibres. If one be exposed in its
course and divided in a living animal, it will be found that
irritating its peripheral stump causes muscular contractions,
and pinching its central stump causes signs of sensation,
showing that the trunk contained both motor and sensory
fibres. If the trunk be followed away from the centre, as
it breaks up into smaller and smaller branches, it will be
found that the^e too are mixed until very near their endings,
where the very finest terminal branches close to the end-
organs, whether muscular fibres, secretory cells, or sensory
apparatuses, are only afferent or efferent. If the nerve-
trunk be one that arises from the spinal cord and be ex-
amined progressively back to its origin, it will still be found
mixed, up to the point where its fibres separate to enter
either a ventral or a dorsal nerve-root. Each of these latter,
however, is pure, all the efferent fibres leaving the cord by
the ventral or anterior roots, and all the afferent entering it by
the posterior or dorsal. This of course could not be learned
from examination of the dead nerves, since the best micro-

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 207

scope fails to distinguish an afferent from an efferent fibre,
but is readily proved by a simple experiment. If an anterior
root be cut and its outer end stimulated, the muscles of the
parts to which the trunk which it helps to form is distributed
will be made to contract, and the skin will be made to sweat
also if the root happen to be one that contains secretory
fibres for the sweat-glands. On the other hand, if the cen-
tral end of the root (that part of it attached to the cord) be
stimulated no result will follow, showing that the root con-
tains no sensory, reflex, or excito-motor fibres. With the
posterior roots the reverse is the case: if one of them be
divided and its outer end stimulated, no observed result fol-
lows, showing the absence of all efferent fibres; but stimula-
tion of its central end will cause either signs of feeling, or
reflex actions, or both. • We might compare a spinal nerve-
trunk to a rope made up of green and red threads with at
one end all the green threads collected into one skein and
the red into another, which would represent the roots. At
its/farthest end we may suppose the rope divided into finer
cords, each of these containing both red and green threads,
down to the very finest branches consisting of only a few
threads, and those all of one kind, either red or green, one
representing efferent, the other afferent, fibres.

The Cranial Nerves. Most of these are mixed also, but
with one exception (the fifth pair, the small root of which is
efferent and the large gangliated one afferent) they do not
present distinct motor and sensory roots, like those of the
spinal nerves. At their origin from the brain most of them
are purely afferent or purely efferent, and the mixed character
which their trunks exhibit is due to cross-branches with
neighboring nerves, in which afferent and efferent fibres are
interchanged. The olfactory, optic, and auditory nerves re-
main, however, purely afferent in all their course, and others,
though not quite pure, contain mainly efferent fibres (as the
facial) or mainly afferent (as the glosso-pharyngeal).

The Intercommunication of Nerve-Centres. From the
anatomical arrangement of the nervous system it is clear that
it forms one continuous whole. No subdivision of it is
isolated from the rest, but nerve-trunks proceeding from the
centres in one direction bind them to various tissues and,
proceeding in another, to other nerve-centres, which in turn
are united with other tissues and other centres. Since the

208 THE HUMAN BODY.

physiological character of a nerve-fibre is its conductivity —
its power of propagating a disturbance when once its mo-
lecular equilibrium has been upset at any one point — it is
obvious that through the nervous system any one part of the
Body, supplied with nerves, may react on all other parts
(with the exception of such as hairs and nails and cartilages,
which are not known to possess nerves) and excite changes in
them. Pre-eminently the nervous system forms a uniting
anatomical and physiological bond through the agency of
which unity and order are produced in the activities of differ-
ent and distant parts. We may compare it to the Western
Union Telegraph, the head office of which in New York
would represent the brain and spinal cord, the more impor-
tant central offices in other large cities the sympathetic
ganglia, and the minor offices in country stations the sporadic
ganglia; while the telegraph-wires, directly or indirectly
uniting all, would correspond to the nerve-trunks. Just as
information started along some outlying wire may be trans-
mitted to a central office, and from it to others, and then,
according to what happens to it in the centre, be stopped
there, or spread in all directions, or in one or two only, so
may a nervous disturbance reaching a centre by one nerve-
trunk merely excite changes in it or be radiated from it
through other trunks more or less widely over the Body and
arouse various activities in its other component tissues. In
common life the very frequency of this uniting activity of the
nervous system is such that we are apt to entirely overlook
it. We do not wonder how the sight of pleasant food will
make the mouth water and the hand reach out for it; it
seems, as we say, "natural," and to need no explanation.
But the eye itself can excite no desire, cause the secretion of
no saliva, and the movement of no limb. The whole com-
plex result depends on the fact that the eye is united by the
optic nerve with the brain, and that again by other nerves
with saliva-forming cells, and with muscular fibres of the
arm; and through these a change excited by light falling
into the eye is enabled to produce changes in far-removed
organs, and excite desire, secretion, and movement. In cases
of disease this action exerted at a distance is more apt to ex-
cite our attention: vomiting is a Trery common symptom of
certain brain diseases, and most people know that a disordered
stomach will produce a headache; while the pain consequent

GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 209

upon the hip-disease of children is usually felt, not at the hip-
joint, but at the knee.

The Degeneration of Nerve-Fibres separated from their
Centre. A nerve-fibre may in its course be connected with
more than one nerve-cell, but one cell always has a special
influence in maintaining its normal structure and functional
activity. If cut off from this cell the separated portion
undergoes degenerative changes, easily recognized in medul-
lated fibres by a breaking up and, later, a disappearance of
the medullary sheath. If, for example, the sciatic nerve of
a warm-blooded animal be completely cut across, all of the
nerve and its branches beyond the point of section will show
marked changes in three days or less: the medullary sheath
separates into small cuboidal pieces, these in a day or two
more round off at their corners and then are gradually ab-
sorbed, so/that at the end of ten days or a fortnight they have
entirely disappeared. Meantime the nuclei of the internodes
multiply and the usually sparse protoplasm around them in-
creases, and encroaches upon and causes the absorption of the
axis cylinder, so that after some weeks little or no trace of
true nervous elements can be found. Some three or four days
after making the section the peripheral portion of the nerve
ceases to be excitable. If the part of the nerve above the sec-
tion be examined, its fibres will be found to have undergone
no degeneration except close to the place of section, and it re-
mains excitable; pinching it causes pain, and if any muscle
branch arising from it be irritated, the muscles contract. If
instead of cutting a whole mixed nerve-trunk, such as the sci-
atic, we divide only a ventral spinal root (as 5, c, Fig. 71), it is
found that all the fibres in that part of the root which is cut
off from the spinal cord degenerate and become unirritable,
and degenerated fibres can be found in the mixed trunk into
which the cut root is continued; while the fibres of the part
of the root still attached to the cord do not degenerate.
The nutritional integrity of the anterior root-fibres depends
therefore on anatomical continuity with the spinal cord, and
probably with cells there, of the type shown in Fig. 81. On
the other hand, if the dorsal root only be cut across, the por-
tion of it attached to the cord degenerates, while that still
connected to the spinal ganglion and the fibres beyond the
ganglion remain unaltered : the nutritive centres for the dor-
sal root-fibres are the cells of the corresponding root-ganglion.

210 THE HUMAN BODY.

After complete section of the nerve-trunk supplying a
region of the Body that region is for a time paralyzed, but
feeling and the power of movement may return to it. It
used to be thought that in such cases the divided nerve-fibres
grew together again. Such is not the case : all those parts of
the fibres which have been cut oif from their centres com-
pletely disappear, and when function is restored it is by the
formation of new nerve-fibres around outgrowths from the
cut ends of those parts of the fibres still united to their cen-
tres, whether these be in brain, spinal cord, spinal ganglia, or
elsewhere.

Nerves, as we have seen, often give fibres to one another
by means of uniting branches, as in various plexuses and
elsewhere; and when a nerve-branch may contain fibres de-
rived from some one of two or more original trunks which
have communicating branches, it is often of importance to
determine in which original trunk its fibres left the brain or
spinal cord. In such cases the determination may often be
made by dividing one of the possible sources of origin and
after a few days examining the branch for degenerated
fibres, which are easily recognized by the microscope. If
such are found, then they left the centre in the divided trunk;
if not, the branch gets no fibres from that trunk. This
method of tracking the nerve-fibres of a given original trunk
to their final distribution in one or more of many possible
branches is known as the Wallerian method. Instances of
its application will be given in later chapters.

CHAPTER XIV.

THE ANATOMY OF THE HEART AND BLOOD-VESSELS.

General Statement. During life the blood is kept flow-
ing with great rapidity, through all parts of the Body (except
the few non-vascular tissues already mentioned) in definite
paths prescribed for it by the heart and blood-vessels.
These paths, which under normal circum-
stances it never leaves, constitute a con-
tinuous set of close^tubes (Fig. 87)
beginning at and ending again in the
heart, and simple only close to that organ.
Elsewhere it is greatly branched, the most
numerous and finest branches (I and a)
being the capillaries. The heart is essen-
tially a bag with muscular walls, internally
divided into four chambers (d, g, e, /).
Those at one end (d and e) receive blood
from vessels opening into them and known
as the veins. From there the blood passes
on to the remaining chambers (g and/)
which have very powerful walls and, for-
cibly contracting, drive the blood out into
vessels (m and b) which communicate with
them and are known as the arteries. The
big arteries divide into smaller; these into smaller again
(Fig. 88) until the branches become too small to be traced by
the unaided eye, and these smallest branches end in the
capillaries, through which the blood flows and enters the
commencements of the veins ; and these convey it again to
the heart. At certain points in the course of the blood-paths
valves are placed, which prevent a back-flow. This alternat-
ing reception of blood at one end by the heart and its ejec-
tion from the other go on during life steadily about seventy
times in a minute, and so keep the liquid constantly in
motion.

211

sented-

THE HUMAN BODY.

The vascular system is completely closed except at two
points in the neck where lymph- vessels open into the veins;
there some lymph is poured in and mixed directly \^th the
blood. Accordingly everything which leaves. the blood must-
do so by ooiing through the walls of the blood-vessels, and
everything which enters it. must do the same, except matters

mdu

del

mvs

mvp

imv

FIG. 88. — The arteries of the hand, showing the communications or anastomose,
of different arteries and the fine terminal twigs given off from the larger trunks;
these twigs end in the capillaries which would only become visible if magnified, .ff,
the radial artery on which the pulse is usually felt at the wrist ; U, the ulnar ar-
tery.

conveyed in by the lymph at the points above mentioned.
This interchange through the walls of the vessels takes place
only in the capillaries, which form a sort of irrigation systerh
all through the Body. The heart, arteries, and veins are all'
merely arrangements for keeping the capillaries full and
renewing the blood within them. It is in the capillaries
alone that the blood does its ulivsiological workv

ANATOMY OF THE I1EA11T AND BLOOD-VESSELS. 213

The Position of the Heart. The heart (h, Fig. 1) lies
in the chest immediately above the diaphragm and opposite
the low^r two thirds of the breast-bone. It is conical in
form with its base or broader end turned upwards and pro-
jecting a little on the right of the sternum, while its narrow
end or apex, turned downwards, projects to the left of that
bone, where it may be felt beating between the cartilages of
the fifth and sixth ribs. The position of the organ in the
Body is therefore oblique with reference to its long axis. It
does not, however, lie on the left side as is so commonly sup-
posed but very nearly in the middle line, with the upper part
inclined to the right, and the lower (which may be more
easily felt beating— hence the common belief) to the left.

The Membranes of the Heart. The heart does not lie
bare in the chest but is surrounded by a. loose bag composed
of connective tissue and called the pericardium. This bag,
like the heart, is conical but turned the other way, its broad
part being lowest and attached to the upper surface of the
diaphragm. Internally it is lined by a smooth serous mem-
brane like that lining the abdominal cavity, and a similar
layer (the visceral layer of the pericardium) covers the out-
side of the heart itselX adhering closely to it. Each of the
3erous layers is covered by a stratum of flat cells, and in the
space between them is found a small .quantity of liquid
which moistens the contiguous surfaces, and diminishes the
friction which would otherwise occur during the movements
of the heart. „ *., /

Internally the heart is also lined by a .fibroids membrane,
covered with a single layer of flattened cells* and called the
endocardium. Between the endocardium and- -the -visceral
layer of the pericardium the bulk of- the wall of the -heart
lies and is made up. mainly of striped muscular tissue (myocar-
dium) differing from that of '^he skeletal muscles; but con-
nective tissues, blood-vessels, nerve-cells, and nerve-fibres are
also abundant in it.

Note. — Sometimes the pericardium becomes inflamed, this
affection being known as pericarditis. It is extremely apt to
occur in acute rheumatism, and great care should be taken
never, even for a moment, except under medical advice, to
expose a patient to cold during that disease, since any chill
is then especially apt to set up pericarditis. In the earlier
stages ol pericardiac inflammation the rubbing surfaces on

214

1HE HUMAN BODY.

FIG. 89.— Diagram representing a section
through the heart from base to apex.

the outside of the heart and the inside of the pericardium
become roughened, and their friction produces a sound
which can be recognized through the stethoscope. In later
stages great quantities of liquid may accumulate in the peri-
cardium so as to seriously impede the heart's beat.

The Cavities of the Heart. On opening the heart (see
diagram Fig. 89) it is found to be subdivided by a longi-
tudinal partition or sep-
tum into completely sepa-
rated rigl\t and left halves,-
the partition running from
about the middle of the
base to a point a little on
the right of the apex.
Each of the chambers on
the sides of the septum is
again incompletely divided
transversely, into a thinner
basal portion into which
veins open, known as the
auricle, and a thicker apical portion from which arteries
arise, called the ventricle. The heart thus consists of a right
auricle and ventricle and a left auricle and ventricle, each
auricle communicating by an aunculo-ventricular orifice
with the ventricle on its own side, and there is no direct
communication whatever through the septum between the
opposite sides of the heart. To get from one side to the
other the blood must leave the heart and pass through a set
of capillaries, as may readily be seen by tracing the course of
the vessels in Fig. 87.

The Heart as seen from its Exterior. When the heart
is viewed from the side turned towards the sternum (Fig. 90)
the two auricles, Aid and As, are seen to be separated by a
deep groove from the ventricles, Vd and Vs. A more
shallow furrow runs between the ventricles and indicates the
position of the internal longitudinal septum. On the dorsal
aspect of the heart (Fig. 91) similar furrows may be noted,
and on one or other of the two figures the great vessels
opening into the cavities of Utes heart may be seen. The
pulmonary artery, P, arises from the right ventricle, and
very soon divades into the right and left pulmonary arteries,
Pd and Ps, which break up into smaller branches and enter

ANATOMY OF THE HEART AND BLOOD-VESSELS. 215

the corresponding lungs. Opening into the right auricle are
two great veins (see also Fig. 89), cs and ci, known re-
spectively as the upper and lower venae cavce, or " hollow "
veins; so called by the older anatomists because they are
frequently found empty after death. Into the back of the
right auricle opens also another vein, Vc, called the coronary

\d

FIG. 90.— The heart and the great blood-vessel attached to it. seen from the
.vanisthe sternum. The left cavities and the vessels connected with them
are colored red; the right black. Aid, right auricle; Adx and As, the right and
left aariculaf appendages; Vd, right ventricle; Vs, left ventricle; Act, aorta; Ab,
inn. -ruinate artery; Cs, left common cjfrotid artery ;"Ssft left subclavian artery;
P, i tain trunk of the pulmonary artery, and Pd and Ps, its branches to the right
and left lungs; cs, superior vena cava; Adt and A si, the right and left innominate
veins; pd and ps, the right and left pulmonary veins; crd and crs, the right and
left Coronary arteries.

or sinus, which brings back blood that has circulated
in tlie wall? of the heart itsfclf. Springing from the left ven-
tricle, am1 appearing from beneath the pulmonary artery
tae heart is looked; at from the ventral side, is a great

,, th« aorta. Aa. It forms an arch over the base of the
heart and then runs down be* ind it at the back -of the chest.
Fron< Mio . mvexity of the arch of the aorta several great

216

THE HUMAN BODY.

branches are given off, Ssi, Cs, Ab; but before that, close to
the heart, the aorta gives off two coronary arteries, branches
of which are seen at crd and crs lying in the groove over the
partition between the ventricles, and which carry to the sub-
stance of the organ that blood which comes back through the

FIG. 91. -The heart viewed from its dorsal aspect. Atd, right auricle; ci, in-
ferior vena cava; Fc, coronary vein. The remaining letters of reference have
vhe same signification as in f ig. 90.

coronary sinus. Into the ]eft auricle open two rigM and two
left pulmonary veins, ps and pd, which are formt^ by the
union of smaller vein^ proceeding1 from the lungs.

In the diagram Fig. 89 from which the branches of the
great vessels near the heart have been omitted for the .sake
of clearness, the connection of the various vessels with the

ANATOMY OF THE HEART AND BLOOD - VESSELS. 217

chambers of the heart can be better seen. Opening into the
right auricle are the superior and inferior venae cavae (cs and
ci) and proceeding from the right ventricle tine pulmonary
artery, P. Opening into the left auricle are the right and
left pulmonary veins (pel and ps) and springing from the left
ventricle the aorta, A.

The Interior of the Heart. The communication of each
auricle with jits ventricle is also represented in the diagram
Fig. 89, and the valves which are present at those points
and at the origin of the pulmonary artery and that of the
aorta. Interjnally the auricles are for the most part smooth^
but from eacji a hollow pouch, the auricular appendage, pro-
jects over th^ base of the corresponding ventricle as seen at
Adx and As ,ln Figs. 90 and 91. These pouches have some-
what the shape of a dog's ear and have given their name to
the whole auricle. Their interior is roughened by muscular-
elevations, covered by endocardium, known as the fleshy col-
umns (columned car nee). On the inside of the ventricles (Fig.
92) similar fleshy columns are very prominent.

The Auriculo-Ventricular Valves. These are known as
right and left, or as the tricuspid and mitral valves respec-
tively. The mitral valve (Fig. 92) consists of two flaps of the
endocardium fixed by their bases to the margins of the auric-
ulo-ventricular aperture and with their edges hanging down
into the ventricle when the heart is empty. These unattached
edges are not however free, but have fixed to them a number
of stout connective-tissue cords, the cor dee tendinece, which
are fixed below to muscular elevations, the papillary muscles,
Mpm and Mpl, on the interior of the ventricle. The cords
are long enough to let the valve flaps riae into a horizontal
position and so close the opening between auricle and ven-
tricle which lies between them, and passes up behind the
opened aorta, Sp, represented in the figure. The tricuspid
, valve is like the mitral, but with three flaps instead of two.

Semilunar Valves. These are six in number: three at
the mouth of the aorta, Fig. 92, and three, quite like them,
at the mouth of the pulmonary artery. Each is a strong
crescentic pouch fixed by its more curved border, and with
its free edge turned away from the heart. When the valves
are in action these free edges meet across the vessel and pre-
vent blood from flowing back into the ventricle. In the
middle of the free border of each valve is a little cartilagi-

218

THE HUMAN BODY.

nous nodule, the corpus Arantii, and on each side of this the
edge of the valve is very thin and when it meets its neighbor
turns up against it and so secures the closure.

The Arterial System. All the arteries of the Body arise
either directly or indirectly from the aorta or pulmonary
artery, and the great majority of them from the former vessel.

Mpl

Mprn

FIG. 92.— The left ventricle and the commencement of the aorta laid open.
Mpm, Mpl. the papillary muscles. From their upper ends are seen the cordce
tendinece proceeding to 'the edges of the flaps of the mitral valve. The opening
into the auricle lies between these flaps. At the beginning of the aorta are seen its
three pouch-like semilunar valves.

The pulmonary artery only carries blood to the lungs, to un-
dergo exchanges with the air in them after it has circulated
through the Body generally.

After making its arch the aorta continues back through
the chest, giving off many 'branches on its way. Piercing the

AN ATOM Y OF THE HEART AND BLOOD - VESSELS. 219

diaphragm it enters the abdomen and after supplying the
parts in and around that cavity with branches, it ends oppo-
site the last lumbar vertebra by dividing into the right and
left common iliac arteries, which carry blood to the lower
limbs. We have then to consider the branches of the arch of
the aorta, and those of the descending aorta> which latter is
for convenience described by anatomists as consisting of the
thoracic aorta, extending from the end of the arch to the.,
diaphragm, and the abdominal aorta, extending from the
diaphragm to the final subdivision of the vessel.

Branches of the Arch of the Aorta. From this arise first
the coronary arteries (crd and crs, Figs. 90 and 91) which
spring close to the heart, just above two of the pouches of the
eemilunar valve, and carry blood into the substance of that
organ. The remaining branches^ of the arch are three in
number, and all arise from its convexity. The first is the
innominate artery (AT), Fig. 90), which is very short, imme-
diately breaking up into the right subclavian artery, and the
right common carotid. Then comes the left common carotid,
Cs, and finally flie left subclavian, Ssi.

Each subclavian artery runs out to the arm on its own
side and after giving off a vertebral artery (which runs up
the neck to the head in the vertebral canal of the transverse
processes of the cervical vertebrae), crosses the arm-pit and
takes there the name of the axillary artery. This contin-
ues down the arm as the brachial artery, which, giving off
branches on its way, runs to the front of the arm, and just
below the elbowT-joint divides into the radial and vlnar ar-
teries, the lower ends of which are seen at R and U in Fig. 88. .
These supply the forearm and end in the hand by uniting to
form an arch, from which branches are given off to the fingers.

The common carotid arteries pass out of the chest into the
neck, along which they ascend on the sides of the windpipe.
Opposite the angle of the lower jaw each divides into an
internal arid external carotid artery, right or left as the case
may be. The external ends mainly in branches for the face,
scalp, and salivary glands, one great subdivision of it with a
tortuous course, the temporal artery, being often seen in thin
persons beating on the side of the brow. The internal carotid
artery enters the skull through an aperture in its base and
supplies the brain, which it will be remembered also gets
blood through the vertebral arteries.

220 THE HUMAN BODY.

Branches of the Thoracic Aorta. These are numerous
but small. Some, the intercostal arteries, run out between
the ribs and supply the chest-walls; others, the bronchial ar-
teries, carry blood to the lungs for their nourishment, that
carried to them by the pulmonary arteries being brought
there for another purpose; and a few other small branches
are given to other neighboring parts.

Branches of the Abdominal Aorta. These are both large
and numerous, supplying not only the wall of the posterior
part of the trunk, but the important organs in the abdominal
cavity. The larger are : the cceliac axis which supplies stom-
ach, spleen, liver, and pancreas; the superior mesenteric
artery, which supplies a great part of the intestine; the renal
arteries, one for each kidney; and finally the inferior mes-
enteric artery, which supplies the rest of the intestine. Be-
sides these the abdominal aorta gives off very many smaller
branches.

Arteries of the Lower Limbs. Each common iliac di-
vides into an internal and external iliac artery. The former
mainly ends in branches to parts lying in the pelvis, but the
latter passes into the thighs and there takes the name of the
femoral artery. At first this lies on the ventral aspect of the
limb, but lower down passes to the back of the femur, and
above the knee-joint (where it is called the popliteal artery]
divides into the anterior and posterior tibial arteries, which
supply the leg and foot.

The Capillaries. As the arteries are followed from the
heart, their branches become smaller and smaller, and finally
cannot be traced without the aid of a microscope. Ulti-
mately they pass into the capillaries, the walls of which are
simpler than those of the arteries, and which form very close
networks in nearly all parts of the Body; their immense num-
ber compensating for their smaller size.' The average diame-
ter of a capillary vessel is .016 mm. (j-gVo inch) so that only
two or three blood-corpuscles can pass through it abreast,
and in many parts they are so close that a pin's point could
not be inserted between two of them. It is while flowing in
these delicate tubes that the blood does its nutritive work, the
arteries being merely supply-tubes for the capillaries.

The Veins. The first veins arise from the capillary net-
works in various organs, and like the last arteries are very
small. They soon increase in size by union, and so form

ANATOMY OF THE HEART AND BLOOD-VESSELS. 221

larger and larger trunks. These in many places lie near or
alongside the main artery of the part, but there are many
more large veins just beneath the skin than there are large
arteries. This is especially the case in the limbs, the main
veins of which are superficial, and can in many persons be
seen as faint blue marks through the skin. Fig. 94 repre-
sents the arm at the front of the elbow-joint after the skin
and subcutaneous areolar tissue and fat have been removed.

FIG. 93.— A small portion of the capillary network as seen in the frog's web when
magnified about 25 diameters, a, a small artery feeding the capillaries; v, v, small
veins carrying blood back from the latter.

The brachial artery, B, colored red, is seen lying tolerably
deep, and accompanied by two small veins (venae comites)
which communicate by cross-branches. The great median
nerve, 1, a branch of the brachial plexus which supplies
several muscles of the forearm and hand, the skin over a
great part of the palm and the three inner fingers, is seen
alongside the artery. The larger veins of the part are seen

222

THE HUMAN BODY.

to form a more superficial network, joined here and there, as
for instance at *, by branches from deeper parts. Several
small nerve-branches which supply the skin (2, 3, 4) are seen
among these veins. It is from the vessel, cep, called the

cep

FIG. 94.— The superficial veins in front of the elbow- joint. B', tendon of biceps
muscle; Bi, brachialis interims muscle;,, Pt, pronator teres muscle; 1, median
nerve; 2, 3, 4, nerve-branches to the skin; B, brachial artery, with its small accom-
panying veins; cep, cephalic vein; has, basilic vein; m', median vein; *, junction of
a deep-lying vein with the cephalic.

cephalic vein, just above the point where it crosses the median
nerve, that surgeons usually bleed a patient.

A great part of the blood of the lower limb is brought back
by the long saplienous vein, which can be seen in thin persons
running from the inner side of the ankle to the top of the

ANATOMY OF THE HEART AND BLOOD-VESSELS. 223

thigh. All the blood which leaves the heart by the aorta,
except that flowing through the coronary arteries, is finally
collected into the superior and inferior vence cavce (cs and ci,
Figs. 90 and 91), and poured into the right auricle. The
jugular veins which run down the neck, carrying back the
blood which went out along the carotid arteries, unite below
with the arm-vein (subdavian) to form on each side an in-
nominate vein (Asi and Ade, Fig. 90) and the innominates
unite to form the superior cava. The coronary-artery blood
after flowing through the capillaries of the. heart itself also
returns to this auricle by the coronary veins and sinus.

The Pulmonary Circulation. Through this the blood
gets back to the left side of the heart and so into the aorta
again. The pulmonary artery, dividing into branches for
each lung, ends in the capillaries -of those organs'; From
these it is collected by the pulmonary veins, which carry it
back to the left auricle, whence it passes to the left ventricle
to recommence its flow through tho Body gencutlly.

The Course of the Blood. From what has been said it is
clear that the movement of the blood is a circut"' Start-

ing from any one chamber of the heart it will in time return
to it; but to clo this it must pass through at least two sets of
capillaries; one of these is connected with the aorta and the
other with the pulmonary artery, and in its circuit the blood
returns to the heart twice. Leaving the left side it returns to
the right, and leaving the right it returns to the left: and
there is no road for it from one side of the heart to the other
except through ;t <;upill;i.^ network. Moreover, it always
leaves fro; ricle through an artery, and returns to an

auric! * \ein.

Then- i - really only one circulation; but it is not un-

common to &(.t'ak of two, the flow from the left side of the
heart to the right, through the Body generally, being called
the systemic, circulation, and from the right to the left,
through the lungs, the pulmonary circulation. But since
after completing either of these alone the blood is not back
at the point from which it started, but is separated from it by
the septum of the heart, neither is a "circulation" in the
proper sense of the word.

Tho Portal Circulation. A certain portion of the blood
which leaves the left ventricle of the heart through the aorta
has to pass through three sets of capillaries before it can again

224

THE HUMAN BODY.

return there. This is the portion which goes through the
stomach, spleen, pancreas, and intestines. After traversing
the capillaries of those organs it is collected into the portal
vein which enters the liver, and breaking up in it into finer
and finer branches like an artery, ends in the capillaries of
that organ, forming the second set which this blood passes
through on its course. From these it is collected by the he-
patic veins, which pour it into the inferior vena cava, which
carries it to the right auricle, so that
it has still to pass through the pulmo-
nary capillaries to get back to the left
side of the heart. The portal vein is
the only one in the human Body which
like an artery feeds a capillary net-
•work, and the flow from the stomach
and intestines through the liver to the
vena cava is often spoken of as the
portal circulation.

Diagram of the Circulation. Since
the two halves of the heart are actu-
ally completely separated from one
another by an impervious partition,
although placed in proximity in the
Body, we may conveniently represent
the course of the blood as in the accom-

FIG. 95.-Diaffram of the panying diagram (Fig. 95), in which
e the right and left halves of the heart
Et -are represented at different points in
which fLeal;ePSre°sented ^ the vascular system. Such an arrange-

T^ht^ri^Sd V^ridel meilt makeS lfc Clear th^fc the heaH 1S

la and iv, left auricle and ven- reallv two pumps working ' .side bv side,

tricle; ao, aorta; sc, systemic " , ,"

capillaries; vc, venae cavae; each engaged in forcing the blood to
pa, pulmonary artery; »c, pul- ,, ? ru j- /> ii i £1.

monary capillaries; pv, pui- the other, otarting ironi the lelt au-
ricle, la, and following the flow, we

trace it through the left ventricle' and along the brunches of
the aorta into the systemic capillaries, sc', from thence it
passes back through the systemic veins, vc. Beaching the
right auricle, ra, it is sent into the right ventricle, rv, and
thence through the pulmonary artery, pa, to the lung .capilla-
ries, pc, from which the pulmonary veins, pv, carry i« to the
left auricle, which drives it into the left ventricle, Iv, and this
again into the aorta.

ANATOMY OF THE HEART AND BLOOD-VESSELS. 225

Arterial and Venous Blood. The blood when flowing in
tne pulmonary capillaries gives up carbon dioxide to the air
and receives oxygen from it; and since its coloring matter
(haemoglobin) forms a scarlet compound with oxygen, it flows
to the left auricle through the pulmonary veins of a bright
red color. This color it maintains until it reaches the sys-
temic capillaries, but in these it loses much oxygen to the
surrounding tissues and gains much carbon dioxide from them.
But the blood coloring-matter which has lost its oxygen has a
dark purple color, and since this unoxidized or "reduced"
hemoglobin is now in excess, the blood returns to the heart
by the venae cavae of a dark purple-red color. This hue it
keeps until it reaches the lungs, when the reduced haemoglo-
bin becomes again oxidized. The bright red blood, rich in
oxygen and poor in carbon dioxide, .is known as "arterial
blood" and the dark red as "venous blood:" and it must be
borne in mind that the terms have this peculiar technical
meaning, and that the pulmonary veins contain arterial blood,
and the pulmonary arteries, , venous blood; the change from
arterial to venous taking place in the systemic capillaries, and
from venous to arterial in the pulmonary capillaries. The
chambers of the heart and the great vessels containing arte-
rial blood are shaded red in Figs. 90 and 91.

The Structure of the Arteries. A large artery can by
careful dissection be separated into three coats : an internal,
a middle, and an outer. The internal coat tears readily across
the long axis of the artery and consists of an inner lining of
flattened nucleated cells, enveloped by a variable number of
layers composed of membranes or networks of elastic tissue.
The middle coat is made up of alternating layers of elastic
fibres and plain muscular tissue; the former running for the
most part longitudinally and the latter across the long axis
of the vessel. The outer coat is the toughest and strongest
because it is mainly made up of white fibrous connective
tissue; it contains a considerable amount of elastic tissue also,
and gradually shades off into a loose areolar tissue which
forms the sheath of the artery, or the tunica adventitia, and
packs it between surrounding parts. The smaller arteries
have all the elastic elements less developed. The internal
coat is consequently thinner, and the middle coat is made up
mainly of involuntary muscular fibres. As a result the large
arteries are hiorhly elastic, the aorta being physically much
like a piece of india-rubber tubing, while the smaller arte-

226 THE HITMAN BODY.

ries are highly contractile, in the physiological sense of the
word.

Structure of the Capillaries. In the smaller arteries the
outer and middle coats gradually disappear, and the elastic
layers of the inner coat also go. Finally, in the capillaries
the lining epithelium alone is left, with a more or less de-
veloped layer of connective-tissue corpuscles around it, repre-
senting the remnant of the tunica adventitia. These vessels
are thus extremely well adapted to allow of nitration or dif-
fusion taking place through their thin walls.

Structure of the Veins. In these the same three primary
coats as in the arteries are found: the inner and middle coats
are less developed, while the outer one remains thick, and is
made up almost entirely of white fibrous tissue. Hence the
venous walls are much thinner than those of the correspond-
ing arteries, and the veins collapse when empty while the
stouter arteries remain open. But the toughness of their
outer coats gives the veins great strength.

Except the pulmonary artery and the aorta, which possess
the semilunar valves at their cardiac orifices, the arteries pos-
sess no valves. Many veins on the contrary have such, formed
by semilunar pouches of the inner coat, attached by one
margin and having the edge turned towards the heart free.
These valves, sometimes single, oftener in pairs, and rarely
three at one level, permit blood to flow only towards the
heart, for a current in that direction (as in the upper dia-
gram, Fig. 96) presses the valve close against the side of the

vessel and meets with no obstruction

A from it. Should any back-flow be at-

c j~ T^ H tempted, however, the current closes

up the valve and bars its own passage
_ B_ _ as indicated in the lower figure. These

valves are most numerous in super-

ficial veins and those of muscular parts.

the POI>tal and W\™™J Veins. Ustl-

end of the vessel.' ally the vein is a little dilated opposite

a valve, and hence in parts where the

valves are numerous gets a knotted look. On compressing
the forearm so as to stop the flow in its subcutaneous veins
and cause their dilatation, the points at which valves are
placed can be recognized by their swollen appearance. They
are most frequently situated where two veins communicate.

CHAPTER XV.

v^^

THE WORKING OF THE HEART AND BLOOD-VESSELS.

The Beat of the Heart. It is possible with some little
skill and care to open the chest of a living narcotized ani-
mal, such as a rabbit, and see its heart at work, alternately
contracting and diminishing the cavities within it, and relax-
ing and expanding them. It is then observed that each beat
commences at the mouths of the great veins; from there runs
over the rest of the auricles, and then over the ventricles; the
auricles commencing to dilate the moment the ventricles
commence to contract. Having finished their contraction
the ventricles also commence to dilate, and so for some time
neither they nor the auricles are contracting, but the whole
heart is expanding. The contraction of any part of the heart
is known as its systole and the relaxation as its diastole, and
since the two sides of the heart work synchronously, the au-
ricles together and the ventricles together, we may describe a '
whole "cardiac period" or "heart-beat" as made up succes-
sively of auricular systole, ventricular systole, and pause.
This cycle is repeated about seventy times a minute; and if
the whole time occupied by it be subdivided into 100 parts,
about 9 of these will be occupied by the auricular systole,
about 30 by the ventricular systole, and 61 by the pause:
during more than half of life, therefore, the muscle-fibres of
the heart are at rest. In the pause the heart if taken be-
tween the finger and thumb feels soft and flabby, but during
the systole it (especially its ventricular portion) becomes hard
and rigid.

Change of Form of the Heart. During its systole the
heart becomes shorter and rounder, mainly from a change in
the shape of the ventricles. ' A cross-section of the heart at
the base of these latter during diastole would be elliptical in
outline, with its long diameter from right to left; during the
systole it is more circular, the long axis of the ellipse becom-
ing shortened, while the dorso-ventral diameter remains little

227

228 THE HUMAN BODY.

altered. At the same time the length of the ventricles is
lessened, the apex of the heart approaching the base and be-
coming blunter and rounder.

The Cardiac Impulse. The human heart lies with its
apex touching the chest-wall between the fifth and sixth ribs
on the left side of the breast-bone. At every beat a sort of
tap, known as the "cardiac impulse" or "apex beat/' may be
felt by the finger at that point. There is, however, no actual
" tapping/7 since the heart's apex never leaves the chest-wall.
During the diastole the soft ventricles yield to the chest-wall
where they touch it, but during the systole they become hard
and tense and push it out a little between the ribs, and so
cause the apex beat. Since the heart becomes shorter during
the ventricular systole, it might be supposed that at that time
the apex would move up a little in the chest. This, how-
ever, is not the case, the ascent of the apex towards the base
of the ventricles being compensated for by a movement of
the whole heart in the opposite direction. If water be pumped
into an elastic tube, already moderately full, the tube will be
distended not only transversely but longitudinally. This is
what happens in the aorta : when the left ventricle contracts
and pumps blood forcibly into it, the elastic artery is elongated
as well as widened, and the lengthening of that limb of its arch
attached to the heart pushes the latter down towards the dia-
phragm, and compensates for the upward movement of the
apex due to the shortening of the ventricles. Hence if the
exposed living heart be watched it appears as if during the
systole the base of the heart moved towards the tip, rather
than the reverse.

Events occurring within the Heart during a Cardiac
Period. Let us commence at the end of the ventricular
systole. At this moment the semilunar valves at the orifices
of the aorta and the pulmonary artery are closed, so that no
blood can flow back from those vessels. The whole heart,
however, is soft and distensible and yields readily to blood
flowing into it from the pulmonary veins and the venae cavae;
this passes on through the open mitral and tricuspid valves
and fills up the dilating ventricles, as well as the auricles. As
the ventricles fill, back currents are set up along their walls
and these carry up the flaps of the valves so that by the end
of the pause they are nearly closed. At this moment the au-
ricles contract, and since this contraction commences at and

WORKING OF THE HEART AND BLOOD-VESSELS. 229

narrows the mouths of the veins opening into them, and at
the same time the blood in those vessels opposes some resist-
ance to a back-flow into them, while the still flabby and
dilating ventricles oppose much less resistance, the general
result is that/the contracting auricles send blood into the
ventricles, and not back into the veins. At the same time the
increased direct current into the ventricles produces a greater
back current on the sides, which, when the auricles cease
their contraction and the filled ventricles become tense and
press on the blood inside them, completely closes the auriculo-
ventricular valves. That this increased filling of the ventri-
cles, due to auricular contractions, will close the valves may
be seen easily in a sheep's heart. If the auricles be carefully
cut away from this so as to expose the mitral and tricuspid
valves, and water be then poured from a little height into the
ventricles, it will be seen that as these cavities are filled the
valve-flaps are floated up and close the orifices.

The auricular contraction now ceases and the ventricular
commences. The blood in each ventricle is imprisoned be-
tween the auriculo- ventricular valves behind and the semi-
lunar valves in front. The former cannot yield on account
of the cordae tendinese fixed to their edges: the semilunar
valves, on the other hand, can open outwards from the ven-
tricle and let the blood pass on, but they are kept tightly shut
by the pressure of the blood on their other sides, just as the
lock-gates of a canal are by the pressure of the water on
them. In order to open the canal-gates water is let in or out
of the lock until it stands at the same level on each side of
them; but of course they might be forced open without this
by applying sufficient power to overcome the higher water
pressure on one side. It is in this latter way that the semi-
lunar valves are opened. The contracting ventricle tightens
its grip on the blood inside it and becomes rigid to the touch.
As it squeezes harder and harder, at last the pressure on the
blood within it becomes greater than the pressure exerted on
the other side of the valves by the blood in the arteries, the flaps
are forced open and the blood begins to pass out : the ventri-
cle continues its contraction until it has obliterated its cavity
and completely emptied itself; this total emptying' appears,
at least, to occur in the normally beating heart, but in some
pathological conditions and under the influence of certain
drugs the emptying of the ventricles is incomplete. After

230 THE HUMAN BODY.

the systole the ventricle commences to relax and blood imme-
diately to flow back towards it from the highly stretched ar-
teries. This return current, however, catches the pockets of
the semilunar valves, drives them back and closes the valve
so as to form an impassable barrier; and so the blood which
has been forced out of either ventricle cannot flow directly
back into it.

Use of the Papillary Muscles. In order that the con-
tracting ventricles may not force blood back into the auricles
it is essential that the flaps of the mitral and tricnspid valves
be maintained in position across the openings which they
close, and be not pushed back into the auricles. At the com-
mencement of the ventricular systole this is provided for by
the cordae tendineae, which are of such a length as to keep
the edges of the flaps in apposition, a position which is
farther secured by the fact that each set of cordae tendineae
(Fig. 92) radiating from a point in the ventricle, is not at-
tached around the edges of one flap but on the contiguous
edges of two flaps, and so tends to pull them together. But
as the contracting ventricles shorten, the cordae tendineae, if
directly fixed to their interior, would be slackened and the
valve-flaps pushed up into the auricle. The little papillary
muscles prevent this. Shortening as the ventricular systole
proceeds, they keep the cordae taut and the valves closed.

The mechanism is indeed even better working than this.
The area of the valve-flaps is greater than is sufficient to
stretch across the auriculo-ventricular orifice, so that when
their edges are in apposition they form a cone projecting into
the ventricle. Towards the ends of the systole the papillary
muscles pull this cone down into the ventricular cavity so as
to practically obliterate it and force out from it nearly every
drop of blood. \

Sounds of the Heart. If the ear be placed on the chest
over the region of the heart during life, two distinguishable
sounds will be heard during each cardiac cycle. They are
known respectively as the first and second sounds of the
heart. The first is of lower pitch and lasts longer than the
second and sharper sound: vocally their character may be
tolerably imitated by the words lubb, dup. The cause of the
second sound is the closure, or, as one might say, the"*' click-
ing up," of the semilunar -valves, since it occurs at the
moment of their closure and ceases if they be hooked back in

WORKING OF THE HEART AND BLOOD - VESSELS. 231

a living animal. The origin of the first sound is still uncer-
tain: it takes place during the ventricular systole and is
probably due to vibrations of the tense ventricular wall at
that time. It is not due, at least not entirely, to the auriculo-
ventricular valves, since it may still be heard in a beating
heart empty of blood, and in which there could be no closure
or tension of those valves. In various forms of heart disease
these sounds are modified or cloaked by additional " mur-
murs" which arise when the cardiac orifices are roughened
or narrowed or dilated, or the valves inefficient. By paying
attention to the character of the new sound then heard, the
exact period in the cardiac cycle at which it occurs, and the
region of the chest-wall at which it is heard most distinctly,
the physician can often get important information as to its
cause.

Diagram of the Events of a Cardiac Cycle. In the fol-
lowing table the phenomena of the heart's beat are repre-
sented with reference to the changes of form which are seen
on an exposed working heart. Events in the same vertical
column occur simultaneously; on the same horizontal line,
from left to right, successively.

Auricular
Systole.

Commence-
ment of
Ventricular
Systole.

Ventricular

Systole-

Cessation
of Ven-
tricular
Systole.

Pause.

Auricles
Ventricles
Impulse

Contracting
and
emptying.
Dilating and
filling.

Dilating and
filling.

Contracting.
Apex beat.

Dilating and
filling.

Contracting
and
emptying.

Dilating

and filling.

Dilaliag.

Dilating
and filling.

Dilating
and filling.

Auriculo-vrutric
ular valves .
Semilunar valves

Closing.
Closed.

Closed.
Closed.
First sound

Closed.
Open.

Opening.
Closing.
Second

Open.
Closed.

sound.

Function of the Auricles. The ventricles have to do the
work of pumping the blood through the blood-vessels. Ac-
cordingly their walls are far thicker and more muscular than
those of the auricles; and the left ventricle, which has to
force the blood over the Body generally, is stouter than the
right, which has only to send blood around the comparatively
short pulmonary circuit. The circulation of the blood is in
fact maintained by the ventricles, and we have to inquire
what is the use of the auricles. Not unfrequently the heart's

232 THE HUMAN BODY.

action is described as if the auricles first filled with blood and
then contracted and filled the ventricles; and then the latter
contracted and drove the blood into the arteries. From the
account given above, however, it will be seen that the events
are not accurately so represented, but that during all the
pause blood flows on through the auricles into the ventricles,
which latter are already nearly full when the auricles con-
tract; this contraction merely completing their filling and
finishing the closure of the auriculo-ventricular valves. The
real use of the auricles is to afford a reservoir into which
the veins may empty while the comparatively long-lasting
ventricular contraction is taking place: they also largely
control the amount of work done by the heart.

If the heart consisted of the ventricles only, with valves
at the points of entry and exit of the blood, the circulation
could be maintained. During diastolo the ventricle would
fill from the veins, and during systole empty into the arteries.
But in order to accomplish this, during the systole the valves
at the point of entry must be closed, or the ventricle would
empty itself kito the veins as well as into the arteries; and
this closure would necessitate a great loss of time which
might be utilized for feeding the pump. This is avoided by
the auricles, which are really reservoirs at the end of the
venous system, collecting blood when the ventricular pump is
at work. When the ventricles relax, the blood entering the
auricles flows on into them: but previously, during the -ffo
of the cardiac cycle occupied by the ventricular systole, the
auricles have accumulated blood, and when they at last con-
tract they send on into the ventricles this accumulation.
Even were the flow from the veins stopped during the auric-
ular contraction this would be of comparatively little conse-
quence, since that event occupies so brief a time. But, al-
though no doubt somewhat lessened, the emptying of the
veins into the heart does not seem to be, in health, stopped
while the auricle is contracting. For at that moment the
ventricle is relaxing and receives the blood from the auricles
under a less pressure than it enters the latter from the veins.
The heart in fact consists of a couple of " feed-pumps " — the
auricles — and a couple of "force-pumps" — the ventricles;
and so wonderfully perfect is the mechanism that the supply
to the feed-pumps is never stopped. The auricles are never
empty, being supplied all the time of their contraction, which

WORKING OF THE HEART AND BLOOD-VESSELS. 233

is never so great as to obliterate their cavities; while the ven-
tricles contain no blood at the end of their systole.

The auricles also govern to a certain extent the amount
of work done by the ventricles. These latter contract with
more than sufficient force to completely drive out all the
blood contained in them. If the auricles contract more
powerfully and empty themselves more completely at any
given time, the ventricles will contain more blood at the com-
mencement of their systole, and will have pumped out more
at its end. Now, as we shall see in Chapter XVIII, the con-
traction of the auricles is under the control of the nervous
system, and through the auricles the whole work of the
heart. In fact the ventricles represent the brute force con-
cerned in maintaining the circulation, while the auricles are
part of a highly-developed co-ordinating mechanism, by
which the rate of the blood-flow is governed according to
the needs of the whole Body at the time.

The Work Done by the Heart. This can be calculated
with approximate correctness. At each systole each ven-
tricle sends out the same quantity of blood — a^out 180 grams
(6.3 ounces); the pressure se^errfreffl by the blood in the aorta
against the semilunar jp^Tves, and which the ventricle has to
overcome, is about that which would be exerted on the same
surface by a column of mercury 200 millimeters (8 inches)
high. The left ventricle therefore drives out, seve-hty times
in a minute, 180 grams (6.3 ounces) of blood against this
pressure. Since the specific gravity of mercury is 12.5 and
that of blood may for practical purposes be taken as 1, the
work of each stroke of the ventricle is equivalent to raising
180 grams (6.3 ounces) of blood 200 X 12.5 = 2500 millim.
(8.2 feet); or one gram 450 meters (one ounce 51.66 feet);
or one kilogram 0.45 meter (one Ib. 3.23 feet). Work is
measured by the amount of energy needed to raise a definite
weight a given distance against gravity at the earth's surface,
the unit, called a kilogr ammeter, being either that necessary
to raise one kilogram one meter, or, called a foot-pound, that
necessary to raise one pound one foot. Expressed thus the
work of the left ventricle in one minute, when the heart's
rate is seventy strokes in that time, is 0.45 X 70 — 31 50 kilo-
grammeters (3.23 x 70 = 226.1 foot-pounds) ; in one hour it
is 31.50 X 60 = 1890 kilogrammeters (226.1 X 60 = 13,566
foot-pounds); and in twenty-four hours 1890 X 24 = 45,360

234 THE HUMAN BODY.

kilogrammeters (325,584 foot-pounds). The pressure in the
pulmonary artery against which the right ventricle works is
about ^ of that in the aorta; hence this ventricle in twenty-
four hours will do one third as much work as the left, or
15,120 kilogrammeters (108,528 foot-pounds), and adding
this to the amount done by the left, we get as the total work
of the ventricles in a day the immense amount of 60,480
kilogrammeters (434,112 foot-pounds). If a man weighing
75 kilograms (165 Ibs.) climbed up a mountain 806 meters
(2644 feet) high his skeletal muscles would probably be
greatly fatigued at the end of the ascent, and yet in lifting
his Body that height they would only have performed the
amount of work that the ventricles of the heart do daily
without fatigue.

The Plow of the Blood Outside the Heart. The_blood
leaves the heart intermittently and not in a regular stream,
a quantity being forced out at each systole of the ventricles:
before it reaches the capillaries, however, this rhythmic
movement is transformed into a steady flow, as may readily
be seen by examining under the microscope thin transparent
parts of various animals, as the wreb of a frog's foot, a mouse's
ear, or the tail of a small fish. In consequence of the steadi-
ness with which the capillaries supply the veins the flow in
these is also unaffected, directly, by each beat of the heart;
if a vein be cut the blood wells out uniformly, while a cut
artery spurts out not only with much greater force, but in jets
which are much more powerful at regular intervals corre-
sponding witfi the systoles of the ventricles.

The Circulation of the Blood as Seen in the Prog's Web,
There is no more fascinating or instructive phenomenon than
the circulation of the blood as seen with the microscope in
the thin membrane between the toes of a frog's hind lirnb.
Upon focusing beneath the epidermis a network of minute
arteries, veins, and capillaries, with the blood flowing through
them, comes into view (Fig. 91). The arteries, a, are
readily recognized by the fact that the flow in them is fastest
and from larger to smaller branches. The latter are seen
ending in capillaries, which form networks, the channels of
which are all nearly equal in size. While in the veins aris-
ing from the capillaries the flow is from smaller to larger
trunks, and slower than in the arteries, but faster than in the
capillaries.

WORKING OF THE HEART AND BLOOD - VESSELS. 235

The reason of the slower flow of the capillaries is that
their united area is considerably greater than that of the
arteries supplying them, so that the same quantity of blood
flowing through them in a given time has a wider channel
to flow in and moves more slowly. The area of the veins is
smaller than that of the capillaries but greater than that of
the arteries, and hence the rate of movement in them is also
intermediate. Almost always when an artery divides, the
area of its branches is greater than that of the main trunk,
and so the arterial current becomes slower and slower from
the heart onwards. In the veins, on the other hand, the area
of a trunk formed by the union of two or more branches is
less than that of the branches together, and the flow becomes
quicker and quicker towards the heart. But even at the
heart the united cross-sections of the veins entering the auri-
cles are greater than those of the arteries leaving the ventri-
cles, so that, since as much blood returns to the heart in a
given time as leaves it, the rate of the current in the pul-
monary veins and the verise cava3 is less than in the pulmonary
artery and aorta. We may represent the vascular system as
a double cone, widening from the ventricles to the capillaries
and narrowing from the latter to the auricles. Just as water
forced in at a narrow end of this would flow quickest there
and slowest at the widest part, so the blood flows quickest in
the aorta and slowest in the capillaries, which taken together
form a much wider channel.

The Axial Current and the Inert Layer,, If a small
artery in the frog's web be closely examined it will be seen
that the rate of flow is not the same in all parts of it. In the
centre is a very rapid current carrying along all the red cor-
puscles and known as the axial stream, while near the wall
of the vessel the flow is much slower, as indicated by the rate
at which the pale blood-corpuscles are carried along in it.
This is a purely physical phenomenon. If any liquid be for-
cibly driven through a fine tube which it wets, water for in-
stance through a glass tube, the outermost layer of the liquid
will remain motionless in contact with the tube; the next
layer of molecules will move a little, the next faster still;
and so on until a rapid current is found in the centre. If
solid bodies, as powdered sealing-wax, be suspended in the
water, these will all be carried on in the central faster cur-
rent or axial stream, just as the red corpuscles are in the

236 THE HUMAN BODY.

artery. The white corpuscles, partly because of their less
specific gravity, and partly because of their sometimes irregu-
lar form, due to amoeboid movements, get frequently pushed
out of the axial current, so that many of them are found in
the inert layer.

Internal Friction. It follows from the above-stated facts
that there is no noticeable friction between the blood and
the lining of the vessel through which it flows: since the
outermost blood-layer in contact with the wall of the vessel is
changed only by diffusion. There is great friction between
the different concentric layers of the liquid, since each of
them is moving at a different rate from that in contact with
it on each side. This form of friction is known in hydro-
dynamics as "internal friction," and it is of great importance
in the circulation of the blood. Internal friction increases
very fast as the calibre of the tube through which the liquid
flows diminishes: so that with the same rate of flow it is dis-
proportionately much greater in a small tube than in a larger
one. Hence a given quantity of liquid forced in a minute
through one large tube would experience much less resistance
from internal friction than if sent in the same time through
four or five smaller tubes, the united transverse sections ox
which were together equal to that of the single larger one. In
the blood-vessels the increased total area, and consequently
slower flow, in the smaller channels partly counteracts this
increase of internal friction, but only to a comparatively
slight extent; so that the internal friction, and consequently
the resistance to the blood-flow, is far greater in the capil-
laries than in the small arteries, and in the small arteries than
in the large ones. Practically we may regard the arteries as
tubes ending in a sponge: the united areas of all the channels
in the latter might be considerably larger than that of the
supplying tubes, but the friction to be overcome in the flow
through them would be much greater.

The Conversion of the Intermittent into a Continuous
Flow. Since the heart sends blood into the 'aorta intermit-
tently, we have still to inquire how it is that the flow in the
capillaries is continuous. In the larger arteries it is not,
since we can feel them dilating as the "pulse," on applying
the finger over the radial artery at the wrist, or over the tem-
poral artery on the side of the brow.

The first explanation which suggests itself is that since

WORKING OF THE HEAET AND BLOOD-VESSELS. 237

the capacity of the blood-vessels increases from the heart to
the capillaries, an acceleration of the flow during the ven-
tricular contraction which might be very manifest in the
vessels near the heart would become less and less obvious in
the more distant vessels. But if this were so, then when the
blood was collected again from the wide capillary sponge into
the great veins near the heart, which together are but little
bigger than the aorta, we ought to find a pulse, but we do
not: the venous pulse which sometimes occurs having quite a
different cause, being due to a back-flow from the auricles, or
a checking of the on-flow into them, during the cardiac sys-
tole. The rhythm of the flow caused by the heart is therefore
not merely cloaked in the small arteries and capillaries, but
abolished in them.

We can, however, readily contrive conditions outside the
Body under which an intermittent supply is transformed into
a continuous flow. Suppose we
have two vessels, A and B (Fig.
97), containing water and con-
nected below in two ways:
through the tube a on which
there is a pump provided with
valves so that it can only drive
liquid from A to B', and through
#, which may be left wide open
or narrowed by the clamp c, at
will. If the apparatus be left
at rest the water will lie at the
same level, d, in each vessel. If now we work the pump, at
each stroke a certain amount of water will be conveyed from
A to B, and as a result of the lowering of the level of liquid
in A and its rise in B, there will be immediately a return
flow from B to A through the tube 7). A, in these circum-
stances, would represent the venous system, from which the
heart constantly takes blood to pump it into B, representing
the arterial system ; and I would represent the capillary ves-
sels through which the return flow takes place; but, so far,
we should have as intermittent a flow through the capillaries,
b, as through the heart-pump, a. Now imagine 1) to be nar-
rowed at one point so as to oppose resistance to the back-flow,
while the pump goes on working steadily. The result will be
nn accumulation of water in B, and a fall of its level in A.

— at.

FIG. 97.

238 THE HUMAN BODY.

But the more the difference of level in the two vessels in-
creases, the greater is the force tending to drive water back
through I to A, and more will flow back, under the greater
difference of pressure, in a given time, until at last, when the
water in B has reached a certain level, d', and that in A has
correspondingly fallen to d", the current through b will carry
back in one minute just so much water as the pump sends the
other way, and this back-flow will be nearly constant; it will
not depend directly upon the strokes of the pump, but upon
the head of water accumulated in B^ which head of water
will, it is true, be slightly increased at each stroke of the
pump, but the increase will be very small compared with the
whole driving force, and its influence will be inappreciable.
We thus gain the idea that an incomplete impediment to the
flow from the arteries to the veins (from B to A in the dia-
gram), such as is afforded by internal friction in the capil-
laries, may bring about conditions which will lead to a steady
flow along the latter vessels.

But in the arterial system there can be no accumulation of
blood at a higher level than that in the veins, such as is sup-
posed in the above apparatus; and we must next consider if
the "head of water" can be replaced by some other form of
driving force. It is in fact replaced by the elasticity of the
large arteries. Suppose an elastic bag instead of the vessel B
connected with the pump " a" If there be no resistance to
the back-flow the current through b will be discontinuous.
But if resistance be interposed, then the elastic bag will be-
come distended, since the pump sends in a given time more
liquid into it than it passes back through b. But the more it-
becomes distended the more will the bag squeeze the liquid
inside and the faster will it send that back to A, until at last
its squeeze is so powerful that each minute or two or five min-
utes it sends back into A as much as it receives. Thenceforth
the back-flow through b will be practically constant, being im-
mediately dependent upon the elastic reaction of the bag, and
only indirectly upon the action of the pump which keeps it
distended. Such a state of things represents very closely the
phenomena occurring in the blood-vessels. The highly elastic
large arteries are kept stretched with blood by the heart; and
the reaction of their elastic walls, steadily squeezing on the
blood in them, forces it continuously through the small arte-
ries and capillaries. The steady flow in the latter depends

WORKING OF THE HEART AND BLOOD-VESSELS. 239

thus on two factors: first, the elasticity of the large arteries;
and, secondly, the resistance to their emptying, dependent
upon internal friction in the small arteries and the capillaries,
which calls into play the elasticity of the large vessels. Were
the capillary resistance or the arterial elasticity absent the
blood-flow in the capillaries would be rhythmic.

CHAPTER XVI.
ARTERIAL PRESSURE. THE PULSE.

Weber's Schema. It is clear from the statements made
in the last chapter that it is the pressure exerted by the elas-
tic arteries upon the blood inside them which keeps up the
flow through the capillaries, the heart serving to keep the
big arteries tightly filled and so to call the elastic reaction of
their walls into play. The whole circulation depends prima-
rily, of course, upon the beat of the heart, but this only in-
directly governs the capillary flow, and since the latter is the
aim of the whole vascular apparatus, it is of great importance
to know all about arterial pressure; not only how great it is
on the average, but how it is altered in different vessels in
various circumstances so as to make the flow through the
capillaries of a given part greater or less according to circum-
stances; for, as blushing and pallor of the face (which fre-
quently occur without any change in the skin elsewhere)
prove, the quantity of blood flowing through a given part is
not always the same, nor is it always increased or diminished
in all parts of the Body at the same time. Most of what we
know about arterial pressure has been ascertained by experi-
ments made upon the lower animals, from which deductions
are then made concerning what happens in man, since An-
atomy shows that the circulatory organs are arranged upon
the same plan in all the mammalia. A great deal can, how-
ever, be learnt by studying the flow of liquids through ordi-
nary elastic tubes. Suppose we have a set of such (Fig. 98)
supplied at one point with a pump, c, possessing valves of
entry and exit which open only in the direction indicated by
the arrows, and that the whole system is slightly overfilled
with liquid so that its elastic walls are slightly stretched.
These will in consequence press upon the liquid inside them
and the amount of this pressure will be indicated by the
gauges; so long as the pump is at rest it will be the same
everywhere (and therefore equal in the gauges on B and A),

240

ARTERIAL PRESSURE. THE PULSE. 241

since liquid in a set of horizontal tubes communicating freely,
as these do at D, always distributes itself so that the pressure
upon it is everywhere the same. Let the pump c now con-
tract once, and then dilate : during the contraction it will
empty itself into B and during the dilatation fill itself from
A. Consequently the pressure in B, indicated by the gauge
x, will rise and that in A will fall. But very rapidly the
liquid will redistribute itself from B to A through D, until
it again exists everywhere under the same pressure. Every

FIG. 98.— Diagram of Weber's Schema.

time the pump works there will occur a similar series of
phenomena, and there will be a disturbance of equilibrium
causing a wave to flow round the tubing; but there will be
no steady maintenance of a pressure on the side B greater
than that in A. Now let the upper tube D be closed so that
the liquid to get from B to A must flow through the narrow
lower tubes D', which oppose considerable resistance to its
passage on account of their frequent branchings and the
great internal friction in them ; then if the pump works fre-
quently enough there will be produced and maintained in B
a pressure considerably higher than that in A, which may
even become negative. If, for example, the pump works 60
times a minute and at .each stroke takes 180 cubic centi-
meters of liquid (6 ounces) from A and drives it into B, the
quantity sent in at the first stroke will not (on account of the
resistance to its flow offered by the small branched tubes),
have all got back into A before the next stroke takes place,
sending 180 more cubic centimeters (6 oz.) into B. Conse-
quently at each stroke B will become more and more dis-
tended and A more and more emptied, and the gauge x will

242 THE HUMAN BODY.

indicate a much higher pressure than that on A. As B is
more stretched, however, it squeezes harder upon its con-
tents, until at last a time comes when this squeeze is power-
ful enough to force through the small tubes just 180 cubic
centimeters (6 oz.) in a second. Then further accumulation
in B ceases. The pump sends into it 10,800 cubic centi-
meters (360 ounces) in a minute at one end and it squeezes
out exactly that amount in the same time from its other end ;
and so long as the pump works steadily the pressure in B
will not rise, nor that in A fall, any more. But under such
circumstances the flow through the small tubes will be nearly
constant since it depends upon the difference in pressure pre-
vailing between B and A, and only indirectly upon the pump
which serves simply to keep the pressure high in B and low
in A. At each stroke of the pump it is true there will be
a slight increase of pressure in B due to the fresh 180 cub.
cent. (6 oz.) forced into it, but this increase will be but a
small fraction of the total pressure and so have but an in-
significant influence upon the rate of flow through the small
connecting tubes.

Arterial Pressure. The condition of things just de-
scribed represents very closely the phenomena presented in
the blood-vascular system, in which the ventricles of the
heart, with their auriculo-ventricular and semilunar valves,
represent the pump, the smallest arteries and the capillaries
the resistance at D', the large arteries the elastic tube B, and
the veins the tube A. The ventricles constantly receiving
blood through the auricles from the veins, send it into the
arteries, which find a difficulty in emptying themselves
through the capillaries, and so blood accumulates in them
until the elastic reaction of the stretched arteries is able to
squeeze in a minute through the capillaries just so much
blood as the left ventricle pumps into the aorta, and the right
into the pulmonary artery, in the same time. Accordingly in
a living animal a pressure-gauge connected with an artery
shows a much higher pressure than one connected with a vein,
and this persisting difference of pressure, only increased by a
small fraction of the whole at each heart-beat, brings about
a steady flow from the arteries to the veins. The heart keeps
the arteries stretched and the stretched arteries maintain the
flow through the capillaries, and the constancy of the current
in them depends on two factors: (1) the resistance experi-

ARTERIAL PRESSURE. THE PULSE. 243

enced by the blood in its flow from the ventricles to the
veins, and (2) the elasticity of the larger arteries which allows
the blood to accumulate in them under a high pressure, in
consequence of this resistance.

The Arterial Pressure. This cannot be directly meas-
ured with accuracy in man, but from measurements made on
other animals it is calculated that in the human aorta its
average is equal to that of a column of mercury 200 milli-
meters (8 inches) high. During the systole it rises about 5
millimeters (| inch) above this and during the pause falls the
same amount below it. The pressure in the venae cavae on the
other hand is often negative, the blood being, to use ordinary
language, often " sucked " out of them into the heart, and it
rarely rises above 5 millimeters (^ inch) of mercury except
under conditions (such as powerful muscular effort accom-
panied by holding the breath) which force blood on into the
venae cavae and, by impeding the pulmonary circulation, in-
terfere with the emptying of the right auricle. Hence to •
maintain the flow from the aorta to the vena cava we have
an average difference of pressure equal to 200 — 5 — 195
millimeters (7| inches) of mercury, rising to 205 — 5 = 200
mm. (8 inches) during the cardiac systole and falling to
195 — 5 = 190 mm. (7-f inches) during the pause; but the
slight alterations, only about -^ of the whole difference of
aortic and vena-cava pressures which maintain the blood-
flow, are too small to cause appreciable changes in the rate of
the current in the capillaries. The pressure on the blood in
the pulmonary artery is about £ of that in the aorta.

Since the blood flows from the aorta to its branches and
from these to the capillaries and thence to the veins, and
liquids in a set of continuous tubes flow from points of
greater to those of less pressure, it is clear that the blood-
pressure must constantly diminish from the aorta to the
right auricle; and similarly from the pulmonary artery to
the left auricle. At any point, in fact, the pressure is pro-
portionate to the resistance in front, and since the farther
the blood has gone the less of this, due to impediments at
branchings and to internal friction, it has to overcome in
finishing its round, the pressure on the blood diminishes as
we follow it from the aorta to the venae cavae. In the larger
arteries the fall of pressure is gradual and small, since the
amount of resistance met with in the flow through them is

244 THE HUMAN BODY.

but little. In the small arteries and capillaries the resistance
overcome and left behind is (on account of the great internal
friction due to their small calibre) very great, and conse-
quently the fall of pressure between the medium-sized arteries
and the veins is rapid and considerable.

Modifications of Arterial Pressure by Changes in the
Bate of the Heart's Beat. A little consideration will make
it clear that the pressure prevailing at any timu in a given
artery depends on two things — the rate at which the vessel
is filled, i.e., upon the amount of work done by the heart;
and the ease or difficulty with which it is emptied, that is,
upon the resistance in front. A third factor has to be taken
into account in some cases; namely, that when the muscular
coats of the small arteries contract the local capacity of the
vascular system is diminished, and has to be compensated for
by greater distention elsewhere, and vice versa. This would
of itself of course bring about changes in the pressure ex-
erted on the contained liquid, but for the present it may be
left out of consideration. Returning to the system of elastic
tubes with a pump represented in Fig. 98, let us suppose the
pump to be driving as before 10,800 cub. cent. (360 oz.) per
minute into the tubes B, and that these latter are so dis-
tended that they drive out just that quantity in the same
time. Under such conditions the pressure at any given
point in B will remain constant, apart from the small varia-
tions dependent upon each stroke of the pump. Now, how-
ever, let the latter, while still sending in 180 cub. cent.
(6 oz.) at each stroke, work 80 instead of 60 times a minute
and so send in that time 180 X 80 = 14,400 cub. cent.
(480 oz.) instead of the former quantity. This will lead to
an accumulation in B, since its squeeze is only sufficient,
against the resistance opposed to it, to send out 10,800 cub.
cent. (360 oz.) in a minute. B consequently will become
more stretched and the pressure in it will rise. As this
takes place, however, it will press more powerfully on its
contents until at last its distention is such that its elastic
reaction is able to force out in a minute through the small
tubes D, 14,400 cub. cent. (480 oz.) Thenceforth, so long
as the pump beats with the same force and at the same rate
and the peripheral resistanee remains the same, the mean
pressure in B will neither rise nor fall — B sending into A in
a minute as much as c takes from it, and we would have a

ARTERIAL PRESSURE. THE PULSE. 245

steady condition of things with a higher mean pressure in B
than before.

On the other hand2 if the pump begins to work more
slowly while the resistance remains the same, it is clear that
the mean pressure in B must fall. If, for example, the
pump works only forty times a minute and so sends in that
time 180 X 40 = 7200 cub. cent. (240 oz.) into B, which is
so stretched that it is squeezing out 10,800 cub. cent. (360
oz. ), in that time, it is clear that B will gradually empty
itself and its walls become less stretched and the pressure in
it fall. As this takes place, however, it will force less liquid
in a minute through the small tubes, until at last a pressure is
reached at which the squeeze of B only sends out 7200 cub.
cent. (340 oz.) in a minute ; and then the fall of pressure will
cease and a steady one will be maintained, but lower than
before.

Applying the same reasoning to the vascular system, we
see that (the peripheral resistance remaining unaltered), if the
heart's force remains the same1 but its rate increases, arterial
pressure will rise to a new level, while a slowing of the heart's
beat will bring about a fall of pressure.

Modifications of Arterial Pressure Dependent on
Changes in tha Force of the Heart's Beat. Returning
again to Fig. 98 : suppose that, while the rate of the pump
remains the same, its power alters so that each time it sends
200 cub. cent. (6.6 oz.) instead of 180 (6 oz.) and so in a
minute 12,000 cub. cent (396 oz.) instead of 10,800 (360 oz.)
—the quantity which B is stretched enough to squeeze out
in that time. Water will in consequence accumulate in B
until it becomes stretched enough to squeeze out 12,000 cub.
cent. (396 oz.) in a minute, and then a steady pressure at a
new and higher level will be maintained. On the other
hand if the pump, still beating sixty times a minute, works
more feebly so as to send out only 160 cub. cent. (5.6 oz.) at
each stroke, then B, squeezing out at first more than it
receives in a given time, will gradually empty itself until it
only presses hard enough upon its contents to force 160 x 60
— 9600 cub. cent. (336 oz.) out in a minute.

Similarly, if while the resistance in the small arteries and
capillaries remains the same and the heart's rate unchanged
the stroke of the latter alters, so that at each beat it sends

246 THE HUMAN BODY.

more blood out than it did previously, then arterial pressure
will rise; while if the heart beats more feebly it will fall.

Modifications of Arterial Pressure by Changes in the
Peripheral Resistance. Let the pump c in Fig. 98 still
work steadily sending 10,800 cub. cent. (360 oz.) per minute
into B and the resistance increase, it is clear arterial pressure
must rise. For B is only stretched enough to squeeze out in
a minute the above quantity of liquid against the original re-
sistance, and cannot at first send out that quantity against the
greater. Liquid will consequently accumulate in it until at
last it becomes stretched enough to send out 10,800 cub.
cent. (360 cubic oz.) in a minute through the small tubes, in
spite of the greater resistance to be overcome. A new mean
pressure at a higher level will then be established. If, on the
contrary, the resistance diminishes while the pump's work
remains the same, then B will at first squeeze out in a minute
more than it receives, until finally its elastic pressure is
reduced to the point at which its receipts and losses balance,
and a new and lower mean pressure will be established in B.

Similarly in the vascular system, increase of the peripheral
resistance by narrowing of the small arteries will increase
arterial pressure in all parts nearer the heart, while dilata-
tion of the small arteries will have the contrary effect.

Summary. We find then that arterial pressure at any
moment is dependent upon — (1) the rate of the heart's beat;
(2) the quantity of blood forced into the arteries at each
beat; (3) the calibre of the smaller vessels. All of these,
and consequently the capillary circulation which depends
upon arterial pressure, are under the control of the nervous
system (see Chap. XVII.).

The Pulse. When the left ventricle contracts it forces a
certain amount of blood into the aorta, which is already dis-
tended and on account of the resistance in front cannot
empty itself as fast as the contracting ventricle fills it. As
a consequence its elastic walls yield still more — it enlarges
both transversely and longitudinally and if exposed in a
living animal can be seen and felt to pulsate, swelling out at
each systole of the heart, and shrinking and getting rid of
the excess during the pause. A similar phenomenon can be
observed in all the other large arteries, for just as the con-
tracting ventricle fills the aorta faster than the latter empties
(the whole period of diastole and systole being required by

ARTERIAL PRESSURE. THE PULSE. 247

the aorta to pass on the blood sent in during systole), so the
increased tension in the aorta immediately after the cardiac
contraction drives on some of its contents into its branches.,
and fills these faster than they are emptying, and so causes
a dilatation of them also, which only gradually disappears as
the aortic tension falls before the next systole. Hence after
each beat of the heart there is a sensible dilatation of
all the larger arteries, known as the pulse, which becomes
less and less marked at points on the smaller branches
farther from the heart, but which in health can readily be
recognized on any artery large enough to be felt by the
finger through the skin, etc. The radial artery near the
wrist, for example, will always be felt tense by the finger,
since it is kept overfilled by the heart in the way already de-
scribed. jfBut after each heart-beat it becomes more rigid
and dilates a little, the increased distension and rigidity
gradually disappearing as the artery passes on the excess of
blood before the next heart-beat. The pulse is then a wave
of increased pressure started by the ventricular systole, ra-
diating from the semilunar valves over the arterial system,
and gradually disappearing in the smaller branches. In the
aorta the pulse is most marked, for the resistance there to
the transmission onwards of the blood sent in by the heart is
greatest, and the elastic tube in which it consequently accu-
mulates is shortest, and so the increase of pressure and the
dilatation caused are considerable. The aorta, however,
gradually squeezes out the excess blood into its branches, and
so this becomes distributed over a wider area, and these
branches having less resistance in front find less and less diffi-
culty in passing it on ; consequently the pulse-wave becomes
less and less conspicuous and finally altogether disappears be-
fore the capillaries are reached, the excess of liquid in the
whole arterial system after a ventricular systole being too
small to sensibly raise the mean pressure once it has been
widely distributed over the elastic vessels, which is the case
by the time the wave has reached the small branches which
supply the capillaries.

The pulse-wave travels over the arterial system at the rate
of about 9 metres (29.5 feet) in a second, commencing at the
wrist 0.159 second, and in the posterior tibial artery at the
ankle 0.193 second, after the ventricular systole. The blood
itself does not of course travel as fast as the pulse-wave, for

24:8 THE HUMAN BODY.

that quantity sent into the aorta at each heart- beat does not
immediately rush on over the whole arterial system, but by
raising the local pressure causes the vessel to squeeze out
faster than before some of the blood it already contains, and
this entering its branches raises the pressure in them and
causes them to more quickly fill their branches and raise the
pressure in them; the pulse-wave or wave of increased press-
ure is transmitted in this way much faster than any given
portion of the blood. How the wave of increased pressure
and the liquid travel at different rates may be made clearer
perhaps by picturing what would happen if liquid were
pumped into one end of an already full elastic tube, closed at
the other end. At the closed end of the tube a dilatation and
increased tension would be felt immediately after each stroke
of the pump, although the liquid pumped in at the other end
would have remained about its point of entry; it would cause
the pulsation not by flowing along the tube itself, but by giv-
ing a push to the liquid already in it. If instead of absolutely
closing the distal end of the tube one brought about a state
of things more nearly resembling that found in the arteries
by allowing it to empty itself against a resistance, say through
a narrow opening, the phenomena observed would not be es-
sentially altered; the increase of pressure would travel along
the distended tube far faster than the liquid itself.

The pulse being dependent on the heart's systole, " feeling
the pulse" of course primarily gives a convenient means of
counting the rate of beat of that organ. To the skilled touch,
however, it may tell a great deal more, as for example whether
it is a readily compressible or " soft pulse" showing a low ar-
terial pressure, or tense and rigid ("a hard pulse") indicative
of high arterial pressure, and so on. In adults the normal
pulse rate may vary from sixty-five to seventy-five, the most
common number being seventy-two. In the same individual
it is faster when standing than when sitting, and when sitting
than when lying down. Any exercise increases its rate tem-
porarily, and so does excitement ; a sick person's pulse should
not therefore be felt when he is nervous or excited (as the
physician knows when he tries first to get his patient calm
and confident), as it is then difficult to draw correct conclu-
sions from it. In children the pulse is quicker than in adults,
and in old age slower than in middle life.

The Bate of the Blood-Flow. As the vascular system

ARTERIAL PRESSURE. THE PULSE. 249

becomes more capacious from the aorta to the capillaries the
rate of flow in it becomes proportionately slower, and as the
total area of the channels diminishes again from the capilla-
ries to the venae cavae, so does the rate of flow quicken, just
as a river current slackens when it spreads out, and flows
faster where it is confined to a narrower channel ; a fact taken
advantage of in the construction of Eads' jetties at the mouth
of the Mississippi, the object of which is to make the water
flow in a narrower channel and so with a more rapid current
in that part of the river. Actual measurements as to the rate
of flow in the arteries cannot be made on man, but from ex-
periments on lower animals it is calculated that in the human
carotid the blood flows about 400 millimetres (16 inches) in a
second. In the capillaries the current travels only from 0.5
to 0.75 mm. (-fa to -fa inch) in a second. The total time
taken by a portion of blood in making a complete circulation
has been measured by injecting some easily detected sub-
stance into an artery on one side of the body and noting the
time which elapses before it can be found in a corresponding
vein on the oppos: e side. In dogs this time is 15 seconds,
and it is calculated for man at about 23 seconds. Of this
total time about half a second is spent in the systemic and
another half second in the pulmonary capillaries, as each por-
tion of blood on its course from the last artery to the first
vein passes through a length of capillary which on the aver-
age is 0.5 mm. (^¥ inch). The rate of flow in the great veins
is about 100 mm. (4 inche,s) in a second, but is subject to con-
siderable variations dependent on the respiratory and other
movements of the Body; in the small veins it is much slower.
Secondary Causes of the Circulation. x Wh^le the heart's
beat is the great driving force of the circulation, certain other
things help more or less — viz., gravity, compression of the
veins, and aspiration of the thorax. All of them are, how-
ever, quite subsidiary; experiment on the dead Body shows
that the injection of whipped blood into the aorta under a
less force than that exerted by the left ventricle during life
is more than sufficient to drive it round and back by the venae
cavae. Not infrequently the statement is made in books that,
probably, the systemic capillaries have an attractive force for
arterial blood and the pulmonary capillaries for venous blood,
but there is not the slightest evidence of the correctness of
such a supposition, nor any necessity for making it.

250 THE HUMAN BODY.

The Influence of Gravity. Under ordinary circum-
stances this may be neglected, since in parts of the Body
below the level of the heart it will assist the llow in the ar-.
teries and impede it equally in the veins, while the reverse
is the case in the upper parts of the Body. In certain cases,
however, it is well to bear these points in mind. A part
"congested" or gorged with blood should if possible be raised
so as to make the back-flow in its veins easier; and sometimes
when the heart is acting feebly it may be able to drive blood
along arteries in which gravity helps, but not otherwise. Ac-
cordingly in a tendency to fainting it is best to lie down, and
make it easier for the heart to send blood up to the brain,
bloodlessness of which is the cause of the loss of consciousness
in a fainting-fit. In fact, so long as the breathing continues,
the aspiration of the thorax will keep up the venous flow (see
below), while, in the circumstances supposed, a slight dimi-
nution in the resistance opposed to the arterial flow may be
of importance. The head of a person who has fainted should
accordingly never be raised until he has undoubtedly recov-
ered, a fact rarely borne in mind by spectators, who commonly
rush at once to lift any one whom they see fall in the street
or elsewhere.

The Influence of Transient Compression of the Veins.
The valves of the veins being so disposed as to permit only
a flow towards the heart, when external pressure empties a
vein it assists the circulation. Continuous pressure, as by
a tight garter, is of course bad, since it checks all subsequent
flow through the vessel; but intermittent pressure, such as is
exerted on many veins by muscles in the ordinary move-
ments of the Body, acts as a pump to force on the blood in
them.

The valves of the veins have another use in diminishing
the pressure on the lower part of those vessels in many
regions. If, for instance, there were no valves in the long
saphenous vein of the leg the considerable weight of the
column of blood in it, which in the erect position would be
about a metre (39 inches) high, would press on the lower part
of the vessel. But each set of valves in it carries the weight
of the column of blood between it and the next set of valves
above, and relieves parts below, and so the weight of the col-
umn of blood is distributed and does not all bear on any one
point.

ARTERIAL PRESSURE. THE PULSE. 251

Aspiration of the Thorax. Whenever a breath is drawn
the pressure of the air on the vessels inside the chest is di-
minished, while that on the other vessels of the Body is un-
affected. In consequence blood tends to flow into the chest.
It cannot, however, flow back from the arteries on account of
the semilunar valves of the aorta, but it can readily be pressed,
or in common language " sucked/' into the great veins close
to the heart and into the right auricle of the latter. The
details of this action must be omitted until the respiratory
mechanism has been considered. All parts of the pulmonary
circuit being within the thorax, the respiratory movements do
not directly influence it, except in so far as the distention or
collapse of the lungs alters the calibre of their vessels.

The considerable influence of the respiratory movements
upon the venous circulation can be readily observed. In
thin persons the jugular vein in the neck can often be seen
to empty rapidly and collapse during inspiration, and fill up-
in a very noticeable way during expiration, exhibiting a.
sort of venous pulse. Every one, too, knows that by making,
a violent and prolonged expiration, as exhibited for example'
by a child with whooping-cough, the flow in all the veins of
the head and neck may be checked, causing them to swell up
and hinder the capillary circulation until the person becomes
" black in the face/' from the engorgement of the small ves-
sels with dark-colored venous blood.

In diseases of the tricuspid valve another form of venous,
pulse is often seen in the superficial veins of the neck, since
at each contraction of the right ventricle some blood is driven
back through the right auricle into the veins.

Proofs of the Circulation of the Bloo.cL The ancient
physiologists believed that the movement of the blood was an
ebb and flow, to and from each side of the heart, and out and in
by both arteries and veins. They had no idea of a circulation,,
but thought pure blood was formed in the lungs and impure
in the liver, and that these partially mixed in the heart
through minute .pores supposed to exist in the septum.
Servetus, who was burnt alive by Calvin in 1553, first stated
that there was a continuous passage through the lungs from
the pulmonary artery to the pulmonary veins, but the great
Englishman Harvey first, in lectures delivered in the College
of Phvsicians of London about 1616, demonstrated that the
movement of the blood was a continuous circulation as we

252 THE HUMAN BODY.

now know it, and so laid the foundation of modern Physi-
ology. In his time, however, the capillary vessels had not
been discovered, so that although he was quite certain that
the blood got somehow from the final branches of the aorta to
the radicles of the venous system, he did not exactly know
how.

The proofs of the course of the circulation are at present
quite conclusive, and may be summed up as follows: (1)
Blood injected into an artery in the dead Body will return by
a vein; but injected into a vein will not pass back by an
artery. (2) The anatomical arrangement of the valves of the
heart and of the veins shows that the blood can only flow
from the heart, through the arteries and back to the heart by
the veins. (3) A cut artery spurts from the end next the
heart, a cut vein bleeds most from the end farthest from the
heart. (4) A portion of a vein when emptied fills only from
the end farthest from the heart. This observation can be
made on the veins on the back of the hand of any thin per-
son, especially if the vessels be first gorged by holding the
hand in a dependent position for a few seconds. Select then
a vein which runs for an inch or so without branching, place
a finger on its distal end, and then empty it up to its next
branch (where valves usually exist) by compressing it from
below up. The vessel will then be found to remain empty as
long as the finger is kept on its lower end, but to fill im-
mediately when it is removed; which proves that the valves
prevent any filling of the vein from its heart-end backwards.
(5) If a bandage be placed around the arm, so as to close the
superficial veins, but not tight enough to occlude the deeper-
seated arteries, the veins on the distal side of the bandage
will become gorged and those on its proximal side empty,
showing again that the veins only receive blood from their
ends turned towards the capillaries. (6) In the lower animals
direct observation with the microscope shows the steady flow
of blood from the arteries through the capillaries to the veins,
but never in the opposite direction.

CHAPTER XVII.

THE NERVES OF THE HEART, AND SOME PHYSIOLOGI-
CAL PECULIARITIES OF CARDIAC MUSCLE.

The Co-ordination of Heart and Arteries. We have
hitherto considered the working of the vascular system as if
it were a mere mechanical hydraulic apparatus; and such in
a certain sense it is, and by so regarding it many of the phe-
nomena of the blood-flow can be explained. But life is a
constant adjustment to constantly varying conditions, and
the higher the organism the more numerous the conditions
which influence it and the greater its power of adapting itself
to them ; and this adaptability, this continuous self -adjust-
ment, is nowhere better exhibited than in the heart and
blood-vessels.

The object to be attained is the maintenance of an orderly
current in the capillaries in accordance with the needs of the
whole Body and of each of its organs at the time. This
clearly calls for some means -of interaction between heart anu
blood-vessels: should the heart beat and the arteries relax or
contract, each without reference to the other, no steady capil-
lary flow could result. To secure such a flow the work done
by the heart and the resistance offered in the vessels must
at all times be correlated; so that the heart shall not
by too powerful action over-distend or perhaps burst the
small arteries, nor the latter contract too much and so, by
increasing the peripheral resistance, raise aortic pressure to a
great height and increase unduly the work to be done by the
left ventricle in forcing open the semilimar valves.

Further, the total amount of blood in the Body is not suffi-
cient to keep all its organs simultaneously supplied with the
amount needful for the full exercise of their activity; in fact
the blood-vessels of the spleen, liver, and alimentary canal, if
all fully distended, can themselves contain almost the whole
blood of the Body, so that by paralyzing their coats in an animal
it can be caused to faint, or even be killed, by what has been

253

254 THE HUMAN BODY.

called an "internal bleeding," due to the accumulation of so
much blood in the vessels of the abdomen that not enough is
left over for the supply of the brain and other parts. In the
Body, accordingly, we never find all its parts hard at work at
the same moment. If when one group of muscles was set at
work and needed an extra blood-supply, this should be pro-
vided merely by increasing the heart's activity and keeping
up a faster blood-flow everywhere throughout the Body,
there would be a clear waste — much as if the chandeliers
in a house were so arranged that when a larger flame was
wanted at one burner it could only be obtained by turn-
ing more gas on at all the rest at the same time; besides the
big tap at the gas-meter regulating the general supply of the
house, local taps at each burner are required which regulate
the gas-supply to each flame independently of the others. A
corresponding arrangement is found in the Body. Certain
nerves control the calibre of the arteries supplying different
organs and, when the latter are set at work, cause their arte-
ries to dilate and so increase the amount of blood flowing
through them, while the general circulation elsewhere re-
mains practically unaffected. The resting parts at any mo-
ment thus get enough blood to maintain their healthy nutri-
tion and the working parts get the larger quantity required to
make good used-up material and to wash out wastes : as certain
organs come to rest and others are set in activity, the arteries
of the former narrow and of the latter dilate; in this way the
distribution of the blood in the Body is undergoing constant
changes, parts which at one time contain much blood at an-
other having but little. In addition, then, to nervous organs
regulating the work of the heart and the arteries with refer-
ence to one another, we have to consider another set of vascu-
lar nerves which govern the local blood-supply of different
regions of the Body. How important this is may be illus-
trated by considering what happens when the surface of the
Body is exposed for some time to cold. The skin normally
contains much blood, brought to it in part to be cooled ; but
under the supposed conditions the loss of heat would soon be
so great as to be harmful did not small arteries of the skin
contract, as is indicated by its pallor, and thus lessen the
blood-flow through it. This contraction is not chiefly, if at
all, due to direct action of the cold on the vessels, but to the
stimulation of cutaneous afferent nerves which excite a nerve-

NERVES OF THE HEART. CARDIAC MUSCLE. 255

centre from which efferent impulses are in turn sent to the
muscular coat of the cutaneous arteries. The blood driven
from the skin must find a place elsewhere in the circulatory
system, and so internal organs tend to become over-full and
at the same time general arterial pressure is raised. This,
again through nerves, acts upon the heart, and alters its rate
of beat for a time. But in health some internal arteries soon
dilate sufficiently to compensate for the constriction of the
surface vessels, and arterial pressure and the pulse again be-
come normal, though with a less proportion of the total blood
flowing through the skin than before: this readjustment is
brought about entirely through nerves and nerve-centres
placing all the arteries in connection with one another and
with the heart, so that they exert a mutual control. If the
cold be not too prolonged its cessation is followed by a return
of the blood-flow to its original condition, this action being
brought about by cardiac and vascular nerve apparatuses. We
have to mainly consider in this and the succeeding chapter
the nerves which regulate the heart-beat and those which in-
fluence the calibre of arteries; but it is necessary iirst to study
the muscular tissue of the heart more thoroughly than we have
hitherto done.

Some Physiological Peculiarities of Cirdiac Muscle.
We have already seen that the muscular tissue of the heart,
though striped, differs considerably in structure from the
tissue of the skeletal muscles: it differs also somewhat in
properties, and as the latter differences can be most readily
studied on the heart of the frog, which will beat for a long
time after excision, it will be best to commence with that.
The frog's heart consists of four contractile chambers through
which the blood flows successively, as is indicated in the dia-
gram, Fig. 99, in which no attempt has been made to indicate
the actual appearance of the organ, which is in fact curved
on itself somewhat in the form of a capital si (see Z, Fig. 9.9),
and this is also the shape of the mammalian heart in an early
stage of embryonic development. The main chambers are
incompletely separated by constrictions, at some of which
valves are placed, and are in order — the venous sinus, A,
receiving blood from the systemic veins; the atrium, consist-
ing of two auricles, B, C, of which the right is much the
larger and is supplied from the sinus, while the left gets blood
from the small pulmonary veins, pv, the ventricle, D, sup-

256

THE HUMAN BODY.

plied from both auricles and having projecting into it the two
flaps of the auriculo-ventricular valve, which are continued

from the end of the septum
or partition lying between
the auricles; the bulbus ar
teriosua, E, from which the
systemic and pulmonary arte-
ries are supplied. To describe
the very interesting mechan-
ism by which the arterial and
venous blood supplied to the
single ventricle are kept sep-
arate and sent from the arte-
rial bulb through different
channels would take us be-
yond the limits of this book,
but it is well worth study in
some treatise on comparative
physiology.

The muscular tissue of the
frog's heart consists of cells
which are in form somewhat
like those of involuntary mus-
cle, but they are frequently
forked at their ends, and they
are obscurely cross - striped
like human cardiac muscle
(Fig. 123). The main thick-
ness of the walls of all the
chambers of the heart consists
of this muscle, and is known
as the myocardium. It com-
mences on the ends of the

Flo. 99.— Diagram of the frog's heart, ~raof VPITKS npm* whprp thpv
A, venous sinus; B, C, right and left au- great Veins liey

ricies, together forming the atrium ; p «., -jojn the heart, and is thence

pulmonary veins; a, a, constriction be- J

tween sinus and atrium; D, ventricle; continued to the roots of the
g, 0, constriction between auricles and . .... ,

ventricles; i, auriculo-ventricular valve; great arteries arising irom the
E, arterial bulb; P, pneumogastric or f . ,». , ,-,

vagus nerve; S, sympathetic nerve; N, bulb; but it IS thinner at the
cardiac nerve containing fibres from both . ,. , . -, r -,

vagus and sympathetic. Z shows the COllStriCtlOUS Which 116 De-
natural relative positions of the chief , rn-_ fr._ ^o^ ncnrifipa fVian
chambers of the heart: d, vena cava; e, tween the main Cavities tnail

?SSflBfcf>/f ^*l" ;le; elsewhere, and there is ar-

ranged in rings around the openings.

NERVES OF THE HEART. CARDIAC MUSCLE. 257

A single nerve, N, goes to the heart from each side (only
that of the right side is represented in the diagram). This
nerve is usually spoken of as the cardiac branch of the vagus
or pneumogastric, P, but it is partly made up of fibres from
the sympathetic nerve, S, which join the pneumogastric close
to the skull and run on with its cardiac branch, the two form-
ing the apparently single nerve-trunk, N, which runs to the
venous sinus, breaking up near it into several twigs. On these
twigs and in the plexus which they form in the wall of the sinus
are numerous nerve-cells, forming the sinus ganglion or
ganglion of Remak. From the sinus nerves run down the
walls of the auricles to the auriculo-' ventricular groove, #, and
two comparatively large twigs pass down the auricular septum
to the region of the valve, i, and there enter a collection of nerve
cells which, with other cells lying in the groove, constitute the
auricula-ventricular or Bidder's ganglion. From that gan-
glion nerves are continued to the wrall of the ventricle, and
near its base have nerve-cells mixed with them. A few
nerve-cells are also found among the fibres running down the
auricular septum : in the apex of the ventricle, however, and
in the bulb there are no ganglion-cells, though nerve-fibres
are present. We find then a considerable collection of nerve-
cells in the walls of the venous sinus, a few cells in the au-
ricular septum, a considerable collection at the junction of
atrium with ventricle, and a few scattered cells in the neigh-
boring portions of the ventricle. The cells of the ganglion
of Eemak and some of those in the septum belong to a type
differing somewhat from those hitherto described. Each is
pear-shaped, and has a conspicuous nucleus with a nucle-
olus; from the narrow end of the cell proceeds a branch
which ultimately becomes the axis cylinder of a medullated
nerve-fibre. Another branch arises by two. or more roots
which coil spirally around the straight branch, and finally
unite and proceed as a non-medullated fibre. Most of the
remaining nerve-cells of the frog's heart are spindle-shaped,
and receive a nerve-fibre atrone end and give one off at the
other. They are known as bipolar cells. The cardiac nerve,
N, Fig. 99, contains both gray and medullated fibres, the
latter coming entirely or almost entirely from its vagus root;
as the fibres passing on from the sinus ganglion to the gan-
glion of Bidder contain very few medullated fibres, it is prob-
able that many of the vagus fibres end in the pear-shaped

258 THE HUMAN BODY.

cells from which gray fibres are given off to the rest of the
heart, mingled with the original gray fibres derived from the
sympathetic : in the ventricle and bulb only non-medullated
fibres are found.

The Beat of the Frog's Heart. When both cardiac
nerves are cut in a frog the heart continues its regular rhyth-
mic beat, as it does also when carefully removed from the
body of the animal: this makes it clear that whatever initi-
ates the beat lies in the heart itself, which must therefore be
regarded as an automatic organ; but leaves it still uncertain
whether the exciting cause of each beat is to be sought in the
nervous elements of the heart or in the cardiac muscle itself.
Arguing from the analogy of ordinary striped muscle, which
is not automatic, one would be inclined to ascribe to the
nerve-cells of the isolated heart the origination of nervous
impulses for the myocardium, and certain experiments tend
to support this view; but cardiac muscle differs considerably
from the skeletal muscles in its histology, so it is unsafe to
argue from one to the other, and some experiments show that
we must ascribe to it, in addition to contractility, a certain
amount of automaticity and of conductivity and co-ordinating
power. In physiological properties it combines the character-
istic properties of fully differentiated nerve-cell and nerve-
fibre with those of muscle-fibre.

/ Each beat of the heart of the frog can be seen to com-
mence where the great veins enter the venous sinus, and from
there to spread rapidly over the whole sinus; then there is a
brief check, and the atrium beats; then another check, fol-
lowed by the beat of the ventricle; finally, again after a very
short pause, comes the contraction of the arterial bulb : then
the series of phenomena is repeated in the same unvarying
order as long as. the heart is in good condition and is left to
itself. The fact that each cycle of contractions begins at
the mouths of the venae caves and the sinus, where nerve-cells
are very numerous, and passes on to the ventricle, where they
are few, and to the bulb, where there are none, has been taken
as an evidence of the origination of each beat through stimuli
developed in cardiac nerve-cells; and this opinion gains sup-
port from what is usually seen on an excised heart when it is
gradually dying. The bulb and ventricle cease to beat first,
then the auricles, last the sinus, and this although the ven-
tricle may still be contractile and able to give a good beat or

NERVES OF THE HEART. CARDIAC MUSCLE. 259

a set of several beats when directly stimulated, as by pricking

or by induction shocks. The loss of irritability as the

heart dies also usually appears in the same order: when the

ventricle and auricle have both ceased to beat, it is frequently

possible to excite the auricle by a direct stimulation which is

powerless when applied to the ventricle; and when the whole

heart has ceased to pulsate the venous sinus will sometimes re-

spond to direct stimulation when auricle and ventricle will not.

Still further, if the heart be carefully divided at the level aa,

Fig. 99, so as to separate the sinus from the

rest, the usual result is that the sinus goes on

beating, but the rest of the heart lies for a

time at rest: soon it begins to beat quite

rhythmically, but at a slower rate than the

separated sinus. If the cross-section be made

at the level gg so as to separate the sinus and

auricle from the rest, they go on beating, but

the ventricle and bulb usually lie quiescent

for a considerable time, and then commence.

On account of the anatomical relations of the

parts (Z, Fig. 99) it is not possible to com-

pletely separate the ventricle from the sinus Of a pert uskm

without doing injury to the former; but if separated apex t of

the lower third of the ventricle (which con- f?og'Jeheartefo?fthe

tains no nerve-cells) be cut off from the rest

of the heart along the line oo, this separated jfj

portion never begins to beat spontaneously, «uia

though the remainder of the heart continues its pulsations.

So far the case for the view that the nerve-cells take the in-

itiative in the changes which result in a normal beat, and

that cardiac muscle is not automatic, is a strong one; but

other facts show that it cannot be accepted without modifi-

cation.

Although the separated apex of the ventricle of the frog,
left to itself, does not beat, yet it can be made to beat without
the application to it of anything that we are justified in call-
ing a stimulus: it does under certain conditions exhibit auto-
maticity. If it be tied on the end of a tube divided by a
partition (Fig. 100), and some blood or blood-serum be circu-
lated through it, in from a and out by b, under a slight press-
ure, this bit of ventricle, devoid of nerve-cells, after a time
begins to beat rhythmically. It has been suggested that in

$ Jan-

260 THE HUMAN BODY.

this case the distension of the muscle or some chemical con-
stituent of the liquid acts as a stimulus; but in no other
muscle do we find blood-supply or mere stretching act as a
stimulus, and if they are to be assumed as so acting in this
case their action is uniform, while the resulting contractions
are interrupted and rhythmic: moreover, they are co-ordi-
nated; they are not irregular twitches first of one bundle of
the myocardiac fibres and then of another, but duly combined,
so as by their mutual action to empty the cavity they surround.
The evidence thus obtained as to the possession of some auto-
matic and some co-ordinative properties by the frog's cardiac
muscle is strengthened by experiments on the hearts of tor-
toises and terrapins. In those animals the apical portions of
the ventricle are devoid of nerve-cells, yet narrow strips of
them hung up and slightly loaded will usually begin to beat
after a time. If they do not, all that is necessary is to stimu-
late them rhythmically for a short time; then on ceasing the
stimulation the rhythmic contractions continue. Here, no
doubt, the loading is a favoring condition, but so it is for the
activity of ordinary muscles, on which, nevertheless, it does
not act as a stimulus.

The conclusion to which we are led is that the muscle-
cells of the frog's -heart have retained to some extent those
automatic and co-ordinating faculties of undifferentiated
protoplasm which the more highly evolved fibre of skeletal
muscle -has lost. We find in the presence of certain of the
nerve- cells of the heart a highly favorable condition for the
exhibition of those powers: the nerve-elements perhaps influ-
ence the nutrition, perhaps in some other mode affect the
molecular structure of the muscle-cells connected with them
so as to favor spontaneous contraction, but, like stretching
the isolated strip of ventricle, they merely bring about a state
of things promoting the exercise of powers inherent in the
cardiac muscle tissue itself.

The evidence as to the automaticity of the muscle of the
mammalian heart is not quite as full as in the case of the frog.
In it also there are collections of ganglion-cells where the
great veins join the auricles and near the base of the ventri-
cles; but there are others in the apical region of the ventricles,
so it is not possible to examine an isolated apex free from
ganglion-cells as it is in the frog. The musculature of the
auricles is prolonged for some little way on the ends of the

NERVES OF THE HEART. CARDIAC MUSCLE. 261

vencB cavce and the pulmonary veins, and there each normal
beat commences, the contraction spreading rapidly over the
whole auricle and thence to the ventricle without the brief in-
termediate pause observable in the frog. In the mammal, also,
the ventricles if supplied with blood from the auricles go on
beating although all nerve and muscular continuity between
auricle and ventricle has been destroyed, by passing rigid tubes
through the auriculo-ventricular openings and then tying a
ligature tight on the outside of the heart along the auriculo-
ventricular groove, so as to crush the tissues between the
string and the tubes. If the ligatures be so placed as not to
impede the flow in the coronary vessels the ventricles beat
long and powerfully, but with a rhythm independent of that
of the auricles and usually slower. Also when the mammalian
heart is dying slowly, as in a suffocated animal, the auricles
usually continue to beat after the ventricle has ceased, the
small dog's-ear-shaped projection of the auricles (which it
may be noted has given its name to the whole auricle) usually
being the last portion to come to rest, especially that on the
right side, which was accordingly named ultima moriens by
the old physiologists. On the whole we are perhaps justified
in assuming that the myocardium of the mammal is automatic,
like that of the frog, and that in it also the presence and
influence of ganglion-cells favor the production of a beat,
but do not initiate it.

The muscle of the frog's heart is, we have seen, co-ordi-
native: the isolated ventricular apex can perform a regular
beat. It is probable that this is not the case in the mammal.
When a dog's heart is injured the ventricles sometimes cease
to give true beats though the muscle bundles constituting
them go on contracting, but it is with no combined action
such as would empty the ventricle. Irregular and useless
contractions travel simultaneously over the myocardium in
various directions, so that the whole mass seems trembling.
Such a state (known as "fibrillar contraction") is especially
apt to follow wounds in the region of the main nerve-trunks
running down the ventricles alongside the larger branches of
the coronary arteries, and is probably due to the injury of some
nervous apparatus concerned in securing the proper co-ordi-
nated contractions of the normal beat. In many other
regions wounds may be inflicted on the ventricle with con-
siderable impunity.

262 TEE HUMAN BODY.

The Heart-beat is not a Tetanic Contraction. We have
seen that it is possible by rapidly succeeding stimuli to throw
the skeletal muscles into a prolonged and apparently contin-
uous contraction, and that there is good reason, afforded by
the phenomena of " secondary tetanus," for the belief that
all normal contractions of the voluntary muscles are compound
or tetanic contractions. This is not the case with the heart.
It is possible by repeated stimuli to hurry the beat of a frog's
heart, but not to fuse two or more beats into a single longer
uninterrupted contraction. And as regards the normal beat
of the heart, experiments as to secondary tetanus prove the
same thing. If the heart of an anaesthetized dog or other
mammal be carefully laid bare and the nerve of a nerve-
muscle preparation be laid on it, we get for each beat a single
twitch of the signal muscle, and not a short tetanus lasting
as long as the ventricular contraction, such as must arise
were this contraction tetanic.

The Ventricular Contraction is always Maximal. It
has been pointed out with reference to the skeletal muscles
that within limits the extent of a contraction varies with the
stimulus used: a feeble stimulus giving a small contraction,
a stronger a greater. This is not the case with cardiac mus-
cle. A quiescent ventricle or strip of ventricle taken from
the heart of a frog or turtle can often be made to contract by
stimulation; but provided the stimulus is powerful enough
to cause a beat at all, it always causes the fullest contraction
the piece of heart is capable of at the time.f Increase of
stimulus causes no increase of contraction. There is good
reason to believe that in the physiological working of the
ventricles of the mammalian heart each completely expels
during its contraction all the blood contained in it: the
papillary muscles pulling down the flaps of the auriculo-
ventricular valves so that they finally form a cone on which
the rest of the ventricular boundaries can fit closely so as to
obliterate the cavity they enclose. This being so, the quantity
of blood driven into the arteries by each contraction of the
ventricles depends on the amount in the latter when their
beat commences. This amount depends partly upon the
quantity of blood returned from the great veins during the
preceding diastole and partly upon the force with which
the auricles contract, for they, although each contraction is
probably maximal for their 'condition at the time being, do

NERVES OF THE HEART. CARDIAC MUSCLE. 263

not completely empty themselves at each stroke; they some-
times do so more completely and sometimes less. In this
manner the auricles can to a great extent control the work
done by the ventricles, through influencing the amount of
blood in the latter at the commencement of the ventricular
systole : more complete relaxation of the auricles during i
diastole promotes inflow from the great veins, more extensive
contraction during auricular systole more completely fills the
ventricles. As we shall see, the force and rate of the auricular
beat is much more under the control of nerves reaching the
heart from other parts than is that of the ventricles. The
auricles are a feed-pump adjusting their work, and through
it the work of the whole heart, to the general condition of
the Body; the ventricles are a grosser force-pump driving on
whatever blood is supplied to them, be it much or be it little.
The Extrinsic Nerves of the Mammalian Heart. As in
the frog, these come from two sources, at least so far as indi-
cated by gross anatomy. Their exact anatomical arrangement
differs in various mammals, as the rabbit, dog, and man, and
even somewhat in different individuals of these species, but in
the main is the same. The pneumogastric gives off from its
main stem in the neck several cardiac branches; so do the
lower cervical and the upper thoracic ganglia of the sympa-
thetic chain. Both sets intermingle, and near the heart end
in plexuses containing nerve-cells; from these plexuses nerves
are distributed to that organ. In the heart itself, as already
stated, are collections of ganglion-cells in the auricles near the
ends of the great veins, near the base of the ventricles, and a
few cells scattered over the ventricles even in their apical re-
gions. The nerve-fibres coming through the pneumogastrics
are medullated and consist of a set of small fibres and a group
of large: the smaller lose their medulla in ganglion-cells in or
near the heart; the larger retain the medullary sheath, and may
be traced even over the ventricles, which in this respect differ
from that of the frog; the fibres supplied from the sympathetic
are non-medullated. Broadly speaking, the nerve-fibres fall
into three physiological sets corresponding to the three
anatomical varieties: the small medullated fibres are effer-
ent and inhibitory — when excited they slow the heart-beat;
the large medullated are in part at least afferent, conveying
to the central nervous system impulses which originate in the
heart; the sympathetic fibres are efferent and excitor, and

264 THE HUMAN BODY.

when stimulated quicken or strengthen the heart beat. The
afferent fibres will be more conveniently studied in connection
with nerves of the blood-vessels (Chap. XVIII).

The Cardio-inhibitory Fibres. These, though running
in the neck in what seems to be the main pneumogastric
trunk, do not leave the skull in that nerve, but in the spinal
accessory (XI cranial nerve), which, it will be remembered,
arises in part from the brain and in part from the upper por-
tion of the spinal cord. That nerve gives off near the brain
a small branch which joins the pneumogastric and runs on in
it to near the heart. The fibres may be tracked in the pneu-
mogastric by their small size, but more satisfactorily by the
Wallerian method. It is then found — 1, when the main
pneumogastric trunk is divided in the neck all the medullated
fibres in it distal to the place of section degenerate; 2, if only
the branch joining the spinal accessory to the pneumogastnc
be cut, then only some fibres in the pneumogastric stem do-
generate, and these fibres are the small medullated set; 3, if
the pneumogastric alone be divided above the point where the
branch from the spinal accessory joins it, then the large
medullated fibres of the cardiac branches of the vagus degen-
erate, but the small do not. Hence we conclude that the
small fibres come through the accessory. Physiological ex-
periment confirms this. Immediately after cutting the main
pneumogastric trunk stimulation of its peripheral end checks^
the beat of the heart; but if the stimulation be applied
after several days, it has no effect on the heart. If instead
of cutting the whole pneumogastric stem we divide only
the branch going to it from the accessory, we find similar
results: after two or three days (i.e., when the microscope
reveals degeneration of the small medullated fibres in the
main stem, all the rest being in their normal condition) stim-
ulation of it is as absolutely without direct effect on the heart
as after complete degeneration of the whole nerve-trunk. In
the frog there is no separate spinal accessory nerve; the cardio-
inhibitory fibres pass from the brain directly into the pneumo-
gastric; but in both frog and mammal their centre lies in a
group of nerve-cells of the medulla oblongata known as the
cardio-inliibitory centre.

The cardiac nerve of the frog consists (Fig. 99) of a pneu-
mogastric and a sympathetic portion: if it be stimulated the
usual result is that the heart is slowed when the stimulus is

NERVES OF THE HEART. CARDIAC MUSCLE. 265

feeble, and is stopped when the stimulus is more powerful;
and in this animal it is possible by carefully applied stimula-
tion to keep the heart at rest for a considerable time, during
which it lies distended and flabby; but nearly always it ulti-
mately recommences its beat even though the stimulation of
the nerve be continued. During its inhibition the heart is
irritable and contractile, for it beats if a direct stimulus be
applied to it: the myocardium is therefore not incapable of
action; but either some influence normally proceeding from
its nerve-cells and promoting its automatic contraction is pre-
vented, or the stimulation directly acts on the cardiac muscle
and for the time lowers or removes its spontaneity. If the
stimulus applied to the cardiac nerve be not strong enough to
completely inhibit the heart, it is usually seen that the pulsa-
tions are not only fewer, but more feeble; but this is not always
the case : the beats may be slower and not less powerful than
before, or they may continue with the same rhythm, but be
less powerful; in any case the result is to diminish for the
time the work done by the heart.

In marnmalia the phenomena are essentially the same. If
artificial respiration be maintained in an anaesthetized rabbit
and its heart laid bare, and then the pneumogastric trunk be
divided on one side of the neck and its cardiac end stimu-
lated, the heart comes to rest, distended and soft to the touch;
or, with more feeble stimulation, the pulsations are slowed;
or they may be both slower and feebler, or feebler and not
slower; but the amount of blood driven out by the ventricles
in a given time is usually much less. When the beat is only
weakened it often happens that the effect shows itself much
more markedly on the auricles than on the ventricles, though
this of course diminishes the work done by the ventricles, as
they are then supplied with less blood to pump on; and occa-
sionally it may be seen that the auricles miss a beat, giving only
one for each two of the ventricles, quite contrary to the case
of a dying heart, in which, as we have seen, the auricular beat
is more prominent. This illustrates the fact that the auricles
are more sensitive to external nervous control than the ven-
tricles, and provide, so to speak, the " fine adjustment" of the
cardiac apparatus.

Wh ether the heart is stopped or slowed or its beats weak-
ened, the result must be a fall in arterial pressure, for the
stretched arteries go on driving blood through the capillaries

266

THE HUMAN BOD Y.

to the veins, while their supply from the heart is cut off or
lessened. Hence a pressure-gauge attached to an artery
shows readily the influence of stimulation of the cardio-in-
hibitory fibres; and in order to avoid the serious operation of
opening the thorax to observe the heart directly, it is usual
to study indirectly the cardiac effect of stimulation of the
pnemogastric by observing its influence on arterial pressure.

FIG. 101.— Manometer for recording variations in arterial pressure, ddggg, glass?
U-tube partly filled with mercury, o; its limb, gg, is open to the air, and a float bear-
ing the light stem e on which is the pen / rests on the mercury: the limb dd is
filled above the mercury with magnesium sulphate solution and connected water-
tight by tubes and the cannula a with the heart end of a divided artery. The pen
writes on a horizontally travelling surface and rises and falls with the mercury on
the side gg, a rise indicating increase of arterial pressure, a fall the reverse: the
pressure 'in the artery at any moment is indicated by the vertical distance between
the top of the mercury in dd and that in gg. due allowance being made for the
weight of the magnesium sulphate and some other possible sources of error.

For this purpose a small glass tube or cannula, a, filled with
solution of magnesium sulphate (to check blood-clotting) is in-
troduced into the cardiac end of a divided artery, say the fem-
oral, of a living animal, the artery being clamped at a place
nearer the heart than the p'oint where the cannula is tied on.

NERVES OF THE HEART. CARDIAC MUSCLE. 267

The cannula is (Fig. 101) connected by an inelastic tube, c,oi
convenient length, also filled with magnesium sulphate, to
one end of a U-shaped glass pressure-gauge or manometer,
ddgg, containing mercury. On the top of the mercury in the
limb gg of the manometer floats a light stem e carrying a pen
which writes on a travelling surface. Above the mercury, o,
on the side dd, the tube is filled with magnesium sulphate
solution. When the pressure on each side of the manometer
is alike the mercury stands at the same level in both limbs,
but when it is increased on the side dd by taking the clamp
off the artery and throwing in the pressure of the blood the
mercury in gg rises, carrying the float and pen with it and
draws a line such as that at yz, Fig. 102, on the travelling

g

FIG. 102. — Tracing: of arterial pressure during vagus inhibition of the heart. To
be read from right to left: yzpq, blood pressure-line traced by the manometer pen;
o indicates on the tracing the instant at which the nerve was stimulated; p, the
instant at which the stimulation ended; ae, line traced by a pen marking half
seconds; xg, line of no pressure, that is, level at which the pen would write were
there no arterial pressure; the distance between it and the part of the manometer
line directly above it multiplied by two gives the actual pressure in mercury in the
artery at that moment. The small variations of pressure seen on the curve are
due to beats of the heart; they are absent during the inhibition and slow for a
short time after it.

surface, the small curves (pulse-waves) on which correspond to
the slight increases of arterial pressure following each contrac-
tion of the left ventricle. The number of these small curves in
a given time gives us therefore the pulse-rate. The pneu mo-
gastric is meanwhile exposed in the neck and cut across: the
object of dividing it is to prevent stimuli travelling to the
brain by the afferent fibres in it, as they would act on the nerve-
centres and lead to complicated results. The peripheral end
of the cut nerve is then stimulated, the excitation commencing
at, say, the instant corresponding to the point o on the tracing.

268 THE HUMAN BODY.

It is seen that the heart does not stop at once but gives a beat
or two and then stops as indicated by the sudden fall of arte-
rial pressure and the absence of all pulse-waves from the
tracing. ( If the stimulation be stopped at the instant indi-
cated by p, the heart does not begin immediately to beat, but
when it does, the beats are powerful and soon bring the arte-
rial pressure back to its former level, or in many cases to a
point above it for some time before the previous pressure and
pulse-rate are regained. Such a tracing shows among other
things that a certain "latent period" elapses before the
stimulation of the inhibitory fibres influences the heart-
beat^ and that the influence of the stimulus once established
continues a short time after the stimulation is stopped; and
that the first beats after cessation of the inhibition are slow
and powerful. Of course without any manometer one can
detect the effect of cardio-inhibitory stimulus by a finger
placed over the -pulse of an animal or by listening to the
heart-sounds, but the graphic method above described allows
of much more accurate study.

It has been stated in a previous paragraph that stimulation
of the cardiac nerve usually stops or slows the heart-beat of a
frog. The reason for the qualifying term is that sometimes
the stimulation quickens the beat. This is due to the fact
that the nerve (see Fig. 99) is a mixed one and that the
fibres it receives from the sympathetic are directly antago-
nistic in action to those derived from the vagus. In most
cases when the whole trunk is stimulated the vagus fibres get
tlie upper hand, but to be sure of pure cardio-inhibitory results
the vagus must be stimulated before the sympathetic branch
joins it. Then the action is always inhibitory; and certain
other important phenomena may be observed, showing that
the vagus contains fibres which tend to throw the heart into a
better working state. When an exposed frog's heart is dying
and has ceased to beat, or when the ventricle has come to rest
though the sinus and auricles still work, it not unfrequently
happens that a period of vagus stimulation is followed by a
set of beats : or similarly that when the whole heart is beating
feebly stimulation of the vagus is after a time followed
by more forcible contractions. Hence it has been suggested
that the nerve contains fibres which tend to promote the
nutrition of the cardiac muscle, fibres which are anabolic
and favor constructive chemical processes. Whether these fibres

NERVES OF THE HEART. CARDIAC MUSCLE. 269

are the same as the cardio-inhibitory or are a distinct set is
still uncertain. In mammals, also, it is frequently noticeable
that vagus inhibition of the heart is followed by a period of.
unusually powerful pulsation.

The Cardio-inhibitory Centre. This consists of nerve-
cells lying in the medulla oblongata and giving origin to the
cardio-inhibitory fibres. In some animals it seems to be nor-
mally always in a state of slight' activity, sending out feeble
impulses which exert a slighUfcheck on the rate of pulse.
This is the case in the dog, for in that animal division of
both pneumogastric nerves in the neck is followed by a
quicker heart-beat: in the rabbit, on the other hand, the
centre appears usually at rest, as section of the pneumogas-
trics in that animal has no effect on the pulse-rate. Whether
normally in action or not the centre can readily be excited,
especially by afferent impulses reaching it through abdominal
nerves. If the intestines of a frog (the brain of which in
front of the medulla oblongata has been entirely removed so
as to make consciousness impossible) be exposed and sharply
struck, the heart stops in diastole; but if both cardiac nerves
have been previously divided this result does not follow.
The stoppage is clearly then a reflex inhibition through the
cardio-inhibitory centre and nerves, and the afferent tract can
be readily traced. The afferent impulses from the intestine
pass through the mesenteric branches of the sympathetic, for
if these be cut no cardiac standstill follows the mechanical
stimulation of the intestine, although the vagi be intact. If
only the communicating branches from the sympathetic gan-
glia to the spinal cord be cut or only the anterior roots of the
corresponding spinal nerves, or only the spinal cord above the
place of entry of these roots, or only the medulla oblongata
destroyed, yet, in each case, the intestinal stimulation causes
no stoppage of the heart. When the standstill does result it
is therefore reflex, the afferent path being — sensory nerve-end-
ings in intestine, mesenteric nerves, sympathetic ganglion,
communicating branches, anterior spinal roots, spinal cord
to centre in medulla; the efferent fibres are the inhibitory
in the vagus. The fainting which in man not infrequently
follows a severe blow on the pit of the stomach is due to
similar reflex excitation of the cardio-inhibitory centre: and
the fainting seen during severe pain and that which certain
odors cause in some persons are due to similar stimulation of

270 THE HUMAN BODY.

the cardio-inhibitory centre through sensory nerves, and serve
to illustrate the many afferent fibres from different regions of
the Body which can influence the heart-beat.

The cardio-inhibitory centre may also be stimulated
directly (as by piercing it with a needle) and stop the heart.
But a more interesting instance is its excitation by high
arterial pressure. Nearly always a very high pressure in the
aorta is accompanied by a slow pulse due to cardio-inhibitory
nerve-impulses, for if the vagi be cut under such circum-
stances the heart-rate immediately increases. The slower
beat, of course, by lessening the work of the heart tends to
bring back the high arterial pressure to a more normal level,
providing an adjustment of the heart's work to the condition
of the arterial system at the time. The brain, enclosed in
the rigid skull-cavity, is especially likely to be affected by
increased arterial tension, for distension of the intra-cranial
arteries must bring about greater pressure on all the other
contents of the skull; and the cardio-inhibitory centre is very
sensitive to increased pressure. If a small hole be bored
through the skull of a dog and a little innocuous fluid in-
jected so as to cause pressure on the brain, the beat of the
heart is promptly slowed and weakened, but if the pneumo-
gastrics have been previously cut the heart-beat is not
influenced. In man similar stimulation of the cardio-inhibi-
tory centre is shown in apoplexy, which is due to the bursting
of some vessel inside the skull and the effusion of blood, which
by pressure on the brain causes the unconsciousness and pa-
ralysis which characterize the stroke. During such a fit the
pulse is almost invariably very slow from the action of the
increased pressure on the cardio-inhibitory cells. This is
clearly a preservative action, for the resulting lower arterial
pressure makes the haemorrhage less, and more likely to come
to an end. Among conditions of the blood which stimulate
the cardio-inhibitory apparatus may be mentioned deficient
oxygenation, which will be referred to again when the phe-
nomena of suffocation are described.

The Cardio-accelerator or Augmentor Nerves. The
influence of these on the heart is to quicken or strengthen
its beat or both: but only for a time, their final action being
to hasten exhaustion; they are essentially katabolic in their
influence on the nutrition of the organ.

Both in frog and mammal they pass to the heart from

NERVES OF THE HEART. CARDIAC MUSCLE. 271

the sympathetic, taking somewhat different paths in different
animals. In the frog their course is shown in Fig. 99; in
mammals most of them come from the upper thoracic gan-
glion of the sympathetic and the neighboring parts of the
main sympathetic chain. If the heart of a frog be exposed
and watched while the branch s, Fig. 99, is stimulated its
beat is seen to be quickened, especially if the previous rate
were slow: and quite similar phenomena may be observed
when the corresponding nerves are stimulated in a rabbit or
dog. And the beat is not merely made more rapid: it is dis-
tinctly more powerful for the time, the heart driving out more
blood at each stroke (even though pressure in the aorta may
be high) and thus doing increased work.

Though the augmentor fibres reach the heart through the
sympathetic they have their centre (cardio-accelerator centre)
in the medulla oblongata, from which in mammalia they pass
down the spinal cord to the anterior roots of the upper tho-
racic spinal nerves, to the communicating branches, to the
sympathetic ganglia, and thence to the cardiac plexus and
the heart. Their centre, like the inhibitory, may be reflexly
excited : powerful stimulation of a sensory nerve, after section
of the vagi, usually quickens the pulse if the accelerator fibres
passing from the thoracic ganglia be intact, but has no effect
if these be previously divided. If the vagi are not cut the
result is not so certain, as the afferent impulses may also
excite the cardio-inhibitory centre and cause a mixed action:
but speaking generally afferent impulses which in a conscious
animal would cause acute but not extreme pain cause increase
of the heart- beat. This by raising general arterial tension
would for the time put the animal in good condition to make
a vigorous effort, and so is obviously an unconscious adaptation
of the organism for the preservation of its safety. While ex-
treme pain or extensive injury involving many afferent nerves
tends to cause fainting and loss of consciousness, the cardio-
inhibitory centre getting the upper hand.

The Influence of Temperature Changes and of Calcium
Salts on the Heart-beat. If the excised heart of a frog be
cooled it beats more slowly; if heated, more quickly; until
the temperature approaches the limit at which muscle passes
into rigor. The observation is more difficult with mammals,
but if the heart of a dog be completely separated from all
the rest of the body except the lungs and supplied with blood

272 THE HUMAN BODY.

it is possible to keep it alive for some hours, beating regularly
and powerfully, and on such a heart it is easy to observe that
cooler blood causes slower beat and vice versa. While the
quick pulse observed in fevers may therefore be in part due
to paralysis of the cardio-inhibitory centre or stimulation
of the cardio-accelerator, it is in part at least due solely to
the hotter blood circulating through the coronary vessels.
Whether the higher temperature in this case acts primarily
on the nerve-cells of the heart or on the muscle is not known.
If circulation be kept up through a frog's heart by the
perf usion method (Fig. 100), the organ may be kept beating for
a very long time if the liquid supplied be blood or serum. If
only dilute solution (0.75$) of sodium chloride be given, the
beat continues for some time, but not so long as if no liquid
be circulated ; the salt apparently washes out something
which the heart needs. The beat of such a " washed-out "
heart may be restored by substituting milk or serum or de-
fibrinated blood for the saline solution, or even by adding to
the sodium chloride a very little of a soluble calcium salt.
Serum, blood, and milk all contain calcium salts, and albu-
minous solutions free from calcium (as paraglobulin) do not
restore the beat; nor do serum or milk or blood deprived of
calcium. Hence the presence of some salt of that metal
seems to have a close relation to the functional activity of
the heart, as indeed it has to muscular activity in general.

CHAPTER XVIII.
THE VASO-MOTOR NERVES AND NERVE-CENTRES.

The Nerves of the Blood-vessels. The arteries, as
already pointed out, possess a muscular coat composed of
fibres arranged around them, so that their contraction can
narrow the vessels. This coat is most prominent in the
smaller vessels, — those of the size which go to supply separate
organs, — but disappears again in the smallest branches, which
are about to divide into capillaries for the individual tissue
elements of an organ. These vascular muscles are under the
control of certain special nerves called vaso-motor, and these
latter can thus govern the amount of blood reaching any
organ at a given time. Most of the vascular nerve-fibres
have their origin in the cerebro-spinal centre, though they
pass through sympathetic ganglia on their way to the vessels.
In a few regions ganglion-cells are found lying close to the
arteries, and some of the vaso-motor fibres are probably con-
nected with them, but as a rule they end directly in the mus-
cular coat.

In the heart we had to consider a rhythmically contract-
ing organ the force of whose contractions could be increased
or diminished by the influence of extrinsic nerves; in the
arteries, speaking broadly, we have to deal with muscle in a
condition of tonic or constant contraction, which contraction
can be increased by impulses coming through excitor or vaso-
constrictor nerves, and diminished through the activity of
inhibitory or vaso-dilator nerves. The general tonic con-
traction of the arterial muscle is, however, much more de-
pendent on the vaso-constrictor nerve-fibres than is the beat
of the heart on the cardio-excitor nerves. The inhibitory
set of vaso-motor nerves have a much less extensive distribu-
tion over the arterial system than the constrictor.

The Vaso-constrictor Nerves. If the ear of a white rab-
bit be held up against the light while the animal is kept quiet
and not alarmed, the red central artery can be seen coursing

273

274 THE HUMAN B( >D Y.

along the translucent organ, giving off branches which by
subdivision become too small to be separately visible, and the
whole ear has a pink color and is warm from the abundant
blood flowing through it. Attentive observation will show
also that the calibre of the main artery is not constant; at
somewhat irregular periods of a minute or more it dilates and
contracts a little.

If the sympathetic trunk have been previously divided on
the other side of the neck of the animal, the ear on that side
will present a very different appearance. It arteries will be
much dilated and the whole ear fuller of blood, redder, and
distinctly warmer; the slow alternating variations in arterial
diameter also have disappeared. We get thus evidence that
the normal mean calibre of the artery is maintained by influ-
ences reaching its muscular coat through the cervical sym-
pathetic. Stimulation of the upper end of the cut nerve
confirms this opinion. It is then seen that the arteries of
the corresponding ear gradually contract until even the main
vessel can hardly be seen, and in consequence the whole ear
becomes pale and cold. After the stimulation is stopped the
arteries again slowly dilate until they have regained their full
paralytic size, and they usually remain permanently in that
condition. Sometimes they regain after some days almost
the size of those in the ear on the uninjured side, even when
the nerve has not only been cut, but the upper cervical
sympathetic ganglion extirpated; this seems to indicate that
the arterial muscle has a small automaticity of its own tending
to keep it in a moderate state of contraction, but it is less
marked than the automaticity of the myocardium.

Quite similar phenomena can be observed in transparent
parts of other living animals, as in the web of a frog's foot,
the arteries of which dilate after section of the sciatic nerve
and constrict when the peripheral end of the nerve is stimu-
lated. In the case of other parts changes in temperature
may be used to detect alterations in the flow of blood. In a
dog or cat, for example, a sensitive thermometer placed be-
tween the toes indicates a rise of temperature, owing to in-
creased flow of warm blood through the skin, after section of
the chief nerve of the limb, and a fall of temperature (usu-
ally) during stimulation of the peripheral end of the divided
nerve.

When the vaso-constrictor.nerves cut are those controlling a

VASO-MOTOR NERVES AND NERVE-CENTRES. 275

large number of arteries, the dilatation of the latter so much
diminishes peripheral resistance to the blood-flow as to lead
to a marked fall of general arterial pressure; and, due care
being taken to avoid or to allow for concomitant variations in
the rate or force of the heart's beat, this gives us another use-
ful method of studying the distribution of the nerves con-
cerned. For example, the splanchnic nerves are branches
which spring from the thoracic portion of the sympathetic
chain and pass through the diaphragm to end in the gan-
gliated solar plexus from which nerves pass to the arteries of
most of the abdominal viscera. When the splanchnic nerves
are cut on both sides arterial pressure falls enormously, from
say 120 millimetres of mercury in the carotid of a dog to 15
or 20 millimetres, most of the blood of the body lying almost
stagnant in the dilated blood-vessels of the abdomen. On the
other hand, stimulation of the splanchnic nerves so diminishes
the paths open for the circulation of the blood as to enor-
mously increase general blood-pressure; especially if the
cardio-inhibitory nerves be first divided so that raised blood-
pressure inside the skull-chamber may not slow the heart-
beat.

The skin and the abdominal organs seem to be the pre-
dominant localities of distribution of the vaso-constrictor
nerves: other parts have them, but not in quantity sufficient
to bring about any great general change in the blood-flow.
In the abdomen is warmer, in the skin cooler blood: and
according to the amount of heat produced in the Body and
the temperature of the surrounding medium, the vessels of
abdomen and skin contract or relax so as to control the pro-
portion of blood sent to the skin to lose heat.

The Vaso-constrictor Centre. The constrictor nerves of
the arteries do not originate in the sympathetic system. If
all the branches of the latter be left intact, the phenomena of
paralytic dilatation of the blood-vessels can be fully brought
about by dividing the communicating branches between certain
spinal nerves and the corresponding sympathetic ganglia,
or by dividing the anterior roots of certain spinal nerves.
In this way it can be shown that the fibres all proceed
from the thoracic and lumbar regions of the spinal cord,
but have not their origin in the cord. If it be cut anywhere
in the cervical region, all arteries having a constrictor nerve
supply are paralyzed, while stimulation of the posterior end

276 THE HUMAN BODY.

of the divided cord causes widespread arterial constriction.
The main centre for the vaso-constrictors must then lie as
far forward as the medulla: and as all the brain in front of
the medulla oblongata can be removed without any con-
sequent arterial paralysis, the centre must lie in the medulla
itself. This centre is often named the vaso-motor centre, but
it is better to distinguish it as the vaso-constr ictor from the
centre for the dilator efferent nerves.

The Control of the Vaso-constrictor Centre. The vaso-
constrictor centre is automatic; it maintains a certain amount
of activity of its own, independently of any stimuli reaching
it through afferent nerve-fibres. Nevertheless, like nearly all
automatic nerve-centres, it is under reflex control, so that its
activity may be increased or lessened by afferent impulses
conveyed to it. Nearly every sensory nerve of the Body is in
connection with it; any stimulus giving rise to pain, for
example, excites it, and thus constricting the arteries, in-
creases the peripheral resistance to the blood-flow and raises
arterial pressure. On the other hand, certain fibres conveying
impulses from the heart inhibit the centre and dilate the
arteries, lower blood-pressure, and diminish the resistance to
be overcome by the heart. These afferent fibres, which have
been already referred to as the large medullated fibres (p. 263)
of the pneumogastric, are known as the depressor fibres, or in
certain animals, for example the rabbit, where they are all
collected into one branch, as the depressor nerve. If this
nerve be divided and its cardiac end stimulated no effect is
produced, but if its central end (that still connected with the
rest of the pneumogastric trunk and through it with the
medulla oblongata) be stimulated, arterial pressure gradually
falls; this result being dependent upon a dilatation of the
small arteries, and consequent diminution of the peripheral
resistance, following an inhibition of the vaso-constrictor
centre brought about by the depressor nerve. Through the de-
pressor nerve the heart can therefore influence the calibre of
the small arteries and, by lowering aortic pressure, diminish its
own work if need be. In Fig. 103 is reproduced a tracing of
the great but slow fall of blood-pressure which results from
stimulation of the depressor fibres. It shows the slow fall of
pressure and slightly changed pulse-rate accompanying the
slow dilatation of the arteries, and may be compared with the
rapid fall and slow pulse brought about (Fig. 102) by excita-

VASO-MOTOR NERVES AND NERVE-CENTRES. 277

tion of the cardio-inhibitory nerves. The latent period is
also noticeably long and the effect of the stimulus outlasts
considerably the time of its application.

Blushing. The depressor nerves control a great part of the
vaso-constrictor centre (especially that portion of it connected
with the splanchnic nerves) and so can bring about dilatation
of a large number of arteries — their influence is accordingly
called into play when general arterial pressure is to be lowered,
but is useless for controlling local blood-supply. This is
managed in part by other afferent nerves, each of which
inhibits a small part only of the vaso-constrictor centre, gov-
erning the arteries of a limited tract of the Body; the dilata-

FIG. 103.— Tracing of pressure from femoral artery of a rabbit showing the influ-
ence of stimulation of the central end of the depressor nerve; to he read from right
to left: a b c d e, tracing of arterial pressure, the small variations indicating heart-
beats; op, tracing of seconds pen; s, moment of commencement of stimulation; £,
cessation of stimulation; xg, line of no pressure.

tion of these increases the amount of blood flowing through the
particular region to which they are .distributed, but does not
affect the total resistance to the blood-flow sufficiently to
influence noticeably the general pressure in the arterial system.
In blushing, for example under the influence of an emotion,
that part of the vase-motor centre which supplies constrictor
nerves to the arteries of the skin of the neck and face, is
inhibited by nerve-fibres proceeding from the cerebrum to the
medulla oblongata, and the. face and neck consequently be-
come full of blood and flush up. Quite similar phenomena
occur under other conditions in many parts of the Body,
although when not visible on the surface we do not usually
call them blushes. The mucous membrane lining the empty
stomach is pallid and its arteries contracted, but as soon as
food enters the organ it becomes red and full of blood ; the
food stimulating afferent nerve-fibres there, which inhibit

278 THE HUMAN BODY.

that part of the vaso-motor centre which governs the gastric
arteries.

Taking Cold. This common disease is not unfrequentlj
caused through undue reflex excitement of the vaso-motor
centre. Cold acting upon the skin stimulates, through the
afferent nerves, the portion of the vaso-motor centre governing
the skin arteries, and the latter become contracted, as shown
by the pallor of the surface. This has a twofold influence —
in the first place, more blood is thrown into internal parts,
and in the second, contraction of the arteries over so much of
the Body considerably raises the general blood-pressure.
Consequently the vessels of internal parts become overgorged
or " congested," a condition which readily passes into inflam-
mation. The action is of course primarily protective, to
prevent too great loss of heat from the Body; but if internal
organs be weak or diseased or if the exposure to wet or cold
be prolonged, it is apt to be followed by catarrh or inflam-
mation of more or less of the respiratory tract causing bron-
chitis, or of the intestines causing diarrhoea. In fact the com-
mon summer diarrhoea is far more often due to a chill of the
surface, causing intestinal catarrh, than to the fruits eaten
in that season which are so often blamed for it. The best
preventative is to wear, when exposed to great changes of tem-
perature, a woollen or at least a cotton garment over the trunk
of the Body; linen is so good a conductor of heat that it
permits any change in the external temperature to act almost
at once upon the surface of the Body. After an unavoidable
exposure to cold or wet the thing to be done is of course to
restore the cutaneous circulation; for this purpose movement
should be persisted in, and a thick dry outer covering put on,
until warm and dry underclothing can be obtained.

For healthy persons a temporary exposure to cold, as a
plunge in a bath, is good, since in them the sudden contrac-
tion of the cutaneous arteries soon passes off and is succeeded
by a dilatation causing a warm healthy glow on the surface.
If the bather remain too long in cold water, however, this
reaction passes off and is succeeded by a more persistent
chilliness of the surface, which may even last all day. The
bath should therefore be left before this occurs, but no abso-
lute time can be stated, as the reaction is more marked and
lasts longer in strong persons, and in those used to cold bath-
ing, than in others.

VASO- MOTOR NERVES AND NERVE-CENTRES. 279

Vasodilator Nerves. We have already noticed, in the
case of the stomach, one method by which a locally increased
blood-supply may be brought about in an organ while it is
at work, viz., by inhibition of local vaso- constrictor fibres.
Frequently, however, in the Body this is managed in
another way; by efferent vaso-dilator nerves which inhibit
or paralyze, not the vaso-constrictor centre, but the muscles
of the blood-vessels directly. The nerves of the skeletal
muscles for example contain two sets of efferent fibres: one
motor proper and the other vaso-dilator. When the muscle
contracts in a reflex action or under the influence of the
will both sets of fibres are excited ; so that when the organ is
set at work its arteries are simultaneously dilated and more
blood flows through it. But if the animal have previously
administered to it such a dose of curare as to just paralyze
the true motor-fibres, stimulation of the nerve produces
dilatation of the arteries without a corresponding muscular
contraction. Quite a similar thing occurs in the salivary
glands. Their cells, which form the saliva, are aroused to
activity by special nerve-fibres; but the gland-nerve also
contains a quite distinct set of vaso-dilator fibres which nor-
mally cause a simultaneous dilatation of the gland-artery,
though either can be artificially stimulated by itself and
produce its effect alone. Through such arrangements the
distribution of the blood in the Body at any moment is gov-
erned : so that working parts shall have abundance and other
parts less, while at the same time the general arterial pressure
remains the same on the average; since the expansion of a
few small local branches but little influences the total periph-
eral resistance in the vascular system. Moreover, commonly
when one set of organs is at work with its vessels dilated,
others are at rest with their arteries comparatively contracted,
and so a general average blood -pressure is maintained. Few
persons, for example, feel inclined to do brain-work after a
heavy meal: for then a great part of the blood of the whole
Body is led off into the dilated vessels of the digestive organs,
and the brain gets a smaller supply. On the other hand, when
the brain is at work its vessels are dilated and often the whole
head flushed : and so excitement or hard thought after a meal
is very apt to produce an attack of indigestion, by diverting
the blood from the abdominal organs, where it ought to be at
that time. Young persons, whose organs have a superabuu-

280 THE HUMAN BODY.

dance of energy enabling them to work under unfavorable
conditions, are less apt to suffer in such ways than their eld-
ers. One sees boys running actively about after eating, when
older people feel a desire to sit quiet and ruminate — or even
to go to sleep.

When the nerve of a limb is cut and its peripheral end is
stimulated the usual result is arterial constriction, because
the constrictor fibres are more numerous and more powerful
than the dilator; a day or two after section, when the nerve
has begun to degenerate, stimulation, however, causes dilata-
tion, apparently because the constrictor fibres degenerate
more quickly: and when the stimuli (as induction shocks)
given to the nerve are repeated at only a slow rate the dilator
effect frequently overcomes the constrictor.

The Vaso-dilator Centre. The vaso-dilator nerves, like
the vaso-constrictor, seem to originate primarily in a centre
in the rnedullaoblongata. In regard to the arteries in general,
they play a much less conspicuous part than their analogues,
the cardio-inhibitory fibres, do in regard to the heart.

The Vaso-motor Nerves of the Veins. Most veins have
a muscular coat, though it is much less developed than in
the arteries, and this coat is probably under the control of
nerve-fibres. Satisfactory evidence of their existence is still
wanting.

The Vascular Phenomena of Inflammation. When
some transparent portion of an animal (for example the
mesentery of a mouse or guinea-pig) is carefully exposed arid
studied with a microscope, the normal flow in the small ves-
sels may be studied for some time, much as in the web of the
frog. If an irritant be applied, the immediate result is a
widening of the small arteries and a greater and more rapid
flow through them and the capillaries and veins. This seems
dependent mainly on a direct paralysis of the arteries, and if
the irritant be transient in its influence the congested con-
dition soon passes off. If the irritant be more powerful, the
vascular dilatation continues and other circulatory changes
are seen. The corpuscles, instead of keeping, as is usual in
arteries of microscope size, to the central part of the tube
(axial current), spread more evenly, and the white cor-
puscles especially tend to pass into the layer of liquid in im-
mediate contact with the inner coat of the artery, and at the
same time to exhibit much more marked amoeboid move-

VASO-MOTOR NERVES AND NERVE-CENTRES. 281

merits than they commonly do while travelling in the blood-
current. The platelets, also, which are normally confined to
the axial currents, now pass towards the sides. If this stage
of very early inflammation pass on to the next, it is observed
that white corpuscles and platelets both stick to the inside of
the vessels. The platelets next adhere together and break
down into granular masses, and the white corpuscles thrust
amoeboid processes between the lining-cells of the capillaries
and smallest veins, and begin to push their way through. By
these means a considerable impediment to the blood-flow is
caused, and the circulation becomes slower, though all the
vessels of the part may be dilated. If the inflammation con-
tinue, many white corpuscles pass quite out of the vessels
(migration) and enter the neighboring lymph-spaces: the
red corpuscles get blocked and squeezed together into a mass
in which their individual boundaries are indistinguishable,
and some of them may even be squeezed through the walls
of the capillaries (diapedesis). Next all blood-flow in the
area under observation may be stopped, while more lymph
than normal collects in it. From this state recovery may
take place; or continued inflammation may lead to destruc-
tion of the part. The primary local disturbances in the cir-
culation seem due to changes in the inner coats of the vessels
of the irritated region; but an extensive continued inflam-
mation produces fever and many other secondary general
results, partly through the absorption of disease products
from the inflamed part and partly through irritation of
afferent nerve-fibres which throw various nerve-centres into
abnormal action.

CHAPTER XIX.
THE SECRETORY TISSUES AND ORGANS.

Definitions. .In its broad etymological meaning a secre-
tion is any substance separated or derived from the blood, so
that in a certain sense all the solid tissues of the Body, built
up from materials supplied by the blood, are secretions. In
practice the name has a more limited application and is given
to two classes of substances, distinguished as true or external
secretions and internal secretions.

Internal secretions are the results of the vital activities of
various organs, their by-products, passed out directly into
the lymph and blood; and in many cases are simple wastes,
sent to the blood-stream for conveyance to other organs which
get rid of them: such, for example, is the carbon dioxide
formed in every part of the Body. In other cases the by-
products of certain organs, after absorption into the blood,
have to be further changed in a second organ before elimina-
tion, and are probably of use to this second — a part of its
pabulum: as an instance we may take leucin (amido-caproic
acid), which is formed in many organs and, given by them, to
the blood, is carried to the liver, the cells of which convert it
(or at least a great part of it) into urea,' to. be subsequently
eliminated by the kidneys. A third very important class of
internal secretions consists of substances form-ed only in one
organ or one pair of organs and yielded by them to the blood
which flows through them, the presence of which substances
in the blood is essential to the healthy nutrition and the con-
tinuance of the life of the Body : in such cases removal or
extensive disease of the producing organ results in death.
Examples are to be found in substances which the thyroid
body and suprarenal capsules produce; they will be consid-
ered more fully in Ch&pter XXIII.

Excluding such things as cast hairs and epidermic scales,
the true or external secretions may be denned as gases or
liquids, often of very complex composition, passed out on

THE SECRETORY TISSUES AND ORGANS. 283

some free surface of the Body, either that of the general
exterior or of some internal cavity, or into recesses commu-
nicating with such a surface. The true secretions fall into two
classes : one in which the product is of no further use in the
Body and is merely separated for removal, as the urine; and
one in which the product is intended to be used, for instance
as a solvent in the digestion. of food. The former group are
sometimes distinguished as excretions and the latter as secre-
tion* proper, but there is no real difference between them, the
organs and processes concerned being fundamentally alike in
each case. A better division is into transudata and secretions,
a transudation being a product which contains nothing which
did not previously exist in the blood, and only in such quan-
tity as might be derivable from it by merely physical processes;
while a secretion in addition to transudation elements contains
a specific element, due to the special physiological activity of
the secretory organ; being either something which does not
exist in the blood at all or something which, existing in the
blood in small quantity, exists in the secretion in such a high
proportion that it must have been actively picked up and
conveyed there by the secretory tissues concerned. For in-
stance, the gastric juice contains free hydrochloric acid which
does not exist in the blood ; and the urine contains so much
urea that we must suppose the kidney-cells to have a peculiar
power of removing that body from the liquids flowing near
them. This subdivision is also justifiable oh histological
grounds; wherever there is a secreting surface or recess it is
lined by cells, but these cells where transudata are formed (as
on the serous membranes) are mere flat scales, with little or
no protoplasm remaining in them (Fig. HB), while the cells
which line a true secreting organ are cuboidal, spherical, or
columnar, and still retain, with their high physiological activ-
ity, a good deal of their primitive protoplasm.

Organs of Secretion. The simplest form in which a
secreting organ occurs (A, Fig. 104) is that of a flat membrane
provided with a layer of cells, a, on one side (that on which
the secretion is poured out) and with a network of capillary
blood-vessels, c, on the other. The dividing membrane, b, is
known as the basement membrane and is usually made up of
flat, closely fitting connective-tissue corpuscles; supporting it
on its deep side is a layer of connective tissue, d, in which the
blood-vessels and lymphatics are supported. Such simple forms

284 THE HUMAN BODY.

of secreting surfaces are found on the serous membranes, but
are not common; in most cases an extended area is required
to form the necessary amount of secretion, and if this were
attained simply by spreading out plane surfaces, these from
their number and extent would be hard to pack conveniently
in the Body. Accordingly in most cases, the greater area is
attained by folding the secreting surface in various ways so
that a large area can be packed in a small bulk, just as a
Chinese lantern when shut up occupies much less space than
when extended, although its actual surface remains of the
same extent. In a few cases the folding takes the form of
protrusions into the cavity of the secreting organ as indicated
at (7, Fig. 104, and found on some synovial membranes; but
much more commonly the surface extension is attained in
another way, the basement membrane, covered by its epithe-
lium, being pitted in or involuted as at B. Such a secreting
organ is known as a gland.

Forms of Glands. In some cases the surface involutions
are uniform in diameter, or nearly so, throughout (B, Fig.
104). Such glands are known as tubular; examples are found
in the lining coat of the stomach (Fig. 113); also in the skin
(Fig. 135), where they form the siveat-glands. In other cases
the involution swells out at its deeper end and becomes more or
less sacculated (E) ; such glands are racemose or acinous. The
small glands which form the oily matter poured out on the
hairs are of this type. In both kinds the lining cells near the
deeper end are commonly different in character from the rest;
and around that part of the gland the blood-vessels form a
closer network. These deeper cells form the true secreting
elements of the gland, and the passage, lined with different
cells, leading from them to the surface, and serving merely to
carry off the secretion, is known as the gland-duct. When
the duct is undivided the gland is simple ; but when, as is
more usual, it is branched and each branch has a true secret-
ing part at its end, we get a compound gland, tubular (&) or
racemose (F, If) as the case may be. In such cases the main
duct, into which the rest open, is often of considerable length,
,so that the secretion is poured out at some distance from the
main mass of the gland.

A fully formed gland, H, thus comes to be a complex
structure, consisting primarily of a duct, c, ductules, dd, and
secreting recesses, ee. The ducts and ductules are lined witli

THE SECRETORY TISSUES AND ORGANS. 285

epithelium which is merely protective and differs in charac-
ter from the secreting epithelium which lines the deepest

FIG. 104. — Forms of glands. A. a simple secreting: surface ; a. its epithelium ;
b, basement membrane ; c, capillnries ; B, a simple tubular gland ; C, a secreting
surface increased by protrusions ; E. a simple racemose gland ; D and Gr, com-
pound tubular glands ; F, a compound racemose gland. In all but A, B, and C
the capillaries are omitted for the sake of clearness. H, halt of a highly developed
racemose gland ; c, its main duct.

parts. Surrounding each subdivision and binding it to its
neighbors is the gland stroma formed of connective tissue, a

286 THE HUMAN BODY.

layer of which also commonly envelops the whole gland, as
its capsule. Usually on looking at the surface of a large
gland it is seen to be separated by partitions of its stroma,
coarser than the rest, into lobes, each of which answers to a
main division of the primary duct; and the lobes are often
similarly divided into smaller parts or lobtiles. In the con-
nective tissue between the lobes and lobules blood-vessels
penetrate, to end in fine capillary vessels around the terminal
recesses. They never penetrate the basement membrane.
Lymphatics and nerves take a similar course; there is reason
to believe that the nerve-fibres penetrate the basement mem-
brane and become directly united with the secreting cells of
some glands.

The Physical Processes in Secretion. From the struc-
ture of a gland it is clear that all matters derived from the
blood and poured into its cavity must pass not only through
the walls of the capillary blood-vessels, but also, by filtra-
tion or dialysis, through the basement membrane and the
lining epithelium. By filtration is meant the passage of a
fluid under pressure through the coarser mechanical pores
of a membrane, as in the ordinary filtering processes of a
chemical laboratory ; and the higher the pressure on the
liquid to be filtered the greater the amount which, other
things being equal, will pass through in a given time. Since
in the living Body the liquid pressure in the blood-capillaries
is nearly always higher than that outside them, filtration is
apt to take place everywhere to a greater or less extent, and
will be increased in amount in any region by circum-
stances raising blood-pressure there, and diminished by those
lowering it. To a certain extent also the nature of the
liquid filtered has an influence. True solutions, as those of
salt in water, passed through unchanged ; but solutions con-
taining substances such as boiled starch or raw egg-albumen,
which swell up greatly in water rather than truly dissolve,
are altered by filtration ; the filtrate containing less of the
imperfectly dissolved body than the unfiltered liquid. The
higher the pressure the greater the proportion of such sub-
stances which gets through ; and if the pressure is slight the
water or other solvent may alone pass, leaving all the rest
behind on the filter. Under moderate pressure the blood
may thus lose by filtration only such bodies as water and
salines ; while an increase 'of arterial pressure may lead to

THE SECRETORY TISSUES AND ORGANS. 287

the passage of albumen and fibrinogen. , Under healthy con-
ditions, for example, the urine contains no albumen, but any-
thing considerably increasing the capillary pressure in the kid-
neys will cause it to appear. Dialysis or osmosis has already
been considered (p. 42); by it substances pass through the in-
termolecular pores of a membrane independently of the press-
ure on either side, and for its occurrence two liquids of dif-
ferent chemical constitution are required, one on each side of
the membrane. At least if diffusion takes place, as is proba-
ble, between two exactly similar solutions, the amount and
character of the substances passing opposite ways in a given
time are exactly equal, so that no change is produced by the
dialysis; which practically amounts to the same thing as if
none occurred. When a solution is placed on one side of a
membrane allowing of dialysis, and pure water on the other,
it is found that for every molecule of the dissolved body that
passes one way a definite amount of water, called the en-
dosmotic equivalent of that body, passes in the opposite
direction. Crystalline bodies as a rule (haemoglobin is an
exception) have a low endosmotic equivalent or are readily
dialyzable; while colloids, such as gum and proteids, have a
very high one, so that to get, by dialysis, a small amount of
albumen through a membrane, a practically infinite amount
of water must pass the other way. Accordingly, if we find
such bodies in a secretion we cannot suppose that they have
been derived from the blood by mere osmosis.

The Chemical Processes of Secretion. As above pointed
out certain secretions, called transudata, seem to be products
of filtration and dialysis alone, containing only such sub-
stances as those which are found in the blood-plasma, more
or less altered in relative quantity by the ease or difficulty
with which they severally passed through the layers met
with on their way to the surface. But in many cases the
composition of a secretion cannot be accounted for in this
way ; it contains some specific element, either a substance
which does not exist in the blood at all and must therefore
have been added by the secreting membrane, or some body
which, although existing in the blood, does so in such minute
proportion, compared with that in which it is found in the
secretion, that some special activity of the secreting cells is
indicated : some affinity in them for these bodies by which
they actively pick them up.

288 THE HUMAN BODY.

Each living cell, we have seen, is the seat of constant
chemical activity, taking up materials from the medium
about it, transforming and utilizing them, and sooner or
later restoring their elements, differently combined, to the
outer medium. By such means it builds up and maintains
its living substance, and obtains energy to carry on its daily
.work. While this is true of all cells in the Body, we find
certain groups in which chemical metabolism is the promi-
nent fact — cells which are specialized for this purpose just
as muscular fibre is for contraction or nerve-fibre for con-
duction; and certain of these prominently metabolic tissues
exist in the true glands and produce or collect the specific
elements of their secretions. Their chemical processes are
no doubt primarily directed to their own nutritive mainte-
nance; they live primarily for themselves, but their nutritive
processes are such that the bodies formed in them and sent
into the secretion are such as to be useful to the rest of the
cells of the community; or the bodies which they specially
collect, and in a certain sense feed on, are those the removal
of which from the blood is essential for the general good.
Their individual nutritive peculiarities are utilized for the
welfare of the whole Body.

The Mode of Activity of Secretory Cells. If we con-
sider the modes of activity of living cells in general, it be-
comes clear that secretory cells may produce the specific
element of a secretion in either of two ways. They may,
as a by-result of their living play of forces, produce chemical
changes in the surrounding medium ; or they may build up
certain substances in themselves and then set them free as
specific elements. Yeast, for example, in a saccharine solu-
tion causes the rearrangement into carbon dioxide, alcohol,
glycerine and succinic acid, of many atoms of carbon, hydro-
gen and oxygen which previously existed as sugar; and
a very considerable quantity of sugar may be broken up by
the activity of a few living yeast-cells. How the latter act
we do not know with certainty, but most likely by picking cer-
tain atoms out of the sugar molecule, and leaving the rest to
fall down into simpler compounds. On the other hand, we find
cells which form and store up in themselves large quantities
of substances, which they afterwards liberate; starch, for
instance, being formed and laid by in many fruit-cells, and

THE SECRETORY TISSUES AND ORGANS. 289

afterwards dissolved and sent in solution to nourish the
young plant.

Gland-cells might a priori give rise to the specific ele-
ments of secretions in either of these ways, and we have to
seek in which manner they work. Do they simply act as fer-
ments (however that is) upon the surrounding medium; or
do they form or collect the bodies characterizing their
secretion, first within their own substance, and then liberate
them, either disintegrating or not at the same time ? At
present there is a large and an increasing mass of evidence
in favor of the second view. There is, no doubt, some
reason to believe that every living cell can act more or
less as a ferment upon certain solutions should they come
into contact with it. Not always, of course, as an alcoholic
ferment, though even as regards that one fermentative power
it seems very generally possessed by vegetable cells, and there
is some evidence that alcohol is normally produced in small
amount (and presumably by the fermentation of sugar) under
the influence of certain of the living tissues of the Human
Body. As regards distinctively secretory cells, however, the
evidence is all the other way, and in many cases we can see
the specific element collecting in the gland-cells before it is
set free in the secretion. For example, in the oil-glands of
the skin (Chapter XXVIII) we find the secreting cells, at
first granular, nucleated, and protoplasmic, gradually under-
going changes by which their protoplasm disappears and is
replaced by oil-droplets, until finally the whole cell falls to
bits and its detritus forms the secretion; the cells being re-
placed by new ones constantly formed within the gland. In
such cases the secretion is the ultimate product of the cell-
life? the result of degenerative changes of old age occurring
in it.

In other cases, however, the liberation of the specific ele-
ment is not attended with the destruction of the secreting
cell; as an example we may take the pancreas, which is a
large gland lying in the abdomen and forming a secretion
used in digestion. Among others, this secretion possesses
the power, under certain conditions, of dissolving proteids
and converting them into dialyzable peptones (p. 10). This
it owes to a specific element known as trypsin, the formation
of which, or rather of its forerunner trypsinoyen, within the
gland-cells can be traced with the microscope.

290 THE HUMAN BODY.

The pancreas, like the majority of the glands connected
with the alimentary canal, has an intermittent activity, de-
termined by the presence or absence of food in various parts
of the digestive tract. If the organ be taken from a recently
killed dog which has fasted thirty hours and, after proper
preparation, be stained with carmine and examined micro-
scopically, we get specimens of what we may call the " rest-
ing gland " — a gland which has not been secreting for some
time. In these it will be seen that the cells lining the secret-
ing recesses present two very distinct zones: an outer, next
the basement membrane which combines with the coloring
matter and is not granular, and an inner which is granular
and does not pick up the carmine. The granules we shall
find to be indications of the presence of a trypsin-yielding
substance formed in the cells.

If another dog be kept fasting until it has a good appetite
and be then allowed to eat as much meat as it will, the animal
will commonly take so much that the stomach will only be emp-
tied at the end of about twenty hours. This period may, so
far as the pancreas is concerned, be divided into two. From
the time the food enters the stomach and on for about ten
hours, the gland secretes abundantly; after that the secretion
dwindles, and by the end of the second ten hours has nearly
ceased. We have, then, a time during which the pancreas is
working hard, followed by a period in which its activity is
very little, but during which it is abundantly supplied with
food-materials. The pancreas taken from an animal at the
end of the first period and prepared for microscopic exami-
nation will be found different from that taken from a dog
killed at the end of the second digestion period, and also
from the resting gland. Towards the end of the period of
active work the gland-cells are diminished in size and the
proportions of the granular and non-granular zones are quite
altered. The latter now occupies most of the cell, while
the granular non-staining inner zone is greatly diminished.
During the secretion there is, therefore, a growth of the non-
granular and a destruction of the granular zone; and the
latter process rather exceeding the former, the whole secret-
ing cell is diminished in size. During the second digestive
period, when secretion is languid, exactly a reverse process
takes place. The cells increase in size so as to become larger
than those of the resting gland; and this growth is almost

THE SECRETORY TISSUES AND ORGANS. 291

entirely due to the granular zone which now occupies most
of the cell.

These facts suggest that during secretion the granular
part of the cells is used up : but that, simultaneously, the
deeper non - granular zone, being formed from materials
yielded by the blood, gradually renews the granular. Dur-
ing active secretion the breaking down of the latter to
yield the specific element occurs faster than its regenera-
tion; in a later period, however, when the secretion is ceas-
ing, the whole cell grows and, especially, the granular zone is
formed faster than it is disintegrated; hence the great in-
crease of that part of the cell. If this be so, then we ought
to find some relationship between the digestive activity of an
infusion or extract of the gland and the size of the granular
zones of the cells; and it has been shown that such exists;
the quantity of trypsin which can be obtained from a pan-
creas being proportionate to the size of that portion of its
cells.

The trypsin, however, does not exist in the cells ready
formed, but only a body which yields it under certain cir-
cumstances, and called trypsinogen.

If a perfectly fresh pancreas be divided into halves and
one portion immediately minced and extracted with glyce-
rine, while the other is laid aside for twenty-four hours in a
warm place and then similarly treated, it will be found that
the first glycerine extract has no power of digesting proteids,
while the second is very active. In other words, the fresh
gland does not contain trypsin, but only something which
yields it under some conditions; among others, on being
kept. The inactive glycerine extract of the fresh gland is,
however, rich in trypsinogen : for if a little acetic acid be
added to it, trypsin is formed and the extract becomes
powerfully digestive.

We may, then, sum up the life of a pancreas-cell in this
way. It grows by materials derived from the blood and first
laid down in the non-granular zone. This latter, in the ordi-
nary course of the cell-life, gives rise to the granular zone;
and in this is a store of trypsinogen produced by the active
metabolisms of the cell. When the gland secretes, the tryp-
sinogen is converted into trypsin and set free in the secre-
tion; but in the resting gland this transformation does not
occur. During secretory activity, therefore, the chemical

292 THE HUMAN BODY.

processes taking place in the cell are different from those at
other periods; and we have next to consider how this change
in the mode of life of the cells is brought about.

Influence of the Nervous System upon Secretion.
When the gland is active it is fuller of blood than when at
rest: its arteries are dilated and its capillaries gorged so that
it gets a brighter pink color; this extra blood-supply might
be the primary cause of the altered metabolism. Again, the
activity of the pancreas is under the influence of the nervous
system, as proved not only by the reflex secretion called forth
when food enters the stomach, but also by the fact that
electrical stimulation of the medulla oblongata will cause the
gland to secrete. The nervous system may, however, only
act through the nerves governing the calibre of the gland
arteries, and so but indirectly on the secreting cells; while
on the other hand it is possible that nerve-fibres act directly
upon the gland-cells and, controlling their nutritive pro-
cesses, govern the production of the trypsin. To decide be-
tween the relative importance of these possible agencies we
must pass to the consideration of other glands; since the
question can only be decided by experiment upon the lower
animals, and the position of the pancreas and the difficulty
of getting at its nerves without such severe operations as
upset the physiological condition of the animal furnish ob-
stacles to its study which have not yet been overcome.

In certain other glands, however, we find conclusive evi-
dence of a direct action of nerve-fibres upon the secreting
elements. When the sciatic nerve of a cat is stimulated
electrically, the balls of its feet sweat. Under ordinary cir-
cumstances they become at the same time red and full of
blood; but that this congestion is a factor of subsidiary im-
portance as regards secretion is proved by the facts that stim-
ulation of the nerve is still able to excite the gland-cells and
cause sweating in a limb which has been amputated ten or
fifteen minutes (and in which therefore no circulatory changes
can occur) and also by the cold sweats, with a pallid skin, of
phthisis and the death-agony. It is, however, with reference
to the submaxillary and parotid salivary glands that our in-
formation is most precise.

When the mouth is empty and the jaws at rest the sali-
vary secretion is comparatively little : but a sapid substance
placed on the tongue will -cause a copious flow. The phe-

THE SECRETORY TISSUES AND ORGANS. 293

nomenon is closely comparable to the production of a reflex
muscular contraction. A stimulus acting upon an irritable
tissue excites through it certain afferent nerve-fibres; these
excite a nerve-centre, which in turn stimulates efferent fibres;
going to a muscle in the one case, to a gland in the other.
It will be useful to consider again for a moment what occurs
in the case of the muscle, taking account only of the efferent
fibres and the parts they act upon.

When a muscle in the Body is made to contract reflexly,
through its nerve, two events occur in it. One is the short-
ening of the muscular fibres; the other is the dilatation of
the muscular arteries; every muscular nerve contains two
sets of fibres, one motor and one vaso-dilator, and normally
both act together. In this case, however, it is clear that the
activities of both, though correlated, are essentially inde-
pendent. The contraction is not due to the greater blood-
flow, for not only can an excised muscle entirely deprived of
blood be made to contract by stimulating its nerves, but in
an animal to which a small dose of curari — the arrow-poison
of certain South American Indians — has been given, stimu-
lation of the nerve will cause the vascular dilatation but no
muscular contraction: the curari paralyzing the motor fibres,
but, unless in large doses, leaving the vaso-dilators intact.
The muscular fibres themselves are unacted upon by the poi-
son, as is proved by their ready contraction when directly
stimulated by an electric shock.

Now let us return to the salivary glands and see how
far the facts are comparable. The main nerve of the sub-
maxillary gland is known as the chorda tympani. If it be
divided in a narcotized dog, and a tube placed in the gland-
duct, no saliva will flow. But on stimulating the peripheral
end of the nerve (that end still connected with the gland)
an abundant secretion takes place. At the same time there
is a great dilatation of the arteries of the organ, much more
blood than before flowing through it in a given time: the
chorda obviously then contains vaso-dilator fibres. Now in
this case it might very well be that the process was different
from that in a muscle. It is conceivable that the secretion
may be but a filtration due to increased pressure in the gland
capillaries, consequent on dilatation of the arteries supplying
them. If a greater filtration into the lymph spaces of the
gland took place, this liquid might then merely ooze on

294 THE HUMAN BODY.

through the secreting cells into the commencing ducts
as it passed through, dissolve out and carry on from the cells
the specific organic elements of the secretion. Of these, in
the submaxillary of the dog at least, mucin is the most im-
portant and abundant. That, however, the process is quite
different, and that there are in the gland true secretory fibres
in addition to the vaso-dilator, just as in the muscle there are
true motor fibres, is proved by other experiments.

If the flow of liquid from the excited gland were merely
the outcome of a filtration dependent on increased blood-
pressure in it, then it is clear that the pressure of the secre-
tion in the duct could never rise above the pressure in the
blood-vessels of the gland. Now it is found, not only that
the gland can be made to secrete in a recently decapitated
animal, in which of course there is no blood-pressure, but
that, when the circulation is going on, the pressure of the
secretion in the duct can rise far beyond that in the gland
arteries. Obviously, then, the secretion is no question of
mere filtration, since a liquid cannot filter against a higher
pressure. Finally, the proof that the vascular dilatation is
quite a subsidiary phenomenon has been completed by show-
ing that we can produce all the increased blood-flow through
the gland without getting any secretion — that just as in a
muscle nerve we can, by curari, paralyze the motor fibres
and leave the vaso-dilators intact, so we can by atropin, the
active principle of deadly nightshade, get similar phenomena
in the gland. In an atropized animal stimulation of the
chorda produces vascular dilatation but not a drop of secretion.
Bringing blood to the cells abundantly will not make them
drink; we must seek something more in the chorda than the
vaso-dilator fibres — some proper secretory fibres; that the
atropin acts upon them and not upon the gland-cells is shown,
as in the muscle, by the fact that the cells still are capable of
activity when stimulated otherwise than through the chorda
tympani. For example, by stimulation of the sympathetic
fibres going to the gland.

So far, then, we seem to have good evidence of a direct
action of nerve-fibres upon the gland-cells. But even that is
not the whole matter. It is extremely probable, if not cer-
tain, that there are two sets of secretory fibres in the gland-
nerves: a set which so acts upon the cells as to make them
pass on more abundantly the transudation elements of the

THE SECRETORY TISSUES AND ORGANS.

secretion (the water and mineral salts), and another, quite
different, which governs the chemical transformations of the
cells so as to make them produce mucin from mucigen pre-
viously stored in them, in a way comparable to the production
of trypsin from trypsinogen in the active pancreas. These
latter fibres may be called " trophic," since they directly con-
trol the cell metabolism: while the former may be called
" transudatory " fibres. Some of the evidence which leads to
thisconclusionis a little complex, but it is worth while to con-
sider it briefly. In the first place, on stimulation of the
chorda of an unexhausted gland (that is, a gland not over-
fatigued by previous work) the following points can be
noted : —

With increasing strength of the stimulus the quantity of
the secretion, that is of the water poured out in a unit of
time, increases ; at the same time the mineral salts also in-
crease, but more rapidly, so that their percentage in a rap-
idly formed secretion is greater than in a more slowly
formed, up to a certain limit. The percentage of organic
constituents of the secretion also increases up to a limit; but
soon ceases to rise, or even falls again, while the water and
salts still increase. This of course is readily intelligible;
since the water and salts can be derived continually from the
blood, while the specific elements, coming from the gland-
cells, may be soon exhausted ; and so far the experiment
gives no evidence of the existence of distinct nerve-fibres
for the salts and water, and for the specific elements: all
vary together with the strength of the stimulus applied to
the nerve. But under slightly different circumstances their
quantities do not run parallel. The proportion of specific
elements in the secretion is largely dependent on whether
the gland has been previously excited or not. Prior stimula-
tion, not carried on of course to exhaustion, largely increases
the percentage of organic matters in the secretion produced
by a subsequent stimulation; but has no effect whatever on
the quantities of water and salts. These are governed en-
tirely by the strength of the second stimulation. Here,
then, we find that under similar circumstances the transuda-
tory and specific elements of the secretion do not vary to-
gether; and are therefore probably dependent upon different
exciting causes. And the facts might lead us to suspect
that there are in the chorda, besides the vaso-dilator, two

396 THE HUMAN BODY.

other sets of fibres : one governing the salts and water, and
the other the specific elements of the secretion. So far the
evidence is, perhaps, not quite conclusive; but experiments
upon the parotid gland of the dog put the matter beyond a
doubt.

The submaxillary gland receives fibres from the sympa-
thetic system, as well as the chorda tympani from the cerebro-
spinal. Excitation of the sympathetic fibres causes the
gland to secrete, but the saliva poured out is different from
that following chorda stimulation, which is in the dog abun-
dant and comparatively poor in organic constituents, and
accompanied by vascular dilatation : while the " sympathetic
saliva," as it is called, is less abundant, very rich in inucin,
and accompanied by constriction of the gland arteries.
According to the above view we might suppose that the
chorda contains many trausudatory and few trophic fibres,
and the sympathetic many trophic and few transudatory.
It might, however, well be objected that the greater rich-
ness in organic bodies of the sympathetic saliva was really
due to the small quantity of blood reaching the gland, when
that nerve was stimulated. This might alter the nutritive
phenomena of the cells and cause them to form mucin in
unusual abundance, in which case the trophic influence of
the nerve would be only indirect. Experiments on the
parotid preclude this explanation. That gland, like the sub-
maxillary, gets nerve-fibres from two sources: a cerebral and
a sympathetic. The latter enter the gland along its artery,
while the former, originating from the glosso-pharyngeal,
run in a roundabout course to the gland. Stimulation of the
cerebral fibres causes an abundant secretion, rich in water
and salts, but with hardly any organic constituents. At the
same time it produces dilatation of the gland arteries. Stim-
ulation of the sympathetic causes contraction of the parotid
gland arteries and no secretion at all. Nevertheless it causes
great changes in the gland-cells. If it be first stimulated
for a while and then the cerebral gland-nerve, the resulting
secretion may be ten times as rich in organic bodies as that
obtained without previous stimulation of the sympathetic;
and a similar phenomenon is observed if the two nerves be
stimulated simultaneously. So that the sympathetic nerve,
though unable of itself to cause a secretion, brings about
great chemical changes in the gland-cells. It is a distinct

THE SECRETORY TISSUES AND ORGANS. 297

trophic nerve. This conclusion is confirmed by histology.
Sections of the gland after prolonged stimulation of the sym-
pathetic show its cells to be quite altered in appearance, and
in their tendency to combine with carmine, when compared
either with those of the resting gland or of the gland which
has been made to secrete by stimulating its glosso-pharyngeal
branch alone.

We have still to meet the objection that the sympathetic
fibres may be only indirectly trophic, .governing the meta-
bolism of the cells through contraction of the blood-vessels.
If this were so, cutting off or diminishing the blood-supply
of the gland in any way ought to have the same result as
stimulation of its sympathetic fibres. Experiment shows that
such is not the case and reduces us to a direct trophic influ-
ence of the nerve. When the arteries are closed and the cere-
bral gland-nerve stimulated, it is found that the percentage of
organic constituents in the secretion is as low as usual; it re-
mains almost exactly the same whether the arteries are open
or closed or have been previously open or closed. We must
conclude that the peculiar influence of the sympathetic does
not depend upon its vaso-constrictor fibres.

These observations make it clear that the phenomena of
secretion are dependent on very complex conditions, at least
in the salivary glands and presumably in others. Primarily
dependent upon filtration and dialysis from the blood-vessels
and upon the physiological character of the gland-cells, both
of these factors are, we find, controlled by the nervous system,
such secreting cells being no more automatic than striped
muscle; and the facts also give us important evidence of the
power of the nervous system to influence cell nutrition directly.
In other simpler cases, secretion seems to be a mere direct re-
sult of the growth and life of the secreting cell; for example
the formation, storage and discharge of fatty matters by the
oil-glands of the skin.

Summary. By secretion proper is meant the separation
of such substances from the blood as are poured out on free
surfaces of the Body, whether external or internal. In its
simplest form it is merely a physical process dependent on fil-
tration and dialysis; for example, the elimination of carbon
dioxide from the surfaces of the lungs, and very watery liquid
poured out on the surface of the serous membranes. Such
secretions are known as transudata, and their amount is only

298 THE HUMAN BODY.

indirectly controlled by the nervous system, through the in-
fluence of the latter upon the circulation of the blood; they
are however dependent on the structure of the cells concerned,
so that the characters of the transudata and their quantity are
altered when the cells are diseased. After death, too, the
process of dialysis through such cells is different from that
during life, for the living cell controls to a certain extent the
nature and amount of the substances which it will allow to
pass through it. The cells lining such surfaces are not, how-
ever, secretory tissues in any true sense of the word. In
other cases the lining cells are thicker, and more actively
concerned in the process; they are then usually spread over
the recesses of a much folded membrane, so that the whole
is rolled up into a compact organ called a gland, the secre-
tion of which may contain only transudation elements (as
for example that of the lachrymal glands which form the
tears) or may contain a specific element, formed in the
gland by its cells, in addition to transudation elements.
In both cases the activity of the organ may be influenced
by the nervous system, usually in a reflex manner (e.g. the
watering of the eyes when the eyeball is touched and the
saliva poured into the mouth when food is tasted), but may
also be otherwise excited, as for example the flow of tears
under the influence of those changes of the central nervous
system which are associated with sad emotions, or the water-
ing of the mouth at the thought of dainty food. The nerves
going to such glands, besides controlling their blood-vessels,
act upon the gland-cells; one set governing the amount of
transudation of water and salines which shall take place
through them, and another (in the case of glands producing
secretions with one or more specific elements) controlling the
production of these, by starting new chemical processes in
the cells by, which a substance built up in them during rest
is converted into the specific element, which is soluble in and
carried off by the transudation elements. What the specific
element of a gland shall be, or whether its secretioii contain
any, is dependent on the nature of its special cells; how
much transudation and how much specific element shall be
secreted at any time is controlled by the nervous system;
just as the contractility of a muscle depends on the endow-
ments of muscular tissue, and whether it shall rest or con-
tract— and if the latter, how powerfully — upon its nerve.

CHAPTER XX. ,

THE INCOME AND EXPENDITURE OF THE BODY.

The Material Losses of the Body. All day long while
life lasts each of us is losing something from his Body. The
air breathed into the lungs becomes in them laden with
carbon dioxide and water vapor, which are carried off with
it when it is expired. The skin is as constantly giving off
moisture, the total quantity in twenty-four hours being con-
siderable, even when the amount passed out at any one time
is so small as to be evaporated at once and so does not collect
as drops of visible perspiration. The kidneys again are con-
stantly at work separating water and certain crystalline ni-
trogeneous bodies from the blood, along with some mineral
salts. The product of kidney activity, however, not being
forthwith carried to the surface but to a reservoir, in which
it accumulates and which is only emptied at intervals, the ac-
tivity of those organs appears at first sight intermittent. If to>
these losses we add certain other waste substances passed into-
the alimentary canal and got rid of along with the undigested
residue of the food, and the loss of hairs and of dried cells
from the surface of the skin, it is clear that the total amount
of matter daily removed from the Body is considerable. The
actual quantity varies with the individual, with the work
done, and with the nature of the food eaten; but the follow-
ing table (p. 300) gives approximately that of the more im-
portant daily material losses of an average man.

The living Body thus loses daily in round numbers 4 kilo-
grams of matter (9 Ibs.) and, since it is unable to create new
matter, this loss must be compensated for from the exterior
or the tissues would soon dwindle away altogether; or at least
until they were so impaired that life came to an end. After
death the losses would be of a different kind, and their quan-
tity much more dependent upon surrounding conditions; but
except under very unusual circumstances the wasting away
would still continue in the dead Body. Moreover, the compo-

299

300

THE HUMAN BODY.

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INCOME AND EXPENDITURE OF THE BODY. 301

sition of the daily wastes of the living Body is tolerably con-
stant; it does not simply lose a quantity. of matter weighing
so much, but a certain amount of definite kinds of matter,
carbon, nitrogen, oxygen, and so on; and these same sub-
stances must be restored to it from outside, in order that life
may be continued. To give a stone to one asking for bread
might enable him, if he swallowed it, to make up the weight
of matter lost in twenty-four hours; but bread would be
needed to keep him alive. The Body not only requires a
supply of matter from outside, but a supply of certain definite
kinds of matter.

The Losses of the Body in Energy. The daily expendi-
ture of matter by the living Body is not the only one: as
continuously it loses in some form or another energy, or the
power of doing work; often as mechanical work expended in
moving external objects, but even when at rest energy is con-
stantly being lost to the Body in the form of heat, by radia-
tion and conduction to surrounding objects, by the evaporation
of water from the lungs and skin, and by removal in warm
excretions. Unless the Body can make energy it must there-
fore receive a certain supply of it also from the exterior, or it
would very soon cease to carry on any of its vital work ; it
would be unable to move and woui/l cool down to the temper-
ature of surrounding objects. The discoveries of this century
having shown that energy is as inuestructible and uncreatable
(see Physics) as matter, we are led to look for the sources of
the supply of it to the Body; and finding that the living Body
daily receives it and dies when the supply is cut off, we no
longer suppose, with the older physiologists, that it works by
means of a mysterious vital force existing in or created by it;
but that getting energy from the outside it utilizes it for its
purposes — for the performance of its nutritive and other living
work — and then returns it to the exterior in what the physi-
cists know as a degraded state; that is, in a less utilizable
condition. While energy like matter is indestructible it is,
unlike matter, transmutable; iron is always iron and gold
always gold; neither can by any means which we possess be
converted into any other form of matter; and so the Body,
needing carbon, hydrogen, oxygen, and nitrogen to build it
and to cover its daily losses, must be supplied with those very
substances. As regards energy this is not the case. While
the total amount of it in the universe is constant, its form is

302 THE HUMAN BODY.

constantly subject to change — and that one in which it enters
the Body need not be that in which it exists while in it, nor
that in which it leaves it. Daily losing heat and mechanical
work the Body does not need, could not in fact much utilize
energy, supplied to it in these forms; but it does need energy
of some form and in amount equivalent to that which it loses.
The Conservation of Energy. The forms of energy yet
discovered are not nearly so numerous as the kinds of matter.
Still we all know several of them; such as light, heat, sound,
electricity, and mechanical work; and most people nowadays
know, that some of these forms are interconvertible, so that
directly or indirectly we can turn one into another. In such
changes it is found that a definite amount of one kind always
disappears to give rise to a certain quantity of the other; or,
in other words, that so much of the first form is equivalent
to so much of the second. In a steam-engine, heat is pro-
duced in the furnace; when the engine is at work all of this
energy does not leave it as heat; some goes as mechanical
work, and the more work the engine does the greater is the
difference between the heat generated in the furnace and that
leaving the machine. If, however, we used the work for rub-
bing two rough surfaces together we could get the heat back
again, and if (which of course is impossible in practice) we
could avoid all friction in the moving parts of the machine,
the quantity thus restored would be exactly equal to the
excess of the heat generated in the furnace over that leaving
the engine. Having turned some of the heat into mechanical
work we could thus turn the work back into heat again, and
find it yield exactly the amount which seemed lost. Or we
might use the engine to drive an electro-magnetic machine
and so turn part of the heat liberated in its furnace first into
mechanical work and that into electricity; and if we chose, to
use the latter with the proper apparatus, we could turn more
or less of it into light, and so have a great part of the energy
which first became conspicuous as heat in the engine furnace,
now manifested in the form of light at some distant point.
In fact, starting with a given quantity of one kind of energy,
we may by proper contrivances turn all or some of it into
one or more other forms; and if we collected all the final
forms and retransformed them into the first, we should have
exactly the amount of it which had disappeared when the
other kinds appeared. This law, that eneryv can

INCOME AND EXPENDITURE OF THE BODY. 303

form but that its amount is invariable, that it cannot. V>P nrpafpd
or destroyed but simply transmuted, is known as the law of
tlie Uonservaiion of Energy (see Physics), and, like the inde-
structibility of matter, lies at the basis of all scientific con-
ceptions of the universe, whether concerned with animate or
inanimate objects.

Since all forms of energy are interconvertible it is con-
venient in comparing amounts of different kinds to express
them in terms of some one kind, by saying how much of that
standard form the given amount of the kind spoken of would
give rise to if completely converted into it. Since the most
easily measured form of energy is mechanical work this is
commonly taken as the standard form, and the quantities of
others are expressed by saying how great a distance against
the force of gravity at the earth's surface a given weight could
be raised by the energy in question, if it were all spent in
lifting the weight. The units of mechanical work being the
kilogrammeter or the foot-pound, the mechanical equivalent
of any given kind of energy is the number of kilogrammeters
or foot-pounds of work its unit quantity would perform if
converted into mechanical work and used to raise a weight.
FW example the Uhlt quantity oi heaTTs that necessary to
raise one kilogram of water one degree centigrade in temper-
ature; or sometimes, in books written in English, the quan-
tity necessary to warm one pound of water one degree Fahren-
heit. When therefore we say that the mechanical equivalent
of heat is 423 kilogrammeters we mean that the quantity of
heat which would raise one kilogram of water in temperature
from 4° C. to 5° C. would, if all turned into mechanical work,
be able to raise one kilogram 423 meters against the attraction
of the earth; and conversely, that this amount of mechanical
work if turned into heat would warm a kilogram of water
one degree centigrade. The mechanical equivalent of heat,
taking the Fahrenheit thermometric scale and using feet and
pounds as measures, is 772 foot-pounds.

Potential and Kinetic Energy. At times energy seems
to be lost. Ordinarily we only observe it when it is doing
work and producing some change in matter:, but sometimes
it is at rest, stored away and producing no changes that we
recognize and thus seems to have been destroyed. Energy at
work is known as kinetic energy; energy at rest, not produc-
ing changes in matter, is called potential energy. Suppose a

304 THE HUMAN BODY.

stone pulled up by a string and left suspended in the air.
We know a certain amount of energy was used to lift it; but
while it hangs we have neither heat nor light nor mechanical
work to represent it. Still the energy is not lost; we know
we have only to cut the string and the weight will fall, and
striking something give rise to heat. Or we may wind up a
spring and keep it so by a catch. In winding it up a certain
amount of energy in the form of mechanical work was used
'to alter the form of the spring. Until the catch is removed
this energy remains stored away as potential energy : but we
know it is not lost. Once the spring is let loose again it may
drive a clock or a watch, and in so doing will perform again
just so much work as was spent in coiling it; and when the
watch has run down this energy will all have been turned
into other forms — mainly heat developed in the friction of
the parts of the watch against one another: but partly also
in producing movements of the air, a portion of which we
ean readily observe in the sound of its ticking. The law of
the conservation of energy does not say, then, that either the
total potential or the total kinetic energy in the universe is
constant in amount: but that the sum of the two is invariable,
while constantly undergoing changes from kinetic to potential
and vice versa : and from one form of kinetic to another.

The Energy of Chemical Affinity. Between every two
chemical atoms which are capable of entering into combina-
tion there exists a certain amount of potential energy; when
they unite this energy is liberated, usually in the form of heat,
and once they have combined a certain amount of kinetic
energy must be spent to pull them apart again; this being
exactly the amount which was liberated when they united.
The more stable the compound formed the more kinetic
energy appears during its formation, and the more must be
spent to break it up again. One may imagine the separated
atoms as two balls pushed together by springs, the strength
of the spring being proportionate to the degree of their
chemical affinity. Once they are let loose and permitted to
strike together the potential energy previously represented by
the compressed springs disappears, and in its place we have
the kinetic energy, represented by the heat developed when
the balls strike together. To pull them apart again, against
the springs, to their original positions, just so much mechani-
cal work must be spent as is the equivalent of that amount

INCOME AND EXPENDITURE OF THE BODY. 305

of heat which appeared when they struck; and thus kinetic
energy will again become latent in breaking up the compound
represented by the two in contact. The energy liberated in
chemical combination is the most important source of that
used in our machines: and also of that spent by the living
Body.

The Relation between the Matters Removed from the
Body daily and the Energy Spent by it. A working loco-
motive is, we know, constantly losing matter to the exterior
in the form of ashes and gaseous products of combustion, the
latter being mainly carbon dioxide and water vapor. The
engine also expends energy, not only in the form of heat
radiated to the air, but as mechanical work in drawing the
cars against the resistance offered by friction or sometimes,
np an incline, by gravity. Now the engine-driver knows that
there is a close relationship between the losses of matter and
the expenditure of energy, so that he has to stoke his furnace
more frequently and allow a greater draft of air through it in
going up a gradient than when running on the level. The
more work the engine does the more coals and air it needs to
make up for its greater waste. If we seek the cause of this
relationship between work and waste, the first answer natu-
rally is that the engine is a machine the special object of
which is to convert heat into mechanical work, and so the
more work it has to do the more heat is required for conver-
sion, and consequently the more coals must be burnt. This,
however, opens the question of the source of the heat — of all
that vast amount of kinetic energy which is liberated in the
furnace; and to answer this we must consider in what forms
matter and energy enter the furnace, since the energy liber-
ated there must be carried in somehow from outside. For
present purposes coals may be considered as consisting of
carbon and hydrogen, both of which substances tend to
forcibly combine with oxygen at high temperatures, forming
in the one case carbon dioxide and in the other water. The
oxygen necessary to form these compounds being supplied
by the air entering the furnace, all the potential energy of
chemical affinity which existed between the uncombined
elements becomes kinetic, and is liberated as heat when the
combination takes place. The energy utilized by the engine
is therefore supplied to it in the form of potential energy,
associated with the uncombined forms of matter which reach

306 THE HUMAN BODY.

the furnace. Once the carbon and hydrogen have combined
with oxygen they are no longer of any use as liberators of
energy; and the compounds formed if retained in the furnace
would only clog it and impede farther combustion; they are
therefore got rid of as wastes through the smoke-stack. The
•engine, in short, receives uncombined elements associated
with potential energy; and loses combined elements (which
have lost the energy previously associated with them) and
kinetic energy: it, so to speak, separates the energy from the
matter with which it was connected, utilizes it, and gets rid
of the exhausted matter. The amount of kinetic energy
liberated during such chemical combinations is very great:
a kilogram of carbon uniting with oxygen to form carbon
dioxide sets free 8080 units of heat, or calories. During the
combination of oxygen and hydrogen to form water even
more energy is liberated, one kilogram of hydrogen when
completely burnt liberating more than thirty-four thousand
of the same units. The mechanical equivalent of this can be
calculated if it is remembered that one heat unit = 423
kilogrammeters.

Turning now to the living Body we find that its income
and expenditure agree very closely with those of the steam-
engine. It receives from the exterior substances capable of
entering into chemical union; these combine in it and liber-
ate energy; and it loses kinetic energy and the products of
combination. From the outside it takes oxygen through the
lungs, and oxidizable substances (in the form of foods)
through the alimentary canal; these combine under the con-
ditions prevailing in the living cells just as the carbon and
oxygen, which will not unite at ordinary temperatures, com-
bine under the conditions existing in the furnace of the
engine; the energy liberated is employed in the work of the
Body, while the useless products of combination are got rid
of. To explain, then, the fact that our Bodies go on working
we have no need to invoke some special mysterious power
resident in them and capable of creating energy, a vital force
having no relation with other natural forces, such as the
older physiologists used to imagine. The Body needs and
gets a supply of energy from the exterior just as the steam-
engine does, food and air being to one what coals and air are
to t-he other; each is a machine in which energy is liberated
by chemical combinations and then used for special work;

INCOME AND EXPENDITURE OF THE BODY. 307

the character of which depends upon the peculiarities of
mechanism which utilizes it in each case, and not upon any
peculiarity in the energy utilized or in its source. The Body
is, however, a far more economical machine than any steam-
engine; of all the energy liberated in the latter only a small
fraction, about one eighth, is usefully employed, while our
Bodies can utilize for the performance of muscular work
alone one fifth of the whole energy supplied to them; leaving
out of account altogether the nutritive and other work carried
on in them, and the heat lost from them.

The Conditions of Oxidation in the Living Body. Al-
though the general principles applied in the Body and the
steam-engine for getting utilizable energy are the same, in
minor points obvious differences are found between the two.
In the first place the coals of an engine are oxidized only at
a very high temperature, one which would be instantly fatal
to our Bodies, which, although warm when compared with
the bulk of inanimate objects, are very slow fires when com-
pared with a furnace. Chemistry and physics, however,
teach us that this difference is quite unimportant so far as
concerns the amount of energy liberated. If magnesium
wire be ignited in the air it will become white-hot, flame, and
leave at the end of a few seconds only a certain amount of
incombustible rust or magnesia, which consists of the metal
combined with oxygen. The heat and light evolved in the
process represent of course the energy which, in a potential
form, was associated with the magnesium and oxygen before
their combination. We can, however, oxidize the metal in a
different way, attended with no evolution of light and no
very perceptible rise of temperature. If, for instance, we
leave it in wet air it will become gradually turned into mag-
nesia without having ever been hot to the touch or luminous
to the eye. The process will, however, take days or weeks;
and while in this slow oxidation just as much energy is liber-
ated as in the former case, it now all takes the form of heat;
and instead of being liberated in a short time is spread over
a much longer one, as the gradual chemical combination
takes place. The slowly oxidizing magnesium is, therefore,
at no moment noticeably hot, since it loses its heat to sur-
rounding objects as fast as it is generated. The oxidations
occurring in our Bodies are of this slow kind. An ounce of
arrowroot oxidized in a fire, and in the Human Body, would

308 THE HUMAN BODY.

liberate exactly as much energy in one case as the other, but
the oxidation would take place in a few minutes and at a
high temperature in the former, and slowly, at a lower tem-
perature, in the latter. In the second place, the engine dif-
fers from the living Body in the fact that the oxidations in it
all take place in a small area, the furnace, and so the tem-
perature there becomes very high; while in our Bodies the
oxidations take place all over, in each of the living cells;
there is no one furnace or hearth where all the energy is lib-
erated for the whole and transferred thence in one form or
another to distant parts : and this is another reason why no
one part of the Body attains a very high temperature.

The Fuel of the Body. This is clearly different from
that of an ordinary engine: no one could live by eating coals.
This difference, again, is subsidiary; a gas-engine requires
different fuel from an ordinary locomotive; and the Body re-
quires a somewhat different one from either. It needs, as
foods, substances which can, in the first place, be absorbed
from the alimentary canal and carried to the various tissues;
and, in the second, can be oxidized at a low temperature in
the blood or tissues, or can be converted by the living cells
into compounds which can be so oxidized. With some trivial
exceptions, all substances which fulfil these conditions are
complex chemical compounds, and to understand their utili-
zation in the Body we must extend a little the statements
above made as to the liberation of energy in chemical com-
binations. The general law maybe stated thus: Energy is
liberated whenever chemical union takes place: and whenever
more stable compounds are formed from less stable ones, in
which the constituent atoms were less firmly held together.
Of the liberation by simple combination we have already seen
an instance in the oxidation of carbon in a furnace; but the
union need not be an oxidation. Every one knows how hot
quicklime becomes when it is slaked; the water combining
strongly with the lime, and energy being liberated in the
form of heat during the process. Of the liberation of energy
by the breaking down of a complex compound, in which the
atoms are only feebly united, into simpler and stabler ones,
we get an example in alcoholic fermentation. During that
process grape-sugar is broken down into more stable com-
pounds, mainly carbon dioxide and alcohol, while oxygen is
at the same time taken up. To pull apart the carbon, hydro-

INCOME AND EXPENDITURE OF THE BODY. 309

gen, and oxygen of the sugar molecule requires a certain
expenditure of kinetic energy: but in the simultaneous for-
mation of the new and stabler compounds a greater amount of
energy is set free, and the difference appears as heat, so that
the brewer frequently has to cool his vats with ice. It is by
processes like this latter, rather than by direct combinations,
that most of the kinetic energy of the Body is obtained; the
complex proteids and fats and starches and sugar taken as
food being broken down (usually with concomitant oxida-
tion) into simpler and more stable compounds.

Oxidation by Successive Steps. In the furnace of an
engine the oxidation takes place completely at once. The
carbon and hydrogen leaving it, if it is well managed, are
each in the state of their most stable oxygen compound.
But this need not be so: we might first oxidize the carbon so
as to form carbon monoxide, CO, and get a certain amount of
heat; and then oxidize the carbon monoxide farther so as to
form carbon dioxide, C02, and get more heat. If we add
together the amounts of heat liberated in each stage, the sum
will be exactly the quantity which would have been obtained
if the carbon had been completely burnt to the state of car-
bon dioxide at first. Every one who has studied chemistry
will think of many similar cases. As the process is impor-
tant physiologically, we may take another example, say the
oxidation of alcohol. This may be burnt completely and di-
rectly, giving rise to carbon dioxide and water —

C2H60 + 06 = 2C02 + 3H20

1 Alcohol. 6 Oxygen. 2 Carbon dioxide. 3 Water.

But instead of this we can oxidize the alcohol by stages, get-
ting at each stage only a comparatively small amount of heat
evolved. By combining it first with one atom of oxygen, we
get aldehyde and water —

C,H60 -h 0 = C2H40 + H,0

1 Alcohol. 1 Oxygen. 1 Aldehyde. 1 Water.

Then we add an atom of oxygen to the aldehyde and get
acetic acid (vinegar)—

C2H40 + 0 - C2H40,

1 Aldehyde. 1 Oxygen. 1 Acetic acid.

310 THE HUMAN BODY.

And finally we may oxidize the acetic acid so as to get carbon
dioxide and water —

We get, in both cases, from one molecule of alcohol, two
of carbon dioxide and three of water; and six atoms of oxy-
gen are taken up. In each stage of the gradual oxidation a
certain amount of heat is evolved; and the sum of these is
exactly the amount which would have been evolved by burn-
ing the alcohol completely at once.

The food taken into the Body is for the most part oxi-
dized in this gradual manner; the products of imperfect
combustion in one set of cells being carried off and more
completely oxidized in another set, until the final products,
no longer capable of further oxidation in the Body, are car-
ried to the lungs, or kidneys, or skin, and got rid of. A great
object of physiology is to trace all intermediate compounds
between the food which enters and the waste products which
leave; to find out just how far chemical degradation is carried
in each organ, and what substances are thus formed in vari-
ous parts : but at present this part of the science is very im-
perfect.

The Utilization of Energy in the Human Body. In
the steam-engine energy is liberated as heat; some of the heat
is used to evaporate water and expand the resulting steam ;
and then the steam to drive a piston. But in the living Body
it is very probable (indeed almost certain) that a great part
of the energy liberated by chemical transformations does not
first take the form of heat; though some of it does. This,
again, does not affect the general principle: the source of
energy is essentially the same in both cases; it is merely the
form which it takes that is different. In a galvanic cell
energy is liberated during the union of zinc and sulphuric
acid, and we may so arrange matters as to get this energy as
heat; but on the other hand we may lead much of it off, as
a galvanic current, and use it to drive a magneto-electric
machine before it has taken the form of heat at all. In fact,
that heat may be used to do mechanical work we must reduce
some of it to a lower temperature: an engine needs a con-
denser of some kind as well as a furnace; and, other things
being equal, the cooler the condenser the greater the proper-

INCOME AND EXPENDITURE OF THE BODY. 311

tion of the whole heat liberated in the furnace which can be
used to do work. Now in a muscle there is no condenser; its
temperature is uniform throughout. So when it contracts
and lifts a weight, the energy employed must be liberated in
some other form than heat — some form which the muscular
fibre can use without a condenser.

Summary. The living Body is continually losing mat-
ter and expending energy. So long as we regard it as work-
ing by virtue of some vital force, the power of generating
which it has inherited, the waste is difficult to account for,
since it is far more than we can imagine as due merely to
wear and tear of the working parts. When, however, we con-
sider the nature of the income of the Body, and of its ex-
penditure, from a chemico-physical point of view, we get the
clue to the puzzle. The Body does not waste because it
works, but works because it wastes. The working power is
obtained by chemical changes occurring in it, associated with
the liberation of energy which the living cells utilize; and
the products of these chemical changes, being no longer
available as sources of energy, are passed out. The chemical
changes concerned are mainly the breaking down of complex
and unstable chemical compounds into simpler and more
stable ones, with concomitant oxidation. Accordingly the
material losses of the Body are highly or completely oxidized,
tolerably simple, chemical compounds; and its material in-
come is mainly nncombined oxygen and oxidizable substances,
the former obtained through the lungs, the latter through
the alimentary canal. In energy, its income is the potential
energy of uncombined or feebly combined elements, which
are capable of combining or of forming more stable com-
pounds; and its final expenditure is kinetic energy almost
entirely in the form of mechanical work and heat. Given
oxygen, all oxidizable bodies will not serve to keep the Body
alive and working, but only those which (1) are capable of
absorption from the alimentary canal and (2) those which
are oxidizable at the temperature of the Body under the influ-
ence of protoplasm. Just as carbon and oxygen will not
unite in the furnace of an engine unless the fire be lighted by
the application of a match but, when once started, the heat
evolved at one point will serve to bring about the conditions
of combination through the rest of the mass, so the oxida-
tions of the Body only occur under special conditions; and

312 THE HUMAN BODY.

these are transmitted from parent to offspring. Every new
Human Being starts as a portion of protoplasm separated
from a parent and affording the conditions for those chemi-
cal combinations which supply to living matter its working
power: this serves, like the energy of the burning part of a
fire to start similar 'processes in other portions of matter. At
present we know nothing in physiology answering to the
match which lights a furnace: those manifestations of energy
which we call life are handed down from generation to gen-
eration, as the sacred fire in the temple of Vesta from one
watcher to another. Science may at some time teach us how
to bring the chemical constituents of protoplasm into that
combination in which they possess the faculty of starting
oxidations under those conditions which characterize life;
then we shall have learnt to strike the vital match. For
the present we must be content to study the properties of
that form of matter which possesses living faculties; since
there is no satisfactory proof that it has ever been produced,
within our experience, apart from the influence of matter
already living. How the vital spark first originated, how
molecules of carbon, hydrogen, nitrogen and oxygen first
united with water and salts to form protoplasm, we have no
scientific data to ground a positive opinion upon, and such as
we may have must rest upon other grounds.

CHAPTER XXj.
FOODS.

Foods as Tissue-formers. Hitherto we have considered
foods merely as source of energy, but they are also required
to build up the substance of the Body. From birth to man-
hood we increase in bulk and weight, and that not merely by
accumulating water and such substances, but by forming more
bone, more muscle, more brain, and so on, from materials
which are not necessarily bone or muscle or nerve-tissue.
Alongside of the processes by which complex substances are
broken down and oxidized and energy liberated, constructive
processes take place by which new complex bodies are formed
from simpler substances taken as food. A great part of the
energy liberated in the Body is in fact utilized first for this
purpose, since to construct complex unstable molecules, like
those of protoplasm, from the simpler compounds taken into
the Body, needs an expenditure of kinetic energy. Even
after full growth, when the Body ceases to gain weight, the
same synthetic processes go on; the living tissues are steadily
broken down and constantly reconstructed, as we see illus-
trated by the condition of a man who has been starved for
some time, and who loses not only his power of doing work
and of maintaining his bodily temperature but also a great
part of his living tissues. If again fed properly he soon
makes new fat and new muscle and regains his original mass.
Another illustration of the continuance of constructive
powers during the whole of life is afforded by the growth of
the muscles when exercised properly.

Since the tissues, on ultimate analysis, yield mainly car-
bon, hydrogen, nitrogen and oxygen, it might be supposed
a priori that a supply of these elements in the uncombined
state would serve as material for the constructive forces of
the Body to work with. Experience, however, teaches us
that this is not the case, but that the animal body requires,
for the most part, highly complex compounds for the con-

314 THE HUMAN BODY.

struction of new tissue elements. All the active tissues yield
on analysis large quantities of proteids which, as pointed out
in Chapter I, enter always into the structure of protoplasm.
Now, so far as we know at present,* the animal body is unable
to build up proteids from simpler compounds of nitrogen,
although when given one variety of them it can convert that
one into others, and combine them with other things to form
protoplasm. Hence proteids are an essential article of diet,
in order to replace the proteid of the living cells which is
daily broken down and eliminated in the form of urea and
other waste substances. Even albuminoids (p. 10), although
so nearly allied to proteids, will not serve to replace them
entirely in a diet; a man fed abundantly on gelatine, fats,
and starches would starve as certainly, though not so quickly,
as if he got no nitrogenous food at all: his tissue waste would
not be made good, and he would at last be no more able to
utilize the energy-yielding materials supplied to him than a
worn-out steam-engine could employ the heat of a fire in its
furnace. So, too, the animal is unable to take the carbon for
the construction of its tissues, from such simple compounds
as carbon dioxide.* Its constructive power is limited to *he
utilization of the carbon contained in more complex and less
stable compounds, such as proteids, fats or sugars.

Nearly all the tissue-forming foods must therefore consist
of complex substances, and of these a part must be proteids,
since the Body can utilize nitrogen for tissue formation only
when supplied with it in that form.* The bodies thus taken
in are sooner or later broken down into simpler ones and
eliminated; some at once in order to yield energy, others
only after having first been built up into part of a living cell.
The partial exceptions afforded by such losses to the Body as
milk for suckling the young, or the albuminous and fatty
bodies stored for the same purpose in the egg of a bird, are
only apparent; the chemical degradation is only postponed,
taking place in the body of the offspring instead of that of
the parent. In all cases animals are thus, essentially, proteid
consumers or wasters, and breakers down of complex bodies;
the carbon, hydrogen, and nitrogen which they take as foods
in the form of complex unstable bodies, ultimately leaving

* There is some reason to believe tliat some few of the lower animals
which contain chlorophyl can manufacture proteids and utilize carbon
dioxide.

FOODS. 315

them in the simpler compounds, carbon dioxide, water, and
urea; which are incapable of either yielding energy or build-
ing tissue for any other animal and so of serving it as food.
The question immediately suggests itself — How, since animals
are constantly breaking up these complex bodies and cannot
again build them, is the supply kept up ? For example, the
supply of proteids. substances which cannot be made arti-
ficially by any process which we know, and yet are necessary
foods for all animals, and daily destroyed by them.

The Food of Plants. As regards our own Bodies the
question at the end of the last paragraph might perhaps be
answered by saying that we get our proteids from the flesh
of the other animals which we eat. But, then, we have to
account for the possession of them by those animals; since
they cannot make them from urea and carbon dioxide and
water any more than we can. The animals eaten get them,
in fact, from plants which are the great proteid formers of
the world, so that the most carnivorous animal really depends
for its most essential foods upon the vegetable kingdom ; the
fox that devours a hare in the long-run lives on the proteids
of the herbs that the hare had previously eaten. All animals
are thus, in a certain sense, parasites; they only do half of
their own nutritive work, just the final stages, leaving all the
rest to the vegetable kingdom and using the products of its
labor ; and plants are able to meet this demand because they
can live on the simple compounds of carbon, hydrogen, and
nitrogen eliminated by animals, building up out of them new
complex substances \vhich animals can use as food. A green
plant, supplied with ammonium salts, carbon dioxide, water,
and some minerals, will grow and build up large quantities
of proteids, fats, starches, and similar things; it will pull the
stable compounds eliminated by animals to pieces, and build
them up into complex unstable bodies, capable of yielding
energy when again broken down. However, to do such work,
to break up stable combinations and make from them less
stable, needs a supply of kinetic energy which disappears in
the process, being stored away as potential energy in the new
compound ; and we may ask whence it is that the plant gets
the supply of energy which it thus utilizes for chemical con-
struction, since its simple and highly oxidized foods can yield
it none. It has been proved that for this purpose the green
plant uses the energy of sunlight : those of its cells which con-

316 THE HUMAN BODY.

tain the substance called chloropUyl (leaf green) have the power
of utilizing energy in the form of light for the performance
of chemical work, just as a steam-engine can utilize heat for the
performance of mechanical work. Exposed to light, and re-
ceiving carbon dioxide from the air, and water and ammonia
(which is produced by the decomposition of urea) and other
simple nitrogen compounds from the soil, the plant builds
them up again, with the elimination of oxygen, into complex
bodies like those which animals broke down with fixation of
oxygen. Some of the bodies thus formed it uses for its own
growth and the formation of new protoplasm, just as an animal
does; but in sunlight it forms more than it uses, and the
excess stored up in its tissues is used by animals. In the long-
run, then, all the energy spent by our Bodies comes through
millions of miles of space from the sun; but to seek the source
of its supply there would take us far out of the domain of
Physiology.

Non-oxidizable Foods. Besides our oxidizable foods, a
large number of necessary food-materials are not oxidizable,
or at least are not oxidized in the Body. Typical instances
are afforded by water and common salt. The use of these is
in great part physical : the water, for instance, dissolves ma-
terials in the alimentary canal, and carries the solutions
through the walls of the digestive tube into the blood and
lymph vessels, so that they can be carried from part to part ;
and it permits interchanges to go on by diffusion. The
salines also influence the solubility and chemical interchanges
of other things present with them. Serum albumen, the
chief proteid of the blood, for example, is insoluble in pure
water, but dissolves readily if a small quantity of neutral salts
is present. Besides such uses the non-oxidizable foods have
probably others, in what we may call machinery formation.
In the salts which give their hardness to the bones and teeth,
we have an example of such an employment of them : and to
a less extent the same may be true of other tissues. The
Body, in fact, is not a mere store of potential energy, but
something more — it is a machine for the disposal of it in cer-
tain ways; and, wherever practicable, it is clearly advanta-
geous to have the purely energy-expending parts made of
non-oxidizable matters, and so protected from change and
the necessity of frequent renewal. The Body is a self -build-
ing and self-repairing machine, and the material for this

FOODS. 317

building and repair must be supplied in the food, as well as
the fuels, or oxidizable foods, which yield the energy the
machine expends; and while experience shows us that even
for machinery construction oxidizable matters are largely
needed, it is nevertheless a gain to replace them by non-oxi-
dizable substances when possible; just as if practicable it
would be advantageous to construct an engine out of mate-
rials which would not rust, although other conditions deter-
mine the use of iron for the greater part of it.

Definition of Foods. Foods may be defined as substances
which, when taken into the alimentary canal, are absorbed
from it, and then serve either to supply material for the
growth of the Body, or for the replacement of matter which
has been removed from it, either after oxidation or without
having been oxidized. Foods to replace matters which have
been oxidized must be themselves oxidizable; they are force-
generators, but may be and generally are also tissue-formers;
and are nearly always complex organic substances derived
from other animals or from plants. Foods to replace matters
not oxidized in the Body are force-regulators, and are for the
most part tolerably simple inorganic compounds. Among
the force-regulators we must, however, include certain organic
foods which, although oxidized in the Body and serving as
liberators of energy, yet produce effects totally dispropor-
tionate to the energy they set free, and for which effects they
are taken. In other words, their influence as stimuli in excit-
ing certain tissues to liberate energy, or as inhibitory agents
checking the activity of parts, is more marked than their
direct action as force -generators. As examples, we may take
condiments: mustard and pepper are not of much use as
sources of energy, although they no doubt yield some; we
take them for their stimulating effect on the mouth and
other parts of the alimentary canal, by which they promote
an increased flow of the digestive secretions or an increased
appetite for food. Thein and caffein, the active principles of
tea and coffee, are taken for their stimulating effect on the
nervous system, rather than for the amount of energy yielded
by their own oxidation.

Conditions which a Pood must Fulfil. (1) A food
must contain the elements which it is to replace in the Body:
but that alone is not sufficient. The elements leaving the
Body being usually derived from the breaking down of com-

318 THE HUMAN BODY.

plex substances in it, the food must contain them either in
the form of such complex substances, or in forms which the
Body can build up into them, Free nitrogen and hydrogen
are no use as foods, since they are neither oxidizable under
the conditions prevailing in the Body (and consequently can-
not yield it energy), nor are they capable of construction by
it into its tissues. (2) Food after it has been swallowed is
still in a strict sense outside the Body; the alimentary canal
is merely a tube running through it, and so long as food lies
there it does not form any part of the Body proper. Hence
foods must be capable of absorption from the alimentary
canal; either directly, or after they have been changed by the
processes of digestion. Carbon, for example, is useless as
food, not merely because the Body could not build it up into
its own tissues, but because it cannot be absorbed from the
alimentary canal. (3) Neither the substance itself nor any
of the products of its transformation in the Body must be
injurious to the structure or activity of any organ. If so it
is a poison, not a food.

Alimentary Principles. The articles which in common
language we call foods are, in most cases, mixtures of several
foodstuffs, with substances which are not foods at all. Bread,
for example, contains water, salts, gluten (a proteid), some
fats, much starch, and a little sugar; all true foodstuffs: but
mixed with these is a quantity of cellulose (the chief chemical
constituent of the walls which surround vegetable cells), and
this is not a food since it is incapable of absorption from the
alimentary canal. Chemical examination of all the common
articles of diet shows that the actual number of important
foodstuffs is but small: they are repeated in various propor-
tions in the different things we eat, mixed with small quan-
tities of different flavoring substances, and so give us a pleas-
ing variety in our meals; but the essential substances are
much the same in the fare of the workman and in the
" delicacies of the season." These primary foodstuffs, which
are found repeated in so many different foods, are known as
" alimentary principles" ; and the physiological value of any
article of diet depends on them far more than on the traces
of flavoring matters which cause certain things to be espe-
cially sought after and so raise their market value. The
alimentary principles may be conveniently classified into

FOODS. 319

proteids, albuminoids, hydrocarbons, carbohydrates, and inor-
ganic bodies.

Proteid or Albuminous Alimentary Principles. Of the
nitrogenous foodstuffs the most important are proteids: they
form an essential part of all diets, and are obtained both from
animals and plants. The most common and abundant are
myosin and syntonin, which exist in the lean of all meats; egg
albumen; casein, found in milk and cheese; gluten and vege-
table casein from various plants.

Gelatinoid or Albuminoid Alimentary Principles. These
also contain nitrogen, but cannot replace the proteids entirely
as foods; though a man can get on with less proteids when he
has some albuminoids in addition. The most important is
gelatin, which is yielded by the white fibrous tissue of animals
when cooked. On the whole the gelatinoids are not foods of
high value, and the calf's-foot jelly and such compounds,
often given to invalids, have not nearly the nutritive value
they are commonly supposed to possess.

Hydrocarbons (Fats and Oils). The most important are
stearin, palmatin, and olein, which exist in various propor-
tions in animal fats and vegetable oils; the more fluid contain-
ing more olein. Butter contains also a little of a fat named
butyrin. Fats are compounds of glycerine and fatty acids,
and any such substance which is fusible at the temperature
of the Body will serve as a food. The stearin of beef and
mutton fats is not by itself fusible at the body temperature,
but is mixed in those foods with so much olein as to be melted
in the alimentary canal. Beeswax, on the other hand, is a
fatty body which will not melt in the intestines and so passes
on unabsorbed; although from its composition it would be
useful as a food could it be digested. A distinction is some-
times made between fats proper (the adipose tissue of ani-
mals consisting of fatty compounds inclosed in albuminous
cell-walls) and oils, or fatty bodies which are not so organized.

Carbohydrates. These are mainly of vegetable origin.
The most important are starch, found in nearly all vegetable
foods ; dextrin ; gums ; grape-sugar, called also dextrose or
glucose (into which starch is converted during digestion) ; and
cane-sugar. Sugar of milk and glycogen are alimentary prin-
ciples of this group, derived from animals. All of them, like
the fats, consist of carbon, hydrogen and oxygen; but the per-

320 THE HUMAN BODY.

centage of oxygen in them is much higher, there being
one atom of oxygen for every two of hydrogen in their
molecule.

Inorganic Foods. Water; common salt; and the chlo-
rides, phosphates, and sulphates of potassium, magnesium
and calcium. More or less of these bodies, or the materials
for their formation, exists in all ordinary articles of diet, so
that we do not swallow them in a separate form. Phosphates,
for example, exist in nearly all animal and vegetable foods;
while other foods, as casein, contain phosphorus in combina-
tions which in the Body yield it up to be oxidized to form
phosphoric acid. The same is true of sulphates, which are
partially swallowed as such in various articles of diet, and are
partly formed in the Body by the oxidation of the sulphur of
various proteids. Calcium salts are abundant in bread, and
are also found in many drinking-waters. Water and table-
salt form exceptions to the rule that inorganic bodies are
eaten imperceptibly along with other things, since the Body
loses more of each daily than is usually supplied in that way.
It has, however, been maintained that salt, as such, is an
unnecessary luxury; and there seems some evidence that
certain savage tribes live without more than they get in the
meat and vegetables they eat. Such tribes are, however,
said to suffer especially from intestinal parasites; and there
is no doubt that to civilized man the absence of salt is a great
privation.

Calcium seems to be an essential constituent of all living
cells and in some way closely connected with the manifestation
of their activity. As previously mentioned the heart of a
frog after thorough irrigation with dilute solution of sodium
chloride ceases to beat, but resumes its pulsations when a
minute trace of calcium chloride is added to the solution;
and while ordinary serum restores the beat of such a washed-
out heart, serum from which all its calcium has been removed
does not. Moreover if defibrinated blood to which a little
more sodium oxalate than is sufficient to precipitate all its
calcium has been added, be circulated through the vessels of a
muscle, the latter loses, its contractility, apparently because
the slight excess of oxalate precipitates the calcium of the
muscle-fibres; for the contractility may be restored by sup-
plying some dissolved calcium chloride. Nerves treated simi-
larly lose their irritability"; and the eggs of some aquatic

FOODS. 321

animals will not develop normally in water from which all
calcium salts have been removed.

Mixed Poods. These, as already pointed out, include
nearly all common articles of diet; they contain more than
one alimentary principle. Among them we find great differ-
ences; some being rich in proteids, others in starch, others in
fats, and so on. The formation of a scientific dietary depends
on a knowledge of these characteristics. The foods eaten by
man are, however, so varied that we cannot do more than
consider the most important.

Flesh. This, whether derived from bird, beast, or fish,
consists essentially of the same things — muscular fibres,
connective tissue and tendons, fats, blood-vessels, and nerves.
It contains several proteids, especially rnyosin; gelatin-yield-
ing matters in the white fibrous tissue; stearin, palmatin,
and olein as representatives of the fats; and a small amount of
carbohydrates in the form of glycogen and grape-sugar, or
some chemically allied substances. Flesh also contains much
water and a considerable number of salines, the most important
and abundant being potassium phosphate. Osmazome is a
crystalline nitrogenous body which gives much of its taste to
flesh; and small quantities of various similar substances
exist in different kinds of meat. There is also more or less
yellow elastic tissue in flesh ; it is indigestible and useless as
food.

When meat is cooked its white fibrous tissue is turned
into gelatin, and the whole mass becomes thus softer and
more easily disintegrated by the teeth. When boiled some
of the proteid matters of the meat pass out into the broth,
and there in part coagulate and form the scum : this loss may
be prevented in great part by putting the raw meat at once
into boiling water which coagulates the surface albumen be-
fore it dissolves out, and this keeps in the rest, while the
subsequent cooking is continued slowly. In any case the
myosin, being insoluble in water, remains behind in the boiled
meat. In baking or roasting, all the solid parts of the flesh are
preserved and certain agreeably flavored bodies are produced,
as to the nature of which little is known.

Eggs. These contain a large amount of egg albumen
and, in the yolk, another proteid, known as vitellin. Also
fats, and a substance known as lecithin, which is important
as containing a considerable quantity of phosphorus. Leci-

322 THE HUMAN BODY.

thin, or rather a substance yielding it, is an important con-
stituent of the nervous tissues.

Milk contains a proteid, caseinogen; several fats in the
butter; a carbohydrate, milk-sugar; much water; and salts,
especially potassium and calcium phosphates. Butter consists
mainly of the same fats as those in beef and mutton; but has
in it about one per cent of a special fat, butyrin. In the milk
it is disseminated in the form of minute globules which, for
the most part, float up to the top when the milk is let stand
and then form the cream. In this each fat-droplet is sur-
rounded by a pellicle of albuminous matter; by churning,
these pellicles are broken up and the fat-droplets then run to-
gether to form the butter. Caseinogen is insoluble in water;
in milk it is dissolved by the alkaline salts present. When
milk is kept, its sugar ferments and gives rise to lactic acid,
which neutralizes the alkali and precipitates the caseinogen
as curds. In cheese-making the caseinogen is acted upon by
a ferment (rennin) present in the extract of stomach used,
and converted into tyrein which is precipitated : this clotting
does not take place unless a calcium salt be present. Tyrein,
which forms the main bulk of a true cheese, is different from
the curd precipitated from milk by acids; cheese made from
the latter does not " ripen.'' Caseinogen is frequently called
casein, which name should be given to the tyrein formed from
caseinogen by ferment action.

Vegetable Foods. Of these wheat affords the best. In
1000 parts it contains 135 of proteids, 568 of starch, 46 of
dextrin (a carbohydrate), 49 of grape-sugar, 19 of fats, and
32 of cellulose, the remainder being water and salts. The
proteid of wheat is mainly gluten, which when moistened
with water forms a tenacious mass, and this it is to which
wheaten bread owes its superiority. When the dough is
made yeast is added to it, and produces a fermentation by
which, among other things, carbon dioxide gas is produced.
This gas, imprisoned in the tenacious dough, and expanded
during baking, forms cavities in it and causes it to "rise"
and make "light bread," which is not only more pleasant to
eat but more digestible than heavy. Other cereals may con-
tain a larger percentage of starch, but none have so much
gluten as wheat; when bread is made from them the carbon
dioxide gas escapes so readily from the less tenacious dough
that it does not expand the mass properly. Corn contains in

FOODS. 323

1000 parts, 79 of proteids, 637 of starch, and from 50 to 87
of fats; much more than any other kind of grain. Rice is
poor in proteids (5G parts in 1000) but very rich in starch
(823 parts in 1000). Peas and leans are rich in proteids
(from 220 to 260 parts in 1000), and contain about half their
weight of starch. Potatoes are a poor food. They contain a
great deal of water and cellulose, and only about 13 parts of
proteids and 154 of starch in 1000. Other fresh vegetables,
as carrots, turnips, and cabbages, are valuable mainly for the
salts they contain ; their weight is mainly duQ to water, and
they contain but little starch, proteids, or fats. Fruits, like
most fresh vegetables, are mainly valuable for their saline
constituents, the other foodstuffs in them being only present
in small proportion. Some fruit or vegetable is, however, a
necessary article of diet ; as shown by the scurvy which used
to prevail among sailors before fresh or canned vegetables
and lime-juice were supplied to them.

The Cooking of Vegetables. This is of more importance
even than the cooking of flesh, since in most the main ali-
mentary principle is starch, and raw starch is difficult of
digestion. In plants starch is nearly always stored up in the
form of solid granules, which consist of alternating layers of
starch cellulose and starch yranulose. The digestive fluids
turn the starch into sugars which are soluble and can be
absorbed from the alimentary canal, while starch itself can-
not. These fluids act slowly and imperfectly on raw starch,
and then only on the granulose; but when boiled, the starch
granules swell up, and become more readily converted into
sugars, and the starch cellulose is so altered that it too un-
dergoes that change. When starch is roasted it is in part
turned into a substance known as soluble starch which is read-
ily dissolved in the alimentary canal. There is, therefore, a
scientific foundation for the common belief that the crust of
a loaf is more digestible than the crumb, and toast than ordi-
nary bread.

Alcohol. There are perhaps no common articles of diet
concerning which more contradictory statements have been
made than alcoholic drinks. This depends upon their pe-
culiar position: according to circumstances alcohol may be a
poison or be useful; when useful it may be regarded either
as a force-regulator or a force-generator. It is sometimes a
valuable medicine, but it does no good to the healthy body.

324 THE HUMAN BODY.

If not more than two ounces (which would be contained in
about four ounces of whiskey or two quarts of lager-beer) are
taken in the twenty-four hours, they are completely oxidized
in the Body and excreted as water and carbon dioxide. In
this oxidation energy is of course liberated and can be util-
ized. Commonly, however, alcohol is not taken for this pur-
pose but as a force-regulator, for its influence on the nervous
system or digestive organs, and it is in this capacity that it
becomes dangerous. For not only may it be taken in quan-
tities so great that it is not at all oxidized in the Body but is
passed through it as alcohol, or even that it acts as a narcotic
poison instead of a stimulant, but when taken in what is
called moderation there can be no doubt that the constant
" whipping up " of the flagging organs, if continued, must be
dangerous to their integrity. Hence the daily use of alcohol
merely in such quantities as to produce slight exhilaration or
to facilitate work is by no means safe; though in disease
when the system wants rousing to make some special effort,
the physician cannot dispense with it or some other similarly
acting substance. In fact, as a force-generator alcohol may
be advantageously replaced by other foods in nearly all cases;
and there is no evidence that it helps in the construction of
the working tissues, though its excessive use often leads to an
abnormal accumulation of fat. Its proper use is as a " whip,"
and one has no more right to use it to the healthy Body than
the lash to overdrive a willing horse. The physician is the
proper person to determine whether it is wanted under any
given circumstances.

If alcohol is used as a daily article of diet it should be
borne in mind that when concentrated it may chemically alter
the proteids of the cells of the stomach with which it comes
in contact, in the same sort of way, though of course to a
much less degree, as it shrivels and dries up an animal pre-
served in it. Dilute alcoholic drinks, such as claret and beer,
are therefore far less baneful than whiskey or brandy, and
these are, so far as direct action on the stomach is con-
cerned, worse the less they are diluted. For the same reason
alcoholic drinks are far more injurious on an empty stomach
than after a meal. When the stomach is full the liquor
is diluted, is more slowly absorbed, and, moreover, is largely
used up in coagulating the proteids of the food instead
of those of the gastric lining membrane. The old "three

FOODS. 325

bottle " men who drank their port-wine after a heavy dinner,
got off far more safely than the modern tippler who is taking
'•'nips" all day long, although the latter may imbibe a
smaller quantity of alcohol in the twenty-four hours. By far
the best way, however, is to avoid alcohol altogether in health.
If the facts lead us to conclude that under some conditions
it may be to a certain extent a food, it is a dangerous one:
even in what we may call "physiological " quantities, or such
amounts as can be totally oxidized in the Body.

The Advantage of a Mixed Diet. The necessary quan-
tity of daily food depends upon that of the material daily lost
from the Body, and this varies both in kind and amount with
the energy expended and the organs most used. In children
a certain excess beyond this is required to furnish materials
for growth. Although it is impossible to lay down with per-
fect accuracy how much daily food any individual requires,
still the average quantity may be derived from the table of
daily losses given on page 300, which shows that a healthy
man needs daily in assimilable forms about 274 grams (4220
grains) of carbon and 19 grams (292 grains) of nitrogen.
The daily loss of hydrogen, which is very great (352 grams
or 5428 grains), is for the most part made good by water which
has been drunk and, so to speak, merely filtered through the
Body, after having assisted in the solution and transference
through it of other substances. About 300 grams (4620
grains) of water containing 33.3 grams (513 grains) of hy-
drogen are, however, formed in the Body by oxidation, and
the hydrogen for this purpose must be supplied in the form
of some oxidizable foodstuff, whether proteid, fat, or carbo-
hydrate. The oxygen eliminated is mainly received from the
air through the lungs, but some is taken in combination in
the food.

Since proteid foods contain carbon, nitrogen and hydro-
gen, life may be kept up on them alone, with the necessary
salts, water and oxygen; but such a form of feeding would
be anything but economical. Ordinary proteids contain in
100 parts (p. 9) about 52 of carbon and 15 of nitrogen, so a
man fed on them alone would get about 3| parts of carbon
for every 1 of nitrogen. His daily losses are not in this ratio,
but about that of 274 grams (4220 grains) of carbon to 20
grams (308 grains) of nitrogen, or as 13.7 to 1; and so to get
enough carbon from proteids far more than the necessary

326 THE HUMAN BODY.

amount of nitrogen must be taken. Of dry proteids 52?
grams (8116 grains) would yield the necessary carbon, but
would contain 79 grams (1217 grains) of nitrogen; or four
times more than is required to cover the necessary daily
losses of that element. Fed on a purely proteid diet a man
would, therefore, have to digest a vast quantity to get enough
carbon, and in eating and absorbing it, as well as in getting
rid of the extra nitrogen which is useless to him, a great deal
of unnecessary labo,r would be thrown upon the various or-
gans of his Body. Similarly, if a man were to live on bread
alone he would burden his organs with much useless work.
For bread contains but little nitrogen in proportion to its
carbon, and so, to get enough of the former, far more carbo-
naceous substances than could be utilized would have to be
eaten, digested and eliminated daily.

Accordingly, we find that mankind in general employ a
mixed diet when they can get it, using richly proteid sub-
stances to supply the nitrogen needed, but deriving the car-
bon mainly from non-nitrogenous foods of the fatty or carbo-
hydrate groups, and so avoiding excess of either. For instance,
lean beef contains about \ of its weight of dry proteid, which
contains 15 per cent of nitrogen. Consequently the 133
grams (2048 grains) of proteid which would be found in 532
grams (1 Ib. 3 oz.) of lean meat would supply all the nitrogen
needed to compensate for a day's losses. But the proteid
contains 52 per cent of carbon, so the amount of it in the
above weight of fatless meat would be 69 grams (1062 grains)
of carbon, leaving 205 grams (3157 grains) to be got either
from fats or carbohydrates. The necessary amount would be
contained in about 256 grams (3942 grains) of ordinary fats
or 460 grams (7084 grains) of starch; hence either of these,
with the above quantity of lean meat, would form a far better
diet, both for the purse and the system, than the meat alone.

As already pointed out, nearly all common foods contain
several foodstuffs. Good butcher's meat, for example, con-
tains nearly half its dry weight of fat; and bread, besides
proteids, contains starch, fats and sugar. In none of them,
however, are the foodstuffs mixed in the physiologically best
proportions, and the practice of employing several of them at
each meal, or different ones at different meals, during the day,
is thus not only agreeable to the palate but in a high degree
advantageous to the Body. The strict vegetarians who do

FOODS. 327

not employ even such substances as eggs, cheese and milk,
but confine themselves to a purely vegetable diet (such as is
always poor in proteids), daily take far more carbon than they
require, and are to be congratulated on their excellent diges-
tions which are able to stand the strain. Those who use eggs,
cheese, etc., can of course get on very well, since such sub-
stances are extremely rich in proteids, and supply the nitro-
gen needed without the necessity of swallowing the vast bulk
of food which must be eaten in order to get it from plants
directly.

CHAPTER XXII.
THE ALIMENTARY CANAL AND ITS APPENDAGES.

General Arrangement. The alimentary canal is essen-
tially a tube running through the Body (Fig. 2) and lined by
a vascular membrane, most of which is specially adapted for
absorption; it communicates with the exterior at three points
(the nose, the mouth, and the anal aperture), at which the
lining mucous membrane is continuous with the general outer
integument. Supporting the absorbent membrane are layers
which strengthen the tube, and are in part muscular and, by
their contractions, serve to pass materials along it from one
end to the other. In the walls of the canal are numerous
blood and lymphatic vessels which carry off the matters ab-
sorbed from its cavity; and there also exist in connection with
it numerous glands, whose function it is to pour into it various
secretions which exert a solvent influence on such foodstuffs as
would otherwise escape absorption. Some of these glands are
minute and imbedded in the walls of the alimentary tube it-
self, but others (such as the salivary glands) are larger and lie
away from the main channel, into which their products are
carried by ducts of various lengths.

The alimentary tube is not uniform but presents several
dilatations on its course; nor is it straight, since, being much
longer than the Body, a large part of it is packed away by
being coiled up in the abdominal cavity.

Subdivisions of the Alimentary Canal. The mouth-
opening leads into a chamber containing the teeth and
tongue, the mouth-chamber or buccal cavity. This is suc-
ceeded by the pharynx or throat-cavity, which narrows at
the top of the neck into the gullet or oesophagus; this runs
down through the thorax and, passing through the dia-
phragm, dilates in the upper part of the abdominal cavity
into the stomach. Beyond the stomach the channel again
narrows to form a long and greatly coiled tube, the small
intestine, which terminates -by opening into the large intes-

328

THE ALIMENTARY CANAL AND ITS APPENDAGES. 329

tine, much shorter although wider than the small, and ter-
minating by an opening on
the exterior.

The Mouth - cavity (Fig.
105) is bounded in front and
on the sides by the lips and
cheeks, below by the tongue,
k, and above by the palate ;
which latter consists of an an-
terior part, I, supported by
bone and called the hard pal-
ate, and a posterior, /, con-
taining no bone, and called
the soft palate. The two can
readily be distinguished by ap-
plying the tip of the tongue
to the roof of the mouth and
drawing it backwards. The
hard palate forms the parti-
tion between the mouth and
nose. The soft palate arches
down over the back of the
mouth, hanging like a cur-
tain between it and the pharynx, FIG. 105.— The mouth, nose and pha-

i i -IT j.i rynx. with the commencement of the
as Can be Seen by holding the gullet and larynx, as i exposed by

mouth open in front of
looking-glass. From the mid-
dle of its free border a conical
process, the uvula, hangs
down.

The Teeth. Immediately
within the cheeks and lips are two semicircles, formed by the
borders of the upper and lower jaw-bones, which are covered
by the gums, except at intervals along their edges where
they contain sockets in which the teeth are implanted.
During life two sets of teeth are developed; the first or milk
set appears soon after birth and is shed during childhood,
when the second or permanent set appears.

The teeth differ in minor points from one another, but
in each three parts are distinguishable; one, seen in the mouth
and called the crown of the tooth ; a second, imbedded in the
jaw-bone and called the root or fang; and between the two,

a

section, a little to the left of the me-
& dian plane of the head, a, vertebral
column ; 6, gullet : c, windpipe ; d,
larynx ; e, epiglottis ; f. soft palate ;
g, opening of Eustachian tube ; fc,
tongue ; I, hard palate ; m, the sphe-
noid bone on the base of the skull ; n,
the fore part of the cranial cavity;
<>. p, q, the tubinate bones of the out-
er side of the left nostril-chamber.

330

THE HUMAN BODY.

embraced by the edge of the gum, is a narrowed portion, the
neck or cervix. From differences in their forms and uses
the teeth are divided into incisors, canines., bicuspids and
molars, arranged in a definite order in each jaw. Beginning
at the middle line we meet in each half of each jaw with,
successively, two incisors, one canine, and two molars in the
milk set; making twenty altogether in the two jaws. The
teeth of the permanent set are thirty-two in number, eight in
each half of each jaw, viz. — beginning at the middle line-
two incisors, one canine, two bicuspids, and three molars.
The bicuspids, or premolars, of the permanent set replace the
milk molars, while the permanent molars are new teeth added
on as the jaw grows, and not substituting any of the milk-
teeth. The hindmost permanent molars are often called the
wisdom-teeth.

Characters of Individual Teeth. The incisors (Fig. 106)
are adapted for cutting the food. Their crowns are chisel-
shaped and have sharp horizontal cutting edges, which be-
come worn away by use so that they are bevelled off behind
in the upper row, and in the opposite direction in the lower.
Each has a single long fang. The canines (Fig. 107) are
somewhat larger than the incisors. Their crowns are thick
and somewhat conical, having a central point or cusp on the
cutting edge. In dogs, cats and other carnivora the canines
are very large and adapted for seizing and holding prey.
The bicuspids or premolars (Fig. 108) are rather shorter than

FIG. 106.

FIG. 1

FIG. 108.

FIG. 109.

FIG. 106.— An incisor tooth.

FIG. 107.— A canine or eye tooth.

FIG. 108.— A bicuspid tooth seen from its outer side; the inner cusp is, accord-

ingly, not visible.

FIG. 109.— A molar tooth.

i*E.

the canines and their crowns are somewhat cuboidal^Each
has two cusps, an outer towards the cheek, and an inner on
the side turned towards the interior of the mouth. The fang

THE ALIMENTARY CANAL AND ITS APPENDAGES. 331

is compressed laterally, and has usually a groove partially
subdividing it into two. At its tip the separation is often
complete. The molar teeth or grinders (Fig. 109) have large
crowns with broad surfaces, on which are four or five project-
ing tubercles, which roughen them and make them better
adapted to crush the food. Each has usually several fangs.
The mil fc -teeth only differ in subsidiary points from those of
the same names in the permanent set.

The Structure of a Tooth. If a tooth be broken open, a
cavity extending through both crown and fang will be found
in it. This is filled during life with a soft vascular pulp, and
hence is known as the "pulp-cavity" (<", Fig. 110). The hard
parts of the tooth disposed around the pulp-cavity consist of
three different tissues. Of these one immediately surrounds
the cavity and makes up most of the bulk of the tooth; it is
dentine (2, Fig. 110); covering the dentine on the crown is
the enamel (1, Fig. 110) and, on the fang, the cement
(3, Fig. 110).

The pulp-cavity opens below by a narrow aperture at the
tip of the fang, or at the tip of each if the tooth have more
than one. The pulp consists mainly of connective tissue, but
its surface next the dentine is covered by a layer of columnar
cells. Through the opening on the fang blood-vessels and
nerves enter the pulp.

The dentine (ivory) yields on analysis the same materials
as bone but is somewhat harder, earthy matters constituting
72 per cent of it as against CG per cent in bone. Under the
microscope it is recognized by the fine dentinal tiibules
which, radiating from the pulp-cavity, perforate it through-
out, finally ending in minute branches which open into
irregular cavities, the interglobular spaces, which lie just
beneath the enamel or cement. At their widest ends, close
to the pulp-cavity, the dentinal tubules are only about 0.005
millimeter (T-Vo °^ an inch) in diameter. The cement is
much like bone in structure and composition, possessing
lacunae and canaliculi, but rarely any Haversian canals. It is
thickest at the tip of the fang and thins away towards the
cervix. Enamel is the hardest tissue in the Body, yielding
on analysis only from two per cent to three per cent of
organic matter, the rest being mainly calcium phosphate and
carbonate. Its histological elements are minute hexagonal
prisms, closely packed, and set on vertically to the surface of

332

THE HUMAN BODY.

the subjacent dentine. It is thickest over the free end of the
crown, until worn away by use. Covering the enamel in

FIG. 110. — Section through a premolar tooth of the cat still imbedded in its
socket. 1, enamel; 2, dentine; 3, cement ; 4, the gum ; 5, the bone of the lower
jaw ; c, the pulp-cavity.

unworn teeth is a thin structureless horny layer, the enamel
cuticle.

The Tongue (Fig. Ill) is a muscular organ covered by
mucous membrane, extremely mobile, and endowed not
only with a delicate tactile sensibility but with the terminal
organs of the special sense .of taste; it is attached by its root
to the hyoid bone. On its upper surface are numerous small

THE ALIMENTARY CANAL AND ITS APPENDAGES. 333

eminences or papilla, such as are found more highly devel-
oped on the tongue of a cat, where they may he readily felt.
On the human tongue there are three forms of papillae, the
circumvallate, i\\Q fungi form, and the filiform. The circum-
vallate papillae, 1 and 2, the largest and least numerous, are
from seven to twelve in number and lie near the root of the
tongue arranged in the form of a V with its open angle turned

Fm. 111.— The upper surface of the tongue with part of the pillars of the fauces
and the tonsils. 1, 2, circumvallate papillae ; 3, fungiform papillae ; 4, filiform
papillae ; 6, mucous glands ; 7, tonfeils ; 8, tip of epiglottis.

forwards. Each is an elevation of the mucous membrane,
covered by epithelium, and surrounded by a trench. On the
sides of these papillae, imbedded in the epithelium, are many
small oval bodies richly supplied with nerves and sup-

334 THE HUMAN BODY.

posed to be concerned in the sense of taste, and hence called
the taste-buds (Chap. XXXV). The fungiform papillcB, 3,
are rounded elevations attached by somewhat narrowed stalks,
and found all over the middle and fore part of the upper
surface of the tongue. They are easily recognized on the
living tongue by their bright red color. ^\^Q filiform papillae,
most numerous and smallest, are scattered all over the dorsum
of the tongue except near its base. Each is a conical emi-
nence covered by a thick horny layer of epithelium. It is
these papillae which are so highly developed on the tongues
of Carnivora, and serve them to scrape bones clean of even
such tough structures as ligaments.

In health the surface of the tongue is moist, covered by
little " fur," and in childhood of a red color. In adult life
the natural color of the tongue is less red, except around the
edges and tip; a bright-red glistening tongue being then,
usually a symptom of disease. When the digestive organs
are deranged the tongue is commonly covered with a thick
yellowish coat, composed of a little mucus, some cells of
epithelium shed from the surface, and numerous microscopic
organisms known as bacteria; and there is frequently a " bad
taste in the mouth." The whole alimentary mucous mem-
brane is in close physiological relationship; and any thing'
disordering the stomach is likely to produce a "furred
tongue."

The Salivary Glands. The saliva, which is poured into
the mouth and which, mixed with the secretion 'of minute
glands imbedded in its lining membrane, moistens it, • is
secreted by three pairs of glands, the parotid, the submaxil-
lary and the suUingual. The parotid glands lie in front of
the ear behind the rarnus of the lower jaw; each sends its
secretion into the mouth by a tube known as 8 tenon's duct,
which crosses the cheek and opens opposite the second upper
molar tooth. In the disease known as mumps * the parotid
glands are inflamed and enlarged. The subm axillary glands
lie between the halves_of the lower |aw-bope, near its angles,
and their ducts open beneath the tongue near the middle line.
The sublingual glands lie beneath the floor of the month,
covered by its mucous membrane, between the back part of
the tongue and the lower jaw-bone. Each has many ducts

* Parotitis, in technical language.

THE ALIMENTARY CANAL AND ITS APPENDAGES. 335

(8 to 20), some of which join the submaxillary duct, while
the rest open separately in the floor of the mouth.

The Fauces is the name given to the aperture which can
be seen at the back of the mouth below the soft palate (Fig.
105), and leading into the pharynx. It is bounded above by
the soft palate and uvula, below by the root of the tongue,
and on the sides by muscular elevations covered by mucous
membrane, which reach from the soft palate to the tongue.
These elevations are the pillars of the fauces. Each bifur-
cates below, and in the hollow between its divisions lies a
tonsil (7, Fig. Ill), a soft rounded body about the size of an
almond, and containing numerous minute glands which form
mucus.

The tonsils not unfrequently become enlarged during a
cold or sore throat, and then pressing on the Eustachiau tube
(Chap. XXXIV), which leads from the pharynx to the mid-
dle ear, keep it closed and produce partial deafness.

The Pharynx or Throat-cavity (Fig. 105). This por-
tion of the alimentary canal may be described as a conical
bag with its broad end turned upwards towards the base of
the skull, and its narrow end downwards and passing into the
gullet. Its front is imperfect, presenting openings which
lead into the nose, the mouth, and (through the larynx
and windpipe) the lungs. Except during swallowing or
speech the soft palate hangs down between the mouth and
pharynx; during deglutition it is raised into a horizontal
position and separates an upper or respiratory portion of the
pharynx from the rest. Through this upper part, therefore,
air alone passes, entering it from the posterior ends of the
two nostril- chambers; while through the lower portion both
food and air pass, one on its way to the gullet, b, Fig. 105,
the other through the larynx, d, to the windpipe, c\ when
a morsel of food " goes the wrong way " it takes the latter
course. Opening into the upper portion of the pharynx on
each side is an Eustachian tube, g: so that the apertures
leading out of it are seven in number; the two posterior
nares, the two Eustachian tubes, the fauces, the opening of
the larynx, and that of the gullet. At the root of the tongue,
over the opening of the larynx, is a plate of cartilage, the
epiglottis, e, which can be seen if the mouth is widely opened
and the back of the tongue pressed down by some such thing
as the handle of a spoon. During swallowing the epiglottis

336 THE HUMAN BODY.

is pressed down like a lid over the air-tube and helps to keep
food or saliva from entering it. In structure the pharynx
consists essentially of a bag of connective tissue lined by
mucous membrane, and having muscles in its walls which
drive the food on.

The (Esophagus or Gullet is a tube commencing at the
lower termination of the pharynx and which, passing on
through the neck and chest, ends below the diaphragm by
joining the stomach. In the neck it lies close behind the
windpipe. It consists of three coats — a mucous membrane
within; next, a submucous coat of areolar connective tissue;
and, outside, a muscular coat made up of two layers, an inner
with transversely and an outer with longitudinally arranged
fibres. In and beneath its mucous membrane are numerous
small mucous glands whose ducts open into the tube. v

The Stomach (Fig. 112) is a somewhat conical bag placed
transversely in the upper part of the abdominal cavity. Its

larger end is turned to the
left and lies close beneath
the diaphragm; opening
into its upper border,
through the cardiac orifice
at a, is the gullet d. The
narrower right end is con-
tinuous at c with the small
intestine; the aperture be-
tween the two is the pyloric

OTlfice. The pyloric end of

the stomach lies lower in the
abdomen than the cardiac,

the pylorus and d, the lesser curvature. an(j jg separated from the

diaphragm by the liver (see Fig. 1). The concave border be-
tween the two orifices is known as the small curvature, and
the convex as the great curvature, of the stomach. From
the latter hangs down a fold of peritoneum (ne, Fig. 1)
as the great omentum. It is spread over the rest of
the abdominal contents like an apron. After middle life
much fat frequently accumulates in the omentum, so that it
is largely responsible for the " fair round belly with good
capon lin'd." The protrusion b to the left side of the cardiac
orifice, Fig. 112, is the fund us or great cul de sac. The size
of the stomach varies greatly "with the amount of food in it;

THE ALIMENTARY CANAL AND ITS APPENDAGES. 337

just after a moderate meal it is about ten inches long, by five
wide at its broadest part.

Structure of the Stomach. This organ has four coats,
known successively from without in as the serous, the mus-
cular, the submucous, and the mucous. The swous coat is
formed by a reflection of the peritoneum, a double fold of
which slings the stomach : after separating to envelop it the
two layers again unite and, hanging down beyond it, form the
great omentum. The muscular coat t (Fig. 59) consists of
unstriped muscular tissue arranged in three layers: an outer,
longitudinal, most developed about the curvatures; a circu-
lar, evenly spread over the whole organ, except around the
pyloric orifice where it forms a thick ring; and an inner,
oblique and very incomplete, radiating from the cardiac
orifice. The submucous coat is made up of lax areolar tissue
and binds loosFlTTlie mucous coat to the muscular. The
mucous coat is a moist pink membrane which is inelastic, and
rar^e-etttrtrfli to line the stomach evenly when it is fully dis-
tended. Accordingly, when the organ is empty and shrunken,
this coat is thrown into folds, which disappear when the organ
is distended. During digestion the arteries supplying the
stomach become dilated and, its capillaries being gorged, its
mucous membrane is then much redder than during hunger.

The blood-vessels of the stomach run to it between the
folds of peritoneum which sling it. After giving off a few
branches to the outer layers, most of the arteries break up
into small branches in the submucous coat, from which twigs
proceed to supply the close capillary network of the mucous
membrane.

The nerves of the stomach are chiefly derived from the
Tmeumoffastrica. In the lower part of the thorax these nerves
consist mainly of nonmedullated. fibres, and lie on the sides_
of the gullet, across which they interchangej^bres by means
of several branches. On entering the'abdomen the left pneu-
mogastric passes to the ventral side of the..^JQinacfa., in which
ft ends: the right supplies the dorsal side of the stomach, but
a considerable portion of it passes on to enter the solarjrtexus,
which lies behind the stomach and ^contains several large
ganglia. The sympathetic also supplies gastric nerves which
mainly go to the blood-vessels. In the muscular coat of the
stomach are many nerve-cells.

Histology of the Gastric Mucous Membrane. Examina-

338

THE HUMAN BODY.

lion of the inner surface of the stomach with a hand lens
shows it to be covered with minute shallow pits. Into these
open the mouths of minute tubes, the gastric glands, wliich
are closely packed side by side in the mucous membrane;
something like the cells of a honeycomb, except that each is
open at one end. Between them lie a small amount of con-
nective tissue, a close network *f lymph-channels, and capil-
lary blood-vessels. The connective tissue is of a peculiar
variety closely packed with lymph-cells and will be more mi-
nutely described later (Chap. XXIII). The whole surface of
the mucous membrane is lined by a single layer of columnar
mucus-making epithelium cells (Fig. 113). These dip down
and line the necks of the tubular
glands. The deeper portions of the
glands are lined by a layer of
shorter and somewhat cuboidal cells,
the central or chief cells. In speci-
mens taken from a healthy animal
killed during digestion these cells are
large and do not stain deeply with
carmine. Similar specimens taken
from an animal an hour or two

FIG. 113. — A thin section

through the gastric mucous alter a good meal has been swallowed

membrane, perpendicular to its , .? 7 . ,. ,, , , ,

surface, magnified about 25 di- SllOW tllC CniCJ Cells Shrunken and
anieters. a, a simple gastric , • • -i i rni ,1

gland ; 6, a compound gastric staining more deeply. They, thus,

store up during rest a material which
they get rid »f when the gastric juice
is being secreted. This material 5s,
in part, pepsinogen, which during activity of the gland is
changed, giving rise among other things to pepsin, the chief
enzyme of gastric juice. The deeply staining protoplasmic
portion of the cell which is left behind, forms and stores more
pepsinogen during the next period during which the stomach
is not digesting. In the pyloric end of the stomach only the
chief jlells line the glands, but elsewhere there is found out-
side them, in most of the glands, an incomplete layer of larger
tvtl cells (d, Fig. 113). These are sometimes called the
txinitic cells, from the belief that they are especially con-
cerned in secreting the acid of the gastric juice. The glands
frequently branch at their deeper ends.

The Pylorus. If the stomach be opened it is seen that
the mucous membrane projects in a fold around the pyloric

vc

; °' retif°rm

THE ALIMENTARY CANAL AND ITS APPENDAGES, 339

orifice and narrows it. This is due to a thick ring of the
circular muscular layer there developed, and forming around
the orifice a sphincter muscle, which, by its contraction, keeps
the passage to the small intestine closed except when portions
of food are to be passed on from the stomach to succeeding
divisions of the alimentary canal.

Since the cardiac end of the stomach lies immediately be-
neath the diaphragm, which has the heart on its upper side,
its over-distension, due to indigestion or flatulence, may im-
pede the action of the thoracic organs, and cause feelings of
oppression in the chest, or palpitation of the heart.

The Small Intestine (Fig. 1*20), commencing at the py-
lorus, ends, after many windings, in the large. It is about six
meters (twenty feet) long, and about five centimeters (two
inches) wide at its gastric end, narrowing to about two thirds
of that width at its lower portion. Externally there are no
lines of subdivision on the small intestine, but anatomists
arbitrarily describe it as consisting of three parts; the first
twelve inches being the duodenum, D, the succeeding two
fifths of the remainder the jejunum, J, and the rest the
ileum, I.

Like the stomach, the small intestine possesses four coats 3
a serous, a muscular, a submucous, and a mucous. The!
serous coat is formed by a dup'licature of the peritoneum, but
presents nothing answering to the great omentum; this
double fold, slinging the intestine as the small omenturn
slings the stomach, is named the mesentery. The muscular
coat is composed of plain muscular tissue arranged in two
strata, an outer longitudinal, and an inner transverse or cir-
cular. The submucous coat is like that of the stomach; con-
sisting of loose areolar tissue, binding* together the mucous
and muscular coats, and forming a bed in which the blood
and lymphatic vessels (which reach the intestine in the fold
of the mesentery) break up into minute branches before en-
tering the mucous membrane.

The Mucous Coat oFthe Small Intestine. This is pink,
soft and extremely vascular. It does not present temporary
or effaceable folds like those of the stomach, but is, through-
out a great portion of its length, raised up into permanent
transverse folds in the form of crescentic ridges, each of
which runs transversely for a greater or less way round the
tube (Fig. 114). These folds are the valvulce connivences.

340 THE HUMAN BODY.

They are first found about two inches from the pylorus, and
are most thickly set and largest in the upper half of the
jejunum, in the lower half of which they become gradually
less conspicuous; and they finally disappear altogether about
the middle of the ileum. The folds serve greatly to increase
the surface of the mucous membrane both for absorption and
secretion, and they also delay the food somewhat in its pas-
sage, since it must collect in the hollows between them, and
so be longer exposed to the action of the digestive liquids.
Examined closely with the eye or, better, with aid of a lens,
the mucous membrane of the small intestine is seen to be not
smooth but shaggy, being covered everywhere (both over the
valvulae conniventes and between them) with closely packed
minute processes, standing up somewhat like the4' pile "on
velvet, and known as the villi. Each villas is from 0.5 to 0.7
millimeter (-fa to fa inch) in length; some are conical and
rounded, but the majority are compressed at the base in one
diameter (Fig. 115). In structure a villus is somewhat com-
plex. Covering it is a single layer of columnar epithelial cells,
the exposed ends of the majority having a peculiar bright
striated border and being probably of great importance in ab-
sorption. Mixed with these cells are others in which most of
the cell has become filled with a clear mass which does not
stain readily with reagents; the deep narrow end of the cell
stains easily and contains the nucleus. From time to time the
clear substance (mucigen) is converted into mucus and dis-
charged into the intestine, leaving behind only the nucleus and
the protoplasm around it. These reconstruct the cell and form
more mucigen. These mucus-forming cells are named goblet*
cells, from their shape. Beneath the epithelium the villus may

FIG. 114.— A portion of the small intestine opened to show the valvulae, conniventes.

be regarded as made up of a framework of connective tissue,
mainly of the adenoid variety (Chap. XXIII), supporting the

THE ALIMENTARY CANAL AND ITS APPENDAGES. 341

more essential constituents. Near the surface is an incomplete
layer of plain muscular tissue, continuous below with a mus-
cular stratum forming the deepest layer of the mucous mem-
brane and named the muscularis mucosce. In the centre is an
offshoot of the lymphatic system; sometimes in the form of a
single vessel with a closed dilated end, and sometimes as a net-
work formed by two main vessels with cross-branches. During
digestion these lymphatics are filled with a milky-white liquid
absorbed from the intestines, and they are accordingly called
the lacteals. They communicate with larger branches in the
submucous coat, which end in trunks that pass out through
the mesentery to join the main lymphatic system. Finally,
in each villus, outside the lacteals and beneath the muscular
layer of the villus, is a close network of blood-vessels.

Opening on the surface of the small intestine, between
the bases of the villi, are small glands, the crypts of Lieber-
kiilin. Each is a simple unbranched tube lined by a layer of
columnar cells some of which have a striated free border,
though less marked than that on the corresponding cells of

FIG. 115.— Villi of the small intestine: magnified about 80 diameters. In the
right-hand figure the lacteals, a, 6, c, are filled with white injection ; d, blood-ves-
sels. In the left-hand figure the lacteals alone are represented, filled with a dark
injection. The epithelium covering the villi, and their muscular fibres, are omitted.

the villi, and others are goblet-cells. The crypts of Lieber-
kdhn are closely packed, side by side, like the glands of the
stomach. In the duodenum are found other minute glands,
the glands of B runner. They lie in the submucous coat

342 THE HUMAN BODY.

•

and send their ducts through the mucous membrane to open
on its inner side.

The Large Intestine (Fig. 120), forming the final por-
tion of the alimentary canal, is about 1.5 meters (5 feet)
long, and varies in diameter from about G to 4 centimeters
(2^ to 1£ inches). Anatomists describe it as consisting of
the ccecum with the vermiform appendix., the colon, and the
rectum. The small intestine does not open into the com-
mencement of the large but into its side, some distance from
its closed upper end, and the ca3cum, CO, is that part of the
large intestine which extends beyond the communication.
From it projects the vermiform appendix, a narrow tube
not thicker than a cedar pencil, and about 10 centimeters
(4 inches) long. The colon commences on the right side of
the abdominal cavity where the small intestine communicates
with the large, runs up for some way on that side (ascending
colon, AC), then crosses the middle line (transverse colon,
TO) below the stomach, and turns down (descending colon,
DC) on the left side and there makes an S-shaped bend
known as the sigmoid flexure, SF; from this the rectum, R,
the terminal straight portion of the intestine, proceeds to
the anal opening, by which the alimentary canal communi-
cates with the exterior. In structure the large intestine
presents the same coats as the small. The external stratum
of the muscular coat is not, however, developed uniformly
around it, except on the rectum, but occurs in three bands
separated by intervals in which it is wanting. These bands
being shorter than the rest of the tube cause it to be puck-
ered, or sacculated, between them. The mucous coat pos-
sesses no villi or valvula3 conniventes, but is usually thrown
into effaceable folds, like those of the stomach but smaller.
It contains numerous closely set glands much like the crypts
of Lieberkiihn of the small intestine.

The Ileo-colic Valve. Where the small intestine joins
the large there is a valve, formed by two flaps of the mucous
membrane sloping down into the colon, and so disposed as to
allow matters to pass readily from the ileum into the large
intestine but not the other way.

The Nerves of the Intestines. These, like those of the
heart with which we shall later have to compare them
physiologically, are intrinsic and extrinsic. The former are
connected with small ganglia found abundantly on the

THE ALIMENTARY CANAL AND ITS APPENDAGES. 343

plexus of Auerbach which lies between the two muscular
coats, and the plexus of Meissner found in the submucous
coat. The extrinsic fibres proceed immediately from the
gangliated solar plexus already referred to and from a similar
mesenter ic plexus which lies lower in the abdomen; except a
few branches to the longitudinal muscular coat of the rectum
which pass directly from some of the sacral spinal nerves.
Some of the fibres distributed from the solar plexus are
those running from the brain in the right pneumogastric,
and probably also from the left, having crossed over to the
left in branches joining the two. Others reach the solar
plexus by means of the splanchnics and other nerves pro-
ceed ing from the thoracic parts of the two sympathetic chains.
These are partly vaso-constrictor fibres (Chap. XVIII.), but
in part go to the muscular coats of the intestine. They may
be traced back through the communicating branches from
sympathetic ganglia to the corresponding spinal nerves and
thence by the ventral nerve-roots into the spinal cord.
The fibres passing to the intestines from the mesenteric
plexus reach that plexus from the posterior thoracic and
anterior lumbar sympathetic ganglia, and can also be tracked
by experiment to the spinal cord.

The Liver. Besides the secretions formed by the glands
imbedded in its walls, the small intestine receives those of
two large glands, the liver and the pancreas, which lie in the
abdominal cavity. The ducts of both open by a common
aperture into the duodenum about 10 centimeters (4 inches)
from the pylorus.

The liver is the largest gland in the Body, weighing from
1400 to 1700 grams (50 to 64 ounces). It is situated in the
upper part of the abdominal cavity (le, U'9 Fig. 1), rather
more on the right than on the left side and immediately
below the diaphragm, into the concavity of which its upper
surface fits, and reaches across the middle line above the
pyloru; end of the stomach. It is of dark reddish-brown
color, and of a soft friable texture. A deep fissure incom-
pletely divides the organ into right and left lobes, of which
the risrht is much the larger; on its under surface (Fig. 116)
shallower grooves mark off several minor lobes. Its upper
surface is smooth and convex. The vessels carrying blood
to the liver are the portal vein, Vp, and the hepatic artery;
both enter it at a fissure (the portal fissure) on its under side,

344

THE HUMAN BODY.

and there also a duct passes out from each half of the organ.
The ducts unite to form the hepatic duct, Dli, which meets
at an acute angle, the cystic duct, DC, proceeding from the
gall-bladder, Vf, a pear-shaped sac in which the bile, or gall,
formed by the liver, accumulates when food is not being
digested in the intestine. The common bile-duct, Dck,

Lv

FIG. 116.— The under surface of the liver, d, rigrht, and s, left lobe; Vh. hepatic
vein; Vp< porta.l vein; Vc. vena cava "inferior; Dc/i, 'common bile-duct; DC, cystic
duct; Dh, hepatic duct; Vf, gall-bladder.

formed by the union of the hepatic and cystic ducts, opens
into the duodenum. The blood which enters the liver by
the portal vein and hepatic artery passes out by the hepatic
veins, Vh, which leave the posterior border of the organ close
to the vertebral column, and there open into the inferior vena
cava just before it passes up through the diaphragm.

The Structure of the Liver. On closely examining the
surface of the liver, it will be seen to be marked out into
small angular areas from one to two millimeters (^ to TV
inch) in diameter. These are the outer sides of the super-
ficial layer of a vast number of minute polygonal masses, or
lobules, of which the liver is built up; similar areas are seen
on the surface of any section made through the organ.
Each lobule (Fig. 117) consists of a number of hepatic
cells supported by a close, network of capillaries; and is
separated from neighboring lobules by connective tissue,

THE ALIMENTARY CANAL AND ITS APPENDAGES. 345

larger blood-vessels, and branches of the hepatic duct. The
hepatic cells are the proper tissue elements of the liver, all
the rest being subsidiary arrangements for their nutrition
and protection. Each is polygonal, nucleated and very
granular, and has a diameter of about .025 millimeter (ToVo^
of an inch). In each lobule they are arranged in rows or
strings, which form a network, in the meshes of which the
blood-capillaries run. Covering the surface of the liver
is a layer of the peritoneum, beneath which is a dense

FIG. 117.— A lobule of the liver, magnified, showing the hepatic cells radiately
arranged around the central imralobulur vein, and the lobular capillaries inter-
laced with them.

connective-tissue layer, forming the capsule of Glisson. At
the portal fissure offsets from this capsule run in, and line
canals, the portal canals, which are tunnelled through the
organ. These, becoming smaller and smaller as they branch,
finally become indistinguishable close to the ultimate
lobules. From their walls and from the external capsule,
connective-tissue partitions radiate in all directions through
the liver and, support its other parts. In each portal canal
lie three vessels — a branch of the portal vein, a branch of
the hepatic artery, and a branch of the hepatic duct; the
division of the portal vein being much the largest of the
three. These vessels break up as the portal canals do, and
all end in minute branches around the lobules. The blood
carried in by the portal vein (which has already circulated
through the capillaries of the stomach, spleen, intestines and

346 THE HUMAN BODY.

pancreas) is thus conveyed to a fine vascular interlobular
plexus around the Hver-lobnies, from which it flows on
through the capillaries (lobular plexus] of the lobules them-
selves. These (Fig. 117) unite in the centre of the lobule to
form a small intralobular vein, which carries the blood out
and pours it into one of the branches of origin of the hepatic
vein, called the sublobular vein. Each of the latter has
many lobules emptying blood into it, and if dissected out
with them (Fig. 118) would look something like a branch of
a tree with apples attached to it by short stalks, represented

FIG. 118.— A small portion of the liver, injected, and magnified about twenty
diameters. The blood-vessels are represented white; the large vessel is a sub-
lobular vein, receiving the intralobular veins, which in turn are derived from the
capillaries of the lobules.

by the intralobular veins. The blood is finally carried, as
already pointed out, by the hepatic veins into the inferior vena
cava. The hepatic artery, a direct offshoot of the coeliac
axis, ends mainly in Glisson's capsule and the walls of the
blood-vessels and bile-ducts, but some of its blood reaches
the lobular plexuses; it all finally leaves the liver by the
hepatic veins.

The bile-ducts can be readily traced to, the periphery
of the lobules, and there communicate with a network of
extremely minute commencing bile ducts, ramifying in the
lobule between the hepatic cells composing it.

The Pancreas or Sweetbread. This is an elongated
soft organ of a pinkish yellow color, lying along the great
curvature of the stomach. Its right end is the larger,
and is embraced by the duodenum (Fig. 119), which there

THE ALIMENTARY CANAL AND ITS APPENDAGES. 347

makes a curve to the left. A duct traverses the gland and
joins the common bile-duct close to its intestinal opening.
The pancreas produces a watery-looking secretion which is of
great importance in digestion; the gland also (Chap. XXIII)
exerts an important influence on the general nutritional
processes of the Body. It is of the compound racemose type.

FIG. 119 — The stomacn. pancreas, liver, and duodenum, with part of the rest
of the small intestine and the mesenterv; the stomach and liver have been
turned up so as to expose the pancreas. V. stomach : D, D'. "D". duodenum ; L,
spleen ; P. pancreas: R. rigrht kidney : T. jejunum"; Vf, 'gfell-Bladderr'/i, hepatic
duct: c. cystic duct ; ch. common bile-duct: 1. aorta; 2. an artery (left coronary)
of the stomach; 3. hepatic artery: 4. splenic artery; 5. superior rnesenteric artery;
6, superior mesenteric vein; 7, splenic vein; Fp, portal vein.

The Blood-vessels of Alimentary Canal, Liver, Spleen
and Pancreas The portal vein ( Vp. Fig. 119) has already

348

THE HUMAN BODY.

been referred to as differing from all other veins in that it
not only receives blood from a system of capillaries but ends
in a second set of capillaries, which lie in the liver. The
quantity of blood brought to supply the hepatic capillaries
by the hepatic artery is in fact much less than that brought
by the portal vein. The stomach, the intestines, the pancreas

and the spleen are supplied
with arterial blood from
three great branches of the
aorta. The most anterior
of these, the cceliac axis,
springs from the aorta close
beneath the diaphragm and
divides into the hepatic
artery, splenic artery, and
arteries for the stomach;
some of these divisions may
be seen in Fig. 119. The
pancreas is supplied partly
from the hepatic, partly
from the splenic artery.
The two other branches
(superior and inferior mes-
enteric artery) are given oif
from the aorta lower down
in the abdominal cavity;
the former (5, Fig. 119)
supplies the small intestine
and half of the large, the
FIG. 120.— Diagram of abdominal part of latter the remainder of the

alimentary canal. C, the cardiac, and P, ,„, -, , -,

the pyloric end of the stomach; A the large. Ihe blood paSSing

duodenum; J, I. t,he convolutions of the , -, T, ,1 o*.f/-»-mnei

small intestine: cc, the caecum with the through all these arteries

vermiform appendix; AC. ascending, TO, Vi^orimoa vcmrm« in thp
transverse, and DC, descending colon; R, 06001868 VCllOUS 6

the rectum. capillaries of the organs

they supply, and is gathered into corresponding veins (Fig.
119) which unite near the liver to form the portal vein.
The further course of the blood carried to the liver (partly
.arterial from the hepatic artery, partly venous from the portal
system) has been described already (p. 345),

CHAPTER XXIII.
THE LYMPHATIC SYSTEM AND THE DUCTLESS GLANDS.

The Lymphatics or Absorbents are very widely distrib-
uted in the Body. Most organs, as has been pointed
out (p. 63), possess a sort of internal skeleton made up
of connective tissue, which consists mainly of bundles of
fibres, united together and covered-in by a " cement" sub-
stance. In this substance are found numerous cavities, usu-
ally branched, and communicating with one another by their
branches. They frequently contain connective-tissue cor-
puscles, which, however, do not completely fill them; and
they thus, with their branches, form a set of intercommuni-
cating channels known as the " lymph-canaliculi" because
they are filled with lymph. As the connective tissues accom-
pany blood-vessels wherever the latter run, the canaliculi,
which are frequently relatively large around the blood-capil-
laries, take up the liquid which transudes through their walls,
and this transudation liquid is the origin of the lymph.
Even where blood-vessels and connective tissue do not pene-
trate, as in bone between the Haversian canals, lymph-canal-
iculi penetrate, being connected with the cavities in which
the bone-corpuscles lie; and in the deeper layers of the
epidermis the cells are covered with prickle-like projections
and unite by the tips of these so as to leave minute channels
which apparently are lymph-canaliculi. These very minute
channels, with no definite lining cells, but mere crevices be-
tween tissue elements, or tubes hollowed out in the matrix of
connective tissue, bone and (possibly) cartilage, constitute
the origin of the lymphatic system. The transudation liquid
which enters them from the blood-vessels is rapidly altered
by interchange with the neighboring tissues, losing certain
materials and gathering others; and as the substances taken
and the waste and other products returned vary very much
in different organs, the lymph leaving them must differ also.
Nevertheless it retains certain common features, histological

349

350 THE HUMAN BODY.

and chemical (pp. 49, 62), which justify us in speaking of it
in general as the lymph. The lymphatic vessels collect this
lymph or at least such part of it as does not pass back locally
by diffusion into the blood, and pour it into the veins.

The Structure of Lymph-vessels. The smallest lymph-
vessels proper are the lymph -capillaries; tubes rather wider
than the blood-capillaries, but like them having a wall con-
sisting of a single layer of flattened epithelium cells. The
cells have, however, a wavy margin and are not as a rule much
longer in one diameter than another, in both of which respects
they differ from the cells of the corresponding blood-vessels.
In some regions, as in many glands, the lymph-capillaries are
much dilated and form irregular lymph lacunce, everywhere
bounded by their peculiar wavy cells, lying in the interstices
of organs; and sometimes they form tubes around small blood-
vessels, as in the brain (perivascular lympJt-channel). In
some places they commence by blind ends as in the lacteal
vessels of the villi of the small intestine (Fig. 115) which are
lymph-capillaries; but usually they branch and join to form
networks. Lymph from the canaliculi enters them (whether
by passing through their boundary cells or through clefts left
between these is not certain) and is passed on to larger vessels
which much resemble veins of corresponding size, having the
same three coats, and being abundantly provided with valves.

The Thoracic Duct. The lymph-vessels proceeding from
the capillaries in various organs become larger and fewer by
joining together, and all end finally in two main trunks which
open into the venous system on the sides of the neck, at the
point of junction of the jugular and subclavian veins. The
trunk on the right side is much smaller than the other and
is known as the " right lymphatic duel." It collects lymph
from the right side of the thorax, from the right side of the head
and neck, and the right arm. The lymph from all the rest
of the Body is collected into the thoracic duct. It com-
mences at the upper part of the abdominal cavity in a dilated
reservoir (the receptaculum cht/li), into which the lacteals
from the intestines, and the lymphatics of the rest of the
lower part of the Body, open. From thence the thoracic
duct, receiving tributaries on its course, runs up the thorax
alongside of the aorta and, passing on into the neck, ends on the
left side at the point already indicated; receiving on its way
the main stems from the left arm and the left side of the

LYMPHATIC SYSTEM AND DUCT/LESS GLANDS. 351

head and neck. The thoracic duct, thus, brings back to the
blood much more lymph than the right lymphatic duct.

The Serous Cavities. These are great dependencies of
the lymphatic system and may be regarded as large lacunae.
Each of them (peritoneal, plenral, arachnoidal and pericar-
diac) is lined by a definite epithelioid layer of close-fitting
polygonal cells. At certain points, however, openings or
stomata occur, surrounded by a ring of smaller cells, and
leading into tubes which open into subjacent lymphatic
vessels. The liquid moistening these cavities is, then, really
lymph: in some dropsical diseases it collects in great excess
in them.

Lymphoid or Adenoid Tissue is the name given to cer-
tain aggregations of slightly differentiated cells (leucocytes)
supported by a peculiar form of tissue and found in con-
nection with the lymphatic system in many parts of the body.
The cells much resemble white blood--corpuscles, though their
nuclei usually have a more distinct network, and they are
capable of executing amoeboid movements. Many of them
ultimately are carried by the lymph into the blood to be-
come pale corpuscles, and from the blood some again pass
back into the lymph by migrating through the walls of
the blood-capillaries. By amoeboid movement these lymph-
corpuscles can take up foreign particles into themselves
and creep with the absorbed material along lymph-canaliculi
and lymph-capillaries. Lymphoid tissue is extensively devel-
oped in the mucous membrane of a great part of the ali-
mentary canal.

The deepest layer of the mucous membrane of stomach
and intestines, lying next to the submucous coat is the mus-
cular is mucosce, a thin layer of unstriped muscular tissue quite
distinct from the proper muscular coats of those viscera. Above
it and forming the main bulk of the mucous membrane lying
between the glands (o, Fig. 112) and, in the small intestine,
the main mass of the villi, is a delicate connective tissue con-
sisting of very fine fibres which originated by the branch-
ing of cells; in many places the nuclei of these cells have quite
disappeared, and the original central part of the cell is only
recognizable as the place from which the branches spread : such
tissue is reticular connective tissue. Its meshes contain many
leucocytes, and the mixture of reticular tissue with these cells
constitutes adenoid or lymphoid tissue. At numerous spots,

352 THE HUMAN BODY.

especially in the small intestine, the cells are peculiarly abun-
dant, forming local aggregations of about the size of the head
of a small pin: these are named dosed or solitary follicles.
A minute artery enters each and gives rise to a capillary net-
work in it, from which the blood is carried off by a small vein.
The follicle lies in, or rather projects into, a lymph-lacunae
which closely invests it, and is in direct communication with
other lymphatic vessels of the neighborhood. The central
leucocytes of the follicle are smaller than the outer, and their
nuclei are often found in various stages of karyokinesis.
Each follicle must therefore be regarded as a seat of forma-
tion of new leucocytes, new-made ones being pushed to the
outside, growing, and finally being cast out into the sur-
rounding lymph-lacuna, to be carried away in the lymph-
current.

Near the lower part of the ileurn large numbers of solitary
follicles are closely collected side by side at intervals along
the part of the bowel opposite to that at which the mesentery
joins it: these aggregations are known as Peyer's patches; and
are easily recognizable by the unaided eye, as villi are absent
from the part of the mucous membrane opposite them, and
they also cause a bulging, visible on the outside of the intes-
tine. They disappear after middle life.

The Lymphatic Glands are essentially Peyer's patches
more complicated in structure by the fact that the constitu-
ent follicles are more closely united and are gathered into
roundish masses instead of being spread out in a single layer.
They are found in various regions on the course of lymphatic
vessels; especially in the mesentery, groin and neck. In the
latter position they often inflame and give rise to abscesses,
especially in tuberculous persons; and still more often enlarge,
harden and become more or less tender, so as to attract at-
tention to them. In common parlance it is then frequently
said that the person's " kernels have come down/' or that he
has " waxing kernels." Each lymphatic gland is enveloped
in a connective-tissue capsule, partitions of which incomplete-
ly divide it into chambers in which the lymphoid tissue lies.
The partitions are more complete in the outer parts of the
gland (cor.tical portion), which accordingly looks different
from the central portion (medulla) in sections. In the lym-
phoid tissue are contained many leucocytes in process of
division. " Afferent " lymphatic vessels open into the pe-

LYMPHATIC SYSTEM AND DUCTLESS GLANDS. 353

riphery of the gland, and efferent vessels arise in its centre.
Hence the lymph in its flow traverses the cellular gland sub-
stance, and in its course picks up extra corpuscles which it
carries on to the blood. In the lymphoid tissue there is a
close network of blood-capillaries. It is clear that these
organs are not true glands, in the proper sense of the word:
they are sometimes called lymphatic ganglia, but that sug-
gests a connection with nerve-centres; a good name for them
is lymphatic nodes. In Fig. 12| is given a diagrammatic rep-
resentation of a lymphatic node.

tr

Fm. 121 —Diagram of cross-section of a lymphatic gland : al, afferent lymphatic
vessels; el, efferent lymphatic vessel; tr. one of the connective-tissue bands sub-
dividing: the gland ; C, cortical portion: M. medullary portion. The leucocytes are
represented only in a part of the right half of the figure, where they are seen, Ih.
to lie closely packed in the centre of a gland-chamber, while towards the walls of
the chamber, 7.9, where they are naturally less closely packed, they have been
washed away, as often happens in preparing a specimen, leaving the reticular sup-
porting tissue conspicuous.

The Movement of the Lymph. This is no doubt some-
what irregular in the commencing vessels, but, on the whole,
sets on to the larger trunks and through them to the veins.
In many animals (as the frog) at points where the lymphatics
communicate with the veins, there are found regularly con-
tractile "lymph-hearts" which beat with a rhythm independ-
ent of that of the blood-h'eart, and pump the lymph into a
vein. In the human Body, however, there are no such hearts,
and the flow of the lymph is dependent on less definite
arrangements. It seems to be maintained mainly by three
things. (1) The pressure on the blood-plasma in the capil-
laries is greater than that in the great veins of the neck;
hence any plasma filtered through the capillary-walls will be

354 THE HUMAN BODY.

under a pressure which will tend to make it flow to the ve-
nous termination of the thoracic or the right lymphatic duct.
(2) On account of the numerous valves in the lymphatic
vessels (which all only allow the lymph to flow past them to
larger trunks) any movement compressing a lymph-vessel will
cause an onward flow of its contents. The influence thus
exerted is very important. If a tube be put in a large lym-
phatic, say at the top of the leg of an animal, it will be seen
that the lymph only flows out very slowly while the animal
is quiet; but as soon as it moves the leg the flow is greatly
accelerated. (3) During each inspiration the pressure on the
thoracic duct is less than that in the lymphatics in parts of
the Body outside the thorax (see Chap. XXV). Accord-
ingly, at that time, lymph is pressed, or, in common phrase,
is "sucked," into the thoracic duct. During the succeeding
expiration the pressure on the thoracic duct becomes greater
again, and some of its contents are pressed out; but on
account of the valves of the vessels which unite to form, the
duct, they can only go towards the veins of the neck.

During digestion, moreover, contractions of the villi press
on the lymph or chyle within them and force it on; and in
certain parts of the Body gravity, of course, aids the flow,
though it will impede it in others.

The Ductless G-lands— Spleen, Thyroid, Thymus, Pit-
uitary Body, Suprarenals. — There are in the Body several
organs of such considerable size and so constantly present
in vertebrate animals that a priori they would seem to be of
functional importance. Until quite recently, however, the
functions of nearly all of them were quite problematical, al-
though it has long been known that pathological changes in
some of them were associated with grave conditions of general
disease. Even yet their physiology is very incompletely known.

When we speak of a true gland we mean an organ that
forms some definite secretion which it pours out in a separate
form, but the organs we are about to consider have no secret-
ing recesses and no ducts: nevertheless some of them un-
doubtedly make, and pass into the lymph and blood, substances
of great importance to the healthy working of the Body.
Some true glands indeed do this, quite apart from the manu-
facture of what is usually spoken of as their secretion. Why
so large an organ as the liver should be set apart for the for-
mation of so comparatively unimportant a digestive fluid as

LYMPHATIC SYSTEM AND DUCTLESS GLANDS. 355

the bile was long a puzzle. We now know that the chief use
of the liver is connected with the storage and formation of car-
bohydrate materials (see Chap. XXIX), and that, quite apart
from the use of bile in digestion or the elimination of part of
the bile as waste, the liver exerts an essential influence on the
whole normal nutritional processes of the Body. Again, in
the pancreas we have an organ which forms a very important
digestive secretion, and it might well be that this was its sole
use in the economy. But when the pancreas is carefully re-
moved from an animal great nutritional disturbances follow,
as shown, among other things, by diabetes, i.e., the presence
of sugar in the urine. Since the pancreatic secretion poured
into the intestine by the gland duct has much to do with the
digestion of starch and its conversion into sugar, it might be
supposed that mere digestive disturbances due to its absence
led to the diabetic and general changes. But this is not so.
If a piece of living pancreas be transplanted from one animal
to beneath the skin of another, and left until it has grown
there, the pancreas of the second animal may be removed
without causing diabetes. Moreover it is possible by inject-
ing melted paraffin into the pancreatic duct of an animal not
only to prevent the gland secretion from reaching the intes-
tine, but to cause atrophy of the true gland-cells. Yet
animals so treated do not become diabetic. It is then clear
that there is some material necessary to health and quite
distinct from pancreatic juice formed by pancreatic tissue and
taken up from it by the circulating liquids. Scattered through
the pancreas, and quite distinct from its proper gland tissue?
are peculiar patches of cells very richly supplied with blood-
vessels. Probably these cells are concerned in the antidiabetic
function of the gland; but whether through special cells
or not, the organ has an important internal secretion to
blood and lymph, in addition to its external secretion to its
duct. This fact may have a very wide bearing: it may be
that all organs, or many organs, in addition to their more ob-
vious functions, do, as the result of the chemical processes
taking place in them, manufacture substances a supply of
which, to lymph or blood, is required for the life or health of
distant parts of the Body. The waste of one organ before its
final conversion into carbon dioxide, water, or urea, for elimi-
nation from the system, may be a necessary food of another.
It is, for example, quite possible that the kreatin formed in

356 THE HUMAN BODY.

muscles and passed from them to the circulating fluid is
essential to the general health of the Body. There are, how-
ever, so many muscles that the removal of some of them, as
when a limb is amputated, does not cut off the kreatin supply,
and so disease does not result. When, on the other hand, an
organ is unique, as the thyroid, or exists only in a single pair,
as the suprarenals, then removal or extensive disease, by de-
priving the system of the peculiar internal secretion of the
organ concerned or, possibly, from the accumulation within
the blood of substances which it is the function of the missing
part to absorb and destroy, may, often in fact does, lead to
widespread nutritional changes, resulting in death.

The Spleen. This is an organ situated at the left end of
the stomach (//, Fig. J-H1)) and is about 110 grains (G oz. ) in
weight. Its size is, however, very variable; it enlarges dur-
ing digestion and shrinks after it until the next meal. In
many fevers, especially in those of malarial nature, it also
becomes enlarged, frequently to a very great extent, and this
enlargement may become permanent, constituting the so-
called " ague-cake." In color the spleen is dark red, but if
cut across numerous white spots of about 1 mm. (^ inch)
diameter are seen scattered over the surface of the section : it
is very richly supplied with blood which is carried away by
the splenic vein (7, Fig. 119) and poured into the portal vein.
The spleen possesses on its exterior a connective-tissue capsule
very rich in elastic fibres and giving off numerous bands
(trabeculcB] which branch and interlace throughout the organ
forming a spongy mass, in the spaces of which is contained a
soft red pulp of peculiar structure. The arteries of the organ
by frequent branching are reduced to almost capillary size,
and these terminal twigs enter into the pulp, and there, los-
ing all coats but the lining epithelium, assume the structure
of capillaries. The cells forming the walls of these ca-
pillaries next separate from one another so as to leave
clefts between them, and at the same time become irregu-
larly branched and, joining by their branches, form a sup-
porting framework or reticulum through the pulp, into
which latter the blood is poured freely through the spaces
between the cells. The main mass of the splenic pulp con-
sists of red blood-corpuscles, some normal in appearance,
some appearing partly broken down; mixed with these are
some white corpuscles, arid some larger colorless amreboid

LYMPHATIC SYSTEM AND DUCTLESS GLANDS. 357

cells in which are often found one or more red corpuscles
which have apparently been swallowed by them. There are
also many pigmented granules, some free and some within
amosboid cells; they are apparently the debris of red corpus-
cles which have been broken down. In early life the splenic
pulp also contains granular colorless cells within which red
corpuscles are seen in the process of development. The whole
histological structure of the adult pulp suggests that in it
many red blood-corpuscles are finally destroyed, setting free
haemoglobin and other coloring matters derived from it. This
breaking down of haemoglobin must also give rise to proteids
and substances derived from the chemical degradation of
proteids, and the spleen is extremely rich in nitrogenous
crystallhie^^ubatances. The increase in size of the spleen
during Tngestion, when the veins of the alimentary canal are
pouring great quantities of blood laden with absorbed mat-
.ters into the portal system, suggests that the spleen supplies
things to the liver at that time which, are of importance to it.
There is reason to believe that the main coloring matter
of the bile (bilirubvii) is derived from the haemoglobin of red
corpuscles which have completed their life-period and been
destroyed, and it may be that the spleen takes the first steps in
the preparation of bilirubin for its elimination from the Body
as a waste product. There still is, however, much doubt as
to the real function of the spleen; it almost certainly plays
an important part in the proteid metabolisms of the Body.
Though so large an organ it is not essential; animals from
whom it has been completely removed can live a long time
in good health. The red marrow of spongy bone greatly re-
sembles the splenic pulp in histological characters and may
have similar functions and be able to entirely take the place of
the spleen when the organ has been excised. The white spots
seen on the cut surface of a spleen are sections of masses of
adenoid tissue attached to the smaller splenic arteries and
named MalpigMan corpuscles; they resemble the elosed fol-
licles of the intestine in structure.

The Thyroid Body or Gland. This organ lies in the
neck on the sides of the windpipe and consists usually of a
right and a left lobe united by a narrow isthmus across the
front of the air-tube. It is about thirty grams (two ounces)
in weight; in the disease known as goitre it is greatly en-
larged and its structure altered. The thyroid is dark red in

358 THE HUMAN BODY.

color and very vascular, richly supplied with nerves, and is
subdivided by connective tissue into cavities or alveoli, the
largest of which are just visible to the unaided eye. Each
alveolus is lined by a single layer of cuboidal cells, and filled
by a glairy fluid which appears to contain mucin.

The very abundant blood-supply of the thyroid suggests
that it is the seat of important metabolic or chemical changes,
and observation and experiment confirm this. Extensive
disease of the thyroid leads to great changes in the general
nutrition of the Body, ending in the condition named
myxodosma; muciginous liquid collects in the connective tis-
sues, nervous and muscular activity are much impaired,
tremors and convulsions occur, and finally a semi idiotic con-
dition (cretinism) comes on and is followed by death if all the
gland be diseased. Quite similar symptoms follow the com-
plete removal of the thyroid body from animals, or from man
for tumors; but if even a small part of healthy gland-tissue
be left behind the symptoms do not occur. Moreover, if a
portion of living thyroid from one animal be grafted beneath
the skin of another, the thyroid of the latter can be com-
pletely removed without influencing the general health. It
would seem then that the gland is the place of formation of
some substance essential to the healthy working of the Body,
but that under ordinary conditions of life the whole organ
is not required to produce the necessary minimum of this
substance. This view is strengthened by the fact that in
patients with thyroid disease and in animals deprived of the
organ the symptoms of myxodoema may be relieved or removed
by adding raw thyroid tissue to the food, or by subcutaneous
injection of the expressed juice of a fresh gland. When in-
jected into a healthy animal extract of thyroid causes arterial
dilatation, and a lowering of blood pressure.

The Thymus. This is a temporary organ of unknown
function. It has its greatest size in proportion to the whole
weight of the Body a short time before birth. After birth
it grows in absolute weight for some time, but then begins
to dwindle away and has usually completely disappeared by
the twelfth or fourteenth year. It lies in front of the wind-
pipe in the lower part of the neck and the upper part of the
thorax, and is the "neck" sweetbread of the butcher as dis-
tinguished from the true sweetbread or pancreas. The

LYMPHATIC SYSTEM AND DUCTLESS GLANDS. 359

thymus essentially consists of adenoid tissue, and is well sup-
plied with blood-vessels and lymphatics.

The Pituitary Body (Fig. 75) is in part an offshoot of the
brain, and probably that portion of it is, like the pineal body,
a remnant of a once functionally important ancestral organ.
The anterior lobe of the pituitary body, however, is derived
in development from the pharynx, of which it is an embryonic
outgrowth. This part of it somewhat resembles the thyroid
in structure. Complete removal of the pituitary body in the
case of cats and dogs causes a lowering of temperature, mus-
cular twitchings and spasms, difficulty in breathing, general
lassitude, and death within a fortnight. These symptoms
improve when extract of the gland is injected. The.prgan
has therefore been supposed to form an internal secretion use-
ful in maintaining the nutrition of the muscular and nervous
systems. Disease of the pituitary body in man has been found
to be associated with the curious condition named acromegaly,
in which there is hypertrophy of the bones of the limbs and
face, and of parts of the skin and mucous membranes. In-
jection of the extract of the gland causes, in a normal animal,
a more powerful but not quicker heart-beat, and constriction
of the arteries.

The Suprarenal Capsules or Adrenals are a pair of
small organs, weighing together about 12 grams (f oz.) placed
one on the top of each kidney. They have, however, no inti-
mate connection with the kidneys, and in many animals are
placed at some distance from them. Each consists of a denser
less colored external cortex, and a central deep yellow-brown
softer medulla. The cortex is subdivided into chambers by
connective tissue, and the chambers are filled by closely
packed, polygonal nucleated cells. Similar cells are found
in the medulla, which is, moreover, closely connected with
the sympathetic system and is richly supplied with nerves.

It was noticed some fifty years ago by a physician named
Addisonthat certain obscure diseased conditions characterized
by great debility and by the appearance of bronzed patches
on the skin, and leading to death, were found on post-mortem
examination to be accompanied by disease of the adrenals.
The disease has hence been named Addison's disease. When
the suprarenal capsules are completely removed from animals
a similar fatal diseased condition results, death taking place
in warm-blooded animals within two or three days, and be-

360 THE HUMAN BODY.

ing preceded by muscular weakness, dilatation of the arteries,
mental feebleness and general prostration. The exact role
played in the organism by these small but essential organs is
still unknown, but they form substances which have a pro-
found effect on the nerves of the heart and blood-vessels.
A very minute portion of the watery or alcoholic extract of a
suprarenal capsule when injected into avein of an animal causes
a very slow heart-beat, or even complete inhibition of the
auricles. If the cardio-inhibitory nerves have first been cut,
on the other hand, the injection causes a great increase in
the rate of heart- beat and a great increase of its force, espe-
cially that of the auricles. The small arteries become greatly
contracted, and this combined with the powerful heart-beats
leads to a very great increase of arterial pressure. The arterial
constriction is not due to stimulation of the vaso-con stricter
centre, but to a direct action on the muscular coats of the
arteries: it is very transient. The skeletal muscles are also
affected, the period of a simple muscular contraction being
greatly prolonged, and this effect lasts much longer than the
changes produced in the organs of circulation. The active
material exists only in the medulla of the adrenal, is efficient
in extremely minute doses, is dialyzable, and its efficacy is not
impaired by short boiling.

It would appear then that the suprarenals are constantly
forming and passing into the blood minute quantities of a
substance which is of great importance for the maintenance
of the " tone " of the muscles, especially of the cardiac and
arterial muscles. Whether in addition they also remove
noxious substances from the blood, the accumulation of which
after their removal is one cause of the death which results, is
still undecided. The blood of such animals acts as a poison
to other animals, and this has been supposed to be due to the
presence in it of a specific poison which the adrenals normally
pick up and destroy: but it is clear that the blood of an ani-
mal dying from extensive malnutrition produced in any way
would be quite abnormal, ana might well be poisonous to other
animals. The same remark may be made as to the poisonous
character of the blood of animals dying as a result of removal
of the thyroid: there is no satisfactory evidence that it is due
to the accumulation of any one special toxic substance which
it is a function of the thyroid to remove: still, it maybe.
The symptoms produced by its injection are quite different
from those produced by injection of thyroid extract

CHAPTER XXIV.
DIGESTION.

The Object of Digestion. Of the various foodstuffs swal-
lowed, some are already in solution and ready to dialyze at
once into the lymphatics and blood-vessels of the alimentary
canal; others, such as a lump of sugar, though not dissolved
when put into the mouth, are readily soluble in the liquids
found in the alimentary canal, and need no further digestion.
In the case of many most important foodstuffs, however,
special chemical changes have to be wrought, either with the
object of converting insoluble bodies into soluble, or non-
dialyzable into dialyzable, or both. The different_secretions
poured into the alimentary tube act in various ways upon
different foodstuffs, and at last get them into a state in which
they can pass into the circulating medium and be carried to
all parts of the Body.

The Saliva. The first solvent that the food meets with
is the saliva, which, as found in the mouth, is a mixture of
pure saliva, formed in paroticlr-subm axillary, and sublingual
glands, with the mucus secreted by small glands of the buccal
mucous membrane. This mixed saliva is a colorless, cloudy,
feebly alkaline liquid, "ropy" from the mucin present in 4t,
arid usually containing air-bubbles. Pure saliva, as obtained
by putting a fine tube in the duct of one of the salivary
glands, is more fluid and contains no imprisoned air.

Usually but little saliva is secreted ; the presence of food
in the mouth, especially highly flavored or acid food, leads
to a more abundant flow : the mere chewing of a tasteless
inert substance will, however, excite some secretion. The secre-
tion thus brought about is reflex: the afferent fibres running
to the brain in the glossopharyngeal and lingual nerves, and
exciting there the centre from which the efferent secretory
nerve-fibres for the glands arise. The centre may be excited
in other ways: as by nausea, or through the nerves of eye or
nose when the sight or smell of desirable food makes " the
mouth water."

361

362 THE HUMAN BODY.

The uses of the saliva are for the most part physical and
mechanical. It keeps the mouth moist and allows us to speak
with comfort; most young orators know the distress occa-
sioned by the suppression of the salivary secretion through
nervousness, and the imperfect efficacy under such circum-
stances of the traditional glass of water placed beside public
speakers. The saliva, also, enables us to swallow dry food ;
such a thing as a cracker when chewed would give rise merely
to a heap of dust, impossible to swallow, were not the mouth
cavity kept moist. This fact used to be taken advantage of
in the East Indian rice ordeal for the detection of criminals.
The guilty person, believing firmly that he cannot swallow
the parched rice given him, and fearful of detection, is apt to
have the nerve-centres of his salivary glands inhibited or
paralyzed by terror, and does actually become unable to swal-
low the rice; while in those with clear consciences the nerv-
ous system excites the usual reflex secretion, and the dry
food gives rise to no difficulty in its deglutition. The saliva,
also, dissolves such bodies as salt and sugar, when they are
taken into* the mouth in solid form, and enables us to taste
them; undissolved substances are not tasted, a fact which any
one can verify for himself by wiping his tongue dry and
placing a fragment of sugar upon it. No sweetness will be
felt until a little moisture has exuded and dissolved part of
the sugar.

In addition to such actions the saliva, however, exerts a
chemical one on an important foodstuff. Starch (although
it swells up greatly in hot water) is insoluble, and could not
be absorbed from the alimentary canal. The saliva contains
an enzyme, ptyalin, which has the power of turning starch
into soluble substances. Until recently the chief product was-
believed to be grape sugar (glucose)', but it is now ascertained
that it is maltose, belonging to the cane-sugar chemical series.
In the small intestine the maltose is changed into glucose and
absorbed ; so the chemical action of ptyalin upon starch is at
most but a preparatory one. In effecting the change the ptyalin
is not altered; a very small amount of it can convert a vast
amount of starch, and does not seem to have its activity im-
paired in the process. The starch is made to combine with
the elements of one or more molecules of water, and the
ptyalin is unchanged.

This faculty of ptyalin is known as q/niiilolytic : and since

DIGESTION. 363

it is associated with the taking up of a molecule of water is
a hydrolytic action. Ptyalin is a typical enzyme ; it differs
from the true ferments, such as yeast, in the fact that it is
not a living organism, and does not multiply during the oc-
currence of the change which it sets up; its activity belongs
to the obscure chemical category of contact actions.

In order that the ptyalin may act upon starch certain
conditions are essential. Water must be present, and the
liquid must be neutral or feebly alkaline; acids retard, or if
stronger, entirely stop the process. The change takes place
most quickly at about the temperature of the human- Body,
and is greatly checked by cold. Boiling the saliva destroys
its ptyalin and renders it quite incapable of converting starch.
Cooked starch is changed more rapidly and completely than
raw.

Saliva has another important but indirect influence in
promoting digestion. Weak alkalies stimulate the mucous
membrane of the stomach and cause it to pour forth more
gastric juice. Hence the efficacy of a little carbonate of soda,
taken before meals, in some forms of dyspepsia. The saliva
by its alkalinity exerts such an action ; and this is one reason
why food should be well chewed before being swallowed; for
then its taste, and the movements of the jaws, cause the
secretion of more saliva.

Deglutition. A mouthful of solid food is broken up "
the teeth, and rolled about the mouth by the tongue, untij6
is thoroughly mixed with saliva and made into a soft pa&
mass. The muscles of the cheeks keep this from gettii
between them and the gums; persons with facial paralysL
have, from time to time, to press out with the finger food
which has collected outside the gums, where it can neither be
chewed nor swallowed. The mass is finally sent on from the
mouth to the stomach by the process of deglutition, which is
described as occurring in three stages. The first stage in-
cludes the passage from the mouth into the pharynx. The
food being collected into a heap on the tongue, the tip of
that organ is placed against the front of the hard palate, and
then the rest of the tongue is raised from before back, so as
to press the food mass between it and the palate, and drive it
back through the fauces. This portion of the act of swallow-
ing is voluntary, or at least is under the control of the will,
although it commonly takes place unconsciously. The second

364 . THE HUMAN BODY. -

' stage of deglutition is that in which the food passes through
• the pharynx; it is the most rapid part of its progress, since
, the pharynx has to be emptied quickly so as to clear the
.opening of the air-passages for breathing purposes. The
•food mass, passing back over the root of the tongue, pushes
-down the epiglottis; at the same time the larynx (or voice-
'box at the top of the windpipe) is raised, so as to meet it,
•and thus the passage to the lungs is closed ; muscles around
'the aperture probably also contract and narrow the opening.
•The raising of the larynx can be readily felt by placing the
finger on the large cartilage forming "Adam's apple" in the
neck, and then swallowing something. The soft palate is at
the same time raised and stretched horizontally across the
pharynx, thus cutting off communication with its upper, or
respiratory portion, leading to the nostrils and Eustachian
tubes. Finally, the isthmus of the fauces is closed as soon as
the food has passed through, by the contraction of the mus-
cles on its sides and the elevation of the root of the tongue.
All passages out of the pharynx except the gullet are thus
blocked, and when the pharyngeal muscles contract the food
can be squeezed only into the oesophagus. The muscular
movements concerned in this part of deglutition are all re-
flexly excited; food coming in contact with the mucous mem-
brane of the pharynx stimulates afferent nerve-fibres in it;
jpse excite the centre of deglutition which is placed in the
dulla oblong ata, and from it efferent nerve-fibres proceed
, the muscles concerned and (under the co-ordinating influ-
ice of the centre) cause them to contract in proper sequence,
rhe pharyngeal muscles, although of the striped variety, are
but little under the control of the will; it is extremely diffi-
cult to go through the movements of swallowing without
something (if only a little saliva) to swallow and thus excise
the movements reflexly. Many persons, after having got the
mouth completely empty cannot perform the movements of
the second stage of deglutition at all. On account of the re-
flex nature of the contractions of the pharynx, any food which
has once entered it must be swallowed: the isthmus of the
fauces is a sort of Rubicon; food that has passed it must
continue its course to the stomach, although the swallower
learnt immediately that he was taking poison. The third
stage of deglutition is that in which the food is passing along
the gullet, and is comparatively slow. Even liquid substances

•DIGESTION: 365

do not fall or flow down this tube, but have their passage
controlled by its muscular coats, which grip the successive
portions swallowed and pass them on. Hence the possibility
of performing the apparently wonderful feat of drinking a
glass of water while standing upon the head, often exhibited
by jugglers; the onlookers forget that the same thing is^done
every day by horses, and other animals, which drink with the
pharyngeal end of the gullet lower than the stomach. The
movements of the oesophagus are of the kind known as ver-
micular or peristaltic. Its circular muscular fibres contract
behind the morsel and narrow the passage there; and the con-
striction then travels along to the stpmacjh, pushing the food
in front of it. Simultaneously tire longitudinal fibres, at the
point where the food-mass is at any moment and immediately
in front of that, contracting, shorten and widen the passage.
The Gastric Juice. — The food having entered the stom-
ach is subjected to the auction of the gastric juice, which is a
thin, colorless or pale yellow liquid, of a strongly acid reac-
tion. It contains as specific elements free hydrochloric acid
(about .2 per cent), and an enzyme called pepsin which, in
acid liquids, has the power of converting the ordinary non-'
dialyzable proteids which we eat, into closely allied bodies,
some of which are dialyzable and named peptones. It also
dissolves solid proteids, changing them similarly. Dilute
acids will by themselves produce the same changes in the,
course of several days, but in the presence of pepsin and at
the temperature of the Body the conversion is far more
rapid. In neutral or alkaline media the pepsin is inactive;
and cold checks its activity. Boiling destroys it. In addi-
tion to pepsin, gastric juice contains another enzyme (rennin)
which coagulates the casein ogen of milk, as illustrated by
the use of "rennet," prepared from the mucous membrane
of the calf s digestive stomach, in cheese-making. The acid
of the natural gastric juice would, it is true, precipitate the
casein, but such precipitate is quite different from the true
tyre'm, and neutralized gastric juice still possesses this power;
moreover, boiled gastric juice loses the milk-clotting property,
and a very little normal juice can coagulate a great quantity
of milk. The curdled condition of the milk regurgitated by
infants is, therefore, not any sign of a disordered state of the
stomach, as nurses commonly suppose. It is proper for milk

366 THE HUMAN BODY.

to undergo this change, before the pepsin and acid of the
gastric juice digest it.

The most important change effected by the gastric juice
is that of the proteids. This may be studied either on natu-
ral juice obtained from the stomach of an animal through an
opening (gastric fistula) or on an artificial juice prepared by
extracting the mucous membrane of a fresh stomach with
glycerine, and 'adding a large quantity of dilute (0.2$) hydro-
chloric acid. If blood-fibrin or boiled white of egg be placed
in such a mixtttre and kept at a temperature of about 38° C.
| (100° F.) these bodies swell, become transparent, and soon
dissolve ; and all other solid proteids undergo similar changes.
If the solution be now neutralized a small white precipitate
of parapeptone (which is probably only ordinary acid albu-
min) is obtained. The filtrate from this gives no precipitate
on boiling, but an abundant one of albumose on the addition
of ammonium sulphate. The filtrate from this precipitate
yields an abundant precipitate of peptone when alcohol is
added. Peptone is dialyzable, though not so easily as saline
bodies, and in this differs from albumose and parapeptone
and all other proteids. The parapeptone is probably a bye-
product due to the action of the acid of the juice alone: the
albumose and peptone are true products of peptic digestion of
proteids, due to their breaking up with concomitant hy-
dration, the peptone being the more finished or complete
digestive product. If instead of solid proteids we use solu-
tion of white of egg or of serum albumin, the earlier stages of
the process cannot be followed by the eye, but the final prod-
ucts are the same: the original proteid disappears, giving
origin to some parapeptone, to albumose, and to peptone; and
prolonged artificial peptic digestion causes no further breaking
up of the albumose or peptone. Peptone is very soluble in
water, and its solutions are not coagulated by boiling. A
very small amount of pepsin can, if some acid be added from
time to time, convert a very large amount of proteid: it is de-
stroyed by boiling.

Gastric Digestion. The process of swallowing is contin-
uous, but in the stomacli the onward progress of the food is
stayed for some time. The pyloric sphincter, remaining con-
tracted, closes the aperture leading into the intestine, and the
irregularly disposed muscular layers of the stomach keep its
semi-liquid contents in constant movement, maintaining a

DIGESTION. 367

sort of churning by which all portions are brought into con-
tact with the mucous membrane, and thoroughly mixed with
the secretion of its glands. The gelatin-yielding connective
tissue of meats is dissolved away, and the proteid-containing
fibres, left loose, are dissolved and changed. The albuminous
walls of the fat-cells are dissolved and their oily contents set
free; but the gastric juice does not act upon the latter. Cer-
tain mineral salts (as phosphate of lime, of which there is
always some in bread) which are insoluble in water but solu-
ble in dilute acids, are also dissolved in the stomach. On
the other hand, the gastric juice has itself no action upon
starch, and since ptyalin does not act at all, or only imper-
fectly, in an acid medium, the activity of the saliva in con-
verting starch is stayed in the stomach. By the solution of
the white fibrous connective tissue, that disintegration of ani-
mal foods commenced by the teeth, is carried much farther
in the stomach, and the food-mass, mixed with much gastric
secretion, becomes reduced to the consistency of a thick soup,
usually of a grayish color. In this state it is called cliyme.
Chyme contains, after an ordinary meal, much peptone, though
some of this has been already dialyzed into the gastric mucous
membrane and carried off along with other dissolved dialyz-
able bodies, such as salts and sugar. The albumose, fats, and
starch still remain in the chyme. After the food has re-
mained in the stomach some time (one and a half to two
hours) the chyme begins to be passed on into the intestine
in successive portions. The pyloric sphincter relaxes at in-
tervals, and the rest of the stomach, contracting at the same
moment, injects a quantity of chyme into the duodenum;
this is repeated frequently, the larger undigested fragments
being at first unable to pass the orifice. At the end of about
three or four hours after a meal the stomach is again quite
emptied, the pyloric sphincter finally relaxing to a greater
extent and allowing any larger indigestible masses, which the
gastric juice cannot break down, to be driven into the in-
testine.

The Chyle. When the chyme passes into the duodenum
it finds preparation made for it. The pancreas is in reflex
connection with the stomach, and its nerves cause it to qom-
mence secreting as soon as food enters the latter; hence a
quantity of its secretion is already accumulated in the intes-
tine when food enters. The gall-bladder is distended with

368 THE HUMAN BODY.

bile, secreted since the* last meal; this passing down the
hepatic duct has been turned back up the cystic duct (Dc,
Fig. 115) on account of the closure of the common bile-duct.
The acid chyme, stimulating"' nerve-endings in the duodenal
mucous membrane, causes reflex contraction of the muscular
coat of the gall-bladder, and a relaxation of the orifice of the
common bile-duct; and so a gush of bile is poured out on the
chyme. From this time on, both liver and pancreas continue
secreting actively for some hours, and pour their products
into the intestine. The glands of Brunner and the crypts
of Lieberkiilm are also set at work, but concerning their
physiology we know very little. All of these secretions are
alkaline, and they suffice very soon to more than neutralize
the acidity of the gastric juice^and to convert the acid chyme
into alkaline chyle, which, after an ordinary meal, will con-
tain a great variety of things: mucus derived from the ali-
mentary canal; ptyalin from the saliva; pepsin from the
stomach; water, partly swallowed and partly derived from
the salivary and other secretions; the peculiar constituents of
the b^e and pancreatic juice and of the intestinal secretions;
some™! digested proteids; unchanged starch; oils from the
fats eaten; peptones formed in the stomach but not yet ab-
sorbed; albumose; parapeptone; possibly salines and sugar
which have also escaped absorption in the stomach; and in-
digestible substances taken with the food.

The Pancreatic Secretion is clear, watery, alkaline, and
much like saliva in appearance. The Germans call the pan-
creas the "abdominal salivary gland." In digestive prop-
erties, however, the pancreatic secretion is far more impor-
tant than the saliva, or even the gastric juice. Starch
it changes as the saliva does, but converts it into maltose
more quickly : and it acts also on proteids and fats. It
is by far the most important of all the digestive secretions.
All proteids not already converted into peptone or albumose
are acted upon by the pancreatic juice even more ener-
getically than in the stomach, being not only converted into
peptone, but in part further broken up, if the digestion (arti-
ficial) be prolonged, and converted into crystallizable nitrog-
enous bodies which, unlike peptone, retain no proteid-like
characters: the chief of these are leucin and tyrosin, the
former allied chemically to the fatty acids, the other to bodies
of the aromatic series. In normal digestion, however, it is

DIGESTION. 369

probable that but little of the proteid is broken up beyond
the peptone stage, and all of it never is; an album ose is
formed as an intermediate product. The enzyme concerned is
trypsin ; it is active only in an alkaline or neutral medium,
and before dissolving solid proteids does not cause them to
swell and become transparent as pepsin does. Like the other
digestive ferments, it is most active at about the temperature
of the Body, and is destroyed by boiling. On fats the pan-
creatic secretion has a double action. To a certain extent it
breaks them up, with hyd ration, into free fatty acids and
glycerin; for example—

(C'CHH0)' 1 °3 + 3H'° = 3(°18Hs H 1 °) + °3H6

1 Stearin + 3 Water = 3 Stearic acid + 1 Glycerine.

The fatty acid then combines with some of the alkali present
to make a soap, which being soluble in water is capable of
absorption. Glycerin, also, is soluble in water and dialyz-
able. The greater part of the fats are not, however, so broken
up, but are simply mechanically separated into
which remain suspended in the chyle and give it
color, just as the cream -drops are suspended in milk, or the
olive-oil in mayonnaise sauce. This is effected by the help of
a quantity of albumin which exists dissolved in the pancreatic
secretion. In the stomach, the animal £its eaten have lost
their cell-walls, and have become melted by the temperature
to which they were exposed. Hence their oily part floats free
in the chyme when it enters the duodenum. If oil be shaken
up with water, the two cannot be got to mix; immediately
the shaking ceases, the oil floats up to the top; but if some
raw egg be added, a creamy mixture is readily formed, in
which the oil remains for a long time evenly suspended in
the watery menstruem. The reason of this is that each oil-
droplet becomes surrounded by a delicate pellicle of albumin,
and is thus prevented from fusing with its neighbors to make
large drops, which would soon float to the top. Such a mix-
ture is called an emulsion, and the albumin of the pancreatic
secretion emulsifies the oils in the chyle, which becomes
, white (for the same reason as milk is that color) because the
innumerable tiny oil-drops floating in it reflect all the light
which falls on its surface.

In brief, the pancreatic secretion converts starch into

370 THE HUMAN BODY.

maltose ; dissolves proteids (if necessary) and converts them
into peptones; emulsifies fats, and, to a certain extent, breaks
them up into glycerin and fatty acids; the latter are then
saponified by the alkalies present.

The Bile. — Human bile when quite fresh is a golden
brown liquid; it becomes green when kept. As formed in
the liver it contains hardly any mucin, but if it make any
stay in the gall-bladder it acquires much from the lining mem-
brane of that bag, and becomes slimy and " ropy." It is
alkaline in reaction and, besides coloring matters (the more
important of which, bilirubin, is probably a waste product
derived from haemoglobin), contains mineral salts and water,
and the sodium salts of two nitrogenized acids, taurocliolic
and glycliocholic, the former predominating in human bile.

Pettenkofer's Bile Test. If a small fragment of cane
sugar be added to some bile, and then a large quantity of strong,
sulphuric acid, a brilliant purple color is developed, by cer-
tain products of the decomposition of the bile acids; the
physician can by this test, in disease, detect their presence in
the urine or other secretions of the Body. Gmetin's Bile
Test. Whe bile-coloring matters, treated with yellow nitric
acid, go through a series of oxidations, accompanied with
changes of color from yellow-brown to green, then to blue/
violet, purple, red, and dirty yellow.

Bile has no digestive action upon starch or proteids./ji It
does not break up fats, but to a limited extent emulsifies
them, though far less perfectly than the pancreatic secretion.
It is even doubtful whether this action is exerted in the in-
testines at all. In many animals, as in man, the bile and
pancreatic ducts open together into the duodenum, so that,
on killing a dog during digestion and finding emulsified fats
in the chyle, it is impossible to say whether or no the bile
had a share in the process. In the rabbit, however, the pan-
creatic duct opens into the intestine about a foot farther
from the stomach than the bile-duct, and it is found that if a
rabbit be killed after being fed with oil, no milky chyle is
found down to the point where the pancreatic duct opens.
In this animal, therefore, the bile alone does not emulsify
fats, and, since the bile is pretty much the same in it and
other mammals, it probably does not emulsify fats in them
either. From the inertness of bile with respect to most food-
stuffs it has been doubted whether it be of any digestive use at

DIGESTION. 371

all, and whether it should not be regarded merely as an excre-
tion, poured into the alimentary canal to be got rid of. But
there are many reasons against such a view. In the first place,
the entry of the bile into the upper end of the small intestine
where it has to traverse a course of more than twenty feet
before getting out of the Body, instead of its being sent into
the rectum, close to the final opening of the alimentary canal,
makes it probable that it has some function to fulfil in the
intestine. Moreover, a great part of the bile, including prac-
tically all the bile salts, poured into the intestines is again
absorbed from them ; this seems to show that part of the bile
is secreted for some other purpose than mere elimination
from the Body. One subsidiary use is to assist, by its alka-
linity, in overcoming the acidity of the chyme, and so to
allow the trypsin of the pancreatic secretion to act upon pro-
teids. Constipation is, also, apt to occur in cases where the
bile-duct is temporarily stopped, so that bile probably helps to
excite the contractions of the muscular coats of the intestine;*
under similar circumstances putrefactive decompositions are
apt to occur in the intestinal contents. Apart from such sec-
ondary influences,however, the bile probably has some influence
in promoting the absorption of fats. If one end of a capillary
glass tube, moistened with water, be dipped in oil, the latter will
not ascend in it, or but a short way; but if the tube be moist-
ened with bile, instead of water, the oil will ascend higher in
it. So, too, oil passes through a plug of porous clay kept moist
with bile, under a much lower pressure than through one wet
with water. Hence bile, by soaking the epithelial cells lining
the intestine, may facilitate the passage into the villi of oily
substances. At any rate, experiment shows that if the bile
be prevented from entering the intestine of a dog, the animal
eats an enormous amount of food compared with that
amount which it needed previously; and that of this food a
great proportion of the fatty parts passes out of the alimen-
tary canal unabsorbed. There is no doubt, therefore, that
the bile somehow aids in the absorption of fats, but exactly
how is uncertain. Its possible action in exciting the muscles
of the villi to contract will be referred to presently.

The Intestinal Secretions or Succus Entericus. These
consist of the secretions of the glands of Brunner and the
crypts of Lieberkiihn. It is difficult to obtain them pure; in-
deed the product of Brunner's glands has never been obtained

372 THE HUMAN BODY.

unmixed. That of the crypts of Lieberkiihn is watery and
alkaline, and poured out more abundantly during digestion
than at other times. It has no special action on starches,
most proteids, or on fats; but is said to dissolve blood fibrin
and convert it into peptone, and it changes maltose into
grape sugar; so that this cane sugar is turned into a grape
sugar before being absorbed. Mucus is also formed and
poured out abundantly by the epithelium cells of the intes-
tinal lining membrane. It is more especially secreted during
fasting, and by its stickiness collects debris and keeps the
mucous membrane clean.

Intestinal Digestion. Having considered separately the
actions of the secretions which the food meets with in the
small intestine we may now consider their combined effect.

The neutralization of the chyme, followed by its conver-
sion into alkaline chyle, will prevent any further action of
the pepsin on proteids, but will allow the ptyalin of the
saliva (the activity of which was stopped by the acidity of the
gastric juice) to recommence its action upon starch. More-
over, in the stomach there is produced, alongside of the albu-
mose and true peptone, the parapeptone, which agrees very
closely with syntonin in its properties, and this passes into
the duodenum in the chyme. As soon as the bile meets
the chyme it precipitates the parapeptone, and this carries
down with it any peptones which, having escaped absorption
in the stomach, may be present; it also precipitates the pep-
sin. In consequence, one finds in an animal killed during
digestion, a granular precipitate over the villi, and in the
folds between the valvulge conniventes of the duodenum.
This is redissolved by the pancreatic secretion, which also
changes into peptone the proteids (usually a considerable pro-
portion of those eaten at a meal) which have passed through
the stomach unchanged, or as albumose or parapeptone. The
conversion of starch into maltose will go on very rapidly under
the influence of the pancreatic secretion. Fats will be split
up and saponified to a certain extent, but a far larger pro-
portion will be emulsified and give the chyle a whitish appear-
ance. Later cane sugar, which may have escaped absorption in
the stomach, and maltose will be converted into grape sugar
and absorbed, along with such salines as may, also, have hith-
erto escaped. Elastic tissue from animal substances eaten,

DIGESTION. 373

cellulose from plants, and mucin from the secretions of the
alimentary tract, will all remain unchanged.

Absorption from the Small Intestine. The chyme leav-
ing the stomach is a semi-liquid mass which, mixed in the
duodenum with considerable quantities of pancreatic secre-
tion and bile, is further diluted. Thenceforth it gets the
intestinal secretion added to it, but the absorption more
than counterbalancing the addition of liquid, the food-
mass becomes more and more solid as it approaches the ileo-
colic valve. At the same time it becomes poorer in nutritive
constituents, these being gradually removed from it in "its
progress; most dialyze through the epithelium into the sub-
jacent blood and lymphatic vessels, and are carried off.
Those passing into the blood capillaries are taken by the por-
tal vein to the liver; while those entering the lacteals are
carried into the left jugular vein by the thoracic duct. As
to which foodstuffs go one road and which the other, there is
still much doubt; sugars probably go by the portal system,
while the fats, mainly, if not entirely, go through the lacteals.
How the fats are absorbed is not clear, since oils will not dia-
lyze through membranes, such as that lining the intestine,
moistened with watery liquids. Most of them, nevertheless,
get into the lacteals as oils and not as soluble soaps; for one
finds these vessels, in a digesting animal, filled with white
milky chyle; while at other periods their contents are watery
and colorless like the lymph elsewhere in the Body. The
little fat-drops of the emulsion formed in the intestine, go
through the epithelial cells and not between them, for during
digestion these cells are loaded with oil-droplets; as their
free ends are striated and probably devoid of any definite
cell- wall, it is possible that the intestinal movements squeeze

Lrops into them, but the cells may play a more active part.
yThe \triation of the border is due to closely-set rods which
fiown to be able to change their form, and it is possible

'they actively seize oil-droplets and other minute solid
particles. The cell passes the fat to its deeper end
thence, out into the subjacent lymphoid tissue. It is
probable that here certain amoeboid cells of the adenoid
tissue pick it up, and carry it into the central lacteal of a villus,
where they break up and set it free. In the villus there are
all the anatomical arrangements for a mechanism which shall
actively suck substances into it. Each is more or less

374 THE HUMAN ItODY.

elastic, and moreover, its capillary network when filled with
blood will distend it. If its plain muscular layer contracts
and compresses it, causing its central lacteal to empty into
vessels lying deeper in the intestinal wall, the villas will
actively expand again so soon as its muscles relax. In so
doing it cannot fill its lacteals from the deeper vessels on
account of the valves in the latter, and, accordingly, must
tend to draw into itself materials from the intestines; much
like a sponge re-expanding in water, after having been
squeezed dry. The liquid thus sucked up may draw oil-drops
with it, into the free ends of the cells and between them ; and
by repetitions of the process it is possible that considerable
quantities of liquid, with suspended oil-drops, might be car-
ried into the epithelial cells covering a villus. The bile
moistening the surface of the villus may facilitate the passage
of oil, and it is also said to stimulate the contractions of the
villi; if so, its efficacy in promoting the absorption of fats
will be explained, in spite of its chemical inertness with re-
spect to those bodies. There is also reason to believe that a
good deal of the emulsified fat is also directly picked up by
amoeboid corpuscles, which push their way between the
epithelial cells and thrusting processes into the intestine, pick
up oil-droplets, and then travel back and convey their load
to the lacteal.

The path taken by peptones is uncertain. They seem to
be very rapidly converted into proteids (? serum albumin) after
absorption as they cannot be found, or only traces of them,
in the thoracic duct or the portal vein blood of a digesting
animal. Moreover, peptones directly injected into the blood
are poisonous. Probably they are seized upon
formed by the cells of the lymphoid tissue.

Digestion in the Large Intestine. The contractions of
the small intestine drive on its continually diminishing con-
tents until they reach the ileo-colic valve, through which
they are ultimately pressed. As a rule, when the mass enters
the large intestine its nutritive portions have been almost
entirely absorbed, and it consists merely of some water, with
the indigestible portion of the food and of the secretions of
the alimentary canal. It contains cellulose, elastic tissue,
mucin, and somewhat-altered bile pigments; some fat if a
large quantity has been eaten; and some starch, if raw vege-
tables have formed part of -the diet. In its progress through

DIGESTION. 375

the large intestine it loses more water, and the digestion of
starch and the absorption of fats is continued. Finally the
residue, with some excretory matters added to it in the large
intestine, collects in the sigmoid flexure of the colon and in
the rectum, and is sent out of the Body from the latter.

The Digestion of an Ordinary Meal. We may best sum
up the facts stated in this chapter by considering the diges-
tion of a common meal; say a breakfast consisting of bread
and butter, beefsteak, potatoes and milk. Many of these
substances contain several alimentary principles, and, since
these are digested in different ways and in different parts of
the alimentary tract, the first thing to be done is to consider
what are the proximate constituents of each. We thus sepa-
rate the materials of the breakfast as in table on next page.

From such a meal we may first separate the elastin, cellu-
lose, and calcium sulphate, as indigestible and passed out of
the Body in the same state and in the same quantity as they
entered it. Then come the salines which need no special
digestion, and, taken either in solution or dissolved in the
saliva or gastric juice, are absorbed from the mouth, stomach,
and intestines without further change. Cane and grape
sugars experience the same lot, except that any cane sugar
or maltose reaching the intestines before absorption is
changed into grape sugar by the succus entericus. Calcium
phosphate will be dissolved by the free acid in the stomach,
yielding calcium chloride, which will be absorbed there or in
the intestine. Starch will be partially converted into maltose
ng mastication and deglutition, and it is possible that

e of this sugar may be absorbed from the stomach.

great part of the starch will, however, be passed on
into the intestine unchanged, since the action of saliva is
suspended in the stomach; and its conversion will be com-
pleted by the pancreatic secretion, and perhaps by the ptyalin,
though this is probably destroyed in the stomach by the gastric
juice; but in any case the starch will only have been changed
to maltose, and will need further digestive treatment. '-/The
various proteids will be partially dissolved in the stomach
and converted into peptone, which will in part be absorbed
there; the residue, with the undigested proteids, will be
passed on to the intestines. There the bile will precipitate
the peptones and parapeptones and, with the pancreatic
secretion, render the chyme alkaline, and so stop the activity

376

HUMAN BODY.

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DIGESTION. 377

of the gastric pepsin. The pancreatic secretion will, how-
ever, redissolve the precipitated peptone, and the unchanged
proteids and parapeptone and the albumose, and turn the
three last into peptone, breaking up part of this into leucin
and tyrosin ; these will be absorbed as they pass along the
small intestine; a small quantity perhaps passing into the
large intestine, to be taken up there. The fats will remain
unchanged until they enter the small intestine, except that
the proteid cell-walls of the adipose tissue of the beefsteak
will be dissolved away. In the small intestine some of these
little oil masses will be in part saponified, but most will be
emulsified and taken up into the lacteals in that condition.
Gelatin, from the white fibrous tissue of the beefsteak, will
undergo changes in the stomach and intestine, and be dis-
solved and absorbed.

The substances leaving the alimentary canal after such a
meal would be, primarily, the indigestible cellulose and
elastin, and some water. But there might also be some uriab-
sorbed fats, starch, and salts. To these would be added, in
the alimentary canal, mucin, some of the ferments of the di-
gestive secretions, some slightly altered bile pigments, and
other bodies excreted by the large intestine.

Dyspepsia is the common name of a number of diseased
conditions attended with loss of appetite or troublesome
digestion. Being often unattended with acute pain, and if
it kills at all doing so very slowly, it is pre-eminently suited
for treatment by domestic quackery. In reality, however,
the immediate cause of the symptoms, and the treatment
called for, may vary widely; and their detection and the
choice of the proper remedial agents often call for more than
ordinary medical skill. A few of the more common forms
of dyspepsia may be mentioned here, with their proximate
causes, not in order to enable people to undertake the rash
experiment of dosing themselves, but to show how wide a
chance there is for any unskilled treatment to miss its end,
and do more harm than good.

Appetite is primarily due to a condition of the mucous
membrane of the stomach which, in health, comes on after a
short fast, and stimulates.its sensory nerves; and loss of appe-
tite may be due to either of several causes. The stomach
may be apathetic and lack its' normal sensibility, so that the
empty condition does not act, as it normally does, as a sum*-

378 THE HUMAN BODY.

cient excitant. When food is taken it is a further stimulus
and may be enough ; in such cases " appetite comes with eat-
ing." A bitter before a meal is useful as an appetizer to
patients of this sort. On the other hand,, the stomach may
be too sensitive, and a voracious appetite be felt before a,
meal, which is replaced by nausea, or even vomiting, as soon
as a few mouthfuls have been swallowed; the extra stimulus
of the food then overstimulates the too irritable stomach,
just as a draught of mustard and warm water will a healthy
one. The proper treatment in such cases is a soothing one.
When food is taken it ought to stimulate the sensory gastric
nerves, so as to excite the reflex centres for the secretory
nerves, and for the dilatation of the blood-vessels of the
organ; if it does not, the gastric juice will be imperfectly
secreted. In such cases one may stimulate the secretory
nerves by weak alkalies, as certain mineral waters or a little
carbonate of soda, before meals; or give drugs, as strychnine,
which increase the irritability of reflex nerve-centres. The
vascular dilatation may be helped by warm drinks, and this is
probably the rationale of the glass of hot water after eating
which has often been found useful; the usual small cup of
hot coffee after dinner is a more agreeable form of the same
aid to digestion. In states of general debility, when the
stomach is too feeble to secrete under any stimulation, the
administration of weak acids and artificially prepared pepsin
is needed, to supply gastric juice from outside, until the im-
proved digestion strengthens the stomach up to the point of
being able to do its own work.

Enough has probably been said to show that dyspepsia is
not a disease, but a symptom accompanying many pathologi-
cal conditions, requiring special knowledge for their treat-
ment. From its nature — depriving the Body of its proper
nourishment — it tends to intensify itself, and so should never
be neglected; a stitch in time saves nine.

The Movements of the Intestines. When the abdomen
of a living anaesthetized animal is opened, especially during
digestion, contractions are seen slowly travelling along the
bowels, which have in consequence somewhat the appearance
of a writhing mass of worms, hence the name vermicular
often given to these movements: they are also called peri-
staltic. On observing a portion of the gut a narrowing due
to contraction of its circular muscular coat will be seen to

DIGESTION. 379

pass slowly along it, normally in a direction towards the
rectum; these contractions push before them part of the con-
tents of the intestine. The simultaneous contractions of the
outer longitudinal layer of the muscular coat are not so
marked or so easily directly observable. If the bowels be
entirely removed from the body of the animal the movements
go on for some time, so they are obviously not directly de-
pendent on extrinsic nerves. They are probably primarily
due to a slight automaticity of the muscle itself, which as in
the case of the heart (Chap. XVII) is favored by distension, but
they may be due to nerve impulses arising in the cells of the
plexus of Auerbach. As in the case of the heart these move-
ments are under control of extrinsic nerve-fibres, originating
in the cerebro-spinal centre, and these fibres are excitor and
depressor. Exactly contrary to that which we find in the
case of the heart, the fibres reaching the intestines through
the pneumogastrics are excitor, causing more powerful con-
tractions, and the fibres coming from the sympathetic through
the splanchnics (where they are mixed with but quite dis-
tinct from the vaso-constrictor fibres) are inhibitory. Stimu-
lation of the splanchnic nerves will bring actively contract-
ing intestines to rest. The influence of the central nervous
system on the motions of the bowel is shown by the contrac-
tions caused by fright or other strong emotions, illustrated
by the Hebrew phrase "bowels moved with compassion."
Deficiency of arterial blood excites powerful intestinal con-
tractions. The various purgative medicines act in very differ-
ent ways ; some directly on the intestinal neuro-muscular
apparatus; some on the extrinsic nerve centres concerned;
some (as Epsom salts) mainly by causing a great secretion of
liquid into the bowel and so distending it.

CHAPTER XXV.

THE RESPIRATORY MECHANISM.

.
3J

Definitions. Tttie blood as it flows from the right ventri-
cle of the heart, through the lungs, to the left auricle, loses
carbon dioxide and gains oxygen. In the systemic-- circula-
tion exactly the reverse changes take place, oxygen leaving
the blood to supply the living tissues; and carbon dioxide, gen-
erated in them, passing back into the blood capillaries. The
oxygen loss and carbon dioxide gain are associated with a
change in the color of the blood from bright scarlet to purple
red, or from arterial to venous; and the opposite changes in
the lungs restore to the dark blood its bright tint. The whole
set of processes through which blood becomes venous in the
systemic circulation and arterial in the pulmonary — in
other words the processes concerned in the gaseous reception,
distribution and elimination of the Body — constitute the
function of respiration ; so much of this as is concerned in
the interchanges between the blood and air being known as
external respiration / while the interchanges occurring in
the systemic capillaries, and the processes in general by
which oxygen is fixed and carbon dioxide formed by the liv-
ing tissues, are known as internal respiration. When the
term respiration is used alone, without any limiting adjective,
the external respiration only, is commonly meant.

Respiratory Organs. The blood being kept poor in oxy-
gen and rich in carbon dioxide by the action of the living
tissues, a certain amount of gaseous interchange will nearly
always take place when it comes into close proximity to the
.surrounding medium: whether this be the atmosphere itself
.or water containing air in solution. When an animal is
.small there are often no special organs for its external res-
piration, its general surface being sufficient (especially in
aquatic animals with a moist skin) to permit of all the gas-
•eous exchange that is necessary. In the simplest creatures,
.indeed, there is even no blood, the cell or cells composing

380

THE EE8PIRATORT MECHANISM. 381

them taking up for themselves from their environment the
oxygen which they need, and passing out into it their car-
bon dioxide waste; in other words, there is no differen-
tiation of the external and internal respirations. When,
however, an animal is larger many of its cells are so far from
a free surface that they cannot transact this give-and-take
with the surrounding medium directly, and the blood, or
some liquid representing it in this respect, serves as a mid-
dleman between the living tissues and the external oxygen;
and then one usually finds special respiratory organs devel-
oped, to which the blood is brought to make good its oxygen
loss and get rid of its excess of carbon dioxide. In aquatic
animals such organs take commonly the form of gills; these
are protrusions of the body over which a constant current of
water, containing oxygen in solution, is kept up; and in
which blood capillaries form a close network immediately be-
neath the surface. In air-breathing animals a different ar-
rangement is usually found. In some, as frogs, it is true, the
skin is always moist and serves as an important respiratory
organ, large quantities of venous blood being sent to it for
aeration. But for the occurrence of the necessary gaseous
diffusion, the skin must be kept very moist, and this, in a
terrestrial animal, necessitates a great amount of secretion by
the cutaneous glands to compensate for evaporation; accord-
ingly in most land animals the air is carried into the body
through tubes with narrow external orifices and so the drying
up of the breathing surfaces is greatly diminished; just as
water in a bottle with a narrow neck will evaporate much more
slowly than the same amount exposed in an open dish. In
insects (as bees, butterflies, and beetles) the air is carried by
tubes which split up into extremely fine branches and ramify
all through the body, even down to the individual tissue ele-
ments, which thus carry on their gaseous exchanges without
the intervention of blood. But in the great majority of
air-breathing animals the arrangement is different; the air-
tubes leading from the exterior of the body do not subdivide
into branches which ramify all through it, but open into one
or more large sacs to which the venous blood is brought, and
in whose walls it flows through a close capillary network.
Such respiratory sacs are called lungs, and it is a highly de-
yeloped form of them which is employed in the Human
Body.

The Air-Passages and Lungs. In our own Bodies some

382

THE HUMAN BODY.

small amount of respiration is carried on in the alimentary

canal, the air swallowed with food
or saliva undergoing gaseous ex-
changes with the blood in the gas-
tric and intestinal mucous mem-
branes. The amount of oxygen
thus obtained by the blood is
however very trivial, as is that
absorbed through the skin, cov-
ered as it is by its dry horny non-
vascular epidermis. All the really
essential gaseous interchanges be-
tween the Body and the atmos-
phere take place in the lungs, two
large sacs (lu, Fig. 1) lying in the
thoracic cavity, one on each side
of the heart. To these sacs the
air is conveyed through a series of
passages. Entering the pharynx
through the nostrils or mouth,
it passes out of this by the open-
ing leading into the larynx, or
voice-box (a, Fig. 122), lying in
the upper part of the neck (the communication of the two
is seen in Fig. 107) ;' from the larynx passes back the trachea
or windpipe, b, which, after entering the chest cavity, divides
into the right and left bronchi, d, e. Each bronchus divides
up into smaller and smaller branches, called bronchial tubes,
within the lung on its own side; and the smallest bronchial
tubes end in sacculated dilatations, the infundibula of the
lungs, the sacculations (Fig. 124) being the alveoli; the word
" cell " being here used in its prim-
itive sense of a small cavity, and
not in its later technical significa-
tion of a morphological unit of
the Body. On the walls of the
air-cells the pulmonary capillaries
ramify, and it is in them that the
interchanges of thje external res-
piration take place.

Structure of the Trachea and
Bronchi. The windpipe may
readily be felt in the middle line of the neck, a little below

Vis. 122. — The lungs and air-
passnges seen from the front. On
the left of the figure the pulmo-
nary tissue lias been dissected
away to show the ramifications of
the bronchial tubes, a, larynx ;
6, trachea ; d, right bronchus.
The left bronchus is seen entering
the root of its lung.

FIG. 123.— A small bronchial tube,
a, dividing into its terminal branch-
es, c ; these have pouched or saccu-
lated walls and end in the saccu-
lated infundibula, b.

THE RESPIRATORY MECHANISM. 383

Adam's apple, as a rigid cylindrical mass. It consists funda-
mentally of a fibrous tube in which cartilages are imbedded,
so as to keep it from collapsing; and is lined internally by a
mucous membrane covered by several layers of epithelium
cells, of which the superficial is ciliated. The elastic car-
tilages imbedded in its walls are imperfect rings, each some-
what the shape of a horse shoe, and the deficient part of
each ring being turned backwards, it comes to pass that the
deeper or dorsal side of the windpipe has no hard parts in it.
Against this side the gullet lies, and the absence there of the
cartilages no doubt facilitates swallowing. The bronchi re-
semble the windpipe in structure.

The Structure of the Lungs. These consist of the bron-
chial tubes and their terminal dilatations; numerous blood-
vessels, nerves and lymphatics; and an abundance of connec-
tive tissue, rich in elastic fibres, binding all together. The
bronchial tubes ramify in a tree-like manner (Fig. 122). In
structure the larger ones resemble the trachea, except that
the cartilage rings are not regularly arranged so as to have
their open parts all turned one way. As the tubes become
smaller their constituents thin away; the cartilages become
less frequent and finally disappear; the epithelium is re-
duced to a single layer of cells which, though still ciliated,
are much shorter than the columnar superficial cell-layer of
the larger tubes. The terminal
alveoli (a, a, Fig. 124), and the
air-cells, ft, which open into them,
have walls composed mainly of
elastic tissue and lined by a
single layer of flat, non-ciliated
epithelium, immediately beneath
which is a very close network
of capillary blood-vessels. The
air entering by the bronchial tube
is thus only separated from the
blood by the thin capillary wail
and the thin epithelium, both of
which are moist, and well adapted

,.~ . FIG. 124.— Two infuudibula of the

to permit gaSeOUS diffusion. lung: much magnified 6, 6. the air-

m, -TV, -™ -i i • cells, or hollow protrusions of the

The Pleura. Each lung IS alveolus, opening into its central

covered, except at one point, by ™'

an elastic serous membrane which adheres tightly to it and

384 THE HI MAN BODY.

c_/^

is called the pleura ; that point at which the pleura is
wanting is called the root of the lung and is on its
median side; it is there that its bronchus, blood-vessels and
nerves enter it. At the root of the lung the pleura turns
back and lines the inside of the chest cavity, as represented
by the dotted line in the diagram Fig. 3. The part of the
pleura attached to each lung is its visceral, and that attached
to the chest-wall its parietal layer. Each pleura thus forms
a closed sac surrounding a pleural cavity, in which, during
health, there are found a few drops of lymph, keeping its
surfaces moist. This lessens friction between the two layers
during the movements of the chest-walls and the lungs; for
although, to insure distinctness, the visceral and parietal
layers of the pleura are represented in the diagram as not in
contact, that is not the natural condition of things; the lungs
are in life distended so that the visceral pleura rubs against
the parietal, and the pleural cavity is practically obliterated.
This is due to the pressure of the atmosphere exerted through
the air-passages on the interior of the lungs. The lungs are
extremely elastic and distensible, and when the chest cavity
is perforated each shrivels up just as an indian-rubber blad-
der does when its neck is opened; the reason being that then
the air presses on the outside of each with as much force as
it does on the inside. These two pressures neutralizing one
another, there is nothing to overcome the tendency of the
lungs to collapse. So long as the chest-walls are whole, how-
ever, the lungs remain distended. The pleural sac is air-tight
and contains no air, and the pressure of the air around the
Body is borne by the rigid walls of the chest and prevented
from reaching the lungs; consequently no atmospheric pres-
sure is exerted on their outside. On their interior, however,
the atmosphere presses with its full weight,
equal to about 90 centigrams on a square
centimeter (14.5 Ibs. on the square inch), and
this is far more than sufficient to dis-
tend the lungs so as to make them com-
pletely fill all the parts of the thoracic cav-
ity not occupied by other organs. Suppose

'

me mngs'Khl^ho- bJ a cork through which two tubes pass,
rax. one of which, £, leads into an elastic bag,

d, and the other, c, provided with a stop-cock, opens freely

THE RESPIRATORY MECHANISM. 385

below into the bottle. When the stop-cock, c, is open
the air will enter the bottle and press there on the out-
side of the bag, as well as on its inside through b. The bag
will therefore collapse, as the lungs do when the chest -cavity
is opened. But if some air be sucked out through c the pres-
sure of that remaining in the bottle will diminish, and of that
inside thebag will be unchanged, and the bag will thus be blown
up, because the atmospheric pressure on its interior will not be
balanced by that on its exterior. At last, when all the air is
sucked out of the bottle and the stop-cock on c closed, the
bag, if sufficiently distensible, will be expanded so as to com-
pletely fill the bottle and press against its inside, and the
state of things will then answer to that naturally found in
the chest. If the bottle were now increased in size without
letting air into it, the bag would expand still more, so as to
fill it, and in so doing would receive air from outside through
6; and if the bottle then returned to its original size, its
walls would press on the bag and cause it to shrink and
expel some of its air through b. Exactly the same must of
course happen, under similar circumstances, in the chest, the
windpipe answering to the tube b through which air enters
or leaves the elastic sac.

The Respiratory Movements. The air taken into the
lungs soon becomes laden in them with carbon dioxide, and
at the same time loses much of its oxygen; these interchanges
take place mainly in the deep recesses of the alveoli, far from
the exterior and only communicating with it through a long
tract of narrow tubes. The alveolar air, thus become unfit
to any longer convert venous blood into arterial, could only
very slowly be renewed by gaseous diffusion with the atmos-
phere through the long air-passages — not nearly fast enough
for the requirements of the Body, as one learns by the sensa-
tion of suffocation which follows holding the breath for a
short time with mouth and larynx open. Consequently co-
operating with the lungs is a respiratory mechanism, by
which the air within them is periodically mixed with fresh
air taken from the outside, and also the air in the alveoli is
stirred up so as to bring fresh layers of it in contact with the
walls of the air-cells. This mixing is brought about by the
breathing movements, consisting of regularly alternating in-
spirations^ during which the chest cavity is enlarged and
fresh air enters the lungs, and expirations, in which the cav-

386 THE HUMAN BODY.

ity is diminished and air expelled from the lungs. When the
chest is enlarged the air the lungs contain immediately dis-
tends them so as to fill the larger space ; in so doing it be-
comes rarefied and less dense than the external air; and since
gases flow from points of greater to those of less pressure,
some outside air at once flows in by the air-passages and
enters the lungs. In expiration the reverse takes place. The
chest cavity, diminishing, presses on the lungs and makes the
air inside them denser than the external air, and so some
passes out until an equilibrium of pressure is restored. The
chest, in fact, acts very much like a bellows. When the bel-
lows are opened air enters in
consequence of the rarefaction
of that in the interior, which
is expanding to fill the larger
space; and when the bellows

Fi«. 126.-D5agramto illustrate the en- are cl°Sed again & is expelled.

try of air to the lungs when the thoracic To make the bellows Quite
cavity enlarges.

like the lungs we must, how-
ever, as in Fig. 126, have only one opening in them, that of
the nozzle, for both the entry and exit of the air; and this
opening should lead, not directly into the bellows cavity, but
into an elastic bag lying in it, and tied to the inner end of
the nozzle-pipe. This sac would represent the lungs and the
space between its outside and the inside of the bellows, the
pleural cavities.

We have next to see how the expansion and contraction
of the chest cavity are brought about.

The Structure of the Thorax. The thoracic cavity has
a conical form determined by the shape of its skeleton (Fig.
127), its narrower end being turned upwards. Dorsally, ven-
trally, and on the sides, it is supported by the rigid frame-
work afforded by the thoracic vertebrae, the breast-bone, and
the ribs. Between and over these lie muscles, and the
whole is covered in, air-tight, by the skin externally, and the
parietal layers of the pleurae inside. Above, its aperture is
closed by muscles and by various organs passing between the
thorax and the neck; and below it is bounded by the dia-
phragm, which forms a movable bottom to the, otherwise,
tolerably rigid box. In inspiration this box is increased in
all its diameters — dorso-ventrally, laterally, and from above
down.

THE RESPIRATORY MSCSANI8M,

381

The Vertical Enlargement of the Thorax. This is
brought about by the contraction of the diaphragm which
(Figs. 1 and 128) is a thin muscular sheet, with a fibrous
membrane, serving as a tendon, in its centre. In rest, the
diaphragm is dome-shaped, its concavity being turned towards
the abdomen. From the tendon on the crown of the dome
striped muscular fibres radiate, downwards and outwards, to
all sides; and are fixed by their inferior ends to the lower
ribs, the breast-bone, and the vertebral column. In expiration
the lower lateral portions of the diaphragm lie close against
the chest-walls, no lung intervening between them. In in-
spiration the muscular fibres, shortening, flatten the dome

FIG. 127.— The skeleton of the thorax, a, g, vertebral column; 6, first rib; c,
clavicle; d, third rib; i, glenoid fossa.

and enlarge the thoracic cavity at the expense of the ab-
dominal ; and at the same time its lateral portions are pulled
away from the chest-walls, leaving a space into which the
lower ends of the lungs expand. The contraction of the
diaphragm thus increases greatly the size of the thorax cham-
ber by adding to its lowest and widest part.

The Dorso-Ventral Enlargement of the Thorax. The
ribs on the whole slope downwards from the vertebral
column to the breast-bone, the slope being most marked
in the lower ones. During inspiration the breast-bone

388

THE HUMAN BOD7

and the sternal ends of the ribs attached to it are raised,
and so the distance between the sternum and the vertebral

a

Ql
FIG. 128.— The diaphragm seen from below.

column is increased. That this must be so will readily be
seen on considering the diagram Fig. 129, where db repre-
sents the vertebral column, c and d two
ribs, and st the sternum. The continu-
jl \sf ous lines represent the natural position
of the ribs at rest in expiration, and the
dotted lines the position in inspiration.
It is clear that when their lower ends
are raised, so as to make the bars lie in
a more horizontal plane, the sternum is
pushed away from the spine, and so the
chest cavity is increased dorso-ventrally.
The inspirator? elevation of the ribs is

FIG. 129.— Diagram illus- . f -

tratiug the dorso-ventrai mainly due to the action of the scalene

increase in the diameter of , . 7 . . , , _ rri,

the thorax when the ribs and external intercostal muscles. The
scalene muscles, three on each side, arise
from the cervical vertebras, and are inserted into the upper
ribs. The external intercostals (Fig. 113, A) lie between the
ribs and extend from the vertebral column to the costal carti-
lages; their fibres slope downwards and forwards. During
an inspiration the scalenes contract and fix the upper ribs
firmly; then the external intercostals shorten and each raises
the rib below it. The muscle, in fact, tends to pull together
the pair of ribs between which it lies, but as the upper one of
these is held tight by the scalenes and other muscles above,

THE RESPIRATORY MECHANISM. 389

the result is that the lower rib is pulled up, and not the upper
down. In this way the lower ribs are raised much more than
the upper, for the whole external intercostal muscles on each
side may be regarded as one great muscle with many bellies,
each bell)'' separated from the next by a tendon, represented
by the rib. When the whole muscular sheet is fixed above
and contracts, it is clear that its lower end will be raised more
than any intermediate point, since there is a greater length
of contracting muscle above it. The elevation of the ribs
tends to diminish the vertical diameter of the chest; this is

B

FIG. 130. — Portions of four ribs of a dog with the muscles between them, a, a,
ventral ends of the ribs, joining at c the rib cartilages, 6, which are fixed to carti-
laginous portions, d. of the sternum. A, external intercostal muscle, ceasing be-
tween the rib cartilages, where the internal intercostal, B. is seen. Between the
middle two ribs the external fntercostal muscle has been dissected away, so as to
display the internal which was covered by it.

more than compensated for by the simultaneous descent of
the diaphragm.

The Lateral Enlargement of the Chest is mainly due to
the diaphragm, which, when it contracts, adds to the lowest
and widest part of the conical chest cavity. Some small
widening is, however, brought about by a rotation of some of
the middle ribs which, as they are raised, roll round a little
at their vertebral articulations and twist their cartilages.
Each rib is curved and. if the bones be examined in their
natural position in a skeleton, it will be seen that the most

390 THE HUMAN BODY.

curved part lies below the level of a straight line drawn from
the vertebral to the sternal attachment of the bone. By the
rotation of the rib, during inspiration, this curved part is
raised and turned out, and the chest widened. The mech-
anism can be understood by clasping the hands opposite the
lower end of the sternum and a few inches in front of it,
with the elbows bent and pointing downwards. Each arm
will then answer, in an exaggerated way, to a curved rib, and
the clasped hands to the breast-bone. If the hands be sim-
ply raised a few inches by movement at the shoulder-joints
only, they will be separated farther from the front of the
Body, and rib elevation and the consequent dorso-ventral en-
largement of the cavity surrounded will be represented. But
if, simultaneously, the arms be rotated at the shoulder-joints
so as to raise the elbows and turn them out a little, it will be
seen that the space surrounded by the two arms is consider-
ably increased from side to side, as the chest cavity is in in-
spiration by the similar elevation of the most curved part or
'*' angle " of the middle ribs.

Expiration. To produce an inspiration requires consid-
erable muscular effort. The ribs and sternum have to be
raised; the elastic rib cartilages bent and somewhat twisted;
the abdominal viscera pushed down ; and the abdominal wall
pushed out to make room for them. In expiration, on the
contrary, but little, if any, muscular effort is needed. As
soon as the muscles which have raised the ribs and sternum
relax, these tend to return to their natural unconstrained
position, and the rib cartilages, also, to untwist themselves
and bring the ribs back to their position of rest; the elastic
abdominal wall presses the contained viscera against the
under side of the diaphragm, and pushes that up again as
soon as its muscular fibres cease contracting. By these means
the chest cavity is restored to its original capacity and the
air sent out of the lungs, rather by the elasticity of the parts
which were stretched or twisted in inspiration, than by any
special expiratory muscles.

Forced Respiration. When a very deep breath is drawn
or expelled, or when there is some impediment to the entry
or exit of the air, a great many muscles take part in produc-
ing the respiratory movements; and expiration then becomes,
in part, an actively muscular act. The main expiratory mus-
cles are the internal intercastals which lie beneath the exter-

THE RESPIRATORY MECHANISM. 391

nal between each pair of ribs (Fig. 130 B}, and have an oppo-
site direction, their fibres running upwards and forwards. In
forced expiration the lower ribs are fixed or pulled down by
muscles running in the abdominal wall from the pelvis to
them and to the breast-bone. The internal intercostals, con-
tracting, pull down the upper ribs and the sternum, and so
diminish the thoracic cavity dorso-ventrally. At the same
time, the contracted abdominal muscles press the walls of
that cavity against the viscera within it, and pushing these
up forcibly against the diaphragm make it very convex
towards the chest, and so diminish the latter in its vertical
diameter. In very violent expiration many other muscles
may co-operate, tending to fix points on which those muscles
which can directly diminish the thoracic cavity, pull. In
violent inspiration, also, many extra muscles are called into
play. The neck is held rigid to give the scalenes a firm at-
tachment; the shoulder-joint is held fixed and muscles going
from it to the chest-wall, and commonly serving to move the
arm, are then used to elevate the ribs; the head is held firm
on the vertebral column by the muscles going between the
two, and then other muscles, which pass from the collar-bone
and sternum to the skull, are used to pull up the former.
The muscles which are thus called into play in labored but
not in quiet breathing are called extraordinary muscles of
respiration.

The Respiratory Sounds. The entry and exit of air
are accompanied by respiratory sounds or murmurs, which
can be heard on applying the ear to the chest wall. The
character of these sounds is different and characteristic over
the trachea, the larger bronchial tubes, and portions of lung
from which large bronchial tubes are absent. They are vari-
ously modified in pulmonary affections, and hence the value
of auscultation of the lungs in assisting the physician to
form a diagnosis.

The Capacity of the Lungs. Since the chest cavity
never even approximately collapses, the lungs are never com-
pletely emptied of air: the space they have to occupy is
larger in inspiration than during expiration, but is always
considerable, so that after a forced expiration they still con-
tain a large amount of air which can only be expelled from
them by opening the pleural cavities; then they entirely col-
lapse, just as the bag in Fig. 125 would if the bottle inclosing

392 THE HUMAN BODY.

it were broken. The capacity of the chest, and therefore of
the lungs, varies much in different individuals, but in a man
of medium height there remain in the lungs after the most
violent possible expiration, about 1640 cub. cent. (100 cub.
inches) of air, called the residual air. After an ordinary
expiration there will be in addition to this about as much
more supplemental air ; the residual and supplemental to-
gether forming the stationary air, which remains in the
chest during quiet breathing. In an ordinary inspiration 500
cub. cent. (30 cub. inches) of tidal air are taken in, and
about the same amount is expelled in natural expiration.
By a forced inspiration about 1600 cub. cent. (98 cub. inches)
of complemental air can be added to the tidal air. After a
forced, inspiration therefore the chest will contain 1640 -f-
1640 -f- 500 -f 1600 = 5380 cubic centimeters (328 cubic
inches) of air. The amount which can be taken in by the
most violent possible inspiration after the strongest possible
expiration, that is, the supplemental, tidal, and complemental
air together, is known as the vital capacity. For a healthy
man 1.7 meters (5 feet 8 inches) high it is about 3700 cub.
cent. (225 cub. inches) and increases 60 cub. cent, for each
additional centimeter of stature; or about 9 cubic inches for
each inch of height.

The Quantity of Air Breathed Daily. Knowing the
quantity of air taken in at each breath and expelled again
(after more or less thorough admixture with the stationary air)
we have only to know, in addition, the rate at which the
breathing movements occur, to be able to calculate how
much air passes through the lungs in twenty-four hours.
The average number of respirations in a minute is found by
counting on persons sitting quietly, and not knowing that
their breathing rate is under observation, to be fifteen in a
minute. In each respiration half a liter (30 cubic inches) of
air is concerned; therefore 0.5 X 15 X 60 X 24 = 10,800
liters (375 cubic feet) is the quantity of air breathed under
ordinary circumstances by each person in a day.

Hygienic Remarks. Since the diaphragm when it con-
tracts pushes down the abdominal viscera beneath it, these
have to make room for themselves by pushing out the soft
front of the abdomen which, accordingly, protrudes when the
diaphragm descends. Hence breathing by the diaphragm,
being indicated on the exter.ior by movements of the abdo-

THE RESPIRATORY MECHANISM. 393

men, is often called " abdominal respiration," as distinguished
from breathing by the ribs, called " costal " or " chest breath-
ing." In both sexes the diaphragmatic breathing is the
most important, but, as a rule, men and children use the ribs
less than adult women. Since both abdomen and chest alter-
nately expand and contract in healthy breathing, anything
which impedes their free movement is to be avoided; and the
tight lacing which used to be thought elegant a few years
back, and is still indulged in by some who think a distorted
form beautiful, seriously impedes one of the most important
functions of the Body, leading, if nothing worse, to shortness
of breath and an incapacity for muscular exertion. In ex-
treme cases of tight lacing some organs are often directly
injured, weals of fibrous tissue being, for example, not unfre-
quently found developed on the liver, from the pressure of
the lower ribs forced against it by a tight corset.

The Aspiration of the Thorax. As already pointed out,
the external air cannot press directly upon the contents of
the thoracic cavity, on account of the rigid framework which
supports its walls; it still, however, presses on them indi-
rectly through the lungs. Pushing on the interior of these
with a pressure equal to that exerted on the same area by a
column of mercury 760 mm. (30 inches) high, it distends
them and forces them against the inside of the chest-walls,
the heart, the great thoracic blood-vessels, the thoracic-duct,
and the other contents of the chest-cavity. This pressure is
not equal to that of the external air, since some of the total
air-pressure on the inside of the lungs is used up in overcom-
ing their elasticity, and it is only the residue which pushes
them against the things outside them. In expiration this
residue is equal to that exerted by a column of mercury 754
mm. (29.8 inches) high. On most parts of the Body the at-
mospheric pressure acts, however, with full force. Pressing
on a limb it pushes the skin against the soft parts beneath,
and these compress the blood and lymph vessels among them;
and the yielding abdominal walls do not, like the rigid tho-
racic walls, carry the atmospheric pressure themselves, but
transmit it to the contents of the cavity. It thus comes to
pass that the blood and lymph in most parts of the Body are
under a higher atmospheric pressure than they are exposed
to in the chest, and consequently these liquids tend to flow
into the thorax, until the extra distention of the vessels in

394 THE HUMAN BODY.

which they there accumulate compensates for the less exter-
nal pressure to which those vessels are exposed. An equilib-
rium would thus very soon be brought about were it not for
the respiratory movements, in consequence of which the
intra-thoracic pressure is alternately increased and dimin-
ished, and the thorax comes to act as a sort of suction-pump
on the contents of the vessels of the Body outside it; thus
the respiratory movements influence the circulation of the
blood and the flow of the lymph.

Influence of the Respiratory Movements upon the Cir-
culation. Suppose the chest in a condition of normal expira-
tion and the external pressure on the blood in the blood-ves-
sels within it and in the heart, to have come, in the manner
pointed out in the last paragraph, into equilibrium with the
atmospheric pressure exerted on the blood-vessels of the neck
and abdomen. If an inspiration now occurs, the chest cavity
being enlarged the pressure on all of its contents will be di-
minished. In consequence, air enters the lungs from the
windpipe, and blood enters the venae cavae and the right au-
ricle of the heart. Thus not only the lungs, but the right
side of the heart, and the intra-thoracic portions of the sys-
temic veins leading to it, are expanded during an inspiration;
but the lungs being much the most distensible take far the
greatest part in filling up the increased space. The left side
of the heart is not much influenced as it is filled from the
pulmonary veins; and the whole vessels of the lesser circula-
tion lying within the chest, and being all affected in the
same way at the same time, the blood-flow in them is not di-
rectly influenced by the aspiration of the thorax. Distention
of the lungs seems, however, to diminish the capacity of their
vessels, and so to a certain extent the flow is influenced; as
the lungs expand blood is forced out of their vessels into the
left auricle, and when they again contract their vessels fill
up from the right ventricle. The pressure on the thoracic
aorta being diminished in inspiration, blood tends to flow back
into it from the abdominal portion of the vessel, but cannot
enter the heart on account of the semilunar valves; and the
back-flow does not in any case equal the onflow due to the
beat of the heart; so what happens in the aorta is but a
slight slowing of the current. The general result of all this
is that the circulation is considerably assisted. When the
next expiration occurs, and the pressure in the thorax again

THE RESPIRATORY MECHANISM. 395

rises, air and blood both tend to be expelled from the cavity.
The aorta thus regains what it lost during inspiration; the
pressure on it is increased and it empties itself faster into its
abdominal portion. The semilunar valves having prevented
any regurgitation into the heart, there is neither gain nor
loss so far as it is, concerned. With the systemic intra-tho-
racic veins, however, this is not the case; the extra blood en-
tering them has already in great part gone on beyond the
tricuspid valve, and cannot flow back during expiration ; and
the pressure in the auricle being constantly kept low by its
emptying into the ventricle, the increased pressure on the
venae cavae tends rather to send blood on into the heart, than
back into the extra-thoracic veins. Moreover, whatever
blood tends to take the latter course cannot do it effectually
since, although the venae cavae themselves contain no valves,
the more distant veins which open into them do. Conse-
quently, whatever extra blood has, to use the common phrase,
been '* sucked" into the intra-thoracic venae cavae in inspira-
tion and has not been sent already on into the right ventricle
before expiration occurs, is, on account of the venous valves,
imprisoned in the cavae under an increased pressure during
expiration; and this tends to make it flow faster into the au-
ricle during the diastole of the latter. How much the alter-
nating respiratory movements assist the venous flow is shown
by the dilation of the veins of the head and neck which oc-
curs when a person is holding his breath ; and the blackness
for the face, from distention of the veins and stagnation of
the capillary flow, which occurs during a prolonged fit of
coughing, which is a series of expiratory efforts without any
inspirations.

On the whole the influence of the respiratory movements
on the blood-flow is such as to favor it in inspiration and to
impede it during expiration. This influence very often shows
itself on tracings of arterial pressure taken as described in
Chap. XVIII. Such tracings usually show in addition to the
pulse waves, slower and greater rises and falls of pressure
which have the same rhythm as the respiration. In general,
the rise of pressure in these respiratory waves of blood-pres-
sure is synchronous with inspiration and the fall with expira--
tion, but not exactly. The changes manifest themselves on
the blood-pressure curve a little later than the commencement
of the thoracic movement which leads to them; the rise be-

396

THE HUMAN BODY.

ginning a little after the beginning of inspiration, the fall a
little later than the commencement of expiration.

In still another way the aspiration of the thorax assists
the heart. The heart and lungs are both distensible, though
in different degrees, and each is stretched in the chest some-
what beyond its natural size; the one by the atmospheric
pressure directly, the other by that pressure indirectly ex-
erted through the blood exposed to it in the extra-thoracic
veiiis. Supposing, therefore, the heart suddenly to shrink, it
would leave more space in the chest to be filled by the lungs;
these must accordingly, at each cardiac systole, expand a lit-
tle to fill the extra room, just as they do when the space
around them is otherwise enlarged, as during an inspiration.
The elasticity of the lungs, however, causes them to resist
this distention and oppose the cardiac systole. The matter
may be made clear by an arrangement like that in Fig. 131.
A is an air-tight vessel with a tube, e, provided with a stop-
cock, leading from it; b is a highly distensible elastic bag in
free communication through d with the
exterior; and c, representing the heart,
is a less extensible sac, from which a
tube leads and dips under water in the
vessel B. If air be pumped out through
e both bags will dilate, b filling with air,
and c with water driven up by atmos-
pheric pressure. Ultimately, if suffi-
ciently extensible, they would fill the
whole space, the thinner- walled, b, occu-
pying most of it. If then the stop-cock

be cl°Sed> thingS WJU. remain in ^quilib-

rium, each baff striving to collapse and

r

FIG. 131.-Diagram il-

lustrating the influence of

aspiration of the thorax

on the circulation of the so exerting a pull on the other, lor it o

blood. , T

shrinks c must expand and vice versa.
If c suddenly shrink, as the heart does in its systole, b will
dilate; but as soon as the systole of c ceases, b will shrink
again and pull c out to its previous size. In the same way,
after the cardiac systole, when the heart-walls relax, the lungs
pull them out again and dilate the organ. The contracting
heart thus expends some of its work in overcoming the elas-
ticity of the lungs, which opposes their expansion to fill the
space left by the smaller heart; but during the diastole of
the heart this work is utilized to pull out its walls again, and

THE RESPIRATORY MECHANISM. 397

draw blood into it. Since the normal heart has muscular
power, and to spare, for its systole, this arrangement, by
which some of the work then spent is stored away to assist
the diastole, which cannot be directly performed by cardiac
muscles, is of service to it on the whole. It is a physiological
though not a mechanical advantage; no work power is
gained, but what there is, is better distributed.

Influence of the Respiration on the Lymph-Flow.
During inspiration, when intra-thoracic pressure is lowered,
lymph is pressed into the thoracic duct from the abdominal
lymphatics. In expiration, when thoracic pressure rises
again, the extra lymph cannot flow back on account of the
valves in the lymphatic vessels, and it is consequently driven
on to the cervical ending of the thoracic duct. The breath-
ing movements thus pump the lymph on.

CHAPTER XXVI.
THE CHEMISTRY OF RESPIRATION.

Nature of the Problems. The study of the respiratory
process from a chemical standpoint has for its object to dis-
cover what are, in kind and extent, the interchanges between
the air in the lungs and the blood in the pulmonary capilla-
ries; and the nature and amount of the corresponding gaseous
•changes between the living tissues, and the blood in the sys-
temic capillaries. Neglecting some oxygen used up otherwise
than in forming carbon dioxide, and some carbon dioxide elim-
inated by other organs than the lungs, these processes in the
long-run balance, the blood losing as much carbon dioxide gas
in the lungs as it gains elsewhere, and gaining as much oxygen
in the lungs as it loses in the systemic capillaries. To compre-
hend the matter it is necessary to know the physical and chemical
conditions of these gases in the lungs, in the blood, and in the
tissues generally; for only so can we understand how it is that
in different localities of the Body such exactly contrary pro-
cesses occur. So far as the problems connected with the
•external respiration are concerned our knowledge is tolerably
•complete; but as regards the internal respiration, taking
place all through the Body, much has yet to be learnt;
we know that a muscle at work gives more carbon dioxide
to the blood than one at rest and takes more oxygen from
it, but how much of the one it gives and of the other it
takes is only known approximately; as are the conditions
under which this greater interchange during the activity
of the muscular tissue is effected: and concerning nearly
all the other issues we know even less than about muscle.
In fact, as regards the Body as a whole, it is compara-
tively easy to find how great its gaseous interchanges with
the air are during work and rest, waking and sleeping,

THE CHEMISTRY OF RESPIRATION. 399

while fasting or digesting, and so on ; but when it comes to
be decided what organs are concerned in each case in pro-
ducing the greater or less exchange, and how much of the
whole is due to each of them, the question is one far more
difficult to settle and still very far from completely answered.

The Changes Produced in Air by being once Breathed.
These are fourfold — changes in its temperature, in its mois-
ture, in its chemical composition, and its volume.

The air taken into the lungs is nearly always cooler than
that expired, which has a temperature of about 36° 0. (97°
F.). The temperature of a room is usually less than 21° C.
(70° F.). The warmer the inspired air the less, of course, the
heat which is lost to the Body in the breathing process; its
average amount is calculated as about equal to 50 calories in
twenty-four hours; a calorie being as much heat as will raise
the temperature of one kilogram (2.2 Ibs.) of water one degree
centigrade (1.8° F.).

The inspired air always contains more or less water vapor,
but is rarely saturated; that is, rarely contains so much but it
can take up more without showing it as mist ; the warmer air is,
the more water vapor it requires to saturate it. The expired
air is nearly saturated for the temperature at which it leaves
the Body, as is readily shown by the water deposited when it
is slightly cooled, as when a mirror is breathed upon; or by
the clouds seen issuing from the nostrils on a frosty day,
these being due to the fact that the air, as soon as it is cooled,
cannot hold all the water vapor which it took up when
warmed in the Body. Air, therefore, when breathed once,
gains water vapor and carries it off from the lungs; the
actual amount being subject to variation with the tempera-
ture and saturation of the inspired air: the cooler and drier
that is, the more water will it gain when breathed. On an
average the amount thus carried off in twenty-four hours is
about 255 grams (9 ounces). To evaporate this water in the
lungs an amount of heat is required, which disappears for
this purpose in the Body, to reappear again outside it when
the water vapor condenses. The amount of heat taken off in
this way during the day is about 148 calories. The total daily
loss of heat from the Body through the lungs is therefore
198 calories, 50 in warming the inspired air and 148 in the
evaporation of water.

The most important changes brought about in the

400 THE HUMAN BOD 7

breathed air are those in its chemical composition. .Pure
air when completely dried consists in each 100 parts of —

By Volume. By Weight.

Oxygen 20.8 23

Nitrogen 79.2 77

Ordinary atmospheric air contains in addition 4 volumes
of carbon dioxide in 10,000, or 0.04 in 100, a quantity which,
for practical purposes, may be neglected. When breathed
once, such air gains rather more than 4 volumes in 100 of
carbon dioxide, and loses rather more than 5 of oxygen.
More accurately, 100 volumes of expired air after drying give
98.9 volumes, which consist of —

Oxygen 15.4

Nitrogen 79.2

Carbon dioxide 4.3

The expired air also contains volatile organic substances
in quantities too minute for chemical analysis, but readily
detected by the nose upon coming into a close room in which
a number of persons have been collected.

Since 10,800 litres (375 cubic feet) of air are breathed in
twenty-four hours and lose 5.4 per cent of oxygen, the total
quantity of this gas taken up in the lungs daily is 10,800 X
5.4 ~ 100 = 583.2 litres (20.4 cubic feet). One litre of
oxygen measured at 0° C. (32° F.) and under a pressure equal
to one atmosphere, weighs 1.43 grams, so the total weight of
oxygen taken up by the lungs daily is 583.2 X 1.43 = 833.9
grams. Or, using inches and grains as standards, 44.5 cubic
inches of oxygen at the above temperature and pressure
weigh almost exactly 16 grains, so the 20.4 cubic feet ab-
sorbed in the lungs daily weigh 20.4 X 1728 -f- 44.5 X 16 =
12,818 grains.

The amount of carbon dioxide excreted from the lungs
being 4.3 per cent of the volume of the air breathed daily, is
10,800 X 4.3 -^ 100 = 464.4 litres (16.25 cubic feet) measured
at the normal temperature and pressure. This volume
weighs 910 grams, or 14,105 grains.

If the expired air be measured as it leaves the Body its
bulk will be found greater than that of the inspired air, since
it not only has water vapor added to it, but is expanded in
consequence of its higher temperature. If, however, it be
dried and reduced to the same temperature as the inspired

THE CHEMISTRY OF RESPIRATION. 401

air its volume will be found diminished, since it has lost 5.4
volumes per cent of oxygen and gained only 4.3 of carbon
dioxide. In round numbers, 100 volumes of dry inspired air
at zero, give 99 volumes of dry expired air measured at the
same temperature and pressure.

Ventilation. Since at every breath some oxygen is taken
from the air and some carbon dioxide given to it, were the
atmosphere around a living man not renewed he would, at
last, be unable to get from the air the oxygen he required; he
would die of oxygen starvation or be suffocated, as such a
mode of death is called, as surely, though not quite so fast, as
if he were put under the receiver of an air-pump and all the
air around him removed. Hence the necessity of ventilation
to supply fresh air in place of that breathed, and clearly the
amount of fresh air requisite must be determined by the
number of persons collected in a room; the supply which
would be ample for one person would be insufficient for two.
Moreover fires, gas, and oil lamps, all use up the oxygen of
the air and give carbon dioxide to it, and hence calculation
must be made for them in arranging for the ventilation of a
building in which they are to be employed.

In order that air be unwholesome to breathe, it is by no
means necessary that it have lost so much of its oxygen as to
make it difficult for the Body to get what it wants of that
gas. The evil results of insufficient air-supply are rarely, if
ever, due to that cause even in the worst-ventilated room for,
as we shall see hereafter, the blood is able to take what
oxygen it wants from air containing comparatively little of
that gas. The headache and drowsiness which come on from
sitting in a badly ventilated room, and the want of energy
and general ill-health which result from permanently living in
such, are dependent on a slow poisoning of the Body by the
reabsorption of the things eliminated from the lungs in
previous expirations. What these are is not accurately
known; they doubtless belong to those volatile bodies men-
tioned above, as carried off in minute quantities in each
breath; since observation shows that the air becomes injuri-
ous long before the amount of carbon dioxide in it is suffi-
cient to do any harm. Breathing air containing one or two
per cent of that gas produced by ordinary chemical methods
does no particular injury, but breathing air containing one
per cent of it produced by respiration is decidedly injurious,

402 THE HUMAN BODY.

because of the other things sent out of the lungs at the same
time. Carbon dioxide itself, at least in any such percentage
as is commonly found in a room, is not poisonous, as used to
be believed, but, since it is tolerably easily estimated in air,
while the actually injurious substances evolved in breathing
are not, the purity or foulness of the air in a room is usually
determined by finding the percentage of carbon dioxide
in it: it must be borne in mind that to mean much this
carbon dioxide must have been produced by breathing; the
amount of it found is in itself no guide to the quantity
of really important injurious substances present. Of course
when a great deal of carbon dioxide is present the air is
irrespirable : as for example sometimes at the bottom of
wells or brewing-vats.

In one minute .5 X 15 = 7.5 liters (0.254 cubic feet) of
air are breathed and this is vitiated with carbon dioxide
to the extent of rather more than four per cent; mixed
with three times its volume of external air, it would give
thirty liters (a little over one cubic foot) vitiated to the
extent of one per cent, and such air is not respirable for
any length of time with safety. The result of breathing it
for an evening is headache and general malaise; of breath-
ing it for weeks or months a lowered tone of the whole Body
—less power of work, physical or mental, and less power of
resisting disease; the ill effects may not show themselves at
once, and may accordingly be overlooked, or considered scien-
tific fancies, by the careless; but they are nevertheless there
ready to manifest themselves. In order to have air to breathe
in a fairly pure state every man should get for his own
allowance at least 23,000 liters of space to begin with
(about 800 cubic feet) and the arrangements for ventilation
should, at the very least, renew this at the rate of 30 litres
(one cubic foot) per minute. The nose is, however, the best
guide, and it is found that at least five times this supply of
fresh air is necessary to keep free from odor a small room
inhabited by one adult. In the more recently constructed
hospitals, as a result of experience, twice the above minimum
cubic space is allowed for each bed in a ward, and the re-
placement of the old air at a far more rapid rate is also
provided for.

Ventilation does not necessarily imply draughts of cold
air, as is too often supposed. In warming by indirect radia-

THE CHEMISTRY OF RESPIRATION. 403

tion it may readily be secured by arranging, in addition to
the registers from which the warmed air reaches the room,
proper openings at the opposite side, by which the old air
may pass off to make room for the fresh. An open fire in a
room will always keep up a current of air through it, and is
the healthiest, though nojb the most economical, method of
warming an apartment.

Stoves in a room, unless constantly supplied with fresh
air from without, dry its air to an unwholesome extent. If
no appliance for providing this supply exists in a room, it
can usually be got, without a draught, by fixing a board about
four inches wide under the lower sash and shutting the win-
dow down on it. Fresh air then comes in by the opening
between the two sashes and in a current directed upwards,
which gradually diffuses itself over the room without being
felt as a draught at any one point. In the method of heating
by direct radiation, the apparatus employed provides of itself
no means of drawing fresh air into a room, as the draught up
the chimney of an open fireplace or of a stove does; and
therefore special inlet and outlet openings are very necessary.
Since few doors and windows, fortunately, fit quite tight,
fresh air gets even into closed rooms, in tolerable abundance
for one or two inhabitants, if there be outlets for the air
already in them.

Changes undergone by the Blood in 'the Lungs. These
are the exact reverse of those undergone by the breathed air
— what the air gains the blood loses, and vice versa. Con-
'sequently, the blood loses heat, and water, and carbon dioxide
in the pulmonary capillaries; and gains oxygen. These
gains and losses are accompanied by a change of color from
the dark purple which the blood exhibits in the pulmonary
artery, to the bright scarlet it possesses in the pulmonary
veins.

The dependence of this color change upon the access of
fresh air to the lungs while the blood is flowing through
them, can be readily demonstrated. If a rabbit be rendered
unconscious by chloroform, and its chest be opened, after a
pair of bellows has been connected with its windpipe, it is
seen that, so long as the bellows are worked to keep up arti-
ficial respiration, the blood in the right side of the heart (as
seen through the thin auricle) and that in the pulmonary
artery, is dark colored, while that in the pulmonary veins

404 THE HUMAN BODY.

and the left auricle is bright red. Let, however, the artificial
respiration be stopped for a few seconds and, consequently,
the renewal of the air in the lungs (since an animal cannot
breathe for itself when its chest is opened), and very soon the
blood returns to the left auricle as dark as it left the right.
In a very short time symptoms of suffocation show them-
selves and the animal dies, unless the bellows be again set at
work.

The Blood Gases. If fresh blood be rapidly exposed to
as complete a vacuum as can be obtained, it gives off certain
gases, known as the gases of the Mood. These are the same
in kind, but differ in proportion, in venous and arterial
blood; there being more carbon dioxide and less oxygen ob-
tainable from the venous blood going to the lungs by the
pulmonary artery, than from the arterial blood coming back
to the heart by the pulmonary veins. The gases given off by
venous and arterial blood, measured under the normal pres-
sure and at the normal temperature, amount to from 58 to 62
volumes for every 100 volumes of blood, and in the two cases
are about as follows —

Venous Blood. Arterial Blood.

Oxygen 10 20

Carbon dioxide 46 40

Nitrogen 2 2

It is important to bear in mind that while arterial blood
contains some carbon dioxide that can be removed by the
air-pump, venous blood also contains some oxygen removable
in the same way; so that the difference between the two is
only one of degree. When an animal is killed by suffocation,
however, the last trace of oxygen which can be yielded up in
a vacuum disappears from the blood before the heart ceases
to beat. All the blood of such an animal is what might be
called suffocation blood, and has a far darker color than
ordinary venous blood.

The Cause of the Bright Color of Arterial Blood. The
color of the blood depends on its red corpuscles, since pure
blood plasma or blood serum is colorless, or at most a very
faint straw yellow. Hence the color change which the blood
experiences in circulating through the lungs must be due to
some change in its red corpuscles. Now, minute solid bodies
suspended in a liquid reflect more light when they are more
dense, other things being equal; and the first thing that sug-

THE CHEMISTRY OF RESPIRATION. 405

gests itself as the cause of the change in color of the blood is
that its red corpuscles have shrunk in the pulmonary circula-
tion, and so reflect more light and give the blood a brighter
look. This idea gains some support from the fact that, as
seen under the microscope, the red blood corpuscles of some
animals, as the frog, do expand somewhat when exposed to
carbon dioxide gas and shrink up a little in oxygen. But
that this is not the chief cause of the color change is readily
proved. By diluting blood with water the coloring matter of
the red corpuscles can be made to pass out of them and go
into solution in the plasma, and it is found that such a
solution, in which there can be no question as to the reflect-
ing powers of colored solid bodies suspended in it, is brighter
red when supplied with oxygen than when deprived of that
gas. This suggests that the coloring matter or hcemoglobin
of the red corpuscles combines with oxygen to form a scarlet
compound, and when deprived of that gas has a darker and
more purple color; and other experiments confirm this.
Haemoglobin combined with oxygen is known as oxyhcemo-
globin, and it is on its predominance that the color of arterial
blood depends. Haemoglobin uncombined with oxygen,
sometimes named reduced hmmoglolin, predominates in
venous blood, and is the only kind found in the blood of a
suffocated mammal.

The Laws Governing the Absorption of Gases by a
Liquid. In order to understand the condition of the gases
in the blood liquid it is necessary to recall the general laws
in accordance with which liquids absorb gases. They are as
follows :

1. A given volume of a liquid at a definite temperature if
it absorbs any of a gas to which it is exposed, and yet does
not combine chemically with it, takes up a definite volume
of the gas. If the gas be compressed the liquid will still, at
the same temperature, take up the same volume as before,
but now it takes up a greater weight; and a weight exactly
as much greater as the pressure is greater, since one volume
of a gas under any pressure contains exactly twice as much
of the gas by weight as the same volume under half the pres-
sure; and so on. A liter or a quart of water, for example,
exposed to the air will dissolve a certain amount of oxygen.
If the air (and therefore the oxygen in it) be compressed to
one fourth its bulk then the water will dissolve exactly the

406 THE HUMAN BODY.

same volume of oxygen as before, but this volume of the-
compressed gas will contain exactly four times as much
oxygen as did the same volume of the gas under the original
pressure; and if, now, the pressure be again diminished the
oxygen will be given off exactly in proportion as its pressure
on the surface of the water decreases. Finally, when a com-
plete vacuum is formed above the surface of the water, it will
be found that the latter has given off all its dissolved oxygen.
This law, that the quantity of a gas dissolved by a liquid
varies directly as the pressure of that gas on the surface of
the liquid is known as Dalton's law.

2. The amount of a gas dissolved by a liquid depends, not
on the total pressure exerted by all the gases pressing on its
surface, but on the fraction of the total pressure which is
exerted by the particular gas in question. For example, the
average atmospheric pressure is equal to that of a column
of mercury 760 mm. (30 inches) high. But 100 volumes of
air contain approximately 80 volumes of nitrogen and 20 of
oxygen : therefore -J of the total pressure is due to oxygen
and f to nitrogen : and the amount of oxygen absorbed by
water is just the same as if all the nitrogen were removed
from the air and its total pressure therefore reduced to -J- of
760 mm. (30 inches) of mercury; that is, to 152 mm. (6 inches)
of mercury pressure. It is only the fraction of the total
pressure exerted by the oxygen itself which affects the
quantity absorbed by water at any given temperature. So,
too, of all the atmospheric pressure ^ is due to nitrogen, and
all the oxygen might be removed from the air without affect-
ing the quantity of nitrogen which would be absorbed from
it by a given volume of water. The atmospheric pressure
would then be f of 760 mm. of mercury, or 608 mm. (24
inches), but it would all be due to nitrogen gas — and be
exactly equal to the fraction of the total pressure due to that
gas before the oxygen was removed from the air. When
several gases are mixed together the fraction of the total pres-
sure exerted by each one is known as the partial pressure of
that gas; and it is this partial pressure which determines the
amount of each individual gas dissolved by a liquid. If a
liquid exposed to the air for some time had taken up all the
oxygen and nitrogen it could at the partial pressures of
those gases in the air, and were then put in an atmosphere
in which the oxygen had all been replaced by nitrogen, it

THE CHEMISTRY OF RESPIRATION. 407

would now give off all its oxygen, since, although the total
gaseous pressure on it was the saihe, no part of it was any
longer due to oxygen; and at the same time it would take
up one fifth more nitrogen, since the whole gaseous pressure
on its surface was now due to that gas, while before only four
fifths of the total was exerted by it. If, on the contrary, the
liquid were exposed to pure hydrogen under a pressure of one
atmosphere it would give off all its previously dissolved oxygen
and nitrogen, since none of the pressure on its surface would
now be due to those gases; and would take up as much
hydrogen as corresponded to a pressure of that gas equal to
760 mm. of mercury (30 inches).

3. A liquid may be such as to combine chemically with a
gas. Then the amount of the gas absorbed is independent
of the partial pressure of the gas on the surface of the liquid.
The quantity absorbed will depend upon how much the
liquid can combine with. Or, a liquid may partly be com-
posed of things which simply dissolve a gas and partly of
things which chemically combine with it. Then the amount
of the gas taken up under a given partial pressure will de-
pend on two things; a certain portion, that merely dissolved,
will vary with the pressure of the gas in question; but an-
other portion, that chemically combined, will remain the
same under different pressures. The amount of this second
portion depends only on the amount of the substance in the
liquid which can chemically combine with it, and is totally
independent of the partial pressure of the gas.

4. Bodies are known which chemically combine with
certain gases when the partial pressure of these is consider-
able, forming compounds which break up, or dissociate,
liberating the gas, when its partial pressure falls below a
certain limit. Oxygen forms such a compound with heemo-
globin.

5. A membrane, moistened by a liquid in which a gas is
soluble, does not essentially alter the laws of absorption, by
a liquid on one side of it of a gas present on its other side,
whether the absorption be due to mere solution or to chem-
ical combinations or to both.

The Absorption of Oxygen by the Blood. Applying
the physical and chemical facts stated in the preceding
paragraph to the blood, we find that the blood contains (1)
plasma, which simply dissolves oxygen, and (2) JicKrm.oglobin>

408 THE HUMAN BODY.

which combines with it under some partial pressures of that
gas, but gives it up under lower.

Blood plasma or, what comes to the same thing, fresh'
serum, exposed to the air, takes up no more oxygen than so
much water: about 0.56 volumes of the gas for every 100 of
the liquid, at a temperature of 20° 0. At the temperature
of the Body the volume absorbed would be still less. This
quantity obeys Dalton's law.

If fresh whipped blood be employed, the quantity of oxy-
gen taken up is much greater; this extra quantity must be
taken up by the red corpuscles (in possessing which whipped
blood alone differs from blood serum) and it does not obey
Dalton's law. If the partial pressure of oxygen on the sur-
face of the whipped blood be doubled, only as much more
oxygen will be taken up as corresponds to that dissolved in
the serum ; and if the partial pressure of oxygen on its sur-
face be reduced to one half, only a very small amount of
oxygen (one half of that dissolved by the serum) will be given
off. All the much larger quantity taken up by the red corpus-
cles will be unaffected and must therefore be chemically com-
bined with something in them. Since 90 per cent of their
dry weight is haemoglobin, and this body when prepared
pure is found capable of combining with oxygen, there is
no doubt that it is the haemoglobin in the circulating blood
which carries around most of its oxygen. The red corpuscles
are so many little packages in which oxygen is stowed away.

The compound formed between oxygen and haemoglobin
is, however, a very feeble one; the two easily separate, and
always do so when the oxygen pressure in the liquid or gas
to which the oxyhaemoglobin is exposed falls below 25 mil-
limeters of mercury. Hence, in an air-pump, the blood only
gives off some of its small portion of merely dissolved oxygen,
until the pressure falls to about -J- of an atmosphere, that is
to -3$o. z= 125 mm. (5 inches) of mercury, of which total
pressure one fifth (25 millimeters or 1 inch) is due to the
oxygen present. As soon as this limit is passed the haemo-
globin gives up its oxygen with a rush.

Consequences of the Peculiar Way in which the Oxy-
gen of the Blood is Held. The first, and most important,
is that the blood can take up far more oxygen in the lungs
than would otherwise be possible. Since blood serum ex-
posed to pure oxygen takes up only 3 volumes for 100, blood

THE CHEMISTRY OF RESPIRATION. 409

exposed to the air would take up one fifth only of that amount
at ordinary temperatures, and still less at the temperature of
the Body, were it not for its haemoglobin. In the lungs even
less would be taken up, since the air in the air-cells of those
organs is poorer in oxygen than the external air; and conse-
quently the partial pressure of that gas in it is lower. The
tidal air taken in at each breath serves merely to renew
directly the air in the big bronchi; the deeper we examine
the pulmonary air the less oxygen and more carbon dioxide
would be found ; in the layers farthest from the exterior and
only renewed by diffusion with the air of the large bronchi,
it is estimated that the oxygen only exists in such quantity
that its partial pressure is equal to 130 millimeters of mer-
cury, instead of 152 as in ordinary air. In the second place,
on account of the way in which haemoglobin combines with
oxygen, the quantity of that gas taken up by the blood is
independent of such variations of its partial pressure in the
atmosphere as we are subjected to in daily life. At the top
of a high mountain, for example, the atmospheric pressure
is greatly diminished, but still mountaineers can breath
freely and get all the oxygen they want; the distress felt for
a time by persons unused to living in high altitudes is due
mainly to circulatory disturbances resulting from the low
atmospheric pressure. So long as the partial pressure of that
gas in the lung air-cells is above 25 millimeters of mercury,
the amount of it taken up by the blood depends on how much
haemoglobin there is in that liquid and not on how much
oxygen there is in the air. So, too, breathing pure oxygen
under a pressure of one atmosphere, or air compressed to
one half or a fourth its normal bulk, does not increase the
quantity of oxygen absorbed by the blood, apart from the small
extra quantity dissolved by the plasma. The widespread state-
ments as to the exhilaration caused by breathing pure oxygen
are erroneous, being founded on experiments made with im-
pure gas.

The General Oxygen Interchanges in the Blood. Sup-
pose we have a quantity of arterial blood in the aorta. This,
fresh from the lungs, will have its haemoglobin almost fully
combined with oxygen and in the state of oxyhsemoglobin.
In the blood plasma some more oxygen will be dissolved, viz.,
so much as answers to a pressure of that gas equal to 130
mm. (5.2 inches) of mercury, which is the partial pressure of

410 THE HUMAN BODY.

oxygen in the pulmonary air-cells. This tension of the gas
in the plasma will be more than sufficient to keep the haemo-
globin from giving off its oxygen. Suppose the blood now
enters the capillaries of a muscle. In the liquid i»oistening
this organ the oxygen tension is almost nil, since the tissue
elements are steadily taking -the gas up from the lymph
around them. Consequently, through the capillary walls,
the plasma will give off oxygen until the tension of that gas
in it falls below 25 millimeters of mercury. Immediately
some of the oxyhaemoglobin is decomposed, and the oxygen
liberated is dissolved in the plasma, and from there next
passed on to the lymph outside; and so the tension in the
plasma is once more lowered and more oxyhaemoglobin decom-
posed. This goes on so long as the blood is in the capillaries
of the muscle, or at any rate so long as the muscular fibres
keep on taking oxygen from the lymph bathing them; if
they cease to do so of course the tension of that gas in the
lymph will soon come to equal that in the plasma: the latter
will therefore cease to yield oxygen to the former; and so
maintain its tension (by the oxygen received from the last
decomposed oxyhaemoglobin) at a point which will prevent
the liberation of any more oxygen from such red corpuscles
as have not yet given all of theirs up. The blood will now go
on as ordinary venous blood into the veins of the muscle
and so back to the lungs. It will consist of (1) plasma with
oxygen dissolved in it at a tension of about 25 millimeters
(1 inch) of mercury. (2) A number of red corpuscles con-
taining reduced haemoglobin. (3) A number of red corpus-
cles containing oxyhaemoglobin. Or perhaps all of the- red
corpuscles will contain some reduced and some oxidized
haemoglobin. The relative proportion of reduced and un-
reduced haemoglobin will depend on how active the muscle
had been; if it worked while the blood flowed through it, it will
have used up more oxygen, and the blood leaving it will con-
sequently be more venous, than if it rested. This venous
blood, returning to the heart, is sent on to the pulmonary
capillaries. Here, the partial pressure of oxygen in the air-
cells being 130 mm. (5.2 inches) and that in the blood
plasma much less, oxygen will be taken up by the latter, and
the tension of that gas in the plasma tend to be raised above
the limit at which haemoglobin combines with it. Hence, as
fast as the plasma gets oxygen those red corpuscles which

THE CHEMISTRY OF RESPIRATION. 411

•contain any reduced haemoglobin rob it, and so its oxygen
tension is kept down below that in the air-cells until all the
haemoglobin is satisfied. Then the oxygen tension of the
plasma rises to that of the gas in the air-cells; no more
oxygen is absorbed, and the blood returns to the left auricle
of the heart in the same condition, so far as oxygen is con-
cerned, as when we commenced to follow it.

The Carbon Dioxide of the Blood. The same general
laws apply to this as to the blood oxygen. The gas is partly
merely dissolved and partly in a loose chemical combination
much like that of oxygen with haemoglobin, but the body
with which it combines probably exists in the plasma more
than in the red corpuscles; what it may be is not certainly
known. Besides this, some more carbon dioxide is stably
combined and is only given off on the addition of a stronger
acid. The partial pressure of carbon dioxide in the pulmo-
nary air-cells is about 40 mm. (1.6 inches) of mercury. There-
fore the tension of that gas in the pulmonary capillaries
must be more than this. On the other hand its tension in
arterial blood must be less than that in the lymph around
the tissues; otherwise it could not enter the blood in the
systemic circulation, which it does, as proved by the fact
that 100 vols. of venous blood give off 46 of this gas, and 100
vols. of arterial only 40.

The nitrogen dissolved in the blood is, so far as we know,
quite unimportant.

Internal Respiration. As to the amount of oxygen used
by each tissue and the quantity of carbon dioxide produced
by it we know but little; the following points seem, however,
tolerably certain:

1. The amount of carbon dioxide produced in an organ
in a given time bears no constant ratio to the amount of
oxygen taken up by it simultaneously. This is certainly
true of muscle, for experiment shows that muscular work
if really severe leads to an elimination of carbon dioxide
containing more oxygen than the total oxygen taken up from
the lungs at the same time. The balance is of course made
up in subsequent periods of rest, when more free oxygen is
taken up than is eliminated in combination during the same
time. Moreover, a frog's muscle excised from the body and
put in an atmosphere containing no oxygen and made there
•to contract, will evolve with each contraction considerable

412 THE HUMAN BODY.

quantities of carbon dioxide— although from the conditions
of the experiment it can receive from outside no uncombined
oxygen, and other experiments show that it contains none.
Hence the living muscular fibre must contain a substance
which is decomposed during activity and yields carbon
dioxide as one product of decomposition ; and this quite in-
dependent of any simultaneous direct oxidation.

2. What is true of muscle is probably true of most of the
tissues. During rest they take up oxygen and fix it in the
form of complex compounds, bodies which, like nitro-glycer-
ine, are readily decomposed into simpler, and in such decom-
positions liberate energy which is used by the working tissue.
One product of the decomposition is the highly oxidized
carbon dioxide, and this is eliminated; other products are
less oxidized, and possibly are not eliminated but built up
again, with fresh oxygen taken from the blood and fresh
carbon from the food, into the decomposable substance.

3. During the day a man gives off from his lungs more
oxygen in carbon dioxide, than he takes up by the same
organs from the air. During the night the reverse is the
case. This, however, has nothing to do with the alternating
periods of light and darkness, as it has in the case, of a green
plant, which in the light evolves more oxygen than it con-
sumes, and in the dark the contrary. It depends, rather, on
the fact that during the day more muscular effort is exerted
than at night, and the meals are then taken and digested.
The activity of the muscles and the digestive glands is de-
pendent on processes which give rise to a large production of
carbon dioxide and, during the night, when both are at rest,
more oxygen is taken up than is contained in the carbon
dioxide eliminated. If a man works and takes his meals at
night, and sleeps in the day, the usual ratios of his gaseous
exchanges with the exterior are entirely reversed.

4. The amount of work that a man's organs do, is not
dependent on the amount of oxygen supplied to them, but
the amount of oxygen used by him depends on how much he
uses his organs. The quantity of oxygen supplied must of
course always be, at least, that required to prevent suffoca-
tion; but an excess above this limit will not make the tissues
work. Just as a man must have a certain amount of food
to keep him alive, so he must have a certain amount of
oxygen; but as extra food will not make his tissues or Jiim

THE CHEMISTRY OF RESPIRATION. 413

(who is physiologically the sum of all his tissues) work,
apart from some stimulus to exertion, so it is with oxygen.
Highly arterialized blood, or an abnormal amount of blood,
flowing through an organ will nof arouse it to activity; the
working organ, muscle, or gland, for example, usually gets
enough more blood to supply its extra needs — just as a healthy
man who works will have a better appetite than an idle one;
but as taking more food by an idle man will not of itself
make him more energetic, so neither will sending more arterial
blood through an organ excite it to activity.

5. The preceding statement is confirmed by experiments
which show that an animal uses no more oxygen in an hour
when made to breathe that gas in a pure state, than when
allowed to breathe ordinary air. In other words, the amount
of oxygen an animal uses (provided it gets the minimum
necessary for health) is dependent only on how much it uses
its tissues. These (the rest in most cases subject to a certain
amount of control from the nervous) determine their own
activity, and this, in turn, how much oxygen shall be used in
the systemic circulation and restored in the pulmonary. In
other words, the physiological work of an animal, which of
course is largely dependent upon how external forces act upon
it, determines how much oxygen it uses daily; and not the
supply of oxygen how much its tissue activity shall be, unless
the supply sinks below* the starvation limit.

CHAPTER XXVII.

THE NERVOUS FACTORS OF THE RESPIRATORY
MECHANISM. ASPHYXIA.

The Respiratory Centre. The respiratory movements
are to a certain extent under the control of the will; we can
breathe faster or slower, shallower or more deeply, as we
wish, and can also " hold the breath " for some time — but the
voluntary control thus exerted is limited in extent; no one
can commit suicide by holding his breath. In ordinary quiet
breathing the movements are quite involuntary; they go- on
perfectly without the least attention on our part, and, not
only in sleep, but during the unconsciousness of fainting or
of an apoplectic fit. The natural breathing movements are
therefore either reflex or automatic.

The muscles concerned in producing the changes in the
chest which lead to the entry or exit of air are of the ordinary
striped kind; and these, as we have seen, only contract in the
Body under the influence of the nerves going to them; the
nerves of the diaphragm are the two phrenic nerves, one for
each side of it ; the external intercostal muscles are supplied
by certain branches of the thoracic spinal nerves, called the
intercostal nerves. If the phrenic nerves be cut the diaphragm
ceases its contractions, and a similar paralysis of the external
intercostals follows section of the intercostal nerves.

Since the inspiratory muscles only act when stimulated
by nervous impulses reaching them, we have next to seek
where these impulses originate; and experiment shows that
it is in the medulla oblongata. All the brain of a cat or a
rabbit in front of the medulla can be removed, and it will
still go on breathing; and children are sometimes born with
the medulla oblongata only, the rest of the brain being un-
developed, and yet they breathe for a time. If, on the
other hand, the spinal cord be divided immediately below
the medulla of an animal, all breathing movements of the

414

TEE RESPIRATORY MECHANISM. 415

chest cease at once. We conclude, therefore, that the nerv-
ous impulses calling forth contractions of the respiratory
muscles arise in the medulla oblongata, and travel down the
spinal cord and thence out along the phrenic and intercostal
nerves. This is confirmed by the fact that if the spinal cord
be cut across below the origin of the fourth pair of cervical
spinal nerves (from which the phrenics mainly arise) but
above the first thoracic spinal nerves, the respiratory move-
ments of the diaphragm continue, but those of the intercostal
muscles cease; this phenomenon has sometimes been observed
on men so stabbed in the back as to divide the spinal cord in
the region indicated. - Finally, that the nervous impulses ex-
citing the inspiratory muscles originate in the medulla, is
proved by the fact that if a small portion of that organ, the
so-called vital point, be destroyed, all the respiratory move-
ments cease at once and forever, although all the rest of the
brain and spinal cord may be left uninjured. This part of
the medulla is known as the respiratory centre. The im-
pulses proceeding from it probably do not pass directly to
the motor nerve-fibres concerned, but first to subsidiary
centres in the cord, from which properly co-ordinated impulses
are sent to the muscles concerned. Occasionally in young
animals, especially after a small dose of strychnia has been
administered, a few respiratory movements are seen after
•section of the cord high up in the neck. But the broad
general fact remains, that in. the normal working of the Body
the spinal respiratory centres only send out respiration-caus-
ing impulses when excited by impulses descending to them
from the main respiratory centre in the medulla.

In the above statements, attention has been chiefly con-
fined to the diaphragm and the intercostal muscles; but
what is said of them is true of the respiratory innervation of
all other breathing muscles, whether expiratory or inspira-
tory, normal or extraordinary.

Is the Respiratory Centre Reflex? Since this centre
goes on working independently of the will, we have next to
inquire is it a reflex centre or not ? are the efferent discharges
it sends along the respiratory nerves due to afferent impulses
reaching it by centripetal nerve-fibres ? or does it originate
efferent nervous impulses independently of excitation through
afferent nerves?

We knew, in the first place, that the respiratory centre is

416 THE HUMAN BODY.

largely under reflex control; a dash of cold water on the
skin, the irritation of the nasal mucous membrane by snuff,
or of the larynx by a foreign body, will each cause a modifi-
cation in the respiratory movements — a long indrawn breath,
a sneeze, or a cough. But, although thus very subject to
influences reaching it by afferent nerves, the respiratory
centre seems essentially independent of such. In many ani-
mals, as rabbits (and in some men), marked breathing move-
ments take place in the nostrils, which dilate during inspira-
tion ; and when the spinal cord of a rabbit is cut close to the
medulla, thus cutting off all afferent nervous impulses to the
respiratory centre except such as may reach it through cranial
nerves, the respiratory movements of the nostrils still con-
tinue until death. The movements of the ribs and dia-
phragm of course cease, and so the animal dies very soon
unless artificial respiration be maintained. Moreover, if
after cutting the spinal cord as above described, the chief
sensory cranial nerves be divided, so as to cut off the respira-
tory centre from almost all possible afferent nervous im-
pulses, the regular breathing movements of the nostrils con-
tinue. It is, therefore, nearly certain that the activity of the
respiratory centre, however much it may be capable of modi-
fication through sensory nerves, is essentially independent of
them ; in other words the normal respiratory movements are
not reflex.

What it is that Excites the Respiratory Centre. The
thing that, above all others, influences the respiratory centre
is the greater or less venosity of the blood flowing through it.
If this blood be very rich in oxygen and comparatively poor
in carbon dioxide the respiratory centre acts but feebly, and
the respirations are shallow. If, on the other hand, this
blood be highly venous the respiratory movements are more
rapid than normal, and are forced, the extraordinary muscles
of respiration being called into play; this state of violent
labored respiration, due to deficient aeration of the blood is
called dyspnma. Normal quiet breathing is eupnosa. If
active artificial respiration be kept up on an animal for a
short time, it is found, on its cessation, that the creature
(dog or rabbit) makes no attempt to breathe for a period
which may extend to one and a half minutes. TMs breath-
less condition, in which an animal with no hindrance opposed
to its breathing makes no respiratory movement is apncea.

THE RESPIRATORY MECHANISM. 417

Apnoea used to be ascribed solely to an overloading of the
blood with oxygen, but the haemoglobin of the blood leaving
the lungs is normally so nearly saturated with that gas that
this explanation is not sufficient. The apnceic state is in
part due no doubt to the high percentage of oxygen in the
air-cells of the lungs, brought about by the active artificial
ventilation. The blood, as it flows through the lungs, is thus
able to supply itself with oxygen for some time without any
renewal of the air within them. But even this is not the
whole matter, for an animal made apnceic will often continue
so after its arterial blood has become distinctly venous in
color; and an animal may, if its pneumogastric nerves be
intact, be rendered apnceic for a short time by rapid insuffla-
tion of its lungs with an indifferent gas. In fact, there is
evidence that distention of the lungs tends to inhibit the
sending out of impulses to the inspiratory muscles, the
afferent fibres exerting this inhibitory action on the centre
taking their course in the pulmonary branches of the pneu-
mogastric; and this inhibition plays a part in the production
of apnoea. It should be noted that by apncea physicians
usually mean only extreme dyspnoea.

How venous blood causes great excitation of the respira-
tory centre is not certainly known. We may make the
following provisional hypothesis: the chemical changes
occurring in the respiratory centre produce a substance
which stimulates its nerve-cells; when the. blood is richly
oxygenated this substance is oxidized as fast as it is formed,
and the centre is not excited ; but when the blood is poor in
oxygen, the stimulating body accumulates and the respiratory
discharges become powerful. Under normal circumstances
the oxygen is not kept up to the point of entirely removing
this exciting substance, and 'the centre is stimulated so as to
produce the natural breathing movements. That the stimu-
lant acts upon the respiratory centre itself, and not upon
other organs of the Body and through their sensory nerves
upon the medulla, is proved by experiments which show that
the circulation of venous blood through the trunk and limbs
of an animal, while its respiratory centre is supplied with
arterial blood, does not produce dyspnoea.

Why are the Respiratory Discharges Rhythmic ? Every
complete respiratory act consists of an inspiration, an expira-
tion and a pause; and then follows the inspiration of the

418 THE HUMAN BODY.

next act. In natural quiet breathing there is no essential
difference between the expiration and the pause. The in-
spiration is the only active part; the expiration and the
pause are dependent on muscular inactivity and, there-
fore, on the cessation of the discharge of nervous impulses
from the respiratory centre. But then, we may ask, if in
accordance with the hypothesis made in the last paragraph,
the respiratory centre is constantly being excited, why is it
not always discharging ? why does it only send out nervous
impulses at intervals? This question, which is essentially
the same as that why the heart beats rhythmically, belongs
to the higher regions of Physiology and can only at present
be hypothetically answered. Let us consider, for a moment,
ordinary mechanical circumstances under which a steady
supply is turned into an intermittent discharge. Suppose a
tube closed water-tight below by a hinged bottom, which is
kept shut by a spring. If a steady stream of water is poured
into the tube from above, the water will rise until its weight
is able to overcome the pressure of the spring, and the bottom
will then be forced down and some water flow out. The
spring will then press the bottom up again, and the water
accumulate until its weight again forces open the bottom of
the tube, and there is another outrush; and so on. By
opposing a certain resistance to the exit we could thus turn
a steady inflow into a rhythmic outflow. Or, take the case
of a tube with one end immersed in water and a steady
stream of air blown into its other end. The air will emerge
from the immersed end, not in a steady current, but in a
series of bubbles. Its pressure in the tube must rise until
it is able to overcome the cohesive force of the water, and
then a bubble bursts forth; after this the air has again to
get up the requisite pressure in the tube before another
bubble is ejected; and so the continuous supply is trans-
formed into an intermittent delivery. Physiologists sup-
pose something of the same kind to occur in the respiratory
centre. Its nerve-cells are always, under usual circum-
tances, being excited; but, to discharge a nervous impulse
along the efferent respiratory nerves, they have to overcome
a certain resistance. The nervous impulses have to accumu-
late, or " gain a head," before they travel out from the
centre, and, after their discharge, time is required to attain
once more the necessary level of irruption before a fresh in-

THE RESPIRATORY MECHANISM. 419

nervation is sent to the muscles. This method of account-
ing for the respiratory rhythm is known as the " resistance
theory." If not altogether satisfactory it is at least far
preferable to the older mode of considering the question
solved by assuming a rhythmic character or property of the
respiratory centre. It gives a definite hypothesis, which
accords with what is known of general natural laws outside
of the Body, and the validity of which can be subjected to
experiment: and so serves very well to show how scientific
differs from pre-scientific, or mediaeval, physiology. The
latter was content with observing things in the Body and
considered it explained a phenomenon when it gave it a
name. Now we call a phenomenon explained, when we have
found to what general category of natural laws it can be
reduced as a special example; and this reducing a special
case to a particular manifestation of some one or more
general properties of matter already known is, of course, all
that we ever mean when we say we explain anything. We
explain the fall of an apple and the rise of the tides by
referring them to the class of general results of the law of
gravitation; but the why of the law of gravitation we do not
know at all; it is merely a fact which we have found out.
So with regard to Physiology; we are working scientifically
when we try to reduce the activities of the living Body to-
special instances of mechanical, physical, or chemical laws
otherwise known to us, and unscientifically when we lose
sight of that aim. Certain vital phenomena, as those of
blood-pressure, we can thus explain, as much as we can ex-
plain anything; others, as the rhythm of the respiratory
movements, we can provisionally explain, although not yet
certain that our explanation is the right one; and still
others, as the phenomena of consciousness, we cannot explain
at all, and possibly never shall, by referring them to general
properties of matter, since they may be associated only with
that particular kind of matter called protoplasm, and per-
haps only with some varieties of it.

The Relation of the Pneumogastric Nerves to the Re-
spiratory Centre. We have next to consider if any phenom-
ena presented by the living Body give support to the resist-
ance theory of the respiratory rhythm. A very important
collateral prop to it is given by the relation of the pneu mo-
gastric nerves to the rate and force of the respiratory move-

420 THE HUMAN BODY.

ments. These nerves give branches to the larynx, the wind-
pipe, and the lungs, and might therefore be suspected to
have something to do with breathing. Indeed at one time it
was maintained that the breathing movements were purely
reflex, the afferent fibres running in the pneumogastrics
from the lungs to the respiratory centre. That the vagi are
not concerned in influencing the respiratory muscles directly
is shown by the fact that all of these muscles (except certain
small ones in the larynx) contract as usual in breathing after
both pneumogastric nerves have been divided. Still, the
section of both nerve's has a considerable influence on the
respiratory movements; they become slower and deeper.
We may understand this by supposing that the resistance to
the discharges of the respiratory centre is liable to variation.
It may be increased, and then the discharges will be fewer
and larger; or diminished, and then they will be more fre-
quent but each one less powerful. If the spring, in the
illustration used in the preceding paragraph, be made stronger,
while the inflow of water to the tube remains the same, the
outflows will be less frequent but each one greater; and vice
versa. The effect of section of the pneumogastric trunk
may, therefore, be explained if we suppose that, normally, it
carries up, from its lung branches, nervous impulses which
diminish the resistance to the discharges of the respiratory
centre ; when the nerves are cut these helping impulses are
lost to the centre, and its impulses must gather more head
before they break out, but will be greater when they do.
This view is confirmed by the fact that stimulation of the
central ends of the divided pneumogastrics, if weak, brings
back the respirations to their normal rate and force; if
stronger makes them more rapid and shallower; and when
stronger still, abolishes the respiratory rhythm altogether,
with the inspiratory muscles in a steady state of feeble con-
traction. That is to say, the resistance to the discharges of
the centre being entirely taken away (which is equivalent to
the total removal of the spring in our example), the centre
sends out uninterrupted and non -rhythmic stimuli to the
inspiratory muscles.

The pneumogastric nerve gives two branches to the
larynx; known respectively as the superior and inferior (re-
current) laryngeal nerves; the action of these on the respira-
tory centre is opposite to that of the fibres from the lungs

THE RESPIRATORY MECHANISM. 421

coming up in the main pneumogastric trunk. If the supe-
rior laryngeal branch be divided and its central end stimu-
lated, the respirations become less frequent but each one
more powerful; hence this nerve appears to contain fibres
which increase the resistance to inspiratory discharges from
the respiratory centre. The same, but to a less degree, is true
of the inferior laryngeal branch. Both are inhibitory fibres
so far as inspiration is concerned; whereas the main vagus
stem when its central end is electrically stimulated is acceler-
ator or augmentor.

The Expiratory Centre. Hitherto we have considered
breathing as due to the rhythmically alternating activity and
rest of an inspiratory centre — and such is the case in normal
quiet breathing, in which the expirations are passive. But
in dyspnoea expiration is a muscular act, and so there must
be a section of the respiratory centre controlling the expira-
tory muscles, and we may regard the whole centre as consist-
ing really of two; an inspiratory and expiratory. The latter
part of the respiratory centre, however, is less irritable than
the inspiratory part, and hence when the blood is in a normal
state of aeration never gets stimulated up to the discharging
point. In dyspncea the stimulus becomes sufficient to cause
it also to discharge, but only after the more irritable inspira-
tory centre; hence the expiration follows the inspiration.
This alternation of activity is, moreover, promoted by the fact
that the pneumogastric nerve-fibres coming up from the
lungs are of two kinds. The predominant sort are the
accelerator set already referred to, which favor discharge of
the inspiratory centre, and perhaps also increase the resist-
ance to the expiratory discharge. This set is excited when
the lungs diminish in bulk, as in expiration; and when the
whole nerve is stimulated electrically they usually get the
better of the other set, which carry up to the medulla im-
pulses which inhibit inspiratory discharges. This set is
stimulated by expansion of -the lungs, even in quiet breath-
ing: and they play a part in producing the phenomenon
of apncea. When the distention of the lungs is con-
siderable these fibres not only check inspiration but favor
expiratory movements. Hence, every expansion of the lungs
(inspiration) tends to promote an expiration, and every col-
lapse of the lungs (expiration) tends to produce an inspira-

422 THE HUMAN BODY.

tion; and so, through the pneumogastric nerves, the respira-
tory mechanism is largely self -regulating.

Asphyxia. Asphyxia is death from suffocation, or want
of oxygen by the tissues. It may be brought about in
various ways; as by strangulation, which prevents the entry
of air into the lungs; or by exposure in an atmosphere con-
taining no oxygen; or by putting an animal in a vacuum;
or by making it breathe air containing a gas which has a
stronger affinity for haemoglobin than oxygen has, and which,
therefore, turns the oxygen out of the red corpuscles and
takes its place. The gases which do the latter are very in-
teresting since they serve to prove conclusively that the Body
can only live by the oxygen carried round by the haemoglobin
of the red corpuscles; that amount dissolved in the blood
plasma being insufficient for its needs. Of such gases carbon
monoxide is the most important and best studied; in the fre-
quent French mode of committing suicide by stopping up all
the ventilation holes of a room and burning charcoal in it, it
is poisoning by carbon monoxide which causes death.

The Relations of Carbon Monoxide to Haemoglobin.
If aerated whipped blood, or a solution of oxyhyaemoglobin,
be exposed to a gaseous mixture containing carbon monoxide,
the liquid will absorb the latter gas and give off oxygen.
The amount of carbon monoxide taken up will (apart from
a small amount dissolved in the plasma) be independent of
the partial pressure of that gas in the gaseous mixture to
which the blood is exposed ; the quantity absorbed depends
on the quantity of haemoglobin in the liquid, and is replaced
by an equal volume of oxygen liberated. This equivalence of
volume, of itself, proves that the phenomenon is due to the
chemical replacement of oxygen in some compound, by the
carbon monoxide; for if the carbon monoxide were merely
dissolved in the liquid in proportion to its partial pressure on
the surface, it would turn out no oxygen; the quantity of
dissolved gases held by a liquid being dependent only on the
partial pressure of each individual gas on its surface, and
unaffected by that of all others. During the taking up of
carbon monoxide the blood changes color in a way that can
be recognized by a practised eye; it becomes cherry-red in-
stead of scarlet. This shows that some new chemical com-
pound has been formed in it; examination with the spectro-
scope confirms this, and shows the color change to be due to

THE RESPIRATORY MECHANISM. 423

the formation of carbon-monoxide haemoglobin which has a
different color from oxyhaemoglobin. A dilute solution of
reduced haemoglobin absorbs all the rays of light in one
region about the green of the solar spectrum, and so pro-
duces there a dark band; a thin layer of the blood of an
asphyxiated animal does the same. Dilute solution of oxy-
haemoglobin absorbs the rays in two narrow regions of the
solar spectrum at the • confines of the yellow and green, and
arterial blood does the same. Dilute solution of carbon-
monoxide haemoglobin, or blood which has been exposed to
this gas, also absorbs the light in two narrow bands of the
>.olar spectrum; but these are a little nearer the blue end of
the spectrum than the absorption bands of oxyhaemoglobin.
Pure blood serum saturated with oxygen gas or with carbon
monoxide does not specially absorb any part of the spectrum;
therefore the absorptions when haemoglobin is present must
be due to chemical compounds of those gases with that body.

Since carbon-monoxide haemoglobin has a bright-red color,
we find, in the Bodies of persons poisoned by that gae, the
blood all through the Body cherry-red; the tissues being
unable to take carbon monoxide from haemoglobin in the
systemic circulation. Hence the curious fact that, while
death is really due to asphyxia, the blood is almost the color
of arterial blood, instead of very dark purple, as in ordinary
cases of death by suffocation. Experiments with animals
show that in poisoning by carbon monoxide persistent ex-
posure of the blood to oxygen, by means of artificial respira-
tion, will cause the poisonous gas to be slowly replaced again
by oxygen; hence if the heart has not yet quite stopped
beating, artificial respiration, kept up patiently, should be
employed in the case of poisoning by carbon monoxide unless
transfusion of blood be possible.

The Phenomena of Asphyxia. As soon as the oxygen
in the blood falls below the normal amount the breathing
becomes hurried and deeper, and the extraordinary muscles
of respiration are called into activity. The dyspnoea be-
comes more and more marked, and this is especially the case
with the expirations which, almost or quite passively per-
formed in natural breathing, become violently muscular. At
last nearly all the muscles in the Body are set at work; the
rhythmic character of the respiratory acts is lost, and general
convulsions occur, but, on the whole, the contractions of the

424 THE HUMAN BODY.

expiratory muscles are more violent than those of the inspira-
tory. Thus undue want of oxygen at first merely brings
about an increased activity of the respiratory centre, and
especially of its expiratory division which is not excited in
normal breathing. Then it stimulates other portions (the
convulsive centre) of the medulla oblongata also, and gives
rise to violent and irregular muscular spasms. That the
convulsions are due to excitation of nerve-centres in the
medulla (and not, as might be supposed, to poisoning of the
muscles or of the fore parts of the brain by the extremely
venous blood) is shown of the facts (1) that they do not
occur in the trunk' of an animal when the spinal cord has
been divided in the neck so as to cut off the muscles from
the medulla; and (2) that they still occur if (the spinal cord
remaining undivided) all the parts of the brain in front of
the medulla have been removed.

The violent excitation of the nerve-centres soon exhausts
them, and all, the more readily since their oxygen supply
(which they like all other tissues need in order to continue
their activity) is cut off. The convulsions therefore gradu-
ally cease, and the animal becomes calm again, save for an
occasional act of breathing when the oxygen want becomes
so great as to lead to efficient stimulation even of the dying
respiratory centre: these final movements are inspirations
and, becoming less and. less frequent, at last cease, and the
animal appears dead. Its heart, however, though gorged
with extremely dark venous blood still makes some slow
feeble pulsations. So long as it beats artificial respiration can
restore the animal, but once the heart has finally stopped
restoration is impossible. There are thus three distinguish-
able stages in death from asphyxia. (1) The stage of
dyspnoea. (2) The stage of convulsions. (3) The stage of
exhaustion; the convulsions having ceased but there being
from time to time an inspiration. The end of the third
stage occurs in a mammal about five minutes after the
oxygen supply has been totally cut off. If the asphyxia be
due to deficiency, and not absolute want of oxygen, of course
all the stages take longer.

Circulatory Changes in Asphyxia. During death by
suffocation characteristic changes occur in the working of
the heart and blood-vessels. The heart at first beats quicker,
but very soon, before the end of the dyspnoeic stage, more

THE RESPIRATORY MECHANISM. 425

slowly, though, at first, more powerfully. This slowing is
due to the fact that the unusual want of oxygen leads to
stimulation of the cardio-inhibitory centre in the medulla
and this, through the pneumogastric nerves, slows the
heart's beat. Soon, however, the want of oxygen affects
the heart itself and it begins to beat more feebly, and also
more slowly, from exhaustion, until its final stoppage. Dur-
ing the second and third stages the heart and the venae cavae
become greatly overfilled with blood, because the violent
muscular contractions facilitate the flow of blood to the
heart, while its beats become too feeble to send it out again.
The overfilling is most marked on the right side of the heart
which receives the venous blood from the Body generally.

During the first and second stages of asphyxia arterial
pressure rises in a marked degree. This is due to excita-
tion of the vaso-motor centre by the venous blood, and
the consequent constriction of the muscular coats of the
arteries and increase of the peripheral resistance. In the
third stage the blood-pressure falls very rapidly, because the
feebly acting heart then fails to keep the arteries tense, even
although their diminished calibre greatly slows the rate at
which they empty themselves into the capillaries.

Another medullary centre unduly excited during asphyxia
is that from which proceed the nerve-fibres governing those
muscular fibres of the eye which enlarge the pupil. During
suffocation, therefore, the pupils become widely dilated.
At the same time all reflex irritability is lost, and touching
the eyeball causes no wink; the reflex centres all over the
Body being rendered, through want of oxygen, incapable of
activity. The same is true of the higher nerve-centres; un-
consciousness comes on during the convulsive stage, which,
horrible as it looks, is unattended with suffering.

Modified Respiratory Movements. Sighing is a deep
long-drawn inspiration followed by a shorter but correspond-
ingly large expiration. Yawning is similar, but the air is
mainly taken in by the mouth instead of the nose, and the
lower jaw is drawn down in a characteristic manner. Hic-
cough depends upon a sudden contraction of the diaphragm,
while the aperture of the larynx closes; the entering air,
drawn through the narrowing opening, causes the peculiar
sound. Coughing consists of a full inspiration followed by a
violent and rapid expiration, during the first part of which

426 THE HUMAN BODY.

the laryngeal opening is kept closed ; being afterwards sud-
denly opened, the air issues forth with a rush, tending to
carry out with it anything lodged in the windpipe or larynx.
Sneezing is much like coughing, except that, while in a
cough the isthmus of the fauces is held open and the air
mainly passes out through the mouth, in sneezing the fauces
are closed and the blast is driven through the nostrils. It is
t commonly excited by irritation of the nasal mucous mem-
brane, but in many persons a sudden bright light falling into
the eye will produce a sneeze. Laugh ing consists of a series
of short expirations following a single inspiration; the
larynx is open all the time, and the vocal cords (Chap.
XXXVII.) are set in vibration. Crying is, physiologically,
much like laughing and, as we all know, one often passes
into the other. The accompanying contractions of ,the face
muscles giving expression to the countenance are, however,
different in the two.

All these modified respiratory acts are essentially reflex
and they serve to show to what a great extent the discharges
of the respiratory centre can be modified by afferent nerve
impulses; but, with the exception of hiccough, they are to a
certain extent, like natural breathing, under the control of
the will. Most of them, too, can be imitated more or less
perfectly by voluntary muscular movements; though a good
stage sneeze or cough is rare.

CHAPTER XXVIII.
THE KIDNEYS AND SKIN.

General Arrangement of the Urinary Organs. These
consist of (1) the kidneys, the glands which secrete the
urine; (2) the ureters or ducts of the "kidneys, which carry
their secretion to (3) the urinary bladder, a reservoir in
which it accumulates and from which it is expelled from
time to time through (4) an exit tube, the urethra. The
general arrangement of these parts, as seen from behind, is
represented in Fig. 132. The two kidneys, R, lie in the
dorsal part of the lumbar region of the abdominal cavity,
one on each side of the middle line. Each is a solid mass,
with a convex outer and a concave inner border, and its
upper end a little larger than the lower. From the ab-
dominal aorta, A, a renal artery, Ar, enters the inner border
of each kidney, to break up within it into finer branches,
ultimately ending in capillaries. The blood is collected from
these into the renal veins, Vr, one of which leaves each kid-
ney and opens into the inferior vena cava, Vc. From the
concave border of each kidney proceeds also the ureter, U, a
slender tube from 28 to 34 cm. (11 to 13.5 inches) long,
opening below into the bladder, Vu, on its dorsal aspect, and
near its lower end. From the bladder proceeds the urethra,
at ffrt. The channel of each ureter passes very obliquely
through the wall of the bladder to open into it; accordingly
if the pressure inside the latter organ rises above that of the
liquid in the ureter, the walls of the oblique passage are
pressed together and it is closed. Usually the bladder,
which has a thick coat of unstriped muscular tissue lined by
a mucous membrane, is relaxed, and the urine flows readily
into it from the ureters. While urine is collecting, the be-
ginning of the urethra is kept closed, in part at least, by
bands of elastic tissue around it: some of the muscles which
surround the commencement of the urethra assist, being kept
in reflex contraction ; it is found that in a dog the urinary

427

428

2 HE HUMAN BODY.

bladder can retain liquid under considerably higher pressure
when the spinal cord is intact than after destruction of its

Ua

Fio. 132.— The renal organs, viewed from behind. R, right kidney; A, aorta; Ar,
right renal artery; Fc, inferior vena cava; Vr, right renal vein; £7, right ureter;
Fw, bladder; C7a, commencement of urethra.

lumbar portion. The contraction of these urethra-con-
stricting muscles can be reinforced voluntarily. When some
amount of urine has accumulated in the bladder, it contracts

THE KIDNEYS AND SKIN. 429

and presses on its contents; the ureters being closed in the
way above indicated, the elastic fibres closing the urethral
exit are overcome, and the urethral muscles simultaneously
relaxing, the liquid is forced out.

Naked Eye Structure of the Kidneys. These organs
have externally a red-brown color, which can be seen through
the transparent capsule of peritoneum which envelops them.
When a section is carried through a kidney from its outer to
its inner border (Fig. 133) it is seen that a deep fissure, the
hilus, leads into the latter. In the hilus the ureter widens
out to form the pelvis, D, which breaks up again into a
number of smaller divisions, the cups or calices. The cut
surface of the kidney proper is seen to consist of two distinct
parts; an outer or cortical portion, and an inner or medul-
lary. The medullary portion is less red and more glistening
to the eye, is finely striated in a radial direction, and does not
consist of one continuous mass but of a number of conical
portions, the pyramids of Malpighi, 2', each of which is
separated from its neighbors by an inward prolongation, *, of
the cortical substance: this, however, does not reach to the
inner end of the pyramid, which projects, as the papilla, into
a calyx of the ureter. At its outer end each pyramid sepa-
rates into smaller portions, the pyramids of Ferrein, 2",
separated by thin layers of cortex and gradually spreading
everywhere into the latter. The cortical substance is redder
and more granular looking and less shiny than the medullary,
and forms everywhere the outer layer of the organ next its
capsule, besides dipping in between the pyramids in the way
described.

The renal artery divides in the hilus into branches (5)
which run into the kidney between the pyramids, giving off
a few twigs to the latter and ending finally in a much richer
vascular network in the cortex. The branches of the renal
vein have a similar course.

The Minute Structure of the Kidney. The kidneys
are compound tubular glands, composed essentially of
branched microscopic urimferous tubules, lined by epithe-
lium. Each tubule commences at a small opening on a
papilla and from thence has a very complex course to its
other extremity: usually about twenty open, side by side,
on one papilla, where they have a diameter of about 0.125
mm. (5Jo i»ch). Running from this place into the pyramid

430

THE HUMAN BODY.

each tubule divides several times. At first the branches are
smaller than the main tube; but as soon as they have come
down to about 0.04 mm. (vfa inch) this diminution in size
ceases, and division continuing while the tubules retain the
same diameter, the pyramid thus gets, in part, its conical form.
Ultimately each branch runs out of the pyramid somewhere,
either from its base or side, into the cortex and there dilates

FIG. 133. — Section through the right Kidney trom its outer to its inner border.
I, cortex; 2, medulla: 2', pyramid of Malpiglii; <!", pyramid of Ferreiu; 5, small
branches of the renal artery entering between the pyramids: A, a branch of the
renal artery; Z>, the pelvis of the kidney; U, ureter; C, a calyx.

and is twisted. It then narrows and doubles back into one of
the pyramids of Ferrein and runs as a straight tube towards
the papilla, but before reaching it makes a loop (loop of Henle],
and turns back again as a straight tube towards the base of
the pyramid, where it once more enters the cortex, dilates
and becomes contorted, and then ends in a spherical capsule,

THE KIDNEYS AND SKIN. 431

containing a tuft of small blood-vessels. Or, followed the
other way, each tubule commences in the cortex with a
globular dilatation, the Malpigliian capsule. From this it
continues as a convoluted tubule in the cortex; this passes
into a pyramid of Ferrein, becomes straight, and runs to near
a pyramid of Malpighi as the descending limb of a loop of
Henle. Turning at the loop, it continues as its ascending
limb, and this passes out again into the cortex and becomes
the convoluted junctional tubule, which passes as a straight
collecting tubule into a pyramid of Ferrein, where it joins
others to form an excretory tubule; the excretory tubules
run into the main pyramid and unite to form the discharging
tubules which open on the papilla. Throughout its course
the tubule is lined by a single layer of epithelium cells differ-
ing in character in its different sections: they are flat and
clear in the capsules, and very granular in both the convo-
luted parts, where their appearance suggests that they are
not mere lining cells but cells with active work to do; they
are non-granular and flat in the descending limb of the loop
of Henle, clear and columnar in most of the ascending, and
in both are probably only protective; in the collecting and
discharging tubules they are somewhat cuboidal in form and
have no active secretory function. All the tubes are bound
together by a sparse amount of connective tissue and by
blood-vessels to form the gland. The lymph spaces are large
and numerous, especially about the convoluted portions of the
tubules.

The Blood-flow through the Kidney. The amount of
blood brought to the kidney is large relatively to the size of
the organ and enters under a very high pressure almost direct
from the aorta, and leaves under a very low, into the inferior
cava (Fig. 132). The final twigs of the renal artery in the
cortex, giving off a few branches which end in a capillary
network around the convoluted tubules and in the pyramids,
are continued as the afferent vessels of Malpighian capsules,
the walls of which are doubled in before them (Fig. 134);
there each breaks up into a little knot of capillary vessels
called the glomerulus, from which ultimately an efferent vessel
proceeds. Where the wall of the glomerulus, w, Fig. 134, is
doubled in before the blood-vessels, its lining cells continue
as a covering, c, to the latter, closely adhering to the vascular
walls. A space, A, is left between the epithelial cells of the

432 THE HUMAN BODY.

outside of the capsule and those involuted on the vessels, as
there would be in the interior of a rubber ball one side of
which was pushed in so as to nearly meet the other; this
cleft, into which any liquid transuded from the vessels must
enter, opens by a narrow neck, d, into the commencement of
the first contorted part of an uriniferous tubule. The effer-
ent vein, carrying blood away from the glomerulus, breaks
up into a close capillary network around the neighboring

tubules of the cortex. From these
capillaries the blood is collected
into the renal vein. Most of the
blood flowing through the kidney
thus goes through two sets of capil-
laries; one found in the capsules,
and the second formed by the
breaking up of their efferent veins.
The capillary network in the pyra-
mids is much less close than that
in the cortex, which gives reason to
suspect that most of the secretory
work of the kidneys is done in the

FIG. 134.— Diagram showing a capsules arid convoluted tubules.
nanSdnffe^ The pyramidal blood flows only

through one set of capillaries, there
**™S no glomeruli in the kidney

vein e issues; c, involuted epithe- rnprlnlla
Hum covering: the vascular tuft; "

for the sake of distinctness it is The Renal Secretion. The

represented as a general wrapping

for the whole tuft, but in nature amount of this carried off from the

it forms a close investment .

around each vessel of the glo- Body in 24 hours IS SU Dject to CO11-

merulus; A. space in capsule into . , , , ... , . ,,

which liquid transuded from siderable variation, being especially
ng diminished by anything which pro-
: motes perspiration, and increased
"£££ by conditions, as cold to the sur-
brane- face, which diminish the skin ex-

cretion. Its average daily quantity varies from 1200 to 1750
cub. cent. (40 to 60 fluid ounces). The urine is a clear
amber-colored liquid, of a slightly acid reaction; its specific
gravity is about 1022, being higher when the total quantity
excreted is small than when it is greater, since the amount of
solids dissolved in it remains nearly the same in health; the
changes in its bulk being dependent mainly on changes in
the amount of water separated from the blood by the kidneys.

THE KIDNEYS AND SKIN.

433

Normal urine consists, in 1000 parts, of about 960 water
and 40 solids. The latter are mainly crystalline nitrogenous
bodies (urea and uric acid], but small quantities of pigments
and of non-nitrogenous organic bodies are also present, and a
considerable quantity of mineral salts. The following table
gives approximately, in the first column, the average compo-
sition of the urine excreted in twenty-four hours expressed im
grams; in the second column the same expressed in grains.
The third column gives the composition of 1000 parts of
urine.

Urine in 24 hours.

1500 grams.

23,250 grains.

In 1000 parts.

Water

1428 00

22 134.00

952 00

Solids

72.00

1116.00

4800

The solids consist of —
Urea

33 00

511 50

22.00

Uric acid

0 50

7 75

0.33

Hippuric acid

0 40

6.20

0.27

Kreiitinin

\ 00

15 50

0.67

Pigments and fats . . ....

10 00

155 00

0.66

Sulphuric acid

2 00

31 00

1.33

Phosphoric acid

3.00

46.50

200

Chlorine

7 00

108 50

4.67

Ammonia

0 75

12 00

0.50

Potassium. . .

2.50

38 75

1.67

Sodium

11.00

170.50

7.33

Calcium . ...

0 25

3 80

0.17

Magnesium

0 20

3.00

0.13

71.60

1110.00

47.73

The urine, however, even in health is subject to consid-
erable variation in composition; not only as regards the
amount of water in it, but also in respect to its solid con-
stituents; the latter are especially modified by the quantity
and nature of the food taken.

The Crystalline Nitrogenous Constituents of the Urine
are of great interest as they represent the final result of the
breaking down in the Body of albuminous and gelatinaginous
substances, whether due to tissue waste or to the destruction
in the bodily liquids of proteids and albuminoids existing in
them in solution. Their chemical relationships tend to cast
some light on the structure of an albumen molecule and on
the metabolisms it undergoes in the living organism.

434 THE HUMAN BODY.

Urea (N2H4CO) is the chief nitrogenous waste product
of the human Body and is related to the ammonia group,
being readily converted into ammonium carbonate by hydra-
tion, a change which occurs under the influence of some
living ferments when stale urine becomes alkaline and ac-
quires its well-known offensive ammoniacal odor —

N2H4CO + 2H20 = (NH4)2C03.

On another side urea is allied to the cyanogen group of sub-
stances, being isomeric with ammonium cyanate, which is
converted into it by simple heating.

Uric acid (C5H4N403) is present in but small quantity in
normal human urine, but is the chief nitrogenous excretion
of birds and reptiles. Its molecular structure is more com-
plex than that of urea, and when it is decomposed by various
methods urea is very frequently one of the products. It is a
less complete product of proteid degradation than urea.
Some of its decompositions indicate relations to oxalic acid
and to amido-acetic acid (glycin), and through this latter to
the ammonias and the fatty acids series. In human urine
uric acid exists chiefly in the form of salts of potassium and
sodium; these are less soluble in cold than in warm water,
and are sometimes deposited as a flocculent precipitate when
originally clear urine is left to cool. The precipitate dis-
appears on reheating the liquid.

Hippuric acid (C7H602) is scanty in normal human urine
but abundant in the urine of herbivora. Chemically it is
related to the aromatic series, being formed when benzoic
acid and glycin are made to unite with dehydration; and it
is broken up into those substances when boiled with mineral
acids. Certain aromatic bodies allied to benzoic acid are
found in hay and similar foods and account for the large
amount of hippu rates in herbivorous urine. But proteids
when broken up by putrefaction also yield bodies of the ben-
zoic group, and the hippu ric acid of human urine probably
has its origin in the liberation of benzoic residues in metabolic
activities of some of the living cells of the Body; these
residues being then combined with glycin to form hippu ric
acid. That glycin is formed in the Body is shown by the j
fact that benzoic acid given in food reappears in human urine I
as hippuric acid, having been somewhere united to a glycin
residue.

THE KIDNEYS AND SKIN. 435

Kreatinin (C4H7N,0) is closely allied tokreatin (C4H9N302),
of which it is a simple dehydration product. Kreatin is a
normal constituent of muscle (0.2-0.3$), being, indeed, most
conveniently prepared from Liebig's extract; it is also known
that kreatin introduced into the Body is converted into
kreatinin; for if given in the food it causes an equivalent
increase of the kreatinin excreted in the urine. Kreatin
formed in the muscles has accordingly been supposed to be a
source of the kreatinin of the urine, but this does not appear
to be the case, as all kreatinin disappears from the urine
during starvation. The kreatinin of normal urine probably
has its source in the kreatin of flesh eaten as food.

The Urinary Pigments are still very imperfectly known,
but appear in part to be derived from uro-bilin, which, as we
have seen (Chap. XXIV), is itself probably a derivative of
haemoglobin.

Of the inorganic salts sodium chloride is by far the most
abundant, but the phosphates deserve notice because the
acidity of normal fresh urine is dependent on the presence of
acid sodium phosphate.

In various diseases abnormal substances are found in the
urine: the more important are albumens in albuminuria or
Bright's disease; grape sugar or glucose in diabetes; bile
salts ; bile pigments.

The Secretory Actions of Different Parts of an Urinif-
erous Tubule. — The microscopic structure of the kidneys is
such as to suggest that in those organs we have to do with
two essentially distinct secretory apparatuses : one represented
by the glomeruli, with their capillaries separated only by a
single layer of flat epithelial cells from the cavity of the
capsule and especially adapted for filtration and dialysis; the
other represented by the contorted portions of the tubules,
with their large granular cells, which clearly have some more
active part to play than that of a mere passive transudation
membrane. And we find in the urine substances which like
the water and mineral salts may easily be accounted for by
mere physical processes, and others, urea especially, which are
present in such proportion as must be due to some active
physiological work of the kidney, whether a merely selective
activity of its cells or a constructive one. More direct evi-
dence does, in fact, justify us in saying that in general the
glomeruli are transudation organs, the contorted portions of

436 THE HUMAN BODY.

the tubuli secretory organs, while the loops of Henle and the
collecting and discharging tubules are merely passive channels
for the gathering and transmission of liquid. Even in the
glomeruli, however, the renal cells provide something more
than a merely passive physical membrane for dialysis and
filtration : to a certain extent they control the passage of
substances through them; while they are in health no serum
albumen or paraglobulin passes through them into the urine,
though egg albumen injected into the blood of a living
mammal does. But when they are altered in disease or even
by a temporary stoppage of their blood -supply, then they
allow the normal blood proteids to transude: if the blood-
supply of a kidney be cut off for some minutes by clamping
the renal artery, then the urine secreted for some time after
the clamp is removed is albuminous.

The structure of the glomerular epithelium and its rela-
tion to the blood-vessels are such as to make it almost certain
that when albumen appears in the urine it enters through
them and not through other parts of the tubule; but in some
amphibia we get direct evidence of the entry of substances other
than salts and water into the renal secretion by the path of
the Malpighian capsules. In amphibia the blood carried to
the kidney, like that supplying the mammalian liver, has two
sources, one venous and one arterial ; the arterial supply
comes from the renal arteries, the venous from the veins of
the leg by the reniportal vein. Both, bloods leave the organ
by the renal veins, but their distribution in it is in great part
distinct; the arteries supply the glomeruli, the reniportal
vein the tubules of the cortex, though mixed there with blood
from the efferent vessels of the glomeruli. In some small
amphibia it is, in fact, possible to observe the circulation in
the living kidney and to see that all blood-flow in the glomer-
uli ceases when the renal arteries are tied, though it con-
tinues elsewhere throughout the organ. When sugar or
peptone is injected into the blood of such an animal those
substances appear in the urine; but if the renal arteries be pre-
viously tied they do not. It is true that under those circum-
stances all secretion of urine usually ceases, but it may be
excited by administering certain drugs, and then is found to
be free from sugar and peptone. Grape-sugar when present
in the blood of mammals beyond a certain small percentage
passes out in the urine; and the same is true of peptone:

THE KIDNEYS AND SKIN. 487

indeed, the absence of peptone (or of all but the merest traces
of it) from healthy human urine is one of the main reasons
for believing that peptone absorbed from the alimentary canal
is converted at once by the lymphoid tissues of the mucous
membrane into the proper proteids of the blood plasma.
When sugar appears in the urine either in disease or, as some-
times happens temporarily, in health, after a meal rich in
starchy substances we have from the results of experiment
on amphibia reason to believe that its path of excretion is
through the glomeruli. In hcvmoglobinuria, a condition in
which haemoglobin is found in solution in urine (not in blood-
corpuscles, for in that case it may have come from ruptured
vessels anywhere in the renal apparatus), the haemoglobin also
passes out through the Malpighian bodies: for when some
laky blood (Chap. IV) is injected into the vessels of an animal
and the secretion of urine at the same time made slow, col-
lections of haemoglobin may be found in the cavities of the
capsules. While, however, we have evidence that the epithe-
lium of the capsule has a certain selective power and is the
special seat of transmission of particular, especially abnormal,
urinary constituents, yet on the whole the glomeruli provide
a merely physical apparatus. Through them most of the
bulk of the urine passes out, and, flushing the more active
portions of the tubules on its course to the pelvis of the kid-
ney, picks up from them the more specific urinary con-
stituents.

Urea is the most important and most abundant of the
characteristic ingredients of urine, and it has a very marked
influence on kidney activity, the injection of some of it into
blood causing a greatly increased secretion of urine, in which
the injected urea is quickly passed out. Judging from ex-
periments on amphibia, urea is not excreted or at any rate not
chiefly excreted by the glomeruli. On tying the renal arte-
ries of one of these animals urinary secretion ceases, there
being then no blood-pressure in the glomeruli to cause the
transudation of liquid; but if some urea be now injected
into the blood the ephithelial cells of other parts of the
tubules are stimulated to secrete, and urine rich in urea is
formed; but in these circumstances it cannot come from the
Malpighian bodies. It would seem then that urea is a special
stimulant to some cells of the tubules, and that an excess
of it in the blood can stir them up to its elimination along

438 THE HUMAN BODY.

with some water, quite independently of any formation of
transudation urine. In mammalia we cannot separate the
glomerular secretion from the tubular as in amphibia; and
the diuresis which administration of urea causes in them is
in part due to increased glomerular activity, as urea dilates
the kidney vessels and causes more blood flow through the
glomeruli, which causes the transudation of more water
through them; but the simultaneous increase of urea is
almost certainly due to special activity of the other parts of
the tubules.

The proteids and albuminoids of food may while within
the organism have been built up into tissue or may have
remained in solution in the liquids; but in either case they
are sooner or later broken up and oxidized, the main final
products being carbon dioxide, water, and urea. But this
breaking down may and does occur in many stages and by
different modes in the various tissues; and there is no doubt
that most of the intermediate processes in the chemical
degradation of albuminous compounds take place outside
the kidneys. It was, however, at one time believed that the
urea itself was a kidney product : that the penultimate ni-
trogenous products of proteid degradation were brought to
the kidneys, and that there the final formation of urea took
place. But if this were so there could not be less urea in the
blood leaving the kidneys by their veins than in that brought
to them by the renal arteries; yet such is the case. And
further, if urea be made in the kidneys it ought not to
accumulate in the blood of animals from whom both kidneys
have been removed, as it is now known to do, though not
the immediate cause of the symptoms of so-called urcemic
poisoning seen in persons with extensive kidney disease.
So far, then, as urea is concerned the cells of the kidney
tubules are not its producers; they have a special affinity for
it and pick it up from the lymph of the kidney, which in
turn gets it from the blood. The cells then pass it on with
some water, and no doubt other things, into the tubules
which they line. That it is the epithelial cells of the
contorted portions of the tubules which especially exer-
cise this selective power is, so far as urea is concerned,
a presumption based on their histological characters, but
there is evidence that these cells have a special selective
power for some other substances circulating in the blood.

. THE KIDNEYS AND SKIN. 439

A blue substance known as sodium sulphindogate after in-
jection, in solution, into a vein of an animal is excreted in
the bile and urine. If the animal be killed during the
excretion no traces of this body can under normal circum-
stances be detected in any special part of the kidney; it is
in fact washed away by the urine as fast as the cells pick it
up and pass it into the tubuli. But if the blood-pressure of
the animal be made so low (as by cutting the main vaso-
constrictor nerves) as to bring the renal secretion to a stand,
and the animal be killed some time after injection of the
indigotate, the glomeruli and most of the tubules are found
free of the blue, which lies only in the contorted portions,
just where the cells which gathered it from the circulating
liquid had passed it out.

Though the renal epithelium does not make urea it
has constructive powers as regards some other urinary constit-
uents. As already stated, benzoic acid taken with the food
leaves the Body as hippuric acid, having been combined
with glycin. If blood containing benzoic acid and glycin be
artificially circulated through a perfectly fresh still living
kidney, the renal vein blood will contain hippuric acid.
Even if no glycin be provided in the blood injected through
the renal artery the returning blood will still yield hippuric
acid. So living kidney cells can not only perform the
synthesis, with dehydration, necessary to form hippuric acid,
but can also form and supply the required glycin. The
process is closely dependent on the vitality of the cells; the
experiment fails if the organ be not perfectly fresh and unin-
jured, and if the blood supplied be not properly arterialized.

The Influence of Renal Blood-flow on the Amount of
Urine Secreted. From the structure of the glomeruli and
the fact that most of the water of the urine is derived
from them it is a priori probable that anything tending to
increase the pressure of blood in them will increase the bulk
of urine secreted, and anything diminishing that pressure
decrease the urine. This is confirmed by experiment. The
kidney is supplied with both vaso-constrictor and vaso-dilator
nerves which reach it mainly through the solar plexus,
though both sets come ultimately from the spinal cord.
When the spinal cord is cut in the neck region of a dog the
kidney vessels as well as those of the rest of its body dilate
and blood-pressure everywhere is very low. Under these

440 THE HUMAN BODY.

circumstances the secretion of urine is suppressed. If the
lower end of the cut cord be stimulated the vessels all over
the body of the animal contract, and blood-pressure every-
where becomes very high. But the kidney vessels being
constricted with the rest allow very little blood to enter
the glomeruli in spite of the high aortic pressure, and little
or no urine is secreted. If, however, the vasoconstrictor
nerves of the kidney be cut before the stimulation of the
cord, we get a dilatation of the kidney vessels with a con-
striction of vessels elsewhere, and abundant blood flows
through the glomeruli under high pressure : the whole
kidney swells and abundant urine is formed. When the
skin vessels contract on exposure to cold, more blood flows
through internal organs, the kidneys included, and the blood-
pressure in these is if anything increased, the expansion of
internal arteries not at the most more than counterbalancing
the constriction of the cutaneous. Hence the greater secre-
tion of urine in cold weather. Injection of a little water
into a vein of an animal causes a very transient constriction
of the kidney vessels followed by a dilatation; and general
blood-pressure not being at the same time lowered, pressure
in the renal glomeruli is high and the secretion of urine
increased. Urea introduced into the blood acts in a similar
way, but more markedly , so that this substance causes
diuresis not merely, as we have seen, by stimulating the cells
of the tubuli, but also by exciting the vaso-dilator nerves of
the glomerular arteries. Solution of sodium acetate or even
of common salt injected into the veins causes very marked
local vascular dilatation in the kidney, and hence great flow
through the organ under high pressure and a marked in-
crease in the quantity of urine excreted. Even if the nerves
going to the kidney be first cut, the above results follow,
these salts appearing to act directly on a local renal vaso-
dilator mechanism. They may of course also, like urea,
directly stimulate the cells of the contorted tubules, but this
is not proved. The increased secretion of urine after drink-
ing much water is probably produced by the dilution of the
blood by the liquid absorbed by the alimentary canal, essen-
tially in the same manner as the extra secretion caused by
direct injection of water into the blood-vessels. That the
central nervous system may influence the renal secretion is
well known, fear, for example, stimulating it. Probably

THE KIDNEYS AND SKIN. 441

such influence is mainly due to vaso-motor changes — either
paralysis of the renal vaso-constrictor nerves or stimulation
of the vaso-dilator. Such changes would account for the
phenomenon, and there is no evidence of the existence of
true secretory nerves acting directly on the cells of the
organ as certain fibres of the chorda tympani (Chap. XIX)
do on the cells of the submaxillary gland.

The Skin, which covers the whole exterior of the Body,
consists everywhere of two distinct layers ; an outer, the cuti-
cle or epidermis, and a deeper, the dermis, cutis vera, or
corium. A blister is due to the accumulation of liquid be-
tween these two layers. The hairs and nails are excessively
developed parts of the epidermis.

The Epidermis, Fig. 135, consists of cells, arranged in
many layers, and united by a small amount of cementing
substance. The deepest layer, d, is composed of elongated
or columnar cells, set on with their long axes perpendicular
to the corium beneath. To it succeed several layers of round-
ish cells, b, the deepest of which, prickle cells, are covered by
minute processes (not indicated in the figure) which do not
interlock but join end to end so as to leave narrow spaces
between the cells ; in more external layers the cells become
more and more flattened in a plane parallel to the surface.
The outermost epidermic stratum is composed of many layers
of extremely flattened cells from which the nuclei (conspicu-
ous in the deeper layers) have disappeared. These super-
ficial cells are dead and are constantly being shed from the
surface of the Body, while their place is taken by new cells,
formed in the deeper layers, and pushed up to the surface
and flattened in their progress. The change in the form of
the cells as they travel outwards is accompanied by chemical
changes, and they finally constitute a semitransparent dry
horny stratum, a, distinct from the deeper, more opaque and
softer Mfilpiffhiftn or mucous layer, b and d, of the epider-
mis. The cells of this latter, in spite of their name, are not
muceginous; they are soluble in acetic acid; those of the
horny stratum not.

The rolls of material which are peeled off the skin in the
"shampooing^ of the Turkish bath, or by rubbing with a
rough towel after an ordinary warm bath, are the dead outer
scales of the horny stratum of the epidermis.

442

THE HUMAN BODY.

In dark races the color of the skin depends mainly on
minute pigment granules lying in the cells of the deeper part
of the Malpighian layer.

No blood or lymphatic vessels enter the epidermis, which
is entirely nourished by matters derived from the subjacent

x— a

FIG. 135.— A section through the epidermis, somewhat diagrammatic, highly
magnified. Below is seen a papilla of the dermis, with its artery, /, and veins, gg ;
a, the horny layer of the epidermis ; b, the rete mucosum or Malpighian layer; d,
the layer of columnar epidermic cells in immediate contact with the dermis ; h, the
duct of a sweat-gland.

corium. Fine nerve-fibres run into it and end there among
the cells.

The Corium, Cutis Vera, or True Skin, Fig. 136, consists
fundamentally of a close feltwork of elastic and white fibrous
tissue, which, becoming wider meshed below, passes gradually
into the subcutaneous areolar tissue (Chap. VIII) which
attaches the skin loosely to parts beneath. In tanning it is the

THE KIDNEYS AND SKIN.

443

cutis vera which is turned into leather, its white fibrous tissue
forming an insoluble and tough compound with the tannin
of the oak-bark employed. Wherever there are hairs, bun-
dles of plain muscular tissue are found in the corium ; it
contains also a close capillary network and numerous lym-
phatics and nerves. In shaving, so long as the razor keeps
in the epidermis there is no bleeding; but a deeper cut shows
at once the vascularity of the true skin.

The outer surface of the corium is almost everywhere
raised into minute elevations, called the papilla, on which

a

h
m

f

•so

FIG. 186. — A section through the skin and subcutaneous areolar tissue, ft,
horny stratum, and m, deeper more opaque layer of the epidermis ; d, dermis;
passing below into sc, loose areolar tissue, with fat, /, in its meshes : above, dermic
papillae are seen, projecting into the epidermis which is moulded on them, a,
opening of a sweat-gland ; gl, the <rland itself.

the epidermis is moulded, so that its deep side presents pits
corresponding to the projections of the dermis. In Fig. 135
is shown a papilla of the corium containing a knot of. blood-
vessels, supplied by the small artery, f, and having the blood
carried off from them by the two little veins, g g. Other
papillae contain no capillary loops but special organs connected
with nerve-fibres, and supposed to be concerned in the sense

444 THE HUMAN BODY.

of touch (Chap. XXXV). On the palmar surface of the hand
the dermic papillae are especially well developed (as they are
in most parts where the sense of touch is acute) and are fre-
quently compound, or branched at the tip. On the front of
the hand, they are arranged in rows; the epidermis fills up the
hollows between the papillae of the same row, but dips down
between adjacent rows, and thus are produced the finer ridges
seen on the palms. In many places the corium is also fur-
rowed, as opposite the finger-joints and on the palm. Else-
where such furrows are less marked, but they exist over the
whole skin. The epidermis closely follows all the hollows,
and thus they are made visible from the surface. The
wrinkles of old persons are due to the absorption of subcu-
taneous fat and of other soft parts beneath the skin, which,
not shrinking itself at the same rate, is thrown into folds.

Hairs. Each hair is a long filament of epidermis devel-
oped on the top of a special dermic papilla, seated at the
bottom of a depression reaching down from the skin into the
tissue beneath, and called the hair-follicle. The portion of
a hair buried in the skin is called its root ; this is succeeded
by a stem which, in an uncut hair, tapers off to a point. The
stem is covered by a single layer of overlapping scales form-
ing the hair-cuticle ; the projecting edges of these scales are
directed towards the top of the hair. Beneath the hair-cuti-
cle comes the cortex, made up of greatly elongated cells
united to form fibres; and in the centre of the shaft there
is found, in many hairs, a medulla, made up of more or less
rounded cells. The color of hair is mainly dependent upon
pigment granules lying between the fibres of the cortex.
All hairs contain some air cavities, especially in the medulla.
They are very abundant in white hairs and cause the white-
ness by reflecting all the incident light, just as a liquid beaten
into fine foam looks white because of the light reflected from
the walls of all the little air cavities in it. In dark hairs the
air cavities are few.

The hair-follicle (Fig. 137) is a narrow pit of the dermis,
projecting down into the subcutaneous areolar tissue, and
lined by an involution of the epidermis, At the bottom of
the follicle is a papilla, and the epidermis, turning up over
this, becomes continuous with the hair. On the papilla epi-
dermic cells multiply rapidly so long as the hair is growing,
and the whole hair is there made up of roundish cells. As

THE KIDNEYS AND SKIN. 445

these get pushed up by fresh ones formed beneath them, the
outermost layer become fiat-
tened and form the hair
cuticle; several succeeding

layers elongate and form MaM—m—aHgaHH^g^s u
the cortex; while, in hairs
with a medulla, the middle
cells retain pretty much
their original form and size.
Pulled apart by the elongat-
ing cortical cells, these cen-
tral ones then form the *O

medulla with its air Cavities. FIG. |137.— Parts of two hairs imbedded

rni . , , » , , in their follicles, a, the skin, which is seen

1 lie innermost layer OI the to dip down and line the follicle; 6, the sub-

'A^*, ' v ' 4-1* f IT i cutaneous tissue; c, the muscles of the hair-

epidermis lining the lOlllCle, follicle, which by their contraction can

has its Cells projecting, With erect the hair ; o, sebaceous gland.

overlapping edges turned downwards. Accordingly these inter-
lock with the upward directed edges of the cells of the hair-
cuticle; consequently when a hair is pulled out the epidermic
lining of the follicle is usually brought with it. So long as the-
dermic papilla is left intact a new hair will be formed, but not.
otherwise. Slender bundles of unstriped muscle (c, Fig. 137}
run from the dermis to the side of the hair-follicles. The latter-
are in most regions obliquely implanted in the skin so that
the hairs lie down on the surface of the Body, and the mus-
cles are so fixed that when they shorten, they erect the hair
and cause it to bristle, as may be seen in an angry cat, or
sometimes in a greatly terrified man. Opening into each hair-
follicle are usually a couple of sebaceous or oil glands. Hairs are
found all over the skin except on the palms of the hands and
the soles of the feet; the back of the last phalanx of the fingers
and toes, the upper eyelids, and one or two other regions.

Nails. Each nail is a part of the epidermis, with its
horny stratum greatly developed. The back part of the nail
fits behind into a furrow of the dermis and is called its root.
The visible part consists of a body, fixed to the dermis be-
neath (which forms the bed of the nail), and of a free edge.
Near the root is a little area whiter than the rest of the nail
and called the lunula. The whiteness is due in part to the
nail being really more opaque there and partly to the fact
that its bed, which seen through the nail causes its pink
color, is in this region less vascular.

446

THE HUMAN BODY.

The portion of the corium on which the nail is formed
is called its matrix. Posteriorly this forms a furrow lodging
the root, and it is by new cells added on there that the nail
grows in length. The part of the matrix lying beneath the
body of the nail, and called its bed, is highly vascular and
raised up into papillae which, except in the region of the
lunula, are arranged in longitudinal rows, slightly diverging
as they run towards the tip of the finger or toe. It is by
new cells formed on its bed and added to its under surface
that the nail grows in thickness, as it is pushed forward by
the new growth in length at its root. The free end of a
nail is therefore its thickest part. If a nail is " cast " in
consequence of an injury, or torn off, a new one is produced,
provided the matrix is left.

The Glands of the Skin are of two kinds, the sudo-
riparous or sweat glands, and the sebaceous or oil glands.
The former belong to the tubular, the latter to the race-
mose type. The siv eat- glands, Fig. 138, lie in the subcu-
taneous tissue, where they form little globular masses com-
posed of a coiled tube. From the coil a duct (sometimes
double) leads to the surface, being usually
spirally twisted as it passes through the epi-
dermis. The secreting part of the gland
consists of a connective-tissue tube, continu-
ous along the duct with the dermis; within
this is a basement membrane; and the final
secretory lining consists of several layers of
gland-cells. ' A close capillary network inter-
twines with the coils of the gland. Sweat-
glands are found on all regions of the skin,
but more closely set in some places, as the
palms of the hands and on the brow, than
elsewhere: there are altogether about two
and a half millions of them opening on the
surface of the Body.

The sebaceous glands nearly alwavs open

.,,.„,,., , -i I gland. d, horny

into hair-follicles, and are lound wherever layer of cuticle; c,

there are hairs. Each consists of a duct

opening near the mouth of a hair-follicle

and branching at its other end : the final

branches lead into globular secreting saccules, mis-

which, like the ducts, are lined with epithelium. In the

FIG. 138.— A sweat

iiB

THE KIDNEYS AND SKIN. 447

saccules the substance of the cells becomes charged with oil-
drops, the protoplasm disappearing; and finally the whole
cell falls to pieces, its detritus constituting the secretion.
New cells are, meanwhile, formed to take the place of those
destroyed. Usually two glands are connected with each hair-
follicle, but there may be three or only one. A pair of seba-
ceous glands are represented on the sides of each of the hair-
follicles in Fig. 137.

The Skin Secretions. The- skin besides forming a pro-
tective covering and serving as a sense-organ (Chap. XXXV)
also plays an important part in regulating the temperature of
the Body, and, as an excretory organ, in carrying off certain
waste products.

The sweat poured out by the sudoriparous glands is a
transparent colorless liquid, with a peculiar odor, varying in
different races and, in the same individual, in different regions
of the Body. Its quantity in twenty-four hours is subject to
great variations, but usually lies between 700 and 2000 grams
(10,850 and 31,000 grains). The amount is influenced mainly
by the surrounding temperature, being greater when this is
high; but it is also increased by other things tending to
raise the temperature of the Body, as muscular exercise.
The sweat may or may not evaporate as fast as it is secreted;
in the former case it is known as insensible, in the latter as
sensible perspiration. By far the most passes off in the in-
sensible form, drops of sweat only accumulating when the
secretion is very profuse, or the surrounding atmosphere so
humid that it does not readily take up more moisture. The
perspiration is acid, and in 1000 parts contains 990 of water
to 10 of solids. Among the latter are found urea (1.5 in
1000), fatty acids, sodium chloride, and other salts. In dis-
eased conditions of the kidneys the urea may be greatly
increased, the skin supplementing to a certain extent defi-
ciencies of those organs.

The Nervous and Circulatory Factors in the Sweat
Secretion. It used to be believed that an increased flow of
blood through the skin would suffice of itself to cause in-
creased perspiration; but against this view are the facts that,
in terror for example, there may be profuse sweating with a
cold pallid skin; and that in many febrile states the skin may
be hot and its vessels full of blood, and yet there may be no
sweating.

448 THE HUMAN BODY.

Direct experiment shows that the secretory activity of
the sweat-glands is under immediate control of nerve-fibres,
and is only indirectly dependent on the blood-supply in their
neighborhood. Stimulating the sciatic nerve of the freshly
amputated leg "of a cat will cause the balls of its feet to
sweat, although there is no blood flowing through the limb.
On the other hand, if the sciatic nerve be cut so as to para-
lyze it, in a living animal, the skin arteries dilate and the
foot gets more blood and becomes warmer; but it does not
sweat. The siveat-fibres originate in certain sweat-centres in
the spinal cord, which may either be directly excited by
blood of a higher temperature than usual flowing through
them or, reflexly, by warmth acting on the exterior of the
Body and stimulating the sensory nerves there. Both of
these agencies commonly also excite the vaso-dilator nerves
of the sweating part, and so the increased blood-supply goes
along with the secretion; but the two phenomena are funda-
mentally independent.

The Sebaceous Secretion. This is oily, semifluid, and
of a special odor. It contains about 50 per cent of fats (olein
and palmatin). It lubricates the hairs and usually renders
them glossy, even in persons who use none of the various
compounds sold as " hair-oil." No doubt, too, it gets spread
more or less over the skin and makes the cuticle less permea-
ble by water. Water poured on a healthy skin does not wet
it readily but runs off it, as " off a duck's back " though to a
less marked degree.

Hygiene of the Skin. The sebaceous secretion, and the
solid residue left by evaporating sweat, constantly form a
solid film over the skin, which must tend to choke the
mouths of the sweat-glands (the so-called " pores " of the
skin) and impede their activity. Hence the value to health
of keeping the skin clean: a daily bath should be taken by
every one. Women cannot well wash their hair daily as it
takes so long to dry, but a man should immerse his head
when he takes his bath. As a general rule, soap should only
be used occasionally; it is quite unnecessary for cleanliness,
except on exposed parts of the Body, if frequent bathing be a
habit and the skin be well rubbed afterwards until dry,
Soap nearly always contains an excess of alkali which in itselJ
injures some skins, 'and, besides, is apt to combine chemically
with the sebaceous secretion and carry it too freely away

THE KIDNEYS AND SKIN. 449

Persons whose skin will not stand soap can find a good sub-
stitute, for washing the hands and face, in a little cornmeal.
No doubt many folk go about in very good health with very
little washing; contact with the clothes and other external
objects keeps its excretions from accumulating on the skin
to any very great extent. But apart from the duty of per-
sonal cleanliness imposed on man as a social animal in daily
intercourse with others, the mere fact that the healthy Body
can manage to get along under unfavorable conditions is no
reason for exposing it to them. A clogged skin throws more
work than their fair share on the lungs and kidneys, and the
evil consequences may be experienced any day when some-
thing else puts another extra strain on them.

Animals, a considerable portion of whose skin has been
varnished, die within a few hours. This used to be thought
due to poisoning by retained ingredients of the sweat. But
the main cause of death seems to be an excessive radiation
of heat from the surface of the body, dependent mainly on
dilatation of the cutaneous vessels caused by the varnish,
though possibly the retention of some poisonous substance
usually excreted by the skin may have an influence. The
bodily temperature falls in consequence of the great loss of
heat until it reaches the fatal point, about 20° 0. (68° F.) for
rabbits. If the animal be packed in raw cotton or kept in
an atmosphere at a temperature of 30° 0. (86° F.) it does not
die as a consequence of the varnishing, or at least not nearly
so soon as it would otherwise die.

Bathing. The general subject of bathing may be consid-
ered here. One object of it is that above mentioned — to
cleanse the skin; but it is also useful to strengthen and in-
vigorate the whole frame. For strong healthy persons a cold
bath is the best, except in extremely severe weather, when the
temperature of the water should be raised to 15° C. (about
60° F.), at which it still feels quite cold to the surface. The
first effect of a cold bath is to contract all the skin-vessels
and make the surface pallid. This is soon followed by a
reaction, in which the skin becomes red and congested, and a
glow of warmth is felt in it. The proper time to come out is
while this reaction lasts, and after emersion it should be pro-
moted by a good rub. If the stay in the cold water be too
prolonged the state of reaction passes off, the skin becomes
cold and pale and the person feels chilly, uncomforta-

450 THE HUMAN BODY.

ble, and depressed all day. Then bathing is injurious instead
of beneficial; it lowers instead of stimulating the activities
of the Body. How long a stay in the cold water may be
made with benefit depends greatly on the individual: a vigor-
ous man can bear and set up a healthy reaction after much
longer immersion than a feeble one; moreover, being used to
cold bathing renders a longer stay safe, and, of course, the
temperature of the water has a great influence: water called
" cold " may vary within very wide limits of temperature, as
indicated by the thermometer; and the colder it is the shorter
is the time which it is wise to remain in it. Persons who in
the comparatively warm water of Narragansett during the
summer months stay with benefit and pleasure in the sea,
have to content themselves with a single plunge on parts of
the coast where the water is colder. The nature of the water
has some influence; the salts contained in sea-water stimu-
late the skin-nerves and promote the afterglow. Many per-
sons who cannot stand a simple cold fresh-water bath take
one with benefit when some salines are previously dissolved
in the water. The best for this purpose are probably those
sold in the shops under the name of " sea-salts."

It is perfectly safe to bathe when warm, provided the skin
is not perspiring profusely, the notion commonly prevalent to
the contrary notwithstanding. On the other hand, no one
should enter a cold bath when feeling chilly, or in a depressed
vital condition. It is not wise to take a bath immediately
after a meal, since the afterglow tends to draw away too
much blood from the digestive organs, which are then ac-
tively at work. The best time for a long bath is about three
hours after breakfast; but for an ordinary daily dip, lasting
but a short time, there is no better period than on rising and
while still warm from bed.

The shower-bath abstracts less heat from the skin than an
ordinary cold bath and, at the same time, gives it a greater
stimulus: hence it has certain advantages.

Persons in feeble health may diminish the shock to the
system by raising the temperature of the water they bathe in
up to any point at which it still feels cool to the skin. Bath-
ing in water which feels hot is not advisable : it tends gen-
erally to diminish the vital activity of the Body. Hence warm
baths should only be taken occasionally and for special pur-
poses, other than mere luxury.

CHAPTER XXIX.

NUTRITION.

The Problems of Animal Nutrition. We have in pre-
ceding chapters traced certain materials, consisting of foods
more or less changed by digestion, into the Body from the
alimentary canal, and oxygen into it from the lungs. We
have also detected the elements thus taken into the Body in
their passage out of it again by lungs, kidneys, and skin; and
found that for the most part their chemical state was differ-
ent from that in which they entered; the difference being
expressible in general terms by saying that more oxidized
forms of matter leave the Body than go into it. We have now
to consider what happens to each food during the journey
through the Body : is it changed at all ? is it oxidized ? if so
where ? what products does its oxidation give rise to ? Is
the oxidation direct and complete at once, or does it occur in
successive steps ? Has the food been used first to make part
of a living tissue and is that then oxidized; or has it been
oxidized without forming part of a living tissue ? if so,,
where ? in the blood stream, or outside of it ? Finally, if
the chemical changes undergone in the Body are such as
to liberate energy, how has this energy been utilized ? to
maintain the temperature of the Body or to give rise to mus-
cular work, or for other purposes ? This is a long string of
questions, the answers to many of which Physiology has still
to seek.

The Seat of the Oxidations of the Body. According
to elder views oxidation mainly took place in the blood while
flowing through the lungs. Those organs were considered a
sort of furnace in which heat was liberated by blood oxidation,
and then distributed by the circulation. But if this were so
the lungs ought to be the hottest parts of the Body, and the
blood leaving them by the pulmonary veins much hotter than
that brought to them by the pulmonary artery after it had
been cooled by warming all the tissues; and neither of these

451

452 THE HUMAN BODY.

things is true. A small amount of heat is liberated when
haemoglobin combines with oxygen in the pulmonary capil-
laries, but the affinities thus satisfied are so feeble that the
•energy liberated is trivial in amount when compared with
that set free when this oxygen subsequently forms stabler com-
pounds elsewhere. There is good reason to believe that
hardly any of this latter class of oxidations occurs in the
living circulating blood at all; its cells do, no doubt, use up
some oxygen and set free some carbon dioxide; but not
enough to be detected by ordinary methods of analysis. The
percentage of oxygen liberated in a vacuum by two specimens
of the blood of an animal, taken one from an artery near the
heart, and the other from a distant one, are practically the
same; showing that during the time occupied in flowing two
or three feet through an artery the blood uses up no appreci-
able quantity of its own oxygen; while in its brief capillary
transit it almost suddenly loses so much oxygen as to become
venous. The difference is explained by the fact that the
blood gives off oxygen gas through the thin capillary walls
to the surrounding tissues; and in the latter the oxidation
takes place. As we have already seen, a freshly excised
muscle deprived of blood can still be made to contract; and for
some considerable time if it be the muscle of a cold-blooded
animal. During its contraction it evolves large amounts
of carbon dioxide, although the resting fresh muscle contains
hardly any of that gas. Here we have direct evidence of
oxidation taking place in a living tissue and in connection
with its functional activity; and what is true of a muscle
is probably true of all tissues: the oxidations which supply
them with energy take place within the living cells themselves.
The statement frequently made that the oxygen in the cir-
culating blood exists as ozone, rests on no sufficient basis;
decomposing haemoglobin does ozonize some oxygen when
exposed to the air, but there is no ozone in fresh blood. Ex-
periments made by adding various combustible substances, as
sugar, to newly drawn blood, also fail to prove the occurrence
of any oxidation of such bodies in that liquid.

Tissue-Building and Energy-Yielding Foods. The
Human Body, like that of other animals, is, on the whole,
chemically destructive; it takes in highly complex substances
as food, and eliminates their elements in much simpler
compounds, which can again be built up to their original

NUTRITION. 453

condition by plants. Nevertheless the Body has certain con-
structive powers: it, at least, builds up protoplasm from
proteids and other substances received from the exterior;
and there is reason to believe it does a good deal more of the
same kind of work, though never an amount equalling its
chemical destructions. Given one single proteid in its food,
say egg albumen, the Body can do very well; making serum
albumen and paraglobuliri out of it for the blood, myosinogen
for the muscles, and so on : in such cases the original proteid
must have been taken more or less to pieces, remodelled, and
built up again by the living tissues; and it is extremely
doubtful if anything different occurs in other cases, when
the proteid eaten happens to be one found in the Body. In
fact, during digestion the proteids arc broken down some-
what and turned into peptones; in this state they are absorbed
and must somewhere again be built up into the proteids of
the tissues.

The constructive powers of the Body used to be rather
too much ignored. Foods were divided into assimilable and
combustible, the former serving directly to renew the organs
or tissues as they were used up, or to supply materials for
growth; these were mainly proteids and fats; no special
chemical synthesis was thus supposed to take place, the living
cells being nourished by the reception from outside of mole-
cules similar to those they had lost. Fat-cells, it was sup-
posed, grew by picking up fatty molecules like their contents,
received from the food; and albumen-rich tissues by the re-
ception of ready-made proteid molecules, needing no further
manufacture in the cell. The combustible foods, on the other
hand, were the carbohydrates and some fat: the carbohy-
drates, according to the hypothesis, were incapable of being
made into parts of a living tissue, and were merely oxidized
in order to maintain the bodily warmth. It having been
proved, however, that more fat might accumulate in the body
of an animal than was taken in its food, this excess was ac-
counted for by supposing it was due to excess of com-
bustible foods, converted into fats and stored away as oil-
droplets in various cells; but not actually built up into true
living adipose tissue. Liebig, somewhat similarly, classed all
foods into plastic, concerned in making new tissue, and
respiratory, directly oxidized before they ever constituted
part of a tissue. The plastic foods were the proteids, but

454 THE HUMAN BODY.

these also indirectly gave rise to the energy expended ii>
muscular work, and to some heat : the proteid muscular fibre
being broken first into a highly nitrogenous part (urea, or
some body well on the road to become urea) and a non-nitro-
genized richly hydrocarbonous part; and this latter was then
oxidized and gave rise to heat. Several facts may be urged
against this view: (1) Men in tropical climates live mainly
on non-proteid foods, yet their chief needs are not heat pro-
duction, but tissue formation and muscular work : according
to Liebig's view their diet should be mainly nitrogenous.
(2) Carnivorous animals live on a diet very rich in proteids,
nevertheless develop plenty of animal heat, and that without
doing the excessive muscular work which, on Liebig's theory,
must first be gone through in order to break up the proteids,
with the production of a non-azotized part which could then
be oxidized for heat-production. (3) Great muscular work
can be done on a diet poor in proteids; beasts of burden are
for the most part herbivorous. (4) Further, we know exactly
how much energy can be liberated by the oxidation of pro-
teids to that stage which occurs in the Body; and it is pos-
sible to estimate with considerable accuracy the amount of
urea and uric acid excreted in a given time; from their sum
the amount of proteid oxidized arid the amount of energy
liberated in that oxidation can be calculated; if this be done
it is found that, nearly always, the muscular work done* dur-
ing the same period represents far more energy expended
than could be yielded by the proteids broken down.

The Source of the Energy Expended in Muscular Work.
This important question, which was postponed in the chap-
ters dealing with the physiology of the muscular tissues,
needs now consideration. It may be put thus : Does a
muscle-fibre work by the oxidation of its proteids, i.e. by
breaking them down into compounds which are then re-
moved from it and conveyed out of the Body ? or does it
work by the energy liberated by the oxidation of carbon and
hydrogen compounds only ? The problem may be attacked
in two ways : first, by examining the excretions of a man, or
other animal, during work and rest; second, by examining
directly the chemical changes produced in a muscle when it
contracts. Both methods point to the same conclusion, viz.,
that proteid oxidation is not the source of the mechanical
energy expended by the Body.

NUTRITION. 455

One gram (15.5 grains) of pure albumen when completely
burnt liberates, as heat, an amount of energy equal to 2117
kilogrammeters (15,270 foot-pounds). But in the Body pro-
teids are not fully oxidized ; part of their carbon is, to form
carbon dioxide, and part of the hydrogen, to form water;
but some carbon and hydrogen pass out, combined with ni-
trogen and oxygen, in the incompletely oxidized state of urea.
Therefore all of the energy theoretically obtainable is not de-
rived from proteids in the Body: from the above full amount
for each gram of proteid we must take the quantity carried
off in the urea, which will be the amount liberated when that
urea is completely oxidized. Each gram (15.5 grains) of
proteid oxidized in the Body gives £ of a gram (5.14 grains)
of urea ; since one gram of urea liberates, on oxidation,
energy amounting to 934 kilogrammeters (6740 foot-pounds),
each gram of proteid oxidized, so far as is possible in the
Body, will yield during the process 2117 — *f * = 1805.7 kilo-
grammeters (13,037 foot-pounds) of energy. Knowing that
urea carries off practically all the nitrogen of proteids broken
up in the Body, and contains 46.6 per cent of nitrogen, while
proteids contain 16 per cent, it is easy to find that each gram
of urea represents the decomposition of about 2.80 grams of
proteid and, therefore, the liberation of 5060.00 kilogram-
meters (36,533.0 foot-pounds) of energy. If, therefore, we
know how much urea a man excretes during a given time,
and how much mechanical work he does during the same
time, we can readily discover if the latter could possibly have
been done by the energy set free by proteid decomposition.
Let us take a special case. Tick and Wislecenus, two Ger-
man observers, climbed the Faulhorn mountain, which is
1956 meters (about 6415 feet) high. Fick weighed 66 kilo-
grams and, therefore, in lifting his Body alone, did during
the ascent 129,096 kilogrammeters (932,073 foot-pounds) of
work. Wislecenus, who weighed 76 kilograms, did similarly
148,656 kilogrammeters (1,073,296 foot-pounds) of work.
But during the ascent, and for five hours afterwards, Fick
secreted urine containing urea answering only to 37.17 grams
of proteid, and Wislecenus urea answering to 37 grams.
Since each gram of proteid broken up in the Body liberates
1805.7 kilogrammeters (13,037 foot-pounds) of energy, the
amount that Fick could possibly have obtained from such a
source is 1805.7 X 37.17 = 67,117 kilogrammeters (484,584

456 THE HUMAN BODY.

foot-pounds), and Wislecenus 1805.7 X 37 = 66,810 kilo-
grammeters. If to the muscular work done in actually rais-
ing their bodies, we add that done simultaneously by the
heart and the respiratory muscles, and in such movements
of the limbs as were not actually concerned in lifting the
Aveight, we should have, at least, to double the above total
muscular work done ; and the amount of energy liberated
meanwhile by proteid oxidation, becomes utterly inadequate
for its execution. ^ It is thus clear that muscular work is not
wholly done at the expense of the oxidation of muscle pro-
teid; and it is very probable that none is so done under ordi-
nary circumstances, for the urea excretion during rest is
about as great as that during work, if the diet remain the
same :\f the work be very severe, as in long-distance walking-
matches, the urea quantity is sometimes temporarily raised,
but this increase, which no doubt represents an abnormal
wear and tear of muscle-fibre, is probably independent of the
liberation of energy in the form in which a muscle can use it,
more likely taking the form of heat ; and is, moreover, com-
pensated for afterwards by a diminished urea excretion.
Thus, hourly, before the ascent Fick and Wislecenus each
excreted on the average about 4 grams (62 grains) of urea ;
during the ascent between 7 and 8 grams (108-124 grains) ;
but during the subsequent 16 hours, when any urea formed
in the work would certainly have reached the urine, only an
average of about 3 grams (46.5 grains) per hour.

It may still be objected, however, that a good deal of the
muscle work may be done by the energy of oxidized muscle
proteid ; that the amount of this oxidation occurring in a
muscle during rest or ordinary work is pretty constant and
simply takes different forms in the two cases, much as a
steam-engine with its furnace in full blast will burn as much
coal when resting as when working, but in the former case
lose all the generated energy in the form of heat, and in the
latter partly as mechanical work. Thus the smallness of in-
crease in urea excretion as a consequence of muscular activity
could be explained, while still a good deal of utilizable energy
might come from proteid degradation. But if this were so,
then the working Body should eliminate no more carbon
dioxide than the resting; the amount of chemical changes in
its muscles being by hypothesis the same, the carbon dioxide
eliminated should not be increased. Experiment, however,

NUTRITION. 457

shows that it is, and that to a very large extent, even when
the work done is quite moderate and falls within the limits
which could be performed by the normal proteid degradation
of the Body. Quite easy muscular work doubles the carbon
dioxide excreted in twenty-four hours, and in a short period
of very hard work it may rise to five times the amount elimi-
nated during rest. Since the urea is not increased, or but
slightly increased, at the same time, this carbon dioxide can-
not be due to increased proteid metamorphosis; and it there-
fore indicates that a muscle works by the oxidation of car-
bonaceous non-nitrogenous compounds. Since all the carbon
compounds oxidized in the Body contain hydrogen this
element is also no doubt oxidized during muscular work; but
the estimation of the amount so used is difficult and has
not been satisfactorily made, because the Body contains so
much water ready formed that a large quantity is always
ready for increased evaporation from the lungs and skin,
whenever the respirations are quickened, as they are by
exercise. It, thus, is very difficult to say how much of the
extra water eliminated from the Body during work is due
merely to this cause and how much to increased hydrogen
oxidation.

The conclusion we are led to is, then, that a muscle
works by the oxidation mainly, if not entirely, of carbon and
hydrogen';, and that the proteid constituents of the living
muscle .substance are essentially the machinery determining
in what way the energy shall be spent: they may and do
suffer some wear and tear, but this bears no direct proportion
to the work done; as a steam-engine may rust, so muscle
proteid may and does oxidize, but not to supply the organ
with energy for use. This conclusion, arrived at by a study
of the excretions of the whole Body, is confirmed by the re-
sults obtained by the chemical study of a single muscle.
A fresh frog's muscle (which agrees in all essential points
with a man's) contains practically no carbon dioxide, yet
made to work in a vacuum gives off that gas, and more the
more it works. Some carbon dioxide is therefore formed in
the working muscle. If the muscle, after contracting as long
as it can be made to do so, be thrown into death rigor it
gives off more carbon dioxide; and if taken perfectly fresh
and sent into rigor mortis without contracting, it gives off
carbon dioxide also, in amount equal to the sum of that

458 THE HUMAN BODY.

which it would have given off in two stages, if first worked
and then sent into rigor. The muscle must, therefore, con-
tain a certain store of a carbon-dioxide-yielding body, and
the decomposition of this is associated with the occurrence
both of muscular activity and death stiffening. Similar
things are true of the acid simultaneously developed; the
muscle when it works produces some sarcolactic acid, and
when thrown into rigor mortis still more. No increase of
urea or kreatin or any similar product of nitrogenous de-
composition is found in a worked muscle when compared
with a rested one, but the total carbohydrates are rather less
in the former. These facts make it clear that muscular work
is not done at the expense of proteid oxidation; and we have
already seen (Chap. XXVI) that the oxygen a muscle uses in
contracting is not taken up by it at the time it is used, since
a muscle containing no oxygen will still contract in a vacuum
and form carbon dioxide. It is probable that the chemical
phenomena occurring in contraction and rigor are essentially
the same; the death stiffening results when they occur to an
extreme degree. Provisionally one may explain the facts as
follows: A muscle in the Body takes up from the blood,
oxygen, proteids, and non-nitrogenous (carbohydrate or fatty)
substances. These it builds up into a highly complex and
very unstable compound, comparable, for example, to nitro-
glycerine. When the muscle is stimulated this falls down
into simpler substances in which stronger affinities are satis-
fied; among these are carbon dioxide and sarcolactic acid and
a proteid (myosinogen). The energy liberated is thus in-
dependent of any simultaneous taking up of oxygen; the
amount possible depends only on how much of the decom-
posable body existed in the muscle. Under natural condi-
tions the carbon dioxide is carried off in the blood and per-
haps the sarcolactic acid also, the latter to be elsewhere
oxidized further to form water and more carbon dioxide.
The myosinogen remains in the muscle-fibre and is combined
with more oxygen, and with compounds of carbon and hydrogen
taken from the blood, and built up into the unstable energy-
yielding body again; no increased quantity of nitrogenous
material, under ordinary circumstances, leaves the working
muscle. If, however, the blood-supply be deficient, myosin
forms from myosinogen and clots (Chap. IX) before this
restitution takes place, and cannot be directly rebuilt into

NUTRITION. 459

living muscle material; in excessive work the same thing
partially occurs, decomposition occurring faster than recom-
position; clotted myosin is then broken up into simpler
bodies as kreatin, and these are somewhere turned into urea
and excreted. In rigor mortis all the myosinogen passes into
clotted myosin and causes the rigidity. A working muscle
takes up more oxygen from the blood than a resting one, as
is shown by a comparison of the gases of the venous blood
of the two; this oxygen assumption is not necessarily pro-
portionate to the carbon-dioxide elimination at the same
time.; for the latter depends on the breaking down of a
material already accumulated in the muscle during rest, and
this breaking down may occur faster than the reconstruction.
We are thus enabled, also, to understand how, during exercise,
the carbon dioxide evolved from the lungs may contain more
oxygen than that taken up at the same time; for it is largely
oxygen previously stored during rest which then appears in
the carbon dioxide of the expired air. The kreatin which
can always be found even in muscles suddenly killed after
long rest, represents the breaking down of proteid in the
chemical processes of the living fibres, in their vital meta-
bolisms, which are not necessarily similar to the special
chemical changes associated with a contraction.

Are any Foods Respiratory in Liebig's Sense of the
Term ? We find, then, that Liebig's classification of foods
cannot be accepted in an absolute sense. There is no doubt
that the substance broken down in muscular contraction is
proper living muscular tissue; and if this (its proteid con-
stituent being retained) be reconstructed from foods con-
taining no nitrogen (whether carbohydrates or fats) then
the term plastic or tissue-forming cannot be restricted to the
proteids of the diet. We must rather conclude that any
alimentary principle containing carbon may be used to re-
place the oxidized carbon, and any containing hydrogen to
replace the oxidized hydrogen, of a tissue; and so even non-
proteid foods may be plastic. A certain proportion of the
foods digested may perhaps be oxidized to yield energy,
before they ever form part of a tissue; and so correspond
pretty much to Liebig's respiratory foods; but no hard and
fast line can be drawn, making all proteid foods plastic and
all oxidizable non-proteid foods respiratory.

Luxus Consumption. Not only, as above pointed out,

460 THE HUMAN BODY.

may non-nitrogenous foods be plastic but, on the other hand,
it is certain that if any foods are oxidized at once before
being organized into a tissue, proteids are under certain
circumstances; namely, when they are contained in excess in
a diet. If an animal be starved it is found that its non-
nitrogenous tissues go first; an insufficiently fed animal loses
its fat first, and if it ultimately dies of starvation is found to
have lost 97 per cent of its adipose tissue and only about 30
per cent of its proteid-rich muscular tissue, and almost none
of its brain and spinal cord; all of course reckoned by their
dry weight. It is thus clear that the proteids of the tissues
resist oxidation much better than fat does. But, on the
other hand, if a well-fed animal be given a very rich proteid
diet all the nitrogen of its food reappears in its urine, and
that when it is laying up fat; so that then we get a state of
things in which proteids are broken up more easily than
fats. This indicates that proteid in the Body may exist
under two conditions ; one, when it forms part of a living
tissue and is protected to a great extent from oxidation,
and another, in which it is oxidized with readiness and is
presumably in a different condition from the first, being not
yet built up into part of a living cell. The use of proteids
for direct oxidation is known as luxus consumption ; how
far it occurs under ordinary circumstances will be considered
presently. The main point now to be borne in mind is that
while all organic non-nitrogenous foods cannot be called
respiratory, neither can proteids under all circumstances be
called plastic, in Liebig's sense.

The Antecedents of Urea. In the long-run the pro-
genitors of the urea excreted from the Body are the proteids
taken in the food; but it remains still to be considered what
intermediate steps these take before the excretion of their
nitrogen in the urine.

In seeking antecedents of urea one naturally turns first
to the muscles, which form by far the largest mass of pro-
teid tissues in the Body. Analysis shows that they always
yield kreatin, the quantity of this in muscles being practically
unaffected by work, and from 0.2 to 0.3 per cent of the dry
weight of the muscle. Since it is readily soluble and dialyz-
able, and therefore fitted to pass rapidly out of the muscles
into the blood stream, it is a fair conclusion that a good deal
of it is formed in the muscles daily and carried off from them.

NUTRITION. 461

Kreatin, too, exists in the brain, and probably there and else-
where in the nervous system is produced by chemical degra-
dation of protoplasm; the spleen also contains a good deal
of kreatin, and so do many glands. This substance would
therefore seem to be constantly produced in considerable
quantities by the protoplasmic tissues generally; and since
it belongs to a group of nitrogenous compounds which the
Body is unable to utilize for reconstruction into proteids, it
must be carried off somehow. The urine, however, contains
no kreatin and but little of its immediate derivative, krea-
tinin, and what kreatinin it does contain depends mainly on
the feeding, since its amount varies with the diet and it
disappears during starvation. Kreatin can readily be chem-
ically broken up with hyd ration, yielding urea and sarkosin;
and sarkosin in turn can be decomposed so as to yield its
nitrogen in the form of urea. Hence there are no great
chemical difficulties in regarding kreatin as the main im-
mediate source of the urea of normal urine. There are some
reasons for thinking that kreatin is not the form of the
actual nitrogen waste in living muscle but a post-mortem
product from that waste; but that is not of importance in-
the present connection. Whatever the original form of the'
waste substance be, if it be not kreatin it is certainly very
easily converted into it. The formation of the final product,,
urea, does not occur in the muscles. They never contain
urea; and very little of it, if any, can be extracted from the;
brain.

Where the kreatin is finally changed into urea is doubt-
ful. We have seen (Chap. XXVIII) that it is not formed in
the kidneys but merely separated by them from the blood.
A good deal of urea is found in the liver, which suggests some
part played by that organ in urea formation. Further, in
certain cases of hepatic disease (acute yellow atrophy) in
which the liver cells are profoundly changed, the urea of the
urine is greatly diminished and a quite different substance,
leucin, takes its place; and this favors the view that the liver
has much to do with the final elaboration of urea. It may
also be noted in this connection that, quite apart from kreatin
as a source of urea, there may be another in leucin, for leucin
is very widely distributed through the Body, and when proteids
are decomposed by various chemical methods leucin is very
constant among the products. It is therefore a possible form

462 THE HUMAN BODY.

for the primary nitrogen waste of many tissues. Chemically
leucin is an ammonium derivative, being the amide of caproic
(a fatty) acid.

"While the urea resulting from further changes in the
kreatin, leucin, or similar substances formed in the tissues, is
a measure of the wear and tear of their protoplasm, part of
the urea excreted has probably a different source; being due
to the oxidation of proteids as energy liberators or respira-
tory foods, before they have ever formed a tissue. When
abundant proteid food is taken the urea excretion is largely
increased and that very rapidly, within a couple of hours for
example, and before we can well suppose the proteids eaten
to have been built up into tissues, and these in turn broken
down; in fact there need be, and usually is, under such cir-
cumstances no sign of any special activity of any group of
tissues, such as one would expect to see if the urea always
came from the breaking down of formed histological ele-
ments. This urea is thus indicative of a utilization of pro-
teids for other than plastic purposes; and the same fact is
indicated by the storage of carbon and elimination of all
the nitrogen of the food when a diet very rich in proteid
alimentary principles is taken. This luxus consumption may
be compared to the paying out of gold by a merchant instead
of greenbacks when he has an abundance of both. Only the
gold can be used for certain purposes, as settling foreign
debts, but any quantity above that needed for such a purpose
is harder to store than the paper money, and not so con-
venient to handle; so it is paid out in preference to the
paper money, which is really somewhat less valuable, as
available at par only for the settlement of domestic debts.
Similarly, only proteids can be used for certain final stages of
tissue building, but an excess of them is more difficult to
store than fats or carbohydrates, and so is eliminated in pref-
erence to them.

In artificial pancreatic digestions, when long carried on,
two bodies, called leucin and tyrosin, are produced from
proteids. It is found that when leucin is given to an ani-
mal in its food, it reappears in the urine as urea; so the Body
can turn leucin into that substance. Hence a possible source
of some of the luxv.s-consumption urea is leucin produced dur-
ing intestinal digestion; and this is very likely turned into
urea in ^the liver. Mammalia rapidly die when the liver is

NUTRITION. 463

removed, but some birds survive for a time. In them it has
been found that the uric acid (which in avian urine has the
predominance which urea takes in mammalian) excreted is
diminished after extirpation of the liver; and also that leucin
which when given to the normal bird reappears in the urine
as uric acid, in the bird from which the liver has been removed
is excreted unaltered.

Circulating and Fixed Proteid. When an animal is
fed on food deficient in proteids, or containing none of them
at all, its urea excretion falls very rapidly during the first day
or two, but then much more slowly until death: there is thus
indicated a double source of urea, apart resulting from tissue
wear and tear, and always present; and a part resulting from
the breaking down of proteids not built up into tissue, and
ceasing when the amount of this proteid in the Body (in the
blood for example) falls below a certain limit as a result of
the starvation. As the nitrogen-starved Body wastes, its
bulk of proteid tissues is slowly reduced and the urea result-
ing from their degradation diminishes also. How well pro-
teid built up into a tissue resists removal is shown by the
tacts already mentioned as to the relative losses of the pro-
teid-rich and proteid-poor tissues during starvation.

On the other hand, if an animal be taken while starving
and losing weight and have a small amount of flesh given it,
it will continue to lose weight, and more urea than before
will appear in the urine; increased proteid diet increases the
proteid metamorphosis, and the animal still loses, though
less rapidly than it did. A little more proteid still increases
proteid metamorphosis in its body and its urea elimination,
and so on for some time; but each increment of proteid in
the food increases the nitrogenous metamorphosis in propor-
tion to itself somewhat less than the preceding one did, until,,
finally, a point is reached at which the nitrogen egesta and
ingesta balance: in a dog this occurs when the animal gets
daily -^ its weight of lean meat, along with the necessary
water. More flesh if then given is at first stored up and the
animal increases in weight; but very soon the greater wear
and tear of the larger mass of tissues shows itself as increased
urea excretion, and again the egesta and ingesta balance, and
the animal comes to a new weight equilibrium at the higher
level. More meat now causes a repetition of the phenomenon:
at first increase of tissue, and nitrogen storage; and then a

464 THE HUMAN BODY.

cessation of the gain in weight, and an excretion in twenty-
four hours of all the nitrogen taken. And so on, until the
animal refuses to eat a larger quantity.

These facts seem, very clearly, to show that proteids can-
not be built up quickly into tissues. Meat given to the
starving animal has its proteids, at first, used up mainly in
luxits consumption — while a little is stored as tissue, though
at first not enough to counterbalance the daily tissue waste.
When a good deal more proteid is given than answers to the
nitrogen excretion during starvation, the animal builds up
as much into living tissue as it breaks down in the vital
processes of these, the rest going in luxus consumption ; it
thus neither gains nor loses. Still more proteid if now given
does not all appear in the urine at once; some is used to
build up new tissue, but only slowly; then, after some daysr
the increased metabolism of the increased mass of living
tissues balances the excess of nitrogen in the diet, and equi-
librium is again attained. But, all through, it seems clear
that the tissue formation is slow and gradual; and so it be-
comes additionally probable that the increased urea excretion
soon after a meal is not due to rapidly increased tissue forma-
tion and degradation, but to a more direct proteid destruction.
The more stable proteid, that which breaks down slowly in
starvation and is rebuilt slowly when food is given, has been
distinguished as fixed or tissue albumen from the less stable
portion, which from the belief that it mainly exists in the
liquids of the Body has been named circulating albumen
Feeding experiments further show the important fact that
the gelatinous or albuminoid foods cannot be converted
into fixed proteid; for its formation true albumens are
required. The tissues of an animal deprived of all proteid
food -stuffs waste, no matter how much albuminoids be given :.
but given some of the latter the Body can build tissues and
maintain their integrity with less true proteid than would
otherwise be necessary, so the gelatin-yielding foods are by
no means without nutritive value.

The Storage Tissues. Every healthy cell of the Body
contains at any moment some little excess of material laid'
by in itself, above what is required for its immediate neces-
sities. The capacity of contracting, and the concomitant
evolution of carbon dioxide, exhibited by an excised muscle
in a vacuum, seem to show that even oxygen, of which

NUTRITION. 465

warm-blooded animals have but a small reserve, may "be
stored up in the living tissues in such forms that they can
utilize it, even when the air-pump fails to extract any from
them. But in addition to the supplies for immediate spend-
ing, contained in all the cells, we find special food reserves
in the Body, on which any of the tissues can call at need.
These, especially the oxygen and proteid reserves, are found
for most part in the blood. Special oxygen storage is, however,
rendered unnecessary by the fact that the Body can, except
under very unusual circumstances, get more from the air at
any time, so the quantity of this substance laid by is only
small; hence death from asphyxia follows very rapidly when
the air-passages are stopped; while, on account of the re-
serves laid up, death from other forms of starvation is a
much slower occurrence. Proteids, also, we have learnt from
the study of muscle, are probably but little concerned in
energy-production in the tissues. Speaking broadly, the
work of the Body is carried on by the oxidation of carbon
and hydrogen, and we find in the Body, in correspondence
with this fact, two great storehouses of fatty and carbo-
hydrate foods, which serve to supply the materials for the
performance of work and the maintenance of the bodily
temperature in the intervals between meals, and during
longer periods of starvation. One such store, that of car-
bohydrate material, is found in the liver-cells; the other,
or fatty reserve, is laid by in the adipose tissue and to a cer-
tain extent in oil droplets found in other cells, and sometimes
in blood and lymph. That such substances are true reserves,
not for any special local purpose but for the use of the Body
generally, is shown by the way they disappear in starvation;
the liver reserve in a few days, and the fat somewhat later
and more slowly, but very largely before any of the other
tissues has been seriously affected. By using these accumu-
lated matters the Body can work and keep warm during
several days of more or less deficient feeding; and the fatter
an animal is at the beginning of a starvation period the
longer will it live; which would not be the case could not its
fat be utilized by the working tissues. Hibernating animals
prove the same thing; bears, before their winter sle p, are
very fat, and at the end of it commonly very thin; while
their muscular and nervous systems are not noticeably
diminished in mass. During the whole winter, then, the

466 THE HUMAN BODY.

energy needed to keep the heart and respiratory muscles at
work, and to maintain the temperature of the body, must
have been obtained from the oxidation of the fat reserve
with which the animal started.

Glycogen. The size of the liver was long a stumbling-
block to physiologists: it was difficult to understand why so
large an organ should be developed for the mere secretion of
some bile, a not very important digestive liquid. But even
centuries ago some glimmering of the truth was guessed,
and the liver was believed to be concerned in the elaboration
of nutritive blood, which was distinguished from the blood,
charged with vital spirits, which came from the lungs and
the left side of the heart. Harvey's discovery of the real
course of the circulation, and Lavoisier's interpretation of
the meaning of respiration, upset these crude doctrines; and
for long the germ of truth which they contained was lost to
view in the glare of the new light. We have now learned,
on a new basis of actual experiment, that the liver is very
largely concerned in the nutritive processes of the Body : its
relation to proteid metabolism and urea formation has
already been considered, and we have now to study its
activity in regard to the formation, and storage, and trans-
mission of a carbohydrate substance, glycogen.

If a liver be cut up two or three hours after removal from
the body of a healthy well-fed animal, and thoroughly ex-
tracted with water, it will yield much grape-sugar. If, on the
other hand, a perfectly fresh liver be heated rapidly to the tem-
perature of boiling water, and be then pounded up and ex-
tracted, it will yield a milky solution, containing little grape-
sugar, but much glycogen; a substance which chemically
has the same empirical formula as starch (C6H100B), and in
other ways is closely allied to that body. The salivary and
pancreatic secretions rapidly convert it into the sugar maltose,
as they do starch. The transformation of glycogen into glucose
(grape-sugar) which occurs in the liver after death and prob-
ably also during life is then quite different from that brought
about by the digestive enzymes; and in fact no enzyme has
been extracted from fresh liver. The change is apparently
not a fermentative one, but one dependent on some vital
metabolic activity of the liver-cells, which activity is greatly
accelerated during their period of dying: hence the need of
killing them rapidly by boiling, if any considerable amount of

NUTRITION. 467

glycogen is to be obtained from the organ. Pure glycogen
is a white amorphous inodorous powder, readily soluble in
water, forming an .opalescent milky solution; insoluble in
alcohol, and giving with iodine a red coloration which dis-
appears on heating and reappears on cooling again.

About four per cent of glycogen can be obtained from
the liver of a well- nourished animal (dog or rabbit). This
for the human liver, which weighs about 1500 grams (53
oz.), would give about 60 grams (2.1 oz.) of glycogen at any
one moment. The quantity actually formed daily is, how-
ever, much in excess of that, since glycogen is constantly
being removed from the liver and carried elsewhere, while a
fresh supply is formed in the organ. Its quantity is subject,
also, to considerable fluctuations; being greatest about two
hours after a good meal, and falling from that time until the
next digestion period commences, when it begins to rise
until it again attains its maximum. If a warm-blooded
animal be starved glycogen disappears from its liver in the
course of four or five days. We are, thus, led to believe that
glycogen is being constantly used up, and that its mainte-
nance in normal quantity depends on food supply.

The accumulation and disappearance of glycogen can be
demonstrated histologically. The liver is essentially a com-
pound tubular gland, but its structure is obscured by the fact
that the hepatic cells are very large in proportion to the
tubules wnich they surround, so that these are reduced to
in^re ductules, formed by the apposition of grooves on the
adjacent sides of two cells; and by the fact that cells and
ductules form an irregular network interlaced with the capil-
laries of the lobule (Chap. XXII), which capillaries are far
larger than the interlobular bile-ducts. When properly pre-
pared hepatic cells, taken from a healthy well-fed animal,
are examined, the side of the cell nearest the bile-ductule is
seen to be granular, and it also picks up readily most of the
ordinary protoplasmic stains. * The rest of the cell contains
few granules and does not stain with carmine, but it does
stain red with iodine. It is in fact mainly filled with glyco-
gen, and if this be dissolved out by digestion with saliva there
is left a loose protoplasmic network. If sections from the
liver of a starved animal be compared with those from a
well-fed, the liver-cells are seen to be considerably smaller, to
be granular throughout, and to stain everywhere with carmine

468 THE HUMAN BODY.

and not at all with iodine: they contain no glycogen and may
be compared with the cells of the pancreas in a late stage of
digestion (Chap. XIX).

In the liver we have to deal with cells of twofold function;
the granular portion of each especially concerned with bile
secretion, and the larger portion of the cell with the making
of glycogen. In a salivary gland we have cells whose sole
apparent function is the formation of secretion to be poured
into the gland ducts; in the thyroid and suprarenal bodies
we find cells forming special materials which are passed into
blood or lymph. The hepatic cells do both, and it should be
borne in mind that possibly all gland-cells do. In fact it has
already been pointed out that the pancreas has still an-
other function than the formation of pancreatic juice. As
regards the liver-cells, we naturally ask whether the two
processes, bile-making and glycogen-rnaking, are distinct and
independent activities, or whether bile and glycogen are
simultaneous products of a single metabolic activity, as soap
and glycerine are of the chemical process of soap-making :
but to this question it is not possible yet to give a satisfactory
answer.

The Source and Destination of Liver Glycogen. All
foods are not equally efficacious in keeping up the stock of
glycogen in the liver; fats by themselves are useless; proteids
by themselves give a little; by far the most is formed on
a diet rich in starch and sugar ; so it would seem tliat glyco-
gen is mainly formed from carbohydrate materials absorbed
from the alimentary canal and carried to the hepatic cells by
the portal vein. The chief of these materials is probably
glucose, since, although saliva and the amylolytic ferment of
the pancreas convert starch into maltose (CiaHMOn + H20),
of the cane-sugar group, the intestinal secretion rapidly con-
verts this into grape-sugar or glucose. This is taken up by
the liver-cells, modified by them and stored as glycogen; and
by their further activity from time to time reconverted into
glucose and passed into the blood according to the needs of
the Body in general. The cells then do distinctly chemical
work on the carbohydrate material: possibly, indeed even
probably, they build that supplied into their own living sub-
stance and then by partial breaking down of this, deposit some
of it for a time as glycogen: and by further living activity
turn this into glucose and send it on to the blood, when the

NUTRITION. 469

sugar in that liquid falls below a certain percentage. That
the chief part of the glycogen found in the normal liver has
its ultimate source in carbohydrate foods is shown by several
facts. (1) Sugar if it exist in the blood in above a certain
small percentage, passes out by the kidneys and appears in the
urine, constituting the characteristic symptom of the disease
called diabetes. In health, however, even after a meal very
rich in carbohydrates, sugar rarely appears in the urine, and
then but temporarily; so that the large quantity of it absorbed
from the alimentary canal within a brief time under such cir-
cumstances, must be stopped somewhere before it reaches the
general blood-current. (2) Glucose injected into one of the
general veins of an animal, if in any quantity, soon appears
in the urine; but the same amount injected into the portal
vein, or one of its radicles, causes no diabetes, but an accumu-
lation of glycogen in the liver. We may therefore conclude
that the sugar absorbed from the alimentary canal is taken
by the portal vein to the liver, and there converted into
glycogen and stayed for a time ; and later slowly passed on
into the hepatic veins during the intervals between meals.
Thus in spite of the intervals which elapse between meals the
carbohydrate content of the blood is kept pretty constant:
during digestion it is not suffered to rise very high, nor dur-
ing ordinary periods of fasting to fall very much below the
average.

In what form glycogen leaves the liver is not certain ; it
might be dissolved out and carried off as such, or previously
turned again into glucose and sent on in that form ; since the
fresh liver-cells are capable of changing glycogen into glucose
the latter view is the more probable. Analyses of portal and
hepatic bloods, made with the view of determining whether
more sugar was carried out of the liver during fasting than
into it, are conflicting; and considering the great amount of
blood which flows through the liver in twenty-four hours, a
very slight increase of sugar (falling within the limits of
error of the difficult quantitative determination of that sub-
stance in the blood) in the hepatic vein would represent a
large total amount during the whole day. The main fact,
however, remains that somehow this carbohydrate reserve in
the liver is steadily carried off to be used elsewhere: and
animal glycogen thus answers pretty much to vegetable starch,
which, made in the green leaves, is dissolved and carried away

470 THE HUMAN BODY.

by the sap currents to distant and not green parts (as the
grains of corn or tubers of a potato, which cannot make starch
for themselves) and in them is again laid down in the form of
solid starch grains, which are subsequently dissolved and used
for the growth of the germinating seed or potato. Eeasons
have been given in an early part of this chapter for believing
that the carbohydrate leaving the liver is not oxidized in the
blood, but only after it has passed out of that into the organ-
ized tissue. Among these the muscles at least seem to get
some, since a fresh muscle always contains glycogen, and even
to retain it in normal amount after an animal has been starved
for some time; the muscle-fibres then, so to speak, drawing on
the balance with their banker (the liver) so long as there is
any. When a muscle contracts, this glycogen disappears and
some glucose appears, but not an amount equivalent to the
glycogen used up; so that the working muscle, it is probable,
uses this substance, among others, for its repair after each
contraction.

How it is that the glycogen, which is so rapidly converted
into grape-sugar by the dying liver, escapes such rapid con-
version during life has not been satisfactorily answered. It
may be that the metabolisms of the dying hepatic cell include
processes which are an exaggeration of those occurring dur-
ing normal life; in some such way as the production of myo-
sin in dying muscle is apparently an exaggeration of chemical
changes occurring in norma^contracting muscle: or the gly-
cogen in the living cell may not exist free, but combined
with other portions of the cell substance so as to be pro-
tected ; while, after death, post-mortem changes may rapidly
liberate it in a condition to be acted upon.

Diabetes. The study of this disease throws some light
upon the history of glycogen. Two distinct varieties of it
are known; one in which sugar appears in the urine only
when the patient takes carbohydrate foods; the other in
which it is still excreted when he takes no such foods, and
mnst therefore form sugar in his Body from substances not at
all chemically allied to it. The more probable source of the
sugar in the latter case is proteids; since some glycogen is
found in the livers of animals fed 011 proteids only, while fats
by themselves give none of it. It seems that the proteid
molecule, in some complex way, is split up in the liver into a
highly nitrogenized part (urea or an antecedent of urea) and

NUTRITION. 471

a nonazotized part, giycogen. On this view the more severe
form of diabetes would be due to an increased activity of a
normal proteid-decomposing function of the hepatic cells; and
sometimes the urea and sugar in the urine of diabetics rise
and fall together, thus seeming to indicate a community of
origin. Diabetes dependent on carbohydrate food might be
produced in several ways. The liver-cells might cease to
stop the sugar and, letting it all pass on into the general cir-
culation, suffer it to rise to such a percentage in the blood
after a meal, that it attained the proportion in which the
kidneys pass it out; or the tissues might cease to use their
natural amount of sugar, and this, sent on steadily out of the
liver, at last rise in the blood to the point of excretion. Or
the liver might transform (into glucose) and pass on its giy-
cogen faster than the other tissues used it, and so diabetes
might arise; but this would only be temporary, lasting until
the liver stock was used up by the rapid conversion. Arti-
ficially we can, in fact, produce diabetes in several of these
ways; curari poisoning, for example, paralyzing the motor
nerves, makes the skeletal muscles lie completely at rest, and
so diminishes the giycogen consumption of the Body and pro-
duces diabetes. Carbon-monoxide poisoning produces dia-
betes also, .presumably by checking bodily oxidation. Fi-
nally, pricking a certain spot in the medulla oblongata causes
a temporary diabetes. This might conceivably be due to the
fact that the operation injures that part of the vaso-motor
centre which controls the muscular coat of the hepatic artery,
and this artery, then dilating, carries so much blood through
the liver that an excess of giycogen is carried off by the
hepatic veins; and in favor of this opinion is the fact that if
the splanchnic nerves be cut the whole arteries of the ab-
dominal viscera dilate no diabetes follows. This has been
explained as due to the fact that so many vessels are dilated
that a great part of the blood of the Body accumulates in
them, and there is in consequence no noticeably increased
flow through the liver. Others, however, maintain that the
"piqiire " diabetes (as that due to pricking the medulla is
called) is due to irritation of trophic nerve-fibres originating
there, and governing the rate at which the liver-cells produce
giycogen or convert it into glucose. This latter view,
though perhaps the less commonly accepted, is probably the
more correct. The hepatic cells do not merely hold back

472 THE HUMAN BODY.

glucose carried through the liver so that it is there to be
washed out by a greater blood-flow, but they feed on sugar
and proteids and make glycogen; and this is later converted
into glucose and carried off. Glycogen, except for its dis-
charge into the blood instead of a gland duct, would then be
comparable to the materials stored in the cells of the salivary
and some other glands (Chap. XIX); and the transforma-
tion of such bodies into the specific element of a secretion
we have already seen to be directly under the control of the
nervous system, and almost entirely or quite independent of
the simultaneous blood-flow.

The History of Fats. While glycogen forms a reserve
store of material which is subject to rapid alterations, deter-
mined by meal-times, the fats are much more stable; their
periods of fluctuation are regulated by days, weeks, or months
of good or bad nutrition, and during starvation they are not
so readily, or at least so rapidly, called upon as the hepatic
glycogen. If we carry on the simile by which we compared
the reserve in each cell to pocket-money, the glycogen
would answer somewhat to a balance on the right side
with a man's banker; while the fat would represent assets or
securities not so rapidly realizable; as capital in business, or
the cargoes afloat in the argosies of Antonio, the " Merchant
of Venice." Fat, in fact, is slowly laid down in fat-cells and
surrounded in these by a cell-wall, and, being itself insoluble
in blood plasma or lymph, it must undergo chemical changes,
which no doubt require some time, before it can be taken
into the blood and carried off to other parts.

When adipose tissue is developing it is seen that undif-
ferentiated cells in the connective tissues (especially areolar)
show minute oil-drops in their protoplasm; these increase
in size and ultimately fuse together and form one larger
oil-droplet, while most of the original protoplasm disappears.

The oily matter would thus seem due to a chemical meta-
morphosis of the cell protoplasm, during which it gives rise
to a non-azotized fatty residue which remains behind, and *a
highly nitrogenous part which is carried off. In many parts
of the Body protoplasmic masses are subject to a similar but
less complete metamorphosis; fatty degeneration of the heart,
for example, is a more or less extensive replacement of the
proper substance of its muscular fibres by fat-droplets; and
the cream of milk and the oily matter of the sebaceous secre

NUTRITION. 473

tion are due to a similar fatty degeneration in gland-cells.
Moreover, careful feeding experiments undoubtedly show that
fat can come from proteids; when an animal is very richly
supplied with these all the nitrogen taken in them reappears
in its excretions, but all the carbon does not; it is in part
stored in the Body: and, since such feeding produces but
little glycogen, this carbon can only be stored as fat.

While there is, then, no doubt that some fat may have a
proteid origin, it is not certain that all has such. During
digestion a great deal of fat is ordinarily absorbed, in a
chemically unchanged state, from the alimentary canal; it is
merely emulsified and carried off in minute drops by the chyle
to be poured into the blood; and this fat might conceiveably
be directly deposited, as such, in adipose tissue. There are,
however, good reasons for supposing that all the fat in the
Body is manufactured. The fat of a man, of a dog, and of a
cat varies in the proportions of palmatin, stearin, margarin,
and olein in it; and varies in just the same way if all be fed
on the same kind of food, which could not be the case if the
fat eaten were simply deposited unchanged. Moreover, if
an animal be fed on a diet containing one kind of fat only,
say olein, but a very slightly increased percentage of that
particular fatty substance is found in its adipose tissue,
which goes to show that if fats come from fats eaten, these
latter are first pulled to bits by the living cells and built up
again into the forms normal to the animal; so that, even with
fatty food, the fats stored up seem to be in most part manu-
factured in the Body.

In still another way it is proved that fats can be con-
structed in the Body. In animals fed for slaughter, the total
fat stored up in them during the process is greatly in excess
of that taken with their food during the same time. For
example, a fattening pig may store up nearly five hundred
parts of fat for every hundred in its food, and this fat must be
made from proteids or carbohydrates. Whether it can come
from the latter is still perhaps an open question ; for, while
all fattening foods are rich in starch or similar bodies, there
are considerable chemical difficulties in supposing an origin
of fats from such; and it is on the whole more probable that
they simply act by sparing from use fats simultaneously
formed or stored in the body, and which would have other-
wise been called upon. They make glycogen, and this

474 THE HUMAN BODY.

shelters the fats. Liebig, indeed, in a very celebrated dis-
cussion, maintained that fats were formed from carbohydrates
He showed that a cow gave out more butter in its milk than
it received fats in its food; and Huber, the blind naturalist,
showed that bees still made wax (a fatty body) for a time
when fed on pure sugar; and indefinitely when fed on honey.
Consequently, for a long time, an origin of fats from carbo-
hydrates was supposed to be proved; but their possible origin
from proteids (a possibility now shown to be a certainty) was
neglected, and the validity of the above proofs of their carbo-
hydrate origin is thus upset. The cow may have made its
butter from proteids; the bees, fed on sugar, their wax for a
time from proteids already in their bodies; and, indefinitely,
when fed on honey, from the proteids in that substance.
Moreover, animals (ducks) fed on abundant rice, which con-
tains much carbohydrate but very little proteid or fat, remain
lean; while if some fat be added they lay up fat.

Persons who fatten cattle for the butcher find that the
foods useful for the purpose all contain proteids, carbohy-
drates, and fats, and that rapid fattening is only obtained
with foods containing a good deal of fat; as oilcake, milk,
or Indian corn. Taking all the facts into account we shall
probably not be wrong in concluding that nearly all the
bodily fat is manufactured either from fats or proteids;
from fats easier than from anything else, but when much
proteid is eaten some is made from it also. Carbohydrates
alone do not fatten; the animal body cannot make its pal-
matin, etc., out of them. Nevertheless they are, indirectly,
important fattening foods when given with others, since,
being oxidized instead of it, they protect the fat formed.

Dietetics. That "one man's meat may be another man's
poison" is a familiar saying, and one that, no doubt, ex
presses a certain amount of truth; but the difference probably
depends on the varying digestive powers of individuals
rather than on peculiarities in their laws of cell nutrition :
we all need about the same amount of proteids, fats, and
carbohydrates for each kilogram of body weight; but all of
us cannot digest the same varieties of them equally well : it
is also a matter of common experience that some foods have
peculiar, almost poisonous, effects on certain persons. Some
people are made ill by mutton, which the majority digest
better than beef.

NUTRITION. 475

The proper diet must necessarily vary, at least as to
amount, with the work done; whether it should vary in kind
with the nature of the work is not so certain. Provided a
man gets enough proteids to balance those lost in the wear
and tear of his tissues, it probably matters little whether he
gets for oxidation and the liberation of energy either fats or
carbohydrates, or even excess of proteids themselves; any one
of the three will allow him to work either his brain or his
muscles, and to maintain his temperature. Proteids, how-
ever, are wasteful foods for mere energy-yielding purposes:
in the first place, they are more costly than the others;
secondly, they are incompletely oxidized in the Body; and,
thirdly, it is probably more laborious to the system to get rid
of urea than of the carbon dioxide $nd water, which alone
are yielded by the oxidation of fats and carbohydrates. Be-
tween fats and carbohydrates similar considerations lead to
a use of the latter when practicable: starch is more easily
utilized in the Body than fats, as shown by the manner in
which it protects the latter from oxidation; and a given
weight of starch fully oxidized in the Body will liberate
about one half as much energy as the same amount of butter,
while it costs considerably less than half the money. Also,
starch is more easily digested than fats by most persons:
children especially are apt to be fond of starchy or saccharine
foods and to loathe fats; and the appetite in ouch cases is
a good guide. As a rule the people of the United States
differ very markedly from the English in their love of sweet
foods of all kinds; whether this is correlated with their char-
acteristic activity, calling for some food that can be rapidly
used, is an interesting question.

It is certain that no general rules for the best dietary
for all persons can be formulated, but on broad principles
the best diet is that which contains just the amount of pro-
teid necessary for tissue repair, and so much carbohydrates
as can be well digested; the balance needed, if any, being
made up by fats and platinoids. Such a food would be
the cheapest; that is, the supplying of it would call for less
of the time and energy of the nation using it, and leave more
work to spare for other pursuits than food production — for
all the arts which make life agreeable and worth living, and
which elevate civilized man above the merely material life of
the savage whose time is devoted to catching and eating.

476 THE HUMAN BODY.

We have high authority for saying that man does not live by
bread alone; in other words, his highest development is
impossible when he is totally absorbed in " keeping body and
soul together/' and the more labor that can be spared from
getting enough food the better chance has he, if he use his
leisure rightly, of becoming a more worthy man. While
there is, thus, a theoretically best diet, it is nevertheless
impossible to say what that is for each individual; but what
the general experience is may be approximately gathered by
taking an average of the dietaries of a number of public
institutions in which the health of many people is main-
tained as economically as possible. Such an examination
made by Moleschott gives us as its result a diet containing
daily — . •

Proteids 30 grams or 465 grains

Fats 84 " or 1,300 "

Carbohydrates 404 " or 6,262 "

Salts 30 " or 465

Water 2800 " or 43,400 "

People in easy circumstances take as a rule more proteids
and fats and less amyloids; and this selection, when a choice
is possible, probably indicates that such a diet is the better
one : the proteids in the above table seem especially deficient.
Experimenting on himself the physiologist Ranke found that
when he was in good health, neither gaining nor losing
weight, and excreting daily as much nitrogen as he took in
food, he maintained this condition of equilibrium on a diet
containing

Proteids 100 grams ( 1550 grains)

Fats 100 " ( 1550 " )

Carbohydrates 240 " ( 3720 " )

Salts 25 " ( 437 " )

Water 2600 " (40,400 " )

Other experimenters have since arrived at very similar re-
suits; and such a diet is probably about the normal for per-
sons of our race living in a temperate climate.

CHAPTEE XXX.

THE PRODUCTION AND REGULATION OF THE HEAT OF
THE BODY.

Cold- and Warm-blooded Animals. All animals, so
long as they are alive, are the seat of chemical changes by
which heat is liberated; hence all tend to be somewhat
warmer than their ordinary surroundings, though the differ-
ence may not be noticeable unless the heat production is
considerable. A frog or a fish is a little hotter than the air
or water m which it lives, but not much; the little heat that
it produces is lost, by radiation or conduction, almost at once.
Hence such animals have no proper temperature of their own;
on a warm day they are warm, on a cold day cold, and are
accordingly known as changeable-temperatured (poikilo-tlier-
mous) or, in ordinary language, " cold-blooded " animals.
Man and other mammals, as well as birds, on the contrary,
are the seat of very active chemical changes by which much
heat is produced, and so maintain a tolerably uniform tem-
perature of their own, much as a fire does whether it be burn-
ing in a warm or a cold room; the heat production during
any given time balancing the loss, a normal body temperature
is maintained, and usually one considerably higher than that
of the medium in which they live; such animals are com-
monly named " warm-blooded. " This name, however, does
not properly express the facts; a lizard basking in the sun
on a warm summer's day may be quite as hot as a man usu-
ally is; but on the cold day the lizard becomes cold, while
the average temperature of the healthy Human Body is,
within a degree, the same in winter or summer; within the
arctic circle or on the equator. Hence it is better to call
such animals " homothermous " or of uniform temperature.

Moderate warmth accelerates protoplasmic activity; com-
pare a frog dormant in the winter with the same animal ac-
tive in the warm months: what is true of the whole frog is
true of each of its living cells. Its muscles contract more

477

478 THE HUMAN BODY,

rapidly when warmed, and the white corpuscles of its blood
when heated up to the temperature of the Human Body are
seen (with the microscope) to exhibit much more active amoe-
boid movements than they do at the temperature of frog's
blood. In summer a frog or other cold-blooded animal uses
much more oxygen and evolves much more carbon dioxide
than in winter, as shown not only by direct measurements of
its gaseous exchanges, but by the fact that in winter a frog
can live a long time after its lungs have been removed (being
able to breathe sufficiently through its moist skin), while in
warm weather it dies of asphyxia very soon after the same
loss. The warmer weather puts its tissues in a more active
state; and so the amount of work the animal does, and there-
fore the amount of oxygen it needs, depend to a great extent
upon the temperature of the medium in which it is living.
With the warm-blooded animal the reverse is the case. Within
very wide limits of exposure to heat or cold it maintains its
temperature at that at which its tissues live best ; accordingly
in cold weather it uses more oxygen and sets free more carbon
dioxide because it needs a more active internal combustion to
compensate for its greater loss of heat to the exterior. And
it does not become warmer in warm weather, partly because
its oxidations ore less than in cold (other things being equal),
and partly because of physiological arrangements by which it
loses heat faster from its body. In fact the living tissues of
a man may be compared to hothouse plants, living in an arti-
ficially maintained temperature; but they differ from the
plants in the fact that they themselves are the seats of the
combustions by which the temperature is kept up. Since,
within wide limits, the Human Body retains the same temper-
ature no matter whether it be in cold or warm surroundings,
it is clear that it must possess an accurate arrangement for
heat regulation ; either by controlling the production of heat
in it, or the loss of heat from it, or both.

The Temperature of the Body. The parts of the Body
are all either in contact with one another directly or, if not,
at least indirectly through the blood, which, flowing from
part to part, carries heat from warmer to colder regions.
Thus, although at one time one group of muscles may espe-
cially work, liberating heat, and at other times another, or
the muscles may be at rest and the glands the seat of active
oxidation, the temperature of the whole Body is kept pretty
much the same. The skin, however, which is in direct con-

THE HEAT OF THE BODY. 479

tact with external bodies, usually colder than itself, is cooler
than the internal organs; its temperature in health is from
36° to 37° C. (96.8-98.5° F.), being warmer in more protected
parts, as the hollow of the armpit. In internal organs, as
the liver and brain, the temperature is higher; about 43° C.
(107° F.) in health. In the lungs there is a certain quantity
of heat liberated when oxygen combines with haemoglobin, but
this is more than counterbalanced by loss of the heat carried
out by the expired air and that used up in evaporating the
water carried out in the breath, so the blood returned to the
heart by the pulmonary veins is slightly colder than that
carried from the right side of the heart to the lungs.

The Sources of Animal Heat. Apart from heat received
from its surroundings in hot food and drink the sources of
heat in the Body are twofold — direct and indirect. Heat is
directly produced wherever oxidation is taking place; and,
since almost invariably the chemically degrading or katabolic
processes going on in a living organ exceed the anabolic, the
living tissues at rest produce heat as one result of the chemical
changes supplying them with energy for the maintenance of
their vitality: and whenever an organ is active and its chemi-
cal metamorphoses are increased it becomes hotter: a secret-
ing gland or a contracting muscle is warmer than a resting
one, and the venous blood leaving noticeably warmer than
•the arterial supplied to it. Indirectly, heat is developed
within the Body by the transformation of other forms of en-
ergy: mainly mechanical work, but also of electricity. All
movements of parts of the Body which do not move it in
space or move external objects, are transformed into heat
within it; arid the energy they represent is lost in that form.
Every cardiac contraction sets the blood in movement, and
this motion is for the most part turned into heat within the
Body by friction within the blood-vessels. The same trans-
formation of energy occurs with respect to the movements of
the alimentary canal, except in so far as they expel matters
from the Body; and every muscle in contracting has part of
the mechanical energy expended by it turned into heat by
friction against neighboring parts. Similarly the movements
of cilia and of amoeboid cells are for the most part converted
in the Body into heat. The muscles and nerves are also the
seats of manifestations of electricity, which, though small in
amount, for the most part do not leave the Body in that form
tout are first converted into heat.

480 THE HUMAN BODY.

The Energy Lost by the Body in Twenty-four Hours

Practically speaking, the Body only loses energy in two
forms; as heat and mechanical work: by applying conduct-
ors to different parts of its surface small amounts of elec-
tricity can be carried off, but the amount is quite trivial in
comparison with the total daily energy expenditure. During
complete rest, that is, when no more work is done than that
necessary for the maintenance of life, nearly all the loss takes
the form of heat. The absolute amount of this will vary
with the surrounding temperature and other conditions, but
on an average a man loses, during a day of rest, 2700 calories;
that is enough to raise 2700 kilograms (5940 Ibs.) of water
from 0° to 1° C. (from 32° to 33 8° F.); otherwise expressed,
this amount of heat would boil 27 kilos (59.4 Ibs.) of ice-cold
water. This does not quite represent all the energy lost by
the Body in that time: since a small proportion is lost aa
mechanical work in moving the clothes and air by the respir-
atory movements, and even by the beat of the heart, which <it
each systole pushes out the chest-wall a little and moves the
things in contact with it. The working Body liberates and
loses much more energy; part as mechanical work done on
external objects, part as increased heat radiated or conducted
from the surface, or carried off by the expired air in. the
quickened respirations. Every one knows that he feels
warmer when he takes exercise, and this is due to the greater
amount of blood then carried to the skin and raising for the
time its temperature. The general temperature of the Body
as measured in the mouth is not at all or only very slightly
raised, however, as the greater loss of heat from the skin keeps
the average temperature of the blood at its normal level. This
greater loss corresponding to the greater production has been
measured on persons enclosed in specially constructed calori-
meters; and though there are considerable difficulties in the
way of getting quite accurate results, the measurements show
that the heat produced and lost in a day of moderate work is
about one third greater than that in a day of rest. The fol-
lowing table gives more definite numbers:

Day of Rest. Day of Work.

Rest 16 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs.
Heat-units (calo- )
ries) produced \ 24~QA 320 1235.2 2169.6 320.

10,885 Fah.-lb. \ „,,„, R / 14,528 Fah.-lb.

heat-units. ) 3724.8 ( |ieat-units.

THE HEAT OF THE BODY. 481

The mechanical work done on the working day represented
in addition an expenditure of energy of 213,344 kilogram-
meters, which is equal to 502 calories. Of the excess heat in
the working day, part is directly produced by the increased
chemical changes in the quicker working heart and respira-
tory muscles, and the other muscles set at work; while part
is indirectly due to heat arising from increased friction in the
blood-vessels as the blood is driven faster around them, and
to friction of the various muscles used. The average cardiac
work in twenty-four hours is about 60,000 kilogrammeters;
that of the respiratory muscles about 14,000; and since nearly
all of both is turned finally into heat within the Body, we
have 74,000 kilogrammeters of energy answering to about 174
calories (6786 Fah.-lb. units) indirectly produced in the rest-
ing Body daily from these sources.

Of 100 parts of heat lost from the resting Body, about
74.7 are carried off in radiation or conduction from the skin.
14.5 are carried off in evaporation from the skin.
5.4 " " " " " " lungs.

3.6 " " " . expired air.
1.8 " " « the excretions.

In a day of average work, of every 100 parts of energy lost
in any form from the Body —
1-2 go as heat in the excreta.
3-4 iii heating the expired air.

20-30 in evaporating water from the lungs and skin.
60-75 in heat radiated or conducted from the surfaces and in
external mechanical work.

It is obvious, however, that such numbers are only rough
approximations and must vary greatly with the temperature
and moisture of the surrounding air, the rate of respiration,
and other circumstances.

The Superiority of the Body as a Working Machine.
During eight hours of work we find (see table) the Body
loses 2169.6 calories of energy as heat, and can do simul-
taneously work equivalent to 502 calories. So of all the
energy lost from it in that time about \ may take the form of
mechanical work; this is a very large proportion of the total
energy expended, being a much higher percentage than that
given by ordinary machines. The best steam-engines can
utilize as mechanical work only about | of the total energy
liberated in them and lost from them :n a given time, the

482 THE HUMAN BODY,

remainder is transmitted directly as heat to the exterior, and
is lost to the engine for all useful purposes.

The Maintenance of an Average Temperature. This is
necessary for the continuance of the life of a warm-blooded
animal; should the temperature rise above certain limits
chemical changes, incompatible with life, occur in the tissues ;
for example at about 49° C. (120° F.) the muscles begin to
become rigid. On the other hand, death ensues if the Body
be cooled down to about 19° 0. (66° F.). Hence the need
of means for getting rid of excess heat, and of protection
from excessive cooling. Either end may be gained in two
ways: by altering the rate at which heat is lost or that at
which it is produced. As regards heat-loss, by far the most
important regulating organ is the skin : under ordinary cir-
cumstances nearly 90 per cent of the total heat given off from
the Body in 24 hours goes by the skin (73 by radiation and
conduction, 14.5 by evaporation). This loss may be con-
trolled—

1. By clothing ; we naturally wear more in cold and less
in warm weather; the effect of clothes being, of course, not
to warm the Body but to dimmish the rate at which the heat
produced in it is lost.

2. Increased temperature of the surrounding medium in-
creases the activity of the heart and lungs. A hastened cir-
culation by itself does not, as already pointed out (Chap.
XXVI), increase the general tissue activity of the Body, or
the oxidations occurring in it, and so, apart from the harder-
working heart itself, does not influence the amount of heat
liberated in the Body during a given time: but the more rapid
blood-flow through the skin carries more of that fluid through
this cool surface in each minute and in that wray increases
the loss of heat. The quickened respirations, too, increase the
evaporation of water from the lungs and, thus, the loss of heat.

3. Warmth, mainly through reflex vaso-moter actions leads
to dilatation of the skin-vessels and cold to contraction. In
a warm room the vessels on the surface dilate as shown by its
redness, while in a cold atmosphere they contract and the
skin becomes pale. But the more blood that flows through
the skin the greater will be the heat lost from the surface —
and vice versa.

4. Heat induces sweating and cold checks it; the heat
appears to act, partly, reflexly through afferent cutaneous

THE HEAT OF THE BODY. 483

nerve fibres exciting the sweat-centres from which the
secretory nerves for the sudoriparous glands arise and, partly,
directly on those centres, as they are thrown into activity, at
least in health, as soon as the temperature of the blood flow-
ing through the spinal cord is raised. In fever of course we
may have a high temperature with a dry non-sweating skin.
The more there is sweat poured out, the more heat is used
up in evaporating it and the more the Body is cooled.

5. Our sensations induce us to add to or diminish the
heat in the Body according to circumstances; as by cold or
warm baths, and iced or hot drinks.

As regards temperature-regulation by modifying the rate
of heart production in the Body, the following points may be
noted ; on the whole, such regulation is far less important
than that brought about by changes in the rate of loss, since
the necessary vital work of the Body always necessitates the
continuance of oxidative processes which liberate a tolerably
large quantity of heat. The Body cannot therefore be cooled
by diminishing such oxidations; nor, on the other hand, can
it be safely warmed by largely increasing them. Still, within
certain limits, the heat production may be controlled in
several ways :

1. Cold increases hunger; and increased ingestion of
food increases bodily oxidation, as shown by the greater
amount of carbon dioxide excreted in the hours succeeding
a meal. This increase is probably due to the activity into
which the digestive organs and such metabolic organs as the
liver are thrown; hepatic-vein blood is about one degree cen-
tigrade (nearly two degrees Fahrenheit) warmer than portal-
vein blood, and during digestion much more blood flows
through the liver.

2. Cold inclines us to voluntary exercise; warmth to
muscular idleness; and the more the muscles are worked the
more heat is produced in the Body.

3. Cold tends to produce involuntary muscular move-
ments, and so increased heat production; as chattering of
the teeth and shivering.

4. Cold applied to the skin increases the bodily chemical
metamorphoses and heat production. At least the tem-
perature in the armpit rises at first on entering a cold bath,
though the heat carried off from the surface soon over-
balances its increased production. The phenomenon may,

484 THE HUMAN BODY.

•N.

however, be explained in another way, the rise being at-
tributed to a sudden diminution of loss from more exposed
parts of the skin, dependent on contraction of the cutaneous
arteries. In some cases, however, the temporary rise is accom-
panied by an increased excretion of carbon dioxide, which
would indicate that the surface cooling does really increase
the oxidations of the Body.

5. Certain drugs, as salicylic acid, and perhaps quinine,
diminish the heat production of the Body. Their mode of
action is still obscure.

On the whole, however, the direct heat-regulating mech
anisms of the Human Body itself are not very efficient,
especially as protections against excessive cooling. Man
needs to supplement them by the use of clothing, fuel, and
exercise.

Local Temperatures. Although, by the means above
described, a wonderfully uniform bodily temperature is
maintained, and by the circulating blood all parts are kept
at nearly the same warmth, variations in both respects do
occur. The arrangements for equalization are not by any
means fully efficient. External parts, as the skin, the lungs
(which are really external in the sense of being in contact
with the air), the mouth, and the nose chambers, are always
cooler than internal; and even all parts of the skin have not
the same temperature, such hollows as the armpit being
warmer than more exposed regions. On the other hand, a
secreting gland or a working muscle becomes warmer, for
the time, than the rest of the Body, because more heat is
liberated in it than is carried off by the blood flowing
through. In such organs the venous blood leaving is warmer
than the arterial coming to them; while the reverse is the
case with parts, like the skin, in which the blood is cooled.
An organ colder than the blood is of course warmed by an
increase in its circulation, as seen in the local rise of temper-
ature in the skin of the face in blushing.

Thermogenic Nerves. All nerves, such as motor or
secretory, which can throw working tissues into activity are
in a certain sense thermogenic nerves, since they excite in-
creased oxidation and heat production in the parts under
their control. A true, purely thermogenic nerve would be
one which increased the heat production in a tissue without
otherwise throwing it into activity; and whether such exist

THE HEAT OF THE BODY. 485

is still undecided. Certain phenomena of disease, however,
seem to render their existence probable. If we return for a
moment to our former comparison of the working Body to
a steam-engine, such nerves might be regarded as agencies
increasing its rate of rusting without setting it at work.
The oxidation of the iron would develop some heat, but by
processes useless to the steam-engine, although such are, in
moderation, essential to living cells; the vitality of these,
even when they rest, seerns to necessitate a constant, if small,
breaking down of their substance. In an amoeboid cell no
doubt such processes occur quite independently of the ner-
vous system; but in more differentiated tissues they may be
controlled by it. Just as a muscle does not normally con-
tract unless excited through its nerve, although a white
blood-corpuscle does, so may the natural nutritive processes
of the muscle-fibre in its resting condition be dependent on
the nerves going to it. If these be abnormally excited the
muscle will break down its protoplasm faster than it con-
structs it, and consequently waste; at the same time the
increased chemical degradation of its substance will elevate
its temperature. Febrile conditions, in which many tissues
waste, without any unusual manifestation of their normal
physiological activity, would thus be readily accounted for
as due to superexcitation of the thermogenic nerves and
nerve-centre.

The condition of fever or pyrexia, as an abnormally high
temperature is named, could conceivably be brought about by
increased heat production, decreased heat loss, or both; or
by a greater increase of production than of loss. Direct ex-
periments on animals prove that there is always increased
production of heat, in febrile diseases. This is shown by the
fact that the animal uses more oxygen and gives off more
carbon dioxide in a given time than when in health. It also
usually gives off more heat, but not enough to compensate for
the increase of oxidative processes going on in its body, and
so its temperature rises. The regulating mechanism which
in health keeps heat production and heat dissipation propor-
tionate is out of gear. As regards the increased heat formation
in pyrexial conditions, there is some reason to believe that it
is usually due to excitation by morbid products of thermogenic
centres lying in the corpora striata or optic thalami. Prick-
ing those regions of the brain of an animal causes greatly in-

486 THE HUMAN BODY.

creased heat formation in its body. This has been interpreted
either as due to the excitation of thermogenic nerve-centres
which then stir up increased katabolisms in the tissues or to
injury and paralysis of inhibitory centres which normally
hold tissue metabolisms in check. The fact that a similar
result may be obtained by electrical stimulation of this region
of the brain is in favor of the excitation theory, but the possi-
bility of the existence also of febrile paralysis of nerve-cells
which normally inhibit a heat-production centre should be
borne in mind.

Clothing. While the majority of other warm-blooded
animals have coats of their own, formed of hairs or feathers,,
over most of man's Body his capillary coating is merely rudi-
mentary and has lost nearly all physiological importance as a
protection from cold; except in tropical regions he has to
protect himself by artificial garments, which his aesthetic
sense has led him to utilize also for purposes of adornment.
Here, however, we must confine ourselves to clothes from a.
physiological point of view. In civilized societies every one
is required to cover most of his Body with something, and
the question is what is the best covering; the answer will
vary, of course, with the climatic conditions of the country
dwelt in. In warm countries, clothing, in general terms,,
should allow free radiation or conduction of heat from the
surface; in cold it should do the reverse; and in temperate-
climates, with varying temperatures, it should vary with the
season. If the surface of the Body be exposed so that cur-
rents of air can freely traverse it much more heat will be
carried off (under those usual conditions in which the air is
cooler than the skin) than if a stationary layer of air be main-
tained in contact with the surface. As every one knows, a
" draught " cools much faster than air of the same tempera-
ture not in motion. All clothing, therefore, tends to keep
up the temperature of the Body by checking the renewal of
the layer of air in contact with it. Apart from this, how-
ever, clothes fall into two great groups: those which are
good, and those which are bad, conductors of heat. The-
iormer allow changes in the external temperature to cool or
heat rapidly the air stratum in actual contact with the Body,
while the latter only permit these changes to act more slowly.
Of the materials used for clothes, linen is a good conductor;

THE HEAT OF THE BODY. 487

calico not quite so good; and silk, wool, and fur are bad con-
ductors.

Whenever the surface of the Body is suddenly chilled
the skin-vessels are contracted and those of internal parts
reflexly dilated; hence internal organs tend to become con-
gested ; this within limits is a protective physiological pro-
cess, but if excessive it readily passes into the diseased state
known as inflammation. When hot, therefore, the most
unadvisable thing to do is to sit in a draught, throw off the
clothing, or in other ways to strive to get suddenly cooled.
Moreover, while in the American summer it is tolerably safe
to wear good-conducting garments, and few people take cold
then, this is by no means safe in the spring or autumn, when
the temperature of the air is apt to vary considerably within
the course of a day. A person going out, clad only for a
warm morning, may have to return in a very much colder
evening; and if his clothes be not such as to prevent a sud-
den surface chill, will get off lightly if he only " take " one
of the colds so prevalent at those seasons. In the great
majority of cases, no doubt, he suffers nothing worse, but
persons, especially of the female sex, often thus acquire far
more serious diseases. When sudden changes of temperature
are at all probable, even if the prevailing weather be warm,
the trunk of the Body should be always protected by some
tolerably close-fitting garment of non-conducting material.
Those whose skins are irritated by anything but linen should
wear immediately outside the under-garments a jacket of
silken or woollen material. In mid-winter comparatively few
people take cold, because all then wear thick and noncon-
ducting clothing of some kind.

CHAPTER XXXI.
SENSATION AND SENSE-OKGANS.

The Subjective Functions of the Nervous System.

Changes in many parts of our Bodies are accompanied or
followed by those states of consciousness which we call sen-
sations. All such sensitive parts are in connection, direct
or indirect, with the brain, by certain afferent nerve-fibres
called sensory. Since all feeling is lost in any region of the
Body when this connecting path is severed, it is clear that
all sensations, whatever their primary exciting cause, are
finally dependent on conditions of the central nervous system.
Hitherto we have studied this as its activities are revealed
through movements which it excites or prevents; we have
seen it, directly or reflexly, cause muscles to contract, glands
to secrete, or the pulsations of the heart to cease; we have
viewed it objectively, as a motion-regulating apparatus. Now
we have to turn to another side and consider it (or parts of
it) as influencing the states of consciousness of its possessor:
this study of the subjective activities of the nervous system is
one of much greater difficulty.

It may be objected that considerations concerning states
of feeling have no proper place in a treatise on Anatomy
and Physiology; that, since we cannot form the beginning
of a conception how a certain state of the nervous system
causes the feeling redness, another the feeling blueness, and
a third the emotion anger, all examination of mental phe-
nomena should be excluded from the sciences dealing with
the structure and properties of living things. But, although
we cannot imagine how a nervous state (neurosis) gives rise
to a conscious state (psychosis), we do know this, that dis-
tinct phenomena of consciousness never come under our
observation apart from a nervous system, and so are pre-
sumably, in some way, endowments of it; we are, therefore,
justified in calling them properties of the nervous system;

488

SENSATION AND SENSE-ORGANS. 489

and their examination, especially with respect to what nerve-
parts are concerned with different mental states, and what
changes in the former are associated with given phenomena
in the latter, forms properly a part of Physiology. Whether
masses of protoplasm, before the differentiation of definite
nerve-tissues, possess some ill-defined sort of consciousness,
as they possess an indefinite contractility before they have
been modified into muscular fibres, may for the present be
left undecided: though those who accept the doctrine of
evolution will be inclined to assent to the proposition.

' While, however, the physiologist has a right to be heard
on questions relating to our mental faculties, it is neverthe-
less true that many laws of thought have been established
concerning which our present knowledge of the laws of the
nervous system gives as no clue; the science of Psychology
has thus a well-founded claim to an independent existence.
But, in so far as its results are confined merely to the succes-
sions and connections of mental states, as established by
observation, they are merely descriptions, and not explana-
tions in a scientific sense: we know that so many mental phe-
nomena have necessary material antecedents and concomi-
tants in nervous changes, that we are justified in believing
that all have such, and in continuing to seek for them. We
do not know at all how an electric current sent round a bar
of soft iron makes it magnetic; we only know that the one
change is accompanied by the other; but we say we have
explained the magnetism of a piece of iron if we have found
an electric current circulating around it. Similarly, we do
not know how a nervous change causes a mental state, but
we have not explained the mental state until we nave found
the nervous state associated with it and how that nervous
state was produced.

As yet it is only with respect to some of the simplest
states of consciousness that we know much of the necessary
physiological antecedents, and among these our sensations
are the best investigated. As regards such mental phenom-
ena as the Association of Ideas and Memory, physiology
can give us some light; but so far as others, such as the Will
and the Emotions, are concerned, it has at present little to
offer. The phenomena of Sensation, therefore, occupy at
present a much larger portion of physiological works than
all other mental facts put together.

490 THE HUMAN BODY.

Common Sensation and Organs of Special Sense. A

sensory nerve is one which, when stimulated, arouses, or may
arouse, a sensation in its possessor. The stimulant is in all
cases some form of motion, molar (e.g., mechanical pressure)
or molecular (as ethereal vibrations or chemical changes).
Since all our nerves lie within our Bodies as circumscribed
by the skin, and are excited within them, one might a priori
be inclined to suppose that the cause of all sensations would
appear to be within our Bodies themselves; that the thing
felt would be a modified portion of the feeler. This is the
case with regard to many sensations; a headache, toothache,
or earache gives us no idea of #ny external object; it merely
suggests to each of us a particular state of a sensitive portion
of myself. As regards many sensations, however, this is not
so; they suggest to us external causes, to properties of which,
and not to states of our Bodies, we ascribe them; and so they
lead us to the conception of an external universe. A knife
laid on the skin produces changes in it which lead us to
think not of a state of our skin, but of states of some object
outside the skin; we believe we feel a cold heavy hard thing
in contact with it. Nevertheless we have no sensory nerves
going into the knife and informing us directly of its condi-
tion; what we really feel are the modifications of our Body
produced by it, although we irresistibly think of them as
properties of the knife — of some object that is no part of the
Body, and not of them as states of the latter. Let now the
knife cut through the skin; we feel no more knife, but ex-
perience pain, which we think of as a condition of ourselves.
We do not say the knife is painful, but that our finger is, and
yet we have, so far as sensation goes, as much reason to call
the knife painful as cold. Applied one way it produced
local changes arousing a sensation of cold, and in another
local changes causing a sensation of pain. Nevertheless in
the one case we speak of the cold as being in the knife, and
in the other of the pain as being in the finger.

Sensitive parts, such as the surface of the skin, through
which we get, or believe we get, information about outer
things, are of far more intellectual value to us than sensitive
parts, such as the subcutaneous tissue into which the knife
may cut, which give us only sensations referred to conditions
of our Bodies. The former are called Sense-organs proper,

SENSATION AND SENSE-ORGANS. 491

or Organs of Special Sense / the latter are sensitive parts,
or Organs of Common Sensation.

The Peripheral Reference of our Sensations, The fact
that we refer certain sensations to external causes is only
one case of a more general law, in accordance with which
we do not ascribe our sensations, as regards their locality, to
the brain, where the neurosis is accompanied by the sensa-
tion, but to a peripheral part. With respect to the brain,
other parts of the Body are external objects as much as the
rest of the material universe, yet we locate the majority of
our common sensations at the places where the sensory
nerves concerned are irritated, and not in the brain. Even
if a nerve-trunk be stimulated in the middle of its course,
we refer the resulting sensation to its outer endings. A blow
on the inside of the elbow-joint, injuring the ulnar nerve,
produces not only a local pain, but a sense of tingling
ascribed to the fingers to which the ends of the fibres go.
Persons with amputated limbs have feelings in their fingers
and toes long after they have been lost, if the nerve-trunks
in the stump be irritated. To explain such facts we must
trench on the ground of Psychology, and so they cannot be
fully discussed here; but they are commonly ascribed to the
results of experience. The events of life have taught us that
in the great majority of instances the sensory impulses which
excite a given tactile sensation, for example, have acted upon
the tip of a finger. The sensation goes when the finger is
removed, and returns when it is replaced; and the eye con-
firms the contact of the external object with the finger-tip
when we get the tactile sensation in question. We thus
come firmly to associate a particular region of the skin with
a given sensation, and whenever afterwards the nerve-fibres
coming from the finger are stimulated, no matter where in
their course, we ascribe the origin of the sensation to some
thing acting on the finger-tip.

The Differences between Sensations. In both groups
of sensations, those derived through organs of special sense
and those due to organs of common sensation, we distinguish
kinds which are absolutely distinct for our consciousness,
and not comparable mentally. We can never get confused
between a sight, a sound, and a touch, nor between pain,
hunger, and nausea; nor can we compare them with one
another: each is sui generis. The fundamental difference

492 THE HUMAN BODY.

which thus separates one sensation from another is its
modality. Sensations of the same modality may differ; but
they shade imperceptibly into one another, and are com-
parable between themselves in two ways. First, as regards
quality: while a high and a low pitched note are both
auditory sensations, they are nevertheless different and yet
intelligibly comparable; and so are blue, purple, and red ob-
jects. In the second place, sensations of the same modality are
distinguishable and comparable as to amount or intensity: we
readily recognize and compare a loud and a weak sound of
the same pitch; a bright and feeble light of the same color;
an acute and a slight pain of the same general character.
Our sensations thus differ in the three aspects of modality,
quality within the same modality, and intensity. Certain
sensations also differ in what is known as the " local signs"
a difference by which we tell a touch on one part of the skin
from a similar touch on another; or an object exciting one
part of the eye from an object like it, but in a different loca-
tion in space and exciting another part of the visual surface.
As regards modality, we commonly distinguish five senses,
those of sight, sound, touch, taste, and smell; to these, tem-
perature must be added. The varieties of common sensation
are also several; for example, pain, hunger, satiety, thirst,
nausea, malaise, Men etre (" feeling good "), fatigue. The
muscular sense stands on the intermediate line between
special and common sensations; we gather by it how much
our various muscles are contracted: and so learn the position
of various parts of the Body, on the one hand, and the re-
sistance opposed to bodily movement by external objects, on
the other. In fact, we cannot draw a sharp line between the
special senses and common sensations : all the Body, we con-
clude from observations on the lower animals, is, at an early
stage of its development, sensitive; very soon its cells sepa-
rate themselves into an outer layer exposed to the action of
external forces and an inner layer protected from them : and
some of the former cells become especially sensitive. From
them, as development proceeds, some are separated and
buried beneath the surface to become the brain and spinal
cord ; of those which remain superficial, some are modified
so that they (in the eye) become especially excited by ethereal
vibrations; others (in the ear) become especially responsive
to sound vibrations; others to slight chemical changes (in

SENSATION AND SENSE-ORGANS. 493

mouth and nose), and others (in the skin) to variations in
pressure or temperature.

All our sensations are thus modifications of one common
primary sensibility, represented by that of the skin, or rather
by the primitive representative of the skin in such an animal
as the Hydra (see Zoology). The cutaneous sensations, being
less differentiated, shade off more readily into the common
sensibility of the other living tissues than do the activities of
the highly differentiated cells in the eye and ear. We find,
accordingly, that while a powerful pressure or a high tem-
perature acting on the skin readily arouses a sensation of
pain, that this is not the case with the more specialized visual
and auditory organs. Their super-excitement may be dis-
agreeable, but never passes into pain, in the ordinary sense
of the word. Similarly the special skin sensations, touch
and temperature, may sometimes be confounded, while a
sound and a sight cannot be : the modality of the less modi-
fied skin-senses is less complete.

The study of comparative anatomy and development
shows that the irritable parts of our sense-organs are but
special differentiations of the primary external layer of cells
which covered the Body when it was very young. Some of
these cells become nerve end-organs in the eye, others end-
organs in the ear, and so on; while others, less changed, re-
main in the skin as organs of touch and temperature; and
so, from a general exterior surface responding equally readily
to many external natural forces, we get a surface modified so
that its various parts respond with different degrees of read-
iness to different external forces; and these modified parts
constitute the essential portions of our organs of special sense,
^very sense organ thus comes to have a special relationship to
some one natural force or form of energy — is a specially
irritable mechanism by which such a force is enabled to excite
sensory nerves; and is, moreover, commonly supplemented by
arrangements which, in the ordinary circumstances of life,
prevent other forces from stimulating the nerves connected
with it. Not all natural forces have sense-organs with ref-
erence to them developed in the Human Body; for example,
we have no organ standing to electrical changes in the same
relation that the eye does to light or the ear to sound.

The Essential Structure of a Sense-organ. In every
sense-organ the fundamental part is one or more end-organs,

494 THE HUMAN BODY.

which are highly irritable tissues (p. 31), so constructed and
so placed as to be normally acted on by some one of the
modes of motion met with in the external world. A sensory
apparatus requires in addition at least a brain-centre and a
sensory nerve-fibre connecting this with the terminal appa-
ratus; but one commonly finds accessory parts added. In
the eye, e.g., we have arrangements for bringing to a focus
the light rays which are to act on the end organs of the
nerve-fibres; and in the ear are found similar subsidiary
parts, to conduct sonorous vibrations to the end apparatus of
the auditory nerve.

Seeing and hearing are the two most specialized senses:
the stimuli usually arousing them are peculiar and quite dis-
tinct from the group of general nerve stimuli (Chap. XIII),
while those most frequently, or naturally, acting upon our
other sense-organs are not so peculiar; they are forces
which act as general nerve stimuli when directly applied to
nerve-fibres. The end-organs, however, as already pointed
out, so increase the sensitiveness of the parts containing
them that degrees of change in the exciting forces, which
would be totally unable to directly stimulate the nerve-fibres,
are appreciated. These terminal apparatuses are therefore
as truly mechanisms enabling changes, which would not
otherwise stimulate nerves, to excite them, as are the end-
organs in the eye or ear.

The Cause of the Modality of our Sensations. Seeing
that the external forces usually exciting our different sensa-
tions differ, and that the sensations do also, we might at first
be inclined to believe that the latter difference depended on
the former: that brightness differed from loudness because
light was different from sound. In other words, we are apt
to think that each sensation derives its specific character
from some property of its external physical antecedent, and
that our sensations answer in some way to, and represent
more or less accurately, properties of the forms of energy
arousing them. It is, however, quite easy to show that we
have no sufficient logical warrant for such a belief* Light
falling into the eye causes a sensation of luminosity, a feel-
ing belonging to the visual group or modality; and, since
usually nothing else excites such feelings and light entering
the healthy eye always does, we come to believe that the
physical agent light is something like our sensation of

SENSATION AND SENSE-ORGANS 495

luminosity. But, as we have already seen, no matter
how we stimulate the optic nerve we still get visual sensa-
tions; close the eyes and press with a finger-nail on one eye-
lid; a sensation of touch is aroused where the finger meets
the skin; but the pressure on the eyeball distorts it and
stimulates the optic nerve-fibres in it also, and the result is
a luminous patch seen in front of the eye in such a position
as a bright body must occupy in space to radiate light to the
stimulated part of the expansion of the optic nerve. Finding,
then, the same kind of sensation, a visual one, produced by
the totally different causes, pressure and light, we are led to
doubt if the differences of modality in our sensations depend
upon the differences of the natural forces arousing them;
and this doubt is strengthened when we find still other forces
giving rise to visual sensations. But then, since light
and pressure, electricity and cutting, all cause visual sensa-
tions, we have no valid reason for supposing that light, more
than either of the others, is really in any way like our sensa-
tion of light: or that sight-feeling differs from sound-feeling
because objectively light differs from sound. The eye is an
organ specially set apart to be excited by light, and accord-
ingly so fixed as to have its nerve-fibres far more often ex-
cited by that form of force than by any other; but the fact
that light sensations can be otherwise aroused shows plainly
that their kind or character has nothing directly to do with
any property of light. Just as by pinching or heating or
galvanizing a motor nerve we can make the muscles attached
to it contract, and the contraction has nothing in common
with the excitant, so the visual sensation, as such, is inde-
pendent of the stimulus arousing it and, of itself, tells us
•nothing concerning the kind of stimulus which has operated.
Differences in kind between external forces being thus
eliminated as possible causes of the modalities of our sensa-
tions, we next naturally fall back upon differences in the
sense-organs themselves. They do undoubtedly differ both
in gross and microscopic structure, and the fact that pressure
on the closed eye arouses a touch-feeling where the skin is
compressed, and a sight-feeling where optic nerve-fibres are,
might well be due to the fact that a peripheral touch-organ
was different from a peripheral sight-organ, and the same
force might therefore produce totally different effects on
them and so cause different kinds of feelings. However,

496 THE HUMAN BODY.

here also c.oser examination shows that we must seeK farther.
Sensation is not produced in a sense-organ, but far away
from it in the brain; the organ is merely an apparatus for
generating nervous impulses. If the optic nerves be divided,
no matter how perfect the eyeballs, no amount of light will
arouse visual sensations; if the spinal cord be cut in the
middle of the back no pressure on the feet will cause a tactile
or other feeling; though the skin, and its nerves and the
lower half of the spinal cord be all intact. In all cases we
find that if the nerve-paths between a sense-organ and the
brain be severed no stimulation of the organ will call forth a
sensation. The final production of this clearly depends,
then, on something occurring in the brain, and so the kind
of a sensation is presumably dependent upon brain events
rather than on occurrences in sense-organs. Still it might
be that something in the sense-organ caused one sensa-
tion to differ from another. Each organ might excite the
brain in a different way and cause a different sensation, and
so our sensations differ because our sense organs do. Such
a view is, however, negatived by observations which show
that perfectly characteristic sensations can be felt in the
absence of the sense-organs through which they are normally
excited. Persons whose eyeballs have been removed by the
surgeon, or completely destroyed by disease, have frequently
afterwards definite and unmistakable visual sensations., quite
as characteristic as those which they had while still possess-
ing the visual end organs. The tactile sensations felt in am-
putated limbs, already referred to, afford another example
of the same fact. The persons still feel things touching
their legs or lying between their long-lost toes; and the sen-
sations are distinctly tactile and not in any way less different
from visual or auditory sensations than are the touch-feelings
following stimulation of those parts of the skin which are still
possessed. It is, then, clear that the modality of our sensa-
tions is to be sought deeper than in properties of the end-
organs of the nerves of each sense.

Properties of external forces and properties of periph-
eral nerve-organs being excluded as causes of differences in
kind of sensation, we come next to the sensory nerve-fibres
themselves. Is it because optic nerve-fibres are different
from auditory nerve-fibres that luminous sensations are dif-
ferent from sonorous ? This question must be answered in

SENSATION AND SENSE-ORGANS. 497

the negative, for we have already seen reason to believe
that all nerve-fibres are alike in essential structure and that
their properties are everywhere the same; that all they do is
to transmit "nervous impulses" when excited, and that, no
matter what the excitant, these impulses are molecular move-
ments, always alike in kind, though they may differ in
amount and in rate of succession. Since, then, all that the
optic nerve does is to send nervous impulses to the brain,
and all that the auditory and gustatory and tactile and olfac-
tory nerve-fibres do is the same, and these impulses are all
alike in kind, we cannot explain the difference in quality of
visual and other sensations by any differences in property of
the nerve-trunks concerned, any more than we could attempt
to explain the facts that, in one case, an electric current sent
through a thin platinum wire heats it, and, in another, sent
through a solution of a salt decomposes it, by assuming that
the different results depend on differences in the conducting
copper wires, which may be absolutely alike in the two cases.
We are thus driven to conclude that our sensations pri-
marily differ because different central nerve-organs in the
brain are concerned in their production. That just as an
efferent nerve-fibre will, when stimulated, cause a secretion if
it go to a gland-cell, and a contraction if it go to a muscle-
fibre, so an optic nerve-fibre, carrying impulses to one brain
apparatus and exciting it, will cause a visual sensation, and a
gustatory nerve-fibre, connected with another brain-centre, a
taste sensation. In other words, our kinds of sensation
depend fundamentally on the properties of our own cerebral
nervous system. For each special sense we have a nervous
apparatus with its peripheral terminal organs, its nerve-fibres,
and its brain-centres ; and the excitement of this apparatus, no
matter in what way, causes a sensation of a given modality,
determined by the properties of its central portion. Usually
the apparatus is excited by one particular force acting first
on its peripheral organs, but it may be aroused by stimulat-
ing its nerve-fibres directly or, as in certain diseased states
(delirium), or under the action of certain drugs, by direct
excitation of the centres. The sensations of dreams, fre-
quently so vivid, and hallucinations, are also probably in
many cases due to direct excitation of the central organs of
sensory apparatuses, though no doubt also often due to periph-
eral stimulation. But no matter how or where the appa

498 THE HUMAN BODY.

ratus is excited, provided a sensation is produced it is always
of the modality of that sense apparatus.

While in the more specialized senses the modality of the
sensation can be ascribed only to brain properties (so that
we may be pretty sure that a man, the inner end of whose
optic nerve was in physiological continuity with the outer
end of his auditory, and the inner end of his auditory with
the outer end of his optic, would hear a picture and see a
symphony), yet, conceivably, differences in the rhythm or
intensity of afferent nervous impulses might cause differ-
ences in modality in less differentiated senses. Until quite
recently it has been considered possible that tactile and tem-
perature sensations were but extremes of one general kind of
feeling; that they were of the same "modality;" and com-
parable, for example, to the sensations of yellow and blue in
the visual set of feelings. This view has now been definitely
proved to be inadmissible (Chap. XXXV). The points of the
skin which arouse in us the sensations of touch, heat, and cold
are all distinct; each one when stimulated gives rise to only
one kind of sensation, if any; and always the same kind. A
heavy pressure, gradually increased, arouses sensations which
pass imperceptibly from touch to pain, and this result may
be due to the fact that regular and orderly afferent impulses,
determined through tactile nerve-endings, excite the centre
in one way; while irregular, disorderly, and violent impulses,
originated when the pressure is great enough to directly
excite nerve-trunks beneath the skin, may cause a different
sensation; much as musical notes properly combined may
cause pleasure, but all clashed together may cause suffering,
although the same brain-centres are stimulated in the two
cases. The pain from a heavy weight may, however, be due
to the fact that it excites a different set of nerve-fibres than
those connected with tactile feeling, and gives rise to impulses
which excite new centres, the modality of which is a pain
sensation so great as to cloak concomitant touch sensations.

However differences in nervous rhythm may account for
minor differences in sensation, it remains clear that the
characters of our sensations are creations of our own organ-
i^m; they depend on properties of our Bodies and not on
properties of external things, except in so far as these may
or may not be adapted to arouse our different sensory appa-
ratuses to activity. From the kind of the sensation we can-

SENSATION AND SENSE-ORGANS. 499

not, therefore, argue as to the nature of the excitant: we
have no more warrant for supposing that light- is like our
sensation of light than that the knife that cuts us is like our
sensation of pain. All that we know with certainty is states
of our own consciousness, and although from these we form
working hypotheses as to an external universe, yet, granting
it, we have no means of acquiring any real knowledge as to
the properties of things about us. What we want to know,
however, for the practical purposes of life is, not what things
are, but how to use them for our advantage, or to prevent
them from acting to our disadvantage; and our senses en-
able us to do this sufficiently well.

The Psycho-Physical Law. Although our sensations
are, in modality or kind, independent of the force exciting
them, they are not so in degree or intensity, at least within
certain limits. We cannot measure the amount of a sensa-
tion and express it in foot-pounds or calories, but we can get
a sort of unit by determining how small a difference in sensa-
tion can be perceived. Supposing this smallest perceptible
difference to be constant within the range of the same sense
(which is not proved), it is found that it is produced by dif-
ferent amounts of stimuli, measured objectively as forces;
and that there exists in some cases a relation between the two-
which can be expressed in numbers. The increase of stimu-
lus necessary to produce the smallest perceptible change in a
sensation is proportional to the strength of the stimulus
already acting; for example, the heavier a pressure already
acting on the skin the more must it be increased or dimin-
ished in order that the increase or diminution may be felt.
Expressed in another way the facts may be put thus: sup-
pose three degrees of stimulation to bear to one another ob-
jectively the ratios 10, 100, 1000, then their subjective ef-
fects, or the amounts of sensation aroused by them, will be
respectively as 1, 2, 3: in other words, the sensation increases
proportionately to the logarithm of the strength of the stimu-
lus. Examples of this, which is known as " Weber's " or
" Fechner's psycho-physical law" will be hereafter pointed
out, and are readily observable in daily life; we have, for
example, a luminous sensation of certain intensity when a
lighted candle is brought into a dark room; this sensation is
not doubled when a second candle is brought in; and is
hardly affected at all by a third. The law is only true, how-

500 THE HUMAN BODY.

ever (and then but approximately), for sensations of medium
intensity; it is applicable, for example, to light sensations of
all degrees between those aroused by the light of a candle
and ordinary clear daylight : but it is not true for luminosi-
ties so feeble as only to be seen at all with difficulty, or so
bright as to be dazzling.

Besides their variations in intensity, dependent on varia-
tions in the strength of the stimulus, our sensations also vary
with the irritability of the sensory apparatus itself; which is
not constant from time to time or from person to person.
In the above statements the condition of the sense-organ and
its nervous connections is presumed to remain the same
throughout.

Perceptions. In every sensation we have to carefully
distinguish between the pure sensation and certain judg-
ments founded upon it; we have to distinguish between what
we really feel and what we think we feel; and very often
firmly believe we do feel when we do not.

The most important of these judgments is that which
leads us to ascribe certain sensations, those aroused through
organs of special sense, to external objects — that outer refer-
ence of our sensations which leads us to form ideas concern-
ing the existence, form, position, and properties of external
things. Such representations as these, founded on our senses,
are called perceptions. Since these always imply some
mental activity in addition to a mere feeling, their full dis-
cussion belongs to the domain of Psychology. Physiology,
however, is concerned with them so far as it can determine
the conditions of stimulation and neurosis under which a
given mental representation concerning a sensation is made.
It is quite certain that we can feel nothing but states of our-
selves, but, as already pointed out, we have no hesitation in
saying we feel a hard or a cold, a rough or smooth body.
When we look at a distant object we usually make no demur
to saying that we perceive it. What we really feel is, how-
ever, the change produced by it in our eyes. There are no
parts of our Bodies reaching to a tree or a house a mile off —
and yet we seem to feel all the while that we are looking at
the tree or the house and feeling them, and not merely ex-
periencing modifications of our own eyes or brains. When
reading we feel that what we really see is the book; and yet

SENSATION AND SENSE-ORGANS. 501

the existence of the book is a judgment founded on a state
of our Body, which alone is what we truly feel.

We have the same experience in other cases, for example
with regard to touch.

Hairs are quite insensible, but are imbedded in the sensi-
tive skin, which is excited when they are moved. But
if the tip of a hair be touched by some external object we
believe we feel the contact at its insensible end, and not in
the sensitive skin at its root. So, the hard parts of the teeth
are insensible; yet when we rub them together we refer the
seat of the sensation aroused 'to the points where they touch
one another, and not to the sensitive parts around the sockets
where the sensory nerve impulse is really started.

Still more, we may refer tactile sensations, not merely to
the distal ends of insensible bodies implanted in the skin,
but to the far ends of things which are not parts of our
Bodies at all; for instance, the distant end of a rod held
between the finger and a table while the finger is moved a little
from side to side* We then believe we feel touch or pressure
in two places; one where the rod touches our finger, and the
other where it conies in contact with the table. A blind
man gropes his way along by feeling at the end of his stick.
If the rod is attached immovably to the table we feel only
its end next the finger. If we could fix it immovably on the
finger while the other end was movable on the table, we
would lose the sensation at the finger and refer the sensa-
tion of pressure to where the rod touched the table. When a
tooth is touched with a rod we only feel the contact at its
end, unless it is loose in its socket; and then we get two
sensations on touching its free end with a foreign body.

This irresistible mental tendency to refer certain of our
states of feeling to causes outside of our Bodies, and either in
contact with them or separated from them by a certain space,
is known as the phenomenon of the extrinsic reference of our
sensations. The discussion of its origin belongs properly to
Psychology, and it will suffice here to point out that it seems
largely to depend on the fact that the sensations extrinsically
referred can be modified by movements of our Bodies.
Hunger, thirst, and toothache all remain the same whether
we turn to the right or left, or move away from the place we
are standing in. But a sound is altered. We may find that
in a certain position of the head it is heard more by the

502 THE HUMAN BODY.

right ear than the left; but on turning round the reverse is
the case; and half way round the loudness in each ear is the
same. Hence we are led, by mental laws outside of the
physiological domain, to suspect that its cause is not in our
Body, but outside of it; and depends not on a condition of
the Body but on something else And this is confirmed
when going in one direction we find the sound increased,
and in the other that it is diminished. This implies that we
have a knowledge of our movements, and this we gain
through the muscular sense. It constitutes the reactive side
of our sensory life, associated with the changes we produce
in external things; and is correlated and contrasted with the
passive side, in which other things produce sensations by act-
ing upon us.

As regards our common sensations we find something of
the same kind. The more readily they can be modified by
movement the more definitely do we localize them in space,
though in this case within the Body instead of outside it.
Hunger and nausea can be altered by pressure on the pit of
the stomach; thirst by moistening the throat with water;
the desire for oxygen (respiration-hunger) by movements of
the chest; and so we more or less definitely ascribe these
sensations to conditions of those parts of the Body. Other
general sensations, as depression, anxiety, and so on, are not
modifiable by any particular movement, and so appear to us
rather as mental states, pure and simple, than bodily sensa-
tions.

Sensory Illusions. " I must believe my own eyes " and
" we can't always believe our senses " are two expressions
frequently heard, and each expressing a truth. No doubt a
sensation in itself is an absolute incontrovertible fact: if I
feel redness or hotness I do feel it, and that is an end of the
matter: but if I go beyond^ the fact of my having a certain
sensation and conclude from it as to properties of something
else — if I form a judgment from my sensation — I may be
totally wrong; and in so far be unable to believe my eyes or
skin. Such judgments are almost inextricably woven up
with many of our sensations, and so closely that we cannot
readily separate the two; not even when we know that the
judgment is erroneous.

For example, the moon when rising or setting appears
bigger than when high in the heavens — we seem to feel

SENSATION AND SENSE- ORGANS. 503

directly that it arouses more sensation, and yet we know cer-
tainly that it does not. With a body of a given brightness
the amount of change produced in the end organs of the eye
will depend on the size of the image formed in the eye, pro-
vided the same part of its sensory surface is acted upon.
Now the size of this image depends on the distance of the
object; it is smaller the farther off it is and greater the
nearer, and measurements show that the area of the sensitive
surface affected by the image of the rising moon is no larger
than that affected by it when overhead. Why then do we,
even after we know this, see it bigger ? The reason is that
when the moon is near the horizon we imagine, unconsciously
and irresistibly, that it is farther off; even astronomers who
know perfectly well that it is not, cannot help forming this
unconscious and erroneous judgment — and to them the moon
appears in consequence larger when near the horizon, just as
it does to less well-informed mortals. In fact we have a con-
ception of the sky over which the moon seems to travel, not as
a half sphere but as somewhat flattened, and hence when the
moon is at the horizon we unconsciously judge that it is
farther off than when overhead. But any body which ex-
cites the same extent of the sensitive surface of the eye at a
great distance that another does at less, must be larger than
the latter; and so we conclude that the moon at the horizon
is larger than the moon in the zenith, and are ready to de-
clare that we see it so.

So, again, a small bit of pale gray paper on a white
sheet looks gray: but placed on a large bright green surface
it looks purple; and on a bright red surface looks blue-
green. As the same bit of gray paper is shifted from one to
the other we see it change its color: it arouses in us different
feelings, or feelings which we interpret differently, although
objectively the light reflected from it remains the same.
Similarly a medium-sized man alongside of a very tall one
appears short, but when walking with a very short one, tall.

Such erroneous perceptions as these are known as sensory
illusions; and we ought to be constantly on guard against
them.

CHAPTER XXXII.

THE EYE AS AN OPTICAL INSTRUMENT.

The Essential Structure of an Eye. Every visual organ
consists primarily of a nervous expansion, provided with end-
organs by means of which light is enabled to excite nervous
impulses, and exposed to the access of objective light; such
an expansion is called a retina. By itself, however, a retina
would give no visual sensations referable to distinctly limited
external objects ; it would enable its possessor to tell light
from darkness, more light from less light, and (at least in its
highly developed forms) light of one color from light of an-
other color; but that would be all. Were our eyes merely
retinas we could only tell a printed page from a blank one by
the fact that, being partly covered with black letters (which
reflect less light), it would excite our visual organ less power-
fully than the spotless white page would. In order that dis-
tinct objects and not merely degrees of luminosity may be
seen, some arrangement is needed which shall bring all light
entering the eye from one point of a luminous surface to a
focus again on one point of the sensitive surface. If A and
B (Fig. 139) be two red spots on a black surface, K, and rr
be a retina, then rays of light diverging from A would fall
equally on all parts of the retina and excite it all a little ; so
with rays starting from B. The sensation aroused, suppos-
ing the retina in connection with the rest of the nervous
visual apparatus, would be one of a certain amount of red
light reaching the eye; the red spots, as definite objects,
would be indistinguishable. If, however, a convex glass lens
L (Fig. 140) be put in front of the retina, it will cause to
converge again to a single point all the rays from A falling
upon it; so, too, with the rays from B : and if the focal dis-
tance of the lens be properly adjusted these points of conver-
gence will both lie on the retina, that for rays from A at «,
and that for rays from B at b. The sensitive surface would
then only be excited at two limited and separated points by

504

THE EYE AS AN OPTICAL INSTRUMENT.

505

the red light emanating from the spots; consequently only
some of its end-organs and nerve-fibres would be stimulated
and the result would be the recognition of two separate red

FIG. 189. —Diagram illustrating the indistinctness of vision with a retina alone.
K, a surface on which are two spots, A and B; r i\ the retina. The diverging
lines represent rays of light spread uniformly over the retina from each spot.

objects. In our eyes there are certain refracting media
which lie in front of the retina and take the place of the lens
L in Fig. 140. That portion of physiology which treats of

FIG. 140.— Illustrating the use of a lens in giving definite retinal images. A, B,
K, r r, as in Fig. 139. Z,, a biconvex lens so placed that it l>rings to a focus on the
points a and b. of the retina, rays of light diverging from ,-1 and B respectively.

the physical action of these media or, in other words, of the
eye as an optical instrument, is known as the dioptrics of the
eye.

The Appendages of the Eye. The eyeball itself con-
sists of the retina and refracting media, together with sup-
porting and nutritive structures and other accessory appa-
ratuses, as, for example, some controlling the light-converg-
ing power of the media, and others ^regulating the size of the
aperture (pupil] by which light enters. Outside the ball lie
muscles which bring about its movements, and other parts
serving to protect it.

Each orbit is a pyramidal cavity occupied by connective
tissue, muscles, blood-vessels and nerves, and in great part by
fat, which forms a soft cushion on which the back of the eye-
ball lies and rolls during its movements. The contents of

506 THE HUMAN BODY.

the orbit being for the most part incompressible, the eye can-
not be drawn into its socket. It simply rotates there, as
the head of the femur does in the acetabulum. When the
orbital blood-vessels are gorged, however, the eyeball may
protrude (as in strangulation) ; and when these vessels empty
it recedes somewhat, as is commonly seen after death. The
front of the eye is exposed for the purpose of allowing light
to reach it, but can be covered up by the eyelids, which are
folds of integument, movable by muscles and strengthened
by plates of fibro-cartilage. At the edge of each eyelid the
skin which covers its outside is turned in, and becomes con-
tinuous with a mucous membrane, the conjunctiva, which
lines the inside of each lid, and also covers all the front of
the eyeball as a closely adherent layer.

The upper eyelid is larger and more mobile than the
lower, and when the eye is closed covers all its transparent
part. Ifc has a special muscle to raise it, the levator palpebrm
superioris. The eyes are closed by a flat circular muscle,
the orbicularis palpebrarum which, lying on and around the
lids, immediately beneath the skin, surrounds the aperture
between them. At their outer and inner angles (canthi) the
eyelids are united, and the apparent size of the eye depends
upon the interval between the canthi, the eyeball itself being
nearly of the same size in all persons. Near the inner can-
thus the line of the edge of each eyelid changes its direction
and becomes more horizontal. At this point is found a small
eminence, the lachrymal papilla, on each lid. For most of
their extent the inner surfaces of the eyelids are in contact
with the outside of the eyeball, but near their inner ends a
red vertical fold of conjunctiva, the semilunar fold (plica
seniilunaris] intervenes. This is a representative of the third
eyelid, or nictitating membrane, found largely developed in
many animals, as birds, in which it can be drawn all over the
exposed part of the eyeball. At the inner or nasal corner is a
reddish elevation, the caruncula laclirymalis, caused by a
collection of sebaceous glands imbedded in the semilunar
fold. Opening along the edge of each eyelid are from
twenty to thirty minute compound sebaceous glands, named
the Meibomian follicles. Their secretion is sometimes ab-
normally abundant, and then appears as a yellowish matter
along the edges of the eyelids, which olten dries in the night
and causes the lids to be glued together in the morning.

\

THE EYE AS AN OPTICAL INSTRUMENT. 507

The eyelashes are short curved hairs, arranged in one or two
rows along each lid where the skin joins the conjunctiva.

The Lachrymal Apparatus consists of the tear-gland in
each orbit, the ducts which carry its secretion to the upper eye-
lid, and the canals by which the tears, unless when excessive,
are carried off from the front of the eye without running down
over the face. The lachrymal or tear gland, about the size
of an almond, lies in the upper and outer part of the orbit,
near the front end. It is a compound racemose gland, from
which twelve or fourteen ducts run and open in a row at the
outer corner of the upper eyelid. The secretion there poured
out, is spread evenly over the exposed part of the eye by the
movements of winking, and keeps it moist; finally the tear is
drained off by two lachrymal canals, one of which opens by a
small pore (punctum lachrymalis) on each lachrymal papilla.
The aperture of the lower canal can be readily seen by ex-
amining the corresponding papilla by the aid of a looking-
glass. The canals run inwards and open into the lachrymal
sac, which lies just outside the nose, in a hollow where the
lachrymal and superior maxillary bones (L and Ms, Fig.
30) meet. From the sac the nasal duct .proceeds to open
into the nose-charnber, below the inferior turbinate bone
and within the nostril.

Tears are constantly being secreted, but ordinarily in
such quantity as to be drained off into the nose, from which
they flow into the pharynx and are swallowed. When the
lachrymal ducts are stopped up, however, their continual
presence makes itself unpleasantly felt, and may need the aid
of a surgeon to clear the passage. In weeping the secretion
is increased, and then not only more of it enters the nose,
but some flows down the cheeks. The frequent swallowing
movements of a crying child, sometimes spoken of as " gulp-
ing down his passion/7 are due to the need of swallowing the
extra tears which reach the pharynx.

The Muscles of the Eye (Fig. 141). The eyeball is
spheroidal in form and attached behind to the optic nerve, n,
somewhat as a cherry might be to a thick stalk. On its ex-
terior are inserted the tendons of six muscles, four straight
:and two oblique. The straight muscles lie, one (superior
rectus), s, above, one (inferior rectus) below, one (external
rectus), a, outside, and one (internal rectus), i, inside the
•eyeball. Each arises behind from the bony margin of the

508

THE HUMAN BODY.

foramen through which the optic nerve enters the orbit. In
the figure, which represents the orbits opened from above,
the superior rectns of the right side has been removed. The
superior oblique or pulley (trocUlear) muscle, t, arises behind
near the straight muscles and forms anteriorly a tendon, u,
which passes through a fibre-cartilaginous ring, or pulley,
placed at the notcli in the frontal bone where it bounds
superiorly the front end of the orbit. The tendon then turns

FIG. 141.— The eyeballs and their muscles as seen when the roof of the orbit
has been removed and ilie fat in the cavity has been partly cleared away. On the
right side the superior rectus muscle has been cut away, a, external rectus ; s,
superior rectus ; i, internal rectus; /. superior oblique.

back and is inserted into the eyeball between the upper and
outer recti muscles. The inferior oblique muscle does not
arise, like the rest, at the back of the orbit, but near its front
at the inner side, close to the lachrymal sac. It passes thence
outwards and backwards beneath the eyeball to be inserted
into its outer and posterior part.

The inner, upper, and lower straight muscles, the inferior
oblique, and the elevator of the upper lid are supplied by
branches of the third cranial nerve. The sixth cranial nerve
goes to the outer rectus; and the fourth to the superior oblique.

The eye may be moved from side to side; up or down;
obliquely, that is neither truly vertically nor horizontally,
but partly both ; or, finally, it may be rotated on its antero-
posterior axis. The oblique movements are always accom-

THE EYE AS AN OPTICAL INSTRUMENT. 509

panied by a slight amount of rotation. When the glance is
turned to the left, the left external rectus and the right in-
ternal contract, and vice versa; when up, both superior recti;
when down, both the inferior. The superior oblique muscle
acting alone will roll the front of the eye downwards and
outwards with a certain amount of rotation; the inferior
oblique does the reverse. In oblique movements two of the
recti are concerned, an upper or lower with an inner or
outer; at the same time one of the oblique also always con-
tracts. Movements of rotation rarely, if ever, occur alone.

The natural combined movements of the eyes by which
both are directed simultaneously towards the same point de-
pends on the accurate adjustment of all its nervo-muscular
apparatus. When the co-ordination is deficient the person is
said to squint. A left external squint would be caused by
paralysis of the inner rectus of that eye, for then, after the
eyeball had been turned out by the external rectus, it would
not be brought back again to its median position. A left
internal squint would be caused, similarly, by paralysis of
the left external rectus; and probably by disease of the sixth
cranial nerve or its brain-centres. Dropping of the upper
eyelid (ptosis) indicates paralysis of its special elevator muscle
and is often a serious symptom, pointing to disease of the
brain -parts from which it is innervated.

The Globe of the Eye is on the whole spherical, but
consists of segments of two spheres (see Fig. 142), a portion
of a sphere of smaller radius forming its anterior transparent
part and being set on to the front of its posterior segment,
which is part of a larger sphere. From before back it
measures about 22,5 millimeters (^ inch), and from side to
side about 25 millimeters (1 inch). Except when looking at
near objects, the antero-posterior axes of the eyeballs are
nearly parallel, though the optic nerves diverge considerably
(Fig. 141); each nerve joins its- eyeball, not at the centre, but
about 2.5 mm. (^ inch) on the nasal side of the posterior end
of its antero-posterior axis. In general terms'the eyeball may
be described as consisting o± three coats and three refracting
media.

The outer coat, 1 and 3, Fig. 142, consists of the sclerotic
and the cornea, the latter oeing transparent and situated in
front; the iormei is opaque and white and covers the back
and sides of the globe and part of the front, where it is seen

510

THE HUMAN BODY.

between the eyelids as the ivliite of the eye. Both are tough
and strong, being composed of dense connective tissue. The
white of the eye and the cornea are covered by a thin layer of
the conjunctiva, 4 and 5. Behind the proper connective-
tissue layer, 3, of the cornea is a thin structureless membrane,

FIG. 142. — The left eyeball in horizontal section from before back. 1, sclerotic;
2, junction of sclerotic and cornea; 3, cornea: 4. 5, conjunctiva; 6, posterior
elastic layer of cornea; 7. ciliary muscle; 10, choroid; 11, 13, ciliary processes;
14, iris; 15, retina; 16, optic nerve; IT, artery entering retina in optic nerve; 18,
fovea centralis; 19, region where sensory part of retina ends; 22, suspensory
ligament; 23 is placed in the canal of Petit and the line from 25 points to it; 24,
the anterior part of the hyaloid membrane; 26, 27, 28 are placed on the lens; 28
points to the line of attachment around it of the suspensory ligament; 29, vitreous
humor; 30, anterior chamber of aqueous humor; 31, posterior chamber of aqueous
humor.

6, lined inside by a single layer of epithelial cells; it is the
membrane of Descemet, or the posterior elastic layer.

The second coat consists of the choroid, 9, 10, the ciliary
processes, 11, 13, and the iris, 14. The choroid is made
up of blood-vessels supported by loose connective tissue
containing numerous corpuscles, which in its inner layers
.are richly rilled with dark-brown or black pigment granules.
Towards the front of the eyeball, where it begins to diminish
in diameter, the choroid is thrown into plaits, the ciliary
processes, 11, 13. Beyond these it continues as the iris,
which forms the colored part of the eye seen through the
cornea; and in the centre of the iris is a circular aperture,

THE EYE AS AN OPTICAL INSTRUMENT. 511

the pupil : so its second coat does not, like the outer one,
completely envelop the eyeball. In the iris is a ring of plain
muscular tissue encircling the aperture of the pupil : when its
fibres contract they narrow the pupil. Radial fibres can be
found passing from the ring to the outer edge of the iris,
and they have been supposed to be muscular and concerned
in dilating the pupil. They are probably merely elastic and,
being stretched when the circular muscle contracts, by
mere physical elasticity dilate the pupil when the muscle
relaxes. The circular or sphincter muscle appears to be
normally in a state of tonic contraction; this is increased
by impulses travelling in fibres of the third cranial nerve
and is diminished or inhibited by impulses travelling along
fibres of the sympathetic, which, however, have their origin in
the medulla oblongata and run down the spinal cord to the
lower part of the neck, where they pass out in anterior spinal
nerve-roots to reach the sympathetic. The pigment in the
iris is yellow, or of lighter -or darker brown, according to the
color of the eye, and more or less abundant according as the
eye is black, brown, or gray. In blue eyes the pigment is
confined to the deeper layers, and modified in tint by light
absorption in the anterior colorless strata through which the
light passes.

The third coat of the eye, the retina, 15, is its essential
portion, being the part in which the light produces those
changes that give rise to impulses in the optic nerve. It is
a still less complete envelope than the second tunic, extend-
ing forwards only as far as the commencement of the ciliary
processes, at least in its typical form. It is extremely soft
and delicate; and, when fresh, transparent. Usually when
an eye is opened the retina is colorless; but when the eye has
been cut open in faint yellow light and the exposed retina
quickly examined in white light it is seen to be purple. The
coloring substance (visual purple) very rapidly bleaches when
a dead eye is exposed to daylight. On front or inner surface
of the human retina two special areas can be distinguished in
a fresh eye. One is the point of entry of the optic nerve, 16, the
fibres of which, penetrating the sclerotic and choroid, spread
out in the retina. At this place the retina is whiter than
elsewhere and presents an elevation, the optic mound. The
other peculiar region is the yellow spot (macula luted], 18,
which lies nearly at the posterior end of the axis of the eye-

512 THE HUMAN BODY.

ball and therefore outside the optic mound; in its centre the
retina is thinner than elsewhere and so a pit (fovea cen-
tralis), 18, is formed. This appears black, the thinned
retina there allowing the choroid to be seen through it more
clearly than elsewhere. In Fig. 143 is represented the left
retina as seen from the front, the elliptical darker patch
about the centre indicating the yellow spot, and the white circle
on one side, the optic mound. The vessels of the retina
arise from an artery (17, Fig. 142) which runs in with the
optic nerve and from which branches diverge as shown in
Fig. 143.

The Optic Nerves, Commissure, and Tracts. The optic
nerves converge to meet in the optic commissure (m, Fig.
141), from which the optic tracts pass to the region of the
mid brain. They terminate mainly in the anterior corpora
quadrigemina (Chap. XII) and in masses of gray nerve matter
lying to the outer sides and in front of these, and known as
the corpora geniculata. At the commissure (m, Fig. 141) many
fibres cross the middle line, so that fibres from each optic nerve
are found in both optic tracts. In general, fibres from the
right (that is, the outer or temporal) side of the right retina
and the right (i.e. nasal) side of the left retina pass on to the
brain in the right optic tract; and similarly for the left sides
of the two retinas. Catting the right optic nerve, therefore,
causes total blindness of the right eye, but cutting of the
right optic tract blindness of the right half of each retina
(hemianopia). It will later be seen that rays of light cross in'
the eye so that objects to the left in space form images on
the right sides of the retinas; and vice versa (Figs. 153, 154).
Consequently section or extensive disease of the right optic
tract causes left hemianopia; that is, blindness to objects on
the left of the line of vision.

The incomplete crossing of the optic nerve-fibres in man
is correlated with the fact that his eyes are so placed that
part of the field of vision is common to both. In mammals
whose eyes are so laterally placed that at any given moment
the objects seen by the two eyes are quite different, the cross-
ing at the commissure is complete; when the eyes are placed
so that some oojects can be seen simultaneously by the two
eyes, some fibres cross, and a greater number cross the larger,
the common part of the visual fields. Even in man more of
the fibres cross than go direct to the same side of the brain.

THE EYE AS AN OPTICAL INSTRUMENT. 513

The Microscopic Structure of the Retina. A simpli-
fied stratum, continuous with the proper retina, and formed
of a layer of nucleated columnar cells, is continued over the
ciliary processes; elsewhere the membrane has a very com-
plex structure, and a section taken, except at the yellow spot
or the optic mound, shows ten layers, partly sensory appa-
ratuses and nerve-tissues, and partly accessory structures.

Beginning (Fig. 144) on the front side we find, first, the
internal limiting membrane, 1, a thin structureless layer.
Next comes the nerve-fibre, layer, 2, formed by radiating
fibres of the optic nerve; third, the nerve-cell layer, 3; fourth,

FIG. 143. — The right retina as it would be seen if the front part of tho eyeball
with the lens and vitreous humor were removed.

the inner molecular layer, 4, consisting partly of very fine
nerve-fibrils, and largely of connective tissue; fifth, the
inner nuclear layer, 5, composed of nucleated cells, with a
small amount of protoplasm at each end, and a nucleolus.
These cells, or at any rate the majority of them, have an
inner process running to the inner molecular layer and an
outer running to, 6, the outer molecular layvr, which is
thinner than the inner. Then comes, seventh, the rod and
cone fibre layer, 7, or outer nuclear layer; composed of thick
and thin fibres in each of which is a conspicuous nucleus
with a nucleolus. Next is the thin external limiting mem-
brane, 8, perforated by apertures through which the rods and
cones, 9, of the ninth layer join the fibres of the seventh.
Outside of all, next the choroid, is the pigmentary layer, 10;

514

THE HUMAN BODY.

the cells of this layer send processes between the rods and
cones. The processes contain dark pigment and in eyes
which have been exposed to bright light reach a long way,
sometimes even as far as the external limiting membrane.
If, however, the animal have been kept in the dark for some

10

FIG. 144.— A section through the retina from its anterior or inner surface, 1, in
contact with the hyaloid membrane, to its outer, 10, in contact with the chqroicl.
1, internal limiting membrane; 2, nerve-fibre layer; 3, nerve-cell layer; 4, inner
molecular layer: 5, inner nuclear layer; 6, outer molecular layer; 7, rod and cone
fibres or outer nuclear layer; 8, external limiting membrane; 9, rod and cone
layer; 10, pigment-cell layer.

time before its eye is removed, the processes of the pigment-
cells are short and extend only a short distance between the
outer ends of the rods. In addition, certain fibres run verti-
cally through the retina from the inner to the outer limiting
membrane; they are known as the radial fibres of Mutter

THE EYE AS AN OPTICAL INSTRUMENT. 515

and give off lateral branches, which are especially numerous
in the molecular layers. Like the limiting membranes they
are merely supporting tissues.

On account ol the way in which the supporting and essen-
tial parts are interwoven in the retina it is not easy to track
the latter through it. There is, however (Chap. XXXIII),
good evidence that light first acts upon the rod and cone
layer, traversing all the thickness of inner strata of the retina
to reach it, before starting those changes which result in
visual sensations; and it is therefore probable that the rods
and cones are in direct continuity with the optic nerve-fibres.
The limiting membranes, with the fibres of Miiller and their
branches, are undoubtedly merely accessory and supporting.

Each rod and cone consists of an outer and an inner seg-
ment. The outer segments of both tend to split up trans-
versely into disks and are very similar, except that those of
the rods are longer than those of the cones and do not taper
as the latter do. Moreover, the visual purple is entirely con-
fined to the outer segments of the rods, the cones containing
none of it. The inner segments of the cones are swollen,
while those of the rods are narrow and nearly cylindrical.
Over most of the retina the rods are longer and much more
numerous than the cones, but near the ciliary processes they
cease before the cones do; and in the yellow spot elongated
cones alone are found. In this region the whole retina is
modified; at its margin all the layers are thickened but
especially the nerve-cell layer, which becomes six or seven
thick, while elsewhere the cells are found in but one or two
strata. Most of the fibres run obliquely, reaching in to become
continuous with the cones of the central pit, which are long,
slender, and very closely packed. In the fovea itself all the
layers, except that ot the cones, thin away, and thus the depres-
sion is produced. The fovea is the seat of most acute vision;
when we look at an object we always turn our eyes so that the
light proceeding from it shall be focussed on the two foveae.
Where the optic nerve enters, all the layers but the nerve-
fibre layer (which is there very thick), and the internal limit-
ing membrane, are absent.

The blood-vessels of the retina lie almost entirely in the
nerve-fibre and nerve-cell layers.

The Refracting Media of the Eye are, in succession from
before back, the cornea, the aqueous humor, the crystalline
lens, and the vitreous humor.

516 THE HUMAN BODY.

The aqueous humor fills the space between the front of
the lens, 28, and the back of the cornea. This space is in-
completely divided by the iris into an anterior chamber, 30,
and a posterior, 31 (Fig. 142). Chemically, the aqueous humor
consists of water holding- in solution a small amount of solid
matters, mainly common salt.

The crystalline lens ( 28, 26, 27) is colorless, transparent,
and biconvex, with its anterior surface less curved than the
posterior. It is surrounded by a capsule, and the inner edge
of the iris lies in contact with it in front. In consistence it
is soft, but its central layers are rather more dense than the
outer.

The vitreous humor is a soft jelly enveloped in a thin
capsule, the hyaloid membrane* In front, this membrane
splits into two layers, one of which, 22, passes on to be fixed
to the lens a little in front of its edge. This layer is known
as the suspensory ligament of the lens; its line of attachment
around that organ is not straight but sinuous as represented
by the curved line between 28 and 26 in Fig. 142. The space
between the two layers into which the hyaloid splits is the
canal of Petit. The vitreous humor consists mainly of water
and contains some salts, a little albumin, and some mucin.
It is divided up, by delicate membranes, into compartments
in which its more liquid portions are imprisoned.

The Ciliary Muscle. Running around the eyeball where
the cornea joins the sclerotic is a lymph-vessel called the
canal of Schlemm; it is seen in section at 8 in Fig. 142.
Lying on the inner side of this canal, just where the iris and
the ciliary processes meet, there is some plain muscular tissue,
imbedded mainly in the middle coat of the eyeball arid form-
ing the ciliary muscle, which consists of a radial and a
circular portion (Fig. 149). The radial part is much the
larger, and arises in front from the inner surface of the scler-
otic; the fibres pass back, spreading out as they go, and are
inserted into the front of the choroid opposite the ciliary
processes. The circular part of the muscle lies around the
outer rim of the iris. The contraction of the ciliary muscle
tends to pull forward (radial fibres) and press inward (circu-
lar fibres) the front part of the choroid, to which the back
part of the suspensory ligament of the lens is closely at-
tached. When this occurs the tension exerted on the margin
of the lens by its ligament is diminished.

The Properties of Light. Before proceeding to the

THE EYE AS AN OPTICAL INSTRUMENT. 517

study of the eye as an optical instrument, it is necessary to
recall briefly certain properties of light.

Light is considered as a form of movement of the particles
of an hypothetical medium, or ether, the vibrations being in
planes at right angles to the line of 'propagation of the light.
When a stone is thrown into a pond a series of circular waves
travel from that point in a horizontal direction over the
water, while the particles of water themselves move up and
down, and cause the surface inequalities which we see as
the waves. Somewhat similarly, light-waves spread out from
a luminous point, but in the same medium travel equally in
all directions so that the point is surrounded by shells of
spherical waves, instead of rings of circular waves travelling
in one plane only, as those on the surface of the water.
Starting from a luminous point light would travel in all
directions along the radii of a sphere of which the point is
the centre; the light propagated along one such radius is
called a ray, and in each ray the ethereal particles swing
from side to side in a plane perpendicular to the direction of
the ray. Taking a particle on any ray it would swing aside a
certain distance from it, then back to it again, and across for
a certain distance on the other side; and then back to its
original position on the line of the ray. Such a movement is
an oscillation, and takes a certain time; in lights of certain
kinds the periods of oscillation are all the same, no matter
how great the extent or amplitude of the oscillation; just as
a given pendulum will always complete its swing in the same
time no matter whether its swings be great or small. Light
composed of rays in which the periods of oscillation arc all
equal is called mono chromatic or simple light, while light
made of a mixture of oscillations of different periods is called
mixed or compound light.

If monochromatic light is steadily emitted from a point,
we come at definite distances along a ray, to particles in
the same phase of oscillation, say at their greatest distance
from their position of rest; just as in the concentric waves
seen on the water after throwing in a stone we would along
any radius meet, at intervals, with water raised most above
its horizontal plane as the crest of a wave, or depressed most
below it as the hollow of a wave. The distance along the ray
from crest to crest is called a wave-length and is always the
same in any given simple light; but it is different in simple

518

THE HUMAN BODY.

lights of different colors; the briefer the time of an oscillation
the less the wave-length.

When light falls on a polished surface separating two
transparent media, as air and glass, part of it is reflected or
turned back into the first medium; part goes on into the
second medium, and is commonly deviated from its original
course or refracted. The original ray falling on the surface
is the incident ray.

Let A B (Fig. 145) be the
surface of separation; a x the
incident ray; and C D the
perpendicular or normal to the
surface at the point of inci-
dence : a x C will then be the
angle of incidence. Then the
reflected ray makes an angle
of reflection with the normal
which is equal to the angle of
incidence; and the reflected
ray lies in the same plane as
the incident ray and the nor-
mal to the surface at x. The
refracted ray lies also in the
same plane as the normal and
equal to the angle a x c. foQ indent ray, but does not

continue in its original direction, x /; if the medium below
A B be more refractive than that above it, the refracted ray
is bent, as x d, nearer to the normal, and making with it an
angle of refraction, D xd, smaller than the angle of inci-
dence, a x C. If, on the contrary, the second medium is less
refracting than the first, the refracted ray x g is bent away
from the normal, and makes an angle of refraction, D x g,
greater than the angle of incidence. The ratio of the sine of
the angle of incidence to that of the angle of refraction is
always the same for the same two media with light of the
same wave-length. When the first medium is air the ratio of
the sine of the angle of refraction to that of the angle of in-
cidence is called the refractive index of the second medium.
The greater this refractive index the more is the refracted
ray deviated from its original course. Rays which fall per-
pendicularly on the surface of separation of two media pass
on without refraction.

D

FIG. M 45.— Diagram illustrating the
refraction of light. A B, surface of
separation between two transparent
media; C D, the perpendicular to the
surface at the point of incidence, #;
a x, incident ray; x d, refracted ray,
if the second medium be denser than
the first ; x <?, refracted ray, if the
second medium is less refractive than
the first. The reflected ray is not

with

THE EYE AS AN OPTICAL INSTRUMENT. 519

The shorter the oscillation periods of light-rays the more
they are deviated by refraction. Hence mixed light when

fi" B

FIG. 146.— Diagram illustrating the dispersion of mixed light by a prism. -~

sent through a prism is spread out, and decomposed into- its
simple constituents. For let a x (Fig. 146) be a ray of mixed
light composed of a set of short and a set of long ethereal
waves. When it falls on the surface A B of the prism, that
portion which enters will be refracted towards the normal
E D, but the short waves more than the longer. Hence the
former will take the direction x y, and the latter the direc-
tion x z. On emerging from the prism both rays will again
be refracted, but now from the normals Fy and G z, since
the light is passing from a more to a less refracting medium.
Again the ray x y, made up of shorter waves, will be most
deviated, as in the direction y v, and the long wraves less, in
the direction z r. If a screen were put at S Sf, we would re-
ceive on it at separate points, v and r, the two simple lights
which were mixed together in the compound incident ray
a x. Such a separation of light-rays is called dispersion.

Ordinary white light, such as that of the sun, is composed
of ethereal vibrations of every rate, mixed together. When
such light is sent through a prism it gives a continuous band
of light-rays, known as the solar spectrum,, reaching from the
least refracted to the most refracted and shortest waves. The
exceptions to this statement due to Frauenhofer's lines (see
Physics) are unessential for our present purpose. All of the
simple lights into which the compound solar light is thus

1
\

520 THE HUMAN BODY.

separated do not, however, excite in us visual sensations when
they fall into the eye, but only certain middle ones. If solar
light were used, with the prism, Fig. 146, certain least re-
fracted rays between r and 8' would not be seen, nor the
most refracted between v and S', while between v and r
would stretch a luminous band exciting in us the series of
color sensations from red (due to the least refracted visible
rays), through orange, yellow, green, bright blue, and indigo, to
violet, which latter is the sensation aroused by the most re-
frangible visible rays. The still shorter waves beyond the
violet can only be seen under special conditions; they are
known mainly by their chemical effects and are called the
actinic rays; the invisible waves beyond the red exert a
powerful heating influence and compose the dark-heat rays.
The eye, as an organ for making known to us the existence
of ethereal vibrations, has, therefore, only a limited range.

Refraction of Light by Lenses. In the eye the refract-
ing media have the form of lenses thicker in the centre than
towards the periphery; and we may here confine ourselves
therefore to such converging lenses. If simple light from a
point A, Fig. 140, fall on such a lens its rays, emerging on
the other side, will take new directions after refraction and
meet anew at a point, #, after which they again diverge. If
a screen, r r, be held at a it will therefore receive an image
of the luminous point A. For every converging lens there
is such a point behind it at which the rays from a given point
in front of it meet: the point of meeting is called the conju-
gate focus of the point from which the rays start. If instead
of a luminous point a luminous object be placed in front of
the lens an image of the object will be formed at a certain
distance behind it, for all rays proceeding from one point of
the object will meet in the conjugate focus of that point be-
hind. The image is inverted, as can be readily seen from

Fig. 147. All rays from the point
A of the object meet at the point
a of the image; those from B at 6,
and those from intermediate" points
at intermediate positions. If the

sin§'le lens were rep]aced by sev-

converging lens. eral combined so as to form an

optical system the general result would be the same, provided
the system were thicker in the centre than at the periphery.

THE EYE AS AN OPTICAL INSTRUMENT. 521

The Camera Obscura, as used by photographers, is an in-
strument which serves to illustrate the formation of images
by converging systems of lenses It consists of a box blackened
inside and Having on its front face a tube containing the
lenses; the posterior wall is made of ground glass. If the
front of the instrument be directed on exterior objects, in-
verted and diminished images of them will be formed on the
ground glass; those images only are well defined, at any one
time, which are at such a distance in front of the instrument
that the conjugate foci of points on them fall exactly on the
glass behind the lens: objects nearer or iarther off give con-
fused and indistinct images; but by altering the distance be-
tween the lenses and the ground glass, in common language
" focussing the instrument," either can be made distinct. For
near objects the lenses must be farther from the surface on
which the image is to be received, and for distant nearer.
The reason of this may readily be seen from Fig. 148. If the
system of lenses brings the parallel rays a c and b d, proceed-
ing from an infinitely distant object, to a focus at x9 then the
diverging rays f c and f d, proceeding from a nearer point,
will be harder to bend round, so to speak, and will not meet
until a point y, farther behind the system than x is. The
more divergent the rays, or what amounts to the same thing,
the nearer the point they proceed from, the farther behind
the refracting system will y be.

ment.

FIG. 148.— Diagram illustrating the need of " focussing " m an optical

int-.

The refracting media of the eye form a convergent optical
system, made up of cornea, aqueous. humor, lens, and vitreous
humo.r. These four media are reduced to three practically,
by the tact tha^ the indices of refraction of the cornea and
aqueous -humor are the same, so that they act together as one
converging lens. The surfaces at which refraction occurs
are — (1) that between the air and the cornea, (2) that between
the aqueous humor and the front of the lens, (3) that between
the vitreous humor and the back of the lens. The refractive

522 THE HUMAN BODY.

indices of those media are — the air, 1; the aqueous humor,
1.3379; the lens (average), 1.4545; the vitreous humor,
1.3379. From the laws of the refraction of light it therefore
follows that (Fig. 149) the rays C d will at the corneal surface
be refracted towards the normals N9 N, and take the course d e.
At the front of the lens they will again be refracted towards
the normals to that surface and take the course ef; at the
back of the lens, passing from a more refracting to a less re-
fracting medium, they will be bent from the normals N" and
take the course /#. If the retina be there, these parallel rays
will therefore be brought to a focus on it. In the resting
condition of the natural eye this is what happens to parallel
rays entering it : and, since distant objects send into the eye
rays which are practically parallel, such objects are seen dis-
tinctly without any effort, because all rays emanating from a
point of the object meet again in one point 011 the retina.

Accommodation. Points on near objects send into the eye
diverging rays : these therefore would not come to a focus on
the retina but behind it, and would not be seen distinctly,
did not some change occur in the eye; since we can see them

FIG. 149.-Diagram illustrating the surfaces at which light is refracted in the eye.

quite plainly if we choose (unless they be very near indeed),
there must exist some means by which the eye is focussed or
accommodated for looking at objects at different distances.
That some change does occur one can, also, readily prove by
observing that we cannot see distinctly, at the same moment,
both near and distant objects. For example standing behind

THE EYE AS AN OPTICAL INSTRUMENT. 523

a lace curtain, at a window, we can as we choose look at
the threads of the lace or at the houses across the street; but
when we look at the one we see the other only indistinctly;
and if, after looking at the more distant object, we look at the
nearer we experience a distinct sense of elfort. It is clear,
then, that something in the eye is different in the two cases.
The resting eye, suited for distinctly seeing distant objects,
might conceivably be accommodated for near vision in several
ways. The refracting indices of its media— might be in-
creased; that of course does not happen; the physical prop-
erties of the media are the same in both cases: or the dis-
tance of the retina from the refracting surfaces might be in-
creased, for example by compression of the eyeball by the
muscles around it; however, experiment shows that changes
of accommodation can, by stimulating the third cranial nerve,
be brought about in the fresh excised eyes of animals from
which the muscles lying outside the eyeball have been re-
moved, in which no such compression is possible; we are thus
reduced to the third explanation, that the refracting surfaces,
or some of them, become more curved, and so bring diverging
rays sooner to, a focus; for a lens of smaller curvature is more
converging than one of greater curvature composed of the
same material. Observation shows that this is what actually
happens: the corneal surface remains unchanged when a near
object is looked at after a distant one, but
the anterior surface of the lens becomes con-
siderably more convex and the posterior
slightly so. As already pointed out, when
light meets the separating surface of two
media some is reflected and some refracted.
If, therefore, a person be taken into a
dark room and a candle be held on one side
of his eye while he looks at a distant object, a Je°s-
an observer can see three images of the flame {^Sed fronHhe re-
in his pupil, due to that portion of the light tracting media of the
reflected from the surfaces between the
media. One image (a, Fig. 150) is erect and bright, reflected
from the convex mirror formed by the cornea; the next, b, is
dimmer and also erect; it comes from the front of the lens.
The third, c, is dim and inverted, being reflected from the
concave mirror (see Physics) formed by the back of the lens.
When the curvature of a curved mirror is altered the size of

524

THE HUMAN BODY.

the image reflected from it is also altered, becoming smaller
when the radius of curvature of the mirror is lessened and
vice versa. If the three images be carefully watched while
the observed eye looks at a near object in the same line as the
distant point previously looked at, it is seen that the image
due to corneal reflection remains unchanged; that due to light
from the front of the lens becomes smaller and brighter; the
image from the back of the lens also becomes very slightly
smaller. The change in the curvature of the front of the
lens can be calculated from the change in size of the image
reflected from it when the eye changes from distant to near
accommodation. When a distant object is looked at the
radius of curvature is 10 mm. (f inch), when a very near
about 6 mm. (/-inch), and this change is sufficient to ac-
count for the range of accommodation of the normal eye.

When the eye is focussed for seeing a near object the cir-
cular muscle of the iris contracts, narrowing the pupil, but this
has nothing directly to do with the accommodation.

Accommodation is brought about mainly by the ciliary
muscle (Fig. 151). In the resting eye it is relaxed and the
suspensory ligament of the lens is taut, and, pulling on its
edge, drags it out laterally a little and flattens its surfaces,

ch

v, ,.FIG. 151.— Diagram to illustrate the mechanism of accommodation; on the right
half ot the figure for a near, on the left for a distant, object: cs, canal of Schlemm;
cf, circular portion of ciliary muscle: rf. radial portion of ciliary muscle: ch, ciliary
process of choroid; si, suspensory ligament; i, iris.

especially the anterior, since the ligament is attached a little
in front of the edge. To see a nearer object the ciliary muscle
is contracted, and according to the degree of its contraction
slackens the suspensory ligament, and then the elastic lens,
relieved from the lateral drag, bulges out a little in the centre.

THE EYE AS AN OPTICAL INSTRUMENT. 525

Short Sight and Long Sight. In the eye the range of
accommodation is very great, allowing the rays from points
infinitely distant up to those from
points about eight inches in front
of the eye to be brought to a
focus on the retina. In the nor-
mal eye parallel rays meet on the
retina when the ciliary muscle is
completely relaxed (A, Fig. 152).
Such eyes are emmetropic. In
other eyes the eyeball is too long
from before back; in the resting
state parallel rays meet in front
of the retina (B}. Persons with

Such eyes, therefore, Cannot See and a hypermetropic (C) eye.

distant objects distinctly without the aid of diverging (con-
cave) spectacles; they are short-sighted or myopic. Or the
eyeball may be too short from before back; then, in the rest-
ing state, parallel rays are brought to a focus behind the
retina (C). To see even infinitely distant objects, such per-
sons must therefore use their accommodating apparatus to
increase the converging power of the lens; and when objects
are near they cannot, with the greatest effort,, bring the di-
vergent rays proceeding from them to a focus soon enough.
To get distinct retinal images of near objects they therefore
need converging (convex) spectacles. Such eyes are called
hypermetropic, or in common language long-sighted.

Hygienic Remarks. Since muscular effort is needed by
the normal eye to see near objects, it is clear why the pro-
longed contemplation of such is more fatiguing than looking
at more distant things. If the eye be hypermetropic still
more is this apt to be the case, for then the ciliary muscle
has no rest when the eye is used, and to read a book at a dis-
tance such that enough light is reflected from it into the eye
in order to enable the letters to be seen at all, requires an ex-
traordinary effort of accommodation. Such persons complain
that they can read well enough for a time, but soon fail to be
able to see distinctly. This kind of weak sight should always
lead to examination of the eyes by an oculist, to see if
glasses are needed; otherwise severe neuralgic pains about
the eyes are apt to come on, and the overstrained organ
may be permanently injured. Old persons are apt to have

526 THE HUMAN BODY.

such eyes; but young children frequently also possess them,
and if so should at once be provided with spectacles.

Short-sighted eyes appear to be much more common now
than formerly, especially in those given to literary pursuits.
Myopia is rare among those who cannot read or who live
mainly out of doors. It is not so apt to lead to permanent
injury of the eye as is the opposite condition, but the effort
to see distinctly objects a little distant is apt to produce head-
aches and other symptoms of nervous exhaustion. If the
myopia become gradually worse the eyes should be rested for
several months. Short-sighted persons are apt to have, or
acquire, peculiarities of appearance : their eyes are often
prominent, indicative of the abnormal length of the eyeball.
They also get a habit of " screwing " up the eyelids, probably
an indication of an effort to compress the eyeball from before
back so that distant objects may be better seen. They often
stoop, too, from the necessity of getting their eyes near ob-
jects they want to see. The acquirement of such habits may
be usually prevented by the use of proper glasses. On the
other hand " it is said that myopia even induces peculiarities
of character, and that myopes are usually unsuspicious and
easily pleased ; being unable to observe many little matters in
the demeanor or expression of those with whom they con-
verse, which, being noticed by those of quicker sight, might
induce feelings of distrust or annoyance."

In old age the lens loses some of its elasticity and becomes
more rigid. This leads to the long-sightedness of old people,
known as presbyopia. The stiffer lens does not become as
convex as it did in early life, when the ciliary muscle con-
tracts and the suspensory ligament is relaxed. A special
effort of accommodation is therefore needed in order to adapt
the eye to see near objects distinctly; and convex glasses are
required.

In all forms of deficient accommodation too strong glasses
will injure the eyes irreparably, increasing the defects they
are intended to relieve. Skilled advice should therefore be
invariably obtained in their selection, except perhaps in the
long-sightedness of old age, when the sufferer may tolerably
safely select for himself any glasses that allow him to read
easily a book about 30 centimeters (12 inches) from the eye.
As age advances stronger lenses must usually be obtained.

Optical Defects of the Eye. The eye, though it answers

THE EYE AS AN OPTICAL INSTRUMENT. 527

admirably as a physiological instrument, is by no means per-
fect optically; not nearly so good, for example, as a good
microscope objective. The main defects in it are due to—

1. Chromatic Aberration. As already pointed out, the
rays at the violet end of the solar spectrum are more refran-
gible than those at the red end. Hence they are brought to a
focus sooner. The light emanating from a point on a white
object does not, therefore, all meet in one point on the retina;
but the violet rays come to a focus first, then the indigo, and
so on to the red, farthest back of all. If the eye is accommo-
dated so as to bring to a focus on the retina parallel red rays,
then violet rays from the same source will meet half a milli-
meter in front of it, and crossing and diverging there make
a little violet circle of diffusion around the red point on the
retina. In optical instruments this defect is remedied by
combining together lenses made of different kinds of glass;
such compound lenses are called achromatic.

The general result of chromatic aberration, as may be seen
in a bad opera-glass, is to cause colored borders to appear
around the edges of the images of objects. In the eye we
usually do not notice such borders unless we especially look
for them; but if, while a white surface is looked at, the edge
of an opaque body be brought in front of the eye so as to
cover half the pupil, colorations will be seen at its margin.
If accommodation be inexact they appear also when the
boundary between a white and a black surface is observed.
The phenomena due to chromatic aberration are much more
easily seen if light containing only red and violet rays be used
instead of white light containing all the rays of intermediate
refrangibility. Ordinary blue glass only lets through these two
kinds of rays. If a bit of it be placed over a very small hole
in an opaque shutter and sunlight be admitted through the
hole, it will be found that with one accommodation (that for
the red rays) a red point is seen with a violet border, and
with another (that at which violet rays are brought to a focus
on the retina) a violet point is seen with a red aureole.

2. Spherical Aberration. It is not quite correct to state
that ordinary lenses bring to a focus in one point behind them
rays proceeding from a point in front, even when these are
all of the same refrangibility. Convex lenses whose surfaces
are segments of spheres, as are those of the eye, bring to a
focus sooner the rays which pass through their marginal than

528 THE HUMAN BODY.

those passing through their central parts. If rays proceeding
from a point and traversing the lateral part of a lens be
brought to a focus at any point, then those passing through
the centre of the lens will not meet until a little beyond that
point. If the retina receive the image formed by the periph-
eral rays the others will form around this a small luminous
circle of light — such as would be formed, by sections of the
cones of converging rays in Fig. 140, taken a little in front
of r r. This defect exists in all glass lenses, as it is found
impossible in practice to grind them of the non-spherical
curvatures necessary to avoid it. In our eyes its effect is to
a large extent corrected in the following ways — (a) The
opaque iris cuts off many of the external and more strongly
refracted rays, preventing them from reaching the retina.
(b) The outer layers of the lens are less refracting than the
central; hence the rays passing through its peripheral parts
are less refracted than those passing nearer its axis.

3. Irregularities in Curvature. The refracting surfaces
of our eyes are not even truly spherical; this is especially the
case with the cornea, which is very rarely curved to the same
extent in its vertical and horizontal diameters. Suppose the
vertical meridian to be the most curved; then the rays pro-
ceeding from points along a vertical line will be brought to a
focus sooner than those from points on a horizontal line. If
the eye is accommodated to see distinctly the vertical line, it
will see indistinctly the horizontal and vice versa. Few
people therefore see equally clearly at once two lines crossing
one another at right angles. The phenomenon is most obvi-
ous, however, when a series of
concentric circles (Fig. 153) is
looked at : then when the lines
appear sharp along some sec-
tors, they are dim along the
rest. When this defect, known
as astigmatism, is marked it
causes serious troubles of vis-
ion and requires peculiarly
shaped glasses to counteract
it.

4. Opaque Bodies in the
FIG. 153. Refracting Media. In dis-

eased eyes the lens may be opaque (cataract) and need

THE EYE AS AN OPTICAL INSTRUMENT. 529

removal; or opacities from ulcers or wounds may exist
on the cornea. But even in the best eye there are apt to be
small opaque bodies in the vitreous humor causing musccz
volitantes; that is, the appearance of minute bodies floating
in space outside the eye, but changing their position when
the position of the eye changes, by which fact their origin in
internal causes may be recognized. Many persons never see
them until their attention is called to their sight by some
weakness of it, and then they think they are new phenomena.
Visual phenomena due to causes in the eye itself are called
entoptic; the most interesting are those due to the retinal
blood-vessels (Chap0 XXXIII. ). Tears, or bits of the secre-
tion of the Meibomian glands, on the front of the eyeball
often cause distant luminous objects to look like ill-defined
luminous bands or patches of various shape. The cause of
such appearances is readily recognized, since they disappear
or are changed after winking.

CHAPTER XXXIII.

THE EYE AS A SENSORY APPARATUS.

The Excitation of the Visual Apparatus. — The excitable
visual apparatus for each eye consists of the retina, the optic
nerve, and the brain-centres connected with the latter; how-
ever stimulated, if intact, it causes visual sensations. In the
great majority of cases its excitant is objective light, and sx>
we refer all stimulations of it to that cause, unless we have
special reason to know the contrary. As already pointed
out pressure on the eyeball causes a luminous sensation
(phosphene), which suggests itself to us as dependent on a
luminous body situated in space where such an object must
be in order to excite the same part of the retina. Since all
rays of light penetrating the eye, except in the line of its
long axis, cross that axis, if we press the outer side of the
eyeball we get a visual sensation referred to a luminous body
on the nasal side; if we press below we see the luminous
patch above, and so on.

Of course different rays entering the eye take different
paths through it, but on general optical principles, which
cannot here be detailed, we may trace all oblique rays through
the organ by assuming that they meet and leave the optic
axis at what are known as the nodal points of the system;
these (Jck* ', Fig. 154) lie near together in the lens. If we
want to find where rays of light from A will meet the
retina (the eye being properly accommodated for seeing an
object at that distance) we draw a line from A to Jc (the first
nodal point) and then another, parallel to the first, from Tc'
(the second nodal point) to the retina. The nodal points of
the eye lie so near together that for practical purposes we
may treat them as one (k, Fig. 155), placed near the back of
the lens. By manifold experience we have learnt that a
luminous body (A, Fig. 155) which we see, always lies on the
prolongation of the line joining the excited part of the retina,

530

THE EYE AS A SENSORY APPARATUS.

531

a, and the nodal point Jc. Hence any excitation of that part
of the retina makes us think of a luminous body somewhere
on the line a A, and, similarly, any excitation of b, of a body

B

FIG. 154. — Diagram illustrating the points at which incident rays meet the retina.
xx, optic axi£ ; fc, first nodal point; fc', second nodal point; 6, point where the im-
age of B would be formed, were the eye properly accommodated for it ; a, the
retinal point where the image of A would be formed.

on the line b B or its prolongation. It is only other conflict-
ing experiences, as that with the eyes closed external bodies
do not excite visual sensations, and the constant connection

• •

FIG. 155.- Diagrammatic section through the eyeball, xx, optic axis ; k, nodal
point.

of the pressure felt on the eyelid with the visual sensation,
that enable us when we press the eyeball to conclude that, in
spite of what we seem to see, the luminous sensation is not
due to objective light from outside the eye.

The Idio-Retinal Light.— The eyelids are not by any
means perfectly opaque ; in ordinary daylight they still allow
a considerable quantity of light to penetrate the eye, as any
one may observe by passing his hand in front of the closed
eyes. But even in a dark room with the eyes completely
covered up so that no objective light can enter them, there is
still experienced a small amount of visual sensation due to

532 THE HUMAN BODY.

internal causes. The field of vision is not absolutely dark
but slightly luminous, with brighter fleeting patches travers-
ing it. These are especially noticeable, for example, in try-
ing to see and grope one's way with the eyes open up a per-
fectly dark staircase. Then the luminous patches attract
special attention because they are apt to be taken for the
signs of objective realities; they become very manifest when
any sudden jar of the Body, due for example to knocking
against something, occurs; and have no doubt given rise to
many ghost stories. These visual sensations felt in the ab-
sense of all external stimulation of the eyes, may for conveni-
ence be spoken of as due to the idio-retinal light.

The Excitation of the Visual Apparatus by Light. —
Light only excites the retina when it reaches its nerve end
organs, the rods and cones. The proofs of this are several.

FIG. 156.

1. Light does not arouse visual sensations tvhen it falls
directly on the fibres of the optic nerve. Where this nerve
enters there is a retinal part possessing only nerve-fibres,
and this part is blind. Close the left eye and look steadily
with the right at the cross in Fig. 156, holding the book verti-
cally in front of the face, and moving it to and fro. It will
be found that at about 25 centimeters (10 inches) off the
white circle disappears ; but when the page is nearer or
farther, it is seen. During the experiment the gaze must be
kept fixed on the cross. There is thus in the field of vision a
Uind spot, and it is easy k> show by measurement that it lies
where the optic nerve enters.

When the right eye is fixed on the cross, it is so directed
that rays from this fall on the yellow spot (y, Fig. 157).
The rays from the circle then cross the visual axis at the
nodal point, n, and meet the retina at o. If the distance of
the nodal point of the eye from the paper be/, and from

THE ETE AS A SENSORY APPARATUS.

533

the retina (which is 15mm.) be F, then the distance, on
the paper, of the cross from the circle will be
to the distance of y from o as/ is to F. Meas-
urements made in this way show that the circle
disappears when its image is thrown on the
entry of the optic nerve, which lies to the nasal
side of the yellow spot.

2. The above experiment having shown that
light does not act directly on the optic nerve-
fibres any more than it does on any other nerve-
fibres, we have next to see in what part of the
retina those changes do first occur which form
the link between light and nervous impulses.
They occur in the outer part of the retina, in
the rods and cones. This is proved by what is
called Purkinje's experiment. Take a candle in-
to a dark room and look at a surface not covered
with any special pattern, say a whitewashed wall
or a plain window-shade. Hold the candle to the side of one
eye and close to it, but so far back that no light enters the
pupil from it; that is so far back that the flame just can-
not be seen, but so that a strong light is thrown on the white
of the eye as far back as possible. Then move the candle a
little to and fro. The surface looked at will appear luminous
with reddish-yellow light, and on it will be seen dark branch-
ing lines which are the shadows of the retinal vessels. Now
in order that these shadows may be seen the parts on which
the light acts must be behind the vessels, that is in the outer
layers of the retina since the blood vessels lie in its inner
strata. The experiment may be more satisfactorily performed
by getting another person to focus with a lens the light of
the candle as a bright spot as far back as possible on the white
of the observer's eye.

If the light be kept steady the vascular shadows soon dis-
appear ; in order to continue to see them the candle must be
kept moving. The explanation of this fact may readily be
made clear by fixing the eyes for ten or fifteen seconds on the
dot of an"i" somewhere about the middle of this page : at
first the distinction between the slightly luminous black
letters and the highly luminous white page is very obvious ;
in other words, the different sensations arising from the
strongly and the feebly excited areas of the retina. But if

534 THE HUMAN BODY.

the glance do not be allowed to wander, very soon the letters
become indistinct and at last disappear altogether ; the whole
page looks uniformly grayish. The reason of this is that the
powerful stimulation of the retina by the light reflected from
the white part of the page soon fatigues the part of the visual
apparatus it acts upon ; and as this fatigue progresses the
stimulus produces less and less effect. The parts of the
retina, on the other hand, which receive light only from the
black letters are but little stimulated and retain much of their
original excitability, so that, at last, the feebler excitation act-
ing upon these more irritable parts produces as much sensa-
tion as the stronger stimulus acting upon the fatigued parts;
and the letters become indistinguishable. To see them con-
tinuously we must keep shifting the eyes so that the parts of
the visual apparatus are alternately fatigued and rested, and
the general irritability of the whole is kept about the same.
So, in Purkinje's experiment, if the position of the shadows
remain the same, the shaded part of the retina soon becomes
more irritable than the more excited unshaded parts, and its
relative increase of irritability makes up for the less light
falling on it, so that the shadows cease to be perceived. It is
for this reason that we do not see the retinal vessels under ordi-
nary circumstances. When light, as usual, enters the eye
from front through the pupil the shadows always fall on the
same parts of the retina, and these parts are thus kept suffi-
ciently more excitable than the rest to make up for the less light
reaching them through the vessels. To see the latter we
must throw the light into the eye in an unusual direction,
not through the pupil but laterally through the sclerotic. If
v, Fig. 158, be the section of a retinal
vessel, ordinarily its shadow will fall
at some point on a line prolonged
through it from the centre of the pupil.
If a candle flame be held opposite b it
illuminates that part of the sclerotic
and from there light radiates and illu-
mines the interior of the eye. The
resulting sensation we refer to light
entering the eye in the usual manner
through the pupil, and accordingly see
the surface we look at as if it were illuminated. The shadow
of v, is now cast on an unusual spot c, and we see it as if at the

THE E7E AS A SENSORY APPARATUS. 535

point d on the wall, on the prolongation of the line joining
the nodal point, k, of the eye with c. If the candle be moved
so as to illuminate the point V of the sclerotic, the shadow of
v will be cast on c' and will accordingly seern on the wall to
move from d to d' . It is clear that if we know how far b is
from b', how far the wall is from the eye, and how far the
nodal point is from the retina (15 mm. or 0.6 incJa}, and
measure the distance on the wall from d to d', we c^f calcu-
late how far c is from c' : and then how far the vessel throwing
the shadow must be in front of the retinal parts perceiving
it. fin this way it is found that the part seeing the shadow,
that is the layer on which light acts, is just abouJMs far be-
hind the retinal vessels as the main vascular trumts of the
retina are in front of the rod and cone layer. It is, there-
fore, in that layer that the light initiates those changes which
give rise to nervous impulses ; which is further made obvious
by the fact that the seat of most acute vision iaihefovea cen-
tral is, where only this layer and the cone-fibres diverging
from it are present. When we want to see anything dis-
tinctly we always turn our eyes so that its image shall fall on
the centres of the yellow spots.J

The Vision Purple. How light acts in the retina so as to
produce nerve stimuli is still uncertain. Recent observations
show that it produces chemical changes in the rod and cone
layer, and seemed at first to indicate that its action was to
produce 'substances which were chemical excitants of nerve-
fibres ; but although there can be little doubt that these
chemical changes play some important part in vision, what
their role may be is at present quite obscure. If a perfectly
fresh retina be excised rapidly, its outer layers will be found
of a rich purple color. In daylight this rapidly bleaches, but
in the dark persists even when putrefaction has set in. In
pure yellow light it also remains unbleached a long time, but
in other lights disappears at different rates. If a rabbit's eye
be fixed immovably and exposed so that an image of a window
is focused on the same part of its retina for some time, and
then the eye be rapildy excised in the dark and placed in
solution of potash alum, a colorless image of the window is
found on the retina, surrounded by the visual purple of the
rest which is, through the alum, fixed or rendered incapable
of change by light. Photographs, or optograms, are thus ob-
tained which differ from the photographer's in that he uses

536 THE HUMAN BODY.

light to produce chemical changes which give rise to colored
bodies, while here the reverse is the case. If the eye be not
rapidly excised and £ut in the alum after its exposure, the
optogram will disappear ; the vision purple being rapidly re-
generated at the bleached part. This reproduction of it is
due mainly to the cells of the pigmentary layer of the retina,
which in living eyes exposed to light thrust long processes
betwee^he rods and cones. Portions of frogs' retinas raised
from this, bleach more rapidly than those left in contact with
it, but become soon purple again if let fall back upon the
pigment-cells. Experiments show, however, that animals
(frogs) exited for a long time to a bright light may have
their retinW completely bleached and still see very well; they
can still unerringly catch flies that come within their reach ;
and they can also distinguish colors, or at least some colors,
as green. Moreover, the vision purple is only found in the
outer segments of the rods ; there is none in the cones, and
yet these alone exist in the yellow spot of the human eye,
which is the seat of most acute vision; and animals, such as
snakes, which have only cones in the retina, possess no vision
purple and nevertheless see very well.

It may be that other bodies exist in the retina which are
also chemically changed by light, but the changes of which
are not accompanied by alterations in color which we can see;
and, in the absence of the vision purple, seeing might be
carried on by means of these, which may be less quickly
destroyed by light and so still persist in the bleached retinas
of the frogs above mentioned. For the present, however, the
question of the part, if any, played in vision by such bodies
must be left an open one : and the possibility that the rods
and cones form an apparatus which directly converts ethereal
vibrations into nerve stimuli without any intervening chemi-
cal process must be borne in mind.

The Intensity of Visual Sensations. Light considered as
a form of energy may vary in quantity ; physiologically, also,
we distinguish quantitative differences in light as degrees of
brightness, but the connection between the intensity of the
sensation excited and the quantity of energy represented by
the stimulating light is not a direct one. In the first place, ""
some rays excite our visual apparatus more powerfully than
others : a given amount of energy in the form of yellow light,
for example, causes more powerful visual sensations than the

THE EYE AS A SENSORY APPARATUS. 537

same quantity of energy in the form of violet light; and ultra-
violet rays only become visible, and then very faintly, when
all others are suppressed; but if they be passed through some
fluorescent substance (see Physics), such as an acid solution
of quinine sulphate, which, without altering the amount of
energy, turns it into ethereal oscillations of a longer period,
then the light becomes readibly perceptible. ^

Even with light-rays of the same oscillation periot^ur sen-
sation is not proportional to the amount of energy in th^
light; to the amount of heat, for example, to which it would
give rise if all transformed into it. If objective- light increase
gradually in amount our sensation increases also, £|| to a limit
beyond which it does not go, no matter how strong the light
becomes; but the increase of sensation takes place far more
slowly than that of the light, in accordance with the psycho-
physical law already mentioned. If we call the amount of
light given out by a single candle a, then that emitted by two
candles will be 2a; and so on. If the amount of sensation
excited by the single candle be A, then that due to two can-
dles will not be 2 A, and that by three will be far less than 3 A.
If a white surface, P, Fig 159, be illuminated by a candle at
c and another elsewhere, and a rod, a, be placed so as to in-
tercept the light from c, but not - Q
that from the other candle, we see
clearly a shadow, since our eyes
recognize the difference in luminos-

£5

ity of this part of the paper, reflect-
ing light from one candle only, from
that of the rest which is illuminated

by two: that is wo can tell the sensation due to the stimulus
ah-om that due to the stimulus 2 a.) If now a bright lamp be
brought in and placed alongside, and its light be physically
equal to that of 10 candles, we cease to perceive the shadows.
We find the sensation aroused by objective light = 12« (due
to the lamp and two candles) cannot be told from that due to
light = llaj although the difference of objective light is still
la as before. Most persons must have observed illustrations
of this. Sitting in a room with three lights not unfrequently
some object so intercepts the light from two as to cast on the
wall two shadows which partly overlap. Where the shadows
overlap the wall gets light only from the third candle; around
that, where each shadow is separate, it is illuminated by this

538 THE HUMAN BODY.

and one other candle; and the wall in the neighborhood of
the shadows by all three. Objectively, therefore, the differ-
ence between the deep shadow and half shadow is that
between the light of one candle and that of two. The differ-
ence between the half shadows and the wall around is that
between the light of two and three candles. But as .a matter
of sensation the difference between the half shadow and the
full s^low seems much greater than that between the
half shadow and the rest of the wall; in other words the
difference, a, between a and 2a, is a more efficient stimulus
than the same difference, a, between 2a and 3«. When the
total stim^bs increases the same absolute difference is less
felt or mlP be entirely unperceived. An example of this
which every one will recognize is afforded by the invisibility
of the stars in daytime.

On the other hand, as the total stimulus increases or de-
creases the same fractional difference of the whole is per-
ceived with the same ease; i.e., excites the same amount of
sensation. In reading a book by lamplight we perceive
clearly the difference between the amount of light reflected
from the black letters and the white page. If we call the
total lamplight reflected by the blank parts 10« and that by
the letters 2#, we may say we perceive with a certain distinct-
ness a luminous difference equal to one fifth of the whole.
If we now take the book into the daylight the total light re-
flected from the letters and the imprinted part of the page
increases, but in the same proportion. Say the one now is
50a and the other 10#; although the absolute difference be-
tween the two is now 40& instead of 8a we do not see the
letters any more plainly than before. The smallest difference
in luminous intensity which we can perceive is about T^ of
the whole, for all the range of lights we use in carrying on
our ordinary occupations. For strong lights the smallest per-
ceptible fraction is considerably greater; finally we reach a
limit where no increase in brightness is felt. For weak
illumination the sensation is more nearly proportioned to the
total differences of the objective light. Thus in a dark room
an object reflecting all the little light that reaches it appears
almost twice as bright as one reflecting only half; in a
stronger light it would so appear. Bright objects in general
obscurity thus appear unnaturally bright when compared
with things about them, and indeed often look self-luminous.

THE EYE AS A SENSORY APPARATUS. 539

A cat's eyes, for example, are said to "shine in the dark";
and painters to produce moonlight effects always make the
bright parts of a picture relatively brighter, when compared
with things about them, than would be the case if a sunny
scene were to be represented ; by a relatively excessive use of
white pigment they produce the relatively great brightness of
those things which are seen at all in the general obscurity of
a moonlight landscape.

The Duration of Luminous Sensations.— This is greater
than that of the stimulus, a fact taken advantage of in mak-
ing fireworks : an ascending rocket produces the sensation of
a trail of light extending far behind the position of the bright
part of the rocket itself at the moment, because the sensation
aroused by it in a lower part of its course still persists. So,
shooting stars appear to have luminous tails behind them.
By rotating rapidly before the eye a disk with alternate white
and black sectors we get for each point of the retina on
which a part of its image falls, alternating stimulation (due
to the passage of white sector) and rest (when a black sector
is passing). If the rotation be rapid enough the sensation
aroused is that of a uniform gray, such as would be produced
if the white and black were mixed and spread evenly over
the disk. In each revolution the eye gets as much light as if
that were the case, and is unable to distinguish that this
light is made up of separate portions reaching it at intervals :
the stimulation due to each lasts until the next begins and so
all are fused together. If, while looking at the flame, one
turns out suddenly the gas in a room containing no other
light, the image of the flame persists a short time after the
flame itself is extinguished.

The Localizing Power of the Retina. — As already pointed
out a necessary condition of seeing definite objects, as distin-
guished from the power of recognizing differences of light
and darkness, is that all light entering the eye from one point
of an object shall be focused on one point of the retina.
This, however, would not be of any use had we not the faculty
of distinguishing the stimulation of one part of the retina
from that of another part. This power the visual apparatus
possesses in a very high degree; while with the skin we can-
not distinguish from one, two points touching it less than 1
mm. (-£s inch) apart, with our eyes we can distinguish two
points whose retinal images are not more than .004 mm.

540 THE HUMAN BODY.

(.00016 inch) apart. The distance between the retinal images
of two points is determined by the "visual angle " under
which they are seen; this angle is that included between
lines drawn from them to the nodal point of the eye. If a
and b (Fig. 160) are luminous points, the image of a wiU b&

formed at a' on the prolongation of the line a n joining a
with the node, n. Similarly the image of b will be formed
at b'. If a and b still remaining the same distance apart, be
moved nearer the eye to c and d, then the visual angle
under which they are seen will be greater and their retinal
images will be farther apart, at c' and d'. If a and b are the
highest and lowest parts of an object, the distance between
their retinal images will then depend, clearly, not only on the
size of the object, but on its distance from the eye; to know
the discriminating power of the retina we must therefore
measure the visual angle in each case. In the fovea centralis
two objects seen under a visual angle of 50 to 70 seconds can
be distinguished from one another; this gives for the distance
between the retinal images that above mentioned, and corre-
sponds pretty accurately to the diameter of a cone in that
part of the retina. We may conclude, therefore, that when
two images fall on the same cone or on two contiguous cones
they are not discriminated; but that if one or more unstimu-
lated cones intervene between the stimulated, the points may
be perceived as distinct. The diameter of a rod or cone, in
fact, marks the anatomical limit up to which we can by prac-
tice raise our acuteness of visual discrimination; and in the
yellow spot which we constantly use all our lives in looking
at things which we want to see distinctly, we have educated
the visual apparatus up to about its highest power. Else-
where on the retina our discriminating power is much less
and diminishes as the distance from the yellow spot increases.
This is partly due, no doubt, to a less sensibility of those reti-
nal regions, such as, by other facts, is proved to exist, but in
part, no doubt is also due to a want of practice. The more

THE EYE AS A SENSORY APPARATUS. 541

peripheral the retinal region the less we have used it for such
purposes. It is probable, therefore, that outlying portions of
the retina are capable of education to a higher discriminating
power, just as we shall find the skin to be for tactile stimuli.

While we can tell the stimulation of an upper part of the
retina from a lower, or a right region from a left, it must be
borne in mind that we have no direct knowledge of which is
upper or lower or right or left in the ocular image. All our
visual sensations tell us is that they are aroused at different
points, and nothing at all about the actual positions of these
on the ratina. There is no other eye behind the retina look-
ing at it to see the inversion of the image formed on it.
Suppose I am looking at a pane in a second-story window
of a distant house: its image will then fall on the fovea cen-
tralis ; the line joining this with the pane is called the. visual
axis. The image of the roof will be formed on a part of the
retina below the fovea, and that of the front door above it. I
distinguish that the images of all these fall on different parts
of the retina in certain relative positions, and have learnt, by
the experience of all my life, that when the image of any-
thing arouses the sensation due to excitation of part of the
retina below the fovea the object is above my visual axis, and
vice versa ; similarly with right and left. Consequently 1 in-
terpret the stimulation of lower retinal regions as meaning
high objects, and of right retinal regions as meaning left ob-
jects, and never get confused by the inverted retinal image
about which directly I know nothing. A new-born child,
even supposing it could use its muscles perfectly, could not,
except by mere chance, reach towards an object which it saw;
it would grasp at random, not yet having learnt that to reach
an object exciting a part of the retina above the fovea needed
movement of the hand towards a position in space below the
visual axis ; but very soon it learns that things near its brow,
that is up, excite certain visual sensations, and objects below
its eyes others, and similarly with regard to right and left; in
time it learns to interpret retinal stimuli so as to localize
accurately the direction, with reference to its eyes, of outer
objects, and never thenceforth gets puzzled by retinal inver-
sion.

Color Vision. — Sunlight reflected from snow gives us a
sensation which we call white. The same light sent through
a prism and reflected from a white surface excites in us, mo

542 THE HUMAN BODY.

white sensation but a number of color sensations, gradating
insensibly from red to violet, through orange, yellow, green,
blue-green, blue, and indigo. The prism separates from one
another light-rays of different periods of oscillation and each
ray excites in us a colored visual sensation, while all mixed
together, as in sunlight, they arouse the entirely different
sensation of white. If the light fall on a piece of black
velvet we get still another sensation, that of black; in this
case the light-rays are so absorbed that but few are reflected
to the eye and the visual apparatus is left at rest. Physically
black represents nothing: it is a mere zero — the absence of
ethereal vibrations; but, in consciousness, it is as definite a
sensation as white, red, or any other color. We do not feel
blackness or darkness except over the region of the possible
visual field of our eyes. In a perfectly dark room we only
feel the darkness in front of our eyes, and in the light there
is no such sensation associated with the back of our heads or
the palms of our hands, though through these we get no
visual sensations. It is obvious, therefore, that the sensation
of blackness is not due to the mere absence of luminous
stimuli, but to the unexcited state of the retinas, which are
alone capable of being excited by such stimuli when present.
This fact is a very remarkable one, and is not paralleled in any
other sense. Physically, complete stillness is to the ear what
darkness is to the eye; but silence impresses itself on us as
the absence of sensation, while darkness causes a definite
feeling of " blackness."

Young's Theory of Color Vision. — Our color sensations
insensibly fade into one another; starting with black we can
insensibly pass through lighter and lighter shades of gray
to white: or beginning with green through darker and darker
shades of it to black or through lighter and lighter to white:
or beginning with red we can by imperceptible steps pass to
orange, from that to yellow and so on to the end of the solar
spectrum: and from the violet, through purple and carmine,
we may get back again to red. Black and white appear to be
fundamental color sensations mixed up with all the rest: we
never imagine a color but as light or dark, that is as more or
less near white or black; and it is found that as the light
thrown on any given colored surface weakens, the shade be-
comes deeper until it passes into black; and if the illumina-
tion be increased, the color becomes "lighter" until it passes

THL EYE AS A SENSORY APPARATUS. 543

into white. Of all the colors of the spectrum yellow most
easily passes into white with strong illumination. Black and
white, with the grays which are mixtures of the two, thus
seem to stand apart from all the rest as the fundamental
visual sensations, and the others alone are in common par-
lance named " colors." It has even been suggested that the
power of differentiating them in sensation has only lately
been acquired by man, and a certain amount of evidence has
been adduced from passages in the Iliad to prove that the
Greeks in Homer's time confused together colors that are
very different to most modern eyes; at any rate there seems
to be no doubt that the color sense can be greatly improved
by practice; women whose mode of dress causes them to pay
more attention to the matter, have, as a general rule, a more
acute color sense than men.

Leaving aside black, white, gray, and the various browns
(which are only dark tints of other colors), we may enumer-
ate our color sensations as red, orange, yellow, green, blue,
violet, or purple; between each there are, however, numerous
transition shades, as yellow-green, blue-green, etc., so that
the number which shall have definite names given to them is
to a large extent arbitrary. Of the above, all but purple are
found in the spectrum given when sunlight is separated by a
prism into its rays of different refrangibility; rays of a cer-
tain wave-length or period of oscillation cause in us the feel-
ing red; others yellow, and so on; for convenience we may
speak of these as red, yellow, blue, etc., rays; all together, in
about equal proportions, they arouse the sensation of white.
A remarkable fact is that most color feelings can be aroused
in several ways. White, for example, not only by the above
general mixture, but red and blue-green rays, or orange and
blue, or yellow and violet, taken in pairs in certain propor-
tions, and acting simultaneously or in very rapid succession
on the same part of the retina, cause the sensation of white:
snch colors are called complementary to one another. The
mixture may be made in several ways; as, for example, by
causing the red and blue-green parts of the spectrum to
overlap, or by painting red and blue-green sectors on a disk
and rotating it rapidly; they cannot be made, however, by
mixing pigments, since what happens in such cases is a very
complex phenomenon. Painters, for example, are accustomed
to produce green by mixing blue and yellow paints, and some

544 THE HUMAN BODY.

may be inclined to ridicule the statement that yellow and
blue when mixed give white. When, however, we mix the
pigments we do not combine the sensations of the same name,
which is the matter in question. Blue paint is blue because it
absorbs all the rays of the sunlight except the blue and some
of the green; yellow is yellow because it absorbs all but the
yellow and some of the green, and when blue and yellow are
mixed the blue absorbs all the distinctive part of the yellow
and the yellow does the same for the blue; and so only the
green is left over to reflect light to the eye, and the mixture
has that color. Grass-green has no complementary color in
in the solar spectrum; but with purple, which is made by
mixing red and blue, it gives white. Several other colors
taken three together, give also the sensation of white. If
then we call the light-rays which arouse in us the sensation
red, «, those giving us the sensation orange #, yellow c, and
so on, we find that we get the sensation white with a, b, c, d,
e, f, and g all together; or with b and e, or with c and/*, or
with #, d, and e ; our sensation white has no determinate re-
lation to ethereal oscillations of a given period, and the same
is true for several other colors; yellow feeling, for example,
may be excited by ethereal vibrations of one given wave-
length (spectral yellow), or by a light containing only such
waves as taken separately cause the sensations red and grass-
green; in other words a physical light in which there are no
waves of the " yellow " length may cause in us the sensation
yellow, which is only one more instance of the general fact
that our sensations, as such, give us no direct information as
to the nature of external forces; they are but signs which we
have to interpret. The doctrine of specific nerve energies
makes it highly improbable that our different color sensa-
tions can all be due to different modes of excitation of exactly
the same nerve-fibres; a fibre which when excited alone gives
us the sensation red will always give us that feeling when
so excited. The simplest method of explaining our color
sensations would therefore be to assume that for each there
exists in the retina a set of nerve-fibres with appropriate
terminal organs, each excitable by its own proper stimu-
lus. But we can distinguish so innumerable and so finely
graded colors, that, on such a supposition, there must be an
almost infinite number of different end organs in the retina,
and it is more reasonable to suppose that there are a limited

THE EYE AS A SENSORY APPARATUS. 545

number of primary color sensations, and that the rest are due
to combinations of these. That a compound color sensation
may be very different from its components when these are,
regarded apart, is clearly shown by the sensation white
aroused either by what we may call red and blue-green, or
green and purple, stimuli acting together; or of yellow due
to grass-green and red. To account for our various color sen-
sations we may, therefore, assume a much smaller number of
primary sensations than the total number of color sensations
we experience; all can in fact be explained by assuming any
three primary color sensations which together give white, and
regarding all the rest as due to mixtures of these in various
proportions; there may be more than three, but three will
account for all the phenomena, black being a sensation expe-
rienced when all visual stimuli are absent. This is known as
Young's theory of color vision, and is that at present most
commonly accepted. The selection of the three primary sen-
sations is decided by the phenomena of color-blindness, which
show that if this theory of color vision be correct red must
be one of the primary color sensations: if so, then green
and violet must be the other two. The theory further
assumes that all kinds of light stimulating the end appa-
ratuses give rise to all three sensations, but not necessarily in
the same proportion. When all are equally aroused the sen-
sation is white or some shade of gray when the red and green
are tolerably powerfully excited and the violet little, the sen-
sation is yellow; when the green powerfully and the red and
violet little, the sensation is green, and so on. In this way
we can also explain the fact that all colored surfaces when
intensely illuminated pass into white. A red light, for ex-
ample, excites the primary red sensation most, but green and
violet a little; as the light becomes stronger a limit is
reached beyond which the red sensation cannot go, but the
green and violet go on increasing with the intensity of the
light, until they too reach their limits; and all three primary
sensations being then equally aroused, the sensation white is
produced.

Color Blindness. Some persons fail to distinguish colors
which are to others quite different; when such a deficiency is
well marked it is known as "colorblindness," and, assuming
Young's theory to be correct, it may be explained by an ab-
sence of one or more of the three primary color sensations;

546 THE HUMAN BODY.

observation of color-blind persons thus helps in deciding
which these are. The most common form is red color blind-
ness; persons afflicted with it confuse reds and greens. .Red
to the normal eye is red because it excites red sensation
much, green some, and violet less; and a white page white,
because it excites red, green, and violet sensations about
equally. In a person without red sensation a red object
would arouse only some green and violet sensation and so would
be indistinguishable from a bluish green; in practice it is
found that many persons confound these colors. Cases of
green and violet color blindness are also met with, but they
are much rarer than the red color blindness or " Daltonism."

The detection of color blindness is often a matter
of considerable importance, especially in sailors and railroad
officials, since the two colors most commonly confounded, red
and green, are those used in maritime and railroad signals.
Persons attach such different names to colors that a decision
as to color blindness cannot be safely arrived at by simply
showing a color and asking its name. The best plan is to^
take a heap of worsted of all tints, select one, say a red, and
tell the man to put alongside it all those of the same color,
whether of a lighter or a darker shade; if red blind he will
select not only the reds but the greens, especially the paler
tints. About one man in eight is more or less red blind.
The defect is much rarer in women.

Fatigue of the Retina. The nervous visual apparatus is
easily fatigued. Usually we do not observe this because its
restoration is also rapid, and in ordinary life our eyes, when
open, are never at rest; we move them to and fro, so that
parts of the retina receive light alternately from brighter and
darker objects and are alternately excited and rested. How
constant and habitual the movement of the eyes is can be
readily observed by trying to fix for a short time a small spot
without deviating the glance; to do so for even a few seconds
is impossible without practice. If any small object is steadily
" fixed " for twenty or thirty seconds it will be found that the
whole field of vision becomes grayish and obscure, because
the parts of the retina receiving most light get fatigued, and
arouse no more sensation than those less fatigued and stimu-
lated by light from less illuminated objects. Or look steadily
at a black object, say a blot on a white page, for twenty
seconds, and then turn the eye on a white wall; the latter

THE E7E AS A SENSORY APPARATUS. 547

will seem dark gray, with a white patch on it; an effect due
to the greater excitability of the retinal parts previously
rested by the black, when compared with the sensation
aroused elsewhere by light from the white wall acting on the
previously stimulated parts of the visual surface. All persons
will recall many instances of such phenomena, which are es-
pecially noticeable soon after rising in the morning. Similar
things may be noticed with colors; after looking at a red
patch the eye turned on a white wall sees a blue-green patch;
the elements causing red sensations having been fatigued, the
white, mixed light from the wall now excites on that region
of the retina only the other primary color sensations. The
blending of colors so as to secure their greatest effect depends
on this fact; red and green go well together because each
rests the parts of the visual apparatus most excited by the
other, and so each appears bright and vivid as the eye wan-
ders to and fro; while red and orange together, each exciting
and exhausting mainly the same visual elements, render dull,
r or in popular phrase " kill," one another.

Contrasts. If a well-defined black surface be looked at on
a larger white one the parts of the latter close to the black look
whiter than the rest, and the parts of the black near the
white blacker than the rest; so, also, if a green patch be
looked at on a red surface each color is heightened near where
they meet. This phenomenon is largely due to fatigue and
deficient fixation : the retinal parts not excited and fatigued
by the black or the green are brought by a movement of the
organ so as to receive light from the white or red surface;
phenomena due to this cause are known as those of successive
contrast. Even in the case of perfect fixation, however, some-
thing of the same kind is seen; black looks blacker near
white and green greener near red when the eye has not
moved in the least from one to the other. A small piece of
light gray paper put on a sheet of red, which latter is then
covered accurately with a sheet of semi-transparent tissue-
paper, assumes the complementary color of the red, i.e., IOOKS
bluish green ; and gray on a green sheet under similar cir-
cumstances looks pink. Such phenomena are known as those
of simultaneous contrast, and are explained on psychological
grounds by those who accept Young's theory of color vision.
Just as a medium-sized man looks short beside a tall one, so,
it is said, a black surface looks blacker near a white one, or a

548 THE HUMAN BODY.

gray (slightly luminous white) surface, which feebly excites
red, green, and violet sensations, looks deficient in red (and so
bluish green) near a deeper red surface. There are, however,
certain phenomena of simultaneous contrast which cannot be
satisfactorily so explained, and these have led to other theories
of color vision, the most important of which is that described
in the next paragraph.

Hering's Theory of Vision. Contrasts can be seen with
the eyes closed and covered. If we look a short time at a
bright object and then rapidly exclude light from the eye, we
see for a moment a positive after-image of the object, e.g.f
a window with its frame and panes after a glance at it and
then closing the eyes. In these positive after-images the
bright and dark parts of the object which was looked at retain
their original relationship; they depend on the persistence of
retinal excitement after the cessation of the stimulus and
usually soon disappear. If an object be looked at steadily for
some time, say twenty seconds, and the eyes be then closed, a
negative after-image is seen. In this the lights and shades of
the object looked at are reversed. Frequently a positive
after-image becomes negative before disappearing. The
negative images are explained commonly by fatigue; when the
eye is closed some light still enters through the lids and ex-
cites less those parts of the retina previously exhausted by
prolonged looking at the brighter parts of the field of vision;
or, when all light is rigorously excluded, the self stimula-
tion of the visual apparatus itself, causing the idio-retinal
light, affects less the exhausted portions, and so a negative
image is produced. If we fix steadily for thirty seconds a
point between two white squares about 4 mm. (£ inch)
apart on a large black sheet, and then close and cover our
eyes, we get a negative after-image in which are seen two
dark squares on a brighter surface; this surface is brighter
close around the negative after-image of each square, and
brightest of all between them. This luminous boundary is
called the corona, and is explained usually as an effect of
simultaneous contrast; the dark after-image of the square it
is said makes us mentally err in judgment and think the
clear surface close to it brighter than elsewhere; and it is
brightest between the two dark squares, just as a middle-sized
man between two tall ones looks shorter than if alongside one
only. If, however, the after-image be watched it will often

THE EYE AS A SENSORY APPARATUS. 549

be noticed not only that the light band between the squares
is intensely white, much more so than the normal idio-retinal
light, but, as the image fades away, often the two dark after-
images of the squares disappear entirely with all of the
corona, except that part between them which is still seen as a
bright band on a uniform grayish field. Here there is no
contrast to produce the error of judgment, and from this and
other experiments Hering concludes that light acting on one
part of the retina produces inverse changes in all the rest,
and that this has an important part in producing the phe-
nomena of contrasts. Similar phenomena may be observed
with colored objects; in their negative after-images each tint
is represented by its complementary, as black is by white in
colorless vision.

Endeavoring to exclude such loose general explanations as
" errors of judgment," Hering proposes a theory of vision
•which can only be briefly stated here. We may put all
our colorless sensations in a continuous series, passing through
grays from the deepest black to the brightest white; some-
where half-way between will be a neutral gray which is as
black as it is white. We may do something similar with our
color sensations; as in gray we see black and white so in
purple we see red and blue, and all colors containing red and
blue may be put in a series of which one end is pure red, the
other pure blue. So with red and yellow, blue and green,
yellow and green. If we call to mind the whole solar spec-
trum from yellow to blue, through the yellow-greens, green,
and blue -greens, we get a series in which all but the ter-
minals have this in common that they contain some green.
Green itself forms, however, a special point; it differs from
all tints on one side of it in containing no yellow, and from
all on the other in containing no blue. In ordinary language
this is recognized : we give it a definite name of its own and
call it green. Its simplicity compared with the doubleness
of its immediate neighbors entitles it to a distinct place in
the color-sensation series. There are three other color sensa-
tions which like green are simple and must have specific
names of their own; they are red, blue, and yellow. Green
may be pure green or yellow-green or blue-green, but never
yellow and bluish at once, or reddish. Red may be pure or
yellowish or bluish, but never greenish. Red and green are
thus mutually exclusive; yellow and blue stand in a similar

550 THE HUMAN BODY.

relationship. All other color sensations, as orange, suggest
two of the above, and may be described as mixtures of them;
but they themselves stand out as fundamental color sensa-
tions. Moreover, it follows from the above, that more than
two simple color sensations are never combined in a com-
pound color sensation.

Since red always excludes green, and yellow blue, we may
call them anti-colors (the complementary colors of Young's
theory), and are led to suspect that in the visual organ there
must occur, in the production of each, processes which pre-
vent the simultaneous production of the other, since there is
no a priori reason in the nature of things why we should not
see red and green simultaneously, as well as red and yellow.
Along with our color sensations there is always some color-
less from the black- white series; which we recognize in speak-
ing of lighter and darker shades of the same color.

Hering assumes, then, in the retina or some part of the-
nervous visual apparatus, three substances answering to the
black-white, red-green, and yellow-blue sensational series, the
construction of each substance being attended with one sen-
sation of its pair, and its destruction with the other. Thus,
when construction of the black-white substance exceeds de-
struction, we get a blackish-gray sensation; when the pro-
cesses are equal the neutral gray; when destruction exceeds
construction a light-gray, and so on. In the other color
series similar things would occur; when construction of red-
green substance exceeded destruction in any point of the
retina we would get, say, a red feeling; if so, then excess of
destruction would give green sensation. The intensity of
any given simple sensation would depend on the ratio of the
difference between the construction and destruction of the
corresponding substance, to the sum of all the constructions
and destructions of visual substances going on in that part of
the visual apparatus : in this way anabolic and katabolic
nutritive processes would be the material basis of visual sen-
sations. The intensity of a mixed color sensation would be
the sum of the intensities of its factors, and its tint and
shade dependent on the relative proportion of these factors.
When the construction and destruction of the red-green sub-
stance are equal no color sensation is aroused by it; and we
get gray, due to those simultaneously occurring changes in
the black- white substance which are always present, but were

THE EYE AS A SENSORY APPARATUS. 551

previously more or less cloaked by the results of the changes
in the red-green substance. Ked and green in certain pro-
portions cause then a white or gray sensation, not because
they supplement one another, as on Young's theory, but be-
cause they mutually cancel; and so for other complementary
colors.

Moreover, according to Hering, destruction of a visual sub-
stance going on in one region of the retina promotes con-
struction and accumulation of that substance elsewhere, but
especially in the neighborhood of the excited spot. Hence,
when a white square on a black ground is looked at. destruc-
tion of the black-white substance overbalances construction
in the place, on which the image of the square falls, but
around this construction occurs in a high degree. When the
eyes are shut, this latter retinal region, with its great accumu-
lation of decomposable material, is highly irritable and,
under the internal stimuli causing the idio-retinal light,
breaks down comparatively fast, causing the corona, which
may be intensely luminous; for with the closed eye the total
constructive and destructive processes in the visual apparatus
are small, and so the excess of destruction in the coronal
region bears a large ratio to the sum of the whole processes.
The student must apply this theory for himself to the other
phenomena of contrasts and negative images, as also to the
gradual disappearance of differences between light and dark
objects when looked at for a time with steady fixation; the
general key being the principle that anything leading to the
accumulation of a visual substance increases its decomposi-
tions under given stimulation, and vice versa. The main
value of Bering's theory is that it attempts to account
physiologically for phenomena previously indefinitely ex-
plained psychologically by such terms as "errors of judg-
ment," which really leave the whole matter where it was,
since if (as we must believe) mind is a function of brain, the
errors of judgment have still to be accounted for on physio- '
logical grounds, as due to conditions of the nervous system.

The three visual substances, the anabolisms and katabol-
isms of which according to Hering give rise to color sensa-
tions, need not necessarily be in the retina itself: they may
be in the central nerve portions of the visual apparatus,
being excited through different nerve fibres excited by dif-
ferent lights falling on the retina.

552 THE HUMAN BODY.

There are difficulties in the way of the full acceptance of
either the Young (often called the Young-Helmholtz) theory
or the theory of Hering, and the whole doctrine of color
vision is still in a very unsettled state.

Visual Perceptions. The sensations which light excites
in us we interpret as indications of the existence, form, and
position of external objects. The conceptions which we
arrive at in this way are known as visual perceptions. The
full treatment of perceptions belongs to the domain of
Psychology, but Physiology is concerned with the conditions
under which they are produced.

The Visual Perception of Distance. With one eye our
perception of distance is very imperfect, as illustrated by the
common trick of holding a ring suspended by a string in
front of a person's face, and telling him to shut one eye and
pass a rod from one side through the ring. If a pen -holder
be held erect before one eye, while the other is closed, and
an attempt be made to touch it with a finger moved across
towards it, an error will nearly always be made. (If the
finger be moved straight on towards the pen it will be
touched because with one eye we can estimate direction accu-
rately and have only to go on moving the finger in the proper
direction till it meets the object.) In such cases we get the
only clue from the amount of effort needed to " accommo-
date" the eye to see the object distinctly. When we use
both eyes our perception of distance is much better; when
we look at an object with two eyes the visual axes are con-
verged on it, and the nearer the object the greater the con-
vergence. We have a pretty accurate knowledge of the
degree of muscular effort required to converge the eyes on
all tolerably near points. When objects are farther off, their
apparent size, and the modifications of their retinal images
brought about by aerial perspective, come in to help. The
relative distance of objects is easiest determined by moving
the eyes; all stationary objects then appear displaced in the
opposite direction (as for example when we look out of
the window of a railway car) and those nearest most rapidly;
from the different apparent rates of movement we can tell
which are farther and nearer. We so inseparably and uncon-
sciously bind up perceptions of distance with the sensations
aroused by objects looked at, that we seem to see distance;
it seems at first thought as definite a sensation as color.

TEE EYE AS A SENSORY APPARATUS. 553

That it is not is shown by cases of persons born blind, who
have had sight restored later in life by surgical operations.
Such persons have at first no visual perceptions of distance:
all objects seem spread out on a flat surface in contact with
the eyes, and they only learn gradually to interpret their
sensations so as to form judgments about distances, as the
rest of us did unconsciously in childhood before we thought
about such things.

The Visual Perception of Size. The dimensions of the
retinal image determine primarily the sensations on which
conclusions as to size are based; and the larger the visual
angle the larger the retinal image : since the visual angle de-
pends on the distance of an object the correct perception of
size depends largely upon a correct perception of distance;
having formed a judgment, conscious or unconscious, as to
that, we conclude as to size from the extent of the retinal
region affected. Most people have been surprised now and
then to find that what appeared a large bird in the clouds
was only a small insect close to the eye; the large apparent
size being due to the previous incorrect judgment as to the
distance of the object. The presence of an object of toler-
ably well-known height, as a man, also assists in forming
conceptions (by comparison) as to size; artists for this pur-
pose frequently introduce human figures to assist in giving
an idea of the size of other objects represented.

The Visual Perception of a Third Dimension of Space.
This is very imperfect with one eye; still we can thus arrive
at conclusions from the distribution of light and shade on an
object, and from that amount of knowledge as to the relative
distance of different points which is attainable monocularly;
the different visual angles under which objects are seen also
assist us in concluding that objects are farther and nearer,
and so are not spread out on a plane before the eye, but
occupy depth also. Painters depend mainly on devices of
these kinds for representing solid bodies, and objects spread
over the visual field in the third dimension of space.

Single Vision with Two Eyes. When we look at a flat
object with both eyes we get a similar retinal image in each.
Under ordinary circumstances we see, however, not two ob-
jects but one. In the habitual use of the eyes we move them
so that the images of the object looked at fall on the two
yellow spots. A point to the left of this object forms its

554 THE HUMAN BODY.

image on the inner (right) side of the left eye and the outer
(right) side of the right. An object vertically above that
looked at would form an image straight below the yellow
spot of each eye; an object to the left and above, its image
to the inner side and below in the left eye and to the
outer side and below in the right eye; and so on. We
have learned that similar simultaneous excitations of these
corresponding points mean single objects, and so interpret
our sensations. This at least is the theory of the experi-
ential or empirical school of psychologists, though others be-
lieve we have a sort of intuition on the subject. When the
eyes do not work together, as in the muscular incoordination
of one stage of intoxication, then they are not turned so that
images of the same objects fall on corresponding retinal
points, and the person sees double. When a squint comes
on, as from paralysis of the external rectus of one eye, the
sufferer at first sees double for the same reason, but after a
time he makes new associations of corresponding retinal
points, and this is in favor of the empirical theory.

When a given object is looked at, lines drawn from it
through the nodal points reach the fovea centralis in each
eye. Lines so drawn at the same time from a more distant
object diverge less and meet each retina on the inner side of
its fovea; but as above pointed out the corresponding points
for each retinal region on the inside of the left eye, are on
the outside of the right, and vice versa. Hence the more
distant object is seen double. So, also, is a nearer object, be-
cause the more diverging lines drawn from it through the
nodal points lie outside of the fovea in each eye. Most
people go through life unobservant of this fact; we only pay
attention to what we are looking at, and nearly always this
makes its images on the two foveae. That the fact is as
above stated may, however, be readily observed. Hold one
finger a short way from the face and the other a little farther
off; looking at one, observe the other without moving the
eyes; it will be seen double. For every given position of the
eyes there is a surface in space, all objects on which produce
images on corresponding points of the two retinas: this sur-
face is called the horopter for that position of the eyes: all
objects in it are seen single; all others in the visual field,
double.

The Perception of Solidity. When a solid object is

THE EYE AS A SENSORY APPARATUS.

555

looked at the two retinal images are different. If a truncated
pyramid be held in front of one eye its image will be that
represented at P, Fig. 161. If, however, it be held midway

FIG. 161.

between the eyes, and looked at with both, then the left-eye
image will be that in the middle of the figure, and the right-
eye image that to the right. The small surface, bdca, in
one answers to the large surface, V df c' a', in the other.
This may be readily observed by holding a small cube in
front of the nose and alternately looking at it with each eye.
In such cases, then, the retinal images do not correspond,
and yet we combine them in consciousness so as to see one
solid object. This is known as stereoscopic vision, and the
illusion of the common stereoscope depends on it. Two
photographs are taken of the same object from two different
points of view, one as it appears when seen from the left, and
the other when seen from the right. These are then mounted
for the stereoscope so that each is looked at by its proper eye,
and the object appears in distinct relief, as if, instead of flat
pictures, solid objects, occupying three dimensions of space,
were looked at. Of course in many stereoscopic views the dis-
tribution of light and shade, etc., assist, but these are quite
unessential, as may be readily observed by copying the draw-
ings of Fig. 161 and mounting them on a card the size of a
stereoscopic slide, and placing it in the instrument. A solid
pyramid standing out into space will be distinctly perceived ;
if the pictures be reversed the pyramid appears hollow. The
pictures must not be too different, or their combination to give
the idea of a single solid body will not take place. Many
persons, indeed, fail entirely to get the illusion with ordinary
stereoscopic slides. The phenomena of stereoscopic vision
militate strongly against the view that there are any anatom-
ically prearranged corresponding points in the two retinas.
The Perception of Shine. When we look at a rippled

556 THE HUMAN BODY.

lake in the moonlight, we get the perception of a "shiny " or
brilliant surface. The moonlight is reflected from the waves
to the eyes in a number of bright points: these are not ex-
actly the same for both eyes, since the lines of light-reflection
from the surface of the water to each are different, The
perception of brilliancy seems largely to depend on this
slight non-agreement of the light and dark points on the two
retinas. A rapid change of luminous points, to and fro be-
tween neighboring points on one retina, seems also to pro-
duce it.

CHAPTER XXXIV.
THE EAK AND HEARING.

The External Ear. The auditory organ in man consists
of three portions, known respectively as the external ear, the
middle ear or tympanum, and the internal ear or labyrinth;
the latter contains the end organs of the auditory nerve.
The external ear consists of the expansion seen on the ex-
terior of the head, called the concha, M, Fig. 162, and a pas-
sage leading in from it, the external auditory meatus, 6r.

FIG. 162. — Semidiagrammatic section through the right ear (Czermak). M,
concha; (?, external auditory meatus ; T, tympanic membrane ; P, tympanic
cavity ; o, oval foramen ; r, round foramen ; R, pharyngeal opening of Eusta-
chian tube ; V, vestibule ; B, a semicircular canal ; S, the cochlea ; Vt, scala ves-
tibuli ; Pt, scala tympani ; A, auditory nerve.

This passage is closed at its inner end by the tympanic or
drum membrane, T. It is lined by skin, through which
numerous small glands, secreting the wax of the ear, open.

The Tympanum (P, Fig. 162) is an irregular cavity in
the temporal bone, closed externally by the drum membrane*

557

558 THE HUMAN BODY.

From its inner side the EustacMan tube (R) proceeds to the
pharynx, and the mucous membrane of that cavity is con-
tinued up the tube to line the tympanum; the proper tym-
panic membrane composed of connective tissue is therefore
covered by mucous membrane on its inner, as it is by very
thin skin on its outer, side. In the bony inner wall of the
tympanum are two small apertures, the oval and round fora-
mens, o and r, which lead into the labyrinth. During life the
round aperture is closed by the lining mucous membrane, and
the oval in another way, to be described presently. The tym-
panic membrane, T, stretched across the outer side of the
tympanum, forms a shallow funnel with its concavity out-
wards. It is pressed by the external air on its exterior, and
by air entering the tympanic cavity through the Eustachian
tube on its inner side. If the tympanum were closed the
pressures on the inner and outer sides of the drum membrane
would not be always equal when barometric pressure varied,
and the membrane would be bulged in or out according as
the external or internal pressure on it were the greater. On
the other hand, were the Eustachian tube always open the
sounds of our own voices would be loud and disconcerting, so
it is usually closed; but every time we swallow it is opened,
and thus the air-pressure in the cavity is kept equal to that
in the external auditory meatus. By holding the nose, keep-
ing the mouth shut, and forcibly expiring, air may be forced
under pressure into the tympanum, and will be held in part
imprisoned there until the next act of swallowing. On
making a balloon ascent or going rapidly down a deep mine,
the sudden and great change of aerial pressure outside fre-
quently causes painful tension of the drum membrane, which
may be greatly alleviated by frequent swallowing movements.
The Auditory Ossicles. Three small bones lie in the
tympanum forming a chain from the drum membrane to the
oval foramen. The external bone (Fig. 163) is the malleus
or hammer; the middle one, the incus or anvil; and the
internal, the stapes or stirrup. The malleus, M, has an
upper enlargement or head, which carries on its inner
side an articular surface for the incus; below the head is
a constriction, the neck, and below this two processes com-
plete the bone; one, the long or slender process, is im-
bedded in a ligament which reaches from it to the front
wall of the tympanum; the other process, the handle,

THE EAR AND HEARING. 559

reaches down between the mucous membrane lining the

inside of the drum membrane

and the membrane proper,

and is firmly attached to the

latter near its centre and keeps Mcp

the membrane dragged in there

so as to give it its peculiar MC

concave form, as seen from

the outside. The incus has a MI^

body and two processes, and is 4

much like a molar tooth with ~f~

two fangs. On its body is an

articular hollow to receive the Mm

head of the malleus; its short FlG i63.-The auditory ossicles of the
v»vnr>pc<3 ( 77) \ io tiffnnliorl Kirlirra right ear. seen from the front. M, mal-

piocess (Jo) is attached byiiga- le*s . j incus . s stapes . Mcp^ head of

mpnf fn HIP h«oV \vnll nf flip the malleus ; Me, neck of ditto; Af J, long

ment to tne Dack wall oi tne process. ^nl, handle ;jc, body,X short,
tympanum; the long process and <n< long process of incus; jpi, os

* r orbiculare ; Scp, head ot stapes.

(J7) is directed inwards to the

stapes; on the tip of this process is a little knob, which rep-
resents a bone (05 orbiculare) distinct in early life. The
stapes (S) is extremely like a stirrup, and its base (the foot-
piece of the stirrup) fits into the oval foramen, to the margin
of which its edge is united by a fibrous membrane, allowing
of a little play in and out.

From the posterior side of the neck of the malleus a liga-
ment passes to the back wall of the tympanum : this, with
the ligament imbedding the slender process and fixed to the
iront wall of the tympanum, forms an antero-posterior axial
ligament, on which the malleus can slightly rotate, so that the
handle can be pushed in and the head out and vice versa.
If a pin be driven through Fig. 163 just below the neck of
the malleus and perpendicular to the paper it will very fairly
represent this axis of rotation. Connected with the malleus
is a tiny muscle, called the tensor tympani; it is inserted on
the handle of the bone below the axis of rotation, and when
it contracts pulls the handle in and tightens the drum mem-
brane. Another muscle (the stapedius] is inserted into the
outer end of the stapes, and when it contracts fixes the bone
so as to limit its range of movement in and out of the fenestra
ovalis.

The Internal Ear. The labyrinth consists primarily of
chambers and tubes hollowed out in the temporal bone and

£60

THE HUMAN BODY.

inclosed by it on all sides, except for the oval and round
foramens on- its exterior, and certain apertures on its inner
side by which blood-vessels and branches of the auditory
nerve enter; during life all these are closed water-tight in one
way or another. Lying in the bony labyrinth thus consti-

Vc

FIG. 164.— Casts of the bony labyrinth. A, left labyrinth seen from the outer
side ; B. right labyrinth from the inner side ; C. left labyrinth from above ; Fc,
round foramen ; Fv. oval foramen ; h, horizontal semicircular canal ; ha, its
ampulla ; vaa, ampulla of anterior vertical semicircular canal ; rpa, ampulla of
posterior vertical semicircular canal ; re, conjoined portion or the two vertical
canals.

tuted, are membranous parts, of the same general form but
smaller, so that between the two a space is left; this is filled
with a watery fluid, called the perilympli; and the mem-
branous internal ear is filled by a similar liquid, the endo-
lymph.

The Bony Labyrinth. The bony labyrinth is described
in three portions, the vestibule, the semicircular canals, and
the cochlea ; casts of its interior are represented from differ-
ent aspects in Fig. 164. The vestibule is the central part
and has on its exterior the oval foramen (Fv) into which the
base of the stirrup-bone fits. Behind the vestibule are three
bony semicircular canals, communicating with the back of
the vestibule at each end, and dilated near one end to form
an ampulla (vpa, vaa, and ha). The horizontal canal lies in
the plane which its name implies, and has its ampulla at the
front end. The two other canals lie vertically, the anterior
at right angles, and the posterior parallel, to the median
antero-posfcerior vertical plane of the head. Their ampulltiry
ends are turned forwards and open close together into the
vestibule; their posterior ends unite (vc) and have a common
vestibular opening.

The bony cochlea is a tube coiled on itself somewhat like
a snail's shell, and lying in front of the vestibule.

THE EAR AND HEARING. 561

The Membranous Labyrinth. The membranous vesti-
bule, lying in the bony, consists of two sacs communicating
by a narrow aperture. The posterior is called the utriculus,
and into it the membran-
ous semicircular canals
open. The anterior, called
the sacculus, communi-
cates by a tube with the
membranous cochlea. The
membranous semicircular
canals much resemble the
bony, and each has an-
ampulla; in most of their
extent they are only united

by a few irregular COUneC- . FIG .165.— A section through the cochlea
J m the hue of its axis.

tive-tissue bands with the

periosteum lining the bony canals; but in the ampulla one
side of the membranous tube is closely adherent to its bony
protector; at this point nerves enter the former. The rela-
tions of the membranous to the bony cochlea are more com-
plicated. «•*" A section through this part of the auditory appa-
ratus (Fig. 165) shows that its osseous portion consists of a
tube wound two and a half times (from left to right in the
right ear and vice versa) around a central bony axis, the
modiolus. From the axis a shelf, the lamina spiralis, pro-
jects and partially subdivides the tube, extending farthest
across in its lower coils. Attached to the outer edge of this
bony plate is the membranous cochlea (scala media), a tube
triangular in cross-section and attached by its base to the
outer side of the bony cochlear spiral. The spiral lamina
and the membranous cochlea thus subdivide the cavity of the
bony tube (Fig. 166) into an upper portion, the scala vesti-
buliy &V, and a lower, the scala tympani, ST. Between these
lie the lamina spiralis (Isoi) and the membranous cochlea (CO),
the latter being bounded above by the membrane of Keissner
(R) and below by the basilar membrane (b). The free edge
of the lamina spiralis is thickened and covered with con-
nective tissue which is hollowed out so as to form a spiral
groove (the sulcus spiralis, ss) along the whole length of the
membranous cochlea. The latter does not extend to the tip
of the bony cochlea; above its apex the scala vestibuli and
scala tympani join; both are filled with perilymph, and the

562

THE HUMAN BODY.

former communicates below with the perilymph cavity of the
vestibule, while the scala tympani abuts below on the round
foramen, which, as has already been pointed out, is closed by
a membrane. The membranous cochlea contains certain

FIG. 166.— Section of one coil of the cochlea, magnified, SV, scala vestibnli ;
R, membrane of Reissuer; CC, membranous cochlea (scala media); Us, limbus
lamince spiralis; t, rectorial membrane; ST,- scala tympani; also, spiral lamina;
Co, rods of Corti ; 6, basilar membrane.

solid structures seated on the basilar membrane and forming
the organ of Cor^i • the rest of its cavity is filled with endo-
lymph, which has free passage to that in the sacculus. ^

The Organ of Corti. This contains the end organs of
the cochlear nerves. Lining the sulcus spiralis are cuboidal
cells; on the inner margin of the basilar membrane the cells
become columnar, and then are succeeded by a row which bear
on their upper ends a set of short stiff hairs, and constitute

A B

4' **

1

\ -f FiG'i!67V~Tile rods of Co'1*1'- ^4i a pair of rods separated from the rest ; B, a
m ot tne basnar membrane with several rods on it, showing how they cover in
tne tunnel of Corti ; i, inner, arid e, outer rods ; 6, basilar membrane : r, reticular
membrane.

the inner hair-cells, which are fixed below by a narrow apex
to the basilar membrane ; nerve-fibres enter them. To the
inner hair-cells succeed the rods of Corti (Co, Fig. 166),
which are represented much magnified in Fig. 167. These
rods are stiff and arranged side by side in two rows, leaned

THE EAR AND HEARING.

563

against one another by their upper ends so as to cover in a
tunnel; they are known respectively as the inner and outer
rods, the former being nearer the lamina spiralis. Each
has a somewhat dilated base, firmly fixed to the basilar mem-
brane; an expanded head where it meets its fellow (the inner
rod presenting there a concavity into which the rounded
head of the outer fits); and a slender shaft uniting the two,
slightly curved like an italic /. The inner rods are more
slender and more numerous than the outer, the numbers
being about 6000 and 4500 respectively. Attached to the
external sides of the head of the outer rods is the reticular
membrane (r, Fig. 167), which is stiff and perforated by
holes. External to the outer rods come four rows of outer
hair-cells, connected like the inner row with nerve-fibres;
their bristles project into the holes of the reticular mem-
brane. Beyond the outer hair-cells is ordinary columnar
epithelium, which passes gradually into cuboidal cells lining
most of the membranous cochlea. The upper lip of the
sulcus spiralis is uncovered by epithelium, and is known as
the Iwibus lamince spiralis; from it projects the tectorial
membrane (t, Fig. 166) which extends over the rods of Corti
and the hair-cells.

Nerve-Endings in the Semicircular Canals and the
Vestibule. Medullated fibres (/, Fig. 168) from the vestib-
ular branch of the auditory nerve are distributed along a line
across the ampulla of each
semicircular canal. They lose
their medullary sheath close
to the basement membrane,
a, which the axis cylinders
pierce. The axis cylinders
branch among the epithelium
cells, which at this place are
several rows thick, but have
not yet been traced into direct b-
continuity with any of them.
The cells of the epithelium
are of two varieties. The
columnar cells or hair cells, c,
do not reach the
membrane, are
slightly granular : from the free end of each projects a rigid

FIG. 168. — Diagram of epithelium in
basement nervous region of ampulla of a semicir-
cular canal.

nucleated or

564 THE HUMAN BODY.

hair process, d. The remaining cells, rod cells, ~b, are in
several rows: each has a slender inner process extending to
the basement membrane and an outer which reaches to the
bases of the columnar cells and appears there to end in a
rigid membrane, e, which is perforated for the passage of the
hairs. They probably are mere supporting structures an-
swering somewhat to the fibres of Miiller of the retina.
After death the hairs tend to break up into a bunch of fila-
ments, and they are found imbedded in a sticky mucus-like
material, which is probably a post-mortem product: it has
been named the cupula terminalis. In some parts of the
utricle and saccule is a region of epithelium very similar to
that above described, and also supplied with nerve-fibres. In
connection with them are found minute calcareous particles,
— otoliths or ear-stories.

The Loudness, Pitch, and Timbre of Sounds. Sounds,
as sensations, fall into two groups — notes and noises. Physi-
cally, sounds consist of vibrations, and these, under most
circumstances, when they first reach our auditory organs, are
alternating rarefactions and condensations of the air, or
aerial waves. When the waves follow one another uni-
formly, or periodically, the resulting sensation (if any) is a
note; when the vibrations are aperiodic it is a noise. In
notes we recognize (1) loudness or intensity; (2) pitch; (3)
quality or timbre, or, as it has been called, tone color; a note
of a given loudness and pitch produced by a flute and by a
violin has a different character or individuality in each case;
this quality is its timbre. Before understanding the work-
ing of the auditory mechanism we must get some idea of the
physical qualities in objective sound of which the subjective
differences of auditory sensations are signs.

The loudness of a sound depends on the force of the aerial
waves ; the greater the intensity of the alternating condensa-
tions and rarefactions of these in the external auditory
meatus, the louder the sound. The pitch of a note depends
on the length of the waves, that is the distance from one
point of greatest condensation to the next, or (what amounts
to the same thing) on the number of waves reaching the ear
in given time, say a second. The shorter the waves the
more rapidly they follow one another, and the higher the
pitch of the note. When audible vibrations bear the ratio
1:2 to one another, we hear the musical interval called an

THE EAR AND HEARING. 565

octave. The note c on the unaccented octave is due to 132
vibrations in a second. The note c', the next higher octave
of this, is produced by 264 vibrations in a second; the next
lower octave (great octave, 0), by 66; and so on. Sound
vibrations may be too rapid or too slow in succession to pro-
duce sonorous sensations, just as the ultra-violet and ultra-
red rays of the solar spectrum fail to excite the retina. The
highest-pitched audible note answers to about 38,016 vibra-
tions in a second, but it differs in individuals; many persons
cannot hear the cry of a bat nor the chirp of a cricket, which
lie near this upper audible limit. On the other hand, sounds
of vibrational rate about 40 per second are not well heard,
and a little below this become inaudible. The highest note
used in orchestras is the dv of the fifth accented octave, pro-
duced by the piccolo flute, due to 4752 vibrations in a second;
and the lowest-pitched is the JEJf of the contra octave, pro-
duced by the double bass. Modern grand pianos and organs
go down to 0, in the contra octave (33 vibrations per second)
or even A", (27i), but the musical quality of such notes is
imperfect; they produce rather a "hum." than a true tone
sensation, and are only used along with notes of higher
octaves to which they give a character of greater depth.

Pendular Vibrations. Since the loudness of a tone de-
pends on the vibrational amplitude of its physical antece-
dent, and its pitch on the vibrational rate, we have still to
seek the cause of timbre; the quality by which we recognize
the human voice, the violin, the piano, and the flute, even
when all sound the same note and of the same loudness.
The only quality of periodic vibrations left to account for
this, is what we may call wave- form. Think of the movement
of a pendulum; starting slowly from its highest point, it
sweeps faster and faster to its lowest, and then slower and
slower to its highest point on the opposite side; and then
repeats the movements in the reverse direction. Graphically
we may represent such vibrations by the outer continuous
curved line in Fig. 169. Suppose the lower end of the pen-
dulum to bear a writing point which marked on a sheet of
paper travelling down uniformly behind it, and at such a rate
as to travel the distance 0-1 in two seconds. If the pendu-
lum were at rest the straight vertical line would be drawn.
But if the pendulum were swinging we would get a curved
line, compounded of the vertical movement of the paper and

566

THE HUMAN BODY.

the to-and-fro movement of the pendulum, writing sometimes
on one side of the line 0-1-2 and sometimes on the other.
Starting at a moment when the pendulum crosses the middle,
0, we would get described the curve 0, a^a^a^ at first sepa-
rating fast from the vertical line, then slower, then return-
ing, at first gradually, then faster, until it crossed the vertical
again at the end of a second, and commenced a similar ex-
cursion on its other side, at the end of which it would be
back at 1, and in just the same position, and ready to repeat
exactly the swing, with which we commenced. A pendulum
thus executes similar movements in equal
periods of time, or its vibrations are periodic.
A full swing on each side of the position of
rest constitutes a complete vibration, so the
vibrational period of a seconds pendulum is
two seconds: at the end of that time it is
precisely where it was two seconds before,
and moving in the same direction and at the
same rate. It is clear that by examining
such a curve ^we could tell exactly how the
pendulum moved, and also in what period, if
we knew the rate at which the paper on
which its point wrote was moving. The
vertical line 0-1-2 is called the abscissa;
perpendiculars drawn from it and meeting
the curve are ordinates : equal lengths on
the abscissa represent equal times; where an
ordinate from a given point of the abscissa
meets the curve, there the writing point was
at that moment; where successive ordinates
increase or decrease rapidly the pendulum
moved fast from or towards its position of
rest, and vice versa. Similarly, any other
periodic movement may be perfectly repre-
sented by curves; and since the form of the
curve tells us all about the movement, it is
common to speak of the " form of a vibration," meaning the
form of the curve which indicates its characters. Periodic
vibrations (Fig. 169), whose ordinates at first grow fast, then
more slowly, next diminish slowly and then faster, and
represented by a symmetrical curve on one side the abscissa,

THE EAR AND HEARING. 567

which is repeated exactly on the other side of the abscissa,
are known as pendular vibrations.

The Composition of Vibrations. The vibrations of a
seconds pendulum set the air-particles in contact with it in
similar movement, but the aerial waves succeed one another
too slowly to produce in us the sensation of a musical note.
If, for the pendulum, we substitute a tuning-fork (the prongs
of which move in a like way), and the fork vibrates 132 times
per 1", then 132 aerial waves will fall on the tympanic mem-
brane in that time, and we will hear the note c of the unac-
cented octave. If the larger continuous curve in Fig. 169
represent the aerial vibrations in this case, the distance 0 to
1 on the abscissa will represent T^¥ of a second. Let, simul-
taneously, the air be set in movement by a fork of the next
higher octave, c', making 264 vibrations per 1"; under the
influence of this second fork alone, the aerial particles would
move as represented by the line 0, b1, b3, and so on, the
waves being half as long and cutting the abscissa twice as
often. But when both forks act together the aerial move-
ment will be the algebraic sum of tjhe movements due to
each fork; when both drive the air one way they will rein-
force one another, and vice versa; the result will be the
movement represented by the dotted line, which is still
periodic, repeating itself at equal intervals of time, but no
longer pendular, since it is not alike on the ascending and
descending limbs of the curves. We thus get at the fact
that non-pendular vibrations may be produced by the fu-
sion of pendular, or, in technical phrase, by their compo-
sition.

Suppose several musical instruments, as those of an or-
chestra, to be sounded together. Each produces its own
effect- on the air-particles, whose movements, being the alge-
braical sum of those due to all, must at any given instant be
very complex; yet the ear can pick out at will arid follow the
tones of any one instrument. From the complex aerial
movement it can select that fraction of it which one vibrat-
ing body produces. The air in the external auditory meatus
at any given moment can only be in one state of rarefaction
or condensation and at one rate and in one direction of move-
ment, this being the resultant of all the forces acting upon
it; all clashing, and some pushing one way and others an-
other. If the resultant movement be not periodic it will be

568 THE HUMAN BODY.

recognized as due to noises or to several simultaneous in-
harmonic musical tones; this is commonly the case when
musical tones are not united designedly, and the ear thus
gets one criterion for distinguishing movements of the air
due to several simultaneous musical tones. However, a com-
posite set of tones will give rise to periodic vibrations when
all are due to vibrations of rates which are multiples of the
same whole number. In such cases the movement of the
air in the auditory meatus has no property except vibrational
form by which the ear could distinguish it from a simple
tone; when the two tuning-forks giving the forms of vibra-
tion (with rates as 1 to 2), represented in Fig. 169 by con-
tinuous lines, are sounded together, we get the new form of
vibration represented by the dotted line, and this has the
same period as that of the lower-pitched fork; yet the ear
can clearly distinguish the resultant sound from that of this
fork alone, as a note of the same pitch but of different
timbre; and with practice can recognize exactly what simple
vibrations go to make it up.

The Analysis of Non-Pendular Vibrations. If a per-
son with a trained ear listens attentively to any ordinary
musical tone, such as that of a piano, he hears, not only the
note whose vibrational rate determines the pitch of the tone
as a whole, but a whole series of higher notes, in harmony
with the general or fundamental tone; this latter is the
primary partial tone, and the others are secondary partial
tones; nearly all tones used in music contain both. If the
prime tone be due to 132 vibrations a second (c), its first
upper partial is c' (= 264 vibrations per second); the next is
the fifth of this octave (g' — 396 — 132 X 3 vibrations per 1');
the next is the second octave, c" (132x4 = 528 vibrations per
1'); the next is the major third of the c" (— 132 X 5 = 660
vibrations per second = e"), and so on. The only form of
vibration which gives no upper partial tones is the pendular;
we may call notes due to such vibrations simple tones; and
we, consequently, recognize in music tones which are simple
(such as those of tuning-forks) and those which are com-
pound ; these latter are non-pendular in form.

We find, then, that the form of aerial vibrations deter-
mines in our sensations the occurrence or non-occurrence of
upper partial tones. It also, as we have seen, determines the
quality or timbre of the tone, since vibrational amplitude and

THE EAR AND HEARING. 569

rate are otherwise accounted for in sensation by loud ness
and pitch.

It can be proved, by the employment of the higher mathe-
matics, that every periodic non-pendular movement can be
analyzed (as the dotted curve of Fig. 169 may be) into a
given number of pendular vibrations, that is,-every compound
vibration into a set of simple ones; and that every periodic
non-pendular vibration can be made by the combination of
pendular. Moreover, any given compound vibration can be
analyzed into but one set of simple ones; no other combina-
tion will produce it. Consequently a vibrational movement
of the air in the external auditory passage, producing a com-
pound musical tone sensation, can be exhibited in every case,
but only in one way, as the sum of a number of simple vibra-
tions, whose rates are multiples of that which determines the
pitch of the tone.

Now when the trained ear listens to a tone with the ob-
ject of detecting upper partials if present, it hears them only
when the vibrations producing the tone are non-pendular,
i.e. when upper partials, theoretically, might be expected;
and those heard are exactly those demanded by theory; by
the help of instruments their detection is made easy even to
untrained ears. In ordinary circumstances we do not heed
secondary partial tones; we hear a note of the pitch of the
primary partial and of a certain timbre; and whenever the
upper partials present are different, or of different relative
intensities, the timVre of the note varies. Hence it becomes
probable that, jnst as the ear can at will follow any instru-
ment in an orchestra, analyzing the aerial movement so as to
select and follow the fraction of the whole due to that one,
so it can and does analyze compound tones when proceeding
from one instrument, and that the upper partials, not rising
into consciousness as definite tones, but present as subdued
sensations, give its character to the whole tone and determine
its timbre. It might be, however, that the composition of
non-pendular vibrations from pendular was a mere mathe-
matical possibility, having no real existence in nature. Be-
fore we nan accept the above explanation of timbre, we must
see if there is any evidence that, as a matter of fact, non-
pendnlar vibrations, not only mat/ be, but are, made up by
the combination of pendnlar.

Sympathetic Resonance. Imagine slight taps to be

570 THE HUMAN BODY.

given to a pendulum; if these be repeated at such intervals
of time as to always help the swing and never to retard it,
the pendulum will soon be set in powerful movement. If
the taps are irregular, or when regular come at such inter-
vals as sometimes to promote and sometimes retard the move-
ment, no great swing will be produced; but if they always
push the pendulum in the way it is going at that instant,
they need not come every swing in order to set up a powerful
vibration; once in two, three, or four swings will do. A
stretched string, such as that of a piano, is so far like a
pendulum that it tends to vibrate at one rate and no other;
if aerial waves hit it at exactly the right times they soon set
it in sufficiently powerful vibrations to cause it to emit an
audible note. By using such strings we might hope to de-
tect the separate pendular vibrations in any non-pendular
aerial periodic movement if such really existed; certain
strings would pick out the pendular component agreeing
in rate with their own vibrational period and be soon set
in powerful movement; while those not vibrating in the
same period as any of the pendular components, would
remain practically at rest, like the pendulum getting taps
which sometimes helped and sometimes impeded its swing.
If the dampers of a piano be raised and a note be sung
loudly to it, it will be found that several strings are set
in vibration, such vibrations being called sympathetic. The
human voice emits compound tones which can be mathe-
matically analyzed into simple vibrations, and if the piano
strings set in movement by it be examined, they will be
found to be exactly those which answer to these pendular
vibrations and to no others. We thus get experimental
grounds for believing that compound tones are really made
up of a number of simple vibrations, and get an additional
justification for the supposition that in the ear each note is
analyzed into its pendular components; and that the differ-
ence of sensation which we call timbre is due to the effect of
the secondary partial tones thus perceived. If so, the ear
must have in it an apparatus adapted for sympathetic reso-^
nance.

It may be asked why, if the ear analyzes vibrations in
this way, do we not commonly perceive it ? How is it that
what we ordinarily hear is the timbre of a given tone and not

THE EAR AND HEARING. 571

the separate upper partials which give it this character?
The explanation is more psychological than physiological, and
belongs to the same category as the reason why we do not
ordinarily notice the blind spot in the eye, or the doubleness
of objects out of the horopter, or the duplicity of stereoscopic
images. We only use our senses in daily life when they can
tell us something that may be useful to us, and we neglect so
habitually all sensations which would be useless or confusing,
that at last it needs special attention to observe them at all.
The way in which tones are combined to give timbre to a
note is a matter of no importance in the daily use of them,
and so we fail entirely to observe the components and note
only the resultant, until we make a careful and scientific
examination of our sensations.

The Functions of the Tympanic Membrane. If a
stretched membrane, such as a drum-head, be struck, it will
be thrown into periodic vibration and emit for a time a note
of a determined pitch. The smaller the membrane and the
tighter it is stretched the higher the pitch of its note; every
stretched membrane thus has a rate of its own at which it
tends to vibrate, just as a piano or violin string has. When
a note is sounded in the air near such a membrane, the alter-
nating waves of aerial condensation and rarefaction will
move it; and if the waves succeed at the vibrational rate of
the membrane the latter will be set in powerful sympathetic
vibration; if they do not push the membrane at the proper
times, their effects will neutralize one another: hence such
membranes respond well to only one note. The tympanic
membrane, however, responds equally well to a large number
of notes; at the least for those due to aerial vibrations of rates
from 60 to 4000 per second, running over eight octaves and
constituting those commonly used in music. This faculty
depends on two things: (I) the membrane is comparatively
loosely and not uniformly stretched; (2) it is loaded by the
tympanic bones.

The drum-membrane is a shallow funnel with its sides con-
vex towards the external auditory meatus; something like an
umbrella turned inside out ; in such a membrane the tension is
not uniform but increases towards the centre, and it has accord-
ingly no proper note of its own. Further, whatever tendency
such a membrane may have to vibrate rather at one rate than

572 THE HUMAN BODY.

another, is almost completely removed by " damping" it; i.e.
placing in contact with it something comparatively heavy and
which has to. be moved when the membrane vibrates. This
is effected by the tympanic bones, fixed to the drum-membrane
by the handle of the malleus. Another advantage is gained
by the damping; once a stretched membrane is set vibrating it
continues so doing for some time; but if loaded its movements
cease almost as soon as the moving impulses. The dampers
of a piano are for this purpose; and violin-players have to
"damp" with the fingers the strings they have used when
they wish the note to cease. The tympanic bones act as
dampers.

Functions of the Auditory Ossicles. When the air in
the external auditory meatus is condensed it pushes in the
handle of the malleus. This bone then slightly rotates on
the axial ligament and, locking into the incus where the
two bones articulate, causes the long process (Jl, Fig. 163)
of the latter to move inwards. The incus thus pushes-in the
stapes ; the reverse occurs when air in the auditory passage is
rarefied. Aerial vibrations thus set the chain of bones swing-
ing, and push in and pull out the base of the stapes, which
sets up waves in the perilymph of the labyrinth, and these
are transmitted through the membranous labyrinth to the
endolymph. These liquids being chiefly water, and practi-
cally incompressible, the end of the stapes could not work in
and out at the oval foramen, were the labyrinth elsewhere
completely surrounded by bone : but the membrane covering
the round foramen bulges out when the base of the stapes is
pushed in, and vice versa ; and so allows of waves being set
up in the labyrinthic liquids. These correspond in period
and form to those in the auditory meatus; their amplitude is
determined by the extent of the vibrations of the drum mem-
brane.

The form of the tympanic membrane causes it to transmit
to its centre, where the malleus is attached, vibrations of its
lateral parts in diminished amplitude but increased power; so
that the tympanic bones are pushed only a little way but with
considerable force. Its area, too, is about twenty times as
great as that of the oval foramen, so that force collected on
the large area is, by pushing the tympanic bones, all concen-
trated on the smaller. The ossicles also form a bent lever
(Fig. 163) of which the fulcrum is at the axial ligament and

THE EAR AND HEARING. 573

the effective outer arm of this lever is about half as long again
as the inner, and so the movements transmitted by the drum-
membrane to the handle of the malleus are communicated
with diminished range, but increased power, to the base of
the stapes.

Ordinarily, sound-waves reach the labyrinth through the
tympanum, but they may also be transmitted through the
bones of the head; if the handle of a vibrating tuning-fork
be placed on the vertex, the sounds heard by the person ex-
perimented upon seem to have their origin inside his own,
cranium. Similarly, when a vibrating body is held between
the teeth, sound reaches the end organs of the auditory nerve
through the skull-boDes; and persons who are deaf from dis-
ease or injury of the tympanum can thus be made to hear, as
with the audiplione. Of course if deafness be due to disease
of the proper nervous auditory apparatus no device can make
the person hear.

Function of the Cochlea. We have already seen reason
to believe that in the ear there is an apparatus adapted for
sympathetic resonance, by which we recognize different musi-
cal tone-colors; the minute structure of the membranous
cochlea is such as to lead us to look for it there. An old view
was that the rods of Corti, which vary in length, were like so
many piano-strings, each tending to vibrate at a given rate
and picking out and responding to pendular aerial vibrations
of its own period, and exciting a nerve which gave rise to a
particular tone sensation. When the labyrinthic fluids were
set in non-pendular vibrations, the rods of Corti were thought
to analyze these into their pendular components, all rods of
the vibrational rate of these being set in sympathetic move-
ment, but that rod most whose period was that of the primary
partial tone; this rod would determine the pitch of the note,
and the less-marked sensation due to the others affected would
give the timbre. The rods, however, do not differ in size
sufficiently to account for the range of notes which we hear;
they are absent in birds, which undoubtedly distinguish differ-
ent musical notes; and the nerve-fibres of the cochlea are not
connected with them but with the hair-cells.

On the whole it seems probable that the basilar membrane
is to be looked upon as the primary arrangement for sympa-
thetic resonance in the ear. It increases in breadth twelve
times from the base of the cochlea to its tip (the less width of:

574 THE HUMAN BODY.

the lamina spiralis at the apex more than compensating for
the less size of the bony tube there) and is stretched tight
across, but loosely in the other direction. A membrane so
stretched behaves as a set of separate strings placed side by
side, somewhat as those of a harp but much closer together;
and each string would vibrate at its own period without in-
fluencing much those on each side of it. Probably, then,
each transverse band vibrates to simple tones of its own
period, and excites the hair-cells which lie on it, and through
them the nerve-fibres. Perhaps the rods of Corti, being stiff,
and carrying the reticular membrane, rub that against the
upper ends of the hair-cells which project into its apertures
and so help in a subsiduary way, each pair of rods being
especially moved when the band of basilar membrane carrying
it is set in vibration. The tectorial membrane is probably a
"damper;" it is soft and inelastic, and suppresses the vibra-
tions as soon as the moving force ceases.

Function of the Vestibule and Semicircular Canals.
Many noises are merely spoiled music; they are due to tones so
combined as not to give rise to periodic vibrations; these are
probably heard by the cochlea. If a single violent air-wave
ever cause a sound sensation (which is doubtful, since any vio-
lent push of an elastic substance, such as the air, will cause it
to make several rebounds before coming to rest) we perhaps
hear it by the vestibule ; the otoliths, there in contact with
the auditory hairs, are imbedded in a tenacious gummy mass
quite distinct from the proper endolymph, and are not
adapted for executing regular vibrations, but they might
yield to a single powerful impulse and transmit it to the hair-
cells, and through them stimulate the nerves. There is reason
to believe that the semicircular canals have nothing to do
with hearing; their supposed function is described in Chapter
XXXVI.

Auditory Perceptions. Sounds, as a general rule, do not
seem to us to originate within the auditory apparatus; we
refer them to an external source, and to a certain extent can
judge the distance and direction of this. As already men-
tioned, the extrinsic reference of sounds which reach the laby-
rinth through the general skull-bones instead of through the
tympanic chain is imperfect or absent. The recognition of
the distance of a sounding body is possible onlv when the
sound is well known, and then not very accurately; from its

THE EAR AND HEARING. 575

faintness or loudness we may make in some cases a pretty
good guess. Judgments as to the direction of a sound are
also liable to be grossly wrong, as most persons have experi-
enced. However, when a sound is heard louder by the left
than the right ear we can recognize that its source is on the
left; when equally with both ears, that it is straight in front
or behind ; and so on. The concha has perhaps something to
do with enabling us to detect whether a sound originates be-
fore or behind the ear, since it collects, and turns with more
intensity into the external auditory meatus, sound-waves
coming from the front. By turning the head and noting
the accompanying changes of sensation in each ear we can
localize sounds better than if the head be kept motionless.
The large movable concha of many animals, as a rabbit or a
horse, which can he turned in several directions, is probably
an important aid to them in detecting the position of the
source of a sound. That the recognition of the direction of
sounds is not a true sensation, but a judgment, founded on
experience, is illustrated by the fact that we can estimate
much more accurately the direction of the human voice,
which we hear and heed most, than that of any other sound.

CHAPTER XXXV.

TOUCH. TEMPERATURE SENSATIONS. PATN. COMMON

SENSATIONS. SMELL. TASTE. THE MUSCULAR SENSE.

THE skin is very abundantly supplied with afferent nerve-
fibres, and from it we get several very distinct kinds of sen-
sations; it is therefore not surprising that nerve-fibres are
found to end in it in different ways, but at present we are not
able to associate satisfactorily any one particular variety of
cutaneous nerve-ending with the origination of the impulses
which lead to the occurrence of any one kind of the skin sen-
sations.

Many cutaneous afferent nerve-fibres end in a very simple
way: they form plexuses in the outermost layer of the dermis
and then, losing the medullary sheath, the axis cylinders enter
the epidermis and there break up into extremely minute fila-
ments which ramify among the cells of the Malpighian layer
and terminate there without any special end organs. Other
fibres have special terminal apparatuses, known as (1) tactile
cells; (2) end bulbs ; (3) tactile corpuscles ; (4) Pacinian
bodies.

The Tactile Cells lie usually in the deepest layer of the
epidermis, but sometimes are found also in the dermis. They
ace larger and more granular than the neighboring epidermic
cells, more oval, and stain more deeply with some reagents,
especially gold chloride. Minute axis-cylinder branches can
be traced into close relation to them, and according to some
histologists end in flat expansions closely applied to the tactile
cells, while others believe the nerve-filament to be directly
continuous with the cell substance. These cells are especially
abundant in the epidermis lining the root-sheaths of such
tactile hairs as the ' ' whiskers " of a cat, but they exist in
many if not most regions of the human skin.

The End Bulbs lie in the dermis of certain regions as the
lips, but they are mainly confined to the conjunctiva and to
the mucous membrane lining the mouth and that of the lowest

576

TOUCH. TEMPERATURE SENSATIONS. 577

part of the rectum, all of which possess tactile sensibility.
Very similar bodies are found in the synovial membranes of
some joints. In man they are spheroidal and vary in diameter
from .03 to 0.1 m.m. (-g^-^ inch). Each has an external
capsule of connective tissue within which is a core consisting
of polygonal nucleated ill-defined cells. The nerve-fibre
loses its medullary sheath close to the end bulb and the axis
cylinder enters the core and there usually breaks up into fila-
ments which ramify between the cells of the core and end in
little knobs: sometimes the axis cylinder does not branch.
The tactile corpuscles (Fig. 170) are found especially in the

Fife. 170.— Section of skin showing two papillae of the dermis and some of the
deeper cells of the epidermis ; a, papilla containing blood vessels; 6, papilla con-
taining a tactile corpuscle, t ; d, niedullated nerve-fibres going to the corpuscle;
at/, optical cross-sections of the fibres are seen as they wind round the outside of
the corpuscle; the general transverse direction of the connective-tissue bundles of
the capsule of the corpuscle is shown.

skin of the hands and feet, but also on the inner surface of
the forearm, on the nipple, the lips, and mucous membrane
of the tip of the tongue. They lie in dermic papillae and are
oval in form, measuring about 0.8 m.m. (^fa inch) in the long
and 0.3 m.m. (¥JT inch) in the transverse diameter. Each
has an outer capsule of connective tissue from which many
transverse or oblique dissepiments enter and divide the in-
terior into many small chambers. Two or three medullated
nerve-fibres go to each corpuscle, and after winding around ifc
obliquely several times penetrate the capsule at various levels,
at the same time becoming non-medullated. The axis cylin-
ders run in the clefts between the connective-tissue dissepi-
ments and after branching many times end in pear-shaped or
spherical enlargements, which are always placed near the out-
side of the corpuscle.

578

THE HUMAN BODY.

The Pacinian Bodies or Corpuscles (Fig. 171) are found
in large numbers in the subcutaneous areolar tissue of the hand
and foot, and occasionally in other regions of the skin. But
they are also found in internal parts, as on the nerves of
tendons and ligaments and on some branches of the solar
plexus; and they are very abundant and easily seen in the
mesentery of the cat, so that though almost certainly organs
in which afferent nerve impulses originate, they are not organs
of touch. The corpuscles are oval, often curved on the long
and from 1.5 to 2.5 m.m. (TV-TV inch) in length.

axis,

When fresh they have a whitish translucent appearance and
are somewhat more opaque in the centre. When magnified
each Pacinian body is seen to consist of an almost structure-
less core surrounded by many concentric capsules. Each
capsule is a layer of imperfectly developed connective tissue
having a few very fine fibres, the interstices between which
are filled with liquid : each surface of each capsule is formed
by a well-marked layer of flat nucleated cells, and the cell
layer on the inner side of one capsule is separated from the
layer on the outer side of the next by a narrow cleft, which

is a lymph lacuna. The capsules
are usually so closely applied to
one another that the lymph spaces
between them are almost oblit-
erated. A medullated nerve-
fibre runs to one pole of each Pa-
cinian body and the axis cylinder
and medullary sheath are contin-
ued through the capsules to the
core; the medullary sheath be-
coming thinner on the way. The
axis cylinder enters the core and
runs to near its opposite end,
where it ends in a rounded en-
largement or sometimes divides
into several short branches, each
with a knobbed end.

Touch, or the Pressure
Sense. Through the skin we
get several kinds of sensation; touch proper, heat and cold,
and pain; and we can with more or less accuracy localize
chem on the surface of the Body. The interior of the mouth

•6

FIG. 171. — A Pacinian corpuscle,
magnified.

TOUCH TEMPERATURE SENSATIONS. 579

possesses also these sensibilities. Through touch proper we
recognize pressure or traction exerted on the skin, and the
force of the pressure ; the softness or hardness, roughness or
smoothness, of the body producing it; and the form of this,
when not too large to be felt all over. When to learn the form
of an object we move the hand over it, muscular sensations
are combined with proper tactile, and such a combination of
the two sensations is frequent; moreover, we rarely touch
anything without at the same time getting temperature sen-
sations ; therefore pure tactile feelings are rare.

From an evolution point of view, touch is probably the first
distinctly differentiated sensation, and this primary position
it still largely holds in our mental life ; we mainly think of the
things about us as objects which would give us certain tactile
sensations if we were in contact with them. Though the eye
tells us much quicker, and at a greater range, what are the
shapes of objects and whether they are smooth, rough, and so
on, our real conceptions of round and square and rough
bodies are derived through touch, and we largely translate
unconsciously the teachings of the eye into mental terms of
the tactile sense.

The delicacy of the pressure sense varies on different parts
of the skin; it is greatest on the forehead, temples, and back
of the forearm, where a weight of 2 milligr. (.03 grain) press-
ing on an area of 9 sq. millim. (.0139 sq. inch) can be felt.
On the front of the forearm 3 milligr. (.036 grain) can be
similarly felt, and on the front of the forefinger 5 to 15 milligr.
(.07-0.23 grain).

In order that the sense of touch may be excited neighboring
skin areas must be differently pressed ; when we lay the hand
on a table this is secured by the inequalities of the skin, which
prevent end organs, lying near together, from being equally
compressed. When, however, the hand is immersed in a
liquid, as mercury, which fits into all its inequalities and
presses with practically the same weight on all neighboring
immersed areas, the sense of pressure is only felt at a line along
the surface, where the immersed and non-immersed parts of
the skin meet.

It was in connection with the tactile sense that the facts on
which so-called psycho-physical law (Chap. XXXI.) is based,
were first observed. The smallest perceptible difference of
pressure recognizable when touch alone is used, is about J;

580 THE HUMAN BODY.

i.e., we can just tell a weight of 20 grams (310 grains) from
one of 30 (465 grains) or of 40 grams (620 grains) from one
of 60 (930 grains) ; the change which can just be recognized
being thus the same fraction of that already acting as a stimu-
lus. The ratio only holds good, however, for a certain mean
range of pressures ; it is not true for very small or very great
pressures. The experimental difficulties in determining the
question are considerable; muscular sensations must be rigidly
excluded; the time elapsing between laying the different
weights on the skin must always be equal ; the same region
and area of the skin must be used; the weights must have
the same temperature; and fatigue of the organs must be
eliminated. Considerable individual variations are also ob-
served, the least perceptible difference not being the same in
all persons.

The Localizing Power of the Skin. When the eyes are
closed and a point of the skin is touched we can with some
accuracy indicate' the region stimulated; although tactile
feelings are in general characters alike, they differ in some-
thing (local sign) besides intensity by which we can distin-
guish them ; some sensation quality must be present enabling
us to tell from one another two precisely similar contacts of
an external object when applied, say, to the tips of the fore
and ring fingers respectively. The accuracy of the localizing
power is not nearly so great as in the retina and varies widely
in different skin regions; it may be measured by observing
the least distance which must separate two objects (as the
blunted points of a pair of compasses) in order that they may
be felt as two. , The following table illustrates some of the
differences observed—

/r

Tongue-tip 1.1 mm. (.04 inch) \

Palm side of last phalanx of finger 2. 2 mm. (.08 inch) \

Red part of lips 4.4mm. (.16 inch) \

Tip of nose 6.6mm. (.24 inch) \

Back of second phalanx of finger 11.0 mm. (.44 inch)

Heel". 22.0 mm. (.88 inch)

Back of hand 30.8 mm. (1.23 inches)

Forearm 39.6mm. (1.58 inches),

S^fnum 44.0mm. (1.76 inches)

Back of neck 52.8mm. (2.11 inches)

MMdleofback 66.0mm. (2.64 inches)

The localizing power is a little more acute across the long

TOUCH. TEMPERATURE SENSATIONS. 581

axis of a limb, and is better when the pressure is only strong
enough to just cause a distinct tactile
sensation, than when it is more power-
ful ; it is also very readily and rapidly
improvable by practice.

It might be thought that this local-
izing power depended directly on nerve c — "'
distribution ; that each touch nerve had
connection with a special brain-centre
at one end (the excitation of which
caused a sensation with a chaiWri^ristic
local sign), and at the other end was
distributed over a certain skin area, and
that the larger this area the farther
apart might two points be and still give
rise to only one sensation. If this were so, however, the
peripheral tactile areas (each being determined by the ana-
tomical distribution of a nerve-fibre) must have definite un-
changeable limits, which experiment shows that they do not
possess. Suppose the small areas in Fig. 172 to each repre-
sent a peripheral area of nerve distribution. If any two
points in c were touched we would according to the theory
get but a single sensation; but if, while the compass points
remained the same distance apart, or were even approximated,
one were placed in c and the other on a contiguous area, two
fibres would be stimulated and we ought to get two sensa-
tions; but such is not the case; on the same skin region the
points must be always the same distance apart, no matter how
they be shifted, in order to give rise to two just distinguish
able sensations.

It is probable that the nerve areas are much smaller thac
the tactile ; and that several unstimulated must intervene be-
tween the excited, in order to produce sensations which shall
be distinct. If we suppose twelve unexcited nerve areas
must intervene, then, in Fig. 172, a and # will be just on the
limits of a single tactile area; and no matter how the points
are moved, so long as eleven, or fewer, unexcited areas come
between, we would get a single tactile sensation ; in this way
we can explain the fact that tactile areas have no fixed boun-
daries in the skin, although the nerve distribution hi any part
must be constant. We also see why the back of a knife laid
on the surface causes a continuous linear sensation, although

582 THE HUMAN BODY.

it touches many distinct nerve areas ; if we could discriminate
the excitations of each of these from that of its immediate
neighbors we would get the sensation of a series of points
touching us, one for each nerve region excited ; but in the
absence of intervening unexcited nerve areas the sensations are
fused together.

The ultimate differentiation of tactile areas takes place in
the central organs, as will be more fully pointed out in the
next chapter. Afferent nerve impulses reaching the spinal
cord from a finger-tip enter the gray matter and tend to
spread or radiate in it; from the gray region through which
they spread, impulses are sent on to perceptive tactile centres,
in the brain ; if two skin-points are so close that their regions
of irradiation in the cord overlap, then the two points touched
cannot be discriminated in consciousness, since the brain region
excited is in part common to both. The more powerful the
stimulus the wider the irradiation in the cord, and hence the
less accurate the discriminating power. The more often an
impulse has travelled, the more does it tend to keep its own
proper tract through the gray matter of the cord, and get
on to its own proper brain-centre alone; hence the increase
of tactile discrimination with practice, for we cannot suppose
it to be due to a growth of more nerve-fibres down to the
skin, and a rearrangement of the old, with smaller areas of
anatomical distribution. As a general rule, more movable
parts have smaller tactile areas; this probably depends on
practice, since they are the parts which get the greatest
number of different tactile stimulations.

The Temperature Sense. By this we mean our faculty
of perceiving cold and warmth ; and, with the help of these
sensations, of perceiving temperature differences in external
objects. Its organ is the whole skin, the mucous membrane
of mouth and fauces, pharynx and upper part of gullet, and
the entry of the nares. /Direct heating or cooling of a sensory
nerve may stimulate it and cause painjbut not a trite 'tem-
perature sensation; and the amount oTheat or cold requisite
is much greater than that necessary when a temperature-
perceiving surface is acted upon; hence we must assume the
presence /of temperature end organs.

In a/ comfortable room we feel at no part of the Body
either heat or cold, although different parts of its surface are

TOUCH. TEMPERATURE SENSATIONS. 583

at different temperatures; the fingers and nose being cooler
than the trunk which is covered by clothes, and this, in turn,
cooler than the interior of the mouth. The temperature
which a given region of the temperature organ has (as
measured by a thermometer), when it feels neither hot nor
cold is its temperature-sensation zero for that time, and is
not associated with any one objective temperature; for not
only, as we have just seen, does it vary in different parts of
the organ, but also on the same part from time to time.
Whenever a skin region passes with a certain rapidity to a
temperature above its sensation zero we feel warmth; and
vice versa : the sensation is more marked the greater the dif-
ference, and the more suddenly it is produced; touching a
metallic body, which conducts heat rapidly to or from the*
skin, causes a more marked hot or cold sensation than touching
a worse conductor, as a piece of wood, of the same temperature.

The change of temperature in the organ may be brought
about by changes in the circulatory apparatus (more blood
flowing through the skin warms it and less leads to its cool-
ing), or by temperature changes in gases, liquids, or solids in
contact with it. Sometimes we fail to distinguish clearly
whether the cause is external or internal; a person coming in
from a windy walk often feels a room uncomfortably warm
which is not really so; the exercise has accelerated his circu-
lation and tended to warm his skin, but the moving outer
air has rapidly conducted off the extra heat ; on entering the
house the stationary air there does this less quickly, the skin
becomes hotter, and the cause is supposed to be oppressive
heat of the room. Hence, frequently, opening of windows
and sitting in a draught, with its concomitant risks; whereas
keeping quiet for five or ten minutes, until the circulation
had returned to its normal rate, would attain the same end
without danger.

The acuteness of the temperature sense is greatest at tem-
peratures within a few degrees of 30° 0. (86° F.); at these ,
differences of less than .1° C. can be discriminated. As a
means of measuring absolute temperatures, however, the skin
is very unreliable, on account of the changeability of its sen-
sation zero. We can localize temperature sensations much
as tactile, but not so accurately.

Are Touch and Temperature Sensations of Different
Modality ? Tactile and temperature feelings are ordinarily so

584 THE HUMAN BODY.

very different that we can no more compare them than lumi-
nous and auditory; and if we accept the modern modified
form of the doctrine of specific nerve energies (Chap. XIII),
in accordance with which the same sensory fibre when ex-
cited always arouses a sensation of the same quality, if any,
because it excites the same brain-centre, it is hard to conceive
how the same fibre could at one time arouse a tactile, and at
another a temperature sensation. It has, however, been
maintained that touch and temperature feelings sometimes
pass into one another insensibly. If a half dollar cooled to 5°
C. (41° F.) be placed on a person's brow, and then two (one
on the other) warmed to 37° C. (98.5° F.), he commonly
thinks the weight in the two cases is equal ; i. e. , the tempera-
ture difference leads to errors in his pressure judgments. But
this does not prove an identity in the sensations; the cold
half-dollar may produce contraction of the cutaneous tissues,
leading to compression of the tactile end organs, which is
mistaken, in mental interpretation, for a heavier pressure.
When sensations are combined in other cases, as red and blue-
green to produce white, or partial tones to form a compound,
we either cannot, or can but with difficulty, recognize the
components ; in this case the person feels both the cold and
pressure distinctly when the half-dollar is laid on him.

In certain cases a person mistakes the contact of a piece of
raw cotton with his skin, for the approach of a warm object;
this has been taken to prove that touch and temperature feel-
ings graduate into one another. However, the feeble touch
of the raw cotton might well be less felt than the increased
temperature of the skin, due to diminished radiation when it
was covered by this non-conducting substance ; and the con-
stancy with which, in the ordinary circumstances of life, we
feel and discriminate clearly, on the same skin region at the
same time, both temperature and touch sensations, is a strong
argument against any transition of one into the other.

Moreover, there is direct evidence that three different ap-
paratuses in the skin or at least differently located apparatuses,
are concerned in arousing touch, heat and cold sensations. If
a metal point, lightly weighted, be slowly and evenly moved
along the skin by clockwork, it gives rise to sensations of
touch at some places and if hotter or cooler than the skin
to sensations of temperature at others; but never when in
contact with one point to more than one sensation. If the

r

PAIN. COMMON SENSATIONS. 585

points at which the observed person says I feel touch or I
feel cold or I feel heat, be carefully marked on the skin and
the experiment repeated on one or more subsequent days
the contact points for the three sensations are found to be
unchanged. In certain cases of spinal -cord disease, moreover,
it has been noticed that tactile sensibility may be lost while
temperature sensibility remains; and in others that the capac-
ity of feeling warmth may be nearly or completely lost while
cold sensation remains normal. Excluding pain (' ' abnormal
sensation"), we must conclude that there are in the skin
three distinct sets of nerve-fibres : — One, when excited, arouses
" touch " sensation; a second, " warm " sensation; the third,
11 cold " sensation.

Pain and Common Sensibility. When the skin is power-
fully stimulated by heat, cold or pressure, or is inflamed, we
get a new sensation which we call pain. This is something
quite different from the unpleasantness caused by a dazzling
light or a musical discord or a disagreeable odor or taste.
We recognize these as being still sight or sound or smell or
taste sensations. Pain on the one hand is different from any
of the normal skin sensations and, on the other, is recog-
nized in consciousness as often proceeding from diseased in-
ternal organs from which normally we get no noticeable sen-
sations. An exposed healthy tendon is quite insensible to
touch, but if it be inflamed the slightest pressure may give
rise to nerve impulses causing very acute pain, and pain which
to the consciousness is similar to cutaneous pains or pains of
other organs. Since direct stimulation of the sensory nerves
proceeding from the skin in any way except through their
end organs gives rise to feelings of pain rather than to the
special skin sensations, and pressure and temperature feelings
do insensibly give way to pain feelings when the stimuli ap-
plied to the skin are gradually increased, it has been supposed
that pain is not due to excitation of a special nerve apparatus
of its own, but to over-excitation of the tactile apparatus.
On this theory it would be hard to account for the fact that
skin pain is so very different in modality from a touch or tem-
perature feeling, and to understand why it gives rise in con-
sciousness to conceptions concerning a condition of the Body
and not of some external object : it is not extrinsically referred
by the mind to a quality of anything but the painful part itself,
as a dazzling light sensation or a fetid odor is. There is also

THE HUMAN BODY.

experimental and pathological evidence that the paths taken in
the spinal cord by nerve impulses causing pain are different
from those leading to a consciousness of touch. If certain
parts of the cord are cut in the thoracic region of a rabbit,
gentle touches on the hind limb appear to be felt ; the animal
erects its ears or moves its head: but powerful stimulation of
the sciatic nerve causes no signs of pain, while if the posterior
white columns be cut the animal still can feel stimuli applied
to the hind limb and sufficient to cause pain under normal
conditions, but it appears insensible to gentle pressure on the
skin. In human beings very similar phenomena have been
observed in cases of spinal-cord disease : and in a certain stage
of chloroform or ether narcosis the patient feels the surgeon's
hand or his knife where it touches the skin, but he experiences
no pain when deeper parts are cut.

Such considerations seem to lead to the conclusion that the
nerve-fibres and sense apparatuses concerned with painful
sensations are quite distinct from those of all the special senses.
If that be so we must also assume that there are "pain"
fibres very widely distributed over the skin and through most
other parts of the Body, and usually not so stimulated as to
cause sensations which are present in consciousness. In acci-
dent or disease the afferent impulses become powerful enough
to arouse perception and imperiously call attention to danger.
The nerve-fibres concerned may be named " fibres of common
sensibility," and there is reason to believe that, normally,
feeble afferent impulses travel along them from nearly all
organs to the fore brain ; but so weak and so uniform as not
to excite a perceived feeling: these impulses would thus form
a great background of subconscious feeling, on which special
points from time to time become conspicuous as one or other
nerve of special sense is stimulated or some fibre of common
sensibility is abnormally excited. So far as the epidermis is
concerned, the axis-cylinder branches, which end in it with-
out any special terminal apparatus, may be specially fibres of
common sensibility.

Pains can be localized though but only imperfectly, and
the less perfectly the more severe they are. The exact place
of a needle prick after removal of the needle (so that there is
no guiding concomitant touch sensation) cannot be recognized
as well as pin touch on the same region of the skin, but still
fairly well ; while the acute pain caused by a small abscess

PAIN. COMMON SENSATIONS. 587

(bone felon) under the periosteum of a finger bone is often
felt all over the forearm; and a single diseased tooth may
cause pain felt over the whole of that side of the face. This
is probably due to imperfection in brain and spinal cord of the
isolation of the paths of conduction of the nervous impulses
concerned.

Common Sensations^ These agree with pain sensations in
calling attention to conditions of our Bodies and not of outer
things. Some of them, as general malaise and ' ' feeling well, ' '
are probably due to modifications of the general inflow of im-
pulses through the apparatus of common sensibility, not suffi-
cient to cause a feeling of definite pain or pleasure. Others,
as hunger, thirst and nausea, may have similar origin, but in
a more localized region.

Hunger and Thirst. These sensations, which regulate
the taking of food, are peripherally localized in consciousness,
the former in the stomach and the latter in the throat, and
local conditions no doubt play a part in their production;
though general states of the Body are also concerned.

Hunger in its first stages is probably due to a condition of
the gastric mucous membrane which comes on when the stom-
ach has been empty some time, and it may be temporarily
stilled by filling the organ with indigestible substances. But
soon the feeling comes back intensified and can only be allayed
by the ingestion of nutritive substances; provided these are
absorbed and reach the blood, their mode of entry is unessen-
tial ; the hunger may be stayed by injections of food into the
rectum as well as by putting it into the stomach.

Similarly, thirst may be temporarily relieved by moisten-
ing the throat without swallowing, but then soon returns;
while it may be permanently relieved by water injections into
the veins, without wetting the throat.

While both sensations depend in part on local peripheral
conditions, they may also be, and more powerfully, excited by
poverty of the blood in foods and water; such deficiency
probably directly stimulates hunger and thirst brain-centres.

Smell. The region of the nostril nearest its outer end
possesses the sense of touch, and most of its lining mucous
membrane has common sensibility, which can be aroused by
such substances as ammonia vapor: the nerve-fibres concerned
in these feelings are derived from the superior maxillary branch
of the fifth nerve.

588

THE HUMAN BOD T.

The olfactory organ proper consists of the upper portions of
the two nasal cavities, over which the endings of the olfactory
nerves are spread and where the mucous membrane has a
brownish-yellow color. This region (regio olfactoria) covers
the upper and lower turbinate bones, which are expansions of
the ethmoid on the outer wall of the nostril chamber, the
opposite part of the partition between the nares, and the part
of the roof of the nose separating it from the cranial cavity.
The epithelium covering the mucous membrane contains three
varieties of cells (2, Fig. 173). The cells of one set are much
like ordinary columnar epithelium, but with long branched
processes attached to their deeper ends; mixed with these are

peculiar cells, each of which has
a large nucleus surrounded by a
little protoplasm; a slender ex-
ternal process reaching to the sur-
face ; and a very slender deep one.
The latter cells have been sup-
posed to be the proper olfactory
end organs, and to be connected
with the fibres of the olfactory
nerve, which enter the deeper
strata of the epithelium and there
divide. In Amphibia the corre-
sponding cells have fine filaments
on their free ends. The cells of
the third *kind are irregular in
form and lie in several rows in the
deeper parts of the epithelium.
It may be that the cylindrical cells
if not (as is possible) directly con-

K.i73.-ceiis from the olfactory cerned in olfaction, have import-
ant functions in regard to the
nourishment of the olfactory cells
outer processed, its slender central which they surround i they may

process. 3, gray nerve-fibres of the ' ..?

olfactory nerve, seen dividing into for example Supply them With
fine peripheral branches at a. -, „ , - • , , -i

needful material, as the pigment-
cells of the retina are concerned in the formation of visual
purple in the rods.

Odorous substances, the stimuli of the olfactory apparatus,
are always gaseous and frequently act powerfully when present
in very small amount. We cannot, however, classify them by

TASTE. 589

the sensations they arouse, or arrange them in series; and
mells are but minor sensory factors in our mental life, al-
though very powerful associations of memory are often aroused
by odors. AVe commonly refer them to external objects, since
we find that the sensation is intensified by ' ' sniffing ' ' air into
the nose, and ceases when the nostrils are closed. Their
peripheral localization is, however, imperfect, for we confound
many smells with tastes (see below) ; nor can we well judge of
the direction of an odorous body through the olfactory sen-
sations which it arouses.

Taste. The organ of taste is the mucous membrane on
the dorsum of the tongue and, in some persons, of the soft
palate and fauces. The nerves concerned are the glosso-
pharyngeals, distributed over the hind part of the tongue,
and the lingual branches of the inferior maxillary division of
the trigeminals on its anterior two thirds.

On the tongue most of the sensory nerves run to papillae ;
the circumvallate have the richest supply, and on these are
peculiar end organs (Fig. 174) known as taste buds, they

FIG. 174.— Taste-buds.

are oval and imbedded in the epidermis covering the side
of the papilla. Each consists, externally, of a number of flat,
fusiform, nucleated cells and, internally, of six or eight so-
called taste-cells. The latter are much like the olfactory cells
of the nose, and are probably connected with nerve-fibres at
their deeper ends. The capsule formed by the enveloping
cells has a small opening on the surface ; each taste-cell termi-
nates in a very fine thread which there protrudes. Taste-
buds are also found on some of the fungiform papillae, and
it is possible that simpler structures, not yet recognized, and
consisting of single taste-cells are widely spread over the

590 THE HUMAN BODY.

tongue, since the sense of taste exists where no taste-buds can
be found. The filiform papillae are probably tactile.

That substances be tasted they must be in solution : wipe
the tongue dry and put a crystal of sugar on it; no taste
will be felt until exuding moisture has dissolved some of the
crystal. Excluding the feelings aroused by acid substances,
tastes proper may be divided into sweet, bitter, acid, and
saline. Although contributing much to the pleasures of
life, they are intellectually, like smells, of small value; the
perceptions we attain through them as to qualities of external
objects being of little use, except as aiding in the selection of
food, and for that purpose they are not safe guides at all
times.

Many so-called tastes (flavors) are really smells ; odoriferous
particles of substances which are being eaten reach the olfac-
tory region through the posterior nares and arouse sensations
which, since they accompany the presence of objects in the
mouth, we take for tastes. Such is the case, e.g., with most
spices; when the nasal chambers are blocked or inflamed by
a cold in the head, or closed by compressing the nose, the so-
called taste of spices is not perceived when they are eaten ; all
that is felt, when cinnamon, e.g. , is chewed under such cir-
cumstances is a certain pungency due to its stimulating nerves
of common sensation in the tongue. This fact is sometimes
taken advantage of in the practice of domestic medicine when-
a nauseous dose, as rhubarb, is to be given to a child. Tactile
sensations play also a part in many so-called tastes.

As the tongue, in addition to taste functions, possesses
lactile, temperature, and general sensibility, its nerve ap-
paratus must be complex; and there is even reason to be-
lieve that different nerve-fibres with presumably different end
organs are concerned in the different true tastes. Most
persons taste bitter things better with the back part of the
tongue and sweet things with the tip, and in some persons
the separation of function is quite complete. Chemical com-
pounds are known which in such persons cause a pure sweet
sensation if placed on the tongue tip and a pure bitter sensa-
tion if placed in the region of the circumvallate papillse;
these facts seem to show that the fibres concerned in bitter
and sweet sensation are distinct. Again, if leaves of a certain
plant (Gymnema sylvestre) be chewed, the capacity to taste
sweet or bitter things is lost for some time, but salts and acids

THE MUSCULAR SENSE. 591

are tasted as well as usual; and most persons taste salines
better at the sides of the tongue than elsewhere ; so that the
salt and acid sensations seem to have a different apparatus, not
only from the sweet and bitter, but from one another,

The Muscular Sense. The muscles are endowed with com-
mon sensibility, as proved by the pains of cramp and fatigue,
but in connection with them we have other sensations of great
importance, although they do not often Kecome so obtrusive
in consciousness as to arouse separate attention. Certain of
these feelings (muscle sensations proper] are due to the ex-
citation of sensory nerves ending within the muscles them-
selves : others (innervation sensations) have possibly a central
origin and accompany the starting of volitional impulses from
brain-cells; they are only felt in connection with the voluntary
skeletal muscles.

We have at any moment a fairly accurate knowledge of the
position of various parts of our Bodies, even when we do not
see them ; and we can also judge fairly accurately the extent
of a movement made with the eyes shut. The afferent nerve
impulses concerned in the development of such judgments may
be various ; different parts of the skin are pressed or creased ;
different joints are subjected to pressure; different tendons
are put on the stretch and different muscles are in different
states of contraction, and it is by no means easy to determine
• the part played in each case by the sensory nerves of the
different organs. Moreover, when we push against an object,
or lift it, we are able to form a judgment as to the amount of
effort exerted ; but here again pressure on skin and joints and
tension of tendons come in. Although under normal circum-
stances the skin sensations are undoubtedly of importance, they
are not necessary : persons with cutaneous paralysis can, apart
from sight, judge truly the position of a limb and the extent
of movement made by it ; and in many movements change in
joint pressure must be very little if any. We have then to
look to muscles and tendons themselves for an important
part of the sensations, and in both muscles and tendons there
are organs in connection with nerve-fibres which are certainly
sensory in nature : moreover, muscle sensory nerves, whether
through the organs of Golgi or some other end organ, appear
to be excited by mere passive change of form in the muscle :
with the eyes closed each of us can tell how much another
person has lifted one of our arms.

592 THE HUMAN BODY.

Whether, in addition to the true muscle sense, dependent
on afferent impulses sent to the brain from the contracted
muscle or its tendons, we have a more direct consciousness of
the amount of will exerted to produce a given muscular con-
traction, and can form thereby a judgment as to the extent
of the movement or effort, is a question still in dispute. A
main argument in favor of the existence of such centrally origi-
nating ' ' imiervatioii sensations ' ' is based on phenomena ob-
served in persons afflicted with paresis. They frequently
judge erroneously for a time as to the extent of movements made
by them, thinking that the movement is greater than it really
is. It is argued that in such cases the error cannot be based
on peripheral sensations, but must be due to the fact that
the person judges by the amount of volitional effort he has
made, which was such as in his previous condition of health
would have produced a greater muscular contraction than it
now does in his paretic condition. It is especially in connec-
tion with eye muscles that such errors have been noticed.
When we follow a moving object with the eyes we judge of
the rate of movement by the degree of contraction of the ocular
muscles needed to keep its image on the two f oveas : if the eye
muscles become suddenly enfeebled the person at first thinks
he turns his eyeballs faster than he really does and hence that
the object is moving faster than it actually does: or he may not
move his eye at all when he has willed to do so, and hence
conclude that stationary objects are in motion because their
images are still formed on the same region of the retina, which
could not be the case with stationary objects if the position of
the eyes were changed.

Whether the sensations by which we judge the extent of a
muscular movement be entirely peripheral or in part central,
they enable us to determine very minute differences of con-
traction: the ocular determination of the distance of an object
not too far off to have its absolute distance determined with
considerable accuracy, depends almost entirely upon judg-
ments based upon very small changes in the degree of con-
traction of the internal and external straight (recti) muscles,
converging or diverging the eyeballs; and of the ciliary muscle
determining the necessary accommodation of the lens. A
singer, too, must be able to judge with great minuteness the
degree of contraction of the small muscles of the larynx nec-
essary to produce a certain tension of the vocal cords. It may

THE MUSCULAR SENSE. 593

be well to point out that we do not refer a muscular sensation
to any given muscle or muscles : it is merely associated with a
certain movement or position, and a person who knows noth-
ing about his ocular muscles can judge distance through sen-
sations derived from them, quite as well as any anatomist.
This fact is of course correlated with the fact that in voluntary
movement we do not make a conscious effort to contract any
particular muscles : the higher nerve centres are merely con-
cerned with the initiation of a given movement of a given ex-
tent, and all the details are carried out by lower co-ordinating
centres. In ordinary daily life in fact we have no interest
whatever in a muscular contraction per se ; all we are con-
cerned with is the result, and consciousness has never had need
to trouble itself, if it could, with associating a particular feel-
ing or a particular movement with any individual muscle.

Muscular feelings are, as already pointed out, frequently
and closely combined not only with visual but also with tactile,
in providing sensations on which to base judgments : in the
dark, when an object is of such size and form that it cannot
be felt all over by any one region of the skin, we deduce its
shape and extent by combining the tactile feelings it gives rise
to, with the muscular feelings accompanying the movements
of the hands over it. Even when the eyes are used the sen-
sations attained through them mainly serve as short-cuts which
we have learned by experience to interpret, as telling us what
tactile and muscular feelings the object seen would give us if
felt ; and, in regard to distant points, although we have learnt
to apply arbitrarily selected standards of measurement, it is
probable that distance, in relation to perception, is primarily
a judgment as to how much muscular effort would be needed
to come into contact with the thing looked at.

When we wish to estimate the weight of an object we al-
ways, when possible, lift it,'and so combine muscular with
tactile sensations. By this means we can form much better
judgments. While with touch alone just perceptibly differ-
ent pressures have the ratio 1:3, with the muscular sense
added differences of ^ can be perceived.

CHAPTER XXXVI.
THE SPINAL CORD AND REFLEX ACTIONS.

The Special Physiology of Nerve-Centres. We have al-
ready studied the general physiological properties of nerves
and nerve-centres (Chap. XIII) and found that while the
former are mere transmitters of nervous impulses, the latter
do much more than merely conduct. In some cases the centres
are automatic ; they originate nerve impulses, as illustrated by
the rhythmic impulses starting from the respiratory centre.
In other cases a feeble impulse reaching the centre gives rise to
a great discharge of energy from it (as when an unexpected
noise produces a violent start, due to many impulses sent out
from the excited centre to numerous muscles), so that certain
centres are irritable. Such nerve-centres contain a store of
energy-liberating material which only needs a slight disturb-
ance to upset its equilibrium and initiate powerful efferent
impulses as the result of one feeble afferent. Further, the im-
pulses thus liberated are co-ordinated. When mucus in the
larynx tickles the throat and excites afferent nerve impulses,
these, reaching a centre, cause discharges along many efferent
fibres, so combined in sequence and power as to produce, not
a mere aimless spasm, but a cough which clears the passage.
In very many cases the excitation of centres, with or without
at the same time some of the above phenomena, is associated
with sensations or other states of consciousness.

We have now to study which of these powers is manifested
by different parts of the central cerebro-spinal nervous system,
and under what circumstances and in what degree : and also
some of the phenomena of conduction in spinal cord and
brain.

Conduction in the Spinal Cord. The spinal cord, form-
ing (except the slender sympathetic) the only direct com-
munication between the brain and most of the nerves of the
Body, was considered by the older physiologists as merely a
huge nerve-trunk, into which the various spinal nerves were

594

THE SPINAL CORD AND REFLEX ACTIONS. 595

collected on their way to the encephalon. It does, it is true,
contain the paths for the conduction of all those impulses
which, originating in the cerebrum, give rise to voluntary
movements of the trunk and limbs ; also for all the centrally
travelling impulses which give rise to sensations ascribed to
those parts ; and it is also the path for certain impulses^giving
rise to involuntary movements as, for example, those which,
originating in the respiratory centre, travel to the phrenic and
intercostal nerves.

If, however, the cord were merely collected and continued
nerve-roots it ought to increase considerably in bulk as it ap-
proached the skull, and this it does not do in anything like
the required proportion; a histological enumeration also shows
that the total number of fibres cut across in a transverse sec-
tion of the cord in the upper cervical region is far less than
the total number of fibres in all the spinal nerve-roots. Most
of the root-fibres, in fact, pass at once into the central gray
mass and their axis cylinders end in its cells, or lose their in-
dividuality by joining its network of cell branches and fine
non-medullated fibres. Most of the fibres of the anterior root
end in nerve-cells near the level at which they join the cord,
especially in the cells of the anterior horns : many of the fibres
of the posterior roots also join the gray network, either at or
a little above or a little below the level at which they reach
the cord, but some appear to run on to the brain without en-
tering the gray core. Those which do pass into it probably
break up in its network and are not directly continued into
a cell, but this is still uncertain. In correspondence with the
fact that most of the spinal nerve-fibres have their primary ter-
mination in it near their point of entry, is the fact that the
amount of gray matter at any level is greater or less accord-
ing as the nerve-roots at that level are large or small: the
cervical and lumbar enlargements for example are almost
entirely due to increase of gray matter in those regions.
When we make a voluntary movement of a limb the impulse
orginating in the brain does not pass directly to the motor
nerves of the muscles concerned, but to a mechanism in the
gray matter of the cord, which is in connection with those
muscles ; and when we feel an object touching the finger, the
afferent impulses probably, though not so certainly, first enter
the gray core of the cord and thence make a fresh start to
the brain. When the blood-vessels constrict on painful stimu-

596 THE HUMAN BODY.

latiou of the sciatic nerve, impulses must travel from the
lumbar enlargement of the cord to the vaso-constrictor centre
in the medulla and reflex afferent impulses from it down the
cord to the region of the gray matter from which the anterior
roots conveying motor fibres for the blood-vessels pass out
Although part of the whole course of such impulses lies
in the gray core, yet most of it, in the normal physiological
working of the Body lies, so far as the cord is concerned,
in its white columns, and we have now to try and track
these paths: as also paths of special conduction between
different regions of the spinal gray matter themselves. The
gray matter of the cord being directly continuous with the
gray matter of the medulla oblongata and through it with that
of some other parts of the brain can transmit impulses after
all the white columns of the cord have been divided, but with
such conduction we are not for the present concerned.

To determine the special paths in the white substance of
the cord from and to the brain several methods have been
employed. Experiment on animals as* to loss of sensation or
the power of voluntary movement in parts supplied by nerves
arising from the cord posterior to a partial transverse section
give on the whole unsatisfactory results : partly because of the
difficulty in exactly limiting the section and partly because
of the general shock to the nervous system resulting from the
operation. Still something has been learned in that way,
and something also from observations on persons suffering
from more or less localized diseases of the spinal cord. Direct
stimulation of parts of the cord exposed by transverse section
have also given some results; but more satisfactory evidence
as to tracts of conduction between the brain and cord have
been obtained by the Wallerian method and by the study
of development. Removal or disease of certain parts of
the brain and partial cross-sections of portions of it or of
the cord itself, give rise to degeneration of localized groups of
fibres in parts of the cord posterior to the disease or injury:
these are tracts of descending degeneration. Partial cross-
section of other parts of the cord or of the posterior spinal
roots lead to degenerations above the injury or ascending de-
generations: and in general all the fibres which degenerate as
a result of a given injury acquire in embryonic development
their medullary sheaths at the same time, which is different
from the period at which other groups acquire theirs. Finally,

THE SPINAL CORD AND REFLEX ACTIONS. 597

some regions of the white substance of the cord undergo no
degeneration as the result of injuries above or below them.

The details as to the result of sections or injuries at various
levels differ considerably, but their broad features are indi-
cated in Fig. 175, in which tracts of descending degeneration

FIG. 175.— Diagram of a cross-section of the spinal cord near the upper part of
the cervical enlargement to indicate the main tracts of ascending and descending
degenerations. The gray matter is in solid black; tracts of descending degenera-
tion are shaded in vertical and of ascending in horizontal lines; pt, pyramidal or
crossed pyramidal tract; dpt, direct pyramidal tract; dc-al, descending antero-
lateral tract; ct, comma tract; cbt. cerebellar tract; ac-al, ascending antero-lateral
tract; s, e, t, c, posterior median tract: It, tract of Lissauer-, epc, external posterior
column; t'.aJinternal antero-lateral column.

are shaded in vertical lines, and of ascending degeneration in
black. It represents a cross-section of the cord at about the
level of the fifth cervical nerve. The descending area of de-
generation, pt, is the pyramidal tract or crossed pyramidal
tract; its fibres degenerate posterior to any hemisection of the
cord on the same side, and to section of the anterior pyramid
of the medulla oblongata, or of the cms cerebri on the
opposite side, or as a result of disease or lesions of certain
parts of the convolutions of the cerebral hemisphere of the
opposite side. This tract is large in the upper part of the
cord and becomes smaller the further down it is examined,
because fibres are all the way separating from it to end in the
gray matter of the cord, where they join the mechanisms from
which the motor fibres of the anterior spinal roots arise. The
fibres of the pyramidal tract originate and have their nutri-
tive centres in what is known as the motor area of the
cerebral cortex : from there they converge and are collected
into the ventral portion of the cms cerebri and pass through
it and the pons Varolii to the ventral median portion (an-
terior pyramid) of the medulla oblongata, and there cross the

598 THE HUMAN BODY.

middle line at what is known as the decussation of the pyr-
amids and enter the spinal cord on the opposite side. The
area of descending degeneration, dpi, lying close to the an-
terior fissure is the direct pyramidal tract. Its fibres arise in
the same cerebral region as those of pyramidal tract, and have a
similar course and ending, except that they do not cross to the
other side in the medulla oblongata, but gradually pass over
in the spinal cord itself, to end in the gray matter connected
with the origin of the anterior spinal roots: the direct pyram-
idal tract does not extend so far down the cord as the crossed
tract, pt. Another tract of descending degeneration is dc.al,
the descending antero-lateral : it represents rather an area over
which are to be found some degenerated fibres scattered among
many undegenerated, than a distinct group of fibres. The
same may be said of ct, the comma tract: it only extends
a short way down in the external posterior column of the cord
after a hemisection has been made on the same side. Its fibres
are posterior root-fibres running back in the white matter a
little distance before entering the gray core.

A conspicuous tract of ascending degeneration is the cere-
bellar tract cb.t. It lies on the outer side of the pyramidal
tract and can be traced along the dorsal side of the medulla
oblongata to the cerebellum. It commences in the lumbar
region of the cord, and seems to contain two sets of fibres;
some originating in the gray matter and passing on to re-enter
it at a higher level of the cord ; and others continued to the
cerebellum. The nerve-fibres of this tract are very large.
Another important ascending tract, s, e, f, c, lies in the median
portion of the posterior white column and is named the median
posterior tract. It commences in the lower portion of the
cord and gradually increases in size upwards. Its degenera-
tion follows not only section of the posterior column, but
section of the dorsal roots of the spinal nerves : sections of
these roots in the sacral, lumbar, thoracic, and cervical nerves
cause degenerations in the areas marked respectively s, e,t, c\
hemisection of the cord is followed above the section by de-
generations in this tract corresponding to all tiie spinal nerves
which join the cord below the section. The posterior median
tract is lost as a distinct tract in the medulla oblongata: its
fibres are nearly all small. The ascending antero-lateral tract,
ac.al, contains many fibres which undergo degeneration after
section of the cord on the same side, mixed with many fibres

THE SPINAL CORD AND REFLEX ACTIONS. 599

which do not degenerate. It resembles the cerebellar tract
and differs from the median posterior in only undergoing de-
generation after section of the cord itself, and not of the pos-
terior roots also. The upward ending of its fibres is still
uncertain, but is probably in the cerebellum: the origin of the
fibres is in the gray matter of the cord.

Allowing for all the tracts of degeneration above described
it will be seen that considerable portions of the white col-
umns of the cord (left unshaded in Fig. 175) are un-
affected: at the most, trifling degenerations extending a little
way above or below the point of cross-section are found.
Some of these are due to bundles of posterior root-fibres which
run for a short distance in the external posterior column, epc,
before separating into two sets, one entering the gray matter
of the posterior cornu, the other passing into the internal
median tract. Another bundle of posterior root-fibres runs
up in the cord a short way in what is named the column of
Lissauer, It, and gives rise to an ascending degeneration ex-
tending a short way above any hemi section. The main bulk
of the unshaded parts of the figure, however, represents longi-
tudinal fibres which do not degenerate up or down after section
of the cord : they appear, therefore, to have nutritive centres
at each end ; and probably are fibres uniting different levels
of the gray matter. In addition to the longitudinal are of
course some horizontal : these are partly fibres of spinal roots
passing into the gray core, partly medullated fibres crossing
the middle line in the anterior white commissure; and in
addition to fibres of the gray matter proper uniting its halves
across the middle line are many fine medullated fibres which
run dorso-ventrally and from side to side in it.

As regards longitudinal conduction in the white columns of
the cord, we may sum up the main facts as follows : Tho py-
ramidal and direct pyramidal tracts consist of efferent fibres
uniting the cerebral cortex with various levels of the gray
matter of the cord from which motor fibres for the voluntary
muscles pass out. The descending antero-lateral tract prob-
ably also contains efferent fibres uniting the brain with
different parts of the gray matter of the cord. The cerebellar
and ascending lateral tracts convey afferent impulses from the
gray matter of the cord to the brain, but are only indirectly
connected with the fibres of the dorsal spinal roots. The
median posterior tract is afferent and mainly made of fibres

600 THE HUMAN BODY.

which pass directly from the dorsal spinal roots to the brain
without intervention of the gray matter of the cord ; but some
of its fibres pass into the gray matter of the cord before
reaching the medulla oblongata. Finally, the tracts which
show no special ascending or descending degenerations are
mainly made of longitudinal commissural fibres uniting differ-
ent regions of the gray matter of the cord.

The Spinal Cord as a Reflex Centre. In order to explain
physiological facts we must assume in addition to the special
paths of union between parts of the gray matter of the cord
afforded by certain fibres of the white columns, first, that a
nervous impulse entering the gray network at any point may,
under certain conditions, travel all through it, and give rise
to efferent impulses emerging at any level; and, on the other
hand, that there are certain lines or paths of easiest propa-.
gation between different points in this network, which the
impulses keep to under ordinary conditions.

When a frog is decapitated it lies down squat on its belly
instead of assuming the more erect position of the uninjured
animal; its respiratory movements cease (their centre being
removed with the medulla) ; the hind legs at first remain
sprawled out in any position into which they may happen to
fall, but after a time are drawn up into their usual position,
with the hip and knee-joints flexed : having made this move-
ment the animal, if protected from external stimuli, makes 110
other by its skeletal muscles ; it has lost all spontaneity, and
only stirs under the influence of immediate excitation. Nev-
ertheless the heart goes on beating for hours; the muscles
and nerves, when examined, are found to still have all their
usual physiological properties; and, by suitable irritation, the
animal can be made to execute a great variety of complex
movements. But it is no longer a creature with a will, doing
tilings which v/e cannot predict ; it is an instrument which
can be played upon, giving different responses to different
stimuli (as different notes are produced when different keys
of a piano are struck), and always the same reaction to the
same stimulus; so that we can say beforehand what will hap-
pen when we touch it. Such actions are called reflex or excito-
molor and fall into two groups : (1) orderly or purpose-like
reflexes, which are correlated to the stimulus and are often
defensive, tending, for instance, to remove an irritated part
from the irritating object ; (2) disorderly or convulsive reflexes.

THE SPINAL CORD AND REFLEX

not tending to produce any definite result, and affecting either
a limited region or all the muscles of the body.

In higher animals similar phenomena may be observed. If
a rabbit's spinal cord be divided at the bottom of the neck the
animal is at first thrown into a flaccid limp condition like the
frog, but it soon recovers. Voluntary movements in muscles
supplied from the spinal cord behind the section are never seen
again ; but 011 pinching the hind foot it is forcibly withdrawn.
Men, whose spinal cord has been divided by stabs or disease
below the level of the fifth cervical spinal roots (above which
the fibres of the phrenic nerve, which are necessary for breath-
ing, pass out) , sometimes live for a time, but can no longer move
their legs by any effort of the will, nor do they feel touches,
pinches, or hot things applied to them ; if, however, the soles
of the feet be tickled the legs are thrown into vigorous move-
ment. As a rule, however, orderly reflexes are less marked
and less numerous in the higher animals ; in them the organ-
ization is less machine-like, the spinal cord being more the
servant of the larger brain, and less capable of working with-
out directions. Such animals, when intact, can to a greater
extent control the muscular responses which shall be made to
stimuli under various conditions; they have less automatic
protection in the ordinary risks of life, but a greater range of
possible protection. The human spinal cord, controlled by
the brain, can adapt the reactions of the Body, with great
nicety, to a vast variety of conditions ; the frog's cord by itself
does this for a smaller number of possible emergencies without
troubling at all such brain as the animal has, but is less com-
pletely under the control of the higher centres for adaptation
to other and more complex conditions. The difference being,
however, but one of degree and not of kind, it is best to
approach the study of the reflex actions of the human spinal
cord through an examination of tjiose exhibited by the frog.

The Ordinary Reflex Movements of a Decapitated
Prog. For the occurrence of these the following parts must be
intact : (a) the end organs of sensory nerve-fibres ; (b) afferent
fibres from these to the cord; (c) efferent fibres from the
cord to the muscles; (d) the part of the spinal cord between
the afferent and efferent fibres ; (e) the muscles concerned in
the movement. If the decapitated animal be suspended ver-
tically after the shock of the operation is over, it makes a few
attempts to hold its hind legs in their usual flexed position ;

602 THE HUMAN BODY.

these soon cease, the legs hang down, and the creature comes
to rest. If one flank be now gently scratched with the point
of a pencil a reflex movement occurs, limited to the muscles
of that region; they twitch, somewhat as a horse's neck when
tickled by flies. If a pinch with small forceps be given
at the same spot, more muscles on the same side come
into play; a harder pinch causes also the hind leg of that
side to be raised to push away the offending object; more
violent and prolonged irritation causes all the muscles of
the body to contract, and the animal is convulsed. Here
then we see that a feeble stimulation causes a limited and
purpose-like response; stronger causes a wider radiation of
efferent impulses from the cord and the contraction of
more muscles, but still the movements are co-ordinated to
an end; while abnormally powerful stimulation of the sen-
sory nerves throws all the motor fibres arising from the
cord into activity, and calls forth inco-ordinate spasmodic
action. The orderly movements are very uniform for a given
stimulation; if the anal region be pinched, both hind legs are
raised to push away the forceps ; if a tiny bit of bibulous paper
moistened with dilute vinegar be put on the thigh, the lower
part of that leg is raised to wipe it off; if on the middle of the
back near the head, both feet are wiped over the spot; if on
one flank, the leg and foot of that side are used, and so on ;
in fact, by careful working, the frog's skin can be mapped
into many regions, the application of acidulated water to each
causing one particular movement, due to the co-ordinated
contractions of muscles in different combinations, and never,
under ordinary circumstances, any but that one movement.
The above purpose-like reflex movements may all be charac-
terized as defensive, but all orderly reflexes are not so. For
example, in the breeding season the male frog clasps the female
for several days with his fore limbs. If a male at this season
be decapitated and left to recover from the shock, it will be
found that gently rubbing his sternal region with the finger
causes him to clasp it vigorously.

Disorderly Reflexes or Reflex Convulsions. These come
on when an afferent nerve-trunk is stimulated instead of the
tactile end organs in the skin ; or when the skin is very power-
fully excited; or, with feeble stimuli, in certain diseased states
(pathological tetanus), and under the influence of certain
poisons, especially strychnine. If a frog or a warm-blooded

THE SPINAL CORD AND REFLEX ACTIONS. 603

:animal be given a dose of the latter drug, a stimulus, such as
normally would excite only limited orderly reflexes, will excite
the whole cord, and lead to discharges along all the efferent
fibres so that general convulsions result. It has been clearly
proved that, in such cases, not the skin, or afferent or efferent
nerves, or the muscles, but the spinal cord itself is affected by
the poison (at least primarily), unless unnecessarily large doses
have been given.

The Least-Resistance Hypothesis. In order to compre-
hend reflex acts we mast assume a manifold union of afferent
with efferent nerve-fibres; this is anatomically afforded by the
minute plexus of the gray network, which is continuous through
the whole cord, and in which many fibres of the anterior and
posterior nerve-roots directly or indirectly end. The contin-
uity of this network serves to explain general reflex convul-
sions, and the spread of an afferent impulse, or its results,
through the whole cord, with the consequent emission of effe-
rent impulses through many or all the anterior roots ; but, on
the other hand, it renders it difficult to understand limited
and orderly reflexes, in which only a few efferent fibres are
stimulated. To explain them we have to assume a great re-
sistance to conduction in the gray network, so that a nerve
impulse entering it is soon blocked and transmuted into some
other form of energy; hence it only reaches efferent fibres
originating near the point at which it enters, or fibres placed
in specially easy communication with that. When the frog's
flank is tickled, only muscles innervated from anterior roots
on the same side of the body, and springing from the same
level of the cord, are made to contract ; when the stimulus is
more powerful, the stronger afferent impulse radiates farther,
but mainly in directions determined by lines of conductivity
in the cord ; e.g. , to the origin of the efferent fibres which
cause lifting of the hind leg to the irritated spot. These
paths of easiest conduction, or of least resistance, in some
cases lie in the gray matter itself, in others in the inter-central
or commissural fibres of the highly conductive medullated
kind, which, passing out of the gray substance at one level,
run in the white columns to it at another, where the efferent
fibres of the muscles called into play originate. A still stronger
afferent impulse radiates wider still, and, liberating energy
from all the nerve-cells in the gray matter, produces a useless
.general convulsion. Under the influence of strychnine and

604 THE HUMAN BODY.

in pathological tetanus (as observed, for example, in hydro-
phobia), the conductivity of the whole gray matter is so in-
creased that all paths through it are easy, and so a feeble
afferent impulse spreads in all directions.

To account for the phenomena of localized skin sensations
and of limited voluntary movements we must make a similar
hypothesis. If the nervous impulses entering the gray net-
work of the cord or, through fibres of the posterior median
tract, the gray matter of the medulla oblongata, when the
tip of a finger is touched spread all through it irregularly,
we could not tell what region of the skin had been stimu-
lated, for the central results of stimulating the most varied
peripheral parts would be the same. From each region of the
gray network where a sensory skin-nerve enters there must,
therefore, be a special path of conduction to an anterior brain
region, producing results which differ recognizably in con-
sciousness from those following the stimulation of a different
skin region. Possibly for true touch and temperature sensa-
tions these paths are in the post-median tract. The acuteness of
the localizing power will largely depend on the definiteness
of the path of least resistance in the gray matter, since while
traveling in a medullated nerve-fibre from the skin to the
cord, or (in the white columns) from the gray matter of the
latter to the brain, the nervous impulse is confined to a definite
track. Hence anything tending to let the afferent impulse
radiate when it enters the gray network will diminish the ac-
curacy with which its peripheral origin can be located. This
we see in violent pains; a whitlow on the finger affects only
a few nerve-fibres, but gives rise to so powerful nerve impulses
that when they reach the cord they spread widely and, break-
ing out of the usual track of propagation to the brain, give
rise to ill-localized feelings of pain often referred all the way
up the arm to the elbow. Such cases are comparable to the
transformation of an orderly reflex into a general convulsion
when the stimulus increases.

As animals exhibit no, or at most limited, spontaneous move-
ments when their whole cerebral hemispheres are removed, we
conclude that the nerve impulses giving rise to such movements
normally start in those parts of the brain. Thence they travel
down the pyramidal tracts of the cord to its gray matter, which
they enter at different levels, each in the neighborhood of a
centre for producing a given movement. If they there radiated

TEE SPINAL CORD AND REFLEX ACTIONS. 605

far and wide no definite movement could result, for all the
muscles supplied from the cord would be made to contract,
and not morely those necessary to bend the index finger, for
example. We must here again, therefore, assume a path of
least resistance for the propagation of nerve impulses from a
given fibre coming down from the brain, to the efferent fibres
going to a certain muscle or group of muscles. The path
between the two is almost certainly not direct ; a co-ordinating
spinal centre intervenes, and all that the brain has to do is
to excite this centre, which then secures the proper muscular
co-ordination. If the hand be laid flat on the table and its
palm be rolled over, many muscles, including thousands of
muscular fibres, have to contract in definite order and sequence.
Persons who have not studied anatomy and who are quite
ignorant of the muscles to be used can perform the movement
perfectly; and even a skilled anatomist and physiologist, if
he knew them all and their actions, could not by conscious,
effort combine them so well as the cord does without such
direct interference. We have then to look on the cord as
containing a host of co-ordinating centres for different muscles..
These centres are put in nervous connection, on the one hand,,
with certain regions of the skin, and, on the other, with regions
of the brain, and may be excited from either ; in the former
case the movement is called reflex ; in the latter it may be
reflex, or may be accompanied with a feeling of " will " and
is then called voluntary. The more accurately the required
centre, and no other, is excited, the more definite and precise
the movement.

The Education of the Cord. Much of what is called edu-
cating our touch or our muscles is really education of the
spinal cord. A person who begins to play the piano finds at
first much difficulty in moving his fingers independently; the
nervous impulses from the brain to the cord radiate from the
spinal centres of the muscle which it is desired to move, to
others. But with practice the independent movements be-
come easy. So, too, the localizing power of the skin can be
greatly increased by exercise as one observes in blind per-
sons, who often can distinguish two stimuli on parts of the
skin which are so near together as to give only one sensation
to other people. Such phenomena depend on the fact that
the more often a nervous impulse has traveled along a given
road in the gray matter, the easier does its path become, and

606 THE HUMAN BODY.

the less does it tend to wander from it into others. We
may compare the gray matter to a thicket ; persons seeking to
beat a road through from one point to another would keep the
same general direction, determined by the larger obstacles in
the way, but all would diverge more or less from the straight
path on account of undergrowth, tree trunks, etc. , and would
meet with considerable difficulty in their progress. After
some hundreds had passed, however, a tolerably beaten track
would be marked out, along which travel was easy and all
after-comers would take it. If instead of one entry and one
exit we imagine thousands of each, and that the paths between
certain have been often traveled, others less, and some hardly
at all, we get a pretty good mental picture of what happens in
the passage of nervous impulses through the gray matter of
the cord; the clearing of the more trodden path? answering
to the effects of use and practice. The human cord and that
of the frog must not, however, be looked upon ns pathless
thickets at the commencement ; each individual inherits cer-
tain paths of least resistance determined by the stricture of
the cord, which is the transmitted material result of the life
experiences of a long line of ancestors.

The Inhibition of Reflexes. Since it is possible, as by
strychnine, to diminish the resistance in the gray matter, it
is conceivably also possible to increase it, and diminish or
prevent reflexes. Such is found to be actually the case. We
can to a great extent control reflexes by the will ; for example,
the jerking of the muscles which tends to follow tickling: and
it is found that after a frog's brain is removed it is much
easier to get reflex actions out of the spinal cord. Certain
drugs, as bromide of potassium, also diminish reflex excit-
ability. If a frog's brain be removed and the animal's toe be
dipped into very dilute acid, it will be removed after a few
seconds; the time elapsing between the immersion and the
lifting of the foot is known as the reflex time; anything
diminishing reflex excitability increases this, as the stimulus
(which has a cumulative effect on the centre) has to act longer
before it arouses the cord to the discharging point. If the
sciatic nerve of the other leg be stimulated while the toe is in
the acid the reflex time is increased, or the reflex may fail
entirely to appear. This is one case of a general law, that
any powerful stimulation of one sensory nerve tends to in-
hibit orderly reflexes due to the excitation of another. A

THE SPINAL CORD AND REFLEX ACTIONS. 607

common example is the well-known trick of pinching the
nose or upper lip to prevent a sneeeze. The whole question
of reflex inhibition is at present very obscure. It may be due
to the excitation of special fibres which inhibit reflex centres,
as the fibres of the depressor nerve do the activity of the vaso-
constrictor centre ; or to tlie fact that one nerve impulse in
the cord in some cases blocks or interferes with another; or
partly to both.

Psychical Activities of the Cord. Since we can get quite
marked reflex movements in the lower part of the Body of a
man whose cord is divided and who cannot voluntarily move
his lower limbs, and on questioning him find that he feels
nothing and is quite ignorant of his movements unless he sees
his legs, it is most probable that the spinal cord in all cases is
devoid of centres of consciousness and volition: this is not
certain, however; for there might well be a less division of
physiological labor between the cord and brain of a frog, than
between those of a man. Still we are entitled to good evi-
dence before we admit that things so similar as the human
cord and that of the frog possesses different properties. Co-
ordinated movements following a given stimulus, or cries
emitted by an animal, will not suffice to prove that it is con-
scious, since we know these may occur entirely unconsciously
in men, who alone can tell us of their feelings. We must
look for something that resembles actions only done by men
consciously. In the frog it has been maintained that we have
evidence of such. If a bit of acidulated paper be put on the
thigh of a decapitated frog, the animal will bend its knee and
use its leg to brush off the irritant; always using this same
leg if the stimulus be not so strong as to produce disorderly
reflexes. If now the foot be tied down so that the frog can-
not raise it, after a few ineffectual efforts it will move the
other leg, and may wipe the paper off with it. This it has
been said shows a true psychical activity in the cord; a con-
scious and voluntary employment of new procedures under
unusual circumstances. Bat a close observation of the phe-
nomenon shows that it ,/ill hardly bear this interpretation;
the movements of the other leg are very irregular and inco-
ordinate, and much resemble reflex convulsions stirred up by
the prolonged action of the acid, which goes on stimulating
the skin nerves more and more powerfully. Even if new
muscles came, in an orderly way, into play under the stronger

608 THE HUMAN BODY.

stimulus, that would not prove a volitional conscious use of
them ; we see quite similar phenomenon when there is nothing
purpose-like in the movement. Many dogs reflexly kick vio-
lently the hind leg of the same side when one flank is tickled.
If this leg be held and the tickling continued, very frequently
the opposite hind leg will take on the movements, which it
never does in ordinary circumstances. This is quite compar-
able to the frog's use of its other leg under the circumstances
above described, but here it would be obviously absurd to talk
of a volitional source for such a senseless movement.

Reflex Time. This is the time elapsing between the stimu-
lation of a sensory surface and the resulting reflex contraction
of a muscle. It contains, of course, several elements — the
time taken in the origination and afferent course of the nerve
impulse, the time occupied in the centre, and that in the
eiferent nerve-fibres, and the period of latent excitement of
the muscles. Since the rate of travel of nerve impulses and
the time of latent excitement are known with tolerable ac-
curacy they can be estimated; and their sum subtracted from
the whole time gives the time taken up in the central organ.
This, as might be expected, when we consider the highly
complex nature of the processes required to produce a co-
ordinated reflex movement, is very much greater than the
time occupied in traversing an equal length of nerve trunk.
An electric shock given to one eyelid causes a reflex wink of
both, and by suitable apparatus the time lapsing between
stimulation of one eyelid and movement of the other can be
measured. It is about .0660 sec. ; the calculated time for the
passage of the afferent impulse to the centre in the gray matter
of the fourth ventricle and of the efferent to the orbicularis
muscle of the other eyelid, or the period of latent excitation,
is about .0160 sec., leaving .0500 sec. for the central processes.
Reflex time varies considerably. It is longer for more com-
plicated reflex movements; also the strength of the stimulus
has an influence ; if one toe of a decapitated frog be immersed
in very dilute acid the time which elapses before it is with-
drawn is greater than when the acid is a little stronger.

CHAPTER XXXVII.
THE PHYSIOLOGY OF THE BRAIN.

The Functions of the Brain in General. The brain, at
least in man and the higher animals, is the seat of conscious-
ness and intelligence ; these disappear when its blood-supply
is cut off, as in fainting ; pressure on parts of it, as by a tumor
or by an effusion of blood in apoplexy, has the same result ;
inflammation of it causes delirium; and when the cerebral
hemispheres are unusually small idiotcy is observed. The
brain has, however, many other important functions ; it is the
seat of many reflex, automatic, and co-ordinating centres,
which can act as entirely apart from consciousness as those
of the spinal cord. It is also traversed by many paths of con-
duction, some uniting it with the spinal cord and numerous
others putting its own parts in anatomical connection.

The psychical activities, at least in man, seem to be depend-
ent on the forebrain, the rest of the complex mass having
other non-mental functions or at most being only concerned
in very simple mental states. After the cerebral hemispheres
have been removed from a frog it is still able to perform every
movement as before, but. it no longer performs any spontane-
ously. Suitably stimulated it will leap, swim, crawl, climb,
turn off its back to its normal position ; and if the optic thalami
have not been injured will in leaping forward avoid an ob-
stacle placed between it and the light. Its whole essential
mechanism of movement is clearly intact, and can be thrown
into action and to a certain extent be guided by afferent
nervous impulses. Quite similar phenomena may be observed
in pigeons ; they not only can stand, but walk, fly if thrown
into the air, and preen their feathers, after removal of the
cerebral hemispheres; and if carefully tended will live for
months. Mammals bear badly extensive operations on the
forebrain and usually die before fully recovering from the
shock of the operation; but rats survive some hours, and
then exhibit very similar phenomena. However it has been

609

610 THE HUMAN BODY.

possible by repeated operations, taking away only a part at
a time, to successfully remove almost all the surface gray matter
of the cerebral hemispheres from dogs ; and the animals have
recovered so as to perform many ordinary movements so well
that a person observing them only for a short time would
notice nothing abnormal. But in such cases not only some
cerebral cortex has been left but also the deeper lying corpora
striata and optic thalami: when these gray masses and all
the cerebral cortex are removed, as is possible in frogs and
birds, the animal does not move unless directly stimulated,
or so rarely that movements which appear due to a spontane-
ous volition are probably due to some unobserved irritation or
stimulation. In addition to loss of willed movements there
is loss or nearly complete loss of perception, that is, of the
power of mentally interpreting and giving a meaning to in-
coming nervous impulses. The pigeon or rat will start at a
loud noise, but makes no attempt to escape, as if it conceived
danger; it will follow a light with the eyes but make no at-
tempt to escape from a hand stretched out to seize it ; it can
and does swallow food placed in its mouth, but will starve if
left alone with plenty of it, the sight of edible things seeming
to arouse no idea or conception. It has been doubted whether
the animals have any true sensations; they start at sounds,
avoid opaque objects in their road, and cry when pinched; but
all these may be unconscious reflex acts : on the whole it seems
more probable, however, that they have sensations but not
perceptions ; they feel redness and blueness, hardness and soft-
ness, and so on; but sensations, as already pointed out, tell
in themselves nothing; they are but signs which have to be
mentally interpreted as indications of external objects or of
conditions of the Body: it is this interpreting power which
seems deficient in the animal deprived of its forebrain. In
some cases a like state appears to occur in man in connection
with abnormal states of parts of the cerebral hemispheres.
The patient may have eye, retina, optic nerves and all the
endings of these in the optic thalami and corpora quadrigemina
intact, and his pupils react to light, and the eyes follow a
bright object, yet the object arouses in the patient no idea as
to its nature : apparently he sees it, but he is mind blind.

The Medulla Oblongata. Lying on the ventral aspect of
this (Chap. XII) on the sides of the continuation of the anterior
fissure of the cord are the two masses of nerve-fibres known

THE PHYSIOLOGY OF THE BRAIN. 611

as the anterior pyramids : most of the fibres of these are con-
tinuations of the pyramidal tracts of the cord and here cross
the middle line, forming thus the decussation of the pyramids.
The fibres of the direct pyramidal tract pass on in the pyramid
of the same side, only crossing in the cord. The pyramidal
fibres pass on through the pons Yarolii and along the ventral
or basal side of the crura cerebri (Fig. 176), and enter the
cerebral hemispheres. In the medulla are a number of masses
of gray matter (often named nuclei) which have the same
relation to the motor fibres of cranial nerves as areas of gray
matter in the cord have to the motor fibres of the spinal roots,
and from these motor nuclei medullated fibres join the pyr-
amids and go with them into the forebrain. Such fibres of
ascending degeneration in the cerebellar tract of the cord and
of the ascending antero-lateral tract as extend above the cord
run on the 'dorsal side of the medulla oblongata as the restiform
ladies ; they diverge in front so as to lie on the sides of the
fourth ventricle and enter the cerebellum. The fibres of the
posterior median column terminate in a mass of gray matter
in the medulla known as the nucleus gracilis : those of the
exterior median column in a similar nucleus cuneatus. These
nuclei in turn give origin to many fibres, a large number
crossing the middle line, and some of these are then continued
as the fillet along the dorsal side of the crus cerebri to the fore
brain; others join the restiform body and through it the
opposite side of the cerebellum: these crossings constitute
the sensory decussation, as distinguished from the pyramidal
or motor. The fibres of the antero-lateral descending tract
which do not undergo descending degeneration probably join
the pyramids ; all their fibres entering the medulla from the
cord end in gray matter of the medulla. By the word " end-
ing " is meant, of course, only that they cannot be further
traced as individual fibres, not that no physiological represen-
tatives of them arise in the gray matter of the medulla and
pass to other parts of the brain.

The central canal of the spinal cord passes (Chap. XII)
into the medulla oblongata, in the anterior portion of which it
expands to form the fourth ventricle. The gray matter of the
cord is continued around the canal and on the floor and sides of
the ventricle ; and in connection with it are special thickenings,
rich in nerve-cells forming the nuclei or deep origins of most
of the cranial nerves: some of these nerves arise from more

612 THE HUMAN BODY.

than one nucleus and some of the nuclei are separated from
the gray matter around the central cavity, but a minute ana-
tomical description would be here out of place. The olivary
capsules, however, placed in the olivary bodies which lie on
the outer side of each anterior pyramid may, however, be
mentioned. The nerves having their nuclei in the medulla
oblongata are the hypoglossal (xn), the spinal accessory (xi)
except its spinal portion, the vagus (x), the glossopharyngeal
(ix) (some fibres of which perhaps come from the cord), the
auditory (vm) (by two distinct bundles of fibres, cochlear
and vestibular, connected with distinct nuclei), the facial
(vn), the patheticus (vi), part of the trigeminal. Some of
the trigeminal arises from gray matter in the corpora quad-
rigemina. The nucleus of the abducens (iv) lies just under
the floor of the aqueduct of Sylvius (Fig. 176), opposite the
posterior border of the anterior corpora quadrigemina. The
oculo-motor (in) arises from gray matter under the front
of the aqueduct and from the posterior part of the third ven-
tricle. All the fibres of the above ten nerves arise, then,
from gray matter around the cerebral continuation of the gray
matter of the cord, and most of them behind the midbrain.

Besides its functions as affording paths between the cord
and the rest of the brain and as the seat of many relay and
junction centres the medulla has important reflex and auto-
matic activities. As in the case of the cord, its motor centres
may be thrown into reflex activity by afferent impulses from
below, as well as by efferent travelling down from cerebrum
or cerebellum. It is especially concerned with nervous con-
trol of the organs more immediately connected with circu-
lation, respiration, and mastication. The physiological action
of most of the medullary centres has already been described ;
the more important are — 1. The respiratory centre. 2. The
cardio-inhibitory centre ; the centre of the accelerator heart-
fibres lies in the medulla. 3. The vaso-motor centres. 4.
The centre for the dilator muscle-fibres of the pupil. 5.
The centre for the muscles of chewing and swallowing, which
are commonly thrown into action reflexly, though they may
be made to contract voluntarily. 6. The convulsive centre.
7. The diabetic centre. 8. The centre reflexly exciting activ-
ity in the salivary glands, when sensory nerves in the mouth
are stimulated. 9. Certain centres for complex bodily move-
ments; an animal with its medulla oblongata can execute

THE PHYSIOLOGY OF THE BRAIN. 613

much more complicated reflex acts than one with its spinal
cord alone.

The Cerebellum and Pons Varolii. (Figs. 74, 75). The
anterior part of the medulla oblongata is covered above by
the cerebellum and below by the pons, the latter of which is
mainly a transverse commissure uniting the hemispheres of
the cerebellum, though the pyramidal and other longitudinal
commissural fibres run through it ; and in it are many gray
nuclei. The halves of the cerebellum are also united with
one another by transverse fibres of its middle lobe ; and, be-
hind, by the posterior peduncles with the restiform bodies
and the medulla, and, in front, by the anterior peduncles,
with the cerebrum. Besides its gray surface with small nerve-
cells and the cells of Purkinje (Fig. 82) it contains other
more central gray matter. The most striking anatomical fact"
in relation to the cerebellum is its close connection with the
afferent tracts of the spinal cord, nearly all of which except
the fibres of the fillet are only connected with the cerebrum
through the intervention of the cerebellum. The same is true
of the vestibular portion of the auditory nerve and probably
also of most of the afferent fibres of all the posterior cranial
nerves. The cerebellum is thus subjected to influences from
many regions of the Body ; the skin, the muscles, the ears, and
probably also the eyes are sources of impulses streaming into
it all the time, and modifying the conditions of its gray matter
and the nature of the impulses in turn issued from that. The
most marked result of extensive injury of the cerebellum is
muscular inco-ordination ; it seems to be a chief organ of what
we may call personally acquired reflexes, as distinguished from
inherited.

Every one has to learn to stand, walk, run, and so on; at
first all are difficult, but after a time become easy and are
performed unconsciously. In standing or walking very many
muscles are concerned, and if the mind had all the time to
look directly after them we could do nothing else at the same
time; we have forgotten how we learnt to walk, but in ac-
quiring a new mode of progression in later years, as skating,
we find that at first it needs all our attention, but when once
learnt we have only to start the series of movements and they
are almost unconsciously carried on for us. At first we had
to learn to contract certain muscle groups when we got par-
ticular sensations, either tactile, from the soles, or muscular,

614 THE HUMAN BODY.

from the general position of the limbs, or visual, or others
(equilibrium sensations, see below) from the semicircular
canals. But the oftener a given group of sensations has been
followed by a given muscular contraction the more close be-
comes the association of the two ; the path of connection be-
tween the aiferent and efferent fibres becomes easier the more
it is travelled, and at last the afferent impulses arouse the
proper movement without volitional interference at all, and
while hardly exciting any consciousness; we can then walk or
skate without thinking about it. The will, which had at first
to excite the proper muscular nerve-centres in accordance with
the felt directing sensations, now has no more trouble in the
matter; the afferent impulses stimulate the proper motor
centres in an unconscious and unheeded way. Injury or dis-
ease of the cerebellum produces great disturbances of locomo-
tion and insecurity in maintaining various postures. After a
time the animals (birds, which bear the operation best) can
walk again, and fly, but they soon become fatigued, probably
because the movements require close mental attention and
direction all the time.

Sensations of Equilibrium. In order to make proper
movements of balancing or locomotion we need a knowledge
of the space relations of the Body to its surroundings. When
eyes, muscles, and skin send in concordant afferent impulses,
movements are precise ; if sensations of any one of these groups
are wanting (excluding blind persons who have learned to do
without some of them) or abnormal the whole mechanism is
thrown out of gear. Persons who have lost muscular or tactile
sensibility stand 'and walk with difficulty; those who have
nystagmus (jerking unconscious movements of the eyeballs
which cause the visual field to seem to move in space) do the
same and feel giddy ; and, if a person be rapidly rotated with
his eyes open he soon becomes giddy ; the succession of retinal
images suggests that he is moving in space, but the muscular
and tactile afferent impulses are in conflict with that ; and
though this discordance hardly comes into direct consciousness
as a definite contradiction between sensations, the want of
harmony in the afferent impulses throws co-ordinating motor
mechanisms out of gear, with resulting uncertainty in loco-
motion. An important group of afferent impulses concerned
with the maintenance of bodily equilibrium in addition to
those above referred to is probably derived through the semi-

THE PHYSIOLOGY OF THE BRAIN. 615

circular canals of the ear, which are supplied by the vestibular
portion of the auditory nerve; and it has, as we have seen, a
special cerebellar connection. An old view was that, lying
in three planes at right angles to one another, they served to
distinguish the direction of sound-waves reaching the ear ; but
as the direction of oscillation of the tympanic ossicles is the
same, no matter what that of the sound-waves entering the
external auditory meatus may be, such an hypothesis has no
foundation. The cochlea sufficiently accounts for the appre-
ciation of notes, and such noises as are due to inharmonically
combined tones; while the sacculus will suffice for other
noises : and it is found that disease of the semicircular canals
does not interfere with hearing, but often causes uncertainty
of movements and feelings of giddiness.

Experiment shows that cutting a semicircular canal is fol-
lowed by violent movements of the head in the plane of the
canal divided; the animal staggers, also, if made to walk;
and, if a pigeon and thrown into the air, cannot fly. All its
muscles can contract as before, but they are no longer so co-
ordinated as to enable the animal to maintain or regain a
position of equilibrium. It is like a creature suffering from
giddiness; and similar phenomena follow, in man, electrical
stimulation of the regions of the skull in which the semicir-
cular canals lie.

If, moreover, a person lie perfectly quiet with closed eyes
on a table which can be rotated, he is able to tell when the
table is turned and in which direction, and often with con-
siderable accuracy through what angle. If the rotation be
continued for a time the feeling of it is lost, and then when
the movement ceases there is a sense of rotation in the oppo-
site direction. In such case neither tactile, muscular, nor
visual sensations can help, and in the semicircular canals we
seem to have a mechanism through which rotation of the head
could give origin to afferent impulses, whether the head be
passively moved with the rest of the Body or independently
by its own muscles. Movements of endolymph in relation
to the walls of the canals may act as stimuli by causing a
swaying of the projecting hairs of the ampullae (Fig. 167).
Place a few small bits of cork in a tumbler of water, and rotate
the tumbler ; at first the water does not move with it ; then it
begins to go in the same direction, but more slowly; and,
finally, moves at the same angular velocity as the tumbler.

616 THE HUMAN BODY.

Then stop the tumbler, and the water will go on rotating for
some time. Now if the head be turned or rotated in a hori-
zontal plane similar phenomena will occur in the endolympli
of the horizontal canal ; if it be bent sidewise in the vertical
plane, in the anterior vertical canal ; and if nodded, in the
posterior vertical; the hairs moving with the canal would
meet the more stationary water and be pushed and so, possibly,
excite the nerves at the deep ends of the cells which bear
them, and generate afferent impulses which will cause the
general nerve-centres of bodily equilibration to be differently
acted upon in each case. Under ordinary circumstances the
results of these impulses do not become prominent in con-
sciousness as definite sensations ; out they are probably always
present. If one spins round for a time, the endolympli takes
up the movement of the canals, as the water in the tumbler
does that of the glass ; on stopping, the liquid still goes on
moving and stimulates the hairs which are now stationary;
and we feel giddy, from the ears telling us we are rotating
and the eyes that we are not; hence difficulty in standing
erect or walking straight. A common trick illustrates this
very well : make a person place his forehead on the handle
of an umbrella, the other end of which is on the floor, and
then walk three or four times round it, rise, and try to go out
of a door ; he will nearly always fail, being unable to combine
his muscles properly on account of the conflicting afferent
impulses. This and the feeling of rotation in the contrary
direction when a previous rotation ceases become readily intel-
ligible if we suppose feelings to be excited by relative move-
ments of the endolymph and the canals inclosing it.

The Midbrain. The general arrangement of these parts
has been already described (Fig. 82). Cross-sections show
(Fig. 176) the aqueduct of Sylvius, 8, traversing the mid-
brain near its upper part and surrounded by a thin layer of
gray matter, in close connection with which are the origins
of the third and fourth cranial nerves, iv, and of part of the
fifth. The crura cerebri form the main mass of the midbrain.
Each is divided by gray matter (locus niger, Ln) into a ven-
tral portion (pes or crusta, P), which forms the semicylin-
drical portion of the cms seen on the base of the brain and a
dorsal portion, the tegmentum, Tg. The pes consists mainly
of the fibres of the pyramidal tract, Py, but some fibres of the
fillet, /, also run forward in it, as do fibres, fr and oc, connecting

THE PHYSIOLOGY OF THE BRAIN.

617

it with the frontal and occipital lobes of the cerebral hemisphere.
The tegmentum contains gray masses and many transverse
and longitudinal fibres. Many of the fibres, cp, come from the
anterior peduncle of the cerebellum ; these cross in the pos-
terior part of the tegmenturn ; most of them end in a large
mass of gray matter in the front of the tegmentum named the
red nucleus ; others run forward to the optic thalamus direct.
Other longitudinal fibres are continued from the fillet some of

FIG. 176.— Diagram of cross-section of midhrain in region of posterior corpora
quadrigemina : P, pes; Ln, locus riiger; Tg, tegmentum; .S, aqueduct of Sylvius;
cq, right corpus quadrigeminum. In the pes on left side are indicated by /r, fibres
from frontal lobe of cerebral hemisphere; /, fibres of fillet; py, fibres of pyramidal
tract; oc, fibres from occipital lobe of cerebral hemisphere. In tegmental region,
/', fillet fibres to anterior corpus quadrigeminum; /", fillet fibres for posterior
corpus quadrigeminum; cp. fibres from cerebellar peduncle. Parts containing gray
nerve matter are shaded in horizontal lines.

these,/", end in the posterior corpora quadrigemina; others,
/', in the superior corpora quadrigemina and the occipital re-
gion of the cerebral hemispheres. The corpora quadrigemina
are covered by a layer of medullated fibres, but their main
mass is gray matter. The anterior pair are closely connected
with the optic tracts, and therefore with the optic nerves and
the retinas : to^ikh' outer sides and in front are the external
corpora geniculata, gray masses closely associated with the
optic tracts.

The structure of the midbrain shows that it is in great part
merely a commissure between the parts in front of and behind
it: but its connection with fibres of the optic tract shows
that it has a close relation to visual sensations; and the origin
in it of the oculo-motor and abducens nerves, that from it the
eye muscles, and the iris and ciliary muscle are innervated.

618

THE HUMAN BODY.

The Brain Regions in Front of the Midbrain. It would
be quite a hopeless task to attempt in a few pages any detailed
account of the topography of these, but in addition to the
facts already stated a few points of special physiological signifi-
cance may be indicated. These portions of the brain may
be in general described as consisting of three masses of gray
matter on each side ; optic thalamus, corpus striatum, cerebral

Fl

FIG. 177. — Diagram to illustrate cerebral distribution of fibres proceeding from
the pes of the crus cerebri. For description see texr,.

cortex. They are united in manifold ways by the transverse,
longitudinal, and oblique fibres of the white substance of the
cerebral hemisphere. Their more fundamental relations to
the midbrain and to one another are shown in a very sim-
plified and diagrammatic manner in Figs. 177 and 178. The
tier, or aqueduct of Sylvius, i, is seen passing into the pos-
terior end of the third ventricle, 3, which is separated by
only a very thin layer of white matter from the large ovoid
gray mass o£, which is the optic thalamus. Connected by the

THE PHYSIOLOGY OF THE BRAIN. 619

foramen of Monro, fM, with the third ventricle is the left
lateral ventricle, 2, bounded on the inner side by the thin
septum lucidum. Between the septa lucida is the fifth ven-
tricle, 5. The gray mass, JVc, to the side of the lateral ven-
tricle is the caudate nucleus and the mass Ln the lenticular
nucleus of the corpus striatum. In front and at a level dif-
erent from that of the diagram the two are continuous.

The band of white fibres, ic, lying here between the
lenticular nucleus on the outer side and the caudate nucleus
.and optic thalamus on the inner side is the internal capsule :
ec is the external capsule. Fl is the cortical gray matter of
tH frontal lobe of the cerebrum; PI, of the parietal lobe;
Oc.l. of the occipital lobe: Ro, the fissure of Rolando; Po, the
parieto-occipital fissure. The course of many fibres in the
forebrain is still uncertain, but some important paths have
been traced by anatomical and microscopic work, and still
more by following tracts of degeneration resulting from cer-
tain lesions, as in the case of the spinal cord ; and also by
noticing the results of stimulation or removal of definite areas.

Taking first the pes of the crus cerebri (Fig. 176), which
consists entirely of longitudinal fibres, we find that the py-
ramidal tract, py, Fig. 177, is continued through the internal
capsule and radiates beyond it, to end in the cortex of the
frontal and parietal lobes in the region of the fissure of Rolando.
These fibres are all efferent and degenerate to their endings in
the gray matter of the cord or the motor nuclei of cranial nerves
when the cortex in the Rolandic region is removed. A second
collection of fibres in the pes is the frontal, and its fibres, /r,
can be traced to the frontal region of the cortex; when that
is removed the fibres degenerate as far as the gray matter of
the pons, from which they are probably connected by other
fibres with the opposite side of the cerebellum. A third set
of fibres in the pes is the temporo-occipital, oc: they also pass
through the internal capsule to the corresponding region of
the cortex: they can be traced as far as the gray matter of
the pons, and appear to be fibres of descending degeneration.
Another set of fibres, ca, of descending degeneration in the
internal capsule has no immediate connection with the cortex:
it arises in the caudate nucleus ; the course of the fibres be-
yond the pons is not known. The lenticular nucleus also
gives off fibres, #, to the internal capsule, which probably con-
nect the corpus striatum through the pes with the pons and

620 THE HUMAN BODY.

medulla oblongata, but they are so mingled with other fibres
that they have not been satisfactorily traced.

Passing now to the tegmentum, it is first to be borne in
mind that many fibres (including most of those of the anterior
cerebellar peduncle) entering it from behind, end in the large
red nucleus lying in its anterior portion and in its other gray
masses. From these (Fig. 178) numerous fibres, rn, pass to
the optic thalamus: so that the majority of the tegmental
fibres differ from the pedal in that they only have indirect
connection with the cortex through the thalamal and other
gray matter. The thalamus is united with nearly all regions
of the cortex by fibres, «/, passing from its outer side into
the internal capsule, and distributed in special abundance to
the occipital lobe. Since the thalamus receives fibres through
the tegmentum from the anterior quadrigemina and the
lateral geniculata (which we have seen to bave close connec-
tion with the optic nerves), and there is independent reason
for believing parts of the occipital lobe to be closely associated
with visual perceptions, the close anatomical association of
that lobe with the thalamus is significant. Another group of
fibres, d, connects the thalamus with the temporal and occip-
ital cortex, but does not take its path through the internal
capsule. Some fibres of the tegmentum reach the cortex
without primary connection with the thalamus : of these is a
set, e, which passes through the lenticular nucleus (but with-
out any communication with its gray substance) on its way
to the frontal and parietal lobes. At ce is indicated a set of
fibres of the tegmentum which there is some reason to believe
run to the fore f>art of the cortex direct, having no connection
with the thalamus and passing ventral to the internal capsule.

Most of the fibres of the fillet, we have seen, end in the
red nucleus or corpora quadrigemina: fibres arising in these
gray masses connect them with the thalamus and through it
with the cortex.

Besides fibres connecting the cortex with other parts are
many which unite different cortical areas directly. A vast
number (Ca, Fig. 177) cross the middle line in the corpus
callosum and are believed to join corresponding parts of the
two hemispheres. Others pass over in the small white an-
terior commissure and unite the two olfactory lobes and
portions of the temporal lobes. The posterior commissure
unites mainly the optic thalami and the front ends of the

THE PHYSIOLOGY OF THE BRAIN.

621

tegmenta. The soft commissure is mainly gray matter.
Finally a large number of associational fibres, as, unite
different parts of the cortical substance of the same hemi-
sphere.

The different gray masses on the same side of the forebrain
are also united by fibres. They are either so scattered among
others that they cannot be tracked out along special tracts of
degeneration; or, as is possible, resemble some of the com-

FI

— i

FIG 178.— Diagram to illustrate cerebral distribution of the fibres proceeding
from the tegmentum. For description see text.

missural fibres uniting upper and lower regions of gray matter
in the spinal cord in having nutritive, centres at each end,
and therefore not degenerating on either side of a section. In
any case very little is known as to their numbers or paths :
their existence is indicated by the dotted lines in Figs. 177,
178.

Omitting the associational and the cross commissure fibres
and those uniting the corpora striata and optic thalami,.

622 THE HUMAN BODY.

it may be said in general that the systems of fibres represented
in Fig. 177 are all almost certainly concerned in conveying
impulses from the cortex, and those in Fig. 178 in the trans-
mission of afferent impulses. It will be noted that both affer-
ent and efferent fibres are abundant in the internal capsule ;
and that the corpus striatum and pes are more especially con-
nected with efferent and the tegmentum and thalamus with
afferent impulses. It can hardly be necessary to add that
each line in the diagrams represents hundreds of thousands
of nerve-fibres.

The Functions of the Cerebral Cortex. That this part of
the nervous system is in close association with the intellect and
with the initiation of voluntary movements seems beyond
doubt: but it may have other functions quite apart from any
states of consciousness ; and intelligence and every volition may
not entirely depend on it. The experiments made in recent
years on the lower animals tend to the conclusion that some
will and some intellect may remain in animals all or almost all
of whose gray cerebral surfaces have been removed ; the more
complete loss of those powers described by earlier workers
being due to the fact that the animals were not kept alive long
enough after the operation. It has been observed that a dog
whose cerebral cortex (as verified by subsequent post-mortem
examination) had been nearly completely removed did learn
after some months to walk about to all appearance voluntarily,
and to find and eat his food ; he even learned not to take the
food of other dogs after he had been severely bitten several
times for so doing. But more complex perceptions were lost :
before the operation, for example, he was greatly terrified by
seeing a man fantastically dressed, but afterwards no such
appearance aroused in him so complex a conception as that of
a strange or dangerous object. He also never recovered the
trick of "giving paw," which had previously been taught
him. But on the whole a person casually observing him would
not have thought him very different from any other dog, ex-
cept perhaps that he was rather stupid : put into a low open
box, for example, he would not jump out of it when called,
though he easily could do so and clearly desired to. Such
simple and fundamental perceptions and volitions as remained
in this and some similar cases probably have their seats in the
optic thalami and corpora striata, and indeed embryology shows
that the corpora striatum is morphologically a part of the

THE PHYSIOLOGY OF THE BRAIN. 623

cerebral cortex : it is therefore probable that in man some of
the lower and simpler mental faculties are associated also with
those parts. There are, however, great and obvious chances
of error in arguing from the actions of the lower animals as to
their mental state : and these are increased by the compara-
tively small proportion the cerebral cortex bears to the whole
cerebro-spinal centre in these animals when compared with its
ratio in man, showing its less importance in the management
of their actions. Hence the most useful observations are
those made of late years on apes and monkeys and on men
suffering from local brain disease. By utilizing these it has
been possible to map out certain areas of the brain surface
as having special, though possibly not absolutely unshared
association, with volitional movement and with groups of
sensations and sensory interpretations. In addition to facts
obtained by removal or local disease of parts of the brain we
have others obtained by electrical stimulation of certain
parts of the cortex, which although quite insensible to cut-
ting or mechanical irritation does in some places respond to
application of the interrupted or constant electric current.
The more important results obtained are indicated in a
general way in Figs. 179 and 180, representing respectively the
outer and inner surfaces of the right cerebral hemisphere;
these diagrams should be compared with the more detailed
figures in Chapter XI.

The shaded area beginning on the top of the brain and
extending down the sides of the fissure of Rolando or central
fissure, Ro, and beyond its ventral end is the motor area of
the cortex. It also extends to the inner side of the hemi-
sphere, as shown in Fig. 179. Electric stimulation of dif-
ferent parts of this area causes movements of leg, arm, or
face as indicated. Removal of the region marked " arm " in
the monkey causes motor paralysis and some loss of sensi-
bility in the arm on the opposite side of the body. It is
also followed by degenerations extending from the re-
moved region of cortex through the internal capsule to
some pyramidal fibres in the pes and thence back through
the pyramids to the crossed pyramidal and direct pyram-
idal tracts in the cord as far as the cervical enlarge-
ment. Localized disease of this area in man is followed
by paralysis of voluntary movements of the opposite «'irm
and by similar degenerations. Similar statements are true

624

THE HUMAN BODY.

for the areas marked leg, foot, and face, except that the re-
sulting degeneration would extend in the one case to the
lumbar enlargement of the cord, in the other to the nucleus
of the vn nerve in the medulla. Moreover, each of these
areas can be mapped out into smaller ones, giving origin
to a more limited movement when stimulated and a more
limited paralysis and tract of degeneration when removed.
Thus areas especially associated with the eyelids, with the
muscles of the angle of the mouth, with the flexor muscles of

Pa

Fv

FIG. 179.— Diagram of outer surface of left cerebral hemisphere to illustrate the
localization of functions. The motor area is shaded in vertical and transverse
lines: .Vj/, fissure of Sylvius; an, angular gyrus or convolution; Ro, fissure of
Rolando; Fv, frontal lobe; Pa, parietal lobe; Te, temporal lobe. Only a very few
of the more important fissures are indicated. Compare with Fig. 180.

the wrist, all have their definite places in the general face or
arm region. So definite are the positions of these areas that
in cases of localized paralysis, diagnosed as due to lesions of
the cerebral cortex, surgeons now have no hesitation in open-
ing the skull in order if possible to remove the cause of
trouble, as a small tumor: they know precisely in what spot
they will find it. Although the localization is therefore
tolerably precise, yet the limits of neighboring areas are not
as sharp cut as the boundaries of neighboring countries on a
map: as shown in Fig. 179, the arm area in its lower part
overlaps part of the face area; and the minor areas within
the main ones also overlap one another at their margins.

The general interpretation put upon the above facts, and
one which seems justified, is that in making definitely willed

THE PHYSIOLOGY OF THE BRAIN. 625

movements the cortical area connected through the pyram-
idal tract with the muscles concerned is the place from
which efferent impulses start throwing into action lower
centres which more immediately co-ordinate the muscles:
these lower centres in mid brain, cerebellum, medulla or cord
may of course be thrown into reflex action by afferent im-
pulses having no connection with the cortex, and to the eye
the resulting movement would be exactly the same as a
willed one. In another person, and still more in a dog or
monkey, we must often be in doubt whether an action is or
is not intentional; and as already pointed out, many move-
ments of our own which were at one time even painfully in-
tentional become quite unconscious after* practice and are
carried out by lower centres. It is also to be borne in mind
that the cortical area from which the efferent processes of a
willed movement make their start is in connection by as-
sociational and other commissural fibres with many other
regions of the cortex, and with fibres from the optic thalamus
which may bring nerve impulses exciting it, and it is also in
connection with the whole gray cortical network, so that the
brain antecedents or excitants leading to a given movement,
either alone or in combination with others, may be very
different, and may be associated or not with concomitant
sensations or emotions.

Take such a movement as clenching the fist. On a corpse
this might be brought about by pulling on the flexor tendons
of the digits, but in an imperfect way; or, again in a very
imperfect manner by stimulation of the motor nerves of the
flexor muscles in the arm of a living person. If, however, we
knew exactly the proper sensory fibres in spinal nerve-roots
to stimulate and could thus act on the centre co-ordinating
the proper muscles, there is no doubt we could bring about
reflexly, and apart from all consciousness, a quite normal
clenching movement. Next suppose a person struggling for
breath : as his extraordinary muscles of respiration come into
play his fists are clenched; here impulses from the medulla
oblongata travel down the cord and throw the " clenching "
spinal centre into activity along with many other muscles,
and co-ordinating them all so as to give as good a pull as pos-
sible to ail muscles which can help an inspiration. In a
higher but still not volitional stage, more groups of muscles
are concerned, and centres of co-ordination in the pons and

626 THE HUMAN BODY.

cerebellum come into action also; take a man preparing for a
high jump: as he crouches and puts himself in bal/nce for
the spring he clenches his fists, quite unconsciously of course.
Here the immediate clenching centre is thrown into activity
along with the muscles of breathing, and of all parts of the
trunk and limbs. Each subsidiary peripheral centre plays its
part and the instreaming afferent impulses from the skin of the
feet, from the fibres of the muscular sense, from the semicir-
cular canals, from the eyes, are all concerned (without the
person's perception of them) in throwing the motor mechanisms
of midbrain, cerebellum, medulla, and cord into harmonious
activity, so that when the jump is actually willed it shall be
accomplished. But that in this case the volition plays a very
secondary part is obvious; it merely acts on an apparatus all
ready to discharge in a given way when a suitable additional
nerve impulse reaches it. A runner all tense for the start of
a hundred-yard race can hardly be said to start voluntarily
when he hears the signal; the case is comparable more to the
self-balancing of a pigeon deprived of its cerebral hemi-
spheres, when its perch is tilted. Next, suppose I clench my
fist "involuntarily," as we commonly say, when I see some-
thing that arouses my indignation; here clearly a mental
element is in play, but not a volitional one, and so far as
the movement is concerned probably the motor area of the
cortex has little or nothing to do with it: it is more in accord
with what is seen on animals to suppose that such simple
emotions and their characteristic movements may be carried
out by nerve apparatuses lying no higher than the thalami
and -corpora striata. If, however, I strike a man with the
intention to punish him, there can be little doubt that the
"clenching" centre is excited by fibres from the cortex and
passing down in the pyramidal tract. But this cortical area
may in turn be thrown into activity and may have its ten-
dency to discharge modified in many ways. My anger may
be the culminating result of many long past received and
interpreted and remembered sensations, and whether I shall
give the blow or restrain myself also be dependent on many
antecedents of experience. Again, I clench my hand to
knock doAvn a madman, as the only immediate method of
preventing him from committing a murder: here the same
motor cortical area no doubt would be thrown in action as
when the blow was struck in anger, but it is clear that the

THE PHYSIOLOGY OF THE BRAIN. 627

antecedent nerve processes arousing its activity would be
quite different in the two cases; and they would yet again be
different if I clenched the fist in order to explain to a child
the meaning of the word clench. We see then that the im-
mediate motor centres may be excited in various ways and in
various combinations quite apart from the cortex of the cere-
brum and by fibres not connected with the pyramidal tracts;
and that when excited from the cortical area of the cere-
brum through fibres of the pyramidal tract, that area itself
may be excited or controlled in its activity by a vast number
of other parts of the cortex, and by non-cortical parts of the
nervous system. The motor area cannot properly be spoken
of as the seat of volition : an act of willing is the final out-
come of changes in other and often numerous other regions
of the cortex, the resultant of whose material processes is a
discharge of efferent impulses from some region of the motor
area.

The permanent effects of local lesions of the Rolandic
region differ with the development of the brain. In dogs
removal of the left brain region connected with the fore
paw causes only temporary motor paralysis of the limb on
the other side; after a time the animal learns to walk again
as well as before: then removal of the corresponding area on
the right side of the brain is followed by paralysis of both fore
limbs. This has been supposed to show that the centre on
the right side had taken up the duty of control for both sides
after that on the left had been removed. However that may
be, the second paralysis is also only temporary, disappearing
in some weeks or months; and as has been already stated,
even after removal of all the motor area the animal occasion-
ally learns in the course of time to walk nearly as well as
ever. This must be due to lower centres (corpora striata ?),
and the question is whether the movements in such cases are
truly volitional, for definite acts of willing a movement prob-
ably play a very small part in a dog's life: most of its move-
ments are the immediate efferent expression of afferent im-
pulses and true volitions have but a small part in them. In
the lower monkeys definite motor effects of removal of part
of the cortical motor area are also temporary, but last longer
than in dogs; and in the anthropoid apes the same is the case
in a greater degree, and according to some experimenters
certain delicate combined movements are permanently lost

628 THE HUMAN BODY.

.after destruction of the motor area. These facts are correl-
.ated with the relatively larger size of the cortical motor area
.and of the pyramidal tracts in monkeys as compared with dogs,
and the anthropoid apes as compared with other monkeys.
The larger and more highly organized the brain area the
greater the part it plays in the life-work of the animal and
more noticeable are the results of its absence. In man local
paralysis due to local cortical lesion is often only temporary:
this may be due to disappearance of the disease; or to the
primary paralysis being only a " shock " effect, and not due to
actual disease of the motor centre, for it is well known that
in animals injury to one region of the brain will often for a
considerable time inhibit the activity of other parts: or it may
be due to the hemisphere of the opposite side assuming con-
trol. Different observers attribute very various values to these
three possible factors. \ In this connection reference may be
made to cases of what is called aphasia, which in its fully de-
veloped state is a loss of the power to apply words to express
ideas. The power of speech may, of course, be lost through
disease of the larynx or paralysis of the nerves or muscles of the
voice organs, but such a condition is not true aphasia: the
aphasic person can often articulate perfectly well, but he can-
not attach a meaning to his spoken word : in some cases he
can write words with meaning, though he cannot say them; in
other cases (agr aphid) the power of using written words to
express ideas is also lost, though the person can write, and his
general conduct shows that he is still guided by .his intelli-
gence; he knows quite well what he wants to say, but he can-
not set the proper motor apparatus in action to utter the
word : if he speaks, the word has no connection with that in
his mind, and as soon as he hears himself speaking it he often
knows that the word he uses is quite wrong. We find in such
cases the power to understand words, and to form ideas of
words, and to utter words, but some link between the origin
of the idea and the discharge of the motor impulses willed to
express it is out of gear. It is as if an injured reflex centre
should give a wrong or inco-ordinate efferent response to an
afferent impulse. Aphasia is almost invariably connected
with disease of the area marked SP in Fig. 179 and known
as the third or lower frontal convolution, and the pathological
change is on the left side of the brain only. The area, as will
be seen, is closely associated with the face area and the tongue

THE PHYSIOLOGY OF THE BRAIN. 629

and partly overlaps them, or rather is intermixed with them ; as
pointed out above, the lesion is not one of motor speech cen-
tres, but of the connection between these and other cerebral
areas in which have occurred changes accompanied by the
desire of verbal expression; something wrong probably in the
gray network. Very rarely aphasia has been known to follow
disease or injury of the corresponding convolution on the
right side; so that in it we have an example of a very definite
nexus between a limited area of the cortex and the expres-
sion of will through movements. Cases of recovery from
aphasia have occurred, but are extremely rare. In the ex-
ceptional cases it has been supposed that the right side of the
brain takes up the duty of connecting the material changes
in the gray network which accompany the origination of an
idea in one or more cortical areas, with the other changes
which result in speech. This view gains some support from
the fact that in certain cases of recovery due to left-side dis-
ease, subsequent disease in the third right frontal convolution
has been followed by a fresh aphasia. But however that may
be we have in aphasic persons definite evidence of the limita-
tion of definite function to a very limited area or areas of
the cerebral cortex.

Much less is known as to other regions of the cortex than
of the motor area: most of them do not respond to electrical
stimulation at all, and those areas that do, only show it by
movements lacking in precision. We are reduced, therefore,
to observation on animals from whom certain cortical parts
have been removed, and to observations on diseased persons.
Certain broad regions have in this way been mapped out as
connected with certain main groups of sensations (Figs. 179,
180), probably rather with the combining and interpreting
of sensations, with their ideation, than with the mere raw
sensation itself. The latter is probably more dependent on
the lower brain centres; in most cases it is secondary changes
in these which lead to impulses which are passed on to excite
the cortical sensory areas.

There is considerable evidence that removal or extensive
injury of the left occipital lobe causes blindness of the left
half of each retina, and vice versa. Also, that stimulation of
this region of the brain may cause movements of the eyes
and eyelids which have been described as such as an animal
would make if it thought it saw something, though obviously

630

THE HUMAN BODY.

that must be a very uncertain deduction. Also, the optic
tract of each side has through the anterior corpus quadri-
geminum and some other gray masses a close connection with
the cortex of the occipital lobe. Probably, therefore, that
region has some close 'connection with vision. There is also
some evidence that the angular gyrus (an, Fig. 179) has con-
nection with sight.

The sense of smell has been supposed especially connected
with the uncinate gyrus of median side of the temporal lobe
(un, Fig. 180), and the sense of taste with a neighboring

Ro

FIG. 180.— Diagram of inner surface of left cerebral hemisphere to illustrate
cerebral localization. Sq, fissure of Sylvius; Ro, fissure of Rolando; Fr, frontal
lobe; Of, occipital lobe: Te, temporal Jobe; C.cZ, corpus callosum; ZZ7, third ven-
tricle. Compare with Fig. 179.

area, but the evidence is unsatisfactory; and the same may be
said of the reasons which led to designation of the region of
the temporal lobe close behind the fissure of Sylvius with
hearing. The region marked on the diagram as that of
cutaneous sensations has also a doubtful claim : there is some
reason to believe that the motor area of the cortex has con-
nection with the muscular sense; also to some extent with
tactile feelings.

Tactile and temperature impulses cross the middle line
somewhere on their path from the skin to the brain. An
apoplectic effusion in one cerebral hemisphere causes loss of
sensation and of voluntary movement on the other side of
the Body.

The frontal lobes are quite irresponsive to excitation, and
considerable parts of them have been removed without ap-

THE PHYSIOLOGY OF THE BRAIN. 631

parent diminution of motor or sensory faculty. By a sort of
process of exclusion, the rest of the cortex being allotted
(though on unsatisfactory evidence) to motion and sensation
the frontal regions have been supposed to have special con-
nection with the higher intellectual faculties.

Mental Habits. Movements which are commonly exe-
cuted together tend to become so associated that it is difficult
to perform one alone; many persons, e.g., cannot close one
eye and keep the other open. From frequent use, the paths
of conduction between the co-ordinating centres for both
groups of muscles have become so easy that a volitional im-
pulse reaching one centre spreads to the other and excites
both. This association of movements, dependent on the
modification of brain structure by use, finds an interesting
parallel in the psychological phenomenon known as the asso-
ciation of ideas; and all education is largely based on the
fact that the more often brain regions have acted together
the more readily, until finally almost indissolubly, do they so
act. If we always train up the child to associate feelings of
disgust with wrong actions and of approbation with right,
when he is old he will find it very hard to do otherwise: such
an organic nexus will have been established that the activity
of the one set of centres will lead to an excitation of that
which habit has always associated with it. The higher nerve-
centres are throughout eminently plastic; it is that which
marks them out for a far higher utility and greater adaptation
to the varying experiences of individual life than the more
fixed and machine-like lower centres: every thought leaves
in them its trace for good or ill; and the moral truism that
the more often we yield to temptation — the more often an
evil solicitation, sensory or otherwise, has resulted in a wrong
act — the harder it is to resist the repetition of it, has its par-
allel (and we can hardly doubt its physical antecedent) in the
marking out of a path of easier conduction from perceptive
to volitional centres in the brain. The knowledge that every
weak yielding degrades our brain structure and leaves its trail
in that organ through which man is the " paragon of animals/'
while every resistance makes less close the bond between the
thought and the act for all future time, ought surely to
"give us pause :" on the other hand, every right action helps
to establish a " path of least resistance," and makes its sub-
sequent performance easier.

632 THE HUMAN BODY.

The brain, like the muscles, is improved and strengthened
by exercise and injured -by overwork or idleness; and just as
a man may specially develop one set of muscles and neglect
the rest until they degenerate, so he may do with his brain;
developing one set of intellectual faculties and leaving the
rest to lie fallow until, at last, he almost loses the power of
using them at all. The fierceness of the battle of life nowa-
days especially tends to produce such lopsided mental de-
velopments; how often does" one meet the business man, so
absorbed in money-getting that he has lost all power of ap-
preciating any but the lower sensual pleasures; the intel-
lectual joys of art, science, and literature have no charm for
him; he is a mere money-making machine. One, also, not
unfrequently meets the scientific man with no appreciation
of art or literature; and literary men utterly incapable of
sympathy with science. A good collegiate education in early
life, on a broad basis of mathematics, languages, and the
natural sciences, is a great security against such imperfect
mental growth; one danger in American life is the tendency
to put lads in a technical college, or to start them in business
before they have attained any broad general education. An-
other danger, no doubt, is the opposite one of making the
training too broad; a man who knows one or two literatures
fairly well, and who has mastered the elements of mathemat-
ics and of one of the observational or experimental sciences,
is likely to have a better and more utilizable brain than he
who has a smattering of half a dozen languages and a con-
fused idea of all the " ologies." The habits of mental sloven-
liness, the illogical thinking, and the incapacity to know
when a thing really is mastered and understood, which one so
often finds as the results of such an education, are far worse
than the narrowness apt to follow the opposite error, which
is often associated with the power of accurate logical thought.
Those who are deprived of the advantages of a general colle-
giate education may now, more easily than at any previous
period, cultivate mental breadth by reading some of the many
excellent general reviews and magazines, and the readable
but exact popular expositions now available on nearly all
subjects, which are such a feature of our age. Associating,
out of working hours, with those whose special pursuits are
different from our own is almost necessary to those who
would avoid such an asymmetrical development as almost
amounts to intellectual deformity.

CHAPTER XXXVIII.
VOICE AND SPEECH.

Voice consists of sounds produced by the vibrations of
two elastic bands, the true vocal cords, placed in the larynx,
an upper modified portion of the passage which leads from the
pharynx to the lungs. When the vocal cords are put in a cer-
tain position, air driven past them sets them in periodic vibra-
tion, and they emit a musical note; the lungs and respiratory
muscles are, therefore, accessory parts of the vocal apparatus :
the strength of the blast produced by them determines the
loudness of the voice. The larynx itself is the essential voice-
organ: its size primarily determines the pitch of the voice,
which is lower the longer the vocal cords; and, hence, shrill
in children, and usually higher pitched in women than in
men: the male larynx grows rapidly at commencing man-
hood, causing the change commonly known as the " breaking
of the voice." Every voice, while its general pitch is de-
pendent on the length of the vocal cords, has, however, a
certain range, within limits which determine whether it shall
be soprano, mezzo-soprano, alto, tenor, baritone, or bass.
This variety is produced by muscles within the larynx which
alter the tension of the vocal cords. Those characters of
voice which we express by such phrases as harsh, sweet, or
sympathetic, depend on the structure of the vocal cords of
the individual; cords which in vibrating emit only harmonic
partial tones (Chap. XXXV) are pleasant; while those in
which inharmonic partials are conspicuous are disagreeable.

The vocal cords alone would produce but feeble sounds;
those that they emit are strengthened by sympathetic reso-
nance of the air in the pharynx and mouth, the action of
which may be compared to that of the sounding-board of a
violin. By movements of throat, soft palate, tongue, cheeks,
and lips the sounds emitted from the larynx are altered or

634 THE HUMAN BODY.

supplemented in various ways, and converted into articulate
language or speech.

The Larynx lies in front of the neck, beneath the hyoid
bone and above the windpipe; in many persons it is promi-
nent, causing the projection known as "Adam's apple." It
consists of a framework of cartilages, partly joined by true
synovial joints and partly bound together by membranes;

•Ci

It^-^BP^**

FIG. 181.— The more important cartilages of the larynx from behind, t, thy-
roid; Cs, its superior, and Ci. its inferior, horn of the right side; #*, cricoid carti-
lage; t, arytenoid cartilage; Pv, the corner to which the posterior end of a vocal
cord is attached; Pm, corner on which the muscles which approximate or sepa-
rate the vocal cords are inserted; co, cartilage of Santorini.

muscles are added which move the cartilages with reference
to one another; and the whole is lined by a mucous mem-
brane.

The cartilages of the larynx (Fig. 181) are nine in. num-
ber; three single and median, and three pairs. The largest
(t) is called the thyroid, and consists of two halves which
meet at an angle in front, but separate behind so as to inclose
a V-shaped space, in which most of the remaining cartilages
lie. The epiglottis (not represented in the figure) is fixed
to the top of the thyroid cartilage and overhangs the entry
from the pharynx to the larynx; it may be seen, covered
by mucous membrane, projecting at the base of the tongue,
if the latter be pushed down while the mouth is held open in
front of a mirror; and is, similarly covered, represented, as
seen from behind, at a in Fig. 182. The cricoid, the last

VOICE AND SPEECH. 635

of the unpaired cartilages, lias the shape of a signet-ring; its
broad part („,„,, Fig. 181) is on the posterior side and lies at
the lower part of the opening between the halves of the
thyroid; in front and on the sides it is narrow, and a space,
occupied by the crico-tJiyroid membrane, intervenes between
its upper border and the lower edge of the thyroid cartilage.
The angles of the latter are produced above and below into
projecting horns (Cs and Ci, Fig. 181), and the lower horn
on each side forms a joint with the cricoid. The thyroid can
be rotated on an axis, passing through the joints on each
side, and rolled down so that its lower front edge shall come
nearer the cricoid cartilage, the membrane there intervening
being folded. The arytenoids (f,Fig. 181) are the largest of
the paired cartilages; they are seated on the upper edge of
the posterior wide portion of the cricoid, and form true
joints with it. Each is pyramidal with a triangular base,
and has on its tip a small nodule (co, Fig. 181), the cartilage
of Santorini. From the tip of each arytenoid cartilage the
aryteno-epiglottidean fold of mucous membrane (10, Fig. 182)
extends to the epiglottis; the cartilage of Santorini causes a
projection (8, Fig. 182) in this; and a little farther on (9)
is a similar eminence on each side, caused by the remaining
pair of cartilages, known as the cuneiform, or cartilages of
Wrisberg.

The Vocal Cords are bands of elastic tissue which reach
from the inner angle (Pv, Fig. 181) of the base of each aryte-
noid cartilage to the angle on the inside of the thyroid where
the sides of the V unite; they thus meet in front but are
separated at their other ends. The cords are not, however,
bare strings, like those of a harp, but covered over with the
lining mucous membrane of the larynx, a slit, called the
glottis (c, Fig. 182), being left between them. It is the pro-
jecting cushions formed by them on each side of this slit
which, are set in vibration during phonation. Above each
vocal cord is a depression, the ventricle of the larynx (bf,
Fig. 182); this is bounded above by a somewhat prominent
edge, the false vocal cord. Over most of the interior of the
larynx its mucous membrane is thick and covered by ciliated
epithelium, and has many mucous glands imbedded in it.
Over the vocal cords, however, it is represented only by a
thin layer of flat non-ciliated cells, and contains no glands.
In quiet breathing, and after death, the free inner edges of the

636

THE HUMAN BODY.

vocal cords are thick and rounded, and seem very unsuitable
for being readily set in vibration. They are also tolerably
widely separated behind, the arytenoid cartilages, to which
their posterior ends are attached, being separated. Air under

11

11

FIG. 182.— The larynx viewed from its pharyngeal opening. The back wall of
the pharynx has been divided and its edges (11) turned aside. 1, body of hyoid;
2, its small, and 3, its great, horns; 4, upper and lower horns of thyroid cartilage;
5, mucous membrane of front of pharynx, covering the back of the cricoid carti-
lage; 6. upper end of gullet; 7, windpipe, lying in front of the gullet; 8, eminence
caused by cartilage of Santorini; 9, eminence caused by cartilage of Wrisberg;
both lie in, 10, the aryteno-epiglottidean. fold of mucous membrane, surrounding
the opening (aditus larynyis) from pharynx to larynx, a, projecting tip of epi-
glottis; c, the glottis, the lines leading from the letter point to the free vibratory
edges of the vocal cords. 6', the ventricles of the larynx: their upper edges, mark-
ing them off from the eminences fe, are the false vocal cords.

these conditions passes through without producing voice. If
they are watched with the laryngoscope during phonation, it
is seen that the cords approximate behind so as to narrow the
glottis; at the same time they become more tense, and their
inner edges project more sharply and form a better-defined
margin to the glottis, and their vibrations can be seen.
These changes are brought about by the delicately co-

VOICE AND SPEECB. 637

ordinated activity of a number of small muscles, which move
the cartilages to which the cords are fixed.

The Muscles of the Larynx. In describing the direc-
tion and action of these it is convenient to use the words
front or anterior and back or posterior with reference to the
larynx itself (that is, as equivalant to ventral and dorsal) and
not with reference to the head, as usual. The base of each
arytenoid cartilage is triangular and fits on a surface of the
cricoid, on which it can slip to and fro to some extent, the
ligaments of the joint being lax. One corner of the tri-
angular base is directed inwards and forwards (i.e. towards
the thyroid) and is called the vocal process (Pv, Fig. 181), as
to it the vocal cords are fixed. The outer posterior angle
(Pm, Fig, 181) has several muscles inserted on it and is
called the muscular process. If it be pulled back and
towards the middle line the arytenoid cartilage will rotate on
its vertical axis, and roll its vocal processes forwards and out-
wards, and so widen the glottis; the reverse will happen if
the muscular process be drawn forwards. The muscle pro-
ducing the former movement is the posterior crico-arytenoid
(Cap, Fig. 183); it arises from the back of the cricoid carti-
lage, and narrows to its insertion into the muscular process
of the arytenoid on the same side. The opponent of this
muscle is the lateral crico-arytenoid, which arises from the
side of the cricoid cartilage, on its inner surface, and passes
upwards and backwards to the muscular process. The pos-
terior crico-arytenoid s, working alone, pull inwards and down-
wards the muscular processes, turn upwards and outwards
the vocal processes, and separate the posterior ends of the
vocal cords. The lateral crico-thyroid, working alone, pulls
downwards and forwards the muscular process, and rotates
inwards and upwards the vocal process, and narrows the
glottis; it is the chief agent in producing the approximation
of the cords necessary for the production of voice. When
both pairs of muscles act together, however, each neutralizes
the tendency of the other to rotate the arytenoid cartilage;
the downward part of the pull of each is, thus, alone left, and
this causes the arytenoid to slip downwards and outwards, off
the eminence on the cricoid with which it articulates, as far
as the loose capsular ligament of the joint will allow. The
arytenoid cartilages are thus moved apart and the glottis
greatly widened and brought^ into its state in deep quiet

638

THE HUMAN BODY.

breathing. Other muscles approximate the arytenoid carti-
lages after the cartilages have been separated. The most im-
portant is the transverse arytenoid (A, Fig. 183), which runs
across from one arytenoid cartilage to the other. Another is
the oblique arytenoid (Taep), which runs across the middle
line from the base of one arytenoid to the tip of the other;

FIG. 183.— The larynx seen from behind and dissected so as to display some of
its muscles. The mucous membrane of the front of the pharynx (5, Fig. 157) has
been dissected away, so as to display the laryngeal muscles beneath it. Part of
the left half of tne thyroid cartilage has been cut away, co, cartilage of San-
tor iui; c«, cartilage of Wrisberg.

thence certain fibres continue in the aryteno-epiglottidean
fold (10, Fig. 182) to the base of the epiglottis; this, with its
fellow, embraces the whole entry to the larynx; when they
contract they bend inwards the tips of the arytenoid car-
tilages, approximate the edges of the aryteno-epiglottidean
fold, and draw down the epiglottis, and so close the passage
from the pharynx to the larynx. When the epiglottis has
been removed, food and drink rarely enter the larynx in
swallowing, the folds of mucous membrane being so brought
together as to effectually close the aperture between them.

Increased tension of the vocal cords is produced by the
crico-tliyroid muscles, one of which lies on each side of the
larynx, over the crico- thyroid membrane. Their action may

VOICE AND SPEECH.

be understood by help of the diagram, Fig. 184, in which t
represents the thyroid cartilage, c
the cricoid, a an arytenoid, and vc
a vocal cord. The muscle passes
obliquely backwards and upwards

from about d near the front end of \a } vc

c, to t, about /, near the pivot (which
represents the joint between the
cricoid cartilage and the inferior
horn of the thyroid). When the
muscle contracts it pulls together
the anterior ends of t and c; either
by depressing the thyroid (as rep-
resented by the dotted lines) or by raising the front end of
the cricoid; and thus stretches the vocal cord, if the ary-
tenoid cartilages be held from slipping forwards. The an-
tagonist of the crico-thyroid is the tltyro-arytenoid muscle;
it lies, on each side, imbedded in the fold of elastic tissue
forming the vocal cord, and passes from the inside of the
angle of the thyroid cartilage in front, to the anterior angle
and front surface of the arytenoid behind. If the latter be
held firm, the muscle raises the thyroid cartilage from the
position into which the crico-thyroid pulls it down, and so
slackens the vocal cords. If the thyroid be held fixed by the
crico-thyroid muscle, the thyro-arytenoid will help to approxi-
mate the vocal cords, rotating inwards the vocal processes of
the arytenoids.

The lengthening of the vocal cords when the thyroid
cartilage is depressed tends to lower their pitch; the in-
creased tension, however, more than compensates for this
and raises it. There seems, however, still another method
by which high notes are produced. Beginning at the bot-
tom of his register, a singer can go on up the scale some dis-
tance without a break; but, then, to reach his higher notes,
must pause, rearrange his larynx, and begin again. What
happens is that, at first, the^vocal processes are turned in, so
as to approximate but not to meet; the whole length of each
edge of the glottis then vibrates, and its tension is increased,
and the pitch of the note raised, by increasing contraction of
the crico-thyroid. At last this attains its limit and a new
method has to be adopted. The vocal processes are more
rolled in, until they touch. This produces a node (see

640 THE HUMAN BODY.

Physics) at that point and shortens the length of vocal cord
which vibrates. The shorter string emits a higher note; so
the crico- thyroid is relaxed, and then again gradually tight-
ened as the notes sung are raised in pitch from the new
starting-point. To pass easily and imperceptibly from one
such arrangement of the larynx to another is a great art in
singing. There is some reason to believe that a second node
may, for still higher notes, be produced at a more anterior
point on the vocal cords.

The method of production of falsetto notes is uncertain;
during their emission the free border of the vocal cords
alone vibrates.

The range of the human voice is about three octaves,
from e (80 vib. per 1") on the unaccented octave, in male
voices, to c on the thrice-accented octave (1024 vib. per 1"),
in female. Great singers of course go beyond this range;
basses have been known to take a on the great octave (55
yib. per 1") ; and Nilsson in " II Flauto Magico " used to take
/on the fourth accented octave (1408 vib. per 1"). Mozart
heard at Parma, in 1770, an Italian songstress whose voice
had the extraordinary range from g in the first accented
octave (198 vib. per 1") to c on the fifth accented octave
(2112 vib. per 1"). An ordinary good bass voice has a com-
pass from /(88 vib. per 1") to d" (297 vib. per 1"); and a
soprano from bf (248 vib. per 1") to g"' (792).

Vowels are, primarily, compound musical tones produced
in the larvnx. Accompanying the primary partial of each,
which determines its pitch when said or sung, are a number
of upper partials, the first five or six being recognizable in
good full voices. Certain of these upper partials are rein-
forced in the mouth to produce one vowel, and others for
other vowels; so that the various vowel sounds are really
musical notes differing from one another in timbre. The
mouth and throat cavities form an air-chamber above the
larynx, and this has a note of its own which varies with its
size and form, as may be observed by opening the mouth !
widely, with the lips retracted and the cheeks tense; then I
gradually closing it and protruding the lips, meanwhile tap- \
ping the cheek. As the mouth changes its form the note
produced changes, tending in general to pass from a higher
to a lower pitch and suggesting to the ear at the same time a ;
change from the sound of a (father) through 6 (more) to 06 j

VOICE AND SPEECH. 641

(moor). When the mouth and throat chambers are so ar-
ranged that the air in them has a vibratory rate in unison
with any partial in the laryngeal tone, it will be set in sym-
pathetic vibration, that partial will be strengthened, and the
vowel characterized by it uttered. As the mouth alters its
form, although the same note be still sung, the vowel changes.
In the above series (a, 6, 06) the tongue is depressed and the
cavity forms one chamber; for a this has a wide mouth open-
ing; for 6 it is narrowed; for 56 still more narrowed, and the
lips protruded so as to increase the length of the resonance
chamber. The partial tones reinforced in each case are, ac-
cording to Helmholtz —

In other cases the mouth and throat cavity is partially sub-
divided, by elevating the tongue, into a wide posterior and a
narrow anterior part, each of which has its own note; and
the vowels thus produced owe their character to two rein-
forced partials. This is the case with the series a (man), e
(there), and i (machine), the tones reinforced by resonance
in the mouth being —

The usual I of English, as in spire, is not a true simple
vowel but a diphthong, consisting of a (pad) followed by e
(feet), as may be observed by trying to sing a sustained note
to the sound T; it will then be seen that it begins as a and
ends as ee. A simple vowel can be maintained pure as long
as the breath holds out.

In uttering true vowel sounds the soft palate is raised so
as to cut off the air in the nose, which, thus, does not take
part in the sympathetic resonance. For some other sounds
(the semi-vowels or resonants) the initial step is, as in the
case of the true vowels, the production of a laryngeal tone;

642 THE HUMAN BODY.

but the soft palate is not raised, and the mouth exit is more
or less closed by the lips or the tongue; hence the blast partly
issues through the nose, and the air there takes part in the
vibrations and gives them a special character; this is the case
with D), n, and ng.

Consonants are sounds produced not mainly by the vocal
cords, but by modifications of the expiratory blast on its way
through the mouth. The current may be interrupted and
the sound changed by the lips (labials); or, at or near the
teeth, by the tip of the tongue (dentals); or, in the throat, by
the root of the tongue and the soft palate (gutturals). Con-
sonants are also characterized by the kind of movement
which gives rise to them. In explosives an interruption to
the passage of the air-current is suddenly interposed or re-
moved (P, T, B, D, K, G). Other consonants are continuous
(as F, S, R), and may be subdivided into — (1) aspirates, char-
acterized by the sound produced by a rush of air through a
narrow passage, as when the lips are approximated (F), or the
.teeth (S), or the tongue is brought near the palate (Sh), or its
tip against the two rows of teeth, they not being quite in
contact (Th). For L the tongue is put against the hard
palate and the air escapes on its sides. For Ch (as. in the
proper Scotch pronunciation of loch) the passage between the
back of the tongue and the soft palate is narrowed. To
many of the above pure consonants answer others, in whose
production true vocalization (i.e. a laryngeal tone) takes a
part. F with some voice becomes V; S becomes Z, Th soft
(tee^) becomes Th hard; and Ch becomes Gh. (2) reso-
nant s; these have been referred to above. (3) vibratories
(the different forms of R). which are due to vibrations of
parts bounding a constriction put in the course of the air-
current. Ordinary R is due to vibrations of the tip of the
tongue held near the hard palate; and guttural R to vibra-
tions of the uvula and parts of the pharynx.

The consonants may physiologically be classified as in the
following table (Foster) :

Explosives. Labials, without voice P.

" with voice B.

Dentals, without voice T.

" with voice D.

Gutturals, without voice . . . K.

" with voice G (hard).

VOICE AND SPEECH. 643

Aspirates. Labials, without voice F.

" with voice V.

Dentals, without voice S, L, Sh, Th (hard).

" with voice Z, Zh (azure), Th (soft).

Gutturals, without voice. . .Ch (loch).

" with voice Ch.

Resonants. Labial M.

Dental N.

Guttural NG-.

Vibratories. Labial — not used in European languages.

Dental .R (common).

Guttural R (guttural).

H is a laryngeal sound : the vocal cords are separated for
its production, yet not so far as in quiet breathing. The air-
current then produces a friction sound but not a true note,
as it passes the glottis; and this is again modified when the
current strikes the wall of the pharynx. Simple sudden
closure of the glottis, attended with no sound, is also a
speech element, though we do not indicate it with a special
letter, since it is always understood when a word begins with
a vowel, and only rarely is used at other times. The Greeks
had a special sign for it, ', the soft breathing; and another,
\ the hard breathing, answering somewhat to our h and indi-
cating'that the larynx was to be held open, so as to give a
friction sound, but not voice.

In whispering there is no true voice; the latter implies
true tones, and these are only produced by periodic vibra-
tions; whispering is a noise. To produce it the glottis is
considerably narrowed but the cords are not so stretched as to
produce a sharply defined edge on them, and the air driven
past is then thrown into irregular vibrations. Such vibra-
tions as coincide in period with the air in the mouth and
throat are always present in sufficient number to characterize
the vowels; and the consonants are produced in the ordinary
way, though the distinction between such letters as P and B,
F and V, remains imperfect.

CHAPTER XXXIX.
REPRODUCTION.

Reproduction in General. In all cases reproduction
consists, essentially, in the separation of a portion of living
matter from a parent; the separated part bearing with it, or
inheriting, certain tendencies to repeat, with more or less
variation, the life history of its progenitor. In the more
simple cases a parent merely divides into two or more pieces,
each resembling itself except in size; these then grow and
repeat the process; as, for instance, in the case of Amoeba and
our own white blood corpuscles (pp. 23, 44). Such a process
may be summed up in two words as discontinuous growth;
the mass, instead of increasing in size without segmentation,
divides as it grows, and so forms independent living beings.
In some tolerably complex multicellular animals we find
essentially the same thing; at times certain cells of the fresh-
water Polype multiply by simple division in the manner
above described, but there is a certain concert between them:
they build up a tube projecting from the side of the parent,. a
mouth-opening forms at the distal end of this, tentacles
sprout out around it, and only when thus completely built up
and equipped is the young Hydra set loose on its own career.
How closely such a mode of multiplication is allied to mere
growth is shown by other polypes in which the young, thus
formed, remain permanently attached to the parent stem, so
that a compound animal results. This mode of reproduction
(known as gemmation or budding) may be compared to the
method in which many of the ancient Greek colonies were
founded; carefully organized and prepared at home, they
were sent out with a due proportion of artificers of various
kinds; so that the new commonwealth had from its first sep-
aration a considerable division of employments in it, and was,
on a small scale, a repetition of the parent community. In
the great majority of animals, however (even those which at
times multiply by budding),'a different mode of reproduction

644

REPRODUCTION. 645

occurs, one more like that by which our western lands were
settled and gradually built up into Territories and States.
The new individual in the political world began with little
differentiation; it consisted of units, separated from older and
highly organized societies, and these units at first did pretty
much everything, each man for himself, with more or less
efficiency. As growth took place development also occurred;
persons assumed different duties and performed different
work until, finally, a fully organized State was formed.
Similarly, the body of one of the higher animals is, at an
early stage of life, merely a collection of undiiferentiated
cells, each capable of multiplication by division, and more or
less retaining all its original protoplasmic properties; and
with no specific individual endowment or function. The
mass (Chap. III.) then slowly differentiates into the various
tissues, each with a predominant character and duty; at the
same time the majority of the cells lose their primitive powers
of reproduction, though exactly how completely is a problem
not yet sufficiently studied. In adult Vertebrates it seems
certain that the white blood corpuscles multiply by division:
and in some cases (in the newts or tritons, for example) a
limb is reproduced after amputation. But exactly what cells
take part in such restorative processes is uncertain; we do
not know if the^ old bone corpuscles left form new bones, old
muscle-fibres new muscles, and so on; though it is probable
that the little-differentiated leucocytes build up most of the
new limb. In Mammals no such restoration occurs; an am-
putated leg may heal at the stump but does not form again.
In the healing processes the connective tissues play the main
part, as we might expect; their cellular elements being but
little modified from their primitive state (p. 102) can still
multiply and develop. New blood capillaries, however, sprout
out from the sides of old, and new epidermis seems only to be
formed by the multiplication of epidermic cells; hence the
practice, frequently adopted by surgeons, of transplanting
little bits of skin to points on the surface of an extensive
burn or ulcer. In blood capillaries and epidermis the de-
parture from the primary undifferentiated cell is but slight;
and, as regards the cuticle, one of the permanent physiologi-
cal characters of the cells of the rete mucosum is their multi-
plication throughout the whole of life; that is a main physio-
logical characteristic of the tissue: the same is very probably

646 THE HUMAN BODY.

true of the protoplasmic cells forming the walls of the capil-
laries. When a highly differentiated tissue is replaced in
the body of mammals after breaking down or removal, it is
usually by the activity of special cells set apart for that pur-
pose, or by repair or outgrowth of the cells affected and not
by their division. The red blood corpuscles are constantly
being broken down and replaced, but the new ones are not
formed by the division of already fully formed corpuscles but
by certain special licematoblastic cells retained throughout
life in the red marrow of bone and perhaps in the spleen.
The nervous tissues are highly differentiated and a nerve is
often regenerated after division, but this is by outgrowth of
the ends of axis cylinders still attached to their cells and by
secondary formation of a medullary sheath around these, and
not by division or multiplication of already existing fibres.
A striped muscle when cut across is healed by the formation
of a band of connective tissue; after a very long time it is
said that true muscular fibres may be found in the cicatrix,
but their origin is not known; it is probably not from pre-
viously developed muscle fibres. On the other hand, the less
differentiated unstriated muscle has been observed to be re-
paired in some cases after injury by true karyokinetic division
of previously formed muscle cells. Although many gland-
cells in the performance of their physiological work are par-
tially broken down and lost in their secretion, and then
repaired by the 'residue of the cell, multiplication by division
of fully differentiated gland-cells does not appear to occur, if
we except such organs as the testes, the secretion of which
consists essentially of cells. An excised portion of a salivary
or parotid gland is never regenerated : the wound is repaired
by connective tissues.

We find, then, as we ascend in the animal scale a diminish-
ing reproductive power in the tissues generally: with the
increasing division of physiological labor, with the changes
that fit pre-eminently for one work, there is a loss of other
faculties, and this one among them. The more specialized a
tissue the less the reproductive power of its elements, and the
most differentiated tissues are either not reproduced at all after
injury, or only by the specialization of amoeboid cells, and not
by a progenitive activity of survivors of the same kind as those
destroyed. In none of the higher animals, therefore, do we
find multiplication by simple division, or by budding : no one

REPRODUCTION. 647

cell, and no group of cells used for the physiological mainte-
nance, of the individual, can build up a new complete living
being; but the continuance of the race is specially provided
for by setting apart certain cells which shall have this one
property — cells whose duty is to the species and not to any one
representative of it — an essentially altruistic element in the
otherwise egoistic whole.

Sexual Reproduction. In some cases, especially among
insects, the specialized reproductive cells can develop, each for
itself, under suitable conditions, and give rise to new indi-
viduals; such a mode of reproduction is called parthenogenesis:
but in the majority of cases, and always in the higher animals,
this is not so; the fusion of two cells, or of products of two
cells, is a necessary preliminary to development. Commonly
the coalescing cells differ considerably in size and form, and
one takes a more direct share in the developmental processes;
this is the egg-cell or ovum; the other is the sperm-cell or
spermatozoon. The fusion of the two is known as fertiliza-
tion. Animals producing both ova and spermatozoa are
hermaphrodite; those bearing ova only, female; and those
spermatozoa only, male : hermaphrodiBsm is not found in
Vertebrates, except in rare and doubtful cases of monstrosity.

Accessory Reproductive Organs. The organ in which
ova are produced is known as the ovary, that forming sperma-
tozoa, as the testis or testicle; but in different groups of animals
many additional accessory parts may be developed. Thus,
in all but the very lowest Mammalia, the offspring is nourished
for a considerable portion of its early life within the body of
its mother, a special cavity, the uterus or womb, being pro-
vided for this purpose: the womb communicates with the
exterior by a passage, the vagina; and two tubes, the oviducts
or Fallopian tubes, convey the eggs to it from the ovaries.
In addition, mammary glands provide milk for the nourish-
ment of the young in the first months after birth. In the
male mammal we find as accessory reproductive organs, vasa
defer entia which convey from the testes the seminal fluid con-
taining spermatozoa; vesiculce seminales (not present in all
Mammalia) , glands whose secretion is mixed with that of the
testes or is expelled after it in the sexual act ; a prostate gland,
whose secretion is added to the semen; and an erectile organ,
the penis, by which the fertilizing liquid is conveyed into the
vagina of the female.

648

THE HUMAN BODY.

The Male Reproductive Organs. The testes in man are
paired tubular glands, which lie in a pouch of skin called the
scrotum. This pouch is subdivided internally by a partition
into right and left chambers, in each of which a testicle lies.
The chambers are lined inside by a serous membrane, the
tunica vaginalis, and this doubles back (like the pleura round
the lung) and covers the exterior of the gland. Between the
external and reflected layers of the tunica vaginalis is a space
containing a small quantity of lymph.

The testicles develop in the abdominal cavity, and only
later (though commonly before birth) descend into the scrotum,
passing through apertures in the muscles, etc. , of the abdom-
inal wall, and then sliding down over the front of the pubes,
beneath the skin. 'The cavity of the tunica vaginalis at first
is a mere offshoot of the peritoneal cavity, and its serous mem-
brane is originally a part of the peritoneum. In the early years
of life the passage along which the testis passes usually becomefe
nearly closed up, and the communication between the peri-
toneal cavity and that of the tunica vaginalis is also obliterated.
Traces of this passage can, however, readily be observed in
male infants; if the skin inside the thigh be tickled a muscle
lying beneath the skin of the scrotum is
made to contract reflexly, and the testis
is jerked up some way towards the
abdomen and quite out of the scrotum.
Sometimes \he passage remains per-
manently opeii and a cpil of intestine
may descend along it and enter the
scrotum, constituting an inguinal
hernia or rupture. A hydrocele is an
excessive accumulation of liquid in the
serous cavity of the tunica vaginalis.

Beneath its covering of serous mem-
brane each testis has a proper fibrous
tunic of its own. This forms a thick
mass on the posterior side of the gland,
from which partitions or septa (i, Fig.
185) radiate, subdividing the gland
into many chambers. In each chamber
lie several greatly coiled seminiferous tubules, a, a, averaging in
length 0.68 metre (27 inches) and in diameter only 0.14 mm.
(T^_ inch) . Their total number in each gh nd is about 800.

FIG. 185.— Diagram of a
Vertical section through the
testis. a, a, tubuli semini-
feri; b, vasa recta: d, vasa
efferentia endii^g in the
coni vasculosi; e, e, epidi-
dyiriis. &, vas deferens.

EEPROD UCTION. 649

Near the posterior side of the testis the tubules unite to form
about 20 vasa recta (#), and these pass out of the gland at its
upper end, as the vasa efferentia (d), which become coiled up
into conical masses, the coni vasculosi; these, when unrolled,
are tubes from 15 to 20 cm. (6-8 inches) in length; they taper
somewhat from their commencements at the vasa efferentia,
where they are 0.5 mm. (-£$ inch) in diameter, to the other
end where they terminate in the epididymis (e, e, Fig. 185).
The latter is a narrow mass, slightly longer than the testicle,
which lies along the posterior side of that organ, near the lower
end of which it passes (g) into the vas defer ens , h. If the
epididymis be carefully unravelled it is found to consist of a
tube about 6 metres (20 feet) in length, and varying in diam-
eter from 0.35 to 0.25 mm. (T^ to -fa inch).

The vas deferens (A, Fig. 185) commences at the lower
part of the epididymis as a coiled tube, but it soon ceases to be
convoluted and passes up beneath the skin covering the inner
part of the groin, till it gets above the pelvis and then, passing
through the abdominal walls, turns inwards, backwards, and
downwards, to the under side of the urinary bladder, where it
joins the duct of the seminal vesicle; it is about 0.6 meters
(2 feet) in length and 2.5 mm. (fa inch) in diameter. Its
lining epithelium is ciliated.

The vesiculcB seminales, two in number, are membranous
receptacles which lie, one on each side, beneath the bladder,
between it and the rectum. They are commonly about 5 cm.
(3 inches) long and a little more than a centimetre wide (or
about 0.5 inch) at their broadest part. The narrowed end of
each enters the vas deferens on its own- side, the tube formed
by the union being the ejaculatory duct, which, after a course
of about an inch, enters the urethra near the neck of the
bladder. In some animals the vesiculce seminales form a liquid
Avhieh IP added to the secretion of the testis. In man they
appear to be merely reservoirs in which the semen collects.

The prostate gland is a dense body, about the size of a
large chestnut, which surrounds the commencement of the
urethra; the ejaculatory ducts pass through it. It is largely
made up of fibrous and unstriped muscular tissues, but con-
tains also a number of small secreting saccules whose ducts
open into the urethra. The prostatic secretion though small
in amount would appear to be of importance: at least the
gland remains undeveloped in persons who have been castrated

650 THE HUMAN BODY.

in childhood ; and atrophies after removal of the testicles later
in life.

The male urethra leads from the bladder to the end of the
penis, where it terminates in an opening, tlie meatus urinarius.
It is described by anatomists as made up of three portions,
the prostatic, the membranous, and the spongy. The first is
surrounded by the prostate gland and receives the ejaculatory
ducts. On its posterior wall, close to the bladder, is an eleva-
tion containing erectile tissues (see below) and supposed to be
dilated during sexual congress, so as to cut off the passage to
the urinary receptacle. On this crest is an opening leading
into a small recess, the utricle, which is of interest, since the
study of its embryology shows it to be an undeveloped male
uterus. The succeeding membranous portion of the urethra
is about 1.8 cm. (f inch) long; the spongy portion lies in the
penis.

The penis is composed mainly of erectile tissue, i.e., tissues
so arranged as to inclose cavities which can be distended by
blood. Covered outside by the skin, internally it is made up
of three elongated cylindrical masses, two of which, the corpora
cavernosa, lie on its anterior side; the third, the corpus spongi-
osum, surrounds the urethra and lies on the posterior side of
the organ for most of its length; it, however, alone forms
the terminal dilatation, or glans, of the penis. Each corpus
cavernosiun is closely united to its fellow in the middle line
and extends from the pubic bones, to which it is attached
behind, to the glans penis in front. It is enveloped in a dense
connective-tissue capsule from which numerous bars, contain-
ing white fibrous, elastic, and unstriped muscular tissues,
radiate and intersect in all directions, dividing its interior into
many irregular chambers called venous sinuses. Into these
blood is conveyed partly through open capillaries, partly
directly by the open ends of small arteries ; this blood is car-
ried off by veins proceeding from the sinuses.

The arteries of the penis are supplied with vaso-dilator
nerves, the nervi erigentes, derived from the sacral plexus.
Under certain conditions these are stimulated and, the
arteries expanding, blood is poured into the venous sinuses
faster than the veins drain it off ; the latter are probably also
at the same time compressed where they leave the penis by the
contraction of certain muscles passing over them. Simul-
taneously the involuntary muscular tissue of the bars ran if y-

REPRODUCTION. 651

ing through the erectile masses relaxes. As a result the whole
organ becomes distended and finally rigid and erect. The
co-ordinating centre of erection lies in the lumbar region of the
spinal cord, and may be excited reflexly by mechanical stimu-
lation of the penis, or under the influence of nervous impulses
originating in the brain and associated with sexual emotions.
The corpus spongiosum resembles the corpora cavernosa in
essential structure and function.

The skin of the penis is thin and forms a simple layer for
some distance; towards the end of the organ it separates and
forms a fold, the foreskin or prepuce, which doubles back,
and, becoming soft, moist, red, and very vascular, covers the
glands to the meatus urinarius, where it becomes continuous
with the mucous membrane of the urethra; in it, near the
projecting posterior rim of the glans, are imbedded many
sebaceous glands. It possesses nerve end organs (genital
corpuscles) which much resemble end bulbs in structure.

The Seminal Fluid. The essential elements of the tes-
ticular secretion are much modified cells, the spermatozoa,
which are passed out with some albuminous liquid. The
spermatozoa (Fig. 186) are motile bodies about
0.04 m.m. (-g-J^ inch) in length. They have a
flattened clear body or head and a long vibratile
tail or cilium ; the portion of the tail nearest to
the head is thicker than the rest, and is known
as the neck. The mode of development of a
spermatozoon shows that the head is a cell-nu-
cleus and the neck and tail a modified cell-

hnrlv 'n s'tle view, a,

DC1y- head; 6, neck; c.

On cross-section a seminiferous tubule pre- tail-
sents externally a well-marked basement membrane, upon
which are borne several layers of cells; the lumen or bore of
the tubule is in great part occupied by the tails of sper-
matozoa projecting from some of the lining cells. The outer
cells, those next the basement membrane, are arranged in a
single layer, and are usually found in one or other stage of
active karyokinetic division (p. 19). The result of the divi-
sion is an outer cell, which remains next the basement mem-
brane to repeat the process, and an inner, which is the mother -
cell of spermatozoa. The latter cell by repeated mitotic divi-
sion give rise to a number of cells lying side by side and each
having a relatively large nucleus and small cell-body. These

** 3 -T .<

652 THE HUMAN BODY.

cells elongate, the nucleus remaining near the deeper end and
the protoplasm extending towards the lumen of the tubule,
into which it ultimately projects. Such cells are sperma-
toUasts, and lie in bunches side by side and several rows
deep. Interlaced among them are other granular supporting
cells of the epithelium, which are probably concerned with
the nutrition of the essential cells. The final step by which
the spermatoblast is converted into a spermatozoon is a kary-
okinetic division into two unequal cells: a part of the nu-
cleus with a little of the protoplasm separates and appears to
have no further function; the remaining part of the nucleus
(male pronucleus] remains as the head of the spermatozoon
and the cell protoplasm develops into the neck and tail.
The spermatozoa appear frequently to be cast off before
their development is completed : at least many spermato-
blasts which have not gone through the final stages are
found in the vasa recta, and even in the vas deferens.
Probably the secretion normally collects in the vesiculse
seminales, and there undergoes its final elaboration.

The Reproductive Organs of the Female. Each ovary
(0, Fig. 187) is a dense oval mass about 3.25 cm. (1.5 inches)
in length, 2 cm. (0.75 inch) in width, and 1.27 cm. (0.5 inch)
in thickness; it weighs from 4 to 7 grams (60-100 grains).
The organs lie in the pelvic cavity enveloped in a fold of
peritoneum (the broad ligament), and receive blood-vessels
and nerves along one border. From time to time ova reach
the surface, burst through the enveloping peritoneum, and
are received by the wide fringed aperture, j#, of the oviduct
or Fallopian tube, od. This tube narrows towards its inner
end, where it communicates with the uterus, and is lined by
a mucous membrane, covered by ciliated epithelium; plain
muscular tissue is also developed in its wall. The uterus
(u, c, Fig. 187) is a hollow organ, with relatively thick mus-
cular walls (left unshaded in the figure); it contains the
foetus during pregnancy and expels it at birth; it lies in the
pelvis between the Trrhrary bladder and~fhe"rectum (Fig. 188);
the Fallopian tubes open into its anterior corners. It is free
above, but its lower end is attached to and projects into the
vagina. In the fully developed virgin state the organ rs
somewhat pear-shaped, but flattened from before back; about
7.5 cm. (3 inches) in length, 5 cm. (2 inches) in breadth at
its upper widest part, and 2.5 cm. (1 inch) in thickness; it

REPRODUCTION.

655

The vulva is a general term for all the portions of the female
generative organs visible from the exterior. Over the front of
the pelvis the skin is elevated by adipose tissue beneath it, and
forms the mom Veneris. From this two folds of skin (I, Fig.
188), the labia major a, extend downwards and backwards on
each side of a median cleft, beyond which they again unite.
On separating the labia majora a shallow genito-urinary sinus,
into which the urethra and vagina open, is exposed. * At the
upper portion of this sinus lies the clitoris, a small and very
sensitive erectile organ, resembling a miniature penis in struc-
ture, except that it has no corpus spongiosum and is not
traversed by the urethra. From the clitoris descend two folds
of mucous membrane, the nymphce or labia internet, between
which is the vestibule, a recess containing, above, the opening
of the short female urethra, and, below, the aperture of the
vagina, which is in the virgin more or less closed by a thin
duplicature of mucous membrane, the hymen.

FIG. 189.— k section of a Mammalian ovary, considerably magnified. 1, outer
capsule of ovarv 2 3 3', atroma; 4, blood-vessels; 5, rudimentary Graaflan fol-
licles: 678 follicles bejdnninff to Pnlargp and mature, and receding: from the sur-
face; 9 a nearly ripe follicle which is extending towards the surface preparatory to
discharging the ovum; «, membrana granules*: b. discus proifgernfl: c, ovum, with
d. ^prminal vesicle, and e. eprminal snot. The general cavity of thf follicle (in
which 9 is printed) is filled with lymph-like, transudation liquid during life.

Microscopic Structure of the Ovary. The main mass of
the .ovary consists of a dense connective-tissue stroma, con-
ta'ninof unstriped muscle, blood-vessels, and nerves; it is
covered externally by a peculiar germinal epithelium, and con-
tains imbedded in it many minute cavities, the Graafian folH-
r7/x, in which omlie. If a thin section of an ovary be examined
the microscope many hundreds of small Graafian follicles,

656 THE HUMAN BODY.

each about 0.25 mm. (TJ¥ incli) in diameter, will be found
imbedded in it near the surface. These are lin^d by cells, and
each contains a single ovum. In a woman Sf child-bearing
age there will be found also, deeper in, larger follicles (7, 8,
9, Fig. 189), their cavities being distended, during life, by
liquid ; in these the essential structure may be more readily
made out. Each has an external fibrous coat constituted by
a dense and vascular layer of the ovarian stroma; within this
•come several layers of lining cells (9, &, Fig. 189) constituting
the membrana granulosa. At one point, #, the cells of this
layer are heaped up, forming the discus proligerus, which
projects into the liquid filling the cavity of the follicle. Buried
.among the cells of the discus proligerus the ovum, c, lies.

The Mammalian Ovum. As the Graafian follicles enlarge
the ova grow but not proportionately, so that they occupy
relatively less of the cavities of the larger follicles: the cells of
the discus proligerus probably elaborate food for the egg cell
from material derived from the blood-vessels which form a
•close network around most of each enlarging Graafian follicle
and transude crude nutritive matter into the liquid filling
most of the follicle. The fully formed
ovum (Fig. 19Q) is about 0.2 mm.
(yi-o inch) in diameter: it has a well-
marked outer coat or sac, a, the zona
radiata, zona pellucida or vitelline
membrane, surrounding a very granu-
lar cell-body or viteiius, b, in which is a
conspicuous nucleus, c, here named
the germinal vesicle and possessing
FIG. 190.— A human ovum; a nucleolus or germinal spot. The

•somewhat diagrammatic, a, .....

zona peiiucida; b, viteiius; c, zona pellucida exhibits distinct radial

germinal vesicle, with distinct .

reticuium of karyopiasm and markings which probably are due to
flue tubes traversing it. The main bulk

of the viteiius or yelk consists of highly refracting spheroidal
particles of nutritive matter (deutoplasm) imbedded in and
concealing a true protoplasmic reticuium. In the eggs of birds
and reptiles the deutoplasm is in very large amount and forms
nearly all of the yelk, the protoplasm being for the most part
aggregated around the germinal vesicle at a small area on one
side of the yelk. It is in this area that new cell-formation
occurs and the embryo is built up, the rest of the yelk being
gradually absorbed by it: such eggs are known as mesoblastic

REPROD UCTION. 657

or partly-dividing eggs. In all the higher mammalia the
dentoplasm is relatively sparse and tolerably evenly mingled
with the protoplasm, and the whole fertilized ovum divides to
form the first cells of the embryo: such eggs are named
holoblastic.

The Maturation of the Ovum. From time to time,
usually at intervals of about four weeks, in a woman of child-
bearing age, certain ova after attaining the size and struc-
ture described in the preceding paragraph undergo further
changes by which the egg-cell is rendered capable of fertiliza
tion. These phenomena, known as the maturation of the
ovum, result in separation of small parts of the nucleus or
germinal vesicle and cell protoplasm from the rest. They are
essentially typical cases of indirect cell division (p. 19). The
cell-body shrinks a little so as to not quite fill the zona pellu-
cida, and the germinal vesicle approaches one side. Meanwhile
the nuclear membrane and karyoplasm form the chromatic
loop and this divides into the usual two sets of Vs. One
set of these, with part of the nuclear plasm, now separates
with a little of the cell protoplasm
to form a small cell, the first polar
globule or directive corpuscle (c, Fig
191). The much larger cell result
ing from the division and represent-
ing the remainder of the vitellus
and nucleus now repeats the process,
and gives rise to the second polar
globule. In Fig. 191 the first polar
globule is shown at c. as already

* FIG. 191. -An ovum about to

Separated, and the nucleus, d. IS form the second polai globule.

, . . ,. a, zona pellucida ; b. space filled

dividing, preparatory to the iorma- with liquid and left by the shrfok-

, • » ,i -i" -i • , • age of the vitellus: c, first polar

tlOIl OI the Second directive COr- globule; d, nucleus of ovum divid-

puscle. The stage of karyokmesis
is more advanced than those repre-
sented m Fig. 10. The two polar of the nuclear spindle,
globules lie for a time (Fig. 192) within the zona pellucida
in the space left by the shrinkage of the vitellus, but take no
part in the formation of the embryo and soon disappear. The
rest of the original ovum is now mature and ready for fertili-
zation; its nucleus is known as the female pronucleus, fn,
Fig. 192. It passes towards the centre of the ovum and forms

658 THE HUMAN BODY.

the usual recticulum of karyoplasm found in normal resting
nuclei (Fig. 8).

Ovulation. From puberty, during the whole child-bearing
period of life, certain comparatively very large Graafian follicles
may nearly always be found either close to the surface of the
ovary or projecting on its exterior. These, by accumulation
of liquid within them, have become distended to a diameter
of about 4 mm. (-J- inch) ; finally, the thinned projecting por-
tion of the wall of the follicle, which differs from the rest in
containing few blood-vessels, gives way and the ovum is dis-
charged, surrounded by some cells of the discus proligerus.
The emptied follicle becomes filled up with a reddish-yellow
mass of cells, and constitutes the corpus luteum, which recedes
again to the interior of the ovary and disappears in three or
four weeks, unless pregnancy occur; in that case the corpus
luteum increases for a time, and persists during the greater
part of the gestation period.

Menstruation. Ovulation occurs during the sexual life of
a healthy woman at intervals of about four weeks, and is
attended with important changes in other portions of the gen-
erative apparatus. The ovaries and Fallopian tubes become
congested, and the fimbrias of the latter are erected and come
into contact with the ovary so as to receive any ova discharged.
Whether the fimbrise embrace the ovary and catch the ovum,
or merely touch it at various points and the ova are swept along
them by their cilia to the cavity to the oviduct, is not certain.
Having entered the Fallopian tube the egg slowly passes on
to the uterus, probably moved by the cilia lining the oviduct ;
its descent probably takes about four or five days; if not fertil-
ized, it dies and is passed out. In the womb important changes
occur at or just before the periods of ovulation; its mucous
membrane becomes swollen and soft, and minute hemorrhages
occur in its substance. The superficial layers of the uterine
mucous membrane are broken down, and discharged along with
more or less blood, constituting; the menses, or monthly sick-
ness, which commonly lasts from three to five days. During
this time the vaginal secretion is also increased, and, mixed with
the blood discharged, more or less alters its color and usually
destroys its coagulating power. Except during pregnancy and
while suckling, menstruation occurs at the above intervals,
from puberty up to about the forty-fifth year; the periods
then become irregular, and finally the discharges cease ; this

REPRODUCTION. 659

is an indication that ovulatioii has come to an end, and that
the sexual life of the woman is completed. This time, the
climacteric or ' ' turn of life, " is a critical one ; various local
disorders are apt to supervene, and even mental derangement.

Hygiene of Menstruation. During menstruation there is
apt to be more or less general discomfort and nervous irrita-
bility; the woman is not quite herself, and those responsible
for her happiness ought to watch and tend her with special
solicitude, forbearance, and tenderness, and protect her from
anxiety and agitation. Any strong emotion, especially of a
disagreeable character, is apt to check the flow, and this is
always liable to be followed by serious consequences. A sudden
chill often has the same eff ect ; hence a menstruating woman
ought always to be warmly clad, and take more than usual
care to avoid draughts or getting wet. At these periods, also,
the uterus is enlarged and heavy, and being (as may be seen
in Fig. 188) but slightly supported, and that near its lower
end, it is especially apt to be displaced or distorted; it may
tilt forwards or sideways (versions of the uterus], or be bent
where the neck and body of the organ meet (flexion). Hence
violent exercise at this time should be avoided, though there
is no reason why a properly clad woman should not take her
usual daily walk.

The absence of the menstrual flow (amenorrhcea) is normal
during pregnancy and while suckling; and in some rare cases
it never occurs throughout life, even in healthy women capa-
ble of child-bearing. Usually, however, the non-appearance
of the menses at the proper periods is a serious symptom, and
one which calls for prompt measures. In all such cases it
cannot be too strongly impressed upon women that the most
dangerous thing to do is to take drugs tending to induce the
discharge, except under skilled advice; to excite the flow, in
many cases, as for example occlusion of the os uteri, or in
general debility (when its absence is a conservative effort of
the system), may have the most disastrous results.

Fertilization. As the ovum descends the Fallopian tube
the changes of menstruation are taking place in the uterus.
Fertilization usually takes place in a Fallopian tube. The
spermatozoa are carried along partly, perhaps, by the contrac-
tions cf the muscular walls of the female cavities, but mainly
by thei~ own activity. Occasionally the ovum is fertilized

660

THE HUMAN BODY.

before reaching the Fallopian tube and fails to enter it, giving
rise to an extra-uterine pregnancy.

The actual process of the fertilization of the ovum has only
been observed in the lower animals, but there is no doubt that
the phenomena are the same in all essentials in all cases. Some
of the spermatozoa penetrate the zona pellucida and the head
of one of them enters the ovum, when it grows and forms the

male pronucleus (mn, Fig. 192).
This travels towards the nucleus
of the matured ovum or female
pronucleus, fn, and in each pro-
nucleus a karyoplastic filament
forms and breaks up into a set
of V's; in the pronuclei repre-
sented in Fig. 192 this has not
yet taken place, the karyoplasm
being still arranged in a retic-
ulum. The tail of the sperma-
tozoon (which represents, it will

i^ rmn<miTwiW1 fliA rkvnfnr»locTYi
* 3a? T

of a male cell) disappears;
whether it is cast off when the
head enters the vitellus or min-
gles with the protoplasm of the
latter is not known. As the pronuclei approach one another
two attraction particles, p, p, appear in the protoplasm of the
ovum; around these the granules of the vitellus show a radial
arrangement and a nuclear spindle (p. 19) unites them. The
spindle lies with its long axis at right angles to a line joining
the pronuclei. The latter next completely fuse across the
middle of the spindle and form a new single nucleus: Fertili-
zation is then complete, and the ovum capable of dividing or
segmenting (Fig. 11) to form the cells which by multiplication
and differentiation build up the embryo. The zona pellucida
takes no part in the segmentation and is gradually absorbed.

The Signification of the Polar Globules. The union of
the male and female pronuclei is the essential fact in fertiliza-
tion and the material basis of all the phenomena of heredity;
therefore everything pertaining to it is of very great interest.
There is reason to believe that each half of the nucleus of the
fertilized egg contains karyoplasm from both pronuclei, and
that in all subsequent cell-divisions each new cell gets nuclear

FIG. 192.— An ovum shortly before
the fusion of the pronuclei. a, zona
pellucida; 6, polar globules; fn, female
pronucleus; nin, male pronucleus; pp,
attraction bodies, with the nuclear
spindle lyin£ between them; s, sper-
matozoa which have not taken part in
fertilization.

REPRODUCTION. 661

karyoplasm from both, and therefore contains both male and
female morphological elements. If this be so, every cell of the
adult Body contains a material representative of both father
and mother, and may be regarded as hermaphrodite. Upon
this supposition explanations of the unequal cell-divisions of
the ovum giving rise to the polar globules have been based.
The ovum before maturation and the spermatoblast before final
formation of the spermatozoon being bisexual, each must, it
has been suggested, get rid of material derived from one
parent before it can fuse with a residuum of the other to make
a new cell. The spermatoblast therefore in its first cell-
division separates female nuclear matter, and the spermatozoon
is a purely male cell ; the ovum on the other hand gets rid of
male material in the polar globules, and the mature ovum is a
solely female cell ; the union of the two makes a complete her-
maphrodite cell from which the new animal develops. This
view was supported by the belief that certain insect eggs which
develop parthenogenetically did not separate polar globules
before commencing to form the embryo. It is now known,
however, that such eggs do separate one polar globule, so the
theory requires modification. We cannot here go into the dis-
cussion of this matter, which is one of the most interesting
biological questions. The argument gathers mainly round
the theory (Weismann) that each complete cell apart from
male and female elements contains two kinds of living mate-
rial : one (nuclear plasma) with controlling, reproducing, and
hereditary functions; the other (nutritive plasma) with as-
similative duties and other powers in various cells, as con-
tractility, irritability, and so forth, but exercising these under
the influence and direction of the nuclear plasma. In the
nuclear plasma itself are two distinct substances — a germinal
plasma with hereditary functions, and alone found in the just
fertilized ovum, and a histogenetic or tissue-building plasma,
which is formed by and from the germinal plasma and controls
cell-growth, division, and differentiation. The ovum in the
first polar globule gets rid of some of its histogenetic plasma,
and then in the second polar globule of the male portion of
its germinal plasma, and these are replaced by the material
brought by the spermatozoon, which is a cell that has in a
similar way got rid of some of its histogenetic and germinal
plasma. On this theory, moreover, the proportion of the
ovum extruded in the polar globules and the ratio of that

662 THE HUMAN BODY.

remaining to the germ plasma brought by the spermatozoon
may be supposed to differ in different instances and account
for individual differences in the offspring: thus some physical
basis for the facts of variation as well as of heredity would
be obtained.

Impregnation. The fertilized ovum continues its descent
to the uterine cavity, but, instead of lying dormant like the
unfertilized, segments (p. 29), and forms a morula. This,
entering the womb, becomes imbedded in the soft, vascular
mucous membrane from which it imbibes nourishment, and
which, instead of being cast off in subsequent menstrual dis-
charges, is retained and grows during the whole of pregnancy,
having important duties to discharge in connection with the
nutrition of the embryo.

Sexual congress is most apt to be followed by pregnancy if
it occur immediately after a menstrual period; at those times
a ripe ovum is usually in the Fallopian tube, near the upper
end of which it is probably fertilized in the majority of cases.
There is some difference of opinion as to whether the rupture
of the Graafian follicle occurs most frequently immediately
before the appearance of the menstrual flow, or towards its
close ; but the preponderance of evidence favors the latter view.
The menstrual process probably is a special preparation of the
womb for the reception of an embryo and its nourishment.
There is, however,, evidence that ova are ocfisionally discharged
at other than the Tegular monthly periods of ovulation and
may be fertilized and cause a pregnancy.

Pregnancy. When the mulberry mass reaches the uterine
cavity the mucous membrane lining the latter grows rapidly
and forms a new, thick, very vascular lining to the womb,
known as the decidua. At one point on this the morula be-
comes attached, the decidua growing up around it. As preg-
nancy advances and the embryo grows, it bulges out into the
uterine cavity and pushes before it that part of the decidua
which has grown over it (the decidua reflexa) ; at about the
end of the third month this coalesces with the decidua lining
the opposite sides of the uterine cavity so~that the two can no
longer be separated. That part of the decidua (decidua
serotina) against which the morula is first attached subsequently
undergoes a great development in connection with the forma-
tion of the placenta (see below). Meanwhile the whole uterus
enlarges; its muscular coat especially thickens. At first the

REPRODUCTION. 663

organ still lies within the pelvis, where there is but little room
for it ; it accordingly presses on the bladder and rectum (see
Fig, 188) and the nerves in the neighborhood, frequently
causing considerable discomfort or pain; and. reflexly, often
exciting nausea or vomiting (the morning sickness of preg-
nancy). Later on, the pregnant womb escapes higher into the
abdominal cavity, and although then larger, the soft abdominal
walls more readily make room for it, and less discomfort is
usually felt, though there may be shortness of breath and
palpitation of the heart from interference with the diaphrag-
matic movements. All tight garments should at this time be
especially avoided; the woman's breathing is already suffi-
ciently impeded, and the pressure may also injure the develop-
ing child. Meanwhile, changes occur elsewhere in the Body.
The breasts enlarge and hard masses of developing glandular
tissue can be felt in them ; and there may be mental symptoms :
depression, anxiety, and an emotional nervous state.

During the whole period of gestation the woman is not
merely supplying from her blood nutriment for the foetus, but
also, through her lungs and kidneys, getting rid of its wastes;
the result is a strain on her whole system which, it is true,
she is constructed to bear and will carry well if in good health,
but which is severely felt if she be feeble or suffering from dis-
ease. The healthy married woman who endeavors to evade
motherhood because she thinks she will thus preserve her per-
sonal appearance, or because she dislikes the trouble of a
family, deserves but little sympathy ; she is trying to escape ?
duty voluntarily undertaken, and owed to her husband, he'
country, and her race ; but she whose strength is undermine
and whose life is made one long discomfort for the sexu
gratification of her husband deserves every consideration, ai ,
the family physician ought perhaps to warn the husband mo
frequently than he does of the risk to a delicate wife's healt ^
or even life, of frequent pregnancies: and the husband shoui
control himself accordingly. The professor of gynaecology ii
a leading medical school, gives it as his deliberate opinion
that the majority of American women must at some periods of
their lives choose between freedom from pregnancy or early
death.

Apart from pregnancy, moreover, a woman's health is oft *
injured by frequent sexual intercourse. A physician who 1
unusual opportunities of knowing states that he has reason ^*

664 m# HUMAN BODY.

believe that not only is the act of sexual congress at best, from
a physical point _of view, a mere nuisance to the majority of
women belonging to the more luxurious classes of society after
they attain the age of twenty-two or twenty-three, but that a
very considerable proportion suffer acute pain from it such as,
if frequent, breaks down the general health. A Ipyin^ woman,
finding her highest happiness in suffering for those dear to her,
is very unlikely to let her husband know this, so long as she
can bear it ; but if the possibility is known it will not, per-
haps, need much acuteness in him to discover such suffering
when it exists, nor very much real affection to direct him-
self accordingly. In the class of cases referred to, rest of the
over-irritable and congested female organs is above all essen-
tial. The cause is frequently removable by simple, but skilled,
treatment; the desirability of rendering this available to a
woman in members of her own sex is now generally recog-
nized.

The Intra-TJterine Nutrition of the Embryo. At first

the embryo is nourished by absorption of materials from the

soft vascular lining of the womb; as it increases in size this

is not sufficient, and a new organ, the placenta, is formed for

the purpose. A foetal outgrowth, the allantois, plants itself

firmly against the decidua serotina, and villi developed on it

burrow from j£s surface into the uterine mucous inSmbrane.

In the deeper layer of this latter are large sinuses through

which the maternal blood flows, and into which the allantoic

Tilli project. Blood is brought from the foetus to the allantois

y arteries and carried back by veins after traversing the

cpillaries of the villi, arid while flowing through these re-

aives, by dialysis, oxygen and food materials from the mater-

kj blood, and gives up to it carbon dioxide, urea, and other

cistes. There is thus no direct intermixture of the two

noods; the embryo is from the first an essentially separate

\id independent organism. The allantois and decidua sero-

,ina becoming inseparably united together form the placenta,

which in the human species is, when fully developed, a round

"hick mass about the size of a large saucer, connected to the

mbryo by a narrow stalk, the umbilical cord, in which blood-

essels run to and from the placenta.

Ul Parturition. At the end of from 275 to 280 days from
^'•tilization of the ovum (conception') pregnancy terminates,
ei)d the child is expelled by powerful contractions of the

REPRODUCTION. 665

uterus, assisted by those of the muscles in the abdominal
walls. When the child is born, it has attached to its navel
the umbilical cord, which is then usually ligatured and cut
across: some good authorities, however, maintain that this
should not be done until after the contractions which expel
the placenta, as otherwise a quantity of the infant's blood
remains in that organ; the loss of which might be serious to
a feeble infant. Shortly after the birth of the child renewed
uterine contractions detach and expel the placenta, both its
foetal or allantoic and maternal or decidual part, as the after-
birth. Where it is torn loose from the uterine wall large
blood sinuses are left open; hence a certain amount of bleed-
ing occurs, but in normal labor this is speedily checked by
firm contraction of the uterus. Should this fail to take
place profuse haemorrhage occurs (flooding] and the mother
may bleed to death in a few minutes unless prompt measures
are adopted.

For a few days after delivery there is some discharge (the
locJiia) from the uterine cavity: the whole decidna being
broken down and carried off, to be subsequently replaced by
new mucous membrane. The muscular fibres developed in
the uterine wall in such large quantities during pregnancy
undergo rapid fatty degeneration and are absorbed, and in a
few weeks the organ returns almost to its original size. The
parturient woman is especially apt to take infectious diseases;
and these, should they attack her, are fatal in a very large
percentage of cases'. Very special care should therefore be
taken to keep all contagion from her.

There is a current impression that a pregnancy, once
commenced, can be brought to a premature end, especially in
its early stages, without any serious risk to the woman. That
belief is erroneous. Premature delivery, early or late in
pregnancy, is always more dangerous than natural labor at
the proper term; the physician has sometimes to induce it,
as when a malformed pelvis makes normal parturition impos-
sible, or the general derangement of health accompanying
the pregnancy is such as to threaten the mother's life; but
the occasional necessity of deciding whether it is his duty to
procure an abortion is one of the most serious responsibilities
he meets with in the course of his professional work.

Dr. Storer, an eminent gynascologist, states emphatically,

666 THE HUMAN BODY.

from extended observation, that despite apparent and isolated
instances to the contrary —

1. »A larger proportion of women die during or in con-
sequence of an abortion, than during or in consequence of
child-bed at the full term of pregnancy.

2. A very much larger number of women become con-
firmed invalids, perhaps for life; and—

3. The tendency to serious and often fatal organic disease,
as cancer, is rendered very much greater at the so-called
"turn of life," by previous artificially induced premature
delivery.

During pregnancy there is a close connection between the
placenta and uterus; nature makes preparation for the safe
dissolution of this at the end of the normal period, but " its
premature rupture is usually attended by profuse haemor-
rhage, often fatal, often persistent to a greater or less degree
for many months after the act is completed, and always at-
tended with more or less shock to the maternal system, even
though the full effect of this is not noted for years." The
same authority states again: " Any deviation from this proc-
ess at the full term" (i.e., the process, associated with lacta-
tion, by which the uterus is restored to its small non-gravid
dimensions) "lays the foundation of, and causes, a wide
range of uterine accidents and disease, displacements of
various kinds; falling of the womb downwards or forwards
or backwards, with the long list of neuralgic pains in the
back, groin, thighs, and elsewhere that they occasion ; con-
stant and inordinate leucorrhcea; sympathetic attacks of
ovarian irritation, running even into dropsy," etc., etc.
There is, thus, abundant reason for bearing most things
rather than the risks of an avoidable abortion.

Lactation. The mammary glands for several years after
birth remain small, and alike in both sexes. Towards
puberty they begin to enlarge in the female, and when fully
developed form in that sex two rounded eminences, the
breasts, placed on the thorax. A little below the centre of
each projects a small eminence, the nipple, and the skin
around this forms a colored circle, the areola. In virgins
the areolse are pink; they darken in tint and enlarge during
the first pregnancy and never quite regain their original hue.
The mammary glands are , constructed on the compound
racemose type. Each consists of from fifteen to twenty

REPRODUCTION. 667

distinct lobes, made up of smaller divisions; from each main
lobe a separate galactophorous duct, made by the union of
smaller branches from the lobules, runs towards the nipple,
all converging beneath the areola. There each dilates
and forms a small elongated reservoir in which the milk
may temporarily collect. Beyond this the ducts narrow
again, and each continues to a separate opening on the nip-
ple. Imbedding and enveloping the lobes of the gland is a
quantity of firm adipose tissue which gives the whole breast
its rounded form.

During maidenhood the glandular tissue remains imper-
fectly developed and dormant. Early in pregnancy it begins
to increase in bulk, and the gland lobes can be felt as hard
masses through the superjacent skin and fat. Even at par-
turition, however, their functional activity is not fully estab-
lished. The oil-globules of the milk are formed by a sort of
fatty degeneration of the gland-cells, which finally fall to
pieces; the cream is thus set free in the watery and albu-
minous secretion formed simultaneously, while newly de-
veloped gland-cells take the place of those destroyed. In the
milk first secreted after accouchment (the colostrum) the cell
destruction is incomplete, and many cells still float in the
liquid, which has a yellowish color; this first milk acts as a
purgative on the infant, and probably thus serves a useful
purpose, as a certain amount of substances (biliary and
other), excreted by its organs during development, are found
in the intestines at birth.

Human milk is undoubtedly the best food for an infant in
the early months of life ; and to suckle her child is useful to
the mother if she be a healthy woman. There is reason to
believe that the processes of involution by which the large
mass of muscular and other tissues developed in the uterine
walls during pregnancy are broken down and absorbed, take
place more safely to health if the natural milk secretion is
encouraged. Many women refuse to suckle their children
from a belief that so doing will injure their personal appear-
ance, but skilled medical opinion is to the contrary effect; the
natural course of events is the best for this purpose, unless
lactation be too prolonged. Of course in many cases there are
justifiable grounds for a mother's not undertaking this part of
her duties ; a physician is the proper person to decide.

In a healthy woman, not suckling her child, ovulation and

668 THE HUMAN BOD '7.

menstruation recommence about six weeks after childbirth; a
nursing mother usually does not menstruate for ten or twelve
months; the infant should then be weaned.

When an infant cannot be suckled by its mother or a wet-
nurse an important matter is to decide what is the best food
to substitute. Good cow's milk contains rather more fats than
that of a woman, and much more casein ; the following table
gives averages in 1000 parts of milk :

Woman. Cow.

Casein 28.0 54.0

Butter 33.5 43.0

Milk sugar 44.5 42.5

Inorganic matters 4.75 7.75

The inorganic matters of human milk yield, on analysis, in
100 parts — calcium carbonate 6.9; calcium phosphate, 70. 6 ;
sodium chloride, 9.8; sodium sulphate, 7.4; other salts, 5.3.
The lime salts are of especial importance to the child, which
has still to build up nearly all its bony skeleton.

When undiluted cow's milk is given to infants they rarely
bear it well; the too abundant casein is vomited in loose
coagula. The milk should therefore be diluted with half or,
for very young children, even two thirds its bulk of water.
This, however, brings down the percentage of sugar and
butter below the proper amount. The sugar is commonly
replaced by adding cane sugar; but sugar of milk is readily
obtainable and is better for the purpose. If used at all it
should, however, be employed from the first ; it sweetens much
less than cane sugar, and infants used to the latter refuse milk
in which milk sugar is substituted. Cream from cow's milk
may be added to raise the percentage of fats to the normal, but
must be perfectly fresh and only added to the milk immediately
before it is given to the child. While milk is standing for the
cream to rise it is very apt to turn a little sour ; the amount of
this sour milk carried off with the cream is itself no harm
when mixed with a large bulk of fresh milk ; it carries with it,
however, some of the fungus whose development causes the
souring, and this will rapidly develop and sour all the milk it
is added to if the mixture be let stand. As the infant grows
older less diluted cow's milk may gradually be given; after
the seventh or eighth month no addition of water is necessary.

In the first weeks after birth it is no use to give an infant
starchy foods, as arrowroot. The greater part of the starch

REPRODUCTION. 669

passes through the bowels unchanged; apparently because the
pancreas has not yet fully developed, and has not commenced
to make its starch-converting ferment. Later on, starchy
substances may be added to the diet with advantage, but it
should be borne in mind that they cannot form the chief part
of the child's food; it needs proteids for the formation of
its tissues, and amyloid foods contain none of these. Many
infants are, ignorantly, half starved by being fed almost en-
tirely on such things as corn-flour or arrowroot.

Puberty. The condition of the reproductive orgarcy
each sex described in preceding pages is that found in a* in
although mapped out, and, to a certain extent, deveihe
before birth and during childhood, these parts grow c
slowly and remain functionally incapable during the early
years of life ; then they comparatively rapidly increase in size
and become physiologically active; the boy or girl becomes man
or woman.

This period of attaining sexual maturity, known as puberty,
takes place from the eleventh to the sixteenth year, and is
accompanied by changes in many parts of the Body. Hair
grows more abundantly on the pubes and genital organs, and
in the armpits; in the male also on various parts of the face;
The lad's shoulders broaden; his larynx enlarges, and lengthen-
ing of the vocal cords causes a fall in the pitch of his voice ;
all the reproductive organs increase in size ; fully formed seminal
fluid is secreted, and erections of the penis occur. As these
changes are completed spontaneous nocturnal seminal emis-
sions take place from time to time during sleep, being usually
associated with voluptuous dreams. Many a young man is
alarmed by these ; he has been kept in ignorance of the whole
matter, is too bashful to speak of it, and getting some quack
advertisement thrust into his hand in the street is alarmed to
learn that his strength is being drained off, and that he is on
the high-road to idiocy and impotence unless he place himself
in the hands of the advertiser. Lads at this period of life
should have been taught that such emissions, when not too
frequent and not excited by any voluntary act of their own,
are natural and healthy. They may, however, occur too often ;
if there is any reason to suspect this, the family physician
should be consulted, as the healthy activity of the sexual
organs varies so much in individuals as to make it impossible
to lay down numerical rules on the subject. The best preven-

670 THE HUMAN BODY.

tives in any case are, however, not drugs, but an avoidance of
too warm and soft a bed, plenty of muscular exercise, and
keeping out of the way of anything likely to excite the sexual
instincts.

In the woman the pelvis enlarges considerably at puberty,

and, commonly, more subcutaneous adipose tissue develops

over the Body generally, but especially on the breasts and hips ;

consequently the contours become more rounded. The exter-

al generative organs increase in size, and the clitoris and

^phae become erectile. The uterus grows considerably, the
tes enlarge, some Graafian follicles ripen, and menstruation

mences.

The Stages of Life. Starting from the ovum each human
being, apart from accident or disease, runs through a life-cycle
which terminates on the average after a course of from 75 to
80 years. The earliest years are marked not only by rapid
growth but by differentiating growth or development ; then
comes a more stationary period, and finally one of degenera-
tion. The life of various tissues and of many organs is not,
however, coextensive with that of the individual. During life
all the formed elements of the Body are constantly being
broken down and removed; either molecularly (i.e., bit by bit
while the general size and form of the cell or fibre remains
unaltered), or in mass, as when hairs and the cuticle are shed.
The life of many organs, also, does not extend from birth to
death, at least in a functionally active state. At the former
period numerous bones are represented mainly by cartilage.
The pancreas has not attained its full development; and some
of the sense-organs seem to be in the same case; at least new-
born infants appear to hear very imperfectly. The reproductive
organs only attain full development at puberty, and degenerate
and lose all or much of their functional importance as years ac-
cumulate. Certain organs have even a still shorter range of
physiological life ; the thymus, for example, attains its fullest
development at the end of the second year and then gradually
dwindles away, so that in the adult scarcely a trace of it is to
be found. The milk-teeth are shed in childhood, and their
so-called permanent successors rarely last to ripe old age.

During early life the Body increases in mass, at first very
rapidly, and then more slowly, till the full size is attained,
except that girls make a sudden advance in this respect at
puberty. Henceforth the woman's weight (excluding excep-

REPRODUCTION. 671

tional cases of accumulation of non-working adipose tissue)
remains about the same until the climacteric. After that
there is often an increase of weight for several years due
mainly to increased formation of fat; a man's weight usually
slowly increases until forty.

As old age comes on a general decline sets in, the rib car-
tilages become calcined, and lime salts are laid down in the
arterial walls, which thus lose their elasticity ; the refracting
media of the eye become more or less opaque ; the physiological
irritability of the sense-organs in general diminishes ; and fatty
degeneration, diminishing their working power, occurs in
many tissues. In the brain we find signs of less plasticity ; the
youth in whom few lines of least resistance have been firmly
established is ready to accept novelties and form new associa-
tions of ideas ; but the longer he lives, the more difficult does
this become to him. A man past middle life may do good,
or even his best work, but almost invariably in some line of
thought which he has already accepted: it is extremely rare
for an old man to take up a new study or change his views,
philosophical, scientific, or other. Hence, as we live, we all
tend to lag behind the rising generation.

Death. After the prime of life the tissues dwindle (or at
least the most important ones) as they increased in childhood ;
it is conceivable that, without death, this process might occur
until the Body was reduced to its original microscopic dimen-
sions.

Before any great diminution takes place, however, a break-
down occurs somewhere, the enfeebled community of organs
and tissues forming the man is unable to meet the contingen-
cies of life, and death supervenes. " It is as natural to die as
to be born," Bacon wrote long since; but though we all
know it, few realize the fact until the summons comes. To
the popular imagination the prospect of dying is often asso-
ciated with thoughts of extreme suffering ; personifying life,
men picture a forcible and agonizing rending of it, as an
entity, from the bodily frame with which it is associated. As
a matter of fact, death is probably rarely associated with any
immediate suffering. The sensibilities are gradually dulled us
the end approaches; the nervous tissues, with the rest, loye
their functional capacity, and, before the heart ceases to beat,
the individual has commonly lost consciousness.

The actual moment of death is hard to define : that of the-

672 THE HUMAN BODY.

Body generally, of the mass as a whole, may be taken to be the
moment when the heart makes its last beat ; arterial pressure
then falls irretrievably, the capillary circulation ceases, and
the tissues, no longer nourished from the blood, gradually die,
not all at one instant, but one after another, according as their
individual respiratory or other needs are great or little.

While death is the natural end of life, it is not its aim — we
should not live to die, but live prepared to die. Lifer'has its
duties and its legitimate pleasures, and we better play our part
by attending to the fulfilment of the one and the enjoyment
of the other, than by concentrating a morbid and paralyzing
attention on the inevitable, with the too frequent result of
producing indifference to the work which lies at hand for each.
Our organs and faculties are not talents which we may justifi-
ably leave unemployed; each is bound to do his best with
them, and so to live that he may most utilize them. An active,
vigorous, dutiful, unselfish life is a good preparation for death ;
when that time, at which we must pass from the realm con-
trolled by physiological laws, approaches, when the hands
tremble and the eyes grow dim, when " the grasshopper shall
be a burden and desire shall fail," then, surely, the conscious-
ness of having quitted us like men in the employment of our
faculties while they were ours to use, will be no mean consola-
tion.

INDEX.

Abdomen, contents of, 5.

Abdominal respiration, 393.

Abducens nerve, 174.

Aberration, chromatic, 527.

Aberration, spherical, 527.

Absorbents, 349.

Absorption from intestines, 373.

Absorption of gases, 405.

Absorption of oxygen by blood,
407.

Accelerator nerves of heart, 271.

Accessory reproductive organs, 647.
V Accommodation, 522.

Acetabulum, 77.

Achromatin filaments, 20.

Acid, acetic, 13; butyric, 13; car-
bonic, see carbon dioxide ; for-
mic, 13 ; glycero-phosphoric,
13 ; glycocholic, 12 ; hippuric,
434 ; lactic, 13; oleic, 12 ; pal-
mitic, 12 ; sarcolactic, 13, 123 ;
stearic, 12 ; taurocholic, 370 ;
uric, 434.

Action current (negative varia-
tion), 139. 199.

Actions, reflex, 188, 600.

Addison's Disease, 359.

Adenoid tissue, 103, 353.

Adipose tissue, 107.

Adrenals ( supra-renal capsules),
359.

Advantage of mixed diet, 326, 474.

-After-birth (placenta), 665.

After images, 548.

Air, chemical composition of, 400.

Air-cells, 383.

Air, changes produced in, by

breathing, 399.
, Air, complemental, tidal, etc., 392.

Air-passages, 381.

Albumin, serum, 59.

Albuminoids (gelatinoids), 10, 319.

Albuminous bodies, 9.

Albumose, 366.

Alcohol, 323.

Alimentary canal, 328.

Alimentary principles, 819.

Amoeboid cells, 110.

Amoeboid movements, 23, 48.

Amyloids (carbohydrates), 12, 319.

Amyloids, digestion of, 362, 368,
375.

Anabolism, 22.

Anaemia, 60.

Anatomical systems, 39.

Anatomy of alimentary canal, 328;
of brain, 166, 610; of ear, 557
of eye, 504; of joints, 92; of
lymphatic system, 349; of mus-
cular system, 112; of nervous
system, 158, 172; of respiratory
organs, 380; of skeleton, 63; of
skin, 441; of urinary organs,
427; of vascular system, 211.

Animal heat, source'of , 477.

Anvil bone, 558.

Agraphia, 628.

Aorta, 215, 219.

Aphasia, 628

Apoplexy, 170.

Appendicular skeleton, 77.

Appendix vermiformis, 342.

Appetite, 377.

Aqueduct of Sylvius, or iter, 171,
618.

Aqueous humor, 516.

Arachnoid, 5, 161.

Arbor vitae, 171.

Area, motor, 623.

Areolar tissue, 100.

Areolar tissue, subcutaneous, 442.

Arm, skeleton of, 77.

673

674

INDEX.

Arterial blood, 225, 404.

Arterial pressure, 242, 267.

Arteries, distribution of, 21C.

Arteries, structure of, 225.

Artery, axillary, 219 ; brachial,
219 ; bronchial, 220 ; carotid,
219; cceliac, 220; coronary, 216,
219; femoral, 220; hepatic, 345;
iliac, 220 ; innominate, 219 ; in-
tercostal, 220 ; mesenteric, 220 ;
radial, 219 ; renal, 220, 427 ;
subclavian, 219; ulnar, 219; ver-
tebral, 219.

Articular cartilage. 92.

Articulations, 64, 91.

Arytenoid cartilages, 635.

Ascending antero-lateral tract, 598.

Asphyxia, 422.

Aspiration of thorax, 251, 393.

Assimilation, 22.

Assimilative tissues, 32.

Associated movements, 631.

Association of ideas, 631.

Astigmatism, 528.

Astragalus, 81.

Atlanto-axial articulation, 95.

Atlas vertebra, 69.

Attraction particle, 19.

Auditory nerve, 174.

Auditory ossicles, 558.

Auditory perceptions, 574.

Augmentor nerve-fibres, 270.

Auriculo-ventricular valves, 217.

Automatic centres, 189.

Automatic movements, 25.

Automatic tissues, 34.

Automatism of heart, 258.

Axial current, 235.

Axial ligament, 559.

Axial skeleton, 64, 67.

Axillary artery, 219.

Axis vertebra, 69.

Axis, visual, 541.

Ball-and-socket joints, 94.

Basement membrane, 103, 283.

Basilar membrane, 561, 573.

Bathing, 449.

Beat of heart, 227, 258.

Beef -tea, 125.

Biceps muscle of arm, 113.

Bile, 370.

Bilirubin, 357, 370.

Blackness, sensation of, 542.

Bladder, urinary, 427.

Blind spot, 532.

Blood, 41 ; arterial and venous,

225, 380 ; composition of, 59 ;

clotting of, 51 ; crystals, 47 ;

gases of, 404 ; histology of, 44 ;
laky, 46 ; quantity of, 61 ; se-
rum, 51. v

Blood corpuscles, 44, 60.

Blood-now in capillaries, 234 ; in
kidneys, 431 ; in liver, 347.

Blood-vessels, anatomy of, 211.

Blood-vessels, nerves of, 273.

Blood-vessels, structure of, 225.

Blushing, 277.

Bone, composition of , 89; histology
of, 87; gross structure of, 85.

Bones of face, 75; of fore-limb,
77; of hind-limb, 78; of pectoral
arch, 77 ; of pelvic arch, 77 ; of
skull, 72.

Brachial artery, 219.

Brachial plexus, 164.

Brain, anatomy of, 166, 610; physi-
ology of, 609 ; membranes of,
160.

Bread, 323.

Breast-bone, 72.

Bronchial arteries, 220.

Bronchial tubes, 382.

Bronchus, 382.

Brunner's glands, 341.

Buccal cavity, 328.

Buffy coat on blood-clot, 52.

Caecum, 342.

Calcaneum, 79.

Calcium salts, relation to blood-
clotting, 55; to heart-beat, 271.

Camera obscura, 521.

Canals, semicircular, 560, 563, 574,
615.

Capacity of lungs, 391.

Capillaries, blood, 220, 226.

Capillaries, lymphatic, 352.

Capillary circulation, 234.

Capsule, internal, 619.

Capsule of Glisson, 345.

Carbamide, see Urea.

Carbohydrates, see Amyloids.

Carbon dioxide, 13; m blood, 411;
production of, in muscle, 457.

Carbon monoxide haemoglobin, 422.

Cardiac impulse, 228.

Cardiac muscular tissue, 123, 255.

Cardiac nerves, 257.

Cardiac orifice of stomach, 338.

Cardiac plexus, 176.

Cardio-inhibitory nerves, 264, 269.

Carotid artery, 219.

Carpus, 77.

Casein, 10.

Caseinogen, 10.

Cartilage, 98; articular, 92; elas-

INDEX.

675

tic, 104 ; fibre-, 104 ; histology
of, 99 ; inter-articular, 104.

Cartilages of larynx, 634.

Cataract, 528.

Cauda equina, 164.

Caudate nucleus, 619.

Cells, 17; amoeboid, 110; ciliated;
35, 110; division of, 18; differ-
entiation of, 29 ; growth of, 17 ;
oxyntic, 3o8 ; secretory, 288 ;
vaso-motor, 273.

Cement, of tooth, 331.

Central fissure, 623.

Centre, cardio-inhibitory, 269 ;
cerebro-spinal, 5, 159 ; convul-
sive, 424; respiratory, 414.

Centre of gravity of body, 149.

Centres, nerve, general functions
of, 188.

Centrosome, 19.

Cephalic vein, 222.

Cerebellar tract, 598.

Cerebellum, 167, 613.

Cerebral cortex, 622 ; motor areas
in, 623; sensory areas in, 629.

Cerebral hemispheres, 166.

Cerebral hemispheres, functions
of, 622.

Cerebral localization, 623.

Cerebral ventricles, 168

Cerebro-spinal centre, 5, 159.

Cerebro-spinal liquid, 161, 169.

Cervical plexus, 164.

Cervical vertebrae, 68.

Characteristics of human skeleton,
83. ^

Chemical changes in breathed air,
399.

Chemical combinations, energy
liberated in, 304.

Chemical composition of body, 7.

Chemistry, of bile, 370 ; of blood,
59 ; of bone, 89; of fats, 108; of
gastric juice, 365; of lymph, 62;
of muscle, 123, 457; of pancreatic
secretion, 868 ; of respiration,
398; of secretion, 287; of teeth,
331 ; of urine, 433 ; of white
fibrous tissue, 102 ; of working
muscle, 454.

Chest, see Thorax.

Chondrin, 11.

Chorda tympani nerve, 293.
Choroid, 510.
Chorojd plexus, 169.
Chromatic aberration, 527.
Chromoplasm, 19.
Chyle, 367.
Chyme, 367.

Ciliary muscle, 516, 524.
Ciliary, processes, 510.
Ciliated cells, 110.
Circulation, 211, 223, 234; during
asphyxia, 423 ; influence of re-
spiratory movements on, 394; in-
fluence of nerves on, 253; portal,
223, 347; pulmonary, 223; renal,
431.

Circulatory organs, 211.
Circumvallate papillae, 333.
Classification of the tissues, 31.
Classification of nerve-fibres, 191.
Clavicle, 77.
Clothing, 486.
Coagulated proteid, 10.
Coagulation of blood, 51.
Coccyx, 71.
Cochlea, 560, 573.
Coeliac axis, 220.
Cold-blooded animals, 477.
Collagen, 105.
Collar-bone, 77.
Colon, 342.
Color blindness, 545.
Color mixing, 543.
Color vision, 541.

Comma tract, 598.

Combustible foods, 453.

Commissure, optic, 512.

Commissures, cerebral, 170, 620.

Common bile-duct, 344.

Common sensation, 490, 585, 587.

Complemental air, 392.

Complementary colors, 543.

Concha, 557.

Conduction in spinal cord, 594, 599.

Conductive tissues, 35

Conductivity, physiological, 24.

Congestion, 487.

Conjunctiva, 506.

Connective tissue, 63, 100, 106.

Connective-tissue corpuscles, 102.

Conservation of energy, 302.

Consonants, 642.

Contractile tissues, 35, 117.

Contractility, 23, 127.

Contraction, muscular, 130.

Contrasts, visual, 547.

Convulsive centre, 424.

Cooking of meats, 321 ; of vege-
tables, 323.

Co-ordinating tissues, 34.

Co-ordination, 24, 188.

Cord, spinal, 161, 594.

Cords, vocal, 635.

Corium, 6, 442.

Corn, 322.

Cornea, 509.

676

INDEX.

Coronary artery, 216; sinus, 215.
Corpora albicantia, 173.
Corpora geniculata, 617.
Corpora quadrigemina. 167, 617.
Corpora striata, 167, 618.
Corpus callosum, 170.
Corpuscles of blood, red, 44 ; color-
less, 47; platelets, 49.
Corresponding retinal points, 554.
Cortex of cerebrum, 622.
Corti, organ of, 562.
Costal cartilages, 72.
Costal respiration, 393.
Coughing, 425.
Cranial nerves, 172.
Cranium, 73.
Cream, 322.
Cretinism, 358.
Cricoid cartilage, 634.
Crura cerebri, 167.
Crying, 426.

Crypts of Lieberkiihn, 341.
Crystalline lens, 516.
Curare poisoning, 129.
Cutaneous secretions, 447.
Cutis vera, 442.
Cystic duct, 344.

Daltonism, 546.

Death, 671.

Death stiffening, 123, 457.

Defects (optical) of eye, 525, 526.

Degeneration of nerve-fibres, 209.

Degenerations, in brain, 619 ; in
spinal cord, 596.

Deglutition, 363.

Dentine, 331.

Depressor nerve, 276.

Dermis, 5, 576.

Descernet, membrane of, 510.

Deutoplasm, 26.

Development, 29.

Diabetes, 355, 470

Dialysis, 42.

Diapedesis, 281.

Diaphragm, 4, 386.

Dietetics, 474.

Diet, mixed, advantages of, 325.

Differentiation of the tissues, 29.

Digestion, 361...

Digestion of a typical meal, 375.

Diploe. 89.

Dislocation, 96.

Dispersion of light, 519.

Dissimilation, 22.

Distance, perception of, 552, 574.

Division of physiological employ-
ments, 30.

Dorsal (neural) cavity, 5.

Dorsal (thoracic) vertebrae, 66.

Drum of ear, 557.

Ductless glands, 354.

Duodenum, 339.

Dura mater, 160.

Duration of luminous sensations,

539.
Dyspepsia, 377.

Ear, 557.

Eggs, 321.

Elasticity of muscle, 137.

Elastic cartilage, 104.

Elastic tissue, 102.

Electrical currents, of muscle, 138;
of nerve, 198.

Elements found in body, 8.

Eliminative (excretory) tissues, 32.

Emrnetropia, 525.

Emulsification, 369.

Enamel, 331.

End bulbs, 576.

Endocardium, 213.

Endo- lymph, 560.

Endo-skeleton, 63.

End plates, 121.

Energy, conservation of, 302 ; ki-
netic, 303; lost from body "daily,
300, 480; of chemical affinity,
304; potential, 303 ; muscular,
source of, 140, 454; source of,
in body, 304 ; utilization of, in
body, 310.

Energy-yielding foods, 452.

Enzymes, 11, 363.^—

Epidermis, 5, 441.

Epiglottis, 634.

Epithelium, 6, 36.

Epithelium, ciliated, 110.

Equilibrium sensations, 614,

Erect posture, 149.

Ethmoid bone, 75.

Eustachian tube, 558.

Excretion, '282.

Exercise, 154.

Exoskeleton, 63.

Expiration, 390.

Expiratory centre, 421.

External auditory meatus, 557.

External ear, 557.

External respiration, 380.

Extrinsic reference of sensations,
501.

Eye, 504; appendages of, 505; op-
tical defects of, 526 ; physiology
of, 530 ; refraction of light in,
521.

Eyeball, 509.

Eyeball, muscles of, 507.

INDEX.

677

Eyelids, 506.

Facial nerve, 174.

False vocal cords, 635.

Fat, 12, 108.

Fat, source of, in body, 472.

Fatigue of retina, 546.

Fatty tissue, 107.

Fauces, 335.

Feclmer's law, 499.

Feeding of infants, 667.

Femoral artery, 220.

Femur, 78, 93.

Ferments, 11, 363.

Fertilization, 659.

Fever, 485.

Fibrin, 10, 52, 54.

Fibrin ferment, 55.

Fibrinogen, 10, 54.

Fibro-cartilage, 104.

Fibula, 79.

Fick and Wislecenus, 455.

Filiform papillae, 334.

Flesh foods, 321.

Follicles of hairs,x444.

Fontanelles, 92.

Food of plants, 315.

Foods, definition of, 317 ; energy
yielding, 452 ; flashy, 321 ;
oxidizable, 316, 320; ti
forming, 313, 452; vegetable,
322.

Foot, skeleton of, 79.

Foramen, intervertebral, 71; mag-
num, 75 ; of Monro, 171, 619 ;
oval, 558 ; round, 558 ; thyroid,
78; vertebra!

Forebrain, 166, 009, 618.

Forelimb, skeleton of, 77.

Fornix, 170.

Frog, heart of, 256.

Frontal bone, 75.

Fuel of body, 308.

Fundamental physiological actions,
28.

Fungiform papillae, 333.

Fur on tongue, 334.

Gall bladder, 344.

Ganglia, 160, 175, 176, 184.; of

cranial nerves, 175 ; of heart,

257, 263 ; of spinal nerve-roots,

163, 184 ; sporadic 176.
Ganglion, Gasserian, 174 ; spinal,

184.

Gases of blood, 404.
Gasserian ganglion, 174.
Gastric digestion, 366.
Gastrc glands, 338,

Gastric juice, 365.

Gelatin, 11.

Gelatinoids, 10, 319.

Gemmation, 644.

Glands, 284.

Glenoid fossa, 77.

Gliding joints, 96.

Glisson. capsule of, 346.

Globe of eye, 509.

Globulin, 10, 47.

Glossopharyngeal nerve, 174.

Glottis, 635.

Glucose (grape sugar), 12.

Gluten, 322.

Glycerine, 12.

Glycocholic acid, 12.

Glycogen, 12, 466.

Gmelin's test, 370.

Goitre, 357.

Golgi's tendon organs, 122.

Grape sugar, 12.

Great omenturn, 339.

Growth, 17.

Gullet, 336.

Hsernal (ventral) cavity, 4, 6.

Haematin, 12, 47.

Haeniatoblasts, 62.

Haemoglobin, 46, 405.

Hairs, 444.

Hair-cells, 562.

Hammer-bone, 558.

Hand, skeleton of, 77.

Haversian canals, 87.

Hearing, 557.

Heart, 213, 271 ; beat of, 227, 256,
262 ; nerves of, 253.

Heat production and regulation in
Body, 477.

Heat lost from lungs, 399.

Heel-bone, 79.

Hemianopia, 512.

Hemispheres, cerebral, 166 ; func-
tions of, 622.

Hensen, band of, 119.

Hepatic artery, 343.

Hepatic cells, 344, 467.

Hepatic duct, 344.

Hepatic veins, 224, 344.

Heredity, theories of, 661.

Hering's theory of color vision,
548-

Hiccough, 425.

Hind limb, skeleton of, 78.

Hinge joints, 95.

Hip-joint, 92.

Hippuric acid, 434.

Histology, 1 ; of adipose tissue
107 ; of areolar tissue, 100 ; of

678

INDEX.

blood, 44; of bone, 87; of cardiac
muscle, 123 ; of cartilage, 99 ; of
connective tissues, 100 ; of ear,
561 ; of elastic issue, 102 ; of
hairs, 444; of heart, 123; of kid-
ney, 429; of liver, 344; of lungs,
383, of lymph, 49; of lymph
glands, 352 ; of nails, 445 ; of
nerve-cells, 179, 184 ; of nerve-
fibres, 176 ; of nose, 588 ; of
plain muscular tissue, 122 ; of
skin, 441; of small intestine,
339 ; of retina, 511; of spinal
cord, 181; of stomach, 122, 337;
of striped muscle, 117; of teeth,
331; of tactile organs, 576; of
tongue, 589 ; of white fibrous
tissue. 101.

Homologies of supporting tissues,
105.

Homology, 68; of limbs, 80.

Horopter, 554.

Humerus, 77, 86.

Humor, aqueous, 516 ; vitreous,
516.

Hunger, 587.

Hyaloplasm, 19, 120.

Hydrocarbons. See Fats.

Hydrogen, 9.

Hygiene, 1; of blood, 60; of brain,
632; of clothing, 392, 486; of
exercise, 153; of eye, 525; of
growing skeleton, 90, 106 ; of
joints, 96 ; of muscles, 153 ; of
respiration, 392, 401 ; of sight,
525 ; of skeleton, 90; of skin,
448; of supporting tissues, 106.

Hyoid bone, 76.

Hypermetropia, 525.

Hypoglossal nerve, 175.

Ideas, association of, 631.

Idio-retinal light, 531.

Ileum, 339.

Ileo-colic valve, 342.

Iliac artery, 220.

Ilium, 77.

Illusions, sensory, 502.

Images, after, 548.

Impulse, cardiac, 228.

Impregnation, 662.

Impulse, nervous, 203.

Incus, 558.

Indigestion, 377.

Inert layer, 235.

Inferior laryngeal nerve, 420.

Inferior maxillary nerve, 174.

Inferior mesenteric artery, 220.

Inferior vena cava, 215.

Inflammation, 280.

Infundibuluin, 171.

Inhibition of reflexes, 606.

Inhibitory nerves, 190.

Innervation sensations, 591.

Innominate artery, 219.

Innominate bone, 77.

Innominate vein. 223.

Inogen, 141.

Inorganic constituents of Body, 13;
foods, 320.

Inosit, 13.

Inspiration, how effected, 385.

Intensity of sensations, 499.

Interarticular cartilage, 104.

Intercostal arteries, 220.

Intercostal muscles, 388.

Internal ear. 559.

Internal medium, 40.

Internal respiration, 411.

Intervertebral disks, 92.

Intervertebral foramina, 71.

Intestinal digestion, 372; move-
ments, 378.

Intestines, 339.

Intrinsic heart-nerves, 257, 263.

Intussusception, 18.

Iris, 510.

Irritability, 23.

Irritability, muscular, 128.

Irritable tissues, 33.

Ischium, 77.

Iter, 618.

Jaw-bones, 75.

Jejunum, 339.

Jelly-like connective tissue, 103.

Joints, 92.

Jugular vein, 223.

Karyokinesis, 18.
Karyoplasm, 19.
Katabolism, 22.
Kidneys, 427.
Kinetic energy, 303.
Knee-cap or knee-pan, 79.
Kreatin, 11, 123, 460.

Labyrinth, 560.
Lachrymal apparatus, 507.
Lachrymal bone, 76.
Lactation, 6ti6.
Lacteals, 44, 341.
Lactose, 13.

Lacunae, lymphatic, 350.
Lamina spiral is, 561.
Large intestine, 342, 374.
Laryngeal nej ves, 420.
Larynx, 634.

INDEX.

679

Laughing, 426.

Law, the psycho-physical, 499.

Leaping, 153.

Least-resistance hypothesis, 603.

Lecithin, 13, 321.

Lens, crystalline, 516.

Lenses, refraction of light by, 520.

Lenticular nucleus, 619.

Leucin, 368, 462.

Leucocytes, 49.

Levers in the Body, 145.

Lieberkiihn, crypts of, 341.

Liebig's classification of foods, 453.

Liebig's extract, 125.

Ligament, round, 93.

Ligament, suspensory, of lens, 516,

Ligaments, 93.

Light, dispersion of, 519.

Light, properties of, 516.

Light, refraction of, 518.

Limbs, 7.

Limbs, skeleton of, 77.

Liquid extract of meat, 125.

Liquor sanguinis, 44.

Liver, 344; glycogenic function of,

466; histology of, 346, 467.
Local sign of sensations, 492.
Local temperatures, 484.
Localization of cerebral functions,

623.

Localizing powers of retina, 539.
Localizing power of skin, 580.
Locomotion, 151.
Locus niger, 616.
Long saphenous vein, 222.
Long sight, 525.
Losses of energy daily, 301, 480.
Losses of material from Body, 299.
Lower maxilla, 75.
Lumbar plexus, 165.
Lumbar vertebrae, 69.
Lungs, 383. ^
Lungs, capacity of, 391.
Luxus consumption, 459, 462.
Lymph, 42 ; canaliculi, 103, 351 ;

chemistry of, 62; hearts, 353;

histology of, 49; lacunas, 350;

movement of, 353, 397; renewal

of, 42; vessels, 43, 350.
Lymphatic glands, 352.
Lymphatic system, 349.
Lymphoid tissue, 49, 351.

Macula lutea, 511,
Malar bone, 76.
Malleus, 558.

Malpighian corpuscles of spleen,
357.

Malpighian layer of epidermis, 441,
Malpighian pyramids of kidney,

429.

Maltose, 362.
Mammalia, 4.
Mandible, 75.
Manometer, 267.
Marrow of bone, 87.
Marrow, spinal (spinal cord), 160,

181, 594.

Material daily losses of Body, 299.
Maxilla, 75.
Measurement of arterial pressure,

266.

Meatus, external auditory, 557.
Mechanisms, physiological, 38.
Media, refracting, in eye, 515.
Median posterior tract, 598.
Medulla oblongata, 167, 610.
Medullary cavity, 87.
Membrane, basilar, 561; of Desce-

met, 510; of Krause, 119; refic-

ular, 563; synovial, 93; tectorial,

563; tympanic, 558.
Menstruation, 658.
Mesentery, 339.
Metabolic tissues, 33, 288.
Metacarpus, 77.
Metatarsus, 79.
Microscopic anatomy, 2, see Histol-

Mid-brain, functions of, 616.

Midriff, see Diaphragm.

Migration, 281.

Milk, 322; for infants, 667.

Millon's test, 9.

Mitosis, 18.

Mixed diet, advantage of, 325.

Modality of sensation, 492, 494.

Modiolus, 561.

Monro, foramen of, 171, 619.

Morula, 29.

Motion, 144.

Motor area of cortex, 623.

Motor organs, 109.

Motor tissues, 3o.

Motores oculi, 172.

Mouth, 329.

Movements, associated, 63, 593 ;
voluntary, 622.

Movements, intestinal, 378 ; re-
spiratory, 385.

Mucin, 11.

Mucous layer of epidermis, 441.

Mucous membranes, 6, 339, 341.

Mulberry mass, 29.

Mumps, 334.

Muscse volitantes, 529.

Muscle, biceps, 113; cardiac, 123,

680

INDEX.

255, 262 ; ciliary, 516; stape-
dius, 559; tensor tyinpani, 559.

Muscles, chemistry of, 123, 457;
histology of, 117, 122; of eyeball,
507, of larynx, 637; of respi-
ration, 387; physiology of, 127;
skeletal, 113; structure of, 112,
117; visceral, 122.

Muscle spindle, 121.

Muscular contraction, 130.

Muscular energy, source of, 140.

Muscular sense, 591.

Muscular" ttsSueT" 35, 117, 123,
127.

Muscular work, 135, 454.

Myocardium, 256.

Myopia, 525.

Myosin, 10, 124.

Myosinogen, 124.

Nails, 445.

Nasal bone, 76.

Negative variation, 139, 199.

Nerve-cells, 179.

Nerve-centres, 158, 181, 594.

Nerve-fibres, 35, 176.

Nerve-fibres, classification of, 191.

Nerve plexuses, 158.

Nerve stimuli, 194.

Nerve trunks, 158.

Nerves, 158; accelerator or aug-
nientor, 270; cranial, 172, 207;
cardiac, 253, 264 ; laryngeal,
420 ; optic, 512 ; respiratory,
414; secretory, 292; spinal, 163;
sympathetic, 160, 175; thermo-
genic, 484; trophic, 192, 295;
vaso- constrictor, 273; vaso-dila-
tor, 279, 293; vaso-motor, 273,
280.

Nervous impulses, 203.

Nervous system, anatomy of, 158.

Nervous system, physiology of,
186.

Neural tube (dorsal cavity), 5.

Neurilemma, 177.

Neuroglia, 180.

Nitrogenous compounds in body,
9.

Nodal points of eye, 530.

Noises, 564.

Non-vascular tissues, 41.

Notes, musical, 564.

Nuclear spindle, 20.

Nuclein, 27.

Nucleo-albumins, 28.

Nucleolus, 17, 19.

Nucleoplasm, 19.

Nucleus, 17, 20.

Nucleus, caudate, 619; lenticular,

619.

Nucleus, red, of mid-brain, 617.
Nutrition, 451.
Nutrition of embryo, 664.
Nutritive tissues, 32.
Nystagmus, 614.

Occipital bone, 75.
Oculo-motor nerves, 172.
Odontoid process, 95.
Odorous bodies, 588.

(Esophagus, 336.

Olecranon, 81.

Olein, 12.

Olfactory lobe, 167.

Olfactory nerves, 172.

Olfactory organs, 588.

Omentum, 339.

Ophthalmic nerve, 174.

Optical defects of eye, 526.

Optic commissure, 512.

Optic nerves, 172, 512.

Optic thalarni, 167, 170, 618.

Optic tracts, 512.

Organ of Corti, 562.

Organs, 1, 36; of animal life, 110;
of circulation, 211; of common
sensation, 587; of digestion, 328;
of movement, 109; of relation,
110; of reproduction, 647; of
respiration, 380; of secretion,
282; of special sense, 490, 493;
urinary, 427; of vegetative life,
110.

Os calcis, 79.

Os innominatum, 77.

Os orbiculare, 559.

Os pubis, 79.

Osmazome, 321.

Ossicles, auditory, 558.

Otoliths, 564.

Oval foramen, 558.

Ovary, 652, 655.

Over-tones (upper partial tones),
568.

Ovulation, 658.

Ovum, 29, 656.

Oxidation by stages, 309.

Oxidations in the body, 307, 451,
454.

Oxygen in the blood, 407.

Oxygen consumed daily, 400.

Oxyhaemoglobin, 405.

Oxyntic cells, 338.

Pacinian bodies, 576, 578.
Pain, 585.
Palate, 329.

INDEX.

681

Palate bones, 75.

Palmatin, 12.

Pancreas, 290, 346, 358.

Pancreatjcjsecretion , 368.

PapiliifSyiiiusclesT 217, 230.

Papillae of tongue, 334.

Paraglobulin, 10. 54.

Parapeptone, 366.

Paraplasm, 26.

Parietal bone, 75.

Parotid gland, 296, 334.

Partial tones. 568.

Parturition, 664.

Patella, 79.

Patbeticus, 173.

Pathology, 1.

Peas, 323.

Pectoral arcb, 77.

Pelvic arch, 77.

Pendular vibrations, 565.-

Pepsin, 365.

Peptones, 10, 365, 372.

Perceptions, 500 ; visual, 552 ;
auditory, 574.

Pericardium, 213.

Perichondrium, 98.

Perilymph, 560.

Perimysium, 117.

Perineurium, 177.

Periosteum, 85.

Peripheral reference of sensations,
491, 501.

Peristaltic movements, 365.

Peritoneum, 5, 337.

Pes of crus cerebri, 616.

Pettenkofer's test, 370.

Peyer's patches, 352.

Phagocytes, 48.

Phalanges of fingers and toes, 77,
79.

Pharynx, 335.

Phrenic nerve, 165, 390.

Physiological chemistry, 8.

Physiological mechanisms, 38.

Physiological properties, 16.

Physiology, 1 ; of blood-vessels,
227; of brain, 609; of connective
tissues, 102; of digestion, 361:
of ear, 564, 571; of eye, 530; of
heart, 253; of kidneys, 435; of
muscles, 127, 144 ; of nerves,
186; of nerve-centres, 186, 594;
of nutrition, 451; of respiration,
385, 398, 414; of skin, 446, 578;
of smell, 587; of spinal cord,
594; of taste, 589; of touch, 578. ,

Pia mater, 160.

Pineal gland, 171.

Pitch of notes, 564.

Pitch of voice, 640.

Pituitary body, 171, 359.

Pivot-joints, 95.

Placenta, 665.

Plain muscular tissue, 123, 142.

Plastic foods, 453.

Platelets, or plaques of blood, 49.

Pleura. 5, 383.

Plexus, 158; brachial, 164; car-

diac, 176; cervical, 164; choroid,

169; lumbar, 165; sacral, 166;

solar, 176.
Pneumogastric nerves, 174, 264,

419.

Polar globules, 660.
Pons Varolii, 167, 613.
Popliteal artery, 220.
Portal circulation, 223, 347.
Portal vein, 343.
Posterior tibial artery, 220.
Postures, 149.
Potatoes, 323.
Potential energy, 303.
Pregnancy, 662.
Presbyopia, 526.
Pressure, arterial, 242.
Pressure, intra-thoracic, 384.
Pressure sense, 578.
Primates, 2.

Production of heat in body, 477o
Pronation, 96.
Proofs of circulation, 251.
Prostate, 649.
Protective tissues, 36.
Proteids, 9, 314, 453.N
Proteids, oxidation of, 455.
Protoplasm, 19, 26.
Psychical activities of cord, 607.
Psycho-physical law, 499.
Ptosis, 509.
Ptyalin, 362.
Puberty, 669.
Pubis, 77.

Pulmonary artery, 214.
Pulmonary circulation, 223.
Pulmonary veins, 216.
Pulse, 246.

Purkinje's experiment, 533.
Pus, 48.

Pylorus, 336, 338.
Pyramidal tracts, 597.
Pyramids of Malpighi, 429.
Pyrexia, 485.

Qualities of sensation, 491.
Quantity of air breathed daily, 392.
Quantity of blood, 61.
Quantity of food needed daily, 326,
476.

682

INDEX.

Radial artery, 219.

Radio-ulnar articulation, 95.

Radius, 77.

Range of voice, 640.

Rate of blood now, 248.

Receptaculum ckyli, 350.

Receptive tissues, 32.

Rectum, 342.

Red blood-corpuscles, 44, 60.

Reduced haemoglobin, 405.
-Jjeflex actions, 188, 600.

Reflex convulsions, 602.

Reflex time, 608.

Reflexes, acquired, 613.

Reflexes, inhibition of, 606.

Refracting media of eye, 515.

Refraction of lenses, 520.

Refraction of light, 518.

Refraction in the eye, 521.

Regulation of temperature, 482.

Renal artery, 220, 427.

Renal organs, 427.

Renal secretion, 432.

Rennet, 365.

Reproduction, 22, 644.

Reproductive tissues, 36.

Residual air, 392.

Resistance theory, 419.

Resonance, sympathetic, 569.

Respiration, 22, 380.

Respiration chemistry of, 398.

Respiration, nerves of, 414.

Respiratory centre, 414.

Respiratory foods, 453.

Respiratory movements, 385; influ-
>mce of, on circulation and on

/flow of lymph, 394.
/Respiratory sounds, 391.
jr Reticular membrane, 563.

Reticulum of cells, 19.

Retiform (adenoid) connective tis-
sue, 103.

Retina, 511, 514.

Rhythmic movements, 417.

Ribs, 72.

Rib cartilage, 72.

Rice, 323.

Right lymphatic duct, 350.

Rigor mortis, 123, 457.

Rods and cones, 513, 532.

Rolando, fissure of, 623.

Round foramen, 558.

Running, 152.

Sacculus, 561.
Sacral plexus, 166.
Sacral vertebrae, 69.
Sacrum, 69.
Saliva, uses of, 361.

Salivary glands, 363.

Salivary glands, nerves of, 293.

Salivin (ptyalin), 362.-

Santorini, cartilages of, 635.

Sarcolactic acid, 13, 123.

Sarcolemma, 118.

Sarcoinere, 119.

Sarcoplasm, 120.

Sarcous element, 120.

Sarcosome, 120.

Sarcostyle, 119.

Scalae of cochlea, 561.

Scalene muscles, 388.

Scapula, 77.

Sciatic nerve, 166.

Sclerotic, 509.

Sebaceous glands, 446.

Secondary (acquired) reflexes, 613.

Secretion, 282.

Secretion, cutaneous, 447.

Secretion, renal, 432.

Secretory nerves, 292.

Secretory tissues, 32, 283.

Sections of Body, 6.

Segmentation of ovum, 29.. *

Segmentation-of skeleton, 67.

Semicircular canals, 560, 563, 574,
615.

Semilunar valves, 217.

Sensation, 488; color, 541; com-
mon, 490, 585; intensity of, 499;
of equilibrium, 614; of hunger,
587; special, 490; of thirst, 587;
pain, 585; peripheral reference
of, 491, 501; qualities of, 491.

Sense-organs, 493.

Sense, muscular, 591; of hearing,
557; of pain, 585; of sight, 530;
of smell, 587; of taste, 589; of
temperature, 582; of touch, 576.

Sensory illusions, 502.

Serous or lymph canaliculi, 103,
351.

Serous cavities, 351.

Serous membranes, 5.

Serum, 51, 59.

Serum albumin, 9, 59.

Shin-bone, 79.

Shine, 555.

Shingles, 192.

Short sight, 525.

Shoulder-blade, 77.

Shoulder-girdle, 77.

Sighing, 425.

Sight, sense of, 530.

Sight, hygiene of, 525.

Sigmoid flexure, 342.

Single vision, 553.

Size, perception of, 553.

INDEX.

683

Skeleton, 63 ; appendicular, 77 ;
axial, 64, 67 ; of face, 75 ; of
skull. 72; peculiarities of human,
83; of thorax, 386.

Skin, 5, 441; glands of, 446; hy-
giene of, 448; nerve endings iu,
576

Skull, 72.

Small intestine, 339.

Smell, 587.

Sneezing, 426.

Solar plexus, 176.

Solar spectrum, 519.

Solidity, visual perception of, 554.

Soluble ferments, 11.

Sounds, 564.

Sounds of the heart, 230.

Sounds, respiratory, 391.

Source of animal heat, 479.

Source of fats, 472.

Source of glycogen, 468.

Source of muscular work, 454.

Source of urea, 460.

Sources of energy to Body, 304.

Special senses, 490.

Specific elements, 283.

Specific nerve energies, 197.

Spectacles, 526.

Speech, 633.

Spermatozoa, 651.

Sphenoid bone, 75.

Spherical aberration, 527.

Spinal column, 71.

Spinal cord, 161, 594 ; conduction
in, 594 ; functions of, 187, 594,
600 ; histology of, 181 ; mem-
branes of, 160; psychical activi-
ties of, 607.

Spinal access9ry nerve, 174.

Spinal marrow, see Spinal cord.

Spinal nerves. 163.

Spinal nerve-roots, 163, 206.

Spleen, 356.

Spongioplasm, 19.

Spontaneity, "25.

Sporadic ganglia, 176.

Sprains, 97.

Squinting, 509.

Stages of life, 670.

Stapedius muscle, 559.

Stapes, 558.

Starch, 323; digestion of , 362, 367.

Starvation, proteid, 463.

Stationary air, 392.

Stearin, 12.

Stereoscopic vision, 555.

Sternum, 73.

Stimuli, muscular, 128 ; nervous,
194.

Stimulus, 24.

Stirrup-bone, 558.

Stomach, 122, 336.

Stomata, lymphatic, 351.

Storage tissues, 33, 464.

Strabismus (squinting), 509.

Structure of bone, 85.

Strychnine poisoning, 602.

Subclavian artery, 219.

Subcutaneous areolar tissue, 442.

Sublingual gland, 336.

Submaxillary gland, 293, 334.

Succus entericus, 371.

Sudoriparous glands, 446.

Superior laryngeal nerve, 420.

Superior maxillary nerve, 174.

Superior mesenteric artery, 220.

Supination, 96.

Supplemental air, 392.

Supporting tissues, 32, 105.

Supra-renal capsules, 359.

Sutures, 92.

Swallowing, 363.

Sweat, 446.

Sweat-glands, 446.

Sweat-glands, nerves of, 292.

Sweetbread, 290, 346, 358.

Sympathetic nervous system, A5,
160, 175.

Sympathetic resonance, 569.

Sympathetic resonance in ear, 573.

Synovial membranes, '93.

Syntonin, 125.

System, alimentary, 328 ; circula-
tory, 211; muscular, 116, 127;
nervous, 158, 186 ; osseous, 63 ;
respiratory, 380 ; renal, 427.

Systemic circulation, 223.

Systems, anatomical, 39.

Tactile organs, 576.

Taking cold, 278.

Tarsus, 79.

Taste, 589.

Taste-buds. 589.

Taurocholic acid, 12.

Tear-glands, 507.

Tectorial membrane, 563.

Teeth, 329.

Teeth, structure of, 331.

Tegmentum, 616

Temperature of Body, 478.

Temperature, bodily, regulation of,

482.
Temperature, influence of, on pulse

rate, 271.

Temperature sense, 582.
Temperatures, local, 484.
Temporal artery, 219.

684

INDEX.

Temporal bone, 75.

Tendons, 113.

Tension of blood gases, 407.

Tensor tyinpani muscle, 559.

Testis, 648.

Tests for proteids, 10.

Tetanus, 133, 140.

Thalarnencephalon, 171, 618.

Theory, resistance, 419.

Theory of color vision, 542, 548.

Thermogenic nerves, 484.

Thigh-bone, 78.

Thirst, 587.

Thoracic duct, 350.

Thoracic vertebrae, 66.

Thorax, aspiration of, 251, 393 ;
contents of, 5.; movements of, in
respiration, 385 ; skeleton of,
387.

Throat, 335.

Thyroid body, 357.

Thyroid cartilage, 634.

Thyroid foramen, 78.

Thymus, 358.

Tibia, 79.

Timbre, 564.

Tissues, 1 ; adenoid, 103; adipose;
107 ; areolar, 100 ; assimilative,
32; automatic, 34; bony, 87; car-
tilaginous, 100; classification of,
31; conductive, 35; connective,
63, 100; contractile, 35, 117; co-
ordinating, 34, 594; elastic, 102;
eliminative, 32; excretory, 32; ir-
ritable, 33; jelly-like connective,
103, lymphoid; 49; metabolic, 33,
288 ; motor, 35, 121 ; muscular,
35, 127; nervous^ 176; nutritive,
32 ; plain '"muscular, 123, 142 ;
protective, 36; receptive, 32; re-
productive, 36; respiratory, 33;
retiform or adenoid, 103 ; socre-
tory, 32, 283 ; storage, 33, 464 ;
supporting, 32; undifferentiated,
32 ; white fibrous, 100.

Tissue-forming foods, 314, 452.

Tone, sensations of, 564, 568.

Tone color (timbre), 564.

Tongue, 332.

Tonsil, 335.

Touch-organs, 576.

Touch, sensations of, 578.

Trachea, 382.

Tracts of degeneration in spinal
cord, 596.

Training, 157.

Transudata, 283.

Trigeminal nerve, 173.

Trophic nerves, 192, 295.

Trypsin, 289, 369.
Tunica adventitia, 225.
Turbinate bones, 76.
Tympanic bones, 557, 571.
Tympanic membrane, 557, 571.
Tyrein, 10.
Tyrosin, 368, 462.

Ulna, 77.

Ulnar artery, 219.

Undifferentiated tissues, 32.

Upper maxilla, 75.

Urea, 11, 434, 460.

Ureter, 427.

Uric acid, 11, 434.

Urinary organs, 427.

Urine, 432.

Uterus, 652.

Utilization of energy in Body, 310.

Utriculus, 561.

Uvula, 329.

Vagus nerve,

Valve, ileoco

Valves, auriculo-ventricular, 217 ;
of veins, 226; semilunar, 217.

Valvulae conniventes, 339.

Vaso- constrictor centre, 276.

Vaso-dilator centre, 280.

Vaso-dilator nerves, 273, 279, 293.

Vaso-motor nerves, 273, 280.

Vegetable foods, 322.

Veins, 220, 226; cephalic, 222;
coronary, 215; hepatic, 345; in-
nominate, 223; jugular, 223; long
saphenous, 222; portal, 224, 345;
pulmonary, 216.

Velum interposituin, 169.

Vena cava, 215. ""-

Venous blood, 225.

Ventilation, 401.

Ventral cavity, 3, 6,

Ventricles of brain, 168; of larynx.
635.

Vermicular (peristaltic) move-
ments, 365, 378.

Vermiform appendix, 342.

Vertebra?, 64; cervical, 68; coccy-
geal, 70; dorsal or thoracic, 66;
lumbar, 69; sacral, 69.

Vertebral artery, 219.

Vertebral column, 3, 64, 71.

Vertebral foramen, 68.

Vertebrata, 3.

Vestibule, 560, 561, 574.

Vibrations, analysis of, 568; com-
position of, 567; pendular, 565;
sonorous, 564.

Villi of intestine, 341.

INDEX.

685

Vision, color, 541.
Vision, purple, 511, 535.
Vision, stereoscopic, 555,
Visual axis, 541.
Visual contrasts, 547.
Visual perceptions, 551.
Visual sensations, 530, 536; dura-
tion of, 539; intensity of, 536.
Vital capacity, 392.
Vitreous humor, 516.
Vocal chords, 635.
Vocal chords, false, 635.
Voice, 633.

Voluntary movements, 622.
Vorner, 75.
Vowels, 640.

Walking, 151.
Wallerian method, 210, 596.
Wandering cells, 103.
Warm-blooded animals, 477.
Water, constituent, 27.

Water, percentage of, in body, 13.
Weber's law, 499.
Weber's schema, 240.
Weissman's theory of heredity,

661.

Wheat, 323.
Whipped blood, 52.
Whispering, 643.
White blood-corpuscles. 17, 47, 60.
White fibrous tissue, 101.
Windpipe, 382.
Work, muscular, 135.
Wrisberg, cartilage of, 635.
Wrist, 77.

Xantho-proteic test, 9.

Yawning, 425.

Yellow spot, 511

Young's theory of color vision,

542.

Zoological position of man, 2.

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