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OUTLINES OF PHYSIOLOGY

JONES AND BUNCE

Cranium.

7 Cervical Vertebrae.

Clavicle.
Scapula.

Humerus.

Ilium.

Ulna.
Radius.

Pelvis.

Bones of the Carpus.
Bones of the Meta-
carpus.
Phalanges of Fingers.

Fatella.

Tibia.
Fibula.

Bones of the Tarsus.
Bones of the Metatarsus.
Phalanges of Toes.

THE SKELETON (AFTER HOLDEN).

OUTLINES

PHYSIOLOGY

EDWARD GROVES JONES, A.B., M.D., F.A.C.S.

PROFESSOR OF SURGERY, EMORY UNIVERSITY
(ATLANTA MEDICAL COLLEGE)

AND

ALLEN H. BUNCE, A.B., M.D.

ASSOCIATE IN MEDICINE, EMORY UNIVERSITY
(ATLANTA MEDICAL COLLEGE)

FOURTH EDITION, REVISED
in ILLUSTRATIONS

PHILADELPHIA

P. BLAKISTON'S SON & CO.

1012 WALNUT STREET

, • * -: c ^ • J

>,» - • e -• •- * s

TO
DOCTOR WILLIAM S. KENDR1CK

SENIOR PROFESSOR OF MEDICINE, EMORY UNIVERSITY
(ATLANTA MEDICAL COLLEGE)

THESE PAGES ARE AFFECTIONATELY DEDICATED

'Oi^v-41 r~***j

o7157

PREFACE TO FOURTH EDITION

IN preparing this revision the majority of changes have
been of details, the chapters and general arrangement of the
book have been kept as in the third edition. A number of
new illustrations have been added and others have been re-
engraved. Every effort has been made to bring the subject
matter up to date and to keep the book up to the highest
standard in every way.

We appreciate the reception which has been accorded the
previous editions and hope that this will prove even more
valuable to the student and practitioner than those which

have preceded it.

EDWARD G. JONES.

ALLEN H. BUNCB.
ATLANTA, GA.

PREFACE TO FIRST EDITION

THIS volume has been prepared with the view of present-
ing, in as convenient form as possible, the essential facts of
modern physiology as related to the practice of medicine. In
the execution of this purpose brevity has been made a prime
consideration; therefore, such details as are of secondary
importance are omitted, theories are avoided, and conclusions
are recorded without argument. There is no short road to
knowledge, and it would be unfortunate should such a book
as this in any way discourage extended research; but stu-
dents in college have none too much time to devote to any one
subject, and any simple collection of pertinent facts, however
brief, can, if reliable, be used to great advantage. I have en-
deavored, however, to make the work sufficiently exhaustive
to be self-explanatory, believing that otherwise economy of
expression is practised at the expense of the reader's interest.

A maximum of space has been given to those subjects
which seem of most practical importance. The chemistry of
the body, the special senses and embryology have not been
treated in great detail. It has been thought undesirable to
omit a brief anatomical description of the separate organs
discussed.

In the preparation of this volume no claim to original in-
vestigation is made. The writings of various authorities
have been freely drawn upon. Especial acknowledgment
is due to the following authors: Howell (American Text-
book), Halliburton (Kirkes5 Handbook), Flint, Verworn
and Stewart.

I am under obligations to Dr. J. Clarence Johnson, whose
lectures have been of great value to me, and to Dr. Frank K.
Boland, who has written the whole of Chapter II., read the
proof sheets, and rendered other valuable assistance in con-
nection with the work. E. G. J.

ATLANTA, GA.

CONTENTS

PAGE

INTRODUCTION xv

CHAPTER I.

THE CELL - . . i

CHAPTER II.

THE ELEMENTARY TISSUES . . 7

The epithelial tissues 7

The connective tissues n

The muscular tissues 19

The nervous tissues 22

CHAPTER III.

PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 23

CHAPTER IV.

SECRETION 27

Sebaceous glands 3°

Mammary glands • 31

Thyroid gland 32

Adrenal glands 33

Pituitary body 34

Testis and ovary 34

CHAPTER V.

THE BLOOD 35

CHAPTER VI.

THE CIRCULATION OF THE BLOOD 41

The heart . . 42

Circulation in Wood- vessels . . . . . .... . . . 46

Xll CONTENTS

PAGE

Structure of the blood-vessels 47

The lymph 57

CHAPTER VII.

THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 63

Foods 63

Digestion ' . 68

Prehension 70

Digestion in the mouth 71

The salivary glands and their secretion 71

Deglutition 78

Digestion and absorption in the stomach 81

The gastric glands 85

Digestion and absorption in the intestines 96

The small intestine 96

The large intestine 117

Absorption in general 122

Absorption from the alimentary canal 126

CHAPTER VIII.

RESPIRATION 131

Anatomy of the respiratory organs 132

Mechanism of respiration 139

CHAPTER IX.

NUTRITION, DIETETICS AND ANIMAL HEAT 171

Nutrition 171

Dietetics 181

Animal heat 184

CHAPTER X.

EXCRETION 192

The kidneys 192

The skin 208

CHAPTER XL

THE NERVOUS SYSTEM 214

The cerebro-spinal axis 235

CONTENTS Xlll

PAGE

The spinal cord 238

The encephalon 251

The medulla oblongata 251

The pons varolii 255

The crura cerebri 256

The cerebrum 260

The cerebellum 274

The cranial nerves 276

The spinal nerves 296

The sympathetic system 298

CHAPTER XII.

THE SENSES 305

Common sensations 305

Special sensations 306

The sense of touch 306

The sense of smell 307

The sense of sight 308

The sense of taste . . . .' 317

The sense of hearing 318

The production of the voice 325

CHAPTER XII F.

REPRODUCTION 328

INDEX 365

INTRODUCTION

THE science which treats of the structure, function and
organization of living forms, both vegetable and animal, is
called biology. That branch of biology which describes ani-
mal life exclusively is termed zoology, while that branch
which describes vegetable life exclusively is termed botany.

The study of the form of organisms, both vegetable and
animal, is termed morphology. Morphology is further di-
vided into ( i ) histology, which treats of the formed elemen-
tary constituents of organisms, and (2) anatomy, which
treats of the parts and organs of the organism.

After the form and structure of an organism has been
studied, the next step is the study of the work which the or-
ganism has to perform. This study of the vital phenomena,
or life, of the organism is called physiology. Physiology
may be either animal or vegetable. Human physiology is
that branch of physiology which treats of the vital phenom-
ena occurring in man.

The structural unit of the body is the cell. Myriads of
cells are grouped together to form organs. An organ may be
defined as a group of cells combined together to perform
some special function, e. g., the kidney is an organ whose
special function is the secretion of urine. The organs are
further grouped together to form systems. Thus we have
the circulatory system composed of the heart, arteries, veins,
and capillaries. Now the study of the function which the
circulatory system has to perform is the physiology of circu-
lation. Likewise we may subdivide physiology into the phy-

XVI INTRODUCTION

siology of the nervous system, the physiology of the digestive
system, the physiology of the respiratory system, etc.

Thus we have seen the relation physiology bears to the
other sciences dealing with life and the special part human
physiology plays in the whole physiology. . In the following
pages we have endeavored to give a brief outline of human
physiology.

OUTLINES OF PHYSIOLOGY

CHAPTER I./'*

THE CELL.

ALL the tissues of the body are made up of cells and inter-
cellular substance. All the cells are descended from one
parent cell, called the ovum, while the intercellular substance
is created through the medium of the cells.

Nuclear
membrane.

Linin. .,.

Nuclear fluid
(matrix).

Nucleolus.

Chromatin cords

(nuclear

network).

Nodal enlarge-
ments of the
chromatin.

Cell membrane.
~\ - ----• Exoplasm.

Microsomes.
Centrosome.

-* Spongioplasm.

y Hyaloplasm.

--i»—- Foreign inclosures.

FIG. i. — Diagram of a cell.

Microsomes and spongioplasm are only partly drawn. (Brubaker.)

A cell, which is the histologic unit of the body, may be de-
nned as an irregular round or oval mass of protoplasm of mi-
croscopic size, enclosing a small indistinct spherical body, the
nucleus.

2 THE CELL

The essential parts of the cell are, (i) the cytoplasm,
which is, a special name given to the protoplasm forming the
cell-body and (2) the nucleus, which is a small round or
oval body embedded in the cytoplasm. A great many cells
are surrounded by- a cell-wall or cell-membrane, but this
c;aaiftot be. regarded ;as one of the essential elements since all
cells do not possess such membranes.

-,*(i)' The Cytoplasm. — This is a gelatinous or semi-fluid,
granular substance, transparent and generally colorless.
Chemically it consists of water and salts, together with vari-
ous organic substances, called proteids, which are complex
combinations of carbon, hydrogen, oxygen and nitrogen, and
sometimes phosphorus and sulphur. The proteids of the cy-
toplasm contain little phosphorus, while those of the nucleus
are rich in it.

The cytoplasm does not always present the same structural
appearance since its constituents vary in their condition and
arrangement. In some cells it has a clear homogeneous ap-
pearance, while in others it contains fine spherical particles
which give it a granular structure. When these granules are
large and clear, and are surrounded by denser areas they give
to the cytoplasm an alveolar outline. But most frequently
the cytoplasm contains in its structure a mesh work of threads
or fibrils which give it a reticular appearance. This network
of fibrils is called the spongioplasm which encloses a less firm
portion, the hyaloplasm ( Fig. i ) .

However,, in all these varieties, the cytoplasm has both an
active and a passive structure. In young granular cells the
active substance is represented by small spherical particles,
called microsomes (Fig. i). These are not always evenly dis-
tributed throughout the cytoplasm, but are grouped in an area
near the nucleus, while the area next the cell-wall is almost
free from granules. The dense inner area is called the endo-
plasm, while the clear outer area is called the exoplasm

(Fig. i).

(2) The nucleus, which is the second essential part of a

THE VITAL PHENOMENA OF CELLS 3

typical cell, is a small round or oval body contained within
the cell. It is usually surrounded by a distinct nuclear mem-
brane, except during division. Nuclei play an important role
in cell reproduction and cell nutrition. They are character-
ized by their affinity for certain stains, e. g.} hematoxylin.

The substance of the nucleus, the karyoplasm, may be di-
vided into two parts — the nuclear fibrils which form an ir-
regular reticulum, and the nuclear matrix which forms the
intervening semi-fluid mass. The nuclear fibrils, when prop-
erly stained, are found to consist of minute irregular masses
of a deeply colored substance, called chromatin, in recogni-
tion of their affinity for certain stains. The chromatin par-
ticles are supported within delicate and colorless threads of
linin. The nuclear matrix, which is semi-fluid in character
and which occupies the spaces between the nuclear fibrils,
possesses a very weak affinity for the stains used to color the
chromatin. Hence, it usually appears clear and untinted.
Chemically, the chromatin contains a substance nuclein,
which is rich in phosphorus.

The nucleolus ordinarily appears as a small spherical mass
among the nuclear fibrils. It is supposed to be of little sig-
nificance in so far as the vital phenomena of the cell are con-
cerned.

The Vital Phenomena of Cells. — The vital phenomena of
the cell include all those processes and changes which it un-
dergoes during its life and which take place in the perform-
ance of its various functions. They include (i) metabolism,
(2) growth, (3) reproduction and (4) irritability.

(i) Metabolism, includes all those processes by which the
cell is enabled to select from the various substances furnished
it and convert them into its own substance, and, secondly,
those processes whereby the cell is enabled to cast off the
waste products set free by its activity. The first process, that
by which the cell takes the simple substances furnished it and
converts them into its .complex compounds, is called anabol-
ism, or constructive metabolism.

4 THE CELL

The second process, that by which the cell breaks up these
complex compounds formed by anabolism and discharges
them from its substance, is called katabolism, or destructive
metabolism. A good example of anabolism is that by which
the vegetable cells take such substances as carbon dioxide,
water and inorganic salts and prepare food-material for the
nutritive and katabolic processes in animals.

(2) Growth is the natural sequence of the nutritive
changes effected by metabolism and may be unrestricted and
equal in all directions. However, this is not usually the case
as is shown by the fact that cells are so intimately associated
with other structural elements as to influence and modify
their growth. These result in unequal growth, to which
the specialization of cells is due. Examples of the unequal
growth of cells are shown in the columnar cells of epithe-
lium, the neurones of nervous tissue, the fibers of muscle
tissue, etc.

(3) Reproduction may be regarded as the culmination of
the activities of the cell, for by this process the cell loses its
individuality and continues its life in that of its offsprings.
There are two methods by which cells may reproduce them-
selves, (i) by direct cell division or amitosis and (2) by
indirect cell division or mitosis.

(4) Irritability is that property of cells whereby they are
enabled to respond to stimuli, i. e., to change their form and
shape in response to these* stimuli. The various stimuli
which affect cells may be mechanical, thermal, nervous,
chemical or electrical.

Cell Division. — (i) Direct cell division, or amitosis, is the
simplest form of cell division. In this form of cell division
the nucleus and protoplasm constrict in the middle until two
new cells are formed. This form of cell division does not
occur in the higher animals except as a secondary process.

(2) In the higher animals cell division takes place chiefly
by the indirect or mitotic method. This may be described

CELL DIVISION

briefly as follows : In the beginning of this phenomenon, the
nucleus, which plays the most important role, grows larger.
Its chromatin greatly increases, becomes contorted so as
to form a dense convolution, the close skein, or spireme.
Then the chromatin fibrils further thicken, become less con-
voluted and form irregularly arranged loops, the loose skein.

Close Skein

(viewed from the

side); Polar field.

Loose Skein

(viewed from above — i. e., from
the pole).

Mother Stars

viewed from

the side).

Mother Star Daughter Star
(viewed from above).

Beginning. Completed.

Division of the Protoplasm.

FIG. 2. — Karyokinetiq figures observed in the epithelium of the oral
cavity of a salamander.

The picture in the upper right-hand corner is from a section though a dividing
egg of Siredon pisciformis. Neither the centrosomes nor the first stages of the
development of the spindle can be seen by this magnification. X 560. (From
Brubaker.)

During the formation of these skeins the nuclear membrane
and the nucleoli disappear. The fibrils of the loose skein now
separate at their peripheral turns into a score of loops, the
closed ends of which point toward a common center — a clear
space called the polar Held. When seen from above these
loops of chromatin make a wreath, called the mother wreath;

O THE CELL

when seen from the side, they make a star, called the mother
star or aster. While the loose skeins are forming, delicate
striae appear within the achromatin, so disposed as to make
their bases within the polar field and directed toward one
another, and their apices directed toward the future new nu-
clei. These achromatin figures constitute the nuclear
spindle. They then arrange themselves into two daughter
wreaths, or asters, similar to the mother star. At this junc-
ture the cell protoplasm begins to divide by becoming con-
stricted in the center. The daughter stars are converted into
two new nuclei, in the inverse order to that by which the
original nucleus was broken up. Nuclear membranes and
nucleoli appear, the cell protoplasm divides into two new
cells, and the cycle is completed.

Derivation of Tissues. — The primary parent cell divides
into an innumerable mass of cells which is called the blasto-
derm. The blastoderm soon divides into two more or less
distinct layers, an outer and an inner, named ectoderm and
entoderm, between which a middle layer develops, the meso-
derm. From these three primary layers all of the various
tissues of the body are later developed. (See Embryology.)

CHAPTER II.
THE ELEMENTARY TISSUES.

THE tissues which make up the various organs and parts
of the body may be divided into the following groups: (i)
Epithelial tissue, (2) connective tissue, (3) muscular tissue
and (4) nervous tissue.

The Epithelial Tissues.

The epithelial tissues include those which form the cover-
ing for the body, the lining of the digestive canal, the respira-
tory tract and the genito-urinary tract. They also constitute
the derivatives of the epidermis, such as nails, hair, seba-
ceous glands, and the lining of the glands connected with the
digestive and genito-urinary systems.

These tissues are composed of cellular and intercellular
elements and perform various functions in different parts of
the body. In the skin where they constitute the epidermis,
they protect the delicate surface of the true skin beneath ; in
the alimentary and genito-urinary canals they aid in secre-
tion and excretion ; in the respiratory system they preserve
an equable temperature, while in all internal parts they yield
lubricants.

These tissues are characterized by the preponderance of
the cellular over the intercellular elements. The intercellular
structure consists of a cement substance which holds the cells
together and through which the food for the cells is ab-
sorbed. They contain no blood-vessels and no nerves. The
tissue usually rests upon a basement membrane, or membrana
propria, which is a modification of the connective tissue be-
neath.

8 THE ELEMENTARY TISSUES

Varieties. — The varieties of epithelium may be classified
as follows: (i) Squamous, (a) simple, consisting of a single
layer, (b) stratified, consisting of several layers; (2) Col-
umnar, (a) simple, (b) stratified; (3) Modified, (a) ciliated,
(b) goblet, (c) pigmented, (d) glandular, (e) neuro-epi-
t helium.

(i) Squamous Epithelium. — {a) Simple squamous epithe-
lium consists of a single layer of cells which, when viewed

FIG. 3. — From a section of the lung of a cat, stained with silver

nitrate.

N, Alveoli or air-cells, lined with large, flat, nucleated cells, with some
smaller polyhedral nucleated cells. (Halliburton after Klein and Noble Smith,)

from above, appear as flattened polyhedral nucleated plates
like a regular mosaic. It occurs in but a few places, lining
the air sacs of the lungs, the mastoid cells, the membranous
labyrinth and crystalline lens (Fig. 3).

(b) Stratified squamous epithelium is composed of several
layers of epithelial cells placed upon one another. The deep-
est layer, which rests upon the basement membrane, is com-
posed of irregularly columnar cells which have their nuclei

EPITHELIAL TISSUES 9

near the lower border of the cells. As they approach the
surface the layers become flatter and more scale-like and
possess less vitality. As the outer layers are worn away the
lower more vigorous layers push upward to the surface to
take their place. In the middle strata, where the cells are
polyhedral in shape, we find the layer of prickle cells which

FIG. 4. — Vertical section of the stratified epithelium of the rabbit's

cornea.

a, anterior epithelium, showing the different shapes of the cells at various
depths from the free surface; b, a portion of the substance of cornea. (Kirkes
after Klein.)

have minute projecting spines by which they are connected
together. This layer is sometimes called the stratum spin-
osum. Just below this is the stratum germinativum, or ger-
minating layer.

(2) Columnar Epithelium. — (a) The simple columnar va-
riety consists of a single layer of column or rod-shaped cells,
set upright, longitudinally striated and containing oval-shaped
nuclei. This variety is found in the lining of the stomach
and intestines.

(b) In the stratified columnar variety the single layer of
cells is replaced by several layers and the superficial elements
alone are typical. This type is found in the vas deferens.
Ciliated, it occurs in the Eustachian tube, lachrymal ducts,
respiratory part of the nasal fossae, ventricle of larynx, tra-
chea and bronchi, epididymis and first part of vas deferens.

(3) Modified Epithelium. — (a) Ciliated epithelium is
more common with the columnar variety than with any other.

THE ELEMENTARY TISSUES

Each of the ciliated epithelial cells presents on its free sur-
face twenty or more small, hair-like, protoplasmic append-
ages, called cilia. During life these small processes are in
constant rapid motion, waving in a direction toward the out-

FIG. 5. — Ciliated epithelium of the human trachea.

a, layer of longitudinally arranged elastic fibers; b, basement membrane; c,
deeoest cells, circular in form; d, intermediate elongated cells; e, outermost
layer of cells fully developed and bearing cilia. X 350. (Kirkes after Kollikcr.)

let of the cavity in which th.ey are found. In the genital or-
gans they are important in bringing together the male and
female elements of reproduction, while in the respiratory
tract they are concerned in aiding the
passage of the mucus and in the ex-
pulsion of foreign bodies.

(b) Goblet cells are found on all
surfaces covered by columnar epithe-
lium, but especially in the large intes-
tine. They secrete mucin, the main
constituent of mucus, which so dis-
tends the cell that it ultimately bursts
and sets free its contents.

(c) Pigment ed epithelium is ordi-
nary epithelium, the protoplasm of which has become in-
vaded and colored by foreign matter, such as fat, proteid,
etc. Such cells are constant in the deeper layers of the epi-

FIG. 6. — Goblet cells.

(Halliburton after Klein.)

CONNECTIVE TISSUES II

dermis, especially of certain races, as the negro. It is also
found in the choroid coat of the eye.

(d) Glandular epithelium may be columnar, spherical or
polyhedral in shape. It is found lining the terminal recesses
of secreting glands. The protoplasm of the cell usually con-
tains the material which the gland secretes.

(e) N euro-epithelium is the name given to that covering
those parts toward which the nerves of special sense are di-
rected, and is epithelium of the highest specialization. It
occurs in the retina, the membranous labyrinth and in the ol-
factory and taste cells.

The Connective Tissues.

All these tissues are developed from the same embryonal
elements, but present varieties differing widely in appearance
and properties. They are characterized by the preponder-
ance of the inter-cellular over the cellular elements. The
physical characteristics of these tissues are very important
and depend mostly upon the intercellular elements. Their
purpose in the animal economy is to furnish a supporting
and connecting framework for the body. In the embryonal
state the intercellular substance is semi-fluid and gelatinous.
Later, in adult connective tissue it becomes more definitely
formed, although it is still soft. In adult areolar tissue the
intercellular substance becomes tough and yielding. When
this intercellular substance becomes impregnated with cal-
careous salts we hav6 bone. However, during all these
changes in the intercellular substance little or no change has
taken place in the cellular structure. The bone-corpuscle, the
cartilage-cell, the tendon-cell and the connective tissue-cell
are all essentially identical.

The divisions of connective tissue are: (i) Mucous Tis-
sue, (2) Reticular Tissue, (3) Fibrous Tissue, (4) Adipose
Tissue, (5) Cartilage, (6) Bone.

THE ELEMENTARY TISSUES

(i) Mucous Tissue.— This is the most immature form of
connective tissue and consists of a loose protoplasmic net-
work having a gelatinous intercellular substance. It

FIG. 7. — Tissue of the jelly of Wharton from umbilical cord.

a, Connective-tissue corpuscles; b, fasciculi of connective-tissue fibers; c,
spherical cells. (Halliburton after Frey.)

FIG. 8. — Reticular tissue from a lymphatic gland, from a section

which has been treated with dilute potash. {Halliburton after

S chafer.}

found in Wharton's jelly in the embryo and in certain tu-
mors, known as myxomata.

(2) Reticular Tissue. — This is composed chiefly of a net-
work of connective-tissue cells which enclose a mass of lym-

CONNECTIVE TISSUES 13

phoid elements. It forms the connecting layer beneath the
skin, the submucous and subserous tissues, and the layer be-
tween the muscles. It receives its name on account of the
areolse or spaces within its substance, which permit the adja-
cent parts to move easily upon one another. It consists of
white and yellow fibers in about an equal proportion.

(3) Fibrous Tissue. — This variety includes all the more
usual forms of connective tissue found in the various parts
of the body. It may be further subdivided into : (a) White
fibrous tissue, (b) yellow elastic, and (3) loose fibrous or
areolar tissue.

FIG. o.— Bundles of the white fibers of areolar tissue partly
unravelled. (Kirkes after Sharpey.}

(a) White fibrous tissue is composed of groups or bundles
of fibers which have a wavy longitudinal striation. It is
tough and inelastic and forms ligaments, tendons and mem-
branes in various parts of the body. Chemically this tissue
is composed of a complex albuminoid substance, collagen.
Upon being treated with acetic acid the fibers become swollen
and transparent and finally invisible.

14 THE ELEMENTARY TISSUES

(b) Yellow elastic tissue is composed of bundles of long,
regular and branched fibers. It is characterized by its
marked elasticity. It is found in the vocal cords, longitudi-
nal coat of the trachea and bronchi, inner coat of blood-ves-
sels, especially the large arteries, and in some ligaments. Its
yellow-tinted fibers are seen in parallel waves and are
larger than those in the white tissue. They sometimes form
a web-like layer, as in the fenestrated layer of Hienle in the
arteries.

FIG. 10. — Elastic fibers FIG. 11.— Group of fat-cells (F c)

from the ligamenta stibflava. with capillary vessels (c). (Kirkcs

X200. (Halliburton after after Noble Smith.}
Sharpey.]

(4) Adipose Tissue. — This tissue exists in nearly all parts
of the body except the subcutaneous tissue of the eyelids, the
penis and scrotum, the nymphse and in certain parts of the
lungs. It is nearly always found within the meshes of are-
olar tissue, where it forms lobules of fat. Fatty matter in
the form of oily tissue is found in the brain, liver, blood and
chyle. The tissue is densest beneath the skin, especially of

CONNECTIVE TISSUES

the abdomen, around the kidneys^ between the furrows on
the surface of the heart and in bone marrow. It has a rich
blood supply.

(5) Cartilage. — Those tissues in which the intercellular
substance has undergone condensation until it appears homo-
gqneous are classified as cartilage.
Consequent upon the differences
exhibited by the intercellular ma-
trix it is divided into the follow-
ing varieties: (a) Hyaline, (b)
elastic, and (c) fibrous.

(a) Hyaline cartilage is of firm
consistence, considerable elasticity
and is pearly blue in color. It is
enveloped in a fibrous membrane,
the perichondrium, from the vessels
of which it derives its nutrition. It
is composed of cells, irregular in
outline and arranged in patches of
various shapes, which are embedded
in a homogeneous matrix. The
articular surfaces of bones, the
costal cartilages, and the larger
cartilages of the larynx, trachea and
bronchi, and also, those of the
nose and Eustachian tube are
formed of this variety. In the em-
bryo this cartilage forms nearly
the whole of the future bony
skeleton.

(b) Elastic cartilage is characterized by the presence of an
abundance of elastic fibers in the matrix. These resemble
those found in the yellow variety of elastic tissue. This va-
riety or cartilage is found in the external ear, epiglottis,
cornicula laryngis and Eustachian tube.

FIG. 12. — Sections of
Hyaline cartilage.

a, Fibrous layer^ of peri-
chondrium ; b, genetic layer of
perichondrium; c, youngest
chondroblasts ; d, older chon-
droblasts; e, capsule; f, cells;
g, lacuna. (Radasch.)

THE ELEMENTARY TISSUES

large amount of white fibrous tissue in the matrix. It com-
bines the toughness and flexibility of fibrous tissue with the
firmness and elasticity of cartilage. It is found chiefly in the
intervertebral disks, the symphyses and interarticular disks
of certain joints, and lining, bony grooves for tendons.
Chemically, cartilage is complex, consisting of a mixture

FIG. 13. — Elastic fibro-cartilage,

Showing cells in capsules and elastic fibers in matrix. (From Yeo after Cadiat).

of collagen, chondro-mucoid and albuminoid substances. On
boiling, it yields a substance known as chondrin, which on
cooling turns to gelatin.

FIG. 14. — 'White fibro-cartilage. (Radasch.}

(6) Bone. — Bone is a dense form of connective tissue con-
stituting the skeleton or framework of the body. It serves to

CONNECTIVE TISSUES I/

protect vital organs in the skull and trunk and acts as levers
which are worked by the muscles in the limbs. The tissue
is characterized by the deposit of calcareous or lime salts
within its intercellular substance, to which its well-known
hardness is due. Most bones may be divided into an outer
layer of compact bone and an inner layer of spongy or can-
cellated bone.

FIG. 15. — Transverse section of compact bony tissue (of humerus).

Three of the Haversian canals are seen, with their concentric rings; also the
lacunae, with the canaliculi extending from them across the direction of the
lamellae. The Haversian apertures were filled with air and debris in grinding
down the section, and therefore appear black in the figure, which represents
the object as viewed with transmitted light. The Haversian systems are so
closely packed in this section that scarcely any interstitial lamellae are visible.
X 150. (Kirkes after Sharpey.)

Microscopically bone is seen to consist of numbers of os-
seous layers or lamellae, arranged as, (a) circumferential la-
mellae which are arranged parallel to the inner and outer
surfaces of the bone, (b) Haversian lamella which are ar-
ranged concentrically around the Haversian canals and (c)
interstitial lamella, which are arranged irregularly so as to

1 8 THE ELEMENTARY TISSUES

fill in the spaces which the other lamellae do not fill. The
Haversian canals are minute longitudinal channels, each sur-
rounded by its lamellae within which run still smaller longi-
tudinal channels, called lacunas. Connecting the main chan-
nel and the lacunae, and radiating in all directions between
them are other very minute channels known as canaliculi.
Each Haversian canal with its surrounding lamellae, lacunae
and canaliculi composes an Haversian system.

A fibrous membrane, the periosteum, forms the outer
covering of all bones except when they are covered with car-
tilage. It consists of two layers, an outer fibrous and an in-
ner fibre-elastic layer. However, during the period of devel-
opment a third layer, the osteogenetic layer, lies to the inte-
rior. It possesses a rich blood supply which nourishes the
subjacent bone, and contains cells which later become bone-
forming elements — the osteoblasts.

Bone marrow is the highly vascular substance found
within the central cavity of the long bones and the Haversian
canals. It may be divided into two classes: (i) Red bone
marrow, and (2) yellow marrow. In early childhood all the
marrow in the bones is red or has a reddish tint, but in adult
life we find two kinds — the red and the yellow.

( i ) Red bone marrow is classed as one of the blood-
forming organs since it plays an important role in the for-
mation of the blood. When stained and examined under the
microscope it is found to consist of a delicate connective-
tissue reticulum which supports the blood-vessels and con-
tains in its meshes numerous cells. On the outside of the
marrow, next to the bone, we find a thin fibrous-tissue coat,
the endosteum, which lines the medullary cavity and extends
into the larger Haversian canals. The more numerous of
the cells found in the red marrow are, (a) the myelocytes,
which are very numerous and contain several different va-
rieties of granules, (b) the eoslnophiles, which are few in
number, but which are conspicuous by the presence of coarse
granules within the cytoplasm, which are colored intensely

MUSCULAR TISSUES

by acid stains, such as eosin, (c) the
giant cells, which are very large, but
contain only one nucleus, (d) the
erythroblasts, which are nucleated
red blood-cells. In addition to these
the red marrow contains mast-cells,
fat-cells and osteoclasts, or multinu-
clear giant-cells.

(2) Yellow bone marrow is formed
from red marrow by the infiltration of
fat-cells which convert it into adipose
tissue. When examined in section
yellow marrow resembles ordinary
fat-tissue, consisting chiefly of large
compressed spherical fat-cells which
are supported by a recticulum of con-
nective tissue. Yellow marrow is
found in all the adult long bones, ex-
cept at their extremities.

The Muscular Tissues.

The chief characteristic of muscu-
lar tissue which distinguishes it from
all other tissues is its marked contrac-
tility. This variety of tissue may be
divided into three large groups: (i)
Striated muscle, (2) cardiac muscle,
and (3) smooth muscle.

(i) Striated or voluntary muscle
makes up the greater part of all the
skeletal muscles by means of which
all voluntary movements are made.
In addition to this, it constitutes the
walls of the abdomen, and a few of
the muscles connected with the mid-
dle ear, tongue, pharynx, larynx, dia-

FIG. 1 6. — Two fibers
of striated muscle,

In which the contractile
substance, m, has been rup-
tured and separated from
the sarcolemma, a and j;
p, space under sarcolemma.
(From Yeo after Ranvier.)

2O THE ELEMENTARY TISSUES

phragm, and generative organs. This group of muscular tis-
sue is composed of bundles of fibers, each fiber of which is
derived from a single cell which has many nuclei. Each
fiber is enclosed in a thin, homogeneous, elastic membrane,
the sarcolemma. The fibers are composed of a semi-fluid
and viscous material which is called the muscle plasma. The
muscle plasma consists of two elements, the fibrils and the
sarcoplasm. The fibrils which are long and thread-like, run-
ning the entire length of the .fiber, consist of alternating light

FIG. 17. — Striated muscular tissue of the heart,

Showing the trelliswork formed by the short branching cells, with central
^ nuclei. (Yeo.)

and dark segments which fall together in the different fibrils
and give the muscle its characteristic striated appearance.
The sarcoplasm, which varies greatly in the striated muscle
of different animals, fills in the space between the fibrils.
From a study of comparative physiology it is assumed that
the fibrils are the contractile element of the muscle fiber,
while the sarcoplasm serves a general nutritive function.

Striated muscular tissue is very richly supplied with .blood-
vessels. The larger arteries and their accompanying veins

MUSCULAR TISSUES

enter the muscle along connective tis-
sue septa and then break up into
smaller branches and, finally, into a
capillary network which supplies the
individual muscle fibers. They are
also supplied with lymphatics which
occupy the clefts in the connective-tis-
sue septa around the fibers. There
are also definite lymph-vessels which
accompany the blood-vessels within
the muscle. This tissue is also sup-
plied with both motor and sensory
nerves, by means of which the stimuli
are carried to and from the muscle
fibers.

(2) Heart muscle occupies an in-
termediate position between the stri-
ated voluntary muscle and the non-
striated involuntary muscle tissue. It
is characteristic in that it is striated
and involuntary. The following is a
brief summary of its chief distin-
guishing features: (i) Its fibers are
united with each other at frequent in-
tervals by short branches, (2) its
fibers are smaller and their striation is
less marked than in voluntary mus-
cle, (3) it has no sarcolemma, and
(4) its nuclei are situated within the
substance of the fiber and not upon it.

(3) Smooth or involuntary muscle
occurs in bundles and thin sheets
chiefly in viscera and blood-vessels.
Its general distribution may be out-
lined as follows: (i) It is found in
the digestive tract from the middle of

FIG. 18. — Cells of
smooth muscle tissue
from the intestinal
tract of rabbit. (From
Yeo after Ranvier.}

A and B, muscle-cells in
which differentiation of the
protoplasm can be well seen.

22 THE ELEMENTARY TISSUES

esophagus to the anus, (2) in the capsule of the pelvis of the
kidney, (3) in the trachea and bronchi, (4) in the ducts of
glands, (5) in the gall-bladder, (6) in the vas deferens and
seminal vesicles of the male reproductive organs, (7) in the
uterus, vagina and oviducts of the female reproductive or-
gans, (8) in the blood-vessels and lymphatics, (9) in the
iris, ciliary bodies and eye-lids, and ( 10) in the hair follicles,
sweat glands, and skin of the scrotum and in some other
places throughout the body.

The structural unit of smooth muscle is the fiber-cell
which is a delicate spindle with its nucleus usually situated
nearer one end than the other. The nuclei of the fiber-cells
are usually elongated and oval. These fiber-cells are held
together by a delicate connective-tissue network which is
composed of both white and elastic fibers. Smooth muscle
is very poorly supplied with blood-vessels in comparison to
striated muscle. The blood-vessels run along the connective-
tissue septa and small branches are distributed to the fiber-
cells. The lymphatics, also, follow the connective-tissue
septa. The nerves which supply the smooth muscle are from
the sympathetic system.

The Nervous Tissues.

These tissues will be considered under the chapter on the
Physiology of the Nervous System.

CHAPTER III.
PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE.

WHEN a muscle is acted upon by a weight it extends
quite readily, but as soon as the weight is removed the mus-
cle resumes its normal shape. This illustrates the extensi-
bility and elasticity of muscular tissue. The muscles all over
the body are in a constant state of elastic tension, which
causes them to be of greater value as -a support to the body
skeleton. A muscle which is in a state of elastic tension con-
tracts more readily and forcibly than one which is relaxed.

Under ordinary conditions a muscle receives the stimulus
which causes it to contract through its motor nerve from the
central nervous system. If this nerve be cut, the muscle is
paralyzed. However, it has been demonstrated that a mus-
cle which has its nerve cut may still be made to contract by
applying an artificial stimulus, as an electrical shock. But
such a muscle would still have its nerve endings in the mus-
cle undestroyed, and hence, this would not prove that the
muscle has independent contractility. Still, if the nerve is
severed and the nerve endings are destroyed, e. g., by a drug,
we find that the muscle will still respond to an electrical stim-
ulus. This shows that muscular tissue has independent irri-
tability. Hence, striated muscular tissue possesses indepen-
dent contractility, by which is meant that its power of short-
ening is due to active processes developed in its own tissue,
and independent irritability, by which is meant that it may
enter into contraction by artificial stimuli applied directly to
its own substance.

If we isolate a muscle and stimulate it, we get a simple
contraction. If the end of this muscle is attached to a lever

24 PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE

connected with a revolving drum, we get a simple muscle
curve (Fig. 19). The time required for a simple contraction
varies with the muscles of different animals, and also with
different muscles of the same animal. After the muscle is
stimulated (Fig. 19), an appreciable time elapses, the latent
period, before it contracts, which is about Moo second. Then
the muscle passes into the stage of contraction, during which
time the lever rises. Immediately it relaxes and elongates
and the lever again descends to the base line. The whole
contraction occupies about Mo second.

FIG. 19. — Simple muscle curve. (Halliburton.)

Those factors which modify the character of a simple
muscle curve are, (a) the strength of the stimulus, (b) the
amount of the load, (c) the influence of fatigue, (d) the ef-
fect of temperature, and (e) the effect of veratrine.

(a) A stimulus which is just strong enough to produce a
contraction is called a minimal stimulus. As the strength of
the stimulus is increased the amount of the contraction,
which is represented by the height of the curve, is increased.
This continues until a certain point is reached, the maximal

PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 25

stimulus, then an increase in the stimulus produces no in-
crease in the contraction.

(b) As the weight of the load is increased the contrac-
tion becomes less until a weight is reached which the muscle
is unable to raise. Also, the latent period is longer with a
heavy than with a light load.

(c) If we apply a series of successive stimuli to a muscle
we notice that at first the contractions improve with each
successive stimulus which is due to the beneficial effect of
contraction. Later the contractions get less and less. As the
contractions get less, the period of contraction becomes
longer, the latent period is increased and the period of re-
laxation becomes very much longer. As the period of relax-
ation becomes longer, the muscle fails to return to its nor-
mal length before a second stimulus arrives, so that the orig-
inal base line is not reached at all. This condition is known
as contracture.

(d) By varying the temperature of a muscle we find that it
causes a variation in the extent and duration of its contrac-
tions. Thus, by beginning at o° C.. and increasing the tem-
perature, we find that the contractions increase up to 5°-9°
C. and then decrease up to i5°-i8° C. After this point is
reached they again increase reaching their maximum at
26°-3O° C. This maximum is much greater than the first
maximum which was reached at 5°-9° C. As the tempera-
ture is still increased, the contractions decrease rapidly until
at about 37° C. irritability is entirely lost. If the tempera-
ture is increased to about 42° C. heat rigor makes its appear-
ance due to the coagulation of the muscle plasma.

(e) Veratrine is an alkaloid which exerts a peculiar effect
upon the contraction of muscle. By injecting it into an ani-
mal before the muscle is removed the following effects are
noted: (i) The phase of shortening is not altered, but the
period of relaxation is very much prolonged, and (2) there
is a secondary rise in the curve of relaxation.

PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE

Effect of Two or More Successive Stimuli.— If a muscle
receives two successive stimuli a sufficient length of time
apart, two curves of contraction are produced, the second
being a little higher than the first (beneficial effect of con-
traction). However, if the second stimulus arrives before
the period of relaxation is complete, a secondary rise is pro-
duced which is called superposition or summation of effects.

FIG. 20.

I, Two successive submaximal contractions. //, A series of contractions in-
duced by 12 induction-shocks in a second. ///, Marked tetanus induced by
rapid shocks. (Landrois.)

If the two stimuli occur close enough together, the result will
be one curve which is greater than either would have pro-
duced separately. This is called summation of stimuli. If,
instead of just two stimuli, a number of stimuli are applied
very close together, we get the effect shown in (II). If
these stimuli occur still closer together the effect shown in
(III) is produced which is called tetanus. When the stim-
uli occur so as to allow partial relaxation between each
stimulus, (II) the effect is called incomplete tetanus, but
when no relaxation occurs as in (III) the effect is complete
tetanus.

CHAPTER IV.
SECRETION.

Secretion and Excretion. — Ordinarily the product of
glandular activity is spoken of as a secretion. On the one
hand, glands may take from the blood substances which are
formed in that fluid, which would accumulate and pro-
duce detrimental effects if not removed, and which are dis-
charged from the body. On the other hand, glands may
form out of materials furnished by the blood substances
which are peculiar to that gland's activity, which have an
office to perform in the economy, which do not accumulate
on removal of the gland, and which are not discharged from
the body. The product in the first case is an excretion, in
the second case a secretion. But when it comes to naming
an exclusively excretory or exclusively secretory gland,
the task is found to be practically impossible. Probably the
most typical excretion of the body is the urine, yet there' are
in the urine substances, like hippuric acid, etc., which are
undoubtedly formed by the kidney, and which do not pre-
exist in the blood. The succus entericus, e. g., would seem
as typical a secretion as it is possible to find, but not infre-
quently it contains urea when the activity of the kidney is
impaired, to say nothing, under normal conditions, of the
water and salts which are taken as such from the blood. The
liver is notable in its secreto-excrementitious action. While
the desirability of thus separating the glands into secretory
and excretory and their products into secretions and excre-
tions is granted, the impossibility of such a division is appar-
ent.

It is possible in most cases to apply the distinction to the
separate constituents of the product of a particular gland,

28 SECRETION

but not to the product as a whole. In view of these facts, at-
tention will be given in this chapter to several glands which
manifestly produce excretions as well as secretions. The
action of the kidney and sweat glands is so predominantly
excretory that they are treated separately. In what follows
the term "secretion" cannot always be taken as meaning a
true secretion, for it is customary and convenient to speak
of the "secretion of urine," for example.

Glands. — If we conceive of a single layer of secreting epi-
thelial cells supported by a thin basement membrane, and
then this structure invaginated or folded in upon itself, so
that the two layers of epithelium face each other with a
greater or less interval between them, with the basement
membrane constituting the external support for both, we
will have in mind the essential structure of a gland proper.
The invaginated cells are the gland cells, and the interval
between the two layers of cells is the lumen. Whether the
invaginated structure sends off from itself secondary or ter-
tiary folds similar to the original, or whether the lumen of
any of these folds is in the shape of a simple tube or sac, or
both, is immaterial. They may all be considered as identical
in nature with the original invagination and only modifica-
tions of its architecture.

However, these modifications are more or less distin-
guished by names. Those which become complex by numer-
ous branchings of the involuted tube are usually termed
compound, as opposed to a single simple fold ; glands are
further classified, as tubular, racemose, or tubulo-racemose,
according as the termination of the lumen has the shape of a
tube, or sac, or both. Thus a simple or a compound gland
may belong to any one of the three last-named varieties.
The crypts of Lieberkuhn are simple tubular glands. The
glands of Brunner are usually described as compound tubulo-
racemose structures.

In a compound gland that portion which communicates
with the surface is called the duct and is supposed not to be

GLAND SECRETION 29

concerned in actual secretion, but simply in carrying the pro-
duct away from the secreting terminal ramifications of the
subdivisions of the involution — which terminations are called
acini or alveoli. It follows, of course, that a collection of
acini may discharge their secretion into the main duct by a
smaller duct — that is, that the gland may have various subdi-
visions of the duct proper.

Furthermore, secretions are classified as external when
they are discharged upon a surface communicating with the
external air, such as the alimentary canal, or skin, and
internal when they are discharged upon surfaces not in
communication with the exterior, such as blood-vessels.
Both external and internal secretions are liquid or semi-
liquid in character, for they must contain water as a vehicle
for the salts and organic substances which are present in all
of them and which, in fact, distinguish them from one an-
other.

Glands in general have been divided into serous and
mucous by Heidenhain, according as the secreted fluid is
watery and thin, or viscid and stringy from the presence of
mucin. This division is further warranted by histologic
differences in the cells concerned in each kind of secretion.
The cells in a serous gland are small and finely granular, and
are in close apposition to each other. Those of mucous
glands are larger, almost square and are definitely separated.
Many glands contain both kinds of cells, but since their se-
cretion contains mucin, such glands are usually spoken of as
belonging to the mucous variety. It will be seen that the
salivary glands illustrate these varieties.

Gland Secretion. — Underneath the basement membrane of
a gland (that is, on the side opposite the epithelial cells)
ramifies an abundant network of blood and lymph capillaries.
This anatomical arrangement favors osmotic transudation
from the vessels, especially since the pressure in the vessels
is normally greater than in the acini and ducts of the gland.
Numerous experiments, however, prove the inadequacy of

30 SECRETION

simple osmosis to explain all the processes of glandular se-
cretion, especially those connected with the presence of or-
ganic constituents; while the undoubted presence of secre-
tory nerves (besides the vaso-motor nerves to the vessels)
would seem to give a priori evidence that the glandular epi-
thelium takes some active part in the formation of the se-
cretion. Such an office is granted to these cells, but whether
it is of chemical, or a physical, or a "vital" character is not
evident.

The physiology of the salivary glands, the gastric and in-
testinal glands, the pancreas and liver is taken up under the
chapter on Digestion in which they are vitally concerned.

Sebaceous Glands.

The sebaceous glands (see Hair-follicles) are chiefly asso-
ciated with hair-follicles and, existing wherever hair is to be
found, cover well-nigh the whole cutaneous surface. They
are of the simple or compound tubular type, and discharge
their secretion into the hair- follicle near its outer extremity.
The alveoli are lined by several layers of cuboidal epithelial
cells. The cells of the layer nearest the lumen contain fatty
matter, and are thought to form the secretion by breaking
down and being thrown off themselves. Their place is taken
by cells from the deeper layers, which undergo similar
changes and disintegrate.

Composition and Properties of Sebum. — Chemically se-
bum is largely made up of fatty matters. It also contains
cholesterin, which is in combination with a fatty acid. It
forms a thin coating over the cutaneous surface, accounting
for the normal oiliness of the skin. It also contributes to the
characteristic softness of the hairs, and prevents their break-
ing off from brittleness. Its presence over the body surface
may have some influence in regulating the loss of heat by
evaporation.

Cerumen, smegma and the secretion from the Nabothian

MAMMARY GLANDS 31

glands are only modified forms of sebum, and the structures
producing these secretions belong to the class of sebaceous
glands.

Mammary Glands.

Structure. — The mammary glands are two in number in
the human being, and are loosely attached to the great pec-
toral muscles. They are rudimentary in both sexes until
puberty, and in men throughout life. At puberty the gland
in the female enlarges markedly, but is never fully developed
before pregnancy. At this time the gland vesicles make their
appearance, and the rudimentary ducts come to be more and
more ramified. These ramifications do not reach their full
development, however, until lactation begins. The skin cov-
ering the areola of the nipple is dark, especially during preg-
nancy, and much thinner than over other parts. The dark
color is due to a deposit of pigment.

The mammary gland belongs to the compound tubulo-race-
mose type, and consists of fifteen or twenty lobes bound
together by areolar connective tissue. Each lobe is made up
of a number of lobules, containing the alveoli or secreting
portions. The secretion from all the alveoli and lobules of a
lobe converges to a single duct, which discharges its contents
upon the surface of the nipple without anastomosis with any
duct. There are, therefore, some fifteen or twenty ducts
thus opening upon the surface. Each of them has a dilata-
tion beneath the nipple, and it is in these sinuses largely that
the milk accumulates during lactation. When lactation has
ceased the ducts retract, the sinuses disappear, the alveoli
undergo retrograde changes, and the whole gland is inclined
to become flabby and pendulous. It does not regain after
pregnancy the firmness which characterized it before.

Secretion of Milk. — After parturition the first discharge
from the gland is colostrum, a liquid resembling milk in some
respects. In two or three days the true milk appears. Be-
sides water and salts, all the constituents of milk are formed

32 SECRETION

by the cells of the mammary gland. During the period of
gestation the cells lining the alveoli are flat and have only a
single nucleus. When they begin -to secrete they increase in
height, the nuclei divide and that portion of the cell toward
the lumen undergoes fatty degeneration. This fatty ma-
terial is extruded into the lumen and apparently constitutes
a part of the secretion. The liquid constituents taken out
of the blood probably hold the proteid and carbohydrate
portions in solution, while the fatty particles constitute the
fat of the milk. Thus secreted, the liquid accumulates in the
ducts and sinuses until removed by the infant or otherwise.
The fact that the secretion of milk in woman is influenced
by emotions of fear, grief, etc., is strong evidence of a ner-
vous control of the procedure, but proof of secretory fibers
to the cells has not been established.

The quantity of food required by the mother during the
time the child is nursed is increased, but no particular kind
of food seems to be especially required. The larger demand
for liquids is marked, however, and when the quantity of
milk is increased by a large ingestion of liquids, the solids in
the secretion are not relatively diminished.

Composition and Properties of Milk. — Human milk has
specific gravity of about 1030, and is not so white or so
opaque as cow's milk. Besides water, its chief constituents
are fats, lecithin, cholesterin, casein and lactose, of which
the two last named are the most important. Casein is the
main proteid constituent. Lactose is very abundant, and is
responsible for the sweet taste and for a large part of the
nutritive value of the fluid.

Thyroid Gland.

The thyroid gland consists of two glandular masses united
by an isthmus .of the same structure. It lies in front of the
trachea at the lower end of the larynx. It consists of a large
number of vesicles bound together by connective tissue.

THYROID AND ADRENAL GLANDS 33

Each vesicle is lined by cuboidal epithelial cells, which secrete
a semi-gelatinous substance, colloid.

It has long been known that the removal of the whole thy-
roid gland, including the parathyroid, occasioned marked in-
terference with nutrition and other changes, the chief of
which are disturbances of muscular coordination, possibly
convulsions, emaciation, apathy, and subsequent death. There
is no duct connected with the gland, and the secretion is
therefore an internal one. Very little is known of it except
that it is necessary to the maintenance of life. If a very little
of the gland be left, or if, after its complete removal, a small
bit of it be transplanted in some other part of the body, or if
the animal be fed on the thyroid extract or the fresh gland,
the characteristic symptoms do not ensue.

The muscular disturbances direct the attention to the cen-
tral nervous system when an attempt is made to explain the
occurrences and it is not improbable that the effect of the
thyroid secretion is in some way exerted upon or through
the central system. It seems generally agreed that the thy-
roid does discharge a secretion into the blood and that it is
the withdrawal of some part of that secretion from the cir-
culation which is responsible for the remarkable train of
symptoms sequent upon its removal. This essential constit-
uent is regarded by some as being an agent which destroys
certain toxic principles in the blood, by others as being requi-
site to the metabolic functions in the body without destroy-
ing anything. .Baumann has isolated from the gland sub-
stance a material containing a large proportion of iodine, to
which he gives the name iodothyrin, and it is very probable
that this is one, at least, of the beneficial substances in the
thyroid secretion.

Adrenal Glands.

The adrenal gland or suprarenal capsules, resting upon
the upper ends of the kidneys, are ductless glands whose re-

34 SECRETION

moval is followed by weakness, impaired nutrition and dis-
turbances in the circulation. Death usually supervenes in
two to four days. These bodies must produce an internal
secretion which is removed by way of ,the adrenal veins. It
may destroy toxic substances in the blood. A solution in-
jected into the circulation certainly affects the middle wall
of the vessels, causing contraction, and a heightened pres-
sure. The heart is also notably inhibited. It is not thought
that the effect on the vessels is brought about through the
vaso-motor nerves, but by direct excitation of the muscular
substance. Little in fact is known about the secretion, ex-
cept that it is necessary to life. Abel has isolated an alkaloid,
epinephrine, which is claimed to be the active principle.
These glands are the seat of lesions in Addison's disease, and
many cases of this malady are at least favorably influenced
by the use of adrenal extract.

Pituitary Body.

The pituitary body lying in the sella turcica on the superior
surface of the sphenoid bone, also produces an internal se-
cretion of physiological value. Its removal is regarded as
causing death. Hbwell has shown that injection of extract
from the posterior division occasions a rise of temperature
and slowing of the heart. Its situation makes satisfactory
experiments very difficult.

Testis and Ovary.

The testes and ovaries, though not probably true glands,
also may produce an internal secretion of obscure physio-
logical value. It is not essential to life. Injections of ex-
tracts from these bodies are claimed to have a remarkable
stimulating effect upon the nervous and muscular systems.
In mental and physical disturbances occasionally following
removal of the ovaries, gynecologists often find administra-
tion of the ovarian extract to be beneficial.

CHAPTER V.
THE BLOOD.

General Characteristics. — The blood is a red, opaque and
viscid fluid having a characteristic stale odor and a salty
taste. The blood is heavier than water, having a specific
gravity in the adult male of 1.041 to 1.067, tne average being
about 1.055.

The reaction of the blood is neutral. The nature of the
diet, either meat or vegetable, causes this neutrality to turn
to either an acid or an alkaline reaction.

The blood temperature is that of the body. In the periphery
it is about 99° F. ; in deeper vessels it varies from 100° F.
to 107° F. ; and in the hepatic veins it is about 107° F.

The Function of the Blood. — The most important physio-
logical functions of the blood are: (i) It carries to the tis-
sues food-stuffs after they have been digested, (2) it trans-
ports to the tissues oxygen which it has absorbed from the
air in the lungs, (3) it carries off from the tissues the waste
products of metabolism, (4) it transmits the internal secre-
tions of glands to the different parts of the body, and (5) it
aids in equalizing -the body temperature.

Quantity and Distribution of the Blood. — The quantity of
the blood in the body is estimated at about 7.5 per cent, of
the body weight. A man weighing 150 pounds has a fraction
over eleven pounds of blood, which is about one-thirteenth
of the body weight.

The distribution is generally given as, one-fourth in the
heart, large arteries, lungs, and veins ; one-fourth in the liver ;
one- fourth in the muscles attached to the skeleton ; and the
other one- fourth variously distributed to the other organs of
the body.

36 THE BLOOD

Composition of Blood.

The blood is composed of a fluid part, the plasma, in which
float a great mass of small bodies, the blood corpuscles. The
plasma may be defined as the blood minus the corpuscles.
These are of three varieties : ( I ) The red corpuscles, or ery-
throcytes, (2) the white corpuscles, or leukocytes, and (3)
the blood platelets, or thrombocytes. The plasma is a thin
slightly yellowish fluid with a specific gravity of 1.026 to
1.029. Hence, 'the bright red color of the blood is due to the
red corpuscles which are held in suspension in the plasma.
The proportion of plasma to corpuscles is about two to one
(Ho well).

Plasma.

Chemically, plasma is composed of water and about 10
per cent, of solids, together with oxygen, carbon dioxide and
nitrogen. A thousand parts of plasma contain: (Hallibur-
ton.)

Water , 902.90

Solids 97-io

Proteins: I. Yield of fibrin 4.05

2. Other proteins 78.84

Extractives (including fat) 5.66

Inorganic salts 8.55

The most important solids are the proteins, the chief of
which are: (i) Fibrinogen, (2) serum globulin, and (3)
serum albumin. Fibrinogen belongs to the globulin class of
proteins, but differs from serum globulin and may be separ-
ated from it. Fibrinogen is the least abundant of the pro-
terns. Serum globulin and serum albumin form the chief
proteins of the plasma. They may be separated by the use
of neutral salts.

The extractives are substances other than proteins which
may be extracted from the dried residue by the use of water,
alcohol, or ether. The principal extractives are fats, sugar,
lecithin, cholesterin, lactic acid and urea.

RED BLOOD CORPUSCLES 37

The most abundant salt of the plasma is sodium chloride.
It forms from 60 to 90 per cent, of the total mineral matter
of plasma. Potassium chloride is present in much smaller
amount. Other salts are the carbonates, sulphates and phos-
phates.

Corpuscles.

Suspended in the plasma of the blood we have a cellular
formed element moving and functionating. This element is
the corpuscular element and is composed of (a) the red
blood corpuscles, (b) the white blood corpuscles, (c) the
blood platelets.

(a) Red Blood Corpuscles or Erythrocytes.

General Description. — The red blood corpuscles are circu-
lar, bi-concave discs with rounded edges. They are from 7
to 8 micra in diameter and 2 micra in thickness, so can only
be seen with the aid of the microscope. When looked at
singly they appear to have a yellowish-green color, collec-
tively they are red.

Number. — In males there are about 5,000,000 red cells per
cubic millimeter ; in females about 4,500,000. The propor-
tion of reds to whites is one white to every 500 red.

Origin and Destruction. — The red corpuscles are continu-
ally being destroyed in the body. It appears that this de-
struction occurs principally in the liver. As the red cells
are thus destroyed it is natural to look for a place of manu-
facture. In the embryo we find that this generation takes
place in the liver and in the spleen ; in the adult it seems that
the manufacture takes place only in the red marrow of the
bones.

The red corpuscles are formed from colored, nucleated
cells called hemoblasts.

Constituents of Red Blood Corpuscles. — The red blood
corpuscles are made up of 65 per cent, water and 35 per

3o THE BLOOD

cent, solids. The principal solid constituents are (a) hemo-
globin (oxyhemoglobin) 87-95 Per cent., (b) stroma, com-
posed of fat, lecithin, and cholesterin, and (c) salts, princi-
pally potassium chloride, and potassium phosphate.

Hemoglobin. — Hemoglobin is the coloring matter of the
red cells, and is composed of (i) hematin, a pigment con-
taining iron, and (2) globin, a proteid. Hemoglobin is of
great physiological importance because of its ability to unite
with oxygen and thus form oxyhemoglobin. By it the blood

A, human colored blood corpuscles — i, on the flat; 2, on edge; 3, rouleau of
colored corpuscles. B, amphibian colored blood corpuscles — i, on the flat; 2, on
edge. C, ideal transverse section of a human colored blood corpuscle magnified
5,000 times linear — a, b, diameter; c, d, thickness. (Landois.)

carries its oxygen from the lungs to the tissues. It also
unites to some extent with carbon dioxide and it is thus that
carbon dioxide is brought from the tissues. We find oxy-
hemoglobin chiefly in the arterial blood, while in venous
blood we find both hemoglobin and oxyhemoglobin. In as-
phyxiated blood we find only hemoglobin.

The stroma is the colorless framework of the corpuscles

BLOOD PLATELETS 39

after the coloring matter is dissolved out. The hemoglobin
is ensnared in the stroma.

(b) White Blood Corpuscles or Leukocytes.

General Description. — The white blood corpuscles or leu-
kocytes are large, colorless, nucleated cells with no general
form, but which are capable of changing their form by ame-
boid movement.

Number. — The number of leukocytes varies from seven to
ten thousand per cubic millimeter.

Function. — The white corpuscles are not under the control
of the central nervous system, but are controlled by some
chemotaxic force. They are able to go and come by ame-
boid movement through the stromata of capillary walls and
wander here and there in the tissues. It is this that gives
them their name of wandering cells.

White blood corpuscles are of importance from a physio-
logical standpoint, because of this ability to wander. They
can transfer undissolved substances from one part of the
body to another and can destroy and remove foreign sub-
stances and harmful microorganisms.

The power they have of ingesting foreign substances is
called phagocytosis. They will migrate in large numbers
and surround a foreign object and endeavor to remove it
from the tissue. They have the power of liquefying tissue
and it is this liquefied tissue mixed with the dead bodies of
white corpuscles that is known as pus.

These are colorless discs about one-third to one-fourth the
size of red blood corpuscles. Some claim for them the full
value of blood cells, while others insist they are the nuclear
remains of destroyed leukocytes. There are about 635,000

40 THE BLOOD

to one cubic millimeter of blood. As to their function little
is known. Some claim they play an important part in the
coagulation of the blood. Nothing definite is known of their
origin.

The Coagulation of the Blood.

When blood is allowed to stand after being shed it rapidly
becomes more viscous and later sets into a firm jelly.
Later, as the fibrin contracts, a clear straw colored fluid, the
serum, is set free. The formation of fibrin is the essential
factor in coagulation. It is contained in the plasma in the
form of fibrinogen.

The relation of plasma, serum and clot is shown by the fol-
lowing table :

Plasma

Blood \ dot

Corpuscles

CHAPTER VI.
THE CIRCULATION OF THE BLOOD.

General. — We have seen that the composition of the blood
fits it for its function of carrying foodstuffs to the tissues
and removing the products of combustion ; but, for the blood
to exercise these offices, it is necessary that it be in communi-
cation wi'th the outside world and the tissues. The move-
ment it makes through its network of vessels in order to
carry products from the exterior to the interior and from
the interior to the exterior is what is meant by circulation.

Pulmonary and Systemic Circulation. — Two systems of
circulation are generally distinguished. The first is the pul-
monary, and is the circulation of the blood through the lungs
in order to get rid of carbon dioxide and to get a fresh sup-
ply of oxygen by aeration. The second is the systemic and
is the circulation through the great masses of body tissue in
order, by means of the lymph, to supply the tissues with
different solid, liquid, and gaseous nutritive material and
take from the tissues the products no longer needed but
which must be eliminated. These systems are also called
respectively the lesser and greater circulation.

Discovery. — The circulation of the blood was an unknown
fact up to 1628 when the discovery of its movements was
made and proved by Sir iWilliam Harvey, an English physi-
cian prominent in his time and now famous for this dis-
covery.

The Circulatory Apparatus. — The blood circulates through
a series of closed tubes known as blood-vessels, which divide
up, ramify, and go to all parts of the body. These vary from
large, macroscopic vessels to tiny, little, hair-like tubes, the
capillaries, which cannot be seen with the naked eye.

42 THE CIRCULATION OF THE BLOOD

The central organ of the circulatory system is the heart.
From this lead off the arteries, these in turn connect with the
capillaries, and these with the veins, which lead back to the
heart.

I. THE HEART.

The heart is a hollow, muscular organ divided by a mus-
cular septum into two distinct compartments designated for
convenience, the right and left heart. The right side, and
similarly the left, is divided by a muscular septum into two
chambers, the upper called the auricle and the lower the ven-
tricle. There is an opening between the right auricle and
the right ventricle and one between the left auricle and the
left ventricle and each opening is guarded and can be closed
by a thin membranous flap called a valve.

Situation. — The heart is located in the thoracic cavity be-
hind the sternum. It is placed in a diagonal position and its
base is in the middle line and looks backward, upward, and
to the right. Its apex is three inches to the left of the median
line, a half inch internal to the nipple, and in the fifth inter-
costal space.

Covering and Lining. — A serous sac, called the pericar-
dium, covers the heart. It hugs the muscle of the heart
closely, completely enveloping the organ, then turns back on
itself leaving a space between the outer layer and the layer
next to the muscle. In this space is a fluid which acts as a
lubricant.

The heart is lined by a membrane called the endocardium,
which is composed of endothelial tissue.

Structure. — The muscle of the heart is striated, but con-
trary to the usual rule, is involuntary in its action. The mus-
cle fibers run circularly, obliquely, and some in the form of
the figure eight, thus giving the power to contract and pump
the blood on into the circulation.

THE HEART 43

Contraction. — The physiological contraction of the car-
diac muscle is called systole, the relaxation is called diastole.
The contraction of the heart starts at the mouth of the
veins and, with a uniform rhythm glides along through the
auricles and along to the ventricles, each part relaxing as the
rhythmic contraction passes on. The whole time of contrac-

OIASTOLE
OF

UR1CLE&VENTRICLE.

FIG. 22. — Scheme of cardiac cycle.

The inner circle shows the events which occur within the heart; the outer the
relation of the sounds and pauses to these events. (Kirkes after Sharpey and
Gairdner.)

tion, from 'one beginning in the veins to another beginning,
is called the cardiac cycle. It lasts about .86 second.

The cycle may be divided thus : the auricles contract (sys-
tole) and ventricles are relaxed (diastole) which occupies
.16 second; the ventricles contract (systole) and the auricles
are relaxed (diastole) and this occupies .3 second; both au-
ricles and ventricles then rest and this occupies .4 second.

Number of Beats. — In an adult the heart beats on an aver-
age of 72 times per minute, in children it is higher. The fre-
quency of 'beat is influenced by age, sex, disease, drugs, phy-
sical causes and digestion.

44 THE CIRCULATION OF THE BLOOD

Valves and Openings.

Right Auricle. — Leading off from the right auricle anter-
iorly and superiorly is a sinus that bears the name of the
auricular appendix. It is a little hollow pouch capable of
distention with blood.

Opening into the right auricle we find the coronary veins,
the two venae cavae, and the auriculo-ventricular opening.
Guarding these openings are valves to prevent the backward
flow of the blood current.

Right Ventricle. — Opening into the right ventricle are the
pulmonary artery and the right auricle.

The tricuspid valve guards the auriculo-ventricular open-
ing. It is composed of three triangular shaped membranes
attached to the base of the circumference of the opening and
the apices of the triangles coming together when closed.

The semi-lunar valves guard the pulmonary .opening.
They are three entirely separate segments of semi-lunar
shape and are attached by their long curved margins to the
circumference of the artery just where it springs from the
muscular substance of the ventricles.

Left Auricle. — Like the right auricle, this cavity has a
small sinus leading off from it anteriorly and superiorly—
the auricular appendix. The openings into the left auricle
are the four pulmonary veins and the left ventricle.

Left Ventricle. — This ventricle has the thickest walls and
does the most work of any of the chambers of the heart, be-
cause it forces the fresh arterial blood out into the aorta and
thence through the entire systemic circulation.

The aorta and the left auricle open into • this ventricle.
The aortic semi-lunar valves guard the aortic opening. They
are three distinct semi-lunar shaped membranes to close the
aortic opening at the end of the systole. The mitral or bi-
cuspid valve closes the left auriculo-ventricular opening. It
is somewhat like the tricuspid except that it has' only two
flaps instead of three.

VALVES AND OPENINGS 45

Functions of Valves. — The valves are arranged at the
openings of the different chambers of the heart so the blood
will be forced in a constant direction. When the auricles
are at systole the auriculo-ventricular valves are open thus
letting the flow of blood go from auricles to ventricles ; but
as soon as auricular diastole and ventricular systole begin
these valves shut and the blood is kept from flowing back-
ward into the auricles. Then the semi-lunar valves are open
and the blood is forced into the aorta and pulmonary artery.
When ventricular diastole begins these semi-lunar valves are
closed and thus blood is prevented from running back into
the heart from the arteries.

Work of the Heart. — The work done by the heart is equal
to the weight of a column of blood multiplied by the height
or distance to which this column is carried by the heart force.
The column of blood is that amount that is sent by a single
contraction of the heart and the height to which it is carried
is equal to the pressure in the aorta and pulmonary arteries.

The amount of blood thrown into the aorta at each con-
traction of the ventricles weighs about 87 grams (about 3
oz.) and the height to which it is forced is about 1.5 meters
or 5 feet in man.

In estimating the work of a machine the English express
the result in foot pounds. The French in grammetres. A
foot pound is the energy expended in raising a unit weight
(i Ib.) through a unit distance (i ft.). A grammetre is the
force expended in raising one gram one meter. Thus the
work of the left ventricle at each contraction is 130.5 gram-
metres (or 15 foot pounds). Add 45 grammetres as the
work done by the right ventricle in contracting. If the
heart beats 72 times per minute it will, in twenty-four hours,
do 18,000 kilogramme-metres of work.

Sounds of the Heart. — Listening to the heart's action
through the thoracic wall we hear two distinct sounds. The
first is a slightly elongated sound and comes immediately
after the beat of the radial pulse. It is characterized by the

46 THE CIRCULATION OF THE BLOOD

syllable lub. The cause of this sound is supposedly the
closure of the auriculo-ventricular valves combined with
the sound made by the contracting muscle. It can best be
heard over the apex of the heart.

The second sound is shorter and sharper than the first and
is heard just before the impulse of the radial pulse. It is
characterized by the shorter syllable dup.

The cause of this sound is supposedly the closure of the
aortic semi-lunar valves along with those of the pulmonary
artery. It is best heard in the right second intercostal
space, as the aortic current transmits it.

Certain diseases affect the heart valves and the sounds
then depart from the normal. Thus it is of importance to
know the cause and sound of the normal vibrations so as to
detect the diseased conditions.

Heart Innervation. — The nerves that inhibit the action of
the heart are the two vagi; cutting these results in an in-
crease of the frequency of the heart beats.

The nerves that accelerate the action of the heart are the
nervi accelerantes, which are branches of the sympathetic
system. Stimulation of these causes increase in force
and frequency of heart beats.

II. CIRCULATION IN BLOOD-VESSELS.

Taking the heart as a central station for supplying force,
we find the blood current constantly going from a place of
higher pressure to a place of lower pressure.

The highest pressure is in the muscular center, the heart.
Blood-vessels connect with both auricles and ventricles.
Those connecting with the ventricles and carrying blood
away from the heart are called arteries and the pressure in
these is high, but lower than in the heart. Those vessels
connecting with the auricles and carrying blood back to the
heart are called veins and the pressure is lowest of all in
these.

STRUCTURE OF THE BLOOD-VESSELS 47.

The minute vessels that connect the arteries and veins and
collect waste from and supply nutritive material to the lymph
stream are called capillaries. The pressure in these is lower
than in the arteries but higher than in the veins.

The blood is thus kept in motion, constantly going from
place of higher to lower pressure.

The completed circulation is thus : —

(Beginning with the right auricle of the heart.) The two
venae cavae pour venous blood into the
right auricle and it in turn empties its
contents into the right ventricle. From
here the blood is driven into the pulmon- -pic. 23.— Aor-
ary artery (carrying venous blood) to be tic regurgitation.
aerated in the lungs. From the lungs it (Greene.}
comes by pulmonary veins (carrying arterial blood) to the
left auricle. This is the lesser or pulmonary circulation.

From the left auricle the blood goes into the left ventricle
and from here it is forced into the aorta and thus into the
systemic arteries, then through the capillaries to the veins
and back by means of the venae cavae into the right auricle.

The complete cycle in man takes about twenty-two sec-
onds.

STRUCTURE OF THE BLOOD-VESSELS.

Arteries. — The arteries have three coats : ( i ) the external
coat called the tunica adventitia, which is composed of
fibrous tissue with a little plain muscular tissue, (2) middle
coat or tunica media, composed of yellow, elastic tissue, and
(3) the inner coat or tunica intima, composed of endothe-
lium.

Veins. — The veins also have three coats, the external, the
middle and internal, as the arteries ; but the middle coat is
composed chiefly of inelastic, fibrous tissue. Thus the veins
lack the elasticity and contractility given to the arteries by
the middle coat.

THE CIRCULATION OF THE BLOOD

The Capillaries. — As the arteries get smaller we find them
still composed of the three above named coats. Finally,
though, in the minutest vessels we find only the innermost

layer remaining. These one-
coated vessels are the capillaries,
and they have only one layer of
endothelial cells on a basement
membrane. This is in order to
render possible the interchange
of material between the blood cur-
rent and the lymph stream, so the
tissues may be nourished and the
waste products removed.

IMPORTANCE OF ARTERIAL
ELASTICITY.

If an amount of fluid corre-
sponding to that of the "pulse
volume" be suddenly injected into
the end of a rubber tube already
distended with liquid, the tube
will be further distended by the
liquid injected, but if a like
amount of fluid be allowed to es-
cape at the other end the tube will
resume its original caliber. Thus
the pulse -volume enters -with
much force (the aorta or pulmon-
ary artery; the artery is very
elastic and expands under this in-
fluence, but immediately recoils
with a great pressure on the con-
tents. The pressure tends to
vessel in both directions, but its
is effectually prevented by the

FIG. 24. — Scheme of the
circulation.

a, right, b, left, auricle; A,
right, B, left, ventricle; i, pul-
monary artery; 2, aorta; i, area
of pulmonary, K, area of syste-
mic, circulation; o, the superior
vena cava; G, area supplying the
inferior vena cava; u; d, d, in-
testine; m, mesenteric artery; q,
portal vein; L, liver; h, hepatic
vein. (Landois.)

force the blood along the
return into the ventricle

IMPORTANCE OF ARTERIAL ELASTICITY 49

closure of the semi-lunar valves. Consequently it can go
only toward the periphery.

Now it is evident that the flow in the beginning of the
aorta is intermittent ; but it is found that, in vessels as
large as the carotids the flow has resumed a remittent char-
acter. The smaller the vessel the nearer the flow becomes
continuous until this condition is established in the capil-
laries.

It is the elastic coat of the arteries which allows them

FIG. 25. — Transverse section of part of the wall of the posterior
tibial artery. (Man.) (From Yeo after Shafer.}

a, endothelium lining the vessel, appearing thicker than natural from the con-
traction of they outer coats; b, the elastic layer of the intima; c, middle coat
composed of muscle fibers and elastic tissue; d, outer coat consisting chiefly of
white fibrous tissue.

to expand and contract, thus forcing the contents onward.
Furthermore it is this elasticity that causes the intermit-
tent and remittent flow to become continuous. So the func-
tion of the elastic coat is two-fold ; first, it forces the blood
current continuously toward the periphery, and second, it is
chiefly the cause of the change from an intermittent flow
to a constant flow, which is of so much importance in the
capillaries

Rate of Flow. — The velocity of the blood current is equal
to the volume flowing through a determined section in one
second divided by the cross section. The rate is determined
by the pressure, the friction in the vessels, and the cross sec-
tion of the vessels.

5O THE CIRCULATION OF THE BLOOD

The combined cross section of the capillaries is greater
than the combined cross section of the arteries or the veins,
so the rate of flow must be greater in the arteries and veins
than in the capillaries. The friction is greater in the smaller
vessels than in the larger which retards the flow. The
pressure is greater in the arteries than in the capillaries
and veins. From these facts it is evident that the velocity
is greater in the arteries than in the capillaries and veins,
but increases in the veins as compared to the capillaries.

In the large arteries the rate is 200-400 mm. per second,
in the capillaries, 6-8 mm. and in the large veins it is but
little less than in the arteries.

Valves in the Veins. — At frequent intervals in the course
of the veins are found small folds of membrane protruding
into the lumen of the vessels. The flow of the blood in the
veins is more sluggish than in the arteries, because, as we
have seen, the pressure lessens in the veins while gravity
and friction tend to cause a stoppage. These protruding folds
of the endothelial membrane or valves found in the veins
aid in the circulation by overcoming gravity and preventing
a backward flow of blood, by holding the blood until a
fresh impulse can impel it forward. They are found in
pairs and are most abundant in the veins of the extremities
where gravity impedes the onward flow of the current.

Capillary Importance. — The capillaries are the smallest
blood-vessels and the most important as to function. Being
of only one thickness of endothelium and in direct com-
munication with the lymph flow, we can readily see that the
food products brought by the arterial blood can be ex-
changed here for waste brought by the lymph. The flow in
the capillaries is constant, as we have already sfeen. We
can understand the importance of this when we take into
consideration the rapidity with which the tissues use oxy-
gen, the necessity of a constant supply, and the importance
of removing the carbon dioxide poisons.

Innervation of Vessels. — The blood-vessels are controlled

INNERVATION OF VESSELS 51

by the sympathetic nervous system by means of the vaso-mo-
tor nerves. These are composed of the vaso-constrictors
which cause the vessels to contract, and the vaso-dilators
which cause them to dilate. The entire physiological distribu-
tion of blood is regulated by the vaso-motor system of nerves.
It is by their means that the blood is increased to any part
of the body where physiological activity is going on, as

FIG. 26.

A, vein with valves open. B, with valves closed; stream of blood passing off by
lateral channel. (Kirkes after Dalton.)

when the gastro-intestinal tract is active during digestion,
when a muscle is in motion, or a gland in activity. Paraly-
sis of (the vaso-constrictors causes blushing, paralysis of the
dilators causes pallor as from fright. Outside influences
will cause the constrictors to act, as cold ; while alcohol will
cause the dilators to act and paralyzes the constrictors.

The chief vaso-motor center is in the medulla oblongata,
while subordinate centers exist in the cord. The vaso-motor
fibers reaching the vessels proceed from ganglia in the sym-

52 THE CIRCULATION OF THE BLOOD

pathetic system, but these ganglia are influenced by the cells
in the vaso-motor center.

Amount of Blood Important. — When there is a small loss
of blood from a slight injury the entire vascular system con-
tracts and the current supplying this diminished area is
sufficient; but at other times the loss of blood is so great that
the amount remaining is not sufficient to carry on a complete
circulation. Unless remedied this results in death. In such

FIG. 27. — Capillaries.

The outlines of the nucleated endothelial cells with the cement blackened by the
action of silver nitrate. (Landois.)

cases of great loss the deficit may be supplied by a normal
salt solution, thus giving an amount of fluid sufficient to
maintain the heart action. But in cases where as much as
two-thirds of the blood is lost, the injection of fluid does no
good. The amount of fluid necessary to cause the heart's
action to continue may be supplied, but the amount of hemo-
globin necessary for life is lost and this cannot be sup-
plied. Asphyxiation is the result.

PULSE

Pulse. — If a finger be placed on any artery in the body
there will be transmitted to it a perceptible impulse. This
impulse is what is called the pulse. It is caused by the force
of the heart's action against the elastic arterial wall, 'and the

FIG. 28. — Interior of right auricle and ventricle exposed by the
removal of a part of their walls. (From Yeo after Allen-
Thompson.}

i, superior vena cava; 2, inferior vena cava; 2', hepatic veins; 3, 3', 3", inner
wall of right auricle; 4, 4, cavity of right ventricle; 4', papillary muscle; 5,
5', 5", flaps of tricuspid valve; 6, pulmonary artery in the wall of which_ a
window has been cut; 7, on aorta near the ductu's arteriosus; 8, 9, aorta and its
branches; 10, u, left auricle and ventricle.

subsequent contraction of this wall against the current it
contains.

THE CIRCULATION OF THE BLOOD

The impulse is an index to the condition of the circulation.
Its frequency normally in an adult is about 72 times per

FIG. 29. — The left auricle and ventricle opened and part of their

walls removed to show their cavities. (From Yeo

after Allen Thompson.)

i, right pulmonary vein cut short; i', cavity of left auricle; 3, 3", thick wall
of left ventricle; 4, portion of the same with papillary muscle attached; 5, the
other papillary muscles; 6, 6', the segments of the mitral valve, 7, in aorta is
placed over the semi-lunar valves.

minute, in children it is higher, and it is more frequent in
woman than in man. Its frequency is affected by age, sex,
exercise, disease, drugs, and psychical causes, as fear, sor-

PULSE

row, etc. We feel the pulse to learn several things: — (i)
Its frequency, which tells how many times the heart is beat-
ing.

(2) Its tension, which is the state of the arterial walls

FIG. 30. — Portion of the wall of ventricle.

d, d', and aorta, a, b, c, showing attachments of one flap of mitral and the
aortic valves; h and g, papillary muscles; e, e' and f, attachment of the tendi-
nous cords. (From Yeo after Allen Thompson.)

and is the resistance offered in peripheral vessels. We
judge the tension by the force necessary to obliterate the
impulse.

50 THE CIRCULATION OF THE BLOOD

(3) Regularity, which tells whether the heart is regular
in either its force or rhythm.

(4) Its strength, which tells as to the force with which
the heart is acting.

(5) Its length, whether the beat is long or slow and con-
tinuous.

(6) The condition of the vessel wall, whether sclerotic

FIG. 31. — Dudgeon sphygmograph.

or not. In the study of the pulse an instrument called the
sphygmograph is used, which receives the impulse from a
beating artery and transmits it by means of a finely ad-
justed lever to a smoked surface of paper. Thus a graphic
representation of the impulse .is given, the height to which
the writing end of the lever goes denoting the force of the
impulse of the heart beat at the time of the writing.

THE LYMPH 57

THE LYMPH.

The lymph is a clear colorless fluid contained in the lym-
phatic vessels and tissue spaces. It resembles plasma in gen-
eral appearance and does not differ greatly from it in com-
position.

The Lymph Vessels.— These vessels originate in at least
three different ways. ( i ) All cells may be said to be bathed
in lymph, being surrounded by that fluid lying in the irregu-
larly shaped spaces between them. These spaces communi-
cate with each other and finally converge to the lymph ca-
pillaries. The intervals are called the "extravascular lymph
spaces!' (2) In certain situations, particularly in the ner-
vous centers, the small blood-vessels are completely sur-
rounded by and included in larger tubes, the "perivascular
lymph canals." These likewise pass on to the lymph capil-
laries proper. ( 3 ) The large serous cavities, like those lined
by the peritoneum, pleura, tunica vaginalis, etc., have large
numbers 'of lymphatic radicles opening abruptly into them,
or rather originating from them, and these may be consid-
ered as great extravascular lymph spaces.

The course of the lymph is from the tissues to the sub-
clavian veins, where it enters the vascular circulation. The
lymphatic vessels from the right arm and the right side
of the face, head and chest converge to form the ductus lym-
phaticus dexter, which enters the right subclavian vein at its
junction with the internal jugular. The lymphatics from all
other parts of the body converge to form the thoracic duct,
which enters the left subclavian vein at its junction with the
internal jugular. The thoracic duct begins by a dilated
pouch lying upon the second lumbar vertebra. This pouch
receives the lymphatic branches which have converged from
the lacteals, and is called the receptaculum chyli. The lac-
teals pass through the mesenteric lymphatic glands on their
way to the receptaculum chyli.

The distribution of the lymphatics needs no comment when

THE CIRCULATION OF THE BLOOD

FIG. 32. — Diagram showing the course of the main trunks of the
absorbent system.

The lymphatics of lower extremities, D, meet the lacteals of intestines, LAC,
at the receptaculum chyli, R.C., where the thoracic duct begins. The superficial
vessels are shown in the diagram on the right arm and leg, S, and the deeper
ones on the left arm, D. The glands are here and there shown in groups. The
small right duct opens into the veins on the right side. The thoracic duct opens
into the union of the great veins of the left side of the neck, T. (Yco.)

THE LYMPHATIC GLANDS 59

it is known that they receive the plasma which has been
passed out of the vascular capillaries and thus collect fluid
from well-nigh every tissue in the body.

The structure of the lymph- vessels is quite similar to that
of the veins, though they are more delicate. The lymph
capillaries probably contain only a single coat like the venous
capillaries. In the large vessels this thin endothelial coat is
supplemented by conne'ctive tissue fibers together with some
elastic and non-striated muscle fibers. They are very abun-
dantly supplied with valves which operate in the same way
as the venous valves. The vessel wall is quite elastic and has
some contractile power.

Lymphatic Glands. — 'All the lymphatics pass through one
or more lymphatic glands on their way to the Larger trunks.
These bodies are not true glands. Their structure is adenoid.
There are some six or seven hundred in the body, varying in
size from a pinhead to a large bean. The superficial glands
are especially abundant about the groin, axilla, neck and
other flexures. The deep glands are most numerous about
the great vessels. The mesenteric glands are found between
the folds of the mesentery.

The lymphatic glands are of irregular shape and contain
within their substance large numbers of lymph spaces or
canals through which the incoming lymph must pass. The
vasa efferentia are usually fewer in number and larger in
size than the vasa afferentia. The current must be consid-
erably delayed in the glands. They are concerned in the pro-
duction of leucocytes, while their retention of toxic materials
— even to their own hurt — is a common pathological occur-
rence.

Properties and Composition of Lymph. — Lymph is a com-
paratively clear liquid containing leucocytes. After meals
the color becomes whitish from the admixture of chyle, and
numerous fat droplets are present. Neither red corpuscles
nor platelets are found in lymph except accidentally. The
specific gravity is lower than that of the blood. Lymph

60 THE CIRCULATION OF THE BLOOD

coagulates when drawn, since the fibrin factors are present ;
but the process is less prompt and the clot is less firm than
in the case of blood.

In order to form an idea as to the constituents of lymph
it is only necessary 'to say that its ultimate origin is the blood
plasma, except in so far as its composition is changed during
digestion. The plasma makes its way through the capillary
walls out to the tissues bringing nourishment to them and re-
moving waste products from them. In thus coming in con-
tact with the tissues the plasma finds itself in the extravas-
cular lymph spaces and its name is simply changed to lymph.
lit thus appears that lymph may enter the extravascular
spaces by the direct passage of plasma out of the vessels or
by being excreted, as it were, from the tissue cells.

In any case the constituents of lymph are not very differ-
ent from those of plasma, except, of course, when intestinal
digestion is in progress and chyle is introduced into the lym-
phatic circulation. It contains the three plasma proteids,
urea, fat, lecithin, cholesterin, sugar and inorganic salts.
The proteids are less abundant than in plasma, as might be
supposed when it is remembered that they possess little os-
motic power. The inorganic saks are in about the same
proportion in both fluids. It is significant that the amount
of urea and related excrementitious products is more abun-
dant in lymph than in plasma ; their source is the destructive
metabolism going on in the cells to which the plasma has been
supplied, this plasma finding its way back as lymph. It is by
no means certain, however, that all the plasma escaping from
the capillaries is carried away by the lymphatic system.
Some may reenter the blood-vessels.

There is no unanimity of opinion as to the exact method
of passage of plasma through the capillary walls into the
lymph spaces. Some maintain that the phenomena can be
explained by the ordinary physical laws of diffusion, filtra-
tion and osmosis when existing conditions of pressure, etc.,
are taken into consideration. Others hold that these laws are

THE FLOW OF LYMPH 6 1

insufficient in themselves to account for various occurrences
in this connection, and ascribe to the capillary endothelium
some active secretory power governing, or at least influenc-
ing, the outward passage of the plasma.

The Flow of Lymph. — There is no organ corresponding to
the heart to keep the lymph current in motion. The main
causes for its direction from the extravascular spaces toward
the veins in the neck is the degree of pressure to which it is
subjected in those spaces as compared with the inferior, or
even "negative," pressure obtaining near the terminations of
the great ducts. It is known that at all times the venous
pressure in the subclavian veins is low and that it may even
fall below the atmospheric pressure, so that "suction" is ex-
erted upon the lymphatic ducts where they enter those ves-
sels. The lymph pressure in the extravascular spaces is esti-
mated to be one-half the capillary blood-pressure. Friction
and gravity (where the course of the vessels is upward) op-
pose the passage of the fluid. Consequently it accumulates
in the spaces and in the smaller lymphatics until the pressure
there becomes greater than the resistance of these forces,
when it passes onward. Since lymph is being continually
produced this superior pressure in the extravascular spaces
and small lymphatics is a fairly constant factor and keeps up
a correspondingly constant current.

There are two factors which are accessory to this peri-
pheral pressure : ( i ) Thoracic aspiration by bringing about
negative pressure in the veins in and near the chest brings
about a like condition in the tributary lymphatic ducts ; fur-
thermore, the effect of aspiration makes itself felt directly
upon the thoracic duct since its greatest extent is in the tho-
rax. (2) The valves of the lymphatics act in a similar man-
ner to those of the veins and constitute a very necessary
factor in the lymphatic circulation. Although the lymph
flow resembles that of the venous blood, it is less regular and
more sluggish, but probably not so slow as might be sup-
posed. Properly colored solutions injected into the blood

62 THE CIRCULATION OF THE BLOOD

have been demonstrated in the lymph of the thoracic duct
"in from four to seven minutes."

Lymph and Chyle. — It is scarcely necessary to refer to the
differences between these two fluids. Chyle is the intestinal
lymph during digestion. In the intervals of digestion the
contents of the lacteals do not differ materially from lymph
in other localities. Chyle has a whitish milky appearance due
to the presence of emulsified and saponified fats. Its specific
gravity naturally depends largely upon the amount of fat in-
gested, but is always higher than that of ordinary lymph and
lower than that of blood. Not only is there more fat in the
chyle than in lymph, but the other solids are also increased.
The proteid constituents are considerably more abundant.
For the most part the higher specific gravity is explained by
the absorption of solids in solution from the alimentary
canal.

Chyle is forced out of the lacteals by contraction of the
non-striated muscle fibers which run along by the vessel.
When relaxation of the fibers occurs, return of chyle into
the lacteal is prevented by a valve at the base of the villus.

CHAPTER VII.
THE PHYSIOLOGY OF DIGESTION AND ABSORPTION.

FOODS.

IT is evident that all the tissues of the body are continually
undergoing "physiological wear" — that the materials of
which they are intrinsically composed are being changed into
effete matter and discharged from the system. This is a
process going on in the substance of every cell in the body,
and obviously, for these cells to continue to live and func-
tionate, there must be a continual appropriation of new mat-
ter to take the place of the materials which have served
their physiological purpose, and are of no further value to
the body. This supply of material is made directly to the
tissues by the blood, but lest this fluid be impoverished, it
must in turn be furnished with an approximate constant
quantity of nutritive matter. The ultimate source of that
matter is in the food which we eat. However, it must pass
through the processes of digestion and absorption before it
can be utilized by the tissues. This conception of a food
must be understood to embrace all substances contributing,
either directly or indirectly, to body nutrition, including,
therefore, the oxygen of the air as well as all articles usually
classed as drinks.

An animal whose weight remains about the same must eat
and digest a certain quantity of food to keep up the body
temperature, to supply mechanical energy, and to repair the
wastes which are continually going on in the body. An ani-
mal which is growing and increasing in weight must eat
enough not only to supply the demands just mentioned, but
also to form the new tissue.

64 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

The articles we eat, besides being largely insoluble, differ
very materially in their composition from any substances
found as parts of the body tissues. Even those undigested
substances most closely resembling living tissue will no.1 be
utilized by the cells when presented to them by being injected
into the blood. All the articles which we use for food must
undergo a special process, called digestion, before they can
be absorbed by the tissues.

Seat of Hunger. — Food is taken into the body in obedience
to an expressed want on the part of the system. The desire
for food — the sensation of hunger — is referred, in a rather
indefinite way, to 'the stomach. That sensation is ordinarily
satisfied by the introduction of food into the stomach. How-
ever, this does not necessarily mean that its seat is in that
organ, since removal of the stomach by no means prevents
hunger. .But, if nutritious material be introduced in suffi-
cient quantity into the circulation, as by rectal enemata, hun-
ger is relieved. The true seat of this sensation is undoubt-
edly in the cells themselves, it being simply a call from them
for more material to take the place of their worn-out con-
stituents.

Cold weather demands an increase in the amount of food,
as also do physical and psychical activity, certain drugs, etc.

Seat of Thirst. — The demands of the cells for water is re-
ferred to the fauces and throat, but this is no more the seat
of thirst than is the stomach of hunger. The taking of
water into the mouth alone will not quench thirst, except in
so far as absorption may take place from its mucous mem-
brane. But, if water in sufficient amount be placed into the
circulation in any way satisfaction ensues. Next to the de-
mand for oxygen, that for water is the most imperative
which comes from the tissues; that is, they can live much
longer without solid food than without water. The amount
necessary is manifestly subject to many conditions, such as
external moisture and temperature, exercise, etc.

Classification of Foods. — A very large number of sub-

FOODS 65

stances are taken into the alimentary canal as food ; but ex-
amination reveals that all such materials contain one or more
of a very few classes of food stuffs. These may be divided
as follows:

I. Water.
II. Inorganic or mineral salts.

III. Carbohydrates.

IV. Fats.

V. Proteids.

I. Water is scarcely looked upon as food in the common
acceptation of the term, but it is quite as necessary to cell life
as any of the other classes. It is found in all foods and in
all tissues and fluids of the body. It forms about 70 per cent,
of the entire body weight and acts as a solvent upon vari-
ous ingredients of the food, liquefying them and rendering
them capable of absorption.

II. The mineral salts which are chiefly necessary for nutri-
tion are:

Chlorides "1

Sujhater f Of sodium and P°tassium-

Carbonates

Phosphates V Qf ca]cium and esium

Carbonates J

Of these salts, sodium chloride, or common table-salt, is the
most important and abundant in the foods we eat. It is
present in nearly all the tissues and fluids of the body, es-
pecially the blood. Of the other salts, those of calcium exist
in the largest quantity in the body. They are especially im-
portant on account of the part they play in the formation of
the bones, teeth and cartilages. The remaining salts exist
in larger or smaller quantities in the tissues and fluids of the
body.

III. The carbohydrates include principally the starches

66 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

and sugars. They are of definite chemical composition con-
taining carbon, hydrogen and oxygen, but no nitrogen. The
hydrogen and oxygen which they contain are always in the
proportion to form water, i. e., two atoms of hydrogen to one
of oxygen. The starches are found chiefly in wheat, corn,
oats and other grains ; also in potatoes, peas, beans and in the
roots and stems of many plants, and in some fruits. Starch
is found in a pure state, as a white powder, in arrowroot and
corn-starch. The sugars are of several kinds, the principal
being : cane sugar, beet sugar, maple sugar, grape sugar which
is found in grapes, peaches and other fruits, and malt sugar
which is obtained from malt. These are all obtained from
vegetable tissue, however a few are found in or formed by
the animal organisms, as glycogen, dextrose and lactose.
They are the cheapest foods from financial and digestive
standpoints and constitute the main bulk of articles eaten.
They contain more oxygen than do the fats, and are more
easily oxidized and converted into heat and muscular energy.
In fact, their great physiological value lies in the ease with
which they are burned up in the body. They furnish the
main part of the fuel necessary to the running of the animal
mechanism. They may also be converted into fatty tissue by
the body.

IV. The fats are ingested with both animal and vegetable
diets. They are compounds of carbon, hydrogen and oxy-
gen. The principal fats are stearin, palmatin, margarin and
olein. These exist in varying proportions in the fat of ani-
mals, in the various vegetable oils and in milk, butter, lard
and in other foods and vegetable substances. The fats contain
no nitrogen, and, Jike carbohydrates, their great physiological
value lies in the fact that they are destroyed in the organism
to produce energy, whether in the form of heat or muscu-
lar exercise. They are handled and converted less readily by
the system than the carbohydrates, and consequently tax the
digestive powers more. But it is found that, weight for
weight, they are the more efficient in the production of

FOODS 67

energy than are the carbohydrates. They also furnish fuel
for the running of the body mechanism.

V. The proteids form a large part of all living organisms
and are absolutely necessary to animal life. They are very
stable compounds and are found in both animal and vegetable
foods. They contain carbon, hydrogen, oxygen and nitro-
gen, together with, usually, a small quantity of sulphur and
phosphorus. They occur in the form of casein in milk and
cheese, myosin and syntonin in muscle, vitellin in the yolk of
eggs, glutein in flour, legumin in peas, beans and lentils, and
in some other forms. Proteids may be used by the body to
produce heat and energy, but being more stable in composi-
tion than carbohydrates and fats, they are more often used
to build up tissue. In fact the proteids are absolutely essen-
tial to life while this is not true of carbohydrates and fats,
since the proteids must be used to build up new cells to take
the place of those being constantly worn out and eliminated.

The animal foods which are richest in proteids are lean
meat, milk, eggs, cheese and all kinds of fish, while the vege-
table are wheat, beans, peas and oatmeal. It has been found
that the animal proteid foods are split up and digeste'd much
more easily than are the vegetable. Hence the great ma-
jority of the people rely upon the animal foods for their sup-
ply of proteid material which is necessary to life.

The composition of a few of the more important articles
used as food is shown by the following tables.*

Milk : Woman, Cow,

Per cent. Per cent.

Protein (chiefly caseinogen) 1.7 3.5

Butter (fat) '..... 3.4 3.7

Lactose 6.2 4.9

Salts 0.2 0.7

Eggs :

Total amount of solid 13.3 per cent

These tables are taken from Halliburton's Handbook of Physi-
ology.

68 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

Protein 12.2 per cent.

Sugar 0.5 per cent.

Fats -}

Lecithin t Traces.

Choiesterin |

Inorganic salts 0.6 per cent.

Meats:

Ox. Calf. Pig. Fowl. Pike.

Water 76.7 75.6 72.6 70.8 79.3

Solids 23.3 24.4 27.4 29.2 20.7

Proteins 20.0 19.4 19.9 22.7 18.3

Fats ...s 1.5 2.9 6.2 4.1 0.7

Carbohydrates . . 0.6 0.8 0.6 1.3 0.9

Salts 1.2 1.3 i.i i.i 0.8

Vegetable Foods:

Wheat. Barley. Oats. Rice. Peas. Potatoes.

Water 13.6 13.8 12.4 13.1 14.8 76.0

Protein 12.4 n.i 10.4 7.9 23.7 2.0

Fat 1.4 2.2 5.2 0.9 1.6 0.2

Starch 67.9 64.9 57.8 76.5 49.3 20.6

Cellulose ... 25 5.3 11.2 0.6 7.5 0.7

Mineral salts 1.8 2.7 3.0 i.o 3.1 i.o

DIGESTION.

Object. — Digestion is largely a chemical process. Certain
physical phenomena are auxiliary. The foods not yielding
energy are not affected in a chemical way by digestion. They
are simply dissolved, if not already in solution, and are dis-
charged from the body in the same condition in which they
entered. But the other classes of food must either be separ-
ated from innutritions substances with which they enter, or
undergo certain changes themselves, or both, before they
can be absorbed and assimilated. This necessitates a com-
plicated digestive apparatus and the subjecting of different
classes of food to different digestive fluids and other gastro-
intestinal influences. The object of digestion is therefore
twofold, first, to convert the foods into soluble materials and,

DIGESTION 69

second, to bring about such changes in their composition as
will insure their absorption and appropriation by the tissues.

Enzymes. — The chemical changes taking place in digestion
are of a peculiar nature, in that they are effected largely by
the presence of substances known as enzymes, correspond-
ing in an obscure way with ordinary chemical reagents.
These have been called unorganized or unformed ferments,
to distinguish them from such organized ferments as bac-
teria, yeast, fungi, etc. They are not themselves possessed
of any vital activity, though formed in living organisms, like
plants or animals. They are of indefinite chemical composi-
tion, contain nitrogen and are supposed to be of proteid
structure. The characteristic point in their action has been
supposed to be that they produce a chemical change without
themselves being affected by that change. This is doubtless
practically true, but it is found in experimental work that "a
given solution of enzyme cannot be used over and over again
indefinitely." It finally loses its identity.

According to the foods on which they act and the effects
they produce, enzymes are classified as : ( i ) Proteolytic
enzymes, which convert proteids into soluble peptones ; ex-
amples are pepsin and trypsin. (2) Amylolytic enzymes,
which convert starches into sugar ; examples are ptyalin and
amylopsin. (3) Fat-splitting enzymes, which convert neu-
tral fats into glycerine and fatty acids ; an example is steap-
sin. (4) Sugar-splitting enzymes, which convert the non-
absorbable (saccharose) into absorbable (dextrose) sugar;
an example is invertase. (5) Coagulating enzymes, which
precipitate soluble proteids ; an example is rennin.

Characteristics of Enzymes. — Some of the characteristics
of enzymes are as follows : ( I ) They are soluble in water
and in glycerine. (2) In solution they are destroyed before
the boiling point is reached (140° to 180° Fahrenheit). Very
low temperatures do not destroy them, but suspend their ac-
tion. (3) They never completely convert the substance upon
which they act. It is supposed that the substance produced,

JO THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

as peptones for example, have an inhibitory action upon the
enzyme. If these substances be removed as they are formed,
the action of the enzyme continues. (4) The particular re-
sult is independent of the amount of the enzymes (unless
it be very small) no matter how large a quantity of the sub-
stance to be acted upon is present.

Manner of Action. — These enzymes are supposed to bring
about their respective changes through hydrolysis — that is,
by causing water to be taken up by the molecules of the af-
fected substance and by the subsequent splitting of the
newly formed molecule into two or more simpler ones. How
they cause this appropriation of water is as yet undeter-
mined. It was formerly supposed to be brought about by
contact merely, and the enzymes were called catalytics; but
this term offers no explanation of the real change which oc-
curs.

Digestive Processes. — The digestive processes may be con-
sidered under the heads of (i) prehension, (2) masti-
cation, (3) salivary digestion, (4) deglutition, (5) gastric
digestion, and (6) intestinal digestion. Prehension, mastica-
tion and deglutition cannot properly be looked upon as di-
gestive processes, inasmuch as they involve no chemical
change. They are, however, necessary occurrences, and can-
not be disregarded. Of course, absorption and "internal di-
gestion" follow gastro-intestinal or "external digestion," and
assimilation or cell appropriation follows absorption.

Prehension.

Prehension is simply the taking of food into the mouth.
Its mechanism in the human adult is so familiar that it needs
no description. In the sucking child it is more complex.
The buccal cavity is closed posteriorly by the application of
the velum palati to the base of the tongue. The tip of the
tongue is applied to the hard palate, and successive portions
of it (going backward) being applied in the same way leave

SALIVARY GLANDS 7 1

a partial vacuum in front, and liquids are drawn into the
mouth. The mechanism of drinking is the same.

Digestion in the Mouth.

Mastication. — The object of mastication is to grind up the
food so that it may be swallowed more easily and the various
digestive fluids, particularly the saliva and gastric juice, may
have more ready access to its parts. The proper mastication
of the food is an important factor in its. complete digestion
later on.

Mechanically, mastication is effected by the action of the
lower jaw, aided by the tongue, lips and cheeks. This re-
mark presumes of course that the teeth are intact. Lateral
and antero-posterior movements of the lower jaw combine
with its simple elevation to compress and grind the food be-
tween the teeth. The muscles which depress the lower jaw
are the diagastric, mylohyoid, geniohyoid and platysma.
Those which elevate it are' the temporal, masseter, internal
and external pterygoids. The attachments of the external
pterygoids are such that by their simultaneous action the
mandible can be thrown forward and, by their alternate con-
traction, from side to side. The tongue is active during mas-
tication in carrying the mass of food to this or that part of
the buccal cavity so that it may be ground up completely.
It also gives accurate information as to the size (of the mass)
and stage of mastication. The cheeks, as is shown in facial
palsy, are quite important in keeping the food from between
them and the teeth. The lips prevent the escape of liquids
from the mouth, in addition, to assisting in prehension.

The Salivary Glands and Their Secretion.

The first of the digestive juices with which the food comes
in contact is the saliva which is the mixed secretion of the
large salivary glands and the various smaller mucous and

72 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

serous glands which open into the mouth cavity. The chief
salivary glands are three in number on each side of the
mouth — the parotid, submaxillary and sublingual. Besides
these, there are, throughout the buccal mucous membrane, a
number of smaller glands of similar structure contributing
to the formation of saliva. The parotid gland is situated
just beneath and in front of the lobe of the ear; the sub-
maxillary beneath the mandible about the center of the base
of the submaxillary triangle, and the sublingual beneath the
mucous membrane of the mouth, just lateral to the lingual
frenum.

FIG- 33. — Cells of the alveoli of a serous or watery salivary gland.
(Brubaker after Yeo.}

A, after rest; B, after a short period of activity; C, after a prolonged period
of activity.

The duct from the parotid, Stenson's duct, runs beneath
the mucous membrane of the cheek to a point opposite the
second upper molar tooth, where is its opening into the
mouth. The duct from the submaxillary, Wharton's duct,
discharges the secretion from that gland into the mouth by
the side of the frenum of the tongue. The secretion from
the sublingual reaches the mouth by a. number of small ducts
(Rivinus) which open also by the side of the frenum, and
sometimes as well by a larger duct, Bartholin's, which runs
parallel with Wharton's and empties near it.

Histology. — In structure the salivary glands have been
shown to be of the compound tubular variety, the secreting
part being tubular. The parotid is a serous gland, the other

SALIVARY GLANDS

two are usually said to be mucous, though they contain both
serous and mucous cells. The ducts subdivide into smaller
ducts and tubes, until a distinct tubule is distributed to every
acinus and becomes the lumen of that acinus. The whole
arrangement resembles the branchings of a tree.

The flow from these glands is greatly increased by masti-
cation. From the parotid the flow is much more abundant
on that side upon which the mastication takes place. During
activity it can be shown that the granules of the serous cells
accumulate toward the lumen of the acinus while the outer

FIG. 34. — Section of a mucous gland. (Brubaker after Lavdowsky.}

A, in a state of rest; B, after it has been for some time actively secreting.

segment of the cells becomes comparatively clear. It is sup-
posed that this is an essential step in the production of the
organic constituents of the secretion — that the granules con-
tain either the ptyalin or the substance necessary to its for-
mation. It is also supposed that at the same time that the
ptyalin is being thus produced and discharged, very active
constructive changes are occurring in the clear zone of the
cells. During activity some at least of the mucous cells seem
to break down, but it is probable that the granules in the cell
protoplasm become converted into mucin, which, being ex-
truded, seem to destroy the cell itself.

74 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

Composition and Properties of Saliva. — While it is possi-
ble to draw certain distinctions between the saliva from the
different glands, these distinctions are comparatively unim-
portant, so far as digestion is concerned; for the secretions
from the three pairs of glands become mixed in the mouth,
and it is their combined effect which, in any particular case,
is observed. Saliva contains in 1,000 parts about 994 of
water, the remaining six parts being organic and inorganic
solids.

These solids are chiefly mucin, ptyalin, albumin and salts.
The salts are mainly the chlorides of sodium and potassium,
the sulphates of potassium, the phosphates of potassium, so-
dium, calcium and magnesium, and sulphocyanide of potas-
sium. The mucin gives the ropy consistence to the fluid and
serves a mechanical purpose only. The sulphocyanide of po-
tassium is unusual in the body secretions and its presence
here is interesting. It may represent an end product of pro-
teid metabolism. The true digestive value of saliva is due to
ptyalin, an amylolytic enzeme.

Were it not for the presence of epithelial cells in suspen-
sion, saliva would be clear and transparent.- Its reaction is
alkaline, its specific gravity is about 1004 to 1008, and the
average amount of daily secretion is about 2^4 pounds.

The parotid saliva is much more watery and mixes much
more readily with the food than the submaxillary and sub-
lingual, which latter is mucilaginous and gives to the bolus a
glairy coating. The sublingual saliva is thicker and more
viscid than the submaxillary.

Nerve Supply. — The connection of the nervous system
with salivary secretion deserves particular attention, since the
phenomena presented under its influence are typical, and,
if not explanatory of occurrences elsewhere in the body, are
at least very suggestive.

Each one of the three glands is supplied with both cere-
bro-spinal and sympathetic fibers. Each one of them has
three kinds of nerve fibers, secretory, vaso-dilator and vaso-

SALIVARY GLANDS 75

constrictor. The secretory and vaso-dilator reach the gland
in the cerebro-spinal trunks ; the vaso-constrictor in the sym-
pathetic. The vaso-constrictors and vaso-dilators are dis-
tributed to the walls of the blood vessels, and influence secre-
tion indirectly only by increasing or diminishing the amount
of blood going to the glands. The secretory fibers exert their
influence directly upon the gland cells. It is claimed also
that the secretory fibers are divided into sets controlling the
production of the energy-yielding constituents and sets con-
trolling the production of water and salts.

The parotid gland receives its cerebro-spinal fibers through
a branch of the fifth nerve, but when they are traced back-
ward it can be shown that they are in the tympanic branch
of the ninth, and pass from this branch to the small super-
ficial petrosal nerve and thence to the optic ganglion — from
which ganglion they run to the parotid gland by the way of
the auricula-temporal branch of the third division of the
fifth. The cervical sympathetic also sends fibers to this gland.

The submaxillary and sublingual glands are supplied by
the same nerves. Their cerebro-spinal fibers leave the brain
by way of the facial, follow the chorda tympani as far as a
short distance beyond its junction with the lingual nerve,
and then leave it to reach the submaxillary ganglion and
run thence to the submaxillary and sublingual glands. These
glands receive sympathetic fibers from the superior cervical
ganglion.

Influence of Nerve Supply. — Taking the parotid as an ex-
ample, it is found that stimulation of its cerebro-spinal fibers
produces an abundant watery flow of saliva ; the gland be-
comes decidedly redder, pulsation is sometimes apparent,
and it is evident that the amount of blood is locally increased.
When the sympathetic supply of the parotid is stimulated, the
secretion is inhibited or reduced to a minimum, the gland be-
comes pale and the amount of blood in it is evidently dimin-
ished.

Similar corresponding results are occasioned in the sub-

/6 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

maxillary and sublingual glands by stimulation of the chorda
tympani and the sympathetic fibers.

It would seem at first, in the light of the vascular changes
accompanying stimulation of the two supplies to all these
glands, that the resultant phenomena could be explained en-
tirely by variations in the amount of blood, and that the ner-
vous system influences their secretion only by contraction and
dilatation of the vessels. However, a number of circum-
stances, which it is unnecessary to relate here, prove that the
secretory fibers exert an influence directly upon the cells
themselves, causing them to secrete. The mere distribution
of these fibers to the gland cells presupposes some such func-
tion on their part ; and it can actually be shown that the se-
cretion can be increased when the blood supply is cut off, or
without dilatation of the vessels. Such action, however, is
of course only temporary, for the materials for secretion
must be supplied by the blood. The exact method of ter-
mination of the secretory fibers has not been determined.
It is probable that they end between and around the cells and
do not penetrate their substance.

Section of the chorda tympani causes a continuous flow
of saliva from the submaxillary and sublingual glands for
several weeks. This has been termed paralytic secretion,
and is supposed to be due to the fact that the chorda fibers
do not themselves run directly to the glands, but are distrib-
uted to sympathetic ganglia (the submaxillary or others in
the gland substance). Section of the chorda, then, causes
degeneration of its fibers only as far as these ganglia, and
their cells are thought to be subject, in some obscure way, to
continuous irritation (luring the period for which the para-
lytic secretion continues.

Function. — The function of this secretion is twofold, (a)
mechanical, and (b) chemical.

(a) From a mechanical standpoint (i) it facilitates pho-
nation, mastication and gustation by maintaining a proper
degree of moisture in the mouth; (2) its more watery parts

SALIVARY GLANDS 77

(parotid) mix with the food, dissolving part of it, so that it
may be more easily masticated and swallowed while its more
viscid parts ( submaxillary and sublingual) spread over the
surface of the bolus to aid in deglutition.

(b) From a chemical standpoint, the function of the saliva
is to convert starch into sugar. It does this through the
agency of its enzyme, ptyalin, which conforms to the char-
acteristics of enzymes already noted. Maltose (Ci2H22Oii
H-HaO) is the form of sugar produced, but there are several
intermediate substances formed before maltose finally re-
sults. The starch molecule (CeHioOs) was formerly sup-
posed to simply appropriate a molecule of water to form
dextrose (grape sugar, glucose, CeH^Oe), but it is now
thought that there is a succession of hydrolytic changes with
the production of dextrin and maltose. That is, the starch
molecule appropriates a molecule of water; this new mole-
cule splits into a certain kind of dextrin and maltose* the
dextrin left itself appropriates water and splits up into an-
other kind of dextrin and maltose; this last dextrin goes
through a similar process with a like result, until finally only
maltose is produced. Some dextrose may be produced. It
will be seen under gastric digestion that mineral acids will
also convert starch into sugar, but in this case the form of
sugar is dextrose.

The effect of temperature on the action of enzymes has
been noticed. The optimum for ptyalin is 100° Fahrenheit
The reaction of saliva is alkaline and its effect on starch is
stopped by an acid medium, since the enzyme is thereby de-
stroyed. However, ptyalin has been shown to act even a
little better in perfectly neutral than in alkaline solutions
(Chittenden). The action of this substance on starch is very
much facilitated if the starch be cooked; in fact, its action
on uncooked starch is so slow that probably it is inconse-
quential in digestion. Cooked starch becomes hydrated, and
furthermore has its cellulose capsule removed from the
granulose, both of which circumstances make if much more
susceptible to salivary influences.

78 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

However, it must be admitted that the practical effect of
ptyalin in digestion is not very considerable in the mouth
mainly because the food is not kept in the mouth long
enough. However, large quantities of saliva are swallowed
with the food and it continues its action in the stomach
while the food is stored in the cardiac end and only ceases
its activity when the food is thoroughly mixed with the acid
gastric juice. The conversion of starch into sugar is con-
tinued and concluded in the small intestine.

Deglutition.

The act of deglutition is commonly divided into three pe-
riods, depending upon the part through which the food is
passing. During the first period the bolus passes from the
mouth through the isthmus of the fauces, during the second
through the pharynx, and during the third through the
esophagus into the stomach. A brief reference to the anat-
omy of these parts is necessary.

Fauces. — The isthmus of the fauces is the opening at the
back of the mouth, bounded below by the base of the tongue,
and above by the soft palate and uvula, and laterally by the
pillars of the fauces, between which are the tonsils. The
anterior pillars are easily seen when the mouth is opened
widely, and consist of the palatoglossi muscles with their
covering mucous membrane. The posterior pillars approach
each other more nearly than the anterior, and consist
of the palatopharyngei muscles and their covering mucous
membrane.

Pharynx. — The pharynx extends from the basilar process
of the occipital bone above about four and a half inches
downward. It communicates with the posterior nares, the
mouth, the Eustachian tubes, the larynx and esophagus. The
tube is made up of two coats, an external muscular and an
internal mucous. The muscular coat consists of the three con-
strictors and the stylopharyngeus. The mucous coat is cov-

DEGLUTITION 79

ered in its upper part with columnar ciliated and its lower
part by pavement epithelium.

Esophagus. — The esophagus runs a course of about nine
inches from the end of the pharynx, at a point behind the
cricoid cartilage, to the stomach, which it enters a little to the
left of the median line. The coats of the esophagus are two,
an external muscular and an internal mucous. The external
coat has its fibers disposed in two layers, longitudinal
and circular. The circular layer is internal. In the upper
third of the esophagus the fibers of the muscular coat are all
striped, but at the beginning of the middle third they begin to
give place to plain fibers, and these latter progressively in-
crease, to constitute virtually the whole muscular coat at the
diaphragm. The internal mucous coat is lined by squamous
epithelium. This is thrown into longitudinal folds except
during the passage of substances through the esophagus.
The outside fibrous tissue attaches the whole esophagus to
the surrounding tissue.

Mechanism of Deglutition. — The first period of degluti-
tion is voluntary but automatic, like respiration. The mor-
sel of food is forced toward and through the fauces by the
tongue, which presses from before backward against the
hard palate, with the bolus above it. That the tongue is
mainly concerned in this act is shown by inability to swallow
when this organ is absent, unless the food is pushed far back
into the mouth by the finger or other means.

The mechanism of the second period is much more com-
plex. The food must pass through the pharynx into the
esophagus, and must not be allowed to enter any of the other
openings communicating with the pharynx. The larynx es-
pecially is to be protected. Since the air enters through the
posterior nares above the isthmus and must enter through
the larynx in front of the esophagus, it follows that the cur-
rent of air would cross the current of food if swallowing
and respiration took place together. Consequently respira-
tion is suspended during deglutition. A.S soon as the food

8O THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

has passed the fauces, the elevators of the hyoid raise that
bone, and with it the larynx. It is at the same time pulled
a little forward, and since the pharynx is attached to the
larynx posteriorly, the former necessarily follows the move-
ment of the latter, and is thus slipped under the base of the
tongue and the entering bolus. With elevation of the larynx
the superior constrictor of the pharynx contracts upon the
food, and passes it quickly to the grasp of the middle con-
strictor, which in turn hands it to the inferior constrictor and
thence to the esophagus.

The posterior nares are protected by contraction of the
posterior pillars and the superior constrictor. The laryngeal
opening is protected by the epiglottis. When the tongue is
forced back and the larynx raised the natural effect would
be to fold the epiglottis down over the laryngeal opening.
At the same time contraction of the pharyngeal muscles
draws together the sides of the larynx and aids in closing
the glottis. Furthermore, the vocal cords fall together (as
they always lie except during inspiration — and inspiration is
now suspended).

The third period passes the food through the esophagus
into the stomach by contraction from above downward of
successive portions of its muscular wall. Contraction of the
longitudinal fibers draws the mucous membrane above the
bolus. Then the circular fibers, contracting in successive
segments from above downward, force the bolus before
them. These movements are continued until the food
reaches the stomach. The time consumed in swallowing a
given article is about six seconds.

This is the mechanism which carries all materials through
the alimentary canal from the esophagus to the anus. It is
called peristalsis, or vermicular (worm-like) action.

Nervous Control. — While nearly all the muscular tissue
concerned in deglutition is of the striated variety, the whole
process, except the first, which is automatic, must be consid-
ered as reflex. The mechanism of deglutition is one of the

DIGESTION AND ABSORPTION IN THE STOMACH 8l

best examples of finely coordinated muscular action to be
found. The afferent fibers concerned are from the 5th, gth,
and loth, and the superior laryngeal branch of the last. The
efferent fibers are from the 5th, 7th, gth, loth, and I2th.
The center for the reflex is supposed to be far forward in
the medulla.

It ought to be added that the Kronecker-Meltzer theory
of deglutition assails with considerable plausibility the me-
chanism of deglutition as above given. In a word, this the-
ory holds that when the bolus of food rests upon the dor-
sum of the tongue, and the tip of that organ prevents, by its
apposition to the hard palate, the escape of the food forward,
the mylohyoids contract with great force, compress the food,
and it escapes by the route of least resistance, which is back-
ward. It is thus shot into the esophagus, and the contraction
of the pharyngeal muscles only supplements that of the my-
lohyoids.

Digestion and Absorption in the Stomach.

Anatomy. — The istomach is situated beneath the dia-
phragm in the upper part of the abdominal cavity, and is
moored by the esophagus and folds of the peritoneum. Its
general shape has been compared to that of the bagpipe. Its
large, or fundic, end is to the left; its small, or pyloric, to
the right. By far the greater part of the organ is to the left
of the median line. A very considerable portion is to the
left of the esophageal opening. Except when distended, its
anterior and posterior walls hang in an approximately ver-
tical direction, and are usually in contact by their mucous
surfaces. Its greatest length when moderately distended is
about fourteen inches, its transverse diameter about five
inches, and its capacity about five pints. At the point where
the anterior and posterior walls meet inferiorly, the great
omentum (the peritoneum from the two walls) is given off.
This is the greater curvature and has the gastro-epiploica

82 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

FIG. 35. — Human alimentary canal.

a, esophagus; b, stomach; c, cardiac orifice; d, pylorus; e, small intestine; f,
biliary duct; g, pancreatic duct; h, ascending colon; i, transverse colon; ;', de-
scending colon; k, rectum. (Collins & Rockwell.)

DIGESTION AND ABSORPTION IN THE STOMACH 83

dextra and the gastro-epiploica-sinistra arteries running
along it between the two folds of the omentum. Where the
anterior and posterior walls meet superiorly, the stomach is
joined by the lesser omentum, the two layers of which are
continued in front and behind as the serous covering of the
stomach. This is the lesser curvature, and has the gastric
and pyloric branch of the hepatic arteries running along be-
tween the two layers of the lesser omentum. The large left
hand portion of the stomach cavity 'is called the fundus or
greater pouch. The opposite portion of the cavity is called
the lesser pouch or antrum pylori. At one end is the
cardiac or esophageal opening, at the other the pyloric.

Histology. — The coats of the stomach walls are four.
From without inward these are the (i) peritoneal, or serous,
(2) muscular, (3) submucous, and (4) mucous.

1. The peritoneal coat covers the whole of the organ ex-
cepting an inconsiderable linear area, where the two layers
of the lesser (gastro-hepatic) omentum join it along the
lesser curvature, and a similar area along the greater curva-
ture, where the serous coats of the anterior and posterior
walls leave the organ to form the great omentum. This coat
is simply a fold given off from the peritoneum to envelop the
stomach in practically the same manner as the other abdomi-
nal viscera. Its structure is that of serous membranes in
general.

2. The muscular coat, varying in thickness from %o in.
over the fundus to M.2 in. at the pylorus, is disposed in three
layers, (a) the external longitudinal, (b) middle circular,
and (c) internal oblique. The longitudinal fibers are con-
tinued from the corresponding fibers of the esophagus. They
are marked along the lesser curvature, but not very distinct
over other parts. The circular fibers are not abundant to the
left of the esophageal opening. They progressively increase
toward the right, and at the pyloric opening constitute a dis-
tinct and powerful muscular ring, the pyloric sphincter,
which, projecting into the lumen presents a more or less

84 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

flat surface on the duodenal side to prevent the regurgita-
tion of food. The oblique fibers are supposed to be contin-
uous with the circular fibers of the esophagus. They extend
over the greater pouch from a point just to the left of the
esophageal opening to a point on the greater curvature, about
the junction of the middle and pyloric thirds. Here, at the

Serosa. _:

FIG. 36.— V. S. Wall of human stomach.
E, epithelium; G, glans; Mm, muscularis mucosae. 15. (Stirling.)

fight hand limit of the oblique fibers, the stomach is said
during digestion to be considerably constricted, so that a tem-
porary sphincter is established. This is the point of separa-
tion between the fundus and the antrum pylori, and is some-
times called the sphincter antri pylorici. The fibers of the
muscular coat are of the plain variety, as is all the gastro-in-

GASTRIC GLANDS 85

testinal muscular tissue from the lower end of the esophagus
to the external sphincter.

3. The submucous coat consists of loose fibro-elastic con-
nective tissue which allows free movement between the mus-
cular and mucous coats. It contains rather large blood-ves-
sels and a nerve-plexus, the plexus of Meissner.

4. The mucous coat has an average thickness of about ^5
in., is loosely attached to the submucous coat, and, except
during gastric digestion, is thrown into longitudinal rugae.
It consists of columnar epithelium resting upon a basement
membrane, beyond (underneath) which is the capillary blood
supply. Throughout the greater part of the stomach the
mucous membrane can be shown to be divided by delicate
connective tissue into numerous polygonal depressions, from
the bottom of which extend the gastric glands.

The Gastric Glands.

In the mucous membrane of the stomach are found two
kinds of glands. According to their relative position with
reference to the two ends of the stomach they are called
fundic and pyloric. It is to be noted, however, that neither
of these divisions is confined strictly to that portion of the
stomach which its name would seem to indicate. Accord-
ing to their secretion the glands are called acid and peptic.
The fundic and acid, and the pyloric and peptic are consid-
ered to be identical. But attention is called to the fact that
while peptic (pyloric) glands secrete pepsin only, the acid
(fundic) secrete both acid and pepsin.

Structure. — Some of the gastric glands are simple tubules,
while others may be bifurcated, so that two (or more) tu-
bules communicate with the surface by a single canal. They
may all, however, be classified as belonging to the simple
tubular variety. They have a deep secreting portion and a
superficial non-secreting portion. The latter is lined by
columnar epithelium, and is the duct proper. The former is

THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

lined by cuboidal epithelium which discharges its secretion
into the lumen, this lumen being only a continuation of the
duct. These cuboidal cells are called peptic cells because

FIG. 37. — Vertical section of the gastric mucous membrane.

S, g, pits on the surface; p, neck of a fundus-gland opening into a duct, g; x,
parietal, and y, chief cells; a, v, c, artery, vein, capillaries; d, d, lymphatics,
emptying into a large trunk, e. (Landois.)

they produce pepsin, or its forerunner, pepsinogen. The
fundic (acid) glands are found to have lying close to the
basement membrane a number of large cells at intervals be-

GASTRIC GLANDS 87

tween the cuboidal cells and not extending outward to the
central lumen. They are thought to communicate with the
lumen by capillary ducts, which may even penetrate their
substance. They are supposed to secrete hydrochloric acid,

FIG. 38. — Section of the pyloric mucous membrane. (Landois.)

and are called acid cells from this fact, or parietal cells
from their position. (See Fig. 37.)
Method of Secretion,— When food is ingested gastric

88 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

movements very soon begin, carrying the food in this direc-
tion or that, as described later. At the same time, the gastric
mucous membrane changes from a pale pink to a congested
red, and soon drops of gastric juice begin to appear. They
run to the dependent portions of the cavity and become in-
corporated with the alimentary mass. It is believed that if
the gastric movements did not occur, this secretion would be
limited for fifteen or thirty minutes to a very small area,
namely, that with which the food is in contact. But it is
comparatively general because the movements bring practic-
ally all parts, at least of the fundic mucous membrane, in
contact with the food before this time has elapsed. The idea
is that up to fifteen or thirty minutes after the introduction
of food, the glands are made to secrete by direct mechanical
stimulation of the food, and after this time the secretion
becomes general, whether mechanical irritation becomes gen-
eral or not.

It ought to be added, however, that in recent years secre-
tion by mechanical stimulation has been denied, and the de-
nial is supported by good evidence. Besides direct proof by
experiments, it is shown that this early secretion occurs
without mechanical irritation, as when food is chewed and
made to pass through an esophageal fistula, or even by the
sight of food. These observers (Pawlow) state that food
introduced into the stomach through a fistula produces abso-
lutely no flow if the animal experimented upon does not
know of the introduction. Under this view the secretion is
a distinct reflex, the impressions being carried to the center
by afferent nerves distributed to the mouth, or by nerves o/
special sense.

Whether as a reflex or as a result of mechanical stimula-
tion, the fact remains undisputed that the flow begins a few
minutes after the introduction of food, and lasts until gas-
tric digestion is completed. After a time it is supposed that
chemical changes in the food itself further stimulate the gas-
tric glands, through their influence on the secretory nerves.

GASTRIC GLANDS 89

These stimulating chemical products are not developed alike
from all foods; and the conclusion is warranted that some
substances do not undergo gastric digestion so readily as
others. Ordinary bread and the whites of eggs, for example,
are said not to develop them. It has been further shown that
fats, oils, etc., actually develop substances which chemically
inhibit gastric secretion. There appears also to be a kind of
chemical regulation of the amount and quality of juice, ac-
cording as much or little, or varying acidity, is needed in the
digestion of the substance in the stomach.

Conditions influencing digestion operate mainly by produc-
ing changes in the quantity or quality of gastric juice, and
these changes in turn are largely effected through the ner-
vous system. Fever, overeating, depressing emotions, stren-
uous physical or mental exercise, etc., decrease the secretion
and correspondingly interfere with digestion.

Changes During Activity. — Like the salivary cells, the cu-
boidal peptic cells can be shown to undergo changes during
secretory activity. When at rest they contain abundant gran-
ules, but during secretion these granules disappear, first from
the base and later from well-nigh the whole cell. The gran-
ules are supposed to contain pepsin, or rather pepsinogen,
for it is thought that pepsin is not-formed by the cell directly,
but is made out of pepsinogen, which is the product of the
peptic cells, probably under the action of hydrochloric acid.
The rennin is also supposed to exist in the cells as some pre-
liminary material corresponding to pepsinogen. This ma-
terial may be termed rennin zymogen.

Changes in the acid cells during activity also occur, but
are more obscure than those in the peptic cells. The source
of hydrochloric acid is a decomposition of the neutral chlor-
ides of the blood and the union of the chlorine thus liber-
ated with hydrogen, but how or why this occurs is not ex-
plained by phenomena so far observed.

Secretory Nerves. — While it has been impossible to de-
monstrate secretory fibers to the cells of the gastric glands,

9O THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

such fibers' must exist in the vagifc. Section of it (and the
sympathetic), however, does not entirely stop the secretion,
but incidents referred to in a preceding section, such as se-
cretion at sight of food, or when food is chewed and not
swallowed, certainly point to an influence of the central sys-
tem over secretion. Of course the sympathetic fibers to the
vessel walls are indirectly concerned.

Condition of Food on Entering Stomach. — The food en-
ters the stomach in the same condition in which it left the
mouth. It has been more or less completely triturated by
mastication ; the whole has been moistened, and a part dis-
solved by the saliva. All the materials taken in have been
thoroughly mixed with each other, and some of the starch
has been converted into sugar. The reaction is now alka-
line, unless the acidity of the articles taken has been too
great to be overcome by the alkalinity of the saliva — in which
case there would be no amylolytic change. Excepting starch,
all foods entering the stomach are chemically unaffected. It
remains to see what happens to the foods under the influence
of gastric digestion. These changes are brought about by
the gastric juice aided by muscular movements of the stom-
ach.

Properties and Composition of Gastric Juice. — The secre-
tion of the glands of the stomach is called gastric juice. Gas-
tric juice may be secured in several ways, but the most reli-
able article for experimentation is taken from a previously
established gastric fistula in one of the lower animals. It is
a thin, almost colorless liquid of an acid reaction, and a spe-
cific gravity of 1005 to 1009. Chemically it contains per
thousand about 973 parts water and 27 solids. Proteid sub-
stances compose some 17 of the 27 parts of solid matter.
These substances are mainly mucin, pepsin and rennin. The
most important non-nitrogenous constituent is free hydro-
chloric acid. The others are chiefly the chlorides of sodium,
potassium, calcium, and ammonium, and the phosphates of
iron, calcium, and magnesium, The amount of gastric juice

GASTRIC GLANDS 91

secreted in twenty-four hours is from six to fourteen pounds.
Gastric juice will resist putrefaction for a long time, prob-
ably on account of the free acid. Its digestive properties
are due to the proteolytic enzyme, pepsin, the milk-curdling
enzyme rennin, and the free hydrochloric acid.

Hydrochloric Acid — The amount of free hydrochloric
acid present in normal gastric juice is from two-tenths to
three-tenths of one per cent. It has been frequently claimed
that the acidity of this secretion is due to lactic acid, but
while it cannot be denied that lactic acid, from the fermenta-
tion of carbohydrates is, or may be, normally in the stomach
during ingestion, yet hydrochloric acid is undoubtedly the
free acid proper to the gastric juice. Digestion, however,
will proceed under a proper (variable) degree of an acidity
from almost any acid.

Beyond an insignificant effect in converting cane sugar
into dextrose, its function is a passive one, namely, that of
simply making the secretion acid, so that pepsin may act upon
the proteids.

Pepsin. — Pepsin is a proteolytic enzyme, the composition
of which has not been determined. From the definition, it
converts proteids into peptones. It operates only in an acid
medium. Hence its action is contingent upon the presence
of another constituent of the gastric juice, namely, hydro-
chloric acid. Pepsin is a typical enzyme, and reference to
the characteristics of those bodies will avoid repetition of its
properties here.

Rennin. — Rennin has the property of coagulating milk.
It acts upon the soluble proteid of milk (casein), changing it
into an insoluble product, which is precipitated. Acids also
will coagulate casein. Milk when left standing at ordinary
temperature has lactic acid produced by the action of bac-
teria upon the lactose in it, and this acid precipitates the
curd. The acid of the gastric juice might be sufficient to
bring about this result, but the quick coagulation of milk
when it is introduced into the stomach is probably not due

92 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

to the acid, since neutral extracts of the gastric mucous mem-
brane will themselves curdle milk. After coagulation the
action of pepsin begins and the casein is converted into pep-
tones in the usual manner. The value of the curdling pro-
cess is not apparent.

Action of Gastric Juice on Foods. (A) On Proteids. — A
familiar test for the proper performance of gastric digestion
is the observation of the effect of the juice in a given case
upon the white of an egg (proteid). In normal gastric
juice, or in a properly prepared artificial solution, the egg is
seen to swell up and dissolve. This soluble proteid is now
called peptone, and it differs from the proteid of the egg in
certain important respects, to be noted later. .But, although
peptone is the final product of pepsin-hydrochloric action,
there are certain substances produced intermediate between
the initial proteid and the final peptone, just as in the case
of the formation of maltose by ptyalin. Some of these sub-
stances have been called acid-albumin, parapeptone, propep-
tone, etc. But whatever they may be, the nomenclature of
Kuhne is being largely followed at present. He supposes
that the first product is an acid albumin which he calls syn-
tonin ; that syntonin under the influence of pepsin undergoes
hydrolysis, taking up water and splitting into primary pro-
teoses; that each of these primary proteoses takes up water
and splits into secondary proteoses; that these last undergo
a similar change with the production of peptones; so that
the successive substances are proteid, syntonin (acid-albu-
min), primary proteoses, secondary proteoses, peptones.

Peptones can be shown to be different from syntonin and
the proteoses by chemical reaction. The chief object of pro-
teolytic digestion is to get the proteids into a diffusible
condition. Peptones differ from proteids in at least three im-
portant respects: (i) They can pass through animal mem-
branes, that is, can be absorbed; (2) they are no longer co-
agulable by heat or many acids; (3) they are capable of as-
similation by the cells after they have been absorbed.

GASTRIC GLANDS 93

(B) On Carbohydrates. — There is no enzyme furnished
by the stomach to affect any of the carbohydrates. It is
true that salivary digestion proceeds in some small degree
in the stomach. Saliva is swallowed with the food, and until
the reaction becomes acid (which cannot be immediately),
there is no reason why the conversion of starch into maltose
should not proceed. It is also true that the mere acid of the
gastric juice can slowly convert cane sugar into dextrose.
Simple acidulated water will do the same.
• (C) On Fats. — Neither is there any fat-splitting enzyme
in the gastric secretion. So far as any chemical change is
concerned the fats leave the pylorus in exactly the same con-
dition as they entered the mouth. Their physical condition,
however, undergoes some change in the stomach. The body
temperature is sufficient to liquefy them, the vesicles in
which the droplets are contained are dissolved, and thus set
free, they become a part of the mechanical mixture, chyme,
and are made easier subjects of intestinal digestion.

(D) On Albuminoids. — The albuminoids are acted upon
by pepsin and hydrochloric acid in much the same way as are
the proteids. Taking gelatin as a type, gelatoses are formed
instead of proteoses. It is stated that peptic digestion does
not go further than the gelatose stage with the albuminoids,
conversion into peptones taking place under the influence of
trypsin.

Resistance of Stomach Wall to Digestion. — It would be
interesting to know why the stomach (or the intestine) does
not digest itself. If a portion of the stomach of another
animal be placed in that of a living animal, it will be di-
gested ; or if the circulation be cut off from a limited area of
the stomach, the secretion will frequently digest that part of
the organ and bring about a perforation ; or further, if any
living part of an animal, as the leg of a frog, be fastened in
the stomach of another animal, it will likewise be digested.
The last instance would seem to lead to the conclusion that
living matter can be digested, but in reality it is shown (Ber-

94 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

nard) that the tissue is first killed by the acid, and that no
digestion takes place in the alkaline intestinal juice. But
why the stomach is not thus attacked when other living tissue
is remains obscure. The most plausible theory is that the
gastric epithelium is possessed of some power, mechanical
^or physical, the nature of which is unknown, inhibiting the
action of the gastric juice, most probably by preventing its
absorption.

"A nearer approach to an explanation seems to have been
attained in the discovery of an antipeptic and antitryptic ac-
tion of the stomach and intestinal mucosa. This action is
probably due to antienzymes which are found throughout
the whole animal scale and occur not only in the intestinal
tract, but also in cells of other organs. " (Tigerstadt.)

Movements of the Stomach. — Whether the exact details of
the muscular movements of the stomach be known or not, the
essential fact to be remembered is that the organ is in a more
or less continuous state of muscular activity for several
hours after the ingestion of an ordinary meal, and that this
activity results in the physical disintegration of most of the
solids introduced, in the thorough mixing of all classes of
foods with each other and with the gastric juice, and in the
passage from time to time of such parts as have been re-
duced to a pultaceous condition through the pylorus into the
duodenum, until finally the stomach is empty.

In considering the mechanism of these movements a di-
vision of the organ into two segments, fundic and pyloric, by
the sphincter antri pylorici is to be kept in mind. When food
has entered the stomach the peristaltic wave of contraction
begins at the splenic end and passes toward the right. This
contraction is comparatively weak, is mainly evident along
the greater curvature, and increases in strength as it passes
toward the pylorus. Its wave-like character is due to the
contraction and subsequent relaxation of successive bands of
circular and oblique fibers. Regurgitation of food is pre-
vented by a rhythmical contraction of the lower end of the

GASTRIC GLANDS 95

esophagus, and the effect of this muscular wave (peristalsis)
in the fundus is to force the food toward the pylorus. But
when the right end is reached, the rather firm contraction of
the sphincter antri pylorici prevents the entrance into the an-
trum of all except the liquid or semi-liquid parts. The food,
thus denied admission to the antrum, takes a course along the
lesser curvature to the splenic end, then back along the
greater curvature, and such parts of it as have, during this
revolution, been sufficiently dissolved pass into the antrum.
These revolutions continue until the fundus has been emp-
tied.

It is not to be supposed that food has been accumulating
meantime in the antrum. Indeed, it is certain that muscular
contractions are here much more active than in the fundus,
where the movements are slow and of a rather compressing
nature. It is thought that very soon after the entrance of
food from the fundus the entire muscular wall of the antrum
undergoes very strong contraction of a peristaltic nature, and
the pultaceous parts of its contents are sent with some force
into the duodenum. Those which are not sufficiently dis-
solved to pass the pyloric sphincter are said to excite an anti-
peristaltic movement, whereby they are thrown back into the
fundus for further digestion — the sphincter antri pylorici
having now relaxed. However, substances which the gastric
juice and contractions cannot dissolve will finally pass the
pylorus, but they are probably delayed for a considerable
time.

This succession of movements is continued with a rapidity
and regularity varying with the condition of the organ and
the nature of its contents. They last until the organ is emp-
tied in part by the absorption of its contents, but mainly by
their passage into the small intestine. Each circuit in the
fundus probably occupies about three minutes, and gastric
digestion as a whole lasts usually from two to five hours.
The contraction and relaxation of plain muscle is much
slower than that of striped.

96 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

It is the fundus, and not the pylorus, which serves as a
reservoir and in which the greater part of gastric digestion
occurs. The precise condition of the pyloric sphincter dur-
ing gastric digestion is unknown. It may have simply an ex-
alted degree of tonicity which does not completely close the
opening and which can be overcome by pressure, or it may be
tightly contracted and require a distinct nervous dispensa-
tion to effect its relaxation for the passage of fluids as well
as solids. It would seem that the length of time for which
food is detained in the stomach depends more upon its phy-
sical condition than upon its chemical — that is, that upon
any stage of digestion which it may have reached ; for it can
be shown that fluids pass very quickly into the intestine.

The secretory occurrences during these movements are of
the greatest importance (see pp. 86-88).

Nerve Supply. — The stomach is supplied with pneumo-
gastric and sympathetic fibers. The latter can be traced
through the solar plexus, splanchnics and cervical ganglia to
the spinal cord. They exert an inhibitory effect on the mus-
cular tissues; their stimulation causes relaxation. The
vagus fibers are motor ; their stimulation causes contraction.
But these nerves serve only to regulate the muscular move-
ments. It is the stimulus of food in the stomach which ex-
cites gastric peristalsis. It is not stopped by section of the
nerves, though it may be interfered with. This stimulation
is exerted either directly upon the nerve fibers or upon the
ganglia of the stomach wall.

The conditions influencing gastric digestion operate mainly
through changes in the quality and quantity of gastric juice.

Digestion and Absorption in the Intestines.
The Small Intestine.

Anatomy. — The small intestine extends from the pylorus
to the caput coli, and is about twenty feet in length. It lies
in numerous coils which are held loosely in place by a fold of

DIGESTION AND ABSORPTION IN THE INTESTINES 97

peritoneum running from one side of the great abdominal
vessels, enveloping the gut, and returning to the parietal wall
on the opposite side of the vessels. The fold thus attaching
the intestine to the abdominal wall is the mesentery. The
distance along the mesentery from this parietal region to the
gut is three or four inches, except at the beginning and end of
the small intestine, where it is shorter, to bind the tube more
firmly in place. The upper eight or ten inches of the small
gut is called the duodenum, the next eight feet the jejunum,
and the remainder the ileum. No anatomical peculiarity sep-
arates these parts. The average diameter is about one and a
quarter inches.

Histology. — The wall of the intestine is in four layers,
serous, muscular, submucous and mucous. The serous layer
consists of the enveloping fold of peritoneum and needs no
description, except that, like serous membranes elsewhere, it
furnishes a lubricating secretion to provide for the easy glid-
ing of the intestines over each other and over the other vis-
cera. The muscular coat has its muscular fibers disposed in
two layers, an external longitudinal and an internal circular.
The latter is the stronger. Between the two muscular layers
is the nervous plexus of Auerbach. Between the circular
layer and the mucous coat is the submucous layer which con-
tains the nerve plexus of Meissner. These communicate
with others by fibers of extension. The mucous coat pre-
sents several points deserving mention. These are (i) val-
vulse conniventes; (2) villi ; (3) secreting glands, (a) of
Brunner and (b) of Lieberkuhn; (4) solitary and agminate
glands.

i . The valvulae conniventes are simply tpansverse folds or
tucks of the entire mucous membrane, each of which extends
from one-third to one-half around the circumference of the
tube and projects by its middle portion sometimes to the cen-
ter of the lumen. The small folds, 800 to 1,000 in number,
extend from about the middle of the duodenum to the begin-
ning of the last third of the ileum and greatly increase the

98 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

length of the mucous membrane over that of the gut proper.
They are not effaced by the passage of food or by other cir-
cumstances, for the two surfaces of the fold which are in ap-
position are bound together by loose connective tissue. The
fold as a whole, however, is freely movable upward or down-
ward in the intestine and has no tendency to obstruct the
canal. The only function of the valvulae conniventes is to
furnish a greater secreting surface and, by somewhat re-

FIG. 39. — Diagram of a longitudinal section of the wall of the small

intestine.

a, villi; b, Lieberkuhn's glands; c, tunica muscularis mucosae, below which
lies Meissner's nerve plexus; d, connective tissue in which many blood and
lymph vessels lie; e, circular muscle fibers cut across with Auerbach's nerve
plexus, below it; f, longitudinal muscle fibers; g, serous coat. (Yeo.)

tarding the passage of the alimentary mass, to subject it for
a longer time to the digestive fluids.

2. The villi are especially important in connection with
absorption, and their description properly belongs under that
head. They are conical elevations responsible for the velvety
character of the 'mucous membrane. They exist in great
numbers from the pylorus to the ileo-cecal valve, covering
the valvulse conniventes as well as the general surface of the
mucous membrane. The largest are about ^o in. long and ^o
in. in diameter at their base. They are only elevations of the
mucous membrane containing a central tube, the lacteal,
which is nothing but an intestinal lymphatic. The structure

DIGESTION AND ABSORPTION IN THE INTESTINES

from without inward — that is, from the surface of the villus
inward to its center — is (i) a layer of columnar epithelium
resting upon a delicate basement membrane; (2) lymphoid

FIG. 40. — Portion of the wall of the small intestine laid open to
show the valvulae conniventes. (From Yeo after Brinton.)

tissue containing abundant capillaries and connective tissue
cells; (3) a thin layer of plain muscle fibers continuous from

FIG. 41.— Vertical section of a villus of the small intestines of a cat.

a, striated border of the epithelium; b, columnar epithelium; c, goblet cells;
d, central lymph-vessel; e, smooth muscular fibers; f, adenoid stroma of the
villus in which lymph corpuscles lie. (Kirkes after Klein.)

the intestinal wall; (4) the lacteal, whose endothelial wall
contains many stomata.

3. The glands of Brunner and the crypts of Lieberkuhn,
or intestinal tubules, are supposed to produce the succus en-

IOO THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

tericus. The former are chiefly limited to the upper half of
the duodenum. The latter exist throughout the small and
large intestine.

4. The solitary and agminate glands are not supposed to
contribute to the production of the intestinal juice. They are
alike in structure, the agminate glands being only a collection
of solitary glands. The former are the Peyer's patches, so
important in the pathology of typhoid fever. These patches
are usually about twenty in number and confined to the lower
two-thirds of the ileum, where they occupy that portion of
the circumference of the tube opposite the attachment of the
mesentery. Their average dimensions are I X il/2 in. They
consist essentially of lymphoid tissue, the separate follicles of
which are surrounded by lymphatics and penetrated by
blood-vessels. They are covered by villi, but the valvulae
conniventes cease at their edges. The solitary glands are
more widely distributed than the agminate.

The chyme, having passed from the stomach to the small
intestine, encounters three digestive fluids, pancreatic juice,
bile and intestinal juice. These are, of course, mixed to-
gether, but none interferes with the action of the other.

The Pancreas. — The pancreas is a large gland lying in the
upper part of the abdominal cavity behind the stomach. It
has the general shape of a hammer, its head being embraced
by the bend of the duodenum and its opposite extremity
reaching to the spleen. It weighs some four or five ounces,
and is about seven inches long. Its duct, the duct of Wir-
sung, usually joins the common bile duct just where this lat-
ter penetrates the wall of the duodenum, so that the bile and
pancreatic juice enter the small intestine together. Some-
times the two ducts do not join, and sometimes a second
smaller duct from the pancreas penetrates the duodenum a
little below the larger one. The duct of Wirsung traced
backward divides and subdivides until its final ramifications
end in the alveoli, or secreting portions.

Histology. — This is a compound tubular gland. The cells

THE PANCREAS IOI

in the alveoli are of the serous type aiid-'sxe gr&nular V>\v;ird
the central lumen. During activity they undergo- .changes
very similar to the salivary cells; Ihe'il6il->&#nuter,'£&ite to-
ward the basement membrane increasing and extending and
the granular zone becoming correspondingly smaller. Here,
as in the salivary glands, it is believed that the granules are
made from the clear protoplasm, and contain the enzymes or

a
A

FIG. 42. — One sacctile of the pancreas of the rabbit in different states
of activity. (From Brubaker after Yeo.)

A, after a period of rest, in which case the outlines of the cells are indistinct
and the inner zone — i. e., the part of the cells (a) sext the lumen (c) — is broad
and filled with fine granules. B, after the gland has poured out its secretion,
when the cell outlines (d) are clearer, the granular zone (a) is smaller, and the
clear outer zone is wider.

their formative materials. The formative material in all
these glands is given the name of zymogen, although the zy-
mogen in a particular gland may have a particular name, as
pepsinogen, the forerunner of pepsin, or trypsinogen, the
forerunner of trypsin.

Properties and Composition of Pancreatic Juice. — The
pancreatic juice is a colorless liquid, alkaline in reaction, and
has -a specific gravity of about 1040 if taken from a recent
fistula. It coagulates when heated and is prone to putrefac-
tion on exposure. With a specific gravity of about 1040, it
contains per thousand about 900 parts of water, the remain-
der being different solid food materials in solution. These
constituents are a proteid and three very important digestive

IO2 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

f erments, tryp&ift, steapsin and amylopsin. The phosphates
and carbonates are plentiful and give the fluid its alkaline
reaction.

Trypsin. — Trypsin, like pepsin, converts proteids into pep-
tones. Nothing positive is known of its composition, but it
is possessed of the usual characteristics of enzymes regard-
ing temperature, etc. It differs from pepsin in that its pro-
teolytic action is more powerful and can take place in alka-
line media. It will also act in neutral or weakly acid media.
The opinion is advanced that while the gastric juice is capa-
ble of converting proteids into peptones, as a matter of fact
it does not usually carry the process further than the pro-
teose stage, and thus prepares the proteoses for tryptic di-
gestion.

It was seen that the successive products of pepsin-hydro-
chloric digestion are syntonin, primary proteoses, secondary
proteoses and peptones. In tryptic digestion it seems that, in
the splitting process, the syntonin (here alkali-albumin) and
primary proteose stages are omitted, and the first product is
secondary proteoses, which are split into peptones. Further-
more trypsin goes a step beyond with some of the peptones
and converts them into simpler compounds, the best known
of which are leucin and tyrosin. These are found normally
in the intestinal canal, but the physiological importance of
this conversion is not apparent. The opinion that it is a
useless sacrifice of useful peptones does not seem warranted.

Amylopsin. — The amylolytic enzyme, amylopsin, is iden-
tical in its action with ptyalin. This enzyme is very impor-
tant, for it has been remarked that the action of ptyalin is
probably rather inconsequential, and by far the greater por-
tion of the starch, which constitutes a large part of our ordi-
nary food, must be digested in the small intestine — and al-
most entirely by amylopsin.

Steapsin. — Under the influence of steapsin neutral fats
take up water and undergo hydrolysis, with the production
of glycerine and the fatty acid corresponding to the kind

INTERNAL PANCREATIC SECRETION IO3

of fat which is split up. In the intestine it is probable that
only a part of the neutral fats are thus split in glycerine and
fatty acids. The fatty acids thus formed unite with the alka-
line salts to form soaps, and these soaps, aided by intestinal
peristalsis, convert the remaining fats into an emulsion. The
products of fat digestion are therefore glycerine, soaps, and
emulsions, all of which can be absorbed in a way to be noted
later. While the emulsification of fats under the influence of
soaps (fatty acids and alkaline salts) is an undoubted effect,
the method of procedure is unknown. It is certain that the
emulsification is aided by the presence of bile, although this
fluid possesses no fat-splitting enzyme.

Method of Secretion. — It can be shown that the secretion
begins to be discharged into the duodenum very soon after
the entrance of food into the stomach, and continues as long
as intestinal digestion is in progress. Consequently the flow
will be intermittent if the meals are far enough apart. It is
almost certain that the secretion is a reflex act as a result of
impressions upon the mucous membrane of either the stom-
ach or duodenum. The acidity of the gastric juice seems to
be the natural stimulus and to exert its influence upon the
duodenal mucous membrane. This is not incompatible with
the early flow after the ingestion of food, for it will be seen
later that at least a small quantity of that food passes quickly
to the duodenum and carries gastric juice with it. The com-
position of the secretion seems to be influenced in some de-
gree by the character of the food. It is interesting that oils
increase the pancreatic flow.

Nerve Supply. — The pancreas has, besides vaso-motor
fibers to its vessels, distinct secretory fibers, like those of the
salivary glands. These fibers probably run in both the sym-
pathetic and the vagus.

Internal Pancreatic Secretion. — Circumstantial evidence
leaves scarcely any doubt that the pancreas produces some
substance which is discharged into the blood and markedly
influencees nutrition. Removal of the gland is followed by

IO4 THE- PHYSIOLOGY OF DIGESTION AND ABSORPTION

death from inanition in two or three weeks ; and previous to
that sequel the most striking phenomenon is marked glyco-
suria, with the ordinary symptoms of diabetes mellitus. Re-
tention of a comparatively small portion of the gland obviates
this condition. Sugar does not exist normally in the blood,
and this internal secretion may contain some ferment which
effects its consumption.

The Liver.

The liver is the largest gland in the body. Its function is
to produce bile, glycogen and urea.

FIG. 43. — The under surface of the liver.

i, lobus hepatis sinister; 2, lobus henatis dexter; 3, quadrate lobe; 4, caudate
lobe; 5, lobus caudatus; 6, hepatic artery; 7, portal vein; 8, fossa ductus venosi;
9, fossa vesicae fellae; 10, cystic duct; n, hepatic duct; 12, fossa venae cavae;
13, vena cava.

Anatomy. — The liver is situated in the upper part of the
abdominal cavity, chiefly in the right hypochondrium. Its
weight in the average adult is about four and a half pounds.
It is covered, except for a small area behind, by peritoneum,
processes of which run from it at several points and consti-
tute its supporting ligaments. The proper coat of the liver
lies underneath the peritoneum, and at the transverse fissure

THE LIVER IO5

is continued into the gland as a sheath, embracing the struc-
tures entering there and ramifying with them in their distri-
bution. This is the capsule of Glisson. It is fibrous in struc-
ture, is closely attached to the liver substance, and rather
loosely adherent to the structures which it envelops. The
walls of the portal vein are seen collapsed on section, while
those of the hepatic veins, which are not surrounded by Glis-
son's capsule, and which are closely adherent to the gland
substance, stand well open.

A general idea of the liver's anatomy is obtained by noting
that it has five lobes, five fissures, five ligaments and five
structures passing through the transverse fissure. The lobes
are right, left, caudate, quadrate and Spigelian. The fissures
are transverse, umbilical, that for the ductus venosus, the
fossa for the vena cava and the fossa vesicalis. The liga-
ments are coronary, right lateral, left lateral, round and sus-
pensory or longitudinal. The structures passing through the
transverse fissure are the portal vein, the hepatic artery, the
hepatic duct, the nerves and the lymphatics.

Blood-vessels. — Of the two blood-vessels entering the fis-
sure the portal vein is decidedly the larger. It has collected
the blood from the abdominal organs by the radicles of its
tributaries, the gastric, splenic, superior and inferior mesen-
teric veins, while the hepatic artery is a branch of the celiac
axis. These, having been distributed in a manner to be noted
presently, discharge their blood into the radicles of the
hepatic veins, which, usually three in number, enter the as-
cending vena cava, where that vessel passes through the liver
behind. Again, it is to be remembered that these two vessels,
as well as the nerves and lymphatics, are enveloped in the
vagina, or capsule of Glisson.

The portal vein and the hepatic artery give off branches to
the capsule of Glisson, constituting the vaginal plexus. The
portal vein, still ensheathed, then divides and subdivides until
its branches run directly between the lobules, and are called
interlobular veins. These direct subdivisions of the portal

106 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

vein are not the only interlobular veins, however. Those
branches of this vein which were given off to the capsule of
Glisson, having received the corresponding branches from
the hepatic artery, also here run between the lobules and
make part of the interlobular plexus. The interlobular veins,
thus surrounding the lobules and having lobules on either

FIG. 44. — Diagram of the portal vein.

(pv) arising in the alimentary tract and spleen (s) and carrying the blood
from these organs to the liver. (From Brubaker after Yeo.)

side of them, giving off in both directions branches (lobular
branches) which penetrate the lobules, to break up into ca-
pillaries. The capillaries finally converge to three or four
small radicles, which in turn unite to form a small vein in the
center of the lobule. This is the intralobular vein, which at
the base of the lobule joins the sublobular vein. These sub-
lobular veins join each other to form hepatic veins, which

THE LIVER

107

become larger and larger until they have collected all the
blood which has entered the liver. They finally enter the
ascending vena cava.

FIG. 45. — Section of lobule of liver of rabbit in which the blood
capillaries and bile canaliculi have been injected. (From Yeo after
Cadiat.}

a, intralobular vein; b, interlobular veins; c, biliary canals beginning in fine
capillaries.

But what has become of the hepatic artery? As soon as it
has entered the sheath, it gives off branches to the capsule

IO8 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

forming part of the vaginal plexus and entering into the vag-
inal branches of the portal vein just before these run be-
tween the lobules. It also furnishes branches to the wall of
the portal vein, to the wall of the larger divisions of the ar-
tery itself, and to the hepatic duct.

Histology of a Lobule. — The liver is made up of a large
number of lobules about one-t wenty-.fi fth of an inch in di-
ameter, separated by vessels, nerves and radicles of the he-
patic duct. Such a lobule in certain of the lower animals has
a distinct polygonal shape, but in man the outlines are not
clear. In the lobule are the hepatic cells, ovoid in shape,
possessed of small granules and one or two nuclei. They
are disposed in columns radiating from the central intralob-
ular vein. These cells belong to the epithelial type, and the
liver is not essentially different from other glands, such as
the salivary, except in the complexity of its arrangement.
The analogy is established by the origin of the bile ducts in
the lobules between the cells.

Bile Ducts. — It is not difficcult to demonstrate the inter-
lobular ducts, but to follow them as such into the lobule is
less easy. However, there is no doubt at all that they do
originate between the hepatic cells. It is probable that here
they have no distinct lining membrane, but are mere tubular
intercellular spaces, into which the bile is poured and car-
ried into the interlobular duct. Typically a liver cell has
one of these bile capillaries on one side and a blood capillary
on the other, and while this relation does not always hold
good, every cell does communicate with both kinds of capil-
laries. The interlobular bile ducts consist of epithelium rest-
ing upon a very thin basement membrane. As they increase
in size they gain fibrous inelastic and elastic tissue, and the
largest, some non-striated muscular elements. Gradually as
the ducts become larger the lining epithelium changes from
the columnar to the pavement form. Mucous glands exist in
the largest ducts. The interlobular ducts join each other and
gradually increase in size as they merge from all parts of the

THE LIVER

109

liver, to leave its substance in two divisions — one from the
right and one from the left lobe. These two unite to form
the hepatic duct which, running a course of about one and a
half inches, is joined at an acute angle by the cystic duct
to form the common bile duct, or the ductus communis
choledochus. The last penetrates obliquely the duodenal

Branch of portal vein.

Large interlobular bile duct.

Interlobular connective

Central veins

Central vein.

FIG. 46. — From a horizontal section of human liver. X4°.

Three central veins, cut transversely, . represent each a center of as many
hepatic lobules, that at the periphery are but slightly denned from their neigh-
bors. Below and to the right of the section the lobules are cut obliquely and
their boundaries cannot be distinguished. (From Stohr.)

wall and discharges the bile into the intestine. The cystic
duct has its origin at the apex of the gall bladder, and is
about one inch long. The common bile duct has an average
length of three inches. (See Fig. 43.)

Gall Bladder. — The gall bladder has an oval shape with its
large end forward. It is on the under surface of the liver,
the peritoneum running over (or rather under) it. It has a
mucous lining and the remainder of its structure is chiefly

I IO THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

fibrous. A little plain muscular tissue may exist. Its capac-
ity is about one and a half ounces. Mucous glands are found
in its lining, as in that of the large ducts, and these are re-
sponsible for the mucin of the bile.

Hepatic Nerves. — With regard to the exact destination of
the nerves entering the liver, little is known. Evidence going
to establish the termination of fibers in the cells, that is, the
existence of distinct secretory fibers is meager. There is
little doubt that secretory fibers for the glycogenic func-
tion of the liver do exist. It is known that fibers from the
vagus, phrenic and solar plexus enter the fissure, but they
cannot be followed farther than the ramifications of Glisson's
capsule between the lobules. Of course, vaso-motor fibers
go to the vessels, as elsewhere. Fibers acting similarly go
also to the muscular tissue of the large ducts and of the gall
bladder. The contraction of the gall bladder is thought to be
reflex, afferent impressions being conveyed by the vagus
from the mucous membrane of the duodenum.

Hepatic Lymphatics. — The lymphatics are abundant, and
those not originating in the connective tissue are thought to
originate by perivascular canals surrounding the blood-ves-
sels of the lobules. The fact that when the outflow of bile is
occluded it passes, not into the vascular, but into the lym-
phatic circulation is a curious circumstance. It may be due
to the absence of a definite wall for the intralobular ducts
and their comparatively free communication with the lym-
phatics in those localities.

Properties and Composition of Bile. — Human bile is of a
dark greenish-red color, has a bitter taste and is practically
odorless when fresh. It undergoes putrefaction easily, but
is not coagulable by heat. It is viscid, chiefly on account of
the mucin it contains. It has an alkaline reaction, and a spe-
cific gravity of about 1030. Besides water, which consti-
tutes more than ninety per cent, of its bulk, it contains the
sodium salts of taurocholic acid and glycocholic acid (the
biliary salts), cholesterin, bilirubin, lecithin, fats, soaps, mu-

THE LIVER III

cin and various inorganic salts, such as sulphates, carbonates,
phosphates, etc., and a quantity of carbon dioxide. The
quantity of bile secreted in twenty-four hours is about two
and a half pounds.

In human bile sodium taurocholate largely predominates
over glycocholate. These are formed as acids by the liver
cells, are absorbed in their passage down the intestine, and
are presumably those parts of the bile which are concerned
in its digestive action, particularly in the absorption of fats.
So far as these constituents are concerned, the bile is a typi-
cal secretion.

Cholesterin, on the other hand, seems to be simply re-
moved from the blood by the liver cells, and is discharged in
the feces, where, however, it exists in a slightly changed
form, stercorin. It is thought to be held in solution by the
bile acids, glycocholic and taurocholic. So far as this con-
stituent is concerned, therefore, the bile is a typical excretion.
It is produced in many of the body tissues, and no function
has been discovered for it.

Bilirubin is the characteristic coloring matter of the hu-
man bile; that of herbivorous animals is biliverdin, and a
little of this latter is also present in human bile. These pig-
ments originate from hemoglobin. It is supposed that when
the red corpuscles break down, "the hemoglobin is brought
to the liver, and then under the influence of liver cells is con-
verted into an iron-free compound, bilirubin, or biliverdin."
(Howell.)

The lecithin is probably an end product of physiological
activity in the tissues, and is apparently an excretion.

The mucin gives the fluid its viscid character.

The production of bile is continuous, but this does not
mean that its discharge into the duodenum is continuous, for
in the intervals of digestion it is not admitted (freely at
least) into the intestine, but regurgitates from the ductus
communis choledochus through the cystic duct into the gall
bladder, which acts as a reservoir until its contents are

112 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

needed. The secretion is more active, however, during in-
testinal digestion than at other times. This appears to be
reflex, but may be simply a result of the increased amount of
blood passing through the portal vein to the liver during that
period, for the whole alimentary canal is congested while di-
gestive activity is in progress. Again, it is known that the
best cholagogue is bile itself, and some of the bile is ab-
sorbed in its passage down the intestine. Its presence in the
blood may account for the accelerated flow.

Method of Secretion and Discharge. — The bile is a pro-
duct of the liver cells. How they receive their normal stimu-
lus is obscure. But it is reasonable to suppose that a larger
supply of blood means a more abundant secretion. Such an
increase of blood supply occurs during digestion.

The cells discharge the bile into the bile capillaries, which
pass it onward either to the intestine directly, or, during
the intervals of digestion, to the gall bladder. When food
enters the duodenum, a reflex influence causes the wall of the
gall bladder to contract and compress its contents. The only
outlet is through the cystic duct into the common duct, thence
into the duodenum. This reflex does not take place until
food has entered the duodenum, and of different foods it is
found that proteids (peptones) and fats are the most effi-
cient stimuli.

The secretion of bile is not stopped by ligation of either
the portal vein or the hepatic artery, showing that both of
these vessels contain bile materials. But it would be unrea-
sonable to suppose that the blood of the portal vein does not
furnish the bulk of secreting material.

Glycogenic Function. — The formation of glycogen is con-
nected with nutrition, but will receive some notice here.
This is an internal secretion. It is produced by the liver
cells, and can be demonstrated in their substance by the mi-
croscope and by chemical reagents. It can also be shown to
increase markedly after eating, and to decrease notably when
eating is refrained from for some time.

THE LIVER 113

Glycogen is a carbohydrate very similar to starch, and
when ingested it is acted upon by the same enzymes and un-
dergoes the same conversions. Furthermore, the amount of
glycogen in the liver is very greatly increased by restricting
the diet to carbohydrate foods and is lessened considerably
below the normal (that is, its amount on a mixed diet), but
is not reduced to zero, when proteids alone are taken. This
points to the conclusion that the source of glycogen is car-
bohydrates, but that it can be formed to some extent from
proteids. Let it be said now that practically all carbohy-
drates are converted by digestion into maltose, or maltose
and dextrin and furthermore that during absorption these
sugars are converted into dextrose or dextrose and levulose.
It is customary to assume that the digestion of a carbohy-
drate means its conversion into dextrose (glucose, levulose).
It is, then, this sugar which is carried to the liver by the
portal vein.

We may say that the formula for dextrose is CeH^Oe and
for glycogen CeHioCte, though neither of these formulae is
probably exactly correct. It will be seen, therefore, that the
abstraction of one molecule of water (HsO) from dextrose
will produce glycogen, and this is the change which the liver
cells are supposed to effect. Again, when the conversion of
dextrose into glycogen has taken place, the glycogen is stored
up in the liver cells, to be given off continuously to the blood
only in such quantities as the system may demand. The liver
thus becomes a warehouse for the storage of all carbohy-
drates.

It will be seen under Nutrition that the carbohydrates fur-
nish the chief material to be burned up in the body for the
purpose of liberating heat and furnishing energy, and if they
should be consumed as soon as they enter the circulation,
there would be not only an unnecessary waste during their
quick consumption, but also an unfortunate lack of energy-
producing materials before another meal. This storing up
brings about a kind of conservation of energy and an eco-
8

1 14 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

nomical regulation of its distribution. The amount of sugar
in the circulation at any time is very small, and a single car-
bohydrate meal may, by the action of the liver be made to
supply the carbohydrate demands of the tissues for a consid-
erable period.

Now, it was just said that the sugar of the blood is dex-
trose; if the dextrose of the portal blood is converted into
glycogen to be stored up, it must be reconverted into dex-
trose before it can leave the liver, since it leaves by the blood.
The cells do effect the second conversion, and this is the sec-
ond part of the glycogenic function. It may be that the liver
cells produce an enzyme corresponding to ptyalin, which con-
verts the glycogen. Dextrose does not normally exist in the
liver cells. At the very moment of its formation it is car-
ried away by the blood.

The fact that the liver can form glycogen out of pro-
teids shows, of course, that nitrogen is eliminated from the
proteid molecule in some way. A carbohydrate molecule is
left to be oxidized in the usual manner. This is thought to
be the initial step in the final consumption of proteids in nu-
trition. The fats have no influence on glycogen formation.

Glycogen also exists in other parts of the body, particu-
larly in the voluntary muscular substance. The cells of the
tissue in which it is found must also have a glycogenic func-
tion.

Urea Formation. — But the liver has another function be-
sides the production of bile and glycogen, and that is to form
urea. It will be seen later that the chief end product of pro-
teid metabolism is urea, and that it is eliminated almost en-
tirely by the kidneys. The liver is much more active in the
production of thfs substance when the portal blood is charged
with digested materials, but it also forms urea in fasting
animals. The liver must, therefore, be capable of forming
urea from some of the products of digested foods. With
reference to its formation in fasting animals, suffice it to say
here that it seems that as long as proteid metabolism goes

THE INFLUENCE OF THE BILE ON DIGESTION 115

on in other tissues, there are produced in those tissues ma-
terials (ammonia compounds) which, when carried to the
liver, are converted by it into urea. Further notice will be
given to this phase of the subject under Nutrition.

The liver cells produce urea ; it enters the blood, is carried
to the kidneys and eliminated by those organs. In the me-
chanism of its production and discharge from the liver, it
thus corresponds to the internal secretions, though urea is
distinctly an excretion.

It must not be supposed, however, that the liver is the only
organ producing urea. There are other organs which cer-
tainly produce it, while there are those who maintain that it
is produced directly wherever proteid metabolism is in pro-
gress.

The Influence of the Bile on Digestion.

The bile is not, properly speaking, a digestive fluid, for
it contains no enzyme capable of effecting digestive changes
in any of the foods ; but it so materially affects the
action of some of the other fluids that it cannot be overlooked
in a discussion of intestinal digestion.

So far as the bile acids, glycocholic and taurocholic (com-
bined to form salts of sodium) are concerned, the fluid is a
secretion, and it is these which are mainly concerned in the
digestive process. The production of bile is continuous, but
the gall bladder acts as a reservoir in which a part at least of
the secretion is stored in the intervals of digestion, to be dis-
charged in greater abundance when chyme enters the duo-
denum. While the action of bile in most of the digestive
functions to be mentioned is obscure, it is known to have at
least these uses :

1. It promotes intestinal peristalsis. '

2. It has an inhibitory effect on putrefaction in the intesti-
nal tract. By this it is not to be understood that the bile is
directly antiseptic, for it undergoes putrefaction very read-
ily itself, but only that in some way its withdrawal from the

Il6 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

substances passing through the alimentary canal allows their
more ready disintegration.

3. It aids in the emulsification of fats.

4. It promotes the absorption of fats. Recently the state-
ment that the bile promotes all kinds of absorption has appar-
ently been successfully disproved, but it seems certain that
"the bile acids enable the bile to hold in solution a consider-
able quantity of fatty acids, and possibly this fact explains
its connection with fat absorption." (American Text-Book.)

The Secretion of the Intestines.

The intestinal secretion, or succus entericus, is a product
of the crypts of Lieberkuhn and Brunner's glands. It is
scanty, of a yellow color and an alkaline reaction. Opinions
vary as to what foods are affected by this fluid, but since the
more recent experiments have overcome some difficulties in
obtaining specimens, the conclusions based upon them seem
most reliable. It is said to have no effect on proteids or
fats. It contains an amylolitic enzyme, which aids the pan-
creatic juice in converting starch into maltose. It also has
an enzyme, invertase, which converts cane sugar 'into dex-
trose and levulose, as well as an allied enzyme, maltose,
which converts maltose into dextrose. The carbohydrates
are absorbed as dextrose, with the probable exception of
lactose. It is mainly cane sugar, maltose (from starch)
and lactose that are in the alimentary tract and require to be
thus changed to dextrose.

It is not out of place to say that ptyalin produces maltose
and a little dextrose, and that the pancreatic juice and succus
entericus produce maltose and considerable dextrose. The
maltose is converted into dextrose during the process of
absorption. It is, therefore, customary to say that the carbo-
hydrates are absorbed only as dextrose.

Movements of the Small Intestine. — The effect of intesti-
nal movements is to force the contents onward through the

LARGE INTESTINE 117

ileocecal valve. Here it is that typical peristalsis is found.
The main factor in the passage is the layer of circular fibers.
Contraction of these fibers in the upper duodenum may at
least be conceived to begin upon the introduction of chyme.
The contraction passes down the gut in a wave-like manner,
the wave being produced by the contraction of segment after
segment of the circular fibers with relaxation just behind the
advancing contraction. The tendency of such a movement is
to force the alimentary mass along the canal. The longitu-
dinal fibers are probably chiefly concerned in changing the
position of the intestine and in shortening the tube, and thus
slipping the mucous membrane above the bolus, so that it
can be grasped by the circular fibers. A continuation and
repetition of these movements, which are slow, gentle and
gradual in character, is finally effectual in passing the con-
tents into the colon. It is not probable that antiperistaltic
movements take place normally.

Nerve Supply. — Very probably the intestinal movements
are naturally excited by the food and by the bile. It is prob-
able also that these stimuli exert their influence through the
ganglia of the plexuses of Auerbach and Meissner. The in-
testine receives fibers from the right vagus and the sympa-
thetic. The former are probably motor (contractors) and
the latter inhibitory (dilators). Here, as in the stomach,
they are probably only regulators of the movements, without
being actually necessary to peristalsis.

The Large Intestine.

Anatomy. — The Jarge intestine, known as the colon, is
about five feet in length and is divided into ascending, trans-
verse and descending portions. The sigmoid flexure is the
terminal extremity of the descending colon and empties into
the rectum. The small intestine communicates with the
colon at right angles a little above the beginning of the latter,
leaving below the opening a blind pouch, the cecum, or

Il8 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

caput coli. From the posterior and inner aspect of the cecum
runs off the appendix vermiformis. The diameter of the
colon gradually decreases from two and a half to three and
a half inches in the cecum to the beginning of the rectum.
The ascending colon passes upward from its beginning in the
right iliac fossa to the under surface of the liver, where it
bends upon itself almost at a right angle (hepatic flexure).
The transverse colon runs directly across the upper part of
the abdominal cavity to the lower border of the spleen, where
an abrupt turn downward (splenic flexure) begins the de-
scending colon. The lower part of the descending colon oc-
cupies the left iliac fossa in the shape of the letter S, and is
the sigmoid flexure.

The rectum, which receives the contents of the sigmoid, is
not straight, as its name indicates. It curves ( i ) to the right
to reach the median line, (2) forward to follow the contour
of the sacrum, and (3) backward in the last inch of its
course. It has the shape of a dilated pouch, its lower ter-
mination at the anus being guarded by the powerful external
sphincter of striated muscle. Its diameter is greatest below.

The vermiform appendix has the three coats common to
the intestine, but its muscular coat is ill-developed. The
peritoneal coat generally forms a short meso-appendix at the
root of the organ. The blood supply of the organ is not
abundant. It is greater in the female than in the male, a
part of it coming through the appendiculo-ovarian ligament.
The appendix has no function.

The ileo-cecal valve, guarding the opening between the
large and small intestines, is made of two folds, upper and
lower, of the muscular and mucous coats, which folds pro-
ject into the large intestine. The serous coat runs directly
over from the small to the large intestine at their point of
junction, without being folded inward upon itself, as are
the others. This prevents obliteration of the folds by dis-
tention. By this arrangement the two portions of the gut
communicate only by a buttonhole slit, which is easily

LARGE INTESTINE 119

opened by pressure from the direction of the ileum but
which pressure from the cecum tends to close more firmly.

Structure. — The large intestine has the usual three coats.
The peritoneal, however, is lacking on the posterior part of
the cecum, ascending and descending colons, these parts
being bound down closely and having no meso-colon. The
sigmoid is entirely covered as is the upper third of the rec-
tum. The middle third of the rectum has no serous coat be-
hind, being firmly held in place, while the lower third lacks
this coat entirely. The muscular coat is peculiar, in that its
longitudinal fibers are collected into three quite strong bands,
evident to the eye. When the rectum is reached they spread
out over the whole circumference of that part of the canal.
These bands are shorter, as it were, than the wall proper, and
the consequence is that the whole length of the large intes-
tine is gathered up into a number of pouches. The mucous
coat is paler than that of the small intestine, presents no villi
and is rather closely adherent to the subjacent parts. In it
are found glands corresponding in appearance to the crypts
of Lieberkuhn, and they are so classed; but they probably
secrete mucus only. Some solitary lymphoid follicles also
usually exist here.

Changes Taking Place in the Alimentary Mass in the
Large Intestine. — Most of the substances which enter the
large intestine have resisted the action of the various diges-
tive fluids and are on their way to be discharged in defeca-
tion. Doubtless, though, some materials undergo digestive
changes in the colon, not under the influence of any secretion
there formed, but of the intestinal juice with which they are
incorporated on leaving the ileum. The secretion of the
mucous membrane of the large intestine furnishes no diges-
tive enzyme, and the changes going on in the alimentary
mass (now feces) are chiefly due to absorption. By some
unknown process, however, rectal aliments of an easily di-
gestible nature are absorbed, and that in a nutritive form.
The consistence of the fecal matter increases in its passage

I2O THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

through the colon, owing to the absorption of its more fluid
portions. The bile pigment is responsible for the character-
istic color. The odor is mainly due to bacterial decomposi-
tion, but partly to the secretion of the mucous membrane.

Bacteria in Intestinal Digestion. — The entrance of the
bile and pancreatic juice into the duodenum changes to alka-
line the previously acid reaction of the chyme. But it is
found that, when an ordinary mixed diet is given, the mass
leaving the ileo-cecal valve has an acid (proteid) reaction,
and that the proteids have not undergone putrefaction. The
alkaline medium of the upper intestine favors bacterial ac-
tivity, and it would seem that proteid putrefaction would en-
sue. But it is supposed that in health these bacteria set up
fermentative changes in the carbohydrates, with the produc-
tion of acids which inhibit proteid putrefaction, and account
for the acid reaction at the ileo-cecal valve. When the mass
has entered the colon the acidity is soon overcome and putre-
faction is the usual consequence. It can be seen how readily
this delicately adjusted balance may be disturbed by errors in
the proper kind and proportion of food, etc. Some of the
products of bacterial activity upon carbohydrates and pro-
teids are luecin, tyrosin, indol, skatol, phenol, lactic and bu-
tyric acid. The object of the production of these substances
is unknown.

Composition of Feces. — It seems at present that the main
bulk of fecal matter is made up of substances which are con-
tained in the intestinal secretions, and the alimentary canal is
more important in excretion than was formerly supposed.
These substances are waste matters from tissue metabolism.
Besides these materials, feces normally contain indigestible
and undigested matters, inactive salts, stercorin, mucus, epi-
thelium from the intestinal wall, coloring matter and sub-
stances resulting from bacterial activity. Stercorin is the
converted form of cholesterin, a constituent of the bile. The
coloring matter is from the pigment (bilirubin) of the same
fluid. Of the bacterial products the most important are in-

LARGE INTESTINE 121

dol and skatol. They represent proteid putrefaction; they
are responsible for the fecal odor; hence the characteristic
difference in the odor of the contents of the ileum and colon.
The reaction of fecal matter varies. The amount for the av-
erage person is about four and a half ounces per day.

Gases. — Hydrogen, nitrogen and cafbon dioxide are found
normally in the small intestines. They serve to keep the tube
patulous, and avoid obstruction, and also to prevent con-
cussion. In the large intestine bacterial activity increases the
number of gases present. Here, in addition to those found
in the small intestine, there are carbitretted and sulphuretted
hydrogen, with others at times.

Movements of the Large Intestine. — The muscular con-
tractions of the colon forcing the feces onward are of the
same general character as those of the small intestine, though
less violent. The contents thus passed analward by peristal-
sis accumulate gradually in the sigmoid flexure until defeca-
tion occurs.

Defecation. — The act of defecation is both voluntary and
involuntary — voluntary in the relaxation of the external
sphincter and involuntary in the peristalsis which brings the
fecal matter to present at that muscle. It is probable that the
rectal pouch does not usually contain feces, but the desire to
defecate is brought about by the entrance of the mass into it
from the sigmoid. Then, if the desire is obeyed, peristalsis
of the non-striated muscular coat continues, the internal
sphincter of plain muscle relaxes, as does also the external
of striped muscle, and evacuation takes place.

Usually, by an effort of will, evacuation can be voluntarily
prevented by maintaining the tonic contraction o'f the exter-
nal sphincter. If the desire to defecate be disregarded, the
fecal accumulation probably returns to the sigmoid, leaving
the rectum comparatively empty. The act of evacuation is
commonly aided further by voluntary contraction of the
diaphragm and abdominal muscles. The lungs are filled,
"the breath is held" (forcing down and holding the dia-

122 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

phragm), and the abdominal muscles likewise contract
powerfully to compress the viscera and force the feces into
the rectum. Pressure on the afferent nerves of the rectum
probably sets up the desire to defecate, and the contraction of
its walls, as well as the relaxation of the internal sphincter is
a reflex act. The center is in the lower segment of the cord,
but it is connected with the cerebrum, as is shown by emo-
tional influences on the act.

The average time occupied in the passage of the residue
of an ordinary meal from the mouth to the rectum is about
24 hours. Something like 12 hours of this is thought to be
spent in the large intestine.

While it has been endeavored to establish clearly the sep-
arate action of each fluid with which the aliment comes in
contact, it is to be remembered that they form a mixture, the
combined activity of whose component parts results in the
extraction of all the nutritive material from the bolus in its
long journey through the gastro-intestinal tract. It can
hardly be said to be still at any time during that passage,
the continual peristalsis to which it is subjected facilitating
both the chemical action of the enzymes and the physical
phenomenon of absorption.

ABSORPTION IN GENERAL.

Obviously digested materials are of no service in the vital
economy until they are absorbed — first by the circulation and
then by the tissues themselves. Here we will consider only
their absorption from the alimentary canal, which process, in
contradistinction to the other, may be termed external ab-
sorption.

While it is known that the laws of diffusion and osmosis
outside the body are largely responsible for absorption within
the organism there are many phenomena in connection with
that process which cannot be explained under these laws,
and which are indeed, in some cases, at variance with them.

ABSORPTION IN GENERAL 123

The only explanation at present to be offered of anomalous
action is to refer it to some peculiar property inherent in the
cells themselves — the epithelium in case of the alimentary
canal. So profoundly important in connnection with physio-
logical activity are the laws of osmosis outside of the body,
and what is known concerning the mutability of those laws
inside the body, that a brief consideration of the subject
seems necessary to an intelligent conception of many vital
phenomena.

Osmosis. — When two different kinds of gases are brought
in contact they mingle with each other, making a homogen-
eous mixture. This is due to the continual motion of their
molecules. When two different kinds of liquids are brought
in contact, a homogeneous mixture results for the same rea-
son— unless the liquids be non-miscible, as oil and water.
If now the liquids happen to be separated by a membrane
permeable by both, the result, while it may be delayed, will
be the same. If, further, these liquids hold in solution sub-
stances the molecules of which can penetrate the interposed
membrane, there will likewise be an interchange of these sub-
stances, and the fluids on both sides will come ultimately to
have the same composition. This passage of liquids and dis-
solved matters through an animal membrane is known as
osmosis.

It must be remembered that in the body particularly the
interposed membrane may be permeable to the solvent,
water, and less so, or not at all, to the dissolved substances.
Materials which will in solution pass through a membrane
are called crystalloids; those which will not, colloids. If
simple water be on both sides of the membrane, the inter-
change continues because of incessant molecular motion ; but
the currents equalize each other, and no alteration in volume
or composition becomes apparent. But if to the water on
one side there be added a solution of some crystalloid, as
sugar, the excess of water will pass to that side. The crys-
talloid in solution is said to exert an osmotic pressure, and

124 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

that pressure depends upon the density of the solution. In
course of time, however, the crystalloid passing itself through
the membrane, conditions of equal volume and density will
be established on the two sides of the membrane, and osmosis
in either direction will cease to be apparent. But if the
membrane be nonpermeable to the dissolved substance, an
excess of water will pass to the colloid side and will continue
so to pass until finally it will be inhibited by hydrostatic
pressure on that side. This is taken as the measure of os-
motic pressure for the colloid.

All substances in solution, whether crystalloids or colloids,
exert a certain osmotic pressure ; that is, they may be said to
interfere with the passage of a current from their side of the
membrane, and that interference depends on the number of
molecules in solution, or, in other words, upon the density of
the fluid. A fanciful but striking illustration refers the ex-
planation to the continual molecular motion: the molecules
of the dissolved substance act as a screen to protect the
membrane from the water molecules, which are incessantly
moving against it, and consequently, in a given time, more
molecules of water will strike and pass through the mem-
brane on the unscreened than upon the partially screened
side. Evidently the number of molecules in solution (the
density) has a material influence upon the escape of water
from that side. Of course, since a crystalloid finally passes to
. the less dense side in sufficient quantity to establish an equi-
librium, the effect of its osmotic pressure is only temporary ;
but while the osmotic pressure of a colloid may be less than
that of a crystalloid, its effect is inclined to be permanent.
For instance, if a hypertonic solution (one whose density is
greater than that of blood serum) of sodium chloride be in-
jected into the blood, the first effect is to cause an increased
flow of water to the vessels, but soon enough sodium chloride
passes out by osmosis to raise the density of the extravascu-
lar fluids, and thus to cause an escape of water from the ves-
sels. On the other hand, the osmotic pressure exerted by the

ABSORPTION IN GENERAL 125

proteids of the blood is comparatively small. But since they
are here chiefly as colloids and tend to maintain the concen-
tration of the circulating fluid, their effect is a permanent
factor influencing absorption into the blood-vessels.

Isotonic and hypotonic solutions are those having equal and
less densities respectively as compared to blood serum. Hy-
potonic solutions are most easily absorbed ; isotonic least
easily. Application of these principles explains the rationale
of giving some medicines in dilute and others in concentrated
form. As to the direction of the current, the one of greater
volume may be called the endosmotic and the one of lesser
volume may be called ex osmotic. For example, the current
in ordinary absorption from the alimentary canal is usually
termed endosmotic, though it may be reversed, as when
magnesium sulphate is given.

When it is said that the greater current is from the less
dense to the more dense fluid, no reference is had to the di-
rection of the solids in solution. If there be only one solid
concerned, it will be the one responsible for the difference in
density and if it be a crystalloid, it will pass through the
membrane until the density on the two sides is equal, and its
direction will be opposite to that of the water. If on the
side of less density there be another crystalloid in solu-
tion, but in less quantity than the solid on the side of greater
density, it will pass in the direction of the greater current of
water until conditions of equal concentration with respect
to this solid are established. In the laboratory the final re-
sult in any case of dissolved crystalloid or crystalloids is two
liquids absolutely identical in composition. A rectal enema,
hypertonic with sodium chloride, will give up sodium chlor-
ide to the blood, but it may at the same time draw upon that
fluid for urea, for example. This is suggestive when an at-
tempt is made to explain the products of glandular secretion,
excretion, etc. It may be that the capillary walls are per-
meable to certain substances in certain situations and not in
others.

126 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

In the body it may be said that well-nigh all the vital func-
tions are dependent upon osmosis. There are fluids separ-
ated by animal membranes everywhere. In the alimentary
canal, for instance, is a fluid containing matters fit to be ab-
sorbed; ramifying in the wall of that canal are blood and
lymph capillaries filled with fluid ; while separating the two
is an animal membrane consisting of the alimentary epithe-
lium, a little connective tissue and the endothelial lining of
the capillaries. These are conditions most favorable for os-
mosis, but the osmotic laws of the laboratory are by no means
immutable in the body.

From what has been said of osmosis in general, and con-
sidering variations due to conditions of circulation, etc., the
following facts seem clear as to absorption in the body : ( i )
The substance must be in a liquid or gaseous state; (2) it
must be diffusible; (3) the membrane must be permeable;

(4) the greater current is toward the more dense solution

(5) the less dense the solution the more quickly will it be ab-
sorbed; (6) the greater the pressure in the vessels the less
rapidly will absorption into them take place; (7) absorption
is more rapid the more rapid the blood current (continually
preventing "saturation" of the adjacent blood) ; (8) the
higher the temperature the more rapid is absorption; (9)
the "vital condition" of the cells is the most important fac-
tor of all.

A thorough grasp of these principles and probabilities will
do much to clarify almost all the phenomena of vital activity,
and many questions of a pathological nature.

Absorption from the Alimentary Canal.

It has been said that all digested materials must find their
way into the blood. It is to be remembered that there are
two ways by which they reach the vascular circulation ; first,
by direct absorption into the capillaries of this system, and
second, indirectly, by absorption into the lymphatic circula-

ABSORPTION FROM THE ALIMENTARY CANAL 1 27

tion and passage thence to the left subclavian vein. Those
lymph capillaries which are concerned in this absorption oc-
cupy the villi, and are called lacteals.

(A) From the Stomach. — Since all classes of food except
fats have been partly digested in the stomach, it follows that
all except fats may be absorbed here. However, as a mat-
ter of observation, the stomach is of much less importance
in absorption than was once thought. Practically, it is found
that water and salts are passed quickly on toward the duo-
denum and are not largely absorbed in the stomach. Sugar
and peptones are also found to be absorbed rather sparingly
here. All these substances can undoubtedly be absorbed by
the gastric mucous membrane, and their complete absorption
is prevented only by their removal through the pylorus. It
is interesting to note that alcohol and condiments, like pep-
per and mustard, greatly hasten absorption, either by in-
creasing the blood flow or by directly stimulating the "vital
activity" of the epithelium.

(B) From the 'Small Intestine. — Here absorption of all
classes of food is possible, and here in fact most of the foods
are absorbed. The digestive influences are more active upon
all the aliments, the mucous membrane is well adapted to
absorption by reason of its valvulae conniventes and its villi,
and the food necessarily remains in the small intestine for a
considerable time. The fats are absorbed in the upper
part of the small intestine ; for they pass into the lacteals of
the villi, and these do not exist in the lower ileum. The
fluids swallowed are almost completely absorbed here, but
their place is taken by the intestinal secretions. The pro-
teids are absorbed to the extent of 85 per cent., more or
less, before reaching the large intestine, and the carbohy-
drates almost entirely disappear.

(C) From the Large Intestine. — The absorption process
in the large intestine is quite active. The passage of the
mass through it is slower, and even occupies an absolutely
greater time than the journey through the much longer small

128 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

intestine. The consistence of the contents progressively in-
creases owing to continual absorption of the fluid portions,
until the pultaceous mass received by the cecum becomes
almost solid in the sigmoid. The degree of consistence may
be said to be greater the longer the sojourn in the large in-
testine. The proteids and carbohydrates which have es-
caped absorption in the small intestine are disposed of here,
partly by bacterial decomposition, and do not appear as such
in the feces. The absorption of easily digestible substances
in solutions, such as eggs, etc., from the lower bowel, al-
though there is no digestive enzyme there, is a matter of
common observation, but one which lacks explanation.

Forms in Which the Different Classes Are Absorbed, i.
Water and Salts. — Of course, water is absorbed in connec-
tion with all the foods as a vehicle for them, but water and
salts as such have been shown to be absorbed sparingly in
the stomach. They are soon conveyed to the small intestine,
where their rapid disappearance ensues. Hbwever, they
may be absorbed anywhere in the alimentary canal. The loss
of the water from the alimentary mass in the upper small in-
testine is compensated for by the secretions, so that the flu-
idity of the contents is not materially affected until the colon
is reached. Here absorption of water is active, and the
mass becomes more and more solid as the rectum is ap-
proached.

2. Proteids. — It is agreed that the first object of proteid
digestion is to render the nitrogenous foods more diffusible.
It is also agreed that the end products of such digestion, so
far as alimentary absorption is concerned, are proteoses and
peptones; and the natural conclusion, supported by experi-
mental evidence, is that these represent the forms in which
the proteids are absorbed. True, leucin, ty rosin, etc., fur-
ther end products of proteolysis, are formed, but these can
not be absorbed. The opinion that proteoses and peptones
are the absorbable forms of proteids is correct, for by far
the largest part of these foods are absorbed in this shape.

ABSORPTION FROM THE ALIMENTARY CANAL I2Q

It is supposed also that syntonin at least can itself be spar-
ingly absorbed from the alimentary canal, while the phe-
nomena of rectal absorption would point to the conclusion
that proteid absorption in other shapes is possible. Prac-
tically, however, proteoses and peptones may be regarded as
the products cf proteid digestion, and their production as
the object of proteolysis.

But, although these substances are absorbed by the blood-
vessels, the artificial injection of them into the veins occa-
sions untoward effects, or at least their rejection through
the organs of excretion. Furthermore, proteoses and pep-
tones cannot be detected in the blood during alimentary ab-
sorption. It follows, then, that in their passage from the
alimentary canal to the blood they undergo some change
whereby they lose their identity and are no longer recognis-
able as such. It is claimed that they are converted into
serum-albumin, and this is probably true. One effect at
least of the change is that they are now (in the blood) less
diffusible, more complex, and consequently remain more
easily a constituent part of that fluid.

The proteids enter the radicles of the portal vein.

3. Carbohydrates. — The sugar of the blood is dextrose,
and if cane sugar be introduced into the -veins it is rejected
by the urine without being changed. It may be said that,
with a few exceptions, all the carbohydrates are converted
into dextrose or dextrose and levulose, before entering the
blood. This form of sugar is easily oxidized in the tissues.
It is conveyed directly to the liver by the portal vein.

4. Fats. — The digestive end of the fats has been seen to
be emulsions and soaps. . They pass into the intestinal lym-
phatics, or lacteals. Their absorption is a mechanical pro-
cess. They enter and pass through the epithelial cells and
basement membrane of the villus. Having thus passed into
the stroma of the villus, their entrance into the lacteal is
easy ; for undoubtedly lymph spaces in the stroma .are con-
nected with the stomata of the central lymph capillary, and

-9

130 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION

there is a more or less constant flow of lymph through these
spaces toward the lacteal. The tendency, therefore, of the
fats to enter the lacteal is physically natural. It is a curious
fact that the peptones and sugars, having penetrated the lin-
ing epithelium of the villus, enter the blood instead of the
lymph capillaries.

A number of circumstances, such as the rate of absorp-
tion, the persistent direction of the current toward the blood
in the face of superior pressure, the disappearance of non-
osmotic substances from the canal, etc., are frequently at
variance with laboratory experiments. Application of
the laws of osmosis to the vital processes is seemingly sub-
ject to many variations, and explanation of many of the phe-
nomena of absorption in the body waits upon a clearer un-
derstanding of the so-called "vital activity" of the tissues.

CHAPTER VIII.
RESPIRATION.

Object. — The object of respiration is to furnish oxygen to
the tissues and remove carbon dioxide from them. The in-
tervention of the lungs and blood is necessary to accom-
plish this end. At each inspiration a certain volume of air
is taken into the lungs, and from it, while in these organs, is
removed a certain amount of oxygen which enters the blood
of the pulmonary capillaries. At each expiration there is re-
moved from the lungs a certain volume of air, and it con-
tains a proportion of carbon dioxide over and above that
contained in the ordinary atmosphere, i. e., in the inspired
air ; this carbon dioxide is removed from the blood of the pul-
monary capillaries and enters the air in the lungs. The en-
trance and exit of air to and from the lungs, in obedience to
movements to be noticed later, constitutes what is commonly
called respiration; but the mere tide of the air inward and
outward is of no significance unless the interchange of oxy-
gen and carbon dioxide takes place.

Internal Respiration. — Nor is this interchange of value
unless another occurs in the tissues. The oxygen which has
entered the pulmonary blood is conveyed by the circulation
to a point where the fluid is brought into very close relation-
ship with the tissues (namely, in the capillaries), and is here
given up to the cells; furthermore, at. the same place the
cells give up carbon dioxide to the capillary blood. It is only
for the purpose of effecting this last interchange that there
is any respiration, or any respiratory apparatus. Inspira-
tion and expiration, the pulmonary interchange of gases, the
transportation of oxygen and carbon dioxide to and away
from the. cells, are all equally immaterial except as being
means to the accomplishment of this end. It would make no
difference whether pulmonary respiration were kept up or

132 RESPIRATION

not if oxygen could be introduced into the blood and carbon
dioxide removed from it in some other equally efficient way.
So far as the cell is dependent on the acquisition of oxygen
and the removal of carbon dioxide, it would make no differ-
ence if there were no respiration and no circulation if these
materials could be acquired and removed in some other
equally efficient way.

On the other hand, it would be useless to keep up artificial
respiration or to inject oxygen into the lungs if the cells,
through some disability, cannot take up the oxygen fur-
nished, or if the circulation cannot absorb or convey the
oxygen.

It is seen that, from the standpoint of the blood, the inter-
change of gases on the lungs is exactly opposite to that in the
tissues ; that is to say, in the lungs it loses carbon dioxide and
gains oxygen, while in the tissues it loses oxygen and gains
carbon dioxide. The pulmonary interchange is properly
termed external respiration in contradistinction to that in
the tissues which is termed internal respiration.

It is needless to comment upon the universal necessity of
oxygen to the life of cells. Its appropriation is to be looked
upon as a part of the nutritive process; and, indeed, while
in the long run, cells are certainly dependent upon the nu-
triment furnished by the ordinary aliments, they will retain
their vital activity for a longer time when deprived of any
or all of these than when deprived of oxygen alone. This
gas is more immediately necessary to the maintenance of life
than is any other substance.

Since, in order to bring about internal respiration in the
human being, the lungs and circulation happen to be nec-
essary, attention will have to be directed to the respiratory
phenomena taking place in both.

ANATOMY OF THE RESPIRATORY ORGANS.

It will be considered that the air has passed through the
posterior nares into the pharynx and is ready to enter the
larynx.

ANATOMY OF THE RESPIRATORY ORGANS

133

The Larynx. — This lies in front of the esophagus, its
upper opening communicating with the middle pharynx. It
is composed of four cartilages and the muscles and liga-
ments which hold them together. The cartilages keep its
lumen constantly open, while the muscles effect movements
concerned in deglutition, respiration and phonation. The
cartilages are the thyroid, cricoid and two arytenoids. The

The wind
bronchi, whic

FIG. 47. — Diagram of the respiratory organs.

pipe leading down from the larynx is seen to branch into two large
lich subdivide after they enter their respective lungs. (Yeo.)

two alae of the thyroid meet at an acute angle in front to
form the Adam's apple. The cricoid is at the lower end of
the larynx, completely surrounding it. The arytenoids are
movable and rest upon the back of the cricoid. (Fig. 48.)

The vocal cords, two ligamentous bands covered by a thin
layer of mucous membrane, stretch antero-posteriorly across
the upper end of the larynx, while the false vocal cords,
having nothing to do with phonation, and pinker in color, are

134'

RESPIRATION

FIG. 48. — Outline showing the general form of the larynx, trachea,
and bronchi, as seen from behind.

h, great cornu of the hyoid bone; *, superior, and t' the inferior, cornu of
the thyroid cartilage; e, epiglottis; a, points to the back of both the arytenoid
cartilages, which are surmounted by the cornicula; c, the middle ridge on the
back of the cricoid cartilage; tr, the posterior membranous part of the trachea;
b, b', right and left bronchi. (Kirkes after Allen Thomson.)

THE TRACHEA 135

above and parallel with the true cords. A small triangular
leaflet of fibre-cartilage is attached by its base to the base
of the tongue and to the upper anterior part of the larynx.
This is the epiglottis. It fits accurately over the opening of
the larynx, and during the act qf deglutition is closed to pre-
vent the entrance of food, saliva, etc. Except during deglu-
tition the epiglottis is raised and there is free passage of air
into and out of the laryngeal cavity. The vocal cords are
fixed anteriorly to a point between the alse of the thyroid
and posteriorly to the movable arytenoids. Intrinsic muscles
have the power of so moving the arytenoids as to separate
and approximate the posterior attachments of the cords and
thus increase or decrease the size of the rima glottidis. Dur-
ing inspiration these muscles act to separate the cords and
allow free entrance of air into the trachea. When this act
has ceased they relax and the cords are passively approxi-
mated. The expiratory act separates the cords and they af-
ford no obstruction to the exit of air. The inspiratory act,
on the other hand, tends to draw the cords together and the
active intervention of the muscles is necessary to keep the
glottis open.

The Trachea. — The trachea succeeds the larynx in the re-
spiratory tract. It begins at the cricoid cartilage and
extends downward for about four and a half inches where it
bifurcates to form the right and left bronchi, one of which
goes to each lung. The trachea consists of an external
fibrous membrane, between the layers of which are a num-
ber of cartilaginous rings, and an internal mucous mem-
brane. The rings are the most striking part of the tra-
chea. They serve to keep the canal open at all times. The
inspiratory effort would otherwise collapse the walls and
prevent the entrance of air. These rings are sixteen to
twenty in number, and are lacking in the posterior third or
fourth of the circumference. They are, therefore, not true
rings. The interval between their ends is filled with fibrous
and non-striped muscular tissue. The mucous membrane is

136

RESPIRATION

lined by ciliated epithelium, and has mucous glands in its
substance (Figs. 47, 48).

The Bronchi. — The primitive bronchi are of the same es-
sential structure as the trachea. The right is the larger,
shorter, and more nearly horizontal. This probably ac-
counts for the more frequent lesions in the right lung. Pen-

BrorteMal Musc/e.

Bronchial ' flrtery.

G/anc/ acini & cfucf.

FIG. 49.

B, intra-pulmonary bronchus of cat; P. A. and P.V., pulmonary artery and
vein; bv, bronchial vein; V ', air vesicles. (Stirling.)

etrating the lung substance they divide and subdivide until
each, by its ramifications, communicates with every air vesi-
cle in that lung. When the primitive bronchus has divided, the
incomplete cartilaginous rings are replaced by irregular
plates of cartilage, which are so arranged as to completely
encircle the tube. These extend as far as the division of
the tubes into branches %o in. in diameter.

Surrounding the tubes in the lung substance is a circular
layer of plain muscular fibers; these cease only at the air

AIR VESICLES 137

vesicles. Elastic fibrous tissue is also present everywhere in
the bronchial walls and is continued over the vesicles them-
selves.

Bronchial tubes above Vm in. in diameter have in their
walls cartilaginous plates, muscular tissue, fibrous elastic
and inelastic tissue and a lining membrane of ciliated epi-
thelium.

Bronchial tubes l/5o in. in diameter, and smaller, have in
their walls the same elements except the cartilage; but as the
tubes subdivide, their walls grow continuously thinner and
the inelastic tissue becomes less and less in amount until it
finally practically disappears; the ciliated epithelial cells
gradually give place to a single layer of squamous cells in
the smallest tubes. The smallest bronchial tubes, the bron-
chioles, are from ^20 to M"o in. in diameter. O'f course ev-
erywhere in the walls there are vessels and nerves.

The Air Vesicles. — Each bronchiole opens into a collection
of air vesicles, or cells, called a pulmonary lobule. The term
lobulette will be here applied to it, however, reserving the
word lobule for a collection of lobulettes about *% in. in di-
ameter. The bronchiole entering the lobulette becomes the
infundibulum (Fig. 50), a slightly dilated canal from which
are given off from eight to sixteen oblong vesicles, the true
air cells. The cells are a little deeper than they are wide and
end in blind extremities. The diameter of the lobulette is
about %o-%2 in. ; that of the vesicle about ^oo-^o in. It has
been estimated that there are some 725,000,000 of these ves-
icles in the lungs and that their combined area is something
over two hundred square yards.

The walls of the air cells are very thin, being composed of
a single layer of -flattened epithelium together with highly
elastic fibrous tissue. Ramifying in this latter is a most
abundant supply of capillaries, which are larger here than
anywhere else in the body. The physical conditions are most
favorable for the exchange of gases between the blood and
air, each capillary being exposed to vesicles on both sides,

138 RESPIRATION

and the air and blood being separated only by the very thin
walls of the capillary and vesicle. The elastic tissue is very
important in expelling the air from the cells when the in-
spiratory effort has ceased.

For the nutrition of the bronchi and lung substance ar-

FIG. 50.-— Terminal branch of a bronchial tube, with its infundib-
ula and air-sacs, from the margin of the lung of a 'monkey, injected
with quicksilver.

a, terminal bronchial twig; b, b, air-sacs; c, c, infundibula. X 10. (Kirkes
after E. E. Schulse.)

terial blood is furnished by the bronchial artery, which en-
ters and ramifies with the bronchi. The entire mass of
venous blood passes directly from the heart through the
pulmonary artery to the lungs to be arterialized, and it is
the capillaries of this artery which furnish the abundant net-
work between the air cells.

The lungs have the shape of irregular cones, their bases
resting on the diaphragm and their apices extending to
points a little above the clavicles. They are completely sep-
arated from each other by the mediastinum and their exter-
nal surfaces are covered by the pleura, a serous membrane
similar to the peritoneum and reflected from the thoracic
wall. The right lung is divided by fissures into three lobes
and the left into two. Superficially the lung substance is
seen to be subdivided into areas about Y$ in. in diameter
called the lobules. Each lobule is composed of a number of
lobulettes as above mentioned.

MECHANISM OF RESPIRATION 139

MECHANISM OF RESPIRATION.

Respiration implies the more or less regular entrance and
exit of air to and from the lungs. The entrance is inspira-
tion; the exit expiration. Now, the thorax is a closed cav-
ity, notwithstanding the fact that the lungs have an opening
(the trachea) by which they communicate with the external
air; and, so far as the simple ingress and egress of air is
concerned, the question of pulmonary respiration resolves
itself into one of pure mechanics. The lungs may be looked
upon as a bag (or two bags) in the thoracic cavity. Inspired
air does not enter the thoracic cavity, but this bag which is
in it. This fact is of the greatest importance.

Furthermore, the lungs are everywhere in contact with the
thoracic wall by their pleural surfaces. They are composed
very largely of highly developed elastic tissue, but are per-
fectly passive themselves. That is to say, they possess no
power of expansion except in obedience to extraneous in-
fluences. As 'found in the thorax they possess a contractile
power, but only because certain forces have put their elastic
tissue on the stretch, and the contraction is a simple effort
of the tissue to return to the condition which characterized
it before it was subjected to t]je expanding force.

Before birth there is no air in the lungs, and this is the
only time when the elastic tissue is not on the stretch. The
bronchioles and air cells are collapsed, but the thorax is con-
tracted and the pulmonary and thoracic walls are in contact
by their respective pleural surfaces. When the child is born
an inspiration fills the lungs and they are never thereafter
devoid of air. They collapse to a certain extent and leave
the thoracic wall when the chest is opened, but cannot empty
themselves entirely because the walls of the bronchioles col-
lapse before all the air can escape. This collapse of the
lungs when the chest wall is opened shows that the lung
structure is in a constant state of tension, which tension has
always a tendency to empty the lungs, but cannot do so be-

I4O RESPIRATION

cause the thorax can contract only so far, and when its con-
traction has reached its limit, for the lung to contract far-
ther would mean a separation of the pulmonary and tho-
racic walls and the formation of a vacuum between them.
The additional reason above given, namely the collapse of
the bronchioles before all the air can escape, is inoperative
under normal conditions and need not be considered.

Causes of Respiratory Movements. — Seeing that the lung
structure has always a tendency to empty itself of air, it
follows that inspiration cannot be dependent upon the lung
itself. Granting, from the physical conditions present, that
the lungs and thorax must expand and contract together, the
expansion of the lungs in inspiration is a consequence and
not a cause of the thoracic expansion, and contraction of the
lungs in expiration is a cause and not a consequence of tho-
racic contraction. This statement as to expiration applies
only to ordinary tranquil respiration, as will be seen later.
Speaking broadly then, inspiration is an active and expira-
tion a passive process. That is, inspiration occurs as a re-
sult of the activity of certain muscles which operate to ex-
pand the thorax, and expiration, as a consequence, simply
of the cessation of activity on the part of those muscles and
the passive contraction of the lung tissue.

The relation of the thorax and lungs and the action of
each in respiration may be illustrated. Suppose a bellows,
which, say for some mechanical reason, cannot completely
collapse and which is itself air-tight, to contain a thin rubber
bag. communicating by a tube with the external air ; suppose
the bag conforms in general outline to the shape of the bel-
lows, and under a moderate degree of distention completely
fills the cavity of the bellows when the latter is collapsed as
far as possible. Now, it being understood that the bag was
somewhat distended to cause it to fill the bellows, and that
all air .has been allowed to escape by a temporary opening
from between the walls of the two and the bellows itself
made air-tight afterwards, it follows that unless the bellows

INSPIRATION 141

can contract the bag will remain distended and will not leave
the bellows wall, although it will have a constant tendency
to do so. It is also apparent that, since the bag exerts a con-
tinual compressing effect on its contents, the pressure inside
it will be greater than that outside between it and the bellows
wall. Under these conditions there will be a constant ten-
dency on the part of the bellows to collapse, and some active
force will be necessary to expand it ; when it is made to ex-
pand the contained bag will expand with it. Suppose
the expansion should be stopped at a certain point and the
bellows held (to prevent contraction) ; it is obvious that now
the pressure inside the bag is greater, while that outside be-
tween its walls and those of the bellows is less, than when
the expansion began; that is, the bag has become distended
more and is exerting a greater compressing effect upon its
contents. If now the bellows be simply released, both the
bag and the bellows will contract and the former will empty
itself so far as the latter will allow; but when the bellows
has reached the limit of its contraction the bag also ceases
to contract, although it remains in a constant state of ten-
sion. If at any time air be admitted to the bellows proper
the bag will at once collapse.

This illustration can be applied to the mechanical princi-
ples obtaining in ordinary respiration. The bellows is the
air-tight thorax which cannot contract beyond a certain
point ; the rubber bag is the elastic lungs under constant ten-
sion, communicating by the trachea with the external air
and following, or being followed by, the movements of the
thorax; the pressure in the bag and between it and the bel-
lows wall represents the intrapulmonary and intrathoracic
pressures respectively.

It will be noticed later that this illustration does not go
quite far enough to explain a few of the phenomena of ex-
piration, but it could very easily be made to do so.

Inspiration. — Any force which expands the thorax aids
in inspiration; and any muscles which increase any of the

142 RESPIRATION

thoracic diameters expand the thorax. The diameters in-
creased are chiefly the (i) vertical, and (2) ant ero- posterior.

The vertical is increased by descent of the diaphragm,
which descent is caused by its contraction, since, owing to
the intra-thoracic "pull" exerted upon it, it is normally
vaulted upward.

The antero-posterior diameter is increased chiefly by the
elevation of the ribs. Since these bones, attached posteriorly
to the spinal column, run not only forward but also down-
ward to join the sternum by the costal cartilages, it follows
that the elevation of their anterior ends will increase the di-
ameter in question.

Muscles of Inspiration. — Elevation of the ribs is effected
by a number of muscles. The three scaleni are attached
above to the cervical vertebrae and below to the first and sec-
ond ribs; their action elevates not only these ribs but the
whole anterior chest wall.

The action of the intercostales externi is still a subject of
dispute in connection with the physiology of respiration.
These muscles are attached externally to the adjacent bor-
ders of the ribs, and thus occupy the intercostal spaces.
Their fibers are directed downward and forward, and the
effect of contraction of any single intercostal muscle would
be to approximate the two ribs to which it is attached ; but
if it can be assumed that the first rib is fixed, then, from
the direction of their fibers, the external intercostals will
render the ribs more nearly horizontal by raising their an-
terior movable extremities. It seems that the first rib is pre-
vented from descending, probably by the simultaneous con-
traction of the scaleni. The intercostales interni have a di-
rection almost at right angles to that of the externi ; the ster-
nal portions of these act from the sternum and also elevate
the anterior extremities of the ribs. The levatores costarum
are attached to the transverse processes of the dorsal verte-
brae and to the upper borders of the ribs posteriorly. The
transverse processes are fixed points and the ribs are mov-

EXPIRATION 143

able on their spinal articulations. Contraction of these mus-
cles is, therefore, very efficient in elevating the anterior ends
of the ribs.

The action of the diaphragm is the most notable of the
muscular phenomena connected with respiration, and it de-
serves to be called the "muscle of respiration."

These are the muscles which are chiefly concerned in ordi-
nary inspiration. Their combined action also increases
slightly the transverse diameter of the chest. But there are
certain others, known as auxiliary muscles of inspiration,
which are called into play during profound or forced in-
spiration. Their action is evident from their attachments —
all operating chiefly to increase the antero-posterior diame-
ter. They are the serratus posticus superior, sterno-mastoi-
deus, levator anguli scalpula, trapezius, pectoralis minor,
pectoralis major (costal portion), serratus magnus, rhom-
boidei and erectores spines. It will be noticed that several of
these which usually take their point on the chest, as, for ex-
ample, the sterno-mastoideus, pectorales, etc., must, in order
to aid inspiration, take their fixed points at their other ex-
tremities.

Expiration. — When the force which expands the chest
during inspiration ceases to operate, expiration follows.
Not only does the elastic (i) lung tissue force out the air,
but the (2) thoracic walls, by their costal cartilages and their
intercostal tissues, are themselves elastic, and this elasticity,
aided by the (3) "tone" of the muscles which have been put
upon the stretch during inspiration and which are now seek-
ing to return to their normal condition, tends to restore the
thorax to the dimensions it had previous to the inspiratory
act. So far no actual muscular contraction has been brought
into play, and it is here assumed that none is usually con-
cerned in the expiratory act of ordinary tranquil respiration.

Some maintain that the costal portions of the intercostales
interni particularly are expiratory in quiet breathing; they
do contract and the ribs approach each other during the

144 RESPIRATION

act, but it is probable that they serve only to maintain the
proper degree of tension of the intercostal tissues.

Although the elastic reaction of the lung tissue during ex-
piration operates together with the elasticity of the thoracic
wall in diminishing the antero-posterior diameter of the
chest, it is chiefly effective in diminishing the vertical diam-
eter by raising the diaphragm. It exerts a certain "suction"
upon that muscle, causing it to arch upward in following the
contracting lungs. It is seen, therefore, that during inspira-
tion the chest wall and diaphragm exert "suction" upon the
lungs, causing them to follow, and during expiration the
lungs exert "suction" upon the chest wall and diaphragm,
causing them to follow.

Forced Expiration. — It is evident that, while ordinary ex-
piration is a passive process, a person can voluntarily force
out of his lungs more air than is ordinarily expelled, as in
singing, blowing, talking, etc. This is effected by certain
muscles whose contraction diminishes the thoracic capacity,
chiefly by depressing the ribs and elevating the diaphragm.
Those which depress the ribs are the intercostales internl,
infracostales and triangularis sterni. Those which elevate
the diaphragm do so by compressing the abdominal contents
and forcing them up against that muscle. They are the ob-
liquus externus, obliquus internus transver sails and rectus
abdominis. These depress the chest wall as well.

Rhythm of Respiration. — Under ordinary conditions in-
spiration and expiration follow each other in a regular rhyth-
mical fashion. Some hold that an interval follows inspira-
tion before expiration begins, but this is probably not cor-
rect. Indeed, it is doubtful if there be an interval following
expiration, though it will be here considered that there is a
brief one. Expiration is a little longer than inspiration. The
inspiratory act is of uniform intensity throughout, while the
expiratory act gradually diminishes in intensity as it ap-
proaches completion — a circumstance to be expected from
the physical condition causing it.

RATE OF RESPIRATION 145

After every six to ten respiratory acts a more profound
(sighing) inspiration than usual is taken, the effect being a
more thorough changing of the pulmonary contents. Cough-
ing, sneezing, hiccoughing, laughing, etc., all interfere with
rhythmical respiration.

Modified Respiration. — In coughing and sneezing a pro-
found inspiration precedes a violent convulsive contraction
of the expiratory muscles. Sighing is an expression on the
part of the tissues that more oxygen is needed and that,
therefore, the contents of the lungs must be more completely
changed. Yawning is a phenomenon similar to sighing, but
may not represent deficient oxygenation, as when it occurs
from contagion. Except in the contraction of different fa-
cial muscles, sobbing and laughing are identical from a re-
spiratory standpoint; in both there is a succession of quick
contractions of the diaphragm. Hiccough is an involuntary
contraction of the diaphragm accompanied by closure of the
glottis. It takes place during inspiration. In hawking the
glottis is open and a continuous expiratory current is sent
through the narrowed passage between the base of the
tongue and the soft palate. Snoring occurs with the mouth
open ; the current of air throws the uvula into vibration and
produces the characteristic sounds.

Sounds of Respiration. — When the ear is applied to the
chest there is heard during inspiration a breezy expansive
sound of slightly increasing intensity throughout, and ceas-
ing abruptly at the end of the act. Immediately begins the
expiratory sound, very short, lower in pitch than the inspira-
tory, and gradually decreasing in intensity until it is lost
before expiration is more than one-fourth finished. When
listening over a large bronchus this sound is prolonged and
has a higher pitch than usual. Respiratory sounds are more
pronounced in the female than in the male chest, owing to
the predominance of costal breathing in the former sex.

Rate of Respiration. — The respiratory rate sustains a fair-
ly constant relation to the cardiac rate, the ratio being about
10

146 RESPIRATION

one to four. This makes the average number of respirations
about eighteen per minute for adults. In a general way this
rate is subject to variations from the same causes as that of
the pulse. Any appreciable .fall in the amount of oxygen in
the inspired air will increase the number of respirations for
obvious reasons. The frequency and depth usually bear an
inverse ratio to each other.

Types of Respiration. — (i) Costal respiration is that car-
ried on by the chest walls; (2) diaphragmatic, that effected
by the diaphragm. In the former type movements of the
thorax are concerned ; in the latter, movements of the abdo-
men. According as the movements in costal respiration are
more pronounced in the upper or lower segment of the chest,
that type is subdivided into (a) superior costal, and (b) in-
ferior costal.

In young children the diaphragmatic, or abdominal, type
prevails ; in adult males a combination of the inferior costal
and abdominal; in adult females the superior costal. The
last circumstance is probably due in part to the mode of
dress in civilized countries, and in part to the provision
against encroachment of the uterus upon the abdominal
cavity during pregnancy.

Intrapulmonary and Intrathoracic Pressure. — It is evi-
dent that during inspiration the pressure inside the lungs
(intrapulmonary) is less than the ordinary atmospheric
pressure ; this, in fact, is the immediate cause of the entrance
of air. It is also evident that during expiration the intrapul-
monary pressure, owing to the compressing effect of the
lung tissue and the thoracic walls, is greater than the outside
atmospheric pressure ; this is the immediate cause of the exit
of air. In both acts the air rushes in or out, as the case may
be, in an effort to maintain the same pressure inside the
lungs as exists in the surrounding atmosphere. It is con-
venient to call the pressure which is less than atmospheric
negative, and that which is greater positive pressure.

The intrapulmonary pressure is negative during inspira-

PULMONARY CAPACITY 147

tion and positive during expiration. Now, owing to condi-
tions already referred to, as the chest and lungs expand dur-
ing inspiration, the pressure between the adjacent walls of
the two (intrathoracic) becomes less and less and reaches
a minimum at the end of that act. Furthermore, owing to
the continuous "pull" of the elastic lungs upon the chest
walls the intrathoracic pressure remains negative even at the
end of expiration. But it can be made to become positive
under forced action of the expiratory muscles, as in cough-
ing, blowing, etc. The constantly increasing negative con-
dition of intrathoracic pressure is evidenced by a drawing
in of the intercostal tissues during inspiration; when the
pressure assumes a positive character, as in the expiratory
acts of the pulmonary emphysema, these tissues bulge out-
ward.

Pulmonary Capacity. — It is evident that the most forcible
expiration cannot completely empty the lungs of air. The
air remaining after such an effort is the residual air. It
amounts to about 100 cubic inches. But in ordinary respira-
tion at the end of the expiratory act there is more than 100
cubic inches of air in the lungs, because in such cases all the
air possible is not forced out. In fact about 200 cubic inches
usually remain ; this consists of the residual plus another 100
cubic inches, which is called the reserve or supplemental
air. It can be forced out, but is not in tranquil respiration.
The amount of air which is taken into the lungs by an ordi-
nary respiratory act amounts to about 20 cubic inches, and is
termed tidal air. It is the only volume used in quiet breath-
ing. At the end of the inspiratory act in tranquil respira-
tion it is obvious that the expansion may continue still far-
ther, and a certain amount of air, over and above the tidal
air, be taken into the lungs. The maximum amount which
can be so inspired (beyond the tidal) is about no cubic
inches, and is the complemental air.

It is seen, then, that the entire lung capacity is equal to
about 330 cubic inches. But the residual air cannot under

148 RESPIRATION

any circumstances be called into use, and consequently the
vital capacity is equal to the total capacity minus the residual
air (100 cubic inches), or 230 cubic inches. It is the volume
which can be expelled by the most forcible expiration after
the most forcible inspiration.

The capacity of the trachea and larger bronchi is known
as the bronchial capacity, and amounts to about 8 cubic
inches.

The quantity of air in the small bronchioles and air vesi-
cles is increased by inspiration and decreased by expiration;
it is called alveolar capacity, and at the end of ordinary ex-
piration amounts to about 150 cubic inches. Quiet inspira-
tion increases it to about 180 cubic inches.

All these estimates, of course, represent only an aver-
age. The vital capacity is increased by stature, by any oc-
cupation which calls for active physical work and by various
other conditions.

Composition of Air. — Ordinary atmospheric air contains,
in round numbers, about 21 parts of oxygen to 79 parts of
nitrogen. These two gases make up the main bulk of the at-
mosphere. In addition, the atmosphere always contains a
little carbon dioxide (about .04 per cent.), ammonia, mois-
ture, organic material, dust, nitric acid, etc. All except the
oxygen and nitrogen are of minor importance in respiration
when they are not present in amounts beyond the usual. It
will be seen that the striking difference between inspired and
expired air is in the proportions of oxygen and carbon diox-
ide.

Diffusion in the Lungs. — The expired air contains much
more CO and much less O than the inspired air. The inter-
change of gases between the alveolar air and the blood is
responsible for the difference.

The question is what forces cause the O of the air to enter
the alveoli and the CCte to leave it. As might be supposed,
the air escaping during the first part of expiration differs
very little in composition from the inspired air, for it has

DIFFUSION IN THE LUNGS 149

been occupying the upper air passages where no interchange
occurs. The bronchial capacity is only about one-third large
enough to accommodate the tidal air, and consequently the
greater part of it must come from lower down in the lung
structure, and the CO2 in the expired air continuously in-
creases until the end of the act. At each inspiration at least
two-thirds of the tidal air must pass into the small bronchi,
or lower. Thus it is that inspiration and expiration them-
selves, taking into and bringing out of the vesicles (or at
least the bronchioles) air fresh with O and air vitiated
with CO2, aid very materially in keeping constant the com-
position of the alveolar air.

In the second place, the cardiac movements have a similar
effect, each systole decreasing the size of the heart and in-
ducing a fresh atmospheric current toward the deep alveoli,
and each diastole forcing a like current of vitiated air toward
the trachea. This force is not inconsequential.

In the third place, the diffusibility of gases under known
physical laws, without the aid of any such movements as
have been described, is an occurrence in connection with the
phenomenon in question. Every gas, under ordinary atmos-
pheric conditions, exerts a certain pressure. In every me-
chanical mixture of gases (such as the atmosphere) each in-
dividual gas exerts a part of the total pressure — a part pro-
portional to its percentage in that mixture. This has been
called the "partial pressure" of that gas. Since O is
present in ordinary atmosphere to the extent of 21 parts per
hundred, the partial pressure of oxygen in the atmosphere
is 21/ioo of the total pressure.

Now, in the air of the alveoli O is present to a less extent
than 21 parts per hundred, and consequently its partial pres-
sure in that situation is less than in the trachea and bronchi.
The result is that O continually makes its way from the
point of higher pressure (trachea and bronchi) toward the
point of lower pressure (alveoli). The tendency is thus to
establish a uniform partial pressure throughout the whole

I5O RESPIRATION

respiratory tract; but this is never done during life because
the partial pressure above is being continually increased by
the introduction of new O, and below is being continually
diminished by the removal of that gas from the alveoli by
the blood.

In case of CO2 opposite conditions prevail. This gas is
being continually introduced into the alveolar air from the
blood, and consequently it is present there in much larger
quantities than in the trachea and bronchi, which contain
newly inspired air. The partial pressure, therefore, of CO2
in the alveoli is much higfrer than in the upper respiratory
passages, and a continual current of it diffuses upward to
equalize the pressure; this is never accomplished, however,
for reasons of similar nature to those keeping up the con-
stantly unequal pressure of O.

These three factors — respiratory and cardiac movements
and the natural diffusion of gases — are, therefore, in con-
tinual operation to get O to and CO2 away from the alveoli.
Under their influence the composition of the alveolar air re-
mains fairly uniform.

Alterations of Air in the Lungs. — These are chiefly : (a)
Loss of oxygen, (b) gain of carbon dioxide, (c) elevation
of temperature, (d) gain of water, (e) gain of ammonia, (/)
gain of organic matter, (g) gain of nitrogen, (h) loss of
(actual) volume. The capital changes are loss of O and
gain of CO2.

(a) Loss of Oxygen. — The air in passing through the
lungs loses of O nearly 5 per cent, of its total volume. That
is, whereas on entering it contains 21 parts, on leaving it
contains only about 16 parts per hundred of this gas. Nearly
25 per cent, of the total volume of O inspired, therefore, is
lost in the lungs.

When the respirations are 18 to the minute, and 20 cu. in.
of air are inspired at each breath, the amount inspired in an
hour will be 21,600 cu. in. Since a little more than one-fifth
of this air is O, and since only 'one-fourth of the inspired O

ALTERATIONS OF AIR IN THE LUNGS

is consumed, the total amount necessary for an hour will be
about 1,100 cu. in. This allows, however, for no muscular,
digestive or other activity, and the amount actually necessary
is larger than this.

The circumstances which call for an increase in O almost
invariably cause an increase in the output of C(X

(b) Gain of Carbon Dioxide. — The amount of CCte in in-
spired air is about .04 part per hundred (fioo per cent.) ; the
amount in expired air is something more than 4 parts per
hundred. In round numbers then, the air in passing through
the lungs gains of CO2 4 per cent, of its entire volume.
This is in periods of rest from exercise, digestion, etc. The
total amount discharged in one hour is, on an average, about
1,000 cu. in. This estimate should probably be raised to
1,200 cu. in. for ordinary activity, and varies according to
many conditions, some of which are rapidity and depth of
respiration, age, sex, digestion, diet, sleep, exercise, mois-
ture, temperature, season, integrity of the nerve supply, etc.

The subjoined table from Kirkes' Physiology compares
the composition of inspired and expired air.

Inspired Air.

Expired Air.

Oxygen . . .

2096 vols per cent.

1 6 03 vols per cent

Nitrogen

79 vols. per cent.

79 vols per cent

Carbonic acid

o 04 vols per cent.

4 4 vols per cent

Watery vapor

variable

saturated

Temperature

variable

that of body (36° C )

Conditions Influencing Output of CO 2. — When the ra-
pidity of respiration is increasing, the depth remaining con-
stant, the percentage of CCte in the expired air is reduced
because more air is respired, but the total quantity in any
given time is increased. The same result follows an in-
creased depth and a constant rate. With a diminished ra-

152 RESPIRATION

pidity and increased depth more CO2 is exhaled than under
opposite conditions.

The amount of CO exhaled is small in very young in-
fants. But soon the output begins to increase, and in males
continues to do so up to about thirty years ; there is then a
slight decrease up to sixty, and afterward a considerable de-
crease to death.

In the female the output is less than in the male. In the
former sex the increase is said to cease at puberty and to
remain constant until the menopause, after which time it in-
creases to sixty and diminishes subsequently.

During digestion the quantity is considerably increased.
This is probably due to the muscular activity of the alimen-
tary tract, to glandular metabolism and to changes taking
place in the food products.

As to diet, it may be said in general that the exhaled CQz
is increased in quantity by the taking of nitrogenized foods,
tea and coffee.

The influence of sleep is to diminish the output.

Muscular exercise is very efficient in increasing the
amount of CCte exhaled ; in fact, this explains partly the va-
riations in connection with sex, digestion, sleep, etc.

A high degree of moisture increases the exhalation, as
does a rise in body temperature. A rise in external tempera-
ture, however, has an opposite effect.

The output is increased in spring and decreased in autumn.

When the efferent nerve supplying a part is severed the
production of COs in that part is at once diminished.

The consumption of O and the exhalation of CCte bear a
fairly constant relation to each other — any condition in-
creasing one increasing the other, and vice versa. The
facts, therefore, which have been mentioned as governing
the exhalation of CO2 may be applied to the consumption
of O.

OXYGE-N CONSUMED AND CARBON DIOXIDE EXHALED 153

70° F., it is found that air inspired through the nose and ex-
pired through- the mouth has its temperature raised from 70°
to about 95° ; the rise is less when the inspiration takes place
through the mouth. The last air of expiration is warmer
than the first. This gain of heat while the air is in the lungs
needs no explanation when it is remembered that the aver-
age temperature of the tissues with which it is in contact is
98.5° F., or higher.

(d) Gain of Water. — This water is in the form of vapor.
It is natural that the air should absorb water from the moist
surfaces with which it is in contact. The capillary network
with which it is in close relation supplies moisture to the mu-
cous membrane not only of the alveoli but of the entire
respiratory tract. One or two pounds of water are elimi-
nated thus daily.

(e) Gain of Ammonia. — Ammonia is exhaled in small
quantity by the lungs. It is insignificant except in cases of
suppressed kidney action.

(/) Gain of Organic Matter. — The quantity of organic
matter exhaled by the lungs is inconsequential (unless venti-
lation be bad), but such exhalation does occur to a small ex-
tent. It gives the odor to the breath.

(g) Gain of Nitrogen. — The exhalation of this gas by the
lungs is of no respiratory importance. The amount is said
to be Moo4£o the amount of oxygen consumed. An occa-
sional loss of nitrogen has been observed.

(h) Decrease of (Actual) Volume. — When the external
temperature is below about 90° F. the volume of expired air
is a little greater than that of the inspired air, because of the
increase of temperature it undergoes in passing through the
lungs. But the actual volume of the expired air, when re-
duced to the same temperature as the inspired, is found to be
always a little less than that of the latter. It is estimated
that from #o-%o of the total volume of the inspired air is
thus lost in respiration.

Besides the substances mentioned as being exhaled from

154 RESPIRATION

the lungs, it is well known that odorous emanations proceed
from them after garlic, onions, turpentine, alcohol, certain
drugs, etc., have been taken into the stomach.

Relation Between Oxygen Consumed and Carbon Dioxide
Exhaled. — A given volume of O will combine with carbon
to form the same volume of CO ; or the amount of O in a
given volume of CO is equivalent to that volume when set
free from the carbon. A cubic foot of O will unite with
carbon to form a cubic foot of CCte; or a cubic foot of
CO2 will yield, on dissociation, a cubic foot of O.

This being the case, if all the O consumed in the lungs
were exhaled therefrom in the form of CO, the amount of
CO exhaled would just equal the amount of O consumed.
But the amount of consumed O is about 5 per cent, of the
inspired air, while the amount of exhaled CO is only about
4 per cent, of the expired air. It follows, therefore, that I
per cent, of the volume of inspired air is not represented by
the CO2 exhaled from the lungs and skin. The relation be-
tween the consumed O and the exhaled COv is usually ex-
pressed as the "respiratory quotient" — the division of the
latter by the former giving the quotient. This quotient is
made to vary by many circumstances, though for any con-
siderable period its average is about the same.

While it has been stated that the O absorbed and the CO2
produced vary together usually, they are in a certain meas-
ure independent of each other. For CO does not result
from the immediate union of O with carbon of the carbo-
hydrates and fats, but may be stored in the shape of com-
plex compounds, which may later split up with the formation
of CO2, either by oxidation or by intramolecular cleavage.
Furthermore, more O is necessary to oxidize (that is, to
form carbon dioxide) some molecules than others. A fat
requires considerably more O to produce CO2 than does a
carbohydrate; so that the kind of food in store would
also affect the respiratory quotient.

With respect to the O which, in the long run, is not repre-

CONDITION OF CO2 IN THE BLOOD 155

sented in the CO2 exhaled from the lungs and skin, it is
certain that when various of the food stuffs are broken down
at least a part of it is appropriated by hydrogen to form
water.

Source of Exhaled Carbon Dioxide. — The increase of CO2
in expired air over the small amount contained in inspired
air is derived from the venous blood circulating through the
lungs. It exists in that blood under a constant tension, as is
demonstrated by its escape when the blood is placed in a
vacuum. The total amount escapes when the blood intact is
placed in vacua : when the corpuscles alone are so treated
they yield up all their OCte, though it is small in amount;
but the plasma alone in vacuo yields a less amount than
when it contains corpuscles. If now corpuscles be added to
the plasma the total amount of OO2 is forthcoming. The
corpuscles must, therefore, act as an acid causing the liber-
ation of this gas from the plasma. It is probably the hemo-
globin, or oxyhemoglobin, which has this effect, though in
the laboratory the phosphates and certain proteids of the
corpuscles produce a like reaction when brought in contact
with the carbonates and bicarbonates of soda.

Condition of CCte in the Blood. — About 5 per cent, of the
total amount of OCte in venous blood is in simple solution
in the plasma; about 75-85 per cent, is in loose chemical
combination in both corpuscles and plasma; the remaining
10-20 per cent, is in comparatively stable combination in the
plasma. Of the 75-85 per cent., by far the largest part is in
the plasma, probably in a condition of loose association with
sodium to form carbonates and bicarbonates ; the small part
in the corpuscles may exist in a similar state, but it is now
thought to exist in combination with the proteid portion of
hemoglobin. The total 75-85 per cent, in corpuscles and
plasma is so loosely combined that the mere diminution in
pressure in the lungs is probably sufficient to liberate it.
The 10-20 per cent, in firm chemical combination is that part
which cannot be extracted from plasma alone in vacuo, but

156 RESPIRATION

which is dissociated on the addition of an acid, or corpuscles,
or hemoglobin, etc. It may be that as the blood passes
through the lungs there is set free, in the formation of oxy-
hemoglobin, an acid which immediately unites with the bases
holding the CO in combination — the liberation of the latter
being the consequence.

The O being thus in the air vesicles, and the CO thus
free, or set free, in the blood, with the very thin animal mem-
brane consisting of the vesicular and capillary walls between
them, it remains to be seen what forces are concerned in the
interchange of these gases. It has been noted that only one-
fourth of the O entering the lungs in the air is taken up by
the blood ; so it is to be remembered that not all the CCte
entering the lungs in the venous blood is taken up by the air.

Interchange of Oxygen and Carbon Dioxide in the Lungs.
—The condition of "partial pressure" of gases in mixture
has been mentioned. Each gas exerts a pressure in propor-
tion to its percentage in the mixture, and this is called its
"partial pressure." Now, the extraction of O and COs from
the blood by placing it in a vacuum shows that both these
gases exist in the blood under a certain degree of tension.

The tension of a gas in solution being only the pressure
necessary to keep it in solution, it follows that if the pres-
sure be diminished the gas will partly escape. If an atmos-
phere containing, say, O at a certain partial pressure be
Drought in contact with a fluid containing O at a certain
tension, unless the partial pressure of the O in -the air be
equal to its tension in the fluid there will be an escape of the
gas from the point of higher to the point of lower pressure
or tension. If the partial pressure of the gas be less in the
atmosphere than its tension in the fluid, the current will be
from the latter to the former and vice versa. This will be
the case whether the media are in actual contact or separ-
ated by an animal membrane.

This is the condition which obtains in the pulmonary alve-
oli. The partial pressure of O in the alveolar air is much

CONDITION OF OXYGEN IN THE BLOOD 157

greater than the tension of O in the blood ; consequently the
current is from the air to the blood. The tension of CO2 in
the venous blood is much greater than the partial pressure
of the CQz in the alveolar air; consequently the current is
from the blood to the air.

But, here, as in the last analysis of almost all physiolog-
ical phenomena, it is found that, while these purely physical
laws are certainly concerned in the pulmonary interchange
of gases, they are insufficient to explain the occurrence in
full. For the blood will take from the alveolar air more than
enough O to establish an equilibrium of tension and partial
pressure; the tension of O in arterial blood is higher
than its partial pressure in alveolar air. So it is
found that the alveolar air will remove more than enough
CO2 to establish a similar equilibrium of this gas. It is
known that the avidity (chemical) of corpuscles for O to
form oxyhemoglobin causes the blood to appropriate more
O than it would otherwise do, but even then we are driven to
the usual ultimatum of ascribing some peculiar office to the
living epithelium of the intervening membrane.

Condition of Oxygen in the Blood. — Almost all the oxy-
gen is conveyed in the blood by the red corpuscles, where
it exists in rather unstable composition with hemoglobin
(probably with its pigment portion) under the name of oxy-
hemoglobin. Only a comparatively small part is held in so-
lution by the plasma. Dissociation of oxyhemoglobin oc-
curs when the pressure is sufficiently reduced.

Alterations in Blood in Passing Through the Lungs. —
The sum total of the changes taking place in the blood as it
passes through the lungs is represented by the term arteriali-
zation. In general, it may be said that the blood undergoes
changes exactly opposite to those of the air in circulating
through the pulmonary structure, and reference to the list
of substances gained and lost by the air will suggest the
main alterations in the blood.

Of course the most striking phenomena are the loss of

158 RESPIRATION

CO2 and the gain of O. In 100 volumes of arterial or venous
blood there are found to be, on an average, 60 volumes of O
and CO2. This total remains approximately constant, though
the relative amount of each gas varies according as the
blood is venous or arterial, and in venous blood under the
influence of several conditions to be mentioned. In arterial
blood the O will represent about 20, and the COn about 40,
of the total 60 volumes per hundred of gas. In ordinary
venous blood the O will represent about 7 volumes less (13)
and the CO about 7 volumes more (47) of the total 60. In
both venous and arterial blood there is an insignificant
amount of nitrogen, which is usually present to the extent
of 1.5 volumes per hundred.

The proportion of gases is about the same in arterial
blood taken from any part of the system. In blood coming
from actively secreting glands the ratio of O to CO2 is
nearly the same as in arterial blood ; in fact, such blood may
have a red (arterial) instead of a blue (venous) color. This
is because during activity blood is sent to the gland in in-
creased amount to furnish materials for secretion, while the
demand for oxygen is not relatively increased in that gland.

Besides the changes which are apparent on referring to
the alterations in the air passing through the lungs, there
are certain other general characteristics which distinguish
arterial from venous blood. The most noticeable is color.
Venous blood is changed in the lesser circulation from a dark
blue, or black, to a bright red. This is due to the formation
of oxyhemoglobin. The change of color does not occur
when the appropriation of O is interfered with, as when the
air is excluded from the lungs, or when carbon monoxide is
inhaled. . Again, there is every reason to believe that venous
blood coming from different organs differs in composition
according to the special materials which have been extracted
from it by those organs; the portal blood during digestion
must certainly be different in composition from the general
venous blood, and so it may be conceived that the blood com-

INTERNAL RESPIRATION 159

ing from no two different sets of capillaries is identical.
When all this meets in the right side of the heart and is sent
thence into the lungs it has a nearly uniform composition,
and needs only to receive O before it can supply the wants
of any particular tissue in the body. Arterial blood is also
more coagulable than venous.

Internal Respiration. — It has been said that the object of
external respiration and the transportation of O and CCte is
to make internal respiration possible. Oxygen, leaving the
alveoli in a manner already described, enters the blood and
at once combines with hemoglobin of the red corpuscles
to form oxyhemoglobin. A small portion of the O is used
up by the corpuscles in transit, with the production of CO2
and other metabolic materials — the corpuscles requiring O
in their metabolism just as do other cells. But by far the
largest portion is carried to the capillaries, where it is taken
up by the cells. At the same time the cells give up to the
blood CO2 — a result of their metabolic activity. The blood,
having thus given up its O, is changed in color, and carries
the CO2 back to the lungs to be exhaled.

To furnish O and to remove CO is the only object of
respiration. Living tissue exposed to an atmosphere con-
taining O will consume O and exhale CO even if no blood
be circulating through it. The exact manner in which a cell
uses O is not apparent. It is evidently an oxidation process,
which produces CC)2, and O is directly necessary to this pro-
cess. But the amount of CCte produced in any given time
may not correspond to the amount of O consumed in that
time ; it may be greater or less. "It is probable that during
rest O is utilized to some extent in oxidations which are not
at once carried to their final stage and in which relatively
little CO2 is formed ; hence during activity comparatively
little O is required to cause a final disintegration of the now
partially broken down substances, and thus to give rise to a
relatively large formation of COz" (Reichert).

The absorption of O is to be looked upon as a part of the

l6o RESPIRATION

nutritive process just as the absorption of proteid, e. g.,
and COs as one of the products of destructive metabolism
just as urea. There is small probability that the O unites
directly with the carbon of any of the food stuffs — although
this is the final result.

Interchange of Oxygen and Carbon Dioxide in the Tis-
sues.— Here application of the principles governing the in-
terchange of these gases in the lungs applies. It is found
that the tissues act as very strong reducing agents upon oxy-
hemoglobin, setting free the O. Now the tension of O in the
arterial capillaries is much higher than in the tissues ; in fact,
it is practically nothing in the latter situation, for the O en-
ters so quickly into combination that there is very little to be
found here at any time. Consequently physical laws en-
courage the passage of this gas out of the capillaries into
the tissue.

On the other hand, the tension of COa in the tissues is
much higher than in the blood, and the same physical laws
"encourage a current of CO2 toward the blood. Neverthe-
less, these laws do not explain all the phenomena of inter-
change ; the activity of the cells is an important agent, though
their influence may be of a chemical nature only.

Cutaneous Respiration. — Cutaneous respiration in man is
insignificant and not essential to life. The skin absorbs a
little O and exhales a little more CCte. It is estimated by
Scharling that the skin performs about %o of the respiratory
function. Death following the covering of the body surface
with an impermeable coating is not due to interference with
cutaneous respiration.

Ventilation. — Persons breathing in a confined space grad-
ually consume the O and increase the OOa of the atmosphere.
When the amount of O has been decreased to fifteen parts
per hundred it is insufficient for the respiratory demands.
When the OCte is increased to .07 part per hundred the air
becomes disagreeable and close; this is not, however, from
the accumulation of CO so much as from organic emana-

RESPIRATION OF VARIOUS GASES l6l

tions and disagreeable odors from the body, clothing, etc.
It is only that the amount of CCte serves as an indication of
the extent of accumulation of these materials that the
amount of .07 per cent, is fixed as the limit beyond which it
ought not to be present. This percentage of COs in air free
from emanations, etc., is not deleterious.

Since 1,200 cu. in. of O are consumed per hour, about 15
cu. ft. will be necessary for a day ; and since the 1,200 cu. in.
consumed represent only about one-fourth of the O inspired,
60 cu. ft. will be necessary ior inspiration during twenty-
four hours. This amount represents some 300 cu. ft. of at-
mospheric air — which an ordinary person must have in that
time.

But this estimate allows nothing for increased respiratory
activity, which inevitably occurs from some of the numerous
conditions influencing it. It is found that in prisons and
other institutions of confinement it is not safe to allow each
person less than 1,000 cu. ft. of atmospheric air. In crowded
houses, where this space per individual cannot be obtained,
it is necessary, in order to avoid unpleasant results, to change
the air continuously, or at frequent intervals. Natural and
artificial means are employed to accomplish this end.

Respiration of Various Gases. — The inhalation of pure
oxygen is not deleterious unless it be under higher tension
than in atmospheric air, when it becomes a local irritant.
The blood will not, however, appropriate more than the us-
ual amount. Nitrous oxide will sustain respiration for a
time, but soon produces unconsciousness and asphyxia, prob-
ably because it unites so firmly with the hemoglobin of the
corpuscles. Hydrogen may be inhaled with impunity if it
contain also oxygen in the atmospheric proportion. Carbon
monoxide is poisonous because it unites with hemoglobin to
the exclusion of oxygen and will not dissociate itself. Sul-
phuretted, phosphoretted and arseniuretted hydrogen are de-
structive of hemoglobin and consequently poisonous. Pure
carbon dioxide cannot be inhaled for any length of time,
ii

1 62 RESPIRATION

Abnormal Respiration. — The term eupnea is used to de-
scribe normal, tranquil breathing. Apnea is suspended res-
piration. Hyperpnea is exaggerated respiration. Dyspnea
is labored breathing. Asphyxia is essentially a want of O
characterized by convulsive respirations, and later by irregu-
lar shallow breathing. The last two named deserve some at-
tention.

Dyspnea may be due to either a deficiency of O or an ex-

FIG. 51. — The heart in the first stage of asphyxia.

The left cavities are seen to be distended; the left ventricle partly overlaps
the right; La., left auricle; l.v., left ventricle; a, aorta; p.a., pulmonary artery;
p.v., pulmonary vein; r.a., right auricle; r.v., right ventricle; v.c.d.. descending
vena cava; v.c.a., ascending vena cava. (Kirkes after Sir George Johnson.)

cess of CO2 in the blood. When an animal is made to
breathe in a small, confined space the amount of O soon be-
comes insufficient even though the amount of CO2 in the
blood be not increased. Again, if an animal be caused to
breathe air containing the usual amount of O and a large
amount of CO2, it will suffer from dyspnea also. In either
case the manifestations are practically the same — slow, deep
and labored respiration. In cardiac disease, hemorrhage,
pulmonary diseases, etc., dyspnea is from lack of O in
the tissues, because of enfeebled action of the heart, deficient

ASPHYXIA 163

quantity of blood, insufficient exposure of the blood in the
lungs, etc.

Asphyxia may be looked upon as exaggerated dyspnea.
The labored breathing of dyspnea becomes convulsive, and
finally collapse ensues. Respiration becomes shallow, con-
sciousness is lost, the pupils are dilated, opisthotonus devel-
ops, the reflexes disappear, and at last the heart stops beat-
ing. The skin and mucous membranes become blue from

FIG. 52. — 'The heart in the final stage of asphyxia.

The letters have the same meaning as in Fig. 51; in addition, p.c. represents
the pulmonary capillaries. The right auricle and ventricle, and the pulmonary
artery, are fully distended, while the left cavities of the heart and the aorta
are nearly empty. (Kirkes after Sir George Johnson.)

non-oxygenation of the blood. Asphyxia from submersion
is harder to overcome than from simple deprivation of air
outside the water. Resuscitation is extremely doubtful when
a person has been submerged as long as five minutes.

While the phenomena of dyspnea and asphyxia are refer-
able to the lungs, it is not the need of air in these organs, but
of O in the tissues, which gives rise to the symptoms. The
non-oxygenated blood in asphyxia will not circulate through
the capillaries except with the greatest difficulty, and the
result is that it accumulates in the arterial system, dams

i64

RESPIRATION

back upon and distends the heart, so that this organ is finally
paralyzed and ceases to beat. This is the cause of death
from asphyxia.

Effect of Respiration on Blood-Pressure. — The lowest
blood-pressure is just after the beginning of inspiration,
from which time it increases during inspiration to reach its
maximum just after the beginning of expiration; it gradu-

FIG. 53. — Carotid blood-pressure tracing of a dog.

Vagi not divided; I, inspiration; E, expiration. (Stirling.')

ally decreases from this time to the minimum just after the
beginning of inspiration. The general effect, then, of in-
spiration is to increase blood-pressure and of expiration to
decrease it. This remark applies to general arterial tension.

Taking inspiration, the increase in arterial tension is, in its
last analysis, due to the larger amount of blood sent into
the arterial system at each ventricular systole. The explana-
tion is somewhat complex, but if the mechanics of respira-
tion be understood it may be made satisfactory.

It was seen that the lungs are contained in an air-tight
cavity, the chest, and that they expand with the chest be-
cause of negative pressure ("suction") exerted upon them.
The heart is also a hollow organ situated in this cavity; it
has connected with it, and lying also in the thoracic cavity,
large vessels communicating with smaller extrathoracic ves-
sels. The heart and these great thoracic vessels are elastic
and distensible. Consequently the expansion of the thorax

RESPIRATION AND BLOOD-PRESSURE 165

also expands them slightly and tends to draw blood from
the extrathoracic into the intrathoracic vessels and
heart; in fact inspiration is one of the main forces
causing a flow of venous blood toward the
heart. Now all this, so far as it goes, tends to keep the
blood out of the extrathoracic vessels, and so to contradict
the statement that inspiration increases arterial tension.

But, remembering that we are dealing with arterial ten-
sion and that our effort is to prove that more blood is sent
into the aorta during inspiration than during expiration, it
is of value to note that since the walls of the aorta are more
resistant than those of the venae cavae there is less expansion
of the former than of the latter during inspiration, and con-
sequently less tendency for the arterial blood to regurgitate
into the thoracic aorta than for the venous blood to enter
the thoracic venae cavse. The same expanding force dilates
the pulmonary capillaries, pulmonary artery and pulmonary
veins — the artery least of these. Taking it for granted that
more blood is being received by the right side of the heart
from the incoming venae cavae, the somewhat dilated pul-
monary artery receives more from the right ventricle ; the
pulmonary capillaries are more dilated than the artery and
this fact greatly encourages (by a suggestive "suction") the
increased flow from the pulmonary artery ; they, therefore,
receive more blood than usual. The pulmonary veins, being
likewise dilated, exert "suction" upon the capillaries, and
thus receive and pass on to the heart a larger supply of
blood than usual. The heart, receiving more blood, must
send more into the aorta, thereby increasing arterial tension
in the extrathoracic vessels, unless, by expansion of the
chest, the thoracic aorta be so dilated as to accommodate
the increased amount — which is not true.

Then, finally, the validity of this argument will hinge on
the relative dilatation of the thoracic aorta and of the thor-
acic venae cavae. If the veins be less dilated by inspiration
than the artery, then they will receive an increase of blood

1 66 RESPIRATION

which will not completely occupy the increase of space in the
dilated thoracic aorta, and there will be a backward "suc-
tion" made upon the contents of the arterial tree with a
consequent decrease in pressure; but a condition just oppo-
site to this seems to obtain.

During expiration contrary conditions in general are op-
erative with contrary results. The intrapulmonary vessels
and heart are compressed, but the veins and capillaries more
than the aorta, with the result that less blood reaches the
heart than during inspiration, and the thoracic aorta being,
relatively to the thoracic venae cavae, more dilated now than
during inspiration can easily accommodate the decreased
amount of blood which it receives. Of course expiration in-
creases venous pressure in the veins which enter the thorax
back as far as the valves.

The reason the pressure does not rise with the beginning
of inspiration is because a short time is consumed in filling
the flaccid intrapulmonary veins, and the first increase of
blood is delayed for that purpose instead of passing on to
the left side of the heart. Similarly, the pressure continues
to rise for a short time after expiration has begun because
the large veins are being emptied by pressure during this
time and their contents are reaching the heart and being
forced into the aorta.

Movements of the diaphragm and abdominal muscles dur-
ing respiration also lend themselves to create like changes in
arterial pressure, but the main factors are intrathoracic.

The fact that the cardiac rate is increased during inspira-
tion and decreased during expiration may also have to do
with the variations in pressure.

All the foregoing remarks relative to arterial tension are
meant to apply to tranquil respiration. During forced in-
spiration, or forced expiration, the results may be modified,
or even- reversed, by circumstances not necessary to mention.

Nervous Mechanism of Respiration. — Although the mus-
cles of respiration are of the striated variety, it is by no ef-

NERVOUS MECHANISM OF RESPIRATION 167

fort of the will that the movements are kept up. They belong
to the class known as automatic ; that is, they are, up to cer-
tain limits, under the control of the will, but recur in a reg-
ular, coordinate and orderly manner without the active inter-
vention of volition. Respiration represents the activity of a
self-governing apparatus. These movements constitute a
finely coordinated set of contractions — contractions which
are regulated by means of afferent and efferent nerves under
the supervision of the respiratory center.

The respiratory center is in the lower part of the medulla
oblongata. Destruction of the encephalon above, or the cord
below, the center does not arrest respiration. It is bi-
lateral— a center for each side — and these are more or less
independent of each other, but are so intimately connected
by commissural fibers that any impression made upon one
usually produces a like effect upon the other. Each half pre-
sides over the lungs and respiratory muscles of its own side,
but acts synchronously with its fellow of the opposite side.
Furthermore, each of these lateral centers may be regarded
as consisting of two parts, one for inspiration and one for
expiration. Stimulation of the inspiratory center not only
strengthens the inspiratory act, but also accelerates respira-
tion. Stimulation of the expiratory center strengthens ex-
piration and also retards the respiratory rate. The acceler-
ator portion of the center seems more sensitive than the in-
hibitory, and the result of stimulation of the whole center ist
therefore quickened respiration.

Subsidiary respiratory centers are said to exist in the tuber
cinereum, optic thalamus, corpora quadrigemina, pons Va-
rolii and spinal cord ; but the existence of at least some of
these is doubtful.

Rhythm of Respiration. — 'What agency excites the center
to keep up the respiratory movements with such regularity
is a matter of interest. The chief circumstances which seem
to affect the rate and rhythm are (i) the will, (2) emotions,
(3) composition of the blood, and (4) afferent impressions.

1 68 RESPIRATION

i, 2. The effect of the will and emotions are too apparent
to call for comment. I and 2 are properly included in 4.

3. A deficiency of O or an excess of CO2 in the blood will
increase the rate. Increase in temperature of the blood, as
in fever, will produce a similar effect.

4. The most important of these agencies is found in affer-
ent impressions conveyed to the center. The fibers carrying
these impressions are chiefly in the pneumo gastric, glosso-
pharyngeal, trigeminal and cutaneous nerves. Of these the
pneumogastric is by far the most important.

Section of a single pneumogastric is followed by variable
respiratory disturbances which usually disappear in less than
an hour. Section of both nerves is followed, after a short
interval of increased respiratory activity, by slow and pow-
erful inspirations, by forced expiration and an appreciable
interval before the next inspiration. Irritation of the cen-
tral end of the cut nerve by a very weak current seems to
stimulate the inhibitory part of the center, for the rate is
slowed, the expirations are strenuous and the inspirations
weak. When the current is increased to a moderate strength
opposite results are obtained, the accelerator portion of the
center being stimulated. These facts show that the pneu-
mogastrics possess both inspiratory and expiratory fibers,
and that the former are stimulated more by a moderate cur-
rent and the latter more by a very weak one. The rhythm
of respiration, therefore, includes the regular sequence of
inspiratory and expiratory movements upon each other.

Now what is it that, under normal conditions, irritates the
terminals of the pneumogastrics and causes them to convey
inspiratory and expiratory impressions? It has been held
that a change in the composition of the alveolar air — an ac-
cumulation of carbon dioxide — irritates the nerve terminals
and explains the conveyance of the inspiratory impressions,
while the stretching of the lung tissue originates the expira-
tory impressions. Others ascribe both inspiratory and ex-
piratory impressions to lung movements — movements of in-

NERVOUS MECHANISM OF RESPIRATION 169

spiration exciting expiratory fibers, and movements of ex-
piration exciting inspiratory -fibers. These observers cite
the fact that artificial inflation and aspiration excite expira-
tion and inspiration respectively.

Stimulation of the superior laryngeal, as when foreign
bodies accidentally enter the larynx, excites violent expira-
tion.

The glosso-pharyngeal contains afferent fibers especially
important in arresting respiration — at any stage whatever —
during the act of deglutition.

Stimulation of the sensory fibers of the trigeminal in the
nose, as by irritating vapors, may arrest respiration.

Irritation of the cutaneous nerves in general, as by cold or
hot water, slapping, etc., stimulates respiratory movement.

There are, of course, running from the cortex to the res-
piratory center intracranial fibers whereby the organ of the
will makes its presence felt in respiration.

But when all the afferent nerve connections are severed,
respiration continues with modified rhythm and rate, at least
for a time. It is thought that, under these conditions, it is
the circulation through the center of blood deficient in oxy-
gen which causes the cells to discharge ; that is, after every
inspiration and subsequent expiration there is not another
inspiration until the blood has become sufficiently deoxygen-
ated, or charged with carbon dioxide, to irritate the respira-
tory center.

We may conclude that "the rhythmical discharges from
the center are due primarily to an inherent quality of peri-
odic activity of the nerve cells constituting the respiratory
center, and maintained by the blood, and that the rhythm,
rate, and other characters of these discharges may be af-
fected by the will and the emotions, by the composition, sup-
ply and temperature of the blood, and by various afferent im-
pulses. The chief factors are the quantities of O and
CO2 in the blood, and the impulses conveyed from the lungs
by the fibers of the pneumogastric nerves." (Am. Text-
.Book.)

170 RESPIRATION

The efferent nerves of respiration control the muscular
movements of that act. They are chiefly the facial, hypo-
glossal and spinal accessory controlling the respiratory
movements about the face and throat, the pneumo gastric
going to the larynx and the phrenic to the diaphragm.

To the lungs proper fibers are distributed by the vagus,
the dorsal sp.inal and the sympathetic nerves. Besides the
expiratory and inspiratory fibers already noticed, the vagus
supplies the lungs with broncho-motor, general sensory, tro-
phic and secretory (mucous) fibers. The sympathetic fur-
nishes trophic and vaso-motor fibers, which latter come from
the cord by the roots of the dorsal nerves mentioned to join
the sympathetic ganglia.

CHAPTER IX.

NUTRITION, DIETETICS AND ANIMAL HEAT.
NUTRITION.

ALL the processes of the body as digestion, absorption, se-
cretion, circulation, respiration, etc. — have a single object,
viz., the nutrition of the cells of the body.

The ultimate source of all nutriment is, of course, food
and oxygen. The oxygen has been followed from the lungs
to the tissues as oxyhemoglobin of the blood. The various
foods have been seen to disappear from the digestive tract
and to be conveyed to the tissues by the great nutritive fluid,
some in recognizable and some in unrecognizable form. If,
now, we shall be able to discover in what way these different
materials thus furnished the cells are utilized and appropri-
ated by them, and in what condition they subsequently es-
cape from the system, the study of nutrition will have been
rendered much clearer. The intake is through the lungs and
alimentary canal ; the output is mainly by the lungs, skin, kid-
neys, and intestines. To show for the changes which take
place while the food is in the body there is the growth of the
body, the maintenance of tissue integrity, secretion, heat,
motion and nervous energy.

It may be said at once, however, that the exact method of
appropriation of nutritive material by the tissues is a sub-
ject of speculation, since it involves the question of life
itself ; and we shall have to be content with recounting some
of the conditions influencing and some of the phenomena
attendant upon the process.

171

172 NUTRITION, DIETETICS AND ANIMAL HEAT

Metabolism. — By metabolism is meant those processes in
the body whereby food products are appropriated, their
stored-up energy utilized, and the waste discarded.

Metabolism is divided into, (i) anabolism, and (2) kata-
bolism. Anabolism is the process of building up tissue by
cell appropriation of food stuffs. Katabolism is the process
of destroying tissue in order to set free energy that the or-
gans of the body may perform their various functions.

When the anabolic processes are equal to the katabolic
there is no excessive storage of material, but an individual
remains of uniform size, weight, and strength. If the ana-
bolic are in excess of the katabolic processes, the excessive
products are stored up in cells and an individual increases in
size, weight and strength. If the katabolic processes are in
excess of the anabolic there is a call on the tissues for the
matter already stored there and there is a decrease in
strength, weight and size.

Death. — As long as a cell appropriates enough to supply
the deficit caused by the destruction of material in the ex-
penditure of energy, the cell will live; but when the intake
cannot make up for the output lost the cell ceases to func-
tionate and this is called death.

Problems Involved in the Nutritive Process. — Since the
actual changes occurring and the method of their produc-
tion cannot be understood, the question of nutrition resolves
itself into a consideration of the final fate of the various
aliments, of their relative value in nutrition, of conditions in-
fluencing the process, and of the explanation of certain
facts connected with the destruction of the food stuffs, par-
ticularly the production of heat.

The change which the foods finally undergo in the body
is one of oxidation. It is therefore chemical changes which
give rise to physical activity. Oxidation is accompanied by
the production of heat. The same sum total of heat is de-
veloped when a piece of iron rusts completely away in five
years as when it is consumed in an atmosphere of oxygen in

FOODS IN NUTRITION 1/3

five minutes. In both cases it is oxidized. In the cell oxi-
dation is continually going on with the production of heat
and of certain excrementitious (oxidation) products de-
pending on the kind of food stuffs.

Fate of Different Foods in the Organism. — In the first
place, the foods may be divided, into (I) those which pass
through the organism unchanged, and (II) those which lose
their identity and are discharged as bodies different from
those which entered. The first class includes the foods fur-
nishing no energy; the second those furnishing energy.

Only a few foods undergo in the body reactions which
alter their identity. They may be regarded as already di-
gested and, in fact, when dissolved, ready for discharge from
the body. They are, however, useful and necessary constitu-
ents of the body, and if they do not take a considerable active
part in nutrition, their favorable influence on that process
makes them essential to health. The foods producing no
energy may be dismissed with a repetition of the statement
that they are largely introduced in connection with the pro-
teid foods from which they cannot be separated without de-
struction of the proteid molecule. Indeed, all the proteid
food introduced, whether animal or vegetable, contains inert
constituents as a part of the molecule, and these seem as
necessary to nutrition as do the energy furnishing constitu-
ents. The foods furnishing energy and those furnishing no
energy enter, are deposited, and seem to be discharged both
together. The few reactions which the inert foods undergo
in the body do not materially affect the supply of energy.

(II) The proteids, carbohydrates and hydrocarbons are
all consumed in the organism, none (unless they have acci-
dentally escaped digestion) being discharged as they entered.

i. The nitrogenous foods are changed into peptones in the
alimentary canal, undergo some unknown change in their ab-
sorption therefrom, appear in the blood as the proteid con-
stituents of that fluid, and are offered to the tissues through
the medium of the lymph. The complex proteid molecule is

174 NUTRITION, DIETETICS AND ANIMAL HEAT

broken down into simpler but more stable ones. These end
products are carbon dioxide, water and urea, together with
some sulphates and phosphates, the production of which is
comparatively immaterial. The urea is distinctive. Heat,
which is equivalent to so much energy, is evolved in the oxi-
dation process.

It is probable that not all the proteid, under the ordinary
diet, is actually built up into cell substance. A part of it
seems to be destroyed without being transformed into pro-
toplasmic material, but the destruction always takes place
through the agency of the cells, and the end products are al-
ways the same whether disassimilation of the proteid occurs
with or without its becoming an intrinsic part of the cell.

Nitrogenous Equilibrium — Circulating and Tissue Pro-
teids. — The fact, however, that the characteristic function of
the nitrogenous foods is to furnish protoplasmic material
should not be lost sight of. A certain amount is necessary
to maintain "nitrogenous equilibrium" ; that is, to keep the
intake of nitrogen up to the output. When nitrogenous food
is withdrawn there continues to be a discharge of urea, which
is the chief nitrogenous excretion and the amount of which
represents the amount of nitrogenous disassimilation in the
body. The urea eliminated under these conditions must rep-
resent the actual destruction of cell substance, and, since the
supply is zero and the output is considerable, there is not a
state of nitrogenous equilibrium; the animal is suffering
destruction of its protoplasm without a compensatory con-
structive process. On the other hand, the supply of nitro-
genous material may be, and usually is, in excess of the de-
mands of the cells for the actual regeneration of their sub-
stance. This excess may be termed "circulating proteid,"
and is that just referred to as being oxidized under the in-
fluence of the cells, but without being transformed into pro-
toplasm. That part of the nitrogenous supply which is
built up into a part of the cell has been called "tissue pro-
teid" Whether any given molecule of proteid food pass

FOODS IN NUTRITION 175

through the system as circulating or tissue proteid is only an
accident — provided the supply be above the demand of the
cells for tissue proteid; these demands are the first to be
supplied by the nitrogenous material at hand.

From this it is not to be inferred that the exigencies of nu-
trition will be met as well without as with circulating pro-
teid. When the diet consists of just enough proteid to
supply the tissue wastes and of ample carbohydrate and hy-
drocarbon materials, the nutritive process is impaired. It
seems necessary to perfect health that the supply of nitro-
genous food be sufficient to allow for the oxidation of some
of it as circulating proteid in a manner analogous to oxida-
tion of the non-nitrogenized materials. Life can be main-
tained on nitrogenous food alone, but it is obvious that when
this is done the amount of circulating proteid must be enor-
mously increased so that it may be oxidized to furnish energy
for the body; for those substances, the oxidation of which
corresponds to oxidation of circulating proteids and which
furnish the main supply of energy for doing work (viz., the
carbohydrates and hydrocarbons), are now withdrawn from
the economy. It follows, conversely, that the ingestion of
carbohydrates and hydrocarbons lessens the amount of pro-
teid necessary to nutrition.

The albuminoids, such as gelatin (not meant to be in-
cluded under the term "nitrogenous" foods, though they con-
tain nitrogen), cannot take the place of tissue proteid; they
may be burnt in lieu of the circulating proteids and supply
energy just as the carbohyrdates and fats do.

It is to be remembered that any excess of proteid or al-
buminoid food is not discharged as such in the excreta, but
undergoes oxidation, the end products of which are always
the same, water, carbon dioxide and urea, or related sub-
stances ; the development of heat is also an invariable accom-
paniment of their destruction.

While a person may live on proteid food, the amount
necessary taxes the digestive and excretory organs to such

176 NUTRITION, DIETETICS AND ANIMAL HEAT

an extent that life is probably shortened. Since the total
amount of urea is discharged by the kidney, that organ, un-
der an excess of proteid diet, is particularly prone to degen-
erative changes of a most serious nature.

2. The carbohydrates enter the blood from the alimentary
canal as dextrose, are conveyed to the liver and converted
into glycogen, which is stored up there to be dealt out to the
blood gradually, after being reconverted into dextrose. Dex-
trose exists in the blood for a short time only, being con-
verted into other substances, but its final oxidation is ef-
fected by the tissues. Its end products are carbon dioxide
and water, with heat. Sugar (dextrose) injected into the
blood soon disappears. It is thought by some to be con-
verted into alcohol in the blood and then oxidized. At any
rate, the formation of the end products just mentioned is
the final fate of the carbohydrates, through whatever split-
ting processes the sugar molecule may pass before it is con-
verted into these substances.

The removal of the pancreas occasions diabetes mellitus,
and the inference is that this gland gives off to the blood
some internal secretion which splits up the sugar molecule
in the blood. Hbw this lesion causes the disease in question
is not clear, but the retention of a small part of the gland
enables the oxidation of sugar by the tissues to proceed in
the proper way and it is not discharged in the urine.

Value of the Carbohydrates in Nutrition. — The distinctive
function of the carbohydrates is to act as fuel for the body
machine ; they are burnt up to supply heat, and heat repre-
sents energy. Hydrogen and oxygen exist already in the
proportion to form water — one of the end products — and
only enough O is required to unite with the carbon of the
carbohydrates to form CO2 — the other end product. The
burning (oxidation) of a carbohydrate outside the body re-
sults in the formation of CCte and H2O and the elimination
of heat, which last, if properly utilized, can be converted into
energy — the power to do work. The result of the oxidation

FOODS IN NUTRITION • 177

of a carbohydrate in the body is the same. Since this class
of food is easily handled by the alimentary canal, requires
little extra O for its destruction, and is very abundantly sup-
plied by the vegetable world, it is the most economical from
digestive, absorptive, respiratory and financial standpoints.
Carbohydrates may also be deposited as adipose tissue as
will be seen presently.

3. The fats have the same general office in nutrition as the
carbohydrates, viz., the furnishing of energy by oxida-
tion. They leave the alimentary canal by way of the lacteals,
are conveyed by the blood to the tissues and there oxidized
with the formation of carbon dioxide and water and the lib-
eration of heat. Though more O is necessary to burn up the
fat than the carbohydrate molecule, oxidation of the fat is
attended with the liberation of the greater amount of heat —
i. e., of energy. This would seem to indicate that it would
be more economical to eat fats to the exclusion of carbo-
hydrates, since a smaller quantity of the former will supply
the requisite amount of energy. This is theoretically true,
but considerations of digestion render it not practically
so, since fats tax the digestive apparatus much more than
carbohydrates.

The fat deposited in the body — the adipose tissue — what-
ever may be its source, it is to be looked upon as so much
stored-up energy. When the supply of blood is cut off it is
the first part of the organism to be consumed. Hence, a fat
animal will survive starvation longer than a lean one.

The individuality, the functional activity, and the proper-
ties involved in regeneration of protoplasm are ultimately
dependent upon its nitrogenous characters. The other con-
stituents are more or less passive. The oxidation of fats and
carbohydrates, however, takes place under the influence and
through the agency of the cells. It is scarcely necessary to
add that neither fats nor carbohydrates, nor both together,
are sufficient to sustain life; for life is embodied in proto-
plasm and protoplasm must have nitrogen, which element
these foods cannot furnish.
12

178 NUTRITION, DIETETICS AND ANIMAL HEAT

Formation of Adipose Tissue. — The adipose tissue in the
body is not the result of direct deposition of the oleaginous
foods. The amount of fat taken on in a given time by some
animals, as hogs, is often far in excess of the quantity of
fat in the ingesta. Adipose tissue is, under normal condi-
tions, the result always of changes due to protoplasmic ac-
tivity. It is formed by the tissues chiefly from the carbohy-
drates, but also to a less extent from the proteids. The
chemical changes by which sugar is converted into fat are as
yet undetermined, but there are so many evidences of an in-
crease in body fat upon an excess of carbohydrate food that
the fact itself that this class of food is the main source of
fat is no longer disputed.

As regards the formation of fat from proteids, it is
thought that the molecule is split up into a nitrogenous mole-
cule, which is discharged as urea, and a non-nitrogenous,
which at once, or after undergoing other changes, is depos-
ited as fat. Experimental observations demonstrate that the
liver produces gyycogen on a purely proteid diet. Since
glycogen is a carbohydrate, and carbohydrates are the chief
source of body fat, it is not improbable that the non-nitro-
genous molecule of the proteid dissociation takes the form
of glycogen and is later converted into fat after the manner,
whatever it may be, of the glycogen introduced in carbohy-
drate form. When the carbon discharged is less than the
carbon ingested the deficit is thought to be retained to form
fat, which is deposited as a reserve to be used whenever its
oxidation may become necessary as a supply of energy.

It follows that to reduce body fat the carbohydrates should
be largely interdicted, while to increase it they should be
taken in excess. In human beings proper regulation of the
diet is more efficacious in reducing than increasing the
amount of adipose tissue.

Adipose Tissue a Reserve Supply of Energy. — The carbo-
hydrates and fats are preeminently the energy-producing
foods, and of these the carbohydrates, for reasons indicated,

CONDITIONS INFLUENCING METABOLISM 1/9

are the more important. They not only furnish energy
which is immediately used up in running the machinery of
the body, but they deposit, or attempt to deposit, a reserve
supply to protect the proteid portions of the organism
against accidents to temporary deprivation of food, demands
for an unusual amount of energy, malnutrition from vari-
ous causes, etc. — savings laid by for the proverbial rainy day.
This reserve supply takes the form first of glycogen, which
is soon used up, meeting as it were only the demands of the
hour, and second of fat, which begins to be drawn upon when
the glycogen is exhausted, and which lasts for a length of
time depending upon its amount.

Conditions Influencing Metabolism. — Regular exercise is
undoubtedly favorable to the nutrition of any part, as e. g.,
the muscles, the brain, etc. Exercise may mean increased dis-
assimilation, but if so it also means increased assimilation.
With regard to muscular exercise of average severity and
reasonable duration, the results of cellular activity seem at
first a little surprising, but are really to be expected if the
concluding remarks of the previous paragraph are true. The
amount of urea under such exercise is not appreciably in-
creased— which means that disassimilation in the protoplasm
of the muscle cells is not increased. This remark holds good
however, only when the supply of sugars, starches and fats
is abundant ; if they are not present in sufficient quantity to
meet the increased demand for energy-supplying materials,
then the proteids must be oxidized to furnish it, and the urea
discharged is increased. In striking contrast to the constant
output of urea is the largely increased output of CO2, repre-
senting oxidation of the carbohydrates and fats.

During sleep the nitrogenous output is not materially di-
minished, while that of CO2 is markedly less. This is ex-
plained by the fact that there is less energy needed and cor-
respondingly less oxidation of the energy-producing mate-
rials. Proteid metabolism is undisturbed.
f A low external temperature does not increase the output

I-80 NUTRITION, DIETETICS AND ANIMAL HEAT

of urea ; it increases the output of CCte. These two facts to-
gether mean again that only the carbohydrates and fats are
being oxidized in increased amount. This increased oxida-
tion, the effect of which is to maintain the normal body tem-
perature is usually dismissed with the statement that it is a
reflex nervous act. It is claimed by Johannson that the CO
output is not increased until shivering occurs (Reichert).
That being the case, the increase is explained on the ground
of increased energy and heat production incident to muscular
exercise, and shivering assumes the dignity of a physiological
factor in keeping up the temperature of the body. This is
perfectly reasonable when it is remembered how effective ac-
tive muscular exercise is in keeping the body warm. But the
fact that a person when cold shivers and is restless involun-
tarily does not allow us to escape the unsatisfactory "reflex
action" explanation of the phenomenon in question. Within
ordinary and reasonable limits proteid metabolism is undis-
turbed; it is still being protected by the fats and carbohy-
drates.

During starvation nothing is supplied from the outside
world except oxygen, and the animal must live on the mater-
ials already in his body. The glycogen is first consumed ; it is
the surplus on hand ; but at best it is all gone in a very few
days. Then the fat stored up as adipose tissue is drawn
upon ; it is the reserve fund ; but it is likewise soon con-
sumed; the animal becomes progressively emaciated. When
this is exhausted the tissue proteid is attacked; this is the
capital and is the last to be touched ; but there must be heat
and at least some energy, and there is no other source. When
the proteid capital has at least been so impaired that it can no
longer furnish heat to maintain the body temperature and
energy to carry on the necessary organic functions, the or-
ganism is physiologically bankrupt and assignment follows
— death is at hand.

REQUISITES OF DIET l8l

DIETETICS.

The appetite, under normal conditions, may be depended
upon to regulate both quantity and quality of diet in a fairly
satisfactory manner. Different peoples require different pro-
portions and amounts of the various food stuffs and the
same is true of any given individual for varying conditions
of temperature, exercise, etc. But in any case the object of
eating is to prevent the loss, in aggregate, of proteid tissue,
fat, etc. — to replace the wastes, and that in the most conveni-
ent and economical way.

When the ingesta exceed the excreta the animal is gaining
in weight ; when opposite conditions obtain he is losing ; when
there is a balance between the two the body equilibrium is
being maintained.

Determination of the Requisites of a Diet. — The usual
method of determining, in a scientific manner, the requisites
of a normal diet for persons in general is to estimate the
amount of the various excretions from the bodies of a lim-
ited number of persons in health, and from this knowledge
to calculate the amount and kind of food which will supply
the demands in the most satisfactory way, it being assumed
that these excretions represent the normal and necessary
metabolism going on in the body. The results of such ex-
amination are found to correspond with the actual demands
of the system.

It has been seen that the organism demands some fifteen
or more chemical elements for use to keep itself in good run-
ning order ; it has been seen also that its demands, so far as
quantity is concerned, are chiefly confined to carbon, hydro-
gen, oxygen and nitrogen. The other elements deserve no
attention here since they (excepting sodium chloride) are
unconsciously introduced with the ordinary foods in amounts
sufficient to satisfy the requirements of the system. More-
over, the air we breathe and the water we drink furnish an
ample supply of hydrogen and oxygen when to this supply is

l82 NUTRITION, DIETETICS AND ANIMAL HEAT

added the quota of these elements contained in the necessary
quantities of other aliments. So, therefore, if we fix upon
a diet which will furnish the requisite amounts of carbon and
nitrogen no attention need t^e paid to the other elements.
The supply of the others may be said to regulate itself if the
supply of carbon and nitrogen be regulated.

The object, then, of food may be said to be the replace-
ment of carbon and nitrogen — the carbon and nitrogen in the
excreta. Of these two elements, carbohydrates and fats
will furnish only carbon ; proteid food will furnish both.

Amount of C and N Necessary. — It is found that the daily
discharge of nitrogen is about 18 grams (4^5) and of car-
bon about 281 grams (8^2§). These are the amounts, there-
fore, which must be supplied by food. We may accept, as
representing the proteid molecule in general, the formula, Ci2
HmOifflNisS. Then it is evident that an amount of proteid
food which would furnish the necessary 18 grams of nitro-
gen would furnish only 72 grams of carbon — only about one-
fourth enough. If, now, the proteid food be increased to
supply 281 grams of carbon, the system will have to handle
four times as much nitrogen as it needs ; and this is a tax to
the digestive apparatus and the excretory organs, particu-
larly the kidney — a tax which is rendered unnecessary by the
availability of the carbohydrates and fats as food. These
contain abundance of carbon, and it is far better to eat only
enough proteid food to supply the 18 grams of nitrogen, and
make up the deficit of carbon with non-nitrogenized articles
of diet. One can supply all the demands by eating nitro-
genous food alone, and life will be preserved indefinitely
perhaps, but the prediction would be warranted that in such
a case the person would probably die prematurely — as a re-
sult of kidney or liver disease.

Articles Which Will Supply the Necessary Amounts of
C and N. — The conclusion (modified) of Moleschott is that
the average man needs daily about 120 grams of proteid, 90
grams of fat, and 320 grams of carbohydrate food, estimated

REQUISITES OF DIET

dry ; and that with this, in the usual state in which such food
is taken, he will consume unconsciously, or as a result of
craving, some 30 grams of salt and 2,800 grams of water.
These proportions are supposed to satisfy the demands of
the system in an economical way. The estimates of Ranke
vary somewhat from this as indicated in the subjoined table
which shows also the balance kept up in the body.

Income.

Expenditure.

Foods.

Nitrogen.

Carbon.

Excretions.

Nitrogen.

Carbon.

Proteid, loogm.
Fat, loogm.
Carbohydrates,
250 gm.

iSogm.
o.o "
o.o "

53-0 gm.
79.0 "
93-0 '

Urea, 31.5 gm.
Uric acid, 0.5
gm.
Feces
Respiration
(C02)

1 14.4-

i.i
o.o

6.16

10.84
208.00

15-5 gm.

225.0 gm.

15-5

225.00

The actual amounts of given substances which it is neces-
sary to eat in order to supply the requirements of these esti-
mates depend, of course, on the composition of those sub-
stances, and would have to be settled by reference to a table
giving analyses of the common articles of diet. Two pounds
of bread and 3/4 pound (when uncooked) of lean meat, to-
gether with water and salt, will supply the demands ; but this
is an unusual diet. Or i pound of meat, I pound of bread
and l/4 pound of butter, or other fat, with water and salt is
probably preferable.

In any case if nutrition is to be properly performed the
diet must be varied. It could not be held that the above
supply of food would keep a person indefinitely in good
health, His demands for nitrogen and carbon are always

184 NUTRITION, DIETETICS AND ANIMAL HEAT

v approximately the same, but the organism revolts at being
supplied with them from exactly the same source for any
considerable length of time.

As a diet is necessary (Schenck and Gurber) :

Proteid. Fat. Carbohydrates.

Resting man 100 gm. 60 gm. 400 gm.

Resting woman go gm. 40 gm. 350 gm.

Working man 130 gm. 100 gm. 500 gm.

It need scarcely be added that any condition, such as exer-
cise, temperature, etc., which increases the excreta, calls for
a larger supply of ingesta. Ordinary exercise is allowed
for in the estimates just given.

ANIMAL HEAT.

The Temperature. — The average temperature of the hu-
man body, taken under the tongue, is 98.5° F. It varies in
different parts, the mean being about 100°. The metabolic
activity in different parts of the body is changeable, and con-
sequently the heat production in all parts is not the same.

The fact that -the temperature is nearly identical through-
out the body is due to the distribution of heat, which distri-
bution is mainly effected through the agency of the circulat-
ing fluids. The rectal temperature is a little higher than that
obtained in the mouth. The temperature of arterial is higher
than that of venous blood. The warmest blood is in the hepat-
ic veins ; the coolest is that which has just passed through the
most exposed peripheral parts, as the helix of the ear. The
mean body temperature is a little lower in the morning than
in the evening, in the female than in the male, on a restricted
than on an abundant diet, in cold than in hot climates, and,
in general, in conditions of diminished than of exalted met-
abolic activity.

But in health these variations are of trivial importance and
do not represent a sweep of more than 2° F. The body tern-

HEAT AND FORCE 185

perature may be looked upon as being a fairly constant
quantity. It varies scarcely at all with variations of exter-
nal temperature, so long as the heat-regulating apparatus is
in order. An external (dry) temperature of 212° F., or the
extremely low temperature of some regions, can be borne
with very slight fluctuations in that temperature of the body.
The actual limits of internal temperature consistent with the
preservation of life are given by Flint as 83° and 107° F.
These temperatures cannot be long endured.

The fundamental fact to be kept constantly in mind is
that there is a continual production and a continual dissipa-
tion of heat, in ways to be indicated presently. These two
processes are known as thermogenesis (heat production) and
thermolysis (heat loss) respectively. The preservation of
the proper balance between heat production and heat dissipa-
tion is known as thermotaxis.

Supply of Heat and its Relation to Force. — It is a matter
of common observation that the burning (oxidation) of
any substance, as a piece of wood or an article of diet, is ac-
companied by the evolution of heat. It is also known that
heat may be converted into force — may be made to do work.
The burning of a fat or a sugar produces CO2 and H2O ; the
burning of a proteid produces CCte and H2O, and additional
substances. The final products, and the amount of heat
evolved, are precisely the same whether the oxidation be
rapid or slow. Now, the oxidation of food is exactly what
occurs in the human organism, though that of the proteids is
not completely effected ; CO2 and H2O are produced from
them, and the "additional substances" mentioned are repre-
sented by urea. This process, then, is the source of body
heat. To the supply thus furnished may be added a little
from reactions between non-energy producing materials in
the body, from warm foods and drinks, and from friction in
the vessels, joints, etc.

The foods thus possess a certain potential energy, an en-
ergy which may be converted directly or indirectly into heat,

l86 NUTRITION, DIETETICS AND ANIMAL HEAT

or its equivalent. The potential energy of the foods keeps
up the body temperature and supplies force for doing work.
It is converted into heat and kinetic energy. Kinetic energy
is working energy, and is represented in the body chiefly
by muscular contractions. But, since this kinetic energy has
its source in the transformation of food stuffs, and since
kinetic energy and heat are mutually convertible, it may be
assumed that all the potential energy of the foods is con-
verted into heat. The kinetic energy may be taken as rep-
resenting so much heat, and the total production of heat
(including kinetic energy) as representing the total produc-
tion of energy. Or, to state the case differently, the potential
energy of the food is converted into heat, a part of which ap-
pears as kinetic energy. .By far the largest part of this po-
tential energy, however, is converted directly into heat. Not
more than one-fifth of the heat produced in the body can be
utilized to do work, and a part of that work is actually con-
verted indirectly into heat, and contributes to the total heat
of the body, by overcoming friction incident to respiration,
circulation, movements of the joints, muscles, etc.

Potential Value of Foods. — It is estimated that the oxida-
tion in the body of one gram of fat produces 9,300 calories
of heat, one gram of carbohydrate 4,100 calories, and one
gram of proteid 4,100 calories. These figures represent the
potential energy of the several foods. Fats, it is seen, pro-
duce, weight for weight, more than twice as much energy as
other foods, but reasons have been given why they cannot be
used exclusively.

A calorie is the amount of heat necessary to raise i Kg
of water from o° to i° C. A grammeter is the amount of
energy necessary to raise I gram i meter. Now since heat
and work are only different forms of energy, these two units
— calorie and grammeter — have each equivalents in terms of
the other. One calorie equals 424.5 grammeters ; 'that is, the
force represented by one calorie will raise one gram 424.5
meters. The terms kilo-calorie, or kilogramdegree, and kilo-

TOTAL AND SPECIFIC HEAT l/

grammeter are used sometimes, and represent 1,000 times
the calorie and grammeter respectively.

Total and Specific Heat. — The temperature of a body in-
dicates nothing as to the quantity of the heat it contains.
The degree of heat requires only a thermometer to deter-
mine it, but the quantity depends on the temperature, the
weight and the specific heat of the substance in question.

Specific heat is analogous to specific gravity. Water is
taken as the standard in both cases. If it require only .5 ca-
lorie to raise I gram of a certain substance I degree C., the
specific heat of that substance is said to be .5. The specific
heat of the body is .8 ; that is, whereas it requires a certain
amount of heat to raise 150 pounds of water to a certain
temperature, it would require only .8 as much to raise a hu-
man body weighing the same to the same temperature. To
find the total heat in calories in any body it is only necessary
to multiply the weight (in grams) by the specific heat and
by the temperature C. Estimates made by calorimetry from
these data and from the potential value of the different foods
give the total daily heat production as about 2,500,000 ca-
lories for the average individual. This is equal to about i,-
400 calories per hour per kilo weight.

The English heat unit is the pound-degree F. It is the
amount of heat necessary to raise I pound of water i degree
F. Its mechanical equivalent is the force necessary to raise
i pound 772 feet. The estimates just given in the metric
system when translated to English nomenclature give the to-
tal heat production for 24 hours as about 8,400 pound-
degrees, or 2.5 per hour per pound weight. These figures are
given as only approximate and are subject to change by many
causes, such as sex, cardiac and respiratory activity, internal
and external temperature, exercise, digestion, age, nervous
influences, the body weight, etc.

Thermogenesis. — Thermogenesis, or the production of
heat, is the result of activity on the part of the tissues, nerves
and centers. Now, the potential energy of the food stuffs is

l88 NUTRITION, DIETETICS AND ANIMAL HEAT

the ultimate source of all bodily heat no matter how it may
be manifested, and it is evident from what has been said al-
ready that all the tissues of the body are heat-producing tis-
sues, because oxidation processes go on in them all. But
muscular tissue seems to be endowed with special heat-pro-
ducing capabilities, so much so that it is said to generate
heat as a specific product, and not as a mere incident of its
metabolism. Muscle will reproduce heat when entirely at
rest — when the nutritive metabolic changes are practically
nothing. The process seems to be regulated in accordance
with the needs of the economy by means of a nervous me-
chanism, making the production of heat analogous to secre-
tion. Separation of a muscle from its nerve does not stop
thermogenesis, but markedly interferes with it in that part.
The existence of distinct thermogenic nerves has not been
demonstrated. The existence of specific thermogenic centers
seems certain. Some of them increase and some decrease
thermogenesis.

The general thermogenic centers are in the spinal cord.
Centers increasing thermogenesis are probably in the cau-
date nuclei of the corpora stria, the optic thalami, pons and
medulla. Irritation of these regions causes a rise in temper-
ature. The location of the thermo-inhibitory centers is a
matter of speculation. The general thermogenic centers in
the cord probably maintain a fairly constant pro-
duction of heat independently, but they are subservient to
encephalic centers which excite them to increased or de-
creased activity by reason of certain impressions, cutaneous
or otherwise, which they have received.

Heat Loss. — About 85 per cent, of animal heat, dis-
charged as such, is lost by radiation and evaporation from
the skin; about 12 per cent, is dissipated in the lungs by
evaporation and in warming the inspired air ; the remainder
is discharged in the urine and feces (disregarding the small
amount which goes to warm ingested articles).

Hbat is radiated from the body just as from a hot stove.

CONDITIONS INFLUENCING HEAT DISSIPATION 189

Radiation is affected by the conductivity of the surrounding
medium. For instance, in media of water and air of the
same temperature the radiation is greater in water, because
it is a better conductor of heat.

Evaporation from the skin is of very great importance in
increasing heat dissipation. 582 calories of heat are con-
sumed when one gram of water is vaporized ; and when this
evaporation takes place on the skin the heat is abstracted
largely from the body. This is said to represent nearly 15
per cent, of the total heat dissipation. Hence the value of
perspiring in hot weather. Evaporation also takes place
from the moist surfaces of the lungs and, moreover, when
as is usually the case, the inspired air is cooler than the lung
structure, a certain amount of heat is consumed in warming
it.

But it is not to be inferred that loss of heat takes place
from the body just as from an inanimate object. On the
other hand, it is intimately connected with and influenced by
circulation, respiration, secretion and other functions. When
there is a tendency for the body temperature to rise, the cir-
culation becomes more active and sends more blood to the
periphery to be cooled; respiration is augmented, causing a
greater abstraction in the lungs ; the secretion of sweat, for
instance, is increased.

There may be distinct centers governing the loss of heat.

Conditions Influencing Heat Dissipation. — These have
been suggested in a previous section. Heat dissipation is
greater in proportion to weight in small than in large ani-
mals because the radiating surface is relatively larger. It is
less in the female than in the male because she has, as a rule,
a larger proportion of subcutaneous fat, which is a poor con-
ductor of heat. It is less when the body is covered with
clothing which is a poor conductor of heat than when the
covering conducts heat readily. It is increased when the
internal temperature is raised and when the external temper-
ature is lowered. Any general increase in vascular or

I9O NUTRITION, DIETETICS AND ANIMAL HEAT

respiratory activity increases heat dissipation for reasons al-
ready given. When the external temperature is high and
the air is dry evaporation is more abundant, and conse-
quently heat dissipation is greater than when the air is al-
ready impregnated with moisture. Hence the oppressiveness
of the high external temperature with high humidity. In
fever heat dissipation is usually increased, but to a less de-
gree than the production.

Thermotaxis. — Thermo taxis is the regulation of heat
production and heat dissipation so that the temperature of
the body may remain the same. It is evident that there is
frequently a transient increase or decrease of thermogen-
etic activity ; unless there be a corresponding change in ther-
molytic activity the temperature will be disturbed.

The temperature of the body is not necessarily raised
when heat production is increased, or lowered when it is de-
creased ; for heat loss may be, and in health is, correspond-
ingly increased or diminished. Conversely, a change in heat
loss does not necessarily mean an opposite change in the
body temperature. Alterations which do occur in the tem-
perature are the result of the improper regulation of the
heat at hand. For instance, fever may result from average
heat production and deficient heat loss ; from increased heat
production and heat loss when the latter is increased less than
the former; from diminished heat production and heat loss
when the latter is diminished less than the former, etc. A
subnormal temperature is caused by opposite conditions.
The temperature remains constant when heat production and
loss are normal, or when they are increased or decreased
correspondingly.

Thermotactic activity is the result of changes in the tem-
perature of the blood, or of cutaneous impressions. A rise
in the temperature of the blood excites heat loss, as indi-
cated. A cold atmosphere increases heat loss, but at the
same time it makes impressions on the cutaneous nerves
which, when carried to the centers, excite heat production

THERMOTAXIS igi

and thus compensation is established. A cold bath lowers
the temperature because heat loss is increased more than
heat production. There is increased radiation because of
the relatively increased difference in the temperature of the
body and of the surrounding medium. On the other hand,
the cold contracts the capillaries, diminishing the amount
of blood exposed to the cooling influence of the water and
decreasing the amount of sweat ; but these influences tend-
ing to inhibit heat loss are not equal to those augmenting
it. However, in health, thermotaxis prevents the disturb-
ance of the balance between thermogenesis and thermolysis
to any great extent, and the temperature cannot be lowered
very much. These are only examples of the reciprocal reT
lations maintained between the production and dissipation
of heat, a disturbance of which relations is prevented under
normal conditions by thermotaxis. Any change in one pro-
cess is followed at once by a compensatory change in the
other.

CHAPTER X.
EXCRETION BY THE KIDNEYS AND SKIN.

EXCRETION of the various foods after they have dis-
charged their several functions in the body is effected
mainly by the kidneys, skin, lungs and alimentary canal.
The excretory action of the last two named is considered
under Respiration and Digestion. Attention is again called
to the fact that it is impossible to differentiate strictly be-
tween a secretory and excretory fluid. The urine is as typi-
cal of the excretions as any fluid to be found. But it will
be convenient to speak of- the "secretion" of urine when
reference is made to the act of separating its constituents
from the blood.

THE KIDNEYS.

Anatomy. — The kidneys, one on each side of the body,
are behind the peritoneum in the lumbar region. The right
is usually a little lower and a little lighter than the left. The
hilum from which the ureter springs looks inward and for-
ward. The kidney, as found behind the peritoneum, is cov-
ered with a considerable amount of fat, but the substance
proper of the organ is closely surrounded by a somewhat re-
sistant fibrous capsule which in health can be easily stripped
away. At the hilum the capsule is continued inward to line
the pelvis, infundibula and calyces.

The kidney belongs to the class of compound tubular
glands. If it be cut into two halves by an incision passing
through the two borders (and, therefore, through the hilum)
an idea of its gross divisions is objtained. The renal sub-
stance is seen to be divided into an outer layer, known as the

192

STRUCTURE OF THE KIDNEY

193

cortical substance, and an inner, or pyramidal, portion. In-
ternally the incision reveals a cavity into which the ureter
opens. This is the pelvis.

FIG. 54. — Longitudinal section through the kidney, the pelvis of the

kidney, and a number of renal calyces.

(From Brubaker, after Tyson.)

A, branch of the renal artery; U, ureter; C, renal calyx; i, cortex; i', medul-
lary rays; i", labyrinth, or cortex proper; 2, medulla; 2', papillary portion of
medulla, or medulla proper; 2", border layer of the medulla; 3, 3, transverse
section through the axes of the tubules of the border layer; 4, fat of the renal
sinus; 5, 5, arterial branches; *, transversely coursing medulla rays in column
of Bertin.

Tracing the divisions of the pelvis toward the kidney sub-
13

194

EXCRETION BY THE KIDNEYS AND SKIN

stance, it is found to be continued by three short canals, one
toward the upper, one toward the lower and one toward the
central portion of the organ. These are the three infun-
dibula. Each infundibulum, passing outward, subdivides

Cortex.

Boundary or
1 ^marginal
zone.

^Papillary

zone.

FIG. 55- •

LSt of a pyramid of Malpighi; PF, pyramids of Ferrein; RA, branch of renal
artery with an interlobular artery; RV, lumen of a renal vein receiving an in-
terlobular vein; VR, vasa recta; PA, apex of a renal papilla; b, b, embrace the
bases of the lobules. (Stirling.)

into two or three, or more, short cylinder-like canals which
receive the apices of the pyramids. These are the calyces,
each of which receives the apex of one or more pyramids.
The urine thus escaping from the pyramidal tubules passes
in succession through the calyces, infundibula, pelvis, and
thence into the ureter.

STRUCTURE OF THE KIDNEY 195

The cortical substance constitutes the outer layer of the
kidneys and is about % inch thick. It is reddish and granu-
lar in appearance. From it pass in between the Malpighian
pyramids columns known as the columns of Bertin. The
cortical substance contains the glomeruli and convoluted tu-
bules together with blood-vessels and lymphatics supported
by connective tissues.

The pyramidal substance, also called the medullary sub-
stance, consists of a number of pyramids, about 12-15, whose
bases look outward and rest on the cortical substance and
whose apices look inward and are received into the calyces.
These are called the pyramids of Malpighi. They contain
uriniferous tubules, vessels, etc., supported by connective
tissue. It will be seen that these tubes converge and join
each other in passing from the base to the apex of the pyra-
mid, so that the very large number entering the base is rep-
resented by only 10-25 at the apex. Thus it is that the Mal-
pighian pyramid is divided into a number of smaller pyra-
mids. These latter are the pyramids of Ferrein, and cor-
respond in number to the number of tubes radiating from
the apex of the larger pyramid. The medullary substance is
marked by striae which have the direction of tubules and
which are caused by them. Its consistence is firmer and its
color is darker than that of the cortical substance.

Malpighian Bodies. — These are scattered throughout the
cortical substance, and are M.oo-^50 inch in diameter. They
consist of a bunch of capillaries in the shape of a ball, the
glomerulus, surrounded by the extremity, or rather the be-
ginning, of one of the renal tubules. At the point where the
tubule joins the Malpighian tuft it is constricted; running
then over the glomerulus it reaches the afferent artery and
the efferent vein on the opposite side ; when it has reached
these vessels it is reflected over the whole network of capil-
laries so that really the tuft is outside the tube, but practic-
ally it is covered by a double layer of the tube wall. A space,
the beginning of the tubule, is left between these two layers

196

EXCRETION BY THE KIDNEYS AND SKIN

and into it the glomerular secretion passes. The outer
layer is the capsule of Bowman (or Miiller). Both layers
consist of a single stratum of flattened epithelial cells ; those
of the inner layer are applied closely to the glomerulus and

FIG. 56. — Transverse section of
a developing Mialpighian capsule
and tuft (human) X 300.

From a fetus at about the fourth
month; a, flattened cells growing to
form the capsule; b, more rounded
cells continuous with the above, re-
flected round c, and finally enveloping
it; c, mass of embryonic cells which
will later become developed into blood-
vessels. (Kirkes after W. Pye.)

FIG. 57. — Epithelial elements
of a Malpighian capsule and
tuft.

With the commencement of a urinary
tubule showing the afferent and effer>
ent vessels; a, layer of flat epithelium
forming the capsule; b, similar but
rather larger epithelial cells, placed in
the walls of the tube; c, cells covering
the vessels of the capillary tuft; d,
commencement of the tubule, some-
what narrower than the rest of it.
(Kirkes after W. Pye.)

are thought to be very important in secretion. The incom-
ing artery breaks up to form the capillary tuft; the corre-
sponding outgoing vein has a smaller caliber than the artery.
The vein, having left the glomerulus, breaks up into a sec-
ondary network around the convoluted tubes. This arrange-

STRUCTURE OF THE KIDNEY 197

ment of the Malpighian body furnishes a most favorable
opportunity for the passage of substances out of the blood
current into the beginning of the tube.

Uriniferous Tubules. — These begin at the glomeruli and
end at the apices of the Malpighian pyramids. From their
tortuous course in the cortical portion they are there called
convoluted tubules, in contradistinction to the straight tubes
of the medullary portion. This, however, is only a general di-
vision ; further divisions are to be noted.

The constricted portion of the tube where it leaves the
glomerulus is the (i) neck; passing away from the neck the
tubule becomes very tortuous and is known as the (2) pri-
mary convoluted tubule, which, having run for a variable
distance, becomes narrow near the base of the pyramid, and
taking a comparatively straight course downward enters the
pyramid under the name of the (3) descending limb of
Henle's loop ; some of these run nearly as far as the apex,
but most of them near the base or middle of the pyramid
turn upward forming thus (4) Henle's loop and beginning
the (5) ascending limb of Henle's loop; the tube having re-
entered the cortical substance becomes convoluted again,
(6) secondary convolution, which, by a less tortuous con-
tinuation, the (7) intermediate tube, communicates with the
collecting tubules, or the (8) straight tubes of Bellini; these
last beginning in the cortex, and receiving in their course
large numbers of intermediate tubes, enter the base of the
pyramid and run in a nearly straight direction toward the
apex. About 100 of these straight tubes entering at the base
join in their course downward until at the apex they are
represented by a single tube. These collections constitute
the pyramids of Ferrein ; there are about 12-18 pyramids of
Ferrein to each Malpighian pyramid, and as many tubal ori-
fices at the apex. The so-called zigzag and spiral tubules
are here considered parts of the first and second convoluted
tubules. (See Fig. 58.)

Before they reach the collecting tubules the tubes vary in

198

EXCRETION BY THE KIDNEYS AND SKIN

FIG. 58. — A diagram of the sections of uriniferous tubes.

A, cortex limited externally by the capsule; a, subcapsular layer not containing
Malpighian corpuscles; a', inner stratum of cortex, also without Malpighian cap-
sules; B, boundary layer; C, medullary part next the boundary layer; i, Bow-
man's capsule of Malpighian corpuscle; 2, neck of capsule; 3, firsfl convoluted
tubule; 4, spiral tubule; 5, descending limb of Henle's loop; 6, the loop proper;
7, thick part of the ascending limb; 8, spiral part of ascending limb; g, narrow
ascending limb in the medullary ray; 10, the zigzag tubule; u, the second con-
voluted tubule; 12, the junctional tubule; 13, the collecting tubule of the medul-
lary ray; 14, the collecting tube of the boundary layer; 15, duct of Bellini.
(Kirkes after Klein.)

STRUCTURE OF THE KIDNEY IQ9

diameter from M.WO to ^ooo inch; the collecting tubules pro-
gressively increase in diameter from %oo to ^oo inch. The
cells lining the convoluted and intermediate tubules are in-
clined to the pyramidal shape. Their bases present the ap-
pearance of fibers at right angles to the basement membrane
(hence "rodded" cells), while their opposite extremities are
granular. The tubes of Henle are lined by flattened epi-
thelium for the most part.

The division is somewhat arbitrary, but the secreting por-
tion of the tubules is supposed to be confined to the cortical
substance, while the tubes of the medullary substance only
carry away the fluid.

Blood Supply. — The renal artery, having entered the hi-
lum, divides into branches, two of which usually enter each
column of Bertin. Running upward in these columns the
branches give off small arterial twigs to the substance of
the column. When a point opposite the bases of the Malpig-
hian pyramids is reached each branch follows the convex
base of the pyramid to which it is adjacent, the one branch
going in an opposite direction to the other. Each meets a
corresponding branch from the other side of the pyramid,
and thus a convex arterial arch covers the base of the pyra-
mid from which arch branches go inward to supply the me-
dullary substance and outward to furnish branches to the
glomeruli. The arrangement of the vessels in relation to the
Malpighian bodies has been noticed. In the glomerulus the
capillaries do not form a true anastomosis, but this is not
true of the network surrounding the convoluted tubes.

Mechanism of Urinary Secretion. — Histologists have been
unable to demonstrate the presence of distinct secretory
fibers for the glomerular or tubal cells. This leaves the me-
chanism of secretion to be explained by (i) the vascular
supply and by (2) the "vital activity" of the cells — both op-
erating in conjunction with osmosis.

Irritation of a certain part of the floor of the fourth ven-
tricle occasions certain marked changes in the quantity and

2OO

EXCRETION BY THE KIDNEYS AND SKIN

quality of the urine ; secretion of the upper dorsal cord tem-
porarily arrests the secretion; mental emotions, such as
fright, anxiety, etc., also modify the flow. All these circum-
stances, and many others, indicate some control over the ac-

\
FIG. 59. — Blood-vessels of the kidney.

A, capillaries of cortex; B, of medulla; a, interlobular artery; i. vas afferens;
2, vas efferens; i, e, vasa recta; VV , interlobular vein; S, origin of a stellate
vein; i, i, Bowman's capsule and glomerules; P, apex of papilla; C, capsule of
kidney; e, vasa recta from lowest vas efferens. (Stirling.)

STRUCTURE OF THE- KIDNEY 2OI

tivity of the kidneys by the nervous system; but that influ-
ence is probably exerted only through vaso-constrictor and
vaso-dilator fibers to the vessels.

Assuming for the present that nearly all the constituents
of urine preexist in the blood and are simply taken out of
the circulation in the kidney, it may be stated that, for the
most part, the water and salts are extracted by the cells of
the Malpighian bodies, while the urea and related nitrogen-
ous solids are removed by the cells of the convoluted tubes ;
so that the specific gravity of the fluid is raised in passing
down the tubes. While the histology of the kidney, and
especially the arrangement of the glomeruli, is most favor-
able for the exercise of simple osmosis, and while this pro-
cess is doubtless mainly responsible for the phenomena
which occur, it seems highly probable that the cells them-
selves modify osmotic action by taking an active part in the
secretion of urine. They undoubtedly exercise a selective
affinity accounting for the different materials handled by the
glomeruli and the tubes. Moreover, morphological changes
in the tubal cells during activity have been microscopically
demonstrated. Vesicles are described as forming in the body
of the cell, approaching the lumen, bursting and discharging
their contents — which are supposed to include the urea and
such other materials as may be here extracted from the
blood.

As regards the elimination of water and salts by the
glomerular epithelium, it must also be admitted that the
cells take some obscure but active part. Were this only an
osmotic process the amount eliminated would vary exactly
as the pressure. While usually a rise in renal blood-pressure
is accompanied by an increased flow of urine and a fall by
a correspondingly decreased flow, the rule does not always
hold good. For instance, compression of the renal vein
raises the pressure but does not increase the amount of
urine.

Another fact, which seems almost if not quite as invari-

2O2 EXCRETION BY THE KIDNEYS AND SKIN

able as the effect of blood-pressure, is that the amount of
urine varies directly as the amount of blood passing through
the kidney, independently of the pressure; and these two
facts constitute about all that is definitely known concerning
the local conditions affecting the amotmt of urine. Whether
diuretics increase the urinary flow by simply drawing water
from the tissues into the blood and thus increasing the
amount and pressure, or by stimulating the cells of the
glomeruli to increased functional activity is a matter as yet
undetermined.

Properties and Composition of Urine. — When an ordi-
nary amount of liquid is ingested and when the skin is
moderately active the urine, in normal conditions, has a clear
reddish amber color and a specific gravity of about 1020.
The more fluid ingested the paler will be the color and the
lower the specific gravity ; the more active the skin the higher
will be the color and specific gravity. The urine is diluted
in the first case and concentrated in the second. The fact is,
the amount of solids (represented by urea) to be eliminated
in 24 hours remains approximately the same, and those solids
will cause a high or low specific gravity according as little
or much water is eliminated with them. The average amount
of urine for a day is 2 or 3 pints. Normally it has an acid
reaction from the presence, not of a free acid, but of acid
salts — chiefly acid sodium phosphate. The odor is not dis-
agreeable on ejection, but decomposition soon begins and a
characteristic offensive, ammoniacal odor develops.

The kidney is the most important excretory organ in the
body and the large number of urinary constituents is not sur-
prising. The chief organic constituents are urea, uric acid,
hippuric acid, xanthin, hypoxanthin, creatinin, phenol, indi-
can, oxalic acid, lactates, etc. The phosphates, nitrates,
sodium chloride, and carbon dioxide are the chief inorganic
materials.

Urea is the most important of the nitrogenous constitu-
ents. It contains a large amount of nitrogen. Nearly all

FORMATION OF UREA 2O3

of it is removed from the body by the kidneys, and double
nephrectomy means death from its retention. Its formation
is constant and its removal necessary. Its presence in the
blood seems to be the normal stimulus exciting the activity
of the cells of the convoluted tubes.

Whether urea is produced directly in the tissues, or
whether only certain substances antecedent to it are there
formed, it cannot be doubted that it is the chief final pro-
duct of nitrogenous ingesta and nitrogenous dissimilation.
It is practically the only way in which the nitrogen of pro-
teid foods can escape from the body. It exists not only in
the blood but in the lymph, vitreous humor, sweat, milk, sa-
liva, etc. It has been stated that the taking of large quanti-
ties of liquids lowers the specific gravity of the urine by di-
luting it ; this is true, but the actual amount of urea is in-
creased somewhat by such a procedure. It is not surprising
that the quantity of urea is largely increased when much
nitrogenous food is taken, and that it is greatly decreased
by an exclusively vegetable diet. Anything, like exercise,
which will increase actual tissue metabolism, will increase
the output of urea, while anything retarding tissue metabol-
ism, like alcohol, will decrease the output. The average
amount of urea for 24 hours is 350 to 450 grains.

Formation of Urea. — Seeing that urea is the typical end
product of the physiological oxidation of the proteids, it be-
comes of interest to determine, if possible, where urea for-
mation takes place. It is known that the liver is very active
in producing this substance ; but it is not alone by this organ
that urea is formed. At the present time the prevailing
opinion is that, for the most part, the proteids under destruc-
tive metabolism in the tissues do not reach the urea stage of
transformation, but are converted into ammonia compounds
(which differ very slightly from the urea in chemical compo-
sition), and these compounds are conveyed by the blood to
the liver, where the slight change necessary to make them
urea is effected under the influence of this organ. Ammon-

204 EXCRETION BY THE KIDNEYS AND SKIN

mm carbamate seems the typical compound, but ammonium
carbonate and others are probably likewise converted. Ar-
tificial circulation of these compounds through the liver gives
rise to urea; removal of the liver increases the ammonia
compounds and decreases the urea in the urine ; ammonia
compounds are normally very much more abundant in the
portal blood than in the arterial, but when the liver is re-
moved they are evenly distributed throughout the circula-
tion, and the animal dies in a few days of symptoms which
can be aggravated by administration of the ammonia com-
pounds;— all of which circumstances go to show that it is
ammonia compounds which the tissues produce, and that
they are changed to urea in the liver.

Still, removal of the liver does not suspend entirely the
output of urea. Consequently this substance must be formed
elsewhere, but by what organs is unknown. It is not impos-
sible that it is formed to some extent in all organs where
proteid dissociation is progressing. This is practically, if
not really, the case in health at any rate, even under the
theory above mentioned.

It is to be noted that urea is not fidl oxidized; it can be
oxidized outside the body. Thus the heat-producing capac-
ity of the proteids is not completely utilized. If they have
been broken down in the body into substances simpler than
urea, then the amount of heat liberated in such dissocia-
tion is consumed in building up the urea molecule to be dis-
charged.

Uric acid is combined in normal urine to form the urates
of sodium, potassium, magnesium, calcium and ammonium.
The urate of sodium is by far the most abundant of these,
and, besides urate of potassium, only traces of the others
are found. Free uric acid in human urine is pathological.
The urates, like urea, come ultimately from oxidation of the
nitrogenous constituents of the body. They are not formed
in the kidney, but pass out as such from the blood. About
9-14 gr. are discharged daily. The amount is increased in
gout.

HIPPURIG ACID 2O5

111 some animals uric acid takes the place of urea, none of
the latter being formed. In these cases it is manufactured
by the liver from ammonia compounds. This does not, how-
ever, seem to be the origin of uric acid in human urine. It
has been looked upon as unconverted urea, i. e., as a product
antecedent to urea ; but at present such does not seem to be
the case. A theory that it is the end product of the destruc-
tion of certain materials in the nuclei of cells has consider-
able support.

Hippuric acid exists in the urine as hippurates. It differs
from most of the other urinary constituents in being formed
in the kidney ; it does not preexist in the blood. The daily
output of this substance is about 10 grains, though the
amount may be considerably increased on a vegetable diet.
The benzoic acid of vegetables seems to be synthesized into
hippuric. In proteid dissimilation some benzoic acid may be
produced and eliminated in this shape.

The various lactates are not formed by the kidney, but
pass unchanged into it from the blood. The lactic acid from
which they are formed probably results from the transfor-
mation of dextrose.

Creatinin is normally present in the urine. It is a nitro-
genous body differing from creatin only by a molecule of
water. It is eliminated to the extent of about 15 grains per
day. A part comes from proteid destruction in the body, and
another part is said to come directly, without metabolism,
from creatin which is a constituent of ordinary meat. It is
not formed in the kidney.

Xanthin, hypoxanthin, etc., are to be regarded as nitro-
genous excreta allied to uric acid and resulting in some way
from proteid metabolism. They are regarded by some as
having the same probable origin as uric acid, viz., the disin-
tegration of cell nuclei.

The non-nitrogenous constituents scarcely deserve separ-
ate mention. It is through the kidney that the largest variety
of these materials are discharged. Certain of these are con-

206 EXCRETION BY THE KIDNEYS AND SKIN

stant, but the wide variety of such materials taken into the
alimentary canal accounts for the same wide variety in the
urine. The proportion of inert substances in the blood is
approximately constant — kept so by the removal of any
excess by the kidneys chiefly.

Sodium Chloride is eliminated thus to the extent of about
151 grains daily. The sulphates are unimportant. About
25 grains are excreted daily. The phosphates are more im-
portant, the acid sodium phosphate being mainly responsible
for the acid reaction of the urine. Nitrogen and carbon di-
oxide are the chief gases to be found. The color of urine is
due to a substance, urochrome, which is probably formed
from hemoglobin. Some mucus from the bladder is also
in the urine.

Variation in Amount and Composition of Urine. — "Its
constitution is varying with every different condition of nu-
trition, with exercise, bodily and mental, with sleep, age, sex,
diet, respiratory activity, the quantity of cutaneous exhala-
tion, and indeed with every condition which affects any part
of the system. There is no fluid in the body that presents
such a variety of constituents as a constant condition, but
in which the proportion of these constituents is so vari-
able" (Flint).

Prolonged bodily exercise will increase the amount of
urea, but the urine is generally decreased in quantity because
perspiration is more active. The young child discharges rel-
atively much more urea and urine than the adult. The fe-
male discharges relatively more urine, but less urea, than the
male. Digestion increases the urinary flow. Climate and
season act chiefly though increasing or diminishing cutaneous
activity. Emotions of various kinds may give rise to an
abundant flow of pale urine.

Discharge of Urine. — On leaving the pelvis of the kidney
the urine enters the ureters and passes through them to the
bladder, whence it is discharged per urethram.

The ureters run, one from each kidney, downward and

THE BLADDER 2O7

slightly inward behind the peritoneum, a distance of some 18
inches to the base of the bladder. In the female the cervix
uteri lies between the two ureters just before they enter the
bladder. They penetrate the bladder wall obliquely, their
course therein being nearly an inch long. The effect of this
arrangement is that distention of the bladder closes the open-
ing more closely instead of causing regurgitation into the
ureter. The ureter is composed of three coats. The outer
is fibrous, the middle muscular, and the internal mucous.

The bladder serves as a reservoir for the urine until such
time as it is convenient for it to be evacuated. This organ,
when empty, lies deep in the pelvis in front of the rectum
in the male and of the uterus in the female. When moder-
ately distended it will hold about a pint, has an ovoid shape
and rises to the brim of the pelvis. It also has three coats.
The outer is peritoneal, and covers the posterior and small
parts of the lateral and anterior surfaces only. Its lower
limit posteriorly is the entrance of the ureters. The middle
layer is muscular. The fibers, which are non-striped, are
disposed in three sheets. Their contraction compresses the
contents from all directions. Embracing the neck (outlet)
of the bladder is a thick band of plain muscle tissue known
as the sphincter vesicce. The tonic contraction of this mus-
cle prevents the continual escape of urine. The inner coat
of the bladder is mucous. It is rather thick, and loosely ad-
herent to the subjacent muscular coat except over the corpus
trigonum where it is closely attached. The corpus trigonum
is a triangular body of fibrous tissue just underneath the
mucous membrane; its apex is at the origin of the urethra,
and its other angles are at the vesical openings of the uret-
ers.

Absorption from the intact mucous membrane of the blad-
der takes place very sparingly, if at all. Abrasions of the
membrane from any cause allow absorption to occur; and
this fact may be made use of to locate lesions giving rise to
hematuria. Iodide of potassium injected into the bladder

2O8 EXCRETION BY THE KIDNEYS AND SKIN

can be detected in the saliva if the bladder is the source of
the blood.

Micturition. — When the bladder has become moderately
full the desire to expel its contents arises. The act of mic-
turition involves relaxation of the sphincter vesica and
contraction of the muscular walls of the bladder aided by
the abdominal muscles and those of the urethra. A slight
contraction of the abdominal muscles compresses the blad-
der; after a short interval the sphincter relaxes and allows
the stream to pass out through the urethra. When the act
has been begun contraction of the bladder will suffice to
nearly empty the organ, but complete evacuation is finally
brought about by a series of convulsive contractions on the
part of the muscles of the abdomen.

The center controlling the reflex nervous phenomena of
micturition is opposite to the fourth lumbar vertebra in the
spinal cord.

THE SKIN.

Functions. — The functions of the skin from a physical
standpoint are sufficiently apparent. It furnishes protection
to the underlying parts, preserves the general contour of the
body, affords lodgment for afferent nerve terminations, and
thus establishes relations between ourselves and our sur-
roundings. As an organ of excretion it is very important,
and in fact essential to life. While various materials, such
as urea and CO, are thus discharged from the body, their
amount is more or less inconsequential, and it appears that it
is the action of the skin as a regulator of heat "excretion"
which is vital. It furnishes one of the three chief routes
for the discharge of water from the body, and it will be
seen that it is largely through the output of water that the
output of heat is regulated. So necessary is the skin in this
respect that the covering with impermeable substances of as
much as half the body surface is followed by death.

The skin excretions are contained in the products of the

STRUCTURE OF THE SKIN

2O9

sebaceous and sweat glands. These products correspond al-
together to neither the secretions nor the excretions, and the
sebaceous glands have been described under the head of
secretion. It is to be remembered, however, that the sweat
usually represents part of the sebaceous as well as the sudo-

Stratum corneum.

Stratum lucidum.
Stratum granulosum.

Stratum Malpighii.

FIG. 60. — Vertical section of the human epidermis.

The nerve-fibrils, n, b, stained with gold chloride. (Landois.)

riparous secretion, because the mixture of the two is a
physical necessity. It is the water of the sweat which is the
most important excretion from the skin, although the elimi-
nation of CO2 and inorganic salts, and especially of urea in
some pathological conditions, is not to be overlooked.

2IO EXCRETION BY THE KIDNEYS AND SKIN

Structure. — The skin consists of an external covering, the
epidermis, with its modifications, hair and nails, and of the
cutis vera. Imbedded in the cutis vera are sebaceous and
sweat glands and hair- follicles. (Fig. 61.)

Epidermis. — The epidermis consists of at least four lay-
ers of epithelial cells. From above downward these are ( I )
the stratum corneum} (2) the stratum lucidum, (3) the
stratum granulosum, (4) the rete mucosum or Malpighii.
All these except the stratum corneum have a fairly constant
thickness. The stratum corneum is thick or thin according
to location and degree of exposure, and its cells are flat and
horny. The lowest cells of the rete mucosum are columnar.
From this last-named layer the cells pass gradually upward,
and as gradually assume the shape of the horny layer. The
horny cells are thrown off and their place is taken by others
from beneath. (Fig. 60.)

Hairs are to be found on almost all parts of the cutaneous
surface. They consist of a bulb and a shaft. A depression
of the skin involving both epidermis and cutis vera consti-
tutes the hair-follicle in which the bulb rests. A projection
at the bottom of the follicle corresponds to a papilla, and
upon it the bulb is placed. The shaft has an oval shape in
cross section. It is composed of fibrous tissue, outside
which is a layer of imbricated cells.

Nails consist of a superficial layer of horny cells and a
deeper one corresponding to the rete mucosum. The root of
the nail is received into the matrix — a specialized portion
of the cutis vera.

Cutis Vera. — The cutis vera is tough but elastic. It rests
upon cellular and adipose tissue. Its structure is areolar
with some non-striated muscle fibers. Projecting from the
cutis vera into the epidermis are minute conical elevations,
the papilla. Many of them contain sensory nerve terminals.

Sweat Glands. — Practically the whole cutaneous surface
contains sweat glands. Some two and a half mil-
lions are thought to exist in the skin of the av-

SWEAT GLANDS

211

erage individual. They are particularly abundant in
the skin of the palms of the hands and soles

FIG. 61. — Vertical section of skin.

A, sebaceous gland opening into hair- follicle; B, muscular fibers; C, sudorif-
erous or sweat-gland; D, subcutaneous fat; E, fundus of hair- follicle, with hair-
papilla. (Kirkes after Klein.)

of the feet. They belong to the simple tubular type,
and consist of a secreting portion and an excretory duct.

212 EXCRETION BY THE KIDNEYS AND SKIN

The secreting part lies just underneath the true skin and, as
a whole, resembles a small nodule ; however, the nodule con-
sists of an intricate coiling of the tube itself which is of ap-
proximately uniform diameter throughout. It curls upon
itself some 6-12 times and ends by a blind extremity. It is
lined by epithelial cells.

The duct passes away from the glandular coil, runs
through the cutis vera. in a comparatively straight course and
assumes a spiral shape as it traverses the epidermis to open
obliquely on the surface. With the ducts of the larger
glands are connected a few non-striped muscular fibers
which may aid in the discharge of the secretion. (Fig. 61.)

Properties and Composition of Sweat. — The secretion is
colorless, has a slight characteristic odor, and a salty taste.
Its specific gravity is about 1003-4, and its reaction is usu-
ally acid when just discharged. It contains a large propor-
tion of water, a little urea and fatty matter, and quite a
quantity of inorganic salts of which the chief is sodium
chloride. All the constituents in health are of subsidiary im-
portance except the water. Under average conditions of
temperature and exercise the amount secreted in 24 hours
is about 2 pounds. But the quantity is very variable — as
much so as~the urine, and may be said in a general way to
vary inversely as the urinary secretion.

Mechanism of the Secretion of Sweat. — Sweat is pro-
duced continuously, though up to a certain point it passes
off as vapor or "insensible perspiration." Beyond that point
it accumulates on the skin as an evident fluid and becomes
"sensible perspiration." Whether it escapes as sensible or
insensible perspiration, it is secreted as a fluid.

The activity of the cells lining the glandular coils in sep-
arating sweat from the blood is undoubted. Distinct secre-
tory fibers are distributed to them, and through the influence
of these fibers the glands will secrete sweat even without an
increase in the blood supply. But usually a determination
of blood to the surface means an increase of perspiration.

SECRETION OF SWEAT 213

This occurs during violent exercise, e. g. However, that
the production of sweat is not altogether dependent on this
factor is shown by profound sweating in shock, nausea and
like conditions when the skin is pale and cold, and by dry-
ness of the flushed skin in febrile diseases. Furthermore,
experiments on inferior animals have revealed fibers which
influence the secretion of sweat without affecting the blood
flow.

Practically, in health, the only conditions which increase
the flow of perspiration are muscular exercise and a high ex-
ternal temperature. Of these, exercise probably works
through the nerve centers; external heat does not stimulate
the glands directly, but irritates the cutaneous terminations
of afferent fibers which convey impressions to the sweat cen-
ters, whence messages are sent out by secretory (fibers to the
glandular epithelium and their activity begins. In both cases
ihere is accompanying dilatation of the superficial vessels
under the influence of the vaso-dilator fibers.

It is supposed that the chief center is in the medulla ob-
longata and that secondary centers exist in the lumbar re-
gion of the cord.

The amount of CO2 eliminated by the skin is inconsider-
able in the human being.

CHAPTER XL
THE NERVOUS SYSTEM.

General Functions of the System as a Whole. — The ner-
vous system is the most delicately organized part of the ani-
mal body. Its sensory terminations receive impressions
which are conducted to the centers; it conveys impulses
from the centers to the different parts of the body, control-
ling and regulating their action. Connecting, as it does, all
parts of the organism into a coordinate whole, it is the only
medium through which impressions are received, and is the
only agency through which are regulated movement, secre-
tion, calorification and all the processes of organic life. This
system, ramified throughout the body, connected with and
passing between its various organs, serves them as a bond
of union with each other, as well as with the brain. The
mind influences the corporeal organs through the instru-
mentality of this system, as when volition calls them into ac-
tion; on the other hand, changes in the organs of the body
may affect the mind through the same channel, as when, for
instance, pain is mentally perceived when the finger is
burned. Thus it is that the nervous system becomes the
main agent in what is known as the "life of relation" ; for
without some medium for the transmission of its mandates,
or some means of receiving those impressions which exter-
nal objects are capable of exciting, the mind would be com-
pletely isolated, and could hold no communion with the ex-
ternal world.

It should not be understood, however, that the nervous
system cannot operate independently of mental influence. All
those manifestations of nervous activity connected with the

214

GENERAL FUNCTIONS 215

performance of the so-called "organic functions" of life as
digestion, circulation, etc., are not directly influenced by vo-
lition; indeed an essential character of these functions is
that they are completely removed from the influence of the
will ; to be conscious subjectively of their performance is an
evidence of abnormality.

The first step in every voluntary act is a mental change,
in which the act of volition consists. If this mental change
be of such nature as to direct its influence upon a muscle,
or a particular set of muscles, the contraction of those mus-
cles immediately supervenes, so as to bring about the prede-
termined voluntary act. But the influence of the will
could not possibly be exerted upon those muscles except
through intervention of the nerves.

Furthermore, a certain mental state, in cases of common
or special sensation, is induced by an impression made upon
certain bodily organs. But in no case could the mental state
be produced unless a particular part of the nervous system
were present to convey the impression received to the center
capable of recognizing it. If the hand be burned pain is felt,
but were the nerves not present to convey the impression
made by the heat no degree of temperature could make the
mind cognizant of injury. When light is admitted to the
eye a corresponding mental sensation is produced, but for
the production of this the integrity of the optic nerve is a
necessary condition.

It will be gathered from the foregoing remarks that the
nervous system is not only capable of conveying communi-
cations, but that it has the power, in certain of its divisions,
of receiving impressions and of giving rise to stimulating in-
fluences— that is, that it is capable of generating a peculiar
power known as "nerve force." It thus becomes the seat of
distribution of energy to all the cells. These generating
parts of the system are the reservoirs of force — force which
has been derived from the cells and is distributed to them.
This nervous force, having its origin in the living activity

2l6 THE NERVOUS SYSTEM

of the cells, is the highest manifestation of vital energy.
The nervous structure is divided into two great systems:

1. The Cerebro-spinal System consists of the brain, the
spinal cord and all the nerves which run off from these. This
system is especially concerned with the functions of relation,
or of animal life. It presides over general and special sen-
sation, over voluntary movements, over intellection, over all
conscious activity, and over all other functions which are
peculiar to the animal. It is by this system that we know
of and deal with the other great system.

2. The Sympathetic, or Ganglionic System is especially
connected with the functions relating to nutrition — func-
tions similar to those occurring in the vegetable kingdom.
It presides over all organic life — over all unconscious ac-
tivity. While the operations over which this system holds
sway are quite different from those under the supervision
of the cerebro-spinal system, it must not be concluded that
the two are not anatomically and physiologically related.
Neither is independent of the other, as was once thought, but
both are parts of the same great apparatus.

Divisions of the Nervous Substance as a Whole. — The
nervous matter, irrespective of the two systems, may be
studied as consisting of two divisions. The first is made up
of cells; the second of tubes, or fibers. Although the tissue
may be thus divided into nerve cells and nerve fibers, the
present conception of the arrangement of the nervous
substance is that these two are only different parts of the
same element known as the neuron, supported by tissue ele-
ments known as neuroglia, which, though not identical with
connective tissue, is comparable to it in its function of sup-
port. The neuron, thus considered, consists of a proto-
plasmic body which sends out a number of branching pro-
cesses called dendrites, one of which becomes the axis cylin-
der. While, therefore, it is to be understood that the cell
and the fiber in the nervous system are both portions of an
identical whole, a description of them as separate parts is

NERVE FIBERS 217

warranted for the sake of convenience and by differences in
their general characteristics.

The nerve cells are the only organs capable, under any cir-
cumstances, of generating nerve force. As a rule they are
stimulated to generate this force by the reception of an im-
pression through the nerve fiber, but they may in some cases
be directly excited by mechanical, electrical or chemical
means. They also frequently act as conductors, as will be
seen later.

Under no circumstances can nerve fibers generate force.
Their office is exclusively to conduct impressions and im-
pulses, and they usually receive these impressions and im-
pulses at their terminal extremities* in the case of afferent
nerves, and from the centers in the case of efferent nerves ;
but in many instances they may be stimulated in any part of
their course. Some fibers are incapable of being thus di-
rectly stimulated. The nerves of special sense are insensi-
ble to direct stimulation.

Nerve Fibers. — Nerve fibers are of two kinds: (A) white
or medullated fibers and (B) gray or non-medullatcd fibers.
The non-medullated fibers possess the conducting elements
alone, while the medullated possess certain accessory ana-
tomical elements.

(A) Each medullated fiber has (i) an external envelop-
ing membrane called the ncurilemma, or the primitive nerve
sheath, or the sheath of Schwann; (2) an intermediate sub-
stance known as the myeline sheath, or the white substance
of Schwann, or the medullary substance; (3) a central
fiber, the true conducting element, which usually goes under
the name of the axis cylinder, or axone.

The sheath of Schwann is analogous to the sarcolemma
of muscle fibers. It is a structureless protective membrane,
somewhat elastic, and presents oval nuclei with their long
diameter corresponding to the direction of the fiber. This
sheath is wanting over the medullated fibers in the white sub-
stance of the brain and spinal cord,

218

THE NERVOUS SYSTEM

Node of Ranvier.

Primitive sheath.

Nerve corpuscles.

Axis cylinder.

White substance
of Schwann.

Node of Ranvier.

FIG. 62. — Scheme of a
medullated nerve fiber of
a rabbit acted on by
osmic acid.

The incisures are omitted.
X 400. (Landois.)

It is the white substance of
Schwann which gives to the nerve
its peculiar whitish appearance.
This is a fatty substance of a semi-
fluid consistence. It fills the tube
made by the sheath of Schwann
and surrounds the axis cylinder.
It is wanting at the origin of the
fibers in the centers and at their
peripheral distribution. It is prob-
ably not necessary to conductivity.
In fresh nerves this substance is
'strongly refractive, and the optical
effect produced by its varying
thickness in the center and at the
edges is the appearance of dark
borders. It easily coagulates into
an opaque mass. The idea that the
myeline sheath acts as an insulator
lacks supporting evidence. The
theory that it is nutritional is
plausible; but no sufficient differ-
ence in the medullated and non-
medullated fibers in this respect
has been found to establish the the-
ory as a fact. At certain points in
in the course of medullated fibers
there are seen constrictions called
the nodes of Ranvier. At these
points the medullary substance is
wanting and the sheath of
Schwann is in contact with the
axis cylinder. It is not improbable
that these nodes furnish a mode of
access for the nutrient plasma.
Certain it is that they are most

NERVE TRUNKS

2IQ

numerous where the physiological activity is supposed to be
most active.

The axis cylinder is composed of
a large number of primitive fibrillae.
This band occupies about one-fourth
the diameter of the tube and is the
true conducting element, as is shown
by its invariable presence, its contin-
uity and other considerations equally
conclusive. It is demonstrated under
the microscope with difficulty in fresh
specimens. It is directly connected
with a nerve cell, and is the essential
part of the fiber. The process of the
cell which becomes the axis cylinder
is not, as was once thought unbranch-
ed, but itself sends off "collaterals"
in the gray substance. These collat-
erals, however, do not actually join
any other nerve cells or fiber.

The average diameter of medullated
fiber is about ^ooo in., though all are
said not to preserve the same diam-
eter throughout their course.

(B) The non-medullated fibers
(fibers of Remak) seem to be simple
axis cylinders without the other atom-
ical elements peculiar to medullated
fibers. They make up a large part of
the trunks and branches of the sym-
pathetic system, and represent the fil-
aments of origin and distribution of FIG. 63.— Non-medullated
all nerves. They are thought by some nerve fiber-

to possess a neurilemma. They are u> VnucfeuSf; d^
pale gray in color. surrounding it.

Nerve Trunks. — The above remarks apply to a single

22O

THE NERVOUS SYSTEM

nerve fiber. These fibers seldom run an extended course
alone, but are bound together in large numbers to make a
nerve trunk. This trunk is composed of a number of
bundles of fibers, and is surrounded by a connective tissue
membrane known as the epineurium; the separate bundles,
or funiculi, are surrounded each by a similar membrane
called the perineurium; while inside the funiculi, between

FIG. 64. — Transverse section of a nerve. (Median.)

ep, epineurium; pe, perineurium; ed, endoneurium. (Landois.)

the primitive fasciculi, is a delicate supporting tissue known
as the end on curium, or the sheath of Henle. In connection
with this sheath there are nuclei belonging to the connective
tissue and to the nerve fibers themselves. The sheath be-
gins where the nerve fibers emerge from the white portion
of the centers, is interrupted by the ganglia in the course of
the fibers, branches as the bundle branches, and is lost before
the terminal distribution is reached. It is seldom found sur-
rounding single fibers. It is likewise rare for capillaries to

NERVE CELLS 221

penetrate it and reach the fibers themselves. There are nu-
merous lymph spaces around the individual fibers as well
as around the funiculi. In situations where the nerves are
well protected, as in the cranium, the amount of fibrous
tissue in the trunks is small, but where opposite conditions
prevail, as in muscular substance, this tissue is largely in-
creased in amount as regards both that which surrounds the
trunk and that which is sent" in between the funiculi and
fiber. This tissue has ramifying in it a network of fibers
known as neri nervorum. The blood supply is not large.

Individuality of Nerve Fibers. — It is to be remembered
that so far as can be determined every nerve fiber, having
entered a trunk, proceeds without interruption to the part to
which it is finally distributed, whether that part be the skin,
or a viscus, or a muscle, or a gland, or some organ of special
sense, or another nerve cell, or what not. Collections of
fibers forming bundles run together in the same trunk, may
leave that trunk together, may send out part of their fibers to
another bundle or trunk, or may receive other fibers from
other funiculi ; but everywhere the relation of the primitive
fibers to each other is simply one of contiguity. Hbwever,
as the axis cylinder approaches the seat of its final distribu-
tion, it breaks up into several fibrillse, such divisions always
taking place at the nodes of Ranvier.

Nerve Centers. — The nerve centers include the gray mat-
ter of the brain and cord and the ganglia in both the cerebro-
spinal and sympathetic systems. These centers have a gray
color due to the presence of a pigmentary substance in the
cells and surrounding tissue. The ganglionic centers are
simple collections of nerve cells with their usual accessory
elements — myelocytes, intercellular granular matter, delicate
membranes covering some of the cells, connective tissue ele-
ments, blood-vessels and lymphatics.

Nerve Cells. — These are irregular in shape and may be
unipolar, bipolar or multipolar. They also vary much in
size. The unipolar cell has a single prolongation which be-

222

THE NERVOUS SYSTEM

comes the axis cylinder. Bipolar cells are prolonged in two
directions, and may be looked upon as simply protoplasmic
enlargements of the nerve fiber. This cell is frequently cov-
ered by a connective tissue envelope which is continuous in
both directions with the sheath of Schwann. Multipolar

Dendrites.

Nerve-cell. «

Nerve process,
or axone.

Neurilemma. —

Neurilemma.

• Neive-cell.

FIG. 65.

A, efferent neuron; B, afferent neuron. (Brubaker.)

cells have three or more prolongations, one of which always
becomes continuous with the axis cylinder and is called the
axis-cylinder process, the neuraxon, or the axone. The
other poles branch in various irregular directions like the
limbs of a tree, and are hence called dendrites. They also
go under the name of protoplasmic prolongations. Some of
these unite the cells to contiguous cells by interlacing with,

NEURONS 223

but not actually joining, similar poles from those cells. The
multipolar cells in the anterior cornua of gray matter of the
cord are said to be larger in size and to present more poles
than corresponding cells in the posterior column.

The diameter of nerve cells varies from M.250 to %oo in.
The nucleus is usually single, and most cells have no true
surrounding membrane. If a nerve fiber be followed toward
the center which gives it origin it will be found first to lose
its sheath and later its medullary substance ; this medullary
substance may continue for some distance after the sheath
is lost, as in the white substance of the encephalon, but never
penetrates the gray substance proper. Every nerve fiber
is connected with a cell by that cell's axis-cylinder prolon-
gation.

Certain retrograde changes take place in the neurons in
old age — morphological changes agreeing with the physio-
logical decrease in energy-producing- power at that time.
The cell body becomes smaller, the dendrites atrophy, and
the axones diminish in mass. Nerve "fatigue" can also be
demonstrated by the microscope. The nuclei of the sheath
are flattened, the protoplasm is shrunken and vacuolated
and the nucleus is crenated. The quantity and quality of
the food may be perfect, but the power of the cell to utilize
it is impaired, and this means diminished physiological
power.

Communication Between Different Neurons. — Every neu-
ron is anatomically independent of every other neuron.
There is no actual joining of fibers or dendrites — simply an
interlacement of the end arborizations. This is illustrated
in Figs. 62 and 63. In the latter the afferent fiber is joined
to no cell except G, one of the cells of the spinal root gang-
lion. Its end arborizations simply interlace with the dend-
rites of the motor cell M. C. and cause it to send out an
efferent impulse to the muscle M.

Furthermore, there are frequent relays in the transmission
of nerve messages. By no means do all the fibers from the

224

THE NERVOUS SYSTEM

B.C.

M,C:

FIG. 66. — Reflex action; old idea. (Kirkcs.)

FIG. 67. — Reflex action; modern idea. (Kirkes.)

NERVE FIBERS

motor area of the brain pass them-
selves out as parts of the anterior
roots. The relay service is illus-
trated in Fig. 64. Here again, it is
seen that there is no actual joining of
the neurons. Whenever it is said that
a nerve cell is "joined" to another, or
that the axis cylinder of a cell "joins"
another cell, no actual continuity of
tissue is meant. Different neurons
communicate only by contiguity.

Peripheral Nerve Terminations. —
Nerves terminate peripherally (r) in
muscles, (2) in glands, (3) in special
organs connected with the senses of
sight, hearing, smell and taste, (4)
in hair-follicles, (5) in simple free
extremities passing between epithelial
and other cells, and (6) in several
kinds of so-called tactile corpuscles.

The motor nerves passing to vol-
untary muscles form first a "ground
plexus" for each group of muscle
bundles — this plexus being made of
the axis-cylinder fibrillae. From this
plexus fibrils pass to form an "inter-
mediary plexus" corresponding to
each muscle bundle. These fibrils are
still medullated, and when a branch
from the intermediary plexus enters a
muscle fiber its sheath becomes con-
tinuous with the sarcolemma of that
fiber, and the axis-cylinder fibrils
form a network on the surface of
the muscle fiber. This is called an
end motorial plate. It contains a

s.c;

FIG. 68. — Diagram of an
element of the motor path

U. S., upper segment; L.
S., lower segment; C.C.,
cell of cerebral cortex; S.C.,
cell of spinal cord, in ante-
rior cornu; M, the muscle;
S, path from sensory nerve
roots. (Kirkes after Cowers.)

226 THE NERVOUS SYSTEM

number of nuclei, and sends off from its under surface fine
fibrillae which are said to pass between the muscular fibrillse
which make up the fiber. Sensory fibers are somewhat
scantily distributed to the voluntary muscles.

In plain muscle tissue the motor nerves are distributed
after the same general manner as in the striped muscles,

Nerve-fibre.

End-plate.

Muscle nucleus.

FIG. 69. — Termination of a nerve fiber in end-plate of a lizard's
muscle. (Stirling.)

though with some differences. Here the fibers are not me-
dullated, and primitive fibrils passing from the intermediary
plexus finally enter the nuclei of the muscle cells.

Medullated fibers have been traced to the cells of glands,
but not farther. It is thought by some that, having formed
a plexus, non-medullated fibers pass in to terminate in the
nucleoli of the gland cells, though such endings have not
been demonstrated.

The peripheral distribution of nerves connected with the
special senses will be discussed elsewhere.

The remaining methods of termination above noted apply
to afferent nerves. It is claimed that a very large number of
sensory nerves terminate in hair-follicles. If such be the
case it will account for sensory terminations in by far the
greater part of the cutaneous surface. It is supposed that

NERVE FIBERS

227

nerve fibrillse form a plexus beneath the true skin and send
branches thence to the follicles, though the exact mode of
termination is a question of
some obscurity.

Terminations between epi-
thelial cells are probably more
common than any other meth-
od of sensory distribution.
The fibers, having passed to
the surface of the skin or mu-
cous membrane, lose every-
thing excepting the axis cylin-
der, which, dividing into mi-
nute ramifications, passes, by
means of these fibrillae,
among the epithelial cells.
This mode of termination is
held by some to prevail in the
glands. It certainly prevails
in parts other than the skin
and mucous membranes.

Sensory nerves further ter-
minate in (a) the corpuscles
of P acini or Vater, (b) the end
bulbs, or tactile corpuscles of
Krause, (c) the tactile corpus-
cles of Meissner, (d) the
tactile menisques, and (e) the
corpuscles of Golgi.

(a) The Pacinian Corpus-
cles are oval elongated bodies-
Each corpuscular body has a

FIG. 70. — Vater's or Pacini's
corpuscle.

a, stalk; b. nerve fiber entering it;

- P" , , -,, r~ • i c, d., connective-tissue envelope; e,
length of about 1/12 of an inch, axis-cylinder with its end divided

and is about half as broad. It

at /. (Landois.)

is made up of a number of concentric layers of connective
tissue in a hyaline ground substance and is attached by a

228

THE NERVOUS SYSTEM

pedicle to the nerve whose termination it is. Through this
pedicle passes a single (occasionally more) nerve fiber
which, piercing the several concentric layers constituting
the corpuscle, gradually loses its myeline substance and runs
longitudinally through the center of the body to terminate at
the distal end of the central cavity in a knob-like enlarge-
ment. These corpuscles are found in great abundance on the
palmar and plantar surfaces of the hands and feet, being far
more numerous on the first phalanx of the index finger than
elsewhere. About six hundred are said to be present in each
hand and foot. They are also to be found on the dorsal sur-
faces of the hands and feet, over parts of the forearm, arm
and neck, in the nipples, in the substance of muscles, in all

the great plexuses of the sympa-
thetic system, and in numerous
other situations. These bodies can-
not be considered true tactile cor-
puscles because they are situated
beneath the skin ; neither can they
be positively said to have any "spe-
cial sensory" function such as the
appreciation of temperature,
weight, etc.

(b) The end bulbs of Krause
exist in great number in the con-
junctiva, the glans penis and cli-
toris, the lips, and in other situa-
tions. They bear some resem-
blance to the corpuscles of Pacini,
but are much less elaborate in their arrangement; the num-
ber of concentric layers is much smaller, while the contained
mass is larger. The shape is spherical. From one to three
medullated fibers pass from the underlying plexus to wind
through the corpuscle and break up in free extremities. The
sheath of the fiber is continuous with the outer covering of
the corpuscle, and the medulla is gradually lost as the fiber

FIG. 71. — End bulb from
human conjunctiva, treat-
ed with osmic acid, show-
ing cells of core. (From
Yeo after Longivorth.)

a, nerve fiber; b, nucleus of
sheath; c, nerve fiber within
core; d, cells of core.

NERVE FIBERS

229

enters the bulb. The end bulb of Krause measures from
Hooo to ^so of an inch in diameter.

(c) The tactile corpuscles of Meissner have to do with the
sense of touch, and are situated largely in the papillae of the
skin covering the palmar surfaces of the hands and the
plantar surfaces of the feet; they also exist in other situa-
tions, corresponding in general to the distribution of the
Pacinian corpuscles. The largest number is found over the
distal phalanges of the fingers and toes on their palmar and

FIG. 72. — Drawing from a section of injected skin.

Showing three papillae, the central one containing a tactile corpuscle, a, which
is connected with a medullated nerve, and those at each side are occupied by
vessels. (From Yeo after Cadiat.)

plantar surfaces ; they diminish in number proximally from
these points. They may be simple or compound according
as the enclosing capsule contains one or more collections of
nucleated cells. Their form is oblong with the long axis
in the direction of the papillae. They vary in thickness with
the papillae of the region in which they are located. They
may have a transverse diameter of from %oo to M.50 of an
inch, and probably in most instances occupy the secondary
eminences of the papillae in which they are found. A simple
papilla does not generally possess both vascular and nervous
loops.

(d) The tactile menisques are found in certain cutaneous
regions. Nerves in the superficial layer of the skin lose their

230 THE NERVOUS SYSTEM

medullary substance and divide to form arborization which
are flattened into the form of a leaf.

(e) The corpuscles of Golgi are situated at the point of
union of tendons with muscles, and are believed by some to
have to do with the muscular sense. They are flattened fusi-
form bodies composed of granular substance enclosed in
layers of hyaline membrane and containing nervous fibrillae.

Properties and Classification of Nerve Fibers. — Nerve
fibers are for the purpose of conveying messages either peri-
pherally or centrally. They may be stimulated to action by
anything capable of suddenly increasing their irritability. In
any case the effect of the stimulus, whether normal or ab-
normal, is manifested at the peripheral distribution of the
stimulated fiber. So far as most external manifestations are
concerned, nerves may be classified as motor and sensory.
That is to say, stimulation, for instance, of a cerebro-spinal
nerve (except those of special sense) is followed, under or-
dinary conditions, by one of two results — there is either
pain or contraction of a muscle to which the nerve is dis-
tributed. This is a typical illustration of the action of motor
and sensory fibers, and the manifestation of nerve action,
whether it consists in pain or motion, is a result only of the
conduction of an impression of an impulse to the center or
the periphery. It is to be noted that the result of thus stim-
ulating a nerve fiber is manifested at one extremity only of
that fiber, and always at the same extremity.

However, since there are nerve fibers the stimulation of
which is not followed by pain or motion, the division into
sensory and motor fibers is not comprehensive enough to in-
clude all the fibers in the body. But since, as above stated,
the only office of fibers is to conduct, and since they always
conduct in a direction either tozvard or away from the cen-
ter, all nerves may be classified as either centripetal or cen-
trifugal. A corresponding division is into afferent and
efferent. It will be seen that all motor fibers are centrifu-
gal or efferent, but not all centrifugal or efferent fibers are

EFFERENT NERVES 23!

motor. It will likewise be seen that all sensory fibers are
centripetal or afferent, but not all centripetal or afferent
fibers are sensory. For impressions made upon the termina-
tions, or upon the trunk, of a centripetal nerve may cause
(i) pain, or some other kind of sensation; (2) special sen-
sation; (3) renex action of any kind; (4) inhibition. Simi-
larly impressions made upon a centrifugal nerve may (i)
cause contraction of a muscle (motor nerve) ; (2) influence
nutrition (trophic nerve) ; (3) control secretion (secretory
nerve) ; (4) inhibit, augment, or stop any other efferent ac-
tion (Kirkes).

To these two classes, efferent and afferent, should be
added a third, the intercentral fibers which connect different
parts of the nervous centers. Most of these even can be
called either afferent or efferent.

Characteristics of Efferent Nerves. — In case of these
nerves a force is generated in the centers and conveyed by
the nerves to the periphery, where it manifests itself in one
of the ways mentioned above as characteristic of centrifugal
fibers. Division of these fibers, or interference with their
conductivity by disease or otherwise, renders impossible the
manifestation of nervous force generated in the center, for
the simple reason that the organ to which the fibers are di-
tributed cannot receive the message intended for it. For in-
stance, a muscle cannot, by the most persistent effort of the
will be made to contract if the motor fibers running to that
muscle are divided. In case, however, of division of effer-
ent nerves, if the peripheral end be irritated, thus roughly
counterfeiting normal stimulation, the ordinary effects of
normal stimulation will be brought about, provided (as is
usually the case) that particular nerve can be thus directly
stimulated. Stimulation, however, of the central end of
such a cut nerve produces no effect. No matter whether
such efferent nerves receive their stimulus directly from the
center or artificially, as by mechanical or electrical means,
the effect is produced in the end organs, whatever they may

232 THE NERVOUS SYSTEM

be. It is an invariable law to which reference has already
been made, that a nerve fiber thus conducting a message in
either direction is not interfered with by the proximity of
other fibers, similar or dissimilar. Such message is not in
any way imparted to a neighboring fiber or diffused through
the fasciculus, but is conveyed uninterruptedly to its destina-
tion. It is possible that the myeline sheath has an insulating
effect upon the contained axis cylinder, just as an electric
wire may be insulated by non-conducting substances like
silk, but this is doubtful.

Interesting manifestations of motor centrifugal impulses
are seen in certain movements associated with correspond-
ing muscles on different sides of the body and with sets of
muscles on the same side. It is almost impossible to effect
certain movements with a single finger or toe without causing
similar movements in other fingers and toes; a part of a
muscle cannot be made to contract separately ; it is doubtful
if it be possible to move one eye-ball without the other, even
by the most persistent practice. Other similar examples are
numerous. It is quite probable that in most cases these as-
sociated movements are solely matters of habit. But the
connection by commissural fibers of the cells in the centers
controlling and regulating the movement of these muscles
and sets of muscles would offer a not unreasonable explana-
tion of the phenomena in question, since such an arrange-
ment might render impossible separate and individual action
by the cells thus connected. Excepting, perhaps, the move-
ments of the eye-balls, these associated movements can be
greatly modified by education.

Characteristics of Afferent Nerves. — Impressions received
by these fibers, although they are conveyed toward the cen-
ter and must reach a center before there is any nervous
manifestation, are always referred to the periphery. A most
common illustration of this fact is furnished by injury to the
ulnar nerve as it passes the elbow — such injury being mani-
fested not usually by any pain at the point of infliction, but on

AFFERENT NERVES 233

the ulnar side of the hand where the nerve is distributed. A
person whose limb has been amputated often seems to feels
pain in the extremity although it has been removed from the
body — such pain coming from compression by the cicatrix
(or otherwise) of the nerves which before the amputation
were distributed to the severed limb. Htere, as in the case of
efferent nerves, division of the fibers between the seat of im-
pression and the center precludes the possibility of any ner-
vous manifestation. That is to say, no pain will be felt, no
matter how great the injury be, if the sensory fibers running
from the seat of injury be divided. Stimulation of the peri-
pheral end of a divided afferent fiber produces no effect;
but stimulation of the central end is followed by the ordi-
nary manifestation — by pain if the nerve stimulated be a
common sensory one. This remark, of course, applies only
to those nerves which can be thus directly stimulated —
typically to true sensory fibers.

Impressions conveyed by nerves of special sense must be
received through the intervention of certain complex or-
gans, consideration of which belongs elsewhere.

Although a division has been made of nerve fibers into
afferent and efferent, each with definite, proper and dissim-
ilar functions so far as the direction of conduction is con-
cerned, it has been impossible to discover any actual differ-
ence in the composition, appearance, or other properties, of
the actual fibers themselves. In fact, it may be even consid-
ered as only an accident that one fiber conveys a message
peripherially and another centrally — an accident dependent
upon the kind of center with which the fiber is connected
and the kind of termination it has in the periphery.

Direction of the Current in Nerve Fibers. — It has long
been understood that in no case will a fiber in situ convey
a message at one time in one direction and at another in an
opposite one, that no individual fiber can be both afferent
and efferent; and so far as practical action is concerned
this is true, but "experiment has shown that if a nerve

234 THE NERVOUS SYSTEM

trunk be stimulated at a given point, then the nerve impulse
can be demonstrated as passing away from the point of
stimulation in both directions" (American Text-book).
However, only the message traveling in the physiological di-
rection is manifest, for it is the only one which finds a suit-
able terminal.

It is not to be concluded, however, that in any nerve
trunk, as the ulnar nerve, there may not be both afferent and
efferent fibers. Such, in fact, is the usual arrangement. Any
nerve trunk may contain all kinds of fibers — sensory, spe-
cial sensory, vaso-motor, motor, trophic, secretory — but the
presence of all these does not interfere with the individu-
ality and the individual action of each fiber. A nerve trunk
containing more than one kind of fibers is called a mixed
nerve.

Speed of Nervous Conduction. — It is stated that afferent
impressions are conveyed by nerves at the rate of about 120
feet per second; the rate for efferent impulses is somewhat
less rapid, probably no feet. In the spinal cord tactile im-
pressions are conveyed a little faster than in the nerves
proper, and painful impressions somewhat less than one-
half as fast. The rate of motor conduction in the cord is
said to be one-third the rate in the nerves. It has also been
demonstrated that an act of volition requires a definite
time for the inception of its performance; this is stated to
be about ^s of a second. The recognition of a simple im-
pression (conveyed in the opposite direction, of course) re-
quires about ^5 of a second. Furthermore, the part played
by the spinal cord in reflex action (to be considered later)
also consumes an appreciable period; this is found to be
more than twelve times the period occupied in the transmis-
sion of the impression to the cord or the impulse back to the
muscles. '

Action of Electricity Upon Nerves. — A nerve may be irri-
tated in any one of several ways; but mechanical, thermal
and chemical irritants, besides working injury to the tissues,

THE CEREBRO-S FINAL AXIS 235

are much less easily managed and regulated than is elec-
tricity. This agent may be applied time after time to a nerve
trunk without causing any permanent change in its conduc-
tivity, and the strength, time and duration of application,
etc., can be accurately governed.

It has been noticed that the uninterrupted flow of an elec-
tric current through a nerve is unattended by muscular con-
traction; it has likewise been seen that very slow changes
in the strength of the current are similarly unaccompanied
by the manifestations of ordinary stimulation; but sudden
changes in the strength, whether in the direction of increase
or decrease, act as stimuli. However, while the passage of
a constant current through a nerve does not manifest itself
by contractions except at making and breaking, such a- pas-
sage brings about a change in the tissue of the nerve known
as electrotonus. It may be considered a state of electric
tension. In the anodic area the excitability is diminished
(anelectrotonus) ; in the kathodic area- it is increased (katel-
ectrotonus). Nor is the electrotonic condition restricted to
that portion of the nerve between the poles. Between the
poles there is a point where the two influences — anelectro-
tonus and katelectrotonus — meet and there is neither in-
creased nor decreased excitability. With weak currents this
point is nearer the anode ; with strong ones nearer the ka-
thode. A descending current diminishes the excitability of
a nerve; an ascending increases it. Prolonged application
of electric stimuli will exhaust nervous excitability, but it
may be restored by rest, or more quickly by an opposite cur-
rent.

THE CEREBRO-SPINAL AXIS.

The cerebro-spinal axis embraces the nervous matter in
the cranial cavity and in the spinal canal, excepting the roots
of the cranial and spinal nerves. This axis consists of both
white and gray matter. The white matter is made up of
conducting elements ; the gray matter consists of a number

236 THE NERVOUS SYSTEM

of connected ganglia. In the cord the white matter is situ-
ated externally; in the brain the gray. The encephalon is
situated in the cranial cavity and consists of the cerebrum,
the cerebellum, the pons Varolii, and the medulla oblongata.
These different parts are connected with each other and
with the cord by nerve fibers, and all the cranial and spinal
nerves are connected with gray matter either in the brain or
in the cord, or in both. This gray matter exists for the pur-
pose of receiving impressions and generating nerve force.

Membranes. — The encephalon and cord are covered by
membranes for protection and for the support of vessels be-
longing thereto. These are (i) the dura mater, (2) the ar-
achnoid, and (3) the pia mater.

The dura mater is a dense fibrous structure surrounding
the encephalon and adherent to the inner surfaces of the
cranial bones. At certain points .the two layers of which it
is composed separate to form the venous sinuses. Processes
of the internal layers also are sent inward between the two
lobes of the cerebrum (falx cerebri), between the cerebrum
and cerebellum (tentorium cerebelli) and between the lateral
halves of the cerebellum (falx cerebelli). This membrane
passes through the foramen magnum to cover also the spinal
cord, and to follow as a sheath the spinal nerves at their
foramina of exit.

The arachnoid resembles the serous membranes: It cov-
ers the brain and cord underneath the dura' mater without
dipping into the sulci of the brain. Between it and the pia
mater is what is known as the subarachnoid space containing
the subarachnoid fluid. This fluid serves a mechanical pur-
pose, equalizing pressure in different parts of the cerebro-
spinal axis and protecting the nervous substance from in-
jury by concussion, etc. Besides being found in the subarach-
noid space, it occupies the ventricles of the brain and the
central canal of the cord, communication between these being
furnished by a small opening at the inferior angle of the
floor of the fourth ventricle.

THE PIA MATER

237

The pia mater is a very delicate structure dipping between
the convolutions of nervous matter and lying in close con-
tact with the external surface of the encephalon and cord.
It is exceedingly vascular,; indeed its main function is to
support vessels belonging to the nervous substance under-
neath. Both the arachnoid and the pia mater pass out at
the foramen magnum with the dura to cover the cord.

PIG 73. — .Different views of a portion of the spinal cord from the
cervical region, with the roots of the nerves. (Slightly enlarged.)

In A, the anterior surface of the specimen is shown; the anterior nerve-root
of its right side is divided; in B, a view of the right side is given; in C, the
upper surface is shown; in D, the nerve-roots and ganglion are shown from
below, i, the anterior median fissure; 2, posterior median fissure; 3, anterior
lateral depression, over which the anterior nerve-roots are seen to spread; 4,
posterior lateral groove, int9 which the posterior roots are seen to sink; 5, ante-
rior roots passing the ganglion; 5', in A, the anterior root divided; 6, the poste-
rior roots, the fibers of which pass into the ganglion 6'; 7, the united or com-
pound nerve; 7', the posterior primary branch, seen in A and D to be derived
in part from the anterior and in part from the posterior root. (Kirkes after
Allen Thomson.)

238 THE NERVOUS SYSTEM

The Spinal Cord.

The spinal cord occupies the spinal canal and is about
eighteen inches long, extending from the foramen magnum
to the lower border of the first lumbar vertebra. Its distal
extremity is in the shape of a slender filament known as
iilum terminate, which is gray in color. The sacral and
coccygeal nerves, having taken origin from the cord in the
dorsal region, pass downward in the canal to find exit
through the sacral and coccygeal foramina. This collection
of nerves thus passing down is known as the cauda equina.

Gross Divisions of the Spinal Cord in Section. — Cross sec-
tion of the cord reveals the division of its substance into
two lateral halves connected by the anterior and posterior
commissures. In the center of the cord, and between these
commissures, is a small opening, the central canal of the
cord, communicating with the fourth ventricle above. This
division of the substance of the cord into lateral halves is
effected by the two median fissures, anterior and posterior.
The former is the more clearly marked, and is lined through-
out with pia mater. It is bounded posteriorly by the an-
terior white commissure. The posterior median fissure is
not lined with pia mater and extends anteriorly as far as the
posterior gray commissure. It is to be noted that there are
both anterior and posterior gray commissures, but only one
white commissure (anterior), which is bounded posteriorly
by the anterior gray commissure.

Besides the anterior and posterior median fissures there
are also on each side antero-lateral and postero-lateral fis-
sures, marking the lines of exit of the anterior and posterior
roots of the spinal nerves. These are not well defined.
They divide the cord into anterior, posterior and two lateral
columns.

Arrangement of Gray Substance. — The disposition of the
gray substance in the cord (in transverse section) is some-
what after the manner of the letter H, each lateral portion

THE SPINAL CORD 239

representing the anterior and posterior cornua of gray mat-
ter for that side, and being connected to the corresponding
portion of the other side by the commissures embracing the
central canal. The anterior cornua are shorter and thicker
than the posterior. From these issue the anterior and pos-
terior roots respectively of the spinal nerves. The cells are :
(i) Those in the anterior cornu; (2) those in the posterior
cornu; (3) those in the lateral aspect of the gray matter;
(4) those at the inner base of the posterior cornu (Clarke's
vesicular column).

The gray substance is made up of cells with, of course,
the usual neuroglia and blood-vessels. The cells in the an-
terior cornua are large in size and possess a greater number
of poles than those in the posterior cornua ; from their con-
nection with the anterior (motor) spinal nerve roots they
are called motor cells in contradistinction to the sensory
cells in the posterior cornua which are connected indirectly
with the posterior (sensory) nerve roots.

Degeneration. — Nerve fibers when separated from the
cells of which they are outgrowths degenerate. Fibers have
been said to degenerate in the direction in which they carry
messages, but this is by no means always so. For instance,
the parent cells for the fibers of the posterior spinal roots
are in the ganglia on those roots near the cord, and section
of the root beyond the ganglion causes degeneration of its
fibers peripherally — which is in the opposite direction to the
passage of impressions in them. Section of the posterior
root between the ganglion and cord is followed by centripetal
degeneration, and there is no centrifugal degeneration. The
anterior spinal root fibers are outgrowths of cells in the an-
terior cornua of gray matter. Section of this root anywhere
occasions centrifugal degeneration (Fig. 74).

Arrangement of the White Substance. — It is scarcely
necessary to state that the white substance of the cord con-
sists of nerve fibers with their usual accompaniments. It is
external to the gray. The fibers are medullated, but have
no sheath of Schwann.

240

THE NERVOUS SYSTEM

The divisions of the cord already referred to are purely
anatomical. Physiological and pathological researches war-
rant the further division of the white substance of the cord
into eight columns for each side. The course of all the fibers
in the white matter of the cord is by no means certain. The
division here given may not be strictly correct, but it prob-

FIG. 74. — Diagram to illustrate wallerian degeneration of nerve-
roots. (Kirkes.)

ably receives as little adverse criticism as any of the others.
Classified according to the direction in which their fibers
degenerate after section the paths are: (I) Degenerating
downward, (a) the column of Turck and (b) the crossed
pyramidal tract; (II) degenerating upward, (a) the col-
umn of Goll, and (b) the direct cerebellar tract; (III) de-
generating in neither direction, (a) the anterior fundamental
fasciculus, (b) the anterior radicular zone, (c) the mixed
lateral column, and (d) the column of Burdach.

I. (a) The column of Turck occupies a position just lat-
eral to the anterior median fissure and extends downward to
the lower dorsal region. Its fibers decussate high up in the
cord. This column is sometimes called the direct, or un-

THE SPINAL CORD 24!

crossed, pyramidal tract, as distinguishing it from the other
descending column, (b) The crossed pyramidal tract is ex-
ternal to the posterior cornu of gray matter and internal to
the direct cerebellar tract. Its fibers decussate in the an-
terior pyramids of the medulla oblongata.

II. (a) The direct cerebellar tract occupies the outer pos-
terior part of the lateral column. Its fibers reach the cere-

...Ji

FIG. 75. — Scheme of the conducting paths in the spinal cord at the
third dorsal nerve.

The black. part is the gray matter, v, anterior, hw, posterior root; a, direct,
and g, crossed, pyramidal tracts; b, anterior fundamental fasciculus; c, Goll s
column; d, column of Burdach; e, anterior radicular zone; /, mixed lateral tract;
h, direct cerebellar tracts. (Landois, modified.)

bellum through the inferior peduncles, after having trav-
ersed the posterior pyramids of the medulla. This tract
exists throughout the length of the cord, (b) The column
of Goll (postero-internal column) is situated posteriorly in
a position corresponding to the column of Turck anteriorly
— just lateral to the posterior median fissure. Fibers in this
column extend from the upper lumbar region to the funi-
culi graciles of the medulla.

III. (a) The anterior fundamental fasciculus lies between

the column of Turck internally and the anterior cornu and

anterior roots of the spinal nerves externally. Its fibers are

lost in 'the medulla above. (b) The anterior radicular

242

THE NERVOUS SYSTEM

zone is external to the anterior roots of the spinal nerves
and anterior to the crossed pyramidal tract and the direct

FIG. 76. — Course of the fibers for voluntary movement.

ab, path for the motor nerves of the trunk; c, fibers of the facial nerve; B,
corpus callosum; Nc, nucleus caudatus; Gi, internal capsule; N, I, lenticular
nucleus; P, pons; N. f., origin of the facial; Py, pyramids and their* dccussa-
tion; Ol, olive; Gr, restiform body; P.R., posterior root; A.R., anterior root; x,
crossed, and s, direct pyramidal tracts. (Landois.)

cerebellar fasciculus. Its fibers are lost in the medulla
above, (c) The mixed lateral column is just external to the

MOTOR PATHS IN THE CORD 243

main body of gray matter and does not reach the surface of
the cord. Its fibers are likewise lost in the medulla ob-
longata. (d) The' column of Burdach (postero-external
column) is situated posteriorly in a location corresponding
to the anterior fundamental fasciculus anteriorly — external
to the column of -Goll and internal to the posterior cornu.
Its fibers reach the cerebellum through the inferior pe-
duncles, having passed through the restiform bodies.

Functions of the Columns. — Remarks already made touch-
ing the direction of degeneration in the separate columns
throw some light upon the physiological function of the
fibers in each.

Motor impulses pass downward from the brain through
certain fibers to the cells of the anterior cornua of gray mat-
ter in the cord, and are sent thence through the spinal nerves
to the muscles. The paths in the cord conveying these im-
pulses are found to be the columns of Turck and the crossed
pyramidal tracts, and these are the only parts of the cord
known so to act. Impulses to the upper segment of the cord
may be conveyed by either of these columns, but impulses to
the lower segment must follow the crossed pyramidal tract,
since the column of Turck ceases to exist in the dorsal re-
gion. Only some 3-7 per cent, of motor fibers from the cor-
tex are thought to enter the columns of Turck. The others
decussate in the medulla and enter the crossed pyramidal
tracts. In any case motor impulses originating in the brain
and so conveyed are manifested on the side opposite their
cerebral origin, since the fibers in both these tracts decussate
in passing downward. It is a well known pathological fact
that the lesions of motor areas in the brain, or section of one
lateral half of the cord, are followed by paralysis on the side
opposite the lesion.

Following a motor fiber (A, Fig. 77) through the anterior
root of a spinal nerve, it is found to originate from one of
the large multipolar cells (3) in the anterior cornu of gray
matter. Around these anterior horn cells (i, 2, 3, 4) arbor-

244

THE NERVOUS SYSTEM

ize the end filaments of fibers which have come down
through the cord from the brain. Some fibers have come
down in the uncrossed pyramidal tract (column of Turck)
on the side opposite the cells, i, 2, 3, 4, and crossed over to
the same side through the anterior white commissure ap-

FIG. 77. — Course of nerve fibers in spinal cord. (Kirkes after
S chafer.)

proximately on a level with the cells ; others have decussated
in the medulla, and come down in the crossed pyramidal
tract on the same side as the cells. In both cases the fibers
originated in the brain on the side opposite the cells around
which they arborize in the cord. This is the connection
which exists between the brain and the anterior root fibers.
Not all fibers in the anterior nerve roots are thus pro-

SENSORY PATHS IN THE CORD 245

longed upward in the pyramidal tracts. The number of
fibers in these roots is much larger than in the pyramidal
tracts, and consequently some of them must end (originate)
directly in the cells of the anterior cornua. Furthermore, it
seems that some fibers pass from the anterior nerve roots
directly into the pyramidal tracts without being interrupted
by motor cells.

The column of Turck and the crossed pyramidal tract
are, therefore, the motor paths in the cord.

Fibers entering the cord by the posterior roots send pro-
longations both upward and downward in the gray matter of
the cord, and communicate by end arborizations with the
small sensory cells in the posterior cornua and with cells in
several other localities. (See Figs. 77, 84.) Reference to
Fig. 77 will show that the connection of the anterior nerve
fibers with the gray matter of the cord is simple, while that
of the posterior is comparatively complex, i, 2, 3, 4 are an-
terior horn cells. Each of these gives rise to an efferent
fiber, one of which (A) is shown distributed to a muscle
(M). Each of these cells also is surrounded by the end ar-
borization of a fiber (P) from the cortex.

A fiber from the posterior root is also shown. It origin-
ates in a cell of the sensory ganglion (G). It bifurcates,
one branch going to the surface (S), the other enters the
cord and itself bifurcates. The branch (E) is short and
arborizes around a small cell (Pi) in the posterior cornu,
from which a new axis cylinder arises to arborize around
the anterior horn cell (4). The other branch (D) travels
upward in the posterior column of the cord. A collateral (5)
is seen going to the anterior horn cell (2), one to the pos-
terior horn cell (P2) and another to a cell (C) in the inner
base of the posterior cornu (in Clarke's column) ; from C
an axis cylinder enters the direct cerebellar tract. The
main fiber (8) may terminate in the gray matter of the
cord above, or in the medulla. Impressions brought thus to
the cord are carried to the opposite side and pass up

246

THE NERVOUS SYSTEM

through the gray matter in most part. The fibers decussate
at no particular point, but throughout the length of the cord.
However, some fibers bearing sensory impressions pass to
the column of Goll and thus upward, while some also go to

FIG. 78. — Transverse section through half the spinal cord, showing

the ganglia.

A, anterior cornuaJ cells; B, axis-cylinder process of one of these going to
posterior root; C, anterior (motor) root; D, posterior (sensory) root; E, spinal
ganglion on 'posterior root; F, sympathetic ganglion; G, ramus communicans;
H, posterior branch of spinal nerve; /, anterior branch of spinal nerve; a, long
collaterals from posterior root fibers reaching to anterior horn; b, short collater-
als passing to Clarke's column; c, cell in Clarke's column sending an axis-cylin-
der process (d) to the direct cerebellar tract; e, fiber of the anterior root; f,
axis cylinder from sympathetic ganglion cell, dividing into two branches, one
to the periphery, the other toward the cord; g, fiber of the anterior root termi-
nating by an arborization in the sympathetic ganglion; h, sympathetic fiber pass-
ing to periphery. (Kirkes after Romany Cajal.)

the encephalon by the direct cerebellar fasciculi and the col-
umns of Burdach. Experimentally, decussation of sensory
fibers is demonstrated (i) by longitudinal section of the
spinal cord in the median line, which is followed by anes-

THE COLUMNS OF BURDACH 247

thesia on both sides below the section; and (2) by horizontal
section of one-half of the cord, which is followed by anes-
thesia on the opposite side below the section. It is claimed
that pain and temperature sensations decussate at once on
reaching the gray matter, while sensations of touch, pres-
sure and equilibration pass up on the same side until the me-
dulla is reached. Some afferent fibers are probably not con-
tinued upward to the brain either directly or indirectly.

It thus appears that we have no very accurate knowledge
of the sensory paths in the cord. The gray matter seems
principally concerned ; but the columns of Goll and Burdach
and the direct cerebellar fasciculi also convey afferent im-
pressions.

The columns of Burdach have been said to present no de-
generation secondary to section. Trophic centers for their
fibers must, therefore, exist above and below any given point
of section. It is found that the fibers constituting these col-
umns pass in and out along the cord between cells in differ-
ent planes and acting as longitudinal commissural fibers. In
locomotor ataxia the characteristic symptom is inability to
coordinate the muscular movements — especially of the lower
extremities ; the characteristic lesion has been found to be in
the columns of Burdach. This is of importance in deter-
mining the function of these columns, and, in fact, leads to
the conclusion that their fibers assist in regulating and co-
ordinating the voluntary movements. This opinion is fur-
ther supported by the connection of these fibers with the
cerebellum, which contains the center for muscular coordi-
nation— if such a center exist. The sense of pressure and
the so-called muscular sense are probably connected with the
fibers of this column, and these may be the only sensory im-
pressions conveyed through the columns of Burdach.

The anterior fundamental fasciculi, the anterior radicular
zones, and the mixed lateral paths degenerate in neither
direction after section, their trophic cells existing at both ex-
tremities. They connect cells in the gray matter of the cord.

243 THE NERVOUS SYSTEM

Functions of the Spinal Cord. — These are (i) conduc-
tions, (2) transference, (3) reflex action, (4) augmenta-
tion, (5) coordination, (6) inhibition of reflex acts, (7)
special centers (Collin and Rockwell, modified).

i.* Conduction. — This has been referred to in discussing
the white columns of the cord. This function makes it pos-
sible for the brain to receive impressions from and send im-
pulses to the periphery. It is to be remembered that most of
these impressions and impulses are interrupted by spinal
nerve cells in their passage between brain and periphery.

2. Transference. — An impression reaching the gray mat-
ter of the cord may be transferred (not as in typical reflex
action) so as to be felt in an entirely different region from
that in which the irritation takes place. Hip joint disease
often gives pain in the knee alone.

3. Renex Action. — The cord may act as a center without
the cooperation of the brain. Indeed, by no means do mus-
cular movements cease immediately on removal of the en-
cephalon if the cord and its nerves be left intact. An ani-
mal so mutilated possesses no sensation or volition, but for a
time the sensory nerves will continue to convey impressions
and the motor nerves impulses. Under these conditions im-
pressions (as of heat) are conveyed to the cord by the affer-
ent nerves ; the gray matter of the cord receives the impres-
sions and generates motor force which is sent out through
the corresponding efferent nerves, and movements result.
This is reflex action. The impression is reflected through
the cord and manifested in motion without the intervention
of sensation or volition. Reference to Figs. 77 and 80
shows how reflex action is anatomically possible through the
cord connections. Typical reflex action requires anatomic-
ally (i) something to produce an impression, (2) a nerve
terminal to receive it, (3) a centripetal fiber to convey it,
(4) a center to receive and transform it, (5) a centrifugal
fiber to convey it to the periphery, and (6) a muscle to con-
tract. This remark applies to reflex action connected with

REFLEX ACTION 249

the cord, but by common consent reflex action is not limited
to the cord and its connections.

If reflex action be defined as any involuntary manifestation
of nerve force consequent upon the reception of an impres-
sion (general or special) by a nerve center, the term must be
made to include such phenomena as intestinal peristalsis,
contraction and dilatation of the pupil, certain mental op-
erations, etc. In reality most reflex acts are of a complex
nature, involving associated action on the part of several
neurons and being manifested frequently at several points.
For example, a foreign body in the larynx causes reflexly
not only closure of the glottis, but also the convulsive mus-
cular contractions incident to coughing. The realm of reflex
action is obviously a wide one.

It may be said that ordinary reflexes are usually under the
direction of the cord, but this does not imply that the brain
may not be concerned. Pricking the sole of the foot of a
sleeping person will cause him to draw up his leg without
the intervention of consciousness. Probably were he awake
the withdrawal would still be a reflex but he would certainly
be conscious of the pain, though after the act of withdrawal
zvas accomplished. Nor is reflex action by any means lim-
ited to the cerebro-spinal system. Either of the two sys-
tems, or both, may be concerned.

Now in order for reflex movements to occur, there must
be a transference of impressions received by sensory cells to
cells capable of giving origin to motor impulses. The cells
communicate by their collaterals, which may be short or
long, depending on the distance between the cells concerned.
Cells in the gray matter of the cord are "connected" by such
fibers, and they run largely in the white matter of the cord
joining cells on different planes. They constitute the larger
part of the anterior fundamental fasciculi, the anterior radic-
ular zones, and the mixed lateral tracts, and it is these paths
which are mainly concerned in reftex action of the cord,

4. Augmentation.— Sensory fibers, on reaching the cord,

250 THE NERVOUS SYSTEM

send prolongations both upward and downward in the gray
matter. These prolongations, by their end arborizations,
seem to communicate indirectly with several motor cells. In
the simplest reflex movements connected with the spinal cord
the muscular activity is limited to the area corresponding
to the distribution of the afferent nerve which has been irri-
tated; but if the irritation be sufficiently increased other
muscles in the same locality, or the corresponding muscles
on the opposite side of the body, or even the whole muscu-
lature, may be thrown into action. This is explained on the
ground that under favorable conditions of central excita-
bility, strength of peripheral irritation, etc., the afferent im-
pression is disseminated by collaterals throughout a large
area of the cord (for example), and a large number of effer-
ent cells are made to discharge. The reflex excitability of
the cord is markedly increased by the administration of
such drugs as strychnin. An animal so poisoned will be
thrown into the most violent convulsions by so slight a sen-
sory impression as a simple breath of air. Removal of the
encephalon in inferior animals also exaggerates reflex ex-
citability.

5. Coordination. — This has been referred to under the
columns of Burdach. Coordination is "a repetition of ordi-
nary reflex acts for our daily lives." No effort is necessary
to coordinate the muscular movements of deglutition, res-
piration, walking, etc. These movements may be performec^
when the cerebrum is removed.

6. Inhibition of Refiex Acts. — This is not a function of
the cord proper, but is directed by the cerebrum. A great
many reflex movements may be inhibited by an act of the
will, providing always they are due to contraction of striped
muscle. The reflex acts of coughing or sneezing, or those
resulting from tickling, for example, can be largely con-
trolled. These are usually performed as reflex cord acts,
but the brain may evidently assert its superiority over the
cord and inhibit them.

THE ENCEPHALON 25!

7. Special Centers. — In the gray matter of the cord are
found various centers for distinct acts such as defecation,
parturition, micturition, etc. These are all connected with
each other and with the encephalon and obey the usual laws
of reflex action.

THE ENCEPHALON.

The encephalon is situated within the cranial cavity and
is commonly called. the brain. Its gross divisions are the
medulla oblongata, the pons Varolii, the cerebellum, and the
cerebrum. All the other divisions are in a measure subordi-
nate to the cerebrum, though each division has individual
functions. The human brain weighs about 49^/2 ounces in
the male and about 44 in the female.

The Medulla Oblongata.

Anatomy. — The medulla oblongata, or bulb, joins the
upper extremity of the spinal cord and extends to the pons
above. It has a pyramidal shape, lies in the basilar groove
of the occipital bone, and is slightly flattened antero-poster-
iorly. It is about an inch and a quarter in length, half an
inch thick, and three-quarters of an inch broad above. The
anterior and posterior median fissures of the cord are con-
tirmed upward in the medulla ; the central canal terminates
in the inferior angle of the fourth ventricle. The anterior
columns appear to be continuous with the anterior pyramids
of the medulla. These pyramids are situated just lateral to
the anterior median fissure. The innermost fibers of the
pyramids are the continuations upward of the crossed pyra-
midal tracts, and are seen to decussate in the median line ;
the outermost fibers are the prolongations of the uncrossed
pyramidal tracts. The olivary bodies, oval in shape, are
just external to the anterior pyramids separated from them
by a groove. The restiform bodies make up the postero-

252 THE NERVOUS SYSTEM

lateral portion of the medulla, and are external to the oli-
vary bodies. They contain fibers from the columns of .Bur-
dach, and contribute largely to the formation of the inferior
peduncles of the cerebellum. The restiform bodies, diverg-
ing as they ascend, form the lateral boundaries of the in-
ferior division of the fourth ventricle. Beneath the olivary

FIG. 79. — Floor of the 4th ventricle and the connections of the
cerebellum.

On the left side the three cerebellar peduncles are cut short; on the right the
connections of the superior and inferior peduncles have been preserved, while
the middle one has been cut short, i, median groove of the 4th ventricle with
the fasciculi teretes; 2, the striae of the auditory nerve on each side emerging
from it; 3, inferior peduncle; 4, posterior pyramid and claya, with the calamus
scriptorius above it; 5, superior peduncle; 6, fillet to the side of the crura cer-
ebri; 8, corpora quadrigemina. (Landois.)

bodies, and between the anterior pyramids and the restiform
bodies, are the lateral fasciculi, or the funiculi of Rolando.
They constitute the upward prolongation of all the antero-
lateral portion of the cord which does not go to the forma-
tion of the auterior pyramids. Their chief importance is in
the fact that they contain the centers for respiration. The
posterior pyramids are sometimes called the funiculi graciles.
They join the restiform bodies and pass to the cerebellum.

THE MEDULLA OBLONGATA 253

The fourth ventricle deserves particular attention. It is
a cavity on the posterior aspect of the pons and medulla ex-
tending from the upper limit of the former to a point on the
latter opposite the lower border of the olivary body. It has
the shape of two isosceles triangles placed base to base. The
apex of the inferior triangle is at the calamus scriptorious,
and its lateral boundaries are the diverging restiform bodies.
The superior peduncles of the cerebellum form the lateral
boundaries of the superior triangle. The inferior triangle
is covered by the cerebellum; the superior by the valve of
Vieussens, which stretches between the superior peduncles.
This ventricle communicates above with the third ventricle
by the aqueduct of Sylvius, or the iter a tertio ad quartum
ventriculum; below, with the central canal of the cord and
with the subarachnoid space. The floor of the ventricle
presents a longitudinal median fissure and numerous small
elevations indicating the position of the nuclei of origin of
certain of the cranial nerves.

The gray matter of the medulla has the same general dis-
tribution as that in the cord, but is by no means so regular in
its disposition. The direction of the white fibers is not so
uniform as in the cord. They run not only longitudinally, but
transversely to connect the lateral halves, and in other direc-
tions to connect various centers situated in this part of the
encephalon and to connect the medulla with other parts of
the brain. The following is the relation of the columns of
the cord to the medulla :

The direct and crossed pyramidal tracts pass to the ence-
phalon constituting, in the medulla, the anterior pyramids —
the direct, having decussated below, occupying here the outer
portion of the pyramid, and the crossed decussating in the
medulla and occupying the inner portion of the pyramid.

Those columns concerned in reflex action, the anterior
fundamental fasciculi, the anterior root zones and the mixed
lateral tracts do not continue farther upward than the gray
matter of the medulla.

254 THE NERVOUS SYSTEM

The columns of Coll are continuous with the funiculi
graciles.

The columns of Burdach and the direct cercbellar fasci-
culi pass to the cerebellum through the restiform bodies of
the medulla.

Functions. — The functions of the medulla are (i) con-
duction, (2) reflex action, (3) to furnish centers for special
acts.

i. As a conductor the medulla is absolutely necessary as a
means of connection between the brain and the cord. Sen-
sory impressions to and motor impulses from the brain
must all pass through by this route.

As a reflex nerve center the medulla also resembles the
cord, though impressions reflected through this organ are
frequently much less simple than those reflected through the
cord. Reflex action in the medulla is dependent on (3), to
be noticed now.

3. The most important center presiding over coordinated
movements is that for respiration. The encephalon may be
cut away down as far as the medulla, and life will continue
for a certain time. It is also true that the medulla itself may
be gradually cut away from above downward until a certain
point is reached, when respiration suddenly ceases. Likewise
the spinal cord may be cut away upward till this point is
reached, when the same results will follow. This is the true
respiratory center, and is situated at the site of origin of the
vagi. Its destruction is followed by an immediate suspension
of respiration and consequent death by asphyxia, though
there is no manifestation of the distress usually accompany-
ing this condition. The sense of want of air is simply lost.
There is one of these centers for each side, but they act syn-
chronously, being connected by commissural fibers. Probably
the usual mode of stimulation of the respiratory center is by
afferent impressions, but it may also be stimulated directly,
as by deoxygenated blood. Mutilation of the medulla, on
account of the presence of this center, is followed by the

THE PONS VAROLIJ 255

nearest approach to instantaneous death, and the respiratory
center has, therefore, been called the "vital spot," though
death from any cause cannot be instantaneous.

Some other reflex centers are for deglutition, sucking, se-
cretion of saliva, vomiting, coughing, sneezing, dilatation of
the pupil, secretion of sweat, secretion of glycogen, etc.
Typical of these is the reflex act of sneezing, in which case
impressions are conveyed to the medulla by the nasal
branches of the fifth nerve.

Additional centers in the medulla are those which preside
over inhibition and acceleration of the heart, vaso-motor
centers for the vessel walls, and centers for special senses
like hearing and taste. There is also said to be here a center
controlling the production of heat by the tissues.

The Pons Varolii.

Anatomy. — The pons is situated just above the medulla
oblongata at the base of the brain, and is frequently called
the great commissure, for the reason that it contains white
fibers connecting the two lateral halves of the cerebellum
and the different portons of the cord and medulla with the
parts of the brain above. It resembles the cord in having its
white matter situated externally, while within its substance
are a number of collections of gray matter. The longitudi-
nal fibers are continuations upward of fibers from the oli-
vary bodies and the anterior pyramids of the medulla and
also of parts of the posterior and lateral columns of the cord.
They pass through the crura cerebri to the brain.

Functions. — The anatomical structure and situation of the
pons at once suggest that its function is to transmit motor
impulses from and sensory impressions to the cerebrum.

The gray centers, however, indicate a further function of
this organ. It is found that the removal of all parts of the
encephalon above the pons does not deprive an animal of
voluntary motion and general sensibility. It will be seen

256 THE NERVOUS SYSTEM

later that the integrity of the cerebrum is essential to any
intellectual operation, and manifestly, under the conditions
mentioned, there can be no voluntary motion which indi-
cates any degree of intelligence; but the fact remains that
the animal can perform movements which are different from
the reflex movements depending on the presence of the cord
when all other parts of the cerebro-spinal axis have been
removed. The pons is apparently "an organ capable of
originating impulses giving rise to voluntary movements,
when the cerebrum, corpora striata and optic thalami have
been removed, and it probably regulates the automatic vol-
untary movements of station and progression." (Flint.)

Nor can it be doubted that an animal thus mutilated feels
pain. It is probable that the sensory impression is received
by some of the gray centers in the pons itself, but not being
conveyed to the cerebrum, is not remembered.

The Crura Cerebri, Corpora Striata, Optic Thalami, Inter-
nal Capsule and Corpora Quadrigemina.

It will be well before discussing the cerebrum to consider
briefly other collections of gray and white matter in the
neighborhood of the upper part of the pons.

The crura cerebri, passing upward from the anterior part
of the pons, diverge to run apparently underneath the cor-
pora striata and optic thalami iri the direction of the cere-
bral hemispheres. They are about % mcn l°ng and slightly
broader above than below. The main bulk of each crus con-
sists of white fibers, but a collection of gray matter (locus
niger) divides the band into a lower or superficial section,
called the crusta, and an upper or deep section, called the
.tegmentum. There is also some gray matter in the tegmen-
tum proper. The fibers of the tegmentum are supposed to
convey afferent impressions chiefly, and end for the most
part in the optic thalamus, though some are continued to the
cerebrum through the internal capsule. The fibers of the

THE CRURA CEREBRI

257

crusta are supposed to convey efferent impulses, and pass
to the corpus striatum and the cerebrum.

It is evident that the function of the cfura is mainly to

Nuclear loitKomli

Clauitnln*

FIG. 80. — Human brain, with the hemispheres, removed by a
horizontal incision on the right side.

4, trochlear; 8, acoustic nerve; 6, origin of the abducens; F, A, L, position of
the pyramidal (motor) fibers for the face, arm and leg; S, sensory fibers. (Lan-
dois. )

conduct messages to and from the parts above. It is said
that the locus niger is concerned in coordination of the move-
ments of the eye-ball and iris.

258 THE NERVOUS SYSTEM

The Corpora Striata, Optic Thalami and Internal Capsule
are closely related and are best considered together.

Each corpus striatum is pear-shaped with its large end
forward and near the median line ; the posterior small ex-
tremities are divergent from each other and embrace the two
optic thalami. Externally they are white; internally white
and gray elements are mixed. Each is separated by the an-
terior limb of the internal capsule into two divisions, exter-
nal and internal, known respectively as the lenticular and
caudate nuclei. (See Fig. 80.)

The optic thalami, one on either side, have an oval shape
and rest upon the crura cerebri between the posterior ex-
tremities of the two corpora striata. Most of their external
surface is white ; internally each possesses six gray nuclei.

Separating the two nuclei of the corpus striatum anteri-
orly, and the lenticular nucleus from the optic thalamus pos-
teriorly, is a band of white fibers known as the internal cap-
sule. The part between the two nuclei is the interior limb ;
that between the lenticular nucleus and the optic thalamus is
the posterior limb. These limbs, joining at an obtuse angle,
constitute a bend in the internal capsule which is called the
genu, or knee. The fibers of the capsule pass to the frontal,
parietal and occipital lobes of the cortex, and in their course
to these parts they diverge to form the corona radiata.

External to the lenticular nucleus is a band of white fibers
known as the external capsule. In it is a longitudinal mass
of gray matter, the claustrum. Fig. 76 shows the relations
of these parts.

Functions. — The exact function of the corpora striata is
a matter of some doubt. They have been considered the
great motor ganglia of the base of the brain ; but, although
lesions here are followed by paralysis on the opposite side of
the body, it is held that this phenomenon is due to the prox-
imity of the internal capsule. The further fact that irrita-
tion of this organ is followed by muscular contraction does
not prove that it ordinarily generates motor force, for many

THE CORPORA QUADRIGEMINA 259

of the fibers from the motor cortical zone pass to or through
the corpus striatum. This may be only a relay station, and
the corpus may be quite subsidiary. It undoubtedly, how-
ever, is connected with motion in some way.

The precise function of the optic thalami is equally ob-
scure. The relation of these organs to the tegmenta would
suggest that they have something to do with the sensory
fibers on their way to the cortex. It cannot be denied that
they are concerned in sensation, since their removal is fol-
lowed by crossed anesthesia. They may likewise be relay
stations. Each sends fibers to the cerebellum and contains
one of the nuclei of origin of the optic nerve.

Regarding the function of the internal capsule it may be
said that its fibers are in main part prolongations from the
crusta and from the gray matter of the corpora striata;
fibers also pass upward through it from the tegmentum and
the optic thalamus. As a matter of fact, most of the fibers
of the crura go directly into the corpora striata (motor) and
the optic thalami (sensory), but s*ome pass directly upward
through the capsule. It is to be noted, however, that the
capsule does not consist of these last named fibers alone, but
of fibers from the corpora striata and optic thalami as well.
Observations show that pathological lesions affecting the
anterior two-thirds of the posterior division of the internal
capsule are followed by paralysis of motion; that lesions
affecting only the posterior one-third of the posterior divi-
sion are followed by anesthesia; and that lesions affecting
the entire posterior limb are followed by both paralysis and
anesthesia — these phenomena always manifesting themselves
on the side opposite the lesion only. This leads to a definite
conclusion ; viz., that efferent fibers occupy the anterior two-
thirds and afferent fibers the posterior one-third of the pos-
terior limb of the capsule.

Nothing conclusive can be said about the function of the,
external capsule or of the claustrum.

The Corpora Quadrigemina, two on each side, are promi-

260 THE NERVOUS SYSTEM

nences on the dorsal surface of the pons and crura above the
aqueduct of Sylvius. They contain white and gray matter.
The posterior tubercles are connected with the eighth nerve,
the sensory tract, the temporal region of the brain, and the
lateral corpora geniculata. The anterior tubercles are con-
nected with the optic nerve, with the occipital region, and
with the median corpora geniculata.

The function of the anterior of these bodies is mainly
connected with the eye; the posterior are associated with
the sense of hearing.

The Cerebrum,

The great size of the cerebral hemispheres in man ob-
scures the fact that the different parts of the brain are dis-
posed in a linear series ; these, from before backward, are,
the olfactory lobes, cerebral hemispheres, optic thalami,
corpora quadrigemina, cerebellum, medulla oblongata. This
arrangement exists in theliuman fetus, and persists through-
out life in some of the lower animals.

Anatomy. — The substance of each hemisphere is divided
by fissures into five lobes — (a) frontal, (b) parietal, (c)
occipital, (d) temporo-sphenoidal and (e) central. The
main fissures are four in number — (i) The fissures of Syl-
vius running from the front and under part of the brain
backward, outward and upward; (2) the fissures of Rolando
running from the median line near the center of the longi-
tudinal fissure forward, outward and downward; (3) the
parie to -occipital fissure, little of which is evident upon the
surface of the brain, but which appears on longitudinal sec-
tion separating the occipital and parietal lobes; (4) the
calloso-marginal fissure, also evident only on the internal
aspect of the hemisphere, parallel with and above the cor-
pus callosum. (Figs. 81, 82.)

(a) The frontal lobe is bounded internally by the longitu-
dinal fissure, posteriorly by the fissure of Rolando and be-

THE CEREBRUM

26l

low by the fissure of Sylvius. On its surface are seen three
convolutions, approximately parallel, called the superior,
middle and inferior frontal convolution, and occupying po-
sitions which their names indicate. In addition the posterior

-cm

FIG. 81.— Left side of the. human brain (diagrammatic).

F, frontal; P, parietal; O, occipital; T, tempero-sphenoidal lobe; S, fissure of
Sylvius; S' , horizontal; S", ascending ramus of S; c, sulcus centralis, or fissure
of Rolando; A, ascending frontal, and B, ascending parietal convolution; F\,
superior, FZ, middle, and FS, inferior frontal convolutions; fi, superior, and fz,
inferior frontal fissures; fz, sulcus precentralis; P, superior parietal lobule; PZ,
inferior parietal lobule, consisting of PZ, supra-marginal gyrus, and PZ' , angular
gyrus; ip, sulcus interparietalis; cm, termination of callpso-marginal fissure;
O, first, Oz, second, Oz, third occipital convolutions :po, parietal-occipital fissure;
o. transverse occipital fissure; 02, inferior longitudinal occipital fissure; T\, first,
TZ, second, T%, third temporo-sphenoidal convolutions; t\, first, tz, second tem-
pero-sphenoidal fissures. (Landois.)

262

THE NERVOUS SYSTEM

portion of this lobe is occupied by the ascending frontal, or
the anterior central convolution, lying just in front of the
Rolandic fissure.

(b) The parietal lobe is bounded anteriorly by the fissure

FIG. 82. — Median aspect of the right hemisphere.

CC, corpus callosum divided longitudinally; Gf, gyrus fornicatus; H, gyrus
hippocampi; h, sulcus hippocampi; U, uncinate gyrus; cm, calloso-marginal fis-
sure; F, first frontal convolution; c, terminal portion of fissure of Rolando; A,
ascending frontal; B, ascending parietal convolution and paracentral lobule; P\' ,
parecuneus or quadrate lobule; Os, cuneus; Po, parieto-occipital fissure; o',
transverse occipital fissure; oc, calcarine fissure; oc' , superior, oc" , inferior
ramus of the same; G' , gyrus descendens; T±, gyrus occipito-temporalis lateralis
(lobulus fusiformis) ; T§, gyrus occipito-temporalis medialis (lobulus lingualis).
(Landois.)

of Rolando, internally by the longitudinal fissure, posteriorly
by the parieto-occipital fissure and below by the fissure of
Sylvius. Just behind the fissure of Rolando is the ascending
parietal, or posterior central convolution, above, this is con-
tinuous with the upper parietal convolution, below which is
the inferior parietal lobule separated from the preceding by
the intra-parietal sulcus. This inferior parietal lobule winds
around the posterior part of the fissure of Sylvius, and is

THE CEREBRUM 263

divided into the supra-marginal convolution, embracing the
short arm of this fissure, and the angular convolution con-
necting below with the temporal lobe.

(c) The occipital lobe is situated posteriorly below the
parieto-occipital fissure and external to the median fissure.
It presents three convolutions, the superior, middle and in-
ferior.

(d) The temporo-sphenoidal lobe is below the fissure of
Sylvius in front of the occipital lobe. It likewise presents
superior, middle and inferior convolutions.

(e) The central lobe, or island of Reil, presents the gyrus
fornicatus, a convolution curving around the corpus cal-
losum ; the marginal convolutions beyond the calloso-mar-
ginal fissure from the preceding and between it and the edge
of the longitudinal fissure; the continuation of the parieto-
occipital fissure running downward and forward to meet the
calcarine fissure, between which is the cuneus; the internal
aspect of the temporal lobe, the uncinate gyrus.

Structure. — The cerebral hemispheres are composed of
white and gray matter, but here the gray matter is situated
externally. To increase its amount, with economy of space,
the gray matter is thrown into many convolutions, to some
of which reference has been made. The sulci separating
these convolutions have a depth in the average human brain
of about one inch. The thickness of the gray matter of the
cortex varies from 1/i2 to % in., being thinnest in the occipital
and thickest in the front parietal region.

The cells found in the superficial and deep portions of the
gray matter are not uniform in size or shape. In a general
way it may be said that they increase in size as the surface
is left, but in addition to the comparatively large cells in the
deep parts there are also numbers of small ones. Passing in
the same direction there are found in succession small pyra-
midal, larger pyramidal, and irregular branching cells.

Fibers from the Cerebrum. — Fibers pass from each cere-
bral hemisphere to (a) the spinal cord, (b) the cerebellum,

264 THE NERVOUS SYSTEM

(a) Fibers converge from the anterior and middle (par-
ticularly the latter) parts of the cortex to pass by the corona

FIG. 83. — Scheme of the projection fibers within the brain. (Starr.)

Lateral view of the internal capsule; A, tract from the frontal gyri to the
pons nuclei, and so to the cerebellum; B, motor tract; C, sensory tract for touch
(separated from B for the sake of clearness in the scheme); D, visual tract; E,
auditory tract; F, G, H, superior, middle, and inferior cerebellar peduncles; /,
fibers between the auditory nucleus and the inferior quadrigeminal body; K,
motor decussation in the bulb; At, fourth ventricle. The numerals refer to the
cranial nerves. The sensory radiations are seen to be massed toward the
occipital end of the hemisphere. (Am. Text-book.)

radiata to the corpora striata, from which fibers are con-
tinued to the crusta, pons, pyramids of the medulla and
pyramidal tracts of the cord; most of these pass down
through the internal capsule to reach the corpora striata.
From the same regions also some fibers pass directly through
the internal capsule, without connection with the corpora
striata, to be actually continuous themselves with fibers
which, following the same course downward, are found in
the pyramidal tracts of the cord. All fibers passing from
these cortical areas mentioned through the internal capsule

THE CEREBRUM

265

FIG. 84. — Scheme of relationship of cells and fibers of brain and cord.

(Kirkes.)

Pyr, cell of Rolandic area; Ax, its axis cylinder crossing the middle line AB,
to enter one of the pyramidal tracts; the collateral Call goes to the cortex of
the opposite hemisphere, while another, str, enters the corpus striatum. The axis
cylinder arborizes around an anterior horn cell, whence a motor fiber goes to the
muscle.

The axis cylinder from the spinal ganglion cell is represented as bifurcating
and sending one branch to the periphery and one to the cord; the latter itself
bifurcates, the lower division ending as shown better in Fig. 77. N.G., cell in
posterior cornu of the cord or posterior column of the bulb. The distance of
this cell from the point of entrance of the axis cylinder into the cord may be
great or small. Note the collaterals from it in Fig. 77. I. A., decussating fiber
ending at cell in optic thalamus, O.T., from which a fiber passes to the cortex.
A collateral is shown passing from the ascending sensory fiber to a cell of
Clarke's column, whence a fiber passes to a cell, P, of the cerebellum.

266 THE NERVOUS SYSTEM

occupy the anterior two-thirds of the posterior division of
that tract. Furthermore, fibers from the posterior cortical
area pass through the posterior one-third of the posterior
division of the internal capsule to the optic thalamus, from
which fibers pass through the tegmentum to the pons and
medulla and are continuous with fibers from the sensory
tracts of the cord. The decussation of all these fibers has
been mentioned.

FIG. 85. — Diagram of the motor areas on the outer surface of a
monkey's brain. (Landois after Horsley and Schafer.)

Fig. 84 taken in conjunction with Fig. 77 illustrates the
most recent ideas of the motor and sensory connections be-
tween brain and cord and the motor and sensory paths in
the cord.

(b) Fibers from the anterior portion of the frontal lobe
pass through the anterior limb of the internal capsule and
seem to end in the gray matter of the pons and there to com-
municate with the cerebellum through the middle peduncles.
Fibers also pass from the temporo-sphenoidal lobes and
from the caudate nuclei of the corpora striata to the cere-
bellum on the opposite side. The connection is crossed in all
these cases.

(r) Transverse fibers in the corpus callosum connect all
parts of the two lateral hemispheres. Besides these com-

THE CEREBRUM

FIG. 86. — Side view of the brain of man, with the areas of the cerebral
convolutions according to Ferrier. (Brubaker.)

The figures are constructed by marking on the brain of man, in their respec-
tive situations, the areas of the brain of the monkey as determined by experi-
ment, and the description of the effects of stimulating the various areas refers
to the brain of the monkey.

i, advance of the opposite hind limb, as in walking; 2, 3, 4, complex move-
ments of the opposite leg and arm, and of the trunk, as in swimming; a, b, c, d,
individual and combined movements of the fingers and wrist of the opposite
hand. Prehensile movements. 5, extension forward of the opposite arm and
hand; 6, supination and flexion of the opposite forearm; 7, retraction and ele-
vation of the opposite angle of the mouth by means of the zygomatic muscle;
8, elevation of the alae nasi and upper lip, with depression of the lower lip on
the opposite side; 9, 10, opening of the mouth, with (9) protrusion and (10)
retraction of the tongue; region of aphasia, bilateral action; n, retraction of
the opposite angle of the mouth, the head turned slightly to one side; 12, the
eyes open widely, the pupils dilate, and the head and eyes turn toward the oppo-
site side; 13, 13', the eyes move toward the opposite side, with an upward (13)
or downward (13') deviation; the pupils are generally contracted; 14, pricking
of the opposite ear, the head and eyes turn to the opposite side, and the pupils
dilate widely.

268 THE NERVOUS SYSTEM

missural fibers there are those of the anterior and posterior
white commissures. Fibers in the anterior connect the tem-
poro-sphenoidal lobes and probably the corpora striata with
each other ; fibers in the posterior connect the temporo-sphe-
noidal lobes with the optic thalami of the opposite side.

(d) The arcuate fibers connect different convolutions of
the same lobe and the convolutions of different lobes with
each other. Some of these are the jornix, in the corpus
callosum} and in the other parts, as well as running along
the concave surface of the cortex.

Cerebral Localization. — There are certain cortical areas
which have certain fixed functions. There are certainly such
areas for motion and for the reception of impressions con-
veyed by the nerves of special sense; areas for the reception
of impressions conveyed by the nerves of general sensation
have not been definitely determined.

Motor Centers. — Electrical stimulation of the convex sur-
face of the cerebrum shows that the anterior part is motor
and the posterior part non-motor; that stimulation of the
motor portion produces muscular contractions on the oppo-
site side of the body, that stimulation in the same spot is al-
ways followed by the same contractions; and that when the
current is quite weak the contractions are limited to distinct
muscles or sets of muscles. It may be further said that while
the experiments establishing these facts have been largely
limited to inferior animals, the deductions have been made
applicable to man by pathological observations and by the
fact that in different animals stimulation of anatomically
corresponding parts is followed by corresponding results.
Destruction of motor areas is followed by descending sec-
ondary degeneration of fibers through the corona radiata,
internal capsule, crura cerebri (crusta), anterior pyramids
of the medulla and the pyramidal tracts of the cord; the
resulting paralysis is on the side opposite the lesion.

The motor cortical zone, so far as can now be said, cor-
responds to the ascending frontal and parietal convolutions

THE CEREBRUM 269

on either side of the fissure of Rolando, to the paracentral
lobule, and possibly to a small area in front of the ascending
frontal convolution. From above downward, on either side
o.f the Rolandic fissure are areas presiding over the move-
ments of the leg, arm and face.

More specific information as regards areas controlling
various movements may be obtained by reference to Fig. 86.

Various kinds of monoplegia (crossed) are caused by
lesions, as hemorrhage, in localized parts of the motor area ;
there may be facial, brachial, crural, bracho-facial monople-
gia, etc. There can be no doubt that from the motor cortical
zone pass the fibers which constitute the pyramidal tracts of
the cord.

Sensory Centers. — Centers for the reception of impres-
sions giving rise to general sensation may exist. Fibers from
the temporo-sphenoidal and occipital lobes pass through the
posterior third of the posterior division of the internal cap-
sule, and it may, therefore, be assumed that these parts of
the cerebrum are connected with general sensation.

Special Centers. — Besides these areas for motion and gen-
eral sensation, special centers certainly exist.

The Optic Center is in the occipital lobe, probably in the
cuneus. Removal of the right occipital lobe is followed by
left hemiopia and vice versa; removal of both causes total
blindness.

The Olfactory Center is probably on the inner surface of
the anterior extremity of the uncinate gyrus (inner extrem-
ity of the temporal lobe).

The Gustatory Center is supposed to be in the temporal
lobe very near the preceding.

The Auditory Center is located in the superior and middle
convolutions of the temporo-sphenoidal lobe.

The Center for Cutaneous Sensations cannot be strictly
limited, though it is said to correspond with the motor area.

The Center for Muscular Sensations is thought to be in
the lower parietal region.

2/O THE NERVOUS SYSTEM

The Speech Center. — One may not be able to speak be-
cause he cannot control the muscles usually involved in such
an act, or because he has no comprehension of the meaning
of words, or because he is incapable of forming the idea
which links the reception of the impression and the muscu-
lar act. Aphasia is the term generally applied to inability to
express one's self by language. It is to be distinguished,
however, from aphonia, which is simply a loss of voice.
Ataxic aphasia is an inability to express ideas only by reason
of muscular incoordination ; a person so affected may use
words, but he cannot tell what sounds he is going to utter ;
his ability to receive ideas is unimpaired, and he can express
his own ideas in writing. When there is inability to express
ideas in writing, because of muscular incoordination, a con-
dition of agraphic aphasia is said to exist. There are also
cases in which a person cannot comprehend ideas expressed
in language and cannot express himself by either speaking
or writing; this is known as amnesic aphasia. It is not im-
possible that in some instances ideas may be received and
there still be an inability to express one's self in any way.
It is noted that when the hemiplegia accompanying the
aphasia is marked the form is usually ataxic ; when there is
no hemiplegia the aphasia is usually amnesic.

The part of the brain presiding over speech is in the left
third frontal convolution near the island of Reil. In left-
handed persons its usual situation is almost certainly at a
corresponding point on the right side. Why the center is
unilateral has -not been explained. It may be that it was
originally bilateral, and the growth of the right has been
stopped by the superior development of the left side of the
brain. It is at least noticed that the right instead of the left
side of the brain is heavier in left-handed persons. Fibers
from this center (Broca's convolution) pass through the an-
terior part of the posterior division of the internal capsule
to reach the left crus, leaving which they enter the pons to
decussate and go to the right side of the medulla.

THE CEREBRUM 2JI

Functions of the Cerebrum. — The superior development
of the intellect in man is the most predominant characteristic
distinguishing him from the lower animals. That many such
animals are possessed of a certain degree of intelligence is
not usually denied ; and the nature of their mental oper-
tions, though they are insignificant as compared with man's,
may be admitted as identical with his. The most striking
difference in the nervous system of man as compared with
that of inferior animals is the large size of the cerebrum in
the former. This is not surprising when it is admitted that
in the substance of this part of the encephalon is the seat of
those faculties which manifest themselves in mental opera-
tions.

The seat of the changes, if they be changes, which result
in mental operations is supposed to be in the frontal lobes;
these are insensible and inexcitable, but severe injury to
them, as by hemorrhage, is followed by a cessation of mental
activity; congenital defects also cause a corresponding de-
crease in the mental caliber.

From what has been said it is evident that the cerebral
hemispheres are capable of generating motor impulses and
receiving impressions general and special; but predominat-
ing in importance over these functions is the fact that the
gray substance of the cerebrum is essential to the exercise
of the intellect — even to the existence of that indefinite some-
thing called the mind.

It is by the cerebrum that we perceive and retain impres-
sions, that we understand, imagine, reflect, reason and judge,
and thus concoct and issue the mandates of our will. It is
the link which connects our impressions and our purposeful
actions.

In animals upon which experiments have been made it is
found that life may persist for a time after the removal of
the hemispheres, and that, outside of the cessation of men-
tal activity, the results are not so marked as one would on
first thought suppose. Stupor and absence of the ordinary

2/2 THE NERVOUS SYSTEM

instinctive acts (as corresponding in a way with ac.ts of the
will in man) are noted, but voluntary motion and general
sensibility are not destroyed, and may be but little inter-
fered with. Of course there is no voluntary motion in the
sense of carrying out the behests of the will, for the organ
of the will is destroyed ; nor is there any record of painful
impressions, for the organ of memory is absent. But the
animal can perform various consecutive and coordinate
movements, such as walking, swimming, etc. For example,
a pigeon thus mutilated will fly when thrown into the air.
This does not argue any mental operation. A person does
not ordinarily apply his mind to the act of walking or stand-
ing; his mental faculties may be as completely engaged with
the deepest thoughts of psychology, literature, medicine or
other subjects while walking as at any other time. True, he
probably started with some fixed purpose to go in some par-
ticular direction to some definite place, but the act of pro-
gression does not per se require fixed attention on his part.
So in the case of the pigeon ; it does not make up its mind
to fly at all; and it will not fly without being thrown into
the air, or the application of some other similar stimulus ;
nor does it fly in any particular direction, or to any par-
ticular place. It is reduced to the condition of a "mechan-
ism without spontaneity." It can perform voluntary move-
ments but cannot originate them without external interven-
tion.

Animals which have been subjected to the operation men-
tioned undoubtedly feel pain. They move away or cry out
on being burned, for example. The coordination of their
movements and the cries contrast with the phenomena (re-
flex) following such stimulation when only the cord is left.
It was noted above that the impressions in these cases are
probably received by the gray matter of the pons and not
recorded.

The special senses of sight and hearing remain after the
removal of the cerebrum. The same is probably true of
taste and smell.

THE CEREBRUM 273

It would seem that the cerebrum is a kind of storehouse
in which are kept all the materials necessary for the per-
formance of all kinds of pro-determined acts, whether they
manifest themselves in speech, or thought, or muscular
action. What excites these materials to activity — i. e., what
excites a voluntary act — is not clear. We know certain
things will usually excite a certain train of thought, or cause
us to will to do or say certain things. Such phenomena* are
akin to, if not identical with, reflex action. These manifes-
tations of our voluntary power are due to impressions con-
veyed by afferent fibers to the cortex ; indeed it may be that
every afferent fiber in the system exerts an influence thus
indirectly upon the organ of the will, "and the impressions
conveyed by them are reflected in one's character and life.
But it cannot be said that all voluntary activity is thus of a
reflected nature; there is some cause other than the recep-
tion of afferent impressions which sets the will in operation.

Connection Between the Brain and Intelligence. — It is
claimed that a single hemisphere is capable of performing all
the ordinary intellectual acts as well as both; and atrophy,
or destruction otherwise, of one hemisphere has frequently
been noticed to entail no mental defect. But whether the
mind under such conditions would be equal to the highest
intellectual attainments is doubtful. It would seem that in
health the brain unites the impressions received by the two
sides (as e. g., through the optic nerves), and the resulting
idea is a single one; that is to say a person does not have
two opposing ideas about the same thing the same time ; the
two hemispheres seem to agree.

In a general way, it may be stated that the degree of in-
telligence corresponds to the weight of the brain, though
to this rule there are many exceptions. It may be more prop-
erly said that the development of the intellectual faculties is
greater as the area of gray matter is increased by the convo-
lutions of the cortex. Idiots' brains are usually, though not
by any means invariably, much below the average weight.

274 THE NERVOUS SYSTEM

A difference in intellectual vigor may be present in per-
sons whose brains have the same weight and even the same
amount of gray matter. A difference in the quality of the
gray substance may in such cases account for the varying
results. It is a matter of common observation that mental
exercise increases mental vigor and capacity, just as muscu-
lar exercise develops muscular strength. It is difficult to
reach a conclusion as to whether there is an increase in the
amount of gray substance or whether that already present is
endowed with additional power.

The Cerebellum.

Anatomy. — The cerebellum, or little brain (see Fig. 79),
is situated beneath the occipital lobes of the cerebrum,
weighs some 5*4 ounces in the male to 4^ ounces in the fe-
male, and consists of a central and two lateral lobes. It is
composed of white and gray matter, the latter being, with
the exception of the corpora dentata in the lateral lobes, sit-
uated externally. The convolutions on its surface are much
finer than are those on the cerebral surface. It is separated
from the parts above by the tentorium cerebelli, a pro-
cess of the dura mater.

Fibers. — The fibers passing away from the cerebellum are
collected into three bundles on each side, known as the su-
perior, middle and inferior peduncles. The superior pe-
duncle has a direction forward and upward to reach the crus
and optic thalamus ; fibers in it connect the cerebellum with
the cerebrum. Certain of these decussate underneath the
corpora quadrigemina with corresponding fibers from the
opposite side, so that each side of the cerebellum is connected
with both sides of the cerebrum. Attention has been called
to fibers passing down from the cerebrum through the pons
to the cerebellum. Fibers in the middle peduncle connect
the two lateral halves of the cerebellum through the pons.
Fibers in the inferior peduncle are continuous below with

THE CEREBELLUM 275

fibers in the posterior columns of the cord through the resti-
form bodies of the medulla.

Function. — The only characteristic phenomenon invariably
following removal of the cerebellum is an inability to coor-
dinate the voluntary muscular movements. The foot, for
example, can be raised, and the voluntary muscular act con-
cerned in raising it may be as vigorous as ever, but the ani-
mal cannot so govern his movements as to know where he put
it down. Even the coordination necessary in standing is lost,
and the maintenance of the equilibrium is very difficult, if not
impossible. The so-called muscular sense is abolished, and,
while the power to contract the muscles remains, the animal
cannot contract them in a regular or coordinate manner.
When it is remembered that wellnigh every voluntary act
requires concerted or consecutive muscular movements some
idea is gotten of the helpless condition sequent upon such
a lesion. If it be granted that there is a center presiding
over the coordination of the voluntary muscles, that center
is in the cerebellum, and an animal deprived of this organ is
as powerless, so far as this function is concerned, as a per-
son is to see when the optic centers are destroyed. Its action
is crossed.

It has been noted already that lesions of the posterior
white columns of the cord are followed by disturbances of
coordination, and that the cerebellum is connected with these
columns through the inferior peduncles and restiform bodies.
Fibers in these columns serve only as anatomical connec-
tions by which the coordinating center communicates with
the muscles whose movements it is to regulate, and of ne-
cessity any lesion of these fibers destroying that connection
is followed by the loss of control of the center over the mus-
cles. However, in degeneration of the posterior columns
(locomotor ataxia) an effort at coordination can be made,
so that progression is possible by the aid of fixed attention.
It is possible also that the coordinating messages are carried
in such cases by the motor fibers, though in an unsatisfactory
manner.

276 THE NERVOUS SYSTEM

It has been supposed that the cerebellum is in some way
connected with the generative function, and this much is
probably true, though the evidence submitted is not suffi-
cient to warrant the assumption that the cerebellum is the
seat of the sexual instinct.

THE CRANIAL NERVES.

The cranial nerves, twelve in number on each side, take
their origin from some part of the encephalon, pierce the
dura mater and leave the skull by various openings. They
have been numbered from before backward in the order in
which they pass through the dura mater. Their names,
indicating something of their function, and corresponding to
their numbers, are as follows :

I. Olfactory.
II. Optic.

III. Motor Oculi Communis.

IV. Patheticus (Trochlearis).
V. Trifacial (Trigeminus).

VI. Abducens.
VII. Facial.
VIII. Auditory.
IX. Glosso-pharyngeal.

X. Pneumogastric (Vagus).
XI. Spinal Accessory.
XII. Hypoglossal.

The point at which one of these nerves can be seen to issue
from the brain tissue is the apparent origin, while the gray
nucleus, or nuclei, to which the fibers can be traced in the
brain substance is the deep origin.

First Nerve (Olfactory).

Origin. — This is a nerve of special sense. Its apparent
origin is by three roots. The internal root issues from the

THE CRANIAL NERVES 2/7

gyrus fornicat.us; the middle from the under surface of the
frontal lobe anterior to the anterior perforated space; the
external from the temporo-sphenoidal lobe. These three
roots unite to pass forward underneath the frontal lobe near
the longitudinal fissure as the olfactory tract. The deep
origin is unsettled.

Course and Distribution. — Reaching the upper surface of
the cribriform plate of the ethmoid, the olfactory tract ex-
pands into the olfactory bulb, from the under surface of
which are given off the special nerve fibers of the sense of
smell. They are about twenty in number and pass through
the foramina in the cribriform plate to be distributed to the
mucous membrane (Schneiderian) of the nose in three sets
— an inner to the upper third of the septum, a middle to the
roof of the nares, and an outer to the superior and middle
turbinated bones and the ethmoid in front of them. The
fibers are non-medullated.

Function. — The olfactory nerves are insensible and inex-
citable. They are concerned with the sense of smell alone
and their integrity is necessary to the preservation of that
sense. They convey to the brain impressions which are rec-
ognized as odors only. Removal of the olfactory bulb in a
dog is evidently followed by a loss of the sense so charac-
teristic of the animal. Furthermore, the olfactory bulbs in
lower animals are shown to be developed in proportion to the
acuteness of the sense of smell.

Second Nerve (Optic).

Origin. — This is the nerve of sight. Its apparent origin is
from the anterior part of the optic cgmmissure. The optic
commissure occupies the optic groove 'on the superior sur-
face of the sphenoid. It represents the union of the two
optic tracts each of which, traced backward, is found to
divide into two bands ; the external takes its origin from the
external geniculate b.ody, from the pulvinar of the optic thai-

2/8 THE NERVOUS SYSTEM

amus and from the superior corpus quadrigeminum ; the in-
ternal comes from the internal geniculate body. These two,
uniting, cross the crusta obliquely to reach the optic commis-
sure, or chiasm. In the commissure the fibers from the inner
margin of each optic tract pass to the other side of the brain,
and may be called commissural fibers between the internal
geniculate bodies. Some fibers anteriorly connect the two
optic nerves with each other and are not properly part of
the chiasm, but connect the two retinae. The outer fibers of
each tract pass to the nerve of the same side, while the
central fibers decussate in the commissure with similar fibers
from the other tract and pass thus- to the optic1 nerve of the
opposite side. The deep origin is indicated above.

Course and Distribution. — Each optic nerve leaves the
front of the optic chiasm to pass out of the cranium and
enter the orbital cavity by the optic foramen. Having
pierced the sclerotic and choroid coats of the ball it expands
into the retina.

Function. — The optic nerves have no properties other
than the conveying to the brain of the special impressions of
sight. Stimulation produces neither pain nor motion.

Third Nerve (Motor Oculi Communis).

Origin. — The third is a motor nerve. Its apparent origin
is from the inner surface of the crus just in front of the
pons Varolii. Its deep origin is in a nucleus just lateral to
the median line beneath the aqueduct of Sylvius. Here de-
cussation with fibers from the opposite side occurs. The
fibers pass forward from this place through the locus niger
and tegmentum to the point of apparent origin.

Course and Distribution. — Having traversed .the outer
aspect of the cavernous sinus, the third nerve divides into
two branches which leave the cranial cavity by the sphe-
noidal fissure between the two heads of the external muscle
of the eye. The superior division is distributed to the su-

THE CRANIAL NERVES 2/9

perior rectus and levator palpebrae superioris; the inferior
separates into three branches, one of which is distributed to
the inferior rectus, another to the internal rectus, and a
third to the inferior oblique. From this last a branch is given
off to the lenticular ganglion to form its inferior root.

Functions. — This nerve has no function other than to
supply motion to the parts to which it is distributed. It is
insensible at its root, but receives filaments from the fifth
in the cavernous sinus, beyond which point stimulation pro-
duces pain as well as muscular contractions. The phenomena
sequent upon section of the nerve are suggested in its distri-
bution. ( i ) There is ptosis, or dropping of the upper lid ;
for the lid is kept open by the levator palpebrae superioris.
(2) There is external strabismus, because the external
rectus is not supplied by this nerve and is unopposed by the
internal rectus, the action of which is paralyzed. Diplopia
is the consequence. (3) There is inability to turn the ball
except in an outward direction because the muscles produc-
ing movements on the vertical and horizontal axes are de-
prived of innervation. (4) There is inability to rotate the
eye in certain directions on the antero-posterior axis. The
antagonist of the inferior oblique is the superior oblique,
the tendency of which latter is to rotate the globe so as to
make the pupil look downward and outward. When the
inferior oblique is paralyzed the superior oblique is unop-
posed, it is impossible to rotate the ball as is usual in side-
wise movements of the head, and double vision is the result.
(5) There is slight protrusion of the whole ball from relax-
ation of the muscles. (6) The pupil is dilated and move-
ments of the iris are interfered with. Stimulation of the
third nerve contracts the pupil, but when it is cut the pupil
does not respond to light. The ciliary nerves controlling
the movements of the iris come from the ophthalmic gang-
lion of the sympathetic ; to this ganglion goes a branch from
the third nerve. It is known that the action of the sympa-
thetic cannot be divorced from that of the cerebro-spinal

28O THE NERVOUS SYSTEM

system; and whether this influence of the third nerve is
exerted directly upon the iris or indirectly, through the oph-
thalmic ganglion is a matter of some obscurity. The fact
that the action of the iris is not instantaneous strongly sug-
gests control by the sympathetic.

The decussation under the aqueduct of Sylvius is evi-
denced by the reflex contraction of the pupil on the opposite
side when the central end of a divided optic nerve is stimu-
lated. The impulse is reflected through the third nerve. It
is not to be understood, however, that the motor oculi is
the only nerve capable of influencing movements of the iris.
Section of the sympathetic in the neck contracts the pupil,
even after section of the third.

Fourth Nerve (Patheticus).

Origin. — This is a purely motor nerve. Its apparent
origin is behind the corpora quadrigemina from the valve of
Vieussens. The two nerves decussate above this valve. Its
deep origin is just below that of the third nerve beneath the
aqueduct of Sylvius.

Course and Distribution. — Emerging from the valve of
Vieussens the nerve winds around the superior peduncle of
the cerebellum and the crusta immediately above the pons,
and passes forward near the outer wall of the cavernous
sinus to find exit from the cranial cavity by the sphenoidal
fissure. Having entered the orbit, it runs forward to be dis-
tributed to the orbital surface of the superior oblique. In
the cavernous sinus it receives fibers from the ophthalmic
division of the fifth and from the sympathetic, and occasion-
ally gives off a branch to the lachrymal nerve.

Function. — It supplies motor power to the superior ob-
lique muscle alone. Remembering the origin and attachment
of this muscle it is not difficult to foretell the consequence of
lesions of the nerve. The action of the superior oblique is
to rotate the ball upon an oblique horizontal axis so that

THE CRANIAL NERVES 28l

the pupil will look downward and outward. This move-
ment cannot be accomplished when the nerve is cut, and the
inferior oblique asserts itself unduly to bring about an op-
posite effect. The ball cannot accommodate itself to move-
ments of the head toward the shoulder, and double vision
supervenes — -unless the object be brought in the involuntary
line of vision of the affected eye.

Fifth Nerve (Trifacial, Trigeminus).

The fifth is analogous to the spinal nerves (i) in rising
by two roots, (2) in having a ganglion on its posterior root,
and (3) in having a mixed function. The anterior root is
small and motor; the posterior large and sensory.

Origin. — Its apparent origin is from the side of the pons
above the median line. The deep origin of the large, sen-
sory root is in the pons immediately below the floor of the
fourth ventricle and just internal to its marginal boundary.
The small, motor root rises from a point just internal to the
large root.

Course and Distribution. — The two roots, taking their
origin as above described, pass through the dura above the
internal auditory meatus and run along the superior border
of the petrous portion of the temporal bone to a point near
its apex, where a large ganglion, the semilunar or Gasserian,
is developed on the posterior root and occupies a depression
on the bone for its reception. The motor root passes be-
neath the ganglion without being connected with it.

The posterior root will be first followed to its distribu-
tion.

From the anterior surface of the Gasserian ganglion are
given off three branches — (i) ophthalmic, (2) superior
maxillary, (3) inferior maxillary. After the inferior max-
illary has left the cranial cavity it receives fibers from the
small or motor root, but the other branches are composed
entirely of fibers from the. sensory root.

282 THE NERVOUS SYSTEM

1. The Ophthalmic Branch passes forward along the outer
wall of the cavernous sinus, divides into three branches —
(a) lachrymal, (b) frontal, (c) nasal — and enters the orbit
by the sphenoidal fissure. It communicates with the cavern-
ous sympathetic, third and sixth nerves, (a) The lachrymal
branch, running along the outer wall of the orbit, reaches
the lachrymal gland, gives off filaments to it and to the con-
junctiva, and pierces the tarsal ligament to be finally dis-
tributed to the integument of the upper lid. (b) The
frontal branch runs along the upper wall of the orbit and
separates into the supra-trochlear and supra-orbital
branches. The former of these leaves the orbit in front and
turns up over the bone to supply the integument of the
lower forehead ; the latter traverses the supra-orbital canal,
escapes by. the foramen of the same name, and supplies
the skin as far back as the occiput as well as the peri-
cranium in the frontal and parietal regions, (c) The nasal
branch, crossing to the inner wall of the orbit, enters the
anterior ethmoidal foramen, passes thus into the cranium
again, runs in a groove on the cribriform plate of the eth-
moid and finds exit into the nose through a slit by the side
of the crista galli. Here it gives off branches which supply
common sensation to the mucous membrane of the fore part
of the nose, and then running in a groove on the posterior
surface of the nasal bone, it leaves the cavity at the lower
border of that bone to supply the integument of the ala and
tip of the nose. From the nasal nerve pass fibers to the oph-
thalmic ganglion and to the ciliary muscle, iris and cornea.

2. The Superior Maxillary Branch passes away from the
Gasserian ganglion and leaves the cranium by the foramen
rotundum. Crossing the spheno-maxillary fossa it enters
the orbit through the spheno-maxillary fissure and traverses
the infra-orbital canal to emerge upon the face at the infra-
orbital foramen. In the cranium it gives off a meningeal
branch to supply the neighboring dura mater. In the
spheno-maxillary fossa it supplies branches (a) to the

THE CRANIAL NERVES 283

integument over the temporal and post-frontal regions and
over the cheeks; (b) to the spheno-palatine ganglion; (c)
the posterior superior dental branches (generally two),
which enter the posterior dental canals in the zygomatic
fossa, and, passing forward in the substance of the superior
maxilla, give off twigs to the fangs of the molar teeth, sup-
plying them with sensation. In the infra-orbital canal the
superior maxillary nerve gives off (a) the middle superior
dental, which runs downward and forward in the outer wall
of the antrum to reach the roots of the bicuspid teeth; (b)
the anterior superior dental, which likewise runs in the
outer wall of the antrum to supply the incisor and canine
teeth. After its exit from the infra-orbital canal the nerve
divides into palpebral, nasal and labial branches, which sup-
ply sensation to the regions indicated by their names.

3. The Inferior Maxillary Branch after its exit from the
cranium is a mixed nerve, supplying motion to the muscles
of mastication as well as common sensation to the parts
presently to be noted, and special sense to a part of the
tongue. Its large or sensory root comes from the Gasserian
ganglion to be joined just beneath the base of the skull by
the small motor root which has passed -under the ganglion.
Almost immediately this common trunk divides into (a)
anterior and (b) posterior branches, but first gives off a re-
current meningeal branch and a branch to the internal ptcry-
goid muscle.

(a) The anterior of the two divisions of the inferior max-
illary nerve receives nearly the whole of the motor root and
divides into branches which supply the muscles of mastica-
tion, excepting the internal pterygoid and the buccinator.

(b) The posterior division, chiefly sensory, divides into
the auriculo-temporal, lingual and inferior dental branches.
The auriculo-temporal branch runs backward to a point in-
ternal to the neck of the condyle of the inferior maxilla, then
passing upward under the parotid gland divides into
^branches, which are distributed to the external auditory

284 THE NERVOUS SYSTEM

meatus, parotid gland, integument of the temporal region
and of the ear and surrounding parts. It communicates with
the otic ganglion. The lingual branch is joined by the
chorda tympani, passes to the inner side of the ramus of the
jaw, crosses Wharton's duct, and is distributed to the pa-
pillae and mucous membrane of the tongue and mouth. It
communicates with the facial through the chorda tympani,
with the hypoglossal, and with the submaxillary ganglion.
The inferior dental branch passes between the internal lat-
eral ligament and ramus of the jaw to enter the inferior
dental foramen. Thence it traverses the dental canal in the
inferior maxilla to issue at the mental foramen. Here it di-
vides into incisor and mental branches ; the former con-
tinues in the bone to supply the incisor and canine teeth ;
the latter supplies the skin of the chin and lower lip. In its
course the inferior dental gives off the mylo-hyoid (before
entering the canal) to the mylo-hyoid and anterior belly of
the digastric, and dental branches to supply the molar and
bicuspid teeth.

Four small ganglia, usually classed as part of the sympa-
thetic system, are connected with the three divisions of the
tri facial nerve. The ophthalmic, or lenticular, ganglion is
connected with the first division ; the spheno-palatine or
Meckel's with the second ; the otic and submaxillary with
the third. All these receive sensory fibers from the trifacial
and motor fibers from various sources.

Functions. — It is seen from the foregoing description that
the trifacial is the great sensory nerve of the head and face,
and the motor nerve of the muscles of mastication. The
small, or motor, division has properly been called the "nerve
of mastication." It is insensible upon stimulation before
it is joined by the third division of the sensory root. Its sec-
tion causes paralysis of the muscles of mastication on that
side. It cannot be doubted that the large root is exclusively
sensory at its origin, and the acuteness of that sensibility, as
c. g., in the teeth, is a matter of common observation. Im-

THE CRANIAL NERVES 285

mediate loss of sensibility in the area of its distribution fol-
lows section, and even the cornea, which is normally ex-
quisitely sensitive, can be touched without exciting pain.
Both roots are usually cut at the same time, and besides a
loss of motion and general sensibility, section of this nerve
produces a decided effect upon the eye, the sense of taste,
deglutition and the nutrition of the parts to which the nerve
is distributed. The flow of tears is increased, the pupil be-
comes temporarily contracted and the ball protrudes. In a
few hours congestion is marked, and in a day or two the
cornea sloughs and the eye is destroyed. Section of the
fifth before its lingual branch is joined by the chorda tym-
pani from the facial causes a loss of general sensation, but
not of taste, in the anterior part of the tongue ; section of
the lingual branch after it has received the chorda is fol-
lowed by loss of general sensation and of taste. This
shows that the special^ sensibility distributed to the tongue
by the lingual branch of the fifth is furnished by the chorda
timpani. The fifth nerve sends filaments to give sensibility
to the velum palati. The reflex act of deglutition is due to
impressions carried from the velum and neighboring parts
to the centers ; when the fifth nerve is cut no such impres-
sions are conveyed and the reflex act cannot be excited.

Regarding nutrition it is noticed that, besides the slough-
ing of the cornea, there is also, about the same time, the ap-
pearance of ulcers in the mouth and on the tongue, and ani-
mals thus experimented upon soon die. These lesions are
much less marked when the section is behind the semilunar
ganglion. Explanations of this difference are not altogether
satisfactory, but it is rational to suppose that section of sym-
pathetic fibers when the nerve is cut in front of Gasser's
ganglion is responsible for the disturbances of nutrition ; for
this is the system of nutrition, and changes following its sec-
tion in other parts of the body are not unlike those under,
discussion. Why, however, the changes should be inflam-
matory in character is not explained by this hypothesis, un-

286 THE NERVOUS SYSTEM

less it be an explanation to say that the inflammation is set
up by the impairment of nutrition in these structures — the
impairment resulting in part from the impoverished condi-
tion of the blood as a consequence of the inability of the
animal to chew.

Sixth Nerve (Abducens).

Origin. — This is a motor nerve entirely. Its apparent
origin is from the lower border of the pons in the groove
separating it from the anterior pyramid of the medulla. Its
deep origin is close to the median line beneath the floor of
the fourth ventricle a little below the motor root of the fifth.

Course and Distribution. — The nerve enters the cavern-
ous sinus, runs forward to enter the orbit by the sphenoidal
fissure, passes between the two heads of the external rectus,
and is distributed to the ocular surface of that muscle. In
the cavernous sinus it receives fibers from the first division
of the fifth and from the sympathetic.

Function. — The function is indicated in its distribution.
It is insensible at its origin. Stimulation produces contrac-
tion of the external rectus ; section causes paralysis of that
muscle and consequent internal strabismus and diplopia.

Seventh Nerve (Facial).

Origin. — The apparent origin of the seventh is from the
upper end of the medulla in the groove between the olivary
and restiform bodies. Its deep origin is in the pons beneath
the floor of the fourth ventricle a little external to the nu-
cleus of the sixth.

Course and Distribution. — The seventh nerve passes out-
ward and forward with the auditory nerve (on its inner
side) to enter the internal auditory meatus. From their
relative firmness and texture and their close relation here,
the seventh and eighth nerves have been called respectively

THE CRANIAL NERVES 287

the portio dura and the portio mollis. Running between
them is a fasciculus from the medulla known as the inter-
mediary nerve of Wrisberg, or the portio inter duram et
mollem; most of its fibers join the facial in the internal audi-
tory meatus. The facial nerve enters the Fallopian aque-
duct at the bottom of the meatus and follows it to issue at
the stylo-mastoid foramen, runs forward in the substance of
the parotid gland and divides behind the ramus of the jaw
into temp or -o- -facial and cervico -facial branches.

Its branches of communication are numerous. ( i ) In the
internal auditory meatus it communicates with the auditory
nerve; (2) in the aqueductus Fallopii with the otic and
spheno-palatine ganglia, with the sympathetic and with the
auricular branch of the pneumogastric ; (3) after leaving
the stylo-mastoid foramen, with the fifth, ninth, tenth and
sympathetic.

Its branches of distribution are also quite numerous, (i)
In the aqueductus Fallopii it gives off (a) the tympanic
branch to the stapedius muscle, and (b] the chorda tympani,
which passes through the cavity of the tympanum and
emerges by a foramen at the inner end of the Glasserian
fissure to go to the lingual branch of the fifth. (2) At its
exit from the stylo-mastoid foramen it gives off (a) a
posterior auricular branch which, receiving a filament from
the auricular branch of the tenth, is distributed to the retra-
hens aurem and the occipital portion of the occipito-fron-
talis; (6) a digastric branch to the posterior Ipelly of the di-
gastric muscle; (c) a stylo-hyoid branch to the muscle of
that name. (3) On the face it divides into (a) a temporo-
facial branch, which is distributed to the muscles over the
temple and upper face; and (b) a cervico-facial branch,
which is distributed to the lower face and upper cervical
region.

Functions. — This is the motor nerve of the muscles of ex-
pression, of the platysma, buccinator, digastric (posterior
belly), stylo-hyoid, the muscles of the external ear and the

288 THE NERVOUS SYSTEM

stapedius. Communicating freely with the fifth, it also con-
tains sensory fibers, but it is in all probability insensible at
its root. Its section causes paralysis of the muscles which it
supplies, but no marked changes in sensation. The branches
to the otic and spheno-palatine ganglia in the aqueductus
Fallopii constitute their motor roots; the branch given off
in this situation to the tenth supplies it with motor filaments,
and probably also here pass sensory fibers from the tenth to
the seventh. In facial paralysis when the lesion is in the
aqueductus Fallopii or behind it, there is paralysis also of
the muscles of the palate and uvula, the uvula is drawn "to
the opposite side and there is trouble in deglutition. The
fibers to the azygos uvulae and levator palati pass from the
aqueductus Fallopii through Meckel's ganglion.

The effect of paralysis of the facial upon the superficial
muscles of the face is suggested in its distribution. The
brow cannot be corrugated; the eye is constantly open and
there may be consequent inflammation from exposure ; the
nostril cannot be dilated, and inspiration and possibly ol fac-
tion are interfered with ; the cheek is flaccid ; the lips are im-
mobile and saliva may flow from that corner of the mouth ;
the buccinator is paralyzed, and there is often great diffi-
culty in mastication because of the accumulation of food be-
tween the cheek and the teeth ; the unopposed action of the
muscles of the opposite side greatly distort the facial fea-
tures, the affected side being quite expressionless. Facial
monoplegia is common ; facial diplegia is very uncommon.

The Chorda Tympani. — This branch of the seventh is con-
cerned especially in gustation. The fibers of which it is
composed undoubtedly come from nerve of Wrisberg. Sec-
tion of the seventh involving also the nerve of Wrisberg
causes not only facial palsy but also a loss of the sense of
taste in the anterior two-thirds of the tongue. The sense of
taste will receive later notice.

THE CRANIAL NERVES 289

Eighth Nerve (Auditory).

Origin. — This is a nerve of special sense. Its apparent
origin is by two roots — one from the groove between the
olivary and restiform bodies at the lower border of the pons,
the other coming around the upper end of the restiform
body to join the first in the groove. The deep origin of the
two roots is different. That of the median root is the dor-
sal auditory nucleus in the floor of the fourth ventricle;
that of the lateral root is mainly from the ventral auditory
nucleus in front of the restiform body between the two
roots.

Course and Distribution. — Crossing the posterior border
of the middle peduncle of the cerebellum, it enters the in-
ternal auditory meatus in company with the facial nerve and
the nerve of Wrisberg. At the bottom of the meatus it re-
ceives fibers from the seventh, and divides into branches
which pass to the cochlea, semi-circular canals and vestibule.

Function. — This nerve receives and conveys to the brain,
impressions produced by sound waves; it is the nerve of
hearing and is in all probability not sensible to stimulation
in any other way.

Ninth Nerve (Glosso-pharyngeal).

Origin. — The apparent origin of this nerve is from the
upper part of the medulla in the groove between the olivary
and restiform bodies. Its deep origin is in the lower part of
the floor of the fourth ventricle above the nucleus of the
tenth.

Course and Distribution. — Leaving the skull by the jugu-
lar foramen, it passes forward between the internal jugu-
lar vein and the internal carotid artery, descends in front of
the latter to the lower border of the stylo-pharyngeus where
it curves inward, runs beneath the hyoglossus, and is distrib-

290 THE NERVOUS SYSTEM

uted to the fauces, posterior third of the tongue, and the
tonsil.

It communicates with the seventh, tenth and sympathetic.

Its branches of distribution go to the mucous membrane
and muscles of the pharynx, the stylo-pharyngeus, the ton-
sil and soft palate, the circumvallate papillae and the mucous
membrane at the base and side of the tongue and on the an-
terior surface of the epiglottis. Some of its branches join
branches from the pharyngeal and external laryngeal
branches of the pneumogastric to form the pharyngeal
plexus.

Functions. — It is the nerve of sensation to the pharynx
and fauces and a nerve of taste to the base of the tongue.
Its sensibility at its root is dull, but stimulation produces no
motion. Although this nerve is distributed to the mucous
membrane over the base of the tongue, palate and pharynx,
these parts receive the greater portion of their general sen-
sibility from filaments of the fifth, and section of the ninth
produces no marked effect upon the reflex phenomena of
deglutition. The sense of taste is distributed to the anterior
two-thirds of the tongue by the chorda tympani, and it has
nothing to do with general sensation, while the glosso-
pharyngeal, endowing the posterior third with gustatory
power, also furnishes to it a degree of general sensibility.

Tenth Nerve (Pneumogastric, Vagus).

Origin. — This is a mixed nerve. Its apparent origin is
from the groove between the olivary and restiform bodies
below the ninth! Its deep origin is in the floor of the fourth
ventricle just below that of the glosso-pharyngeal.

Course and Distribution. — As it leaves the cranium by the
jugular foramen it presents a ganglionic enlargement, the
jugular ganglion, or ganglion of the root, just below which
it is joined by the accessory portion of the spinal accessory.
Below the junction is a second ganglion, the ganglion of the

THE CRANIAL NERVES 291

trunk. The accessory part of the eleventh passes through
this ganglion, and below unites with the vagus trunk to pass
chiefly into its pharyngeal and superior laryngeal branches.
The pneumogastric passes down the neck behind and. be-
tween the internal jugular* vein and the internal and com-
mon carotid arteries, and sends motor and sensory fibers to
the organs of voice and respiration, and motor fibers to the
pharynx, esophagus, stomach and heart.

The branches of the pneumogastric are numerous, (i)
In the jugular fossa it gives off (a) a meningeal branch to
the dura mater of the posterior fossa of the skull; (b) an
auricular branch which, traversing the substance of the tem-
poral bone, emerges by the auricular fissure to supply the in-
tegument of the back part of the pinna and external auditory
meatus. (2) In the neck it gives off (a) a pharyngeal
branch, which consists mainly of fibers from the accessory
portion of the eleventh and is the chief motor nerve of the
pharynx and soft palate; (b) a superior laryngeal branch,
which also consists mainly of fibers from the accessory part
of the eleventh and is the chief sensory nerve of the larynx ;
it also animates the crico-thyroid muscle; (c) a recurrent
laryngeal branch, which, on the right side, winds round the
subclavian artery and, on the left, round the aorta to re-
turn to the muscles of the larynx whose motor nerve it is ;
(d) cervical cardiac branches, which communicate with the
cardiac branches of the sympathetic and pass to the deep
cardiac plexus. (3) In the thorax it gives off (a) thoracic
cardiac branches, which pass to the deep cardiac plexus ; (b)
anterior pulmonary branches, which go to the roots of the
lungs in front; (c) posterior pulmonary branches, which go
to the roots of the lungs behind and send some filaments to
the pericardium; filaments from (b) and (c) follow the air
passages through the lungs; (d) esophageal branches, which
unite with fibers from the opposite nerve to form the esopha-
geal plexus. (4) In the abdomen are the gastric branches;
those from the left nerve are distributed to the anterior

292 THE NERVOUS SYSTEM

surface of the stomach, and those from the right to the pos-
terior ; the right vagus is also distributed to the liver, spleen,
kidneys and entire small intestine.

Throughout its whole course the pneumogastric communi-
cates with other nerves, especially the sympathetic.

Functions. — The root of the tenth in the medulla is
purely sensory, but the nerve communicates with at least five
motor nerves, and is distributed to mucous membranes and
to voluntary and involuntary muscle tissue. The auricular
branches contain both motor and sensory fibers, and their
function is indicated in their distribution. The pharyngeal
branches are mixed, receiving motor filaments from the
spinal accessory. Sensibility is supplied to the pharynx not
by this nerve alone, but by the branches of the fifth and
probably of the ninth ; indeed it seems that the pharyngeal
branches of the tenth have little to do with the reflex phe-
nomena of deglutition. The superior laryngeal branches,
mainly sensory, supply also motor power to the crico-thy-
roids. Stimulation of the filaments of these branches pre-
vents the entrance of foreign bodies into the larynx by reflex
closure of the glottis, and also excites movements of deglu-
tition. Their section produces hoarseness. The recurrent,
or inferior laryngeal, branches, chiefly motor, supply the
muscular tissue of the upper esophagus and trachea, as well
as the muscles of the larynx. Section of them causes em-
barrassed phonation, though the fibers thus influencing the
vocal sounds come to the recurrent laryngeal from the spinal
accessory. The uses of the cardiac branches have been no-
ticed under discussion of the heart's action. The pulmonary
branches are both motor and sensory and go to the lower
trachea, the bronchi and lung substance. Section of the
tenth destroys the sensibility of the mucous membrane of
the trachea and bronchi and the contractile power of the
muscular fibers of the tubes. The esophageal branches are
mixed, though motor fibers predominate. Food will not pass

THE CRANIAL NERVES 293

readily into the stomach on section of the tenth because of
the absence of muscular contractions in the esophagus.

Influence of the Vagus on Respiration. — Section of both
these nerves temporarily increases the number of respira-
tions which soon, however, become exceedingly slow until
death ensues. Inspiration is very profound — indeed so pro-
found as to produce rupture of some of the pulmonary
capillaries with consequent hemorrhage and coagulation of
the blood and consolidation of the lung in part or whole.
Section of only one of the vagi is not usually followed by
death. Further notice of the relation of the pneumogastric
to respiration is given elsewhere.

Influence of the Vagus on the Stomach, Intestine and
Liver. — Stimulation of the pneumogastric causes contraction
of the stomach; but since the contraction is not immediate,
the impulse is probably carried to it by fibers of the sympa-
thetic running with the gastric branches of the tenth. When
the vagus is cut during digestion in the stomach the contrac-
tions of the muscular wall are impaired and the sensibility of
the organ is abolished. Secretion is interfered with, but not
stopped.

Section of the vagus seems also to impair intestinal secre-
tion and movements, but it is not improbable that this is be-
cause sympathetic fibers joining the vagus high in the neck
are distributed with it to the intestine.

Simple division of the pneumogastrics inhibits the forma-
tion of glycogen in the liver; but when the central ends of
the cut nerves are stimulated there is an increased pro-
duction of sugar even to the point of glycosuria. The irri-
tation is probably reflected through the sympathetic; indeed
it is not supposed that the vagi are concerned in the glyco-
genic function of the liver, except reflexly ; its section only
prevents the conduction cephalad of the impressions which
usually give rise to a secretion of glycogen.

The connection of the vagus with the kidneys, spleen and
suprarenal capsules is obscure.

294 THE NERVOUS SYSTEM

Eleventh Nerve (Spinal Accessory).

Origin. — This nerve consists of a cranial portion, acces-
sory to the tenth, and a spinal portion. The apparent origin
of the cranial root is from the side of the medulla just below
the vagus. Its deep origin is in the medulla to the posterior
and outer side of the nucleus of the ninth. The apparent
origin of the spinal portion is by several filaments from the
side of the cord as low down as the sixth cervical nerve. Its
deep origin is from a column of cells in the anterior cornu
of gray matter of the cord.

Course and Distribution (Accessory Portion). — Passing
out to the jugular foramen it is joined by the spinal portion,
and sends a few filaments to the ganglion of the root of the
tenth ; then leaving the spinal portion it finds exit from the
cranium by the jugular foramen, passes over the ganglion of
the trunk of the tenth (adherent to it), and is continued
chiefly in the pharyngeal and superior laryngeal branches of
that nerve (Gray), but in the recurrent laryngeal as well.

Spinal Portion. — Running upward between the two roots
of the spinal nerves the spinal portion enters the cranial cav-
ity by the foramen magnum, passes outward to the jugular
foramen, where it joins the accessory portion to separate
from it on passing through that foramen. After leaving the
skull it takes a course backward, pierces the sterno-mastoid,
crosses the occipital triangle and terminates in the trapezius.
It gives branches to the sterno-mastoid and to the cervical
plexus.

Functions. — Both roots of this nerve are purely motor, but
communication with other nerves gives it a degree of sensi-
bility. The fibers from the medulla (accessory) go exclu-
sively to the muscles of the larynx and pharynx, while
those from the cord (spinal) go exclusively to the sterno-
mastoid and trapezius; and section of either root separately
is followed by phenomena corresponding to these facts.
When both roots are divided there is loss of voice, disturb-

THE CRANIAL NERVES 295

ance of deglutition, loss of cardiac inhibition and partial
paralysis of the sterno-mastoid and trapezius. The loss of
voice and disturbance in deglutition are explained by the dis-
tribution of the fibers of the eleventh. with the pharyngeal
and laryngeal branches of the tenth. The loss of the power
of the vagus to inhibit cardiac action is because the fibers of
the tenth which convey the inhibitory impulses are received
from the spinal accessory. The sterno-mastoid and trape-
zius are only partially paralyzed because they receive motor
fibers also from the cervical plexus.

Twelfth Nerve (Hypoglossal).

Origin. — This nerve supplies motion to the tongue. Its
apparent origin is by 1015 filaments in the groove between
the anterior pyramid of the medulla and the olivary body.
Its deep origin is in the floor of the fourth ventricle under
the lower border of the fasciculus teres.

Course and Distribution. — The nerve passes through the
anterior condyloid foramen in two bundles which unite to
form a common trunk below. Running downward in com-
pany with the internal carotid artery and internal jugular
vein, it reaches a point opposite the angle of the jaw, then
runs forward, crosses the external carotid, lies on the hyo-
glossus and is continued forward in the genio-hyoglossus
to the tip of the tongue.

It communicates with the tenth, sympathetic, .first and sec-
ond cervical and the lingual branch of the fifth.

Its branches of distribution are ( I ) meningeal to the dura
mater in the posterior fossa of the skull; (2) descendens
hypoglossi, which running downward across the sheath of
the great vessels, meets branches of the second and third
cervical nerves to form a loop from which are supplied the
sterno-hyoid, the omo-hyoid and the sterno-thyroid muscles ;
(3) thyro-hyoid to the muscle of that name; (4) muscular
to the muscular substances of the tongue and to the stylo-

296 THE NERVOUS SYSTEM

glossus, hyoglossus, genio-hyoid and genio-hyoglossus mus-
cles.

Functions. — This nerve posseses no sensibility at its root,
but receives sensory fibers from anastomoses with other
nerves. Its stimulation, therefore, causes movements of the
tongue and some pain. Section of both nerves causes difficult
deglutition, loss of power over the tongue and consequent
disturbances in mastication and articulation. When the
twelfth is affected in hemiplegia the tongue, on protrusion,
deviates to the affected side because it is pushed out by the
genio-hyoglossus.

It will be seen from the foregoing that, classified accord-
ing to their properties at their roots, the L, II. and VIII.
are nerves of special sense ; the III., IV., VI., XI and XII.
are motor; the X. is sensory; and the V., VII. and IX. are
mixed. It is to be remembered, however, that most of these
(excepting the nerves of special sense) are mixed in their
distribution by reason of the reception of fibers from other
nerves. The term "mixed" in the above classification is used
as meaning the association of special sensory fibers with
motor or common sensory fibers as well as the associa-
tion of these latter with each other. The VII. is classed as
a mixed nerve only by allowing the intermediary nerve of
Wrisberg is to be considered a part of it. Its own proper
root is purely motor.

THE SPINAL NERVES.

The spinal nerves, thirty-one on each side, are so called
from the fact that they originate in the spinal cord and es-
cape from the spinal canal by the intervertebral foramina.
Eight pairs come from the cervical region of the column,
twelve from the dorsal, five from the lumbar, five from the
sacral, and one from the coccygeal. They are numbered ac-
cording to their foramina of exit.

Each nerve rises by two roots — an anterior which can be

THE SPINAL NERVES

297

traced to the anterior cornu of gray matter and a posterior
which goes (apparently) to the posterior cornu — and these
emerge respectively from the antero-lateral and postero-lat-
eral fissures of the cord. .Before leaving the spinal canal
these two roots join to pass through the corresponding in-
tervertebral foramen as a single trunk which, however, just
beyond that foramen divides into anterior and posterior

FIG. 87.

A, bipolar cell from spinal ganglion of a 4% weeks' embryo (after His), n,
nucleus; the arrows indicate the direction in which the nerve processes grow,
one to the spinal cord, the other to the periphery. B, a cell from the spinal gan-
glion of the adult; the two processes have coalesced to form a T-shaped junc-
tion. (Kirkes.)

branches to be distributed to the anterior and posterior parts
of the body.

The posterior root (inside the spinal canal) is sensory,
and has a ganglion developed upon it. The fibers of the
posterior root are outgrowths of cells in the ganglion of that
root, as indicated in Fig. 87. This accounts for the arborisa-
tion of the different fibers around cells in the cord instead
of an actual connection with them. These facts should not
be lost sight of though it is customary to speak of an efferent

298 THE NERVOUS SYSTEM

fiber as passing directly to a cord cell itself. The anterior
root is entirely motor except for a degree of "recurrent"
sensibility which is due to the presence in it of posterior root
fibers which have passed backward from the point of junc-
tion of the two probably to supply the membranes of the
cord. The common trunk is, of course, mixed, as are the
anterior and posterior branches passing from it.

These spinal nerves are distributed to the muscles of the
trunk and extremities, to the integument of almost the entire
body and to some mucous membranes; and from what has
been said in speaking of the cord about the connection be-
tween it and these nerves, and their connection through it
with the higher centers, it is evident that they are most im-
portant factors which, acting under the guidance of the sen-
sorium, on the one hand, tell of the condition of the or-
ganism— its relations and environments — and, on the other,
control the voluntary movements of the body.

The spinal nerve fibers come in part directly from the
brain and in part from the gray cells of the cord.

THE SYMPATHETIC SYSTEM.

The sympathetic has been separated from the cerebro-
spinal system only for the sake of convenience. The
former sends filaments to the latter and receives both motor
and sensory .fibers in return, while the cooperation of the two
systems, regulating in harmony all the physiological pro-
cesses going on in the body, is too evident to be questioned.

The sympathetic system is remarkable for the number of
ganglia connected with it. These may be divided into (a)
those along the vertebral column, as the thoracic, (b) those
in close proximity to the viscera and from which those vis-
cera are to be directly supplied, as the semilunar, and (r) ter-
minal ganglia which the fibers reach just before final distri-
bution, as the cardiac, intestinal, etc. The sympathetic is,
therefore, frequently known as the ganglionic system.

THE SYMPATHETIC SYSTEM 299

Arrangement. — There is on each side of the spinal column,
extending from the lenticular ganglion above to the gang-
lion impar below, a chain of ganglia all of which are united
to each other and to the ganglia of the opposite chain by
commissural fibers. From these ganglia go fibers to form
numerous plexuses and to be distributed to the various parts.
In the skull there are four of these ganglia, the otic, oph-
thalmic, submaxillary and spheno-palatine or Meckel's; in
the cervical region there are three; in the dorsal twelve; in
the lumbar four ; in the sacral four or five ; and in front of
the coccyx the single ganglion impar.

Connections between the cranial nerves and cranial sym-
pathetic ganglia have already been noted.

The cervical ganglia are of special interest as furnishing
the chief sympathetic supply to the heart.

The thoracic or dorsal ganglia give rise to the sympa-
thetic supply for the great abdominal viscera. From the
sixth, seventh, eighth and ninth springs the great splanchnic
nerve, which passes through the diaphragm to the semilunar
ganglion. This is the largest of the sympathetic ganglion,
and is sometimes called the abdominal brain. It has been in-
accurately called the center of the sympathetic system. The
two ganglia occupy positions on opposite sides of the celiac
axis, and give rise to fibers which supply most of the abdom-
inal viscera. The tenth and eleventh thoracic ganglia give
rise to the lesser splanchnic nerve. From the last thoracic
springs the renal splanchnic nerve. The radiating fibers
from the semilunar ganglia form the solar plexuses for the
two sides.

The lumbar ganglia give off fibers to form the aortic lum-
bar and hypogastric plexuses.

The sacral and coccygeal ganglia supply the pelvic vessels.

Properties. — The ganglia and nerves are slightly sensitive.
Contraction of involuntary muscular tissue follows stimula-
tion— not immediately, but after a considerable interval, and
the subsequent relaxation is tardy. Some of the ganglia are

3OO THE NERVOUS SYSTEM

dependent for power upon their fibers from the cerebro-
spinal system, while others seem capable of acting indepen-
dently, at least for a time.

Functions. — Little is known of the functions of the sym-
pathetic except as regards efferent fibers. They are dis-
tributed in general to the non-striped musculature of the cir-
culatory apparatus and of the viscera, to secreting glands
and to the heart. The heart furnishes the only example of a
direct sympathetic supply to striated muscle. The sympa-
thetic has a very definite effect upon secretion, nutrition and
the local production of heat. Section of the sympathetic
fibers going to any part causes hyperemia, an increased
amount of secretion (sweat, e. g.), and a rise of temperature
in that part. The last two conditions are caused by the first,
and it in turn is due to a paralysis of the muscular coat of the
vessels, allowing an abrogation of their usual tonic condition
and, consequently, dilatation and an increased amount of
blood with exaggerated nutritive activity. This statement
confronts us with the question of vaso-motor action.

Vaso-motor Phenomena. — By vaso-motor nerves is meant
those fibers which convey to the muscular coat of the vessel
walls impulses causing them to contract and decrease the
caliber, or to relax and increase it. Those causing contrac-
tion are called vaso-constrictors ; those causing relaxation
vaso-dilators. It is mainly through the operation of vaso-
motor nerves that the sympathetic system influences nutri-
tion in a particular part, though all vaso-motor fibers are not
confined to the sympathetic cords. However, it is not
through the operation of the vaso-motor nerves alone that
the sympathetic lays claim to be the "system of nutrition,"
for all the parts to which its other fibers are distributed con-
tribute also very materially to nutrition, though perhaps in
not so direct a manner as do the muscular coats of the ar-
teries. While intestinal peristalsis, the secretion of many
glands, as, for example, the production of glycogen, bile, etc.,
cannot be shown to be absolutely dependent on sympathetic

THE SYMPATHETIC SYSTEM 3OI

connections, yet all these processes — nutritive in nature —
have their normal activity seriously impaired by with-
drawal of the sympathetic influence.

The chief vaso-motor center is in the medulla, though ac-
cessory centers exist also in the cord ; all vaso-motor fibers
pass out from these centers and leave the cerebro-spinal
axis with the cranial or spinal nerves.

The most usual mode of action of the vaso-motor nerves
is reflex, as when the mucous membrane of the stomach be-
comes hyperemic upon the introduction of food ; or when the
salivary secretion increases during mastication, or even
sometimes at the sight or thought of food; or when emo-
tions are evidenced by paling or blushing.

Raising blood-pressure by stimulating the vaso-constric-
tors and lowering it by stimulating the vaso-dilators are sim-
ply mechanical results, and require no comment.

Sleep. — Sleep is closely associated with vaso-motor action.
Every part of the body has a function to perform, but it
must have some rest from that performance or it will begin
to act inefficiently and finally cease altogether. For most
organs these periods of rest occur at approximately uniform
intervals, as in case of the stomach, heart or respiratory
muscles ; but notably in case of the involuntary muscles these
periods of repose have no regularity — i. e., a person exer-
cises them at no regular time except by accident of occupa-
tion or otherwise. But, in any case, there comes a time
when repose must be had, for during activity the destructive
processes far exceed the constructive, and in order for the
balance to be preserved there must be a time when the op-
posite is true.

Now we may say that it is the function of the brain to fur-
nish consciousness — if we can allow that consciousness em-
braces all the various manifestations of nerve force peculiar
to the brain. For the brain to suspend this function at fre-
quent intervals like the heart (e. g.) would be manifestly im-
possible if one is to do any consecutive work depending upon

302 THE NERVOUS SYSTEM

this organ. The brain works longer, and must, therefore,
rest longer at a time than most of the other organs of the
body. True, so far as the voluntary muscles are concerned
they rest best probably when the brain is resting, but the lat-
ter condition is not a necessary one for the maintenance of
their physiological integrity. This repose of the brain — this
temporary abolition of the cerebral functions— is sleep.
While, of course, the activity of that organ during wakeful-
ness may be increased or diminished by volition, and it may,
therefore rest from a comparative standpoint — as when one
ceases to think actively upon a subject and becomes men-
tally listless — still the brain can never, under such circum-
stances, rest properly, and sleep finally becomes imperative.

Vascular Phenomena of Sleep. — Coma is analogous to
sleep in that consciousness is lost ; but in this case the brain
is congested and the condition is unnatural. It was long
supposed that this was the vascular condition during natural
sleep, but application of the physiological principles prevail-
ing in other parts of the body would rather presuppose a
condition of cerebral anemia; for the brain receives blood
for two purposes — first, to supply nutrition to the nervous
substance, and second, to bring supplies which, by the ac-
tion of the brain cells, may be converted into nerve force —
and during sleep only the first of these purposes is to be
served. This is true in case of glands, muscles, etc., during
their intervals of repose. As a matter of fact, the cerebral
vessels are contracted and there is much less blood in the
brain during sleep than during consciousness.

Dreams. — In explanation of the phenomena of dreams and
somnambulism, it is said that what we call sleep may occur in
one part of the brain and not in another, or in different de-
grees in different parts of the nervous centers. "In the
former case [dreams] the cerebrum is still partially active ;
but the mind products of its action are no longer corrected
by the reception, on the part of the sleeping sensorium, of
impressions of objects belonging to the outer world ; neither

THE SYMPATHETIC SYSTEM 303

can the cerebrum, in this half-awake condition, act on the
centers of reflex action of the voluntary muscles, so as to
cause the latter to contract — a fact within the painful experi-
ence of all who have suffered from nightmare. In somnam-
bulism the cerebrum is capable of exciting that train of re-
flex nervous action which is necessary for progression, while
the nerve center of muscular sense (in the cerebellum?) is
presumably fully awake; but the sensorium is still asleep,
and impressions made on it are not sufficiently felt to rouse
the cerebrum to a comparison of the difference between
mere ideas or memories and sensations derived from exter-
nal objects" (Kirkes).

Relation Between the Cerebro-spinal and Sympathetic
Systems. — A brief resume may help to clarify the association
between the two systems.

1. Anatomically. — The two are developed from the same
embryological tissue ; the vaso-motor sympathetic fibers obey
centers in the medulla and cord, and must, therefore, be con-
nected with those centers either directly or indirectly ; char-
acteristic small medullated fibers pass at intervals from the
cord through the roots into the sympathetic ganglia; they
send fibers each to the trunks of the other to be distributed
directly, or to form plexuses and then be distributed to-
gether; their fibers are found together in all organs which
receive cerebro-spinal nerves (unless they be non- vascular) ;
in some of these organs just named the sympathetic fibers
are there only as vaso-motor nerves, while in others, as
glandular structures like the liver and salivary glands, sym-
pathetic fibers are distributed to the gland cells themselves,
and both have a definite but associated influence on secretion.

2. Physiologically. — The physiological relation is best indi-
cated by examples. A great many, if not all, the sympathetic
ganglia seem to receive their power to generate nerve force
from the cerebro-spinal system ; there can be no proper nu-
trition of the parts animated by cerebro-spinal fibers with-
out the associated aqtion of vaso-motor sympathetic fibers —

3O4 THE NERVOUS SYSTEM

not even of the nerve cells and fibers themselves; in reflex
action the afferent impression may be conveyed by a cerebro-
spinal fiber and reflected through a sympathetic, or vice
versa; when one hand is thrust into hot or cold water the
temperature of the opposite hand may be raised or lowered,
impressions having been carried to the center by cerebro-
spinal and reflected by sympathetic fibers, not only to the
immersed hand, but to the other as well ; food is taken into
the mouth, impressions are carried by nerves of common
sensation to the brain and are reflected through the sympa-
thetic system, an increased amount of blood is thereby sent
to the salivary glands and an increased secretion supervenes ;
one smells savory articles and the mouth waters, etc.

Examples could be multiplied ad infinitum to establish
the cooperation existing between the two systems. What
has been incidentally and indirectly said on this point in con-
sidering secretion, digestion, circulation, respiration, etc.,
serves to emphasize their connection.

CHAPTER XII.
THE SENSES.

IT is evident from preceding remarks that it is through the
intervention of the nervous system that we have a "sense" of
existence, of the existence and condition of different parts of
our bodies and of our relations to the external world. The
knowledge we thus obtain is based upon sensations of various
kinds, all of which are carried to the centers by afferent
fibers. Such sensations may be what are termed (A) Com-
mon, or (B) Special, including (i) Touch, (2) Smell, (3)
'Sight, (4) Taste f (5) Hearing. It is to be remembered that
the seat of sensation is in the brain, and not in any organ
which primarily receives or conveys the impression. We do
not in reality see with the eye or hear with the ear ; these are
only complex organs so arranged that rays of light or sound
waves produce upon them such impressions as, when trans-
mitted to the sensorium, will give rise to the sensations of
sight or hearing.

(A) COMMON SENSATIONS.

As regards the uses of the fibers conveying impressions
which result in these sensations, they (unless it be those con-
cerned with tactile impressions) are distinct from those of
special sense. That is to say, the fibers of the olfactory,
optic, gustatory and auditory nerves do not convey general
impressions; but it is almost certain that fibers conveying
tactile impressions convey also painful impressions — and the
sensation of pain is taken as typical of common sensations.
It is known that very painful impressions sometimes over-
come tactile sensibility, and that very frequently tactile sen-

20 305

306 THE SENSES

sibility remains in parts which receive no painful impres-
sions, as e. g., under anesthesia by cocain ; but it may be that
the power in the same fiber to convey, in the first case tactile,
and in the second painful impressions is destroyed without
destroying its power to convey the other.

The varieties of common sensation are too numerous to
even mention. Thirst, hunger, fatigue, discomfort, satiety,
etc., are everyday examples, as are also the desire to urin-
ate or defecate. Numerous subdivisions of the sensation of
pain might be mentioned, such as itching, burning, aching,
etc. The so-called muscular sense — by which we become
aware of the condition, relation, coordination and degree of
activity or repose of the muscles — will be considered as be-
longing here.

(B) SPECIAL SENSATIONS,
i. The Sense of Touch.

The sense of touch is closely related to common sensation.
Its distribution over the body is as uniform as that of com-
mon sensation, but it is most highly developed in those parts
where general sensibility is most marked (as in the skin),
and attains its highest degree of perfection only in those
situations in which tactile corpuscles exist, for example, on
the palmar surfaces of the tips of the fingers. The teeth,
hair, nails, etc., are rather surprisingly endowed with tactile
sensibility. Leaving pain and the muscular sense as part of
general sensibility, the sense of touch may be considered
under two heads — (a) Tactile Sensibility proper and (b)
Temperature.

(a) Tactile sensibility proper is most marked where the
epidermis over the papillae is thin. When the epidermis is
removed and the cutis is touched there is pain instead. Tac-
tile sensibility is much decreased where the epidermis is
thickened, as over the heel. The terminal tactile organs

THE SENSE OF SMELL. 307

have been described in connection with afferent nerves.
They are chiefly the end bulbs of Krause and the tactile
corpuscles of Meissner. (See Figs. 71 and 72). Besides,
tactile impressions are received by the free extremities of
afferent nerves situated over the body at large. Numbness
from cold is due to interference with cutaneous circulation
— upon which the sense of touch is directly dependent. It is
almost impossible to distinguish mere touch from pressure.

Acuteness. — How the sense of touch is capable of devel-
opment by practice is well illustrated in the case of many
blind persons. They learn to read with comparative facility
by passing the hand over raised letters ; or they frequently
make the sense of touch take the place of the lost sense in
other almost incredible ways. The acuteness of this sense
in different portions of the body has been made the subject
of observation by touching two different parts in the same
region with finely pointed instruments and noting how near
the points can be brought together and still be recognized as
two. This distance is found to vary from ^4 inch on the tip
of the tongue to 2^2 inches in the dorsal region.

(b) It is not improbable that there are special nerve end-
ings concerned in the reception of temperature impressions,
though this has not been definitely proven. Decisions as to
temperature are only relative; the surface temperature of
the part upon which the impression is made is the standard,
and one can only tell absolutely whether the object is hotter
or colder than the skin, and, within certain limits, approxi-
mate how much hotter or colder. The delicacy of the tem-
perature sense agrees with that of touch as regards the thick-
ness or absence of the epidermis.

2. The Sense of Smell.

Regarding the mechanism of olfaction it is found that one
of the .first conditions necessary is the presence of particular
cells. Between the epithelial cells of the mucous membrane

308 THE SENSES

to which the olfactory fibers are distributed are delicate
spindle-shaped cells known as olfactory cells, and to them
pass the terminal filaments from the olfactory bulbs. These
cells are stimulated by contact with odorous substances, and
from them go, by way of the nerve fibers, impressions which
are recognized as odors of different kinds. The olfactory
fibers are the only ones which will convey such impressions.
True, the same substance may, at the same time, excite other
sensations, as of pain or taste, but the impressions giving
rise to these latter sensations are conveyed by different fibers
altogether. The substances which excite olfaction must
come in actual contact with the nerve terminals and to do
this must be dissolved in the mucus of the nasal mucous
membrane; hence dryness of the nasal cavities (as in the
first stage of nasal catarrh) interferes with olfaction. It is
also said that odorous substances introduced in solution into
the nasal cavities will not excite the sense of smell, but that
they must be introduced by a current of air.

Whether an odor is pleasant or unpleasant is largely a
relative matter ; odors most disgusting to some animals are
not offensive to others. This same difference may also
hold good among different men. Impairment of the sense
of taste, for some reason, follows a loss of the sense
of smell.

3. The Sense of Sight.

It is not intended to go into a detailed consideration of the
sense of sight, but some remarks on the normal eye and its
action are in order.

Protection of the Ball. — The orbital cavity has a pyra-
midal shape with its base forward. It contains the eye-ball,
its muscles, some adipose tissue and most of the lachrymal
apparatus. Above the orbit, the eye-brows prevent a flow of
perspiration from the forehead on to the lid, and also shade
the eye to some extent. The lids, when closed, entirely ob-

MOVEMENTS OF THE BALL 309

scure the balls and protect them in front. On their free bor-
ders are rows of hairs (eye-lashes) curling away from the
globe and shading and protecting it from dust. The lids
are closed by the orbicular es palp ebr arum and opened by the
levatores palpebrarum superiores. In the ordinary closing
of the lids only the upper one is moved, but the lower one is
raised in forcible contraction of the orbicularis. Interven-
tion of the will is not necessary to the action of these mus-
cles, though they are striated. Except during fatigue, the
eyes are kept open involuntarily, but when the cornea is
touched no effort of the will can prevent contraction of the
orbicularis palpebrarum. During sleep the globes are
rotated upward.

The Lachrymal Apparatus. — This consists of the lachry-
mal glands, canal, duct and sac, and the nasal duct. The
secretion of the lachrymal gland keeps the cornea and con-
junctiva constantly bathed in a thin fluid. It is situated in
the orbital cavity at its upper and outer portion. Its secre-
tion is discharged upon the conjunctiva by several little
ducts. The excess of secretion is carried into the nose
through the nasal duct. Near the inner canthus is a small
opening in each lid; these openings are the orifices of the
lachrymal canals, which canals join at the inner angle of the
eye to form the lachrymal sac ; the sac is continued below as
the nasal duct, opening into the inferior meatus of the nose.
The secretion of tears is much diminished during sleep. The
influence of the nervous system on lachrymal secretion is
well known. Emotional disturbances operate through the
sympathetic to increase the flow. Irritation of the mucous
membrane of the nose or eye is followed by a like result.

Movements of the Ball. — The capsule of Tenon, a fibrous
membrane outside the sclerotic, holds the ball loosely in
place. A small amount of adipose tissue behind the globe is
never absent. Movements of the ball are effected through
the action of the internal and external recti, the superior
and inferior recti, and the superior and inferior oblique;

3IO THE SENSES

of these, all but the two last named arise from the apex of
the orbital cavity. The recti are inserted into the sclerotic
just back of the cornea. The superior oblique runs along
the inner aspect of the orbital cavity to a point near the
supero-internal angle; here it becomes tendinous, passes
through a fibro-cartilaginous ring, and then turns backward
and outward to be inserted into the sclerotic between the
superior and external recti just behind the center of the
globe. The inferior oblique arises just within the orbital
cavity near the anterior inferior angle, and passes around the

FIG. 88. — Muscles of the eye and tendon or ligament of Zinn.

i, tendon of Zinn; 2, external rectus divided; 3, internal rectus; 4, inferior
rectus; 5, superior rectus; 6, superior oblique; 7, pulley for superior oblique;
8, inferior oblique; 9, levator palpebrse superioris; 10, 10, its anterior expansion;
n, optic nerve. (Sappey.)

anterior part of the globe to be inserted in the sclerotic just
below the superior oblique.

The effect these muscles have upon the movements of the
ball is indicated by their origin and attachment. The exter-
nal and internal recti rotate it outward and inward, the su-
perior and inferior recti upward and downward. The su-

ANATOMY OF THE BALL 3! I

perior and inferior oblique antagonize each other. The
former rotates the globe so that the pupil is directed out-
ward and downward ; the latter so that it looks outward and
upward. The associated action of all these muscles can pro-
duce almost any variety of movements, and no effort of the
will is necessary to properly associate them when it is desired
to direct the line of vision toward a certain object. For in-
stance, when it is desired to look at an object on the right it
takes no distinct voluntary effort to contract the external
rectus of the right eye and the internal rectus of the left. It
will be seen later that vision for the two eyes is normal only
when impressions are made upon exactly corresponding
parts of the two retinae, so that they may act as a single or-
gan ; and for this to be done not always the same movements
are called for in both balls.

Anatomy of the Ball. — The eye-ball is a globular body
consisting of several coats enclosing refracting media. Of
these coats the external is the sclerotic, dense and fibrous,
covering the posterior five-sixths of the organ and continu-
ous with the cornea, which covers the anterior one-sixth. It
is not well supplied with blood-vessels. The cornea is trans-
parent, and upon its external surface are several layers of
delicate nucleated epithelium; underneath this layer of cells
is a thin membrane, the anterior elastic lamella, which is a
continuation of the conjunctiva. The substance proper of
the cornea is composed of pale interlacing fibers among
which are connective tissue corpuscles and quite a quantity
of fluid. These fibers are continuous from the sclerotic, but
they lose their opacity at the corneo-sclerotic margin. On
the posterior surface of the cornea is the transparent elastic
membrane of Descemet, a part of which, at the circumfer-
ence of the iris, passes into the ciliary muscle. The cornea
is very sensitive, but contains no blood-vessels.

Next inside the sclerotic is the choroid coat of the eye. It
does not lie under the cornea, but is confined to the sclerotic
area of the ball Behind the optic nerve penetrates it, and in

3I2

THE SENSES

front it is connected with the iris. The choroid is very vas-
cular. Its color is dark brown on account of the abundance
of pigment in the cells on the inner surface of the mem-
brane. Anteriorly the choroid is folded in upon itself to
form the ciliary processes, which project inward around the
margin of the crystalline lens.

The ciliary muscle is important in accommodation. It is

FIG. 89. — Diagram of a vertical section of the eye. (From Yeo
after H olden.}

i, anterior chamber filled with aqueous humor; 2, posterior chamber; 3,
canal of Petit; a, hvaloid membrane; b, retina (dotted line); c, choroid coat
(black line); d, sclerotic coat; e, cornea; f, iris; g, ciliary processes; h, canal of
Schlemm or Fontana; i, ciliary muscle.

in the shape of a muscular ring surrounding the margin of
the choroid just outside the ciliary processes. In front it is
attached to the line of junction of the cornea and sclerotic
and to the ligament on the anterior surface of the iris ; be-
hind it is lost in the substance of the choroid. Its contrac-
tion, therefore, compresses the vitreous humor and relaxes
the suspensory ligament of the lens. The iris is a circular
veil hanging in front of the lens. It presents a perforation a

ANATOMY OF THE BALL 313

little to the nasal side of its center, the pupil. It is attached
to the corneo-sclerotic line. It contains circular and radi-
ating fibers. The iris divides the space between the cornea
and lens into two chambers, anterior and posterior — the lat-
ter of which is very small. The "color of the eyes" depends
on the color of the anterior surface of the iris ; its posterior
surface has a constant dark purple hue. The size of the pu-
pil is subject to variations to be noted later.

Inside the choroid is the retina, which is that part of the
eye capable of receiving impressions of sight. Anteriorly it
reaches nearly to the ciliary processes. Externally it is in
contact with the choroid, and internally with the hyaloid
membrane of the vitreous humor. It is penetrated by the
optic nerve a little within and below the center of the pos-
terior hemisphere. Just external to the point of entrance of
the nerve is the macula lutea, a small yellow area in the cen-
ter of which is the fovea centralis; this last is exactly in the
axis of distinct vision. Nine layers of cells are usually de-
scribed as composing the retina. From without inward they
are (i) the pigment layer, (2) rods and cones, (3-6) the
four granular layers, (7) nerve cells, (8) expansion of fibers
of the optic nerve, (9) the limitary membrane. Of these, the
most important is the layer of rods and cones. The rods, or
cylinders, extend through the thickness of the membrane and
have between them, at intervals, flask-shaped bodies, the
cones. At the macula lutea only the cones exist. Elsewhere
the rods are more abundant than the cones. . The length of
the cones is about half that of the rods, and they occupy the
inner aspect of the membrane. The layer of nerve cells pre-
sents cells communicating on the one hand with the rods and
cones and on the other with fibers of the optic nerve. The
rods and cones are the only parts of the retina possessing
special sensibility, impressions being conveyed from them
to the brain by the optic nerve. The fibers of the second
nerve, composing one layer, are pale and transparent. The
blood supply of the retina is from the arteria centralis

314 THE SENSES

retinae, which enters the optic nerve just before it expands,
and, running in its substance, is distributed as far as the
ciliary processes anteriorly.

The Crystalline Lens is a biconvex transparent body situ-
ated just behind the iris. Its function is to refract the rays
of light, and its action in this respect is similar to such
lenses in optical instruments. It is held in place by the sus-
pensory ligament. Its anterior convexity is more marked
than its posterior. It is enveloped by a thin transparent cap-
sule.

The Suspensory Ligament is a continuation of the an-
terior layer of the hyaloid membrane of the vitreous humor.
When this layer reaches the edge of the lens (coming for-
ward) it divides into two parts, one passing in front of and
the other behind that body ; the divisions are continuous re-
spectively with the anterior and posterior portions of the cap-
sule of the lens. The ligament supports the lens.

The Aqueous Humor is behind the cornea and in front of
the lens and suspensory ligament. The iris has been said to
separate this cavity into anterior and posterior chambers
communicating through the pupillary opening. The aqueous
humor is colorless and perfectly transparent. It serves to
refract the rays of light, having for that purpose the same
index as the cornea.

The Vitreous Humor occupies about the posterior two-
thirds of the globe, and is back of the lens and suspensory
ligament surrounded by the delicate hyaloid membrane. It
is of a gelatinous consistence, and is divided into numerous
compartments by very delicate membranes radiating from
the point of entrance of the optic nerve. It is a transparent
refracting medium.

Ocular Refraction. — In order for the image of an object
to be distinct the rays passing from it must fall on a single
portion of the retina, viz., the fovea centralis. The sensi-
bility of the retina to light decreases in passing away from
the fovea. All rays would not meet on the retina unless they

ACCOMMODATION 315

were refracted; and for this purpose there are the cornea,
the aqueous humor, the lens and the vitreous humor. The
surfaces of the cornea and lens are the most important of
these. Since the two surfaces of the cornea are parallel, the
external surface alone is concerned in refraction. The cen-
ter of distinct vision (fovea) is in the axis of the lens
precisely in the plane upon which the rays of light are
brought to a focus by the refracting media. Refraction by
the cornea alone would focus the rays behind the retina;
hence the necessity of convex lenses before the eye after op-
erations for cataract. Rays leaving the cornea are refracted
by the anterior surface of the lens, by its substance to a cer-
tain extent, and again by its posterior surface, the normal
mechanism being such that all rays are focused on the fovea.
The rays cross each other after refraction, and the image is
inverted, but the brain takes no notice of this fact, and ob-
jects are seen in their natural positions.

Accommodation. — Accommodation means a change in the
convexity of the lens, whereby images are focused on the
retina, whether the object be far away from or near the eye.
Rays of light from distant objects strike the eye practically
parallel, and we may assume that there is a certain "passive"
condition of the refracting media which will bring such rays
to a focus at the proper point. But when the object ob-
served is near the eye a change in the arrangement of the
media, or of the convexity of their surfaces, is necessary to
prevent the focusing of the rays behind the retina. The de-
sired end is accomplished by increasing the convexity of the
lens. When the ciliary muscle is "passive" the capsule com-
presses the lens, decreasing its convexity to a minimum;
from the attachments of this muscle, already noted, its con-
traction is attended by a relaxation of the suspensory liga-
ment, which in turn relieves in some degree the compression
of the capsule upon the lens and allows its antero-posterior
diameter to increase ; the result is increased convexity of the
lens.

THE SENSES

When distant objects are looked at the lens becomes flatter
as a result of the contraction of the suspensory ligament,
which contraction is a consequence of the relaxation of the
ciliary muscle. Accommodation for distant objects seems
a passive process entirely.

The ciliary muscle is the "muscle of accommodation."

The contraction of the pupil for near objects is not, prop-
erly speaking, a part of accommodation.

Then, granting special sensibility to the retina and optic
nerve the formation and appreciation of an image is simple.
Rays of light having passed through the cornea and aqueous
humor are admitted by the pupil to pass through the lens and
vitreous humor. By all these objects they are refracted so
that they cross each other and fall upon the retina, producing
an inverted image there. The size of the pupil, other things
being equal, is regulated by the intensity of the light, the
opening being contracted to admit less when the light is
strong.

Myopia, Hyperopia and Presbyopia. — Sometimes the an-
tero-posterior diameter of the eye-ball is too long and the
rays of light are brought to a focus in front of the retina.
Such a condition is known as myopia; the person will be
near-sighted. He brings objects near his eyes so that the
rays may have a greater divergence and thus be focused far-
ther back. Or the rays may be scattered by placing concave
lenses before the eyes. Sometimes, too, the antero-posterior
diameter may be too short and the rays come to a focus be-
hind the retina. Such a condition is known as hyperopia;
the person will be far-sighted. He holds objects far away
from his eyes that the rays from them may strike the ball
with less divergence and thus be focused farther forward.
Or the same end may be accomplished by placing convex
lenses before the eyes. In old age the lens becomes flattened
and accommodates itself less easily. This tends to focus light
behind the retina and objects have to be held far away from

THE SENSE OF TASTE 317

the eye. This is known as presbyopia. Its remedy is the
same as that for hyperopia.

Reaction to Light. — Regarding the reaction of the pupil
to light, it is evident that this is mainly a reflex nervous phe-
nomenon, though direct light will cause the muscular tissue
of the iris to contract. The direct influence of the third
nerve on the action of the iris has been referred to under a
consideration of that nerve. Reflexly, the pupil is con-
tracted by light by the conveyance of an impression to the
brain through the optic fibers, a message is sent to the pro-
per center, and a stimulus is reflected through the third nerve
to the sphincter of the iris causing it to contract. When the
optic nerve is cut the circuit is broken, and movements of
the iris do not occur from the admission of light. Practic-
ally, then, when much or little light reaches the retina the pu-
pil contracts or dilates, as the case may be, in an effort to
keep the amount constant.

Binocular Vision. — It is evident that when a person looks
at an object two images are formed — one on each retina —
but they are combined in his consciousness and he sees but
one object. If one of the balls be thrown out of the proper
axis, by pressure, e. g., objects appear double. The same is
true in strabismus, at least until the person has grown ac-
customed to the defect. In normal vision the rays from
an object are formed on the fovea centralis of each eye — that
is, upon corresponding points which are, for each, the centers
of distinct vision.

4. The Sense of Taste.

In order that gustatory sensation may be exercised it is
necessary (i) that there be specially endowed nerves and
nerve centers; (2) that the nerve terminals be excited by
sapid (tastable) materials; (3) that these substances be in
solution. It has already been seen that the special nerves of
taste are (a) the chorda tympani distributed to the anterior

31.8 THE SENSES

two-thirds of the tongue, and (b) the glosso-pharyngeal to
the posterior third of that organ. It is probable that only the
dorsum of the tongue, the lateral parts of the soft palate, the
uvula and the upper pharynx are concerned in gustation. On
the tongue are found special papillae, ( i ) the circumvallate,
large and few in number, near the base of the organ, and (2)
the fungiform, about 200 in number, over the remaining area.
The circumvallate and some of the fungiform papillae contain
taste beakers, true gustatory organs. They are ovoid col-
lections of cells beneath the epithelial covering of the, mu-
cous membrane. Sapid substances enter these beakers in so-
lution and come in contact with the taste cells, which are
connected with the filaments of the gustatory nerves. Thus
are produced specific impressions which are conveyed to the
gustatory center, and the sense of taste is excited. The lim-
ited distribution of the taste beakers makes it impossible that
they should be the only organs capable of receiving special
gustatory impressions. The taste center has been indefi-
nitely located in the uncinate gyrus near the olfactory center.

Since it is necessary to the tasting of substances that they
come in actual contact with the taste organs, and since to do
so they must be in solution, it follows that dryness of the
mouth interferes with, or abolishes, this sense.

The most marked tastes are the sweet, bitter, saline, and
alkaline. The more delicate flavors involve also the special
sense of smell, and it has been seen that dissociation of the
two kinds of impressions is often impossible. Taste is also
subject to variations by reason of education, age, association,
caprice, etc. Bitters are most easily appreciated at the back,
salts and sweets at the tip, and acids at the sides at the
tongue.

5. The Sense of Hearing.

The ear consists of a complicated apparatus for the pur-
pose of the reception of special impressions which are appre-
ciated by the brain as sounds. Anatomically it consists of

THE EXTERNAL EAR

319

the external, the middle and the internal ear ; the last con-
tains the essentials of the auditory apparatus, the external
and middle divisions serving only to concentrate the sound
waves upon the parts of the internal.
The External Ear. — This consists of the pinna and the

FIG. 90. — Scheme of the organ of hearing.

AG, external auditory meatus; T, tympanic membrane; K, malleus with its
head (/i)» short process (kf) and handle (»t) ; a, incus, its short process (x)
and its long process united to the staples (s) by means of the Sylvian ossicle
(Z); P, middle ear; o, fenestra ovalis; r, fenestra rotunda; x, beginning of
the lamina spiralis of the cochlea; pt, scala tympani, and vt, scala vestibuli; V ,
vestibule; S, saccule; U, utricle; H, semicircular canals; TE, Eustachian tube.
The long arrow indicates the line of traction of the tensor tympani; the short
curved one, that of the stapedius. (Landois.)

external auditory canal. The pinna is the external visible
portion, and consists of the large cavity, the concha, into
which the external auditory canal opens externally ;
of two prominent ridges partly surrounding the concha, the
helix outside and the antehelix internal to this; and of a
fibro-cartilaginous process projecting backward in front of
the concha, the tragus. The external auditory canal runs

32O THE SENSES

inward and slightly forward from the concha to terminate
at the membrana tympani, or drum. Its inner part is in the
petrous portion of the temporal bone; its external part is
fibro-cartilaginous in structure. The whole is lined by in-
tegument.

The Middle Ear (Tympanum). — This is a cavity at the
bottom of the external auditory canal in the petrous portion
of the temporal bone, containing ossicles for the conduction
of sound waves to the internal ear. The cavity communi-
cates, through the Eustachian tube, with the pharynx, and
this is its only direct connection with the external air, though
it does communicate with the mastoid air cells. It is lined by
mucous membrane. The membrana tympani, separating it
from the external auditory canal, is fibrous in structure. It
is lined externally by skin and internally by mucous mem-
brane.

The three ossicles of the middle ear are the malleus, incus
and stapes. The malleus, shaped like a hammer, is attached
in a vertical direction to the upper radius of the
membrana tympani, and articulates by its head with the
incus. The incus has the shape of an anvil ; its base articu-
lates with the malleus, while its small extremity curves
downward to articulate with the neck of the stapes. The
base of the stapes is applied to the membrane covering the
fenestra ovalis. The tensor and laxator tympani are at-
tached to the neck of the malleus ; the stapedius to the neck
of the stapes. These bones constitute a chain, which con-
veys the vibrations of the membrana tympani to the fenestra
ovalis.

The Internal Ear (Labyrinth). — This consists of a series
of cavities in the petrous portion of the temporal bone lined
by a peculiar membrane. When the bony substance sur-
rounding these cavities is carefully removed it is found
that that portion immediately outside them is harder than
the adjacent structure. This constitutes the bony labyrinth,
while the membrane inside the bony walls is the membranous
labyrinth.

THE BONY LABYRINTH

32I

The bony labyrinth consists of the vestibule, cochlea and
semicircular canals. The vestibule occupies the mid-portion

FIG. 91.

/, Transverse section of a turn of the cochlea; II, A, ampulla of a semicircu-
lar canal with the crista acustica; a, auditory cells; p, provided with a fine hair;
T, otoliths; ///, scheme of the human labyrinth; IV, scheme of a bird's laby-
rinth; V, scheme of a fish's labyrinth. (Landois.)

of the labyrinth, and is that part with which the middle ear
communicates by the fenestra ovalis; it communicates also

THE SENSES

with the cochlea and semicircular canals, and on its internal
aspect are openings for the entrance of some of the branches
of the auditory nerve. The cochlea, shaped like a snail shell,
runs off from the front of the vestibule, winds about two
and a half times around a cone-shaped central axis — the
modiolus — and ends in a blind apex. The canal of the coch-
lea is partially separated into two compartments by a bony
plate, the lamina spiralis.

The basilar membrane completes the septum and divides
the lumen of the cochlea into two canals, the scala tympani
and the scala vestibuli, corresponding in name to the tym-
panic and vestibular openings of the cochlea. The semicircu-
lar canals, three in number — superior, external and posterior
— describe arches from the posterior aspect of the vestibule,
communicating by both their extremities with that cavity.

The membranous labyrinth consists of a special mem-
brane lying inside the bony labyrinth and corresponding in
general outline to the walls of the cavity. It is, however, sep-
arated from the walls by perilymph, and encloses a similar
fluid, the endolymph. It covers the sides of the lamina spir-
alis in the cochlea and completes the septum, besides follow-
ing the wall proper ; and on one side it sends a distinct pro-
cess from the tip of the lamina spiralis to the wall of the ca-
nal, so that there are in reality three divisions of the lumen of
the cochlea. This process is the membrane of Reissner, and
the third canal is the scala media the true membranous coch-
lea. (See Fig. 91.)

Termination of Auditory Nerve. — The membranous laby-
rinth, containing and being suspended in fluid, receives the
terminal filaments of the eighth nerve as well as all the so-
norous vibrations intended for that nerve. When the audi-
tory nerve has reached the base of the internal auditory
meatus it enters the internal ear by two divisions, one for the
vestibule and semicircular canals and the other for the coch-
lea. The vestibular portion again subdivides, sending one
branch to the utricle and superior and horizontal semicircular

FUNCTIONS OF THE COCHLEA 323

canals, and another to the saccule and posterior semicircular
canal. The fibers of the eighth nerve spread out over the
inner surface of the membrane to end in a way somewhat
obscure. The membrane is lined internally by epithelium
whose character differs in different areas. In the region of
distribution of the vestibular portion of the nerve the cells
are of two kinds, hair cells and rod cells. From the inner
ends of the hair cells ciliated processes project into the en-
dolymph; to their outer ends pass the axis cylinders of the
nerve fibers, though the exact mode of connection is not
clear. The rod cells are much more numerous than the hair
cells, but their precise connection with audition is not ap-
parent.

Upon the basilar membrane are the rods of Corti. They
consist of two sets of pillars of varying length, slanting to-
ward each other, thus leaving at their base a space which be-
comes a canal by a longitudinal succession of these pillars.
There are supposed to be about 4,500 elements in the outer
and 6,500 in the inner set of these rods. Intimately associ-
ated with the pillars are large numbers of hair cells with
which the auditory nerve filaments may communicate; it is
certain that these filaments are closely connected in some way
with the pillars.

Functions of the Semicircular Canals. — The use of these
is obscure. Their destruction is not followed by interference
with hearing, although auditory filaments are distributed
to some parts of them. Curiously enough, however, this
lesion is one of the three chief ones interfering so markedly
with equilibrium — the phenomena following it being not un-
like those sequent upon lesions of the cerebellum and the pos-
terior white columns of the cord.

Functions of the Cochlea. — While the exact mechanism of
the production of auditory impressions is unknown, there
seems to be no doubt that such mechanism takes place almost
entirely in the cochlea, and that fibers which convey to the
auditory centers impressions of sound are distributed to the

324 THE SENSES

•

organ of Corti therein. That is to say, loss of the sense of
hearing supervenes upon destruction of this part of the in-
ternal ear. In physics it is known that for a sound, for ex-
ample of a piano string, to be heard the membrana tympani
must vibrate in unison with the sonorous vibrations of the
cord; that is, "consonating bodies" repeat sonorous vibra-
tions, giving them their proper pitch and quality. It has
been supposed that the thousands of rods of Corti, of vary-
ing length and size, in the cochlea are made to vibrate separ-
ately or in correctly associated collections (like the strings of
a harp), and thus reproduce communicated vibrations, and
so -give rise to impressions which, conveyed by the auditory
nerve to the center, are there recognized as sounds of differ-
ent degrees of intensity, pitch and quality. This theory may
be true, but its correctness is probably beyond the range of
experimental proof.

While the usual mode of conduction of sound waves to the
cochlea is through the external ear, they may reach it in
other ways, as through the bones of the head, or through the
Eustachian tube. Nor is the integrity of the membrana tym-
pani actually necessary to the production of sound ; although
practically speaking a person in whom this organ is de-
stroyed is deaf, he can hear if the ossicles can in some way
be placed in vibration by sound waves, as by the intervention
of an artificial membrane. Indeed it has already been seen
that none of the parts of the external or middle ear are actu-
ally necessary to hearing. They are only accessory conveni-
ences for the better transmission of impressions to the fila-
ments of the auditory nerve.

The (so-called) tensor and laxator tympani muscles
make tense or lax the membrana tympani, thus influencing
the rapidity and amplitude of its vibrations, and therefore
the pitch and intensity of the sound. The stapedius pre-
vents too great movements of the stapes. The free com-
munication of the air in the tympanum with that in the mas-
toid cells and pharynx insures an approximately constant

THE PRODUCTION OF THE VOICE 325

internal pressure upon the membrane, and thus precludes ac-
cidents which would otherwise interfere with its proper vi-
bration.

The auditory center in man is in the first and second tem-
poral convolution of the temporo-sphenoidal lobe.

Briefly then, the physiology of hearing is as follows:
Sound waves collected by the pinna enter the external audi-
tory canal and impinge upon the membrana tympani. The
drum is thus set to vibrating and communicates its move-
ments to the ossicles, which in turn hand them over through
the fenestra ovalis to the fluids of the internal ear, through
which media they reach the auditory filaments, are conducted
to the brain and given proper recognition.

The Production of the Voice.

The production of the voice is not connected with the spe-
cial senses, but its consideration will be introduced here for
the sake of convenience.

The Larynx is the organ of voice. It is a cavity closed
except for its openings above and below. It consists of four
cartilages — cricoid, thyroid and two arytenoid — joined to-
gether by ligaments and muscles. The vocal cords are at-
tached posteriorly to the bases of the movable arytenoid car-
tilages and anteriorly to the angle between the alae of the thy-
roid. The muscles serve to move the cartilages and thus to
separate or approximate and to render lax or tense the vocal
cords.

Production of Sound. — The human voice is produced by
vibrations of the vocal cords, which vibrations are set up by
currents of expired air.

Movements of the Vocal Cords. — These are those taking
place (i) in respiration, and (2) during vocalization.

i. In Respiration. — When the cords are "passive" they are
approximated anteriorly and separated posteriorly, so that
the interval between them (rima glottidis) is triangular. This

THE SENSES

interval becomes a little wider during inspiration and a little
narrower during expiration.

2. In Vocalization. — The production of sound in the larynx
involves an approximation of the cords and an increase in
their tension. They are made more nearly parallel by the
approach of the arytenoids to each other, and the rima glot-
tidis assumes the shape of a mere chink. The tenser the
cords, the higher the note produced ; usually also the closer
the cords are brought together, the higher the note. The
range of the voice depends principally on the degree of ten-
sion which the cord can be made to assume.

Varieties of Vocal Sounds. — These are mainly (i) mo-
notonous, (2) transitional, (3) musical.

1. In monotonous sounds the notes have all nearly the
same pitch, as in reading.

2. In transitional sounds there is a gradual change in the
tension and approximation of the cords, so that the notes be-
come successively higher or lower, as in the howling of a
dog.

3. In musical sounds the vocal cords have a definite num-
ber of vibrations for each successive note — a number corres-
ponding to the production of that note in the musical scale.

The range of the average human voice is from one to
three octaves. The highest and lowest notes of females are
about one octave higher than the corresponding notes of
males. The chief difference between male and female
voices is, therefore, one of pitch; but they also differ materi-
ally in tone. The difference in pitch is a result of the differ-
ent length, and therefore the different rate of vibration, of
the cords in the two sexes. The female cords are about
two-thirds the length of the male.

Before puberty the male larynx resembles the female, but
at that period the alae of the thyroid becomes more promi-
nent in the male and the cords increase in length, thus ac-
counting for the change of voice.

In old age control of the musculature of the larynx is

SPEECH 327

partly lost, the cords become altered and the cartilages ossify.
These circumstances make the voice weak and unsteady.

Speech. — Modifications and alterations of the sounds pro-
duced in the larynx during and after their production result,
under the influence of the sensorium, in articulate speech.
These modifications are made chiefly by the tongue, teeth,
and lips.

The speech sounds are divided into vowels and consonants.
The distinction is that the vowel sounds are generated in the
larynx, while the consonant sounds are produced by altera-
tions in the current of air above the larynx, and cannot be
pronounced except constantly with a vowel. The current is
modified mainly by the tongue and teeth in the formation of
linguals and dentals, by the cavity of the nose in case of na-
sals, and by changes in the shape and size of the oral cavity
in the production of other sounds.

Nervous Supply of the Larynx. — The superior laryngeal
branch of the teeth is the sensory nerve, which guards the
glottis to prevent the entrance of foreign bodies. Impres-
sions made on the filaments of this nerve are reflected
through the medulla and inferior laryngeal branch of the
tenth to the muscles which close the glottis. The inferior
laryngeal also innervates the muscles that vary the tension
of the cords, and the superior laryngeal keeps the mind in-
formed of the state of the muscles and of the necessity for
forced expiration or coughing.

•*»-•• I

CHAPTER XIII.
REPRODUCTION.

VERY many facts in our knowledge of reproduction de-
pend on observations made upon lower animals, but there is
sufficient analogy between the known facts connected with
human reproduction and development and those of the same
stages in other groups of beings to enable us to present, as at
least approximately accurate, certain broad principles regard-
ing the process as it pertains to the human race.

In order that a human being may be brought into exist-
ence it is necessary that there be a union of the male ele-
ment, the spermatozoon, and the female element, the ovum.
Both these sexual cells are developed from epithelium — the
spermatozoon from that of the seminiferous tubules of the
male, and ovum from the germinal layer of the ovary.

In what follows reference will be had to reproductive pro-
cesses in the human being.

Spermatozoa. — Human spermatozoa (Fig. 92) are elon-
gated bodies, about one five-hundredth of an inch in length,
and consist of three parts, head, mid-portion and tail. The
last-named part is about four-fifths the length of the entire
spermatozoon. The head is egg-shaped and much the thick-
est part of the element. A slender filament, the axial fiber,
extends throughout its length from head to tail and projects
slightly beyond the latter. Spermatozoa are possessed of
wonderful vitality. They live for several weeks in the geni-
tal passages of the female. In the male genital passages they
may live for months in a quiescent state. The nucleus is the
fertilizing agent. Spermatozoa are also remarkable for
their power of locomotion, which is effected by lashing?
and rotary movements of the tail.

328

OVA

329

Ova. — The ovum (Fig. 93), or female sexual cell, is the
largest cell to be found in the human body. Its diameter is
about M.25 of an inch. Its structure is that of a typical cell.
When the ovary is developing a part of its covering epithel-

Ttl

FIG. 92. — Spermatozoa.

i, human ( X 600), the head seen from the side; 2, on edge; k, head; m.
middle piece; f, tail; e, terminal filament; 3, from the mouse; 4, bothriocephalus
latus; 5, deer; 6, mole; 7, green woodpecker; 8, black swan; 9, from a cross be-
tween a goldfinch (m.) and a canary (f.) ; 10, from cobitis. (Landois.")

ium dips down into the substance of the organ and become?
walled off by the union of the surface cells above it. A
part of this ball of epithelium becomes the ovum, and a part
the Graafian follicle for that ovum. The youngest ova are
thus found nearest the surface of the ovary. The cell has an
enveloping membrane, the vitelline membrane, a protoplasm,
the mtellus, a nucleus, the germinal vesicle, and a nucleolus,

330

REPRODUCTION

the germinal spot. Outside the ovum, but not strictly a part
of it, is the zona pellucida, a transparent envelope, and out-
side the zona pellucida a collection of cells, the corona radi-
ata. The perivitelline space is between the ovum proper and
the zona pellucida. The zona presents a radial striae, which
may facilitate the entrance of the spermatozoon.

Ova are capable of being impregnated as long as '7-9 days
after their discharge from the ovary. Their formation be-
gins early in fetal life. The ovum possesses no power of in-
dependent motion. It is pass-
ive in fecundation ; it is sought
by the male element. Its vitel-
lus, or yolk (protoplasm),
contains nutritive non-living
material, deutoplasm, whose
function is to furnish food
substance to the impregnated
ovum until the fetal circulation
is established. Deutoplasm in
the human ovum is scarcely to

,-, ,_, be distinguished from the liv-

FIG. 93.— Ovum. (From Yeo . &. 11-

after Robin.) mg protoplasm, though in the

ova of birds, e. g.. it is clearly

a, zona pellucida and yitelhne mem- i • •

brane; b, yolk; c, germinal vesicle or marked Off, and Constitutes the
nucleus; a, germinal spot or nucleo- • r n r .1

lus; e, interval left by the retraction main bulk of the mature egg,

of the viteHus from the zona pellucida. c;_ ,^ developing embryo

receives no blood from the mother.

Graafian Follicles.-The Graafian follicles are directly
concerned in the development and maturation of ova. These
are small vesicles in the cortical ovarian substance sur-
rounded by a capsule of thickened ovarian stroma, the
tunica vasculosa. Inside the tunica vasculosa, lining the
spherical cavity of the vesicle, are several layers of epithelial
cells making up the membrana granulosa. The cavity is
filled with an albuminous liquid, the liquor folliculi. At one
point in its circumference the membrana granulosa is much

GRAAFIAN FOLLICLES

331

thickened, and in this thickened portion is imbedded the
ovum, The epithelial cells of the membrana completely sur-
round the ovum, constituting the discus proligerus. The

FIG. 94. — Section of the ovary of a cat, showing the origin and devel-
opment of Graafian follicles. (From Yeo after C'adiat.)

a, germ epithelium; b, Graafian follicle partly developed; c, earliest form of
Graafian follicle; d, well-developed Graafian follicle; e, ovum; f, vitelline mem-
brane; g, veins; h, i., small vessels cut across.

cells of the discus next the ovum have their long axes at
right angles to the circumference of the egg, and this layer

332 REPRODUCTION

is the corona radiata already mentioned. The zona pel-
lucida is just underneath the corona.

Usually a Graaiian follicle contains only one ovum. The
follicles and their contained ova begin to be formed early in
fetal life. Probably none are newly formed after the child
is two years old, but they are undeveloped before puberty.
It is estimated that some 72,000 follicles and ova exist in
the two ovaries of the average woman; but of these not
more than 400 reach full development, the others undergoing
retrograde changes and disappearing.

Up to puberty the follicles and ova are small, but at that
time some of them begin to enlarge, and at more or less
regular intervals one of these follicles bursts and allows the
escape of its contained ovum into the fimbriated extremity of
the Fallopian tube — a process known at ovulation. Previ-
ous to its rupture the Graafian follicle has been enlarging.
It is always located in the cortical part of the ovary, but it
may now not only form a distinct protrusion above the sur-
face of the organ, but may by its size encroach upon the me-
dullary portion. It may at this time have a diameter of half
an inch. Meantime the more superficial part of the tunica
vasculosa has been undergoing fatty degeneration, has lost
its blood supply and become very thin. Hlere rupture oc-
curs, and the mature ovum, ready for impregnation, escapes
upon the surface of the ovary.

Corpus Luteum. — When the ovum has been extruded hem-
orrhage occurs, filling the empty follicle with blood. By
contraction of the extra-vesicular adjacent tissue the walls
of the Graafian follicle become folded into the cavity. Soon
proliferation of the cells of the follicular wall takes place
into the blood clot, vascular loops are formed, and the tunica
vasculosa itself becomes greatly hypertrophied. The clot
.later disappears and the mass then has a yellowish color and
is known as the corpus luteum.

Whether or not the ovum that escaped from the follicle
which was the antecedent of any given corpus luteum was

CORPUS LUTEUM

333

impregnated, has an influence upon the growth of that cor-
pus. If the ovum failed of fecundation the corpus luteum
will reach its highest development in about fifteen days, and
will then assume the character of cicatrical tissue and be ab-
sorbed in a few weeks. If the ovum is fecundated, the cor-
pus luteum will increase in size for some three months, until
it may be half the size of the ovary. At labor it has been re-
duced to a white cicatrix, which probably persists as a small
nodule throughout life. The differences between the cor-
pora lutea pf menstruation and pregnancy are shown by the
following table from Dalton:

Corpus Luteum
of Menstruation.

Corpus Luteum
of Pregnancy.

At the end of
three weeks.

One month.

Two months.

Four months.

Six months.

Nine months.

Three-quarters of an inch in diameter; central
clot reddish ; convoluted wall pale.

Smaller ; convoluted
wall bright yellow ; clot
still reddish.

Reduced to the con-
dition of an insignificant
cicatrix.

Absent or unnotice-
able.

Absent.

Larger; convoluted wall
bright yellow; clot still
reddish.

Seven-eighths of an inch
in diameter ; convoluted ;
wall bright yellow; clot
perfectly decolorized.

Seven-eighths of an inch
in diameter; clot pale and
fibrinous; convoluted wall
dull yellow.

Still as large as at the
end of second month; clot
fibrinous ; convoluted wall
paler.

Half an inch in diame-
ter; central clot converted
into a radiating cicatrix;
external wall tolerably
thick and convoluted, but
without any bright yellow
color.

334 REPRODUCTION

Maturation. — But previous to its discharge from the
Graafian follicle, the ovum undergoes certain changes — a
ripening process — whereby it is made ready to receive and
be impregnated by the spermatozoon. This maturation con-
sists in the discharge from the cell proper of a part of its
nucleus and a part of its protoplasm. The nucleus
moves toward the periphery, and the perinuclear
membrane is lost. As the nucleus approaches the surface
of the egg it undergoes karyokinesis, and a part of it, to-
gether with a little surrounding protoplasm, is extruded and

FIG. 95. — The 'fertilized ovum, or blastophere. (Kirkcs.)

finds itself in the perivitelline space. This is the first polar
body. A second polar body is likewise later discharged by
karyokinetic division. (See Fig. 95.)

The object of this extrusion and the final fate of the polar
bodies are matters of speculation. That portion of the nu-
cleus which remains after the polar bodies have been thrown
off finds its way back to the center of the ovum. It soon de-
velops a covering membrane, and is now the female pronu-
cleus, ready for union with the male pronucleus. It is about
the time of the completion of this process that the follicle
ruptures and the discharge of the ovum — ovulation — occurs.

Ovulation. — It is supposed that from puberty to the meno-
pause one (or more?) ovum is discharged at tolerably regu-

MENSTRUATION 335

lar intervals of about four weeks. It should, and usually
does, enter the outer end of the Fallopian tube, to be con-
veyed toward the uterus. Obviously only a few, and some-
times none, are ever impregnated. Should the ovum fail to
reach the uterus and become fecundated, ectopic gestation
will be the result.

The patent fimbriated extremity of the tube may grasp
the ovary at the time of rupture of the Graafian follicle, but
this is not probable. One of the tubal fimbriae is attached to
the outer extremity of the ovary and has on its surface a
small linear depression lined by ciliated epithelium and lead-
ing to the tube. The ovum very likely in most cases drops
into this depression, and the -influence of the cilia is to carry
it toward the tube.

Menstruation. — Usually between the fourteenth and sev-
enteenth years of female life menstruation begins. It is a
discharge of blood, epithelium and other parts of the mu-
cous membrane of the uterine cavity, together with mucus
from the glands of the uterus and vagina. About the be-
ginning of menstrual life there are marked changes in bodily
development, Graafian follicles enlarge and begin to approach
the surface, ovulation is begun, and the female is capable of
being impregnated.

In most cases menstruation occurs at regular intervals of
twenty-eight days. The function is suspended during preg-
nancy and usually during lactation. When it is first estab-
lished it is frequently irregular in its occurrence for several
months ; a like irregularity usually accompanies the cessation
of the function between the fortieth and fiftieth years — when
the menopause, or climacteric, is established. The normal
female may be impregnated during menstrual life, but not
before or after.

The average length of time for which the menstrual flow
continues is four days. There are many exceptions in both
directions for different women, but the time for any one
woman probably varies little under normal conditions. The

REPRODUCTION

discharge for each period averages some five ounces. It does
not usually coagulate, on account of the presence of alkaline
mucus. For five or six days preceding the flow, the uter-
ine mucous membrane gradually thickens, the glands be-
come longer and more tortuous, the connective tissue cells
multiply and the blood-vessels are greatly increased in- size.
This is apparently a preparation for the reception of the im-
pregnated ovum. A short time before the flow begins there
is hemorrhage into the subepithelial tissue, possibly by dia-
pedesis, possibly by rupture. In a day or so the super jacent
mucous membrane becomes disintegrated and is discharged
with the included parts of the glands. The underlying ves-
sels, being thus exposed, rupture and the sanguineous dis-
charge carries away the debris.

For three or four days subsequent to the cessation of the
flow the uterine mucosa is being repaired. The deeper lay-
ers, including the deeper portions of the glands, were not
cast off, and the whole is reconstructed from the intact parts.
Following the reconstructive period there is a stage of qui-
escence lasting some two weeks, until six or seven days prior
to the next menstruation.

At the beginning of each menstrual flow there is general
congestion of the pelvic viscera and mammary glands, ac-
companied usually by headache and a sense of pelvic oppres-
sion. The congestion and discomfort begin to disappear
when the flow is established.

Ovulation probably in most cases takes place just before
the menstrual flow begins, but neither occurrence is depen-
dent upon the other. Ovulation has frequently been shown
to take place in the inter-menstrual period, but the conges-
tion of the reproductive organs incident to menstruation
probably hastens the rupture of any -Graafian follicle which
at that time happens to be near the completion of its devel-
opment.

The relations between ovulation, menstruation and im-
pregnation are not definitely determined. Pregnancy lasts

IMPREGNATION 337

for ten lunar months and dates from the time of impreg-
nation (conception), but that time cannot in any case be
fixed upon with precision. The vitality of the ovum is
thought not to last longer than seven days unless impreg-
nated, and if impregnation is to occur, it must take place
within the first week after ovulation. Since, therefore, ovu-
lation and menstruation usually occur together, and since im-
pregnation probably occurs about the beginning of menstru-
ation, we reckon from the first day of the last menstruation
280 days forward to determine the probable time of labor.
This is equivalent to adding nine calendar months and seven
days to the first day of the last menstrual period. It is evi-
dent that this calculation at best gives only the approximate
time.

While fertilization probably occurs at the time mentioned,
the spermatozoon effecting fecundation may have been in the
female genital tract for weeks. Its vitality here is so pro-
longed that the time of its deposit with reference to men-
struation very probably has little to do with whether or not
conception shall occur.

Impregnation. — The term impregnation, or fertilisation,
or fecundation, is used to signify that union of the male and
female sexual cells which makes possible the development
of a new human being. Normally impregnation takes place
in the Fallopian tube, and almost always in the outer third.
The male element, the spermatozoon, seeks and penetrates
the female element, the ovum. It is the blending of the nu-
clei (pronuclei) which is essential. Spermatozoa in large
numbers swarm around the ovum and several at least enter
the perivitelline space. Only one, however, is destined usu-
ally to enter the ovum. As it approaches the vitelline mem-
brane, head first, the protoplasm of the ovum swells up into
a prominence to meet it. The fertilizing spermatozoon makes
its way through the vitelline membrane, losing its tail in the
passage, and becomes the male pronucleus. The female
pronucleus now advances from its central position to meet

REPRODUCTION
\

the male element, and they coalesce to become the segmenta-
tion nucleus. Impregnation has now taken place. The seg-
mentation nucleus represents a new being. It contains ana-
tomical elements from both parents, and it is not surprising
that the child should resemble both, anatomically and other-
wise.

The term "ovum" has so far been used to signify the un-
impregnated sexual cell discharged from the female ovary.
It is also used to signify the fertilized cell, and is in fact
often applied without much precision to the product of con-
ception at almost any stage of its intrauterine development.

The fertilized ovum is carried through the tube to the
uterus, arriving there some seven days after its fecundation.
In its passage it becomes covered with a coating of albu-
minous material. This layer is probably impervious to sper-
matozoa— which fact may account for the practical univer-
sality of fecundation in the outer part of the tube, if at all.
The coating corresponds to the white of an egg, in that it
penetrates the perivitelline membrane and furnishes nutritive
material to the vitellus. On reaching the uterus the ovum
becomes attached to and covered by the thickened mucous
membrane of that organ in a way to be noted presently.
Here it remains until expelled during parturition.

Segmentation. — As soon as union of male and female
pronuclei has taken place, cleavage of the ovum begins. The,
nucleus (segmentation nucleus) and protoplasm divide kary-
okineses to form two nearly similar cells. These two divide
into four, these four into eight and so on, till a large number
of cells occupy the vitelline space and are all surrounded by
the perivitelline membrane. As division proceeds, cells ar-
range themselves around others, so that the former occupy
the circumference and the latter the center of the vitelline
cavity. Later, while the outer cells constitute a layer cover-
ing the entire inner surface of the perivitelline membrane, the
inner cells group to form a mass which is in contact with the
outer layer at one point only — like a ball lying in a relatively

SEGMENTATION

339

large hollow sphere. The space thus left between the two
kinds of cells is called the segmentation cavity. Soon the sur-
rounding cells become attenuated (Rauber's 'cells) and dis-

FIG. 96.— Sections of the ovum of a rabbit, showing the formation o«t
the blastodermic vesicle. (From Yeo after E. Van Beneden.)

a, b, c, d,- are ova in successive stages of development; z.p., zona pellucida;
ect, ectomeres, or outer cells; ent, entomeres, or inner cells.

appear. Their place, as a surrounding envelope, is taken
by some of the cells of the inner layer. This second sur-
rounding layer is the epiblast, or ectoderm; the surrounded
mass is the hypoblast, or entoderm.

340

REPRODUCTION

Before long the entoderm spreads out over a larger area,
and from it and from the ectoderm is developed a layer of
cells, the mesoblast, or mesoderm, which occupies a position
between the other two layers. The three-layered germ is
now the blastodermic vesicle, or the gastrula, and its cavity
is the archenteron, or celenteron. From these three germ
layers are developed all the parts of the body by the forma-
tion of folds, ridges, constrictions, etc., and by various meta-
morphoses which have as their end the adaptation of struc-
ture to function.

Derivatives of the Germ Layers. — According to Heisler
these are :

From the ectoderm: (i) The epidermis and its append-
ages, including the nails, the hair, the epithelium of the se-
baceous and sweat glands and the epithelium of the mam-
mary gland. (2) The infoldings
of the epidermis, including the
epithelium of the mouth and
salivary glands, of the nasal
tract and its communicating cav-
ities, of the external auditory
canal, of the anus and anterior
urethra, of the conjunctiva and
anterior part of the cornea, the
anterior lobe of the pituitary
body, the crystalline lens and the
enamel of the teeth. (3) The
spinal cord and brain with its
outgrowths, including the optic
nerve, the retina and the pos-
terior lobe of the pituitary body.
(4) The epithelium of the inter-
nal ear.

From the entoderm: The epithelium of the respiratory
tract, of the digestive tract (from the back part of the phar-
ynx to the anus, including its associated glands, the liver and

FIG. 97.— Impregnated egg.

With commencement of forma-
tion of embryo; showing the area
germinativa or embryonic spot,
the area pellucida, and the primi-
tive groove and streak. (Kirkes
after Dalton.)

DEVELOPMENT OF MESODERM 341

pancreas), of the middle ear and Eustachian tube, of the
thymus and thyroid bodies, of the bladder and first part of
the male urethra and of the entire female urethra.

From the mesoderm: (i) Connective tissue in all its
forms, such as bone, dentine, cartilage, lymph, blood, fibrous
and areolar tissue; (2) muscular tissue; (3) all endothelial
cells; (4) the spleen, kidney and ureter, testicle and its ex-
cretory ducts, uterus, Fallopian tube, ovary and vagina.

The Embryonal Area. — Soon after the germ reaches the
uterus (probably) there appears on its surface an oval whit-
ish spot, the embryonal area. The impregnated ovum is still
in the shape of a vesicle. It is from the embryonal area
alone that the body is developed. The other parts are acces-
sory. Longitudinal division of this area is supposed to give
rise to twins of the same sex and of almost identical struc-
ture. Running in the long diameter of the embryonal area
is a marking, the primitive streak, in which is a longitudinal
depression, the primitive groove. (Fig. 97).- These sur-
face markings are caused by thickening of the ectoderm.
(Fig. 98.)

Development of Mesoderm. — It is about this time that the
mesoderm makes its appearance. It begins under the primi-
tive groove and extends in all directions. It originates from
both ectoderm and entoderm, and lies between them. In the
median line the three layers are closely united to each other.
(Fig. 98). At first the mesoderm does not completely em-
brace the germ, but is deficient opposite the embryonal area.

Fig. 94 shows that the cells of the mesoderm make up a
thickened mass near the median line, but farther away they
constitute two distinct lamellae. The mass near the median
line is the vertebral or axial plate. The outer of the lateral
lamellae is the somatic mesoderm,; the inner is the splanchnic
mesoderm. The ectoderm and somatic mesoderm unite to
form the somatopleure ; the entoderm and splanchnic meso-
derm unite to form the splanchnopleure. The interval left
between the somatopleure and splanchnopleure is the

342

REPRODUCTION

or body cavity. (Fig. 98.) The great serous cavities of the
body are developed from it.

Beginning Differentiation.— It thus appears that the em-

bryo is beginning to develop from the simple vesicle into
specialized parts.

We shall notice briefly the development of the body pro-

NEURAL CANAL

343

per, and the extra-embryonic accessory structures, the um-
bilical vesicle, amnion, allantois and placenta. As regards
the embryonic body, some of the most prominent occurrences

connected with its development consist in the formation of
the neural canal, chorda dorsalis, or notochord, and meso-
blastic somites.
Neural Canal. — About the fourteenth day, along under-

344

REPRODUCTION

neath the primitive groove, the cells of the ectoderm become
thickened to form the medullary plate. The edges of this
longitudinal plate soon begin to curl up, and thus form the
medullary furrow, or groove. (Fig. 99.) The margins of
the adjacent ectoderm are carried up with the curling edges,
and constitute the medullary folds. Later the edges of the
medullary plate meet each other, and join to form a closed
canal, the neural, or medullary canal. The edges of the

a.O

FIG. 100. — Transverse section through dorsal region of embryo chick

(45 hours).

One-half of the section is represented; if completed it would extend as far to
the left as to the right of the line of the medullary canal (Me). A, epiblast;
C, hypoblast, consisting of a single layer of flattened cells; Me, medullary canal ;
Pv, protovertebra; IV d, Wolffian duct; So, somatopleure; Sp, splanchnopleure;
pp, pleuroperitoneal cavity; eh, riotochord; ao, dorsal aorta, containing blood-
cells; v, blood-vessels of the yolk-sac. (Kirkes after Foster and Balfour.)

medullary folds unite above, so that the neural canal comes
to lie underneath the surface ectoderm. (Fig. 100.) The
neural canal is the forerunner of the whole nervous system.
Chorda Dorsalis. — The method of formation of the chorda
dorsalis, or notochord, is very similar to that of the neural
canal. It is a solid, instead of a cylindrical, longitudinal col-
lection of cells, extending along the dorsal aspect of the
celom. It is developed from the entoderm. A thickening of
the cells of this layer constitutes the chordal plate. Its edges
curl up in a direction opposite to those of the medullary plate

BODY CAVITY 345

and carry with them chordal folds of the entoderm. When
the curling edges have joined to form a solid cylinder of
cells, the chordal folds unite over the ventral surface of the
cylinder. Figures 99 and 100 illustrate these facts. The
notochord is in the line of the future vertebral bodies, but it
is not developed into any adult structure.

Somites. — These are masses of cells developed from the
axial plates of the mesoderm, lying parallel with and on each
side of the notochord. (Fig. 100.) They are in segments,
the formation of which begins in the neck and proceeds
caudad and cephalad. They are sometimes called the pro-
tovertebra. They represent the primitive vertebrae.

The body begins to assume shape and the fetal append-
ages to be developed at the same time. The latter are for
the protection and nutrition of the embryo. The essential
parts of a vertebrate are a vertebral column with a neural
canal above and a body cavity below it. The body cavity
contains the alimentary canal. The somites representing the
vertebral column and the formation of the neural canal have
been noticed.

Body Cavity. — At first the embryo, as represented by the
embryonal area, is on a level with the remaining surface of
the blastoderm. Soon, however, there appears, marking the
head of the embryo and with its concavity backward, a cres-
centic folding in of the blastodermic wall. It is evident on
the surface as a simple furrow. This tucking-in finally sur-
rounds the whole embryonal area, and the surface fissure,
now oval, becomes deeper and deeper, until those portions
of the wall which are being tucked under the embryo ap-
proach each other on its ventral aspect and divide the yolk
into two communicating cavities. (See Figs. 102 and 103.)

The layers of the blastoderm thus folded underneath the
embryo are the visceral plates. They form the boundaries
of a cavity which still communicates in front, at the site of
the future umbilicus, with the yolk-sac. This narrow canal
is the vitelline duct, and the two cavities communicating

346 REPRODUCTION

through the vitelline duct are the future alimentary canal
and the yolk-sac, or umbilical vesicle. It is to be noticed
that the visceral plates embrace both somatopleure and

FIG. 101. — Diagrammatic section showing the relation in a mammal

between the primitive alimentary canal and

the membrane of the ovum.

The stage represented in this diagram corresponds to that of the fifteenth or
seventeenth day in the human embryo, previous to the expansion of the allantois;
c, the villqus chorion; a, the amnion; a', the place of convergence of the amnion
and reflexion of the false amnion; a", a", outer or corneous layer; e, the head
and trunk of the embryo, comprising the primitive vertebrae and cerebro-spinal
axis ; i, i, the simple alimentary canal in its upper and lower portions. Immedi-
ately beneath the right hand * is seen the fetal heart, lying in the anterior part
of the pleuroperitoneal cavity; v, the yolk-sac or umbilical vesicle; vi, the vitello-
intestinal opening; u, the allantois connected by a pedicle with the hinder por-
tion of the alimentary canal. (Kirkes after Quain.)

splanchnopleure, and that it is the ectodermic layers of the
splanchnopleure which finally join to form the gut tract, and
the somatopleure which forms the ventral ancj lateral walls

FETAL MEMBRANES 347

of the body cavity. The gut tract has the shape of a straight
tube occupying the long axis of the embryo and opening into
the umbilical vesicle.

Fetal Membranes.

Umbilical Vesicle. — The umbilical vesicle represents that
part of the vitellus which has not been constricted off to

FIGS. 102 AND 103.

a, chorion with villi. The villi are shown to be best developed in the part of
the chorion to which the allantois is extending; this portion ultimately becomes
the placenta; b, space between the true and false amnion; c, amniotic cavity;

d, situation of the intestine, showing its connection with the umbilical vesicle;

e, umbilical vesicle; f, situation of heart and vessels; g, allantois. (Kirkes.)

form the gut tract. (Figs. 101, 102, 103.) It furnishes
nutriment to the embryo for a short time and is then
largely cut off from the body. It gradually shrivels (Figs.
107, 108), and with that part of the duct external to the ab-
domen is cast off either before or at parturition. Vessels
develop in its walls and absorb the nourishment in it to be
conveyed to the embryo. But in the human being more sat-
isfactory arrangements for nutrition are soon made and its
function ceases.
Amnion. — When the embryo has become depressed, as it

348

REPRODUCTION

were, into the substance of the blastoderm, and while the
body cavity is being formed, the layers of the somatopleure
grow up over the embryo to meet and blend dorsally. (Figs.
107, 108.) The two layers of which the somatopleure is
composed separate, the outer forming the false amnion and

FIG. 104. — Diagram of
fecundated egg.

a, umbilical vesicle; b,
amniotic cavity; c, allan-
tois. (Kirkes after Dai-
ton.)

FIG. 105. — Fecundated egg with allantois
nearly complete.

a, inner layer of amniotic fold; b, outer layer of
ditto; c, point where the amniotic folds come in
contact. The allantois is seen penetrating between
the outer and inner layers of the amniotic folds.
This figure, which represents only the amniotic
folds and the parts within them, should be compared
with Figs. 99, 100, in which will be found the struc-
tures external to these folds. (Kirkes after Dalton.)

the inner the true amnion. The false amnion now coalesces
with the original vitelline membrane to constitute the false
chorion. Evidently there is thus formed a closed cavity, the
amniotic cavity, between the true amnion and the body of
the embryo.

At first the amnion and the embryo are in close contact,
but soon the cavity begins to be distended with the fluid, the
liquor amnii, which increases until it reaches a considerable
quantity. It affords mechanical protection to the fetus dur-
ing intrauterine life, and at labor serves to evenly dilate the
cervix. When this has been accomplished is the usual time
at which the sac ruptures and the liquor amnii escapes. It
also supplies the fetal tissues with water, parts of it being
swallowed from time to time.

THE ALLANTOIS

349

The cavity between the false amnion and the true amnion
is continuous, with the body cavity at the umbilicus.

Allantois. — The allantois grows out from the back part of
the intestinal canal into the celom or the body cavity. (Figs.

FIG. 106. — This and the two following wood-cuts are diagrammatic
views of sections, through the developing ovum, showing the forma-
tion of the membranes of the chick. (Yeo after Foster and Balfour.)

A, B, C, D, E, and F, are vertical sections in the long axis of the embryo at
different periods, showing the stages of development of the amnion and of the
yolk-sac; /, II, III, and IV, are transverse sections at about the same stages of
development; i, ii, and in, give only the posterior part of the longitudinal sec-
tion to show three stages in the formation of the allantois; e, embryo; y, yolk;
pp, pleuroperitoneal fissure; vt, vitelline membrane; af, amniotic fold; al, allan-
tois.

104, 105.) It is of splanchnopleuric origin. It soon be-
comes a membranous sac, the walls of which are very vascu-
lar. It fills the space between the two amniotic folds and
joins the false amnion. Its vessels thus reach the chorion,
which is already establishing vascular connections with the

350

REPRODUCTION

mother. Finally they are distributed only to a certain part
(placenta) of the chorion ; and as the allantoic vessels anas-

FIG. 107.

e, embryo; a, amnion; a', alimentary canal; vt, vitelline membrane; af, amniotic
fold; ac, amniotic cavity; y, yolk; al, allantois.

tomose more and more freely with those of the chorion, the
umbilical vesicle shrivels, as it is no longer needed. The
vessels of the allantois are the two allantoic arteries and the

CHORION

351

same number of allantoic veins. The allantois also receives
the fetal urine.

As the true placental circulation is established and the vis-
ceral plates close the abdominal cavity, the allantois is con-
stricted at the umbilicus so as to be divided into two parts.

FIG. 108. — Diagrammatic sections of embryo.

Showing the destiny of the yolk-sac, ys. vt, vitelline membrane; pp, pleuro-
peritoneal cavity; ac, cavity of the amnion; a, amnion; a', alimentary canal;
ys, yolk-sac.

That outside the body shrivels and is cut away with the um-
bilical cord at birth, while that inside the body becomes the
first part of the male and the whole of the female urethra,
the bladder and the urachus.

Chorion. — The chorion is the outer surrounding mem-
brane of the embryo after the appearance of the amnion. It
consists of three layers. From without inward these are the

352 REPRODUCTION

original vitelline membrane, the false amnion and the allan-
tois. The allantois has been seen to extend around between
the two amniotic folds and to blend with the outer.
From its formation from these several membranes, the cho-
rion evidently consists of the outer ectodermic, inner ento-
dermic and intervening mesodermic strata.

By the time the impregnated ovum reaches the uterus, the
chorion (false at this time) has numerous spike-like projec-
tions— villi — over its whole surface. (Fig. 101.) These are
at first non-vascular, but soon become vascular by the pro-
jection into them of capillaries from the vessels of the allan-
tois. These capillaries probably absorb nutrient matter se-
creted by the uterine glands. .But at the beginning of the
third month the villi become much more highly developed
over a certain part of the surface of the chorion than at
other points, and a more intimate relation is established be-
tween their vessels and those of the mother; here the pla-
centa is to be formed.

The Decidua. — The decidua of pregnancy consists of the
hypertrophied mucous membrane lining the cavity of the
uterus and reflected at a certain point entirely over the de-
veloping ovum. Before the ovum reaches the uterus, the
mucous membrane of the latter has been undergoing
changes, such as are mentioned under Menstruation. If fe-
cundation has not taken place, menstruation occurs and the
mucosa is discharged under the name of the decidua men-
strualis. But if conception has occurred, menstruation does
not ensue and the uterine mucosa becomes much more thick
and spongy. Whether or not it shall be discharged as the
decidua of menstruation or be retained to form the decidua
of pregnancy is probably a point which is decided while the
ovum is yet in the tube.

When the fecundated ovum reaches the uterus it becomes
attached to the mucous membrane, usually a little to one side
of the median line on the posterior wall. The mucous mem-
brane extends over and completely envelops it. This re-

THE DECIDUA

353

fleeted portion is the decidua reflexa; that lining the whole
uterine cavity is decidua vera, while that part of the decidua
vera intervening between the ovum and the uterine wall is

FIG. 109. — Diagrammatic view of a vertical transverse section of the
uterus at the seventh or eighth week of pregnancy.

c, c, c' , cavity of uterus,

the decidua, opening at

is, which becomes the cavity of the ut^UUa,

c, c, the cornua, into the Fallopian tubes, and at c' into the cavity of the cervix,
which is closed by a plug of mucus; dv, decidua vera; dr, decidua reflexa, with
the sparser villi imbedded in its substance; ds, decidua serotina, involving the
more developed chorionic villi of the commencing placenta. The fetus is seen
lying in the amniotic sac; passing up from the umbilicus is seen the umbilical
cord and its vessels passing to their distribution in the villi of the chorion; also
the pedicle of the yolk-sac, which lies in the cavity between the amnion and
chonon. (Kirkes after Allen Thomson.)

the decidua serotina and becomes the maternal part of the
placenta.

354 REPRODUCTION

Of course there is at first a considerable cavity left be-
tween the reflex and the vera, but as the embryo increases in
size the space becomes smaller and is obliterated by the end
of the fifth month. After this time both vera and reflexa
undergo retrograde changes due to pressure and become
closely attached to the chorion. They are discharged with
the membranes at birth.

Placenta. — The placenta is the organ of nutrition for the
fetus after about the end of the third month. Through it the
vessels of the fetus and those of the mother are brought
into most intimate relations.

It has been said that the villi of the chorion in one locality
become very highly developed. This is at the site of the
reflection of the decidua serotina and is the chorion fron-
dosum. The union of these, with certain other develop-
ments, constitutes the placenta.

The decidua serotina becomes very spongy. It is filled
with sinuses, into which the enlarged villi of the chorion
frondosum project. The sinuses are filled with maternal
blood, while the capillaries of the villi contain fetal blood.
There is no direct connection between the vessels of mother
and child, but the thin lining of the villi and sinuses allows
free interchange of materials by osmosis.

It seems that the interchange is under the influence of two
sets of cells, each disposed in a single layer — one belonging
to the maternal and the other to the fetal part of the pla-
centa. These layers of cells are situated on either side of
the membrane of the villus. They seem to take out of the
maternal blood materials needed for the nutrition of the fe-
tus, and out of the fetal blood materials which require re-
moval. The maternal blood performs both alimentary and
respiratory functions for the fetus.

The placenta as a whole is discoid in shape. Its fetal sur-
face is concave and covered by the amnion. The mass has a
diameter of 4-5 in., and a thickness of half an inch. The
villi receive blood from the allantoic or umbilical arteries ; it
is returned by the umbilical vein.

UMBILICAL CORD 355

At labor uterine contractions detach the placenta and the
decidua and expel them from the womb. The separation
takes place in the deeper part of the maternal placenta, or
decidua serotina, so that the mass discharged represents both
the fetal and maternal portions. The vessels entering the
sinuses do so obliquely ; consequently uterine contractions at
birth very effectually check the hemorrhage which separa-
tion of the placenta occasions.

Umbilical Cord. — The umbilical cord is made up of the
vessels which convey blood between the placenta and fetus,
together with the remnants of the umbilical vesicle and allan-
toic stalk, all of which are held together by the jelly of
Wharton, a species of connective tissue.

The outgrowing allantois has developed in it the two al-
lantoic arteries and veins. By the time the placenta is
formed the allantoic stalk has become much elongated, and
the allantoic vessels extend into the fetal placenta (chorion
f rondosum) and become now the umbilical vessels. The two
veins blend to constitute a single umbilical vein, but the ar-
teries remain separate. The vein enters the fetal body at the
umbilicus, passes to the under surface of the liver and di-
vides in a manner to be noted presently. After birth the
intra-abdominal portion atrophies, and is the round liga-
ment of the liver. The two umbilical arteries issue at the
umbilicus. Their intra-abdominal portions are the fetal hy-
pogastric arteries.

The average length of the umbilical cord is about twenty-
one inches. It appears to be twisted on account of the spiral
course of its relatively long arteries. It is usually attached
near the center of the fetal surface of the placenta.

Condition of the Fetal Membranes at Birth. — The mem-
branes discharged with the placenta at birth are, from with-
out inward, the decidua vera, decidua reftexa, chorion and
amnion. The amniotic fluid, in which the fetus floats,
reaches its maximum amount at about the sixth month. It
is sufficient then to force the amnion closely against the cho-

356 REPRODUCTION

rion, covered by the decidua reflexa; these last named (cho-
rion and reflexa) are in turn forced everywhere against the
decidua vera. The result is that all four become practically
one membrane, though the union between amnion and cho-
rion is not so close as that between the other layers. These
membranes constitute, then, a sac filled with fluid. The sac
is ruptured in labor, and the child escapes through the rent.
Afterward the decidua vera and placenta are detached, and
escape together as the "after birth."

Development of the Circulation. — The development of the
circulation may be considered in these stages : ( i ) Vitelline
circulation, (2) placental circulation, (3) adult circulation.
The heart is the propelling organ in all these.

i. Vitelline Circulation. — The blood and the vessels make
their appearance almost as early as the primitive groove.
Certain blastodermic cells are transformed into both red and
white corpuscles. They are larger than the adult's cells and
both are nucleated. Blastodermic cells also group to form
small tubes, which constitute the area vasculosa. At the
same time mesoblastic cells develop two tubes, one along
each side of the body, which soon unite to form a single one,
representing the heart. It becomes enlarged and twisted
upon itself, and pulsations begin in it at a very early date.
The heart is in the median line and gives off two arches
which unite below to form the abdominal aorta. From the
arches pass branches to the area vasculosa, which now form
a nearly circular plexus around the embryo. Two of these
branches, larger than the others, enter the umbilical vesicle
and become the omphalo-me sent eric arteries; these are two
corresponding veins. This circulation through the ompha-
lo-mesenteric vessels and the area vasculosa does not con-
tinue long in the human being. As soon as the allantois
is formed and the placental circulation begins to be set up,
the omphalo-mesenteric vessels are obliterated and the place
of the first circulation is taken by the second.

Development of the Heart. — The tube just mentioned as

PLACENTAL CIRCULATION 357

representing the heart has communicating with it two veins
at its lower extremity and two arteries at its upper. Soon
the tube becomes twisted upon itself so that the upper (ar-
terial) is thrown in front of the lower (venous). The loop
is V-shaped and is the outline of the future ventricles. Af-
terward a constriction forms the auricle. At this time the
heart consists of a single ventricle and a single auricle.
Later the ventricular and auricular septa are formed. The
latter appears after the former and is incomplete ; the open-
ing left between the auricles is the foramen ovale.

2. Placental Circulation. — As the allantois is developed
and the vitelline circulation is abolished, the hypogastric ar-
teries are given off first from the aorta, but later (with the
development of the vessels of the lower extremities) they
are pushed down, as it werej so that they take origin from
the internal iliacs. They pass to the umbilicus and thence to
the placenta by the cord. Blood is at first returned from the
placenta by two umbilical veins, but these soon fuse into one.

Object of Placental Circulation. — Since the activity of the
respiratory and alimentary tracts has not been established,
their functions must be performed by those of the mother
and the necessary materials supplied from her blood. Con-
sequently there must be a continual passage of fetal blood to
and from the placenta to discharge effete matter and to ab-
sorb nutriment. Certain modifications of the circulatory ap-
paratus, not requisite after birth, are necessary to bring this
about.

Course of Fetal Circulation. — The umbilical vein contain-
ing blood enriched with oxygen and other materials enters
the body at the umbilicus and passes to the under surface of
the liver. Hbre it divides into two branches. The larger
joins the portal vein and enters the liver ; the smaller is the
ductus venosus, which enters the ascending vena cava.

The ascending vena cava, when it enters the right auricle,
therefore, contains blood from the lower extremities, blood
which has come from the placenta directly through the

358

REPRODUCTION

ductus venosus, and blood which has come from the pla-
centa indirectly through the liver. Considering that blood

FIG. no. — Diagram illustrating the circulation through the heart and
the principal vessels of a fetus. (From Yeo after Cleland.)

a, umbilical vein; b, ductus venosus; f, portal vein; e, vessels to the viscera;
d, hypogastric arteries; c, ductus arteriosus.

from the body of the fetus is venous and that blood directly
from the placenta is arterial, the contents of the ascending
vena cava are mixed when they enter the heart. The Eus-

PLACENTAL CIRCULATION 359

tachian valve, together with the direction of the entering
current, causes the blood from the ascending vena cava to
pass through the foramen ovale into the left auricle.

Blood from the upper extremities (impure) enters the
right auricle through the descending vena cava. The Eus-
tachian valve and the direction of the current here again
cause this blood to enter the right ventricle. There is sup-
posed to be very little mingling of blood from the two venae
cavae as it passes thus through the right auricle. At the
same time the blood which has entered the left auricle
through the foramen ovale, augmented slightly by blood
from the ill-developed pulmonary veins, passes into the left
ventricle. The ventricles now contract simultaneously.

Blood from the right ventricle (impure) passes in small
part through the pulmonary artery to the lungs, but chiefly
through a tube, the ductus arteriosus, into the descending
part of the aortic arch.

Blood from the left ventricle (mixed) enters the aorta and
goes to the system at large.

The vessels going to the head and upper extremities are
given off from the aortic arch before it is joined by the
ductus arteriosus. Since the ductus arteriosus contains im-
pure blood, the supply going to the upper extremities is
purer than that going to the lower.

Of the blood which passes down the aorta a part leaves
by the hypogastric arteries, to go again to the placenta, while
the other part is distributed to the trunk and lower extremi-
ties.

It thus appears that the liver is the only organ of the
fetus which receives pure blood, and that the head and upper
extremities are better provided for in this respect than are
the lower parts. This may account for the relatively large
liver of the fetus, and for the fact that the upper extremities
are better developed than the lower.

The ductus arteriosus, ductus venosus, foramen ovale,
Eustachian valve, hypogastric (umbilical) arteries and the

360 REPRODUCTION

umbilical vein are the organs which distinguish the placental
circulation, and they all partially disappear after birth, as
will be immediately seen.

3. Adult Circulation. — The circulation as it exists in the
adult has been described. It is only necessary to see what
changes mark its establishment.

When the child is born detachment of the placenta, or
ligation of the cord, stops the placental circulation. The
first noticeable effect comes from the consequent deoxygen-
ation of the blood. The respiratory center is stimulated and
the child gasps to fill the hitherto collapsed lungs with air.
Owing to the diminished resistance in the expanded lungs,
the pulmonary artery begins to carry most of the blood from
the right ventricle, and the ductus arteriosus commences to
atrophy. .Before birth, too, the Eustachian valve becomes
less distinct and the foramen ovale partly closes. At labor
a kind of valve guards the opening of the foramen ovale and
allows the escape possibly of a little blood from the right
into the left auricle, but none in the opposite direction. It
commonly closes about the tenth day of extra-uterine life.
The ductus arteriosus is reduced to the condition of an im-
pervious fibrous cord between the third and tenth days after
birth.

The hypogastric arteries, umbilical vein and ductus ven-
osus are closed between the second and fourth days. That
part of each hypogastric artery between the internal iliac
and the upper lateral part of the bladder remains in adult life
as the superior vesical artery; the part between this point
and the umbilicus is that which atrophies. The umbilical
vein remains as the round ligament of the liver. The ductus
venosus is represented by a fibrous cord in the fissure for
the ductus venosus in the liver.

The Skeleton. — The appearance of the notochord and of
the protovertebrse, or somites, has been observed. The noto-
chord becomes a thin line of soft cartilage, around which the
bodies of the vertebra are developed, though it does not

NERVOUS SYSTEM 361

itself become those bodies. The protovertebrse were seen to
lie longitudinally on either side of the notochord. These
grow around the neural canal dorsally and the notochord
ventrally to form the vertebrae. From them also are de-
veloped the muscles and skin of the back.

The cranium is developed as a modification of the verte-
bral column.

All the bones are in early fetal life cartilaginous or mem-
branous. Centers of ossification appear at one or more
points in each bone.

The bones of the extremities are not at first separate.
They bud out from the upper and lower parts of the trunk,
to be subdivided later.

Nervous System. — The origin of the nervous system has
been indicated in describing the neural canal. The meso-
dermic cells multiply and fill the tube, until only the canal of
the spinal cord is left. Headward the neural canal termin-
ates in a dilated extremity, which soon becomes divided into
three vesicles, anterior, middle and posterior. From these
are developed the different parts of brain. Some of these
parts develop much more rapidly than others, and we thus
account for the predominant size of the cerebrum. At first
there are no cerebral convolutions, but later the cavity of
the cranium seems too small for the brain and the charac-
teristic infoldings occur.

The eye is formed by the projection of the optic vesicle
from the side of the anterior brain vesicle.

The internal ear is formed by the projection of the audi-
tory vesicle from the posterior brain vesicle.

The alimentary canal is formed by being pinched off
from the mesodermic layer of splanchnopleure. It com-
municates for some time by means of the vitelline duct with
the umbilical vesicle. When cut off from the latter it is a
straight tube, occupying the long axis of the body just in
front of the vertebral column, and is divided into the fore-
gut, hindgut and a central part. Later it communicates above

362 REPRODUCTION

with the pharynx and mouth and opens below upon the ex-
ternal body surface (anus). The liver and pancreas are de-
veloped from protrusions from the sides of the duodenum.

The bladder has been seen to be that part of the allantois
which is constricted off and remains in the body.

The lungs are developed from the esophagus and at first
lie in the abdominal cavity; but the formation of the dia-
phragm fixes them in the thorax.

The kidneys are developed from the Wolffian bodies.
These bodies are 'embryonic structures only. Each is a tube
lying parallel to the vertebral column on either side of it.
This tube consists of a collection of tubules, which unite to
form a common excretory duct. This duct joins the corres-
ponding one from the opposite side to empty into the alimen-
tary canal opposite the allontoic stalk. Outside the Wolffian
bodies are two other ducts, the ducts of Miiller. They also
enter the intestine.

The WolfBan body finally gives place to the kidney, from
which the ureter is developed.

In the female the ducts of Miiller become the tube, uterus
and vagina. In the male they atrophy.

Just behind the Wolffian bodies are developed the ovaries
or the testes, as the case may be.

The development of a few of the organs has thus been
simply referred to.

Satisfactory explanation of these procedures can be given
only in extended works on embryology, and this section may
be closed with the subjoined table of development, which is
abbreviated from one by Heisler:

First Week. — Segmentation and passage of ovum to
uterus.

Second Week. — Ovum in uterus. Decidua reflexa present.
Entoderm and ectoderm layers formed — also mesoderm.
Embryonal area, primitive streak in primitive groove. Cho-

FETAL DEVELOPMENT 363

rion and villi. Amnion folds. Umbilical vesicle partly
formed. Vascular area. Two primitive heart tubes. Gut
tract partly formed.

Third Week. — Body indicated. Dorsal outline concave.
Vitelline duct. Amnion. Allantoic stalk. Visceral arches.
Heart divides. Vitelline circulation begins. Gut tract still
connected with umbilical vesicle. Liver evagination begins.
Anal plate. Pulmonary protrusion. Wolffian bodies. Neu-
ral canal. The brain vesicles. Optic and otic vesicles. Ol-
factory plates. Notochord.

Fourth Week. — Flexion of body. Yolk-sac largest size.
Somites well formed. Allantois grows. Vitelline circulation
complete. Allantoic vessels developing. Pharynx, esopha-
gus, stomach and intestine differentiated. Pancreas begins.
Pulmonary protrusion bifurcates. Ventral roots of spinal
nerves. Limb buds apparent.

Fifth Week. — Umbilical vesicle begins to shrink. Cord
longer and spiral. Length of fetus two-fifths of an inch.
Primitive aorta divides into aorta and pulmonary artery.
Intestine shows loops. Bronchi divided. Ducts of Miiller.
Epidermis. Olfactory lobe. Eyes move forward. Limb
buds segment. Digitation indicated.

Sixth Week. — Umbilical vesicle shrunken. Amnion
larger. Vitelline circulation supplanted by allantoic^ Teeth
indicated. Duodenum, cecum. Rectum. Larynx. Genital
folds and ridges. Dorsal roots of spinal nerves. Eye-lids.
Lower jaw and clavicle begin to ossify. Vertebrae and ribs
cartilaginous. Fingers separate.

Seventh Week. — Body and limbs well defined. Heart
septa complete. Transverse and descending colon. Nails
indicated. Cerebellum indicated. Muscles recognizable.
Ossification in cranium and vertebrae begins.

Eighth Week. — Hiead somewhat elevated. Parotid gland.
Gall bladder. Miillerian ducts unite. Genital groove. Mam-
mary glands begin. Sympathetic nerves. Nose discernible.
Additional centers of ossification.

364 REPRODUCTION

Ninth Week.-^Weight, three-fourths of an ounce.
Length, one and a quarter inches. Pericardium. Anal ca-
nal. External genitals begin to indicate sex. Ovary and
testis distinguishable. Kidney characteristic. External ear
indicated.

Third Month. — Weight, four ounces. Length, two and
three-quarter inches. Chorion frondosum. Placental ves-
sels. Tonsil. Stomach rotates. Vermiform appendix.
Liver large. Epiglottis. Ovaries descend. Testes in false
pelvis. Hair and nails. Development of different parts of
brain. Limbs have definite shape.

Fourth Month. — Weight, seven and three-quarter ounces.
Length, five inches. Head one-fourth of entire body. Germs
of permanent teeth. Distinction of external genitals well
marked. Spinal cord ends at end of coccyx. Eye-lids and
nostrils closed.

Sixth Month. — Weight, two pounds. Length, twelve
inches, Amnion at maximum size. Trypsin in pancreatic se-
cretion. Air vesicles. Eye-lashes. Lobule of ear charac-
teristic.

Seventh Month. — Weight, three pounds. Length, four-
teen inches. Meconium. Ascending colon. Testes at inter-
nal rings. Cerebral convolutions evident. Differentiation
of muscular tissue.

Eighth Month. — Weight, four to five pounds. Length,
sixteen inches. Body more plump. Ascending colon larger.
Testes in inguinal canal. Skin brighter color. Nails pro-
ject beyond finger tips.

Ninth Month. — Weight, six to seven pounds. Length,
twenty inches. Meconium dark green. Testes in scrotum.
Labia majora in contact. Spinal cord ends at last lumbar
vertebra. Ossification centers completed.

INDEX

Abducens nerve, 286
Absorption, from stomach, 81

from intestines, 96
Accommodation, ocular, 315
Adipose tissue, 14

formation of, 178

value of, 178, 179
Adrenal glands, 33
Afferent nerves, 234
Air, amount necessary, 161

alterations of, in lungs, 150

cells, 137

composition of, 148

diffusion of, in lungs, 148

vesicles, 137
Albuminoids, 175
Allantois, 349
Alveoli, capacity of, 147
Ammonia compounds antece-
dents of urea, 203, 204
Amnion, the 347
Amniotic cavity, 348
Amylopsin, 102
Anabolism, 172
Animal heat, 184

loss of, by evaporation, 189

radiation of, 189

relation of, to force, 185

source of, 185

specific, 187

total, 187
Antehelix, 319
Anterior chamber of eye, 313

elastic lamella, 311

fundamental fasciculus, 241

radicular zone, 241
Aphasia, 270
Aqueous humor, 314
Arachnoid, 236
Archenteron, 340
Arteria centralis retinae, 313, 314
Arterial circulation, 47

effect of respiration on, 164

Arteries, 47

elasticity of, 48
Arytenoid cartilages, 133, 325
Asphyxia, 162
Auditory canal, external, 319

center, 325
Auditory nerves, 289

terminations of, 322
Auerbach, plexus of, 97
Auricle, left, 44

right, 44
Axis cylinders, 222

Bacteria in digestion, 120
Bartholin's duct, 72
Bellini, straight tubes of, 197
Bertin, columns of, 195
Bile ducts, 108
Bile, in digestion, 105

functions of, 115

properties and composition

of, no

Bilirubin, in
Binocular vision, 317
Bladder, 207

absorption of, 207

structure of, 207
Blastoderm, 340
Blood, the, 35

alterations of, in lungs, 157

amount in body, 35

arterialization of, 150

coagulation of, 40

color of, 35

composition of, 36

functions of, 35

plasma, 36

platelets, 39

serum, 36
Bone, 16

Bone, marrow, 18
Bowman's capsule, 196
Brain, the, 251

365

366

INDEX

membranes of, 236
Breathing (see Respiration, 131)
Broca's convolution, 270
Bronchi, 136

capacity of, 148
Bronchioles, 137
Burdach, columns of, 247

Capillaries, the, 48
Capillary, importance, 50
Carbohydrates, 176

final products of, 176

value of, in nutrition, 176
Carbon, amount in excreta, 182

dioxide, amount exhaled, 154
amount in bipod, 157
condition of, in blood, 155
discharge, 151
gain of, in lungs, 151
inhalation of, 161
interchange of, in lungs,

156
source of, exhaled, 154

monoxide, inhalation of, 161
Cardiac cycle, 43

length of, 43
Cartilage, 15

Hyaline, 15

white fibrous, 15, 16

yellow elastic, 15
Cauda equina, 238
Cecum, 117
Celenteron, 340
Celom, 344
Cell, i

properties of, 3

structure of, 2
Centrifugal nerves, 230
Centripetal nerves, 230
Cerebellum, the, 274

anatomy of, 274

fibers of, 274

function of, 275

peduncles of, 274
Cerebral localization, 268
Cerebro-spinal axis, 235

system, 216
Cerebrum, the, 260

cells of, 263

Cerebrum, convolutions of, 262,
263

fibers of, 263

fissures of, 260

functions of, 271

lobes of, 260

motor centers in, 268
paths from, 268

sensory centers in, 269
paths to, 269, 270

special centers in, 269
Cerumen, 30
Cervical ganglia, 299
Cholesterin, in
Chorda dorsalis, 344

tympani, 288
Chordal folds, 345

plates, 344
Chorion, 351
Choroid coat of eye, 311
Chyme, 100
Ciliary muscle, 312

processes, 312
Circulation, the, 41

pulmonic, 41

systemic, 41

Circumvallate papillae, 318
Claustrum, 258
Cochlea, 322, 323
Colloids, 123
Colon, 117
Colostrum, 31
Common bile duct, 109
Complemental air, 147
Conjunctiva, 311
Connective tissue, n
Convoluted tubules, 197
Cornea, 311
Corona radiata, 258
Corpora quadrigemina, 259

striata, 256, 258
Corpus luteum, 332
Corti, rods of, 323
Coughing, 145
Cranial nerves, 276
Creatin, 205
Cricoid cartilage, 133.
Crossed pyramidal tracts, 245
Crura cerebri, 256

INDEX

367

Crystalloids, 123
Cutaneous respiration, 160

sensations, center for, 269
Cutis vera, 210
Cystic duct, 109

Death, 172

Decidua menstrualis, 352

of pregnancy, 352
Defecation, 121
Deglutition, 78

mechanism of, 79

nervous control of, 80
Dendrites, 216, 222
Descemet, membrane of, 311
Descendens hypoglossi, 295
Diet, amount of, 183

determination of, 181

necessary constituents of, 182
Dietetics, 181
Diffusion in lungs, 148
Digestion, 68

gastric, 81

intestinal, 96

object of, 68

processes in, 70
Direct cerebellar tract, 241
Discus proligerus, 331
Dreams, 302
Ductus arteriosus, 359

communis choledochus, 109

venosus, 359
Dura mater, 236
Dyspnea, 162

Ear, the, 318

drum, 320

external, 319

internal, 320

middle, 320
Ectoderm, 339
Efferent nerves, 231
Eighth nerve (see Auditory, 289)
Elasticity of arteries, 48
Electrical stimulation of nerves,

234
Elementary tissues, 7

derivation of, 7

varieties of, 8

Eleventh nerve (see Spinal acces-
sory, 294)

Embryonal area, 341
Encephalon (see Brain, 251)
Endocardium, 42
Entoderm, 340
Enzymes, 69

characteristics of, 69

classification of, 69

manner of action of, 70
Epiblast, 339
Epidermis, 208
Epiglottis, 135
Epinephrine, 34
Epithelial tissue, 7

ciliated, 9

columnar, 9

glandular, II

modified, 9

neuro, n

squamous, 8

stratified, 8

varieties of, 8
Esophagus, 79
Eupenea, 162
Eustachian valve, 359
Excretion, 192
Expiration, 143

causes of, 143

forced, 144

effect of, in blood pressure,

166

Expired air, composition of, 151
External capsule, 258
Eye-ball, anatomy of, 311

movements of, 309

protection of, 308

Facial nerve, 286
Fats, as foods, 66

end products of, 177
Fauces, 78

Feces, composition of, 120
Fecundation, 337
Ferrein, pyramids of, 195
Fertilization, 337
Fetal membranes, 347
Fibrin, 40
Fibrous tissue, 13

368

INDEX

Fifth nerve (see Trifacial, 281)
Filum terminate, 238
First nerve (see Olfactory, 276)
Foods, 63

classification of, 64

fate of in body, 173

how absorbed, 128

potential energy of, 185

where absorbed, 128
Fourth nerve (see Patheticus,
280)

ventricle, 253
Fovea centralis, 313, 314
Fungiform papillae, 318
Funiculi graciles, 252

of Rolando, 252

Gall bladder, 109
Gases in intestine, 127
Gastric glands, cells of, 85
nerve supply of, 89
structure of, 85
varieties of, 85

juices, action of on foods, 92
properties and composition

of, 90

secretion of, 87
Gastrula, 340
Glands, 27

adrenal, 33

agminate, 100

intestinal, 96

gastric, 85

mammary, 31

of Brunner, 99

of Lieberkuhn, 99

parotid, 72

salivary, 71

sebaceous, 30

secretion in, 29

solitary, TOO

sublingual, 72

submaxillary, 72

sweat, 210

thyroid, 32
Glisson's capsule, 105
Glomeruli, renal, 195
Glosso-pharyngeal nerve, 289
Glycocholic acid, in

Glycogen, formation of in liver,

112

Golgi, corpuscles of, 230
Goll, column of, 247
Gustatory center, 269
Graafian follicles, 330

Hairs, 210
Haversian canals, 17

systems, 18
Hawking, 145
Hearing, sense of, 318
Heart, anatomy of, 42

beats of, 43

contractions of, 43

development of, 356

diastole of, 43

innervation of, 46

sounds of, 46

systole of, 43

valves and openings of, 44

work of, 45
Heat of body (see Animal heat,

184)

Hemoglobin, 38
Henle, loops of, 197

sheath of, 220
Hepatic artery, 105
Hiccough, 145
Hippuric acid, 205
Hunger, seat of, 64
Hydrochloric acid, 90
Hydrogen, inhalation of, 161
Hyperopia, 316
Hyperpnea, 162
Hypoblast, 339
Hypoglossal nerve, 295
Hypoxanthin, 205

Ileo-cecal valve, 118
Impregnation, 337
Incus, 320
Infundibula, 137
Innervation of vessels, 50, 51
Inspiration, 141

causes of, 142

effects of on blood pressure.
164

muscles of, 143

INDEX

369

Inspired air, composition of, 151
Interlobular veins, 106
Internal capsule, 259

respiration, 159
Intestinal glands, 99
Intestine, digestion in, 96

divisions of, 97

movements of, 121

nerve supply of, 117

structure of, 119
Intralobular veins, 106
Intrapuimonary pressure, 146
Intrathoracic pressure, 146
Iris, the, 312
Katabolism, 172
Kidney, blood supply of, 199

structure of, 192
Kinetic energy, 186
Krause, end bulbs of, 228

Labyrinth, bony, 321

membranous, 322
Lacrymal apparatus, 309

duct, 309

glands, 309

sac, 309

Lactates, discharge of, 205
Lacteals, 98
Large intestine, 117

digestive changes in, 119

divisions of, 117

movements of, 121

structure of, 119
Larynx, 133, 325

nerve supply of, 327
Laughing, 145
Lenticular ganglion, 284
Leukocytes (see White cor-

T • u i t»scles> 39)
Lieberkuhn, crypts of, 99
Liquor amnii, 348

sanguinis (see Blood plasma

36)
Liver, anatomy of, 104

histology of, 108

lymphatics of, no

nerve supply of, no

vessels of, 105
Lumbar ganglia, 299

Lungs, 138

capacity of, 147
Lymphatic glands, 59
Lymph, 57

course of, 57

flow of, 61

properties and composition

of, 59
Lymph vessels, origin of, 57

Macula lutea, 313
Malleus, 320
Malpighian bodies, 195

pyramids, 195
Mammary glands, 31
Mastication, 71
Maturation, 32
Meckel's ganglion, 284
Medulla oblpngata, 251
centers in, 254, 255
functions of, 254
gray matter of, 253
pyramids of, 251
relation of cord tracts to, 253
white fibers of, 253
Meissner, corpuscles of, 229

plexus of, 97
Membrana tympani, 320
Menstruation, 335
Mesoblast, 340
Mesoderm, 341
Metabolism, 172

conditions influencing, 179
Micturition, 208

center for, 208
Milk, human, 32
Mitral valve, 44
Mixed lateral column, 240, 242
Motor oculi communis, 278

paths from cerebrum, 266
Muscular contractions, 23

physiological characteristics,

tissues, 19
Myopia, 316

Nails, the, 210
Nasal duct, 309

370

INDEX

Nerve cells, 221
centers, 221
fibers, 217
action of electricity upon,

234

afferent, 232
classification of, 230
degeneration of, 239
directions of currents in,

233

efferent, 231
individuality of, 221
medullated, 217
non-medullated, 219
properties of, 230
speed of conduction in, 234
terminals, 225
between epithelial cells, 227
in bulbs of Krause, 228
in Golgi's corpuscles, 230
in glands, 226
in hair-follicles, 226
in Meissner's corpuscles,

229

in Pacinian corpuscles, 227
in plain muscle, 226
Nerve terminals, in striped mus-
cle, 225

in tactile menisques, 229
trunks, 219

Nervous system, the, 214
development of, 361
divisions of, 216
general functions of, 214
Neural canal, 343
Neuroglia, 216
Neurons, 216

communication between, 223
Ninth nerve (see Glosso-pharyn-

geal, 289)

Nitrogen, amount necessary, 182
Nitrogenous equilibrium, 174
Nitrous oxide, inhalation of, 161
Nutrition, 171

Olfactory bulb, 277
cells, 276
center, 269
nerve, 276

Olivary bodies, 251
Omphalo-mesenteric vessels, 356
Ophthalmic ganglion, 284
Optic center, 269

commissure, 277

nerve, 277

thalami, 258

functions of, 258

tracts, 277
Osmosis, 123
Otic ganglion, 284
Ova, 329

Ovary, secretion of, 34
Ovulation, 334
Oxidation in the body, 171
Oxygen, amount consumed, 154

amount in blood, 157

condition of in blood, 157

entrance of into tissues, 159

loss of in lungs, 148

Pacini, corpuscles of, 227
Pancreas, anatomy of, 100

histology of, 100

internal secretion of, 103

nerve supply of, 103

secretion in, 103
Pancreatic juice, 101
Partial pressure of gases, 156
Patheticus nerve, 280
Pepsin, 91
Peptones, 92
Pericardium, 42
Periosteum, 18
Perspiration, 212
.Pharynx, 132
Pia mater, 237
Pinna, 319
Pituitary body, 34
Placenta, 354
Placental circulation, 357
Pneumogastric. nerve, 290

influence of on respiration,

168
stomach and intestines,

168
Pons Varolii, 255

functions of, 255
Posterior chamber of eye, 313

INDEX

371

Prehension, 70
Presbyopia, 316
Pronucleus, female, 334

male, 337
Proteids as foods, 67

circulating, 174

final products of, 174

tissue, 174
Proteoses, 92
Protovertebrse, 345
Ptyalin, 74
Pulse, the, 53
Pupil, the, 313

Ranvier, nodes of, 218
Reaction of pupil, 316
Receptaculum chyli, 57
Rectum, 118
Red corpuscles, 37
Reflex action, 248
Refraction, ocular, 314
Reil, island of, 263
Renal tubules, 197
Rennin, 91
Reproduction, 328
Reserve air, 147
Residual air, 147
Respiration, 131

abnormal, 161

afferent nerves of, 167

center for, 167

costal, 146

cutaneous, 160
Respiration, diaphragmatic, 146

effect of, on blood pressure,

164
, Efferent nerves of, 170

external, 132

internal, 131, 158

mechanism of, 139

modified, 145

nervous control of, 166

object of, 131

organs of, 132

rate of, 145

rhythm of, 144

sounds of, 145

types of, 146
Restiform" "bodies, 251

Retina, 313

Rolando, fissures of, 260

Saliva, functions of, 71

properties and composition

of, 74
Salivary glands, 71

histology of, 72

nerve supply of, 74

secretion in, 76
Salts, 65
Schwann, sheath of, 217

white substance of, 218
Sclerotic coat of eyes, 311
Sebaceous glands, 30
Second nerve {see Optic nerve,

. 277)
Secretion, 27

external, 29

internal, 29

paralytic, 76
Segmentation, 338

cavity 338

nucleus, 338

Semicircular canals, 322
Semilunar valves, 44
Sensations, common, 305

special, 306
Senses, the, 305
Serum-albumin, 36

-globulin, 36

Seventh nerve (see Facial, 286)
Sighing, 145
Sight, sense of, 308
Sigmoid flexure, 117
Sixth nerve (see Ab due ens, 286)
Skin, excretion of, 208

functions of, 208

structure of, 210
Sleep. 301

vascular phenomena of, 302
Smegma, 30
Smell, sense of, 307
Sneezing, 145
Snoring, 145
Sobbing, 145
Sodium salts, 65

functions of, 65
Solar plexus, 299

372

INDEX

Somatopleure, 341
Somites, 345
Speech, 325, 327
Spermatozoa, 328
Spheno-palatine ganglion, 284
Spinal accessory nerve, 294
cord, 238

columns of, 240
commissures of, 238
cross section of, 238
degeneration in, 240
functions of, 248
gray matter in, 238
motor paths in, 243
sensory paths in, 245
special centers in, 251
nerves, 296

Splanchnic nerves, 299
Splanchnopleure, 34*
Starvation, effects of, 180
Steapsin, 102
Stenson's duct, 72
Stercorin, in
Stomach, the, 81
histology of, 83
movements of, 94
nervous supply of, 96
Straight tubules (renal), 197
Striated muscle, 19

characteristics of, 19
Stapes, the, 320
Sublobular veins, 106
Submaxillary ganglion, 284
Succus entericus, 116
Supplemental air, 147
Suspensory ligament, 314
Sweat glands, 210
Sweat, properties, composition

of, 212

secretion of, 212
Sylvius, aqueduct of, 253

fissures of, 267 •
Sympathetic system, 216
Syntonin, 92

Tactile sensibility, 306

acuteness of, 307
Taste beakers, 318

sense of, 3J7

Taurocholic acid, in
Temperature impressions, 307

of body, 184
Tenon, capsule of, 309
Tenth nerve (see Pneumo gastric,

290)

Testes, secretion of, 34
Thermogenesis, 188
Thermotaxis, 190
Third nerve (see Motor oculi

communis, 278)
Thirst, seat of, 64
Thoracic duct, 57

ganglia, 299
Thorax, 139.

Thyroid cartilage, 133
gland, 32

Tidal air, 147

Touch, sense of, 305, 306

Trachea, 135

Tragus, 319

Tricuspid valve, 44

Trifacial nerve, 281

Trigeminal nerve, 281

Trypsin, 102

Turck, column of, 245

Twelfth nerve (see Hypoglossal,

295)
Tympanum, 320

Umbilical cord, 355

vesicle, 347
Urea, 202

daily discharge of, 203

formation of, 203
Ureters, 206
Uric acid, 204

daily discharge of, 204
Urine, constituents of, 202

discharge of, 206

properties of, 202

salts of, 206

secretion of, 199

variations in amount of, 200

in composition of, 206
Uriniferous tubules, 197

secretory changes in, 202
Vaginal plexus, 105

INDEX

373

Vagus nerve (see Pneumo gastric,

290)

Valvulse conniventes, 97
VasQ-motor nerves, 51

centers for, 51
Vater, corpuscles of, 227
Veins, 48

valves of, 50
Ventilation, 160
Ventricle, left, 44

right, 44

Vermiform appendix, 118
Vestibule of ear, 321
Villi, 98
Vitelline, circulation, 356

duct, 345 '
Vitreous humor, 314

Vocal cords, 133, 325

sounds, varieties of, 326
Voice, production of, 325

Water, 65

elimination of by kidney, 201
Wharton's duct, 72
White corpuscles, 39
Wirsung, duct of, 100
Wolffian bodies, 362

Xanthin, discharge of, 205
Yawning, 145
Zymogen, 101

MOLOGY

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